Compositing Semiconductor Photocatalysts and Their Microstructure Modulation

International Journal of Photoenergy

Guest Editors: Baibiao Huang, Stéphane Jobic, Xuxu Wang, and Weifeng Yao Compositing Semiconductor Photocatalysts and Their Microstructure Modulation International Journal of Photoenergy

Compositing Semiconductor Photocatalysts and Their Microstructure Modulation

This is a special issue published in “International Journal of Photoenergy.” All articles are open access articles distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Editorial Board

Mohamed Abdel-Mottaleb, Egypt Shahed Khan, USA Avigdor Scherz, Israel Nihal Ahmad, USA Cooper H. Langford, Canada Lukas Schmidt-Mende, Germany Nicolas Alonso-Vante, France Yuexiang Li, China Panagiotis Smirniotis, USA Wayne A. Anderson, USA Stefan Lis, Poland Zoﬁa Stasicka, Poland Vincenzo Augugliaro, Italy Niyaz M. Mahmoodi, Iran Juliusz Sworakowski, Poland Detlef W. Bahnemann, Germany Dionissios Mantzavinos, Greece Nobuyuki Tamaoki, Japan Mohammad A. Behnajady, Iran Ugo Mazzucato, Italy Gopal N. Tiwari, India Ignazio Renato Bellobono, Italy Jacek Miller, Poland Nikolai V. Tkachenko, Finland Raghu N. Bhattacharya, USA Kazuhiko Mizuno, Japan Veronica Vaida, USA Gion Calzaferri, Switzerland Jarugu N. Moorthy, India Roel van De Krol, The Netherlands Adriana G. Casas, Argentina Franca Morazzoni, Italy Mark van Der Auweraer, Belgium Wonyong Choi, Korea Fabrice Morlet-Savary, France Man Shing Wong, Hong Kong Veraˇ Cimrova, Czech Republic Ebinazar B. Namdas, Australia David Worrall, UK Vikram L. Dalal, USA Maria da Grac¸a P.Neves, Portugal F. Yakuphanoglu, Turkey Dionysios D. Dionysiou, USA Leonidas Palilis, Greece Minjoong Yoon, Korea Mahmoud M. El-Nahass, Egypt Leonardo Palmisano, Italy Hongtao Yu, USA Ahmed Ennaoui, Germany Ravindra K. Pandey, USA Jimmy C. Yu, Hong Kong David Ginley, USA David Lee Phillips, Hong Kong Klaas Zachariasse, Germany Beverley Glass, Australia Pierre Pichat, France Lizhi Zhang, China Juozas Grazulevicius, Lithuania Xie Quan, China Jincai Zhao, China Shinya Higashimoto, Japan Tijana Rajh, USA Yadong Jiang, China Peter Robertson, UK Contents

Compositing Semiconductor Photocatalysts and Their Microstructure Modulation, Baibiao Huang, Stephane´ Jobic, Xuxu Wang, and Weifeng Yao Volume 2012, Article ID 595021, 4 pages

Nitrogen Incorporation in TiO2: Does It Make a Visible Light Photo-Active Material?, B. Viswanathan and K. R. Krishanmurthy Volume 2012, Article ID 269654, 10 pages

Simultaneous Elimination of Formaldehyde and Ozone Byproduct Using Noble Metal Modiﬁed TiO2 Films in the Gaseous VUV Photocatalysis, Pingfeng Fu, Pengyi Zhang, and Jia Li Volume 2012, Article ID 174862, 8 pages

Synthesis of Neutral SiO2/TiO2 Hydrosol and Its Application as Antireﬂective Self-Cleaning Thin Film, Chiahung Huang, Hsunling Bai, Yaoling Huang, Shuling Liu, Shaoi Yen, and Yaohsuan Tseng Volume 2012, Article ID 620764, 8 pages

Enhanced Photocatalytic Activity of Hierarchical Macro-Mesoporous Anatase by ZrO2 Incorporation, M. L. Garc´ıa-Benjume, M. I. Espitia-Cabrera, and M. E. Contreras-Garc´ıa Volume 2012, Article ID 609561, 10 pages

Doped Titanium Dioxide Films Prepared by Pulsed Laser Deposition Method, Juguang Hu, Huabin Tang, Xiaodong Lin, Zhongkuan Luo, Huiqun Cao, Qiwen Li, Yi Liu, Jinghua Long, and Pei Wang Volume 2012, Article ID 758539, 8 pages

Solar Photocatalytic Removal of Chemical and Bacterial Pollutants from Water Using Pt/TiO2-Coated Ceramic Tiles, S. P. Devipriya, Suguna Yesodharan, and E. P. Yesodharan Volume 2012, Article ID 970474, 8 pages

Synthesis and Photocatalytic Activity of TiOX PowderswithDiﬀerent Oxygen Defects, Leini Wang, Fei Lu, and Fanming Meng Volume 2012, Article ID 208987, 7 pages

The Synthesis of Anatase Nanoparticles and the Preparation of Photocatalytically Active Coatings Based on Wet Chemical Methods for Self-Cleaning Applications,DejanVerhovsek,˘ Nika Veronovski, Urska˘ Lavrenci˘ c˘ Stangar,˘ Marko Kete, Kristina Zagar,˘ and Miran Ceh˘ Volume 2012, Article ID 329796, 10 pages

High-Eﬃciently Photoelectrochemical Hydrogen Production over Zn-Incorporated TiO2 Nanotubes, Gayoung Lee, Min-Kyeong Yeo, Myeong-Heon Um, and Misook Kang Volume 2012, Article ID 843042, 10 pages

Statistical Optimization of Operational Parameters for Enhanced Naphthalene Degradation by TiO2/Fe3O4-SiO2 Photocatalyst, Aijuan Zhou, Jing Peng, Zhaobo Chen, Jingwen Du, Zechong Guo, Nanqi Ren, and Aijie Wang Volume 2012, Article ID 607283, 9 pages

Photoetching of Immobilized TiO2-ENR50-PVC Composite for Improved Photocatalytic Activity, M.A.Nawi,Y.S.Ngoh,andS.M.Zain Volume 2012, Article ID 859294, 12 pages Synthesis, Characterization, and Photocatalysis of Fe-Doped TiO2: A Combined Experimental and Theoretical Study, Liping Wen, Baoshun Liu, Xiujian Zhao, Kazuya Nakata, Taketoshi Murakami, and Akira Fujishima Volume 2012, Article ID 368750, 10 pages

Enhanced Photoactivity of Fe + N Codoped Anatase-Rutile TiO2 Nanowire Film under Visible Light Irradiation, Kewei Li, Haiying Wang, Chunxu Pan, Jianhong Wei, Rui Xiong, and Jing Shi Volume 2012, Article ID 398508, 8 pages

Enhanced Visible Light Photocatalytic Activity of V2O5 Cluster Modiﬁed N-Doped TiO2 for Degradation of Toluene in Air, Fan Dong, Yanjuan Sun, and Min Fu Volume 2012, Article ID 569716, 10 pages

Preparation and Characterization of Visible-Light-Activated Fe-N Co-Doped TiO2 and Its Photocatalytic Inactivation Eﬀect on Leukemia Tumors, Kangqiang Huang, Li Chen, Jianwen Xiong, and Meixiang Liao Volume 2012, Article ID 631435, 9 pages

Synthesis and Characterization of Iron Oxide Nanoparticles and Applications in the Removal of Heavy Metals from Industrial Wastewater, Zuolian Cheng, Annie Lai Kuan Tan, Yong Tao, Dan Shan, Kok Eng Ting, and Xi Jiang Yin Volume 2012, Article ID 608298, 5 pages

Nitrogen-Doped TiO2 Nanotube Arrays with Enhanced Photoelectrochemical Property, Shipu Li, Shiwei Lin, Jianjun Liao, Nengqian Pan, Danhong Li, and Jianbao Li Volume 2012, Article ID 794207, 7 pages

Photocatalytic Activity of Titanate Nanotube Powders in a Hybrid Pollution Control System, Sun-Jae Kim, Young-Seak Lee, Byung Hoon Kim, Seong-Gyu Seo, Sung Hoon Park, and Sang Chul Jung Volume 2012, Article ID 901907, 6 pages

Synthesis of Core-Shell Fe3O4@SiO2@TiO2 Microspheres and Their Application as Recyclable Photocatalysts, Zhenghua Wang, Ling Shen, and Shiyu Zhu Volume 2012, Article ID 202519, 6 pages

Enhanced Hydrogen Production over C-Doped CdO Photocatalyst in Na2S/Na2SO3 Solution under Visible Light Irradiation, Quan Gu, Huaqiang Zhuang, Jinlin Long, Xiaohan An, Huan Lin, Huaxiang Lin, and Xuxu Wang Volume 2012, Article ID 857345, 7 pages

Synthesis of Hollow CdS-TiO2 Microspheres with Enhanced Visible-Light Photocatalytic Activity, Yuning Huo, Jia Zhang, Xiaofang Chen, and Hexing Li Volume 2012, Article ID 907290, 5 pages

Eﬀects of Calcination Temperature on Preparation of Boron-Doped TiO2 by Sol-Gel Method, Wenjie Zhang, Bo Yang, and Jinlei Chen Volume 2012, Article ID 528637, 8 pages Synthesis, Characterization, and Photocatalytic Activity of TiO2 Microspheres Functionalized with Porphyrin, Jin-Hua Cai, Jin-Wang Huang, Han-Cheng Yu, and Liang-Nian Ji Volume 2012, Article ID 348292, 10 pages

Preparation and Characterization of (I2)n Sensitized Nanoporous TiO2 with Enhanced Photocatalytic Activity under Visible-Light Irradiation, Juzheng Zhang, Xin Liu, Shanmin Gao, Quanwen Liu, Baibiao Huang, and Ying Dai Volume 2012, Article ID 536194, 8 pages

Facile Low-Temperature Synthesis of Carbon Nanotube/TiO2 Nanohybrids with Enhanced Visible-Light-Driven Photocatalytic Activity, Yunlong Xie, Huanhuan Qian, Yijun Zhong, Hangming Guo, and Yong Hu Volume 2012, Article ID 682138, 6 pages

Preparation, Characterization, and Activity Evaluation of CuO/F-TiO2 Photocatalyst, Zhang Jinfeng, Yang Yunguang, and Liu Wei Volume 2012, Article ID 139739, 9 pages

Facile Preparation and Photoinduced Superhydrophilicity of Highly Ordered Sodium-Free Titanate Nanotube Films by Electrophoretic Deposition, Minghua Zhou and Huogen Yu Volume 2012, Article ID 830321, 6 pages

Light-Driven Preparation, Microstructure, and Visible-Light Photocatalytic Property of Porous Carbon-Doped TiO2, Xiao-Xin Zou, Guo-Dong Li, Jun Zhao, Juan Su, Xiao Wei, Kai-Xue Wang, Yu-Ning Wang, and Jie-Sheng Chen Volume 2012, Article ID 720183, 9 pages

One-Pot Template-Free Hydrothermal Synthesis of Monoclinic BiVO4 Hollow Microspheres and Their Enhanced Visible-Light Photocatalytic Activity, Bei Cheng, Wenguang Wang, Lei Shi, Jun Zhang, Jingrun Ran, and Huogen Yu Volume 2012, Article ID 797968, 10 pages Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 595021, 4 pages doi:10.1155/2012/595021

Editorial Compositing Semiconductor Photocatalysts and Their Microstructure Modulation

Baibiao Huang,1 Stephane´ Jobic,2 Xuxu Wang,3 and Weifeng Yao4

1 State Key Lab of Crystal Materials, Shandong University, Jinan 250100, China 2 Institut des Mat´eriaux Jean Rouxel, Universit´e de Nantes, BP 32229, 44322 Nantes Cedex 3, France 3 Research Institute of Photocatalysis, Fuzhou University, Fuzhou 350002, China 4 College of Energy and Environmental Engineering, Shanghai University of Electric Power, Shanghai 200090, China

Correspondence should be addressed to Baibiao Huang, [email protected]

Received 17 July 2012; Accepted 17 July 2012

Copyright © 2012 Baibiao Huang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Photocatalytic materials have attracted increasing interests The special issue contains twenty-nine papers, which owing to the potential applications on solving global mainly focused on the synthesis, microstructure modulation, energy and environmental problems. However, the rapid and practical applications of compositing semiconductor recombination of photogenerated charge carriers and the photocatalysts. Among them eight papers are dealing with limited spectral response range greatly restricted their further hetero-structured composite photocatalysts. Five papers are development and practical applications. Compositing semi- related to modified photocatalysts with noble metal or other conductor photocatalysts, by combining semiconductors sensitizing materials. Eleven papers are regarding the doping with other materials, can not only effectively expand the or co-doping of wide band gap semiconductor photocata- visible-light absorption, but also improve the photogen- lysts. Finally, five papers addresses the applications of pho- erated charge carriers’ separation, which would effectively tocatalysts on solving environmental and energy problems. solve the above problems and improve the photocatalytic In the paper entitled “One-pot template-free hydrothermal activities. Moreover, it also provides a way to develop an synthesis of monoclinic BiVO4 hollow microspheres and their efficient photocatalyst by tailoring the band gaps and band enhanced visible-light photocatalytic activity,” B. Cheng et al. positions to meet the requirements for specific applications. present a one-pot template-free hydrothermal synthesis of The microstructures of photocatalysts also greatly affect their BiVO4 hollow microspheres via a localized Ostwald ripening photocatalytic performances. Therefore, it has become a hot mechanism. The reaction duration and urea concentration topic in photocatalysis on the synthesis, photocatalytic char- played important roles in the formation of BiVO4 hollow acterization, and microstructure modulation of compositing microspheres. And photocatalytic properties of BiVO4 under semiconductor photocatalysts. visible-light irradiation were also discussed. The selected topics and papers in this special issue In the paper entitled “Preparation, characterization, and presented the recent progresses on the synthesis of composit- activity evaluation of CuO/F-TiO2 photocatalyst,”W.Liu ing semiconductor photocatalysts, as well as the practical et al. present the synthesis of CuO/F-TiO2 photocatalysts by applications on solving energy and environmental problems. ball milling process. The light absorption range of CuO/F- Limited by the numbers of the papers,they cannot cover TiO2 was effectively expanded comparing to pure TiO2.And the whole areas in photocatalysis, but they also provide rich the photocatalytic reduction activity was greatly improved information and knowledge that we would like to share with by increasing the amount of doped p-CuO in CuO/F- the readers. Moreover, we would like to thank the authors TiO2 composites. The effects of ball milling time and the for their excellent contributions and patience in assisting us. photocatalytic mechanism were also discussed. Finally, the fundamental work of all reviewers on the special In the paper entitled “Simultaneous elimination of formal- issue is also warmly acknowledged. dehyde and ozone byproduct using noble metal modified TiO2 2 International Journal of Photoenergy

films in the gaseous VUV photocatalysis,”P.Y.Zhangetal. indicated the photocatalytic activities were significantly present asystematical investigations on the removal of low enhanced in presence of APEMPP-MPS-TiO2 compared concentration formaldehyde and ozone byproduct in a with the nonmodified TiO2. gaseous VUV (vacuum ultraviolet) photocatalytic system In the paper entitled “Synthesis, characterization, and by using noble metal modified TiO2 films. Metallic Pt or photocatalysis of Fe-doped TiO2: a combined experimental and + − Au could reduce the recombination of h /e pairs, thus, theoretical study,”B.S.Liuetal.preparedFe-dopedTiO2 simultaneously increase the elimination of HCHO and nanoparticles using hydrothermal method. The photocat- ozone, but PdO oxide seemed to inhibit the HCHO oxidation alytic activity of as-prepared Fe-doped TiO2 first increases in the UV254nm photocatalysis. And the effects of O3 on the and then decreases as the Fe concentration increases. And decomposition of HCHO and ozone under VUV were also the different effects of bulk dopant and surface dopant on investigated. photocatalytic activity were investigated systematically. In the paper entitled “Synthesis of core-shell Fe3O4@ In the paper entitled “Enhanced photoactivity of Fe + N SiO2@TiO2 microspheres and their application as recyclable Co-doped anatase-rutile TiO2 nanowire film under visible- photocatalysts,” Z. H. Wang et al. report the fabrication light irradiation,” R. Xiong et al. synthesized Fe + N codoped of Fe3O4@SiO2@TiO2 core-shell microspheres through a rutile-anatase mix-phase TiO2 nanowires by a two-step wet-chemical approach. The TiO2 nanoparticles on the anodic oxidation method. The co doping with nitrogen surfaces of microspheres can degrade organic dyes under and iron could enhance the visible-light absorption and the illumination of UV light, and the samples can be easily improved the photocatalytic activity of TiO2 nanowires. The separated from the solution after the photocatalytic process reasons for the increase of photocatalytic activity in the Fe + due to the ferromagnetic Fe3O4 core, which provided a way N codoped sample were also discussed. to recycle the photocatalysts for practical applications. In the paper entitled “Effects of calcination temperature In the paper entitled “Synthesis and photocatalytic activ- on preparation of boron-doped TiO2 by sol-gel method,”W. ity of TiOx powder with different oxygen defects,”F.M. J. Zhang et al. present the synthesis of boron-doped TiO2 Meng et al. reported the synthesis of TiOx powders by by a modified sol-gel method. The effects of calcination mechanochemical technique and heating process. Carbon temperature on the properties of the boron-doped TiO2 were and chromium were used to control the degree of oxygen investigated. The photocatalytic activity of the samples were vacancies in TiOx by doping into the TiOx crystal matrix. The examined by degradation of methyl orange and the sample photocatalytic measurements indicate that the TiOx sample with the optimal activity was obtained after being calcinated ◦ with 66.93% mass fraction of Ti7O13 is the most efficient at 400 C. on the degradation of MO dye. Moreover, the origin for In the paper entitled “Preparation and characterization ff the visible-light absorption and the e ect of band gap on of (I2)n sensitized nanoporous TiO2 with enhanced photo- photocatalytic activities were also discussed in this paper. catalytic activity under visible-light irradiation,”S.M.Gao In the paper entitled “Nitrogen incorporation in TiO2— et al. present an unique template-free synthetic strategy to does it make a visible-light photoactive material?,”B.Viswa- prepare(I2)n sensitized nanoporous TiO2.I2 hydrosol is used nathan et al. examined the state and location of nitrogen as nucleation center and sensitizer, while TiCl4 was used as doped in TiO2 lattice and the optical absorption induced by the precursor for TiO2 and the calcination temperature has nitrogen doping. They found that the surface of N-doped acrucial role in the amount of (I2)n sensitized nanoporous TiO2 adopted a non-native configuration, while the bulk TiO2. The as-prepared samples shows enhanced photocat- material was still in the native configuration of pure TiO2. alytic activity in visible region and the possible mechanism Though N-doped TiO2 showed visible-light response, the for the enhanced photocatalytic activity was also proposed. improvement on photocatalytic activity is only marginal in In the paper entitled “Enhanced visible-light photocat- most of the cases. alytic activity of V2O5 cluster modified N-doped TiO2 for In the paper entitled “The synthesis of anatase nanopar- degradation of toluene in air,”F.Dongetal.reported ticles and the preparation of photocatalytically active coating the fabrication of V2O5 cluster modified N-doped TiO2 based on wet chemical methods for self-cleaning applications,” nanocomposites photocatalyst. The conduction band (CB) D. Verhovsekˇ et al. reported an improved sol-gel method for potential of V2O5 is lower than the CB level of N-doped TiO2, producing highly photocatalytic anatase TiO2 nanoparticles which favors the photogenerated electron transfer from CB and tested the coating for self-cleaning glass and ceramic of N-doped TiO2 to V2O5 clusters. This helps promote the surfaces. Both the sol-gel synthesis and the coating prepa- transfer and separation of photogenerated; thus, the visible- ration are based on a wet chemical process, which provided light activity of N-doped TiO2 was significantly enhanced by an economic, facile, and non-toxic process for numerous loading V2O5 clusters. applications. In the paper entitled “Statistical optimization of opera- In the paper entitled “Synthesis, characterization, and tional parameters for enhanced naphthalene degradation by photocatalytic activity of TiO2 microspheres functionalized TiO2/Fe3O4-SiO2 Photocatalyst,”A.J.Wangetal.opti- with phorphyin”, J.-H.Cai et al. reported a way to expand mize the operational parameters for enhanced naphthalene the visible-light absorption of TiO2 by loading 5-(4-allylox- degradation by TiO2/Fe3O4-SiO2 (TFS) photocatalyst using y)phenyl-10,15,20-tri(4-methylphenyl)porphyrin (APTMPP) statistical experimental design and analysis. The central on TiO2 surfaces. The photocatalytic experiment by the composite design method was adopted and the experi- oxidation of α-terpinene under visible-light irradiation mental results showed that irradiation time, pH, and TFS International Journal of Photoenergy 3 photocatalyst loading had significant influence on naphtha- transmission auto-recorder method to evaluate the photo- lene degradation. catalytic properties. The results indicated the as-prepared In the paper entitled “Synthesis and characterization N-doped TiO2 film exhibits high photocatalytic activity on of iron oxide nanoparticles and applications in the removal decomposing MO dye under visible-light irradiation. of heavy metals from industrial wastewater,”Z.L.Cheng In the paper entitled “Nitrogen-doped TiO2 nanotube et al. synthesized maghemite (γ-Fe2O3) nanoparticles of arrays with enhanced photoelectrochemical property,”S.W. 60 nm using a co-precipitation method and investigated the Lin et al. report a fabrication of N-doped TiO2 nanotube applicability of maghemite nanoparticles for the removal (NTN) arrays by electrochemical anodization in glycerol of Pb2+ from electroplating wastewater. This study showed electrolyte. Systemic studies are done in electrochemical that the prepared γ-Fe2O3 nanoparticles could be used as anodization process. And the prepared N-doped NTN arrays an alternate to the conventional adsorbents for the removal exhibit high visible-light-driven photocatalytic activity. of metal ions from wastewater with high removal efficiency In the paper entitled “Facile preparation and photoin- within a very short time. duced superhydrophilicity of highly ordered sodium-free titan- In the paper entitled “Enhanced photocatalytic activity of ate nanotube films by electrophoretic deposition,” H. G. Yu hierarchical marcro-meso porous anatase by ZrO2 incorpora- et al. prepared highly ordered sodium-free titanate nanotube tion,”M.E.Contreras-Garc´ıa et al. fabricated TiO2/ZrO2 films by one-step preparation on F-doped SnO2-coated macro-mesoporous nanostructure by sol-gel hydrothermal (FTO) glass via an electrophoretic deposition method by method. The mesoporous sizes of the binary oxides could using sodium titanate nanotubes as precursor. The self- be tailored by the variation of the molar ratios of the metal assembled formation of highly ordered sodium titanate precursors. With high surface areas, the meso-macroporous nanotube films was accompanied with the effective removal binary oxides exhibit high photocatalytic activities and are of sodium ions in the nanotubes during the EPD process, potential for photocatalytic applications. resulting in the final formation of protonated titanate In the paper entitled “Synthesis of neutral SiO2/TiO2 nanotube film. The effects of calcination temperatures on the hydrosol and its application as antireflective self-cleaning surface morphology, microstructures as well as the properties thin film,” H. L. Bai et al. present a synthesis of neu- of superhydrophilicity were studied. tral SiO2/TiO2 composite hydrosol by a coprecipitation- In the paper entitled “High -efficiently photoelectrochemi- peptization method. The addition of SiO2 not only decreases cal hydrogen production over Zn-incorporated TiO2 nanotubes the refractive index, but also suppresses the aggregation (Zn-TNT),” M. Kang et al. designed Zn-incorporated TiO2 of TiO2, by forming Ti–O–Si bond with TiO2. And the nanotube (Zn-TNT) photocatalyst to investigate the Zn as-prepared SiO2/TiO2 thin film also demonstrates anti- dopant and nanotube morphology effects of TiO2 in elec- reflection and photocatalytic self-cleaning effect. trochemical hydrogen production from the photosplitting of In the paper entitled “Preparation and characterization of methanol/water solution. The hydrogen production over the visible-light activated Fe-N co-doped TiO2 and Its photocat- Zn-TNT photocatalysts was higher than that over the TNT, alytic inactivation effect on leukemia tumors”, J . W. X i o n g e t a l . which was attributed to the shift toward the visible region presented a study of inactivation effect on leukmia tumors by and increased number of excited electrons and holes. using Fe-N co-doped TiO2 as photocatalysts under visible- In the paper entitled “Enhanced hydrogen production light irradiation. Fe-N co-doped TiO2 exhibit a significant over C-doped CdO photocatalyst in Na2S/Na2SO3 solution inhibition on the growth of HL60 cells and high inactivation under visible-light irradiation,” J. L. Long et al. report the efficiency in photo-deconstruction of HL60 cancer cells. fabrication of the C-doped CdO photocatalysts by high- In the paper entitled “Facile low-temperature synthesis temperature solid-state process. The doping of C results in of carbon nanotube/TiO2 nanohybrids with enhanced visible- the red-shift of the optical absorption of CdO. The C-doped light-driven photocatalytic activity,”Y.L.Xieetal.report CdO photocatalysts have higher photocatalytic activity over a facile and novel low-temperature chemical precipitation parent CdO under visible-light irradiation. The results route to synthesize CNT/TiO2 nanohybrids which exhibit indicate that the H2 production is due to the existence of CdS high photocatalytic activity in decomposition of RhB under and the enhancement of visible-light photocatalytic activity visible-light irradiation. of H2 production is originated from the doping of carbon In the paper entitled “Light-driven preparation, into the CdO lattice. The probably reaction mechanism was microstructure, and visible-light photocatalytic property also discussed and proposed. of porous carbon-doped TiO2,” G.-D. Li et al. report an In the paper entitled “Photo-etching of Immobilized TiO2- unusual light-driven strategy for the preparation of highly ENR50-PVC Composite for Improved Photocatalytic Activity”, ◦ porous C-doped TiO2. With thermal treatment at 200 C, M. A. Nawi et al. present the preparation of a highly reusable Ti–O–C bonds and the coke species were formed, which were immobilized TiO2-ENR50-PVC composite via simple dip- regarded to play a critical role on enhancing the visible-light coating method. It exhibited better photocatalytic efficiency photocatalytic activity for the as-prepared C-doped TiO2. than the TiO2 powder in a slurry. The photo-etched catalyst In the paper entitled “Doped titanium dioxide films and the photo-mineralization capability of the product are prepared by pulsed laser deposition method,”X.D.Lin also investigated. Such efficient immobilized photocatalyst et al. presented the fabrication of N-doped TiO2 film by system offers excellent advantage of use and reuse without pulsed laser deposition method using novel ceramic target of the need to filter the treated water after the treatment and mixture of TiN and TiO2 and developed a continuous optical can be easily adapted for continuous flow reactor. 4 International Journal of Photoenergy

In the paper entitled “Photocatalytic activity of titanate nanotube powders in a hybrid pollution control system,”S.- C. Jung et al. present a novel microwave/UV/photocatalyst hybrid system to evaluate the photocatalytic activity of the titanate nanotubes. The effects of each element technique as well as the synergy effects on decomposition of organic material were investigated. In the paper entitled “Synthesis of hollow CdS-TiO2 microspheres with enhanced visible-light photocatalytic activity,” Y. N. Huo et al. synthesized CdS-TiO2 hollow microsphere via the hard-template preparation with polystyrene microspheres followed by ion-exchange approach. The introduction of CdS can effectively extend the photo-response of TiO2 to visible-light region and improve the separation of photoinduced charges, thus enhances photocatalytic activity of decomposition of rhodamine B (RhB) in aqueous solution. In the paper entitled “Solar photocatalytic removal of chemical and bacterial pollutants from water using Pt/TiO2- coated ceramic tiles,” E. P. Yesodharan et al. immobilized Pt/TiO2 on a ceramic tile in order to enhance the commercial viability of the catalyst by recycling. Optimum loading of Pt on TiO2 was found to be 0.5%. This catalyst was found effective for the solar photocatalytic removal of chemical and bacterial pollutants from water. Once the parameters are optimized, the Pt/TiO2/tile can find application in swimming pools, hospitals, water theme parks, and even industries for the decontamination of water. Baibiao Huang Stéphane Jobic Xuxu Wang Weifeng Yao Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 269654, 10 pages doi:10.1155/2012/269654

Review Article NitrogenIncorporationinTiO2:DoesItMakeaVisibleLight Photo-Active Material?

B. Viswanathan and K. R. Krishanmurthy

National Centre for Catalysis Research, Indian Institute of Technology Madras, Chennai 600 036, India

Correspondence should be addressed to B. Viswanathan, [email protected]

Received 18 January 2012; Revised 11 April 2012; Accepted 26 April 2012

Academic Editor: Xuxu Wang

Copyright © 2012 B. Viswanathan and K. R. Krishanmurthy. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The possibility of hydrogen production by photo-catalytic decomposition of water on titania has provided the incentive for intense research. Titania is the preferred semiconductor for this process, in spite of its large band gap (∼3.2 eV) that restricts its utility only in the UV region. Various sensitization methodologies have been adopted to make titania to be active in the visible region. Doping of TiO2 with nitrogen is one such method. The purpose of this presentation is to examine the state and location of nitrogen introduced in TiO2 lattice and how far the shift of optical response to visible radiation can be beneficial for the observed photo-catalysis. The specific aspects that are discussed in this article are: (i) N-doped titania surface adopts a non-native configuration, though the bulk material is still in the native configuration of pure TiO2 (ii) Though the nitrogen doped materials showed optical response in the visible region, the changes/improvements in photo-catalytic activity are only marginal in most of the cases. (iii) The exact chemical nature/state of the introduced nitrogen, and its location in titania lattice, substitutional and/or interstitial, is still unclear (iv) Is there a limit to the incorporation of nitrogen in the lattice of TiO2?

1. Introduction methods for incorporation of nitrogen in TiO2 have been reported in literature [2, 3]. In titanium-dioxide-promoted photocatalysis or photoelectrochemical applications, the primary objective appears to be “how to make this system respond to visible radiation?” This aspect can be generally termed as sensitization of the 2. Role of Heteroatoms in Titania semiconductor either intrinsically (conventionally termed as In recent times, there are many reports dealing with het- doping even though this may not be the correct connotation eroatom,(typically,S,C,F,P,B,etc.)substituted(doped when the dopant concentration exceeds a certain limit) or by or implanted) materials examined as catalysts. The driving using visible light absorbing materials (usually dyes or some force for these studies is the possibility of generating extra complex species) [1]. These studies have given rise to many allowed energy levels in the wide band gap (e.g., TiO2) spin offs like understanding of the defect chemistry of the of the semiconductors. These additional allowed energy materials, new synthesis strategies for generating these useful levels in the band gap of the semiconductor will not only semiconducting oxides, and above all the understanding of promote absorption of visible light photons but also bring in the physics of the alternate energy levels that are induced by alternate pathways for the electron-hole recombination, thus the incorporation of dopants in the original semiconductor. altering their life times which are essential for an effective It should be remarked that the efforts to push the photoactive photocatalyst. Such electronic structure alterations could be range of these materials to visible region (so that solar induced by these heteroatoms. Though the analysis of such spectrum could be effectively utilized) have been the prime effects may be interesting, this aspect is not considered in motive. It is therefore natural that a variety of synthesis this presentation. The scope of this presentation is therefore 2 International Journal of Photoenergy

restricted to considering incorporation of nitrogen in TiO2 4. Preparation Methods for Incorporation of especially from the points of view of: Nitrogen in TiO2

(i) what is the exact chemical nature of the substituted A number of preparative procedures have been adopted for and interstitial nitrogen? introduction of nitrogen in TiO2 lattice, which are briefly considered below. (ii) what is the net effect observed in shifting the absorption edge of the semiconductor, and if it does, is there (1) Synthesis from solution phase (sol gel method): [20, any relationship with the extent of substitution? 21] Titanium isopropoxide (Ti(OPr)4) was dissolved (iii) the net changes that have been observed in the in ethanol with dodecylamine and refluxed to get a photocatalytic activity of substituted systems clear solution. After cooling the solution is neutralized with acetic acid followed by hydrolysis by water. (iv) are there any qualitative correlations that can be This method has also been followed for multilayer derived from these observations? formation of porous TiO2 by dip coating method and then treated with nitrogen source. The preformed The reason for this self-imposed restriction is that there TiO2 films were usually annealed in the presence of is considerable literature already available on these systems. nitrogen precursors (e.g., NH3)[22]. In the last five years alone, a simple search with “Nitrogen doped TiO2” search term in Web of Knowledge returned (2) Reaction with ammonia or other N-containing 940 articles with 93 proceedings and 25 reviews published. organic substrates like urea, guanidine hydrochlo- Publications, year wise, indicate continued interest in the ride, or guanidine carbonate: [23, 24]. A dried topic, with 123 in 2007, 146 in 2008, 138 in 2009, 174 commercial TiO2 powder was treated in a flow of in 2010, and 153 in 2011. Proper evaluation of these dry air or inert gas mixtures containing nearly 50% systems may provide some directions on catalysis promoted ammonia or other nitrogen containing substrates ◦ by these systems, especially for important reactions like and, the sample is heated to 500–700 C. the photo-catalytic decomposition of water or the photo- (3) Preparation by plasma treatment [25]: the solid is catalytic reduction of carbon dioxide. However, most of fixed to a quartz disk and placed in a plasma chamber the investigations pertain to applications for degradation of with heating (400◦C). Purified nitrogen was metered organic pollutants in aqueous streams, model compounds and introduced into the chamber, and the plasma therein, and typical dye chemicals. treatment was carried out for a definite time period (oftheorderof10minutes). 3. Status on Nitrogen-Incorporated Titania (4) Oxidation of TiN [26–29]: thin film of TiNx was The doped nitrogen assuming substituting anionic positions deposited using DC magnetron sputtering (PVD) in (this is already an assumption!) titanium dioxide has and then the film was oxidized. Instead, TiO2 is first attracted attention due to its possible photo-catalytic activity sputtered and then annealed in nitrogen. If suitable in the visible region [4]. Even though the visible light chemicals are chosen then TiO2 wascoatedona response of anion doped TiO2 hasbeenreportedasearly substrate and it is then treated with suitable nitrogen as 1986 by Sato [5], the recent work by Asahi et al. [6] source like ammonia. have reopened the interest in this system. In the recent past, (5) Electrochemical anodization [30]oftitaniumfoils few innovative preparation methods have been evolved in with suitable nitrogen source has also shown to be a literatureforN,S,P,F,andBdopedTiO2 catalysts [7–10]. method for controlled incorporation. Simultaneously, there are also attempts to rationalize the observed results from theoretical calculations on this system (6) Low ion implantation method [31] has also been [11–18]. The complexities involved in N-doped titania, used to introduce nitrogen in TiO2 single crystals. especially with respect to the type of N species, their effects on visible light absorption, and the moderate influence on It is not clear as to which of the methods give substituted photocatalytic activity have been covered by Emeline et al. N and which other methods result in interstitial N. According [19]. The current review addresses some of the issues that to one report [32], plasma-enhanced chemical vapor depo- have emerged from the earlier presentation. sition yields substitutional nitrogen, while sol-gel method, Besides, it is also observed that the state of N in titania and annealing in NH3 and chemical methods in general produced its effectiveness in extending the light absorption edge depend predominantly interstitial nitrogen. upon the way it is introduced through several preparation None of the methods mentioned above could, however, methods/techniques. Since this presentation mainly deals with give clear clues regarding the state of substituted nitrogen. nitrogen introduction in TiO2,itisappropriatetolistthe This has to be elucidated by other structural techniques. In typical preparation methods employed in literature, for most of these studies the amount of nitrogen incorporated incorporation of nitrogen in titania and look for possible is small and hence the X-ray patterns for doped samples correlations between nature of N introduced and preparation resembled that of pure titanium dioxide, and no clear cut methods. changes in the structure could be discerned. International Journal of Photoenergy 3

The observations of N-1s-binding energy >400 eV may be CB e− attributed to the possible O–N–Ti linkages or some surface oxidation. In a subsequent paper [46] the same authors have Visible Ti3+ Ti-O-N tried to substantiate the presence of different types of N species like Ti–N and O–Ti–N in N–TiO2 system. It is clear Ultraviolet N 2p that the XPS results reported on this system require a better 0.73 eV 0.14 eV understanding. VB h+ In order to comprehend the situation, especially the bonding of substituted nitrogen in TiO2, it is necessary that one attempts to summarize the different results reported in Figure 1: Schematic energy level diagram for nitrogen substituted literature on XPS studies. This technique probes the core TiO2 [Bare semiconductor absorbs UV radiation while the localized level binding energies of the constituent species, and the energy levels of nitrogen above valence band facilitate the visible value of the binding energy is a reflection of the valence light absorption]. state and charge density around each of the atoms. Since the objective of this presentation is to get clarity on the nature of the nitrogen species, the data on the binding energy of the 5. The Chemical Nature of Substituted 1s level of nitrogen in substituted TiO2 are summarized in Table 1. Nitrogen in TiO2 The data presented in Table 1 give rise to a number of The primary motive for nitrogen substitution in TiO2 is to points, which are listed as follows. increase the visible light absorption by the semiconductor It appears that there is some consensus on the state [33, 34]. In this sense, the reduction in band gap value of nitrogen in substituted systems as Ti–N with nitrogen with attendant increase in photo-catalytic activity has been substituting for the anion. However, there is no definite reported [4–6, 10, 35–38]. This could have arisen due to information on the valence state of this substituted species. either mixing of the nitrogen p states with oxygen 2p states It has been assumed in some cases as N− anion and in some on the top of the valence band or by the creation of N- of the other reports the valence state is not exactly mentioned induced mid gap levels as proposed by Asahi et al. [6] except stating that there is Ti–N bonding. This can give rise (Figure 1). Though these are the generally stated conceptual to the following contemplations. alterations in the energy states of the doped semiconductor, (a) If N− is the species then the valence state of Ti has it is necessary to see the extent of validity of these postulates. to be different from Ti4+, which has been recognized However, it must be realized that the possibility of nitrogen in some of the reports, but they have not exactly substitution in TiO2 provides opportunities for other appli- accounted for the valence state of Ti so that electro- cations hitherto not examined, namely, oxidation of CO, neutrality is maintained. ethanol, acetaldehyde, and NO removal at room temperature [6, 39, 40].Thesenewerpossibilitiesmayhavesomefar (b) It has been tacitly assumed that the N-1s-binding ∼ reaching implications. energy is around 396-397 eV, and this peak is There seems to be no consensus among the reports on mostly present when the nitrogen content in substituted systems is very small. It is generally observed the state of doped nitrogen in N–TiO2 even though it is considered to be in the anionic form [4, 6, 41]. XPS analyses that with increasing nitrogen content the higher ∼ of N–TiO2 show that the N 1s core level binding energy binding energy ( 400 eV) peak appears which is nor- lies between 396 and 397 eV, and it has been claimed that mally considered to be due to chemisorbed molecular the state of nitrogen to be either nitrogen anion [41]or species or interstitial N or due to the nitrogen of atomic nitrogen atoms [6, 40]. Additional N 1s peaks on the precursor species employed for N substitution in N–TiO2 observed at binding energies 400 and 402 eV have TiO2. been attributed to chemisorbed molecular dinitrogen or (c) The acceptable binding energy value of N 1s level adsorbed organic compounds [6, 42]. Sakthivel and Kisch in substituted systems is around 396-397 eV. If this [43] observed no anionic-like nitrogen species with binding were to be the 1s binding energy of the nitrogen energy around 396 eV but only observed nitrogen 1s peak species then the species cannot be either in positive at around 404 eV which has been assigned to hyponitrite or multiple negative valence states. If nitrogen were to type nitrogen species. Valentin et al. [35–37] recognized this assume anionic states (as is generally believed) then controversy in the assignment of N 1s peak position. They nitrogen-1s-binding energy should be around 394 eV. observed N 1s core level at 400 eV and hinted at a lower This ionic state can also be expected on the basis of valence state for N. Recently Chen and Burda [44]observed electronegativity difference between that of titanium N 1s level at 401.3 eV from a detailed XPS investigations and nitrogen. If it were to be in cationic state, it of nano-TiO2 and suggested that there is N–Ti–O bond should be around 400 eV which is less likely on the formation due to nitrogen doping and no oxidized nitrogen basis of size and charge. If on the other hand the Ti–N is present. Sathish et al. [45] have examined this system bond were to assume covalent character, the observed and attributed the observed N-1s-binding energy at 398.2 eV nitrogen-1s-binding energy can vary with extent of to be due to N–Ti–O type species present in the system. loading and possibly account for the variations in 4 International Journal of Photoenergy

Table 1: Nitrogen-1s-binding energies in nitrogen substituted TiO2.

Nitrogen-1s-binding energy in S. No Assigned to species like Reference N-TiO2 in eV 1 ∼397 T–N bonding IBAD [12] 2 396, 400 Ti–N bond chemisorbed dinitrogen [13]

3 396.2, 398.3, 400.4 Chemically bound, O–Ti–N, chemisorbed N2 [47]

4 396.7, 398.5 N–Ti bonding; symmetric to asymmetric with % N2 [15] 5 398.6, 401 N substituted for oxide, Ti–N–O, incorporated nitrogen [48]

6 395.4, 400.5 Ti–O–N unreacted NH3, adsorbed nitrogen [49]

7 396, 400 Ti–N, adventitious N2 [14]

8 396.5, 402 B–N (bonded to Ti) and molecular N2 [50] 9 397.4 −403.7, 401.3, 396 O–Ti–N, chemically bound N [44] − 10 396, 400, 402 N species, chemisorbed N2 [6] 11 396 Chemically bound N− [51] 12 397 Ti–N concentration dependent [52]

13 396, 400 O–Ti–N, (TiO2−xNx) 14 398.2 Covalency of Ti–N bond, anionic N, N–Ti–O [45] 15 396 β-N [39]

16 396.7, 399.6 Bulk N in two states nitride species N2-anions NHx species chemically bound to H [41] 17 398.7, 400.1 C–N bonds, hyponitrite, O–Ti–N, C–Ti–O, Ti–O–N–C [53] − 18 396, 399.9 N substituted, chemisorbed N2 [30] 19 396.8 β-N, substitution [54]

20 397, 400, 403 Ti–N, chemisorbed N2 [55] 21 396.2, 400.2 β-N and γ-N [56]

22 396, 400 Incorporated, trapped N2 [27] 23 395.6, 399.6 N3− in TiN, Ti–N–O, Ti–O–N [11] 24 396, 400 O–Ti–N [57]

25 396, 398.8, 400.5 Substitution, molecular N2 [20] 26 397–402 Substituted nitrogen [23] 27 397.4, 400.3, 402.5 Ti–N–Ti, Ti–O–N–O/Ti–O–N–N [58] 28 395.8, 400 β-N, N–O, N–C, N–N [26]

29 396, 400 β-N, molecular N2 [4] 30 396, 400, 402 β-N, others not assigned [29]

31 396.1, 402 N–Ti–O, γ-N2 [59] 32 396, 400 β-N, N–O, N–C, N–N [24]

33 396, 399.5 β-N, chemisorbed N2, interstitial O–N–Ti [28] 34 396.2, 398.4, 400, 401.5 β-N, chemisorbed nitrogen [46]

35 396, 400, 402 β-N (396 eV) and molecularly chemisorbed γ-N2 (400 and 402 eV) [6] 36 399.6, 396 Interstitial nitrogen and substituted nitrogen [60] 37 400.5 No substitution nitrogen [61] 38 399 Both substitution and interstitial nitrogen [62] Interstitial nitrogen doping at low levels and nitrogen doping at higher levels; the 39 399.6 [63] limit for this transition is 1.2 at%. 40 400 O–Ti–N bonding [64]

41 399.2, 400.5 C=N–C, N–Csp2,–C=N[65] 42 400, 402 Chemisorbed nitrogen [25] 43 399.5 Valence state of nitrogen between −2and−3[66] 44 402, 399.5 Chemisorbed molecular nitrogen, interstitial nitrogen [67] International Journal of Photoenergy 5

the binding energy values reported in literature. The literature is unfortunately silent on this aspect for some unknown reasons. (d) Another important aspect is that the type of species envisaged, namely, either Ti–N or Ti–O–N, could have been present mostly on the surface as they have been detected only in XPS measurements with no indications in the X-ray diffraction patterns, which are nearly the same as the starting TiO2. This means that the surface layers have a non-native configuration as compared to the native configuration that is present in the bulk of the material. If this were to be true, then the reactivity (or even the photocatalytic consequences reported) of the surface should be different from what is generally observed on bare TiO2. Though this has been observed in many publications, they have not been assigned to the changes observed due to the nonnative configurations that may be present on the surfaces of substituted systems [68]. Figure 2: Model of the cluster (Ti5O14H8) [the position of the replaced oxygen is shown by an arrow]. (e) Even though there are reports on the changes in the intensity of the emission due to N 1s level in the substitution state (namely, around 396-397 eV), it is not clear from the available literature to what per- TiO2. In any of the studies reported, this aspect has not been centage the nitrogen substitution in anionic position clearly stated. This could be deduced by the shift of the band will take place and when the signal due to molecularly gap value as a function of the extent of nitrogen substitution. chemisorbed nitrogen species starts to appear. The A rough estimate of the data available in literature shows that changes observed or expected in the optical spectrum the substituted nitrogen cannot exceed 10%. However, it is are critically dependent on this parameter. necessary to examine the band structure alterations that can take place by the substitution of anions in titanium dioxide (f) Observation of two XPS peaks at 396–398 eV and by nitrogen anion. 400–402 eV possibly indicates the presence of substitutional as well as interstitial nitrogen, respectively, in titania lattice though the exact chemical nature is not 6. N Substitution Theoretical Approaches clear. A few theoretical studies have been reported on this aspect (g) Nitrogen incorporation in titanium dioxide has thus on the basis of the calculation of the density of states (DOS). been claimed through a number of preparation The salient points that emerge out of these theoretical studies methods [65, 69]. It is generally presumed that the are [12–15, 17, 18] visible light photo activity of the resultant material is due to incorporation of nitrogen in the lattice. An (1) the nitrogen 2p states give rise to allowed energy alternate postulate is that nitrogen precursors could states just above the valence band of the semiconduc- give rise to light active molecular species on the tor (Figure 1). surface (dye like) which function as sensitizers for the (2) the 3d states of the metal provide allowed energy observed visible light photo-activity [70–73]. How- levels near the conduction band. ever, this concept cannot account wherein molecular nitrogen or other molecules, which on decomposi- (3) the transition from the allowed 2p states of nitrogen tion yield molecular nitrogen, are employed for the to the conduction band accounts for the visible light activity of these doped systems. preparation of nitrogen substituted TiO2. (4) Asahi et al. [6] believe that the Nitrogen 2p and The data presented in Table 1 show that the nitrogen oxygen 2p states hybridize and thus account for incorporated in TiO2 is mostly in the substitution position the band gap narrowing. This statement has been for the anion conventionally designated as β-N species. substantiated by the calculations of Sakai et al. [13] Several investigations have reported peaks at 400–402 eV and that of Li et al. [12]Shangetal.[14], and Sathish indicating presence of interstitial nitrogen as well. However, et al. [18]. The model calculations have been made on there is no clear cut statement on the valence state of the a typical cluster [Ti5O14H8] (Figure 2). nitrogen. Is it a trivalent anion? Or at least an anion with negative charge on substituted nitrogen? The N substituted The position of the substituted nitrogen in the place of systems on further incorporation of nitrogen gives rise to oxygen atom is shown by arrow. This corresponds to an molecularly chemisorbed species. This observation leads one anionic position in the TiO2 lattice. The density of states to make an estimate of the possible extent of substitution in (DOS) calculation has shown the reduction in the band 6 International Journal of Photoenergy

Table 2: 2p3/2 binding energy values for Ti in N-substituted TiO2.

Binding energy of 2p level of S. No 3/2 Assigned as Reference Ti In N–TiO2 (in eV) 1 458 Ti in 4+ oxidation state [27] 2 459.3, 458.5 Electronic interaction is diﬀerent, covalency [45] 3 <459.1 Substitution of N Multiple oxidation states +3+2 [52]

4 +0.9 and +2.1 with respect TiO2 Intrinsic defects implanted nitrogen [31]

5 458 Same as in TiO2 [27] 6 458.6, 456.2 Ti in +3 and +4 states [59] 7 458.2 Shifted to lower value, electron density increases, covalency [26, 53] Decrease by 05–02 eV with respect to TiO , N–Ti–N or O–Ti–N, N–Ti–N, 8 459.2, 456.1 2 [58] O–Ti–N 9 458.1, 463.8 Ti–N bonds shifts to lower values [23] 10 457.5 Ti–N [20] gap which is attributed to the orbital mixing of the hetero not making any significant comment of the corresponding atom with oxygen 2p and Ti 3d states as stated earlier. It features of the Ti 2p XP spectra. This study could have given is presumed that the energetic considerations show that the some information on the state of nitrogen in the substituted nitrogen 2p states can mix with both the 2p state of oxide ions systems. If nitrogen incorporation were to take place similar and also the 3d state of the titanium ions thus accounting for to TiN-like species then Ti could have been present in +3 the reduction in the band gap with the possibility of visible oxidation state and the covalency of the bond could have light active nature of N-doped TiO2. It has been shown that increased, and it could have resulted in the decrease in both the top of the valence band and the bottom of the the binding energy of 2p3/2 level of Ti present in N–TiO2. conduction band are broadened due to nitrogen doping in However, most of the studies have not reported explicitly this TiO2. observation and also in some of the studies, they have not The points of relevance are observed the shift to lower binding energy values with respect to what is observed in TiO2, which possibly implies that (i) does nitrogen doping in TiO2 directly reduces the nitrogen is not either substituted for oxygen or the species value of the band gap of TiO2? generated are not like TiN. (ii) does the substitution of nitrogen give rise to allowed energy states in the band gap of the semiconductor thus accounting for the visible light response of the 8. Nitrogen in Interstitial Sites in Titania doped semiconductor? According to Asahi et al. [6] only substitutional type of N (iii) how does the substitutional/interstitial nitrogen doping is possible. This leads to mixing of the 2p states of influence the activity? oxygen from titania with the 2p states of N and results in intra band states which in turn can effectively narrow down Theseissueshavenotyetbeenresolved. the band gap, thereby facilitating visible light. Interstitial type N doping was found to be ineffective. 7. The Nature of Titanium Species in In contrast, Valentin et al. [35–37] propose that incor- ff N-Substituted TiO2 poration of nitrogen in substitutional state is more e ective in the formation of localized levels within the band gap and Having seen that the exact chemical nature of nitrogen in conclude that both substitutional and interstitial nitrogen N-substituted TiO2 is subject to controversy, it is natural to may exist with energy levels as shown in (Figure 1)and expect some changes in the 2p3/2 binding energy of Ti. This depending on the preparation (oxygen partial pressure) con- is required since nitrogen can either take up the position of ditions. According to Valentin et al. [35–37], substitutional the oxide ions or can also replace the cations. In addition, nitrogen states lie just above valence band, by 0.14 eV, and since the difference in the values of the electronegativity of interstitial nitrogen states lie higher in the gap at 0.73 eV. nitrogen and titanium is 1.5 as against the value of 1.9 for Two N1s XPS peaks, one observed at 396 eV (substitutional) Ti and oxygen, one can expect even if nitrogen were to take and the other at 400 eV (interstitial) nitrogen species lend up the anionic position, the bonding may involve in some credence to this theory. It is also known that nitrogen can covalent character and hence the 2p3/2-binding energy of substitute the Ti site in TiO2. titanium can be different from what is observed for Ti4+. Hence, at present, it is not clear in what state nitrogen The reported XPS results on the binding energy of Ti is introduced in TiO2. No correlations exist between the 2p3/2 in nitrogen-substituted TiO2 are given in Table 2. method adopted for N incorporation and the type of N in Though most of the studies on the N-substituted TiO2 the lattice, substitutional, or interstitial. In the absence of systems have clearly dealt with the XPS of N 1s level, they are clear understanding of the state of nitrogen introduced in International Journal of Photoenergy 7

Table 3: Typical photocatalytic studies reported on N-substituted (c) (a) (b) TiO2. 100

S. No Photo-catalytic studies on N–TiO2 Reference Photo-activity using stearic acid, no 1 [27] visible-light-induced activity (d) Photo degradation of Phenol N–TiO ,anatase 2 2 [55] is higher 50

Degradation of methylene blue, no specific [30, 39, 45, Reflectance 3 trend with respect to nitrogen substitution 46] [48, 53, 4 Photo-degradation of methyl orange 58] 5 Photodecomposition of isopropyl alcohol [52, 75] 0 6 Photo-electrochemical decomposition of water [52] 200 400 600 800 7 Degradation of ethylene glycol [76] Wavelength (nm) 8 Decomposition of 2-chlorophenol [14, 23] 9 Decomposition of VOC and Rhodamine B [29] Figure 3: UV-vis reflectance spectra of (a) TiO2, (b) P-25, (c) undoped hollow TiO sphere, and (d) N-doped hollow TiO sphere 10 Decomposition of acetaldehyde [6] 2 2 [74].

ffi titanium dioxide, it is di cult to interpret the photo-catalytic photo-catalytic decomposition of water for the production activity observed with these systems since the creation of of fuel (namely, hydrogen) may be of greater relevance and new adsorption centers could also be one of the reasons for importance. The studies so far reported in literature on the observed photo-catalytic effect even though the photon nitrogen-doped TiO2 have not shown considerable enhance- absorption range could have shifted to visible range. ment of the decomposition of water by increasing absorption in the visible range. Besides Kitano et al. [77] observed that 9. Photo-Catalytic Studies on N-doped titania film after photo electrolysis of water under visible irradiation showed a decrease in N concentration Nitrogen-Introduced TiO2 at surface layer indicating that the surface was oxidized It is known that the primary reason for substitution of during electrolysis. Such stability issues [77, 78]withN- nitrogeninTIO2 is to extend the photon absorption doped titania usage need to be addressed and improved. range to visible region. Even though, this expectation is mostly fulfilled, as substantiated from the optical response 10. Conclusions of a typical system reproduced in Figure 3, its effects on photocatalytic activity are yet to be realized in expected (1) It is not clear from the results reported in literature terms. that nitrogen introduction in TiO2 lattice can pro- Cho et al. [74] have studied the photo-catalytic activity mote the phase transformation from anatase to rutile. of nitrogen-doped TiO2 hollow nanospheres and concluded It is known that anatase to rutile phase transforma- that the activity under visible light was superior to that of tion is either inhibited or promoted by the addition pure TiO2 or Degussa P-25. of substances like K2O, P2O5,Li2O and carbon in the A compilation of the photocatalytic activity studies on former three cases the transformation was inhibited N-doped titania is presented in Table 3. while in the case of carbon it is accelerated [79–81]. It is surprising to note that the photo-catalytic activity However, the available reported literature is not clear of N–TiO2 has been restricted to the same type of reac- if introduction of nitrogen in TiO2 lattice can alter tions (namely decomposition of organic substrates), and or promote the phase transformation from anatase to considerable increase in the reactivity is not observed for rutile. This can be considered as an indirect evidence visible light irradiation. This casts doubt on the attempts for nitrogen not incorporated in the lattice. that are made for shifting the absorption to visible range (2) Interrelationships between the methods adopted for by sensitizing the semiconductor with the hope that more N introduction and chemical nature and location of of the solar radiation can be utilized. This postulate is NinTiO2 lattice are yet to be understood. not established till now. It appears that one has to re- examine the methodology for the selection of materials for (3) Maximum/optimum doping of N that could be ff photo/photo-electrochemical decomposition of water where achieved, and its e ect on photoactivity is another one is searching for a semiconductor which can be photo aspect that needs further study. active in the visible range. (4) If the X-ray diffraction patterns for N-doped TiO2 Photo-catalytic decomposition of volatile organic com- remain the same as that of pure TiO2, then the effect pounds and decontamination of water, though, may be of N introduction should have been minimal and relevant from environmental control point of view the hence the surface configuration could be different 8 International Journal of Photoenergy

from that of the bulk native structure. If this archi- [13] Y. W. Sakai, K. Obata, K. Hashimoto, and H. Irie, “Enhance- tecture is possible, then optical absorption changes ment of visible light-induced hydrophilicity on nitrogen and observed could have arisen out of the surface layers sulfur-codoped TiO2 thin ﬁlms,” Vacuum,vol.83,no.3,pp. and hence only the adsorptive properties could have 683–687, 2008. altered, accounting for the increased photo-catalytic [14] G. Shang, H. Fu, S. Yang, and T. Xu, “Mechanistic study oxidation of organic substrates. of visible-light-induced photodegradation of 4-chlorophenol by TiO2−xNx (0.021

glass by chemical vapour deposition,” Journal of Photochem- and photocatalytic activity of nitrogen-doped TiO2 nanocat- istry and Photobiology A, vol. 179, no. 1-2, pp. 213–223, 2006. alyst,” Chemistry of Materials, vol. 17, no. 25, pp. 6349–6353, [28] C. J. Tavares, S. M. Marques, T. Viseu et al., “Enhancement 2005. of the photocatalytic nature of nitrogen-doped PVD-grown [46] M. Sathish, B. Viswanathan, and R. P. Viswanath, “Character- titanium dioxide thin films,” Journal of Applied Physics, vol. ization and photocatalytic activity of N-doped TiO2 prepared 106, Article ID 113535, 2009. by thermal decomposition of Ti-melamine complex,” Applied [29] T. Morikawa, R. Asahi, T. Ohwaki, K. Aoki, K. Suzuki, and Y. Catalysis B, vol. 74, no. 3-4, pp. 307–312, 2007. Taga, R&D Review of Toyota CRDI, vol. 40, p. 46, 2005. [47]T.Ma,M.Akiyama,E.Abe,andI.Imai,“High-efficiency dye- [30] D. Chen, Z. Jiang, J. Geng, Q. Wang, and D. Yang, “Carbon sensitized solar cell based on a nitrogen-doped nanostructured and nitrogen co-doped TiO2 with enhanced visible-light titania electrode,” Nano Letters, vol. 5, no. 12, pp. 2543–2547, photocatalytic activity,” Industrial & Engineering Chemistry 2005. Research, vol. 46, no. 9, pp. 2741–2746, 2007. [48] B. Naik, K. M. Parida, and C. S. Gopinath, “Facile synthesis [31] M. Batzill, E. H. Morales, and U. Diebold, “Surface studies of of N-and S-incorporated nanocrystalline TiO2 and direct nitrogen implanted TiO2,” Chemical Physics, vol. 339, no. 1–3, solar-light-driven photocatalytic activity,” Journal of Physical pp. 36–43, 2007. Chemistry C, vol. 114, no. 45, pp. 19473–19482, 2010. [32] A. Fujishima, X. Zhang, and D. A. Tryk, “TiO2 photocatalysis [49] H. M. Yates, M. G. Nolan, D. W. Sheel, and M. E. Pemble, “The and related surface phenomena,” Surface Science Reports, vol. role of nitrogen doping on the development of visible light- 63, no. 12, pp. 515–582, 2008. induced photocatalytic activity in thin TiO2 films grown on [33] A. Hattori, M. Yamamoto, H. Tada, and S. Ito, “A promoting glass by chemical vapour deposition,” Journal of Photochem- effect of NH4F addition on the photocatalytic activity of sol- istry and Photobiology A, vol. 179, no. 1-2, pp. 213–223, 2006. gel TiO2 films,” Chemistry Letters, no. 8, pp. 707–708, 1998. [50] C. Shifu, L. Xuqiang, L. Yunzhang, and C. Gengyu, “The [34] T. Yamaki, T. Sumita, and S. Yamamoto, “Formation of preparation of nitrogen-doped TiO2−xNx photocatalyst coated TiO2−xFx compounds in fluorine-implanted TiO2,” Journal of on hollow glass microbeads,” Applied Surface Science, vol. 253, Materials Science Letters, vol. 21, no. 1, pp. 33–35, 2002. no. 6, pp. 3077–3082, 2007. [35] C. D. Valentin, G. Pacchioni, and A. Selloni, “heory of carbon [51] E. Gyorgy, A. P. D. Pino, P. Serra, and J. L. Morenza, “Depth doping of titanium dioxide,” Chemistry of Materials, vol. 17, profiling characterisation of the surface layer obtained by no. 26, pp. 6656–6665, 2005. pulsed Nd:YAG laser irradiation of titanium in nitrogen,” [36] C. D. Valentin, G. Pacchioni, A. Selloni, S. Livraghi, and Surface and Coatings Technology, vol. 173, no. 2-3, pp. 265– E. Giamello, “Characterization of paramagnetic species in 270, 2007. N-doped TiO2 powders by EPR spectroscopy and DFT [52] M. Kitano, K. Funatsu, M. Matsuoka, M. Ueshima, and M. calculations,” The Journal of Physical Chemistry B, vol. 109, no. Anpo, “Preparation of Nitrogen-Substituted TiO2 Thin Film 23, pp. 11414–11419, 2005. Photo catalysts by the Radio Frequency Magnetron Sputtering [37] D. Valentin, E. Finazzi, G. Pacchioni et al., “N-doped TiO2: Deposition Method and Their Photo catalytic Reactivity under theory and experiment,” Chemical Physics, vol. 339, no. 1–3, Visible Light Irradiation,” The Journal of Physical Chemistry B, pp. 44–56, 2007. vol. 110, no. 50, pp. 25266–25272, 2006. [38] R. Nakamura, T. Tanaka, and Y. Nakato, “Mechanism for [53] J. Yang, H. Bai, X. Tan, and J. Lian, “IR and XPS investigation Visible Light Responses in Anodic Photocurrents at N-Doped of visible-light photocatalysis-nitrogen-carbon-doped TiO2 TiO2 Film Electrodes,” The Journal of Physical Chemistry B, vol. film,” Applied Surface Science, vol. 253, no. 4, pp. 1988–1994, 108, pp. 10617–10620, 2004. 2006. [39]J.L.Gole,J.D.Stout,C.Burda,Y.Lou,andX.Chen, [54] K. Shankar, K. C. Tep, G. K. Mor, and C. A. Grimes, “An “Highly efficient formation of visible light tunable TiO2-xNx electrochemical strategy to incorporate nitrogen in nanos- photocatalysts and their transformation at the nanoscale,” tructured TiO2 thin films: modification of bandgap and Journal of Physical Chemistry B, vol. 108, no. 4, pp. 1230–1240, photoelectrochemical properties,” Journal of Physics D, vol. 39, 2004. no. 11, pp. 2361–2366, 2006. [40] T. Sano, N. Negishi, K. Koike, K. Takeuchi, and S. Matsuzawa, [55] Z. Wang, W. Cai, X. Hong, X. Zhao, F. Xu, and C. Cai, “Preparation of a visible light-responsive photocatalyst from a “Photocatalytic degradation of phenol in aqueous nitrogen- 4+ complex of Ti with a nitrogen-containing ligand,” Journal of doped TiO2 suspensions with various light sources,” Applied Materials Chemistry, vol. 14, no. 3, pp. 380–384, 2004. Catalysis B, vol. 57, no. 3, pp. 223–231, 2005. [41]O.Diwald,T.L.Thompson,T.Zubkov,E.G.Goralski,S.D. [56]R.P.Vitiello,J.M.Macak,A.Ghicov,H.Tsuchiya,L.F.P. Walck, and J. T. Yates, “Photochemical activity of nitrogen- Dick, and P. Schmuki, “N-Doping of anodic TiO2 nanotubes doped rutile TiO2(110) in visible light,” Journal of Physical using heat treatment in ammonia,” Electrochemistry Commu- Chemistry B, vol. 108, no. 19, pp. 6004–6008, 2004. nications, vol. 8, no. 4, pp. 544–548, 2006. [42] N. C. Saha and H. G. Tompkins, “Titanium nitride oxidation [57] Y. Wang, C. Feng, Z. Jin, J. Zhang, Y. Yang, and S. Zhang, chemistry: an X-ray photoelectron spectroscopy study,” Jour- “A novel N-doped TiO2 with high visible light photo catalytic nal of Applied Physics, vol. 72, no. 7, pp. 3072–3079, 1992. activity,” JournalofMolecularCatalysisA, vol. 260, no. 1–3, pp. [43] S. Sakthivel and H. Kisch, “Photocatalytic and photoelec- 1–3, 2006. trochemical properties of nitrogen-doped titanium dioxide,” [58]F.Peng,L.Cai,L.Huang,H.Yu,andH.Wang,“Preparationof ChemPhysChem, vol. 4, no. 5, pp. 487–490, 2003. nitrogen-doped titanium dioxide with visible-light photocat- [44] X. Chen and C. Burda, “Photoelectron spectroscopic investi- alytic activity using a facile hydrothermal method,” Journal of gation of Nitrogen doped Titania Nanoparticles,” The Journal Physics and Chemistry of Solids, vol. 69, no. 7, pp. 1657–1664, of Physical Chemistry B, vol. 108, no. 40, pp. 15446–15449, 2008. 2004. [59] C. Sarra-Bournet, B. Haberl, C. Charles, and R. Boswell, [45] M. Sathish, B. Viswanathan, R. P. Viswanath, and C. S. “Characterization of nanocrystalline nitrogen-containing tita- Gopinath, “Synthesis, characterization, electronic structure, nium oxide obtained by N2/O2/Ar low-field helicon plasma 10 International Journal of Photoenergy

sputtering,” Journal of Physics D, vol. 44, no. 45, Article ID nanospheres,” Journal of Physics and Chemistry of Solids, vol. 455202, 2011. 72, no. 12, pp. 1462–1466, 2011. [60] X. Zhang, K. Udagawa, Z. Liu et al., “Photocatalytic and pho- [75] C.-M.Huang,L.-C.Chen,K.-W.Cheng,andG.-T.Pan,“Effect toelectrochemical studies on N-doped TiO2 photocatalyst,” of nitrogen-plasma surface treatment to the enhancement of Journal of Photochemistry and Photobiology A, vol. 202, no. 1, TiO2 photocatalytic activity under visible light irradiation,” pp. 39–47, 2009. Journal of Molecular Catalysis A, vol. 261, no. 2, pp. 218–224, [61] K. S. Rane, R. Mhalsiker, S. Yin et al., “Visible light-sensitive 2007. yellow TiO2−xNx and Fe-N co-doped Ti1−yFeyO2−xNx anatase [76] T. Tachikawa, Y. Takai, S. Tojo et al., “Visible light-induced photocatalysts,” Journal of Solid State Chemistry, vol. 179, no. degradation of ethylene glycol on nitrogen-doped TiO2 Pow- 10, pp. 3033–3044, 2006. ders,” Journal of Physical Chemistry B, vol. 110, no. 26, pp. [62] Y. Cong, J. Zhang, F. Chen, M. Anpo, and D. He, “Preparation, 13158–13165, 2006. photocatalytic activity, and mechanism of nano-TiO2 Co- [77] M. Kitano, K. Funatsu, M. Matsuoka, M. Ueshima, and M. doped with nitrogen and iron (III),” Journal of Physical Anpo, “Preparation of nitrogen-substituted TiO2 thin film Chemistry C, vol. 111, no. 28, pp. 10618–10623, 2007. photocatalysts by the radio frequency magnetron sputtering [63] J.Wang,D.N.Tafen,J.P.Lewisetal.,“OriginofPhotocatalytic deposition method and their photocatalytic reactivity under Activity of Nitrogen-Doped TiO2 Nanobelts,” Journal of the visible light irradiation,” Journal of Physical Chemistry B, vol. American Chemical Society, vol. 131, no. 341, pp. 2290–12297, 110, no. 50, pp. 25266–25272, 2006. 2009. [78] T. Ihara, M. Miyoshi, Y. Iriyama, O. Matsumoto, and S. [64] J. Yang, H. Bai, X. Tan, and J. Lian, “IR and XPS investigation Sugihara, “Visible-light-active titanium oxide photocatalyst of visible-light photocatalysis-Nitrogen-carbon-doped TiO2 realized by an oxygen-deficient structure and by nitrogen film,” Applied Surface Science, vol. 253, no. 4, pp. 1988–1994, doping,” Applied Catalysis B, vol. 42, no. 4, pp. 403–409, 2003. 2006. [79] B. Grzmil, B. Kic, and M. Rabe, “Inhibition of the anatase— [65] D. Mitorj and N. H. Kisch, “The nature of nitrogen-modified rutile phase transformation with addition of K2O, P2O5,and titanium dioxide photocatalysts active in visible light,” Ange- Li2O,” Chemical Papers, vol. 58, no. 6, pp. 410–414, 2004. wandte Chemie International Edition, vol. 47, no. 51, pp. 9975– [80]A.Chatterjee,S.B.Wu,P.W.Chou,M.S.Wong,andC. 9978, 2008. L. Cheng, “Observation of carbon-facilitated phase transfor- [66] Z. Zhang, X. Wang, J. Long, Q. Gu, Z. Ding, and X. Fu, mation of titanium dioxide forming mixed-phase titania by “Nitrogen-doped titanium dioxide visible light photocatalyst: confocal Raman microscopy,” Journal of Raman Spectroscopy, spectroscopic identification of photoactive centers,” Journal of vol. 42, no. 5, pp. 1075–1080, 2011. Catalysis, vol. 276, no. 2, pp. 201–214, 2010. [81] K. Prasad, D. V. Pinjari, A. B. Pandit, and S. T. Mhaske, [67] Y.-C. Tang, X.-H. Huang, H.-Q. Yu, and L.-H. Tang, “Phase transformation of nanostructured titanium dioxide “Nitrogen-doped TiO2 photocatalyst prepared by from anatase-to-rutile via combined ultrasound assisted sol- mechanochemical method: doping mechanisms and visible gel technique,” Ultrasonics Sonochemistry,vol.17,no.2,pp. photoactivity of pollutant degradation,” International Journal 409–415, 2010. of Photoenergy, vol. 2012, Article ID 960726, 10 pages, 2012. [68] M. Pandey and R. S. Pala, “Stabilization and growth of non- native nano crystals at low and atmospheric pressures,” Journal of Chemical Physics, vol. 136, Article ID 044703, 2012. [69] S. H. Lee, E. Yamasue, H. Okumura, and K. N. Ishihara, “Preparation of N-doped TiOx films as photocatalyst using reactive sputtering with dry air,” Materials Transactions, vol. 50, no. 7, pp. 1805–1811, 2009.

[70] N. Serpone, “Is the band gap of pristine TiO2 narrowed by anion- and cation-doping of titanium dioxide in second- generation photocatalysts?” The Journal of Physical Chemistry B, vol. 110, no. 48, pp. 24287–24293, 2006. [71] V. N. Kuznetsov and N. Serpone, “Visible light absorption by various titanium dioxide specimens,” The Journal of Physical Chemistry B, vol. 110, no. 50, pp. 25203–25209, 2006. [72] V. N. Kuznetsov and N. Serpone, “Photoinduced coloration and photobleaching of titanium dioxide in TiO2/polymer compositions upon UV- and visible-light excitation of color centers’ absorption bands: direct experimental evidence negating band-gap narrowing in anion-/cation-doped TiO2s,” The Journal of Physical Chemistry C, vol. 111, no. 42, pp. 15277–15288, 2007. [73] R. Beranek and H. Kisch, “Tuning the optical and photoelectrochemical properties of surface-modiﬁed TiO2,” Photochem- ical & Photobiological Sciences, vol. 7, no. 1, pp. 40–48, 2008. [74] H.-J. Cho, P.-G. Hwang, and D. Jung, “Preparation and photocatalytic activity of nitrogen-doped TiO2 hollow Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 174862, 8 pages doi:10.1155/2012/174862

Research Article Simultaneous Elimination of Formaldehyde and Ozone Byproduct Using Noble Metal Modiﬁed TiO2 Films in the Gaseous VUV Photocatalysis

Pingfeng Fu,1 Pengyi Zhang,2 and Jia Li2

1 School of Civil and Environment Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, China 2 State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China

Correspondence should be addressed to Pengyi Zhang, [email protected]

Received 30 January 2012; Accepted 16 March 2012

Academic Editor: Baibiao Huang

Copyright © 2012 Pingfeng Fu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Simultaneous removal of low concentration formaldehyde (HCHO) and ozone byproduct was investigated in the gaseous VUV (vacuum ultraviolet) photocatalysis by using noble metal modified TiO2 films. Noble metal (Pt, Au, or Pd) nanoparticles were deposited on TiO2 films with ultrafine particle size and uniform distribution. Under 35 h VUV irradiation, the HCHO gas (ca. 420 ppbv) was dynamically degraded to a level of 10∼45 ppbv without catalyst deactivation, and over 50% O3 byproduct was in situ decomposed in the reactor. However, under the same conditions, the outlet HCHO concentration remained at 125∼178 ppbv in the O3 +UV254 nm photocatalysis process and 190∼260 ppbv in the UV254 nm photocatalysis process. And the catalyst deactivation also appeared under UV254 nm irradiation. Metallic Pt or Au could simultaneously increase the elimination of HCHO and ozone, but the PdO oxide seemed to inhibit the HCHO oxidation in the UV254 nm photocatalysis. Deposition of metallic Pt or Au reduces + − the recombination of h /e pairs and thus increases the HCHO oxidation and O3 reduction reactions. In addition, adsorbed O3 may be partly decomposed by photogenerated electrons trapped on metallic Pt or Au nanoparticles under UV irradiation.

1. Introduction a ppm level is also formed in the gaseous VUV photocatalysis [9, 10]. As ozone is also a hazardous contaminant in indoor The heterogeneous photocatalytic oxidation (PCO) can environment, the elimination of ozone byproduct is neces- eliminate various kinds of volatile organic compounds sary for the safe use of VUV photocatalytic technique. (VOCs) with the potential to improve indoor air quality Ozone-decomposing catalysts (e.g., MnO2)havebeen (IAQ) [1–4]. However, the low degradation rate [2, 5]and employed to remove ozone byproduct in a separate unit fol- possible photocatalyst deactivation [6, 7] limit its practical lowing the photocatalytic process [8, 10]. However, these cat- application. Vacuum ultraviolet (VUV) with high-energy alysts usually lose activity while the humidity is high [13, 14]. photon can dissociate oxygen and water molecules in the Besides the thermal decomposition process, the semicon- gas phase to reactive oxygen species such as O(1D), O(3P), ductor photocatalysis has also been considered to remove • and hydroxyl radicals ( OH) [8, 9]. Recently, several authors gaseous ozone [15, 16]. Modification of TiO2 with noble reported that VUV photocatalysis (i.e., TiO2 photocatalyst metals (e.g., Pt, Ag, Au) can remarkably increase the decom- combined with 185 nm VUV) showed higher decomposition position rate of ozone by trapping photogenerated electrons rates of VOCs than the common UV254 nm photocatalysis [8– with metal nanoparticles [17–19]. Therefore, to eliminate 12]. Furthermore, the catalyst deactivation is alleviated by ozone byproduct in this case, it is an effective way to in situ effective decomposition of nonvolatile oxidation intermedi- photocatalytically decompose O3 via increasing the reactivity ates on catalyst surface [8–10]. However, ozone byproduct at of used photocatalysts. The aim of this study is to evaluate 2 International Journal of Photoenergy × − the feasibility of simultaneous elimination of formaldehyde Q [HCHO]initial [HCHO]steady R = × 100%, and ozone byproduct in the VUV photocatalysis by using V noble metal (Pt, Au, or Pd) modified TiO2 film photocata- (E3) lysts. where the [HCHO]initial was the equilibrium concentration of HCHO before irradiation, the [HCHO]steady was the 2. Experimental steady-state concentration of HCHO under UV irradiation; Noble metal nanoparticles (NPs) modified TiO films, that [O3]initial represented the outlet concentration of VUV gener- 2 ated ozone (without photocatalysts) with a value of 22.5 mg/ is, Pt-TiO2,Au-TiO2,orPd-TiO2 films supported on Ti 3 mesh, were prepared via a low-temperature electrostatic self- m , and the [O3]steady was the steady-state concentration of assembly method as described in our previous paper [20]. ozone under UV irradiation; Q was the flow rate of HCHO gas (m3/min), and V was the effective volume of the pho- The morphologies of the photocatalysts were observed using 3 an ultra-high-resolution field-emission scanning electron toreactor (m ). microscope (FESEM, S-5500, Hitachi). X-ray diffraction (XRD) analysis was carried out with a Rigaku D/max-RB 3. Results and Discussion using Cu Kα radiation. To analyze the crystalline structure of loaded noble metal nanoparticles, the amount of deposited 3.1. Characterization of Pt (Au, Pd)-TiO2 Film Photocatalysts. Pt, Au, or Pd nanoparticles reached ca. 4% (wt). Figure 1 shows the ultra-high-resolution FESEM images of The used VUV lamp (3 W) was an ozone-producing low- Pt,Au,andPdmodifiedTiO2 films. Noble metal (Pt, Au, Pd) nanoparticles (NPs) are physically separated and uniformly pressure mercury lamp with λmax at 254 nm and a minor emission (ca. 5%) at 185 nm. Two VUV lamps were located at dispersed on TiO2 films with an interparticle spacing of ∼ the center of the flow photoreactor with an effective volume 5 20 nm. The average particle size of loaded Pt, Au, and Pd of 0.628 L. Two pieces of Pt (Au, Pd)-TiO /Ti (or TiO /Ti) NPs is 1.9 nm, 4.2 nm, and 3.9 nm, respectively. Each TiO2 2 2 ∼ mesh were, respectively, fixed at both sides of the lamp. The particle is coated with nearly 2 3metalNPs.Thesurface ∼ × 11 UV intensity at the photocatalyst surface was 3.2 mW/ density of noble metal NPs reaches (5 10) 10 particles 254 nm 2 cm2. The effective residence time of HCHO gas was 21 s in per cm . It is obvious that noble metal NPs can be well the reactor with the gas-solid contact time of 0.34 s. Because deposited on TiO2 films in the electrostatic self-assembly the indoor formaldehyde concentration usually lies at a ppbv process. level, typically less than 300 ppbv [21], the inlet concentra- The XRD analysis (Figure 2) reveals that TiO2 films have tion of HCHO was set at ca. 420 ppbv. After the HCHO mixed crystalline phases with 62.5% anatase and 37.5% rutile. As shown in Figure 2, the diffraction peaks at 2θ = concentration reached dynamical equilibrium (2 h), two ◦ ◦ ◦ ◦ VUV lamps were turned on to start the 35 h VUV pho- 39.9 , 46.4 , 67.5 , and 81.3 can be attributed to the (111), (200), (220), and (311) reflections of metallic Pt (JCPDS tocatalysis. To carry out the O3 enhanced UV254 nm (O3 + number 4-0802). For the Au-TiO films, the diffraction peaks UV254 nm) photocatalysis, two UV254 nm lamps (3 W) were 2 at 2θ = 38.3◦, 44.4◦, 64.7◦, 77.5◦, and 81.9◦ can be attributed used, and the HCHO gas mixed with O3 was introduced. 3 to the (111), (200), (220), (311), and (222) reflections of The concentration (ca. 22.5 mg/m ) of introduced O3 is close to that of ozone generated by two VUV lamps. The other metallic Au (JCPDS number 65-8601). For the Pd-TiO2 = ◦ ◦ ◦ ◦ ◦ procedures were as same as the VUV photocatalytic exper- films,thepeaksat2θ 33.9 , 41.9 , 60.3 , 60.9 , and 71.5 can be attributed to the (101), (110), (103), (200), and (202) iments. The 35 h UV254 nm photocatalysis was carried out with the same procedure except no ozone is mixed with reflections of palladium oxide (PdO, JCPDS number 41- 1107), respectively. After annealed in air at 300◦Cfor1.5h, HCHO gas. The O3 concentration was monitored with an Pt and Au NPs are still in metallic form, while Pd NPs have on-line O3 analyzer (Model 49i, Thermo Electron). The formaldehyde concentration was analyzed by MBTH method been oxidized to palladium oxide (PdO). (GB/T18204.28, China). Because the ozone may disturb the formaldehyde analysis with MBTH method, a KI (potassium 3.2. VUV Photocatalytic Degradation of Formaldehyde and iodide) coated annular denuder was used to scrub ozone Simultaneous Removal of Ozone Byproduct. As shown in while sampling. Figure 3, the HCHO can be rapidly decomposed under VUV In this study, the removal rate of HCHO and O3 was irradiation even without the photocatalyst, but ozone calculated according to (E1) and (E2), and the reaction rate byproduct with high concentration (ca. 22.4 mg/m3) (R) of HCHO oxidation was calculated with (E3): appears. Abundant reactive oxygen species such as hydroxyl radical, O(1D), and O(3P) are formed under 185 nm VUV [HCHO] − [HCHO] Removal rate of HCHO = initial steady irradiation. So the VUV photolysis (VUV photochemical [HCHO]initial process) of HCHO can effectively occur. While TiO2 or noble metal modified TiO photocatalysts are presented, the reac- × 100%, 2 tion rate of HCHO decomposition increases from 1.16 to (E1) above 1.4 mg/m3·min. The outlet concentration of residual HCHO lowers enough to 10∼45 ppbv (Table 1). These − [O3]initial [O3]steady steady-state HCHO concentrations are much lower than the Removal rate of O3 = × 100%, (E2) [O3]initial WHO guideline level of indoor formaldehyde (80 ppbv). International Journal of Photoenergy 3

Table 1: The outlet concentration and removal rates of HCHO and O3, and reaction rates of HCHO in the 35 h VUV, O3 +UV254 nm,and UV254 nm photocatalysis, respectively.

Formaldehyde Ozone Kinds of photocatalysis Photocatalysts Concentration Removal rate Reaction rate Concentration Removal rate (ppbv) (%) (mg/m3·min) (mg/m3) (%) VUV photolysis 122.4 72.9 1.16 22.4 0

TiO2 36.1 91.4 1.34 19.2 14.3

VUV photocatalysis Pd-TiO2 20.1 95.3 1.43 14.2 36.6

Pt-TiO2 9.5 97.7 1.47 10.7 52.2

Au-TiO2 27.6 93.6 1.41 15.1 32.6

TiO2 178.8 57.4 0.85 13.8 37.8

Pd-TiO2 150.1 65.7 0.96 9.3 58.1 O3 +UV254 nm photocatalysis Pt-TiO2 125.1 68.6 1.01 6.2 72.1

Au-TiO2 130.6 68.8 1.02 10.8 51.4

TiO2 263.4 37.8 0.57

Pd-TiO2 251.3 37.6 0.56 UV254 nm photocatalysis Pt-TiO2 190.6 56.7 0.89

Au-TiO2 218.1 45.2 0.63

50 nm 100 nm

(a) (b) 40 (d) 35 30 25 20 15

Distribution (%) 10 5 0 50 nm 0 12345678910 Particle size (nm)

Pt d =1.9± 0.5 nm Au d=4.2± 1.2 nm Pd d=3.9± 1.5 nm

Figure 1: FESEM images of deposited Pt (a), Au (b), and Pd (c) nanoparticles on TiO2 ﬁlms. (d) The size distribution histogram of deposited Pt, Au, and Pd nanoparticles. 4 International Journal of Photoenergy

A Photocatalysis. Figures 4(a) and 5 show the outlet concentrations of HCHO versus irradiation time in the O3 + R A UV254 nm and UV254 nm photocatalysis, respectively. As shown A in Figures 4(a) and 5 and Table 1, the outlet concentration Pd A ∼ Pd of HCHO reaches 125 178 ppbv in the O3 +UV254 nm pho- Pd Pd tocatalysis and 190∼260 ppbv in the UV254 nm photocatalysis, ∼ (d) whileitisjust10 45 ppbv in the VUV photocatalysis when the same photocatalyst is used. The reaction rate of HCHO Au Au Au oxidation in the VUV photocatalysis is ca. 1.4 times and 2 times higher than that in the O3 +UV254 nm and UV254 nm (c) photocatalysis, respectively. As listed in Table 1, the HCHO degradation rate in the VUV photolysis (without photocata- Pt Pt Pt lyst) has been much higher than that in the O3 +UV254 nm or UV254 nm photocatalysis. Obviously, the VUV photochemical (b) process in gas phase makes great contribution in the oxidation of HCHO molecules in the VUV photocatalysis. Additionally, the stability of the outlet HCHO con- (a) centration in the long-term VUV photocatalysis is also very different from that in the O3 +UV254 nm or UV254 nm photocatalysis. The HCHO concentration gradually rises up, 20 30 40 50 60 70 80 90 while the reaction time exceeds 25 h in the O3 +UV254 nm or 2 theta (deg) UV254 nm photocatalysis (Figures 4(a) and 5), while it remains at a very low level in the VUV photocatalysis (Figure 3). A: anatase Due to the lower degradation rate of HCHO in the O3 + R: rutile UV254 nm or UV254 nm photocatalysis, the reaction intermediates combined with adsorbed HCHO molecules may per- Figure 2: XRD patterns of as-prepared TiO films (a), Pt-TiO (b), 2 2 sistently accumulate on the catalyst surfaces, which leads to Au-TiO2 (c), and Pd-TiO2 (d) nanocomposite films scratched from Ti wire net. observed catalyst deactivation. Nevertheless, the degradation behaviour under VUV irradiation suggests that no catalyst deactivation occurs in the VUV photocatalysis regardless of modification of TiO2. Due to abundant reactive species In the VUV photocatalysis, the HCHO can be decom- generated in gas phase, the oxidation of reaction inter- posed both in gas phase via VUV photochemical process and mediates and HCHO should been significantly enhanced under VUV irradiation. Therefore, the carbon accumulation on the photocatalyst surface via UV254 nm excited photocatalysis [8–10]. So the presence of photocatalysts under VUV on the catalyst could be alleviated even the HCHO gas is irradiation can remarkably increase the oxidation rate of continuously introduced with very short residence time. The VOCs. However, in the VUV photocatalysis of HCHO, only results in this study are highly consistent with the previously minor improvement of HCHO removal is observed by mod- observed long lifetime of the photocatalysts under VUV irradiation [8–10]. ifying TiO2 films with noble metals. This may be attributed to that most of HCHO molecules are decomposed in gas While the HCHO concentration is at a typical indoor ff phase not on the photocatalyst surface. level, the film-di usional resistance at the interface of gas However, the removal ratio of ozone byproduct increases phase and catalyst film becomes very large, leading to remarkable decrease of HCHO degradation rate in the from 14.3% (TiO2)to32∼52% (Au, Pd, or Pt-TiO2)withan enhancement of 2.3∼3.6-folds (Table 1). At the same time, UV254 nm photocatalysis [22]. Until now, it is still a great the outlet O concentration can keep almost at a same challenge for UV254 nm photocatalysis using semiconductor 3 ff level for a long steady-state period for noble metal modified films to e ectively decompose low concentration HCHO, while the gas residence time becomes short enough to several TiO2, but it gradually rises in the case of pure TiO2.The result suggests that ozone byproduct can be in situ photocat- seconds [22–24]. In this study, the results indicate that the alytically decomposed in the photoreactor, and modification VUV photocatalysis cannot only rapidly decompose HCHO vapor with low concentration but also avoid the deactivation of TiO2 films with noble metals can both enhance the photodegradation of O and alleviate the catalyst deactivation. The of the photocatalysts. Therefore, it can be anticipated that 3 ffi results reveal that simultaneous removal of low concentra- the VUV photocatalysis is an alternative method to e ciently decompose indoor VOCs. tion HCHO and O3 byproduct is surly feasible in the VUV photocatalysis when TiO2 films are modified with Pt, Au, or Pd NPs. 3.4. Enhanced Effects of Deposited Noble Metals on the Decom- position of HCHO and Ozone. As shown in Table 1,depo- 3.3. Comparison of HCHO Degradation in Long-Term VUV sition of noble metals (Pt or Au) on TiO2 surface actually Photocatalysis, O3 +UV254 nm Photocatalysis and UV254 nm increases the decomposition rate of HCHO in the VUV, International Journal of Photoenergy 5

500

400 20

300 15

200 10 HCHO concentration (ppbv) HCHO concentration O3 concentration (mg/m3) O3 concentration 100 80 ppbv 5

0 0 0510 15 20 25 30 35 0 5 10 15 20 25 35 30 Irradiation time (h) Irradiation time (h)

VUV photolysis Pd-TiO2 VUV photolysis Pd-TiO2 TiO2 Pt-TiO2 TiO2 Pt-TiO2 Au-TiO2 Au-TiO2 (a) (b)

Figure 3: Time courses for the outlet concentration of HCHO (a) and ozone byproduct (b) with irradiation time in the 35 h VUV photocatalysis. The VUV photolysis is the VUV photochemical process without the photocatalyst.

O3 +UV254 nm or UV254 nm photocatalysis. For the ozone of enhanced electron-hole separation caused by deposited decomposition, all of these three noble metal NPs (Pt, Au, noble metallic Pt or Au NPs and O3 decomposition routes or Pd) can have evident positive role. Especially, the removal (Figure 6). As presented in Figure 6,locallyformedmetallic ratio of HCHO increases from 57.4% (TiO2) to ca. 68% (Pt, Pt or Au NPs act as the electron trapping center. When or Au-TiO2)andthatofO3 augments from 37.8% (TiO2) the photogenerated electrons are trapped by metallic Pt or to 72.1% (Pt-TiO2) in the O3 +UV254 nm photocatalysis. In Au, electron-hole pairs are efficiently separated, producing • the UV254 nm photocatalysis, the reaction rate of HCHO oxi- more OH on the TiO2 surface [20, 25–28]. Thus, the photo- 3 dation increases from 0.57 mg/m ·min (TiO2)to0.89mg/ catalytic oxidation of HCHO is significantly enhanced. 3 m ·min (Pt-TiO2). However, deposition of Pd NPs in form For the photocatalytic decomposition of O3, the previous of PdO seems to be deleterious to HCHO oxidation in the ESR measurements revealed that ozonide radical anion •− •− UV254 nm photocatalysis (Figure 5). (O3 ), surface O radicals, and hydroperoxyl radical • These results indicate that deposited metallic NPs (Pt, (HO2 ) could be produced at the gaseous O3/TiO2 interface Au) can enhance the photocatalytic decomposition of both under UV irradiation [29]. Therefore, we propose the photo- HCHO and O3 molecules. While the PdO NPs appears, pho- catalytic decomposition of O3 on exposed TiO2 surface of the tocatalytic oxidation of HCHO is inhibited, but the ozone Pt-TiO2 (or Au-TiO2) as follows [15, 29, 30]: decomposition surely enhanced. The O3 removal behaviours − −→ •− (Figures 3 and 4) also reveal that the ozone, in situ generated O3 +e O3 (R1) in the VUV photoreactor or exteriorly added, can be effec- •− −→hν •− tively decomposed similarly. In the tested noble NPs (Au, O3 O2 +O (R2) Pt, or Pd), the metallic Pt NPs modified TiO2 exhibits the •− − • O +H2O −→ OH + OH (R3) highest activity in the HCHO oxidation and O3 reduction reactions. In addition, it can find that noble metal deposition •− − • O3 +H2O −→ OH + OH + O2 (R4) hasmuchmoreevidenteffect in O3 removal than in the − •− HCHO degradation. O2 +e −→ O2 (R5)

•− • •− O2 +O3 −→ OH + O3 (R6) 3.5. Proposed Mechanism of Positive Role of Deposited Noble •− Metallic NPs. In this work, the thermal decomposition of O3 The adsorbed O3 is reduced to O3 radicals by capturing on noble metal modiﬁed TiO2 ﬁlms is proved to be little (less the photogenerated electrons (R1). Under UV irradiation, •− •− than 5%) at room temperature in the dark. So the observed the unstable O3 radicals rapidly split to form O2 and O O3 decomposition should be ascribed to heterogeneous radicals (R2). Then, the adsorbed water molecules are oxi- •− •− • photodegradation. Therefore, we propose the mechanism dized by O3 or O radicals to form OH radicals (R3) and 6 International Journal of Photoenergy

500 16

400 12

300

200

4 O3 concentration (mg/m3) O3 concentration HCHO concentration (ppbv) 100 80 ppbv

0 0 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 Irradiation time (h) Irradiation time (h) TiO2 Pt-TiO2 TiO2 Pt-TiO2 Au-TiO2 Pd-TiO2 Au-TiO2 Pd-TiO2 (a) (b)

Figure 4: Time courses for the outlet concentration of formaldehyde (a) and ozone (b) with irradiation time in the 35 h O3 +UV254 nm photocatalysis.

500 O O3 3 − •− e O3 O2

− − − •− 400 e e O3 •− O2

Metallic 300 Au or Pt hA≥ 3.2eV

TiO2 200 + • h+ OH HCHO concentration (ppbv)

100 80 ppbv − H2O, OH

0 51015 20 25 30 35 Figure 6: Proposed mechanism of enhanced electron-hole separation caused by deposited noble metallic Au or Pt nanoparticles and Irradiation time (h) O3 decomposition routes on the Pt or Au modiﬁed TiO2. TiO2 Pt-TiO2 Au-TiO2 Pd-TiO2

Figure 5: Time courses for the outlet concentration of formalde- enhanced h+/e− separation efficiency by deposited Pt or hyde with irradiation time in the 35 h UV254 nm photocatalysis. Au NPs could improve the photocatalytic decomposition of adsorbed O3 on exposed TiO2. But it should be noted that the exposed TiO2 surface area, available for O3 decomposition, (R4). Alternatively, adsorbed O2 will also capture electrons, is also remarkably reduced owing to covering Pt or Au NPs • •− leading to formation of OH and O3 radicals (R5) and layer. Since the ESR tests have proved that adsorbed O3 can + − •− (R6). Obviously, enhanced separation of h /e pairs allows capture electrons to form O3 radicals on CeO2 [31]orTiO2 more photogenerated electrons to be captured by adsorbed [29] surface even in the dark. We propose here that adsorbed O3 molecules. O3 may also interact with electron-rich surface sites on Pt •− As shown in Table 1, the O3 removalrateisimproved or Au NPs to form O3 radicals (R1) by considering the over 2 times with metallic Pt or Au modified TiO2.The fact that photogenerated electrons have been readily trapped International Journal of Photoenergy 7 by metallic Pt or Au NPs. With the help of UV irradiation, in an air cleaner,” Chemical Engineering Science, vol. 60, no. 1, •− the split of O3 radicals on Pt or Au NPs is enhanced (R2). pp. 103–109, 2005. The similar phenomenon had been observed in the ESR test [6] M. C. Blount and J. L. Falconer, “Steady-state surface species of gaseous O3/TiO2 interface [29]. Then, the chain radical during toluene photocatalysis,” Applied Catalysis B, vol. 39, no. reactions proceeded on Pt or Au NPs may be similar to those 1, pp. 39–50, 2002. [7] H. Einaga, S. Futamura, and T. Ibusuki, “Heterogeneous pho- on UV-irradiated TiO2 (R3) and (R4). tocatalytic oxidation of benzene, toluene, cyclohexene and cyclohexane in humidified air: comparison of decomposition 4. Conclusions behavior on photoirradiated TiO2 catalyst,” Applied Catalysis B, vol. 38, no. 3, pp. 215–225, 2002. In this work, we investigated the feasibility of simultaneous [8] J. Jeong, K. Sekiguchi, and K. Sakamoto, “Photochemical and elimination of gaseous formaldehyde and ozone byproduct photocatalytic degradation of gaseous toluene using short- in the VUV photocatalysis process. The gaseous HCHO wavelength UV irradiation with TiO2 catalyst: comparison of with the inlet concentration of ca. 420 ppbv can be readily three UV sources,” Chemosphere, vol. 57, no. 7, pp. 663–671, degraded to meet the WHO guideline level of indoor 2004. formaldehyde in a short solid-gas contact time (0.34 s). How- [9]P.Zhang,J.Liu,andZ.Zhang,“VUVphotocatalyticdegrada- ever, the O3 +UV254 nm or UV254 nm photocatalysis shows tion of toluene in the gas phase,” Chemistry Letters, vol. 33, no. much lower removal ratio of HCHO with observable catalyst 10, Article ID CL-040801, pp. 1242–1243, 2004. [10] J. Jeong, K. Sekiguchi, W. Lee, and K. Sakamoto, “Pho- deactivation. By modifying TiO2 film with noble metallic Pt or Au, both HCHO removal and ozone decomposition todegradation of gaseous volatile organic compounds (VOCs) are significantly improved. Especially, over 50% ozone using TiO2 photoirradiated by an ozone-producing UV lamp: decomposition characteristics, identification of by-products byproduct can be in situ decomposed by using Pt-TiO2 and water-soluble organic intermediates,” Journal of Photo- films. However, deposition of PdO oxide seems to inhibit chemistry and Photobiology A, vol. 169, no. 3, pp. 279–287, the oxidation of HCHO in the UV254 nm photocatalysis but 2005. enhances the O3 decomposition in the VUV or O3 +UV254 nm [11] J. Jeong, K. Sekiguchi, M. Saito, Y. Lee, Y. Kim, and K. Sakamo- photocatalysis. Loaded metallic Pt or Au particles reduce the to,“RemovalofgaseouspollutantswithaUV-C254+185 nm/TiO2 + − recombination of h /e pairs and thus increase the HCHO irradiation system coupled with an air washer,” Chemical oxidation and O3 photocatalytical reduction on exposed Engineering Journal, vol. 118, no. 1-2, pp. 127–130, 2006. ff TiO2 surface. In addition, adsorbed O3 may also be decom- [12] N. Quici, M. L. Vera, H. Choi et al., “E ect of key parameters posed on electron-rich surface sites of metallic Pt or Au on the photocatalytic oxidation of toluene at low concen- under UV irradiation. trations in air under 254+185 nm UV irradiation,” Applied Catalysis B, vol. 95, no. 3-4, pp. 312–319, 2010. [13] Z. Hao, D. Cheng, Y. Guo, and Y. Liang, “Supported gold Acknowledgments catalysts used for ozone decomposition and simultaneous elimination of ozone and carbon monoxide at ambient temper- The authors gratefully acknowledge the financial support ature,” Applied Catalysis B, vol. 33, no. 3, pp. 217–222, 2001. from National High Technology Research and Development [14] K. Sekiguchi, A. Sanada, and K. Sakamoto, “Degradation of Program of China (2012AA062701), National Nature Science toluene with an ozone-decomposition catalyst in the presence Foundation of China (50908132), and Tsinghua University of ozone, and the combined effect of TiO2 addition,” Catalysis Initiative Scientific Research Program. Communications, vol. 4, no. 5, pp. 247–252, 2003. [15] P. Pichat, J. Disdier, C. Hoang-van, D. Mas, G. Goutailler, and References C. Gaysee, “Purification /deordorization of indoor air and gas effluents by TiO2 photocatalysts,” Catalysis Today, vol. 63, no. [1] S. Wang, H. M. Ang, and M. O. Tade, “Volatile organic com- 2–4, pp. 363–369, 2000. pounds in indoor environment and photocatalytic oxidation: [16] A. Mills, S. K. Lee, and A. Lepre, “Photodecomposition of state of the art,” Environment International,vol.33,no.5,pp. ozone sensitized by a film of titanium dioxide on glass,” Jour- 694–705, 2007. nal of Photochemistry and Photobiology A, vol. 155, no. 1-3, pp. [2]B.Y.Lee,S.W.Kim,S.C.Lee,H.H.Lee,andS.J.Choung, 199–205, 2003. “Photocatalytic decomposition of gaseous formaldehyde using [17] K. C. Cho, K. C. Hwang, T. Sano, K. Takeuchi, and S. Mat- TiO2,SiO2-TiO2 and Pt-TiO2,” International Journal of Pho- suzawa, “Photocatalytic performance of Pt-loaded TiO2 in the toenergy, vol. 5, no. 1, pp. 21–25, 2003. decomposition of gaseous ozone,” Journal of Photochemistry [3]S.V.Awate,A.A.Belhekar,andS.V.Bhagwat,“Effect of and Photobiology A, vol. 161, no. 2-3, pp. 155–161, 2004. gold dispersion on the photocatalytic activity of mesoporous [18] Y. C. Lin and C. H. Lin, “Catalytic and photocatalytic degra- titania for the vapor-phase oxidation of acetone,” International dation of ozone via utilization of controllable nano-Ag modi- Journal of Photoenergy, vol. 5, no. 1, pp. 21–25, 2003. fied on TiO2,” Environmental Progress, vol. 27, no. 4, pp. 496– [4] P. Monneyron, A. De La Guardia, M. H. Manero, E. Oliveros, 502, 2008. M. T. Maurette, and F. Benoit-Marquie,´ “Co-treatment of [19]P.He,M.Zhang,D.Yang,andJ.Yang,“PreparationofAu- industrial air streams using A.O.P. and adsorption processes,” loaded TiO2 by photochemical deposition and ozone photo- International Journal of Photoenergy, vol. 5, no. 3, pp. 167–174, catalytic decomposition,” Surface Review and Letters, vol. 13, 2003. no. 1, pp. 51–55, 2006. [5] C. H. Ao and S. C. Lee, “Indoor air purification by photocat- [20] P. Fu and P. Zhang, “Uniform dispersion of Au nanoparticles alyst TiO2 immobilized on an activated carbon filter installed on TiO2 film via electrostatic self-assembly for photocatalytic 8 International Journal of Photoenergy

degradation of bisphenol A,” Applied Catalysis B, vol. 96, no. 1-2, pp. 176–184, 2010. [21] T. Salthammer, S. Mentese, and R. Marutzky, “Formaldehyde in the indoor environment,” Chemical Reviews, vol. 110, no. 4, pp. 2536–2572, 2010. [22] F. Shiraishi, D. Ohkubo, K. Toyoda, and S. Yamaguchi, “Decomposition of gaseous formaldehyde in a photocatalytic reactor with a parallel array of light sources: 1. Fundamental experiment for reactor design,” Chemical Engineering Journal, vol. 114, no. 1-3, pp. 153–159, 2005. [23] L. Yang, Z. Liu, J. Shi, Y. Zhang, H. Hu, and W. Shangguan, “Degradation of indoor gaseous formaldehyde by hybrid VUV and TiO2/UV processes,” Separation and Purification Technology, vol. 54, no. 2, pp. 204–211, 2007. [24] L. Yang, Z. Liu, J. Shi, H. Hu, and W. Shangguan, “Design consideration of photocatalytic oxidation reactors using TiO2- coated foam nickels for degrading indoor gaseous formaldehyde,” Catalysis Today, vol. 126, no. 3-4, pp. 359–368, 2007. [25] Z. Sheng, Z. Wu, Y. Liu, and H. Wang, “Gas-phase photocatalytic oxidation of NO over palladium modified TiO2 catalysts,” Catalysis Communications, vol. 9, no. 9, pp. 1941– 1944, 2008. [26] M. A. Aramend´ıa, J. C. Colmenares, A. Marinas et al., “Effect of the redox treatment of Pt/TiO2 system on its photocatalytic behaviour in the gas phase selective photooxidation of propan- 2-ol,” Catalysis Today, vol. 128, no. 3-4, pp. 235–244, 2007. [27] P. Falaras, I. M. Arabatzis, T. Stergiopoulos, and M. C. Bernard, “Enhanced activity of silver modified thin-film TiO2 photocatalysts,” International Journal of Photoenergy, vol. 5, no. 3, pp. 123–130, 2003. [28] A. Yamakata, T. A. Ishibashi, and H. Onishi, “Pressure dependence of electron- and hole-consuming reactions in photocatalytic water splitting on Pt/TiO2 studied by time- resolved IR absorption spectroscopy,” International Journal of Photoenergy, vol. 5, no. 1, pp. 7–9, 2003. [29] M. D. Hernandez-Alonso,´ J. M. Coronado, A. Javier Maira, J. Soria, V. Loddo, and V. Augugliaro, “Ozone enhanced activity of aqueous titanium dioxide suspensions for photocatalytic oxidation of free cyanide ions,” Applied Catalysis B, vol. 39, no. 3, pp. 257–267, 2002. [30] M. Nicolas, M. Ndour, O. Ka, B. D’Anna, and C. George, “Photochemistry of atmospheric dust: ozone decomposition on illuminated titanium dioxide,” Environmental Science and Technology, vol. 43, no. 19, pp. 7437–7442, 2009. [31] A. Naydenov, R. Stoyanova, and D. Mehandjiev, “Ozone decomposition and CO oxidation on CeO2,” Journal of Molec- ular Catalysis. A, Chemical, vol. 98, no. 1, pp. 9–14, 1995. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 620764, 8 pages doi:10.1155/2012/620764

Research Article Synthesis of Neutral SiO2/TiO2 Hydrosol and Its Application as Antireﬂective Self-Cleaning Thin Film

Chiahung Huang,1, 2 Hsunling Bai,1 Yaoling Huang,2 Shuling Liu,2 Shaoi Yen,2 and Yaohsuan Tseng3

1 Institute of Environmental Engineering, National Chiao Tung University, Hsinchu 300, Taiwan 2 Green Energy and Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan 3 Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei City 106, Taiwan

Correspondence should be addressed to Hsunling Bai, [email protected]

Received 25 January 2012; Accepted 27 February 2012

Academic Editor: Xuxu Wang

Copyright © 2012 Chiahung Huang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

AneutralSiO2/TiO2 composite hydrosol was prepared by a coprecipitation-peptization method using titanium tetrachloride and silicon dioxide hydrosol as precursors. It is not only an antireflective self-cleaning coating material but also an environmental- ◦ benign material. Even heated at 700 C for 5 minutes in the tempering process, the as-prepared SiO2/TiO2 thin film still demonstrated antireflection and photocatalytic self-cleaning effect. The SiO2/TiO2 thin film increased near 2% of transmittance; however, the TiO2 thin film decreased 5% of transmittance at least. In addition to antireflection, the SiO2/TiO2 thin film decomposed the surface coated oleic acid under ultraviolet light and showed superhydrophilicity under dark for two days. The SiO2/TiO2 thin film also showed good photocatalytic degradation of methylene blue. With these antireflection, persistent superhydrophilicity, and photocatalytic self-cleaning effects, this prepared neutral SiO2/TiO2 hydrosol would be a good coating material for tempered glass and other building materials.

1. Introduction and reduce photocatalytic activity after tempering process, ◦ heating at 700 Cfor5minutes[6, 7]. Therefore, pure TiO2 A solar cell or building glass with antireflective self-cleaning is not a good coating material for solar cell or tempered glass. coating layer on the front glass would be a tendency for solar In general, silicon dioxide is often employed as an addi- energy or building materials [1–3]. The semiconductor- tive to TiO2 because of its chemical inertness, transparency based photocatalyst has been extensively studied since to UV radiation, high thermal stability, large specific surface the discovery of electrochemical photolysis of water on area, and low refractive index [8]. The SiO2-modified TiO2 TiO2 electrodes [4]. Among various photocatalysts, TiO2 material enhances the photocatalytic activity and increases has been widely applied to building materials because of the thermal stability [7, 9]. Zhang et al. [10, 11] used its environmental-benign property and self-cleaning effect electrostatic attraction method to deposit single-layered ff [1, 2]. The self-cleaning e ect is caused by the photoinduced TiO2 particles on SiO2 particles and formed an antireflective superhydrophilicity and photocatalysis on the photocatalytic photocatalytic layer. In the preparation process, an electro- layer [5]. However, the self-cleaning effect is inhibited with- static layer foamed by cationic or anionic polymer should out ultraviolet light or sunlight, and the large refractive index be coated before SiO2 or TiO2 deposition.Liu et al. [12] of anatase TiO2, n ∼ 2.52 would cause high reflection and and Jiang et al. [13] also used solvent-based SiO2 and TiO2 reduce the transmission of visible light. In addition, the ana- colloid solutions to prepare the TiO2/SiO2 bilayer in which tase TiO2 thin film may easily transfer to rutile phase TiO2 was the upper layer and demonstrated antireflection 2 International Journal of Photoenergy and higher photocatalytic activity. In contrast, Lee et al. [14] hydrosols were 0.0, 1.5, 3.0, and 5.0, which were labeled as prepared a SiO2/TiO2 thin film consisting of sequential pairs TiO2,ST1.5, ST 3, and ST 5, respectively. And the solid of positively charged TiO2 layer and negatively charged SiO2 concentration of TiO2 wasfixedat1wt%forallprepared layer via layer-by-layer method without organic binder. An hydrosols. For comparison, a pure SiO2 (3 wt%) hydrosol antireflective, antifogging, and photocatalytic thin film glass was also prepared by directly diluting the SM-30 with deion- could be prepared by increasing the number of SiO2/TiO2 ized water. bilayers. Similarly, Permpoon et al. [15] found that the upper In order to simulate the tempering process, a quartz glass, SiO2 layer of SiO2/TiO2 bilayer showed natural hydrophilic- 10 cm × 6cm× 0.15 cm, was used as the substrate, and the ◦ ity and stable Si–OH surface bond so that SiO2/TiO2 heating condition was controlled at 700 Cfor5minutes bilayer exhibited the optimal persistent superhydrophilicity with increasing rate of 10◦C/min. The quartz glass was under dark. Although literature data demonstrated excellent immersed in 1 M NaOH solution for 30 minutes to remove results, it takes many steps and energy to prepare SiO2/TiO2 oily pollutants and rinsed with deionized water for several or TiO2/SiO2 bilayers. times. After drying with air spray, a dip-coating method was Some researchers have prepared SiO2-TiO2 mixed sol used to deposit TiO2,SiO2,orSiO2/TiO2 thin film on the via sol-gel methods, and an antireflective self-cleaning thin quartz glass with corresponding hydrosol. The quartz glass film could be manufactured directly [16–19]. However, the was pulled up at the rate of 10 cm/min; then, it was dried at ◦ ◦ solvent-based SiO2-TiO2 mixed sol is not benign to environ- 60 Cfor5minutesandheatedat700 C for 5 minutes in air. ment. Zhang et al. [20] dispersed the prepared SiO2/TiO2 powder in water by ultrasonic treatment and formed the 2.2. Characterization of SiO /TiO Hydrosol and Thin Film. SiO -modified TiO hydrosol, but their results did not 2 2 2 2 The zeta potential of various hydrosols was analyzed by show any antireflection and self-cleaning effects. However, Malvern Zetasizer Nano-ZS. The specific surface area of TiO some sediment may appear in the prepared SiO -modified 2 2 or SiO /TiO composite material which had been heated at TiO hydrosol during storage. In this study, a modified 2 2 2 700◦C for 5 minutes was measured by Micromeritics ASAP coprecipitation-peptization method was used to directly 2020 using the Brunauer-Emmett-Teller (BET) method. The synthesize a neutral SiO /TiO hydrosol using titanium 2 2 morphology of TiO ,SiO,andSiO/TiO particles was tetrachloride and SiO hydrosol as raw materials. The char- 2 2 2 2 2 characterized by transmission electron microscopy (TEM: acteristics of the synthesized hydrosol were investigated, and Phillips Tecnai G2 F20, 100 kV). The crystal structure of the optical and self-cleaning effects of as-prepared thin film TiO and SiO /TiO was determined by X-ray diffractometer were evaluated. The optimal one-step-prepared SiO /TiO 2 2 2 2 2 (XRD: Bruker AXS/D2, Cu Kα radiation, 30 kV, 10 mA). composite thin film showed persistent superhydrophilicity, ThechemicalbondsofSiO/TiO composite materials antireflection, and photocatalytic self-cleaning effect. 2 2 were analyzed by Fourier transform infrared spectroscopy (FTIR: HORIBA FT-730). The topography and roughness of SiO2/TiO2 thin film were measured by atomic force 2. Experimental microscopy (AFM: Force Precision Instrument SThM). The thickness and transmittance of coating layer on quartz 2.1. Preparation of Neutral SiO /TiO Hydrosol and Thin 2 2 glass were measured by reflectometry (Micropack NanoCalc- Film. The neutral SiO /TiO hydrosol was synthesized by 2 2 2000) and UV-VIS spectrometer (JASCO V-530), respec- a modified coprecipitation-peptization method using TiCl 4 tively. solution and SiO2 hydrosol as precursors. Titanium tetrachloride (99%, Showa chemical) was added into deionized ◦ water at 4 C to form 1 M transparent TiCl4 solution. In order 2.3. Evaluation of Photocatalytic Self-Cleaning Effect to ensure complete hydrolysis, the NH4OH (4 M) solution was added into the 1 M TiCl4 solution and adjusted the 2.3.1. Contact Angle Measurement. The measurement of pH value to 7.0 [7, 21]. A white precipitate, Ti(OH)4,was water contact angle is a standard method for evaluating the then obtained after aging for 2 hours. The precipitate was self-cleaning performance of photocatalytic material [23]. filtered and washed with deionized water for several times to This method deposits oleic acid on photocatalytic plate as remove ammonium and chloride ions. A proper amount of oily pollutants. While the oleic acid is decomposed by the deionized water was then added to the filtered cake, and an photocatalytic plate, the surface would be more hydrophilic aqueous H2O2 solution (30 wt%) was added to oxidize and and the contact angle decreases. The water contact angle was peptize the Ti(OH)4 precipitate to form titanium peroxide measured by FTA 125, First Ten Angstrom, and the volume solution. The weight ratio of H2O2/TiO2 was controlled to of water droplet was 1 μL. be 2.0, and the peptization process should be kept for two First, the test piece should be irradiated under ultraviolet hours with vigorous stirring [22]. The titanium peroxide light (FL20SBLB, Sankyo Denki Co.) for 24 hours, and solution was heated at 90◦C for 4 hours, and various amount the intensity was adjusted at 2 mW/cm2 measured by UV of crystalline SiO2 hydrosol (30%, Ludox SM-30, Aldrich) photometer (UV-340A, Lutron). Second, the test piece was was then added into the solution. The neutral transparent dipped in a 0.5 vol% oleic acid solution (75093, Sigma- SiO2/TiO2 hydrosols with various SiO2/TiO2 weight ratios Aldrich Co., diluted with n-heptane), pulled up at a rate of were obtained by heating the mixture at 90◦Cfor4hours. 60 cm/min, and then dried at 70◦C for 15 minutes. Next,the The weight ratios of SiO2/TiO2 in the prepared SiO2/TiO2 water contact angle of test piece was measured before International Journal of Photoenergy 3

UV irradiation and periodically under UV irradiation increased with SiO2 content. For example, the surface area of 2 ◦ (1 mW/cm ) until less than 5 . Then, the test piece was put in ST 3 was almost three times that of TiO2. Figure 2 shows the dark, and the persistent hydrophilicity could be evaluated by XRD patterns of TiO2 and SiO2/TiO2 composite materials the periodic measurement of water contact angle. after heating at 700◦C for 5 minutes. All the four XRD patterns showed anatase TiO2 structure, and the peak (2θ ∼ 22◦) as signed for SiO was only observed in SiO /TiO 2.3.2. Decomposition of Methylene Blue. The decomposition 2 2 2 composite materials. Using the Scherrer’s equation (i.e., d = of methylene blue in an aqueous medium is also a standard 0.94λ/β cos θ) to calculate the crystallite size of TiO based method for measuring the self-cleaning activity of photo- 2 on the XRD patterns in Figure 2, it was 40.5 nm, 8.5 nm, catalytic surface [24, 25]. A modified reactor was made of 8.3 nm, and 7.8 nm for TiO ,ST1.5, ST 3, and ST 5, acrylic glass with higher ultraviolet transmittance, and the 2 respectively. On the other hand, the aggregation of TiO at inner size was 7 cm (length) × 1 cm (width) × 6 cm (height). 2 high temperature could be retarded by the added SiO It held the test liquid (35 mL, 10 μmol/L of methylene blue) 2 particles which suppressed the growth of crystallite size. The and a test piece. Two UV lamps irradiated the test piece from higher thermal stability of SiO /TiO composite materials both sides. The UV lamps and photometer were the same as 2 2 may be caused by the Ti–O–Si bond formed in the SiO /TiO contact angle measurement, but the intensity was 2 mW/cm2 2 2 hydrosols, as shown in Figure 3. measured on both sides of the surface of the test piece. The In Figure 3, the FTIR spectra of TiO2,SiO2 and three concentration of methylene blue in test liquid was measured ◦ SiO /TiO hydrosols heated at 100 C for one hour were by a UV-VIS spectrometer (V-530, JASCO) at 664 nm every 2 2 depicted. The interaction between TiO and SiO in the 20 minutes for 3 hours. 2 2 prepared SiO2/TiO2 hydrosols was clearly shown in the Ti– −1 O–Si bond (∼970 cm )[7–9]. The spectra of SiO2/TiO2 3. Results and Discussion hydrosols showed that only part of SiO2 reacted with TiO2 to form Ti–O–Si structure. The Ti–O–Si bond improved the 3.1. Characteristics of SiO2/TiO2 Hydrosols and Thin Films. thermal stability of TiO2 and suppressed the phase transfor- −1 The properties of prepared SiO2/TiO2 hydrosols and as- mation from anatase to rutile. The band at 1100 cm was prepared thin films are summarized in Table 1. The pH value assigned as the asymmetric Si–O–Si stretching vibration, of TiO2 and ST serial hydrosols was around 7 to 8 which observed for SiO2 and all SiO2/TiO2 hydrosols. And the band could be considered as a neutral hydrosol. Because of their at 1400 cm−1 was attributed to Ti–O–Ti vibration, observed neutral property, they could be applied to various substrates for TiO2,ST1.5 and ST 3 hydrosols. The spectra showed without the problem of corrosion. Meanwhile, each absolute that the intensity of Ti–O–Ti band was weaker while the value of zeta potential was higher than 40 mV and showed weight ratio of SiO2/TiO2 was higher, and it could not be good stability even in neutral condition. In our prior study measured at ST 5. The band at 1630 cm−1 was assigned to [7], ST 1.5 and ST 3 hydrosols have been stored at room the bending vibration of the O–H bond of chemisorbed temperature for more than two years without any sediment. water. And the broad band around 3400 cm−1 was due to However, the hydrosol using ultrasonic treatment for suspen- the stretching mode of the O–H bond of free water, both sion only could be stable for 2 weeks [26]. This stability for mainly caused by the hydrophilicity of SiO2. This natural storage may not only be caused by the preparation method wettability attributed to the upper O–H bonds on the ST but also by the addition of SiO2. Because the zero point serial composite thin films would prefer to adsorb moisture of charge was 5.5 for TiO2 and 2.1 for SiO2, the surface than oily pollutant and keep persistent superhydrophilicity. charge of both TiO2 and SiO2 was negative while the pH Figure 4 depicts the transmittance spectra of SiO2,TiO2, value was around 7 to 8. The negatively charged TiO2 and and SiO2/TiO2 composite thin films coated on quartz glass ◦ SiO2 particles tended to separate each other and promote the andheatedat700C for 5 minutes; the heating condition stability of suspension. The TEM micrograph in Figure 1(a) was similar to the tempering process. The spectrum of TiO2 indicates that the shape of TiO2 in TiO2 hydrosol was thin film showed the lowest transmittance because of its arrowhead-like with long axis of 30–40 nm and short axis of highest refractive index, 2.52. In contrast, the spectrum 6–10 nm, and Figure 1(b) shows the spherical SiO2 particles of SiO2 thin film exhibited the highest transmittance and with the size of 10 nm. However, the TiO2 and SiO2 hydrosols antireflection effect because of its lowest refractive index, showed some aggregation. Figures 1(c) and 1(d) indicate 1.46, whereas SiO2 thin film has no photocatalytic activity. very slender rodlike shape TiO2 crystals (short axis of 2– While adding SiO2 into the preparation of neutral TiO2 4 nm) embedded deeply in spherical SiO2 particles and well hydrosol, the prepared neutral SiO2/TiO2 hydrosol had lower dispersed in the ST 1.5 and ST 3 hydrosols. Therefore, the refractive index as presented in Table 1. And the as-prepared addition of SiO2 in the preparation process could decrease SiO2/TiO2 thin film exhibited photocatalytic activity and the particle size of TiO2 and increase the stability of neutral higher transmittance. The ST 1.5 thin film increased by 1.6% SiO2/TiO2 composite hydrosols. of transmittance than TiO2 thin film at 600 nm but still lower A high temperature calcination was usually used to than the substrate. When the SiO2/TiO2 weight ratio was promote the immobility of TiO2 on a substrate but it would raised to 3, the transmittance was increased by 0∼1.6% as decrease the surface area and the photocatalytic activity of compared to that of quartz glass in the range of 440 to TiO2 [27]. The surface area shown in Table 1 demonstrated 800 nm. Further raised SiO2/TiO2 ratio to 5, the trans- that the surface area of SiO2/TiO2 composite material was mittance was raised by 2.7%, whereas some fluctuation 4 International Journal of Photoenergy DegradationofMB(%) d Ra (nm) Roughness thin film 2 /TiO 2 Thickness (nm) (%) c Transmittance at 600 nm b ]. 18 [ 2 and SiO Refractive index 2 hydrosols and as-prepared thin films. 2 a /g) /TiO 2 2 (m BET S Surface area Cfor5minutesfirst. ◦ 1: Properties of SiO 49.442.042.241.045.3 55 122 147 177 — 2.52 1.81 1.67 1.60 1.46 90.5 92.1 94.8 94.4 97.8 65 84 165 216 129 4.25 6.20 4.04 2.36 0.86 54.0 64.9 70.2 90.2 9.7 − − − − − Table hydrosol SiO 2 /TiO 2 SiO marea. μ 5 conc. (wt%) pH Zeta potential (mV) × 2 m 0.0/1.03.0/0.0 7.2 9.0 μ /TiO 2 2 2 1.535 1.5/1.0 3.0/1.0 5.0/1.0 7.7 7.8 7.9 The average roughness in 5 The refractive index of ST serial bulk composite material was calculated by the molar composition of TiO Before the measurement of surface area, the material should be heated at 700 The transmittance of quartz glass at 600 nm was 93.4%. TiO ST ST ST SiO Nomenclature SiO a b c d International Journal of Photoenergy 5

(a) (b)

Figure 1: TEM micrographs: (a) TiO2,(b)SiO2,(c)ST 1.5, and (d) ST 3 hydrosol. happened in the spectrum of ST 5 and even lower than the under dark after the contact angle was lower than 5◦.If glass substrate during 663 to 843 nm. The thickness of ST 5 the photocatalytic thin film could keep the water contact thin film was too thick, may be a reason for the fluctuation. angle lower than 5◦ for long time, it means that the thin Previous studies [28, 29] have reported that the refractive film has better persistent superhydrophilicity. As reported index of an oxide thin film could be reduced by the presence in previous studies [18, 19], SiO2/TiO2 composite materials 2 = had the property of natural wettability and tended to attract of porosity, which can be explained by the equation np 2 − − moisture more than oily pollutants; thus, the surface could (nd 1)(1 P)+1,wherenp and nd are the refractive indices of the oxide on porous and nonporous states and keepcleanevenindark.Figure 6 shows the results of water P is the porosity percentage. Thus, the refractive index contact angle measurement on TiO2,ST1.5, ST 3, and ST 5 thin films. Although the initial contact angle of TiO2 thin of a porous thin film could become much lower than its ◦ nonporous form. The topography of the uncoated quartz film was as high as 58 ,TiO2 thin film still presented the highest decomposition rate of oleic acid, and it only took 6 glass and coated thin films was analyzed by AFM and showed ◦ in Figure 5. The AFM 3D images showed the surface of hours to reach 0 . The natural wettability of ST 1.5, ST 3, uncoated quartz glass was much smoother than other thin- and ST 5 thin film suppressed the adsorption of oleic acid on the surface and exhibited lower initial contact angle of film-coated glasses. The SiO2 thin film had a uniform porous ◦ ◦ ◦ structure; thus it exhibited the highest transmittance and 39 ,36,and27, respectively. It indicated that the natural antireflection effect. Although the porous structure on the wettability may be correlated to the SiO2/TiO2 ratio. This ST 3andST5 thin film was not so uniform, the high was consistent with the previous studies [18, 19]. However, it was very difficult to decompose the oleic acid adsorbed porosity of thin film reduced the value of np and thus enhanced the transmittance and led to higher antireflection on SiO2;thus,ST1.5, ST 3andST5 thin film took 8, 24, effect than the substrate. and 48 hours to decompose oleic acid thoroughly. When the superhydrophilic samples were stored in dark, TiO2 thin ◦ 3.2. Photocatalytic Self-Cleaning Effect of SiO2/TiO2 Thin film only kept the contact angle at 0 for 4 hours; then it Film. The water contact angle was continuously measured increased to 30◦ at the 24th hour and further to 47◦ at the 6 International Journal of Photoenergy

100 SiO 2 ST 5 ST 3 95 Quartz ST 1.5 90 TiO2

a (%) Transmittance 85

80 Intensity (a.u.) Intensity 200 300 400 500 600 700 800 900 1000 1100 b Wavelength (nm) Figure 4: Transmittance spectra of TiO ,SiO,andSTserial c 2 2 composite thin ﬁlms coated on quartz glass and heated at 700◦C for 5 minutes. d

10 20 30 40 50 60 ◦ but the transmittance is the lowest. The SiO2/TiO2 composite 2θ ( ) thin films showed higher degradation rates of methylene blue and higher transmittance. The degradation of methylene Anatase TiO 2 blue was enhanced with increasing the SiO ratio which SiO2 2 was attributed to the higher surface area. Furthermore, the Figure 2: XRD patterns of TiO2 and ST serial composite materials photoinduced negative-charged surface of n-type semicon- ◦ heated at 700 C for 5 minutes: (a) TiO2,(b)ST1.5, (c) ST 3, and ductor TiO2 attracted the cationic dye of methylene blue (D) ST 5. and then adsorbed on the surface of SiO2.Aspresented in Table 1,ST5 thin film had the highest surface area and Ti–O–Ti thickness; thus, it performed the highest degradation ratio 1400 of methylene blue. However, the FTIR spectrum of ST 5 a thin film showed the Ti–O–Ti bond was too weak to be O–H Ti–O–Si detected. The corresponding contact angle measurement of b O–H 1630 970 ST 5 also reveals its lower photocatalytic activity. Thus, it 3400 Si–O–Si c 1100 could be inferred that the degradation of methylene blue over ST 5 may mainly be caused by adsorption instead of photocatalysis effect. As described in the introduction, a Intensity (a.u.) Intensity d good antireflective self-cleaning thin film should have higher transmittance than substrate, fair photocatalytic activity and e persistent superhydrophilicity. As a result, ST 3 thin film should be the optimal choice. 4000 3600 3200 2800 2400 2000 1600 1200 800 Wavenumber (cm−1) 4. Conclusions Figure 3: FTIR spectra of TiO2,SiO2, and ST serial composite ma- Inthisstudy,aneutralSiO/TiO composite hydrosol was terials heated at 100◦C for one hour: (a) TiO ,(b)ST1.5, (c) ST 3, 2 2 2 synthesized by a modified coprecipitation-peptization meth- (d) ST 5, and (e) SiO2. od using TiCl4 and SiO2 hydrosol as precursors. The prepared SiO2/TiO2 composite hydrosol is an environmental 48th hour. In contrast, ST 1.5, ST 3andST5 all kept the benign material and has good stability for keeping in am- ◦ contact angle at 0 for 48 hours and exhibited good persistent bient for more than two years. The additive SiO2 decreased superhydrophilicity. This persistent superhydrophilicity is the refractive index, suppressed the aggregation of TiO2,and very important to the self-cleaning effect for building formed Ti–O–Si bond with TiO2. The lower refractive index material or solar cell, especially in cloudy day or at night. of SiO2/TiO2 thin film could increase the transmittance of The wet decomposition of methylene blue is a typical visible light, and the Ti–O–Si bond could retard the trans- method to evaluate the activity of photocatalytic thin film. formation of TiO2 from anatase to rutile. In addition, SiO2 Figure 7 shows the results which performed by various thin particles separated TiO2 particles and suppressed the growth films prepared by SiO2,TiO2, and various SiO2/TiO2 hy- of TiO2; thus, SiO2/TiO2 composite material had larger drosols. While the SiO2 thin film demonstrated the highest surface area after treating with high temperature. Simul- transmittance, the degradation of methylene blue was almost taneously, the natural wettability of SiO2/TiO2 thin film zero, only adsorption. In contrast, the TiO2 thin film contributed to the persistent superhydrophilicity. While the exhibited photocatalytic decomposition of methylene blue SiO2/TiO2 weight ratio was 3, the prepared SiO2/TiO2 thin International Journal of Photoenergy 7

0 823.167 2503 1646.33 Y 0 2469.5 Y 0 3292.67 2503 823.167 4115.83 1646.33 2469.5 X 3292.67 X 5006 5006 4939 4939 4115.83 (a) (b)

0 0 823.167

1646.33

2469.5 2503 Y Y 3292.67 0 823.167 0 1646.33 4115.83 2469.5 3292.67 2503 4939 4115.83 X 5006 4939 5006 X (c) (d)

Figure 5: AFM 3D images: (a) uncoated quartz glass, (b) SiO2,(c)ST3, and (d) ST 5 thin ﬁlm coated on quartz glass.

60 10 )

◦ 50 Under UV Under dark 8 40

30 mol/L) μ 6 20 4

Water contact angle ( contact Water 10

0 0 1224364860728496108 blue ( Methylene 2 Time (h)

TiO2 ST 3 0 0 20 40 60 80 100 120 140 160 180 ST 1.5 ST 5 Irradiation time (min) Figure 6: Water contact angle of TiO2,ST1.5, ST 3, and ST 5 thin film coated with oleic acid, first 48 hours irradiated with UV, next SiO2 ST 3 60 hours in dark. TiO2 ST 5 ST 1.5 film exhibited antireflection, persistent superhydrophilicity, ◦ and self-cleaning effect even treated at 700 Cfor5minutes Figure 7: Photocatalytic decomposition of methylene blue over for simulating the tempering process. The results showed SiO2,TiO2, and ST serial composite thin films. 8 International Journal of Photoenergy

the neutral SiO2/TiO2 composite hydrosol could be a good [15] S. Permpoon, M. Houmard, D. Riassetto et al., “Natural and antireflective self-cleaning coating material for solar cell or persistent superhydrophilicity of SiO2/TiO2 and TiO2/SiO2 bi- building materials. layer films,” Thin Solid Films, vol. 516, no. 6, pp. 957–966, 2008. [16] S. Permpoon, G. Berthome,´ B. Baroux, J. C. Joud, and M. Acknowledgments Langlet, “Natural superhydrophilicity of sol-gel derived SiO2- TiO2 composite films,” Journal of Materials Science, vol. 41, no. This research was supported by the Industrial Technology 22, pp. 7650–7662, 2006. Research Institute, Taiwan, Republic of China. And the au- [17] K. Guan, “Relationship between photocatalytic activity, hy- ff thors would like to thank Dr. Ching-chin Wu of GEL/ITRI drophilicity and self-cleaning e ect of TiO2/SiO2 films,” Sur- for the support of BET and AFM measurement. face and Coatings Technology, vol. 191, no. 2-3, pp. 155–160, 2005. [18] M. Houmard, D. Riassetto, F. Roussel et al., “Morphology and References natural wettability properties of sol-gel derived TiO2-SiO2 composite thin films,” Applied Surface Science, vol. 254, no. 5, pp. 1405–1414, 2007. [1] A. Fujishima, X. Zhang, and D. A. Tryk, “TiO2 photocatalysis and related surface phenomena,” Surface Science Reports, vol. [19] M. Houmard, D. Riassetto, F. Roussel et al., “Enhanced persis- 63, no. 12, pp. 515–582, 2008. tence of natural super-hydrophilicity in TiO2-SiO2 composite [2] K. Hashimoto, H. Irie, and A. Fujishima, “TiO2 photocatalysis: thin films deposited via a sol-gel route,” Surface Science, vol. a historical overview and future prospects,” Japanese Journal of 602, no. 21, pp. 3364–3374, 2008. Applied Physics 1, vol. 44, no. 12, pp. 8269–8285, 2005. [20] M. Zhang, L. Shi, S. Yuan, Y. Zhao, and J. Fang, “Synthesis [3] G. E. Jonsson, H. Fredriksson, R. Sellappan, and D. Chakarov, and photocatalytic properties of highly stable and neutral “Nanostructures for enhanced light absorption in solar energy TiO2/SiO2 hydrosol,” Journal of Colloid and Interface Science, devices,” International Journal of Photoenergy, vol. 2011, vol. 330, no. 1, pp. 113–118, 2009. Article ID 939807, 11 pages, 2011. [21] Y. H. Tseng, S. L. Liu, C. S. Kuo et al., “Preparation of nano- [4] A. Fujishima and K. Honda, “Electrochemical photolysis of sized TiO2 sol and its visible-light-responsive photocatalysis in water at a semiconductor electrode,” Nature, vol. 238, no. aquatic state,” Micro & Nano Letters, vol. 1, no. 2, pp. 116–119, 5358, pp. 37–38, 1972. 2006. [5] A. Fujishima, T. N. Rao, and D. A. Tryk, “Titanium dioxide [22] N. Sasirekha, B. Rajesh, and Y. W. Chen, “Synthesis of TiO2 sol photocatalysis,” Journal of Photochemistry and Photobiology C, in a neutral solution using TiCl4 as a precursor and H2O2 as an vol. 1, no. 1, pp. 1–21, 2000. oxidizing agent,” Thin Solid Films, vol. 518, no. 1, pp. 43–48, [6] Y. H. Tseng, H. Y. Lin, C. S. Kuo, Y. Y. Li, and C. P. Huang, 2009. “Thermostability of nano-TiO2 and its photocatalytic activ- [23] International Organization for Standardization, “Fine ceram- ity,” Reaction Kinetics and Catalysis Letters, vol. 89, no. 1, pp. ics (advanced ceramics, advanced technical ceramics)-test 63–69, 2006. method for self-cleaning performance of semiconducting [7]C.H.Huang,H.Bai,S.L.Liu,Y.L.Huang,andY.H.Tseng, photocatalytic materials—measurement of water contact “Synthesis of neutral SiO2/TiO2 hydrosol and its photocatalyt- angle,” Tech. Rep. ISO 27448, International Organization for ic degradation of nitric oxide gas,” Micro & Nano Letters, vol. Standardization, Geneva, Switzerland, 2009. 6, no. 8, pp. 646–649, 2011. [24] International Organization for Standardization, “Fine ceram- [8] X. Gao and I. E. Wachs, “Titania-silica as catalysts: molecular ics (advanced ceramics, advanced technical ceramics)— structural characteristics and physico-chemical properties,” determination of photocatalytic activity of surfaces in an Catalysis Today, vol. 51, no. 2, pp. 233–254, 1999. aqueous medium by degradation of methylene blue,” Tech. [9] M. Bellardita, M. Addamo, A. di Paola et al., “Photocatalytic Rep. ISO 10678, International Organization for Standardiza- activity of TiO2/SiO2 systems,” Journal of Hazardous Materials, tion, Geneva, Switzerland, 2010. vol. 174, no. 1–3, pp. 707–713, 2010. [25] J. Yao and C. Wang, “Decolorization of methylene blue [10] X. T. Zhang, O. Sato, M. Taguchi, Y. Einaga, T. Murakami, and with TiO2 sol via UV irradiation photocatalytic degradation,” A. Fujishima, “Self-cleaning particle coating with antireflec- International Journal of Photoenergy, vol. 2010, Article ID tion properties,” Chemistry of Materials, vol. 17, no. 3, pp. 696– 643182, 6 pages, 2010. 700, 2005. [26] J. Xu, L. Li, Y. Yan et al., “Synthesis and photoluminescence of [11] X. Zhang, A. Fujishima, M. Jin, A. V. Emeline, and T. well-dispersible anatase TiO2 nanoparticles,” Journal of Colloid Murakami, “Double-layered TiO2-SiO2 nanostructured films and Interface Science, vol. 318, no. 1, pp. 29–34, 2008. with self-cleaning and antireflective properties,” Journal of [27] J. Yu, W. Wang, B. Cheng, B. Huang, and X. Zhang, Physical Chemistry B, vol. 110, no. 50, pp. 25142–25148, 2006. “Preparation and photocatalytic activity of multi-modally [12] Z. Liu, X. Zhang, T. Murakami, and A. Fujishima, “Sol-gel macro/mesoporous titania,” Research on Chemical Intermedi- SiO2/TiO2 bilayer films with self-cleaning and antireflection ates, vol. 35, no. 6-7, pp. 653–665, 2009. properties,” Solar Energy Materials and Solar Cells, vol. 92, no. [28] B. E. Yoldas, “Investigation of porous oxides as an antireflec- 11, pp. 1434–1438, 2008. tive coating for glass surfaces,” Applied Optics,vol.19,no.9, [13] B. Jiang, X. C. Duan, Z. C. Zhou, Y. J. Cheng, G. C. Liu, pp. 1425–1429, 1980. and Y. L. Liu, “Synthesis and characterization of self-cleaning [29] B. E. Yoldas and D. P. Partlow, “Formation of broad band and anti-reflective SiO2-TiO2 nanometric films,” Journal of antireflective coatings on fused silica for high power laser Inorganic Materials, vol. 26, no. 4, pp. 375–380, 2011. applications,” Thin Solid Films, vol. 129, no. 1-2, pp. 1–14, [14] D. Lee, M. F. Rubner, and R. E. Cohen, “All-nanoparticle thin- 1985. film coatings,” Nano Letters, vol. 6, no. 10, pp. 2305–2312, 2006. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 609561, 10 pages doi:10.1155/2012/609561

Research Article Enhanced Photocatalytic Activity of Hierarchical Macro-Mesoporous Anatase by ZrO2 Incorporation

M. L. Garcıa-Benjume,´ 1 M. I. Espitia-Cabrera,2 and M. E. Contreras-Garcıa´ 1

1 Instituto de Investigaciones Metalurgicas,´ Universidad Michoacana de San Nicolas´ de Hidalgo, Ediﬁcio “U” Ciudad Universitaria, 58060 Morelia, MICH, Mexico 2 Facultad de Ingenier´ıa Qu´ımica, Universidad Michoacana de San Nicolas´ de Hidalgo, Ediﬁcio “U” Ciudad Universitaria, 58060 Morelia, MICH, Mexico

Correspondence should be addressed to M. E. Contreras-Garc´ıa, [email protected]

Received 27 January 2012; Revised 12 March 2012; Accepted 13 March 2012

Academic Editor: Stephane´ Jobic

Copyright © 2012 M. L. Garc´ıa-Benjume et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The effect of the addition of zirconia in the photocatalytic behaviour of titania is analysed. In order to increase the ways for reagent and product diffusion in the material, a sol-gel hydrothermal synthesis route using Tween-20 as a directing agent to obtain a hierarchical macro-mesoporous structure is proposed. Nanostructured macro-mesoporous TiO2/ZrO2 photocatalyst with 0, 10, 20, 30, and 100% mol of ZrO2 were obtained and calcined at different temperatures. The crystalline structure was analyzed by X-ray diffraction and TEM. The porosity was confirmed by SEM, TEM, and nitrogen adsorption-desorption isotherms. The worm-like mesoporous structure was confirmed by TEM. The specific surface areas obtained by Brunauer-Emmet-Teller method (BET) ranged from 125 to 180 m2/g. The Tween-20 total elimination from the structure by thermal treatment was confirmed by infrared spectroscopy and thermogravimetric analysis. Additionally, the photocatalytic effect of the zirconia addition was studied in the methylene blue (MB) degradation reaction, and the best photocatalytic activity was obtained in the sample with 10% mol of ZrO2, degrading up to 92% the MB.

1. Introduction the surface of the material (photocatalytic efficiency)[3–5]. The use of different nanoconjugates has been proposed to Recently, TiO2 has been widely used for the preparation circumvent the partial photocatalytic reactivity of TiO2 due of different types of nanomaterials, including nanoparticles to charge recombination and to increase the photoresponse with different shapes (nanorods, nanowires, nanotubes, and of TiO2, allowing nanoconjugates to be used for sterilization nanoflowers) and also mesoporous and nanoporous TiO2- with sunlight in a most energy-effective way. Multiple containing materials [1]. Regardless of scale, TiO2 maintains additions can be made to TiO2 nanoparticles to create TiO2 its photocatalytic abilities [2]. Nevertheless, TiO2 can present nanoconjugates; these additions include noble metal depo- low photocatalytic activity mainly due to the high electron- sition, doping with metal (Fe3+,Cu2+,Cr3+) and nonmetal hole pair recombination rate and the fact that it can be only ions (C, N, S, F) [6, 7], compositing with a polymer and exited under UV light wavelength. In order to solve these the creation of core-shell magnetic nanoparticles. Currently, problems, titania is combined with other oxides to form TiO2 nanoconjugates are used in multiple antimicrobial, nanoconjugates in order to increase the energetic range at antifungal, and waste decontamination applications [8], and which the nanoparticles can be excited (photoresponse). This can also be used to sterilize medical devices such as catheters allows the TiO2 nanoconjugates to be excited by energies in and dental implants [9, 10]. TiO2 nanoconjugates have also the visible light spectrum and to increase the ability of a been tested for the sterilization of food packaging and food photocatalytic material to overcome charge recombination preparation surfaces to prevent the bacterial contamination and allow separated charges to interact with molecules at of food [11–13]. However, the most often used application 2 International Journal of Photoenergy

of TiO2 nanoconjugates in this area has been for the These hierarchical porous ceramics can be used in catalysis, purification of drinking water and decontamination of waste photocatalysis, separation and for the immobilization of water [8, 14]. biological molecules, and even microorganisms, for filtration The semiconductors (CdS, ZnO, WO3)[15] and other and bioreactor applications [1]. metal oxides ZnO, MnO2,Al2O3 [16, 17], ZrO2 [18–20], Ta 2O5 [21], and In2O3 when coupled to TiO2 nanoparticles have also been shown as efficient in increasing the 2. Experimental Procedure photocatalytic capability of TiO2 nanoparticles [22–25]. The Two solutions were prepared, one of them was with surfac- addition of MnO2 or Al2O3 (charge-transfer catalysts) to tant (Tween-20 Fluka) water in weight ratio of 1 : 10 and TiO2 nanoparticles broadens the excitation spectrum of TiO2 to visible light ranges, as demonstrated by methylene blue the other was a buthanol alcoholic solution of Ti(OC4H9)4 degradation [22]andE. coli destruction [25]. and Zr(OC4H9)4 (both from Sigma-Aldrich) in the ste- quiometric ratios of 0, 10, 20, 30, and 100% mole of ZrO2-TiO2 system has been the object of few studies, and ZrO2. This solution was aggregated to the surfactant-water ZrO2 and TiO2 have similar catalytic behaviors presenting solution and was vigorously stirred. The sol obtained was amphoteric properties, which allows them to behave as either ◦ a Lewis acid or a Lewis base, depending on the pH of the hydrothermally processed at 80 C for 24 hours, dried at 100◦C. Those samples were labeled as T, T10Z, T20Z, T30Z, solution used to wash the TiO2 or ZrO2 material [26]. and Z, respectively, and then calcined at 400◦Cfor3hours Acidic conditions cause the TiO2 to be positively charged ◦ and to exhibit anion-exchange properties [27]. Consequent- at a heating rate of 1 C/min, after calcinations, the samples were labeled as T-400, T10Z-400, T20Z-400, T30Z-400, and ly, samples prepared for analysis on TiO2 or ZrO2 columns Z-400, respectively. The sample T10Z was also calcined at are often dissolved in an acidic solution so as to promote ◦ ◦ electrostatic binding between the positively and negatively 500 C for 3 hours at a heating rate of 1 C/min, this sample charged groups [28] after which they can be desorbed under was labeled as T10Z-500. alkaline conditions [29]. In order to determine the suitable thermal treatment to Concerning the photocatalytic behavior of TiO2 and totally eliminate the organic compounds, thermal analysis ZrO2 semiconductors, the TiO2 band gap is 3.2 eV [30, 31] was carried out using a thermogravimetric (TG) analyzer, a and the ZrO2 is 3.4 [32]. The similar band gap of both oxides Perkin Elmer Pyris Diamond model equipped with “Extar indicates a similar photocatalytic behavior. Nevertheless, the V6.2” software. The fresh samples were heated from room ◦ ◦ use of TiO2-ZrO2 nanoconjugates allows the creation of temperature to 400 Catarateof1 C/minute in air to deter- additional levels between TiO2’s valence and conduction mine the surfactant pyrolisis temperature. To corroborate bands which act as traps that retard the surface migration the successful elimination of organics, Fourier transform of the electron-hole pairs leading to a reactive migration infrared spectroscopy (FTIR) characterization was carried to the surface and the corresponding desorption of the out on pellets of the calcined powders mixed with KBr oxidized or reduced species. An additional advantage in the (spectroscopy grade) and was recorded on a Bruker Tensor −1 use of ZrO2 is that the zirconium stabilizes the anatase phase 27 spectrometer, in the range from 4000 to 400 cm ,atroom allowing its obtention at lower temperatures (<600◦C) [33]. temperature. The crystalline phases of the calcined powders An additional point is that nanosized materials have high were determined by X-ray diffraction (XRD) in a Philips surface area to volume ratios, which allows them to bind to a X’PERT model diffractometer, using Cu Kα radiation λ = ◦ much greater number of targets [2]. 1.54 A,˚ with 0.02 /seg steps. A scanning electron microscope Otherwise, the macro-mesoporous structure proposed in (SEM), JEOL JSM-6400 model, was used to observe the pow- this work for the TiO2-ZrO2 nanoconjugates enhances the der morphology. The nanoporous structure was observed by catalytic efficiency due to the better transport of reactants transmission electron microscopy (TEM) in a field emission and products through the hierarchical porous structure and FEI TECHNAI 200 microscope. The powder samples were also allows better light penetration into the photocatalytic dispersed in isopropyl alcohol and subject to ultrasonic material [34]. Zhang and Yu [30] have shown that the treatment for five minutes, then they were deposited on a presence of extralarge meso-macroporous channels in the hole copper grid covered with an amorphous carbon film, materials represent an efficient way for the reactants molec- by capillary deposition. ular species to reach the active sites on the porous walls. The powder samples were degassed at 200◦Ctoobtain In this paper, we demonstrate a self-formation phe- nitrogen adsorption-desorption isotherms, using Microme- nomenon of hierarchy with multiple-scaled porosity, using ritics ASAP 2010 Model equipment. The Brunauer-Emmet- Tween-20 as the structure direction agent in the photo- Teller (BET) surface areas of the powder samples were catalytic system TiO2-ZrO2. Macrochanneled structures are calculated from the obtained isotherms. The pore size dis- formed, with openings ranging from 0.2 to 5 microns and tributions were determined by the Barrett-Joyner-Halenda wormhole-like mesoporous walls, with various TiO2-ZrO2 (BJH) method. The photocatalytic tests were carried out ceramic compositions that can be prepared just by control- for methylene blue (MB) degradation using the obtained ling the hydrolysis and polycondensation conditions such powders and TiO2 P-25 Degussa powder as a reference, in a as pH and using hydrothermal treatment. The macrochan- batch reactor consisting of a reaction cell with a compressed nels (funnel-like or straight) are parallel to each other air supply in a tightly closed compartment equipped with and perpendicular to the tangent of the particles’ surface. an ultraviolet light using a Ultravg l25 6286 model lamp. International Journal of Photoenergy 3

110 1400 14311390 615 T-400 100 1200 T 90 Z –OH 1000 C) O–Ti–O 80 T10Z ◦ –OH 70 800 (%)

w 60 600 456 50

T20Z 400 Temperature ( T10Z-400 40 O–Zr–O Temperature 200 (a.u.) Transmittance T20Z-400 30 20 0 T30Z-400 434 0 50 100 150 200 250 300 Z-400 Time (min)

Figure 1: Thermograms of the TiO2-ZrO2 mixed oxides system 4000 3000 2000 1000 0 (T10Z-400, T20Z-400, and Z-400 samples). Wavenumber (cm−1)

Figure 2: FT-IR spectra of T10Z-400, T20Z-400, T30Z-400, T-400, and Z-400 samples. Thereactionwascarriedoutatroomtemperatureinan aqueous solution with 10 mg/L of MB. The powder rate was one gram of oxides powder per litre of MB at pH = 7. The Table 1: Wave number of the main registered vibrations. solution had been previously magnetically stirred at 400 rpm Frequency cm−1 Bond Corresponds to for 30 minutes without the use of UV light in order to reach a stable MB absorbance on the photocatalyst surface. Then, 3411 O–H Adsorbed water the solution was irradiated under UV light, magnetically 1630 OH Ti–OH, Zr–OH Tween-20 and stirred, and air was fed in simultaneously when the reaction 2927, 1450 CH yCH organic system was started. Aliquots of powder sample suspensions 2847, 1390 3 2 were taken each hour during 5 h of testing. MB residual precursors concentration was determined at 666 nm for absorbance 615 O–Ti–O TiO2 using a Perkin-Elmer UV-Vis Lambda 20 spectrophotometer. 456, 434 O–Zr–O ZrO2 Prior to the UV-Vis measurement, the suspensions were centrifuged at 2000 rpm by 5 minutes to eliminate the powder oxides interference during the test. The MB concentration Table 1 presents the wave number of the registered vibra- after each experiment was determined quantitatively through tions. The O–Ti–O bonding vibration located at 615 cm−1 is the calibration graph constructed from standard solutions of a broadband, the vibration at 1034 cm−1 corresponds to the MB at various concentrations. ester group present in Tween-20 [39], and this signal is not present in the mixed oxides or in ZrO2 samples. −1 3. Results and Discussion In the Z-400 sample, the bands at 456 and 434 cm are characteristic of Zr–O bond vibration of ZrO2 in its 3.1. Organic Compounds. Figure 1 presents the thermograms tetragonal phase [40]; in the mixed oxides, those bands are low, displaced from 456 to 466 cm−1. As the ZrO content obtained for the mixed oxide system TiO2-ZrO2 (T10Z- 2 400, T20Z-400, T-400, and Z-400 samples). In these ther- increases, the band presents better definition and is located at 456 cm−1. The broadband characteristic of the O–Ti–O bond mograms, all samples continuously lost weight in a linear − ◦ ◦ 1 ff manner from 90 C until 300 C this weight loss corresponds vibration, located at 615 cm , does not allow di erentiation to the adsorbed water and the buthanol desorption [35– of the characteristic bands in the mixed oxides systems. 37]. Another form of weight loss, between 180 and 210◦C, This behaviour is due to the fact, zirconium is the most was present; this is because the alkoxides bonded strongly electropositive transition metal and it presents a greater to the chemisorbed water. The pure zirconia sample, Z-400, attraction to water [41] than to the organic part of Tween- presented a great weight loss, which leads to the conclusion 20. that zirconia has less affinity than titania to these groups. Drastic weight loss was present from 270 to 300◦C, which 3.2. Crystalline Structure. The XRD results are presented corresponds to surfactant and organic precursors pyrolisis in Figure 3(a). Sample T-400 only presents the anatase [37, 38] all other weight loss was registered. These results phase and Z-400 sample shows the tetragonal phase of confirm that surfactant has been removed completely at zirconia (t-ZrO2). Samples T10Z-400, T20Z-400, and T30Z- ◦ 400 C, this temperature is similar to the that previously 400 present both phases, anatase and t-ZrO2. These results reported by the authors for TiO2-Al2O3 system [39]. clearly show the composition effect in the principal peaks Figure 2 shows the FT-IR spectra of T10Z-400, T20Z- intensity. The tetragonal phase t-ZrO2 is observed even at low ◦ 400, T30Z-400, T-400, and Z-400 samples calcined at 400 C. concentration (10% mol de ZrO2), so zirconia crystallization 4 International Journal of Photoenergy

t a t t t Z-400 t T10Z-500 a a a t a T30Z-400 t t

T20Z-400 Intensity (a.u.) a T10Z-400 Intensity (a.u.)

a a a a T10Z-400 T-400

20 30 40 50 60 70 20 30 40 50 60 70 2θ (degrees) 2θ (degrees)

t = t-ZrO2 JCPDS # 50−1089 t = t-ZrO2 JCPDS # 50-1089 a = anatase JCPDS #21-1272 a = anatase JCPDS # 21-1272

(a) (b)

Figure 3: X-ray diﬀraction patterns (a) of T10Z-400, T20Z-400, T30Z-400, T-400, and Z-400 samples and (b) for sample T10Z calcined at 400 and 500◦C.

JCPDS #21-1272

Zone axe [1-11]

Zone axe [11-1] d(101) = 3.5 A˚

d(101) = 2.9 A˚

= ˚ t- ZrO2 JCPDS#50-1089 d(−101) 3.5 A d(011) = 2.9 A˚

(a) (b)

Figure 4: HRTEM images of T20Z-400 sample showing the nanoparticles forming the nanoconjugates, inserts present the FFT and the high-resolution indexed zone. (a) Zirconia nanocrystal indexed as tetragonal phase, (b) titania nanocrystal indexed as anatase phase. was not affected by the presence of titania, and the anatase 3.3. Textural Properties. A SEM micrograph of the T10Z- phase remains stable in the presence of zirconia. Figure 3(b) 400 sample, at low magnification (2000x) is presented in shows the XRD diffraction patter for sample T10Z calcined Figure 5(a). The morphology of this sample presents a at 400 and 500◦C; from this result, it was found that anatase smooth surface “skin-film” type formed by channels; the TiO2 phase is thermally stable and the crystal size apparently fracture surface of the particle allows us to observe the is not affected when the sample was calcined between 400 and inside, showing a structure composed of spherical aggregates 500◦C, because the full width at half maximum is (FWHM) forming the porous walls. This complicated morphology can very similar in both diffractograms. be observed better in Figure 5(b), which shows a micrograph High-resolution transmission electron microscopy at greater magnification (20000x). Figures 5(c) and 5(d) (HRTEM) images of the T20Z-400 sample are shown in present SEM micrographs of the T20Z-400 sample, Figures Figure 4. It is possible to distinguish nanoparticles with 5(e) and 5(f) correspond to the T30Z-400 sample and mean diameters between 7 and 11 nm. Figure 4(a) presents Figures 5(g) and 5(h) correspond to the Z-400 sample. nanoparticles indexed as ZrO2 in the tetragonal phase. Comparing these micrographs, it can be observed that Figure 4(b) presents a particle indexed at the anatase phase, the spherical aggregates size increases as the zirconia content in this image, the particles size diameters are between 4 to is increased. In general, the macroporous structure is formed 12 nm. by channels, and the walls of these channels are formed by International Journal of Photoenergy 5 agglomerated spherical particles, as a consequence of this Table 2: Specific surface areas and Pore mean diameters results. the macroporous size increase too, as can be clearly seen in Figure 5(f). The pure zirconia Z-400 sample presents similar Mean pore Mean pore Specific surface diameter diameter morphological characteristics. Sample 2 area (SBET)m /g (BJH adsorption) (BJH desorption) The obtained macroporous structure will act as addi- nm nm tional paths for light penetration into the oxide powders, Z 45 16.2 15.7 helping to enhance the photocatalytic efficiency. The extinc- T10Z-400 150 11 9 tion of light in semiconductors follows the exponential law I = Io exp(−αl), where lis the distance that light penetrates T10Z-500 182 4 3.8 and α is the mutual penetration, which in titania has a T20Z-400 160 9.2 8 value of 2.6 × 104 cm−1 at 320 nm. This implies, that at a T30Z-400 128 10.4 9.1 wavelength of 320 nm, the light would be extinguished by T-400 136 8.5 6.6 90% after having traversed a distance of 3900 A˚ or 390 nm of the photocatalytic material [42]. Figure 6(a) shows the TEM image obtained by the Z- contrast technique for sample T10Z-400. This image shows a curve at relative pressures between 0.6 and 0.8 presents a porous zone around a sponge-like particle, this porous zone hysteresis loop type H2 corresponding to pores with narrow corresponding to macro-mesoporous textured powders. necks and wider bodies. At high relative pressures between Figure 6(b) presents thin and thick zones forming a 0.8 and 1, the shape of the hysteresis loop is type H3, sponge-like structure; in the thinnest zone, it is possible to associated with plate-like particles given rise to narrow slit distinguish the porosity in the sample, the dark zones are shape porous. Sample Z-400 presents a isotherm with a ff combination of type III and IV; at relative pressures below the mesoporous and the aggregates of di erent sizes. The ffi bright particles have high zirconium concentration and the 0.8, the isotherm corresponds to materials with low a nity grey ones have high titanium concentration. for the adsorbate [49], in this case nitrogen, or otherwise due to the low specific surface area. At high relative pressures, the The mesoporous powder structure was observed by isotherm is type IV with hysteresis loop type H3. the TEM using an out-of-focus technique with underfocus Figure 8(b) presents the isotherms for T10Z-400 and and overfocus approaches, the resultant image can provide T10Z-500 samples. T10Z-500 sample presents a type IV adequate contrast to be able to distinguish the channels isotherm characteristic for mesoporous materials which [43]. This mesoporous structure can be seen in Figure 7 present two hysteresis loops; this characteristic isotherm for the T20Z-400 sample, the image presents a “worm-like is associated to bimodal pore size distributions in the structure” [44], and this structure is typical for transition mesoporous region. At relative pressures between 0.3 and metals oxides where the bright and clear parts are the 0.7, the adsorption curve has the hysteresis loop type H2 channels and the dark zones are the oxide walls. From this corresponding to pores with narrow necks and wider bodies. result, the porous diameters are from 2 to 4 nm. At high relative pressures between 0.8 and 1, the shape The formation mechanism of the hierarchical macro- of the hysteresis loop is type H3, associated with plate- mesoporous structure of TiO2 has been explained by dif- like particles given rise to narrow slit porous shape. This ferent authors, Yu et al. [45] used the model proposed by characteristic isotherm is associated to the bimodal pore Collins et al. [46] to explain the porous structure obtained size distributions in the mesoporous. This is due to the with template-free photocatalytic hierarchical mesoporous presence of nanopores with sizes smaller than 5 nm and the TiO2. Collins has reported that the addition of surfactants correspondent high specific surface areas. Compared with could enhance the stability of microphase-separated inter- the T10Z-400 sample isotherm, in the T10Z-500 sample faces, rather than serve as self-assembled templates. The isotherm, it can be observed the shifting to lower relative mechanism has been also proposed for the porous structures pressures this result indicates the best remotion of the obtained using Tween-20 as template for TiO2-Al2O3 system organic compounds resulting in the increasing of the area in a previous work [39]. The similarity in the morphology and the decreasing of around 50% of the mesoporous size obtained with and without template needs to be further due to the pore walls contraction at higher temperature, investigated. which means that the interparticle porosity decreases the pore size [47]. 3.4. Macro-Mesoporosity. Figure 8(a) presents the adsorp- Specific surface area results obtained by the N2 absorption-desorption N2 isotherms for T-400, Z-400, T10Z-400, tion BET method for all samples are presented in Table 2. T20Z-400, and T30Z-400 samples. T-400 sample presents The specific surface areas increase when zirconia content type IV isotherm characteristic for mesoporous materials is increased up to 20% mol content and diminish with which presents a hysteresis loop type H2 corresponding further ZrO2 content increase. In Figure 6, corresponding to to pores with narrow necks and wider bodies [45, 47]. 20% mol of ZrO2, the TEM image shows white and dense T10Z-400, T20Z-400, and T30Z-400 samples present type IV ZrO2 aggregates according to EDS analysis not shown, this isotherms which present two hysteresis loops; this character- aggregates block out the mesoporous structure and it might istic of the isotherm is associated to the bimodal pore size be the reason of why the highest ZrO2 concentrations of the distributions in the mesoporous region [48]. The adsorption lower specific surface areas are obtained. The highest value 6 International Journal of Photoenergy

(a) (b)

(e) (f)

(g) (h)

Figure 5: SEM micrographs of the T10Z-400 sample ((a) and (b)), T20Z-400 sample ((c) and (d)), T30Z-400 sample ((e) and (f)), and Z-400 sample ((g) and (h)).

(a) (b)

Figure 6: TEM image of sample T10Z-400 obtained by Z-contrast technique, (a) macro-mesoporous “sponge-” like agglomerate, (b) diﬀerent agglomerates, with mesoporous structure. International Journal of Photoenergy 7

(a) (b)

Figure 7: TEM image of T20Z-400 sample, presenting the obtained mesoporous with a “worm like structure” (a) underfocus image and (b) overfocus image.

260 280 T10Z-400 240 T10Z-400 260 T20Z-400 220 240 T30Z-400 220 200 180 200 T-400 /gr) /gr) 180 160 3 3 160 140 140 120 120 100 100 Volume (cm Volume (cm Z-400 80 T10Z-500 80 60 60 40 40 20 20 0 0 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 Relative pressure (P/Po) Relative pressure (P/Po) (a) (b)

Figure 8: Adsorption-desorption N2 isotherms of the (a) T10Z-400, T20Z-400, T30Z-400, T-400, and Z-400 samples and (b) T10Z-400 and T10Z-500. was reached for the T10Z-500 sample, which is probably due 3.5. Photocatalytic Test. The photocatalytic test with the to the better mesoporous structure formation of this sample powders obtained was carried out for the MB degradation. leading to the formation of more and smaller mesopores. Figure 9 shows the plots of MB degradation versus reaction This explanation can be supported by the N2 absorption- time; the concentration of MB diminishes with time for all desorption isotherm corresponding to this sample, shown in samples. The addition of zirconia provokes a drastic change Figure 8(b). The isotherm indicates that this sample adsorbs in the kinetic behavior of the MB degradation reaction, agreatervolumeofN2 at lower partial pressure than the changing it from parabolic for the pure titania sample T- other samples and presents two hysteresis loops, one at 400 and P-25 Degussa powder to lineal for all the mixed high and the other at low partial pressures. This isotherm samples. The T10Z-400 sample degraded 57% in 4 hours, presents the typical form of type IV isotherms, so the T20Z-400 and T30Z-400 samples degraded only 18 and 20% ◦ calcinations temperature of 500 C in the T10Z-500 sample respectively, in 4.5 hours; it might be due to the ZrO2 at high is really important to lead to a better mesoporous structure concentrations increases domains of ZrO2 with band gap formation, and the isotherm suggests that there are also = 5 eV resulting in a decrease in the photocatalytic activity microporosity (pore size < 2 nm). The mean pore diameter because ZrO2 is difficult to exited even by UV light [51]; of the T10Z-500 sample is effectively the lowest one, obtained additionally, for the ZrO2 agglomeration increase and the by the BJH method from the corresponding adsorption and blockout of the porous structure by these agglomerates, see desorption data. The obtained specific surface areas are in Figure 6(b). The T10Z-500 sample showed the best behavior agreement with those reported by other authors [49, 50]. degrading 90% of the MB initial concentration in 4 hours 8 International Journal of Photoenergy

10 In titania, for example, there are oxygen vacancies that +3 9 introduce localized Ti states which are donor impurities which in turn generate traps near the conduction band, 8 and these traps are not usually recombined [55], in ZrO2. 7 Foster et al. [32] found the positions of defect levels with g/L) μ respect to the bottom of silicon conduction band. It is now 6 generally accepted that the addition of transition metals in 5 the structure of titania increases the speed of photooxidation, 4 due to the combination of the electrons to the metal ions in the semiconductor surface according with the following 3

MB concentration ( reaction: 2 1 n+ − −→ (n−1)+ M +ebc M ,(1) 0 0 12345 where Mn+ represents transition metals. Time (hours) This reaction prevents the rapid recombination of those T-400, R2 = 0.99 T10Z-400, R2 =−0.99 electron-hole pairs generated, resulting in an increase in the 2 =− 2 =− − T30Z-400, R 0.99 T10Z-500, R 0.98 rate of formation of OH ions on the TiO2 surface as well 2 T20Z-400, R2 =−0.95 P− 25 Degussa R = 0.98 as in the reaction solution facilitates the formation of OH• radicals [26, 52], consequently the eﬃciency of the process is Figure 9: Kinetic plots of MB concentration versus reaction time. logically enhanced.

4. Concluding Remarks and degraded up to 92% at 5 hours of reaction. As a com- A series of TiO -ZrO photocatalytic powder systems with parison, TiO P-25 Degussa powder presents a parabolic 2 2 2 hierarchically macro-mesoporous structures was successfully behavior in the degradation of MB curve, as pure titania T- synthesized by hydrothermal synthesis using Tween-20 as 400 sample does, it degraded 66% in 4 hours and degraded the directing agent, allowing the formation of the bimodal up to 70% in 5 hours; this result is accordance with Tayade et pore size distribution in the mixed oxide system, which al. [52], so it resulted less effective than T10Z-500 sample this ◦ remained stable under calcinations up to 500 C. For all result could be due to the smaller crystallite size compared the compositions studied, the macropores obtained are in with P-25 Degussa powder [53]. one-dimensional orientation, parallel with each other, and From these results, we conclude that, in order to have the macroporous framework is composed of interconnected better photocatalytic activity, there is an optimal addition of wormhole-like mesopores. The mesoporous sizes of the 10% mole of ZrO , due in part to the specific surface area, 2 binary oxides, as well as macropore sizes, could be tailored by and the macro-mesoporosity according with Schattka et al. the variation of the molar ratios of the metal precursors. The [54] who reported that zirconia additions to titania increased stabilization of the anatase crystalline phase of titania and the specific surface areas of titania, they concluded that the tetragonal crystalline phase of zirconia was observed in these presence of zirconia in the titania network, at a molar ratio ◦ binary oxides. The role of thermal treatment at 500 Con of 1 : 9, resulted in enhanced photocatalytic activity relative the improved mesoporous structure formation, on the total to the pure titania network. A further increase in the ratio elimination of organics and surfactant molecules, and on of zirconia to titania leads to a decrease in photocatalytic the resultant photocatalytic activity has been clearly shown. activity, as can be concluded from the photocatalytic test The best photocatalytic behavior was obtained for the sample results in the present work. Huang et al. [51] also reported T10Z-500 with an addition of 10% mole of zirconia, which that specific surface areas are increased as the ZrO content 2 resulted better than TiO P-25 Degussa powder. These meso- is increased but the photocatalytic degradation does not 2 macroporous binary oxides showed higher surface areas as depend on this parameter. They found that Ti Zr O 0.91 0.09 2 the zirconia content was increased, larger pore volumes and solid solution exhibited the highest photocatalytic activity higher thermal stabilities than single metal oxides, exhibited amongst all their samples with S area of 67.8 m2/g. BET efficient potential for photocatalytic applications. Otherwise Yu et al. [47] reported better behavior for sample 2 with SBET area of 203 m /g and crystallite size of 7.1 nm ◦ obtained by hydrothermal treatment at 180 C; this value is Acknowledgments similar to SBET T10Z-500, see Table 2. The way in which zirconia enhances the photocatalytic This research was financially supported by CONACYT properties of titania can be explained by band theory, which Project no. 83880 and CIC-UMSNH. The authors gratefully says that it is possible to create some additional energy states acknowledge Dr. Marco Antonio Medina, Dr. M. Elena within the band gap which reduces the energy required Nunez,˜ and Dr. Ariosto Medina Flores for their technical to liberate an electron and move to the conduction band. support. International Journal of Photoenergy 9

References [17] M. L. Garc´ıa-Benjume, M. I. Espitia-Cabrera, and M. E. Contreras-Garc´ıa, “Synthesis and characterization of meso- [1] H. Arora, C. Doty, and Y. Yuan, “Titanium dioxide nanocon- porous nanostructured TiO2–Al2O3 photocatalytic system,” in jugates,” in Nanomaterials for the Life Sciences,C.S.S.R. Processing of Nanoparticle Structures and Composites: Ceramic Kumar, Ed., vol. 8, Wiley-VCH, Weinheim, Germany, 2006, Transactions, T. Hinklin and K. Lu, Eds., vol. 208, pp. 79–89, Nanoconjugates. 2009. [2] S. S. Liang, H. Makamba, S. Y. Huang, and S. H. Chen, “Nano- [18] G. Tian, K. Pan, H. Fu, L. Jing, and W. Zhou, “Enhanced pho- titanium dioxide composites for the enrichment of phospho- tocatalytic activity of S-doped TiO2–ZrO2 nanoparticles under peptides,” Journal of Chromatography A, vol. 1116, no. 1-2, pp. visible-light irradiation,” Journal of Hazardous Materials, vol. 38–45, 2006. 166, no. 2-3, pp. 939–944, 2009. [3] C. A. Paez,D.Poelman,J.P.Pirard,andB.Heinrichs,“Un-´ [19] J. Y. Zheng, K. Y. Qiu, and Y. Wei, “Investigation of Zr- predictable photocatalytic ability of H2-reduced rutile-TiO2 incorporated mesoporous titania materials via nonsurfactant xerogel in the degradation of dye-pollutants under UV and templated sol-gel route: synthesis, characterization and stabil- visible light irradiation,” Applied Catalysis B,vol.94,no.3-4, ity,” Journal of Materials Science, vol. 38, no. 3, pp. 437–444, pp. 263–271, 2010. 2003. [4] W. Ren, Z. Ai, F. Jia, L. Zhang, X. Fan, and Z. Zou, “Low tem- [20]F.Fresno,M.D.Hernandez-Alonso,´ D. Tudela, J. M. Coron- perature preparation and visible light photocatalytic activity of ado, and J. Soria, “Photocatalytic degradation of toluene over mesoporous carbon-doped crystalline TiO2,” Applied Catalysis doped and coupled (Ti,M)O2 (M = Sn or Zr) nanocrystalline B, vol. 69, no. 3-4, pp. 138–144, 2007. oxides: influence of the heteroatom distribution on deactiva- [5] Q. Xiao, J. Zhang, C. Xiao, Z. Si, and X. Tan, “Solar photocat- tion,” Applied Catalysis B, vol. 84, no. 3-4, pp. 598–606, 2008. alytic degradation of methylene blue in carbon-doped TiO2 [21] C. Wang, A. Geng, Y. Guo, S. Jiang, X. Qu, and L. Li, “A novel nanoparticles suspension,” Solar Energy,vol.82,no.8,pp. preparation of three-dimensionally ordered macroporous 706–713, 2008. M/Ti (M = Zr or Ta) mixed oxide nanoparticles with enhanced [6] S. K. Samantaray and K. M. Parida, “Effect of anions on the photocatalytic activity,” Journal of Colloid and Interface Science, textural and catalytic activity of titania,” Journal of Materials vol. 301, no. 1, pp. 236–247, 2006. Science, vol. 38, no. 9, pp. 1835–1848, 2003. [22] M. Xue, L. Huang, J. Q. Wang et al., “The direct synthesis [7] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, of mesoporous structured MnO2/TiO2 nanocomposite: a “Visible-light photocatalysis in nitrogen-doped titanium novel visible-light active photocatalyst with large pore size,” oxides,” Science, vol. 293, no. 5528, pp. 269–271, 2001. Nanotechnology, vol. 19, no. 18, Article ID 185604, 2008. [8]J.A.Byrne,P.A.Fernandez-Ibanez,P.S.M.Dunlop,D.M.A.˜ [23] Q. Wang, D. Yang, D. Chen, Y. Wang, and Z. Jiang, “Synthesis Alrousan, and J. W. J. Hamilton, “Photocatalytic enhancement of anatase titania-carbon nanotubes nanocomposites with for solar disinfection of water: a review,” International Journal enhanced photocatalytic activity through a nanocoating- of Photoenergy, vol. 2011, Article ID 798051, 2011. hydrothermal process,” Journal of Nanoparticle Research, vol. [9] Y. Yao, Y. Ohko, Y. Sekiguchi, A. Fujishima, and Y. Kubota, 9, no. 6, pp. 1087–1096, 2007. “Self-sterilization using silicone catheters coated with Ag [24] M. L. Garc´ıa-Benjume, M. I. Espitia-Cabrera, and M. E. and TiO2 nanocomposite thin film,” Journal of Biomedical Contreras-Garc´ıa, “The effect of synthesis parameters on the Materials Research B, vol. 85, no. 2, pp. 453–460, 2008. production of titania nanostructured spherical aggregates,” [10] A. Mo, W. Xu, S. Xian, Y. Li, and S. Bai, “Antibacterial activity Journal of Ceramic Processing Research, vol. 11, no. 2, pp. 198– of silver-hydroxyapatite/titania nanocomposite coating on 203, 2010. titanium against oral bacteria,” Key Engineering Materials, vol. [25] E. Barajas-Ledesma, M. L. Garc´ıa-Benjume, I. Espitia-Cabrera, 330–332, pp. 455–458, 2007, Bioceramics 19 (parts 1 and 2). A. Bravo-Patino,˜ and M. E. Contreras-Garc´ıa, “Biocide activity [11] G. F. Fu, P. S. Vary, and C. T. Lin, “Anatase TiO2 nanocompos- of TiO2 nanostructured films,” Journal of Nano Research, vol. ites for antimicrobial coatings,” Journal of Physical Chemistry 9, pp. 17–24, 2010. B, vol. 109, no. 18, pp. 8889–8898, 2005. [26] J. Sin, S. Lam, A. R. Mohamed, and K. Lee, “Degrading [12] L. F. Liu, B. John, K. L. Yeung, and G. Si, “Non-UV based endocrine discrupting chemicals from wastewater by TiO2 germicidal activity of metal-doped TiO2 coating on solid photocatalysis: a review,” International Journal of Photoenergy, surfaces,” Journal of Environmental Sciences, vol. 19, no. 6, pp. vol. 2012, Article ID 185159, 2012. 745–750, 2007. [27] L. R. Yu, Z. Zhu, K. C. Chan, H. J. Issaq, D. S. Dimitrov, [13] B. Mahltig, E. Gutmann, D. C. Meyer et al., “Solvothermal and T. D. Veenstra, “Improved titanium dioxide enrichment preparation of metalized titania sols for photocatalytic and of phosphopeptides from HeLa cells and high confident antimicrobial coatings,” Journal of Materials Chemistry, vol. phosphopeptide identification by cross-validation of MS/MS 17, no. 22, pp. 2367–2374, 2007. and MS/MS/MS spectra,” Journal of Proteome Research, vol. 6, [14] D. F. Ollis, “Integrating photocatalysis and membrane tech- no. 11, pp. 4150–4162, 2007. nologies for water treatment,” Annals of the New York Academy [28] K. Ashman and E. L. Villar, “Phosphoproteomics and cancer of Sciences, vol. 984, pp. 65–84, 2003. research,” Clinical & Translational Oncology, vol. 11, no. 6, pp. [15] S. Kohtani, A. Kudo, and T. Sakata, “Spectral sensitization 356–362, 2009. of a TiO2 semiconductor electrode by CdS microcrystals and [29] A. Paradela and J. P. Albar, “Advances in the analysis of protein its photoelectrochemical properties,” Chemical Physics Letters, phosphorylation,” Journal of Proteome Research, vol. 7, no. 5, vol. 206, no. 1–4, pp. 166–170, 1993. pp. 1809–1818, 2008. [16] V. Subramanian, Z. Ni, E. G. Seebauer, and R. I. Masel, [30] L. Zhang and J. C. Yu, “A sonochemical approach to hier- “Synthesis of high-temperature titania-alumina supports,” archical porous titania spheres with enhanced photocatalytic Industrial and Engineering Chemistry Research, vol. 45, no. 11, activity,” Chemical Communications, vol. 9, no. 16, pp. 2078– pp. 3815–3820, 2006. 2079, 2003. 10 International Journal of Photoenergy

[31]P.Kluson,H.Luskova,L.Cerveny,J.Klisakova,andT. [48]J.Yu,J.C.Yu,W.Hoetal.,“Effects of alcohol content and Cajthaml, “Partial photocatalytic oxidation of cyclopentene calcination temperature on the textural properties of bimo- over titanium(IV) oxide,” Journal of Molecular Catalysis A, vol. dally mesoporous titania,” Applied Catalysis A, vol. 255, no. 2, 242, no. 1-2, pp. 62–67, 2005. pp. 309–320, 2003. [32]A.S.Foster,V.B.Sulimov,F.LopezGejo,A.L.Shluger,andR. [49] C. Flego, L. Carluccio, C. Rizzo, and C. Perego, “Synthesis M. Nieminen, “Structure and electrical levels of point defects of mesoporous SiO2–ZrO2 mixed oxides by sol-gel method,” in monoclinic zirconia,” Physical Review B, vol. 64, no. 22, Catalysis Communications, vol. 2, no. 2, pp. 43–48, 2001. Article ID 224108, 10 pages, 2001. [50] J. Zhao, Y. Yue, W. Hua, and Z. Gao, “Catalytic activities [33] J. M. Herrmann, “Heterogeneous photocatalysis: state of the and properties of mesoporous sulfated Al2O3–ZrO2,” Catalysis art and present applications,” Topics in Catalysis, vol. 34, no. Letters, vol. 116, no. 1-2, pp. 27–34, 2007. 1–4, pp. 49–65, 2005. [51] Y. Huang, Z. Zheng, Z. Ai, L. Zhang, X. Fan, and Z. Zou, [34] J. Yu, L. Zhang, B. Cheng, and Y. Su, “Hydrothermal prepa- “Core - Shell microspherical Ti1−xZrxO2 solid solution pho- ration and photocatalytic activity of hierarchically sponge-like tocatalysts directly from ultrasonic spray pyrolysis,” Journal of macro-/mesoporous titania,” Journal of Physical Chemistry C, Physical Chemistry B, vol. 110, no. 39, pp. 19323–19328, 2006. vol. 111, no. 28, pp. 10582–10589, 2007. [52] R. J. Tayade, T. S. Natarajan, and H. C. Bajaj, “Photocatalytic [35] J. Escobar, J. A. de Los Reyes, and T. Viveros,´ “Influence of the degradation of methylene blue dye using ultraviolet light emit- synthesis additive on the textural and structural characteristics ting diodes,” Industrial and Engineering Chemistry Research, of sol-gel Al2O3–TiO2,” Industrial and Engineering Chemistry vol. 48, no. 23, pp. 10262–10267, 2009. Research, vol. 39, no. 3, pp. 666–672, 2000. [53] Y. Fan, G. Chen, D. Li et al., “Highly selective deethylation [36] E. Munoz,˜ J. L. Boldu,´ E. Andrade et al., “Intrinsically formed of rhodamine B on TiO2 prepared in supercritical fluids,” trivalent titanium ions in sol-gel titania,” Journal of the International Journal of Photoenergy, vol. 2012, Article ID American Ceramic Society, vol. 84, no. 2, pp. 392–398, 2001. 173865, 2012. [37] G. Q. Liu, Z. G. Jin, X. X. Liu, T. Wang, and Z. F. Liu, “Anatase [54] J. H. Schattka, D. G. Shchukin, J. Jia, M. Antonietti, and R. TiO2 porous thin films prepared by sol-gel method using A. Caruso, “Photocatalytic activities of porous titania and CTAB surfactant,” Journal of Sol-Gel Science and Technology, titania/zirconia structures formed by using a polymer gel vol. 41, no. 1, pp. 49–55, 2007. templating technique,” Chemistry of Materials, vol. 14, no. 12, [38] S. G. Liu, H. Wang, J. P. Li, N. Zhao, W. Wei, and Y. H. Sun, pp. 5103–5108, 2002. “A facile route to synthesize mesoporous zirconia with ultra [55] A. L. Linsebigler, G. Lu, and J. T. Yates Jr., “Photocatalysis on high thermal stability,” Materials Research Bulletin, vol. 42, no. TiO2 surfaces: principles, mechanisms, and selected results,” 1, pp. 171–176, 2007. Chemical Reviews, vol. 95, no. 3, pp. 735–758, 1995. [39] M. L. Garc´ıa-Benjume, M. I. Espitia-Cabrera, and M. E. Contreras-Garc´ıa, “Hierarchical macro-mesoporous structures in the system TiO2–Al2O3, obtained by hydrothermal synthesis using Tween-20 as a directing agent,” Materials Characterization, vol. 60, no. 12, pp. 1482–1488, 2009. [40] F. del Monte, W. Larsen, and J. D. Mackenzie, “Stabilization of tetragonal ZrO2 in ZrO2–SiO2 binary oxides,” Journal of the American Ceramic Society, vol. 83, no. 3, pp. 628–634, 2000. [41] D. A. Ward and E. I. Ko, “Synthesis and structural transformation of zirconia aerogels,” Chemistry of Materials, vol. 5, no. 7, pp. 956–969, 1993. [42] X. Wang, J. C. Yu, C. Ho, Y. Hou, and X. Fu, “Photocatalytic activity of a hierarchically macro/mesoporous titania,” Lang- muir, vol. 21, no. 6, pp. 2552–2559, 2005. [43] U. Ciesla and F. Schuth,¨ “Ordered mesoporous materials,” Microporous and Mesoporous Materials,vol.27,no.2-3,pp. 131–149, 1999. [44] X. He and D. Antonelli, “Recent advances in synthesis and applications of transition metal containing mesoporous molecular sieves,” Angewandte Chemie—International Edition, vol. 41, no. 2, pp. 215–229, 2002. [45] J. Yu, Y. Su, and B. Cheng, “Template-free fabrication and enhanced photocatalytic activity of hierarchical macro- /mesoporous titania,” Advanced Functional Materials, vol. 17, no. 12, pp. 1984–1990, 2007. [46] A. Collins, D. Carriazo, S. A. Davis, and S. Mann, “Sponta- neous template-free assembly of ordered macroporous titania,” Chemical Communications, vol. 10, no. 5, pp. 568–569, 2004. [47] J. Yu, G. Wang, B. Cheng, and M. Zhou, “Effects of hydrothermal temperature and time on the photocatalytic activity and microstructures of bimodal mesoporous TiO2 powders,” Applied Catalysis B, vol. 69, no. 3-4, pp. 171–180, 2007. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 758539, 8 pages doi:10.1155/2012/758539

Research Article Doped Titanium Dioxide Films Prepared by Pulsed Laser Deposition Method

Juguang Hu,1 Huabin Tang,1 Xiaodong Lin,1 Zhongkuan Luo,2 Huiqun Cao,2 Qiwen Li,1 Yi Liu,1 Jinghua Long,1 and Pei Wang1

1 College of Physics Science and Technology, Shenzhen University, Guangdong, Shenzhen 518060, China 2 College of Chemistry and Chemical Engineering, Shenzhen University, Guangdong, Shenzhen 518060, China

Correspondence should be addressed to Xiaodong Lin, [email protected]

Received 28 January 2012; Accepted 2 April 2012

Academic Editor: Baibiao Huang

Copyright © 2012 Juguang Hu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

TiO2 was intensively researched especially for photocatalystic applications. The nitrogen-doped TiO2 films prepared by pulsed laser deposition (PLD) method were reviewed, and some recent new experimental results were also presented in this paper. A new optical transmission method for evaluating the photocatalystic activity was presented. The main results are (1) PLD method is versatile for preparing oxide material or complex component films with excellent controllability and high reproducibility. (2) ◦ ◦ Anatase nitrogen-doped TiO2 films were prepared at room temperature, 200 C, and 400 C by PLD method using novel ceramic target of mixture of TiN and TiO2. UV/Vis spectra, AFM, Raman spectra, and photocatalystic activity for decomposition of methyl orange (MO) tests showed that visible light response was improved at higher temperature. (3) The automatic, continuous optical transmission autorecorder method is suitable for detecting the photodecomposition dynamic process of organic compound.

1. Introduction sunlight. Theoretical calculations have been performed to clarify the effect of anion doping of TiO2 on band gap 1.1. General Description of Preparation of TiO2 Films. In modifications [20–22]. recent years, the field of photocatalysis has became an Nitrogen-doped TiO2 materials were intensively extremely well-researched field due to wide application researched since Asahi et al. proposed that it has narrow interest in self-cleaning surfaces, water or air purification, band gap and little recombination of electrons and holes self-sterilizing surfaces, antifogging surfaces, optical or gas [23]. However, Batzill et al. reported that no band gap sensor [1–4], and so forth. TiO2 is a fascinating material that narrowing is observed for N-doped TiO2 single crystals, has been intensively researched by worldwide researchers. but N-doping induces localized N 2p states within the For photocatalystic applications, much attention has been band gap just above the valence band (VB). N is present paid to prepare and use TiO2 powder, for it has large specific in an N(III) valence state, which facilitates the formation area [5, 6], but it has shortcoming of difficulty to recycle of oxygen vacancies and Ti 3d band gap states at elevated in aqueous fluid. TiO2 films have been prepared by many temperatures. This thermal instability may degrade the technologies, including chemical bath deposition (CBD) catalyst during applications [24]. Socol et al. proposed method [7, 8], electron-beam evaporation [9], reactive that both substitutional N and O vacancies contribute to electron beam evaporation [10], magnetron sputtering [11, the visible light absorption [25]. The width of the TiO2 12], sol-gel [6, 9, 13–17], and thermal oxidation [18, 19]. band gap was not affected by the presence of fluorine Much effort has been given to understanding and altering either, as reported by Todorova et al. [5]. The red shift of the optical properties of titanium dioxide, especially for the absorption edge was attributed to the increased rutile enhancing visible light absorption mainly by narrowing the content in the fluorine-doped TiO2 powers. The codoping band gap (3.2 eV) for using the economical and ecological effect between nitrogen and hydrogen is responsible for the 2 International Journal of Photoenergy

enhanced photoactivity of N-doped TiO2 in the range of and substrate, type and pressure of ambient air, and laser visible light [26]. wavelength, and so forth. Its advantages for the film growth Balek et al. prepared nitrogen and fluorine codoped of oxides and other multicomponent materials include stoi- titania photocatalyst samples for air purification by spray chiometric transfer, growth from an energetic plasma plume, pyrolysis method [27]. A high photocatalytic activity in reactive deposition, good adherence to the substrate surface, a visible light region of spectrum depended on the spray excellent controllability, and high reproducibility. PLD has pyrolysis temperature and can be ascribed to a synergetic played a significant role in advancing our understanding of effect of nitrogen and fluorine doping. Synergetic effect the physics of the thin-film structures, the material science of also happened in Nd2O3 modified TiO2 nanoparticles, a new system, and so forth [31]. formation of the surface anatase/rutile phase junction favors With the use of PLD, TiO2 films doped with metal, tran- photoinduced charge separation and further improves its sition metal, or nonmetallic elements have been prepared, photocatalytic activity [28]. and their properties were controlled by varying the preparing Qu et al. prepared Fe(3+) and Ce(3+) codoped nanos- parameters [25]. Socol et al. have grown crystalline anatase tructure titanium dioxide films via the improved sol-gel phase TiO2 thin films by PLD technique in oxygen, nitrogen, process. The samples had smaller crystal size, larger surface and methane and nitrogen with oxygen mixture. Their area, and larger pore volume. They also found that codoped studies proved the positive influence of anion doping on ions could obviously not only suppress the formation of the photoreduction activity under visible light exposure. The brookite phase but also inhibit the transformation of anatase best photoactivity under visible light exposure was obtained to rutile at high temperature. Fe(3+)/Ce(3+) codoped TiO2 for films deposited in pure nitrogen, which was correlated film showed excellent photocatalytic activity compared with with the highest red-shift (480 nm) of the absorption edge pure TiO2 film, Fe(3+) or Ce(3+) single doped TiO2 film. and the larger nitrogen incorporation characteristic to these They concluded that the surface microstructure of the films films. Quite different evolutions were observed in case of UV and improved sol-gel process ions doping methods are light irradiation. Significant results were obtained in this case responsible for improving the photocatalytic activity [29]. for the films deposited in pure oxygen or methane, while Our group has reported works about hydrophilicity the photoactivity (quantum yield) of the films deposited in between titanium oxide coatings with and without addition nitrogen was lower as compared with the blank. of silica. Through the investigation of change of water Sato et al. prepared N-doped TiO2 films by the atmo- contact angle on the surface after UV exposure and sunlight spheric controlled PLD (AC-PLD) method to generate visible radiation, it can be concluded that hydrophilicity of mixed light active photocatalytic films [32]. For nitrogen doping, coatings with low-temperature heat treatment of titanium theuseofCH(3)CNgaswasfoundtobemoreeffective than oxide and silica is much better than a pure titanium oxide the use of NH(3). The visible light absorption properties coating. This effect makes for an improved self-cleaning of the films were very sensitive to the CH(3)CN partial coating under natural sunlight. The mechanism is that par- pressure during ablation. When using CH(3)CN, nitrogen ticles of titanium oxide separated by silica reduce the contact and an equal quantity of carbon was uniformly doped into chance of recombination of electrons and holes, thereby the TiO2 films. The resultant films showed better catalytic increasing the photocatalytic action on organic compounds. performance than those which were either undoped or doped The addition of silica increases water absorption in the using NH(3). It is also suggested that stronger reducing coating. Water molecules absorbed by silica will be photo- agents such as carbon are required for doping nitrogen into catalyzed to free hydroxyl groups under the illumination of TiO2 films. UV light. These groups benefit the hydrophilicity of coating Metal nanoparticles can act as electron traps due to the [30]. formation of a Schottky barrier at the metal-semiconductor As for photo-induced hydrophilic effect, Fujishima et al. contact. Holes can decompose organic substances more have reached the conclusion that there is an aspect of this efficiently, because it has strong oxidative power. Sauthier et effect that does not involve simply the cleaning of the surface. al. used PLD technique to prepare Ag-TiO2 nanocomposites The precise nature of the effect has not been elucidated even to improve photocatalytic activity and compared with that now, but researchers proposed that the surface species are of bare TiO2 [33]. It was proposed that two distinct mecha- basically the same ones involved with conventional photo- nisms can contribute to the enhanced photoreactivity under catalysis [2]. Hendersonpresented recent research highlights near-UV irradiation. The first is Ag NPs retard electron- of the significant insights obtained from molecular-level hole recombination by photogenerated electron transfer studies of TiO2 photocatalysis. This comprehensive review from TiO2. And the second one is localized surface plasma has illustrated how a surface science perspective on TiO2 resonance absorption of Ag NPs, which can have positive photocatalysis can provide unique insights and motivate effect on the photocatalytic activity. more fundamental research in photocatalysis [3]. The films with more clusters exhibited higher photocatalytic performances than the films with less clusters [34]. 1.2. TiO2 Films Prepared by PLD Method. PLD technique is a The author pointed out that the specific surface area of versatile tool for preparing thin-films, because it is capable of the films was increased by the deposition of clusters. The preparing films with various properties by simply adjusting larger contact area induces high decomposition rate [35]. It the deposition conditions, like the type of target, type of is interesting that the clusters formed in PLD method are substrate and its temperature, distance between the target not desirable in other semiconductor industrial fields, like International Journal of Photoenergy 3

solar cell and so forth, where smooth and uniform surface The photocatalytic activity of the N-doped TiO2 films is desirable [36]. Suda et al. found that the particle size is with surface area of about 18 cm2 was studied by decom- changed with the substrate temperature, and larger particle positing organic methyl orange (MO) dye in aqueous size was obtained at higher temperature [37]. Table 1 lists solution. The initial concentration of MO solution is 2 mg/L, part of the publications about the preparation of TiO2 films and the total solution is about 80 mL. A tungsten halogen by PLD method in recent years. lamp was used as visible light source with 180 mW/cm2 Chen et al. obtained heavily nitrogen doped of about 15% power density on the surface of the MO solution. During anatase TiO2 films by using TiN target. Different from using the photodegradation experiments, the absorbance of the N2 gas as N source, TiN target as solid source might provide solution was measured at 460 nm wavelength, which corre- reactive molecular or cluster species with Ti-N bonds [43]. sponds to the peak absorbance of MO. The intensity of the Suda et al. measured depth profiles of the prepared films by transmitted detecting light was recorded by a data recorder, XPS and found that the film prepared using TiO2 target has whose data sampling interval was set as 2 minutes. This little nitrogen, while the film prepared using TiN target has photocatalystic activity evaluating experimental method has almost 8% atomic ratio of nitrogen [37]. Somekawa et al. not been used before to our knowledge. proposed that the N-doping occurred when N species and TiO2 particles collide on the substrate [46]. We consider that 3. Results and Discussion when the laser pulse irradiate onto the solid N source, N ion with high energy is produced and ejects to the substrate. It is 3.1. Optical Spectra. The sample color is transparent pre- quite easier to migrate and incorporate into the film lattice pared at RT or 200◦C, and light yellow at 400◦C. Figure 1 than using the N2 or NH3 gasasNsource. shows the transmission spectra of N-doped TiO2 films One advantage of PLD technique is that there is sto- prepared under different temperature. The absorption edges ichiometric transfer of material from target to film [31]. shift toward longer wavelengths from 300 nm to 350 nm For preparing N-doped TiO2 films, we used a novel type of with the increase of the substrate temperature, indicating a target, ceramic target mixture of TiN and TiO2 (molar ratio decrease in the band gap of the films, which may due to the N 1:3),different from pure TiN or TiO2 targets used by other composition increase with the increasing temperature. This researchers [37, 43]. Energetic N ion, O ion, and Ti ion, is different from the results suggested by Farkas et al. [47]. produced at the same time with high-intensity pulsed laser Another reason is the grain size increases with increasing ff irradiation may promote the growth of doped TiO2 films. temperature, resulting to weak quantum size e ects causing the red-shift of the absorption edge [48]. X-ray photoelec- 2. Experiments tron spectra measurement should be performed to detect the state and component of N element in the films. The N The N-doped TiO2 thin films were prepared inside a stainless element is usually formed as TiO2−xNx in films prepared by steel reaction chamber. A KrF excimer laser (wavelength: PLD method [42, 43, 47]. 248 nm, pulse frequency: 10 Hz, pulse duration: 25 ns) was used for the irradiation of N : TiO2 targets. The target was 3.2. AFM Measurements. Figure 2 shows the AFM images of ◦ prepared from TiN and TiO2 powders (molar ratio 1 : 3) by N-doped TiO2 films prepared at room temperature, 200 C, pressing at 5 MPa and sintered at 1100◦C for 4 h. The laser and 400◦C. The grain sizes are 18.5, 19.2, and 28.1 nm, beam incidence angle onto the target was chosen of about and their root mean square (rms) of roughness is 3.32, 35◦. The incident laser fluence on the target surface was set 3.96, and 6.73 nm, respectively. This is in agreement with at about 2.5 J/cm2. results obtained by Suda et al. [37]. With the temperature To avoid piercing, the target was rotated at 10 rpm. And increasing, the grain size and roughness increase, which laser spot was scanned on the target surface to prepare large suggests an increase of crystallinity of the films, and inducing area film of 50 mm in diameter. The substrate is a round red-shift in absorption spectrum because of quantum size normal glass slip with 50 mm in diameter, its temperature effect. was controlled from room temperature (RT) to 400◦C. The dynamic ambient gas pressure during the irradiations 3.3. Raman Spectra. The micro-Raman spectra of TiO2 films was kept at 1 Pa by feeding pure oxygen and nitrogen gas are shown in Figure 3. It can be seen that intense Raman peak ◦ (99.9%, ratio: O2 :N2 = 1 : 1) into the chamber for reduction does not occur until temperature reaches 400 C, indicating the desorption loss in vacuum. After the preparation was the crystallization realized at that point. This is in accord completed, the sample was cooled down to RT with the same with that in [48]. In our experiments, only anatase structures oxygen gas pressure. appearing as the typical Raman modes at 145, 198, 396, −1 The sample surface morphology was investigated by 517, and 640 cm are assigned to the Eg, Eg, B1g, A1g,and −1 domestic CSPM5000 atomic force microscopy (AFM) test. Eg modes, respectively. The strongest mode at 145 cm Optical transmission spectra in the near UV and visible indicates that the anatase phase with a long-range order has spectral regions were studied by PerkinElmer Lamda 950 been obtained [49]. UV/VIS spectrometer. The Raman spectra test was performed at room temperature with a Renishaw Invia Reflex 3.4. Photocatalystic Activity. Shinguu et al. proposed a confocal micro-Raman apparatus with He-Cd laser emitting reflectance method to evaluate the photodecomposition at 325 nm. rate of TiO2 films [41]. We have used the conductivity 4 International Journal of Photoenergy

Table 1: Preparation of TiO2 thin ﬁlms by PLD method.

Laser Substrate Target Dopant Crystalline phase Ambient air Photocatalytic activity ∗ Year Ref.

Nd:YAG,532 nm glass TiO2 —rutileO2 — 1999 [38]

ArF 193 nm Mica, quartz, Si TiO2 — Rutile, anatase Ar — 2002 [39]

ArF 193 nm α-Al2O3 Ti — Rutile, anatase O2 — 2004 [40]

Nd:YAG,266 nm Si TiO2 — Rutile, anatase O2 MB, anatase with clusters 2005 [34]

Nd:YAG,532 nm SiO2(corning 7059) Ti,TiO,TiO2,TiN N anatase O2,N2 MB, TiN target 2005 [37]

Nd:YAG,532 nm Glass Ti N anatase NH3/N2/O2 — 2006 [26]

KrF 248 nm Si or quartz glass Ti, TiO2,WO3 multilayer — O2 MB, WO3, 5% 2006 [41]

Nd:YAG,1064 nm quartz glass TiO2 N anatase O2,N2 MB, MO, Eg = 1.0 eV, 2.5 eV 2008 [42]

KrF 248 nm LSAT TiO2,TiN N anatase O2 — 2008 [43] ◦ Nd:YAG,266 nm quartz TiO2,La2O3 La Rutile N2 MB, 900 C postannealing 2009 [44]

KrF 248 nm glass TiO2 C, N anatase O2,N2,CH4 Cr(II), N doped 2010 [25]

KrF 248 nm SiO2 quartz TiO2 N anatase O2,N2 — 2010 [45]

KrF 248 nm SiO2 quartz Ag, TiO2 — anatase O2 MB 2011 [33] ∗ Organic compound for decomposition and optimal conditions obtained. MO: methyl orange, MB: methylene blue, Cr(II): toxic Chromium ion, it can be photoreducted to Cr(III) state.

100 depends on the preparation temperature. MO was almost decomposed completely after 4 hours for sample prepared ◦ 80 at 400 C. This is due to band gap narrowing by nitrogen (a) atom, larger surface area, and better crystallization at higher temperature. 60 (b) 3.5. Discussions. TiO2 film is a versatile material for use in many fields. For photocatalysis applications, the main 40 (c) problems are to narrow the band gap for visible light and Transparence (%) to retard the recombination of electrons and holes. Anion 20 or cation doping, or codoping, noble mental, and multilayer structure modification methods, and so forth have been proposed to improve TiO photocatalystic activity till now. 0 2 300 400 500 600 700 800 PLD technique is a widely used method to prepare oxide Wavelength (nm) materials; it is easy to change the growth parameters to (a) RT get various properties of doped films. Its controllability (b) 200◦C and reproducibility provide much convenience for base (c) 400◦C research of films materials with high melting point or multicomponent. Figure 1: Transmission spectra of N-doped TiO2 films prepared at In our N-doped TiO preparation experiments, laser ff ◦ ◦ 2 di erent temperature, (a) RT, (b) 200 C, (c) 400 C. pulses with intensity density about 1.0 × 1012 W/m2 was irradiated onto the surface of N : TiO2 ceramic target. Plasma plumes were produced. Energetic N ion, Ti ion, and O ion, method to check the rate [35, 50]. Recently, we developed as well as N containing TiO2 micrograins are ejected from a transmittance method to detect the concentration of the the target surface to the glass substrate. The TiO2 crystal MO to evaluate the photocatalystic activity of the N-doped nucleus formed and became larger with the subsequent TiO2 films. The setup is shown in Figure 4. The light source plasma plume until they combined with each other to form can be visible or UV light as demand. The LED light is thin film. During this process, N element was easier to 460 nm or 650 nm wavelength, which corresponds to the incorporate into the lattice of TiO2 as oxygen substitutor or peak absorbance of MO or MB. The relationship between interstitial atoms than that using N2 air as N source, due to it the transmittance and concentration was calibrated. This got energy from the laser irradiation directly [37, 43]. Higher method has the advantages of continuous, automatic check substrate temperature, 400◦C in our procedure, is beneficial without disturbing the reaction process, and avoiding danger to form crystallization, and larger grains as shown in the to operator when use UV light source, and so forth. AFM image. Large grains and high rms of roughness provide Figure 5 shows the decomposition rate with time of MO large surface area resulting to big contact chance of organic using N-doped TiO2 films prepared at different temperatures compound. Mole ratio of TiN and TiO2 in target is 1 : 3, but under visible light irradiation. It is clearly shown that we can speculate that the corresponding ratio inside the film photocatalystic activity of N-doped TiO2 films strongly is smaller due to relative easier desorption of small mass atom International Journal of Photoenergy 5

(a) (b)

(c)

◦ ◦ Figure 2: AFM images of the surface morphology of N-doped TiO2 ﬁlms under diﬀerent temperature, (a) RT, (b) 200 C, and (c) 400 C.

Light source Eg

MO solution

TiO2 ﬁlms (c)

Light Raman intensity Eg Transparent intensity Eg LED tube (a) B1g A1g recorder (b)

Small pump 100 200 300 400 500 600 700 Wave number (cm−1) Figure 4: The schematic diagram of experimental setup for automatic detecting the photodecomposition rate. The whole setup (a) RT is put in an aluminum box. The data record interval can be set from (b) 200◦C ◦ 1 minute to 1 hour. MO solution: methyl orange solution. (c) 400 C

Figure 3: Raman spectra of N-doped TiO2 ﬁlms prepared at diﬀer- ent temperature, (a) RT, (b) 200◦C, (c) 400◦C.

and morphology of TiO2 nanomaterials are crucial factors for establishing efficient photocatalytic reaction systems. from the film surface [31]. MO was almost photodegraded The morphology of TiO2 affects the charge recombination completely using visible light after 4 hours. dynamics, and anisotropic adsorption was found in recent Tachikawa et al. concluded that the adsorption dynamics research [51]. of substrates and organic compound, the electronic interac- Photocatalysis is a complex process involving chemical tion between TiO2 and adsorbents, and the band structure and physical reactions. The researchers should combine 6 International Journal of Photoenergy

References 1 (a) [1] U. Diebold, “The surface science of titanium dioxide,” Surface Science Reports, vol. 48, no. 5-8, pp. 53–229, 2003. 0.8 (b) [2] A. Fujishima, X. Zhang, and D. A. Tryk, “TiO2 photocatalysis and related surface phenomena,” Surface Science Reports, vol. 0.6 63, no. 12, pp. 515–582, 2008. 0

C [3] M. A. Henderson, “A surface science perspective on TiO2 pho- / (c) C tocatalysis,” Surface Science Reports, vol. 66, no. 6-7, pp. 185– 0.4 297, 2011. [4]N.E.Stankova,I.G.Dimitrov,T.R.Stoyanchov,andP.A. 0.2 (d) Atanasov, “Optical and gas sensing properties of thick TiO2 films grown by laser deposition,” Applied Surface Science, vol. 254, no. 4, pp. 1268–1272, 2007. 0 0 0.5 1 1.5 2 2.5 3 3.5 4 [5] N. Todorova, T. Giannakopoulou, G. Romanos, T. Vaimakis, J. Irradiation time (hour) Yu, and C. Trapalis, “Preparation of fluorine-doped TiO2 photocatalysts with controlled crystalline structure,” International (a) Blank Journal of Photoenergy, vol. 2008, Article ID 534038, 9 pages, (b) RT 2008. (c) 200◦C ◦ [6] M. Farbod and M. Khademalrasool, “Synthesis of TiO (d) 400 C 2 nanoparticles by a combined sol-gel ball milling method and Figure 5: Decomposition rate with time of MO using N-doped nvestigation of nanoparticle size effect on their photocatalytic TiO2 films prepared at different temperature under visible light activities,” Powder Technology, vol. 214, pp. 344–348, 2011. ◦ ◦ irradiation, (a) without TiO2, (b) RT, (c) 200 C, (d) 400 C. C0 is [7] A. M. More, T. P. Gujar, J. L. Gunjakar, C. D. Lokhande, and O. the initial concentration of Methyl orange about 2 mg/L, and C is S. Joo, “Growth of TiO2 nanorods by chemical bath deposition the concentration changing with time. method,” Applied Surface Science, vol. 255, no. 5, pp. 2682– 2687, 2008. [8] H. Zhu, J. Yang, S. Feng, M. Liu, J. Zhang, and G. Li, “Growth of TiO2 nanosheet-array thin films by quick chemical bath chemical methods and physical methods to overcome prob- deposition for dye-sensitized solar cells,” Applied Physics A, lems from photocatalystic material modification to degrade vol. 105, pp. 769–774, 2011. organic compound. For example, Rimeh et al. prepared [9] J. K. Yao, H. Y. Li, Z. X. Fan et al., “Comparison of TiO2 and Ti/TiO2 electrode by PLD technique and obtained a degra- ZrO2 films deposited by electron-beam evaporation and by dation rate of almost 75% of chlortetracycline within 2 hours sol-gel process,” Chinese Physics Letters, vol. 24, no. 7, article [52]. 049, pp. 1964–1966, 2007. [10] O.¨ Duyar, F. Placido, and H. Zafer Durusoy, “Optimization of 4. Conclusions TiO2 films prepared by reactive electron beam evaporation of Ti3O5,” Journal of Physics D, vol. 41, no. 9, Article ID 095307, 2008. Fascinating TiO2 films were worldwide researched using various preparing method. PLD technique is a versatile [11] B. Cojocaru, S¸. Neat¸u, E. Sacaliuc-Parvulescu,ˆ F. Levy,´ V. I. Parvulescu,ˆ and H. Garcia, “Influence of gold particle method for preparing films of oxide materials. Its advantages size on the photocatalytic activity for acetone oxidation of of controllability and reproducibility are suitable for basis Au/TiO2 catalysts prepared by dc-magnetron sputtering,” research for preparing various properties of TiO2 films. Applied Catalysis B, vol. 107, pp. 140–149, 2011. Some recent experimental results obtained in our group were [12]D.Y.Chen,C.C.Tsao,andC.Y.Hsu,“PhotocatalyticTiO2 presented. N-doped TiO2 anatase films were prepared at thin films deposited on flexible substrates by radio frequency ◦ substrate temperature from RT to 400 C by PLD method (RF) reactive magnetron sputtering,” Current Applied Physics, using a novel ceramic target of mixture of TiN and TiO2 and vol. 12, pp. 179–183, 2012. were characterized by UV/Vis optical spectra, AFM, Raman [13]J.C.Yu,W.Ho,J.Yu,S.K.Hark,andK.Iu,“Effects of tri- spectra, and photocatalystic activity for decomposition of fluoroacetic acid modification on the surface microstructures methyl orange. It was found that the film crystallinity, the and photocatalytic activity of mesoporous TiO2 thin films,” visible light response, and decomposition rate were signif- Langmuir, vol. 19, no. 9, pp. 3889–3896, 2003. icantly improved at higher temperature. New method of [14] S. Zhan, D. Chen, X. Jiao, and C. Tao, “Long TiO2 hollow continuous autodetecting the solution optical transmission fibers with mesoporous walls: sol-gel combined electrospun for evaluating the photodecomposition dynamic process was fabrication and photocatalytic properties,” JournalofPhysical Chemistry B, vol. 110, no. 23, pp. 11199–11204, 2006. developed. [15] J. Zhu, F. Chen, J. Zhang, H. Chen, and M. Anpo, “Fe3+-TiO2 photocatalysts prepared by combining sol-gel method with Acknowledgment hydrothermal treatment and their characterization,” Journal of Photochemistry and Photobiology A, vol. 180, no. 1-2, pp. 196– Financial support by Shenzhen basic research project of 204, 2006. science and technology (JC201005280419A) is gratefully [16] N. Arconada, A. Duran,S.Su´ arez´ et al., “Synthesis and pho- acknowledged. tocatalytic properties of dense and porous TiO2-anatase thin International Journal of Photoenergy 7

films prepared by sol-gel,” Applied Catalysis B,vol.86,no.1-2, of nitrogen doped TiO2 films prepared using the AC-PLD pp. 1–7, 2009. method,” Topics in Catalysis, vol. 52, no. 11, pp. 1592–1597, [17] Z. Luo, H. Cai, X. Ren, J. Liu, W. Hong, and P. Zhang, 2009. “Hydrophilicity of titanium oxide coatings with the addition [33] G. Sauthier, A. PerezdelPino,A.Figueras,andE.Gy´ orgy,¨ of silica,” Materials Science and Engineering B, vol. 138, no. 2, “Synthesis and characterization of Ag nanoparticles and Ag- pp. 151–156, 2007. loaded TiO2 photocatalysts,” Journal of the American Ceramic [18] J. L. Falconer and K. A. Magrini-Bair, “Photocatalytic and Society, vol. 94, no. 11, pp. 3780–3786, 2011. thermal catalytic oxidation of acetaldehyde on Pt/TiO2,” [34] T. Yoshida, Y. Fukami, M. Okoshi, and N. Inoue, “Improve- Journal of Catalysis, vol. 179, no. 1, pp. 171–178, 1998. ment of photocatalytic efficiency of TiO2 thin films prepared [19] J. Zhang, C. Pan, P. Fang, J. Wei, and R. Xiong, “Mo + by pulsed laser deposition,” Japanese Journal of Applied Physics, C codoped TiO2 using thermal oxidation for enhancing Part 1, vol. 44, no. 5, pp. 3059–3062, 2005. photocatalytic activity,” ACS Applied Materials and Interfaces, [35] Z.Luo,L.Song,H.Cai,J.Liu,W.Hong,andJ.Huang,“Photo- vol. 2, no. 4, pp. 1173–1176, 2010. catalytic De-chlorination of chlorinated methane by titanium [20] R. Long and N. J. English, “Electronic properties of anatase- oxide sol,” Journal of Inorganic Materials,vol.21,no.1,pp. TiO2 codoped by cation-pairs from hybrid density functional 145–149, 2006. theory calculations,” Chemical Physics Letters, no. 513, pp. [36] R. K. Thareja and A. Mohanta, “Gas suspended ZnO clusters 218–223, 2011. and pulsed laser deposition of ZnO thin film,” Physica Status [21] D. J. Mowbray, J. I. Martinez, G. J. M. Lastra, K. S. Thygesen, Solidi, no. 5, pp. 1413–1416, 2010. andK.W.Jacobsen,“Stabilityandelectronicpropertiesof [37] Y. Suda, H. Kawasaki, T. Ueda, and T. Ohshima, “Prepa- TiO2 nanostructures with and without B and N doping,” ration of nitrogen-doped titanium oxide thin film using a Journal of Physical Chemistry C, vol. 113, no. 28, pp. 12301– PLD method as parameters of target material and nitrogen 12308, 2009. concentration ratio in nitrogen/oxygen gas mixture,” Thin [22] R. Long and N. J. English, “First-principles calculation of Solid Films, vol. 475, no. 1-2, pp. 337–341, 2005. ff nitrogen-tungsten codoping e ects on the band structure of [38] L. Escobar-Alarcon,E.Haro-Poniatowski,M.A.Camacho-´ anatase-titania,” Applied Physics Letters, vol. 94, no. 13, Article Lopez,´ M. Fernandez-Guasti,´ J. J´ımenez-Jarqu´ın, and A. ID 132102, 2009. Sanchez-Pineda,´ “Structural characterization of TiO2 thin [23] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, films obtained by pulsed laser deposition,” Applied Surface “Visible-light photocatalysis in nitrogen-doped titanium Science, vol. 137, no. 1–3, pp. 38–44, 1999. oxides,” Science, vol. 293, no. 5528, pp. 269–271, 2001. [39] N. Koshizaki, A. Narazaki, and T. Sasaki, “Preparation of [24] M. Batzill, E. H. Morales, and U. Diebold, “Influence of nitro- nanocrystalline titania films by pulsed laser deposition at gen doping on the defect formation and surface properties of room temperature,” Applied Surface Science, vol. 197-198, pp. TiO rutile and anatase,” Physical Review Letters, vol. 96, no. 2, 2 624–627, 2002. Article ID 026103, 4 pages, 2006. [40] S. I. Kitazawa, Y. Choi, and S. Yamamoto, “In situ optical spec- [25] G. Socol, Y. Gnatyuk, N. Stefan et al., “Photocatalytic activity troscopy of PLD of nano-structured TiO ,” Vacuum, vol. 74, of pulsed laser deposited TiO thin films in N ,O and CH ,” 2 2 2 2 4 no. 3-4, pp. 637–642, 2004. Thin Solid Films, vol. 518, no. 16, pp. 4648–4653, 2010. [26] P. Xu, L. Mi, and P. N. Wang, “Improved optical response [41] H. Shinguu, M. M. H. Bhuiyan, T. Ikegami, and K. Ebihara, /WO multilayer thin film by PLD for N-doped anatase TiO films prepared by pulsed laser “Preparation of TiO2 3 2 method and its catalytic response to visible light,” Thin Solid deposition in N2/NH3/O2 mixture,” Journal of Crystal Growth, vol. 289, no. 2, pp. 433–439, 2006. Films, vol. 506-507, pp. 111–114, 2006. [27] V.Balek, D. Li, J. Subrtˇ et al., “Characterization of nitrogen and [42] L. Zhao, Q. Jiang, and J. Lian, “Visible-light photocatalytic fluorine co-doped titania photocatalyst: effect of temperature activity of nitrogen-doped TiO2 thin film prepared by pulsed on microstructure and surface activity properties,” Journal of laser deposition,” Applied Surface Science, vol. 254, no. 15, pp. Physics and Chemistry of Solids, vol. 68, no. 5-6, pp. 770–774, 4620–4625, 2008. 2007. [43] T. L. Chen, Y. Hirose, T. Hitosugi, and T. Hasegawa, “One unit- cell seed layer induced epitaxial growth of heavily nitrogen [28] M. Yuan, J. Zhang, S. Yan et al., “Effect of Nd2O3 addition on doped anatase TiO2 films,” Journal of Physics D, vol. 41, no. the surface phase of TiO2 and photocatalytic activity studied by UV Raman spectroscopy,” Journal of Alloys and Compounds, 6, Article ID 062005, 2008. vol. 509, no. 21, pp. 6227–6235, 2011. [44] T. Ando, T. Wakamatsu, K. Masuda et al., “Photocatalytic [29] Y. Z. Qu, M. M. Yao, F. Li, and X. H. Sun, “Microstructures and behavior of heavy La-doped TiO2 films deposited by pulsed Photocatalytic Properties of Fe3+/Ce3+ Codoped Nanocrys- laser deposition using non-sintered target,” Applied Surface talline TiO2 Films,” Water, Air and Soil Pollution, vol. 221, no. Science, vol. 255, no. 24, pp. 9688–9690, 2009. 1–4, pp. 13–21, 2011. [45] G. Sauthier, F. J. Ferrer, A. Figueras, and E. Gyorgy,¨ “Growth [30] Z. Luo, H. Cai, X. Ren, J. Liu, W. Hong, and P. Zhang, and characterization of nitrogen-doped TiO2 thin films pre- “Hydrophilicity of titanium oxide coatings with the addition pared by reactive pulsed laser deposition,” Thin Solid Films, of silica,” Materials Science and Engineering B, vol. 138, no. 2, vol. 519, no. 4, pp. 1464–1469, 2010. pp. 151–156, 2007. [46] S. Somekawa, Y. Kusumoto, M. Ikeda, B. Ahmmad, and Y. [31] J. Schou, “Physical aspects of the pulsed laser deposition Horie, “Fabrication of N-doped TiO2 thin films by laser technique: the stoichiometric transfer of material from target ablation method: mechanism of N-doping and evaluation of to film,” Applied Surface Science, vol. 255, no. 10, pp. 5191– the thin films,” Catalysis Communications,vol.9,no.3,pp. 5198, 2009. 437–440, 2008. [32] N. Sato, M. Matsuda, M. Yoshinaga, T. Nakamura, S. Sato, and [47] B. Farkas, J. Budai, I. Kabalci, P. Heszler, and Z. Geretovszky, A. Muramatsu, “The synthesis and photocatalytic properties “Optical characterization of PLD grown nitrogen-doped TiO2 8 International Journal of Photoenergy

thin films,” Applied Surface Science, vol. 254, no. 11, pp. 3484– 3488, 2008. [48] X. Wang, J. Shen, and Q. Pan, “Raman spectroscopy of sol- gel derived titanium oxide thin films,” Journal of Raman Spectroscopy, vol. 42, no. 7, pp. 1578–1582, 2011. [49] S. Sahoo, A. K. Arora, and V. Sridharan, “Raman line shapes of optical phonons of different symmetries in anatase TiO2 nanocrystals,” Journal of Physical Chemistry C, vol. 113, no. 39, pp. 16927–16933, 2009. [50] Z. Luo, M. Li, H. Cai, and J. Liu, “Conductivity characteristics of DCM solution under photo-oxidation of titanium oxide sol,” Journal of Materials Science & Engineering,vol.22,no.4, pp. 523–526, 2004. [51] T. Tachikawa, M. Fujitsuka, and T. Majima, “Mechanistic insight into the TiO2 photocatalytic reactions: design of new photocatalysts,” Journal of Physical Chemistry C, vol. 111, no. 14, pp. 5259–5275, 2007. [52] D. Rimeh, D. Patrick, K. Ibrahima, E. Khakani, and M. Ali, “Photoelectrocatalytic degradation of chlortetracycline using Ti/TiO2 nanostructured electrodes deposited by means of a Pulsed Laser Deposition process,” Journal of Hazardous Materials, vol. 199-200, pp. 15–24, 2012. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 970474, 8 pages doi:10.1155/2012/970474

Research Article Solar Photocatalytic Removal of Chemical and Bacterial Pollutants from Water Using Pt/TiO2-Coated Ceramic Tiles

S. P. Devipriya,1, 2 Suguna Yesodharan,1 and E. P. Yesodharan1

1 School of Environmental Studies, Cochin University of Science and Technology, Kochi 682022, India 2 Department of Ecology and Environmental Sciences, Pondicherry University, Puducherry 605014, India

Correspondence should be addressed to E. P. Yesodharan, [email protected]

Received 24 January 2012; Revised 22 March 2012; Accepted 27 March 2012

Academic Editor: Baibiao Huang

Copyright © 2012 S. P. Devipriya et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Semiconductor photocatalysis has become an increasingly promising technology in environmental wastewater treatment. The present work reports a simple technique for the preparation of platinum-deposited TiO2 catalysts and its immobilization on ordinary ceramic tiles. The Pt/TiO2 is characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX), and diffuse reflectance spectroscopy (DRS). Deposition of Pt on TiO2 extends the optical absorption of the latter to the visible region which makes it attractive for solar energy application. Optimum loading of Pt on TiO2 was found to be 0.5%. The Pt/TiO2 is coated on ceramic tiles and immobilized. This catalyst was found effective for the solar photocatalytic removal of chemical and bacterial pollutants from water. Once the parameters are optimized, the Pt/TiO2/tile can find application in swimming pools, hospitals, water theme parks, and even industries for the decontamination of water.

1. Introduction reported [19–21].DepositionofnoblemetalssuchasPt,Pd, Au, and Ag, on TiO2 enhances the catalytic oxidation of Semiconductor-mediated photocatalysis is fast becoming an organic pollutants [22–26]. ffi e cient advanced oxidation process (AOP) for the removal Pt/TiO2 nanocomposites have been shown to have high of chemical and bacterial pollutants from water [1–5]. The photocatalytic activity for the decomposition of organic most widely studied catalyst in this respect is TiO2 in view of compounds. In this case, the enhancement is attributed to its favorable physicochemical properties, low cost, easy avail- the increased light absorption and retarding of the photogen- ability, high stability, and low toxicity. However, it is active erated electron-hole recombination [25–27]. However, since only in the UV range which constitutes less than 5% of sun- Pt is expensive, this type of catalysts will be unattractive from light. Photocatalytic reactions take place when particles of commercial application point of view unless they can be the semiconductor absorb photon of energy equal to or recycled. The problem can be overcome, at least partially by greater than its band gap and the electrons get excited from immobilizing the catalyst on suitable stable supports. the valence band to the conduction band. This results in the It is reported that nanoparticles of noble metals such as formation of an electron-hole pair which promotes oxida- Au, Ag, and Pt are capable of absorbing visible light due to tion/reduction of the adsorbed substrate. In aqueous solu- the surface plasmon resonance (SPR) in which their conduct- tion, the reactive OH radicals can promote the oxidation and ing electrons undergo a collective oscillation induced by the eventual mineralization of organic compounds. electric field of visible light [28–30]. Ag and Au nanoparticles A number of studies have been reported on the modi- supported on insulators such as ZrO2 and SiO2 yield fication of semiconductor oxides in order to extend the visible light active photocatalysts capable of promoting both absorption of light to the visible range. These include dye oxidative and reductive reactions [31, 32]. Zheng et al. [28] sensitization, semiconductor coupling, impurity doping, use have recently reported a facile in situ synthesis of visible light of coordination metal complexes, and metal deposition plasmonic photocatalysts M-TiO2 (M = Au, Pt, Ag) and their [6–18]. Composites such as TiO2/carbon have also been evaluation for the oxidation of benzene to phenol in aqueous 2 International Journal of Photoenergy phenol. Other efficient and stable plasmonic catalysts such as Photocatalytic experiments with dye solution were per- Ag/AgCl [33] and Ag/AgBr/WO3·H2O[34] are also reported. formed as follows. In such catalysts, visible light is absorbed by nanoparticles The coated tile (12 × 8 × 0.5 cm) is placed in a jacketed of the noble metal, and the photogenerated electrons and glass vessel of approximately 20 cm diameter and 3 cm holes are separated by the metal-semiconductor interface. height. The experimental dye solution of Rhodamine B Thus, the electron-hole recombination is prevented, and the (200 mL, 10 mg/L) is slowly added into the vessel. Water from photocatalytic process is accelerated. In the case of Au/TiO2 a thermostat is circulated through the jacket to maintain the plasmonic photocatalysts, the mechanism of visible-light- temperature at 27 ± 1◦C. For UV experiments, the dye solu- induced photocatalytic oxidation involves the absorption of tion is irradiated with a 400 W medium pressure mercury light by Au nanoparticles which causes electron transfer vapor lamp. Solar experiments were performed by placing from them to the conduction band of TiO2 particles. Con- the same system on the roof top of our laboratory at Kochi, sequently, the oxidation of the organics will take place at the Kerala, India (9◦ 57 51 N, 76◦16 59 E) during sunny electron-deficient Au nanoparticles [28]. days in February–May 2010. Periodically, the dye solution is

In the present study, we have immobilized Pt/TiO2 on a mixed gently. Samples were drawn periodically and analyzed ceramic tile in order to enhance the commercial viability of for the dye concentration by spectrophotometry (555 nm). the catalyst by recycling. The photocatalytic activity of this Solution kept under identical conditions in the dark is used catalyst is tested for the removal of a typical chemical pol- as the reference in each case to eliminate the contribution lutant dye Rhodamine B and a bacterial pollutant E. coli. from adsorption towards the reduction in the dye concentration. For bacterial disinfection studies, Escherichia coli (ATCC 2. Experimental 11775) was used as the test organism. Cells of E. coli were subcultured under sterile conditions at 37◦Cfor24hrin Degussa P-25 TiO is used as such without further treatment. 2 100 mL nutrient broth (pH ∼ 7.2) containing 0.1 g of yeast It is 99% pure and consisted of approximately 70% anatase extract, 0.5 g of peptone, and 0.5 g of NaCl. The cells were and 30% rutile forms. The average particle size was around sedimented by centrifugation at 5000 rpm for 10 minutes, 200 nm, and the BET surface area was ∼15 m2/g. Analytical washed, and resuspended in sterile distilled water. Absorb- grade chloroplatinic acid (H PtCl ·6H O) and Rhodamine 2 6 2 ance was measured using Shimadzu UV-1601 UV-VIS spec- B were from Sigma-Aldrich, India. All other chemicals used trophotometer. Cell cultures corresponding to ∼107 cells/mL were of AR grade. Doubly distilled water was used in all were used to inoculate the reaction systems and controls. the experiments. The immobilized Pt/TiO photocatalyst 2 Control experiments run in parallel consisted of samples (Pt/TiO /tile) is prepared as follows [26]. 2 exposed to sunlight without catalyst and those in the dark Aqueous suspension containing specific quantity of with and without catalysts, all maintained under otherwise Degussa P-25 TiO2 was taken in the photoreactor and purged identical conditions. In catalytic experiments, the coated tiles with nitrogen to remove dissolved oxygen. Specified amounts were placed in aquarium jars containing 10 liters of the · of H2PtCl6 6H2O dissolved in 20 mL methanol were then inoculated water. The jars were placed in a trough through added to the aqueous suspension of TiO2 andagitatedwitha which water at the required temperature is circulated and magnetic stirrer. The suspension under nitrogen atmosphere irradiated by sunlight. Sampling was done at predetermined is then irradiated with 400 W UV lamp for 8 hr. The milky intervals by pipetting out 1 mL of the experimental solution white suspension turns grayish with the deposition of Pt. The into 9 mL sterile saline which was serially diluted. After suspension is then filtered, and after repeated washings with mixing, 0.1 mL aliquots of each dilution were spread onto doubly distilled water, it is dried and powdered. McConkey agar plates. The cell inactivation was monitored Common glazed tiles normally used for flooring are by counting the colony forming units (CFU) after 24-hour etched with dilute HF. The surface is then cleaned and incubation at 37◦C. coated uniformly with the paste of Pt/TiO2 and polymethyl methacrylate in chloroform. The tile is then dried at 110◦C ◦ 3. Results and Discussions and later heated at 350 C for 3 hr. A thin film of Pt/TiO2 is formed on the surface of the tile. For control experiments, 3.1. Surface Characterization of the Photocatalyst. The photo tiles were prepared exactly as above, one with TiO2 only and deposition of Platinum on the TiO2 surface was indicated by another without any catalyst. The amount of catalyst coated color change of the particles from white to grey. XRD analysis was determined by measuring the weight difference of the tile is done to study the phase structure of the catalyst. Figure 1 before and after the coating. shows characteristic peaks of TiO2 only with no indication X-ray diffraction pattern of the coated catalyst is deter- of the presence of Pt. This may be because either Pt is not mined by using Rigaku X-ray diffractometer with CuKα´ radi- getting deposited on TiO2 or the concentration of Pt in the ation. Scanning electron microscopy (SEM) measurements TiO2 matrix is too small to be detected by XRD. As the results were performed with JEOL Model JSM-6390 LV. Energy presented later in this paper show, reflectance spectrum and dispersive X-ray spectroscopy (EDAX) measurements were enhanced photocatalytic activity of the prepared material made using JOEL model JED-2300 attached to SEM. Diffuse reveal modification in the properties of TiO2, which can be reflectance spectra (DRS) were recorded with Varian Cary attributed to the presence of small amounts of highly 5000 using BaSO4 as the reference. dispersed Pt. Scanning electron microscopy (SEM) of International Journal of Photoenergy 3

1000 K

900 iK i TiKTi T TiKa 800 700 600 (a.u.) 500 Counts 400 K OKa OKa Intensity TiLa T 300 TiLa TiLl TiLl TiKbT TiKb

200 Ka NKa PtMr P PtMr PtMa P PtM PtMa t PtMz P PtMz PtLa PtLa P PtLl 100 PtLl 0 012345678910 3 10 20 30 40 50 60 70 80 (keV) 2θ (degrees) Figure 3: EDAX spectrum of Pt/TiO2. Anatase Rutile

Figure 1: XRD pattern Pt/TiO2 coated on the surface of the ceramic tile. 80 1

60 (%) R 40 2

200 300 400 500 600 700 800 Wavelength (nm)

Figure 4: DRS spectra of (1) TiO2 (P-25) and (2) Pt/TiO2.

Figure 2: SEM of Pt (0.5 wt%)—deposited on TiO2. nanoparticles in the TiO2 matrix can also produce some energy levels in the band gap of the latter [25]. Sakthivel et al. [23] also concluded from similar studies that defect sites on 3+ the sample with 0.5 weight % of Pt dispersed on TiO2 the TiO2 surface, identified as Ti , are necessary for adsorp- (Figure 2) shows that the particles are approximately spher- tion and photoactivation of oxygen which are the primary ical in nature. Elemental mapping of Pt by EDAX (Figure 3) steps in photocatalysis. In the present case, the deposition of shows that the metal particles are uniformly distributed on Pt was made in the absence of air. Hence, the in situ transfor- 3+ 4+ the surface of TiO2, even though the characteristic peaks are mation of Ti to Ti by oxygen can be ruled out. However, not quite distinct possibly due to the very low concentration in presence of the platinum salt, the photogenerated Ti3+ ions of Pt. reduce the noble metal cations to neutral Pt atoms. These The DRS spectrum of the Pt/TiO2 sample shown in atoms can nucleate to grow into clusters, eventually forming Figure 4 indicates that the optical absorption of TiO2 is nanoparticles of Pt on the TiO2 surface (Pt/TiO2). The enhanced significantly from 360 nm onwards. This extension surface plasmon resonance in Pt/TiO2 under visible light towards the visible region of the optical spectrum is con- irradiation enables the transfer of photogenerated electrons sistent with the findings reported by earlier investigators from the metal particles to TiO2 conduction band by crossing [25, 26]. the metal-semiconductor interface and surmounting the According to F. B. Li and X. Z. Li [25]andChenetal. Schottky barrier [28]. This will reduce the recombination of [26], deposition of Pt on TiO2 results in the formation of the electron-hole pair. The electrons are taken up by 3+ Ti , possibly due to the interaction between Pt and TiO2 adsorbed oxygen forming active species which interact with during photo reduction. The presence of Ti3+ sites in the the organic substrate, leading to eventual mineralisation. In lattice may form a defect energy level in the band gap of TiO2 the process, the substrate can compensate the depleted thereby absorbing visible light more efficiently and resulting electron, and the SPR is fully restored, thereby ensuring in enhanced photocatalytic activity. The dispersion of Pt continued efficient photocatalytic activity. 4 International Journal of Photoenergy

−• 110 O2 +Dye −→ Intermediates −→ Degradation products 100 (2) 90 [Rh B]: 10 mg/L 80 Pt conc: 0.5% In the case of pure TiO2 also, the degradation of the dye is pH: 5.5 70 signiﬁcant in sunlight unlike in the case of substrates like (%) phenol, alcohol, cresol, and so forth where the degradation is 60 much slower [35]. This might be because the dye Rhodamine 50 B can absorb visible light and act as a sensitizer thereby 40 transferring electrons from the excited dye molecule to the Degradation 30 conduction band of TiO2, 20 −→ ∗ 10 Dye(ads) Dye(ads) ∗ + − 0 Dye + Pt-TiO2 −→ Dye + Pt-TiO2(eCB ) 0 50 100 150 200 250 (ads) (ads) + −→ −→ Time ( min) Dye(ads) Intermediates Oxidation/Degradation (3)

Pt/TiO2/tile/UV Pt/TiO2/tile/SL TiO2/tile/UV TiO2/tile/SL The electrons are scavenged by the oxygen adsorbed on the UV only SL only surface of TiO2 as in (1) above. Thus, the dye behaves like an electron donor at the excited state and injects electron Figure 5: Photocatalytic degradation of Rhodamine B under directly to the conduction band upon irradiation. Anandan various conditions in presence of UV and sunlight (SL). et al. [36] also suggested similar pathway for the degradation of dyes on Ag/TiO2 catalysts. The study further shows that, in the presence of sunlight, 3.2. Photocatalytic Degradation of Rhodamine B on Pt/TiO2- the photocatalytic activity of dye sensitized TiO2 is much less Coated Tiles. Photocatalytic degradation of the highly sol- compared to Pt/TiO2. Deposition of Pt enhances the pho- uble basic red dye of the xanthenes class, Rhodamine B, is tocatalytic activity of TiO2 in two ways, that is, by the pre- evaluated using TiO2/tile and Pt/TiO2/tile. Earlier studies vention of electron hole recombination and the extension of reported on the degradation of the dye involved the use of the light absorption range as discussed earlier. Modification catalyst suspensions, which are not quite convenient in terms of the surface of TiO2 by Pt can also result in enhancement of practical application. The advantage of the immobilized of the number of active sites for the dye-catalyst interaction catalyst over suspension is that it needs not be separated from which in turn can increase the photodegradation rate. On the water every time and can be recycled. Degradation of the dye other hand, dye sensitization of TiO2 leads to only visible- takes place even in the absence of catalyst, though slowly. The light absorption which is highly concentration dependent. At degradation is investigated in both UV light as well as in sun- lower concentration, when there is only monolayer coverage light (SL). The pH of the system is maintained at 5.5 which of the dye, light absorption may not be adequate to acquire is close to the natural pH of the dye solution under the reac- sufficient energy to be transferred to the semiconductor. At tion conditions used in the study. The results are plotted in the same time, multilayer adsorption of the dye does not Figure 5. enhance the light absorption or photocatalytic efficiency As expected, Pt/TiO2 is more active in the visible sunlight significantly because the inner layers will tend to act as as well as in the UV region compared to TiO2.Degrada- insulators with respect to outer layers [37]. tion/decolorisation in the absence of catalysts may be due to The effect of concentration of Pt in Pt/TiO2 on the rate of the natural self-fading of the dye. degradation of the dye is tested by varying the loading of The percentage degradation of the dye in presence of the former in TiO2, that is, 0.1, 0.25, 0.75, and 1.0%. The Pt/TiO2/tile is about 60% more compared to TiO2/tile under rate constant of degradation in sunlight at various Pt con- UV irradiation. However, the degradation is approximately 5 centrationsisplottedinFigure 6. The rate increases with times more in the case of Pt/TiO2/tile compared to TiO2/tile Pt loading initially. However, it levels off or even decreases in presence of sunlight. The comparative enhancement in slightly beyond the optimum. this case increases as the irradiation progresses, possibly due The detrimental effect of higher content of Pt in pho- to the inability of pure TiO2 to get activated significantly in tocatalytic degradation was reported earlier also [25]. At sunlight. At the same time, Pt nanoparticles absorb visible lower concentration, Pt clusters can act as charge separation light resulting in the formation of electron-hole pairs and centres. As explained earlier, one of the reasons for the subsequent activation of adsorbed oxygen as explained enhanced photocatalytic activity of Pt/TiO2 is the separation −• • earlier. The superoxide/hydroperoxide (O2 /HO2 ) radicals of the photogenerated electrons and holes by the metal-semi- thus formed initiate degradation of the dye. Repeated attacks conductor interface which prevents their recombination. −• • by the (O2 /HO2 ) radicals on the dye nuclei will lead to However, at higher concentrations of Pt, the average distance mineralization producing mainly CO2 and water. of separation between the electrons and holes decreases, and the Pt clusters themselves can act as recombination centers. - −• Pt-TiO2 (eCB ) +O2 −→ O2 (1) Choi et al. [38] also observed that there is an optimal metal International Journal of Photoenergy 5

×103 40 6 35 [RhB]: 10 mg/L Pt conc: 0.5% 5 Time: 30 min 30 (%)

4 [Rh B]: 10 mg/L 25 ) 1 pH: 5.5 3

(min 20 Degradation k

2 15

1 10 024681012 (pH) 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Figure 8: Eﬀect of pH on the solar photocatalytic degradation of

Pt in Pt-TiO2 (%) Rhodamine B in presence of Pt/TiO2/Tile.

Figure 6: Effect of Pt concentration in Pt/TiO2/Tile on the rate constant of Rhodamine B degradation under sunlight. absence of significant continued degradation, the adsorbed dye will not leave the surface sites, thus preventing the ×103 adsorption of new molecules and consequent degradation. 10 Thus, the light absorption and number of adsorption sites are the two factors affected by higher concentration of the dye. It is also known that the dye itself will be absorbing 8 more solar light at higher concentration. The path length of light entering the system also decreases with increase in concentration [39]. Pt conc: 0.5% ) 6 The pH is an important factor in the case of wastewater, 1 − pH: 5.5 and hence, its effect on the photocatalytic degradation of the

(min dye in water is investigated. The degradation remains more k 4 or less the same in the range of 3–6.5 and increases thereafter. The results are shown in Figure 8. The increased degradation at higher pH is not due to 2 any change in the absorption of light as the λmx for the dye changes very little (551–553 nm) in the pH range of 1–13 [40], even though Rhodamine exists in two principal forms 0 in water, that is, cationic (RhB+) or in zwitterionic (RhB±). 0 1020304050 At pH value less than the point of zero charge (PZC) of TiO2 Concentration of Rhodamine B (mg/L) (6.5), the surface will be positively charged. In the acidic Figure 7: Effect of concentration of the Rhodamine B on its range, the dye will be in cationic form (RhB+). Hence, in this photocatalytic degradation in presence of Pt/TiO2/Tile under range, due to electrostatic repulsive forces, the adsorption of sunlight. the dye on the catalyst is less. Thus, the surface-promoted degradation is less, and the observed degradation is primarily taking place in the solution. At higher pH value, the RhB+ dopant concentration above which the photocatalytic activ- gets deprotonated and its zwitterion is formed. This can ity decreases. get adsorbed onto the negatively charged catalyst surface Studies on the effect of concentration of the dye on the resulting in increased degradation of the dye. Further, under rate of photocatalytic degradation showed that the degrada- alkaline conditions, more OH radical formation is possible tion decreased with increasing initial concentration. The from the abundant hydroxide ions, which also enhances the results are plotted in Figure 7. degradation. Since the degradation is accelerated by both catalyst and light, the negative effect of increasing concentration implies 3.3. Photocatalytic Disinfection of E. coli on Pt/TiO2/Tile. that at higher concentration, the dye is inhibiting the action Heterogeneous photocatalysis has been found to be an effec- of catalyst and/or light. At higher concentration, there will be tive method for the deactivation of bacteria in water [41]. stronger adsorption of the dye on the surface of the catalyst. Conventional semiconductors such as ZnO and TiO2 which This will inhibit the direct absorption of light by Pt/TiO2, are extensively investigated as photocatalysts can be activated thereby affecting its ability to generate active radicals. In the only by UV light, which make them inefficient for solar 6 International Journal of Photoenergy

×105 The reason often cited for the convenience of chemical 12 decontamination over other techniques such as photocatalysis is that in the former, the destruction of microorganisms is complete, and hence, the reemergence is not signiﬁcant. To 10 compare the eﬃcacy of chemical treatment with photocatalysis, the reemergence of E. coli in water treated with 8 TiO2/tile/light, Pt/TiO2/tile/light, and light alone is examined. After complete inactivation of the bacteria, in experiments with and without catalyst, the reemergence was tested 6 after 2, 24, and 48 hr. There was no bacterial presence

CFU/100mL detected in any of the systems after 2 hr. However, significant 4 reemergence was noticed after 24 hr in the light alone system. The reemergence was much slower in the system with catalyst, the comparative rate of reemergence being in the order 2 TiO2/tile/light Pt/TiO2/tile/light. This indicates that the destruction in presence of the catalyst Pt/TiO2 is almost as 0 irreversible as in the case of chemical decontamination, while 0 50 100 150 200 250 300 the sunlight alone may not be sufficient for complete destruc- Time (min) tion. The mechanism of deactivation of E. coli is not known ff Dark TiO2/sunlight clearlyyet.Freeradicalshaveanadversee ect on cell DNA Sunlight Pt/TiO2/sunlight replication and the modification of cellular membrane. Cho • ff et al. [45] demonstrated linear correlation between OH Figure 9: Solar deactivation of E. coli under di erent photocatalytic concentration and E. coli inactivation. Direct photocatalytic conditions. oxidation of intracellular coenzyme is also proposed by some authors [42]. The H2O2 formed in the system also is a good bactericide [44, 46]. Higher concentration of OH and H2O2 energy application. Since Pt/TiO2/tile is effective for the in presence of catalyst will lead to higher irreversible destruc- removal of chemical pollutants in sunlight, the same is tested tion of the bacteria. Exposure to reactive oxygen species such −• • for the removal of E. coli from water. as O2 and OH generated during photocatalysis can result Results presented in Figure 9 show that the bacteria in oxidative damage to cellular components, DNA and RNA remain practically unaffected in the dark. However, in pres- [47–49]. The disruption of cell membrane is attributed to ence of sunlight, the population is reduced gradually even in peroxidation of the unsaturated phospholipids leading to the absence of the catalyst. In the case of Pt/TiO2/tile, total loss of respiratory activity [50, 51]. Subsequent loss of inactivation takes place in 50 minutes, while it takes 90 fluidity and increased ion permeability permit oxidative minutes in the case of TiO2/tile. In the absence of catalyst but attack of internal cellular components resulting in cell death. in presence of light, there is an initial induction period during which the deactivation is negligible. This may be the 4. Conclusion reason why some of the earlier researchers [42] did not observe any inactivation of E. coli in the 60 minutes duration Deposition of noble metal Pt on TiO2 extends the light of their study. The deactivation is not due to temperature absorption of the latter from UV to the visible range which rise in presence of light as reported by Melian et al. [43] makes the composite a good photocatalyst for solar decon- since in our case the temperature was maintained constant tamination of polluted water. In this study, Pt/TiO2 is at 27 ± 1◦C by circulating water from a thermostat. Since the immobilized by coating on an ordinary ceramic tile, and its visible light induced deactivation is more in the presence of photocatalytic activity is tested for the removal of a chemical Pt/TiO2/tile compared to direct irradiation in the absence of pollutant, Rhodamine B, and a bacterial pollutant, E. coli. any catalyst, it is clear that the catalyst plays a significant role Pt/TiO2 is more effective than TiO2 alone for the removal in harnessing solar energy for bacterial decontamination. of both types of pollutants. The catalyst is characterized by Comparative evaluation of the catalyst in immobilized form XRD, DRS, SEM, and EDAX. Higher pH favours the removal as well as in suspension showed that the suspension is of Rhodamine B, while increase in concentration of the dye more effective [44]. The number of surviving bacteria in the decreases the rate of removal. There is an optimal con- case of the Pt/TiO2/tile and the suspended catalyst under centration of Pt beyond which the rate of enhancement identical conditions at different time intervals show that the decreases. Suspension system is more efficient compared to suspension is more effective throughout for bacteria removal. the immobilized catalyst. However, problems associated with The pH of natural waters and wastewater will normally the separation of the catalyst make the latter more promising be in the range of 5–9. Hence, the effect of pH on the for field application. The bacterial contaminant E. coli can be deactivation of E. coli mediated by Pt/TiO2 in presence of removed by light even in the absence of the catalyst. However, sunlight is examined. It is observed that pH has no significant the presence of catalyst accelerates the decontamination effect on the rate of deactivation in this pH range. significantly. Also, the reemergence of the bacteria after International Journal of Photoenergy 7 the light is off is inhibited to a great extent by the catalysts. A oxide semiconductors,” Accounts of Chemical Research, vol. 42, tentative mechanism for the chemical and bacterial decon- no. 12, pp. 1966–1973, 2009. tamination is proposed and discussed. [15]S.Sakthivel,B.Neppolian,M.V.Shankar,B.Arabindoo,M. Palanichamy, and V. Murugesan, “Solar photocatalytic degra- ffi Acknowledgments dation of azo dye: comparison of photocatalytic e ciency of ZnO and TiO2,” Solar Energy Materials and Solar Cells, vol. 77, Financial support from the Department of Science and Tech- no. 1, pp. 65–82, 2003. [16] Y. Xu, W. Zheng, and W. Liu, “Enhanced photocatalytic nology (DST), Government of India to one of the authors ff (S.P.Devipriya)undertheWOS-Aschemeisgratefully activity of supported TiO2: dispersing e ectofSiO2,” Journal of Photochemistry and Photobiology A, vol. 122, no. 1, pp. 57– acknowledged. 60, 1999. [17] H. Tada, A. Hattori, Y. Tokihisa, K. Imai, N. Tohge, and S. Ito, References “A patterned-TiO2/SnO2 bilayer type photocatalyst,” Journal of Physical Chemistry B, vol. 104, no. 19, pp. 4585–4587, 2000. [1] R. W. Matthews, “Photocatalytic oxidation of organic contam- [18] Y. Li, S. Sun, M. Ma, Y. Ouyang, and W. Yan, “Kinetic study inants in water: an aid to environmental preservation,” Pure and model of the photocatalytic degradation of rhodamine B and Applied Chemistry, vol. 64, pp. 1285–1290, 1992. (RhB) by a TiO2-coated activated carbon catalyst: effects of [2] S. Devipriya and S. Yesodharan, “Photocatalytic degradation initial RhB content, light intensity and TiO2 content in the of pesticide contaminants in water,” Solar Energy Materials and catalyst,” Chemical Engineering Journal, vol. 142, no. 2, pp. Solar Cells, vol. 86, no. 3, pp. 309–348, 2005. 147–155, 2008. [3] M. Muneer and C. Boxall, “Photocatalysed degradation of [19] H. Uchida, S. Itoh, and H. Yoneyama, “Photocatalytic decom- a pesticide derivative glyphosate in aqueous suspensions of position of propyzamide using TiO2supported on activated titanium dioxide,” International Journal of Photoenergy, vol. carbon,” Chemistry Letters, vol. 22, pp. 1995–1998, 1993. 2008, Article ID 197346, 7 pages, 2008. [20] D. K. Lee, S. C. Kim, I. C. Cho, S. J. Kim, and S. W. Kim, [4] P. A. Deshpande and G. Madras, “Photocatalytic degradation “Photocatalytic oxidation of microcystin-LR in a fluidized bed of phenol by base metal-substituted orthovanadates,” Chemi- reactor having TiO2-coated activated carbon,” Separation and cal Engineering Journal, vol. 161, no. 1-2, pp. 136–145, 2010. Purification Technology, vol. 34, no. 1–3, pp. 59–66, 2004. [5] D. Ollis, P. Pichat, and N. Serpone, “TiO2 photocatalysis-25 [21] D. K. Lee, S. C. Kim, S. J. Kim, I. S. Chung, and S. W. Kim, years,” Applied Catalysis B, vol. 99, no. 3-4, p. 377, 2010. “Photocatalytic oxidation of microcystin-LR with TiO2- [6]J.Moon,C.Y.Yun,K.W.Chung,M.S.Kang,andJ.Yi, coated activated carbon,” Chemical Engineering Journal, vol. “Photocatalytic activation of TiO2 under visible light using 102, no. 1, pp. 93–98, 2004. Acid Red 44,” Catalysis Today, vol. 87, no. 1–4, pp. 77–86, 2003. [22] X. Z. Li and F. B. Li, “Study of Au/Au3+-TiO photocatalysts [7] D. Pei and J. Luan, “Development of visible light-responsive 2 toward visible photooxidation for water and wastewater sensitized photocatalysts,” International Journal of Photoen- treatment,” Environmental Science and Technology, vol. 35, no. ergy, vol. 2012, Article ID 262831, 13 pages, 2012. 11, pp. 2381–2387, 2001. [8] S. Kim and W. Choi, “Visible-light-induced photocatalytic [23] S. Sakthivel, M. V. Shankar, M. Palanichamy, B. Arabindoo, degradation of 4-chlorophenol and phenolic compounds D. W. Bahnemann, and V. Murugesan, “Enhancement of in aqueous suspension of pure titania: demonstrating the photocatalytic activity by metal deposition: Characterisation existence of a surface-complex-mediated path,” Journal of and photonic efficiency of Pt, Au and Pd deposited on TiO Physical Chemistry B, vol. 109, no. 11, pp. 5143–5149, 2005. 2 catalyst,” Water Research, vol. 38, no. 13, pp. 3001–3008, 2004. [9] H. Chen, W. Li, H. Liu, and L. Zhu, “Performance enhance- [24] V. M. Mendez-Florez, D. Friedmann, and D. W. Bahnemann, ment of CdS-sensitized TiO2 mesoporous electrode with two different sizes of CdS nanoparticles,” Microporous and “Durability of Ag-TiO2 photocatalysts assessed for the degra- Mesoporous Materials, vol. 138, no. 1–3, pp. 235–238, 2011. dation of dichloroacetic acid,” International Journal of Pho- [10] M. M. Rahman, K. M. Krishna, T. Soga, T. Jimbo, and M. toenergy, vol. 2008, Article ID 280513, 11 pages, 2008. Umeno, “Optical properties and X-ray photoelectron spectro- [25] F. B. Li and X. Z. Li, “The enhancement of photodegradation efficiency using Pt-TiO2 catalyst,” Chemosphere, vol. 48, no. 10, scopic study of pure and Pb-doped TiO2 thin films,” Journal of Physics and Chemistry of Solids, vol. 60, no. 2, pp. 201–210, pp. 1103–1111, 2002. ff 1999. [26]H.W.Chen,Y.Ku,andY.L.Kuo,“E ectofPt/TiO2 charac- [11] C. G. Wu, C. C. Chao, and F. T. Kuo, “Enhancement of the teristics on temporal behavior of o-cresol decomposition by photo catalytic performance of TiO2 catalysts via transition visible light-induced photocatalysis,” Water Research, vol. 41, metal modification,” Catalysis Today, vol. 97, no. 2-3, pp. 103– no. 10, pp. 2069–2078, 2007. 112, 2004. [27] J. M. Herrmann, J. Disdier, and P. Pichat, “Photoassisted [12] E. Bae and W. Choi, “Highly enhanced photoreductive platinum deposition on TiO2 powder using various platinum degradation of perchlorinated compounds on dye-sensitized complexes,” JournalofPhysicalChemistry, vol. 90, no. 22, pp. metal/TiO2 under visible light,” Environmental Science and 6028–6034, 1986. Technology, vol. 37, no. 1, pp. 147–152, 2003. [28] Z. Zheng, B. Huang, X. Qin, X. Zhang, Y. Dai, and M. H. [13] Y. Pellegrin, L. Le Pleux, E. Blart et al., “Ruthenium polypyri- Whangbo, “Facile in situ synthesis of visible-light plasmonic dine complexes as sensitisers in NiO based p-type dye- photocatalysts M@TiO2 (M = Au, Pt, Ag) and evaluation of sensitized solar cells: effects of the anchoring groups,” Journal their photocatalytic oxidation of benzene to phenol,” Journal of Photochemistry and Photobiology A, vol. 219, no. 2, pp. 235– of Materials Chemistry, vol. 21, no. 25, pp. 9079–9087, 2011. 242, 2011. [29] P. V. Kamat, “Photophysical, photochemical and photocat- [14] W. J. Youngblood, S. H. Anna Lee, K. Maeda, and T. E. alytic aspects of metal nanoparticles,” Journal of Physical Mallouk, “Visible light water splitting using dye-sensitized Chemistry B, vol. 106, no. 32, pp. 7729–7744, 2002. 8 International Journal of Photoenergy

[30] S. Eustis and M. A. El-Sayed, “Why gold nanoparticles are [46] M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. Bah- more precious than pretty gold: Noble metal surface plasmon nemann, “Environmental applications of semiconductor pho- resonance and its enhancement of the radiative and nonradia- tocatalysis,” Chemical Reviews, vol. 95, no. 1, pp. 69–96, 1995. tive properties of nanocrystals of different shapes,” Chemical [47] Z. Huang, P. C. Maness, D. M. Blake, E. J. Wolfrum, S. L. Society Reviews, vol. 35, no. 3, pp. 209–217, 2006. Smolinski, and W. A. Jacoby, “Bactericidal mode of titanium [31] H. Zhu, X. Chen, Z. Zheng et al., “Mechanism of supported dioxide photocatalysis,” Journal of Photochemistry and Photo- gold nanoparticles as photocatalysts under ultraviolet and biology A, vol. 130, no. 2-3, pp. 163–170, 2000. visible light irradiation,” Chemical Communications, no. 48, [48] A. Erkan, U. Bakir, and G. Karakas, “Photocatalytic microbial pp. 7524–7526, 2009. inactivation over Pd doped SnO2 and TiO2 thin films,” Journal [32] X. Chen, Z. Zheng, X. Ke et al., “Supported silver nanoparticles of Photochemistry and Photobiology A, vol. 184, no. 3, pp. 313– as photocatalysts under ultraviolet and visible light irradia- 321, 2006. tion,” Green Chemistry, vol. 12, no. 3, pp. 414–419, 2010. [49] D. M. Blake, P. C. Maness, Z. Huang, E. J. Wolfrum, J. Huang, [33] P. Wang, B. Huang, X. Qin et al., “Ag@AgCl: a highly efficient and W. A. Jacoby, “Application of the photocatalytic chemistry and stable photocatalyst active under visible light,” Ange- of titanium dioxide to disinfection and the killing of cancer wandte Chemie - International Edition, vol. 47, no. 41, pp. cells,” Separation and Purification Methods,vol.28,no.1,pp. 7931–7933, 2008. 1–50, 1999. [34] P. Wang, B. Huang, X. Qin, X. Zhang, Y. Dai, and M. [50] A. G. Rincon,´ C. Pulgarin, N. Adler, and P. Peringer, H. Whangbo, “Ag/AgBr/WO3·H2O: visible-light photocatalyst “Interaction between E. coli inactivation and DBP-precursors- for bacteria destruction,” Inorganic Chemistry, vol. 48, no. 22, dihydroxybenzene isomers - in the photocatalytic process of pp. 10697–10702, 2009. drinking-water disinfection with TiO2,” Journal of Photochem- [35] S. G. Anju, S. Yesodharan, and E. P. Yesodharan, “Sonopho- istry and Photobiology A, vol. 139, no. 2-3, pp. 233–241, 2001. tocatalytic degradation of phenol over semiconductor oxides,” [51] A. G. Rincon´ and C. Pulgarin, “Field solar E. coli inactivation Chemical Engineering Journal, vol. 189-190, no. 1, pp. 84–93, in the absence and presence of TiO2: is UV solar dose an 2012. appropriate parameter for standardization of water solar dis- [36] S. Anandan, P. Sathish Kumar, N. Pugazhenthiran, J. Madha- infection?” Solar Energy, vol. 77, no. 5, pp. 635–648, 2004. van, and P. Maruthamuthu, “Effect of loaded silver nanoparticles on TiO2 for photocatalytic degradation of Acid Red 88,” Solar Energy Materials and Solar Cells, vol. 92, no. 8, pp. 929– 937, 2008. [37] H. Gerischer, “Photodecomposition of semiconductors,” Berichte der Bunsengesellschaft fur¨ physikalische Chemie, vol. 77, pp. 771–779, 1973. [38]W.Choi,A.Termin,andM.R.Hoffmann, “The role of metal ion dopants in quantum-sized TiO2: correlation between photoreactivity and charge carrier recombination dynamics,” Journal of Physical Chemistry, vol. 98, no. 51, pp. 13669–13679, 1994. [39] L. You-Ji and C. Wei, “Photocatalytic degradation of Rho- damine B using nanocrystalline TiO2-zeolite surface composite catalysts: effects of photocatalytic condition on degradation efficiency,” Catalysis Science and Technology,vol.1,no.5,pp. 802–809, 2011. [40] S. Merouani, O. Hamdaoui, F. Saoudi, and M. Chiha, “Sono- chemical degradation of Rhodamine B in aqueous phase: effects of additives,” Chemical Engineering Journal, vol. 158, no. 3, pp. 550–557, 2010. [41] A. Mills and S. Le Hunte, “An overview of semiconductor photocatalysis,” Journal of Photochemistry and Photobiology A, vol. 108, no. 1, pp. 1–35, 1997. [42] C. Wei, W. Y. Lin, Z. Zainal et al., “Bactericidal activity of TiO2 photocatalyst in aqueous media: toward a solar-assisted water disinfection system,” Environmental Science and Technology, vol. 28, no. 5, pp. 934–938, 1994. [43] J. A. H. Melian, J. M. D. Rodriguez, A. V. Suarez et al., “The photocatalytic disinfection of urban wastewaters,” Chemo- sphere, vol. 41, pp. 323–327, 2000. [44] S. P. Devipriya, R. Kanjur, and S. Yesodharan, “Inactivation of Escherichia coli in water using TiO2 as photocatalyst,” Pollution Research, vol. 24, no. 1, pp. 87–91, 2005. [45] M. Cho, H. Chung, W. Choi, and J. Yoon, “Linear correlation between inactivation of E. coli and OH radical concentration in TiO2 photocatalytic disinfection,” Water Research, vol. 38, no. 4, pp. 1069–1077, 2004. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 208987, 7 pages doi:10.1155/2012/208987

Research Article Synthesis and Photocatalytic Activity of TiOX Powders with Different Oxygen Defects

Leini Wang,1 Fei Lu,1 and Fanming Meng1, 2, 3

1 Anhui Key Laboratory of Information Materials and Devices, School of Physics and Materials Science, Anhui University, Hefei 230039, China 2 Department of Physics and Astronomy, University of Arkansas at Little Rock, Little Rock, AR 72204, USA 3 Key Laboratory of Materials Modiﬁcation by Laser, Ion and Electron Beams Dalian University of Technology, Ministry of Education, Dalian 116024, China

Correspondence should be addressed to Fanming Meng, [email protected]

Received 10 February 2012; Revised 28 March 2012; Accepted 30 March 2012

Academic Editor: Weifeng Yao

Copyright © 2012 Leini Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The novel carbon- or chromium-doped TiOX photocatalysts with different oxygen defects were synthesized by mechanochemical technique and heating process. The samples were characterized by X-ray diffraction, UV-vis spectrophotometer, and fluorescence spectrometer. Carbon and chromium species were incorporated into TiOX crystal matrix. The mass fraction of Ti7O13 in TiOX photocatalysts could be tunable through carbon or chromium doping. The mass fraction of Ti7O13 could be an indication of 3+ the degree of oxygen defects (the concentration of Ti ) in the TiOX. The degree of oxygen defects increased for carbon doping, while the degree of oxygen defects decreased for chromium doping. The photocatalytic activity measurement results showed that photodegradation rate of methyl orange reached the maximum value with mass fraction of Ti7O13 of about 66.93%, but the photodegradation rate decreased when mass fraction of Ti7O13 is raised further. In addition, the origin of absorption in the visible spectral range for carbon-doped TiOX as well as the effect of band gap on photocatalytic activity has also been discussed in this paper.

1. Introduction including metal or nonmetallic ions doping [10, 11], semiconductor coupling [12], and deposition of noble metals Currently, extensive research has been carried out on oxide [13], have been devoted to extend the spectral response semiconductor photocatalysis in the conditions of aggrava- region of titania and enhance its photocatalytic activity. tion of environmental pollution and resources shortage [1– The photocatalytic activity of doped TiO is a complex 5]. In this sense, semiconductor photocatalysis can change 2 function of the dopant concentration [14], crystal structure solar energy to electrical and chemical energy, drive redox [11], surface area [15], and lattice defects. Especially oxygen reactions, degrade organic substance, and improve environ- vacancies play an important role in photocatalytic efficiency ment. As the leading candidate semiconductor photocatalyst, of TiO [16–18]. However, the photocatalyst of the TiO titania has attracted most attention due to its unique physic- 2 X ochemical properties, including good chemical stability, in- with oxygen defects was little reported in previous literatures. expensiveness, relatively good reactivity, and notoxicity [6, In this paper, TiOX powder including Ti7O13 and TiO2 7]. However, the large band gap (3.2ev) of titania allows it was used as precursor. Carbon or chromium doping was to absorb only the ultraviolet light, which seriously restricts used to control the degree of oxygen defects of samples. The its utilization efficiency for the solar photons. Furthermore, photocatalytic activities dependence on the degree of oxygen the low-quantum yield that results from high frequency defects was evaluated in terms of the photodegradation of of recombination of photoinduced current carriers limits methyl orange (MO) under UV light irradiation. Further- its practical applications [8, 9]. Hence, significant efforts, more, a facile green synthetic route of C-doped TiOX was 2 International Journal of Photoenergy developed, which could provide an effective technique for in- The UV-vis absorption spectroscopy of the sample dustrial production due to its low cost. was measured using Ultraviolet-Visible-Near Infrared Spec- trophotometer (U-4100), while BaSO4 wasusedasarefer- 2. Experimental ence. Photoluminescence (PL) spectra were obtained using a fluorescence spectrophotometer (F-4500) at room tempera-

2.1. Preparation of Samples. The TiOX powder and chromic ture. oxide (Cr2O3) were purchased from Tianjin Guangfu Fine Chemical Research Institute, China. Glucose (C6H12O6)was 2.3. Photocatalytic Activity. Photocatalytic activity of sam- purchased from Tianjin Guangfu technology development ples was characterized by decolorization of methyl orange Co. Ltd., China. All the reagents were of analytical grade (MO). The photocatalyst (0.3 g) was dispersed in 25 mL MO purity and used without any further purification. Distilled aqueous solution with a concentration of 5 ppm in a dish water was used in all experiments. A planetary ball mill was with diameter of 8 cm. The mixture was kept in the dark used for sample synthesis. for 30 min to obtain the absorption-desorption equilibrium Carbon- and chromium-codoped TiOX sample was pre- before UV-light irradiation. A 100 W mercury lamp was used pared by following process. Appropriate amount of C6H12O6 as a light source. The distance between the lamp and the and Cr2O3 were added to TiOX powder. The mass ratio of reaction solution was 9 cm. The absorbance of MO solution Cr2O3/C6H12O6/TiOX was kept constant at 2 : 9 : 200. Total at 464 nm was measured with UV-vis spectrophotometer at 40 g of above mixture was put into the bottle, and then 25 mL intervals of 15 min and the total irradiation time was 45 min. distilled water was added. After being milled for 180 min at a speed of 400 rpm, the wet powder was dried in air over at 90◦Cfor10h.Thegroundpowderwassubsequentlyheated 3. Results and Discussion at 200◦C for 300 min. The prepared catalyst was named C/Cr-TOV. Carbon-doped and chromium-doped and pure Figure 1 (a, b, c, d) shows the XRD patterns of the samples. It is found that all the samples consist of mixed phases of TiOX samples were also prepared through the same method, without adding the corresponding dopant, named as C-TOV, anatase, Ti7O13, and brookite. For doped TiOX (b, c, d), the Cr-TOV, and TOV, respectively. Namely, for the synthesis of characteristic peaks of carbon and Cr2O3 are not observed. It may be attributed to the small amount of dopant or carbon, Carbon-doped TiOX , the mass ratio of C6H12O6/TiOX was kept constant at 9 : 200. Total 40 g of above mixture was put and Cr2O3 is dispersed uniformly into the TiOX matrix into the bottle followed by the addition of 25 mL of distilled [20, 21]. The average crystalline sizes of the samples and the water. mass fraction of three phases are calculated by equations (1)– (4), as listed in Table 1.AscanbeseenfromTable 1, the average crystalline sizes of doped TiO are lower than that 2.2. Characterizations. The crystal phases of the samples X ff of undoped TiOX.This result implies that the existence of were analyzed by X-ray di raction (XRD) with CuKα.The impurity prevents the agglomeration of particles [15]. The crystalline sizes of anatase and Ti7O13 were calculated by samples show different mass fraction of three phases for Scherrer formula. The average crystalline size was obtained different doping. Especially the mass fraction of Ti O is using following equation [15]: 7 13 changed through carbon and Cr2O3 doping, which suggested that carbon or chromium doping can change the degree of = Ia Iv Dave Da + Dv ,(1)oxygen defects for samples. This is because that the mass Ia + Iv Ia + Iv fraction of Ti7O13 can be an indication of the degree of 3+ where Dave is average crystalline size, Da and Dv are oxygen defects (the concentration of Ti ) in the TiOX . crystalline sizes of anatase and Ti7O13,respectively.Ia and From Figure 1 (e, f, g, h), it can be seen that, compared Iv are peak intensity of anatase (1 0 1) and Ti7O13 (125), with TOV, anatase peaks (1 0 1) and Ti7O13 peaks (125)of respectively. other samples shift to lower 2θ value, which imply that the d- The mass fraction of anatase (Wa), Ti7O13 (Wv), and spacing of TiOX increases. Furthermore, it is suggested that brookite (Wb) were calculated using following equations some of C/Cr atoms or ions have entered into the interstitial [11, 19]: sites of TiOX host and that results in the expansion of TiOX lattice [20, 22]. I W = a ,(2)Figure 2 shows the UV-vis absorption spectra of all the a ka I /ka + I /kv + I /kb a a a v a b a samples. In contrast to the pure TiOX ,carbon-orchromium- modified TiO show, broader absorption shoulders in the I X = v visible light region. The relation between absorption coef- Wv v a v b ,(3) ka Ia/ka + Iv/ka + Ib/ka 1/2 ficient (α) and band gap (Eg )canbewrittenas(αhv) ∝ hv-Eg ,wherev is the frequency and h is Planck’s constant = Ib Wb b a v b ,(4)[14]. The band gap energies can be estimated by the plot k Ia/k + Iv/k + Ib/k a a a a of (αhv)1/2 versus photon energy (hv), as shown in Figure 3 where Ia, Iv,andIb are peak intensity of anatase (1 0 1), and listed in Table 1. It can be seen that the band gap a v Ti7O13 (1 2 5), and brookite (2 1 1), respectively. ka, ka,and for carbon- or Cr2O3-doped TiOX is less than that of b ka are constant (taken as 1, 0.1963, and 0.3354, resp.). undoped TiOX. Two small absorption peaks located at 450 International Journal of Photoenergy 3

(h) (d)

(g) (c) Intensity (a.u.) Intensity (f) (b)

(a) (e)

10 20 30 40 50 60 70 80 25 26 27 28 29 30 2θ (deg) 2θ (deg)

Anatase Ti7O13 Brookite

Figure 1: XRD patterns of samples with diﬀerent oxygen defects: (a) undoped TiOX , (b) C-doped TiOX , (c) Cr-doped TiOX ,and(d)C/Cr- doped TiOX . (e)–(h) are magniﬁed XRD spectra of (a)–(d), respectively.

Table 1: Physicochemical properties of the TiOX samples.

Samples Crystalline Size (nm) Anatase (wt %) Ti7O13 (wt %) Brookite (wt %) Band gap (eV) TOV 42.3 20.01 66.93 13.06 3.13 Cr-TOV 40.5 19.79 66.87 13.34 3.09 C-TOV 38.4 22.02 67.73 10.25 3.02 C/Cr-TOV 38.5 18.41 68.19 13.40 2.99 and600nmcanbeobservedclearlyforCr-TOV.Basedon 1.2 related documents [11, 23, 24], the peak located around 3+ 450 nm can be attributed to the charge transfers band Cr → 1 4+ 4 4 3+ Ti or A2g → T1g of Cr in an octahedral environment, 4 4 3+ another due to A2g → T2g d-d transitions of Cr .However, 0.8 in the case of carbon-doped TiO , there is controversial 2 0.6 reports in the literature for the origin of absorption in the visible spectral range. Some researchers proposed that this 0.4 red shift was ascribed to the presence of localized states of (a.u.) Absorbance the dopants in the band gap [14, 25]. While others have 0.2 suggested that the formation of oxygen vacancies and the 0 appearance of color centers were responsible for obvious absorption in the visible light range for nonmetal doped 300 400 500 600 700 800 titania [16, 26]. It is well known that carbon atoms or ions Wavelength (nm) that diffused into the interstitial sites of TiOX host will TOV C-TOV “plunder”oxygeninTiOX attributed to the chemical bond strength of C–O (1076 kJ/mol) stronger than that of Ti–O Cr-TOV C/Cr-TOV (662 kJ/mol) [21]. Oxygen vacancies state between the conduction and valence bands will be easier to form in the Figure 2: UV-vis absorption spectra of the samples with different carbon-doped TiOX [16]. We assume that carbon doping is oxygen defects. consistent with the increase of oxygen vacancies that result in red shift for C-TOV and C/Cr-TOV. Similar results have been observed in nonmetal doped TiO2 [20, 27, 28]. spectraofTOVandCr-TOVmeasuredatroomtemperature PL spectrum analysis is an effective tool to discern defect- at an excitation wavelength of 320 nm. It is found that related transitions in the samples. To further explore the the emission peaks are mainly centered on 400, 470, and degree of oxygen defects in the samples, PL measurements 530 nm. The emission peaks around 400 nm are related were done for all of the samples. Figure 4(a) shows the PL to the band-edge free excitons. Peaks around 470 and 4 International Journal of Photoenergy

10 TOV 8 Cr-TOV 8

6 2

6 2 / / 1 1 ) ) A A αh αh 4 ( 4 (

2 2

Eg = 3.09 eV Eg = 3.13 eV 0 0 3 3.2 3.4 3.6 3.8 3 3.2 3.4 3.6 3.8 Energy (eV) Energy (eV) (a) (b) 10 10 C-TOV C/Cr-TOV 8 8 2 2

/ 6 / 6 1 1 ) ) A A αh αh ( 4 ( 4

2 2 = Eg = 3.02 eV Eg 2.99 eV 0 0 2.8 3 3.2 3.4 3.6 3.8 4 2.8 3 3.2 3.4 3.6 3.8 4 Energy (eV) Energy (eV) (c) (d)

Figure 3: The plot of (αhv)1/2 versus photon energy of the samples with diﬀerent oxygen defects.

530 nm originate from the bound excitons [29, 30]. The 0.112, 0.104, and 0.081 for TOV, Cr-TOV, C-TOV, and C/Cr- luminescence bands ranging from 460 to 580 nm for TiO2 TOV, respectively. Obviously, the loss of oxygen plays a sample are ascribed to the transition from different exciton significant role in improving the photodegradation rate for energy levels arising from oxygen vacancies to the valence various catalysts. band [29–31]. Obviously, the emission intensity increases The ln(C0/C) dependence of the mass fraction of Ti7O13 with the degree of oxygen defects. Compared with the Cr- phase in the samples is showed in Figure 6. The mass fraction TOV sample, the enhanced PL intensity for TOV reflects the of Ti7O13 can be an indication of the degree of oxygen 3+ increases of the degree of oxygen defects in the TOV sample. defects (the concentration of Ti ) in the TiOX .Ascan The result is in good accord with the XRD analysis about be seen from Figure 6, the photocatalytic activity increases the mass fraction of Ti7O13 phase between TOV and Cr-TOV and then decreases with the increase of the mass fraction (Table 1). Also, it can be concluded from Figure 4(b) that the of Ti7O13. The photocatalytic reaction rate of MO reaches degree of oxygen defects of C/Cr-TOV is more than that of the maximum value with mass fraction of Ti7O13 of about C-TOV. Combined with the XRD analysis, it is believed that 66.93%. The relationship between the decolorization of MO the degree of oxygen defects of the samples is determined in and the degree of oxygen defects (the concentration of Ti3+) the following order: C/Cr-TOV > C-TOV > TOV > Cr-TOV. can be explained as follows. For TOV sample, appropriate Figure 5 shows the reaction constant k for MO pho- degree of oxygen defects (or appropriate concentration of todegradation of samples under UV light irradiation. The Ti3+) would act as electron-trapping centers, which inhibit reaction constant is evaluated by equation: ln(C0/C) = the recombination of photoinduced electron-hole pairs. kt [32], where C0 and C are the initial concentration And thus more photoinduced holes are removed to the and the reaction concentration of MO, t is the time of surface of photocatalyst to produce more hydroxyl radicals light irradiation. It is clear that the samples with different and participate in the redox reaction that results in the oxygen defects exhibit the superior photocatalytic activity enhancement of photocatalytic activity [17, 20, 26]. For Cr- compared with blank sample (the absence of photocatalyst) TOV sample, lower concentration of oxygen defects (or lower for decolorization rate of MO. In addition, the k is 0.135, concentration of Ti3+) will lead to higher photocatalytic International Journal of Photoenergy 5

300 400

350 250 C/Cr-TOV TOV 300 200 Cr-TOV 250 Intensity (a.u.) Intensity Intensity (a.u.) Intensity 150 200 C-TOV

100 150 400450 500 550 600 400450 500 550 600 Wavelength (nm) Wavelength (nm) (a) (b)

Figure 4: Photoluminescence spectra of samples: (a) TOV and Cr-TOV, (b) C/Cr-TOV and C-TOV at room temperature (λex = 320 nm).

16 2

) 14 2 −

10 12 1.8 × 1 − )

10 C / 0 1.6 8 C ln(

6 1.4

4 1.2

Reaction constant (min constant Reaction 2 66.9 67.2 67.5 67.8 68.1 0 C/Cr-TOV C-TOV TOV Cr-TOV Blank Mass fraction of Ti7O13 (wt%)

Figure 5: The comparison of reaction constant of different samples. Figure 6: The photocatalytic activity as a function of the mass fraction of Ti7O13. activity. However, in the case of C-TOV and C/Cr-TOV samples, the highest degree of oxygen defects (or the highest concentration of Ti3+) results from carbon doping that heating process. The mass fraction of Ti O phase of suppresses the photocatalytic activity. This is because excess 7 13 samples can be changeable by C/Cr doping. The mass amount of oxygen vacancies would become recombination fraction of Ti O increased for carbon- doping, while the centers of the photoinduced charge carriers, leading to the 7 13 mass fraction of Ti O decreased for chromium doping. depressed quantum yield [20]. 7 13 The photocatalytic activity of samples is closely related to In addition, there are also many factors [3] such as phase the degree of oxygen defects (the concentration of Ti3+) structure, band gap, and surface state that affect the pho- and band gap. However, the photodegradation rate will be tocatalytic activity. Combining absorption spectra analysis suppressed with excess oxygen defects. Furthermore, carbon (band gap) with photocatalysis mechanism, photocatalytic doped TiO shows a broader absorption shoulder in the activity of TiO is also dependent on energy gap. From X X visible light region. It is attributed to the increase of oxygen above band gap of Table 1, it can be reasonably deduced that vacancies and the advent of color centers. proper band gap (around 3.13 eV) is beneficial to creation of electronic-hole pairs that make excellent photocatalytic activity of TiOX . When energy gap is smaller than 3.13 eV, Acknowledgments it is beneficial to creation of electronic-hole pairs, but at the same time, recombination chance of electronic-hole This work was supported by the Natural Science Foundation increases, as a result, photocatalytic activity is lower. of Anhui province of China (no. 1208085ME81), the Scien- tific Research Foundation of Education Ministry of Anhui 4. Conclusions province of China (nos. KJ2011A010 and KJ2012A029), the Teaching Research Foundation of Education Ministry In summary, some of C/Cr atoms or ions are successfully of Anhui province of China (no. 20100185), the Doctor incorporated into the TiOX host by mechanochemical and Scientific Research Starting Foundation of Anhui University 6 International Journal of Photoenergy of China, the Foundation of “211 Project” of Anhui Uni- [15] I. C. Kang, Q. W. Zhang, S. Yin, T. Sato, and F. Saito, “Prepa- versity of China (no. KJTD004B), and the Foundation of ration of a visible sensitive carbon doped TiO2 photo-catalyst Construction of Quality Project of Anhui University of China by grinding TiO2 with ethanol and heating treatment,” Applied (nos. XJ200907, xj201140, and 39020012). Catalysis B, vol. 80, no. 1-2, pp. 81–87, 2008. [16] V. N. Kuznetsov and N. Serpone, “On the origin of the spectral bands in the visible absorption spectra of visible-light-active References TiO2 specimens analysis and assignments,” Journal of Physical Chemistry C, vol. 113, no. 34, pp. 15110–15123, 2009. [1] M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. [17] P. Sangpour, F. Hashemi, and A. Z. Moshfegh, “Photoen- Bahnemann, “Environmental applications of semiconductor hanced degradation of methylene blue on cosputtered M:TiO2 photocatalysis,” Chemical Reviews, vol. 95, no. 1, pp. 69–96, (M = Au, Ag, Cu) nanocomposite systems: a comparative 1995. study,” Journal of Physical Chemistry C, vol. 114, no. 33, pp. [2] F. M. Meng and F. Lu, “ure and silver (2.5–40 vol%) modified 13955–13961, 2010. TiO2 thin films deposited by radio frequency magnetron [18] Y. Z. Li, D. S. Hwang, N. H. Lee, and S. J. Kim, “Synthesis sputtering at room temperature: surface topography, energy and characterization of carbon-doped titania as an artificial gap and photo-induced hydrophilicity,” Journal of Alloys and solar light sensitive photocatalyst,” Chemical Physics Letters, Compounds, vol. 501, no. 1, pp. 154–158, 2010. vol. 404, no. 1–3, pp. 25–29, 2005. [3] F. Meng and F. Lu, “Characterization and photocatalytic activ- [19] R. A. Spurr and H. Myers, “Quantitative analysis of anatase- ff ity of TiO2 thin films prepared by RF magnetron sputtering,” rutile mixtures with an X-ray di ractometer,” Analytical Vacuum, vol. 85, no. 1, pp. 84–88, 2010. Chemistry, vol. 29, no. 5, pp. 760–762, 1957. [4] A. Fujishima and K. Honda, “Electrochemical photolysis of [20] Y. M. Wu, M. Y. Xing, J. L. Zhang, and F. Chen, “Effective visi- water at a semiconductor electrode,” Nature, vol. 238, no. ble light-active boron and carbon modified TiO2 photocatalyst 5358, pp. 37–38, 1972. for degradation of organic pollutant,” Applied Catalysis B, vol. [5] F. M. Meng, L. Xiao, and Z. Q. Sun, “Thermo-induced 97, no. 1-2, pp. 182–189, 2010. hydrophilicity of nano-TiO2 thin films prepared by RF [21] J. Zhang, Z. Zhao, X. Wang et al., “Increasing the oxygen magnetron sputtering,” Journal of Alloys and Compounds, vol. vacancy density on the TiO2 surface by La-doping for dye- 485, no. 1-2, pp. 848–852, 2009. sensitized solar cells,” Journal of Physical Chemistry C, vol. 114, [6] M. Ksibi, S. Rossignol, J. M. Tatibouet,¨ and C. Trapalis, no. 43, pp. 18396–18400, 2010. “Synthesis and solid characterization of nitrogen and sulfur- [22]Y.W.Ma,J.Ding,J.B.Yi,H.T.Zhang,andC.M.Ng, doped TiO2 photocatalysts active under near visible light,” “Mechanism of room temperature ferromagnetism in ZnO Materials Letters, vol. 62, no. 26, pp. 4204–4206, 2008. doped with Al,” Journal of Applied Physics, vol. 105, no. 7, [7] W. Wang, J. Zhang, F. Chen, D. He, and M. Anpo, “Preparation Article ID 07C503, 3 pages, 2009. 3+ and photocatalytic properties of Fe -doped Ag@TiO2 core- [23] J. F. Zhu, Z. G. Deng, F. Chen, J. L. Zhang, and H. J. Chen, 3+ shell nanoparticles,” Journal of Colloid and Interface Science, “Hydrothermal doping method for preparation of Cr -TiO2 vol. 323, no. 1, pp. 182–186, 2008. photocatalysts with concentration gradient distribution of [8] H. G. Yu, S. C. Lee, J. G. Yu, and C. H. Ao, “Photocatalytic Cr3+,” Applied Catalysis B, vol. 62, no. 3-4, pp. 329–335, 2006. activity of dispersed TiO2 particles deposited on glass fibers,” [24] X. X. Fan, X. Y. Chen, S. P. Zhu et al., “The structural, physical Journal of Molecular Catalysis A, vol. 246, no. 1-2, pp. 206–211, and photocatalytic properties of the mesoporous Cr-doped 2006. TiO2,” Journal of Molecular Catalysis, vol. 284, no. 1-2, pp. [9] J. G. Yu, G. H. Wang, B. Cheng, and M. H. Zhou, “Effects 155–160, 2008. of hydrothermal temperature and time on the photocatalytic [25] X. B. Chen and C. Burda, “The electronic origin of the visible- activity and microstructures of bimodal mesoporous TiO2 light absorption properties of C-, N- and S-doped TiO2 powders,” Applied Catalysis B, vol. 69, no. 3-4, pp. 171–180, nanomaterials,” Journal of the American Chemical Society, vol. 2007. 130, pp. 5018–5019, 2008. [10] H. Wang, Z. Wang, and Y. Liu, “A simple two-step template [26] N. Serpone, “Is the band gap of pristine TiO2 narrowed by approach for preparing carbon-doped mesoporous TiO2 anion- and cation-doping of titanium dioxide in second- hollow microspheres,” JournalofPhysicalChemistryC, vol. generation photocatalysts?” Journal of Physical Chemistry B, 113, no. 30, pp. 13317–13324, 2009. vol. 110, no. 48, pp. 24287–24293, 2006. [11] J. Choi, H. Park, and M. R. Hoffmann, “Effects of single metal- [27] Y. Wang, C. X. Feng, M. Zhang, J. J. Yang, and Z. J. Zhang, ion doping on the visible light photoreactivity of TiO2,” The “Enhanced visible light photocatalytic activity of N-doped Journal of Physical Chemistry C, vol. 114, pp. 783–792, 2010. TiO2 in relation to single-electron-trapped oxygen vacancy [12] W. Ho, J. C. Yu, J. Lin, J. Yu, and P. Li, “Preparation and doped-nitrogen,” Applied Catalysis B, vol. 100, no. 1-2, pp. and photocatalytic behavior of MoS2 and WS2 nanocluster 84–90, 2010. sensitized TiO2,” Langmuir, vol. 20, no. 14, pp. 5865–5869, [28] Q. Xiao, J. Zhang, C. Xiao, Z. Si, and X. Tan, “Solar photocat- 2004. alytic degradation of methylene blue in carbon-doped TiO2 [13] L. L. Ren, Y. P. Zeng, and D. L. Jiang, “Preparation, character- nanoparticles suspension,” Solar Energy,vol.82,no.8,pp. ization and photocatalytic activities of Ag-deposited porous 706–713, 2008. TiO2 sheets,” Catalysis Communications,vol.10,no.5,pp. [29] K. Das, S. N. Sharma, M. Kumar, and S. K. De, “Morphology 645–649, 2009. dependent luminescence properties of Co doped TiO2 nanos- [14] Y. M. Wu, J. L. Zhang, L. Xiao, and F. Chen, “Properties of tructures,” Journal of Physical Chemistry C, vol. 113, no. 33, pp. carbon and iron modified TiO2 photocatalyst synthesized at 14783–14792, 2009. low temperature and photodegradation of acid orange 7 under [30] W. F. Zhang, M. S. Zhang, Z. Yin, and Q. Chen, “Photolu- visible light,” Applied Surface Science, vol. 256, no. 13, pp. minescence in anatase titanium dioxide nanocrystals,” Applied 4260–4268, 2010. Physics B, vol. 70, pp. 261–265, 2000. International Journal of Photoenergy 7

[31] L. D. Zhang and C. M. Mo, “Luminescence in nanostructured materials,” Nanostructured Materials, vol. 6, no. 5–8, pp. 831–834, 1995. [32] F. Dong, W. Zhao, Z. Wu, and S. Guo, “Band structure and visible light photocatalytic activity of multi-type nitrogen doped TiO2 nanoparticles prepared by thermal decomposition,” Journal of Hazardous Materials, vol. 162, no. 2-3, pp. 763–770, 2009. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 329796, 10 pages doi:10.1155/2012/329796

Research Article The Synthesis of Anatase Nanoparticles and the Preparation of Photocatalytically Active Coatings Based on Wet Chemical Methods for Self-Cleaning Applications

Dejan Verhovsek,ˇ 1 Nika Veronovski,1 Urskaˇ Lavrenciˇ cˇ Stangar,ˇ 2 Marko Kete,2 Kristina Zagar,ˇ 3 and Miran Cehˇ 3

1 Cinkarna Celje, d.d., Kidriceva 26, 3001 Celje, Slovenia 2 Laboratory for Environmental Research, University of Nova Gorica, Vipavska 13, 5001 Nova Gorica, Slovenia 3 Department for Nanostructured Materials, Jozef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia

Correspondence should be addressed to Dejan Verhovsek,ˇ [email protected]

Received 27 January 2012; Revised 28 March 2012; Accepted 28 March 2012

Academic Editor: Stephane´ Jobic

Copyright © 2012 Dejan Verhovsekˇ et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

We report on an improved sol-gel method for the production of highly photocatalytic titanium dioxide (TiO2)anatase nanoparticles which can provide appropriate control over the final characteristics of the nanoparticles, such as particle size, crystallinity, crystal structure, morphology, and also the degree of agglomeration. The synthesized anatase nanoparticles were characterized using various techniques, such as X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), and were tested in coatings for self-cleaning glass and ceramic surfaces. The coatings were prepared using a soft chemistry route and are completely transparent to visible light and exhibit a high photocatalytic effect, which was determined by contact-angle measurements. Finally, it is worth mentioning that both the sol-gel synthesis method and the coating-preparation method are based on a wet chemical process, thus presenting no risk of handling the TiO2 anatase nanoparticles in their potentially hazardous powder form at any stage of our development. Low-price, easy-to-handle, and nontoxic materials were used. Therefore, our work represents an important contribution to the development of TiO2 anatase nanoparticle coatings that provide a high photocatalytic effect and can thus be used for numerous applications.

1. Introduction applications in photocatalysis [6–16]. The photocatalytic effect is a well-known characteristic of the nanosized TiO2, Pigmentary titanium dioxide (TiO2) exhibits a high refrac- having a large surface area capable of absorbing incident tive index making it a unique white pigment that is not UV radiation and transforming it into electrons (e−)and expected to be replaced in either the long or short term. It is positive holes (h+), which migrate to the surface, resulting mostly used in the production of paints and coatings of every in chemical reactions with adsorbed pollutants and oxygen ff kind, as well as rubbers, plastics, ceramics, papers, foodstu s, from the air [17]. The preferred crystal structure of the TiO2 medicine, and many more [1]. photocatalyst is anatase, which can be prepared with larger With the advent of nanotechnology, the so-called “nano- surface areas than rutile, although having a slightly larger ”formofTiO2 has received a great deal of attention, since bandgap (3.2 eV) than the latter. nanosized particles have many distinctive and enhanced The sulphate production of pigmentary TiO2 is based on physical and chemical characteristics. Nanosized TiO2 is the hydrolysis of a titanyl sulphate solution, which results in known for its many applications, which include dye- the formation of a white precipitate, the so-called metatitanic sensitized solar cells [2], UV absorption for surface protec- acid, which is an agglomerate of anatase nanoparticles. In tion [3], electrochemical splitting of water [4, 5], and various our paper we refer to an agglomerate as a particle in which 2 International Journal of Photoenergy the subunits are connected with each other mainly through 0.5 M. The concentration of barium chloride was varied physical bonding and can be broken when subjected to between 0.25 and 1.4 M in order to prepare barium sulphate stronger attraction forces or changes in the surface charge. particles of varying sizes. Barium chloride was added to An aggregate, on the other hand, is a consolidated particle the metatitanic acid dropwise at room temperature while in which smaller crystallites are connected mainly with mixing at 200 rpm. The reaction mixture was stirred for crystalline bridges. An aggregate cannot be broken down into around 15 minutes. Afterwards, the barium sulphate was individual subunits. In view of this we refer to metatitanic removed by means of centrifugation (30 min at 3000 rpm). acid as an agglomerate, being made up of smaller subunits, After the centrifugation a precipitate of barium sulphate aggregates, which themselves are made up of very small was present on the bottom of the vial, while the TiO2 anatase crystallites. The crystallite mean size is about 5- suspension exhibited no sedimentation, thus allowing a 6 nm, the aggregate size ranges from 40 to 100 nm, and the simple separation via decantation. agglomerate size is in the order of 1-2 μm[18, 19]. Since the aggregate size is in the nanoregion, we refer to them as 2.3. Preparation of Anatase Self-Cleaning Coating. In order to nanoparticles. It is very interesting that the size of the anatase prepare a stable colloid used for the final coating application nanoparticles can be easily adjusted, simply by controlling that exhibits good adhesion, mechanical stability, and also the hydrolysis reactions of titanyl sulphate solution [19]. The superhydrophilicity, a soft chemical method was employed. aggregates that form the metatitanic acid agglomerate are The anatase was used in the form of an acidic suspension mainly connected through sulphate bridges. The sulphate prepared as described above. To the acidic anatase suspen- bridges are electrostatic bonds which arise due to the negative sion (5 mL), water (40 mL), ethanol (Fluka) (40 mL), and a charge of the sulphate ion [19]. When subjected to water- silica binder solution (400 μL) (made from either of the two soluble barium salts, the barium ion, having a greater silica sources, that is, colloidal silica Levasil (200/30%) and affinity for the sulphate ion, draws the sulphate ions holding TEOS precursor) were added and stirred until a homogenous agglomerates together and thus breaks them apart into mixture formed [20]. The molar ratio of Ti and Si in the individual anatase nanoparticles. Since the final suspension final suspension is 1.3 : 1. The as-prepared suspension is very is acidic, the nanoparticles within it are well dispersed. The stable and remains so for a period of about six months. The suspension is, therefore, very stable, enabling the separation as-prepared suspension was used to prepare self-cleaning of the nanoparticles from the barium sulphate precipitate coatings using various techniques, such as spraying and dip which is a result of the reaction between the barium ions coating onto various substrates. No thermal treatment of the and the sulphate ions [19]. There has been very little or resulting coatings at elevated temperatures was required. no research on determining the photocatalytic activity of The coatings prepared differ in the size of anatase anatase nanoparticles in the as-prepared suspension, nor has nanoparticles used. NTi1 and both NTi2 and NTi3 were much research been done on its possible applications. Since prepared with anatase nanoparticles synthesized with a metatitanic acid is an intermediate in the production of seeding volume of 0.3 and 0.6%, respectively. pigmentary TiO2, and thus prepared in very large amounts Coatings NTi4, NTi5, and NTi6 were prepared with when the sulphate process of production is used, it would anatase nanoparticles synthesized with a seeding volume of be very useful if a procedure could be developed to enable 1.8, 0.6%, and 0.3%, respectively, and were used to test the the harvesting of anatase nanoparticles directly from it. In effect of particle size on the photocatalytic activity of the final our paper we report on such a procedure, enabling an easy coating. The three coating types were also thermally treated and inexpensive production of a stable, acidic suspension of at 450◦C (30 min) to achieve a good adhesion on the glass anatase nanoparticles, having a good photocatalytic activity substrate. and self-cleaning applications. 2.4. Characterization Methods. For X-ray powder diffraction 2. Experimental Details (XRD) investigations the suspensions were neutralized after the reaction, filtered, washed with distilled water, and dried ◦ 2.1. Metatitanic Acid Preparation. The metatitanic acid was at 80 C in order to acquire TiO2 powder that was used for prepared using the sulphate process [1] in which ilmenite is characterization. The TiO2 powder was pressed into a pellet dissolved with concentrated sulphuric acid (98%, Cinkarna and the spectra recorded from 10◦ to 70◦ (2-theta angle) with Celje Inc.) and then hydrolysed in the presence of anatase aCuKα radiation (λ = 1.5418 A˚ ) using a CubiX PRO PW seeds. The anatase seeds are prepared separately by hydroly- 3800 instrument (PANalytical). The TiO2 crystal structure sing a titanyl sulphate solution at 80◦C and are added into the was identified using the X’Pert Data Viewer software. dissolved ilmenite solution. The amount added was varied Scanning Electron Microscopy (SEM) studies (Jeol JSM- through 0.1–1.8% in order to produce different metatitanic 7600F, Jeol Ltd., Tokyo, Japan) and Transmission Elec- types. tron Microscopy (TEM) studies (Jeol JEM-2100, Jeol Ltd., Tokyo, Japan) were used to estimate their morphology 2.2. Preparation of a Stable Anatase Nanoparticle Suspension. and dimensions. The samples for the TEM observations A stable anatase nanoparticle suspension was prepared by were ultrasonically dispersed and placed onto lacey, carbon- mixing the metatitanic acid and a barium chloride solution coated nickel grids. (p.a., Fluka) in the molar ratio Ti : Ba ∼ 30 : 1. The molar The optimum amount of barium chloride to be added concentration of the barium chloride used in this case was was determined by using Dynamic Light Scattering (DLS) International Journal of Photoenergy 3 measurements (Zeta PALS, Brookhaven). The samples for The inoculation of a reaction mixture with suitable seeds DLS measurement were prepared by dispersing a single TiO2 provides all the above-stated benefits through a process suspension droplet into 50 mL of distilled water. The final of heterogeneous nucleation. Since the seeds used in our sample was translucent. The sample was poured into a plastic reactions are of the same crystal structure as the final vessel (10 mL) which was transferred to the instrument cell product, we call such heterogeneous nucleation a secondary where it was analyzed at a fixed angle of 90◦ using a laser nucleation. with a wavelength of 660 nm. In order to better understand the effect of seeding on the The zeta potential of TiO2 nanoparticles was determined final product, we conducted experiments where we varied by Zeta PALS (Brookhaven). The samples for zeta potential the quantity of seeds added to the titanyl sulphate solution. measurement were prepared by dispersing a single TiO2 According to the literature, the variation of the seeding vol- suspension droplet into 50 mL of distilled water and mixing ume should directly affect the size of the primary aggregates it to form a translucent suspension. The electrophoretic (nanoparticles) [19]. The SEM results in Figures 1(a)–1(e) mobility was converted to zeta potential using a PALS indeed showed that larger seeding volumes tend to produce Zeta Potential Analyzer software and was based on the smaller anatase nanoparticles, which are composed of a Smoluchowski approximation. smaller number of crystallites. On the other hand, smaller Photocatalytic activity measurements were performed seeding volumes produce anatase nanoparticles composed of using the contact-angle method [21, 22]. The TiO2 anatase a larger number of crystallites. It was concluded that one can suspension was used to prepare an anatase nanoparticle layer, effectively control the size of the anatase nanoparticles by which was done by dip-coating microscope glass slides (76 × changing the amount of added anatase seeds. This result also 26 mm, Menzel-Glaser)¨ into an anatase suspension (100 g/L) demonstrates that the seeds directly control the nucleation (dipping speed of 70 or 100 mm/min). Dip coating of the process via secondary nucleation mechanisms. microscope glass slides was performed only once. The layers The theory of heterogeneous nucleation states that the were used as-prepared or were additionally thermally treated free-energy change needed for the nucleation to occur (450◦C, 30 min) in order to achieve better adhesion onto depends on the wetting angle in a liquid-solid system. In the glass substrate and also to improve the crystallinity of the case of secondary nucleation, the seeds of the same the anatase nanoparticles. Methyl stearate (p.a., Fluka)(0.2 M material as the one being crystallized are used; therefore, the solution in hexane) was used as a test organic pollutant affinity in the liquid-solid system is complete. This means and was additionally deposited onto the anatase layers using that the contact angle becomes zero and that the free energy dip coating (100 mm/min). After the addition of the methyl of nucleation is also zero. In this case no additional nuclei stearate the contact angle was measured, while the layers were are formed in the supersaturated reaction medium when it illuminated with UVA (CLEO light source, 20 W, 438 mm is seeded [24]. This is in direct agreement with the observed × 26 mm, λ 355 nm, Philips) of ca. 2 mW/cm2 irradiation results of our experiments. Since no additional nuclei were intensity. Real-time contact-angle measurements provided produced after the inoculation with the seeds, they are solely a quantitative measurement of the photocatalytic activity responsible for particle formation and thus also determine of different anatase samples. It was determined for anatase their size. As stated previously [19] the seeds added could act nanoparticles of various sizes. as centres for nucleation of the newly formed nanoparticles. It was shown that the number of nanoparticles formed is 3. Results and Discussion of the same order of magnitude as the number of crystals inoculated as seeds. This suggests that every nanoparticle 3.1. Metatitanic Acid Preparation. Metatitanic acid was pro- arises from a single seed particle, as shown in Figure 2. duced when a titanyl sulphate solution hydrolysed in the The seeds used in the synthesis are large agglomerates presence of a certain amount of anatase seeds. Anatase seeds of very small anatase crystallites, as shown in Figure 3(a).It are crucial for the hydrolysis of the titanyl sulphate solution, is very likely that they break up when added to the titanyl since seeding provides better control over the final hydrolysis sulphate solution, since their surface charge changes because product. These seeds can have many distinctive benefits of the change in the pH value. [18, 19, 23]: Zeta potential measurement of the anatase seeds was performed by using a diluted water suspension (∼5g/L) (i) they promote the precipitation process; which was titrated with sodium hydroxide solution (1 M). The titration was done starting at an acidic pH value (∼2.5) (ii) they control particle formation and ensure a narrow and raised to ∼10.5. As seen in the diagram of zeta potential size distribution of the synthesised nanoparticles; measurements in Figure 3(b), the seeds exhibit the isoelectric (iii) they increase the reaction yield; point (IEP) at a pH value of about 5, which is very near to the actual pH of the seed suspension that was used. The IEP (iv) they provide an active surface, which can greatly measured for anatase seeds is lower than expected for anatase accelerate the crystallization process, since crystal- which is around 6 [25]. The observed difference could have lization on a solid-solid interface is much easier than arisen because of surface impurities such as anions that were nucleation in a liquid phase; adsorbed onto the TiO2 surface. Sulphate anions adsorb (v) they decrease the activation energy of a reaction, thus onto the TiO2 surface and may reduce the IEP value, as was further promoting crystallization. observed in our measurements [26]. 4 International Journal of Photoenergy

100 nm 100 nm

(a) (b)

100 nm 100 nm

100 nm

(e)

Figure 1: SEM images of anatase nanoparticles produced by the hydrolysis of a titanyl sulphate solution when diﬀerent seeding volumes were used. The quantity of seeds (regarding the TiO2 content in titanyl sulphate in wt%) was (a) 1.8%, (b) 0.9%, (c) 0.6%, (d) 0.3%, and (e) 0.1%.

Figure 2: Scheme representing a possible mechanism of particle formation via secondary nucleation using seed inoculation [18]. International Journal of Photoenergy 5

100 nm 5 nm 5 nm

(a) (a)

50 101

40 Anatase seeds

(a.u.) 10

(mV)

004 0 200 105

234567891011 Intensity

potential −

Zeta −20 −30

−40 pH −50 20 30 40 50 2θ (◦) (b) (b)

Figure 3: (a) Bright-ﬁeld TEM image of anatase seed agglomerate. Figure 4: (a) TEM image of an anatase nanoparticle showing its Inset shows high-resolution TEM image of anatase seeds in which polycrystalline (aggregated) nature. The individual crystallites are smaller, approximately 5 nm crystallites of anatase are clearly visible. approximately 5 nm in size and they are aggregated into an anatase (b) Zeta potential measurement of anatase seeds used to inoculate nanoparticle. (b) X-ray powder diﬀractogram of the anatase nano- titanyl sulphate solution. The isoelectric point of the anatase seeds particles. is around the pH value of 5, which is also the pH value of the seed suspension used for inoculation.

larger than the critical nucleus size. Afterwards, the formed Nevertheless, it is reasonable to assume that the particles nuclei can be swept away from the surface by shear forces in the anatase seed suspension are highly agglomerated since which are the result of solution mixing and the consequent its pH value is near the pH value of the observed IEP. collisions between particles. The freed nuclei can then initiate But when they are added into the acidic titanyl sulphate secondary nucleation or they can remain attached to the solution, the pH drops rapidly (below 1). Consequently the seed and form a polycrystalline aggregate (nanoparticle). zeta potential becomes more positive, which may give rise to In any case, highly consolidated polycrystalline particles deagglomeration of the seed particles into smaller particles, are formed with a narrow size distribution (Figure 1 and or even individual crystallites. Figure 4(a)). A high-resolution TEM image of an individual The mechanism of particle formation during seeding anatase particle is shown in Figure 4(a), where individual has been researched extensively [27–29]. Qian and Botsaris crystallites of approximately 5 nm in size are clearly visible. [29] presented the so-called “embryo coagulation secondary From the X-ray diﬀractogram in Figure 4(b) it is evident that nucleation (ECSN)” model, which combines classical nucle- the produced particles have the anatase crystal structure. It ation theory with colloid science. The model states that the also has to be noted that a small amount of rutile TiO2 is supersaturated solution already contains a large number of present. The shoulder on the right-hand side of the (101) embryos. When the seeds are inoculated the embryos migrate anatase peak corresponds to (110) peak for the rutile TiO2. to the surface of seeds, where they gather in large numbers A small amount of rutile is present in metatitanic acid and is which then promotes the formation of clusters that are the consequence of production procedure. 6 International Journal of Photoenergy

3.2. Formation of a Stable Nanoparticle Suspension. The Volume distribution metatitanic acid that was used to carry out the experiments was made by 0.6% seed inoculation amount. As was already 100 mentioned in Section 1, the metatitanic acid is made up of the anatase aggregates presented in Figure 4, which are 80 connected into large agglomerates through sulphate ion

(a.u.) bridges. In order to produce a stable suspension of anatase 60 nanoparticles, sulphate bridges have to be removed which 40 is possible with the addition of an appropriate amount Intensity of barium chloride. Since the barium ion bonds with the sulphate ion that holds the anatase aggregates together, it 20 is very important to add enough of the barium chloride 0 solution in order to remove most of the sulphate ion bridges 0 50 100 150 200 250 300 and thus form a finely dispersed and stable suspension. The Diameter (nm) optimal addition of barium chloride solution was defined by performing DLS measurements as shown in Figure 5. If the 400 + 110 400 + 130 volume added is too low, the agglomerate breakage is not 400 + 120 400 + 140 sufficient and the average particle size is around 110 nm (red Figure 5: DLS measurements of nanoparticle suspensions that curve). By increasing the volume addition of barium chloride were made from metatitanic acid (0,6% seed quantity) and the solution, the agglomerates break and the average size attained subsequent addition of a barium chloride solution. The curves is around 50 nm, which coincides very well with the actual correspond to DLS measurements of nanoparticle suspensions particle size observed by SEM (Figure 1(c)). prepared using different amounts of barium chloride solution that From the obtained results it can be concluded that the were added to the metatitanic acid. Namely, 400 mL of metatitanic optimal amount of barium chloride is in the molar ratio acid suspension was mixed with 110 mL of 0,5 M solution of barium chloride, 120 mL, 130 mL, and 140 mL, respectively. TiO2 : Ba of approximately 30 : 1. This ratio enables the production of very stable anatase nanoparticle suspensions, which can be effectivelyseparatedfrombariumsulphatevia 50 various separation procedures, such as centrifugation. 40 The separation of barium sulphate and TiO2 nanoparticles is quantitative since TiO2 nanoparticles exhibit a high 30 zeta potential at acidic pH values which prevents their 20 sedimentation. The zeta potential of the TiO2 nanoparticles

(mV) 10 is shown in Figure 6. On the other hand, the barium sulphate particles are 0 larger and settle when centrifugation is applied. This was 234567891011 potential − 10 also confirmed with XRD (Figure 7) analysis of the settled material which is pure barium sulphate. Zeta −20 This is important since the method allows the production −30 of a very stable TiO2 acidic suspension without a byproduct −40 pH being present. The concentration of TiO2 in the final suspension is dependent only on the amount of metatitanic −50 acid that is used. The transformation of the metatitanic acid into TiO2 nanoparticle suspension has a very high yield (∼ Figure 6: The change in zeta potential for anatase nanoparticles 100%) since only a sufficient amount of barium chloride has when titrating the acidic suspension with sodium hydroxide to be added in order to remove the sulphate ions holding solution. The zeta potential is large for the acidic suspension but the anatase nanoparticles together in the metatitanic acid drops when sodium hydroxide is added. The IEP seems to be at the agglomerate. pH value of about 6 which is in accordance with the results found in literature [25]. It is also interesting to note that by careful addition of a barium chloride solution almost monodispersed particles of barium sulphate are formed (Figure 8). They are in the size range of 0.5 μmto1μm, which depends on the formed. On the other hand, if a diluted barium chloride concentration of barium chloride used. As expected, the solution was used (Figure 8(b)), then the particles were barium sulphate particles form according to the classical bigger since supersaturation was lower. theory of nucleation, where control over the particle size can easily be achieved by controlling the solute concentra- 3.3. Characterization of the Photocatalytic Activity of Anatase tion (supersaturation). If a concentrated barium chloride Nanoparticle Suspensions and Their Application as Self- solution was used (Figure 8(a)), smaller particles formed Cleaning Surfaces. The photocatalytic activity of anatase since supersaturation was larger and thus more particles were nanoparticles in self-cleaning coatings was determined using International Journal of Photoenergy 7

TiO2. The sample coating NTi1 performed slightly better than the sample coating NTi2, which can be attributed to the fact that the layer thickness diﬀers for the two samples. This could be due to the fact that the anatase nanoparticles

(a.u.) differ in size since they were prepared using 0.3 and 0.6% seeding volume, respectively. However, the difference in layer ff Intensity thickness was not examined. The slight di erence in the photocatalytic activity of the NTi1 and NTi2 coatings could also be a direct result of the fact that anatase nanoparticles differ in size. The anatase nanoparticles used for NTi1 20 30 40 50 coating are depicted in Figure 1(d), the anatase nanoparticles 2 (◦) θ used for NTi2 coating in Figure 1(c). Our results indicate Figure 7: The X-ray powder diffractogram of the material collected that the anatase nanoparticles depicted in Figure 1(d) could after the centrifugation cycle was finished. The diffractogram be slightly more photocatalytically active than the anatase exhibits all the peaks for barium sulphate while no observable peaks nanoparticles depicted in Figure 1(c). However, raw anatase for anatase are evident. nanoparticles were not analysed for their photocatalytic activity, and therefore the difference in the photocatalytic activity for the NTi1 and NTi2 coating cannot be addressed adequately. Sample coating NTi3 performed worse even though the coatings were additionally thermally treated in order to enhance the particle crystallinity and adhesion to the substrate. Such worsening in performance can be attributed to the fact that a thermal treatment was used. It is possible that during the treatment the nanoparticles present in the coating became larger due to sintering, which lowered their specific surface and consequently the photocatalytic activity of the coating as a whole. Since anatase nanoparticles are well crystallised at the beginning, the thermal treatment does not contribute to their crystallinity and thus does not enhance it (Figure 9). Asonecansee(Figure 10) the photocatalytic activity of (a) the anatase nanoparticles depends greatly on the synthesis procedure, that is, on the seeding volume used for the secondary nucleation during the nanoparticle synthesis. Although the nanoparticles synthesized using the 1.8% seeding volume (NTi4 coating) are smaller in size and thus have a greater specific surface, they performed worse in comparison to the nanoparticles that were synthesized using the 0.6% seeding volume (NTi5 coating). This can be explained by the fact that the hydrolysis and polycondensation reactions during the synthesis are carried out more rapidly when using a greater seeding volume, which consequently influences the overall crystallinity of the final product. Since the hydrolysis/polycondensation reactions were very fast when the 1.8% seeding volume was used, it can be assumed that the product crystallinity is worse since the kinetics of (b) the reaction do not allow the crystal structure to be fully developed during the synthesis. Therefore, smaller seeding Figure 8: SEM image of barium sulphate particles. (a) Barium volumes are necessary and provide a better nanoparticulate sulphate particles formed when a concentrated solution of barium anatase which is suitable for self-cleaning applications. chloride was used. (b) Barium sulphate particles formed when a diluted solution of barium chloride was used. However, it seems that a smaller seeding volume of 0.3% produces a photocatalytically more active anatase than the 0.6% seeding volume since the NTi6 coating (anatase contact-angle measurements. Figure 7 shows that the sam- prepared with 0.3% seeding volume) performs better than ples NTi1 and NTi2 decompose the methyl stearate test the NTi5 coating (anatase prepared with 0.6% seeding pollutant rapidly, since the water contact angle fell in less volume). than one hour of UVA irradiation from initial hydrophobic Self-cleaning coatings were also tested in real-life envi- value, characteristic for the fatty deposit, to the final ronmental conditions (8-month testing period with seasonal hydrophilic value, characteristic for clean and hydroxylated climate changes (summer-spring)). Self-cleaning coatings 8 International Journal of Photoenergy

120

100 ) ◦

( 80 e angl 60 act 40 Cont

0 020 40 60 80 100 120 (a) t (m in)

N-Ti1 (an atase, 10 cm /min, dried at ro o m tem perature) N-Ti2 (an atase, 7 cm /min, dried at ro o m tem perature) N-Ti3 (an atase, 10 cm /min, dried at 400 ◦C 30 min)

Figure 9: Contact-angle measurements for self-cleaning coatings prepared from anatase nanoparticles presented in Figure 1(c) using a soft chemical method. Water contact angles were measured after diﬀerent times of irradiation of the coatings covered by methyl stearate layer.

100 (b)

80 Figure 11: Glass substrates treated with self-cleaning coating and placed in real-life conditions. One can see that the coatings exhibit )

◦ the superhydrophilic eﬀect, which is depicted clearly in image (b), (

e 60 where a clear distinction between the treated (superhydrophilic)

angl and untreated (water droplets) surface is shown.

act 40 Cont TiO2 coating was to lose its activity or was even mechanically 20 washed away due to improper and inadequate adhesion, the resazurin dye would not decompose and no observable change in colour would occur (Figure 12). 0 05 20 25 30 As seen from Figure 12, the resazurin test has proven that the TiO coating is still present and photocatalytically t (h ) 2 active after eight months of exposure to real-life weather N-Ti4 (1.8% seeding volum e, dried at 450 ◦C) ◦ conditions. N-Ti5 (0.6% seeding volum e, dried at 450 C) Nevertheless, it has to be noted that the contact-angle N-Ti6 (0.3% seeding volum e, dried at 450 ◦C) measurement technique that was used does not provide an Figure 10: Contact-angle measurements for different coatings entirely accurate impression on the photocatalytic activity prepared from anatase nanoparticles using a soft chemical method. of the coatings since the change in contact angle can also ff The anatase nanoparticles were prepared using di erent seeding arise from the fact that TiO2 becomes superhydrophilic when volumes during the synthesis and therefore differ in size. Water illuminated with UV light. Therefore the change in contact contact angles were measured after different times of irradiation of angle value cannot be attributed to the methyl stearate the coatings covered by methyl stearate layer. decomposition solely, but also to the fact that the layer itself changes its surface properties. A more straightforward technique to determine the photocatalytic activity of the were applied on a glass surface (Figure 11) and exposed to coatings should be used as will be the case in our future changing weather conditions over a period of 8 months. The research activities of self-cleaning coatings. coatings remained photocatalytically active and mechanically stable over the whole testing period. 4. Conclusions This was tested by performing swift quantitative tests for photocatalytic performance using the resazurin dye. The In summary, anatase nanoparticles were synthesized based dye was stained onto the coated glass substrates depicted in on a well-established sulphate process for TiO2 pigment Figure 11 which were then subjected to UV irradiation. If the production in which metatitanic acid is produced. The International Journal of Photoenergy 9

(a) (b)

(e) (f)

Figure 12: The resazurin decomposition test after (a) 0 min, (b) 2.5 min, (c) 5 min, (d) 7.5 min, (e) 12.5 min and (f) 20 min. As can be seen, the resazurin dye decomposes when UV illumination is applied which clearly indicates that the TiO2 layer remained active and mechanically stable.

metatitanic acid can easily be converted into an anatase thesis of D. Verhovsekˇ under Grant P-MR-07/26 by the TIA nanoparticulate suspension by a simple precipitation reac- of Republic of Slovenia. The authors would also like to greatly tion using a water-soluble barium salt. The anatase nanopar- acknowledge Cinkarna Celje Inc. which provided the raw ticle suspension was used to prepare a self-cleaning-coating material needed for performing the research presented in this colloidal solution using a soft chemical method, which allows paper. The electron microscopy was performed at the Center the production of transparent and highly photocatalytically for Electron Microscopy at the Jozefˇ Stefan Institute. active coatings. The coatings were tested using contact- angle measurements and perform well even without thermal treatment, which enables a low-cost application on various References substrates. The testing of self-cleaning coating is still in [1] J. Winkler, Titanium Dioxide, Vincentz, Hanover, Germany, progress but shows great promise, since it exhibits good 2003. photocatalytic activity and mechanical characteristics. Fur- [2] B. O’Regan and M. Gratzel,¨ “A low-cost, high-efficiency solar thermore, it is based on a wet/soft chemical method, which cell based on dye-sensitized colloidal TiO2 films,” Nature, vol. makes it suitable for large-scale production using inexpensive 353, no. 6346, pp. 737–740, 1991. equipment and technology. [3] A. Jaroenworaluck, W. Sunsaneeyametha, N. Kosachan, and R. Stevens, “Characteristics of silica-coated TiO2 and its UV absorption for sunscreen cosmetic applications,” Surface and Acknowledgments Interface Analysis, vol. 38, no. 4, pp. 473–477, 2006. [4] A. Fujishima and K. Honda, “Electrochemical photolysis of This work is partially financed by the European Union, water at a semiconductor electrode,” Nature, vol. 238, no. European Social Fund and is based in part on the Ph.D. 5358, pp. 37–38, 1972. 10 International Journal of Photoenergy

[5] S. U. M. Khan, M. Al-Shahry, and W. B. Ingler, “Efficient [23] Y. Li, Y. Fan, and Y. Chen, “A novel method for preparation photochemical water splitting by a chemically modified n- of nanocrystalline rutile TiO2 powders by liquid hydrolysis of TiO2,” Science, vol. 297, no. 5590, pp. 2243–2245, 2002. TiCl4,” Journal of Materials Chemistry, vol. 12, no. 5, pp. 1387– [6] M. Schiavello, Photocatalysis and Environment: Trends and 1390, 2002. Applications, Kluwer Academic, Dodrecht, The Netherlands, [24] J. W. Mullin, Cystallization, Butterworth-Heinemann, Lon- 1988. don, UK, 3rd edition, 1993. [7] N. Serpone and E. Pelizetti, Photocatalysis: Fundamentals and [25] G. D. Parfitt, “Surface chemistry of oxides,” Pure and Applied Applications, Wiley, New York, NY, USA, 1989. Chemistry, vol. 48, pp. 415–418, 1976. [8]D.F.OllisandH.Al-Ekabi,Photocatalytic Purification and [26] G. D. Parfitt, J. Ramsbotham, and C. H. Rochester, “An Treatment of Water and Air, Elsevier, Amsterdam, The Nether- electrophoretic investigation of the effect of chloride and of lands, 1993. silanol groups on the properties of the surface of rutile,” [9] M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. Bahne- Journal of Colloid And Interface Science, vol. 41, no. 3, pp. 437– mann, “Environmental applications of semiconductor photo- 444, 1972. catalysis,” Chemical Reviews, vol. 95, no. 1, pp. 69–96, 1995. [27] T. W. Evans, A. F. Sarofim, and G. Margolis, “Models of [10] J. M. Herrmann, “Heterogeneous photocatalysis: fundamen- secondary nucleation attributable to crystal-crystallizer and tals and applications to the removal of various types of crystal-crystal collisions,” AIChE Journal,vol.20,no.5,pp. aqueous pollutants,” Catalysis Today, vol. 53, no. 1, pp. 115– 959–966, 1974. 129, 1999. [28] E. G. Denk and G. D. Botsaris, “Mechanism of contact [11] D. Bahnemann, “Photocatalytic detoxification of polluted nucleation,” Journal of Crystal Growth, vol. 15, no. 1, pp. 57– waters,” in Environmental Photochemistry, Springer, Berlin, 60, 1972. Germany, 1999. [29] R. Y. Qian and G. D. Botsaris, “A new mechanism for nuclei [12] A. Fujishima, K. Hashimoto, and T. Watanabe, TiO2 Photo- formation in suspension crystallizers: the role of interparticle catalysis: Fundamentals and Applications,BKC,Tokyo,Japan, forces,” Chemical Engineering Science, vol. 52, no. 20, pp. 3429– 1999. 3440, 1997. [13] P. Pichat, J. Disdier, C. Hoang-Van, D. Mas, G. Goutailler, and C. Gaysse, “Purification/deodorization of indoor air and gaseous effluents by TiO2 photocatalysis,” Catalysis Today, vol. 63, no. 2–4, pp. 363–369, 2000. [14] M. Muneer, S. Das, V. B. Manilal, and A. Haridas, “Pho- tocatalytic degradation of waste-water pollutants: titanium dioxide-mediated oxidation of methyl vinyl ketone,” Journal of Photochemistry and Photobiology A, vol. 63, no. 1, pp. 107–114, 1992. [15] S. G. Botta, D. J. Rodr´ıguez, A. G. Leyva, and M. I. Litter, “Features of the transformation of HgII by heterogeneous photocatalysis over TiO2,” Catalysis Today, vol. 76, no. 2–4, pp. 247–258, 2002. [16]S.C.Moon,Y.Matsumura,M.Kitano,M.Matsuoka,and M. Anpo, “Hydrogen production using semiconducting oxide photocatalysts,” Research on Chemical Intermediates, vol. 29, no. 3, pp. 233–256, 2003. [17] J. H. Carey, J. Lawrence, and H. M. Tosine, “Photodechlorina- tion of PCB’s in the presence of titanium dioxide in aqueous suspensions,” Bulletin of Environmental Contamination and Toxicology, vol. 16, no. 6, pp. 697–701, 1976. [18] S. Sathyamoorthy, G. D. Moggridge, and M. J. Hounslow, “Controlling particle size during anatase precipitation,” AIChE Journal, vol. 47, no. 9, pp. 2012–2024, 2001. [19] S. Sathyamoorthy, G. D. Moggridge, and M. J. Hounslow, “Particle formation during anatase precipitation of seeded titanyl sulfate solution,” Crystal Growth and Design, vol. 1, no. 2, pp. 123–129, 2001. [20] U. Cernigojˇ and U. L. Stangar,ˇ “Preparation of TiO2/SiO2 and use thereof for deposition of self-cleaning anti-fogging,” International patent application number PCT/SI2009/000052. International patent publication number WO 2010/053459 A1; 2009. [21] U. Cernigoj,ˇ M. Kete, and U. L. Stangar,ˇ “Development of a fluorescence-based method for evaluation of self-cleaning properties of photocatalytic layers,” Catalysis Today, vol. 151, no. 1-2, pp. 46–52, 2010. [22] U. Cernigoj,ˇ M. Kete, U. L. Stangar,ˇ and V. Ducman, “Testing of photocatalytic activity of self-cleaning surfaces,” Advances in Science and Technology, vol. 68, pp. 126–134, 2010. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 843042, 10 pages doi:10.1155/2012/843042

Research Article High-Efﬁciently Photoelectrochemical Hydrogen Production over Zn-Incorporated TiO2 Nanotubes

Gayoung Lee,1 Min-Kyeong Yeo,2 Myeong-Heon Um,3 andMisookKang1

1 Department of Chemistry, College of Science, Yeungnam University, Gyeongsan, Gyeongbuk 712-749, Republic of Korea 2 Department of Environmental Science and Environmental Research, College of Engineering, Kyung Hee University, Yongin, Gyeonggi 446-701, Republic of Korea 3 Department of Chemical Engineering, College of Engineering, Kongju National University, Cheonan, Chungnam 330-717, Republic of Korea

Correspondence should be addressed to Misook Kang, [email protected]

Received 3 February 2012; Revised 21 March 2012; Accepted 22 March 2012

Academic Editor: Baibiao Huang

Copyright © 2012 Gayoung Lee et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

To investigate the Zn dopant and nanotube morphology eﬀects of TiO2 in electrochemical hydrogen production from the photosplitting of methanol/water solution, we have designed a Zn-incorporated TiO2 nanotube (Zn-TNT) photocatalyst. The TNT and Zn-TNT materials had a width of 70∼100 nm. The hydrogen production over the Zn-TNT photocatalysts was higher than that over the TNT; speciﬁcally, 10.2 mL of H2 gas was produced after 9 hours when 0.5 g of 0.01 mol% Zn-TNT was used. The zeta-potential values in aqueous solution determined by electrophoretic light scattering (ELS) had negative surface charges, which was related to the surface stability, and the absolute value was the largest in 0.01 mol% Zn-TNT. On the basis of UV-visible and photoluminescence (PL) spectra results, the high photoactivity of Zn-TNT was attributed to the shift toward the visible region and increase of PL intensity due to the increased number of excited electrons and holes.

1. Introduction hydrogen production remains uneconomically low, and noble metals are too expensive. Therefore, new and inexpensive Hydrogen is likely to become an increasingly used energy photocatalysts need to be developed that are environmentally source due to its environmental friendliness. Therefore, the friendly and possess greater hydrogen-producing activity technology for generating hydrogen by the photosplitting under visible light irradiation. of water using a photocatalyst has attracted considerable Nanotubes materials have recently been applied on pho- research attention. The photocatalytic formation of hydro- tocatalysis [15–17]. Xu et al. [15]reportedaneﬃcient Cu- gen and oxygen on semiconductors such as TiO2 [1–5]and incorporated TiO2 nanotube (Cu-TNT) photocatalyst for MTiO3 [6–10] has been studied extensively due to the low H2 production, with an average H2 generation rate re- bandgap and high corrosion resistance of these semiconduc- corded across a 4-hour reaction of between 15.7 and tor materials. However, the photocatalytic decomposition 40.2 mmol h−1 g−1 catalyst, depending on the initial Cu/Ti of water on a TiO2 photocatalyst is ineﬀective because the ratio in the solution, which was optimized at 10 atom%. Sang amount of hydrogen produced is limited by the rapid recom- et al. [16] have also researched the photoelectrochemical bination of holes and electrons, resulting in the formation hydrogen evolving over C- and N-doped TNT arrays elec- of water. In order to overcome this rapid recombination, the trode: C-TNT could harvest more light and produce more noble metals (Ag, Pd, Pt, and Ga) loaded TiO2 photocatalysts photoactive sites than N-TNT, which also made the charge [11–14] have used in methanol or ethanol photodecompo- transfer resistance in C-TNT larger than that in N-TNT. sition, rather than water which has relatively high activity As the result, under UV-vis light irradiation, the average and chemical stability under UV irradiation. However, the hydrogen generation rate of C-TNT was 282 μLh−1 cm−2. 2 International Journal of Photoenergy

However, little research has been conducted on metal-TNT, generalization of Langmuir’s treatment of the unimolecular and the correlation between TNT properties and hydro- layer. The BET surface area was measured using a Belsorp genation performance has barely been addressed. II-mini (BEL, Japan inc.) equipped with a TCD instrument. In this study, Zn-incorporated TNT (Zn-TNT) was syn- The CV results were obtained using a BAS 100B instrument thesized using Zn-TiO2 nanosized particles as starting mat- at room temperature and a scan rate of 100 mV/s with erials with three molar ratios of Ti/Zn. The relationship 0.1 M KCl as the supporting electrolyte, platinum wires as between the spectroscopic properties of the nanotube par- the working and counter electrodes, and Ag/AgCl as the ticles and the photocatalytic performance for the production reference electrode. Zeta potentials of the TNT and Zn- of H2 was examined by X-ray diffraction (XRD), transmis- TNT were determined by electrophoretic mobility using an sion electron microscopy (TEM), UV-visible spectroscopy, electrophoresis measurement apparatus (ELS 8000, Otsuka photoluminescence (PL), electrophoretic light scattering Electronics, Japan) with a plate sample cell. ELS determina- (ELS), and cyclic voltammetry (CV). tions were performed with the reference beam mode and a laser light source wavelength of 670 nm, modular frequency of 250 Hz, and scattering angle of 15◦. The standard error 2. Experimental of the zeta potentials, converted from the experimentally determined electrophoretic mobility according to the Smolu- To synthesize the TNT and Zn-TNT, all the nanosized parti- chowsk [19] limit of the Henry equation, was typically <1.5% cles of TiO2 and Zn-incorporated TiO2 were prepared using and the percent error <5%. To measure the zeta potentials, a hydrothermal treatment as shown in Figure 1(a). First, 0.1 wt% of each sample was dispersed in deionized water the nanometer-sized TiO2 and Zn-TiO2 with various mol and the solution pH was adjusted with HCl or NaOH. The fractions of zinc (0.1, 0.01, and 0.001 mol-%) were prepared relative molecular diameter size distributions of the various using a solvothermal treatment: titanium tetraisopropoxide solutions were also measured by this equipment. The zeta (TTIP, 99.95%, Junsei Chemical, Japan) and zinc acetate potential distributions were obtained by averaging 2 or 3 (Zn(CH3COO)2·2H2O, 99.99%, Junsei Chemical, Japan) runs. were used as the titanium and zinc precursors, respectively, The photosplitting of methanol/water was carried out and ethanol was used as the solvent. After 0.1, 0.01, and using a liquid photoreactor designed in our laboratory. For 0.001 mol-% of zinc acetate and 1.0 mol TTIP were added water photosplitting, 0.5 g of the powdered TNT and Zn- stepwise to 250 mL of ethanol, the mixture was stirred TNT photocatalysts was added to 1.0-L of distilled water in a homogeneously for 2 h. Acetic acid was added and the 2.0-L Pyrex reactor. UV-lamps (6 × 3Wcm−2 = 18 W cm−2, pH was maintained at 4.0 for rapid hydrolysis. The final 30 cm length × 2.0 cm diameter; Shinan, Korea) emitting solution was stirred homogeneously for 2 h and moved to an radiation at 365 nm were used. Water photosplitting was autoclave for thermal treatment. TTIP and zinc acetate were carried out for 9 h with constant stirring. Hydrogen evo- ◦ hydrolyzed during thermal treatment at 200 Cfor8hunder lution was measured after 1 h. The hydrogen gas (H2) a nitrogen environment with a pressure of approximately produced during water photosplitting was analyzed using a 15.0 atm. The resulting precipitate was washed with distilled TCD-type gas chromatograph (GC, model DS 6200; Donam water until the pH was neutralized at 7.0 and then dried Instruments Inc., Korea). To determine the products and at 80◦C for 24 h. Next, the general preparation methods intermediates, the GC was connected directly to the water of TNT and Zn-TNT powders have been described in the decomposition reactor under the following GC conditions: literature [18]. Prepared TiO2 or Zn-TiO2 particles were TCD detector, Carbosphere column (Alltech, Deerfield, IL), added in 10.0 M NaOH aqueous solution (Figure 1(b)), and 413 K injection temp., 393 K initial temp., 393 K final temp., the mixture was stirred homogeneously for 2 h and moved and423Kdetectortemp. to an autoclave for thermal treatment at 130◦Cfor24hThe resulting precipitate was washed with HCl solution until the ◦ pH = 7.0 and then dried at 80 C for 24 h. The TNT or Zn- 3. Results and Discussion TNT was in the form of multiwalled scroll nanotubes with an average width of about 70–100 nm. Figures 2(a) and 2(b) show the XRD patterns of the Zn- The synthesized powders were examined by XRD (MPD, TiO2 nanoparticles (Zn-TNP) and Zn-TNT. The anatase PANalytical) with nickel-filtered CuKα radiation (30 kV, structure in both TNP and TNT without Zn element had 30 mA). The sizes and shapes of the TNT and Zn-TNT were main peaks at 24–25.3, 38.0, 48.2, 54, 63, and 68◦2θ,which measured by a transmission electron microscope (TEM; H- were assigned to the (d101), (d004), (d200), (d105), (d211), and 7600, Hitachi) operated at 120 kV. UV-visible spectra were (d204) planes, respectively [20, 21]. The peak intensities of obtained using a Cary500Scan (Varian) spectrometer with the anatase structures were decreased and slightly shifted a reflectance sphere in the range of 200∼800 nm. PL spec- to high angles with increasing Zn content. Two special troscopy measurements were obtained using a LabRamHR peaks (JSPDF#251164), which were assigned to Zn2TiO4 at (Jobin Yvon) spectrometer in the range of 200∼700 nm to 2 θ = 28.5(d220) and 34.5(d311) in 0.1 mol% Zn-TNP and examine the number of photoexcited electron hole pairs for Zn-TNTs, were attributed to the good connection between all of the samples. The specific surface area was calculated Zn and the TiO2 framework. Following a literature [18], an according to the Brunauer-Emmett-Teller (BET) theory that H2Ti2O5 (d003, JSPDF#360654) peak also appears at same ◦ gives the isotherm equation for multilayer adsorption by location to 28∼29 of Zn2TiO4 in TNT and Zn-TNT because International Journal of Photoenergy 3

Ethanol in beaker H2O in beaker Stirring Stirring Addition of TTIP Addition of NaOH Stirring Stirring

Addition of Zn(CH3COO)2·2H2O Addition of Zn-incorporated TiO2 Stirring Stirring

Addition of acetic acid until pH = 4 Thermal treatment at 130 C for 2 h

Washing with HCl solution Thermal treatment at 200 C for 8 h (until pH = 7)

Washing and drying Drying

(a) (b)

Figure 1: Synthesis procedure of (a) the Zn-incorporated TiO2 nanoparticles (Zn-TNP) and (b) the Zn-incorporated TiO2 nanotubes (Zn- TNT).

(h)

(d) (g)

Intensity (cps) Intensity (c) (cps) Intensity

(f) (b)

(e) (a)

20 30 40 50 60 70 20 30 40 50 60 70 2θ/Cu Ka 2θ/Cu Ka

Anatase Anatase Zn2TiO4 Zn2TiO4 (a) (b)

Figure 2: XRD patterns of synthesized Zn-TNP (a) and Zn-TNT (b): (a) TNP, (b) 0.001 mol% Zn-TNP, (c) 0.01 mol% Zn-TNP, (d) 0.1 mol% TNP, (e) TNT,(f) 0.001 mol% Zn-TNT, (g) 0.01 mol% Zn-TNT, and (h) 0.1 mol% Zn-TNT.

treating TiO2 or Zn-TiO2 with concentrated NaOH solution particles. The (–Ti–O–Ti–O–Ti–O–)x chains included in the preferentially ruptured the longer Ti–O bonds due to the planar fragments were rendered ﬂexible by the covalent action of the OH− ions. Linear fragments were formed link between their end groups, so that they formed a and linked together by O−–Na+–O− ionic bonds to form thermodynamically stable product, that is, solid nanotube ﬀ planar fragments that peeled o from the TiO2 crystalline Na2Ti2O4(OH)2, indicating that the monolayer nanotube 4 International Journal of Photoenergy

20 nm 500 nm 20 nm 500 nm

(a) (b) (c) (d)

20 nm 500 nm 20 nm 500 nm

(e) (f) (g) (h)

Figure 3: TEM images of synthesized Zn-TNP and Zn-TNT: (a) TNP, (b) TNT, (c) 0.001 mol% Zn-TNP, (d) 0.001 mol% Zn-TNT, (e) 0.01 mol% Zn-TNP, (f) 0.01 mol% Zn-TNT, (g) 0.1 mol% Zn-TNP, and (h) 0.1 mol% Zn-TNT.

may have acted as the template for the multilayer nanotubes. trend. The pore size distribution (PDS) is an important Such a formation process of nanotube Na2Ti2O4(OH)2 was characteristic for porous materials. Among the various evident. After replacing Na+ with H+ using HCl solution at methods that have been reported by researchers to identify pH 1, the nanotube H2Ti2O5 finally appeared due to the the PDS of porous materials, BJH and Comparison Plots water elimination. [24, 25] are the most suitable for the range of nanopores. The TEM images (Figure 3) show the particle shape of the relative pressure at which pore filling takes place by capillary TNP and Zn-TNP, and the pore shape and size of the TNT condensation can be calculated from Kelvin’s equation [26], and Zn-TNT. TNP exhibited a mixture of rhombic and according to which the pore radius in which the capillary spherical particles of size distribution within the range of 5– condensation occurs actively can be determined as a function 10 nm, and increasing size and cubic shape as the Zn content of the relative pressure (P/P0). The mean pore diameter, Dp, was increased. The tube pore dimensions observed by TEM was calculated from Dp = 4VT /S,whereVT is the total were a width of about 70–100 nm, otherwise, the tube widths volume of pores, and S the BET surface area. This was shown were wide corresponds of Zn addition. by the variation in the average pore diameter value, Dp, Figures 4(a)–4(d) show the adsorption-desorption iso- between 7.66 and 15.58 nm (Table 1). therms of N2 at 77 K for TNT and the three Zn-TNT Figure 5(b) summarizes the evolution of H2 from meth- samples. The figure illustrates the shape and behavior of anol/water photosplitting over the TNT and Zn-TNT photo- the N2 adsorption isotherms for nonporous and nanotube catalysts in a batch-type, liquid photoreactor system (Fig- materials. All the isotherms belong to IV types in the IUPAC ure 5(a)). A very small amount of H2 was collected over TNT classification [22]: the isotherm has been widely used, and after methanol/water photodecomposition for 9 h, while a a certain hysteresis slope can be observed at intermediate significant amount of H2 gas was collected over Zn-TNT. and high relative pressures in TNT, which is indicative of The amount of H2 producedreached10.2mLover0.5gof the presence of large nanopores (tubes) (type IV in the 0.01 mol% Zn-TNT, and consequently the emitted rate for IUPAC classification). The adsorption and desorption lines hydrogen production was 0.225 mmol h−1 g−1 in 0.01 mol% for TNT and Zn-TNT overlapped completely in the low Zn-TNT photocatalytic system. This result is significant − relative pressure range, while the hysteresis loop was in the compared to the results that 120∼281 μLh 1 of hydrogen high relative pressure region (P/P0 > 0.4), mainly due to gases were produced over H-, N-, and C-TNT photocatalysts the presence of ink-bottle pore types that have a larger inreportsbySangetal.[16, 17]. While the amount produced pore size in the bottle body, which induces hysteresis in was lower on the 0.1 mol% Zn-TNT catalysts with higher the high relative pressure region [22, 23]. The BET surface loadings, possibly because of the structural damage. The areas of Zn-TNT were increased with increasing Zn content, use of two semiconductors, Zn and Ti oxides, with dif- although the total pore volume did not show any consistent ferent redox energy levels, in contact, can be considered International Journal of Photoenergy 5

180 450 1 1 − − 120 300 (STP)g (STP)g 3 3 /cm /cm p p 60 150 V V

0 0 0 0.5 1 0 0.5 1

P/P0 P/P0

ADS ADS DES DES (a) (b) 360 270 1 1 − − 240 180 (STP)g (STP)g 3 3 /cm /cm p p 120 90 V V

0 0 0 0.5 1 0 0.5 1

P/P0 P/P0

ADS ADS DES DES (c) (d)

Figure 4: Adsorption-desorption isotherms of N2 at 77 K for TNT and Zn-TNT: (a) TNT, (b) 0.001 mol% Zn-TNT, (c) 0.01 mol% Zn-TNT, and (d) 0.1 mol% Zn-TNT.

Table 1: Surface areas and average pore-diameter for TNT and Zn-TNT calculated by BET method.

BET multipoint surface Total pore volume Average pore diameter 2 −1 3 −1 area [m g ] (P/P0 = 0.990) [cm g ] [nm] TNT 104.21 0.2325 8.9262 0.001 mol% Zn-TNT 118.21 0.4605 15.583 0.01 mol% Zn-TNT 135.71 0.428 12.615 0.1 mol% Zn-TNT 153.01 0.293 7.66 a promising method to improve charge separation and in- in the semiconductor remain there and cannot migrate crease the lifetime of the charge carriers. Consequently, it to TNT. These thermodynamic conditions favor electron enhances the efficiency of the interfacial charge transfer to injection. Additionally, the tube-shaped TNT offers a larger the adsorbed substrate. For efficient inter-particle electron surface area, and with the increasing affinity of methanol, transfer between the semiconductor, which is considered the the performance degradation is eventually considered to be ZnO and TNT, the conduction band of TNT must be more higher. anodic than the corresponding band of the ZnO. Under Tables 2 and 3 and Figure 6 depict the influence of pH visible irradiation, only the ZnO is excited, and electrons on the position of the zeta potential distribution median of generated to their conduction band are injected into the TNT and Zn-TNT. The zeta potential of TNT suspension inactivated TNT conduction band. If the valance band of the was significantly decreased with increasing pH. The surface ZnO is more cathodic than that of TNT, the holes generated charges were transferred from positive in acidic solution 6 International Journal of Photoenergy

11 (C) 10 Pump 9 8 7 (D) Gas chromatography Ar Air 6 -TCD detector 5 -Carbonspere column IL separator 4 Reactor 3 Hydrogen production (mL) production Hydrogen 2 Reaction conditions 1 (A) UV-lamp: 3 ea (B) (6 W/cm2, 365 nm) 0 Solution: MeOH (500 mL 1 23456789 + DI water (500 mL)) Time (h) Reaction time: 1–9 TNT 0.01 h (stirring) Catalyst: 0.5 g 0.001 0.1 (a) (b)

Figure 5: The evolution of H2 from methanol/water photosplitting over the TNT and Zn-TNT photocatalysts in a batch-type liquid photoreactor system. (a) Photoreactor system for hydrogen production from methanol aqueous solution and (b) hydrogen production from methanol aqueous solution according to the time on stream: (A) TNT, (B) 0.001 mol% Zn-TNT, (C) 0.01 mol% Zn-TNT, and (D) 0.1 mol% Zn-TNT.

15 Table 2: Zeta-potential values and aggregated diameter sizes for Zeta-potential curves 0.01 mol% Zn-TNT according to pH, as determined by ELS. 10 Zeta potential (mV) ELS diameter (nm) = 5 pH 3 9.28 3641.8 pH = 5 2.87 8385.4 0 pH = 7 −21.41 709.2 357911pH = 9 −18.44 687.5 −5 pH = 11 −18.81 664.5

−10 Table 3: Zeta-potential values and aggregated diameter sizes for all samples at pH = 7.0, as determined by ELS. −15 Samples at pH = 7 Zeta potential (mV) ELS diameter (nm) −20 TNT −18.44 468.2 0.001 mol% Zn-TNT −18.86 728.0 − 25 0.010 mol% Zn-TNT −21.41 709.2 0.100 mol% Zn-TNT −19.07 618.2 Figure 6: Zeta-potential curve for 0.01 mol% Zn-TNT according to pH.

suspended with high charge with the increase of the Zn gradient. The zeta-potential was maximized at −21.41 mV to negative in alkali solution. For 0.01 mol% Zn-TNT, the in the 0.01 mol% Zn-TNT suspension. The zeta potential isoelectric point was at pH 5.3 with large aggregation, but and mobility of the Zn-TNT particles were relatively high for otherwise at pH = 7.0. The surface charge had the highest electrophoretic display compared to those of TNT. absolute value of −21.41 mV, which indicated that the The absorption band of TNT and Zn-TNT for the 0.01 mol% Zn-TNT colloidal was stable [27, 28] having less tetrahedral symmetry of Ti4+ normally appeared at approx- aggregation. Table 3 summarize the zeta-potential values imately 350∼400 nm (Figure 7(a)). The maximum absorp- and aggregated diameter sizes for all samples at pH = 7.0 tion bands in Zn-TNT were slightly shifted to a shorter as determined by ELS. The Zn-TNT particles ranged from wavelength than those of TNT but a broadened tail appeared −18.86 to −21.41 mV on the surface and the particles were in Zn-TNT. Bandgaps in semiconductor materials are International Journal of Photoenergy 7

1.5 3.5 400 nm

2.5 1

Emission 1.5 Absorbance (a.u.) Absorbance 0.5 1

0.5

0 0 300 400 500 600 300 400 500 600 Wavelength (nm) Wavelength (nm) 0.1 mol% Zn-TNT 0.001 mol% Zn-TNT TNT 0.01% Zn-TNT 0.01 mol% Zn-TNT TNT 0.001% Zn-TNT 0.1% Zn-TNT (a) (b)

Figure 7: UV-visible and PL spectra of TNT and Zn-TNT: (a) UV-visible spectra of TNT and Zn-TNT and (b) photoluminescence (PL) spectra of TNT and Zn-TNT. closely related to the wavelength range absorbed, where the potential window. It is desirable to display a reversible wave bandgap decreases with increasing absorption wavelength. in order to provide the following information. The reversible The bandgaps obtained by extrapolation based on the Tauc reactions display a hysteresis of absolute potential between equation [29] in TNT and 0.01 mol% Zn-TNT were about the reduction (Epc) and oxidation (Epa) peaks. Reversible 3.54 eV and 3.44 eV, respectively. A shorter bandgap eases reactions show a ratio of the peak currents passed at the movement of excited electrons from the valence band to reduction (ipc) and oxidation (ipa) that is near unity the conduction band on the surface, which occurs despite (1 = ipa/ipc). When such reversible peaks are observed, ther- the weaker visible radiation. Otherwise, the recombination modynamic information in the form of half-cell potential, 0 between electron and hole is faster for a smaller bandgap, E1/2(Epc + Epa/2) can also be determined [30, 31]. In partic- which decreases the photocatalytic performance. Figure 7(b) ular when, waves are semi-reversible, such as when ipa/ipc is presents the PL spectra of TNT and Zn-TNT. The PL less than or greater than 1, it can be possible to determine curve indicates that the electrons in the valence band were more information about the kinetic processes. In this study, transferred in an excited state to the conduction band and the oxidation potentials were measured by means of CV then stabilized by photoemission. In general, an increased in distilled water solutions of the 0.2 mM complex using number of emitted electrons resulting from the recombina- a pelletized sample as the working electrode, Ag/AgCl as tion between excited electrons and holes increases the PL the reference electrode, and 0.1 M KCl as the supporting intensity and thus decreases the photoactivity. Therefore, electrolyte. The electronic parameters for CV are listed in there is a strong relationship between PL intensity and Table 4. The synthesized TNT and Zn-TNT exhibited the photoactivity. However, in the presence of a metal that can Ti(IV) → Ti(III) redox process for a reversible reaction, capture excited electrons or exhibit conductivity, known as and the absolute potentials between the reduction (Epc)are the relaxation process, the PL intensity decreases to a greater seen at −0.79, −0.43, −0.46, and −0.41 eV in TNT, and extent. In Figure 7(b), the PL curve of Zn-TNT was similar to 0.001 mol%, 0.01 mol%, and 0.1 mol% Zn-TNT samples, that of TNT with an emission at 400 nm. The PL intensity of respectively. Zn-TNT was larger and maximized in 0.01 mol% Zn-TNT. Recently, some researchers have reported a useful equa- The utility of the CV results shown in Figure 8(a) is tion that can be used to determine the energy levels of the highly dependent on the analytic condition being studied, highest occupied molecular orbital (HOMO) and the lowest and it has to be redox active within the experimental unoccupied molecular orbital (LUMO) using CV [31]. The 8 International Journal of Photoenergy

− −2 0.01 −0.79 eV 0.01 0.43 eV

− 0.005 0.005 1.5

0 0 −1 Current (A) Current Current (A) Current −0.005 TNT −0.005 −0.5

− − + − 0.01 0.01 4H +4e → 2H2 0 2 102 −15−10 −50 5101510 −15−10 −50 51015 E (mV) versus Ag/AgCl E (mV) versus Ag/AgCl 0.5 1.23 eV (A) (B) 0.01% Zn-TNT 0.01 0.01 1 TNT 3.44 eV − 3.54 eV 0.46 eV − + 2H2O → 4e +4H +O2 0.005 0.005 −0.41 eV 1.5 Potential energy SHE) (versus Potential 0 0 2 Current (A) Current Current (A) Current −0.005 −0.005 2.5 −0.01 −0.01

2 2 −15−10 −50 5101510 −15−10 −50 510 1510 3 E (mV) versus Ag/AgCl E (mV) versus Ag/AgCl (C) (D) 3.5 (a) (b)

Figure 8: CV of TNT and Zn-TNT (a) and an expected potential energy diagram based on UV-visible spectra and CV (b): (A) TNT, (B) 0.001 mol% Zn-TNT, (C) 0.01 mol% Zn-TNT, and (D) 0.1 mol% Zn-TNT.

Table 4: Bandgap and HOMO-LUMO values based on CV and UV-visible spectroscopy. eV TNT 0.001 mol% Zn-TNT 0.01 mol% Zn-TNT 0.1 mol% Zn-TNT Ag/AgCl (LUMO) −0.79 −0.43 −0.46 −0.41 HOMO 2.75 3.02 2.98 2.96 Bandgap 3.54 3.45 3.44 3.37 results of the CV curve and UV-visible spectra are presented TEM observation, the morphology of Zn-TNT was a regular in Table 4, and Figure 8(b) is plotted based on these data. tube shape with a length of 1∼2 μm and width of 70– The bandgap of Zn-TNT was shorter than that of pure TNT, 100 nm. The UV-visible spectra and CV results revealed particularly 0.01 mol% Zn-TNT, and its valence and conduc- that the bandgap in TNT and 0.01 mol% Zn-TNT was tion bands were at lower energy levels than those of TNT. about 3.54 and 3.44 eV, and the LUMO energy levels This indicated that the energy level of the electrons excited were calculated as −0.79 and −0.43 eV, respectively. The from the valence band during photocatalysis was further zeta potential and mobility of the Zn-TNT particles were elevated, which increased the hole eﬀect at the valence band relatively high for electrophoretic display, compared to those to a much greater extent than that in TNT. These phenomena of TNT. The zeta-potential was maximized at −21.41 mV consequently increased the evolution of OH radicals formed in the 0.01 mol% Zn-TNT suspension. These spectroscopic from the electrons and holes, and the resulting increase properties facilitated the collection of a signiﬁcant amount in the degradation rate of methanol and water eventually of H2 gas (10.2 mL) over 0.5 g of the 0.01 mol% Zn-TNT enhanced the photocatalytic performance. Therefore, the catalyst. photocatalytic activity was enhanced over Zn-TNT.

Acknowledgment 4. Conclusions This research was supported by a Yeungnam University Zn-TNT was synthesized using Zn-TNP loaded with three research grant, no. 211-A-380-136, and the authors are very diﬀerent molar ratios on nanosized TiO2. As shown by grateful for this support. International Journal of Photoenergy 9

References [15] S. Xu, J. Ng, A. J. Du, J. Liu, and D. D. Sun, “Highly efficient TiO2 nanotube photocatalyst for simultaneous hydrogen pro- [1] H.-W. Wang, H.-W. Chung, H.-T. Teng, and G. Cao, “Gener- duction and copper removal from water,” International Journal ation of hydrogen from aluminum and water—effect of metal of Hydrogen Energy, vol. 36, no. 11, pp. 6538–6545, 2011. oxide nanocrystals and water quality,” International Journal of [16]L.X.Sang,Z.Y.Zhang,G.M.Bai,C.X.Du,andC.F.Ma,“A Hydrogen Energy, vol. 36, pp. 15136–15144, 2011. photoelectrochemical investigation of the hydrogen-evolving [2] Y. Liu, L. Guo, W. Yan, and H. Liu, “A composite visible- doped TiO2 nanotube arrays electrode,” International Journal light photocatalyst for hydrogen production,” Journal of Power of Hydrogen Energy, vol. 37, pp. 854–859, 2012. Sources, vol. 159, no. 2, pp. 1300–1304, 2006. [17] L. X. Sang, Z. Y. Zhang, and C. F. Ma, “Photoelectrical and [3] M. Antoniadou, P. Bouras, N. Strataki, and P. Lianos, charge transfer properties of hydrogen-evolving TiO2 nan- “Hydrogen and electricity generation by photoelectrochemical otube arrays electrodes annealed in different gases,” Interna- decomposition of ethanol over nanocrystalline titania,” Inter- tional Journal of Hydrogen Energy, vol. 36, no. 8, pp. 4732– national Journal of Hydrogen Energy, vol. 33, no. 19, pp. 5045– 4738, 2011. 5051, 2008. [18] H.-H. Ou and S.-L. Lo, “Review of titania nanotubes synthe- [4] N.-L. Wu, M.-S. Lee, Z.-J. Pon, and J.-Z. Hsu, “Effect of sized via the hydrothermal treatment: fabrication, modifica- calcination atmosphere on TiO2 photocatalysis in hydrogen tion, and application,” Separation and Purification Technology, production from methanol/water solution,” Journal of Photo- vol. 58, no. 1, pp. 179–191, 2007. chemistry and Photobiology A, vol. 163, pp. 277–280, 2004. [19] J. Skvarla, “Evaluation of mutual interactions in binary min- [5] M. I. Badawy, M. Y. Ghaly, and M. E. M. Ali, “Photocatalytic eral suspensions by means of electrophoretic light scattering hydrogen production over nanostructured mesoporous titania (ELS),” International Journal of Mineral Processing, vol. 48, pp. from olive mill wastewater,” Desalination, vol. 267, no. 2-3, pp. 95–109, 1996. 250–255, 2011. [20] C. Li, J. Yuan, B. Han, L. Jiang, and W. Shangguan, “TiO2 [6] S. Xu, J. Ng, X. Zhang, H. Bai, and D. D. Sun, “Fabrication nanotubes incorporated with CdS for photocatalytic hydrogen production from splitting water under visible light irradia- and comparison of highly efficient Cu incorporated TiO2 photocatalyst for hydrogen generation from water,” International tion,” International Journal of Hydrogen Energy, vol. 35, no. 13, JournalofHydrogenEnergy, vol. 35, no. 11, pp. 5254–5261, pp. 7073–7079, 2010. 2010. [21] S. A. Setiadi, “Photocatalytic hydrogen generation from glycerol and water using Pt loaded N-doped TiO2 nanotube,” [7] R. Dholam, N. Patel, and A. Miotello, “Efficient H2 production International Journal of Engineering and Technology, vol. 11, by water-splitting using indium-tin-oxide/V-doped TiO2 multilayer thin film photocatalyst,” International Journal of Hydro- no. 3, pp. 91–95, 2011. gen Energy, vol. 36, no. 11, pp. 6519–6528, 2011. [22] W. Stefaniak, J. Goworek, and B. Bilinski,´ “Pore size analysis by nitrogen adsorption and thermal desorption,” Colloids and [8] T. Puangpetch, T. Sreethawong, and S. Chavadej, “Hydrogen Surfaces A, vol. 214, no. 1–3, pp. 231–237, 2003. production over metal-loaded mesoporous-assembled SrTiO 3 [23] S. Onsuratoom, S. Chavadej, and T. Sreethawong, “Hydrogen nanocrystal photocatalysts: effects of metal type and loading,” production from water splitting under UV light irradiation International Journal of Hydrogen Energy, vol. 35, no. 13, pp. over Ag-loaded mesoporous-assembled TiO -ZrO mixed 6531–6540, 2010. 2 2 oxide nanocrystal photocatalysts,” International Journal of Hy- [9] S. Onsuratoom, S. Chavadej, and T. Sreethawong, “Hydrogen drogen Energy, vol. 36, no. 9, pp. 5246–5261, 2011. production from water splitting under UV light irradiation [24] M. Iza, S. Woerly, C. Danumah, S. Kaliaguine, and M. over Ag-loaded mesoporous-assembled TiO -ZrO mixed ox- 2 2 Bousmina, “Determination of pore size distribution for meso- ide nanocrystal photocatalysts,” International Journal of Hy- porous materials and polymeric gels by means of DSC meas- drogen Energy, vol. 36, no. 9, pp. 5246–5261, 2011. urements: thermoporometry,” Polymer, vol. 41, no. 15, pp. [10] R. Sasikala, A. Shirole, V. Sudarsan et al., “Highly dispersed 5885–5893, 2000. phase of SnO2 on TiO2 nanoparticles synthesized by polyol- [25] H. Y. Zhu and G. Q. Lu, “Estimating pore size distribution mediated route: photocatalytic activity for hydrogen genera- from the differential curves of comparison plots,” Studies in tion,” International Journal of Hydrogen Energy,vol.34,no.9, Surface Science and Catalysis, vol. 128, pp. 243–250, 2000. pp. 3621–3630, 2009. [26] A. C. Mitropoulos, “The Kelvin equation,” Journal of Colloid [11] M.-S. Park and M. Kang, “The preparation of the anatase and Interface Science, vol. 317, no. 2, pp. 643–648, 2008. and rutile forms of Ag–TiO2 and hydrogen production from [27] D. L. Liao, G. S. Wu, and B. Q. Liao, “Zeta potential of shape- methanol/water decomposition,” Materials Letters, vol. 62, pp. controlled TiO2 nanoparticles with surfactants,” Colloids and 183–187, 2008. Surfaces A, vol. 348, no. 1–3, pp. 270–275, 2009. [12] B. S. Kwak, J. Chae, J. Kim, and M. Kang, “Enhanced hydrogen [28] K. Maeda, M. Eguchi, W. J. Youngblood, and T. E. Mal- production from methanol/water photo-splitting in TiO2 louk, “Niobium oxide nanoscrolls as building blocks for dye- including Pd component,” Bulletin of the Korean Chemical sensitized hydrogen production from water under visible light Society, vol. 30, pp. 1047–1053, 2009. irradiation,” Chemistry of Materials, vol. 20, no. 21, pp. 6770– [13] X.-J. Zheng, L.-F. Wei, Z.-H. Zhang et al., “Research on pho- 6778, 2008. tocatalytic H2 production from acetic acid solution by Pt/TiO2 [29] A. W. Burton, K. Ong, T. Rea, and I. Y. Chan, “On the esti- nanoparticles under UV irradiation,” International Journal of mation of average crystallite size of zeolites from the Scherrer Hydrogen Energy, vol. 34, no. 22, pp. 9033–9041, 2009. equation: a critical evaluation of its application to zeolites with [14]J.Chae,J.Lee,J.H.Jeong,andM.Kang,“Hydrogenpro-duc- one-dimensional pore systems,” Microporous and Mesoporous tion from photo splitting of water using the Ga-incorporated Materials, vol. 117, pp. 75–90, 2009. TiO2s prepared by a solvothermal method and their character- [30] K. B. Oldham and J. C. Myland, “Modelling cyclic voltamme- istics,” Bulletin of the Korean Chemical Society, vol. 30, p. 302, try without digital simulation,” Electrochimica Acta, vol. 56, 2009. no. 28, pp. 10612–10625, 2011. 10 International Journal of Photoenergy

[31] M. Wang, D.-J. Guo, and H.-L. Li, “High activity of novel Pd/TiO2 nanotube catalysts for methanol electro-oxidation,” Journal of Solid State Chemistry, vol. 178, no. 6, pp. 1996–2000, 2005. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 607283, 9 pages doi:10.1155/2012/607283

Research Article Statistical Optimization of Operational Parameters for Enhanced Naphthalene Degradation by TiO2/Fe3O4-SiO2 Photocatalyst

Aijuan Zhou,1 Jing Peng,2 Zhaobo Chen,3, 4 Jingwen Du,1 Zechong Guo,1 Nanqi Ren,1, 3 and Aijie Wang1, 3

1 School of Municipal and Environmental Engineering, Harbin Institute of Technology (HIT), 202 Haihe Road, Harbin 150090, China 2 Institute of Architecture Design and Research, Harbin Institute of Technology, Harbin 150090, China 3 State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (SKLUWRE, HIT), Harbin 150090, China 4 Collage of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China

Correspondence should be addressed to Aijie Wang, [email protected]

Received 11 December 2011; Revised 13 February 2012; Accepted 15 February 2012

Academic Editor: Stephane´ Jobic

Copyright © 2012 Aijuan Zhou et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The optimization of operational parameters for enhanced naphthalene degradation by TiO2/Fe3O4-SiO2 (TFS) photocatalyst was conducted using statistical experimental design and analysis. Central composite design method of response surface methodology (RSM) was adopted to investigate the optimum value of the selected factors for achieving maximum naphthalene degradation. Experimental results showed that irradiation time, pH, and TFS photocatalyst loading had significant influence on naphthalene degradation and the maximum degradation rate of 97.39% was predicted when the operational parameters were irradiation time 97.1 min, pH 2.1, and catalyst loading 0.962 g/L, respectively. The results were further verified by repeated experiments under optimal conditions. The excellent correlation between predicted and measured values further confirmed the validity and practicability of this statistical optimum strategy.

1. Introduction electron beam irradiation [10], electrolytic aeration [11], and photocatalysis [12–14]. Among these techniques, biodegra- Naphthalene, as the first member of polycyclic aromatic hy- dation is expected to be an economical and energy-efficient drocarbon (PAH) and one of the 16 PAHs classified as prior- approach which attracts more and more attentions to inves- ity pollutants by the Environmental Protection Agency (EPA) tigate the application to treat PAHs. Nevertheless, because of of the United States, is a class of persistent organic pollutants its toxicity and low water solubility, the efficacy of bioreme- of special concern. Naphthalene can be frequently found diation still remains a critical point [15]. in many anthropogenic fluxes, such as combustion fumes, Photocatalysis has been proposed as an alternative to used oil, and bilge water, which is exceedingly recalcitrant to degrade refractory organic compounds unquestionably due degradation due to its inhibitory nature [1–3]. Since it is the to the specificity of hydroxyl radicals which represents high most water-soluble PAH, naphthalene is the dominant one in reaction rate and low selectivity [16–19]. Although this tech- water which has been considered as possibly a carcinogen to nique presents critical advantages over other techniques, humans (EPA 1998, The International Agency for Research therearesomeproblemstoberesolvedurgently.Among on Cancer (IARC) 2002) and has both acute and chronic these, the commonly mentioned problems are the designs effects on human and animal health. of adequate reactors for efficient utilization of photons and A solution to this naphthalene pollution problem has the required higher degradability to persistent organic pol- now become urgent. Removing naphthalene from water is lutants. Moreover, this technology has not been successfully possible via many techniques, including biofiltration [4], commercialized, in part because of the difficulty in separa- microbial degradation [5, 6], anaerobic degradation [7–9], ting TiO2 nanoparticles from the suspension [20]. To resolve 2 International Journal of Photoenergy

this problem, TiO2 dispersed on magnetic oxide support where Xi is the coded value of the independent variable, Ai (Fe3O4-SiO2) is used in this study as the catalyst for pho- is the actual value of independent variable, A0 is the actual tooxidative degradation of naphthalene which could be re- value of the Ai at the center point, and Ai is the step change claimed using ferromagnetic separation processes. of independent variable. In order to enhance the naphthalene degradation per- In the study, catalyst loading, pH, and irradiation time formance, an optimization approach should be employed. were taken as the independent variable and naphthalene Response surface methodology (RSM) is an efficient stan- degradation rate was the dependent variable or response of dard and well-established mathematical optimization proce- the design experiments. By means of multilinear regression dure which can achieve such an optimization by analyzing method [28, 29], a second-order polynomial function was and modeling the effects of multiple variables and their fitted to correlate relationship between independent variable responses and finally optimizing the process [21]. This and response. Quadratic equation for the independent var- method has been successfully employed for optimization in iable was expressed as follows. some photocatalytic oxidation processes [22–26]. However, = 2 to the best of our knowledge, the optimization of photocat- y β0 + β1x1 + β2x2 + β3x3 + β11x1 (2) alytic degradation of naphthalene solution by TiO2/Fe3O4- 2 2 + β22x2 + β33x3 + β12x1x2 + β13x1x3 + β23x2x3, SiO2(TFS) catalyst has not been reported. Therefore, the objective of this study was to investigate the effect of where y represents the predicted response; β0 is the intercep- TiO2/Fe3O4-SiO2(TFS) photocatalyst on the treatment of a tion coefficient; β1, β2,andβ3 are the linear coefficients; β11, simulated high-concentration wastewater polluted by naph- β ,andβ are the quadratic coefficients; β , β ,andβ thalene. The central composite circumscribed (CCC) design 22 33 12 13 23 are the interactive coefficients; x1, x2,andx3 represent the method of RSM was employed to determine the optimal pro- independent variable studied. cess condition for maximizing naphthalene degradation rate. The Design Expert (Version 7.4.1.0, Stat-Ease Inc., Minneapolis, MN, USA) software was used for regression 2. Materials and Methods and graphical analyzes of the data obtained. All experimental designs were randomized, and mean values were applied. 2.1. Reagents and Photocatalyst Preparation. FeCl2,FeCl3, NaOH, HNO3,Fe3O4, absolute alcohol, and isopropanol 2.4. Procedure. As the solubility of naphthalene is 25– (Tianjin Kermel Chemical Reagent Co. Ltd.) were of analyti- 30 mg/L at ambient temperature, this study chose to work cal grade; naphthalene (Aldrich), Ti(OC4H9)4, 3-aminopro- with the highest concentration in order to show the unique pyltriethoxysilane, and tetraethoxysilane (J&K Chemical Ltd removal efficiency of TFS photocatalyst [3]. In all the pho- Co.) were of reagent grad; all the reagents mentioned above tocatalytic experiments, the reaction temperature was kept were used as received without further purification. Millipore constant at 25 ± 0.1◦C. Unless required, pH was not initially deionized water was used for dilution. modified or controlled in the reactor. When required, initial For now, the TFS photocatalyst has been prepared suc- pH values were adjusted using 4 mol/L NaOH or H2SO4. cessfully, and the specific preparation and characterization All experiments were carried out in 250 mL quartzose have been published in other paper [27]. The synthesis route beakers comprised of 100 mL aqueous naphthalene solution of TFS photocatalyst is shown in Figure 1. and the appropriate amount of the TFS photocatalyst powder, stirring at 1000 rpm (higher rotated speed may result 2.2. Photoreactor. A schematic representation of the photore- in naphthalene volatilization without degradation). Before actor is shown in Figure 2. The reactor mainly consisted irradiation, the reaction mixture was premixed in the dark of mechanical stirrer, mercury lamp, quartz cold trap, and for 20 min to reach adsorption equilibrium. beaker. The irradiation experiments were carried out in four parallel 250 mL quartz beakers. The light source was a high- 2.5. Analytical Methods. The residual naphthalene was deter- pressure mercury lamp (HPK 125W, Philips) setting in a mined by a high-performance liquid chromatography (LC- quartzose cold trap, emitting the near-UV (mainly around 10A, Shimadzu Corporation, Kyoto, Japan) equipped with a 365nm). The warp of photoreactor was made of polymethyl diode array detector and an Uptisphere C18 HDO column methacrylate (PMMA), inner surface of which clings silver (stationary phase 3 μm, dimensions 150 mm × 3 mm) using paper in order to return UV light. a mixture of methanol and deionized water (ratio 80 : 20) as the mobile phase at a flow rate of 1.0 mL/min. 2.3. Multivariate Experimental Design. A 3-factor CCC with six replicates at the center point leading to 20 experiments 3. Results and Discussion was employed to optimize the operational parameters for improving naphthalene degradation. For statistical calcula- 3.1. Effect of Catalyst Loading on Degradation of Naphthalene. tion, the relation between the coded values and actual values The catalyst loading in slurry photocatalytic processes was an of independent variable is described as follows. important factor. Figure 3 presents the variation of the first order rate constant and the value of Ct/Co as a function of the Ai − A0 catalyst loading, where Ct is the naphthalene concentration Xi = ,(1) Ai at t time and Co is the initial concentration of naphthalene. International Journal of Photoenergy 3

HO HO TiO Si OH Ti OH 2 OH O SiO2 O SiO O O 2 Si(OC2H5)4 Ti(OC H ) Fe O 4 9 4 3 4 OH Fe3O4 OSiOH Fe3O4 OTiOH Fe3O4 Hydrolysis, NH OH Hydrolysis, HCl 4 O O OH O Si OH O Ti OH HO HO

Figure 1: Synthesis route of TiO2/Fe3O4-SiO2 photocatalyst.

1 0.36

) 0.32 1 − s

0.8 3 0.28 − 0.24 (10

k 0.2 0.6

0 0.16 C

/ 0.25 0.5 0.75 1 1.25 1.5 t

C Catalyst loading (g/L) 0.4

0.2

0 0204060 80 100 120 Irradiation time (min)

0.25 g/L R2 = 0.9945 1.25 g/L R2 = 0.9981 0.75 g/L R2 = 0.9944 1.5 g/L R2 = 0.9888 Figure 2: Schematic diagram of the photoreactor. 1 g/L R2 = 0.9988

Figure 3: Effect of TFS catalyst loading on the degradation of naphthalene. Inset graph: rate constant (k) versus TFS catalyst loading. The optimum concentration of the TFS photocatalyst was examined by varying the catalyst amount from 0.25 to 1.50 g/L. The reaction rates (k) were estimated by fitting the When TiO surface was hydrated, the principal surface func- time-course curves using the first-order kinetics [14, 30], 2 tionality was the amphoteric “Titanol” moiety, −TiOH, which was presented in the inset of Figure 3.Itcanbe which took part in the following acid-base equilibrium [31]: seen from Figure 2 that, as the TFS photocatalyst loading increased, the photocatalytic efficiency of the degradation + + S −TiOH + H = TiOH 2,pK,(3) increased firstly and then decreased. At 1.00 g/L catalyst a1 loading, the first-order rate constant k reached maximum − TiO +H+ =−TiOH, pKS ,(4) and the rate of degradation reached a saturation value. By a2 further increasing the catalyst loading, it would lead to the where pKS and pKS represented the negative log of the aggregation of the catalyst particles, which, in turn, resulted a1 a2 acidity constants of the first and second acid dissociation, in the decrease of active sites and the naphthalene photo- respectively. The pH of zero point of charge, pH ,wasgiven degradation. Consequently, the operating range of catalyst zpc as the following equation: loading for multivariate experimental design was 0.8–1.0 g/L, taking costs into account simultaneously. = S S pHzpc 0.5pKa1 +pK a2 . (5)

3.2. EffectofpHonDegradationofNaphthalene. It is known In the TFS photocatalyst, TiO2 spreads as a layer over Fe3O4- that the influence of pH to the photodegradation is substan- SiO2 core (Figure 1). Thus as far as surface properties be tially complex, which can directly affect the surface charged concerned, TFS was similar to pure TiO2.ForTiO2 Degussa properties of photocatalyst particles and, by extension, influ- P-25, the corresponding surface acidity constants are found S S ence the adsorption behavior of substrate molecules on to be pK a1 = 4.5 and pK a2 = 8.0, and pHzpc has been the photocatalyst surface. In the present study, therefore, determined by titration: pHzpc = 6.3 [32]. it was chosen as a predominant operational factor for the Figure 4 presents the variation of the first-order rate photooxidative degradation of naphthalene. Metal oxide par- constant and the value of Ct/Co as a function of pH. The ticles suspended in water behaved similar to diprotic acids. catalyst loading was 1.0 g/L. It can be noticed that a higher 4 International Journal of Photoenergy

1.2 ﬀ 0.6 Table 1: E ect of irradiation time on the degradation of naphtha-

) 0.5 lene. 1

− 0.4 s 3 1 − 0.3 Irradiation time (min) 20 40 60 80 100 120 (10 0.2 k 0.1 Degradation rate (%) 51.6 63.6 81.0 89.2 91.7 93.3 0 0.8 246810 pH 0 C / t 0.6 ﬀ

C 3.3. E ect of Irradiated Time on Degradation of Naphthalene. Table 1 shows the variation of the degradation rate in the 0.4 condition of optimal catalyst loading (1.00 g/L) and pH (2) as a function of irradiation time. 0.2 As displayed in Table 1, the increase of degradation rate went ﬂatter after 80 min. The degradation rate enhanced ∼ 0 2.5% in the time range of 80 100 min, while the degradation 0204060 80 100 120 rate enhanced only 1.51% in the time range of 100∼120 min Irradiation time (min) which was much lower than the previous three ranges. The irradiation time counted little to the degradation rate of pH 2 R2 = 0.9899 pH 8 R2 = 0.9944 naphthalene after 80 min. Consequently, the operating range pH 3 R2 = 0.9927 2 = pH 7 R 0.9997 of irradiation time for multivariate experimental design was pH 6 R2 = 0.9991 pH 10 2 = 0 981 R . 80–100 min, taking costs into account simultaneously.

ff Figure 4: E ect of pH on the degradation of naphthalene. Inset 3.4. Optimization of Operational Parameters for Enhancing the graph: rate constant (k)versuspH. Naphthalene Degradation. Experimental design and results are shown in Table 2. By applying multiple regression analysis on the experimental data, a second-order polynomial equa- photodegradation effect existed in acid environment, which tion was obtained to describe the correlation between the was assumed to be linked with the surface characteristic of independent variable and the naphthalene degradation rate: TFS photocatalyst particles. At low pH, hydroxyl radicals =− were formed on the surface of TFS particles by the reaction y 1262.32 + 15.63x1 +54.20x2 + + of hole (hvb ) with adsorbed H2Omolecule:hvb +H2O → • − + 1130.84x − 0.11x x − 2.56x x OH +H+, which made electron (e ) assemble to the surface 3 1 2 1 3 (7) −• of TFS particles and then reacted with adsorbed O2 to O2 , 2 2 2 +19.5x2x3 − 0.067x1 − 14.81x2 − 480.26x3 , leading to longer lifetime of the electron/hole pair [33]. Moreover, the increased hydrogen ions was beneficial to the where y is the predicted naphthalene degradation rate (%); • production of H2O2 and OH radicals. The specific reactions x1, x2,andx3 are irradiation time (min), pH value, and were given as follows. catalyst loading (g/L), respectively. The analysis of variance (ANOVA) was conducted to test − −→ − • e +O2ads O2ads the significance of the fit of the second-order polynomial equation for the degradation rate as shown in Table 3. − • + −→ • O2ads +H HO2 The F value, represented the likelihood of two different (6) groups being statistically different, was 551.40 which implied • −→ 2HO2 O2 +H2O2 that the model was significant because values of “Prob > F” less than 0.05 were considered to be significant. There − • −→ • − H2O2 +O2ads OH +OH +O2 wasonlyless0.01%chancethatamodelF value this large could occur; the regression mathematical model was a good At high pH, the TFS particles’ surface was negatively fit to experimental data. In addition, the model did not charged which increased the work function for electron show lack of fit. The Pred R-squared was 0.9827 which was abjection and also trapped of valence-band holes. It made in reasonable agreement with the Adj R-squared (0.9964). against the photocatalytic reaction. But the reaction rates in The disparity of R-squared (0.9982) and Adj R-squared was pH 8 and 10 were a little increased than those in neutral in probably originated in the insignificance of x1x2 term. The this case, because the solutions at different pH had different Adeq Precision was 64.080 which indicated an adequate ionic strength: HNO3 was present at acid solution and NaOH signal, measuring the signal to noise ratio. Myers pointed that was present at alkaline solution, whereas no ionic species model adequacies should be checked by R2,Adj-R2,Pre-R2, were added at neutral pH 7, and additionally the quantities Adeq.Precision,andCV(lackoffit> 0.1; R2 > 0.95; (Adj- of ionic also affected the ionic strength. R2–Pre-R2) < 0.2; CV < 10; Pre-R2 > 0.7; Adeq. Precision > As seen in the inset of Figure 4, the first-order rate 4) [34]. After calculated, R2,Adj-R2,Pre-R2, Adeq. Precision constant of the degradation (k) at pH 2 was higher than those and coefficient of variation (CV, 1.00) all satisfied the request at other pH values. As a consequence, the operating range of of model adequacy; therefore, (7) properly described the pH for multivariate experimental design was 1.5–2.5. degradation rate of naphthalene in this study. From (7), International Journal of Photoenergy 5

Table 2: The central composite experimental design with three independent variables and results.

Factors (coded value) Run Irradiation time Code pH Code Catalyst loading Code Degradation rate y (%)

(min) x1 x2 (g/L) x3 1 80 +1 1.5 +1 0.8 +1 54.5 2 100 −1 1.5 +1 0.8 +1 81.3 380+12.5−1 0.8 +1 55.3 4 100 −12.5−1 0.8 +1 82.4 580+11.5+11.0−1 71.3 6 100 −11.5+11.0 −1 90.3 780+12.5−11.0−1 78.5 8 100 −12.5−11.0−1 92.8 9 90 0 2.0 0 0.9 0 91.0 10 90 0 2.0 0 0.9 0 90.5 11 90 0 2.0 0 0.9 0 91.1 12 90 0 2.0 0 0.9 0 90.2 13 90 0 2.0 0 0.9 0 91.1 14 90 0 2.0 0 0.9 0 91.6 15 73.18 −a 2.0 0 0.9 0 52.6 16 106.82 a 2.0 0 0.9 0 91.4 17 90 0 1.16 −a 0.9 0 78.7 18 90 0 2.84 A 0.9 0 82.0 19 90 0 2.0 0 0.7318 −α 64.3 20 90 0 2.0 0 1.0682 α 90.2

Table 3: Analysis of variance (ANOVA) for the model regression representing degradation rate.

P value Source Sum of squares df Mean square F value (Prob >F) Model 3557.63 9 395.29 551.40 <0.0001 x1 1707.76 1 1707.76 2382.18 <0.0001 x2 21.57 1 21.57 30.09 0.0004 x3 774.28 1 774.28 1080.06 <0.0001 x1x2 2.49 1 2.49 3.47 0.0955 x1x3 52.33 1 52.33 72.99 <0.0001 x2x3 7.64 1 7.64 10.66 0.0098 2 x1 638.82 1 638.82 891.10 <0.0001 2 x2 197.46 1 197.46 275.44 <0.0001 2 x3 332.13 1 332.13 463.30 <0.0001 Residual 6.45 9 0.72 Lack of ﬁt 5.84 5 1.17 7.64 0.036 Cor total 3565.48 19

the optimal values of x1, x2,andx3 in the actual units were varied within the experimental range. As can be seen from found to be 97.1 min irradiation time, 2.1 pH, and 0.962 g/L Figures 4–6, the response surface of degradation rate showed catalyst loading, respectively. The maximum predicted value a clear peak, indicating that the optimum conditions were of degradation rate obtained was 97.39%. inside the design boundary well. Degradation rate increased Figures 5–7 show the response surface plots and corre- with irradiation time, pH, and catalyst loading to optimum sponding contour plots based on (7) with one variable being conditions, respectively, and then decreased with a further kept constant at its optimum level and the other two variables increase. This result indicates that irradiation time, pH, and 6 International Journal of Photoenergy

2.5

2.25 96 92.84 95 88.75

2 75 79 87 81.25

pH 83 91

74.25 Degradation rate (%) rate Degradation

67 Degradation rate 1.75

2.5 100 52.56 71 2.25 95 2 90 1.5 pH 1.75 85 80 85 90 95 100 1.5 80 Irradiation time (min) Irradiation time (min) (a) (b)

Figure 5: The response surface plot and the corresponding contour plot showing the eﬀects of irradiation time and pH on naphthalene degradation rate.

0.95 97 98 92.84 88 93 0.9 78 81 89 85 68

Catalyst loading (g/L) 77 58 Degradation rate 0.85 73 (%) rate Degradation 69 1 52.56 65 100 Catalyst0.95 loading (g/L) 95 0.9 90 0.8 0.85 80 85 90 95 100 85 0.8 80 Irradiation time (min) Irradiation time (min) (a) (b)

Figure 6: The response surface plot and the corresponding contour plot showing the effects of irradiation time and catalyst loading on naphthalene degradation rate. catalyst loading all had individual significant influences on 3.5. Validation of the Model. In order to confirm the vali- degradation rate. In addition, the angle of inclination of the dity of the statistical experimental strategy, the repeated principal axis was evidently towards either irradiation time experiments under optimal conditions were carried out or catalyst loading in Table 1 and Figure 3,respectively,and (Table 4). As a consequence, the maximum standard error this indicated that the positive effect of increased irradiation between the observed value and the predicted value was time or catalyst loading levels on degradation rate was more less than 4% which indicated that the quadratic model can pronounced than pH increased. predict experimental results well. Furthermore, the standard International Journal of Photoenergy 7

0.95 95 92.84 89.75 91 0.9 84.5

88 79.25 Catalyst loading (g/L)

Degradation rate (%) rate Degradation 74 Degradation rate 0.85 85 1 82 2.5 52.56 Catalyst0.95 loading (g/L) 2.25 76 79 0.9 2 0.8 76 0.85 1.75 1.5 1.75 2 2.25 2.5 pH 0.8 pH 1.5 (a) (b)

Figure 7: The response surface plot and the corresponding contour plot showing the eﬀects of pH and catalyst loading on naphthalene degradation rate.

Table 4: Conﬁrm analyses for the model on naphthalene removal.

Factors (coded value) Degradation rate y (%) Codes Irradiation time Catalyst loading Standard error (%) pH value Actual value Predicted value (min) (g/L) 1 90 1.5 0.80 71.9 74.6 3.63 2 100 2 0.90 92.8 95.5 2.79 3 120 2.5 0.80 55.7 54.8 2.26 4 97.1 2.1 0.962 95.9 97.4 1.55 5 97.1 2.1 0.962 96.3 97.4 1.07 6 97.1 2.1 0.962 97.0 97.4 0.37 error was only 2.26% in code 3 when the irradiation time statistical design methodology offers an efficient and feasible (120 min) was beyond the operating range for multivariate approach for optimization the operational parameters of experimental design, from which we can see that the model naphthalene degradation by TiO2/Fe3O4-SiO2 photocatalyst. had extrapolation ability. As been shown, the degradation rate of naphthalene was 95.88%, 96.34%, and 97.03% in the Acknowledgment optimal experimental condition, respectively. The observed values enhanced about 4% than nonoptimal experimental The authors gratefully acknowledge the financial support values at the same temperature. by the National Natural Science Foundation of China (no. 51078100), by the National Creative Research Groups 4. Conclusions Project (Grant no. 51121062), by the Heilongjiang Science Foundation for Distinguished Young Scholars (Grant no. The present study focused on the optimization of opera- JC201003), and by the State Key Laboratory of Urban Water tional parameters for enhancing naphthalene degradation by Resource and Environment (Grant no. 2010DX11). TiO2/Fe3O4-SiO2 photocatalyst using the statistical methodology. Based on the central composite design, the maximum predicted value of degradation rate was obtained when References the operational parameters were 97.1 min irradiation time, [1] S. C. Wilson and K. C. Jones, “Bioremediation of soil con- 2.1 pH, and 0.962 g/L catalyst loading, respectively. The taminated with polynuclear aromatic hydrocarbons (PAHs): high correlation between the predicted and observed values a review,” Environmental Pollution, vol. 81, no. 3, pp. 229–249, indicated the validity of the model. This result suggested that 1993. 8 International Journal of Photoenergy

[2] B. Guieysse, M. D. Cirne, and B. Mattiasson, “Microbial degra- photocatalytic oxidation of naphthalene,” Applied Catalysis A, dation of phenanthrene and pyrene in a two liquid phase- vol. 244, no. 2, pp. 383–391, 2003. partioning bioreactor,” Applied Microbiology and Biotechnolo- [18] B. Pal and M. Sharon, “Photodegradation of polyaromatic gy, vol. 56, pp. 796–802, 2001. hydrocarbons over thin film of TiO2 nanoparticles; a study of [3] A. Lair, C. Ferronato, J. M. Chovelon, and J. M. Herrmann, intermediate photoproducts,” JournalofMolecularCatalysisA, “Naphthalene degradation in water by heterogeneous photo- vol. 160, no. 2, pp. 453–460, 2000. catalysis: an investigation of the influence of inorganic anions,” [19]N.Barrios,P.Sivov,D.D’Andrea,andO.Nu´ nez,˜ “Conditions Journal of Photochemistry and Photobiology A, vol. 193, no. 2-3, for selective photocatalytic degradation of naphthalene in tri- pp. 193–203, 2008. ton X-100 water solutions,” International Journal of Chemical [4] K. Maillacheruvu and S. Safaai, “Naphthalene removal from Kinetics, vol. 37, no. 7, pp. 414–419, 2005. aqueous systems by Sagittarius sp,” Journal of Environmental [20] M. J. Garc´ıa-Mart´ınez, L. Canoira, G. Blazquez,´ I. Da Riva, Science and Health Part A, vol. 37, no. 5, pp. 845–861, 2002. R. Alcantara,´ and J. F. Llamas, “Continuous photodegradation [5] H. Pathak, D. Kantharia, A. Malpani, and D. Madamwar, of naphthalene in water catalyzed by TiO2 supported on glass “Naphthalene degradation by Pseudomonas sp. HOB1: in Raschig rings,” Chemical Engineering Journal, vol. 110, no. 1–3, vitro studies and assessment of naphthalene degradation effi- pp. 123–128, 2005. ciency in simulated microcosms,” Journal of Hazardous Mate- [21] J.-P. Wang, Y.-Z. Chen, Y. Wang, S.-J. Yuan, and H.-Q. Yu, rials, vol. 166, no. 2-3, pp. 1466–1473, 2009. “Optimization of the coagulation-flocculation process for [6] Y. Jia, H. Yin, J. S. Ye et al., “Characteristics and pathway of pulp mill wastewater treatment using a combination of uni- naphthalene degradation by pseudomonas sp. N7,” Journal of form design and response surface methodology,” Water Re- Environmental Science, vol. 29, no. 3, pp. 756–762, 2008. search, vol. 45, no. 17, pp. 5633–5640, 2011. [7] F. Musat, A. Galushko, J. Jacob et al., “Anaerobic degradation [22] I. H. Cho and K. D. Zoh, “Photocatalytic degradation of azo of naphthalene and 2-methylnaphthalene by strains of marine dye (Reactive Red 120) in TiO2/UV system: optimization and sulfate-reducing bacteria,” Environmental Microbiology, vol. modeling using a response surface methodology (RSM) based 11, no. 1, pp. 209–219, 2009. on the central composite design,” Dyes and Pigments, vol. 75, [8] J. Dou, X. Liu, and A. Ding, “Anaerobic degradation of no. 3, pp. 533–543, 2007. naphthalene by the mixed bacteria under nitrate reducing con- [23] A. F. Caliman, C. Cojocaru, A. Antoniadis, and I. Poulios, ditions,” Journal of Hazardous Materials, vol. 165, no. 1–3, pp. “Optimized photocatalytic degradation of Alcian Blue 8 GX in 325–331, 2009. the presence of TiO2 suspensions,” Journal of Hazardous Mat- [9] M. Safinowski and R. U. Meckenstock, “Methylation is the erials, vol. 144, no. 1-2, pp. 265–273, 2007. initial reaction in anaerobic naphthalene degradation by a [24] J. F. Fu, Y. Q. Zhao, X. D. Xue, W. C. Li, and A. O. Babatunde, sulfate-reducing enrichment culture,” Environmental Microbi- “Multivariate-parameter optimization of acid blue-7 wastew- ology, vol. 8, no. 2, pp. 347–352, 2006. ater treatment by Ti/TiO photoelectrocatalysis via the Box- [10] W. J. Cooper, M. G. Nickelsen, R. V. Green, and S. P. Mezyk, 2 Behnken design,” Desalination, vol. 243, no. 1–3, pp. 42–51, “The removal of naphthalene from aqueous solutions using 2009. high-energy electron beam irradiation,” Radiation Physics and [25] A. R. Khataee, M. Zarei, and S. K. Asl, “Photocatalytic treat- Chemistry, vol. 65, no. 4-5, pp. 571–577, 2002. ment of a dye solution using immobilized TiO 2 nanoparticles [11] R. K. Goel, J. R. V. Flora, and J. Ferry, “Mechanisms for combined with photoelectro-Fenton process: optimization of naphthalene removal during electrolytic aeration,” Water operational parameters,” Journal of Electroanalytical Chem- Research, vol. 37, no. 4, pp. 891–901, 2003. istry, vol. 648, no. 2, pp. 143–150, 2010. [12] E. Pramauro, A. B. Prevot, M. Vincenti, and R. Gamberini, [26] M. Fathinia, A. R. Khataee, M. Zarei, and S. Aber, “Compara- “Photocatalytic degradation of naphthalene in aqueous TiO2 ff tive photocatalytic degradation of two dyes on immobilized dispersions: e ect of non ionic surfactants,” Chemosphere, vol. ff 36, no. 7, pp. 1523–1542, 1998. TiO 2 nanoparticles: e ect of dye molecular structure and [13] S. Dass, M. Muneer, and K. R. Gopidas, “Photocatalytic degra- response surface approach,” Journal of Molecular Catalysis A, dation of wastewater pollutants. Titanium-dioxide-mediated vol. 333, no. 1-2, pp. 73–84, 2010. oxidation of polynuclear aromatic hydrocarbons,” Journal of [27] L. Y. Wang, H. X. Wang, A. J. Wang, and M. Liu, “Surface mod- Photochemistry and Photobiology A, vol. 77, no. 1, pp. 83–88, ification of a magnetic SiO2 support and immobilization of a 1994. nano-TiO2 photocatalyst on it,” Chinese Journal of Catalysis, [14] L. Hykrdova,´ J. Jirkovsky,´ G. Mailhot, and M. Bolte, “Fe(III) vol. 30, no. 9, pp. 939–944, 2009. photoinduced and Q-TiO2 photocatalysed degradation of [28] S. L. Akhnazarova and V. V. Kafarov, Experiment Optimization naphthalene: comparison of kinetics and proposal of mecha- in Chemistry and Chemical Engineering, Mir Publishers, Mos- nism,” Journal of Photochemistry and Photobiology A, vol. 151, cow, Russia, 1982. no. 1–3, pp. 181–193, 2002. [29] H. G. Neddermeijer, G. J. van Oortmarssen, N. Piersma, and [15] S. V. Mohan, T. Kisa, T. Ohkuma, R. A. Kanaly, and Y. R. Dekker, “Framework for Response Surface Methodology for Shimizu, “Bioremediation technologies for treatment of PAH- simulation optimization,” in Proceedings of the 32nd Winter contaminated soil and strategies to enhance process efficien- Simulation Conference Proceedings, pp. 129–136, Orlando, Fla, cy,” Reviews in Environmental Science and Biotechnology, vol. USA, December 2000. 5, no. 4, pp. 347–374, 2006. [30] E. Pramauro, A. B. Prevot, M. Vincenti, and R. Gamberini, [16] M. Lindner, J. Theurich, and D. W. Bahnemann, “Photocat- “Photocatalytic degradation of naphthalene in aqueous TiO2 alytic degradation of organic compounds: accelerating the dispersions: effect of non ionic surfactants,” Chemosphere, vol. process efficiency,” Water Science and Technology, vol. 35, no. 36, no. 7, pp. 1523–1542, 1998. 4, pp. 79–86, 1997. [31] M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. Bahne- [17] T. Ohno, K. Tokieda, S. Higashida, and M. Matsumura, mann, “Environmental applications of semiconductor photo- “Synergism between rutile and anatase TiO2 particles in catalysis,” Chemical Reviews, vol. 95, no. 1, pp. 69–96, 1995. International Journal of Photoenergy 9

[32] N. Jaﬀrezic-Renault, P. Pichat, A. Foissy, and R. Mercier, “Study of the eﬀect of deposited platinum particles on the surface charge of titania aqueous suspensions by potentiom- etry, electrophoresis, and labeled-ion adsorption,” Journal of Physical Chemistry, vol. 90, pp. 2733–2738, 1986. [33] D. Duonghong, J. Ramsden, and M. Gratzel,¨ “Dynamics of interfacial electron-transfer processes in colloidal semiconductor systems,” Journal of the American Chemical Society, vol. 104, no. 11, pp. 2977–2985, 1982. [34] R. H. Myers, D. C. Montgomery, and C. M. Anderson- Cook, Response Surface Methodology: Process and Product Opti- mization Using Designed E xperiments, John Wiley & Sons, Ho- boken, NJ, USA, 2002. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 859294, 12 pages doi:10.1155/2012/859294

Research Article Photoetching of Immobilized TiO2-ENR50-PVC Composite for Improved Photocatalytic Activity

M.A.Nawi,Y.S.Ngoh,andS.M.Zain

School of Chemical Sciences, Universiti Sains Malaysia, Minden 11800, Malaysia

Correspondence should be addressed to M. A. Nawi, [email protected]

Received 23 February 2012; Accepted 26 March 2012

Academic Editor: Stephane´ Jobic

Copyright © 2012 M. A. Nawi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Commercially acquired TiO2 photocatalyst (99% anatase) powder was mixed with epoxidized natural rubber-50 (ENR50)/polyvinyl chloride (PVC) blend by ultrasonication and immobilized onto glass plates as TiO2-ENR50-PVC composite via a dip-coating method. Photoetching of the immobilized TiO2-ENR50-PVC composite was investigated under the irradiation of a 45 W compact ﬂuorescent lamp and characterized by chemical oxygen demand (COD) analysis, scanning electron microscopy-energy dispersive X-ray (SEM-EDX) spectrometry, thermogravimetry analysis (TGA), and fourier transform infrared (FTIR) spectroscopy. The BET surface area of the photoetched TiO2 composite was observed to be larger than the original TiO2 powder due to the systematic removal of ENR50 while PVC was retained within the composite. It also exhibited better photocatalytic eﬃciency than the TiO2 powder in a slurry mode and was highly reproducible and reusable. More than 98% of MB removal was consistently achieved for 10 repeated runs of the photo-etched photocatalyst system. About 93% of the 20 mg L−1 MB was mineralized over a period of 480 min. 2− − − The presence of SO4 ,NO3 ,andCl anions was detected in the mineralized solution where the solution pH was reduced from 7 to 4.

1. Introduction major challenge in industrializing this technology seems to lay in the effective immobilization of photocatalyst on solid Semiconductor TiO2 has been broadly studied as a hetero- support without decreasing its photocatalytic activity. geneous photocatalyst for the decomposition of hazardous The use of rubber-related polymer for the immobiliza- ffl compounds in water e uents, including organic dyes [1– tion of TiO2 powder has been explored by a number of ff 3]. TiO2 is highly e ective for producing oxidizing species, researchers. Jin et al. [8]formedTiO2 layer on a natural specifically HO• radicals, which possess significant oxidation rubber substrate via liquid phase deposition while Sriwong potential. Other advantages of TiO2 include its biological et al. [9]incorporatedTiO2 powder into rubber sheet and chemical stability, nontoxicity, low cost, and availability inordertoimmobilizeTiO2 powder. As for Silva et al. [3]. However, as the use of this catalyst for photocatalytic [10], they compounded silicone rubbers with TiO2 for the reactions is usually carried out in slurry modes, the need for purpose of immobilizing TiO2. While most of these works posttreatment or filtration step makes its practical applica- reported good photocatalytic activities in removing their tions tedious and costly. Therefore, effective immobilization respective pollutants, no efforts have been made to monitor technique of the catalyst for industrial scale application is what happened to the rubber additive during irradiation highly needed. However, immobilization of the catalyst onto of their photocatalytic systems. In other works, Nawi et solid supports poses its own intrinsic problems. Due to the al. [11, 12] had previously reported the use of ENR50 and fixed surface area, the photocatalytic activity of the immobi- ENR50/phenol formaldehyde (PF) blend, respectively, for lized catalyst is often significantly reduced [4–6]. Some other the immobilization of TiO2 powder over aluminum using inherent problems created by immobilization of the photo- electrophoretic deposition and dip-coating method on glass catalyst are potential loss of TiO2 and decreased adsorption plates for the removal of phenol. As reported by Nawi et al. of organic substances on the TiO2 surface [7]. Thus, the [12], the ENR additive was highly degradable and could be 2 International Journal of Photoenergy photocatalytically photoetched out and served as the pre- water. For chemical oxygen demand (COD) analysis, digester cursor for the formation of pores and also once completely and COD reagent solutions were purchased from HACH. removed from the immobilized photocatalyst could increase the surface area, pore volume of the immobilized P-25 TiO2 2.2. Preparation of Immobilized TiO -ENR -PVC Composite particles. 2 50

Polymer blends have become a subject of interest in the 2.2.1. Preparation of TiO2-ENR50-PVC Dip-Coating Formu- field of polymeric materials since their individual properties lation. The TiO2-ENR50-PVC dip-coating formulation was can be modified to obtain desired new properties. Polymer prepared by dissolving 0.80 g of PVC powder in 35 mL blending is known to improve mechanical, environmental, of dichloromethane and sonicated using ultrasonic cleaner and rheological properties of the polymers. One widely (Crest Ultrasonics, 50 kHz) for 1 h. Then, 2 g of 12% (w/w) studied polymer blend is epoxidized natural rubber-50 ENR50 solutionand65mLofacetonewereaddedbefore (ENR50)/poly(vinyl) chloride (PVC), which is found to adding 12 g of TiO2 powder. The mixture was then homoge- form miscible blends at any compositions ratio [13]. The nized via ultrasonication for 5 h at 30–40◦C. The prepared excellent miscibility between ENR50 and PVC is believed to formulation was kept in an amber bottle to avoid the be induced by the highly polar epoxide groups within the exposure of the formulation to light. ENR50 molecules [14]. PVC is also anticipated to provide high tensile strength and good chemical resistance while ENR 2.2.2. Deposition of TiO -ENR -PVC Composite onto Glass can act as a plasticizer for PVC [15]. In fact, the mechanical 2 50 Plates. For coating the TiO -ENR -PVC composite onto properties of ENR /PVC have been widely studied and 2 50 50 the glass plates, the formulation was poured into a custom reported in the literatures [13, 16]. made coating cell. The dip-coating process was then done by The main objective of this work was to prepare a highly immersing a preweighed cleaned glass plate with the dimen- reusable immobilized TiO -ENR -PVC composite having 2 50 sion of 4.0 cm × 7.5 cm into the dip-coating formulation comparable or better photocatalytic activity than the catalyst with uniform pulling rates. Before weighing the coated glass powder applied in a slurry mode. In order to achieve this ◦ plate, it was left to dry in an oven at 50 Cfor5minutes objective, the surface area of the immobilized catalyst com- in order to allow the evaporation of the solvents. The pro- posite would be improved via photoetching of the polymer cess of coating-drying-weighing was done repeatedly until additive under light irradiation. The photo-etched catalyst the desired amount of TiO -ENR -PVC composite was composite would also be systematically characterized in 2 50 deposited onto the glass plate. The TiO2-ENR50-PVC com- order to understand the surface transformation process that − posite loading used throughout this work was 1.00 mg cm 2. occurred during the etching process. We also evaluated the The optimum composition of the dip-coating formu- photomineralization capability of the prepared immobilized lation (i.e., the amount of ENR solution, PVC powder TiO composite by using methylene blue (MB) as the 50 2 and TiO powder) was obtained upon systematically varying model pollutant. The reusability and reproducibility of the 2 each component of the formulation for optimum photocat- TiO composite in the degradation of MB were also tested. 2 alytic activity and acceptable adhesion of the photocatalyst Therefore, this method was developed as a new approach in composite onto the glass plates. The adhesion test of the producing immobilized TiO photocatalyst with comparable 2 series of immobilized TiO composites with different ratio or better photocatalytic efficiency than the slurry mode but 2 of ENR and PVC was carried out using a sonication test. In with long-term reusability and without the cumbersome 50 this test, the respective glass plates coated with TiO -ENR - filtration step of the treated water. 2 50 PVC composite were first immersed in a beaker filled with ultrapure water and then were subjected to an interval of 5 s sonication in an ultrasonic cleaner until 30 s. After each 2. Materials and Methods successive intermittent sonication, the remaining coating that still adhered onto the glass plate was dried and weighed. 2.1. Chemicals. All chemicals were of analytical grades and The glass plate with the largest remaining TiO2-ENR50-PVC used without further purification. Titanium (IV) oxide composite coating would be considered as having the highest (TiO2) (99% anatase) was purchased from Sigma Aldrich. degree of adhesion. The adhesion of the TiO2-ENR50-PVC Epoxidized natural rubber (50% epoxidation) (ENR50)was composite with different amounts of ENR50 and PVC was from Guthrie Group Limited. The 12% (w/w) ENR50 then compared against the reference plate, which was made solution was prepared by refluxing 25.80±0.05 g solid ENR50 ◦ of TiO2-ENR50 or TiO2-PVC in order to find out their rel- in 250 mL toluene (from BDH AnalaR) at 88–90 Covera ativestrength.Therelativestrengthvalueswereobtainedby period of 80 h until it was completely dissolved. Poly (vinyl) manipulating the following equation (1): chloride (PVC) powder was purchased from Petrochemicals (Malaysia) Sdn. Bhd. The K value of the PVC powder was 67. SP − RP Acetone was obtained from Systerm and dichloromethane × 100 = relative strength (%), (1) CPi was a product from R&M Chemicals. Methylene blue (MB) (∼98%, Colour Index Number: 52015, chemical formula: where SP is the remaining weight of the TiO2-ENR50-PVC C16H18ClN3S·2H2O, λmax: 661 nm) was purchased from composite after 30 s of sonication. RP is the reference plate −1 Unilab, Ajax Chemicals. The 1000 mg L MB stock solution that could be either TiO2-ENR50 or TiO2-PVC composite was prepared by dissolving 1.00 g MB powder in 1 L ultrapure also after 30 s of sonication, respectively, while CPi is International Journal of Photoenergy 3

the initial weight of control plates that could be TiO2-ENR50- (b) PVC, TiO2-ENR50,orTiO2-PVC plate where each of them was controlled to have a similar weight value. (a) (c)

2.3. Photo-Etching of the TiO2-ENR50-PVC Composite and Its Characterizations. Chemical oxygen demand (COD) test was (g) carried out to detect the photodegradation of ENR50/PVC within the TiO2 composite. In this test, the glass plate with (d) immobilized TiO2 composite was immersed in ultra pure water in a glass cell and irradiated under a 45 W household fluorescent lamp (see Figure 1 for the reactor design). The (e) COD analyses of the ultrapure water samples after predefined irradiation times were determined by refluxing the treated (f) solution in a COD digester (model 45600-02, HACH). Mean- while, the changes in the morphology of the photo-etched TiO2 composite were observed using a scanning electron microscope (SEM, model LEO SUPRA 50 VP Field Emis- sion) while energy dispersive X-ray (EDX) and CHN ana- Figure 1: The experimental setup for photocatalytic studies consists lyzer were used to determine the element composition of the of (a) 45 W fluorescent lamp, (b) aeration supply from aquarium photo-etched TiO composites. pump, (c) custom made glass cell, (d) model pollutant, (e) 2 immobilized TiO composite, (f) scissor jack, and (g) power supply. The photocatalytic degradation of the polymer blends 2 within the TiO2 composite was further investigated by sub- jecting the non-etched and the photo-etched TiO2 composite to a thermogravimetry analyzer (TGA, STARe model Mettler Adsorption study and photolysis experiment were done in Toledo) in order to detect the remaining polymer content of a similar manner except that the cell reactor was placed in the composites before and after irradiation. For comparison a box and without the presence of the catalyst. For the purpose of performance comparisons, photocatalytic evaluation purposes, ENR50 and PVC were also, respectively, analyzed involving TiO slurry mode was carried out using a similar with TGA. The TGA analysis was performed in N2 atmo- 2 sphere at a heating rate of 10◦Cmin−1 from 30◦C to 800◦C. experimental set-up except that the TiO2 powder was Fourier transform infrared (FTIR, Perkin Elmer FTIR System employed and the degraded MB solution was filtered using Model 2000) spectrometry was also carried out from 400– 0.20 μm nylon filter to separate the catalyst particles at every 4000 cm−1 to see the alteration of the functional groups of interval of 15 minutes before its absorbance reading was taken. the TiO2 composites after subsequent irradiation. The KBr pellets for IR analysis were prepared using well-dried samples For the reproducibility and reusability study of the by leaving the samples in an oven at 60◦Cfor5days. photo-etched immobilized TiO2-ENR50-PVC composite, the immobilized photocatalyst composite was subjected for 10 repeated cycles of MB degradation where each cycle took 2.4. Photocatalytic Degradation of MB. Photocatalytic activ- 90 min. The percentage of MB that remained at each interval ity of the photoetched immobilized TiO2-ENR50-PVC com- was calculated according to the following equation (2): posite was investigated using MB solution as the model pollutant and by employing the reactor shown in Figure 1. Co − Ct MB remained (%) = ∗ 100, (2) A 45 W compact fluorescent lamp with a UV leakage Co −2 irradiance of 4.35 Wm was placed in contact with the outer −1 −1 surface of the (5.0 cm × 8.0 cm × 1.0 cm) glass reactor cell where Co (mg L )andCt(mgL ) were concentrations of containing 20 mL of 12 mg L−1 MB solution. The UV leakage MB before and after treatment at time t,respectively.The irradiance of the light source was measured using a Solar degradation of MB by the photo-etched TiO2-ENR50-PVC light co. PMA 2100 radiometer connected with UV-A composite was best fitted by pseudo-first-order kinetics. The related kinetic model is shown in: and UV-B broadband detector (PMA 2107). The photo- etched immobilized TiO2-ENR50-PVC composite was placed Co ln = kt, (3) uprightly inside the photoreactor cell and was exposed Ct directly to the light source. The MB solution was bubbled with air supplied by an aquarium pump and was maintained where Co and Ct are the initial MB concentration and the at a flow rate of 40 mL min−1 throughout the photocatalytic concentration at time t (min), respectively, t is the irradiation treatment using a Gilmont direct reading flow meter. time (min), and k is the pseudo-first-order rate constant −1 The photocatalytic efficiency of the photo-etched immo- (min ). bilized TiO2-ENR50-PVC composite was assessed by monitoring the degradation of the MB solution at every interval 2.5. Photocatalytic Mineralization of MB. The photocat- of 15 min for 90 min using the UV-Vis spectrophotometer alytic mineralization of 20 mg L−1 MB by the photo-etched (HACH model DR2000) at the wavelength of 661 nm. immobilized TiO2-ENR50-PVC composite was studied over 4 International Journal of Photoenergy

2− − − a period of 8 h and the COD, SO4 ,NO3 ,andCl 90 concentrations as well as the change of solution pH in the treated MB were noted hourly. The COD analyses of 80 the MB solution were carried as elaborated in Section 2.3 whereby the COD digester and water samples were reﬂuxed. 70 The photomineralization eﬃciency of the photo-etched 60

immobilized TiO2-ENR50-PVC composite was calculated ) 1 −

according to the following equation: L

50 − COD COD (mg i f COD remained (%) = ∗ 100, (4) 40 CODi COD −1 −1 where CODi (mg L )andCODf (mg L )werecon- 30 centrations of COD in solution before and after hourly photo-mineralization. The generated inorganic anions were 20 detected using ion chromatograph (IC, Metrohm model 792 Basic IC). A pH meter (Orion) was used to monitor the 10 changes in pH of the treated MB solutions. 0 12345678910 Time (h) 3. Results and Discussion Figure 2: COD concentration of water sample with photo-etched TiO -ENR -PVC composite over the span of 10 hours. 3.1. TiO2-ENR50-PVC Dip-Coating Formulation. Results 2 50 from the optimization study (see Table 1) indicated that although the degree of adhesion of the composite onto the support increased proportionately with the amount of either L−1 and it subsequently decreased over the evaluated period polymer, the trend showed that addition of beyond 0.8 g until it became essentially zero. As the polymer blend was PVC and 2 g ENR50 reduced its photocatalytic efficiency. As subsequently photo-etched out, the organic residual within will be discussed later in Section 3.3, this was due to the the composite decreased and since the exposed water was extensive coverage of the polymer upon the surface of the changed every hour, its COD concentration continued to photocatalyst which inadvertently lowered its photocatalytic decrease until negligible leaching of DOM was observed activity.Apparently0.8gPVCand2gof12%ENR50 solution beyond the 8th hour of irradiation. It can be assumed from were the optimum polymeric loadings since this blend pro- Figure 2 that the etching process was completed in the 9th vided the optimum pseudo-first-order rate constant values hour of irradiation. for the degradation of MB and acceptable relative adhesion ± strength. As 2 g of ENR50 solution was equivalent to 0.24 3.3. Characterization of the Photo-Etched Immobilized TiO2- 0.05 g solid ENR50, the ratio of the optimum mixture of TiO2, ENR50-PVC Composite. The surface morphology of TiO2 PVC, and ENR50 in the dip-coating formulation in term of powder as shown by the SEM micrograph in Figure 3(a) was solidweightwas50:3:1. made up of evenly distributed particles with no apparent signs of agglomerations. The particles were held closely 3.2. Photo-Etching of the Immobilized TiO2-ENR50-PVC Com- to one another, giving its surface a smooth-like texture. posite. Many polymers are not resistant towards light and The SEM micrographs of the TiO2-ENR50-PVC composites oxidizing environment. In the presence of light, they tend to before and after etching process are shown in Figures 3(b) undergo photolysis and degrade. Furthermore, it has been and 3(c) while their related elemental analyses results are reported that TiO2 particles have significant photocatalytic provided in Table 2, respectively. The micrograph of the non- effects on polymers such as polyethylene and polyvinyl etched TiO2-ENR50-PVC composite showed in Figure 3(b) chloride due to the generation of highly oxidizing species exhibits extensive aggregations of TiO2 particles with irreg- under UV light [17, 18]. Therefore, ENR50/PVC polymers ular emergence of pores with visible depths. The catalyst can be systematically photoetched out to modify the sur- particles were heavily agglomerated and are seen to be face of the immobilized catalyst. This etching process was covered by whitish layers which are believed to be the poly- expected to enhance the surface area of the immobilized mer blends. As seen in Figure 3(c), the photo-etched TiO2- catalyst composite. The etching process was evaluated by ENR50-PVC composite via irradiation for 9 h clearly shows irradiating the immobilized composite in ultrapure water for the elimination of the sticky whitish layers that enveloped a predefined time and the subsequent COD concentration the TiO2 particles even though aggregations of TiO2 particles of the water was monitored. The detected COD values in and porous depths are still observed. the treated water indicated that the organic polymers were The EDX elemental analyses results of the composites in degraded to produce dissolved organic matter (DOM). Table 2 show that the amount of carbon dropped drastically Figure 2 shows that upon one hour of irradiation of the from 9.14 ± 0.87 to 1.50 ± 0.18% after the photo-etching immobilized TiO2/ENR50/PVC composite, the COD con- process. The amount of Cl element detected within the centration of the water sample was found to be 73 ± 3mg photo-etched composite also decreased significantly. These International Journal of Photoenergy 5

Table 1: (a) Pseudo-first-order rate constants for the adsorption and photocatalytic degradation of 12 mg L−1 MB in aqueous solution by TiO2-5 g ENR50-PVC formulations with different amount of PVC and (b) TiO2-ENR50-0.8 g PVC with different amount of ENR50 as well as their respective relative adhesion strength upon the glass plates. TiO2 powder was maintained at 12 g.

Pseudo-ﬁrst-order rate constants (min−1) PVC (g) ENR sol. (g) Relative strength (%) Adsorption Photocatalysis (a) 0.0 5 0.0049 0.0180 0.00 0.1 5 0.0047 0.0165 — 0.2 5 0.0048 0.0176 5.97 0.3 5 0.0050 0.0180 — 0.4 5 0.0054 0.0184 12.62 0.5 5 0.0055 0.0186 — 0.6 5 0.0056 0.0189 15.75 0.7 5 0.0064 0.0198 — 0.8 5 0.0068 0.0205 29.86 0.9 5 0.0034 0.0172 — 1.0 5 0.0028 0.0094 20.50 (b) 0.8 0.0 0.0045 0.0207 0.00 0.8 1.0 0.0048 0.0208 15.21 0.8 2.0 0.0049 0.0276 18.21 0.8 3.0 0.0045 0.0189 22.54 0.8 4.0 0.0052 0.0185 25.26 0.8 5.0 0.0068 0.0205 29.86

Table 2: (a) EDX and (b) CHN analyses of the nonetched TiO2-ENR50-PVC composite and the photoetched TiO2-ENR50-PVC composites.

Weight (%) Sample Carbon Hydrogen Oxygen Titanium Chloride (a) EDX

TiO2 composite 9.14 ± 0.87 — 40.20 ± 2.10 49.93 ± 3.54 1.04 ± 0.14 Etched TiO 2 1.50 ± 0.18 — 40.06 ± 0.60 58.20 ± 0.74 0.25 ± 0.04 composite (b) CHN

TiO2 composite 4.61 0.51 Etched 3 h 3.22 0.09 Etched 6 h 3.15 0.15 Etched 9 h 1.39 0.01 Etched 10 h 1.20 0.05

results further conﬁrmed that the ENR50/PVC blends within Figure 4 illustrates the TGA and DTG curves of ENR50, the TiO2 composite were degraded to a certain degree by PVC and TiO2-ENR50-PVC composites before and after the applied light source. These results support the COD photo-etching. The corresponding thermogravimetric data analyses obtained earlier in Section 3.2. A detailed stepwise is provided in Supplementary Table 1 (in Supplementary monitoring of the etching process of the immobilized TiO2- Material available online at doi: 10.1155/2012/859294). The ENR50-PVC composite via CHN analysis is provided in (b) degradation of ENR50 shown in Figure 4(a) occurred in one of Table 2. The value of C element decreased with time of stage with the maximum weight loss temperature (Tmax)at irradiation indicating the continuous removal of the polymer 400◦C. The weight loss was determined to be 98.56%. The blend. It can be inferred from here that the etching process degradation of ENR50 was initiated by the fragmentation of was completed after 9 h of irradiation since the value of C polyisoprene chains and the epoxy polar groups, producing element remained more or less constant beyond this time volatile components such as isoprene and dipentene [14]. which was once again in good agreement with the previous On the other hand, Figure 4(b) shows that the degradation EDX and COD analyses. of PVC occurred in two stages. The ﬁrst stage occurred at 6 International Journal of Photoenergy

(a) (b)

(c)

Figure 3: SEM micrographs of (a) TiO2 powder, (b) nonetched TiO2-ENR50-PVC composite, and (c) photo-etched TiO2-ENR50-PVC composite under 10,000 x magniﬁcation.

◦ ◦ 230–350 CwithTmax at 300 C while the second stage of The TGA and DTG curves of the photo-etched TiO2 compos- ◦ the PVC degradation happened at 380−500 CwithTmax at ite are shown in Figure 4(d). It exhibits only one degradation ◦ ◦ 450 C.The first stage of the degradation process corresponds stage with Tmax at 300 C. This can be ascribed to the to the elimination of HCl molecules from the polyene chains dehydrochlorination of the PVC. In short, after the etching whereas the second dominant stage refers to the degradation process of the TiO2 composite, it may be deduced that PVC is of the polyene structure, yielding volatile aromatic and still predominantly present within the TiO2 composites. This aliphatic compounds from the conjugated sequences [19]. is in line with Table 2 whereby elemental C was detected even It was also observed from the DTG curve of PVC, that after 10 h of irradiation and this could be due to the presence there was a shoulder peak after the first stage, indicating the of PVC within the composite as supported by the detection pyrolysis of the HCl residuals of PVC [19]. This peak became of Cl within the composite. more obvious in the DTG curve of the TiO2-ENR50-PVC The FTIR spectra of TiO2 powder, TiO2-ENR50, TiO2- composite shown in Figure 4(c) where three degradation PVC, TiO2-ENR50-PVC composite and the photo-etched stages of the composite were observed. The first and second TiO2-ENR50-PVC composite are shown in Figures 5(a)–5(e), degradation stage can be associated with the dehydrochlo- respectively. The presence of ENR50 and PVC within the TiO2 rination of PVC while the last degradation stage was due composite were qualitatively identified by comparing the to the degradation of both the ENR50 and PVC. Nair et FT-IR spectra of TiO2-ENR50 (Figure 5(b)) and TiO2-PVC al. [19] observed two dominant degradation stages during (Figure 5(c)) with TiO2 powder (Figure 5(a)) and TiO2- the TGA analysis of polymer blend ENR50/PVC where they ENR50-PVC composite (Figure 5(d)). The bands attributed attributed the first degradation stage to PVC and the second to the presence of ENR50 in the composite are confirmed by degradation stage to the decomposition of ENR50 and PVC. the C–H stretching vibrations of the polymer main chains International Journal of Photoenergy 7

100 0 100 0 90 90 −0.02 −0.02

80 )

80 ) 1 1 − −0.04 − −0.04 70 70 min min

Degradation of PVC (%) (%)

−0.06 −0.06 60 (stage 2) (mg

60 (mg

wt. wt.

Weight 50 Weight 50 −0.08 −0.08 Deriv. 40 Deriv. 40 −0.1 −0.1 30 30 Degradation of ENR50 − −0.12 0.12 20 20 Degradation of PVC (stage 1) 10 10 100 150 200 250 300 350 400 450 500 550 100 150 200 250 300 350 400 450 500 550 Temperature (◦C) Temperature (◦C) (a) (b)

100 0 100 0 90 Degradation of 90 − − ENR50 and 0.02 0.02

80 PVC ) 80 Degradation of PVC (stage 1) ) 1 1 −0.04 − −0.04 − 70 70 min min

(%) (%) −0.06 −0.06

60 (mg 60 (mg

wt. wt.

Weight 50 −0.08 Weight 50 −0.08

40 Deriv. 40 Deriv. − − Degradation of PVC (stage 1) 0.1 0.1 30 30 −0.12 −0.12 20 20

10 10 100 150 200 250 300 350 400 450 500 550 100 150 200 250 300 350 400 450 500 550 Temperature (◦C) Temperature (◦C) (c) (d)

Figure 4: TGA and DTG curves of (a) ENR50, (b) PVC, (c) non-etched TiO2-ENR50-PVC composite, and (d) photo-etched TiO2-ENR50- PVC composite.

–CH2– (asymmetric) and –CH3, C–H bending vibrations in ENR50 was reduced. The photoetching of ENR50was also and the O–H stretching of hydroxyl groups at peak 2937, indicated by the absence of the C–H bending vibrations band 1378, and 3000–3500 cm−1, respectively. Meanwhile, the at 1376 cm−1. The photoetching of PVC in the irradiated −1 absorption bands at 1330, and 1257 cm show the deforma- immobilized TiO2-ENR50-PVC composite was also indicated tion of –CH2– and CH rocking vibration of the CH2Cl bonds by the reduction in the absorption band at 1331 and −1 only when PVC was included into the composite [14]. It can 1254 cm , signifying the destruction of CH2 deformation be concluded from Figure 5 that the spectrum of the TiO2- and CH2Cl bonds, respectively. Figure 5(e) also shows the ENR50-PVC composite is a combined spectra of the TiO2- appearance of two new absorption bands at 1192 and −1 ENR50 and TiO2-PVC. 1151 cm . These absorption bands may be attributed to After irradiation for 10 h, the photo-etched TiO2 com- the formation of aliphatic ether which suggests the cross- posite produced the IR spectrum shown in Figure 5(e). It linking interactions between the two polymers. It has been can be observed that the intensity of the absorption band reported that ENR50 and PVC are capable of forming self at 2900 cm−1 region which represents the C–H stretching cross-linkable blends. Ratnam and Zaman [16] also observed −1 vibrations of the polymer –CH2– (asymmetric) and –CH3 the emergence of two absorption bands at 1100 cm region 8 International Journal of Photoenergy

Table 3: Microstructures of TiO powder, nonetched TiO -ENR - (a) 2 2 50 PVC composite, and photoetched TiO2-ENR50-PVC composite.

(b) Samples BET surface Total pore Average pore 1378 1457 area (m2 g−1)volume(cm3 g−1) diameter (nm) 2956 (c) TiO2 powder 10.17 0.0237 9.348

(%) 1331 TiO2 T 1434 1254 5.44 0.0161 11.846 composite (d) Etched TiO2 1379 1257 15.24 0.0244 6.418 2937 1433 1330 composite 1264 (e) 1192 1434 1151 2923 1728 30 600 400 800 1200 4000 3600 3200 2800 2400 2000 1800 1600 1400 1000 25 − 1 STP) (cm ) at 1

− 20

Figure 5: FTIR spectra of (a) TiO2 powder, (b) TiO2/ENR50,(c) g

TiO2/PVC, (d) non-etched TiO2-ENR50-PVC composite, and (e) (cc photo-etched TiO2-ENR50-PVC composite. 15

adsorbed 10 when studying the effect of cross-linking agent on the 5 properties of ENR50/PVC. Thus, the exposure of the TiO2- Volume ENR50-PVC composite to irradiation and oxidizing condi- 0 tion did not only induce the degradation of the polymers 0 0.2 0.4 0.6 0.8 1 but also initiated the cross-linking of the polymers within the Relative pressure (P/P0) composite. The photocatalytic degradation of the polymer blend Figure 6: Nitrogen adsorption (coloured) and desorption (open) within the composite occurred due to the presence of highly isotherm of TiO2 powder (red square), non-etched TiO2-ENR50- oxidizing radicals. As the TiO particles of the composite PVC composite (green circle) and etched TiO2-ENR50-PVC com- 2 posite (blue diamond). are stimulated by UV irradiation from the light source, •− • • highly oxidizing radicals such as O2 ,HOO ,andHO were generated (5). These active oxygen species then induced the degradation reactions by attacking the neighbouring poly- pore volumes and surface area. It was also observed that merchainsorbydiffusingintothepolymermatrixtoform the non-etched TiO2-ENR50-PVC composite adsorbed the carbon-centered radicals. Their following reactions with the least nitrogen indicating that its surface area was the lowest. photogenerated oxidizing radicals lead to the production of The BET surface area, total pore volumes, and average pore oxidized species such as carbonyl intermediates [8, 17]and diameters are listed in Table 3. The pure TiO2 powder used 2 −1 eventually to CO2 and H2O: in this work has a small BET surface area (10.17 m g ), which was consistent with a literature value [20]. When −→ ∗ − + TiO2 + h TiO2 ecb +hvb ENR50/PVC polymer blend was added to form TiO2-ENR50- PVC composite, the surface area decreased significantly − −→ •− ecb +O2 O2 by almost half of the original value. This is consistent with the •− • − results of SEM whereby most of the non-etched immobilized O2 +H2O −→ HOO +OH TiO2-ENR50-PVC surface was covered with the polymer • (5) 2HOO −→ H2O2 +O2 blend. However, compared with pure TiO2, the surface areas of the photo-etched immobilized TiO -ENR -PVC −→ • 2 50 H2O2 + h 2HO composite were increased while its pore diameter decreased. + • These smaller pores possibly account for the larger surface hvb +H2O −→ HO area of the photo-etched immobilized TiO2 composite. This result amplifies the useful technique of using ENR50/PVC The nitrogen adsorption isotherms for the TiO2 powder, blend as the adhesive for immobilizing the TiO2 powder onto TiO2-ENR50-PVC composite and the photo-etched TiO2- solid supports such as glass plates since their subsequent ENR50-PVC, composite are provided in Figure 6. As can be seen, the isotherms for all cases resemble type II based on the etching process enhances the surface properties of the IUPAC system, which correlates with the nonporous solids. immobilized system. Their shape indicates that no significant differences in surface texture exist. However, isotherms for the photo-etched 3.3.1. Photocatalytic Degradation of MB Solution by the Photo- samples absorbed slightly more nitrogen at lower pressure Etched Immobilized TiO2-ENR50-PVC Composite. As shown than the TiO2 powder which is due to the presence of larger in Figure 7,only10.53 ± 1.15% of MB was photolysed within International Journal of Photoenergy 9

100 and sustainability of the photo-etched immobilized TiO2- ENR50-PVC composite in the photocatalytic degradation of 90 MB is shown in Figure 8. The photocatalytic efficiency of 80 TiO2 composite was consistent throughout the 10 subjected cycles whereby more than 98% of the MB was consistently 70 degraded from the first cycle until the tenth cycle of applications. The pseudo-first-order rate constants were also fairly (%)

60 consistent throughout the reuse of the immobilized photo- 50 catalyst plate whereby the average pseudo-ﬁrst-rate constant was 0.054±0.003 min−1. The cleaning-up process of the used remained

40 photo-etched immobilized TiO2 system was done by subject- MB ing the photocatalyst plate to 30 min of irradiation with light 30 in ultra pure water after each run in order to clear up the 20 accumulation of MB dye substrates and any intermediates. The photo-etched immobilized TiO2-ENR50-PVC composite 10 system therefore possesses good sustainability and reusability upon its recycled applications. It was also noted that the 0 0 153045607590 fabricated photocatalyst plate was essentially intact after 10 Time (min) cycles of applications and can certainly be reused for many more applications. Figure 7: Degradation of MB by photolysis (black circle), TiO2 powder (black square), non-etched TiO2-ENR50-PVC (white diamond), and photo-etched TiO2-ENR50-PVC composite (black diamond) via irradiation under 45 W compact fluorescent lamp with UV leakage of 4.35 Wm−2 (continous line) and adsorption in 3.3.3. Mineralization of MB by the Photo-Etched Immobi- the dark (dashed line). lized TiO2-ENR50-PVC Composite. Preliminary mineralization study of MB solution under photocatalytic treatment by TiO2 slurry mode and the photoetched immobilized TiO2 90 min in the absence of the photocatalyst while the adsorp- composite was carried out via COD analyses. The kinetics tion of MB by the photo-etched TiO2 composite achieved of the COD removal at different reaction times is shown in 35% removal. The photocatalytic degradation of MB by Figure 9. It was observed that in the course of 8 h of irradi- the non-etched immobilized TiO2 composite achieved 94% ation, the reduction of the COD concentration of 20 mg L−1 removal within 90 minutes of irradiation. However, about MB by TiO2 in the slurry mode achieved 99% as compared 99% of MB was degraded by the photo-etched immobilized to 95% when the photo-etched TiO2 composite system was TiO2 composite. In fact, the photocatalytic degradation of used. These values are not far apart which once again MB dye by the photo-etched immobilized TiO2 composite proved that immobilization of TiO2 via this technique did was found to be better than the TiO2 powder mode (Figure 7) not reduce much of the mineralization efficiency of the whereby 97% of the dye was degraded by that system. photocatalyst system with respect to its slurry counterpart. The pseudo-first-order rate constant calculated from the In fact, Figure 9 shows that COD reduction for photo-etched −1 linear plots based on (2) was 0.047 min for the photo- TiO2 composite was faster than the slurry system during the etched TiO2 composite. This value is slightly higher than first 4 h of irradiation but became slower than the slurry sys- −1 0.040 min , which is the pseudo-first-order rate constant tem onwards. This phenomenon can be due to the interfer- produced by the TiO2 slurry mode. A non-etched TiO2 com- ence from the formed intermediates that might get adsorbed posite system produced a pseudo-first-order rate constant and accumulate on the fixed surface of the catalyst and −1 of 0.036 min . This improved performance of the photo- reduced the efficiency of the catalyst. It was also observed that etched immobilized TiO2 system was definitely due to the the total decolourization of MB in both systems was achieved bigger BET surface area achieved during its preparation. in less than 4 h, as shown in Supplementary Figure 1. As As can be seen in Figure 7, the photo-etched photocatalyst expected, the decolourization of the dye occurred much system has a better adsorption behavior of MB as compared faster than the mineralization process since the former to the rest of the system. Since photocatalytic degradation of process only involved the breakdown of the chromophore the dye happens principally on the photocatalyst surface, the while the latter required conversion of all organics into CO2 adsorption of the dye molecules from aqueous solution on and H2O. TiO2 surface is very important. This would allow the photo- The photomineralization process also yielded anions ff 2− − − generated oxidizing radicals to react e ectively with the such as SO4 ,NO3 ,andCl as detected by ion chromatog- organic pollutants. It is suggested that the reasonably good raphy. The result, shown in Figure 10, depicts the concentra- adsorption capacity of the photo-etched TiO2 composite can tion of each of the anions found in the treated MB solution. be associated with its bigger surface area. In addition, the solution also became acidic with time indicating the production of H+ ions, as discussed by Lachheb 3.3.2. Reusability and Sustainability of the Photo-Etched et al. [21]. The solution pH decreased from pH 7 to 4 after the Immobilized TiO2-ENR50-PVC Composite. The reusability photocatalytic treatment. As suggested by Fabiyi and Skelton 10 International Journal of Photoenergy

100 0.07 12

90 ) 1

0.06 − 10 80 ) (min 1

− L

(%) 70 8 0.05 (mg

60 constant 6

50 0.04 rate

degradation

40 4 MB 0.03 30 Concentration Total 2 20 0.02 Pseudo-ﬁrst-order 10 0 Chloride Sulphate Nitrate 0 0.01 12345678910 Anions Number of cycle Figure 10: Concentration of anions chloride, sulphate, and nitrate found in MB solution after photocatalytic treatment by photo- Figure 8: Total percentage of MB solution degradation in each etched TiO -ENR -PVC composite (coloured) and TiO in slurry cycle up to 10 cycles of photocatalytic activity over the reused 2 50 2 mode (opened). photo-etched TiO2-ENR50-PVC composite (black square) and their corresponding pseudo-ﬁrst-order rate constants (black diamond).

100 2− containing intermediates that lead to the evolution of SO4 90 ions [23]. One possible pathway of this reaction is as presented by the following equations [23]: 80 − + • 70 OH +h −→ OH • 60 R–S+=R +OH −→ R-S(=O)–R +H+ (%)

• 50 NH2–C6H3(R)–S(=O)–C6H4-R + OH

COD (7) 40 −→ NH2–C6H4–R+SO2–C6H4–R • 30 SO2–C6H4–R + OH −→ R–C6H4–SO3H

20 • • 2− + R–C6H4–SO3H+OH −→ R–C6H4 +SO4 +2H 10 − Another product of the mineralized MB is NO3 ions. The 0 − NO3 ions are produced by the subsequent oxidization of 0123456789 + Time (h) NH4 ions which are the product of successive attacks by H+ atoms on nitrogen atoms in MB structure. The oxidation ﬃ + − Figure 9: Reduction e ciencies of COD by TiO2 powder (black of NH4 , leading to the formation of NO3 correlates with square) and photo-etched TiO2-ENR50-PVC composite (black the stable oxidation of nitrogen (+5) [21]. This formation of diamond). − NO3 is provided by the following equations [21]:

+ + NH2–C6H4–R + H −→ C6H4–R +NH3 [22] and Lachheb et al., [21], the stoichiometric equation of + −→ − + MB total mineralization is as follows (6): NH3 +2H2O+6h NO2 +7H (8)

− + − + 1 TiO ,Air, hv> 3.2eV NO +H O+2h −→ NO +2H C H N SCl + 25 O −−−−−−−−−−−→2 16CO +6H O 2 2 3 16 18 3 2 2 2 2 The carbonaceous components of the intermediates were + 3HNO3 +H2SO4 then continued to be mineralized eventually into CO2 and +HCl H2O. (6) 4. Conclusion The initial step of MB degradation can be ascribed to the attack on the R–S+=R functional group in MB by the The paper showed that a reusable photo-etched immobilized • − photogenerated OH radicals, producing sulfonyl or SO3 , TiO2-ENR50-PVC composite that possessed slightly better International Journal of Photoenergy 11

photocatalytic efficiency than the TiO2 slurry mode can be [6] G. Mascolo, R. Comparelli, M. L. Curri, G. Lovecchio, A. successfully prepared via a simple dip-coating method. This Lopez, and A. Agostiano, “Photocatalytic degradation of ffi low cost but effective method utilized a dip-coating emulsion methyl red by TiO2: comparison of the e ciency of immobilized nanoparticles versus conventional suspended catalyst,” prepared by mixing TiO2 powder with ENR50/PVC in mixed solvent system via ultrasonication before being immobilized Journal of Hazardous Materials, vol. 142, no. 1-2, pp. 130–137, 2007. on glass plates. Photodegradability of ENR50/PVC within the [7]K.V.S.Rao,M.Subrahmanyam,andP.Boule,“Immobilized TiO2 composite under the irradiation of the applied light source (45 W compact fluorescent lamp) allowed us to etch TiO2 photocatalyst during long-term use: decrease of its activity,” Applied Catalysis B, vol. 49, no. 4, pp. 239–249, 2004. out the polymer blend thus enhanced the surface properties [8] M. Jin, X. Zhang, A. V. Emeline, T. Numata, T. Murakami, of the system. It was observed that this technique produced and A. Fujishima, “Surface modification of natural rubber by a slightly bigger surface area than the initial TiO2 powder. TiO2 film,” Surface and Coatings Technology, vol. 202, no. 8, The photo-etched immobilized TiO2-ENR50-PVC composite pp. 1364–1370, 2008. performed at par or better than the TiO2 slurry mode system. [9] C. Sriwong, S. Wongnawa, and O. Patarapaiboolchai, “Pho- It was highly reusable with sustainable photocatalytic effi- tocatalytic activity of rubber sheet impregnated with TiO2 ciencies in the photocatalytic degradation of MB solution. particles and its recyclability,” Catalysis Communications, vol. The photomineralization capability of the photo-etched 9, no. 2, pp. 213–218, 2008. TiO2-ENR50-PVC composite was also comparable to the [10] V. P. Silva, M. P. Paschoalino, M. C. Gonçalves,M.I.Felisberti, TiO2 slurry mode system. Therefore, it can be said that the W. F. Jardim, and I. V. P. Yoshida, “Silicone rubbers filled with photo-etching technique is a potential new approach in fab- TiO2: characterization and photocatalytic activity,” Materials ricating immobilized TiO2 system without facing the usual Chemistry and Physics, vol. 113, no. 1, pp. 395–400, 2009. reduction in photocatalytic activity as commonly observed [11] M. A. Nawi, L. C. Kean, K. Tanaka, and M. S. Jab, “Fabrication for immobilized photocatalyst systems. Such efficient immo- of photocatalytic TiO2-epoxidized natural rubber on Al plate bilized photocatalyst system offers excellent advantage of use via electrophoretic deposition,” Applied Catalysis B, vol. 46, no. and reuse without the need to filter the treated water after 1, pp. 165–174, 2003. the treatment and can be easily adapted for continuous flow [12]M.A.Nawi,A.H.Jawad,S.Sabar,andW.S.W.Ngah,“Immo- reactor. bilized bilayer TiO2/chitosan system for the removal of phenol under irradiation by a 45 watt compact fluorescent lamp,” Desalination, vol. 280, no. 1–3, pp. 288–296, 2011. Acknowledgments [13] C. T. Ratnam and K. Zaman, “Stabilization of poly(vinyl chloride)/epoxidized natural rubber (PVC/ENR) blends,” Polymer The authors thank Ministry of Science and Technology Degradation and Stability, vol. 65, no. 1, pp. 99–105, 1999. (MOSTI) of Malaysia and Universiti Sains Malaysia (USM) [14] U. S. Ishiaku and Z. A. M. Ishak, “Developments in poly (vinyl for their generous financial assistance under Grant FRGS: chloride)/Epoxidized Natural Rubber Blends,” in Polymer 203/227/PKIMIA/67/027 and IRPA: 305/229/PKIMIA/ Blends and Alloys, G. O. Shonaike and G. P. Simon, Eds., 613402. The authors are also grateful to the Institute of Marcel Dekker, New York, NY, USA, 1999. Postgraduate Studies (IPS) USM for Fellowship Scheme and [15] M. C. S. Perera, U. S. Ishiaku, and Z. A. M. Ishak, “Thermal the facilities provided by the USM throughout this research. degradation of PVC/NBR and PVC/ENR50 binary blends and PVC/ENR50/NBR ternary blends studied by DMA and solid state NMR,” Polymer Degradation and Stability, vol. 68, no. 3, References pp. 393–402, 2000. [16] C. T. Ratnam and K. Zaman, “Modification of PVC/ENR [1] O. Prieto, J. Fermoso, Y. Nunez,˜ J. L. Del Valle, and R. Irusta, blend by electron beam irradiation: effect of crosslinking “Decolouration of textile dyes in wastewaters by photocatalysis agents,” Nuclear Instruments and Methods in Physics Research, with TiO2,” Solar Energy, vol. 79, no. 4, pp. 376–383, 2005. Section B, vol. 152, no. 2, pp. 335–342, 1999. [2] M. Saquib, M. Abu Tariq, M. Faisal, and M. Muneer, [17] S. Cho and W. Choi, “Solid-phase photocatalytic degradation “Photocatalytic degradation of two selected dye derivatives in of PVC-TiO polymer composites,” Journal of Photochemistry aqueous suspensions of titanium dioxide,” Desalination, vol. 2 and Photobiology A, vol. 143, no. 2-3, pp. 221–228, 2001. 219, no. 1–3, pp. 301–311, 2008. [18] S. S. Chin, T. M. Lim, K. Chiang, and A. G. Fane, “Factors [3] R. Jain and M. Shrivastava, “Photocatalytic removal of ff hazardous dye cyanosine from industrial waste using titanium a ecting the performance of a low-pressure submerged mem- dioxide,” Journal of Hazardous Materials, vol. 152, no. 1, pp. brane photocatalytic reactor,” Chemical Engineering Journal, 216–220, 2008. vol. 130, no. 1, pp. 53–63, 2007. [4] C. Guillard, H. Lachheb, A. Houas, M. Ksibi, E. Elaloui, and J. [19] M. N. Radhakrishnan Nair, G. V. Thomas, and M. R. M. Herrmann, “Influence of chemical structure of dyes, of pH Gopinathan Nair, “Thermogravimetric analysis of PVC/ELNR and of inorganic salts on their photocatalytic degradation by blends,” Polymer Degradation and Stability,vol.92,no.2,pp. 189–196, 2007. TiO2 comparison of the efficiency of powder and supported TiO2,” Journal of Photochemistry and Photobiology A, vol. 158, [20] K. Khuanmar, W. Wirojanagud, P. Kajitvichyanukuk, and S. no. 1, pp. 27–36, 2003. Maensiri, “Photocatalysis of phenolic compounds with syn- [5] C. M. Ling, A. R. Mohamed, and S. Bhatia, “Performance of thesized nanoparticles TiO2/Sn2,” Journal of Applied Sciences, photocatalytic reactors using immobilized TiO2 film for the vol. 7, no. 14, pp. 1968–1972, 2007. degradation of phenol and methylene blue dye present in [21] H. Lachheb, E. Puzenat, A. Houas et al., “Photocatalytic de- water stream,” Chemosphere, vol. 57, no. 7, pp. 547–554, 2004. gradation of various types of dyes (Alizarin S, Crocein Orange 12 International Journal of Photoenergy

G, Methyl Red, Congo Red, Methylene Blue) in water by UV- irradiated titania,” Applied Catalysis B, vol. 39, no. 1, pp. 75– 90, 2002. [22]M.E.FabiyiandR.L.Skelton,“Photocatalyticmineralisation of methylene blue using buoyant TiO2-coated polystyrene beads,” Journal of Photochemistry and Photobiology A, vol. 132, no. 1-2, pp. 121–128, 2000. [23] A. Houas, H. Lachheb, M. Ksibi, E. Elaloui, C. Guillard, and J. M. Herrmann, “Photocatalytic degradation pathway of methylene blue in water,” Applied Catalysis B, vol. 31, no. 2, pp. 145–157, 2001. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 368750, 10 pages doi:10.1155/2012/368750

Research Article Synthesis, Characterization, and Photocatalysis of Fe-Doped TiO2: A Combined Experimental and Theoretical Study

Liping Wen,1 Baoshun Liu,1, 2 Xiujian Zhao,1 Kazuya Nakata,2, 3 Taketoshi Murakami,2 and Akira Fujishima2, 3

1 State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan City, Hubei Province 430070, China 2 Photocatalyst Group, Kanagawa Academy of Science and Technology, KSP East 412, 3-2-1 Sakado, Takatsu-ku, Kawasaki, Kanagawa 213-0012, Japan 3 Division of Photocatalyst for Energy and Environment, Research Institute for Science and Technology, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan

Correspondence should be addressed to Baoshun Liu, [email protected] and Kazuya Nakata, [email protected]

Received 6 January 2012; Revised 25 February 2012; Accepted 27 February 2012

Academic Editor: Baibiao Huang

Copyright © 2012 Liping Wen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Fe-doped TiO2 was prepared by hydrothermal treating Ti peroxide sol with different amount of iron nitrate. Fe ions can enter 4+ TiO2 lattice by substituting Ti ions, which significantly affect the crystallinity and morphology of TiO2 nanoparticles. Fe doping also influences the UV-Vis absorption and photoluminescence of TiO2, due to the change of electronic structure. It is shown that Fe ions are more easily doped on TiO2 surface than in bulk. The theoretical computation based on the density functional theory (DFT) shows that the Fe ions in TiO2 bulk are localized and mainly act as the recombination centers of photoinduced electrons and holes. Some results support that the Fe3+ ions on surface can form intermediate interfacial transfer pathway for electrons and holes, which is beneficial for increasing the photocatalytic activity of TiO2. The photocatalytic activity first increases and then decreases as the Fe concentration increases, which is coaffected by the bulk-doped and surface-doped Fe ions.

1. Introduction It is always reported that the photocatalytic activity under UV light illumination will firstly increase and then Since the publishing of photocatalytic water splitting [1], decrease with the increase of doped Fe concentration [17, photocatalysis has drawn much attention in the world for 19, 20]. It is considered that the Fe ions can trap holes or many years [2–4]. Compared with other photocatalysts, electrons for low-level doping, while they will change to TiO2 is extensively studied because of low-cost, non-toxicity, recombination centers for high-level doping [17, 19, 22]. and high chemical-stability. Generally, pure TiO suffers 2 However, the physical reason that the doped Fe3+ ions mainly from problems of large band gap (E )[5, 6] and low g act as recombination centers is not well known. Although quantum yield. Doping is proved to be effective to narrow tunneling recombination is said to become significant for E and increases photocatalytic activity [7–14]. Transition- g high doping level, no experimental proofs can support metal-doped TiO2 have been widely studied due to their d electronic configuration [15–18]. Fe is the most studied this conclusion. The research 20 devoted to studying the because it has the radium identical to Ti4+ and the half-filled photocatalytic mechanism of transition-metal-doped Q- d electronic configuration [19–21]. It was reported that the TiO2 particles of 3-4 nm. In this case, Fe ions are mainly on photocatalytic activity of Fe-doped titanate nanotubes is 2 surface due to the small particle size, which can form effective times higher than P25 under visible light illumination [19], pathways for charge interfacial transfer. For example, Wang 3+ and the Fe-doped Q (quantum)-TiO2 particles can remove et al. reported that the Fe ions can combine with the chloroform under UV light illumination due to the trapping hydroxyl groups on TiO2 nanoparticle surface and form of photoinduced electrons and holes [20]. (OH)Fe3+ complexes, which is a good way for electron 2 International Journal of Photoenergy

2+ transfer [23]. Other ions, such as Cu ions, on TiO2 surface The surface composition and element chemical states of Fe- can also promote the electron transfer, and the mechanism doped samples were characterized by an X-ray photoelectron of multielectron transfer is suggested [24, 25]. Compared spectroscopy(XPS)inanESCAsystemwithmonochro- with the Q-particles, the specific surface area decreases for matic Mg kα source and a charge neutralizer. The particle large particles (>15 nm), and it is possible for the Fe ions morphologies ware observed with a JEM-2001 transmittance to enter TiO2 bulk beside surface. Since the carrier exciton electron microscope (TEM) at an acceleration voltage of Bohr radius in TiO2 is ca. 2 nm [20], the holes or electrons 200 kV. Diffused reflectance spectra were recorded on a Shi- trappedatFeionsinbulkaredifficult to migrate to surface madzu UV2550-UV spectrophotometer equipped with an before going to recombination. It is also reported that the integrating sphere assembly in the range of 190–700 nm, with optimal Fe-doped concentration is dependent on the particle BaSO4 used as the reference. The photoluminescence (PL) size [26], and the increase of particle sizes will result in the spectra were measured at room temperature with Shimadzu decrease of optimal concentration, which may be due to that RF-5301 fluorospectrophotometer by using 320 nm line of more Fe ions that enter the bulk for larger particles can Xe lamp as excitation source. The energy-dispersive X-ray increase the recombination of electrons and holes. It predicts (EDX) equipped on the JSM-7500F field emission scanning that the Fe ions on surface may increase the photocatalytic electron microscope was used to detect the real element activity, while the Fe ions in bulk will have negative effect. concentrations in the Fe-doped TiO2 samples. However, the different effect of bulk dopant and surface dopant on photocatalysis is not systematically investigated. 2.3. Computational Detail. The computational calculation In this work, we aim to discuss the different functions of has been performed within DFT plane wave pseudo potential Fe ions on surface and in bulk during photocatalysis under method. The general gradient approximation (GGA) with UV light illumination. It is found that the surface doping is the Perdew-Burke-Ernzerhof (PBE) functional and ultra- dominant for low-level doping, and more Fe ions will enter soft pseudopotentials was used to describe the exchange- TiO bulk with the increase of doping concentration. The 2 correlation effects and electron ion interactions. The kinetic ab initio computation based on density functional theory energetic cutoffs of 24 and 288 Ry for the smooth part (DFT) shows that the Fe ions in TiO bulk are localized, indi- 2 of electronic wave functions and augmented electron den- cating that they mainly act as the recombination centers of sity were used. Quantum-ESPRESSO code, Pwscf package electrons and holes. It is suggested that the role change of Fe [27], was used to perform ab initio quantum calculation. ions from trapping to recombination centers as the doped Fe Monkhorst-Pack (M-P) k-space sampling was adopted for concentration increases is related to the transformation from geometry optimization and density of the electronic states surface to bulk doping, and this transformation explains the (DOS) calculation. The convergence threshold for the self- photocatalytic behavior of Fe-doped TiO observed by us 2 consistency energy calculation was set to 10−8 Ry, and all and other researchers. atomic relaxations were carried out until all components of the residual forces were less than 10−3 Ry/Bohr. A 3 × 3 × 2. Experimental 1 supercell (i.e., 108 atoms of TiO2 anatase matrix) with two adjacent Fe ions and an oxygen vacancy between them 2.1. Sample Preparation. The TiO2 samples were synthesized was employed to simulate Fe-doped reduced anatase TiO2. using hydrothermal method. TiCl4 and Fe(NO3)3 were A2× 2 × 1 anatase supercell with one Fe ion substituting used as the precursors of TiO2 and Fe ions. 4.0 mL TiCl4 one titanium ion models the Fe-doped TiO2 without oxygen was added to 400 mL ice distilled water under vigorous vacancy. A 2 × 2 × 1supercellwithoneoxygenvacancywas stirring, and then, the resulted solution was stirred for constructed to model the reduced anatase. In addition, pure 24 h at room temperature. Subsequently, ammonia aqueous TiO2 unit cell was also calculated for comparison. 4 × 4 × solution (1 : 9) was added to adjust the solution pH to 3and2× 2 × 3 M-P grids for 48 supercell and 108 atom ca. 7 to get Ti(OH)4 precipitation. The precipitation was supercell were used for geometry optimization, and 8 × 8 × filtered and cleaned using distilled water for many times. 6and4× 4 × 6 M-P grids for 48 supercell and 108 atom Afterwards, the Ti(OH)4 precipitation was dispersed in ice- supercell were used for DOS calculation. For the unit cell distilled water. 30% H2O2 wasdroppeduntilitbecame calculation (pure TiO2),a6× 6 × 3and12× 12 × 6M- yellow and transparent. A designed amounts of Fe(NO3)3 P grids were used for geometry optimization and density of were added under stirring, and then, the solutions were states (DOSs) calculation. heated in enclosed autoclave for 12 h at 160◦CtogetFe- doped TiO hydrosols. A series of Fe-doped TiO samples 2 2 2.4. Photocatalytic Activity Evaluation. The photocatalytic were prepared with the Fe: Ti molar ratio being 0, 0.1, 0.3, activities of the Fe-doped TiO samples under UV were 0.5, 0.8, 1.0, 2.0, and 5.0 at. %, respectively. Finally, the 2 ◦ evaluated on the basis of the degradation rate of methyl hydrosols were dried at 40 C to obtain the powders. orange (MO) in a 100 × 20 self-designed quartz reactor containing 0.3 g of catalyst and 30 mL of 10 mg/L MO 2.2. Characterization. The X-ray diffraction (XRD) patterns aqueous solution. Prior to photocatalytic reactions, the were collected in the range of 20–80◦(2θ) with a HZG4- suspension was allowed to reach adsorption equilibrium by PC diffractometer using Cukα radiation with an accelerating stirring in dark about 60 min. The irradiation was performed −2 voltage of 40 kV and current of 40 mA (λx = 1.5406 A). with a 365 nm 1300 μWcm . International Journal of Photoenergy 3

4 k 2500 3.5 k

2000 3 k

2.5 k 1500 2 k

1000 Intensity 1.5 k Intensity 1 k 500 500

0 0

10 20 30 40 50 60 70 80 23 23.5 24 24.5 25 25.5 26 26.5 27 2θ (deg) 2θ (deg) Pure 1% Pure 1% 0.1% 2% 0.1% 2% 0.3% 5% 0.3% 5% 0.5% 0.8% (a) (b)

Figure 1: (A) The XRD patterns of the Fe-doped TiO2 samples and (B) the magniﬁed XRD (101) peak.

UV florescent lamp. In addition, the blank experiment in accordance with the XRD analysis. This is easy to be without using UV illumination and TiO2 photocatalyst was understood because doping foreign ions in TiO2 lattice can also conducted for comparison. Generally, the photocatalytic actually prohibit the crystal growth. In addition, it can degradation follows a Langmuir-Hinshelwood (LH) mecha- also be seen that the TiO2 nanoparticles change to leaf-like nism for the low concentration [28, 29], and the reaction rate morphology after Fe doping, indicating that Fe doping may equation can be expressed as ln(C0/C) = kt,whereC0 and also affect the particle growth. As shown in Figure 2,some C are the initial concentration and reaction concentration, white arrows are labeled to show the [001] crystal direction respectively. of anatase nanoparticles. The nanoneedles and sticks of the pure TiO2 are long in [001] direction. After doping Fe, the length of [001] becomes shorter and shorter as the Fe-doped 3. Results and Discussion content increases. This result indicates that Fe doping may prohibit the nanoparticle growth in [001] direction. 3.1. Characterization. Figure 1(a) shows the XRD patterns of Figure 3 shows the O1s, Ti2p, and Fe2p core spectra of the Fe-doped TiO2 samples. All of the as-prepared samples the pure and Fe-doped samples (2.0 and 5.0 at. %). Ti2p are mainly composed of anatase TiO2. There is a little peaks contain Ti2p1/2 and Ti2p3/2 subpeaks due to the spin- brookite, and its content becomes less and less with the orbital coupling effect [32, 33]. The Fe doping almost has increase of Fe concentration, indicating that Fe doping can no effect on the Ti2p peak, indicating that Fe doping does restrain the brookite formation to some extent. The average not change the coordination and chemical state of Ti4+ ions. anatase crystallite sizes decrease with the increase of Fe Compared with the symmetrical O1s peak of pure TiO concentration because the doping restrains the grain growth 2 sample, a new broad peak at ca. 531.5 eV after Fe doping [30, 31]. It can be seen from Figure 1(b) that the anatase shows the absorption of hydroxyl groups on TiO2 surface, (101) peak shifts to low 2θ direction as the Fe concentration 3+ increases, which indicates the (101) plane spacing becomes which may indicate the formation of Fe (OH) groups. larger and larger when more and more Fe ions are doped. It is reported that the Ti2p peak of N-doped TiO2 shifts Because the radius of Fe3+ is a little smaller than that of to low binding energy (BE) due to the bond weakening Ti4+, the widening of (101) plane spacing indicates that it is between nitrogen and titanium, implying that the nitrogen possible for some Fe3+ ions to enter the interstitial voids of enters TiO2 matrix by replacing oxygen ions [34]. In present TiO2 lattice. research, the shifting of O1s peaks to high BE shows that the Figure 2 shows the TEM photos of pure (A-1), 2.0 at. Fe doping changes the chemical coordination of oxygen. In % (B-1), and 5.0 at. % Fe-doped TiO2 (C-1), with A-2, B- addition, the Fe2p spectra show a positive shift as compared 2, C-2 being their high-resolved photos. The pure sample is to those in Fe2O3, probably indicative of more positively mainly composed of needles, sheets, and sticks. The TEM charged surface Fe3+ [22]. The slight enhancement of Fe2p 3+ photos clearly show that the nanoparticles become smaller BE is due to the Fe diffusion into TiO2 lattice and the and smaller as the doped Fe content increases, which is formation of Fe–O–Ti bond. 4 International Journal of Photoenergy

(A1) (A2) (B1)

[001] [001]

[101] [001]

[001]

(B2) (C1) (C2)

[001]

[101]

Figure 2: The TEM photos of pure TiO2 (A) and the Fe-doped TiO2 samples (2.0% (B) and 5.0% (C)).

The EDX area scanning technique was used to detect the the increase of Fe concentration due to the increase of non- average Fe on surface and in bulk. The 2.0 at. % and 5.0 irradiative recombination. The electrons and holes trapped at. % Fe-doped TiO2 samples contain ca. 1.9 and ca. 4.9 on surface contribute to the PL band from 3.0 to 3.2 eV at. % Fe ions. It can be seen that the Fe concentrations are [39]. This band intensity decreases with the increases of very close to the designed values. The Fe content on surface Fe concentration, which shows that that Fe doping has a checked using XPS is 8.2 at. % and 14.1 at. %, respectively. significant effect on surface electronic structure. Since the Therefore, it can be seen that the Fe ions are not uniformly PL signal is from the recombination, lower PL intensity may distributed in but enrich on TiO2 surface, in agreement with indicate lower recombination [40, 41], which indicates that the study [35]. Since XPS is a detecting technique of surface the surface Fe ions may increase the probability of interfacial chemical information of one and two atomic layers, it can be transfer of photoinduced electrons and holes. The PL peaks known that the Fe ions are enriched on TiO2 surface. The within 2.6 and 2.75 eV are suggested to be from the indirect ratio between the surface Fe concentrations and the designed recombination via bulk defects [42]. The PL shoulders from values is ca. 4.31 and 2.82 for the 2.0 at. % and 5.0 at. 2.2 to 2.45 eV are from the deactivation of the photoinduced ffi % samples. Therefore, the surface doping becomes di cult electrons in [TiO6]groupsonTiO2 surface [39], which also gradually with the increase of doping concentration, which show that the Fe doping has little effect on the Ti–O chemical will make more Fe ions enter TiO2 bulk. environment, in agreement with the XPS result. Figure 4 shows the DRS spectra of the Fe-doped TiO2 samples. The onset of the absorption edge for pure TiO2 is at ca. 390 nm [36]. Those spectra show an obvious red 3.2. Electronic Structure Investigation. Based on the experi- shift after Fe doping, which is induced by the electron ments, DFT computations were performed. The projected density of states (PDOSs) is shown in Figure 6.Itcanbe transition from Fe3d orbitals to TiO2 conduction band (CB). The absorption peak at 480 nm is from the d → d seen that the VB and CB of TiO2 are mainly composed of transition (T2g → A2g)[37], and it becomes clearer and O2p orbitals and Ti3d orbitals, respectively [42, 43]. For clearer with the increase of Fe concentration. Figure 5 pure TiO2 (Figure 6(a)), the Fermi level (EF) locates in the shows the normalized PL spectra of Fe-doped TiO2 samples. middle of the forbidden band, indicating that VB is full filled, Thepeakat3.43eVisfromtheCB→ valence band (VB) while CB is empty. The Eg of pure TiO2 isca.1.9eV,which recombination [38], and its density decreases gradually with is underestimated due to the well-known shortcoming of International Journal of Photoenergy 5

Ti2p 55000 30000 Ols 50000 25000 45000 40000 20000 35000 30000 15000 Intensity

Intensity 25000 10000 20000 15000 5000 10000

0 5000 450 455 460 465 470 524 526 528 530 532 534 Binding energy (eV) Binding energy (eV) Pure Pure 2% 2% 5% 5% (a) (b)

14200 Fe2p 14000 13800 13600 13400 13200 13000

Intensity 12800 12600 12400 12200 12000 11800 700 710 720 730 740 Binding energy (eV) 2% 5% (c)

Figure 3: The O1s, Ti2p, and Fe2p core XPS spectra of 0.0, 2.0, and 5.0% Fe-doped TiO2 samples. exchange-correction functional in describing excited states. T2g band, and the A2g band is empty. Therefore, T2g → A2g The theoretical study indicates that oxygen vacancy can transition is possible, which is the d-d transition in the UV- be easily introduced by Fe doping [44], so the electronic Vis DRS spectra. In addition, the hybrid A2g band partly structure of reduced anatase was also investigated, which is contributes to the DRS red shift. When an oxygen vacancy is 4+ shown Figure 6(b). The EF shifts to CB because some Ti introduced in the Fe-doped anatase (Figure 6(d)), the Fe3d ions are reduced as Ti3+, and some energy levels related to orbitals are still composed of two subbands, one midgap oxygen defects appeared at the CB bottom (labeled as the band (T2g) and one hybrid band (A2g). However, the T2g arrow). The Eg is narrowed by downshifting the CB bottom band is ca. 1.2 eV above the VB top, and the T2g-A2g distance ca. 0.2 eV, which is in agreement with other reports [45, 46]. is reduced to 0.9 eV due to the weakening of the crystal ﬁeld 3+ For the supercell containing a Fe ion, the Fe3d orbitals split eﬀect. The EF is almost at the top of A2g band because the into two bands (Figure 6(c)). The upper hybrid band with CB electrons in oxygen vacancy are shared by the two adjacent Fe 3+ is mainly composed of dz2 and dx2−y2 (A2g), and the inter- ions, which may be the reason that no Ti ions are detected gap band consists of dzx,dxy,anddzy (T2g), ∼0.5 eV above from XPS spectra. Consequently, DFT computation gives a the VB top. The T2g-A2g distance is about 1.8 eV. Because clear result that the Fe doping will induce localized bandgap EF is in the middle of T2g band, the electrons mainly occupy states, namely, T2g band. 6 International Journal of Photoenergy

1.8 2 1.6 3 1.4 1.2 1 1 0.8 4 Pure Intensity 0.1% Absorbance 0.6 0.3% 0.4 0.8% 1% 0.2 2% 5% 0 −0.2 2 2.5 3 3.5 200300 400 500 600 700 800 Photon energy (eV) Wavelength (nm) Figure 5: The PL spectra Fe-doped TiO2 nano particles. 0.1 at. % 1 at. % 0.3 at. % 2 at. % 5 at. % 0.8 at. % spectra. Therefore, the bulk-doped Fe ions mainly form the Figure 4: The DRS UV-Vis spectra of Fe-doped TiO2 nanoparticles. recombination centers of electrons and holes; more Fe ions entering TiO2 bulk cannot contribute to the photocatalysis. Compared with the Fe3+ ions in bulk, the Fe3+ ions on 3.3. Photocatalytic Performance Evaluation. Figure 7(a) surface have different coordination environment. They can shows the dependence of ln(C0/C) on time of the Fe-doped directly combine with the oxygen molecule (Fe–O2) or the TiO2 samples photodecomposing MO aqueous solution. hydroxyl groups (Fe–OH), so the electrons or holes trapped Figure 7(b) shows the UV-Vis absorption spectra of MO 3+ ff on surface Fe ions are possible to transfer to O2 and –OH, aqueous solution for di erent periods of UV light illumina- resulting in the formation of O2− and •OH, which has also tion time for 0.3 at. % doped TiO2. The blank experiment been reported by others. Therefore, the Fe ions on surface shows that the MO cannot be degraded in the absence of can form intermediate pathways for the interfacial transfer. either TiO2 or UV light. The Fe doping obviously influences Because the surface recombination of the trapped electrons the photocatalytic activity under UV light illumination. The and holes is generally much slower than the interfacial 0.3 at. % Fe-doped sample has the best activity, better transfer [48], the presence of Fe3+ ions on surface is useful than the photocatalytic activity of P25. An optimal Fe to improve photocatalytic speed. concentration for the Fe-doped TiO2 to present the best 3+ photocatalytic activity was observed, which is not a surprise For the present experiment, the Fe ions are not because this has been seen for many times. Although it is uniformly distributed in TiO2 nanoparticles. In the case of stated that the Fe ions mainly form trapping centers for holes low-level doping, they are mainly on TiO2 surface. According and electrons for low doping level, while they will become to the above discussion, it can be known that the Fe ions recombination centers for high doping [47], the mechanism mainly form the pathway for the electron-hole transfer for of this trapping to recombination transformation is still low doping concentration, so the photocatalytic activity will unclear and in debate. increase with the increase of doping concentration. For 3+ As shown by DFT calculation, the Fe3d T2g band is half- high-level doping, many Fe ions can be doped in bulk filled, and thus, The Fe3+ ions not only can trap electrons beside those on the surface, so the recombination through 3+ but also can trap holes. From the viewpoint of crystal field the bulk Fe ions cannot be neglected. Therefore, the theory [17, 19, 20], the Fe ions after trapping a hole (Fe4+) photocatalytic activity firstly increases and then decreases or an electron (Fe2+) are unstable, and can favorably go with the increase of doped Fe content. It is widely accepted 3+ back to Fe3+ ions by releasing an electron and hole. In that the doped Fe ions will change from the trapping general, there are some ways to resume Fe3+ ions. Firstly, centers to recombination centers as the doped concentration increases, how this change takes place is still not clear. It is the trapped electrons in Fe ions can migrate to TiO2 surface via multitrapping. However, this multitrapping needs to claimed that the tunnel recombination through Fe ions is the overcome the potential barrier between Fe3d T2g band reason of this trapping to recombination changing. We give new explanation of the trapping to recombination changing and TiO2 CB, and the DFT calculation shows that this barrier is too high for the multitrapping to take place. In with the increase of Fe concentration, which is related to addition, the trapped electrons can migrate to the adjacent surface doping and bulk doping, as illustrated above. Fe site via hopping. Obviously, the hopping is difficult because Fe3d T2g states are almost localized according to 4. Conclusion the DFT calculation. Finally, the only possible pathway for 4+ 2+ 3+ Fe and Fe ions to regenerate Fe ions is to recombine The Fe-doped TiO2 nanoparticles are prepared using with the holes in VB, which are also supported by the PL hydrothermal method. It is confirmed that Fe3+ ions are International Journal of Photoenergy 7

25 80

Fermi level 70 Fermi level 20 60

50 15 40

10 PDOS (a.u.) PDOS (a.u.) 30

20 5 10

0 0 −10 −5 0 5 10 15 −10 −5 0 5 10 15 Energy (eV) Energy (eV) Total Ti3d Total Ti3d O2p Ti3s O2p Ti3s (a) (b)

180 180

160 160 Fermi level Fermi level 140 140

120 120

100 100

80 80 PDOS (a.u.) PDOS (a.u.) 60 60

40 40

20 20

0 0 −10 −5 0 5 10 15 −10 −5 0 5 10 15 Energy (eV) Energy (eV)

Total Ti3s Total Ti3s O2p Fe3d O2p Fe3d Ti3d EF Ti3d EF (c) (d)

Figure 6: The PDOS of pure TiO2 (a), 48 atom supercell containing one oxygen defect (b), 48 atom supercell containing a Fe ion (c), and 108 atom supercell containing two adjacent Fe ions and oxygen defect (d).

mainly doped on TiO2 surface for low doping concentration, is confirmed that this changing is related to the increase of and some Fe ions will enter TiO2 bulk for high doping bulk doping as the doped concentration increases. concentration. The experiments and DFT calculation show 3+ that the Fe ions in TiO2 bulk are localized, so they mainly Acknowledgments form the recombination centers. The Fe3+ ions on surface can form bridges for the transfer of electrons and holes, This research was partly financially supported by NFSC so surface doping is beneficial to the photocatalysis. It (no. 50702041), and thanks to the supporting of Wuhan is observed that the photocatalytic activity of as-prepared Young Scientists Chenguang Plan (no. 20091j0080) and the Fe-doped TiO2 firstly increases and then decreases with National Basic Research Program of China (2009CB939704). the increase of doping concentration, which is due to the This research was also supported by Japan Society for the trapping to recombination changing of doped Fe3+ ions. It Promotion of Science (JSPS) for a postdoctoral fellowship 8 International Journal of Photoenergy

1.8 2 1.6 1.8 1.4 1.6 1.2 1.4 1 1.2 /C) 0 0.8 1 Ln (C 0.6 Absorbance 0.8 0.4 0.6 0.2 0.4 0 0.2 −0.2 0 0 50 100 150200 250 300 400 450 500 550 600 650 700 Time (min) Wavelength (nm)

P25 0.8% Fe-TiO2 0 min 160 min Pure TiO2 1% Fe-TiO2 40 min 200 min 0.1% Fe-TiO2 2% Fe-TiO2 80 min 240 min 0.3% Fe-TiO2 5% Fe-TiO2 120 min 280 min (a) (b)

Figure 7: (a) The dependence of ln(C0/C) on the UV illumination time of Fe-doped TiO2 and (b) the absorption spectra of MO aqueous solution at diﬀerent time span of UV light illumination.

for foreign researchers, and a scientific research (B) from [8] D. Chen, Z. Jiang, J. Geng, Q. Wang, and D. Yang, “Carbon the Ministry of Education, Culture, Sports, Science and and nitrogen co-doped TiO2 with enhanced visible-light Technology (MEXT), Japan. photocatalytic activity,” Industrial and Engineering Chemistry Research, vol. 46, no. 9, pp. 2741–2746, 2007. [9] S. In, A. Orlov, R. Berg et al., “Effective visible light-activated References B-doped and B,N-codoped TiO2 photocatalysts,” Journal of the American Chemical Society, vol. 129, no. 45, pp. 13790–13791, [1] A. Fujishima and K. Honda, “Electrochemical photolysis of 2007. water at a semiconductor electrode,” Nature, vol. 238, no. [10] S. Tojo, T. Tachikawa, M. Fujitsuka, and T. Majima, “Iodine- 5358, pp. 37–38, 1972. doped TiO2 photocatalysts: Correlation between band struc- [2] U. I. Gaya and A. H. Abdullah, “Heterogeneousphotocatalyt- ture and mechanism,” Journal of Physical Chemistry C, vol. icdegradation of organic contaminants over titanium dioxide: 112, no. 38, pp. 14948–14954, 2008. ff a review of fundamentals, progress and problems,” Journal of [11] H. Ozaki, S. Iwamoto, and M. Inoue, “E ects of amount of Photochemistry and Photobiology C, vol. 9, no. 1, pp. 1–12, Si addition and annealing treatment on the photocatalytic 2008. activities of N- and Si-codoped titanias under visible-light irradiation,” Industrial and Engineering Chemistry Research, [3] J. R. Jennings, A. Ghicov, L. M. Peter, P. Schmuki, and vol. 47, no. 7, pp. 2287–2293, 2008. A. B. Walker, “Dye-sensitized solar cells based on oriented [12] N. Lu, X. Quan, J. Li, S. Chen, H. Yu, and G. Chen, “Fab- TiO2 nanotube arrays: transport, trapping, and transfer of electrons,” Journal of the American Chemical Society, vol. 130, rication of boron-doped TiO2 nanotube array electrode and no. 40, pp. 13364–13372, 2008. investigation of its photoelectrochemical capability,” Journal of Physical Chemistry C, vol. 111, no. 32, pp. 11836–11842, 2007. [4] D. Kuang, J. Brillet, P. Chen et al., “Application of highly [13] T. Hirakawa and Y. Nosaka, “Selective production of super- ordered TiO2 nanotube arrays in flexible dye-sensitized solar oxide ions and hydrogen peroxide over nitrogen- and sulfur- cells,” ACS Nano, vol. 2, no. 6, pp. 1113–1116, 2008. doped TiO photocatalysts with visible light in aqueous ff 2 [5] M. R. Ho mann, S. T. Martin, W. Choi, and D. W. suspension systems,” Journal of Physical Chemistry C, vol. 112, Bahnemann, “Environmental applications of semiconductor no. 40, pp. 15818–15823, 2008. photocatalysis,” Chemical Reviews, vol. 95, no. 1, pp. 69–96, [14] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, 1995. “Visible-light photocatalysis in nitrogen-doped titanium [6] C. Kormann, D. W. Bahnemann, and M. R. Hoffmann, “Pho- oxides,” Science, vol. 293, no. 5528, pp. 269–271, 2001. tolysis of chloroform and other organic molecules in aqueous [15] P. E. Lippens, A. V. Chadwick, A. Weibel, R. Bouchet, and TiO2 suspensions,” Environmental Science and Technology, vol. P. Knauth, “Structure and chemical bonding in Zr-doped 25, no. 3, pp. 494–500, 1991. anatase TiO2 nanocrystals,” Journal of Physical Chemistry C, [7]S.Sakthivel,M.Janczarek,andH.Kisch,“Visiblelight vol. 112, no. 1, pp. 43–47, 2008. activity and photoelectrochemical properties of nitrogen- [16] J. Yu and H. Yu, “Preparation, characterization and photocat- doped TiO2,” Journal of Physical Chemistry B, vol. 108, no. 50, alytic activity of in situ Fe-doped TiO2 thin films,” Thin Solid pp. 19384–19387, 2004. Film, vol. 496, no. 2, pp. 273–280, 2006. International Journal of Photoenergy 9

[17]M.Zhou,J.Yu,andB.Cheng,“Effects of Fe-doping on the [32] Y. Lv, Y. Ding, J. Zhou, W. Xiao, and Y. Feng, “Preparation, photocatalytic activity of mesoporous TiO2 powders prepared characterization, and photocatalytic activity of N, S-codoped by an ultrasonic method,” Journal of Hazardous Materials, vol. TiO2 nanoparticles,” Journal of the American Ceramic Society, 137, no. 3, pp. 1838–1847, 2006. vol. 92, no. 4, pp. 938–941, 2009. [18] A. Mattsson, M. Leideborg, K. Larsson, G. Westing, and [33] B. Liu, L. Wen, and X. Zhao, “The surface change of TiO2 L. Osterlund,¨ “Adsorption and solar light decomposition of film induced by UV illumination and the effects on UV-vis acetone on anatase TiO2 and niobium doped TiO2 thin films,” transmission spectra,” Applied Surface Science, vol. 255, no. 5, Journal of Physical Chemistry B, vol. 110, no. 3, pp. 1210–1220, pp. 2752–2758, 2008. 2006. [34] H. Choi, M. G. Antoniou, M. Pelaez, A. A. De La [19] J. Yu, Q. Xiang, and M. Zhou, “Preparation, characterization Cruz, J. A. Shoemaker, and D. D. Dionysiou, “Mesoporous and visible-light-driven photocatalytic activity of Fe-doped nitrogen-doped TiO2 for the photocatalytic destruction of titania nanorods and first-principles study for electronic the cyanobacterial toxin microcystin-LR under visible light structures,” Applied Catalysis B, vol. 90, no. 3-4, pp. 595–602, irradiation,” Environmental Science and Technology, vol. 41, 2009. no. 21, pp. 7530–7535, 2007. [20]W.Choi,A.Termin,andM.R.Hoffmann, “The role of metal [35] Z. Ambrus, N. Balazs,´ T. Alapi et al., “Synthesis, structure ion dopants in quantum-sized TiO2: correlation between and photocatalytic properties of Fe(III)-doped TiO2 prepared photoreactivity and charge carrier recombination dynamics,” from TiCl3,” Applied Catalysis B: Environmental, vol. 81, no. Journal of Physical Chemistry, vol. 98, no. 51, pp. 13669–13679, 1-2, pp. 27–37, 2008. 1994. [36] B. Liu, X. Wang, G. Cai, L. Wen, Y. Song, and X. Zhao, 3+ [21] J. Zhu, F. Chen, J. Zhang, H. Chen, and M. Anpo, “Fe -TiO2 “Low temperature fabrication of V-doped TiO2 nanoparticles, photocatalysts prepared by combining sol-gel method with structure and photocatalytic studies,” Journal of Hazardous hydrothermal treatment and their characterization,” Journal of Materials, vol. 169, no. 1–3, pp. 1112–1118, 2009. Photochemistry and Photobiology A, vol. 180, no. 1-2, pp. 196– [37]K.S.W.Sing,D.H.Everett,R.A.W.Hauletal.,“Reporting 204, 2006. physisorption data for gas/solid systems with special reference [22] T. Tong, J. Zhang, B. Tian, F. Chen, and D. He, “Preparation of 3+ to the determination of surface area and porosity,” Pure and Fe -doped TiO2 catalysts by controlled hydrolysis of titanium Applied Chemistry, vol. 57, no. 4, pp. 603–619, 1984. alkoxide and study on their photocatalytic activity for methyl [38] B. Liu, L. Wen, and X. Zhao, “The photoluminescence orange degradation,” Journal of Hazardous Materials, vol. 155, spectroscopic study of anatase TiO prepared by magnetron no. 3, pp. 572–579, 2008. 2 sputtering,” Materials Chemistry and Physics, vol. 106, no. 2-3, [23] J. Wang, Z. Liu, and R. Cai, “A new role for Fe3+ in TiO 2 pp. 350–353, 2007. hydrosol: accelerated photodegradation of dyes under visible [39] B. Liu, X. Zhao, and L. Wen, “The structural and photolumi- light,” Environmental Science and Technology, vol. 42, no. 15, nescence studies related to the surface of the TiO sol prepared pp. 5759–5764, 2008. 2 by wet chemical method,” Materials Science and Engineering B, [24] H. Yu, H. Irie, and K. Hashimoto, “Conduction band energy vol. 134, no. 1, pp. 27–31, 206. level control of titanium dioxide: toward an efficient visible- light-sensitive photocatalyst,” Journal of the American Chemi- [40] Y. Cong, J. Zhang, F. Chen, M. Anpo, and D. He, “Preparation, cal Society, vol. 132, no. 20, pp. 6898–6899, 2010. photocatalytic activity, and mechanism of nano-TiO2 Co- [25]H.Irie,S.Miura,K.Kamiya,andK.Hashimoto,“Efficient doped with nitrogen and iron (III),” Journal of Physical visible light-sensitive photocatalysts: grafting Cu(II) ions onto Chemistry C, vol. 111, no. 28, pp. 10618–10623, 2007. [41] H. Tang, K. Prasad, R. Sanjines,P.E.Schmid,andF.L` evy,´ TiO2 and WO3 photocatalysts,” Chemical Physics Letters, vol. 457, no. 1–3, pp. 202–205, 2008. “Electrical and optical properties of TiO2 anatase thin films,” [26] Z. Zhang, C. C. Wang, R. Zakaria, and J. Y. Ying, “Role of Journal of Applied Physics, vol. 75, no. 4, pp. 2042–2047, 1994. [42] T. L. Villarreal, R. Gomez,´ M. Gonzalez,´ and P. Salvador, “A particle size in nanocrystalline TiO2-based photocatalysts,” Journal of Physical Chemistry B, vol. 102, no. 52, pp. 10871– kinetic model for distinguishing between direct and indirect 10878, 1998. interfacial hole transfer in the heterogeneous photooxidation [27] http://www.quantum-espresso.org/ of dissolved organics on TiO2 nanoparticle suspensions,” [28] S. Ardizzone, C. L. Bianchi, G. Cappelletti, S. Gialanella, C. Journal of Physical Chemistry B, vol. 108, no. 52, pp. 20278– Pirola, and V. Ragaini, “Tailored anatase/brookite nanocrys- 20290, 2004. talline TiO2. The optimal particle features for liquidand gas- [43] K. Yang, Y. Dai, and B. Huang, “First-principles calculations phase photocatalytic reactions,” Journal of Physical Chemistry for geometrical structures and electronic properties of Si- C, vol. 111, no. 35, pp. 13222–13231, 2007. doped TiO2,” Chemical Physics Letters, vol. 456, no. 1–3, pp. [29] A. Testino, I. R. Bellobono, V. Buscaglia et al., “Optimizing the 71–75, 2008. photocatalytic properties of hydrothermal TiO2 by the control [44] A. Roldan,M.Boronat,A.Corma,andF.Illas,“Theoretical´ of phase composition and particle morphology. A systematic confirmation of the enhanced facility to increase oxygen approach,” Journal of the American Chemical Society, vol. 129, vacancy concentration in TiO2 by iron doping,” Journal of no. 12, pp. 3564–3575, 2007. Physical Chemistry C, vol. 114, no. 14, pp. 6511–6517, 2010. [30] J. G. Yu, M. H. Zhou, B. Cheng, and X. J. Zhao, “Preparation, [45] I. Justicia, P. Ordejon, G. Canto et al., “Designed self- characterization and photocatalytic activity of in situ N,S- doped titanium oxide thin films for efficient visible-light codoped TiO2 powders,” Journal of Molecular Catalysis A, vol. photocatalysis,” Advanced Materials, vol. 14, no. 19, pp. 1399– 246, no. 1-2, pp. 176–184, 2006. 1402, 2002. [31] B. Liu, X. Zhao, Q. Zhao, C. Li, and X. He, “The effectofO2 [46] F. Zuo, L. Wang, T. Wu, Z. Zhang, D. Borchardt, and P. partial pressure on the structure and photocatalytic property Feng, “Self-doped Ti3+ enhanced photocatalyst for hydrogen of TiO2 films prepared by sputtering,” Materials Chemistry and production under visible light,” Journal of the American Physics, vol. 90, no. 1, pp. 207–212, 2005. Chemical Society, vol. 132, no. 34, pp. 11856–11857, 2010. 10 International Journal of Photoenergy

[47] M. Gratzel,¨ Heterogeneous Photochemical Electron Transfer Reactions, CRC Press, Boca Raton, Fla, USA, 1987. [48] B. Liu, K. Nakata, X. Zhao, T. Ochiai, T. Murakami, and A. Fujishima, “Theoretical kinetic analysis of heterogeneous photocatalysis: the eﬀects of surface trapping and bulk recombination through defects,” The Journal of Physics and Chemistry, vol. 115, no. 32, pp. 16037–16042, 2011. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 398508, 8 pages doi:10.1155/2012/398508

Research Article Enhanced Photoactivity of Fe + N Codoped Anatase-Rutile TiO2 Nanowire Film under Visible Light Irradiation

Kewei Li,1, 2 Haiying Wang,1, 2, 3 Chunxu Pan,3 Jianhong Wei,3 Rui Xiong,3, 4 and Jing Shi3

1 Department of Communication Engineering, Chengdu Institute of Technology, Chengdu 611730, China 2 Intel-cec joint Lab, Institute of IC Mannufacturing Engineering, Chengdu 611730, China 3 Key Laboratory of Artiﬁcial Micro- and Nanostructures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, China 4 Ministry of Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei University, Wuhan 430072, China

Correspondence should be addressed to Haiying Wang, [email protected] and Rui Xiong, [email protected]

Received 9 January 2012; Revised 22 February 2012; Accepted 22 February 2012

Academic Editor: Baibiao Huang

Copyright © 2012 Kewei Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Rutile-anatase phase mixed Fe + N codoped TiO2 nanowires were designed and prepared by a two-step anodic oxidation method. The results of X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy confirm that the prepared Fe + N codoped TiO2 nanowires exhibit intimately contacted anatase-rutile heterostructure with the rutile content of 21.89%. The X-ray photoelectron spectroscopy measurements show that nitrogen and iron atoms are incorporated into the titania oxide lattice, and the UV-visible absorption spectra show that the codoping of iron and nitrogen atoms could extend the absorption to visible light region. The photocatalytic activities of all the samples were evaluated by photocatalytic degradation of methylene blue under visible light irradiation. The Fe + N codoped sample achieves the best response to visible light and the highest photocatalytic activities. The enhancement of photocatalytic activity for Fe + N codoped sample should be ascribed to the synergistic effects of codoped nitrogen and iron ions and the anatase-rutile heterostructure.

1. Introduction the monodoping can generate the recombination center inside the TiO2, which goes against the light-induced charge Titanium oxide is a multifunctional material with a wide carriers’ migration to the surface [16, 20]. In compensated n- variety of potential applications including solar cell [1], p codoping systems, the defect bands can be passivated and photocatalyst [2], water splitting [3], and gas sensors [4]. will not be effective as carrier recombination centers [16, 21] However, the wide band gap of TiO2 (3.2 eV for the anatase and the Columbic attraction between the n- and p-type phase and 3.0 eV for the rutile phase) requires ultraviolet dopants with opposite charge substantially enhances doping (UV) light for electron-hole separation, which is only 5% concentration [22]. Recently, noncompensated codoping was of the natural solar light [5]. It is of great significance proposed by Zhu et al., for its distinctive merit of ensuring to develop photocatalysts that can be used in visible light the creation of intermediate electronic bands in the gap region to improve the photocatalytic efficiency. Numerous region and enhancing photoactivity manifested by efficient attempts have been made to optimize the band gap of TiO2 electron-hole separation in the visible-light region [23]. by different doping schemes, for example, nonmetal doping It is well-known that powders consisting of anatase (suchasC,S,N,I[6–11]), metal doping (such as Fe, V, and rutile (a classical reference materials is Degussa P25, Cr, Nb [12–15]), and nonmetal-metal codoping (such as a mixture of 20% rutile, and 80% anatase) possess higher Mo + C codoping [16], Mo + N codoping [17], Pr + N photoactivity than those of pure anatase. It was thought codoping [18], and Cd + N codoping [19]), to improve that the anatase-rutile heterojunction inside the TiO2 was the photocatalytic activity. As a result, it was found that the main reason for enhanced photoactivity [24]. Deak´ et 2 International Journal of Photoenergy al. calculated the band structure of rutile and anatase and validated that both bands of rutile lie higher than those of (004)A (002)Ti (103)Ti (101)Ti (110)R (101)A anatase, so in mixed systems, the holes are accumulated in the (101)R (211)A (102)Ti (110)Ti (200)A former while the electrons in the latter [25]. As pointed out (112)Ti earlier, the charge separation will enhance the photochemical Fe+N codoped TiO2 activity of mixed phase TiO2 powders. (a.u.) New interesting issue: does the noncompensated codoping with rutile-anatase mixing phase improve the visible N-doped TiO2 light photocatalytic activity? It was reported that Fe-doped Intensity TiO2 and N-doped TiO2 exhibit enhanced visible light Fe-doped TiO2 photocatalytic activity as compared to undoped TiO2, while Fe + N codoping is the noncompensated codoping, but the light photocatalytic activity is seldom reported. Thus, in this Undoped TiO2 study,Fe+NcodopedTiO2 nanowires with rutile-anatase 20 40 60 80 mixed phase were designed and prepared. Compared with 2θ (deg) the undoped, Fe-doped and N-doped TiO2,Fe+Ncodoped Figure 1: X-ray diffraction patterns of undoped, Fe-doped, N- TiO2 gains the best photocatalytic activity under visible light doped, and Fe + N codoped TiO2 nanowires. irradiation and the mechanism of enhanced visible-light photocatalytic activity is discussed in detail. to the C 1s peak at 284.6 eV. The UV-visible absorption 2. Experimental Section spectra were measured using a Cary 5000 UV-Vis-NIR spectrophotometer; BaSO4 was used as a reectance standard in a Titanium foils (0.6 mm thick, 99.5% purity, and cut in UV-visible diffuse reectance experiment. The photocatalytic 1cm× 2 cm) were used as the substrates for the growth activities under visible light irradiation were evaluated by of the TiO2 nanowire arrays. The titanium sheets were the degradation of methyl blue irradiated by a 450 W xenon cleaned by sonicating in 1 : 1 acetone and ethanol solution, lamp. In the process, a TiO2 nanowire array with dimensions followed by being rinsed with deionized water and dried of 0.5 cm × 0.5 cm was immersed into 3 mL 10 mg/L methyl in airstream. The anodization was carried out in a two- blue (MB) solution and placed below xenon lamp with electrode electrochemical cell with a graphite sheet as the 10 cm distance. The solution in the photoreactor was placed cathode at a constant potential 60 V. A DC power supply in dark for 30 minutes to reach the absorption-desorption (WYK-6010, 0–60 V, and 0–10 A) was used to control the equilibrium of the dye molecules on the sample surface. experimental current and voltage for 1 h. The electrolyte contained 0.5 wt% NH4F, 5 mL H 2O, and 195 mL ethylene 3. Results and Discussions glycol. After anodization, the specimens were cleaned in 10% HCl by ultrasonic immediately for 10 minutes and dried in Figure 1 shows the XRD patterns of the undoped, Fe-doped, airstream. The anodization process was repeated for several N-doped, and Fe + N codoped polycrystalline TiO2 samples. times. Compared with the standard patterns of the anatase phase The samples were then rinsed by ethanol for 5 minutes, TiO2 (JCPDS 21-1272) and the rutile phase (JCPDS 21- after which some of them were submerged in 1.5×10−3 mol/L 1276), no peaks of impurities are detected except the peaks Fe(NO3)3 solutions for 40 minutes for Fe doping. Postan- of metal titanium coming from the substrates. The contents nealing in air at 700◦C was employed to remove most of the of rutile phase and anatase phase are calculated by the XRD organic and inorganic species encapsulated in the arrays and results, using the method described by Zhu et al. [26]and transform the amorphous titania to nanocrystalline TiO2. listed in Table 1. The results show that the content of rutile Nitrogen doping was carried out by annealing the samples phase in Fe-doped TiO2 (19.66 wt%) is lower than that of ◦ in ammonia atmosphere at 500 C. The reagents—acetone the undoped TiO2 (21.88 wt%), that is to say, the content (CH3COCH3), ethanol (C2H5OH), ammonium fluoride of rutile decreases for the iron doping after the heating (NH4F), Fe(NO3)3, and ethylene glycol (C2H6O2)wereof treatment, which indicates that the doping of iron may retard analytical-grade without further purification. the transition from anatase to rutile. The content of rutile Thecrystalstructuresofsampleswerecharacterizedby in the N-doped and Fe + N codoped TiO2 are 22.48 wt% X-ray diffraction (XRD, Bruker AXS D8 Advance diffrac- and 21.89 wt%. The average crystallite sizes were calculated tions) using Cu Kα radiation. The surface morphologies by Scherrer’s formula (D = kλ/β cos θ, k = 0.89, λ = of the nanowire arrays were observed by scanning electron 0.15418 nm) and listed in Table 1, choosing the peaks at microscopy (SEM, S-4800) and high-resolution transmission scattering angles of 25◦ and 27.5◦ for anatase and rutile, electron microscopy (TEM 2010FEF HRTEM, JEOL, Japan). respectively. The average crystallite sizes for anatase phase The X-ray photoelectron spectroscopy (XPS) experiments in the undoped TiO2 and samples doped with Fe, N, Fe + were performed on a VG Multilab2000 spectrometer to N are 32.92 nm, 25.27 nm, 27.60 nm, and 27.78 nm, while obtain the information on chemical binding energy of the the average crystallite sizes for rutile phase are 30.57 nm, TiO2 nanowires which was calibrated with the reference 25.30 nm, 26.93 nm, and 28.78 nm, respectively. These results International Journal of Photoenergy 3

(a) (b)

Figure 2: SEM images of (a) the undoped, (b) Fe-doped, (c) N-doped, and (d) Fe + N codoped TiO2 nanowires.

Table 1: The average crystallite size of anatase TiO2 and the content of rutile, anatase for undoped, Fe-doped, nitrogen-doped, and Fe + N codoped TiO2 nanowires.

Undoped TiO2 Fe-doped TiO2 N-doped TiO2 Fe + N codoped TiO2 Content of rutile (wt%) 21.88 19.66 22.48 21.89 Content of anatase (wt%) 78.12 80.34 77.52 78.11 Average crystallite size of anatase (nm) 32.92 25.27 27.60 27.78 Average crystallite size of rutile (nm) 30.57 25.30 26.93 28.78

reveal that doping with Fe and N element will retard the growth of the crystallite sizes, which is similar to those described in [27, 28]. Figures 2(a), 2(b), 2(c),and2(d) present the illustrative top SEM images of typical undoped, Fe-doped, N-doped, and Fe + N codoped TiO2 samples. The right part of Figure 2(d) is the SEM image of cross section of the Fe + N codoped sample. As it can be seen, the length and the diameter of all the samples are about 5 μm and 90 nm. The nanowires are arranged one by one closely, ensuring that light can be eﬀectively scattered by the rutile phase in the mixing-phase ﬁlms [29]. The crystalline structure of TiO2 is further examined by HRTEM. The HRTEM images of a single Fe + N codoped TiO2 nanowire and the magnied local image in the white rectangle zone is shown in Figure 3. HRTEM observations

Figure 3: The HRTEM images of Fe + N codoped TiO2 with mag- reveal that the diameter of the nanoparticles is in a size range niﬁed local image in the right. of 70–100 nm and TiO2 nanoparticles can be discriminated 4 International Journal of Photoenergy

O 1s 12200 800000 12100 N 1s 12000 2

/ 11900 3 2p 2s

600000

2p 11800

Fe Ti

Ti 11700 400.2 eV (a.u.) (a.u.)

2 396.04 eV / 400000 1 11600 2p 11500 Ti Counts Counts 11400 200000 11300 1s

3s 3p

1s C 11200 Ti Ti N 11100 0 11000 1000 800 600 400 200 0 10900 Binding energy (eV) 388 390 392 394 396 398 400 402 404 406 408 Binding energy (eV)

Raw 396.04 eV Total 400.2 eV Baseline (a) (b)

74000 2p3/2 Fe 2p 2p1/2 72000

(a.u.) 70000

Counts 68000

66000

64000 700 710 720 730 740 Binding energy (eV) (c)

Figure 4: XPS spectra of (a) survey spectrum, (b) N 1s, and (c) Fe 2p for Fe + N codoped TiO2. and exhibit a perfect lattice. As shown in Figure 3, the in general, is a signal of adventitious elemental carbon as clear anatase-rutile interface is conrmed by HRTEM, where reported in other works [32, 33]. No other impurity peaks the lattice spaces are measured to be 0.189 nm along the were observed, which reveals that the sample contains Ti, O, (200) plane of anatase phase and 0.248 nm along the (101) Fe, and N four elements. plane of rutile phase, respectively. The formation of intimate Figure 4(b) is the N 1s core level spectra of Fe + N contact between anatase and rutile phases is realized so codoped TiO2. It is found that there are two peaks with that the charge transfer between the two phases can occur similar intensity at the bonding energies of 396.04 eV and smoothly [30]. The anatase-rutile interface is the active 400.20 eV, respectively. Generally, the peak at 396 eV reﬂects site for photocatalysis and is one of the dominate factors the formation of N–Ti–O bonds, indicating the substitution for the enhancement of the photocatalytic activity reported of N ion for O ion [34–36]. The peak at 400 eV can be elsewhere [31]. assigned to molecular nitrogen bonded to the surface defects To investigate the composition and the chemical state or the N atoms bonding to O sites in TiO2, forming Ti– of ions, XPS measurements were performed. Figure 4(a) is O–N bonds [37]. Figure 4(c) shows the high-resolution the XPS survey spectrum of the Fe + N sample. The sharp Fe 2p core level spectrum. As it can be seen, the binding peaks at 284.7 eV, 458.60 eV, 530.15 eV are corresponding to energies of the Fe 2p XPS peaks are at 710.41 eV, indicating the binding energy of C 1s, Ti 2p3/2, O1s, and the two weak that the Fe element in the sample exists mainly in the +3 photoelectron peaks at 710.41 eV and 396.00 eV correspond oxidation state [38]. The peak position of Fe + N codoped to Fe 2p3/2, and N 1s, respectively. The peak at 284.7 eV, TiO2 exhibits a shift about 0.3 eV to lower energy compared International Journal of Photoenergy 5

1.2 0.8

1 0.6 0.8

0.6 0.4 Absorbance

Absorbance 0.4 0.2 396.3 nm 0.2 394 nm 384 nm 405 nm 0 0 200 300 400 500 600 700 340 360 380 400 420 440 Wavelength (nm) Wavelength (nm)

Undoped TiO2 N-doped TiO2 Undoped TiO2 N-doped TiO2 Fe-doped TiO2 Fe + N codoped TiO2 Fe-doped TiO2 Fe + N codoped TiO2 (a) (b) (a.u.) 2 ) A ah ( 3.13 eV 3.06 ev 3.15 eV 3.23 eV

3 3.2 3.4 3.6 3.8 4 hA(eV)

Undoped TiO2 N-doped TiO2 Fe-undoped TiO2 Fe + N codoped TiO2 (c)

Figure 5: UV-vis absorption spectroscopy of undoped, Fe-doped, N-doped, and Fe + N codoped TiO2 with the wavelength in the range of (a) 200–700 nm, (b) 330–450 nm, and (c) (αhv)2 versus hv curves.

with that in Fe2O3 (710.7 eV for Fe 2p3/2)[38]. The variation significant enhancement of absorption visible light range in the elemental binding energy due to the difference in is observed in N-doped and Fe + N codoped samples. As the chemical potential and polarizability of Fe ions in Fe + itcanbeseenfromFigure5(a), the spectrum of Fe + N 3+ NcodopedTiO2 and Fe2O3 indicates that Fe ion has codoped sample has the largest red-shift. The local spectra incorporated into the TiO2 lattice and Fe–O–Ti bonds form (Figure 5(b)) indicate that the onset of the absorption in the Fe + N codoped sample [39]. The calculations of the spectrum appears at 384 nm, 394 nm, 396.3 nm, and 405 nm, N 1s core level spectrum and Fe 2p core level spectrum show for undoped, Fe-doped, N-doped, and Fe + N codoped TiO2, that the N and Fe content is 3.15% and 1.09%, respectively. respectively. According to the equation λ = 1240/Eg , the Figure 5(a) illustrates the UV-vis absorption spec- band gaps of the undoped, Fe-doped, N-doped, and Fe + troscopy of the undoped, Fe-doped, N-doped, and Fe + N NcodopedTiO2 are 3.23 eV, 3.15 eV, 3.13 eV, and 3.06 eV. 2 codoped TiO2.TheundopedTiO2 exhibits the characteristic Figure 5(c) shows the plots of (αhv) versus hv deduced from spectrum of TiO2 with its fundamental absorption sharp Figure 5(a). The band gap of each samples is determined by edge around 380 nm. Compared with the undoped TiO2, fitting the absorption spectra data according to the equation 2 the absorption edge is shifted toward visible light range for (αhv) = B(hv − Eg )(α is the absorption coefficient; hv Fe-doped, N-doped, and Fe + N codoped samples and a is the photo energy; B is a constant number; and Eg is 6 International Journal of Photoenergy the absorption band gap energy). As it can be seen that 1 the band gaps of the undoped, Fe-doped, N-doped, and Fe + N codoped TiO2 are 3.23 eV, 3.15 eV, 3.13 eV, and 0.9 3.06 eV, which are consistent with the results of Figure 5(b) and similar to those reported in [40, 41],butmuchhigher 0.8 0

than the values calculated by Romero-Gomez et al. [23]. The C / reason for this deviation may be that the doping content C 0.7 − in experiments is much lower than that in the theoretical 1 model. 0.6 As to the origin of visible-light sensitivity in Fe, N 0.5 and Fe + N codoped TiO2, the possible mechanism can be proposed. It is known that the band gap of TiO can be 2 0.4 tailored by doping with some nonmetal or metal elements. Herein, for the Fe-doped sample, the absorption edge red- 0 30 60 90 120 150 180 210 240 shift could be understood by the band gap narrowing, Time (min) which is the result of the induced sub-band-gap transition Undoped TiO N-doped TiO corresponding to the excitation of 3d electrons of Fe3+ to 2 2 Fe-doped TiO2 Fe + N codoped TiO2 TiO2 conduction band [41, 42]. In nitrogen doped sample, the doped N ions could induce an add-on shoulder on the Figure 6: Degradation curves of MB for the undoped, Fe-doped, edge of the valence band maximum and the localized N N-doped, and Fe + N codoped samples. 2p states above the valence band [40]. On the other hand, it is known that N has a lower valence state than O so that the incorporation of N must promote the synchronous The Fe3+ energy level is just below the conduction band formation of oxygen vacancies for the charge equilibrium of anatase [41, 42]. The nitrogen doping could induce an in TiO2 [43]. Therefore, the band narrowing due to newly add-on shoulder on the edge of the valence band maximum formed oxygen vacancies by N-doping in TiO2 cannot be and the localized N 2p states above the valence band [40]. neglected. As a result, the visible light response for N-doped Therefore, after Fe + N codoping, the anatase-rutile mixed TiO2 is attributed to both oxygen vacancies and the N 2p TiO2 nanowires have complex band lineup, similar with the states. For Fe + N codoped sample, compared with the N- band lineup of rutile and anatase [25]. This complex stag- doped TiO2, the Fe element is introduced into the lattice ﬀ 3+ gered alignment of the bands e ectively enhances the charge of TiO2 as the oxide state of Fe by above discussion, so separation with migrating holes accumulating in rutile while that Fe-doping is a p-type doping and can increase the electrons in anatase, thus enhancing photocatalytic activity, oxygen vacancies induced by the hole doping of N, similar accordingly. as [43]. Consequently, the codoped TiO2 results in the best response to visible-light and the largest red-shift because of 4. Conclusions the synergistic eﬀect of Fe and N codopant.

To evaluate the photocatalytic activities of the undoped, In conclusion, TiO2 nanowires with mixed rutile-anatase Fe-doped, N-doped, and Fe + N codoped samples, experi- phase were synthesized by a two-step anodic oxidation mentswerecarriedoutforMBdegradationinanaqueous method. Doping with nitrogen or iron could enhance the vis- solution under visible light irradiation by insert a ﬁlter (λ ible light absorption and the photocatalytic activity of TiO2 400 nm) between the Xe-lamp and the samples. Figure 6 nanowires. The Fe + N codoped TiO2 nanowire possesses the shows the photocatalytic degradation curves of MB catalyzed highest visible light absorption and the best photocatalytic by the samples. The photocatalytic activity is in the order activity for catalyzing the degradation of methylene blue. The of the Fe + N codoped TiO2 > N-doped TiO2 > Fe-doped increase of the quantity of O vacancy and the enhancement TiO2 > undoped TiO2.TheFe+NcodopedTiO2 shows the of the charge separation in the nanowires due to the complex best photocatalytic activity and leads to a removal of MB up staggered alignment of the bands leading by Fe and N to 57.42% in 4 hours. codoping may be two of the reasons for the increase of It has been reported that oxygen vacancy induced by photocatalytic activity in the Fe + N codoped sample. N doping or self-doping plays an important role in the photocatalytic activity of TiO2 catalyst by trapping the Acknowledgments photoinduced electron and acting as a reactive center for the photocatalytic process [44, 45]. Fe + N codoping on the pho- The authors would like to acknowledge the ﬁnancial support tocatalytic activity in anatase-rutile mixed TiO2 nanowires, from the National Basic Research Program (973 Program) p-type Fe doping can increase the oxygen vacancies in N- (no. 2009CB939704 and no. 2009CB939705), the National doped TiO2 so that the photocatalytic activity of the sample Natural Science Funds (no. 51172166, 10974148, 61106005), is enhanced after introducing Fe3+ to form the Fe + N National Science Fund for Talent Training in Basic Science codoped sample. It is known that the minimum of the (no. J0830310), and Ministry of Education Key Laboratory conduction band of the rutile phase lies at higher in energy for the Green Preparation and Application of Functional than that of the anatase phase by about 0.3-0.4 eV [25]. Materials (Hubei University). International Journal of Photoenergy 7

References [17] H. Liu, Z. Lu, L. Yue et al., “(Mo+N) codoped TiO2 for enhanced visible-light photoactivity,” Applied Surface Science, [1] C.-C. Wu, Y.-J. Zhuo, P.-N. Zhu, B. Chi, J. Pu, and J. Li, vol. 257, no. 22, pp. 9355–9361, 2011. “Tunable fabrication of TiO2 nanotube arrays with high aspect [18] J. Yang, J. Dai, and J. Li, “Synthesis, characterization and ratio and its application in dye sensitized solar cell,” Journal of degradation of Bisphenol A using Pr, N co-doped TiO2 with Inorganic Materials, vol. 24, no. 5, pp. 897–901, 2009. highly visible light activity,” Applied Surface Science, vol. 257, [2] S. Sreekantan, R. Hazan, and Z. Lockman, “Photoactivity no. 21, pp. 8965–8973, 2011. of anatase-rutile TiO2 nanotubes formed by anodization [19] H. Gao, B. Lu, F. Liu, Y. Liu, and X. Zhao, “Photocatalytical method,” Thin Solid Films, vol. 518, no. 1, pp. 16–21, 2009. properties and theoretical analysis of N, Cd-codoped TiO2 [3]Y.Liu,B.Zhou,J.Baietal.,“Efficient photochemical water synthesized by thermal decomposition method,” International splitting and organic pollutant degradation by highly ordered Journal of Photoenergy, vol. 2012, Article ID 453018, 9 pages, TiO2 nanopore arrays,” Applied Catalysis B,vol.89,no.1-2, 2012. pp. 142–148, 2009. [20] T. Umebayashi, T. Yamaki, H. Itoh, and K. Asai, “Analysis of [4]M.H.Seo,M.Yuasa,T.Kida,J.S.Huh,K.Shimanoe,andN. electronic structures of 3d transition metal-doped TiO2 based Yamazoe, “Gas sensing characteristics and porosity control of on band calculations,” Journal of Physics and Chemistry of nanostructured films composed of TiO2 nanotubes,” Sensors Solids, vol. 63, no. 10, pp. 1909–1920, 2002. and Actuators B, vol. 137, no. 2, pp. 513–520, 2009. [21] K.-S. Ahn, Y. Yan, S. Shet, T. Deutsch, J. Turner, and M. Al- [5] X. Chen and S. S. Mao, “Titanium dioxide nanomaterials: syn- Jassim, “Enhanced photoelectrochemical responses of ZnO thesis, properties, modifications and applications,” Chemical films through Ga and N codoping,” Applied Physics Letters, vol. Reviews, vol. 107, no. 7, pp. 2891–2959, 2007. 91, no. 23, Article ID 231909, 2007. [6] X. Yang, C. Cao, K. Hohn et al., “Highly visible-light active C- [22] W. Zhu, X. Qiu, V. Iancu et al., “Band gap narrowing of and V-doped TiO2 for degradation of acetaldehyde,” Journal of titanium oxide semiconductors by noncompensated anion- Catalysis, vol. 252, no. 2, pp. 296–302, 2007. Cation codoping for enhanced visible-Light photoactivity,” [7] T. Ohno, M. Akiyoshi, T. Umebayashi, K. Asai, T. Mitsui, and Physical Review Letters, vol. 103, no. 22, Article ID 226401, 2009. M. Matsumura, “Preparation of S-doped TiO2 photocatalysts and their photocatalytic activities under visible light,” Applied [23] P. Romero-Gomez, A. Borras, A. Barranco, J. P. Espinos, and Catalysis A, vol. 265, no. 1, pp. 115–121, 2004. A. R. Gonzalez-Elipe, “Enhanced photoactivity in bilayer films with buried rutile-anatase heterojunctions,” ChemPhysChem, [8] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, vol. 12, no. 1, pp. 191–196, 2011. “Visible-light photocatalysis in nitrogen-doped titanium oxides,” Science, vol. 293, no. 5528, pp. 269–271, 2001. [24] P.Deak,´ B. Aradi, and T. Frauenheim, “Band lineup and charge carrier separation in mixed rutile-anatase systems,” Journal of [9] C.-C. Hu, T.-C. Hsu, and L.-H. Kao, “One-step cohydrother- Physical Chemistry C, vol. 115, no. 8, pp. 3443–3446, 2011. mal synthesis of nitrogen-doped titanium oxide nanotubes [25] K. Y. Jung and S. B. Park, “Anatase-phase titania: preparation with enhanced visible light photocatalytic activity,” Interna- by embedding silica and photocatalytic activity for the decom- tional Journal of Photoenergy, vol. 2012, Article ID 391958, 9 position of trichloroethylene,” Journal of Photochemistry and pages, 2012. Photobiology A, vol. 127, no. 1–3, pp. 117–122, 1999. [10] Y.-C. Tang, X.-H. Huang, H.-Q. Yu, and L.-H. Tang, [26] J. Zhu, W. Zheng, B. He, J. Zhang, and M. Anpo, “Characteri- “Nitrogen-doped TiO photocatalyst prepared by mechan- 2 zation of Fe-TiO photocatalysts synthesized by hydrothermal ochemical method: doping mechanisms and visible pho- 2 method and their photocatalytic reactivity for photodegra- toactivity of pollutant degradation,” International Journal of dation of XRG dye diluted in water,” Journal of Molecular Photoenergy, vol. 2012, Article ID 960726, 10 pages, 2012. Catalysis A, vol. 216, no. 1, pp. 35–43, 2004. ˇ [11] V. Stengl and T. M. Grygar, “The simplest way to iodine- [27] J. Yu, M. Zhou, H. Yu, Q. Zhang, and Y. Yu, “Enhanced pho- doped anatase for photocatalysts activated by visible light,” toinduced super-hydrophilicity of the sol-gel-derived TiO2 International Journal of Photoenergy, vol. 2011, Article ID thin films by Fe-doping,” Materials Chemistry and Physics, vol. 685935, 13 pages, 2011. 95, no. 2-3, pp. 193–196, 2006. [12] S. Klosek and D. Raftery, “Visible light driven V-doped TiO2 [28] W. Q. Peng, M. Yanagida, and L. Y. Han, “Rutile-anatase photocatalyst and its photooxidation of ethanol,” Journal of TiO2 photoanodes for dye-sensitized solar cells,” Journal of Physical Chemistry B, vol. 105, no. 14, pp. 2815–2819, 2002. Nonlinear Optical Physics and Materials, vol. 19, no. 4, pp. 673– [13] H. Kato and A. Kudo, “Visible-light-response and photocat- 679, 2010. alytic activities of TiO2 and SrTiO3 photocatalysts codoped [29] G. Liu, X. Yan, Z. Chen et al., “Synthesis of rutile-anatase core- with antimony and chromium,” Journal of Physical Chemistry shell structured TiO2 for photocatalysis,” Journal of Materials B, vol. 106, no. 19, pp. 5029–5034, 2002. Chemistry, vol. 19, no. 36, pp. 6590–6596, 2009. [14] K. T. Ranjit and B. Viswanathan, “Synthesis, characterization [30] T. Miyagi, M. Kamei, T. Mitsuhashi, T. Ishigaki, and A. and photocatalytic properties of iron-doped TiO2 catalysts,” Yamazaki, “Charge separation at the rutile/anatase interface: Journal of Photochemistry and Photobiology A, vol. 108, no. 1, a dominant factor of photocatalytic activity,” Chemical Physics pp. 79–84, 1997. Letters, vol. 390, no. 4–6, pp. 399–402, 2004. [15] W. Li, Y. Wang, H. Lin et al., “Band gap tailoring of Nd3+- [31] W. Ren, Z. Ai, F. Jia, L. Zhang, X. Fan, and Z. Zou, “Low doped TiO2 nanoparticles,” Applied Physics Letters, vol. 83, no. temperature preparation and visible light photocatalytic activ- 20, pp. 4143–4145, 2003. ity of mesoporous carbon-doped crystalline TiO2,” Applied [16] Y. Gai, J. Li, S.-S. Li, J.-B. Xia, and S.-H. Wei, “Design Catalysis B, vol. 69, no. 3-4, pp. 138–144, 2007. of narrow-gap TiO2: a passivated codoping approach for [32] H. Irie, Y. Watanabe, and K. Hashimoto, “Carbon-doped enhanced photoelectrochemical activity,” Physical Review Let- anatase TiO2 powders as a visible-light sensitive photocata- ters, vol. 102, no. 3, Article ID 036402, 2009. lyst,” Chemistry Letters, vol. 32, no. 8, pp. 772–773, 2003. 8 International Journal of Photoenergy

[33]J.L.Gole,J.D.Stout,C.Burda,Y.Lou,andX.Chen, “Highly efficient formation of visible light tunable TiO2-xNx photocatalysts and their transformation at the nanoscale,” Journal of Physical Chemistry B, vol. 108, no. 4, pp. 1230–1240, 2004. [34] C. S. Gopinath, “Comment on ”photoelectron spectroscopic investigation of nitrogen-doped titania nanoparticles”,” Jour- nal of Physical Chemistry B, vol. 110, no. 13, pp. 7079–7080, 2006. [35] C. Burda and J. Gole, “Reply to “comment on ‘photoelectron spectroscopic investigation of nitrogen-doped titania nanoparticles”’,” Journal of Physical Chemistry B, vol. 110, no. 13, pp. 7081–7082, 2006. [36]J.F.Molder,W.F.Stickle,P.E.Sobol,andK.D.Bomben, Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, Minn, USA, 2nd edition, 1992. [37]C.D.Wagner,W.M.Riggs,L.E.Davis,J.F.Moulder, andG.E.Muilenberg,Handbook of Xray Photoelectron Spec- troscopy, PerkinElmer Corp., Physical Electronics Division, Eden Prairie, Minn, USA, 1979. [38] J. Li, J. Xu, W.-L. Dai, H. Li, and K. Fan, “Direct hydro-alcohol thermal synthesis of special core-shell structured Fe-doped titania microspheres with extended visible light response and enhanced photoactivity,” Applied Catalysis B,vol.85,no.3-4, pp. 162–170, 2009. [39]J.Wang,D.N.Tafen,J.P.Lewisetal.,“Originofphotocatalytic activity of Nitrogen-doped TiO2 nanobelts,” Journal of the American Chemical Society, vol. 131, no. 34, pp. 12290–12297, 2009. [40] J. Yu, Q. Xiang, and M. Zhou, “Preparation, characterization and visible-light-driven photocatalytic activity of Fe-doped titania nanorods and first-principles study for electronic structures,” Applied Catalysis B, vol. 90, no. 3-4, pp. 595–602, 2009. [41]W.Choi,A.Termin,andM.R.Hoffmann, “The role of metal ion dopants in quantum-sized TiO2: correlation between photoreactivity and charge carrier recombination dynamics,” Journal of Physical Chemistry, vol. 98, no. 51, pp. 13669–13679, 1994. [42] K. Tan, H. Zhang, C. Xie, H. Zheng, Y. Gu, and W. F. Zhang, “Visible-light absorption and photocatalytic activity in molybdenum- and nitrogen-codoped TiO2,” Catalysis Com- munications, vol. 11, no. 5, pp. 331–335, 2010. [43] T. Ihara, M. Miyoshi, Y. Iriyama, O. Matsumoto, and S. Sugihara, “Visible-light-active titanium oxide photocatalyst realized by an oxygen-deficient structure and by nitrogen doping,” Applied Catalysis B, vol. 42, no. 4, pp. 403–409, 2003. [44] I. Nakamura, N. Negishi, S. Kutsuna, T. Ihara, S. Sugihara, and K. Takeuchi, “Role of oxygen vacancy in the plasma-treated TiO2 photocatalyst with visible light activity for NO removal,” Journal of Molecular Catalysis A, vol. 161, no. 1-2, pp. 205–212, 2000. [45] I. Justicia, P. Ordejon,´ G. Canto et al., “Designed self- doped titanium oxide thin films for efficient visible-light photocatalysis,” Advanced Materials, vol. 14, no. 19, pp. 1399– 1402, 2002. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 569716, 10 pages doi:10.1155/2012/569716

Research Article Enhanced Visible Light Photocatalytic Activity of V2O5 Cluster Modiﬁed N-Doped TiO2 for Degradation of Toluene in Air

Fan Dong, Yanjuan Sun, and Min Fu

College of Environmental and Biological Engineering, Key Laboratory of Catalysis Science and Technology of Chongqing Education Commission, Chongqing Technology and Business University, Chongqing 400067, China

Correspondence should be addressed to Fan Dong, [email protected]

Received 2 January 2012; Revised 19 February 2012; Accepted 20 February 2012

Academic Editor: Xuxu Wang

Copyright © 2012 Fan Dong et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

V2O5 cluster-modified N-doped TiO2 (N-TiO2/V2O5) nanocomposites photocatalyst was prepared by a facile impregnation- calcination method. The effects of V2O5 cluster loading content on visible light photocatalytic activity of the as-prepared samples were investigated for degradation of toluene in air. The results showed that the visible light activity of N-doped TiO2 was significantly enhanced by loading V2O5 clusters. The optimal V2O5 loading content was found to be 0.5 wt.%, reaching a removal −1 ratio of 52.4% and a rate constant of 0.027 min , far exceeding that of unmodified N-doped TiO2. The enhanced activity is due to the deposition of V2O5 clusters on the surface of N-doped TiO2. The conduction band (CB) potential of V2O5 (0.48 eV) is lower than the CB level of N-doped TiO2 (−0.19 V), which favors the photogenerated electron transfer from CB of N-doped TiO2 to V2O5 clusters. This function of V2O5 clusters helps promote the transfer and separation of photogenerated electrons and holes. The present work not only displays a feasible route for the utilization of low cost V2O5 clusters as a substitute for noble metals in enhancing the photocatalysis but also demonstrates a facile method for preparation of highly active composite photocatalyst for large-scale applications.

1. Introduction doped TiO2 exhibiting enhanced visible light activities [10– 14]. Among the various types of nonmetal doped TiO2,N- Environmental pollution and energy crisis are the two major doped TiO2 is the most typical and has been intensively global challenges faced by human beings. Semiconductor investigated for solar energy conversion and pollutants deg- photocatalysis is green technology that allows the use of sun- radation [15–18]. However, nitrogen doping intrinsically light for the destruction of pollutants and conversion of solar favored the formation of defects that act as recombination energy to hydrogen, thus providing an attractive route to center, which largely limited the photoactivity of N-doped potentially solve the both problems [1–3]. The widely used TiO2 under visible light illumination [19–22]. photocatalyst TiO2 is, however, only UV active due to its In order to improve the visible light photocatalytic per- relatively large band gap [4, 5]. As UV light accounts for a formance of N-doped TiO2, further modification has been small fraction (4%) of the sun’s energy compared to visible employed, including codoping with nonmetals (C, S, F) [23– light with a large fraction of 45%, the shift in the optical ab- 25], co-doping with metal ions (Fe, La, V) [26–28], noble sorption of TiO2 from the UV to the visible region is of metal deposition (Pt, Au) [29, 30] and metal oxide coupling significance for practical application of photocatalyst [6–8]. (WO3,PdO,ZrO2)[31–33]. The modifications usually show Visible light driven photocatalysis is one of the hottest promotive effects on visible light photocatalytic activity of topics worldwide [9–22]. Pioneered by Asahi et al. who N-doped TiO2. The activity promotion is related to phase reported that nitrogen doping can extend the optical ab- structure, optical absorption, charge transfer, and surface sorption of TiO2 into visible region and enhance the pho- properties. Recently, nanoscale clusters have been utilized tocatalytic activity under visible light irradiation [9], many to modify different types of photocatalysts [34–42]. Highly efforts have been devoted afterwards to developing nonmetal enhanced photocatalytic activities over the investigated 2 International Journal of Photoenergy

cluster/photocatalystcomposites systems,includingNi(OH)2/ photoluminescence spectra were measured with a fluoros- TiO2,Cu(OH)2/TiO2,CuO/self-dopedTiO2,V2O5/C-doped pectrophotometer (PL: Fluorolog-3-Tau, France) using a TiO2,V2O5/TiO2, vanadium species/N-doped TiO2,Fe2O3/ Xe lamp as excitation source with optical filter. Nitrogen WO3, and CdS/graphene have been observed [34–42]. How- adsorption-desorption isotherms were obtained on a nitro- ever, to our knowledge, there are few reports on the enhanced gen adsorption apparatus (ASAP 2020, USA) with all ◦ photocatalytic activity over V2O5 cluster-modified N-doped samples degassed at 200 C prior to measurements. The BET TiO2 up to now. surface area was determined by a multipoint BET method In our previous study, N-doped TiO2 (N-TiO2) pho- using the adsorption data in the relative pressure (P/P0) tocatalyst was prepared by partial oxidation of TiN in air range of 0.05–0.3. [43]. In the present study, we report the facile preparation of V2O5 cluster-modified N-doped TiO2 nanocomposite for the 2.3. Visible Light Photocatalytic Activity. The visible light first time through an impregnation-calcination method and photocatalytic activity was evaluated by removal of toluene in enhanced visible light photocatalytic activity for degradation air in a continuous flow reactor at ambient temperature. The of toluene in air. The microstructure, optical, and surface volume of the rectangular reactor, made of stainless steel and properties of the resulted nanocomposites photocatalysts covered with Saint-Glass, was 4.5 L (30 cm × 15 cm × 10 cm). were investigated systematically by XRD, Raman, TEM, XPS, A 300 W commercial Xe lamp was vertically placed outside UV-vis DRS, and PL. Based on the characterization results, the reactor. Four minifans were used to cool the lamp. For a new mechanism on the promotive effects of V2O5 cluster the visible light photocatalytic activity test, a UV cut-off modification on the charge transfer and visible light photo- filter (420 nm) was adopted to remove UV light in the light catalysis of N-doped TiO2 was discussed and proposed. beam. Photocatalyst (0.20 g) was coated onto a dish with a diameter of 12.0 cm. The coated dish was then treated at 2. Experimental Section 70◦C to remove water in the suspension. The toluene gas was acquired from a compressed gas cylinder at a concentration 2.1. Preparation of Photocatalysts. N-doped TiO2 (N-TiO2) of 100 ppm of toluene. The initial concentration of toluene was prepared by partial oxidation of TiN. In a typical process, was diluted to 1.0 ppm at indoor level by the air stream 3.0 g TiN powder was loaded in a ceramic crucible, and then supplied by compressed gas cylinder. The relative humidity placed in the middle of muffle furnace open to the atmos- (RH) level of the flow system was controlled at 50%. The phere. The temperature was slowly ramped up to 450◦Cata gas streams were premixed completely by a gas blender, and ◦ rate of 15 C/min and kept for 2 h to obtain N-doped TiO2. the flow rate was controlled at 1.0 L/min by a mass flow V2O5 cluster modification was performed by incipient wet- controller. After the adsorption-desorption equilibrium was ness impregnation of N-doped TiO2 with aqueous solutions achieved, the lamp was turned on. The concentration of of NH4VO3 at room temperature, followed by stirring for 1 h, toluene was continuously measured by GC-FID (Shanghai, treating at 150◦C for water evaporation and heating at 300◦C 7890II). The removal ratio (η) of toluene was calculated as for 1 h. The amount of loaded vanadium was controlled at 0, η(%) = (1 − C/C0 ) × 100%, where C and C0 are con- 0.01, 0.05, 0.2, 0.5, and 1.0 wt.%. The as-prepared samples centrations of toluene in the outlet steam and the feeding were labeled as N-TiO2/V2O5–x,wherex represented the stream, respectively. The kinetics of photocatalytic tolu- content of vanadium. For comparison, V2O5 (vanadium: ene removal reaction is a pseudo first-order reaction as 0.5 wt.%) was also loaded on SiO2 instead of N-doped TiO2 ln(C0/C) = kt,wherek is the initial apparent rate constant by the same process. The reference sample was labeled as [44, 45]. SiO2/V2O5–0.5%. Pure V2O5 was prepared accordingly in the absence of N-doped TiO2 and SiO2. 3. Results and Discussion 2.2. Characterization. The crystal phase of the samples was 3.1. Phase Structure. The XRD patterns of as-prepared sam- analyzed by X-ray diffraction with Cu Kα radiation (XRD: ples are shown in Figure 1.ThephasestructureofN- model D/max RA, Rigaku Co., Japan). Raman spectra were TiO2 and V2O5 cluster-modified N-TiO2 samples consists of recorded at room temperature using a micro-Raman spec- mixed phases of anatase (JCPDS file No. 21–1272) and rutile trometer (Raman: RAMANLOG 6, USA) with a 514.5 nm (JCPDS, file no. 77–442). In the absence of substrate N-TiO2, + Ar laser as the excitation source in a backscattering geom- pure orthorhombic V2O5 phase (JSPD file no. 72–433) was etry. The morphology and structure of the samples were produced. No characteristic diffraction peaks of V2O5 are examined by transmission electron microscopy (TEM: JEM- observed in N-TiO2/V2O5 composite samples because of its 2010, Japan). X-ray photoelectron spectroscopy with Al Kα lower loading content, on the other hand, also indicating that X-rays (hν = 1486.6 eV) radiation (XPS: Thermo ESCALAB V2O5 clusters were well dispersed on the N-TiO2 surface [34, 250, USA) was used to investigate the surface properties of 35]. Figure 1 also shows that V2O5 cluster loading has almost the samples. The shift of the binding energy was corrected no influence on the phase structure of N-TiO2. By using using the C1s level at 284.8 eV as an internal standard. The the Debye-Scherrer equation, the crystallite sizes of anatase UV-vis diffuse reflection spectra were obtained for the dry- and rutile phase are calculated to be 18.3 and 22.8 nm, re- pressed disk samples using a Scan UV-vis spectrophotometer spectively. The BET surface areas of N-doped TiO2 are 2 (UV-vis DRS: UV-2450, Japan) equipped with an integrating measured to be 62.8 cm /g, and V2O5 cluster loading is found sphere assembly, using BaSO4 as reflectance sample. The to have little influence on the surface areas of N-doped TiO2. International Journal of Photoenergy 3

V2O5 clusters, elemental mapping images of O, Ti, and V are demonstrated in Figures 3(d)–3(f). It can be seen that V2O5 V2O5 clusters are uniformly deposited on the surface of N-TiO2 nanoparticles, which is also favorable for the interaction N-TiO2/V2O5-1% between V2O5 clusters and N-TiO2.

N-TiO2/V2O5-0.5% 3.4. XPS Analysis. XPS is used to determine the chemical N-TiO2/V2O5-0.2% composition and surface properties of catalysts. Figure 4(a)

Intensity (a.u.) Intensity N-TiO2/V2O5-0.05% shows the binding energy (Eb) for the N1s region of N-TiO2 and N-TiO2/V2O5-0.5% samples. For both samples, Eb of N-TiO2/V2O5-0.01% N1s at around 400 eV can be observed. This Eb is a typical 3− N-doped TiO2 feature of substitutional lattice nitrogen (N ) for oxygen, forming N-Ti-O structure [43]. The formation of N-Ti-O bond is the natural result of partial oxidation of TiN by 10 20 30 40 50 60 70 80 oxygen. The content of doped nitrogen is determined to be 2θ (degree) 0.87 at % and have little change after modification with V2O5 Figure 1: XRD patterns of N-doped TiO2,V2O5 cluster modified clusters according to the XPS result. N-TiO2 and V2O5 samples. Figure 4(b) shows the XPS spectra for O1s region. It can be seen that the O1s region can be fitted into three peaks for both samples, which can be attributed to Ti-O (529.9 eV), -OH hydroxyl groups (531.3 eV), and chemisorbed H2O 3.2. Raman Analysis. Figure 2(a) shows the Raman spectra (532.7 eV), respectively [44]. Further observation in Figure of N-doped TiO2, selected V2O5 cluster modified N-TiO2, 4(b) indicates that the molar ratio of oxygen in hydroxyl and V2O5 samples. The observed characteristic Raman bands groups to all kinds of oxygen contributions increases after at 144, 196, 395, 515, and 638 cm−1 for samples containing V2O5 cluster modification. N-TiO2, assigned to the E , B1 , A1 , B2 ,andE vibrational g g g g g Figure 4(c) shows the Eb for Ti 2p3/2 for both samples. modes of anatase phase TiO2 [46]. The typical Raman bands It can be seen that Ti2p peak at 458.85 eV of N-TiO /V O - −1 2 2 5 of rutile phase TiO2 appear at 143, 235, 447, and 612 cm , 0.5% sample shifted positively by 0.25 eV in comparison with which can be ascribed to the B , two-phonon scattering, 1g that of the Ti2p peak in N-TiO2 sample (458.60 eV). The E ,andA1 modes of rutile phase, respectively [47]. Raman g g shifting of Eb can be ascribed to the interaction between host bands of rutile phase at 143 cm−1 are overlapped by 144 cm−1 N-TiO2 and guest V2O5 clusters, as also confirmed by Raman band of anatase phase. The inset in Figure 2(a) shows the spectra [20]. enlarged Raman bands at 447 cm−1 of rutile phase for Figure 4(d) shows the Eb for V2p3/2 for both samples. The samples containing N-TiO2. As the content of rutile phase is V2p3/2 peak of V2O5 cluster samples can be fitted into two low, other Raman bands of rutile phase cannot be observed peaks, located at 517.3 and 516.2 eV. These two Eb can be directly in Figure 2(a). Raman mode of V2O5 is also shown assigned to V5+ and V4+,respectively[49]. The variation of in Figure 2(a) [48]. No Raman bands relevant to V2O5 are vanadium oxidation state is frequently observed in catalysis. observed for the composite samples, which imply the absence In XPS measurement, N-TiO2 can be exited by the high of a separate crystalline V2O5 phase on the N-TiO2/V2O5 energy of X-rays (hν = 1486.6 eV) to produced electrons samples, consistent with XRD results. The enlarged view in conduction band (CB). The V5+ then accepts the CB −1 of Raman bands in the range of 110–180 cm in Figure electrons to produce V4+. These results also confirm that the 2(b) shows that the bands at 144 cm−1 shift to lower wave electrons transfer from the CB of N-TiO2 to V2O5 clusters on numbers as the loading content of V2O5 increases, suggesting the surface and the partial reduction of V5+ to V4+ [37]. the strong interaction between N-TiO2 and V2O5 clusters. 3.5. UV-Vis DRS. Figure 5(a) shows UV-vis DRS of N-doped 3.3. TEM-EDS Analysis. TEM observation (Figures 3(a) and TiO2,N-TiO2/V2O5,andV2O5 samples and P25. Compared 3(b)) reveals that the N-TiO2 and N-TiO2/V2O5-0.5% sam- with undoped TiO2 P25, optical absorption of N-doped TiO2 ple consists of agglomerates of primary particles of 20– shifts into visible light region (400–550 nm) as the localized 30 nm in diameter. For V2O5 modified sample, some V2O5 N doping level was formed in the band gap. When the clusters with size of ca. 1–3 nm are observed and dispersed amount of vanadium loaded was less than 0.20 wt.%, there on the surface of N-TiO2 (Figure 3(b) and see Figure S1 was no obvious change in visible light absorption compared in supplementary material available on line at doi: 10.1155/ with N-TiO2. The optical absorption in visible region was 2012/569716). The direct contact between V2O5 cluster and significantly increased when the content of vanadium loaded N-TiO2 favors the formation of heterojunction between the was higher than 0.50 wt.%. Pure V2O5 exhibits broad visible two components. Figure 3(c) illustrates the energy-dispersive light absorption. The enhanced absorption of V2O5 cluster- X-ray spectroscopy (EDS) spectra of N-TiO2/V2O5-0.5% modified N-TiO2 nanocomposite can be ascribed to d-d sample. The amount of vanadium content obtained from transition of vanadium species. EDS (Table inset in Figure 3(c)) is in agreement with the Bandgap (Eg ) energies can be estimated from UV-vis modification content. To further reveal the dispersity of DRS spectra. Semiconductors are classified to be indirect or 4 International Journal of Photoenergy

V2O5 V2O5-0.5% 447, R V2O5 N-TiO2/V2O5-0.5% V2O5-0.05% N-TiO2/V2O5-0.5% N-TiO2/V2O5-0.05% N-doped TiO2 N-TiO2/V2O5-0.05% N-doped TiO2 N-doped TiO2 300 350 400 450 500 550 − Wavenumber (cm 1) 638, A 395, A 196, A 515, A Raman intensity (a.u.) Raman intensity (a.u.) 144, A 100 200 300 400 500 600 700 800 110 120 130 140 150 160 170 180 Wavenumber (cm−1) Wavenumber (cm−1) (a) (b)

Figure 2: Raman spectra of N-doped TiO2, selected V2O5 cluster modified N-TiO2 and V2O5 samples (A: anatase, R: rutile). direct according to the lowest allowed electronic transition. direct recombination of electrons and holes, as also con- The relation between absorption coefficient (α) and incident firmed by XPS result on V2p (Figure 4(d)). n photon energy (hν)canbewrittenasαhν = Bd(hν − Eg ) for allowed transitions (n = 2 for indirect transition, n = 1/2 3.7. Photocatalytic Activity and Proposed Mechanism. To ff direct transition), where Bd is the absorption constants. Plot elucidate the e ects of V2O5 cluster modification on the of (αhν)1/2 and (αhν)2 versus hν from the spectra data of photoactivity of N-doped TiO2, visible light photocatalytic degradation of gaseous toluene at indoor level was per- N-TiO2 and V2O5 in Figure 5(a) presented in Figure 5(b) formed. Figure 7 shows the visible light photocatalytic deg- [44, 45]. The Eg estimated from the intercept of the tangents radation curves (Figure 7(a)) and apparent reaction rate to the plots is 2.75 and 2.24 eV for N-TiO2 and V2O5,respectively. The estimated E for V O is consistent with constant k (Figure 7(b))ofSiO2/V2O5,N-TiO2,andN- g 2 5 ff previous report [50]. It is generally recognized that nitrogen TiO2/V2O5 samples with di erent loading content of V2O5. SiO2/V2O5 exhibits negligible activity, which implies that doping could reduce the bandgap of TiO2 by uplifting the position of valence band (VB) while keeping the position of V2O5 aloneisnotactiveundervisiblelightprobablydueto the rapid recombination between CB electrons and VB holes. CB unchanged [15–18]. N-doped TiO2 shows decent visible light activity toward toluene degradation with removal ratio η of 27.5% and k of −1 3.6. PL Spectra. Photoluminescence (PL) emission spectra 0.009 min . With the loading of V2O5 clusters in rang of have been widely used to investigate the efficiency of 0.01∼1.0 wt.%, N-TiO2/V2O5 samples exhibit significantly charge carrier trapping, migration, and transfer in order to enhanced visible light photocatalytic activity than that of understand the fate of electron/hole pairs in semiconduc- unmodified N-TiO2. After loading only 0.01 wt.% of V2O5 tors since PL emission results from the recombination of on N-TiO2, the visible light activity of N-TiO2/V2O5-0.01% photogenerated charge carriers. Figure 6 shows the room- is markedly enhanced with a η 39.7% and k of 0.0165 min−1. temperature PL spectra of selected V2O5 cluster-modified With further increasing V2O5 loading from 0.01 wt.% to N-TiO2 samples under the excitation of 300 nm light and 0.5 wt.%, the visible light activity on N-TiO2/V2O5 is further 425 nm light. As the PL emission reflects the recombination increased and achieves a maximum η of 52.4% and k of −1 rate of excited electrons and holes, a lower PL intensity 0.027 min . When the V2O5 loading content is higher indicates decreased charge recombination rate and enhanced than 0.5 wt.%, a further increase in V2O5 loading leads to charge separation rate [17–20]. It can be seen from Figure 6 a obvious reduction of the photocatalytic activity. This is that the PL intensity of V2O5 cluster modified N-TiO2 is probably due to the following reasons: (i) deposition of ex- lower than that of N-TiO2. As the loading content of V2O5 cessive V2O5 clusters resulted in the decrease (or shield) of increases, the PL intensity decreases correspondingly. This the N-TiO2 surface active sites; (ii) disappearance of surface result indicates that V2O5 loaded on N-TiO2 surface (Inset effect due to the increase of particle size [34, 35]. in Figure 2(a))caneffectively enhance the separation of From what has been observed and discussed above, sev- electron/hole pairs. This also implies that N-TiO2/V2O5 has eral conclusions can be drawn: (1) V2O5 is inactive for pho- a lower recombination rate of electron/hole pairs under both tocatalytic degradation of toluene under visible light irradi- UV and visible light irradiation. This is ascribed to the fact ation although the band gap of V2O5 is suitable for visible that the electrons are excited from the VB to the CB of N- light excitation. (2) After V2O5 cluster modification, the TiO2 and then migrate to V2O5 clusters, which prevents the visible light activities of N-TiO2/V2O5 samples are highly International Journal of Photoenergy 5

(a) (b)

O Ti V Element Weight/% O 66.76 Ti 32.76 V 0.48 Ti

V Ti V 0123456789 (keV) Full scale 385 cts cursor: −0.044 (2231 cts)

(c)

1 mm 1 mm 1 mm

(d) (e) (f)

Figure 3: TEM image of N-doped TiO2 sample (a), TEM image (b), EDS (c) and elemental mapping image (d, e, f) of of N-TiO2/V2O5-0.5% sample.

enhanced. (3) The content of V2O5 cluster signiﬁcantly in- N-TiO2 to V2O5 cluster is directly responsible for the highly ﬂuences visible light activity of N-TiO2. Obviously, the en- enhanced visible light photocatalytic activity. hanced separation of electrons and holes pairs on N-TiO2/ Here comes a fundamental issue. What is the driven force V2O5 nanocomposite due to the CB electron migration from for CB electron of N-TiO2 to migrate to V2O5 clusters? The 6 International Journal of Photoenergy

N-doped TiO2

–OH H2O N-doped TiO2 Intensity (a.u.) Intensity (a.u.)

N-TiO2/V2O5-0.5%

406 404 402 400 398 396 394 536 534 532 530 528 526 524 Binding energy (eV) Binding energy (eV) (a) (b)

N-doped TiO2

N-doped TiO2 Intensity (a.u.) Intensity (a.u.) V5+ N-TiO2/V2O5-0.5% N-TiO2/V2O5-0.5% V4+

468 466 464 462 460 458 456 454 522 521 520 519 518 517 516 515 514 513 512 Binding energy (eV) Binding energy (eV) (c) (d)

Figure 4: XPS spectra for N-doped TiO2 and N-TiO2/V2O5-0.5% samples of N1s (a), O1s (b), Ti2p, (c) and V2p (d).

14 1.4 V2O5 2.5 N-doped TiO2 N-TiO2/V2O5-1% 12 2 Eg = 2.75 eV

N-TiO2/V2O5-0.5% 2 1.2 / 1

) 1.5 N-TiO2/V2O5-0.2% A

10 αh 1 N-TiO2/V2O5-0.005% ( 1 N-TiO2/V2O5-0.01% 0.5

2 8 0.8 N-TiO ) 2 A 0

P25 αh 1.5 2 2.5 3 3.5 4 4.5 5 ( 6 Abs (a.u.) 0.6 Band gap (eV) V O 0.4 4 2 5 Eg = 2.24 eV 0.2 2 0 0 300 400 500 600 700 800 1.6 1.8 2 2.2 2.4 2.6 2.8 3 Wavelength (nm) Band gap (eV) (a) (b)

2 Figure 5: UV-vis DRS of N-doped TiO2,N-TiO2/V2O5,V2O5 samples and P25 (a). Plot of (αhν) versus photon energy of V2O5 (direct 1/2 semiconductor) (b), inset in (b) shows the plot of (αhν) versus photon energy of N-TiO2 (indirect semiconductor). International Journal of Photoenergy 7

N-TiO2 N-TiO2 N-TiO2/V2O5-0.05% N-TiO2/V2O5-0.05% N-TiO2/V2O5-0.5% N-TiO2/V2O5-0.5% N-TiO2/V2O5-1% N-TiO2/V2O5-1% Intensity (a.u.) Intensity (a.u.)

350 400 450 500 550 600 650 700 750 800 500 550 600 650 700 750 Wavelength (nm) Wavelength (nm) (a) (b)

Figure 6: PL spectra of N-TiO2 and selected V2O5 cluster modiﬁed N-TiO2 samples under the excitation of 300 nm light (a) and 425 nm light (b).

0.03 50 (6) 0.025 ) 1

40 − (5) 0.02 (4) (7) (3) 30 0.015

20 0.01 (2) Removal ratio (%)

10 (min constant Reaction 0.005

0 (1) 0 0 20 40 60 80 100 120 140 0.5 0 0.01 0.05 0.2 0.5 1

Irradiation time (min) Weight ratio of V2O5 (%)

SiO2/V2O5 N-TiO2/V2O5-0.2% N-TiO2/V2O5-0% N-TiO2/V2O5-0.5% N-TiO2/V2O5-0.01% N-TiO2/V2O5-1% N-TiO2/V2O5-0.05% (a) (b)

Figure 7: Visible light photocatalytic degradation curves (a) and comparison of the apparent rate constant (k) of as-prepared samples (b), (1) SiO2/V2O5 sample, (2) N-TiO2, (3)–(7) V2O5 cluster modified N-TiO2 samples. band edge positions of CB and VB of semiconductor can can be determined based on the calculated CB position of be determined with the following approach. The CB edge undoped TiO2 P25 and Eg of N-doped TiO2, as also shown 0 (ECB) of a semiconductor at the point of zero charge (pHZPC) in Table 1. 0 = − C − can be predicted by the equation ECB X E 1/2Eg The schematic enhanced charge transfer and separation [35, 50], where X is the absolute electronegativity of the in the V2O5 cluster modified N-doped TiO2 system are illus- semiconductor (for V2O5, X is 6.10 eV [51]; for P25 TiO2, X trated in Figure 8. Under visible light irradiation, electrons is 5.81 eV [51]; X is unknown for N-doped TiO2). EC is the can be excited to the CB of N-TiO2, leaving holes in the VB − energy of free electrons on the hydrogen scale (∼4.5 eV). Eg (1). These holes will react with OH on the catalyst surface is the band gap energy of the semiconductor. The calculated to form •OH radicals (2). Because the potential of V2O5 0 = 0 = positions of CB and VB of V2O5 and P25 are listed in Table (ECB 0.48 eV) is lower than the CB level of N-TiO2 (ECB 1. It is well known that nonmetal doping does not change −0.19 V), the photogenerated CB electrons in N-TiO2 can the CB position of TiO2. The VB position of N-doped TiO2 transfer rapidly to V2O5 clusters and then effectively reduce 8 International Journal of Photoenergy

Table 1: Absolute electronegativity, calculated conduction band (CB) edge, calculated valence band (VB) position, and bandgap energy for P25, V2O5, and N-doped TiO2 at the point of zero charge.

Absolute electronegativity Calculated CB position Calculated VB position Semiconductors Band gap energy E (/eV) (X)(/eV) (/eV) (/eV) g P25 5.81 −0.19 2.81 3.0

V2O5 6.10 0.48 2.72 2.24

N-doped TiO2 — −0.19 2.56 2.75

V2O5 clusters Cu(OH)2,andNi(OH)2 cluster modified TiO2 with highly enhanced photocatalytic activity [34–36]. O The effect of V O cluster modification on the charge 2 Ti 2 5 3d transfer and separation of other semiconductor is, to some e− e− e− − extent, similar to that of the role of noble metals in •− e E =−0.19 eV O2 CB photocatalysis [45, 52–54]. The present work not only = displays a feasible route for the utilization of low cost V2O5 = Eg 2.75 eV ECB 0.48 eV clusters as a substitute for noble metals in enhancing the A N-doped TiO2 h , visible photocatalysis but also demonstrates a facile method for preparation of highly active composite photocatalysts for OH• environmental application. EVB = 2.56 eV h+ h+ h+

O2p +N2p 4. Conclusion –OH In order to enhance the visible light photocatalytic activity

V2O5 clusters of N-doped TiO2 for practical environmental application, a facile impregnation-calcination method is developed for Figure 8: Schematic illustration for the enhanced charge transfer the preparation of highly active V2O5 cluster modified N- and separation in the V2O5 cluster modified N-doped TiO2 system doped TiO2 nanocomposites photocatalyst for degradation under visible light irradiation. of toluene in air. The V2O5 clusters can act as electron mediatortoeffectively inhibit the recombination of photogenerated electron/hole pairs. The optimal V2O5 loading partial V5+ to V4+ (see Figure 4(d)), thus promoting the content is determined to be 0.5 wt.%, and the corresponding separation of and transfer of photogenerated electrons and toluene removal ratio is 52.4%, which largely exceeds that holes pairs. The PL experiments further confirm the transfer of unmodified N-doped TiO2. The CB potential of V2O5 of photogenerated electrons from N-TiO2 to V2O5 clusters (0.48 eV) is lower than the CB level of N-doped TiO2 (Figure 6). The electrons accepted by V5+ can then be (−0.19 V), which is the driven force for the electron transfer transferred quickly to oxygen molecules under the aerated from CB of N-doped TiO2 to V2O5 clusters, thus greatly 4+ •− condition to regenerate V to produce O2 superoxide promoting the visible light activity of N-doped TiO2. This anion radicals. The process can be defined as V5+/V4+ redox work not only provides a feasible route for utilizing low-cost cycle, as shown in the following equations (3)-(4): V2O5 clusters as a substitute for noble metals in enhancing the visible light photocatalysis but also demonstrates a ν −→ + − N-TiO2 + (visible) h +e (1) facile method for preparation of highly active composites photocatalyst for large scale application. OH− +h+ −→ • OH (2)

5+ − 4+ Acknowledgment V (V2O5) +e −→ V (3) This research is ﬁnancially supported by the National Natural 4+ 5+ •− Science Foundation of China (51108487), the Program for V +O2 −→ V (V2O5) +O2 . (4) Young Talented Teachers in Universities (Chongqing, 2011), •− The •OH and O2 radicals are known to be the most the National High Technology Research and Development oxidizing species in photocatalysis reaction [1–3]. These Program (863 Program) of China (2010AA064905), The key results suggest that the separation of electrons and holes can discipline development project of CTBU (1252001), Projects be eﬀectively enhanced by V2O5 clusters, which will greatly from Chongqing Education Commission (KJTD201020, enhance the visible light photocatalytic activity of N-doped KJZH11214, KJ090727), and Natural Science Foundation of TiO2. Similar phenomena have also been observed on CuO, Chongqing (CSTC, 2010BB0260). International Journal of Photoenergy 9

References [18] F. Dong, W. Zhao, Z. Wu, and S. Guo, “Band structure and visible light photocatalytic activity of multi-type nitrogen [1] C. Chen, W. Ma, and J. Zhao, “Semiconductor-mediated pho- doped TiO2 nanoparticles prepared by thermal decomposi- todegradation of pollutants under visible-light irradiation,” tion,” Journal of Hazardous Materials, vol. 162, no. 2-3, pp. Chemical Society Reviews, vol. 39, no. 11, pp. 4206–4219, 2010. 763–770, 2009. [2] A. V. Emeline, V. N. Kuznetsov, V. K. Rybchuk, and N. Ser- [19] Y. Cong, J. Zhang, F. Chen, M. Anpo, and D. He, “Preparation, pone, “Visible-light-active titania photocatalysts: the case of N photocatalytic activity, and mechanism of nano-TiO2 Co- doped TiO2-properties and some fundamental issues,” Inter- doped with nitrogen and iron (III),” Journal of Physical Chem- national Journal of Photoenergy, vol. 2008, Article ID 258394, istry C, vol. 111, no. 28, pp. 10618–10623, 2007. 19 pages, 2008. [20] F. Dong, H. Wang, Z. Wu, and J. Qiu, “Marked enhancement [3]J.Mo,Y.Zhang,Q.Xu,J.J.Lamson,andR.Zhao,“Photo- of photocatalytic activity and photochemical stability of N- 3+ 2+ catalytic purification of volatile organic compounds in indoor doped TiO2 nanocrystals by Fe /Fe surface modification,” air: a literature review,” Atmospheric Environment, vol. 43, no. Journal of Colloid and Interface Science, vol. 343, no. 1, pp. 200– 14, pp. 2229–2246, 2009. 208, 2010. [4] S. Liu, J. Yu, and M. Jaroniec, “Anatase TiO2 with dominant [21] J. Lu, Y. Dai, H. Jin, and B. Huang, “Effective increasing of high-energy 001 facets: synthesis, properties, and applica- optical absorption and energy conversion efficiency of anatase tions,” Chemistry of Materials, vol. 23, no. 18, pp. 4085–4093, TiO2 nanocrystals by hydrogenation,” Physical Chemistry 2011. Chemical Physics, vol. 13, no. 40, pp. 18063–18068, 2011. [5] H. Zhang, G. Chen, and D. W. Bahnemann, “Photoelectro- [22] J. Zhang, Y. Wu, M. Xing, S. A. K. Leghari, and S. Sajjad, catalytic materials for environmental applications,” Journal of “Development of modified N doped TiO2 photocatalyst with Materials Chemistry, vol. 19, no. 29, pp. 5089–5121, 2009. metals, nonmetals and metal oxides,” Energy and Environmen- [6] X. Chen, S. Shen, L. Guo, and S. S. Mao, “Semiconductor- tal Science, vol. 3, no. 6, pp. 715–726, 2010. based photocatalytic hydrogen generation,” Chemical Reviews, [23] F. Dong, W. Zhao, and Z. Wu, “Characterization and pho- vol. 110, no. 11, pp. 6503–6570, 2010. tocatalytic activities of C, N and S co-doped TiO2 with 1D [7] D. Zhang, G. Li, and J. C. Yu, “Inorganic materials for pho- nanostructure prepared by the nano-confinement effect,” Na- tocatalytic water disinfection,” Journal of Materials Chemistry, notechnology, vol. 19, no. 36, Article ID 365607, 2008. vol. 20, no. 22, pp. 4529–4536, 2010. [24] P. Periyat, D. E. McCormack, S. J. Hinder, and S. C. Pillai, [8] X. Nie, S. Zhuo, G. Maeng, and K. Sohlberg, “Doping of TiO2 “One-pot synthesis of anionic (nitrogen) and cationic (sulfur) polymorphs for altered optical and photocatalytic properties,” codoped high-temperature stable, visible light active, anatase International Journal of Photoenergy, vol. 2009, Article ID Photocatalysts,” Journal of Physical Chemistry C, vol. 113, no. 294042, 22 pages, 2009. 8, pp. 3246–3253, 2009. [9] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, [25] Y. huo, Y. jin, J. zhu, and H. li, “Highly active TiO2−x−yNxFy “Visible-light photocatalysis in nitrogen-doped titanium ox- visible photocatalyst prepared under supercritical conditions ides,” Science, vol. 293, no. 5528, pp. 269–271, 2001. in NH4F/EtOH fluid,” Applied Catalysis B,vol.89,no.3-4,pp. [10] S. U. M. Khan, M. Al-Shahry, and W. B. Ingler, “Efficient 543–550, 2009. photochemical water splitting by a chemically modified n- [26] M. Xing, J. Zhang, and F. Chen, “Photocatalytic performance 3+ TiO2,” Science, vol. 297, no. 5590, pp. 2243–2245, 2002. of N-doped TiO2 adsorbed with Fe ions under visible light [11] S. Sakthivel and H. Kisch, “Daylight photocatalysis by carbon- by a redox treatment,” Journal of Physical Chemistry C, vol. modified titanium dioxide,” Angewandte Chemie, vol. 42, no. 113, no. 29, pp. 12848–12853, 2009. 40, pp. 4908–4911, 2003. [27] H. Wei, Y. Wu, N. Lun, and F. Zhao, “Preparation and pho- ff [12] Y. Huang, W. Ho, S. Lee, L. Zhang, G. Li, and J. C. Yu, “E ect tocatalysis of TiO2 nanoparticles co-doped with nitrogen and of carbon doping on the mesoporous structure of nanocrystal- lanthanum,” Journal of Materials Science,vol.39,no.4,pp. line titanium dioxide and its solar-light-driven photocatalytic 1305–1308, 2004. degradation of NOx,” Langmuir, vol. 24, no. 7, pp. 3510–3516, [28]D.E.Gu,B.C.Yang,andY.D.Hu,“VandNco-dopedna- 2008. nocrystal anatase TiO2 photocatalysts with enhanced pho- [13] W. Haiqiang, W. Zhongbiao, and L. Yue, “A simple two-step tocatalytic activity under visible light irradiation,” Catalysis template approach for preparing carbon-doped mesoporous Communications, vol. 9, no. 6, pp. 1472–1476, 2008. TiO2 hollow microspheres,” Journal of Physical Chemistry C, [29]L.H.Huang,C.Sun,andY.L.Liu,“Pt/N-codopedTiO2 vol. 113, no. 30, pp. 13317–13324, 2009. nanotubes and its photocatalytic activity under visible light,” [14] Z. Ai, L. Zhu, S. Lee, and L. Zhang, “NO treated TiO2 as an Applied Surface Science, vol. 253, no. 17, pp. 7029–7035, 2007. efficient visible light photocatalyst for NO removal,” Journal of [30] Y. Wu, H. Liu, J. Zhang, and F. Chen, “Enhanced photo- Hazardous Materials, vol. 192, no. 1, pp. 361–367, 2011. catalytic activity of nitrogen-doped titania by deposited with [15] Y. Cong, J. Zhang, F. Chen, and M. Anpo, “Synthesis and char- gold,” Journal of Physical Chemistry C, vol. 113, no. 33, pp. acterization of nitrogen-doped TiO2 nanophotocatalyst with 14689–14695, 2009. high visible light activity,” Journal of Physical Chemistry C, vol. [31] B. Gao, Y. Ma, Y. Cao, W. Yang, and J. Yao, “Great enhance- 111, no. 19, pp. 6976–6982, 2007. ment of photocatalytic activity of nitrogen-doped titania by [16] K. Yang, Y. Dai, and B. Huang, “Density functional study of coupling with tungsten oxide,” Journal of Physical Chemistry boron-doped anatase TiO2,” Journal of Physical Chemistry C, B, vol. 110, no. 29, pp. 14391–14397, 2006. vol. 114, no. 46, pp. 19830–19834, 2010. [32] Q. Li, N. J. Easter, and J. K. Shang, “As(III) removal by [17] X. Chen, X. Wang, Y. Hou, J. Huang, L. Wu, and X. Fu, “The palladium-modified nitrogen-doped titanium oxide nanopar- effect of postnitridation annealing on the surface property and ticle photocatalyst,” Environmental Science and Technology, photocatalytic performance of N-doped TiO2 under visible vol. 43, no. 5, pp. 1534–1539, 2009. light irradiation,” Journal of Catalysis, vol. 255, no. 1, pp. 59– [33] X. C. Wang, J. C. Yu, Y. Chen, L. Wu, and X. Z. Fu, “ZrO2- 67, 2008. modified mesoporous manocrystalline TiO2−xNx as efficient 10 International Journal of Photoenergy

visible light photocatalysts,” Environmental Science & Technol- [48] F. Chen, J. Wang, J. Q. Xu, and X. P. Zhou, “Visible light ogy, vol. 40, no. 7, pp. 2369–2374, 2006. photodegradation of organic compounds over V2O5/MgF2

[34] J. Yu, Y. Hai, and B. Cheng, “Enhanced photocatalytic H2- catalyst,” Applied Catalysis A, vol. 348, no. 1, pp. 54–59, 2008. production activity of TiO2 by Ni(OH)2 cluster modification,” [49] M. Heber and W. Grunert,¨ “Application of ultraviolet pho- Journal of Physical Chemistry C, vol. 115, no. 11, pp. 4953– toelectron spectroscopy in the surface characterization of 4958, 2011. polycrystalline oxide catalysts. 2. Depth variation of the [35] J. Yu and J. Ran, “Facile preparation and enhanced photoreduction degree in the surface region of partially reduced catalytic H2-production activity of Cu(OH)2 cluster modified V2O5,” Journal of Physical Chemistry B, vol. 104, no. 22, pp. TiO2,” Energy and Environmental Science,vol.4,no.4,pp. 5288–5297, 2000. 1364–1371, 2011. [50] H. Jiang, M. Nagai, and K. Kobayashi, “Enhanced photocat- [36] M. Liu, X. Qiu, M. Miyauchi, and K. Hashimoto, “Cu(II) alytic activity for degradation of methylene blue over V2O5/ 3+ oxide amorphous nanoclusters grafted Ti self-doped TiO2: BiVO4 composite,” Journal of Alloys and Compounds, vol. 479, an efficient visible light photocatalyst,” Chemistry of Materials, no. 1-2, pp. 821–827, 2009. vol. 23, no. 23, pp. 5282–5286, 2011. [51] X. Yong and M. A. A. Schoonen, “The absolute energy po- [37] Z. Wu, F. Dong, Y. Liu, and H. Wang, “Enhancement of the sitions of conduction and valence bands of selected semicon- visible light photocatalytic performance of C-doped TiO2 by ducting minerals,” American Mineralogist,vol.85,no.3-4,pp. loading with V2O5,” Catalysis Communications, vol. 11, no. 2, 543–556, 2000. pp. 82–86, 2009. [52]H.Park,W.Choi,andM.R.Hoffmann, “Effects of the prepa- [38] M. A. Rauf, S. B. Bukallah, A. Hamadi, A. Sulaiman, and ration method of the ternary CdS/TiO2/Pt hybrid photocata- F. Hammadi, “The effect of operational parameters on the lysts on visible light-induced hydrogen production,” Journal of photoinduced decoloration of dyes using a hybrid catalyst Materials Chemistry, vol. 18, no. 20, pp. 2379–2385, 2008. V2O5/TiO2,” Chemical Engineering Journal, vol. 129, no. 1–3, [53] H. Tada, T. Mitsui, T. Kiyonaga, T. Akita, and K. Tanaka, “All- pp. 167–172, 2007. solid-state Z-scheme in CdS-Au-TiO2 three-component nano- [39] K. Bhattacharyya, S. Varma, A. K. Tripathi, S. R. Bharadwaj, junction system,” Nature Materials, vol. 5, no. 10, pp. 782–786, andA.K.Tyagi,“Effect of vanadia doping and its oxidation 2006. state on the photocatalytic activity of TiO2 for gas-phase [54] H. Wang, Z. Wu, Y. Liu, and Y. Wang, “Influences of various oxidation of ethene,” Journal of Physical Chemistry C, vol. 112, Pt dopants over surface platinized TiO2 on the photocatalytic no. 48, pp. 19102–19112, 2008. oxidation of nitric oxide,” Chemosphere, vol. 74, no. 6, pp. 773– [40] S. Higashimoto, W. Tanihata, Y. Nakagawa, M. Azuma, H. 778, 2009. Ohue, and Y. Sakata, “Effective photocatalytic decomposition of VOC under visible-light irradiation on N-doped TiO2 modified by vanadium species,” Applied Catalysis A, vol. 340, no. 1, pp. 98–104, 2008.

[41] D. Bi and Y. Xu, “Improved photocatalytic activity of WO3 through clustered Fe2O3 for organic degradation in the presence of H2O2,” Langmuir, vol. 27, no. 15, pp. 9359–9366, 2011. [42] Q. Li, B. Guo, J. Yu et al., “Highly eﬃcient visible-light-driven photocatalytic hydrogen production of CdS-cluster-decorated graphene nanosheets,” Journal of the American Chemical Soci- ety, vol. 133, no. 28, pp. 10878–10884, 2011. [43] Z. Wu, F. Dong, W. Zhao, and S. Guo, “Visible light induced electron transfer process over nitrogen doped TiO2 nanocrystals prepared by oxidation of titanium nitride,” Journal of Hazardous Materials, vol. 157, no. 1, pp. 57–63, 2008. [44] F. Dong, S. Guo, H. Wang, X. Li, and Z. Wu, “Enhancement of the visible light photocatalytic activity of C-doped TiO2 nanomaterials prepared by a green synthetic approach,” Jour- nal of Physical Chemistry C, vol. 115, no. 27, pp. 13285–13292, 2011. [45] F. Dong, H. Wang, G. Sen, Z. Wu, and S. C. Lee, “Enhanced visible light photocatalytic activity of novel Pt/C-doped TiO2/ PtCl4 three-component nanojunction system for degradation of toluene in air,” Journal of Hazardous Materials, vol. 187, no. 1–3, pp. 509–516, 2011. [46] Y. Su, J. Yu, and J. Lin, “Vapor-thermal preparation of highly crystallized TiO2 powder and its photocatalytic activity,” Jour- nal of Solid State Chemistry, vol. 180, no. 7, pp. 2080–2087, 2007. [47] J. Zhang, M. Li, Z. Feng, J. Chen, and C. Li, “UV raman spectroscopic study on TiO2 I. phase transformation at the surface and in the bulk,” Journal of Physical Chemistry B, vol. 110, no. 2, pp. 927–935, 2006. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 631435, 9 pages doi:10.1155/2012/631435

Research Article Preparation and Characterization of Visible-Light-Activated Fe-N Co-Doped TiO2 and Its Photocatalytic Inactivation Effect on Leukemia Tumors

Kangqiang Huang,1 Li Chen,2 Jianwen Xiong,1 and Meixiang Liao3

1 Laboratory of Quantum Information Technology, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou 510006, China 2 Department of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, China 3 School of Physics and Engineering, Sun Yat-Sen University, Guangzhou 510275, China

Correspondence should be addressed to Jianwen Xiong, [email protected]

Received 16 January 2012; Revised 19 February 2012; Accepted 19 February 2012

Academic Editor: Baibiao Huang

Copyright © 2012 Kangqiang Huang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The Fe-N co-doped TiO2 nanocomposites were synthesized by a sol-gel method and characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), ultraviolet-visible absorption spectroscopy (UV-vis) and X-ray photoelectron spectroscopy (XPS). Then the photocatalytic inactivation of Fe-N-doped TiO2 on leukemia tumors was investigated by using Cell Counting Kit-8 (CCK-8) assay. Additionally, the ultrastructural morphology and apoptotic percentage of treated cells were also studied. The experimental results showed that the growth of leukemic HL60 cells was significantly inhibited in groups treated with TiO2 nanoparticles and the photocatalytic activity of Fe-N-TiO2 was significantly higher than that of Fe-TiO2 and N-TiO2, indicating that the photocatalytic efficiency could be effectively enhanced by the modification of Fe-N. Furthermore, when 2 wt% Fe-N-TiO2 nanocomposites at a final concentration of 200 μg/mL were used, the inactivation efficiency of 78.5% was achieved after 30-minute light therapy.

1. Introduction nanoparticle in this case is regarded as a potential anticancer drug or photosensitizer for photodynamic therapy (PDT) to Titanium dioxide (TiO2) as photocatalyst has been widely improve the traditional photodynamic effect. However, TiO2 used for industrial and medical applications due to its useful was only excited by UVlight due to its wide band gap (ap- physical and biological properties in the last two decades, proximately 3.2 eV). Additionally, the photogenerated holes such as disposal of wastewater, decontamination of air pol- are easy to recombine with the photoinduced electrons, lutants, and sterilization of bacteria [1–4]. It has been well which greatly reduce the photocatalytic efficiency of TiO2 known that photoinduced electrons and holes could be nanoparticles and hinder its practical applications [16–19]. generated on the TiO2 surface under exposure to ultraviolet Fortunately, it has been demonstrated that the visible light (UV) light [5, 6]. These excited electrons and holes have absorption and photocatalytic activity of TiO2 can be ef- strong reduction and oxidation activities and could further fectively improved by the method of metal or nonmetal el- react with hydroxyl ions or water resulting in the formation ements doping [20–24]. We aim to enhance the photocat- of various reactive oxygen species (ROS) which have been alytic inactivation efficiency of TiO2 on tumor cells by the proved to significantly damage cancer cells [7–9]. modification of Fe-N. Recently, with the rapid development of nanotechnology, In the present work, Fe-N-doped TiO2 nanocomposites research involving use of TiO2 nanoparticle in biomedical were used as a “photosensitizer” of PDT for tumor treatment fields has also drawn attention [10, 11]. TiO2 for photother- in vitro. Up to our knowledge, there are still no previous apy of cancer cells has been noticed [12–15]. Therefore, TiO2 reports on the study of photocatalytic inactivation effects of 2 International Journal of Photoenergy

Fe-N-doped TiO2 on leukemia HL60 cells. The aims of the automated cell counter and adjusted to the required final present study were focused on the possible use of co-doped concentration. Cells viability before treatment was always TiO2 as an anticancer agent in the presence of visible light over 95%. and its potential therapeutic effect on leukemia-tumor-based PDT. 2.4. Cell Viability Assay. The immediate cytotoxicity of the HL60 cells after treatment was assessed using the trypan blue 2. Materials and Methods exclusion test. The viable/dead cells were counted using a countess automated cell counter. Viability for the samples 2.1. Chemicals and Apparatus. HL60 cells were kindly pro- were further evaluated by Cell Counting Kit-8 assays (CCK- vided by the Department of Medicine of Sun Yat-Sen Uni- 8 assay). Cell suspension (200 μL) was seeded onto 96-well versity. Fluo-3 AM was purchased from Sigma (USA). Cell plate and incubated with 20 μL CCK-8 solution at 37◦Cin Counting Kit-8 (CCK-8) assays were purchased from a humidified 5% CO2 atmosphere. After 4 h incubation, the Dojindo (Japan). RPMI medium 1640 was obtained from absorbance (OD values) at 490 nm was determined using the Gibco BRL (USA). All chemicals used were of the highest Model 680 Microplate Reader. The percentage of viability purity commercially available. The stock solutions of the was determined by comparison with untreated cells. compounds were prepared in serum-free medium immediately before using in experiments. 2.5. Statistical Analysis. Data are presented as means ± These apparatuses, including ZEISS Ultra-55 scanning SD (standard deviation) from at least three independent electron microscope (Carl Zeiss, Germany), JEM-2100HR experiments. Statistical analysis was then performed using transmission electron microscope (JEOL, Japan), BRUKER the statistical software SPSS11.5, and values of P<0.05 were ff D8 ADVANCE X-ray powder di ractometer (XRD) (Bruker, considered statistically significant. Germany), U-3010 UV-visible spectrophotometer (Hitachi, Japan), AXIS Ultra X-ray photoelectron spectroscopy (XPS) (Kratos, UK), the Countess Automated Cell Counter (Invit- 3. Results and Discussion rogen, USA), a photodiode (Hitachi, Japan), Model 680 3.1. Characterization of Fe-N-TiO2 Nanocomposites Microplate Reader (Bio-Rad, USA), HH.CP-TW80 CO2 incubator, and PDT reaction chamber, were used. 3.1.1. SEM and TEM Studies. The morphology and particle size of the pure TiO2 andFe-Nco-dopedTiO2 nanocom- 2.2. Preparation of Fe-N Co-Doped TiO2 Nanocomposites posites were observed with a scanning electron microscope. Solutions. The 2 wt% Fe-N-TiO2 nanocomposites were syn- TEM analysis was also performed using a JEM-2100HR thesized using sol-gel method [25, 26]. Firstly, 19 mL of tetra- microscope to obtain further information and support SEM butyl titanate (55.6 mmol) and 0.32 g Fe(NO3)2 (0.4 mmol results. was dissolved in 60 mL of anhydrous ethanol at room tem- ThemorphologiesofpureTiO2and Fe-N-TiO2 prepared perature to prepare solution A. Meanwhile, the appropriate by sol-gel method at 400◦C are shown in Figure 1.Itcanbe amount of hydroxylamine hydrochloride was mixed with seen that the average size of pure TiO2 particles is significant- 2 mL of doubly distilled water and 16 mL of anhydrous ly larger than that of Fe-N-TiO2 (Figures 1(a) and 1(b)). It ethanol to prepare solution B. Afterwards, solution A was appears that the Fe-N-TiO2 particles are spherical or square slowly added to solution B at a rate of 2 mL per minute shaped with a primary particle size of from 18 to 20 nm. under vigorous stirring within 10 min. The solution was sub- Figures 1(c) and 1(d) display the TEM images of consequently stirred for further 30–60 min. The prepared TiO2 trolled TiO2 and Fe-N-doped TiO2. As shown in Figure 1(d), ◦ gels after washing with deionized water were dried at 120 C the particle size of most of Fe-N-doped TiO2 samples is for 12 h. The 2 wt% Fe-N co-doped TiO2 nanocomposites approximately 19.0 nm, which is basically consistent with the were obtained after calcination at 400◦Cfor2handfinally SEM observation. Furthermore, with careful observation we grinded for 15 min. Additionally, the pure TiO2,2wt%Fe- can find that there are some fuscous points on the doped TiO2, and 2 wt% N-TiO2 were prepared through a similar sample surfaces; maybe this can be explained by the fact that procedure. Fe and N have been successfully incorporated into the lattice The prepared samples were encapsulated in four bottles, of TiO2 structure. respectively, and then placed in YX-280B-type pressure steam sterilizer to sterilize for 30 minutes. Finally, an appro- 3.1.2. X-Ray Diffraction. XRD was used to further investigate priate amount of culture medium was added to fully dissolve the crystalline structural properties of the Fe-N-doped TiO2, the nanoparticles. All solutions were filtered through a and the XRD patterns of TiO2,2wt%Fe-TiO2, 2wt%N-TiO2, 0.22 μm membrane filter and stored in the dark at 4◦Cbefore 2wt%Fe-N-TiO2 are presented in Figure 2. being used in the experiments. Figure 2 shows that the four synthesized samples have the highest diffraction peak in (101) crystal plane (2θ = 2.3. Cell Culture. Human leukemia HL60 cells were cultured 25.3◦), and the other diffraction peaks are consistent with in RPMI 1640 medium supplemented with 10% fetal bovine crystalline phases of (004), (200), (105), (211), and (204). serum (FBS) in a humidified incubator with 5% CO2 at Thus, the doped TiO2 nanocomposites obtained by the sol- 37◦C. The cell concentration was measured using a countess gel method have primarily the anatase phase. Additionally, International Journal of Photoenergy 3

(a) (b)

Figure 1: The SEM and TEM images of TiO2 particles. (a) and (c): pure TiO2;(b)and(d):2wt%Fe-N-TiO2.

approximately 21.5 nm, 20.7 nm, 21.0 nm, and 19.3 nm for pure TiO2,Fe-dopedTiO2, N-doped TiO2, and Fe-N-doped TiO2, respectively. As can be seen from the values, the crystallite sizes of Fe-N-doped TiO2 are smaller than those of others, indicating that Fe-N-doped TiO2 nanocomposites are highly crystallized which is in good accordance with the results of the TEM shown in Figure 1. Intensity (a.u.) Intensity

3.1.3. UV-Vis Spectroscopy. The Fe-N-TiO2 nanocomposites have been also identified with UV-vis adsorption spectra. As is shown in Figure 3, the spectrum obtained from the controlled TiO2 has a sharp absorption edge at 393 nm due 20 30 40 50 60 70 80 to its intrinsic band gap showing that absorption only in 2θ (deg) the ultraviolet light region (less than 400 nm). However, the TiO2 N-TiO2 absorption thresholds of the doped TiO2 are slightly shifted Fe-TiO2 Fe-N-TiO2 towards the visible region of the spectrum. These results demonstrate that the absorption for the doped TiO2 in the Figure 2: The XRD patterns of pure and doped TiO2 calcined at 400◦C. visible light region is significantly enhanced compared with that of pure TiO2. Additionally, as can be seen in Figure 3, the onset of absorption edge of Fe-N-doped TiO2 is extended from 393 nm to visible range 425 nm, indicating that the compared to the undoped TiO2, there are no indications visible light absorption of TiO2 nanoparticles has been ef- of peaks corresponding to Fe or N observed; the reason fectively enhanced by the incorporation of Fe and N, which may be attributed to the amount of Fe and N being little in turn may considerably increase the photocatalytic activity and well dispersed in the TiO2 surface. Furthermore, the of TiO2 under visible light irradiation. average crystallite sizes of the samples were roughly estimated In order to reach a high photocatalytic inactivation using Scherer’s equation [27], and the obtained sizes are efficiency, a built lamp with many high-power light-emitting 4 International Journal of Photoenergy

1.5 1.1 1 1.05 0.8 1 1.2 410.25 nm 0.6 0.95 0.9 0.4 0.9 0.85 0.2 0.8 0 0.75

0.6 intensity (a.u.) Normalized 200 300 400 500 600 700 0.7

Wavelength (nm) viability of Relative cells 0.65 Absorption intensity (a.u.) Absorption 0.3 0.6 0.55 0 0.5 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800 900 1000 1100 Wavelength (nm) Concentration of nanoparticle (μg/mL)

TiO2 N-TiO2 TiO2 N-TiO2 Fe-TiO Fe-N-TiO Fe-TiO2 Fe-N-TiO2 2 2

Figure 3: The UV-vis absorption spectra of TiO with different Figure 5: The influence of Fe-N-TiO2 concentrations on the rela- 2 ± doping. Inset is the normalized emission spectra of the blue LEDs. tive viability of HL60 cells. Data are presented as the means SD from five independent measurements. ∗P values are less than 0.05 as compared with untreated control cells.

Ti O1s O the doped photocatalyst, which may cause a change in the charge distribution of the atoms on the photocatalyst surface and resulting in enhancing the photocatalytic activity. We Fe2p can also ﬁnd the characteristic peak at 399.7 eV which is Ti2p Ti2p corresponding to N1s. It has been clearly determined that the Fe and N have already been incorporated into Fe-N-TiO2

Intensity (cps) Intensity nanocomposites. Additionally, the concentrations of Fe and N, determined by XPS, of the Fe-N-TiO2 are 0.91 wt% and N1s 0.97 wt%, respectively, which is basically consistent with the C1s theoretical expectation.

3.2. Cytotoxicity of TiO2 Nanoparticles or Doped TiO2 Nano- 200 400 600 800 1000 1200 composites on Leukemia Tumor Cells. It is well known that Binding energy (eV) the photosensitive drugs used for cancer treatment not only have high photocatalytic inactivation capability under light Figure 4: The XPS spectra of 2 wt% Fe-N-TiO2 nanocomposites calcined at 400◦C. irradiation, but also have no toxicity in the dark. Therefore, it is very important to investigate the cytotoxicity of TiO2 nanocomposites without light treatment. The toxicity of Fe- N-TiO2 was measured by exposing HL60 cells in the medium diodes (LEDs), emitting light in the visible-light region (390– containing various concentrations of Fe-N-TiO (0 μg/mL, 425 nm) with a peak at 410.25 nm, was used as light sources. 2 2 50 μg/mL, 100 μgl/mL, 150 μg/mL, 200 μg/mL, 300 μg/mL, The light density at the position of the sample was 5 mW/cm 500 μgl/mL, 1000 μg/mL) for 48 hours in dark, respectively. as measured with a photodiode. As shown in the inset, The obtained OD values of HL60 cells at different concen- the blue LEDs can better meet the needs of the following trations were normalized by the OD values of control group experiments. (the final concentration of nanoparticles was 0 μg/mL). The relative viability of HL60 cells is presented in Figure 5. 3.1.4. XPS Analysis. To determine whether the implemen- As shown in Figure 5, the relative viabilities of the groups tation of Fe-N co-doping is successful, the surface of Fe- in the presence of TiO2 or doped TiO2 are obviously lower N-TiO2 nanocomposites has been investigated using XPS than those of the control group (0 μg/mL) under the same analysis. As it can be observed from Figure 4, the signal conditions, indicating that TiO2 or doped TiO2 has a certain for Fe with a weaker peak at 710.5 eV was observed, and degree of inhibitory or toxic effects on the proliferation of the binding energies in the range 710–712 eV were assigned HL60 cells. Moreover, the inhibition effect of Fe-N-TiO2 to Fe2p3/2 of Fe3+ cation. The results indicate that the nanocomposites on HL60 cells is much more obvious than 3+ 4+ presence of Fe is in the form of Fe by replacing Ti in that of the other three TiO2. International Journal of Photoenergy 5

100 ones. However, treating cells with the combination of TiO2 and light exposure resulted in significant decrease in cell viability compared with the control ones. It can also be found 80 that the viability of HL60 cells in the presence of doped TiO2 is significantly lower than that of TiO2 after light treatment. 60 These results reveal that the modification of Fe or N can greatly enhance the photocatalytic inactivation effect of TiO2. Additionally, the Fe-N-doped TiO2 nanocomposites 40 present a higher efficiency in photokilling HL60 cancer cells

Cell viability (%) compared with that of Fe-TiO2 or N-TiO2 under the same conditions. When 200 μg/mL Fe-N-TiO2 (2 wt%) nanocom- 20 posites were used, the inactivation efficiency of HL60 cells can be increased to 78.5% after a 30-minute irradiation. 0 Control TiO2 Fe/TiO2 N/TiO2 Fe-N-TiO2 3.4. Ultrastructural Morphology of the Treated Cells. After light treatment (PDT), the treated cells were fixed in 2.5% Without light treatment ff Light treatment glutaraldehyde in 0.1 M phosphate bu er (pH 7.4) for 1 h. They were then washed three times thoroughly with Figure 6: The effect of light irradiation on cell viability at an triple-distilled water before being freeze-dried using K750 2 intensity of 5.0 mW/cm for 30 minutes in the presence of TiO2 turbo freeze drier. The samples were coated with platinum or doped TiO2. Each data point represents mean ± SD (n = 3). ∗ using automatic high-vacuum coating system (Quorum P<0.05. Q150T ES) before observing with a ZEISS Ultra-55 scanning electron microscope. The ultrastructural morphology of the HL60 cells Figure 5 also shows that with the increasing concentra- exposed to light at an intensity of 5.0 mW/cm2 for 30 minutes tion of nanoparticles, the viability of HL60 cells is decreased in the presence Fe-N-TiO2 nanocomposites is shown in gradually. At a concentration of 1000 μg/mL, the four relative Figure 7. Untreated control cells show numerous microvilli viabilities of HL60 cells for TiO2,Fe-TiO2,N-TiO2,andFe- on their membrane surface (Figure 7(a)), whereas the cells N-TiO2 have decreased to 77%, 73%, 71.5%, and 67.3%, exposed to light in the presence of Fe-N-TiO2 nanocom- respectively. However, when the concentrations of TiO2 posites display a markedly reduced number of microvilli nanoparticles or doped-TiO2 nanocomposites were in the compared with control cells (Figure 7(b)). The treated cells range from 0 to 200 μg/mL, the survival rates of HL60 were seriously damaged with apparent deformation; some cells were always greater than 90%. In this case, the TiO2 papillous protuberances are observed on the surface of cells nanoparticles and doped TiO2 nanocomposites could be where the cytoplasm seemed to have extruded through the considered as nontoxic materials for cancer cells in the dark, membrane boundary. which is in agreement with the suggestions reported in references [28, 29]. 3.5. Apoptosis Detection Based on the Induction of Fe-N- TiO Nanocomposites. To determine whether the observed ff 2 3.3. Photocatalytic Inactivation E ect of Fe-N-TiO2 Nanocom- reduced cell viability is related to apoptotic cell death, posites on Leukemia Tumor Cells. The HL60 cells were inoc- we investigated apoptotic cells after 24 h treatment by the ulated into two 96-well plates marked with A or B. The cell number and the sizes of dead cells obtained from the cell suspensions of plate A were exposed to light after incubating counter in combination with the nondestructive testing for 24 hours and then preincubated for another 24 hours methods [30]. in the dark. The HL60 cells in plate B were incubated for The obtained data are shown in Figure 8; no significant 48 hours in the incubator without light treatment. The final difference in the number of apoptotic cells was observed with concentration of TiO2or doped TiO2 used was 200 μg/mL. light treatment alone and Fe-N-TiO alone compared with ff 2 The photocatalytic e ect of Fe-N-TiO2 nanocomposites on control untreated cells. The percentage of apoptotic cells after leukemic HL60 cells was evaluated by measuring OD values. light therapy at an intensity of 5.0 W/cm2 for 30 minutes in Thecellviabilitywascalculatedasfollows: the presence of Fe-N-TiO2 was 7.3 times greater than that of control untreated cells. The results revealed that reduced OD cell viability (%) = treated · 100%, (1) cell viability was the result of apoptotic induction, which are ODuntreated in agreement with recent publications reporting that TiO2 nanoparticles induce death by apoptosis in different types of where the ODtreated and ODuntreated are the mean absorption values at 490 nm for the treated and untreated samples, re- cells [31, 32]. spectively. The obtained results are summarized in Figure 6. As can be observed in Figure 6, when the cells were 3.6. Alteration of Ca2+ in HL60 Cells during Light Treat- treated with TiO2 alone or with light irradiation alone, cell ment. The treated cells were incubated with Fluo-3-AM at viability was basically unchanged as compared to untreated a concentration of 500 μmol/L for 30 min. They were then 6 International Journal of Photoenergy

(a) (b)

Figure 7: The ultrastructural morphology of the cultured HL60 cells before and after light therapy. (a) The control cells. (b) The treated cells.

50 1.8 45 1.5 40 35 1.2 30 25 0.9 20 0.6 Apoptotic cells (%) cells Apoptotic 15 Fluorescence intensity (a.u.) Fluorescence 10 0.3 5 0 0 ControlFe-N-TiO2 Light Fe-N-TiO2 + light 0 min 10 min 20 min 30 min 50 min 60 min 40 min Control Figure 8: Change of percentage of apoptotic cells after light therapy 120 min 2 at an intensity of 5.0 mW/cm for 30 minutes in the presence of Fe- Figure 9: Alteration of Ca2+ in HL60 cells with Fe-N-TiO nano- ± = ∗ 2 N-TiO2. Each data point represents mean SD (n 3). P<0.05. composites during light therapy. Each data point represents mean ± SD (n = 3). ∗P<0.05.

washed three times with PBS before being detected using 3.7. Mechanism of Photocatalysis in Fe-N-TiO2 Nanocompos- 2+ fluorescence spectrometer. The change of Ca in HL60 cells ites. The photocatalytic mechanisms of Fe-N-TiO2 nano- during light treatment is presented in Figure 9. composites are initiated by the absorption of the photon hv4 It is now well established that cell apoptosis is executed by with energy lower than the band gap of TiO2 (3.2 eV for the family of caspases, and the activation of Ca2+contributes the anatase phase), and photoinduced electrons and holes to the morphological and functional changes associated with could be produced on the surface of TiO2 as schematized apoptosis [33, 34]. As can be observed from Figure 9,Ca2+ in Scheme 1. An electron is promoted to the conduction concentration in cells rapidly increased at the beginning band (CB) while a positive hole is formed in the valence of 10-minute and reached the maximum after 30-minute band (VB). Excited-state electrons can reduce the dissolved − light treatment in the presence of Fe-N-TiO2. It also can be O2 to produce the superoxide anion O . Meanwhile, the found that there were no significant changes in Ca2+ con- photo-generated holes in the valence band can further react centration during the time from 10 minutes to 60 minutes. with water to generate powerful hydroxyl radicals (OH•) Moreover, according to our previous studies, the most and other oxidative radicals, which are playing an important efficient inactivation of Fe-N-TiO2 nanocomposites on HL60 role in destroying the membrane and component of tumor cells is located at the time of 30 minutes, which suggests that cells [35, 36]. Additional benefit of the dispersion of Fe-N the increased concentration of Ca2+ promotes cell apoptosis is the improved trapping of electrons to inhibit electron- through activation of apoptosis signaling pathways, to a hole recombination during irradiation, as suggested in [37– certain extent. 39]. Decrease of charge carriers recombination results in International Journal of Photoenergy 7

Light hv CB O2 e− e−

− − e e− e− H2O2 O2 hv3

3.2 eV hv1 hv OH• 2 hv4 H2O h+ h+ h+ OH• h+ h+ VB

Scheme 1: Schematic representation of the mechanism of photocatalytic titanium dioxide particles (TiO2:hv1,Fe-TiO2:hv2,N-TiO2:hv3, Fe-N-TiO2: hv4).

significantly enhanced photocatalytic activity of TiO2 nano- References particles. [1] K. Esquivel, L. G. Arriaga, F. J. Rodriguez, L. Martinez, and L. A. Godinez, “Development of a TiO2 modified optical fiber 4. Conclusion electrode and its incorporation into a photoelec-trochemical reactor for wastewater treatment,” Water Research, vol. 43, no. 14, pp. 3593–3603, 2009. In this paper, Fe-N-TiO2 nanocomposite has been successfully synthesized by sol-gel method and for the first time [2] M. Guarino, A. Costa, and M. Porro, “Photocatalytic TiO2 used as a new “photosensitizer” in photodynamic therapy coating-to reduce ammonia and greenhouse gases concentra- for cancer cell treatment. Then they were characterized by tion and emission from animal husbandries,” Bioresource Tech- nology, vol. 99, no. 7, pp. 2650–2658, 2008. scanning electron microscope (SEM), transmission electron [3] Y. H. Tsuang, J. S. Sun, Y. C. Huang, C. H. Lu, W. H. S. Chang, microscope (TEM), X-ray diffraction (XRD), UV-vis adsorp- and C. C. Wang, “Studies of photokilling of bacteria using tion spectra, and X-ray photoelectron spectroscopy (XPS), titanium dioxide nanoparticles,” Artificial Organs, vol. 32, no. respectively. Additionally, the ultrastructural morphology of 2, pp. 167–174, 2008. 2+ treated cells and alteration of Ca in cells during PDT [4] C. L. Cheng, D. S. Sun, W. C. Chu et al., “The effects of the were also studied. The experimental results show that the bacterial interaction with visible-light responsive titania pho- absorption of TiO2 nanoparticles in the visible light region tocatalyst on the bactericidal performance,” Journal of Biomed- could be enhanced effectively by the method of (Fe, N) co- ical Science, vol. 16, no. 1, pp. 1–7, 2009. 2+ doping, and both pure TiO and doped TiO2 nanocompos- [5] T. Zubkoy, D. Stahl, T. L. Thompson, D. Panayotov, O. Diwald, ites at high concentrations have a significant inhibition on and J. T. Yates, “Ultraviolet light-induced hydrophilicity effect the growth of HL60 cells. It is also found that Fe-N-TiO2 on TiO2(110) (1-1). Dominant role of the photooxidation of nanocomposites present much higher inactivation efficiency adsorbed hydrocarbons causing wetting by water droplets,” Journal of Physical Chemistry B, vol. 109, no. 32, pp. 15454– in photokilling HL60 cancer cells than TiO2 nanoparticles under the same conditions, indicating that the photocatalytic 15462, 2005. [6] J. Wang, Y. Guo, B. Liu et al., “Detection and analysis of inactivation effects of TiO2 could be greatly improved by the modification of Fe-N. Moreover, the PDT efficiency reactive oxygen species (ROS) generated by nano-sized TiO2 powder under ultrasonic irradiation and application in sono- of Fe-N-TiO2 nanocomposites on HL60 cells can reach catalytic degradation of organic dyes,” Ultrasonics Sonochem- 78.5% at a concentration of 200 μg/mL after a 30-minute istry, vol. 18, no. 1, pp. 177–183, 2011. ff light treatment. The high photocatalytic inactivation e ects [7] N. Shimizu, C. Ogino, M. F. Dadjour, K. Ninomiya, A. Fuji- of Fe-N-TiO2 nanocomposites on tumor cells suggest that hira, and K. Sakiyama, “Sonocatalytic facilitation of hydroxyl it may be an important potential photosensitizer-based radical generation in the presence of TiO2,” Ultrasonics Sono- photodynamic therapy for cancer treatment [40–42]. chemistry, vol. 15, no. 6, pp. 988–994, 2008. [8] Y. Chihara, K. Fujimoto, H. Kondo et al., “Anti-tumor effects of liposome-encapsulated titanium dioxide in nude mice,” Acknowledgments Pathobiology, vol. 74, no. 6, pp. 353–358, 2007. [9] J. Yu, L. Qi, and M. Jaroniec, “Hydrogen production by pho- This work has been financially supported by the National tocatalytic water splitting over Pt/TiO22 nanosheets with ex- Natural Science Foundation of China (61072029), the posed (001) facets,” Journal of Physical Chemistry C, vol. 114, Natural Science Foundation of Guangdong Province no. 30, pp. 13118–13125, 2010. (10151063101000025), and Science and Technology Plan- [10] E.A.Rozhkova,I.Ulasov,B.Lai,N.M.Dimitrijevic,M.S.Les- ning Project of Guangzhou City (2010Y1-C111). niak, and T. Rajh, “A high-performance nanobio photocatalyst 8 International Journal of Photoenergy

for targeted brain cancer therapy,” Nano Letters, vol. 9, no. 9, [25] U. G. Akpan and B. H. Hameed, “The advancements in sol- pp. 3337–3342, 2009. gelmethodofdoped-TiO2 photocatalysts,” Applied Catalysis [11] C. M. Sayes, R. Wahi, P.A. Kurian et al., “Correlating nanoscale A, vol. 375, no. 1, pp. 1–11, 2010. titania structure with toxicity: a cytotoxicity and inflammatory [26] R. S. Sonawane and M. K. Dongare, “Sol-gel synthesis of Au/ response study with human dermal fibroblasts and human TiO2 thin films for photocatalytic degradation of phenol in lung epithelial cells,” Toxicological Sciences, vol. 92, no. 1, pp. sunlight,” Journal of Molecular Catalysis A, vol. 243, no. 1, pp. 174–185, 2006. 68–76, 2006. [12] J. Xu, Z. D. Chen, Y. Sun, C. M. Chen, and Y. Z. Jiang, “Photo- [27] H. Li, Z. Bian, J. Zhu, Y. Huo, H. Li, and Y. Lu, “Mesoporous catalytic killing effect of gold-doped TiO2 nanocomposites on Au/TiO2 nanocomposites with enhanced photocatalytic activ- humancoloncarcinomaLoVocells,”Acta Chimica Sinica, vol. ity,” Journal of the American Chemical Society, vol. 129, no. 15, 66, no. 10, pp. 1163–1167, 2008. pp. 4538–4539, 2007. [13] J. J. Wang, B. J. S. Sanderson, and H. Wang, “Cyto- and gen- [28] R. Cai, Y. Kubota, T. Shuin, H. Sakai, K. Hashimoto, and A. otoxicity of ultrafine TiO2 particles in cultured human lym- Fujishima, “Induction of cytotoxicity by photoexcited TiO2 phoblastoid cells,” Mutation Research, vol. 628, no. 2, pp. 99– particles,” Cancer Research, vol. 52, no. 8, pp. 2346–2348, 1992. 106, 2007. [29] E. Fabian, R. Landsiedel, L. Ma-Hock, K. Wiench, W. Wohlle- [14] Y. S. Lee, S. Yoon, H. J. Yoon et al., “Inhibitor of differentiation ben, and B. van Ravenzwaay, “Tissue distribution and toxicity 1 (Id1) expression attenuates the degree of TiO2-induced cyto- of intravenously administered titanium dioxide nanoparticles toxicity in H1299 non-small cell lung cancer cells,” Toxicology in rats,” Archives of Toxicology, vol. 82, no. 3, pp. 151–157, Letters, vol. 189, no. 3, pp. 191–199, 2009. 2008. [15] K. Q. Huang, L. Chen, M. X. Liao, and J. W. Xiong, “The [30] J. W. Xiong, L. Chen, M. S. Liu et al., “Optical manner for photocatalytic inactivation effect of fe-doped TiO2 nanocom- estimation viability of cells based on their morphological posites on leukemic HL60 cells based photodynamic therapy,” characteristics,” Journal of Optoelectronics Laser, vol. 16, no. 12, International Journal of Photoenergy, vol. 2012, Article ID pp. 1514–1518, 2005. 367072, 12 pages, 2012. [31] A. Kathiravan and R. Renganathan, “Photoinduced interac- [16] J. Xu, Y. Sun, J. Huang et al., “Photokilling cancer cells using tions between colloidal TiO2 nanoparticles and calf thymus- highly cell-specific antibody-TiO2 bioconjugates and electro- DNA,” Free Radical Biology and Medicine,vol.28,no.7,pp. poration,” Bioelectrochemistry, vol. 71, no. 2, pp. 217–222, 1374–1378, 2009. 2007. [32] A. P. Zhang and Y. P. Sun, “Photocatalytic killing effect of [17] H. Choi, E. Stathatos, and D. D. Dionysiou, “Sol-gel prepa- TiO2 nanoparticles on Ls-174-t human colon carcinoma cells,” ration of mesoporous photocatalytic TiO2 films and TiO2/ World Journal of Gastroenterology, vol. 10, no. 21, pp. 3191– Al2O3 composite membranes for environmental applications,” 3193, 2004. Applied Catalysis B, vol. 63, no. 1-2, pp. 60–67, 2006. [33] D. E. J. G. J. Dolmans, A. Kadambi, J. S. Hill et al., “Vascular [18] J. Yu, Y. Hai, and B. Cheng, “Enhanced photocatalytic H2- accumulation of a novel photosensitizer, MV6401, causes production activity of TiO2 by Ni(OH)2 cluster modification,” selective thrombosis in tumor vessels after photodynamic Journal of Physical Chemistry C, vol. 115, no. 11, pp. 4953– therapy,” Cancer Research, vol. 62, no. 7, pp. 2151–2156, 2002. 4958, 2010. [34] H. Hui, F. Dotta, U. Di Mario, and R. Perfetti, “Role of caspases [19] J. Wang, J. Wu, Z. Zhang et al., “Sonocatalytic damage of in the regulation of apoptotic pancreatic islet beta-cells death,” bovine serum albumin (BSA) in the presence of nanometer Journal of Cellular Physiology, vol. 200, no. 2, pp. 177–200, anatase titanium dioxide (TiO2),” Ultrasound in Medicine and 2004. Biology, vol. 32, no. 1, pp. 147–152, 2006. [35] J. F. Reeves, S. J. Davies, N. J. F. Dodd, and A. N. Jha, “Hydroxyl [20]A.DiPaola,S.Ikeda,G.Marc`ı, B. Ohtani, and L. Palmisano, radicals (·OH) are associated with titanium dioxide (TiO2) “Transition metal doped TiO2: physical properties and photo- nanoparticle-induced cytotoxicity and oxidative DNA damage catalytic behaviour,” International Journal of Photoenergy, vol. in fish cells,” Mutation Research, vol. 640, no. 1-2, pp. 113–122, 3, no. 4, pp. 171–176, 2001. 2008. [21] H. Fu, L. Zhang, S. Zhang, Y. Zhu, and J. Zhao, “Electron spin [36] K. Yang, Y. Dai, and B. Huang, “Study of the nitrogen con- resonance spin-trapping detection of radical intermediates in centration influence on N-doped TiO2 anatase from first- N-doped TiO2-assisted photodegradation of 4-chlorophenol,” principles calculations,” Journal of Physical Chemistry C, vol. Journal of Physical Chemistry B, vol. 110, no. 7, pp. 3061–3065, 111, no. 32, pp. 12086–12090, 2007. 2006. [37] X. Chen and S. S. Mao, “Titanium dioxide nanomaterials: syn- [22] V. Iliev, D. Tomova, R. Todorovska et al., “Photocatalytic thesis, properties, modifications and applications,” Chemical properties of TiO2 modified with gold nanoparticles in the Reviews, vol. 107, no. 7, pp. 2891–2959, 2007. degradation of oxalic acid in aqueous solution,” Applied Catal- [38] J. A. Rengifo-Herrera and C. Pulgarin, “Photocatalytic activ- ysis A, vol. 313, no. 2, pp. 115–121, 2006. ity of N, S co-doped and N-doped commercial anatase TiO2 [23]R.Bacsa,J.Kiwi,T.Ohno,P.Albers,andV.Nadtochenko, powders towards phenol oxidation and E. coli inactivation “Preparation, testing and characterization of doped TiO2 under simulated solar light irradiation,” Solar Energy, vol. 84, active in the peroxidation of biomolecules under visible light,” no. 1, pp. 37–43, 2010. Journal of Physical Chemistry B, vol. 109, no. 12, pp. 5994– [39]Y.Kesong,D.Ying,H.Baibiao,andH.Shenghao,“Theoretical 6003, 2005. study of N-doped TiO2 rutile crystals,” Journal of Physical [24] X. H. Li, J. B. Lu, Y. Dai, M. Guo, and B. B. Huang, Chemistry B, vol. 110, no. 47, pp. 24011–24014, 2006. “The synthetic effects of Iron with sulfur and fluorine on [40] N. P. Huang, M. H. Xu, C. W. Yuan, and R. R. Yu, “The photoabsorption and photocatalytic performance in codoped study of the photokilling effect and mechanism of ultrafine TiO2 ,” International Journal of Photoenergy, vol. 2012, Article TiO2 particles on U937 cells,” Journal of Photochemistry and ID 203529, 8 pages, 2012. Photobiology A, vol. 108, no. 2-3, pp. 229–233, 1997. International Journal of Photoenergy 9

[41] J. B. Lu, H. Jin, Y. Dai, K. S. Yang, and B. B. Huang, “Eﬀect of electronegativity and charge balance on the visible-light- responsive photocatalytic activity of nonmetal doped anatase TiO2 ,” International Journal of Photoenergy, vol. 2012, Article ID 928503, 8 pages, 2012. [42] W. H. Suh, K. S. Suslick, G. D. Stucky, and Y. H. Suh, “Nan- otechnology, nanotoxicology, and neuroscience,” Progress in Neurobiology, vol. 87, no. 3, pp. 133–170, 2009. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 608298, 5 pages doi:10.1155/2012/608298

Research Article Synthesis and Characterization of Iron Oxide Nanoparticles and Applications in the Removal of Heavy Metals from Industrial Wastewater

Zuolian Cheng,1 Annie Lai Kuan Tan,1 Yong Tao, 2 Dan Shan,2 Kok Eng Ting,1 and Xi Jiang Yin2

1 School of Chemical & Life Sciences, Singapore Polytechnic, 500 Dover Road, Singapore 139651 2 Advanced Material Technology Centre, Singapore Polytechnic, 500 Dover Road, Singapore 139651

Correspondence should be addressed to Zuolian Cheng, [email protected]

Received 16 January 2012; Accepted 17 February 2012

Academic Editor: Stephane´ Jobic

Copyright © 2012 Zuolian Cheng et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This study investigated the applicability of maghemite (γ-Fe2O3) nanoparticles for the selective removal of toxic heavy metals from electroplating wastewater. The maghemite nanoparticles of 60 nm were synthesized using a coprecipitation method and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDX). Batch experiments were carried out for the removal of Pb2+ ions from aqueous solutions by maghemite nanoparticles. The effects of contact time, initial concentration of Pb2+ ions, solution pH, and salinity on the amount of Pb2+ removed were investigated. The adsorption process was found to be highly pH dependent, which made the nanoparticles selectively adsorb this metal from wastewater. The adsorption of Pb2+ reached equilibrium rapidly within 15 min and the adsorption data were well fitted with the Langmuir isotherm.

1. Introduction larger surface area-volume ratio, diminished consumption of chemicals, and no secondary pollutant. However, with With heavy metal pollution becoming one of the most another special property of this kind magnetic materials serious environmental problems, various methods for heavy are realized and utilized in the context of environmental metal removal from wastewater have been extensively studied remediation. More and more magnetic separation has been during the past decades, such as chemical precipitation, elec- combined with adsorption for the removal of heavy metals trochemical techniques, membrane filtration, ion exchange, from contaminated water at laboratory scales [13–15]. Espe- and adsorption [1]. To date, considerable research attention cially in industries, magnetic separation is desirable because has been paid to the removal of heavy metals from contami- it can overcome many drawbacks occurring in the membrane nated water via adsorption process. In theory, the adsorption filtration, centrifugation, or gravitational separation and is process can offer flexibility in design and operation and easy to achieve a given level of separation just via external in many cases will produce high-quality treated effluent. magnetic field. In addition, as the adsorption is sometimes reversible and Iron oxides exit in many forms in nature, with magnetite adsorbent can be regenerated by suitable desorption process, (Fe3O4), hematite (α-Fe2O3) and maghemite (γ-Fe2O3), various types of adsorbents have found application in being most probably common and important technologically the removal of heavy metals, including activated carbon [16]. It has been reported that surface effects have a strong [2, 3], carbon nanotubes [4–6], polymeric adsorbents [7], influence on the magnetic properties of iron oxide nanopar- metal oxides [8], and bioadsorbents [9–12]. Among these ticles [17]. As the surface area of iron-oxide-based magnetic adsorbents, iron-based magnetic nanomaterials have dis- materials decreased, their responses to external magnetic tinguished themselves by their unique properties, such as field decreased, making it difficult to recover the adsorbents 2 International Journal of Photoenergy after treatment has been completed [14]. On the other hand, values of water samples were stable over the the experiment it has also been noted that the adsorption capacities of period. All the adsorption experiments were carried out at adsorbents rely largely on the available surface areas, and aroomtemperatureof22± 2◦Candwereperformedin the increase of the surface area is normally obtained by triplicate. The total aqueous concentrations of Pb2+ were the decrease of the particle size of adsorbents. As a result, measured using an inductively coupled plasma-optical emis- there is a need to synthesize such absorbents with proper sion spectrometer (ICP-OES, Thermo, icap 6000). Sample particles sizes for the removal of heavy metals from industrial dilution was conducted before the ICP-OES measurement, wastewater. where necessary. Up to now, there are several methods that can be used to synthesize iron-oxide-based nanomaterials. These methods include hydrothermal synthesis [18, 19], thermal 3. Results and Discussion decomposition [20, 21], co-precipitation [22, 23], sol-gel 3.1. Synthesis and Characterization of γ-Fe O . The crys- method [15], and colloidal chemistry method [24]. Among 2 3 talline grain size is mainly determined by both the formation these synthesis methods, coprecipitation has proven to be the energy of growth unit and the lattice energy, besides different most promising method for the production of nanomaterials synthesis conditions. In the present study, Fe O nanopar- as the procedure is relatively simple and the particles can be 2 3 ticles were synthesized by varying pH values, ageing time, obtained with controlled particle size. the mass ratio of FeCl and urea, and so forth. Figure 1(a) The specific objective of the present study was (1) to 3 shows an SEM image of the synthesized γ-Fe O , confirming synthesize γ-Fe O nanoparticles using a modified method, 2 3 2 3 that the particles obtained were indeed in the nanometer which involved urea as a uniformity precipitation reagent, range. Upon ageing for different durations, it was observed (2) to characterize γ-Fe O nanoparticles synthesized using 2 3 that the grain size increased with increasing ageing time. different kinds of analytical instruments, and (3) to evaluate The smallest grain size (63.20 ± 0.928 nm) was obtained synthesized γ-Fe O nanoparticles as adsorbents to remove 2 3 after ageing for 45 minutes. In addition, strong peaks for heavymetalssuchasPb2+ from industrial wastewater. Fe and O can be observed in the spectrum illustrated in Figure 1(b) for particles after ageing time of 45 min. The 2. Experimental insert of Figure 1(b) reveals that the O/Fe atomic ratio of the γ-Fe2O3 analyzed was 1.56, which was relatively consistent 2.1. Preparation and Characterization of γ-Fe2O3 Nanopar- with the theoretical O/Fe atomic ratio of 1.50. ticles. The synthesis of γ-Fe2O3 nanoparticles involves the The magnetization with respect to applied field was following steps: (1) a designated molar ratio of iron chloride recorded at room temperature. The hysteresis loop, shown and urea was dissolved in deionized water; (2) the mixture in Figure 2, suggested a weak magnetic nature of the samples was continuously stirred for 45 min at 90◦Cinawaterbath with little hysteresis. The weak magnetism might be caused before it was cooled to room temperature; (3) the produced by the presence of α-Fe2O3 as detected by XRD. From precipitation was centrifuged and washed by deionized water Figure 2, the MS was calculated to be 0.025 emu/g and the ◦ and followed by ethanol; (4) after being dried at 75 Cfor4 HC to be 1250 Oe. hours, the collected powder was slowly calcined to 650◦Cin Figure 3 shows the results of XRD analysis for the air and dwelt for 2 hours. The resultant product of γ-Fe2O3 synthesized nanoparticles that were obtained with different nanoparticles was obtained for subsequent characterization. initial concentrations of FeCl3. It indicates that the particles A scanning electron microscope (SEM, JEOL JEM2010, consist of γ-Fe2O3 (peaks denoted by ∗)andα-Fe2O3 (peaks Japan) was used to characterize the structure properties of denoted by #). It also indicates that a concentration of 0.02 M the synthesized materials. The element composition of the of FeCl3 resulted in the particles with the smallest Z-average synthesized materials was identified by an energy dispersive diameter. Therefore, 0.02 M was the optimized concentration X-ray spectroscopy system (EDX) coupled to the SEM. The for FeCl3. crystallization phase analysis was executed by a powder X-ray diffraction (XRD) (Philips PW-1830, Netherlands). 3.2. Effect of Adsorption Time and Initial Concentration. Magnetization measurement was carried out with a vibration The adsorption of Pb2+ onto γ-Fe O nanoparticles was sample magnetometer (VSM) at room temperature. 2 3 monitored for 120 min. The initial Pb2+ concentrations were 50, 100, and 150 mg/L, respectively. The initial pH value of 2.2. Removal of Heavy Metals from Wastewater. Astock water samples was 5.5, and the solution temperature was solution containing Pb2+ was prepared by dissolving a 22 ± 2◦C. As seen in Figure 4,Pb2+ ions were adsorbed known quantity of lead nitrate (Pb(NO3)2) in deionized onto γ-Fe2O3 nanoparticles rapidly, and equilibrium was water. Batch adsorption studies were performed by mixing established within 30 minutes. This could be due to the small 2+ 0.5 g of γ-Fe2O3 with 50 mL of solutions of varying Pb size of γ-Fe2O3 nanoparticles, which was favorable for the concentrations (50, 100, and 150 mg/L) in 100 mL glass vials. diffusion of Pb2+ ions from bulk solution onto the active The adsorption on γ-Fe2O3 wasfirststudiedatpHvalues sites of the solid surface. External adsorption dominated of 2.5 to 6.5 to investigate the effects of pH values on the and no pore diffusion was observed to slow down the Pb2+ adsorption. 0.1 M HCl and 0.1 M NaOH solutions were adsorption rate. Despite the short equilibrium time, a 24-h used to adjust the pH values of water samples. The pH contact time was adopted for the subsequent experiment to International Journal of Photoenergy 3

O K Element Wt% At% Au M Fe L O K 15.47 38.89 Fe K 84.53 61.02 Fe L Fe K Total 100 100

Au M Au M Fe K

2 4 6 8 10 12 14 16 (KeV) (a) (b)

Figure 1: SEM image and EDX spectrum for γ-Fe2O3 nanoparticles.

0.3

14000 0.2 47.686 [330] 38.73 [320] 27.057 [211] 0.1 63.182 [440] 7000 # 23.383 [012] # 69.622 [208] 45.622 [410] 0 Intensity 55.937 [430] (emu/g) # 89.044 [226] 82.604 [710] M # 61.17 [214] − 0.1 # 74.855 [217] 0 − 0.2 10 20 30 40 50 60 70 80 90 − 0.3 2 (deg) − 10000 − 5000 0 5000 10000 Denotes 0.05 M Denotes 0.01 M H (Oe) Denotes 0.02 M Denotes 0.005 M

Figure 2: VSM measurements for γ-Fe2O3 nanoparticles. Figure 3: XRD patterns of synthesized γ-Fe2O3 nanoparticles with different concentrations of FeCl3. ensure that adsorption equilibrium was achieved. The short equilibrium time was in agreement with that reported by to find the optimum pH on the adsorption of Pb2+ ions other researchers for the adsorption of other metal ions onto onto γ-Fe2O3 nanoparticles using different initial pH values iron oxide nanoparticles [25–27]. This is in contrast to other of 2.5 to 6.5. The experimental pH values of up to 6.5 were conventional porous adsorbents in which adsorption occurs ff chosen as precipitation of lead hydroxide would occur at through pore di usion steps, which in turn slow down the pH values equal to or higher than 7.0, even though other adsorption rate. The γ-Fe2O3 nanoparticles are nonporous variables such as the amount of nanoparticles were fixed [29]. adsorbents, as confirmed by the surface area and porosity Therefore, it was not feasible to carry out the adsorption measurements, where only external adsorption occurred. experiments for Pb2+ at pH > 6.5 without introducing some This type of adsorption mass transfer requires less time uncertainties to the results. Figure 5 shows the effects of pH to reach the equilibrium [28]. This result is promising as on the adsorption of Pb2+. As observed in the graph, the equilibrium time plays a major role in economic viability removal efficiency of Pb2+ ions from watersamples by the for wastewater treatment plant. Furthermore, as shown in 2+ γ-Fe2O3 nanoparticles was clearly pH dependent and the Figure 4, the amount adsorbed of Pb increased with the ffi ≥ 2+ highest adsorption e ciency was obtained at pH 5.5. Lee increase in the initial concentration of Pb in water samples. et al. also observed a similar pH effect for the adsorption of This can be attributed to the increase in the ion occupancy Pb2+ onto bulk iron oxides in water samples [30]. It indicated number, which favors the adsorption process. that a water sample with a higher pH value was favorable for the deprotonation of sorbent surface [31, 32]. Increased 3.3. Effect of pH. It is well known that pH is one of the most deprotonation could result in the increase of negatively important factors that affect the adsorption process of heavy charged sites, which enhanced the attractive forces between metals in water samples. The experiments were carried out the sorbent surface and the Pb2+ ions. Therefore, it will result 4 International Journal of Photoenergy

100 4. Conclusions 90 This study showed that the prepared γ-Fe2O3 nanoparticles 80 could be used as an alternate to the conventional adsorbents 70 for the removal of metal ions from wastewater with high ffi 60 removal e ciency within a very short time. The removal of Pb2+, as a typical metal ion commonly present in 50 wastewater, by adsorption onto γ-Fe2O3 nanoparticles was 40 successfully accomplished. Adsorption was very rapid and 30 equilibrium was achieved within 15 min. It also showed that Removal eRemoval ciency (%) 20 adsorption was highly dependent on the initial concentration of Pb2+ and initial pH value. Maximum removal efficiency 10 was achieved at pH 5.5 at room temperature. Increasing 0 NaCl from 0% to 3.5% (the salinity of seawater) had no 2+ 0 102030405060708090100110120 effects on the adsorption of Pb on γ-Fe2O3 nanoparticles. Time (min) The adsorption isotherms were also determined and were 50 mg/L appropriately described by both Langmuir and Freundlich 100 mg/L models, with a better fitting to the Langmuir model than the 150 mg/L Freundlich model. Therefore, γ-Fe2O3 nanoparticles were ff Figure 4: Effect of contact time on the removal of Pb2+ at recommended as fast, e ective, and inexpensive nanoadsor- different initial concentrations. Adsorbent dose: 10 g/L, shaking bents for rapid removal and recovery of metal ions from rate: 200 rpm, pH: 5.5, T:22± 2oC. industrial wastewater.

Acknowledgment 100 This work was supported by the Ministry of Education 80 Innovation Fund of Singapore (MOE2008-IF-1-010).

60 References [1] F. Fu and Q. Wang, “Removal of heavy metal ions from 40 wastewaters: a review,” Journal of Environmental Management, vol. 92, no. 3, pp. 407–418, 2011.

Removal eRemoval ciency (%) [2] K. C. Kang, S. S. Kim, J. W. Choi, and S. H. Kwon, “Sorption 20 of Cu2+ and Cd2+ onto acid- and base-pretreated granular activated carbon and activated carbon fiber samples,” Journal of Industrial and Engineering Chemistry, vol. 14, no. 1, pp. 131– 0 135, 2008. 02468[3] A. Jusoh, L. Su Shiung, N. Ali, and M. J. M. M. Noor, pH “A simulation study of the removal efficiency of granular activatedcarbononcadmiumandlead,”Desalination, vol. Figure 5: EffectofpHonPb2+ adsorption onto γ-Fe O nanoad- 2 3 206, no. 1–3, pp. 9–16, 2007. sorbents. Initial Pb2+ concentration: 50 mg/L. Adsorbent dose: [4] Y. Li, F. Liu, B. Xia et al., “Removal of copper from aqueous 10 g/L, shaking rate: 200 rpm, T:22± 2oC. solution by carbon nanotube/calcium alginate composites,” Journal of Hazardous Materials, vol. 177, no. 1–3, pp. 876–880, 2010. in the increase in the adsorption capacity. On the other hand, [5] M. I. Kandah and J. L. Meunier, “Removal of nickel ions from water by multi-walled carbon nanotubes,” Journal of in a water sample with lower pH, the positively charged Hazardous Materials, vol. 146, no. 1-2, pp. 283–288, 2007. sites dominate and this could enhance the repulsion forces [6] Y. H. Li, Z. Di, J. Ding, D. Wu, Z. Luan, and Y. Zhu, 2+ existing between the sorbent surface and the Pb ions and “Adsorption thermodynamic, kinetic and desorption studies 2+ therefore decrease the adsorption of Pb ions. of Pb2+ on carbon nanotubes,” Water Research, vol. 39, no. 4, pp. 605–609, 2005. 3.4. Effects of Salinity. Increasing NaCl salinity from 0% to [7] A. Dabrowski, Z. Hubicki, P. Podkocielny, and E. Robens, “Selective removal of the heavy metal ions from waters and 3.5% (the salinity of seawater) had no effects on the removal 2+ industrial wastewaters by ion-exchange method,” Chemo- of Pb by γ-Fe2O3 nanoadsorbents. This suggested that sphere, vol. 56, no. 2, pp. 91–106, 2004. 2+ no interaction occurred among NaCl, Pb and γ-Fe2O3 [8] Z. Ai, Y. Cheng, L. Zhang, and J. Qiu, “Efficient removal 2+ − nanoadsorbents, and the complexation of Pb ,andCl was of Cr(VI) from aqueous solution with Fe@Fe2O3 core-shell much weaker than the coordination between Pb2+ and the nanowires,” Environmental Science and Technology, vol. 42, no. adsorptive sites on the surfaces of γ-Fe2O3 nanoadsorbents. 18, pp. 6955–6960, 2008. International Journal of Photoenergy 5

[9]T.Aman,A.A.Kazi,M.U.Sabri,andQ.Bano,“Potatopeels nanoparticles,” Journal of Colloid and Interface Science, vol. as solid waste for the removal of heavy metal copper(II) from 301, no. 2, pp. 402–408, 2006. waste water/industrial effluent,” Colloids and Surfaces B, vol. [26] A. Uheida, G. Salazar-Alvarez, E. Bjorkman,¨ Z. Yu, and 63, no. 1, pp. 116–121, 2008. M. Muhammed, “Fe3O4 and γ-Fe2O3 nanoparticles for the [10] S. J. Kohler,¨ P. Cubillas, J. D. Rodr´ıguez-Blanco, C. Bauer, adsorption of Co2+ from aqueous solution,” Journal of Colloid and M. Prieto, “Removal of cadmium from wastewaters by and Interface Science, vol. 298, no. 2, pp. 501–507, 2006. aragonite shells and the influence of other divalent cations,” [27] H. Sun, X. Zhang, Q. Niu, Y. Chen, and J. C. Crittenden, Environmental Science and Technology, vol. 41, no. 1, pp. 112– “Enhanced accumulation of arsenate in carp in the presence of 118, 2007. titanium dioxide nanoparticles,” Water, Air, and Soil Pollution, [11] P. Pavasant, R. Apiratikul, V. Sungkhum, P. Suthiparinyanont, vol. 178, no. 1–4, pp. 245–254, 2007. S. Wattanachira, and T. F. Marhaba, “Biosorption of Cu2+, [28] P. K. Dutta, A. K. Ray, V. K. Sharma, and F. J. Millero, Cd2+,Pb2+,andZn2+ using dried marine green macroalga “Adsorption of arsenate and arsenite on titanium dioxide Caulerpa lentillifera,” Bioresource Technology, vol. 97, no. 18, suspensions,” Journal of Colloid and Interface Science, vol. 278, pp. 2321–2329, 2006. no. 2, pp. 270–275, 2004. [12] M. Souiri, I. Gammoudi, H. B. Ouada et al., “Escherichia [29] T. K. Naiya, A. K. Bhattacharya, and S. K. Das, “Adsorption coli-functionalized magnetic nanobeads as an ultrasensitive of Pb(II) by sawdust and neem bark from aqueous solutions,” biosensor for heavy metals,” Procedia Chemistry, vol. 1, no. 1, Environmental Progress, vol. 27, no. 3, pp. 313–328, 2008. pp. 1027–1030, 2009. [30] S. Z. Lee, L. Chang, H. H. Yang, C. M. Chen, and M. C. [13] J. T. Mayo, C. Yavuz, S. Yean et al., “The effect of nanocrys- Liu, “Adsorption characteristics of lead onto soils,” Journal of talline magnetite size on arsenic removal,” Science and Tech- Hazardous Materials, vol. 63, no. 1, pp. 37–49, 1998. nology of Advanced Materials, vol. 8, no. 1-2, pp. 71–75, 2007. [31] P. Roonasi and A. Holmgren, “An ATR-FTIR study of sulphate [14]C.T.Yavuz,J.T.Mayo,W.W.Yuetal.,“Low-fieldmagnetic sorption on magnetite; rate of adsorption, surface speciation, separation of monodisperse Fe3O4 nanocrystals,” Science, vol. and effect of calcium ions,” Journal of Colloid and Interface 314, no. 5801, pp. 964–967, 2006. Science, vol. 333, no. 1, pp. 27–32, 2009. [15] J. Hu, G. Chen, and I. M. C. Lo, “Removal and recovery of [32] S. B. Johnson, G. V. Franks, P. J. Scales, D. V. Boger, and Cr(VI) from wastewater by maghemite nanoparticles,” Water T. W. Healy, “Surface chemistry-rheology relationships in Research, vol. 39, no. 18, pp. 4528–4536, 2005. concentrated mineral suspensions,” International Journal of [16] A. S. Teja and P. Y. Koh, “Synthesis, properties, and applica- Mineral Processing, vol. 58, no. 1–4, pp. 267–304, 2000. tions of magnetic iron oxide nanoparticles,” Progress in Crystal Growth and Characterization of Materials, vol. 55, no. 1-2, pp. 22–45, 2009. [17] T. Neuberger, B. Schopf,¨ H. Hofmann, M. Hofmann, and B. Von Rechenberg, “Superparamagnetic nanoparticles for biomedical applications: possibilities and limitations of a new drug delivery system,” Journal of Magnetism and Magnetic Materials, vol. 293, no. 1, pp. 483–496, 2005. [18] V. Sreeja and P. A. Joy, “Microwave-hydrothermal synthesis of γ-Fe2O3 nanoparticles and their magnetic properties,” Materials Research Bulletin, vol. 42, no. 8, pp. 1570–1576, 2007. [19]T.J.Daou,G.Pourroy,S.Begin-Colin´ et al., “Hydrothermal synthesis of monodisperse magnetite nanoparticles,” Chem- istry of Materials, vol. 18, no. 18, pp. 4399–4404, 2006. [20] K. Simeonidis, S. Mourdikoudis, M. Moulla et al., “Controlled synthesis and phase characterization of Fe-based nanoparticles obtained by thermal decomposition,” Journal of Magnetism and Magnetic Materials, vol. 316, no. 2, pp. e1–e4, 2007. [21] W. Zhou, K. Tang, S. Zeng, and Y. Qi, “Room temperature synthesis of rod-like FeC2O4 · 2H2O and its transition to maghemite, magnetite and hematite nanorods through controlled thermal decomposition,” Nanotechnology, vol. 19, no. 6, Article ID 065602, 2008. [22] W. Yu, T. Zhang, J. Zhang, X. Qiao, L. Yang, and Y. Liu, “The synthesis of octahedral nanoparticles of magnetite,” Materials Letters, vol. 60, no. 24, pp. 2998–3001, 2006. [23] I. Nedkov, T. Merodiiska, L. Slavov, R. E. Vandenberghe, Y. Kusano, and J. Takada, “Surface oxidation, size and shape of nano-sized magnetite obtained by co-precipitation,” Journal of Magnetism and Magnetic Materials, vol. 300, no. 2, pp. 358– 367, 2006. [24] P. D. Cozzoli, E. Snoeck, M. A. Garcia et al., “Colloidal synthesis and characterization of tetrapod-shaped magnetic nanocrystals,” Nano Letters, vol. 6, no. 9, pp. 1966–1972, 2006. [25] A. Uheida, M. Iglesias, C. Fontas` et al., “Sorption of palladium(II), rhodium(III), and platinum(IV) on Fe3O4 Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 794207, 7 pages doi:10.1155/2012/794207

Research Article Nitrogen-Doped TiO2 Nanotube Arrays with Enhanced Photoelectrochemical Property

Shipu Li, Shiwei Lin, Jianjun Liao, Nengqian Pan, Danhong Li, and Jianbao Li

Key Laboratory of Ministry of Education for Application Technology of Chemical Materials in Hainan Superior Resources, School of Materials and Chemical Engineering, Hainan University, Haikou 570228, China

Correspondence should be addressed to Shiwei Lin, [email protected]

Received 22 January 2012; Revised 22 February 2012; Accepted 27 February 2012

Academic Editor: Xuxu Wang

Copyright © 2012 Shipu Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

N-doped TiO2 nanotube arrays were prepared by electrochemical anodization in glycerol electrolyte, followed by electrochemical deposition in NH4Cl solution. An orthogonal experiment was used to optimize the doping conditions. Electrolyte concentration, reaction voltage, and reaction time were the main factors to influence the N-doping effect which was the determinant of the visible range photoresponse. The optimal N-doping conditions were determined as follows: reaction voltage is 3 V, reaction time is 2 h, and electrolyte concentration is 0.5 M. The maximal photocurrent enhanced ratio was 30% under white-light irradiation. About 58% improvement of photocatalytic efficiency was achieved in the Rhodamine B degradation experiment by N doping. The kinetic constant of the N-doped TNT arrays sample was almost twice higher than that of the undoped sample. Further analysis by X-ray photoelectron spectroscopy supported that electrochemical deposition is a simple and efficient method for N doping into TiO2 nanotube arrays.

1. Introduction overcome this problem, two different strategies have been typically taken: element doping and surface modification. Since the discovery of water photolysis on TiO2 electrode Asahi et al. reported that improved photocatalytic activity by Fujishima and Honda in 1972 [1], TiO2 has been exten- of TiO2 under visible light irradiation can be achieved by sively studied. It is one of the most promising oxide semicon- nitrogen doping [12]. Since then, many preparation meth- ductors for photoelectrochemical applications, particularly ods, such as ion implantation, hydrothermal, chemical vapor due to its low cost, nontoxicity, and stability against photo- deposition, chemical liquid deposition, and anodic oxida- corrosion. In contrast to nanoparticle, highly ordered TiO 2 tion of Ti/N alloy [13–19], have been reported to dope nitro- nanotube (TNT) arrays not only have high surface-to- ff volume ratios and adsorptive capacity, but also have good gen into TiO2 using di erent N-doped precursors to enhance the photoelectrochemical properties of TiO2. However their photocatalytic properties and high photoelectrical conver- ff sion efficiency. Also, the nanotubes produced by anodization doping e ects are contradictory, and the underlying mecha- can permit a careful control over their nanotube diameter, nisms are also lack of a consensus. layer thickness, and wall thickness, obtaining structures ver- In our previous work, we reported the successful con- tically oriented from the surface. So the TNT arrays have trollable growth of highly ordered TiO2 nanotube arrays by widespread application prospect in dye sensitization solar adjusting the anodization voltage and duration [20]. In this cells, sensors, hydrogen generation by water photoelec- paper, we investigate a simple and efficient nitrogen-doping trolysis, photocatalytic degradation of pollutants, and bio- approach, that is, electrochemical deposition. Despite the medicines [2–11]. vast reports on TNT arrays, there is little research on the in- However, due to the comparably large bandgap (anatase, fluence of doping parameters on electrochemical nitrogen- Eg ∼ 3.2eV),TiO2 can only respond to UV light irradiation doping process. Thus, we take an orthogonal experiment to (λ<380 nm) which possesses about 4% of the solar optimize the doping conditions in order to obtain an ex- spectrum. This greatly limits its application prospects. To cellent visible range photoresponse. Then, the N-doping 2 International Journal of Photoenergy

Table 1: Factors and levels for orthogonal experiments. of the samples was analyzed by X-ray photoelectron spectroscopy (XPS, PHI-5300) with Al Kα radiation source. The Levels Factors absorption properties of the samples were recorded using a 123diffuse reflectance UV-vis spectrometer (Persee, TU-1901) (A) Electrolyte concentration (M) 0.5 1 1.5 with the wavelength range between 250 and 750 nm. (B) Reaction voltage (V)123 (C) Reaction time (h)1232.4. Photoelectrochemical and Photocatalytic Properties. Pho- tocurrent densities were measured using an electrochemical workstation (Zahner, Zennium) in a standard three- effect on the photocatalytic activity is further characterized electrode configuration with a platinum foil as the counter- by the degradation of Rhodamine B under simulated sun- electrode, a saturated calomel electrode (SCE) as the refer- light irradiation. ence electrode, and the TNT arrays sample as the working electrode. 1 M KOH was used as the electrolyte. A self- contained white light source with intensity of 12 mW/cm2 2. Experimental was utilized as an excitation source (dominant wavelength: 553 nm, half intensity line width: Δ119 nm). 2.1. Preparation of TiO2 Nanotube Arrays. The TiO2 nanotube arrays were prepared by electrochemical anodization To compare the photocatalytic activity of the undoped of titanium foils (0.5 mm thickness, 99.4% purity). Firstly, and N-doped TiO2 photocatalysts, photocatalytic degrada- the titanium foils were cut into small pieces (1.5 × 5cm) tion experiments were performed. Rhodamine B (Rh B) was chosen as a target compound. The initial concentration and polished by chemical polishing fluid (HF : HNO3= 1:4, in volumetric ratio). Then all of the foils were degreased of the dye was 5 mg/L and the pH value of the Rh B by sonication in acetone, methanol, deionized (DI) water, solution was 7. A 500 W xenon lamp was used to produce the respectively, and finally dried in air. The anodization was per- simulated sunlight. Before the photocatalytic degradation, × formed in a two-electrode configuration with titanium foil the photocatalyst (3.0 cm 1.5cm)wassoakedin10mLRh as the working electrode and stainless steel foil as the count- B solution for 30 min to establish the adsorption/desorption erelectrode. The distance between two electrodes was kept at equilibrium. The concentration of Rh B was then determined 2.5 cm. The samples were anodized at 30 V and in solutions at 554 nm every 30 min by the UV-vis spectrophotometer (Persee, TU-1901). The blank test was also carried out by containing 0.27 M NH4F consisting of mixtures of DI water and glycerol (1, 2, 3-propanetriol) prepared in volumetric irradiating Rh B solution without TiO2 photocatalyst for ratio of 50 : 50% for 3 h. After the electrochemical treatment checking the self-photolysis of Rh B. the samples were rinsed with DI water and dried in air. 3. Results and Discussion 2.2. N-Doping Approach. The as-prepared TiO2 nanotube arrays (on Ti sheet) were used as the cathode and plat- 3.1. SEM Analysis. Figure 1 shows FESEM images of the inum as the anode. The electrolyte was NH4Cl solution. undoped and N-doped TNT arrays samples. Since the mor- The distance between two electrodes was kept at 2.5 cm. phologies of all the N-doped TNT arrays samples are similar, The electrochemical deposition process was performed at we chose a representative sample, with the reaction voltage different potentials for various reaction durations. Then, of 3 V, reaction time of 3 h, and electrolyte concentration the sample was taken out and rinsed with DI water. of 0.5 M, to make the results more clear and simple. The Thermal annealing was performed in ambient air at 450◦C obtained nanotube arrays show highly ordered and vertically for 3 h. An orthogonal experiment was used to investigate oriented morphology with inner diameter approximately the optimal doping condition. As seen from Table 1, the 140 nm, and the nanotube length is about 2.1 μm. It can doping experiments were carried out with 3 factors and 3 be clearly seen that the nanotube arrays keep its structural levels, namely, electrolyte concentration (0.5, 1, and 1.5 M), integrity after the nitrogen doping electrochemical process. reaction voltage (1, 2, and 3 V), and reaction time (1, 2, and 3 h), respectively. The range of each factor level was 3.2. XRD Analysis. The XRD patterns of the as-prepared determined based on the results of preliminary experiments. TNT arrays sample, the annealed undoped sample, and the The photocurrent enhancement ratio of the doped samples annealed N-doped TNT arrays sample are shown in Figure 2. under visible light irradiation relative to the undoped sample The as-prepared TNT arrays film exhibits an amorphous was used as the dependent variable to determine the optimal structure with the existence of only typical diffraction peaks doping condition. of metallic titanium. Hence, the annealing process is necessary to transfer the amorphous TiO2 film into a well- 2.3. Morphology and Structure Characterization of TNT crystallized phase. Upon thermal annealing the samples at Arrays. The morphologies of the as-prepared samples were 450◦C for 3 h, two anatase diffraction peaks at 25.5◦ (101), observed using a field-emission scanning electron micro- and 48.1◦ (200) can be clearly observed, indicating that the scope (FESEM, Hitachi, S-4800) and their crystalline phase TNT arrays samples had been crystallized. In addition, the was identified using an X-ray diffraction (XRD, Bruker AXS, undopedsamplehasmoreobviousrutilepeaksthantheN- D8 Advance) technique. The surface chemical composition doped sample. This suggests that the phase transition from International Journal of Photoenergy 3

Table 2: Results of the orthogonal experiment.

No. (A) Electrolyte concentration (M)(B) Reaction voltage (V)(C) Reaction time (h) Photocurrent enhancement ratio (%) 10.51 1 9.0 2 0.5 2 2 11.5 3 0.5 3 3 30.0 4 1 1 2 16.0 5 1 2 3 13.2 6 1 3 1 18.5 71.51 3 9.0 81.52 1 6.2 9 1.5 3 2 25.9

K1 16.8 11.3 11.2

K2 15.9 10.3 17.8

K3 13.7 23.0 17.4 R 3.1 12.7 6.6

T A T T T A T (c) T Intensity (a.u.) Intensity

(b) R (a) R

(a)

20 30 40 50 60 70 80 2θ (degree) Figure 2: XRD patterns of (a) the as-prepared TNT arrays sample, (b) the undoped TNT arrays sample annealed at 450◦Cfor3h, and (c) N-doped TNT arrays sample after annealing at 450◦Cfor 3 h. The doping conditions are that reaction voltage is 3 V, reaction time 3 h, and electrolyte concentration 0.5 M. A, R,andT represent anatase, rutile, and titanium, respectively.

(b)

Figure 1: FESEM top images of the (a) undoped and (b) N- the visible light region is observed for the N-doped sample. doped TNT arrays samples. The doping conditions are that reaction The broad absorption peaks between 400 and 600 nm are voltage is 3 V,reaction time 3 h, and electrolyte concentration 0.5 M. attributed to the subbandgap states of the TNT arrays due The insets show the corresponding cross-sectional images. to the special nanotube structure [21], which makes it difficult to accurately determine the UV-vis absorption for the pure TNT arrays. So, further photoeletrochemical characterization and measurements should be performed to clarify the anatase to rutile during annealing might be inhibited by the N-doping effects. incorporation of N dopant in the TiO2 lattice [17]. 3.4. Photoelectrochemical Property and Orthogonal Test 3.3. UV-Vis Absorption Spectroscopic Analysis. Figure 3 com- Results. The photocurrent response measurements were per- pares the UV-visible diffuse reflectance spectra of the un- formed under visible light pulsed irradiation to investigate doped and the N-doped TNT arrays. TiO2 absorption thre- the photoinduced charge separation efficiency of the un- shold localizes between 350 and 380 nm, and a shift toward doped and N-doped samples. The results are shown in 4 International Journal of Photoenergy

1.6 1 1.5 0.96 1.4 0.92 1.3 0 C

1.2 / 0.88 C

Abs (a.u.) Abs Undoped TNT 1.1 0.84

1 0.80 0.9 N-doped TNT 0.76 0.8 0 0.5 1 1.5 2 2.5 3 3.5 300 400 500 600 700 Irradiation time (h) Wavelength (nm) RhB photolysis Figure 3: UV-vis absorption spectra of the undoped and N-doped Undoped TNT TNT arrays. The doping conditions are that reaction voltage is 3 V, N-doped TNT reaction time 3 h and electrolyte concentration 0.5 M. (a) 0.32

0.4 0.28 0.35

) 0.24 2 0.3 0.2 A/cm (II) ) μ

} 0

0.25 C /

C 0.16

0.2 ln ( (I) 0.12 0.15 Light on 0.08 Light off 0.1

Photocurrent density Photocurrent ( 0.04 0.05 0 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Irradiation time (h) 0 60 120 180 240 300 360 420 − 1 Time (s) k1 = 0.00976 h k = 0.04877 h − 1 Figure 4: Photocurrent response of (I) undoped TNT and (II) N- 2 k = 0.08271 h − 1 doped TNT arrays samples. 3 (b)

Figure 5: Comparison of (a) degradation rates and (b) kinetic Figure 4. The working electrode potential was set at 0 V curves by the Rh B degradation using the undoped TNT and N- to keep the same working condition. All the N-doped doped TNT arrays samples. The doping conditions are that reaction samples, treated under different doping conditions, possess voltage is 3 V, reaction time 3 h and electrolyte concentration 0.5 M. higher photocurrents than the undoped ones. We can see that the highest photocurrent of the N-doped TNT arrays (0.316 μA/cm2) is much higher than the undoped sample, 2 which is 0.243 μA/cm . However, as well known pure TiO2 N-doped samples should have better photocatalytic activity can only respond to UV light, as the excitation source as compared to the undoped sample. (dominant wavelength: 553 nm, half intensity line width: The photoelectrochemical property of N-doped TNT Δ119 nm) contains a little UV light, the undoped sample arrays was also found to be strongly dependent on the doping got a small photocurrent. Higher photocurrent means more conditions. Electrolyte concentration, reaction voltage, and photoinduced carriers can transfer from N-doped TNT to reaction time are the three most important factors that in- the counterelectrode via external circuit. The photocatalytic fluence the N-doping effect during the electrochemical de- activity of titania greatly depends on the electron-hole position process. The independent variables with three varia- transfer ability [22]. Therefore, it can be expected that the tion levels, X1 (electrolyte concentration: 0.5, 1, and 1.5 M), International Journal of Photoenergy 5

O1s Ti2p 2p3/2

Ti2p

Intensity (a.u.) Intensity 2p1/2 Intensity (a.u.) Intensity

C1s

N1s

0 200 400 600 800 1000 1200 450 455 460 465 470 Banding energy (eV) Banding energy (eV) (a) (b)

O1s N1s Intensity (a.u.) Intensity Intensity (a.u.) Intensity

525 530 535 540 392 396 400 404 408 Banding energy (eV) Banding energy (eV) (c) (d)

Figure 6: XPS spectra of the N-doped TNT arrays sample. (a) A survey spectrum, (b) a spectrum of Ti 2p, (c) a spectrum of O 1s, and (d) a spectrum of N 1s. The doping conditions are that reaction voltage is 3 V, reaction time 3 h and electrolyte concentration 0.5 M.

X2 (reaction voltage: 1, 2, and 3 V), and X3 (reaction time: 1, electrolyte concentration. When the reaction voltage is 2, and 3 h) are listed in Table 1. To compare the N-doping 3 V, the enhancement ratio has the maximum value. And effect more clearly, the photocurrent enhancement ratio the photocurrent enhancement ratio increases with the of the N-doped samples relative to the undoped one was increasing reaction time and reaches the peak value at 2 h and chosen as the dependent variable. The analysis results of the then drops from 2 to 3 h. So, with respect to the photocurrent orthogonal experiment are presented in Table 2. Although enhancement ratio, the optimal condition is obtained when the maximum photocurrent enhancement ratio was 30%, electrolyte concentration, reaction voltage, and reaction time we cannot choose the corresponding extraction condition are B3C2A1,thatare3V,2hand0.5M,respectively.However, as the best condition. In view of orthogonal analysis, we according to the K value, the difference between 2 and 3 h is calculated the values of K and R. Ki (i = 1, 2, 3) is the very little, and the reaction time is relatively secondary factor, average of the sum of photocurrent enhancement ratio for so we chose the sample, with the reaction voltage of 3 V, certain factor at level i, and R refers to the result of extreme reaction time of 3 h, and electrolyte concentration of 0.5 M, analysis. According to the R value, the factors which influence to perform further characterization and measurements. the photocurrent enhancement ratio are listed in a decreasing order: B>C >A. Based on the K value, we can see that the 3.5. Photocatalytic Activity. In order to further study the N- photocurrent enhancement ratio decreases with increasing doping effect, the photocatalytic activity of the undoped 6 International Journal of Photoenergy and N-doped TNT arrays samples was studied using the The XRD patterns suggest that the N doping might sup- degradation of Rh B solution, and the results are shown in press the anatase-rutile phase transition. A systematic re- Figure 5. Direct photodegradation ability was also studied, search of the doping conditions was made through an ortho- and only less than 4% of Rh B was decomposed. After 3.5 h gonal experiment. The experimental results indicate that the visible light irradiation, the Rh B degradation efficiencies effect of reaction voltage is dominant, and the next fac- were determined as 16% and 25.7%, for the undoped and tors are reaction time and electrolyte concentration, respec- N-doped TNT arrays samples, respectively. The doping tively. The optimal N-doping condition was then determined conditions are reaction voltage 3 V, reaction time 3 h, and as follows: reaction voltage is 3 V, reaction time 2 h, and elec- electrolyte concentration 0.5 M. About 58% improvement of trolyte concentration 0.5 M. Both the photoelectrochemical photocatalytic efficiency is achieved by N doping. properties and photocatalytic activity under visible light irra- The apparent first-order kinetic equation is ln(C0/C) = diation were enhanced after N doping into TiO2 nanotube kt,wherek is the reaction rate constant, t is the irradiation arrays. XPS results indicate that the nitrogen element could time, and C0 and C are the initial and reaction concentrations be successfully doped into TiO2 lattice, leading to visible of Rh B aqueous solutions, respectively. The variations in light response. This, thus, supports the practicability of the ln(C0/C) as a function of irradiation time are given in electrochemical deposition method using for element doping Figure 5(b). It can be seen that the Rh B photodegradation into TNT arrays. clearly obeyed the first-order reaction kinetics. And the apparent rate constant k of the TiO2 nanotube photocatalyst is significantly higher than that of Rh B photolysis, indicating Acknowledgments that TiO2 plays an important role in the photocatalytic This work was supported by Program for New Century process. The k value of the undoped TNT arrays sample Excellent Talents in University (NCET-09-0110), National is 0.04877 h−1, while the reaction rate constant of the N- −1 Nature Science Foundation of China (51162007), the Key doped sample is 0.08271 h . The kinetic constant of the Project of Chinese Ministry of Education (210171), and N-doped sample is almost twice higher than that of the Hainan Natural Science Foundation (511110). undoped sample, suggesting that its photocatalytic activity driven by visible light can be enhanced by the electrochemical N-doping deposition method. References

[1] A. Fujishima and K. Honda, “Electrochemical photolysis of 3.6. XPS Analysis. The elemental composition of the water at a semiconductor electrode,” Nature, vol. 238, no. N-doped TNT arrays sample was determined by XPS. 5358, pp. 37–38, 1972. Figure 6(a) shows that the surface of the TNT arrays is [2] K. Xu, S. Paul, and L. L. Cao, “Ordered TiO2 nanotube arrays composed of Ti, O, C, and N elements. The residual carbon on transparent conductive oxide for dye-sensitized solar cells,” from electrolyte and adventitious element carbon may cause Chemistry of Materials, vol. 22, no. 1, pp. 143–148, 2010. the presence of C element [23]. From Figure 6(b),itcanbe [3] J. Wang and Z. Q. Lin, “Dye-sensitized TiO2 nanotube solar seen that Ti 2p core level appears at 458.6 eV. The binding cells with markedly enhanced performance via rational surface 3/2 engineering,” Chemistry of Materials, vol. 22, no. 2, pp. 579– energy in the N-doped TNT arrays sample is lower than 584, 2010. the undoped sample (458.7 eV, not shown here), suggesting [4] Q. Zheng, B. X. Zhou, and J. Bai, “Self-organized TiO2 nano- that TiO2 lattice is modified due to N-substitution [24, 25]. tube array sensor for the determination of chemical oxygen Oxygen 1s core level peak appears around 530 eV, as shown demand,” Advanced Materials, vol. 20, no. 5, pp. 1044–1049, in Figure 6(c). A feature extending to higher binding energy 2008. at about 532 eV is clearly visible, suggesting the presence of [5] S. Erdem, C. Zeliha, K. Necmettin, and Z. Z. Ozt¨ urk,¨ “Syn- another type of oxygen due to the more covalent nature of thesis of highly-ordered TiO2 nanotubes for a hydrogen sen- the N-doped TNT arrays sample. This might be due to the sor,” International Journal of Hydrogen Energy,vol.35,no.9, presence of oxygen and nitrogen from the same lattice units pp. 4420–4427, 2010. [6] Z. H. Hu, K. Du, and Y. Cao, “Fabrication and photocatalytic in TiO2 [25]. Figure 6(d) shows the XPS spectrum of N 1s core level of the N-doped TNT arrays. Only a single peak at application of highly ordered TiO2 nanotube arrays,” Journal of Materials Science and Engineering, vol. 4, no. 8, pp. 45–50, about 400 eV can be found, which was due to the N− anion 2010. incorporated in the TiO2 as N-Ti-O structural feature [24– [7]J.Bai,J.H.Li,andY.B.Liu,“Anewglasssubstratephoto- 27]. This is in good agreement with the XPS findings by Chen electrocatalytic electrode for efficient visible-light hydrogen et al. [24] and Sathish et al. [25]. production: CdS sensitized TiO2 nanotube arrays,” Applied Catalysis B, vol. 95, no. 3-4, pp. 408–413, 2010. [8] J. W. Ng, X. W. Zhang, and T. Zhang, “Construction of self- 4. Conclusions organized free-standing TiO2 nanotube arrays for effective disinfection of drinking water,” Journal of Chemical Technology Nitrogen-doped TiO2 nanotube arrays were prepared by a and Biotechnology, vol. 85, no. 8, pp. 1061–1066, 2010. simple electrochemical deposition method. By comparing [9] X. W. Zhang, J. H. Du, and P. F. Li, “Grafted multifunctional the FESEM images of undoped and N-doped TNT, the TiO2 titanium dioxide nanotube membrane: separation and pho- nanotube arrays kept their structural integrity with inner todegradation of aquatic pollutant,” Applied Catalysis B, vol. diameter approximately 140 nm and length about 2.1 μm. 84, no. 1-2, pp. 262–267, 2008. International Journal of Photoenergy 7

[10] Y. Y. Song, S. S. Felix, and B. Sebastian, “Amphiphilic TiO2 transformation at the nanoscale,” The Journal of Physical nanotube arrays: an actively controllable drug delivery sys- Chemistry B, vol. 108, no. 4, pp. 1230–1240, 2004. tem,” Journal of the American Chemical Society, vol. 131, no. 12, pp. 4230–4232, 2009. [11] M. Kalbacova, J. M. Macak, and S. F. Schmidt, “TiO2 nanotubes: photocatalyst for cancer cell killing,” Physica Status Soli- di, vol. 2, no. 4, pp. 194–196, 2008. [12] R. Asahi, T. Ohwaki, and K. Aoki, “Visible-light photocatalysis in nitrogen-doped titanium oxides,” Science, vol. 293, no. 5528, pp. 269–271, 2001. [13] A. Ghicov, J. M. Macak, and H. Tsuchiya, “Ion implantation and annealing for an efficient N-doping of TiO2 nanotubes,” Nano Letters, vol. 6, no. 5, pp. 1080–1082, 2006. [14] Y. Y. Chen, S. M. Zhang, and Y. Yu, “Synthesis, characterization, and photocatalytic activity of N-doped TiO2 nanotubes,” Journal of Dispersion Science and Technology,vol.29,no.2,pp. 245–249, 2008. [15] Z. Jiang, F. Yang, and N. J. Luo, “Solvothermal synthesis of N- doped TiO2 nanotubes for visible-light-responsive photocatalysis,” Chemical Communications, vol. 11, no. 4, pp. 6372–6374, 2008. [16] X. Feng, Y. Wang, and Z. S. Jin, “Photoactive centers responsible for visible-light photoactivity of N-doped TiO2,” New Journal of Chemistry, vol. 32, no. 6, pp. 1038–1047, 2008. [17] Y. K. Lai, J. Y. Huang, H. F. Zhang et al., “Nitrogen-doped TiO2 nanotube array films with enhanced photocatalytic activity under various light sources,” Journal of Hazardous Materials, vol. 184, no. 1–3, pp. 855–863, 2010. [18] J. Q. Geng, D. Yang, and J. H. Zhu, “Nitrogen-doped TiO2 nanotubes with enhanced photocatalytic activity synthesized by a facile wet chemistry method,” Materials Research Bulletin, vol. 44, no. 1, pp. 146–150, 2009. [19] D. Kim, S. Fujimoto, and P. Schmuki, “Nitrogen doped anodic TiO2 nanotubes grown from nitrogen-containing Ti alloys,” Electrochemistry Communications, vol. 10, no. 6, pp. 910–913, 2008. [20] S. P.Li, S. W. Lin, and J. J. Liao, “Effects of synthesis parameters on controllable growth of highly-ordered TiO2 nanotube arrays,” Advanced Materials Research, vol. 284–286, pp. 791–795, 2011. [21] Y. K. Lai, L. Sun, and C. Chen, “Optical and electrical characterization of TiO2 nanotube arrays on titanium substrate,” Applied Surface Science, vol. 252, no. 4, pp. 1101–1106, 2005. [22]J.J.Xu,Y.H.Ao,andM.D.Chen,“Photoelectrochemicalpro- perty and photocatalytic activity of N-doped TiO2 nanotube arrays,” Applied Surface Science, vol. 256, no. 13, pp. 4397– 4401, 2010. [23] S. Sakthivel and H. Kisch, “Daylight photocatalysis by carbon- modified titanium dioxide,” Angewandte Chemie, vol. 42, no. 40, pp. 4908–4911, 2003. [24] Y. Y. Chen, S. M. Zhang, and Y. Yu, “Synthesis, characterization, and photocatalytic activity of N-doped TiO2 nanotubes,” Journal of Dispersion Science and Technology,vol.29,no.2,pp. 245–249, 2008. [25] M. Sathish, B. Viswanathan, R. P. Viswanath, and C. S. Gopinath, “Synthesis, characterization, electronic structure, and photocatalytic activity of nitrogen-doped TiO2 nanocat- alyst,” Chemistry of Materials, vol. 17, no. 25, pp. 6349–6353, 2005. [26] J. W. Zhang, Y. Wang, and Z. S. Jin, “Visible-light photocatalytic behavior of two different N-doped TiO2,” Applied Surface Science, vol. 254, no. 15, pp. 4462–4466, 2008. [27]J.L.GoleandJ.D.Stout,“Highlyefficient formation of visible light tunable TiO2-xNx photocatalysts and their Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 901907, 6 pages doi:10.1155/2012/901907

Research Article Photocatalytic Activity of Titanate Nanotube Powders in a Hybrid Pollution Control System

Sun-Jae Kim,1 Young-Seak Lee, 2 Byung Hoon Kim,3 Seong-Gyu Seo,4 Sung Hoon Park,5 and Sang Chul Jung5

1 Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 143-747, Republic of Korea 2 Department of Fine Chemical Engineering and Applied Chemistry, Chungnam National University, Daejeon 305-764, Republic of Korea 3 Department of Dental Materials, School of Dentistry, Chosun University, Gwangju 501-759, Republic of Korea 4 Department of Civil and Environmental Engineering, Chonnam National University, Yosu 550-749, Republic of Korea 5 Department of Environmental Engineering, Sunchon National University, Sunchon 540-742, Republic of Korea

Correspondence should be addressed to Sang Chul Jung, [email protected]

Received 26 January 2012; Revised 21 February 2012; Accepted 24 February 2012

Academic Editor: Weifeng Yao

Copyright © 2012 Sun-Jae Kim et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The photocatalytic activity on decomposition of Rhodamine B (RB) of titanate nanotubes (TNTs) synthesized by alkali hydrothermal treatment method was evaluated using a microwave/UV/photocatalyst hybrid system. The effects of each element technique as well as the synergy effects on decomposition of organic material were investigated. When TNTs were ion-exchanged with HCl, Na+ content was reduced from 8.36 wt% to 0.03 wt%, whereas the TNTs calcined at 723 K was phase-transformed into anatase structure. The RB decomposition rate increased with TNTs dosage as well as with microwave intensity. Effect of addition of auxiliary oxidants on photocatalytic decomposition of RB was also investigated. When ozone was added, the decomposition rate increased with the amount of ozone added. When H2O2 was used as the auxiliary oxidant, however, addition of H2O2 exceeding a threshold amount caused reduction of decomposition rate. A synergy effect was observed when H2O2 addition was combined with microwave-assisted photocatalysis.

1. Introduction Yu et al. [2] synthesized hydrogen titanate nanowires at various heat treatment temperatures and investigated their phase Titanate nanotube (TNT), having unique properties, for ex- structure, crystallite size, morphology, specific surface area, ample, tubular structure, large surface area, strong ion ex- pore structure, and photocatalytic activity. Baiju et al. [3] change, and high sedimentation rate, is a material with high investigated the photocatalytic activity of a mixed nanobelts- potential in various applications such as supports, ion ex- nanotubes titanate obtained through hydrothermal method change, adsorption, and dye sensitized solar cells. Among at 423 K with 30 h of treating time on degradation of meth- others, application as photocatalyst is of special interest be- ylene blue. They reported that photocatalytic activity of the cause of its material property combined with nanoscale catalyst was very low although it exhibited high adsorptivity structure. Nevertheless, data reported for the photocatalytic towards methylene blue. On the other hand, Nakahira activity of synthesized TNTs are inconsistent. According to et al. [4] reported a significantly high activity of TNT on Yu et al. [1], who used TNT synthesized hydrothermally at degradation of HCHO under UV light. Xiao et al. [5] also 423 K with 48 h of treatment time to enhance the photo- found that TNT synthesized hydrothermally at 403 K with catalytic oxidation of acetone in air, TNT showed no pho- 24 h of treating time had a high photocatalytic activity on tocatalytic activity before calcination. It has been reported degradation of Rhodamine B (referred to as RB hereafter), that nanotubes do not have photocatalytic activity towards with the degradation efficiency similar to that of P25. This amaranth degradation regardless of its sodium content. inconsistency found in the literature may be attributed, at 2 International Journal of Photoenergy

least partly, to different precursors used (amorphous TiO2, It was then again dispersed in 0.1-N HCI solution at 333 K anatase, rutile, and their mixture) and reaction conditions and stirred for a long time to allow Na+ ions trapped on applied (temperature and time). Another possibility is that the surface and interior of the particles during the hy- the produced TNT may have contained unreacted anatase drothermal reaction to be ion-exchanged with H+and then phase which could have provided unexpected photocatalytic removed. During the ion-exchange processing, precipitate activity. was sampled to measure the Na+ content. It was confirmed It is of growing interest to use microwave energy for syn- by the analysis of the precipitate that the Na+ content of thetic organic chemistry because it can accelerate reactions the precipitate was 0.03 wt% at pH = 7.0. The precipitate and improve yields and selectivity. Many researchers have sample was dried for 24 h using a freeze-dryer to remove reported improved photocatalytic performance achieved by moisture from the sample. The ion-exchanged nanotubes microwave irradiation. For example, Kataoka et al. [6]re- were calcined in air at 723 K for 30 min and then were ported that microwave irradiation increased the photocat- ground in an agate mortar before being used in the pho- alytic oxidation rate of ethylene by 83.9%. Horikoshi et tocatalytic experiments. Scanning electron microscope (S- al. [7] found from their experiments using electron spin 4700, Hitachi) and transmission electron microscope (JEM- resonance (ESR) that generation of OH radicals during 2010F, JEOL) observations showed that the powders had photocatalytic reaction was enhanced by about 20% by nanotubular shape. The Na+ ion content was analyzed by microwave irradiation. One difficulty in combining UV light EDX (Oxford INCA energy detector, equipped with SEM). source and microwave irradiation is electrode spoilage. This Crystal structure of the powder was analyzed using X- problem can be solved if microwave electrodeless lamp is ray diffractometer (D/MAX-2500/PC, Rigaku) and ambient used as the light source. Furthermore, microwave electrode- Raman Spectroscopy (inVia Raman macroscope, Renishow). less lamp brings additional benefits: good photochemical Photocatalytic activity of TNT prepared in this way on efficiency, long lifetime, low cost, and simple equipment decomposition of RB in its aqueous solution was evaluated setup [8]. Horikoshi et al. [9] argued that photocatalysis using a microwave/UV/photocatalyst hybrid process system. with electrodeless lamp (a double quartz cylindrical plasma RB (Junsei Chem. Co., Ltd.) was chosen because it is not photoreactor) was about 10-times more efficient than the adsorbed well on TNT powders. Refer to our previous paper photocatalysis using traditional lamp. [11] for detailed description on the experimental apparatus. In a previous study of ours, the TNTs were calcined at Microwave irradiation was created by a microwave/UV sys- several different temperatures and their photocatalytic activi- tem manufactured by Korea microwave instrument Co., ties were compared with those of Degussa P-25 powders [10]. Ltd. It is composed of a microwave generator (frequency, The TNT calcined at 673 K showed a lower activity than P- 2.45 GHz; maximal power, 1 kW), a three-stub tuner, a power 25, whereas the activity of the TNT calcined at 698 K was monitor, and a reaction cavity. Microwave (actual power similar to that of P-25. On the other hand, all the TNTs used, 200∼600 W) irradiated on the RB aqueous solution calcined at 723 K or higher exhibited higher photocatalytic (5.0 × 10−6 mM, 1 L) containing TNT powders (loading, activities than P-25. In this study, the photocatalytic activity 0.05∼0.15 g) was delivered through a wave guide. Microwave of the TNT calcined at 723 K was evaluated using a novel irradiation was continuous and its intensity was controlled microwave/UV/photocatalyst hybrid system. A microwave- by a power monitor. Optimal low reflection of the microwave assisted electrodeless lamp is used as the UV source. The radiation was achieved using the three-stub tuner. The UV effect of microwave irradiation on photocatalytic reaction source and the microwave generator were located on the efficiency is investigated. To improve the pollutant removal right-hand and left-hand sides, respectively, of the device. A rate further, two auxiliary oxidants, O3 and H2O2,areadded stirrer was installed on the back side in the reaction cavity to in the reaction and their effects on the reaction efficiency enhance the microwave transfer. are investigated based on the experimental results. The role A double-tube type microwave discharge electrodeless of each element technique and interaction among them are lamp (referred to as MDEL hereafter), with 170 mm length, discussed. 44 mm inner diameter, and 60 mm outer diameter, emitting UV upon irradiation of microwave was used in this study. It 2. Experimental wasmadeofquartztomaximizethereactionefficiency. Small amount of mercury was doped between the tubes inside the Commercial TiO2 P25 (Evonik, former Degussa, Germany), double-tube UV lamp that was kept vacuumed. The lamp composed of 75% of anatase and 25% of rutile, was used used in this study is UV-C type lamp although a little amount as raw material for TNT production. The BET surface area of UV-A and UV-B wavelength lights is emitted as well. and the anatase crystallite size of P25 are about 50 m2/g Because microwave irradiation heats up the reactant so- and 25 nm, respectively. P25 was put into a Teflon-lined lution, the reaction temperature could not be maintained at autoclave containing 10-M NaOH. The suspension was a constant without a cooling system. Therefore, the reactant treated ultrasonically for 1 h to have TiO2 particles dissolution was put in a stainless steel beaker installed in a persed well. Then the autoclave was heated to maintain constant-temperature equipment. The heated reactant so- the temperature at 423 K for 48 h. After the reaction was lution was circulated through a cooling system by a roller completed, nanotube precipitate with a high pH level was pump to keep the reaction temperature constant at 298 K. washed repeatedly using distilled water to remove NaOH. To investigate the effect of addition of auxiliary oxidants on The washed precipitate had 8.36 wt% of Na+ at pH = 7.0. the decomposition organic compounds, ozone and hydrogen International Journal of Photoenergy 3

A 145

A A A 390 506 638

10 20 30 40 50 60 70 200 400 600 800 1000 2θ (Deg) Raman shift (cm−1) A: anatase (a) (b)

Figure 1: X-ray diﬀraction pattern (a) and Raman spectra (b) of TNT powders calcined for 30 min at 723 K in O2 atmosphere.

(a) (b)

Figure 2: SEM and TEM images of TNT powders calcined for 30 min at 723 K in O2 atmosphere. peroxide were added. Ozone was produced by feeding oxygen of the titanate phase was detected at 280 cm−1, which is not with the flow rate of 500 cc/min to an ozone generator (Lab- clearly shown in normal scale. 1,OzoneTechCo.,Ltd).Theozoneproductionratewas Figure 2(a) shows an SEM image of calcined TNT pow- controlled between 0.75 and 3.26 g/hr. The reactant solution ders. The powders are shown to be nanotubes growing was prepared using double distilled water. The decompo- along axial direction. The tubular shape of the powder is sition rate of RB was calculated from the change in its observedfromaTEMimageaswell(Figure 2(b)). It is concentration measured at the reactor outlet as a function also observed that the powders were broken into nanorods of reaction time. The RB concentration was determined by and then became nanoparticles. Figure 2 shows that thermal measuring absorbance at 550 nm using a spectrophotometer treatment for 30 minutes at 723 K in O2 atmosphere did not (UV-1601, Shimadzu). destroy the nanotubular structure of TNT. Figure 3 shows the decay in RB concentration obtained with three different TNT powder dosages. For all experi- 3. Results and Discussion ments, initial concentration of RB was about 5.0 × 10−6 mM Figure 1(a) shows the XRD results for TNT powders calcined and 1 L of solution was circulated into the reactor with a flow at 723 K for 30 min in O atmosphere. Characteristic peaks rate of 400 cc/min. The intensity of microwave irradiation 2 was 0.4 kW. It is shown that the RB decomposition rate of anatase TiO2 phase at 2θ = 25.35, 37.84, 48.14, 53.97, and 55.18◦ are clearly observed due to transformation of increases with TNT powder dosage. The results for the the titanate into anatase phase. This is in good agreement three cases were all fitted well by linear lines indicating that with Raman spectra shown in Figure 1(b) that demonstrates decomposition of RB over TNT catalyst can be approximated anatase characteristic peaks at 145, 390, 506, and 638 cm−1. by a pseudo-first-order reaction model: It is clearly shown in Figure 1 that the titanate phase C = exp(−Kt),(1) transformation was almost complete at 723 K although trace C0 4 International Journal of Photoenergy

1 0.3

0.25 0

0.2

−1 ) ) 1 0 − C / 0.15 C (min

ln( −2 K 0.1

−3 0.05

− 4 0 0 5 10 15 20 0 0.75 1.78 3.26 Reaction time (min) Ozone injection rate (g/hr)

0.05 g 0.15 g Microwave/UV/O3/TiO2 Microwave/O3 0.1 g O3 Figure 3: Eﬀect of TNT powder dosages on decomposition of RB. Figure 5: Eﬀect of ozone injection on decomposition of RB.

1 intensity results in a higher UV intensity. UV, which carries intense energy, enhances the photocatalytic reaction rate by 0 exciting photo-catalyst. Therefore, it was not possible to identify the mechanism by which microwave irradiation con- − ) 1 tributed to enhancing the photocatalytic reaction rate in this 0 C

/ study. It can be either one of the effects described above or C − combination of two or all of them. ln( 2 Ozone is a strong oxidant with the electric potential difference of 2.07 V. It has widely been used in water − 3 treatment because it can effectively remove taste, odor and precursors of trihalomethanes (THMs). The effect of ozone −4 in oxidation of organic compounds is, however, relatively 0 5101520 selective because ozone has very low reactivity on single- Reaction time (min) bond chemicals and aromatic compounds with specific 0.2 kW 0.6 kW functional groups such as –COOH and –NO2. On the 0.4 kW contrary, the hydroxyl radical (OH), which has a higher oxidation potential (2.80 V) than ozone, can react with Figure 4: Effect of microwave intensity on photocatalytic degrada- organic compounds unselectively. Large attention has been tion of RB. given to the advanced oxidation processes (AOPs), in which the organic compounds decomposition is enhanced by OH radicals. One of the strengths of the microwave/UV/TiO2 where C is the RB concentration, C0 the initial concentration, hybrid system used in this study is that combined microwave K the over-all reaction rate constant, and t the reaction time. and UV irradiations activate photo-catalyst to maximize the K is determined from the slope of the lines shown in Figure 3. formation of OH radicals when ozone is introduced. Figure 4 compares the RB concentration decay obtained Figure 5 compares the reaction rate constants obtained with three different microwave intensities: 0.2, 0.4, and with three different combinations of element techniques, that 0.6 kw. For all experiments shown in this figure, 0.1 g of is, ozone injection only, ozone injection combined with mi- TNT powders was used. It is clearly shown in this figure crowave irradiation, and ozone injection combined with that the degradation rate increases with microwave intensity. microwave-assisted UV photocatalysis, with four different Microwave irradiation contributes to enhancing the pollu- ozone injection levels: 0, 0.75, 1.78, and 3.26 g/hr. For all tant degradation rate in two ways [7]. It increases temper- the experiments shown in this figure, TNT powder dosage ature selectively, quickly, and uniformly leading to higher and microwave irradiation intensity were 0.1 g and 0.4 kW, reaction rate (thermal effect). Microwave also increases the respectively, when they were applied. collision frequency between reactants (nonthermal effect). The RB decomposition rate was highest when ozone was In addition, microwave intensity affects UV intensity as well injected to the microwave/UV/TiO2 photocatalyst system. in this study because the UV irradiation was created by The reaction rate increased with ozone dosage. Even the MDEL upon the irradiation of microwave: higher microwave ozone-only experiments showed high RB decomposition rate International Journal of Photoenergy 5

0.3 0.3

0.25 0.25

0.2 )

0.2 1 − ) 1 0.15 − 0.15 (min K (min 0.1 K 0.1 0.05

0.05 0 M OHMH MO MUP MUPO MUPH Experiment methods 0 0 1.16e−5 1.16e−4 1.16e−3 1.16e−2 1.16e−1 Figure 7: Reaction rate constants obtained under various experi- H2O2 conc. (M) mental conditions.

Figure 6: Effect of injection H2O2 for decomposition of RB in aqueous solutions. hydroxyl radicals was not possible in this study due to the difficulty of installation of a sensor in the microwave cavity. with the rate constant of 0.1354, which was a little higher Figure 7 compares the decomposition rate constants ob- than that obtained by microwave/UV/TiO2 photocatalyst tained under different experimental conditions. The results system without ozone injection (0.1284). When microwave of eight different experiments are shown in this figure: mi- irradiation combined with ozone injection was used, the crowave irradiation only (M), ozone injection only (O), reaction rate constant was in between the results of the other H2O2 injection only (H), microwave irradiation combined two conditions. When only microwave irradiation was used, with ozone injection (MO), microwave irradiation com- very little RB decomposition was obtained (not shown). bined with H2O2 injection (MH), microwave-assisted pho- Reports on the effect of H2O2 on pollutant removal are tocatalysis by MDEL (MUP), MUP combined with ozone abundant in the literature. There is a general consensus that injection (MUPO), and MUP combined with H2O2 injection addition of H2O2 enhances the photocatalytic degradation of (MUPH). For all the experiments shown in this figure, TNT organic pollutants [12]. The increased reaction rate caused powder dosage of 0.1 g, microwave irradiation intensity of by H2O2 addition can be explained by a couple of reasons. 0.4 kW, circulation flow rate of 400 mL/min, ozone injec- First, it takes surface-trapped electrons, and hence reduces tion rate of 0.75 g/h, and H2O2 addition amount of 1 mL the electron-hole recombination rate and makes more holes (1.1632 × 10−2 mol) were used. A relatively low ozone injec- available for reactions creating OH radicals (e.g., OH− + tion rate of 0.75 g/h was used because the reaction proceeded + h → OH). Second, H2O2 may split into two OH radicals too fast under some conditions with a higher ozone injection directly by photocatalytic reaction. Because H2O2 can be an rate. efficient electron acceptor in TiO2 photocatalytic systems, its As shown in Figure 7, RB was hardly decomposed under effect on photocatalytic pollutant degradation was evaluated M(K = 0.0060) and H (K = 0.0135) conditions. The [13]. Figure 6 compares the reaction rate constants obtained reaction constant was low also under MH condition (K = with different amount of H2O2 added to the microwave- 0.0430). When these element techniques were all combined assisted photocatalytic system. H2O2 addition increased the (MUPH), however, the reaction rate was about six times that reaction rate while the added amount was smaller than a obtained under MH condition (K = 0.2568). Completely threshold value, 1.16 × 10−2 here, but further addition of different results were obtained with addition of ozone. While H2O2 exceeding the threshold reduced the reaction rate. the reaction rate constants of condition O (K = 0.1354) This is in good agreement with the results reported in the and condition MO (K = 0.1438) were high compared to literature [14]. H2O2 is known to form a surface complex those obtained with other element techniques, the reaction on TiO2 [15], which can cause blocking of surface sites of rate obtained under MUPO in which all these techniques catalysts if the amount of H2O2 is too large. Another possible were combined was not very high (K = 0.1798) showing no reason for the reduction of photocatalytic reaction rate due synergy effect. to excessive H2O2 is OH radical scavenging by H2O2 (H2O2 + Microwave, which is an electromagnetic wave with a very OH → HO2 +H2O). In order to examine the function short wavelength, excites polar molecules to cause them to of H2O2 as the scavenger of OH radicals, it is required to rotate and vibrate back and forth rapidly. For example, water observe the change in OH radicals. For instance, Xiang et al. molecules can vibrate about 2.45 × 109 times every second [16] quantitatively measured the hydroxyl radicals produced upon microwave irradiation. The original objective of this from various semiconductor photocatalysts contained in study was to enhance the decomposition reaction rate by aqueous solutions using the photoluminescence technique. exciting pollutant molecules using microwave irradiation. Unfortunately, however, quantitative measurement of the According to the experimental results shown above (M), 6 International Journal of Photoenergy however, the effect of excitement of pollutant molecules [4] A. Nakahira, W. Kato, M. Tamai, T. Isshiki, and K. Nishio, was negligible. Nevertheless, when the microwave-assisted “Synthesis of nanotube from a layered H2Ti4O9· H2Oina photocatalysis was combined with addition of auxiliary hydrothermal treatment using various titania sources,” Journal of Materials Science, vol. 39, no. 13, pp. 4239–4245, 2004. oxidant such as H2O2,asynergyeffect enhancing the reaction rate considerably was observed. This result suggests that [5] M. W. Xiao, L. S. Wang, X. J. Huang, Y. D. Wu, and Z. Dang, microwave irradiation may promote the production of active “Synthesis and characterization of WO3/titanate nanotubes intermediate products, for example, OH radicals, by exciting nanocomposite with enhanced photocatalytic properties,” Journal of Alloys and Compounds, vol. 470, no. 1-2, pp. 486– the auxiliary oxidant molecules. It should be noted, however, 491, 2009. that this hypothesis cannot be proved based only on the [6]S.Kataoka,D.T.Tompkins,W.A.Zeltner,andM.A.Ander- limited experimental results of this study. Further studies ff ff son, “Photocatalytic oxidation in the presence of microwave under di erent conditions, including di erent pollutants irradiation: observations with ethylene and water,” Journal of ff and di erent auxiliary oxidants, are required to validate the Photochemistry and Photobiology A, vol. 148, no. 1–3, pp. 323– hypothesis further. 330, 2002. [7] S. Horikoshi, H. Hidaka, and N. Serpone, “Environmental 4. Conclusion remediation by an integrated microwave/UV-illumination method. 1. Microwave-assisted degradation of rhodamine-B To use hybrid pollution control system for advanced treat- dye in aqueous TiO2 dispersions,” Environmental Science and ment of RB, a series of experiments were performed in Technology, vol. 36, no. 6, pp. 1357–1366, 2002. which the effects of microwave irradiation and auxiliary [8] J. Literak and P. Klan, “The electrodeless discharge lamp: a oxidants were evaluated. The conclusions obtained from the prospectivetoolforphotochemistry.Part2.Scopeandlimita- experimental results are as follows. tion,” Journal of Photochemistry and Photobiology A, vol. 137, no. 1, pp. 29–35, 2000. + (1) When TNTs was ion-exchanged with HCl, Na con- [9] S. Horikoshi, H. Hidaka, and N. Serpone, “Environmental re- tent was reduced from 8.36 wt% to 0.03 wt%, whereas mediation by an integrated microwave/UV illumination tech- the TNTs calcined at 723 K was phase-transformed nique VI. A simple modified domestic microwave oven in- into anatase structure. tegrating an electrodeless UV-Vis lamp to photodegrade en- (2) The RB decomposition rate increased with TNTs dos- vironmental pollutants in aqueous media,” Journal of Photo- chemistry and Photobiology A, vol. 161, no. 2-3, pp. 221–225, age as well as with microwave intensity. 2004. (3) When ozone was added, the decomposition rate in- [10] N. H. Lee, H. J. Oh, S. C. Jung, W. J. Lee, D. H. Kim, and S. J. creased with the amount of ozone added. Kim, “Photocatalytic properties of nanotubular-shaped TiO2 powders with anatase phase obtained from titanate nanotube (4) When H2O2 was used as the auxiliary oxidant, how- powder through various thermal treatments,” International ever, addition of H2O2 exceeding a threshold amount caused reduction of decomposition rate. Journal of Photoenergy, vol. 2011, Article ID 327821, 7 pages, 2011. (5) When the microwave-assisted photocatalysis was [11]S.H.Park,S.J.Kim,S.G.Seo,andS.C.Jung,“Assessmentof combined with addition of auxiliary oxidant such as microwave/UV/O3 in the photo-catalytic degradation of bro- H2O2,asynergyeffect enhancing the reaction rate mothymol blue in aqueous nano TiO2 particles dispersions,” considerably was observed. Nanoscale Research Letters, vol. 5, no. 10, pp. 1627–1632, 2010. [12] M. Harir, A. Gaspar, B. Kanawati et al., “Photocatalytic reactions of imazamox at TiO2 ,H2O2 and TiO2/H2O2 in water Acknowledgment interfaces: kinetic and photoproducts study,” Applied Catalysis This research was supported by Basic Science Research Pro- B, vol. 84, no. 3-4, pp. 524–532, 2008. gram through the National Research Foundation of Korea [13] R. N. Rao and N. Venkateswarlu, “The photocatalytic degradation of amino and nitro substituted stilbenesulfonic acids (NRF) funded by the Ministry of Education, Science, and 2+ by TiO2/UV and Fe /H2O2/UV under aqueous conditions,” Technology (2010-0007412 and 2011-0016699). Dyes and Pigments, vol. 77, no. 3, pp. 590–597, 2008. [14] S. Kim, H. Park, and W. Choi, “Comparative study of homo- References geneous and heterogeneous photocatalytic redox reactions: −3 PW12O40 vs TiO2,” Journal of Physical Chemistry B, vol. 108, [1] J. Yu, H. Yu, B. Cheng, and C. Trapalis, “Effects of calcination no. 20, pp. 6402–6411, 2004. temperature on the microstructures and photocatalytic activ- [15] J. Q. Chen, D. Wang, M. X. Zhu, and C. J. Gao, “Photocatalytic ity of titanate nanotubes,” Journal of Molecular Catalysis A, vol. degradation of dimethoate using nanosized TiO2 powder,” 249, no. 1-2, pp. 135–142, 2006. Desalination, vol. 207, no. 1–3, pp. 87–94, 2007. [2] H. Yu, J. Yu, and B. Cheng, “Photocatalytic activity of the [16] Q. Xiang, J. Yu, P.K. Wong, and M. Chen, “Facile fabrication of calcined H-titanate nanowires for photocatalytic oxidation of mechanochromic-responsive colloidal crystal films,” Journal of acetone in air,” Chemosphere, vol. 66, no. 11, pp. 2050–2057, Colloid and Interface Science, vol. 357, no. 1, pp. 163–168, 2011. 2007. [3]K.V.Baiju,S.Shukla,S.Biju,M.L.P.Reddy,andK.G.K. Warrier, “Hydrothermal processing of dye-adsorbing one-dimensional hydrogen titanate,” Materials Letters, vol. 63, no. 11, pp. 923–926, 2009. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 202519, 6 pages doi:10.1155/2012/202519

Research Article Synthesis of Core-Shell Fe3O4@SiO2@TiO2 Microspheres and Their Application as Recyclable Photocatalysts

Zhenghua Wang, Ling Shen, and Shiyu Zhu

Anhui Key Laboratory of Functional Molecular Solids, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China

Correspondence should be addressed to Zhenghua Wang, [email protected]

Received 14 January 2012; Revised 2 March 2012; Accepted 2 March 2012

Academic Editor: Xuxu Wang

Copyright © 2012 Zhenghua Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

We report the fabrication of core-shell Fe3O4@SiO2@TiO2 microspheres through a wet-chemical approach. The Fe3O4@SiO2@TiO2 microspheres possess both ferromagnetic and photocatalytic properties. The TiO2 nanoparticles on the surfaces of microspheres can degrade organic dyes under the illumination of UV light. Furthermore, the microspheres are easily separated from the solution after the photocatalytic process due to the ferromagnetic Fe3O4 core. The photocatalysts can be recycled for further use with slightly lower photocatalytic eﬃciency.

1. Introduction the magnetic composites can be conveniently separated and collected from water by applying an external magnetic field. In recent years, an enormous research effort has been Many studies have been carried out on the preparation and dedicated to the study of semiconductors in the area of application of TiO2/magnetic composites [14–20]. In these photocatalysis. Among the semiconductor photocatalysts, studies, Fe3O4 and γ-Fe2O3 nanoparticles were applied as the TiO2 has attracted a lot of attention because of its high core magnetic materials. However, Fe3O4 nanoparticles are photocatalytic efficiency, high stability, and low cost [1– easily oxidized, even at room temperature. The TiO2 shell 5]. For the application of TiO2 in the area of wastewater in the composites was usually prepared by sol-gel methods, purification, TiO2 slurry reactor is the most common where heat treatment was essential. After heat treatment, the device because of its high specific surface area, efficient ferromagnetic γ-Fe2O3 may transform to paramagnetic α- light absorption, and good dispersion [6–8]. However, the Fe2O3. separation of TiO2 particles from treated water, especially In this work, through the medium of SiO2,TiO2 from a large volume of water, is an expensive and time- nanoparticles were successfully coated on Fe3O4 micro- consuming work, which limited the application of TiO2 spheres, and Fe3O4@SiO2@TiO2 microspheres with well- slurry reactor in industrial application. Tosolve this problem, defined core-shell structures were obtained. The prepared many methods including titania beads [9], TiO2-based Fe3O4@SiO2@TiO2 microspheres are multifunctional. The thin film [10, 11], fiberglass loaded with titania [12], and TiO2 nanoparticles are at the outmost of the core-shell encapsulated titania within a zeolite framework [13]have microspheres, which can act as photocatalysts to decompose been developed to immobilize TiO2 in fixed beds. However, organic dyes in wastewater. The Fe3O4 core makes them very the photocatalytic activity of TiO2 is considerably reduced by easy to be separated and recycled from water with the help these immobilization techniques because the effective surface of an external magnet. Furthermore, the Fe3O4 microspheres area of photocatalysts is decreased. are stable at room temperature, and are unchanged after Recently, magnetic core-shell composites have gained a heat treatment under nitrogen atmosphere. Therefore, the great deal of attentions. The magnetic core has good mag- Fe3O4@SiO2@TiO2 microspheres may serve as an ideal netic responsibility and can be easily magnetized. Therefore, photocatalyst for the treatment of wastewater. 2 International Journal of Photoenergy

PEG, EG, FeCl3 Solvothermal TEOS TBT Calcination CH3COONa reaction Hydrolysis Hydrolysis

Fe3O4 Fe3O4@SiO2 Fe3O4@SiO2@TiO2

Scheme 1:ThesyntheticroutetoFe3O4@SiO2@TiO2 core-shell microspheres.

2. Experimental All the chemical reagents were of analytical grade and were used without further puriﬁcation. # ∗

2.1. Synthesis of Fe3O4 Microspheres. The procedure for the synthesis of Fe3O4 microspheres was carried out according to apreviousreport[21], summarized as follows: 1.35 g of iron ∗ chloride (III) hexahydrate, 1.0 g of polyethylene glycol (Mw (a.u.) Intensity # # = 4000), and 3.6 g of sodium acetate trihydrate were added ∗ # ∗ # ∗ #∗ into 40 mL ethylene glycol under constant magnetic stirring # to form a clear solution. Then the solution was transferred # ∗ ∗ into a Teﬂon-lined stainless steel autoclave with a capacity ◦ of 50 mL and heated at 180 C for 19 h. The products were 10 20 30 40 50 60 70 collected and fully rinsed with deionized water and absolute 2θ (deg) ethanol with the help of an external magnet, and then dried ∗ under vacuum at 60◦C for 2 h for further use. TiO2 21–1272 # Fe3O4 75–1609

Figure 1: XRD pattern of the Fe3O4@SiO2@TiO2 microspheres. 2.2. Synthesis of Fe3O4@SiO2 Microspheres. The synthesis of Fe3O4@SiO2 microspheres was carried out according to a previous report [22]. Typically, 0.2 g of as-prepared Fe3O4 microspheres was dispersed into a mixture of 20 mL ethanol solution. Before photocatalytic experiments, the solution was and 4 mL deionized water in a glass beaker, and then, under stirred in the dark to permit the adsorption/desorption equi- constant mechanical stirring, 1 mL of ammonia solution librium until the concentration of methyl orange solution (25%) and 0.8 mL of tetraethyl orthosilicate (TEOS) were was constant. After that, a 300 W column-like low-pressure consecutively added. The mixture was further stirred for 3 h. mercury lamp was placed over the solution with a distance of The resultant products were collected and washed, and then 20 cm, and the solution was irradiated with the lamp under ◦ dried under vacuum at 60 C for 2 h for further use. constant mechanical stirring and fan cooling. At intervals of 10 min, 3 mL of the solution was taken out and analyzed with a UV-vis absorption spectrometer. 2.3. Synthesis of Fe3O4@SiO2@TiO2 Microspheres. The synthesis of Fe3O4@SiO2@TiO2 microspheres was carried out according to following procedure. Firstly, 5 mL tetrabutylti- 2.5. Characterization. X-ray powder diffraction (XRD) pat- tanate was dissolved in 35 mL ethanol to form a clear terns were recorded using a Shimadzu XRD-6000 X-ray solution. Then, 0.1 g of Fe3O4@SiO2 microspheres as dis- diffractometer equipped with graphite monochromatized Cu persed in the above solution with the aid of ultrasonication Kα radiation (λ = 0.15406 nm). The field-emission scanning for 5 min. After that, 20 mL of a 1 : 5 (v/v) mixture of electron microscopy (FESEM) images were captured with a water and ethanol was added dropwise to the suspension of Hitachi S-4800 field-emission scanning electron microscope. Fe3O4@SiO2 microspheres with constant mechanical stirring Transmission electron microscopy (TEM) images were over a period of approximately 15 min. The suspension was taken on a JEOL-2010 high-resolution transmission electron further stirred for about 1 h. The products were collected and microscope. UV-vis absorption spectra were measured with washed with ethanol for three times, then calcined under a Shimadzu UV-3010 spectrometer. nitrogen atmosphere at 500◦C for 3 h. The whole synthesis procedure is shown in Scheme 1. 3. Results and Discussion 2.4. Photocatalytic Degradation of Methyl Orange. The pho- The phase and composition of the as-obtained samples were tocatalytic degradation was conducted as the following pro- examined by XRD. Figure 1 shows a typical XRD pattern of cedure: 40 mg of Fe3O4@SiO2@TiO2 microspheres were dis- the Fe3O4@SiO2@TiO2 microspheres. All of the diffraction persed into an aqueous solution of methyl orange (0.01 g L−1, peaks in this pattern can be classified into two sets. The peaks 50 mL). The pH value of the methyl orange solution that marked with “#” can be indexed to the orthorhombic was adjusted by hydrochloric acid and sodium hydroxide phase of Fe3O4 (JCPDS card no. 75–1609), while the other International Journal of Photoenergy 3

400 nm 400 nm

(a) (b)

400 nm 400 nm

Figure 2: (a)–(c) FESEM images of the Fe3O4 microspheres, Fe3O4@SiO2 microspheres, and Fe3O4@SiO2@TiO2 microspheres, (d) TEM image of the Fe3O4@SiO2@TiO2 microspheres.

∗ peaks that marked with “ ” can be indexed to the tetragonal Figure 3(b). This result shows that the Fe3O4@SiO2@TiO2 phase of TiO2 (JCPDS card no. 21–1272). No diffraction microspheres can be easily separated and collected from peaks corresponding to SiO2 were observed because the SiO2 water with the assistance of external magnetic force. is amorphous. The photocatalytic activities of the Fe3O4@SiO2@TiO2 The morphology and size of the as-prepared products microspheres were investigated by photodegradation of were characterized by FESEM and TEM. Figure 2(a) displays methyl orange in aqueous solution under the irradiation an FESEM image of the Fe3O4 microspheres, in which many of UV light. Figure 4 shows the time-dependent UV-vis nearly monodisperse microspheres with diameters of about absorption spectra of methyl orange aqueous solution −1 400 nm can be seen. The surfaces of the Fe3O4 microspheres (0.01 g L ,pH= 3) in the presence of Fe3O4@SiO2@TiO2 are rough. Figure 2(b) shows an FESEM image of the microspheres under exposure to UV light. The main Fe3O4@SiO2 microspheres. The surfaces of the Fe3O4@SiO2 absorption peak of methyl orange is centered at 506 nm. microspheres are smooth, which is distinctly different from It is clear that the intensity of the main absorption the initial Fe3O4 microspheres that are shown in Figure 2(a). peak of methyl orange gradually decreases with increas- Such a difference can be attributed to the coating of SiO2 ing time due to the concentration decreasing of methyl layer on Fe3O4. Figure 2(c) is an FESEM image of the orange. This result shows that methyl orange is gradually Fe3O4@SiO2@TiO2 microspheres, from which we can see degraded or changes into other structures under the irra- that many tiny TiO2 nanoparticles with diameters of 10– diation of UV light in the presence of Fe3O4@SiO2@TiO2 20 nm are adhered to the SiO2 layer. Figure 2(d) is a TEM microspheres. image of the Fe3O4@SiO2@TiO2 microspheres, which shows As a comparison, the photodegradation of methyl orange many nanoparticles adhered to the microspheres, consistent under different conditions was measured, as shown in with the FESEM obser vations. Figure 5. When the reaction was carried out in the dark, Figure 3(a) shows a digital photograph of the Fe3O4@ there is hardly any degradation of methyl orange occurred. SiO2@TiO2 microspheres that were dispersed in deionized When Fe3O4@SiO2@TiO2 microspheres were substituted by water with the assistance of ultrasonication. The micro- Fe3O4@SiO2 or Fe3O4 microspheres, the degradation rate spheres can be easily dispersed in water, and this suspension of methyl orange is dramatically decreased. This result can be kept constant for several minutes. When a magnet was shows that the TiO2 nanoparticles in the Fe3O4@SiO2@TiO2 placed aside, the black microspheres can be quickly collected microspheres play key role in the photodegradation of in several seconds, leaving a clear solution, as shown in methyl orange. 4 International Journal of Photoenergy

(a) (b)

Figure 3: (a) Digital photograph of Fe3O4@SiO2@TiO2 microspheres dispersed in water, (b) digital photograph of Fe3O4@SiO2@TiO2 microspheres collected by an external magnet.

1.4 100 1.2

1 80

0.8 60 0.6 40 0.4 Absorbance (a.u.) Absorbance Degradation (%) rate 0.2 20

0 0 −0.2 300 350 400 450 500 550 600 650 0 1020304050 Time (min) Wavelength (nm) Fe O @ SiO @TiO 0 min 30 min 3 4 2 2 Fe3O4@SiO2 10 min 40 min Fe3O4 Dark 20 min 50 min Figure 5: Degradation rate of methyl orange aqueous solution ﬀ Figure 4: Time-dependent absorption spectra of a methyl orange in the dark or in the presence of di erent photocatalyst under aqueous solution (0.01 g L−1,pH= 3) in the presence of exposure to UV light. Fe3O4@SiO2@TiO2 composites under exposure to UV light.

photocatalytic reactions are completed; therefore, they can The pH value of solution is an important parameter in be recycled for further use. Here we studied the photocat- photocatalytic degradation reactions that are taking place on alytic efficiency of the Fe3O4@SiO2@TiO2 microspheres for the surfaces of semiconductors. It dictates the surface charge recyclable usage. Figure 7 shows the photocatalytic efficiency properties of the photocatalyst and therefore the adsorption of the Fe3O4@SiO2@TiO2 microspheres after 6 cycles. In behavior of pollutants [23]. Here we examined the photo- each cycle, the methyl orange solution (0.01 g L−1,pH= 3) catalytic activities of the Fe3O4@SiO2@TiO2 microspheres at was irradiated under UV light for 45 min. The results show different pH conditions. The results are shown in Figure 6. that the photocatalytic activity of the Fe3O4@SiO2@TiO2 It is obvious that at acidic conditions the photocatalytic microspheres only decreased a little after 6 cycles of the efficiency of the Fe3O4@SiO2@TiO2 microspheres is much photocatalysis experiments. The reason may be that some higher, and the photocatalytic efficiency decreases with the of the TiO2 nanoparticles are broken away from the increase of pH value. At acidic conditions, the surface of Fe3O4@SiO2@TiO2 microspheres when they are stirred; as a TiO2 will be positively charged, so the negatively charged dye result, the photocatalytic efficiency of the Fe3O4@SiO2@TiO2 anion can be effectively adsorbed, and higher photocatalytic microspheres decreases. However, the degradation rate of efficiency is obtained. methyl orange can still reach 91% after 6 cycles of reuse. The Fe3O4@SiO2@TiO2 microspheres can be conve- So the Fe3O4@SiO2@TiO2 microspheres can be applied as niently separated and collected from the treated water after recyclable photocatalysts. International Journal of Photoenergy 5

Acknowledgment 100 Financial supports from the National Natural Science 80 Foundation of China (20701001, 21171006) are gratefully acknowledged. 60

40 References

Degradation (%) rate 20 [1] A. Fujishima and K. Honda, “Electrochemical photolysis of water at a semiconductor electrode,” Nature, vol. 238, no. 5358, pp. 37–38, 1972. 0 [2]M.R.Hoffmann, S. T. Martin, W. Choi, and D. W. Bahne- mann, “Environmental applications of semiconductor photo- 0 1020304050 catalysis,” Chemical Reviews, vol. 95, no. 1, pp. 69–96, 1995. Time (min) [3] A. Wold, “Photocatalytic properties of TiO2,”Chemistry of pH3 Materials, vol. 5, no. 3, pp. 280–283, 1993. pH12 [4]H.ParkandW.Choi,“Effects of TiO2 surface fluorination on pH7 photocatalytic reactions and photoelectrochemical behaviors,” Journal of Physical Chemistry B, vol. 108, no. 13, pp. 4086– Figure 6: Degradation rate of methyl orange aqueous solution 4093, 2004. at different pH values in the presence of Fe3O4@SiO2@TiO2 [5] V. A. Sakkas, A. Dimou, K. Pitarakis, G. Mantis, and T. composites under exposure to UV light. Albanis, “TiO2 photocatalyzed degradation of diazinon in an aqueous medium,” Environmental Chemistry Letters, vol. 3, no. 2, pp. 57–61, 2005. [6]K.W.Kim,S.H.You,S.S.Park,G.H.Kang,W.T.Bae,andD. W. Shin, “Effect of experimental conditions on photocatalytic 100 efficiency in TiO 2 powder slurry systems,” Journal of Ceramic Processing Research, vol. 9, no. 5, pp. 530–537, 2008. 80 [7] I. Arslan, I. A. Balcioglu, and D. W. Bahnemann, “Het- erogeneous photocatalytic treatment of simulated dyehouse effluents using novel TiO2-photocatalysts,” Applied Catalysis 60 B, vol. 26, no. 3, pp. 193–206, 2000. [8] N. Bouanimba, R. Zouaghi, N. Laid, and T. Sehili, “Factors 40 influencing the photocatalytic decolorization of Bromophenol blue in aqueous solution with different types of TiO2 as

Degradation (%) rate 20 photocatalysts,” Desalination, vol. 275, no. 1–3, pp. 224–230, 2011. [9] S. Yamazaki, S. Matsunaga, and K. Hori, “Photocatalytic 0 degradation of trichloroethylene in water using TiO2 pellets,” Water Research, vol. 35, no. 4, pp. 1022–1028, 2001. 0123456 [10] H. Kominami, H. Kumamoto, Y. Kera, and B. Ohtani, Experiment cycles “Immobilization of highly active titanium(IV) oxide particles ffi A novel strategy of preparation of transparent photocatalytic Figure 7: Relationship between the degradation e ciency of coatings,” Applied Catalysis B, vol. 30, no. 3-4, pp. 329–335, Fe3O4@SiO2@TiO2 photocatalyst and cyclic time. 2001. [11] I. M. Arabatzis, S. Antonaraki, T. Stergiopoulos et al., “Preparation, characterization and photocatalytic activity of 4. Conclusions nanocrystalline thin film TiO2 catalysts towards 3,5- dichlorophenol degradation,” Journal of Photochemistry and In summary, we have successfully prepared Fe3O4@SiO2@ Photobiology A, vol. 149, no. 1–3, pp. 237–245, 2002. TiO2 microspheres with well-defined core-shell structures [12] S. Horikoshi, N. Watanabe, H. Onishi, H. Hidaka, and N. Ser- pone, “Photodecomposition of a nonylphenol polyethoxylate through a wet-chemical method. The Fe3O4@SiO2@TiO2 microspheres are multifunctional. The magnetic core makes surfactant in a cylindrical photoreactor with TiO2 immobi- the microspheres very easy to be separated from solution lized fiberglass cloth,” Applied Catalysis B,vol.37,no.2,pp. 117–129, 2002. with the help of an external magnet, and the titania [13] M. Anpo, S. G. Zhang, H. Mishima, M. Matsuoka, and H. nanoparticles at the outside can act as photocatalyst to Yamashita, “Design of photocatalysts encapsulated within the decompose organic dyes in waste water. Photocatalytic zeolite framework and cavities for the decomposition of NO experiments show that the prepared Fe3O4@SiO2@TiO2 into N2 and O2 at normal temperature,” Catalysis Today, vol. microspheres have good photocatalytic activities under the 39, no. 3, pp. 159–168, 1997. illumination of UV light and can be applied as recyclable [14]D.Beydoun,R.Amal,G.K.C.Low,andS.McEvoy, photocatalysts. “Novel photocatalyst: titania-coated magnetite. Activity and 6 International Journal of Photoenergy

photodissolution,” Journal of Physical Chemistry B, vol. 104, no. 18, pp. 4387–4396, 2000. [15] D. Beydoun, R. Amal, J. Scott, G. Low, and S. McEvoy, “Studies on the mineralization and separation eﬃciencies of a magnetic photocatalyst,” Chemical Engineering & Technology, vol. 24, no. 7, pp. 745–748, 2001. [16] F. Chen, Y. Xie, J. Zhao, and G. Lu, “Photocatalytic degradation of dyes on a magnetically separated photocatalyst under visible and UV irradiation,” Chemosphere,vol.44,no.5,pp. 1159–1168, 2001. [17] D. Beydoun and R. Amal, “Implications of heat treatment on the properties of a magnetic iron oxide-titanium dioxide photocatalyst,” Materials Science and Engineering B, vol. 94, no. 1, pp. 71–81, 2002. [18] Y. Gao, B. Chen, H. Li, and Y. Ma, “Preparation and characterization of a magnetically separated photocatalyst and its catalytic properties,” Materials Chemistry and Physics, vol. 80, no. 1, pp. 348–355, 2003. [19] W. Wu, X. H. Xiao, S. F. Zhang, F. Ren, and C. Z. Jiang, “Facile method to synthesize magnetic iron oxides/TiO2 hybrid nanoparticles and their photodegradation application of methylene blue,” Nanoscale Research Letters,vol.6,no.1,p. 533, 2011. [20] C. F. Chang and C. Y. Man, “Titania-coated magnetic composites as photocatalysts for phthalate photodegradation,” Industrial & Engineering Chemistry Research, vol. 50, no. 20, pp. 11620–11627, 2011. [21] H. Deng, X. Li, Q. Peng, X. Wang, J. Chen, and Y. Li, “Monodisperse magnetic single-crystal ferrite microspheres,” Angewandte Chemie, vol. 44, no. 18, pp. 2782–2785, 2005. [22] W. Stober,¨ A. Fink, and E. Bohn, “Controlled growth of monodisperse silica spheres in the micron size range,” Journal of Colloid And Interface Science, vol. 26, no. 1, pp. 62–69, 1968. [23] V. K. Gupta, R. Jain, A. Mittal et al., “Photo-catalytic degradation of toxic dye amaranth on TiO2/UV in aqueous suspensions,” Materials Science and Engineering C, vol. 32, no. 1, pp. 12–17, 2012. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 857345, 7 pages doi:10.1155/2012/857345

Research Article Enhanced Hydrogen Production over C-Doped CdO Photocatalyst in Na2S/Na2SO3 Solution under Visible Light Irradiation

Quan Gu, Huaqiang Zhuang, Jinlin Long, Xiaohan An, Huan Lin, Huaxiang Lin, and Xuxu Wang

State Key Laboratory Breeding Base of Photocatalysis, Research Institute of Photocatalysis, Fuzhou University, Fuzhou 350002, China

Correspondence should be addressed to Jinlin Long, [email protected] and Xuxu Wang, [email protected]

Received 6 December 2011; Revised 12 February 2012; Accepted 13 February 2012

Academic Editor: Weifeng Yao

Copyright © 2012 Quan Gu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The C-doped CdO photocatalysts were simply prepared by high-temperature solid-state process. The as-prepared photocatalysts were characterized by X-ray powder diffraction (XRD), diffuse reflectance spectroscopy (UV-Vis DRS), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results demonstrated that the carbon was doped into CdO, resulting in the red-shift of the optical absorption of CdO. The photocatalytic behavior of CdO and C-doped CdO was evaluated under the visible light irradiation by using the photocatalytic hydrogen evolution as a model reaction. The C-doped CdO photocatalysts had higher photocatalytic activity over parent CdO under visible light irradiation. The results indicated that the H2 production was due to the existence of CdS and the enhancement of visible light photocatalytic activity of H2 production was originated from the doping of carbon into the CdO lattice. The probably reaction mechanism was also discussed and proposed.

1. Introduction C-, N-, and S-doped TiO2[8–10], N-doped ZrO2 [11], and C-, N-doped In2O3 [12, 13], and C-doped Nb2O5 [14], have Production of hydrogen, through which a green and poten- been reported to show enhanced photocatalytic activities for tial energy can be obtained, has been deemed to be an water splitting under visible-light irradiation compared with alternative to generation energy owing to its own merits such the undoped samples. Carbon has been considered to be one as renewability, environmental friendliness, and high energy of the most promising dopants, and C-doped materials were capacity. Therefore, numerous studies on photocatalytic H2 widely investigated in recent years. As the most qualified C- production and a series of semiconductor photocatalysts doped materials, C-doped TiO2 has showed much higher include metal oxides, nitride, titanates, and niobates [1– photoactivity for hydrogen production than pure TiO2 6] where high photocatalytic activities for H2 production [10, 15]. Jia et al. [16] prepared C-doped LaCoO3 by have been reported. Unfortunately, these photocatalysts can microorganism chelate method using Bacillus licheniformis only work under UV light irradiation. It is essential to R08 biomass as a chelating agent. The photocatalysis tests develop novel visible light responsive photocatalysts and showed that C-doped LaCoO3 had more obvious advantages modification techniques for tuning the photoactivity of UV in harvesting the visible-light than pure LaCoO3.Sunetal. active photocatalysts into the visible-light region. [12] reported that C-doped In2O3 showed highly enhanced Ions doping is one of the most effective ways for modi- photoelectrochemical activity under ultraviolet and visible fying the semiconductor photocatalysts to enhance the pho- light irradiation compared with undoped In2O3.Guoetal. toactivity of hydrogen production and to obtain the visible [17] synthesized C-doped Zn3(OH)2V2O7 nanorods with light responsive photocatalysts [7]. Compared with mental higher photoactivity than ZnO via a one-step method involv- doping, the nonmetal doping shifts the valence band edge ing a hydrothermal process in the presence of polyethylene upward and thus narrows the band gap of the semiconductor. glycol and ethylenediamine tetraacetic acid. Cadmium oxide Currently, various nonmetal-doped photocatalysts, such as as a narrow band-gap semiconductor, with specific optical 2 International Journal of Photoenergy and electrical properties, has been widely used specifically of surface adventitious carbon. The morphologies of the in the field of optoelectronic devices including solar cells, samples were investigated by scanning electron microscopy transparent electrodes, and sensors [18–21]. However, to the (SEM) (XL30ESEM, Philips). best of our knowledge, little work has been attempted to present the photocatalytic property of nonmetal-doped CdO 2.4. Photocatalytic Activity Measurements. The photocat- photocatalysts due to special band structure [22]. alytic activities of the samples for hydrogen evolution under In the present work, the bulk CdO and C-doped CdO visible light irradiation were studied. The photocatalytic photocatalysts were prepared by the high-temperature solid H2 evolution reaction was performed in a closed gas- state process, and their photocatalytic activities for hydro- recirculation system equipped with a side-irradiation Pyrex gen evolution from sodium sulfide/sodium sulfite aqueous reaction vessel. 0.4 g of the catalyst was suspended in 400 mL solution were then compared under visible light irradiation. of water containing 3.12 g sodium sulfide and 1.0 g sodium The results indicated that the photocatalytic activity of sulfite anhydrous by a magnetic stirrer in Pyrex reaction the C-doped CdO was higher than that of pure CdO. In vessel. The pravious suspension was evacuated 50 min to addition, these as-prepared C-doped CdO photocatalysts remove air prior to irradiation under a 300 w xenon lamp were characterized by X-ray powder diffraction (XRD), ff ff with a cuto filter (λ>420 nm). The evolved hydrogen di use reflectance spectroscopy (UV-Vis DRS), infrared gas was circulated with a gas pump and quantified by gas spectroscopy (IR), scanning electron microscopy (SEM), and chromatography (Shimadzu GC-8A, TCD, Ar carrier). X-ray photoelectron spectroscopy (XPS). The enhancement of photocatalytic activity for C-doped CdO was discussed, and the probably reaction mechanism was also discussed and 3. Results and Discussion proposed. 3.1. XRD Measurement. Figure 1(a) shows the XRD patterns of all samples. For the pure CdO, the peaks associated with 2. Experimental planes (111), (200), (220), (311), and (222) are observed, and all XRD peaks can be indexed to CdO (JCPDS 65-2908). 2.1. Reagents and Materials. Cadmium sulfate, 8/3-hydrate When the mixture powders are annealed at 700◦Cfor10h, (CdSO4·8/3H2O, AR grade), cadmium oxide (AR grade), the peaks due to CdO and Cd3O2SO4 are observed as shown and sodium sulfide (AR grade) were purchased from Aladdin in Figure 1(a) curve b. The XRD results of samples obtained Company. Graphite powder (SR grade) and a sodium at above 700◦C indicated that the C-doped CdO maintained sulfite anhydrous (AR grade) were obtained from Sinopharm a cubic structure of CdO. Figure 1(b) is an enlargement of chemical reagent Co. Ltd. All of the reagents were used the (111) plane of all samples. As we can see, compared with without any further purification. parent CdO, the peak position of the (111) plane of C-doped CdO is shifted slightly toward lower 2θ value, demonstrating 2.2. Preparation of Samples. The samples were prepared the distortion of the crystal lattice of doped CdO by the by the high-temperature solid state process. For a typical carbon dopant. Interestingly, the intensity ratio of (111) and experiment, 3.0 g of CdSO4·8/3H2Owasaddedin50mL (200) plane is smaller than that of pure CdO and decreased of H2O with constant stirring to obtain the cadmium with the increase of preparation temperature. Generally, the sulfate solution, and then 1 g of graphite powder was added preferred orientation could destroy the randomness of plane under vigorously magnetic stirring at room temperature. orientation and the normal relative intensity of plane. So, it Stirring of the pravious suspension was continued for 1 seems that the samples synthesized at high temperature grow hour at the same temperature until the graphite powder was preferentially along (111) plane orientation. dispersed uniformly in the cadmium sulfate solution. The ◦ well-distributed suspension was heated at 120 Conoilbath 3.2. UV-Vis DRS Measurement. Figure 2 shows the diffuse to evaporate the water, thus the black solid was obtained. ffl reflectance spectra of the prepared samples. It is clearly The solid was grinded and then calcined on mu efurnace observed that a strong absorption band exists in the range at desired temperature for 10 h. of 200–600 nm for parent CdO and an optical absorption edge of pure CdO is at 620 nm. The parent CdO exhibits a 2.3. Characterization of Samples. The X-ray powder diffrac- weak adsorption at the range of 620–800 nm, which may be tion (XRD) measurements were performed on a Bruker D8 due to the adsorbed C species as indicated in XPS results. Advance X-ray diffractometer using Cu Kα1 radiation (λ = Therefore, the corresponding value of the band-gap energy 1.5406 A˚ ). The accelerating voltage and the applied current is about 2.2 eV, which is smaller about 0.1 eV than that of were 40 kV and 40 mA, respectively. The UV-Vis diffuse the reference value (2.3 eV) of the band-gap energy [20]. It reflectance spectra were recorded on a Varian Cary 500 Scan may be due to the effect of CdO crystallites diameter on UV-Vis-NIR spectrometer with BaSO4 as a reference sample. optical absorption and difference of synthesis procedures X-ray photoelectron spectroscopy analysis was conducted [19, 20, 23]. Compared with parent CdO, the C-doped CdO on an ESCALAB 250 photoelection spectroscopy (Thermo adsorbed the light at the range of 200–800 nm. The broad Fisher Scientific) at 3.0 × 10−10 mbar using Al Kα X-ray absorption band of C-doped CdO is probably associated with beam (15 kv 200 W 500 μmpassenergy= 20 eV). All binding the formation of impurity states in the band gap by the energies were referenced to the C 1s peak at 284.6 eV partial substitution of O with C in the crystal lattice of CdO. International Journal of Photoenergy 3

111 200 e 220 311 222 d Intensity (a.u.) Intensity c (a.u.) Intensity b

10 20 30 40 50 60 70 80 32.4 32.6 32.8 33 33.2 33.4 33.6 2θ (deg) 2θ (deg)

◦ CdO 900 C CdO 900◦C ◦ 700◦C 1000 C 700◦C 1000◦C 800◦C 800◦C (a) (b)

Figure 1: XRD patterns of samples.

10 When samples are calcined at high temperature, crystals grow along a plane into large particles and the as-prepared samples have good crystallinity, which can also be observed 8 in the XRD patterns. The infrared spectra changes of CdO created by C 6 doping are presented in Figure 3(b). The IR spectrum

) of the CdO shows seven bands centered at 3433, 1634, R ( −1 F 1401, 983, 859, 684 and, 591 cm .Thebroadbandnear 4 3433 cm−1 and the weak band at 1634 cm−1 are assigned to OH-stretching vibrations and deformative vibration of 2 adsorbed H2O, respectively. According to [19, 24], CdO phase is characterized by an intense and very broad IR band with poor resolved shoulders at 1400, 1000, and 580 cm−1. 0 Therefore, we believe that the bands at 1401, 983, 859, 684, 200 300 400 500 600 700 800 and 591 cm−1 are assigned to the CdO phase. After C-doping, Wavelength (nm) a new band at 1109 cm−1 is formed in IR spectrum of samples CdO 900◦C compared with the IR spectrum of pure CdO, which is ◦ 800◦C 1000 C thought to be related to the C-Cd stretching. Figure 2: UV-Vis diffuse reflectance spectra of pure CdO and C- ff doped CdO samples prepared under di erent temperatures. 3.4. XPS Analysis. To investigate the chemical state of the possible dopant incorporated into CdO and the effect of dopant on chemical states of other elements, pure CdO and C-doped CdO samples were characterized by XPS, as shown Therefore, the results indicated that C-doping into the CdO in Figure 4. lattice leads to the red-shift of the optical absorption for the Figure 4(a) presents the C1s XPS spectra of CdO and C- C-doped CdO. doped CdO. The C1s XPS spectrum of pure CdO shows two peaks at 284.6 and 289.4 eV and a very small peak at 287.0 eV 3.3. SEM and IR Studies. The scanning electron microscopy (Negligible). These three C 1s peaks are also observed in (SEM) image of C-doped CdO sample prepared by the high C 1s XPS spectra of C-CdO. The peak with a binding temperature calcinations is shown in Figure 3(a). It shows energy of 284.6 eV is assigned to surface adventitious carbon. that the as-prepared C-doped CdO sample form very large The peak at 289.4 eV can be assigned to the chemisorbed massive particles and the average size of block is about carbon species on the surface of CdO [25]. The new peak 10 μm, which is caused by high-temperature calcinations. with the binding energy at 282.5 eV is observed in the C 4 International Journal of Photoenergy

these results, it can be concluded that C is doped into the CdO lattice.

3.5. Visible Light Photocatalytic Activity for H2 Production. The photocatalytic activities for H2 evolution of CdO and C-doped CdO synthesized at different temperatures (800◦C, 900◦C, and 1000◦C) under the visible light irradiation are shown in Figure 5. It can be seen that both the CdO and C-doped CdO could photoinduce production of H2 and the amounts of H2 evolved for all samples increase with the irradiation time. The turnover numbers of CdO and C-doped CdO prepared at 800◦C, 900◦C, and 1000◦C, S4800 5 kV 8.4 mm x5 k SE(M) 10 μm which was calculated from the molar amount of H2 and (a) the molar amount of photocatalysts [27], are 1.4, 3.1, 3.6, 0.65 684 and 7.5, respectively. These values suggest that the reaction 591 proceeded photocatalytically. The pure CdO displays a 3433 859 1401 low activity for H evolution from the aqueous solutions 0.6 983 2 containing Na2S/Na2SO3 as sacrificial reagents. The C- 1109 doped CdO samples prepared at different temperatures have 0.55 1634 higher photocatalytic activity than pure CdO under the visible light irradiation by using Na2S/Na2SO3 as sacrificial 0.5 reagents, and the photocatalytic H2 evolution activity for ◦ Intensity (a.u.) Intensity these samples increased in the order of CdO < 800 C < ◦ ◦ 0.45 900 C < 1000 C. The results demonstrated that the C- doped CdO could significantly improve the photocatalytic H production activity under visible light irradiation by 0.4 2 carbon doping. In addition, it can be seen that the amount 4000 3500 3000 2500 2000 1500 1000 500 of produced H2 over the C-CdO-1000 increases slowly with Wavenumbers (cm−1) irradiation time in 5 h; then the H2 production rate increases after 5 h irradiation. CdO 900◦C 800◦C 1000◦C In general, to facilitate the reduction of H2O by photoexcited electrons, the match of conduction band of (b) semiconductor photocatalyst with the reduction potential of + Figure 3: (a) SEM image of the C-doped CdO samples synthesized H /H2 (0 V versus NHE) is very important. In other words, at 1000◦C. (b) The infrared spectra changes of CdO induced by C the bottom level of the conduction band of photocatalyst doping. has to be more negative than the reduction potential of + H /H2 (0 V versus NHE). However, the energy level of conduction band edge for cadmium oxide is −4.61 eV versus Absolute Vacuum Scale (AVS) [22]; according to E(NHE) = 1s XPS spectrum of C-doped CdO, which suggests that a E(AVS) − 4.50, the ENHE of cadmium oxide is located at new chemical state of carbon species is formed during the 0.11 eV, which is positive than the reduction potential of + doping process. Jia et al. [16] assigned the peak at 282.8 eV H /H2. Hence, theoretically, the photogenerated electrons + to Co–C in C-doped LaCoO3. The C1s peak around 282 eV in the conduction band of CdO cannot reduce the H in was ascribed to the existence of O–Ti–C bonds by Wang the aqueous solution in the absence of sacrificial reagents. [25]. Jang et al. [26] demonstrated that the broad peak Interestingly, the addition of sodium sulfide and cadmium around 280–284 eV could be assigned to carbon interacting sulfate into solution leads to the production of hydrogen with Zn through Zn–C bond formation. The Cd 3d3/2 XPS from water under the visible light irradiation. It is well spectra of parent CdO and C-doped CdO are shown in known that electronic structures of cadmium sulfide match Figure 4(b). The Cd 3d3/2 peak at 411.8 of parent CdO can well with the redox potential of water into hydrogen. So, be assigned to lattice Cd atoms and the peak at 410.2 eV on the basis of experimental results, we hypothesized that may be originated from the interaction between CdO and cadmium sulfide maybe formed on surface of CdO during chemisorbed carbon species, which is in parallel with the the visible light irradiation and photogenerated electrons in + result of C1s XPS spectra. The Cd 3d3/2 XPS spectra of the conduction band of CdS could reduce the H into H2. C-doped CdO show two peaks at 410.2 and 411.8 eV and This is also the reason why the photocatalytic reactivity of C- the intensity ratio of 410.2 and 411.8 eV increased with the CdO-1000 increases after 5 h irradiation. After the formation increase of preparation temperature. We consider that the of CdS on the surface of C-CdO-1000, the interactions peak assigned to C–Cd species is overlapped with the peak between CdS and C-doped CdO promote the activity. To originated from chemisorbed carbon species. According to testify the formation of CdS on the surface of CdO during the International Journal of Photoenergy 5

3 ×10 ×104 65 50 60 284.6 C 1s 405.1 Cd 3d 45 55 289.4 411.8 282.5 287 40 50 ◦ 800 C 35 45 410.2 403.5 Bare CdO 40 30 900◦C 35 25 30 ◦ 20 800 C Intensity (a.u.) Intensity Intensity (a.u.) Intensity 25 ◦ 1000 C 15 20 900◦C 15 10

10 5 Bare CdO 1000◦C 5 0

292 290 288 286 284 282 280 416 414 412 410 408 406 404 402 400 Binding energy (eV) Binding energy (eV) (a) (b)

Figure 4: XPS spectra of the pure CdO and C-doped CdO samples prepared under diﬀerent temperatures.

CdS phase is observed after irradiation in XRD pattern, as 0.012 shown in Figure 6(b). It indicates that the CdS formed on the surface of CdO is amorphous and/or its content is lower 0.01 than the detect limit of XRD apparatus. In our photocatalytic system, the CdO and the CdS formed by photocorrosion are activated by visible light 0.008 simultaneously. The photogenerated h+ would transferred from the valence band of CdO to that of CdS, while the photogenerated e− on the conduction band of CdS would be evolved (mmol) evolved 0.006 2 reduce the H2OtoH2 and/or inject to the conduction band of CdO. Similar to case of other nonmetal-doped catalysts, C 0.004 doping in CdO causes a valence band upward shift to a higher energy level, which narrows the band gap. It is facilitating Amount ofAmount H 0.002 to the electron-hole separation. Therefore, the photoactivity of C-doped CdO for hydrogen evolution in Na2S/Na2SO3 solution is higher than that of pure CdO sample under the 0 same reaction conditions. It can be subsequently concluded that the H2 production is due to the existence of CdS and 012345678910 the enhancement of visible light photocatalytic activity of Time (h) H2 production is originated from the C doping into CdO CdO C-Cdo-900 lattice. The proposed reaction mechanism for accelerating C-CdO-800 C-Cdo-1000 the photocatalytic H2 production process over the C-CdO Figure 5: Photocatalytic hydrogen amount as a function of photocatalysts is demonstrated in Scheme 1. irradiation time. 4. Conclusions C-doped CdO photocatalysts with visible light response photoreaction, the S 2p XPS spectrum of CdO after reaction are prepared by high-temperature solid state process. The are recorded and shown in Figure 6(a). Two peaks at 161.5 experiment characterization results such as XRD, UV-Vis and 162.6 eV are observed, which are consistent with the DRS, IR, and XPS indicate that C is doped into the CdO reported values of CdS in the literatures [28]. It is conﬁrmed lattice. The photocatalytic hydrogen production activities of that CdS were formed on the surface of CdO. However, no as-prepared photocatalysts are studied under the visible light 6 International Journal of Photoenergy

×103 11 600 161.5 550 10 S 2p 500 450 9 162.6 400 8 350 300 After 7 After 250 Intensity (a.u.) Intensity

Intensity (a.u.) Intensity 200 6 Before 150 5 100 50 Before 4 0 166 164 162 160 158 10 20 30 40 50 60 70 80 Binding energy (eV) 2θ (deg) (a) (b)

Figure 6: (a) XPS spectra of the C-doped CdO before and after photocatalytic H2 evolution reaction. (b) XRD patterns of C-doped CdO before and after photocatalytic H2 evolution reaction.

Conduction band e− e− e− e− − H+ e + 0 H /H2 Conduction− − − band− 0)/V e e e e A = h CdS hA 1.23 O2/H2O C-doped CdO h+ h+ h+ Valence band

(versus NHE, pH NHE, (versus +

V h New valence band 2− 2− 2− − + = 2− + S , SO3 SO3 + 2OH +2h SO4 +2H h+ h+ h+ h+ 2− + = 2− Valence band S +2h S2 2− 2− = 2− 2− S2 +SO3 S2O3 +S S2−, S, SO2− 2 4 2− 2− + = 2− SO3 ++2hS S2O3

Scheme 1: Proposed photocatalytic hydrogen evolution process over the C-doped CdO. irradiation. The C-doped CdO photocatalysts exhibit higher Natural Science Foundation of Fujian Province of China photoactivity for hydrogen production over undoped CdO (2010J05024), and the Science and Technology Project of under visible light irradiation by using the Na2S/Na2SO3 as Education Office of Fujian Province of China (JK2010002). sacrificial reagents, which indicated that the formation of CdS is an important factor for the hydrogen production and C doping into CdO lattice results in the enhancement of References photocatalytic hydrogen production activity of CdO. [1] Y. Wang, Z. Zhang, Y. Zhu et al., “Nanostructured VO2 photocatalysts for hydrogen production,” ACS Nano, vol. 2, no. 7, pp. 1492–1496, 2008. Acknowledgments [2]G.N.SchrauzerandT.D.Guth,“Photolysisofwaterand photoreduction of nitrogen on titanium dioxide,” Journal of This work was financially supported by the NSFC (Grants the American Chemical Society, vol. 99, no. 22, pp. 7189–7193, Nos. 21003021, 20873022, and 21173044), the National 1977. High Tech R&D Program of China (863 Program, [3] X. Chen, T. Yu, X. Fan et al., “Enhanced activity of mesoporous 2008AA06Z326), Program for Changjiang Scholars and Nb2O5 for photocatalytic hydrogen production,” Applied Innovative Research Team in University (PCSIRT0818), the Surface Science, vol. 253, no. 20, pp. 8500–8506, 2007. International Journal of Photoenergy 7

[4] K. Maeda, N. Saito, D. Lu, Y. Inoue, and K. Domen, “Photo- [20] O. Vigil, F. Cruz, A. Morales-Acevedo, G. Contreras-Puente, catalytic properties of RuO2-loaded β-Ge3N4 for overall water L. Vaillant, and G. Santana, “Structural and optical properties splitting,” The Journal of Physical Chemistry C, vol. 111, no. 12, of annealed CdO thin films prepared by spray pyrolysis,” pp. 4749–4755, 2007. Materials Chemistry and Physics, vol. 68, no. 1–3, pp. 249–252, [5]J.S.Jang,S.H.Choi,D.H.Kim,J.W.Jang,K.S.Lee,andJ. 2001. S. Lee, “Enhanced photocatalytic hydrogen production from [21] C. H. Bhosale, A. V. Kambale, A. V. Kokate, and K. Y. Rajpure, water-methanol solution by nickel intercalated into titanate “Structural, optical and electrical properties of chemically nanotube,” The Journal of Physical Chemistry C, vol. 113, no. sprayed CdO thin films,” Materials Science and Engineering B, 20, pp. 8990–8996, 2009. vol. 122, no. 1, pp. 67–71, 2005. [6] K. Domen, A. Kudo, M. Shibata, A. Tanaka, K. I. Maruya, and [22] X. Yong and M. A. A. Schoonen, “The absolute energy T. Onishi, “Novel photocatalysts, ion-exchanged K4Nb6O17, positions of conduction and valence bands of selected semi- with a layer structure,” Journal of the Chemical Society, no. 23, conducting minerals,” American Mineralogist,vol.85,no.3-4, pp. 1706–1707, 1986. pp. 543–556, 2000. [7] X. Chen, S. Shen, L. Guo, and S. S. Mao, “Semiconductor- [23] A. Gulino, G. Compagnini, and A. A. Scalisi, “Large third- based photocatalytic hydrogen generation,” Chemical Reviews, order nonlinear optical properties of cadmium oxide thin vol. 110, no. 11, pp. 6503–6570, 2010. films,” Chemistry of Materials, vol. 15, no. 17, pp. 3332–3336, [8]L.Randeniya,A.Murphy,andI.Plumb,“AstudyofS- 2003. [24] A. Askarinejad and A. Morsali, “Syntheses and characteriza- doped TiO2 for photoelectrochemical hydrogen generation from water,” Journal of Materials Science,vol.43,no.4,pp. tion of CdCO3 and CdO nanoparticles by using a sonochem- 1389–1399, 2008. ical method,” Materials Letters, vol. 62, no. 3, pp. 478–482, [9] J. Yuan, M. Chen, J. Shi, and W. Shangguan, “Preparations and 2008. [25] H. Wang, Z. Wu, and Y. Liu, “A simple two-step template photocatalytic hydrogen evolution of N-doped TiO2 from urea and titanium tetrachloride,” International Journal of Hydrogen approach for preparing carbon-doped mesoporous TiO2 Energy, vol. 31, no. 10, pp. 1326–1331, 2006. hollow microspheres,” The Journal of Physical Chemistry C, vol. [10] S. U. M. Khan, M. Al-Shahry, and W. B. Ingler, “Efficient 113, no. 30, pp. 13317–13324, 2009. photochemical water splitting by a chemically modified n- [26]J.S.Jang,C.-J.Yu,S.H.Choi,S.M.Ji,E.S.Kim,andJ.S.Lee, TiO ,” Science, vol. 297, no. 5590, pp. 2243–2245, 2002. “Topotactic synthesis of mesoporous ZnS and ZnO nanoplates 2 and their photocatalytic activity,” Journal of Catalysis, vol. 254, [11] T. Mishima, M. Matsuda, and M. Miyake, “Visible-light no. 1, pp. 144–155, 2008. photocatalytic properties and electronic structure of Zr- [27]A.Kudo,I.Tsuji,andH.Kato,“AgInZnS solid solution based oxynitride, Zr ON , derived from nitridation of ZrO ,” 7 9 2 2 2 photocatalyst for H evolution from aqueous solutions under Applied Catalysis A, vol. 324, no. 1-2, pp. 77–82, 2007. 2 visible light irradiation,” Chemical Communications, no. 17, [12]Y.Sun,C.J.Murphy,K.R.Reyes-Gil,E.A.Reyes-Garcia,J.P. pp. 1958–1959, 2002. Lilly, and D. Raftery, “Carbon-doped In O films for photo- 2 3 [28] S. Rengaraj, S. Venkataraj, S. H. Jee et al., “Cauliflower-like electrochemical hydrogen production,” International Journal CdS microspheres composed of nanocrystals and their phys- of Hydrogen Energy, vol. 33, no. 21, pp. 5967–5974, 2008. icochemical properties,” Langmuir, vol. 27, no. 1, pp. 352–358, [13] K. R. Reyes-Gil, E. A. Reyes-Garc´ıa, and D. Raftery, “Nitrogen- 2011. doped In2O3 thin film electrodes for photocatalytic water splitting,” The Journal of Physical Chemistry C, vol. 111, no. 39, pp. 14579–14588, 2007. [14] S. Ge, H. Jia, H. Zhao, Z. Zheng, and L. Zhang, “First observation of visible light photocatalytic activity of carbon modified Nb2O5 nanostructures,” Journal of Materials Chemistry, vol. 20, no. 15, pp. 3052–3058, 2010. [15] C. Xu, Y. A. Shaban, W. B. Ingler Jr, and S. U. M. Khan, “Nan- otube enhanced photoresponse of carbon modified (CM)-n- TiO2 for efficient water splitting,” Solar Energy Materials and Solar Cells, vol. 91, no. 10, pp. 938–943, 2007. [16] L. Jia, J. Li, W. Fang, H. Song, Q. Li, and Y. Tang, “Visible-light- induced photocatalyst based on C-doped LaCoO3 synthesized by novel microorganism chelate method,” Catalysis Commu- nications, vol. 10, no. 8, pp. 1230–1234, 2009. [17] H. Guo, J. Chen, W. Weng, and S. Li, “Hydrothermal synthesis of C-doped Zn3(OH)2V2O7 nanorods and their photocatalytic properties under visible light illumination,” Applied Surface Science, vol. 257, no. 9, pp. 3920–3923, 2011. [18] Q. Chang, C. Chang, X. Zhang et al., “Enhanced optical limiting properties in suspensions of CdO nanowires,” Optics Communications, vol. 274, no. 1, pp. 201–205, 2007. [19] N. C. S. Selvam, R. T. Kumar, K. Yogeenth, L. J. Kennedy, G. Sekaran, and J. J. Vijaya, “Simple and rapid synthesis of Cadmium Oxide (CdO) nanospheres by a microwave-assisted combustion method,” Powder Technology, vol. 211, no. 2-3, pp. 250–255, 2011. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 907290, 5 pages doi:10.1155/2012/907290

Research Article Synthesis of Hollow CdS-TiO2 Microspheres with Enhanced Visible-Light Photocatalytic Activity

Yuning Huo, Jia Zhang, Xiaofang Chen, and Hexing Li

The Education Ministry Key Lab of Resource Chemistry, Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Normal University, Shanghai 200234, China

Correspondence should be addressed to Yuning Huo, [email protected]

Received 20 January 2012; Accepted 20 February 2012

Academic Editor: Weifeng Yao

Copyright © 2012 Yuning Huo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

CdS-TiO2 composite photocatalyst in the shape of hollow microsphere was successfully synthesized via the hard-template preparation with polystyrene microspheres followed by ion-exchange approach. The hollow CdS-TiO2 microspheres significantly extended the light adsorption into visible light region, comparing to TiO2 microspheres. It led to much higher photocatalytic activities of hollow CdS-TiO2 microspheres than that of TiO2 during the photodegradation of rhodamine B under visible light irradiations. Furthermore, the well-remained hollow structure could achieve light multireflection within the interior cavities and the separation of photo-induced electrons and holes is efficient in CdS-TiO2, which were facilitated to improving the photoactivity.

1. Introduction [10]. Various methods have been developed for preparing hollow microspheres, including spray-drying method, pho- Photocatalysis has attracted considerable scientific interest tochemical preparation, and hard-template method [11–13]. owing to its potential applications in environmental purifica- Among these techniques, hard-template process is a general tion and H2 energy production [1]. TiO2 is the most promis- and effective approach for preparing hollow microspheres via ing photocatalyst owing to its various advantages including using polymer, carbon, and silica microspheres as templates. nontoxicity, low cost, high activity, and strong stability. As we know, the polymeric polystyrene (PS) microspheres However, its practical application has been restricted by the are easy to be removed by calcination. Additionally, it is more ffi wide band gap (3.2 eV) and low quantum e ciency [2]. Up convenient to achieve the uniform diameter of polystyrene to now, various methods have been developed to extend the microspheres than other microspheres. light absorbance of TiO2 into visible light region and thus Herein, we report the preparation of hollow CdS-TiO ffi 2 improve the photocatalytic e ciency, such as doping with microspheres via the hard-template process with PS micro- other impurities [3], depositing noble metals on TiO2 [4], spheres followed by the ion-exchange approach. The ion- and combining TiO2 with other semiconductors [5]. Cad- exchange is a convenient and flexible method to convert CdO mium sulfide (CdS) with band gap of 2.4 eV has been widely into CdS [14, 15]. The introduction of CdS can effectively used to sensitize TiO2 to realize visible-light absorbance and extend the photoresponse of TiO to visible-light region ffi 2 improve the charge separation e ciency [6]. Meanwhile, it and improve the photocatalytic activity of decomposition of is well known that the design of well-defined structure is rhodamine B (RhB) in aqueous solution. a promising way to achieve high photocatalytic activity via increasing specific surface area and promoting light harvest- 2. Experimental Details ing [2]. Considerable efforts have been devoted to obtain the unique morphology, such as wires, tubes, core-shell micro- 2.1. Catalyst Preparation. All the chemicals were of analytic spheres, and hollow microspheres [7–10]. Hollow micro- purity and used without further purification. The synthetic spheres have attracted an immense interest for their high approach included two steps. (1) hollow CdO-TiO2 micro- surface area, low density, and large light-harvesting efficiency spheres prepared using polystyrene microspheres as hard 2 International Journal of Photoenergy template. Monodisperse PS microspheres were synthesized by emulsifier-free batch emulsion polymerization according to the previous report [16]. 0.27 g Cd(Ac)2 ·2H2O was dissolved in the mixture of 80.0 mL 2-propanol and 5.0 mL ethanol. Then a 5.0 mL prepared PS microspheres latex was dropped into the solution followed by stirring at 55 C for 30 min. After the solution was cooled down to room tem- CdS-TiO perature, 4.0 mL 0.2 M aqueous NaOH solution was added. 2

Then, the mixture was cooled down to room temperature (a.u.) Intensity after reacting at 55 C for 20 min followed by the centrifugation and wash in ethanol for three times. 0.10 g obtained TiO2 powder was dispersed into the solution of 1.3 mL water and 5.0 mL ethanol. Another solution was mixed with 76.0 μL Ti(OBu)4 and 20.0 mL ethanol via being refluxed at 75 C 20 30 40 50 60 70 80 for 2 h. Subsequently, two solutions were mixed together 2θ (deg) and reacted for 20 h. The obtained composites were washed in ethanol for three times and dried at 40 C. In order to Anatase TiO2 remove the polymer template, the obtained powder was CdS heated at 550 C for 4 h. (2) Converting CdO to CdS at room temperature. 0.3 g above-prepared powder was dispersed Figure 1: XRD patterns of CdS-TiO2 and TiO2 samples. in 50.0 mL 0.2 M Na2S aqueous solution with stirring for 2 h at room temperature followed by filtration and wash. The collected products were designated as CdS-TiO2.For Here, C0 was the initial concentration of RhB and C was the comparison, TiO2 sample was prepared according to the concentration of RhB remained in the solution. Meanwhile, same process without Cd(Ac)2 ·2H2O. experimental results also confirmed that only less than 6.0% RhB was removed in the absence of catalyst or light irradia- 2.2. Characterization. The sample composition was deter- tion and thus could be neglected in comparison with the mined by inductively coupled plasma (ICP, Varian VISTA- photocatalysis process. MPX). The surface morphologies were observed using field emission scanning electron microscopy (FESEM, HITA-CHI 3. Results and Discussion S4800). X-ray diffraction patterns were recorded to confirm the composition of crystallite using a Rigaku Dmax-3C 3.1. Characterization of CdS/TiO2 Composites. The ICP result = ˚ ff with Cu Kα radiation (λ 1.5418 A). The UV-vis di use demonstrated that the molar ratio of CdS to TiO2 was 12.3% reflectance spectra (DRS) of the samples over a range of 200– in CdS-TiO2 composites. The crystal phase of calcined CdS- 800 nm were collected on a UV-2450 instrument. Photolu- TiO2 and TiO2 samples were characterized by wide-angle minescence measurements were carried out on Varian Cary- powder XRD method as shown in Figure 1. The principal Eclipse 500. peaks of anatase TiO2 were found (JCPDS 21–1272), indicative of the well crystallization. The diffraction peak of CdS 2.3. Photocatalytic Test. Liquid-phase photocatalytic degra- could be corresponded to the hexagonal CdS crystal (JCPDF dation of RhB was carried out at 30 Cinan80mLself- 41–1049). The peak of CdS was weak possibly due to the designed quartz photochemical reactor containing 50.0 mg package of exterior TiO . No other crystalline impurities catalyst and 50.0 mL 10.0 mg/L RhB aqueous solution. The 2 including CdO crystal could be detected, indicative of the suspension was magnetically stirred continually. After adso- complete ion exchange converting from CdO to CdS. Mean- rption-desorption equilibrium for 1 h, the photocatalytic while, no significant shift of principal peaks of either the degradation of RhB was initiated by irradiating the reaction CdS or the TiO was observed, confirming that the CdS was mixture with a one 500 W xenon lamp (CHF-XM500, light 2 present in a separated phase rather than in the TiO lattice. intensity = 600 mW/cm2), a 420 nm glass filter (JB-420) was 2 FESEM images in Figure 2 displayed the formation pro- used to remove the radiation in the UV region. The light cess of CdS-TiO composites in the shape of hollow micro- source was located at 18 cm away from the reaction solut- 2 sphere via the hard template of PS microspheres. All the ion. Each experiment was lasted for 3 h and the concen- microspheres covered with precursor revealed uniform size tration was measured at a time intervals of 30 min. the and smooth surface, indicating that both Cd and Ti precur- RhB concentration was determined by measuring absor- sors were successfully coated on PS microspheres. Further- bance using a UV-vis spectrophotometer (UV-7502PC) at its more, the size of microspheres after coating with Ti precursor characteristic wavelength of 553 nm. Preliminary tests dem- was larger than that of microspheres only coated with Cd onstrated a good linear relationship between the light absor- precursor with the increased diameter from about 300 nm bance and RhB concentration. The degradation yield can be to 340 nm. After calcination and ion exchange from CdO calculated using the following equation: to CdS, CdS-TiO2 sample showed the slightly decreased size C − C with the diameter of about 300 nm. The hollow structure was Degradation yield = 0 × 100%. (1) C0 formed with the shell thickness of about 30 nm. It is clear that International Journal of Photoenergy 3

Figure 2: FESEM micrographs of (a) PS microspheres covered with Cd precursor and (b) further covered with Ti precursor before calcinations and (c) CdS-TiO2 sample after calcinations.

λex = 360 nm 720 nm

TiO2

CdS-TiO CdS-TiO2 2 Intensity (a.u.) Intensity Absorbance (a.u.) Absorbance

TiO2

680 700 720 740 760 200 300 400 500 600 700 800 Wavelength (nm) Wavelength (nm) Figure 4: PL spectra of CdS-TiO2 and TiO2 samples with the excita- Figure 3: UV-vis DRS spectra of CdS-TiO2 Band TiO2 samples. tion wavelength of 360 nm.

e− the hollow structure was maintained well after calcination without any destroying and collapse. The decreased diameter CB of hollow CdS-TiO microspheres was attributed to the CB 2 CdS shrinkage during the removal of PS microsphere templates. It hA was suggested that hollow CdS-TiO2 microspheres could be 2.4 eV successfully fabricated by the polymer hard-template method TiO 3.2 eV combined with ion-exchange approach. 2 h+ VB From Figure 3, UV-vis DRS spectra of CdS-TiO2 and TiO2 samples indicated that TiO2 could only absorb UV light VB with wavelength less than 400 nm for its large energy gap of anatase (3.2 eV). The introduction of CdS in CdS-TiO2 sam- ff ple can e ectively shift the absorption range of TiO2 into vis- Scheme 1: Illustration of electron transfer between CdS and TiO2 ible light region of 400–550 nm due to the narrow band gap in the CdS-TiO2 composite. of CdS (2.4 eV). Therefore, the light-harvesting efficiency of CdS-TiO2 hollow microspheres was larger than that of TiO2 in visible light region, beneficial to the photocatalytic activity. ff The efficiency of charge trapping and recombination of e ectively separate photo-induced electrons from holes in photo-induced electrons and holes in the semiconductor the CdS semiconductor and thus inhibit their recombina- could be verified by PL spectra. According to PL spectra of tion, resulting in the enhanced charge density in the TiO2 conduction (Scheme 1). both CdS-TiO2 and TiO2 samples in Figure 4 with the excitation wavelength of 360 nm, CdS-TiO2 exhibited much weaker intensity of peak at 720 nm than TiO2.Itcouldbe 3.2. Photocatalytic Performance. The photodegradation of ascribed to the lower recombination probability of photo- RhB under visible-light irradiations (λ>420 nm) was used induced electrons and holes [17, 18]. As we know, the effici- as a probe to evaluate the performance of photocatalysts. ent charge transfer from CdS to TiO2 conduction band could According to the dependence of photodegradation rate on 4 International Journal of Photoenergy

100 References

[1] X. Z. Li and H. S. Liu, “Development of an E-H2O2/TiO2 pho- 80 toelectrocatalytic oxidation system for water and wastewater treatment,” Environmental Science and Technology, vol. 39, no. 12, pp. 4614–4620, 2005. 60 CdS-TiO 2 [2]M.R.Hoﬀmann, S. T. Martin, W. Choi, and D. W. Bahne- TiO2 mann, “Environmental applications of semiconductor photo- Blank 40 catalysis,” Chemical Reviews, vol. 95, no. 1, pp. 69–96, 1995. [3]M.Mrowetz,W.Balcerski,A.J.Colussi,andM.R.Hoﬀmann,

Degradation yield (%) “Oxidative power of nitrogen-doped TiO2 photocatalysts 20 under visible illumination,” Journal of Physical Chemistry B, vol. 108, no. 45, pp. 17269–17273, 2004. [4] H. Li, Z. Bian, J. Zhu, Y. Huo, H. Li, and Y. Lu, “Mesoporous 0 Au/TiO2 nanocomposites with enhanced photocatalytic activ- 0 0.5 1 1.5 2 2.5 3 ity,” Journal of the American Chemical Society, vol. 129, no. 15, Time (h) pp. 4538–4539, 2007. [5] K. Lv, J. Li, X. Qing, W. Li, and Q. Chen, “Synthesis and photo- Figure 5: Photocatalytic degradation of RhB with CdS-TiO2 and degradation application of WO3/TiO2 hollow spheres,” Journal TiO2 samples and without catalyst (blank) under visible-light irradiations, respectively. of Hazardous Materials, vol. 189, no. 1-2, pp. 329–335, 2011. [6] W.T.Sun,A.Yu,H.Y.Pan,X.F.Gao,Q.Chen,andL.M.Peng, “CdS quantum dots sensitized TiO2 nanotube-array photoele- ctrodes,” Journal of the American Chemical Society, vol. 130, no. 4, pp. 1124–1125, 2008. [7]B.Liu,J.E.Boercker,andE.S.Aydil,“Orientedsinglecry- reaction time (Figure 5), the removal of RhB in the absence stalline titanium dioxide nanowires,” Nanotechnology, vol. 19, of photocatalyst could be neglected. After reaction for 3 h, no. 50, Article ID 505604, 2008. CdS-TiO2 presented much higher degradation yield than [8] Z. R. Tian, J. A. Voigt, J. Liu, B. McKenzie, and H. Xu, “Large TiO2 sample, which was up to 90%. The degradation result oriented arrays and continuous films of TiO2-based nano- well demonstrated that the introduction of CdS could signif- tubes,” Journal of the American Chemical Society, vol. 125, no. icantly extend light absorbance into visible-light region with 41, pp. 12384–12385, 2003. high intensity and thus promote the photocatalytic activity [9] N. Ghows and M. H. Entezari, “Exceptional catalytic efficiency under visible-light irradiations. The well-remained structure in mineralization of the reactive textile azo dye (RB5) by of hollow microsphere via polymer hard-template approach a combination of ultrasound and core-shell nanoparticles followed by ion-exchange could achieve light multireflection (CdS/TiO2),” Journal of Hazardous Materials, vol. 195, pp. within the interior cavities, leading to more efficient light 132–138, 2011. harvesting [19]. Furthermore, the lower recombination of [10] P. Wang, J. Wu, Y. Ao, C. Wang, J. Hou, and J. Qian, “Pre- photo-induced electrons and holes in CdS-TiO than that paration and enhanced photocatalytic performance of Sn ion 2 modified titania hollow spheres,” Materials Letters, vol. 65, no. in TiO couldleadtotheimprovedquantumefficiency and 2 21-22, pp. 3278–3280, 2011. facilitate the formation of active radicals which decomposed [11] V. Jokanovic,´ A. M. Spasic,´ and D. Uskokovic,´ “Designing of organic compounds. nanostructured hollow TiO2 spheres obtained by ultrasonic spray pyrolysis,” Journal of Colloid and Interface Science, vol. 278, no. 2, pp. 342–352, 2004. 4. Conclusions [12] Y. Huang, F. Sun, T. Wu et al., “Photochemical preparation of CdS hollow microspheres at room temperature and their Hollow CdS-TiO2 microspheres with uniform size were syn- use in visible-light photocatalysis,” Journal of Solid State thesized via hard-template process with PS microspheres fol- Chemistry, vol. 184, no. 3, pp. 644–648, 2011. lowed by ion-exchange. During the photodegradation of RhB [13] C. Wang, Y. Ao, P. Wang, J. Hou, and J. Qian, “Preparation of under visible light irradiations, CdS-TiO2 exhibited much cerium and nitrogen co-doped titania hollow spheres with enhanced visible light photocatalytic performance,” Powder Te- higher photocatalytic activity than TiO2. It was attributed to the introduction of CdS which extended the photorespon- chnology, vol. 210, no. 3, pp. 203–207, 2011. se to the visible-light region and improved the separation of [14] G. S. Li, D. Q. Zhang, and J. C. Yu, “A new visible-light photo- photo-induced charges. catalyst: CdS quantum dots embedded mesoporous TiO2,” Environmental Science and Technology, vol. 43, no. 18, pp. 7079–7085, 2009. [15] N. Biswal, D. P. Das, S. Martha, and K. M. Parida, “Effici- Acknowledgments ent hydrogen production by composite photocatalyst CdS- ZnS/Zirconium-titanium phosphate (ZTP) under visible light This work was supported by National Natural Science illumination,” International Journal of Hydrogen Energy, vol. Foundation of China (20937003), Shanghai Government 36, no. 21, pp. 13452–13460, 2011. (11JC1409000, S30406, 12YZ091, and 10230711600) and [16] J. Wang, Y. Wen, X. Feng, Y. Song, and L. Jiang, “Control Shanghai Normal University (DZL807). over the wettability of colloidal crystal films by assembly International Journal of Photoenergy 5

temperature,” Macromolecular Rapid Communications, vol. 27, no. 3, pp. 188–192, 2006. [17] J. Liqiang, Q. Yichun, W. Baiqi et al., “Review of photoluminescence performance of nano-sized semiconductor materials and its relationships with photocatalytic activity,” Solar Energy Materials and Solar Cells, vol. 90, no. 12, pp. 1773–1787, 2006. [18] H. Zhu, B. Yang, J. Xu et al., “Construction of Z-scheme type CdS-Au-TiO2 hollow nanorod arrays with enhanced photocatalytic activity,” Applied Catalysis B, vol. 90, no. 3-4, pp. 463– 469, 2009. [19] H. Li, Z. Bian, J. Zhu et al., “Mesoporous titania spheres with tunable chamber stucture and enhanced photocatalytic activity,” Journal of the American Chemical Society, vol. 129, no. 27, pp. 8406–8407, 2007. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 528637, 8 pages doi:10.1155/2012/528637

Research Article Effects of Calcination Temperature on Preparation of Boron-Doped TiO2 by Sol-Gel Method

Wenjie Zhang, Bo Yang, and Jinlei Chen

School of Environmental and Chemical Engineering, Shenyang Ligong University, Shenyang 110159, China

Correspondence should be addressed to Wenjie Zhang, [email protected]

Received 5 January 2012; Revised 5 February 2012; Accepted 6 February 2012

Academic Editor: Weifeng Yao

Copyright © 2012 Wenjie Zhang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

◦ Boron-doped TiO2 photocatalyst was prepared by a modiﬁed sol-gel method. Being calcinated at temperatures from 300 Cto ◦ 600 C, all the 3% B-TiO2 samples presented anatase TiO2 phase, and TiO2 crystallite sizes were calculated to be 7.6, 10.3, 13.6, and 27.3 nm, respectively. The samples were composed of irregular particles with rough surfaces in the size range within 3 μm. Ti atoms were in an octahedron skeleton and existed mainly in the form of Ti4+, while the Ti-O-B structure was the main boron existing ◦ ◦ form in the 3% B-TiO2 sample. When calcination temperature increased from 300 C to 600 C, speciﬁc surface area decreased sharply from 205.6 m2/g to 31.8 m2/g. The average pore diameter was 10.53 nm with accumulative pore volume of 0.244 mL/g for ◦ the 3% B-TiO2 sample calcinated at 400 C, which performed optimal photocatalytic degradation activity. After 90 min of UV-light irradiation, degradation rate of methyl orange was 96.7% on the optimized photocatalyst.

1. Introduction [13], I [14], and B [15, 16] has been explored to promote separation of photogenerated charges in TiO . Due to the In recent years, many kinds of metal or nonmetal doped 2 electron deficiency structure of boron, boron-doped TiO TiO photocatalysts were prepared because semiconductive 2 2 has already attracted much attention, and researches have TiO2 was considered to be the most attractive photocatalyst due to its properties of chemically stable, nontoxic, high increasingly focused on the development of boron doped TiO2 systems in recent years. Zhao et al. [17]reported efficient, and relatively inexpensive [1, 2]. TiO2 has attracted much attention in view of its practical applications such that doping with B can extend the spectrum response of as self-cleaning surfaces, wastewater and air purification, TiO2 to visible region and thus can improve its visible light photocatalytic activity. Chen et al. [18] found that B-doped bacteria inactivation, and CO2 photoconversion to methane and low hydrocarbons. Many environmental pollutants can TiO2 showed higher photocatalytic activity than that of pure be degraded by oxidation and reduction processes on TiO2 TiO2 in photocatalytic NADH regeneration. They ascribed surface [3–5]. However, the application of TiO2 is limited the improvement of photocatalytic activity to the formation by its UV activation requirement because of its large band of Ti3+, which can facilitate the separation of photoexcited gap (3.2 eV in the anatase phase), and recombination rate of electrons and holes and slow their recombination rate. Yuan photogenerated electrons and holes is usually very quickly. et al. [19] prepared B and N codoped TiO2 photocatalyst Doping technology is one of the effective means to via sol-gel method and found that interstitial N and [NOB] overcome the disadvantages of TiO2. Since Asahi et al. [6] species in the TiO2 crystal lattice narrowed band gap found out that N doped into TiO2 effectively enhanced the and extended optical absorption of TiO2. Xu et al. [20] photocatalytic activity of TiO2, there has been an explosion indicated that low-temperature hydrothermal method could of interest in TiO2 doping with non-metal ions because be used to prepare boron-doped TiO2, and the photocatalyst of high thermal stability and low carrier recombination showed larger surface area and higher photocatalytic activity centers of nonmetals doped TiO2 nanostructures. A variety than that prepared by sol-gel method. Zaleska et al. [21] of nonmetal ions such as N [7–9], C [10], S [11], F [12], P used a simple surface impregnation method to prepare 2 International Journal of Photoenergy

boron-modiﬁed TiO2, and boron as a B-O-Ti species existed

in the surface of TiO2 grains. (101) Despite the work dedicated to the properties of photoactive B-TiO2, most work focused on eﬀects of B doping

amounts. Very limited published works were related to the (200)

(004) ◦ (211) (105) effects of calcination temperature on properties of B-doped (204) 600 C TiO . In the present work, 3% boron-doped TiO -based 2 2 ◦ photocatalysts were prepared by sol-gel method at different 500 C calcination temperatures using tributyl borate as boron ◦ precursor, and their characteristics were investigated by XRD, Intensity 400 C SEM, FT-IR, XPS, surface area (BET), and porosity determi- 300◦C nation (BJH). Photocatalytic degradation of an organic azo- dye, methyl orange, was investigated under ultraviolet irra- ◦ 400 C TiO2 diation. The effects of calcination temperature on structure, surface area, crystallinity, and photocatalytic activity of B- 10 20 30 40 50 60 70 80 TiO2 photocatalyst were systematically investigated. 2θ (◦)

2. Experimental Figure 1: XRD patterns of 3% B-TiO2 samples with different ◦ calcination temperatures and the pure TiO2 calcinated at 400 C. 2.1. Preparation of B-TiO2 Photocatalysts. 3wt% boron- doped TiO2 photocatalysts were prepared by a modified sol- gel approach. The detailed process was described as follows. Tetrabutyl titanate of chemical pure grade was chosen as to determine pore size distribution using the Barrett, Joyner, the Ti precursor and tributyl borate (99.5%) was used as and Halenda (BJH) method. the boron source. Hydrochloric acid (HCl) and anhydrous ethanol were in the analytical reagent grade. 8 mL anhydrous 2.3. Measurement of Photocatalytic Activity. The effectiveness ethanol and 0.1 mL hydrochloric acid were mixed in a beaker, of B-doped TiO2 nanocrystal was evaluated by degradation and then 2 mL tetrabutyl titanate and desired volume of of methyl orange (MO) solution under UV light irradia- tributyl borate were dropwisely added to the former solution tion. Before photocatalytic experiment, adsorption of MO under constant magnetic stirring to prepare solution 1. solution in the dark on the 3% B-TiO2 photocatalyst was Meanwhile, 1 mL of distilled water was mixed with 4 mL measured in the suspension. 50 mL of 10 mg/L methyl orange anhydrous ethanol to prepare solution 2. After solution 1 aqueous solution was mixed with 30 mg photocatalyst in was stirred for 30 min, solution 2 was dropwisely added into a 250 mL beaker. The suspension was stirred magnetically solution 1. The final mixed solution was continuously stirred for 20 min to reach adsorption equilibrium. After that, until the formation of a gel. After aging for 24 h at room 5 mL suspension was taken out of the reactor and filtrated temperature, the gel was dried at 80◦C for 8 h. Subsequently, through a millipore filter (pore size 0.45 μm) to remove the obtained solid was grinded and calcinated at different the photocatalyst. Finally, absorbency of the solution was temperatures for 3 h, respectively. The obtained 3% boron- measured using a 721E spectrophotometer at the MO doped TiO2 was ascribed as 3% B-TiO2 in the following maximum absorption wavelength of 468 nm. experiments. Photocatalytic activities of the prepared catalysts were evaluated afterwards. A 20 W ultraviolet lamp was located 2.2. Catalyst Characterization. X-ray diffraction (XRD) pat- over the 250 mL beaker with a distance of 11 cm from the terns were obtained by a Rigaku D/Max-rB diffractometer lamp to the surface of the solution. The lamp can irradiate using Cu Ka radiation. The XRD estimation of crystallite UV light at wavelength of 253.7 nm with the intensity of size was based on the Scherrer formula. Scanning electron 1100 μW/cm2. In prior to turn on the lamp, the solution microscopy (SEM, Hitachi, S-3400N) was used for morphol- should ensure adsorption equilibrium according to the above ogy characterization of B-doped TiO2 crystal. The samples process. Irradiation time in the subsequent experiments for SEM imaging were coated with a thin layer of gold film to was set for 30 min except for the prolonged time reaction. avoid charging. FT-IR spectra of the samples were obtained After photocatalytic reaction, 5 mL of the suspension was using a Fourier transform infrared (FT-IR) spectrometer filtrated through millipore filter to measure the change of (WQF-410) with KBr pellets. The samples were analyzed MO concentration. in the wavenumber range of 4000–400 cm−1. The elemental composition of 3% B-TiO2 nanocrytals was determined by 3. Results and Discussion X-ray photoelectron spectroscopy (XPS, MULTILAB2000). Specific surface area measurements were performed using 3.1. Characterization of B-TiO2 Photocatalysts. XRD was car- a surface area and pore size analyzer (F-sorb 3400). The ried out to investigate phase structure of B-TiO2. Figure 1 specific surface area was determined by the multipoint BET shows XRD patterns of 3% B-TiO2 samples that were calci- method using the adsorption data in the relative pressure nated at 300, 400, 500, and 600◦C for 3 h, respectively. All the (P/P0) range of 0.05–0.25. The desorption isotherm was used diffraction peaks in the patterns well match the diffraction International Journal of Photoenergy 3

(a) (b)

◦ ◦ ◦ ◦ Figure 2: SEM images of 3% B-TiO2 samples calcinated at (a) 300 C, (b) 400 C, (c) 500 C, and (d) 600 C, respectively.

peaks of anatase TiO2 crystallite. There are no peaks showing respectively, revealing an obvious crystallite growing under impurities such as B2O3 and TiB2 or other TiO2 phases high-temperature calcination treatment. like rutile and brookite existing in the samples. The XRD Surface morphology is quite essential for photocatalytic analysis can not confirm any formation of boron containing activity of the materials. Figures 2(a) to 2(d) show boron- ◦ compound, probably because it is in the amorphous state doped TiO2 samples calcinated at 300, 400, 500, and 600 C. in the photocatalysts. Grzmil et al. reported formation of The prepared samples are composed of irregular particles ◦ B2O3 when boron-doped TiO2 was calcinated at 1000 C with fairly rough surfaces in the size range within 3 μm. [22]. It indicates that low calcination temperature might be The small particles among the big ones came from grinding the reason for formation of amorphous boron phase in the after calcination. The particle size seems quite suitable samples. for suspending in methyl orange solution under magnetic Being calcinated at temperatures from 300◦C to 600◦C, stirring. There was no strong particles aggregation during all the samples present anatase TiO2 phase, and there is no sol-gel preparation and calcination processes. All of the peak demonstrating transformation from anatase TiO2 to 3% B-TiO2 samples can undergo well dispersion without rutile TiO2. Grzmil et al. [22]andChenetal.[18] pointed apparent deposition during photocatalytic reaction. out that boron-doped TiO2 would undergo anatase-rutile Figure 3 shows FT-IR spectra of 3% B-TiO2 samples with transformation during calcination above 700◦C. Therefore, different calcination temperature. For all samples, the bands −1 −1 the results confirm that no TiO2 phase change occurred at 3384 cm and 1621 cm are assigned to the stretching of ◦ during calcination process at temperatures rising from 300 C hydroxyl groups and the bending vibration of H2Oadsorbed to 600◦C. on the surface of the samples. There are peaks at 469 cm−1 −1 The TiO2 crystallinity and crystallite size increased as and 670 cm for the stretching of Ti–O bond and the calcination temperature increased based on the intensities bending vibration of Ti–O bond, respectively. In the IR of characteristic XRD peaks. The peak intensities of anatase spectra of the samples, another peak appears at 1397 cm−1 TiO2 phase become stronger, and the width of the peaks gets can be ascribed to the vibration of tri-coordinated boron narrower for the samples calcinated at higher temperatures. [23]. Furthermore, neither absorption peak corresponding −1 It indicates that calcination can lead to TiO2 crystal growth to pure B2O3 (1202 cm )[22] nor peak of incorporated −1 with increasing temperature. Crystallite sizes of the samples BO4 (1096 cm )[24] appears in the spectra. It reveals were calculated using the Scherrer formula according to Full that boron is introduced into the titania framework in Width at Half Maximum (FWHM) analysis of the anatase the form of B-O-Ti bond, and this structure is further (101) plane diffraction peak. The crystallite sizes of B- confirmed by XPS measurements later. However, the peak at ◦ −1 TiO2 prepared at calcination temperatures from 300 Cto 1397 cm [23, 25] corresponding to tricoordinated boron is 600◦C were calculated to be 7.6, 10.3, 13.6, and 27.3 nm, weakened after high-temperature calcination, indicating that 4 International Journal of Photoenergy

Table 1: Lattice parameters and BET surface areas of 3% B-TiO2 samples calcinated at diﬀerent temperatures.

Calcination Surface area a(=b) c V temperature /m2/g /10−1 nm /10−1 nm /10−3 nm3 /◦C 300 205.6 3.7852 9.4642 135.6005 400 127.8 3.7997 9.5008 136.2833 600◦C 500 88.6 3.7901 9.5532 137.2303 500◦C 600 31.8 3.7985 9.5678 138.0499

400◦C crystal lattice, and the second peak at 531.7 eV corresponds 1397 469 to the surface hydroxyl groups. The peak at 530.2 eV 1621 300◦C indicates oxygen in the Ti–O–B bond [21], which is in accordance to the result of XPS B1s region. Therefore, XPS 3384 670 analysis confirms that sol-gel synthesis allows incorporation of boron atoms into TiO2 matrix. Figure 4(d) shows XPS 4500 4000 3500 3000 2500 2000 1500 1000 500 C1s spectrum of the 3% B-TiO2 sample. The peak at lower − Wavenumber (cm 1) binding energy of 284.8 eV was used for calibration of the XPS results, and the peak at 289.3 eV is attributed to the Figure 3: FT-IR spectra of 3% B-TiO2 samples with different calcination temperature. adsorbed carbon on the sample. In order to study porous status of the boron-doped TiO2 materials, Brunauer-Emmett-Teller nitrogen sorption measurements were carried out. N2 molecules were in single boron in Ti-O-B form preferably appears at low temperature or multiple layers adsorbed on the internal pore surface calcination. of the materials. As shown in Figure 5(a),N2 desorption Figure 4(a) shows XPS B1s spectrum of 3% boron-doped changed significantly at relative pressure between 0.95 and TiO2 photocatalyst prepared by sol-gel method followed by 0.6 in N2 desorption process, which was mainly caused ◦ calcination at 400 C. Normally, B1s electron-binding energy by capillary aggregation of N2 molecules occurring in peak situates around 188 eV–194 eV for B-TiO2. In the figure, micropores inside the material [32]. Figure 5(b) shows that B1s peak can be separated to three independent peaks using pore size of 3% B-TiO2 mainly distributes in the range from XPSPEAK 4.1 software. This means that different chemical 1.5 nm to 18 nm. The average pore diameter of 3% B-TiO2 ◦ forms of B atoms might exist in B-doped TiO2 nanoparticles. calcinated at 400 C is 10.53 nm, and the accumulative pore The standard binding energy of B1s in B2O3 or H3BO3 equals volume is 0.244 mL/g for the material. to 193.0 eV (B–O bond) and in TiB2 equals to 187.5 eV As shown in Table 1, the specific surface areas of 3% (B–Ti bond) [18]. In the figure, the first peak at 192.9 eV B-TiO2 samples decreased continuously with increasing is related to B–O–B bonds in B2O3 or H3BO3 and the low calcination temperature. When calcination temperature energy peak at 189.6 eV corresponds to boron incorporated increased from 300◦C to 600◦C, surface area decreased 2 2 into the TiO2 lattice through occupying O sites to form O- sharply from 205.6 m /g to 31.8 m /g. The sample calcinated Ti-B band. Su et al. also pointed out that the peak at 189.6 eV at 300◦C had the largest surface area because organic might correspond to B-Ti in TiB2 [26]. The strongest peak at substances did not burn out totally at low temperature, and 191.7 eV is related to boron that is probably weaved into the the residual carbon contributed to the large BET surface interstitial TiO2 and exists in the form of Ti-O-B structure area. Lattice parameters were obtained by using Bragg’s [23, 27, 28]. The Ti-O-B structure is the main boron existing law (2d sin θ = λ) and a formula for a tetragonal system, 2 2 2 2 2 2 form in the 3% B-TiO2 sample. 1/d = (h + k )/a +l/c . As summarized in Table 1, Figure 4(b) shows XPS Ti2p spectra of 3% B-TiO2.There the lattice parameters of all the B-TiO2 samples changed are two isolated symmetrical peaks in the XPS patterns, along with the change of calcination temperature. The cell showing that Ti atoms are in an octahedron skeleton and volumes of 3% B-TiO2 samples became larger at higher existed mainly in the form of Ti4+. The binding energies of calcination temperature due to accelerated crystal growth at Ti2p3/2 and Ti2p1/2 for 3% B-TiO2 sample are at 456.6 eV high temperature. It can also be deduced that crystallite sizes and 462.2 eV, and distance between Ti2p3/2 and Ti2p1/2 of B-TiO2 could grow up with the increase of calcination peaksis5.6eV(Thestandardvalueis5.6to5.7eV[29]). temperature, which is confirmed by XRD analysis. It was reported that boron doping favored the formation of 3+ Ti on the surface of TiO2 [30, 31], but there is no evidence 3.2. Photocatalytic Activity of 3% B-TiO2. Photocatalytic 3+ of Ti formation in Figure 4(b). activity of B-doped TiO2 was evaluated by photocatalytic As shown in Figure 4(c), XPS O1s region is composed degradation of methyl orange under UV light irradiation. of three peaks situating at 532.7 eV, 531.7 eV, and 530.2 eV. 3% B-TiO2 prepared at different calcination temperature The first peak at 532.7 eV is related to oxygen in the TiO2 performed obviously different photocatalytic degradation International Journal of Photoenergy 5

191.7 458.7 Intensity Intensity 192.9 189.6 464.4

195 194 193 192 191 190 189 188 468 466 464 462 460 458 456 454 Binding energy (eV) Binding energy (eV) (a) B1s (b) Ti2p

284.8 530.2 Intensity Intensity 531.7 289.3 532.7

538 536 534 532 530 528 526 524 294 292 290 288 286 284 282 280 278 Binding energy (eV) Binding energy (eV) (c) O1s (d) C1s

◦ Figure 4:XPSpatternsof3%B-TiO2 sample calcinated at 400 C.

140 0.07

120 0.06 /g) /g) 3 100 3 0.05

80 0.04

60 0.03

Pore volume (cm volume Pore 0.02 40

Volume adsorption (cm Volume 0.01 20 0 0 0 0.2 0.4 0.6 0.8 1 0 20 40 60 80 100 120 Relative pressure (P/P0) Pore diameter (nm) (a) (b)

◦ Figure 5: (a) N2 desorption isotherms and (b) BJH pore size distribution of 3% B-TiO2 samples calcinated at 400 C. 6 International Journal of Photoenergy

35 100 30 80 25

20 60

15 40 Degradation (%) Decoloration (%) Decoloration 10 20 5 0 0 0306090 300 400 500 600 700 Irradiation time (min) Calcination temperature (◦C) Adsorption Degradation Degradation Adsorption

Figure 6: Photocatalytic activities of 3% B-TiO2 samples calcinated Figure 7: Photocatalytic activity of 3% B-TiO2 sample with at diﬀerent temperatures. prolonged irradiation time.

0.8 activity, as shown in Figure 6. The optimal degradation 0.7 ◦ 0 min rate was 31.5% on 3% B-TiO2 sample calcinated at 400 C. The highest absorption of the dye was observed on 3% B- 0.6 15 min ◦ TiO2 calcinated at 300 C. As previously indicated (see in 0.5 30 min ◦ Table 1), BET surface area of 3% B-TiO2 calcinated at 300 C 60 min 2 0.4 is 205.6 m /g, which is much more than that prepared at 90 min

◦ Absorbance higher temperatures. Since 300 C was a low calcination tem- 0.3 perature, some organic substances did not burn out totally, 0.2 so that carbon residues caused the large BET surface area and high-absorption capacity. Low calcination temperature 0.1 ffi can be also responsible for insu cient formation of anatase 0 TiO2 crystals as the reason of low photocatalytic activity. AscanbeseenfromFigure 6, the adsorption of methyl 300 400 500 600 700 orange changed slightly on the samples when calcination Wavelength (nm) ◦ ◦ temperature varied from 400 C to 600 C, indicating thor- Figure 8: Absorption spectra of methyl orange solution during oughly burning of organic substances at high temperatures. irradiation in the presence of 3% B-TiO2. The sample that was calcinated at 400◦C presented the optimal photocatalytic degradation activity. As described before, BET surface areas of the samples calcinated at 500◦C ◦ and 600◦C are smaller than that of the sample calcinated at 400 C. The maximum MO absorption peak at 468 nm grad- 400◦C. Meanwhile, FT-IR results show that the formation ually decreased during irradiation process. After 90 min of of Ti-O-B is weakened with the increase of calcination irradiation, the dye was completely decomposed according to temperature. It can be deduced that the sample calcinated at disappearance of the main absorption peak, due to breaking 400◦C represents the optimal physic-chemical and structural up of methyl orange molecules into small parts under characteristics that are suitable for photocatalytic degrada- photocatalytic degradation. tion of methyl orange. Figure 7 presents the adsorption and photocatalytic ◦ 4. Conclusion activities of 3% B-TiO2 calcinated at 400 C with prolonged irradiation time. Adsorption rate did not change noticeably Boron-doped TiO2 photocatalyst was prepared by a modi- after the dye reached its adsorption equilibrium. After 90 min fied sol-gel method. Crystallite sizes of boron-doped TiO2 of UV-light irradiation, degradation rate of methyl orange increased gradually, and BET surface areas of the 3% B- was 96.7% on 3% B-TiO2 powder. It indicates that 3% B- TiO2 samples decreased sharply with increasing calcination TiO2 has satisfactory photocatalytic activity. temperature. Calcination temperature had no apparent Figure 8 shows absorption spectra of methyl orange impact on surface morphology of B-TiO2. XPS and FT- aqueous solution in presence of 3% B-TiO2 calcinated at IR results proved that the B-O-Ti structure in 3% B-TiO2 International Journal of Photoenergy 7 reduced at high calcination temperature. The sample with [13] Y. Y. Lv, L. S. Yu, H. Y. Huang, H. L. Liu, and Y. Y. Feng, the optimal photocatalytic activity was obtained after being “Preparation, characterization of P-doped TiO2 nanoparticles calcinated at 400◦C. Degradation rate of methyl orange was and their excellent photocatalystic properties under the solar 96.7% after 90 minutes of UV irradiation. light irradiation,” Journal of Alloys and Compounds, vol. 488, no. 1, pp. 314–319, 2009. [14] Y. Ma, J. W. Fu, X. Tao, X. Li, and J. F. Chen, “Low Acknowledgments temperature synthesis of iodine-doped TiO2 nanocrystallites with enhanced visible-induced photocatalytic activity,” This work was supported by the National Natural Science Applied Surface Science, vol. 257, no. 11, pp. 5046–5051, 2011. Foundation of China (no. 41071161 and 41130524), National [15]R.Khan,S.W.Kim,T.J.Kim,andC.M.Nam,“Compar- Key Basic Research Foundation of China (2011CB403202), ative study of the photocatalytic performance of boron-iron and Liaoning Science and Technology Project (2010229002). Co-doped and boron-doped TiO2 nanoparticles,” Materials Chemistry and Physics, vol. 112, no. 1, pp. 167–172, 2008. References [16] N. Lu, H. M. Zhao, J. Y. Li, X. Quan, and S. Chen, “Char- acterization of boron-doped TiO2 nanotube arrays prepared [1] A. Fujishima and K. Honda, “Electrochemical photolysis of by electrochemical method and its visible light activity,” water at a semiconductor electrode,” Nature, vol. 238, no. Separation and Purification Technology, vol. 62, no. 3, pp. 668– 5358, pp. 37–38, 1972. 673, 2008. [17]W.Zhao,W.H.Ma,C.H.Chen,J.C.Zhao,andZ.G. [2] H. B. Li, G. C. Liu, S. G. Chen, and Q. C. Liu, “Novel Fe doped Shuai, “Efficient degradation of toxic organic pollutants with mesoporous TiO2 microspheres: ultrasonic-hydrothermal Ni O /TiO − B under visible irradiation,” Journal of the synthesis, characterization, and photocatalytic properties,” 2 3 2 x x American Chemical Society, vol. 126, no. 15, pp. 4782–4783, Physica E, vol. 42, no. 6, pp. 1844–1849, 2010. 2004. [3] N. Sasirekha, S. J. S. Basha, and K. Shanthi, “Photocatalytic [18] D. M. Chen, D. Yang, Q. Wang, and Z. Y. Jiang, “Effects of performance of Ru doped anatase mounted on silica for boron doping on photocatalytic activity and microstructure reduction of carbon dioxide,” Applied Catalysis B, vol. 62, no. of titanium dioxide nanoparticles,” Industrial and Engineering 1-2, pp. 169–180, 2006. Chemistry Research, vol. 45, no. 12, pp. 4110–4116, 2006. [4] G. H. Li, S. Ciston, Z. V. Saponjic et al., “Synthesizing mixed- [19] J. Yuan, E. Wang, Y. Chen, W. Yang, J. Yao, and Y. Cao, “Doping phase TiO2 nanocomposites using a hydrothermal method for mode, band structure and photocatalytic mechanism of B–N- photo-oxidation and photoreduction applications,” Journal of codoped TiO2,” Applied Surface Science, vol. 257, no. 16, pp. Catalysis, vol. 253, no. 1, pp. 105–110, 2008. 7335–7342, 2011. [5] S. Guo, Z. B. Wu, H. Q. Wang, and F. Dong, “Synthesis of [20]J.J.Xu,Y.H.Ao,M.D.Chen,andD.G.Fu,“Low-temperature mesoporous TiO2 nanorods via a mild template-free sono- preparation of boron-doped titania by hydrothermal method chemical route and their photocatalytic performances,” Catal- and its photocatalytic activity,” Journal of Alloys and Com- ysis Communications, vol. 10, no. 13, pp. 1766–1770, 2009. pounds, vol. 484, no. 1-2, pp. 73–79, 2009. [6] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, [21] A. Zaleska, E. Grabowska, J. W. Sobczak, M. Gazda, and “Visible-light photocatalysis in nitrogen-doped titanium J. Hupka, “Photocatalytic activity of boron-modified TiO2 oxides,” Science, vol. 293, no. 5528, pp. 269–271, 2001. under visible light: the effect of boron content, calcination [7]Q.Shen,W.Zhang,Z.P.Hao,andL.D.Zou,“Astudy temperature and TiO2 matrix,” Applied Catalysis B, vol. 89, no. on the synergistic adsorptive and photocatalytic activities 3-4, pp. 469–475, 2009. [22] B. Grzmil, M. Glen,´ B. Kic, and K. Lubkowski, “Preparation of TiO2−xNx/Beta composite catalysts under visible light irradiation,” Chemical Engineering Journal, vol. 165, no. 1, pp. and characterization of single-modified TiO2 for pigmentary 301–309, 2010. applications,” Industrial and Engineering Chemistry Research, vol. 50, no. 11, pp. 6535–6542, 2011. [8] M. Qiao, S. S. Wu, Q. Chen, and J. Shen, “Novel trieth- [23]N.Feng,A.M.Zheng,Q.Wangetal.,“Boronenvironments anolamine assisted sol-gel synthesis of N-doped TiO2 hollow spheres,” Materials Letters, vol. 64, no. 12, pp. 1398–1400, in B-doped and (B, N)-codoped TiO2 photocatalysts: a combined solid-state NMR and theoretical calculation study,” 2010. The Journal of Physical Chemistry C, vol. 115, no. 6, pp. 2709– [9] J. Senthilnathan and L. Philip, “Photodegradation of methyl 2719, 2011. parathion and dichlorvos from drinking water with N-doped [24] S. C. Moon, H. Mametsuka, E. Suzuki, and Y. Nakahara, TiO2 under solar radiation,” Chemical Engineering Journal, vol. “Characterization of titanium-boron binary oxides and their 172, pp. 678–688, 2011. photocatalytic activity for stoichiometric decomposition of [10]Y.Li,M.Y.Ma,X.H.Wang,andX.H.Wang,“Inactivated water,” Catalysis Today, vol. 45, no. 1–4, pp. 79–84, 1998. properties of activated carbon-supported TiO2 nanoparticles [25] H. L. Fei, Y. P. Liu, Y. P. Li et al., “Selective synthesis of borated for bacteria and kinetic study,” Journal of Environmental meso-macroporous and mesoporous spherical TiO2 with Sciences, vol. 20, no. 12, pp. 1527–1533, 2008. high photocatalytic activity,” Microporous and Mesoporous [11] H. Tian, J. F. Ma, K. Li, and J. J. Li, “Hydrothermal synthesis Materials, vol. 102, no. 1–3, pp. 318–324, 2007. of S-doped TiO2 nanoparticles and their photocatalytic ability [26]Y.L.Su,X.W.Zhang,S.Han,X.Q.Chen,andL.C.Lei,“F-B- for degradation of methyl orange,” Ceramics International, vol. codoping of anodized TiO2 nanotubes using chemical vapor 35, no. 3, pp. 1289–1292, 2009. deposition,” Electrochemistry Communications,vol.9,no.9, [12]J.J.Xu,Y.H.Ao,D.F.Fu,andC.W.Yuan,“Low-temperature pp. 2291–2298, 2007. preparation of F-doped TiO2 film and its photocatalytic [27] E. Finazzi, C. D. Valentin, and G. Pacchioni, “Nature of ti activity under solar light,” Applied Surface Science, vol. 254, no. interstitials in reduced bulk anatase and rutile TiO2,” Journal 10, pp. 3033–3038, 2008. of Physical Chemistry C, vol. 113, no. 9, pp. 3382–3385, 2009. 8 International Journal of Photoenergy

[28] X. N. Lu, B. Z. Tian, F. Chen, and J. L. Zhang, “Preparation of boron-doped TiO2 films by autoclaved-sol method at low temperature and study on their photocatalytic activity,” Thin Solid Films, vol. 519, no. 1, pp. 111–116, 2010. [29] X. Q. Chen, X. W. Zhang, and L. C. Lei, “Electronic structures and photocatalysis properties under visible irradiation of F- doped TiO2 nanotube arrays,” Journal of Inorganic Materials, vol. 26, no. 4, pp. 369–374, 2011. [30] M. Fittipaldi, V. Gombac, T. Montini, P. Fornasiero, and M. Graziani, “A high-frequency (95 GHz) electron paramagnetic resonance study of B-doped TiO2 photocatalysts,” Inorganica Chimica Acta, vol. 361, no. 14-15, pp. 3980–3987, 2008. [31] Y. M. Wu, M. Y. Xing, J. L. Zhang, and F. Chen, “Effective visible light-active boron and carbon modified TiO2 photocatalyst for degradation of organic pollutant,” Applied Catalysis B, vol. 97, no. 1-2, pp. 182–189, 2010. [32] J. Yuan, Y. K. Lv, Y. Li, and J. P. Li, “Preparation of mesoporous magnetic photocatalyst and its catalytic activity for degradation of nitrobenzene,” Chinese Journal of Catalysis, vol. 31, no. 5, pp. 597–603, 2010. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 348292, 10 pages doi:10.1155/2012/348292

Research Article Synthesis, Characterization, and Photocatalytic Activity of TiO2 Microspheres Functionalized with Porphyrin

Jin-Hua Cai,1, 2 Jin-Wang Huang,2 Han-Cheng Yu,2 and Liang-Nian Ji2

1 College of Chemistry and Chemical Engineering, Jinggangshan University, Jian 343009, China 2 MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, State Key Laboratory of Optoelectronic Material and Technologies, Sun Yat-Sen University, Guangzhou 510275, China

Correspondence should be addressed to Jin-Hua Cai, [email protected] and Jin-Wang Huang, [email protected]

Received 3 January 2012; Accepted 2 February 2012

Academic Editor: Stephane´ Jobic

Copyright © 2012 Jin-Hua Cai et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In order to utilize visible light more efficiently in the photocatalytic reaction, TiO2 microspheres sensitized by 5-(4- allyloxy)phenyl-10,15,20-tri(4-methylphenyl)porphyrin (APTMPP) were prepared and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nitrogen physisorption, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and UV-vis diffuse reflectance spectroscopy, and so forth, The characterization results indicated that APTMPP-MPS-TiO2 was composed of the anatase crystal phase. The morphology of the composite materials was spheriform with size of 0.3–0.7 μm and the porphyrin was chemisorbed on the surface of TiO2 through a Si–O–Ti bond. The photooxidation of α-terpinene was employed as the model reaction to evaluate the photocatalytic activity of APTMPP-MPS-TiO2 microspheres under visible light. The results indicated that the photodegradation of α-terpinene was significantly enhanced in the presence of the APTMPP-MPS-TiO2 compared with the nonmodified TiO2 under visible light.

1. Introduction only makes up less than 5% of whole sunlight. Therefore, the development of low-bandgap photoactive material, the The application of semiconductors in heterogeneous photo- so-called visible light photocatalyst, is strongly urged for catalysis to eliminate various pollutants in aqueous systems solving environmental problems. To overcome this problem, has gained significant attention in the last decade [1–3]. TiO2 different approaches have been proposed in the literature is considered a preferred and potential semiconductor pho- in which the response of the semiconductor was extended tocatalytic material for applications requiring antimicrobial toward the visible region. Narrowing the bandgap is an and sterilizing characteristics because some of its forms have effective way to enhance the photocatalytic performance of reasonable photoactivity. Besides, it has many advantages titania, and this can be done with metals and nonmetals such as inexpensiveness, easy production, (photo)chemical ion doping [8–10], ion implantation, and photosensitization and biological stability, and innocuity to the environment [11–15]. Among them, dye sensitization is considered to be and to human beings. It has been used for water remediation an efficient method to modify the photoresponse properties ff being e ective for the photodegradation of many harmful of TiO2 particles [16–18]. The dyes used are erythrosine organic pollutant to innocuous inorganic species [4–7]. B, rose bengal, porphyrin, and so forth. Similarly to other However, some drawbacks limit its application especially photosensitizers, porphyrins are recognized to be the most in the large-scale industry. One important disadvantage promising sensitizers. They not only can be involved in of pure titania is characterized by low quantum efficiency the reaction mechanism by transferring electrons to the ∼ because of high bandgap energy ( 3.2 eV) and high rate conduction band of TiO2 or to adsorbed O2, but also they − + of e and h recombination. The light absorption region can cooperate with TiO2 participating in photooxidation of anatase-typed TiO2 particles does not fit with the solar reactions by means of a direct activation of O2 [19– ≤ spectrum because the solar energy above 3.0 eV (λ 410 nm) 24]. Recently, the photocatalytic activity of TiO2 powders 2 International Journal of Photoenergy impregnated with copper porphyrins used as sensitizers for on a 300 MHz Bruker-AMX spectrophotometer with tetram- the decomposition of 4-nitrophenol has been investigated ethylsilane (TMS) as internal reference. Mass spectrometry [25–28]. But Some of them suffer from a reversible binding analysis was performed on a Thermo LCQDECA-XP spec- and the support is broken during the washing of the solvent trometer. An iodine tungsten lamp (200W, Shanghai), ITL, under working conditions. A useful means to avoid these was used as a light source. A distance of about 5 cm between limitations is to establish the stable covalent binding between the lamp and reactor was maintained. porphyrins and TiO2 samples. We recently reported efficient photooxidations using 2.3. Synthesis of TiO2 Microspheres Functionalized with heterogeneous photosensitizers prepared by the cova- APTMPP (APTMPP-MPS-TiO2). The strategy to prepare lent immobilization of metal-free porphyrins on silica TiO2 microspheres functionalized with APTMPP (APTMPP- microspheres [29]. To further study the photosensitiza- MPS-TiO2) is shown in Scheme 1. tion of metal-free porphyrins, one novel metal-free porphyrin derivatives, that is, 5-(4-allyloxy)phenyl-10,15,20- tri(4-chlorophenyl)porphyrin was synthesized and charac- 2.3.1. Synthesis of APTMPP-MPS. APTMPP-MPS was syn- terized. The catalytic activity of metal-free porphyrin com- thesized by the reaction of 5-(4-allyloxy)phenyl-10,15,20- tri(4-methylphenyl)porphyrin (APTMPP) with 3-Mercap- plex covalent binding to TiO2 is studied upon irradiation the sample with visible light. The oxidation of α-terpinene has topropyltrimethoxysilane (MPS), and APTMPP was pre- been chosen as a model reaction. The purpose of this paper pared by the method similar to a previously reported is to study the structural relationship between porphyrin method [32]. A mixture of 5-(4-hydroxyl)phenyl-10,15,20- tri(4-methylphenyl)porphyrin (HPTMPP) (0.6 mmol), allyl complex and TiO2 in the prepared composite by sol-gel processing. Furthermore, the stability of photocatalysts was bromide (0.8 mmol), and anhydrous K2CO3 (1.5 g) in DMF also investigated and discussed. (40 mL) was stirred for 12 h at room temperature. The reaction mixture was poured into water (150 mL) and filtered. The residue was submitted to column chromatography on 2. Experimental Section silica gel using the chloroform as an eluent. The first band was collected and the solvent was evaporated. Purple solid 2.1. Materials and Chemicals. 5-(4-hydroxyl)phenyl-10, APTMPP was obtained in 80% yield. ES-MS [CHCl3,m/z]: + 1 15,20-tri(4-methylphenyl)porphyrin (HPTMPP) was synth- 713[M ]. H NMR (300 MHz, CDCl3), d: −2.68 (s, 2H, NH esized in our laboratory according to [30, 31], 3-mercap- pyrrole), 8.88 (s, 8H, β-pyrroles), 8.11 (d, 8H, 2,6-phenyl), topropyltrimethoxysilane (MPS, 95.8%), tetraethyl ortho- 7.61(d, 6H, 3,5-phenyl), 7.29 (d, 2H, 3,5-phenyl), 6.21–6.34 silicate, and allyl bromide (96%) were purchased from (t, 1H, =CH–), 5.62 (d, 1H, H–C=C–), 5.45 (1H, H–C=C–), Guangzhou Jun-ye Chemistry plant. α-terpinene was ob- 4.81 (2H, =C–CH2–O), 2.73 (s, 9H, –CH3). tained from Aldrich without further purification. Silica gel A mixture of MPS (40 mg, 0.2 mmol), APTMPP used to purify porphyrin by chromatograph was obtained (143 mg, 0.2 mmol), toluene(15 mL), and 10 mg 2,20-azo- from Qingdao. Toluene and acetonitrile were dried with bis-isobutyronitrile (AIBN) was degassed and sealed under calcium hydride and redistilled. All other reagents and nitrogen. Then, the solution was stirring at 75◦C. The solvents were obtained from commercial sources and used progress of the reaction was monitored by TLC (chloroform without further purification. as eluent). After completion of the reaction, the solvent was removed in vacuo and the pasty residue was washed with cold hexane, then a purple oil of APTMPP-MPS was obtained. ES- 2.2. Physical Measurements. The scanning electron micro- + 1 scopy (SEM) was performed with a JSM-6330F Field MS [CHCl3, m/z]: 908[M ], H NMR (300 MHz, CDCl3), d: − Emission Scanning Electron Microscope. The XRD curves 2.71 (s, 2H, NH pyrrole), 8.87 (s, 8H, β-pyrroles), 8.10 (d, were got from Rigaku D/max 2550 VB/PC (Japan). The 8H, 2,6-phenyl), 7.55 (d, 6H, 3,5-phenyl), 7.28 (d, 2H, 3,5- surface property of samples was characterized by X-ray phenyl), 4.35 (2H, H7), 3.63 (9H, H1), 2.89 (2H, H5), 2.73(s, photoelectron spectroscopy (XPS) on a PerkinElmer PHI 9H, –CH3), 2.35 (2H, H4), 2.27 (2H, H6), 1.30 (2H, H3), 0.89 1600 ESCA system. The specific surface areas of the samples (2H, H2). were measured using nitrogen adsorption method at 77 K and the Brunauer-Emmett-Teller (BET) analysis using a 2.3.2. Synthesis of APTMPP-MPS-TiO2. TiO2 microspheres Flow Sorb 2300 apparatus (Micromeritics International). were prepared by sol-gel processes [33]. 10 mL of Ti(OBu)4 The pore size distributions were calculated with the DFT is dissolved in 10 mL of anhydrous C2H5OH (99.7%) to Plus software (Micromeritics), applying the Barrett-Joyner- produce Ti(OBu)4-C2H5OH solution. Meanwhile, 5 mL of Halenda (BJH) model considering cylindrical geometry of water and 3 mL ammonia were added to another 10 mL the pores. Thermal analysis was performed with a Netzsch of anhydrous C2H5OH in turn to form an ethanol-NH3- TG-209 Thermogravimetric Analyzer. UV-vis and IR spectra water solution. After the two resulting solutions are stirred were recorded on a Shimadzu UV-3150 spectrophotometer for 30 min, respectively, the Ti(OBu)4-C2H5OH solution is and an EQUINOX 55 Fourier transformation infrared slowly added dropwise to the ethanol-NH3-water solution spectrometer, respectively. Diffuse reflectance spectra in the under vigorously stirring to carry out a hydrolysis. Thus, 200–800 nm were obtained by using a Shimadzu UV-3150 a semitransparent sol is gained after continuously stirring UV-vis spectrophotometer. 1H NMR spectra were recorded for 1 h. Then, 2 mL ammonia was added into the mixture International Journal of Photoenergy 3

O 7 1 5 4 2 O Ti O O O O S 3 Si O O O OH S O Si O Ti O OH HO Ti O O NH N O Ti C2H5OH H2O Ti(C4H9O)4 HO Ti OH + H3C CH3 C2H5OH H2O NH N O NH3 N HN O H3C CH3 O NH3 N HN HO Ti OH OH CH3 CH3 TiO2 APTMPP-MPS APTMPP-MPS-TiO2 CH3

NH N H3C O N HN S

CH3

APTMPP-MPS-TiO2

Scheme 1: Synthesis route of APTMPP-MPS-TiO2.

(pH 11) and the solution was stirred vigorously another separate the photocatalyst particles. The concentration of α- 12 h at room temperature. After completion of the reaction, terpinene was analyzed by UV-vis spectroscopy. The pho- the resulting precipitate was collected by centrifugation, todegradation percentage of α-terpinene can be calculated washed repeatedly with alcohol, and then dried in a vacuum using the changes of the absorbance of α-terpinene at and white powder was obtained. To complete the surface 265 nm. The reaction products were purified by extraction + modification with APTMPP-MPS, 40 mg APTMPP-MPS was from the reaction mixtures. ESMS [CHCl3, m/z]: 168 (M ).δ dissolved in 30 mL of CHCl3 and 1 g finely TiO2 powder 0.98∼1.00 (d, 6H CH3), 1.36 (s, 3H, CH3), 1.51∼1.56 (2H, was transferred into a weighing bottle. Subsequently, After methylene, CH2), 1.89∼2.04 (2H, methylene, CH2), 1.93 (H, ultrasonication for 2 min and stirring for 20 min vigorously, isopropyl, CH), 6.37∼6.49 (d, 4H, olefinic, CH) [35]. the mixture bottle was kept at 80◦C in the water bath to vaporize the liquid under stirring. The product was 2.5. Approach of Calculation. The degradation rate of α- washed with CHCl3 in a Soxhlet extractor overnight to terpinene in the reaction process could be calculated by the remove unbound APTMPP. Finally, the solids were dried at following formula: 110◦C for 12 h. Unbound APTDCPP was quantified through spectrophotometric measurement at 518 nm, by using a cal- (A0 − At) decolorization rate = × 100%, (1) ibration curve obtained by using a suitably diluted solution. A0 Thus, the amounts of bound porphyrins were determined by difference between the two measurements. APTMPP in where At is the absorbance of α-terpinene measured at 265 nm at time t,andA0 is the initial absorbance prior to APTMPP-MPS-TiO2 was 0.0202 mmol/g obtained by this method. reaction. The residual concentrations of α-terpinene could be calculated by the following formula:

2.4. Photocatalytic Activity Tests. Photooxidation experi- At Ct = × C0,(2) ments of α-terpinene were carried out in a 30 mL self- A0 designed jacketed reactor maintained at a certain temperature by circulation of thermostated water [29, 34]. In where Ct is residual concentration of α-terpinene at time t a typical experiment, aqueous slurries were prepared by and C0 is the initial concentration of α-terpinene. adding a certain amount of TiO2 or APTMPP-MPS-TiO2 − to 20 mL solution containing α-terpinene at 3.7 × 10 4 M. 3. Results and Discussion Irradiations were performed with a 200 W iodine tungsten lamp in which the UV light was ﬁltered by a 410 nm cut 3.1. Preparation and Characterization of APTMPP-MPS- ﬁlter. The aqueous slurries were stirred and bubbled with TiO2. The strategy used to prepare TiO2 microspheres humid oxygen for 10 min prior to the irradiation. At 10 min functionalized with porphyrin is shown in Scheme 1.The intervals, the dispersion was extracted and centrifuged to porphyrin was immobilized on the TiO2 microspheres 4 International Journal of Photoenergy

(a) (b)

Figure 1: Scanning electron micrograph of TiO2 (a) and APTMPP-MPS-TiO2 (b).

1.6 a

1.2 b b c 1396 0.8 1630 Absorbance Absorbance

0.4 928

a 0 300400 500 600 700 40003500 3000 2500 2000 1500 1000 500 −1 Wavelength (nm) Wavenumbers (cm )

Figure 2: UV-vis DRS spectra of TiO2 (a) APTMPP-MPS-TiO2 (b). Figure 3: FT-IR spectra of APTMPP-MPS (a), TiO2 (b), APTMPP- MPS-TiO2(c).

by the condensation reaction of APTMPP-MPS and TiO2 microspheres. above 400 nm for pure TiO2.ItcanbefoundthatAPTMPP- The morphology of TiO2 and APTMPP-MPS-TiO2 was MPS-TiO2 composites exhibit a broader absorption range obtained by scanning electron micrograph as shown in than pure TiO2. Figure 1. The micrograph shows irregularly shaped spherical Figure 3 shows the FT-IR spectra of APTMPP-MPS- −1 particles with loose and discrete structure. Distribution of TiO2. The peak around 1630 cm is due to the bending particle size of TiO2 and APTMPP-MPS-TiO2 over a broad vibration of the O–H bond of chemisorbed water, and the range was observed with the average size of 0.7 μm. No peak around 3400 cm−1 is due to the stretching mode of the −1 change has happened in the diameter of TiO2 after APTMPP O–H bond of free water. The IR band at 400–850 cm and bonding on it. 1400 cm−1corresponds to the Ti–O–Ti stretching vibration TheUV-visDRSspectraofAPTMPP-MPS-TiO2 and mode in crystal TiO2. Unlike the pure TiO2 [37], the pure TiO2 are shown in Figure 2. It is worth noting that modiﬁed TiO2 samples have a new broad IR band at about −1 no shift of the bandgap edge of TiO2 can be observed in 928 cm corresponding to the vibrations of Ti–O–Si bonds loaded sample. APTMPP-MPS-TiO2 exhibits typical por- [38]. The existence of Ti–O–Si bond veriﬁes the formation phyrin absorption features with a strong Soret band at of chemical bonding between titania and organic compo- 419 nm and moderate Q bands at 519, 552, 595, and 650 nm, nent. That at around 1000 cm−1 is assigned to symmetric the strong band appears at 419 nm arising from the transition and asymmetric Si–O–Si stretching vibrations in siloxane −1 of a1u(π)–eg (π), and the less intense bands in the 500– network. Some new bands between 1300 and 1400 cm 650 nm region corresponding to the a2u(π)–eg (π) transition attributed to the pyrrole C=N stretching also indicate that attributed to porphyrin [36] while there is no absorption the presence of porphyrins in the hybrid materials. International Journal of Photoenergy 5

0.5 100 DTG 70 •

0 50 90 40

Intensity 30 • Weight loss (%) Weight −0.5 • • 80 a 20 TG 10 b 70 −1 200 400 600 800 0 Temperature (◦C) 20 30 40 50 60 2θ/(◦) Figure 4: TG-DTG curve for APTMPP-MPS-TiO2. • Anatase

Figure 5: XRD spectra of TiO2 (a), APTMPP-MPS-TiO2 (b).

Figure 4 shows the TG and DTG for APTMPP-MPS-TiO2 sample. From the TG-DTG analysis of the APTMPP-MPS- ◦ TiO2, the weight of particles sharply decreases up to 450 C In order to analyze the chemical composition of the ◦ and slowly decreases from 450 to 800 C. TG-DTG analysis of APTMPP-MPS-TiO2, we performed X-ray photoelectron the sample also shows the endothermic peak at 100◦Cand spectroscopy (XPS) analyses. Figure 8 shows XPS survey ◦ two exothermic peaks at 290 and 450 C. It is thought that spectrum for the surface of APTMPP-MPS-TiO2 particles. the peak at 100◦C is due to free adsorbed water, and the The XPS spectrum reveals characteristic peaks from Ti, C, Si, ◦ peak at 290 C is due to the decomposition of the porphyrin S, N, and oxygen. Parameters of the XPS peak for Ti(2P3/2) and residual hydroxy group. In addition, the peak at 450◦C and O(1S) are listed in Table 1. The peaks of APTMPP- corresponds to the crystallization of the amorphous phase MPS-TiO2 are shifted to higher binding energies by 0.28 and ◦ into the anatase phase. Above 650 C, it can be assumed that 0.62 eV for Ti(2P3/2) and O(1S) than TiO2 [40], respectively. the product completely transforms into the anatase phase The increase of binding energy for Ti(2P3/2)andO(1S) because there is no change in particle weight. while APTMPP-MPS adsorbed on TiO2 indicates chemical From the X-ray diffraction patterns of the samples shown bonding at the interface of the silica coating layer and in Figure 5, we can see only that the anatase crystalline titania particle surface. Figure 9 shows the XPS spectra of phase was present in both samples. The major phase of the O(1S) peak for APTMPP-MPS-TiO2. The shape of a wide photocatalyst is pure anatase without brookite. No rutile and asymmetric peak of XPS O(1S) spectra indicated that peaks were observed for all titania. The peaks appearing at there would be more than one chemical state according 2θ = 25.3◦, 38.7◦, 47.6◦, and 54.8◦ are attributed to anatase to the binding energy. It includes crystal lattice oxygen − − − phase. It was confirmed that Ti(OC4H9)4 was oxidized (OTi O or OSi O) and chemisorbed oxygen (OO H). The into TiO2, and the formed TiO2 was mostly anatase by main contribution is attributed to Ti–O in TiO2, and the the calcination in the air. In addition, immobilization of other two kinds of oxygen contributions can be ascribed APTMPP-MPS on TiO2 has little effects on the interlayer to the –OH in Ti–OH and the Si–O in SiO2,respectively spacing. This demonstrates that the porphyrins grafted on [41, 42]. Usually, hydroxyl groups measured by XPS are TiO2 surface does not crystallize. ascribed to the chemisorbed water. As seen, the hydroxyl Figure 6 shows the nitrogen adsorption-desorption content increased in the sample. This is probably due to the curves measured by BET method and the pore-size distri- fact that the films easily adsorb water vapor in air, leading to bution calculated from desorption isotherms of APTMPP- the formation of hydroxyl on the films. This also corresponds . MPS-TiO2 prepared by sol-gel method. As seen, the isotherm to the results of previous TG and FTIR of the modified sample shows a combination of type I

[39]. At low relative pressure, the isotherm exhibits high 3.2. Photocatalytic Activity of APTMPP-MPS-TiO2 adsorption, indicating that as-prepared samples contain micropores (type I). While at high relative pressure from 0.6 3.2.1. α-Terpinene Photooxidation. The photocatalytic activ- to 1.0, the curve exhibits a little biger adsorption, indicating ities for the degradation of α-terpinene under visible light the bigger crystallites aggregation form bigger pores. The irradiation using prepared photocatalysts were tested. α- BET surface area and speciﬁc pore volume of the composite terpinene degradation occurred promptly in the visible 2 −1 3 −1 are 266.3 m g and 0.23 cm g . Figure 7 shows the pore irradiation under the adding of the APTMPP-MPS-TiO2 as size distribution curve of the corresponding products by the conﬁrmed by the UV-vis spectra changes of APTMPP-MPS- BJH method. It can be seen that the diameter of pore ranged TiO2 (Figure 10). The absorption spectrum of α-terpinene from 1.8 to 4.0 nm, and the largest pore diameter was 2.1 nm. in ethanol was characterized by the band of 265 nm in 6 International Journal of Photoenergy

100 Table 1: XPS data for APTCPP-MPS-TiO2. Orbit Binding energy (eV) FEHM Content (atom%) 80 Ti2p3/2 458.76 1.16 16.56

/g) C1s 284.82 1.87 27.99 3 60 O1s 530.28 1.38 49.41 Si2p 101.48 1.36 1.96 40 S2p 163.29 1.97 1.62 adsorbed (cm

2 N1s 401.94 1.51 6.46 N 20 FWHM: full width at a half of the maximum height of peaks. BE: binding energy.

0 0 0.2 0.4 0.6 0.8 1

Relative pressure (P/P0) O1s

Figure 6: Nitrogen adsorption isotherms of APTMPP-MPS-TiO2. a

0.8 b C/S

0.7

0.6

/g) c 3 0.5

0.4 528 530 532 534 0.3 Binding energy (eV)

Pore volume (cm volume Pore 0.2 (a) 529.9 eV O1s (Ti–O) 0.1 (b) 530.9 eV O1s (–OH) (c) 532.4 O1s (Si–O) 0 0 2 4 6 8 101214161820 Figure 9: XPS spectra of O1s peak for APTMPP-MPS-TiO2. Pore diameter (nm)

Figure 7: Pore diameter distribution of APTMPP-MPS-TiO2.

the UV-vis region. As observed in Figure 10, the intensity of the 265 nm absorption band decreased rapidly under visible light irradiation, indicating the degradation of α- terpinene in the presence of APTMPP-MPS-TiO2 micro- O1s spheres. For comparison, blank experiments, in which the photooxidation experiment of α-terpinene was performed in dark in the presence of APTMPP-MPS-TiO2 or irradiated with visible light with TiO2, were done and no obvious Ti2p

Ti2s catalytic results were observed. It believes that both visible light and APTMPP-MPS-TiO2 were indispensable to the Intensity (a.u.) Intensity photooxidation of α-terpinene.

C1s A tentative photocatalytic mechanism of APTMPP-MPS- TiO2 was deduced and shown in Scheme 2. When the visible N1s S2p Si2p light irradiates on the surface of APTMPP-MPS-TiO2, the porphyrin molecules adsorbed on the surface of TiO2 can be 1000 800 600 400 200 0 excited by visible light, and then the photoinduced electrons Binding energy (eV) inject into the conduction band of TiO2. Subsequently, the reactive electrons in the conduction band can reduce O2 Figure 8: XPS survey spectrum for the surface of APTMPP-MPS- − adsorbed on the surface of TiO2 to O2 , which can further TiO2. • transform into H2O2 and OH, resulting in the oxidation of α-terpinene. International Journal of Photoenergy 7

2.5 2.5 Irradiation time (min) Irradiation time (min) 0 0 2 10 2 10 20 20 30 30 1.5 40 1.5 40 50 50 60 60 1 1 Absorbance Absorbance

0.5 0.5

0 0 200 250 300 200 250 300 Wavelength (nm) Wavelength (nm) (a) (b) 2.5 Irradiation time (min)

2 0 10 20 1.5 30 40 50 1 60 Absorbance

0.5

0 200 250 300 Wavelength (nm) (c)

−4 Figure 10: The temporal UV-vis spectral changes of α-terpinene (3.7 × 10 M): (a) under visible irradiation with APTMPP-MPS-TiO2 (1.0 mg/mL), (b) under visible irradiation with TiO2 (1.0 mg/mL), (c) dark reaction with APTMPP-MPS-TiO2 (1.0 mg/mL).

+ H H2O2 • OH e− O•− 2 − e Porphyrin•+ − e CB O2 Porphyrin hA TiO2

Visible light VB

Scheme 2: Tentative mechanism of α-terpinene photooxidation on the surface of APTMPP-MPS-TiO2.

3.2.2. E ect of Catalyst Amount on the Photocatalytic Activity. constant above a certain level. This has been explained as The photocatalytic activities are also inﬂuenced by the cata- the concentration of the APTMPP-MPS-TiO2 increased; the lyst amount, as shown in Figure 11. It reveals that APTMPP- number of photons absorbed and the number of α-terpinene MPS-TiO2 hybrids have higher photocatalytic activity than molecules adsorbed increased with respect to an increase in that of only TiO2 particles and the photooxidation eﬃ- the number of APTMPP-MPS-TiO2 molecules. The density ciency increases with an increase in APTMPP-MPS-TiO2 of the molecule in the area of illumination also increases and concentration up to 2.0 mg/mL and then remains almost thus the rate gets enhanced. After a certain level, the dye 8 International Journal of Photoenergy

5 100

4 80

) 3

60 C / 0

40 ln(C 2 -terpinene (%) α

20 1

0 0 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Illumination time (min) Illumination time (min) (a) (b)

Figure 11: (a) Effect of catalyst amount on the photooxidation of α-terpinene (b)The pseudo-first-order photocatalysis rate constants of α-terpinene ( ) TiO2(1 mg/mL) and APTMPP-MPS-TiO2 (•) 0.5 mg/mL ( ) 1 mg/mL ( ) 1.5 mg/mL ( ) 2 mg/m. molecules available are not sufficient for adsorption by the metal-free porphyrin on titania sol particles. The metal- increased number of catalyst molecule. Hence, the additional free porphyrin can extend the light absorption of TiO2 catalyst powder is not involved in the photocatalytic activity into visible region and make α-terpinene be effectively and the rate does not increase with increase in the amount photooxidated under visible light. The degradation reaction of catalyst beyond certain limit. Replotting Figure 11(a) process of α-terpinene completely obeys the first-order law. in the ln(C0/Ct)−t coordinates (Figure 11(b)). It is found Furthermore, the photocataysts are stable, harmless, and they that α-terpinene’s photooxidation follows the pseudo-first- canbereusedwithhighefficiency. order kinetic in the lower initial APTMPP-MPS-TiO2 amount (0.5–1.5 mg/mL). However, when APTMPP-MPS- TiO2 amount was greater than 1.5 mg/mL, the photocatalytic Acknowledgments oxidation would deviate from the pseudo-first-order kinetic with respect to APTMPP-MPS-TiO2 amount. The authors are grateful to the supports of Guangzhou Municipality Science and Technology Bureau of China, the 3.2.3. Stability and Reusability of the Catalysts. For the assess- National Natural Science Foundation of China and National ment of catalyst stability and reusability, APTMPP-MPS- Key Foundation Research Development Project (973) Item of China (no. 2007 CB815306), the foundation for Doctors of TiO2 sample was used in the degradation of different solutions of α-terpinene in four consecutive experiments. Jianggangshan University (JZ10044), the Education Depart- Catalyst was recovered using centrifuge during sampling and ment Foundation of the Guangxi Zhuang Autonomous aftereachexperiment.TheUV-visandIRspectraaswellas Region of China (201012MS203), the Education Department the morphology of the recovered hybrid material did not Foundation of the Jiangxi Province (GJJ12472), and the show any substantial change compared to fresh material. Young Science Foundation of Guangxi Province of China All reaction solutions analyzed after catalytic reactions (0832090). did not show the characteristic soret band of porphyrins, which indicated that no APTMPP lost from TiO2. Using the recovered APTMPP-MPS-TiO2 as catalyst to catalyze References α-terpinene photooxidation in the same conditions, they [1] S. U. M. Khan, M. Al-Shahry, and W. B. Ingler, “Efficient retain their high catalytic activity after being recycled four photochemical water splitting by a chemically modified n- times. For example, the photooxidation percentages of α- TiO ,” Science, vol. 297, no. 5590, pp. 2243–2245, 2002. terpinene for four consecutive experiments are as follows: 2 [2]M.R.Hoffmann, S. T. Martin, W. Choi, and D. W. Bahne- 87, 84, 80, 79, and 81% in the same conditions (1 mg/mL mann, “Environmental applications of semiconductor photo- × −4 APTMPP-MPS-TiO2,3.7 10 M α-terpinene, 60 min). catalysis,” Chemical Reviews, vol. 95, no. 1, pp. 69–96, 1995. These results indicate that APTMPP-MPS-TiO has highly 2 [3] A. L. Linsebigler, G. Lu, and J. T. Yates, “Photocatalysis on chemical stability and is recoverable and reusable. TiO2 surfaces: principles, mechanisms, and selected results,” Chemical Reviews, vol. 95, no. 3, pp. 735–758, 1995. 4. Conclusions [4] D. C. Schmelling and K. A. Gray, “Photocatalytic transformation and mineralization of 2,4,6-trinitrotoluene (TNT) in In this work, novel APTMPP-MPS-TiO2 hybrids with high TiO2 slurries,” Water Research, vol. 29, no. 12, pp. 2651–2662, photocatalytic activity were synthesized by immobilizing 1995. International Journal of Photoenergy 9

[5] J. Hong, C. Sun, S. G. Yang, and Y. Z. Liu, “Photocatalytic [20] D. Li, W. Dong, S. Sun, Z. Shi, and S. Feng, “Photocatalytic degradation of methylene blue in TiO2 aqueous suspensions degradation of acid chrome blue K with porphyrin-sensitized using microwave powered electrodeless discharge lamps,” TiO2 under visible light,” Journal of Physical Chemistry C, vol. Journal of Hazardous Materials, vol. 133, no. 1–3, pp. 162–166, 112, no. 38, pp. 14878–14882, 2008. 2006. [21] S.-J. Moon, E. Baranoff, S. M. Zakeeruddin et al., “Enhanced [6] G. Zhanqi, Y. Shaogui, T. Na, and S. Cheng, “Microwave light harvesting in mesoporous TiO2/P3HT hybrid solar cells assisted rapid and complete degradation of atrazine using using a porphyrin dye,” Chemical Communications, vol. 47, no. TiO2 nanotube photocatalyst suspensions,” Journal of Haz- 29, pp. 8244–8246, 2011. ardous Materials, vol. 145, no. 3, pp. 424–430, 2007. [22]J.E.Kroeze,R.B.M.Koehorst,andT.J.Savenije,“Singlet [7] R. D. Barreto, K. A. Gray, and K. Anders, “Photocatalytic and triplet exciton diffusion in a self-organizing porphyrin degradation of methyl-tert-butyl ether in TiO2 slurries: a antenna layer,” Advanced Functional Materials, vol. 14, no. 10, proposed reaction scheme,” Water Research,vol.29,no.5,pp. pp. 992–998, 2004. 1243–1248, 1995. [23] M. Y. Duan, J. Li, G. Mele et al., “Photocatalytic activity ff [8] S. Yang, Y. Guo, N. Yan et al., “Remarkable e ect of the of novel tin porphyrin/TiO2 based composites,” Journal of incorporation of titanium on the catalytic activity and SO2 Physical Chemistry C, vol. 114, no. 17, pp. 7857–7862, 2010. poisoning resistance of magnetic Mn-Fe spinel for elemental [24] C.-L. Wang, Y.-C. Chang, C.-M. Lan, C.-F. Lo, E. W.-G. Diau, mercury capture,” Applied Catalysis B, vol. 101, no. 3-4, pp. and C.-Y. Lin, “Enhanced light harvesting with π-conjugated 698–708, 2011. cyclic aromatic hydrocarbons for porphyrin-sensitized solar [9] Y. Xie, Y. Li, and X. Zhao, “Low-temperature preparation and cells,” Energy and Environmental Science,vol.4,no.5,pp. visible-light-induced catalytic activity of anatase F-N-codoped 1788–1795, 2011. TiO2,” Journal of Molecular Catalysis A, vol. 277, no. 1-2, pp. [25] C. Wang, G. M. Yang, J. Li et al., “Novel meso-substituted 119–126, 2007. porphyrins: synthesis, characterization and photocatalytic [10] Y. H. Xu and Z. X. Zeng, “The preparation, characterization, activity of their TiO2-based composites,” Dyes and Pigments, and photocatalytic activities of Ce-TiO2/SiO2,” Journal of vol. 80, no. 3, pp. 321–328, 2009. Molecular Catalysis A, vol. 279, no. 1, pp. 77–81, 2008. [26] G. Mele, R. D. Sole, G. Vasapollo et al., “TRMC, XPS, and EPR [11] C. C. Yen, D. Y. Wang, M. H. Shih, L. S. Chang, and H. C. characterizations of polycrystalline TiO2 porphyrin impreg- Shih, “A combined experimental and theoretical analysis of Fe- nated powders and their catalytic activity for 4-nitrophenol implanted TiO2 modified by metal plasma ion implantation,” photodegradation in aqueous suspension,” Journal of Physical Applied Surface Science, vol. 256, no. 22, pp. 6865–6870, 2010. Chemistry B, vol. 109, no. 25, pp. 12347–12352, 2005. [12] A. Ghicov, J. M. Macak, H. Tsuchiya et al., “Ion implantation [27] C. Wang, J. Li, G. Mele et al., “Efficient degradation of 4- ffi and annealing for an e cient N-doping of TiO2 nanotubes,” nitrophenol by using functionalized porphyrin-TiO2 photo- Nano Letters, vol. 6, no. 5, pp. 1080–1082, 2006. catalysts under visible irradiation,” Applied Catalysis B, vol. 76, [13] J. K. Park, H. R. Lee, J. Chen, H. Shinokubo, A. Osuka, no. 3-4, pp. 218–226, 2007. andD.Kim,“Photoelectrochemicalpropertiesofdou- [28] G. Mele, R. Del Sole, G. Vasapollo, E. Garc´ıa-Lopez,´ L. bly β-functionalized porphyrin sensitizers for dye-sensitized Palmisano, and M. Schiavello, “Photocatalytic degradation of nanocrystalline-TiO2 solar cells,” Journal of Physical Chemistry 4-nitrophenol in aqueous suspension by using polycrystalline C, vol. 112, no. 42, pp. 16691–16699, 2008. TiO2 impregnated with functionalized Cu(II) -porphyrin or [14] H. Shen, L. Mi, P. Xu, W. Shen, and P. N. Wang, “Visible-light Cu(II)-phthalocyanine,” Journal of Catalysis, vol. 217, no. 2, photocatalysis of nitrogen-doped TiO2 nanoparticulate films pp. 334–342, 2003. prepared by low-energy ion implantation,” Applied Surface [29] J. H. Cai, J. W. Huang, P. Zhao, Y. J. Ye, H. C. Yu, and L. Science, vol. 253, no. 17, pp. 7024–7028, 2007. N. Ji, “Silica microspheres functionalized with porphyrin as [15] M. C. Neves, J. M. F. Nogueira, T. Trindade, M. H. Mendonça, a reusable and efficient catalyst for the photooxidation of 1,5- M. I. Pereira, and O. C. Monteiro, “Photosensitization of TiO2 dihydroxynaphthalene in aerated aqueous solution,” Journal of by Ag2S and its catalytic activity on phenol photodegradation,” Photochemistry and Photobiology A, vol. 207, no. 2-3, pp. 236– Journal of Photochemistry and Photobiology A, vol. 204, no. 2-3, 243, 2009. pp. 168–173, 2009. [30] J. W. Huang, W. J. Mei, J. Liu, and L. N. Ji, “The cataly- [16] A. Kathiravan and R. Renganathan, “Effect of anchoring group sis of some novel polystyrene-supported porphyrinatoman- on the photosensitization of colloidal TiO2 nanoparticles with ganese(III) in hydroxylation of cyclohexane with molecular porphyrins,” Journal of Colloid and Interface Science, vol. 331, oxygen,” Journal of Molecular Catalysis A, vol. 170, no. 1-2, pp. no. 2, pp. 401–407, 2009. 261–265, 2001. [17] O. Ozcan, F. Yukruk, E. U. Akkaya, and D. Uner, “Dye [31]B.Fu,H.C.Yu,J.W.Huang,P.Zhao,J.Liu,andL.N. sensitized artificial photosynthesis in the gas phase over thin Ji, “Mn(III) porphyrins immobilized on magnetic polymer and thick TiO2 films under UV and visible light irradiation,” nanospheres as biomimetic catalysts hydroxylating cyclohex- Applied Catalysis B, vol. 71, no. 3-4, pp. 291–297, 2007. ane with molecular oxygen,” Journal of Molecular Catalysis A, [18] D. Chatterjee and A. Mahata, “Visible light induced pho- vol. 298, no. 1-2, pp. 74–80, 2009. todegradation of organic pollutants on dye adsorbed TiO2 [32] J. H. Cai, J. W. Huang, P. Zhao, Y. J. Ye, H. C. Yu, and L. N. Ji, surface,” Journal of Photochemistry and Photobiology A, vol. “Silica-metalloporphyrins hybrid materials: preparation and 153, no. 1–3, pp. 199–204, 2002. catalysis to hydroxylate cyclohexane with molecular oxygen,” [19] D. Chen, D. Yang, J. Geng, J. Zhu, and Z. Jiang, “Improving Journal of Sol-Gel Science and Technology,vol.50,no.3,pp. visible-light photocatalytic activity of N-doped TiO2 nanopar- 430–436, 2009. ticles via sensitization by Zn porphyrin,” Applied Surface [33] J. Yan, X. Li, S. Cheng, Y. Ke, and X. Liang, “Facile synthesis of Science, vol. 255, no. 5, pp. 2879–2884, 2008. titania-zirconia monodisperse microspheres and application 10 International Journal of Photoenergy

for phosphopeptides enrichment,” Chemical Communications, no. 20, pp. 2929–2931, 2009. [34] J. H. Cai, J. W. Huang, P. Zhao, Y. H. Zhou, H. C. Yu, and L. N. Ji, “Photodegradation of 1,5-dihydroxynaphthalene catalyzed by meso-tetra(4-sulfonatophenyl)porphyrin in aerated aqueous solution,” Journal of Molecular Catalysis A, vol. 292, no. 1-2, pp. 49–53, 2008. [35] S. M. Ribeiro, A. C. Serra, and A. M. D. A. Rocha Gonsalves, “Immobilised porphyrins in monoterpene photooxidations,” Journal of Catalysis, vol. 256, no. 2, pp. 331–337, 2008. [36] E. F. Aboelfetoh and R. Pietschnig, “Preparation and catalytic performance of Al2O3, TiO2 and SiO2 supported vanadium based-catalysts for C-H activation,” Catalysis Letters, vol. 127, no. 1-2, pp. 83–94, 2009. [37] M. Zhang, L. Shi, S. Yuan, Y. Zhao, and J. Fang, “Synthesis and photocatalytic properties of highly stable and neutral TiO2/SiO2 hydrosol,” Journal of Colloid and Interface Science, vol. 330, no. 1, pp. 113–118, 2009. [38] M. Houmard, D. Riassetto, F. Roussel et al., “Morphology and natural wettability properties of sol-gel derived TiO2-SiO2 composite thin films,” Applied Surface Science, vol. 254, no. 5, pp. 1405–1414, 2007. [39] S. Zhan, D. Chen, X. Jiao, and Y. Song, “Mesoporous TiO2/SiO2 composite nanofibers with selective photocatalytic properties,” Chemical Communications, no. 20, pp. 2043–2045, 2007. [40]T.H.Meen,W.Water,W.R.Chen,S.M.Chao,L.W.Ji,andC. J. Huang, “Application of TiO2 nano-particles on the electrode of dye-sensitized solar cells,” Journal of Physics and Chemistry of Solids, vol. 70, no. 2, pp. 472–476, 2009. [41] J. Xu, Y. Ao, D. Fu, and C. Yuan, “A simple route for the preparation of Eu, N-codoped TiO2 nanoparticles with enhanced visible light-induced photocatalytic activity,” Journal of Colloid and Interface Science, vol. 328, no. 2, pp. 447–451, 2008. [42]J.G.Yu,H.G.Yu,B.Cheng,X.J.Zhao,J.C.Yu,and W. K. Ho, “The effect of calcination temperature on the surface microstructure and photocatalytic activity of TiO2 thin films prepared by liquid phase deposition,” Journal of Physical Chemistry B, vol. 107, no. 50, pp. 13871–13879, 2003. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 536194, 8 pages doi:10.1155/2012/536194

Research Article

Preparation and Characterization of (I2)n Sensitized Nanoporous TiO2 with Enhanced Photocatalytic Activity under Visible-Light Irradiation

Juzheng Zhang,1 Xin Liu,1 Shanmin Gao,1, 2 Quanwen Liu,1 Baibiao Huang,2 and Ying Dai2

1 School of Chemistry and Materials Science, Ludong University, Yantai 264025, China 2 State key labs of Crystal Materials, Shandong University, Jinan 250100, China

Correspondence should be addressed to Shanmin Gao, [email protected] and Baibiao Huang, [email protected]

Received 18 December 2011; Accepted 31 January 2012

Academic Editor: Xuxu Wang

Copyright © 2012 Juzheng Zhang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A yellow/brown powder of (I2)n sensitized nanoporous TiO2 was obtained via an hydrolysis with TiCl4 and iodine hydrosol as raw material. I2 nanoparticles in the hydrosol were used as seeds to initiate the nucleation of a precursory TiO2 shell. The hybridized jumbles were further calcinated at different temperatures. The structure, crystallinity, morphology, and other physical-chemical properties of the samples are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N2 adsorption- ff desorption isotherms measurements, and UV-vis di use reflectance spectroscopy (DRS). The formation mechanism of these (I2)n sensitized nanoporous TiO2 is discussed. Methylene blue solutions were used as model wastewater to evaluate the visible light photocatalytic activity of the samples. The results indicate that iodine can exist even in high-temperature calcination for iodine being encapsulated in the nanocavities inside TiO2. The degradation of methylene blue (MB) accorded with the first-order reaction model.

1. Introduction through intercalation of proper atoms into anatase form. For example, by doping carbon [11, 12] and nitrogen [13– Semiconductor-catalyzed photooxidation using solar light as 15] atoms into the TiO2 lattice, a new, weak absorption an energy source is one of the most efficient processes for is generated at lower energies and promising approach for a rapid and low-cost degradation of organic pollutants. The inducing effective Vis-absorption. excellent chemical stability, nontoxicity, and photoactivity of In addition to the narrow bandgap, the enhancement of TiO2 has attracted much attention for its potential use as surface area of the TiO2 is another important aspect for the a solid photocatalyst for environmental purification [1–3]. efficiency of the materials because the desired photocatalytic However, because of its large bandgap of 3.20 eV, only a small reactions occur at the interfaces of the catalyst-pollute solu- UV fraction of solar light, about 2-3%, can be utilized to tion [16, 17]. Mesoporous TiO2 with high specific surface generate electron-hole pairs. Hence, it is crucial to enhance area can provide abundant active sites to adsorb reactant the light harvesting of TiO2 in the visible region which molecules and facilitate the accessibility of reactants to the accounts for more than 43% of the total solar energy [4, 5]. catalysts [18, 19]. Light efficiency can also be enhanced due In general, two methods are utilized to enhance the to the increased specific surface area and multiple scattering light harvest of TiO2 in the visible region. One method [20]. is to sensitize TiO2 with color centers that have proper As a photosensitizer, element iodine not only has the energy levels for photoelectron transfer between the color absorption ability in the visible light region but also has low centers and TiO2. A cost-effective sensitization of TiO2 is toxicity and can be dissolved in many solvent. Therefore, typically achieved by adding nonmetals such as I2 [6–8] the elemental iodine can be used as a photosensitizer and and S [9, 10]. The other way is to form a doping level prepared visible light photocatalytic materials. Titanium(IV) 2 International Journal of Photoenergy alkoxides have been frequently studied as titanium precur- microscope (TEM, Hitachi model H800) using an acceler- sors, but their high cost and the need to handle organic ating voltage of 100 kV. Ultraviolet-visible (UV-vis) diffuse solvents are important drawbacks. Inorganic precursors are reflection spectra (DRS) were recorded in the range 200– preferable, and TiO2 powders have been often prepared by 800 nm at room temperature (Shimadzu UV-2550 UV-vis hydrolysis of TiCl4, generally in aqueous acidic solutions [21] spectrophotometer). Porous structure and BET surface area and in pure water [22, 23]. were characterized by an N2 adsorption-desorption isotherm Based on the above state-of-the-art designing strategies (ASAP-2020 Micromeritics Co.). Note that the samples were ◦ for TiO2-based photocatalysts, this paper reports the syn- degassed at 180 C prior to the BET measurements. The pore thesis of high surface area (I2)n sensitized nanoporous TiO2. volume and pore diameter distribution were derived from The I2 sensitized is confined preferentially in the surface/near desorption branches of the isotherms by the Barrett-Joyner- surface region of TiO2 structure in order to accommodate Halenda (BJH) model. The BET surface area was calculated hot carriers in the surface region. The I2 nanoparticles from the linear part of the Brunauer-Emmett-Teller (BET) in the I2-hydrosol serve as seeds for nucleation of small plot. TiO2 particles formed from the hydrolysis of the TiCl4. This template-free soft synthetic strategy allows the doping 2.3. Adsorption and Photocatalytic Activity Measurements. and pore formation to be accomplished simultaneously The adsorption capability and visible light photocatalytic during calcination, while the amount and position of doping activity of the (I ) sensitized porous TiO were evaluated elements can be readily controlled under mild conditions. 2 n 2 by measuring the adsorption and decomposition rate of methylene blue (MB) solution at room temperature. For 2. Experimental comparison, the same measurements were also performed on the commercial P25. For a typical absorption and photocatalytic experiment, a total of a 200 mg sample was 2.1. Materials and Sample Preparation. All chemicals used × −4 in this study were reagent grade (Tianjin Ruijinte Chemical added to 150 mL MB aqueous solution (5.0 10 mol/L) Reagent Corp.) without further purification. Water used was in a custom-made quartz reactor. The MB concentration deionized and doubly distilled. The (I ) sensitized porous was monitored by the UV-vis spectroscopy during the entire 2 n experiment. The solution was irradiated by visible light TiO2 samples were synthesized via the hydrolysis of TiCl4 in iodine hydrosol without using any templates or surfactants, supplied by a 300 W tungsten-arc lamp (Zhejiang Electric followed by calcination at elevated temperatures. Co. Ltd.) with a glass filter (transparent for λ>400 nm). After 5 minute intervals during adsorption and visible A typical procedure for preparing the samples is light illumination, about 3 mL aliquots were taken out and described as follows. First, 0.80 g iodine powders were centrifuged to remove the trace particles. The absorbance dissolved in 40 mL absolute ethanol and slowly added drop- ◦ of the centrifuged solution was measured in the range 500– wisely into 100 mL of distilled water at 60 Cwithcontinuous 800 nm using a UV-vis spectrophotometer (Shimadzu UV- stirring. The I2 hydrosol was formed after stirring for 2550). 0.5 h and then cooled to room temperature. 8 mL TiCl4 was carefully added into 40 mL-deionized water with gentle stirring in ice-water bath to avoid a drastic hydrolysis of TiCl4 3. Results and Discussions in water (designated solution A). Solution A was then added dropwise with vigorous stirring into 100 mL of I2 hydrosol at 3.1. Sample Structure. The porous nanostructures were room temperature, then the mixture solution was heated to prepared via the hydrolysis of TiCl4 in I2 hydrosol, followed 40◦C and keet for 2 h under closed conditions. After that, a by calcinated in air at elevated temperatures for 3 h. Powder 20% (w/w) solution of ammonia was added dropwise with X-ray diffraction (PXRD) is used to investigate the changes of vigorous stirring until pH = 7, then the mixed solution structure and crystallite sizes of the prepared porous samples was cooled down to room temperature and aged further to at different calcination temperatures. Figure 1 shows the precipitate growth. Finally, the precipitates were centrifuged, XRD patterns of the samples after being calcinated at various ◦ ◦ washed with distilled water three times, and dried in an oven temperatures from 200 C to 600 C. The XRD pattern ◦ at 50◦C. The precipitates obtained were calcined at different for the sample obtained at 200 C indicated the sample temperatures in air for 3 h. The samples after heat treatment predominantly amorphous titania besides trace amounts of ◦ ◦ ◦ were denoted as IT-T, where the second T indicates the anatase. The diffraction peaks at 2θ = 31.6 , 33.5 and 36.0 calcination temperatures (in ◦C). In addition, commercial should be the iodine (JCPDS no. 71-1370). As the calcination P25 (∼80% anatase and ∼20% rutile, Deguass) was used as a temperatures increase, the XRD results show the pure anatase reference. TiO2 phase (JCPDS file no. 21-172), indicating that the crystallization of the samples is achieved. In addition, as the calcination temperature increases, the peaks assigned 2.2. Characterization. The phases of the final products to anatase become sharper and more intense due to the were identified using X-ray diffractometer (XRD) (Rigaku formation of larger grains as summarized in Table 1.The D/max-2500VPC) with a Ni-filtered Cu Kα radiation at a average crystallite sizes of (I2)n sensitized nanoporous TiO2 scanning rate of 0.02◦ s−1 from 20◦ to 70◦. The morphology are 4.1, 7.2, 9.8, and 13.2 nm for IT-300, IT-400, IT-500, and of the samples was observed with transmission electron IT-600, respectively, which are estimated from the full width International Journal of Photoenergy 3

a Table 1: Physicochemical properties of IT-T samples from N2 sorption analysis and XRD results.

S Pore volume Average pore size Crystal size Sample BET (m2/g) (cm3/g) (nm) (nm) IT-200 508.04 1.287 9.863 IT-300 287.10 0.241 3.634 4.1 IT-400 247.64 0.180 16.350 7.2 IT-500 175.02 0.152 20.071 9.8 IT-600 163.71 0.148 26.851 13.2 a BET surface areas were calculated from the linear part of the BET plots. Total pore volumes were obtained from the volume of N2 adsorbed at P/P0 = 0.995 Average pore diameters were estimated using the desorption branch of the isotherm and the Battett-joyner-Halenda (BJH) formula. Crystal size was determined by XRD using Scherrer equation.

diameter). As a result, the interstitial voids among these primary nanoparticles constitute a short-range disordered nanoporous structure. We also notice that the size of the primary nanoparticles grows with the increasing calcination temperature. IT-600 Pore size and speciﬁc surface area of the as-prepared samples were characterized by nitrogen adsorption-desorption isotherm measurements. Figure 3 shows the results obtained IT-500 from N2 adsorption-desorption isotherm studies and analyzed by BET method for surface area and BJH pore

Intensity (a.u.) Intensity size distribution. The surface area and porosity have been IT-400 measured for all of the IT-T samples, and the results are given in Table 1. Calcination treatment showed a signiﬁcant IT-300 inﬂuence on the isotherm behavior of the samples. The VI-type isotherms for sample IT-200 and IV like Isotherm IT-200 for samples IT-300, IT-400, IT-500, and IT-600 shown in Figure 3(a) demonstrate typical characteristics of mesoporous structure in the materials [24]. From the isotherms, it 20 25 30 35 40 45 50 55 60 65 70 ◦ can be seen that the monolayer adsorption is complete when 2θ ( ) the relative pressure reaches 0.7 for sample IT-600. However,

Figure 1: XRD patterns of the corresponding (I2)n sensitized IT-T the monolayer adsorption for other samples is not complete samples after calcinated at 200◦C, 300◦C, 400◦C, 500◦C, and 600◦C. until reaching a relative pressure of 0.8 for sample IT-300 and 0.78 for IT-400. This indicates that sample IT-600 possesses much smaller pore size than other samples. Although mesopores contribution could be observed in Figure 3(b) at half maximum of the diffraction peak using the Scherrer for IT-600, overall pore volume measured is 0.148 cm3g−1, equation. indicating that the contribution from mesoporous is small. Figure 2 shows the TEM images of IT-T samples before Less mesoporosity in IT-600 may be attributed to the large and after being calcinated at different temperatures. The size of the particles. This result agrees with the XRD and average particle size of the precursor is estimated to be TEM results. 21 nm, as presented in Figure 2(a).Noporousstructure Table 1 summarizes the BET-specific surface area and was observed at this point. The wormhole-like irregular the apparent average pore size of our samples. The pore mesopores for (I2)n sensitized porous TiO2 can be observed volume and specific surface area decrease with increasing ◦ in Figures 2(c)–2(f). The sample obtained at 200 C shows calcination temperature. However, the samples still maintain the noncrystalline nature of amorphous (Figure 2(b)), but relatively high specific surface area and pore size even for there were porous structure. The amorphous structure is sample IT-600, which is calcined at 600◦C. Consequently, consistent with the XRD result, and the porous structure this may enhance catalytic activity, as demonstrated later in is caused by the I2 and H2O gas escape at heat treatment the measurement of photocatalytic activity on degradation process. It is the subsequent heat treatment that creates of methylene blue. the mesoporous structures. The apparent common feature of the four samples after heat treatment at 300◦C, 400◦C, ◦ ◦ 500 C and 600 C is that they all consist of pseudospheri- 3.2. Possible Formation Mechanism of (I2)n Sensitized Nanopo- cal TiO2 particles. Furthermore, each individual sphere is rous TiO2. Different from the conventional doping method, composed of a large number of much smaller and loosely in which the iodine (or its precursors) is added to the matrix packed TiO2 nanoparticles (about 4∼10 nanometers in of TiO2,weadoptacounterstrategytoaddprecursors 4 International Journal of Photoenergy

60 nm 100 nm 100 nm

(a) (b) (c)

50 nm 50 nm 50 nm

(d) (e) (f)

◦ ◦ Figure 2: TEM images of the precursor of (I2)n-sensitized nanoporous TiO2 (a), and IT-T calcined for 3 h at (b) 200 C; (c) 300 C; (d) 400◦C, (e) 500◦C and (f) 600◦C.

800 5 ) 1 − g ) 3 4 1 −

600 g 3 3 (cm

400 W 2 log d /

200 dV 1 Volume adsorbed (cm Volume

0 0 0 0.2 0.4 0.6 0.8 1 1 10 100 Relative pressure (P/P0) Pore diameter (nm)

◦ ◦ 200 C 500 C 200◦C 500◦C ◦ ◦ 300 C 600 C 300◦C 600◦C 400◦C 400◦C (a) (b)

Figure 3: N2 adsorption/desorption isotherms (a) and the corresponding BJH pore size distributions (b) of the as-prepared IT-T samples.

of TiO2 into the matrix of I2 nanoparticles formed in the the I2 nanoparticles [23]. These primary TiO2 precursors I2 hydrosol. A possible multistep mechanism, illustrated formed as a shell on the surface of I2 particles. In other in Figure 4, is proposed to interpret the formation of the words, the I2 particles appear as nucleation sites for TiO2 (I2)n sensitized nanoporous TiO2. When TiCl4 solution spheres via the hydrolysis of TiCl4.Atroomtemperature, was added to I2 hydrosol, the heat of the exothermic the hydrolysis of TiCl4 is relatively slow. When the starting reaction explosively generated the formation of orthotitanic solution was heated in an isothermal water bath of 40◦C acid (Ti(OH)4) to become a titania precursory layer on for 2 h and add ammonia solution, it became supersaturated International Journal of Photoenergy 5

TiCl4 TiO2 shell TiO2 particle

Hydrolysis Or Nucleation

I2 particles in I2-hydrosol Aggregation (a.u.) Absorbance

I2 smoke, water vapor

400◦C in air Crystallization 200 300 400 500 600 700 800 Wavelength (nm)

◦ 200◦C 500 C 300◦C 600◦C Figure 4: Proposed formation mechanism of (I2)n sensitized 400◦C P25 nanoporous TiO2. Figure 5: UV-vis diﬀuse reﬂectance spectra of IT-T catalysts at various calcination temperatures compared to P25.

and precipitated. After drying the precursor turn into (I2)n- confirmed by the color change of the samples during the heat H2TiO3. The subsequent calcinated of the precipitated at treatment. The color of the samples changed from puce, to elevated temperatures leading to a series of reactions among brown, slight brown and fawn, and finally to light gray when ◦ I2 and TiO2 precursors with the presence of O2.Webelieve the temperature increased from 200 to 600 C. that the (I2)n-sensitized nanoporous TiO2 is a product of the following synergetic processes, including (1) the I2 sublimes 3.4. Photocatalytic Activity on the Decomposition of Methy- and the thermal decomposition H2TiO3 resulting gaseous lene Blue. Methylene blue (MB) is a brightly colored products pulverize the TiO2 shell, leading to the formation blue cationic thiazine dye and is often used as a test of the primary pores. (2) Part of the I2 attaches to surface model pollutant in semiconductor photocatalysis. A blank of the TiO2 primary particles. The further oxidation of I2 experiment was also carried out. The pure MB solution is more or less prevented by the shell of primary TiO2 cannot decompose without catalyst. Figure 6(a) shows the nanoparticles [7]. (3) The amorphous TiO2 crystallizes into adsorption and photodegradation ability on MB solutions of anatase TiO2. our IT-T samples in comparison to P25. Prior to the turn-on of the light, the concentrations of MB in the presence of the 3.3. UV-Vis Di use Reflectance Spectra. Figure 5 collects the IT-T samples deplete much faster than P25. This is especially UV-Vis diffuse reflectance spectra of the as-synthesized seen for IT-200 sample; nearly 70% of the dye molecules were adsorbed within 30 min. We ascribe this effect to the samples, the P25, as well as the I2 hydrosol. In comparison much larger surface area of the IT-T samples than that of to the pure TiO2, the spectra of (I2)n sensitized nanoporous P25 as characterized and discussed above. For IT-T samples, TiO2 samples exhibit a strong broad absorption band between 400 and 750 nm, covering nearly the entire visible their adsorption capacity increases with the decrease of the range. This band is centered around 500 nm, which is the calcination temperatures. This can be credited to the reduced spectral range where iodine absorbs [7]. Apparently, the surface area. absorbance between 400 and 700 nm decreases for sample Furthermore, Figure 6(a) shows, under visible light prepared at higher temperatures. This can be ascribed to the illumination, the MB solutions containing the IT-T sam- additional loss of I2 by sublimation at higher temperatures. ples underwent significant degradation and became nearly But the sample which is obtained at 600◦C still has a strong transparent within 50 min. In contrast, the MB solutions absorption in the visible region. Nonetheless, the absorption containing P25 show very limited degradation. It should be in the visible-light region implies that the prepared (I2)n noted that IT-400 sample showed the highest photocatalytic sensitized nanoporous TiO2 samples can be activated by vis- oxidation on methylene blue. This is because the crystalline ible light, and that more photogenerated electrons and holes of IT-400 is well and has fewer defects, so the recombination can be created and they can participate in the photocatalytic of electron and hole is very low. In addition, the sample oxidation reactions. We believe that, with the increase of still has larger surface area, so IT-400 has good absorption the temperature during heat treatment, the concentration of capacity. iodine molecules on the surface of the IT samples decreased The photocatalytic activity of the powders can be quanti- due to sublimation of I2 at high temperature, which is also tatively evaluated by comparing the apparent reaction rate 6 International Journal of Photoenergy

Dark Visible light 1 4 )

0 0.8 C / C ) 0.6 0 C / 2 C

0.4 ln( Relative conc. ( conc. Relative

0.2 0

0 −60 −40 −200 204060 0 204060 Irradiation time (min) Irradiation time (min) Blank MB 500◦C ◦ 200◦C 600◦C 200◦C 500 C ◦ 300◦C P25 300◦C 600 C 400◦C 400◦C P25 (a) (b)

Figure 6: (a) The blank MB solution, photodegradation of methylene blue solutions by using IT-T and P25 under visible light in a neutral suspension. (b) The variation of normalized ln(C0/C) of MB concentration as a function of visible light irradiation time.

Visible hυ 100 e−

VB − 80 e (I2)n induced continuous − 60 e states CB h+ 40 Visible hυ −

Degradation yield e photo-excited electrons e− conducting electrons in CB 20 TiO2 (I2)n h+ conducting holes in VB Figure 8: Proposed visible light-driven photocatalytic mechanism 0 on the (I ) sensitized nanoporous TiO nanoparticle. 123456 2 n 2 Recycling number (times) Figure 7: Recycling test result by using the IT-400 sample. to follow first-order kinetics [27].Therateconstantfor the photodegradation of the MB using IT-400 is 21 times constants. It is well known that photocatalytic oxidation higher than that of P25. All together, we can conclude that of organic pollutants in aqueous suspension follows the IT-T samples exhibited a strong photocatalytic activity for Langmuir-Hinshelwood model [25], and it can be described decomposition of MB under visible light irradiation. as follows [26]: The cyclic stability test under the same conditions of IT-400 sample was also examined and shows excellent C0 photocatalytic activity in degrading MB solution, as shown ln = kapp × t,(1) Ct in Figure 7. No significant decrease in its photocatalytic oxidation activity is observed after being used 6 times, which where C is the concentration of MB aqueous at reaction time t makes it very promising for practical application. t, C0 is the initial MB concentration, kapp is apparent rate constant, and t is reaction time. The variations in ln(C0/C) as a function of irradiation time are given in Figure 6(b). 3.5. Proposed Mechanism for the Enhanced Photocatalytic Since all the curves can be fitted roughly to a straight Activity. The enhanced photocatalytic oxidation activity in line, the photocatalytic degradation reaction can be assumed IT-T samples is a product of several factors. (1) The International Journal of Photoenergy 7

sensitized of (I2)n introduces continuous states residing in activity of TiO2,” Angewandte Chemie, vol. 47, no. 9, pp. 1766– between the VB and CB of TiO2 [28]. As schematically 1769, 2008. elucidated in Figure 8, we believe these I-induced continuous [3] H. F. Cheng, B. B. Huang, P. Wang et al., “In situ ion states can either accept visible light-excited electrons from exchange synthesis of the novel Ag/AgBr/BiOBr hybrid with highly efficient decontamination of pollutants,” Chemical the VB of TiO2 and/or provide visible-light excited electrons from these I -induced states to the CB of TiO .(2)Dueto Communications, vol. 47, no. 25, pp. 7054–7056, 2011. 2 2 [4] H. P. Li, W. Zhang, and W. Pan, “Enhanced photocat- the nature of our preparing method, the sensitized I2 must alytic activity of electrospun TiO2 nanofibers with optimal be preferentially located in the surface region of the TiO2 anatase/rutile ratio,” Journal of the American Ceramic Society, nanoparticles. Consequently, the I-induced defective states vol. 94, no. 10, pp. 3184–3187, 2011. are preferentially concentrated in the surface region instead [5] X. N. Wang, B. B. Huang, Z. Y. Wang et al., “Synthesis of of the bulk of the TiO2, which facilitate the trap of hot carrier Anatase TiO2 tubular structures microcrystallites with a high in the surface region where the desired photodegradation percentage of {001} facets by a simple one-step hydrothermal reactions occur [6, 29]. (3) Because the photo-degradation template process,” Chemistry, vol. 16, no. 24, pp. 7106–7109, occurs at the surface of TiO2, the organic pollute molecules 2010. must be preconcentrated at the TiO2 surface in order to react [6] P. Xu, J. Lu, T. Xu, S. M. Gao, B. B. Huang, and Y. Dai, with the trap hot carriers and the reactive radicals. Thus, “I2-hydrosol-seeded growth of (I2)n-C- codoped meso/ nanoporous TiO2 for visible light-driven photocatalysis,” the high surface areas of the nanoporous TiO2 enhance the adsorption of methylene blue molecules onto the surface Journal of Physical Chemistry C, vol. 114, no. 20, pp. 9510– 9517, 2010. of TiO2 particles. This is evidenced by the pronounced [7] S. Usseglio, A. Damin, D. Scarano, S. Bordiga, A. Zecchina, and more and faster deletion of methylene blue molecules in C. Lamberti, “(I2)n encapsulation inside TiO2: a way to tune solution under darkness (Figure 6(a))incomparisontoP25 photoactivity in the visible region,” Journal of the American sample. In addition, large surface area can accommodate Chemical Society, vol. 129, no. 10, pp. 2822–2828, 2007. more surface-adsorbed water and hydroxyl groups that act [8] P. C. Deng, H. Z. Wang, B. W. Sun, L. X. Fan, and L. Li, as photoexcited hole traps and produce hydroxyl radicals for “Hydrothermal preparation and photocatalytic performance the degradation of organic pollute molecules [30]. of (I2)n Sensitized and I(V) Doped TiO2 Supported on SiO2,” Journal of Advanced Oxidation Technologies,vol.12,no.2,pp. 226–230, 2009. 4. Conclusions [9] P. Xu, T. Xu, J. Lu et al., “Visible-light-driven photocatalytic S- and C- codoped meso/nanoporous TiO2,” Energy and In summary, a unique template-free synthetic strategy is Environmental Science, vol. 3, no. 8, pp. 1128–1134, 2010. used to prepare (I2)n sensitized nanoporous TiO2 through [10] H. X. Li, X. Y. Zhang, Y. N. Huo, and J. Zhu, “Supercritical a hydrolysis-calcination process. I2 hydrosol is used as preparation of a highly active S-doped TiO2 photocatalyst for nucleation center and sensitizer, while the TiCl4 was used methylene blue mineralization,” Environmental Science and as the precursor for TiO2. The calcination temperature has Technology, vol. 41, no. 12, pp. 4410–4414, 2007. a crucial role in the amount of (I2)n sensitized nanoporous [11] S. Sakthivel and H. Kisch, “Daylight photocatalysis by Carbon- TiO2. The calcination enhanced the phase transformation of modified Titanium Dioxide,” Angewandte Chemie, vol. 42, no. the TiO2 powders form amorphous to anatase phases and 40, pp. 4908–4911, 2003. crystallization of anatase. The photocatalytic activity of the [12] G. S. Wu, T. Nishikawa, B. Ohtani, and A. C. Chen, “Synthesis as-prepared samples was higher than that of P25 for the and characterization of carbon-doped TiO2 nanostructures degradation of MB. The sample calcined at 400◦C shows with enhanced visible light response,” Chemistry of Materials, vol. 19, no. 18, pp. 4530–4537, 2007. the highest photocatalytic activity in the decomposition of [13] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, methylene blue under visible light due to the enhanced “Visible-light photocatalysis in nitrogen-doped titanium absorption in visible region and the large specific surface oxides,” Science, vol. 293, no. 5528, pp. 269–271, 2001. area. [14] J. Wang, D. N. Tafen, J. P.Lewis et al., “Origin of photocatalytic activity of Nitrogen-doped TiO2 nanobelts,” Journal of the Acknowledgments American Chemical Society, vol. 131, no. 34, pp. 12290–12297, 2009. S. Gao and B. Huang thank the National Basic Research [15]Q.Wang,C.C.Chen,W.H.Ma,H.Y.Zhu,andJ.C. Program of China (973 Program, no. 2007CB613302), Doc- Zhao, “Pivotal role of fluorine in tuning band structure and toral Foundation of Shandong Province (2007BS04040), and visible-light photocatalytic activity of nitrogen-doped TiO2,” Chemistry, vol. 15, no. 19, pp. 4765–4769, 2009. Project of Shandong Province Higher Educational Science [16] Z. Y. Liu, H. W. Bai, and D. Sun, “Facile fabrication of and Technology Program (J10LF02). hierarchical porous TiO2 hollow microspheres with high photocatalytic activity for water purification,” Applied Catalysis B, References vol. 104, no. 3–4, pp. 234–238, 2011. [17] Y. Ide, Y. Nakasato, and M. Ogawa, “Molecular recognitive [1] A. L. Linsebigler, G. Q Lu, and J. T. Yates, “Photocatalysis on photocatalysis driven by the selective adsorption on layered TiO2 surfaces: principles, mechanisms, and selected results,” titanates,” Journal of the American Chemical Society, vol. 132, Chemical Reviews, vol. 95, no. 3, pp. 735–758, 1995. no. 10, pp. 3601–3604, 2010. [2] J. Zhang, Q. Xu, Z. C. Feng, M. J. Li, and C. Li, “Importance [18] Y. Shiraishi, N. Saito, and T. Hirai, “Adsorption-driven pho- of the relationship between surface phases and photocatalytic tocatalytic activity of mesoporous titanium dioxide,” Journal 8 International Journal of Photoenergy

of the American Chemical Society, vol. 127, no. 37, pp. 12820– 12822, 2005. [19] P. Raveendran, M. Eswaramoorthy, U. Bindu et al., “Template- free formation of meso-structured anatase TiO2 with spherical morphology,” Journal of Physical Chemistry C, vol. 112, no. 50, pp. 20007–20011, 2008. [20]J.Fang,F.Wang,K.Qian,H.Bao,Z.Q.Jiang,andW.X. Huang, “Bifunctional N-doped mesoporous TiO2 photocatalysts,” Journal of Physical Chemistry C, vol. 112, no. 46, pp. 18150–18156, 2008. [21] C. A. Nolph, D. E. Sievers, S. Kaewgun et al., “Photocatalytic study of polymorphic titania synthesized by ambient condition sol process,” Catalysis Letters, vol. 117, no. 3–4, pp. 102– 106, 2007. [22] Y. Z. Li, Y. N. Fan, and Y. Chen, “A novel method for preparation of nanocrystalline rutile TiO2 powders by liquid hydrolysis of TiCl4,” Journal of Materials Chemistry, vol. 12, no. 5, pp. 1387–1390, 2002. [23] A. D. Paola, M. Bellardita, R. Ceccato, L. Palmisano, and F. Parrino, “Highly active photocatalytic TiO2 powders obtained by thermohydrolysis of TiCl4 in water,” Journal of Physical Chemistry C, vol. 113, no. 34, pp. 15166–15174, 2009. [24] K. S. W. Singh, D. H. Everett, R. A. W. Haul et al., “Reporting physisorption data for gas/solid systems,” Pure and Applied Chemistry, vol. 57, no. 4, pp. 603–619, 1985. [25] C.-H. Wu, H.-W. Chang, and J.-M. Chern, “Basic dye decomposition kinetics in a photocatalytic slurry reactor,” Journal of Hazardous Materials, vol. 137, no. 1, pp. 336–343, 2006. [26]J.G.Yu,G.H.Wang,B.Cheng,andM.H.Zhou,“Effects of hydrothermal temperature and time on the photocatalytic activity and microstructures of bimodal mesoporous TiO2 powders,” Applied Catalysis B, vol. 69, no. 3–4, pp. 171–180, 2007. [27] J. M. Wu and T. W. Zhang, “Photodegradation of rhodamine B in water assisted by titania films prepared through a novel procedure,” Journal of Photochemistry and Photobiology A, vol. 162, no. 1, pp. 171–177, 2004. [28] S. Tojo, T. Tachikawa, M. Fujitsuka, and T. Majima, “Iodine- doped TiO2 photocatalysts: correlation between band structure and mechanism,” Journal of Physical Chemistry C, vol. 112, no. 38, pp. 14948–14954, 2008. [29] M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. Bahnemann, “Environmental applications of semiconductor photocatalysis,” Chemical Reviews, vol. 95, no. 1, pp. 69–96, 1995. [30] V. Subramanian, E. E. Wolf, and P. V. Kamat, “Catalysis with TiO2/gold nanocomposites. Effect of metal particle size on the fermi level equilibration,” Journal of the American Chemical Society, vol. 126, no. 15, pp. 4943–4950, 2004. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 682138, 6 pages doi:10.1155/2012/682138

Research Article Facile Low-Temperature Synthesis of Carbon Nanotube/TiO2 Nanohybrids with Enhanced Visible-Light-Driven Photocatalytic Activity

Yunlong Xie, 1 Huanhuan Qian,1 Yijun Zhong,1 Hangming Guo,2 and Yong Hu1

1 Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China 2 Department of Chemistry, Jinhua College of Vocation and Technology, Jinhua 321007, China

Correspondence should be addressed to Yong Hu, [email protected]

Received 6 December 2011; Revised 18 January 2012; Accepted 19 January 2012

Academic Editor: Baibiao Huang

Copyright © 2012 Yunlong Xie et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

We demonstrate a facile and novel chemical precipitation strategy for the accurate coating of TiO2 nanoparticles on the surface of carbon nanotubes (CNTs) to form CNT/TiO2 nanohybrids, which only requires titanium sulfate and CNTs as starting materials and reacts in the alkaline solution at 60◦C for 6 h. Using this process, the as-prepared hybrid structures preserved the good dispersity and uniformity of initial CNTs. Furthermore, the CNT/TiO2 nanohybrids show a broad blue luminescence at 469 nm and exhibit signiﬁcantly enhanced photocatalytic activity for the degradation of rhodamine B (RhB) under visible-light irradiation, which is about 1.5 times greater than that of commercial Degussa P25 TiO2 nanoparticles. It is believed that this facile chemical precipitation strategy is scalable and its application can be extended to synthesize other CNT/oxide nanohybrids for various applications.

1. Introduction drawback, that is, an additional separation step is usually required to remove the adsorbent from the solution. More- Titanium oxide (TiO2), with a wide bandgap of 3.2 eV, is over, pure TiO2 nanoparticles often show lower removal a very promising material and has received considerable efficiencies owing to the problem of self-aggregation [11], attention both in fundamental and photocatalysis due to which causes serious fast diminishment of the active surface its natural abundance, nontoxicity, photostability, low-cost, area. To meet these requirements, TiO nanoparticles are physical and chemical stability, availability, and unique 2 frequently immobilized on carbonaceous materials [12–15]. electronic and optical properties [1–4]. Recent studies have Particularly, carbon nanotubes (CNTs) can act as scaffolds also revealed that TiO2 displays excellent photocatalytic activity toward the photodegradation of rhodamine-B (RhB) to anchor light harvesting assemblies, due to their unique under the irradiation of ultraviolet (UV) light. Due to electrical and electronic properties, wide electrochemical the poor utilization of solar energy and the short diffu- stability window, and high surface area [16–18]. By virtue of their advantageous structural features to extend the light- sion length of a photogenerated electron-hole pair, TiO2 nanoparticles show low quantum efficiency in photocatalytic response range and other distinctive features, such as facile reactions. Therefore, it is of great importance to extend the charge separation, and increased adsorption of pollutants photoresponse of the TiO2 to the visible region. Recently, [19–22], we design a unique hybrid structure by directly some extensive efforts to modify the electronic band struc- anchoring TiO2 nanoparticles onto CNTs for degradation tures of TiO2 for visible-light harvesting have been con- of RhB under visible-light irradiation. This novel strategy ducted, including coupling with secondary semiconductors for preparation of CNT/TiO2 nanohybrids only requires [5, 6], metal/nonmetal doping [7, 8], and photosensiti- titanium sulfate and CNTs as starting materials and reacts zation with dyes [9, 10]. Additionally, the application of in the alkaline solution at 60◦C for 6 h. As expected, the as- TiO2 nanoparticles for wastewater treatment has a major prepared CNT/TiO2 nanohybrids demonstrate the excellent 2 International Journal of Photoenergy photocatalytic activity toward the decomposition of RhB when exposed to visible-light irradiation. In addition, the 002 101 ∗ photoluminescence (PL) properties of the as-prepared TiO2 nanoparticles and CNT/TiO2 nanohybrids were also studied, which exhibit a broad blue luminescence at 469 nm.

2. Experimental 004 CNT/TiO2 200 105 100 Intensity (a.u.) Intensity ∗ Allreagentswereofanalyticalgrade,purchasedfromthe 204 Shanghai Chemical Reagent Factory, and used as received without further puriﬁcation. TiO2

2.1. Synthesis of CNT/TiO2 Nanohybrids and TiO2 Nanoparti- cles. CNTs with 40–60 nm in diameter were synthesized 10 20 30 40 50 60 70 following our previous method [23]. For a typical synthesis, 2θ (deg) 30 mg of as-prepared CNTs were dissolved in 40 mL of distilled water with the assistance of ultrasonication for ∗ CNTs 10 min, and the appropriate quantities of NaOH (0.1 M) TiO2 solution were added to adjust the pH value to 11.0. Then, Figure 1: XRD patterns of the as-prepared CNT/TiO2 nanohybrids 0.012 g of titanium sulfate was added, and the mixed solution and TiO2 nanoparticles. was continuously stirred at 60◦C for 6 h. Subsequently, the as-prepared products were washed repeatedly with distilled water and ethanol for several times and then dried in an oven ◦ at 60 C for 6 h. For comparison, the pure TiO2 nanoparticles of as-prepared CNTs/TiO2 nanohybrids as photocatalysts were obtained without addition of CNTs in the reaction were added into 100 mL of RhB solution in distilled water process, while the other conditions remain unchanged. (concentration: 5 mg/L), respectively. After being dispersed in an ultrasonic bath for 5 min, the solution was stirred for 2.2. Characterization. Powder X-ray diffraction (XRD) mea- 2 h in the dark to reach adsorption equilibrium between surements of the samples were performed with a Philips the catalyst and the solution and then was exposed to PW3040/60 X-ray diffractometer using CuK radiation at visible-light irradiation. The samples were collected by a scanning rate of 0.06◦s−1. Scanning electron microscopy centrifugation at given time intervals to measure the RhB (SEM) was performed with a Hitachi S-4800 scanning degradation concentration by UV-vis spectroscopy. electron microanalyzer with an accelerating voltage of 15 kV. Transmission electron microscopy (TEM) and high- resolution TEM (HRTEM) were conducted using a JEM- 3. Results and Discussion 2100F field emission TEM. Energy dispersive X-ray spec- The XRD patterns of the as-prepared CNT/TiO2 nanohy- trometry (EDS) was performed with a spectroscope attached brids and TiO2 nanoparticles are shown in Figure 1.From to HRTEM. Samples for TEM measurements were prepared the XRD patterns, the peaks at 2 values of 26.0◦, 43.0◦ for TEM by dispersing the products in ethanol and placing were associated with the (002) and (100) diffractions of the several drops of the suspension on holey carbon films hexagonal graphite structure, similar to pristine CNTs. The supported by copper grids. PL spectra were recorded on peaks at 2 values of 25.5◦, 37.8◦, 48.1◦, 54.0◦, and 62.8◦ can an Edinburgh FLSP920 fluorescence spectrometer and the be perfectly assigned to the crystal planes of the (101), (004), absorption spectra were measured using a PerkinElmer (200), (105), and (204) of the body-centered tetragonal TiO2 Lambda 900 UV-vis spectrophotometer at room temper- (JCPDS standard card no.89-4921) with a cell constant of a ff o ature. UV-vis di use reflectance spectra (UV-vis DRS) of = 0.3777 nm, and co = 0.9501 nm. In addition, the average the as-prepared samples and Degussa P25 TiO nanopar- 2 size of TiO2 nanoparticles on the surface of CNTs calculated ticles were recorded over the range of 200–800 nm in the using the Debye-Scherrer equation based on the full width at absorption mode using a Thermo Nicolet Evolution 500 UV- half-maximum of the diffractionpeakisabout3.5nm. vis spectrophotometer equipped with an integrating sphere The typical SEM and TEM images of the as-prepared attachment. CNT/TiO2 nanohybrids and TiO2 nanoparticles are shown in Figure 2. A panoramic view by SEM reveals that the 2.3. Photocatalytic Activity of CNT/TiO2 Nanohybrids. Pho- sample consists of sinuous and highly entangled CNTs tocatalytic activities of CNTs/TiO2 nanohybrids were eval- uniformly decorated with TiO2 nanoparticles on the entire uated by the degradation of RhB under visible-light irradi- surface (Figure 2(a)). The formation of CNT/TiO2 nanohy- ation of a 500 W Xe lamp (CEL-HXF300) with a 420 nm brids is further evidenced by TEM observation (Figure 2(b)), cutoff filter. The reaction cell was placed in a sealed black as characterized by radial assembly of high density TiO2 box with the top opened, and the cutoff filter was placed to nanoparticles with an average diameter of about 3.5 nm on provide visible-light irradiation. In a typical process, 0.05 g the long CNT surface. From these images, it can be seen International Journal of Photoenergy 3

Ti C

Cu CPS O Cu Ti Cu

0246810 Energy (KeV)

Figure 2: (a) Typical SEM image, (b) TEM image and EDS pattern (inset) of as-prepared CNT/TiO2 heterostructures. (c) SEM image of the as-prepared TiO2 nanoparticles.

that the as-prepared hybrid structures preserved the good fundamental process of electron/hole pair formation under dispersibility and uniformity of the initial CNTs, which may the visible-light illumination [24]. Thus, the photocatalytic endow the hybrids a high specific surface area and the appli- activity of as-prepared CNT/TiO2 nanohybrids could be cation potential as the photocatalyst for water treatment. improved. Figure 3(b) shows the PL spectra of the as- Additionally, the EDS pattern (inset in Figure 2(b)) of surface prepared TiO2 nanoparticles and CNT/TiO2 nanohybrids, layer of as-prepared CNT/TiO2 also confirms the existence which all feature a broad blue emission at 469 nm under of TiO2 nanoparticles on the CNTs. From SEM image an excitation wavelength of 320 nm. According to previ- (Figure 2(c)), the as-prepared TiO2 nanoparticles show some ous reports, the blue luminescence at 469 nm may result agglomerated forms. from the surface defects on the TiO2 nanoparticles [25]. To investigate the optical properties of the as-prepared The as-prepared CNT/TiO2 nanohybrids show obviously CNT/TiO2 heterostructures and their potential applica- diminished PL intensity as compared to TiO2 nanoparticles, tion as photonic materials, the UV-vis DRS spectrum indicating reduced charge recombination [21]. Therefore, (in Figure 3(a)) were recorded at room temperature. For CNTs in here have an important role to act as an electron comparison, the optical property of the as-prepared TiO2 reservoir to trap electrons emitted from TiO2 particles, which nanoparticles and Degussa P25 were also obtained at the result in hindering electron-hole pair recombination and same conditions. From Figure 3(a),TiO2 nanoparticles improving the photocatalytic activity of TiO2. and Degussa P25 show the characteristic spectra with Figure 4(a) displays the photodegradation behaviour of typical absorption sharp edge rising at 400 nm, which is RhB dye in the absence of any photocatalyst (the blank test), attributed to the band-band transition. However, CNT/TiO2 and in the presence of the as-prepared TiO2 nanoparticles, nanohybrids exhibit extended absorption range to the commercial Degussa P25 and CNT/TiO2 hybrids after expo- visible region. This result implies an increase of surface sure to visible light irradiation, where C is the concentration electric charge of the TiO2 in the composite catalysts due of RhB after different light irradiation times and C0 is the to CNTs introduction, resulting in modifications of the initial concentration of RhB before dark adsorption. About 4 International Journal of Photoenergy

0.8

CNT/TiO 0.6 2

0.4 Intensity (a.u.) Intensity Absorbance (a.u.) Absorbance 0.2 TiO2

0 P25 200 300 400 500 600 700 800 450 500 550 600 Wavelength (nm) Wavelength (nm)

TiO2 CNT/TiO2 (a) (b)

Figure 3: (a) UV-vis DRS spectra of the as-prepared TiO2 nanoparticle, CNT/TiO2 nanohybrids, and P25. (b) PL spectra of TiO2 nanoparticle and CNT/TiO2 nanohybrids.

1 100 Blank 0.8 80

0.6 60 0 C

/ TiO2 C 0.4 P25 40 Absorption Degradation (%) rate 0.2 20

CNT/TiO2 0 0 −10 0 10 20 30 40 50 60 70 123456 Time (min) Cycle number (a) (b)

Figure 4: (a) The photocatalytic degradation of RhB in the absence of any photocatalysts (the blank test) and in the presence of diﬀerent photocatalysts. (b) 6 cycles of degradation of RhB using CNT/TiO2 nanohybrids as the photocatalyst.

11.3% of the RhB is adsorbed for TiO2 nanoparticles and and loss during washing. Thus, the as-prepared CNT/TiO2 Degussa P25 after stirring for 2 h in the dark. However, nanohybrids used as photocatalyst are quite stable and have the as-prepared CNT/TiO2 nanohybrids adsorb about 34.6% great potential application in water treatment. of the RhB under the same condition. After exposure to The schematic diagram representing the charge trans- visible-light irradiation for 50 min the decolorization rates of fer process in CNT/TiO2 heterostructures is illustrated in TiO2 nanoparticles, Degussa P25, the CNT/TiO2 hybrids as Scheme 1. First, the dye of RhB as a sensitizer can absorb photocatalysts are 42.8%, 55.3%, 92.3%, indicating the sig- visible light to be excited. Then, the excited species can niﬁcantly enhanced photocatalytic activity of the CNT/TiO2 directly inject electrons into the conduction band or through nanohybrids. The reusability of the CNT/TiO2 nanohybrids CNT indirect inject electrons into the conduction band of as photocatalyst is also investigated by collecting and reusing the TiO2 semiconductor, forming conduction band electrons − the same photocatalyst for multiple cycles. As shown in (ecb) and the oxidized RhB to realize the charge separation Figure 4(b), after 6 runs of photodegradation of RhB, [9, 26]. This process extends the photoresponse of wide the photocatalytic activity of the CNT/TiO2 nanohybrids bandgap TiO2 semiconductors from the UV to the visible shows a slight deterioration due to incomplete recollection region and opens a unique route to utilize visible-light from International Journal of Photoenergy 5

e− References [1]M.R.Hoffmann, S. T. Martin, W. Choi, and D. W. Bahn- emann, “Environmental applications of semiconductor pho- ∗ hA − RhB − e tocatalysis,” Chemical Reviews, vol. 95, no. 1, pp. 69–96, 1995. e [2]T.L.ThompsonandJ.T.YatesJr.,“TiO2 photocatalysis: sur- O2 face defects, oxygen and charge transfer,” Topics in Catalysis, RhB vol. 35, no. 3-4, pp. 197–210, 2005. [3] A. Fujishima and K. Honda, “TiO2 photoelectrochemistry and TiO2 •− photocatalysis,” Nature, vol. 37, p. 238, 1972. O2 H2O [4] A. Fujishima, T. N. Rao, and D. A. Tryk, “Titanium dioxide photocatalysis,” Journal of Photochemistry and Photobiology C, HO vol. 1, no. 1, pp. 1–21, 2000. [5] Y. Yu, J. C. Yu, C. Y. Chan et al., “Enhancement of adsorption Scheme 1: Schematic diagram representing the charge-transfer and photocatalytic activity of TiO2 by using carbon nanotubes process in CNT/TiO2 nanohybrids. for the treatment of azo dye,” Applied Catalysis B, vol. 61, no. 1-2, pp. 1–11, 2005. [6] X. Liu, Y. Gu, and J. Huang, “Hierarchical, titania-coated, carbon nanofibrous material derived from a natural cellulosic the sun. Furthermore, the photogenerated electrons can be substance,” Chemistry A, vol. 16, no. 26, pp. 7730–7740, 2010. captured by the adsorbed oxygen molecules and holes can be [7] W. Choi, A. Termin, and M. R. Hoffmann, “The role of metal trapped by the surface hydroxyl, both resulting in formation ion dopants in quantum-sized TiO2: correlation between of high oxidative hydroxyl radical species (·OH) [27]. The photoreactivity and charge carrier recombination dynamics,” ·OH shows little selectivity for the attack of dye molecules Journal of Physical Chemistry, vol. 98, no. 51, pp. 13669–13679, and can oxidize the pollutants due to their high oxidative 1994. ability. Additionally, high surface area and strong adsorption [8] X. Chen and C. Burda, “The electronic origin of the visible- light absorption properties of C-, N- and S-doped TiO2 ability of CNTs will also help to enhance photocatalytic nanomaterials,” Journal of the American Chemical Society, vol. activity. Thus, the as-prepared hybrids exhibit the superior 130, no. 15, pp. 5018–5019, 2008. photocatalytic performance. [9] K. Woan, G. Pyrgiotakis, and W. Sigmund, “Photocatalytic carbon-nanotube-TiO2 composites,” Advanced Materials, vol. 21, no. 21, pp. 2233–2239, 2009. 4. Conclusions [10] E. Reisner, D. J. Powell, C. Cavazza, J. C. Fontecilla-Camps, and F. A. Armstrong, “Visible light-driven H production by In summary, we demonstrated a facile and novel low- 2 hydrogenases attached to dye-sensitized TiO2 nanoparticles,” temperature chemical precipitation route to synthesize Journal of the American Chemical Society, vol. 131, no. 51, pp. CNT/TiO2 nanohybrids. This method only requires titanium 18457–18466, 2009. sulfate and CNTs as starting materials and reacts in the [11]A.Rachel,M.Subrahmanyam,andP.Boule,“Comparison ◦ NaOH solution at 60 C for 6 h. With this method, the as- of photocatalytic efficiencies of TiO2 in suspended and prepared hybrids show good dispersity and uniformity of immobilised form for the photocatalytic degradation of initial CNTs. In addition, these nanohybrids exhibited a nitrobenzenesulfonic acids,” Applied Catalysis B, vol. 37, no. broad blue luminescence at 469 nm and a significantly higher 4, pp. 301–308, 2002. photocatalytic activity on the degradation of rhodamine B [12] Y. Hu, Y. Liu, H. Qian, Z. Li, and J. Chen, “Coating colloidal (RhB) in water, which was 1.5 times greater than that of the carbon spheres with CdS nanoparticles: microwave-assisted commercial Degussa P25 nanoparticles under visible-light synthesis and enhanced photocatalytic activity,” Langmuir, vol. 26, no. 23, pp. 18570–18575, 2010. irradiation. It is expected that this strategy can be scalable [13] Y. Guo, H. Wang, C. He, L. Qiu, and X. Cao, “Uniform carbon- and its application can be extended to synthesize other coated ZnO nanorods: microwave-assisted preparation, cyto- ff CNT/oxide nanostructures for di erent applications, and the toxicity, and photocatalytic activity,” Langmuir,vol.25,no.8, CNT/TiO2 nanohybrids should be ideal photocatalysts in the pp. 4678–4684, 2009. removal of dyes and other organic pollutants. [14] W. Chen, Z. L. Fan, B. Zhang et al., “Enhanced visible-light activity of Titania via confinement inside carbon nanotubes,” Journal of the American Chemical Society, vol. 133, no. 38, pp. Authors’ Contribution 14896–14899, 2011. [15] H. H. Qian, Y. Hu, Y. Liu, M. J. Zhou, and C. F. Guo, Y. Xie and H. Qian contributed equally to this work. “Electrostatic self-assembly of TiO2 nanoparticles onto carbon spheres with enhanced adsorption capability for Cr(VI),” Materials Letters, vol. 68, pp. 174–177, 2012. Acknowledgments [16] I. Robel, B. A. Bunker, and P. V. Kamat, “Single-walled carbon nanotube-CdS nanocomposites as light-harvesting assem- Financial support from Natural Science Foundation of China blies: photoinduced charge-transfer interactions,” Advanced (21171146), Zhejiang Provincial Natural Science Foundation Materials, vol. 17, no. 20, pp. 2458–2463, 2005. of China (Y4110304, Y4080532), and Zhejiang Qianjiang [17] P. V. Kamat, “Harvesting photons with carbon nanotubes,” Talent Project (2010R10025) is gratefully acknowledged. Nano Today, vol. 1, no. 4, pp. 20–27, 2006. 6 International Journal of Photoenergy

[18] A. Kongkanand and P. V. Kamat, “Electron storage in single wall carbon nanotubes. Fermi level equilibration in semiconductor-SWCNT suspensions,” ACS nano, vol. 1, no. 1, pp. 13–21, 2007. [19] Y. Liu, L. Zhou, Y. Hu et al., “Magnetic-field induced formationof1DFe3O4/C/CdS coaxial nanochains as highly efficient and reusable photocatalysts for water treatment,” Journal of Materials Chemistry, vol. 21, pp. 18359–18364, 2011. [20] S. Barazzouk, S. Hotchandani, K. Vinodgopal, and P. V. Kamat, “Single-wall carbon nanotube films for photocurrent generation. A prompt response to visible-light irradiation,” Journal of Physical Chemistry B, vol. 108, no. 44, pp. 17015– 17018, 2004. [21] Y. J. Xu, Y. Zhuang, and X. Fu, “New insight for enhanced photocatalytic activity of TiO2 by doping carbon nanotubes: a case study on degradation of benzene and methyl orange,” Journal of Physical Chemistry C, vol. 114, no. 6, pp. 2669–2676, 2010. [22] P. Zabek, J. Eberl, and H. Kisch, “On the origin of visible light activity in carbon-modified titania,” Photochemical and Photobiological Sciences, vol. 8, no. 2, pp. 264–269, 2009. [23]Y.Hu,T.Mei,L.Wang,andH.Qian,“Afacileandgeneric strategy to synthesize large-scale carbon nanotubes,” Journal of Nanomaterials, vol. 2010, Article ID 415940, 2010. [24] W. Wang, P. Serp, P. Kalck, and J. L. Faria, “Visible light photodegradation of phenol on MWNT-TiO2 composite catalysts prepared by a modified sol-gel method,” Journal of Molecular Catalysis A, vol. 235, no. 1-2, pp. 194–199, 2005. [25] L. D. Zhang and C. M. Mo, “Luminescence in nanostructured materials,” Nanostructured Materials, vol. 6, no. 5–8, pp. 831– 834, 1995. [26] C. Chen, W. Ma, and J. Zhao, “Semiconductor-mediated photodegradation of pollutants under visible-light irradiation,” Chemical Society Reviews, vol. 39, no. 11, pp. 4206–4219, 2010. [27] W. Lu, G. Liu, S. Gao, S. Xing, and J. Wang, “Tyrosine-assisted preparation of Ag/ZnO nanocomposites with enhanced photocatalytic performance and synergistic antibacterial activities,” Nanotechnology, vol. 19, no. 44, Article ID 445711, 2008. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 139739, 9 pages doi:10.1155/2012/139739

Research Article Preparation, Characterization, and Activity Evaluation of CuO/F-TiO2 Photocatalyst

Zhang Jinfeng,1 Yang Yunguang,2 and Liu Wei2

1 Department of Physics, Huaibei Normal University, Anhui, Huaibei 235000, China 2 Department of Chemistry, Huaibei Normal University, Anhui, Huaibei 235000, China

Correspondence should be addressed to Liu Wei, [email protected]

Received 21 November 2011; Accepted 31 December 2011

Academic Editor: Baibiao Huang

Copyright © 2012 Zhang Jinfeng et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

CuO/F-TiO2 nanoparticle photocatalyst was prepared by ball milling. The photocatalyst was characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, UV-Vis diffuse reflectance spectroscopy, and photoluminescence emission spectroscopy. The photocatalytic activity was evaluated by photocatalytic oxidation of rhodamine B 2− and reduction of Cr2O7 . The results showed that, for F-TiO2 photocatalyst, the photooxidation activity increases remarkably with the increasing amount of NH4F up to 1.0 g, and the photoreduction activity decreases gradually with the increase in the amounts of NH4F. For the CuO/F-TiO2 photocatalyst, the photoreduction activity increases greatly with the increase in the amount of doped p-CuO up to 1.0 wt.%, and the photooxidation activity decreases rapidly with the increase in the amounts of doped p-CuO. Compared with pure TiO2, the photoabsorption wavelength range of the CuO/F-TiO2 and F-TiO2 photocatalysts red shifts and improves the utilization of the total spectrum. The effect of ball milling time on the photocatalytic activity of the photocatalysts was also investigated. The mechanisms of influence on the photocatalytic activity of the photocatalysts were also discussed.

1. Introduction Recently, fluorinated TiO2 (F-TiO2) had been investigated extensively. The results showed that the photocatalytic Since Fujishima and Honda discovered the photocatalytic oxidation activity of the F-TiO2 is much higher than that splitting of water on titanium dioxide (TiO2) electrodes in of TiO2 by reason of the extension of the photoabsorption 1972 [1], TiO2 as a photocatalyst has been extensively stud- wavelength as a result of doped F element [25–27]. The p-n ied because it has relatively high photocatalytic activity, bio- junction photocatalysts NiO/TiO and p-ZnO/n-TiO have logical and chemical stability, low cost, nontoxicity, and long- 2 2 been studied in our laboratory [21, 28]. The results showed term stability against photocorrosion and chemical corrosion that, compared with pure TiO2, the p-n junction photocata- [2–9]. However, the photocatalytic activity of TiO2 is limited to irradiation wavelengths in the UV region, thus the effective lysts have higher photocatalytic reduction activity, but lower utilization of solar energy is limited to about 3–5% of the photocatalytic oxidation activity. It is known that CuO is a p- total solar spectrum. Some problems still remain to be solv- type semiconductor [29]. When p-type CuO and fluorinated ed in its application, such as the fast recombination of photo- n-type TiO2 integrates, a p-n heterojunction photocatalyst generated electron-hole pairs. Therefore, improving photo- CuO/F-TiO2 will be formed, which may improve charge se- catalytic activity by modification has become a hot topic paration and photocatalytic activity of the photocatalyst. among the researchers in the near decade [10, 11]. Many in- In this paper, CuO/F-TiO2 powder was prepared by ball vestigators have quested for various methods, such as doping milling using NH4F solution as a disperser. The photo- transition metals [12–15], doping nonmetallic elements [16– catalytic activity of the photocatalyst was evaluated by pho- 20], and forming composite photocatalysts from different tocatalytic oxidation of rhodamine B (RhB) and reduction 2− semiconductors [21–24], and so forth, to enhance the photo- of Cr2O7 . The desired result was obtained. The effect catalytic activity of TiO2 and to improve the utilization of of ball milling time on the photocatalytic activity of the visible light. photocatalysts was also investigated. The mechanisms of 2 International Journal of Photoenergy

2− −4 −5 influence on the photocatalytic activity of the CuO/F- Cr2O7 and RhB were 1.0 × 10 and 1.0 × 10 mol/L, re- TiO2photocatalyst were also discussed. spectively. In order to disperse the photocatalyst powder, the suspensions were ultrasonically vibrated for 10 or 20 min prior to irradiation. After illumination, the samples taken 2. Experimental from the reaction suspension were centrifuged at 7000 rpm for 20 min and filtered through a 0.2 μmmilliporefiltertore- 2.1. Materials. The TiO2 powder (Anatase 90%, Rutile 10%, with crystallite size of about 50–60 nm) used in the exper- move the particles. The filtrate was then analyzed. In order iments was supplied by Sinopharm Chemical Reagent Co, to determine the reproducibility of the results, at least dup- licated runs were carried out for each condition for averaging Ltd. Cu(NO3)2 · 3H2O was supplied by Shanghai Reagent Factory (purity 99.9%). NH F, Rhodamine B, and other the results, and the experimental error was found to be 4 ± chemicals used in the experiments were purchased from within 4%. Shanghai and other China Chemical Reagent Ltd. They were of analytically pure grade. Deionized water was used 2.4. Characterization. In order to determine the crystal phase throughout this study. composition and the crystallite size of the photocatalysts, X-ray diffractionmeasurementwascarriedoutatroom ff 2.2. Preparation of F-TiO and CuO/F-TiO . Nanoparticle temperature using a DX-2000 X-ray powder di ractometer 2 2 with Cu Kα radiation and a scanning speed of 3◦/min. The CuO was synthesized by heat treatment of Cu(NO3)2 · 3H2O at 500◦C for 6 h in air, and the crystallite size is about 80 nm. accelerating voltage and emission current were 40 kV and 30 mA, respectively. The crystallite size was calculated by X- The preparation of F-TiO2 photocatalyst was carried out in a ND2-2L ball mill (made in Tianzun Electronics Co, Ltd., ray line broadening analysis using the Scherrer equation. Nanjing University). The procedures for the preparation of The microcrystalline structure and surface characteristics of the photocatalysts were also investigated by using (X-650 F-TiO2 are as follows: 5.0 g TiO2 powder and three different sizes of zirconia balls were mixed in the zirconia tank, Japan) scanning electron microscope. and then a certain amount of NH F (0 g, 0.2 g, 0.5 g, 1.0 g, Transmission electron microscopy and high-resolution 4 transmission electron microscopy (HR-TEM) images were 1.5 g, 2.0 g, 2.5 g) and H2O (5 mL) was added. After being milled for a certain time (0–24 h) at the speed of 550 rpm, performed with a JEOL-2010 transmission electron micro- ◦ scope, using an accelerating voltage of 200 kV. the wet powder was dried at 110 C in air. The final sam- ff ples were used for the determination of photocatalytic ac- UV-Vis di use reflectance spectra measurements were carried out using a Hitachi UV-365 spectrophotometer tivity and characterization. CuO/F-TiO2 photocatalyst was prepared in the same procedure. For each sample, 1.0 g equipped with an integrating sphere attachment. The analysis range was from 250 to 650 nm, and BaSO4 wasusedasa NH4F, 5.0 g TiO 2,and5.0mLH2O were added in a zirconia tank, varying the weight ratio of CuO(0 wt.%, 0.1 wt.%, reflectance standard. 0.3 wt.%, 0.5 wt.%, 1.0 wt.%, 3.0 wt.%, and 5.0 wt.%), and Photoluminescence emission spectra were recorded on a JASCO FP-6500 type fluorescence spectrophotometer over a then different CuO/F-TiO2 powder samples were prepared, respectively. The specific surface area of the different CuO/F- wavelength range of 360–500 nm. TiO photocatalysts has no obvious change and it is about 2 − 2 2 24.7 m /g. The flow chart of preparation of the CuO/F-TiO2 2.5. Analysis. The concentration of Cr2O7 in the solution is given in Scheme 1. is determined spectrophotometrically using diphenylcar- bazide reagent as a developer. The concentration of rhodamine B (RhB) in the solution is determined spectrophoto- 2.3. Photoreaction Apparatus and Procedure. Experiments − metrically. The photoreduction efficiency of Cr O 2 and the were carried out in a photoreaction apparatus [21, 28]. The 2 7 photooxidation efficiency of rhodamine B were calculated photoreaction apparatus consists of two parts. The first part from the following expression: is an annular quartz tube. A 375 W medium pressure mercury lamp (Institute of Electric Light Source, Beijing), with − = (C0 Ct) × a maximum emission at about 365 nm, was used as light η 100%, (1) C0 sources. The lamp is laid in the empty chamber of the annular tube, and running water passes through an inner thim- where η is the photocatalytic efficiency; C0 is the concentra- ble of the annular tube. Owing to continuous cooling, the tion of reactant before illumination; Ct is the concentration temperature of the reaction solution is maintained at approx- of reactant after illumination time t. imately 30◦C. The second part is an unsealed beaker with a diameter of 12 cm. At the start of the experiment, the re- 3. Results and Discussion action solution (volume, 300 mL) containing reactants and photocatalyst was put in the unsealed beakers, and a mag- 3.1. Characterization of Photocatalysts netic stirring device was used to stir the reaction solution. The distance between the light source and the surface of the 3.1.1. XRD Analysis. The fixed ball milling time is 6 h. reaction solution is 11 cm. In the experiments, the initial pH The XRD patterns of different photocatalysts are shown in of the reaction solution was 5.0, the amount of the photo- Figure 1. It is clear that, when the amount of doped CuO is catalyst used was 2.0 g/L, and the initial concentrations of less than 5.0 wt.%, the diffraction peaks of CuO cannot be International Journal of Photoenergy 3

Cu(NO3)2·3H2O Calcination (a certain amount) 500 C, 6 h

TiO2 NH F H2O 4 CuO (5 g) (1 g) (5 mL)

Ball milling (550 rpm)

Dry (110 C)

CuO(x wt.%)/F-TiO2

Scheme 1: The ﬂow chart of preparation of the CuO/F-TiO2.

40000

35000 (111) (-111) 30000 (-202) CuO g 25000

20000 TiO2 (101) (200)

(004) f 15000 (200) Intensity (C.P.S.) Intensity (004) (101) 10000 (-111) e d 5000 c b 0 a

20 30 40 50 60 70 80 2θ (deg)

TiO2 CuO

Figure 1: XRD patterns of diﬀerent photocatalysts. (a) F-TiO2, (b) CuO (0.1 wt.%)/F-TiO2, (c) CuO (1.0 wt.%)/F-TiO2, (d) CuO (5.0 wt.%)/F-TiO2, (e) CuO (10.0 wt.%)/F-TiO2,(f)TiO2, and (g) CuO.

found in XRD patterns. This illustrates that CuO is highly 3.1.2. SEM Analysis. SEM was used to investigate the mor- dispersed in the bulk phase of the catalyst. When the amount phology of the samples. Figure 2 shows SEM images of CuO of doped CuO is higher than 5.0 wt.%, the diﬀraction peaks (1.0 wt.%)/F-TiO2 photocatalysts ball milled for 6 and 24 h, of CuO can be found in XRD patterns. Since no new crystal respectively. It can be seen that the appearance is shapeless phases are found, it can be concluded that a new solid is not sheet, and the average diameter of the photocatalyst is about formed in the ball milling process of TiO2,NH4F, and CuO, 50–60 nm. The result is the same as that of XRD. From or probably because the new materials content are too low Figure 2, it also can be seen that, when the ball milling time to allow detection of their reﬂection peaks. The similar result is 6 h, the dispersion degree of the sample is higher than was reported in [30]. It is known by the calculation from the that of the sample ball milled for 24 h. Namely, when the Scherrer equation that the diameter of the photocatalyst is ball milling time is longer than the optimum time, with the not obviously changed. The crystallite size is about 50 nm. increase in the ball milling time, the fresh surfaces formed by 4 International Journal of Photoenergy

(a) (b)

Figure 2: SEM images of CuO (1.0 wt.%)/F-TiO2 photocatalysts. (a) CuO (1.0 wt.%)/F-TiO2, ball milling time 6 h, (b) CuO (1.0 wt.%)/F- TiO2, ball milling time 24 h.

(a) (b)

Figure 3: TEM and HR-TEM images of CuO (1.0 wt.%)/F-TiO2photocatalyst: (a) TEM image, (b) HR-TEM image.

high-energy ball milling possess high surface energy and heterojunction will be formed by ball milling between CuO prefer to agglomerate. The similar result was reported in and TiO2. [30]. 3.1.4. UV-Vis Analysis. Figure 4 shows the UV-Vis diﬀuse 3.1.3. TEM Analysis. In order to investigate the interface of reﬂectancespectraofF-TiO2 and a series of CuO/F-TiO2 the sample, the CuO (1.0 wt.%)/F-TiO2 was chosen for TEM photocatalysts. The samples were ball milled for 6 h, respec- and high-resolution TEM characterization. tively. It is known that the bandgap of TiO2 is about 3.2 eV Figure 3(a) gives an overview of the typical TEM image of and it can be excited by photons with wavelengths below the CuO (1.0 wt.%)/F-TiO2 photocatalyst. It clearly exhibits 387 nm. From Figure 4, it can be seen that, compared with the existence of CuO nanoparticles with mean sizes of about pure TiO2, the absorption wavelength ranges of the F-TiO2 50–70 nm dispersing over the particle of TiO2. Figure 3(b) and CuO/F-TiO2 are extended greatly towards visible light. shows the HR-TEM image of the sample corresponding to Compared with the F-TiO2, the absorption edge of the the rectangle region of the TEM image in Figure 3(a).The photocatalyst CuO/F-TiO2 extends a little to longer wave- upper part depicts the (101) plane of TiO2 with a spacing length, revealing the good contact between CuO and F- value of 3.510 nm. The lower part depicts the (−111) plane of TiO2crystallites as a consequence of the interdispersion of CuO with a spacing of 2.520 nm. The good crystalline quali- the two phases produced by ball milling process. For F-TiO2, ty and the clear interface between CuO and TiO2 are ad- the presence of a strong absorption band at a low wave- vantageous for the separation of the photogenerated charge length in the spectra near 350 nm indicates that the Ti carriers. Based on the above results, it is suggested that the species are tetrahedral Ti4+.Thisabsorptionbandisgenerally International Journal of Photoenergy 5

In the experimental condition, the results of UV-Vis diﬀuse 100 reﬂectance spectra are consistent with the evaluation of photoreduction activity. There were similar results in previous 80 reports [28].

3.1.5. Photoluminescence Emission Spectra. The photolumi- 60 nescence emission spectra have been widely used to investi- ffi f gate the e ciency of charge carrier trapping, immigration, 40 e and transfer and to understand the fate of electron-hole d pairs in semiconductor particles, since photoluminescence Absorbance (a.u.) Absorbance c emission is resulted from the recombination of free carriers 20 [31, 32]. In this study, using an ultraviolet light with a 260 nm b wavelength as the excitation source, the fluorescence emis- a 0 sion spectra of the different samples are shown in Figure 5.It can be seen that the samples have a stronger emission peak 300 350 400 450 500 550 600 650 700 at around 410 nm and a weaker emission peak at around Wavelength (nm) 470 nm [21, 33]. ff ff From Figure 5, it is clear that the relative intensity of Figure 4: UV-Vis di use reflectance spectra of di erent the emission spectra of TiO has the greatest relative inten- photocatalysts. (a) TiO ,(b)F-TiO, (c) CuO(0.1 wt.%)/F- 2 2 2 sity, which means that electrons and holes of TiO are easy to TiO , (d) CuO(0.3 wt.%)/F-TiO , (e) CuO(0.5 wt.%)/F-TiO ,(f) 2 2 2 2 recombine. The relative intensity of the photocatalyst CuO/ CuO(1 wt.%)/F-TiO2. F-TiO2is lower than that of TiO2, showing that doping CuO is helpful to inhibit the recombination of electrons and holes 200 and to improve the photocatalytic activity. The amount of CuO can influence the thickness of the superficial space- 180 a charge layer of TiO2. When the amount of CuO is 1.0 wt %, 160 b the relative intensity of emission spectra is the lowest, which shows that a 1.0 wt.% doping quantity of CuO can effectively 140 c d restrain the recombination of photoexcited electrons and 120 holes. When the CuO content is too small, due to the absence e of adequate traps, the recombination rate of electron-hole f 100 pairs is higher. When the doping quantity is considerably 80 high, the absorption of light and the generation of electrons-

PL intensity (counts) holes are both decreased. 60

40 3.2. Evaluation of the Photocatalytic Activity

20 3.2.1. Effect of the Amount of NH4F on the Photocatalytic 340 360 380 400 420 440 460 480 Activity of F-TiO2. The photocatalytic activity was investi- Wavelength (nm) gated by photocatalytic oxidation of RhB and photocatalytic 2− reduction of Cr2O7 . The dark absorption and blank tests Figure 5: Photoluminescence emission spectra of photocatalysts. are carried out simultaneously. The dark absorption test de- (a) TiO2, (b) CuO (0.1 wt.%)/F-TiO2, (c) CuO (0.3 wt.%)/F-TiO2, monstrated that the adsorption on the catalysts varied de- (d) CuO (0.5 wt.%)/F-TiO2, (e) CuO (3 wt.%)/F-TiO2,(f)CuO pending on the samples, while the blank experiment showed (1 wt.%)/F-TiO2. that the substrate with no photocatalyst was hardly degraded after exposure to radiation for 1 h, illustrating that photoinduced self-sensitized photodegradation has little influence associated with the electronic excitation of the valence band on the results of the experiment. All of the data obtained O2p electron to the conduction band Ti3d level. The photo- are corrected for absorption after stirring for 30 min in the excited wavelength range of the CuO/F-TiO2 photocatalyst dark. The detailed procedure was described in [34]. The fixed is connected with the amount of CuO. It increases with ball milling time is 3 h, and fixed illumination times for the 2− the increase in the amount of CuO. In theory, because the photocatalytic reduction of Cr2O7 and photocatalytic oxi- absorption wavelength range is extended greatly towards dation of RhB were 20 min and 10 min, respectively. Figure 6 visible light and the absorption intensity increases, the shows the effect of the amount of NH4F on the photocatalytic 2− formation rate of electron-hole pairs on the photocatalyst reduction of Cr2O7 and photocatalytic oxidation of RhB. surface also increases greatly, which results in the photocata- From Figure 6, it can be seen that, without NH4Fpre- lyst exhibiting higher photocatalytic activity. From Figure 7, sent, namely, the pure TiO2 powder photocatalyst, its photo- it is clear that the photocatalytic activity of photocatalyst oxidation activity is the lowest, and the photooxidation effi- CuO/F-TiO2 is strongly dependent on the amount of CuO. ciency for RhB is 15.3%. The photooxidation activity of 6 International Journal of Photoenergy

60 65

60 50 55

50 40 45 (%) (%) η 30 η 40

20 30

25 10 0 0.5 1 1.5 2 2.5 20 Amount of doped NH4F (g) 01 2345 Amount of doped CuO (wt.%) RhB RhB Cr6+ Cr6+

Figure 6: Eﬀect of the amount of NH4F on the photocatalytic 2− Figure 7: Eﬀect of the amount of doped CuO on the photocatalytic reduction of Cr2O7 and photocatalytic oxidation of RhB. 2− reduction of Cr2O7 and photocatalytic oxidation of RhB.

F-TiO2 increases remarkably with the increasing amount of NH4F up to 1.0 g. When the amount of NH4Fis1.0g, and 10 min, respectively. The amounts of TiO2 and NH4F the photocatalytic activity of the F-TiO2 photocatalyst is were 5.0 g and 1.0 g, respectively. It can be seen that, for ffi 2− at its peak, and the photooxidation e ciency for RhB is the photocatalytic reduction of Cr2O7 , the photoreduc- 55.7%. When the amount of doped NH4F is higher than tion activity of CuO/F-TiO2 increases remarkably with the optimal amount, the photooxidation activity of F-TiO2 de- increase in the amount of doped CuO up to 1.0 wt.%. The creases gradually. At the same time, it is clear that the photo- optimum amount of doped CuO is 1.0 wt.%. When the oxidation activity of F-TiO2 is higher than that of pure TiO2 amount of doped is higher than the optimal amount, the photocatalyst in the experimental condition. F-TiO2 enhanc- photoreduction activity of the CuO/F-TiO2 photocatalyst ed the photodegradation of RhB in aqueous solutions. It decreases gradually as the amount of doped p-CuO increases. was proposed that the fluorinated surface favored the gener- The results also show that, without CuO present, namely, the ation of free OH radicals, which was responsible for the F-TiO2 powder photocatalyst, its photoreduction activity is enhanced photocatalytic oxidation activity. The result has the lowest, and the photoreduction efficiency is 22.2%. When been reported in earlier literature [27]. the amount of doped p-CuO is 1.0 wt.%, the photoreduction However, under the same condition, for the photocat- activity of CuO/F-TiO2 is at its peak, and the photoreduction 2− alytic reduction of Cr2O7 , the photocatalytic reduction efficiency is 61.2%. From Figures 6 and 7, it is clear that activity of F-TiO2 decreases gradually with the increase in the photoreduction activity of CuO/F-TiO2is higher than the amounts of doped NH4F. For the pure TiO2 powder pho- that of pure TiO2 and F-TiO2 photocatalyst. It is proposed tocatalyst, its photocatalytic reduction activity is at its peak, that, when the amount of CuO is lower than the optimum and the photocatalytic reduction efficiency is 32.2%. When amount, the increase of the amount of CuO can increase the amounts of NH4F are 0.2 and 2.0 g, the photoreduction trapping sites of the carriers, which prolongs the lifetime of efficiencies are 26.3 and 19.8%, respectively. the carriers, thereby improving the photocatalytic activity. The other important reason is that CuO is a p-type semicon- 3.2.2. Effect of Amount of Doped CuO on the Photocatalytic ductor. When the F-TiO2 doped with suitable amount of p- Activity. In order to obtain the optimum concentration of CuO, both p-CuO and F-TiO2 can form the p-n junction CuO, the effect of the amount of CuO on the photocat- photocatalyst by ball milling. Therefore, the photoreduction alytic activity of CuO/F-TiO2photocatalyst was studied. The activity increases [33]. But when the amount of p-CuO experiments were carried out with different concentrations is higher than the optimum amount of doping, the high of CuO varying from 0% to 5 wt.%. Figure 7 shows the concentration dopant ions act as recombination centers of effects of the amount of doped CuO on the photocatalytic re- electrons and holes, decrease the thickness of the space- 2− duction of Cr2O7 and photocatalytic oxidation of RhB. charge layer on the surface of TiO2 particle, and reduce the The fixed ball milling time for each sample was 3 h, and the photon absorption [21]. However, under the same condition, fixed illumination times for the photocatalytic reduction of for the photocatalytic oxidation of RhB, the photooxidation 2− Cr2O7 and photocatalytic oxidation of RhB were 20 min activity of p-CuO/F-TiO2 decreases rapidly with the increase International Journal of Photoenergy 7

80 the p-n heterojunction photocatalyst CuO/F-TiO2 has higher 75 photocatalytic reduction activity, but lower photocatalytic oxidation activity. 70

65 3.2.3. Eﬀect of Ball Milling Time on the Photocatalytic Activity. The eﬀect of ball milling time on the photocatalytic 60 activity of CuO (1.0 wt.%)/F-TiO2 photocatalyst is shown in

(%) 55 Figure 8. It can be seen that the ball milling time influences η 50 the photocatalytic activity strongly. Without ball milling, the photoreduction efficiency is 39.2%, and the photooxi- 45 dation efficiency is 51.3%. The photoreduction efficiency of 2− 40 Cr2O7 increases gradually with the increase in ball milling time up to 6 h. When the ball milling time is 1, 3, 6, 12, and 35 24 h, the photoreduction efficiency is 45.5, 61.2, 76.8, 74.2, 30 and 70.1%, respectively. For the photocatalytic oxidation of 0 5 10 15 20 25 RhB, the photooxidation efficiency decreases rapidly with the Ball milling time (h) increase in the ball milling time. When the ball milling time ffi RhB is 1 h, the photooxidation e ciency is the lowest, and it is Cr6+ 32.7%. However, when the ball milling time is longer than 1 h, the photooxidation efficiency increases gradually with Figure 8: Effect of ball milling time on the photocatalytic activity. the increase in ball milling time up to 6 h. When the ball milling time is 6 h, the photooxidation efficiency of RhB is 40.6%. When the ball milling time is longer than 6 h, the photoin the amounts of doped p-CuO. For the pure F-TiO2 photooxidation efficiency decreases gradually. The reason is that, catalyst, its photooxidation activity is at its peak, and the without ball milling, F-TiO2 and p-CuO only play their own photooxidation efficiency of F-TiO2 is 55.7%. When the photocatalytic role, and the p-n junction photocatalysts are amounts of doped p-CuO are 0.3 and 1.0 wt.%, the photo- not formed. However, after ball milling, TiO2 and p-CuO oxidation efficiencies are 49.5 and 38.1%, respectively. can form p-n junction photocatalyst, which results in the The photocatalyst CuO/F-TiO2 has higher photocatalytic increase of the photoreduction activity and the decrease of reduction activity than that of TiO2 and F-TiO2. The possible the photooxidation activity rapidly. Another reason is that, reasons are as follows. with the increase in the ball milling time, the specific surface First, it can be explained by the p-n junction principle area of the photocatalyst increases. Correspondingly, the [21, 28]. It is known that CuO is p-type semiconductor, and number of active sites per unit weight of photocatalyst also TiO2 is n-type semiconductor. When p-type CuO and n-type increases [21]. But when the ball milling time is longer than TiO2 integrate p-n junctions will be formed between CuO the optimum time, it is proposed that, with the increase in and TiO2, and the inner electric field will be also formed at the ball milling time, the fresh surface formed by high-energy the same time in the interface. So a number of micro p-n he- ball milling possesses high surface energy and prefers to terojunction CuO/F-TiO2photocatalysts will be formed after agglomerate [36, 37], resulting in the decrease of the photo- doping CuO powder into TiO2 granule. At the equilibrium, catalytic activity. The assumption is proved by the results of the inner electric field formed made p-type semiconductor SEM. CuO region have the negative charge while TiO2 region have the positive charge. The electron-hole pairs will be 3.2.4. Hydroxyl Radical Analysis. The formation of hydroxyl • created under UV light illumination. With the effect of the radicals ( OH) on the surface of CuO (1.0 wt.%)/F-TiO2 inner electric field, the holes flow into the negative field photocatalyst is detected by a photoluminescence (PL) and the electrons move to the positive field. Therefore, the technique with terephthalic acid as a probe molecule. The photogenerated electron-hole pairs will be separated effec- method is rapid, sensitive, and specific, needing only simple tivelybythep-nheterojunctionformedinCuO/F-TiO2. standard PL instrumentation. Terephthalic acid readily reacts • Second, compared with F-TiO2, the photoabsorption with OH to produce a highly fluorescent product, 2-hydro- wavelength range of the CuO/F-TiO2 photocatalyst red shifts xyterephthalic acid, whose PL peak intensity is in proportion and extends the wavelength range of photoexcitation and to the amount of OH radicals produced in water. Experimen- enhances the utilization of the total spectrum. The results talprocedureswerereportedinearlierreports[27]. After UV also show that the photoluminescence emission intensity of irradiation for 10 min, the reaction solution was filtrated to the CuO/F-TiO2 photocatalyst is lower than that of the pure measure the PL intensity at 425 nm excited by 315 nm light F-TiO2. It indicates that doping CuO is helpful to inhibit the of 2-hydroxyterephthalic acid. recombination of electrons and holes and improve the pho- Figure 9 shows the changes of PL spectra of different tocatalytic activity. At the same time, it was reported that p- samples from 5 × 10−4 mol/L terephthalic acid solution in type semiconductor is a collector of photoexcited holes [35]. 2 × 10−3 mol/L NaOH. From Figure 9, it can be seen that, Therefore, there are enrichment electrons in the interface to when the ball milling time is 6 h, the PL peak intensity of 2− react with Cr2O7 adsorbed on the photocatalyst surface, so the sample is the highest. It means that the formation rate of 8 International Journal of Photoenergy

350 porous indium hydroxide nanocrystals,” Environmental Sci- ence and Technology, vol. 44, no. 4, pp. 1380–1385, 2010. 300 [3] X. Xin, M. Scheiner, M. Ye, and Z. Lin, “Surface-treated TiO a 2 nanoparticles for dye-sensitized solar cells with remarkably 250 b enhanced performance,” Langmuir, vol. 27, no. 23, pp. 14594– 14598, 2011. c 200 [4] X. Xu, X. Li, P. Lin et al., “A general templated method d to homogeneous and composition-tunable hybrid TiO2 150 nanocomposite fibers,” Chemical Communications, vol. 47, no. e 9, pp. 2538–2540, 2011. f 100 [5] Z. J. Zhou, J. Q. Fan, X. Wang, W. H. Zhou, Z. L. Du, and S. X. Wu, “Effect of highly ordered single-crystalline TiO2 nanowire Fluorescence intensity (a.u.) Fluorescence length on the photovoltaic performance of dye-sensitized solar 50 cells,” ACS Applied Materials & Interfaces, vol. 3, pp. 4349– 4353, 2011. 0 [6] K. Lv, J. Yu, J. Fan, and M. Jaroniec, “Rugby-like anatase titania hollow nanoparticles with enhanced photocatalytic activity,” 380 400 420 440 460 480 500 520 540 CrystEngCommunity, vol. 13, no. 23, pp. 7044–7048, 2011. Figure 9: Effect of ball milling time on the changes of PL spectra. [7] D. Yang, C. Chen, Z. Zheng et al., “Grafting silica species on (a) 6 h, (b) 3 h, (c) 12 h, (d) 24 h, (e) 1 h, (f) pure terephthalic acid. anatase surface for visible light photocatalytic activity,” Energy and Environmental Science, vol. 4, no. 6, pp. 2279–2287, 2011. [8] Y. Wang, D. Zhao, H. Ji et al., “Sonochemical hydrogen production efficiently catalyzed by Au/TiO2,” Journal of Physical OH radicals on its surface is much larger than that of other Chemistry C, vol. 114, no. 41, pp. 17728–17733, 2010. samples. The PL peak intensities of the samples decrease [9] R. Shi, J. Lin, Y. Wang, J. Xu, and Y. Zhu, “Visible-light photo- are as follows: ball milling time 6 h > 3h> 12 h > 24 h > 1h. catalytic degradation of BiTaO4 photocatalyst and mechanism The result of PL peak intensity is consistent with that of the of photocorrosion suppression,” Journal of Physical Chemistry photooxidation activity. C, vol. 114, no. 14, pp. 6472–6477, 2010. [10] R. J. Wang, G. H. Jiang, Y. W. Ding et al., “Photocatalytic 4. Conclusions activity of heterostructures based on TiO2 and halloysite nanotubes,” ACS Applied Materials & Interfaces, vol. 3, pp. The CuO/F-TiO photocatalyst was prepared by ball milling 4154–4158, 2011. 2 [11] D. Y. Wu and M. Long, “Realizing visible-light-induced self- method. The photooxidation activity of F-TiO2 increases cleaning property of cotton through coating N-TiO2 film and greatly with the increase in the amount of NH4Fupto loading AgI particles,” ACS Applied Materials & Interfaces, vol. 1.0 g. The photoreduction activity of CuO/F-TiO2 increases 3, pp. 4770–4774, 2011. remarkably with the increase in the amount of doped p- [12] M. Liu, X. Qiu, M. Miyauchi, and K. Hashimoto, “Cu(II) 3+ CuO up to 1.0 wt.%. The ball milling time has a significant oxide amorphous nanoclusters grafted Ti self-doped TiO2: influence on the photocatalytic activity of the photocatalyst. an efficient visible light photocatalyst,” Chemistry of Materials, The optimum ball milling time is 6 h. Compared with pure vol. 23, no. 23, pp. 5282–5286, 2011. [13] N. Zhang, S. Liu, X. Fu, and Y.-J. Xu, “Synthesis of M@TiO TiO2, the photoabsorption wavelength range of the F-TiO2 2 (M = Au, Pd, Pt) core-shell nanocomposites with tunable and CuO/F-TiO2 photocatalysts red shifts and improves the utilization of the total spectrum. As the formation of the p- photoreactivity,” Journal of Physical Chemistry C, vol. 115, no. 18, pp. 9136–9145, 2011. n heterojunction and p-type CuO species acts as holes traps [14] T. A. Kandiel, A. A. Ismail, and D. W. Bahnemann, “Meso- and collector, the photogenerated electron-hole pairs of the porous TiO2 nanostructures: a route to minimize Pt loading CuO/F-TiO2 photocatalyst are separated by the inner electric on titania photocatalysts for hydrogen production,” Physical field and the photocatalytic reduction activity is enhanced Chemistry Chemical Physics, vol. 13, no. 45, pp. 20155–20161, greatly. 2011. [15] P. Sangpour, F. Hashemi, and A. Z. Moshfegh, “Pho- Acknowledgments toenhanced degradation of methylene blue on cosputtered M:TiO2 (M = Au, Ag, Cu) nanocomposite systems: a com- This study was supported by the Natural Science Foundation parative study,” Journal of Physical Chemistry C, vol. 114, no. of China (nos. 20973071, 51172086, and 21103060) and the 33, pp. 13955–13961, 2010. Key Project of Science and Technology Research of Ministry [16] K. Yang, Y. Dai, and B. Huang, “Density functional study of of Education of China (208062). boron-doped anatase TiO2,” Journal of Physical Chemistry C, vol. 114, no. 46, pp. 19830–19834, 2010. [17] P. Xu, J. Lu, T. Xu, S. Gao, B. Huang, and Y. Dai, “I2-hydrosol- References seeded growth of (I2)n-C-codoped meso/nanoporous TiO2 for visible light-driven photocatalysis,” Journal of Physical [1] A. Fujishima and K. Honda, “Electrochemical photolysis of Chemistry C, vol. 114, no. 20, pp. 9510–9517, 2010. water at a semiconductor electrode,” Nature, vol. 238, no. [18]W.Guo,Y.Shen,L.Wu,Y.Gao,andT.Ma,“Effect of N dopant 5358, pp. 37–38, 1972. amount on the performance of dye-sensitized solar cells based [2] T. Yan, J. Long, X. Shi, D. Wang, Z. Li, and X. Wang, “Efficient on N-Doped TiO2 electrodes,” Journal of Physical Chemistry C, photocatalytic degradation of volatile organic compounds by vol. 115, no. 43, pp. 21494–21499, 2011. International Journal of Photoenergy 9

[19] J. Lin, J. Lin, and Y. Zhu, “Controlled synthesis of the ZnWO4 [37] J. Wang, S. Yin, M. Komatsu, Q. Zhang, F. Saito, and T. Sato, nanostructure and effects on the photocatalytic performance,” “Preparation and characterization of nitrogen doped SrTiO3 Inorganic Chemistry, vol. 46, no. 20, pp. 8372–8378, 2007. photocatalyst,” Journal of Photochemistry and Photobiology A, [20]A.T.Kuvarega,R.W.M.Krause,andB.B.Mamba,“Nitro- vol. 165, no. 1–3, pp. 149–156, 2004. gen/palladium-codoped TiO2 for efficient visible light photocatalytic dye degradation,” Journal of Physical Chemistry C, vol. 115, no. 45, pp. 22110–22120, 2011. [21] S. Chen, W. Zhao, W. Liu, and S. Zhang, “Preparation, characterization and activity evaluation of p-n junction photocatalyst p-ZnO/n-TiO2,” Applied Surface Science, vol. 255, no. 5, pp. 2478–2484, 2008. [22] Z.-R. Tang, F. Li, Y. Zhang, X. Fu, and Y.-J. Xu, “Composites of titanate nanotube and carbon nanotube as photocatalyst with high mineralization ratio for gas-phase degradation of volatile aromatic pollutant,” Journal of Physical Chemistry C, vol. 115, no. 16, pp. 7880–7886, 2011. [23] V. Stengl,D.Popelkovˇ a,´ and P. Vlaćil,ˇ “TiO2-graphene nanocomposite as high performace photocatalysts,” Journal of Phy- sical Chemistry C, vol. 115, no. 51, pp. 25209–25218, 2011. [24] J. Zhang, J. Yu, Y. Zhang, Q. Li, and J. R. Gong, “Visi- ble light photocatalytic H2-production activity of CuS/ZnS porous nanosheets based on photoinduced interfacial charge transfer,” Nano Letters, vol. 11, no. 11, pp. 4774–4779, 2011. [25] H. Park and W. Choi, “Effects of TiO2 surface fluorination on photocatalytic reactions and photoelectrochemical behaviors,” Journal of Physical Chemistry B, vol. 108, no. 13, pp. 4086– 4093, 2004. [26] W. Choi, “Pure and modified TiO2 photocatalysts and their environmental applications,” Catalysis Surveys from Asia, vol. 10, no. 1, pp. 16–28, 2006. [27] J. Yu, W. Wang, B. Cheng, and B. L. Su, “Enhancement of photocatalytic activity of Mesporous TiO2 powders by hydrothermal surface fluorination treatment,” Journal of Physical Chemistry C, vol. 113, no. 16, pp. 6743–6750, 2009. [28] S. F. Chen, S. J. Zhang, W. Liu, and W. Zhao, “Preparation and activity evaluation of p-n junction photocatalyst NiO/TiO2,” Journal of Hazardous Materials, vol. 155, no. 1-2, pp. 320–326, 2008. [29] H. Yu, J. Yu, S. Liu, and S. Mann, “Template-free hydrothermal synthesis of CuO/Cu2Ocompositehollowmicrospheres,” Chemistry of Materials, vol. 19, no. 17, pp. 4327–4334, 2007. [30] S. F. Chen, L. Chen, S. Gao, and G. Y. Cao, “The preparation of nitrogen-doped photocatalyst TiO2—XN x by ball milling,” Chemical Physics Letters, vol. 413, no. 4-6, pp. 404–409, 2005. [31] T. Cai, M. Yue, X. Wang, Q. Deng, Z. Peng, and W. Zhou, “Preparation, characterization, and photocatalytic performance of NdPW12O40/TiO2 composite catalyst,” Chinese Jour- nal of Catalysis, vol. 28, no. 1, pp. 10–16, 2007. [32] J. W. Tang, Z. G. Zou, and J. H. Ye, “Photophysical and photocatalytic properties of AgInW2O8,” The Journal of Physical Chemistry B, vol. 107, pp. 14265–14269, 2003. [33] S. Kang, K. Shin, K. Prabakar, and C. Lee, “Optical and electrical properties of ZnO doped with nitrogen,” Physica Status Solidi B, vol. 241, no. 12, pp. 2830–2834, 2004. [34]S.F.Chen,X.L.Yu,H.Y.Zhang,andW.Liu,“Preparationand photocatalytic activity evaluation of composite Fe-TiO2/TiO2 photocatalyst,” Journal of the Electrochemical Society, vol. 157, no. 5, pp. K96–K102, 2010. [35] A. Lohner, M. Woerner, T. Elsaesser, and W. Kaiser, “Picosec- ond capture of photoexcited holes by shallow acceptors in p- type GaAs,” Physical Review Letters, vol. 68, no. 26, pp. 3920– 3923, 1992. [36] S. F. Chen, L. Chen, G. Shen, and C. Gengyu, “The preparation of coupled WO3/TiO2 photocatalyst by ball milling,” Powder Technology, vol. 160, no. 3, pp. 198–202, 2005. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 830321, 6 pages doi:10.1155/2012/830321

Research Article Facile Preparation and Photoinduced Superhydrophilicity of Highly Ordered Sodium-Free Titanate Nanotube Films by Electrophoretic Deposition

Minghua Zhou1, 2 and Huogen Yu2

1 Lab of Medical Chemistry, Hubei University of Medicine, Hubei, Shiyan 442000, China 2 Department of Chemistry, School of Science, Wuhan University of Technology, Wuhan 430070, China

Correspondence should be addressed to Huogen Yu, [email protected]

Received 30 December 2011; Accepted 9 January 2012

Academic Editor: Baibiao Huang

Copyright © 2012 M. Zhou and H. Yu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Highly ordered sodium-free titanate nanotube films were one-step prepared on F-doped SnO2-coated (FTO) glass via an electrophoretic deposition method by using sodium titanate nanotubes as the precursor. It was found that the self-assembled formation of highly ordered sodium titanate nanotube films was accompanied with the effectiveremovalofsodiumionsin the nanotubes during the electrophoretic deposition process, resulting in the final formation of protonated titanate nanotube film. With increasing calcination temperature, the amorphous TiO2 phase is formed by a dehydration process of the protonated ◦ ◦ titanate nanotubes at 300 C and further transforms into anatase TiO2 when the calcination temperature is higher than 400 C. Compared with the as-prepared titanate nanotube film, the calcined titanate nanotube film (300–600◦C) exhibits attractive photoinduced superhydrophilicity under UV-light irradiation. In particular, 500◦C-calcined films show the best photoinduced superhydrophilicity, probably due to synergetic effects of enhanced crystallization, surface roughness, and ordered structures of the films.

1. Introduction Ti substrate in concentrated NaOH solution for certain time. Tokudome and Miyauchi [17] developed a layer-by- Among various oxide semiconductors, titania appears prom- layer growth technique to prepare thin titanate nanotube ising and important for the use in environmental purification films. Nevertheless, the construction of the titanate nanotube due to its strong oxidizing power, nontoxicity and long-term films with controllable textures is still remained as a great photostability [1–10]. In addition to the conventional photo- challenge, and alternative techniques for films are highly catalytic activity, photoinduced superhydrophilic properties desirable. Recently, electrophoretic deposition (EPD) was of two-dimensional (2D) TiO2 thin films have attracted successfully developed to fabricate various thin-film mate- much attention in recent years [11–13]. By utilizing the su- rials [18–20]. There are many advantages of this technique perhydrophilic surface of TiO2 thin films, we can develop over other deposition methods. For example, higher depo- antifogging or self-cleaning glass, mirrors, and other eco- sition rate can be easily achieved with a quick deposition, logical building materials [14, 15]. However, the further in addition to its reproducibility of deposition. It is also an enhanced photoinduced hydrophilicity is necessary for prac- efficient technique to control the thickness and morphology tical uses. To achieve this goal, great attentions should of the films by controlling current or potential. Specially, the be focused on the textural design of the 2D TiO2 thin EPD has a flexibility of depositing films on different shapes films. and sizes and can be extended to a large scale for commercial Recently, titanate-based films have been intensively inves- applications. Another advantage of this method is that it can tigated due to the large specific surface areas and unique be carried out at room temperature, thereby reducing the structural features [16–23]. For example, Tian et al. [16]re- possibility of deterioration of the surrounding. ported a simple hydrothermal and seeded growth process Here, we developed the EPD method for constructing to fabricate titanate nanotube film by directly immersing ordered titanate nanotube films using preformed titanate 2 International Journal of Photoenergy nanotubes as raw materials. The effects of calcination tem- technique using terephthalic acid (TA) as a probe molecule. peratures on the surface morphology, microstructures, as TA readily reacts with •OH to produce highly fluorescent well as the properties of superhydrophilicity were studied. product, 2-hydroxyterephthalic acid (TAOH) [26, 27]. PL It was demonstrated that the calcination of those titan- spectra of generated TAOH were measured on a Fluorescence ate nanotube films above 300◦C converted into ordered Spectrophotometer (F-7000, Hitachi, Japan) using a excita- −1 TiO2 nanorod films with attractive photoinduced superhy- tion wavelength 315 nm at a scan speed of 1200 nm min drophilicity. with the PMT voltage of 700 V. The width of excitation slit and emission slit was 1.0 nm. 2. Experimental 2.1. Preparation of Samples. Sodium titanate nanotubes were 3. Results and Discussion prepared according to our previous reported methods [24, Figure 1(a) shows the SEM image of the as-prepared titanate 25]. TiO2 source used for the titanate nanotubes was com- nanotube powder prepared by a hydrothermal process, mercial-grade TiO2 powder (P25, Degussa AG, Germany) indicating that the titanate nanotube shows a diameter with crystalline structure of ca. 20% rutile and ca. 80% of 7–15 nm and a length of several hundred nanometers. anatase and primary particle size of ca. 30 nm. In a typical The TEM image (inset in Figure 1(a)) clearly indicates that preparation, 1.5 g of the TiO2 powder was mixed with the as-prepared nanotubes possess a uniform inner and 140 mL of 10 M NaOH solution, followed by hydrothermal ◦ outer diameter along their length. The corresponding XRD treatment of the mixture at 150 C in a 200 mL Teflon- pattern of the titanate nanotubes is shown in Figure 2(a). lined autoclave for 48 h. After hydrothermal reaction, the The diffractionpeakataround10◦ is due to the layered precipitate was separated by filtration and washed with structure of nanotubes along their radial direction, while distilled water for three times. The washed samples were ff ◦ ◦ ◦ ◦ other di raction peaks at around 24 ,28 ,and48 are related dried in a vacuum oven at 80 Cfor8h. to the sodium titanate phase. Therefore, the as-prepared Ordered titanate nanotube films were deposited on FTO sample can be ascribed to sodium titanate nanotubes. glass using a modified EPD technique [18, 20]. The pH of When the sodium titanate nanotubes were deposited on electrolyte solution is adjusted to about 9.0 using ammonia. the surface of FTO substrate by EPD method, it is interesting The isoelectric point of titanate nanotubes was about 5.5, to find that the as-prepared films show an ordered assembled which was lower than that of electrolyte solution. Therefore, structure of the nanotubes along their long-axis direction titanate nanotubes are negatively charged particles at pH 9. (Figure 1(b)). Moreover, in addition to the diffraction peaks During EPD, the cleaned FTO glass was kept at positive of FTO, only one diffraction peak at ca. 10◦ can be observed potential (anode) while pure silver (Ag) foil was used as and other peaks at ca. 24◦,28◦,and48◦ are completely counter (cathode) electrode. The EPD was carried out at disappeared (Figure 2(b)). This indicates that the phase ± a voltage of 10.0 0.10 V (current up to 0.01 A), and structure of the nanotubes is changed while the layered deposition time was 20 min. Subsequently, the coated films structure of nanotubes is well preserved after EPD. To further were rinsed with distilled water, dried in air, and calcined at ◦ investigate the phase structure and element composition 300, 400, 500, and 600 C in air for 1 h, respectively. of the ordered films, XPS analysis was performed and shown in Figure 3. It can be seen that the sodium titanate 2.2. Characterization. Morphology observation was per- nanotube powders (Figure 3(a)) contain Ti, O, C, and formed on S-4800 field emission scanning electron micro- Na elements, with sharp photoelectron peaks appearing at ff scope (FESEM, Hitachi, Japan). X-ray di raction (XRD) pat- binding energies of 458 (Ti 2p), 531 (O 1s), 285 (C 1s), and ff terns were obtained on a D/MAX-RB X-ray di ractometer 1071 eV (Na 1s), respectively. The carbon peak is attributed (Rigaku, Japan). Transmission electron microscopy (TEM) to the adventitious hydrocarbon from XPS instrument itself. analyses were conducted with a JEM-2010F electron micro- After the sodium titanate nanotube powder is deposited on scope (JEOL, Japan). X-ray photoelectron spectroscopy the surface of FTO to form a film, it is found that no XPS (XPS) measurements were done with a Kratos XSAM800 peaks corresponding to Na element are recorded in the as- XPS system; all the binding energies were referenced to the prepared film (Figure 3(b)). These results indicate that EPD C1s peak at 284.8 eV of the surface adventitious carbon. is an effective way to remove the intercalated sodium in the sodium titanate nanotubes and the Na+ can be detached 2.3. Hydrophilicity Test. The sessile drop method was used from the nanotube when the nanotubes move towards anode for the measurements of water contact angle (θ) on the and are coated on the surface of FTO during EPD, which is surface of as-prepared films with a contact angle meter in good agreement with the previous studies [28]. Therefore, (JC2000A, China) [14, 15]. The droplet size used for each the phase structure of highly ordered film can be attributed measurement was 3 μL. Water droplets were placed at five to protonated titanate with a similar crystal structure of ff di erent positions of each film, and the average value was H2Ti3O7 [29, 30]andHxTi2−x/4 x/4O4(x = 0.75) [31]. adopted as the contact angle. Using a simple hydrothermal treatment of crystalline TiO2 particles with NaOH aqueous solutions, high-quality 2.4. Hydroxyl Radical Analysis. The formation of hydroxyl nanotubes with uniform diameter of around 7–15 nm were radical (•OH) on the surface of photoilluminated as- obtained and their specific surface area reached more than prepared films was detected by photoluminescence (PL) 400.0 m2/g [24]. Unfortunately, the obtained nanotubes are International Journal of Photoenergy 3

50 nm

μ 200 nm 1 m

(a) (b)

500 nm 300 nm

Figure 1: FESEM images of (a) as-prepared sodium titanate nanotube powders, (b) as-prepared titanate nanotube films, (c) 300◦C-calcined titanate nanotube films, and (d) 500◦C-calcined titanate nanotube films. Inset in (a) is the corresponding TEM image.

sodium titanate phase and showed no photocatalytic activity A S in our previous experiments [24, 25]. To prepare the high- S active photocatalyst, the sodium titanate nanotubes should S (f) S S be carefully washed with HCl solution to remove the sodium ions intercalated in the nanotubes. However, the above ion- (e) exchange method is a time-wasting process and a lot of HCl (d) solution is required [24, 25]. In this study, the EPD method provides a more effective and easy strategy for the removal (c) of sodium in the titanate nanotubes. Based on the above results, it is clear that the self-assembly of highly ordered T Relative intensity (a.u.) Relative (b) sodium titanate nanotube films was accompanied with the effective removal of sodium ions in the nanotubes during the T (a) T T EPD process, resulting in the final formation of protonated titanate nanotube film. The effects of calcination temperature on the surface 10 20 30 40 50 60 70 morphology of the titanate nanotube films are investigated. 2 (deg) At 300◦C, the calcined nanotube films still maintain a similar Figure 2: XRD patterns of the (a) as-prepared sodium titanate ordered morphology of self-assembled titanate nanotubes. nanotube powders, (b) as-prepared titanate nanotube films, and However, some broken and short rods can be clearly after calcination at (c) 300◦C, (d) 400◦C, (e) 500◦C, and (f) 600◦C. observed (Figure 1(c)). According to the XRD results shown ff The labeled S, T, and A are related to crystal phase of SnO2, titanate in Figure 2(c), no new di raction peaks can be observed in nanotube, and anatase TiO2,respectively. addition to the substrate of FTO, suggesting the formation of 4 International Journal of Photoenergy

In the dark UV illumination 80 (a) (b) O1s p 2 Na1s OKLL s Uncalcined Ti 2

Ti 60 (c) C1s NaKLL ◦ 600 C

40 ◦ (b) 300 C Relative intensity (a.u.) Relative

Contact angleContact (deg) ◦ 400 C 20 (a) ◦ 500 C

200 400 600 800 1000 0 Binding energy (eV) 0 30 60 20 40 60 Storage time (days) UV illumination time (min) Figure 3: XPS survey spectra of (a) as-prepared sodium titanate nanotube powders, (b) as-prepared titanate nanotube films, and (c) Figure 4: Changes in the water contact angle of the various titanate 500◦C-calcined titanate nanotube films. nanotube films: (a) stored in a dark room for 60 days and (b) subsequently illuminated by UV light for 60 min. amorphous phase after calcination at 300◦C. Therefore, the the films could be remained much longer. Fortunately, the formation of 300◦C-calcined ordered structure (Figure 1(c)) superhydrophilicity of the calcined titanate nanotube films is probably due to the phase transformation from titanate couldbefullyrecoveredbyUVillumination(Figure 4(b)). to amorphous TiO2. When the calcination temperature However, the superhydrophilicity of the as-prepared titanate increases to 400◦C(Figure 2(d)), the diffractionpeakatca. nanotube films cannot be recovered, suggesting that the 25◦ is formed, indicating the further phase transformation conversion of those titanate nanotube films into ordered from amorphous to anatase TiO2. With further increase of titania nanorods films upon calcination is significant for the calcination temperature to 500◦C, the titanate nanotubes the recovering process. Moreover, the recovering rate of are transformed into anatase TiO2 nanorods with a length of hydrophilicity of the calcined titanate nanotube films is 20–70 nm (Figure 1(d)). When the calcination temperature obviously dependent on the calcination temperature. In ◦ is 600◦C, both the one-dimensional structures and their particular, 500 C-calcined films show the best photoinduced ordered assemblies are completely destroyed (data not superhydrophilicity. This may be related to the synergetic shown), while XRD patterns suggest that the film samples effects of enhanced crystallization, surface roughness, and show an enhanced crystallization with increasing calcination orderly assembled structures of the films. The enhanced temperature. On the other hand, it should be noted that, crystallization of anatase film is beneficial for the separation when the calcination temperature is over 500◦C, the XPS of photogenerated charge carriers and thus showing a better peak of Na element (Figure 3(c)) at the binding energy photocatalytic activity. The subsequent faster degradation of 1071 eV appears again, which is associated with the of the surface-enriched pollutant molecules is an important migration of some sodium ions from the FTO glass into the factor responsible for the more rapid change of the surface to films surface during heat treatment, in good agreement with the superhydrophilic state. Moreover, the favored separation our previous results [32]. and diffusion of photogenerated charge carriers facilitates 2− Figure 4 shows the changes of water contact angle for the transfer of the photogenerated holes towards lattice O the titanate nanotube films before and after calcination at anions, creating more oxygen vacancies [33]: various temperatures. It can be seen that the water contact ◦ + 2− 1 angles for all the freshly prepared films were about 5 . This 2h +O −→ O2 + oxygen vacancy (1) 2 may be attributed to the fact that all the freshly prepared films had a clear and roughened surface. However, when these film Consequently, water molecules may coordinate into the samples were stored in the dark in air for about 2 months, oxygen vacancy sites, resulting in more abundant surface the water contact angles increased gradually to about 60∼65◦ hydroxyl, which is confirmed to some extent by the greater (Figure 4(a)). This is ascribed to the fact that the surface formation rate of •OH. The formation of •OH on the surface is contaminated by adsorbing some gaseous contaminants of the samples after UV irradiation can be detected by a PL from air and the surface defect sites can be healed or replaced technique using TA as a probe molecule (see below). gradually by oxygen atoms, which changes the surface wet- The ready reaction of photogenerated •OH with TA tability from a superhydrophilic to a hydrophilic state [33]. gave rise to highly fluorescent TAOH with typical PL signal From the viewpoint of practical use and commercialization, centered at 425 nm. In addition, this PL intensity was pro- it would be more valuable if the superhydrophilicity of portional to the amount of •OH produced. Figure 5 showed International Journal of Photoenergy 5

12 the Science Research Project of Hubei University of Medicine ◦ 500 C (2009QDJ25), and the Fundamental Research Funds for the Central Universities (Grant 2011-1a-039). ◦ 400 C ◦ 8 600 C References ◦ 300 C [1]M.R.Hoffmann, S. T. Martin, W. Choi, and D. W. Bahne- mann, “Environmental applications of semiconductor photo- Uncalcined catalysis,” Chemical Reviews, vol. 95, no. 1, pp. 69–96, 1995. 4 [2] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, “Visible-light photocatalysis in nitrogen-doped titanium Fluorescence intensity (a.u.) Fluorescence oxides,” Science, vol. 293, no. 5528, pp. 269–271, 2001. [3] H. Yu, H. Irie, Y. Shimodaira et al., “An efficient visible- light-sensitive Fe(III)-grafted TiO2 photocatalyst,” Journal of 0 Physical Chemistry C, vol. 114, no. 39, pp. 16481–16487, 2010. 400 450 500 550 [4] H. Yu, H. Irie, and K. Hashimoto, “Conduction band energy Wavelength (nm) level control of titanium dioxide: toward an efficient visible- light-sensitive photocatalyst,” Journal of the American Chemi- Figure 5: The fluorescence spectra related to formed fluorescent cal Society, vol. 132, no. 20, pp. 6898–6899, 2010. TAOH for the various films under UV-light irradiation at a fixed [5] H. Yu, J. Yu, B. Cheng, and S. Liu, “Novel preparation and 60 min. photocatalytic activity of one-dimensional TiO2 hollow structures,” Nanotechnology, vol. 18, no. 6, Article ID 065604, 2007. [6] Y.-C. Liu, Y.-F. Lu, Y.-Z. Zeng, C.-H. Liao, J.-C. Chung, and the PL spectra for the various samples at fixed irradiation T.-Y. Wei, “Nanostructured mesoporous titanium dioxide thin time (60 min). Clearly, the order of the formation rate of film prepared by sol-gel method for dye-sensitized solar cell,” International Journal of Photoenergy, vol. 2011, Article ID •OH for these films is showed as follows: 500◦C- > 400◦C-> ◦ ◦ 619069, 9 pages, 2011. 600 C- > 300 C-calcined > uncalcined, which is consistent [7] J. Subrt,ˇ J. M. Criado, L. Szatmary´ et al., “Mechanochemical with the change rate of hydrophobic to hydrophilic state synthesis of visible light sensitive titanium dioxide photocata- illustrated in Figure 4. In addition, the increased surface lyst,” International Journal of Photoenergy, vol. 2011, Article ID roughness coupled with abundant surface hydroxyl groups 156941, 9 pages, 2011. can adsorb and fix more water [15]toproducemore•OH. [8] J. Yao and C. Wang, “Decolorization of methylene blue Moreover, small micropores in the films can also produce a with TiO2 sol via UV irradiation photocatalytic degradation,” two-dimensional capillary phenomenon, which is supposed International Journal of Photoenergy, vol. 2010, Article ID to be enhanced by the ordered assemblies of nanorod as 643182, 6 pages, 2010. reflected in Figure 1. [9] N. Hafizah and I. Sopyan, “Nanosized TiO2 photocatalyst powder via sol-gel method: effectofhydrolysisdegreeonpow- der properties,” International Journal of Photoenergy, vol. 2009, 4. Conclusions Article ID 962783, 9 pages, 2009. [10] Z. Zhang and J. Gamage, “Applications of photocatalytic Highly ordered sodium-free titanate nanotube films were disinfection,” International Journal of Photoenergy, vol. 2010, one-step prepared on FTO substrate via an EPD method Article ID 764870, 11 pages, 2010. by using sodium titanate nanotubes as the precursor. The [11] H. Irie, S. Washizuka, N. Yoshino, and K. Hashimoto, “Visible- self-assembled formation of highly ordered sodium titanate light induced hydrophilicity on nitrogen-substituted titanium nanotube films was accompanied with the effective removal dioxide films,” Chemical Communications, vol. 9, no. 11, pp. of sodium ions in the nanotubes during the EPD process, 1298–1299, 2003. resulting in the final formation of protonated titanate nan- [12] A. Y. Nosaka, E. Kojima, T. Fujiwara, H. Yagi, H. Akutsu, and ff Y. Nosaka, “Photoinduced changes of adsorbed water on a otube film. The calcination temperature has an obvious e ect TiO photocatalytic film as studied by 1H NMR spectroscopy,” on the morphology and photoinduced hydrophilicity of the 2 ◦ Journal of Physical Chemistry B, vol. 107, no. 44, pp. 12042– titanate nanotube films. At 500 C, the prepared film shows 12044, 2003. the best photoinduced superhydrophilicity and the fastest [13] N. Sakai, A. Fujishima, T. Watanabe, and K. Hashimoto, “En- formation rate of hydroxyl radicals, probably due to syner- hancement of the photoinduced hydrophilic conversion rate of getic effects of good crystallization, surface roughness, and TiO2 film electrode surfaces by anodic polarization,” Journal of orderly assembled structures of the films. Physical Chemistry B, vol. 105, no. 15, pp. 3023–3026, 2001. [14] J. Yu and X. Zhao, “Effect of surface treatment on the photocatalytic activity and hydrophilic property of the sol-gel Acknowledgments derived TiO2 thin films,” Materials Research Bulletin, vol. 36, no. 1-2, pp. 97–107, 2001. This work was partially supported by the National Natural [15]J.Yu,J.C.Yu,W.Ho,andZ.Jiang,“Effects of calcination Science Foundation of China (20803055). This work was temperature on the photocatalytic activity and photo-induced also supported by the Key Science Research Project of the super-hydrophilicity of mesoporous TiO2 thin films,” New Hubei Provincial Department of Education (D20112107), Journal of Chemistry, vol. 26, no. 5, pp. 607–613, 2002. 6 International Journal of Photoenergy

[16]Z.R.Tian,J.A.Voigt,J.Liu,B.McKenzie,andH.Xu,“Large [32]H.Yu,J.Yu,andB.Cheng,“FacilepreparationofNa-free oriented arrays and continuous films of TiO2-based nan- anatase TiO2 film with highly photocatalytic activity on soda- otubes,” Journal of the American Chemical Society, vol. 125, no. lime glass,” Catalysis Communications, vol. 7, no. 12, pp. 1000– 41, pp. 12384–12385, 2003. 1004, 2006. [17] H. Tokudome and M. Miyauchi, “Titanate nanotube thin films [33] J. C. Yu, W. Ho, J. Lin, H. Yip, and P. K. Wong, “Photocatalytic via alternate layer deposition,” Chemical Communications, vol. activity, antibacterial effect, and photoinduced hydrophilicity 10, no. 8, pp. 958–959, 2004. of TiO2 films coated on a stainless steel substrate,” Environ- [18] L. Zhao, J. Yu, J. Fan, P. Zhai, and S. Wang, “Dye-sensitized mental Science and Technology, vol. 37, no. 10, pp. 2296–2301, solar cells based on ordered titanate nanotube films fabricated 2003. by electrophoretic deposition method,” Electrochemistry Com- munications, vol. 11, no. 10, pp. 2052–2055, 2009. [19] Y. Lai, Y. Chen, Y. Tang, D. Gong, Z. Chen, and C. Lin, “Elec- trophoretic deposition of titanate nanotube films with extremely large wetting contrast,” Electrochemistry Communica- tions, vol. 11, no. 12, pp. 2268–2271, 2009. [20] J. Yu and M. Zhou, “Effects of calcination temperature on microstructures and photocatalytic activity of titanate nanotube films prepared by an EPD method,” Nanotechnology, vol. 19, no. 4, Article ID 045606, 2008. [21] Y. Wu, M. Long, W. Cai et al., “Preparation of photocatalytic anatase nanowire films by in situ oxidation of titanium plate,” Nanotechnology, vol. 20, no. 18, Article ID 185703, 2009. [22] N.-H. Lee, H.-J. Oh, S.-C. Jung, W.-J. Lee, D.-H. Kim, and S.-J. Kim, “Photocatalytic properties of nanotubular-shaped TiO2 powders with anatase phase obtained from titanate nanotube powder through various thermal treatments,” International Journal of Photoenergy, vol. 2011, Article ID 327821, 7 pages, 2011. [23] L.Zhao,J.Ran,Z.Shu,G.Dai,P.Zhai,andS.Wang,“Effects of calcination temperatures on photocatalytic activity of ordered titanate nanoribbon/SnO2 films fabricated during an EPD process,” International Journal of Photoenergy, vol. 2012, Arti- cle ID 472958, 7 pages, 2012. [24]J.G.Yu,H.G.Yu,B.Cheng,andC.Trapalis,“Effects of calcination temperature on the microstructures and photocatalytic activity of titanate nanotubes,” Journal of Molecular Catalysis A, vol. 249, no. 1-2, pp. 135–142, 2006. [25] H. G. Yu, J. G. Yu, B. Cheng, and J. Lin, “Synthesis, characterization and photocatalytic activity of mesoporous titania nanorod/titanate nanotube composites,” Journal of Hazardous Materials, vol. 147, no. 1-2, pp. 581–587, 2007. [26] K. I. Ishibashi, A. Fujishima, T. Watanabe, and K. Hashimoto, “Detection of active oxidative species in TiO2 photocatalysis using the fluorescence technique,” Electrochemistry Communi- cations, vol. 2, no. 3, pp. 207–210, 2000. [27] J. Yu, Q. Xiang, and M. Zhou, “Preparation, characterization and visible-light-driven photocatalytic activity of Fe-doped titania nanorods and first-principles study for electronic structures,” Applied Catalysis B, vol. 90, no. 3-4, pp. 595–602, 2009. [28] G. S. Kim, V. P. Godbole, H. K. Seo, Y. S. Kim, and H. S. Shin, “Sodium removal from titanate nanotubes in electrodeposi- tion process,” Electrochemistry Communications, vol. 8, no. 3, pp. 471–474, 2006. [29] G. H. Du, Q. Chen, R. C. Che, Z. Y. Yuan, and L. M. Peng, “Preparation and structure analysis of titanium oxide nanotubes,” Applied Physics Letters, vol. 79, no. 22, pp. 3702–3704, 2001. [30] Q. Chen, W. Zhou, G. H. Du, and L. M. Peng, “Trititanate nanotubes made via a single alkali treatment,” Advanced Materials, vol. 14, no. 17, pp. 1208–1211, 2002. [31]R.Ma,Y.Bando,andT.Sasaki,“Nanotubesoflepidocrocite titanates,” Chemical Physics Letters, vol. 380, no. 5-6, pp. 577– 582, 2003. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 720183, 9 pages doi:10.1155/2012/720183

Research Article Light-Driven Preparation, Microstructure, and Visible-Light Photocatalytic Property of Porous Carbon-Doped TiO2

Xiao-Xin Zou,1, 2 Guo-Dong Li,2 Jun Zhao,2 Juan Su,1, 2 Xiao Wei,1 Kai-Xue Wang,1 Yu-Ning Wang, 1, 2 and Jie-Sheng Chen1

1 School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China 2 State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China

Correspondence should be addressed to Guo-Dong Li, [email protected] and Jie-Sheng Chen, [email protected]

Received 12 November 2011; Accepted 19 December 2011

Academic Editor: Xuxu Wang

Copyright © 2012 Xiao-Xin Zou et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Highly porous carbon-doped TiO2 (C-TiO2) has been prepared, for the first time, through a light-driven approach using crystalline titanium glycolate (TG) as the single-source precursor. Although the nonthermally prepared porous C-TiO2 is amorphous, it shows a remarkable visible-light photocatalytic activity higher than that of nitrogen-doped TiO2 (N-TiO2) due to its significant surface area (530 m2/g) and pore-rich structure. X-ray photoelectron, electron paramagnetic resonance, and UV-Vis diffuse reflectance spectroscopy reveal that the as-prepared porous C-TiO2 photocatalyst contains Ti–O–C bonds which result in visible- light absorption of the material at wavelengths less than 550 nm. Furthermore, it is discovered that the Ti–O–C bonds in the as- ◦ prepared C-TiO2 is easily transformed to coke-type species under mild thermal treatment (200 C). The resulting coke-containing porous TiO2 is an even better visible-light photocatalyst, almost twice as effective as N-TiO2, because of its stronger visible-light absorption. The Ti–O–C and the coke-containing porous TiO2 materials follow two different mechanisms in the visible-light photocatalysis process for degradation of methylene blue.

1. Introduction Asahi on nitrogen-doped TiO2 (N-TiO2)[3], nonmetal- doped TiO2 has attracted a great deal of attention [4–9], The elimination of hazardous organic pollutants from the for the nonmetal doping can lead to formation of intragap environment has become a major and perennial issue. localized states or bandgap narrowing, improving the visible- Heterogeneous photocatalysis proves to be a green and light photocatalytic activity of the material considerably. In ffi e cient approach to photodecompose organic pollutants by particular, carbon-doped TiO2 (C-TiO2)turnsouttobe solar energy [1]. For this application, typical photocatalysts remarkably effective under visible-light irradiation [10–21], commonly used are semiconductor metal oxides and sulfides and there has been report that C-TiO2 is even superior to N- such as TiO2, ZnO, CdS, and ZnS, among which titanium TiO2 in visible-light photocatalysis [21]. dioxide is regarded as the most promising material due to Introduction of porous structures, which increases the its chemical stability, nontoxicity, and low cost [2]. However, surface area of the photocatalyst to a great extent, is the widespread use of TiO2 is limited by its wide bandgap believed to be an effective approach to further enhance the energy, which causes the catalyst to exploit only a very small photocatalytic performance of C-TiO2. The large surface proportion (about 3 ∼ 5%) of solar radiation. Therefore, it area in combination with the porous feature can facilitate is highly desired to develop strategies of shifting the photo- the diffusion and adsorption of reactant molecules [22, 23], responsive range of TiO2 to visible spectral region. One of offer more surface-active sites [24, 25], and enhance light- the most efficientstrategiesistodopetheTiO2 compound harvesting [24, 25]. Moreover, the transfer path of photogen- with nonmetals. Since the pioneering work reported by erated charges from bulk to surface can be shortened, and as 2 International Journal of Photoenergy a result, the recombination of the photogenerated charges is experiment was a 400 W high-pressure mercury lamp (main greatly suppressed [25]. output at 313 nm). To our knowledge, only a few reports have demonstrated the preparation of porous C-TiO2 visible-light photocata- 2.4. Control Experiments. The as-prepared porous C- lysts [26–31], and the carbon-doping was usually accom- ◦ ◦ TiO2(UV) was heated at 200 C and 500 Cinairfor2h, plished through calcination or hydrothermal treatment using respectively, and the corresponding products were desig- organic species as the carbon sources. In this paper, we nated C-TiO2(200) and TiO2(500). The brown sample C- report a facile light-driven preparation route [32] that TiO2(200) contained 0.86 wt% carbon, and the white sample leads to the successful formation of highly porous C-TiO2 TiO2(500) was carbon-free on the basis of elemental analysis. material without any thermal treatment, using crystalline The N-TiO2 containing 2.3 wt% nitrogen was prepared titanium glycolate (TG) as the single-source precursor. The through a previously reported method using urea as the as-prepared porous C-TiO2 containing only Ti–O–C bonds nitrogen source [34], and more characterization results exhibits distinct visible-light photocatalytic activity. In view about this sample are provided (see XRD in Figure S1, UV- of the amorphous feature of the as-prepared C-TiO2, the Vis in Figure S2, and XPS in Figure S3 in Supplementary superior photocatalytic activity can be attributed to the C- Material available online at doi:10.1155/2012/720183). doping in combination with the large surface area (530 m2/g) of the solid, which is unprecedented among the C-TiO2 photocatalytic materials reported so far. Furthermore, it is 2.5. Photocatalytic Activity. The photocatalytic activity was found that the carbon in Ti–O–C bonds in the as-prepared assessed in aqueous solution in a water-cooled quartz cylindrical cell. Generally, the reaction mixture in the cell was porous C-TiO2 is transformed to coke species after mild ◦ thermal treatment, and the resulting coke-containing porous maintained at about 20 C by a continuous flow of water, and was illuminated with an external light source. The visible- TiO2 shows visible-light photocatalytic performance, even superior over the as-prepared C-TiO . The photocatalysis light source was a 500 W Xe lamp (main output > 400 nm), 2 ff processes for the Ti–O–C and the coke-containing porous with a glass optical filter used to cut o the short wavelength part (λ<420 nm). TiO2 materials follow two different mechanisms. The as-prepared C-TiO2(UV) photocatalyst (0.3 g) was mixedwithanaqueoussolutionofmethyleneblue(MB) 2. Experimental (300 mL, 1 × 10−5 mol/L). The aqueous system was magnetically stirred in dark for at least 2 h to establish an adsorp- 2.1. Materials. Absolute ethanol, ethylene glycol, titanium tion/desorption equilibrium of MB on the particle surface of sulfate, urea, methylene blue (MB), and aqueous ammonia the material and then subjected to visible-light irradiation. were purchased from Beijing Chemical Factory. All the Each reaction cycle lasted for about 4 h during which oxygen reagents were of analytical grade and used as received. was bubbled through the solution. At given irradiation time Titanium n-butoxide was purchased from Tianjin Guangfu intervals, a series of aqueous solution samples (3 mL) were Fine Chemical Research Institute. Deionized water was used collected and separated from the suspended catalyst particles throughout. for analysis. The concentration of the MB was determined on a UV-Vis spectrophotometer by monitoring its characteristic absorption at 665 nm. For comparison, the photocatalytic 2.2. Synthesis of Titanium Glycolate (TG). The TG precursor activities of C-TiO2(200), TiO2(500), and N-TiO2 were also was prepared on a large scale according to the reported pro- measured under the same condition. The weights of all cedure with minor modifications [33]. Typically, titanium the catalyst samples were identical (0.3 g). Considering that n-butoxide (15 mL) was added to ethylene glycol (150 mL) ff ◦ MB can absorb visible light above 600 nm, a cuto filter and heated at 180 C for 2 hours under vigorous stirring (λ>600 nm) was used to ensure that only MB was excited, to form the TG compound. After cooling down to room and as a result, there was no significant change in the MB temperature, the white TG precipitate was washed several concentration after 4 h irradiation even in the presence of C- times with ethanol and dried in an oven at 60◦C. TiO2(UV) or C-TiO2(200).

2.3. Light-Driven Preparation of Porous C-TiO2. The TG 2.6. General Characterization. The powder X-ray diffraction precursor (4.0 g) was dispersed in water (400 mL) and (XRD) patterns were recorded on a Rigaku D/Max 2550 then exposed to the UV-light irradiation for 2 h. After X-ray diffractometer with Cu Kα radiation (λ = 1.5418 A)˚ the irradiation, the color of the solid sample turned from whereas the TEM images were obtained on a JEOL JSM- white (TG) to intense blue because of the presence of Ti3+ 3010 TEM microscope. The UV-Vis diffuse reflectance [32]. Finally, the blue solid product was separated from the spectra were recorded on a Perkin-Elmer Lambda 20 UV/Vis mixture and dried in air, the O2 molecules of which oxidize spectrometer, and the absorbance spectra were obtained 3+ 4+ the Ti to Ti . The obtained light yellow TiO2 product, from the reflectance spectra by means of Kubelka-Munk designated C-TiO2(UV), was amorphous and porous on the transformation. The IR spectra were acquired on a Bruker basis of X-ray diffraction and adsorption measurement. The IFS 66 v/S FTIR spectrometer. The carbon contents of elemental analysis indicated that the content of carbon in the obtained samples were determined through elemental the material was 1.08 wt%. The UV-light source used in the analysis on a Perkin-Elmer 2400 elemental analyzer. The International Journal of Photoenergy 3

Table 1: Surface area, pore size, and pore volume of the TiO2 samples.

Surface area Pore size Pore volume Sample 2 −1 3 −1 TiO2(500) (m g ) (nm) (cm g )

C-TiO2(UV) 530 1.8 0.3

C-TiO2(200) 340 2.6 1.2

C-TiO2(200) TiO2(500) 89 3.7 0.1

Intensity (a.u.) Intensity N-TiO2 89 8.2 0.2

C-TiO2(UV)

C-TiO2(200), respectively. Interestingly, the mild thermal treatment at 200◦C results in not only obvious loss of BET- 20 30 40 50 60 70 80 specific surface area (from 530 to 340 m2 g−1), but also 2θ (deg) an unusual increase of pore size (from 1.8 to 2.6 nm) and 3 −1 Figure 1: Powder XRD patterns of C-TiO2(UV), C-TiO2(200), and pore volume (from 0.3 to 1.2 cm g )(Table 1). In general, TiO2(500). surface area, pore size, and pore volume should decrease simultaneously after thermal treatment due to structural shrinkage. The increase of pore size and pore volume of our material after the thermal treatment may result from a IR spectra were acquired on a Bruker IFS 66 v/S FTIR structural rearrangement of pore wall of the porous C-TiO . spectrometer whereas the X-ray photoelectron spectra (XPS) 2 For TiO2(500), the N2 adsorption measurement leads to a were recorded on a VG ESCALAB MK II electron spec- − BET surface area of 89 m2 g 1, a pore size of 3.7 nm, and trometer. The nitrogen adsorption and desorption isotherms − aporevolumeof0.1cm3 g 1. Obviously, at this treatment were measured using a Micromeritics ASAP 2020 M system. temperature, the pore structure of the TiO material is The electron paramagnetic resonance (EPR) spectra were 2 damaged to a considerable extent although its crystallinity recorded on a JEOL JES-FA 200 EPR spectrometer. The is increased significantly. concentration of MB was analyzed with a Shimadzu UV-2450 To further verify the presence of porous structures in spectrophotometer. the obtained TiO2 samples, TEM and HRTEM have been performed and the corresponding images are presented in 3. Results and Discussion Figure 3.TheTEMimages(Figure 3(a)–3(c)) reveal that both C-TiO2(UV) and C-TiO2(200) possess a uniform 3.1. General Structural Characterization. The powder X-ray worm-like porous structure, while TiO2(500) displays only ff di raction (XRD) patterns of the as-prepared porous TiO2 an irregular porous structure formed by randomly arranged photocatalysts are presented in Figure 1. The sample C- and interconnected nanocrystals. The lattice spacing in TiO2(UV) directly obtained by the light-driven technique Figure 3(d), obtained by HRTEM, is about 0.352 nm, which is noncrystalline on the basis of the X-ray diffraction. After is in accordance with the distance between (101) crystal ◦ thermal treatment at 200 C, the product, C-TiO2(200), is planes of the anatase phase. still dominated by an amorphous phase, whereas at 500◦C the obtained material TiO2(500) is identified as pure anatase. ThebroadXRDpeaksforTiO2(500) indicate that this 3.2. Chemical Nature of Carbon Species. To elucidate the material is composed of nanoparticles and the corresponding nature of the carbon species in the obtained porous C- particle size, estimated by the Scherrer formula, is about TiO2 photocatalysts, infrared (IR) spectroscopy has been 8nm. performed in combination with electron paramagnetic res- The specific surface area and the pore structure of the onance (EPR) and X-ray photoelectron spectroscopy (XPS). porous TiO2 solids were evaluated through N2 adsorption The IR spectra of C-TiO2(UV), C-TiO2(200), and the measurements. In Figure 2, the N2 adsorption/desorption precursor TG are shown in Figure 4. Upon UV-irradiation of isotherms and the corresponding BJH pore-size distribution TG, the IR absorption peaks related to organic species in the for the three samples are presented. It is seen that the N2 TG precursor almost completely disappear, and only a very adsorption/desorption isotherms (Figure 2(a))arecharac- weak absorption at 1067 cm−1, which is associated with the teristic type IV curves, demonstrating the presence of a presence of a small quantity of Ti–O–C bonds, remains. The porous structure in all the three materials. For C-TiO2(UV) above observation indicates that UV-irradiation of TG leads and C-TiO2(200), the negligible hysteresis loop at high to the removal of organic species form TG and the formation relative pressures (P/P0) indicates that the pore size of these of a final product C-TiO2(UV) with a nominal formula TiO2 two samples is uniform and small. This result is in agreement and a small amount of Ti–O–C species. Further mild thermal with the pore-size distribution measurement (Figure 2(b)), treatment (200◦C) leads to complete elimination of the Ti– which shows a narrow pore-size distribution with average O–C absorption. This result demonstrates that the Ti–O– pore sizes of 1.8 nm and 2.6 nm for C-TiO2(UV) and C bonds in C-TiO2(UV) are not thermally stable, and can 4 International Journal of Photoenergy

180 0.8 0.7

) 150 1 − ) 0.6 1 g − 3

120 nm 0.5 1 − g

3 0.4 90 (cm 0.3 60 dV/dD 0.2 Volume adsorbed (cm Volume

30 0.1

0 0 0 0.2 0.4 0.6 0.8 1 03691215

P/P0 Pore size (nm) (a) (b)

Figure 2: (a) N2 adsorption/desorption isotherms and (b) the corresponding BJH pore size distributions of ( ) C-TiO2(UV), ()C- TiO2(200), and ()TiO2(500).

(a) (b)

Figure 3: TEM images of (a) C-TiO2(UV), (b) C-TiO2(200), (c) TiO2(500), and (d) HRTEM image of TiO2(500). International Journal of Photoenergy 5

results indicate that the presence of carbon species aﬀect the TG local chemical circumstances of surface elements (Ti4+ and O2−), and strong interaction between carbon species and surface elements is present. To reveal the nature of the remained carbon species in C- HO C-H [

C-H TiO2(200), EPR spectroscopy has been employed to examine

C-TiO2(UV) C-O-Ti C-C-O the C-TiO2(UV) and C-TiO2(200) samples (Figure 6). No EPR signals for paramagnetic species are observed for C- TiO2(UV) whereas C-TiO2(200) shows a distinct singlet

Transmittance (a.u.) Transmittance signal at g = 2.0023 assignable to coke-type carbon species.

C-TiO2(200) Similar EPR signals have also been observed for other porous materials containing coke species [30, 36–38]. Taking into account the existence of coke in C-TiO2(200), it is believed HO

HO that the weak signals of C–O and C=O bonds (Figure 5) observed in this case are derived from the coke through its partial oxidization during the thermal treatment [38]. The 4000 3500 3000 2500 2000 1500 1000 500 XPS signal for the coke itself should be located at 284.8 eV, −1 Wavenumber (cm ) covered by the adventitious elemental carbon peak from the XPS instrument, and thus cannot be distinguished. From the Figure 4: The IR spectra of TG, C-TiO2(UV), and C-TiO2(200). above results, it is easily concluded that the carbon species (Ti–O–C bonds) in C-TiO2(UV) are driven off from the TiO2 framework during the mild thermal treatment, forming be easily transformed to other forms of carbon in the TiO2 a new type of carbonaceous matter (coke), on the surface material. of the C-TiO2(200) material. The signal of coke disappears The X-ray photoelectron spectroscopy (XPS) has been for the carbon-free TiO2(500) sample, which is obtained ◦ used to obtain valuable information about the chemi- after thermal treatment of C-TiO2(UV) at 500 Cinair,asat cal nature of surface elements for the C-TiO2 materials. this temperature the coke species is completely oxidized and Figures 5(a) and 5(b) show the high-resolution C1s spectra removed from the solid sample. of C-TiO2(UV) and C-TiO2(200) samples. The peak at 284.8 eV for both samples are due to adventitious elemental 3.3. Photoresponsive Range. The UV-Vis diffuse reflectance carbon from the XPS instrument [28], and this peak is spectra (Figure 7) demonstrate that the light-yellow C- also observed for the carbon-free TiO (500) sample. Besides 2 TiO (UV) shows two optical absorption thresholds at the peak at 284.8 eV, a shoulder peak associated with Ti– 2 385 nm in the ultraviolet region and 550 nm in the visible O–C bonds at about 285.9 eV is detected for C-TiO (UV). 2 region. In comparison, no visible-light absorption but an Furthermore, no peaks appear at around 282 eV (Ti-C absorption threshold at 400 nm appears for the white bonds) and 288.5 eV (C=O bonds), suggesting that except TiO (500). The ultraviolet absorption thresholds for both for Ti–O–C bonds, neither O–Ti–C bonds nor carbonate 2 C-TiO (UV) and TiO (500) correspond to the inherent species are present in C-TiO (UV). After the mild thermal 2 2 2 bandgap absorptions of TiO , and the small difference treatment (at 200◦C), the peak of Ti–O–C bonds disappears, 2 between these threshold values are due to the difference in and only two weak peaks ascribed to C–O (286.4 eV) and crystal structure (amorphous for C-TiO (UV), anatase for C=O (288.5 eV) bonds are observed for C-TiO (200). The 2 2 TiO (500)). The visible-light absorption band between 400 simultaneous presence of C–O and C=O bonds was con- 2 and 550 nm for C-TiO (UV) arises from the Ti–O–C bonds sidered to be characteristic of carbonate species previously, 2 which form localized occupied states in the bandgap of TiO , but the carbonate species were not chromophores in nature 2 as predicted by density function theory (DFT) calculations [35]. Thus, the strong visible-light response of the brown C- [39]. The energies of these intragap states are higher than TiO (200) may arise from other carbon species, which were 2 that of the top of the TiO valence band, and as a result, the not detected by XPS. Figure 5(c) shows the high-resolution 2 electron transitions from these intragap states to the TiO XPS spectra of Ti2p for C-TiO (UV), C-TiO (200), and 2 2 2 conduction band absorb energies distinctly lower than the TiO (500). It is seen that the XPS spectrum of Ti2p for the 2 TiO bandgap, corresponding to visible-light radiation. For TiO (500) sample exhibit two peaks at 464.4 and 458.8 eV, 2 2 the UV-Vis diffuse reflectance spectrum of the brown C- which are assigned to the 2p and 2p core level of Ti4+, 1/2 3/2 TiO (200), a broad and strong band covering the whole respectively. In comparison with XPS peaks of the TiO (500) 2 2 visible region appears and this absorption is attributed to the sample, an obvious peak shift towards high binding energy presence of coke-type species in the material. (0.2 eV) in the Ti2p spectra of C-TiO2(UV) and C-TiO2(200) is observed. Similar peak shift was also observed in the O1s spectra of C-TiO2(UV) and C-TiO2(200), as demonstrated 3.4. Photocatalytic Performance. The photocatalytic perfor- in Figure 5(d).TiO2(500) gives an XPS peak related to mances of the obtained C-TiO2 materials have been eval- Ti–O–Ti oxygen at 530.0 eV, whereas C-TiO2(UV) and C- uated by testing the degradation of methylene blue (MB), TiO2(200) exhibit this XPS peak at 530.3 eV. The above which is often used as a model pollutant in semiconductor 6 International Journal of Photoenergy

C-TiO2(UV) C-TiO2(200) Intensity (a.u.) Intensity Intensity (a.u.) Intensity

278 280 282 284 286 288 290 292 278 280 282 284 286 288 290 292 Binding energy (eV) Binding energy (eV) (a) (b)

2p3/2 Ti2p Ti-O O1s

2p1/2 TiO2(500) H–O TiO2(500)

C-TiO2(200) C-TiO2(200) Intensity (a.u.) Intensity Intensity (a.u.) Intensity

C-TiO2(UV) C-TiO2(UV)

454 456 458 460 462 464 466 468 470 472 526 528 530 532 534 536 Binding energy (eV) Binding energy (eV) (c) (d)

Figure 5:High-resolutionXPSspectraofC1sfor(a)C-TiO2(UV) and (b) C-TiO2(200); high-resolution XPS spectra of (c) Ti2p and (d) O1s for C-TiO2(UV), C-TiO2(200), and TiO2(500).

C-TiO2(UV)

C-TiO2(200) = C-TiO2(200) g 2.0023 C-TiO2(500) K-M units Intensity (a.u.) Intensity

325 330 335 340 345 350 355 300 400 500 600 700 800 Magnetic ﬁeld (mT) Wavelength (nm)

Figure 6: The EPR spectra of C-TiO2(UV) and C-TiO2(200) Figure 7: UV-Vis diﬀuse reﬂectance spectra of C-TiO2(UV), C- measured at room temperature. TiO2(200), and TiO2(500). International Journal of Photoenergy 7

100 1

80 0.8

0 0.6 C / t

40 A B C

Adsorption ofAdsorption MB (%) 0.4 20

C D 0.2 0

Figure 8: The absorption capacity of (A) C-TiO2(UV), (B) C- TiO2(200), (C) TiO2(500), and (D) N-TiO2 for MB. 0 0 60 120 180 240 Figure 9: The residual fraction of MB in solution as a function of irradiation time with ()C-TiO2(UV), ()C-TiO2(200), () photocatalysis, under visible light (λ>420 nm) irradiation. TiO2(500), and (•)N-TiO2. During the whole process of photocatalysis, the reaction system is saturated by oxygen, which can prevent MB from reduction to colorless leuco form (LMB) [40]. For comparison, the photocatalytic performance of N-TiO2 has also been measured under the same condition. N-TiO2,C-TiO2(UV), and C-TiO2(200), respectively. The It is generally believed that the capacity to adsorb photocatalytic activities of the two C-doped TiO2 materials reactant molecules on the surface of a solid material is a key are distinctly superior to that of the N-TiO2 sample. In parameter for its photocatalytic activity [41–43]. Therefore, contrast, only slight degradation of MB (<5%) has been the MB absorption capacities of the obtained samples were observed for TiO2(500) after visible-light irradiation of 4 assessed before light irradiation, and the results are shown hours, indicating that the TiO2(500) material is not a good in Figure 8. The obtained sample (0.3 g) was mixed with visible-light photocatalyst. In addition, the difference of an aqueous solution of methylene blue (MB) (300 mL, 1 × visible-light photocatalytic performances of the obtained −5 10 mol/L). After the adsorption/desorption equilibration TiO2 materials can be easily explained on the basis of the is reached, about 44% and 45% of the dye are removed UV-Vis diffuse reflectance spectra of the samples (Figure 7). from the respective aqueous solutions by adsorption on The coke-containing C-TiO2(200) and the C-TiO2(UV) with the C-TiO2(UV) and C-TiO2(200) surfaces, while only 11% Ti–O–C bonds both absorb visible light, and consequently, and 10% of the initial dye are absorbed by TiO2(500) they are visible-light responsive in the photocatalytic process. and N-TiO2, respectively. The high adsorption capacities In contrast, the TiO2(500) sample absorbs no visible light, for C-TiO2(UV) and C-TiO2(200) can be attributed to and therefore this material is not photocatalytically active 2 −1 their significant surface areas (530 m g for C-TiO2(UV), under visible-light irradiation. The higher photocatalytic 2 −1 340 m g for C-TiO2(200)). It is of interest to note that C- activity of C-TiO2(200) in comparison with that of C- TiO2(200) shows an adsorption capacity similar to that of C- TiO2(UV) is rationalized by its stronger visible-light TiO2(UV) whereas the surface area of C-TiO2(200) is smaller absorption. The recycling experiments demonstrate that than that of C-TiO2(UV). This unusual observation can be both C-TiO2(UV) and C-TiO2(200) are rather stable when explained by the fact that the pore volume (1.2 cm3 g−1)ofC- used as photocatalysts and there is no loss of photocatalytic 3 −1 TiO2(200) is about 4-times as large as that (0.3 cm g )ofC- activity after at least five cycles of degradation testing of MB. TiO2(UV). In addition, it has been reported that coke matter Considering that MB can absorb visible light above contains polyaromatic structures [30, 36–38], and the π-π 600 nm (the absorption peak is at about 665 nm), a cutoff interactions between coke and the aromatic rings of MB may filter (λ>600 nm) was used to ensure that only MB also contribute to the adsorption of MB by the TiO2(200) was excited during the photocatalysis process. Whereas the material. photocatalysts we used nearly do not absorb the visible light The photocatalytic performances of the samples with a wavelength longer than 600 nm. With the use of (Figure 9) are measured by the changes of MB concentration the cutoff filter, no significant change in MB concentration (Ct/C 0) during the process of photodegradation reaction was observed for the photocatalytic reaction even after 4 h under visible-light irradiation (λ>420 nm), where Ct is the irradiation in the presence of C-TiO2(UV) or C-TiO2(200). concentration of MB at the irradiation time t and C 0 is the This observation indicates that the decoloration of MB under initial concentration of MB after an adsorption/desorption visible light with use of cutoff filter (λ>420 nm) is attributed equilibrium is reached before irradiation. 37%, 50%, to the photocatalytic effect of the obtained C-doped TiO2 and 75% of MB are degraded after 4 h irradiation for samples, rather than MB photosensitization. 8 International Journal of Photoenergy

•− O 2 Acknowledgments ∗ C•+ C − Ti 3de Ti 3d O This paper was financially supported by the National Basic CB CB 2 Vis Research Program of China (2007CB613303), the National C Natural Science Foundation of China. The authors thank UV Vis UV Mingyi Guo for TEM measurement. VB VB O 2p O 2p C-TiO 2(U V ) C-TiO 2(200) References Scheme 1: The photocatalytic mechanisms for C-TiO (UV) and C- 2 [1] M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. TiO (200). 2 Bahnemann, “Environmental applications of semiconductor photocatalysis,” Chemical Reviews, vol. 95, no. 1, pp. 69–96, 3.5. Photocatalytic Mechanism. The carbon species in C- 1995. [2] X. Chen and S. S. Mao, “Titanium dioxide nanomaterials: syn- TiO (UV) and C-TiO (200) are very different in nature, 2 2 thesis, properties, modifications and applications,” Chemical and therefore, they contribute to the photocatalytic perfor- ff Reviews, vol. 107, no. 7, pp. 2891–2959, 2007. mances of the corresponding materials in di erent manners [3] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, (Scheme 1). “Visible-light photocatalysis in nitrogen-doped titanium For C-TiO2(UV), the carbons are incorporated in the oxides,” Science, vol. 293, no. 5528, pp. 269–271, 2001. TiO2 lattice to form Ti–O–C bonds. As demonstrated earlier [4] W. Zhao, W. Ma, C. Chen, J. Zhao, and Z. Shuai, “Efficient by the UV-Vis spectroscopy, the electron transitions from degradation of toxic organic pollutants with Ni2O3/TiO2−xBx the localized states associated with these Ti–O–C bonds to under visible irradiation,” Journal of the American Chemical the TiO2 conduction band absorb visible light, and hence Society, vol. 126, no. 15, pp. 4782–4783, 2004. generating electrons and holes upon visible-light irradiation. [5] H. Luo, T. Takata, Y. Lee, J. Zhao, K. Domen, and Y. Yan, The photogenerated electrons on the conduction band of “Photocatalytic activity enhancing for titanium dioxide by co- TiO interact with O molecules to form oxidative species doping with bromine and chlorine,” Chemistry of Materials, 2 2 vol. 16, no. 5, pp. 846–849, 2004. such as superoxide radicals which degrade the MB molecules. ff [6] G. Liu, Y. Zhao, C. Sun, F. Li, G. Q. Lu, and H. M. For C-TiO2(200), the photocatalytic mechanism di ers Cheng, “Synergistic effects of B/N doping on the visible- to a certain extent. In this material, there exists coke on the light photocatalytic activity of mesoporous TiO2,” Angewandte surface of TiO2. As demonstrated previously in the literature Chemie—International Edition, vol. 47, no. 24, pp. 4516–4520, [30, 44], the coke itself can be photoexcited under visible- 2008. light irradiation, and the excited carbon species inject the [7] W. Ho, J. C. Yu, and S. Lee, “Synthesis of hierarchical photogenerated electrons into the conduction band of TiO2. nanoporous F-doped TiO2 spheres with visible light photocat- The injected electrons move to the surface of the TiO2,where alytic activity,” Chemical Communications, no. 10, pp. 1115– ·− 1117, 2006. they are captured by O2 to form superoxide ions (O2 ), which finally lead to the degradation of MB. In addition, the [8] D. Mitoraj and H. Kisch, “The nature of nitrogen-modified photogenerated holes located in coke can also directly oxidize titanium dioxide photocatalysts active in visible light,” Ange- wandte Chemie—International Edition, vol. 47, no. 51, pp. and degrade MB. 9975–9978, 2008. [9] G. Liu, Z. Chen, C. Dong et al., “Visible light photocatalyst: 4. Conclusions iodine-doped mesoporous titania with a bicrystalline framework,” JournalofPhysicalChemistryB, vol. 110, no. 42, pp. An unusual light-driven strategy is explored for the prepa- 20823–20828, 2006. ration of highly porous C-doped TiO2.Ithasbeendemon- ffi ffi [10] S. U. M. Khan, M. Al-Shahry, and W. B. Ingler, “E cient strated that the obtained material exhibits high e ciency photochemical water splitting by a chemically modified n- in visible-light photocatalysis for degradation of methylene TiO2,” Science, vol. 297, no. 5590, pp. 2243–2245, 2002. blue. The carbon species, which are responsible for the [11]J.H.Park,S.Kim,andA.J.Bard,“Novelcarbon-dopedTiO2 visible-light photocatalytic activity, in the porous C-TiO2 are nanotube arrays with high aspect ratios for efficient solar found to exist in the form of Ti–O–C bonds. The Ti–O– water splitting,” Nano Letters, vol. 6, no. 1, pp. 24–28, 2006. C bonds are thermally nonstable and can be transformed [12] J. Graciani, Y. Ortega, and J. F. Sanz, “Carbon doping of the ◦ to coke matter after mild thermal treatment at 200 C. The TiO2 (110) rutile surface. A theoretical study based on DFT,” Chemistry of Materials, vol. 21, no. 8, pp. 1431–1438, 2009. resulting coke-containing TiO2 is proved to be an even better visible-light photocatalyst, almost twice as effective as N- [13] K. Yang, Y. Dai, B. Huang, and M. Whangbo, “Density functional characterization of the visible-light absorption in TiO2. Our experiments reveal that both the Ti–O–C bonds substitutional C-anion- and C-cation-doped TiO2,” Journal of and the coke species play a role in visible-light photocatalysis, Physical Chemistry C, vol. 113, no. 6, pp. 2624–2629, 2009. providing new insights into the origin of visible-light pho- [14] B. Neumann, P. Bogdanoff,H.Tributsch,S.Sakthivel,and tocatalytic performance of C-TiO2 materials. Moreover, the H. Kisch, “Electrochemical mass spectroscopic and surface strategy reported here is anticipated to open vistas for light- photovoltage studies of catalytic water photooxidation by driven preparation of inorganic materials with advanced undoped and carbon-doped titania,” Journal of Physical Chem- functions. istry B, vol. 109, no. 35, pp. 16579–16586, 2005. International Journal of Photoenergy 9

[15] H. Liu, A. Imanishi, and Y. Nakato, “Mechanisms for pho- [30] C. Lettmann, K. Hildenbrand, H. Kisch, W. Macyk, and W. F. tooxidation reactions of water and organic compounds on Maier, “Visible light photodegradation of 4-chlorophenol with carbon-doped titanium dioxide, as studied by photocurrent a coke-containing titanium dioxide photocatalyst,” Applied measurements,” JournalofPhysicalChemistryC, vol. 111, no. Catalysis B, vol. 32, no. 4, pp. 215–227, 2001. 24, pp. 8603–8610, 2007. [31] J. A. Toledo-Fernandez,´ R. Mendoza-Serna, V. Morales- [16]C.Xu,R.Killmeyer,M.L.Gray,andS.U.M.Khan,“Photocat- Florez´ et al., “Aerogels with applications in biomedicine and alytic effect of carbon-modified n-TiO2 nanoparticles under environment,” Boletin de la Sociedad Espanola de Ceramica y visible light illumination,” Applied Catalysis B,vol.64,no.3-4, Vidrio, vol. 46, no. 3, pp. 138–144, 2007. pp. 312–317, 2006. [32] X. X. Zou, G. D. Li, K. X. Wang, L. Li, J. Su, and J. S. [17] G. Wu, T. Nishikawa, B. Ohtani, and A. Chen, “Synthesis and Chen, “Light-induced formation of porous TiO2 with superior characterization of carbon-doped TiO2 nanostructures with electron-storing capacity,” Chemical Communications, vol. 46, enhanced visible light response,” Chemistry of Materials, vol. no. 12, pp. 2112–2114, 2010. 19, no. 18, pp. 4530–4537, 2007. [33] X. X. Zou, G. D. Li, M. Y. Guo et al., “Heterometal alkoxides [18]Y.Park,W.Kim,H.Park,T.Tachikawa,T.Majima,andW. as precursors for the preparation of porous Fe- and Mn-TiO2 Choi, “Carbon-doped TiO2 photocatalyst synthesized without photocatalysts with high efficiencies,” Chemistry—A European using an external carbon precursor and the visible light Journal, vol. 14, no. 35, pp. 11123–11131, 2008. activity,” Applied Catalysis B, vol. 91, no. 1-2, pp. 355–361, [34] Y. Wu, H. Liu, J. Zhang, and F. Chen, “Enhanced photo- 2009. catalytic activity of nitrogen-doped titania by deposited with [19] E. M. Rockafellow, X. Fang, B. G. Trewyn, K. Schmidt-Rohr, gold,” Journal of Physical Chemistry C, vol. 113, no. 33, pp. andW.S.Jenks,“Solid-state13C NMR characterization of 14689–14695, 2009. carbon-modified TiO2,” Chemistry of Materials, vol. 21, no. 7, [35] Y. Li, D. S. Hwang, N. H. Lee, and S. J. Kim, “Synthesis and pp. 1187–1197, 2009. characterization of carbon-doped titania as an artificial solar [20] H. Kamisaka, T. Adachi, and K. Yamashita, “Theoretical study light sensitive photocatalyst,” Chemical Physics Letters, vol. of the structure and optical properties of carbon-doped rutile 404, no. 1–3, pp. 25–29, 2005. and anatase titanium oxides,” Journal of Chemical Physics, vol. [36] J. P. Lange, A. Gutsze, and H. G. Karge, “Coke formation 123, no. 8, Article ID 084704, 9 pages, 2005. through the reaction of olefins over hydrogen mordenite. [21] S. Sakthivel and H. Kisch, “Daylight photocatalysis by I. EPR measurements under static conditions,” Journal of carbon-modified titanium dioxide,” Angewandte Chemie— Catalysis, vol. 114, no. 1, pp. 136–143, 1988. International Edition, vol. 42, no. 40, pp. 4908–4911, 2003. [37] H. G. Karge, J. P. Lange, A. Gutsze, and M. Łaniecki, “Coke [22] Z. Liu, D. D. Sun, P. Guo, and J. O. Leckie, “One-step formation through the reaction of olefins over hydrogen fabrication and high photocatalytic activity of porous TiO 2 mordenite. II. In situ EPR measurements under on-stream hollow aggregates by using a low-temperature hydrothermal conditions,” JournalofCatalysis, vol. 114, no. 1, pp. 144–152, method without templates,” Chemistry—A European Journal, 1988. vol. 13, no. 6, pp. 1851–1855, 2007. [38] C. L. Pieck, E. L. Jablonski, J. M. Parera, R. Frety, and F. [23] X. Wang, J. C. Yu, C. Ho, Y. Hou, and X. Fu, “Photocatalytic Lefebvre, “Characterization of residual coke during burning,” activity of a hierarchically macro/mesoporous titania,” Lang- Industrial and Engineering Chemistry Research, vol. 31, no. 4, muir, vol. 21, no. 6, pp. 2552–2559, 2005. pp. 1017–1021, 1992. [24] H. Li, Z. Bian, J. Zhu et al., “Mesoporous titania spheres [39] C. D. Valentin, G. Pacchioni, and A. Selloni, “Theory of carbon with tunable chamber stucture and enhanced photocatalytic doping of titanium dioxide,” Chemistry of Materials, vol. 17, activity,” Journal of the American Chemical Society, vol. 129, no. 26, pp. 6656–6665, 2005. no. 27, pp. 8406–8407, 2007. [40] A. Mills and J. Wang, “Photobleaching of methylene blue [25] Z. Bian, J. Zhu, S. Wang, Y. Cao, X. Qian, and H. Li, sensitised by TiO2: an ambiguous system?” Journal of Photo- “Self-assembly of active Bi2O3/TiO2 visible photocatalyst chemistry and Photobiology A, vol. 127, no. 1–3, pp. 123–134, with ordered mesoporous structure and highly crystallized 1999. anatase,” Journal of Physical Chemistry C, vol. 112, no. 16, pp. [41] W. Morales, M. Cason, O. Aina, N. R. De Tacconi, and 6258–6262, 2008. K. Rajeshwar, “Combustion synthesis and characterization [26] F. Dong, H. Wang, and Z. Wu, “One-step “Green” synthetic of nanocrystalline WO3,” Journal of the American Chemical approach for mesoporous C-doped titanium dioxide with effi- Society, vol. 130, no. 20, pp. 6318–6319, 2008. cient visible light photocatalytic activity,” Journal of Physical [42] Y. C. Hsu, H. C. Lin, C. W. Lue, Y. T. Liao, and C. M. Yang, Chemistry C, vol. 113, no. 38, pp. 16717–16723, 2009. “A novel synthesis of carbon-coated anatase nanocrystals [27] D. E. Gu, Y. Lu, B. C. Yang, and Y. D. Hu, “Facile prepara- showing high adsorption capacity and photocatalytic activity,” tion of micro-mesoporous carbon-doped TiO2 photocatalysts Applied Catalysis B, vol. 89, no. 3-4, pp. 309–314, 2009. with anatase crystalline walls under template-free condition,” [43] H. Zhang, X. Lv, Y. Li, Y. Wang, and J. Li, “P25-graphene Chemical Communications, no. 21, pp. 2453–2455, 2008. composite as a high performance photocatalyst,” ACS Nano, [28] W. Wang, Z. Wu, and Y. Liu, “A simple two-step template vol. 4, no. 1, pp. 380–386, 2010. approach for preparing carbon-doped mesoporous TiO2 [44] X. Yang, C. Cao, K. Hohn et al., “Highly visible-light active C- hollow microspheres,” JournalofPhysicalChemistryC, vol. and V-doped TiO2 for degradation of acetaldehyde,” Journal of 113, no. 30, pp. 13317–13324, 2009. Catalysis, vol. 252, no. 2, pp. 296–302, 2007. [29]Y.Huang,W.Ho,S.Lee,L.Zhang,G.Li,andJ.C.Yu, “Effect of carbon doping on the mesoporous structure of nanocrystalline titanium dioxide and its solar-light-driven photocatalytic degradation of NOx,” Langmuir, vol. 24, no. 7, pp. 3510–3516, 2008. Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 797968, 10 pages doi:10.1155/2012/797968

Research Article One-Pot Template-Free Hydrothermal Synthesis of Monoclinic BiVO4 Hollow Microspheres and Their Enhanced Visible-Light Photocatalytic Activity

Bei Cheng,1 Wenguang Wang,1 Lei Shi,1 Jun Zhang,1 Jingrun Ran,1 and Huogen Yu2

1 State Key Laboratory of Silicate Materials for Architectures, Materials College, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China 2 Department of Chemistry, School of Science, Wuhan University of Technology, Wuhan 430070, China

Correspondence should be addressed to Bei Cheng, [email protected]

Received 24 October 2011; Accepted 4 November 2011

Academic Editor: Baibiao Huang

Copyright © 2012 Bei Cheng et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Monoclinic-phase BiVO4 hollow microspheres with diameters of about 2–4 μm have been successfully fabricated in high yield by a one-pot template-free hydrothermal route. The reaction duration and urea concentration are shown to play important roles in the formationoftheBiVO4 hollow microspheres. X-ray diffraction, scanning electron microscopy, nitrogen adsorption-desorption isotherms, fourier transform infrared spectrometry, and UV-visible diffuse reflectance spectroscopy are used to characterize the products. The results show that all the as-prepared BiVO4 samples have monoclinic phase structure and exhibit good crystallinity. A formation mechanism for the BiVO4 hollow spherical structure via a localized Ostwald ripening is proposed based on the experimental observations. In addition, studies of the photocatalytic properties by exposure to visible light irradiation demonstrate • that the as-obtained BiVO4 hollow spheres show potential photocatalytic application. Hydroxyl radicals ( OH) are not detected • on the surface of visible-light-illuminated BiVO4 by the photoluminescence technique, suggesting that OH is not the dominant photooxidant and photogenerated hole could directly take part in photocatalytic reaction. The prepared BiVO4 hollow spheres are also of great interest in pigment, catalysis, separation technology, biomedical engineering, and nanotechnology.

1. Introduction trolled preparation of hollow nanostructures with rationally designed parameters is highly attractive [15]. Currently, one- In recent years, hollow micro/nanostructures have attracted pot template-free methods for hollow structures have been considerable attention because of their novel physicochem- developed mainly based on direct solid evacuation arising ff ical properties that di er markedly from those of bulk from Ostwald ripening, the Kirkendall effect, or chemically materials and potential applications such as nanoscale induced self-transformation [16–19]. ffi chemical reactors, e cient catalysis, drug-delivery carriers, Bismuth vanadate (BiVO4), which has been recognized as low dielectric constant materials, acoustic insulation, and an effective visible-light photocatalyst for photocatalytic O2 photonic building blocks [1–5]. Conventional methods for evolution from aqueous AgNO3 solutions, but also for pho- the preparation of hollow spheres usually require removable tocatalytic degradation of organic pollutants under visible- or sacrificial templates, including hard ones [6–9]orsoft light irradiation, has received significant attention [20–27]. ones [10–14] to direct the formation of inorganic nanopar- On the other hand, the properties of BiVO4 are strongly ticles on their surfaces via adsorption or chemical reactions. dependent on its morphology and microstructure [24, 26, However, use of templates usually suffers from disadvantages 28–33]. According to previous reports, BiVO4 appears in related to high cost and tedious synthetic procedures, which three main crystalline phases: monoclinic scheelite, tetrag- may prevent them from being used in large-scale applica- onal zircon, and tetragonal scheelite [34–36]. Among the tions. Compared with these methods involving multistep above three crystal phase, monoclinic BiVO4 is the best procedures, a one-pot template-free method for the con- visible-light-driven photocatalyst for the degradation of 2 International Journal of Photoenergy

organic pollutants and O2 production from water splitting present and their crystallite size. The accelerating voltage due to the transition from a valence band formed by Bi6s and applied current were 40 kV and 80 mA, respectively. The or a hybrid orbital of Bi6s and O2p to a conduction band average crystallite size of BiVO4 was quantitatively calculated of V3d and its narrow band gap (ca. 2.4 eV), while the using Scherrer formula (d = 0.9λ/Bcosθ,whered, λ, B, photocatalytic activity of tetragonal BiVO4 is negligible [28]. and θ arecrystallitesize,CuKα wavelength (0.15418 nm), Moreover, BiVO4 powders with various morphologies have full width at half maximum intensity (FWHM) of (121) been synthesized through different templating routes. For for BiVO4 peak in radians and Bragg’s diffraction angle, example, Yin and coworkers have synthesized monoclinic resp.) after correcting the instrumental broadening. The BiVO4 hollow spheres with a size of about 700 nm by Brunauer-Emmett-Teller (BET) specific surface area (SBET) employing colloidal carbon spheres (CCSs) as hard templates of the powders was analyzed by nitrogen adsorption in a [37]. Recently, by using mesoporous silica KIT-6 as a hard Micromeritics ASAP 2020 nitrogen adsorption apparatus ◦ template, Yu and coworkers synthesized monoclinic BiVO4 (USA). All the samples were degassed at 180 Cpriorto ordered mesoporous nanocrystals [38]. Microspheric and nitrogen adsorption measurements. The BET surface area lamellar BiVO4 powders were selectively prepared through was determined by a multipoint BET method using the a hydrothermal process by using cetyltrimethylammonium adsorption data in the relative pressure (P/P0)rangeof bromide (CTAB) as a template-directing reagent [39]. How- 0.05∼0.3. Infrared (IR) spectra on pellets of the samples ever, the template-assisted method not only increased the mixed with KBr were recorded on a IRAffinity-1 FTIR product cost but also made it more difficult to scale up spectrometer (Shimadzu, Japan) at a resolution of 4 cm−1. production due to its complexity and the capability of a The concentration of the samples was kept at about 0.25– template. The template-free synthesis of well-crystallized 0.3%. UV-vis diffused reflectance spectra of BiVO4 powders monoclinic BiVO4 hollow spheres still remains a great were obtained for the dry-pressed disk samples using a UV- challenge. vis spectrometer (UV2550, Shimadzu, Japan). BaSO4 was In this work, we fabricate monoclinic BiVO4 hollow used as a reflectance standard in a UV-vis diffuse reflectance microspheres by a one-pot template-free hydrothermal experiment. method using NH4VO3 and Bi(NO3)3 as precursors and urea as additive at 180◦C for 24 h. The influence of urea 2.3. Measurement of Photocatalytic Activity. The photocat- concentration on the morphology and photocatalytic activity alytic activity of the BiVO samples was characterized by of BiVO is studied and discussed. To the best of our knowl- 4 4 the photocatalytic decolorization of methylene blue (MB) edge, this is the first report on template-free fabrication of aqueous solution at ambient temperature. Experimental BiVO hollow spheres. This work will provide new insights 4 details were as follows: 0.1 g of the prepared BiVO powder and understanding on the control of morphology and 4 was dispersed in a 20 mL MB aqueous solution with a enhancement of photocatalytic activity of BiVO and should − 4 concentration of 3 × 10 5 M in a 9.0 cm culture dish. The be of significant interest in pigment, catalysis, separation solution was allowed to reach an adsorption-desorption technology, biomedical engineering, and nanotechnology. equilibrium among the photocatalyst, MB, and water before visible-light irradiation. A 350 W xenon lamp with a 420 nm 2. Experimental cutoff filter positioned 25 cm above the dish was used as a visible-light source to trigger the photocatalytic reaction. The 2.1. Preparation of Sample. All chemicals used in this study integrated visible-light intensity striking on the surface of the were of analytical grade and were used without further reaction solution measured with a visible-light radiometer purification. Deionized water was used in all experiments. (model: FZ-A, China) was 95 mW/cm2 with the wavelength 1.4 g of NH4VO3,5.8gofBi(NO3)3·5H2Oandacertain range of 420–1000 nm. The concentration of MB was amount of urea were dissolved in 50 mL of nitric acid determined by a UV-visible spectrophotometer (UV-2550, aqueous solution (2.0 M). The amount of the added urea was Shimadzu, Japan). After visible-light irradiation for every changed from 0, 3, 6, to 9 g, and the obtained BiVO4 powders 30 min, the reaction solution was filtrated to measure the were labeled as samples A, B, C, and D, respectively. After concentration change of MB. being stirred for 30 min, the mixed solution was transferred into a 100-mL teflon-lined stainless steel autoclave and kept ◦ 2.4. Analysis of Hydroxyl Radicals. The formation of hydroxyl at 180 C for 24 h. The hydrothermal products were collected, radicals (•OH) on the surface of BiVO under visible-light washed with deionized water and anhydrous alcohol for three 4 ◦ irradiation was detected by PL method using terephthalic times, and then dried at 80 Cfor12h. acid as a probe molecule. Terephthalic acid readily reacts with •OH to produce a highly fluorescent product, 2- 2.2. Characterization. Morphological observations were per- hydroxyterephthalic acid [40, 41]. This technique has been formed by an S-4800 field emission scanning electron micro- used in radiation chemistry, sonochemistry, and biochem- scope (FESEM, Hitachi, Japan) and linked with an Oxford istry for the detection of •OH generated in water. The Instruments X-ray analysis system. X-ray diffraction (XRD) method relies on the PL signal at 425 nm arising from patterns obtained on a D/MAX-RB X-ray diffractometer the hydroxylation of terephthalic acid with •OH generated (Rigaku, Japan) using Cu Kα radiation at a scan rate (2θ)of at the water/catalyst interface. The PL intensity of 2- 0.05◦ s−1 were used to determine the identity of any phase hydroxyterephthalic acid is proportional to the amount of International Journal of Photoenergy 3

Table 1: Experimental conditions for preparation of the samples and their physical properties.

a 2 b No. Urea content Phase Bandgap (eV) Crystalline size (nm) SBET (m /g) Relative crystallinity A 0 M 2.24 31.1 2.85 1.00 B 3 M 2.22 45.2 1.64 1.48 C 6 M 2.21 50.6 1.20 1.80 D 9 M 2.20 62.5 1.05 2.13 a M denote monoclinic. bRelative monoclinic crystallinity: the relative intensity of the diﬀraction peak from the monoclinic (121) plane (reference = the sample A prepared in pure water).

•OH radicals produced in water [40, 41]. The method 121)

is rapid, sensitive, and specific and needs only a simple − standard PL instrumentation. Experimental procedures were ( similar to the measurement of photocatalytic activity except × that the MB aqueous solution was replaced by the 5 (011) (040) (002) (211) (150) (042) (202) (161) (321) D 10−4 M terephthalic acid aqueous solution with a concentration of 2 × 10−3 M NaOH. PL spectra of generated 2- hydroxyterephthalic acid were measured on an Hitachi F- 7000 fluorescence spectrophotometer. After UV irradiation C for every 30 min, the reaction solution was filtrated to intensity Relative measure the increase of the PL intensity at 425 nm excited by 315 nm light [15]. B

A 3. Results and Discussion ff 20 30 40 50 60 70 3.1. XRD Study. XRD was used to investigate the e ects of 2θ (degree) the amount of urea on phase structure and crystallite size of the prepared samples. Figure 1 shows the XRD patterns Figure 1: XRD patterns of BiVO4 samples prepared with different ◦ of BiVO4 samples obtained with different amount of urea. amount of urea ranging from 0 (A), 3 (B), 6 (C) to 9 g (D) at 180 C for 24 h. It is clear that all the BiVO4 samples have a monoclinic scheelite structure (JCPDS Card no. 14-0688). No diffraction peaks of any other phases or impurities are detected, and the narrow line widths indicate a high degree of crystallinity. It of greater BiVO4 crystallites. The average crystalline sizes is reasonable to infer that hydrothermal treatment at 180◦C change from 31.1, 45.2, 50.6 to 62.5 nm (see Table 1). This for 24 h is sufficient for the preparation of pure monoclinic result is in good agreement with the previous report that urea scheelite BiVO4, which is consistent with the previous reports added in the reaction system enhanced the crystallization of [39, 42]. Usually, monoclinic scheelite BiVO4 is usually monoclinic phase and promoted the growth of crystallites obtained by the high-temperature process, while tetragonal [43]. The formation of greater BiVO4 crystallites may be BiVO4 (zircon structure) is prepared in aqueous media by related to the fact that urea coordinates to bismuth ions the low-temperature process. The phase transition from to form complex, the dissociation of the complex in situ 3+ − tetragonal BiVO4 (zircon structure) to monoclinic BiVO4 generates Bi , the combination of VO3 and the released 3+ (scheelite structure) irreversibly occurs at 670–770 K [28]. Bi results in the formation of the greater BiVO4 crystallites Thus, it can be concluded that the phase transition from [45]. The coordinating strength of complexes formed under tetragonal BiVO4 to monoclinic BiVO4 is thermodynamically different conditions would affect the release speed and the favored, and hydrothermal environment promotes this phase monomer concentration of free Bi3+ in the solution. Gen- transformation due to a nonequilibrium pressure environ- erally, the aggregation behavior is strongly correlated to the ment in hydrothermal treatment [43]. Moreover, monoclinic nucleation rate of the primary nanoparticles, which can be BiVO4 is a thermodynamically more stable phase while adjusted by controlling the reaction rate [46]. The stronger 3+ tetragonal BiVO4 is a kinetically one [29, 36]. In this reaction coordination led to lowering free Bi concentration in the system, the tetragonal BiVO4 is a metastable phase and can be solution, which prevented the plosive production of BiVO4 easily transformed to the monoclinic scheelite BiVO4 with an and suppressed the nucleation. As a result, their progressive increase in hydrothermal time (24 h) [44]. crystal growth is favored, rather than aggregation. In this Further observation shows that with increasing the case, the more urea added, the lower concentration of free 3+ amount of urea, XRD peak intensities of monoclinic BiVO4 Bi , which slow nucleation rate and then increase the crystal become steadily stronger, and the width of XRD diffraction growth. Therefore, it is not surprising that with increasing peaks of monoclinic BiVO4 become slightly narrower, indi- the amount of urea, the products consist of greater BiVO4 cating that the enhancement of crystallization and formation crystallites. 4 International Journal of Photoenergy

3.2. UV-Vis and FTIR Spectra. Figure 2 shows the UV-visible diffuse reflectance spectra of samples A and D. The two 1.2 samples exhibit a strong absorption in the UV-visible light region, and sample D with hollow structure shows the 1 enhanced visible-light absorption. The steep shape of the spectra indicates that the visible-light absorption was not due 0.8 AD to the transition from impurity levels but to the bandgap transition. The absorption edge for sample A is at ca. 550 nm, 0.6 which is consistent with the characteristic absorption of monoclinic BiVO4 [28]. For monoclinic scheelite BiVO4, the (a.u.) Absorbance 0.4 valence band (VB) is formed by a hybridization of the Bi6s and O orbital, whereas the conduction band is composed D 2p 0.2 of V3d orbital [43]. The presence of Bi6s in the top of the A valence bands results in a more negative energy level of the valence band and then a decrease in the band gap [47]. It is 0 300 400 500 600 700 800 apparent that the diffuse reflectance spectra of BiVO4 hollow microspheres exhibit a red shift and increased absorption in Wavelength (nm) the visible-light range, which is ascribed to the unique hollow Figure 2: UV-visible absorption spectra and corresponding colors configuration of sample D. Hollow structure allows multiple (inset) of sample A and D. scattering of UV-vis light within their frameworks, leading to a longer optical path length for light transport and a greater absorbance than those for large irregular aggregates sample. reaction system, the as-synthesized products are hollow It also leads to the enhanced photocatalytic activity of BiVO4 microspheres with the diameter of about 3 μm, coexisting hollow structure. Similar results were also reported by Zhou with solid microspheres (Figure 3(b)). Figure 3(c) clearly et al. [36]. The bandgap of a semiconductor can be estimated illustrates that most of the microspheres appear hollow struc- from a plot of (αh )2 versus photon energy (h )[48–50]. The tures with the wall thinness of 0.8–1.0 μm when the amount estimated Eg of sample A and D from the intercept of the of urea increases to 6 g. As the amount of urea further tangents to the plots were 2.24 eV and 2.20 eV,respectively. In increases to 9 g, SEM image (Figure 3(d)) indicates that the addition, the conduction band (CB) edge of a semiconductor rough external shells of the hollow microspheres consist of at the point of zero charge (pHzpc) can be predicted by (1): loosely packed nanoparticles and progressively thinned. The above results indicate that hollow microspheres could not 1 form without the addition of urea, contrarily, BiVO hollow E 0 = X − Ec − E ,(1) 4 CB 2 g microspheres are easily produced in the presence of urea, and the hollowing rate is strongly related to the amount where X is the absolute electronegativity of the semicon- of urea added. With increasing the amount of urea, the ductor, expressed as the geometric mean of the absolute hollowing rate increases. Urea enhances the dissolution of electronegativity of the constituent atoms, which is defined the interior and mass transfer from interior of the spheres as the arithmetic mean of the atomic electron affinity and to outer surface during reactive process, accompanied by an c the first ionization energy; E is the energy of free electrons enhancement of crystallization of monoclinic BiVO4 [19]. on the hydrogen scale (ca. 4.5 eV); Eg is the band gap of To investigate the formation mechanism of BiVO4 hollow the semiconductor [21]. For BiVO4, the value of X is 6.035. microspheres, a detailed time-dependent evolution experi- According to the values of Eg estimated above and the ment was carried out. Figure 4 shows the XRD patterns of ◦ foregoing formula, the calculated ECB and EVB of sample A the samples obtained with 9 g urea at 180 C for varying were 0.415 eV and 2.655 eV, respectively. For sample D, ECB hydrothermal time. It can be seen that tetragonal BiVO4 and EVB were 0.435 eV and 2.635 eV, respectively. These data is formed after hydrothermal reaction for 1 h, along with monoclinic BiVO , but tetragonal BiVO is completely clearly demonstrate that the electronic structures of BiVO4 4 4 were slightly changed due to the varying pH caused by the transformed into monoclinic as the time is prolonged to decomposition of urea [43]. 24 h. The corresponding SEM images (Figure 5(a)) show that the solid microspheres with the diameter of about 2.7 μm are obtained by self-assembly of precursor nanoparticles 3.3. SEM Images. The morphologies of the samples were after 1 h reaction. After a reaction time of 6 h, hollow observed by SEM. Figure 3 shows the SEM images of the microspheres can be observed in the products (Figure 5(b)). ff powder samples prepared with di erent amount of urea. When the hydrothermal time prolongs to 12 h, the shell It is clearly seen that the amount of urea has a significant of the hollow microspheres becomes thinner (Figure 5(c)). influence on the morphologies of samples. As shown in The thickness of the shell wall changes from 1.3 μmat Figure 3(a), the products are composed of large aggregates 6h, 1μmat12hto0.6μm at 24 h. Therefore, it can be with ill-defined shape and inhomogeneous size in the inferred from the above results that the thickness of shell absence of urea. With the addition of urea, hollow structures walls of BiVO4 hollow spheres could be easily controlled by appear in the products. When 3 g urea is added in this changing hydrothermal time. The longer the reaction time, International Journal of Photoenergy 5

5 μm 5 μm (a) (b)

2 μm

5 μm 5 μm (c) (d)

◦ Figure 3: SEM images of BiVO4 samples prepared with diﬀerent amount of urea ranging from 0 (a), 3 (b), 6 (c) to 9 g (d) at 180 Cfor24h.

The formation of monoclinic-phase BiVO4 hollow spheres M comes from the dissolution and recrystallization of tetragonal BiVO4 phaseasothersreport[36, 43, 55]. This self- transformation is sustained by the higher solubility of the as-formed metastable tetragonal BiVO4 spheres core, which M M M M MMMMM M 24 h resulted in an increase in the local supersaturation such that secondary nucleation and growth of crystalline monoclinic BiVO4 occurred specially on the external surface of the spheres [17]. 12 h

Relative intensity Relative T T

6 h 3.4. Photocatalytic Activity. The photocatalytic activity of the 3 h prepared samples was evaluated by photocatalytic degradation decolorization of MB aqueous solution under visible- 1 h light (λ>420 nm) illumination. Figure 6 displays the 10 20 30 40 50 60 70 80 changes in the absorption spectra of an MB aqueous solution exposed to visible light for various time in the presence 2θ (degree) of sample D. Methylene blue (MB) shows a maximum M Monoclinic phase T Tetragonal phase absorption band at 664 nm. Under visible-light illumination, an apparent decrease of MB absorption at 664 nm was Figure 4: XRD patterns of BiVO4 samples prepared with 9 g urea at observed. This indicates that the degradation rate of MB was diﬀerentreactiontimefrom1,3,6,12to24h. very fast at the beginning of irradiation and then became slow. The color of the dispersion disappeared after 180 min of irradiation, demonstrating that the chromophoric structure the thinner the shell walls of hollow spheres. Based on these of the dye was destroyed. A sharp decrease of the major above results, a localized Ostwald ripening or chemically absorption band within 3 h indicates that sample D exhibits induced self-transformation mechanism can be used to excellent photocatalytic activity in the degradation of MB. explain the formation of BiVO4 hollow spheres. The mech- Figure 7 shows the comparison of photocatalytic activi- anism for this template-free formation of hollow spheres ties of the samples prepared with diﬀerent amount of urea. It has been elaborated in our previous works [19, 51–54]. is demonstrated that the self-degradation of MB is extremely 6 International Journal of Photoenergy

2 μm 2 μm

(a) (b)

4 μm

(c)

Figure 5: SEM images of BiVO4 samples prepared with 9 g urea at diﬀerent reaction time from 1 (a), 6 (b), to 12 h (c).

2 1 MB only

1.6 0.8 0 min 30 min A 0.6

1.2 60 min 0 C 90 min / C

0.8 0.4 B Absorbance

C 0.4 0.2

D 0 0 0 20406080 400 500 600 700 800 Irradiation time (min) Wavelength (nm) Figure 7: Comparison of photocatalytic activities of samples A, Figure 6: Absorption changes of MB aqueous solution at room B, C, and D for the photocatalytic decolorization of MB aqueous temperature in the presence of the sample D under visible-light solution at ambient temperature; C and C denote the reaction and irradiation. 0 initial concentration of MB in the system, respectively. slow; only 6% of MB is photolyzed after 180 min irradiation to the following two factors. First, hollow inner structures (Figure 7). When BiVO4 samples as photocatalysts are added allow light scattering inside their hollow interior, which in the MB solution, the degradation rate of MB significantly enhances light harvesting and thus increases the quantities of increases. Moreover, it is clearly seen that the degradation photogenerated electrons and holes available to participate in rate of the samples obtained in the presence of urea is the photocatalytic decomposition reactions of the contami- higher than that of the samples prepared without urea. Urea nants [15, 48]. Second, specific surface area and crystallinity added in the reaction system has a positive influence on the are two conflicting factors influencing the photocatalytic photocatalytic activity of the BiVO4 samples. This is due activity of photocatalyst. A large surface area is usually International Journal of Photoenergy 7

0.5 100

0.4 80 60 min 60 min 30 min 30 min 0.3 0 min 60 0 min

0.2 40 Fluorescence intensity (a.u.) Fluorescence Fluorescence intensity (a.u.) Fluorescence 0.1 20

0 0 350 400 450 500 550 350 400 450 500 550 Wavelength (nm) Wavelength (nm) (a) (b)

−4 Figure 8: PL spectral changes observed during illumination of (a) as-prepared BiVO4 and (b) TiO2 sample in a 5 × 10 M basic solution of terephthalic acid (excitation at 315 nm). Each fluorescence spectrum was recorded every 30 min of visible light illumination. associated with a large amount of crystalline defects or weak terephthalic acid as a probe molecule. For comparison, the crystallization, which favor the recombination of photo- TiO2 system was also observed under the same conditions. generated electrons and holes, causing a poor photoactivity. Figure 8 shows PL spectral changes observed during visible- Therefore, amorphous TiO2 powders usually show a large light illumination of as-prepared BiVO4 and TiO2 powders specific surface area, but a poor or negligible photocatalytic in a 5 × 10−4 M basic solution of terephthalic acid (excitation activity due to recombination of photoexcited electrons at 315 nm). There is no PL observed when the BiVO4 and positive holes at defects (i.e., imperfections, impurities, suspension is irradiated, indicating that no •OH radical is dangling bonds, or microvoids) located on the surface and in produced. However, in the case of TiO2, a gradual increase the bulk of particles [56, 57]. This indicates that crystallinity in PL intensity at about 425 nm is observed with increasing is another important requirement for high photocatalytic irradiation time, indicating the production of the •OH activity. Hence, it is easy to understand that with increasing radicals. The above results show that the •OH radicals are not the amount of urea, the crystallinity increases, resulting in the main active oxygen species in the photochemical process the enhancement of photocatalytic activity. of MB/BiVO4 system. This can be explained according to Our experiments also indicate that BiVO4 hollow spheres the location of the valence bandedges (EVB)ofBiVO4 and are more readily separated from the slurry system by the normal potential of OH−/•OH couples, their potentials filtration or sedimentation after photocatalytic reaction and are 2.635 and 2.7 V versus SCE, respectively, suggesting reused than conventional nanosized powder photocatalytic that the holes photogenerated on the surface of BiVO4 − • materials due to their large weight, weak Brownian motion, could not react with OH /H2Otoform OH. Conse- and good mobility [15]. After five recycles for the pho- quently, it is not surprising that no hydroxyl radicals are todegradation of MB, the catalyst did not exhibit any signif- observed on the surface of visible light illuminated BiVO4. icant loss of activity, confirming that BiVO4 hollow spheres The interfacial energy scheme at the BiVO4/electrolyte were not deactivated during the photocatalytic oxidation of interface is summarized in Figure 9. Therefore, we have the pollutant molecules. Further investigation indicates that reason to think that the photocatalytic degradation of MB other colorless organic pollutants such as phenol are also in the presence of BiVO4 is due to the direct participation of quickly decomposed by the prepared BiVO4 hollow spheres the photogenerated holes [15]. under visible-light irradiation. To the best of our knowledge, this is the first time to report template-free preparation and enhanced photocatalytic activity of BiVO4 hollow spheres. 4. Conclusion We believe that the prepared BiVO4 hollow spheres are also of great interest in pigment, catalysis, separation technology, Monoclinic BiVO4 hollow microspheres with diameters of biomedical engineering, and nanotechnology. about 2–4 μm are successfully fabricated on a large scale by a simple hydrothermal method without using any templates. Localized Ostwald ripening and chemically induced self- 3.5. Hydroxyl Radical Analysis. To understand the involved transformation mechanism are thought to be the main active species in the photocatalytic process of BiVO4, the for- driving force for the formation of hollow spheres. Urea plays • mation of hydroxyl radicals ( OH) on the surface of visible a key role in the formation of BiVO4 hollow microspheres. light illuminated BiVO4 is detected by the PL technique using The average crystallite size, specific surface areas, hollow 8 International Journal of Photoenergy

−2 [7] J. Yu and X. Yu, “Hydrothermal synthesis and photocatalytic activity of zinc oxide hollow spheres,” Environmental Science −1 and Technology, vol. 42, no. 13, pp. 4902–4907, 2008. [8]S.W.Kim,M.Kim,W.Y.Lee,andT.Hyeon,“Fabrication of hollow palladium spheres and their successful application 0 H /H+ 2 to the recyclable heterogeneous catalyst for suzuki coupling reactions,” Journal of the American Chemical Society, vol. 124, 1 no. 26, pp. 7642–7643, 2002. V versus SCE V versus Eg = 2.2eV [9] J. Gao, B. Zhang, X. Zhang, and B. Xu, “Magnetic- 2 dipolar-interaction-induced self-assembly affords wires of OH−/•OH hollow nanocrystals of cobalt selenide,” Angewandte Chemie— 3 International Edition, vol. 45, no. 8, pp. 1220–1223, 2006. [10]A.D.Dinsmore,M.F.Hsu,M.G.Nikolaides,M.Marquez, BiVO 4 A. R. Bausch, and D. A. Weitz, “Colloidosomes: selectively permeable capsules composed of colloidal particles,” Science, Figure 9: Energy scheme at the BiVO4/electrolyte interface. vol. 298, no. 5595, pp. 1006–1009, 2002. [11] Y. Li, J. Shi, Z. Hua, H. Chen, M. Ruan, and D. Yan, “Hollow spheres of mesoporous aluminosilicate with a interior structures, and photocatalytic activity of BiVO 4 three-dimensional pore network and extraordinarily high hollow spheres could be tuned by changing the amount of hydrothermal stability,” Nano Letters, vol. 3, no. 5, pp. 609– urea. With increasing the amount of urea, the crystallites 612, 2003. grow to larger size, the core hollowing takes place faster, [12] J. H. Park, C. Oh, S. I. Shin, S. K. Moon, and S. G. Oh, and photocatalytic activity increases. The direct hole transfer “Preparation of hollow silica microspheres in W/O emulsions • (instead of OH radicals) plays an important role in the with polymers,” Journal of Colloid and Interface Science, vol. degradation of MB. 266, no. 1, pp. 107–114, 2003. [13] M. Yang and J. J. Zhu, “Spherical hollow assembly composed of Cu2O nanoparticles,” Journal of Crystal Growth, vol. 256, no. Acknowledgments 1-2, pp. 134–138, 2003. [14] D. H. M. Buchold and C. Feldmann, “Nanoscale γ-AIO(OH) This work was partially supported by the National Natural hollow spheres: synthesis and container-type functionality,” Science Foundation of China (51072154 and 51102190), Nat- Nano Letters, vol. 7, no. 11, pp. 3489–3492, 2007. ural Science Foundation of Hubei Province (2010CDA078), [15] X. Yu, J. Yu, B. Cheng, and B. Huang, “One-pot template- National Basic Research Program of China (2009CB939704), free synthesis of monodisperse zinc sulfide hollow spheres and Self-determined and Innovative Research Funds of and their photocatalytic properties,” Chemistry-A European SKLWUT. Journal, vol. 15, no. 27, pp. 6731–6739, 2009. [16] H. G. Yang and H. C. Zeng, “Preparation of hollow anatase TiO2 nanospheres via Ostwald ripening,” Journal of Physical References Chemistry B, vol. 108, no. 11, pp. 3492–3495, 2004. [1] L. P. Zhu, H. M. Xiao, W. D. Zhang, G. Yang, and S. Y. Fu, [17] Y. Yin, R. M. Rioux, C. K. Erdonmez, S. Hughes, G. A. Somor- jal, and A. P. Alivisatos, “Formation of hollow nanocrystals “One-pot template-free synthesis of monodisperse and single- ff crystal magnetite hollow spheres by a simple solvothermal through the nanoscale kirkendall e ect,” Science, vol. 304, no. route,” Crystal Growth and Design, vol. 8, no. 3, pp. 957–963, 5671, pp. 711–714, 2004. 2008. [18] H. G. Yang and H. C. Zeng, “Self-construction of hollow SnO octahedra based on two-dimensional aggregation of [2]Y.Zheng,Y.Cheng,Y.Wang,L.Zhou,F.Bao,andC. 2 nanocrystallites,” Angewandte Chemie—International Edition, Jia, “Metastable γ-MnS hierarchical architectures: synthesis, vol. 43, no. 44, pp. 5930–5933, 2004. characterization, and growth mechanism,” Journal of Physical [19] J. Yu, H. Guo, S. A. Davis, and S. Mann, “Fabrication of Chemistry B, vol. 110, no. 16, pp. 8284–8288, 2006. hollow inorganic microspheres by chemically induced self- [3] J. Yu, W. Liu, and H. Yu, “A one-pot approach to hierarchically transformation,” Advanced Functional Materials, vol. 16, no. nanoporous titania hollow microspheres with high photocat- 15, pp. 2035–2041, 2006. alytic activity,” Crystal Growth and Design,vol.8,no.3,pp. [20] S. Kohtani, S. Makino, A. Kudo et al., “Photocatalytic degra- 930–934, 2008. dation of 4-n-nonylphenol under irradiation from solar sim- [4] J. Yu, H. Yu, H. Guo, M. Li, and S. Mann, “Spontaneous for- ulator: comparison between BiVO4 and TiO2 photocatalysts,” mation of a tungsten trioxide sphere-in-shell superstructure Chemistry Letters, no. 7, pp. 660–661, 2002. by chemically induced self-transformation,” Small, vol. 4, no. [21] M. Long, W. Cai, J. Cai, B. Zhou, X. Chai, and Y. Wu, “Efficient 1, pp. 87–91, 2008. photocatalytic degradation of phenol over Co3O 4/BiVO4 [5] J. Yu and L. Shi, “One-pot hydrothermal synthesis and composite under visible light irradiation,” Journal of Physical enhanced photocatalytic activity of trifluoroacetic acid modi- Chemistry B, vol. 110, no. 41, pp. 20211–20216, 2006. fied TiO2 hollow microspheres,” Journal of Molecular Catalysis [22] H. Q. Jiang, H. Endo, H. Natori, M. Nagai, and K. Kobayashi, A, vol. 326, no. 1-2, pp. 8–14, 2010. “Fabrication and efficient photocatalytic degradation of [6] F. Caruso, R. A. Caruso, and H. Mohwald,¨ “Nanoengineering methylene blue over CuO/BiVO4 composite under visible- of inorganic and hybrid hollow spheres by colloidal templat- light irradiation,” Materials Research Bulletin,vol.44,no.3, ing,” Science, vol. 282, no. 5391, pp. 1111–1114, 1998. pp. 700–706, 2009. International Journal of Photoenergy 9

[23] L. Zhang, D. Chen, and X. Jiao, “Monoclinic structured BiVO4 [38] G. Li, D. Zhang, and J. C. Yu, “Ordered mesoporous BiVO4 nanosheets: hydrothermal preparation, formation mecha- through nanocasting: a superior visible light-driven photocat- nism, and coloristic and photocatalytic properties,” Journal of alyst,” Chemistry of Materials, vol. 20, no. 12, pp. 3983–3992, Physical Chemistry B, vol. 110, no. 6, pp. 2668–2673, 2006. 2008. [24] L. Zhou, W. Wang, S. Liu, L. Zhang, H. Xu, and W. Zhu, [39]D.Ke,T.Peng,L.Ma,P.Cai,andK.Dai,“Effects of “A sonochemical route to visible-light-driven high-activity hydrothermal temperature on the microstructures of BiVO4 BiVO4 photocatalyst,” Journal of Molecular Catalysis A, vol. and its photocatalytic O2 evolution activity under visible 252, no. 1-2, pp. 120–124, 2006. light,” Inorganic Chemistry, vol. 48, no. 11, pp. 4685–4691, [25] K. Sayama, A. Nomura, T. Arai et al., “Photoelectrochemical 2009. decomposition of water into H2 and O2 on porous BiVO4 thin- [40] K. I. Ishibashi, A. Fujishima, T. Watanabe, and K. Hashimoto, film electrodes under visible light and significant effect of Ag “Detection of active oxidative species in TiO2 photocatalysis Ion treatment,” Journal of Physical Chemistry B, vol. 110, no. using the fluorescence technique,” Electrochemistry Communi- 23, pp. 11352–11360, 2006. cations, vol. 2, no. 3, pp. 207–210, 2000. [41] Q. Xiang, J. Yu, B. Cheng, and H. C. Ong, “Microwave- [26] A. Kudo, K. Ueda, H. Kato, and I. Mikami, “Photocatalytic O2 hydrothermal preparation and visible-light photoactivity of evolution under visible light irradiation on BiVO4 in aqueous plasmonic photocatalyst Ag-TiO2 nanocomposite hollow AgNO3 solution,” Catalysis Letters, vol. 53, no. 3-4, pp. 229– 230, 1998. spheres,” Chemistry-A Asian Journal, vol. 5, no. 6, pp. 1466– 1474, 2010. [27] S. Kohtani, J. Hiro, N. Yamamoto, A. Kudo, K. Tokumura, and [42]X.Zhang,Z.Ai,F.Jia,L.Zhang,X.Fan,andZ.Zou,“Selective R. Nakagaki, “Adsorptive and photocatalytic properties of Ag- synthesis and visible-light photocatalytic activities of BiVO loaded BiVO on the degradation of 4-n-alkylphenols under 4 4 with different crystalline phases,” Materials Chemistry and visible light irradiation,” Catalysis Communications, vol. 6, Physics, vol. 103, no. 1, pp. 162–167, 2007. no. 3, pp. 185–189, 2005. [43] J. Yu and A. Kudo, “Effects of structural variation on the [28]A.Kudo,K.Omori,andH.Kato,“Anovelaqueousprocessfor photocatalytic performance of hydrothermally synthesized preparation of crystal form-controlled and highly crystalline BiVO4,” Advanced Functional Materials, vol. 16, no. 16, pp. BiVO4 powder from layered vanadates at room temperature 2163–2169, 2006. and its photocatalytic and photophysical properties,” Journal [44] M. W. Stoltzfus, P. M. Woodward, R. Seshadri, J. H. Klepeis, of the American Chemical Society, vol. 121, no. 49, pp. 11459– and B. Bursten, “Structure and bonding in SnWO ,PbWO, 11467, 1999. 4 4 and BiVO4: ione pairs vs inert pairs,” Inorganic Chemistry, vol. [29] S. Tokunaga, H. Kato, and A. Kudo, “Selective preparation of 46, no. 10, pp. 3839–3850, 2007. monoclinic and tetragonal BiVO4 with scheelite structure and [45] G. Zhu and P. Liu, “Low-temperature urea-assisted hydrother- their photocatalytic properties,” Chemistry of Materials, vol. mal synthesis of Bi2S3 nanostructures with different mor- 13, no. 12, pp. 4624–4628, 2001. phologies,” Crystal Research and Technology,vol.44,no.7,pp. [30] P. Madhusudan, J. Ran, J. Zhang, J. Yu, and G. Liu, 713–720, 2009. “Novel urea assisted hydrothermal synthesis of hierarchical [46] S. Liu, J. Yu, and S. Mann, “Spontaneous construction of BiVO4/Bi2O2CO3 nanocomposites with enhanced visible-light photoactive hollow TiO2 microspheres and chains,” Nanotech- photocatalytic activity,” Applied Catalysis B, vol. 110, pp. 286– nology, vol. 20, no. 32, Article ID 325606, 2009. 295, 2011. [47] M. Oshikiri, M. Boero, J. Ye, Z. Zou, and G. Kido, “Electronic [31] P. B. Avakyan, M. D. Nersesyan, and A. G. Merzhanov, structures of promising photocatalysts InMO4 (M = V, Nb, Ta) “New materials for electronic engineering,” American Ceramic and BiVO4 for water decomposition in the visible wavelength Society Bulletin, vol. 75, no. 2, pp. 50–55, 1996. region,” Journal of Chemical Physics, vol. 117, no. 15, pp. 7313– [32]P.Shuk,H.D.Wiemhofer,¨ U. Guth, W. Gopel,¨ and M. 7318, 2002. Greenblatt, “Oxide ion conducting solid electrolytes based on [48] J. Yu, J. Xiong, B. Cheng, and S. Liu, “Fabrication and Bi2O3,” Solid State Ionics, vol. 89, no. 3-4, pp. 179–196, 1996. characterization of Ag-TiO2 multiphase nanocomposite thin [33] K. Shantha and K. B. R. Varma, “Preparation and charac- films with enhanced photocatalytic activity,” Applied Catalysis terization of nanocrystalline powders of bismuth vanadate,” B, vol. 60, no. 3-4, pp. 211–221, 2005. Materials Science and Engineering B, vol. 60, no. 1, pp. 66–75, [49] E. M. Patterson, C. E. Shelden, and B. H. Stockton, “Kubelka- 1999. Munk optical properties of a barium sulfate white reflectance standard,” Applied Optics, vol. 16, no. 3, pp. 729–732, 1977. [34] J. D. Bierlein and A. W. Sleight, “Ferroelasticity in BiVO ,” 4 [50] N. Serpone, D. Lawless, and R. Khairutdinov, “Size effects Solid State Communications, vol. 16, no. 1, pp. 69–70, 1975. on the photophysical properties of colloidal anatase TiO ff 2 [35] L. Ho art, U. Heider, R. A. Huggins, W. Witschel, R. Jooss, particles: size quantization or direct transitions in this indirect and A. Lentz, “Crystal growth and conductivity investigations semiconductor?” Journal of Physical Chemistry, vol. 99, no. 45, on BiVO4 single crystals,” Ionics, vol. 2, no. 1, pp. 34–38, 1996. pp. 16646–16654, 1995. [36] L. Zhou, W. Wang, L. Zhang, H. Xu, and W. Zhu, “Single- [51] J. Yu, S. Liu, and H. Yu, “Microstructures and photoactivity crystalline BiVO4 microtubes with square cross-sections: of mesoporous anatase hollow microspheres fabricated by microstructure, growth mechanism, and photocatalytic prop- fluoride-mediated self-transformation,” Journal of Catalysis, erty,” Journal of Physical Chemistry C, vol. 111, no. 37, pp. vol. 249, no. 1, pp. 59–66, 2007. 13659–13664, 2007. [52] J. Yu, S. Liu, and M. Zhou, “Enhanced photocalytic activity of [37] W. Yin, W. Wang, M. Shang, L. Zhou, S. Sun, and L. Wang, hollow anatase microspheres by Sn4+ incorporation,” Journal “BiVO4 hollow nanospheres: anchoring synthesis, growth of Physical Chemistry C, vol. 112, no. 6, pp. 2050–2057, 2008. mechanism, and their application in photocatalysis,” European [53] H. Yu, J. Yu, S. Liu, and S. Mann, “Template-free hydrothermal Journal of Inorganic Chemistry, no. 29-30, pp. 4379–4384, synthesis of CuO/Cu2Ocompositehollowmicrospheres,” 2009. Chemistry of Materials, vol. 19, no. 17, pp. 4327–4334, 2007. 10 International Journal of Photoenergy

[54] W. Cai, J. Yu, and S. Mann, “Template-free hydrothermal fabrication of hierarchically organized γ-AlOOH hollow microspheres,” Microporous and Mesoporous Materials, vol. 122, no. 1–3, pp. 42–47, 2009. [55] A. Zhang, J. Zhang, N. Cui, X. Tie, Y. An, and L. Li, “Eﬀects of pH on hydrothermal synthesis and characterization of visible-light-driven BiVO4 photocatalyst,” JournalofMolecular Catalysis A, vol. 304, no. 1-2, pp. 28–32, 2009. [56] J. Yu, Y. Su, and B. Cheng, “Template-free fabrication and enhanced photocatalytic activity of hierarchical macro- /mesoporous titania,” Advanced Functional Materials, vol. 17, no. 12, pp. 1984–1990, 2007. [57] J. Yu, W. Wang, B. Cheng, B. Huang, and X. Zhang, “Preparation and photocatalytic activity of multi-modally macro/mesoporous titania,” Research on Chemical Intermedi- ates, vol. 35, no. 6-7, pp. 653–665, 2009.