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Selective growth of Ag3PO4 submicro-cubes on Ag nanowires to fabricate necklace-like heterostructures for photocatalytic applications†

Yingpu Bi,*a Hongyan Hu,a Shuxin ,bc Zhengbo ,a Gongxuan *a and Jinhua Ye*bc

Received 4th May 2012, Accepted 12th June 2012 DOI: 10.1039/c2jm32800c

Selective growth of Ag3PO4 submicro-cubes on Ag nanowires to Recently, we reported the new use of a Ag3PO4 semiconductor in construct necklace-like hetero-photocatalysts has been demon- photocatalytic applications, which exhibits extremely high photoox- strated. This novel hetero-structure exhibits much higher activities idative capabilities for O2 evolution from water as well as decom- 13 than both pure Ag3PO4 cubes and Ag nanowires for degradation of position of organic dyes under visible light irradiation. Additionally, organic contaminants under visible light irradiation, which may be the facet effect of single-crystalline Ag3PO4 crystals with rhombic primarily ascribed to highly efficient charge separation at the dodecahedral and cubic morphologies on the photocatalytic perfor- 14 contact interfaces as well as rapid electron export through Ag mances has been further studied. To further promote the charge nanowires. separation efficiency of the Ag3PO4 semiconductor, herein we demonstrate a facile and general growth route for coupling Ag3PO4 submicro-cubes and highly conductive Ag nanowires into novel Semiconductor photocatalysts have attracted considerable attention necklace-like coaxial heterostructures. Moreover, the position and

owing to their great potential in solving current energy and envi- number of Ag3PO4 cubes on Ag nanowires can be further rationally 1–5 + ronment problems by using abundant solar light. However, the tailored by simply adjusting the concentration of the [Ag(NH3)2] semiconductor materials available to date are generally limited by complex. The photocatalytic results indicate that this novel necklace-

either poor photocatalytic efficiency in the visible light range or like Ag nanowire/Ag3PO4 cubes heterostructure exhibits much higher 6,7 insufficient charge separation ability. Toaddressthisproblem, activities than both pure Ag3PO4 cubes and Ag nanowires for the much effort has been focused on the exploration and fabrication of degradation of organic contaminants under visible light irradiation. novel hetero-photocatalysts consisting of metals and semiconductors Single-crystalline Ag nanowires were firstly fabricated through a for improving photocatalytic performances.8,9 More specifically, the

Downloaded by University Town Library of Shenzhen on 29 August 2012 modified polyol process, which serve as the starting templates for the Published on 13 June 2012 http://pubs.rsc.org | doi:10.1039/C2JM32800C metal ingredients could promote the charge separation from the subsequent selective growth and assembly of Ag3PO4 crystals. As contact interface and extend the absorption edge into the visible light shown in Fig. S1†, the as-synthesized Ag nanowires possess a novel region.10 However, current hetero-photocatalysts are generally rectangular cross section and average diameter of 50 nm. + obtained by randomly loading metal nanoclusters onto the semi- Interestingly, when these nanowires were reacted with [Ag(NH3)2] 11 conductor surfaces, which only facilitate the surface separation of complex and Na2HPO4 in aqueous solutions at room temperature, photoexcited electron and hole pairs rather than bulk phase separa- uniform and orderly Ag3PO4 submicro-cubes have been formed on tion. Although metal–semiconductor core–shell hetero-structures the above Ag nanowires through a hetero-growth process. It can be 12 could effectively transfer all of the electrons to the metal core, once clearly seen from Fig. 1A and S2† that the Ag3PO4 submicro-cubes the storage maximum has been attained, the electron–hole recombi- with average diameters of 500 nm combined with the Ag nanowires nation in the semiconductor shell will dominate again. Thereby, the to form 1D necklace-like heterostructures, and no individual and

rational design and fabrication of novel heterostructures with both isolated Ag3PO4 cube has been observed. More specifically, the rapid electron transfer and export capabilities is highly desirable. enlarged SEM images (inset in Fig. 1B) reveal that the single-crystal Ag nanowire drills through the two parallel {100} facets of each

Ag3PO4 submicro-cube and joins them together along the longitu- aState Key Laboratory for Oxo Synthesis & Selective Oxidation, and National Engineering Research Center for Fine Petrochemical dinal axis. Additionally, as well as for a single Ag nanowire, this Intermediates, Lanzhou Institute of Chemical Physics, CAS, Lanzhou epitaxial growth process could also be performed on two Ag nano- 730000, China. E-mail: [email protected]; [email protected] wires, indicating that the successful hetero-growth of Ag3PO4 cubes is bInternational Center for Materials Nanoarchitectonics (MANA), and independent of the number of Ag nanowires, and the only require- Research Unit for Environmental Remediation Materials, National Institute for Materials Science (NIMS), Tsukuba, 305-0047, Japan. ment is their direct interface contacts. Fig. 1C shows the X-ray E-mail: [email protected] diffraction (XRD) pattern of both necklace-like Ag/Ag3PO4 hetero- c -NIMS Joint Research Center, School of Materials Science and structure and pure Ag3PO4 cubes (shown in Fig. S3†). It can be Engineering, Tianjin University, 92 Weijin Road, Nankai District, clearly seen that the main diffraction peaks of this heterostructure Tianjin, P.R. China could be primarily indexed to the body-centered cubic structure of † Electronic supplementary information (ESI) available: Experimental procedure and additional figures. See DOI: 10.1039/c2jm32800c Ag3PO4 (JCPDS no. 06-0505), and only a small diffraction peak of

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+ AsshowninFig.2A,withincreasing concentration of [Ag(NH3)2] complex up to 0.2 M, Ag nanowires have been completely capped by

Ag3PO4 crystals and Ag/Ag3PO4 core–shell coaxial hetero-cables + have been fabricated. In contrast, with decreasing [Ag(NH3)2] concentration, the SEM image shown in Fig. 2B clearly indicates that

the numbers of Ag3PO4 cubes formed on the Ag nanowires have been rationally decreased. Further decreasing the concentration, only a few

Ag3PO4 cubes have been formed on the Ag nanowires (shown in Fig. 2C). Furthermore, their crystalline structures have also been studied, and the XRD results are shown in Fig. 2D. As compared

with the Ag/Ag3PO4 necklace-like heterostructure shown in Fig. 1, the diffraction peak of metallic Ag in Ag/Ag3PO4 core–shell hetero- cables (shown in Fig. 2A) has been further reduced. Contrarily, the

diffraction peaks of Ag3PO4 in the heterostructures shown in Fig. 2B and C have been gradually reduced. These demonstrations clearly

indicate that the epitaxial growth process of Ag3PO4 crystals on Ag nanowires, as well as the compositions and structures of Ag/Ag3PO4 hetero-products, can be rationally tailored by simply adjusting the + concentration of [Ag(NH3)2] complex. Fig. 1 (A and B) SEM images of the Ag nanowire/Ag3PO4 cube neck- lace-like heterostructure; (C) XRD patterns, (D) ultraviolet-visible Their photocatalytic behaviors were explored for the degradation diffusive reflectance spectrums. of Rhodamine B (RhB) and Methyl Orange (MO) dyes under visible- light irradiation (shown in Fig. S9†).15,16 Tothebestofourknowl-

edge, this novel Ag/Ag3PO4 necklace-like heterostructure was used, metallic Ag has been observed. Furthermore, the intensity ratios for the first time, as the catalyst for these photocatalytic reactions. between various peaks of the Ag/Ag3PO4 heterostructure have no For comparison, the photocatalytic performances of pure Ag3PO4 obvious changes compared with pure Ag3PO4 cubes, which gives cubes and Ag nanowires have also been studied and are compared in further support to the above SEM observations. Additionally, the Fig.3.AsshowninFig.3A,itcanbeclearlyseenthatexceptfor ultraviolet-visible diffuse reflectance spectra (shown in Fig. 1D) reveal single-crystalline Ag nanowires, both the Ag/Ag3PO4 and pure that the pure Ag3PO4 cubes could absorb visible light with a wave- Ag3PO4 photocatalysts exhibit excellent photocatalytic activities for length shorter than 550 nm. However, for Ag/Ag3PO4 necklace-like RhB degradation. Among them, the necklace-like Ag/Ag3PO4 het- heterostructures, except for the slight decrease of absorption intensity erostructure (shown in Fig. 1) exhibits the highest photocatalytic at wavelengths shorter than 540 nm, a new broad absorbance peak at activity, and can completely degrade RhB dye in only 2 min. 340 nm has been observed, which should be ascribed to the metallic Furthermore, the complete degradation of RhB dye over Ag/Ag3PO4 Ag nanowires (shown in Fig. S4†). core–shell coaxial hetero-nanowires (shown in Fig. 2A) takes about 6 Furthermore, the effects of [Ag(NH ) ]+ concentrationonthe

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Published on 13 June 2012 http://pubs.rsc.org | doi:10.1039/C2JM32800C min, while the pure Ag3PO4 cubes need about 8 min. In contrast, the structures and compositions of Ag/Ag3PO4 heterostructures have Ag nanowires/Ag3PO4 cubes photocatalysts shown in Fig. 2B and C been investigated, and the experimental results are shown in Fig. 2. exhibit relatively low photocatalytic properties for RhB degradation, which may be due to the high ratios of metallic Ag nanowires. Furthermore, their photocatalytic performances for MO degradation under the same conditions have been shown in Fig. 3B. It can be

clearly seen that except for the higher activities of pure Ag3PO4 cubes than those of Ag/Ag3PO4 core–shell coaxial hetero-nanowires, their photoreactivity order is consistent with the above results for the RhB

Fig. 3 Photocatalytic activities of Ag nanowire/Ag3PO4 cube necklace- like heterostructures, Ag nanowires, and pure Ag3PO4 cubes for (A) RhB Fig. 2 SEM images of as-prepared Ag/Ag3PO4 heterostructures with and (B) MO degradation under visible-light irradiation (l > 420 nm): + different morphologies by adjusting the concentration of [Ag(NH3)2] wire/cube-1 sample (Fig. 2C); wire/cube-2 sample (Fig. 2B); wire/cube-3 complex, (A) 0.2 M, (B) 0.1 M, (C) 0.05 M, (D) their XRD patterns. sample (Fig. 1); wire/cube-4 sample (Fig. 2A).

14848 | J. Mater. Chem., 2012, 22, 14847–14850 This journal is ª The Royal Society of Chemistry 2012 View Online

degradation. Thereby, these demonstrations indicate that the selec- This work was supported by the ‘‘Hundred Talents Program’’ of

tively epitaxial growth of Ag3PO4 submicro-cubes on Ag nanowires the Chinese Academy of Sciences; National Science Foundation to construct necklace-like heterostructures has been proved to be an (21173242); 973 Program, 863 Program of China (2012AA051501, available and successful route for improving the photocatalytic 2007CB613305, 2009CB22003, 2009AA05Z117 and

properties of Ag3PO4 crystals. 2012CB7209001), and Solar Energy Project of Chinese Academy of On the basis of the above results, we identified some possible Sciences (grant no. KGCX2-YW-390-1 and KGCX2-YW-390-3); reasons to clarify the higher photocatalytic activity of Ag nanowire/ the World Premier International Research Center Initiative on

Ag3PO4 cube necklace-like heterostructures than both pure Ag3PO4 Materials Nanoarchitectonics, MEXT, and the Global COE cubes and Ag nanowires. As shown in Fig. 4A, the direct coupling of Program of the Tokyo Institute of Technology, Japan.

Ag nanowires and Ag3PO4 cubes causes the Fermi level to equili- brate, and they attain an energy level close to the conduction band of the Ag3PO4 semiconductor. More specifically, Ag nanocrystals Notes and references possess a favorable Fermi level (EF ¼ 0.4 V vs. normal hydrogen electrode (NHE)),17 which can serve as a good electron acceptor for 1(a) H. Fujishima and K. Honda, Nature, 1972, 298, 37; (b) M. A. Fox and M. T. Dulay, Chem. Rev., 1993, 93, 341; (c) R. Wang, facilitating quick electron transfer from both exterior and inner K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, Ag3PO4 cubes under visible-light irradiation. Thereby, the photoex- A. Kitamura, M. Shimohigoshi and T. Watanabe, Nature, 1997, cited electrons can be rapidly transferred to Ag nanowires, and the 388, 431; (d) P. V. Kamat, Chem. Rev., 1993, 93, 267. 2(a) A. Iwase, Y. Ng, Y. Ishiguro, A. Kudo and R. Amal, J. Am. Chem. photoinduced holes can still locate on Ag3PO4 cubes, which promote Soc., 2011, 133, 11054; (b) A. Ishikawa, T. Takata, J. Kondo, the effective separation of photoexcited electron–hole pairs and M. Hara, H. Kobayashi and K. Domen, J. Am. Chem. Soc., 2002, decrease the probability of electron–hole recombination. More 124, 13547; (c) X. Zong, H. , G. , G. Ma, F. , L. Wang importantly, as shown in Fig. 4B, the enriched electrons on the Ag and C. , J. Am. Chem. Soc., 2008, 130, 7176. 3(a) H. , C. Sun, S. , J. Zou, G. , S. Smith, H. and nanowires could be effectively exported due to the novel necklace G. Lu, Nature, 2008, 453, 638; (b) W. Zhao, W. Ma, C. Chen, J. Zhao heterostructure, which facilitates their participation in the multiple- and Z. Shuai, J. Am. Chem. Soc., 2004, 126, 4782. + 18,19 4(a) J. Son, J. , S. Kwon, J. Lee, J. Joo and T. Hyeon, Adv. Mater., electron reduction reaction of oxygen (O2 +2H +2e / H2O2). 2011, 23, 3214; (b) S. , S. Choi, M. Kang, J. Kim, H. Kim, Simultaneously, the high concentration of holes on Ag3PO4 cubes T. Hyeon and Y. Sung, Adv. Mater., 2008, 20, 54; (c) T. Yu, could significantly accelerate the photocatalytic reaction rates of J. Park, J. Moon, K. An and T. Hyeon, J. Am. Chem. Soc., 2007, organic dye degradation. On the other hand, in this necklace-like 129, 14558. heterostructure, numerous Ag PO cubes have been formed on the 5(a) X. Li, N. Kikugawa and J. Ye, Adv. Mater., 2008, 20, 3816; (b) 3 4 D. Wang, T. Kako and J. Ye, J. Am. Chem. Soc., 2008, 130, 2724; different positions of a single Ag nanowire, and they could share the (c) J. Tang, Z. Zou and J. Ye, Angew. Chem., Int. Ed., 2004, 43, 4463. photoexcited electrons and might achieve novel collective photo- 6(a) G. Tian, H. , L. Jing, B. and K. Pan, J. Phys. Chem. C, catalytic properties. However, the photocatalytic mechanism of the 2008, 112, 3083; (b) M. Addamo, M. Bellardita, D. Carriazo, Ag/Ag PO necklace-like heterostructure can not be completely A. Paola, S. Milioto, L. Palmisano and V. Rives, Appl. Catal., B, 3 4 2008, 84, 742; (c) J. , Q. , Z. Feng, M. Li and C. Li, understood and a more detailed study is still underway. Angew. Chem., Int. Ed., 2008, 47, 1766. In summary, we have demonstrated a facile and efficient process 7(a) Y. Lai, M. Meng, Y. Yu, X. Wang and T. Ding, Appl. Catal., B, 2011, 105, 335; (b) F. Lu, W. Cai and Y. Zhang, Adv. Funct. Downloaded by University Town Library of Shenzhen on 29 August 2012

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