Photocatalytic functional materials for environmental remediation 9781119529842, 1119529840, 9781119529897, 1119529891, 9781119529910, 1119529913

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Photocatalytic Functional Materials for Environmental Remediation

Photocatalytic Functional Materials for Environmental Remediation

Edited by

Alagarsamy Pandikumar

Functional Materials Division CSIR‐Central Electrochemical Research Institute Karaikudi, Tamil Nadu, India

Kandasamy Jothivenkatachalam

Department of Chemistry, Bharathidasan Institute of Technology (UCE – BIT Campus) Anna University Tiruchirappalli, Tamil Nadu, India  

 

This edition first published 2019 © 2019 John Wiley & Sons, Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/ permissions. The right of Alagarsamy Pandikumar and Kandasamy Jothivenkatachalam to be identified as the authors of the editorial material in this work has been asserted in accordance with law. Registered Office John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA Editorial Office 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com. Wiley also publishes its books in a variety of electronic formats and by print‐on‐demand. Some content that appears in standard print versions of this book may not be available in other formats. Limit of Liability/Disclaimer of Warranty In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials, or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/ or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Library of Congress Cataloging‐in‐Publication Data Names: Pandikumar, Alagarsamy, editor. | Jothivenkatachalam, Kandasamy, 1973editor. Title: Photocatalytic functional materials for environmental remediation / edited by Alagarsamy Pandikumar (Functional Materials Division, CSIR-Central Electrochemical Research Institute, Karaikudi, Tamil Nadu, India), Kandasamy Jothivenkatachalam (Department of Chemistry, Bharathidasan Institute of Technology (UCE - BIT campus), Anna University, Tiruchirappalli, Tamil Nadu, India). Description: First edition. | Hoboken, New Jersey : John Wiley & Sons, Inc., 2019. | Includes bibliographical references and index. | Identifiers: LCCN 2019008193 (print) | LCCN 2019021592 (ebook) | ISBN 9781119529910 (Adobe PDF) | ISBN 9781119529897 (ePub) | ISBN 9781119529842 (hardcover) Subjects: LCSH: Photocatalysis. | Nanocomposites (Materials)–Environmental aspects. | Nanostructured materials–Environmental aspects. | Carbon dioxide mitigation. Classification: LCC QD716.P45 (ebook) | LCC QD716.P45 P56 2019 (print) | DDC 628.5028/4–dc23 LC record available at https://lccn.loc.gov/2019008193 Cover Design: Wiley Cover Images: © R.Tsubin / Getty Images, © Geir Pettersen / Getty Images, © Jacky Parker Photography / Getty Images, © Valentyn Volkov / Shutterstock, © djgis / Shutterstock Set in 10/12pt WarnockPro by SPi Global, Chennai, India Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

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Contents List of Contributors  xi Preface  xv 1

Titanium Dioxide and Carbon Nanomaterials for the Photocatalytic Degradation of Organic Dyes  1 Nagamalai Vasimalai

Abbreviations  1 1.1 Introduction  2 1.1.1 Impact of Dye Effluents on the Environment and Health  3 1.2 Principles and Mechanism of Photocatalysis  6 1.2.1 Direct Photocatalytic Pathways  7 1.2.1.1 The Langmuir–Hinshel Wood Process  8 1.2.1.2 The Eley–Rideal Process  8 1.2.2 Indirect Photocatalytic Mechanisms  8 1.3 Importance of Titanium Dioxide  9 1.3.1 Rutile  10 1.3.2 Anatase  10 1.3.3 Brookite  10 1.4 Titanium Dioxide for the Photocatalytic Degradation of Organic Dyes  11 1.4.1 Approaches Enhance the Photocatalytic Activity of TiO2  12 1.4.2 Metal and Multi‐Atom Doped TiO2  13 1.5 Carbon Nanomaterials for the Photocatalytic Degradation of Organic Dyes  15 1.5.1 Activated Carbon  16 1.5.2 Graphite  17 1.5.3 Graphene  19 1.5.4 Carbon Nanotubes and Fullerenes  20 1.5.5 Carbon Black  21 1.5.6 Carbon Nanofibers  22 1.5.7 Carbon Quantum Dots  22

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1.5.8 Mesoporous Carbon  24 1.6 Conclusion and Trends  26 References  27 2

Visible Light Photocatalytic Degradation of Environmental Pollutants Using Metal Oxide Semiconductors  41 S. Thangaraj Nishanthi

2.1 Introduction  41 2.2 Photocatalysis  42 2.3 Mechanism and Fundamentals of Photocatalytic Reactions  42 2.4 Synthesis of Different Photocatalysts  44 2.4.1 Hydrothermal/Solvothermal Methods  45 2.4.2 Electrodeposition  46 2.4.3 Chemical Bath Deposition  46 2.4.4 Sol‐Gel Process  47 2.4.5 Chemical Precipitation  47 2.5 Factors Affecting Photocatalytic Degradation  47 2.5.1 Catalyst Loading  47 2.5.2 pH of the Solution  48 2.5.3 Size and Structure of the Photocatalyst  49 2.5.4 Reaction Temperature  49 2.5.5 Concentration and Nature of Pollutants  49 2.5.6 Inorganic Ions  50 2.6 Metal Oxide Semiconductors  50 2.7 Ternary/Quaternary Oxides  54 2.8 Composites Semiconductors  55 2.9 Sensitization  56 2.10 Conclusions  57 References  57 3

Contemporary Achievements of Visible Light‐Driven Nanocatalysts for the Environmental Applications  69 Panneerselvam Sathishkumar, Nalenthiran Pugazhenthiran, Ramalinga V. Mangalaraja, Kiros Guesh, David Contreras, and Sambandam Anandan

3.1 Introduction  69 3.1.1 Langmuir–Hinshelwood Approach  71 3.1.2 The Eley–Rideal Approach  71 3.1.3 Indirect Photocatalytic Approach  72 3.2 Types of Photocatalytic Reactor Models  73 3.3 Modification of Semiconductor Nanoparticles  90 3.3.1 Metal Nanoparticles  90

Contents

3.3.2 Non‐Metal Deposition  91 3.4 Emerging Photocatalysts  95 3.4.1 Perovskite Photocatalysts  95 3.4.2 C3N4‐Supported Photocatalysts  96 3.5 Mechanisms of Photocatalysis  99 3.6 Conclusion  116 References  121 4

Application of Nanocomposites for Photocatalytic Removal of Dye Contaminants  131 Sivaraman Somasundaram, Pitchaimani Veerakumar, King‐Chuen Lin, and Vignesh Kumaravel

4.1 4.2

Nanocomposites and Applications  131 Dyes: Introduction, Classification, and Impacts on the Environment  131 4.3 Strategies of Dye Contaminant Removal  133 4.4 Photodegradation and the Removal of Dyes Using Nanocomposites  134 4.4.1 Zeolite‐Based Nanocomposites  153 4.4.2 Clay‐Supported Nanocomposites  153 4.4.3 Polymer‐Based Nanocomposites  154 4.5 Photocatalytic Reactors for Dye Degradation  156 4.6 Summary  156 References  157 5

Photocatalytic Active Silver Phosphate for Photoremediation of Organic Pollutants  163 Sachin V. Otari and Hemraj M. Yadav

5.1 Introduction  163 5.2 Properties of Ag3PO4  165 5.2.1 Structural Features  165 5.2.2 Antimicrobial Properties  166 5.3 Photoremediation of Organic Pollutants  167 5.3.1 Effect of Morphology  168 5.3.1.1 Size and Structure of the Photocatalyst  168 5.3.1.2 Facet‐Dependent Photocatalysts  171 5.3.2 Effect of Composition  172 5.3.2.1 Carbon Materials  173 5.3.2.2 Semiconductor Materials  176 5.3.2.3 Magnetic Particles  179 5.3.2.4 Metal Particles  179 5.3.3 Doping Effect  182

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5.4 Conclusions and Future Prospects  182 Acknowledgements  183 References  183 6

Plasmonic Ag‐ZnO: Charge Carrier Mechanisms and Photocatalytic Applications  191 Raghavachari Kavitha, Shivashankar Girish Kumar, and Channe Gowda Sushma

6.1 6.2 6.3 6.4 6.4.1 6.4.2 6.4.3 6.5

ZnO‐Based Photocatalysis  191 Why Deposit Silver on ZnO Surface?  192 Methods to Decorate Silver NPs on the Surface of ZnO  193 Mechanism of Charge Carrier Transfer Dynamics in Ag‐ZnO  197 Schottky Barrier and Charge Transfer Process  198 Surface Plasmon Resonance Effects  198 Defect Chemistry of Ag‐ZnO  199 Influence of Silver Content on Optimizing the Photocatalytic Activity  200 6.6 Structure–Morphology Relationship on Photocatalytic Activity  201 6.7 Co‐modification of Ag‐ZnO for Photocatalysis  204 6.8 Conclusion and Future Prospects  207 References  208

7

Multifunctional Hybrid Materials Based on Layered Double Hydroxide towards Photocatalysis  215 Lagnamayee Mohapatra and Dhananjaya Patra

7.1 Introduction  215 7.2 Hybrid LDHs from LDH Precursors  216 7.3 Photocatalytic Applications of Different LDH‐Based Hybrid Materials  217 7.3.1 LDH‐Based Mixed Metal Oxides (MMO)  221 7.3.2 Hybrid MMOs for Dye Degradation  225 7.3.3 LDH Nanocomposites  227 7.3.4 Intercalated LDH  231 7.4 Conclusions  233 References  234 8

Magnetically Separable Iron Oxide‐Based Nanocomposite Photocatalytic Materials for Environmental Remediation  243 Sakthivel Thangavel, Nivea Raghavan, and Gunasekaran Venugopal

8.1 Introduction  243 8.2 Synthesis Techniques for Magnetic Nanophotocatalyst Composites  246

Contents

8.3

Three Types of Semiconductor Magnetic‐Based Nanocomposites  249 8.4 Graphene‐Based Magnetically Separable Composites  251 8.4.1 Metal Di‐Chalcogenides‐Magnetic Nanocomposite Photocatalysts  252 8.4.2 Graphitic Carbon Nitride‐Based Magnetic Photocatalysts  254 8.5 The Effect of Iron Oxide‐Based Photocatalysts on Pollutants  255 8.5.1 Organic Dye Pollutant Degradation  255 8.5.2 Non‐Dye or Colorless Compounds  256 8.5.3 Heavy Metals  258 8.5.4 Pharmaceutical Waste  259 8.6 Summary  260 References  260 9

Photo Functional Materials for Environmental Remediation  267 Pazhanivel Devendran and Meenakshisundaram Swaminathan

9.1 Introduction  267 9.2 Photoelectric Effect  267 9.3 Photo Functional Materials (Photocatalysts)  268 9.4 Photodegradation of Textile Dyes  271 9.5 Semiconductor‐Based Photocatalysts  272 9.6 Carbon Nanotubes (CNTs)  274 9.7 Photo Functional Semiconductors on CNT Hybrid Materials for Tunable Optoelectronic Devices  275 9.8 Fabrication of CdS Quantum Dot Sensitized Solar Cells Using Nitrogen‐Functionalized CNTs/TiO2 Nanocomposites  276 9.9 Graphene Sheet  280 9.10 CdS/G Nanocomposites for Efficient Visible Light Driven Photocatalysis  281 9.11 Graphitic Carbon Nitride (g‐C3N4)  283 9.12 Conclusions  284 References  285 10

Graphitic Carbon Nitride‐Based Nanostructured Materials for Photocatalytic Applications  291 Jayaraman Theerthagiri, Kumaraguru Duraimurugan, Hyun‐Seok Kim, and Jagannathan Madhavan

10.1 Introduction  291 10.2 General Mechanism: Reaction Pathway  292 10.3 g‐C3N4 and Composites in Photocatalytic Degradation  294 10.4 Conclusions and Future Directions  304 Acknowledgements  305 References  305

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Contents

11

Metal–Organic Frameworks for Photocatalytic Environmental Remediation  309 Mohan Sakar and Trong‐On Do

11.1 Introduction  309 11.2 Structural Features of MOFs  310 11.3 Synthesis of MOFs  312 11.3.1 Evaporation Method  313 11.3.2 Vapor Diffusion Method  313 11.3.3 Gel Crystallization Process  313 11.3.4 Solvothermal Synthesis  313 11.3.5 Microwave‐Assisted Synthesis  314 11.3.6 Sonochemical Methods  314 11.3.7 Electrochemical Synthesis  314 11.3.8 Mechanochemical Synthesis  315 11.4 Photocatalytic MOFs by Design  315 11.5 Photocatalytic Applications of MOFs  317 11.5.1 Degradation of Organic Pollutants  317 11.5.2 CO2 Reduction  320 11.5.3 Heavy Metal Reduction  323 11.5.4 Others 326 11.6 Conclusions and Future Prospects  327 Acknowledgements  329 References  329 12

Active Materials for Photocatalytic Reduction of Carbon Dioxide  343 Balasubramanian Viswanathan

12.1 Introduction  343 12.2 CO2 Photoreduction – Essentials  345 12.3 Heterogeneous Photocatalytic Reduction of Carbon Dioxide with Water  348 12.4 Nanomaterials and New Combinations of Materials for Carbon Dioxide Reduction  350 12.5 Selection of Materials  355 12.6 Material Modifications for Improving Efficiency  359 12.7 Perspectives in the Photocatalytic Reduction of Carbon Dioxide  363 Acknowledgements 367 References  367 Index  373

xi

List of Contributors Sambandam Anandan

Kiros Guesh

Nanomaterials and Solar Energy Conversion Lab, Department of Chemistry National Institute of Technology Trichy, India

Department of Chemistry Aksum University Axum, Ethiopia Raghavachari Kavitha

Centre of Biotechnology University of Concepcion Concepcion, Chile

Department of Chemistry Vijaya College (Affiliated to Bangalore University) Basavanagudi, Bengaluru Karnataka, India

Pazhanivel Devendran

Hyun‐Seok Kim

Department of Physics International Research Centre Kalasalingam Academy of Research and Education, Krishnankoil, India

Division of Electronics and Electrical Engineering Dongguk University‐Seoul Seoul, South Korea

Trong‐On Do

Department of Chemistry, School of Engineering and Technology CMR University Bengaluru, Karnataka, India

David Contreras

Department of Chemical Engineering Laval University Quebec, Canada Kumaraguru Duraimurugan

Solar Energy Lab, Department of Chemistry Thiruvalluvar University Vellore, India

Shivashankar Girish Kumar

Vignesh Kumaravel

Department of Environmental Sciences School of Science, Institute of Technology Sligo Sligo, Ireland

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List of Contributors

King‐Chuen Lin

Dhananjaya Patra

Department of Chemistry National Taiwan University Taipei, Taiwan and Institute of Atomic and Molecular Sciences Academia Sinica Taipei, Taiwan

Department of Chemistry Texas A & M University Doha, Qatar

Jagannathan Madhavan

Solar Energy Lab, Department of Chemistry Thiruvalluvar University, Vellore India Ramalinga V. Mangalaraja

Advanced Ceramics and Nanotechnology Laboratory Department of Materials Engineering University of Concepcion Concepcion, Chile Lagnamayee Mohapatra

Energy Storage Department Qatar Environment and Energy Institute Doha, Qatar S. Thangaraj Nishanthi

Electrochemical Power Sources Division CSIR: Central Electrochemical Research Institute Karaikudi, Tamil Nadu, India Sachin V. Otari

Department of Chemical Engineering Konkuk University Seoul, South Korea

Nalenthiran Pugazhenthiran

Advanced Ceramics and Nanotechnology Laboratory Department of Materials Engineering University of Concepcion Concepcion, Chile Nivea Raghavan

Department of Nanosciences and Technology Karunya Institute of Technology and Sciences Coimbatore, Tamil Nadu, India Mohan Sakar

Centre for Nano and Material Sciences Jain University Bengaluru, India Panneerselvam Sathishkumar

Department of Chemistry Aksum University Axum, Ethiopia Sivaraman Somasundaram

Department of Chemistry Kongju National University Kongju, Republic of Korea Channe Gowda Sushma

Department of Chemistry, School of Engineering and Technology CMR University Bengaluru, Karnataka, India

List of Contributors

Meenakshisundaram Swaminathan

Pitchaimani Veerakumar

Department of Chemistry International Research Centre Kalasalingam Academy of Research and Education Krishnankoil, India

Department of Chemistry National Taiwan University Taipei, Taiwan

Sakthivel Thangavel

Key Lab of Advanced Transducers and Intelligent Control System Ministry of Education and Shanxi Province, College of Physics and Optoelectronics Taiyuan University of Technology Taiyuan, Republic of China Jayaraman Theerthagiri

Centre of Excellence for Energy Research Sathyabama Institute of Science and Technology Chennai, India Nagamalai Vasimalai

Department of Chemistry B.S. Abdur Rahman Crescent Institute of Science & Technology Vandalur, Chennai, India

and Institute of Atomic and Molecular Sciences Academia Sinica Taipei, Taiwan Gunasekaran Venugopal

Department of Materials Science, School of Technology Central University of Tamil Nadu Thiruvarur, Tamil Nadu, India Balasubramanian Viswanathan

National Centre for Catalysis Research Indian Institute of Technology‐ Madras Chennai, India Hemraj M. Yadav

Department of Energy and Materials Engineering Dongguk University Seoul, South Korea

xiii

xv

Preface Increasing environmental concerns are driving a growing need for clean and renewable energy sources. Photocatalysis driven by visible light is a promising strategy which can be used in many applications, such as the removal of organic pollutants, hydrogen production, air purification, and biological studies. Photocatalysis has been considered to be one of the most promising tech­ nologies for the production of solar fuel as well as the mineralization of pollutants. As photocatalysts use photon energy, either from sunlight or from simulated illumination, they are relatively inexpensive, non‐toxic and eco­ friendly. Upon illumination, the semiconductor photocatalysts undergo charge separation. Holes are produced in the valance band and electrons are promoted to the conduction band. These electrons and holes are then involved in redox reactions with adsorbed species. Multifunctional photocatalytic materials are of interest in designing and constructing advanced light energy harvesting assemblies for both energy production and environmental remediation. In this book you will find recent developments in multifunctional photocatalytic materials, such as semi­ conductors, nanocomposites, quantum dots, carbon nanotubes, and graphitic materials, along with novel synthetic strategies and details of their physico­ chemical properties. These materials are suitable for the photocatalytic conversion of CO2 into solar fuels and value‐added products. Also, photo­ catalysts are used to generate H2 via the water splitting reaction and are used to remove contamination. The elaboration of molecular systems and interfaces for the conversion of CO2 into energy‐rich molecules is an important technical and environmental challenge, because the abundance of CO2 in the atmosphere contributes significantly to the greenhouse effect. In this perspective, important research activities are directed toward the preparation of photocatalysts containing: (i) a photosensitizer unit, which initiates photochemical one‐ electron transfer events, and (ii) a catalyst, which stores reducing equivalents to achieve multi‐electron reduction of CO2 and produce fuels. This book presents a collection of twelve chapters written by researchers who are the leading experts in their fields of study. In their chapters they

xvi

Preface

explain the strategies to overcome the challenges in photocatalytic functional materials for environmental remediation. The first chapter of this book is a succinct summary of the state‐of‐the‐art of titanium dioxide and carbon‐ based nanomaterials for the photocatalytic degradation of organic dyes in wastewater. Chapters 2 and 3 focus more on the aspects of visible light driven photocatalysts and their impact on energy and environmental applications. Chapters 4, 5, and 6 explore the plasmonic effect of the nanocomposites. Chapters 7, 8, and 9 discuss the details of the multifunctional hybrid materials and their applications. Chapters 10 and 11 address the key challenges in the fabrication of photocatalysts and give possible strategies to improve the efficiency of the photocatalysts. Chapter  12 describes the reduction of CO2 using functionally active materials. Finally, we would like to express our sincere thanks to all the authors for sharing their knowledge on photocatalytic functional materials for environ­ mental remediation. They have made it possible to produce this book for the benefit of those interested in visible light harvesting by functional materials and applications of this process. We are very grateful to all the authors whose chapters make this a valuable book. Dr. Alagarsamy Pandikumar (Editor) Scientist Functional Materials Division CSIR‐Central Electrochemical Research Institute Karaikudi‐630 003, Tamil Nadu, India Dr. Kandasamy Jothivenkatachalam (Editor) Professor Department of Chemistry Bharathidasan Institute of Technology Anna University (UCE – BIT Campus) Tiruchirappalli‐620 024, Tamil Nadu, India

1

1 Titanium Dioxide and Carbon Nanomaterials for the Photocatalytic Degradation of Organic Dyes Nagamalai Vasimalai Department of Chemistry, B.S. Abdur Rahman Crescent Institute of Science & Technology, Vandalur, Chennai, India

Abbreviations A absorbance AC activated carbon Ads adsorption AOP advanced oxidation process BC brilliant blue dye BG brilliant green dye C dye concentration CB carbon black cb conduction band CDs carbon dots CNFs carbon nanofibers CNTs carbon nanotubes CQDs carbon quantum dots DSAC date stone‐activated carbon ETAD Ecological and Toxicological Association of Dyestuffs and Manufacturing Industry eV electron volt GAs graphene aerogels GO graphene oxide GrF graphite felt h Plank’s constant ka the equilibrium constant of the reactant kr the specific reaction rate constant Photocatalytic Functional Materials for Environmental Remediation, First Edition. Edited by Alagarsamy Pandikumar and Kandasamy Jothivenkatachalam. © 2019 John Wiley & Sons, Inc. Published 2019 by John Wiley & Sons, Inc.

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Photocatalytic Functional Materials for Environmental Remediation

L:TiO2 lysozyme‐coated TiO2 nanoparticles LD lethal dose MB methylene blue MO methyl orange N‐doped CDs nitrogen doped carbon dots Ox oxidation PCNFs porous carbon nanofibers PHF polyhydroxy fullerenes PVA polyvinyl alcohol r reaction rate for the oxidation of reactant Red reduction RhB rhodamine B TiO2@C TiO2@activated carbon nanocomposite TiO2@CF carbon felt supported TiO2 vb valence band XRD X‐ray powder diffraction α alpha β beta λem emission wavelength λex excitation wavelength

1.1 ­Introduction The global environment is being polluted by many toxins. In particularly, water pollution is a major concern, because water is very important to all living beings and accounts for around 70–90% of their body weight. Hence, the qual­ ity of water resources will directly affect the life of humans and other living beings. Industrial development is persistently connected with the water pollu­ tion. The World Bank estimates that 17–20% of water pollution is caused by the dyeing and textile industries. India is the second largest manufacturer of dyestuffs. Globally, ~106 tons of synthetic dyes are produced yearly [1]. It is estimated that worldwide annually 280 000 tons of textile dyes are disposed in the effluent of textile industries [2]. In ancient days, dyes were acquired from natural sources. For example, dur­ ing the Roman empire, only ministers and kings wore purple dyed fabrics. During the middle ages, ruby‐red fabrics were reserved for the most important clergy. Most of the natural coloring agents are of inorganic origin (semipre­ cious stones, malachite, clays, minerals, and metal salts) or are organic dyes. Organic dyes include those of animal origin and those of plant origin. There is a broad spectrum of different organic compounds with different physical and chemical properties. Among the different organic dyes, anthraqui­ none (e.g. madder root) is of special interest. Madder is a bright red‐colored

Titanium Dioxide and Carbon Nanomaterials

traditional dyestuff. In eighteenth and nineteenth century, the red pants of Napoleon’s army and British soldiers were dyed with madder [3]. Alizarin, luci­ din, and other compounds are present in madder root extract, but lucidin has a mutagenic nature, which severely restricts the use of madder root [3]. Recently, the synthetic dyes have replaced the traditional natural dyes, due to their low cost and the vast range of new colors offered. In 1856, William Henry Perkin accidentally discovered the world’s first synthetic dye. In the nineteenth century, 10 000 new synthetic dyes were developed [4]. Generally, the light‐absorbing functional groups in the dye molecules are called “chromophores.” Chromophores contain the hetero atoms N, O, and S, which contain non‐bonding electrons. The followings are the examples of chromophores: –N=N–, C=NH, –C=C–C=C–, =C=O, –NO2, CH=N–, N– OH, C=S, and NO–OH groups. The electron‐acceptor groups are called “auxochromes,” and which are generally present on the opposite side of the electron‐donor molecules and their important function is to increase the color. Indeed, the common meaning of the word auxochrome is color enhancer. Some auxochromes are –NH2, –HSO3, –OH, and –COOH. These groups can give a higher affinity to the fibers. The “chromogen” is a part of the chromophore structure, along with auxochrome, and aromatic structure (normally benzene, anthracene, or naphthalene rings). Generally, synthetic dyes exhibit different chemical and physical properties, which are dependent on their structural diversity. Synthetic dyes can be classified as follows: acidic, basic, direct, metallic, dis­ persed, pigment, mordant, reactive, sulfur, solvent, and vat dyes (Chart 1.1) [4]. The synthetic dyes are classified based on their physical, chemical properties, and their structural functionalities (Table  1.1). The usage of synthetic dyes is based on their compatibility with the type of textile substrates that are being processed. Generally, acid, direct, and reactive dyes are anionic aqueous solu­ ble dyes; basic dyes are cationic dyes; and disperse, solvent, and pigment dyes are non‐ionic dyes. More details of dyes and their applications are given in Table 1.1 [3]. 1.1.1  Impact of Dye Effluents on the Environment and Health During the dyeing process around 10–50% of dyes stuffs are released into wastewater [19, 29]. Textile, paper, leather, cosmetics, food processing, drug, paint, printing, pigments, rubber and plastic industries [30] are the major source of harmful dyes and dyestuffs disposal. The Ecological and Toxicological Association of Dyestuffs and Manufacturing Industry (ETAD) has been esti­ mated that out of 4000 screened dyes, 90% dyes that have lethal dose (LD50) values greater than the World Health Organization recommended level. Among the tested dyes, direct, basic, and di‐azo dyes show the higher toxicity rates [31]. Therefore, the disposal of untreated dyes and dye effluents

3

4

Photocatalytic Functional Materials for Environmental Remediation

Dyes Natural dyes

Animal Derived Dyes Ex: Cochineal, Lac insect, Murex snail

Plant Derived Dyes Ex: Catechu, Indigofera plant, Madder root

Synthetic dyes

Acidic dyes

Basic dyes

Vat dyes

Reactive dyes

Solvent dyes

Direct dyes

Dispersed dyes

Azonic dyes

Oxidation dyes

Developed dyes

Sulfur dyes

Indigoid dyes

Chart 1.1  Classification of dyes.

discharged into the  environmental water bodies will highly detrimental to water quality. The dyestuffs can interfere with the penetration of sunlight (visible light) into water, resulting in a hindrance of photosynthesis and a decrease in the dissolved oxygen level. The dye‐contaminated water also increases the biological oxygen demand. On the other hand, most of the syn­ thetic dyes are soluble in organic solvents and these are harmful to the living organisms. Further, synthetic dyes that include aromatic rings in their structure are regarded as toxic, xenobiotic, and carcinogenic [29, 32–34]. Besides, this type of dye can transfer their toxicity into aquatic organisms and cause intense inju­ ries to human beings, including allergy, dermatitis, cancer, skin irritation, and can affect the reproductive system and kidneys, brain, liver, and central nerv­ ous system [35]. Therefore, the degradation of toxic dyes in environmental and wastewaters is now very important to protect the living beings from their haz­ ard [36]. Recently, several techniques have been used for the removal/degradation of dyes from industrial wastewater and environmental water, including chemical precipitation, reverse osmosis, conventional coagulation, electrodi­ alysis, electrolysis, adsorption, ion‐exchange, and photocatalytic degradation [36]. Among the used different techniques, the photocatalytic method is the most efficient method to solve the environmental problem [31].

Titanium Dioxide and Carbon Nanomaterials

Table 1.1  Classification, characteristics, and applications of dyes. Group of dyes

Characteristics and applications

Ref.

Acid dyes

Acid dyes are water‐soluble anionic dyes and are used for the dyeing of synthetic fibers, silk, wool, nylon, leather, modified acrylics, ink‐jet printing, paper, food, and cosmetics.

[5, 6]

Basic dyes

Basic dyes are water‐soluble cationic dyes that can apply directly to cellulosic with no mordants. The positive charge is localized with an ammonium group and applied for dyeing of silk, wool, polyacrylonitrile, cotton, modified polyester, modified nylons, and tannin‐mordanted cotton.

[7, 8]

Vat dyes

Vat dyes are water‐insoluble dyes. They are generally applied as soluble leuco salt, after reduction in an alkaline solution with hydrogen sulfide. The leuco is formed by the reoxidation to the insoluble keto structure. Vat dyes are used to dye cotton, linen, soap, and rayon.

[9, 10]

Reactive dyes

Reactive dyes can directly react with the fiber molecules to form chemical bonds. They have simple chemical structures and are extensively used for the dyeing of cellulosic fabric and fibers.

[11, 12]

Organic pigments

Organic pigments are used in cotton, cellulosic, paper, and blended fabrics. They are negatively charged compounds, made from rocks, minerals, plants, and animals.

[13, 14]

Direct dyes

Direct dyes are water‐soluble anionic dyes that have a high affinity with cellulose fibers. These dyes are used to dye cotton, regenerated cellulose, leather, paper, nylon, and blends.

[15, 16]

Dispersed dyes

Dispersed dyes are water‐insoluble, non‐ionic dyes. They require additional factors such as dye carrier, pressure, and heat to penetrate synthetic dyes. They are easily disperse in aqueous media wherever the dye is dissolved into fibers. Commonly used for the dyeing of synthetic fibers.

[17, 18]

Azonic dyes

Azonic dyes contain one or more azo groups. These dyes are used [19, 20] in printing inks and pigments.

Solvent dyes

These dyes are water insoluble. Carboxylic acid, sulfonic acid, or quaternary ammonium groups can be present in their structure. Solvent dyes are used in solvent inks, wood staining, waxes, plastics, coloring oils, and gasoline.

[21, 22]

Oxidation dyes

Oxidation dyes belong to three major chemical families – diamines, aminophenols, and phenols or naphthols. These dyes are used as colorant materials for hair dyeing.

[23, 24]

Developed dyes

This type of dye can be diazotized and coupled on the fiber after applying to the fibers. They form shades that are faster to washing. The developed dyes are used to dye cellulosic fibers and fabrics.

[25, 26]

(Continued)

5

6

Photocatalytic Functional Materials for Environmental Remediation

Table 1.1  (Continued) Group of dyes

Characteristics and applications

Ref.

Sulfur dyes

Sulfur dyes are made by heating heterocyclic aromatic compounds with species that release sulfur. Sulfur dyes are classified as sulfur bake, polysulfide bake, and polysulfide melt dyes. Disulfide bridges can be formed during the oxidation, because monomeric molecules become cross‐linked into large molecules forming disulfide bridges. Sulfur dyes have a good affinity with cellulose materials such as cotton, viscose, jute, and flex.

[27]

Indigoid dyes

This are expensive dyes and made from Tyrian purple. During the oxidation process of indigoid, phenylacetic acid is formed. Indigoid dyes are used in textiles, linen, cotton, and wool and dyeing of denim jeans jackets.

[28]

1.2 ­Principles and Mechanism of Photocatalysis Photocatalytic degradation technique is an attractive choice for the degra­ dation of organic dyes from wastewater. There are several methods available to  produce hydroxyl radicals, e.g. Fenton‐based processes [37], ozone‐based processes [38, 39], and photocatalytic processes [40–43]. Among them, the photocatalytic process is an environmentally friendly process with signifi­ cant advantages over other existing methods. The photocatalytic degradation method has been reported to be effective for the degradation of organic dyes from wastewaters and soils [44–49]. The photocatalytic decoloration of dyes is believed to take place according to the following mechanistic pathways. During the UV light irradiation on a cata­ lyst, electrons are promoted to the conduction band (cb) from the valence band (vb). As a result, an electron–hole pair is produced (Scheme 1.1) [44].

catalyst h

e cb h vb (1.1)

where, e−cb and h+vb are the electrons in the conduction band and the elec­ tron vacancy in the valence band, respectively. These entities could migrate to the catalyst surface, where they enter in a redox reaction with other species on the surface. Generally h+vb can react with surface‐bound H2O and produce ˙OH radicals. On the other hand, superoxide radical anions of oxygen are produced through the reaction of e−cb and O2 [50].

H2 O h vb



O2

e cb



OH H (1.2)

O2  (1.3)

Titanium Dioxide and Carbon Nanomaterials

Dye*

t

igh

le l

ib Vis

O2 Dye

e–

CB

O2--

H+ HOO-

UV light H2O

h+

VB

H2O2 OH-

OH-

Scheme 1.1  Schematic illustration for the generation of oxidative species in a photocatalyst.

These reactions prevents the recombination of electron and hole that were produced in the first step. The ˙OH and O2˙ are produced in a similar manner, and then react with dye to cause the decoloration of dye. 



O2



H2 O2





H2 O

H2 O2 (1.4)

2 OH (1.5)

OH dye

dyeox (1.6)

dye e cb

dye red (1.7)

The mechanism for the generation of oxidative species in a photocatalytic study is shown in Scheme 1.1. Generally, the photocatalytic mechanism of dye degradation is of two types  –  the direct photocatalytic pathway and the indirect photocatalytic pathway – and these will be discussed next. 1.2.1  Direct Photocatalytic Pathways The best example of direct photocatalytic mechanism is the Langmuir–Hinshel wood process.

7

8

Photocatalytic Functional Materials for Environmental Remediation

1.2.1.1  The Langmuir–Hinshel Wood Process

This process is applicable for heterogeneous photocatalysis. The hole is trapped by the adsorbed dye molecule and forms the reactive radicals, which can decay as a result of recombination of electrons. The Langmuir–Hinshel wood expres­ sion is given below [51]:

1 / r 1 / kr 1 / kr kaC (1.8)

where r is the reaction rate for the oxidation of reactant (mg/l min), ka is the equilibrium constant of the reactant (l/mg), kr is the specific reaction rate constant for the oxidation of the reactant (mg/l min), and C is the dye concentration. 1.2.1.2  The Eley–Rideal Process

In this process, photofragmentation and subsequently trapping of the holes was obtained due to the surface defects of the free carriers. The surface active centers (S) can react with dye (chemisorption) to form an adduct species (S–dye)+, which will be further decomposed or could be the recombined with electrons. The reaction scheme is given below [52]: Photogeneration of free carriers

catalyst h

e

h (1.9)

Hole trapping by surface defects

S h

S (1.10)

Physical decay of active centers

S

e

S (1.11)

Chemisorption

S

dye

(S CP) (1.12)



(S dye)

S products (1.13)

1.2.2  Indirect Photocatalytic Mechanisms In this process, the photogeneration of electron–hole pairs will occur on the surface of the catalyst. Then, the hole is trapped by H2O molecules and leads to form HO˙ and H+ and the electrons are allowed to form H2O2, with further decomposition of more OH− radicals by their reaction with oxygen, which is supplied by the medium. Finally, the obtained HO˙ radicals are responsible for

Titanium Dioxide and Carbon Nanomaterials

the oxidation of the dye molecules, which produce the intermediates CO2 and H2O [53, 54]. The detailed mechanism is given below:

h

e



h



O2

2e



O2



h (1.14) HO. (ads) H (ads) (1.15)

H2 O(ads)

O2  (ads) (1.16)

(ads) H

HO 2 (ads) (1.17)

HO 2 (ads)

H2 O2 (ads) O2 (1.18)



H2 O2 (ads)

2HO. (ads) (1.19)



HO. dye

intermediates



CO2

H2 O (1.20)

Generally, the efficiency of the degradation of organic dyes can be measured by their absorption maximum value from UV‐vis spectrum, because organic dyes have a good light absorption properties. For example, methyl orange dye shows an absorption maximum at 463 nm. Therefore, the photocatalytic degradation process of methyl orange was monitored at 463 nm by UV‐vis spectrophotometry. The photocatalytic degradation efficiency of organic dyes have been calcu­ lated by the following equation: E ( A0 A) / A0 100% (1.21) where A0 is the equilibrium absorption value of the organic dye solution and A is the absorption value of the organic dyes solution at a specific irradiation time. Several nanomaterials are used for the photodegradation of organic dyes, including ZnO, Fe3O4, TiO2, ZnS, CdS, WO3, Fe2O3, Bi2WO4, and carbon nanomaterials [55, 56] etc., Among them, TiO2 and carbon nanomaterials have received much attention because of their fascinating physical and chem­ ical properties.

1.3 ­Importance of Titanium Dioxide Titanium dioxide (TiO2) is an interesting material for photocatalytic applica­ tions because of its photostability, ease of availability, biologically inertness, low energy consumption, high photocatalytic activity, low operating tempera­ ture, relatively high chemical stability, suitable flat band potential, etc. [57–60].

9

10

Photocatalytic Functional Materials for Environmental Remediation

The photocatalytic activity of TiO2 is highly dependent on its surface and other physicochemical properties, such as crystal composition, particle size distribu­ tion, surface area, band gap, porosity, and surface hydroxyl density. Particle size is important for the photocatalytic studies, as the catalyst efficiency is related to the surface area. Lower particle size materials will have a greater surface coverage and increased number of active surface sites, which will enhance the expected activity [61, 62]. There are three different crystalline forms of TiO2 (anatase, rutile, and brookite) and show different physical and chemical properties. Detailed infor­ mation about TiO2 is given next. 1.3.1 Rutile Rutile is a commonly available crystalline structure of titanium dioxide (tita­ nia). The name “rutile” was given by Abraham Gottlob Werner (1800), and it means “reddish color mineral” in Latin [61, 63]. Rutile has lower molecular volume than the other two forms. Most TiO2 that comes from metamorphic and igneous rocks has the rutile crystal structure (Figure 1.1). The unit cell of rutile is a tetragonal cell and its optical band gap is ~3.0 eV [61, 64]. Rutile is used as a white pigment in paints, polymers, and paper. Rutile is the most com­ monly used form of TiO2 in the world [65–67]. 1.3.2 Anatase Anatase is the second natural form of TiO2 (Figure  1.1). Anatase is named from the Greek Anatasis, which means “elongation.” Anatase is metastable at atmospheric temperature and pressure. Anatase can transform irreversibly to rutile at high temperature [68, 69]. Generally, anatase can be prepared by chemical method [70, 71]. The anatase band gap was reported to be ~3.1 to 3.4 eV [72]. This band gap difference is ascribed to the change in the particle size and semiconductor density carriers [73]. 1.3.3 Brookite Brookite is the third natural form of TiO2. Crystallographer and mineralogist Henry James Brooke (1771–1857) has been honored as the basis for the name brookite. Brookite has a orthorhombic (instead of tetragonal) structure and its band gap has been estimated to be ~3.3 eV [74–78] (Figure 1.1). Anatase shows a higher photocatalytic activity than rutile [66, 67]. Possible explanations are as follows: the larger band gap of anatase reduces the light penetration that can be absorbed and increases electron transfer from the anatase to adsorbed molecules [64, 79]. Surface properties play an important

Titanium Dioxide and Carbon Nanomaterials c a

b

c

c

b

b a

a O-2 Ti + 4

(a)

(b)

(c)

Figure 1.1  Crystalline structures of TiO2: (a) anatase, (b) rutile, and (c) brookite. Source: Reproduced from Ref. [58].

role in the adsorption of molecules and consequent charge transfer to the molecule. Surface properties can be subdivided as follows [61]: i. ii. iii. iv.

chemical nature electronic structure interactions with molecules and surface defects surface potential differences that can affect the charge transfer from the photocatalyst to the molecules [80].

1.4 ­Titanium Dioxide for the Photocatalytic Degradation of Organic Dyes In recent years, TiO2 has been extensively used as a photocatalyst, due to its non‐toxicity, strong oxidation potential, and high photostability, etc. [81]. The rutile phase exhibits higher photocatalytic efficiency than the anatase phase. For example, Shiga et al. used TiO2‐modified electrodes to monitor the photoelectrochemical activities which showed that the anatase phase had a higher photoactivity than the rutile phase [82]. The observed high photocata­ lytic activities of the anatase phase are due to the greater hole trapping ability, which shows a lower recombination rate compared to the rutile phase [83, 84]. Therefore, anatase is considered as the most photochemically active phase of TiO2, due to the above‐mentioned effects combined [85]. These days, researchers are using various forms of TiO2, including TiO2 pow­ der, metal doped TiO2, multi‐atom doped TiO2, carbon nanomaterials com­ bined TiO2, etc., for the photocatalytic degradation of organic dyes [68, 86].

11

Photocatalytic Functional Materials for Environmental Remediation

For example, Mulmi et al., have developed lysozyme‐coated TiO2 nanoparti­ cles (L:TiO2) and confirmed the phase is anatase L:TiO2 by X‐ray powder dif­ fraction (XRD). The size of the nanoparticles was reduced from 30 to 17 nm, and the band gap energy also was decreased from 3.3 to 3.1 eV when they used lysozyme. The synthesized L:TiO2 nanoparticles exhibit the better photocata­ lytic degradation performance of methylene blue (MB) and methyl orange (MO) under ultraviolet (UV) irradiation. Both the photocatalytic degradation reactions follow pseudo‐first‐order reaction kinetics (Figure 1.2) [87]. 1.4.1  Approaches Enhance the Photocatalytic Activity of TiO2 There are some common limitations to the various semiconductors used in the photocatalytic degradation of dyes. Examples are low photostability and the irradiation of the catalysts in aqueous media leads to photocorrosion, subse­ quent elution of metal ions into the water, and, finally, the complete dissolution 100

100

60

L:TiO2

40 (MB)

L:TiO2

40

(a) 0

0 30

60

90 120 Time (min)

150

180 1.5

(c)

1.5 1.0 (MB)

0.5

0 min 30 min 60 min 90 min 120 min 150 min 180 min

(MO)

30

60

90 120 Time (min)

150

180

(d)

0 min 30 min 60 min 90 min 120 min 150 min 180 min

(MO)

0.5

0.0

0.0 500

(b) 0

1.0

Absorbance

2.0

60

20

20 0

Control TiO2

80

Control TiO2

C/CO (%)

C/CO (%)

80

Absorbance

12

550

600 650 700 Wavelength (nm)

750

350

400 450 500 Wavelength (nm)

550

600

Figure 1.2  (a) and (b) Photocatalytic performances of L:TiO2 and TiO2 nanoparticles toward the photodegradation of methylene blue and methyl orange dye solutions, respectively. (c) and (d) Absorbance variations of methylene blue and methyl orange dye solution utilizing L:TiO2 nanoparticles. The insets show the color changes of the corresponding dye solution at different irradiation times. Source: Reproduced from Ref. [87].

Titanium Dioxide and Carbon Nanomaterials

of the solid photocatalysts. The next limitation is the tendency to agglomerate the nanoparticles [88, 89]. TiO2 has many merits, such as high activity under UV irradiation, photosta­ bility, biological inertness, relative chemical stability, low operational tempera­ ture, low energy consumption, water insolubility under typical environmental conditions, disinclination to photocorrosion, and natural abundance (which makes it cheaper than other semiconductor photocatalysts) [90]. However, the application of TiO2 for the treatment of wastewater in industrially viable scale continues to face a series of technical challenges. One of the fundamental problems that exists with the utilization of TiO2, in as far as decontamination of environmental pollutants and many other applications are concerned, is the fact that it has a relatively wide band gap (3.2 eV), which means it only displays photoactivity under ultraviolet irradiation at a wavelength of 387 nm. Only 4% of the solar energy incident on Earth can be used when TiO2 is used in wastewater treatment [91, 92]. The photocatalytic activity is determined by the transport property of photo‐excited carriers from the interior to the sur­ face of photocatalysts, by the high rate of electron–hole recombination, and by low interfacial charge‐transfer rates of the photo‐­generated carriers in TiO2 parti­ cles and results in a lowered quantum yield and inefficient photocatalysis [93, 94]. Various time‐resolved techniques are employed to evaluate the photo‐excited carrier dynamics of TiO2; the carrier lifetimes determined are extremely fast, ranging from picoseconds to microseconds [94]. Another issue that hinders the application of TiO2 on an industrial scale is the fine particle size of TiO2, com­ bined with its large surface area‐to‐volume ratio and high surface energy, which makes it inclined to agglomeration and difficult to decant post‐treatment [89]. Therefore, it is important to enhance the photocatalytic efficiency of TiO2. The approaches used to enhance the photocatalytic activity of TiO2 can be  generalized as either chemical modifications or morphological (physical) modifications. Within the chemical and physical modifications, three strate­ gies can be listed as: (a) doping; (b) surface modification; (c) sensitization. The modifications would ideally be achieved in the following ways: (i) tuning the band gap; (ii) reducing the rate of charge carrier recombination; (iii) promoting the target reaction and increasing the surface‐active sites. 1.4.2  Metal and Multi‐Atom Doped TiO2 The photocatalytic activities of TiO2 have been improved after the sur­ face  modification of TiO2 with noble metals to reduce the electron–hole

13

14

Photocatalytic Functional Materials for Environmental Remediation

recombination [68, 95–100]. Further, the scientists have proved that noble metal‐doped TiO2 shows high reproducibility and excellent stability. For exam­ ple, Gupta et  al. prepared Ag‐doped TiO2 and used it as a photocatalyst for the photodegradation of two different dyes – methyl red and violet 3. The Ag‐doped TiO2 decolorized >99% of violet 3 and 86% of methyl red [101]. Seery et al. also decolorized the rhodamine 6G dye using Ag‐modified TiO2 under visible light [102]. The authors confirmed that the observed highly effi­ cient decolorization is due to the increase of visible light absorption in sil­ ver  nanoparticles. Further, Gunawan et  al. have observed the reversible ­photoswitching of silver nanoparticles on TiO2 surfaces [103]. Metallic silver ­nanoparticle‐modified TiO2 formed the Ag+ state during the exposure of visible light. These optical properties of silver nanoparticle‐modified TiO2 could be attractive for further tailoring of the band gap in TiO2 nanomaterials [104]. Many studies have revealed that co‐doping of TiO2 with metals and non‐ metals can reduce the electron–hole recombination, which will effectively improve the photocatalytic activity of TiO2 [105]. Xing et al. have prepared the carbon and lanthanum co‐doped TiO2, using the hydrothermal method, and reported photocatalytic effects under UV and visible light irradiation [106]. Yan et al. have investigated the higher photocatalytic activity from TiO2–SiO2– NiFe2O4 for the degradation of methyl orange [107]. The observed higher pho­ tocatalytic activity is suspected to be due to the role of hydroxyl radical and electron–hole recombination of TiO2–SiO2–NiFe2O4. Yang et  al. synthesized Mo and C co‐doped TiO2 nanoparticles. They have reported that 1% of Mo–C4/TiO2 exhibited an excellent photocatalytic degradation of rhodamine B dye under visible light [108]. The synthesized Mo and C co‐doped TiO2 nanoparticles show higher photocatalytic activity than TiO2 and mono‐doped catalysts. The obtained higher photocatalytic activity is due to the synergistic effect between Mo and C, which helps to increase the absorption of visible light and affects the photo‐induced ­electron–hole separation. The shape of TiO2 can also play an important role to enhance its photo­ catalytic activity. For example, nanocages, nanorods, nanotubes, nanosheets, nanobowls, nanobels, and nanosprings have been applied [109]. The higher photocatalytic activity observed in the case of TiO2 nanotubes is because of their tube diameter and thickness [110, 111]. However, much longer tubes cause a decrease in the photocatalytic degradation rate, whereas the higher thickness of the tube increased the photocatalytic activities. The obtained higher photocatalytic activities are attributed to the effective separation of electron–hole pairs and higher surface area of the nanotube [68, 112–114]. A second method is the doping of dye sensitizer molecules on the TiO2 sur­ face. Ikeda et al. reported TiO2‐coated phenolic compounds and used them as a photocatalyst under visible light [115]. Zhang et  al. also modified TiO2 nanoparticles with catechol (4.0 wt% catechol/TiO2) without affecting the

Titanium Dioxide and Carbon Nanomaterials

Xe Lamp

MO + NPs/NCs

Sampling at various time interval

UV-vis Spectrometer

Figure 1.3  Schematic representation for the nanoparticles (NPs) and nanoclusters (NCs) mixed with methyl orange.

crystalline nature of TiO2 [116]. The catechol‐modified TiO2 enhanced the photocatalytic efficiency to degrade acid orange 7 dye, due to the surface complexation of catechol. Zhang et al. recently reported the selective oxida­ tion of alcohols in the presence of anthraquinonic dye by TiO2 nanoparticles [117]. The same authors have proposed the mechanism for the formation of a  dye radical cation, which was oxidized by the nitroxyl radical. Further, poly(aniline) [118] and poly(thiophene) [119] have also been used as TiO2 dopants and used for the degradation of the dyes. Raliya et al. have synthesized TiO2, ZnO, and graphene oxide (GO), TiO2/ ZnO, TiO2/GO, ZnO/GO, and TiO2/ZnO/GO nanomaterials. Then, they have applied them for the photocatalytic degradation of methyl orange (MO) dye [120]. The scheme for the degradation of MO is given in Figure 1.3. Initially, TiO2, ZnO, and GO are used for the degradation of MO. Among them, higher photocatalytic degradation was observed from ZnO nanoparticles, followed by TiO2 then GO. Then, TiO2/ZnO/GO, TiO2/ZnO, TiO2/GO, and ZnO/GO nanocomposites were used for the photocatalytic degradation of MO. TiO2/ ZnO/GO showed a higher photocatalytic efficiency for the degradation of methyl orange dye. The photocatalytic degradation efficiency was further enhanced when the concentration of ZnO and GO were increased.

1.5 ­Carbon Nanomaterials for the Photocatalytic Degradation of Organic Dyes Carbon nanomaterials have been extensively used in the field of catalysis because of their fascinating physical and chemical properties, such as chemical inertness, stability, adequate mechanical properties, excellent electron mobil­ ity, and high porosity [121–123]. Carbon‐based nanomaterials can enhance the catalytic activity, which can be much better than polymers. For example, car­ bon nanomaterials, including activated carbon, graphite, graphitized carbon, carbon blacks, nanotubes, fullerenes, and nanofibers, nanohorns, nanowalls,

15

16

Photocatalytic Functional Materials for Environmental Remediation

and mesoporous carbon [88, 124–127], have been used for the photocatalytic degradation of organic dyes. Many studies are available in the literature to confirm that the good photo­ catalytic efficiency of TiO2 and applied for different environmental applica­ tions [128, 129]. However, the photocatalytic efficiency of TiO2 is still not up to the mark to achieve very high levels of photocatalytic efficiency. Therefore, researchers are using different technologies to modify the band gap of TiO2 through creating oxygen vacancies, chemical doping, and surface modifica­ tion, etc. [88, 130–137]. They have been taken many efforts to improve the photocatalytic efficiency of TiO2. TiO2‐coated carbon nanomaterials have shown good photocatalytic efficiency to degrade organic dyes, due to their high stability, chemical inert­ ness, good electrical properties, and tunable structural properties. For exam­ ple, TiO2‐doped or TiO2‐coated activated carbon, fullerenes, carbon nanotubes, graphite, graphene oxide, and carbon quantum dots have been used as photo­ catalysts for the degradation of organic dyes [89, 138]. 1.5.1  Activated Carbon Activated carbon (AC) is a promising candidate that is used extensively as a photocatalyst. The channels create the rigid skeleton layers of carbon atoms, and it is linked with chemical bonds that form a highly porous structure. Generally, activated carbon is available in powder and granular forms and its particle size distribution is 5–150 Å. The advantages of activated carbon are easy synthesis, low cost, chemical inertness, etc. Researchers have been coated the carbon nanomaterials with other nanoma­ terials, such as TiO2 and ZnO, to increase their photocatalytic activity. Beak et  al. have synthesized mesoporous TiO2‐spherical activated carbon (meso‐ TiO2/SAC) by an ion‐exchange method along with a heat treatment process [139]. The obtained meso‐TiO2/SAC shows high surface area coverage and good dispersion (Figure 1.4). Xing et al. have developed the photocatalytic method for the photodegrada­ tion of rhodamine B under UV light, using TiO2‐activated carbon nanocom­ posite [140]. They have synthesized the TiO2‐activated carbon nanocomposite by the sol‐gel method. The photocatalytic and adsorption effects of ­rhodamine B have been monitored with respect to the UV light irradiation. The removal and degradation process mainly depends on the TiO2‐activated carbon nanomate­ rials and UV irradiation. The loading cycle of TiO2 on the activated carbon surface has been controlled by the morphology of nanomaterials. Generally, the multi‐loading will block and reduce the size of the micropores of the com­ posite and hinder the dispersion of the TiO2 nanoparticles on the activated carbon surface [140]. Therefore, the morphology, porosity, and dispersibility of nanomaterials are vital to achieve high photocatalytic activity.

Titanium Dioxide and Carbon Nanomaterials (a)

(b)

(c)

(d)

(e)

(f)

Figure 1.4  (a) SEM image, (b) FE‐TEM image, (c) STEM image, and (d–f ) EDS mapping images. Source: Reproduced from Ref. [139].

Very recently, Omri and Benzina have prepared TiO2 particles supported on date stone‐activated carbon (TiO2/DSAC) and on polyvinyl alcohol (TiO2/ PVA) and compared their photocatalytic efficiencies for the degradation of brilliant green (BG) dye. Finally, they found that the photocatalytic activity of date stone‐activated carbon (TiO2/DSAC) was higher than the TiO2/PVA; this is due to the binding between carbon and anatase phase TiO2, the higher num­ ber of hydroxyl groups, and surface area of the catalyst (Figure 1.5) [68, 141]. 1.5.2 Graphite Graphite is a naturally abundant crystalline form of carbon, which is found in meta­morphic and igneous rocks. It is very resistant to heat but has high

17

Photocatalytic Functional Materials for Environmental Remediation Methyl Orange Adsorption

18

Methyl Orange Collision CO2 Intermediates/ CO2

Non-reaction

Activated Carbon (AC)

TiO2

In the presence of AC

TiO2

In the absence of AC

Figure 1.5  Synergistic mechanism between AC‐TiO2 composites.

electrical conductivity. Graphite is a planar layered structure. The individual layers of graphite are called graphene. Generally, graphite is two forms – alpha (hexagonal) and beta (rhombohedral). The alpha form of graphite can be easily converted into the beta form through mechanical treatment. The beta form can be reverted into the alpha form by heating to 1300 °C [142, 143]. Graphite‐­ supported TiO2 was used to degrade organic dyes. For example, Shen et  al. prepared three‐dimensional carbon felt@TiO2 monoliths and used them for the photocatalytic degradation of methyl orange dye [144]. El‐Kacemi et  al. used mesoporous thin TiO2 films on graphite felt (GrF) and GrF‐coated NiO nanoleaflets for the degradation of amido black dye [145]. In both instances, the authors report that the synthesized nanomaterials have shown a large surface area and that their active surface sites exhibit effi­ cient photocatalysis properties. However, these methods have some problems, including the post‐depollution decanting of the powdered photocatalysts, and it is an expensive and time‐consuming process. The water to be decontami­ nated should not be allowed to contain any other photocatalyst remedies par­ ticles and that will take additional treatment and have operational costs. Shen et al. have made a carbon felt‐supported TiO2 (TiO2@CF) monolith by a solvothermal method. The synthesized TiO2@CF monolith has shown a large surface area and exhibits a well‐defined 3D structure. The photocatalytic activ­ ity of the TiO2@CF monolith was evaluated to degrade the methyl orange dye. These results show the high photocatalytic potential, recyclability, and post‐ depollution of TiO2@CF monolith nanomaterials [142]. El‐Kacemi et al. reported a one‐pot synthesis of the homogeneous coating of graphite felt (GrF) substrates with TiO2. Shen et al. also reported the protocol

Titanium Dioxide and Carbon Nanomaterials

for the synthesis of graphite felt coated with TiO2. The GrF had formed nano­ structured mesoporous TiO2 films and was used for the photocatalytic degra­ dation of amido black dye. For a comparison of the photocatalytic effect of TiO2@GrF, the authors prepared a titanium dioxide/nickel oxide terminated graphite felt (TiO2@NiOGrF) and pointed out the slower degradation kinetics. The authors anticipated the high photocatalytic efficiency from the TiO2@ NiOGrF, because NiO can deter electron–hole recombination rates through the NiO‐n‐TiO2 heterojunction, thereby improving the photocatalytic effi­ ciency. But, it is noted that a charge carrier separation is more substantial in TiO2@GrF [142]. El‐Kacemi et al. proposed the mechanism of the observed higher photocata­ lytic activity of GrF/TiO2. Their proposed mechanism is based on the substan­ tial mole fraction of photo‐generated electrons from TiO2 being transferred to GO. It is believed that upon UV irradiation, the photo‐generated electrons are scavenged by GO and transferred to oxygen and form the highly reactive radi­ cals that show the higher photocatalytic activity of GrF/TiO2 samples. Finally, they have concluded that the direct contact between oxidized GO and TiO2 enhanced the photocatalytic activity [142]. 1.5.3 Graphene Graphene is a single atomic layer of graphite organized by the hexagonal lat­ tice tightly bonded carbon atoms. The stability of graphene is due to the tightly bonded carbon atoms of sp2 orbital hybridization. The π–π bond stack­ ing interaction is responsible for graphene’s notable electronic properties. Because of its fascinating physical and chemical properties, graphene has been extensively used in various applications, including fuel cells, batteries, electrochemical energy storage, super capacitors, etc., Graphene is also used in the catalytic field, due to its excellent redox properties. For example, Zhang et al. prepared the P‐25–graphene and graphene oxide nanocomposite by a hydrothermal method [146]. First, graphene oxide was reduced into graphene and coated with P‐25 nanoparticles on the surface of graphene oxide. As pre­ pared, the P‐25–graphene composite has shown greater photocatalytic activ­ ity for the photodegradation of methylene blue than P‐25 nanoparticles. The observed excellent photocatalytic activity is due to the reduced charge recom­ bination enhanced the absorptivity [146]. Moreover, many reports show the 3D‐graphene aerogels (GAs) can provide the higher photocatalytic activities. Qiu et  al. prepared 3D‐structured TiO2/ GA nanocomposites using glucose by a hydrothermal method [147]. Glucose acted as a face‐controlling agent and helped to achieve TiO2 nanocrystals with {001} facets on the GA surface. The observed excellent photocatalytic effi­ ciency to degrade methyl orange dye was due to their high surface area, hydro­ phobic nature, massive appearance, and high recyclability [68].

19

20

Photocatalytic Functional Materials for Environmental Remediation

rGO

UV light

e– MB

O2 rGO(e–)

TiO2

•O2–

h+ CO2+H2O

H2O

CO2+H2O

MB

•OH MB

Figure 1.6  The reaction mechanism for photocatalytic degradation of methylene blue (MB) over TiO2/graphene porous composites under visible light illumination. Source: Reproduced from Ref. [148].

Yang et  al. have developed graphene microsphere colloidal coated TiO2 and used them as photocatalysts for the degradation of methylene blue dye (MB) [148]. The synthesized graphene microsphere colloidal coated TiO2 has shown high photocatalytic efficiency for the degradation of methylene blue dye due to the high visible light absorption, greater adsorption ability, and rapid charge transfer. The photodegradation mechanism is shown in Figure 1.6. 1.5.4  Carbon Nanotubes and Fullerenes Carbon nanotubes (CNTs) are made of graphite layers and have a cylindrical structure. CNTs are of two types (single‐walled CNTs and multi‐walled CNTs) and have diameters of a few nanometers and are 1 mm in length [149]. The bonding in carbon nanotubes is sp2, which gives the amazing mechanical properties to the carbon nanotubes. The stiffness of CNTs is measured by their Young’s modulus. The Young’s modulus of the CNTs is higher than 1000 GPa, which is approximately five times higher than steel [150]. The con­ ductivity of CNTs is higher than that of copper or silicon. Hence, there are several areas of technology where CNTs are already being used. Because of the above‐­mentioned amazing properties of CNTs, theyare extensively used in several applications [68, 151]. On the other hand, fullerenes have also received much attention. Fullerenes are hollow‐core closed carbon molecules, spherical‐like in structure, with a diameter of less than 1 nm. The most stable fullerene, C60, consists of 12 pentagonal and 20 hexagonal rings in a closed‐cage structure [152, 153]. According to the isolated pentagon rule, C60 atom cages are held together by weak forces. It is this reason that increases the stability of fullerenes. The

Titanium Dioxide and Carbon Nanomaterials

unique physicochemical properties of fullerenes can allow them to conjugate with TiO2 to form nanocomposites and use them to decolorize dyes [154]. Krishna et  al. reported the synthesis of polyhydroxy fullerenes (PHF)‐ modified TiO2, which enhanced the photocatalytic efficiency of TiO2 to degrade procion red dye [155]. The adsorbed PHF molecule on the surface of TiO2 enhanced the photocatalytic activities due to the decrease in the electron–hole recombination. Furthermore, the authors show that the ­ obtained enhancement of photocatalytic activities is due to TiO2 reactive facets. Theoretical and experimental studies revealed that anatase TiO2, {001}, compared to the thermodynamically stable {101} facet is more reactive [156]. The {001} facets of TiO2 minimize the surface energy, which decreases the electron–hole recombination. For example, Liu et al. reported the hollow structured {001} facet TiO2 nano­ composite was exhibited with tunable photocatalytic efficiency to decompose the azo dyes in water [157]. Chen et al. also succeeded in the synthesis of 100% exposed {001} faceted ultrathin anatase TiO2 nanosheets by a hydrothermal method. They used them for photocatalytic applications [158]. Recently, Yu et  al. have synthesized TiO2 nanosheets which exhibited {001} facets; TiO2 nanosheets were synthesized through the layer by layer hydrothermal method [159]. The synthesized {001} facet TiO2 nanosheets that showed excellent pho­ tocatalytic activity for rhodamine B dye degradation [68]. 1.5.5  Carbon Black Carbon blacks are colloidal particles which are nearly spherical in shape and are less than 50 nm in diameter [68]. Carbon blacks are widely used as hetero­ geneous electrocatalysts because of their high surface area, stability, porosity, and excellent electrical conductivity. Mao and Weng et al. have prepare the carbon black (CB)‐coated TiO2 nano­ composite (TiO2/CB) catalyst, which is more active than the bare TiO2 and TiO2/activated carbon composite catalysts [160]. First, TiO2/CB nanocom­ posite was prepared by sol‐gel method and calcinated. The synthesized TiO2/ CB nanocomposite was characterized by surface area analysis and electron microscopic techniques, etc. The CB inhibited the formation of rutile TiO2 phase, thus reducing the recombination of photo‐generated electrons and holes; as a result, the photocatalytic activity of TiO2/CB was enhanced. The photocatalytic activity on methyl orange has been tested. The authors have commented that the CB improved the photocatalytic activity of TiO2. Further, they have proposed the mechanism for the obtained excellent photocatalytic activities of CB/TiO2. The CB prevented the aggregation of TiO2 and increased the crystalline nature of TiO2. Then, the strong interaction between CB and TiO2 inhibits the formation of the rutile TiO2 phase. CB/TiO2 exhibits higher photocatalytic  activity when compared with the commercial DeGussa P‐25

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Photocatalytic Functional Materials for Environmental Remediation

catalyst. This work illustrates the importance of the choice of materials for synthesis in order to obtain highly efficient photocatalytic activity. 1.5.6  Carbon Nanofibers Carbon nanofibers (CNFs) are cylindrical with a graphene layer structure; their aspect ratio is greater than 100 nm [161]. Recently, CNF materials are extensively used as catalysts due to their tubular structure and unique proper­ ties, such as low electrical resistivity, porosity, low cost etc. [162]. Li et al. have reported the superior photocatalytic activity of their fabricated TiO2/porous carbon nanofibers (PCNFs) [163]. The PCNFs were synthesized by an electrospinning technique and produced PCNFs with one‐dimensional structure. The synthesized PCNFs were treated with TiCl4 by the solvothermal method; finally highly porous PCNF‐coated TiO2 nanorods were produced. This nanomaterial was then used in the photodegradation of methyl orange dye under visible light. The observed higher photocatalytic activity is due to the effective separation of photo‐generated electrons and holes. Further, the authors have demonstrated their system with the photodegradation depollu­ tion of industrial wastewater. 1.5.7  Carbon Quantum Dots Carbon quantum dots (CQDs) are a new class of fluorescent carbon nanoma­ terial, with a size of 4–10 nm [164, 165]. CQDs are extensively used in several fields, including electronics, environment, and energy, because of their excel­ lent physical and chemical properties, such as easy functionalization, aque­ ous solubility, cost‐effectiveness, lower toxicity, and good biocompatibility [164, 165]. In particular, the CQD field is fast‐growing and has gained momen­ tum for photocatalytic applications due to their photoluminescence activity and photo‐induced electron transferability. They are many reports available in the literature to degrade the dyes by using carbon quantum dot photocatalysts. For example, the photocatalytic efficiency of CQD composites was estimated by the degradation of the organic dye ­rhodamine B under visible light irradiation. Rhodamine B shows an absorption maximum at 553 nm, which was decreased while irradiating under visible light. A Xe lamp (500 W) with a cutoff filter (>420 nm) was used as a visible light source. TNS, P25, and CQDs alone did not degrade the rhodamine B, whereas the composite of CQDs/TNS and CQDs/P25 enhanced the photocatalytic activity to degrade the rhodamine B. Moreover, the composite CQDs/TNS exhibited higher photocatalytic activity than CQDs/P25. Further, the authors observed that the photocatalytic activity increases while increasing the amount of CQDs in the composite of CQDs/TNS; this is due to the visible light absorp­ tion of graphite [166].

Titanium Dioxide and Carbon Nanomaterials

The photocatalytic degradation of rhodamine B was developed by using a N‐CDs/Bi2O3 composite. After irradiation for 70 min, the absorbance of the dye was completely removed when using N‐CDs/Bi2O3 nanocomposite as the catalyst. It also showed good recyclability. The photocatalytic decolorization efficiency of N‐CDs/Bi2O3, CDs/Bi2O3 IOS, Bi2O3 IOS, and Bi2O3 NPs were found to be 98.21%, 90.5%, 76.23%, and 22.56%, respectively. The observed higher efficiency is due to the presence of N‐CDs, which improved the absorp­ tion intensity of light and electron transition of Bi2O3 IOS film [167]. CDs and TiO2 composites have shown the higher photocatalytic activi­ ties. The CDs/rutile TiO2 nanocomposite, under visible light irradiation (at 420 nm), produced a photocurrent. The observed photocurrent is 2.6 times higher than the CDs/anatase TiO2 nanocomposite. The observed higher catalytic efficiency is ascribed to the higher charge separation efficiency of CDs/rutile TiO2. Under the irradiation with visible light (>420 nm), the up‐converted fluores­ cence peak of CDs was observed at 407 nm (3.05 eV), which is larger, for exam­ ple, for that for rutile TiO2 at 3.0 eV (414 nm) and is smaller than that for anatase TiO2 at 3.2 eV (388 nm). Thus, the up‐converted fluorescence of CDs only excited the rutile TiO2 and formed electron–hole pairs. In the control experi­ ment, the authors observed that the excellent photocatalytic activity of CDs/ TiO2 was due to the interaction between TiO2 and CDs (Figure 1.7) [168]. Photocatalytic degradation of methylene blue was also developed by a graphene quantum dots/copper oxide (GQDs/Cu2O) nanocomposite. The

1.0 0.8

C/C0

0.6 0.4

Rutile TiO2/GQDs Caln2 O4 Anatase TiO2/GQDs

0.2

GQDs Rutile TiO2

0.0

Anatase TiO2

0

10

20 30 40 Reaction time/min

50

60

Figure 1.7  Relationship between methylene blue concentration and reaction time for different catalysts. Source: Reproduced from Ref. [168].

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Photocatalytic Functional Materials for Environmental Remediation

photocatalytic efficiency reached 90% by using GQDs/Cu2O nanocomposites under irradiation of near infrared (NIR) light (>700 nm). The obtained higher photocatalytic activities are due to the unique protruding structure of GQDs and Cu2O [169]. Another research group has developed the photodegradation of methyl­ ene blue using a CQDs/MH nanocomposite. The CQDs/MH nanocomposite photocatalytic degradation efficiency of methylene blue was found to be 97.3%, which is higher than other photocatalysts [170]. The observed higher photo­ catalytic activity is due to the presence of unique interconnected wormhole‐ like porous structures [170]. Similarly, N‐doped CDs were used as photocatalysts for the photodegra­ dation of methyl orange. N‐doped CDs are a good candidate for the degrada­ tion of methyl orange. The higher photocatalytic efficiency (90%) with methyl orange was observed using N‐doped CDs after 120 minute irradiation with visible light, whereas undoped CDs showed a photocatalytic efficiency of only 31%. The authors concluded that the observed higher efficiency is due to the N‐doping [171]. Another research group used CQDs/Ag/Ag3PO4 com­ posites as photocatalysts and compared their catalytic efficiency with other photocatalysts, including Ag3PO4 and Ag/Ag3PO4. Methyl orange was com­ pletely decolorized within 10 min. The authors found higher photocatalytic activity against methyl orange with CQDs/Ag/Ag3PO4 than with CQDs/ Ag3PO4, Ag3PO4 and Ag/Ag3PO4. Still, there is a need to develop photocata­ lytic degradation methods for other dyes, which will be beneficial for the purification of wastewater and environmental concerns [172–174]. Pan et  al. synthesized the water‐soluble and amine‐functionalized CQD‐ coated TiO2 (CQDs@TiO2). The CQDs@TiO2 can absorb more O2 on their surface of the TiO2, which then generates more O2− species for the photo­ catalytic degradation of methyl orange. The CQDs@TiO2 have shown higher photocatalytic efficiency than the pure GQDs and TiO2. The authors have proposed a possible degradation mechanism of methyl orange (Figure  1.8). The obtained higher photocatalytic activity is due to the efficient separation and the electron−hole pair recombination of CQDs@TiO2 [175]. 1.5.8  Mesoporous Carbon Mesoporous carbon materials are porous carbon materials and are sized between 2 and 50 nm. Mesoporous carbon materials are of two types [176]: regular‐order mesoporous carbon and irregular‐order mesoporous carbon. Mesoporous carbon is an excellent candidate for catalytic applications, due to their smaller amounts of micropores and larger surface area coverage. Further, mesoporous carbon has good mechanical and chemical stability and good electrical conductivity [177].

Titanium Dioxide and Carbon Nanomaterials (a)

(b)

(c)

(d)

(e)

(f)

(g)

Figure 1.8  (a) AFM image, (b) TEM image, and (c) HR‐TEM image of GQDs. (d) TEM, (e) HR‐TEM, and (f ) SEM images of GQD/TiO2 composite. (g) Proposed photodegradation pathway of methyl orange by 1.0 wt %‐GQD/TiO2 photocatalyst under visible light irradiation. Source: Reproduced from Ref. [175].

Li et al. prepared the mesoporous TiO2/carbon beads by the impregnation carbonization approach [178]. They report that the synthesized mesoporous TiO2/carbon beads exhibited high photocatalytic activity under visible light irradiation to degrade methyl orange. After degradation occurred, the large mesopore size and greater surface area coverage of nanomaterials can faci­ litate the adsorption of dyes. There is no necessity to separate the beads from the reaction solution because the beads are not destroyed during the photocatalysis.

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Photocatalytic Functional Materials for Environmental Remediation

There is still an urgent need to enhance the photocatalytic activity of nano­ materials. Researchers need to concentrate on enhancing the photocatalytic activity of nanomaterials in the following ways: ●● ●● ●●

fast charge carrier transport; introducing co‐adsorbents; reducing the electron–hole recombination rate in carbon‐based systems.

1.6 ­Conclusion and Trends Organic dyes cause much water pollution. The use of hazardous synthetic dyes has increased in Asian countries. The increasing manufacture and application of synthetic dyes, taking into account their impact in wastewater and effluent water, is a major concern. Therefore, it is important to protect human beings and environment from this hazard. There is an urgent need to develop effective technologies for the viable treatment of colored effluents and wastewater. TiO2 and carbon nanomaterials have been widely employed for the photo­ catalytic degradation of organic dyes. Photodegradation of industrial dyes using improved TiO2 and carbon nanomaterials have been used, and they are somewhat promising and effective treatment technologies. However, it is important to improve the sensitivity of the system, looking at operational parameters including pH, dye concentration, temperature, catalyst concentra­ tion, etc. [179]. Therefore, more advanced level research work is needed to improve the photocatalytic efficiency of TiO2 and carbon nanomaterials for the degradation of organic dyes. Some of the adsorptive/photocatalytic materials cannot be regenerated after use and so create secondary pollution. To solve this problem, the adsorptive/ photocatalytic materials should be designed with a porous nature. The nano­ materials can be degraded during the irradiation in a particular wavelength of light. Recently, researchers have been developing TiO2 photocatalytic nanoma­ terials coated with zeolites, silica, clay, and metal organic frameworks. The advantage of these materials is that they can be easily separated from the treated water. On the other hand, Raliya et al. have developed TiO2/ZnO/GO nanomateri­ als for the photocatalytic degradation of methyl orange dye [120]. They found higher photocatalytic efficiencies with TiO2, ZnO, and GO alone and with ZnO/GO nanocomposites. This approach is confirmed that the multi‐atom coated materials enhance the photocatalytic efficiency of the photocatalyst. Study and implementation of these new treatments for the degradation of organic dyes should not only focus on the dye degradation, but also focus on reusing and recycling of the wastewater and on exploitation of the byproducts for other applications.

Titanium Dioxide and Carbon Nanomaterials

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2 Visible Light Photocatalytic Degradation of Environmental Pollutants Using Metal Oxide Semiconductors S. Thangaraj Nishanthi Electrochemical Power Sources Division, CSIR: Central Electrochemical Research Institute, Karaikudi, Tamil Nadu, India

2.1 ­Introduction Organic dyes are one of the important chemicals used in industry for the pro­ duction of fabric, food, furniture, and paint. However, the wastewater from the dye industry has caused a considerable amount of environmental and health problems to both humans and other living beings. Hence, researchers globally are focusing on developing universal methods to treat the wastewater from the dye industry effectively. There are several conventional methods followed for wastewater treatment, such as coagulation, microbial degradation, absorption on activated carbon, biosorption, filtration, and sedimentation [1–3]. Biological treatments are used for pilot‐scale dye wastewater treatment, involving both aerobic and anaerobic processes. In anaerobic conditions, azo dyes are reduced to by‐products that contain potentially hazardous aromatic amines [4]. Recently, a promising method, the advanced oxidation process (AOP), is being used to treat dye wastewater. AOP normally utilizes a strong oxidizing species of ˙OH radicals produced in situ, which trigger a sequence of reactions that break down the dye macromolecule into smaller and less harmful substances (mineralization) [1, 3, 5]. In wastewater treatment, AOPs usually refer to spe­ cific subsets of processes that involve O3, H2O2, and/or UV light. However, AOPs could also be used to refer to a more general group of processes that involve photocatalytic oxidation, ultrasonic cavitation, electron‐beam irradia­ tion, and Fenton’s reaction. Semiconductor photocatalysts (SPs), such as metal oxides, are typically used as activators that catalyze the radical chain reaction in photocatalytic oxidation. Semiconductor photocatalysts are chosen as the best photocatalyst for dye degradation for the following reasons: (i) they are Photocatalytic Functional Materials for Environmental Remediation, First Edition. Edited by Alagarsamy Pandikumar and Kandasamy Jothivenkatachalam. © 2019 John Wiley & Sons, Inc. Published 2019 by John Wiley & Sons, Inc.

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inexpensive; (ii) they contain low to no toxicity; (iii) they exhibit tunable ­properties that can be modified by size reduction, doping, or sensitizers; (iv)  they contain the ability to allow a multi‐electron transfer process; and (v) they are capable of extending their use without substantial loss in photo­ catalytic activity [6]. Several metal oxides have been reported as good photo­ catalysts; examples are titanium dioxide (TiO2), zinc oxide (ZnO), tungsten trioxide (WO3), vanadium oxides (V2O5), molybdenum oxide (MoO2), and other mixed metal oxides [7–9]. The photocatalytic properties of these metal oxides have been studied extensively, and results show that they have active potential as photocatalysts in the degradation of dyes in wastewater. In this chapter we mainly focus on metal oxides as photocatalysts for dye wastewater treatment, particularly in enhancing its degradation efficiency, in its potential as a visible light photocatalyst, as well as in the repeatability and reusability of these metal oxides.

2.2 ­Photocatalysis Exposure of semiconductor photocatalysts to light irradiation equal or greater than the band gap energy excites the valence band (VB) electrons to its con­ duction band (CB), leaving behind holes in the VB. Subsequently, holes in the VB travel to the surface and react with surface‐adsorbed water molecules (H2O) or hydroxyl groups (–OH) to produce the desired reactive ˙OH radicals. Simultaneously, electrons in the CB move to the surface and react with dis­ solved oxygen (O2) and generate superoxide radical anions (O2˙−). These reac­ tive radical species (˙OH and O2˙−) subsequently react with the pollutants, leading to intermediate products and finally mineralized carbon dioxide, water, and other inorganic ions. In some cases, the photo‐generated holes directly oxidize the pollutants remaining adsorbed on the photocatalyst surface. In addition, the electrons indirectly degrade the pollutants using ˙OH radicals produced through hydrogen peroxide (H2O2) decomposition, which is formed by the reaction of superoxide radical anions with protons (H+). On the other hand, the dye in the dye degradation reaction may be excited under light irra­ diation and inject an electron into the CB of the catalyst; they react with o ­ xygen to yield hydroxyl (˙OH) radicals indirectly via the formation of H2O2.

2.3 ­Mechanism and Fundamentals of Photocatalytic Reactions Photoreactions do not occur on illumination with light alone. These reactions often require the use of photocatalyst – a term that implies photon‐assisted generation of the catalytically active species. In the case of semiconductor

Visible Light Photocatalytic Degradation

materials, the role of photons (light energy) is to generate an electron (e−)–hole (h+) pair (Scheme  2.1) that takes part in subsequent dark (thermal) redox ­reactions with the surface adsorbed molecules to yield the ultimate products. The process of semiconductor photocatalysis basically involves the following stages. Light energy of a certain wavelength is made to fall onto a semiconduc­ tor. If the energy of the incident light is equivalent to the band gap energy of the semiconductor, electrons would be excited from the valence band to the con­ duction band of the semiconductor and holes would be left in the valence band. These electrons and holes could undergo subsequent oxidation and reduction reactions with any species that might be adsorbed on the surface of the semi­ conductor, to give the necessary products. In a homogeneous reacting system, the major disadvantage is the energy‐wasting back‐electron transfer reaction to give back the starting material. Studies on organized assemblies (micelles, vesicles, microemulsions, etc.) as a means to mitigate back‐electron transfer have been reported [10]. Although heterogeneous semiconductor particulate/ colloidal systems which act as light‐absorbing units offer an excellent route to overcome this problem, the strong e−–h+ recombination process (Scheme 2.1) is also a formidable obstacle for realizing high efficiency in the photocatalytic processes. The development of effective means to suppress the back‐electron transfer or e−–h+ recombination process is vital for increasing chemical con­ version efficiency. The e−–h+ generated thus can be used to drive chemical reactions provided: a. the energy separation (band gap) between e− and h+ is larger than the energy required for desired reaction; b. the redox potentials of the e− and h+ (and thus the position of the conduc­ tion band and the valence band) are suitable for inducing redox processes; c. the rates of these redox reactions are faster than or at least fast enough to compete with the e−–h+ recombination. An input of ultra‐band gap energy to the semiconductor particle causes a valence band electron to be promoted to the conduction band, causing charge separation. The conduction band electron and valence band hole can then migrate to the surface of the TiO2 semiconductor and participate in the ­oxidation–reduction reactions. The O2 molecule scavenges the e− from the conduction band of the TiO2 semiconductor, forming the superoxide radical (O2−) because the conduction band of TiO2 is nearly isoenergetic with the reduction potential of oxygen [11, 12]. It is often found that photocatalytic activity is nearly completely suppressed in the absence of oxygen, possibly because of the back‐electron transfer from active species present on photo­ catalytic surface, and the steady‐state concentration of oxygen has a profound effect on the relative rate of photocatalyzed decontamination occurring under ambient conditions. This superoxide radical then reacts with a proton, forming

43

44

Photocatalytic Functional Materials for Environmental Remediation

Adsorbed O2

O·2

Adsorbed H2O

H2O2 Conduction Band e– e– e– e– e–

UV irradiation λ UV100 > PC500 > TTP [59–62]. 2.5.4  Reaction Temperature An increase in reaction temperature generally results in increased photocata­ lytic activity; however, a reaction temperature above 80 °C promotes the recombination of charge carriers and disfavors the adsorption of organic com­ pounds on the TiO2 surface [45]. A reaction temperature below 80 °C favors the adsorption whereas further reduction of reaction temperature to 0 °C results in an increase in the apparent activation energy [53]. Therefore, a temperature range between 20 and 80 °C has been regard as the desired temperature for effective photomineralization of organic content. 2.5.5  Concentration and Nature of Pollutants The rate of photocatalytic degradation of a certain pollutant depends on its nature, concentration, and the other existing compounds in the water matrix.

49

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Photocatalytic Functional Materials for Environmental Remediation

A number of studies have reported the dependency of the TiO2 reaction rate on the concentration of contaminants in water [63]. A high concentration of pol­ lutants in water saturates the TiO2 surface, reduces the photonic efficiency, and deactivates the photocatalyst [64]. In addition to the concentration of pollut­ ants, the chemical structure of the target compound also influences the degra­ dation performance of the photocatalytic reactor. For example, 4‐chlorophenol requires a prolonged irradiation time due to its transformation to intermedi­ ates. This can be compared with oxalic acid, which transforms directly to car­ bon dioxide and water, i.e. complete mineralization [65]. Furthermore, if the nature of the target water contaminants is such that they adhere effectively to the photocatalyst surface, the process would be more effective in removing such compounds from the solution. Therefore, the photocatalytic degradation of aromatic compounds is highly dependent on the substituent group or groups. Organic substrates with electron‐withdrawing groups (benzoic acid, nitroben­ zene) strongly adhere to the photocatalyst and therefore are more susceptible to direct oxidation compared to those with electron‐donating groups [66]. 2.5.6  Inorganic Ions Various inorganic ions, such as magnesium, iron, zinc, copper, bicarbonate, phosphate, nitrate, sulfate, and chloride, present in wastewater can affect the photocatalytic degradation rate of the organic pollutants because they can be adsorbed onto the surface of the TiO2 [67–69]. Photocatalytic deactivation has been reported whether the photocatalyst is used in slurry or fixed‐bed configu­ ration, and so is related to the strong inhibition from the inorganic ions on the surface of the TiO2 [70]. A number of studies have been conducted on the effect of inorganic ions (anions and cations) on TiO2 photocatalytic degrada­ tion [56, 71–77].

2.6 ­Metal Oxide Semiconductors TiO2 is used as a photocatalyst in dye wastewater treatment mainly because of its ability to generate a high oxidizing electron–hole pair, its good chemical stability, that it is nontoxic, and has long‐term photostability [4, 6, 78]. TiO2 naturally occurs in three common crystalline polymorphs: anatase, rutile, and brookite. Although the anatase phase is considered the most photocatalytically active phase, rutile is the most thermodynamically stable and is more stable than anatase [79]. Also, the wide energy band gap (Eg > 3.2 eV) in anatase may limit its potential because only UV light with wavelengths less than 387 nm can initiate the electron–hole separation process [80]. Consequently, it is a great challenge to tailor a good photocatalyst from TiO2 that can efficiently harness the energy from natural sunlight, which consists of no more than 5% UV light

Visible Light Photocatalytic Degradation

but is 45% visible light [81]. Zha et al. [82] studied the degradation mechanism of methyl orange using nanostructured TiO2/ZnO heterojunctions. They found that a hedgehog structure exhibited the best degradation activity of 97% methyl orange within 30 min, which may be due to the hierarchical structure with a larger surface area. The morphological images are shown in Figure 2.3. Noble metals (Au, Pt, Ag, and Pd) are usually used as dopants to modify the TiO2 structure. However, recently transition metals (Cr, Mn, Fe, Co, Ni, Cu, and Zn) have been used to replace noble metals, and thus reduce the overall catalyst production cost. Ghasemi et al. [83] found that TiO2‐doped Fe gave the best results, with more than 90% degradation and 75% total organic carbon (TOC) removal efficiency of Acid Blue 92 upon UV light irradiation. El‐Bahy et al. [84] and Zhu et al. [85] took this a step further by synthesizing TiO2 with a sol‐gel method and, at the same time, incorporating rare earth (RE) metal ions into the catalyst structure. They found that a decolorizing efficiency of 91.2% on meth­ ylene blue after 180 min irradiation with sunlight using Ce3+‐doped TiO2‐SiO2. (a)

(b)

10 µm

1 µm (d)

(c) 1.0 0.8

hv

C/C0

0.6

e– e– e– e– – – – Eg = 3.37ev – – – VB + + + + H2O h h h h CB

In the dark No photocatalyst Prepared ZnO Degussa P25 Mixed TiO2(96.82%) + ZnO(3.18%)

0.4 0.2

Prepared TiO2

0.0

Fan blade Hedgehog

0

20

40

60

80 100 120 140

Irradiation Time (min)

Hole transfer

Electron transfer

O2 O2• e– e– e– e– CB + H2O2 + + Eg = 3.2ev + OH• + + VB h+ h+ h+ h+

H•+OH•

e–: electron p-ZnO

n-TiO2

+ h : hole

Figure 2.3  (a, b) Morphological images, (c) concentration versus irradiation time curve, and (d) energy band diagram of ZnO and TiO2. Source: Reproduced with permission from Ref. [82], copyright 2015, Royal Society of Chemistry.

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Photocatalytic Functional Materials for Environmental Remediation

Vimonses et al. [86] used a modified sol‐gel method to synthesize a m ­ icroporous layer of TiO2 nanocrystallites heterocoagulated with structurally modified kao­ lin for use as a photocatalyst (TiO2‐K). Results showed that using TiO2‐K, the decolorization of Congo red (40 ppm) after dark adsorption for 30 min was 55%, followed by 70% and 90% removal after UV irradiation (11 W) for 1 hour one and 2 hours, respectively. Kurinobu et  al. [87] found that TiO2 and SiO2 coated on a Fe3O4 magnetic core could completely degrade three different types of dye in 240 min under UV irradiation. Most of the photocatalytic studies were conducted using commercial ZnO under UV irradiation [7, 85, 88–90], but studies under visible light conditions have also been investigated [91–93]. Other studies compared the ZnO photocatalytic activity with that of TiO2 [91, 94] and other semiconductor photocatalysts (such as SnO2, ZnS, and CdS) [95]. Nishio et al. [89] and Gouvêa et al. [88] found that with ZnO, the model dye could be completely bleached by 60 min of UV light irradiation. Also, high TOC removal of Remazol Brilliant Blue R (up to 90%) could be achieved using ZnO with 120 min of UV light irradiation. Lu et al. [91] and Mai et al. [93] uti­ lized ZnO nanoparticles in the degradation of triphenylmethane dyes (BB‐11) and methyl green, respectively. Both found that complete degradation of dye was achieved after 20 hours reaction. However, Pare et  al. [92] found that if higher intensity (500 W) visible light was used, complete degradation and chemical oxygen demand (COD) reduction of acridine orange could be obtained after only 3 hours irradiation using ZnO powder. Kansal et al. [95] compared the photocatalytic efficiency of different semi­ conductors (ZnO, TiO2, SnO2, CdS, and ZnS) on dye degradation under UV and solar light irradiation. Muruganadham et al. [96] reported a simple synthe­ sis by calcining zinc oxalate produced from zinc nitrate and oxalic acid. The product was tested by UV irradiation using methylene blue as a model dye. However, only a slight increase in treatment efficiency was observed from 26.5% (without any catalyst) to 39.7% (with synthesized ZnO). Chen et al. [97] found that the ZnO (synthesized from zinc acetate using a solvothermal method) doped by Ag could mineralize methyl orange completely in 60 min with UV light irradiation at 254 nm. Subsequently, a report by Pawinrat et al. [98] showed that by doping ZnO with a small amount of Au (3 wt%) using a one‐step flame spray pyrolysis, the efficiency of the synthesized ZnO was enhanced and it degraded up to 71% of methylene blue in 1 hour with UV irra­ diation at 365 nm, close to the visible light region. Guo et  al. [9] found that ms‐BiVO4 exhibited the highest UV light catalytic activity regardless of its morphology; rhodamine B and methylene blue were completely degraded in only 60 and 45 min, respectively. Shang et  al. [99] reported that nanosized BiVO4 exhibited excellent visible light driven photocatalytic efficiency for degrading rhodamine B. Khanchandani et  al. [100] demonstrated the TiO2/ CuS core shell nanostructures for the degradation of methylene blue (MB) under visible light. They optimized the amount of photocatalyst and organic

Visible Light Photocatalytic Degradation

pollutant for degradation of MB and attained an efficiency of 90%; the band edge scheme is shown in Figure 2.4. Liu et al. [101] studied the degradation of rhodamine B using a series of dys­ prosium (Dy)‐doped WO3 and showed significantly better degradation of rho­ damine B, up to 91% within 180 min, compared with undoped WO3. Purwanto et al. [102] studied the size of Pt‐WO3 nanoparticles on the photodegradation of amaranth under solar‐simulated irradiation. They found that the optimal size of Pt‐WO3 to produce the highest photocatalytic activity was between 18 and 28.4 nm for complete degradation of 10 ppm amaranth. Seddigi [2] used laser induction on zinc‐doped WO3 to treat polluted water that contained alizarin yellow. He found that up to 90% of dye could be degraded within 15 min. Sajjad et al. [103] investigated the operational parameters which could affect the degradation of acid orange 7 and methyl orange by WOx‐TiO2 nano­ composites produced by a sol‐gel method. Coupling a WO3 semiconductor to (a)

(b)

50 nm 50 nm

1.0

C/C0

0.8

TiO2

0.6 CuS 0.4

TiO2/CuS Composite

0.2

Degradation Efficiency (%)

(d)

(c)

90

58 45

13

TiO2/CuS Core/Shell

0.0 0

10

20 30 40 50 Irradiation Time (min)

60

Figure 2.4  (a, b) High resolution transmission electron microscope images, (c) concentration versus irradiation time curve, and (d) degradation efficiency graph of TiO2 and TiO2/CuS composite. Source: Reproduced with permission from Ref. [100]. Copyright 2016, American Chemical Society.

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Photocatalytic Functional Materials for Environmental Remediation

TiO2 is also one of the ways to improve photocatalytic degradation of dye wastewater [104]. Photocatalytic activities of the two polymorphs were higher in comparison with WO3 and TiONx after 120 min [105]. Recently, Ungelenk et al. [106] synthesized a very promising photocatalyst using β‐SnWO4 nano­ particles containing certain amounts of α‐Sn. Xu et al. [107] used a hydrothermal method to synthesize Bi2WO6 multilay­ ered disks, which exhibited superior photocatalytic activity for the degradation of rhodamine B (97% degradation after 320 min) compared with Bi2WO6 nano­ plates and those fabricated by solid state reactions. Xu et al. [108] demonstrated that Bi2WO6 exhibited higher visible light photocatalytic activity for degrading rhodamine B (almost 100% degradation after 45 min) in comparison with the sample synthesized hydrothermally. Cui et al. [109] found that Bi2WO6 QDS were stable at lower temperatures, resulting in higher surface areas and improved photocatalytic activity by facilitating the separation of photo‐­ generated carriers. Zhang et al. [110] demonstrated Bi2WO6 as having excel­ lent photocatalytic performance by completely degrading rhodamine B in 75 min. Alfaro et al. [111] evaluated the degradation efficiency of ­rhodamine B using both Bi2WO6 (75%) and Bi2W2O9, (90% rhodamine B degradation). Recently, different types of metal molybdenum oxides (Bi2MoO6, Bi2Mo2O9, ZrMo2O8, and NiMoO4) have been synthesized for use as photocatalysts in dye wastewater treatment under UV or even visible light irradiation [112–116]. Tang et al. [117] used a solid state reaction to synthesize a series of MIn2O4 (M  =  Ca, Sr, Ba) photocatalysts which exhibited higher activity for methylene blue photodegradation under visible light than commercial Degussa P25, espe­ cially when CaIn2O4 was used as photocatalyst. CaIn2O4 rods showed higher photocatalytic activity for methylene blue degradation under visible light irradia­ tion than did CaIn2O4 synthesized by a solid state reaction [118]. Chang et al. [119] reported that core‐shell‐like coupled composite In2O3/CaIn2O4, which was synthesized by sequential calcinations, showed superior visible light induced photocatalytic degradation of methylene blue than the single phase CaIn2O4. Rodriguez‐Gonzalez et  al. [120] used a sol‐gel method to prepare In2O3/TiO2 with different In2O3 contents and studied the degradation of alizarin red. Yang et al. [121] investigated the system further by co‐doping TiO2 with Ag and In2O3 to produce a new three‐component junction photocatalytic system. Song et al. [122] reported that SrTiO3/CeO2 was an effective photocatalyst in the degrada­ tion of azo dyes under UV irradiation. In work by Hernández‐Alonso et al. [123], CeO2 has received attention as a photocatalyst because of its unique properties of stability under illumination and strong absorption for both UV and visible light.

2.7 ­Ternary/Quaternary Oxides The ternary nanohybrid ZnS‐Ag2S‐RGO composite [124], ZnO‐RuO2/RGO [125], and reduced graphene oxide/titanium dioxide/zinc oxide (rGO/TiO2/

Visible Light Photocatalytic Degradation

ZnO) have great efficiency toward photocatalytic oxidation of rhodamine B and methylene blue. The metal oxides of ZnIn (ZnIn‐MMO) in combination with g‐C3N4 show a higher performance for degradation of rhodamine B to that of pure g‐C3N4 and ZnIn‐MMO under visible light [126]. The core shell Ag2S/Ag3PO4 composites were prepared by varying the content of Ag2S. The composite with 5% and 50% Ag2S shows highest activity in visible light and near infrared (NIR) light, respectively [127]. Hussein and Abass [128] tried decolorization of dye without light and without catalyst, and showed that decolorization occurs only in presence of both the catalyst and light. There are various binary, ternary, and quaternary metal oxides, such as TiO2, ZnO, SnO2 [129], WO3, ZnS, ZrO2, CdO, HgO, PbO, PbO2, Fe2O3, BiVO4, and SrTiO3 [130].

2.8 ­Composites Semiconductors Composite formation, i.e. coupling of semiconductors, is another efficient method to make semiconductors photoresponsive in the visible region of the spectrum. Semiconductors that are chosen to prepare a composite must have variable band gaps. A large band gap semiconductor is usually coupled with a small band gap semiconductor having a more negative conduction band level. Consequently, the electrons in the conduction bands are inserted from the small band gap semiconductor to the large band gap semiconductor. Various combinations of photocatalyst have been coupled, such as FeTiO3/TiO2, Ag3PO4/TiO2, W18O49/TiO2, CdS/TiO2, CdSe/TiO2, NiTiO3/TiO2, CoTiO3/ TiO2, and Fe2O3/TiO2 [131]; ZnO/CdS and TiO2/SnO2 [132]; and ZnO/TiO2 and ZnO/Ag2S [133]. Such composites give higher photocatalytic activity than single semicon­ ductors due to the synergistic effect. It was reported that Cr2O3 coupled with SnO2 has been found to exhibit superior photocatalytic activity under visible light irradiation because of good crystalline nanoparticles, smaller crystal size, and a stronger response to visible light. The sample degrades 98% of rhodamine B in 60 min under irradiation of visible light [134]. Samadi et al. reported the co‐doped TiO2 and SiO2 nanocomposites with Nd3+ and Zr4+ and observed the effect of dopant on distribution and monotonous coating of TiO2/SiO2 thin film [135]. Coupling may also involve the modifi­ cation of the surface of mesoporous SnO2 by other metal oxides such as TiO2 or Al2O3 [136]. Yang et al. [137] developed a new TiO2‐based visible light catalyst (Bi2O3/Si‐TiO2) for the degradation of methyl orange and bis­ phenol. The results found that compared to bare TiO2, Bi2O3/Si‐TiO2 showed better degradation efficiency of methyl orange under visible light irradiation. The rate constant results and band edge diagram are shown in Figure 2.5.

55

Photocatalytic Functional Materials for Environmental Remediation (a) vs NHE –1.0

H2O2

O2

0.0 1.0 2.0 3.0

e– e– e–

CB

CB

Si-TiO2

Bi2O3

VB

VB

4.0

h+

h+ h+ h+ h+

(b)

h+

h+

•OH

MO

Degradation

BPA

OH-

(c)

(d) 1.0 0.8

0.020

b

0.6

c

0.4

d f

0.2 0.0

50 nm

0.0227

a

(A) 0

e

30 60 90 120 Irradiation time (min)

k (min–1)

Remaining ratio (C/C0)

56

0.015

0.0135 0.0109

0.010 0.0062

0.005 0.000

0.0026 0.0014

a

e b c d Photocatalysts

f

Figure 2.5  (a) Energy level diagram of Bi2O3/Si‐TiO2, (b) transmission electron microscopy image, (c) concentration versus irradiation time, and (d) apparent rate constant curve for bare Bi2O3 and Bi2O3 coupled Si‐TiO2. Source: Reproduced with permission from Ref. [138]. Copyright 2014, American Chemical Society.

2.9 ­Sensitization Another promising method for surface modification of photocatalysts is sensi­ tization. Dyes or complexes can be used in photocatalytic systems, including solar cells, due to their redox properties and sensitivity toward visible light. On exposure to visible light, dyes or complexes can inject electrons to the conduc­ tion band of the semiconductor so as to initiate a catalytic reaction. Sensitizers are organic and inorganic compounds that get adsorbed on the surface of a semiconductor by chemisorption or physisorption. The phenomenon of coat­ ing the surface of a semiconductor by sensitizers is sensitization. Sensitizers act as antenna that absorb light and transfer it to a semiconductor, which helps in improving the excitation process. Sensitizers can be categorized into three categories: synthetic sensitizer, natural sensitizers, and self‐sensitizer. Synthetic sensitizers include commercial dyes such as porphyrin dyes [138], Ru com­ plexes [139], Ru‐polypyridyl‐complex sensitizers, e.g. cis‐dithiocyanato bis(4,4′‐dicarboxy‐2,2′‐bipyridine)ruthenium(II) [140], and quinoxaline‐based

Visible Light Photocatalytic Degradation

organic sensitizers [141]. The rate of photocatalysis depends on the sensitivity of the photocatalyst to light radiation. In order to enhance the photosensitivity of the semiconductor, some chemical substances having chromophores (such as dyes or natural pigments etc.) could be used. These chemical substances are called photosensitizers. A photosensitizer absorbs light and transfers energy or electrons to the semiconductor. Dyes can be used in photocatalytic systems and in solar cells due to their redox properties and sensitivity toward visible light. On exposure to visible light, dyes can inject electrons to the conduction band of the semiconductor, so as to initiate a catalytic reaction.

2.10 ­Conclusions Several metal oxide semiconductors have been identified to be potential pho­ tocatalysts for dye wastewater treatment. Among the photocatalysts studied, titanium dioxide and zinc oxide are the most widely used metal oxides in AOPs (advanced oxidation processes). However, other metal oxides could also be used as photocatalysts in this context. This chapter reports and summarizes recent modifications applied to metal oxides used for dye wastewater treat­ ment. Most of these were conducted on titanium dioxide because it is the con­ ventionally preferred photocatalyst in AOPs. Through appropriate catalyst modifications, not only can the degradation efficiency of certain dyes and reus­ ability of metal oxides be enhanced, but the band gap can also be lowered to modify the treatment activity under visible light irradiation. Different modifi­ cations of titanium dioxide using metals (e.g. noble metals, transition metals, and lanthanides) and non‐metal compounds (e.g. nitrogen, sulfur, carbon, and phosphorus) have been tested, and the results showed that such modifications enhanced the photocatalytic activity of titanium dioxide, either under UV or visible light irradiation. Metal oxides, such as zinc oxide, vanadium oxide, tungsten oxide, molybdenum oxide, indium oxide, and cerium oxide, could also be used as photocatalysts in dye wastewater treatment, but they are often considered inferior to titanium dioxide. Additional studies are being conducted on other metal oxide semiconductors to determine whether their roles as pho­ tocatalysts can be expanded in the future.

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Visible Light Photocatalytic Degradation

19 Liu, G., Yan, X.X., Chen, Z.G. et al. (2009). Synthesis of rutile–anatase

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3 Contemporary Achievements of Visible Light‐Driven Nanocatalysts for the Environmental Applications Panneerselvam Sathishkumar 1, Nalenthiran Pugazhenthiran 2, Ramalinga V. Mangalaraja 2, Kiros Guesh1, David Contreras 3, and Sambandam Anandan 4 1

Department of Chemistry, Aksum University, Axum, Ethiopia Advanced Ceramics and Nanotechnology Laboratory, Department of Materials Engineering, University of Concepcion, Concepcion, Chile 3 Centre of Biotechnology, University of Concepcion, Concepcion, Chile 4 Nanomaterials and Solar Energy Conversion Lab, Department of Chemistry, National Institute of Technology, Trichy, India 2

3.1 ­Introduction The inexhaustible availability of sunlight on the Earth’s surface should be able to solve human demands for the future needs of energy and environment. However, the present techniques available for the utilization of sunlight need to be improved in order to achieve the 100% utilization of solar light efficiently. Approximately, 47% of visible light (400 > λ  TOT > NOT (abbreviations are explained in the caption to Figure 5.5). The percent­ ages of the {110} facets exposed are calculated to be 99%, 97%, 90%, and 87% for HBT, TDT, TOT, and NOT, respectively. Bi et  al. fabricated single crystal Ag3PO4 rhombic dodecahedrons having only {110} facets exposed and also fabricated cubes with {100} facets without (a)

(b)

(c)

5

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

(d)

HBT TOT TOT NOT NT Dank

0

5

10 15 t (min)

20

(e)

4

25

In(CJC)

OCn

{110}

3

HBT: 1.1225 TOT: 0.6938 TOT: 0.4008 NOT: 0.2199 NT: 0.0075

2 1 0 0

5

10 15 t (min)

20

25

Figure 5.5  Scanning electron microscopy (SEM) micrographs of the (a) three‐dimensional towers (TDT); (b) highly branched tetrapods (HBT); (c) threefold‐overlapped tetrapods (TOT). Scale bar = 5 μm. The degradation curves (d) and apparent reaction kinetic curves (e) of the samples for the degradation of rhodamine B under visible light irradiation (λ > 420 nm): HBT (64 mg), TDT (97 mg), TOT (94 mg), non‐overlapped tetrapods (NOT, 100 mg), and N‐doped TiO2 (NT, 9.6 mg). Source: Li et al. [27] with permission from the Royal Society of Chemistry.

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Photocatalytic Functional Materials for Environmental Remediation

any capping agent. They then demonstrated the superior photocatalytic per­ formance of the rhombic dodecahedrons compared to that of cubes for degra­ dation of organic compounds on visible light exposure [28]. Density functional theory (DFT) calculations were performed to better understand the mecha­ nisms behind the enhanced performance of the {110} facets. The calculations found that it was that oxygen vacancies can be created more easily on {110} facets than on {100} facets. So, it is proposed that Ag3PO4 with a high percent­ age of {110} facets will provide more catalytically active sites for the adsorption of the reactants in photocatalytic reaction, which improves photocatalytic performance. Zheng and co‐workers reported the synthesis of single crystal tetrahedral Ag3PO4 with {111} facets exposed by reacting phosphoric acid and ethanol at 60 °C [29]. Here also, Ag3PO4 with high percentage of {111} facets showed higher photocatalytic activity compared to Ag3PO4 with {110} and {100} facets on exposure to visible light. The insight of elevated photocatalytic perfor­ mance of {111} facets over {110} and {100} facets was analyzed by DFT calcula­ tions. It was found that surface energy, obtained from DFT calculations, for {111} facets of Ag3PO4 is higher than that of {110} and {100} facets in visible light irradiation (Figure 5.6). This study also suggested that the separation of photo‐generated electrons and holes on the {111} facet surface is dependent on the dispersion of valance bands and conduction bands of the {111} facet surface. So, it is clear that different crystal structures with different facets have differ­ ent atomic arrangements which indeed affect the physicochemical properties of the crystals. The different composites of facets of crystals have effect on the photocatalytic performance. Yu et al. demonstrated the effect of the ratio of {001} and {101} facets of anatase TiO2 for the reduction of CO2 [30]. Here also, DFT calculations were performed to explain the mechanism for the photocata­ lytic performances of the various facets. Very few studies have been performed in case of the silver phosphate where highly photocatalytically active Ag3PO4 crystals could be obtained. 5.3.2  Effect of Composition As discussed earlier, morphology and facet engineering can enhance the photocatalytic performance of crystals where the enhancement of the trans­ port of photo‐generated electrons can be modulated by surface engineering. Photocatalysis can also be affected by the composition of the crystals. A single material cannot fulfill the ideal characteristics of a photocatalyst. So, to get the ideal performance from the material, the synergistic effects of different photo­ catalysts can be tested in a composite material. Attempts have been made to construct a photocatalytically active composite to achieve an enhanced perfor­ mance in a single catalyst.

Photocatalytic Active Silver Phosphate for Photoremediation of Organic Pollutants

Ag

(a)

(b)

{111}

(d)

{100}

P O

(c)

{110}

Figure 5.6  (a) The crystal structure of Ag3PO4. Relaxed geometries for the (b) (111), (c) (110), and (d) (100) surfaces of Ag3PO4 based on a 128‐atom slab model. The vacuum region was set as the same thickness as Ag3PO4. Source: Zheng et al. [29] with permission from the Royal Society of Chemistry.

5.3.2.1  Carbon Materials

Carbon composites have been extensively studied for number of applications with various materials. In a photocatalyst composite, carbon materials such as graphene oxide (GO), reduced graphene oxide (RGO), carbon quantum dots (CQDs), and carbon nanotubes (CNTs) have been successfully demonstrated as enhancing the photocatalytic performance and stability of the composite. Carbon materials improve the surface area of the composite, which allows more interaction with the organic molecules, which increases the photocata­ lytic performance. Also, the conducting nature of these materials improves the transport of photo‐generated electron–hole pairs from photocatalysts, result­ ing in an increased photocatalytic activity. Liu et al. demonstrated an ecofriendly approach for the synthesis of tetrahe­ dral silver phosphate‐GO where Ag3PO4 nucleation was allowed to occur on GO sheets [31]. The photocatalytic performance of Ag3PO4 was significantly enhanced by introduction of 5 wt% of GO in the composite. The composite also showed more than 90% reusability in the photocatalytic degradation of

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Photocatalytic Functional Materials for Environmental Remediation

MB under visible light irradiation. Here, very a negligible band gap width and electrical conduction was observed in the Ag3PO4‐GO composite, where GO acted as the acceptor for the photo‐generated electrons. Also, GO sheets slowed the reduction rate of ionic silver to metallic silver by transferring the photo‐generated electrons. This provided stability to the composite, allowing it to be reusable for photodegradation. Zhang et al. also showed the degrada­ tion of tetrabromobisphenol A under visible light exposure by an Ag3PO4/GO composite synthesized by a chemical oxidation process [32]. Yang et al. synthe­ sized an Ag3PO4/GO composite by a novel electrostatically driven hydro­ thermal method [33]. The as‐prepared Ag3PO4/GO demonstrated increased photocatalytic active (two‐fold) compared to the polyhedral Ag3PO4 crystals on visible light exposure for organic compound degradation. X‐ray diffraction (XRD) analysis was performed during the repeated photocatalytic use of Ag3PO4‐GR composites to demonstrate partial decomposition to Ag, which gives evidence of the stability of Ag3PO4 in Ag3PO4‐GR composites and enhanced photocatalytic activity. RGO was also used for the formation of an Ag3PO4‐RGO composite for visible degradation of organic compounds [34]. The photocatalytic activity of the Ag3PO4‐RGO composite follows a Z‐scheme electron path (Figure  5.7), where photo‐generated electrons and holes accu­ mulated in the conduction band of RGO and the valence band of Ag3PO4, respectively. The charge separation can be increased by increasing the wt% of RGO in the Ag3PO4‐RGO composite; the highest obtained was 4 wt% RGO. As the concentration of the RGO in the photocatalyst increased, unwanted recombination of charge carriers on the surface of the catalyst affected the photocatalytic performance. CNTs are also a widely investigated carbon material that is used for Ag3PO4 composites to improve the photocatalytic performance. Zhai et al. prepared a Pickering emulsion‐based photocatalyst phase consisting of an Ag3PO4 photo­ catalyst and multiwalled carbon nanotubes (MWCNTs) and metal oxide semi­ conductor as a hydrophobic conducting phase [35]. As prepared, a Pickering emulsion showed much higher performance compared to a solution‐dispersed photocatalytic system. Like GO and RGO, CNTs also increased the surface area of the composite, facilitating the maximum interaction of the composite with pollutants and improving the charge separation and transfer of the elec­ tron–holes generated. Tang et  al. reported photocatalytic activity of a novel metal‐free photocatalyst formed by a graphitic carbon nitride (g‐C3N4) tetra­ hedron composite with {111} facets. This followed a similar mechanism for the enhancement of photocatalytic performance as did GO or RGO, by increasing the charge transfer [36]. The increased nitrogen content in the Ag3PO4 com­ posite increased the number of reactive sites for the photocatalytic activity. In recent years, carbon quantum dots (CQDs) have gained the attention of investigators as they are a new class of carbon material, having a size less than 10 nm [36–38]. Due to their chemically robust nature, low toxicity, and stability

V/NHE RGO

–0.52 –0.35 0.0 0.45

V/NHE V/NHE

–0.52

–0.52 0.0 0.45

0.0 0.45

1.23

1.23

1.23 2.6 2.74 2.9

2.6

RGO

2.9

(a)

(b) −

+

2.6 RGO

2.9

(c)

Figure 5.7  (a) Schematic representation of the electron/hole (e to h ) transfer process. Representative possible Z‐scheme mechanism for (b) mineralization of dyes and (c) H2 evolution over RGO‐Ag3PO4 heterostructure under visible light illumination. Source: Samal et al. [34] with permission from John Wiley and Sons.

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Photocatalytic Functional Materials for Environmental Remediation

in a photo‐bleaching environment, they are a highly studied material, not only in industrial and catalysis applications also in biomedical applications. Xu et al. reported the synthesis of a CNT/Ag3PO4 heterojunction composite by a two‐ step method. This composite exhibited more photocatalytic activity than pure Ag3PO4 crystals for the degradation of rhodamine B (RhB) [39]. Here the pho­ tocatalytic reaction followed a pseudo first‐order reaction in the presence of the heterojunction photocatalyst. Zhang et  al. demonstrated synthesis of CQD‐Ag3PO4 and CQD‐Ag‐Ag3PO4 photocatalysts by alkali‐assisted electro­ chemical methods for degradation of methyl orange (MO) under visible light illumination [40]. Here, CQDs enhanced the photocatalytic performance of Ag3PO4 by protecting against photocorrosion and the transfer of electrons from the photocatalyst to organic compounds. Nitrogen‐doped carbon quan­ tum dot (NCQD)‐Ag3PO4 complex photocatalysts for degradation of MO (methyl orange) and bis‐phenol A (BPA) under visible light irradiation were reported by Chen et al. [41]. NCQD‐Ag3PO4 exhibited two times higher pho­ tocatalytic activity for the degradation of organic molecules than Ag3PO4 crystals. 5.3.2.2  Semiconductor Materials

The photocatalytic performance of Ag3PO4 is considered to be an ideal candi­ date for photocatalytic activity under visible light irradiation. As Ag3PO4 is susceptible for photo‐generated electrons to cause photocorrosion, it is essen­ tial to control the electron–hole conjugation rate to enhance the photocatalytic activity of the Ag3PO4. It also possesses more positive potentials for both the conduction band and the valence band compared with most semiconductors. The conjugation of Ag3PO4 with other semiconductors facilitates the transfer of photo‐generated electrons from the conduction band of the other semicon­ ductor to the conduction band of Ag3PO4 while transferring electrons in the opposite directions, so avoiding the electron–hole recombination. It is this which increases the photocatalytic performance. One of the most studied semiconductor photocatalyst conjugates for Ag3PO4 is TiO2, as TiO2 is easy to synthesize and is commercially available in abundance. Li et al. reported the synthesis of heterostructures of Ag3PO4‐TiO2 mesoporous spheres for the deg­ radation of organic molecules (Figure 5.8) [42]. Here, though the weight per­ cent of the Ag in Ag3PO4‐TiO2 mesoporous sphere is a third that of Ag3PO4, photocatalytic activity of Ag3PO4‐TiO2 was more effective than Ag3PO4 and TiO2 for degradation of MB under visible light irradiation. Ag3PO4‐TiO2 was also demonstrated for the degradation of gaseous 2‐propanol under visible light illumination [43]. Several studies have reported the degradation of organic compounds under visible light exposure by a TiO2‐Ag3PO4 heterostructure photocatalytic composite [43–46]. Ma et al. studied the photocatalytic activity of a RGO TiO2‐Ag3PO4 compos­ ite, wrapped like a pine cone, for visible light photocatalytic activity to degrade

Photocatalytic Active Silver Phosphate for Photoremediation of Organic Pollutants (a)

15 nm

[Ag(NH3)2]NO3

sol-gel/solvothermal

calcination

HDA

Ti species Ag3PO4 nanoparticles

(b)

Tio2 nanoparticles Mesoporous Tio2

(c)

Ag3PO4 0.24 nm TiO2 0.357 nm 100 nm

5 nm

Figure 5.8  (a) Schematics of growth process of Ag‐P/m‐Ti‐X composites spheres. (b) TEM image of Ag3PO4‐TiO2 mesoporous sphere. (c) HR‐TEM of Ag3PO4‐TiO2 mesoporous sphere. Source: Chen et al. [41] with permission from Elsevier.

organic pollutants [47]. This triple‐component photocatalytic composite exhibited higher photocatalytic activity compared to free Ag3PO4, TiO2, and TiO2‐Ag3PO4 as graphene transfers the electrons at a higher rate from the pho­ tocatalytic center and the recombination of electron and holes is slower. Yang et al. synthesized bifunctional TiO2/Ag3PO4/graphene photocatalytic compos­ ites for degradation of organic compounds and synergistic inactivation of bac­ teria [19]. Though the concentration of Ag in the TiO2/Ag3PO4/graphene composites is very much lower (reducing the cost of the composite), it showed higher photocatalytic activity with antimicrobial activity better than did Ag3PO4. Several investigators studied the graphene‐TiO2‐Ag3PO4 photocata­ lytic composite for reluctant pollutant degradation under visible light exposure [20, 48]. Like graphene‐based materials, 2D transition metal disulfide materials have been studied extensively, as they have a lamellar structure and electron‐ transporting ability as graphene materials. Shao et al. demonstrated photocata­ lytic activity of the 3D Ag3PO4/TiO2@MoS2 composite with anti‐photocorrosion property [49]. Due to the high content of TiO2@MoS2in the Ag3PO4/TiO2@

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Photocatalytic Functional Materials for Environmental Remediation

MoS2 composite, charge transfer occurs at a very high rate and recombination of electrons and holes was very slow, increasing the photocatalytic activity of Ag3PO4/TiO2@MoS2 composite to be higher than Ag3PO4. Introduction of metallic particles having light‐absorbing properties is one of the strategies to enhance the photocatalytic performance of Ag3PO4/TiO2 composites. Teng et al. reported synthesis of an Ag‐decorated Ag3PO4/TiO2 heterostructure pho­ toelectrode where Ag acted as the electron acceptor and charge separator, decreasing the photocorrosion rate of Ag3PO4. The Ag/Ag3PO4/TiO2 photo­ electrode was used for the degradation of 2‐chlorophenol (2‐CP) under visible light exposure. It showed very high photocatalytic performance compared to Ag3PO4 and a Ag3PO4/TiO2 composite. To make an Ag3PO4/TiO2 composite separable by an external magnetic field, Xu et al. synthesized an Fe3O4/Ag3PO4/ TiO2 composite which not only showed enhanced photocatalytic activity for acid orange 7 (AO7) under visible light irradiation but also demonstrated reus­ able effective antimicrobial activity against pathogenic E. coli. Recent work has also focused on changing the concentration of some ions, such as OH−, H+, and PO43−, around the Ag3PO4 to control the photocatalytic activity. Guo et al. reported synthesis of a heterostructure of In(OH)3‐Ag3PO4 by a precipitation method and, under solar light irradiation, it was used for rhodamine B degradation [50]. The photocatalytic activity of the ln(OH)3‐ Ag3PO4 composite was controlled by changing the content of ln(OH)3. Enhancement of photocatalytic activity was observed with the ln(OH)3‐Ag3PO4 composite as compared to Ag3PO4. Synthesis of Pb9(PO4)6/Ag3PO4 composite has been reported by Guo et  al., where Pb9(PO4)6 concentration‐dependent photocatalytic activity for MO degradation under visible light irradiation was shown [51]. Deonikar et al. synthesized Ag3PO4/LaCO3OH (APO/LCO) het­ erostructure photocatalysts for the degradation of RhB under visible light irra­ diation [52]. An APO/LCO heterostructure composite containing 20 wt% of LCO showed excellent photocatalytic performance compared to Ag3PO4. Graphene‐wrapped Ag3PO4/LaCO3OH (APO/GR/LCO) heterostructures are also reported by Patil et al. for photocatalytic degradation of methylene blue (MB) under visible light exposure [53]. Here also, the 20 wt% LCO content showed maximum photocatalytic activity and stable reusability compared to Ag3PO4. Li et  al. demonstrated the photocatalytic activity of an Ag3PO4‐BiVO4 heterostructure composite where Ag3PO4 was deposited on monoclinic BiVO4(040). It has a truncated bipyramid shape and carries out visible light photodegradation of organic compounds [54]. BiVO4 is nontoxic and is a chemically stable photocatalyst having a 2.4 eV band gap. In the composite the highly active facets of BiVO4 were exposed on which Ag3PO4 was deposited. The active facets of the BiVO4 improved the electron–hole separation effi­ ciency, contributing to high catalytic performance and stability. To eliminate gaseous pollutants, Guo et  al. designed an Ag3PO4/nitridized Sr2Nb2O7

Photocatalytic Active Silver Phosphate for Photoremediation of Organic Pollutants

photocatalytic composite for the degradation of iso‐propyl alcohol (IPA) under visible light irradiation [55]. By adjusting the NH3 concentration, nitridation of Sr2Nb2O7 was controlled at different temperatures and the photocatalytic performance of Ag3PO4 could be modified by adjusting structural properties of Sr2Nb2O7. 5.3.2.3  Magnetic Particles

It is essential for separation of the photocatalyst to be easy to allow for reusa­ ble  photocatalytic activity. Efforts have been made to design magnetic an Ag3PO4 photocatalyst to make the catalyst available for consecutive reactions. Abroushan et  al. synthesized magnetically recyclable Ag3PO4/CoFe2O4 by a hydrothermal method [56]. The magnetic Ag3PO4 composite showed photo­ catalytic degradation of RhB under direct sunlight exposure. Gan et al. reported the incorporation of Ag3PO4 in CoFe2O4 core shell structure by a precipitation method [57]. The core shell photocatalyst showed excellent photocatalytic activity for the degradation of RhB and MO under tungsten halogen lamp light. The incorporation of Ag3PO4 in the CoFe2O4 structure lowered the band gap of Ag3PO4, enhancing the photocatalytic performance compared to Ag3PO4. A two‐step method for the synthesis of magnetically separable Fe3O4‐Ag3PO4 photocatalyst for the reusable degradation of MO under visible light degrada­ tion was reported [58]. Patil et al. reported that magnetically separable Ag3PO4/ NiFe2O4 (APO/NFO) composites were prepared by an in situ precipitation method, showing photocatalytic activity was dependent on the NFO (Ni, Fe, and O) content (Figure 5.9) [59]. Incorporation of Ni, Fe, and O not only enhanced photocatalytic degradation of organic dyes under visible light but also increased conductivity of the photocatalyst. Zhou et  al. demonstrated synthesis of magnetically separable Ag3PO4@MgFe2O4 microstructure composites by ion‐exchange deposition method. They were used for the degradation of RhB under visible light illumi­ nation [60]. The photocatalyst containing 10 wt% of MgFe2O4 showed superior photocatalytic performance and reusability over Ag3PO4 as MgFe2O4 inhibited the photocorrosion. By using a solvothermal/liquid‐phase deposition method, Hou et al. synthesized Ag3PO4 nanodot‐decorated ZnFe2O4 flowers [61]. The Ag3PO4/ZnFe2O4 photocatalyst exhibited higher photocatalytic activity than pure ZnFe2O4 and Ag3PO4. Several reports have demonstrated magnetically separable Ag3PO4 photocatalyst for organic compound removal under visible light irradiation [61–64]. 5.3.2.4  Metal Particles

Noble metal nanoparticle‐deposited Ag3PO4 photocatalysts have gained much  interest for the construction of plasmonic semiconductors, as they show  enhanced photocatalytic performance under visible light irradiation. Separation of holes and electrons is essential for catalytic reactions, and this

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Photocatalytic Functional Materials for Environmental Remediation

PO43– Ag+ PO43–

Ag+

PO43–

1

Ag+ Ag+

2

nucleation

PO43– PO43–

Ag3PO4 nuclei 6 NiFe2O4 Ag3PO4/NiFe2O4

5 4

Ag3PO4 rhombic dodecahedrons

3 Reorientation of Ag3PO4 nuclei

Figure 5.9  Schematic representation of growth process of APO/NFO composite. Source: Patil et al. [59] with permission from Elsevier.

is possible for metal/semiconductor composites as there is difference between the Fermi level of the semiconductor and the work function of the metal nanoparticles. There are several reports for the deposition of noble metal nanoparticles (Au, Ag, Pt, and Pd) on semiconductors to for composites which can improve light absorption and transfer the electrons, so improving photocatalytic performance and stability. Gondal et  al. presented the deposition of metallic Ag on the surface of Ag3PO4, which then showed enhanced photocatalytic performance for degra­ dation of RhB under visible and UV light [65]. The plasmonic Ag/Ag3PO4 com­ posite was synthesized by microwave‐assisted chemical reduction and had {111} facets exposed for photocatalytic activity driven by visible light [66]. Under sunlight illumination, the Ag/Ag3PO4 composite showed enhanced photocatalytic degradation of RhB. The composite had 0.05 wt% of Ag. A neck­ lace‐like structure for the Ag/Ag3PO4 was reported by Bi et al. The nucleation of Ag3PO4 cubes was allowed to occur on Ag nanowires [67]. The Ag/Ag3PO4 composite showed improved photocatalytic activity compared to Ag nanow­ ires and Ag3PO4 for the degradation of MO and RhB under visible light irra­ diation. Liu et  al. demonstrated a highly efficient Ag/Ag3PO4 photocatalyst prepared by an ion‐exchange process [68]. The strong visible and UV light absorbance of Ag/Ag3PO4 was determined by diffuse reflectance spectra (DRS), which showed high visible light degradation performance for dye pol­ lutant removal. It had an activity five times higher than Ag3PO4.

Photocatalytic Active Silver Phosphate for Photoremediation of Organic Pollutants

Gold nanoparticles are interesting and much‐studied noble metal nanopar­ ticles. They have been demonstrated for different applications, such as photo­ catalysts and in industrial processes. Liu et al. reported the rational design for the wide spectral responsive Au rod/Ag3PO4 heterostructure for visible light photocatalysis [69]. Au rod/Ag3PO4 also demonstrated a wide range of light absorbance as Au rods can absorb 800 nm wavelength light. This allowed pho­ tocatalytic performance of the heterostructure to degrade organic pollutants at the visible and far visible range. Similarly, Wang et al. reported incorpora­ tion of metallic Au0 to Ag3PO4, forming a highly photocatalytic heterostruc­ ture under visible light irradiation. It is suggested that the enhancement of the catalytic activity is due to the utilization of hydroxyl ions during Au/Ag3PO4 catalyst synthesis. The term “instantaneous catalysis” (IC) was introduced by investigators for the visible light degradation of rhodamine B, amido black 10b, and bromophe­ nol blue dyes. The mechanism for instantaneous catalysis is represented in Figure 5.10, which explains the simultaneous use of ˙OH ions produced by the Ag3PO4 surface and LSPR (localized surface plasmon resonance) property of Au rods for degradation of the organic dyes. Yan et al. designed a Ag3PO4 het­ erostructure with Au, Pd, and Pt using a chemical reduction method using Step 1 HO HO

Au

HAP

HO HO

Au

HO HO

HO HO

HO

HO

Dissociated

HAP

HAP

Au

Au

Ag3PO4

CO2+H2O

HO

HO HO

HAP

HO

HO HO

Step 2

•OH

Activated

Donor l (–OH)

Organic dye

Step 4

Au

HO HO

Au

Ag3PO4

O2

Donor II

•OH

(H2O or OH•)

HO HO

CO2+H2O

Organic dye

HAP

Dissociated O2•

HO

O2•

Au

HAP Ag3PO4

Au O2

HO HO HO

HO

Ag3PO4

Step 3

Activated

•OH

CO2+H2O

Donor I (–OH) Organic dye

Figure 5.10  Schematic representation for the mechanism for the instantaneous catalysis. Source: Wang et al. [70] with permission from Springer Nature.

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NaBH4 as the reducing agent [71]. The visible light degradation at 420 nm of MO, MB, and RhB using this heterostructure was demonstrated and compared to Ag3PO4, metal‐deposited Ag3PO4, where the heterostructure showed higher photocatalytic performance. 5.3.3  Doping Effect It is very well studied that the photocatalytic performance of semiconductors is largely affected by the surface atomic structure, recombination of electrons and holes, and the value of the band gap. Introduction of an impurity or a for­ eign material into the structure can lead to an alteration in all of the above properties of semiconductors, which can enhance the photocatalytic activity by changing the band gap value and lowering the rate of recombination of elec­ trons and holes during photocatalysis. These defects may also act as a center for electron–hole recombination. So, the percentage of doping can influence the photocatalytic performance of the doped semiconductors. Reunchan and Umezawa designed sulfur‐ and silicon‐doped Ag3PO4 where a change in the electrochemical properties of Ag3PO4 was observed on doping with sulfur [72]. Ghazalian et al. demonstrated that gadolinium doping can enhance the photo­ catalytic properties of Ag3PO4 in the visible light region for the degradation of reactive blue 19 (RB 19) [73]. Gd‐doped Ag3PO4 was used for the three con­ secutive photocatalytic reactions. Zhang et  al. reported high photocatalytic activity of Bi3+‐doped Ag3PO4 for the degradation of MO dye under visible light irradiation. 2 wt% of Bi3+‐doped Ag3PO4 showed more than 90% removal of MO, while the removal of MO by Ag3PO4 was 27.3% [74]. A reduction in OH defects was observed on the surface of Ag3PO4. Similarly, lanthanum‐doped Ag3PO4 was used for effective and enhanced photocatalytic activity under vis­ ible light illumination [75]. Surface defects caused due to doping increased the surface area of LaxAg3−xPO4.

5.4 ­Conclusions and Future Prospects Ag3PO4 and its composites show excellent photocatalytic performance for the degradation of various organic pollutants, demonstrating its potential role in the remediation of polluted water. Its composites also show excellent antimi­ crobial activity against pathogenic microorganisms. The only drawback for the use of Ag3PO4 as photocatalysts is photocorrosion. It loses activity at a very fast rate, limiting its reusability in photocatalytic reactions. So, efforts have been made to design Ag3PO4‐based photocatalysts for efficient photocatalytic per­ formance with considerable stability and reusability. In this chapter we have discussed the properties and photocatalytic perfor­ mance of the Ag3PO4‐based materials. There are several problems needing to

Photocatalytic Active Silver Phosphate for Photoremediation of Organic Pollutants

be resolved for improved performance of Ag3PO4 semiconductors. Recently, several reports demonstrated synthesis of inorganic/organic hybrid nanoflow­ ers for the effective enhancement of the activity. Here, enzymes are immobilized in phosphate‐based materials, such as copper phosphate, cobalt phosphate, and magnesium phosphate, etc., for enhanced protein activity [75–77]. Using a bio­ catalyst, an Ag3PO4 photocatalyst can be constructed for effective phytoreme­ diation of organic pollutants where simultaneous degradation of pollutants can be achieved with enzymes and an Ag3PO4 semiconductor. The size of the material greatly enhances the photocatalytic performance. There are very few studies for showing that Ag3PO4 nanoparticles photocata­ lytically degrade pollutants. So, efforts must be concentrated on synthesis of Ag3PO4 nanoparticle‐based photocatalysts with modified structures for the removal of a wide range of pollutants. Facet‐dependent photocatalytic activity was also demonstrated in Ag3PO4 crystals for the degradation of organic com­ pounds. However, very few studies have been done to look at the mix facets of the Ag3PO4 crystals and their photocatalytic performance. Investigation must be carried out to look at the antimicrobial activity of sil­ ver phosphate‐based materials and their mechanism against pathogenic microorganisms must be studied extensively. So, in conclusion, Ag3PO4 is currently one of the most promising photo­ catalytic materials and effective study is needed to make it commercial and applicable in the field. By overcoming several problems related to its stability and photocatalytic activity, the use of Ag3PO4 for photoremediation appli­ cations against organic compounds will be tremendous and commercially feasible.

­Acknowledgments An author (S.V. Otari) acknowledges the 2015‐KU Brain Pool Fellowship of Konkuk University, Seoul, South Korea. Another of the authors (H.M. Yadav) greatly acknowledges Dongguk University, Seoul, for selection to a faculty position.

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6 Plasmonic Ag‐ZnO: Charge Carrier Mechanisms and Photocatalytic Applications* Raghavachari Kavitha1, Shivashankar Girish Kumar 2, and Channe Gowda Sushma 2 1 Department of Chemistry, Vijaya College (Affiliated to Bangalore University), Basavanagudi, Bengaluru, Karnataka, India 2 Department of Chemistry, School of Engineering and Technology, CMR University, Bengaluru, Karnataka, India

6.1 ­ZnO‐Based Photocatalysis Photocatalytic properties of various metal oxides are of significant interest owing to their potential application for the direct conversion of light energy into chemical energy. Among the plentiful semiconductors, ZnO is regarded as a suitable alternative for the wastewater treatment by the virtue of its photocatalytic activity, broader UV light absorption, high photosensitivity, large exciton binding energy (60 meV), ease of preparation, nontoxicity, and chemical inertness, together with environmental sustainability [1]. In spite of great success, ZnO still suffers from intrinsic drawbacks, such as inadequate photoresponse range and larger decay through recombination of photo‐generated charge carriers. The former constrains its function under solar light, while the latter links with the low quantum yields. In addition, its corrosion results in mass loss and collapse of active facets in the course of photocatalysis and this further reduces the efficiency and also leads to non‐reusability [2]. Therefore, structuring the ZnO to harvest abundant visible light flux from the solar spectrum has garnered immense research attention. Several strategies, including foreign ion doping, noble metal deposition, and coupling with narrow gap materials/inorganic metal complexes, were reported to be suitable to overcome the aforementioned demerits [1]. Although metal ion doping leads to visible light absorption, the subsequent changes achieved in charge carrier density * Dedication: This chapter is dedicated to Prof. K.S.R. Koteswara Rao, Department of Physics, Indian Institute of Science, Bengaluru. Photocatalytic Functional Materials for Environmental Remediation, First Edition. Edited by Alagarsamy Pandikumar and Kandasamy Jothivenkatachalam. © 2019 John Wiley & Sons, Inc. Published 2019 by John Wiley & Sons, Inc.

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(depending on p‐/n‐type doping) may not always favor the charge carrier separation. Contrarily, heterocoupling the ZnO with other nanomaterials involves issues such as multi‐step preparation, optimization of each component, and restricted optical response [3]. In recent times, integrating plasmonic metal nanoparticles (NPs) with the ZnO surface is inclined to meet the challenging issue of fabricating visible light‐activated ZnO [4]. From the prospect of tailored structure–electronic properties, metallic Ag decorated on the ZnO ­surface has been the choice for this purpose. This chapter highlights the various methods adopted for reducing the silver ions, the solution chemistry governing the nucleation of both Ag and ZnO, the charge carrier transfer pathways, the defect chemistry of Ag‐ZnO, surface plasmon resonance (SPR) absorption effects, the optimal content of Ag, morphological effects on Ag distribution, and co‐modification of Ag‐ZnO in implication to photocatalysis.

6.2 ­Why Deposit Silver on ZnO Surface? The convenient method of depositing noble metals on a semiconducting surface in laboratory conditions, even at the bulk scale and with a large degree of dispersion in the aqueous medium, promises effective use of metal/semiconductor composites in photocatalysis. Among the many metals (Pt, Au, Ag, and Pd), Ag is the prime choice because of its remarkable catalytic activity, highest electrical and thermal conductivity, and antibacterial activity [1, 4, 5]. Ag is the metal with the highest surface enhanced Raman scattering (SERS) activity, with a panchromatic response toward the solar spectrum. Ag displays a tunable and intensely localized SPR by the virtue of its frequency dependence of the real and imaginary parts of its dielectric function. Alongside this effect, Ag serves as nano antenna for light trapping that spans the visible to near infrared (NIR) region to intensify the electric field within the proximity of metal NPs and also to generate SPR hot electrons [4, 6, 7]. The dispersed Ag islands acts as sinks for photo‐excited electrons, which lowers the carrier recombination within the ZnO and increases the path length of the electrons [1, 8]. The good results from Ag deposition arise from the fact that the crystal structure of ZnO remains largely intact even after loading a high density of Ag. However, some minor variations in the lattice parameter transpire as a result of large mismatch in the thermal expansion coefficient between ZnO and Ag [9]. Because of their minimal lattice mismatch (~2.68%), Ag‐ZnO are often realized via site‐specific recognition between {001} and {111} crystal planes of ZnO and Ag respectively [5, 10]. The magnitude of interaction between Ag and ZnO is reflected by the shift of binding energies of the Ag 3d levels; Ag 3d5/2 and Ag 3d3/2 in Ag‐ZnO shifts to lower values compared to bulk Ag. The new Fermi level of Ag‐ZnO, results in the tendency of Ag to exhibit higher valence. Since the binding energy of Ag+ is lower than for pure Ag, peak of Ag 3d levels in Ag‐ZnO shifts to the low energy side [11, 12]. The segregation of Ag NPs around the ZnO grain

Plasmonic Ag‐ZnO: Charge Carrier Mechanisms and Photocatalytic Applications

boundaries suppresses the grain growth and enriches the active sites, enabling the easy reaction of charge carriers with surface‐adsorbed molecules. As ZnO and Ag NPs possess optical response at UV and visible regions respectively, heterostructuring their discordant crystal phases can produce unique electrical–optical functionalities and modulate the optical phenomena including harvesting, emission, and concentration of electromagnetic radiation [4, 6, 7]. Despite the benefits of Ag‐ZnO in energy applications, their disadvantages are derived from the chemical instability of Ag itself, which has a tendency to oxidize in the open atmosphere and also in the aqueous medium. The Ag deposition process may not be always beneficial, probably due to the secondary effects such as non‐uniform distribution of Ag islands, the larger size of Ag‐ ZnO, and the inefficient transfer of plasmonic hot electrons from Ag to ZnO [1, 13]. The sudden change of ZnO surface states with morphology and preparation method obstructs the easy growth of Ag NPs while the self‐nucleation of metal particles within the solution phase during the reduction of metal ions remains an unresolved issue [14].

6.3 ­Methods to Decorate Silver NPs on the Surface of ZnO Ag‐ZnO can be synthesized by several physical techniques, such as electrospinning [13], flame spray pyrolysis [15], radio frequency magnetron sputtering [16], pulsed laser deposition [17], plasma‐enhanced chemical vapor deposition [18], and electrodeposition [19]. These methods are quite expensive, including multiple steps in the preparative stage, and tailoring the size of Ag NPs as well as obtaining Ag‐ZnO with hierarchical morphology is quite tedious. The thermal decomposition of Zn(CH3COO)2 and CH3COOAg also forms Ag‐ZnO, but the subsequent annealing induces drastic grain growth [20]. On the other hand, the ball milling method does not result in the complete transformation of the ionic silver precursor to metallic silver [21]. Alternatively, wet chemical routes (solution‐phase synthesis) involving the simultaneous nucleation of ZnO and in situ reduction of Ag+ on the ZnO surface are very successful, because the size/shape‐morphology of Ag‐ZnO can be conveniently tailored to achieve the desired functionalities. Since the chemical reactions are controlled at the precursor level [22], tuning the electronic properties of ZnO or Ag is even more viable. There are two approaches to prepare Ag‐ZnO in solution.



(i) Nucleation of ZnO is successively followed by the deposition of Ag using impregnation, chemical reduction, and photodeposition methods [8, 10]. In this strategy, the size and morphology of only Ag NPs can be varied without altering the bulk properties of ZnO. (ii) Simultaneous nucleation of Ag and ZnO is achieved by mixing their respective precursors in a suitable solvent under the specified reaction conditions.

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The approaches pertaining to the latter are solvothermal [23], hydrothermal [24], ultrasonic irradiation [25], microwave irradiation [26], non‐aqueous method [27], and deposition‐precipitation [28]. These protocols are striking as the size, shape, and morphology of Ag and ZnO NPs can be structured simultaneously. However, competition between the growth units of both the nuclei as a result of mutual interferences among them affects the crystallization kinetics. The multiple reactions operating in these scenarios are quite complex since the reducing agents affect the crystallization kinetics of ZnO. As distinct chemical reactions are involved, it is apparent that Ag distribution on the ZnO surface, concentration of oxygen defects, Ag‐ZnO interfacial structure, and dispersion of Ag‐ZnO in the aqueous medium will be varied. The annealing in this protocol (if required) is amended at moderate temperature to prevent grain growth and the formation of an Ag2O impure phase. NaBH4 [2, 29] and N2H4 [10] are the common reducing agents that operate at room temperature and are used only when Ag+ ions need to be reduced on finely crystallized ZnO (ZnO@Ag). Other reducing agents, such as sodium citrate [30], glucose [31], ethanol [23], ethylene glycol [32], dimethyl formamide (DMF) [33], diethanolamine [34], triethanol amine (TEA) [24, 35], neem plant extract (Azadirachta indica) [36], leaf extract (Urtica dioica leaf ) [37], arginine [38], polyvinyl pyrrolidone [9], cotton fabric [39], and hexamethylene tetramine (HMT) [14], are preferred in the temperature controlled reactions. The reducing additives perform multiple roles depending on the reaction conditions: (i) sodium citrate acts as a structure directing agent; (ii) TEA can coordinate to the zinc precursor, while HMT serves as an ammonia source, shifting the solution pH to alkaline medium [24, 35]; (iii) glucose coordinates with metal ions and easily polymerizes under low temperature to incorporate desirable precursors into the polymeric glucose framework [31]; and (iv) DMF is used in acidic medium as Ag2O or AgO are formed with basic water/alcohol mixtures [33]. It was interesting to note that an Ag@ZnO (core@shell) structure was observed with DMF solvent under the ambient conditions [33]. The use of plant/leaf extracts enables the formation of Zn(OH)2 and Ag2O due to the excess alkalinity of the reaction medium [37]. Unfortunately, fine tuning the particle size of Ag NPs with these reagents is still far from scientific discussion. As a technique free from external reducing agents, photodeposition of Ag+ ions onto the ZnO surface under laboratory conditions is more promising, as this approach offers high reproducibility accompanied with environmental protection [40]. In addition, this method does not affect the crystallinity of ZnO even when loading higher amounts of Ag [41]. The only drawback associated with this approach is the abrupt formation of isolated Ag NPs via self‐ nucleation in the solution phase. Contrary to these aforementioned methods, activation of Ag+ reduction by Sn2+ ions also produced stable Ag‐ZnO [42]. However, the impregnation method is accompanied with nucleation of Ag2O

Plasmonic Ag‐ZnO: Charge Carrier Mechanisms and Photocatalytic Applications

at initial stages, which decomposes to AgO and finally Ag islands are formed with an increase in the calcination temperature [43]. In recent years, the biomolecule bovine serum albumin (BSA) was used as a reducing agent to obtain Ag NPs. BSA coordinates with Ag and Zn ions, thereby serving as shape‐controlling agent [44]. Owing to its efficient coordination capacity, BSA is predicted to avoid self‐nucleation of Ag NPs in the solution medium. The Ag+ ions were reduced using an electrochemically active biofilm (EAB) with electron donor CH3COONa; the biofilm provides excess electrons and protons by biologically decomposing CH3COONa [45]. Phytochemicals such as pomegranate peel extract can also be used as reducing agents as well as stabilizing agents in the synthesis of Ag‐ZnO [7]. Depending on the reaction conditions, Ag is formed from the reduction of Ag+ or Ag2O. In the solution approach under alkaline conditions, [Ag(OH)2]− and [Zn(OH)4]2− undergo intermolecular hydrolysis to form Zn–O–Ag bonds. Even if NH3 is used as a complexing agent, the [Zn(NH3)4]2+ and [Ag(NH3)2]2+ react with hydroxyl anions to form the similar hydroxo species. At the growth stage, ZnO crystals grew very fast and Ag2O was reduced in the presence of reducing agent [12, 14, 39, 44, 46].

Zn 2



Ag



4OH 2OH

Zn OH

4

2

Zn OH Ag OH Ag OH 2+

2

4

2

2



Co-dehydroylsis

Ag 2 O / ZnO 2H2 O 4OH



Compared to [Ag(NH3)2] , Ag2O is very sensitive to reduction, as the standard redox potential of Ag+/Ag (+0.8 eV) is greater than the [Ag(NH3)2]2+/Ag potential (+0.38 eV) [14, 44]. The solution pH plays a central role in the reduction step: Ag+ ions should be allowed to adsorb on ZnO nuclei/surface prior to its reduction as the content of Ag formed depends on the adsorption of Ag precursors. If Ag+ ions experience columbic repulsion from the ZnO surface, a low density of Ag will be formed and isolated nucleation of Ag will prevail in the solution. Conversely, Ag+ experiences electrostatic interactions with the negatively charged ZnO surface in a basic medium, but the formation of Zn(OH)2 and [Zn(OH)4]2− would complicate the reduction process. Therefore, solution pH equating to the point of zero charge would be favorable for the reduction of Ag+ ions on ZnO surface, as electrical double layer is shortened because of disappearance of repulsive forces [1, 3, 8]. The reducing medium and the reduction method are very important to obtain fine‐tuned Ag NPs. As the reaction conditions are distinct with each method, the deposited metal diverges in shapes and density distribution on the ZnO [47].

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The morphology and distribution of Ag depends on the preparative method, along with the reducing medium and the reaction temperature. A low reaction temperature promotes the anisotropic growth of Ag as nanoplates, while irregular morphology is commonly observed at elevated temperature [29]. Although obtaining nanostructured Ag is essential, a low density of Ag formed at a lower temperature may not induce the SPR effect. The chemical reduction of Ag+ ions is normally carried out in the aqueous medium under dark conditions; the solvent effects in tailoring the size and morphology of Ag NPs during the reduction step are still not yet extensively investigated. In contrast, solvent effects dominate in the photoreduction method, wherein alcohols are commonly used as sacrificial electron donors to engage the conduction band (CB) electrons for Ag+ ion reduction. The content of Ag reduced on the surface of the ZnO nanorod (NR) using photodeposition method in air, pure water, and a water/alcohol medium were 1.35, 7.68, and 26.1 at.% respectively [47]. This clearly suggests that the reduction process was efficient in a water/ethanol mixture as compared to pure water and air. Strangely, Ag‐ZnO obtained in aqueous medium had a sheet‐rod‐particle multilevel structure, while a large number of Ag nanoclusters were embedded on the ZnO surface in the ethanol/ water mixture (Figure 6.1) [47].

UV

ZnO nanorod arrays

Ag

air wa te r

water

Ag+

+

et

ha

no

l

UV UV +

Ag

Ag

Ag+ Ag

Figure 6.1  Change in the morphology of Ag nanoparticles in different reducing medium using a photodeposition method. Source: Reprinted with permission from Ref. [47]. Copyright 2015, Elsevier.

Plasmonic Ag‐ZnO: Charge Carrier Mechanisms and Photocatalytic Applications

It was further reported that the size of Ag NPs was reduced with increases in the poly ethylene glycol content [48]. The loading of Ag and shift in SPR was more significant when Ag was deposited on ZnO nanoflowers under water/ glycol medium (35.03  wt%, 400–800  nm), compared to water/ethanol (23.62 wt%, 434 nm) and pure water (27.56 wt%, 452 nm) conditions, which is directly related to the Ag content [49]. As alcohols are susceptible for photoreaction, the by‐products formed might affect the surface chemistry of the metal oxide itself. The formation of CO resulting from the partial oxidation of ethanol strongly adsorbs on the surface of noble metals and its subsequent desorption requires high temperature treatment. Conversely, photodeposition in a water‐rich environment suppresses the CO contamination and seems to promote water adsorption at the surface of metallic clusters, which favors hydroxyl radical generation [50]. Despite their importance, a few issues are yet to be addressed properly: (i) deposition yield of Ag is usually low ( CO

[61, 62]

Bi2O3

2.8

methanol nearly 60 μmol g−1h−1

[62]

SiC

3.0

HCOOH, HCHO, and CH3OH are the main products

[7]

u–g–C3N4 (Cu loaded)

2.75

better performance in photocatalytic carbon dioxide reduction, methane is major product

[61, 63]

ZrO2

5.0

rates of gas evolutions were 19.5 μmol h−1 of H2, 10.8 μmol h−1 of O2, and 2.5 μmol h−1 of CO

[64, 65]

Ta2O5‐rG

high methanol yields

[61]

ZnS

3.91

CO2 reduced to HCOOH, ZnS–MMT (montmorillonite) nanocomposite exhibited 5–6‐fold higher efficiency of photocatalytic CO2 reduction

[8, 61, 66]

GaN

3.4

the reduction of CO2 to CO dominates on as‐grown [65] GaN nanowires under ultraviolet light irradiation

GaP

2.16

HCOOH, HCHO, and CH3OH are the reported products

[8]

GaAs

1.43

spontaneously produce CH3OH, even under conditions of no net current

[67]

PbS

0.37

The PbS QDs enhance CO2 photoreduction rates with TiO2 by a factor of ~5 in comparison to unsensitized photocatalysts

[68]

SrTiO3

3.3

7 μmol h−1 methanol and less than 1 μmol h−1 formaldehye

[69]

LiNbO3

4.0

lithium niobate achieved unexpectedly high conversion of CO2 to products despite the low levels of bandgap light available.

[70]

(Continued)

351

352

Photocatalytic Functional Materials for Environmental Remediation

Table 12.3  (Continued)

Semiconductor

Bandgap (eV) Description

MoS2

1.23

[71] compared with the pristine Bi2S3 or MoS2, the as‐synthesized D‐Bi2S3@MoS2 composite has exhibited much higher adsorption behavior and photocatalytic activity under visible light irradiation

BaLa4Ti4O15q

~4 and 5

H2: 10 μmol h−1, O2: 7 μmol h−1, CO: 4.3 μmol h−1, HCOOH: 0.3 μmol h−1

[68]

KTaO3

~3.5

reduce CO2 to CO and oxidize water to O2

[72]

WO3 and 2.8 graphene‐WO3

Ref.

−1

around 5 μmol h methanol

[61, 69]

vertical TiO2 nanotube arrays (NTAs) have shown significant photocatalytic activity of CO2 reduction [24, 25]. However, maintaining the long‐time stability and high activity of the catalyst, especially the co‐catalyst, is still a great challenge. Noble metal Pt nanoparticles (NPs) are easily poisoned by CO during the catalytic process [26] and for non‐noble metal NPs, changes in the surface states [27] are the main reason for the co‐catalyst deactivation. Efficient photocatalytic conversion of CO2 into CO and hydrocarbons by hydrous hydrazine (N2H4·2H2O) is shown on SrTiO3/TiO2 coaxial NTAs loaded with Au/Cu bimetallic alloy. The synergetic catalytic effect of the Au/ Cu alloy nanoparticles and the facile electron transfer in the SrTiO3/TiO2 coaxial nanoarchitecture are the reasons for the efficiency. The reduction products are identified by IR spectroscopy [28]. By varying the fraction of one component in this bimetallic alloy system, a Au3Cu@STO3/TiO2 NTA has been found that is the most reactive photocatalyst in this family to generate hydrocarbons from diluted CO2 with N2H4·2H2O acting as the hydrogen source and electron donor and also providing a reductive atmosphere for maintaining the alloying system. Plasmonic photocatalysts have become popular in recent times [29–31]. This is because nano metals such as Au, Ag, and Pt exhibit strong absorption in the UV‐vis region due to their surface plasmon resonance (SPR) [32]. SPR means the collective oscillations of conduction band electrons in a metal particle and it is driven by the electromagnetic field of incident light [33, 34]. Dispersing metal nanoparticles (10–100 nm) on a semiconductor photocatalyst shows significant enhancement in photocatalytic activity under UV and visible light irradiation due to SPR. The surface plasmonic resonance frequency can be tuned by manipulating the size, shape, and material of the nanoparticles. Noble metal nanoparticles are a combination of surface area and active sites. In the photocatalytic CO2

Active Materials for Photocatalytic Reduction of Carbon Dioxide

reduction with water, it has been reported that depositing Au nanoparticles on TiO2 results in plasmonic enhancement (532 nm) of a 24‐fold enhanced photocatalytic activity. The synthesis, characteristics, and application of plasmonic photocatalysts in CO2 reduction with H2O, where CH4 and CO are produced under visible light irradiation (>400 nm wavelength) has also been reported. The comparative photocatalytic activities toward the CO2 reduction of a different photocatalyst in the presence of water under visible light were evaluated by the quantity of carbon‐containing products. The results show the increasing amount of CH4 and CO formation with time under visible light. There are various ways, as shown above, that carbon dioxide can be reduced to value‐added chemicals and fuels. In all these attempts, it is the selection of materials to promote the reaction in the desired direction that appears to be both critical and challenging [35]. Many developments have been carried out in the photocatalytic reduction of carbon dioxide. Recently, it has been shown that even infrared radiation can be effectively utilized, using an oxygen‐­deficient WO3 system to reduce CO2 [36]. The pictorial representation of this process that has been proposed [36] is reproduced in Figure 12.3. These developments show that though the conversion levels of carbon dioxide is low at present, it is certain that, in the near future, the conversion of carbon dioxide into value‐ added chemicals and fuels will become a commercially viable process. However, this exploitation requires that some fundamental principles are fully understood and the material selection is made on a rational basis. Only a few of the variations in the choice of materials or combinations will be considered in this section. In a recent publication [37], the photosynthetic

Potential vs. NHE (V) at pH = 0

F CB

–1.0

CO/CO2

0.0 IB

1.0

O2/H2O

2.0 VB

3.0 4.0

Oxygen-deficient WO3 atomic layers WO3 atomic layers

Figure 12.3  Energy level diagram of layers and oxygen‐deficient WO3 atomic layers which can be used with the infrared radiation of solar radiation for the photocatalytic reduction of carbon dioxide. Source: Reproduced from Ref. [36].

353

354

Photocatalytic Functional Materials for Environmental Remediation

conversion of carbon dioxide to hydrocarbons (methane and other higher hydrocarbons) on Pd/TiO2 in conjunction with Nafion has been reported. The conversion efficiency of this process is, as usual, not high. Since the one‐electron reduction of CO2 (the initial step in the reduction process) requires high potentials of the order of 2 V (as pointed out earlier in this chapter), a favorable pathway is to reduce CO2 though PCETs. The role of the Nafion layer is to enhance the local proton activity within the layer to facilitate PCET reactions, to stabilize intermediates, and to inhibit the re‐oxidation of the CO2 reduction products. It is observed that Nf/Pd–TiO2 is more active than Pd–TiO2 for the photoproduction of hydrocarbons (e.g. methane, ethane, and propane). The conceptual design of the catalyst system employed in this study is shown in Figure 12.4 [37]. In the light of these results, the following points are presented for conceptual analysis and possible adaptation. i) Is a direct proton source desirable (rather than the protons coming from the decomposition of water), so that the reduction of CO2 is facilitated? ii) It is essential that the species employed as a proton source should not undergo any electrochemical reaction within the potential range for the CO2 reduction reaction. iii) The reactivity of the proton should be as high as that in Nafion (very nearly a bare proton), where the proton is in a highly electronegative environment of fluorine atoms. iv) The available protons should be capable of reacting with carbon dioxide directly, promoted by the light absorbed by semiconductors (TiO2), and the reduction reaction should be carried out on some reactive metal sites. This possibly means the photon absorption and reactive sites may be distinguishable. CO2

Nafion layer H+ CO2

H+ H+

C F2

CH4 H+

Pd e–

CH4

H2O h+

TiO2

H+ O2

F2 C

x

CF O

F2 C

y CF2 CF CF3

O z

C F2

F2 O C S O

OH

H+

Figure 12.4  Pd/TiO2 on Nafion catalyst system for the photochemical reduction of carbon dioxide. Source: Reproduced from Ref. [37] with permission from the Royal Society of Chemistry.

Active Materials for Photocatalytic Reduction of Carbon Dioxide

Considering these aspects, and also based on argumentative considerations, it is possible that alternative PCET catalyst support systems that may sustain more acidic protons can be tried, such as heteropoly acids or super acids (such as sulfated zirconia). Another opinion in material selection in recent times is the hybrid perovskites (CH3NH3PbI3) as a possible alternative for solar cell and photocatalytic applications. These materials have been shown to be nearly two times more active (24%) as compared to kesterite (Cu2ZnSn(S,Se)4) materials [38], while the number of publications about hybrid perovskites is nearly three times the number of those on kesterite. It may be presumed that kesterite may become one of the possible alternative materials for the photocatalytic reduction of carbon dioxide in the near future. In the next decade, there is the possibility that the efficiency of these materials may improve and thus pave the way for the commercialization of this process. Many more variations are being tried to increase the useful photon energy range into the visible and near infrared, as well as to increase the efficiency of the process. These attempts are continuing exercises in research, and will continue to be, until a viable and efficient system has been identified.

12.5 ­Selection of Materials It is realized that in any successful attempt to make this process viable commercially, the selection of photocatalytic material is an important requirement. There have been various attempts in this direction in the past based on various methodologies. However, even today, the search continues to identify a material which will be able to provide the efficiency required. In this connection it has been pointed out earlier that the positions of the conduction band minimum and valence band maximum of the semiconductor represent the reduction and oxidation ability of the system. A compilation of the known semiconductors and their characteristics is attempted in Table 12.4. The data presented in this table may be useful in order to select a semiconductor material based on the logistics of the process instead of on a trial and error basis. This is an important aspect, since the selection of suitable materials is the immediate need for making this process commercially viable and environmentally acceptable. Note that in the two “Band edge” columns in Table 12.4, the figures in brackets are the pH value at which the band edges values are reported if this isn’t pH 7. The values given in Table 12.4 can be used to identify appropriate and suitable semiconductor materials for the simultaneous photocatalytic reduction of carbon dioxide and photodecomposition of water.

355

356

Photocatalytic Functional Materials for Environmental Remediation

Table 12.4  List of semiconductors with data on bandgap, valence band maximum, and conduction band minimum.

Name

Formula

Eg (eV)

CB edge at pH = 7 (eV)

VB edge at pH = 7 (eV)

λ (nm)

aluminum oxide

Al2O3

7.1

−3.6

3.5

175

antimony oxide

Sb2O3

3.00

0.32

3.32

413

antimony sulfide

Sb2S3

1.72

0.22

1.94

arsenic sulfide

As2S3

2.50

0.08

2.58

496

barium titanate

BaTiO3

3.30

0.08

3.38

376

barium titanate

BaTiO3

3.30

−3.4

0.1

376

bismuth niobium oxychloride

Bi4NbO8Cl 2.39

−2.11

0.28

519

bismuth oxide

Bi2O3

2.80

0.33

3.13

443

bismuth oxychloride

BiOCl

3.42

−1.5

1.92

363

bismuth vanadate

BiVO4

2.4

−0.3

2.1

517

boron carbon nitride

h‐BCN

2.82

−1.59

1.23

440

cadmium ferrite

CdFe2O4

2.30

0.18

2.48

539

cadmium oxide

CdO

2.20

0.11

2.31

564

cadmium selenide

CdSe

2.0

1.2

3.2

620

cadmium sulfide

CdS

2.4

−0.6

1.8

517

cadmium sulfide

CdS

2.40

−0.52

1.88

517

cerium oxide

Ce2O3

2.40

−0.50

1.9

517

chromium oxide

Cr2O3

3.50

−0.57

2.93

354

cobalt oxide

CoO

2.01

−0.11

1.9

617

cobalt titanate

CoTiO3

2.25

0.14

2.39

551

copper iron sulfide

CuFeS2

0.35

0.47

0.82

—

copper oxide

CuO

1.70

0.46

2.16

729

copper(II) oxide

CuO

1.35

4.07

5.42

1032

copper titanate

CuTiO3

3.0

−0.18

2.81

413

copper titanate

CuTiO3

2.99

−0.18

2.81

415

cuprous oxide

Cu2O

1.9

−1.30

0.60

620

cuprous oxide

Cu2O

2.20

−0.28

1.92

564

FAPbI3

FAPbI3

1.5

−5.42

−3.92

827

ferric oxide

Fe2O3

2.2

0.13

2.33

560

ferric oxide

Fe2O3

2.20

0.28

2.48

564

ferrous ferric oxide

Fe3O4

0.10

1.23

1.33

—

Active Materials for Photocatalytic Reduction of Carbon Dioxide

Table 12.4  (Continued)

Name

Formula

Eg (eV)

ferrous sulfide

FeS

0.10

CB edge at pH = 7 (eV)

0.47

VB edge at pH = 7 (eV)

λ (nm)

0.57

—

gallium oxide

Ga2O3

4.80

−1.54

3.26

258

indium oxide

In2O3

2.9

−3.88 (at pH 8.64)

−0.98

428

indium oxide

In2O3

2.80

−0.62

2.18

443

indium sulfide

In2S3

2.00

−0.80

1.20

620

indium tantalum oxide

InTaO4

2.6

−0.75

1.85

477

iron disulfide

FeS2

0.95

0.42

1.37

—

iron titanate

FeTiO3

2.80

−0.21

2.59

443

iron(II) oxide

FeO

2.40

−0.17

2.23

517

lanthanum oxide

La2O3

5.50

−1.97

3.53

227

lead copper antimony sulfide

PbCuSbS3

1.50

0.11

1.61

lead oxide

PbO

2.80

−0.48

2.32

lead sulfide

PbS

0.37

0.24

0.61

443

lead titanate

PbTiO3

2.75

−2.75

0.0

451

lithium niobate

LiNbO3

3.50

−0.73

2.77

354

lithium tantalate

LiTaO3

4.00

−0.95

3.05

310

magnesium oxide

MgO

7.3

−3.0

4.3

159

magnesium titanate

MgTiO3

3.70

−0.75

2.95

335

manganese dioxide

MnO2

0.25

1.33

1.58

manganese disulfide

MnS2

0.50

0.49

0.99

—

manganese oxide

MnO

3.60

−1.01

2.59

344

manganese(II) oxide

MnO

3.6

−3.49 (at pH 8.61)

0.11

345

manganese sulfide

MnS

3.00

−1.19

1.81

413

manganese titanate

MnTiO3

3.10

−0.46

2.64

400

mercury oxide

HgO

1.90

0.63

2.53

653

methyl lead bromide

MaPbBr3

2.3

−5.68

−3.38

539

methyl lead chloride

MAPbCl3

3.09

−2.54

0.55

400

methyl lead iodide

MAPbI3

1.55

−5.43

−3.88

800

methyl tin iodide

MASnI3

1.3

−5.47

−4.17

954

molybdenum sulfide

MoS2

1.17

0.23

1.40 (Continued)

357

358

Photocatalytic Functional Materials for Environmental Remediation

Table 12.4  (Continued)

Name

Formula

Eg (eV)

nickel disulfide

NiS2

0.30

CB edge at pH = 7 (eV)

0.89

VB edge at pH = 7 (eV)

λ (nm)

1.19

nickel oxide

NiO

4.3

−0.5

3.8

285

nickel oxide

NiO

3.50

−0.50

3.0

413

nickel sulfide

NiS

0.40

0.53

0.97

nickel titanate

NiTiO3

2.18

0.20

2.38

569

nickel titanate

NiTiO3

2.18

0.20

2.38

569

niobium oxide

Nb2O5

3.40

0.09

3.49

365

niobium pentoxide

Nb2O5

3.40

0.09

3.49

365

palladium oxide

PdO

1.00

0.79

1.79

1240

platinum sulfide

PtS2

0.95

1.03

1.98

potassium niobate

KNbO3

3.30

−0.86

2.44

376

potassium tantalate

KTaO3

3.50

−0.93

2.57

354

rhodium sulfide

Rh2S3

1.50

0.11

1.61

ruthenium sulfide

RuS2

1.38

0.39

1.77

silicon

Si

1.1

−0.6

0.5

silicon carbide

SiC

3.0

−1.5

1.5

415

silver oxide

Ag2O

1.20

0.19

1.49

1033

1125

silver sulfide

Ag2S

0.92

0.0

0.92

1348

strontium titanate

SrTiO3

3.4

−3.24 (at pH 8.6)

0.16

364

strontium titanate

SrTiO3

3.40

−1.26

2.14

365

tantalum oxide

Ta2O3

4.00

−0.17

3.83

310

tin dioxide

SnO2

3.50

0.0

3.5

354

tin oxide

SnO2

3.5

−4.5 (at pH 4.3)

−1.0

354

tin oxide

SnO

4.20

−0.91

3.29

295

tin sulfide

SnS

1.01

0.16

1.17

titanium oxide (anatase) TiO2

3.23

−0.29

2.94

384

titanium oxide (rutile)

TiO2

3.02

−0.52

2.5

410

titanium sulfide

TiS2

0.70

0.26

0.96

tungsten oxide

WO3

2.6

0

2.5–2.8

tungsten oxide

WO3

2.70

0.74

3.44

459

tungsten selenide

WSe2

1.4

−0.25

1.15

1032

495– 442

Active Materials for Photocatalytic Reduction of Carbon Dioxide

Table 12.4  (Continued) CB edge at pH = 7 (eV)

VB edge at pH = 7 (eV)

Name

Formula

Eg (eV)

tungsten sulfide

WS2

1.35

0.36

1.71

vanadium pentoxide

V2O5

2.7

−4.7 (at pH 6.54)

−2.0

458

vanadium pentoxide

V2O5

2.80

0.20

3.0

443

zinc indium sulfide

Zn3In2S6

2.81

−0.91

1.90

441

λ (nm)

zinc oxide

ZnO

3.2

−0.31

2.91

387

zinc oxide

ZnO

3.20

−0.31

2.89

388

zinc sulfide

ZnS

3.7

−3.46 (at pH 1.7)

0.24

335

zinc sulfide

ZnS

3.60

−1.04

2.56

344

zinc titanate

ZnTiO3

3.07

−0.23

2.84

404

zirconium dioxide

ZrO2

5.00

−1.09

3.9

248

zirconium sulfide

ZrS2

1.82

−0.21

1.61

681

Source: data collected from literature, especially from Refs. [4, 73], among others.

12.6 ­Material Modifications for Improving Efficiency According to the given description of the mechanism of carbon dioxide photoreduction, it can be stated that the selection of the semiconductor photocatalyst is the key for the success of this process. The semiconductor photocatalyst suitable for carbon dioxide photoreduction should not only provide appropriate valence bands and conduction bands with suitable energy positions that can induce water decomposition and carbon dioxide reduction reactions simultaneously, but also have the properties of chemical stability and low cost [39]. According to the research reported so far, all these factors have been found in only one material, which is titanium dioxide (TiO2). This is the most widely used semiconductor, and it has been claimed to be one of the best options to act as a photocatalyst for CO2 photoreduction [40, 41]. Two common crystalline structures of TiO2, rutile and anatase, are commonly used in photocatalysis, with anatase showing a higher photocatalytic activity. This may be because of the relatively larger bandgap of anatase (3.2 eV) than that of rutile (3.0 eV), which allows anatase to provide more sufficiently negative and positive redox potentials in CB and VB during photocatalysis. Hence, most of the reports on TiO2 have shown that the anatase form successfully initiates CO2 photoreduction. However, there are aspects of anatase which have to be improved before it can be exploited for use in a commercial process for the conversion of carbon dioxide.

359

360

Photocatalytic Functional Materials for Environmental Remediation

However, there are still disadvantages hindering the use of TiO2 as an effective catalyst for CO2 photoreduction. Firstly, the efficiency of CO2 photoreduction using TiO2 is still too low for practical applications. It can be seen that even the best work could only achieve a CO2 conversion rate at around 26 μmol g−1 h−1 [40]. Such reaction efficiency is obviously too low for practical applications and, therefore, it is necessary to improve the activity of TiO2 or find alternative materials for carbon dioxide photoreduction. Yet another problem when using TiO2 is its relatively large bandgap (3.2 eV) that can only be effectively excited by ultraviolet (UV) light. As only small fraction of the solar spectrum is within the UV region (not higher than 3%), there is a need to modify the light absorption range of TiO2 to efficiently utilize solar energy for CO2 photoreduction. One of the most widely used methods to improve the activity of TiO2 for CO2 photoreduction is by modifying TiO2 with a metal. This is because the added metal is able to act as a trap of the photo‐ generated charge carriers in order to suppress the electron/hole recombination rate and improve the activity of TiO2. Metal loading on or in a semiconductor can either create alternate adsorption sites or alter the electronic energy levels, thus facilitating the electron‐transfer reactions. (Metal can be loaded on the surface and not incorporated into the lattice, or it can be loaded in the lattice, in which case the doped metal can give rise to additional allowed energy levels in the so‐called forbidden gap or can alter the Fermi level of the semiconductor by charge injection into the host lattice.) Thus, metal loading on to a semiconductor may function as electron traps, thereby suppressing electron/hole recombination rate and thus promoting the desired reaction. This process increases the possibility of these charge carriers reacting with the adsorbed species on the metallic sites that are loaded on the semiconductor photocatalyst. If the loaded metal undergoes oxidation due to calcination, then what one gets is a coupled/composite semiconductor system which also experiences Fermi‐level equalization due to intimate contact. When the added metal is doped into the lattice of the semiconductor (TiO2), the doping can be either substitutional or interstitial. Both of these two types of doping can affect the lattice spacing of TiO2. Incorporation in the substitutional position or interstitial position is reflected in the variation of the lattice spacing of the parent semiconductor. Additional allowed energy levels in the forbidden gap of the semiconductor depends on their energy positions, can act as electron or hole traps, and promotes the corresponding reduction or oxidation reactions or facilitates the recombination of the charge carriers. Hence, the ratio of the doped metal to that of the parent semiconductor must be carefully considered, because the presence of the metal dopant can simultaneously suppress and enhance the recombination of the photo‐generated electron and hole within the semiconductor. It has been reported in the literature that incorporation of metals such as Cu, Pt, Pd, Rh, Fe, and Ag into TiO2 can improve the photocatalytic reduction of carbon dioxide. All these studies can be generally understood by

Active Materials for Photocatalytic Reduction of Carbon Dioxide

the “metal‐modified TiO2” terminology instead of specifying them as either doping or metal loading. TiO2 has been chosen as a representative system. The modifications that have been described, namely doping and coupling, are general in nature and applicable to all the semiconductors that have been tried for this photocatalytic reaction. These studies been attempted in many other semiconductor systems, as shown in Table 12.4. In spite of the many variations that have been tried, the efficiency of the resultant systems has not improved to the desired level and thus the exercise to identify the selection of materials with optimum efficiency is still continuing. These studies that covered the TiO2 system are equally applicable to other systems as well and hence they are not covered in this chapter. To summarize the reactions on semiconductor surfaces, the reduction potentials for various reaction products in the photocatalytic reduction of carbon dioxide are shown in Figure 12.5. On the left‐hand side the potential scale is with zero at vacuum and on the right‐hand side the electrochemical scale is shown at pH = 0. E vs Vacuum

E vs NHE (pH 0) –2.5

–2.0

–2.0 SiC

–7

WO3 WS2 MoS2 1.17 eV

1.95 eV

2.4 eV

1.7 eV

1.47 eV

CdSe

–1.0

1.35 eV

ZnO

2.7 eV

TiO2

CdTe

2.4 eV

3.0 eV

CuFeO2

CdS

3.2 eV

–6

GaAs Inp

1.4 eV

–5

2.24 eV

–4

1.1 eV

Si

–1.5

Cu2O

GaP

3.2 eV

–3

CO2/CO2•

CO2/HCO2H

0.0

CO2/CO CO2/HCOH H2O/H2

0.5

CO2/CH3OH

–0.5

1.0

CO2/CH4 O2/H2O

1.5 2.0 2.5 3.0

Figure 12.5  Conduction band potentials (open squares) and valence band potentials (gray squares) of some commonly used semiconductors, along with with the potentials of several carbon dioxide/water redox couples at pH = 0.

361

362

Photocatalytic Functional Materials for Environmental Remediation

The efforts extended in making this process more efficient have not so far yielded the desired results. This probably indicates that the basic choice of materials has not been appropriate. In spite of the concerted efforts to modify and redesign the available known materials, perhaps one now has to focus on completely new materials for this reaction. It has been pointed out already that the energetic position of the conduction band and the valence band controls the reduction and oxidation reactions that can be promoted by these semiconductors. These positions, as given in Table 12.4, are for the static state of the semiconductor under a semiconductor/vacuum interface. These redox values may be altered during the adsorption and activation of the reagents. This has partly been taken care of in the concept of band bending when the interface is changed to the reaction medium [42]. There is a possibility that the energy level positions of the bands of the semiconductor may functionally change depending on the reaction conditions employed. This aspect has not yet been fully and explicitly dealt within the literature. It was pointed out earlier that the adsorption of carbon dioxide should be such that the band angle should reduce from 180° to a nearly tetrahedral angle. There can be various modes of adsorption of carbon dioxide on the catalyst surface, depending on the nature of the surface morphology. Essentially, the possible structural modes of adsorption of carbon dioxide can be visualized as shown in Figure 12.6 [3]. The modes shown in Figure 12.6 are the ideal configurations of the adsorption mode of carbon dioxide. Depending on the nature of the surface site and the adsorption geometry of the substrate, the modes of adsorption of CO2 can vary and one typical configuration was shown in Figure 12.2. As already stated, the reduction of CO2 is limited kinetically since the lowest unoccupied molecular orbital (LUMO) level is an antibonding orbital and hence the reduction process has to be coupled with proton transfer. For this purpose the qualitative molecular orbital energy diagram for carbon dioxide is as shown in Figure 12.7. The reduction of carbon dioxide to hydrocarbons can be carried out on surfaces where proton and electron transfers are simultaneously possible. Thus the search for catalyst systems should be carried out on such surfaces where the transfer is facile for both these species. Carbon dioxide is chemically inert, has a closed shell electronic configuration, and is linear in structure. The addition of a single electron causes the necessary bond angle reduction and bends the molecule structure due to the repulsion between the added electron and the electron pairs on the oxygen atoms. This repulsion contributes to the high energy of the LUMO level of the carbon dioxide molecule and thus accounts for the low electron affinity of the molecule. This situation makes it virtually certain that no semiconductor is capable of transferring the single electron to a free CO2 molecule; the reduction potential is around −1.90 V as stated above. Though single electron reduction thus experiences a higher energy barrier, the situation is manageable with

Active Materials for Photocatalytic Reduction of Carbon Dioxide

Physisorbed CO2 and its vibrations

(Weakly) chemisorbed CO2 (C-down configuration) and its vibrations

O-Down configuration

Mixed configuration

Upright configuration

(Weakly) chemisorbed CO2 (O-down configuration)

Figure 12.6  Possible adsorption modes (physisorbed, chemisorbed with C or O down) of carbon dioxide on surfaces. Source: Reproduced from Ref. [3].

proton‐assisted transfer of multiple electrons. This is clear from the values of reduction potentials given in Table 12.2.

12.7 ­Perspectives in the Photocatalytic Reduction of Carbon Dioxide In essence, the search for suitable semiconductor materials has been focusing on the following aspects: 1) Extending the photoresponse of the semiconductor to the visible region of the solar spectrum. 2) Sensitizing the semiconductor (by the addition of molecular species or coupling/modifying) to effectively utilize the excitons and improve the efficiency.

363

Photocatalytic Functional Materials for Environmental Remediation Carbon

CO2 a1u a1g

Oxygen combination orbitals

e1u

2p Energy

364

(e1u) (a1u)

2s (a1g)

e1g

π

e1u

σ

a1u

σ

a1g a1u a1g

e1g e1u a1u a1g

a1u a1g

2p(degenerate)

2s(degenerate)

Figure 12.7  Qualitative molecular orbital diagram for carbon dioxide. Source: Reproduced from Ref. [3].

3) Altering the valence band maximum in order to change the value of the bandgap. 4) Altering the conduction band minimum to more reductive potential values so as to facilitate hydrogen production. 5) To generate multiple photocatalytically active sites and also to alter the surface area, new nanoscale morphologies have been employed. In spite of these attempts, so far no realizable success in the reduction of carbon dioxide seems to have been achieved. This may mean that the steps so far attempted, though they may logically be in the correct direction, may only marginally improve the efficiency; perhaps the search has to take an alternative, so far untrodden, path. At present, the formation rates of products from the photocatalytic reduction of carbon dioxide on semiconductors rarely exceed tens of mmol g−1 h−1. This means that the efficiency of the process is generally lower than in natural photosynthesis or even less than that achieved in the photocatalytic generation of hydrogen. However, this has not hampered the interest of scientists pursuing this research field. Recent developments are concentrated on the search for new photocatalytic materials and new nanoscale configurations. New photocatalytic materials could be novel materials of metals with d0 or d10 electronic configurations, though this concept that systems with this electronic configuration will be

Active Materials for Photocatalytic Reduction of Carbon Dioxide

appropriate semiconductors for this reaction has not yet been fully established. New nanoscale configurations could offer improved surface area, increased charge separation, and vectorial electron transfers. The mechanism of the process has been studied by both experimental and computational methods. These studies are aimed to answer the unanswered questions concerning the chemical pathways of CO2 reduction. Of particular interest are the approaches to overcome the barrier associated with the activation of a CO2 molecule toward the first one‐electron reduction. This step is rate limiting, because of the highly negative electrochemical reduction potential of CO2 to the anion radical with respect to the conduction band levels of commonly employed semiconductors. Other important aspects of the mechanism that are relevant for a deeper understanding of the process include charge‐carrier dynamics within the semiconductors, the effect of the nanostructure of the photocatalyst, and the impact of the choice of the catalytic metal on the photocatalytic reduction of carbon dioxide. Most of the studies reported on this reaction employ TiO2 and its variations [43, 44], as it is considered a good model system for comparative study. The situation regarding the results reported about this system does not allow a direct comparison of the system as there is no standardized procedure adopted or assigned to report the results. It may be worthwhile if standardization could be achieved on the amount of the catalyst employed and also if there was a proper measure of the photon intensity employed. Another aspect on which there is no clarity or consistency is the counter reaction; this is the reaction involving oxygen or hydrogen peroxide formation in relation to the reduction reaction carried out. Until this is achieved, there will be always doubt on the sustainability of the reaction or long‐term use of the catalysts for commercial exploitation. The system ultimately employed commercially should be based on the cost of the material and also that it is abundantly available. Since ultimately the products of reaction should be separable, photoelectrochemical cells will be advantageous for exploiting this reaction commercially. The feverish attitude in studying this reaction and the consistent attempts to identify the appropriate semiconductors for promoting this reaction can be expected to move this process closer to success. In the future, the direction of research may be to exploit the effects of mixed crystal phase, defect disorders, and modifications in designing the chosen catalyst systems. It is also necessary that the mechanistic details of the charge transfer will have to be elucidated with respect to details of each and every step at the interface. Various configurations and various additives have been tried for the photocatalytic reduction of carbon dioxide. Varghese et al. [45] reported experiments conducted in outdoor sunlight. The relative hydrocarbon production rate of 111 ppm cm−2  h−1, or ≈160 μl g−1  h−1 has been reported when the nanotube array samples are loaded with both Cu and Pt nanoparticles. This rate of CO2

365

366

Photocatalytic Functional Materials for Environmental Remediation

SUN

CH4, H2,O2 CO-CATALYST

CO2, H2O

Figure 12.8  Flow through nanotube array loaded with co‐catalyst for photocatalytic conversion of carbon dioxide and water into hydrocarbons. Source: Reproduced from Ref. [45].

to hydrocarbon production appears to be higher than what is so far reported. The pictorial representation of their system is shown in Figure 12.8. Osterloh, in a recent commentary on recent advances in solar fuels and photocatalysis, said that this area continues to attract interest in the scientific community [46]. Solar photon energy can be used to form fuels. However, harnessing solar photons and utilizing their energy in technologically useful processes is not an easy job. It requires materials that are strong light absorbers, that exhibit long excited state lifetimes, and enable fast charge carrier processes. As has been pointed out earlier, designing appropriate photocatalysts is important as it determines the activity and selectivity of CO2 reduction. Nanostructuring resulted in increasing surface area and an increase in the number of active reaction sites, as pointed out earlier. In recent times interest has been focused on various configurations of composite photocatalysts by introducing metal and metal oxide co‐catalysts, as well as Z‐scheme systems [47]. These systems are mainly aimed to improve the charge separation. Additionally, Z‐scheme photocatalytic systems have shown a strong tendency toward oxidation and reduction, while their narrow bandgap is beneficial to utilize visible light. Although various photocatalysts have shown good performance, the photocatalytic reduction of protons to hydrogen, a competitive reaction in aqueous CO2 photoreduction, remains a major obstacle. Significant improvements in catalyst and reactor design are needed for the photocatalytic functionalization of CO2 to become a viable technology for the production of energy and chemicals. Fundamental strategies for the rational design of materials for effective transformations of CO2 to value‐added chemicals have to be evolved [48].

Active Materials for Photocatalytic Reduction of Carbon Dioxide

In recent times, a number of reviews and perspectives have been published in the literature on the photocatalytic reduction of carbon dioxide [49–51]. In spite of this spate of activity on this reaction, commercially viable processes are yet to evolve. The various reasons for this have been outlined. The reasons for this failure can be many; however, the foremost among them is the choice of material. In the literature, various sensitization methods have been focused toward exploiting the visible range of the solar spectrum. However, even the utilization of the ultraviolet component of the solar spectrum (~3%) [52] alone may be enough to satisfy the energy needs of the Earth. Under these circumstances, the search for photocatalysts active in the visible spectrum has to be justified. Secondly, the various sensitization methods employed in the fast five decades have not yielded the desired results. Hence, this may mean that the search for the selection of suitable materials must be directed elsewhere. In conclusion, in spite of the various tools (both experimental and theoretical) on hand and also having the facility to scan a number of samples at the same time (high throughput analysis), the appropriate material for photocatalytic reduction of carbon dioxide or hydrogen generation from water has not been achieved in the last five decades, though vigorous attempts have been directed to do so. This may mean that nature still holds her secrets, as in photosynthesis. In our anxiety to mimic nature, we have not yet learnt the ways in which nature performs her functions to satisfy all the needs of the humanity in a satisfactory manner.

­Acknowledgement The author’s grateful thanks are due to his colleagues and to the Department of Science and Technology, Government of India, for the creation of the National Center for Catalysis Research at the Indian Institute of Technology, Madras.

­References 1 Aulice Scibioh, M. and Viswanathan, B. (2007). Carbon dioxide a matter of

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373

Index a absorbed  267, 278 absorption  1, 22, 29, 93, 96, 135, 137, 144, 146, 149, 151, 153, 303 acidic  133, 154 activated carbon  15–17, 21 adsorption  133, 146, 153, 156 advanced oxidation process (AOP)  41, 57, 309 alloy systems  352 anatase  10–12, 17, 359 antibiotics  326, 327

b band edge  355 band gap  12, 29, 42, 43, 50, 55, 57, 65, 93, 96, 164–166, 174, 178, 179, 182, 298, 315, 318 binding energy  191, 192 brilliant green  17 brookite 10.11 brucite layers  215, 221, 222

c carbohydrate 343 carbon  2, 3, 13, 30, 96, 304 carbon dots  23, 24 carbon materials  206, 207 carbon nanofibers (CNFs)  22

carbon nanomaterials  9, 11, 15, 16 carbon nanotubes  173 carbon nitride  3, 12, 13, 29, 30, 99, 305 carbon quantum dots (CQDs)  16, 22, 173, 174, 176 catalytic reduction of CO2 363 charge carrier mechanisms  197–199, 205–207 charge resistance  280 charge seperation  164, 168, 174 charge transfer  11, 13, 20, 166, 174, 178, 197, 216, 217, 231, 279, 280, 296, 322–324, 365 chemical oxygen demand (COD)  136, 137, 144, 153 ciprofloxacin 172 clusters  310, 316, 318, 322, 324 C3N4  96, 348 co‐modifications 204 composite  22, 29, 52, 94, 96, 245, 247, 249, 254, 297 conduction band (CB)  2, 93, 164, 172, 174, 176, 223, 225, 301, 309, 316, 319 contaminants  269, 272 core@shell structures  202 counter electrode  280 C1 source material  343

Photocatalytic Functional Materials for Environmental Remediation, First Edition. Edited by Alagarsamy Pandikumar and Kandasamy Jothivenkatachalam. © 2019 John Wiley & Sons, Inc. Published 2019 by John Wiley & Sons, Inc.

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Index

d date stone‐activated carbon  17 defects  194, 199, 200, 202, 316, 317 degradation  2, 25, 29, 32, 48, 52, 57, 94, 96, 259, 297, 316–319, 326, 327 deposition methods  193 ditopic 310 doped TiO2  11, 13, 14 doping  2, 96, 182, 188, 303, 360 dyes  2, 29, 42, 83, 96, 243, 248, 255, 256, 271, 272, 281, 303 dye degradation  7, 21, 26

e ecological and toxicological association of dyestuffs and manufacturing industry (ETAD)  3 efficiency  268, 274, 276 electric field  192, 198, 199, 202, 203 electrocatalysts 21 electron‐hole  135–137, 144, 146, 149, 151, 153, 156, 268, 274, 281 electron hole pairs  12, 29, 42, 93, 99, 168, 173, 301, 347 Eley‐Rideal process  8 energy 267–269 energy conversion  268, 269, 284 energy devices  284, 285 environment  2, 4, 23, 30, 92, 305 environmental remediation  268, 269 ETAD. See ecological and toxicological association of dyestuffs and manufacturing industry (ETAD)

graphene  2, 10, 12, 25, 46, 52, 254 graphene oxide  15, 16, 19 graphite 17 graphitic nitride  225

h hierarchical  46, 51, 58, 59 highest occupied molecular orbital (HOMO) 314–316 HKUST  311, 314, 315, 322 HOMO‐LUMO OMCT  231 hybrid perovskite  355 hydro carbon  344 hydrophilic 200 hydrous hydrazine  352 hydroxyapatite 167

i interlayer anions  216, 221, 231 iodide  136, 146, 148, 357 IR 353 iron oxide  2, 52, 255 irradiation  1, 29, 42, 63, 95, 303

k kesterite 355 kinetics  61, 62 kinetic energy  267

l

facets  171–174, 178, 180, 183 Fenton’s reaction  41 Fermi level  192, 198, 200 fullerene  15, 16, 20 functionalization 204

Langmuir–Hinshel wood process  7 lanthanides 317 layer‐by‐layer (LbL) assembly  217 layered double oxide (LDO)  217, 219, 221, 225, 226 Lewis 312 low cost  3, 16, 22, 44, 47, 216, 247, 248, 254, 268, 270, 292, 294, 304, 359 lowest unoccupied molecular orbital (LUMO)  315, 316, 361

g

m

f

g‐C3N4  24, 54, 62, 257 gold 181

magnetic  137, 152, 178, 179, 187, 188 material 267–269

Index

mechanism  136, 137, 144, 146, 149, 151, 153 mesoporous  55, 60, 63, 67, 146, 150 mesoporous carbon  24 metal ions  310, 314 metal loading in semiconductor  360 metal organic frameworks (MOF) 348 metal oxides  41, 42, 45, 50, 55, 57, 66, 270, 272, 280 metal‐to‐metal charge transfer (MMCT) 231 methylene blue  19, 20, 23, 24, 51, 52, 54, 55, 66, 144, 154 methylne blue  168, 178, 186 methyl orange  9, 12, 15, 24–26, 45, 51–53, 55, 63, 64, 176 microorganisms  164, 167, 183, 184 MIL  311, 315, 316, 319–324, 326, 327 mixed metal oxides  216, 219, 221, 222 MMCT. See metal‐to‐metal charge transfer (MMCT) moieties  310, 317 molecular orbital  364 morphology  1, 3, 19, 20, 22, 62, 165, 168, 171, 172, 185, 204 MoS2 348 multifunctional 268 multiwalled carbon nanotubes (MWCNTs) 174

n nano  314, 315, 324, 327 nanoparticles 352 nanorods  196, 202–204 nanowires 202 nation 354 noble metals  192 nodes  310, 327

o optimal loading  200–202

optoelectronic  275, 276, 281 organic dyes  179, 181, 187 organic ligands  310, 311, 315 organic linkers  310, 311, 316, 327 organic pollutant  163–165, 167, 177, 181–183 oxidation reaction  167 oxo‐bridged linkage  216, 231

p panchromatic film  192 PCET. See proton‐coupled electron transfer (PCET) perovskites 95 photocatalysis  6, 8, 13, 17, 25, 134, 137, 144, 156, 309, 317, 318, 320, 327 photocatalysts  2, 12, 29, 63, 94, 244, 252, 254, 258, 268–270, 304 photocatalytic mechanism  99 photocatalytic reduction  3, 44, 345 photochemical 163–165 photocorrosion  165, 176–179, 182 photodegradation  134, 136, 137, 144, 146, 148, 154, 156, 268, 269, 271 photoelectric effect  267 photoelectrode 178 photoexcitation  71, 116, 227, 347 photo functional materials  267, 268 photolectrodes  44, 46, 59 photooxidation  164, 167, 184 photoreactors 73 photoreduction  194, 196, 316 photosensitizer 345 photosynthesis  3, 43, 344 physical techniques  193 plasmonic  179, 180, 188 plasmonic photocatalysts  352 platinum  69, 358 p‐n junction  111 pollutants  1, 22, 29, 63, 93, 304, 309, 316 polymer  131, 154, 156

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Index

polyoxometalates (POMs)  231 polypyrrole 56 polytopic 310 porous  310–312, 317, 318, 322, 326 properties  268, 271, 274 proton‐coupled electron transfer (PCET)  349, 354

q QDSSCs. See quantum dot sensitized solar cells (QDSSCs) quantum dots  173, 174, 176, 186 quantum dot sensitized solar cells (QDSSCs) 276–280

r radicals  309, 316–318, 324 reactive sites  168, 174 recombination  43, 44, 49, 315–317, 324 reconstruction effect  224 recycling  151, 153 redox electrolytes  28 reduced graphene oxide (RGO) 173–176 reducing agents  194, 195 reduction potentials  360 regeneration  135, 259, 260, 272 RGO. See reduced graphene oxide (RGO) rhodamine  45, 52–55, 59 rhodamine B  14, 16, 21–23, 172, 177, 179, 182, 188 rutile  10, 359

s sacrificial agent  349 scavenger 146 Schottky barrier  198, 200, 201, 207 semiconductor  134, 136, 146, 147, 153, 156, 309, 315–317, 322, 323, 327 semiconductor‐based photocatalysis 215 semiconductors photocatalyst  346

sensitizer  42, 56, 57, 67, 310 separable  216, 243, 245, 251 silver hydroxides  1, 95, 197 silver oxide  194, 195 size effects  200, 201, 208 solar cells  267–269, 283, 284, 355 sol‐gel  136, 156 SPR. See surface plasmon resonance (SPR) stability  3, 22, 29, 73, 95, 304 stepped surface  350 sunlight  316, 327 surface area  22, 29, 52, 94, 98, 136, 144, 152, 153, 156, 301, 312, 326 surface plasmon resonance (SPR)  91, 192, 196–199, 201, 202, 206, 207, 352 surface site  12, 29, 42, 93, 99, 302 sustainable  268, 269, 279, 280, 284 synergism  137, 153 synergistic  1, 29, 52, 93, 96, 303

t TEM. See The transmission electron microscopy (TEM) ternary  54, 55, 66 ternary composites  5, 20, 42, 206 tetrahydrofuran (THF)  171 tetrapods  168, 171, 202, 203 textile industry  2, 3, 6 THF. See tetrahydrofuran (THF) TiO2 9–20 TiO2 nanotube array (NTA)  352 The transmission electron microscopy (TEM)  136, 146, 148 triethanolamine 349 triiodide 28 tritopic 310 two‐dimensional 215

v valence band (VB)  2, 93, 223, 301, 309, 316

Index

visible light  22, 29, 62, 94, 98, 301, 319–324, 326

w waste water  41, 42, 50, 54, 57, 67, 204, 245, 246 water splitting  166

wet chemical route  193 work function  90

z zeolite  153, 156 zinc hydroxides  194, 195, 204 ZnO  9, 15, 16, 26 Z‐scheme 366

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