Synthesis and Study of Photocatalytic and Conducting Nanoparticles And

ABSTRACT

This work entitled “Synthesis and Study of Photocatalytic and Conducting nanoparticles and nanocomposites” presents the synthesis, characterization and study of photo-electrochemical and photocatalytic properties of nanocomposites. This thesis is divided into six (06) chapters and the organization of each chapter is as follows:

Chapter 1: Introduction and Review of Literature This chapter gives an introductory description of nanoparticles and semiconductor nanocomposites. It includes a brief account of the principle of photocatalysis by semiconductor nanomaterials and its advancements in the past few decades. The term “nanomaterials” is employed to describe the designing and exploitation of materials with structural features in between those of atoms and giant materials, having at least one of its dimensions in between 0.1nm and 500nm range (1nm = 10- 9m). The various physical properties (viz; dynamic, thermodynamic, mechanical, optical, electronic, magnetic) and chemical properties of nanomaterials can be significantly altered relative to their bulk counterparts. Semiconductor nanoparticles (SC NPs) exhibit different size-dependent properties like electronic band gap energies, solid-solid phase transition temperatures, melting temperatures and pressure responses. To understand photoconductivity, electrical conductivity and related phenomena viz; photocatalysis, it is necessary to understand the energy bands and doping of semiconductors. The reason was expressed by Reithmaier as follows, “the properties of a solid can change dramatically if its dimensions or the dimensions of the constituent phases, become smaller than some critical length associated with these properties” In semiconductors, when an electron gains the extra energy required to get excited into the nearby higher conduction band (CB), it can move freely carrying an electric current. This leaves a „gap‟ or „hole‟ in the lower valence band (VB) which can also move in direction opposite to an electron. Further, by supplying an extra energy from outside, or by clever designing of the SC, the way the semiconductor conducts electric current can be controlled. The principle of the semiconductor photocatalytic reaction is a light induced photochemical reaction. In a photocatalytic process, the illumination of a semiconductor photocatalyst with ultraviolet (UV) or visible radiation activates the photocatalyst, generating a redox environment in the aqueous solution.

Semiconductors act as sensitizers for light induced redox processes due to their electronic structure, having a completely filled valence band and an empty conduction band. The semiconductor photocatalyst absorbs impinging photons with quantum energy (i.e. wavelength) that hits an electron in the occupied valence band and excites that electron to the unoccupied conduction band leading to excited state conduction band electrons and positive valence band holes. However, the antagonism between charge-carrier recombination and charge-carrier trapping followed by the race between recombination and interfacial charge transfer actually determines the overall quantum efficiency for interfacial charge transfer. Further, the band positions or flat band potentials of the semiconductor material has an important role. These determine the thermodynamic limitations for these photoreactions. There have been a huge number of semiconductor nanomaterials exploited as the photocatalysts and in other photochemical devices. The elements which form these nanomaterial photocatalysts are classified into four groups (i) to form energy structure and crystal structure (ii) to form crystal structure but not energy structure, (iii) to construct impurity levels as dopants and (iv) to be exploited as co-catalysts. The history of heterogeneous photocatalysis indicates that it appeared as a new emerging “Advanced Oxidation Process” (AOP) at the end of 20th century with more than 2000 publications registered on the subject. Currently, more than 1000 articles are being published yearly on the topic. Although, it is important to mention that the discovery of the Honda–Fujishima effect is one of the most important discoveries in chemistry which opened up and extensively promoted the research field of photocatalysis, though, it is not an origin of photocatalysis. Actually, reports on photocatalytic oxidation of organic compounds by titania powders had been published before the discovery of this effect. Titania was considered as the most suitable photocatalyst because of its good photochemical properties and non toxic nature for three decades. However, it is now an established fact that the use of the bare TiO2 phases poses some limitations like; (i) small visible light response, (ii) high recombination rate for the photoinduced charge carriers (iii) doping with foreign species that often act as recombination centres, (iv) difficult to support powdered TiO2 on some materials. As a consequence, the research in heterogeneous photocatalysis has promisingly modified some morphological and electronic properties of TiO2 so as to improve its photocatalytic efficiency.

In order to improve the photocatalytic activity of the colloidal and bulk TiO2 particles, interfacial charge-transfer reactions need to be enhanced. Significant charge separation and inhibition of charge carrier recombination is imperative for improving the overall quantum efficiency by interfacial charge transfer. This can be achieved by modifying the properties of the particles by different methods. The different approaches include surface modification of the semiconductor particles with redox couples or noble metals. Another efficient approach involves the coupling of two semiconductor particles with different electronic energy levels to form the heterostructures. The various other approaches include cationic doping, anionic doping or organic-inorganic hybrids etc.

The Current State of Research in the field of Photocatalysis is of the belief that TiO2 is still the predominant photocatalyst because no satisfactory alternative has been clearly identified and developed. Despite the prominent progress achieved by photocatalysis in the last decade, there are still various challenges ahead for its full development. The different areas of interest during current times are artificial photocatalysis and photocatalytic water splitting. Nevertheless, the obvious interest in the implementation of more durable processes, surely a brilliant trajectory of photocatalysis in the way to its development has continued. The three main trends forward can be outlined with a reasonable degree of confidence in the near future: (i) the fine control of increasingly complex nanoarchitectures, (ii) the use of novel non-oxide materials and (iii) the coupling with photovoltaic components in a single device. During the long four-decade course of this field, several presentations and concepts are erroneous or misleading and have accumulated wrongly in the literature on this topic of photocatalysis. A few examples viz; the concept of quantum efficiency, activity, reaction rate, normalized photocatalytic experiments, Langmuir- Hinshelwood mechanism and the concept of doping have been discussed. All the above aspects related to the field of photochemical and photocatalytic properties of semiconductor nanomaterials have been discussed in this chapter.

Chapter 2: Methodology This chapter presents the various methods of synthesis and characterization of the semiconductor nanoparticles and nanocomposites. It also includes the methods of evaluation of photocatalytic experiments. The various methods of synthesis include

3 the sol gel method, the sol method, the chemical precipitation method, the hydrothermal method and the solvothermal method. The techniques of characterization include the X-Ray diffraction, Electron microscopy (Scanning Electron Microscopy and Transmission Electron Microscopy), selected area electron diffraction (SAED), Fourier Transform Infra Red spectroscopy (FTIR), Electron Dispersive spectroscopy (EDS), EDS Mapping, BET surface analysis, Photoluminescence, Electrochemical Impedance spectroscopy and Cyclic Voltammetry. To evaluate the photocatalytic property of the semiconductor photocatalyst, this chapter presents the methodology for the experimental set up and evaluation of other phenomena like; Optical absorption spectroscopy, Scavenging experiments and chemical oxygen demand (COD) analysis.

Chapter 3: Synthesis, Characterization and Optimization of Photocatalytic

Activity of TiO2/ZrO2 Nanocomposite Heterostructures

This chapter presents the work on the synthesis of TiO2/ZrO2 nanocomposite heterostructures and the evaluation of their photocatalytic activity in presence of UV irradiation. ZrO2 coupled TiO2 photocatalytic nanoparticles were synthesized via a hybrid sol–gel method followed by a suitable calcination treatment. Tetragonal structure of TiO2/ZrO2 nanocomposite particles with stabilized anatase phase was confirmed by XRD studies. The synthesized TiO2/ZrO2 nanocomposite exhibits unique optical properties as the band gap increases on Zr addition but, incorporation of intermediate energy levels expands its absorption edge into the visible light region.

Results showed a considerable decrease in recombination rate on ZrO2 addition and Impedance spectroscopy showed a significant decrease in dielectric characteristics on

ZrO2 addition. The TiO2/ZrO2 composites show an efficient photocatalytic activity for degradation of the organic pollutants such as aqueous PBS. The optimum loading amount of ZrO2 on TiO2 was 6.0 molar% which showed higher photocatalytic activity than that of the pure TiO2 and commercially available TiO2 (Degussa P25). The main cause of higher activity at 6 molar% ZrO2 doping was the increase in anatase phase

TiO2 and the creation of appropriate trapping centres which inhibit recombination. The recyclability experiments depict small decrease in photocatalytic activity on its repetitive use, indicates stability of the synthesized photocatalysts.

Chapter 4: Efficient visible light photocatalytic activity and enhanced stability over BiOBr/Cd(OH)2 Heterostructures This chapter presents efficient photocatalysis and enhanced stability over

BiOBr/Cd(OH)2 nanocomposite heterostructures. Novel BiOBr/Cd(OH)2 heterostructures were synthesized by a facile chemical bath method under ambient conditions. A series of BiOBr/Cd(OH)2 heterostructures were obtained by tuning the Bi/Cd molar ratios. The composites prepared exhibited strong visible light absorption and red shift in the visible light region. The 50% BiOBr loaded Cd(OH)2 could degrade about 99.85% of Rhodamine B dye in only 30 min of irradiation, where the photocatalytic activity did not show significant decrease after four cycles of reuse. Photocatalytic studies on Rhodamine B under visible light irradiation showed that the heterostructures are very efficient photocatalysts in mild basic medium. Comparison of photoluminescence intensity suggested that an inhibited charge recombination is crucial for the degradation process over these photocatalysts. Moreover, relative •− positioning of the valence and conduction band edges of the semiconductors, O2/O2 , • OH/H2O redox potential and HOMO−LUMO levels of RhB appears to be responsible for the hole-specificity of degradation. The study hence concludes that the heterojunction constructed between the Cd(OH)2 and BiOBr interfaces play a crucial role in influencing the charge carrier dynamics and their photocatalytic activity. The high photocatalytic activity was attributed to the enhanced light absorption and the improved separation of photogenerated charge carriers.

Chapter 5: Enhanced visible-light-driven photocatalysis by α-Bi2O3 sensitized

Bi2O3/TiO2-Zr nanocomposites

In this work, a new TiO2-based visible light active photocatalyst (α-Bi2O3/TiO2-Zr) was synthesized by coupling of α-Bi2O3 and Zr doped TiO2 via a chemical bath method followed by hydrothermal method. The products obtained showed efficient visible light photocatalysis with nice consistency and durability. Different characterization techniques confirm the formation of the heterostructure nanocomposites. BET analysis confirmed that the Bi2O3 and Zr have significant effect on the particle size and specific surface area of the composites. Photoluminescence study showed that addition of 10% molar amount of Bi2O3 (BTZ-10) faces least charge recombination while as excessive addition of Bi2O3 leads to fast recombination. The results showed that Bi2O3/TiO2-Zr catalysts held an anatase phase

5 and possessed highly crystalline nature. The doped Zr content had a significant effect on the surface area. The introduced Bi species mainly existed as oxides on the surface of TiO2 particles, and its photosensitization effect extended the light absorption into the visible region. The superior photocatalytic performance was ascribed to the high surface area, the enhanced visible light response, and the efficient charge separation associated with the synergistic effects of appropriate amounts of Zr and Bi2O3 in the prepared samples. Chemical Oxygen Demand analysis inferred efficient mineralization of the degradation products.

Chapter 6: Efficient visible-light-driven photocatalytic activity and enhanced charge transfer properties over Mo-doped WO3/TiO2 nanocomposites This final chapter presents the evaluation of photo-electrochemical and photocatalytic properties of molybdenum doped WO3/TiO2 nanocomposite heterostructures. The nanocomposites were synthesized by a typical sol-gel method followed by a hydrothermal method. The as synthesized products showed efficient visible light photocatalytic activity with nice consistency and stability for the photodegradation of

Methylene Blue and p-chlorophenol. The XRD results showed that WO3/TiO2-Mo catalysts held an anatase phase and possessed highly diffused crystalline structure. HRTEM and SAED analysis confirmed the formation of the heterostructure. Energy Dispersive Spectroscopy and Cyclic Voltammetry confirm the metal composition and the oxidation states of the metal ions in these nanocomposites. FTIR shows that the nanocomposite harbours rich hydroxyl content which is feasible for the efficient photocatalysis. The doping of Mo content and the coupling with WO3 had a significant effect on the photon absorption of the TiO2 nanoparticles, and its absorption was extended considerably into the visible region. WO3 coupling also favoured the separation and transfer of photoinduced charge carriers to inhibit their recombination. Photoluminescence study showed that addition of Mo and WO3 efficiently decreased the charge recombination in TiO2. Electrochemical Impedance

Spectroscopy explains that the WO3/TiO2-Mo nanocomposite exhibits enhanced charge transfer and reduced charge recombination. Phenol based experiment indicated that the dye-sensitized photocatalysis is not the dominant phenomenon in these photocatalysts. Finally, the COD analysis inferred efficient mineralization of the degradation products.

SYNTHESIS AND STUDY OF PHOTOCATALYTIC AND CONDUCTING NANOPARTICLES AND NANOCOMPOSITES

THESIS SUBMITTED FOR THE AWARD OF THE DEGREE OF

Doctor of Philosophy

IN CHEMISTRY

BY BILAL MASOOD PIRZADA

UNDER THE SUPERVISION OF DR. SUHAIL SABIR

DEPARTMENT OF CHEMISTRY ALIGARH MUSLIM UNIVERSITY ALIGARH (INDIA) 2016

Dedicated To My Beloved Parents Acknowledgements

And Allah said, "Let there be light," and there was light! At the outset, I surrender myself to Almighty Allah, for showering His blessings upon me for making me able to sew up this thesis work. “There has come to you from Allah a light and a plain book”! Peace and blessing of Allah be upon the noblest of the prophets and messengers, our Prophet Mohammad (SAW) who has said, “The best form of worship is the pursuit of knowledge”! I am bereft of words to thank my mentor and supervisor, Dr. Suhail Sabir for his keen interest, valuable guidance, strong motivation, constant support and encouragement. Your parental care and enthusiasm for life has been invaluable, Sir! Thank you for always challenging and helping me to achieve this goal. I will be forever grateful of the opportunities you have given me and the doors you have opened. I am immensely grateful to Prof. M. Shakir, Chairman, Department of Chemistry, AMU, Aligarh for providing me all the necessary facilities. I owe profound thanks and would like to express my sincerest appreciation to my seniors Dr. Mohammad Zain Khan, Dr. Nida Qutub and Dr. Niyaz Ahmad Mir for their support, advice, guidance and encouragement throughout completing this work. I am also thankful to the rest of faculty and staff members of our department for their helpful suggestions and encouragement. I would like to express my appreciation to my lab colleagues; Mr. Owais Mehraj, Dr. Saima Zain Khan, Dr. Shabana Noor, Mr. Danish Khan, Ms. Nishat Khan, Ms. Nehal and Mr. Nafees Khan for their sound advices, good company, fruitful discussions and for all the fun; and of course to the wonderful moments we had together during my research tenure. My sincere thanks to Prof. Sartaj Tabassum and Prof. (Mrs.)Farrukh Arjumand for their constant encouragement and parental concern. I gratefully acknowledge Prof. Shabbir Ahmad and Dr. Shahid Husain (Department of Physics, AMU) for the XRD and UV-DRS facilities they provided at ease. I am also highly thankful to Dr. Irshad Bhat and Mr. Sheeraz Bhat for their timely support in terms of instrumental handling and analyses. It gives me enormous pleasure to gratefully acknowledge the financial support of CSIR, New Delhi in terms of JRF and SRF fellowships which buttressed me to perform my work comfortably. I also acknowledge UGC India for providing the financial support; and Department of Chemistry A.M.U, USIF A.M.U. and Centre of Excellence ZHCET A.M.U. for providing all the facilities and technical support I needed during my research work. AIRF JNU and IACS Kolkata are also acknowledged for availing HRTEM and BET facilities respectively. To my wonderful friends at A.M.U: Mr. Shah Tariq, Dr. Imtiyaz Yousuf, Dr. Towseef Mohsin, Mr. Mohammad Irfan, Dr. Muzaffar Ahmad, Dr. Ali Haider, Dr. Nayeem Ahmad, Mr. Imtiyaz Kabir, Dr. Bashir Ahmad Shiekh, Dr. Athar Masoodi, Dr. Mudassir Irfan, Dr. Bilal Ahmad Mir, Dr. Mohammad Ashraf, Dr. Tariq Dar, Dr. Asger Mughal, Mr. Abdul Lateef, Mr. Imran Mustafa, Mr. Bilal Nabi, Mr. Iqbal Lone, Mr. Rashid Saleem, Dr. Mir Hashim, Dr. Zameer Ahmad Shah, Dr. Saifuddin Ansari and Mr. Qamruzzafar: Without a shadow of doubt I can say I would never ever have made this far without you guys. Thanks for always being there, calming me down when I was stressed, for always making me laugh, for making me forget that I was away from my family. These silly words cannot express how I feel. You guys have provided me some of the best memories which I will surely cherish throughout my life. It is never easy to acclimatize readily at a new place. I sincerely thank Mr. Mumtaz Numani and Mr. Showkat Malik for their constant guidance and hospitality which made my life easier here at AMU. Thanks a lot to both of you! My younger brothers at S.Z. Hall deserve special thanks for their care and respect towards me. Thank You Ryhan Abdullah, Junaid Khan, Affan Fazili, Fayaz Ahmad and Tanveer Shiekh! Special gratitude to my room-mates: Mr. Javaid Lone, Mr. Abdul Qadir and Mr. Saddam Malik for sharing the unforgettable moments of life with me. My heartfelt gratitude and love for the Malaysian Friends who visited to our laboratory: Mr. Klosen Cheng, Mr. Nicholas Chan, Mr. Muzammer Hamim, Mr. Sofri, Mr. Haadi, Mr. Mac, Ms. Xiu Xiu Min and Ms. Teh for the wonderful time we had together. I wish I could rewind to those days! To my wonderful classmates and friends: Mr. (Late) Rayees Ahmad Malik, Mr. Wakeel Ahmad, Mr. Muneer, Mr. Shakeel, Mr. Suhaib, Mr. Arshid, Mrs. Ruqaya, Mrs. Saima, Ms. Mehnaz, and Mrs. Aaliya for sharing an unforgettable time with me. We will miss you Rayees at every phase of our life. Allah bless you a high place in heaven. (Aameen)! To my respected teachers: Prof. Bashir Ahmad Rather, Prof. Haris Izhar Tantray, Prof. Bashir Ahmad and Prof. Bilal Ahmad Bhat for their significant contribution in shaping me as a chemist. I owe a lot to you all! To my dear friends at home: Mr. Shamim Shiekh, Mr. Zaffar Shiekh, Mr. Bashir Mir, Mr. Khalid Masoodi, Dr. Mohammad Amin, Mr. Nisar Masoodi, Er. Nazir, Peer Shamsuddin, Peer Shahabuddin, Peer Zaheeruddin, Peer Sajjad, Peer Yahya, Peer Manzoor Kirmani, Peer M. Shafi, Mr. Tariq Kumar, Mr. Javaid Shiekh and Mr. Fayaz Malik: Hey guys; you are awesome! Thanks for being always there, standing by my side and inspiring me throughout this duration. There is too much for you to say, so I will do one brisk sweep and say THANK YOU to all of you! To my dearest sisters; Mrs. Zareena, Mrs. Tasneem Shafqat, Mrs. Zahida, Mrs. Masarat, Mrs. Gulshan, Mrs. Maryam and Ms. Toufeeq for your constant care, support, blessings and encouragement. Good wishes and love to the cute smiles; Hilal ul Eid, Rabia Lateef, Rafia Lateef, Soliha Saeed, Uzma Beti, Hammad bin Taha, Baazigah, Umair Lateef, Zakariyya and Zainul Ibaad. My heartfelt gratitude to my dear sisters; Mrs. Masooda Shahab, Ms. Rayeesa Sayeed and Ms. Shahida Mukhtar for all the care and respect they have given to me during my stay in Srinagar and even now. My greatest appreciation and respect goes to my brothers, Mr. Abdul Lateef Peerzada, Mr. Nazir Ahmad Masoodi, Mr Shabir Ahmad Masoodi, Mr. Hilal Sahib, Mr. Roshan Shehab and Mr. Aijaz Suliaman for always inspiring me to step ahead to achieve my goals and the extra respect they rendered to me. I profoundly render my deep regards to my beloved parents (Mr. Mohammad Saeed Pirzada and Mrs. Hanifa Begum) for their endless patience, countless sacrifices, sincere encouragement and inspiration. Thank you for believing in my vision and providing me the wings of freedom and opportunity to chase my dreams. It was surely very tedious without your blessings and moral support. Finally, my respect and heartfelt thanks goes to the unfailing love and concern of my elder brother, Mr Peerzada Taha Husain, who have been a source of courage and support in my journey of life. I owe a lot to his love, care, affection and blessings. Allah bless you all!

Dated: (Bilal Masood Pirzada)

CONTENTS

Title Page No.

Abbreviations I-II

List of Figures III-X

List of Tables XI

CHAPTER 1: Introduction and Review of Literature 1–61

CHAPTER 2: Methodology 62-94

CHAPTER 3: Synthesis, Characterization and Optimization 95-116 of Photocatalytic Activity of TiO2/ZrO2 Nanocomposite Heterostructures

CHAPTER 4: Efficient visible light photocatalytic activity 117-142 and enhanced stability over BiOBr/Cd(OH)2 Heterostructures

CHAPTER 5: Enhanced visible-light-driven photocatalysis by 143-171 α-Bi2O3 sensitized Bi2O3/TiO2-Zr nanocomposites

CHAPTER 6: Efficient Visible-light-driven Photocatalytic 172-204 activity and enhanced charge transfer properties over Mo-doped WO3/TiO2 nanocomposites

List of Thesis Papers Imprints Abbreviations

AC Alternating Current APS aminopropyltrimethoxysilane BET Brunauer Emmett Teller BJH Barrett-Joyner-Halenda BQ Benzoquinone CB Conduction Band CBM Conduction Band Minimum COD Chemical Oxygen Demand DTA Differential Thermal Analysis DTG Differential Thermal Gravimetry DRS Diffuse Reflectance Spectra eV Electron Volt EDS Electron Dispersive Spectroscopy EDTA ethylenediaminetetraacetate

EF Fermi level Energy

Eg Band gap Energy EIS Electrochemical Impedance Spectroscopy EMA Effective Mass Approximation FT–IR Fourier Transform Infrared FWHM Full Width at Half Maximum g-C3N4 graphitic carbon nitride HOMO Highest Occupied Molecular Orbital HRTEM High Resolution Transmission Electron Microscopy IC Indigo Carmine IPA isopropylalcohol JCPDS Joint Committee on Powder Diffraction Standards KBr Potassium Bromide LCR Inductance Capacitance Resistance LUMO Lowest Unoccupied Molecular Orbital MB Methylene Blue MeOH Methanol MOF Metal Organic Framework

NHE Normal Hydrogen Electrode NIR Near Infra Red NPs Nanoparticles PBS Ponceau Biebrich Scarlet PCP p-chlorophenol PL Photoluminescence PSH Photo-induced Superhydrophilicity QSE Quantum Size Effect Q-sized Quantum sized RhB Rhodamine B ROS Reactive Oxygen Species SAED Selected Area Electron Diffraction SC Semiconductor SC NPs Semiconductor Nanoparticles SEM Scanning Electron Microscopy TEM Transmission Electron Microscopy TGA Thermal Gravimetric Analysis

TiO2 P25 titanium dioxide pigment 25 TOC Total Organic Carbon TTIP titaniumtetraisopropoxide UV ultra-violet UV–Vis ultra–violet and visible VB Valence Band VBM Valence Band Maximum VO Oxygen Vacancy XRD X-Ray Diffraction

List of Figures

Chapter 1 Page No. Figure 1.1 A very simple comparative representation of the size of 1 nanomaterials with respect to other small particles Figure 1.2 (a) A lotus leaf, (b) Pieces of wood (left) treated with 2 BASF’s lotus spray showing water repellent effect and a normal wood (right) showing wetting & water clogging (c) Normal spectacle glass with water droplets and (d) PWR coated clear spectacle glass Figure 1.3 Applications of semiconductor nanomaterials 3 Figure 1.4 Splitting of energy levels in quantum dots due to the 4 quantum confinement effect, semiconductor band gap increases with decrease in size of the nanocrystal Figure 1.5 Proportion of surface atoms for spherical particles 6

comprising Nv atoms with Ns at the surface Figure 1.6 Processes involved in semiconductor particles upon 11 bandgap excitation Figure 1.7 Elements constructing heterogeneous photocatalysts 15 Figure 1.8 Charge transfer in a capped semiconductor system 22 Figure 1.9 Charge transfer in a coupled semiconductor hetero- 22 structure Figure 1.10 Solar hydrogen production from water using a powdered 29 Photocatalyst Figure 1.11 Photosynthesis by green plants and photocatalytic water 30 splitting as an artificial photosynthesis Figure 1.12 Principle of water splitting using semiconductor 30 photocatalysts Figure 1.13 Relationship between band structure of semiconductor and 32 redox potentials of water splitting. The diagram was adapted using data from previous publications Figure 1.14 Schematic of an artificial leaf with Co-based catalysts for 37 oxygen evolution and a npp+- silicon junction separated by a conductive layer of tin-doped indium oxide (ITO)

III

Figure 1.15 Difference in concepts of catalytic and photocatalytic 40 reactions: A catalyst contains active sites of which a substrate is converted into a product, while no active sites are present on a photocatalyst Figure 1.16 Degradation of Indigo Carmine dye under UV-irradiation 41 (A) and electron transfer from excited IC* molecules without hole formation under visible light (B) Figure 1.17 Schematic n- and p-type doping of titania 42 Figure 1.18 Effect of the different physical parameters, which 47 influence the kinetics of photocatalysis: reaction rate r; (A) mass of catalyst m; (B) wavelength λ; (C) initial concentration c of reactant; (D) temperature T; (E) radiant flux φ Chapter 2 Figure 2.1 TEM images of the round-shaped TiO2 nanoparticles 64

Figure 2.2 A typical SEM image of the Bi2O3 nanorods prepared by 66 the hydrothermal method

Figure 2.3 EDS spectrum of TiO2/ZrO2 nanocomposite showing 68 atomic and weight percentage of different elements present in the nanocomposite

Figure 2.4 The XRD pattern of synthesized anatase TiO2 70 nanoparticles showing broad peak indicating the FWHM Figure 2.5 (a) A representation of the DTA curve showing exotherm, 74 endotherm, thermophysical, thermochemical, thermomechanical and thermoelastic changes or transition. (b) Schematic representation of percentage weight loss and the corresponding DTA curve showing the exothermic or endothermic nature of the loss Figure 2.6 A representative curve of DTG showing its relation with 75 TGA curve Figure 2.7 Adsorption isotherm and pore width distribution (inset) for 76

Bi2O3/TiO2 nanocomposite Figure 2.8 The representative curves for (a) Absorption edge and (b) 80

Band gap calculation Figure 2.9 Photoluminescence spectra of different nanocomposite 81 samples giving idea about charge carrier recombination Figure 2.10 EIS Nyquist plots for different nanocomposite samples 83 giving idea about the impedance imposed to charge transfer Figure 2.11 Photograph showing set-up of photocatalytic experiment 84 in our laboratory Figure 2.12 COD and percentage removal efficiency during a 87 photocatalytic degradation process as a function of duration of irradiation Chapter 3 Figure 3.1 XRD patterns of TiO2/ZrO2 samples (TMZ-a, TMZ-b, 100

TMZ-c, TMZ-d, TMZ-e) and pure TiO2 (TM)

Figure 3.2 Designation of peaks in XRD spectrum of pure TiO2 (TM) 101

Figure 3.3 SEM images of TiO2/ZrO2 nanocomposite (TMZ-b) 102 showing irregularity and sphericity of nanoparticles

Figure 3.4 TEM images of TiO2/ZrO2 nanocomposite (TMZ-b) 102 showing the shape and size of nanoparticles

Figure 3.5 EDS spectrum of TiO2/ZrO2 nanocomposite (TMZ-b) with 103 quantification of atomic percentage Figure 3.6 TGA and DTA curves of TMZ-b on heat treatment upto 103 1200oC

Figure 3.7 Variation of dielectric constant with increase in ZrO2 104 amount Figure 3.8 Variation in band gap as estimated from the UV-DRS 105 spectra, obtained by plotting Kubelka-Munk function, F(R) vs. energy, hν

Figure 3.9 Fluorescence emission spectra of TiO2/ZrO2 107

nanocomposites and pure TiO2 (TM) Figure 3.10 Kinetics of Photocatalytic decolorization of Ponceau BS in 108

the presence of pure TiO2 and TiO2/ZrO2 nanocomposite.

(a) Decrease in absorption intensity of PBS at its λmax

505nm at different irradiation time intervals in the presence of TMZ-b under constant stirring and bubbling of

atmospheric oxygen. (b) Change in concentration (Ct/C0) of PBS as a function of irradiation time in the absence and

presence of catalyst TiO2/ZrO2, Pure TiO2 and Degussa

P25. (c) Plot of ln (C0/Ct) = f(t). (d) Cyclic runs of

TiO2/ZrO2 (TMZ-b) for the decolorization of Ponceau BS. (e) Samples showing decrease in Ponceau BS colour at

different irradiation time in the presence of TiO2/ZrO2 (TMZ-b) Figure 3.11 Schematic diagram of electron–hole pair separation and 110

the possible reaction mechanism over TiO2/ ZrO2 photocatalyst under UV light irradiation Chapter 4 Figure 4.1 XRD pattern of the different samples 122 Figure 4.2 SEM images: (a-b) flower like cluster of BiOBr, (c-d) 123

Cd(OH)2 nanoparticles, (e-f) BC-50 nanocomposite

Figure 4.3 HRTEM images: (a) BiOBr, (b) Cd(OH)2 nanowires, (c-d) 124 BC-50 nanocomposite , (e) SAED pattern of BC-50 and (f) Lattice pattern of the BC-50 composite

Figure 4.4 The EDS spectrum obtained for 50% BiOBr/Cd(OH)2 125 (BC-50)

Figure 4.5 Band gap calculation of (a) Cd(OH)2 and (b) BiOBr, (c) 126 UV-DRS spectra of the various samples Figure 4.6 Kinetics of Photocatalytic decolorization of RhB in the 128

presence of pure BB, CDH and and BiOBr/Cd(OH)2 heterostructures. (a) Change in absorption of RhB after regular intervals of light irradiation in presence of BC-50

photocatalyst, (b) Change in concentration (Ct/Co) of RhB during its decolorization in the presence of CDH, BB and

BiOBr/Cd(OH)2 heterostructures, (c) ln(Co/Ct) versus irradiation time for decolorization of RhB in the presence of different catalysts, (d) Cycling runs of BC-50 for

decolorization of RhB under visible light (e) Change in colour of RhB dye at regular intervals of irradiation in the presence of BC-50 Figure 4.7 Kinetics of Photocatalytic degradation of p-chlorophenol 129 in the presence of pure BB, CDH and and BC-50

heterostructures. (a) Change in concentration (Ct/Co) of PCP during its degradation in the presence of CDH, BB

and BC-50 (b) ln(Co/Ct) versus irradiation time for degradation of PCP in the presence of different catalysts Figure 4.8 Change in concentration of cadmium ions in dye solutions 130 after regular intervals of irradiation as a result of leaching in the catalysts Figure 4.9 Effect of various scavengers on the photocatalytic activity 131 of BC-50 for the decolorization of RhB

Figure 4.10 (a) Band gap alignment of Cd(OH)2 and BiOBr 133 nanoparticles before coupling (b) Formation of p-n junction and subsequent energy level alignment on heterostructure formation Figure 4.11 Schematic mechanism and formation of ROS in the 134 photodegradation process by the heterostructure Figure 4.12 Photoluminescence (PL) spectra of various photocatalyst 135 samples Chapter 5 Figure 5.1 XRD patterns of the nanocomposite heterostructures 148

Figure 5. 2 SEM images: (a-b) Rod shaped stacks of Bi2O3, (c-d) BT- 150 10 heterostructures, (e- f) BTZ-10 heterostructures Figure 5. 3 HRTEM images: (a, b) BTZ-10 nanocomposites, (c) the 151 lattice pattern of the BTZ-10 composite and (d) the SAED pattern of BTZ-10 Figure 5. 4 EDS mapping and EDS spectrum of BTZ-10 152 heterostructure showing the elemental composition and distribution of the composite

Figure 5. 5 FT-IR spectra of BTZ-10, BT-10 and Bi2O3 samples 153

VII

Figure 5. 6 (a) BET isotherm and particle size distribution (inset) of 153 BTZ-10 sample (b)BET isotherm and particle size distribution (inset) of BT-10 sample Figure 5. 7 (a) DRS spectra of the samples (b) Band gap energy 155 curves for the samples Figure 5. 8 Kinetics of photocatalytic decolorization of RhB in the 157

presence of pure Bi2O3, BT-10 and BTZ heterostructures (a) Change in absorption of RhB after regular intervals of light irradiation in the presence of the BTZ-10

photocatalyst, (b) change in concentration (Ct/C0) of RhB

during its decolorization in the presence of Bi2O3, BT-10

and BTZ heterostructures, (c) ln(C0/Ct) versus irradiation time for decolorization of RhB in the presence of different catalysts, (d) cycling runs of BTZ-10 for decolorization of RhB under visible light (e) variation in colour of the RhB dye at regular intervals of irradiation in the presence of BTZ-10

Figure 5. 9 (a) Change in concentration (Ct/C0) of PCP during its 158

degradation in the presence of Bi2O3, BT-10 and BTZ

heterostructures, (b) ln(C0/Ct) versus irradiation time for degradation of PCP in the presence of different catalysts

Figure 5. 10 Effect of different ROS scavengers on kapp of BTZ-10 for 159 RhB degradation Figure 5. 11 Hydroxyterepthalic acid fluorescence indicating 160 generation of HO• by BTZ-10 during the photocatalytic process Figure 5. 12 Photoluminescence spectra of the photocatalyst samples 161 indicating the extent of charge recombination Figure 5. 13 Nyquist plots showing the extent of impedance imposed 162 by different samples to charge transfer Figure 5. 14 Alignment of energy bands of the constituents of the 164 nanocomposite before coupling Figure 5. 15 Formation of the heterostructure and the mechanism of 164

VIII

formation of different reactive oxygen species Figure 5. 16 Change in COD of the RhB solution during the irradiation 166 in presence of BTZ-10 sample Chapter 6 Figure 6. 1 XRD of different photocatalyst samples 178

Figure 6. 2 SEM micrographs: (A-B) WO3/TiO2 heterostructures; (C- 180

D) TiO2-Mo nanoparticles; (E-F) WO3/TiO2-Mo nanocomposite

Figure 6. 3 HRTEM images: (a-b) TiO2-Mo nanoparticles; (c) the 181

lattice plane spacing of the TiO2-Mo nanoparticle and (d)

the SAED pattern of TiO2-Mo nanoparticles

Figure 6. 4 HRTEM images: (a-b) WO3/TiO2-Mo nanocomposite; (c) 182

the lattice plane spacing of the WO3/TiO2-Mo nanocomposite and (d) the SAED pattern of the

WO3/TiO2-Mo nanocomposite Figure 6. 5 The EDS spectrum showing the elemental composition of 183

(a) TiO2-Mo nanoparticles; (b) WO3/TiO2 nanocomposite

Figure 6. 6 EDS mapping and EDS spectrum of the WO3/TiO2-Mo 183 nanocomposite showing the elemental composition and distribution of the constituents

Figure 6. 7 FT-IR spectra of TiO2-Mo, WO3/TiO2 and WO3/TiO2-Mo 184 samples Figure 6. 8 (a) DRS spectra of the samples (b) Band gap energy 185 curves obtained for the samples Figure 6. 9 Kinetics of photocatalytic decolorization of MB in the 187

presence of pure TiO2, TiO2-Mo, WO3/TiO2 and

WO3/TiO2-Mo samples (a) Change in absorption of MB at regular intervals of light irradiation in the presence of the

WO3/TiO2-Mo photocatalyst, (b) change in concentration

(Ct/C0) of MB during its decolorization in the presence of

TiO2, TiO2-Mo, WO3/TiO2 and WO3/TiO2-Mo samples,

Recycling experiments of WO3/TiO2-Mo for decolorization of MB under visible light, (e) Variation in colour of the MB dye at regular intervals of irradiation in

the presence of WO3/TiO2-Mo photocatalyst Figure 6. 10 Comparison between the photodegradation of MB dye and 188 PCP in presence of different photocatalysts

Figure 6. 11 Effect of different ROS scavengers on kapp of WO3/TiO2- 189 Mo nanocomposite for MB degradation Figure 6. 12 Photoluminescence spectra of the photocatalyst samples 190 showing the extent of charge recombination Figure 6. 13 Hydroxyterepthalic acid fluorescence indicating 191 • generation of HO by WO3/TiO2-Mo nanocomposite during the photocatalytic process Figure 6. 14 Nyquist plots showing the extent of impedance imposed 191 by different samples to charge transfer

Figure 6.15: Cyclic voltammograms for (a) TiO2-Mo and (b) WO3/TiO2 193 nanocomposite

Figure 6.16 Cyclic voltammograms for (a) WO3/TiO2-Mo 194 nanocomposite at the potential scan rate of 100-250mVs-1;

(b) TiO2-Mo, WO3/TiO2 and WO3/TiO2-Mo nanocomposite at potential scan rate of 250mVs-1 Figure 6.17 Formation of the heterostructure and the mechanism of 197 formation of different reactive oxygen species Figure 6. 18 Change in COD of the MB solution during the irradiation 199

in presence of WO3/TiO2-Mo sample

List of Tables

Page No. Chapter 3 Table 3.1 Lattice Cell parameters as obtained by PowderX (unrefined) 99 Chapter 4 Table 4.1 Percentage decolorization and apparent rate constant by 129 different samples Chapter 5 Table 5.1 Average crystallite size and corresponding apparent rate 149 constant for the different photocatalyst samples

Chapter 1 Introduction and Review of Literature

1 Chapter 1

Introduction and Review of Literature

1.1. Nanotechnology “Nanotechnology literally comprises any technology performing at nanoscale and has applications in the real world”. Broadly, it refers to manipulating matter at the atomic or molecular scale and exploring materials and structures with nanosized dimension, usually ranging from 1nm to 100nm. Nanotechnology encompasses the production, modification and application of physical, chemical, and biological systems at nanoscales ranging from individual atoms or molecules to submicron dimensions, as well as the assembling of the resulting nanostructures into larger systems.1,2 Nanomaterials exhibit a wide variety of unique physico-chemical properties, different from those of their respective bulk counterparts3,4 such as, large surface area to volume ratios or high interfacial reactivity, because of their minimal size. The term “nanomaterials” is employed to describe the designing and exploitation of materials with structural features in between those of atoms and giant materials, having at least one of its dimensions in the range of 0.1-500nm (1nm = 10-9m).2,4-7A simple comparative representation of the size of nanomaterials with respect to other small particles is given below in Figure 1.1.

Figure 1.1: A very simple comparative representation of the size of nanomaterials with respect to various biomolecules and inorganic materials.8

The various physical properties (viz; dynamic, thermodynamic, mechanical, optical, electronic, magnetic) and chemical properties of nanomaterials can be significantly altered relative to their bulk counterparts. These properties are dependent not only on size but also on morphology and spatial arrangement of the nanomaterials. Further, 2 Chapter 1

when the size of materials is reduced to nanometer scale, more and more atoms are being exposed to the surface. Therefore, surface phenomena (like wetting etc.) begin to play a critical role,4 as shown in Figure 1.2.

Figure 1.2: (a) A lotus leaf, (b) Pieces of wood (left) treated with BASF’s lotus spray showing water repellent effect and a normal wood (right) showing wetting & water clogging (c) Normal spectacle glass with water droplets and (d) PWR coated clear spectacle glass.9

The nanomaterials have proved to be very significant in development due to their tremendous economic, technological and scientific benefits anticipated in several areas. There are various forces driving towards nanotechnology; above all, most of the biomolecules and other bioentities are of nanometer size, thus the nanotechnology provides an excellent opportunity to study such bioentities and their interactions with other materials. Another dynamic force is semiconductor industry, which due to its ever-lasting demand for miniaturization, has been driven profoundly into the nano- realm.10 The importance of nanotechnology was explained by Feynman in 1959,11 in his lecture entitled “There is plenty of room at the bottom” at the annual meeting of the American Physical Society, that had become one of the twentieth century’s classic science lectures and today’s most cited one. He presented a technical vision of extreme miniaturization of materials even several years before the word “chip” came into existence. He talked about the scope and problem of manipulating and controlling things on a small scale.12 Just like the semiconductor technology, information 3 Chapter 1

technology or biotechnology, nanotechnology is likely to have a profound impact on our economy and society. Nanotechnology have promising breakthroughs in the areas like materials and manufacturing, nano-electronics, photonics, healthcare, energy, biotechnology, information technology and national security.1,3 Some of the unique properties and possible applications of nanomaterials are summarized below in Figure 1.3.6,13,14

Figure 1.3: Applications of semiconductor nanomaterials

1.2. Semiconductor nanomaterials Semiconductor (SC) materials in nano-range have attracted much interest since last three decades as they possess unique physical, chemical and optical properties that are directly affected by their size.15 Semiconductor nanoparticles (SC NPs) exhibit different size-dependent properties like electronic band gap energies, solid-solid phase transition temperatures, melting temperatures and pressure responses.16 A simplified energy-band diagrams of bulk and nanoparticle materials are shown in Figure 1.4 below.17 To understand photoconductivity, electrical conductivity and related phenomena viz; photocatalysis, it is necessary to understand the energy bands of a SC and the doping of semiconductors. The law of quantum mechanics says that in an isolated atom, electrons can have only certain discrete amount of energy. But, when these isolated atoms are brought together to form a crystal, the electrons are no longer restricted to a 4 Chapter 1

single energy level, but rather they are allowed to quasi-continuous energy levels called bands or energy-bands.

Figure 1.4: Splitting of energy levels in quantum dots due to the quantum confinement effect, semiconductor band gap increases with decrease in size of the nanocrystal.18

In semiconductors, the highest occupied band is completely filled with electrons, and the lowest empty band is just close by.2 The highest filled energy level is called valence band (VB) which is similar to the highest occupied molecular orbital (HOMO) because it contains the valence electrons of the SC. The unoccupied energy level just above this is called a conduction band (CB), which is similar to the lowest unoccupied molecular orbital (LUMO) as it is completely vacant. The gap between CB and VB is known as the forbidden gap or band gap energy.17 Although, there is no space for the electrons to move around within the valence band in a semiconductor, it is not too hard for an electron to gain the extra energy required to excite into the nearby higher band (CB), where it can move freely carrying an electric current. This leaves a ‘gap’ or ‘hole’ in the lower band which can also move in direction opposite to an electron. Further, by supplying an extra energy from outside, or by clever designing of the SC, the way the semiconductor conducts electric current can be controlled. The electric current is carried by both electrons (e-) and holes (h+) in the case of semiconductors.2 5 Chapter 1

The semiconductors are useful due to their property that by just changing their structure or composition they can be made either conductor or nonconductor of the electricity. In addition, their significant interaction with light, which strongly depends on their electronic band structure, that in turn, depends upon their composition. Due to this dependency on composition or structure, we can design semiconductor devices that have variety of useful properties, such as amplifying or detecting electrical current (i.e. the electrons), as well as amplifying, detecting or emitting lights (i.e. the photons). SC devices can be made by putting a piece of preferred semiconductor next to another, in such a way that the electrons can only move in the preferred direction of a material, the concept of ‘diodes’; or by surrounding the preferred semiconductor by a different semiconductor, so that 'traps' can be created for the charges (electrons and holes) leading to the formation of quantum well, quantum wire or a quantum dot.2 These doped semiconductor nanoparticles have tremendous potential in the field of optoelectronic, nonlinear optical devices, microelectronics, photovoltaics, solar devices, imaging and display technologies, sensing devices, thin film coatings and the photocatalytic applications.16,19,20 By making use of the principles of photoelectrochemistry, semiconductor nanoclusters have been successfully employed in the conversion of light energy13,21 and photocatalytic detoxification of air and water.22,23

1.3. Properties of nanomaterials Nanomaterials have growing interest due to their fascinating properties.4,5 Generally, nanomaterials may have globular (hollow microspheres or quantum dots)8, wire like (nanowires)14, ribbon like (nanoribbons), tube like (nanotubes)24, rod-like (nanorods)25 or more complex geometries.3 In the realm of physics, drastic changes are likely to be expected in the nanometer range. The reason was expressed by Reithmaier as follows, “the properties of a solid can change dramatically if its dimensions or the dimensions of the constituent phases, become smaller than some critical length associated with these properties”.10 The size dependent behavior of nanoparticles alter the physical and chemical behaviour as well.13 In particular, it will cause changes in mechanical, optical, electrical, electro-optical, magnetic, and magneto-optical properties. Some of the properties which are of immense interest in our study have been briefly discussed below.

6 Chapter 1

1.3.1. Surface Effect It is important to keep in mind that smaller the particle size, the larger will be the portions of their constituent atoms located at the surface. In semiconductor nanomaterials, this arrangement facilitates electron (e-) and/or hole (h+) transfers to and from acceptors and/or donors localized at the surface. In metallic nanoparticles, a large surface to volume ratio permits an effective charge transfer and induce charge transfer dependent changes in the optical absorption spectra.13 It is evident from Figure 1.5 that, as the size of the object is reduced to the nanometric range (i.e., <10nm), the proportion of surface atoms is no longer negligible.26 This proportion can be estimated for the transition metals by the given relation:

= (1) 푁푠 1 푁푣 2푅 Where, R is the radius in ‘nm’. This empirical law will give a proportion of 100% of surface atoms for a size of 1nm. It is observed that the properties of the object will be modified by the presence of a large fraction of the atoms located at its surface.26, 9

Figure 1.5: Proportion of surface atoms for spherical particles comprising Nv atoms with Ns at the surface.9

The size-dependent phenomenon is also described by surface-to-volume ratio (A/V). The surface-to-volume ratio is inversely proportional to the diameter or thickness of the material structure. For example, for spherical particles of diameter d (radius R) surface-to-volume ratio will be: 7 Chapter 1

= = = 2 (2) 퐴 4휋푅 3 6 3 푉 4휋푅 ⁄3 푅 푑 The inverse dependence of the diameter holds for simple geometries such as cubes, long cylinders or thin plates, but for complicated structures the relation is less straightforward.

1.3.2. Quantum Size Confinement Effect (Effective Mass Approximation) There has been tremendous effort towards understanding the changes occuring in the electronic structure of nanomaterials as the size of the crystallite changes. Qualitatively, it can be understood like a particle-in-a-box-like problem, where the energy level spacing increases as the box dimensions are reduced, this is known as Quantum Size Confinement Effect or Quantum Size Effect (QSE).6,17,27 However, the quantitative understanding involves much more elaborate calculations of the band gap of the nanocrystals as a function of their size. There is a fundamental difference between the calculations of band gap for the nanocrystals and those for the bulk. In the case of bulk materials, for most practical purposes, the size of the material can be considered to be infinite, with negligibly small influence of the surface or boundary effects on the electronic structure of the bulk. In sharp contrast, a nanomaterial is by definition bounded on all sides, i.e. the electronic propagation is confined in all three dimensions. This implies that the lattice periodicity or the translational invariance is hindered in such a finite-sized system and momentum is no longer a good quantum number to provide a quantum mechanical description of the system. Thus, the electronic structure of a nanomaterial has to be necessarily evaluated directly in real space. As a result, such calculations involve a Hamiltonian matrix with dimension equal to the total number of orbitals in the whole of the nanocrystals.6,27 In case of a semiconductor, quantization (decreasing of the size comparable to Bohr atomic radius) effectively increases the band gap, therefore, the photoexcited electrons and holes will have more negative and more positive redox potentials, respectively, at the lowest respective quantum level in the conduction band and valence band. The enhancement of redox potentials in colloidal semiconductor nanoparticles have been

experimentally verified in a number of systems, including TiO2, CdS, HgSe, PbSe, CdSe, PbS, CuS, ZnS and ZnO.4,28-31 The distribution of the confinement energy or increase in band gap between the photoexcited electrons and holes depend on their respective effective masses.4,28 8 Chapter 1

This can be estimated by the following expressions:4,5

/(1 + ) ∗ (3) 푏푢푙푘 푚푒 ∗ 훥퐸푒 ≈ �퐸푒푥 − 퐸푔 � 푚ℎ (4) 푏푢푙푘 훥퐸ℎ ≅ 퐸푒푥 − 퐸푔 − 훥퐸푒 Where Ee and Eh are the increased redox potentials for electrons and holes

respectively, Eex and Eg bulk are the effective band gap of the nanoparticle and band

gap of the bulk semiconductor respectively, me* and mh* are the effective masses of the electron and hole respectively.4 The quantum confinement effect (i.e.; the band gap variation with change in size for nanomaterials) can be qualitatively explained using the Effective Mass Approximation (EMA)4,17,32,33 after solving the Schrödinger equation for the envelope function ψ:6

+ = 2 2 2 2 2 , , (5) ħ 훻푒 ħ 훻ℎ 푒 �− 2푚푒 − 2푚ℎ − 4휋휀0휀푟푒ℎ 푉0� 휓 �푟푒 푟ℎ� 퐸휓 �푟푒 푟ℎ� Where, the subscripts e and h refer to the electron and the hole with m and r being the

mass and position vector, respectively, and reh = [re – rh]. ε0 and ε are the permittivity in vacuum and the relative dielectric constant of the material. The above equation can be solved by approximate methods using a trial wave function. The EMA has been used to calculate the band gap for various semiconductor nanoparticles. Brus33,34, Ekimov and Efros17 and Kayanuma19 has proposed the

following equation for the effective band gap (Eg) of a spherical particle with radius (R):6,17,35

. = + 0.248 2 2 2 (6) ħ 휋 1 1 1 8푒 ∗ 2 훥퐸푔 ≡ 퐸푅 − 퐸푔 2푅 �푚푒 푚ℎ� − 휀푅 − 퐸푅푦 where, Eg is the bulk band gap, ER is the band gap of nanoparticle, R is the radius of

the quantum dot, me and mh are the effective mass of electron and hole respectively, e is the charge of the electron, ε is the dielectric constant of the semiconductor, ħ=h/2π, 36 where h is the planks constant, π=22/7, and E*Ry is the effective Rydberg energy. The first term in Equation (6) referred to as the quantum localization term (i.e. the -2 kinetic energy term), which shifts the Eg to higher energies proportional to R . The second term arises due to the screened Coulomb interaction between the electron and −1 hole, that shifts the Eg to lower energy proportionally to R . The third term is a size- 9 Chapter 1

independent term and it is the solvation energy loss and is usually small and can be ignored. The effective Rydberg energy (in meV) is defined as:

= 13605.8 + (7) −1 ∗ 1 푚0 푚0 2 퐸푅푦 휀 �푚푒 푚ℎ� However, since the first term becomes dominant with the smaller value of R, the effective band gap is expected to increase, especially when R is very small.6,33,34,37 The quantum size confinement effect becomes particularly significant when the size of nanoparticle becomes comparable to or smaller than the Bohr exciton radius (B)4 which is given by:

= 2 (8) 휀0휀ℎ 2 훼퐵 휋휇푒 Where, εo and ε are the permittivity of vacuum and relative permittivity of the semiconductor, μ is the reduced mass of the electron and hole, and e is the electron charge. The reduced mass is given as:

= (9) 푚푒푚ℎ 휇 푚푒+푚ℎ Where, me and mh are the effective masses of electrons and holes respectively. For instance, the Bohr radius of CdS is around 2.4nm, thus the CdS particles with radius smaller or comparable to 2.4nm will show strong quantum confinement effects, which will be indicated by a significant blue shift of their optical absorption relative to that of bulk.4,9 The Q-particles can have a different colour depending on the particle size. For example, CdS normally exists as a yellow material, but becomes colourless when the particle is smaller than 22Å. Cadmium phosphide, which is normally a black material, can be made in various colours depending on the particle size.38-41 Thus, by varying the size of the semiconductor particles, it is possible to tune the band gap and hence enhance the redox potential of the valence band holes and the conduction band electrons.39 Hence, nanosized semiconductor particles can possess enhanced photoredox chemistry, with reduction reactions, which might not otherwise proceed in bulk materials, being able to occur readily using sufficiently small particles.42,43 Another factor which could be advantageous is the fact that the fraction of atoms that are located at the surface of a nanoparticle is very large. One 10 Chapter 1

disadvantage of nanosized particles is the need for light with a shorter wavelength for photocatalyst activation. Thus, a smaller percentage of a polychromatic light source will be usefully available for the initiation of photocatalysis by these particles.38

1.3.3. Interaction with Light Several important areas of applications of nanomaterials involve their interaction with light. These include photochemical, photoelectrochemical, and photocatalytic reactions. The effect of light with above band gap excitation is to produce very reactive electrons and holes in the semiconductor materials that subsequently react with species near or on the surface of the nanomaterial. The chemical reactions involving the photogenerated electrons are photoreduction reactions, while reactions involving photogenerated holes are photooxidations.44,45 A large percentage of the initially created charge carriers are quickly trapped by surface trap states (on the time scale of a few hundred of fermi seconds to a few of pico seconds). Both free and trapped carriers can participate in reactions with species on or near the surface. The trapped carriers are less energetic than free carriers. Electron or hole transfer across the interface region is a critical step in the overall reaction process. Trapping and transfer of free electrons are competing processes and often occur on ultrafast time scales. Another competing process is electron-hole recombination. Electron transfer can take place following trapping as well, but on longer time scales, nanosecond or longer. Similar events take place for the hole. However, the time scale for hole transfer and trapping can be different from that for the electron.45 As shown schematically in Figure 1.6, the different processes involving photoexcited charge carriers are illustrated. The figure showed that (1) electronic cooling within the CB, (2) trapping of electron by trapping states, (3) electron-hole recombination at band- edges, (4) electron-hole recombination of trapped electrons, (5) electron transfer and reduction reaction with an electron acceptor at conduction band and (6) hole transfer and oxidation reaction with a hole acceptor or electron donor at the valence band. 11 Chapter 1

Figure 1.6: Processes involved in semiconductor nanoparticles upon bandgap excitation. ST: Surface Traps, DT: Deep Traps.9

1.3.4. Applications as a Photocatalyst Photocatalysis is a type of catalysis, which covers the range of the reactions proceeding under the action of light. It includes phenomena such as catalysis of photochemical reactions, photo-activation of catalysts, and photochemical activation of catalytic processes. Usually, the most typical processes that are covered by “photocatalysis” are the photocatalytic decomposition (PCD) and the photocatalytic oxidation (PCO) of substrates, which most often belong to the organic class of compounds. The former takes place in the absence of O2, while the later process employs the use of gas-phase oxygen as direct participant to the reaction. In photocatalysis, semiconductor materials are used as catalysts.46,47 Semiconductor photocatalysis has received much attention during last four decades as a promising remedy for both energy generation and environment related problems using the abundant solar light.46 It can decompose harmful organic and inorganic pollutants present in air and water and can also split water to produce clean and recyclable

hydrogen energy. Uptill now, a lot of photocatalysts, such as TiO2, ZnO, Ag3PO4, 22,30-32,46-50 Ag2S, Bi12TiO20, WO3, WS2, Fe2O3, V2O5, CeO2, CuS, CdS, and ZnS have been prepared and demonstrated to be able to produce hydrogen and decompose pollutants under UV or visible light irradiation.23,51-54

12 Chapter 1

1.4. Principle of Photocatalysis The principle of the semiconductor photocatalytic reaction is a light induced photochemical reaction. In a photocatalytic process, the illumination of a semiconductor photocatalyst with ultraviolet (UV) or visible radiation activates the catalyst, generating a redox environment in the aqueous solution.55 Semiconductors act as sensitizers for light induced redox processes due to their electronic structure, having a completely filled valence band and an empty conduction band.56 The semiconductor photocatalyst absorbs impinging photons with quantum energy (i.e. wavelength) that hits an electron in the occupied valence band of the semiconductor atom, excite that electron to the unoccupied conduction band leading to excited state conduction band electrons and positive valence band holes.57 The fate of these charge carriers may take different paths as described in Figure 1.6 above. Firstly, they can get trapped, either in shallow traps (ST) or in deep traps. Secondly, they can recombine, radiatively or non-radiatively, releasing the energy in the form of heat. Finally, they can react with electron donors or acceptors adsorbed on the surface of the photocatalyst.55 Though, it was recently observed that any photoredox chemistry occurring at the catalyst surface, originates from trapped electrons and trapped holes rather than from free ones.41 The antagonism between charge-carrier recombination and charge-carrier trapping followed by the race between recombination and interfacial charge transfer actually determines the overall quantum efficiency for interfacial charge transfer.55 Further, the band positions or flat band potentials of the semiconductor material has an important role. These determine the thermodynamic limitations for these photoreactions.21,38

1.5. Superiority of Photocatalysis Many methods have been proposed over the years to remove organic toxins from wastewaters. Current treatment methods, such as adsorption by activated carbon and air stripping just trap the contaminants present, but they do not degrade them into benign substances. Thus, one of the major advantages of the photocatalytic process over other technologies is that there is no requirement for post-treatment disposal methods. Another advantage of this process is that expensive oxidizing chemicals are not required as ambient oxygen is the oxidant.58 Photocatalysts are also self- regenerating and can be reused or recycled. Finally, the photocatalytic process can also be applied to mitigate foul odours, taste and other naturally occurring organic 13 Chapter 1

matter,which contains the precursors to trihalomethanes generated during the chlorine disinfection step in drinking water treatment.38,59

1.6. Ideal Photocatalysts An ideal photocatalyst should be stable, inexpensive, non-toxic and highly efficient. Another imperative criteria for the degradation of organic compounds is that the redox - − • − 0 potential of the H2O/ OH couple (OH OH + e ; E = −2.8V) lies within the bandgap of the semiconductor photocatalyst.56 Several semiconductors have bandgap energies well for catalysing a wide spectrum of

chemical reactions. These include TiO2, ZnO, WO3, ZnS, Fe2O3, SrTiO3. Titania

(TiO2) is the most thoroughly investigated semiconductor in the past, indicates to be the most promising for photocatalytic remediation of organic pollutants.60 This semiconductor provides the best compromise between catalytic performance and 61 stability in aqueous media. The anatase phase of TiO2 is the material with the highest photocatalytic detoxification.62 Binary metal sulphide semiconductors like CdS, PbS or CdSe are regarded relatively unstable for catalysis in aqueous media as they readily undergo photoanodic corrosion and are also toxic.60 The iron oxides are also unsuitable semiconductors as they readily undergo photocathodic corrosion.56

The band gap for ZnO (3.2eV) is equal to that of anatase TiO2. However, it is also

unstable in water with Zn(OH)2 being formed on the particle surface. This leads into deactivation of the photocatalyst.60 The photocatalytic activity of a photocatalyst is mainly controlled by (i) the light absorption properties (ii) redox reaction rates on the surface by the electron and hole, and (iii) the electron-hole recombination rate. A larger surface area leads to faster photocatalytic reaction rates. However, the surface is also a defective site; therefore, the larger the surface area can also enhance the recombination if recombination sites predominate. Further, the crystallinity results in the higher photocatalytic activity as the defective sites will be less. High temperature treatment is necessary for

crystallinity of TiO2 nanomaterials but it also induces the aggregation of small nanoparticles and decrease the surface area. Observing from the above conclusions, the relation between the physical properties and the photocatalytic activities is quite complex. Optimal conditions are to be taken into account and may vary from case to case.38,63

14 Chapter 1

1.7. Elements Constructing Heterogeneous Photocatalyst Materials Figure 1.7 shows the general view of elements constructing heterogeneous photocatalyst materials. The elements are classified into four groups (i) to form energy structure and crystal structure (ii) to form crystal structure but not energy structure, (iii) to construct impurity levels as dopants and (iv) to be exploited as cocatalysts. Most of the metal oxide semiconductors, like sulfide and nitride photocatalysts compose of metal cations with d0 and d10 configurations. Their conduction bands for the d0 and d10 metal oxide photocatalysts are usually composed of d and sp orbitals, respectively, while their valence bands consist of 2p orbitals of oxygen atom. Valence bands of metal sulfide and nitride semiconductor photocatalysts are usually composed of 3p of S and 2p orbitals of N, respectively. Orbitals of 3d in Cu+, 4d in Ag+, 6s in Pb2+, 6s in Bi3+, and 5s in Sn2+ can also form valence bands in some metal oxide and sulphide semiconductor photocatalysts. Alkali metals, alkaline earth metals and some lanthanides do not directly lead to the band formation and just construct the crystal structure as A site cations in perovskite compounds. The transition metal cations with partially filled d orbitals such as Cr3+, Ni2+ and Rh3+ can generate some impurity levels in band gaps when they are doped into native metal cations. Although, they often act as recombination centres, however, sometimes they play an instrumental role for visible light response. Some transition metals and the oxides such as noble metals (Pt,64,65 Rh65,66 and Au67,68, NiO69 and 70,71 RuO2 function as co-catalysts for H2 evolution. In water splitting, a reverse

reaction to form H2O from evolved H2 and O2 has to be controlled because of an

uphill reaction. Au, NiO and RuO2 are feasible co-catalysts on which the reverse reaction is efficiently controlled. A Cr–Rh oxide has recently been observed as an 72,73 efficient co-catalyst for H2 evolution by oxynitride photocatalysts. IrO2 colloids 74-76,77 work as an O2 evolution co-catalysts. 15 Chapter 1

77 Figure 1.7: Elements constructing heterogeneous photocatalysts.

1.8. History of Photocatalysis Heterogeneous photocatalysis appeared as a new emerging “Advanced Oxidation Process” (AOP) at the end of 20th century78 with more than 2000 publications registered on the subject. Currently, more than 1000 articles are being published yearly on the topic. Heterogeneous photocatalysis is able to be efficient in Green Chemistry and in emerging “Advanced Oxidation Processes” (AOP).79-81 Currently, the last domain is preferentially studied,81-84 however, photocatalysis is able to provide highly selective and mild oxidation for organic fine chemistry.85 Photocatalysis is based on the two tier process of the photocatalysts, it adsorbs

reactants and absorbs efficient photons (hν ≥ Eg), simultaneously. Photocatalysis was initially originated in Europe from different catalysis laboratories. Stone was first to study the photo-adsorption/desorption of oxygen on ZnO86 before studying the photocatalytic oxidation of CO on the same solid, in England.87 He subsequently switched to titania under rutile phase for oxygen photo-adsorption88 and selective isopropanol oxidation in acetone.89 The last reference was believed to be the first one to present hydroxyl (OH•) radicals as oxidizing agents formed by neutralization of surface OH− by photogenerated holes h+. During the same period, in 16 Chapter 1

Germany, Hauffe was also studying the photocatalytic oxidation of CO on ZnO.90,91 He was the first one to mention the term “photocatalysis” in his paper. During the same decade, Juillet and Teichner in France were working upon the sintering of ultra- pure oxide powders for nuclear applications and analysed their solids through the electrical properties. The erratic results obtained in titania based experiments puzzled the scientists as they had no idea that titania was sensitive to daylight, especially in sunny days.92 They subsequently used the photogenerated oxygen species to perform selective oxidations of small alkanes.93,94 In fact, while photocatalysis was developing confidentially in Europe, there was an exponential development in Japan according to Bickley.95 The previous work by Fujishima and Honda on the photoelectrolysis of water using a UV- irradiated titania-based anode96 was then re-published in English.97 This is considered the first breakthrough for globalization of photocatalysis, which had a primary development in Japan, as mentioned by Kaneko et al.(2002).98 However, new comers in the field of photocatalysis cite it as the starting point of photocatalysis, which is obviously erroneous.99 From this work, photocatalysis received valuable inputs from the other chemical sub-disciplines too. Following the breakthrough lead by Fujishima and Honda in 1972, enormous research efforts have been devoted to photocatalysis under UV light in the presence of many

semiconductors and semiconductor oxides such as TiO2, ZnO, ZrO2, CdS, SnO2,

WO3, SiO2, CeO2, Fe2O3, Nb2O3, SrTiO3, Sb2O4, V2O5 and this field developed very fastly during the last three decades. In the past decade, visible light photocatalysis has caught considerable attention, looking for better use of sunlight spectrum which constitutes 40–50% instead of 4–5% for UV.100

1.8.1. Honda–Fujishima Effect The Honda–Fujishima effect is a popular chemical phenomenon closely related to photocatalysis. Photoexcitation of a titania single-crystal electrode put in an aqueous electrolyte solution induces oxygen generation from the titania electrode and hydrogen generation from a platinum counter electrode when an anodic bias is applied to the titania working electrode e.g., making higher pH of an electrolyte solution for the working electrode. Therefore, even when a titania electrode connected to a platinum electrode is assumed to be a photocatalyst, the system cannot be photocatalytic owing to the requirement of bias. Although, it is important to mention 17 Chapter 1

that the discovery of the Honda–Fujishima effect is one of the most important discoveries in chemistry which opened up and extensively promoted the research field of photocatalysis, though, it cannot be an origin of photocatalysis. Actually, reports on photocatalytic oxidation of organic compounds by titania powders had been published before the discovery of this effect.101,102 In other words, the paper published in Nature in 1972 is undoubtedly an origin of research activity of photocatalysis but not an origin of heterogeneous photocatalysis in the bibliographic sense, as discussed above.103

1.8.2. From Titania to third-generation photocatalysts When analysis of the published scientific literature is done, it indicates that research on photocatalytic materials and applications experiences a continuous exponential

growth. TiO2 is still by far the most studied photocatalyst, especially the commercial 104-106 material Degussa-Evonik P25. During the 1990s, the predominance of TiO2 continuously increased in the research papers devoted to photocatalysis. During the last decade, the publications pertaining to binary oxides reached a high, with titania being one of the major components in 80% of the papers. The enthusiasm in developing solar devices with semiconductors activated by visible light led to explore

new materials, but the significant results were obtained with TiO2 based semiconductor photocatalysts.107

At the end of the last decade, the focus on TiO2 started to decay slightly. However, attention is still being paid to the new aspects of this oxide, such as crystal facet engineering, which is set to provide new insights to tune the selectivity and reactivity.108,109 On the other hand, conceptually different dopant-free approaches are recently being proposed to sensitize sunlight absorption in titania. As a way forward, the modification of the semiconductor surface by introduction of disorder through

partial hydrogenation or by formation of paramagnetic oxygen vacancies (Vo) has been reported already. A dramatic shift of the absorption edge to the near infrared was exhibited by disorder-engineered black TiO2 nanocrystals. This modification brought a significant enhancement of solar-driven photocatalytic activity. Pt-loaded black -1 -1 TiO2 with methanol as a sacrificial reagent produced 10,000μmol H2 g h under simulated solar radiation, with 24% energy conversion efficiency, and produced -1 -1 100μmol H2 g h when the radiation below 400nm was cut-off. The catalytic activity was stable for up to 100h of cyclic operation.110 Besides, near-infrared (NIR) radiation 18 Chapter 1

active novel core–shell heterojunction photocatalysts have been synthesized by 3+ 3+ coating TiO2 on Yb /Tm co-doped YF3 nanocrystals. Upon NIR light absorption, 111 the photocatalyst material emits UV radiation, which can further activate TiO2.

Extending the absorption spectrum of TiO2 to the visible region by doping inherently decreases the reactivity of the active sites, and hence, it proves that with these modifications the photoactivity obtained under visible light is only a fraction of that obtained under UV radiation.110 Moreover, it is becoming clearly evident that visible- light-active non-doped photocatalytic materials require coupling with multiple cations, and the search for the so-called “third-generation photocatalysts” with enhanced features is underway.

Despite the fact that there are still four times more research articles on TiO2, the number of publications devoted to other materials is gradually increasing. In this direction, the number of papers on ZnO, another historical photocatalyst, is very significant, and continues to grow exponentially.112,105This is probably due to the fact

that this oxide shows a photocatalytic activity close to that of TiO2, and can be further improved by forming a variety of nanostructures and is also non-toxic. Among non-oxidic materials, sulphides, initially discarded as a consequence of the poor photocatalytic stability, are now being among the most studied compounds. They have been reconsidered as feasible photocatalysts for particular applications using sacrificial agents to hinder photocorrosion. Metal sulphides can also be efficient co- catalysts, showing enhanced effects compared to noble metals in several systems.113

1.8.3. Advancements in the field of Photocatalysis

The use of the bare TiO2 phases poses some limitations as (i) small visible light response, (ii) high recombination rate for the photoinduced charge carriers (iii) doping with foreign species that often act as recombination centers, (iv) difficulty to support

powdered TiO2 on some materials. As a consequence, the research in heterogeneous photocatalysis has advancedly modified some morphological and electronic properties 114 of TiO2 so as to improve its photocatalytic efficiency.

The different approaches included surface modification of the semiconductor particles with redox couples or noble metals63,116 and have shown that the efficiency of charge transfer at the semiconductor–electrolyte can also be enhanced by simultaneous scavenging of holes and electrons by surface adsorbed redox species. Another efficient approach has involved the coupling of two semiconductor particles with different electronic energy levels to form the heterostructures.115 The various approaches are summarized below:

1.8.3.1. Doping with metal ions Incorporating or doping metal ion dopants into the titanium dioxide or other nanoparticles can influence the performance of these photocatalysts. This influences the dynamics of recombination and interfacial charge transfer. The most significant enhancement of photoactivity through doping was observed in nanoparticles, in which the dopant ions are located within 1–2nm of the surface.116 Also, the high surface areas characteristic of nanoparticles (100–500m2g-1) appear to enhance the deposition process and the resulting activity of the catalyst.117 Choi et al. systematically studied the effects of 21 different metal ion dopants on nanocrystalline TiO2. The results

indicated that some doped quantum dot titania (Q-TiO2) particles had significantly greater photoactivity than the undoped ones. Doping with Fe(III), Mo(V), V(IV),

Ru(III), Rh(III), Re(V) and Os(III) at the 0.5 atomic% concentration in the TiO2 matrix, significantly improved the photoreactivity for both oxidation and reduction processes. Choi (1994)116 used laser flash photolysis and time resolved microwave conductivity measurements to correlate the effects of metal ion dopants to the lifetime of the photoexcited electron. In the V(IV), Fe(III), Mo(V) and Ru(III) doped samples, the lifetime of the generated electrons was found to have increased to 50ms compared to <200μs with the undoped Q-TiO2. This type of doping might not always be instrumental in increasing the lifetime of the generated charge carriers. Smith et al.

(1998) showed that in Ru(III) doped TiO2 colloids, the electronic decay was as fast as

or even faster than in undoped TiO2. The studies carried out by Smith et al. (1998) and those carried out by Choi et al. (1994) had a difference that the higher dopant level of Ru (III) of 3 atomic% used by Smith et al., compared with the 0.5 atomic% dopant level used by Choi et al. There could be many reasons for the variations in the effects of the dopant ions. One reason is the location and co-ordination of the dopant ions in the crytal system. These depend critically on the methods of sample 20 Chapter 1

preparation and pre-treatment as well as the concentration of the dopant ions. The dopant ions may be adsorbed on the surface, incorporated into the interior of the particle, or may form separate oxide phases.60 The dopant ions can act both as hole and electron traps or they can mediate interfacial charge transfer only.116 Once incorporated into the interior of the host, the dopant ions may occupy either lattice or interstitial sites. The ability of dopant ions to function as trap sites or to mediate interfacial charge transfer will depend on above factors.60 When incorporated in the interior of the particles, the d-electronic configuration of the dopant and its energy level within the lattice also seem to significantly influence the photoactivity.118 Finally, the site where the electron gets trapped greatly affects the redox chemistry of the doped semiconductor photocatalyst. A dopant ion might act as an electron trap, and this might in fact lead to a lengthening in the life time of the generated charge carriers, improving the photoactivity. However, if an electron is trapped in a deep trapping site, it will have a longer lifetime, but it may also have a lower redox potential. This might result in a decrease in the photoreactivity.56 The work carried out by Zhang et al. (1998)119 shed a new light on the role of dopant ions and their effect on photoactivity. Firstly, these authors provided further support for the existence of an optimum dopant concentration. The system they studied was

Fe3C doped TiO2 for the photocatalytic degradation of CHCl3. They observed that for 6nm particles, the optimum Fe concentration was 0.2 atomic%, while for 11nm particles, the optimum concentration was 0.05 atomic%. They provided the following explanation for their observations.Their first explanation was with respect to the

existence of an optimal Fe3C dopant concentration. Fe3C ions serve as shallow trapping sites for the charge carriers and increase the photocatalytic efficiency by − + separating the transfer time of e and h to the surface. If Fe3C can act as a trap centre for both e− and h+, at higher dopant concentration, the possibility of charge trapping is high, and as such, the charge carriers may recombine through quantum tunneling. If + Fe3C acts as a h trap only, the recombination of the charge carriers is not of great concern at low dopant concentrations. At high concentrations, a hole h+ may be trapped more than once as it tries to transfer to the surface. This hole which had been ‘held back’, might then recombine with an electron which is generated by a subsequent photon before it can reach the surface (i.e. increased incidence of volume recombination).38 21 Chapter 1

1.8.3.2. Doping with anions

There has been an explosion of papers in the literature on anion-doping of TiO2 since Asahi and coworkers (2001) published Science report of visible light activity in 107 nitrogen-doped TiO2. By now, the most extensively studied anion dopant has been N120-123, but other anion dopants (e.g., C107,121,124-131, S107,124,125,127,132-136, halides137-140, P107,135,141 and B142,143 have also been examined, both experimentally and theoretically.107,120,122,127,128,135,137-139,142,144-161 The general understanding is that when an anion which is less electronegative than O is substitutionally doped into the lattice,

it will have some of their valence p-states pushed up out of the TiO2 VB into the band gap. The question is whether these new gap states are localized or are part of the VB structure remains unresolved. The concept of non-substitutional anion doping also remains an important issue. It is understood that doping preparation methods are generally very diverse, from dry methods to wet methods. However, at present, there is little understanding of how these preparation methods are consistent for the formation of doped materials.162

1.8.3.3. Dual semiconductor systems/ Heterostructures Another approach taken to modify the surface of semiconductor colloids, so as to improve charge separation and inhibit charge-carrier recombination, has been to couple with a second semiconductor. Excitation of these dual semiconductors results in an electron transfer into the lower lying conduction band of the second semiconductor. In the composite nanoparticles, electric field is not necessary, as the charge separation is achieved by the tunnelling of electrons.40 Recent studies report that these interparticle electron transfer occur within 500fs–2ps.163 Henglein reported

the first composite photocatalyst when he found that when a small amounts of Cd2C added to ZnS resulted in ZnS fluorescence quenching. Since then there have been many papers published regarding the optical properties of mixed systems. Some of the 164 165 systems studied include ZnS–CdS , CdS–Ag2S , mixed crystals of ZnxCd1−xS, 166 167 168 169 CdS–ZnS , AgI–Ag2S , ZnS–CdSe and CdS–PbS systems. Recently, emphasis has been placed on the development of coupled and capped semiconductor photocatalysts. Various papers have been published regarding coupled 170 165 semiconductors systems. These include CdS–TiO2, CdS–ZnO , CdS–Ag2S , ZnO– 171 172 173 174 ZnS , ZnO–ZnSe , AgI–Ag2S and CdS–HgS. The charge separation mechanism in both capped semiconductor systems and coupled semiconductor 22 Chapter 1

systems involves the transfer of photogenerated electrons in one semiconductor into the lower lying conduction band of the second semiconductor. However, the mode of interfacial charge transfer is significantly different in both.115 The charge-transfer processes involved in capped and coupled semiconductor systems are shown in Figures 1.8 and 1.9 respectively.

Figure 1.8: Charge transfer in a capped semiconductor system

Figure 1.9: Charge transfer in a coupled semiconductor heterostructure

In a coupled semiconductor system the two particles are in contact with each other and both holes and electrons are accessible on the surface for selective oxidation and reduction processes. On the other hand, capped semiconductors have a core and a shell geometry. The electron gets transferred into the energy levels of the core 23 Chapter 1

semiconductor, if it has a conduction band potential which is lower than that of the shell. The electron hence gets trapped within the core particle, and is not readily

accessible for the reduction reaction. Bedja et al. (1995) synthesized TiO2-capped

SnO2 (SnO2@TiO2) and TiO2-capped SiO2 (SiO2@TiO2) nanocrystallites. The photocatalytic properties of the capped semiconductor systems were tested for the − − oxidation of I and SCN . The SnO2@TiO2 colloids were 80–100Å in diameter and

exhibited improved photocatalytic efficiencies compared to the uncapped TiO2 colloids. By changing certain parameters like the thickness of the shell or the radius of the core, important properties, such as photocatalytic, optical, and magnetic properties, of the photocatalyst can be tailored. It may also be important in addressing problems such as photodissolution of the unstable photocatalysts, such as iron oxide.56 Three-layered colloidal particles are another development in the field of surface- modified semiconductor nanoparticles. These consist of a quantum-sized semiconductor particle as the core, covered by several layers of another semiconductor material, onto which several layers of the core material are then deposited, and act as the outermost shell. These particles are called quantum dots or wells.173 The first example described in the literature was the system CdS–HgS– CdS.49

1.8.3.4. Sensitization of TiO2 to visible light response

The sensitization of TiO2 with a second component to enhance activity and shift the wavelength of irradiation into the visible region is the main interest in this field of research. Again, several approaches have been taken. The first involves the

sensitization of TiO2 with organic and organometallic dyes, these however are seen to be less likely to succeed in photocatalysis due to their instability.60 Another approach

has been to utilize narrow bandgap semiconductors to photosensitize TiO2. The sensitizer usually have higher absorption of visible light and transfers electrons into the lower conduction band of the wide-band gap semiconductor. The separated charge carriers can be used to initiate the chemical reactions. This principle has been used to construct photoactive layers, in which charge separation is achieved with an efficiency of 80%.40 Nanocrystalline, narrow band gap semiconductors can be used as sensitizers. These semiconductors can be tailored to suit specific purposes since changing their size can shift their electronic bands (QSE). This idea is being more 24 Chapter 1

attractive alternative for TiO2 sensitization since they are more stable than organic 60 dye. Sensitizing TiO2 with Q-sized narrow-bandgap semiconductors PbS and CdS was carried out by Vogel et al.174 These systems, however, demonstrated a loss in efficiency, with photocorrosion of the narrow-bandgap semiconductors in aqueous media, under illumination with 460nm light, again posing a problem. Howe suggested

the possibility of using such semiconductor sensitized nanocrystallineTiO2 for gas- phase photocatalysis.59 A different approach was introduced by Bahnemann et al. (1993) so as to synthesize particles which can act as visible light photocatalysts. This method included the synthesis of a mixed Ti(IV)/Fe(III) oxide catalyst. This photocatalyst have increased activity for the destruction of DCA (dichloroacetic acid) and it also showed a photoresponse to 450nm light. In order to alleviate the problem

of photodissolution, the authors suggested the introduction of H2O2, as an electron

acceptor, into the system. H2O2 being a better electron acceptor than O2, it can compete more efficiently with the photocatalyst dissolution.62

1.8.3.5. Nanocrystalline films Semiconductor nanocrystalline films is the another area of research that relates the nanotechnology with heterogeneous photochemistry. Nanocrystalline semiconductor films consist of a network, where electronic conduction takes place. The films are highly porous, and the spaces between the particles are filled with an electrolyte.40 The thin films exhibit interesting photocatalytic and photoelectrochemical properties that are inherited from the native colloids.175 Chemical vapour deposition or molecular beam epitaxy has been the main technique for depositing thin semiconductor films.The precursor nanosized particles from which the films are made are in electronic contact allowing for electric charge transfer through these films. This charge transport is highly efficient, with the quantum yield being practically unity.21 One of the major advantages of nanocrystalline semiconductors is their high porosity which facilitates surface modification with redox couples, sensitizers and other semiconductors. Using nanocrystalline semiconductor films also allows the manipulation of the photocatalysis by electrochemical methods.175

1.8.3.6. Photoelectrochemical devices During the past decade, considerable efforts have been made in the preparation of nanoparticle films and their application in photoelectrochemical devices. In an electrochemically assisted photocatalytic process, a thin nanocrystalline 25 Chapter 1

semiconductor film is deposited on a conducting glass surface, with the generated electrons being driven through an external circuit to a counter electrode by applying a positive bias. This leads to better charge separation and the problem of charge recombination can be minimized easily.175 In most of the photocatalytic reactions, oxygen is essential for scavenging electrons from the irradiated semiconductor particle.176 Thus, advantage of this concept is that oxygen is no longer required as an electron scavenger. Hence, it is possible to carry out the photocatalytic reaction under anaerobic conditions if O2 doesn’t play a role in the reaction mechanism of organic degradation.177 One more advantage is that the anodic and cathodic systems are independent. Photoelectrochemical devices can thus allow the isolation of various reactions occurring in photocatalytic systems and provide a means to carry out selective oxidation and reduction in two separate 175,176 compartments. The highest rates were observed with the SnO2/TiO2 coupled semiconductor films, with a ten-fold enhancement in the degradation rate being observed at an applied bias potential of 0.83V versus saturated calomel electrode (SCE). The role of the coupled semiconductor was to further improve the charge separation. The development of multicomponent nanocrystalline semiconductor films is seen as being of extreme importance to the research in photoelectrochemistry.175

1.8.3.7. Organic–inorganic nanocomposites Another recent development has been the emergence of organic–inorganic nanostructured composites. Different interactions between organic and inorganic molecules led to a range of materials for catalytic technologies. Published work by Braun et al.(1996)178 describes the synthesis of stable semiconductor organic superlattices based on CdS and CdSe. By incorporating organic molecules in an inorganic lattice the authors anticipate that the electronic properties of these type of materials can be tailored. Therefore, these novel organic–inorganic nanostructured composites may be suitable for photocatalytic applications. Tenne et al. (1996)179

have also been working on the preparation of inorganic compounds, namely WSe2

and PtS2, with a crystal structure similar to graphite. These compounds can be used to construct nanotubes and fullerene-like structures with potential applications in photocatalysis and nanoelectronics.38 Development of organic–inorganic nanocomposites were often achieved by grafting synthetic polymers on inorganic particles or by adding modified nanoparticles into 26 Chapter 1

polymer matrices, in order to produce composite materials with improved mechanical and other physical properties. Nanocomposites composed of inorganic nanoparticles and organic polymers form a new class of materials that exhibit improved performance as compared to bulk counterparts.180 Surface modification of inorganic nanoparticles has also attracted a great attention because it presents excellent integration and an improved interface between nanoparticles and polymer matrices.181- 184 However, the nanoparticles have a strong tendency to undergo agglomeration into the polymer matrix, degrading the optical and mechanical properties of the nanocomposites.185,186 To enhance the dispersion stability of nanoparticles in aqueous media or polymer matrices, it is imperative to modify the particle surface involving polymer surfactant molecules or other modifiers.

1.8.3.8. Surface modification of inorganic nanoparticles through chemical treatments The surface modification by chemical treatments (such as the absorption of silane coupling agents) is a beneficial method to enhance the dispersion stability of nanoparticles in various liquid media. The concept of silane coupling agents was presented by Plueddemann and his co-workers.187 After that landmark publication, silane modified particle surfaces to improve the compatibility between the particle and polymer surfaces was established.188,189 The modified nanoparticles show comparatively better dispersion in aqueous and polymer media.190 The surface of nanoparticles may also be modified through reactions with metal alkoxides, epoxides, such as propylene oxide, and alkyl or aryl isocyanates.191 192 Recently, Sabzi et al. carried out surface modification of TiO2 nanoparticles with aminopropyltrimethoxysilane (APS) and investigated its effect on the properties of a polyurethane composite coating. He observed improved mechanical and UV- protective properties of the urethane clear coating. In a more recent study, the

dispersion stability of TiO2 nanoparticles in organic solvents was improved by treating the particle surface with a silane coupling agent.193 The silane coupling agent is adsorbed on the surface of the nanoparticles at its hydrophilic end and interacts with hydroxyl groups that are pre-existing on the nanoparticle surface.194 Conjugated polymers with extended π conjugation such as polyaniline, polythiophene, and polypyrrole are very promising due to their high absorption coefficients in the 27 Chapter 1

visible part of the spectrum, high mobility of charge carriers, and good environmental stability.194 Moreover, many conjugated polymers are also efficient electron donors and good hole transporters upon visible-light excitation.195 Hence, conjugated polymers in principle could act as stable photosensitizers to modify wide band gap inorganic semiconductors.196,197 Many conjugated polymer/semiconductor composites with different combinations of the two components have been reported.198-202 In case of the combined system of a conjugated polymer and a semiconductor, the lowest unoccupied molecular orbital (LUMO) level of the conjugated polymer is energetically higher than the conduction band (CB) edge of semiconductor.200,201 Hence, transfer of the electrons generated from the conjugated polymer upon visible- light irradiation to the conduction band of semiconductor is thermodynamically possible leading to interfacial charge transfer and a significant photoresponse to visible light. A relatively efficient photocatalytic activity has been emphasized and the charge transfer from the conjugated polymer to the semiconductor has been extensively demonstrated in these combined systems.38

1.8.4. Current State of Research in the field of Photocatalysis For environmental remediation, the introduction of advance materials is not as

important as for energy applications. TiO2 is still the predominant photocatalyst because no satisfactory alternative has been clearly identified and developed. Numerous binary, ternary and quaternary compounds are effective for the photocatalytic degradation of different pollutants,203 but either the adsorption of the pollutant is too small, or complete mineralization to benign byproducts is not obtained. The evaluation of photocatalytic activity degradation of dyes, regrettably, cannot be considered as a standard for determining the visible light activity.204,205 Thus, it is difficult to decide whether or not there have been significant improvements in the last few years. The different areas of interest during current times are as follows:

1.8.4.1. Artificial Photosynthesis

The possibility of reducing CO2 using sunlight in a plant photosynthesis mimic process is currently a field of great interest. However, in order to achieve a sustainable process, the use of H2O molecules as electron donors is the best choice. Therefore, among others, the following reactions are expected: 28 Chapter 1

2H2O (l) + CO2 (g) CH4 (g) + 2O2 (g) (10)

2H2O (l) + CO2 (g) CH3OH (l) + 3/2O2 (g) (11)

This process mainly leads to molecules with one carbon atom which can be subsequently used as fuels or chemical building blocks. The redox potentials of large

band gap semiconductors are suitable for the photocatalytic CO2 reduction, hence 206 again, TiO2-based photocatalysts have been investigated in this field.

Efficient solar conversion of CO2 and water vapour to methane and other

hydrocarbons has been achieved using N-doped TiO2 nanotube arrays doped with Pt and Cu as co-catalysts. Among the current developments, it looks that the use of sensitizers are the most

promising research lines for solar fuel production. The hybrid enzyme–TiO2 system,

where TiO2 nanoparticles are modified with a Ru-based photosensitizer and the CO2-

reducing enzyme carbon monoxide dehydrogenase, reduces CO2 at a high rate of 250mmolg-1h-1 under visible light.207 The enzyme, which contains a Fe–S cluster, bypasses the one-electron radical pathway and controllably catalyze a two-electron reduction that is highly selective to CO. Nevertheless, long term durability of the catalysts should be assured and some recent advances in materials development needs to be ascertained.

1.8.4.2. Water Splitting Hydrogen can play an important role in the development as it is an ultimate clean energy and can be used in fuel cells. Moreover, hydrogen is used in chemical industries for various applications. For example, a bulk quantity of hydrogen is utilized in industrial ammonia synthesis. Currently, hydrogen is mainly obtained from fossil fuels such as natural gas by steam reforming.

CH4(g) + H2O(l) CO(g) + 3H2(g) (12)

CO(g) + H2O(l) CO2(g) + H2(g) (13)

In this process, fossil fuels are consumed and CO2 is produced. Hydrogen needs to be

produced from H2O using sunlight if one is concerned of energy and environmental issues. Therefore, achievement of solar hydrogen production from water has been sought. The different methods for solar hydrogen production are below. (i) Electrolysis of water using a solar cell. (ii) Reforming of biomass. 29 Chapter 1

(iii) Photoelectrochemical or photocatalytic water splitting.

The advantage of water splitting using a powdered photocatalyst is its simplicity as shown in Figure 1.10. Solar irradiation of the photocatalyst powders dispersed in a pool of water generates hydrogen. However, the problem associated is to separate H2

evolved from O2 during photocatalytic water splitting process. However, the problem is possible to be overcome using a Z-scheme photocatalyst system.

77 Figure 1.10: Solar hydrogen production from water using a powdered Photocatalyst.

Moreover, large-scale application of solar water splitting is possible because of its simplicity. Hence, photocatalytic water splitting is an enthusiastic reaction and can open ways to green sustainable chemistry in solving energy and environmental issues. The solar energy is converted to chemical energy with a large positive change in the Gibbs free energy through this process. This reaction is similar to photosynthesis by green plants because both are uphill reactions (Figure 1.11). Hence, photocatalytic water splitting is named as an artificial photosynthesis and is an attractive and challenging problem in chemistry. Thermodynamically, photocatalytic water splitting is distinguished from photocatalytic degradation reactions such as photo-oxidation of organic compounds using oxygen molecules that are generally downhill reactions.208,209 Various researchers have extensively studied water splitting using semiconductor photoelectrodes and photocatalysts. However, efficient materials for water splitting

into H2 and O2 under visible light irradiation could not be ascertained. 30 Chapter 1

Figure 1.11: Photosynthesis by green plants and photocatalytic water splitting as an artificial photosynthesis.77

The photon energy conversion using photocatalysts by this process had been considered to be pessimistic and sluggish. However, new photocatalyst materials for water splitting have recently been designed one after another. However, the photocatalytic water splitting is still a challenging reaction.

1.8.4.2.1. Processes in photocatalytic water splitting Photocatalytic reactions involved in semiconductor material assisted water splitting are schematically shown in Figure 1.12.

Figure 1.12: Principle of water splitting using semiconductor photocatalysts.210 31 Chapter 1

When an incident light of larger energy than that of a band gap is used, electrons and holes are produced in the conduction and valence bands, respectively. The photoinduced electrons and holes take part in redox reactions similar to electrolysis.

H2O molecules are reduced by the electrons to form H2 and are oxidized by the holes

to form O2 during the water splitting process. The width of the band gap and levels of the conduction and valence bands in the semiconductor photocatalysts play an important role in these processes. The base level of the conduction band has to be more negative than the redox potential of + H /H2 (0V vs. NHE), while the top level of the valence band should be more positive

than the redox potential of O2/H2O (1.23V). Therefore, the theoretical minimum band gap for water splitting is 1.23eV that corresponds to light of about 1100nm.

( ) = (nm) (14) 1240 퐵푎푛푑 푔푎푝 푒푉 푙 Where l is the absorption edge wavelength. Band levels of various semiconductor materials are shown in Figure 1.13. The band 211 levels usually shift with a change in pH(~0.059 V/pH) for oxide materials. TiO2,

ZrO2, KTaO3 and SrTiO3 possess suitable band structures for water splitting and can be suitably modified for water splitting by co-catalysts. Although CdS looks to have a suitable band position and a band gap with visible light response, it is not active for 2- water splitting into H2 and O2. The reason is that S in CdS rather than H2O is oxidized by photogenerated holes accompanied with leaching of Cd2+ according to the Equation (15).211

CdS + 2h+ Cd2+ + S (15)

This process is called photocorrosion and is a limitation of a metal sulfide photocatalyst. ZnO has also this limitation under band gap excitation even if it is an oxide photocatalyst.

+ 2+ ZnO + 2h Zn + 1/2O2 (16)

32 Chapter 1

Figure 1.13: Relationship between band structure of semiconductor and redox potentials of water splitting. The diagram was adapted using data from previous publications.212

However, CdS is an efficient photocatalyst for H2 production under visible light

irradiation if a hole scavenger is present. Contrarily, WO3 is a good photocatalyst for + O2 evolution under visible light irradiation if an electron acceptor such as Ag and 3+ Fe is present and is not active for H2 evolution because of its low conduction band level. The band structure and its positioning is just a thermodynamic requirement but not a sufficient condition. Even if the photoinduced electrons and holes possess thermodynamically sufficient potentials for water splitting, they will have to recombine if the active sites for redox reactions do not exist on the catalyst surface.

Co-catalysts such as Pt, NiO and RuO2 are often loaded to generate active sites for H2 evolution because the conduction band levels of many oxide photocatalysts are not 77 high enough to reduce water to H2 without catalytic support.

1.8.4.3. Passive photocatalytic elements: self-cleaning, pollutant-abating and antifogging surfaces On excitation of a semiconductor by a particular wavelength photon, the wettability of their surface is enhanced in such a way that water no longer forms droplets but forms a continuous film (Figure 1.2). In another sense, on irradiation the contact angle of water on these surfaces drops to virtually zero. This effect is known as photo-induced superhydrophilicity (PSH) and is closely related to the photocatalytic activity in the sense that it originates from electron–hole pair formation. However, all the photocatalytically active materials don’t present PSH behaviour and vice-versa. Although the actual mechanism of PSH is not completely understood, a combined effect of the generation of oxygen vacancies, with subsequent generation of surface 33 Chapter 1

OH groups upon H2O adsorption, and photocatalytic removal of surface organic impurities, has been proposed.213 The combination of photocatalytic and PSH properties has a vast range of applications, from antifogging glass to antibacterial sanitary surfaces, through self-cleaning construction materials and even odour eliminating textile. Nowadays, the photoactive coatings have reached a status of a strong market, especially in glass, metal and ceramic substrates. Self-cleaning glass and ceramic tiles, anti-fogging mirrors, and pollutant-abating paints, asphalts and cements based on this technology are entering the market.214 Sound proof highway walls with a photocatalytic coating for the elimination of NOx were constructed in Osaka, Japan, already in 1999.215 However, further development is still needed for the incorporation of a photocatalytic function in polymers or textiles as the photocatalyst may degrade the organic substrate itself. In this respect, an intermediate layer (i.e.

SiO2 layer) may also help to overcome this drawback, although the manufacturing constraints and cost increases accordingly.215

1.8.5. Future trends in the field of photocatalysis Despite the prominent progress achieved by photocatalysis in the last decade, there are still various challenges ahead for its full development. Nevertheless, the obvious interest in the implementation of more durable processes, surely a brilliant trajectory of photocatalysis in the way to its development has continued. The three main trends forward can be outlined with a reasonable degree of confidence in the near future: (i) the fine control of increasingly complex nanoarchitectures, (ii) the use of novel non- oxide materials and (iii) the coupling with photovoltaic components in a single device.

1.8.5.1. Towards more complex nanoarchitectures An extraordinary interest with intensive and extensive research regarding tuning the size, shape and composition of semiconductor nanoparticles to enhance their performance and widen their spectrum of applications has attracted many researchers.216 The effect of a crystal size nanoparticle on the photocatalytic properties of semiconductors has been known, although there is still research going on specific aspects of this subject.204 It is well established that nano-sized crystals are important for photocatalytic activity upto certain limits, not only because of their high surface-to-volume ratio, but also because of the modification of their physical and chemical properties compared to their bulk counterparts. Hence, it is worth to tune 34 Chapter 1

semiconductor properties, within certain limits, by controlling the crystal dimensions through a variety of synthetic methods.217,218 As a step forward, nanocrystalline semiconductors are nowadays employed for photocatalytic purposes and their design as nanostructured solids is considered to improve photocatalyst performance. Further, the crystal shape may also influence a photocatalyst by defining the number of atoms at the surface and the exposed facets. A large fraction of atoms at the surface of one or two dimensional structures make them interesting for photocatalytic applications.218 The synthesis of nanocrystals with tailored exposed facets is currently a novel strategy

to enhance the efficiency of photocatalysts, especially TiO2. For TiO2, the stable (101) facets are usually well exposed at the surface, but their reactivity is lower than that of (001) facets.219 Accordingly, anatase crystals with the latter facets preferentially exposed should offer a better photocatalytic efficiency. The problem is how to synthesize this type of crystal without the use of capping agents. The use of capping agents generally favour surface reconstruction towards the more stable (101) facets at higher temperatures for its subsequent removal.220

Apart from TiO2, the dependence of the photocatalytic activity on the exposed facets 220 has been studied in Ag3PO4, BiVO4, BiOCl and layered niobate photocatalysts. The increasing control of preparation procedures allows the synthesis of complex heterostructures in the nanoscale apart from facet engineering. Some of the three dimentional structures have been reported to exhibit improved photocatalytic efficiency with respect to other structures. Hierarchically structured materials can provide large surface area and interfacial charge transfer.113

1.8.5.2. New materials: from MOFs to carbon nitride Among the vast number of materials proposed as photocatalysts in the past, three new type of solids are quickly gathering a great deal of interest. The photocatalytic

applications of metal organic frameworks (MOFs), graphene and g-C3N4, which despite being very different have a common feature of carbon-based networks. MOFs constitute a large family of micro-mesoporous crystalline materials that can show extremely large surfaces areas (>5000m2g-1). The structure of these solids is formed by metal clusters connected by molecular building blocks. The ease in the selection of a variety of organic linkers and also a number of possible metal clusters, provides MOFs their highly tunable characteristics. Their enormous potential has not gone 35 Chapter 1

unnoticed in the photocatalytic field, and in the last few years a substantial number of articles have explored the photoactivation of these materials.221 In order to harvest sunlight it is important to add chromophores and charge separation centres in the MOF lattice.222 An impressive example of the importance of these materials is the possibility of performing enantioselective photocatalytic reactions. In this respect, assembling a photoactive moiety (nitrilotribenzoicacid) and a chiral entity (L- or D- proline derivatives) with Zn centres in the same crystalline network results in a MOF with a layered structure, Zn.BCIP, which acts as an asymmetric photoactive catalyst.223 This material can achieve the alkylation of aliphatic aldehydes with high yield (74%) and chiral selectivity (92% enantiomeric excess) upon illumination. Further, MOFs containing Zr, Ru or Al centres with different structures (viz; Ui66) have also been evaluated for hydrogen generation using solutions of electron donors. In some cases, electron acceptors such as methyl viologen were also used.221 Stable hydrogen production using triethanolamine as a sacrificial agent has been achieved using amino-functionalized Ti(IV)MOF under visible-light irradiation (λ >420nm). In order to promote the photoactivity, this material also incorporates Pt nanoparticles as co-catalysts, which were loaded by photodeposition.224 However, despite the significant development in a short time period, the MOFs as photocatalysts still requires significant improvements to become fully competitive. One serious drawback for these materials is their poor stability, particularly in the presence of water and intense radiation.221 Some MOF structures based on Zr4+, Ti4+, and Fe3+ with carboxylate linkers (e.g. UiOs, MIL-125), or Zn2+ with imidazolate linkers (ZIFs), withstand aqueous solutions but long term studies are warranted. Furthermore, a low photocatalytic rate in most of the cases with strong reliance on sacrificial agents for hydrogen generation and CO2 photoreduction delays the practical progress. The huge scope of MOFs for tailoring new characteristics can remove these limitations but may take its course of time. The discovery of graphene225 as a new allotrope of carbon has lead to a revolution in materials science due to its unusual properties, which are expected to result in futuristic electronic applications.226 Graphene is constituted by a single layer of carbon with a network of sp2 bonds. As a consequence, it is endowed with a very high thermal conductivity (ca. 5000Wm-1K-1), with an excellent charge carriers mobility at room temperature (about 200 000cm2V-1s-1), and shows an extremely high specific surface area. Since the pioneering work by P.V. Kamat et al. (2008),227 graphene has 36 Chapter 1

been exploited for photocatalysis applications, usually in the form of TiO2–graphene composites. Initially, photocatalysis was implemented for the reduction of graphene oxide into graphene by UV irradiated composites. Soon after, it was observed that

graphene–TiO2 showed a significant enhancement of reactivity with respect to the unmodified oxide. In this respect, as much as a 4-fold increase of the degradation rate

of dyes and organic pollutants has obtained for TiO2 modified by a small amount of graphene.228-230 More advanced structures such as carbon nanofibers,231,232 carbon nanotubes,233-235 fullerenes,236-238 and new morphologies such as nanohorns,239 nanopetal240 and nanowalls241 complete the scenario of this family of composite materials. It has been proposed that the role of these components is transferring electrons to the semiconductor and acting as a visible light photosensitizer.234 Modification of semiconductors by other small forms of carbon in order to improve electron transfer in composite photocatalysts has also been assayed. In the way forward, graphyne, a 2-D phase constituted by a network of sp and sp2 C–C bonds,

has been incorporated to TiO2. These composites have been tested for methylene blue

degradation and the removal rate is almost 1.5 times higher than that of TiO2– graphene.242 Carbon nitrides are another type of material with great potential for

photocatalytic applications. In particular, graphitic carbon nitride (g-C3N4), which consists of melem units, can be described as three fused triazine rings, forming a layered structure with semiconducting properties and a band gap of 2.7eV.243, 113

1.8.5.3. Artificial leaves: combining photocatalysis and photovoltaics Although silicon is the most explored semiconductor for most photovoltaic cells, it has never been used in photocatalytic applications. The main reason is the significant corrosion of Si in aqueous solutions due to anodic oxidation. As a result, this material usually needs a protective coating and an electrical bias when used in photochemical processes, such as water splitting. However, the significant overlap of the silicon band gap with the solar spectrum, along with its availability and non-toxicity, still makes this semiconductor an important option for photochemical processes. Currently, silicon has been utilized as one of the major components in a novel concept of an unwired macroscopic device developed by Nocera et al. (2012), which has been named as ‘artificial leaf’.244-246 This system, schematically shown in Figure 1.14, consists of a multi-deck sandwich of semiconductors (n-Si; p-Si; p+-Si) and a coating 37 Chapter 1

of catalyst for either hydrogen or oxygen evolution with an ohmic contact across them. The silicon layers with different kinds of dopants can be activated by light due to the difference of potential established and the photogenerated carriers are driven in opposite directions through the conductor, and then they can reach the catalysts.

Figure 1.14: Schematic diagram of an artificial leaf with Co-based catalysts for oxygen evolution and a npp+- silicon junction separated by a tin-doped indium oxide (ITO).244

As the whole device is put in a solution, it splits the water molecules upon illumination. The efficiency can be improved further by selecting the feasible electrolyte (viz; potassium nitrate or potassium borate) and the slightly alkaline pH. This n–p junction like architecture is similar to that of a conventional photovoltaic cell, but the photogenerated carriers are driven to the catalysts instead of an external circuit. The Co phosphate with a Co–O arrangement resembling that of cubane acts as an oxygen-evolving catalyst. On the other hand, a NiMoZn alloy acts as a hydrogen evolving catalyst. Employing this dispositive, a solar-to-fuel conversion efficiency as high as 4.7% has been reported. More recently, a structure based on the same concept but with much more complex morphology, using TiO2 and Si nanowires, has been

proposed for water splitting (in a 0.5M H2SO4 solution) under simulated sunlight irradiation.247 In this system iridium oxide was used as a co-catalyst and deposited on

the TiO2 nanowires, which were attached to much larger Si rods modified with Pt particles, forming a kind of nanoforest. This device is able to produce O2 and H2 at a constant rate in a 5h interval, but the efficiency of energy conversion is modest, 38 Chapter 1

0.12%. This interesting type of unwired device has opened a new route for innovation, which can exploit the accumulated experience with photovoltaic devices. In this respect, the development of more active co-catalysts may contribute to the success of these systems. In this respect, bio-inspired metal moieties have guided the initial efforts but exploration of other catalytic active materials can be fruitful. Further, long term stability and scaling up of these devices should be further investigated.113

1.9. Common Erroneous Features in Literature Several presentations and concept are erroneous or misleading on the topic of photocatalysis. A few examples are given as under.

1.9.1. Quantum yield In photochemistry, the quantum yield (QY) is defined as the number of molecules converted per quantum absorbed by the medium. In case of heterogeneous photocatalysis, the concept is more complex. This definition of quantum yield could be beneficial, for example, to determine the time of irradiation necessary to acquire a certain conversion in a simple reaction under a given UV-photonic flux. The concept of instantaneous QY is in proximity to the kinetic reality and can be defined as equal to the ratio of two rates, the reaction rate r (in molecules converted per second) divided by the efficient photonic flux ϕ (in UV-photons per second) actually absorbed by the catalyst.

< > = (17) 푟 푄푌 휑 This is an instantaneous magnitude linked to the parameters governing the reaction rate, in particular to the concentrations or partial pressures. The maximum values of QY are acquired at maximum coverage of reactants. The high QY values that are obtained at very small concentrations or low pressures, especially in diluted solutions or in trace elimination or with super-powerful UV-lamps, cited in the literature may appear suspicious.

1.9.2. Confusion between reaction rate and conversion In many papers, it is generally reported that the reaction rate r is of the (apparent) first order as expected for diluted reaction media. Hence, one should expect an increase in the rate r with increase in the concentration. However, in various published articles, it is written that the rate decreases with increase in concentration. 39 Chapter 1

This serious contradiction is because of the confusion between reaction rate r and conversion τ. Conversion is defined as:

= = 1 (18) 퐶0−퐶 퐶 휏 퐶0 − 퐶0 and is generally expressed in %. It must be understood clearly that it is faster to eliminate say 50% of a solution of 10−6molL-1 than 50% of a molar solution (1molL-1) since a photocatalytic reaction is controlled by the photon fluxes of the lamps and their corresponding quantum yields. In fact, in a true or apparent first order reaction, the integration of the reaction rate

−dC/dt = kC gives C = C0 exp(−kt) or ln(C/C0) = −kt. Therefore, conversion τ is equal to:

= = 1 = 1 (19) 퐶0−퐶 퐶 −푘푡 휏 퐶0 − 퐶0 − 푒 Consequently, it clearly appears that in a first order reaction, conversion is independent of the initial concentration C0 but, conversely, the rate and the number of

converted molecules per unit time are proportional to C0.

1.9.3. Activity The term “activity” here often refers to “photocatalytic activity”. Although the authors does not know who first started using this term in the field of photocatalysis, people working in the field of photocatalysis were using this term prior to the 1980s, when photocatalysis field had begun to be accelerated by the pioneer work of the so-called “Honda–Fujishima effect” on photoelectrochemical water splitting using a single- crystal titania electrode.248 Most authors use the term “photocatalytic activity”, but almost in all cases the concept is the same as that of relative or absolute reaction rate. One reason to use the term “photocatalytic activity” may be to make readers think of “photocatalytic reaction rate” as one of the characteristics of a photocatalyst i.e., photocatalysts have their individual activity, while “reaction rate” is influenced by the reaction conditions. The term “catalytic activity” has been used to indicate performance or property of a catalyst, since an “active site” on a catalyst accounts for the catalytic reaction (Figure 1.15). The reaction rate per active site can be determined and should be equal to what we call “catalytic activity”. The term “turnover frequency”, i.e., number of turnovers per unit time of reaction, is sometimes 40 Chapter 1

used to describe how many times a reactive site produces a reaction product(s) per unit time. Contrarily, there are no active sites, in the same meaning used for thermal catalysis, i.e., catalytic reaction rate is mainly governed by the density of active sites, on a photocatalyst, and the reaction rate strictly depends on various factors such as the intensity of irradiated light which initiates a photocatalytic reaction. If the dark side of a photocatalyst or suspension is considered not to work for the photocatalytic reaction, the use of the term “active site” is inappropriate, and a relationship of active sites with photocatalytic activities cannot thus be expected. In the kinetic study of general chemical reactions, a rate constant is determined. Assuming that photoexcited electrons (e−) and positive holes (h+) induce a redox reaction, we can estimate the rate constant of these active species. Since e− and h+ recombine with each other during the process, the overall photocatalytic reaction rate depends also on this recombination rate. Assuming that k(redox) and k(recombination) are rate constants of reactions by e− and h+ and their recombination, respectively, i.e., the simplest kinetic model, the ratio k(redox)/k(recombination) should be a measure of intrinsic photocatalytic activity.249 However, we have no method to estimate k(recombination), since the recombination does not generate any chemical species to be detected.203

Figure 1.15: Difference in concepts of catalytic and photocatalytic reactions: A catalyst contains active sites of which a substrate is converted into a product, while no active sites are present on a photocatalyst.

1.9.4. Normalized photocatalytic tests In order to commercialize the photocatalytic devices (air purifiers, domestic refrigerators, self-cleaning materials, etc.), photocatalytic normalized tests have to be clearly defined and applied. A real photocatalytic activity test can be claimed erroneously if a non-catalytic side-reaction or an artifact does occur. Many 41 Chapter 1

photocatalytic tests are based on dyes decolorization, which is easy to determine with a UV–visible spectrophotometer. However, these tests can represent faulty results, hiding the actual non-catalytic nature of the reaction involved. This was quantitatively demonstrated with the apparent photocatalytic “disappearance” of indigo carmine dye.250

Whereas indigo carmine IC was completely degraded by UV-irradiated TiO2, its color also disappeared using visible light. Infact, IC was decolorized but its corresponding total organic carbon (TOC) remained intact. The loss of colour actually corresponded to a limited stoichiometric transfer of electrons originating from indigo molecules to

TiO2, once photo-excited in the visible as IC*. This is quite feasible since the

electronic energy level of IC* is higher than that of the conduction band of TiO2.This electron transfer degrades the regular distribution of conjugated bonds within the dye molecule and causes its decolorization. Once transferred to TiO2, the electron takes − part in an additional ionosorption of molecular oxygen as O2 . This is described by the following equations and illustrated in Figure 1.16.

Hν(vis) + IC(ads) IC (ads) (20) ∗ + − IC (ads) IC + e (TiO2) (21) ∗ − − e (TiO2) + O2(ads) O2 (ads) (22)

Figure 1.16: Degradation of Indigo Carmine dye under UV-irradiation (A) and electron transfer from excited IC* molecules without hole formation under visible light (B) 42 Chapter 1

When the same reaction was done with a higher concentration of IC, the solution didn’t decolorize with indeed the same constant initial TOC value. As a consequence, all standardization tests, exclusively based on dye decolorization, should be avoided.

1.9.5. Problematic doping Comyns (2009) complains that photocatalytic studies are sometimes focusing too much on peculiar subjects.251 It perhaps points to the case of doping with numerous, even too huge number of publications on it. When they started 30 years ago with cationic doping, no significant improvement in photo-activity could be recorded since it requires an improvement by at least a factor of two, or even better by one order of magnitude. This was seldom observed in literature.252,253

1.9.5.1. Cationic doping It is understood that doping deals with dissolving controlled and moderate quantities of heterovalent cations in lattice sites of the host cations to apply the “induction valence law” defined in electronics254 and is illustrated in Figure 1.17.

Figure 1.17: Schematic n- and p-type doping of titania

It is now generally accepted that cationic doping is detrimental for photocatalysis. Cr doping was extremely inefficient for oxygen chemisorptions.255 This was clearly and quantitatively explained by the fact that doping cations act as recombination 80,81 3+ centers. In the case of substitutional doping of TiO2 by M dissolved trivalent cations (M= Fe, Cr, Ga), according to Figure 1.17, each doping agent generates one acceptor center A:

(–Cr3+–) + e− [(–Cr3+–)e−] (23) 43 Chapter 1

or A + e− A (24) The filled acceptor centers attract photo-holes and become empty after neutralization: A− + h+ A (25)

Since Cr-doping contains 0.86 atomic%, there results: [Cr3+] = 2.50×1020 ions cm-3. Even if cationic doping is by definition low in atomic %, the concentration in Cr is much larger than the instantaneous concentration of electrons and holes, [e−] and [h+]. In the undoped samples, the recombination rate R is given by:

= [ ] [ ] = [ ]2 (26) − + − 푅 푅 i.e. it is of second푅 order.퐾 푒 For dopedℎ sample,퐾 푒 the recombination rate becomes:

= ([ ] + [ ])[ ] = ([ ] + [2.50 × 10 ]) [ ] ′ [ ] − − + − 20 + 푅 푘푅 푒 퐴 ℎ 푘푅 푒 (27) ℎ ≈ + 푅 This indicates푘 ℎ that ≫ 푅recombination rate R is strongly enhanced by p-type doping. A similar demonstration shows that n-type doping agents act as electron-hole recombination centers too. Therefore, cationic doping needs to be avoided.

1.9.5.2. Anionic doping Anionic doping has been a new concept for the narrowing of the band gap energy.107 For nitrogen doping, according to the valence induction law,256 it must be determined (i) that nitrogen is present in a nitride state N3−, (ii) that N3− anions are in O2− lattice bulk positions and (iii) that, titania has no tendency to self-clean expulsing N3− anions from the anionic sub-lattice in oxidizing working conditions,via their oxidation with a favorable decrease of the ionic radius of element N from 1.71Å to 0.55, 0.25, 0.16 and 0.13Å characteristic of the oxidation numbers of N equal to −3, 0, +1, +3 and +5, respectively. Concerning anionic doping, some researchers recommend to “Wait and watch”.99

1.9.6. Langmuir–Hinshelwood mechanism In fact, heterogeneous photocatalysis obeys the Langmuir–Hinshelwood mechanism in most cases with the rate of reaction r being proportional to the surface coverages of reactants. In a bimolecular reaction: A + B C + D (28) 44 Chapter 1

rate r varies as: = (29)

Each coverage θi varies as:푟 푘 휃퐴휃퐵 = ( ) (30) 퐾푖푋푖 푖 푖 푖 where Ki is the adsorption휃 constant1+ 퐾 푋(not under illumination) and Xi represents either

the concentration of solution or the partial pressure Pi of the gas. Hence, reaction rate r becomes:

= = = . . . /(1 + ) (1 + ) (31)

퐴 퐵 퐴 퐵 퐴 퐵 퐴 퐴 퐵 퐵 where푟 k 푟is the푘 휃true휃 rate푘 constant.퐾 퐾 푋 It is푋 understood 퐾 푋that besides 퐾 the푋 mass of catalyst, the rate constant k mainly depends on a single parameter, temperature according to the Arrhenius’ law:

= ( / ) (32) −퐸푎 푅푇 퐾 퐾0 푒 where Ea is the true activation energy.

Similarly, adsorption constants Ki vary only with temperature T according to van t’Hoff’s equation:

= ( ) ( / ) (33) −∆퐻푖 푅푇 퐾푖 퐾푖 0푒 where ∆Hi is the enthalpy of adsorption of reactant i. Hence, even if the true photocatalytic rate constant k doesn’t depend on T, reaction rate r depends on T because of the two temperature-dependent coverages A and B. Further, it is common in the literature or in submitted articles that both k and K vary with the concentration of reactant! Generally, one of the two reactants (for instance B) is either in excess or maintained as constant. Hence, B =1 or B = constant. For example, B is equal to unity in a pure reactant liquid phase; alternatively, B can be constant but less than unity, as

for instance B = oxygen in oxidation reactions either in ambient air (PO2≈1/5atm) or in liquid phase through its dissolution via Henry’s law. Therefore: = = = /(1 + ) (34) ′ ′ 푟 푘 휃퐵휃퐵 푘 휃퐴 푘 퐾퐴퐶퐴 퐾퐴퐶퐴 with k΄ = kθB = pseudo-true rate constant.There are two limit cases:

(i) When C = Cmax i.e; θA = 1 and thence r = k΄ 45 Chapter 1

(ii) When C<

= => = + (35) 휕 푙푛 퐾푎푝푝 퐸푎푝푝 1 푎푝푝 푎 퐴 휕�푇� − 푅 퐸 퐸 ∆퐻 1.9.7. The fundamentals of heterogeneous photocatalysis Five physical parameters influence nanoparticle photocatalytic activity, identified by the temporal reaction rate r. Their influences are illustrated in Figure 1.18. They are (i) the mass of catalyst, (ii) the wavelength, (iii) the initial concentration (or pressure) of the reactant, (iv) exceptionally the temperature in extreme conditions with respect to room temperature and (v) the radiant flux.79,81 In Figure 1.18A, one can see that the reaction rate r is proportional to the mass m of catalyst before reaching a maxima due to the full absorption of photons by the photocatalytic bed. The initial proportionality between r and m is the same as that for conventional thermo-activated catalysis. It

shows that the reaction rate is proportional to the total number of active sites nt at the surface of the photocatalyst.

= × × (36)

푛푡 푚 푆퐵퐸푇 푑푠 SBET is the specific area and dS is the areal density of sites whose maximum is estimated to be ≤5×1018m-2.257 It needs to be noted that the curve of Figure 1.18A is quite general. Whatever the design of the photoreactor, the curve r = f(m) will always exhibit an initial linear variation followed by a plateau. For new materials using

deposited sub-micrometric layers of TiO2, the catalytic activity is still proportional to the number of layers before leveling off. Such a curve can be used to determine the mass mopt corresponding to the maximum absorption of photons reaching the catalytic bed. Figure 1.18B represents r = f(λ). Such a curve has to be obtained under monochromatic light and requires a quiet easy and rapid reaction to have a precise measurement of r. It can be observed that this curve parallels that of the light absorption by the solid and enables one to determine the energy band gap Eg by the catalytic measurements. Thermodynamics for such a curve has to be confronted. Figure 1.18C illustrates the Langmuir–Hinshelwood mechanism, whereas the Arrhenius plot of Figure 1.18D directly depends from Figure 1.18C.79,80 The two 46 Chapter 1

extreme cases can be qualitatively expressed as follows. Since adsorption is a spontaneous and exothermic phenomenon, low temperatures favor adsorption including that of the final products which become inhibitors. Contrarily, high temperatures are detrimental for the adsorption of the reactants and the reaction rate declines.79-81 Eventually, Figure 1.18E illustrated the relationship r = f(φ), φ being the radiant flux of the light source (in Wm-2). At moderate radiant fluxes, r is proportional to φ below a maximum value, designated by a dashed line, above which the rate declines from proportionality to follow a square root variation as r 1/2. 80,81 Hermann studied it at the end of the seventies but he published it much later∝φ. However, a similar report had been independently and much earlier published by Egerton (1979).258, which has to be historically considered as the first one on this subject.They clearly observed that too high radiant fluxes greatly increase the identical concentrations in photo-electrons and photo-holes. Consequently, the electron-hole recombination reaction:

e− + h+ N (37)

where N is the neutral centre which has a recombination rate rR of the second kinetic order:

= [ ][ ] = [ ] (38) − + − 2 푟푅 푘푅 푒 ℎ 푘푅 푒 rR increases parabolically with the charge concentration. Consequently, here is a waste in noble UV-light energy and the system does not run in optimum conditions. Further, since electron-hole recombination is exothermic, such high radiant fluxes may increase the temperature of the catalyst, thus entering in the left-hand side domain in Figure 1.18D where increasing temperatures make the reaction rate decrease. In addition, the electron-hole recombination energy dissipates thermally since any chemiluminescence could never be detected.259 It is not feasible to use over-powered lamps especially with small photoreactors as often reported in the literature. In conclusion, all these recommendations needs to be addressed prior to claiming that one deals with a true photocatalytic reaction.99 47 Chapter 1

Figure 1.18: Effect of the different physical parameters, which influence the kinetics of photocatalysis: reaction rate r; (A) mass of catalyst m; (B) wavelength λ; (C) initial concentration c of reactant; (D) temperature T; (E) radiant flux φ.99 48 Chapter 1

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Chapter 2 Methodology

Chapter 2 62

Methodology

This chapter constitutes the methodology used for the synthesis of nanoparticles and nanocomposites and their characteristic assessment. For structural characterization of nano-dimension particles, powerful characterization tools are required for observing each and every phase and examining the complete structures. The following sections provide a brief description of various synthesis and morphological analyses methods. Further, the methodology for the evaluation of photocatalytic activity has also been discussed.

2.1. Methods of Synthesis for Nanostructures The methods used in the present work for the synthesis of nanoparticles and nanocomposites are as follows.

2.1.1. Sol-Gel Method The sol-gel method is a versatile process used in designing various ceramic nanomaterial photocatalysts.1-6 A typical sol-gel process encompasses the formation of a colloidal suspension, or a sol from the hydrolysis and polymerization reactions of the precursors. The precursors are usually inorganic metal salts or metal organic compounds such as metal alkoxides. Complete polymerization with the aid of solvent leads to the transformation from the liquid sol into a solid gel phase. Thin films can also be designed if a piece of substrate is used as template, by spin-coating or dip-coating. A wet gel is formed when the sol is cast into a mould, which is then converted into a dense ceramic by further drying and heat treatment. If the solvent in a wet gel is removed under a supercritical condition, a highly porous and low-density material called an aerogel is obtained. Ceramic fibers can be drawn from the sol if the viscosity of a sol is adjusted into a particular viscosity range. Ultrafine and homogenous ceramic powders are formed by spray pyrolysis, or mechanical techniques and under specific conditions, other nanomaterials can be designed. TiO2 nanoparticle photocatalysts have been synthesized by the sol-gel method from hydrolysis of a titanium precusor.7-34 This method normally proceeds via acid-catalyzed hydrolysis of titanium(IV)alkoxide followed by condensation.7,19,22, 35-47The development of Ti-O-Ti chains is favoured in presence of low content of water, low hydrolysis rates, and excess of titanium alkoxide precursor in the reaction mixture. Three dimensional polymeric skeletons 63 Chapter 2

with close packing can result from the network of Ti-O-Ti chains. The formation of

Ti(OH)4 is favoured with high hydrolysis rates for a medium amount of water as solvent. The excess of Ti-OH and poor development of three-dimensional polymeric skeletons result in loosely packed first-order particles. Closely packed first order particles are obtained through a three dimensionally structured gel skeleton.7,19,22,35- 47 On analysis of the growth kinetics of TiO2 nanoparticles in aqueous solution using titanium tetraisopropoxide (TTIP) as precursor, it is found that the rate constant for coarsening increases with temperature as the viscosity of the solution and the 19 equilibrium solubility of TiO2 depends on temperature. Secondary particles can be acquired by epitaxial self-assembly of primary particles at higher temperatures and

longer times. The average radius of TiO2 nanoparticle increases linearly with time, according with the Lifshitz-Slyozov-Wagner model for coarsening.19

Many efforts have been put to obtain highly crystallized and narrowly dispersed TiO2 nanoparticles using the sol-gel method along with other modifications. In this respect a semicontinuous reaction method was proposed by Znaidi et al.34 which is a two stage mixing method and also a continuous reaction method proposed by Kim et al.9,10

2.1.2. Sol Method The sol method usually refers to the non-hydrolytic sol-gel processes and mostly involves the reaction of metal halides with a variety of different oxygen donor molecules like a metal alkoxide or an organic ether.48-56

TiX4 + Ti(OR)4 2TiO2 + 4RX (1)

TiX4 + 2ROR TiO2 + 4RX (2) The condensation between Ti-Cl and Ti-OR leads to the formation of Ti-O-Ti bridges. The alkoxide groups can be obtained from titanium alkoxides or can be synthesized in situ by reaction of the titanium chloride with alcohols or ethers. In the method by Trentler and Colvin,56 a metal alkoxide was rapidly injected into the hot solution of titanium halide mixed with trioctylphosphine oxide (TOPO) in heptadecane at 300oC under dry inert gas conditions, and reactions were completed within 5min. For a series of alkyl substituents including methyl, ethyl, isopropyl, and tert-butyl, the reaction rate dramatically increased with greater branching of alkyl groups, while average particle sizes remained relatively unaffected. Reaction in pure TOPO was slower and resulted in smaller particles, while reactions without TOPO were much faster and yielded mixtures of anatase, rutile and brookite phases with average particle sizes

Chapter 2 64

greater than 10nm. Figure 2.1 shows typical TEM images of TiO2 nanocrystals developed by Pirzada et al.57

Figure 2.1: TEM images of the round-shaped TiO2 nanoparticles.

2.1.3. Chemical Precipitation Method Chemical precipitation method is considered as an appropriate and easy method due to its simple operation. While other methods often requires sophisticated equipments/instruments, large time interval and relatively extreme environmental conditions (temperature, pH, pressure etc.), the chemical precipitation method usually requires simple lab equipments and ambient environmental conditions and the experiment usually commences faster.58 A chemical precipitation method generally involves a redox reaction. As a rule, metal salts (metal chlorides, metal sulphides, metal nitrates etc.) act as oxidants while aluminohydrides, borohydrides, hypohydrides, formaldehyde and salts of oxalic acids serve as the reducers.59 It is a multifactor process depending upon the choice of a redox pair, concentration of the precursors, temperature and pH of the medium, and diffusion and sorption characteristics. The mechanism involved in a chemical precipitation method involves a temporary discrete nucleation which upon injection of reagent undergoes an abrupt supersaturation leading to the production of mono- dispersed particles, whose growth is subsequently controlled by Ostwald ripening.60 65 Chapter 2

The size and stability of the nanoparticles can be controlled either by restricting the reaction space within matrices such as glasses, zeolites, silica, polymers, reverse micelles, vesicles and Langmuir–Blodgett (LB) films,61,62 or by using stabilizers and capping agents, such as thiourea, thiols, thioglycerol, phosphates, phosphine oxides, mercaptoacetic acid, bulky anions (hexametaphosphate), polyions (polyvinyl alcohol), or by capping with nucleophilic reagents like thiophenols.60,63,64 The substances acting as a reducer as well as a stabilizer has widely been used recently e.g., N-S containing surfactants, thiols, salts of nitrates, and polymers containing functional groups.59 It is generally assumed that chemical precipitation method yields nanoparticles that are homogenously constituted from identical atoms or molecules and are mono-dispersed in the solvent. But in fact, they often agglomerate into larger irregular entities, and rarely have uniform purity. To combat this limitation, nanoparticles are stabilized by using surfactants or large polyions or by embedding them in matrix.60 But, this inexorably alters their surface states.

2.1.4. Hydrothermal Method Hydrothermal synthesis is normally done in steel pressure vessel called autoclave with or without Teflon liners. The system is being operated at controlled temperature and/or pressure with the reaction in aqueous solutions. The reactions are usually performed at elevated temperatures, usually above the boiling point of water, ensuring the pressure of vapour saturation. This method is widely performed for the synthesis of small particles in the ceramics industry and photocatalysis. Many groups have used 65-74 the hydrothermal method to prepare TiO2 nanoparticles. For example; TiO2 nanoparticles were prepared by hydrothermal reaction of titanium alkoxide in an acidic ethanol-water solution.66 Briefly, titanium tetraisopropoxide, was added dropwise to a mixed ethanol and water solution at pH 1.0 with nitric acid, and reacted o at 240 C for 4h in an autoclave. The synthesis of TiO2 nanoparticles under the acidic ethanol-water environment mainly gives primary structure in the anatase phase without secondary structure. The particle sizes were controlled to the range of 7-25nm by adjusting the concentration of Ti precursor and the composition of the solvent

system. Further, TiO2 nanorods have also been synthesized with the hydrothermal 75-80 method using different precursors and conditions. Zhang et al. obtained TiO2

nanorods by treating a dilute TiCl4 solution at 333-423K for 12h in the presence of

Chapter 2 66

75, 77-80 acid or inorganic salts. Figure 2.2 shows a typical SEM image of the Bi2O3 nanorods prepared by the hydrothermal method in our laboratory.

Figure 2.2: A typical SEM image of the Bi2O3 nanorods prepared by the hydrothermal method.

Similarly, nanowires and nanotubes can be synthesized by changing the precursors and the reaction conditions.

2.1.5. Solvothermal Method The solvothermal method is almost similar to the hydrothermal process except that the solvent used in solvothermal process is non-aqueous. However, the temperature in this case can be elevated much higher than that in hydrothermal method, since a variety of high boiling point organic solvents can also be chosen. The solvothermal method has an advantage over hydrothermal methods that the size and shape distributions along with the crystallinity of the nanoparticles can be efficiently controlled. The solvothermal method has been found to be a versatile method for the synthesis of a variety of nanoparticles with narrow size distribution and dispersity.81-83

The solvothermal method has been used to synthesize TiO2 nanoparticles and nanorods with or without the aid of surfactants as templates.81-89 For example, in a typical procedure by Kim and co-workers,88 TTIP was mixed with toluene at the o weight ratio of 1-3:10 and kept at 250 C for 3h. The average particle size of TiO2 powders tend to increase as the composition of TTIP in the solution increased in the

range of weight ratio of 1:10 to 3: 10, while the pale crystalline phase of TiO2 was not produced at 1:20 and 2:5 weight ratios.88 By controlling the hydrolyzation reaction of

titaniumtetraisobutoxide Ti(OC4H9)4 and linoleic acid as support, redispersible TiO2 nanoparticles and nanorods could be obtained, as found by Li et al.1,81 67 Chapter 2

2.2. Methods for Characterization Characterization of nanomaterials is an important part of nanomaterial study and is surely a tedious task because the dimensions and structures of nanomaterials are very small and beyond the resolution of many standard techniques and instruments.58,90 For clear understanding of nanomaterials, the elucidation of their structures, compositions, and chemical properties is important.91 Common techniques used for nanomaterial characterization includes optical spectroscopy (absorption, electronic, Infrared and Raman); X-ray (diffraction and photoelectron spectroscopy); microscopy (scanning electron microscopy and transmission electron microscopy) etc. Each of these instrumental techniques is used to explore some specific aspects of the nanomaterial properties. X-ray and microscopic techniques are usually implemented to determine the structural properties like particle size, shape, and crystal structure, whereas optical spectroscopy is used to study their electronic and optical properties. In addition, surface characteristics were analyzed by using Brunauer, Emmett and Teller (BET) and Barrett Joyner Halenda (BJH) theories. Charge transfer and charge carrier recombination properties were determined by Electrochemical Impedance Spectroscopy and Photoluminescence spectroscopy, respectively. The biggest task in characterization of nanomaterials is to safe guard their natural state during analysis.91,92

2.2.1. Elemental Analysis The elemental purity and composition of nanomaterials was determined by Fourier Transform Infrared (FTIR) Spectroscopy and Energy Dispersive X-Ray (EDX) Spectroscopy (EDS).36

2.2.1.1. Fourier Transform Infrared (FTIR) Spectroscopy FTIR spectroscopy, due to its great molecular specificity, good sensitivity, and high versatility, is one of the most widely used techniques for material characterization. FTIR spectra give us idea about the type of bonds and interactions and also the specific functional groups present on the basis of various vibrational modes in the sample.91 FTIR spectroscopy operates on several working modes.92,93 The most common arrangement is transmission, where a thin tablet of solid sample is kept between the IR beam and the detector as this mode works best for weakly absorbing samples.91 FTIR technique poses a few limitations; a strong absorption of radiation by the nanomaterial usually limits the vibrational energy window available for analysis.

Chapter 2 68

Also, it is difficult to perform the quantitative analysis by the use of IR absorption band intensities. Finally, it is sometimes very difficult to interpret the FTIR spectra, especially in cases of complex molecules with a large number of vibrational modes.91The purity and composition of synthesized nanomaterial photocatalysts were studied by Interspec 2020 FTIR Spectrometer using the pellets of dried and powdered samples mixed with KBr in my work.

2.2.1.2. Energy Dispersive X-Ray (EDX/EDS) Spectroscopy

EDX is considered to be a relatively rapid, inexpensive, and non-destructive approach towards surface elemental analysis. EDX is widely used to study the purity and elemental composition of the products (Figure 2.3). It can be used to survey surface analytical problems, elemental analysis from carbon to uranium with semi- quantitative analysis in the detection limits of ~0.5 weight % for most of the elements.94 EDX studies were performed by INACTA-ACT, Oxford International EDS instrument coupled with Scanning Electron Microscope (JEOL, JSM6510LV) for the elemental analysis of the synthesized nanoparticles using the dry powder samples.

Figure 2.3: EDS spectrum of TiO2/ZrO2 nanocomposite showing atomic and weight percentage of different elements present in the nanocomposite57

2.2.2. Structural Analysis Pertaining to the nanoscale dimensions of the nanocomposite particles, powerful characterization tools are required for observing each phase and determining the structures of the composite materials. Thus, high resolution microscopes have to be used to observe the all features at minute scales. The following sub-sections provide a brief description of various structural and morphological analysis methods.

69 Chapter 2

2.2.2.1. X-Ray Diffraction (XRD) Spectroscopy X-ray diffraction is a versatile technique to determine the crystal structure of the nanomaterials and analyze their crystal properties. XRD explores the average crystal structure in samples, and by precise analysis of diffraction peak shapes, it provides information concerning crystallite size, crystallographic defects as well as compositional and chemical inhomogeneities.95Although XRD is commonly used to determine the structure and composition of nanoparticles and nanocomposites with crystalline structures,91,96 the power of XRD can be employed to investigate the local structure at higher resolution of few angstroms by means of the analysis of the fine structure and the radial distribution function.95 The X-rays used are usually the copper Kα radiations whose characteristic wavelength is about 1.5418Å, which is comparable to the size of atom.97 The theory of XRD based on single crystals was developed by Laue and Bragg in the early 1900s.95 In 1912, Bragg described that scattering will produce diffracted beam only when certain conditions are satisfied, can be expressed in the equation:97-99

= 2 (3)

Where, λ is the wavelength푛휆 of the 푑푠푖푛X-rays,휃 θ is the angle between the incident radiation and crystal plane; d is the spacing between the crystal planes. The constructive interference occurs when n is an integer.97 Thus, XRD involves specially the identification of specific lattice planes that produce peaks at their corresponding angular positions, 2θ, determined by Bragg’s law.91 Regardless of this limitation, the characteristic patterns characteristic of individual solids make XRD significantly useful for the identification of the bulk crystalline structure of solid nanoparticles and also to determine the average crystallite or grain size of nanoparticles.91 The positions of diffraction peak, as well as the position of its maximum intensity (2θ) are the most commonly used parameter to calculate the interplane distances and other lattice parameters. The full width of intensity distribution at half maximum (FWHM, β) of the highest intensity peak (Figure 2.4) is the simplest measure of the peak width.95 The XRD peaks will be sharply intense only if the sample has sufficient long-range order, and will become broader for crystallite sizes below about 100nm.

Chapter 2 70

Figure 2.4: The XRD pattern of synthesized anatase TiO2 nanoparticles showing broad peak indicating the FWHM.

Average crystallite sizes of nanomaterials below 60nm can be roughly estimated by using the Scherrer equation which relates the FWHM, β, and the average size of the crystallites (i.e. coherently diffracting domains) “d”.95,100 The Scherrer formula94,95,101is based on a restricted assumption supposing that the peak shape is dominated by size effects and can be written as:

= ( ) (4) 푘휆 푑 훽 푐표푠 휃 Where d is the average crystallite size, β is the FWHM, θ is Bragg angle, λ the X-ray wavelength (for Cu Kα line λ=1.54Å) and k is a constant and is close to unity which depends on the shape of the crystallites: e.g; k = 0.94 for cubic shape crystallites.94,95 X-Ray Diffraction pattern were recorded with Miniflex-tm II benchtop XRD system (Rigaku Corporation, Tokyo, Japan) using CuKα radiation (λ=1.5418Å). The samples were analyzed in dry powder form.

2.2.2.2. Microscopic Studies Microscopy is a widely used method for determination of the size and morphology of small particles and objects. Electron microscopy is the best method for the direct determination of the size of nanoparticles, nanocrystallites and solid nanomaterials and for assessing the constitution of the nanoparticles produced from colloid solutions.59,91,102 It is widely used to investigate nanomaterials and in nanotechnologies for advanced characterizations.102 Electron microscopy can be utilized in one of two modes: Either by scanning of a well-focused electron beam over the surface of the sample, or in a transmission arrangement.59,91 Microscopic studies 71 Chapter 2

were performed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

2.2.2.2.1. Scanning electron microscopy (SEM) Scanning electron microscopy is typically used to study surface topography of conducting specimens. For examining insulating specimens using SEM, sputtering coating of a thin layer of conductive material is required. Other information such as composition can also be obtained through SEM by mounting with EDX. A scanning electron microscope analyzes a sample by scanning it with a high-energy beam of electrons and images a micrograph. The electrons excite the atoms close to the surface of the sample, producing signals that contain information about the chemical composition, surface morphology and crystallographic orientation of the samples. SEM has the advantage of simple specimen preparation procedures. There are various types of signals produced by the excitation of the incident electron beam, which are detected and processed by specific detectors. Typical signals include back-scattered electrons (BSE), secondary electrons, characteristic X-rays, and specimen current. Secondary electron detectors are commonly employed to generate secondary electron images. A field emission gun gives a very high-magnification images to reveal the details at nanoscale. The other advantage of SEM is that micrographs with a large depth of field can be obtained, which provides the three- dimensional appearance. This is very handy for revealing the surface features of a sample. If a back-scattered electron (BSE) detector is employed, the SEM can generate BSE images through capturing the beam electrons reflected from the sample by elastic scattering. The contrast of BSE images contains information about chemical composition because the intensity of the BSE signal relies on the atomic number of the materials causing elastic scattering of electrons. Therefore, BSE images reveal the distribution of different elements in the sample. BSE are often used in analytical SEM along with the spectra obtained from the characteristic X-rays. Characteristic X-rays are emitted when the electron beam knocks out an inner shell electron from the sample, generating a higher energy electron to fill the shell and release energy.103 These characteristic X-rays can be utilized to assess the composition and measure the abundance of elements in the specimen. This composition measurement technique is called Energy Dispersive X-ray Spectroscopy (EDS) as discussed above. Scanning electron microscopy was pioneered by Manfred von Ardenne.

Chapter 2 72

2.2.2.2.2. Transmission electron microscopy (TEM) Advanced TEM directly captures images of atoms in crystalline specimens at resolutions close to 0.1nm, even smaller than inter-atomic distance. Here, an electron beam can be focused to a diameter smaller than ~0.3nm, featuring quantitative chemical analysis of a single nanocrystal. The principle of TEM is based on amplitude or scattering contrast of the electron beam owing to the fact that the electron beam is scattered by crystalline material. As compared to SEM, TEM needs more complicated procedures for specimen preparation; however, it has even higher resolving power. A transmission electron microscope generates a beam of electrons which passes through an ultra thin specimen so as to interact with it. An image is generated from the interaction of the electrons transmitted through the specimen that can be further magnified and projected onto a screen. The TEM micrograph may be recorded by a charge-coupled device (CCD) camera or a layer of photographic film. The first TEM was designed by Max Knoll and Ernst Ruska in 1931 based on the idea that electrons with much shorter wavelengths as compared to visible light should produce much higher resolution images than optical microscopes. Eight years later, the first commercial TEM was introduced. The contrast of TEM images depends on the magnification and at lower magnifications; it is due to absorption of electrons in the material. Thus, the composition and thickness of the material determine the contrast of the image. The contrast formation of TEM micrographs is more complicated at higher magnifications, because complex wave interactions modulate the intensity of an image.103 Usually two modes of imaging are employed, bright-field (BF) and dark-field (DF). In bright-field (BF) the deflected electrons are obstructed from the optical axis of the microscope by placing the objective aperture to allow only the unscattered electrons to pass through. In bright-field mode a two dimensional image of the density or thickness of the sample is provided by the intensity of the transmitted beam. On the other hand in dark-field mode the diffraction pattern of electrons is recorded and is used to form the micrograph.91,104 Analysis of the image requires the knowledge of electron diffraction. In addition to the regular absorption based surface imaging, analytical TEM can provide information about chemical composition, crystallographic orientation, electronic structure and electron phase shift. Hence, 73 Chapter 2

along with topographic and crystallographic information, particle size distributions can also be obtained.

2.3. Thermal Analyses Thermal analysis is a group of techniques that study the thermodynamic properties of nanomaterials as they get influenced with temperature. In practice, thermal analysis gives information about properties like enthalpy, thermal capacity, mass changes and the coefficient of heat expansion. It can also be used for studying phase transitions, reactions in the solid state, thermal degradation reactions and phase diagrams. In heterogeneous catalysis, it gives idea about the composition (purity, stoichiometry of surface species); thermal stability (melting point, vaporization, sublimation, desorption, decomposition, dehydration); chemical stability (oxidation, reduction, pyrolysis) and physical properties (specific heat capacity, enthalpy changes) of substances. The various thermal analysis techniques which are being used are Thermal Gravimetric Analysis (TGA), Differential Thermal Gravimetry (DTG) and Differential Thermal Analysis (DTA). In my work, thermal analyses were carried out at a constant rate of 20ºC/min and nitrogen atmosphere using powder of the synthesized nanomaterials by Simultaneous DTG-TG Apparatus, 60H, Shimadzu.

2.3.1. Thermal Gravimetric Analysis (TGA) TGA measures changes in weight with respect to changes in temperature. The obtained weight loss curve gives idea about the changes in different phases, thermal stability and kinetic parameters for various reactions occurring in the sample. A TGA curve is obtained by plotting mass against time or temperature. A derivative weight loss curve can be used to find the temperature at which weight loss is most apparent. TGA is also useful for the characterization of material purity, determination of humidity in the materials, examination of corrosion studies, kinetic processes and gasification processes.96 The various peaks obtained in TGA are shown in Figure 2.5.

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Figure 2.5: (a) A representation of the DTA curve showing exotherm, endotherm, thermophysical, thermochemical, thermomechanical and thermoelastic changes or transition. (b) Schematic representation of percentage weight loss and the corresponding DTA curve showing the exothermic or endothermic nature of the loss.105

2.3.2. Differential Thermal Analysis (DTA) DTA is a thermo-analytical technique which records the difference in the amount of heat required to increase the temperature of a sample and the reference material as a function of heat flow.106 DTA involves recording any temperature difference between the test sample and an inert reference while heating or cooling the sample. In DTA the heat flow to the sample and reference remain constant while as the temperature changes according to the property of the materials.107 Differential temperatures can also arise between two inert samples when their response towards the applied heat treatment is not identical.106The differential temperature is plotted against time or temperature so as to obtain a DTA curve. Changes in the sample which result in the absorption or evolution of heat can be recorded relative to the inert reference. A DTA curve can be used as a fingerprint for identification purposes.107 DTA can therefore be used to study thermal properties and phase changes at constant enthalpy.106The plot of DTA shows the endothermic or exothermic nature of the weight loss leading to thermal decomposition which may also correspond to the melting point of the compounds. While TGA only measures changes caused by mass loss, DTA also register changes where no mass loss occurs, e.g; crystal structure changes, melting, glass transition etc.100An idealized representation of the two major processes observable in DTA is illustrated in Figure 2.5(b), where ∆T is plotted on y-axis and T on x-axis. Downward peaks represent endotherms while upward peaks represent exotherms. The temperature of the sample is greater for an exothermic reaction to that of the reference and vice-versa for endothermic phenomena.

75 Chapter 2

2.3.3. Differential Thermogravimetry (DTG) A differential thermo-gravimetric curve is obtained by plotting the rate of change of weight with time against the temperature. It is a derivative curve whose peak corresponds to the maximum slope on the TGA curve which indicates the maximum weight loss. The thermogram obtained in DTG has been clearly resolved into a peak showing temperature of maximum loss.108 Figure 2.6 shows a representative curve of DTG with respect to TGA.

58 Figure 2.6: A representative curve of DTG showing its relation with TGA curve.

2.4. Surface Analyses The specific surface area of a solid powder sample is determined by physical adsorption of a gas on the surface and by determining the amount of adsorbate gas corresponding to a monomolecular layer on the surface. Physical adsorption phenomenon arises from relatively weak forces (van der Waals forces) between the adsorbent surface and the adsorbate gas molecules. The analysis is usually carried out at the temperature of liquid nitrogen. The amount of gas adsorbed can be determined by a volumetric or continuous flow procedure. This method usually encompasses the determination of external area and pore area evaluations to determine the total specific surface area in m2g–1 yielding important results regarding effects of surface porosity and particle size in many applications like catalysis etc. Barrett-Joyner-Halenda (BJH) analysis method can also be employed to determine pore area and specific pore volume by adsorption-desorption techniques. This technique characterizes pore size distribution independent of external area due to particle size of the sample.109

Chapter 2 76

According to the Brunauer, Emmett and Teller (BET) adsorption isotherm equation:

= × + (5) 1 퐶−1 푃 1 푃0 푚 0 푚 �푉푎� 푃 −1�� 푉 퐶 푃 푉 퐶 Where, P = partial vapour pressure of adsorbate gas in equilibrium with the surface at 77.4 K (boiling point of liquid nitrogen), in pascals.

Po = saturated pressure of adsorbate gas, in pascals.

Va = volume of the gas adsorbed at standard temperature and pressure (STP) [273.15 K and atmospheric pressure (1.013 × 105 Pa)], in millilitres.

Vm = volume of gas adsorbed at STP to produce an apparent monolayer on the sample surface, in millilitres. C = dimensionless constant that is related to the enthalpy of adsorption of the adsorbate gas on the powder sample.109

A value of Va is obtained at each of not less than three values of P/Po. Then the BET value on the left side of the Equation (5) is plotted against P/Po. This plot should yield a straight line usually in the approximate relative pressure range 0.05 to 0.3 (Figure 2.7). The data are considered acceptable only if the correlation coefficient, R, of the linear regression is not less than 0.9975, means; R2 is not less than 0.995.

Figure 2.7: Adsorption isotherm and pore width distribution (inset) for Bi2O3/TiO2 nanocomposite

From the resulting linear plot, the slope, which is equal to (C − 1)/VmC, and the

intercept, which is equal to 1/VmC, are evaluated by linear regression analysis. From 77 Chapter 2

these values, Vm is calculated as 1/(slope + intercept), while C is calculated as

(slope/intercept) + 1. From the value of Vm so determined, the specific surface area, S, in units of m2g–1, is calculated by the equation:

= × (6) 푉푚푁푎 푚 22400 푆 23 −1 Na = Avogadro constant (6.022 × 10 mol ).

Vm = volume of gas adsorbed to produce an apparent monolayer on the sample surface, in millilitres, at STP. m = mass of test powder, in grams. 22400 = volume occupied by one mole of the adsorbate gas at STP, in millilitres. A minimum of three data points is required. Extra measurements may be carried out,

especially when non-linearity is acquired at a P/Po value close to 0.3. Since, non-

linearity is often obtained at a P/Po value below 0.05, values in this region are not recommended. The other methods employed are single-point method and the volumetric method. Hence, BET along with the BJH analysis has tremendous applications for the surface characterization of the nanoparticles photocatalysts. It gives insights into: a) Rapid single point and multipoint determinations of specific BET surface area. b) Complete BET surface characterization of disperse, nonporous or macroporous materials of pore diameter >50nm (type II isotherms); mesoporous materials with pore diameter between 2 nm and 50nm (type IV isotherms); microporous materials with pore diameter, <2nm (type I isotherms). c) The pore volume and the pore area distributions in the mesopore and macropore ranges using BJH analysis. d) BJH adsorption and desorption average pore diameter determinations.109

2.5. Optical Analyses 2.5.1. Ultraviolet-Visible (UV-Vis.) Spectroscopy The UV-vis spectroscopy is an efficient tool for determining the optical properties of nanomaterials and the size, providing the explanation for the quantum size effect. The visible region of the electromagnetic wave spectrum corresponds to 800-400nm and ultraviolet region corresponds to 400-200nm. Ultraviolet-visible spectroscopy is also known as Electronic Spectroscopy or Absorption Spectroscopy as it includes the

Chapter 2 78

measurement of absorption of radiations when an electromagnetic radiation of proper energy is used.110 The spectra thus obtained are also known as absorption spectra. These absorption spectra are used in determining the concentration of a solution on the basis of Beer-Lambert law and are important in our study. Reflectance spectroscopy is related to UV-vis spectroscopy in principle, in that both of these techniques deal with excitation of electrons to the upper states. However, the difference in these techniques is that UV-vis spectroscopy records the relative change of transmittance of light as it passes through a solution, whereas, in diffuse reflectance spectroscopy (DRS), the relative change in the intensity of reflected light off of a surface is measured. Since light cannot penetrate opaque powder samples, it reflects on the surface of the samples. The incident light which reflects symmetrically with respect to the perpendicular line is called the "specular reflection," while the scattered one is called "diffuse reflection." The experiment is performed by putting the sample in front of the incident light window, and converging of the reflected light from the sample on the detector comprised of a sphere with a barium sulphate-coating inside. The obtained value is the relative reflectance with respect to the reflectance of the reference standard, which is considered to have 100%. When light is incident upon the sample at an angle of 0°, the specular reflected light exits the integrating sphere and is not observed. Hence, only diffuse reflected light is measured. Various models of integrating spheres containing different angles of incidence are available, enabling measurement of both specular and diffuse reflected light. A solution that is completely clear and colourless has essentially 100% transmission for all visible wavelengths, means that it does not contain any dissolved components. Accordingly, a white powder effectively reflects 100% of all visible wavelengths of light that interacts with it. DRS have a wide application in determination of band-gap energy of a semiconductor photocatalyst (Figure 2.8(a)). The relationship between diffuse reflectance spectra and the band gap energy is described by Tauc's expression, which can be written as:111-113 The band gap of the photocatalysts was calculated according to the formula:114

n\2 (hv.α) = (Ahv – Eg) (7)

Since α is proportional to Kubelka–Munk function F(R), the expression becomes 79 Chapter 2

n\2 hv. F(R) = (Ahv – Eg) (8)

Where v is the light frequency, F(R) is the Kubelka–Munk function, A is the constant

of proportionality and Eg is the band gap energy. The value of n is determined by the type of optical transition (for direct transition, n = 1 and for indirect transition, n = 114 n/2 4). The Eg of the photocatalysts was determined from the plots of (F(R).hv) versus hv (Figure 2.8b). The valence band edge position of the semiconductor photocatalysts at the point of zero potential can be calculated by the following empirical equation:115,116

c EVB =X−E +0.5Eg (9)

Where EVB is the VB edge potential, X is the electronegativity of the semiconductor, and Ec is the energy of free electrons on hydrogen’s scale (4.5eV). Herein, X is the geometric mean of the electronegativity of the constituent atoms. The X value for every semiconductor has a definite value. Moreover, conduction band edge potential

ECB can be determined by:

ECB = EVB − Eg (10)

From the above equations it is hence evident to determine the conduction band edge and the valence band edge, wherefrom, the mechanism of the photocatalytic processes can be ascertained. With the use of band gap value, the particle radius can be calculated using equation proposed by Brus.117- 120 The Brus expression gives a relation between the radius of the crystallite and the band gap value thus explaining the quantum size effect using effective mass approximation. Brus equation

. = + 0.248 2 2 2 (11) ħ 휋 1 1 1 8푒 ∗ 2 훥퐸푔 ≡ 퐸푅 − 퐸푔 2푅 �푚푒 푚ℎ� − 휀푅 − 퐸푅푦 Where Eg is the band gap value of the nanoparticles, E0 is the band gap value of the

bulk material, ∆Eg is the increase in band gap value, me and mh are the effective masses of electrons and holes respectively, ε is the dielectric constant of the

semiconductor, R is the radius of the particle, h is the Plank’s constant, and Ery is the effective Rydberg’s energy.118,121

Chapter 2 80

= 13605.8 + (12) −1 ∗ 1 푚0 푚0 2 퐸푅푦 휀 �푚푒 푚ℎ�

Figure 2.8: The representative curves for (a) Absorption edge and (b) Band gap calculation

The third term related to the solvation energy loss in Equation (11) is the size independent term and is usually small and thus ignored.

. = + 2 2 2 (13) ħ 휋 1 1 1 8푒 2 훥퐸푔 ≡ 퐸푅 − 퐸푔 2푅 �푚푒 푚ℎ� − 휀푅 In case of semiconductors the effect of the second term, the coulomb interaction term in the Equation (11) is very less and hence can be ignored as well.100, 120, 122, 123 Thus, the above equation becomes:

= + 2 2 (14) ħ 휋 1 1 2 훥퐸푔 ≡ 퐸푅 − 퐸푔 2푅 �푚푒 푚ℎ� In the present work, the Equation (11) was therefore used to calculate the particle size of the synthesized semiconductor nanoparticles. For semiconductor

nanomaterials, as the particle size decreases, the λmax must shifts to shorter wavelengths (indicated by a significant blue shift in optical absorption relative to their bulk), due to the increase in band gap of the nanoparticles confirming quantum confinement effect.117, 124 The optical studies were done on Genesys 20, Thermospectronic UV-visible spectrophotometer, using the stable dispersions of nanomaterials in appropriate solvents.

2.5.2. Photoluminescence Study When a light (photon) has energy greater than the band gap energy of a semiconductor nanoparticle, it excites an electron from the valence band up to the 81 Chapter 2

conduction band across the forbidden energy gap. The excited electron generally has excess energy which it loses before coming to rest at the lowest energy in the conduction band and eventually falls back down to the valence band. The energy it releases is converted back into a luminescent photon which is emitted by the material. Thus, the energy emitted as luminescent photon is a direct measure of the band gap

energy, Eg. The process of excitation followed by emission of photon is called photoluminescence (Figure 2.9).

Figure 2.9: Photoluminescence spectra of different nanocomposite samples giving idea about charge carrier recombination

Hence, fluorescence emission intensity is the direct measure of recombination rate of electron–hole pairs in the excited semiconductor photocatalyst. On this basis, we can determine the efficiency of a photocatalyst as recombination of charge carriers is the biggest limitation against efficient photocatalysis.

2.5.3. Electrochemical Impedance Spectroscopy Electrochemical impedance is generally determined by applying an AC potential to an electrochemical cell and then measuring the current through the cell. If we use a sinusoidal potential excitation, the response to it will be an AC signal. This current signal can be analyzed as a sum of sinusoidal functions or a Fourier series.125 Electrochemical impedance is usually measured using a small excitation signal. This is done so that the cell's response is pseudo-linear. In a linear (or pseudo-linear) system, the current response to a sinusoidal potential will be a sinusoid at the same frequency but shifted in phase.

Chapter 2 82

The sinusoidal excitation signal, expressed as a function of time, can be put in the form:

= ( ) (15)

푡 0 Where Et is the potential at퐸 time퐸 t, 푠푖푛E0 is휔푡 the amplitude of the signal, and ω is the radial frequency. The relationship between radial frequency ω (in radians/second) and frequency f (in hertz) is: = 2 (16)

In a linear system, the response휔 휋푓signal, It, is shifted in phase (φ) and has a different amplitude than I0. = ( + ) (17)

퐼푡 퐼0 푠푖푛 휔푡 휑 An expression analogous to Ohm's law allows us to calculate the impedance of the system as:

( ) ( ) = = = ( ) ( ) (18) 퐸푡 퐸0 푠푖푛 휔푡 푠푖푛 휔푡 퐼푡 퐼0 푠푖푛 휔푡+휑 0 푠푖푛 휔푡+휑 푍 = = ( ) = 푍 ( + ) (19) 퐸 0 0 The impedance푍 is therefore퐼 푍 푒푥푝 expressed푖휑 푍in terms푐표푠휑 of 푖a 푠푖푛휑 magnitude, Z0, and a phase shift, φ. As can be observed from the above expression, Z is composed of a real and an imaginary part. If the real part is plotted on the X-axis and the imaginary part on the Y-axis, we get a "Nyquist Plot" (Figure 2.10). It can be observed that in this plot the Y-axis is negative and that each point on the Nyquist Plot is the impedance at one frequency. On the Nyquist Plot the impedance can be represented as a vector of length |Z|.125 Nyquist Plots have one major limitation that when we look at any data point on the plot, we cannot tell what frequency was used to record that point. 83 Chapter 2

Figure 2.10: EIS Nyquist plots for different nanocomposite samples giving idea about the impedance imposed to charge transfer

The semicircle type plots obtained in such experiments is characteristic of a single "time constant". Electrochemical impedance plots usually give several semicircles but generally a portion of a semicircle is seen. The radius of the semicircles usually gives the idea about the impedance being imposed to the charge transfer. The smaller the radius of the semicircle, the lower is the impedance to charge transfer and vice-versa. Thus, electrochemical impedance spectroscopy is an efficient and recent tool in corrosion and solid state laboratories that gives clear idea regarding the charge transfer properties in the semiconductor nanocomposites and corrosion properties thereof. The advantage of impedance spectroscopy lies in the ability to distinguish the dielectric and electric properties of individual contributions of components under investigation.125

2.6. Evaluation of Photocatalytic Activity 2.6.1. Photocatalytic Experiment The photocatalytic activity of the nanocomposites was assessed by monitoring the decolorization of different organic dyes and non-dye pollutants in presence of UV and visible light. The photocatalytic experiments were carried in an immersion well photoreactor (consisting of inner and outer jacket) made up of Pyrex glass, equipped with a magnetic bar, water circulating jacket and a passage for molecular oxygen (Figure 2.11). Irradiations were carried out by a visible light halogen linear lamp

Chapter 2 84

(500W, 9500 Lumens) and UV lamps.126 The reaction temperature was kept constant at 20±0.3oC using refrigerated circulating liquid bath in each experiment.

Figure 2.11: Photograph showing set-up of photocatalytic experiment in our laboratory

Before irradiation experiments, 300mL of the pollutant solution of appropriate concentration containing the desired quantity of the catalyst was magnetically stirred, while the solution was continuously purged with atmospheric oxygen for at least 20min in the dark to attain adsorption–desorption equilibrium between the solution and the catalyst surface. Afterwards, the first sample (at 0min) was taken out and the irradiation was started. During irradiation, samples of 5mL each were withdrawn at constant time intervals, centrifuged and the supernatant was subsequently analyzed.

The change in absorbance of the aliquots was followed at its λmax as a function of irradiation time. The photocatalyst concentration was optimized by a series of photocatalytic experiments. Higher concentrations of photocatalyst were thought to absorb more incident photons and produce more photogenerated charge carriers, but past a particular concentration, the particles suspended in the solution cause shielding and light scattering, thereby, affecting the light transmittance in solution. Moreover, the decreasing transmittance may enhance recombination as the photons could not be continuously injected onto the photocatalyst particles. Similarly the initial dye solution concentration has significant influence on the photocatalytic activity as it 85 Chapter 2

affects the light penetration into the solution. This result was consistent with the literature for the effect of initial concentration on photocatalysis.127

2.6.2. Kinetic Studies of a Photochemical Reaction The observed absorbance spectra are in accordance with Beer-Lambert Law in the range of studied concentrations. The concentration of pollutant was calculated by standard calibration curve (R2>0.98) obtained from the absorbance of the sample at different known concentrations. The photocatalytic experiments were repeated three times in order to check the reproducibility of the experimental results. The degradation efficiency (%) was calculated by the equation:

(%) = × 100 (20) 퐶표−퐶 퐷푒푔푟푎푑푎푡푖표푛 퐸푓푓푖푐푖푒푛푐푦 퐶표 Where C0 and C are the concentrations of the pollutant in solution at times 0 and t of irradiation, respectively. As per the simplified Langmuir–Hinshelwood (L–H) kinetic model, the following first order kinetic equation can be used to describe photocatalytic degradation.128

æöCo ln ç÷= ktapp (21) èøCt

Where C0 and C are the concentrations of the pollutant in solution at times 0 and t −1 respectively, and kapp is the apparent first-order rate constant (min ).

Hence, apparent rate constant of a photocatalytic reaction, kapp, can be easily obtained

from the above equation as slope of the plot between ln(Co/C) and time of irradiation, t. To determine the fate of resultant reactive oxygen species (ROS), various quencher species were added to the reaction system in the manner similar to the photocatalytic experiment. The dosage of quenchers was referred to the previous studies.129, 130 The consistency in activity and stability of the catalysts were analyzed by the recycling experiments. After the first attempt of the photocatalytic experiment, the catalyst was retrieved from the photoreactor and the aliquots by centrifugation. The retrieved catalyst was thoroughly washed by deionised water and distilled ethanol. The catalyst was dried and then reused in the next cycle of the photocatalysis experiment. Same way, the experiment was repeated for a set of cycles in order to monitor the loss in efficiency of the catalyst after repetitive use.

Chapter 2 86

2.6.3. Role of ROS and Scavenging Tests The radical and hole trapping experiments (scavenger tests) with different scavenger molecules are carried out to elucidate the mechanism of photocatalytic degradation by a photocatalyst. Generally, the reactive species such as, hydroxyl radicals (•OH), −• + superoxide radical anions (O2 ) and holes (h ) are expected to be involved in the photocatalytic degradation processes. The roles of the reactive species are investigated through radical and hole trapping experiments. If the degradation rate decreased significantly upon the addition of isopropyl alcohol (IPA, a hydroxyl radicals scavenger),131 it indicates that dissolved •OH radicals were the dominant active species in the process. Likewise, if the addition of disodium ethylenediaminetetraacetate (EDTA, a hole scavenger)132 has a significant negative effect on the degradation, it confirms that the holes were the dominant active species. Upon addition of benzoquinone (BQ),133 a scavenger to −• −• quench O2 , if the kapp dropped significantly, it indicates that O2 radicals were active 134,135 species in the reactive system. The electron scavenger, AgNO3 was similarly used to confirm electron mediated degradation mechanism. On the basis of these tests, we can affirm the generation and subsequent role of different reactive oxygen species. This leads to a sound understanding of the mechanism involved in the degradation processes.

2.6.4. Mineralization The organic pollutants are degraded by the action of photocatalyst, however, the complete mineralization of the degradation products is to be ascertained, whether, the

degradation products are getting eventually mineralized into CO2 and H2O or merely into some other bye products. The extent of mineralization was estimated by using the concept of chemical oxygen demand (COD) and monitoring the COD of the samples obtained in the photocatalytic experiment (Figure 2.12). The COD is commonly used to estimate the amount of organic matter present in water. It is expressed in milligram per litre (mgl-1) of oxygen required to completely oxidize the organic matter. It is the measurement of consumption of oxygen by the organic pollutants for their complete chemical oxidation under specific conditions of oxidizing agent, temperature and time.

87 Chapter 2

Figure 2.12: COD and percentage removal efficiency during a photocatalytic degradation process as a function of duration of irradiation

The organic matter present in a sample is well oxidized by potassium dichromate

(K2Cr2O7) in presence of sulphuric acid (H2SO4), silver sulphate (Ag2SO4) and

mercury sulphate Hg(SO4) to mineralize it into CO2 and H2O. The dichromate consumed by the sample is equivalent to the amount of oxygen required to mineralize the organic matter and can be estimated titrimetrically.136 The percent COD removal efficiency can be obtained according to the following equation:

= × 100 (22) 퐼푛푖푡푖푎푙 퐶푂퐷−퐹푖푛푎푙 퐶푂퐷 푃푒푟푐푒푛푡 퐶푂퐷 푅푒푚표푣푎푙 퐸푓푓푖푐푖푒푛푐푦 퐼푛푖푡푖푎푙 퐶푂퐷

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Chapter 3 Synthesis, Characterization and Optimization of Photocatalytic

Activity of TiO2/ZrO2 Nanocomposite Heterostructures

Chapter 3 95

Synthesis, Characterization and Optimization of Photocatalytic Activity of * TiO2/ZrO2 Nanocomposite Heterostructures

3.1. Introduction The un-abating use of organic dyes in different industries is a serious problem in terms of environmental concerns. Extensive use of organic dyes (mostly azo dyes) in textile industries is a big threat to water bodies and severely affect water quality parameters. To overcome this issue, several treatment methods, such as, Chemical Oxidation, Wet Oxidation, Biological Treatment, Ozonolysis and Activated Carbon Adsorption have been proposed for the removal of organic pollutants from industrial effluents. Moreover, photocatalysis proved to be promising due to the use of easily available solar energy.1-3 In photocatalysis, the catalyst absorbs photons of light to undergo excitation of electrons from the valence band to the conduction band, generating electron-hole pairs. These electron hole pairs act as redox centres and readily initiate oxidation- reduction processes on catalyst surface, resulting in the degradation of toxic pollutants.4-6 To look for a feasible and efficient photocatalyst, metal oxide nanomaterials are being explored as semiconductor photocatalysts for the degradation of organic pollutants. These semiconductor catalysts are able to degrade pollutants into easily biodegradable compounds and eventually mineralize them into carbon dioxide and water. TiO2 has received much attention as a photocatalyst after Fujishima and Honda discovered the

phenomenon of photocatalytic splitting of water on a TiO2 electrode under ultraviolet 7,8 (UV) light. TiO2 gained priority due to its abundance in the geosphere, high photochemical stability, low cost, non-toxicity, biological inertness, reusability and the possibility of its activation by sunlight.9-11 However, a large band gap ( 3.2eV) in

TiO2 makes it responsive in the ultra-violet light which makes only about∼ 5% of the solar spectrum. Hence, efforts are being put to make it active into the visible region, in order to make the best use of solar energy.12 Moreover, pure titania is poor in quantum efficiency due to prevailing photocorrosion by rapid recombination of electrons and holes. To circumvent, development of reliable composite photocatalysts is a priority in today’s research.13 In order to effectively control photo-corrosion, the various attempts include encapsulation of

* Pirzada et al; Materials Science and Engineering B, 2015, 193, 137

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electron acceptors on functionalized semiconductors, immobilization of semiconductor photocatalyst in redox functionalized polymers and electrostatic association of electron acceptors at the semiconductor surface. The efficiency of photocatalyst is considerably enhanced by coupling with a foreign metal oxide. Usually, the coupling of two semiconductors can diminish the recombination probability and also increase stability of photoactive crystalline phase.14-18 Transition metal ion doping is one approach for acquiring a visible response by introducing intermediate impurity energy levels.19,20 Lin et al. reported that Sn

substitution for Ti in rutile TiO2 leads to an increase in its photocatalytic activity upto 15 times for the oxidation of acetone.21 Nagaveni et al. proposed that incorporating

several metal ions (W, V, Ce, Zr, Fe, and Cu) in TiO2 shows higher photocatalytic

activity than pure TiO2. Thus, doping transition metal into TiO2 should be an efficient method to obtain a preferable photocatalyst.21,22

ZrO2 acts as an n-type semiconductor with comparable physico-chemical properties to

TiO2, and is thus expected to be efficient in photodegradation application when 23-25 coupled with TiO2. It has a wide band gap (~5.0eV) with more-negative (-1.0V vs. NHE) and more-positive (4.0V vs. NHE) reducing potentials in its conduction and valence bands respectively, with 4d states above the conduction-band minimum of 26,27 TiO2. ZrO2 and ZrO2–TiO2 binary oxide catalysts have been investigated for their photocatalytic properties with organic pollutants, especially for the remediation of organic dye effluents. However, more information on the mechanisms behind the reactions is necessary to improve the efficiency. It has been reported that the addition

of small amounts of ZrO2 into TiO2 can decrease the crystallite size of TiO2 due to the dissimilar nuclei and coordination geometry. In addition to this, the surface acidity in

the form of OH groups is also enhanced in binary oxide ZrO2–TiO2 catalysts. The OH group is trapped by the holes and suppresses the recombination process to increase the 28,29 quantum yield.

It is also demonstrated that incorporation of ZrO2 in TiO2 influences its dielectric 30 properties which may affect its photocatalytic activity. ZrO2 acts as a hardener (acceptor) which gives higher conductivity, reduces dielectric constant, increases mechanical quality factor and aging effect. The decrease in dielectric constant in the binary oxide can affect the polarizability and adsorption characteristics.

Among the various methods proposed for the preparation of TiO2, the Sol-gel method has received an overwhelming attention from the recent past. Zhang et al. synthesized Chapter 3 97

fine ZrO2 doped TiO2 nanoparticles by the sol-gel method using both aqueous and 13 non-aqueous solvents. Kambur et al. also prepared Zr doped TiO2 by sol-gel and compared it with other methods.29 Pfleiderer et al. studied the crystallization 31 behaviour of ZrO2 doped TiO2 to optimize the extent of doping. The use of the sol- gel process is believed to have several advantages viz; good homogeneity and ease of composition control, low processing temperature, large area coating, low equipment cost and good optical properties.

In the present study, ZrO2 coupled TiO2 nanocomposites were synthesized via a hybrid sol–gel process. All the reactions were carried out in a mixture containing both aqueous and non-aqueous protic solvents. The gel formed was compact and easy to dry. In this method, we obtained a crystalline and homogeneous photocatalyst with good purity and stability. The low temperature requirement in this method is feasible for the good coupling effects between the mixed oxides. The photocatalytic activity was evaluated by decolorization of an azo-dye, Ponceau BS, which is being used to dye wool, silk, cotton, paper and also as a plasma stain instead of acid fuchsin in Masson’s trichome. The period of complete decolorization of the azo-dye was relatively shorter which signifies the efficiency and importance of the synthesized

photocatalyst. The effect of ZrO2 addition on structural, optical and photocatalytic properties was thoroughly studied and compared to attain an ideal photocatalyst.

3.2. Materials and Methods 3.2.1. Materials

Ti(PrOi)4 (TTIP, 98%) and Ponceau BS (PBS) were obtained from Sigma-Aldrich

India. Zirconyl chloride (ZrOCl2, 99%) was obtained from Merck India. TiO2 P25 (a mixture of 70% anatase and 30% rutile, surface area: 50±5m2g-1, primary particle size of ca. 22nm) was purchased from Degussa (Germany). All other chemicals were of analytical grade obtained from Merck India and were used without any further purification.

3.2.2. Synthesis of TiO2-ZrO2 nanocomposites

TiO2-ZrO2 nanocomposites were synthesized using a simple and modified sol-gel 32 method. In a typical synthesis process, a pre-determined amount of ZrOCl2 was dissolved in 50mL of methanol and stirred for 30min to form a homogenous solution. 50mL methanolic solution of TTIP (0.3M) was then added slowly while the mixture was vigorously stirred until the reaction was complete to form a translucent gel. The

98 Chapter 3

pH of the reaction mixture was maintained at 2.0 using 0.5M HCl aqueous solution. The as synthesized gel was dried at 80ºC to obtain amorphous white solid and then calcinated at 450ºC for 4h to attain crystalline phase nanocomposites. Molar

percentage ratio of Zr/Ti was varied by varying the amount of ZrOCl2. As a result, a series of samples were prepared with Zr/Ti molar ratio of 3%, 6%, 9%, 12%, and 15%. The obtained samples were designated as TMZ-a, TMZ-b, TMZ-c, TMZ-d, and

TMZ-e respectively. Pure TiO2 was also synthesized by the same procedure, without

adding the ZrOCl2 and was designated as TM.

3.2.3. Catalyst characterization The crystal structure of the composite was analysed with powder X-ray diffraction (XRD, Miniflex-TM II Benchtop, Rigaku Cooperation, Tokyo, Japan).The size and surface morphology was characterized by Scanning Electron Microscopy (JEOL, JSM6510LV) and Transmission Electron Microscopy (JEOL, JEM2100). The UV–vis absorption spectra were obtained in the range of 300–800nm by UV-vis Spectrophotometer (Shimadzu UV-1601). Photoluminescence of the catalyst was obtained using photoluminescence spectroscopy. Thermal stability of the catalyst was analysed by DTA/TGA and impedance of the catalyst was analysed by Frequency Dependent Impedance Spectroscopy in the frequency range of 75kHz to 5MHz using an LCR meter (Agilent-4285A).

3.2.4. Evaluation of Photocatalytic Activity The photocatalytic activity of the nanocomposite was assessed by monitoring the decolorization of an azo-dye Ponceau BS (Scheme 3.1) in presence of UV light. The photocatalytic tests were performed in an immersion well photoreactor (consisting of inner and outer jacket) made of Pyrex glass equipped with a magnetic bar, a water circulating jacket and an opening for molecular oxygen. Irradiations were carried out using 125W medium pressure mercury lamps (Philips). The light intensity, as measured by UV-light intensity detector (Lutron UV-340), was found to be in the range of 1.86–1.88mWcm-2. Before irradiation experiments, 180mL of the dye solution of desired concentration containing the appropriate quantity of the catalyst (1gL-1) was magnetically stirred, while the solution was purged continuously with atmospheric oxygen for at least 20min in the dark to attain adsorption–desorption equilibrium between dye and catalyst surface. Afterwards first sample (at 0min) was taken out and then irradiation was started. During irradiation experiments, samples of Chapter 3 99

5mL were withdrawn at specific time intervals, centrifuged and the supernatant was subsequently analyzed. The reaction temperature was kept constant at 20±0.3oC using refrigerated circulating liquid bath. The change in absorbance of the dye was followed

at its lmax (505nm) as a function of irradiation time. The observed absorbance is proportional to Beer-Lambert Law in the range of studied dye concentration. The concentration of dye was calculated by standard calibration curve obtained from the absorbance of the dye at different known concentrations. The photocatalytic experiments were repeated three times in order to check the reproducibility of the experimental results. The accuracy of the optical density was found to be within ±5%.

Scheme 3.1: Chemical structure of Ponceau BS azo dye

3.3. Results and Discussion 3.3.1. Crystallographic Analysis For crystallographic studies, XRD patterns of all the samples were compared. All samples were calcinated at 450oC for 4h before the XRD analysis. Tetragonal nature of the samples was confirmed by calculating the lattice cell parameters (Table 3.1).

Table 3.1: Lattice Cell parameters as obtained by PowderX (unrefined) Sample Radial Parameter Angular Parameter TiO2 (TM) a=b=7.89, c=9.45 α=β=ν= 90 TMZ-a a=b=10.76,c=3.14 α=β=ν= 90 TMZ-b a=b=7.71, c=10.67 α=β=ν= 90 TMZ-c a=b=3.63, c=7.11 α=β=ν= 90 TMZ-d a=b=3.63, c=7.11 α=β=ν= 90 TMZ-e a=b=4.09, c=2.41 α=β=ν= 90

As shown in Figure 3.1, pure TiO2 shows mixed peaks corresponding to both anatase and rutile phases.

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Figure 3.1. XRD patterns of TiO2/ZrO2 samples (TMZ-a, TMZ-b, TMZ-c, TMZ-d, TMZ-e) and pure

TiO2 (TM)

The XRD patterns of TMZ-a and TMZ-b show the presence of anatase phase only whereas that of TMZ(c-e) indicates anatase with substantial appearance of tetragonal

ZrO2 (t-ZrO2) peaks. This indicates the probable incorporation of Zr into the TiO2

lattice. When Zr is added to TiO2 , the more photo-active anatase phase starts stabilizing over rutile upto a certain molar percentage of Zr.33 Since, Zr4+ is more 4+ electropositive than Ti , the electronic cloud is loosely held between the TiO2 and

ZrO2 phases favouring the formation of less dense anatase phase. In other words, the tight packing arrangements required for rutile phase formation is fully suppressed by

the addition of ZrOCl2. ZrOCl2 adopts a tetrameric structure, consisting of the cation 8+ [Zr4(OH)8] and chloride ions in protic solvents which enhances the polarity of the solvent, thus facilitating the formation of anatase phase only.34 The existence of the distinguished peaks at 2θ of about 27.5, 36.0, 41.2, 44.1, 54.2, 56.7, 64.2,and 69.0, corresponding to indices (110), (101), (111), (210), (211), (220), (310), and (301) diffraction planes, respectively, indicate the formation of the rutile 35 phases in pure TiO2 (Figure 3.2). Chapter 3 101

Figure 3.2: Designation of peaks in XRD spectrum of pure TiO2 (TM)

With increase in ZrO2 loading beyond a certain limit, t-ZrO2 peaks appear as evident from increasing peak intensities at 37.2, 44.1 64.2 and 77.5.36 The average crystallite size was calculated by applying Scherrer formula on the anatase 101 diffraction peaks by the following formula:

D= kλ/ βCos θ (1) D is the crystallite size, λ is the X-ray wavelength (1.54Ǻ), β is the full width at half maximum (FWHM), k is equal to 0.89 and θ is the diffraction angle. Results show a gradual decrease in crystallite size from TMZ-a to TMZ-b which confirms that

incorporation of Zr inhibits crystal growth in TiO2. This phenomenon is probably due to the existence of Ti–O–Zr in the mixed oxides, inhibiting the growth of anatase 37 crystals by providing dissimilar boundaries. With further increase in ZrO2 content,

however, there is an increase in crystallite size which may be due to increase in t-ZrO2 phase. The crystallite size obtained were 8.85nm (TMZ-a), 7.53nm (TMZ-b), 33.91nm (TMZ-c), 33.92nm (TMZ-d), 33.92nm (TMZ-e) and 15.60nm (TM).

3.3.2. Microscopic Analysis The surface morphology and the shape and size of the nanocomposites sample were determined by the SEM and TEM analyses. Figure 3.3 shows four random view micrographs to emphasize the irregularity and sphericity of the particles in SEM.

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Figure 3.3: SEM images of TiO2/ZrO2 nanocomposite (TMZ-b) showing irregularity and sphericity of nanoparticles

TEM (Figure 3.4) shows aggregated spherical nanoparticles of average size of 10.5nm in TMZ-b. The intragranular location of both oxide phases could provide an intimate contact between them which improves photocatalytic activity of the catalyst.38,39

Figure 3.4: TEM images of TiO2/ZrO2 nanocomposite (TMZ-b) showing the shape and size of nanoparticles Chapter 3 103

Figure 3.5 shows the Electron Dispersive Spectrum (EDS) of TMZ-b as a representative one. The molar percentage ratio of Zr/Ti was found to be 3.038, 5.797, 9.132, 12.045, 13.603 in TMZ-a, TMZ-b, TMZ-c, TMZ-d, and TMZ-e respectively which is in good agreement with the percentage ratios taken during synthesis and make evident the high integration of Zr atoms in the network.

Figure 3.5: EDS spectrum of TiO2/ZrO2 nanocomposite (TMZ-b) with quantification of atomic percentage

3.3.3. Thermal Analysis As shown in Figure 3.6, TGA curve depicts a mere weight loss of 6.07% upto 400oC which may be due to the removal of water molecules or hydroxyl groups and other organic moieties. At higher temperatures, upto 1000oC, no considerable loss in mass was observed, showing efficient thermal stability of the catalyst.

Figure 3.6: TGA and DTA curves of TMZ-b on heat treatment upto 1200oC

DTA results are in accordance with the TGA showing endothermic peak around 76oC corresponding to removal of loosely bound water molecules.40 The broad convex endothermic peak starting around 200oC shall be due to slow transformation of

104 Chapter 3

anatase to rutile phase.41 After 600oC, anatase phase starts quickly converting to rutile phase. The endothermic peak near 207°C is ascribed to the condensation between Ti- 42,43 4+ 4+ Cl and Ti-(OC3H7). Since, Zr is larger in size and more electropositive than Ti , the lattice can exhibit better bonding property due to electrovalent behaviour and thus

higher thermal stability than pure TiO2.

3.3.4. Dielectric Properties The impedance study of the nanocomposite indicated that dielectric constant

decreases with increase in ZrO2 loading (Figure 3.7).

Figure 3.7: Variation of dielectric constant with increase in ZrO2 amount

It implies that ZrO2 acts as hardener (acceptor) which gives higher conductivity, reduces dielectric constant and increases mechanical quality factor and aging effect.30 There is a gradual decrease in dielectric constant in TMZ-a and TMZ-b, but an abrupt decrease resulted in TMZ-c, TMZ-d and TMZ-e. This may be attributed to the

breakdown of dielectric properties on excessive loading of ZrO2, leading to conducting characteristics in the lattice. This may increase the recombination rate due to decrease in polarizability and may be responsible for decrease in photocatalytic

activity at higher ZrO2 loading. The large decrease in dielectric constant of the catalyst may also decrease the adsorption capacity of the catalyst leading to the inefficient interaction of the dye molecules on the catalyst surface.

3.3.5. UV-Visible Absorption

The effect of ZrO2 on shift in absorption edge was determined on the basis of UV-vis DRS spectra of the different samples. Kubelka-Munk plots were obtained from the Diffuse Reflectance Spectra to determine the variation in band gap. Chapter 3 105

The band gap energy of the semiconductor was determined by the following expression:

(hv.α) = (Ahv – Eg)n\2 (2)

Since α is proportional to Kubelka–Munk function F(R), the expression becomes:

hv. F(R) = (Ahv – Eg) n\2 (3)

Where, v is the light frequency, F(R) is the Kubelka–Munk function, A is the

proportionality constant and Eg is the band gap energy. The value of n is determined by the type of optical transition (n = 1 for direct transition and n = 4 for indirect 44 transition). The value of n for TiO2 and ZrO2 is 4, since they belong to indirect band 1/2 gap semiconductors. The Eg of TiO2 was determined from the plot of (F(R).hv)

versus hv (Figure 3.8) and was elicited to be 3.22eV. Accordingly, the Eg of ZrO2 was found to be 4.6eV from the plot of (F(R).hv)1/2 versus hv.

Figure 3.8: Variation in band gap as estimated from the UV-DRS spectra, obtained by plotting Kubelka-Munk function, F(R) Vs energy, hv

The valence band edge position of TiO2/ZrO2 heterostructure was estimated according to the concepts of electronegativity. Herein, the electro-negativity of an atom is the arithmetic mean of the atomic electron affinity and the first ionization energy. The valence band potentials of a semiconductor at the point of zero charge can be calculated by the following empirical equation:45,46

c EVB =X−E +0.5Eg (4)

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TiO2 and ZrO2 are ca. 5.85 and 5.84 eV, respectively. The top of the valence band

EVB of TiO2 and ZrO2 were calculated to be 2.96eV/NHE and 3.64eV/NHE

respectively. Moreover, conduction band edge potential ECB can be determined by:

ECB = EVB − Eg (5)

Thus, the calculated ECB for TiO2 and ZrO2 are -0.26eV/NHE and -1.96eV/NHE, respectively.

As shown in Figure 3.8, the limited addition of ZrO2 into the TiO2 lattice leads to slight increase in band gap. However, a dopant energy level is formed between valence band maximum (VBM) and conduction band minimum (CBM), which mainly acts as a shallow acceptor with a wide area of distribution and enhances absorption in the longer wavelength region. This energy level produces electron capture centres, and helps the separation of electrons and holes.47 It is well known that the conduction and valence bands in un-doped anatase are mainly composed of Titanium 3d and Oxygen 2p states. However, the conduction band of the co-doped anatase is mainly composed of Titanium 3d and Zirconium 4d states and the valence band mainly of Oxygen 2p and Zirconium 4d states. In the case

of ZrO2, the Oxygen 2p band is stabilized more than in TiO2 and hence, there is an increase in the band gap. The presence of metal ions does not modify the position of the valence band edge of anatase but it introduces new energy levels of the transition

metal ions into the band gap of TiO2.

Thus, the excessive addition of ZrO2 creates quasi-continuous energy levels between VBM and CBM, decreasing the energy band gap. The band gap varied from 3.220eV in Degussa P25 to 3.327eV in TMZ-b.

3.3.6. Photoluminescence Study The activity of a photocatalyst is largely affected by the recombination of electrons and holes. The electron–hole recombination dissipates energy in the form of fluorescence emission whose intensity is directly proportional to the recombination rate.48 On applying an ultraviolet light of 300nm wavelength as an excitation source, Chapter 3 107

the fluorescence emission spectra of TiO2/ZrO2 composites were obtained as shown in Figure 3.9.

Figure 3.9: Fluorescence emission spectra of TiO2/ZrO2 nanocomposites and pure TiO2 (TM)

The extinction fluorescence intensity of these samples follow the order: pure TiO2

(TM) > TMZ-e > TMZ-d > TMZ-c > TMZ-a > TMZ-b which suggests that TiO2

coupled with a certain amount of ZrO2 may increase the lifetime of electrons on the conduction band and diminish the radiative recombination process of electrons and holes. It implies that with a small amount of ZrO2 doping, upto 6% as in TMZ-b, there is increase in band gap and decrease in recombination due to the formation of

adequate trapping sites and lattice defects. With further increase in ZrO2 content, the fluorescence intensity increases indicating faster recombination rate which may be

attributed to inefficient diffusion of Zr into TiO2 lattice and subsequent decrease in 49 band gap. Hence, an intermediate doping quantity (6% ZrO2, in this case) can result in the fabrication of an ideal photocatalyst for photocatalytic activity.

3.3.7. Photocatalytic Activity and Kinetic Study The photocatalytic activity of the nanocomposite photocatalysts was studied by monitoring photo-decolorization of an azo-dye, PBS, in the presence of UV light. Figure 3.10(a) shows decrease in absorption intensity of Ponceau BS at its lmax 505nm as a function of irradiation time in the presence of nanocomposite photocatalyst, TMZ-b.

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Figure 3.10: Kinetics of Photocatalytic decolorization of Ponceau BS in the presence of pure TiO2 and

TiO2/ZrO2 nanocomposite. (a) Decrease in absorption intensity of PBS at its λmax 505nm at different

irradiation time intervals in the presence of TMZ-b under constant stirring and bubbling of atmospheric oxygen. Experimental conditions: [PBS] = 0.1 mM, Temp. = 20±0.3oC, V = 180 mL, Photocatalyst -1 TiO2/ZrO2 = 1 gL , immersion well photo reactor made of Pyrex, Light source: 125W medium pressure Hg lamp. Light intensity (1.86 – 1.88 mW/cm2). Irradiation Time = 27 min. (b) Change in

concentration (Ct/C0) of PBS as a function of irradiation time in the absence and presence of catalyst

TiO2/ZrO2, Pure TiO2 and Degussa P25. (c) Plot of ln (C0/Ct) = f(t). (d) Cyclic runs of TiO2/ZrO2 (TMZ- b) for the decolorization of Ponceau BS. (e) Samples showing decrease in Ponceau BS colour

at different irradiation time in the presence of TiO2/ZrO2 (TMZ-b)

The photocatalytic activities of TiO2/ZrO2 composites were compared with

commercially available TiO2 Degussa P25 which is reported to be one of the efficient photocatalysts. The catalyst dosage was optimized by irradiating the aqueous solution -1 -1 of the dye with TiO2/ZrO2 catalyst ranging from 0.2gL to 2.5gL by keeping other experimental parameters constant. The decolorization rate was found to increase linearly with increase in catalyst dosage from 0.2gL-1 to 1gL-1 depicting a typical heterogeneous photocatalytic regime and a further increase in catalyst amount lead to a decrease in decolorization. Blank experiments were also carried out by irradiating the aqueous solution of the dye derivative in absence of the photocatalyst and stirring the dye solution with catalyst Chapter 3 109

under dark condition. Analysis of the samples in both cases did not show any appreciable loss of the dye indicating that the irradiation or adsorption alone is not capable of decolorizing the dye (Figure 3.10(b)). The above results demonstrated that the photocatalytic experiments occurred in a pure photocatalytic regime where photolytic processes can be neglected. Various researchers have reported that photocatalytic decolorization of most dyes follow Langmuir-Hinshelwood kinetic model,50,51 which is commonly expressed as:

dC kKC -= (6) dt1+ KC

Where k is the reaction rate constant (mMmin-1); K is the adsorption coefficient of the reactant (mM-1); and C is the reactant concentration (mM).When the concentration C is very small, KC is negligible with respect to unity so that Equation (6) can be simplified to an apparent pseudo-first-order kinetics.52

dC -=kKC = k C (7) dt app

æöCo ln ç÷= ktapp (8) èøCt -1 Where kapp is the apparent pseudo-first-order rate constant (min ).

For our experimental conditions, data are in good agreement with pseudo first-order reaction as depicted by plotting ln(C0/Ct) versus irradiation time as shown in Figure 3.10(c).The correlation constant for the fitted lines was calculated to be R2= 0.99 for all the experiments.

The percentage decolorization of the dye in the presence of pure TiO2 and TiO2/ZrO2 nanocomposites follow the order: TMZ-b (99.3%) > TMZ-a (97.8%) > TMZ-c (93.6%) > TMZ-d (88.4%) > TMZ-e (75.5%) > TM (24.45%). The corresponding -1 -1 values of rate constant (kapp) were found to be 0.1810min > 0.1397min > 0.0984min-1 > 0.0799min-1 > 0.0520min-1 > 0.0107min-1 respectively. Figure 3.10(e) depicts the decrease in Ponceau BS colour in the presence of TMZ-b catalyst at different irradiation times.

The enhancement of photocatalytic activity in TiO2/ZrO2 composite can be explained as follows. It has been stated that the doping metal atoms possibly cause the formation

of new phases dispersed into TiO2, temporarily trapping the photogenerated charge

110 Chapter 3

carriers and inhibiting the recombination of photo-induced electron–hole pairs when the electron–hole pairs migrate from the inside of the photocatalyst to the surface.53 However, an excess of the defects affect the charge recombination rate inversely. As

reported earlier, electron and hole separation may take place between TiO2 and ZrO2

in the binary oxide, since the energy level of TiO2, both for valence band and

conduction band, correspond well within the band gap of ZrO2 (Figure 3.11). When the electrons are excited from both catalysts, most of the electrons from the CB

of ZrO2 automatically drift to the CB of TiO2 from the thermodynamic consideration,

and thereby the electron-hole pair recombination may be inhibited in ZrO2 coupled

TiO2. Thus, there occurs enhancement in charge separation between electrons and 54 holes and a lot of holes are captured to induce oxidation. Since the amount of ZrO2

is very small as compared to TiO2 in all these samples, a small number of electrons

will be excited to CB of ZrO2 and all will be drifted to CB of TiO2 leading to a strong 29 + oxidizing centre on ZrO2 surface. Hence, the positive hole centres (h ) are trapped in

the VB of ZrO2 which induce oxidation of the dye molecule by the formation of various reactive oxygen species.

Figure 3.11: Schematic diagram of electron–hole pair separation and the possible reaction mechanism

over TiO2/ ZrO2 photocatalyst under UV light irradiation Chapter 3 111

- On the other hand, the electron centres (e ) are created in the CB of TiO2 inducing the reductive processes. The detailed mechanism can be outlined in the form of the following reaction scheme:

_ + (9) TiO2/ZrO2 + hv TiO2/ZrO2 (e + h ) _ . _ TiO2 (e ) + O2 TiO2 + O2 (10) _ . _ + TiO2 (e )+ O2 + 2H TiO2 + H2O2 (11) _ _ . (12 ) TiO2 (e ) + H2O2 TiO2 + OH + OH _ _ . . (13) O2 + H2O2 OH + OH + O2 _ . + . O2 + H HO2 (14 ) _ _ . (15 ) TiO2 (e ) + HO2 TiO2 + HO2 _ + HO2 + H H2O2 (16 ) + . + ZrO2 (h ) + H2Oads ZrO2 + OHads + H (17 )

+ + (18 ) ZrO2 (h ) + 2H2Oads ZrO2 + 2H + H2O2 _ + . (19 ) ZrO2 (h ) + OHads ZrO2 + OHads . _ . . + PBS O2/ H2O2/ OH/ HO2/ H CO2 + H2O + Nitrates (20) + Sulphates

Scheme 3.2: Series of reactions occurring during the photocatalytic degradation over TiO2/ZrO2 nanocomposites

The kinetic results revealed that TMZ-b has highest activity and almost completely decolorizes the solution in a period of only 27min. The highest photoactivity of TMZ- b can be the result of multiple factors. TMZ-b contains pure anatase phase crystal structure which is feasible for photocatalytic activity. Also, due to the largest band gap and appropriate defects and trapping sites, it shows least recombination and photo-corrosion. The higher activity of the TMZ-b can also be attributed to the excessive generation of •OH radicals and at this optimum concentration, the surface barrier becomes higher and the space charge region is extended, leading to an efficient

separation of electrons and holes. For ZrO2 concentrations higher than ~6%, the space charge region becomes very narrow due to abrupt decrease in dielectric constant and

the penetration depth of light into TiO2 greatly exceeds the thickness of the space charged layer, which increases the recombination rate of electrons and holes. The

112 Chapter 3

concentration of the dopant ions becomes optimum when the thickness of the space charge layer equals the light penetration depth. In Kröger and Vink notation, the probable chemical reaction can be represented in a favourable way.

− ZrO2 ZrTi' + VÖ + Oo+ ½ O2 + 2e (21)

where ZrTi' is the Zr ion at a Ti lattice site with a single charge deficiency, VÖ is a

doubly ionized oxygen vacancy, Oo is an oxygen ion in the normal lattice site where the concentration of the intrinsic defect (VÖ) becomes equal to the concentration of the extrinsic impurity.4 The lower activity of TMZ-c, TMZ-d and TMZ-e can be attributed to increase in t-

ZrO2 phase in them (Figure 3.1) which leads to enhanced recombination and hindered

absorption of light due to surface accumulation on TiO2 phases. It is generally accepted that a larger band gap corresponds to more powerful redox 55 ability. Further, with controlled addition of ZrO2, there is formation of lattice defects which create oxygen vacancies. The resulted oxygen vacancies trap the photogenerated holes and prolong their life which enhances the photoactivity.56 Any factor that suppresses the electron-hole recombination, therefore, enhances the photocatalytic activity. The stability and recyclability of any photocatalyst under irradiation is an important issue related to its practical and economical application. In order to test the stability

and recyclability of TiO2/ZrO2 nanocomposite, experiments were carried out under the identical conditions as applied for the evaluation of photocatalytic activity of different samples for the decolorization of Ponceau BS. The decolorization rate of Ponceau BS in the presence of catalyst TMZ-b obtained after different recycling runs as a function of irradiation time is shown in Figure 3.10(d). Results indicate that after 1st recycle, the catalyst showed almost similar photocatalytic efficiency as shown by fresh catalyst. The catalytic efficiency slightly decreased in 2nd and 3rd recycle and around 32% less decolorization was observed during 4th recycle. The observations inferred that there was a small decrease in photocatalytic efficiency and the chemical properties hardly changed after reuse. This small decrease in efficiency is due to partial deactivation of active sites on the catalyst surface. On this basis, it was clear that photocatalyst can be used repeatedly with a low decrease in adsorption capacity. Chapter 3 113

3.4. Conclusion

ZrO2 coupled TiO2 photocatalytic nanoparticles were synthesized via a hybrid sol–gel method followed by a suitable calcination treatment. Tetragonal structure of

TiO2/ZrO2 nanocomposite particles with stabilized anatase phase was confirmed by

XRD studies. XRD reveals that anatase phase stabilizes on Zr incorporation into TiO2 lattice. SEM analysis shows micrographs with irregular and sharp edged particles. TEM analysis shows spherical particles of average diameter 10.5nm. The synthesized

TiO2/ZrO2 nanocomposite exhibits unique optical properties as the band gap increases on Zr addition but, incorporation of intermediate energy levels expands its absorption edge into the visible light region. Results showed a considerable decrease in recombination rate on ZrO2 addition and impedance study showed a significant decrease in dielectric characteristics on ZrO2 addition. The TiO2/ZrO2 composites show an efficient photocatalytic activity for degradation of the organic pollutants such

as aqueous PBS. The optimum loading amount of ZrO2 on TiO2 was 6.0 molar%

which showed higher photocatalytic activity than that of the pure TiO2 (TM) and commercially available TiO2 (Degussa P25). Almost complete degradation of PBS was attained in a short span of 27 minutes. The main cause of higher activity at 6%

molar ZrO2 doping is the increase in anatase phase TiO2 and the creation of appropriate trapping centres which inhibit recombination. Moreover, due to large band gap it has enhanced redox capability and photocatalytic activity. The recyclability experiments depict small decrease in photocatalytic activity on repetitive use of these photocatalysts.

114 Chapter 3

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(23) V. Vishwanathan, H.-S. Roh, J.-W. Kim and K.-W. Jun, Catalysis Letters, 2004, 96, 23. (24) K.V.R. Chary, G.V. Sagar, D. Naresh, K.K. Seela and B. Sridhar, The Journal of Physical Chemistry B, 2005, 109, 9437. (25) X. Fu, L.A. Clark, Q. Yang and M.A. Anderson, Environmental Science & Technology, 1996, 30, 647. (26) A.V. Emeline, A.V. Panasuk, N. Sheremetyeva and N. Serpone, The Journal of Physical Chemistry B, 2005, 109, 2785. (27) C. Karunakaran and S. Senthilvelan, Journal of Molecular Catalysis A: Chemical, 2005, 233, 1. (28) J.H. Schattka, D.G. Shchukin, J. Jia, M. Antonietti and R.A. Caruso, Chemistry of Materials, 2002, 14, 5103. (29) A. Kambur, G.S. Pozan and I. Boz, Applied Catalysis B: Environmental, 2012, 115-116, 149. (30) M. Aparna, T. Bhimasankaram, S.V. Suryanarayana, G. Prasad and G.S. Kumar, Bulletin of Materials Science, 2001, 24, 497. (31) S.J. Pfleiderer, D. Lützenkirchen-Hecht and R. Frahm, Journal of Sol-Gel Science and Technology, 2012, 64, 27. (32) A. Vioux, Chemistry of Materials, 1997, 9, 2292. (33) I. Justicia, P. Ordejón, G. Canto, J.L. Mozos, J. Fraxedas, G.A. Battiston, R. Gerbasi and A. Figueras, Advanced Materials, 2002, 14, 1399. (34) N. Venkatachalam, M. Palanichamy, B. Arabindoo and V. Murugesan, Journal of Molecular Catalysis A: Chemical, 2007, 266, 158. (35) S. Onsuratoom, S. Chavadej and T. Sreethawong, International Journal of Hydrogen Energy, 2011, 36, 5246. (36) M.R. Mohammadi and D.J. Fray, Sensors and Actuators B: Chemical, 2011, 155, 568. (37) J.C. Yu, J. Lin and R.W.M. Kwok, The Journal of Physical Chemistry B, 1998, 102, 5094. (38) D. Li and H. Haneda, Journal of Photochemistry and Photobiology A: Chemistry, 2003, 155, 171. (39) W. Fujita and K. Awaga, Journal of American Chemical Society, 1997, 119, 4563. (40) H. Zou and Y.S. Lin, Applied Catalysis A: General, 2004, 265, 35.

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Chapter 4 Efficient visible light photocatalytic activity and enhanced stability over

BiOBr/Cd(OH)2 Heterostructures

Chapter 4 117

Efficient visible light photocatalytic activity and enhanced stability over * BiOBr/Cd(OH)2 Heterostructures

4.1. Introduction Heterogeneous photocatalysis has evolved as a viable technology for the control of environmental pollution related issues and energy conversion.1 Till date various kinds of catalyst semiconductor materials, including metal oxides,2 sulphides,3 nitrides,4 and their mixed solid solutions5,6 have been tried as photocatalysts responsive to both the

UV and visible light wavelengths. Primarily, TiO2 received wide attention due to its excellent photocatalytic activity, chemical stability, and non-toxicity.7However, it has limited practical application due to large band gap ~3.2eV and rapid recombination of photogenerated charge carriers.8 In order to overcome the limitation and effectively control the photocorrosion, various attempts have been made in the past including encapsulation of electron acceptors on functionalized semiconductors, immobilization of semiconductor photocatalyst in redox functionalized polymers and electrostatic association of electron acceptors at the semiconductor surface. Transition metal ion doping is one approach for acquiring a visible light response by introducing intermediate impurity energy levels.9,10 The efficiency of photocatalyst is considerably enhanced by coupling with a foreign metal oxide.11,12 Currently, coupling of semiconductors with graphene is widely recognized to be a viable strategy to improve the photocatalytic performance for an electronically conductive 2D platform enabling the acceptance and shuttle of photogenerated electrons from band-gap-excitation of semiconductors.13,14,15 Recent studies have also

revealed that some physical and chemical properties of TiO2, such as light absorption, photocatalytic reactivity and selectivity etc., can be modulated by its defect disorder.16 Heterostructure construction between two different semiconductors has also been extensively applied in many fields including photocatalysis and solar energy conversion,17,18 because heterojunctions control behaviour of photogenerated charges, such as the direction of transportation, the distance for separation, and the recombination rate.19,20 Furthermore, the internal electric field built at a heterojunction interface can greatly decrease the charge-carrier recombination and increase their lifetimes, thus enhancing the photocatalytic activity.21

* Pirzada et al; New Journal of Chemistry, 2015, 39, 7153

118 Chapter 4

As a group of V–VI–VII semiconductors, bismuth oxyhalides are of immense importance due to their optical properties and promising industrial applications, such as in photocatalysis, ferroelectric materials, pigments etc.22,23 In particular, BiOBr has received wider interest because of its good visible light photocatalytic activity. Previous research reported BiOBr nanostructures with various architectures, such as nanoparticles,24 nanoplates,25 3D hierarchical structures, including flower-like microspheres and hollow flower-like microspheres.26,27 All were found to exhibit good catalytic activity. It crystallizes in the tetragonal matlockite (PbFCl) structure,

which comprises of a layer of [Bi2O2] slabs interleaved by double slabs of halogen atoms.28 These layered structures with partially distorted polyhedrons are beneficial to the charge transfer and photocatalytic properties.29 However, it is important to further improve the photocatalytic activity of the pure BiOBr for practical applications. One strategy is to decorate the host BiOBr with noble metals, which can increase the rate of transfer of the photo-induced charge carriers and enhance the photocatalytic activity.30 Another attractive strategy is through the combination of BiOBr with other semiconductor moieties to form 31 32 33 heterojunctions such as, C3N4 , AgBr , Bi2WO6 etc., to facilitate the separation of the electrons and holes and to improve photocatalytic activity.

In the present study, novel BiOBr/Cd(OH)2 heterostructures with efficient

photocatalytic activities have been synthesized. Cadmium hydroxide (Cd(OH)2) is a wide band gap (~3.2-3.5eV) semiconductor material and is not responsive to visible 34 light. Hence, our efforts were directed to sensitize the activity in Cd(OH)2 under

visible light. The energy levels of BiOBr and Cd(OH)2 materials are well-matched with a sign of overlapping, and therefore their combination to prepare

BiOBr/Cd(OH)2 heterostructures is thought to be a suitable method to separate the photogenerated charges. The presence of Bi shifts the absorption band edge in the visible region due to the formation of a hybridized valence band comprising of Bi 6s and O 2p states35 and facilitates oxidation reaction because of high mobility of photogenerated holes.36-38 To the best of our knowledge, there is no report on the fabrication of coupled

BiOBr/Cd(OH)2 heterostructures and their photocatalytic performances. Herein, we

report a low-temperature method to synthesize BiOBr/Cd(OH)2 heterostructures in one step. By simply tuning the Bi/Cd molar ratios, the morphology and the subsequent photochemical properties of the heterostructures were controlled. The Chapter 4 119

photocatalysts synthesized were studied for the degradation of potentially toxic and carcinogenic Rhodamine B dye under visible light irradiation.38 On the basis of experimental and theoretical results, a possible mechanism for the photocatalytic degradation by the BiOBr/Cd(OH)2 composites was proposed and corroborated by radical scavenger experiments and photoluminescence spectroscopy.

4.2. Materials and Methods 4.2.1. Synthesis of Photocatalyst All the reagents for synthesis and analyses were of analytical grade, procured from Sigma Aldrich and Merck India, and were used without further treatment or

modification. BiOBr/Cd(OH)2 heterostructures were synthesized by a facile 21 chemical bath method at ambient temperature. Typically, Cd(NO3)2.4H2O was added to 40 mL of deionized water containing a stoichiometric amount of NaOH (Cd/OH molar ratio of 1:2) with constant stirring resulting into a white precipitate. Then, a stoichiometric amount of NaBr was added, and the mixture was stirred for 30min at room temperature. Subsequently, different stoichiometric amounts of

Bi(NO3)3.5H2O were added to the resultant mixture. The Bi/Cd molar percentages were kept at 0, 30, 50, 70 (designated as CDH, BC-30, BC-50, and BC-70), respectively. After being stirred for another 30min at room temperature, the resulting mixtures were heated at 80oC for 2h on a water bath. Finally, the precipitates were collected, washed thoroughly with deionised water and ethanol, and dried at 50oC in air. For comparison, pure BiOBr was prepared by the

same procedure without the addition of Cd(NO3)2 .4H2O and NaOH.

4.2.2. Characterization The crystal structure of the nanocomposites was analysed by powder X-ray diffraction (PXRD, Miniflex-TM II Benchtop, Rigaku Cooperation, Tokyo, Japan). The surface morphology of the composites was characterized by Scanning Electron Microscopy (JEOL, JSM6510LV) and the heterojunction formation was analyzed by High Resolution Transmission Electron Microscopy (JEOL, JEM2100F) and Selected Area Electron Diffraction (SAED). The UV–vis absorption spectra were obtained in the range of 190–700nm by UV-vis Spectrophotometer (Shimadzu UV-1601). UV–vis Diffuse Reflectance Spectra (UV-DRS) were obtained using a Puxi, UV1901 spectrometer with BaSO4 as a reference, and the chemical compositions of the catalysts were examined by energy dispersive X-ray spectroscopy (EDS, JEOL,

120 Chapter 4

JSM6510LV). To study the recombination of photo induced charge carriers, photoluminescence spectra (PL, Hitachi F-7000, 280 nm) were measured. Cadmium ion leaching by the catalysts was analyzed by Atomic Absorption Spectroscopy (GBC- 932 Plus flame atomic absorption spectrometer, Australia).

4.2.3. Evaluation of Photocatalytic Activity The photocatalytic activity of the nanocomposites was assessed by monitoring decolorization of Rhodamine B (Scheme 4.1) in the presence of visible light. The photocatalytic experiments were carried out in an immersion well photoreactor (consisting of inner and outer jacket) made up of Pyrex glass, equipped with a magnetic bar, water circulating jacket and a passage for molecular oxygen.39 A detailed description of photoreactor is given in Chapter 2. Irradiations were carried out using a visible light halogen linear lamp (500W, 9500 Lumens). The reaction temperature was kept constant at 20±0.3oC using refrigerated circulating liquid bath. Before irradiation experiments, 300mL of the dye solution of appropriate concentration containing the desired quantity of the photocatalyst (1gL-1) and a predetermined amount of 2mM NaOH solution, was magnetically stirred, while the solution was continuously purged with atmospheric oxygen for at least 20min in the dark to attain adsorption–desorption equilibrium between the dye solution and the catalyst surface. Afterwards, first sample (at 0min) was taken out and the irradiation was started. During irradiation, samples of 5mL were withdrawn at regular time intervals, centrifuged and the supernatant was subsequently analyzed. The change in

absorbance of the dye aliquots was followed at its λmax (554nm) as a function of irradiation time. The observed absorbance spectra are in accordance with Beer- Lambert Law in the range of examined dye concentration. The concentration of dye was calculated by standard calibration curve obtained from the absorbance of the dye at different known concentrations. The photocatalytic experiments were repeated three times in order to check the reproducibility of the experimental results. The accuracy of the optical density was found to be within ±5%.39 The consistency in activity of the catalyst was analyzed by the recycling experiments. After the first attempt of the photocatalysis experiment, the catalyst was retrieved from the photoreactor and the aliquots by centrifugation. The retrieved catalyst was thoroughly washed with deionised water and distilled ethanol. The catalyst was dried at 40oC for 12h and then reused in the next cycle of the photocatalysis experiment. Chapter 4 121

Likewise, the experiment was repeated for a set of cycles to monitor the loss in efficiency of the catalyst after repetitive use.

Scheme 4.1: Chemical Structure of Rhodamine B dye

The photocatalyst concentration was optimized by a series of photocatalysis experiments. Higher concentrations of photocatalyst were thought to absorb more incident photons and produce more photogenerated charge carriers, but past a particular concentration, the particles suspended in the solution cause shielding and light scattering, affecting the light transmittance in solution. Moreover, the decreasing transmittance may enhance recombination as the photons could not be continuously injected onto the photocatalyst particles.40 Similarly the initial dye solution concentration has significant influence on the activity as it affects the light penetration into solution. This result was consistent with literature for the effect of initial concentration on photocatalysis.41 The degradation efficiency (%) was calculated as follows:

(%) = × 100 (1) 퐶표−퐶 퐷푒푔푟푎푑푎푡푖표푛 퐸푓푓푖푐푖푒푛푐푦 퐶표 Where C0 is the initial concentration of RhB, and C is the time-dependent concentration of dye upon irradiation. According to a simplified Langmuir– Hinshelwood (L–H) kinetic model, the following first order kinetic equation can be used to describe photocatalytic RhB degradation.42

= (2) 퐶0 푙푛 �퐶푡� 푘푎푝푝푡 Where C0 and C are the concentrations of dye in solution at times 0 and t respectively, −1 and kapp is the apparent first-order rate constant (min ).

122 Chapter 4

To determine the effect of resultant reactive oxygen species (ROS), various quencher species were added to the reaction system in the manner similar to the photocatalytic experiment. The dosage of quenchers was referred to the previous studies.43-45

4.3. Results and Discussion 4.3.1. Crystallographic Study

The powder XRD spectra for the as-prepared BiOBr/Cd(OH)2 nanocomposites are

shown in Figure 4.1. Characteristic diffraction peaks for Cd(OH)2 were detected at 2θ angles of 29.38◦, 35.12◦, 48.9◦, 52.2◦, and 55.98◦ and were attributed to the (1 0 0), (1 0

1), (1 0 2), (1 1 0) and (1 1 1) crystal planes of the hexagonal Cd(OH)2 crystal, respectively.46 The characteristic diffraction peaks for BiOBr were detected at 2θ angles of 25.2◦, 31.7◦, 32.2◦, 46.2◦and 57.1◦, and were attributed to the (1 0 1), (1 0 2), (1 1 0), (2 0 0), and (2 1 2) crystal planes of the BiOBr crystal, respectively.47

Figure 4.1: XRD pattern of the different samples

The composites prepared are considered to possess a highly crystalline nature as the diffraction peaks are relatively acute. In the diffraction patterns of the

BiOBr/Cd(OH)2 composites, both Cd(OH)2 and BiOBr were detected, indicating that the synthesized composite materials possess both the constituent phases. However, when the BiOBr content increased from 30mole% to 70mole%, the intensity of the Chapter 4 123

BiOBr peaks was increased, whereas the Cd(OH)2 peak intensities decreased, which

may be caused by the gradual covering of the Cd(OH)2 surface by the BiOBr.

4.3.2. Microscopic Analyses

The surface morphology of the BiOBr/Cd(OH)2 composite samples was studied by SEM (Figure 4.2). As shown in Figure 4.2(a-b), three dimensional (3D) BiOBr hierarchical microspheres (with diameters of 10-15μm) constructed by numerous petal like interlaced nanosheets were observed. The nanosheets are aligned from the sphere centre to the surface, and give a floral appearance. The microstructure of the Cd(OH)2 nanoparticles is shown in Figure 4.2(c-d). Agglomerates were observed, with sizes varying from 200-250nm, due to small size and large surface energy.

Figure 4.2(e-f) shows the SEM image of BC-50 sample. The Cd(OH)2 nanoparticles were observed to be in intimate proximity to the surface of BiOBr, resulting in effective coupling, which was thought to facilitate the charge transfer. In addition, the

Cd(OH)2 adhered to the BiOBr surface showed much smaller grain sizes than the pure

Cd(OH)2 nanoparticles as the BiOBr microspheres suppressed agglomeration of the

Cd(OH)2 nanoparticles.

Figure 4.2: SEM images: (a-b) flower like cluster of BiOBr, (c-d) Cd(OH)2 nanoparticles, (e-f) BC-50 nanocomposite

124 Chapter 4

The HRTEM images in Figure 4.3 show nanosheets of BiOBr (Figure 4.3(a)) and

Cd(OH)2 nanorods (Figure 4.3(b)). Figure 4.3(c-d) exhibits the HRTEM images of

the heterostructured BiOBr/ Cd(OH)2 nanocomposite (BC-50).

The Cd(OH)2 nanowire and BiOBr nanosheet are in proximity, indicating the intimate contact of the two. As depicted in Figure 4.3(e), the corresponding SAED pattern displays a spot pattern, indicating the diffused crystalline characteristic of the obtained nanocomposite. The clear lattice fringe with a spacing of 0.286nm and 0.261nm matched well with the tetragonal (110) and hexagonal (101) plane of BiOBr 48-51 and Cd(OH)2, respectively.

Figure 4.3: HRTEM images: (a) BiOBr, (b) Cd(OH)2 nanowires, (c-d) BC-50 nanocomposite , (e) SAED pattern of BC-50 and (f) Lattice pattern of the BC-50 composite Chapter 4 125

Interestingly, the intimate contact facilitates the heterojunction construction at the interface, which is beneficial to separate the photogenerated charge carriers, thereby improving the photocatalytic activity. In Energy Dispersive Spectroscopy (Figure 4.4), characteristic peaks associated with O, Br, Bi, Cd, and C were observed for BC-50.

Figure 4.4: The EDS spectrum obtained for 50% BiOBr/Cd(OH)2 sample, (BC-50).

The Cd and O peaks resulted due to Cd(OH)2 and Bi, Br, O peaks resulted due to BiOBr, respectively, and hence confirmed that the composite was composed of

BiOBr and Cd(OH)2. The peak of C atom observed in the spectrum could be attributed to the carbon present on the adhesive tape used to hold the sample.46

4.3.3. Optical Properties

The optical properties of the synthesized BiOBr, Cd(OH)2, and BiOBr/Cd(OH)2 samples were investigated using UV–visible Diffuse Reflectance Spectroscopy and the results are shown in Figure 4.5(c). According to the spectra obtained, the BiOBr sample exhibited strong absorption in

the wavelength below 450nm region of visible light, while the Cd(OH)2 sample exhibited strong absorption in the wavelength range below 300nm. The

BiOBr/Cd(OH)2 composite samples undergo red shift as compared with Cd(OH)2, and

this was attributed to BiOBr, being coupled with Cd(OH)2, and hence acting as a sensitizer to extend the optical response. The decreased band gap and formation of matching trapping sites resulted in good visible light response, appropriate redox centres and hence enhanced photocatalytic

126 Chapter 4

performance. These results confirmed that the efficient visible light absorption and

photocatalytic activity was caused by the synergistic effect of Cd(OH)2 and BiOBr,

and the content of BiOBr was thought to play a crucial role in the BiOBr/Cd(OH)2 composites.

Figure 4.5: Band gap calculation of (a) Cd(OH)2 and (b) BiOBr, (c) UV-DRS spectra of the various samples.

The band gap of the photocatalysts was calculated according to the formula:52

(hv.α) = (Ahv – Eg)n\2 (3)

Since α is proportional to Kubelka–Munk function F(R), the expression becomes

hv. F(R) = (Ahv – Eg) n\2 (4)

Where v is the light frequency, F(R) is the Kubelka–Munk function; A is the

proportionality constant and Eg is the band gap energy. The value of n is determined by the type of optical transition (n = 1 for direct transition and n = 4 for indirect 52 transition). The value of n for Cd(OH)2 and BiOBr is 1 and 4 respectively. The Eg of 2 Cd(OH)2 was determined from the plot of (F(R).hv) versus hv (Figure 4.5(a)) and

was elicited to be 3.35eV. Accordingly, the Eg of BiOBr was found to be 2.85eV from the plot of (F(R) hv)1/2 versus hv (Figure 4.5(b)). Chapter 4 127

The valence band edge position of BiOBr/Cd(OH)2 heterostructure at the point of zero charge can be calculated by the following empirical equation:53, 54

c EVB =X−E +0.5Eg (5)

BiOBr and Cd(OH)2 are ca. 6.18eV and 6.65eV, respectively. The top of the valence

band EVB of BiOBr and Cd(OH)2 were calculated to be 3.10eV/NHE and

3.82eV/NHE respectively. Moreover, conduction band edge potential ECB can be determined by:

ECB = EVB − Eg (6)

Thus, the calculated ECB for BiOBr and Cd(OH)2 are 0.25eV/NHE and 0.47eV/NHE, respectively.

4.3.4. Photocatalytic Activity The photocatalytic performance of the as prepared samples for the decolorization of RhB molecules under visible light irradiation was investigated in mild alkaline conditions, and the results are shown in Figure 4.6(a-e). In the control experiments, no decolorization of RhB was observed in the dark, and in the absence of photocatalyst respectively, indicating that RhB was stable and did not undergo a photolytic process. Moreover, the catalytic degradation was negligible, suggesting that a purely photocatalytic reaction mechanism took place.

The final decolorization of RhB was only 21.69% by pure Cd(OH)2 under visible light irradiation. The little efficiency was due to the large band gap and agglomeration of

Cd(OH)2 nanoparticles, reducing the specific surface areas and the visible light absorption. The photocatalysis test performed using pure BiOBr resulted in the decolorization of about 73.63% of RhB under visible light in 30min. As is shown in Figure 4.6(b), the

50% BiOBr/Cd(OH)2 (BC-50) composite catalysts exhibited the highest photocatalytic activities. The decolorization of RhB was 99.85%, which was about 4.60 and 1.35

times greater than that of Cd(OH)2 and BiOBr, respectively. The experimental data was found to fit well with the first order kinetic equation. Also from Figure 4.6(c),

128 Chapter 4

the BC-50 composite catalyst was found to exhibit the highest photodegradation −1 efficiency, where the kapp of BC-50 was 0.2255min , which was 28.5 and 5.1 times −1 −1 higher than Cd(OH)2 (0.0079min ) and BiOBr (0.0439min ) respectively. This suggested that BiOBr nanoparticles contributed to the higher redox potentials with well-aligned band-structures and heterostructure interfaces were favourable for the separation of electrons and holes, leading to an enhanced photocatalytic performance. However, with further increase of BiOBr content, the RhB decolorization rate

decreased and the 70% BiOBr/Cd(OH)2 composite exhibited only 50% decolorization.

Figure 4.6: Kinetics of Photocatalytic decolorization of RhB in the presence of pure BB, CDH and and

BiOBr/Cd(OH)2 heterostructures. (a) Change in absorption of RhB after regular intervals of light

irradiation in presence of BC-50 photocatalyst, (b) Change in concentration (Ct/Co) of RhB during its

decolorization in the presence of CDH, BB and BiOBr/Cd(OH)2 heterostructures, (c) ln(Co/Ct) versus irradiation time for decolorization of RhB in the presence of different catalysts, (d) Cycling runs of BC-50 for decolorization of RhB under visible light (e) Change in colour of RhB dye at regular intervals of irradiation in the presence of BC-50. Chapter 4 129

The effect of BiOBr loading amount on the photocatalytic activity was investigated, and the results are shown in Table 4.1. TABLE 4.1: Percentage decolorization and apparent rate constant by different samples

-1 Catalyst % Decolorization Kapp(min ) Blank 6.7 0.0023 CDH 21.69 0.0079 BC-70 39.58 0.0176 BB 73.63 0.0439 BC-30 98.31 0.1337 BC-50 99.85 0.2255

Although coupling Cd(OH)2 and BiOBr was beneficial for charge separation in the

BiOBr/Cd(OH)2 composite, excessive loading of BiOBr particles caused agglomeration and surface coverage of Cd(OH)2, hinder the transfer of charge, and 55 reduced the density of active sites on the Cd(OH)2 surface. The BC-50 composite was thought to cause an effective contact and separation of electrons and holes, resulting into an optimal photocatalytic activity. In order to validate its photocatalytic activity, it was subjected to the photocatalytic degradation of non-dye pollutant, p- chlorophenol (PCP). It was found that BC-50 showed efficient performance for PCP degradation, ruling out the exclusive dye sensitized photocatalysis by the catalyst

(Figure 4.7). The rate constant (Kapp) for the phenolic degradation was found to be 0.068min-1, 0.044min-1 and 0.0025min-1 for BC-50, BB and CDH respectively.

Figure 4.7: Kinetics of Photocatalytic degradation of p-chlorophenol in the presence of pure BB, CDH

and and BC-50 heterostructures. (a) Change in concentration (Ct/Co) of PCP during its degradation in the presence of CDH, BB and BC-50 (b) ln(Co/Ct) versus irradiation time for degradation of PCP in the presence of different catalysts.

130 Chapter 4

It is worth noting that the Cd(OH)2 and the BiOBr/Cd(OH)2 nanocomposite samples display superior photocatalytic performance under the alkaline condition. These

results clearly demonstrate that the photocatalytic activity of the BiOBr/Cd(OH)2 samples can be enhanced under basic conditions. In view of the practical applications, besides the efficiency, the stability and durability are also indispensable to photocatalysts. To evaluate the stability of the BC-50 composite catalyst, the photocatalytic activity was investigated in cycling runs and the results are shown in Figure 4.6(d). In our recycling experiments for RhB photo-

decolorization in the alkaline pH conditions, the BiOBr/Cd(OH)2 samples exhibited a minimal decrease in activity after four cycles. The stability of the structure and

properties ensures that BiOBr/Cd(OH)2 heterostructures can be used as efficient and stable photocatalysts under mild alkaline conditions. In this study, the possible reasons to explain the enhanced photocatalytic properties and structural stability of

BiOBr/Cd(OH)2 superstructures in the presence of NaOH is the different solubility

behaviour of Cd(OH)2 or BiOBr/Cd(OH)2 in the presence and absence of the base.

The Ksp of Cd(OH)2 is 5.27 in deionized water. In the presence of base, all Cd(OH)2

or BiOBr/Cd(OH)2 samples possess a much lower solubility, leading to their enhanced stability during the photocatalytic process. Further, it also checks the leaching of cadmium ions in the dye solution and hence nullifies its toxic effects as investigated by Atomic Absorption Spectroscopy (Figure 4.8). It can be seen that the concentration of Cd ions in the solution phase is lower at alkaline pH than at neutral pH.

Figure 4.8: Change in concentration of cadmium ions in dye solutions after regular intervals of irradiation as a result of leaching in the catalysts. Chapter 4 131

Further, in the photodegradation process, transport of the photogenerated holes is easier in the presence of a base than in the absence of a base. Currently, the

photoinduced electron transfer to the CB of Cd(OH)2 results in a charge separation, which hinders the recombination of charge carriers. Thus, the BiOBr/Cd(OH)2 photocatalyzed RhB degradation rate may be significantly accelerated via alkaline conditions.46

4.3.5. Possible Photocatalytic Mechanism 4.3.5.1. Role of Reactive Species The radical and hole trapping experiments (scavenger tests) with different scavenger molecules were carried out to elucidate the mechanism of photocatalytic degradation

of RhB under visible light irradiation over BiOBr/Cd(OH)2 nanocomposite. Generally, the reactive species such as, hydroxyl radicals (•OH), superoxide radical −• + anions (O2 ) and holes (h ) are expected to be involved in the photocatalytic dye decolorization processes. The roles of the reactive species were investigated through radical and hole trapping experiments and the results obtained are shown in Figure 4.9.

Figure 4.9: Effect of various scavengers on the photocatalytic activity of BC-50 for the decolorization of RhB.

The decolorization of RhB decreased slightly upon the addition of isopropyl alcohol (IPA, a hydroxyl radicals scavenger56), indicating that dissolved •OH radicals were

132 Chapter 4

not the dominant active species in this process. In contrast, the addition of disodium ethylenediaminetetraacetate (EDTA, a hole scavenger57) had a significantly negative effect on the degradation of RhB, confirming that the holes were the dominant active 58 −• species. Upon addition of benzoquinone (BQ), a scavenger to quench O2 , the kapp −• dropped remarkably, indicating that O2 radicals were another active species in the 59,60 reactive system. The electron scavenger, AgNO3, addition decreased the rate considerably, hence indicating electron mediated degradation mechanism. The substantial electron dependence depict that the electron transfer along the composite catalysts has role in superoxide radical formation. The above results demonstrated that the photocatalytic process was mainly governed −• by direct holes and O2 oxidation reactions while the role of free hydroxyl radicals were negligible.

4.3.5.2. Band gap structures and possible degradation mechanism Photocatalytic performance of a composite photocatalyst depends on its heterojunction interface, and the electronic structures,61 because the photocatalytic activity is closely related to the energy position of the conduction band (CB) and the valence band (VB), as well as to the mobility of the carriers. CB and VB could determine the oxidative and reductive ability of the catalyst, respectively, while the mobility of the charge carriers could determine the photocatalytic efficiency. For metal oxide photocatalysts, the VB consists of the 2p orbital of oxygen, but for bismuth-based semiconductors, the VB is a hybrid of the 2p orbital of oxygen and 6s orbital of Bi, and the CB is composed of the 6p orbital of Bi,29, 62 which possesses a highly reductive capability. The light energy utilization ratio and the effective separation of photogenerated 31 charge carriers are favourable for photocatalytic activity. The BiOBr/Cd(OH)2 composites exhibited a good light energy utilization ratio, which could photoexcite the charge carriers. It was reported that the well-matched overlapping band-structure in the composite could promote the separation of electrons and holes, thereby improving the photocatalytic activity.63

The band edge positions of BiOBr and Cd(OH)2 were estimated according to the 32 methods discussed above. These results confirmed that BiOBr and Cd(OH)2 possessed a good composite structure, which was favourable for the separation of the photogenerated carriers. On the basis of band gap structure of as-prepared Chapter 4 133

BiOBr/Cd(OH)2 and the effects of scavengers, possible pathways for the photocatalytic decolorization were proposed as follows.

In these samples, BiOBr is a p-type semiconductor, whereas Cd(OH)2 is an n-type semiconductor. The formation of the p–n junction could lead to an efficient electron– hole separation that has minimized the recombination of photoexcited electron–hole 64 pairs. The VB and CB of BiOBr (EVB= 3.10eV, ECB= 0.25eV); and Cd(OH)2 (EVB=

3.82eV, ECB= 0.47eV) were provided to clearly clarify the separation and transfer of

electron–hole pairs at the interface of the heterostructures. The Fermi level (EF) of n-

type Cd(OH)2 was close to the conduction band, whereas the EF of p-type BiOBr was

close to the valence band. When p-type BiOBr and n-type Cd(OH)2 formed p–n

junctions, the EF of Cd(OH)2 and BiOBr were aligned, and when the EF reached

equilibration, the internal electric field with the direction from n-type Cd(OH)2 to p- type BiOBr was built. As a result, the energy bands of Cd(OH)2 shifted downward

along with the EF, whereas those of BiOBr shifted upward and the newly formed energy band structure was obtained,65 as depicted in Figure 4.10(b).

Figure 4.10: (a) Band gap alignment of Cd(OH)2 and BiOBr nanoparticles before coupling (b) Formation of p-n junction and subsequent energy level alignment on heterostructure formation.

The electrons in the CB of BiOBr would quickly be injected to that of Cd(OH)2. In such a way, the photogenerated electron–hole pairs would be separated effectively by

the p–n junction formed in the BiOBr/Cd(OH)2. Contrarily, the CB and VB positions

of the single BiOBr and Cd(OH)2 were unfavourable for the separation of electron– hole pairs before contact.

When the BiOBr/Cd(OH)2 composite was irradiated by visible light, only BiOBr could be excited, and the photogenerated electrons and holes could be produced in its

134 Chapter 4

CB and VB, respectively. Due to the matching energy band structures and the closely contacted interface, as well as the driving force from the internal electric field, the

excited electrons produced by BiOBr were injected into the CB of Cd(OH)2. Since the

CBM of BiOBr is more negative than that of Cd(OH)2, the CB of Cd(OH)2 would accept these excited electrons from the CB of BiOBr. Meanwhile, the photogenerated holes were effectively accumulated in the VB of BiOBr, as depicted in Figure 4.11.

Figure 4.11: Schematic mechanism and formation of ROS in the photodegradation process by the heterostructure.

This process promoted the efficient charge separation and inhibited the recombination of electrons and holes. In order to further investigate the transfer of the photogenerated electron–hole pairs between the two semiconductors, the composite materials were investigated by photoluminescence spectra (PL) and are presented in Figure 4.12, for pure BiOBr and

the BiOBr/Cd(OH)2 composite (using an excitation wavelength of 280nm). The main emission peak was observed at about 445nm for the pure BiOBr, which was attributed to recombination of electron–hole pairs in the BiOBr material.47 The intensity of this emission peak gradually decreased upon BiOBr loading upto 50%

BiOBr/Cd(OH)2 molar ratio, without changing the fluorescence emission peak position. This indicated that the recombination of the photo-excited electrons and

holes was greatly reduced by coupling of the BiOBr and Cd(OH)2. Chapter 4 135

The lower PL peak intensity in BC-50 sample than the other suggested the highest separation and transfer efficiency of photogenerated electron–hole pairs and resulted in its improved photocatalytic activity. As can be seen, Cd(OH)2 exhibited lowest PL intensity due to its large band gap and subsequent small response and separation of the charge carriers on excitation.

Figure 4.12: Photoluminescence (PL) spectra of various photocatalyst samples.

Based on the potential of conduction band edge, it can be inferred that the composite

is unable to reduce the dissolved oxygen on Cd(OH)2 surface, that requires −0.28eV/NHE for the formation of superoxide anion radicals. However, the addition of base facilitated the superoxide formation by influencing the redox potential of −• O2/O2 . The photogenerated electrons in the CB of the Cd(OH)2 could thus be −• 46,66 captured by adsorbed O2 to generate reactive O2 radicals. Furthermore, the VB edge standard redox potentials of BiV/BiIII57 were less positive than E0 (•OH/OH−) (+2.38eV),68 confirming that the photogenerated holes could not oxidize OH− to yield •OH. The enriched •OH radical were very active, and could decompose RhB directly.31 As the formed hydroxyl radicals are quite scarce, it indicated that the photogenerated holes would play a dominant role in the photocatalytic system of

BiOBr/Cd(OH)2 heterojunctions. Under photocatalytic conditions, three possible mechanisms might explain the degradation of the dye on the surface of the semiconductor. In addition to the semiconductor photocatalytic mechanism, the dye-sensitized mechanism and dye- photolysis mechanism may also exist.69 The potential of HOMO−LUMO levels of

136 Chapter 4

RhB (EHOMO = 0.95eV and ELUMO = −1.42eV that matches fairly well with the λmax of 553nm).70, 71 It is clear from Figure 4.11 that, although the excited electrons of RhB

can inject into the CB of BiOBr, they are ineffective to further react with absorbed O2, 31 as they readily move down to the CB of Cd(OH)2.

Moreover, the positioning of EVB below the HOMO levels of RhB is suitable for hole transfer from BiOBr to adsorbed dye molecules and supportive for the role of holes in RhB degradation.38 Based on the above assumption, a plausible photocatalytic process was proposed, as illustrated in Figure 4.11. Firstly, the dye molecules are easily adsorbed on the catalyst surface. The adsorbed RhB can be excited by visible light irradiation and −• inject electrons into the conduction band of the catalyst to form O2 . Simultaneously, the photo-induced holes are capable of oxidizing RhB directly72 and the photogenerated electrons was also believed to enhance the oxidation process through −• 73 −• + reduction of absorbed O2 into O2 . Finally, the O2 and hVB can oxidize the RhB

molecules to CO2 and H2O. In general, these dyes are subjected to de-ethylation accompanied by a ring-opening reaction of the benzene rings during the photocatalytic degradation.74,75 Rhodamine B

(λmax 554nm) could convert to Rhodamine (λmax 498nm) via de-ethylation in the photocatalytic process. In the de-ethyl RhB, the benzene rings are constantly attacked by positive holes till the complete degradation of the molecule. The blue-shift of the maximum absorption peak Figure 4.6(a) could correspond to the de-ethylation of RhB. With the de-ethylation step, the absorption peak was gradually blue-shifted from 554 to 552nm, then to 510nm, and finally to 498nm, which was apparently observed with the gradual change of colour from pink to yellow, Figure 4.6(e).29

4.4. Conclusion

Novel BiOBr/Cd(OH)2 heterostructures were synthesized by a facile chemical bath

method under ambient conditions. A series of BiOBr/Cd(OH)2 heterostructures were obtained by tuning the Bi/Cd molar ratios. The composites prepared exhibited strong visible light absorption and red shift in the visible light region. The 50% BiOBr

loaded Cd(OH)2 could degrade about 99.85% of Rhodamine B dye in only 30min of irradiation, where the photocatalytic activity did not show significant decrease after four cycles of reuse. Photocatalytic studies on Rhodamine B under visible light irradiation showed that the heterostructures are very efficient photocatalysts in mild Chapter 4 137

basic medium. Scavenger test studies confirmed that the photogenerated holes and •− superoxide radicals (O2 ) are the main active species responsible for RhB degradation. Comparison of Photoluminescence intensity suggested that an inhibited charge recombination is crucial for the degradation process over these photocatalysts. Moreover, relative positioning of the valence and conduction band edges of the •− • semiconductors, O2/O2 , OH/H2O redox potential and HOMO−LUMO levels of RhB appears to be responsible for the hole-specificity of degradation. The study hence concludes that the heterojunction constructed between the Cd(OH)2 and BiOBr interfaces play a crucial role in influencing the charge carrier dynamics and their photocatalytic activity. The high photocatalytic activity could be attributed to the enhanced light absorption and the improved separation of photogenerated charge carriers, due to the closely contacted interface and the matched energy level structure in the prepared materials.

138 Chapter 4

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Chapter 5 Enhanced visible-light-driven

photocatalysis by α-Bi2O3 sensitized Bi2O3/TiO2-Zr nanocomposites

Chapter 5 143

Enhanced visible-light-driven photocatalysis by α-Bi2O3 sensitized Bi2O3/TiO2-Zr nanocomposites

5.1. Introduction From the last few decades, heterogeneous semiconductor photocatalysis by anatase

TiO2 has been the focus of extensive research due to its potential applications in water 1–3 treatment and air purification. However, a serious limitation is that the anatase TiO2 is effectively initiated only under UV irradiation (< 387nm) due to relatively wide

band gap (Eg = 3.2eV) which limits its practical application. Therefore, scientists are trying for various methods to develop visible light responsive titania-based photocatalysts. Among these methods, ion doping has been widely adopted to adjust

the position of conduction band or valence band of TiO2, which could facilitate the visible light sensitization to produce the photoelectron−hole pair.4,5 Transition metal ion doping is one approach for acquiring a visible-light-response by introducing intermediate impurity energy levels.6 Lin et al. reported that Sn

substitution for Ti in rutile TiO2 leads to an increase in its photocatalytic activity upto 15 times for the oxidation of acetone.7 Nagaveni et al. proposed that incorporating

various metal ions (viz; W, V, Ce, Zr, Fe, and Cu) in TiO2 leads to higher photocatalytic activity than pure TiO2. Thus, doping transition metal into TiO2 should be an efficient method to obtain a preferable photocatalyst. However, present research in the field suspects that metal ion doping can also lead to the formation of charge recombination centres.

Another vital method to enhance the photocatalytic activity of TiO2 is to control the

microstructure of TiO2 particles, including the crystallinity, crystalline phase, crystallite size, and specific surface area.8The crystal phase and the crystallinity affect

the surface properties of TiO2, which strongly influence the recombination rate of photogenerated charge carriers. The crystallite size and surface area contribute to the efficient light absorption and reactant adsorption.9

The anatase crystalline phase of TiO2 has a loosely packed structure (density 3.89gcm-3) as compared to its rutile phase (density 4.25gcm-3),10 which favours the hole transport in the crystal lattice by the available displacement of the O atoms ¯ through the strong vibration associated with O anion. ZrO2 has also been introduced

into TiO2 lattice, leading to superior thermal stability and stability of anatase phase as mentioned in Chapter 3.11

144 Chapter 5

Heterostructure construction between two different semiconductors has also been extensively applied for efficient photocatalysis,12,13 because heterojunctions control the behaviour of photogenerated charge carriers, such as the direction of charge carrier transport, the distance of separation, and the rate of recombination.14 Further, the internal electric field generated at a heterojunction interface can efficiently reduce the charge-carrier recombination and enhance their lifetimes, thus improving the photocatalytic activity.15

As a group of V–VI–VII semiconductors, bismuth oxide (Bi2O3) with band gap varying from 2.1eV to 2.8eV is a prospective candidate for the formation of heterojunction because of its unique properties including adequately high oxidation power of valence band hole ( +3.13V vs. NHE) and nontoxic characteristics like 16,17 TiO2. ∼

Generally, Bi2O3 has four different polymorphs, denoted as monoclinic (α), tetragonal (β), body-centred cubic (γ), and face-centered cubic (δ). Among them, the low- temperature α-phase and the high-temperature δ-phase are stable; while the other two 18 are high temperature metastable phases. Though many researches on α-Bi2O3 have

been reported, there is only a few on β-Bi2O3 due to the difficulty in the synthesis of this metastable phase.16, 19-20 Density functional theory (DFT) calculations, together with the literature,21,22 confirm

that Bi2O3 can serve as an electron carrier owing to its positive CB level (+0.4V vs.

NHE), which lies exactly in the potential range of the multi-electron reduction of O2 - - (O2 + 2H2O + 4e →4OH ; +0.40V vs. NHE). 23 Liu et al. prepared Bi2O3/TiO2 composite photocatalysts with a non-aqueous sol–gel 24 method. Xu et al. reported that the Bi2O3/TiO2 composite films exhibited higher 25 photocatalytic activity as compared to pure TiO2 under solar irradiation. Bian et al. and Bessekhouad et al.26 have reported the inter-particle electron transfer from the

conduction band of Bi2O3 to the conduction band of TiO2. It implies that Bi2O3 operates as a sensitizer and hence could enhance the charge transfer and quantum efficiency.

In Chapter 3, we have observed that small amounts of ZrO2 can control all the

physico-chemical properties of TiO2 which in turn increases the efficiency of TiO2 photocatalysis under UV irradiation. The particle size is especially well-controlled by 11 ZrO2 in binary oxide catalysts. Chapter 5 145

In this study, the photocatalytic performance of α-Bi2O3/TiO2-Zr nanocomposites was

evaluated with respect to that of Bi2O3 and TiO2/Bi2O3 for the degradation of Rhodamine B and p-chlorophenol (PCP) under visible light irradiation. The effect of

Bi2O3 on the physico-chemical properties and activities of the α-Bi2O3/TiO2-Zr photocatalysts was optimized and discussed.

5.2. Materials and Methods 5.2.1. Synthesis of the Photocatalysts The various reagents for the synthesis and analyses were of analytical grade, obtained from Sigma Aldrich and Merck India, and were used without further purification or

modification. α-Bi2O3/TiO2-Zr heterostructures were synthesized by a facile precipitation method followed by a hydrothermal method. Typically, a predetermined amount of Bi(NO3)3.5H2O was added to 40mL of 5mM HNO3 solution and was ultrasonicated for 20min to obtain a transparent solution. On another side, a 4.4ml of titanium tetraisopropoxide (TTIP) was added to 45mL of 5mM HNO3 solution containing a definite molar ratio of zirconyl chloride (ZrOCl2) and vigorously stirred for 30min to homogenize the precipitate. The resulting solution was added to the

Bi(NO3)3.5H2O solution, drop by drop, with vigorous stirring at room temperature resulting in a white suspension. Then a freshly prepared 1M NaOH solution was added, dropwise, till a thick white precipitate was obtained. The precipitate was continued with vigorous stirring at 80oC for 30min. The resultant mixture was then transferred into a Teflon coated autoclave and was kept in the furnace at 160oC for 15h. Finally, the precipitates were collected, washed thoroughly with deionised water to ensure neutral pH and then with distilled ethanol, and dried at 50oC in air for 24h. The obtained powder samples were calcinated at 400oC for 4h.27 The Zr/Ti molar percentages were kept constant to 6% according to our previous 11 studies, however, α-Bi2O3 was tuned to 0, 5, 10, 15 and 20 mole percent (designated as TZ, BTZ-5, BTZ-10, BTZ-15 and BTZ-20 respectively). For comparison, a sample without Zr was prepared and was designated as BT-10.

5.2.2. Characterization Various analytical techniques were used to characterize the as prepared materials. The crystal structures of the nanoparticles and their composites were analysed by powder X-ray diffraction (Miniflex-TM II Benchtop, Rigaku Cooperation, Tokyo, Japan). The particle size and the surface morphology were characterized by Scanning Electron

146 Chapter 5

Microscopy (JSM6510LV, JEOL) and Transmission Electron Microscopy (JEM2100, JEOL). HRTEM-SAED was used to elucidate the diffraction pattern and to confirm the heterostructure formation. The UV–visible absorption spectra were obtained in the range of 190–700nm by UV-visible Spectrophotometer (UV-1601, Shimadzu). UV– visible diffuse reflectance spectra (UV-DRS) were obtained using a spectrometer

(UV1901, Puxi) with BaSO4 as a reference, and the chemical compositions of the catalysts were examined by energy dispersive X-ray spectroscopy (JSM6510LV, JEOL) and Fourier Transform Infra Red (FTIR) Spectroscopy (Interspec 2020 FTIR Spectrometer). The recombination of photo-induced charge carriers was studied using photoluminescence spectroscopy (F-7000, Hitachi). The specific surface area and the

pore size distribution of the as synthesized samples were characterized by N2 adsorption–desorption isotherm using Quantachrome Instruments Autosorb 1C. Adsorption of the samples was done at 77K and the samples were degassed at 150oC for 3h before analysis. The charge carrier efficiency was elucidated by Electrochemical Impedance Spectroscopy (EIS) with the help of Nyquist plots.

5.2.3. Evaluation of Photocatalytic Activity The photocatalytic activity of the nanocomposites was assessed by monitoring the decolorization of Rhodamine B (Scheme 4.1, Chapter 4) under the influence of visible light irradiation. An immersion well photoreactor, consisting of inner and outer jacket (made up of Pyrex glass), equipped with a magnetic bar, a passage for molecular oxygen and a water circulating jacket, was employed for the photocatalytic experiments. A visible light halogen linear lamp (500W, 9500Lumens) was used to carry out the irradiations.28The temperature was kept constant at 20±0.3oC using refrigerated liquid bath circulation. Before irradiation, an appropriate concentration of the dye solution (300mL), containing the desired quantity of the photocatalyst (1gL-1) was stirred with the help of magnetic stirrer, while the atmospheric oxygen was passed continuously into the solution for 20min in the dark to attain adsorption– desorption equilibrium between the dye solution and the photocatalyst. Subsequently, first sample (at 0min) was taken out and the irradiation was applied. During irradiation, samples of 5mL each were withdrawn at constant time intervals. The samples were centrifuged and the supernatant was subsequently analyzed. The

variation in absorbance of the dye aliquots was monitored at its λmax (554nm) as a function of irradiation time. The observed absorbance spectra were in accordance with Chapter 5 147

Beer-Lambert Law in the range of dye concentrations examined. The concentrations of the dye at different time intervals of irradiation were calculated from standard calibration curve obtained by the absorbance of the dye at different known concentrations. The photocatalytic experiments were repeated four times under the same conditions in order to monitor the reproducibility of the results. The accuracy in the optical density was between ±5%. The stability and the consistency in activity of the photocatalyst were analyzed by the recycling experiments. The photocatalyst concentration to be used for the most efficient activity was optimized by a series of photocatalysis experiments using different photocatalyst concentrations. Higher concentrations of the photocatalyst are likely to absorb more incident radiation and produce more photogenerated charge carriers, but above a particular concentration, the particles suspended in the solution, causing shielding and scattering of light in the solution. The decreasing transmittance may also increase the recombination as the photons could not be continuously injected onto the photocatalyst particles.29Same way, the initial concentration of the dye solution has a significant effect on the activity as it affects the light penetration into solution. The results were consistent with the literature on ‘the effect of initial concentration on photocatalysis’.30 The degradation efficiency (%) by the photocatalyst was calculated as follows:

(%) = × 100 (1) 퐶표−퐶 퐷푒푔푟푎푑푎푡푖표푛 퐸푓푓푖푐푖푒푛푐푦 퐶표 Where, C0 is the initial concentration of RhB and C is the concentration at different irradiation times. According to Langmuir–Hinshelwood (L–H) kinetic model, the first order kinetic equation which can be applied to explain the photocatalytic RhB degradation is as follows.25

æöCo ln ç÷= ktapp (2) èøCt

Where C0 is the initial concentration of the dye solution and C is the concentrations of −1 dye at time t, and kapp is the apparent first-order rate constant (min ). To determine the role and fate of resultant reactive oxygen species (ROS), various quencher species (or ROS Scavengers) were added separately to the reaction system

148 Chapter 5

during the photocatalytic experiments. The dosage of quenchers was referred to the previous studies.31,32

5.3. Results and Discussion 5.3.1. Crystallographic Study

The powder X-ray diffraction (XRD) spectra for the as-prepared TZ, Bi2O3 and α-

Bi2O3/TiO2-Zr heterostructure nanocomposites are shown in Figure 5.1.

Figure 5.1: XRD patterns of the nanocomposite heterostructures

The existence of distinguished broad peaks in TZ shows the formation of pure anatase 11 phase TiO2. The characteristic diffraction peaks for Bi2O3 (BX) were detected at 2θ angles of 26.8◦, 27.2◦, 33.1◦ and 46.2◦, and were attributed to the (0 0 2), (1 2 0), (2 0 0) and (0 4 1) crystal planes, of the monoclinic structure according to JCPDS 41- 1449.33 The composites prepared are considered to possess a highly crystalline nature as the diffraction peaks are relatively sharp. From the diffraction patterns of the α-

Bi2O3/TiO2-Zr composites, peaks for both TiO2 and Bi2O3 were detected which indicates that the synthesized composite materials consist of both the constituent phases. The diffraction angles did not shift on addition of Bi, suggesting that Bi species could

not incorporate into TiO2 lattice, but might present as a separate phase on the surface of TZ. This could be attributed to the bigger size of Bi3+ (103pm) than that of Ti4+ 25 (61pm), which inhibited the replacement of Ti by Bi in the TiO2 crystal lattice. Chapter 5 149

The average crystallite size was calculated by applying Debye-Scherrer formula:

D= kλ/ βCos θ (3)

D is the crystallite size, λ is the X-ray wavelength (1.54Ǻ), β is the full width at half maximum (FWHM), k calculates to 0.89 and θ is the diffraction angle. The anatase (1

0 1) diffraction peaks were used for the nanocomposites while for pure Bi2O3 the most intense (1 2 0) peak was used for the calculation of the crystallite size. Results show a gradual decrease in crystallite size from TZ to BTZ-10 which demonstrates that the incorporation of Bi2O3 inhibits crystal growth in TiO2 along with Zr. This

phenomenon is probably due to the high dispersibility of Bi2O3, inhibiting the growth of anatase crystals by providing dissimilar boundaries.34Based on the XRD data, the average crystallite sizes (D) of as-prepared samples are displayed in Table 5.1.

Table 5.1: Average crystallite size and corresponding apparent rate constant for the different photocatalyst samples

Sample Crystallite size Kapp (RhB) Kapp (PCP) (nm) (min-1) (min-1)

Bi2O3 28.12 0.0043 0.0025

TiO2 24.01 ------TZ 12.20 ------BT-10 ------0.0313 0.044 BTZ-10 8.25 0.0819 0.068

The crystallite size of BTZ-10 was much less when compared to that of TiO2 and

Bi2O3. However, the (101) peak intensity decreased significantly with simultaneous broadening, indicating the decrease in crystallite size. The D values of samples 11 change remarkably with the incorporation of Zr species as observed in Chapter 3.

5.3.2. Microscopic Analysis

The surface morphology of the α-Bi2O3/TiO2-Zr composite samples was studied by

SEM (Figure 5.2). Three dimensional (3D) block-shaped Bi2O3 structures were observed in Figure 5.2(a-b). The microstructures of the BT-10 nanocomposites are shown in Figure 5.2(c-d). Particles with varying size and shape were observed due to mixing of different phases. Figure 5.2(e-f) shows the SEM image of BTZ-10 sample.

The change in morphology is due to the inhibition in crystal growth of Bi2O3 during

150 Chapter 5

nanocomposite formation with TZ35 and also mixing up of different metal oxide phases. The efficient dispersion of phases can effectively improve the photocatalytic activity.36

Figure 5.2: SEM images: (a-b) Rod shaped stacks of Bi2O3, (c-d) BT-10 heterostructures, (e- f) BTZ- 10 heterostructures

The HRTEM images in Figure 5.3 (a-b) shows elongated spindle shaped Bi2O3

surrounded by TiO2 nanoparticles, indicating the intimate contact between the two phases. As depicted in Figure 5.3(c), the corresponding selected area electron diffraction (SAED) pattern displays a diffraction pattern, indicating the diffused crystalline characteristic of the obtained nanocomposite. The HRTEM images in Figure 5.3(c-d) confirm that BTZ-10 samples calcinated at 400°C were indeed comprised of anatase phase with the lattice fringe spacing of 0.352nm corresponding

to the (101) crystallographic plane of anatase TiO2 and 0.276nm spacing

corresponding to (002) plane of α-Bi2O3, which is also consistent with the XRD analysis. No obvious Zr particles were observed in TEM images of BTZ-10, implying Chapter 5 151

the extremely high dispersion of Zr in the TiO2 lattice, which coincides with the XRD results.

Figure 5.3: HRTEM images: (a, b) BTZ-10 nanocomposites, (c) the lattice pattern of the BTZ-10 composite and (d) the SAED pattern of BTZ-10

Interestingly, the intimate contact facilitates the heterojunction construction at the interface, which facilitates efficient charge transfer and separation of the photogenerated charge carriers, thereby improving the photocatalytic activity. The results of the Energy Dispersive Spectroscopy (EDS, Figure 5.4) showed characteristic distribution and peaks associated with O, Bi, Ti, Zr and C were observed for BTZ-10.

152 Chapter 5

Figure 5.4: EDS mapping and EDS spectrum of BTZ-10 heterostructure showing the elemental composition and distribution of the composite

The Bi and O peaks resulted due to Bi2O3 and Ti, Zr, O peaks resulted due to TiO2

and Zr, and hence confirm that the as-prepared materials were composed of Bi2O3,

TiO2 and Zr. The peak of C atom observed in the spectrum could be attributed to the carbon present on the adhesive tape used to hold the samples.37

5.3.3. FT–IR spectral analysis of the photocatalysts

Figure 5.5 depicts the FT–IR spectra of the BTZ-10, BT-10 and Bi2O3 catalysts in the wave number range from 400cm−1 to 4000cm−1. As can be seen, a broad absorption bands in the range from 400 cm−1 to 600cm−1 is obtained and is attributed to the Ti-O bond stretching vibration in Ti-O-Ti.38 The peak at 1630cm−1 corresponds to the O-H bond bending vibrations of chemisorbed water, and the broad absorption peak appearing near to 3440cm−1 is assigned to the O-H bond stretching mode of the surface hydroxyl groups or adsorbed water, strongly bound to the catalyst surface.39 A strong peak at 1382cm-1 for BT-10 and BTZ-10 indicates presence of organic solvent

(viz; HO-CH2CH3) adsorbed on the titania surface. As can be inferred from Figure -1 5.5, pure Bi2O3 has smaller and broad hydroxyl peak around 610cm , whereas among

the composites, BT-10 and BTZ-10, the latter has the largest hydroxyl peak. More surface hydroxyl groups are beneficial for the photocatalytic reactions.35,40,41 Chapter 5 153

Figure 5.5: FT-IR spectra of BTZ-10, BT-10 and Bi2O3 samples

5.3.4. BET analysis Since heterogeneous photocatalysis is generally influenced by the surface area and pore structure of catalysts, the effects of introduced Bi and Zr species on the pore structures and BET surface areas of as-synthesized samples were investigated by nitrogen gas adsorption and desorption measurements. The shape of the isotherm obtained for BTZ-10 and BT-10 samples is a type IV isotherm containing a H3 hysteresis loop at high relative pressures which indicates that these samples are mesoporous structures as the pore diameter ranges from 2– 15nm only.42,43 This result can be further confirmed by the corresponding pore-size distribution, as shown in Figure 5.6(a, inset).

Figure 5.6: (a) BET isotherm and particle size distribution (inset) of BTZ-10 sample (b) BET isotherm and particle size distribution (inset) of BT-10 sample

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Quite gentle curves were obtained for both BT-10 and BTZ-10 in the low pressure

region. The adsorption increased slowly with increase in P/Po, and showed a slight jump around 0.5–1.0 which may be assigned to the capillary condensing phenomenon of nitrogen gas in the mesoporous pressure region.44 Among the BET values, the BTZ-10 composite is obviously higher than that of BT- 10, and the nitrogen adsorption isotherm of BTZ-10 composite also obviously has shifted upward45 indicating the superior photocatalytic activity of the composite material. The difference for larger surface area of composites may be attributed to the concentration of precursors.

It can be seen from Figure 5.6(a, b) that the SBET of BTZ-10 is higher as compared to BT-10. It could also be seen that the pore size also increased, even when a small

amount of Zr was introduced into TiO2. For the sample BTZ-10, there was significant increase in surface area from 61m2g−1 in BT-10 to 111m2g−1 in BTZ-10 and the increase of pore size from 4.54nm to 4.87nm,

respectively. The doped Zr entered into the lattice of TiO2, which hampered the

crystal growth of TiO2 and then led to the decrement of particle size and increment of

specific surface area. Coupled Bi2O3 also resulted in the decrease of particle sizes and the increase of surface area to some extent, which might be attributed to the decrease

in the aggregation of TiO2 by Bi2O3. The high mesoporous nature, large surface area and the adequate adsorption capacity of the nanocomposite could efficiently improve the photocatalytic activity in these photocatalysts.46,47 Previous studies show that a suitable conformation of pores channelizes the light waves to penetrate deep inside the photocatalysts which result into high mobility of charge.48,49 It is speculated that the pores in the BTZ-10 and BT-10 heterostructures allow better penetration of light waves and enhanced mass transfer, which has greatly promoted the photocatalytic activity.50

5.3.5. Optical Properties

The optical properties of the synthesized Bi2O3, BT-10, TZ and BTZ samples with different Bi/Ti molar ratio values were obtained by using UV–visible diffuse reflectance spectroscopy (UV-DRS) and the results are shown in Figure 5.7. Chapter 5 155

Figure 5.7: (a) DRS spectra of the samples (b) Band gap energy curves for the samples

According to Figure 5.7(a), there are no obvious reflectance peaks of Bi2O3 and there is only one reflectance band edge, which is attributed to the poor content of Bi.

Compared with TiO2 or TZ sample, BTZ samples are red-shifted to the visible region. The band gap energy of the photocatalysts was calculated according to the formula:51

(hv.α) = (Ahv – Eg)n\2 (4)

Since α is proportional to Kubelka–Munk function F(R), the expression becomes:

hv. F(R) = (Ahv – Eg) n\2 (5)

Where v is the light frequency, F(R) is the Kubelka–Munk function, A is the

proportionality constant and Eg is the Band gap energy. The value of n is determined according to the type of optical transition (for direct transition, n = 1 and for indirect 51 transition, n = 4). The value of n for Bi2O3 and TiO2 is 4 as both belong to the

indirect band gap semiconductor. The Eg of Bi2O3 was determined from the plot of (F(R).hv)1/2 versus hv (Figure 5.7(b)) and was elicited to be 2.80eV. Accordingly, the

Eg of TZ was found to be 3.35eV (Figure 5.7(b), inset). The valence band edge position of BTZ heterostructures at the point of zero charge can be calculated by the following empirical equation:52, 53

c EVB =X−E +0.5Eg (6)

c Where EVB is the VB edge potential and E is the energy of free electrons on hydrogen’s scale (4.5eV). Herein, X is the geometric mean of the electronegativities of the constituent atoms of the semiconductor. The X values for Bi2O3 and TiO2 are

ca. 6.23eV and 5.85eV, respectively. The top of the valence band EVB of Bi2O3 and

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TZ were calculated to be 3.13eV/NHE and 3.02eV/NHE respectively. Moreover, the

conduction band edge potential ECB can be determined by:

ECB = EVB − Eg (7)

Thus, the calculated ECB for Bi2O3 and TZ are 0.33eV/NHE and -0.33eV/NHE, respectively. As shown in Figure 5.7(b), the band-gap energy of the samples decreased with

increase in the α-Bi2O3 content, which means that the BTZ composites can absorb lower energy photons, and the more electron hole pairs were generated in the photocatalytic reaction. Thus the photocatalytic activities of the samples will be improved. However, decreasing band gap energy may also speed up the recombination of electrons and holes in some cases, thereby, reducing the photocatalytic activity.

5.3.6. Kinetic study and mechanism 5.3.6.1. Photocatalytic activity of catalysts The photocatalytic performance of the synthesized photocatalysts for the decolorization of RhB molecules under visible light irradiation was investigated in aqueous medium, and the results are shown in Figure 5.8(a-e). In the control experiments, no decolorization of RhB was observed in the dark and in the absence of photocatalyst, indicates that RhB was stable and did not undergo a photolytic degradation (Figure 5.8(b)). In addition, the catalytic decolorization in dark as a result of adsorption was negligible, suggesting that a purely photocatalytic reaction mechanism took place during photocatalytic experiments. Chapter 5 157

Figure 5.8: Kinetics of photocatalytic decolorization of RhB in the presence of pure Bi2O3, BT-10 and BTZ heterostructures (a) Change in absorption of RhB at regular intervals of light irradiation in the presence of the BTZ-10 photocatalyst, (b) change in concentration (Ct/C0) of RhB during its decolorization in the presence of Bi2O3, BT-10 and BTZ heterostructures, (c) ln(C0/Ct) versus irradiation time for decolorization of RhB in the presence of synthesized photocatalysts, (d) Recycling experiments of BTZ-10 for decolorization of RhB under visible light, (e) Variation in colour of the RhB dye at regular intervals of irradiation in the presence of BTZ-10.

It can be observed from the Figure 5.8(b) that the decolorization rate follow the order: BTZ-10>BTZ-5>BT-10>BTZ-15>BTZ-20>Bi2O3.The decolorization of RhB by BTZ-10 was 98.64%, which was about 4.22 and 1.15 times greater than that of

Bi2O3 (23.37%) and BT-10 (85.15%), respectively. The experimental data was found to fit well with the first order kinetic equation. Also from Figure 5.8(c), the BTZ-10 composite catalyst was found to exhibit the highest photodegradation efficiency, −1 where the kapp of BTZ-10 was 0.0819min , which was 19.0 and 2.6 times higher than −1 −1 Bi2O3 (0.0043min ) and BT-10 (0.0313min ) respectively. This suggested that Bi2O3 nanoparticles contributed to the higher redox potentials with well-aligned band- structures and heterostructure interfaces were favourable for the separation of electrons and holes, leading to an enhanced photocatalytic performance. However, the

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larger increase in Bi2O3 content led to a decrease in RhB decolorization, where the BTZ-20 composite exhibited only 78.8% decolorization. In view of the practical applications besides the efficiency, the stability and durability are also indispensable to photocatalysts. To evaluate the stability of the BTZ-10 composite catalysts, the photocatalytic activity was investigated in cycling runs and the results are presented in Figure 5.8(d). In our recycling experiments of RhB photodegradation, the BTZ-10 sample exhibited a minimal decrease in activity even after four cycles of reuse. In order to validate the photocatalytic activity of these heterostructure photocatalysts, they were applied for the photocatalytic degradation of non-dye pollutant, such as p- chlorophenol (PCP). It was found that the photocatalysts showed consistent performance for PCP degradation, hence, ruled out the exclusive dye sensitized

photocatalysis by these photocatalysts (Figure 5.9(a)). The rate constant (kapp) for the phenol degradation was found as 0.068min-1, 0.044min-1 and 0.0025min-1 for BTZ-10,

BT-10 and Bi2O3, respectively from Figure 5.9(b).

Figure 5.9: (a) change in concentration (Ct/C0) of PCP during its degradation in the presence of Bi2O3,

BT-10 and BTZ heterostructures, (b) ln(C0/Ct) versus irradiation time for degradation of PCP in the presence of different catalysts.

5.3.6.2. Role of reactive Species The radical and the hole trapping experiments (scavenger tests) with different scavengers were carried out to elucidate the mechanism of photocatalytic degradation of RhB under visible light irradiation over BTZ-10 nanocomposite. Generally, the reactive oxygen species (ROS) such as, hydroxyl radicals (•OH), superoxide radical −• + anions (O2 ) and holes (h ) are expected to have role in the photocatalytic dye decolorization processes. Chapter 5 159

The fate and role of the ROS were investigated through radical and hole trapping experiments and the results obtained are shown in Figure 5.10. The decolorization of RhB decreased significantly upon the addition of isopropyl alcohol (IPA, a hydroxyl radicals scavenger54), indicating that dissolved •OH radicals were the dominant active species in this process. The addition of disodium ethylenediaminetetraacetate (EDTA, a hole scavenger55) had no significant negative effect on the degradation of RhB, −• confirming that the holes were not the dominant active species. Upon addition of O2 56,57 quencher, benzoquinone (BQ), the kapp dropped significantly, indicating that −• O2 radicals were also the dominant active species in the reactive system. Similarly, electron scavenger, AgNO3, also indicated that electrons were another dominant active species in the degradation process.

Figure 5.10: Effect of different ROS scavengers on kapp of BTZ-10 for RhB degradation

The above results demonstrated that the photocatalytic process was mainly governed −• • by O2 radicals and OH oxidation reactions while role of holes was negligible. Concentration of the HO• radicals generated during the photocatalytic process of BTZ-10 were determined by the terephthalic acid oxidation method. The emission intensity of dihydroxyterephthalic acid is the direct measure of the HO• concentrations. The photoluminescence patterns of terephthalic acid solution after being illuminated under visible light for 60min with BTZ-10 are shown in Figure 5.11. The increasing emission intensities with irradiation time indicate generation of HO• by BTZ-10 during the photocatalytic process.

160 Chapter 5

Figure 5.11: Hydroxyterepthalic acid fluorescence indicating generation of HO• by BTZ-10 during the photocatalytic process

The photogenerated electrons on the surface of Bi2O3 nanoparticles played a • − significant role in the photocatalytic process by generating O2 radicals and the

photogenerated holes assist this reaction by providing dissolved O2, in addition to the direct oxidation to form HO• radicals.58

5.3.6.3. Photoluminescence emission spectra of the photocatalysts Photocatalysts generate electrons and holes after being irradiated by visible light, and recombination of some electrons and holes can release energy in the form of fluorescence emission. The smaller fluorescence emission indicates the lower rate of recombination of electron–hole pairs and vice-versa. Figure 5.12 presents the comparison of the photoluminescence spectra at room temperature with an excitation

wavelength of 280nm in the range of 300–700nm for the BT-10, Bi2O3 and BTZ samples. The main peaks of the nanocomposites appeared at ~575nm. The PL

intensity of these samples decreased in the order: Bi2O3> BTZ-20> TZ> BT-10 >

BTZ-15>BTZ-5>BTZ-10. The PL intensity of the BT-10 is lower than that of the TZ,

while that of the BTZ-10 sample is the lowest. It indicates that coupling TiO2 with

Bi2O3 is helpful in inhibiting the recombination of electrons and holes, and thus supports the photocatalytic reactions.24 Chapter 5 161

Figure 5.12: Photoluminescence spectra of the photocatalyst samples indicating the extent of charge recombination

Further, the average diffusion time for randomly generated charge carriers from bulk to the surface is given by τ=r2/π2D, where r denotes the grain radius and D denotes the diffusion coefficient of the carrier.59 As the grain radius decreases, the diffusion time is reduced, and the recombination probability of photo-generated electron–hole pairs decreases, hence advocates the role of Zr.

The intensity of the emission peak decreased gradually upon Bi2O3 loading upto 10%, without shifting the peak position. It emphasizes that the recombination of the photo-

excited electrons and holes was greatly reduced by coupling of the Bi2O3 and TiO2-Zr. The lower PL peak intensity in BTZ-10 samples suggested that the highest separation and transfer efficiency of photogenerated electron–hole pairs occurred in this case which in turn resulted in improved photocatalytic activity.

When the p-type α-Bi2O3 was coupled with n-type TiO2 semiconductor, a number of micro p–n heterojunction in the BT-10 and BTZ photocatalysts would have been

formed. Bi2O3 region has the negative charge, while TiO2 region has the positive charge due to the formation of inner electric field. Under visible light illumination, the electron–hole pairs may be produced, where in the holes get attracted into the negative field, and the electrons move towards the positive field. Hence, the photogenerated electron–hole pairs would effectively get separated across the Bi2O3/TiO2 interfaces. Liu et al.23 reported that it was desirable to prepare heterojunction

composite with intimate contact between hetero-phase nanoparticles of Bi2O3 and

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TiO2 to allow inter-particle electron transfer to occur more efficiently and achieve

high photocatalytic performance. When the amount of Bi2O3 is too small, there are no adequate traps for photogenerated electron–hole pairs. However, if the doping

quantity is considerably high, Bi2O3 becomes the recombination centre for electrons and holes.

The amounts of Bi2O3 can also influence the thickness of the superficial space-charge

layer of TiO2. When the space-charge layer thickness becomes almost equal to the penetration depth of light into the sample, the electron–hole pairs can be effectively

separated. It revealed that Bi2O3 amount was crucial for the activity of the photocatalyst.

5.3.6.4. EIS Nyquist plots The charge separation efficiency of photogenerated electrons and holes across the interface is a crucial factor for the photocatalytic activity, which can also be examined

by the EIS Nyquist plots. Figure 5.13 shows the EIS Nyquist plots of Bi2O3, TiO2, BT-10 and BTZ-10 under visible light irradiation.

Figure 5.13: Nyquist plots showing the extent of impedance imposed by different samples to charge transfer

resistance between the semiconductor and the electrolyte. It shows one dominant semicircle, whose diameter is related to the charge-transfer resistance at the semiconductor/electrolyte interface. Smaller diameter of the semicircle means smaller impedance to charge separation and transportation.

The arc radius of the EIS Nyquist plot of BTZ-10 was smaller than that of Bi2O3, TiO2 and BT-10. Since, the arc radius of EIS spectra reflects the interface layer resistance occurring at the surface of electrode, more effective separation of charge carriers and faster interfacial charge transfer occurred on the BTZ-10 photocatalyst.9,60

5.3.6.5. Band gap structures and possible degradation mechanism Photocatalytic performance of the composite photocatalysts depends upon its heterojunction interface, and the electronic structures,61 because the photocatalytic activity is significantly related to the conduction band and the valence band positions, as well as to the mobility of the charge carriers. The energy position of CB and VB could influence the oxidative and reductive ability of the semiconductor photocatalyst, respectively, while the mobility of the charge carriers could influence the photocatalytic efficiency. For metal oxide photocatalysts, the VB consists of the oxygen 2p orbital, while for bismuth-based semiconductors, the VB is a hybrid of the oxygen 2p orbital and Bi 6s orbital, and the CB is composed of the Bi 6p orbital,62 and possesses a high reduction capability. The improved charge separation of the photogenerated carriers and the light energy utilization ratio are favourable for the photocatalytic activity. It was reported that the well-matched overlapping band-structure in the semiconductor nanocomposite could promote the separation of electrons and holes, thereby enhancing the photocatalytic activity.63

The band edge positions of α-Bi2O3 and TiO2 were estimated according to the methods discussed above. The results confirmed that Bi2O3 and TiO2 possessed a composite heterostructure, which was favourable for the charge separation. Pertaining to the band gap structure of as-prepared BTZ-10 and the influence of scavengers, possible mechanistic pathways for the photocatalytic decolorization were proposed.

In these samples, Bi2O3 behaves as a p-type semiconductor, whereas TiO2-Zr as an n- type semiconductor (Figure 5.14). The formation of the heterostructure could lead to an efficient electron–hole separation that could minimize the recombination of photoexcited electron–hole pairs.64

164 Chapter 5

The VB and CB of Bi2O3 (EVB= 2.86eV, ECB= 0.10eV); and TZ (EVB= 3.02eV, ECB= - 0.33eV) were provided to clearly understand the separation and transfer of electron– hole pairs across the interface of the nanocomposite heterostructures.

Figure 5.14: Alignment of energy bands of the constituents of the nanocomposite before coupling

The Fermi level (EF) for n-type TZ was close to the conduction band, whereas the EF

for p-type Bi2O3 was close to the valence band. When p-type Bi2O3 and n-type TZ

form the heterostructure, only the Bi2O3 gets excited under visible light to generate electrons and holes in the CB and VB respectively,65as depicted in Figure 5.15.

Figure 5.15: Formation of the heterostructure and the mechanism of formation of different reactive oxygen species Chapter 5 165

The electrons in the CB of Bi2O3 would take part in the reduction process to generate −• O2 radical while the holes will transfer to the VB of TZ. As a result, the photogenerated charge carriers would be separated efficiently by the heterostructure being formed in the BTZ heterostructures. Normally, the CB and VB positions of the single Bi2O3 and TZ were unfavourable for the separation of electron–hole pairs before coupling. Furthermore, the doped Zr could generate a positive charge difference and the impurity cation (Zr) acted as a Lewis site, which can generate more hydroxyl groups to balance the positive charge. Therefore, more holes could be quickly scavenged by the hydroxyl groups to produce •OH. The adsorbed •OH eventually degraded RhB and

PCP into benign compounds such as CO2, H2O, and the inorganic substrate. Both the transferring and scavenging of the hole prevented the electron-hole recombination, which remarkably improved the photocatalytic efficiency. Three possible mechanisms might explain the degradation of the dye on the surface of the semiconductor under photocatalytic conditions. In addition to the photocatalyst mediated degradation mechanism, the dye-sensitized mechanism and dye-photolysis mechanism may also exist.66

The potential of HOMO−LUMO levels of RhB (EHOMO = 0.95eV and ELUMO = −1.42eV) matches fairly well with the band gap alignment of TZ.67 It is clear from Figure 5.15 that, as the excited electrons of RhB can inject into the CB of TZ, where they further react with adsorbed O2 on TZ conduction band. The holes produced on the HOMO can also take part in degradation of RhB. Based on the above assumption, a plausible photocatalytic mechanism was proposed, as illustrated in Figure 5.15. Primarily, the dye molecules are readily adsorbed on the catalyst surface and get easily excited by visible light irradiation and inject electrons −• into the conduction band of the photocatalyst to form O2 . At the same time, the photo-induced holes are available for oxidizing RhB directly68 and the photogenerated electrons are also believed to enhance the oxidation process through −• 2 −• + reduction of absorbed O2 into O2 . Finally, the O2 and hVB can oxidize the RhB

molecules to CO2 and H2O.

With regard to the photogenerated electrons in Bi2O3, it was not thermodynamically allowed for the one-electron reduction of O2 on Bi2O3, owing to the higher potential •− of the conduction band (+0.33eV vs. NHE) than the redox potential of O2/O2 (−0.16eV vs. NHE).69 Hence, it proceeds through multi-electron process

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- - (O2+2H2O+4e →4OH ; +0.40 V vs. NHE) but with difficulty. Further, the CB level of

TiO2-Zr (-0.33V vs. NHE) is much more negative than that of Bi2O3, once the

electrons transferred to the negative CB, the electrons can reduce O2 by single- - - electron process (O2 + e → O2 ; -0.16V vs. NHE). Generally, these kinds of dyes are subjected to de-ethylation accompanied by ring 70 opening of benzene rings during the photocatalytic degradation. Rhodamine B (λmax

554nm) could convert to Rhodamine (λmax 498nm) via de-ethylation in the photocatalytic process. The de-ethyl RhB is constantly attacked by reactive species on the benzene rings till the complete degradation of the molecule. The obvious blue- shift of the absorption peak maxima could be assigned to the de-ethylation of RhB (Figure 5.8(a)). With the continuous de-ethylation, the absorption peak was gradually blue-shifted from 554nm to 552nm, then to 510nm, and finally to 498nm, which was also observed apparently with the gradual changing of colour from pink to yellow, Figure 5.8(e).62

5.3.6.6. Chemical Oxygen Demand Removal Efficiency Additionally, to investigate the mineralization of organic pollutants during the photocatalytic process, the chemical oxygen demand (COD) removal during the photocatalytic reaction was carried out in accordance with standard American Public Health Association (APHA) method with BTZ-10 sample.71 The decrease in the value of COD determines the degree of mineralization of organic species. As can be observed from Figure 5.16, the COD value decreased significantly as the function of irradiation time. At the end of irradiation, after 60min, the COD was reduced to about 80% of initial value.

Figure 5.16: Change in COD of the RhB solution during the irradiation in presence of BTZ-10 sample Chapter 5 167

5.4. Conclusion

In this work, a new TiO2-based visible light active photocatalyst (α-Bi2O3/TiO2-Zr)

was synthesized by coupling of α-Bi2O3 and Zr doped TiO2 via a chemical precipitation method followed by the hydrothermal method. The products obtained showed efficient visible light photocatalysis with nice consistency and durability. Different characterization techniques confirm the formation of the heterostructure nanocomposites. BET analysis confirmed that the Bi2O3 and Zr have significant effect on the particle size and specific surface area of the composites. Photoluminescence study showed that BTZ-10 faces least charge recombination while as excessive

addition of Bi2O3 lead to fast recombination. The results showed that Bi2O3/TiO2-Zr catalysts held an anatase phase and possessed highly crystalline nature. The doped Zr content had a significant effect on the surface area, the crystallinity and the size of the

Bi2O3/TiO2-Zr nanocomposites. The introduced Bi species mainly existed as oxides

on the surface of TiO2 particles, and it extended the light absorption into the visible

region by photosensitization. Bi2O3 coupling also favoured the separation and transfer of photoinduced charge carriers to inhibit their recombination. The superior performance was ascribed to the high surface area, the enhanced visible light response, and the efficient charge separation associated with the synergistic effects of appropriate amounts of Zr and Bi2O3 in the prepared samples. The highest activity was observed for BTZ-10 samples calcinated at 400°C. Photocatalytic experiments were also consistent with the photoluminescence study as BTZ-10 showed the highest −• • photocatalytic activity. ROS scavenging experiments concluded that O2 radicals, OH radicals and electrons were dominant species for the RhB degradation process. Phenol based experiments confirmed that dye-sensitized photocatalysis is not the dominant phenomenon in these nanocomposite photocatalysts. Finally, the chemical oxygen demand analysis inferred efficient mineralization of the degradation products.

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Chapter 6 Efficient visible-light-driven photocatalytic activity and enhanced charge transfer properties over Mo-doped

WO3/TiO2 nanocomposites 172 Chapter 6

Efficient Visible-light-driven Photocatalytic activity and enhanced charge

transfer properties over Mo-doped WO3/TiO2 nanocomposites

6.1. Introduction In light of the previous chapters, it is almost obvious that many efforts have been put by various researchers to attain improvised photocatalyst properties. The various methods include the cationic and anionic doping; hybridization with the polymers or coupling of two or more semiconductors to form heterostructures. In all these cases, though a promising improvement was achieved, a lot more needs to be done in this direction. Generally, an efficient visible-light-responsive photocatalyst should have fine crystallinity, suitable band gap, enhanced visible light absorption, large surface area, fair chemical stability and controllable charge separation and charge transfer. Amongst these, suitable band gap is the primary decisive factor for the photocatalysts responding to the visible light.1

In this respect, tungsten trioxide (WO3) has been extensively studied as a viable environmental photocatalyst because it is active under visible light, stable, and non- 2-6 toxic. The photocatalytic reactions on WO3 can be initiated by visible-light irradiation, which create conduction-band electrons and valence-band holes. • Subsequently, H2O2 and OH are primarily generated as oxidants through the two- 7-9 electron reduction of di-oxygen and one-electron reduction of H2O2, respectively.

WO3 + hν (visible) ecb¯ + hvb⁺ (1) + 2ecb¯+ O2 + 2H H2O2 (2) • ecb¯ + H2O2 OH + OH¯ (3)

• • Unlike TiO2, the formation of superoxide/hydroperoxyl radical ( O2¯/HO2 ) as an 10-12 oxidant is limited on WO3 because the CB position of WO3 (+0.3–0.5V NHE) is 0 • more positive than the one-electron reduction potential of di-oxygen [E (O2/ O2¯) = – 0 • 13 0.33V NHE and E (O2/HO2 ) = –0.05V NHE]. The one-electron transfer from WO3 • • CB to di-oxygen (i.e., the formation of O2¯/HO2 ) is not thermodynamically • • • favoured. Therefore, three oxidants except O2¯/HO2 (hole, H2O2, and OH) can be 14 involved in the oxidative degradation processes on WO3.

Hence, WO3 is an ideal candidate, and researchers looked to form its composite with 15-19 TiO2, because it has a suitable band gap (Eg≈ 2.7eV) for visible light absorption. 173 Chapter 6

Lv et al. synthesized WO3/TiO2 hollow spheres using a template method, and found

that the apparent rate constant (kapp) for the photodegradation of Methylene Blue

(MB) over WO3/TiO2 nanocomposite was 3.2 and 3.5 fold higher than by TiO2 under both UV and visible light irradiation.20 Leghari et al. reported that the 5.0%

WO3/TiO2 nanocomposite was synthesized by a template-free hydrothermal method which exhibited far superior photocatalytic activity to methyl orange and 2,4- dichlorophenol degradation under UV and visible light.21

Several other authors have reported that the addition of WO3 to TiO2 enhanced the photocatalytic activity. Do et al. found that the photo-degradation of 1,4- 22 dichlorobenzene was improved by addition of WO3 on the surface of TiO2. Enhanced activity was attributed to the inhibition of recombination of the

photogenerated holes and electrons since WO3 possesses a suitable conduction band

potential to allow the transfer of the photogenerated electrons from TiO2. Akurati et

al. reported that the increase in the surface acidity by the addition of WO3 was also responsible for the enhanced photocatalytic activity in the photodegradation of MB.23

Ternary hybrid of CdS/TiO2/WO3 has been investigated in order to enhance the charge separation by introducing a potential gradient at the interface of the semiconductors with different band gaps and positions.24

On the other hand, WO3 has band gap smaller than that of TiO2, leading to the response to a visible light. However, only a few reports exist concerning the

degradation of organic compounds over the WO3 photocatalyst as the photocatalytic activity is very low in absence of suitable co-catalysts such as Pt and CuO.25-28 Such a

low activity can be attributed to the smaller surface area than TiO2 since surface area is generally one of the important factors that affect photocatalytic activity. The 29 presence of the co-catalysts promotes reduction of O2 by multi-electrons. Cationic doping is not considered desirable as it is believed that mono-doped semiconductor nanoparticles exhibit enhanced charge recombination. In various

mono-doped TiO2, the new energy levels generated by the dopant ions in the TiO2 band gap can act as recombination centres for the photo-induced electrons and holes, decreasing their photocatalytic activity.30,31 However impurity bands can be modified and will not act as electron–hole recombination centres if the oxide semiconductor is co-doped with two different elements.32,33

Hence, a lot of work has been done on metal and non-metal co-doping of TiO2 (Mo- C,34 Mo-N,35 I-Br,36 V-N,37 P-N-Mo,38 V-N39). In contrast, metal-metal co-doping (Pt- 174 Chapter 6

Cr, Cr-V, Pt-Ni, Cr-Ni) of TiO2 has not received as much attention, because co-doped photocatalysts activities were selective toward pollutant nature and also were not 40 correlated with any physicochemical property of the co-doped TiO2. In respect of the above discussion, I have put two ideas together to study the influence of doping and heterostructure formation together on the photocatalytic and

photoelectrochemical properties. One is that WO3 can form an adequate heterostructure with TiO2 to inhibit charge recombination and also enhance visible

light sensitization of the TiO2. The another idea is that Mo can easily dope with TiO2 as the radius of titanium ion (0.61Ǻ) was similar with that of molybdenum ions

(0.59Ǻ) and molybdenum ions may be incorporated into the lattice of TiO2 and occupy some of the titanium lattice sites. Li et al.41 have found that Mo introduces only a shallow donor level below the conduction band minimum and it can be an ideal n-type dopant causing a small perturbation on the CBM. Further, contrary to some transition metals, which create strongly localized d states within the band gap and therefore significantly reduce the mobility of the charge carriers, the shallowness of 4d orbital energy compared to that of 3d orbit presents more non-localized character of the 4d orbit.42-44 Furthermore, Mo6+ ions replacing Ti4+ ions in the lattice act as electron traps to facilitate the charge separation to reduce the recombination rate. The photogenerated electrons trapped by Mo6+ ions are transferred to the oxygen − • − 44,45 molecules so as to form H2O2, HO2 and O2 . Also, there should be a certain

amount of oxygen vacancy (VO) on the surface of Mo-doped TiO2 as they play a role as another separating site for electron. Since the presence of VO could trap the photogenerated electron, thus the trapped electron could transfer to the surface of

TiO2 to react with the oxygen molecules. These active species undergo a series of • − reactions to generate a superoxide radical ( O2 ), which could promote the photocatalytic oxidation reaction.

Hence, in the present work, Mo doped WO3/TiO2 nanocomposite was synthesized

basically with the aim to sensitize wide band gap TiO2 to visible light response and

also to check the charge recombination by heterostructure formation between WO3

and TiO2. The nanocomposites were synthesized by a facile sol–gel method followed

by the hydrothermal method. Results show that Mo doped WO3/TiO2 powders presented enhanced visible photocatalytic activity for the degradation of MB and p- chlorophenol. The enhanced photocatalytic activity was achieved due to the composite nature of the powders, small crystallite size and good crystallinity, 175 Chapter 6

enhanced concentration of hydroxyl groups at the photocatalyst surface, increase in surface acidity, presence of dopant impurity levels, oxygen vacancies and Ti+3 centres.

6.2. Materials and Methods 6.2.1. Materials All the chemicals used were of analytical grade and were not further processed before

use. Titanium tetraisopropoxide (Ti(PrOi)4; TTIP, 98%); sodium tungstate

(Na2WO4.2H2O, 99%); p-chlorophenol (ClC6H4OH) and Methylene Blue Dye

(C16H18N3SCl) were obtained from Sigma Aldrich, India while as sodium

molybdenate (Na2MoO4.2H2O, 99%) was obtained from Merck, India.

6.2.2. Synthesis of the photocatalyst

The Mo doped WO3/TiO2 nanocomposite photocatalyst was synthesized by a simple sol-gel method followed by a hydrothermal method. 0.01M Na MoO ·2H O was

dissolved in 50ml of double distilled water and was vigorously stirred₂ for₄ 30min.₂ To this solution was added 0.3M TTIP dropwise to obtain a white suspension. The

suspension was further stirred for 1h and then 0.1M Na2WO4·2H2O solution was added. The stirring was continued for 2 more hours and the reaction temperature was set at 80oC. The precipitate was then transferred into a Teflon coated stainless steel autoclave and was kept in a furnace at 180oC for 12h. The precipitate obtained from the autoclave was thoroughly washed by double distilled water and then with ethanol. The precipitate was dried at 50oC in oven and then calcinated at 450oC in a muffle furnace. The crystalline pale white powder obtained was studied with the help of different characterization techniques.

For comparison, TiO2-Mo and WO3/TiO2 were synthesized by the similar method. o Pure TiO2 was also synthesized by the sol gel method and was calcinated at 450 C.

6.2.3. Characterization Techniques The obtained material photocatalysts were characterized by a set of characterization techniques. FTIR (Interspec 2020 FTIR Spectrometer) was used to elucidate the different interactions in the nanocomposite photocatalysts. SEM (JSM6510LV, JEOL) was done to determine the surface morphology of the material synthesized and Energy Dispersive Spectroscopy (EDS, JSM6510LV, JEOL) was done to determine the elemental composition of the photocatalyst which was further supported by EDS mapping. Cyclic Voltammetry (CV) was done to determine the metal ion oxidation 176 Chapter 6

states in these nanocomposites. TEM (JEM2100, JEOL) was used to observe the particle shape and size at higher resolutions and SAED pattern was done to get the lattice pattern and determine formation of the heterostructure. XRD (Miniflex-TM II Benchtop, Rigaku Cooperation, Tokyo, Japan) was done to determine the crystal structure and crystallite size for different samples. The samples were exploited for the photocatalytic degradation of an organic dye Methylene Blue and non-dye p- chlorophenol. The UV–visible absorption spectra for the pollutant samples were obtained in the range of 190–800nm by UV-visible Spectrophotometer (UV-1601, Shimadzu). Absorption edge and the band gap of the photocatalysts were determined by using diffuse reflectance spectroscopy (UV-DRS, UV1901, Puxi). Photoluminescence (PL, F-7000, Hitachi) of the samples was done to determine the rate of recombination in these samples and the electrochemical impedance spectroscopy (EIS) was done to assess the rate of charge transfer. Terephthalic acid photoluminescence was also done to determine the generation of hydroxyl radicals on the photocatalyst surface.

6.2.4. Photocatalysis Experiment The photocatalytic activity of the nanocomposites was assessed by monitoring the decolorization of a synthetic dye MB (Scheme 6.1) under the influence of visible light irradiation. The experimental set up was same as mentioned in the previous chapters. A visible light halogen linear lamp (500W, 9500Lumens) was used to carry out the irradiations. The temperature was kept constant at 20±0.3oC using refrigerated liquid bath circulation. Before irradiation, an appropriate concentration of the dye solution (300mL), containing the desired quantity of the photocatalyst (1gL-1) was stirred with the magnetic stirrer, while the atmospheric oxygen was passed continuously into the solution for 20min in the dark to attain adsorption–desorption equilibrium between the dye solution and the photocatalyst. Subsequently, first sample (at 0min) was taken out and the irradiation was applied. During irradiation, samples of 5mL each were withdrawn at constant time intervals. The samples were centrifuged and the supernatant was subsequently analyzed. The variation in absorbance of the dye

aliquots was monitored at its λmax (660nm) as a function of irradiation time. The observed absorbance spectra were in accordance with Beer-Lambert Law in the range of dye concentrations examined. The concentrations of the dye at different time intervals of irradiation were calculated from standard calibration curve obtained by 177 Chapter 6

the absorbance of the dye at different known concentrations. The photocatalytic experiments were repeated four times under the same conditions in order to monitor the reproducibility of the results. The accuracy in the optical density was between ±5%.

Scheme 6.1: Chemical Structure of Methylene Blue (MB) dye

The stability and the consistency in activity of the photocatalyst were analyzed by the recycling experiments as mentioned in the previous chapters. The photocatalyst concentration to be used for the most efficient activity was optimized by a series of photocatalysis experiments using different photocatalyst concentrations. The degradation efficiency (%) by the photocatalyst was calculated as follows:

(%) = × 100 (4) 퐶표−퐶 퐷푒푔푟푎푑푎푡푖표푛 퐸푓푓푖푐푖푒푛푐푦 퐶표 Where, C0 is the initial concentration of MB and C is the concentration at different irradiation times. According to Langmuir–Hinshelwood (L–H) kinetic model, the first order kinetic equation which can be applied to explain the photocatalytic MB degradation is as follows.

æöCo ln ç÷= ktapp (5) èøCt

Where C0 is the initial concentration of the dye solution and C is the concentration of −1 dye at time t, and kapp is the apparent first-order rate constant (min ). To determine the role and fate of resultant reactive oxygen species (ROS), various quencher species (or ROS Scavengers) were added separately to the reaction system during the photocatalytic experiments.

178 Chapter 6

6.3. Results and Discussion 6.3.1. Crystallographic Study

XRD patterns of TiO2 and the Mo doped WO3/TiO2 nanocomposite powder calcined at 450°C for 4h are shown in Figure 6.1.

Figure 6.1: XRD pattern of TiO2 and WO3/TiO2-Mo nanocomposite

The diffraction peaks of TiO2 are attributed to both anatase and the rutile phase while as the diffraction peaks for the nanocomposite are mainly of anatase in nature, and there is no other phase detected. Moreover, the XRD patterns reveal that the intensity of diffraction peaks decreases with Mo-doping and WO3 coupling, which may reflect

the decrease in the crystalline quality of anatase TiO2 due to the doping of Mo 44 element. The Mo ions should substitute the position of Ti ions in the TiO2 lattice. ◦ ◦ ◦ The presence of the main peaks of WO3/TiO2-Mo sample at 2θ= 25.3 , 37.7 , 48.0 , 53.8◦, 55.0◦, 62.6◦are indicative of the (1 0 1), (0 0 4), (2 0 0), (1 0 5),(2 1 1) and (2 0

4) crystal planes of anatase phase of TiO2, respectively (JCPDS # 65-5714), which can be seen in Figure 6.1. Nevertheless, it can be seen that the nanocomposite sample

show the same diffraction peaks as that of pure TiO2, and the characteristic WO3 reflections are not observed since the XRD technique is unable to detect lower percentages of a crystalline phase. In addition, there is no any additional characteristic 179 Chapter 6

peak, which confirms that complex metal oxide or other new phases are not formed during the preparation process. Same way the XRD is insensitive to the detection of Mo doping.46 As shown in Figure 6.1, the X-ray diffraction peaks of anatase crystalline plane (1 0 1) and (2 0 0) in the composite powder show a slight shift toward a higher diffraction

angle compared to pure TiO2 with the doping of Mo and WO3 content. This indicated the occurrence of lattice distortion in the anatase structure by the incorporation of the dopants. Further, it can be seen that the peaks in the diffraction pattern of the nanocomposite sample has low intensity and have broadened with respect to the pure

TiO2. This shows that there was decrease in the crystallite size on doping and heterostructure formation. Grain growth inhibition may result because of the decrease in the number of inter-granular contacts among the neighbouring titania grains upon 47, 33 increasing the dopant amount in TiO2.

6.3.2. Microscopic Analysis

The surface morphology of the WO3/TiO2, TiO2-Mo and the WO3/TiO2-Mo nanocomposite samples was studied by SEM (Figure 6.2). Round and elongated shape particles with rough and irregular surfaces can be seen in all the three cases.

The microstructures of the WO3/TiO2 nanocomposite are shown in Figure 6.2(A-B) while as the micrographs in Figure 6.2(C-D) shows the morphology of Mo doped

TiO2. Figure 6.2(E-F) shows the SEM images of the WO3/TiO2-Mo nanocomposite.

The change in morphology is due to the inhibition in crystal growth of TiO2 during

nanocomposite formation with WO3 and also mixing up of different metal oxide phases. The efficient dispersion of phases can effectively improve the photocatalytic activity.48

180 Chapter 6

Figure 6.2: SEM micrographs: (A-B) WO3/TiO2 heterostructures; (C-D) TiO2-Mo nanoparticles; (E-F)

WO3/TiO2-Mo nanocomposite

Figure 6.3 presents the TEM micrographs of TiO2-Mo nanoparticle samples. As can be observed from Figure 6.3(a-b), there are found small round spherical shaped TiO2- Mo nanoparticles aggregated in chains. Figure 6.3(c) shows the lattice pattern with the measured lattice spacing of 0.380nm which is in coincidence with the spacing distance of (101) plane. The corresponding SAED pattern shows the diffraction ring 44 ascribed to the (101) crystalline plane of anatase TiO2, as shown in Figure 6.3(d). 181 Chapter 6

Figure 6.3: HRTEM images: (a-b) TiO2-Mo nanoparticles; (c) the lattice plane spacing of the TiO2-

Mo nanoparticle and (d) the SAED pattern of TiO2-Mo nanoparticles

Figure 6.4 presents the TEM micrographs of the WO3/TiO2-Mo nanocomposite.

Figure 6.4(a-b) shows round and oval shaped particles corresponding to TiO2-Mo

and WO3 respectively. The lattice pattern shows the lattice spacing of 0.380nm and

0.370nm corresponding to (101) and (002) lattice planes of anatase TiO2 and WO3 respectively (Figure 6.4(c)). The SAED pattern of the nanocomposite showed the

diffraction rings ascribed to (101) and (002) crystal planes of TiO2 and WO3 respectively Figure 6.4(d).44 182 Chapter 6

Figure 6.4: HRTEM images: (a-b) WO3/TiO2-Mo nanocomposite; (c) the lattice plane spacing of the

WO3/TiO2-Mo nanocomposite and (d) the SAED pattern of the WO3/TiO2-Mo nanocomposite

The EDS shows characteristic distribution and peaks associated with O, W, Ti, and C

for WO3/TiO2 (Figure 6.5(a)); and O, Mo, Ti, and C were observed for TiO2-Mo (Figure 6.5(b)). Characteristic distribution and peaks associated with O, W, Mo and Ti were observed

for WO3/TiO2-Mo sample (Figure 6.6); hence confirms that the as-prepared material

was composed of WO3 and Mo-doped TiO2. EDS mapping also confirms that the nanocomposite has fair distribution of its constituents. The peak of C atom observed in the spectrum could be attributed to the carbon present on the adhesive tape used to hold the samples. 183 Chapter 6

Figure 6.5: The EDS spectrum showing the elemental composition of (a) TiO2-Mo nanoparticles; (b)

WO3/TiO2 nanocomposite

Figure 6.6: EDS mapping and EDS spectrum of the WO3/TiO2-Mo nanocomposite showing the elemental composition and distribution of the constituents 184 Chapter 6

6.3.3. FT–IR spectral analysis of the photocatalysts

Figure 6.7 depicts the FT–IR spectra of the WO3/TiO2, TiO2-Mo and the WO3/TiO2- Mo nanocomposite catalysts in the wave number range from 400cm−1 to 4000cm−1.

Figure 6.7: FT-IR spectra of TiO2-Mo, WO3/TiO2 and WO3/TiO2-Mo samples

As can be seen, the broad absorption bands in the range from 400cm−1 to 700cm−1 are obtained and are attributed to the Ti-O bond stretching vibration in Ti-O-Ti.49 The peaks at 1628cm−1 corresponds to the O-H bond bending vibrations of chemisorbed water, and the broad absorption peak appearing near to 3440cm−1 is assigned to the O- H bond stretching mode of the surface hydroxyl groups or adsorbed water, strongly 50 -1 -1 bound to the catalyst surface. A doublet between 830cm to 860cm for TiO2-Mo

and the WO3/TiO2-Mo nanocomposites indicate incorporation of Mo in the TiO2

lattice. As can be inferred from Figure 6.7, TiO2-Mo has the smallest hydroxyl peak -1 around 1628cm , whereas the composite WO3/TiO2-Mo has the largest hydroxyl peak indicating that more water molecules and hydroxyl groups were adsorbed on the surface of WO3/TiO2-Mo nanocomposite. More surface hydroxyl groups are beneficial for the photocatalytic reactions, especially for the generation of •OH radicals.51,52

185 Chapter 6

6.3.4. Optical Study

The optical properties of the synthesized TiO2, WO3/TiO2, and the WO3/TiO2-Mo samples were obtained by using UV–visible diffuse reflectance spectroscopy (UV- DRS) and the results are shown in Figure 6.8.

Figure 6.8: (a) DRS spectra of the samples (b) Band gap energy curves obtained for the samples

According to Figure 6.8(a), the WO3/TiO2-Mo nanocomposite is red-shifted towards

the visible region as compared to the TiO2 on addition of Mo which may be caused by the doping energy level of Mo6+ ions below the conduction band, suggesting that the

visible light absorption of Mo-TiO2 arose from the electronic transition from valence

band to the doping energy level. Further, WO3 showed a fair absorption in the visible region (>450nm).53 The Mo+6 dopant has no valence electrons as its electronic configuration is [Kr] 4d04s0. Electrons may be excited from the O 2p valence band into the Mo impurity band (Mo+6/Mo+5) created below the conduction band. Electron transition can take +5 place from Mo impurity level to the conduction band of TiO2 by d(Mo )-d(Ti) transition.54 Thus, the new energy levels which are created below the conduction band

of TiO2 are responsible for the activation of the catalyst under visible light illumination.33 The band gap energy of the photocatalyst samples was calculated according to the Kubelka–Munk method as discussed in Chapter 2.

The optical band gap of TiO2, WO3 and the WO3/TiO2-Mo was determined from the plot of (F(R).hv)1/2 versus hv (Figure 6.8(b)) and was elicited to be 3.1eV, 2.70eV

and 2.90eV respectively. The valence band edge position of WO3/TiO2-Mo heterostructure at the point of zero charge can be calculated by the following empirical equation:55, 56 186 Chapter 6

c EVB =X−E +0.5Eg (6)

The top of the valence band EVB of WO3 and TiO2-Mo were calculated to be 3.44eV/NHE and 2.8eV/NHE respectively. Moreover, the conduction band edge

potential ECB can be determined by:

ECB = EVB − Eg (7)

Thus, the calculated ECB for WO3 and TiO2-Mo are 0.74eV/NHE and -0.10eV/NHE, respectively.

As shown in Figure 6.8(b), the band-gap energy of the WO3/TiO2-Mo samples decreased with respect to pure TiO2 which means that these nanocomposites absorbed lower energy photons, and the more electron hole pairs were generated in the photocatalytic reaction. Thus, the photocatalytic activities of the samples were improved. However, decreasing band gap energy may also speed up the recombination of electrons and holes in some cases, which was though inhibited here

by the heterostructure formation with WO3.

6.3.5. Kinetic study and mechanism 6.3.5.1. Photocatalytic activity of catalysts The photocatalytic performance of the synthesized photocatalysts for the decolorization of MB under visible light irradiation was investigated in aqueous medium, and the results are shown in Figure 6.9(a-e). In the control experiments, no decolorization of MB was observed in the dark and in the absence of photocatalyst, indicates that MB was stable and did not undergo a photolytic degradation (Figure 6.9(b)). In addition, the catalytic decolorization was negligible in dark as a result of adsorption, suggesting that a purely photocatalytic reaction mechanism took place during photocatalytic experiments. It can be observed from the Figure 6.9(b) that the decolorization rate follows the

order: WO3/TiO2-Mo > WO3/TiO2> TiO2-Mo>TiO2. The decolorization of MB by

WO3/TiO2-Mo was 90.65%, which was about 7.83 times, 2.52 times and 1.15 times

greater than that of pure TiO2 (11.57%), TiO2-Mo (35.97%) and WO3/TiO2 (78.45%) 187 Chapter 6

respectively. The experimental data was found to fit well with the first order kinetic

equation. Also from Figure 6.9(c), the WO3/TiO2-Mo composite catalyst was found

to exhibit the highest photo-degradation efficiency, where the kapp of WO3/TiO2-Mo −1 was 0.0512min , which was 17.65, 4.65 and 1.56 times higher than TiO2 −1 −1 −1 (0.0029min ), TiO2-Mo (0.011min ) and WO3/TiO2 (0.0327min ) respectively.

Figure 6.9: Kinetics of photocatalytic decolorization of MB in the presence of pure TiO2, TiO2-Mo,

WO3/TiO2 and WO3/TiO2-Mo samples (a) Change in absorption of MB at regular intervals of light

irradiation in the presence of the WO3/TiO2-Mo photocatalyst, (b) change in concentration (Ct/C0) of

MB during its decolorization in the presence of TiO2, TiO2-Mo, WO3/TiO2 and WO3/TiO2-Mo

samples, (c) ln(C0/Ct) versus irradiation time for decolorization of MB in the presence of synthesized

photocatalysts, (d) Recycling experiments of WO3/TiO2-Mo for decolorization of MB under visible light, (e) Variation in colour of the MB dye at regular intervals of irradiation in the presence of

WO3/TiO2-Mo photocatalyst. 188 Chapter 6

This suggested that WO3 nanoparticles contributed to the higher redox potentials with well-aligned band-structures and heterostructure interfaces were favourable for the separation of electrons and holes, leading to an enhanced photocatalytic performance. Further the co-doping of Mo has also enhanced the MB decolorization by almost one and a half fold.

To evaluate the stability of the WO3/TiO2-Mo composite catalysts, the photocatalytic activity was investigated in cycling runs and the results are presented in Figure

6.9(d). In our recycling experiments of MB photodegradation, the WO3/TiO2-Mo sample exhibited a minimal decrease in activity even after four cycles of reuse. Figure 6.9(e) shows the change in colour of MB on irradiation in presence of

WO3/TiO2-Mo photocatalyst. In order to validate the photocatalytic activity of these photocatalysts, they were applied for the photocatalytic degradation of non-dye pollutant, such as p- chlorophenol (PCP). It was found that the photocatalysts showed consistent performance for PCP degradation, hence, ruled out the exclusive dye sensitized

photocatalysis by these photocatalysts (Figure 6.10). The rate constant (kapp) for the phenol degradation was found as 0.0437min-1, 0.0289min-1, 0.0094 min-1 and -1 0.0011min for WO3/TiO2-Mo, WO3/TiO2, MoO3/TiO2 and TiO2 respectively.

Figure 6.10: Comparison between the photodegradation of MB dye and PCP in presence of different photocatalysts 189 Chapter 6

6.3.5.2. Role of the Reactive Oxygen Species The radical and hole trapping experiments (scavenger tests) with different scavengers were carried out to elucidate the mechanism of photocatalytic degradation of MB

under visible light irradiation over WO3/TiO2-Mo nanocomposite. Generally the ROS, −• • + such as, superoxide radical anions (O2 ), hydroxyl radicals ( OH), and holes (h ) are expected to have role in the photocatalytic dye degradation processes. The fate and the role of ROS were investigated through radical and hole trapping experiments and the results obtained are shown in Figure 6.11. The decolorization of MB decreased significantly upon the addition of isopropyl alcohol (IPA, a hydroxyl radicals scavenger57), indicating that dissolved •OH radicals were the dominant active species in this process. The addition of disodium ethylenediaminetetraacetate (EDTA, a hole scavenger58) had also significant negative effect on the degradation of MB, −• confirming that the holes were also the dominant active species. Upon addition of O2 59,60 quencher, benzoquinone (BQ), the kapp dropped less significantly, indicating that −• O2 radicals were not the dominant active species in the reactive system. Similarly,

electron scavenger, AgNO3, also indicated that electrons were another dominant active species in the degradation process.

Figure 6.11: Effect of different ROS scavengers on kapp of WO3/TiO2-Mo nanocomposite for MB degradation

190 Chapter 6

6.3.5.3. Photoluminescence Study Figure 6.12 presents the photoluminescence spectra of the nanocomposite samples.

As can be seen, the intensity of fluorescence peak follows the order TiO2-Mo

>WO3/TiO2 >WO3/TiO2-Mo. It confirms that WO3 has significantly inhibited the

charge recombination when coupled with Mo-doped TiO2 in the WO3/TiO2-Mo nanocomposite. This attributes to its high photocatalytic efficiency and durability.

However, TiO2-Mo also showed less recombination than pure TiO2 due to the reasons explained in previous sections.

Figure 6.12: Photoluminescence spectra of the photocatalyst samples showing the extent of charge recombination

Further, to determine the generation of HO• radical on the photocatalyst surface, terepthalic acid photoluminescence was done. The HO• radical reacts with terepthalic acid in the photoreactor during the photocatalytic irradiation process to produce dihydroxyterephthalic acid. The photoluminescence emission intensity of dihydroxyterephthalic acid produced is the direct measure of the HO• concentrations. The photoluminescence patterns of terephthalic acid solution after being illuminated

under visible light for 60min in presence of WO3/TiO2-Mo are shown in Figure 6.13. The increasing emission intensities with irradiation time indicated the continuous • generation of HO radicals on the surface of WO3/TiO2-Mo during the photocatalytic process. 191 Chapter 6

• Figure 6.13: Hydroxyterepthalic acid fluorescence indicating generation of HO by WO3/TiO2-Mo nanocomposite during the photocatalytic process

6.3.5.4. Electrochemical Impedance Spectroscopy The charge separation efficiency of photogenerated electrons and holes across the heterostructure interface is a crucial factor for the photocatalytic activity, which can also be examined by the EIS Nyquist plots. Figure 6.14 shows the EIS Nyquist plots

of TiO2, WO3/TiO2, TiO2-Mo and WO3/TiO2-Mo nanocomposite.

Figure 6.14: Nyquist plots showing the extent of impedance imposed by different samples to charge transfer 192 Chapter 6

Because of the faster charge transfer inside the semiconductor than at the semiconductor/electrolyte interface, high frequency response is attributed to the electronic process in the semiconductor and the resistance of the solution, whereas medium and low frequency response is attributed to the interfacial charge-transfer resistance between the semiconductor and the electrolyte. It shows one dominant semicircle, whose diameter is related to the charge-transfer resistance at the semiconductor/electrolyte interface. Smaller diameter of the semicircle means smaller impedance to charge separation and transportation.

Figure 6.14 shows that the arc radius of the EIS Nyquist plot in WO3/TiO2-Mo nanocomposite was smallest than that of the other three. Since, the arc radius of EIS spectra reflects the interface layer resistance occurring at the surface of the electrode, it implies that more effective separation of charge carriers and faster interfacial charge 61,62 transfer has occurred on the WO3/TiO2-Mo nanocomposite photocatalyst.

6.3.5.5. Cyclic Voltammetry Analysis The electrochemical redox properties of the nanocomposites have been studied using cyclic voltammetry in the potential range +1.0 to -1.0V recorded at 100-250mVs-1 scan rate with reference to Ag/AgCl electrode at room temperature in the presence of KCl as supporting electrolyte. The nature of voltammograms recorded at 100- 250mVs-1 scan rates for the different samples, respectively, were nearly identical suggesting that the species generated during the electrochemical processes possess quite good life time.63 The representative voltammograms and the parameters i.e. c a positions of cathodic peak (EP ) and the anodic peak (Ep ) are indicated in Figure 6.15 and Figure 6.16. IV/III In figure 6.15(a), the Mo doped TiO2 showed a quasi-reversible redox couple (Ti ) c a o with EP = 0.47V and Ep = 0.28V. The corresponding half wave potential, E 1/2, was calculated to be 0.09V. A small irreversible anodic peak was also found with Epa = - 0.27V which can be attributed to Mo+6 oxidation state. IV/III Same way in case of WO3/TiO2 nanocomposite, a reversible redox couple (Ti ) c a with EP = 0.47V and Ep = 0.30V was found. However, it showed another reversible VI/IV c a redox couple for W with EP = -0.48V and Ep = -0.24V respectively (Figure o IV/III VI/IV 6.15(b)). The corresponding half wave potential, E 1/2, for Ti and W redox couples was found to be 0.08V and -0.36V. 193 Chapter 6

Figure 6.15: Cyclic voltammograms for (a) TiO2-Mo; (b) WO3/TiO2 nanocomposite

194 Chapter 6

Figure 6.16(a) presents the CV voltammogram for WO3/TiO2-Mo nanocomposite at the scan rates of 100-250mVs-1.

Figure 6.16: Cyclic voltammograms for (a) WO3/TiO2-Mo nanocomposite at the potential scan rate of -1 100-250mVs ; (b) TiO2-Mo, WO3/TiO2 and WO3/TiO2-Mo nanocomposite at potential scan rate of 250mVs-1 195 Chapter 6

Here again, we obtained two reversible redox couple peaks for TiIV/III and WVI/IV

redox systems with the peak potential values in coherence with that in WO3/TiO2 +6 c nanocomposite. In addition, an irreversible peak was also found for Mo with EP = - 0.28V which is due to electrochemical disproportionation process. This indicated that the composite is composed of a three metal system.

Figure 6.16(b) presents the CV voltammogram for TiO2-Mo, WO3/TiO2 and the -1 WO3/TiO2-Mo nanocomposite at the scan rate of 250mVs . It shows that the

WO3/TiO2-Mo nanocomposite has broader voltammogram that indirectly indicated its higher specific capacitance than the other two which is beneficial for the photocatalysis.63,64

6.3.5.6. Band gap structures and possible degradation mechanism Photocatalytic performance of the semiconductor nanocomposites depends upon their heterojunction interface, and the electronic structures,65 because the photocatalytic activity is significantly related to the CB and the VB positions, as well as to the mobility of the charge carriers. The energy position of CB and VB could influence the oxidative and reductive ability of the semiconductor photocatalyst respectively, while the mobility of the charge carriers could influence the photocatalytic efficiency. The improved charge separation of the photogenerated carriers and the light energy utilization ratio are favourable for the photocatalytic activity. It was reported that the well-matched overlapping band-structure in the semiconductor nanocomposite could promote the separation of electrons and holes, thereby enhancing the photocatalytic activity.66

The band edge positions of WO3/TiO2-Mo, WO3 and TiO2 were estimated according

to the methods discussed above. The results confirmed that WO3/TiO2 possessed a composite heterostructure, which was favourable for the charge separation. Pertaining

to the band positioning of as-prepared WO3/TiO2-Mo and the influence of scavengers, possible mechanistic pathways for the photocatalytic decolorization were proposed.

The VB and CB of WO3 (EVB= 3.44eV, ECB= 0.74eV); and TiO2-Mo (EVB= 2.80eV,

ECB= -0.10eV) were provided to clearly understand the separation and transfer of electron–hole pairs across the interface of the nanocomposite heterostructures (Figure 6.17).

The Fermi level (EF) for n-type TiO2-Mo was close to the conduction band, whereas

the EF for WO3 was close to the valence band. When WO3 and TiO2-Mo form the 196 Chapter 6

heterostructure, the WO3 gets excited under visible light to generate electrons and holes in the CB and VB respectively,67 as depicted in Figure 6.17. As Mo doping lead

to the decrease in band gap of TiO2 and also formed an impurity electron trapping level below its conduction band minimum, it also got sensitized to the visible light. The visible light response of the doped powder is due to the electronic transition

between the impurity levels and the conduction band of TiO2. Thus, the new energy

levels which are created below the conduction band of TiO2 are responsible for the activation of the catalyst under visible light illumination. Apart from the impurity energy levels and d-d transitions, there are other factors that may contribute to visible light absorption such as Ti+3 surface defects and oxygen vacancies. These oxygen +3 68,69 vacancies and Ti centres result in new states below the conduction band of TiO2 and help to contribute in the band gap narrowing, because upon visible light

illumination electrons can be excited to these new states from the TiO2 valence band and from the impurity levels. Electrons from these new states (Ti+3, oxygen

vacancies) can also be transferred to the TiO2 conduction band under visible light irradiations.33

The series of reactions which can occur on the Mo doped TiO2 are as below:

+ − Mo-TiO2 + hν hVB + eCB (8) 6+ 5+ Mo + eCB− Mo (9) 5+ 6+ • − Mo + O2 (ads) Mo + O2 (10) • − − + O2 +eCB + 2H H2O2 (11) − • − H2O2 +eCB OH+OH (12) + − • hVB + OH OH (13) • OH+MB/PCP Intermediates CO2 +H2O (14)

As discussed above, the energy level of conduction band of TiO2-Mo (−0.10eV vs. • − NHE) was higher than the O2/ O2 redox potential (about 0.16eV vs. NHE), thus the electron in conduction band of TiO2 can be captured by O2. In this study, the band gap

of Mo doped TiO2 was 2.90eV, while pure TiO2 was 3.10eV, indicating that the doping energy level of Mo was 0.20eV lower than the conduction band of TiO2, and • − 0.04eV higher than the O2/ O2 redox potential. Meanwhile, it was possible for the photogenerated electrons at the conduction band to fall onto the doping energy level of Mo species. Furthermore, the photogenerated electrons can easily transfer from the conduction band and the Mo doping energy level to the surface of the materials and 197 Chapter 6

then captured by the adsorbed O2, thereby enhancing the separation efficiency of photogenerated charge carriers.53

The electrons in the CB of WO3 would not take part in the reduction process to −• generate O2 radical while the holes will transfer to the VB of TiO2-Mo. However,

the electrons trapped in the CB of TiO2-Mo will transfer to WO3 or may take part in −• O2 radical generation due to its more negative reduction potential. As a result, the photogenerated charge carriers would be separated efficiently by the heterostructure being formed in these heterostructures. Normally, the CB and VB positions of the

single WO3 and TiO2-Mo were unfavourable for the separation of electron–hole pairs before coupling.

Figure 6.17: Formation of the heterostructure and the mechanism of formation of different reactive oxygen species

Furthermore, the doped Mo could generate a positive charge difference and the Mo+6 ion acted as a Lewis site, which can generate more hydroxyl groups to balance the positive charge. Therefore, more holes could be quickly scavenged by the hydroxyl groups to produce •OH. The adsorbed •OH eventually degraded MB and PCP into

benign compounds such as CO2, H2O, and the inorganic substrate. Both the transferring and scavenging of the hole prevented the electron-hole recombination, which remarkably improved the photocatalytic efficiency. 198 Chapter 6

It has also been investigated that by the coverage of WO3 on TiO2 surfaces, the TiO2 particles were well suspended in aqueous solution, and the stability of the colloidal

suspension was greatly improved. It is speculated that WO3 on the surface of TiO2 may accommodate more surfactant molecules, or substantially the surface of

WO3/TiO2 has much more hydrophilic character, compared with that of TiO2. Hence,

the Mo doped WO3/TiO2 photocatalyst will have fair access to the H2O molecules which is beneficial for an efficient photocatalysis. Three possible mechanisms might explain the degradation of the dye on the surface of the semiconductor under photocatalytic conditions. In addition to the photocatalyst mediated degradation mechanism, the dye-sensitized mechanism and dye-photolysis mechanism may also exist.70 However, as observed from the phenol based photocatalytic experiments, dye sensitized photocatalysis was not predominant in these cases. Based on the above assumption, a plausible photocatalytic mechanism was proposed,

as illustrated in Figure 6.17. With regard to the photogenerated electrons in WO3, it

was not thermodynamically allowed for the one-electron reduction of O2 on WO3, owing to the higher potential of the conduction band (+0.74eV vs. NHE) than the •− 71 redox potential of O2/O2 (−0.16eV vs. NHE). Hence, it proceeds through multi- − • electron process (O2 + 2H2O+4e →4OH ; +0.40V vs. NHE) but with difficulty.

Further, the CB level of TiO2 (-0.30V vs. NHE) is much more negative than that of

WO3, once the electrons transferred to the negative CB, the electrons may reduce O2 − •− by single-electron process (O2 + e → O2 ; -0.16V vs. NHE) over the CB of TiO2-

Mo; or may transfer to the CB of WO3 and would again reduce O2 by a multi-electron process.

6.3.5.7. Chemical Oxygen Demand Removal Efficiency Further, to assess the extent of mineralization of MB during the photocatalytic process, the chemical oxygen demand (COD) removal during the photocatalytic reaction was carried out in accordance with standard American Public Health 72 Association (APHA) method with WO3/TiO2-Mo sample. The decrease in the value of COD determines the degree of mineralization of organic species. As can be observed from Figure 6.18, the COD value decreased significantly as the function of irradiation time. At the end of irradiation, after 45min, the COD was reduced to about 78% of initial value. 199 Chapter 6

Figure 6.18: Change in COD of the MB solution during the irradiation in presence of WO3/TiO2- Mo sample

6.4. Conclusion This final chapter presents the evaluation of photo-electrochemical and photocatalytic

properties of molybdenum doped WO3/TiO2 nanocomposite heterostructures. The nanocomposites were synthesized by a typical sol-gel method followed by a hydrothermal method. The as synthesized products showed efficient visible light photocatalytic activity with nice consistency and stability for the photodegradation of

efficient photocatalysis. The doping of Mo content and the coupling with WO3 had a

significant effect on the photon absorption of the TiO2 nanoparticles, and its

absorption was extended considerably into the visible region. WO3 coupling also favoured the separation and transfer of photoinduced charge carriers to inhibit their

recombination. Photoluminescence study showed that addition of Mo and WO3

efficiently decreased the charge recombination in TiO2. Electrochemical Impedance

Spectroscopy explains that the WO3/TiO2-Mo nanocomposite exhibits enhanced 200 Chapter 6

charge transfer and reduced charge recombination. Phenol based experiment indicated that the dye-sensitized photocatalysis is not the dominant phenomenon in these photocatalysts. Finally, the COD analysis inferred efficient mineralization of the degradation products.

201 Chapter 6

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1. Bilal Masood Pirzada, Niyaz Ahmad Mir, Nida Qutub, Owais Mehraj and Suhail Sabir, Synthesis, Characterization and Optimization of Photocatalytic

Activity of TiO2/ZrO2 Nanocomposite Heterostructures, Materials Science and Engineering:B, 2015, 193, 137. 2. Bilal Masood Pirzada, Niyaz Ahmad Mir, Owais Mehraj, Mohammad Zain khan and Suhail Sabir, Efficient visible light photocatalytic activity and

enhanced stability of BiOBr/Cd(OH)2 heterostructures, New Journal of Chemistry, 2015, 39, 7153. 3. Bilal Masood Pirzada, Owais Mehraj, Suhail Sabir, Efficient photocatalytic

activity by α-Bi2O3 sensitized Bi2O3/TiO2-Zr nanocomposites, Journal of Photochemistry and Photobiology A: Chemistry (Under Revision)

Materials Science and Engineering B 193 (2015) 137–145

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Materials Science and Engineering B

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Synthesis, characterization and optimization of photocatalytic activity of TiO2/ZrO2 nanocomposite heterostructures

1 ∗

Bilal Masood Pirzada, Niyaz A. Mir , Nida Qutub, Owais Mehraj, Suhail Sabir , M. Muneer

Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India

a r t i c l e i n f o a b s t r a c t

Article history: Titanium dioxide/zirconium dioxide nanocomposites were synthesized using a simple and modiﬁed

Received 17 June 2014

sol–gel method. The synthesized particles were characterized through SEM, TEM, XRD, and DTA/TGA.

Received in revised form

SEM analysis shows micrographs with irregular and sharp edged particles. TEM analysis shows spherical

17 November 2014

particles of average diameter 10.5 nm. Tetragonal structure of TiO2/ZrO2 nanocomposite particles with

Accepted 1 December 2014

stabilized anatase phase was conﬁrmed by XRD studies. The average crystallite size was calculated from

Available online 11 December 2014

XRD, using Scherrer’s formula. Band gap was calculated from the DRS spectra using Kubelka-Munk func-

tion and Photoluminescence (PL) was done to study the recombination rate of charge carriers. Results

Keywords:

Decolorization showed a considerable increase in band gap on ZrO2 addition and subsequent decrease in recombination

rate. Impedance study showed a signiﬁcant decrease in dielectric characteristics on ZrO2 addition. Pho-

Hybrid sol–gel

Photocatalysis tocatalytic activity of the synthesized catalysts was studied by degradation of an azo-dye, Ponceau BS,

Ponceau BS using ultra-violet source of light. Optimum activity was observed on 6.0% ZrO2 loading.

1. Introduction To look for a feasible and efﬁcient photocatalyst, metal oxide

nanomaterials are being explored as semiconductor photocatalysts

The un-abating use of organic dyes in different industries is a for the degradation of organic pollutants. These semiconductor

serious problem in terms of environmental concerns. Extensive use catalysts are able to degrade pollutants into easily biodegradable

of organic dyes (mostly azo dyes) in textile industries is a big threat compounds and eventually mineralize them into carbon dioxide

to water bodies and severely affect water quality parameters. To and water. TiO2 has received much attention as a photocatalyst

overcome this issue, several treatment methods, such as, Chemi- after Fujishima and Honda discovered the phenomenon of photo-

cal Oxidation, Wet Oxidation, Biological Treatment, Ozonolysis and catalytic splitting of water on a TiO2 electrode under ultraviolet

Activated Carbon Adsorption have been proposed for the removal (UV) light [7,8]. TiO2 gained priority due to its abundance in the

of organic pollutants from industrial efﬂuents. Moreover, photo- geosphere, high photochemical stability, low cost, non-toxicity,

catalysis proved to be promising due to the use of easily available biological inertness, reusability and the possibility of its activa-

∼

solar energy [1–3]. tion by sunlight [9–11]. However, a large band gap ( 3.2 eV) in

In photocatalysis, the catalyst absorbs photons of light to TiO2 makes it responsive in the ultra-violet light which makes only

undergo excitation of electrons from the valence band to the con- about 5% of the solar spectrum. Hence, efforts are being put to make

duction band, generating electron-hole pairs. These electron hole it active into the visible region, in order to make the best use of solar

pairs act as redox centres and readily initiate oxidation-reduction energy [12].

processes on catalyst surface, resulting in the degradation of toxic Moreover, pure titania is poor in quantum efﬁciency due to pre-

pollutants [4–6]. vailing photo-corrosion by rapid recombination of electrons and

holes. To circumvent, development of reliable composite photo-

catalyst is a priority in today’s research [13]. In order to effectively

control photo-corrosion, the various attempts include encapsu-

lation of electron acceptors on functionalized semiconductors,

Abbreviations: TTIP, Titanium tetra-isopropoxide; PBS, Ponceau BS.

immobilization of semiconductor photocatalyst in redox function-

∗

Corresponding author at: Tel.: +91 571 2700920x3366.

alized polymers and electrostatic association of electron acceptors

E-mail addresses: [email protected], [email protected] (S. Sabir).

1 at the semiconductor surface. The efﬁciency of photocatalyst is con-

Current Address: Solid State and Structural Chemistry Unit, Indian Institute of

siderably enhanced by coupling with a foreign metal oxide. Usually,

Science, Bangalore-560 012, India.

http://dx.doi.org/10.1016/j.mseb.2014.12.005

PAPER

Eﬃcient visible light photocatalytic activity and enhanced stability of BiOBr/Cd(OH)2 Cite this: New J. Chem., 2015, 39,7153 heterostructures†

Bilal Masood Pirzada,a Owais Mehraj,a Niyaz A. Mir,b Mohammad Zain Khana and Suhail Sabir*a

Novel BiOBr/Cd(OH)2 heterostructures were synthesized by a facile chemical bath method under

ambient conditions. A series of BiOBr/Cd(OH)2 heterostructures were obtained by tuning the Bi/Cd molar ratios. The obtained heterostructures were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). Optical properties were studied by UV-visible spectroscopy, diffuse reflectance spectroscopy and photoluminescence (PL). Photocatalytic studies on rhodamine B (RhB) under visible light irradiation showed that the heterostructures are very efficient photocatalysts in mild basic medium. Scavenger test studies confirmed that the photogenerated holes and superoxide radicals (O2 ) are the main active species responsible for RhB degradation. Comparison of photoluminescence (PL) intensity suggested that an inhibited charge recombination is crucial for the degradation process over these

Received (in Montpellier, France) photocatalysts. Moreover, relative positioning of the valence and conduction band edges of the 8th April 2015, semiconductors, O2/O2 and OH/H2O redox potentials and HOMO–LUMO levels of RhB appear to be Accepted 6th July 2015 responsible for the hole-specificity of degradation. Photocatalytic recycling experiments indicated the DOI: 10.1039/c5nj00839e high stability of the catalysts in the reaction medium without any significant loss of activity. This study

hence concludes that the heterojunction constructed between Cd(OH)2 and BiOBr interfaces play a www.rsc.org/njc crucial role in influencing the charge carrier dynamics and subsequent photocatalytic activity.

1. Introduction electron acceptors on functionalized semiconductors, immobilization of semiconductor photocatalysts in redox function- Heterogeneous photocatalysis has evolved as a viable technology alized polymers and electrostatic associationofelectronacceptors for the control of environmental pollution related issues and at the semiconductor surface were made. Transition metal ion energy conversion.1 To date various kinds of catalyst semicon- doping is one approach for acquiring visible response by ductor materials, including metal oxides,2 sulphides,3 nitrides,4 introducing intermediate impurity energy levels.9,10 The eﬃ- and their mixed solid solutions,5,6 have been used as photo- ciency of the photocatalyst is considerably enhanced by catalysts responsive to both the UV and visible light wavelengths. coupling with a foreign metal oxide.11,12a Currently, coupling

Primarily, TiO2 received wide attention due to its excellent of semiconductors with graphene is widely recognized to be photocatalytic activity, chemical stability, and non-toxicity.7 a viable strategy to improve the photocatalytic performance However, it has limited practical application due to a large band of an electronically conductive 2D platform enabling the gap of B3.2 eV and rapid recombination of photogenerated acceptance and shuttle of photogenerated electrons from charge carriers.8 band-gap-excitation of semiconductors.12b–d Recent studies In order to overcome the limitation and eﬀectively control have also revealed that some physical and chemical properties

photocorrosion, various attempts including encapsulation of of TiO2, such as light absorption, photocatalytic reactivity, selectivity, etc., can be modulated by its defect disorder.12e Heterostructure construction between two diﬀerent semicon- a Department of Chemistry, Aligarh Muslim University, Aligarh – 202002, India. ductors has also been extensively applied in many fields includ- E-mail: [email protected]; Tel: +91-571-2700920. ext. 3366 ing photocatalysis and solar energy conversion,13,14 because b Solid State & Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India heterojunctions control the behaviour of photogenerated † Electronic supplementary information (ESI) available. See DOI: 10.1039/ charges, such as the direction of transportation, the distance 15,16 c5nj00839e of separation, and the recombination rate. Furthermore, the

This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015 New J. Chem., 2015, 39, 7153--7163 | 7153