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Multifunctional Photocatalytic Materials for Energy [1 ed.]
 0081019777, 9780081019771

Table of contents :
Table of Contents
1. Introduction: A perspective-Multifunctional photocatalytic materials for energy
Zhiqun Lin, Meidan Ye, Mengye Wang
2. Metal oxide powder photocatalysts
Mohammad Mansoob Khan
3. Metal oxide electrodes for photo-activated water splitting
Davide Barreca
4. Theoretical insights for improved photocatalytic activity
Victor Antonio de la Pena O'Shea
5. Energy band engineering of metal oxide for enhanced visible light absorption
Jiangtian Li
6. Graphene photocatalysts
Luisa Maria Pastrana Martinez Sr.
7. Carbon Nitride photocatalysts
Hongqui Sun
8. Graphene-based nanomaterials for solar cells
Syed Farooq Adil, Mujeeb Khan
9. Metal-based Semiconductor nanomaterials for solar cells
Dr. Wenxi Guo Sr.
10. Metal-based Semiconductor nanomaterials for photocatalysis
Raffaele Marotta, Roberto Andreozzi, Ilaria Di Somma, Danilo Russo, Laura Clarizia Sr.
11. Conjugated polymer and nanocrystal nanocomposites for photocatalytic hydrogen production and organic contaminants degradation
Bhaghavathi Parambath Vinayan, Eswaraiah Varrla, Rupali Nagar
12. Hybrid noble metal and semiconductor nanocomposites for plasmon-mediated photocatalysis
Yuekun Lai
13. Hybrid Z-scheme nanocomposites for photocatalysis
Kazuhiko Maeda
14. Ferroelectrics for photocatalysis
A Chithambararaj

Citation preview

Multifunctional Photocatalytic Materials for Energy

Related titles Materials for Fuel Cells (ISBN 978-1-84569-330-5) Superconductors in the Power Grid (ISBN 978-1-78242-029-3) Solid Oxide Fuel Cell Technology (ISBN 978-1-84569-628-3)

Woodhead Publishing in Materials

Multifunctional Photocatalytic Materials for Energy Edited by

Zhiqun Lin

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, USA

Meidan Ye

Research Institute for Biomimetics and Soft Matter, Fujian Provincial Key Lab for Soft Functional Materials Research, Department of Physics, College of Physical Science and Technology, Xiamen University, Xiamen, China

Mengye Wang

Department of Applied Physics at The Hong Kong of Polytechnic University

An imprint of Elsevier

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright © 2018 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-08-101977-1 (print) ISBN: 978-0-08-101978-8 (online) For information on all Woodhead publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisition Editor: Christina Gifford Editorial Project Manager: Andrae Akeh Production Project Manager: Sreejith Viswanathan Cover Designer: Miles Hitchen Typeset by SPi Global, India

Contents

List of contributors

ix

1 Introduction: Multifunctional photocatalytic materials: A perspective Meidan Ye, Mengye Wang, Zhiqun Lin

1

2 Metal oxide powder photocatalysts Mohammad M. Khan 2.1 Historical developments and introduction 2.2 Semiconductors and photocatalysis 2.3 Fundamentals of photocatalysis 2.4 Metal oxides as powder photocatalysts 2.5 Applications of powdered metal oxides photocatalysts 2.6 Future perspectives 2.7 Conclusions References

5

3 Metal oxide electrodes for photo-activated water splitting Davide Barreca, Giorgio Carraro, Alberto Gasparotto, Chiara Maccato 3.1 Introduction 3.2 Fundamentals of photoelectrochemical water splitting: An overview 3.3 Relevant case studies for photoanode development 3.4 Conclusions and future trends Acknowledgments References

5 6 7 9 11 15 16 16 19 19 21 23 38 41 41

4 Energy band engineering of metal oxide for enhanced visible light absorption 49 Jiangtian Li, Deryn Chu 4.1 Introduction 49 4.2 Electronic energy band structure of semiconductors 50 4.3 Principle of photocatalysis for solar fuel generation 54 4.4 Metal oxide photocatalysts 58 4.5 Energy band engineering of metal oxides for enhanced visible light absorption 62 4.6 Concluding remarks 72 Acknowledgments 73 References 73 Further reading 78

viContents

5 Graphene photocatalysts Luisa M. Pastrana-Martínez, Sergio Morales-Torres, José L. Figueiredo, Joaquim L. Faria, Adrián M.T. Silva 5.1 Introduction 5.2 Graphene and its derivatives 5.3 Graphene-based semiconductor photocatalysts 5.4 Energy applications 5.5 Conclusions and outlook Acknowledgments References

79 79 80 82 88 94 95 95

6 Carbon nitride photocatalysts Jinqiang Zhang, Hongqi Sun 6.1 Introduction 6.2 Graphitic carbon nitride for hydrogen evolution 6.3 Carbon nitride for reduction of CO2 6.4 Carbon nitride for other energy applications 6.5 Conclusion and outlook References

103

7 Graphene-based nanomaterials for solar cells Syed Farooq Adil, Mujeeb Khan, Dharmalingam Kalpana 7.1 Introduction 7.2 Properties of graphene 7.3 Synthesis of graphene-based materials 7.4 Graphene in dye-sensitized solar cells (DSSCs) 7.5 Conclusion Acknowledgment References

127

8 Metal-based semiconductor nanomaterials for thin-film solar cells Wenxi Guo, Zijie Xu, Teng Li 8.1 Introduction 8.2 Fabrication of metal-based semiconductor nanomaterials 8.3 Semiconductor nanomaterials as interfacial materials for solar cells 8.4 Semiconductor nanomaterials as mesoporous layers for DSSCs 8.5 Concluding remarks and outlook References Further reading

153

9 Metal-based semiconductor nanomaterials for photocatalysis Laura Clarizia, Danilo Russo, Ilaria Di Somma, Roberto Andreozzi, Raffaele Marotta 9.1 Introduction 9.2 Thermodynamics and kinetics of the water splitting process

187

103 105 119 120 120 121

127 130 130 134 145 145 146

153 155 165 170 180 181 185

187 187

Contentsvii

9.3 Photocatalyst requirements 9.4 Catalytic water photosplitting 9.5 Catalytic photoreforming 9.6 Operating variables affecting photocatalyst activity 9.7 Conclusion References

189 193 199 201 205 205

10 Photocatalysts for hydrogen generation and organic contaminants degradation 215 Rupali Nagar, Eswaraiah Varrla, Bhaghavathi P. Vinayan 10.1 Introduction 215 10.2 Hydrogen economy and photocatalytic splitting of water 224 10.3 Photocatalytic degradation of organic contaminants 227 10.4 Conclusion 232 Acknowledgments 233 References 233 11

Multidimensional TiO2 nanostructured catalysts for sustainable H2 generation 237 Jingsheng Cai, Jianying Huang, Mingzheng Ge, Yuekun Lai 11.1 Introduction 237 11.2 Preparations of multidimensional TiO2 nanostructures 238 11.3 Solar WS by nanostructured TiO2 materials 250 11.4 Conclusions and perspectives 265 Acknowledgments 266 References 267 Further reading 288

12 Hybrid Z-scheme nanocomposites for photocatalysis Ryo Kuriki, Kazuhiko Maeda 12.1 Introduction 12.2 Powder-based Z-scheme photocatalysts of metal-complex/semiconductor hybrids 12.3 Photoelectrochemical CO2 reduction using molecular-based photocathode coupled with a semiconductor photoanode 12.4 Photoelectrochemical CO2 reduction using semiconductor electrodes modified with a catalytic metal complex 12.5 Summary and outlook References

289

13 Ferroelectrics for photocatalysis N.R. Yogamalar, S. Kalpana, V. Senthil, A. Chithambararaj 13.1 Introduction 13.2 Ferroelectric fundamentals 13.3 Ferroelectric semiconductor photocatalysts

307

289 292 297 302 304 305

307 307 308

viiiContents

13.4 Synthesis and characterization of ferroelectric photocatalysts 13.5 Theoretical and computational methods proposed for ferroelectric photocatalysts 13.6 Architectural design of ferroelectric semiconductor photocatalysts 13.7 Factors influencing photocatalytic reaction 13.8 Conclusion 13.9 Outlook Acknowledgments References

310 310 312 318 320 320 321 321

Index 325

List of contributors Syed Farooq Adil King Saud University, Riyadh, Saudi Arabia Roberto Andreozzi University of Naples Federico II, Naples, Italy Davide Barreca Padova University, Padova, Italy Jingsheng Cai Soochow University, Suzhou, PR China Giorgio Carraro Padova University and INSTM, Padova, Italy A. Chithambararaj CSIR Central Electrochemical Research Institute, Chennai, India Deryn Chu  US Army Research Laboratory, Sensor and Electronics Device Directorate, Adelphi, MD, United States Laura Clarizia University of Naples Federico II, Naples, Italy Ilaria Di Somma Italian National Research Council (IRC-CNR), Naples, Italy Joaquim L. Faria Universidade do Porto, Porto, Portugal José L. Figueiredo Universidade do Porto, Porto, Portugal Alberto Gasparotto Padova University and INSTM, Padova, Italy Mingzheng Ge Soochow University, Suzhou, PR China Wenxi Guo Xiamen University, Xiamen, China Jianying Huang Soochow University, Suzhou, PR China Dharmalingam Kalpana  CSIR—Central Electrochemical Research Institute, Karaikudi, India S. Kalpana M. S. Ramaiah Institute of Technology, Bangalore, India Mohammad M. Khan Universiti Brunei Darussalam, Gadong, Brunei Darussalam Mujeeb Khan King Saud University, Riyadh, Saudi Arabia Ryo Kuriki Tokyo Institute of Technology, Tokyo, Japan

x

List of contributors

Yuekun Lai Soochow University, Suzhou, PR China Jiangtian Li  US Army Research Laboratory, Sensor and Electronics Device Directorate, Adelphi, MD, United States Teng Li Xiamen University, Xiamen, China Zhiqun Lin Georgia Institute of Technology, Atlanta, GA, United States Chiara Maccato Padova University and INSTM, Padova, Italy Kazuhiko Maeda Tokyo Institute of Technology, Tokyo, Japan Raffaele Marotta University of Naples Federico II, Naples, Italy Sergio Morales-Torres  University of Granada, Granada, Spain; Universidade do Porto, Porto, Portugal Rupali Nagar Symbiosis International University, Pune, India Luisa M. Pastrana-Martínez University of Granada, Granada, Spain; Universidade do Porto, Porto, Portugal Danilo Russo University of Naples Federico II, Naples, Italy V. Senthil SVS College of Engineering, Coimbatore, India Adrián M.T. Silva Universidade do Porto, Porto, Portugal Hongqi Sun Edith Cowan University, Joondalup, WA, Australia Eswaraiah Varrla SRM University, Chennai, India Bhaghavathi P. Vinayan Helmholtz Institute Ulm (HIU), Ulm; Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany Mengye Wang The Hong Kong Polytechnic University, Kowloon, Hong Kong, China Zijie Xu Xiamen University, Xiamen, China Meidan Ye Xiamen University, Xiamen, China N.R. Yogamalar Velammal Engineering College, Chennai, India Jinqiang Zhang Edith Cowan University, Joondalup, WA, Australia

Introduction Multifunctional photocatalytic materials: A perspective

1

Meidan Ye*, Mengye Wang†, Zhiqun Lin‡ * Xiamen University, Xiamen, China, †The Hong Kong Polytechnic University, Kowloon, Hong Kong, China, ‡Georgia Institute of Technology, Atlanta, GA, United States

The development of modern industrial civilization has led to increasing worldwide energy demands and the worsening of environmental problems. Intense research efforts have been devoted to utilizing multifunctional materials for energy and environment applications. Photocatalysis is a special oxidation process that has been successfully employed in hydrogen generation via water splitting and pollutant degradation for environmental protection and remediation. Consequently, photocatalytic materials have been extensively developed. Until now, a variety of multifunctional photocatalytic materials, including metal, metal oxides, carbon materials, and others, have been rationally designed by several routes to be effective for energy applications. In this book, a comprehensive summary of various multifunctional photocatalytic materials for energy applications is presented. Spanning the full range from theoretical research to experimental preparation and performance measurement, this work aims to address all aspects of multifunctional photocatalytic research currently underway. With a universal understanding of the necessary fundamental theories, in-depth background knowledge, status of recent development, and scope for future research, this book will provide general information for undergraduate and graduate students interested in photocatalytic materials for energy use. We hope this book will serve as a handbook and reference for materials scientists and researchers working on photocatalysis or in related fields. An overview of each chapter included in this book is provided in the following sections under different categories, in order to give readers an idea of the content of this book. In Chapter 2 by Mohammad Mansoob Khan et al., the text focuses on semiconductor metal oxides for photocatalysis. Some basic concepts and principles involving metal oxide-based heterogeneous photocatalysis are highlighted first. Then, various types of metal oxide photocatalysts are discussed with regard to their quantum efficiency, light-harvesting abilities, and photochemical stability. Additionally, several representative methods for preparing metal oxide photocatalysts are carefully described. Finally, future perspectives for effective metal oxide-based heterogeneous photocatalysis are suggested.

Multifunctional Photocatalytic Materials for Energy. https://doi.org/10.1016/B978-0-08-101977-1.00001-6 Copyright © 2018 Elsevier Ltd. All rights reserved.

2

Multifunctional Photocatalytic Materials for Energy

In Chapter  3 by Davide Barreca et  al., the authors summarize recent developments in photoelectrochemical water splitting based on semiconductor metal oxides. Fundamentals of photoelectrochemical water splitting are specially emphasized. Then, some representative photoanodes, including Fe2O3, WO3, ZnO, and BiVO4based nanomaterials, which are expected to provide diverse functions, such as light absorption, photogenerated charge separation, and catalysis of the target process, are individually introduced by highlighting some novel nanostructures as well as their performance in photoelectrochemical water splitting. In Chapter 4 by Jiangtian Li et al., it discusses energy band engineering of metal oxides with visible light absorption ability in photocatalytic application. A brief introduction of the electronic band structures of semiconductors and their principal relationship with light absorption is shown first. Then the photocatalysis process, the mandatory energy band requirements for water splitting and CO2 reduction, and the two most important pathways for solar fuel production are elucidated in detail. Thereafter, some representative metal oxide photocatalysts are highlighted with regard to their band structures, electronic parameters, and peak efficiencies. Finally, strategies including doping, alloying, heterojunction, plasmonics, Z-scheme, and so on, which are frequently applied to enhance the light harvesting capability of metal oxides, are fully emphasized. In Chapter  5 by Luisa M. Pastrana-Martinez et  al., the authors pay attention to graphene-based photocatalysis, in which graphene and its derivatives are generally introduced, and then graphene-based semiconductor photocatalysts are discussed in particular, including the synthesis of graphene-based TiO2 and other semiconductor photocatalysts as well as the immobilization of graphene photocatalysts in membranes and films. Finally, energy applications using these graphene-based photocatalysts, especially photocatalytic hydrogen generation and photocatalytic reduction of carbon dioxide, are further highlighted. In Chapter 6 by Hongqi Sun et al., carbon nitride (g-C3N4) photocatalysts are the topic in focus. An overview of recent developments regarding carbon nitride for hydrogen generation is provided first. Summarization of the morphology, copolymerization, doping, hybridization, and sensitization of carbon nitride is then shown in detail. In addition, CO2 reduction and energy storage using modified g-C3N4 are also briefly introduced. At last, challenges and perspectives of carbon nitride in future research are proposed. In Chapter  7 by Syed Farooq Adil et  al., the authors present another promising application of graphene-based materials in solar cells. Several types of solar cells are reported to employ graphene-based materials, such as dye-sensitized solar cells (DSSCs), perovskite solar cells (PSCs), organic solar cells (OSCs), and heterojunction solar cells (HSCs). Several roles of graphene-based materials in solar cells are emphasized, including their roles as transparent conducting materials, non-­transparent electrodes, catalytic counter electrodes, charge transporters, electrolytes, and light-­ harvesting materials. In Chapter 8 by Wenxi Guo et al., the latest developments for several commonly used metal oxide-based semiconductor nanomaterials, mainly TiO2, ZnO, SnO2, Al2O3, and Nb2O5, applied in different solar cells, such as DSSCs, quantum-dot sensitized solar

Introduction3

cells (QDSCs), PSCs, and OSCs are covered in overview. The authors categorize metal oxide-based semiconductor layers into two groups according to their unique features: semiconductor nanomaterials as interfacial materials for solar cells, and semiconductor nanomaterials as mesoporous layers for solar cells. Then, the chapter focuses on fabrication methods for preparing efficient nanostructured photoanodes, the application of these nanostructured photoanode materials and their impact on device efficiency, and the roles of different interfacial layers used in improving the output of these devices. In Chapter 9 by Marotta Raffaele et al., metal-based semiconductor nanomaterials for photocatalysis are comprehensively reviewed. The text first introduces the fundamentals of photocatalytic water splitting and organic photo-reforming, followed by a discussion of metal-based semiconductor nanomaterials (i.e., metal oxides, metal sulfides, and metal nitrides, etc.) used as photocatalysts for hydrogen production. In particular, recent strategies for improving the effectiveness of the most common metal-­ based semiconductors are highlighted as well. Finally, the chapter discusses the main experimental factors involving performance and design of photocatalytic materials. Chapter 10 by Bhaghavathi Parambath Vinayan et al., is especially concerned with recent progress regarding conjugated polymer and nanocrystal nanocomposites for photocatalytic hydrogen production and organic contaminant degradation. The fundamentals of inorganic/organic semiconductors are indicated first. Introduction of nanocrystals and nanocomposites including synthesis routes, optical and electronic properties, physical/chemical characterization, and work mechanism of photocatalysis are comprehensively discussed. Based on these catalysts, the performance of hydrogen evolution and organic contaminant degradation is shown. In Chapter 11 by Yuekun Lai et al., the authors concentrate on nanostructured TiO2 catalysts for hydrogen generation. Multi-dimensional TiO2 nanostructures, from 0D nanoparticles, 1D nanotubes, nanowires, nanorods, and 2D nanosheets, to 3D hollow and hierarchical nanostructures, are elaborately introduced one by one. Then, solar water splitting based on these TiO2 nanostructured materials is discussed. Insight into critical engineering strategies for high-performance TiO2 nanocatalysts is emphasized, such as increasing the active areas of the surface, enhancing visible light absorption, and reducing the recombinant properties of electron-hole pairs. Chapter 12 by Kazuhiko Maeda et al., covers hybrid Z-scheme nanocomposites for photocatalysis. Photocatalytic CO2 reduction using metal complexes and semiconductors is presented. Metal complexes and semiconductor-based systems, as well as their hybrid systems, are introduced one by one. Molecular-based photocathodes coupled with semiconductor photoanodes, and semiconductor electrodes modified with a metal complex catalyst used in photoelectrochemical CO2 reduction are discussed in detail. In Chapter 13 by A. Chithambararaj et al., ferroelectric materials with regards to photocatalytic areas are addressed. Fundamentals and materials (e.g., titanates, niobates, and tantalates) of ferroelectric photocatalysts are first shown. Then, synthesis and characterizations of ferroelectric photocatalysts are subsequently summarized. Theoretical and computation methods proposed for ferroelectric photocatalysts are provided as well. Architectural design of ferroelectric photocatalysts is also emphasized. Factors (e.g., crystal structure, morphology, crystal size, and pH) involved in ferroelectric-based photocatalytic reactions are elaborately discussed.

4

Multifunctional Photocatalytic Materials for Energy

This book is not an attempt to fully cover all topics relevant to multi-functional photocatalytic materials for sustainable energy. Instead, the book focuses on recent advancements of some fascinating nanomaterials for that purpose. We believe that future exploitation of synthesis methods and precise control over nanostructured catalysts will deeply strengthen their photocatalytic activity and rapidly promote their practical application. The editors thank all of the authors who contributed to this book. We also thank Andrae Akeh and Sreejith Viswanathan of Elsevier for their remarkable patience and ensuring a final product.

Metal oxide powder photocatalysts Mohammad M. Khan Universiti Brunei Darussalam, Gadong, Brunei Darussalam

2

2.1 Historical developments and introduction In 1972 Fujishima and Honda first reported photocatalysis by the splitting of water under UV irradiation, and since then research in this field has expanded considerably [1]. Photocatalysis has drawn extra attention from researchers because of its use in a variety of products across a broad range of areas, mainly environmental- and energy-­ related fields. Until recently, metal oxides, such as TiO2, ZnO, SnO2, and so on, were the main choices for most studies in basic research and practical applications because of their high activity, low cost, high stability, nontoxicity, and chemical inertness, which made them suitable for applications in water and air purification, sterilization, hydrogen evolution, and so on [2–5]. Development of the term “photocatalysis” indicated the improvement of some vital concepts of photochemistry. The turning point enabling photochemistry to become a science on its own occurred when it was distinguished from thermal chemistry. In fact, until the beginning of the 20th century, many scientists thought that irradiation or illumination was one of the several existing ways to catalyze a reaction (i.e., to make it faster), such as treating with chemicals, heating, and so on. Giacomo Ciamician was the first scientist to make a systematic effort to understand the chemical effect of light in ascertaining whether “light and light alone” caused the reactions to accelerate, and not, for instance, heat [2]. He properly described these reactions as “photochemical reactions,” whereas “photocatalytic-tagged reactions” were described as being accelerated by light, while maintaining the same course as the thermal reactions. Later, researchers recognized that photochemical reactions involved electronically excited states, “electronic isomers” of ground states that have a reactivity (and thermodynamics) of their own. However, in 1914 Bodenstein observed it, and it became a common concept after several years [2,3]. The word photocatalysis is of Greek origin and comprises two parts: prefix “photo” derived from phos, meaning light; and “catalysis” derived from katalyo, meaning break apart or decompose. Although there is no agreement in the scientific community on a proper definition of photocatalysis, it is generally described as a process in which light is used to activate a substance, the photocatalyst, that modifies the rate of a chemical reaction without being involved itself in the chemical reaction. Thus the main difference between a conventional thermal catalyst and a photocatalyst is that the latter is activated by photons of appropriate energy, whereas the former is activated by heat. Photocatalytic reactions may take place heterogeneously or homogeneously. However, heterogeneous photocatalysis has been more deeply studied in recent years because of its potential use in a range of environmental- and ­energy-related applications as well as in organic Multifunctional Photocatalytic Materials for Energy. https://doi.org/10.1016/B978-0-08-101977-1.00002-8 Copyright © 2018 Elsevier Ltd. All rights reserved.

6

Multifunctional Photocatalytic Materials for Energy

s­ yntheses [6]. In heterogeneous photocatalysis, the reaction pathway involves the formation of an interface among photocatalysts and fluids containing the reactants and products. Processes involving irradiation on adsorbate-metal-oxide interfaces are generally categorized in the branch of photochemistry. Therefore the term “heterogeneous photocatalysis” is mainly used in the cases where a light-absorbing metal oxide photocatalyst is utilized, which is in contact with either a liquid or a gas phase. A material is considered to be a catalyst when it accelerates a chemical reaction without being consumed in the reaction and lowers the free activation enthalpy of the reaction. Photocatalysis is defined as the acceleration of a reaction in the presence of light and a suitable catalyst. A Photocatalyst can be defined as a material that accelerates a chemical reaction in the presence of appropriate light and a suitable catalyst without being utilized and that lowers the free activation enthalpy of a chemical reaction [3]. Photocatalysis is a rapidly emerging area with a high possibility of an extensive range of industrial applications, such as disinfection of water and air, mineralization of organic pollutants, production of renewable fuels, and organic syntheses [3–6]. In this chapter, the discussion will be restricted to the description of metal oxide-mediated photocatalysis.

2.2 Semiconductors and photocatalysis Semiconductors are mainly useful as photocatalysts because of their promising combination of light absorption properties, electronic structure, charge transport characteristics, and excited-state lifetimes. A semiconductor is nonconductive in its undoped ground state because an energy gap, i.e., a wide band gap, exists between the top of the filled valence band (VB) and the bottom of the vacant conduction band (CB). Accordingly, electron transport between these bands must occur only with considerable energy change. In semiconductor photocatalysis, excitation of an electron from the VB to the CB is achieved by absorption of a photon of energy equal to or higher than the band gap energy of the semiconductor. This light-induced formation of an electron-hole pair is a prerequisite step in all semiconductor-assisted photocatalytic reactions. The formed photogenerated species tend to recombine and dissipate energy as heat of photons, because the kinetic barrier for the electron-hole recombination step is low. However, CB electrons and VB holes can be separated well in the presence of an electric field, such as the one generated spontaneously in the space charge layer of a semiconductor-metal or a semiconductor-fluid interface. Therefore the lifetimes of photogenerated carriers increase, and the possibility is offered to these species to exchange charges with substrates adsorbed on the photocatalyst surface and induce primary and secondary chemical reactions. Transfer of an electron to or from a substrate adsorbed onto a light-activated semiconductor (i.e., interfacial electron transfer) is probably the most critical step in photocatalytic reactions, and its efficiency controls to a large extent the ability of the semiconductor to serve as a photocatalyst for a given redox reaction. The efficiency of electron transfer reactions is, in turn, a function of the position of a semiconductor's VB- and CB-edges relative to the redox potentials of the adsorbed substrates. For an anticipated electron transfer step to occur, the potential of the electron donor species should be located above

Metal oxide powder photocatalysts7

(more negative than) the VB of the semiconductor, whereas the potential of the electron acceptor species should be located below (more positive than) the CB of the semiconductor. Interfacial electron transfer processes are then initiating subsequent redox reactions to form free radicals for primary and secondary reactions. The free radicals formed (such as hydroxyl and super oxide radicals) will be used as strong oxidizing agents for decomposing or degrading the organic pollutants, and so on. This chapter is designed to introduce and fully explain the background, theory, and concepts necessary for understanding metal oxides (TiO2, ZnO, SnO2, CeO2, etc.) as photocatalysts [3–10]. In this chapter, emphasis is placed on the optical and electronic properties of the metal oxides, which will be described with the use of the band model, and so on. Later, various examples are used to discuss photocatalysis in detail.

2.3 Fundamentals of photocatalysis Photocatalysis, in short, is defined as “acceleration of a reaction in presence of a suitable catalyst and appropriate light.” A catalyst is not altered or used up during a chemical reaction, and it accelerates the rate of a reaction by lowering the activation energy. It involves photosensitization, which is a process by which a photochemical reaction takes place in one molecular unit as a result of the initial absorption of light energy by another molecular unit, called the photosensitized. Photocatalysis assists in forming strong reducing and oxidizing agents, which helps in breaking down organic matter into CO2 and H2O in the presence of light, a photocatalyst, and water [4,11–14].

2.3.1 Mechanism When photocatalysts (such as TiO2, ZnO, SnO2, CeO2, etc.) absorb suitable light, a pair of electrons and holes in the CB and VB are produced. The electrons of the VB of metal oxides become excited when irradiated by light. The excess energy of the excited electrons promotes the electron to the CB of metal oxides. Therefore negative-­ electron (e−) and positive-hole (h+) pairs are created. This stage is referred to as the semiconductor's “photo-excitation” state. The energy difference between the VB and CB is known as the “band gap energy” (Eg). Fig. 2.1 shows the band gap energy of metals, metal oxides, and insulators [15,16]. CB CB CB

Eg = ∼0.0 eV

VB

Eg = ∼3.2 eV VB

Metals

Eg = >4.0 eV VB

Metal oxide (semiconductor)

Fig. 2.1  Band gaps of metals, metal oxides, and insulators.

Insulators

8

Multifunctional Photocatalytic Materials for Energy

The photoactivation of a metal oxide photocatalyst is based on its electronic excitation by photons (light) with energy (hv) greater than the band gap energy (Eg). The electrons migrate after the excitation, generating vacancies in the VB (holes, h+) and forming regions with high electron density (e−) in the CB [15,17–22]. These holes have pH-dependent and strongly positive electrochemical potentials, ranging between +2.0 and +3.5 V, measured against a saturated calomel electrode [20]. This potential is sufficiently positive to generate hydroxyl radicals (•OH) from water molecules adsorbed on the surface of the metal oxides (Eqs. 2.1–2.3). The photocatalytic efficiency depends on the competition between the formation of e−/h+ pairs and the recombination of these pairs (Eq. 2.4) on the metal oxide surfaces [17,21,22].

(

Metal oxide + hv ® Metal oxide e - CB + h + VB

)

(2.1)

h + + H 2 Oads . ® HO· + H +

(2.2)

h + + OH - ads . ® HO

(2.3)

(

)

Metal oxide e - CB + h + VB ® Metal oxide + D

(2.4)

Although the oxidation reactions caused by the generated holes occur at the VB, the electrons transferred to the CB are responsible for reduction reactions, such as the formation of gaseous hydrogen and the generation of other important oxidizing species such as superoxide anion radicals. In the case of metal oxides, the Eg is between 3.00 and 3.20 eV [15,17,19]. The whole process is demonstrated schematically in Fig. 2.2. The positive-hole formed in metal oxides dissociates the H2O molecules to form hydrogen gas and hydroxyl radicals (•OH). The negative-electron reacts with adsorbed oxygen molecules to form super oxide anions (O2•) [4,7,13–15]. This cycle continues until suitable light is available. The overall mechanism of the photocatalytic reaction of metal oxides that happens at their surface, in the presence of suitable light, are shown in Fig. 2.2. H2O •O2– Light

–e –e –e – e – e –e – e CB Eg = ∼3.2 eV Excitation Recombination

h+ h+ h+ h+ h+h+

O2

Organic pollutants

•OH

VB

Metal oxide

•OH

HO–

Degraded products

Fig. 2.2  Tentative photocatalysis mechanism occurs during photocatalytic reaction at metal oxide's surface.

Metal oxide powder photocatalysts9

Common chemical oxidants, placed in the order of their oxidizing strength Table 2.1 

Compounds/radicals

Oxidation potentials (V)

Relative oxidizing power (Cl2 = 1.0)

Hydroxyl radical Sulfate radical O3 H2O2 MnO4− ClO2 Cl2 O2 Br2 I2

2.8 2.6 2.1 1.8 1.7 1.5 1.4 1.2 1.1 0.76

2.1 1.9 1.5 1.3 1.2 1.1 1.0 0.90 0.80 0.54

The most influential and advanced oxidation reactions are based on the formation of hydroxyl radicals (•OH), which are extremely powerful oxidizing agents, second only to fluorine in power (2.23 in relative oxidizing power). By making use of the strong oxidation strength of •OH radicals, photocatalytic oxidation can efficiently decompose, degrade, disinfect, deodorize, and purify air, water, and different types of surfaces. Table 2.1 shows the common chemical oxidants, placed in the order of their oxidizing strength.

2.4 Metal oxides as powder photocatalysts Photocatalysis is a favorable, environmentally friendly method for the conversion of solar energy to chemical energy or chemical conversion over a metal oxide nanostructure, such as the degradation of pollutants and hydrogen generation. For the past two decades, substantial research efforts have been devoted to the realization of efficient, economical, and green sources for the treatment of pollutants for environmental remediation processes. Metal oxide nanomaterials have played a main role in this attempt because they have an excellent combination of photochemical activity and mechanical and thermal stability. They have long been pursued for photocatalytic applications, such as H2 production through water splitting or decomposition of water pollutants, and so on. The catalytic activities of metal oxide nanomaterials are influenced significantly by the reactive sites present on the surface, owing to some type of defects in the crystal lattice. Transition metal oxide nanomaterials have attracted substantial attention because of their potential applications, such as photocatalysis, H2 production and storage, environmental remediation and energy, and are expected to be the key nanomaterials for further developments of nanoscience and nanotechnology [4,13–15]. Among the metal oxides available, TiO2 has attracted particular attention owing to its exceptional properties, such as low cost, high stability, high chemical inertness, biocompatibility, nontoxicity, and so on. Since the discovery of its photocatalytic properties by Honda-Fujishima

10

Multifunctional Photocatalytic Materials for Energy

[1,5,6,20–22], TiO2 has been examined widely as an efficient photocatalyst for purification of water and degradation of dyes, pesticides, and so on. A significant diversity of semiconducting materials, mainly metal oxides and chalcogenides, have been explored with respect to their photocatalytic behavior, but only a few of them are considered to be effective photocatalysts. Metal oxide (ZnO, TiO2, SnO2, CeO2, Fe2O3, and V2O3) nanomaterials have attracted considerable interest in several areas of materials science, physics, and chemistry owing to their fascinating properties. In general, wide band gap metal oxides, such as TiO2, have proved to be better photocatalysts than low band gap materials, such as cadmium sulfide (CdS), mainly because of the higher free energy of photogenerated charge carriers of the former and the inherently low chemical and photochemical stability of the latter [3,4]. However, low band gap metal oxides are altered better by the solar spectrum, thus they pose significant potential for the utilization of a continuous and readily available power supply, i.e., sunlight. In recent years, substantial work has been done on the growth of more efficient photocatalysts, based on improved harvesting of light as well as increased quantum efficiency. In this way, favorable outcomes have been obtained with the use of several approaches targeted at the optical and/or modification of electronic properties of different metal oxides, including metal deposition, dye sensitization, doping with transition metals or nonmetallic elements, use of composite semiconductor photocatalysts, and so on [4,7,13]. The potential uses of heterogeneous photocatalysis depend mainly on the progress in scaled-up reactor designs, light harvesting abilities, and reduced recombination efficiency of electron-hole pairs. The main task in the design of a photocatalytic reactor is optimization in the mass transfer and efficient light harvesting ability of the catalyst, especially in liquid phase reactions. Mass transfer restrictions can be dealt with by using monolithic reactors, spinning disc reactors, and microreactors, which have proved to be much more efficient than conventional reactors. Photon transfer can be optimized using light-emitting diodes (LEDs) and optical fibers; however, key advances in this field are still lacking. Thus the artificial formation of photons needed for photocatalytic reactions is the most significant basis in terms of operating costs in practical applications, and a considerable amount of research has been done in the development of solar photoreactors. Among the various types of solar reactor configurations evaluated so far, compound parabolic collectors are the most promising ones and have been successfully scaled up for applications related to wastewater treatment and water cleaning and disinfection [22–26]. Metal oxide-based photocatalysis is currently one of the most active interdisciplinary research areas, and it has been examined from the standpoint of catalysis, photochemistry, electrochemistry, organic, inorganic, physical, polymer, and environmental chemistry. Because of the amount of research in these core areas, the fundamental processes of photocatalysis are now understood much better. The applicability of photocatalysis has been shown in laboratory-scale conditions for many different processes, such as air cleaning, water treatment, disinfection applications, production of fuels from water and atmospheric gases, selective organic synthesis, and metal recovery [4,5,14]. However, industrial applications remain restricted and incomplete. The present lack of extensive industrial applications is due mainly to the low photocatalytic efficiency of metal oxide photocatalysts and the lack of efficient and large-scale photoreactor setups.

Metal oxide powder photocatalysts11

2.5 Applications of powdered metal oxides photocatalysts Over the past few decades, metal oxide nanomaterials, such as TiO2, ZnO, SnO2, WO3, Fe2O3, CeO2, and so on, have been largely studied for their photocatalytic properties [1,4,15–22]. Metal oxide nanomaterials used as photocatalysts showed excellent degradation of organic and toxic pollutants because of the materials' high reactivity at low concentrations, low toxicity, and high stability. Metal oxide photocatalysis has attracted significant attention because of its promising applications in various fields, such as environmental remediation by the photodecomposition of hazardous dyes in polluted water, industrial effluents, and solar energy conversion. Therefore metal oxides, such as TiO2, ZnO, SnO2, and CeO2, have been the prime choice for basic research and practical applications owing to their high activity, good stability, easy availability, low cost, nontoxicity, and chemical inertness. In recent years, emerging alarms about energy and environmental problems have encouraged extensive research on solar energy utilization. Dyes are used widely in a range of fields, but their discharge into water can cause environmental pollution. In addition, most dyes are toxic, carcinogenic, and harmful, resulting in adverse impacts on human and animal health. Dyes are used widely in several industries, including textile, plastic, rubber, paper, concrete, and medicine, with the textile industry as the main user. Unfortunately, approximately 10% of dyes used in industry are discharged directly into the environment as a harmful pollutant, which is environmentally unsafe and aesthetically unacceptable. Heterogeneous photocatalysis involves the application of metal oxide catalysts (e.g., TiO2, ZnO, SnO2, WO3, and CeO2) irradiated with light of an appropriate wavelength to generate highly reactive transitory oxidative species (i.e., •OH, and •O2−) for the mineralization of organic contaminants, impurities, and pollutants [5,6]. Therefore a range of approaches have been explored for the photocatalytic degradation of organic dyes using semiconductor photocatalysts [4,7,13–19]. Generally, semiconductor catalysts show relatively low quantum degradation efficiency because of the high recombination rate of light-induced e−/h+ pairs at or near the surface of the photocatalysts, which is considered one of the major limitations to hindering the photocatalytic efficiency [7,14]. The chemistry that occurs at the surfaces of metal oxides has attracted considerable attention for a range of industrial applications (Fig. 2.3), including catalysis, photocatalysis, water purification, deodorization, air purification, self-cleaning, self-­ sterilizing, antifogging surfaces in optical display technology, chemical synthesis, solar energy devices, antibacterial activities, batteries, and energy production and storage [4,10,14–30].

2.5.1 Water purification Water is a well-recognized necessity, because without it, the world would be devoid of life as we know it. Water cleansing and treatment have become worldwide problems, particularly in industrialized countries where wastewater normally contains organic pollutants, such as organic dyes from the textile industries and organic

12

Multifunctional Photocatalytic Materials for Energy

Environment remediation and chemical synthesis

Catalysis and photocatalysis

Sensors, health and automobiles

Applications of metal oxides

Clean energy production, H2 production

Batteries and supercapacitors

DSSCs, Solar cells and fuel cells

Fig. 2.3  Possible applications for metal oxides.

waste materials from the paper industry, the leather and tanning industries, the food industry, agricultural research, and pharmaceutical industries. The release of these colored and toxic compounds in the environment has raised substantial concern because of their toxic effects on the environment, plants, and human beings. In addition, two classes of dyes, azo dyes and thiazine dyes, can cause serious health risks. Some of the azo dyes are highly carcinogenic. The conventional wastewater treatment plants cannot degrade the majority of these pollutants. Thus there has been increasing interest in methods for decontamination of such products over the past few decades [4,16,17]. The waste from the textile industry, paper industry, leather and tanning industries, food industry, agricultural, and pharmaceutical industries has long been considered a serious environmental issue. Most paper mills and textile industries produce wastewater that contains aromatic dyes, which are highly toxic and difficult to decompose because of their relatively stable chemical structures. Many dyes (methylene blue, methyl orange) are used in industry for a range of purposes. Among them, methylene blue (MB) is one of the most frequently used dyes for agriculture, textile, paper-­ making, cosmetic, and pharmaceutical purposes [4,16–21]. Photocatalysts combined with ultraviolet light can oxidize organic pollutants into nontoxic byproducts, such as water and CO2, and can sterilize certain bacteria. This technology is very effective for removing hazardous organic compounds such as dyes, VOCs, and TOCs and for killing a variety of bacteria and some viruses in secondary wastewater treatments. Pilot projects have demonstrated that photocatalytic detoxification systems can effectively kill fecal coliform bacteria in secondary wastewater treatments [4,14–19]. The excessive use of organic chemicals in both industrial manufacturing and normal household uses has led to their leaching into the environment, causing ­shocking

Metal oxide powder photocatalysts13

environmental contamination. Organic chemicals are also present as pollutants in groundwater and surface water, such as wells, ponds, and lakes. To protect the water resources and achieve quality drinking water, pollutants need to be removed. Numerous processes, such as biological and chemical oxidation reactions, adsorption onto supported substrates, ultrasonic irradiation, and electrochemical devices, have been used widely to destroy or remove these toxins. Among these techniques, the photocatalytic detoxification of organic pollutants has attracted considerable attention because of its numerous advantageous features, such as use of very small amounts of catalysts, regeneration of catalysts, utilization of natural sunlight for environmental remediation, and clean energy production.

2.5.2 Deodorizing and air purification During deodorizing processes, hydroxyl radicals accelerate the breakdown of any volatile organic compounds (VOCs) by breaking the molecular bonds. This process helps combine the organic gases to form a single molecule that is not very harmful to the environment, plants, and human beings, thus enhancing efficient cleaning of the air. Some of the examples of odor molecules are tobacco, formaldehyde, nitrogen dioxide, gasoline, urine, and fecal odors as well as many other hydrocarbon molecules in the atmosphere [14,17,23]. The air purifier developed with TiO2 can remove soil and smoke, pollen, bacteria, viruses, and harmful gases, as well as halt the free bacteria in the air by filtering out ~99.9% of them, with the help of the highly oxidizing effect of photocatalysts, i.e., metal oxides (TiO2, etc.). The photocatalytic activity of metal oxides can be applied for the elimination or reduction of polluted compounds in air such as cigarette smoke, automobile smoke, NOx, as well as volatile compounds arising from various industries and construction sites. Also, high photocatalytic reactivity can be applied to protect lamp housing and walls in tunneling, as well as to prevent white tents from becoming sooty and dark. Atmospheric constituents such as greenhouse gases, chlorofluorocarbons (CFCs), substituted CFCs, and nitrogenous and sulfurous compounds undergo photochemical reactions either indirectly or directly in the presence of sunlight. In a polluted region, these pollutants can ultimately be efficiently removed using a suitable metal oxide. Deodorization and air purification may help to keep the air pure and free from bad odor. These processes can help keep plants and human beings fit and healthy [14,23].

2.5.3 Self-cleaning, self-sterilizing, and antifogging surfaces In most industrial countries, many exterior walls of buildings are soiled by automotive exhaust fumes and smoke emitted by industries, which contain oily components. When the original building materials are coated with a photocatalyst such as TiO2, a protective layer of titanium creates a self-cleaning effect on the building by becoming antistatic, super-oxidative, and super-hydrophilic. The hydrocarbon from automotive exhaust is oxidized, and the dirt on the walls washes away with rainfall, keeping the building exterior clean and shining at all times [14,15,23].

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Multifunctional Photocatalytic Materials for Energy

2.5.3.1 Superhydrophilic When the surface of a photocatalytic layer is exposed to light, the contact angle of the photocatalyst surface with H2O is decreased slowly. After sufficient exposure to light, the surface reaches a super-hydrophilic state; i.e., it does not repel water at all, so water cannot exist in the shape of a drop, but spreads flatly on the surface of the substrate. Finally, the water acquires the form of a highly uniform thin film, which acts optically like a clear sheet of glass. The hydrophilic nature of metal oxides, coupled with gravity, will enable the dust particles to be cleaned away following the water stream, thus making the product self-cleaning. TiO2 can degrade organic contaminants at the surface level with the assistance of UV light. This characteristic has led to the application of TiO2 photocatalysis in a “self-­ cleaning” technique where TiO2-coated surfaces maintain cleanliness under UV light, that is, by utilizing readily available sunlight or ultraviolet emissions from fluorescent lamps, which in turn saves maintenance costs and reduces the need to rely on detergents for cleaning [24]. Fujishima et al. confirmed this self-cleaning concept on a titania-coated ceramic tile in 1992 [25]. The first commercialized product using this method was the self-cleaning cover glass for highway tunnel lamps [24,26]. An example is the sodium lamp in Japan that emits UV light through the cover glass, which is mainly used to decompose the contaminants from automobile exhaust because the UV light is not useful for lighting purposes. Therefore the cover glass maintains its transparency for long-term use. Also, Wang et al. added that the self-cleaning effect could be aided with flowing water (e.g., rainfall) on the TiO2 surfaces [27]. This enhancing phenomenon of flowing water such as rainfall was ascribed to the super-hydrophilic property of TiO2 surfaces; i.e., water was able to penetrate the molecular-level space between the stain and the super-hydrophilic TiO2 surface [27–30]. Outdoor materials benefit from the combined self-cleaning effect of TiO2 photocatalytic and superhydrophilic nature. Apart from lamps, other materials on the road, such as spray coatings for cars, rearview mirrors and windshields, tunnel walls, reflectors, and traffic signs, have utilized TiO2 surface coating to achieve self-cleaning and antifogging advantages. Also, application could be found in materials for residential and office buildings, such as exterior tiles, kitchen and bathroom components, interior furnishings, plastic surfaces, aluminum siding, tent material, building stone and curtains, glass windows, and window blinds [26].

2.5.4 Antibacterial effect The advantage of using photocatalysts as antibacterial materials is that they not only kill bacterial cells but also completely decompose the cells. The TiO2 photocatalyst has been found to be more active than any other antibacterial agent, because the photocatalytic process acts even when the cells' surfaces are covered and bacteria are actively propagating. The end toxins produced after the death of the cells are also expected to be decomposed by photocatalytic action, i.e., by the use of •OH and •O2 radicals. TiO2 does not deteriorate and demonstrates a long-term antibacterial effect. According to researchers, decontamination by TiO2 is three-fold greater than that of

Metal oxide powder photocatalysts15

chlorine, and 1.5-fold stronger than ozone, and the TiO2 process is much safer and more efficient than other processes [14,15].

2.5.5 Organic synthesis Photocatalytic organic synthesis using metal oxides are well known, and research in that area is progressing. Choi et al. recently reported the photocatalytic synthesis of 2-hydroxyterephthalic acid (HTPA) using terephthalic acid (TPA). In this this synthesis, HTPA was treated with novel photocatalysts ZnO and ZnS under visible light irradiation for 6 h in-situ, which generated •OH radicals and reacted with HTPA, leading to the formation of TPA [6,14,15,17].

2.5.6 Energy Global demands for sustainable energy sources, production, and storage continue to increase, but these demands may be eased through recent developments in the use of novel nanomaterials that are more energy- and cost-efficient than other materials currently in use. Because of recent advancements in their print-like manufacturing process and because they can be made into flexible rolls rather than discrete panels, nanostructured solar cells such as TiO2 and ZnO nanotube-based, dye-sensitized solar cells have the potential to be less expensive and easier to install than their predecessors [31,32]. Because of the abundance of water resources and sunlight as energy, it is also inevitable that hydrogen derived from water will be a future source of clean energy. There is expanding research on ways to improve the efficiency of hydrogen production and light harvesting using different metal oxide nanostructures such as TiO2, ZnO, Ce/ TiO2, and CeO2-graphene. [1–5,33–35] This means that these materials will become a major, sustainable source of clean energy for future generations.

2.6 Future perspectives This chapter presents an overview of the recent studies related to heterogeneous photocatalysis using metal oxides as photocatalysts. These studies provide an important basis for the understanding of this topic and related areas of concern; however, we believe that many questions remain unanswered. These questions can be answered through a combined effort of critical field studies, thorough laboratory studies, and modeling using novel tools. Some of the future directions for study include i. Metal oxide-based photocatalytic reactions carried out under atmospherically relevant conditions of gas phase concentrations, relative humidity, solar flux, and so on; ii. Metal oxide-based photocatalytic studies that probe in-situ both surface and gas phase species formed under pertinent environmental conditions, as these are best suited for mechanistic studies of photocatalytic reactions; iii. Investigation of the effects of the physicochemical properties of metal oxides on heterogeneous photocatalysis that include shape, size, coatings, and also use as nanocomposites;

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Multifunctional Photocatalytic Materials for Energy

iv. Hands-on field measurements that provide and validate the importance of these photocatalytic reactions in order to better understand them; and v. Proper modeling to get a better understanding of the relative impact of photocatalysis compared to other heterogeneous reactions.

2.7 Conclusions This chapter discussed metal oxide-based heterogeneous photocatalysis and focused on the basic concepts, such as theory and background, needed in order to understand this particular topic. This chapter also dealt with the principles behind metal oxide photocatalysis, thermodynamics, and the kinetic aspects that regulate photocatalytic performance. Different types of photocatalysts were discussed along with issues related to light, photochemical stability, and environmental concerns. Several methods used in oxide-mediated photocatalytic processes were covered, with the main focus on applications related to the environment and energy. Finally, future perspectives were suggested for the pursuit of studies and the use of novel tools that will enhance and adapt the basic principles for metal oxide-based heterogeneous photocatalysis.

References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [2] A. Albini, M. Fagnoni, ChemSusChem 1 (2008) 63–66. [3] D. Ravelli, D. Dondi, M. Fagnoni, A. Albini, Photocatalysis. A multi-faceted concept for green chemistry, Chem. Soc. Rev. 38 (2009) 1999–2011. [4] H.  Chen, C.E.  Nanayakkara, V.H.  Grassian, Titanium dioxide photocatalysis in atmospheric chemistry, Chem. Rev. 112 (2012) 5919–5948. [5] M.M. Khan, S.F. Adil, A.A. Mayouf, Metal oxides as photocatalysts, J. Saudi Chem. Soc. 19 (2015) 462–464. [6] Y.I. Choi, S. Lee, S.K. Kim, Y.I. Kim, D.W. Cho, M.M. Khan, Y. Sohn, Fabrication of ZnO, ZnS, Ag-ZnS, and Au-ZnS microspheres for photocatalytic activities, CO oxidation and 2-hydroxyterephthalic acid synthesis, J. Alloys Compd. 675 (2016) 46–56. [7] M.M. Khan, S.A. Ansari, D. Pradhan, M.O. Ansari, D.H. Han, J. Lee, M.H. Cho, Band gap engineered TiO2 nanoparticles for visible light induced photoelectrochemical and photocatalytic studies, J. Mater. Chem. A 2 (2014) 637–644. [8] S. Kalathil, M.M. Khan, S.A. Ansari, J. Lee, M.H. Cho, Band gap narrowing of titanium dioxide (TiO2) nanocrystals by electrochemically active biofilm and their visible light activity, Nanoscale 5 (2013) 6323–6326. [9] S.A. Ansari, M.M. Khan, S. Kalathil, A. Nisar, J. Lee, M.H. Cho, Oxygen vacancy induced band gap narrowing of ZnO nanostructure by electrochemically active biofilm, Nanoscale 5 (2013) 9238–9246. [10] S.A.  Ansari, M.M.  Khan, M.O.  Ansari, J.  Lee, M.H.  Cho, Highly photoactive SnO2 nanostructures engineered by electrochemically active biofilm, New J. Chem. 38 (2014) 2462–2469.

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[11] M.M. Khan, S.A. Ansari, D. Pradhan, D.H. Han, J. Lee, M.H. Cho, Defect-induced band gap narrowed CeO2 nanostructures for visible light activities, Ind. Eng. Chem. Res. 53 (2014) 9754–9763. [12] S.A. Ansari, M.M. Khan, M.O. Ansari, J. Lee, M.H. Cho, Band gap engineering of CeO2 nanostructure by electrochemically active biofilm for visible light applications, RSC Adv. 4 (2014) 16782–16791. [13] S.A.  Ansari, M.M.  Khan, M.O.  Ansari, M.H.  Cho, Nitrogen-doped titanium dioxide (N-doped TiO2) for visible light photocatalysis, New J. Chem. 40 (2016) 3000–3009. [14] A. Fujishima, X. Zhang, D.A. Tryk, TiO2 photocatalysis and related surface phenomena, Surf. Sci. Rep. 63 (2008) 515–582. [15] A. Hernandez-Ramırez, I. Medina-Ramırez, Photocatalytic Semiconductors: Synthesis, Characterization, and Environmental Applications, ISBN 978-3-319-10999-2 (eBook); DOI https://doi.org/10.1007/978-3-319-10999-2; Springer International Publishing Switzerland 2015. [16] R. Saravanan, M.M. Khan, F. Gracia, J. Qin, V.K. Gupta, A. Stephen, Ce3+-ion-induced visible-light photocatalytic degradation and electrochemical activity of ZnO/CeO2 nanocomposite, Sci Rep 6 (2016) 31641. [17] M.R. Hoffmann, T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1) (1995) 69–96. [18] R.F.P. Nogueira, W.F. Jardim, Fotocatálise heterogênea e sua aplicação ambiental, Quim. Nova 21 (1) (1998) 69–72. [19] S.G. Kumar, L.G. Devi, Review on modified TiO2 photocatalysis under UV/visible light: selected results and related mechanisms on interfacial charge carrier transfer dynamics, J. Phys. Chem. A 115 (2011) 13211–13241. [20] A.R. Khataee, M. Zarei, R. Ordikhani-Seyedlar, Heterogeneous photocatalysis of a dye solution using supported TiO2 nanoparticles combined with homogeneous photoelectrochemical process: molecular degradation products, J. Mol. Catal. A Chem. 338 (1–2) (2011) 84–91. [21] R.L.  Ziolli, W.F.  Jardim, Mechanism reactions of photodegradation of organic compounds catalyzed by TiO2, Quim. Nova 21 (3) (1998) 319–325. [22] M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production, Renew. Sust. Energ. Rev. 11 (3) (2007) 401–425. [23] M.  Pelaez, N.T.  Nolan, S.C.  Pillai, M.K.  Seery, P.  Falaras, A.G.  Kontos, P.S.  Dunlop, J.W. Hamilton, J.A. Byrne, K. O'shea, M.H. Entezari, A review on the visible light active titanium dioxide photocatalysts for environmental applications, Appl. Catal. B Environ. 125 (2012) 331–349. [24] D.  Spasiano, R.  Marotta, S.  Malato, P.  Fernandez-Ibanez, I.  Di Somma, Solar photocatalysis: materials, reactors, some commercial, and pre-industrialized applications. A ­comprehensive approach, Appl. Catal. B Environ. 170 (2015) 90–123. [25] A.  Fujishima, T.N.  Rao, D.A.  Tryk, Titanium dioxide photocatalysis, J Photochem Photobiol C: Photochem Rev 1 (1) (2000) 1–21. [26] A.O. Ibhadon, P. Fitzpatrick, Heterogeneous photocatalysis: recent advances and applications, Catalysts 3 (1) (2013) 189–218. [27] R.  Wang, K.  Hashimoto, A.  Fujishima, M.  Chikuni, E.  Kojima, A.  Kitamura, M. Shimohigoshi, T. Watanabe, Light-induced amphiphilic surfaces, Nature 388 (6641) (1997) 431. [28] K.  Nakata, A.  Fujishima, TiO2 photocatalysis: design and applications, J Photochem Photobiol C: Photochem Rev 13 (3) (2012) 169–189.

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[29] A.  Fujishima, X.  Zhang, Titanium dioxide photocatalysis: present situation and future approaches, C. R. Chim. 9 (5) (2006) 750–760. [30] S.  Banerjee, D.D.  Dionysiou, S.C.  Pillai, Self-cleaning applications of TiO2 by photo-­ induced hydrophilicity and photocatalysis, Appl. Catal. B Environ. 176 (2015) 396–428. [31] B.  Siwach, S.  Sharma, D.  Mohan, Structural, optical and morphological properties of ZnO/MWCNTs nanocomposite photoanodes for Dye Sensitized Solar Cells (DSSCs) application, J. Integr. Sci. Technol. 5 (2017) 1–4. [32] S. Sharma, B. Siwach, S.K. Ghoshal, D. Mohan, Dye sensitized solar cells: from genesis to recent drifts, Renew. Sust. Energ. Rev. 70 (2017) 529–537. [33] J.F.  de Lima, M.H.  Harunsani, D.J.  Martin, D.  Kong, P.W.  Dunne, D.  Gianolio, R.J. Kashtiban, J. Sloan, O.A. Serra, J. Tang, R.I. Walton, Control of chemical state of cerium in doped anatase TiO2 by solvothermal synthesis and its application in photocatalytic water reduction, J. Mater. Chem. A 3 (2015) 9890–9898. [34] A.O.T. Patrocinio, L.G. Paterno, N.Y. Murakami Iha, Role of polyelectrolyte for layer-bylayer compact TiO2 films in efficiency enhanced dye-sensitized solar cells, J. Phys. Chem. C 114 (2010) 17954–17959. [35] M.E.  Khan, M.M.  Khan, M.H.  Cho, Ce3+-ion, surface oxygen vacancy, and ­visible light-­ induced photocatalytic dye degradation and photocapacitive performance of CeO2-graphene nanostructures, Sci. Rep. 7 (2017) 5928, https://doi.org/10.1038/ s41598-017-06139-6.

Metal oxide electrodes for photo-activated water splitting

3

Davide Barreca*, Giorgio Carraro†, Alberto Gasparotto†, Chiara Maccato† *Padova University, Padova, Italy, †Padova University and INSTM, Padova, Italy

3.1 Introduction The utilization of sunlight can provide helpful power necessary to supply to energy needs in the framework of an improved environmental sustainability, provided that it is efficiently harvested, converted, and stored [1–6]. To this aim, an attractive route involves the conversion of radiant energy into chemical fuels, mimicking the elegant example provided by natural photosynthesis [7–10]. In this wide context, over the last two decades, harnessing solar energy for the clean production of molecular hydrogen (H2) by PEC water splitting over inorganic semiconductors has received a great attention [2,3,11–18]. In fact, this route represents a key strategy for sustainable energy generation [19–28], thanks to the possibility of driving the target processes with virtually zero ecological footprint starting from largely available resources, i.e., water, extremely abundant on Earth, and sunlight, inexhaustible and intrinsically renewable [29–32]. Indeed, PEC solar water splitting could be the basis for a sustainable, carbon-free, and cost-effective hydrogen-based economy, paving the way to the progressive substitution of fossil sources, with relevant economic and environmental advantages [2,15,30,33–35]. In fact, hydrogen, the cleanest energy fuel, has a very high energy density (≈130 MJ × kg−1) and is considered as one of the most promising energy carriers for the near future [28,36–39]. At variance with direct photocatalytic systems, which typically involve simultaneous generation of H2 and O2 at the photocatalyst surface, in PEC systems O2 and H2 evolution occurs at the anode and cathode, respectively (see Section  3.2) [37,40]. The collection of the two gases at separated electrodes offers important technical advantages for practical utilization [32]. To date, the development of a promising H2O splitting device is still directly dependent on the use of photoanodes possessing certain essential requirements [1,8,13,21]: (i) low cost and good stability in aqueous solutions; (ii) a band gap (EG) suitable for solar light absorption (Fig. 3.1); (iii) conduction (CB) and valence band (VB) edges matching water oxidation and reduction potentials; and (iv) high conversion efficiency of photogenerated electrons and holes, implying an efficient charge carrier separation and transport [15]. So far, numerous efforts have been devoted to the preparation of various photoelectrodes endowed with suitable properties [5,8,29,35,41,42], particularly n-type semiconducting materials working as photoanodes, because the oxygen evolution reaction (OER) from water remains a main bottleneck in the H2O splitting process. Since the active material is subjected to severe oxidizing conditions [9,10,15], the most Multifunctional Photocatalytic Materials for Energy. https://doi.org/10.1016/B978-0-08-101977-1.00003-X Copyright © 2018 Elsevier Ltd. All rights reserved.

Multifunctional Photocatalytic Materials for Energy

E

E

e-

e-

Cathode

J (mA x cm −2 )

20

H + /H 2 O 2/H 2O

CB EF

hn

B

6 4

1.23 V

2

Dark

0 1.0 1.2 1.4 1.6 1.8 Potential (V)

EG VB

h+

Anode

eLi

A H+

H+

H 2 evolution H2

Cathode

eeeeeeeeee-

t gh

H 2O

O 2 evolution

O2

Anode

Fig. 3.1  (A) Schematic representation of a PEC cell for photoassisted water splitting. The inset shows the corresponding energy level diagram, in the case of photocathode and photoanode based on a metal and an n-type semiconductor, respectively. EF and EG represent the Fermi energy level and the system band gap, respectively. (B) Photocurrent density/ potential curve for a generic photoanode marking the water oxidation potential (1.23 V), at which the recorded J values are usually compared. The current density obtained in the absence of illumination (dark curve) is also shown. A: Adapted with permission from D. Barreca, G. Carraro, V. Gombac, A. Gasparotto, C. Maccato, P. Fornasiero, E. Tondello, Supported metal oxide nanosystems for hydrogen photogeneration: quo vadis? Adv. Funct. Mater. 21 (2011) 2611–2623. Copyright Wiley, 2011.

appealing candidates are metal oxides, thanks to their favorable PEC performances and stability in aqueous environments [1,16,41,43,44]. Based on the pioneering work demonstrating PEC H2O splitting with TiO2 [14,45], several studies have concentrated on the search of oxide photoanodes allowing an improved solar light harvesting than TiO2, which can absorb only a small fraction (≈5%) of the terrestrial solar spectrum owing to the high EG (≈3.2 eV) [13,15,21,40,42,46]. On this basis, research activities have focused on alternative oxide semiconductors (Fe2O3, WO3, ZnO, BiVO4, etc.) [1,27–29,33,47–62]. In this regard, recent advances in nanotechnology and catalysis have significantly increased the prospects of developing functional systems capable of efficiently converting solar light into chemical fuels [39,63], meeting the challenge of sustainable energy production even under Vis light irradiation. Within this general context, the present chapter provides a survey of the research results obtained in the past decade in the preparation, characterization, and functional

Metal oxide electrodes for photo-activated water splitting 21

testing of nanostructured metal oxide photoanodes for PEC water splitting applications. After a brief overview of the basic concepts of photoelectrochemical water splitting, the chapter summarizes the most recent and cutting-edge results obtained for photoanode materials based on selected metal oxides. Rather than providing a comprehensive review on the topic, the main goal is to offer a general overview of important achievements in the field, with particular regard to the most effective actions carried out to improve material photoefficiency: (i) tailoring of morphology/ nano-organization; (ii) doping and functionalization with suitable catalysts/activators; and (iii) fabrication of nanocomposites and heterostructured systems [42,63]. In the discussion of selected experimental results, particular efforts are dedicated to highlighting the interplay between the system features and the ultimate material activity, examined in terms of photocurrent density/potential curves. For additional technical details, the reader is referred to specialized review papers available in the literature [5,14,31,33,35,37,39,40,42,63,64].

3.2 Fundamentals of photoelectrochemical water splitting: An overview In general, photoelectrochemical water splitting occurs through the following half-­ reactions, which, in a PEC cell, take place at two interconnected, albeit physically separated, electrodes [4,6,40,65]: Oxidation : 2H 2 O ® O2 + 4H + + 4e - E °ox = 1.23 V vs. RHE

(3.1)

Reduction : 4H + + 4e - ® 2H 2 E °red = 0.00 V vs. RHE

(3.2)

Overall reaction : 2H 2 O ® 2 H 2 + O2

(3.3)

where RHE denotes the reversible hydrogen electrode. Among the possible configurations of PEC water splitting cells [4,33], in the most common case, the photoanode is based on an n-type semiconductor, capable of absorbing light with energy ≥EG to generate electron-hole pairs [6,31,40]. Upon light absorption, photo-excited holes move toward the semiconductor surface and are transferred to the electrolyte, oxidizing water molecules to yield O2 (Eq. 3.1 and Fig.  3.1A) [5,33,40]. Concomitantly, photoexcited electrons are forced to the back contact and transferred through the external circuit to the counterelectrode, a metal or a p-type semiconductor (cathode) [18,32]. At the interface of the latter with the electrolyte, electrons can react with protons and directly take part in H2 evolution (Eq. 3.2) [37,40]. The photocurrent generated in the external circuit and registered as a function of the applied potential (Fig. 3.1B) is an important parameter for the evaluation and comparison of photoanode performances. Despite being very attractive for sustainable energy generation, PEC water splitting is indeed a difficult approach [29]. As a matter of fact, hydrogen production is hindered by the remarkable stability of water, because of the large positive change in the Gibbs

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free energy required for the overall process (ΔG° = 237 kJ × mol−1) [32,33,38,42]. The main challenge lies in the complexity of the OER (1), which is both kinetically and thermodynamically demanding, requiring an overall four-electron/four-proton process for the evolution of one O2 molecule [1,3,11,18,20,27,40]. Consequently, the design of suitable photoanode materials capable of efficiently driving this reaction is an important hurdle to overcome [65]. Nonetheless, the target semiconductor must possess an adequate electronic band structure, where the potential of the CB edge should be more negative than that of the H2 evolution reaction, and the potential of the VB edge more positive than that of the O2 evolution reaction [13,66]. Fig. 3.2 reports the band edge positions for the most common oxides used as PEC photoanodes, with particular regard to the ones discussed in the present chapter. As can be observed, the application of an external bias is required in particular for α-Fe2O3 and WO3 to operate as water splitting photoanodes. Nonetheless, it is worthwhile noticing that the reported data are referred to bulk systems and that the actual CB/VB edges might indeed differ from the indicated ones, especially in the case of nanomaterials due to the influence of size/ shape and surface effects on their electronic structure [37]. In addition, it should be noted that Fig.  3.2 refers to pH = 0, whereas at different pH values a shift of water redox potentials takes place.

E(V) vs. NHE –1.5 –1.0 –0.5 0.0

ZnO α-Fe2O3

WO3

BiVO4 E°(H+/H2)

0.5 1.0

E°(O2/H2O)

1.5 2.0 2.5 3.0 3.5

Fig. 3.2  Band gaps and band edge positions with respect to the reversible hydrogen electrode (RHE) at pH = 0 of selected semiconducting oxides examined in the present chapter [3,36,39,66–68]. The valence and conduction band edges are marked in red and blue, respectively. The horizontal red lines indicate the water redox potentials. Adapted with permission from D. Barreca, G. Carraro, V. Gombac, A. Gasparotto, C. Maccato, P. Fornasiero, E. Tondello, Supported metal oxide nanosystems for hydrogen photogeneration: quo vadis? Adv. Funct. Mater. 21 (2011) 2611–2623. Copyright Wiley, 2011.

Metal oxide electrodes for photo-activated water splitting 23

3.3 Relevant case studies for photoanode development In this section, as already mentioned in the introduction, attention will be dedicated to four different types of metal oxide photoanodes, starting from Fe2O3, one of the most studied systems, through WO3, ZnO, and BiVO4. In particular, Fe2O3, WO3, and BiVO4 are among the few stable n-type semiconducting oxides capable of harvesting a significant sunlight fraction [9,13,41], whereas ZnO is an amenable candidate because of its excellent electron mobility, as well as its favorable environmental compatibility [28,34]. In each of the target cases, selected recent examples are provided not only for single-phase oxide photoelectrodes, but also for composites, nanoheterostructures, and systems doped or functionalized with oxygen evolution catalysts (OECs), with emphasis on the design and tailoring of material chemico-physical properties as a powerful tool to attain improved functional performances.

3.3.1 Fe2O3-based materials Iron(III) oxide, and, in particular, hematite (α-Fe2O3), its most stable polymorph, is one of the most important functional materials for PEC water splitting photoanodes [5,8,11,18], thanks to its abundance, nontoxicity, economic viability, stability in aqueous mixtures, and suitable EG (≈2.0 eV) to absorb a significant solar spectrum fraction [11,29,69]. Unfortunately, these properties are adversely affected by various disadvantages, among which: (i) an unfavorable conduction band edge position with respect to water reduction potential (see Fig. 3.2) [11]; (ii) sluggish kinetics of OER reaction requiring a large overpotential for water oxidation [1,11]; (iii) a limited majority carrier conductivity; (iv) a relatively low absorption coefficient; and (v) a short hole diffusion length (LD ≈ 2–4 nm) [1,14,29,33,69], much lower than for other oxides (for instance, LD up to 104 and 150 nm for TiO2 and WO3, respectively [1]), due to ultrafast carrier recombination [69]. The combination of these features is responsible for performance losses, resulting in photocurrents much lower than the maximum 12.6 mA × cm−2 predicted for an ideal hematite photoanode [1]. As a result, several studies have concentrated on tailoring Fe2O3 photoelectrode properties in order to improve their functional behavior. In particular, drawback (iii) was circumvented by adding impurity dopants to obtain higher conductivity values, depending on the dopant type and concentration (e.g., Ti(IV), Sn(IV), Zr(IV), among the most used) [1,5]. Various efforts have demonstrated that: (i) optimizing hematite electrode nano-­ organization can increase photocurrent density, and (ii) surface functionalization can lower the required overpotential. The combination of these strategies can enable to approach the performances of an ideal α-Fe2O3 photoanode (Fig. 3.3A) [70]. In this regard, Graetzel et  al. [69] reported on the fabrication of hematite photoelectrodes on FTO substrates via a solution-based route followed by air annealing. As shown in Fig. 3.3B, an increase in the annealing temperature from 400°C to 800°C resulted in the obtainment of larger feature sizes and in a porosity enhancement (Fig. 3.3C). Correspondingly, variations in the optical absorption coefficient (α) spectra took place (Fig. 3.3D). In particular, whereas the absorption band located at ≈540 nm was almost unchanged by the processing temperature, the one at λ ≈ 380 nm underwent a red

24

Multifunctional Photocatalytic Materials for Energy

(C)

(A) 14

J / mA cm–2

12

(B)

Ideal hematite photoanode

10 8

Morphology control

6

Surface chemistry and catalysis

4 2 0 0.4

0.6

0.8

1.0

1.2

1.4

1.6

V / V vs. RHE

(D)

1.4

(E)

1.2

α (cm–1)

150 100

800 C 700 C

400 C

400 C

700 C

800 C

50

–2 J (mA × cm )

3

200×10

1.0

800 C

0.8

700 C

0.6 0.4

400 C

0.2

0

350 400 450 500 550 600 650 700 750 800 850

Photon wavelength (nm)

0.0

800 C dark

0.8

1.0

1.2

1.4

1.6V

E (V vs. RHE)

Fig. 3.3  (A) Sketch of the strategy adopted to improve α-Fe2O3 (hematite) photoelectrode performances. The continuous lines represent the photocurrent density/voltage curves of an ideal hematite photoanode and the typical performance for an hematite photoelectrode under simulated solar illumination (AM1.5G, 100 mW × cm−2). The effects expected by tailoring the system morphology and surface chemistry are also marked. Plane-view scanning electron microscopy (SEM) micrographs of mesoporous hematite films fabricated by a solution-based colloidal approach [69] on F-doped SnO2 (FTO) and annealed in air at (B) 400°C and (C) 800°C. (D) Optical absorption coefficient spectra and digital photographs for α-Fe2O3 systems annealed at different temperatures. Panel (E) shows the corresponding photocurrent density versus potential curves in 1 M NaOH solutions under simulated sunlight and in the dark (continuous and dashed lines, respectively), versus RHE. A: Adapted with permission from K. Sivula, F. Le Formal, M. Grätzel, Solar water splitting: progress using hematite (α-Fe2O3) photoelectrodes, ChemSusChem 4 (2011) 432–449. Copyright Wiley, 2011; E: Adapted with permission from K. Sivula, R. Zboril, F. Le Formal, R. Robert, A. Weidenkaff, J. Tucek, J. Frydrych, M. Grätzel, Photoelectrochemical water splitting with mesoporous hematite prepared by a solution-based colloidal approach, J. Am. Chem. Soc. 132 (2010) 7436–7444. Copyright American Chemical Society, 2010.

shift, and EG values lowered from 2.20 eV to 2.15 eV upon raising the temperature from 400°C to 800°C. These differences have a direct influence on photoanode performances (Fig. 3.3E). As can be noticed, the use of treatment temperatures ≤700°C resulted in negligible photocurrents, whereas upon annealing at 800°C drastic performance improvements occurred. An onset potential (the value at which a current density of 0.02 mA × cm−2 is first reached [71]) of ≈0.9 V was observed, with J values of 0.6 mA × cm−2 at 1.23 V, the standard potential of the H2O/O2 redox couple.

Metal oxide electrodes for photo-activated water splitting 25

The improvement of material photoactivity could be traced back to the increase in porosity and in light harvesting, of crucial importance in determining the recorded J values, and to the diffusion into the film of Sn(IV) centers from the substrate, acting as donor impurities [72]. The latter phenomenon, which took place only at 800°C, exerted a beneficial role on material conductivity and photoactivity [69]. Further efforts were devoted to the surface functionalization of α-Fe2O3 systems by OECs, with the aim of facilitating the complex four-electron water oxidation, overcoming the already mentioned OER kinetic issues [11,14,18]. To this aim, ruthenium(IV) and iridium(IV) oxides have proved to be effective [33,73], but Ru and Ir are precious metals with limited commercial potential. Among noble metal-free OER catalysts, cobalt phosphate (CoPi), exploiting the cyclic valence change of Co ions between Co(II)/Co(III) and Co(III)/Co(IV), has been successfully integrated with various photoanodes, including hematite-based ones [10,13,14,27,62,65,72]. A viable, though less explored, alternative is offered by the use of Mn oxide-based catalysts, thanks to the Mn multi-oxidation states, that play an important role in enhancing the local hole transport [11]. In a recent work, α-Fe2O3 nanorod arrays were fabricated by a hydrothermal process on FTO substrates and functionalized via spin coating with preformed MnO nanoparticles (NPs) [11]. For comparison, CoPi catalysts were photoelectrodeposited onto hematite nanosystems under controlled conditions. Structural and optical analyses on pristine, MnO-loaded, and CoPi-treated hematite nanomaterials revealed no significant variations and a very similar gap value (EG ≈ 2.1 eV). SEM images highlighted the presence of nanorod-like hematite nanostructures on the FTO substrate, and functionalization with MnO resulted in the formation of a very thin top layer (Fig.  3.4A). CoPi-treated systems showed similar morphology and thickness. Fig. 3.4B sketches the mechanism of charge separation at the MnO-hematite interface, whereas the current density/potential curves obtained upon simulated sunlight irradiation are shown in Fig. 3.4C. As can be seen, J values at 1.23 V versus RHE increased from 1.21 mA × cm−2, for the pristine hematite electrode, up to 1.45 and 2.06 mA × cm−2 after functionalization with CoPi and MnO, respectively. Additionally, both CoPi and MnO introduction produced a negative shift of the onset potential. Current density versus time measurements revealed a comparable stability of the three photoanodes, an important prerequisite for practical utilization. Further studies [11] pointed out that, whereas the beneficial MnO effect was attributed to the higher charge injection into the electrolyte, CoPi mainly exerted a surface passivation effect. The possibility of improving hematite PEC performances through passivation of surface states has also been explored in the formation of oxide-Fe2O3 nanoheterostructures via the introduction of various overlayers, among which Al2O3, Ga2O3, FexSn1-xO4, and TiO2 [31,74–79]. Additional benefits of such strategies include the corrosion protection of the underlying iron oxide deposit and proper tailoring of charge transfer processes between the constituent phases. Nevertheless, carrying out PEC water splitting more efficiently than with actual state-of-the-art hematite photoanodes [80] remains an open challenge. In this regard, a viable approach has involved coating Fe2O3 nanostructures prepared via plasma enhanced-chemical vapor deposition (PE-CVD) by atomic layer deposition (ALD) TiO2 overlayers with different thickness [29], taking advantage of ALD repeatability, conformality, and precise thickness control [14,81,82]. Fig. 3.5A and B

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Multifunctional Photocatalytic Materials for Energy

(A)

(B)

a-Fe2O3 e-

5

J (mA ¥ cm–2)

4

Pristine Hematite_Dark Pristine Hematite_Light CoPi Treated Hematite_Dark CoPi Treated Hematite_Light

MnO



(C) hn

MnO Loaded Hematite_Dark MnO Loaded Hematite_Light

O2

3

h+ +

2

H2O

1 0

0.8

1.0 1.2 1.4 1.6 Voltage (V) vs. RHE

1.8

2.0

Fig. 3.4  (A) Cross-sectional field emission (FE)-SEM micrograph of α-Fe2O3 nanorod arrays, prepared by a modified hydrothermal method and spin-coated with previously synthesized MnO particles [11]. A thin MnO layer can be seen on the top of hematite nanostructures. (B) Simplified sketch of the photogenerated processes occurring at a MnO-loaded hematite photoanode-electrolyte interface in PEC water splitting. (C) Photocurrent density/potential curves of bare, MnO-loaded, and CoPi-treated α-Fe2O3 photoanodes deposited on FTO, measured under simulated sunlight (AM1.5G, 100 mW × cm−2) in 1 M NaOH electrolyte solutions. Dark curves are also shown as dashed lines. Adapted with permission from Gurudayal, D. Jeong, K. Jin, H.-Y. Ahn, P.P. Boix, F.F. Abdi, N. Mathews, K.T. Nam, L.H. Wong, Highly active MnO catalysts integrated onto Fe2O3 nanorods for efficient water splitting, Adv. Mater. Interfaces 3 (2016) 1600176. Copyright Wiley, 2016.

show HAADF-STEM cross-sectional micrographs of samples obtained with a different TiO2 thickness, along with compositional EDXS maps. As can be seen, Fe2O3 nanodeposits consisted of hematite lamellae assembled in open arrays, which allowed ALD depositions even in the inner system regions. These results confirmed the formation of Fe2O3-TiO2 nanoheterostructures with an intimate contact between the components (Fig. 3.5C and D). The porosity of the TiO2 top layer was revealed to be a key feature for the ultimate system PEC performances [29], which were investigated in NaOH aqueous solutions (Fig.  3.5E) and compared with those of a bare Fe2O3 photoelectrode. In particular, functionalization of hematite with TiO2 overlayers resulted in an onset potential decrease (from 1.1 V, for bare Fe2O3, to 0.8 V versus RHE, for Fe2O3-TiO2(H)) and in a significant photocurrent enhancement, proportional to TiO2 loading. For Fe2O3-TiO2(H), a J value of 2.0 mA × cm−2 at 1.23 V versus RHE

Metal oxide electrodes for photo-activated water splitting 27

J (mA × cm–2)

10

(B)

(C)

(D)

1.0

(E)

Fe2O3_TiO2(H)

0.8

8 Fe2O3

6

Fe2O3_TiO2(L) Fe2O3_TiO2(H)

4

(F)

Fe2O3

1.8 V

J (mA × cm–2)

12

(A)

0.6 1.23 V 0.4 0.2

2

1.23 V 0.0

0 0.8

1.0 1.2 1.4 1.6 Voltage (V) vs. RHE

1.8

0.6

0.8

1.0

1.2

1.4

1.6

Voltage (V) vs. RHE

Fig. 3.5  Cross-sectional high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) images and energy dispersive X-ray spectroscopy (EDXS) chemical maps for Fe2O3-TiO2 nanoheterostructured photoanodes deposited on FTO with lower [A, Fe2O3-TiO2(L)] and higher [B, Fe2O3-TiO2(H)] titania overlayer thickness (35 and 80 nm, respectively). The systems were obtained by atomic layer deposition (ALD) of TiO2 over α-Fe2O3 nanostructures fabricated by plasma-enhanced chemical vapor deposition (PE-CVD), tailoring TiO2 thickness by changing ALD cycle number under optimized conditions, and final annealing at 650°C [29]. (C) TEM and (D) HAADF-STEM overviews corresponding to (A) [Fe2O3-TiO2(L)] and (B) [Fe2O3-TiO2(H)], respectively. (E and F) J/V curves for the pristine Fe2O3 and Fe2O3-TiO2 photoelectrodes recorded under simulated sunlight (AM1.5G, continuous lines) and in the dark (dashed lines), (E) in 1 M NaOH, and (F) in simulated seawater (35 g × L−1 sea salt). Adapted with permission from D. Barreca, G. Carraro, A. Gasparotto, C. Maccato, M.E.A. Warwick, K. Kaunisto, C. Sada, S. Turner, Y. Gönüllü, T.-P. Ruoko, L. Borgese, E. Bontempi, G. Van Tendeloo, H. Lemmetyinen, S. Mathur, Fe2O3–TiO2 nano-heterostructure photoanodes for highly efficient solar water oxidation, Adv. Mater. Interfaces 2 (2015) 1500313. Copyright Wiley, 2015.

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Multifunctional Photocatalytic Materials for Energy

was obtained, corresponding to a ten-fold increase with respect to pristine Fe2O3. At variance with the latter, TiO2-containing samples did not display any saturation up to 1.80 V versus RHE, a feature that, along with the photocurrent increase, was traced back to the formation of Fe2O3-TiO2 heterojunctions [14], promoting a more efficient charge separation than hematite. In addition, titania overlayers exerted a protective action against α-Fe2O3 photocorrosion [29]. The present performances correspond to the highest ones ever reported for similar materials, especially at high applied potentials. Based on these results, bare Fe2O3 and Fe2O3-TiO2(H) photoelectrodes were tested using simulated seawater solutions as electrolytes, an important technological target for real-world applications, since 97.5% of the overall water supply on Earth corresponds to salt water [83,84]. Even under these conditions, the TiO2-containing photoelectrode enabled to obtain higher J values than bare Fe2O3 (≈0.4 versus 0.2 mA × cm−2 at 1.23 V versus RHE). Despite the performances are lower than those of NaOH solutions, these preliminary results are very promising in view of sustainable energy generation starting from abundant and renewable natural resources through cost-effective nanoheterostructured devices.

3.3.2 WO3-based materials Tungsten(VI) oxide has been investigated for water splitting applications since the early 1970s [85,86]. WO3 is considered as a promising photoanode material, thanks to its low cost, stability in aqueous solutions under O2 evolution [19], band gap (EG = 2.5−2.8 eV) [15,41,43,44] enabling Vis light harvesting, good electron transport properties, and more favorable hole diffusion length (≈150 nm) than hematite [2,33]. On the other hand, WO3 presents sluggish kinetics for photoproduced holes and rapid electron-hole recombination, limiting its functional performances. To increase material photoactivity, efforts have been dedicated to the preparation of nanostructured WO3 systems, with enhanced light absorption, high active area, and optimized carrier transport properties [2,19,21]. To this aim, various synthetic routes have been adopted, including anodization, sol–gel, sputtering, as well as electrochemical, solvothermal, and hydrothermal processes [19,33]. In the latter context, Feng et al. [2] reported on sandwich-structured WO3 nanoplatelet arrays supported on FTO substrates. For these materials, X-ray diffraction (XRD) revealed a direct dependence of the phase composition on annealing temperature [33]. As-prepared specimens contained only orthorhombic WO3·0.33H2O, whereas after treatment at 400°C, the pattern was dominated by hexagonal WO3. In a different way, upon annealing at 475°C, reflections from monoclinic WO3 were detected, and were also revealed at 500°C and 600°C. This structural evolution was accompanied by appreciable morphological variations. For as-prepared systems (Fig.  3.6A), multilayer nanoplatelets were obtained, and their surface increased in roughness after annealing at 400°C (Fig. 3.6B). Upon treatment at 475°C, a convex morphology at the nanoplatelet edges became evident (Fig. 3.6C), whereas at 500°C the nanoplatelets showed a sandwich morphology with a thick middle layer and two thin side layers (Fig. 3.6D) and were almost perpendicular to the substrate surface (Fig. 3.6F). Upon harsher annealing, polycrystalline aggregate arrays were detected (Fig. 3.6E) [2].

Metal oxide electrodes for photo-activated water splitting 29

(B)

(A)

2.0

(G)

500°C

200 nm

200 nm

2 µm

(C)

2 µm

J (mA ¥ cm–2)

1.5

475°C

1.0 0.5

(D)

600°C

400°C

0.0 0.0 200 nm

2 µm

(E)

2 µm

(F)

2 µm

2,3 µm

200 nm

1 µm

60

Gas evolution (mmol)

200 nm

0.2

0.4 0.6 0.8 1.0 E (V vs. Ag/AgCI)

(H)

H2 O2

50

1.2 1.4

40 30 20 10 0

0

1200 2400 3600 4800 6000 7200 Time (s)

Fig. 3.6  Representative SEM images of WO3 nanoplatelet arrays grown on FTO via a hydrothermal process, as-prepared (A), and after annealing in air for 1 h at (B) 400°C; (C) 475°C; (D) 500°C; (E) 600°C, respectively. (F) Cross-sectional SEM micrograph of the specimen treated at 500°C shown in (D). (G) Photocurrent density of WO3 photoanodes annealed at different temperatures, recorded in 0.5 M Na2SO4 solutions under AM1.5G chopped illumination (100 mW × cm−2). (H) H2 and O2 evolution versus time (irradiation area: 1.9 cm2; 0.6 V vs. Ag/AgCl) obtained under simulated sunlight irradiation of a WO3 photoanode annealed at 500°C. Adapted with permission from X. Feng, Y. Chen, Z. Qin, M. Wang, L. Guo, Facile fabrication of sandwich structured WO3 nanoplate arrays for efficient photoelectrochemical water splitting, ACS Appl. Mater. Interfaces 8 (2016) 18089–18096. Copyright American Chemical Society, 2016.

The interplay between morphology and photoelectrochemical activity was studied by J/V measurements (Fig. 3.6G). The recorded J values were progressively enhanced by raising the annealing temperature to 500°C (highest J of 1.88 mA × cm−2 at 1.3 V versus Ag/AgCl), and then decreased upon treatment at 600°C. Detailed analyses revealed that the PEC activity of the 500°C-treated sample, among the best reported for WO3 photoanodes [2], was due to a combination of the highest surface area and the presence of monoclinic WO3, the most active phase for PEC water splitting. For the best performing system, hydrogen and oxygen evolution with a molar ratio of ca. 2:1 was demonstrated (Fig. 3.6H). Nevertheless, the data suggested the occurrence of side alterations related to electrode dissolution. To circumvent this problem, acidic electrolytes were utilized [13], and PEC tests in H2SO4 electrolytes on WO3 photoanodes prepared by a liquid phase route yielded photocurrents comparable to the state-of-the-art (≈2.7 mA × cm−2 at 1.3 V versus RHE) [19]. An alternative route to

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Multifunctional Photocatalytic Materials for Energy

improve WO3 stability is offered by the production of heterostructured photoanodes with external protective overlayers (see also Section 3.3.1). For instance, the development of BiVO4/WO3 electrodes enabled to obtain good stability and enhanced photocurrent values [87,88], thanks also to an improved charge carrier separation (compare Section 3.3.4; [43]). In this regard, coupling of WO3 even with other semiconductors (CdS, TiO2, Cu2O, …) has attracted considerable attention. Among these, WO3/TiO2 thin films prepared by pulsed electrodeposition [89] yielded an improved and very favorable performance with respect to the single oxides, which was traced back to a suppression of recombination phenomena [33]. In addition, the analysis of core-shell WO3/TiO2 and TiO2/WO3 nanorod arrays fabricated by evaporation [21] demonstrated that TiO2-core/WO3-shell structures had a net photoresponse in the UV spectral region (λ ≤ 400 nm), whereas WO3-core/TiO2-shell structures showed stronger Vis light absorption. In another study, Wei et al. prepared WO3/p-Cu2O multilayers by sequential electrodeposition, obtaining a higher photoactivity than pure WO3 and p-Cu2O films [90]. This improvement was related to the formation of p–n junctions at the interface between the two oxides, and, in particular, to the accumulation of electrons and holes in Cu2O conduction band and WO3 valence band, respectively. Other ongoing research activities have been addressed to investigate composite photoanodes widely used to physically separate the light absorption and catalyst function [22]. In this context, recent efforts have been dedicated to plasmonic metal/semiconductor systems [18,41,44,91,92] in which the formation of metal-oxide junctions [92–95] played a key role in the overall process [66,96]. In fact, the localized surface plasmon resonance (SPR), arising from the collective oscillation of free electrons in metal particles upon interaction with resonant photons, enables the excitation energy transfer from metal NPs to the adjacent semiconductor, with an ultimate enhanced formation of electron-hole pairs. In addition, plasmonic metal NPs can improve the semiconductor absorbance by scattering of incident light [18,34,44]. Enhancing light absorption by the SPR of metal NPs is particularly important in the case of indirect band gap systems, such as WO3, for which the 400–500 nm wavelength range critically influences the absorbed solar light amount. In this regard, it has been demonstrated that the dispersion of silver NPs on FTO substrates prior to WO3 deposition (Fig. 3.7A) is a successful mean to enhance performances [66,97]. Nevertheless, the use of silver NPs in photoelectrochemical devices is only possible in such a kind of “embedded” configuration, in which the particles are protected from corrosion phenomena by the semiconductor overlayer [41]. As a consequence, attention was devoted to the use of Au NPs, and an interesting work involved the capping of Au NPs dispersed on WO3 photoanode surface with Keggin-type Mo polyoxometalate (POM) (i.e., PMo12O403−) ions [44], which have also been investigated as highly effective OECs [13]. An inherent advantage brought about by their use is that the interaction between POMs and Au NPs effectively prevents the agglomeration of capped particles on the WO3 surface, thanks to electrostatic repulsion effects. In addition, the negative charge of POM-capped Au NPs promotes their interaction with WO3 surfaces, which are positively charged in acidic media. In this regard, Fig. 3.7B shows the arrangement of the polyoxomolybdate-capped Au NPs around the larger WO3 particles.

Metal oxide electrodes for photo-activated water splitting 31

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Fig. 3.7  (A) SEM micrograph displaying silver NPs dispersed on FTO substrate on which WO3 films were subsequently deposited. (B) Representative TEM micrograph for Au NPs functionalized with PMo12O403− ions deposited onto the surface of WO3 films prepared by a sol-gel route. (C) Photocurrent density/voltage curves recorded in water splitting (simulated AM 1.5 sunlight, 1 M H2SO4 solution) versus a mercury sulfate electrode (MSE) reference, promoted by: bare WO3 photoanode (1); modified WO3 photoanode with Au-PMo12O403− NPs deposited either on the FTO substrate (2) or on the system surface (3). WO3 films on FTO substrates were fabricated by a modified sol-gel approach. Gold nanoparticles modified with · phosphododecamolybdates were prepared by a liquid-phase route starting from HAuCl4xH2O and H3PMo12O40 [44]. Adapted with permission from J. Augustynski, K. Bienkowski, R. Solarska, Plasmon resonance-enhanced photoelectrodes and photocatalysts, Coord. Chem. Rev. 325 (2016) 116–124. Copyright Elsevier B.V., 2016.

PEC water splitting tests were focused on FTO-supported WO3 films, both as such and with Au NPs incorporated in different configurations (Fig. 3.7C). In the first one, a WO3 film was functionalized with PMo12O403−-capped Au NPs, anchored on its surface through a mild annealing (70°C, 10 min). In a second one, Au NPs were deposited on the FTO substrate and subsequently covered by the WO3 film. This kind of Au

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nanostructures embedded in WO3 may act as a reflector and light scatterer upon irradiating the photoanode from the solution side [44]. A comparison of J/V curves for the photoanode with surface-anchored Au– PMo12O403− nanoparticles and for bare WO3 revealed that, in the former case, the onset potential was less positive by ca. 50 mV and the photocurrent plateau was approximately twice higher. This very favorable improvement was traced back to the synergistic combination of various phenomena: (i) a direct catalytic effect of POMcapped Au NPs in the H2O oxidation reaction, taking also into account that the photoactivity improvement was lower for embedded Au NPs; (ii) local SPR-induced effects of the Au NPs; and (iii) a buffer action of the polyoxomolybdate layer capping Au NPs, a phenomenon preventing detrimental recombination processes at WO3 surface [41,44].

3.3.3 ZnO-based materials ZnO, an n-type transparent semiconductor with a high carrier mobility, has been extensively explored as a photoelectrode for PEC water splitting, thanks to its low cost and nontoxicity [26,27,53,59,98–101]. Nonetheless, its photoactivity is limited by its band gap (EG = 3.4 eV) [52,102–107], constraining light absorption to the UV interval [28,106,108–113] and by the rapid charge carrier recombination [62,99,102,104,105,114]. To circumvent these difficulties, a considerable attention has been dedicated to ZnO-based nanosystems, thanks to the possibility of shortening the photocarrier diffusion length exploiting the high surface-to-volume ratio. Unfortunately, the large grain boundary content of nanostructured photoelectrodes worsens the recombination losses and, at the same time, lowers the electron transport rate [25,26]. To tackle these obstacles and improve photoefficiency, efforts have been focused on the development of highly crystalline nanowire arrays, that offer the two-fold advantage of a lower grain boundary content (enhancing hole diffusion) and a fast electron transport perpendicular to the charge-collecting substrate [16,27]. In this regard, a viable alternative is offered by the use of branched 1D arrays that also display an improved light harvesting capability due to an effective internal scattering of the incoming radiation. In particular, an elegant example is offered by Chen et al. [25], who proposed a two-step hydrothermal route for the preparation of single-­crystalline branched 1D systems through the epitaxial growth of ZnO nanodisks (NDs) on ZnO nanowire (NWs) arrays. XRD patterns indicated that the only crystalline phase was hexagonal ZnO, with a preferential orientation. The overall system morphology (Fig. 3.8A) revealed the presence of lateral branches on the main ZnO NWs, whose surface was coarsened by the secondary growth of ZnO NDs characterized by a narrow size distribution. The ordered protruding NDs had a laminated structure and grew perpendicularly with respect to the main NW growth direction (Fig.  3.8B), inducing a beneficial surface area increase (of ≈70%) with respect to the pristine NWs [25]. ZnO NWs/NDs were tested as photoanodes for PEC water splitting against a ZnO NW reference sample (Fig. 3.8C), yielding a significantly higher photocurrent density

Metal oxide electrodes for photo-activated water splitting 33

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Fig. 3.8  (A) Large scale and (B) magnified cross-sectional SEM micrographs of as-prepared ZnO nanowire (NW)/nanodisk (ND) arrays, fabricated through the epitaxial growth of ZnO NDs on ZnO NWs via a hydrothermal process [25]. (C) Current density/potential curves recorded in the dark (D) and under illumination (L; simulated solar radiation; AM1.5G, 100 mW × cm−2) conditions. (D) Magnified curves in the low potential region. Adapted with permission from H. Chen, Z. Wei, K. Yan, Y. Bai, Z. Zhu, T. Zhang, S. Yang, Epitaxial growth of ZnO nanodisks with large exposed polar facets on nanowire arrays for promoting photoelectrochemical water splitting, Small 10 (2014) 4760–4769. Copyright Wiley, 2014.

throughout the investigated potential range. The main reasons for this improvement were the increased surface area, responsible for a more efficient contact with the electrolyte, and the unique system hierarchical morphology, which promoted an enhanced radiation harvesting. In addition, upon increasing the applied potential, the J/V curve for NWs/NDs showed a sharper increase, achieving saturation at ≈0.7 V versus Ag/ AgCl, at variance with the NW sample. This phenomenon, scarcely reported for ZnO photoanodes even in the presence of catalysts, suggested the occurrence of a more effective charge separation and collection in NWs/NDs. In this regard, a key role is played by the presence of NDs with much lower sizes, which enable an efficient transport of photogenerated holes up to the interface with the electrolyte, thus suppressing detrimental recombination losses. It is also worth noting that NWs/NDs showed a net negative shift of the onset potential (−0.11 V) with respect to the pristine NWs (Fig. 3.8D), indicating that a lower bias is needed for PEC water splitting [25].

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Additional efforts to broaden the ZnO spectral response into the Vis range have concerned elemental doping [100,103,110,111,113,115] and incorporation of plasmonic metals, such as Ag [28] and Au [26,28]. In the latter case, the light harvesting efficiency was enhanced by the SPR effect, and, at the same time, undesired recombination processes were restricted, yielding a significant photocurrent enhancement. In another study, ZnO/Au plasmonic structures, prepared by a hydrothermal and photoreduction combined approach, presented a photocurrent density as high as 9.11 mA × cm−2 at a low potential (1.0 V versus Ag/AgCl) and an efficiency 16 times higher than the pristine ZnO nanorod array [34]. These results provide useful insights into the design of plasmonic metal/semiconductor photoanodes for solar harvesting. Other investigators have explored either the integration of OECs, such as CoPi, onto a ZnO nanosystem surface [27], or coupling with other semiconductors, which has been shown to be one of the most effective and inexpensive methods [112]. In this regard, various heterojunction-containing composites based on ZnO-TiO2 [17], ZnONiO [60], ZnO-M(OH)x with M = Co, Ni [61,62], ZnO-BiVO4 (see also Section 3.3.4) [23,112], ZnO-WOx coupled with CdSe-CdS [116], ZnO-ZnS-FeS2 [107], and ZnOCdS-NiO [117] have been fabricated and tested for the target functional application. So far, the sensitization of ZnO with quantum dots (QDs) based on CdS [118], CdSe [101], and CdTe [113] has been used to improve PEC efficiency, but the toxicity of these systems, along with their dissolution/degradation phenomena, has stimulated the search for harmless green sensitizers. For instance, Guo et al. recently reported on unique photoelectrodes obtained by covalently bonding graphene quantum dots (GQDs), functionalized with carboxyl moieties, on amino group-modified ZnO NWs. The use of GQDs was motivated by their peculiar optical and electronic properties, rendering them attractive candidates to build advanced functional nanomaterials [16]. SEM investigation (Fig. 3.9A) confirmed the presence of high density ZnO NW arrays, in which NWs (mean length ≈1 μm, mean diameter ≈50 nm) grew almost perpendicularly to the FTO substrate surface. ZnO nanostructures were uniformly covered by GQDs, which did not significantly alter the pristine morphology, as also confirmed by TEM results. The evaluation of the system photoactivity (Fig.  3.9B) revealed a considerable photocurrent density increase for [email protected] NWs with respect to the original ZnO NWs, indicating the occurrence of a much more efficient photoelectrochemical process in the former case. In addition, the system performances were highly stable upon prolonged cycling (Fig. 3.9B, inset). This result is of utmost importance, taking into account that a poor ZnO photostability has a detrimental effect on material photoefficiency [17,27]. Based on the obtained results and taking into account the mutual ZnO and GQDs energy level positions, the PEC H2O splitting process promoted by GQDs@ ZnO NW photoelectrodes can be interpreted as shown in the scheme presented in Fig. 3.9C. The lower photoresponse for bare ZnO NWs could be traced back to the limited Vis light absorption, due to the large band gap. Conversely, [email protected] NWs enabled a favorable enhancement of Vis light harvesting. After irradiation, the transfer of photogenerated electrons from GQDs to ZnO is enabled by the mutual positions of energy levels, resulting in an enhanced charge separation that further

Metal oxide electrodes for photo-activated water splitting 35

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Fig. 3.9  (A) Cross-sectional SEM micrograph of ZnO NW arrays functionalized with graphene quantum dots ([email protected] NWs). GQDs were synthesized starting from carbon black by a chemical oxidation approach and covalently immobilized on surface-modified ZnO NW arrays grown on FTO substrates by a hydrothermal synthesis route [16]. (B) Photocurrent density vs. potential curves pertaining to ZnO NW and [email protected] NW specimens (0.5 M Na2SO4 aqueous solutions; Xe lamp, 100 mW × cm−2). Current density curves obtained in the dark are also reported for comparison. The inset shows stability test results obtained by comparing responses corresponding to the 1st and 50th utilization cycles. (C) Sketch of the PEC H2O splitting process referred to a cell containing a [email protected] NW photoelectrode and a Pt counterelectrode. Adapted with permission from C.X. Guo, Y. Dong, H.B. Yang, C.M. Li, Graphene quantum dots as a green sensitizer to functionalize ZnO nanowire arrays on F-doped SnO2 glass for enhanced photoelectrochemical water splitting, Adv. Energy Mater. 3 (2013) 997–1003. Copyright Wiley, 2013.

promoted the uniform distribution of GQDs on ZnO. Thus, photogenerated holes can be directly involved in water oxidation, whereas electrons are transferred to the FTO substrate and eventually reach the counter electrode for hydrogen production. Hence, [email protected] NWs provide a promising green photoelectrode for PEC water splitting.

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3.3.4 BiVO4-based materials In spite of the attention dedicated to various photoelectrode materials (encompassing TiO2, Fe2O3, WO3, ZnO) for PEC water oxidation during the past few decades, none of these binary oxides fulfills all the requirements necessary for an eventual technology implementation. As a consequence, ternary semiconductor oxide p­ hotoanodes have also attracted great interest [15]. Most of these systems, with a general formula ABO4, possess Vis light activity, stability in aqueous electrolytes, and low cost. Among them, monoclinic bismuth vanadate (BiVO4, EG = 2.4 eV) [15,24], an n-type semiconductor, is particularly promising since it can yield a maximum theoretical photocurrent of ≈7.5 mA × cm−2 at 1.23 V versus RHE [9,23,65,119]. In fact, it can absorb Vis light and possesses band edges appropriately positioned for OER (see Fig. 3.2) [10,24]. Nevertheless, rapid charge carrier recombination processes and inefficient charge transport [10,65,112] are responsible for photocurrent density values lower than 1.0 mA × cm−2 at 1.23 V versus RHE. This problem, along with the high onset potential, has prompted morphology tailoring or functionalization/doping (for instance, with Cr, Mo, Nb, …) of this material in order to improve the efficiency of the OER reaction [12,15,24]. So far, the slow hole transfer to the solution remains a major bottleneck, requiring BiVO4 modification with OECs, such as CoPi [23,65]. A recent work looked at the liquid phase preparation of BiVO4 films [65], whose porosity was tailored by a proper choice of ex-situ annealing treatments. SEM micrographs evidenced the formation of compact structures for the “dense” BiVO4 (Fig. 3.10A and B), whereas the corresponding “porous” system presented a network of interconnected nanoaggregates (Fig. 3.10C and D). In both cases, XRD analyses revealed the sole presence of monoclinic BiVO4. Film photoactivity was evaluated in PEC water splitting, and the photocurrent/potential curves obtained under chopped illumination (Fig. 3.10E) revealed in both cases a certain extent of charge recombination, as indicated by the current spikes upon light ignition, followed by an exponential decay. Under continuous illumination, the behavior of the two systems was very similar up to ≈1 V versus RHE, and for higher potentials, dense BiVO4 performed better (Fig. 3.10E, inset). This difference, amplified upon chopped illumination (Fig. 3.10E), was mainly related to a higher photocarrier recombination rate in porous BiVO4, a phenomenon attributed to the higher content of grain boundary defects. Additional analyses showed that the charge transfer kinetics in the dense material is about 3-fold faster than in the porous film [65]. In an attempt to enhance the photoactivity of dense BiVO4 systems, functionalization with CoPi was carried out and yielded a ≈6-fold photocurrent increase at 1.23 V, confirming the CoPi role as an effective water oxidation catalyst (Fig. 3.10F). Nevertheless, the above-mentioned exponential decay immediately after illumination turn-on was still observed [10], suggesting that the recombination phenomena were not completely suppressed. Hence, the development of alternative routes to BiVO4 films with improved PEC performances remains an open challenge. In this context, other efforts have been dedicated to the preparation and chemical modification of BiVO4 systems [9,65,120,121], and some of these studies yielded J values up to 2.16 mA × cm−2 even in natural seawater [30], an important result for real-world ­utilization. Other

Metal oxide electrodes for photo-activated water splitting 37

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Fig. 3.10  Plane-view SEM micrographs of dense (A and B) and porous (C and D) BiVO4 thin films deposited by spin-coating onto FTO. After drying at 150°C, specimens were annealed at 400°C for 10 min and then cooled at room temperature for six times, and finally treated in air at 400°C for 2 h. In order to obtain a dense film (A and B), the specimen was introduced into the oven at 150°C and heated up to 400°C at a rate of 2.5°C × min−1 [65]. In a different way, to produce a porous morphology (C and D), the sample was introduced into an oven already at 400°C. (E) Photocurrent density/voltage curves for BiVO4 thin films with dense and porous morphologies recorded under chopped simulated sunlight irradiation [AM 1.5G, 100 mW × cm−2; 0.1 M sodium phosphate (NaPi) buffer, pH ≈ 7]. Inset: responses under continuous illumination; the dark current curve is reported in black for comparison. (F) Chrono-amperometry (CA) measurements under chopped illumination (1.23 V vs. RHE; 0.1 M NaPi electrolyte) for dense BiVO4 and BiVO4-CoPi photoanodes, obtained by CoPi photo-electrodeposition on the pristine system. Adapted with permission from S. Hernández, G. Gerardi, K. Bejtka, A. Fina, N. Russo, Evaluation of the charge transfer kinetics of spin-coated BiVO4 thin films for sun-driven water photoelectrolysis, Appl. Catal. B 190 (2016) 66–74. Copyright Elsevier B.V., 2016.

works have explored the coupling of BiVO4 with various metal oxides. In this respect, the formation of WO3/BiVO4 heterojunctions is effective in enhancing BiVO4 photoanode performances, since the mutual band positions of the two oxides favor the transfer of photogenerated electrons from BiVO4 to WO3 [43]. In another study [24], PEC performances of a W-doped BiVO4/FTO photoanode toward H2O oxidation were boosted by the electrodeposition of an amorphous TiO2 layer, passivating surface defects and thus blocking recombination phenomena. In a different way, Yan et al. developed a hybrid liquid phase route to 1D ZnO/BiVO4 heterojunction photoanodes, with

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the idea of exploiting ZnO nanorods as an electronic transmission channel and BiVO4 as a light absorber [23,112]. The composite photoanode exhibited a better activity in PEC water splitting, with a photocurrent density up to 1.7 mA × cm−2, higher than that of bare ZnO, thanks mainly to (i) the enlarged spectral region response related to BiVO4 and (ii) the formation of ZnO/BiVO4 heterojunctions, promoting a higher electron-hole separation. Another possibility of preparing low cost and stable heterostructured systems concerns the joint use of Fe2O3, BiVO4, and ZrO2. In this context, Shaddad et al. [9] developed an electrodeposition process which enables to improve H2O oxidation kinetics for BiVO4 photoanodes through the sequential addition of Zr and Fe precursors. Upon annealing, these precursors are converted into monoclinic ZrO2 and α-Fe2O3 nanoparticles. Fig. 3.11A and B show representative TEM micrographs for optimized BiVO4ZrO2 and BiVO4-ZrO2-Fe2O3 photoelectrodes. The former comprise highly crystalline NPs (mean diameter ≈5–10 nm), with interplanar distances corresponding to the (111) (Fig. 3.11A) and (1-11) reflections of monoclinic ZrO2. Chemical mapping indicated that an important zirconium fraction is present at the BiVO4 surface. Nevertheless, it is also worthwhile noting that Zr can indeed substitute Bi in the monoclinic BiVO4 lattice, as shown by XRD analyses, indicating a peak shift toward higher angular values upon Bi(III) substitution by the smaller Zr centers. As regards BiVO4-ZrO2-Fe2O3 samples, they additionally contained homogeneously distributed hematite nanoparticles, whose (104) planes are clearly shown in Fig. 3.11B. Fig.  3.11C illustrates the photoelectrochemical behavior of reference Fe2O3 and BiVO4 photoanodes, along with the best performing composite electrodes. Photocurrents densities at 1.23 V versus RHE for different Zr and Fe contents (Fig. 3.11D) were used to identify the optimal preparative conditions pertaining to the systems reported in Fig. 3.11C. The best performance was obtained for 2.5 mol % Zr and, on the other hand, to 2 mC × cm−2 of Fe charge deposition. In Fig. 3.11C, a remarkable increase in the photocurrent is shown for the optimized BiVO4-ZrO2-Fe2O3 photoanode, which was explained by the cooperative catalytic role of monoclinic ZrO2 and α-Fe2O3 nanoparticles on the BiVO4 surface. Although J values in Fig. 3.11C are lower than previous reports on BiVO4 photoanodes, the enhanced efficiency achieved with Zr and Fe additions and the method simplicity appear to be powerful tools for the engineering of more efficient systems [9].

3.4 Conclusions and future trends The use of an abundant form of energy, sunlight, and a readily available chemical, H2O, in natural photosynthesis, offer an elegant example in order to efficiently harvest and utilize solar light for energy production. To this regard, a PEC system to split water and yield H2 can be considered as an ideal platform where the various concepts regarding different photosynthesis aspects can be tested on a single material. In fact, in semiconductor-based PEC reactions, a photoelectrode is expected to simultaneously perform different functions, i.e., light absorption, photogenerated charge separation, and catalysis of the target process.

Metal oxide electrodes for photo-activated water splitting 39

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Fig. 3.11  High resolution TEM images of: (A) BiVO4-ZrO2 photoanodes (2.5% Zr addition); (B) BiVO4-ZrO2-Fe2O3 electrodes (2.5 mole% Zr addition and Fe electrodeposition, 2 mC × cm−2). BiVO4 photoanodes were fabricated by Bi electrodeposition on FTO substrates from ethylene glycol solutions containing Bi(III) ions, followed by vanadium introduction by drop casting from VO(acac)2 (Hacac = 2,4-pentanedione) and annealing at 500°C for 2 h. Different amounts of Zr2Cl2O were incorporated into the Bi(III) plating bath as the Zr source. (C) J/V curves of the synthesized systems obtained in the dark (dashed lines) and under illumination at 100 mW × cm−2 (solid lines) in phosphate buffer solution (pH = 7.6) for the reference Fe2O3 (A) and BiVO4 (B), along with the best modified BiVO4-based electrodes: (C) BiVO4-ZrO2; (D) BiVO4-Fe2O3; (E) BiVO4-ZrO2-Fe2O3. (D) Photocurrents obtained at 1.23 V vs. RHE for different additions of Zr (blue spheres) and Fe (red triangles) in the phosphate buffer solution. The photocurrent density values for the different Zr and Fe amounts shown in (D) were used to determine the optimum synthetic conditions corresponding to (C). Adapted with permission from M.N. Shaddad, M.A. Ghanem, A.M. Al-Mayouf, S. Gimenez, J. Bisquert, I. Herraiz-Cardona, Cooperative catalytic effect of ZrO2 and α-Fe2O3 nanoparticles on BiVO4 photoanodes for enhanced photoelectrochemical water splitting, ChemSusChem 9 (2016) 2779–2783. Copyright Wiley, 2016.

In this chapter, we have presented various research attempts aimed at improving the performances of PEC devices adopted in sunlight-driven water splitting. The main focus has been given to selected oxide-based photoanodes, namely Fe2O3, WO3, ZnO, and BiVO4. The key factors boosting the performances of such materials have been

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proved to be the possibility of controlling, at the nanoscale level, material spatial organization and interface electronic structure, of great interest for the production of systems and devices with improved efficiency. The development of composites and/or heterostructured photoelectrodes has also been recognized as a key issue to achieve an enhanced system activity in PEC H2O splitting, even from sunlight and seawater, key issues for a sustainable hydrogen generation. Overall, the present findings will help future efforts aimed at developing more efficient photoanodes exceeding the current state-of-the-art performances. Indeed, the complexity and variety of examples reported and discussed herein demonstrate that the choice of adequate fabrication and processing conditions is indeed crucial in order to move the target processes “from the lab to the fab”, i.e., toward functional utilization under real-world conditions. Nevertheless, in spite of various research studies, activities in these fields still have a long way to go in terms of both fundamental and applied research. In fact, the obtained photocurrents (often below 5 mA × cm−2) are far from the industrial requirements of high efficiencies, and driving up the photoelectrode activity still requires a better understanding of the physical and chemical processes that occur at the solid/liquid interface [22]. In this regard, the use of ex-situ pre-treatment of pristine electrodes has been proposed as a successful way to boost their PEC performances in water splitting. A recent example concerns the exposure of WO3 photoanodes, prepared by a spin-coating process, to sustained UV illumination in air [22]. This process has been shown to result in a 30% enhancement of the system photoactivity, an effect traced back to an increase in the corresponding surface area. Whereas illumination did not generate any change in the WO3 onset potential, the application of a similar treatment to BiVO4 photoanodes resulted in a cathodic shift of ≈230 mV, as well as in the obtainment of a less porous surface and in a reduced recombination of photogenerated charge carriers due to the suppression of surface defects [12]. These differences yield useful insights into how UV irradiation affects the properties of semiconductor materials used in PEC applications [22]. Additional freedom is offered by ex-situ plasma treatments to control the density of oxygen vacancies and attain improved photoactivity. Recent attempts in this direction have already been successfully carried out on ultra-thin hematite nanoflakes, obtained by annealing of iron foils in air (Fig.  3.12). The increased number of oxygen vacancies after plasma treatment, resulting in an increased carrier density, was interpreted as the main cause for the registered enhancement of the system PEC activity [122]. Nevertheless, it is worth highlighting that, even after reaching the goal of efficient water photosplitting by stable and low cost materials, only the first part of the problem will be solved. In fact, key open challenges to be solved afterwards include the storage, transportation, and utilization of H2. To circumvent these problems, an amenable option to be pursued will concern the use of solar energy to generate other chemicals different from hydrogen [6,14,64]. The development and implementation of these strategies, which are subjects of current intensive research, may open doors to the development of energy technologies based on unexplored light-powered chemical reactions, of great interest for the production of advanced functional devices that might revolutionize current well-established technologies.

Metal oxide electrodes for photo-activated water splitting 41

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(B)

(C)

(D) 4.5 4.0

J (mA ¥ cm–2)

3.5 3.0

As prepared PM PH Co-Pi/PH

2.5 2.0 1.5 1.0 0.5 0 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

Voltage (V) vs. RHE

Fig. 3.12  Representative SEM micrographs of hematite nanoflakes obtained by annealing iron foils at 400°C: (A) as-prepared; (B) after medium power (PM; 10.5 W) plasma treatment; (C) after high power (PH; 18.0 W) plasma treatment. (D) Photocurrent density versus potential curves for the as-prepared, PM and PH α-Fe2O3 photoelectrodes recorded in 1 M NaOH aqueous solutions under illumination (Xe lamp, 285 mW × cm−2; continuous lines) and in the dark (dashed lines). Adapted with permission from C. Zhu, C. Li, M. Zheng, J.-J. Delaunay, Plasma-induced oxygen vacancies in ultrathin hematite nanoflakes promoting photoelectrochemical water oxidation, ACS Appl. Mater. Interfaces 7 (2015) 22355–22363. Copyright American Chemical Society, 2016.

Acknowledgments The authors kindly acknowledge the financial support under the FP7 project “SOLAROGENIX” (NMP4-SL-2012-310333), as well as Padova University ex-60% 2014-2017, P-DiSC #SENSATIONAL BIRD2016-UNIPD projects and ACTION post-doc fellowship.

References [1] K.  Sivula, F.  Le Formal, M.  Grätzel, Solar water splitting: progress using hematite (α-Fe2O3) photoelectrodes, ChemSusChem 4 (2011) 432–449. [2] X. Feng, Y. Chen, Z. Qin, M. Wang, L. Guo, Facile fabrication of sandwich structured WO3 nanoplate arrays for efficient photoelectrochemical water splitting, ACS Appl. Mater. Interfaces 8 (2016) 18089–18096.

42

Multifunctional Photocatalytic Materials for Energy

[3] L. Yang, H. Zhou, T. Fan, D. Zhang, Semiconductor photocatalysts for water oxidation: current status and challenges, Phys. Chem. Chem. Phys. 16 (2014) 6810–6826. [4] J.W. Sun, D.K. Zhong, D.R. Gamelin, Composite photoanodes for photoelectrochemical solar water splitting, Energy Environ. Sci. 3 (2010) 1252–1261. [5] Z. Li, W. Luo, M. Zhang, J. Feng, Z. Zou, Photoelectrochemical cells for solar hydrogen production: current state of promising photoelectrodes, methods to improve their properties, and outlook, Energy Environ. Sci. 6 (2013) 347–370. [6] N. Armaroli, V. Balzani, Solar electricity and solar fuels: status and perspectives in the context of the energy transition, Chem. Eur. J. 22 (2016) 32–57. [7] M. Grätzel, Photoelectrochemical cells, Nature 414 (2001) 338–344. [8] B. Klahr, S. Gimenez, F. Fabregat-Santiago, T. Hamann, J. Bisquert, Water oxidation at hematite photoelectrodes: the role of surface states, J. Am. Chem. Soc. 134 (2012) 4294–4302. [9] M.N.  Shaddad, M.A.  Ghanem, A.M.  Al-Mayouf, S.  Gimenez, J.  Bisquert, I.  HerraizCardona, Cooperative catalytic effect of ZrO2 and α-Fe2O3 nanoparticles on BiVO4 photoanodes for enhanced photoelectrochemical water splitting, ChemSusChem 9 (2016) 2779–2783. [10] T.H. Jeon, W. Choi, H. Park, Cobalt-phosphate complexes catalyze the photoelectrochemical water oxidation of BiVO4 electrodes, Phys. Chem. Chem. Phys. 13 (2011) 21392–21401. [11] Gurudayal, D.  Jeong, K.  Jin, H.-Y.  Ahn, P.P.  Boix, F.F.  Abdi, et  al., Highly active MnO catalysts integrated onto Fe2O3 nanorods for efficient water splitting, Adv. Mater. Interfaces 3 (2016) 1600176. [12] T. Li, J. He, B. Peña, C.P. Berlinguette, Curing BiVO4 photoanodes with ultraviolet light enhances photoelectrocatalysis, Angew. Chem. Int. Ed. 55 (2016) 1769–1772. [13] M. Sarnowska, K. Bienkowski, P.J. Barczuk, R. Solarska, J. Augustynski, Highly efficient and stable solar water splitting at (Na)WO3 photoanodes in acidic electrolyte assisted by non-noble metal oxygen evolution catalyst, Adv. Energy Mater. 6 (2016) 1600526. [14] M.T. Mayer, Y. Lin, G. Yuan, D. Wang, Forming heterojunctions at the nanoscale for improved photoelectrochemical water splitting by semiconductor materials: case studies on hematite, Acc. Chem. Res. 46 (2013) 1558–1566. [15] J. Quiñonero, T. Lana-Villarreal, R. Gómez, Improving the photoactivity of bismuth vanadate thin film photoanodes through doping and surface modification strategies, Appl. Catal. B 194 (2016) 141–149. [16] C.X. Guo, Y. Dong, H.B. Yang, C.M. Li, Graphene quantum dots as a green sensitizer to functionalize ZnO nanowire arrays on F-doped SnO2 glass for enhanced photoelectrochemical water splitting, Adv. Energy Mater. 3 (2013) 997–1003. [17] S.  Hernández, V.  Cauda, A.  Chiodoni, S.  Dallorto, A.  Sacco, D.  Hidalgo, et  al., Optimization of 1D [email protected] core–shell nanostructures for enhanced photoelectrochemical water splitting under solar light illumination, ACS Appl. Mater. Interfaces 6 (2014) 12153–12167. [18] S.C. Warren, E. Thimsen, Plasmonic solar water splitting, Energy Environ. Sci. 5 (2012) 5133–5146. [19] S.  Hilaire, M.J.  Suess, N.  Kranzlin, K.  Bienkowski, R.  Solarska, J.  Augustynski, M.  Niederberger, Microwave-assisted nonaqueous synthesis of WO3 nanoparticles for crystallographically oriented photoanodes for water splitting, J. Mater. Chem. A 2 (2014) 20530–20537. [20] F.M. Toma, A. Sartorel, M. Iurlo, M. Carraro, P. Parisse, C. Maccato, et al., Efficient water oxidation at carbon nanotube–polyoxometalate electrocatalytic interfaces, Nat. Chem. 2 (2010) 826–831.

Metal oxide electrodes for photo-activated water splitting 43

[21] W. Smith, A. Wolcott, R.C. Fitzmorris, J.Z. Zhang, Y. Zhao, Quasi-core-shell TiO2/WO3 and WO3/TiO2 nanorod arrays fabricated by glancing angle deposition for solar water splitting, J. Mater. Chem. 21 (2011) 10792–10800. [22] T. Li, J. He, B. Peña, C.P. Berlinguette, Exposure of WO3 photoanodes to ultraviolet light enhances photoelectrochemical water oxidation, ACS Appl. Mater. Interfaces 8 (2016) 25010–25013. [23] S.J.A. Moniz, J. Zhu, J. Tang, 1D Co-Pi modified BiVO4/ZnO junction cascade for efficient photoelectrochemical water cleavage, Adv. Energy Mater. 4 (2014) 1301590. [24] D. Eisenberg, H.S. Ahn, A.J. Bard, Enhanced photoelectrochemical water oxidation on bismuth vanadate by electrodeposition of amorphous titanium dioxide, J. Am. Chem. Soc. 136 (2014) 14011–14014. [25] H. Chen, Z. Wei, K. Yan, Y. Bai, Z. Zhu, T. Zhang, S. Yang, Epitaxial growth of ZnO nanodisks with large exposed polar facets on nanowire arrays for promoting photoelectrochemical water splitting, Small 10 (2014) 4760–4769. [26] C. Zhang, M. Shao, F. Ning, S. Xu, Z. Li, M. Wei, et al., Au nanoparticles sensitized ZnO [email protected] core–shell arrays for enhanced photoelectrochemical water splitting, Nano Energy 12 (2015) 231–239. [27] C. Jiang, S.J.A. Moniz, M. Khraisheh, J. Tang, Earth-abundant oxygen evolution catalysts coupled onto ZnO nanowire arrays for efficient photoelectrochemical water cleavage, Chem. Eur. J. 20 (2014) 12954–12961. [28] Y. Wei, L. Ke, J. Kong, H. Liu, Z. Jiao, X. Lu, H. Du, X.W. Sun, Enhanced photoelectrochemical water-splitting effect with a bent ZnO nanorod photoanode decorated with Ag nanoparticles, Nanotechnology 23 (2012) 235401. [29] D.  Barreca, G.  Carraro, A.  Gasparotto, C.  Maccato, M.E.A.  Warwick, K.  Kaunisto, et  al., Fe2O3–TiO2 nano-heterostructure photoanodes for highly efficient solar water ­oxidation, Adv. Mater. Interfaces 2 (2015) 1500313. [30] W.  Luo, Z.  Yang, Z.  Li, J.  Zhang, J.  Liu, Z.  Zhao, Z.  Wang, S.  Yan, T.  Yu, Z.  Zou, Solar hydrogen generation from seawater with a modified BiVO4 photoanode, Energy Environ. Sci. 4 (2011) 4046–4051. [31] R. Liu, Z. Zheng, J. Spurgeon, X. Yang, Enhanced photoelectrochemical water-splitting performance of semiconductors by surface passivation layers, Energy Environ. Sci. 7 (2014) 2504. [32] X.  Chen, Z.  Zhang, L.  Chi, A.K.  Nair, W.  Shangguan, Z.  Jiang, Recent advances in ­visible-light-driven photoelectrochemical water splitting: catalyst nanostructures and reaction systems, Nano-Micro Lett. 8 (2016) 1–12. [33] T. Zhu, M.N. Chong, E.S. Chan, Nanostructured tungsten trioxide thin films synthesized for photoelectrocatalytic water oxidation: a review, ChemSusChem 7 (2014) 2974–2997. [34] M. Wu, W.-J. Chen, Y.-H. Shen, F.-Z. Huang, C.-H. Li, S.-K. Li, In situ growth of matchlike ZnO/Au plasmonic heterostructure for enhanced photoelectrochemical water splitting, ACS Appl. Mater. Interfaces 6 (2014) 15052–15060. [35] T.A. Pham, Y. Ping, G. Galli, Modelling heterogeneous interfaces for solar water splitting, Nat. Mater. 16 (2017) 401–408. [36] Q. Lu,Y. Yu, Q. Ma, B. Chen, H. Zhang, 2D transition-metal-dichalcogenide-nanosheet-based composites for photocatalytic and electrocatalytic hydrogen evolution reactions, Adv. Mater. 28 (2016) 1917–1933. [37] D.  Barreca, G.  Carraro, V.  Gombac, A.  Gasparotto, C.  Maccato, P.  Fornasiero, et  al., Supported metal oxide nanosystems for hydrogen photogeneration: quo vadis? Adv. Funct. Mater. 21 (2011) 2611–2623.

44

Multifunctional Photocatalytic Materials for Energy

[38] M.  Cargnello, A.  Gasparotto, V.  Gombac, T.  Montini, D.  Barreca, P.  Fornasiero, Photocatalytic H2 and added-value by-products – the role of metal oxide systems in their synthesis from oxygenates, Eur. J. Inorg. Chem. 2011 (2011) 4309–4323. [39] J.B. Baxter, C. Richter, C.A. Schmuttenmaer, Ultrafast carrier dynamics in nanostructures for solar fuels, Annu. Rev. Phys. Chem. 65 (2014) 423–447. [40] L. Meda, L. Abbondanza, Materials for photo-electrochemical water splitting, Rev. Adv. Sci. Eng. 2 (2013) 200–207. [41] J. Augustynski, K. Bienkowski, R. Solarska, Plasmon resonance-enhanced photoelectrodes and photocatalysts, Coord. Chem. Rev. 325 (2016) 116–124. [42] S.J.A. Moniz, S.A. Shevlin, D.J. Martin, Z.-X. Guo, J. Tang, Visible-light driven heterojunction photocatalysts for water splitting – a critical review, Energy Environ. Sci. 8 (2015) 731–759. [43] M.G. Mali, H. Yoon, M.-w. Kim, M.T. Swihart, S.S. Al-Deyab, S.S. Yoon, Electrosprayed heterojunction WO3/BiVO4 films with nanotextured pillar structure for enhanced photoelectrochemical water splitting, Appl. Phys. Lett. 106 (2015) 151603. [44] R. Solarska, K. Bienkowski, S. Zoladek, A. Majcher, T. Stefaniuk, P.J. Kulesza, et al., Enhanced water splitting at thin film tungsten trioxide photoanodes bearing plasmonic gold–polyoxometalate particles, Angew. Chem. Int. Ed. 53 (2014) 14196–14200. [45] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [46] H.  Han, F.  Riboni, F.  Karlicky, S.  Kment, A.  Goswami, P.  Sudhagar, et  al., α-Fe2O3/ TiO2 3D hierarchical nanostructures for enhanced photoelectrochemical water splitting, Nanoscale 9 (2017) 134–142. [47] M. Chakraborty, D. Roy, A. Biswas, R. Thangavel, G. Udayabhanu, Structural, optical and photo-electrochemical properties of hydrothermally grown ZnO nanorods arrays covered with α-Fe2O3 nanoparticles, RSC Adv. 6 (2016) 75063–75072. [48] S. Palmas, P.A. Castresana, L. Mais, A. Vacca, M. Mascia, P.C. Ricci, TiO2-WO3 nanostructured systems for photoelectrochemical applications, RSC Adv. 6 (2016) 101671–101682. [49] A. Sheikh, A. Yengantiwar, M. Deo, S. Kelkar, S. Ogale, Near-field plasmonic functionalization of light harvesting oxide–oxide heterojunctions for efficient solar photoelectrochemical water splitting: the AuNP/ZnFe2O4/ZnO system, Small 9 (2013) 2091–2096. [50] Q. Liu, F. Cao, F. Wu, W. Tian, L. Li, Interface reacted ZnFe2O4 on α-Fe2O3 nanoarrays for largely improved photoelectrochemical activity, RSC Adv. 5 (2015) 79440–79446. [51] Y.  Guo, Y.  Fu, Y.  Liu, S.  Shen, Photoelectrochemical activity of ZnFe2O4 modified α-Fe2O3 nanorod array films, RSC Adv. 4 (2014) 36967–36972. [52] C. Chen, H. Bai, Z. Da, M. Li, X. Yan, J. Jiang, et al., Hydrothermal synthesis of Fe2O3/ ZnO heterojunction photoanode for photoelectrochemical water splitting, Funct. Mater. Lett. 08 (2015) 1550058. [53] C. Zhang, W. Fan, H. Bai, X. Yu, C. Chen, R. Zhang, et al., Sandwich-nanostructured NiO– ZnO nanowires@α-Fe2O3 film photoanode with a synergistic effect and p–n junction for efficient photoelectrochemical water splitting, ChemElectroChem 1 (2014) 2089–2097. [54] Y. Liu, H. He, J. Li, W. Li, Y. Yang, Y. Li, et al., ZnO nanoparticle-functionalized WO3 plates with enhanced photoelectrochemical properties, RSC Adv. 5 (2015) 46928–46934. [55] R. Solarska, K. Bieńkowski, A. Królikowska, M. Dolata, J. Augustyński, Nanoporous WO3–Fe2O3 films; structural and photo-electrochemical characterization, Funct. Mater. Lett. 07 (2014) 1440006. [56] R.  Solarska, R.  Jurczakowski, J.  Augustynski, A highly stable, efficient visible-light driven water photoelectrolysis system using a nanocrystalline WO3 photoanode and a methane sulfonic acid electrolyte, Nanoscale 4 (2012) 1553–1556.

Metal oxide electrodes for photo-activated water splitting 45

[57] H.  Ishihara, G.K.  Kannarpady, K.R.  Khedir, J.  Woo, S.  Trigwell, A.S.  Biris, A novel tungsten trioxide (WO3)/ITO porous nanocomposite for enhanced photo-catalytic water splitting, Phys. Chem. Chem. Phys. 13 (2011) 19553–19560. [58] W. Chen, Y. Qiu, S. Yang, Branched ZnO nanostructures as building blocks of photoelectrodes for efficient solar energy conversion, Phys. Chem. Chem. Phys. 14 (2012) 10872–10881. [59] D.K. Bora, A. Braun, Solution processed transparent nanoparticulate ZnO thin film electrode for photoelectrochemical water oxidation, RSC Adv. 4 (2014) 23562–23570. [60] Y. Mao, Y. Cheng, J. Wang, H. Yang, M. Li, J. Chen, et al., Amorphous NiO electrocatalyst overcoated ZnO nanorod photoanodes for enhanced photoelectrochemical performance, New J. Chem. 40 (2016) 107–112. [61] M. Shao, F. Ning, M. Wei, D.G. Evans, X. Duan, Hierarchical nanowire arrays based on ZnO core−layered double hydroxide shell for largely enhanced photoelectrochemical water splitting, Adv. Funct. Mater. 24 (2014) 580–586. [62] Y. Mao, H. Yang, J. Chen, J. Chen, Y. Tong, X. Wang, Significant performance enhancement of ZnO photoanodes from Ni(OH)2 electrocatalyst nanosheets overcoating, Nano Energy 6 (2014) 10–18. [63] C.X.  Kronawitter, L.  Vayssieres, S.  Shen, L.  Guo, D.A.  Wheeler, J.Z.  Zhang, et  al., A perspective on solar-driven water splitting with all-oxide hetero-nanostructures, Energy Environ. Sci. 4 (2011) 3889–3899. [64] J.H. Montoya, L.C. Seitz, P. Chakthranont, A. Vojvodic, T.F. Jaramillo, J.K. Norskov, Materials for solar fuels and chemicals, Nat. Mater. 16 (2017) 70–81. [65] S. Hernández, G. Gerardi, K. Bejtka, A. Fina, N. Russo, Evaluation of the charge transfer kinetics of spin-coated BiVO4 thin films for sun-driven water photoelectrolysis, Appl. Catal. B 190 (2016) 66–74. [66] S. Linic, P. Christopher, D.B. Ingram, Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy, Nat. Mater. 10 (2011) 911–921. [67] G.  Carraro, C.  Maccato, A.  Gasparotto, T.  Montini, S.  Turner, O.I.  Lebedev, et  al., Enhanced hydrogen production by photoreforming of renewable oxygenates through nanostructured Fe2O3 polymorphs, Adv. Funct. Mater. 24 (2014) 372–378. [68] S. Xie, Q. Zhang, G. Liu, Y. Wang, Photocatalytic and photoelectrocatalytic reduction of CO2 using heterogeneous catalysts with controlled nanostructures, Chem. Commun. 52 (2016) 35–59. [69] K.  Sivula, R.  Zboril, F. Le  Formal, R.  Robert, A.  Weidenkaff, J.  Tucek, et  al., Photoelectrochemical water splitting with mesoporous hematite prepared by a solution-­ based colloidal approach, J. Am. Chem. Soc. 132 (2010) 7436–7444. [70] M. Cornuz, M. Grätzel, K. Sivula, Preferential orientation in hematite films for solar hydrogen production via water splitting, Chem. Vap. Deposition 16 (2010) 291–295. [71] X.  Yang, R.  Liu, C.  Du, P.  Dai, Z.  Zheng, D.  Wang, Improving hematite-based photoelectrochemical water splitting with ultrathin TiO2 by atomic layer deposition, ACS Appl. Mater. Interfaces 6 (2014) 12005–12011. [72] M.E.A.  Warwick, K.  Kaunisto, D.  Barreca, G.  Carraro, A.  Gasparotto, C.  Maccato, et al., Vapor phase processing of α-Fe2O3 photoelectrodes for water splitting: an insight into the structure/property interplay, ACS Appl. Mater. Interfaces 7 (2015) 8667–8676. [73] M. Marelli, A. Naldoni, A. Minguzzi, M. Allieta, T. Virgili, G. Scavia, et al., Hierarchical hematite nanoplatelets for photoelectrochemical water splitting, ACS Appl. Mater. Interfaces 6 (2014) 11997–12004. [74] W.-H. Hung, T.-M. Chien, A.-Y. Lo, C.-M. Tseng, D. Li, Spatially controllable plasmon enhanced water splitting photocurrent in Au/TiO2-Fe2O3 cocatalyst system, RSC Adv. 4 (2014) 45710–45714.

46

Multifunctional Photocatalytic Materials for Energy

[75] D.P. Cao, W.J. Luo, J.Y. Feng, X. Zhao, Z.S. Li, Z.G. Zou, Cathodic shift of onset potential for water oxidation on a Ti4+ doped Fe2O3 photoanode by suppressing the back reaction, Energy Environ. Sci. 7 (2014) 752–759. [76] F.  Le Formal, N.  Tétreault, M.  Cornuz, T.  Moehl, M.  Grätzel, K.  Sivula, Passivating surface states on water splitting hematite photoanodes with alumina overlayers, Chem. Sci. 2 (2011) 737–743. [77] L. Steier, I. Herraiz-Cardona, S. Gimenez, F. Fabregat-Santiago, J. Bisquert, S.D. Tilley, et al., Understanding the role of underlayers and overlayers in thin film hematite photoanodes, Adv. Funct. Mater. 24 (2014) 7681–7688. [78] L. Xi, S.Y. Chiam, W.F. Mak, P.D. Tran, J. Barber, S.C.J. Loo, et al., A novel strategy for surface treatment on hematite photoanode for efficient water oxidation, Chem. Sci. 4 (2013) 164–169. [79] M.F. Gromboni, D. Coelho, L.H. Mascaro, A. Pockett, F. Marken, Enhancing activity in a nanostructured BiVO4 photoanode with a coating of microporous Al2O3, Appl. Catal. B 200 (2017) 133–140. [80] J.Y.  Kim, G.  Magesh, D.H.  Youn, J.-W.  Jang, J.  Kubota, K.  Domen, J.S.  Lee, Singlecrystalline, wormlike hematite photoanodes for efficient solar water splitting, Sci. Rep. 3 (2013) 2681. [81] D. Barreca, G. Carraro, A. Gasparotto, C. Maccato, F. Rossi, G. Salviati, et al., Surface functionalization of nanostructured Fe2O3 polymorphs: from design to light-activated applications, ACS Appl. Mater. Interfaces 5 (2013) 7130–7138. [82] S.  Hu, M.R.  Shaner, J.A.  Beardslee, M.  Lichterman, B.S.  Brunschwig, N.S.  Lewis, Amorphous TiO2 coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation, Science 344 (2014) 1005–1009. [83] S.M. Ji, H. Jun, J.S. Jang, H.C. Son, P.H. Borse, J.S. Lee, Photocatalytic hydrogen production from natural seawater, J. Photochem. Photobiol. A 189 (2007) 141–144. [84] H. Joo, S. Bae, C. Kim, S. Kim, J. Yoon, Hydrogen evolution in enzymatic photoelectrochemical cell using modified seawater electrolytes produced by membrane desalination process, Sol. Energy Mater. Sol. Cells 93 (2009) 1555–1561. [85] M.A. Butler, R.D. Nasby, R.K. Quinn, Tungsten trioxide as an electrode for photoelectrolysis of water, Solid State Commun. 19 (1976) 1011–1014. [86] G. Hodes, D. Cahen, J. Manassen, Tungsten trioxide as a photoanode for a photoelectrochemical cell (PEC), Nature 260 (1976) 312–313. [87] P.M. Rao, L. Cai, C. Liu, I.S. Cho, C.H. Lee, J.M. Weisse, et al., Simultaneously efficient light absorption and charge separation in WO3/BiVO4 core/shell nanowire photoanode for photoelectrochemical water oxidation, Nano Lett. 14 (2014) 1099–1105. [88] X. Shi, I.Y. Choi, K. Zhang, J. Kwon, D.Y. Kim, J.K. Lee, et al., Efficient photoelectrochemical hydrogen production from bismuth vanadate-decorated tungsten trioxide helix nanostructures, Nat. Commun. 5 (2014) 4775. [89] N.R. de Tacconi, C.R. Chenthamarakshan, K. Rajeshwar, T. Pauporté, D. Lincot, Pulsed electrodeposition of WO3–TiO2 composite films, Electrochem. Commun. 5 (2003) 220–224. [90] S. Wei, Y. Ma, Y. Chen, L. Liu, Y. Liu, Z. Shao, Fabrication of WO3/Cu2O composite films and their photocatalytic activity, J. Hazard. Mater. 194 (2011) 243–249. [91] S. Mubeen, J. Lee, N. Singh, S. Kramer, G.D. Stucky, M. Moskovits, An autonomous photosynthetic device in which all charge carriers derive from surface plasmons, Nat. Nanotechnol. 8 (2013) 247–251. [92] E. Thimsen, F. Le Formal, M. Grätzel, S.C. Warren, Influence of plasmonic Au nanoparticles on the photoactivity of Fe2O3 electrodes for water splitting, Nano Lett. 11 (2011) 35–43.

Metal oxide electrodes for photo-activated water splitting 47

[93] Z.J. Lin, X.H. Wang, J. Liu, Z.Y. Tian, L.C. Dai, B.B. He, et al., On the role of localized surface plasmon resonance in UV-vis light irradiated Au/TiO2 photocatalysis systems: pros and cons, Nanoscale 7 (2015) 4114–4123. [94] X.D. Wang, G.I.N. Waterhouse, D.R.G. Mitchell, K. Prince, R.A. Caruso, Noble metal-­ modified porous titania networks and their application as photocatalysts, ChemCatChem 3 (2011) 1763–1771. [95] Y. Li, H. Yu, C. Zhang, L. Fu, G. Li, Z. Shao, et al., Enhancement of photoelectrochemical response by Au modified in TiO2 nanorods, Int. J. Hydrogen Energy 38 (2013) 13023–13030. [96] L.  Wang, X.  Zhou, N.T.  Nguyen, P.  Schmuki, Plasmon-enhanced photoelectrochemical water splitting using Au nanoparticles decorated on hematite nanoflake arrays, ChemSusChem 8 (2015) 618–622. [97] R. Solarska, A. Królikowska, J. Augustyński, Silver nanoparticle induced photocurrent enhancement at WO3 photoanodes, Angew. Chem. Int. Ed. 49 (2010) 7980–7983. [98] D.M.  Fragua, R.  Abargues, P.J.  Rodriguez-Canto, J.F.  Sanchez-Royo, S.  Agouram, J.P. Martinez-Pastor, Au–ZnO nanocomposite films for plasmonic photocatalysis, Adv. Mater. Interfaces 2 (2015) 1500156. [99] R. Lv, T. Wang, F. Su, P. Zhang, C. Li, J. Gong, Facile synthesis of ZnO nanopencil arrays for photoelectrochemical water splitting, Nano Energy 7 (2014) 143–150. [100] Y. Qiu, K. Yan, H. Deng, S. Yang, Secondary branching and nitrogen doping of ZnO nanotetrapods: building a highly active network for photoelectrochemical water splitting, Nano Lett. 12 (2012) 407–413. [101] N. Chouhan, C.L. Yeh, S.F. Hu, J.H. Huang, C.W. Tsai, R.S. Liu, et al., Array of CdSe QD-sensitized ZnO nanorods serves as photoanode for water splitting, J. Electrochem. Soc. 157 (2010) B1430–B1433. [102] Y. Liu, L. Sun, J. Wu, T. Fang, R. Cai, A. Wei, Preparation and photocatalytic activity of ZnO/Fe2O3 nanotube composites, Mater. Sci. Eng. B 194 (2015) 9–13. [103] S.-M. Lam, J.-C. Sin, A.Z.  Abdullah, A.R. Mohamed, Sunlight responsive WO3/ZnO nanorods for photocatalytic degradation and mineralization of chlorinated phenoxyacetic acid herbicides in water, J. Colloid Interface Sci. 450 (2015) 34–44. [104] S.-M. Lam, J.-C. Sin, A.Z. Abdullah, A.R. Mohamed, ZnO nanorods surface-decorated by WO3 nanoparticles for photocatalytic degradation of endocrine disruptors under a compact fluorescent lamp, Ceram. Int. 39 (2013) 2343–2352. [105] J. Xie, Z. Zhou, Y. Lian, Y. Hao, X. Liu, M. Li, et al., Simple preparation of WO3–ZnO composites with UV–vis photocatalytic activity and energy storage ability, Ceram. Int. 40 (2014) 12519–12524. [106] F. Zheng, H. Lu, M. Guo, M. Zhang, Q. Zhen, Hydrothermal preparation of WO3 nanorod array and ZnO nanosheet array composite structures on FTO substrates with enhanced photocatalytic properties, J. Mater. Chem. C 3 (2015) 7612–7620. [107] M.  Wang, C.  Chen, H.  Qin, L.  Zhang, Y.  Fang, J.  Liu, et  al., Construction of FeS2sensitized [email protected] nanorod arrays with enhanced optical and photoresponse performances, Adv. Mater. Interfaces 2 (2015) 1500163. [108] G.K.  Pradhan, S.  Martha, K.M.  Parida, Synthesis of multifunctional nanostructured zinc–iron mixed oxide photocatalyst by a simple solution-combustion technique, ACS Appl. Mater. Interfaces 4 (2012) 707–713. [109] A.  Thomas, C.  Janáky, G.F.  Samu, M.N.  Huda, P.  Sarker, J.P.  Liu, et  al., Time- and energy-­efficient solution combustion synthesis of binary metal tungstate nanoparticles with enhanced photocatalytic activity, ChemSusChem 8 (2015) 1652–1663. [110] L. Cai, F. Ren, M. Wang, G. Cai, Y. Chen, Y. Liu, et al., V ions implanted ZnO nanorod arrays for photoelectrochemical water splitting under visible light, Int. J. Hydrogen Energy 40 (2015) 1394–1401.

48

Multifunctional Photocatalytic Materials for Energy

[111] M. Wang, F. Ren, J. Zhou, G. Cai, L. Cai, Y. Hu, et al., N doping to ZnO nanorods for photoelectrochemical water splitting under visible light: engineered impurity distribution and terraced band structure, Sci. Rep. 5 (2015) 12925. [112] L. Yan, W. Zhao, Z. Liu, 1D ZnO/BiVO4 heterojunction photoanodes for efficient photoelectrochemical water splitting, Dalton Trans. 45 (2016) 11346–11352. [113] C.K. Chen, Y.-P. Shen, H.M. Chen, C.-J. Chen, T.-S. Chan, J.-F. Lee, et al., Quantumdot-sensitized nitrogen-doped ZnO for efficient photoelectrochemical water splitting, Eur. J. Inorg. Chem. 2014 (2014) 773–779. [114] S.-M.  Lam, J.-C.  Sin, A.Z.  Abdullah, A.R.  Mohamed, Transition metal oxide loaded ZnO nanorods: preparation, characterization and their UV–vis photocatalytic activities, Sep. Purif. Technol. 132 (2014) 378–387. [115] C.-H. Hsu, D.-H. Chen, Photoresponse and stability improvement of ZnO nanorod array thin film as a single layer of photoelectrode for photoelectrochemical water splitting, Int. J. Hydrogen Energy 36 (2011) 15538–15547. [116] H. Kim, M. Seol, J. Lee, K. Yong, Highly efficient photoelectrochemical hydrogen generation using hierarchical ZnO/WOx nanowires cosensitized with CdSe/CdS, J. Phys. Chem. C 115 (2011) 25429–25436. [117] Y. Li, X. Zhang, S. Jiang, H. Dai, X. Sun, Y. Li, Improved photoelectrochemical property of a nanocomposite NiO/[email protected] photoanode for water splitting, Sol. Energy Mater. Sol. Cells 132 (2015) 40–46. [118] C. Li, X. Zhu, H. Zhang, Z. Zhu, B. Liu, C. Cheng, 3D ZnO/Au/CdS sandwich structured inverse opal as photoelectrochemical anode with improved performance, Adv. Mater. Interfaces 2 (2015) 1500428. [119] H.W. Jeong, T.H. Jeon, J.S. Jang, W. Choi, H. Park, Strategic modification of BiVO4 for improving photoelectrochemical water oxidation performance, J. Phys. Chem. C 117 (2013) 9104–9112. [120] H.S. Park, K.E. Kweon, H. Ye, E. Paek, G.S. Hwang, A.J. Bard, Factors in the metal doping of BiVO4 for improved photoelectrocatalytic activity as studied by scanning electrochemical microscopy and first-principles density-functional calculation, J. Phys. Chem. C 115 (2011) 17870–17879. [121] S.K. Cho, H.S. Park, H.C. Lee, K.M. Nam, A.J. Bard, Metal doping of BiVO4 by composite electrodeposition with improved photoelectrochemical water oxidation, J. Phys. Chem. C 117 (2013) 23048–23056. [122] C.  Zhu, C.  Li, M.  Zheng, J.-J.  Delaunay, Plasma-induced oxygen vacancies in ultrathin hematite nanoflakes promoting photoelectrochemical water oxidation, ACS Appl. Mater. Interfaces 7 (2015) 22355–22363.

Energy band engineering of metal oxide for enhanced visible light absorption

4

Jiangtian Li, Deryn Chu US Army Research Laboratory, Sensor and Electronics Device Directorate, Adelphi, MD, United States

4.1 Introduction Since the 1970s, when it was discovered that TiO2 could split water and reduce CO2 [1,2], the pursuit has continued to produce solar fuels via renewable sunlight, by mimicking photosynthesis. However, doing so remains one of the major scientific challenges. This process requires both efficient light absorption and effective charge carrier transfer for chemical reactions. For commercial applications, long-term stability is also a prerequisite. Many catalysts have been reported for this exciting process [3–6]. In practice, metal oxide semiconductors are the most abundant ones in nature, and they are more stable in a variety of harsh conditions when used as photocatalysts [7–12]. Regarding the energetic criteria, only wide band gap semiconductors (e.g., TiO2 and SrTiO3) are thermodynamically able to drive water splitting without applied external bias. However, the wide band gap of such oxides limits their light absorption within the ultraviolet region. Some oxides, such as Fe2O3 (Eg = 2.0 eV), have advantages for absorbing visible light, but suffer from high electron affinities and poor charge carrier mobility and diffusion [13–15]. The major challenge facing metal oxide semiconductors therefore is the balance between light absorption and charge carrier transfer. Both of them depend on the electronic energy band structure and are coupled together to determine the ultimate solar conversion efficiency. Here we summarize the commonly used strategies in this field, with a focus on engineering the electronic energy band to improve visible light absorption. In Section 4.2, we look at the basic concepts of the electronic structure of semiconductors. In Section 4.3, we present the photo-excitation process for photocatalytic reaction in semiconductors and describe applications of solar fuel generation by water splitting and CO2 reduction. In Section 4.4, we highlight benchmark metal oxide semiconductors in terms of electronic band structure, such as TiO2, Fe2O3, BiVO4, and Cu-based p-type oxides. In Section 4.5, we address the recent efforts in electronic modification/engineering of metal oxides for enhanced light absorption. Several representative examples are underlined, including doping, alloying, heterojunction, plasmonic photosensitization, and multijunction systems. Some concluding remarks and future research directions are recommended at the end.

Multifunctional Photocatalytic Materials for Energy. https://doi.org/10.1016/B978-0-08-101977-1.00005-3 Copyright © 2018 Elsevier Ltd. All rights reserved.

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Multifunctional Photocatalytic Materials for Energy

4.2 Electronic energy band structure of semiconductors A thorough exploration of this topic involves principles of solid physics and quantum mechanics that are beyond the scope of this chapter. Readers are strongly encouraged to refer to professional books in these fields. The objective of this section is to briefly present basic information on the electronic energy band structure of a semiconductor.

4.2.1 Electronic energy band of semiconductors A solid consists of a very large number of atoms (N) that are bonded in an ordered atomic arrangement. At a relatively large separation distance, atoms are independent from each other, and each atom exhibits a discrete atomic energy level (Fig. 4.1) [3]. If the atoms are placed in very close proximity, the outer electrons start to perturb each other. As a result, each distinct atomic energy level will split into a series of closely spaced electronic levels. The space interval depends on the number N. When N becomes very large, these energy levels are restricted within a very limited energy width, which could be considered as a continuum of energy state, that is, an electronic energy band [16,17]. Based on the energy states' distribution, there are three distinguished energy regions for each semiconductor: the conduction band (CB), the valence band (VB), and the forbidden band. No energy states exist in the forbidden band. The VB is completely filled, whereas the CB is completely empty. The energy width between

Vacuum

Energy

Atomic orbital N=1

Molecule

Cluster

N=2

N = 10

Q-Size particle N = 2000

Semiconductor N >> 2000

LUMO

CB DE

DE

DE

DE VB

HOMO

Fig. 4.1  Changes in the electronic structure of a semiconductor as the number N of monomeric units increases from unity to clusters of more than 2000 units. CB, conduction band; VB, valence band. Reproduced with permission from A. Mills, S.L. Hunte, An overview of semiconductor photocatalysis, J. Photochem. Photobiol. A: Chem. 108 (1997) 1–35. Copyright © Elsevier.

Energy band engineering of metal oxide for enhanced visible light absorption51

the maximum energy level of the VB and the minimum level of the CB is called the band gap (Eg) of a semiconductor, which is equal to the forbidden band. Electrons cannot move along the filled VB, and there are no intrinsic electrons to move in the CB [16]. An external energy must be applied to promote an electron from the VB to the CB, which depends on and varies with semiconductors, and in reverse determines the physical and chemical properties of a semiconductor, such as electrical, optical, magnetic, among others.

4.2.2 Light absorption of a semiconductor In this section, we start with the relationship between the light absorption and the electronic energy band structure, and thereby highlight the optoelectronic properties of semiconductors which are highly related to their photocatalytic process and performances. As we previously noted, there is a forbidden energy region in which energy states cannot exist. To move an electron from the VB to the CB, an extra energy must be provided, such as thermal or light energy. The light absorption of a semiconductor is therefore determined by its band gap. Only photons with energy greater than or equal to the band gap can excite the semiconductor and generate the hole-electron pairs. The absorption cutting edge can be calculated by the photon energy Eg =

hc 1240 » l l

(4.1)

where Eg is the band gap (eV) of the semiconductor, h is the Planck's constant 4.13566751691 × 10−15 eV s, c is the speed of light 299.79 m s−1, and λ is the wavelength (nm). To enable the visible light absorption >400 nm, the maximum Eg for a semiconductor is around 3.1 eV. The light absorption coefficient is a property of a material that defines the amount of light absorbed by it. The inverse of the absorption coefficient, α−1, is the average distance traveled by a photon before it gets absorbed, that is, the penetration depth. The relationship between absorption coefficient and light intensity can be described by the Lambert-Baer Law, as [17] I ( x ) = I 0 e -a i x

(4.2)

where α is the absorption coefficient, I and I0 are the transmitted and the incident light intensities, respectively, and x is the distance to the surface. Therefore the light intensity decays exponentially when passing through the semiconductor with the rule

a=

1 I0 ln d I

(4.3)

where d is the thickness of the sample. The wavelength-dependent value of α determines how far the light enters the semiconductor [18–20]. Here we distinguish between direct and indirect band gap semiconductors, which are the result of different electronic energy band structures. The apparent difference

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Multifunctional Photocatalytic Materials for Energy

CB Absorption coefficient/α

E

Phonon transition

In

di

re

ct

Direct

CB

VB

Direct

In-direct

k 0.0

0.1

(B)

(A)

0.2

0.3

0.4

0.5

(Eph–Eg)/eV

Fig. 4.2  (A) Band energy level alignment and optical transitions, and (B) wavelengthdependent light absorption coefficient α for direct and indirect band gap semiconductors, respectively.

between them is the alignment of VB/CB in the Brillouin zone, as shown in Fig. 4.2A. If the VB and the CB align with the same K-vector, that is called direct band gap semiconductor. Otherwise, it is an indirect band gap semiconductor. In the former, an electron can be directly excited to the CB and then decay by directly emitting a photon. Whereas in the latter, such excitation and emission processes must pass through an intermediate state and transfer momentum to the crystal lattice, which requires the assistance of a phonon transition [19–21]. Such a difference in electronic structure leads to completely different light absorption for direct and indirect band gap semiconductors (Fig. 4.2B). For a direct band gap transition, the light absorption coefficient is given as

a ~ ( Eph - Eg )

1/ 2

(4.4)

whereas for an indirect band gap transitions, it is given as

a ~ ( Eph - Eg )

2

(4.5)

where Eg is the band gap, and Eph is the photon energy. For a comparison, as shown in Fig.  4.2B, the absorption coefficient is considerably smaller for indirect band gap semiconductors. As a result, light with a photon energy close to the band gap can penetrate much farther before being absorbed in an indirect band gap semiconductor than in a direct band gap one. Based on the light intensity decay trend (Eq. 4.3), an indirect band gap semiconductor needs a much thicker film to absorb more light than a direct band gap one, which theoretically can absorb most of the light within a small range beneath the surface [19,22]. Consequently, the charge carriers generated deep in an indirect semiconductor with a short charge diffusion length have a high risk of recombining before they reach the surface [6]. This fact is very important in designing solar energy devices like photovoltaics and photoelectrochemical solar cells.

Energy band engineering of metal oxide for enhanced visible light absorption53

4.2.3 Excitation and recombination of charge carriers The charge carriers generated in a semiconductor upon light illumination can be extracted for further applications, that is for “optoelectronics.” Various electronic ­transitions are possible upon light excitation because semiconductors' crystals are not perfect and they contain various intrinsic defects or are manually doped with impurities for special purposes (Fig. 4.3) [17–23]. In addition to the band-band transitions (a), an excitation of an electron from a donor state or an impurity level (b) into the CB (or from the VB to a acceptor band, (c) is feasible. If the impurity concentration is very small, the absorption cross section and the corresponding absorption coefficient will be smaller by many orders of magnitude than those for a band-band transition. The excited charge carriers are apt to relax back to their equilibrium states. This is the recombination of charge carriers. There are three popular recombination mechanisms (Fig. 4.3): radiative recombination (d), trap recombination (e), and Auger recombination (f), respectively [19–23]. Radiative recombination is the reverse process of the band-band absorption, where the photogenerated electron drops back to its empty equilibrium energy band and at the same time radiates a photon. The photon emitted may have the energy of the band gap or less. This recombination occurs primarily in direct band gap semiconductors. The trap recombination, also called Shockley–Read–Hall recombination, occurs when an electron falls into an internal band trap state caused by an alien doping or a structural defect. This type of recombination always takes place when the defect level lies near the middle of the forbidden band. Therefore impurities that introduce energy levels near midgap (deep-level) are very effective recombination centers. This also happens on the surface, where there is an abundance of defects that introduce trapping states. Therefore trap recombination by defect levels contributes significantly to charge carriers' loss at surfaces. Auger recombination is similar to radiative recombination, but the excess energy given off by the electron is transferred to a second electron instead of just emitting the energy as a photon. Photocatalysis takes place if the photogenerated charge carriers separate and survive to the surface for chemical reactions without being recombined, which will be discussed in the following sections.

CB EA a

b

c

d

e

f

ED VB

Fig. 4.3  Excitation and recombination of electrons in a semiconductor. a: band-band absorption, b and c: intraband absorptions, d:radiative emission, e: trap recombination, and f: Auger recombination.

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Multifunctional Photocatalytic Materials for Energy

4.3 Principle of photocatalysis for solar fuel generation 4.3.1 Basic principle of photocatalysis A detailed explanation of photocatalysis can be found in many excellent published books and papers [24–30]. Briefly, semiconductor photocatalysis drives heterogeneous photochemical catalytic reactions on the surface of a solid-state semiconductor. A complete photocatalysis process is shown in Fig. 4.4. There are two basic configurations for photocatalytic reactions: the particulate system (a) and the photoelectrochemical (PEC) system (b) [6]. They share the same basic principles. When a semiconductor absorbs photons with energy equal to or greater than its band gap, electrons will be excited from the VB to the CB, leaving holes in the VB. Then the electrons and holes migrate to the semiconductor/electrolyte interface for chemical reactions (electron for reduction and hole for oxidation). This is, however, an ideal process. Many competing processes consume the charge carriers, as discussed in Section  4.2.3. The possible photoinduced chemical and physical processes involved inside and on the surface of a semiconductor photocatalyst are shown in Fig. 4.4, including several critical steps: (i) light absorption, (ii) charge separation, (iii) charge migration, (iv) charge recombination, and (v) redox reactions [6,29,30]. The overall solar energy conversion efficiency is determined by three major processes: (i) light harvesting, (ii–iv) transport of photogenerated carriers, and (v) carrier injection at semiconductor’s surface. Understanding light absorption and charge carrier behaviors at the semiconductor/electrolyte interface is of paramount importance in the efficient design of photocatalysts for solar fuel generation.

Fig. 4.4  Scheme illustration of (A) a particulate photocatalyst, and (B) a photoelectrochemical (PEC) cell with an n-type photoelectrode. Note: (I) light absorption, (II) charge separation, (III) charge migration, (IV) charge recombination, and (V) red-ox reactions. CB, conduction band; VB, valence band; Eg, band gap; A, acceptor; D, donor. Reproduced with permission from J. Li, N. Wu, Semiconductor-based photocatalysts and photoelectrochemical cells for solar fuel generation: a review, Cat. Sci. Technol. 5 (2015) 1360–1384. Copyright © Royal Society of Chemistry.

Energy band engineering of metal oxide for enhanced visible light absorption55

4.3.2 Solar energy conversion to chemical fuels Photocatalysis has numerous applications, including organic pollutants' degradation, self-surface clearance, water treatment, fuel generation, and so on. Using solar energy to produce chemical fuels (H2 by splitting water and hydrocarbons by reducing CO2) is the most attractive application, however, because it provides a way to meet the world's steadily increasing energy demands and also addresses climate changes caused by the emission of greenhouse gases. Hydrogen is a clean, high-density energy because when it is burned with oxygen, water is the only product. Commercial hydrogen is usually produced by the steam reforming of natural gas, which requires a processing temperature greater than 1000 K [31]. Initiating water splitting thermodynamically requires an energy input of 237.18 kJ mol−1. Solar energy is an attractive and renewable energy source. In view of the electrochemical potential, the minimum band gap of a semiconductor for photocatalytic water splitting should be 1.23 eV plus the required overpotentials. Meanwhile, the CB position must be more negative than the H2 generation potential and the VB more positive than the O2 generation potential to allow overall water splitting. Numerous photocatalysts have been explored and developed for this purpose. We do not provide the details on this topic, but related discussions appear throughout the chapter. Another strategy to generate solar fuel is the photocatalytic conversion of CO2 to hydrocarbons. This actually comes from the natural photosynthesis process, where green plants convert CO2 under sunlight to O2 and hydrocarbons. This concept was experimentally confirmed by Fujishima and coworkers in 1970s [1,2]. This approach is currently even more important given the heavy carbon emissions and the climate changes. CO2 has an extremely stable linear structure. Table 4.1 shows the possible reactions and the corresponding thermodynamic potentials for CO2 reduction to various products versus NHE at pH 7. CO2 to CO2− radical needs only a single electron, but undergoes a reorganization from linear to bent radical anion, and thus requires a huge energy input with a very high negative potential at E° = −1.90 V versus NHE. In comparison, the other reactions are more thermodynamically favorable but with protons involved. CO2 reduction is therefore called a “proton-coupled multi-­electron” reaction [32–37]. Theoretically, photocatalytic CO2 reduction to chemical fuels is

Summary of thermodynamic potentials of CO2 reduction to various products Table 4.1 

Reactions +

Potentials (pH 7 vs. NHE) −

CO2 + 2H  + 2e  → CO + H2O CO2 + 2H+ + 2e− → HCO2H CO2 + 4H+ + 4e− → HCOH + H2O CO2 + 6H+ + 6e− → CH3OH + H2O CO2 + 8H+ + 8e− → CH4 + 2H2O CO2 + e - ® CO2 • H2O + e− → H2 + 1/2O2

−0.53 V −0.61 V −0.48 V −0.38 V −0.24 V −1.90 V −0.41V

56

Multifunctional Photocatalytic Materials for Energy

p­ ossible like hydrogen evolution by water splitting owing to these reactions have similar thermal dynamic potentials, as shown in Table 4.1. However, precise control over this multiple-­step process remains a huge kinetic challenge. The main challenges for photo/catalytic CO2 reduction include (i) low conversion efficiency/current of catalysts, (ii) poor product selectivity of the catalysts, (iii) lack of a fundamental understanding of the real reactions during the CO2 multistep reactions, and (iv) lack of reliable way to precisely predict the possible reaction pathways.

4.3.3 Photocatalysts requirements for catalytic reactions An efficient photocatalyst must meet all requirements simultaneously in order to optimize the five steps presented in Section 4.3.1. First, the semiconductor must have a small band gap in order to harvest the sunlight as much as possible. At the same time, regarding the energy band, the CB and the VB of the semiconductor must straddle the redox potentials of the desired photocatalytic reactions (Fig. 4.5A). To minimize the charge recombination, fast charge mobility and long charge diffusion length are essential as well [6,15,38–41]. The configuration commonly used for photoelectrochemical water splitting is to use a single photoelectrode (n-type photoanode for oxidation shown in Fig. 4.5B or p-type photocathode for reduction shown in Fig. 4.5C) and an electrochemically active material, generally platinum, as the counter electrode, which is called a half PEC cell. In half-cell configurations, however, the photogenerated voltage usually is not high enough to initiate these reactions, and an external bias is required regardless of the band gap and light absorption capability of the semiconductor photocatalyst. The ultimate goal of this field is to produce chemical fuels with solar energy input only by coupling the photoanode and photocathode together in a full PEC tandem cell (Fig. 4.5D), which requires consideration of both light absorption and band alignment for two photoelectrodes (see more on this topic in Section 4.5.5).

4.3.4 Solar to chemical conversion efficiency Not all the photogenerated charge carriers can survive and reach the surface for chemical reactions. Photocatalytic solar fuel generation competes with many side processes that consume the charge carriers and dramatically decrease the carriers’ population. Practical solar-to-chemical (STC) conversion efficiency is therefore far below the theoretical values. Primarily, the STC efficiency η is determined by the following processes: (i) light absorption to generate charge carriers in photoelectrode (light absorption efficiency, ηabs), (ii) charge separation and migration to the photoelectrode surface (charge separation efficiency, ηsep), and (iii) charge injection and reaction at the photoelectrode/electrolyte interface (charge injection efficiency, ηinj) [14,42–45]. The overall STC efficiency η can be illustrated as

h µ habs ´hsep ´h inj

(4.6)

Energy band engineering of metal oxide for enhanced visible light absorption57

CO2/CO2•− CO2/HCOOH CO2/CO

e–

CB

CO2/HCOH H2O/H2

0

e– hv

CO2/CH3OH CO2/CH4

1 VB

2

CxHyOz

H 2O

H2O CO2

O2

h+

Cathode

–1

Photoanode

Potential vs. NHE (V) at pH = 1

–2

Semiconductor

3

Photoanode

O2

Anode

H 2O CO2

CxHyOz

H2 O

(C)

H2O

CxHyOz

O2

H2O CO2

Photocathode

(B)

Photocathode

(A)

(D)

Fig. 4.5  (A) Electronic band structure for solar fuel generation (water splitting and CO2 reduction). (B–D) the different configurations that apply to solar fuel generation (B: photoanode half cell; C: photocathode half cell; and D: photoanode/photocathode full tandem cell).

In a PEC cell, the practical photocurrent J is thereby determined by J = J max ´h = J max ´habs ´hsep ´h inj

(4.7)

where Jmax accounts for the maximum photocurrent for photoelectrodes, which is determined by the band gap of the semiconductor. Among these efficiencies, ηabs reflects the capability of photoelectrodes to absorb light and is decreased by the light reflection and transmission [14,44,45]. Narrow band gap semiconductors are thereby highly desired for harvesting sunlight with a broad wavelength range. For a single material, either geometrical engineering (such as patterning and increasing the thickness, etc.) or band energy engineering (by controlling defects and/or alien atom doping) is helpful for increasing ηabs. The former works in the original absorption wavelength range of the semiconductor, whereas the latter

58

Multifunctional Photocatalytic Materials for Energy

aims to extend the light absorption range by introducing new internal band energy levels or by narrowing the band gap. ηsep stands for the separation of the photoexcited electrons and holes, and their following transport to the current collector and the electrolyte interface [44]. ηsep is in reverse proportion to the recombination either in bulk or at internal interfaces. The intrinsic electronic features such as charge carrier mobility and diffusion length determine the ηsep. ηinj represents the efficiency of charge transfer and injection at the photoelectrode/ electrolyte interface, and is decreased by the surface recombination and poor OER/ HER kinetics [44,45]. High values of ηinj have been achieved either by coating a passivation layer on the surface to suppress surface recombination or by depositing co-catalysts to improve the surface reaction kinetics (oxygen evolution reaction/OER catalysts such as Co-Pi, FeOOH, and NiOOH on photoanode or hydrogen evolution reaction/HER catalysts such as MoS2 and Pt on photocathode) [44–49]. In practical devices, particularly for metal oxides, the value of the product of ηabs and ηsep (i.e., ηabs × ηsep) always remains low, which is a challenge because ηabs and ηsep are coupled together inversely: increasing the thickness of photoelectrode, increases ηabs but decreases ηsep, and vice versa. For example, hematite has a very short hole diffusion length of 2 nm to 4 nm but a 180 nm penetration length of 550 nm. This means most photogenerated charge carriers in thick hematite film recombine during the separation process and cannot contribute to surface reactions [13–15].

4.4 Metal oxide photocatalysts Compared with the conventional inorganic semiconductors, metal oxide semiconductors demonstrate a different electronic band structure that endows them with new optoelectronic properties and design concepts, and even novel functions [7,18]. In this section, we briefly review the electronic energy band structure of metal oxide semiconductors and list the most typical benchmark metal oxide photocatalysts applied in solar fuel generation.

4.4.1 Electronic energy band of metal oxide photocatalysts Each semiconductor's electronic energy band has three regions (Fig. 4.1). If we examine further, we see that the VB of metal oxides is composed mainly of the O 2p character, and the CB mainly comes from the metal s character and/or d character depending on the electronic configurations of the metal ions [50,51]. Fig. 4.6 shows the electronic structures of anatase and rutile TiO2. It is clear that the VB-edge of both materials is dominated by O 2p and that the CB-edge is formed from Ti 3d. The O 2p character has a very positive potential as well as a high ionic metric, which generates a large separation between band edges and consequently leads to the wide band gap for most metal oxide semiconductors [50]. Seen another way, these electronic states are very sensitive to changes in the surrounding environment, which provides viable possibilities for tuning the electronic band structure and thus the light absorption for metal

Energy band engineering of metal oxide for enhanced visible light absorption59

Anatase Total DOS VBmax

CBmin

Eg

Op Ti s –7

–6

Ti d

Ti p –5

–4

–3

(A)

–2 –1 0 Energy (eV)

1

2

3

4

5

Rutile Total DOS

–7

–6

CBmin

Eg

Op Ti p

Ti s

(B)

VBmax

–5

–4

–3

–2 –1 0 Energy (eV)

Ti d 1

2

3

4

5

Fig. 4.6  The electronic structure of anatase and rutile TiO2. Comparison of the total and iondecomposed electronic density of states of anatase (A) and rutile (B) TiO2 calculated using the HSE06 hybrid density functional. Reproduced with permission from D.O. Scanlon, C.W. Dunnill, J. Buckeridge, S.A. Shevlin, A.J. Logsdail, S.M. Woodley, C.R.A. Catlow, M.J. Powell, R.G. Palgrave, I.P. Parkin, G.W. Watson, T.W. Keal, P. Sherwood, A. Walsh, A.A. Sokol, Band alignment of rutile and anatase TiO2, Nat. Mater. 12 (2013) 798–801. Copyright © Nature Publishing group.

oxide semiconductors as well. Using either transition metal cations with d/n electronic configurations, such as Fe2O3, or transition metal cations with occupied high binding s states, such as SnO (2.4 eV) and PbO (2.1 eV), could raise the valence band and thus narrow the band gap for visible light absorption [26,51]. Another approach is to introduce anionic species such as N and S that are less electronegative than O, which will reach the same goal for visible light absorption but work in a different way by having new energy levels within the forbidden band region. We will discuss these effects in detail in Section 4.5.

4.4.2 Representative metal oxide photocatalysts 4.4.2.1 TiO2 TiO2 is the best-known wide band gap metal oxide semiconductor. It has three crystal structures: anatase, rutile, and brookite. The commonly used structures for photocatalysis are anatase and rutile, as well as their mixture P25, which has the best

60

Multifunctional Photocatalytic Materials for Energy

p­ hotocatalytic performance reported so far. TiO2 is photochemically stable under harsh conditions, even in very strong acid or basic electrolytes. TiO2’s shortcoming is its wide band gaps (3.2 eV for anatase and 3.0 eV for rutile phase), which restricts its light absorption within the ultraviolent range ( Eg





e

C.B.

e



e Eg

Red V.B.

Eg –

e C.B.

e

H2O O2

II

hv > Eg

Ox

V.B. h

(C)

Photoanode (Fe2O3)

Photocathode (a-Si)

e–

+

e–

h+



H2 H+ H2 evolution photocatalyst

O2 evolution photocatalyst

Fig. 4.13  Configurations, electronic band alignment, and each function in a multifunctional system for overall water splitting to chemical fuels. (A) PV/PEC tandem configuration, (B) Photoanode/Photocathode tandem PEC cell, and (C) Z-scheme systems. Reproduced with permissions from K. Zhang, M. Ma, P. Li, D.H. Wang, J.H. Park, Water splitting in tandem devices: moving photolysis beyond electrolysis, Adv. Energy Mater. 6 (2016) 1600602; J. Jang, C. Du, Y. Ye, Y. Lin, X. Yao, J. Thorne, E. Liu, G. McMahon, J. Zhu, A. Javey, J. Guo, D. Wang, Enabling unassisted solar water splitting by iron oxide and silicon. Nat. Commun. 6 (2015) 8447; J. Brillet, J. Yum, M. Cornuz, T. Hisatomi, R. Solarska, J. Augustynski, M. Graetzel, K. Sivula, Highly efficient water splitting by a dual-absorber tandem cell, Nat. Photonics 6 (2012) 824–828; K. Maeda, K. Domen, Photocatalytic water splitting: recent progress and future challenges, J. Phys. Chem. Lett. 1 (2010) 2655–2661. Copyright © Wiley & Sons, Nature Publishing group and the American Chemical Society.

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4.6 Concluding remarks Overall, metal oxides are earth-abundant and physically and chemically stable, but their photocatalytic applications are limited because of their poor visible light absorption capability due to their wide band gap. So improving the visible light absorption for metal oxide semiconductors has priority in this field (increasing ηabs). At the same time, charge carrier behaviors (increasing ηsep)—that is, charge separation, migration, and recombination—are of equal importance in achieving high solar energy conversion efficiency, which always competes with light absorption ηabs. Engineering the electronic energy band and geometric structures is the key to balancing and decoupling ηabs and ηsep and to achieving an acceptable value for ηabs×ηsep. Doping with alien atoms has long been employed for wide band gap metal oxides in order to enable visible light absorption, but the introduced shallow and deep energy levels act as recombination centers leading to decreased ηsep. It is worth noting that doping-induced band gap narrowing without introducing internal band energy levels is of particular interest. However, this is a very unique case, and no other doping examples have been found so far. More insight and understanding of the origin and cause of this band gap narrowing is needed. Fortunately, through an alloying or solid solution technique, multiple metal-cation oxides (≥2) can be treated as intrinsic narrow band gap semiconductors to improve light absorption ηabs, while apparently not decreasing ηsep. Tuning of the light absorption range also is possible by varying the electronic energy structure in the valence band with different electronic configurations. In a photosensitizer system, traditional sensitizers like dyes and QDs still face the issue of photochemical stability in an aqueous electrolyte. By contrast, plasmonic nanostructures are more flexible and can take on this responsibility in the future. Plasmonic nanoparticles cover the whole sunlight spectrum and transfer the absorbed solar energy to an adjacent semiconductor through variable pathways, i.e., photonic enhancement, hot electron injection, and PIRET. More important, thermodynamic band alignment and intimate contact between a semiconductor and plasmonic nanostructures are no longer required as in traditional photosensitizer/semiconductor systems, which opens up new directions and offers more designing opportunities to improve the light absorption of metal oxides. The fact is that plasmonic energy transfer efficiency remains very low in most cases; therefore a more fundamental understanding of the energy barriers that compete with the plasmonic energy transfer process is in urgent demand. Finally, the multijunction system is highly desirable for unbiased solar fuel generation. Metal oxide photoanode materials have been studied extensively and used either in half cells or in a PV-PEC cell for water oxidation. Stable p-type metal oxide photocathodes are still in high demand. A typical p-Cu2O photocathode is stable for a very short term, even with a multiple film-protecting layer, for example, AZO-TiO2. Long-term stability is still problematic. More affordable techniques for depositing protection layers are also required. Cu-based ternary oxides are potential candidates as photocathodes for H2 evolution or CO2 reduction, not only because of their improved tuning flexibility on the light absorption, but more importantly because photostability is highly possible owing to the new d/s energy levels in the conduction band

Energy band engineering of metal oxide for enhanced visible light absorption73

that accept the excited electrons from VB, thus avoiding the reduction of Cu. More emphasis should be put on these materials from synthesis techniques, to fundamental understanding, to device fabrication. On the other hand, the photocurrents of currently studied photoanode metal oxides like BVO, hematite, and TiO2 are still much lower than those of semiconductor photocathodes and also their theoretical maximum. Novel engineering techniques and tools on the geometry, surface chemistry, and physical understanding of the energy band and charge transfer need to be further explored.

Acknowledgments The authors gratefully acknowledge support from the United States Department of the Army and the United States Army Material Command. J. Li also acknowledges support from the United States Army Research Laboratory Senior Research Fellowship Program, which is administered by the Oak Ridge Associated Universities.

References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [2] T. Inoue, A. Fujishima, S. Konishi, K. Honda, Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders, Nature 277 (1979) 637–638. [3] A.  Mills, S.L.  Hunte, An overview of semiconductor photocatalysis, J. Photochem. Photobiol. A Chem. 108 (1997) 1–35. [4] S. Navalon, A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Photocatalytic CO2 reduction using non-titanium metal oxides and sulfides, ChemSusChem 6 (2013) 562–577. [5] N. Serpone, A.V. Emeline, Semiconductor photocatalysis-past, present, and future outlook, J. Phys. Chem. Lett. 3 (2012) 673–677. [6] J. Li, N. Wu, Semiconductor-based photocatalysts and photoelectrochemical cells for solar fuel generation: a review, Cat. Sci. Technol. 5 (2015) 1360–1384. [7] X. Yu, T.J. Marks, A. Facchetti, Metal oxides for optoelectronic applications, Nat. Mater. 15 (2016) 383–396. [8] K. Sivula, Metal oxide photoelectrodes for solar fuel production, surface traps, and catalysis, J. Phys. Chem. Lett. 4 (2013) 1624–1633. [9] S.F. Adil, A. Al-Mayouf, Metal oxide as photocatalysts, J. Saudi Chem. Soc. 19 (2015) 462–464. [10] R. Marschall, Semiconductor composites: strategies for enhancing charge carrier separation to improve photocatalytic activity, Adv. Funct. Mater. 24 (2014) 2421–2440. [11] X. Xu, C. Randorn, P. Efstathiou, J.T.S. Irvine, A red metallic oxide photocatalyst, Nat. Mater. 11 (2012) 595–598. [12] T. Wang, J.L. Gong, Single-crystal semiconductors with narrow band gaps for solar water splitting, Angew. Chem. Int. Ed. 54 (2015) 10718–10732. [13] B. Iandolo, B. Wickman, I. Zoric, A. Hellman, The rise of hematite: origin and strategies to reduce the high onset potential for the oxygen evolution reaction, J. Mater. Chem. A 3 (2015) 16896–16912.

74

Multifunctional Photocatalytic Materials for Energy

[14] A.G. Tamirat, J. Rick, A.A. Dubale, W. Su, B. Hwang, Using hematite for photoelectrochemical water splitting: a review of current progress and challenges, Nanoscale Horiz. 1 (2016) 243–267. [15] W. Yin, H. Tang, S. Wei, M.M. Al-Jassim, J. Turner, Y. Yan, Band Structure engineering of semiconductors for enhanced photoelectrochemical water splitting: the case of TiO2, Phys. Rev. B 82 (2010), 045106. [16] M.T.  Uddin, Metal Oxide Heterostructures for Efficient Photocatalysts (Ph.D. thesis), Universte Science et Technologies-Bordeaux I, Darmstadt, 2013. https://tel.archivesouvertes.fr/tel-00879226/. [17] W.D.  Callister  Jr., Materials Science and Engineering: An Introduction, fifth ed., John Wiley &Sons, New York, 2011. [18] E. Thimsen, Metal Oxide Semiconductors for Solar Energy Harvesting (All These and Dissertations (ETD)), Washington University, St. Louis, MO, 2009. [19] “III. Absorption of Light and Generation” in “Semiconductor and Solar Interactions” in Section “The Science of Solar”, @ PHOTON LibreTextsTM, https://photon.libretexts.org (accessed 03.17.17). [20] M.A.  Green, Solar Cells: Operating Principles, Technology, and System Applications, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1982. [21] C. Honsberg, B. Stuart, Absorption of Light, PVEducation.org, http://www.pveducation. org/pvcdrom/pn-junction/absorption-of-light, July 2017. [22] B. V. Zeghbroeck. Principles of Semiconductors Devices. University of Colorado Boulder, Boulder. http://ecee.colorado.edu/~bart/book/recomb.htm, https://ecee.colorado. edu/~bart/book/book/title.htm. [23] R. Entner, Modeling and Simulation of Negative Bias Temperature Instability, Technischen Universitat Wien, Österreich, 2007. http://www.iue.tuwien.ac.at/phd/entner/node11.html. [24] S.  Gimenez, J.  Bisquert, Photoelectrochemcial Solar Fuel Production: From Basic Principles to Advanced Devices, Springer Nature, AG Switzerland, 2016. [25] H. Tuysuz, C.K. Chan, Solar Energy for Fuels, Springer, New York, 2016. [26] J.M.  Coronado, F.  Fresno, M.D.  Hernandez-Alonso, R.  Portela, Design of Advanced Photocatalytic Materials for Energy and Environmental Applications, Springer Verlag, London, 2013. [27] J. Zhou, X.S. Zhao, Visible-light-responsive titanium dioxide photocatalysts, in: M. Anpo, P.V.  Kamat (Eds.), Environmental Benign Photocatalysts, Nanostructured Science and Technology, Springer Science, New York, 2010. [28] T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting, Chem. Soc. Rev. 43 (2014) 7520–7535. [29] A.L. Linsebigler, G. Lu, J.T. Yates Jr., Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758. [30] M.G. Walter, E.L. Warren, J.R. Mckone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446–6473. [31] Office of Energy Efficiency & Renewable Energy, US Department of Energy. Hydrogen Production: Natural Gas Reforming. https://energy.gov/eere/fuelcells/ hydrogen-production-natural-gas-reforming. [32] J.  Albero, H.  Garcia, Photocatalytic CO2 reduction, in: Heterogeneous Photocatalysis, Green Chemistry and Sustainable Technology, Springer Verlag, Berlin/Heidelberg, 2016. [33] J. Qiao, Y. Liu, J. Zhang, Electrochemical Reduction of Carbon Dioxide: Fundamentals and Technologies, CRC Press by Taylor & Francis Group, Boca Raton, 2017. [34] W. Fan, Q. Zhang, Y. Wang, Semiconductor-based nanocomposites for photocatalytic H2 production and CO2 conversion, Phys. Chem. Chem. Phys. 15 (2013) 2632–2649.

Energy band engineering of metal oxide for enhanced visible light absorption75

[35] Y.  Tachibana, L.  Vayssieres, J.R.  Durrant, Artificial photosynthesis for solar water-­ splitting, Nat. Photonics 6 (2012) 511–518. [36] S.C. Roy, O.K. Varghese, M. Paulose, C.A. Grimes, Toward solar fuels: photocatalytic conversion of carbon dioxide to hydrocarbons, ACS Nano 4 (2010) 1259–1278. [37] Photoelectrochemical reduction of CO2. https://en.wikipedia.org/wiki/Photoelectrochemical_ reduction_of_CO2. [38] K. Zhang, M. Ma, P. Li, D.H. Wang, J.H. Park, Water splitting in tandem devices: moving photolysis beyond electrolysis, Adv. Energy Mater. 6 (2016), 1600602. [39] J. Jang, C. Du, Y. Ye, Y. Lin, X. Yao, J. Thorne, E. Liu, G. McMahon, J. Zhu, A. Javey, J. Guo, D. Wang, Enabling unassisted solar water splitting by iron oxide and silicon, Nat. Commun. 6 (2015) 8447. [40] J.  Brillet, J.  Yum, M.  Cornuz, T.  Hisatomi, R.  Solarska, J.  Augustynski, M.  Graetzel, K. Sivula, Highly efficient water splitting by a dual-absorber tandem cell, Nat. Photonics 6 (2012) 824–828. [41] S. Licht, B. Wang, S. Mukerji, T. Soga, M. Umeno, H. Tributsch, Over 18% solar energy conversion to generation of hydrogen fuel: theory and experimental for efficient solar water splitting, Int. J. Hydrog. Energy 26 (2001) 653–659. [42] G.G.  Bessegato, T.T.  Guaraldo, M.V.B.  Zanoni, Enhancement of photoelectrocatalysis by using nanostructured electrodes, in: M. Aliofkhazraei (Ed.), Modern Electrochemical Methods in Nano, Surface and Corrosion Science, InTech, Rijeka, Croatia, 2014. [43] A.G. Tamirat, W. Su, A. Dubale, H. Chen, B. Hwang, Photoelectrochemical water splitting at low applied potential using a NiOOH coated codoped (Sn, Zr)α-Fe2O3 photoanode, J. Mater. Chem. A 3 (2015) 5949–5961. [44] L. Zhou, C. Zhao, B. Giri, P. Allen, X. Xu, H. Joshi, Y. Fan, L.V. Titova, P.M. Rao, High light absorption and charge separation efficiency at low applied voltage from Sb-doped SnO2/BiVO4 core/shell nanorod-array photoanodes, Nano Lett. 16 (2016) 3463–3474. [45] J.  Li, S.K.  Cushing, D.  Chu, P.  Zheng, J.  Bright, C.  Castle, A.  Manivannan, N.  Wu, Distinguishing surface effects of gold nanoparticles from plasmonic effect on photoelectrochemical water splitting by hematite, J. Mater. Res. 31 (2016) 1608–1615. [46] T.W. Kim, K.S. Choi, Nanoporous BiVO4 photoanodes with dual-layer oxygen evolution catalysts for solar water splitting, Science 343 (2014) 990–994. [47] F.F. Abdi, N. Firet, R. van de Krol, Efficient BiVO4 thin film photoanodes modified with cobalt phosphate catalyst and W-doping, ChemCatChem 5 (2013) 490–496. [48] W.D. Chemelewski, H.C. Lee, J.F. Lin, A.J. Bard, C.B. Mullins, Amorphous FeOOH oxygen evolution reaction catalyst for photoelectrochemical water splitting, J. Am. Chem. Soc. 136 (2014) 2843–2850. [49] T.W. Kim, Y. Ping, G.A. Galli, K.S. Choi, Simultaneous enhancements in photon absorption and charge transport of bismuth vanadate photoanodes for solar water splitting, Nat. Commun. 6 (2015) 8769. [50] D.O. Scanlon, C.W. Dunnill, J. Buckeridge, S.A. Shevlin, A.J. Logsdail, S.M. Woodley, C.R.A.  Catlow, M.J.  Powell, R.G.  Palgrave, I.P.  Parkin, G.W.  Watson, T.W.  Keal, P.  Sherwood, A.  Walsh, A.A.  Sokol, Band alignment of rutile and anatase TiO2, Nat. Mater. 12 (2013) 798–801. [51] A.  Walsh, Y.  Yan, M.  Huda, M.  Al-Jassim, S.  Wei, Band Edge electronic structure of BiVO4 elucidating the role of the Bi s and V d orbitals, Chem. Mater. 21 (2009) 547–551. [52] J. Kang, Y. Noh, J. Kim, H. Choi, T. Jeon, D. Ahn, J. Kim, S. Yu, H. Park, J. Yum, W. Choi, D.C. Dunand, H. Choe, Y. Sung, Iron oxide photoelectrode with multidimensional architecture for highly efficient photoelectrochemical water splitting, Angew. Chem. Int. Ed. 56 (2017) 6583–6588.

76

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[53] Y. Park, K.J. McDonald, K. Choi, Progress in bismuth vanadate photoanodes for use in solar water oxidation, Chem. Soc. Rev. 42 (2013) 2321–2337. [54] L. Cai, J. Zhao, H. Li, J. Park, I. Cho, H.S. Han, X. Zheng, One-Step hydrothermal deposition of Ni:FeOOH onto photoanodes for enhanced water oxidation, ACS Energy Lett. 1 (2016) 624–632. [55] H. Zhang, C. Cheng, Three-dimensional FTO/TiO2/BiVO4 composite inverse opals photoanode with excellent photoelectrochemical performance, ACS Energy Lett. 2 (2017) 813–821. [56] S. Wang, P. Chen, J. Yun, Y. Hu, L. Wang, An electrochemically treated BiVO4 photoanode for efficient photoelectrochemical water splitting, Angew. Chem. Int. Ed. 56 (2017) 8500–8504. [57] I. Sullivan, B. Zoellner, P.A. Maggard, Copper(I)-based p-type oxides for photoelectrochemical and photovoltaic solar energy conversion, Chem. Mater. 28 (2016) 5999–6016. [58] S.P. Berglund, F.F. Abdi, P. Bogdanoff, A. Chemseddine, D. Friedrich, R. van de Krol, Comprehensive evaluation of CuBi2O4 as a photocathode material for photoelectrochemical water splitting, Chem. Mater. 28 (2016) 4231–4242. [59] Y. Jang, Y. Park, H. Kim, Y. Choi, S. Choi, J. Lee, Oxygen-intercalated CuFeO2 photocathode fabricated by hybrid microwave annealing for efficient solar hydrogen production, Chem. Mater. 28 (2016) 6054–6061. [60] A. Paracchino, V. Laporte, K. Sivula, M. Gratzel, E. Thimsen, Highly active oxide photocathode for photoelectrochemical water reduction, Nat. Mater. 10 (2011) 456–461. [61] H. Qi, J. Wolfe, D. Fichou, Z. Chen, Cu2O photocathode for low bias photoelectrochemical water splitting enabled by NiFe-layered double hydroxide Co-catalyst, Sci Rep 6 (2016), 30882. [62] M.  Schreier, F.  Heroguel, L.  Steier, S.  Ahmad, J.S.  Luterbacher, M.T.  Mayer, J.  Luo, M. Gratzel, Solar conversion of CO2 to CO using Earth-abundant electrocatalysts prepared by atomic layer modification of CuO, Nat. Energy 2 (2017), 17087. [63] Y. Nishi, T. Miyata, T. Minami, Effect of inserting a thin buffer layer on the efficiency in N-ZnO/P-Cu2O heterojunction solar cells, J. Vac. Sci. Technol. 30 (2012), 04D103. [64] C.G. Morales-Guio, S.D. Tilley, H. Vrubel, M. Gratzel, X. Hu, Hydrogen evolution from a copper (I) oxide photocathode coated with amorphous molybdenum sulphide catalyst, Nat. Commun. 5 (2014) 4059. [65] J. Luo, L. Seier, M.K. Son, M. Schreier, M.T. Mayer, M. Gratzel, Cu2O nanowires photocathodes for efficient and durable solar water splitting, Nano Lett. 16 (2016) 1848–1857. [66] S.D. Tilley, M. Schreier, J. Azevedo, M. Stefik, M. Graetzel, Ruthenium oxide hydrogen evolution catalysis on composite cuprous oxide water splitting photocathodes, Adv. Funct. Mater. 24 (2014) 303–311. [67] M.A. Henderson, J.M. White, H. Uetsuka, H. Onishi, Photochemical charge transfer and trapping at the interface between an organic adlayer and an oxide semiconductor, J. Am. Chem. Soc. 125 (2003) 14974–14975. [68] Doping Semiconductor. https://en.wikipedia.org/wiki/Doping_(semiconductor). [69] R.  Asashi, T.  Morikawa, T.  Ohwaki, K.  Aoki, Y.  Taga, Visible light photocatalysis in nitrogen-doped titanium oxide, Science 293 (2001) 269–271. [70] J. Wang, D. Tafen, J. Lewis, Z. Hong, A. Manivannan, M. Zhi, M. Li, N. Wu, Origin of photocatalytic activity of nitrogen-doped TiO2 nanobelts, J. Am. Chem. Soc. 131 (2009) 12290–12297. [71] F. Meng, Z. Hong, J. Arndt, M. Li, M. Zhi, F. Yang, N. Wu, Visible light photocatalytic activity of nitrogen-doped La2Ti2O7 nanosheets originating from band gap narrowing, Nano Res. 5 (2012) 213–221.

Energy band engineering of metal oxide for enhanced visible light absorption77

[72] A.  Kudo, K.  Omori, H.  Kato, A novel aqueous process for preparation of crystal form-controlled and highly crystalline BiVO4 powder from layered vanadates at room temperature and its photocatalytic and photophysical properties, J. Am. Chem. Soc. 121 (1999) 11459–11467. [73] Z. Huang, L. Pan, J. Zou, X. Zhang, L. Wang, Nanostructured bismuth vanadate-based materials for solar-energy-driven water oxidation: a review on recent progress, Nanoscale 6 (2014) 14044–14063. [74] K. Maeda, T. Takata, M. Hara, N. Saito, Y. Inoue, H. Kobayashi, K. Domen, GaN/ZnO solid solution as a photocatalyst for visible-light-driven overall water splitting, J. Am. Chem. Soc. 127 (2005) 8286–8287. [75] S.X. Ouyang, J.H. Ye, β-AgAl1-xGaxO2 solid-solution photocatalysts: continuous modulation of electronic structure toward high-performance visible-light photoactivity, J. Am. Chem. Soc. 133 (2011) 7757–7763. [76] D.F. Wang, T. Kako, J.H. Ye, New series of solid-solution semiconductors (AgNbO3)1-x (SrTiO3)x with modulated band structure and enhanced visible-light photocatalytic activity, J. Phys. Chem. C 113 (2009) 3785–3792. [77] W.F.  Yao, J.H.  Ye, Photophysical and photocatalytic properties of Ca1xBixVxMo1-xO4 solid solutions, J. Phys. Chem. B 110 (2006) 11188–11195. [78] B.  Zoellner, S.  Stuart, C.-C.  Chung, D.B.  Dougherty, J.L.  Jones, P.A.  Maggard, CuNb1−xTaxO3 (x≪0.25) solid solutions: impact of Ta(V) substitution and Cu(I) deficiency on their structure, photocatalytic, and photoelectrochemical properties, J. Mater. Chem. A 4 (2016) 3115–3126. [79] M. Lalanne, A. Barnabé, F. Mathieu, P. Tailhades, Synthesis and thermostructural studies of a CuFe1-xCrxO2 delafossite solid solution with 0≪x≪1, Inorg. Chem. 48 (2009) 6065–6071. [80] J.R. Swierk, T.E. Mallouk, Design and development of photoanodes for water-splitting dye-sensitized photoelectrochemical cells, Chem. Soc. Rev. 42 (2013) 2357–2387. [81] J. Li, M. Hoffmann, H. Shen, C. Fabrega, J.D. Prades, T. Andreu, F. Hernandez-Ramirez, S.  Mathur, Enhanced photoelectrochemical activity of an excitonic staircase in CdS@ TiO2 and [email protected]@rutile TiO2 heterostructures, J. Mater. Chem. 22 (2012) 20472–20476. [82] G.  Konstantatos, I.  Howard, A.  Fischer, S.  Hoogland, J.  Cliford, E.  Klem, L.  Levina, E.H. Sargent, Ultrasensitive solution-cast quantum dot photodetectors, Nature 442 (2006) 180–183. [83] S.A.  Mcdonald, G.K.  Konstantatos, S.  Zhang, P.W.  Cyr, E.J.D.  Klem, L.  Levina, E.H. Sargent, Solution-processed PbS quantum dot infrared photodetectors and photovoltaics, Nat. Mater. 4 (2005) 138–142. [84] X.  Zhang, Y.  Chen, R.  Liu, D.P.  Tsai, Plasmonic photocatalysis, Rep. Prog. Phys. 76 (2013), 046401. [85] T.  Ming, H.J.  Chen, R.B.  Jiang, Q.  Li, J.F.  Wang, R.  Jiang, B.  Li, C.  Fang, J.  Wang, Plasmon-controlled fluorescence: beyond the intensity enhancement, J. Phys. Chem. Lett. 3 (2012) 191–202. Metal/semiconductor hybrid nanostructures for plasmon-enhanced applications, Adv. Mater. 26 (2014) 5274–5309. [86] C. Clavero, Plasmon-induced hot-electron generation at nanoparticle/metal-oxide interfaces for photovoltaic and photocatalytic devices, Nat. Photonics 8 (2014) 95–103. [87] H.A. Atwater, A. Polman, Plasmonics for improved photovoltaic devices, Nat. Mater. 9 (2010) 205–213. [88] W. Ye, R. Long, H. Huang, Y. Xiong, Plasmonic nanostrucutres in solar energy conversion, J. Mater. Chem. C 5 (2017) 1008–1021.

78

Multifunctional Photocatalytic Materials for Energy

[89] H. Robatjazi, H. Zhao, D.F. Swearer, N.J. Hogan, L. Zhou, A. Alabsatri, M.J. McClain, P.  Nordlander, N.J.  Halas, Plasmon-induced selective carbon dioxide conversion on earth abundant aluminum-cuprous oxide antenna-reactor nanoparticles. Nat. Commun. 8 (2017), https://doi.org/10.1038/s41467-017-00055-z. [90] B. Frank, P. Kahl, D. Podbiel, G. Spektor, M. Orenstein, L. Fu, T. Weiss, M. Hoegen, T.J. Davis, F. Heringdorf, H. Giessen, Short-range surface plasmonics: localized electron emission dynamics from a 60-nm spot on an atomically flat single-crystalline gold surface, Sci. Adv. 3 (2017), e1700721. [91] J.  Li, S.  Cushing, P.  Zheng, F.  Meng, D.  Chu, N.  Wu, Plasmon-induced photonic and energy transfer enhancement of solar water splitting by a hematite nanorod array, Nat. Commun. 4 (2013) 3651. [92] J. Li, S.K. Cushing, P. Zheng, T. Senty, F. Meng, A.D. Bristow, A. Manivannan, N. Wu, Solar hydrogen generation by a CdS-Au-TiO2 sandwich nanorod array enhanced with Au nanoparticle as electron relay and plasmonic photosensitizer, J. Am. Chem. Soc. 136 (2014) 8438–8449. [93] S.  Cushing, J.  Li, F.  Meng, T.  Senty, S.  Suri, M.  Zhi, M.  Li, A.  Bristow, N.  Wu, Photocatalytic activity enhanced by plasmonic resonant energy transfer from metal to semiconductor, J. Am. Chem. Soc. 134 (2012) 15033–15041. [94] J. Li, S. Cushing, J. Bright, F. Meng, T. Senty, P. Zheng, A. Bristow, N. Wu, [email protected] core-shell nanoparticles as visible-light plasmonic photocatalysts, ACS Catal. 3 (2013) 47–51. [95] J. Li, S.K. Cushing, F. Meng, T. Senty, A. Bristow, N. Wu, Plasmon-induced resonance energy transfer for solar energy conversion below semiconductor band-edge, Nat. Photonics 9 (2015) 601–607. [96] S. Cushing, N. Wu, Progress and perspective of plasmon-enhanced solar energy conversion, J. Phys. Chem. Lett. 7 (2016) 666–675. [97] K.  Maeda, K.  Domen, Photocatalytic water splitting: recent progress and future challenges, J. Phys. Chem. Lett. 1 (2010) 2655–2661.

Further reading [1] Y.  Ma, C.A.  Mesa, E.  Pastor, A.  Kafizas, L.  Francas, F.  Le Formal, S.R.  Pendlebury, J.R. Durrant, Rate law analysis of water oxidation and hole scavenging on a BiVO4 photoanode, ACS Energy Lett. 1 (2016) 618–623.

Graphene photocatalysts Luisa M. Pastrana-Martínez*,†, Sergio Morales-Torres*,†, José L. Figueiredo†, Joaquim L. Faria†, Adrián M.T. Silva† *University of Granada, Granada, Spain, †Universidade do Porto, Porto, Portugal

5

5.1 Introduction Significant attention is being paid to the development of renewable and green technologies because of the increased energy demand and persistent environmental pollution. From this perspective, the sun, a free, clean, and inexhaustible resource, is regarded as a promising option [1–3]. New photocatalytic technologies are being developed to transform renewable energy (i.e., solar light) into chemical fuels and products, such as photocatalytic water splitting and carbon dioxide (CO2) reduction, among others [4–6]. Photocatalytic water splitting is aimed at the production of hydrogen (H2) using natural renewable resources like water and the sun. Moreover, by using solar energy, the photocatalytic reduction of CO2 can transform a harmful greenhouse gas (i.e., CO2) into valuable solar fuels such as methane (CH4) and methanol (CH3OH) [2,4,5,7]. Various developments over the past four decades, particularly in regard to energy and environmental applications, have shown great potential for semiconductor photocatalysis to be a low-cost, environmentally friendly treatment technology [8,9]. However, some limitations are still associated with photocatalysts, such as their inability to use visible light efficiently and their poor stability. Recent efforts have been devoted to the development of novel composite photocatalysts with the notion of increasing the efficiency of solar energy conversion. Graphene, a 2D monolayer of sp2 carbon atoms with a hexagonal packed lattice structure, was a breakthrough discovery in 2004, and it is now set to exceed all other carbon allotropes in material science and technology [10]. This material exhibits many interesting properties, such as high mechanical strength, superior thermal conductivity, outstanding transparency, a huge specific surface area, and excellent charge transport [11,12]. Graphene and its derivatives (e.g., graphene oxide, GO; and reduced graphene oxide, rGO) have stimulated interest in the design of sophisticated high-performance graphene-based composite materials for different applications, such as sensors [13], energy storage devices [14], bio-applications [15], and particularly graphene-based photocatalysts with improved solar-to-fuel conversion efficiency [16]. Graphene derivatives have been shown to induce some beneficial effects on the photocatalytic performance of semiconductor catalysts by creating synergies between the semiconductor and the carbon phases. This effect is attributed mainly to a decrease in the band gap energy of the composite catalyst, an enhancement of the adsorptive properties, and the charge separation and transportation properties. This chapter focuses on the current status of graphene-based composites applied in photocatalysis for energy applications, including photocatalytic water splitting and Multifunctional Photocatalytic Materials for Energy. https://doi.org/10.1016/B978-0-08-101977-1.00006-5 Copyright © 2018 Elsevier Ltd. All rights reserved.

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photoreduction of CO2. The chapter is organized into three major sections: (i) a brief introduction of graphene and its most common derivatives; (ii) a review of several important synthesis strategies for graphene-based photocatalysts; and (iii) the application of graphene-based semiconductor photocatalysts in photocatalytic water splitting to H2 and photocatalytic reduction of CO2 to hydrocarbon fuels. Finally, the major challenges for the future development of graphene-based photocatalysts to produce solar fuels are identified.

5.2 Graphene and its derivatives The first isolation of graphene was obtained simply by mechanical exfoliation of graphite using the Scotch tape method [10]. However, this method of preparation is not suitable for assembling graphene with other materials and for large-scale production. Many feasible routes have been developed to prepare various types of graphene materials, such as chemical vapor deposition (CVD), epitaxial growth, and so on [17,18]. However, the most popular methods available to produce graphene-based materials involve an initial strong chemical oxidation of natural graphite to graphite oxide, followed by its mechanical, chemical, or thermal exfoliation to GO sheets, which then can be chemically or thermally reduced, resulting in the rGO material [19,20]. The most obvious difference between pristine graphene (hereafter referred to as graphene) and GO is the presence of oxygen-containing chemical functionalities attached to the graphene surface, as shown in Fig. 5.1A and B, respectively. Graphene has a hydrophobic nature, whereas GO is hydrophilic; that is, it is easily dispersible in water and other polar solvents. In addition, GO contains both sp2 (aromatic) and sp3 (aliphatic) hybridizations, which further expands the types of interactions that can occur with its surface [21]. The chemical reduction of GO to rGO is also a promising

Fig. 5.1  Structure of (A) graphene, (B) GO, (C) rGO, and (D) doped graphene derivatives.

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route toward the large-scale production of graphene for different applications [22] (Fig. 5.1C). Furthermore, rGO offers an important advantage, namely the possibility to obtain a tailored hydrophilic surface of graphene decorated with oxygenated functionalities, which results in low production costs [19,20]. These surface groups can be used to facilitate the anchoring of semiconductors and metal nanoparticles and even for the assembly of macroscopic structures, which are relevant to developing highly efficient photocatalysts [23,24]. Heteroatom doping of graphene is a rising research approach toward enhancing the performance of graphene-based materials for a wide range of applications [25], which presents an opportunity to further extend the role of graphene in photocatalysis [26]. In heteroatom-doped graphene materials, a certain percentage of carbon atoms (typically below 10 wt.%) is replaced by other elements, such as nitrogen (N) [27–29], boron (B) [30], phosphorus (P) [31], and sulfur (S) [32,33] (Fig. 5.1D). Several possibilities exist for preparing doped-graphene materials, such as CVD; arc-discharge between two graphite electrodes in the presence of a suitable reagent containing the dopant element, for example, NH3 and H2S; ball-milling; and pyrolysis under inert atmosphere of a natural biopolymer [34]. It is worth noting that the presence of external atoms and defects in graphene is a critical point in catalysis applications. Indeed, the main graphene materials that have been applied in photocatalysis are GO, rGO, and doped-graphene derivatives [25,26].

5.2.1 General properties of graphene-based materials Graphene can be described as a zero-energy band gap semiconductor; this means that the π⁎-state conduction band (CB) and the π-state valence band (VB) of graphene touch each other at the Dirac point [35], as shown in Fig. 5.2 [35,36]. This unique band structure causes graphene to display amazingly high conductivity and

Fig. 5.2  (A) 3D band structure of graphene; (B) approximation of the low energy band structure as two cones touching at the Dirac point. The position of the Fermi level determines the nature of the doping and the transport carrier. Adapted with permission from P. Avouris, Graphene: electronic and photonic properties and devices, Nano Lett. 10 (11) (2010) 4285–4294. Copyright 2010, American Chemical Society.

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electron mobility. Note that the presence of foreign atoms (i.e., O, N, B, P, and S) and defect sites on the lattice of graphene will influence its electrical properties [26]. In fact, the presence of heteroatoms in graphene changes its properties, such as electric conductivity, thermal stability, and chemical reactivity, improving the efficiency and selectivity of the photocatalytic process and avoiding the need for noble metals in the photocatalytic process [37]. For example, the band structure of graphene can be tailored by chemical doping with electron-withdrawing oxygen functionalities or electron-donating nitrogen functionalities, which usually makes graphene a p-type or an n-type semiconductor, respectively (Fig. 5.2B) [38,39]. A tunable band gap, from insulating to conducting, can be achieved by controlling the reduction degree of rGO, as the band gap energy is strongly correlated with the number of oxidized sites, and the oxidization degree of rGO [40]. Understanding the consequences of doping on graphene's electrical performance is thus key to discovering its possible applications in photocatalysis. Generally, the advantages of graphene and its derivative-based photocatalysts can be categorized as (i) ideal electron sink and/or electron transport bridge to suppress photogenerated carrier recombination; (ii) band gap tuning acting as a photosensitizer to extend absorption of light; (iii) remarkable specific surface areas that can significantly increase the specific area of graphene-based photocatalysts; and (iv) good stability for long-term photocatalytic application. Given their remarkable properties, graphene and its derivatives provide a wide range of opportunities to prepare diverse forms of composite materials with extraordinary properties for the photocatalytic splitting of water to H2 and photocatalytic reduction of CO2 to hydrocarbon fuels. Particularly, the preparation of fine-tuned and robust graphene-based photocatalytic materials is necessary to meet the practical requirements for solar fuels generation.

5.3 Graphene-based semiconductor photocatalysts Graphene and its derivatives have been incorporated into many different semiconductors to fabricate graphene-based composites for various photocatalytic applications. These photocatalysts include inorganic and organic semiconductors, among others. Many preparation protocols have been carried out to prepare graphene-based composite photocatalysts, such as mixing and/or sonication, sol-gel, liquid phase deposition, UV-assisted photoreduction, self-assembling, and hydrothermal and solvothermal methods [20]. In this section, we discuss the synthesis of different graphene-based composites and consider different types of semiconductor photocatalysts and synthesis methods.

5.3.1 Synthesis of graphene-based titanium dioxide photocatalysts Since the pioneering work of Fujishima and Honda in 1972 [8], titanium dioxide (TiO2) has been the most widely studied material for synthesizing composites for photocatalytic applications because of its superior photocatalytic properties, easy availability, long-term stability, and low toxicity. Thus composites of graphene derivatives and

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TiO2 have been studied extensively because of the significant enhancement of photocatalytic activity. In the synthesis of graphene-TiO2 composite photocatalysts, TiO2 has been synthetized from different precursors, such as inorganic titanium salts—for example, titanium(IV) sulfate, Ti(SO4)2 [41,42]; titanium(IV) fluoride, TiF4 [43–45]; ammonium hexafluorotitanate(IV), (NH4)2TiF6 [46–48]; and titanium alkoxides that can hydrolyze easily in aqueous solution (e.g., tetrabutyl titanate, Ti(BuO)4 [49–51]; and titanium(IV) isopropoxide, Ti[OCH(CH3)2]4 [23,52]). On the other hand, a GO aqueous suspension is normally used as precursor instead of graphene on its own because of the presence of hydrophilic oxygen-containing surface groups on GO sheets that can be used to facilitate the anchoring of semiconductors. These groups are beneficial for the dispersion of GO layers in water and the heterogeneous nucleation and growth of the TiO2 particles, which are needed to develop highly efficient photocatalysts [53,54]. During the preparation method, GO can be reduced to rGO via the hydrothermal/solvothermal process or by UV light irradiation [20,55–57]. Different TiO2 semiconductors with well-defined morphologies have been constructed on graphene sheets, for example, zero-dimensional TiO2 nanospheres [41], one-dimensional TiO2 nanorods [58], two-dimensional TiO2 nanosheets, and three-­ dimensional macro-/mesoporous TiO2 [59–61]. In general, these TiO2 nanoarchitectures can be fabricated and then anchored onto graphene by carefully controlling the synthesis conditions, such as the additives and hydrothermal parameters, and using titanium salts as precursors [54]. The epoxy and hydroxyl functional groups on GO sheets can act as heterogeneous nucleation sites for anchoring TiO2 nanoparticles, leading to the formation of well-dispersed mesoporous TiO2 nanospheres on the graphene sheets via a template-free self-assembly process [41]. These functionalities (such as epoxy and hydroxyl groups) mediate the efficient and uniform assembly of the TiO2 nanoparticles on the GO sheets, thus avoiding agglomeration and subsequently increasing the surface area of the resulting materials. During the photocatalyst preparation, TiO2 nanoparticles are produced and interact with the surface chemistry of GO by means of hydrogen bonds, resulting in the formation of well-dispersed mesoporous TiO2 nanospheres on the GO sheets. The hydroxyl and epoxy groups are linked to TiO2 particles and should not function as active sites during photocatalysis. Moreover, these functionalities are stable during the photocatalytic process because of the formation of TiOC bonds. TiO2 nanorods were stabilized by oleic acid and self-assembled on GO sheets at the water/toluene interface [58]. The two-phase, self-assembling procedure is simple and reproducible, and it can be widely and easily used for self-assembling other nonpolar organic soluble nanocrystals on GO sheets. Mesoporous graphene-TiO2 nanocomposites have been synthesized via two successive steps of hydrothermal/hydrolysis using Ti(SO4)2 and an acidic GO solution, followed by UV-assisted photocatalytic reduction of GO [59]. On the other hand, hierarchical macro/mesoporous graphene-TiO2 composites have been prepared by a simple one-step hydrothermal method using GO and tetrabutyl titanate as the titanium precursor [60]. A novel simultaneous reduction-­ hydrolysis technique in a binary ethylenediamine/H2O solvent was used in the synthesis of a graphene-TiO2 2D sandwich-like nanostructure using GO nanosheets and titanium(IV), bis(ammonium lactato)dihydroxide [61]. The technique was based on

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the simultaneous reduction of GO into rGO and the formation of TiO2 nanoparticles, resulting in situ loading onto graphene through chemical bonds (TiOC bond) to yield a 2D sandwich-like nanostructure. In another study, rGO-TiO2 nanocomposites were synthesized by a simple and environmentally benign one-step hydrothermal method using GO and TiCl4, as the titania precursor [62]. While conventional approaches mostly utilize multistep chemical methods using strong reducing agents, this method provides the notable advantages of a single-step reaction without employing toxic solvents or reducing agents, thereby providing a novel green synthetic route to produce the nanocomposites of rGO and TiO2. The fabrication of high-quality GO-TiO2 nanorod composites (GO-TiO2 NRCs) on gram scale has also been reported by a two-phase assembly method, exhibiting an improved photocatalytic performance by the effective charge antirecombination on GO [63]. In another report [64], graphene-wrapped anatase TiO2 was synthetized through one-step hydrothermal GO reduction and TiO2 crystallization from GOwrapped amorphous TiO2. Graphene-TiO2 nanoparticles exhibited a red-shift of the band-edge and a significant reduction of the band gap up to 2.80 eV. Recently, the synthesis of high energy (001) facet-exposed crystalline TiO2 on graphene using hydrothermal [42] and solvothermal methods [43] has also attracted much interest. Graphene-modified TiO2 nanosheets with exposed (001) facets (sample G1.0) were prepared using a microwave-hydrothermal treatment of GO and TiO2 nanosheets in an ethanol-water solvent [65]. Because of the interaction between the hydrophilic functional groups (e.g., OH, COOH) on GO and the hydroxyl groups on TiO2, the TiO2 particles were well dispersed on the GO sheets with face-to-face orientation (Fig. 5.3). The corresponding high-magnification TEM image (Fig. 5.3D) clearly shows the lattice fringes, which are parallel to one of the edges of the TiO2 nanosheets. Another procedure for the preparation of graphene-TiO2 composites that has been reported by our group involves the liquid phase deposition method (LPD) using (NH4)2TiF6 and H3BO3 as precursors, followed by a thermal post-treatment in an N2 atmosphere [56]. Graphene-based TiO2 composites were prepared using GO and different chemical rGO samples to assess the effect of the nature and number of oxygen-containing surface groups on the photocatalytic performance of the composite photocatalysts under near-UV/Vis and visible irradiations. The results showed that the presence of the oxygenated groups mediates the efficient and uniform assembly of the TiO2 nanoparticles on the graphene-derivative materials, as shown in Fig. 5.4 [66]. Similarly to TiO2, other inorganic metal oxides, such as ZnO [67–70], Cu2O [71–74], WO3 [75–77], Ag3PO4 [78–80], Fe2O3 [81–83], BiVO4, [84,85] and MnO2 [86,87], have been used successfully to fabricate other graphene-based composite photocatalysts via different synthesis procedures.

5.3.2 Synthesis of other graphene-based semiconductor photocatalysts The synthesis of other graphene-based photocatalysts includes the use of several metal sulfides, such as CdS [88–91], ZnS [92], ZnIn2S4 [93], In2S3, MoS2 [94], metal-free photocatalysts like graphitic carbon nitride (g-C3N4) [95,96], and ­nonmetal-doped ­materials [90,97–101]. Both GO and rGO have been used as p­ recursors in most

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Fig. 5.3  SEM (A) and TEM (B) images of GO, and TEM (C) and HRTEM (D) images of the G1.0 sample. Reproduced with permission from Q. Xiang, J. Yu, M. Jaroniec, Enhanced photocatalytic H2-production activity of graphene-modified titania nanosheets, Nanoscale 3 (9) (2011) 3670–3678. Copyright 2011, Royal Society of Chemistry.

of the graphene-based composites because of their high hydrophilicity and good dispersion in water. For example, rGO/CdS nanorod composites were prepared using a one-step, microwave-assisted hydrothermal method in an ethanolamine-water solution [90]. In another work, CdS/graphene composites were synthesized by a facile solvothermal method in dimethyl sulfoxide (DMSO), in which the formation of CdS nanoparticles and the reduction of GO to rGO occurred simultaneously [91]. It was found that graphene nanosheets can serve as a two-dimensional material where the CdS nanoparticles interact to avoid aggregation. Graphene nanosheets decorated with CdS clusters were prepared via a solvothermal process using GO, Cd(Ac)2 as the source of Cd2+, and DMSO as the source of S2− and solvent [89]. The reduction of GO into rGO and the formation of CdS clusters on the graphene surface occurred simultaneously during the solvothermal process. Similar results were obtained in the case of rGO/ZnIn2S4 nanocomposites prepared by a one-pot solvothermal method and a mixed solvent of N,N-dimethylformamide and ethylene glycol where the formation of ZnIn2S4 nanosheets on highly reductive rGO were simultaneously achieved [93].

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Fig. 5.4  Schematic illustrative and SEM images of TiO2 composites containing rGO and GO materials. Adapted with permission from L.M. Pastrana-Martínez, S. Morales-Torres, V. Likodimos, P. Falaras, J.L. Figueiredo, J.L. Faria, A.M.T. Silva, Role of oxygen functionalities on the synthesis of photocatalytically active graphene–TiO2 composites, Appl. Catal. B: Environ. 158–159 (2014) 329–340. Copyright 2014, Elsevier.

In addition to the growth of 0D CdS nanostructures on rGO nanosheets, 1D CdS nanostructures (e.g., nanorods and nanowires) [90,102] have also been anchored onto 2D rGO nanosheets to synthetize 1D-2D hybrid photocatalysts by a solvothermal method or an electrostatic self-assembly approach. Coupling of 2D CdS nanosheets and rGO sheets has been reported by a surface modification method using 4-­aminothiophenol (4-ATP) [103]. Fig. 5.5 shows a schematic representation of the composite preparation. The composites of positively functionalized CdS nanostructures and negatively charged GO clearly can be fabricated through electrostatically mediated self-assembly. Composite materials based on TiO2 nanocrystals grown in the presence of a layered MoS2/graphene hybrid have also been reported [94]. Graphene/MoS2-layered heterostructures were prepared by hydrothermal treatment of sodium molybdate, thiourea, and an aqueous GO solution at 210°C for 24 h. After that, further hydrothermal treatment of the obtained graphene/MoS2 hybrid with tetrabutyl titanate in ethanol/water solvent resulted in the formation of a graphene/MoS2/TiO2 composite photocatalyst. As a 2D metal-free organic semiconductor, g-C3N4 has a structure similar to those of graphene derivatives. Thus the fabrication of g-C3N4-based, metal-free photocatalysts with layered heterojunctions between GO and g-C3N4 has received significant attention because of their outstanding physicochemical and electrical properties [104]. Graphene/g-C3N4 materials were prepared through an impregnation-chemical reduction route with a subsequent thermal treatment at 550°C in an N2 atmosphere [95]. Melamine was used as a precursor of g-C3N4, and GO and hydrazine hydrate (as reducing agent) were employed to produce rGO. In another work, sandwich-like graphene/g-C3N4 (GCN) nanocomposites were developed through a facile one-pot,

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Fig. 5.5  Schematic representation of the preparation of CdS nanosheet/rGO composite. Adapted with permission from R. Bera, S. Kundu, A. Patra, 2D hybrid nanostructure of reduced graphene oxide–CdS nanosheet for enhanced photocatalysis, ACS Appl. Mater. Interfaces, 7 (24) (2015) 13251–13259. Copyright 2015, American Chemical Society.

impregnation-thermal reduction strategy, but using urea as the precursor of g-C3N4 [96]. Similarly, other monomers, such as cyanamide [105] and dicyandiamide [106], were employed to fabricate the graphene/g-C3N4 composite via this simple one-pot, impregnation-thermal reduction approach. On the other hand, doping with nonmetal atoms has also been explored as an effective strategy to tailor the electrical conductivity and electronic structure of graphenebased materials. Recently, N-doped GO quantum dots (NGO-QDs) were prepared by treating GO in NH3 at 500°C followed by a harsh oxidation step using a modified Hummers' method [99]. The co-doping of N and O atoms in the graphitic structure provided both p-type and n-type conductivities to NGO-QDs at the same time. In a subsequent study by the same authors [100], surface intact N-doped GO quantum dots were similarly prepared by ultrasonic exfoliation of NH3-treated GO sheets in a triethanolamine aqueous solution. N-doped graphene has also been synthetized by pyrolysis under an inert atmosphere of natural chitosan [97]. As a natural N-containing biopolymer, chitosan could be used as a single source of carbon and nitrogen, making the doping process in the graphitic structure more straightforward. The main parameter controlling the residual amount of N of the material was the pyrolysis temperature, its optimum value being established as 900°C. Apart from the incorporation of N atoms, chemical doping of graphene structures with other heteroatoms, such as phosphorous (P), which results in p-type conductive behaviors, can be simultaneously considered for the preparation of a graphene-based photocatalytst. In this context, P-doped

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graphene was developed by alginate modified with various amounts of H2PO4 at a neutral pH, followed by pyrolysis at a high temperature (900°C) under an inert atmosphere [98].

5.4 Energy applications Artificial photosynthesis is regarded as a potential long-term solution to mitigate the world's energy problems because it may produce solar-based fuels via photocatalytic water splitting and/or CO2 photoreduction, among other methods. The incorporation of graphene-based materials with semiconductor photocatalysts can lead to different promoting effects through one or more of the following effects: (i) supports material for enhanced structure stability; (ii) increases adsorption and active sites toward the reagents; (iii) uses electron acceptor and transport channel to suppress the recombination of photo-excited electron-hole pairs; (iv) involves a co-catalyst; (v) involves photosensitization; and (vi) has photocatalyst and band gap narrowing effect [16,107]. In the following section, the main applications of graphene-based semiconductor photocatalysts in the two referred processes are briefly reviewed.

5.4.1 Photocatalytic hydrogen generation During the past decade, graphene has shown great ability to enhance the photocatalytic H2 production performance of semiconductor photocatalysts [38]. GO/TiO2 photocatalysts were studied for water splitting under UV/vis irradiation [49]. XRD results showed that the average crystal size of TiO2 (anatase) was ∼11 nm for all samples with various GO contents. The GO/TiO2 composite with a 5 wt.% of GO exhibited a H2 evolution rate of 8.6 μmol h−1, 1.9 times higher than that obtained for the TiO2 benchmark, P25 (4.5 μmol h−1). The larger surface area as well as the excellent electronic conductivity of graphene that suppressed the recombination of photoinduced electrons and holes were the main factors enhancing the photocatalytic activity of the GO/TiO2 composite. It is known that graphene as a H2-evolution co-catalyst can greatly boost the photocatalytic activity of metal sulfides. CdS/GO photocatalysts with a uniform distribution of CdS clusters led to a more efficient transfer of photoinduced electrons from CdS to GO [89]. A high H2-production rate of 1.12 mmol h−1 was obtained at an optimal GO content of 1.0 wt.% (about 4.87 times higher than that of bare CdS), presenting an apparent quantum efficiency (QE) of 22.5% at λ = 420 nm. Graphene-supported CdS nanoparticles for photocatalytic H2 production were also prepared by a hydrothermal method [88]. In this case, the H2-production rate was 70 μmol h−1 for the graphene/ CdS composite at an optimal mass ratio of 0.01/1 under Xe lamp irradiation (200 W, λ ≥ 420 nm), while bare CdS only showed a rate of 14.5 μmol h−1. Significant band-gap narrowing was observed due to the strong interactions between CdS and graphene. Combined with the advantage of more efficient charge separation, the graphene-­ modified CdS photocatalyst exhibited much better photocatalytic H2-production performance than bare CdS.

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Graphene derivatives serve as excellent electron acceptors and transport channels to suppress the recombination of photoinduced electrons and holes, thus enhancing the efficiency of photocatalytic H2 production. Photocatalytic H2 production over graphene-supported ZnS nanoparticles under visible light illumination (λ > 420 nm) was studied [92]. The composite photocatalyst with an optimum 0.1 wt.% of GO achieved a H2-production rate of 7.42 μmol h−1 g−1, which was eight times higher than that of bare ZnS. The high photocatalytic H2 production activity was attributed to the photosensitization of graphene. In this case, the electrons photogenerated from graphene could be transferred to the CB of ZnS to participate in the photocatalytic process under visible light illumination. In another work [108], rGO was coupled with ZnxCd1−xS photocatalysts for photocatalytic H2 production under simulated solar irradiation using Na2S and Na2SO3 as sacrificial agents. The photocatalytic H2-production rate of the optimized rGO-Zn0.8Cd0.2S photocatalyst (0.25 wt.% rGO content) was 1824 μmol h−1 g−1 with an apparent QE of 23.0% at 420 nm. The performance was even better than that of optimized Pt-Zn0.8Cd0.2S under the same reaction conditions. It was observed that the introduction of rGO could effectively promote the transfer and separation of charge carriers and increase the surface-active sites for water reduction, thus leading to the enhanced performance. This work also indicated the promising potential of graphene to replace noble metals as a co-catalyst in specific photocatalytic systems for H2 production. The presence of a small amount of rGO (1.0 wt.%) could lead to a significant increase of specific surface area in rGO-ZnIn2S4 nanocomposites (e.g., 150 and 99.8 m2 g−1 for 1.0 wt.% rGO-ZnIn2S4 and bare ZnIn2S4, respectively) [93]. This rGO-ZnIn2S4 nanocomposite showed a H2 evolution rate of 40.9 μmol h−1, whereas the rate of bare ZnIn2S4 was only 9.5 μmol h−1 under visible-light illumination. The strong interaction between ZnIn2S4 nanosheets and rGO in the nanocomposites facilitated the electron transfer from ZnIn2S4 to rGO, with the latter serving as a good electron acceptor and mediator, as well as the co-catalyst for H2 evolution. The H2 production efficiency of those graphene-based binary photocatalysts can be improved by introducing an additional component to form graphene-based ternary composite photocatalysts, as reported for NiS/Zn0.5Cd0.5S/rGO ternary composites, where the three components were well-connected with each other [109]. Such a connection enables rGO to be an effective electron acceptor and transporter to capture photoinduced electrons from the CB of Zn0.5Cd0.5S and simultaneously offer reduction-­active centers for H2 evolution. At the optimal amount of 0.25 wt.% rGO and 3 mol % NiS, the ternary composite photocatalyst exhibited a H2-production rate of 376 μmol h−1 with a high apparent QE of 31% at 420 nm. An effective strategy to obtain an intimate and large contact interface is to construct 2D-2D layered junctions to provide abundant surface-active sites and achieve efficient interfacial charge transfer [110]. A graphene/g-C3N4 composite was applied in photocatalytic H2 production under visible light illumination [95]. The successful formation of 2D–2D layered junctions between g-C3N4 and graphene led to a very efficient interfacial charge separation, which enabled spatial accumulation of photoinduced electrons and holes on the sides of graphene and g-C3N4, respectively. The highest photocatalytic H2-production rate (451 μmol h−1 g−1) was achieved with the composite containing 1.0 wt.% graphene.

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The synergetic effect of MoS2 and graphene as co-catalysts for enhanced photocatalytic H2 production activity of TiO2 nanoparticles was also evaluated [94]. It was found that a layered MoS2/graphene (MG) hybrid served as a highly active co-catalyst for photocatalytic H2 production under Xe arc lamp irradiation, using TiO2 as the photocatalyst and ethanol as the sacrificial agent (Fig. 5.6A). The graphene/MoS2/TiO2 composite (TiO2/MG) photocatalyst with an optimal amount of 5.0 wt.% graphene and 0.5 wt.% MoS2 exhibited a high H2-production rate of 165.3 μmol h−1 with an apparent QE of 9.7% at 365 nm (Fig. 5.6B). This is logical because graphene possesses a high work function (∼−0.08 eV versus NHE) to capture photoinduced electrons from the CB of TiO2 [38]. The photogenerated electrons in the CB of TiO2 can be transferred to MoS2 nanosheets through the graphene sheets and then react with the adsorbed H+ ions at the edges of the MoS2 to generate H2. In fact, nanoscale MoS2 is highly active for H2 evolution as a result of the quantum-confinement effect. This indicates that, because of a notable synergetic effect between MoS2 nanosheets and graphene, the composite co-catalyst has several advantages, including suppression of charge recombination, improvement of interfacial charge transfer, and an increase in the number of active adsorption sites, as well photocatalytic reaction centers. Doped graphene materials have been used to improve the photocatalytic activity of graphene-based semiconductor composites in the photocatalytic generation of H2 because of a high intimate interfacial contact between doped graphene and semiconductor nanoparticles [111–114]. The fabrication of TiO2 nanoparticles-functionalized N-doped graphene composites for photocatalytic H2 generation for water (Fig. 5.7) has been studied [113]. N-doped rGO showed higher electrical conductivity than rGO because of its efficient structural restoration and smaller populations of defects in the graphitic structure. Moreover, the N atoms in N-doped graphene played important roles as nucleation and anchor sites for TiO2 nanoparticles, resulting in their uniform distribution on the graphene sheet. The photocatalytic activities for H2

Fig. 5.6  (A) Photocatalytic H2 evolution of TiO2/MG composites with different MoS2 and rGO contents in the MG hybrid as co-catalyst under UV irradiation; (B) schematic illustration of the charge transfer in TiO2/MG composites. Reproduced with permission from Q. Xiang, J. Yu, M. Jaroniec, Synergetic effect of MoS2 and graphene as cocatalysts for enhanced photocatalytic H2 production activity of TiO2 nanoparticles, J. Am. Chem. Soc. 134 (15) (2012) 6575–6578. Copyright 2012, American Chemical Society.

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Fig. 5.7  (A) Time course of H2 production with different catalysts; (B and C) schematic illustration of the strong coupling between TiO2 and N atoms in the rGO sheet and enhanced photo-induced charge transfer and photocatalytic H2 generation. Reproduced with permission from Z. Mou, Y. Wu, J. Sun, P. Yang, Y. Du, C. Lu, TiO2 nanoparticles-functionalized N-doped graphene with superior interfacial contact and enhanced charge separation for photocatalytic hydrogen generation, ACS Appl. Mater. Interfaces 6 (16) (2014) 13798–13806. Copyright 2014, American Chemical Society.

production were assessed under a 150 W Xe lamp. The average rate of H2 production of the N-graphene/TiO2 composite can reach 13.3 μmol h−1, which is higher than that of graphene/TiO2 (8.9 μmol h−1). In addition, improved durability during the photocatalytic process was observed with N-graphene/TiO2. On the other hand, doped graphene materials can also be used as photocatalysts on their own because of their intrinsic band gap. N-doped GO quantum dots (NGO-QDs) were employed as photocatalysts for water splitting under visible light illumination [99]. The band gap of the NGO-QDs was approximately 2.2 eV, the overall water splitting being achieved with a H2:O2 molar ratio of approximately 2:1. Normally, the p-type conductivity of the oxygen functional groups of GO is responsible for the production of H2, whereas the n-type conductivity of N-doped GO can benefit O2 evolution [100]. Later, the synergistic effect of O- and N-functionalities was verified by the same research group to produce H2 in triethanolamine aqueous solution. A phosphorus (P)-doped graphene material has also been studied for the photocatalytic generation of H2, with the P-doping leading to a conversion from zero-energy band gap graphene (0 eV) to semiconducting graphene (2.85 eV), which exhibited both UV and visible light activities toward photocatalytic H2 production [98]. The highest H2-generation rate was 282 μmol h−1 g−1 under UV/Vis light irradiation using triethanolamine as the sacrificial agent and Pt as the co-catalyst.

5.4.2 Photocatalytic reduction of carbon dioxide The massive release of CO2 into the atmosphere is believed to be resulting in significant climate changes; therefore a great deal of effort is being directed at reducing CO2 concentration in the atmosphere and preventing its emissions. The conversion of CO2 into fuel using a solar source has the potential to reduce the consumption of fossil fuels and thus help reduce humanity's impact on global warming and achieve worldwide targets set to reduce the overall carbon footprint [4].

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Several studies have emphasized the high performance of graphene-based photocatalyst composites for the CO2 reduction to solar fuels. For example, our research group [48] reported the effect of pH and copper oxide precursor salt on graphene derivative-TiO2 (GOT) composites for the photocatalytic reduction of CO2 with water. The pH was identified as the key variable determining the product distribution. Thus it was observed that the prepared GOT composite photocatalyst exhibited superior photocatalytic activity for ethanol (EtOH) production (144.7 μmol g−1 h−1) at pH 11.0 and for methanol (MeOH) production at pH 4.0. Moreover, the presence of both GO and copper in the GOT composites extended the absorption to the visible spectral range, enhancing the CO2 photoreduction in the aqueous phase (Fig. 5.8A). The combination of TiO2 with GO generates a synergistic effect that potentially enhances the photoactivity because of the increase of the adsorption capacity and efficient interfacial electron transfer between the two constituent phases (Fig. 5.8B) [56]. Moreover, the combined contribution of the lower band gap energy and the known quenching of photoluminescence determined by Raman spectroscopy for the GOT catalyst are possible explanations for the higher performance observed, as compared with P25. The synthesis of Cu2O/rGO composites has been reported in regard to the photocatalytic reduction of CO2 by using a 150 W Xe lamp as a light source [71]. Given the good electron conductivity and large specific surface area of rGO, the composite exhibited a low electron-hole recombination rate and high number of surface-active sites. As a result, the prepared material showed a photocatalytic activity six times greater than that of the optimized Cu2O for the CO2 reduction. More interesting, however, is the fact that the photocorrosion problem of Cu2O can also be effectively mitigated by the rGO loading. Therefore the loading of graphene not only improved the photocatalytic activity of the photocatalyst but also enhanced its photostability. Graphene exhibited unique properties when it was hybridized with other materials to function as a co-catalyst. The production of MeOH was described by the photocatalytic reduction of CO2 under visible light illumination using a graphene and

Fig. 5.8  (A) CO2 photoreduction over GOT, GOT-500, and Cu-loaded GOT catalysts at pH 11.0 and 180 min; (B) conceptual scheme of the CO2 photoreduction catalyzed by Cu-loaded GOT composites. Reproduced with permission from L.M. Pastrana-Martínez, A.M.T. Silva, N.N.C. Fonseca, J.R. Vaz, J.L. Figueiredo, J.L. Faria, Photocatalytic reduction of CO2 with water into methanol and ethanol using graphene derivative–TiO2 composites: effect of pH and copper(I) oxide, Top. Catal. 59 (15–16) (2016) 1279–1291. Copyright 2016, Springer.

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­tourmaline co-doped TiO2 nanocomposites (GT/T) [115]. GT/T exhibited significantly improved activity compared to the graphene-loaded TiO2, tourmaline-loaded TiO2, and bare TiO2. This enhancement was attributed to the synergistic effect of graphene and tourmaline. Specifically, both graphene and tourmaline can improve electron-hole separation, whereas graphene can reduce the band gap of TiO2. As a result, GT/T led to the enhanced MeOH production rate via photocatalytic CO2 reduction, which was 21 times higher than that of bare TiO2. A well-defined nanocomposite interface, such as a 2D–2D composite, is important in preparing highly efficient graphene-based photocatalysts [110]. Robust hollow spheres consisting of molecular-scale alternating titania (Ti0.91O2) nanosheets and graphene nanosheets were used for the photocatalytic reduction of CO2 using a 300 W Xe arc lamp [116]. Because both TiO2 and graphene nanosheets are 2D structures, graphene and Ti0.91O2 had a close and large contact surface area. The prepared samples exhibited a high CO formation rate via the photocatalytic CO2 reduction, which was nine times higher than that of the commercial P25, because of its fast electron-­hole separation and good light utilization. 2D-2D layered photocatalysts based on sandwich-like graphene-g-C3N4 (GCN) composite showed enhanced visible light photocatalytic CO2 reduction activity [96]. The GCN sample demonstrated high visible-­light photoactivity toward CO2 reduction under ambient conditions, exhibiting a 2.3-fold enhancement over bare g-C3N4 (Fig. 5.9A). This effect was ascribed to the inhibition of the electron-hole pair recombination by graphene, which increased the charge transfer (Fig. 5.9B). Nonmetal doping is another efficient way to tune the physical, optical, and physicochemical properties of graphene for photoreduction of CO2. Boron (B)-doped graphene (B-GR) nanosheets loaded on P25 nanoparticles have been proposed to improve the photocatalytic reduction of CO2 using a 300 W Xe lamp as the light source [101]. B-GR showed a higher Fermi level than pristine graphene, falling between the CB of P25 and the relevant CO2/CH4 redox potential. The tunable band gap of B-GR determined the large potential application of P25/B-GR in the photoreduction of CO2.

Fig. 5.9  (A) Total CH4 yield over the as-prepared photocatalysts; (B) schematic diagram of photogenerated charge transfer in the GCN system for CO2 reduction with H2O to form CH4 under visible light. Reproduced with permission from W.-J. Ong, L.-L. Tan, S.-P. Chai, S.-T. Yong, Graphene oxide as a structure-directing agent for the two-dimensional interface engineering of sandwich-like graphene-g-C3N4 hybrid nanostructures with enhanced visible-light photoreduction of CO2 to methane, Chem. Commun. 51 (5) (2015) 858–861. Copyright 2014, Royal Society of Chemistry.

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Moreover, the presence of graphene derivatives in the composite photocatalysts could also greatly enhance the adsorption capacity of CO2 [90]. A close linear relationship was found between the CO2 adsorption capacity of the composite photocatalysts and the rGO content, which was independent of the specific surface areas. The CO2 adsorption sites can also function as the active sites for CO2 photoreduction, thus facilitating the direct activation of adsorbed CO2 and the enhancement of photocatalytic activity for CO2 photoreduction. Furthermore, the CO2 adsorption capacity and catalytic activity of carbon co-catalysts could be further enhanced by nitrogen doping. It was recently found that N dopants play a crucial role in the photoactivity and stability of GO-TiO2 composites for the photoreduction of CO2 [117]. N-rGO with an appropriate N quantity and N-bonding configuration acted as a dual-function promoter, simultaneously enhancing CO2 adsorption on the photocatalyst surface and facilitating electron-hole separation, and eventually boosted the photocatalytic performance. This work may inspire some new ideas for designing nanocarbon composite co-catalysts with improved CO2 adsorption capacity and catalytic activity for CO2 photoreduction via coupling graphene derivatives.

5.5 Conclusions and outlook Graphene-based photocatalytic composites are robust materials with the potential to solve the world’s increasing energy demand. In this chapter, we summarized recent accounts of the synthesis and energy applications of graphene-based photocatalysts, particularly those prepared with GO, rGO, and heteroatom-doped graphene. The high morphological and electronic versatility of graphene materials offers the possibility of designing novel photocatalytically active materials for solar fuels, including photocatalytic water splitting to H2 and photocatalytic reduction of CO2 to hydrocarbons. The incorporation of graphene derivatives into various semiconductor photocatalysts has demonstrated that this approach can improve photocatalytic performance because of a combination of several factors: (i) suppressed photogenerated carrier recombination; (ii) increased adsorption capacity; (iii) enhanced photostability; and (iv) enhanced light absorption. Despite the considerable, rapid progress, several challenges remain in the synthesis and application of graphene-based photocatalyst composites for highly efficient solar fuel generation. First, improvements need to be made in the large-scale production of graphene-based photocatalysts with controlled morphologies and compositions as well as with an intimate contact interface. In this regard, more efficient synthesis methods to achieve enhanced performance of graphene-derivative materials and graphene-based semiconductor composites are required. Second, the efficiencies of solar fuel generation by photocatalysis are far from being optimal and considerable breakthroughs must be made before this method can be considered as a viable economical process. Finally, current studies using graphene-based materials focus mostly on solar fuel generation in the presence of sacrificial agents. Therefore the development of graphene-based materials with improved photocatalytic efficiency using natural renewable resources like water and sun is highly encouraged if we are to achieve clean and renewable energy.

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Acknowledgments Financial support for this work was provided by “[email protected]—Advanced Industrial Processes and Materials for a Sustainable Northern Region of Portugal 2020,” with the reference NORTE-01-0145-FEDER-000006, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDF) and the Project POCI-01-0145-FEDER-006984, Associate Laboratory LSRE-LCM funded by ERDF through COMPETE2020—Programa Operacional Competitividade e Internacionalização (POCI); and by national funds through FCT—Fundação para a Ciência e a Tecnologia. LMPM, SMT, and AMTS acknowledge the FCT Investigator Programme (IF/01248/2014, IF/00573/2015, and IF/01501/2013, respectively) with financing from the European Social Fund and the Human Potential Operational Programme. LMPM also acknowledges the Spanish Ministry of Economy and Competitiveness (MINECO) for a Ramon y Cajal research contract (RYC-2016-19347).

References [1] S. Perathoner, G. Centi, CO2 recycling: a key strategy to introduce green energy in the chemical production chain, ChemSusChem 7 (5) (2014) 1274–1282. [2] G. Centi, E.A. Quadrelli, S. Perathoner, Catalysis for CO2 conversion: a key technology for rapid introduction of renewable energy in the value chain of chemical industries, Energy Environ. Sci. 6 (6) (2013) 1711–1731. [3] P.D. Tran, L.H. Wong, J. Barber, J.S.C. Loo, Recent advances in hybrid photocatalysts for solar fuel production, Energy Environ. Sci. 5 (3) (2012) 5902–5918. [4] A. Corma, H. Garcia, Photocatalytic reduction of CO2 for fuel production: possibilities and challenges, J. Catal. 308 (2013) 168–175. [5] A. Dhakshinamoorthy, S. Navalon, A. Corma, H. Garcia, Photocatalytic CO2 reduction by TiO2 and related titanium containing solids, Energy Environ. Sci. 5 (11) (2012) 9217–9233. [6] S.C. Roy, O.K. Varghese, M. Paulose, C.A. Grimes, Toward solar fuels: photocatalytic conversion of carbon dioxide to hydrocarbons, ACS Nano 4 (3) (2010) 1259–1278. [7] H. Ahmad, S.K. Kamarudin, L.J. Minggu, M. Kassim, Hydrogen from photo-catalytic water splitting process: a review, Renew. Sust. Energ. Rev. 43 (2015) 599–610. [8] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (5358) (1972) 37–38. [9] T. Inoue, A. Fujishima, S. Konishi, K. Honda, Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders, Nature 277 (5698) (1979) 637–638. [10] K.S.  Novoselov, A.K.  Geim, S.V.  Morozov, D.  Jiang, Y.  Zhang, S.V.  Dubonos, I.V.  Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (5696) (2004) 666–669. [11] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (3) (2007) 183–191. [12] A.K. Geim, Graphene: status and prospects, Science 324 (5934) (2009) 1530–1534. [13] S.S. Varghese, S. Lonkar, K.K. Singh, S. Swaminathan, A. Abdala, Recent advances in graphene based gas sensors, Sensors Actuators B Chem. 218 (2015) 160–183. [14] H. Wu, Y. Zhang, L. Cheng, L. Zheng, Y. Li, W. Yuan, X. Yuan, Graphene based architectures for electrochemical capacitors, Energy Storage Mater. 5 (2016) 8–32. [15] O.  Akhavan, A.  Meidanchi, E.  Ghaderi, S.  Khoei, Zinc ferrite spinel-graphene in magneto-­photothermal therapy of cancer, J. Mater. Chem. B 2 (21) (2014) 3306–3314.

96

Multifunctional Photocatalytic Materials for Energy

[16] Q.  Xiang, B.  Cheng, J.  Yu, Graphene-based photocatalysts for solar-fuel generation, Angew. Chem. Int. Ed. 54 (39) (2015) 11350–11366. [17] J. Low, B. Cheng, J. Yu, M. Jaroniec, Carbon-based two-dimensional layered materials for photocatalytic CO2 reduction to solar fuels, Energy Storage Mater. 3 (2016) 24–35. [18] S.M. Tan, A. Ambrosi, C.K. Chua, M. Pumera, Electron transfer properties of chemically reduced graphene materials with different oxygen contents, J. Mater. Chem. A 2 (27) (2014) 10668–10675. [19] B.F. Machado, P. Serp, Graphene-based materials for catalysis, Catal. Sci. Technol. 2 (1) (2011) 54–75. [20] S.  Morales-Torres, L.M.  Pastrana-Martínez, J.L.  Figueiredo, J.L.  Faria, A.M.T.  Silva, Design of graphene-based TiO2 photocatalysts - a review, Environ. Sci. Pollut. Res. 19 (9) (2012) 3676–3687. [21] S. Pei, H.-M. Cheng, The reduction of graphene oxide, Carbon 50 (9) (2012) 3210–3228. [22] C.K. Chua, M. Pumera, Chemical reduction of graphene oxide: a synthetic chemistry viewpoint, Chem. Soc. Rev. 43 (1) (2014) 291–312. [23] J. Du, X. Lai, N. Yang, J. Zhai, D. Kisailus, F. Su, D. Wang, L. Jiang, Hierarchically ordered macro−mesoporous tio2−graphene composite films: improved mass transfer, reduced charge recombination, and their enhanced photocatalytic activities, ACS Nano 5 (1) (2010) 590–596. [24] P.V.  Kamat, Graphene-based nanoarchitectures. Anchoring semiconductor and metal nanoparticles on a two-dimensional carbon support, J. Phys. Chem. Lett. 1 (2) (2009) 520–527. [25] J. Albero, H. Garcia, Doped graphenes in catalysis, J. Mol. Catal. A Chem. 408 (2015) 296–309. [26] L.K. Putri, W.-J. Ong, W.S. Chang, S.-P. Chai, Heteroatom doped graphene in photocatalysis: a review, Appl. Surf. Sci. 358 (Part A) (2015) 2–14. [27] X. Wang, X. Li, L. Zhang, Y. Yoon, P.K. Weber, H. Wang, J. Guo, H. Dai, N-doping of graphene through electrothermal reactions with ammonia, Science 324 (5928) (2009) 768–771. [28] L.-S. Zhang, X.-Q. Liang, W.-G. Song, Z.-Y. Wu, Identification of the nitrogen species on N-doped graphene layers and Pt/NG composite catalyst for direct methanol fuel cell, Phys. Chem. Chem. Phys. 12 (38) (2010) 12055–12059. [29] Z.-H. Sheng, L. Shao, J.-J. Chen, W.-J. Bao, F.-B. Wang, X.-H. Xia, Catalyst-free synthesis of nitrogen-doped graphene via thermal annealing graphite oxide with melamine and its excellent electrocatalysis, ACS Nano 5 (6) (2011) 4350–4358. [30] H. Wang, Y. Zhou, D. Wu, L. Liao, S. Zhao, H. Peng, Z. Liu, Synthesis of boron-doped graphene monolayers using the sole solid feedstock by chemical vapor deposition, Small 9 (8) (2013) 1316–1320. [31] R.  Li, Z.  Wei, X.  Gou, W.  Xu, Phosphorus-doped graphene nanosheets as efficient metal-­free oxygen reduction electrocatalysts, RSC Adv. 3 (25) (2013) 9978–9984. [32] Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X.A. Chen, S. Huang, Sulfurdoped graphene as an efficient metal-free cathode catalyst for oxygen reduction, ACS Nano 6 (1) (2011) 205–211. [33] H.L. Poh, P. Šimek, Z. Sofer, M. Pumera, Sulfur-doped graphene via thermal exfoliation of graphite oxide in H2S, SO2, or CS2 gas, ACS Nano 7 (6) (2013) 5262–5272. [34] S. Navalon, A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Carbocatalysis by graphenebased materials, Chem. Rev. 114 (12) (2014) 6179–6212. [35] P. Avouris, Graphene: electronic and photonic properties and devices, Nano Lett. 10 (11) (2010) 4285–4294.

Graphene photocatalysts97

[36] A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K. Geim, The electronic properties of graphene, Rev. Mod. Phys. 81 (1) (2009) 109–162. [37] J. Luo, J. Kim, J. Huang, Material processing of chemically modified graphene: some challenges and solutions, Acc. Chem. Res. 46 (10) (2013) 2225–2234. [38] G. Xie, K. Zhang, B. Guo, Q. Liu, L. Fang, J.R. Gong, Graphene-based materials for hydrogen generation from light-driven water splitting, Adv. Mater. 25 (28) (2013) 3820–3839. [39] K.P. Loh, Q. Bao, P.K. Ang, J. Yang, The chemistry of graphene, J. Mater. Chem. 20 (12) (2010) 2277–2289. [40] A. Mathkar, D. Tozier, P. Cox, P. Ong, C. Galande, K. Balakrishnan, A. Leela Mohana Reddy, P.M.  Ajayan, Controlled, stepwise reduction and band gap manipulation of graphene oxide, J. Phys. Chem. Lett. 3 (8) (2012) 986–991. [41] N. Li, G. Liu, C. Zhen, F. Li, L. Zhang, H.-M. Cheng, Battery performance and photocatalytic activity of mesoporous anatase TiO2 nanospheres/graphene composites by template-free self-assembly, Adv. Funct. Mater. 21 (9) (2011) 1717–1722. [42] B. Liu, Y. Huang, Y. Wen, L. Du, W. Zeng, Y. Shi, F. Zhang, G. Zhu, X. Xu, Y. Wang, Highly dispersive {001} facets-exposed nanocrystalline TiO2 on high quality graphene as a high performance photocatalyst, J. Mater. Chem. 22 (15) (2012) 7484–7491. [43] W.-S. Wang, D.-H. Wang, W.-G. Qu, L.-Q. Lu, A.-W. Xu, Large ultrathin anatase TiO2 nanosheets with exposed {001} facets on graphene for enhanced visible light photocatalytic activity, J. Phys. Chem. C 116 (37) (2012) 19893–19901. [44] Y.  Zhang, Z.-R.  Tang, X.  Fu, Y.-J.  Xu, Engineering the unique 2D mat of graphene to achieve graphene-TiO2 nanocomposite for photocatalytic selective transformation: what advantage does graphene have over its forebear carbon nanotube? ACS Nano 5 (9) (2011) 7426–7435. [45] Y. Zhang, N. Zhang, Z.-R. Tang, Y.-J. Xu, Improving the photocatalytic performance of graphene-TiO2 nanocomposites via a combined strategy of decreasing defects of graphene and increasing interfacial contact, Phys. Chem. Chem. Phys. 14 (25) (2012) 9167–9175. [46] Z. Wang, B. Huang, Y. Dai, Y. Liu, X. Zhang, X. Qin, J. Wang, Z. Zheng, H. Cheng, Crystal facets controlled synthesis of [email protected] nanocomposites by a one-pot hydrothermal process, CrystEngComm 14 (5) (2012) 1687–1692. [47] G. Jiang, Z. Lin, C. Chen, L. Zhu, Q. Chang, N. Wang, W. Wei, H. Tang, TiO2 nanoparticles assembled on graphene oxide nanosheets with high photocatalytic activity for removal of pollutants, Carbon 49 (8) (2011) 2693–2701. [48] L.M.  Pastrana-Martínez, A.M.T.  Silva, N.N.C.  Fonseca, J.R.  Vaz, J.L.  Figueiredo, J.L. Faria, Photocatalytic reduction of CO2 with water into methanol and ethanol using graphene derivative–TiO2 composites: effect of ph and copper(I) oxide, Top. Catal. 59 (15-16) (2016) 1279–1291. [49] X.-Y.  Zhang, H.-P.  Li, X.-L.  Cui, Y.  Lin, Graphene/TiO2 nanocomposites: synthesis, characterization and application in hydrogen evolution from water photocatalytic splitting, J. Mater. Chem. 20 (14) (2010) 2801–2806. [50] Y. Liang, H. Wang, H. Sanchez Casalongue, Z. Chen, H. Dai, TiO2 nanocrystals grown on graphene as advanced photocatalytic hybrid materials, Nano Res. 3 (10) (2010) 701–705. [51] X. Cao, G. Tian, Y. Chen, J. Zhou, W. Zhou, C. Tian, H. Fu, Hierarchical composites of TiO2 nanowire arrays on reduced graphene oxide nanosheets with enhanced photocatalytic hydrogen evolution performance, J. Mater. Chem. A 2 (12) (2014) 4366–4374.

98

Multifunctional Photocatalytic Materials for Energy

[52] P. Dong, Y. Wang, L. Guo, B. Liu, S. Xin, J. Zhang, Y. Shi, W. Zeng, S. Yin, A facile one-step solvothermal synthesis of graphene/rod-shaped TiO2 nanocomposite and its improved photocatalytic activity, Nanoscale 4 (15) (2012) 4641–4649. [53] N. Zhang, M.-Q. Yang, S. Liu, Y. Sun, Y.-J. Xu, Waltzing with the versatile platform of graphene to synthesize composite photocatalysts, Chem. Rev. 115 (18) (2015) 10307–10377. [54] X. Xie, K. Kretschmer, G. Wang, Advances in graphene-based semiconductor photocatalysts for solar energy conversion: fundamentals and materials engineering, Nanoscale 7 (32) (2015) 13278–13292. [55] G. Williams, B. Seger, P.V. Kamat, TiO2-graphene nanocomposites. UV-assisted photocatalytic reduction of graphene oxide, ACS Nano 2 (7) (2008) 1487–1491. [56] L.M. Pastrana-Martínez, S. Morales-Torres, V. Likodimos, J.L. Figueiredo, J.L. Faria, P.  Falaras, A.M.T.  Silva, Advanced nanostructured photocatalysts based on reduced graphene oxide–TiO2 composites for degradation of diphenhydramine pharmaceutical and methyl orange dye, Appl. Catal. B Environ. 123–124 (2012) 241–256. [57] Y.H. Ng, A. Iwase, N.J. Bell, A. Kudo, R. Amal, Semiconductor/reduced graphene oxide nanocomposites derived from photocatalytic reactions, Catal. Today 164 (1) (2011) 353–357. [58] J. Liu, H. Bai, Y. Wang, Z. Liu, X. Zhang, D.D. Sun, Self-assembling TiO2 nanorods on large graphene oxide sheets at a two-phase interface and their anti-recombination in photocatalytic applications, Adv. Funct. Mater. 20 (23) (2010) 4175–4181. [59] S. Liu, C. Liu, W. Wang, B. Cheng, J. Yu, Unique photocatalytic oxidation reactivity and selectivity of TiO2-graphene nanocomposites, Nanoscale 4 (10) (2012) 3193–3200. [60] W. Wang, J. Yu, Q. Xiang, B. Cheng, Enhanced photocatalytic activity of hierarchical macro/mesoporous TiO2–graphene composites for photodegradation of acetone in air, Appl. Catal. B Environ. 119–120 (2012) 109–116. [61] W. Tu, Y. Zhou, Q. Liu, S. Yan, S. Bao, X. Wang, M. Xiao, Z. Zou, An in situ simultaneous reduction-hydrolysis technique for fabrication of TiO2-graphene 2D sandwich-like hybrid nanosheets: graphene-promoted selectivity of photocatalytic-driven hydrogenation and coupling of CO2 into methane and ethane, Adv. Funct. Mater. 23 (14) (2013) 1743–1749. [62] M.S.A. Sher Shah, A.R. Park, K. Zhang, J.H. Park, P.J. Yoo, Green synthesis of biphasic TiO2–reduced graphene oxide nanocomposites with highly enhanced photocatalytic activity, ACS Appl. Mater. Interfaces 4 (8) (2012) 3893–3901. [63] J. Liu, L. Liu, H. Bai, Y. Wang, D.D. Sun, Gram-scale production of graphene oxide– TiO2 nanorod composites: towards high-activity photocatalytic materials, Appl. Catal. B Environ. 106 (1–2) (2011) 76–82. [64] J.S.  Lee, K.H.  You, C.B.  Park, Highly photoactive, low bandgap TiO2 nanoparticles wrapped by graphene, Adv. Mater. 24 (8) (2012) 1084–1088. [65] Q.  Xiang, J.  Yu, M.  Jaroniec, Enhanced photocatalytic H2-production activity of graphene-modified titania nanosheets, Nanoscale 3 (9) (2011) 3670–3678. [66] L.M.  Pastrana-Martínez, S.  Morales-Torres, V.  Likodimos, P.  Falaras, J.L.  Figueiredo, J.L. Faria, A.M.T. Silva, Role of oxygen functionalities on the synthesis of photocatalytically active graphene–TiO2 composites, Appl. Catal. B Environ. 158–159 (2014) 329–340. [67] Y. Feng, N. Feng, Y. Wei, G. Zhang, An in situ gelatin-assisted hydrothermal synthesis of ZnO-reduced graphene oxide composites with enhanced photocatalytic performance under ultraviolet and visible light, RSC Adv. 4 (16) (2014) 7933–7943. [68] B. Weng, M.-Q. Yang, N. Zhang, Y.-J. Xu, Toward the enhanced photoactivity and photostability of ZnO nanospheres via intimate surface coating with reduced graphene oxide, J. Mater. Chem. A 2 (24) (2014) 9380–9389.

Graphene photocatalysts99

[69] A. Bhirud, S. Sathaye, R. Waichal, C.-J. Park, B. Kale, In situ preparation of N-ZnO/ graphene nanocomposites: excellent candidate as a photocatalyst for enhanced solar hydrogen generation and high performance supercapacitor electrode, J. Mater. Chem. A 3 (33) (2015) 17050–17063. [70] C. Han, Z. Chen, N. Zhang, J.C. Colmenares, Y.-J. Xu, Hierarchically CdS decorated 1D ZnO nanorods-2D graphene hybrids: low temperature synthesis and enhanced photocatalytic performance, Adv. Funct. Mater. 25 (2) (2015) 221–229. [71] X. An, K. Li, J. Tang, Cu2O/reduced graphene oxide composites for the photocatalytic conversion of CO2, ChemSusChem 7 (4) (2014) 1086–1093. [72] X.  Liu, Z.  Li, W.  Zhao, C.  Zhao, Y.  Wang, Z.  Lin, A facile route to the synthesis of reduced graphene oxide-wrapped octahedral Cu2O with enhanced photocatalytic and photovoltaic performance, J. Mater. Chem. A 3 (37) (2015) 19148–19154. [73] L.  Xu, F.  Zhang, X.  Song, Z.  Yin, Y.  Bu, Construction of reduced graphene oxide-­ supported Ag-Cu2O composites with hierarchical structures for enhanced photocatalytic activities and recyclability, J. Mater. Chem. A 3 (11) (2015) 5923–5933. [74] Y.-T.  Xu, Y.  Guo, C.  Li, X.-Y.  Zhou, M.C.  Tucker, X.-Z.  Fu, R.  Sun, C.-P.  Wong, Graphene oxide nano-sheets wrapped Cu2O microspheres as improved performance anode materials for lithium ion batteries, Nano Energy 11 (2015) 38–47. [75] B.  Weng, J.  Wu, N.  Zhang, Y.-J.  Xu, Observing the role of graphene in boosting the two-electron reduction of oxygen in graphene–WO3 nanorod photocatalysts, Langmuir 30 (19) (2014) 5574–5584. [76] X. An, J.C. Yu, Y. Wang, Y. Hu, X. Yu, G. Zhang, WO3 nanorods/graphene nanocomposites for high-efficiency visible-light-driven photocatalysis and NO2 gas sensing, J. Mater. Chem. 22 (17) (2012) 8525–8531. [77] H. Huang, Z. Yue, G. Li, X. Wang, J. Huang, Y. Du, P. Yang, Ultraviolet-assisted preparation of mesoporous WO3/reduced graphene oxide composites: superior interfacial contacts and enhanced photocatalysis, J. Mater. Chem. A 1 (47) (2013) 15110–15116. [78] C. Cui, Y. Wang, D. Liang, W. Cui, H. Hu, B. Lu, S. Xu, X. Li, C. Wang, Y. Yang, Photoassisted synthesis of Ag3PO4/reduced graphene oxide/Ag heterostructure photocatalyst with enhanced photocatalytic activity and stability under visible light, Appl. Catal. B Environ. 158–159 (2014) 150–160. [79] X.-j. Chen, Y.-Z. Dai, X.-Y. Wang, J. Guo, T.-H. Liu, F.-F. Li, Synthesis and characterization of Ag3PO4 immobilized with graphene oxide (GO) for enhanced photocatalytic activity and stability over 2,4-dichlorophenol under visible light irradiation, J. Hazard. Mater. 292 (2015) 9–18. [80] Q.  Xiang, D.  Lang, T.  Shen, F.  Liu, Graphene-modified nanosized Ag3PO4 photocatalysts for enhanced visible-light photocatalytic activity and stability, Appl. Catal. B Environ. 162 (2015) 196–203. [81] S. Han, L. Hu, Z. Liang, S. Wageh, A.A. Al-Ghamdi, Y. Chen, X. Fang, One-step hydrothermal synthesis of 2D hexagonal nanoplates of α-Fe2O3/graphene composites with enhanced photocatalytic activity, Adv. Funct. Mater. 24 (36) (2014) 5719–5727. [82] Y. Dai, Y. Chai, Y. Sun, W. Fu, X. Wang, Q. Gu, T.H. Zeng, Y. Sun, New versatile Pt supports composed of graphene sheets decorated by Fe2O3 nanorods and N-dopants with high activity based on improved metal/support interactions, J. Mater. Chem. A 3 (1) (2015) 125–130. [83] H. Wang, F. Raziq, Y. Qu, C. Qin, J. Wang, L. Jing, Role of quaternary N in N-doped graphene-Fe2O3 nanocomposites as efficient photocatalysts for CO2 reduction and acetaldehyde degradation, RSC Adv. 5 (103) (2015) 85061–85064. [84] Y. Li, Z. Sun, S. Zhu, Y. Liao, Z. Chen, D. Zhang, Fabrication of BiVO4 nanoplates with active facets on graphene sheets for visible-light photocatalyst, Carbon 94 (2015) 599–606.

100

Multifunctional Photocatalytic Materials for Energy

[85] F. Zhou, Y. Min, J. Fan, Q. Xu, Reduced graphene oxide-grafted cylindrical like W doped BiVO4 hybrids with enhanced performances for photocatalytic applications, Chem. Eng. J. 266 (2015) 48–55. [86] Q.  Yu, J.  Xu, C.  Wu, J.  Zhang, L.  Guan, MnO2 nanofilms on nitrogen-doped hollow graphene spheres as a high-performance electrocatalyst for oxygen reduction reaction, ACS Appl. Mater. Interfaces 8 (51) (2016) 35264–35269. [87] S. Chen, J. Zhu, X. Wu, Q. Han, X. Wang, Graphene oxide−MnO2 nanocomposites for supercapacitors, ACS Nano 4 (5) (2010) 2822–2830. [88] A. Ye, W. Fan, Q. Zhang, W. Deng, Y. Wang, CdS-graphene and CdS-CNT nanocomposites as visible-light photocatalysts for hydrogen evolution and organic dye degradation, Catal. Sci. Technol. 2 (5) (2012) 969–978. [89] Q.  Li, B.  Guo, J.  Yu, J.  Ran, B.  Zhang, H.  Yan, J.R.  Gong, Highly efficient visible-light-driven photocatalytic hydrogen production of CdS-cluster-decorated ­ graphene nanosheets, J. Am. Chem. Soc. 133 (28) (2011) 10878–10884. [90] J. Yu, J. Jin, B. Cheng, M. Jaroniec, A noble metal-free reduced graphene oxide-CdS nanorod composite for the enhanced visible-light photocatalytic reduction of CO2 to solar fuel, J. Mater. Chem. A 2 (10) (2014) 3407–3416. [91] N. Zhang, Y. Zhang, X. Pan, X. Fu, S. Liu, Y.-J. Xu, Assembly of CdS nanoparticles on the two-dimensional graphene scaffold as visible-light-driven photocatalyst for selective organic transformation under ambient conditions, J. Phys. Chem. C 115 (47) (2011) 23501–23511. [92] W. Faze, Z. Maojun, Z. Changqing, Z. Bin, C. Wen, M. Li, S. Wenzhong, Visible light photocatalytic H2-production activity of wide band gap ZnS nanoparticles based on the photosensitization of graphene, Nanotechnology 26 (34) (2015) 345402. [93] L. Ye, J. Fu, Z. Xu, R. Yuan, Z. Li, Facile one-pot solvothermal method to synthesize sheet-on-sheet reduced graphene Oxide (RGO)/ZnIn2S4 nanocomposites with superior photocatalytic performance, ACS Appl. Mater. Interfaces 6 (5) (2014) 3483–3490. [94] Q. Xiang, J. Yu, M. Jaroniec, Synergetic effect of MoS2 and graphene as cocatalysts for enhanced photocatalytic H2 production activity of TiO2 nanoparticles, J. Am. Chem. Soc. 134 (15) (2012) 6575–6578. [95] Q. Xiang, J. Yu, M. Jaroniec, Preparation and enhanced visible-light photocatalytic H2production activity of graphene/C3N4 composites, J. Phys. Chem. C 115 (15) (2011) 7355–7363. [96] W.-J. Ong, L.-L. Tan, S.-P. Chai, S.-T. Yong, Graphene oxide as a structure-directing agent for the two-dimensional interface engineering of sandwich-like graphene-g-C3N4 hybrid nanostructures with enhanced visible-light photoreduction of CO2 to methane, Chem. Commun. 51 (5) (2015) 858–861. [97] C. Lavorato, A. Primo, R. Molinari, H. Garcia, N-doped graphene derived from biomass as a visible-light photocatalyst for hydrogen generation from water/methanol mixtures, Chem. Eur. J. 20 (1) (2014) 187–194. [98] M. Latorre-Sánchez, A. Primo, H. García, P-doped graphene obtained by pyrolysis of modified alginate as a photocatalyst for hydrogen generation from water–methanol mixtures, Angew. Chem. Int. Ed. 52 (45) (2013) 11813–11816. [99] T.-F.  Yeh, C.-Y.  Teng, S.-J.  Chen, H.  Teng, Nitrogen-doped graphene oxide quantum dots as photocatalysts for overall water-splitting under visible light illumination, Adv. Mater. 26 (20) (2014) 3297–3303. [100] T.-F. Yeh, S.-J. Chen, H. Teng, Synergistic effect of oxygen and nitrogen functionalities for graphene-based quantum dots used in photocatalytic H2 production from water decomposition, Nano Energy 12 (2015) 476–485.

Graphene photocatalysts101

[101] M. Xing, F. Shen, B. Qiu, J. Zhang, Highly-dispersed boron-doped graphene nanosheets loaded with TiO2 nanoparticles for enhancing CO2 photoreduction, Sci. Rep. 4 (2014) 6341. [102] S. Liu, B. Weng, Z.-R. Tang, Y.-J. Xu, Constructing one-dimensional silver nanowire-­ doped reduced graphene oxide integrated with CdS nanowire network hybrid structures toward artificial photosynthesis, Nanoscale 7 (3) (2015) 861–866. [103] R. Bera, S. Kundu, A. Patra, 2D hybrid nanostructure of reduced graphene oxide–CdS nanosheet for enhanced photocatalysis, ACS Appl. Mater. Interfaces 7 (24) (2015) 13251–13259. [104] X. Li, J. Yu, S. Wageh, A.A. Al-Ghamdi, J. Xie, Graphene in photocatalysis: a review, Small 12 (48) (2016) 6640–6696. [105] Y. Li, H. Zhang, P. Liu, D. Wang, Y. Li, H. Zhao, Cross-linked g-C3N4/rGO nanocomposites with tunable band structure and enhanced visible light photocatalytic activity, Small 9 (19) (2013) 3336–3344. [106] Y. Fu, J. Zhu, C. Hu, X. Wu, X. Wang, Covalently coupled hybrid of graphitic carbon nitride with reduced graphene oxide as a superior performance lithium-ion battery anode, Nanoscale 6 (21) (2014) 12555–12564. [107] S.  Cao, J.  Yu, Carbon-based H2-production photocatalytic materials, J Photochem. Photobiol. C: Photochem. Rev. 27 (2016) 72–99. [108] J.  Zhang, J.  Yu, M.  Jaroniec, J.R.  Gong, Noble metal-free reduced graphene ­oxide-ZnxCd1–xS nanocomposite with enhanced solar photocatalytic H2-production performance, Nano Lett. 12 (9) (2012) 4584–4589. [109] J.  Zhang, L.  Qi, J.  Ran, J.  Yu, S.Z.  Qiao, Ternary NiS/ZnxCd1-xS/reduced graphene oxide nanocomposites for enhanced solar photocatalytic H2-production activity, Adv. Energy Mater. 4 (10) (2014) 1301925, https://doi.org/10.1002/aenm.201301925. [110] J. Low, S. Cao, J. Yu, S. Wageh, Two-dimensional layered composite photocatalysts, Chem. Commun. 50 (74) (2014) 10768–10777. [111] D.W. Chang, J.-B. Baek, Nitrogen-doped graphene for photocatalytic hydrogen generation, Chem. Asian. J. 11 (8) (2016) 1125–1137. [112] L.  Jia, D.-H.  Wang, Y.-X.  Huang, A.-W.  Xu, H.-Q.  Yu, Highly durable N-doped graphene/cds nanocomposites with enhanced photocatalytic hydrogen evolution from water under visible light irradiation, J. Phys. Chem. C 115 (23) (2011) 11466–11473. [113] Z. Mou, Y. Wu, J. Sun, P. Yang, Y. Du, C. Lu, TiO2 nanoparticles-functionalized N-doped graphene with superior interfacial contact and enhanced charge separation for photocatalytic hydrogen generation, ACS Appl. Mater. Interfaces 6 (16) (2014) 13798–13806. [114] F. Pei, S. Xu, W. Zuo, Z. Zhang, Y. Liu, S. Cao, Effective improvement of photocatalytic hydrogen evolution via a facile in-situ solvothermal N-doping strategy in N-TiO2/Ngraphene nanocomposite, Int. J. Hydrog. Energy 39 (13) (2014) 6845–6852. [115] E.S.  Baeissa, Green synthesis of methanol by photocatalytic reduction of CO2 under visible light using a graphene and tourmaline co-doped titania nanocomposites, Ceram. Int. 40 (8, Part A) (2014) 12431–12438. [116] W. Tu, Y. Zhou, Q. Liu, Z. Tian, J. Gao, X. Chen, H. Zhang, J. Liu, Z. Zou, Robust hollow spheres consisting of alternating titania nanosheets and graphene nanosheets with high photocatalytic activity for CO2 conversion into renewable fuels, Adv. Funct. Mater. 22 (6) (2012) 1215–1221. [117] L.-Y. Lin, Y. Nie, S. Kavadiya, T. Soundappan, P. Biswas, N-doped reduced graphene oxide promoted nano TiO2 as a bifunctional adsorbent/photocatalyst for CO2 photoreduction: effect of N species, Chem. Eng. J. 316 (2017) 449–460.

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Carbon nitride photocatalysts Jinqiang Zhang, Hongqi Sun Edith Cowan University, Joondalup, WA, Australia

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6.1 Introduction The fast industrial pace, which currently relies heavily on fossil fuels, continues to increase the energy crisis and to cause environmental deterioration, making these two issues major concerns in countries all over the world. As a result, the successful exploitation of alternative energy resources plays a major role in enhancing the future of human beings. Hydrogen, the most widely distributed element on Earth, has become one of the most discussed energy resources. Hydrogen energy has the potential to yield greater economic benefits than conventional fossil fuels and, because of its intrinsic nature, to reduce environmental pollution and the greenhouse effect [1]. However, the current production of hydrogen still relies heavily on traditional gas reforming technology, which requires highly critical conditions such as high temperature and pressure. Therefore the increasing energy demands together with the requirement for sustainable energy sources are driving research activities toward producing more hydrogen via cost-effective and environmentally friendly technologies [2]. In 1972 Fujishima and Honda published a pioneering report on using TiO2 as an electrode in photoelectrochemical hydrogen evolution. Since then hydrogen production via photocatalysis has become a hot topic as an innovative way to convert solar energy to clean chemical energy [3]. In the photocatalytic process, H+ in water is reduced to H2, whereas OH− is oxidized into O2 over the surface of a semiconductor with an appropriate band gap energy [4]. As a result, seeking stable and efficient photocatalysts has been at the frontier in the solar energy storage and conversion fields. Semiconductor materials considered for hydrogen evolution are placed in three categories: metal oxides and sulfides (TiO2 [5], ZnO [6], CdS [7], MoS2 [8]), complex metal semiconductors (Bi2MoO6 [9], TaON [10], Ag3PO4 [11]), and nonmetallic semiconductors (graphitic carbon nitride [12] and black phosphorus [13]). TiO2 (3.2 eV) and ZnO (3.3 eV), which have been widely used in hydrogen evolution as typical photocatalysts, however, they respond only to ultraviolet light, which accounts for less than 5% of solar spectrum energy. CdS is considered to be a fascinating candidate because of its moderate band gap (2.4 eV) and relatively high photocatalytic efficiency. Nevertheless, its poor stability, which is due to easy self-oxidation of S2− by photogenerated holes, is still a big issue [14]. Moreover, metal-based photocatalysts cannot meet the requirement of sustainability because of their high cost, which are due to their scarcity and to contamination caused by incorrect disposal. Additionally, visible light, with about 42% of solar energy, has been insufficiently utilized to date because of the lack of suitable photocatalyst materials. Therefore it can be concluded that photocatalytic hydrogen evolution has been restricted mainly because an e­co-friendly Multifunctional Photocatalytic Materials for Energy. https://doi.org/10.1016/B978-0-08-101977-1.00007-7 Copyright © 2018 Elsevier Ltd. All rights reserved.

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photocatalyst with a visible light response, high performance, and low cost have not yet been achieved. Graphitic carbon nitride, as a metal-free and conjugated polymeric semiconductor material, has a number of advantages, including high corrosion resistance, good stability, and an easily controllable structure. Its narrow band gap (2.7 eV) also enables it to respond to visible light [12]. More strikingly, both the HOMO and the LUMO encompass the oxidation and reduction potentials of water. This characteristic ensures that the hole in the valence band (VB) is sufficiently reactive to oxidize OH– into O2, and at the same time, the electron in the conduction band (CB) has the real potential to reduce H+ to hydrogen. The salient features discovered here seem to be in close proximity to the ideal photocatalyst previously mentioned. Thus, upon the pioneering report in which Wang et al. [15] used g-C3N4 for water splitting, it instantly became a hot topic in the field of photocatalysis. Since that time, an ever-increasing number of papers reporting on the use of carbon nitride to directly convert solar energy into hydrogen energy have been published [2]. Also, the versatile photocatalyst also represents an attractive strategy for generation of other renewable energies, for example, hydrocarbon fuels [16]. Although this conjugated polymer shows a great potential for solar energy utilization, pristine carbon nitride still suffers from low photocatalytic efficiency because of its small surface area with limited active sites, its high charge recombination rate, and its weak ability to harvest visible light. As a result, a myriad of modification methods have been proposed to address the preceding issues. In this chapter, as depicted in Fig. 6.1, the essential introduction to g-C3N4 is provided first. Then we present a detailed survey of effective approaches for modification of g-C3N4, including tuning the parameters of polymerization, shape controlling, doping

Hydrogen evolution

gy

lo

or

C

op

ol

ym

er

iz

at

io

n

n

g red 2

tor age

Hybridization

CO

ys

uc

tio

pin

erg

Bulk carbon nitride

Do

En

sit Sen

izati

on

M

o ph

Fig. 6.1  Structure, modification and application of carbon nitride mentioned in this chapter.

Carbon nitride photocatalysts105

with ­metallic and/or nonmetallic elements, hybridizing with carbonaceous materials and other semiconductors, and sensitization with dyes. Then a review of modified carbon nitride for Li-ion batteries and reduction of carbon dioxide to hydrocarbons fuel is given. Last but not the least, we offer our perspectives on future research using carbon-­based photocatalyst for energy. This chapter aims at providing readers with the instructive text and knowledge they need about the design and application of novel carbon-based photocatalysts.

6.2 Graphitic carbon nitride for hydrogen evolution The history of carbon nitride can be traced back to 1834, when it was initially synthesized by Berzelius and then named melon by Liebig [17]. Afterward, theoretical calculations predicted that carbon nitride existed in five allotropes of α-C3N4, β-C3N4, g-C3N4, cubic-C3N4, and pseudo cubic-C3N4, among which graphitic carbon nitride (g-C3N4) was found to be the most stable allotrope. The perfect structure of g-C3N4 comprises only carbon and nitride elements, which are sp2-hybridized to establish trazine (C3N3) or tri-s-triazine (C6N7) rings. In comparison with trazine, the tri-s-triazine system shows a lower energy, thus g-C3N4 is considered as a π conjugated system connecting a large number of tri-s-triazine rings in planar and stacking based on a Van der Waals force interlayer (Fig. 6.2A and B) [12]. Whereas, because of undeveloped experimental techniques, the unconfirmed molecular structure of C3N4 was forgotten for a long time. Carbon nitride was also first reported to be of no application because of its insolubility and chemical inertness. In 2006 Goettmann et al. [19] for the first time demonstrated that mesoporous carbon nitride can be employed for Friedel-Crafts reactions. Three years later another milestone to applying carbon nitride in photocatalysis water splitting was reported by Wang et al. [15] they synthesized graphitic carbon nitride using the thermal polycondensation of a small organic molecular of cyanamide and were first to test its performance in photocatalytic H2 production. Both the CB and VB matched the redox potentials of H2O for hydrogen and oxygen evolution, making it an appealing candidate in the fields of energy storage and conversion. Thereafter, an exponential increase in the number of carbon nitride-based photocatalysts in H2 production and other energy applications were reported, which we review and discuss in detail in the following section.

6.2.1 Tuning the reaction parameters and precursors The photocatalytic performance of carbon nitride is dependent on its polymerization degree varying from different precursors and calcination temperatures. Therefore a surge of nitrogen-rich precursors, such as cyanamide [15], dicyanamide [20], melamine [21], urea [22], and thiourea [18], have been employed for thermal condensation at different temperatures to prepare carbon nitride photocatalysts. Melamine is a common precursor for the synthesis of graphitic carbon nitride because of the relatively high polycondensation degree of the trazine structure, which is prone to forming melem, the essential unit of carbon nitride. It is also regarded as

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Fig. 6.2  (A) Perfect structure and more stable unit of graphitic carbon nitride [15]; (B) XRD pattern reveals the steric configuration of carbon nitride [15]; and (C) Scheme of polymerization processes using thiourea as the precursor [18].

the intermediate during the calcination process of some small organic molecules such as cyanamide and dicyanamide, among others. Using melamine as the precursor for preparation of carbon nitride, Gu et al. [21] studied the influence of different temperatures on the polymerization degree and etching of carbon nitride. Initially, a series consisting of g-C3N4 were synthesized by tuning calcination temperatures from 470°C to 540°C. XRD results verified that the g-C3N4 could be formed at or above 470°C. Moreover, the BET results revealed that, with an increase of polymerization temperature from 470°C to 540°C, the surface area experienced an obvious rise from 6.0 to 210.1 cm2 g−1, which was attributed to the exfoliation and tailored structure as temperatures rose. Additionally, the photoluminescence (PL), UV-vis, and electron paramagnetic resonance (EPR) spectra showed that, with the temperature increasing from 500°C to 520°C, both the visible light response and the charge transfer were enhanced. In the photocatalytic hydrogen production, a H2 evolution rate of 6.89 μmol h−1 over

Carbon nitride photocatalysts107

the sample prepared at 520°C was higher than those synthesized at 470°C, 500°C, and 540°C because of the synergistic effect of the BET area, surface defects, and the visible light absorption threshold. As a low-cost, environmentally benign, and N-rich precursor, urea is also utilized in the preparation of graphitic carbon nitride. In a previous work, Zhang and coworkers [23] reported that calcination of urea at 550°C for 3 h without any assistance can produce a highly efficient photocatalyst of g-C3N4. BET results indicated that a larger surface area (69.6 m2 g−1) was obtained on urea-derived carbon nitride than on thiourea-derived (11.3 m2 g−1) and dicyanamide-derived (12.3 m2 g−1) carbon nitride. The same results were observed in a hydrogen evolution reaction with an enhanced H2 production rate of 47.2 μmol h−1, which was 3.1 times higher than that of thiourea-derived carbon nitride and 2.26 times to that of dicyanamide-derived carbon nitride. The polymerization temperature, the heating rate in condensation of carbon nitride, and the co-catalyst are also of vital importance in terms of the amount of H2 generation. In another report, Tang and coworkers [24] investigated H2 evolution rates using urea-based carbon nitride synthesized at different polymerization temperatures, heating rates, categories, and quantities of co-catalyst. An extraordinary performance (c.20,000 μmol h−1 g−1) and a high turnover number (more than 641 after 6 h) were observed on the optimized photocatalyst. Further experimental and DFT studies found that the high H2 production rate was ascribed to the higher polymerization and the lower proton concentration. In addition to melamine and urea, thiourea is another good candidate for synthesis of carbon nitride. Zhang et al. [18] successfully prepared graphitic carbon nitride by heating thiourea at 550°C in the air. Fig. 6.2C shows the self-polymerization of the thiourea scheme. Similar to the polymerization process for cyanamide, dicyanamide, and urea, melamine is also the intermediate when using thiourea as the precursor. In addition, the influence of heat treatment was investigated by keeping the temperature between 450°C and 650°C. BET results showed that more active sites were obtained because of the formed nanostructure, and the UV-vis spectra showed a blue shift because of quantum confinement effects. For water splitting, the hydrogen generation rate of thiourea-derived carbon nitride at 650°C can reach as high as 157.2 μmol h−1. Acid pretreatment of precursors as a valid means of altering the physiochemical properties of carbon nitride photocatalysts has been of particular interest. Yan and coworkers [25] reported the preparation of graphitic carbon nitride via direct thermal polymerization of H2SO4-treated melamine. Because the sublimation of melamine during polycondensation was efficiently suppressed by sulfuric acid, a lower polymerization degree of the amino groups in pretreated melamine and a higher BET surface area were achieved, compared with pristine melamine. As a result, the rate of hydrogen evolution on carbon nitride via sulfuric acid pretreatment is twice that of untreated carbon nitride. Table 6.1 provides a summary of carbon nitride photocatalysts prepared using different precursors and different polymerization parameters and their corresponding BET areas and H2 production rates in water splitting. It can be concluded that most urea-derived pristine carbon nitride photocatalysts possess larger surface areas than those polymerized via other precursors, which makes the former attractive in terms of

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Photocatalytic performance in hydrogen evolution of carbon nitride derived from different precursors and different polymerization parameters Table 6.1 

Polymerization parameters (annealing temp, duration, and atmosphere)

Surface area/m2 g−1

Photocatalytic performance in hydrogen evolution (TEOA, Pt)

Ref.

Cyanamide Dicyandiamide Dicyandiamide Dicyandiamide Dicyandiamide Dicyandiamide Melamine Melamine Melamine Melamine Urea Urea Urea Urea Urea Urea Urea Urea Thiourea Thiourea Thiourea H2SO4-melamine HNO3-melamine

550°C, 4 h, air 550°C, 4 h, H2 atmosphere 550°C, 2 h, air 550°C, 4 h, N2, subsequently 550oC, 2 h, H2 550°C, 4 h, air, subsequently 510oC, 1 h, NH3 550°C, 3 h, air 470°C, 2 h, air 500°C, 2 h, air 520°C, 2 h, air 540°C, 2 h, air 450°C, 2 h, air 500°C, 2 h, air 550°C, 2 h, air 600°C, 2 h, air 650°C, 2 h, air 600°C, 5°C min−1 ramp rate, 4 h 450°C, 2 h, air 550°C, 3 h, air 550°C, 3 h, air 600°C, 2 h, air 650°C, 2 h, air 600°C, 4 h, Ar 550°C, 2 h, air

10 20.91 10 124 196 12.3 6.0 cm2 g−1 41.5 cm2 g−1 173.6 cm2 g−1 210.1 cm2 g−1 43 49 58 77 97 43.8 135.6 69.6 11.3 27 52 15.6 86.4

770 μmol H2 after 72 h 4.8 times higher 12.1 μmol h−1 4.8 μmol h−1 82.9 μmol h−1 20.9 μmol h−1 0.29 μmol h−1 2.97 μmol h−1 6.89 μmol h−1 3.79 μmol h−1 3.4 μmol h−1 23.5 μmol h−1 79.0 μmol h−1 109.1 μmol h−1 89.8 μmol h−1 19,412 μmol h−1 g−1 1.4 μmol h−1 47.2 μmol h−1 15.1 μmol h−1 151.1 μmol h−1 157.2 μmol h−1 66 μmol h−1 417 μmol h−1 g−1

[15] [20] [18] [26] [27] [23] [21] [21] [21] [21] [18] [18] [18] [18] [18] [24] [22] [23] [23] [18] [18] [25] [28]

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Precursor

Carbon nitride photocatalysts109

their photocatalytic performances in water splitting. In addition to the precursor, the annealing temperature, rate, duration, and atmosphere are also important parameters that will significantly determine the photocatalytic activities of carbon nitride. As previously mentioned, a series of N-rich precursors were used for the synthesis of carbon nitride, and the polymerization parameters were thoroughly optimized. However, because the lower hydrogen production rate does not meet practical requirements, more suitable precursors and optimum synthesis conditions are still in high demand.

6.2.2 Copolymerization As a modification strategy at the molecular level, copolymerization is a fascinating approach to engineering the CB and VB and enhancing either the redox potentials or visible light absorption of the polymeric semiconductors. Because of the presence of functional groups, such as amino and cyano groups, in the termination of precursor molecules, reactions with other organic monomers bearing amino, carboxyl, anhydride, and so forth are possible. Also, the introduction of a monomer into a carbon nitride framework can enhance the charge separation ability to some extent. Thus the exclusive modification of a polymeric semiconductor can effectively extend the delocalization of the π electrons of graphitic carbon nitride and alter the intrinsic physiochemical properties [29]. Based on this concept, Zhang and coworkers [30] successfully incorporated aromatic groups into carbon nitride polymers by synthesizing organic molecules containing amino and/or cyano functionalities. Further analytical results verified that both the optical and the electrical properties were enhanced simultaneously. A remarkable red shift of light absorption to 700 nm and a fast charge immigration were observed from UV-vis and photocurrent spectra, respectively. When the modified g-C3N4 was applied in the hydrogen evolution reaction, a reinforced hydrogen evolution (TOF of 52 h−1 per added Pt atom) and improved stability were obtained. Fig. 6.3A shows the results of a similar work reported by Zou et al. [31], in which a novel carbon nitride network was fabricated through a facile bottom-up strategy of grafting an electron-deficient pyromellitic dianhydride monomer into the tri-s-triazine unit of carbon nitride. The extended delocalization of π electrons led to a H2 production rate three times greater than that of pristine g-C3N4. Also, utilization of the supramolecular for preparation of carbon nitride opens a new pathway to the modification of carbon nitride. To briefly summarize, assembling the precursors of carbon nitride into supramolecules by hydrogen bonding makes the molecules align in a designated direction, thus boosting the electron flow in a specific orientation. As illustrated in Fig. 6.3B, Shalom et al. [32] utilized cyanuric acid, melamine, and barbituric acid in water to establish a new supramolecular complex. An efficient hydrogen-­ generated photocatalyst was obtained through calcination of this complex, with a much higher turnover frequency of almost 6 h−1 per added Pt atom. This fascinating result was attributed to better light harvesting, higher charge separation efficiency, and more active sites. In general, as a unique modification method of polymeric semiconductor, copolymerization with a proper organic monomer can efficiently strengthen the water

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Fig. 6.3  (A) Scheme of extending the delocalization of the π electrons of graphitic carbon nitride using organic molecular [31]; (B) Schematic illustration of preparation of the supramolecular complex for carbon nitride [32].

Carbon nitride photocatalysts111

s­plitting ability of carbon nitride. In consideration of less organic molecules were reported for copolymerization with the precursors of g-C3N4, more and more relevant studies are therefore urged.

6.2.3 Nanostructured carbon nitride Fabrication of nanostructured materials—such as a porous structure [33]; a 2D nanosheet [34]; a 1D, that is, a nanotube [35], a nanofiber [36], etc.; and an 0D, that is, a quantum dot [37] or a hollow nanosphere [38]—is a very effective approach widely used in the modification of semiconductor materials. The fabrication of nanostructures for g-C3N4 has demonstrated the capacity to enhance visible light absorption, enlarge the surface area, and facilitate the electron-hole separation rate in different morphologies. In order to increase the BET areas, mesoporous graphitic carbon nitride and ompg-carbon nitride were prepared by introducing soft or hard templates into the synthesis. Wang et al. [39] introduced 12 nm SiO2 particles as a hard template and cyanamide as the precursor to prepare mesoporous carbon nitride. After removing the template, mpg-C3N4 was obtained. The surface area of mpg-C3N4 could reach as high as 373 m2 g−1, which was dependent on the proportion of precursor to template. As a result, the rate of H2 gas evolution was enhanced, compared with bulk carbon nitride, with a turnover number exceeding 6.5 after 25 h of reaction. In another report, SBA-15 was employed as a hard template to cast an ordered nanostructure of g-C3N4 (Fig.  6.4A). Chen et  al. [40] impregnated the N-rich precursor cyanamide with SBA-15; then the pre-polymer was heated to establish the framework of

Fig. 6.4  TEM images of (A) ompg-carbon nitride [40]; (B) Carbon nitride nanorods [41]; (C, D, and E) Carbon nitride spheres with different thickness [42]; and (F) Carbon nitride quantum dots [43].

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c­ arbon nitride in the pore canal of template. After removal of the template, a larger external surface and ordered nanostructured g-C3N4 was obtained. Application of this kind of photocatalyst into H2 evolution enabled the reaction rate to reach as high as 85 μmol h−1 in the four consecutive runs, which revealed excellent stability in water splitting. Additionally, to avoid hazardous reagents being involved in the hard-templating method, a novel soft-templating route for preparation of porous carbon nitride was reported by Yan et al. [44]. In this method, a surfactant of Pluronic P123 was selected as the soft template, and melamine was chosen as the precursor. Different from hard-templating, which requires an extra reagent/procedure for the removal of templates, soft-templating synthesis can easily remove the templates by way of calcination. The obtained mesoporous carbon nitride showed a large surface area of 90 m2 g−1. Additionally, an extended light absorption of up to 800 nm of solar spectrum made it possible to more efficiently utilize solar light. When it comes to the hydrogen evolution reaction, the rate of as-prepared mesoporous carbon nitride could reach as high as 148.2 μmol h−1. Bulk carbon nitride always faces the challenges of low electron mobility and other optical properties because of the thickness obstacle. To overcome these challenges and mimicking the exfoliation of graphite into graphene, researchers fabricated numerous 2D carbon nitride nanosheets. With this method of graphene preparation in mind, researchers developed a now widely used solvent ultrasonic route because the ultrasonic wave can surmount the Van der Waals force in the interlayer of bulk carbon nitride [45]. Moreover, based on the equation of ΔHMix/VMix = 2(δG − δsol)2 φ/T sheet (ΔH is mixing enthalpy, φ the volume fraction of nanosheet, δ the square root of the component surface energy, and T the thickness of nanosheets), an appropriate solvent with matchable surface energy was desired. Therefore a variety of solvents were studied in the exfoliation of bulk carbon nitride. For example, Yang et al. [46] fabricated carbon nitride nanosheets via exfoliation of bulk carbon nitride in various solvents, such as isopropyl alcohol, N-methyl-pyrrolidone, water, ethanol, and acetone. After sonication of the suspension solution for 10 h, carbon nitride nanosheets were obtained. Further analysis found that isopropyl alcohol is a good solvent for fabricating carbon nitride with a minimal thickness. The hydrogen evolution ability of as-­prepared carbon nitride nanosheets was also investigated. The results showed that layered 2D material with a large surface area (384 m2 g−1) achieved a much higher hydrogen evolution rate (93 μmol h−1) than that of bulk carbon nitride (10 μmol h−1) and even that of mesoporous carbon nitride in the presence of Pt. In addition to organic solvents and water, acid [47] or alkaline conditions [48] were also employed as the medium in the delamination of carbon nitride. It was found that the photocatalysis efficiencies of the obtained 2D nanosheets in acidic and basic conditions were both enhanced in comparison with pristine carbon nitride. A 1D nanostructure of carbon nitride was also constructed for boosting water splitting abilities. For example, Liu and coworkers [41] took advantage of the ­nano-confinement space in a 1D silica template to synthesize carbon nitride nanorods (Fig. 6.4B). The nanostructure not only endowed a larger external surface but also facilitated the electron mobility. When it came to hydrogen generation, the performance of carbon nitride nanorods was almost 10 times greater than that

Carbon nitride photocatalysts113

of bulk carbon nitride. The same strategy was also employed in the fabrication of nanohelical carbon nitride. In this work [49], chiral mesoporous silica was chosen as a sacrificial template. The helical carbon nitride was successfully prepared through a facial nanocasting method. The excellent light-harvesting capability and charge carrier separation rate of helical carbon nitride led to a higher hydrogen generation rate (74 μmol h−1). In addition to the 1D, 2D, and porous structure, establishment of a 0D nanostructure is also a promising pathway in semiconductor development. To prevent the distortion of hollow carbon nitride during polycondensation of a N-rich precursor, Sun and coworkers [42] employed a novel template of silica nanoparticles to confine the polymerization process. By controlling the space of the template, a series of hollow carbon nitride photocatalysts with different thicknesses were achieved (Fig. 6.4C–E). In the presence of a co-catalyst, the hydrogen production capability of hollow spheres was remarkably enhanced, with an overall AQY reaching approximately 7.5%. In addition to nanospheres, quantum dots, which are less than 10 nm in size, endow fascinating optical properties because of the strong quantum confinement effect [50]. For example, Wang and coworkers [43] adopted a thermal-chemical etching process to break a carbon nitride layer down to nanoribbons and quantum dots successively (Fig. 6.4F). Interestingly, the obtained quantum dots showed strong up-conversion behavior that could convert NIR light to visible light. As a result, carbon nitride quantum dots were applied in hydrogen generation together with pristine carbon nitride, and the rate of H2 production was enhanced roughly 2.87 times than occurred without carbon nitride quantum dots. As mentioned, nanostructured carbon nitride in a porous structure, 2D, 1D, and 0D morphologies are fabricated and utilized for hydrogen evolution reaction. Nevertheless, the reported morphologies of carbon nitride are less explored than those of other inorganic semiconductors because of deformation of the polycondensation process. Therefore further endeavors should be made to overcome this obstacle.

6.2.4 Doped carbon nitride Doping plays predominant roles in engineering the band gap, enhancing the intrinsic optical, and improving electrochemical properties by incorporating heteroatoms into the framework of carbon nitride. In the area of carbon nitride modifications, a myriad of studies on metal doping (Fe and Ag, etc.) and non-metal doping (B, O, F, and S, etc.) have been reported. Yue et al. [51] used ZnCl2 as the Zn source to synthesize the Zn-doped carbon nitride. The X-ray photoelectron spectroscopy (XPS) and diffusion reflectance spectra confirmed the successful introduction of Zn metal into g-C3N4 and the extended absorption threshold by the red shift after doping, respectively. Moreover, the enhanced visible light absorption resulted in a higher photocatalytic capability in water splitting, with a maximum quantum yield of 3.2% at 420 nm. In another work, Gao and coworkers [52] successfully incorporated Fe ion into the matrix of carbon nitride without destroying the host. Further measurements proved that ion-doping favored the electron mobility and resulted in an efficient H2 generation (quantum efficiency of 0.8%).

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More interestingly, because of the surface plasmon resonance effect, noble metals always show an excellent capability to capture photo-excited electrons and a strong visible light absorption ability when they are deposited on the surface of carbon nitride. Thus deposition of noble metal(s) onto the catalyst surface has been very popular in the field of catalysis despite the high cost. The modification of carbon nitride using noble metals can be done by either thermal reduction or photodeposition. Employing the former way, Ge et al. [53] obtained Ag0-doped carbon nitride. With the synergic effect between silver and carbon nitride, the H2 evolution rate surpassed that of pristine carbon nitride by more than 11.7-fold. In addition to metal doping [54], nonmetal [55] doping [56] is also an effective approach [57] in the modification of semiconductors and can significantly enhance the intrinsic electronic property and engineer the band structure of carbon nitride photocatalysts. Phosphorus has been used widely to enhance the conductivity of carbon nitride in recent years. For example, Ran and coworkers [58] successfully prepared P-doped carbon nitride by heating a mixture of 2-aminoethylphosphonic acid and melamine. The extended light absorption spectrum caused by the empty midgap states from P was confirmed by further theoretical calculations and experimental studies. Moreover, the reinforced photocatalytic activity endowed the modified photocatalyst with a robust H2 generation rate (1596 μmol h−1 g−1). Also, Li et al. [59] chose H2O2 as the O source to modify carbon nitride. Using a hydrothermal route, O was successfully introduced into the matrix to partially substitute N atoms in g-C3N4. Then the O-doped carbon nitride was applied in the hydrogen evolution reaction, showing 2.5 times higher efficiency than that of unmodified carbon nitride. The enhanced photocatalysis was mainly attributed to the larger surface area, extended visible light absorption, and increased electron mobility. In another study, Liu et  al. [60] obtained sulfur-doped carbon nitride by treating dicyandiamide-­ derived carbon nitride in a H2S atmosphere. More noticeably, the introduction of the S element not only upshifted the VB but also significantly widened the CB minimum because of the reduced particle size after doping. Therefore an 8.0 times higher H2 generation rate at λ > 420 nm than that of bulk carbon nitride was achieved because of the strengthened photoreduction ability. Table  6.2 provides a summary of different elements doped with carbon nitride along with their relevant doping method and the corresponding bandgap energies and their photocatalytic performances in hydrogen evolution. It was found that doping with either metal or non-metal elements can engineer the intrinsic optical properties of pristine carbon nitride, enhancing its visible light absorption. Meanwhile, the introduction of metal and some non-metal elements can improve the conductivity of carbon nitride, accelerating the separation of electron-hole pairs. Therefore the synergistic effect effectively boosts the photocatalytic ability of g-C3N4 in H2 evolution. It has been found that substitution of either C or N atoms [61] or imbedding [62] the surface [63] of carbon nitride [64] with metal [65] or non-metal element [66] can significantly tune the band structure of pristine carbon nitride and boost the H2-generated rate. Whereas, dual doping for carbon nitride has been rarely reported, it can be a sally port in the following works.

Doped element

Doped carbon nitride and its properties

Dopant

Ag

AgNO3

Co K Zn Fe Cu Fe O O S S P

Cobalt phthalocyanine KNO3 ZnCl2 FeCl3·6H2O Cu(NO3)2 Ferric nitrate nonahydrate H2O2 H2O2 Thiourea H2S 2-Aminoethylphosphonic acid Ammonium iodine Citric acid monohydrate NH4F NH4I Potassium iodine Sodium tripolyphosphate

I N F I K, I Na, P

Polymerization parameters (annealing temp, duration, and atmosphere) Photoreduction, subsequently calcination 300°C, 1 h, air 550°C, 4 h 550°C, 4 h, air 400°C, 4 h, N2 Hydrothermal treatment 45°C, 3 h, H2 (5 vol%) and Ar 400°C, 2 h, Ar Hydrothermal treatment 550°C, 2 h, N2 Hard template method 450°C, 1 h, H2S 500°C, 3 h, N2, subsequently 550°C, 5 h 550°C, 4 h, N2 550°C, 4 h, air 550°C, 4 h, air 550°C, 4 h, air 550°C, 4 h, air 550°C, 2 h, air

Band gap/eV

Photocatalytic performance in hydrogen evolution (TEOA, Pt) −1

(Without Pt)

Ref.

2.33

10.1 μmol h

[53]

2.62 2.76 Red-shifted 1.1 eV A red shift 2.42 2.49 2.61 2.61 2.85 2.91

28.0 μmol h−1 ~20 μmol h−1 59.5 μmol h−1 ∼16.2 mmol h−1 g−1 20.5 μmol h−1 g−1 (without Pt) 397 μmol h−1 g−1 (Ni2P as co-catalyst) 37.5 μmol h−1 60.2 μmol h−1 136.0 μmol h−1 ~81 μmol h−1 1596 μmol h−1 g−1

[54] [55] [51] [52] [56] [57] [59] [61] [62] [60] [58]

2.69 A red shift 2.63 2.70 2.67 Slightly red shift

38 μmol h−1 64 μmol h−1 ~13 μmol h−1 ~15.5 μmol h−1 41.23 μmol h−1 191.0 μmol h−1

[63] [64] [65] [55] [55] [66]

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Table 6.2 

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6.2.5 Carbon nitride-based heterojunctions Hybridization of carbon nitride with appropriate candidates is also considered as an efficient technique for accelerating the separation of the photogenerated electron and hole pairs. Inspired by this fact, numerous carbon nitride-based heterojunctions were fabricated and applied in hydrogen evolution reactions. TiO2, a typical inorganic semiconductor, is widely used as a hybridization candidate with carbon nitride because of its non-response ability to visible light region [67]. For example, Wang et al. [68] synthesized TiO2/carbon nitride heterojunctions simply by annealing the mixture of melamine with the precursor of TiO2. UV-vis results revealed that the presence of carbon nitride can make up the non-response ability of TiO2, and the PL spectra indicated that the presence of TiO2 can significantly solve the low electron transportation rate of carbon nitride. Therefore the synergistic effect provided by the semiconductor heterojunctions resulted in an enhancement of hydrogen evolution rate 5 times that of pristine g-C3N4. In addition, CdS [7] is regarded as another promising photocatalyst because of its high efficiency in solar storage and conversion. However, the stability is generally unsatisfactory because the S2− tends to be self-oxidized by the light-­excited holes. To overcome this obstacle, a novel core-shell photocatalyst of hybridized carbon nitride with CdS was reported by Liu and coworkers [14] via a chemisorption method. As illustrated in Fig. 6.5A, the CB and VB of CdS are suitable for that of carbon nitride; thus the photogenerated electron in carbon nitride can transfer through the intimate interface to the surface of CdS. Meanwhile, the corresponding ­photo-excited holes move to carbon nitride, with remarkably improved charge carrier mobility. Thus the self-oxidation process in CdS can be effectively prohibited. As a result, a H2 production rate 2.5 and 2.2 times higher than pure CdS and graphitic carbon nitride, respectively, can be observed in the hydrogen evolution reaction. An excellent reproducibility was also obtained in the [email protected] structure. In addition to metal oxide and sulfide, a metal organic framework (MOF) [72] is attracting increasing attention to construct heterostructures together with carbon nitride. For instance, Wang and coworkers [73] reported a novel heterostructure made by coupling carbon nitride with Zr-containing MOF UiO-66 octahedrons. The as-obtained visible light photocatalyst was first used for water splitting, showing a H2 generation rate more than 17 times higher than that of carbon nitride alone. The significant enhancement was mostly attributed to the boosted electron-hole separation rate within the intimate contact. He and coworkers [74] reported a facial annealing approach to fabricate a novel heterojunction that combined graphitized polyacrylonitrile nanosheets with layered carbon nitride. The results indicated that the introduction of a polymer can effectively accelerate the charge migration; therefore a H2 production rate 3.8 times that of pristine carbon nitride was observed. Apart from the previously mentioned binary composite, studies on ternary heterojunctions have expanded as well in recent years. For instance, a noble metal-free ternary composite was reported by Yan et al. [69] They first constructed a CdS/carbon nitride core/shell structure, loaded Ni(OH)2 on the surface of the core/shell hybrid via a hydrothermal method, and used the as-fabricated composite in the hydrogen evolution reaction. Fig. 6.5B clearly shows that recombination of the photogenerated electron and hole pairs can be effectively reduced. As a result, the

Carbon nitride photocatalysts117

(A)

(B)

H2

–2

NHE (eV)

–1.0

–1.30 eV

LUMO e- e- e- e-

-

-

e e e e

–0.52 eV Vis

-

0.0 1.0 2.0

1.40 eV 1.88 eV

h+ h+ h + h+ + + + + HOMO h h h h

H2O

e

H2

1 2

h+

h+ Ni(OH)2

g-C3N4

CdS

3

(D)

C3N4

–1 0

MgPc e–

–2

Potential vs. NHE /V

(C)

CdS

e-

0

O2 g-C3N4

2 H+

–1

Vis

Energy vs. NHE

–2.0

H+ Pt

H+

Pt

H2

LUMO

e– CB

1.8 eV h+

H+/H2

hv > 600 nm

TEOA HOMO

+1 O2/H2O +2 VB

Fig. 6.5  (A) Transferring path of photogenerated electron and hole in the intimate interface of binary CdS/carbon nitride heterojunction [14]; (B) Mechanism illustration of hydrogen evolution in ternary Ni(OH)2/CdS/carbon nitride composite [69]; (C) TEM image of multiwalled carbon nanotubes/carbon nitride hybrid [70]; and (D) Mechanism illustration of hydrogen generation with MgPc-sensitized carbon nitride system [71].

apparent quantum efficiency reached as high as ~16.7% at a wavelength of 450 nm. Moreover, the presence of Ni(OH)2 exhibited an activity in H2 evolution that was seven times higher than that of the noble co-catalysts.

6.2.6 Carbonaceous/carbon nitride hybrids Carbon hybrids are special heterojunctions without any metals and are well matched for the requirement of sustainable development [75]. In this configuration, carbon serves as an electron transportation medium because of its excellent conductivity [76]. Based on this, Li et  al. [77] successfully synthesized a variety of heteroatom (O, S, B, N)-doped graphenes coupled with graphitic carbon nitride. The enhanced H2-generated rate of the metal-free heterojunctions revealed the positive influence of the presence of graphene. Moreover, N-graphene/carbon nitride was found to possess the best photocatalytic behavior among all the prepared samples in the report. Also, a possible reason proposed was that the introduction of the N element altered the electronic structure of graphene. In addition, a 1D nanotube was used for the hybridization with carbon nitride. Ge et  al. [70] reported that a novel composite made of

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multi-walled carbon nanotubes and carbon nitride was obtained by a facial thermal process (Fig.  6.5C). Further investigation indicated that the presence of the carbon nanotube effectively facilitated the charge migration. Therefore a 3.7-fold hydrogen evolution rate was observed when the optimized composite photocatalyst was entered into water splitting, compared with that of pristine carbon nitride. More strikingly, a ­photocatalyst with a high photocatalytic performance and stability was exploited by Liu and coworkers [78]. With the advantages of 0D carbon dots—for example, excellent visible light absorption ability, high electron transportation capability, and especially the unique H2O2 decomposition—the 0D carbon was successfully immobilized on the surface of carbon nitride. The presence of carbon dots could significantly improves the hydrogen evolution rate, even in the absence of a non-metal co-catalyst. The quantum efficiencies can reach as high as 16% at a wavelength of 420 ± 20 nm. And the low-cost photocatalyst exhibited an excellent stability with no obvious decline after 50 cycles of 24 h each. The synthesis of this metal-free composite can open up a venue for designing and exploiting novel, environmentally benign, and low-cost photocatalysts. Isotype heterostructure, also named homojunction, is another new type of carbon hybrid designed for facilitating charge migration using the same substance with different crystal phases. For the first time, Zhang et al. [79] employed a band alignment approach to combine two different types of carbon nitride into a CNS/CN homojunction. Similarly to heterojunction, these carbon nitrides derived from two different precursors possessed slightly different inherent band structures that could promptly accelerate the electrons’ dissociation and prohibit electron-hole recombination. Therefore, with this advantage, the novel homojunction showed improved H2-generated activity with almost 50 μmol h−1. Thus it is clear that both the heterojunctions and carbon hybridization can significantly improve hydrogen production, an improvement that is attributed to the enhanced electron mobility in the intimate interface. Although a great variety of heterostructures have been fabricated in recent years, there is still room for improvement by using some budding composite (such as MOF-based heterojunction) for energy.

6.2.7 Dye-sensitized carbon nitride Sensitization of a semiconductor with dyes is a classical modification approach that has been widely used in solar cells, photocatalysis, and so forth [80]. Sensitization systems can be constructed by adsorption or immobilization with metals, which can greatly extend the visible light response region and improve the charge separation. The pioneering study on dye sensitization of carbon nitride was done by Takanabe and coworkers [71]. They used magnesium phthalocyanine as the sensitizer to load on the host of Pt/carbon nitride. The introduction of dye remarkably extended the light absorption to a near-IR region at about 820 nm. With the application of this sensitization system, a quantum efficiency of approximately 5.6% at 420 nm was observed in water splitting. Moreover, a mechanism was proposed in which the dye was excited by electrons injected into the CB of carbon nitride for hydrogen reduction. The excited dye

Carbon nitride photocatalysts119

could be reduced to its original state via a sacrificial agent for the next cycle. Fig. 6.5D shows that the introduction of a sensitizer can efficiently prohibit the charge recombination. Based on this mechanism, Eosin Y was selected as the sensitizer by Min et al. [81]. They found that mesoporous carbon nitride sensitized by Eosin Y significantly extended the absorption threshold of the visible light spectrum to 600 nm. Therefore the improved light response ability led to a striking apparent quantum efficiency of 19.4% under 550 nm wavelength. Dye-sensitized carbon nitride is a promising candidate for producing photosynthesis hydrogen energy because it significantly improves the hydrogen production rate. However, only a limited number of appropriate dyes have been discovered, which has hindered further improvement in this field. Therefore development and utilization of a series of robust sensitizers with proper band structures is indeed needed.

6.3 Carbon nitride for reduction of CO2 Carbon dioxide reduction utilizing photocatalysts shows great potential to solve the issues of the greenhouse effect and simultaneously produce hydrocarbon fuels. As shown in Fig.  6.6A, the conversion of carbon dioxide into HCOOH, CO, HCHO, CH3OH, and CH4 requires two, two, four, six, and eight electrons, respectively. Thus the more electrons are needed, the more CB photocatalysts should possess. Because CB highly favors the reduction of half-reaction, carbon nitride has been widely employed as a CO2 photo-fixation catalyst. For the first time, Ong and coworkers [83] used the as-­ prepared carbon hybrid (graphene/carbon nitride) in CO2 reduction for methane formation. The CH4 yield using optimized carbon nitride-based photocatalyst was as high as 5.87 μmol g−1, which was 2.3 times greater than that of pristine carbon nitride. In another work, Ohno and coworkers [84] synthesized a novel composite that consisted of carbon nitride and WO3 using a planetary mill approach and then tested its photocatalytic performance in carbon dioxide reduction for methanol production. After a 24 h reaction, the amount of CH3OH generated reached about 1400 nmol. Also, when an Audoped carbon nitride/WO3 composite was used, the photocatalytic activity in CH3OH production was enhanced 1.7-fold higher than that of the photocatalyst without Au.

Pt4+

CO2, H+ –

e CB

e



Multielectron reduction

Ef

VB

(A)

e

Solar fuels (CH4, CO, CH3OH, etc.)

C2H5O2C

H+ CO2C2H5

H3C N CH3

h+

h+ h+ g-C3N4

Pt

R-C3N4

H2O

H2

+ 2H+

e

H+, O2

(B)

DHP

Fig. 6.6  (A) Schematic illustration of carbon dioxide reduction [2]; (B) Reaction mechanism of dehydrogenation reaction with 1,4-DHP [82].

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6.4 Carbon nitride for other energy applications The dehydrogenation reaction of organic substrate together with carbon nitride opens up another pathway for hydrogen energy production. Via polycondensation, Wu and coworkers [82] employed melamine as a small pitch for repairing the amino and cyano defects that were residuals on the surface of mesoporous carbon nitride. The ninhydrin test results revealed that the defects could be effectively repaired after introducing melamine into the precursor of mesoporous carbon nitride. The remedied carbon nitride was innovatively applied in the dehydrogenation of 1,4-dihydro-2,6-dimethylpyridine-­ 3,5-dicarboxylate, which could produce the bioactive compounds. The repaired carbon nitride showed a H2 production rate 6.5 times greater than that of unmodified mesoporous carbon nitride; the corresponding scheme is shown in Fig. 6.6B. In addition to hydrogen generation, hydrogen storage is also a problem that is receiving major attention. Nair et al. [85] incorporated palladium into the matrix of carbon nitride and investigated its performance in hydrogen storage. The hydrogen adsorption/desorption studies demonstrated that the hydrogen storage capacity of Pb-doped carbon nitride achieved up to 3.4 wt% at 25°C and 4 MPa because of the spillover mechanism, and exhibited excellent potential as an effective hydrogen storage medium. In addition to the production and storage of hydrogen energy, the storage of electric power in Li-ion batteries shows a promising future. To exploit a stable anode material for Li-ion batteries, Subramaniyam and coworkers [86] constructed a 3D structure consisting of exfoliated carbon nitride nanosheets and reduced graphene oxide layers via a hydrothermal method. The obtained sandwiched carbon nitride composite showed a robust capacity of 970 mAh g−1 after 300 cycles, which was 15fold higher than that of pristine g-C3N4. In a similar work, Tao et al. [87] prepared a N, P dual-doped carbon fiber/carbon nitride composite and applied it as an anode in both lithium and sodium ion batteries. For the Li-ion battery, the carbon nitride-based heterojunction showed excellent activities with reversible capacities of 1030 mAh g−1 after 1000 cycles at 1 A g−1 and 360 mAh g−1 after 4000 cycles at 10 A g−1. Moreover, as an anode for the Na ion battery, the reversible capacities reached 345 mAh g−1 after 380 cycles at 0.1 A g−1 and 110 mAh g−1 after 4000 cycles at 1 A g−1. It can be observed that apart from hydrogen evolution reaction, carbon nitride based catalyst can also be applied into many other energy production reactions. Therefore, broadening the application of carbon nitride in generating various renewable energy resources still needs more efforts.

6.5 Conclusion and outlook This chapter summarized recent developments in employing carbon nitride-based photocatalysts for alternative energy production, including hydrogen, hydrocarbon fuels, and batteries. The photocatalytic performance of carbon nitride was found to improve significantly via tuning polycondensation parameters, copolymerization, fabrication of nanostructure, doping, hybridizing, and dye-sensitization. Despite the great progresses in boosting the activities of carbon nitride for energy storage and conversion,

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the modified carbon nitride still suffers from a number of restrictions, such as limited visible light utilization, less active sites, and a high charge carrier recombination rate. Based on these challenges, endeavors are still required and several strategies are proposed for further studies. (1) The purpose of exploiting renewable energy is to achieve sustainable development. Therefore, during the modification process of carbon nitride, less noble metals or toxic materials should be introduced, even if they significantly boost the photocatalytic performances of carbon nitride. (2) Before experimental modification of carbon nitride, molecular models and reaction processes should be optimized by theoretical calculations, which could yield twice the outcome with half the effort. (3) Copolymerization, doping, hybridization, morphology, and sensitization could improve the photocatalytic capability in energy innovation. However, the current output from the use of modified carbon nitride cannot meet practical requirements. Therefore dual doping or doping of more elements, ternary hybridization, a variety of nanostructures, and substantial dye sensitization of carbon nitride systems should receive more attention. Moreover, further studies should integrate these methods. (4) More energy-generation technologies need to be explored, rather than just focusing on hydrogen evolution. Carbon nitride-based photocatalysts possess a highly photocatalytic activity and have multiple applications in energy preparation, and may hold the solutions for relieving the energy crisis and environmental contamination pressures.

References [1] C. George, M. Ammann, B. D’Anna, D.J. Donaldson, S.A. Nizkorodov, Heterogeneous photochemistry in the atmosphere, Chem. Rev. 115 (10) (2015) 4218–4258. [2] W.J. Ong, L.L. Tan, Y.H. Ng, S.T. Yong, S.P. Chai, Graphitic carbon nitride (g-C3N4)based photocatalysts for artificial photosynthesis and environmental remediation: are we a step closer to achieving sustainability? Chem. Rev. 116 (12) (2016) 7159–7329. [3] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [4] L. Zhou, H. Zhang, H. Sun, S. Liu, M.O. Tade, S. Wang, et al., Recent advances in nonmetal modification of graphitic carbon nitride for photocatalysis: a historic review, Cat. Sci. Technol. 6 (19) (2016) 7002–7023. [5] W.  Zhou, W.  Li, J.Q.  Wang, Y.  Qu, Y.  Yang, Y.  Xie, et  al., Ordered mesoporous black TiO(2) as highly efficient hydrogen evolution photocatalyst, J. Am. Chem. Soc. 136 (26) (2014) 9280–9283. [6] X. Lu, G. Wang, S. Xie, J. Shi, W. Li, Y. Tong, et al., Efficient photocatalytic hydrogen evolution over hydrogenated ZnO nanorod arrays, Chem. Commun. 48 (62) (2012) 7717–7719. [7] C.  Li, J.  Yuan, B.  Han, W.  Shangguan, Synthesis and photochemical performance of morphology-­controlled CdS photocatalysts for hydrogen evolution under visible light, Int. J. Hydrog. Energy 36 (7) (2011) 4271–4279. [8] H. Yu, X. Yu, Y. Chen, S. Zhang, P. Gao, C. Li, A strategy to synergistically increase the number of active edge sites and the conductivity of MoS2 nanosheets for hydrogen evolution, Nanoscale 7 (19) (2015) 8731–8738. [9] J. Li, Y. Yin, E. Liu, Y. Ma, J. Wan, J. Fan, et al., In situ growing Bi2MoO6 on g-C3N4 nanosheets with enhanced photocatalytic hydrogen evolution and disinfection of bacteria under visible light irradiation, J. Hazard. Mater. 321 (2017) 183–192.

122

Multifunctional Photocatalytic Materials for Energy

[10] W. Chen, M. Chu, L. Gao, L. Mao, J. Yuan, W. Shangguan, Ni(OH)2 loaded on TaON for enhancing photocatalytic water splitting activity under visible light irradiation, Appl. Surf. Sci. 324 (2015) 432–437. [11] Y. Hou, F. Zuo, Q. Ma, C. Wang, L. Bartels, P. Feng, Ag3PO4 oxygen evolution photocatalyst employing synergistic action of Ag/AgBr nanoparticles and graphene sheets, J. Phys. Chem. C 116 (38) (2012) 20132–20139. [12] Y. Wang, X. Wang, M. Antonietti, Polymeric graphitic carbon nitride as a heterogeneous organocatalyst: from photochemistry to multipurpose catalysis to sustainable chemistry, Angew. Chem. 51 (1) (2012) 68–89. [13] X. Zhu, T. Zhang, Z. Sun, H. Chen, J. Guan, X. Chen, et al., Black phosphorus revisited: a missing metal-free elemental photocatalyst for visible light hydrogen evolution, Adv. Mater. 29 (17) (2017) 1605776. [14] L. Liu, P. Hu, W. Cui, X. Li, Z. Zhang, Increased photocatalytic hydrogen evolution and stability over nano-sheet g-C3N4 hybridized CdS [email protected] structure, Int. J. Hydrog. Energy 42 (27) (2017) 17435–17445. [15] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, et al., A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater. 8 (1) (2009) 76–80. [16] R. Kuriki, K. Sekizawa, O. Ishitani, K. Maeda, Visible–light-driven CO2 reduction with carbon nitride: enhancing the activity of ruthenium catalysts, Angew. Chem. 54 (8) (2015) 2406–2409. [17] J.V. Liebig, About some nitrogen compounds, Ann. Pharmacother. 10 (1834) 10. [18] G. Zhang, J. Zhang, M. Zhang, X. Wang, Polycondensation of thiourea into carbon nitride semiconductors as visible light photocatalysts, J. Mater. Chem. 22 (16) (2012) 8083. [19] F.  Goettmann, A.  Fischer, M.  Antonietti, A.  Thomas, Metal-free catalysis of sustainable Friedel–Crafts reactions: direct activation of benzene by carbon nitrides to avoid the use of metal chlorides and halogenated compounds, Chem. Commun. (43) (2006) 4530–4532. [20] Q. Tay, P. Kanhere, C.F. Ng, S. Chen, S. Chakraborty, A.C.H. Huan, et al., Defect engineered g-C3N4 for efficient visible light photocatalytic hydrogen production, Chem. Mater. 27 (14) (2015) 4930–4933. [21] Q. Gu, Z. Gao, H. Zhao, Z. Lou, Y. Liao, C. Xue, Temperature-controlled morphology evolution of graphitic carbon nitride nanostructures and their photocatalytic activities under visible light, RSC Adv. 5 (61) (2015) 49317–49325. [22] G.  Wu, S.S.  Thind, J.  Wen, K.  Yan, A.  Chen, A novel nanoporous α-C3N4 photocatalyst with superior high visible light activity, Appl. Catal. B Environ. 142–143 (2013) 590–597. [23] Y. Zhang, J. Liu, G. Wu, W. Chen, Porous graphitic carbon nitride synthesized via direct polymerization of urea for efficient sunlight-driven photocatalytic hydrogen production, Nanoscale 4 (17) (2012) 5300–5303. [24] D.J. Martin, K.P. Qiu, S.A. Shevlin, A.D. Handoko, X.W. Chen, Z.X. Guo, J.W. Tang, Highly efficient photocatalytic H2 evolution from water using visible light and structure-­ controlled graphitic carbon nitride, Angew. Chem. 53 (2014) 9240–9245. [25] H. Yan, Y. Chen, S. Xu, Synthesis of graphitic carbon nitride by directly heating sulfuric acid treated melamine for enhanced photocatalytic H2 production from water under visible light, Int. J. Hydrog. Energy 37 (1) (2012) 125–133. [26] X. Li, G. Hartley, A.J. Ward, P.A. Young, A.F. Masters, T. Maschmeyer, Hydrogenated defects in graphitic carbon nitride nanosheets for improved photocatalytic hydrogen evolution, J. Phys. Chem. C 119 (27) (2015) 14938–14946.

Carbon nitride photocatalysts123

[27] Q.  Liang, Z.  Li, Z.-H.  Huang, F.  Kang, Q.-H.  Yang, Holey graphitic carbon nitride nanosheets with carbon vacancies for highly improved photocatalytic hydrogen production, Adv. Funct. Mater. 25 (44) (2015) 6885–6892. [28] Y. Zhong, Z. Wang, J. Feng, S. Yan, H. Zhang, Z. Li, et al., Improvement in photocatalytic H2 evolution over g-C3N4 prepared from protonated melamine, Appl. Surf. Sci. 295 (2014) 253–259. [29] J. Zhang, X. An, N. Lin, W. Wu, L. Wang, Z. Li, et al., Engineering monomer structure of carbon nitride for the effective and mild photooxidation reaction, Carbon 100 (2016) 450–455. [30] J.S.  Zhang, G.G.  Zhang, X.F.  Chen, S.  Lin, L.  Mohlmann, G.  Dolega, et  al., Angew. Chem. Int. Ed. 51 (13) (2012) 3183–3187. [31] S. Chu, Y. Wang, Y. Guo, J. Feng, C. Wang, W. Luo, et al., Band structure engineering of carbon nitride: in search of a polymer photocatalyst with high photooxidation property, ACS Catal. 3 (5) (2013) 912–919. [32] M. Shalom, M. Guttentag, C. Fettkenhauer, S. Inal, D. Neher, A. Llobet, et al., In situ formation of heterojunctions in modified graphitic carbon nitride: synthesis and noble metal free photocatalysis, Chem. Mater. 26 (19) (2014) 5812–5818. [33] W. Tian, H. Zhang, H. Sun, A. Suvorova, M. Saunders, M. Tade, et al., Heteroatom (N or N-S)-doping induced layered and honeycomb microstructures of porous carbons for CO2 capture and energy applications, Adv. Funct. Mater. 26 (47) (2016) 8651–8661. [34] X.  Duan, K.  O’Donnell, H.  Sun, Y.  Wang, S.  Wang, Sulfur and nitrogen co-doped graphene for metal-free catalytic oxidation reactions, Small 11 (25) (2015) 3036–3044. [35] X. Duan, H. Sun, Y. Wang, J. Kang, S. Wang, N-doping-induced nonradical reaction on single-walled carbon nanotubes for catalytic phenol oxidation, ACS Catal. 5 (2) (2015) 553–559. [36] E. Saputra, S. Muhammad, H. Sun, H.M. Ang, M.O. Tade, S. Wang, Different crystallographic one-dimensional MnO2 nanomaterials and their superior performance in catalytic phenol degradation, Environ. Sci. Technol. 47 (11) (2013) 5882–5887. [37] W.  WT, L.Y.  Zhan, W.Y.  Fan, J.Z.  Song, X.M.  Li, Z.T.  Li, et  al., Cu-N dopants boost electron transfer and photooxidation reactions of carbon dots, Angew. Chem. Int. Ed. 54 (22) (2015) 6540–6544. [38] H. Sun, G. Zhou, Y. Wang, A. Suvorova, S. Wang, A new metal-free carbon hybrid for enhanced photocatalysis, ACS Appl. Mater. Interfaces 6 (19) (2014) 16745–16754. [39] X. Wang, K. Maeda, X. Chen, K. Takanabe, K. Domen, Y. Hou, et al., Polymer semiconductors for artificial photosynthesis: hydrogen evolution by mesoporous graphitic carbon nitride with visible light, J. Am. Chem. Soc. 131 (5) (2009) 1680. [40] X.F. Chen, Y.S. Jun, K. Takanabe, K. Maeda, K. Domen, F. XZ, M. Antonietti, X.C. Wang, Ordered mesoporous SBA-15 type graphitic carbon nitride: a semiconductor host structure for photocatalytic hydrogen evolution with visible light, Chem. Mater. 21 (2009) 4093–4095. [41] J. Liu, J. Huang, H. Zhou, M. Antonietti, Uniform graphitic carbon nitride nanorod for efficient photocatalytic hydrogen evolution and sustained photoenzymatic catalysis, ACS Appl. Mater. Interfaces 6 (11) (2014) 8434–8440. [42] J. Sun, J. Zhang, M. Zhang, M. Antonietti, X. Fu, X. Wang, Bioinspired hollow semiconductor nanospheres as photosynthetic nanoparticles, Nat. Commun. (2012) 1139. [43] W.  Wang, Y.  JC, Z.  Shen, D.K.  Chan, T.  Gu, g-C3N4 quantum dots: direct synthesis, upconversion properties and photocatalytic application, Chem. Commun. 50 (70) (2014) 10148–10150.

124

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[44] H.  Yan, Soft-templating synthesis of mesoporous graphitic carbon nitride with enhanced photocatalytic H2 evolution under visible light, Chem. Commun. 48 (28) (2012) 3430–3432. [45] J. Feng, T. Chen, S. Liu, Q. Zhou, Y. Ren, Y. Lv, et al., Improvement of g-C3N4 photocatalytic properties using the Hummers method, J. Colloid Interface Sci. 479 (2016) 1–6. [46] S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, et al., Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light, Adv. Mater. 25 (17) (2013) 2452–2456. [47] J. Xu, L. Zhang, R. Shi, Y. Zhu, Chemical exfoliation of graphitic carbon nitride for efficient heterogeneous photocatalysis, J. Mater. Chem. A 1 (46) (2013) 14766. [48] T. Sano, S. Tsutsui, K. Koike, T. Hirakawa, Y. Teramoto, N. Negishi, et al., Activation of graphitic carbon nitride (g-C3N4) by alkaline hydrothermal treatment for photocatalytic NO oxidation in gas phase, J. Mater. Chem. A 1 (21) (2013) 6489. [49] Y. Zheng, L. Lin, X. Ye, F. Guo, X. Wang, Helical graphitic carbon nitrides with photocatalytic and optical activities, Angew. Chem. 53 (44) (2014) 11926–11930. [50] H. Zhang, W. Tian, L. Zhou, H. Sun, M. Tade, S. Wang, Monodisperse Co3O4 quantum dots on porous carbon nitride nanosheets for enhanced visible-light-driven water oxidation. Appl. Catal. B Environ. (2017) https://doi.org/10.1016/j.apcatb.2017.03.028. in press. [51] B. Yue, Q. Li, H. Iwai, T. Kako, J. Ye, Hydrogen production using zinc-doped carbon nitride catalyst irradiated with visible light, Sci. Technol. Adv. Mater. 12 (3) (2011) 034401. [52] L.F. Gao, T. Wen, X. JY, X.P. Zhai, M. Zhao, H. GW, et al., Iron-doped carbon nitride-type polymers as homogeneous organocatalysts for visible light-driven hydrogen evolution, ACS Appl. Mater. Interfaces 8 (1) (2016) 617–624. [53] L.  Ge, C.  Han, J.  Liu, Y.  Li, Enhanced visible light photocatalytic activity of novel polymeric g-C3N4 loaded with Ag nanoparticles, Appl. Catal. A Gen. 409–410 (2011) 215–222. [54] P.-W. Chen, K. Li, Y. Y-X, W.-D. Zhang, Cobalt-doped graphitic carbon nitride photocatalysts with high activity for hydrogen evolution, Appl. Surf. Sci. 392 (2017) 608–615. [55] Y.  Guo, T.  Chen, Q.  Liu, Z.  Zhang, X.  Fang, Insight into the enhanced photocatalytic activity of potassium and iodine codoped graphitic carbon nitride photocatalysts, J. Phys. Chem. C 120 (44) (2016) 25328–25337. [56] M. Fan, C. Song, T. Chen, X. Yan, D. Xu, W. Gu, et al., Visible-light-drived high photocatalytic activities of Cu/g-C3N4 photocatalysts for hydrogen production, RSC Adv. 6 (41) (2016) 34633–34640. [57] C. Liu, K. Wu, G. Meng, J. Wu, B. Peng, J. Hou, et al., Explore the properties and photocatalytic performance of iron-doped g-C3N4 nanosheets decorated with Ni2P, J. Mol. Catal. 437 (2017) 80–88. [58] J.  Ran, T.Y.  Ma, G.  Gao, X.-W.  Du, S.Z.  Qiao, Porous P-doped graphitic carbon nitride nanosheets for synergistically enhanced visible-light photocatalytic H2 production, Energy Environ. Sci. 8 (12) (2015) 3708–3717. [59] J. Li, B. Shen, Z. Hong, B. Lin, B. Gao, Y. Chen, A facile approach to synthesize novel oxygen-doped g-C3N4 with superior visible-light photoreactivity, Chem. Commun. 48 (98) (2012) 12017–12019. [60] G. Liu, P. Niu, C.H. Sun, S.C. Smith, Z.G. Chen, L. GQ, H.M. Cheng, Unique electronic structure induced high photoreactivity of sulfur-doped graphitic C3N4, J. Am. Chem. Soc. 132 (2010) 11642–11648.

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[61] Z.F.  Huang, J.  Song, L.  Pan, Z.  Wang, X.  Zhang, J.J.  Zou, et  al., Carbon nitride with simultaneous porous network and O-doping for efficient solar-energy-driven hydrogen evolution, Nano Energy 12 (2015) 646–656. [62] J. Hong, X. Xia, Y. Wang, R. Xu, Mesoporous carbon nitride with in situ sulfur doping for enhanced photocatalytic hydrogen evolution from water under visible light, J. Mater. Chem. 22 (30) (2012) 15006. [63] G. Zhang, M. Zhang, X. Ye, X. Qiu, S. Lin, X. Wang, Iodine modified carbon nitride semiconductors as visible light photocatalysts for hydrogen evolution, Adv. Mater. 26 (5) (2014) 805–809. [64] Y.  Zhou, L.  Zhang, W.  Huang, Q.  Kong, X.  Fan, M.  Wang, et  al., N-doped graphitic carbon-incorporated g-C3N4 for remarkably enhanced photocatalytic H2 evolution under visible light, Carbon 99 (2016) 111–117. [65] Y. Wang, Y. Di, M. Antonietti, H. Li, X. Chen, X. Wang, Excellent visible-light photocatalysis of fluorinated polymeric carbon nitride solids, Chem. Mater. 22 (18) (2010) 5119–5121. [66] S. Cao, Q. Huang, B. Zhu, J. Yu, Trace-level phosphorus and sodium co-doping of g-C3N4 for enhanced photocatalytic H2 production, J. Power Sources 351 (2017) 151–159. [67] C.A.  Caputo, L.  Wang, R.  Beranek, E.  Reisner, Carbon nitride-TiO2 hybrid modified with hydrogenase for visible light driven hydrogen production, Chem. Sci. 6 (10) (2015) 5690–5694. [68] J. Wang, J. Huang, H. Xie, A. Qu, Synthesis of g-C3N4/TiO2 with enhanced photocatalytic activity for H2 evolution by a simple method, Int. J. Hydrog. Energy 39 (12) (2014) 6354–6363. [69] Z.  Yan, Z.  Sun, X.  Liu, H.  Jia, P.  Du, Cadmium sulfide/graphitic carbon nitride heterostructure nanowire loading with a nickel hydroxide cocatalyst for highly efficient photocatalytic hydrogen production in water under visible light, Nanoscale 8 (8) (2016) 4748–4756. [70] L. Ge, C. Han, Synthesis of MWNTs/g-C3N4 composite photocatalysts with efficient visible light photocatalytic hydrogen evolution activity, Appl. Catal. B Environ. 117–118 (2012) 268–274. [71] K. Takanabe, K. Kamata, X. Wang, M. Antonietti, J. Kubota, K. Domen, Phys. Chem. Chem. Phys. 12 (40) (2010) 13020–13025. [72] W. Wang, X. Xu, W. Zhou, Z. Shao, Recent progress in metal-organic frameworks for applications in electrocatalytic and photocatalytic water splitting, Adv. Sci. 4 (4) (2017) 1600371. [73] R. Wang, L. Gu, J. Zhou, X. Liu, F. Teng, C. Li, et al., Quasi-polymeric metal-organic framework UiO-66/g-C3N4 heterojunctions for enhanced photocatalytic hydrogen evolution under visible light irradiation, Adv. Mater. Interfaces 2 (10) (2015) 1500037. [74] F. He, G. Chen, Y. Yu, S. Hao, Y. Zhou, Y. Zheng, Facile approach to synthesize g-PAN/ g-C3N4 composites with enhanced photocatalytic H2 evolution activity, ACS Appl. Mater. Interfaces 6 (10) (2014) 7171–7179. [75] M.Z. Rahman, J. Zhang, Y. Tang, K. Davey, S.Z. Qiao, Graphene oxide coupled carbon nitride homo-heterojunction photocatalyst for enhanced hydrogen production, Mater. Chem. Front. 1 (3) (2017) 562–571. [76] K.  Chen, Z.  Chai, C.  Li, L.  Shi, M.  Liu, Q.  Xie, et  al., Catalyst-free growth of t­hreedimensional graphene flakes and graphene/g-C3N4 composite for hydrocarbon ­oxidation, ACS Nano 10 (3) (2016) 3665–3673.

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[77] X.  Li, Y.  Zheng, A.F.  Masters, T.  Maschmeyer, A nano-engineered graphene/carbon nitride hybrid for photocatalytic hydrogen evolution, J. Energy Chem. 25 (2) (2016) 225–227. [78] J. Liu, Y. Liu, N.Y. Liu, Y.Z. Han, X. Zhang, H. Huang, et al., Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway, Science 347 (6225) (2015) 970–974. [79] J. Zhang, M. Zhang, R.Q. Sun, X. Wang, A facile band alignment of polymeric carbon nitride semiconductors to construct isotype heterojunctions, Angew. Chem. 51 (40) (2012) 10145–10149. [80] J. Xu, Y. Li, S. Peng, G. Lu, S. Li, Eosin Y-sensitized graphitic carbon nitride fabricated by heating urea for visible light photocatalytic hydrogen evolution: the effect of the pyrolysis temperature of urea, Phys. Chem. Chem. Phys. 15 (20) (2013) 7657–7665. [81] S.  Min, G.  Lu, Enhanced electron transfer from the excited Eosin Y to mpg-C3N4 for highly efficient hydrogen evolution under 550 nm irradiation, J. Phys. Chem. C 116 (37) (2012) 19644–19652. [82] W.  Wu, J.  Zhang, W.  Fan, Z.  Li, L.  Wang, X.  Li, et  al., Remedying defects in carbon nitride to improve both photooxidation and H2 generation efficiencies, ACS Catal. 6 (5) (2016) 3365–3371. [83] W.J. Ong, L.L. Tan, S.P. Chai, S.T. Yong, Graphene oxide as a structure-directing agent for the two-dimensional interface engineering of sandwich-like graphene-g-C3N4 hybrid nanostructures with enhanced visible-light photoreduction of CO2 to methane, Chem. Commun. 51 (5) (2015) 858–861. [84] T. Ohno, N. Murakami, T. Koyanagi, Y. Yang, Photocatalytic reduction of CO2 over a hybrid photocatalyst composed of WO3 and graphitic carbon nitride (g-C3N4) under visible light, J. CO2 Util. 6 (2014) 17–25. [85] A.A.S. Nair, R. Sundara, N. Anitha, Hydrogen storage performance of palladium nanoparticles decorated graphitic carbon nitride, Int. J. Hydrog. Energy 40 (8) (2015) 3259–3267. [86] C.M.  Subramaniyam, K.A.  Deshmukh, Z.  Tai, N.  Mahmood, A.D.  Deshmukh, J.B.  Goodenough, et  al., 2D layered graphitic carbon nitride sandwiched with reduced graphene oxide as nanoarchitectured anode for highly stable lithium-ion battery, Electrochim. Acta 237 (2017) 69–77. [87] H. Tao, L. Xiong, S. Du, Y. Zhang, X. Yang, L. Zhang, Interwoven N and P dual-doped hollow carbon fibers/graphitic carbon nitride: an ultrahigh capacity and rate anode for Li and Na ion batteries, Carbon 122 (2017) 54–63.

Graphene-based nanomaterials for solar cells

7

Syed Farooq Adil*, Mujeeb Khan*, Dharmalingam Kalpana† * King Saud University, Riyadh, Saudi Arabia, †CSIR—Central Electrochemical Research Institute, Karaikudi, India

Graphene, an atomically thin two-dimensional carbonaceous material, has attracted remarkable attention from scientists and technologists because of its excellent physicochemical properties. Because of its exceptional electronic properties, such as high electron mobility and high conductance by virtue of its zero band gap, graphene has drawn particular attention to various types of solar cell applications, including dyesensitized solar cells (DSSCs) and organic, heterojunction, and perovskite solar cells. Graphene also exhibits superior optical absorption properties, high mechanical strength, and great flexibility and chemical stability. Because of these properties, graphene can be used effectively as transparent conducting materials, transparent electrodes, catalytic counter electrodes, electrolytes, and light harvesting materials in DSSC applications. Numerous reports suggest that implementing graphene in respective components of DSSCs could enhance the performance and efficiency of power conversion. For instance, in organic solar cells, graphene can be used as electron acceptors and hole conductors. In heterojunctions solar cells, it is used to form Schottky junctions with semiconducting materials (CdSe, and n-Si), which facilitates electron-hole separation and diffusions driven by the built-in potential between graphene and semiconductors. Similarly, in perovskite-type solar cells, graphene is applied as an electron-transporting photo electrode and hole-transport layer material. Thus the introduction of graphene in various types of solar cell applications creates an exciting pathway toward achieving highly efficient and stable solar cells that can help obtain greater power conversion efficiency. Furthermore, the combination of graphene with various materials, including different metals and metal oxide nanoparticles, for solar cell applications not only maximizes practical applications of DSSCs; it also enhances the synergetic effects between both of the active materials, helps to increase the light-to-electricity conversion efficiency, and enhances the performance of photoelectrochemical devices. Therefore graphene and graphene-based materials have emerged as a promising solution for the growing demand of fast-response and energy-efficient applications in future energy conversion devices, including solar cells.

7.1 Introduction Our modern lifestyle has significantly amplified the dependence on technology, which over time has increased the demand for energy. However, conventional energy resources (petroleum, coal, etc.) are limited and are being depleted at a fast rate; thus Multifunctional Photocatalytic Materials for Energy. https://doi.org/10.1016/B978-0-08-101977-1.00008-9 Copyright © 2018 Elsevier Ltd. All rights reserved.

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they will fall short of fulfilling the burgeoning global demand for energy. Because of increasing global warming, air pollution, and environmental concerns, greater efforts are being directed toward the development of energy storage and energy conversion devices with high energy and power densities [1]. Therefore, over recent years, the quest has steadily increased to find alternative, unconventional energy sources. Moreover, the production of “clean and green energy” as an alternative to nuclear energy has become a factor of utmost importance in energy production. However, the development of alternative strategies for the production of clean energy is one of the biggest challenges facing the scientific community. In this regard, several alternative energy sources have been extensively explored, including wind, hydro, and solar energies [2,3]. Among the various energy sources, solar energy is considered to be the most outstanding alternative source of renewable energy owing to its unlimited energy supply of ~10000 TW, which is much greater than total worldwide energy consumption [4]. Generally, solar cells convert the incident photon energy into useful electrical energy through the generation and subsequent collection of electron-hole pairs. Based on their performance and cost-effectiveness, solar cells are classified in three main types: (i) silicon-based solar cells, (ii) thin film-based solar cells, and (iii) dye-sensitized solar cells (DSSCs). Silicon-based solar cells were the precursors of photovoltaic cells and formed the first-generation solar cells. These types of solar cells are favored mostly because of an abundance of silicon and a large industrial infrastructure that facilitates bulk production of solar cells. The majority of commercially available solar cells are fabricated by using a solid state p-n heterojunction semiconductor first reported by Chapin et al. in 1954 [5]. When the photon is incident on the semiconductor surface, an electron-hole pair is created and a separation occurs at the junction and charge carrier, which is collected through the p-n terminal of the semiconductor. Light absorption and charge carrier transport are established by the same semiconductor materials. However, silicon types of photovoltaic cells are expensive and require high-purity material, which leads to an unfriendly environmental manufacturing process [5]. In the second type of solar cells, thin film-based solar cells, semiconducting material such as CuInSe, CdTe, CdS, and Cu(In,Ga)Se2 [6–8] are utilized to create a p-n junction. However, thin film-based solar cells exhibit low efficiency, increased toxicity, and low cost-effectiveness, which are of concern in terms of their large-scale production [4]. Therefore much interest is focused on the development of an alternative to conventional solar cells that is highly efficient, that is not expensive to produce, and that is also ecofriendly. The third type of solar cells include, dye-sensitized solar cells (DSSCs), organic solar cells (OVCs), perovskite solar cells (PSCs), and polymer-based solar cells that are different from previous types of solar cells and are considered to be thirdgeneration solar cells [9–11]. These types of solar cells are not affected by the ShockleyQueisser limit and do not rely on p-n heterojunctions to separate photogenerated charge carriers. These types of solar cells generally require low-cost materials and easy device-fabrication and are capable of achieving versatile material synthesis. However, the performance of these types of solar cells still needs to be developed because they still suffer from lower efficiency, stability, and reproducibility as compared with p-n junction solar cells. The literature reveals that efforts have focused primarily on the

Graphene-based nanomaterials for solar cells 129

development of materials based on their physical properties, such as morphology, particle size, surface thickness, materials composition, and materials deposition method, in order to increase the performance of devices. Graphene-based nanomaterials have generated more interest among researchers as a means for improving devices’ performance and also for the development of solar cells with improved flexibility, stability, and lightweight solar cells [12]. In particular, graphene inorganic nanoparticle-based nanocomposites have potential applications in these devices because of their excellent optical, electronic, and electrical properties, and they have been applied as transparent electrode materials in photoelectrochemical and photovoltaic devices [13]. In this chapter we briefly discuss the use of graphene and graphene-based nanomaterials and their application in different types of solar cells. Graphene is a sp2-hybridized single atomic layer of carbon arranged in honeycomb structures. [13] This precisely two-dimensional material exhibits unique high crystal and electronic qualities and has emerged as a promising nanomaterial for a variety of exciting applications, including solar cells [14,15]. Graphene occurs in the plane of the CCσ bonds, which are the strongest bonds. Whereas in the outer plane, it contains a π bond that provides Van der Waal forces between the graphene layers or between the graphene and the substrate, this π bond displays high electron mobility and is responsible for the conduction of graphene [16,17]. The properties of graphene can vary from a single layer of atoms to a few layers, with its quantum size effect encompassing electronic, optical, mechanical, thermal, and charge transport properties. Specifically, the electronic properties of graphene are based largely on the arrangement and number of graphene layers. Graphene with a few layers has an electronic structure different from that of bulk graphite [18]. Because of these notable properties, graphene-based materials have been used as superb materials in various energy storage and conversion devices, such as supercapacitors, solar cells, Li-ion batteries, sensors, and so on (Fig. 7.1) [19–23].

Dye-sensitized solar cell

Perovskite solar cell

Organic photovotaic cell Graphene based solar cell

Tandem solar cell

Schottky junction solar cell

Fig. 7.1  Illustration of graphene-based nanostructured material in different types of solar cells.

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7.2 Properties of graphene Pristine graphene is generally viewed as a key building block of various graphitic carbon allotropes, including 0D buckminsterfullerene, 1D CNTs, and graphite. Although graphite is a naturally occurring material and has been in use for centuries, the first reported method for producing graphene can be traced only as far back as 1970, and free-standing, single-layer graphene was first obtained in 2004 by isolating graphene from graphite via micromechanical cleavage [24–26]. The delay in the discovery of free-standing graphene sheets is partially attributed to their one-atom-thick nature, which was initially believed to be thermodynamically unstable. However, graphene is not only stable but also has excellent electronic and mechanical properties, such as a charge-carrier mobility of 250,000 cm2 V−1s−1 at room temperature, a thermal conductivity of 5000 Wm−1 K−1, an electrical conductivity of up to 6000 S cm−1, and a large theoretical specific surface area of 2630 m2 g−1. In addition, graphene is highly transparent, with absorption of 8%, J. Mater. Chem. A 3 (2015) 12603–12608. [81] M. Ye, X. Xin, C. Lin, Z. Lin, High efficiency dye-sensitized solar cells based on hierarchically structured nanotubes, Nano Lett. 11 (2011) 3214–3220. [82] B. Liu, E.S. Aydil, Growth of oriented single-crystalline rutile TiO2 nanorods on transparent conducting substrates for dye-sensitized solar cells, J. Am. Chem. Soc. 131 (2009) 3985–3990.

Metal-based semiconductor nanomaterials for thin-film solar cells 185

[83] M. Lv, et al., Optimized porous rutile TiO2 nanorod arrays for enhancing the efficiency of dye-sensitized solar cells, Energy Environ. Sci. 6 (2013) 1615–1622. [84] W.Q.  Wu, Y.F.  Xu, H.S.  Rao, C.Y.  Su, D.B.  Kuang, Trilayered photoanode of TiO2 nanoparticles on a 1D–3D nanostructured TiO2-grown flexible Ti substrate for high-­ efficiency (9.1%) dye-sensitized solar cells with unprecedentedly high photocurrent ­density, J. Phys. Chem. C 118 (2014) 16426–16432. [85] J. Li, et al., Nanotube-based hierarchical titanate microspheres: an improved anode structure for Li-ion batteries, Chem. Commun. 48 (2012) 389. [86] L. Xiang, X. Zhao, J. Yin, B. Fan, Well-organized 3D urchin-like hierarchical TiO2 microspheres with high photocatalytic activity, J. Mater. Sci. 47 (2012) 1436–1445. [87] Q.D. Truong, M. Kobayashi, H. Kato, M. Kakihana, Hydrothermal synthesis of hierarchical TiO2 microspheres using a novel titanium complex coordinated by picolinic acid, J. Ceram. Soc. Jpn. 119 (2011) 513–516. [88] J. Yu, J. Zhang, A simple template-free approach to TiO2 hollow spheres with enhanced photocatalytic activity, Dalton Trans. 39 (2010) 5860–5867. [89] D. Wu, et al., Monodisperse TiO2 hierarchical hollow spheres assembled by nanospindles for dye-sensitized solar cells, J. Mater. Chem. 22 (2012) 11665–11671. [90] Y. Zhang, L. Wu, Q. Zeng, J. Zhi, An approach for controllable synthesis of different-­ phase titanium dioxide nanocomposites with peroxotitanium complex as precursor, J. Phys. Chem. C 112 (2008) 16457–16462. [91] J. Liao, L. Shi, S. Yuan, Y. Zhao, J. Fang, Solvothermal synthesis of TiO2 nanocrystal colloids from peroxotitanate complex solution and their photocatalytic activities, J. Phys. Chem. C 113 (2009) 18778–18783. [92] N. Murakami, K. Yu, T. Tsubota, T. Ohno, Shape-controlled anatase titanium(IV) oxide particles prepared by hydrothermal treatment of peroxo titanic acid in the presence of polyvinyl alcohol, J. Phys. Chem. C 113 (2009) 873–882. [93] M.  Dakanali, et  al., A new dinuclear Ti(IV)−peroxo−citrate complex from aqueous solutions. Synthetic, structural, and spectroscopic studies in relevance to aqueous titanium(IV)−peroxo−citrate speciation, Inorg. Chem. 42 (2003) 4632–4639. [94] J. Li, K. Cao, Q. Li, D. Xu, Tetragonal faceted-nanorods of anatase TiO2 with a large percentage of active {100} facets and their hierarchical structure, CrystEngComm 14 (2011) 83–85. [95] F. Bai, et al., A versatile bottom-up assembly approach to colloidal spheres from nanocrystals, Angew. Chem. 46 (2007) 6650. [96] J.T. Park, et al., Preparation of TiO2 spheres with hierarchical pores via grafting polymerization and sol–gel process for dye-sensitized solar cells, J. Mater. Chem. 20 (2010) 8521–8530.

Further reading [1] Q.  Zheng, et  al., Hierarchical construction of self-standing anodized titania nanotube ­arrays and nanoparticles for efficient and cost-effective front-illuminated dye-sensitized solar cells, ACS Nano 5 (2011) 5088–5093. [2] W.G. Yang, F.R. Wan, Q.W. Chen, J.J. Li, D.S. Xu, Controlling synthesis of well-­crystallized mesoporous TiO2 microspheres with ultrahigh surface area for high-­performance dye-­ sensitized solar cells, J. Mater. Chem. 20 (2010) 2870–2876.

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Metal-based semiconductor nanomaterials for photocatalysis

9

Laura Clarizia*, Danilo Russo*, Ilaria Di Somma†, Roberto Andreozzi*, Raffaele Marotta* *University of Naples Federico II, Naples, Italy, †Italian National Research Council (IRC-CNR), Naples, Italy

9.1 Introduction Sunlight is considered a source of energy for long-term sustainability [1]. Among the different techniques to convert and store sunlight energy as fuel, solar production of hydrogen as a green energy vector is particularly interesting [2]. Today hydrogen is generally produced as a constituent of syngas from fossil fuels through steam reforming and water gas shift processes [3]. Other minor technologies for the production of hydrogen include chemical, electrochemical, biological, and thermal processes [4–7]. With regard to solar energy, hydrogen production can be achieved by means of ­thermo-chemical [8], photoelectrochemical, and photochemical processes [9]. In particular, photochemical processes require the adoption of catalysts able to absorb the solar radiation in order to promote electrons to the higher energy levels needed for hydrogen production. Photocatalytic processes for hydrogen production in aqueous solutions are water catalytic photosplitting [10] and catalytic photoreforming [11]. Water catalytic photosplitting consists of water’s decomposition into hydrogen and oxygen. Such decomposition is achieved under near-ambient conditions through the combined use of radiation and catalysts. In catalytic photoreforming, which may be considered as an intermediate process between water photocatalytic splitting and photocatalytic oxidation, selected organics (called sacrificial agents) are oxidized to generate hydrogen without water dissociation.

9.2 Thermodynamics and kinetics of the water splitting process Water splitting via H 2 O( l ) ® H 2 + 0.5O2 is a highly endergonic reaction (ΔGo = 237 kJ mol−1, ΔHo = 286 kJ mol−1, ΔSo = 163 J K−1 mol−1). Because the change in Gibbs free energy of the water splitting reaction is zero for temperatures higher than 1800 K, the equilibrium yields of the Multifunctional Photocatalytic Materials for Energy. https://doi.org/10.1016/B978-0-08-101977-1.00010-7 Copyright © 2018 Elsevier Ltd. All rights reserved.

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transformation are acceptable only at very high temperatures. The value of the Gibbs free energy change of water dissociation at standard conditions (237 kJ/mol H2) corresponds to a minimum applied voltage of +1.23 V or an energy requirement of 1.23 eV, according to DG o = -n × F × DE o

with n = 2, F = Faraday’s constant, DE o = voltage difference

In other words, in order to start water splitting using an electrochemical cell, a minimum necessary cell voltage of +1.23 V is required to move one electron. In the other way, water splitting is thermodynamically feasible using light radiations with wavelengths shorter than 1100 nm: Band gap ( eV ) =

(

) (

h 6.626 × 10 -34 J / s × c 3.0 × 108 m / s

l ( nm )



10 9 nm / m 1.6 × 10 -19 J / eV

However, water molecules are kinetically stable when irradiated with photons having an energy value close to 1.23 eV. Consequently, a more stringent kinetic requirement occurs for water splitting by light irradiation, because the activation energy for HO bond cleavage is about 500 kJ mol−1, which corresponds to a maximum wavelength of 250 nm (UV-C radiation) [12]. In order to overcome the activation barrier for the water splitting reaction, it is necessary to use catalytic materials capable of converting light into chemical energy by absorbing photons. Theoretically, the photocatalyst would be a semiconductor (i) characterized by a band gap higher than 1.23 eV (thermodynamic energy requirement) and (ii) capable of favoring the cleavage of the HO bond by lowering the energy barrier (500 kJ mol−1) of the reaction (kinetic energy requirement) (Fig. 9.1).

Activation energy for uncatalyzed water splitting

500 kJ/mol

Energy

Activation energy for photocatalyzed water splitting

H2 + O2

237 kJ/mol H2O

Reaction progress

Fig. 9.1  Simplified energy diagram of noncatalytic and photocatalytic water splitting.

Metal-based semiconductor nanomaterials for photocatalysis 189

9.3 Photocatalyst requirements Choosing the photocatalyst is a crucial point, as a number of requirements should be addressed. First of all, according to the principle of microscopic reversibility, the catalytic material promotes both the direct (water dissociation) and the reverse reaction (hydrogen-oxygen recombination). Second, photogenerated electron-hole pairs can be involved in three main processes: ●





migration to the surface of semiconductor; capture by defect sites located in the bulk and/or on the surface of the photocatalyst; and parasitic recombination releasing energy as heat or photons (luminescence or phosphorescence).

In order to (i) suppress hydrogen-oxygen recombination, (ii) prevent the electron-hole recombination process (timescale: c.10−9 s), and (iii) promote the migration of photogenerated charge carriers to the surface where they may react with the water molecules or other reagents (timescale: 10−8–10−1 s), the following strategies can be adopted: ●





band gap engineering of the photocatalytic materials (water photosplitting and photoreforming); purging the reactive system with an inert gas (water photosplitting and photoreforming); and use of suitable sacrificial agents (photoreforming).

Third, because water molecules or organics are involved in redox reactions with photogenerated electrons and holes, it is very crucial that the energy value of the band gap and the energetic levels of the conduction/valence bands are consistent with the standard reduction potentials of the half-redox reactions. In particular, in water photosplitting, H2O molecules are simultaneously reduced by photogenerated electrons and oxidized by photogenerated holes: hn

+ Semiconductor ® e cb + h vb + H 2 O + 2 h vb ® 0.5O2 + 2H +

2H + + 2e cb ® H2

EH2 O / O2 = 1.23 V

EH + / H = 0 V 2

Therefore the bottom level of the conduction band of the photocatalyst has to be more negative than the redox potential of the H+/H2(g) semi-couple (0 V vs. NHE at pH = 0), whereas the top level of the valence band should be more positive than the redox potential of O2(g)/H2O(l) (1.23 V vs. NHE at pH = 0). Overall, in order to efficiently carry out the previous semi-reactions, an overpotential (i.e., the redox potential of the H+/H2(g) semi-couple is −0.421 V at pH = 7.0) for the band-edges of the semiconductor is required. The redox potentials (Uredox, V vs. NHE) of the conduction and valence levels, referenced to H+/H2(g) semi-couple (0 V), can be related to the electronic energy levels (Eredox, eV in absolute scale), which are referenced to the energy level of an electron in vacuum (0 eV) through the following relation: Eredox = -4.5eV - U redox

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In other words, the standard potential reduction of the proton solution (0 V) corresponds to −4.5 eV on an absolute scale [13]. This means that to meet the requirements of the energetic levels of valence/conduction bands, the photocatalyst should have a band gap greater than 1.6–1.8 eV. Moreover, with the aim to efficiently use natural solar light radiation, the value of the band gap should be less than about 2.2 eV (Fig. 9.2): On the contrary, in catalytic photoreforming, the sacrificial agent is oxidized by photogenerated holes forming protons, which are reduced by photogenerated electrons to generate hydrogen gas (Fig. 9.3). In this case, the top of the valence band should be more positive than the oxidation potential of the sacrificial species to be oxidized. However, the band gap of the photocatalyst represents the minimum energy thermodynamically required to drive the process. In addition, the lifetime of the photogenerated electron-hole pair should be as long as possible in order to react with oxidant and reductant species rather than recombine.

NHE (V)

Conduction band

1.3

Valence band Photocatalyst

Fig. 9.2  Band gap and potential edges for solar-driven photocatalytic water photosplitting.

H2

H+

e−

H+ h CO2

+

h

e−

+

h+

Sacrificial agents

Fig. 9.3  Schematic representation of photocatalytic reforming.

e−

Metal-based semiconductor nanomaterials for photocatalysis 191

Finally, the catalyst should be (i) photo-hydrostable in an aqueous environment without undergoing photocorrosion processes [14] and (ii) easily recoverable from the mixture at the end of the process. To date, several heterogeneous semiconductors satisfying some of the above-­ mentioned requirements have been developed for photocatalytic production of hydrogen. In this regard, an energy conversion efficiency for the photocatalytic hydrogen production can be defined as [15,16] QE =

Output energy as H 2 R × DGHo 2 O % = H2 % Input energy ( incident light ) I ×S  

æ W ö æ J ö æ mol ö o where RH2 ç , DGH2 O ç , I ç 2 ÷ , and S (cm2) are the rate of hydrogen ÷ ÷ è cm ø è mol ø è s ø generation, the standard Gibbs free energy for generating one mole of hydrogen from water (237 kJ mol−1), the specific power of the radiation source, and the irradiated surface, respectively. This expression is reported if hydrogen is produced through water photosplitting only. A different form of light-to-hydrogen conversion efficiency can be used if photoreforming is also considered, QE =

o RH2 × 2 × DH comb

I ×S

%

where ∆Hocomb is the standard enthalpy change for the combustion reaction of hydrogen with oxygen (−282 kJ mol−1). However, the QE value reported for a certain number of photocatalysts does not take into account the adopted catalyst load. On the other hand, the activities of different photocatalysts (μmol H2⋅(s⋅grcatalyst)−1) cannot be directly compared to each other because the operating conditions adopted in the experiments (e.g., the effective specific light source) are quite different. In the periodic table, shown in Fig. 9.4, the main metallic elements used, mainly in the form of oxides or sulfides, as photocatalysts for photochemical hydrogen production are highlighted [15,17]. Observe that the oxidation states of the elements in these photocatalysts are the highest (Ti4+, Zr4+, Nb5+, Ta5+, W6+, Ce4+, Ga3+, In3+, Ge4+, Sn4+, Sb5+) [14]. In the table, the transition-metal cations with empty d or f orbitals have d 0 and f 0 electronic configuration, respectively, whereas the metal cations with filled d orbitals have a d10 electronic configuration. Doping photocatalytic materials with selected transition-metal elements, used as co-catalysts in zero-valent (Au, Ag, Cu, Pt, etc.) or higher oxidation state (Cr3+, Ni2+, Rh3+, Ru+4), can contribute to the design of novel visible-light-driven materials with an increased quantum efficiency (QE). It is noteworthy to stress that some metal compounds with dn (0  400 nm) and UV-A (λ > 300 nm) Table 9.1 

Photocatalyst

Band gap (eV)

Wavelength used (nm)

TiO2 (anatase)–TiO2 (rutile)

2.78

λ > 300

Tantalates–NiO Perovskites–NiOx Noble metal/TiO2-CdS (Ga0.88Zn0.12)(N0.88O0.12) –Rh2-xCrxO3 Cu1.94S-ZnxCd1−xS (0 ≤ x ≤ 1) CdS/ZnS CdSe/CdS-MoS3

3.6–4.0 3.2–4.7 N/A 2.6 2.57–3.88 N/A c.1.75–2.44

λ > 310 λ  400 λ > 400 λ > 420 λ > 420 450

MoS2/CuInS2 Cu2O/CuO Ni3N/CdS BaZrO3/BaTaO2N Ir/CoOx/Ta3N5-Rh,Ru/SrTiO3 Pt/BaZrO3-BaTaO2N TaOxN: Tantalum oxynitride Ru,Rh/SrTiO3-BiVO4 WO3/BiVO4 CdS-ZnO/RGO RGO: reduced graphene oxide CdS-TaON/RGO Sulfide-based semiconductors

N/A 1.54–2.01 2.54 1.8 ~2.1 1.8–1.9

λ > 420 λ > 400 λ > 420 λ > 420 λ > 420 λ > 420

[58] [46] [59] [17] [60] [17] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70]

N/A ~2.4 N/A

λ > 420 λ > 420 λ > 400

[71] [72] [73]

2.4–2.5 2.0–2.3

λ > 420 λ > 420

[74] [17]

Ref.

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Multifunctional Photocatalytic Materials for Energy

Comprehensive lists of mixed semiconductor heterostructures for water photosplitting are reported by others [76,77], whereas the band gap values for a large number of semiconductors are reported by Luque and Balu [78]. The highest specific rates of hydrogen production are reached over Pt-decorated Cu1.94S-Zn0.23Cd0.77S (>1.3⋅104 μmol H2/h⋅gr [61]), Na,SrTiO3-RhxCr2−xO3 (>2.2⋅104 μmol H2/h⋅gr [79]), and NiO-NaTaO3 (>2.9⋅104 μmol H2/h⋅gr [76]) under UV irradiation (λ > 300 nm). Under visible light radiation, the best specific productivities are obtained over CdS/ZnS (2.4⋅105 μmol H2/h⋅gr [62], ZnS-In2S3-CuS (3.6⋅105 μmol H2/h⋅gr [80]), and ZnFe2O4-SrTiO3 (>4.0⋅105 μmol H2/h⋅gr [81]). In addition to traditional oxide photocatalysts, new complex nanocomposites based on metal sulfides, metal oxysulfides, tantalates, nitrides, and oxynitrides have been investigated. In several studies, water photosplitting was achieved by adopting a combination of photocatalysts for H2 (i.e., doped SrTiO3) and O2 (i.e., BiVO4) generation. Although coupling n-p type semiconductors can reduce an electron-hole recombination, and because the recombination cannot be totally removed, hydrogen and oxygen production is difficult to achieve. Therefore selected inorganic ions, such as S2−/SO32− [82], IO3−/I− [83], and Ce4+/Ce3+ [84], are used in order to increase the lifetime of photogenerated charges, and in some cases to prevent the photocorrosion of metal sulfides used as n- or p-type semiconductors. These species generally act as redox mediators for the n-p type semiconductor pair [17]. The species in the lower oxidation state scavenges the holes generated in one of the two semiconductors; photogenerated electrons are thus free to reduce protons to form hydrogen. On the contrary, the species in the higher oxidation state reacts with electrons of the second semiconductors, thus favoring the oxidation reaction between photogenerated holes and water to generate oxygen [85]. The photocatalytic system involving two or more different semiconductors (n- and p-types) in the presence of a selected shuttle redox mediator is known as Z-scheme (Fig. 9.9). Through the Z-scheme, solar light can be used more efficiently than with NHE (V) H2 H+/H2

e− e− e− e− e− e−

O2 generation photocatalyst

H+

0 e− e− e− e− e− e−

Ox

Ox/red

h+ h+ h+ h+ h+ h+

Red 1.23

H2O

h+ h+ h+ h+ h+ h+

O2/H2O

H2 generation photocatalyst Redox mediator

O2

Fig. 9.9  Schematic model for Z-scheme.

Metal-based semiconductor nanomaterials for photocatalysis 199

conventional systems constituted by a single photocatalyst or metal-semiconductor system, because the proper combination of materials reduces the energy required for water dissociation. Among the nanostructured photocatalytic materials previously reported, the higher values of quantum yields of hydrogen generation are reached for Ba-doped Sr2Nb2O7 (50%, alkaline pure water, UV light) [86] and NiO/NaTaO3 (56%, pure water UV light) [87]. However, these results have a limited value for practical hydrogen productions because, as stated, UV light accounts for only about 5% of terrestrial solar radiation energy. On the other hand, the maximum quantum yields (10%) indicated as the starting point for commercial applications [88,89]. In some cases, the dopant metal limits the efficiency of these materials because it does not favor the migration of charge carriers either in the catalyst bulk or on the surface because of the generation of trapping centers for photogenerated electrons and holes [17].

9.5 Catalytic photoreforming In catalytic water photosplitting, the back reaction of hydrogen and oxygen to regenerate water remains thermodynamically and kinetically favored. In order to prevent a H2/O2 recombination, it is possible to perform the photocatalytic process under an inert atmosphere using an organic sacrificial agent (reductant agent or hole scavenger). Catalytic photoreforming [90] can be considered as an intermediate process between photocatalytic water splitting and photocatalytic oxidation of organic substances. To this purpose, the most commonly used sacrificial organic species are short chain alcohols (i.e., methanol, ethanol, and glycerol) and carboxylic acids (e.g., formic acid and oxalic acid), and carbohydrates such as glucose [11]. Substrates derived from biomass can also be used as sacrificial agents [11,91]. It is important to stress that similar biomass-derived substrates are often constituents of food and paper industry sewage [92,93]. Consequently, catalytic photoreforming bears relevance on the promotion of cleaner technologies for wastewater treatment and production of an energy carrier with high added value. Sacrificial organic species are supposed to have oxidation potential values lower than the corresponding value for water (1.23 V NHE at pH = 0). In catalytic photoreforming, photogenerated holes oxidize the sacrificial agent instead of water molecules with the production of protons that are reduced to hydrogen by photogenerated electrons. Note that oxygen gas is not generated in the medium during photocatalytic reforming. Generally, alcoholic substances are adsorbed on the catalyst surface both in undissociated structures and in forming alcoxy species [94,95]. Moreover, hydrogen production rates increase along with the increasing number of HO groups in the molecule [96]: polyols, such as glycerol and glucose, show higher activity during the photocatalytic reforming. This result has been ascribed to the role of hydroxyl groups that promote the substrate adsorption on the active sites of the photocatalyst. A ­possible

200

Multifunctional Photocatalytic Materials for Energy

mechanism of photocatalytic reforming of alcohols considers the oxidation of the molecules through an alpha-hydrogen abstraction by photogenerated hole forming RCH·OH radical, which is converted to aldehyde [97]: h+

 HOH + H + R — CH 2 OH ® R — C h+

 HOH ® R — CHO + H + R—C The hydrated form of aldehyde can be further oxidized to the respective carboxylic acid. Longer chain alcohols form alkanes as undesired by-products together with hydrogen and carbon dioxide. Short chain carboxylic (i.e., formic acid) and dicarboxylic acids (i.e., oxalic acid) are mineralized to carbon dioxide and water: h+

R — COOH ® R — COO· + H + R — COO· ® CO2 + R · Proton ions generated during the oxidation process are reduced to hydrogen by photoelectrons. The mechanism of carbohydrate photoreforming is more complex and not sufficiently elucidated. Some efforts have been made to define the chemical pathway of glucose oxidation [98,99]. It would proceed via bridge adsorption on the surface of the catalyst through one or more hydroxyl groups [100]. Several examples of photocatalytic reforming of other organic substances, such as carboxylic acids [101,102], aldehydes [103,104], azodyes [105], and cellulose [106] are also reported. Additional information on different classes of sacrificial agents were recently reported by Puga [11]. Most of the systems used for photocatalytic reforming include the same materials reported for photocatalytic water splitting. Noble metals, despite their high cost, are extensively studied as co-catalysts in photoreforming. In general, it appears that platinum is the most effective metallic element, followed by palladium and gold [107,108]. In order to reduce the cost of photocatalytic materials, some earth abundant coinage metals, such as nickel [109], cobalt [110], and copper have been used as co-catalysts. In particular, Cu-based metal oxides have been developed and tested in the presence of different sacrificial agents [22,111], and also in aqueous matrices with excess nitrate [112] and chloride [113] ions. Some reviews report numerous heterojunction-based nanostructured materials used for photocatalytic reforming of different organic species [11,23,41]. As for water photosplitting, all these materials are manufactured following different preparation methods, such as photoelectrochemical deposition, wet impregnation, photodeposition, sol-gel, solvothermal and precipitation and chemical vapor deposition, thermal and hydrothermal, and so on [76]. Generally, under the same experimental conditions, hydrogen production rates more than two orders of magnitude higher than water photosplitting have been recorded [114]. The highest activities have been achieved over metal/niobates (>4.3⋅104 μmol

Metal-based semiconductor nanomaterials for photocatalysis 201

H2/h⋅gr, sacrificial agent: methanol [115]) and Sr/tantalates (1.1⋅104 μmol H2/h⋅gr, sacrificial agent: methanol [116]) under UV-A light irradiation, and over Ni/CdS nanorods (6.3⋅104 μmol H2/h⋅gr, sacrificial agent: ethanol [117]), CdS/RGO (5.6⋅104 μmol H2/h⋅gr, sacrificial agent: 10% lactic acid [118]), Pt/CdSe-CdS (4.0⋅104 μmol H2/h⋅gr, sacrificial agent: isopropanol [119]), and N/Zn,Ga-mixed oxide-Rh/Cr2O3 (>3.7⋅104 μmol H2/h⋅gr, sacrificial agent: methanol [120]) under visible light irradiation. Currently, the best photocatalysts for hydrogen generation by photoreforming under visible light are CdS-based materials [120a].

9.6 Operating variables affecting photocatalyst activity Because oxygen gas competes with protons as electron scavengers and reacts with gaseous hydrogen to form water, the presence of oxygen gas in the reaction system negatively affects the activity during both water photosplitting and photoreforming processes. The main variables affecting the activity of the photocatalyst toward hydrogen generation by water photosplitting and photoreforming are the size of the particles and their structure, morphology and crystallinity, synthetic procedures, band gap energy, loading of the co-catalyst, nature and concentration of the sacrificial agent, pH of the solution, and operating temperature [10,15]. The structure and the morphology of photocatalytic material depend on the synthetic procedure adopted. Different temperatures and reagents used during synthesis generate particles characterized by different sizes, shapes, and crystallinities. The amorphous/crystalline ratio must be considered. The nature of the crystallographic phase can affect the band gap [121]. The presence of structural defects in the bulk of semiconductors affects electrical properties of the material by generating localized electronic states that trap charge carriers [17,122]. Therefore it is desirable to prepare particles without structural defects and impurities. The particle size is crucial for the activity of the materials. First, it affects the band gap of the semiconductor [121,123]. Second, the surface area of the photocatalyst is strongly dependent on the particle size [15]. Particles with a smaller diameter (i.e., nanoparticles) are more active because they have a higher surface area (high density of surface catalytic sites). Moreover, the amount of inner surface defects is proportional to the particle size. Large particles can be represented as consisting of multi-aggregates of ideal crystals with different finite sizes [124]. Smaller particles exhibit larger portions of highly crystalline areas. Nanoparticles consist of only one ideal crystalline particle with finite dimensions (Fig. 9.10). Finally, the likelihood of recombination between charge carriers, before occurrence of surface reactions, is favored in larger-size particles (Fig. 9.10). The calcination process adopted during photocatalyst synthesis may influence its surface area and crystallinity. An effective calcination aims to improve the stability and the crystallinity of the material. Nevertheless, beyond an optimum calcination temperature, the photocatalytic activity generally decreases because of agglomeration

202

3 nm

1.5 nm

1 nm

ion

gra

h+

h+

Oxida

tion Oxida tion

Large particle SA = 108 nm2

Small particle

SA = 162 nm2 Defects

Large particle

Small particle

Nanoparticle

Fig. 9.10  Surface area versus particle size (left); visual relationship between size particle and charge diffusion (right); crystal defects versus particle size (center).

Multifunctional Photocatalytic Materials for Energy

SA = 54 nm2

ction Redu e−

Mi

e−

tio

n

ct Redu

Metal-based semiconductor nanomaterials for photocatalysis 203

processes and sintering damage, which reduces the surface area and could also cause a phase transition of the material (i.e., anatase to rutile). The activity is dependent on the amount and the loading method of the co-catalyst (i.e., zero-valent metal) on the base photocatalyst. Highly dispersed nanoparticles of co-catalyst contribute to increasing the rate of hydrogen production, but excess loading reduces the catalytic activity because it hinders the radiation absorption and, in some cases, can favor the recombination process between electron-hole pairs [125]. Therefore an optimum load of co-catalyst can be found to maximize the catalyst activity (Fig. 9.11). Moreover, the increase in loading determines the increase in reaction rate. However, there is also a limiting load for the base photocatalyst, above which hydrogen production rate does not further increase because of particle agglomeration and radiation shadowing phenomena for high turbidity of particles’ suspensions [126]. The pH of the solution is an additional important variable to be controlled because it affects both the stability of materials and the photocatalytic process. For example, the rates of water photosplitting are favored under alkaline pH conditions in the presence of NiOx-loaded perovskites [87,127,128] and in acidic pH conditions using RuO2-loaded oxynitrides [129,130]. Even in the presence of the same base photocatalyst, the optimal pH conditions differ depending on the co-catalyst used because of corrosion and hydrolysis phenomena [129,131]. Changes in pH of the solution could also affect the position of the band-edge potential of the photocatalyst [132,133].

• Lower density of photoactive sites of base material • Change of polar characteristics of photocatalyst surface • Occurring of aggregation phenomena

Photocatalyst activity

• Increase charge carriers separation • Higher absorption in visible light range • Lower overpotential of redox reactions

Poor load of cocatalyst

Excessive load of cocatalyst Loading of cocatalyst

Fig. 9.11  Effect of co-catalyst loading on the photocatalyst activity. Adapted from K. Maeda, Photocatalytic water splitting using semiconductor particles: history and recent developments, J. Photochem. Photobiol. C Photochem. Rev. 12 (2011) 237–268.

204

Multifunctional Photocatalytic Materials for Energy

Moreover, the charge status of the photocatalyst surface can be changed by varying the pH of the solution, depending on the semiconductor’s pH at the zero point of charge (pHzpc). In particular, if the surface consists of amphoteric groups, such as TiO2, one must take into account the equilibria [134] pKa 1

> OH + H + Û > OH 2 + pKa 2

> OH + OH - Û > O - + H 2 O where >OH, >OH2+, and >O− indicate the neutral, positive, and negative surface hydroxyl groups, respectively. The point of zero charge is estimated as pH zpc =

1 ( pKa1 + pKa 2 ) 2

where pKa1 and pKa2 are the negative logarithms of equilibrium constants under acidic and alkaline conditions, respectively. If the pH  pHzpc, the negative charge predominates, and neutral conditions stand when the pH = pHzpc. The pHzpc values for the most common metal oxides and other semiconductors have been reported in the scientific literature [135,136]. A charge modification of the solid surface leads to a change in the electrostatic interaction between the reagents (i.e., sacrificial agents, redox mediators) and the photocatalyst [137]. The nature of the photocatalyst surface is important in photocatalytic reforming. For example, surface hydroxyl groups play the role of scavenging centers for photogenerated holes and adsorption sites for sacrificial agents [138]. Under acidic conditions, photocatalyst particles may agglomerate, in which case, the surface area available for the adsorption is reduced. In catalytic photoreforming, hydrogen production increases with increasing concentrations of the sacrificial species [139], but at very high organic concentrations, no further beneficial effects on the rates of hydrogen generation are recorded because of the saturation of the adsorption sites of the photocatalyst [12,140]. The influence of temperature on the photocatalytic reaction rates is not negligible, and the activation energy value for each physicochemical step involved in the photocatalytic process is different. It is reported that a moderate increase in temperature enhances the rate of hydrogen production [121,141,142]. Higher temperatures promote electron transfer from the valence band to the conduction band and increase the mobility of charge carriers, thus lowering the likelihood of electron-hole recombinations [10]. A convenient temperature range is about 60−80°C, although the optimal operating temperature varies depending on the photocatalytic system. Temperature values higher than 80°C disfavor the adsorption of the reagents, thus lowering the process’s photocatalytic efficiency.

Metal-based semiconductor nanomaterials for photocatalysis 205

9.7 Conclusion Hydrogen generation using solar energy through catalytic water photosplitting and catalytic organic photoreforming is one of the most promising technologies for clean and sustainable production of energy. The literature surveyed points out that, despite considerable efforts devoted to the synthesis of metal-based composites for hydrogen production, the activity and stability of the water in photocatalytic materials are far from being satisfactory for commercial utilization. Current results still record low efficiencies for visible-light-to-hydrogen conversion (10% at 600 nm). The photocatalytic materials for visible light water splitting and photoreforming should have proper band gap energy (1.6–2.2 eV) and band alignment, high specific activity (>104 μmol H2/h⋅gr), photostability in aqueous electrolyte media, and high crystallinity. It is also important to observe that, in view of future applications of photocatalytic hydrogen production, reduction in costs and toxicity of photocatalysts are key issues. The possibility of hydrogen generation through photoreforming processes requires large amounts of biomass, high organic load rates, and low hydraulic retention times achieving through the use of sludge and sewage. However, considering the pioneering results achieved by Fujishima and Honda [143] in photocatalysis up to those of the present day, technically and economically viable visible light photocatalytic systems for water photosplitting and photoreforming could become available in the future. Therefore engineering a design for photocatalytic nanomaterials that are active under natural sunlight radiation and that have a high photonic efficiency toward hydrogen production will be a key challenge.

References [1] V. Balzani, N. Armaroli, Energy for a Sustainable World: From the Oil Age to a SunPowered Future, Wiley-VCH Verlag, Weinheim, 2010. [2] G.  Centi, R.A.  Van Santen, Catalysis for Renewables: From Feedstock to Energy Production, Wiley-VCH Verlag, Weinheim, 2008. [3] Ullmann’s Encyclopedia of Industrial Chemistry, 2016. Wiley-VCH Verlag, Weinheim, Germany. [4] H.J. Alves, C.J. Bley, R.R. Niklevicz, E.P. Frigo, M.S. Frigo, C.H. Coimbra-Araujo, Overview of hydrogen production technologies from biogas and the applications in fuel cells, Int. J. Hydrog. Energy 38 (13) (2010) 5215–5225.

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[5] J.D. Holladay, J. Hu, D.L. King, Y. Wang, An overview of hydrogen production technologies, Catal. Today 139 (4) (2009) 244–260. [6] R. Kothari, D. Buddhi, R.L. Sawhney, Comparison of environmental and economic aspects of various hydrogen production methods, Renew. Sust. Energ. Rev. 12 (2) (2008) 553–563. [7] A. Molino, S. Chianese, D. Musmarra, Biomass gasification technology: the state of the art overview, J. Energy Chem. 25 (1) (2016) 20–25. [8] A. Steinfeld, Solar thermochemical production of hydrogen––a review, Sol. Energy 78 (5) (2005) 603–615. [9] Z.  Wang, Z.Z.  Roberts, G.F.  Naterer, K.S.  Gabriel, Comparison of thermochemical, electrolytic, photoelectrolytic and photochemical solar-to-hydrogen production technologies, Int. J. Hydrog. Energy 37 (2012) 16287–16301. [10] H. Ahmad, S.K. Kamarudin, L.J. Minggu, M. Kassim, Hydrogen from photo-catalytic water splitting process: a review, Renew. Sust. Energ. Rev. 43 (2015) 599–610. [11] A.V. Puga, Photocatalytic production of hydrogen from biomass-derived feedstocks, Coord. Chem. Rev. 315 (2016) 1–66. [12] M. Bowker, Sustainable hydrogen production by the application of ambient temperature photocatalysis, Green Chem. 13 (2011) 2235–2246. [13] J.O.M. Bockris, S.U.M. Khan, Surface Electrochemistry, Plenum, New York, 1993. [14] M. Kitano, M. Hara, Heterogeneous photocatalytic cleavage of water, J. Mater. Chem. 20 (2010) 627–641. [15] K.  Maeda, Photocatalytic water splitting using semiconductor particles: history and recent developments, J Photochem Photobiol C: Photochem Rev 12 (2011) 237–268. [16] M. Qureshi, K. Takanabe, Insights on measuring and reporting heterogeneous photocatalysis: efficiency definitions and setup examples, Chem. Mater. 29 (1) (2017) 158–167. [17] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253–278. [18] J.S. Jang, H.G. Kim, J.S. Lee, Heterojunction semiconductors: a strategy to develop efficient photocatalytic materials for visible light water splitting, Catal. Today 185 (2012) 270–277. [19] E.  Liu, L.  Kang, Y.  Yang, T.  Sun, X.  Hu, C.  Zhu, H.  Liu, Q.  Wang, X.  Li, J.  Fan, Plasmonic Ag deposited TiO2 nano-sheet film for enhanced photocatalytic hydrogen production by water splitting, Nanotechnology 25 (2014) 1–10. [20] F. Wu, X. Hu, J. Fan, E. Liu, T. Sun, L. Kang, W. Hou, C. Zhu, H. Liu, Photocatalytic activity of Ag/TiO2 nanotube arrays enhanced by surface plasmon resonance and application in hydrogen evolution by water splitting, Plasmonics 8 (2) (2013) 501–508. [21] C.G. Silva, R. Juarez, T. Marino, R. Molinari, H. Garcia, Influence of excitation wavelength (UV or visible light) on the photocatalytic activity of titania containing gold nanoparticles for the generation of hydrogen or oxygen from water, J. Am. Chem. Soc. 133 (2011) 595–602. [22] L.  Clarizia, D.  Spasiano, I.  Di Somma, R.  Marotta, R.  Andreozzi, D.D.  Dionysiou, Copper modified-TiO2 catalysts for hydrogen generation through photoreforming of organics. A short review, Int. J. Hydrog. Energy 39 (2014) 16812–16831. [23] G. Colon, Towards the hydrogen production by photocatalysis, Appl. Catal. A Gen. 518 (2015) 48–59. [24] L. Li, J. Yan, T. Wang, Z.J. Zhao, J. Zhang, J. Gong, N. Guan, Sub-10 nm rutile titanium dioxide nanoparticles for efficient visible-light-driven photocatalytic hydrogen production, Nat. Commun. 6 (2015) 5881–5891.

Metal-based semiconductor nanomaterials for photocatalysis 207

[25] J. Yu, L. Qi, M. Jaroniec, Hydrogen production by photocatalytic water splitting over Pt/TiO2 nanosheets with exposed (001) facets, J. Phys. Chem. C 114 (30) (2010) 13118–13125. [26] A.A. Melvin, K. Illath, T. Das, T. Raja, S. Bhattacharyya, C.S. Gopinath, M–Au/TiO2 (M = Ag, Pd, and Pt) nanophotocatalyst for overall solar water splitting: role of interfaces, Nanoscale 7 (2015) 13477–13488. [27] J.Y. Park, J.R. Renzas, B.B. Hsu, G.A. Somorjai, Interfacial and chemical properties of Pt/TiO2, Pd/TiO2, and Pt/GaN catalytic nanodiodes influencing hot electron flow, J. Phys. Chem. C 111 (2007) 15331–15336. [28] V. Subramanian, E.E. Wolf, P. Kamat, Catalysis with TiO2/gold nanocomposites: effect of metal particle size on the Fermi level equilibration, J. Am. Chem. Soc. 126 (2004) 4943–4950. [29] J.J. Chen, J.C.S. Wu, P.C. Wu, D.P. Tsai, Plasmonic photocatalyst for H2 evolution in photocatalytic water splitting, J. Phys. Chem. C 115 (2011) 210–216. [30] X.  Zhou, G.  Liu, J.  Yu, W.  Fan, Surface plasmon resonance-mediated photocatalysis by noble metal-based composites under visible light, J. Mater. Chem. 22 (2012) 21337–21354. [31] X. Zhang, Y.L. Chen, R.S. Liu, D.P. Tsai, Plasmonic photocatalysis, Rep. Prog. Phys. 76 (2013) 046401. [32] S.C.  Warren, E.  Thimsen, Plasmonic solar water splitting, Energy Environ. Sci. 5 (2012) 5133–5146. [33] A. Bazzo, A. Urakawa, Understanding synergetic effects of Zn and Rh–Cr promotion to wide-bandgap Ga, Ta and Ti oxides in photocatalytic water splitting, Catal. Sci. Technol. 6 (2016) 4243–4253. [34] M. Kitano, K. Tsujimaru, M. Anpo, Hydrogen production using highly active titanium oxide-based photocatalysts, Top. Catal. 49 (2008) 4–17. [35] A. Primo, T. Marino, A. Corma, R. Molinari, H. Garcia, Efficient visible light photocatalytic water splitting by minute amounts of gold supported on nanoparticulate CeO2 obtained by a biopolymer templating method, J. Am. Chem. Soc. 133 (2012) 6930–6933. [36] Q. Wang, T. Hisatomi, Q. Jia, H. Tokudome, M. Zhong, C. Wang, Z. Pan, T. Takata, M. Nakabayashi, N. Shibata, et al., Scalable water splitting on particulate photocatalyst sheets with a solar-to-hydrogen energy conversion efficiency exceeding 1%, Nat. Mater. 15 (2016) 611–615. [37] Q.  Wang, T.  Hisatomi, S.S.K.  Ma, Y.  Li, K.  Domen, Core/shell structured La- and Rh-codoped SrTiO3 as a hydrogen evolution photocatalyst in Z-scheme overall water splitting under visible light irradiation, Chem. Mater. 26 (2014) 4144–4150. [38] S. Hara, M. Yoshimizu, S. Tanigawa, L. Ni, B. Ohtani, H. Irie, Hydrogen and oxygen evolution photocatalysts synthesized from strontium titanate by controlled doping and their performance in two-step overall water splitting under visible light, J. Phys. Chem. C 116 (2012) 17458–17463. [39] L. Liao, Q. Zhang, Z. Su, Z. Zhao, Y. Wang, Y. Li, X. Lu, D. Wei, G. Feng, Q. Yu, X. Cai, J. Zhao, Z. Ren, H. Fang, F. Robles-Hernandez, S. Baldelli, J. Bao, Efficient solar water-splitting using a nanocrystalline CoO photocatalyst, Nat. Nanotechnol. 9 (2014) 69–73. [40] H.G. Kim, D.W. Hwang, J.S. Lee, An undoped, single-phase oxide photocatalyst working under visible light, J. Am. Chem. Soc. 126 (2004) 8912–8913. [41] Y.  Xu, Y.  Huang, B.  Zhang, Rational design of semiconductor-based photocatalysts for advanced photocatalytic hydrogen production: the case of cadmium chalcogenides, Inorg. Chem. Front. 3 (2016) 591–615.

208

Multifunctional Photocatalytic Materials for Energy

[42] H. Wang, L. Zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu, X. Wang, Semiconductor heterojunction photocatalysts: design, construction, and photocatalytic performances, Chem. Soc. Rev. 43 (2014) 5234–5244. [43] B. Sun, A.V. Vorontsov, P.G. Smirniotis, Role of platinum deposited on TiO2 in phenol photocatalytic oxidation, J. Phys. Chem. B 19 (2003) 3151–3156. [44] P. Deak, B. Aradi, T. Frauenheim, Band lineup and charge carrier separation in mixed rutile-anatase systems, J. Phys. Chem. C 115 (2011) 3443–3446. [45] Y. Mi, Y. Weng, Band alignment and controllable electron migration between rutile and anatase TiO2, Sci Rep 5 (2015) 11482. [46] D.O. Scanlon, C.W. Dunnill, J. Buckeridge, S.A. Shevlin, A.J. Logsdail, S.M. Woodley, C.R.A.  Catlow, M.J.  Powell, R.G.  Palgrave, I.P.  Parkin, G.W.  Watson, T.W.  Keal, P.S. Sherwood, A. Walsh, A.A. Sokol, Band alignment of rutile and anatase TiO2, Nat. Mater. 12 (2013) 798–801. [47] W.N. Zhao, S.C. Zhu, Y.F. Li, Z.P. Liu, Three-phase junction for modulating electron– hole migration in anatase–rutile photocatalysts, Chem. Sci. 6 (2015) 3483–3494. [48] X.  Wang, Q.  Xu, M.R.  Li, S.  Shen, X.L.  Wang, Y.C.  Wang, Z.C.  Feng, Y.J.  Shi, H.X. Han, C. Li, Photocatalytic overall water splitting promoted by an α–β phase junction on Ga2O3, Angew. Chem. Int. Ed. 51 (2012) 13089–13092. [49] P. Wang, P.R. Chen, A. Kostka, R. Marschall, M. Wark, Control of phase coexistence in calcium tantalate composite photocatalysts for highly efficient hydrogen production, Chem. Mater. 25 (2013) 4739–4745. [50] O.M.  Ishchenko, V.  Rogé, G.  Lamblin, D.  Lenoble, TiO2- and ZnO-based materials for photocatalysis: material properties, device architecture and emerging concepts, in: Semiconductor Photocatalysis—Materials, Mechanisms and Applications, InTechOpen Ed, Lenoble, 2016. [51] K. Maeda, Z-scheme water splitting using two different semiconductor photocatalysts, ACS Catal. 3 (2013) 1486−1503. [52] X. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications, Chem. Rev. 107 (2007) 2891–2959. [53] A.O. Ibhadon, P. Fitzpatrick, Heterogeneous photocatalysis: recent advances and applications, Catalysts 3 (2013) 189–218. [54] J. Low, B. Cheng, J. Yu, Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2: a review, Appl. Surf. Sci. 392 (2017) 658–686. [55] O. Ola, M.M. Maroto-Valer, Synthesis, characterization and visible light photocatalytic activity of metal based TiO2 monoliths for CO2 reduction, Chem. Eng. J. 283 (2016) 1244–1253. [56] M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, P.S.M. Dunlop, J.W.J. Hamilton, J.A. Byrne, K. O’Shea, M.H. Entezari, D.D. Dionysiou, A review on the visible light active titanium dioxide photocatalysts for environmental applications, Appl. Catal. B Environ. 125 (2012) 331–349. [57] A. Zielinska-Jurek, Progress, challenge, and perspective of bimetallic TiO2-based photocatalysts, J. Nanomater. 2014 (2014) 1–17. [58] R.  Abe, K.  Sayama, K.  Domen, H.  Arakawa, A new type of water splitting system composed of two different TiO2 photocatalysts (anatase, rutile) and a IO3−/I− shuttle redox mediator, Chem. Phys. Lett. 344 (2001) 339–344. [59] A. Kudo, Development of photocatalyst materials for water splitting, Int. J. Hydrog. Energy 31 (2006) 197–202. [60] H. Zhou, J. Pan, L. Ding, Y. Tang, J. Ding, Q. Guo, T. Fan, D. Zhang, Biomass-derived hierarchical porous CdS/M/TiO2 (M = Au, Ag, pt, pd) ternary heterojunctions for photocatalytic hydrogen evolution, Int. J. Hydrog. Energy 39 (2014) 16293–16301.

Metal-based semiconductor nanomaterials for photocatalysis 209

[61] Y. Chen, S. Zhao, X. Wang, Q. Peng, R. Lin, Y. Wang, R. Shen, X. Cao, L. Zhang, G.  Zhou, et  al., Synergetic integration of Cu1.94S-ZnxCd1-xS heteronanorods for enhanced visible-light-driven photocatalytic hydrogen production, J. Am. Chem. Soc. 138 (2016) 4286–4289. [62] D. Jiang, Z. Sun, H. Jia, D. Lu, P.A. Du, A cocatalyst-free CdS nanorod/ZnS nanoparticle composite for high-performance visible-light-driven hydrogen production from water, J. Mater. Chem. A. Mater. Energy Sustain. 4 (2016) 675–683. [63] M.L.  Tang, D.C.  Grauer, B.  Lassalle-Kaiser, V.K.  Yachandra, L.  Amirav, J.  Yano, J.R.  Long, A.P.  Alivisatos, Structural and electronic study of an amorphous MoS3 ­hydrogen-generation catalyst on a quantum-controlled photosensitizer, Angew. Chem. Int. Ed. 50 (2011) 10203–10207. [64] Y. Liang, L. Zhai, X. Zhao, D. Xu, Band-Gap engineering of semiconductor nanowires through composition modulation, J. Phys. Chem. B 109 (2005) 7120–7123. [65] Y.J. Yuan, D.Q. Chen, Y.W. Huang, Z.T. Yu, J.S. Zhong, T.T. Chen, W.G. Tu, Z.J. Guan, D.P. Cao, et al., MoS2 nanosheet-modified CuInS2 photocatalyst for visible-light-driven hydrogen production from water, ChemSusChem 9 (2016) 1003–1009. [66] A.A.  Dubale, C.J.  Pan, A.G.  Tamirat, H.M.  Chen, W.N.  Su, C.H.  Chen, J.  Rick, D.W. Ayele, B.A. Aragaw, J.F. Lee, J.W. Yang, B.J. Hwang, Heterostructured Cu2O/ CuO decorated with nickel as a highly efficient photocathode for photoelectrochemical water reduction, J. Mater. Chem. A 3 (2015) 12482–12499. [67] Z. Sun, H. Chen, L. Zhang, D. Lu, P. Du, Enhanced photocatalytic H2 production on cadmium sulfide photocatalysts using nickel nitride as a novel cocatalyst, J. Mater. Chem. A 4 (2016) 13289–13295. [68] K. Maeda, D.  Lu, K.  Domen, Solar-driven Z-scheme water splitting using modified BaZrO3−BaTaO2N solid solutions as photocatalysts, ACS Catal. 3 (2013) 1026−1033. [69] S.S.K. Ma, K. Maeda, T. Hisatomi, M. Tabata, A. Kudo, K. Domen, A redox-­mediatorfree solar-driven Z-scheme water-splitting system consisting of modified Ta3N5 as an oxygen-evolution photocatalyst, Chem. A Eur. J. 19 (2013) 7480–7486. [70] T. Matoba, K. Maeda, K. Domen, Activation of BaTaO2N photocatalyst for enhanced non-sacrificial hydrogen evolution from water under visible light by forming a solid solution with BaZrO3, Chem. A Eur. J. 17 (2011) 14731–14735. [71] Y. Sasaki, H. Nemoto, K. Saito, A. Kudo, Solar water splitting using powdered photocatalysts driven by Z-schematic interparticle electron transfer without an electron mediator, J. Phys. Chem. C 113 (2009) 17536–17542. [72] Y.  Pihosh, I.  Turkevych, K.  Mawatari, J.  Uemura, Y.  Kazoe, S.  Kosar, K.  Makita, T. Sugaya, T. Matsui, D. Fujita, M. Tosa, M. Kondo, T. Kitamori, Photocatalytic generation of hydrogen by core-shell WO3/BiVO4 nanorods with ultimate water splitting efficiency, Sci. Rep. 5 (2015) 11141–11151. [73] X.Y. Wang, L.C. Yin, G. Liu, Light irradiation-assisted synthesis of ZnO–CdS/reduced graphene oxide heterostructured sheets for efficient photocatalytic H2 evolution, Chem. Commun. 50 (2014) 3460–3463. [74] J.G.  Hou, Z.  Wang, W.B.  Kan, S.Q.  Jiao, H.M.  Zhu, R.V.  Kumar, Efficient visiblelight-driven photocatalytic hydrogen production using [email protected] core–shell composites coupled with graphene oxide nanosheets, J. Mater. Chem. 22 (2012) 7291–7299. [75] T. Jafari, E. Moharreri, A.S. Amin, R. Miao, W. Song, S.L. Suib, Photocatalytic water splitting—the untamed dream: a review of recent advances, Molecules 21 (2016) 900–929. [76] X. Chen, S. Shen, L. Guo, S.S. Mao, Semiconductor-based photocatalytic hydrogen generation, Chem. Rev. 110 (2010) 6503–6570.

210

Multifunctional Photocatalytic Materials for Energy

[77] R.M.  Navarro, F.  Del Valle, J.A.  Villoria de la Mano, M.C.  Alvarez-Galvan, J.L.G. Fierro, Photocatalytic water splitting under visible light: concept and catalysts development, Adv. Chem. Eng.-Progr. Photocatal. React. Eng. 36 (2009) 111–143. [78] R. Luque, A.M. Balu, Producing fuels and fine chemicals from biomass using nanomaterials, CRC Press, Taylor & Francis Group, Boca Raton, FL, 2013. [79] T.  Takata, K.  Domen, Defect engineering of photocatalysts by doping of aliovalent metal cations for efficient water splitting, J. Phys. Chem. C (45) (2009) 19386–19388. [80] Y.  Li, G.  Chen, Q.  Wang, X.  Wang, A.  Zhou, Z.  Shen, Hierarchical ZnS-In2S3-CuS nanospheres with nanoporous structure: Facile synthesis, growth mechanism, and excellent photocatalytic activity, Adv. Funct. Mater. 20 (2010) 3390–3398. [81] S. Boumaza, A. Boudjemaa, A. Bouguelia, R. Bouarab, M. Trari, Visible light induced hydrogen evolution on new hetero-system ZnFe2O4/SrTiO3, Appl. Energy 87 (2010) 2230–2236. [82] A. Koca, M. Sahin, Photocatalytic hydrogen production by direct sun light from sulfide/sulfite solution, Int. J. Hydrog. Energy 27 (2002) 363–367. [83] K. Lee, W.S. Nam, G.Y. Han, Photocatalytic water-splitting in alkaline solution using redox mediator. 1: parameter study, Int. J. Hydrog. Energy 29 (2004) 1343–1347. [84] G.R. Bamwenda, H. Arakawa, The photoinduced evolution of suspension O2 and H2 from a WO3 aqueous suspension in the presence of Ce4+/Ce3+, Sol. Energy Mater. Sol. Cells 70 (2001) 1–14. [85] M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production, Renew. Sust. Energ. Rev. 11 (2007) 401–425. [86] J. Kim, D.W. Hwang, H.G. Kim, S.W. Bae, J.S. Lee, W. Li, S.H. Oh, Highly efficient overall water splitting through optimization of preparation and operation conditions of layered perovskite photocatalysts, Top. Catal. 35 (3–4) (2005) 295–303. [87] H.  Kato, K.  Asakura, A.  Kudo, Highly efficient water splitting into H2 and O2 over ­lanthanum-doped NaTaO3 photocatalysts with high crystallinity and surface nanostructure, J. Am. Chem. Soc. 125 (2003) 3082–3089. [88] D. Jing, L. Guo, L. Zhao, X. Zhang, H. Liu, M. Li, S. Shen, G. Liu, X. Hu, X. Zhang, K. Zhang, L. Ma, P. Guo, Efficient solar hydrogen production by photocatalytic water splitting: from fundamental study to pilot demonstration, Int. J. Hydrog. Energy 35 (2010) 7087–7097. [89] J. Turner, G. Sverdrup, M.K. Mann, P.C. Maness, B. Kroposki, M. Ghirardi, R.J. Evans, D. Blake, Renewable hydrogen production, Int. J. Energy Res. 32 (5) (2007) 379–407. [90] M. Bowker, Photocatalytic hydrogen production and oxygenate photoreforming, Catal. Lett. 142 (2012) 923–929. [91] J.C.  Colmenares, R.  Luque, Heterogeneous photocatalytic nanomaterials: prospects and challenges in selective transformations of biomass-derived compounds, Chem. Soc. Rev. 43 (2014) 765–778. [92] J.C.  Colmenares, A.  Magdziarz, M.A.  Aramendia, A.  Marinas, J.M.  Marinas, F.J. Urbano, J.A. Navio, Influence of the strong metal support interaction effect (SMSI) of Pt/TiO2 and Pd/TiO2 systems in the photocatalytic biohydrogen production from glucose solution, Catal. Commun. 16 (2011) 1–6. [93] M.  Ilie, B.  Cojocaru, V.I.  Parvulescu, H.  Garcia, Improving TiO2 activity in photo-­ production of hydrogen from sugar industry wastewaters, Int. J. Hydrog. Energy 36 (2011) 15509–15518. [94] S. Pilkenton, S.J. Hwang, D. Raftery, Ethanol photocatalysis on TiO2-coated optical microfiber, supported monolayer, and powdered catalysts: an in situ NMR study, J. Phys. Chem. B 103 (1999) 11152–11160.

Metal-based semiconductor nanomaterials for photocatalysis 211

[95] W.  Xu, D.  Raftery, Photocatalytic oxidation of 2-propanol on TiO2 powder and TiO2 monolayer catalysts studied by solid-state NMR, J. Phys. Chem. B 105 (2001) 4343–4349. [96] X. Fu, X. Wang, D.Y.C. Leung, Q. Gu, S. Chen, H. Huang, Photocatalytic reforming of C3-polyols for H2 production: part (I). Role of their OH groups, Appl. Catal. B 106 (2011) 681–688. [97] C.Y.  Wang, J.  Rabani, D.W.  Bahnemann, J.K.  Dohrmann, Photonic efficiency and quantum yield of formaldehyde formation from methanol in the presence of various TiO2 photocatalysts, J. Photochem. Photobiol. A 148 (2002) 169–176. [98] M. Du, J. Feng, S.B. Zhang, Photo-oxidation of polyhydroxyl molecules on TiO2 surfaces: from hole scavenging to light-induced self-assembly of TiO2-cyclodextrin wires, Phys. Rev. Lett. 98 (2007) 1–4. [99] X.  Fu, J.  Long, X.  Wang, D.Y.C.  Leung, Z.  Ding, L.  Wu, Z.  Zhang, Z.  Li, X.  Fu, Photocatalytic reforming of biomass: a systematic study of hydrogen evolution from glucose solution, Int. J. Hydrog. Energy 33 (2008) 6484–6491. [100] G. Balducci, The adsorption of glucose at the surface of anatase: a computational study, Chem. Phys. Lett. 494 (2010) 54–59. [101] S. Shen, L. Guo, Hydrothermal synthesis, characterization, and photocatalytic performances of Cr incorporated, and Cr and Ti co-incorporated MCM-41 as visible light photocatalysts for water splitting, Catal. Today 129 (2007) 414–420. [102] X.J. Zheng, L.F. Wei, Z.H. Zhang, Q.J. Jiang, Y.J. Wei, B. Xie, M.B. Wei, Research on photocatalytic H2 production from acetic acid solution by Pt/TiO2 nanoparticles under UV irradiation, Int. J. Hydrog. Energy 34 (2009) 9033–9041. [103] L. Jia, J. Li, W. Fang, Effect of H2/CO2 mixture gas treatment temperature on the activity of LaNiO3 catalyst for hydrogen production from formaldehyde aqueous solution under visible light, J. Alloys Compd. 489 (2010) L13–L16. [104] A.  Patsoura, D.I.  Kondarides, X.E.  Verykios, Photocatalytic degradation of organic pollutants with simultaneous production of hydrogen, Catal. Today 124 (2007) 94–102. [105] A. Patsoura, D.I. Kondarides, X.E. Verykios, Enhancement of photoinduced hydrogen production from irradiated Pt/TiO2 suspensions with simultaneous degradation of azodyes, Appl. Catal. B 64 (2006) 171–179. [106] A. Caravaca, W. Jones, C. Hardacre, M. Bowker, H2 production by the photocatalytic reforming of cellulose and raw biomass using Ni, Pd, Pt and Au on titania, Proc. R. Soc. 472 (2016) 20160054. [107] Z.H.N. Al-Azri, W.T. Chen, A. Chan, V. Jovic, T. Ina, H. Idriss, G.I.N. Waterhouse, The roles of metal co-catalysts and reaction media in photocatalytic hydrogen production: performance evaluation of M/TiO2 photocatalysts (M=Pd, Pt, Au) in different alcohol– water mixtures, J. Catal. 329 (2015) 355–367. [108] T. Sakata, T. Kawai, K. Hashimoto, Heterogeneous photocatalytic reactions of organic acids and water. New reaction paths besides the photo-Kolbe reaction, J. Phys. Chem. 88 (1984) 2344–2350. [109] D. Jing, Y. Zhang, L. Guo, Study on the synthesis of Ni doped mesoporous TiO2 and its photocatalytic activity for hydrogen evolution in aqueous methanol solution, Chem. Phys. Lett. 415 (2005) 74–78. [110] G. Sadanandam, K. Lalitha, V.D. Kumari, M.V. Shankar, M. Subrahmanyam, Cobalt doped TiO2: a stable and efficient photocatalyst for continuous hydrogen production from glycerol: water mixtures under solar light irradiation, Int. J. Hydrog. Energy 38 (2013) 9655–9664. [111] L. Clarizia, G. Vitiello, G. Luciani, I. Di Somma, R. Andreozzi, R. Marotta, In-situ photodeposited nanoCu on TiO2 as a catalyst for hydrogen production under UV/visible radiation, Appl. Catal. A Gen. 518 (2016) 142–149.

212

Multifunctional Photocatalytic Materials for Energy

[112] R.  Lucchetti, L.  Onotri, L.  Clarizia, F.  Di Natale, I.  Di Somma, R.  Andreozzi, R. Marotta, Removal of nitrate and simultaneous hydrogen generation through photocatalytic reforming of glycerol over “in situ” prepared zero-valent nano copper/P25, Appl. Catal. B Environ. 202 (2017) 539–549. [113] L.  Clarizia, I.  Di Somma, R.  Marotta, P.  Minutolo, R.  Villamaina, R.  Andreozzi, Photocatalytic reforming of formic acid for hydrogen production in aqueous solutions containing cupric ions and TiO2 suspended nanoparticles under UV–simulated solar radiation, Appl. Catal. A Gen. 518 (2016) 181–188. [114] D.I.  Kondarides, V.M.  Daskalaki, A.  Patsoura, X.E.  Verykios, Hydrogen production by photo-induced reforming of biomass components and derivatives at ambient conditions, Catal. Lett. 122 (2008) 26–32. [115] K. Domen, J.N. Kondo, M. Hara, T. Takata, Photo- and mechano-catalytic overall water splitting reactions to form hydrogen and oxygen on heterogeneous catalysts, Bull. Chem. Soc. Jpn. 73 (2000) 1307–1331. [116] Y. Li, G. Chen, H. Zhang, Z. Li, J. Sun, Electronic structure and photocatalytic properties of ABi2Ta2O9 (A =Ca, Sr, Ba), J. Solid State Chem. 181 (2008) 2653–2659. [117] T.  Simon, N.  Bouchonville, M.J.  Berr, A.R.  Vaneski, A.  Adrović, D.  Volbers, R.  Wyrwich, M.  Döblinger, A.S.  Susha, A.L.  Rogach, F.  Jäckel, J.K.  Stolarczyk, J. Feldmann, Redox shuttle mechanism enhances photocatalytic H2 generation on Nidecorated CdS nanorods, Nat. Mater. 13 (2014) 1013–1018. [118] Q.  Li, B.  Guo, J.  Yu, J.  Ran, B.  Zhang, H.  Yan, J.R.  Gong, Highly efficient visible-light-driven photocatalytic hydrogen production of CdS-cluster-decorated ­ graphene nanosheets, J. Am. Chem. Soc. 133 (2011) 10878–10884. [119] L. Amirav, A.P. Alivisatos, Photocatalytic hydrogen production with tunable nanorod heterostructures, J. Phys. Chem. Lett. 1 (2010) 1051–1054. [120] K.M.  Parida, S.  Martha, D.P.  Das, N.  Biswal, Facile fabrication of hierarchical N-doped GaZn mixed oxides for water splitting reactions, J. Mater. Chem. 20 (2010) 7144–7149. [120a] M.R.  Gholipour, C.T.  Dinh, F.  Béland, T.O.  Do, Nanocomposite heterojunctions as ­sunlight-driven photocatalysts for hydrogen production from water splitting, Nanoscale 7 (2015) 8187–8208. [121] Z. Zhang, P.A. Maggard, Investigation of photocatalytically-active hydrated forms of amorphous titania,TiO2·n H2O, J. Photochem. Photobiol. A Chem. 186 (2007) 8–13. [122] O.  Ola, M.M.  Maroto-Valer, Review of material design and reactor engineering on TiO2 photocatalysis for CO2 reduction, J. Photochem. Photobiol. C: Photochem. Rev. 24 (2015) 16–42. [123] S.P. Meshram, P.V. Adhayak, U.P. Mulik, D.P. Amalnerkar, Facile synthesis of CuO nanomorphs and their morphology dependent sunlight driven photocatalytic properties, Chem. Eng. J. 204–206 (2012) 158–168. [124] U. Muller, Symmetry, in: Inorganic Structural Chemistry, John Wiley and Sons, Ltd, Chichester, 2007. [125] G. Liu, X. Zhang, Y. Xu, X. Niu, L. Zheng, X. Ding, The preparation of Zn2+-doped TiO2 nanoparticles by sol–gel and solid phase reaction methods respectively and their photocatalytic activities, Chemosphere 59 (2005) 1367–1371. [126] Q. Xiao, J. Zhang, C. Xiao, Z. Si, X. Tan, Solar photocatalytic degradation of methylene blue in carbon-doped TiO2 nanoparticles suspension, Sol. Energy 82 (2008) 706–713. [127] H.  Kato, A.  Kudo, Water splitting into H2 and O2 on alkali tantalate photocatalysts ATaO3 (A=Li, Na and K), J. Phys. Chem. B 105 (2001) 4285–4292.

Metal-based semiconductor nanomaterials for photocatalysis 213

[128] T. Takata, S. Shinohara, A. Tanaka, M. Hara, J.N. Kondo, K. Domen, A highly active photocatalyst for overall water splitting with a hydrated layered perovskite structure, J. Photochem. Photobiol. A Chem. 106 (1997) 45–49. [129] K. Maeda, T. Takata, M. Hara, N. Saito, Y. Inoue, H. Kobayashi, K. Domen, GaN:ZnO solid solution as a photocatalyst for visible-light-driven overall water splitting, J. Am. Chem. Soc. 127 (2005) 8286–8297. [130] K. Maeda, N. Saito, D. Lu, Y. Inoue, K. Domen, Photocatalytic properties of RuO2loaded β-Ge3N4 for overall water splitting, J. Phys. Chem. C 111 (2007) 4749–4755. [131] K. Maeda, K. Teramura, H. Masuda, T. Takata, N. Saito, Y. Inoue, K. Domen, Efficient overall water splitting under visible light irradiation on (Ga1-xZnx)(N1-xOx) dispersed with Rh−Cr mixed-oxide nanoparticles: Effect of reaction conditions on photocatalytic activity, J. Phys. Chem. B 110 (2006) 13107–13112. [132] T. Bak, J. Nowotny, M. Rekas, C.C. Sorrell, Photo-electrochemical hydrogen generation from water using solar energy. Materials-related aspects, Int. J. Hydrog. Energy 27 (2002) 991–1022. [133] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [134] D. Spasiano, R. Marotta, S. Malato, P. Fernandez-Ibanez, I. Di Somma, Solar photocatalysis: materials, reactors, some commercial, and pre-industrialized applications. A comprehensive approach, Appl. Catal. B Environ. 170 (2015) 90–123. [135] M.  Kosmulski, Isoelectric points and points of zero charge of metal (hydr)oxides: 50 years after Parks’ review, Adv. Colloid Interf. Sci. 238 (2016) 1–61. [136] Y.  Xu, M.A.A.  Schoonen, The absolute energy positions of conduction and valence bands of selected semiconducting minerals, Am. Mineral. 85 (2000) 543–556. [137] A.A. Nada, H.A. Hamed, M.H. Barakat, N.R. Mohamed, T.N. Veziroglu, Enhancement of photocatalytic hydrogen production rate using photosensitized TiO2/RuO2–MV2+, Int. J. Hydrog. Energy 33 (2008) 3264–3269. [138] A.A. Ismail, Mesoporous PdO–TiO2 nanocomposites with enhanced photocatalytic activity, Appl. Catal. B Environ. 117–118 (2012) 67–72. [139] K. Lalitha, J.K. Reddy, M.V.P. Sharma, V.D. Kumari, M. Subrahmanyam, Continuous hydrogen production activity over finely dispersed Ag2O/TiO2 catalysts from methanol:water mixture under solar irradiation: a structure–activity correlation, Int. J. Hydrog. Energy 35 (2010) 3991–4001. [140] O.  Rosseler, M.V.  Shankar, M.  Karkmaz-Le Du, L.  Schmidlin, N.  Keller, V.  Keller, Solar light photocatalytic hydrogen production from water over Pt and Au/TiO2 (anatase/rutile) photo-catalysts: Influence of noble metal and porogen promotion, J. Catal. 269 (2010) 179–190. [141] T. Hisatomi, K. Miyazaki, K. Takanabe, K. Maeda, J. Kubota, Y. Sakata, K. Domen, Isotopic and kinetic assessment of photocatalytic water splitting on Zn-added Ga2O3 photocatalyst loaded with Rh2-yCryO3 cocatalyst, Chem. Phys. Lett. 486 (2010) 144–146. [142] T. Hisatomi, K. Maeda, K. Takanabe, J. Kubota, K. Domen, Aspects of the water splitting mechanism on (Ga1−xZnx)(N1−xOx) photocatalyst modified with Rh2−yCryO3 cocatalyst, J. Phys. Chem. C 113 (2009) 21458–21466. [143] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38.

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Photocatalysts for hydrogen generation and organic contaminants degradation

10

Rupali Nagar*, Eswaraiah Varrla†, Bhaghavathi P. Vinayan‡,§ ⁎ Symbiosis International University, Pune, India, †SRM University, Chennai, India, ‡ Helmholtz Institute Ulm (HIU), Ulm, Germany, §Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany

10.1 Introduction The combined presence of hydrogen and oxygen in the form of water sustains life on Earth. Nature maintains a balance between various ecological systems; however, these systems are being disturbed by human activities. Rockstrӧm and colleagues discuss nine Earth-system processes and define a safe operating space for the nine planetary systems [1]. Table 10.1 shows the planetary boundaries’ proposed limits and draws a comparison between the preindustrial values and the values up to the past decade. Three of the nine boundaries (i.e., climate change, the biodiversity loss rate, and the nitrogen cycle) are believed to have been crossed already and thus require our attention in order for the course to be corrected [1]. An important step toward meeting the world’s energy demands will be to replace fossil-based fuels with green and renewable energy sources. The other step in this direction is to restore the freshwater ecosystem. The Earth’s surface is about 75% water. About 96.5% of the water on the Earth’s surface exists in the form of oceans, seas, or bays, followed by 1.74% in the form of ice caps, glaciers, and permanent snow and 1.7% as ground water; the remaining 0.06% includes moisture, biological forms, and so on [2,3]. Fig. 10.1 depicts the distribution of water on Earth, which suggests that a very limited amount is usable for household, agricultural, and industrial activities. Thus it is very important to conserve freshwater resources and curb their pollution levels. One of the sources of water contamination is the wastewater from industries that contain dyes. Dyes carry carcinogens that pose a threat to aquatic life and human life and that disturb the ecological balance. Decontaminating water and making it free of organic impurities could prove to be another effective step in this regard [4]. These two subjects are the main focus of this chapter, i.e., photocatalytic hydrogen production and organic contaminant degradation, both of which involve water. The former is discussed in Section 10.2 and the latter in Section 10.3. For achieving these targets, we need to understand the chemical reactions, to engineer materials to carry out the desired reactions, and to integrate them into processes such as water splitting, water decontamination systems, and so on.

Multifunctional Photocatalytic Materials for Energy. https://doi.org/10.1016/B978-0-08-101977-1.00011-9 Copyright © 2018 Elsevier Ltd. All rights reserved.

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Table 10.1 

Planetary boundaries Current status

Preindustrial value

(i) Atmospheric carbon dioxide concentration (parts per million by volume) (ii) Change in radiative forcing (watts per meter squared) Extinction rate (number of species per million species per year) Amount of N2 removed from the atmosphere for human use (millions of tonnes per year)

350

387

280

1 10 35

1.5 >100 121

0 0.1–1 0

Quantity of P flowing into the oceans (millions of tonnes per year)

11

8.5–9.5

−1

Concentration of ozone (Dobson unit) Global mean saturation state of aragonite in surface sea water Consumption of freshwater by humans (km3 per year) Percentage of global land cover converted to cropland

276 2.75 4000 15

283 2.90 2600 11.7

290 3.44 415 Low

Parameters

Climate change

Rate of biodiversity loss Nitrogen cycle (part of a boundary with the phosphorus cycle) Phosphorus cycle (part of a boundary with the nitrogen cycle) Stratospheric ozone depletion Ocean acidification Global freshwater use Change in land use Atmospheric aerosol loading Chemical pollution

Overall particulate concentration in the atmosphere, on a regional basis For example, amount emitted to, or concentration of persistent organic pollutants, plastics, endocrine disrupters, heavy metals, and nuclear waste in the global environment, or the effects on ecosystems and functioning of Earth’s system

To be determined To be determined

Reprinted by permission from Macmillan Publishers Ltd: J. Rockstrom, W. Steffen, K. Noone, A. Persson, F.S. Chapin, E.F. Lambin EF, et al.: A safe operating space for humanity. Nature 461:472–475, 2009. Copyright (2009). Available at http://www.nature.com/nature/journal/v461/n7263/full/461472a.html.

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Proposed boundary

Earth-system process

Photocatalysts for hydrogen generation and organic contaminants degradation217 Distribution of Earth’s water Freshwater 3%

Other 0.9% Ground water 30.1%

Rivers 2%

Surface water 0.3%

Swamps 11%

Icecaps and glaciers 68.7%

Saline (oceans) 97%

Earth’s water

Lakes 87%

Fresh surface water (liquid)

Fresh water

Fig. 10.1  Distribution of world’s water, showing that about 98% of the water exists as saltwater, which is unfit for residential use in its naturally occurring form [2,3].

10.1.1 Semiconducting nanocrystals Semiconducting nanocrystals, or quantum dots, are special types of materials because of their unique optoelectronic properties. These materials usually range in size from 1 to 10 nm and have good photostability. It is possible to tune the band gaps of such materials by tailoring their size; the smaller the nanoparticles, the higher the frequency of the absorbed wavelength, and vice versa. The characteristic forbidden energy gaps in the energy band diagram and their tunability provide the most powerful toolbox for tailoring their electronic properties. The band gap in semiconductors arises because of interactions between neighboring atoms of the elemental solid, which causes splitting of individual atoms’ energy levels into closely spaced energy levels called bands. Fig. 10.2 shows this splitting process as more and more atoms are brought together to form a solid. In the case of molecules, the wave-functions of neighboring atoms

Energy

N=

1

2

Eg

10

Eg

1000

Eg

> 2000

Eg

Splitting of energy levels into continuous bands as N increases

Fig. 10.2  Splitting of energy levels takes place as number of atoms N increases. The energy band gap Eg decreases as N increases.

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+ –

Bonding and anti-bonding orbitals formed after overlapping of two s-orbitals

Fig. 10.3  The s-orbitals overlap to form bonding (top) and antibonding (bottom) orbitals.

overlap giving rise to bonding and antibonding orbitals. Fig.  10.3 shows the overlap between wave-functions resulting in the formation of bonding and antibonding orbitals. Because antibonding orbitals have higher energy than bonding orbitals, the electrons preferentially occupy the latter and fill up the energy levels according to the Pauli exclusion principle. Under normal conditions, electrons fill the valence band and on being excited can sufficiently reach the conduction band via external bias, heat, or photon absorption. These “free” electrons can easily flow, and the material starts conducting electrically. To understand the flow of charges, it is important to consider the categorization of semiconductors, that is, inorganic and organic. The inorganic class of semiconductors comprises elemental semiconductors such as silicon, germanium, I-VII, II-VI, and III-V compound semiconductors. The band structures of such semiconductors are calculated by solving the Schrödinger equation for a periodic potential. These solutions are functions of energy-momentum relations, and the splitting of energy levels gives rise to valence, conduction, and forbidden energy gaps. Calculating energy band relations for inorganic semiconductors is relatively easier because there is a long-range order within the crystal and the periodic potential that can be closely approximated by Bloch functions. The electronic structure for the most common types of semiconductors, such as diamond and zinc-blende, consists of bonding and antibonding states of s-type and p-type atomic orbitals. In such semiconductors, the top of the valence band occurs at the zone center and consists of the bonding p levels. If the top of the valence band and bottom of the conduction band have the same value of wavevector, the semiconductor is called a direct band gap semiconductor. In the case of indirect semiconductors, the bottom of the conduction band and the top of the valence band have different values of wavevector. The electronic transitions of charge carriers within the direct band gap semiconducting materials and the ability to control their flow find applications in a relatively new field of photocatalysis. Such semiconductors are responsive to a specific range of the solar spectrum that may trigger charge generation, recombination, or even charge transfer.

10.1.2 Conjugated polymers Organic semiconductors are made up of polymers or π-bonded molecules and can conduct when charge carriers are injected into them. It is the conjugated system of π bonds, the backbone of the polymeric chain; that mediates in the charge transfer

Photocatalysts for hydrogen generation and organic contaminants degradation219

through the polymer chain. The charge flow mechanism in conjugated polymers is attributed to intra-chain charge diffusion and inter-chain charge hopping, the latter being the reason behind their low conductivities [5,6]. As there is no long-range order present, organic semiconductors lack the high translational periodicity and symmetry of an inorganic crystal lattice, and wave-functions cannot be approximated by Bloch functions. The transport of charge carriers through the semi-crystalline polymeric chains can be influenced by chemical defect/dopant or structural discontinuity. The typical energy gaps lie between 1.5 and 3.5 eV. The thermal energy at room temperature is about 25 meV; therefore such polymers possess no free carriers at room temperature and favor flow of charges only when injected by some means.

10.1.3 Role of photocatalytic materials The semiconducting materials enjoy a very special place in material engineering because of their tunable band gaps, which are also size-dependent. If charges (electrons and holes) can be separated by some means within the material, then their chemical energy can be used to carry out redox reactions. Photocatalytic materials work on the principle of charge separation by absorbing solar radiation in consonance with their characteristic band gaps. Semiconductors employed as photocatalytic materials primarily have two responsibilities: carrying out charge separation and charge transfer. Fig.  10.4A shows the solar-to-hydrogen (STH) conversion efficiency [9]. The ­solar-to-hydrogen conversion efficiency is defined as the ratio of usable chemical energy from the produced hydrogen gas to total solar energy delivered to the system [10,11]. Therefore, for higher STH efficiency, materials with lower band gaps are preferred. Fig. 10.4B depicts some compound semiconducting metal oxides employed as photocatalysts. The charge separation caused by photon absorption depends on the optical band gap, absorption efficiency of the photocatalytic material, chemical purity, crystallinity, stability, surface area, and activation range of solar spectrum [12]. In order to effect a redox reaction, the real challenge lies in creating a charge separation and suppressing the recombination until the charge transfer process takes place. The phenomena of charge generation, recombination, and transfer are probabilistic and competitive events in nature. The timescales at which these processes occur are tabulated in Table 10.2. Recombination is not a desirable process for photocatalysis. If the electrons in the conduction band can migrate to the surface, then they can be suitably transferred to an electron-accepting species thereby carrying out a reduction process. Similarly, if a hole from the valence band can be transferred to an electron-donating species, the hole can be transferred, and the species can in turn be oxidized. It is quite clear that keeping the charges separated (~ms) for about 1012 times longer than their generation rate (~fs) is challenging. The band gap and lifetimes of charge carriers depend on the morphology and dimensions of the semiconductors, thereby providing a control on their generation/recombination. If the size of the semiconducting nanoparticle is comparable to the diffusion lengths of the charge carriers, the carriers can migrate to the surface. Sorption of appropriate ­electron-donating or electron-accepting species can further promote the charge transfer away from the semiconductor and contribute toward a redox reaction. The size of

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Fig. 10.4  (A) The solar-to-hydrogen conversion efficiency for UV to IR region of the electromagnetic spectrum establishing that the conversion efficiency is higher for low band gap (~2 eV) materials. (B) Relationship between band structure of semiconductor and redox potentials of water splitting [7,8]. (A) Reproduced from J. Li, N. Wu, Semiconductor-based phototcatalysts and petrochemical cells for solar fuel generation: a review, Catal. Sci. Technol. 5 (2015) 1360–1384, with permission of the Royal Society of Chemistry. http://pubs.rsc.org/ en/content/articlelanding/2015/cy/c4cy00974f#!divAbstract; (B) A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253–278, with permission of the Royal Society of Chemistry. http://pubs.rsc.org/en/content/ articlelanding/2009/cs/b800489g#!divAbstract.

the semiconducting particles has another favorable effect in that the band gap also can be tuned by employing particles of different sizes. In nano-dimensional semiconductors, the exciton (or bound electron-hole pair) lifetime can be modified by designing heterostructures with different energy band gap offsets, which is discussed in the next section. Fig. 10.5 depicts the various pathways by which the charges can recombine or their transfer can take place.

Photocatalysts for hydrogen generation and organic contaminants degradation221

Table 10.2 

Typical timescale of various processes

Process

Timescale

Charge generation Charge trapping Charge recombination Charge transfer

Few femtoseconds 100 ps to 10 ns 10–100 ns 100 ns to few milliseconds

Adapted from M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis. Chem. Rev. 95 (1995) 69–96.

10–15

10–12

10–9

10–3

Seconds

Timescale

Recombination

Trapping

Generation

VB

Charge transfer and separation

CB

Fig. 10.5  Four different processes involving electron and hole charge carrier: generation (~femtoseconds), trapping (pico to nanoseconds), recombination (nanoseconds), and transfer (milliseconds). Whereas trapping can occur at surface defects or at a compositionally different site, recombination can occur at surface or in bulk. The charge transfer is made effective by using co-catalysts [13].

10.1.4 Fundamental approach to hydrogen generation and organic contaminants’ degradation using semiconductors To generate hydrogen from water, the stable water molecule needs to be split into hydrogen and oxygen. Thermodynamic studies reveal that about 237 kJ of energy per mole of water or 1.23 eV per electron is required to carry out the reduction of a water molecule. The intermediate photochemical reactions governing the photocatalytic process is shown in Eqs. (10.1)–(10.4). hn+ Photocatalyst ® e - + h +

(10.1)

h + + H 2 O ® H + + OH •

(10.2)

h + + OH - ® OH •

(10.3)

e - + O2 ® O2 -

(10.4)

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0.00 V

e–

Increasing energy of electrons

Type-I CB

Type-I –

1/2

Type-II –



E +

1.23 V Potential v/s NHE

(A)

h+

Increasing energy of holes

+

+

VB

(B)

Fig. 10.6  (A) The redox potentials for water splitting occur at 0 and 1.23 V with respect to NHE. A semiconductor with straddling gap greater than 1.23 eV can be employed to carry out redox reactions. (B) Core-shell nanoparticles can manifest limiting charge carrier localization regimes as shown [14]. Reproduced from reference CdM. Donegá, Synthesis and properties of colloidal heteronanocrystals. Chem. Soc. Rev. 40 (2011) 1512–1546, with permission of the Royal Society of Chemistry. http://pubs.rsc.org/en/content/articlelanding/2011/cs/ c0cs00055h#!divAbstract

Thus the holes in the valance band can carry out oxidation whereas electrons in the conduction band can carry out reduction. The ideal straddling condition required by a semiconducting material to carry out spontaneous water splitting is shown in Fig. 10.6A. If voltages are measured with respect to the normal hydrogen electrode (NHE), then the conduction band must lie higher (more negative) than the H+/H2 (0 V versus NHE) reduction potential, and the valence band must lie lower (more positive) than the H2O/O2 (1.23 V versus NHE) oxidation potential. Fig. 10.4B shows semiconductors with band gaps and their corresponding redox potentials for water splitting. Popular semiconductors such as Fe2O3 or TiO2 have band gaps ranging from 2.2 to 3.3 eV, which lie in the near-UV or UV region. In order to meet the conditions for redox reactions involved in water splitting, the minimum band gap of the photocatalyst must be 1.23 eV, which lies in the visible region of the electromagnetic spectrum. Therefore, for semiconductors with band gaps lying in the near UV or UV range, it is required that the band gap be suitably engineered. Besides having a suitable band gap, the charge carriers in the semiconductor must have lifetime of about a few milliseconds to participate in the redox reaction. For this, a sufficient number of electrons from the bottom of the conduction band must be transferred to the surface of the semiconducting nanoparticle for hydrogen evolution, and therefore the lifetime of electrons must be greater than the recombination time. Similarly, in order to oxidize the organic contaminants, holes from the valance band of the semiconductor nanoparticle must exist and be transferred to its surface. Different approaches have been followed for charge separation, such as coupling semiconductors with staggered band gaps, straddling type, discontinuous type nanocomposites of metal/semiconductors where metals can serve as sink for electrons/holes, or by adding a hole (or electron) scavenger species. Semiconducting heterostructures with semiconductors with staggered band gaps, straddling type and discontinuous type, are depicted in Fig. 10.6B.

Photocatalysts for hydrogen generation and organic contaminants degradation223

A scavenger species preferentially removes holes (or electrons) and thereby traps electrons (or holes) for a longer duration. The first generation of photocatalytic materials is considered to be that of metal oxides. TiO2 was one of the most studied metal oxides for odor containment, antibacterial and antimicrobial activities, photosplitting of water, generation of H2 gas in the process, and so on [13]. Along with TiO2, other metal oxides used for photocatalytic properties are depicted in Fig. 10.4B. The ­second-generation photocatalytic materials is that of anion-doped (C, S, N) metal oxides [15]. These doped metal oxides display better photocatalytic activities, which is attributed to better overlap of the states between 2p orbitals of oxygen and the states caused by doping. The third-generation photocatalysts comprise nanocomposites of semiconducting materials and two low band gap photo-responsive materials [16]. Fig. 10.7 shows the scheme through which charge generation and transfer takes place in a third-generation photocatalyst [16]. The vaarious types of nanocomposites that have been used vary in their composition or structure. For instance, nanorods, nanofibers, nanoparticles, nanocubes, nanohexagons, nano-octahedrons, and core-shell nanostructures ([email protected] and metal [email protected], [email protected] and [email protected], [email protected] and metal [email protected], etc.), and techniques to immobilize nanostructures using templates or polymers have been employed. The immobilization of photocatalyst nanostructures helps in their recovery, avoids their agglomeration, and in turn maintains a high surface area of ­nano-photocatalysts [17]. The first step in a three-component nanostructure involves a valence to conduction band electronic transition in solid A to generate free electrons and holes, and because the valence band position of solid A is lower than the valence band of a solid C, hole transfer then occurs from A to C, resulting in charge separation in the A-C junction. Photo-excitation of solid B also generates electrons and holes in solid B, and as the conduction band position of solid B is higher in energy than that of solid C, electron transfer will occur from B to C, thereby causing charge separation at the B-C junction. Positioning of the conduction band of solid A higher in energy than the valence band of solid B causes an electron transfer from A to B and a recombination of the electron Scheme 1. Steps in a three-component nanostructureα e CB e CB

e

B CB

e

e VB

C h

A VB

h

Fig. 10.7  Third-generation photocatalysts [16].

h

VB

h

224

Multifunctional Photocatalytic Materials for Energy

in A with the hole generated in B, thus completing the photo-excitation cycle in which the excited state of solid C has been achieved via a lower-energy, two-photon process. As such, photocatalytic activity of the wide band gap material will be similar to the higher-energy, one-photon process. Photo-excitation of components A and B will be very efficient because the two solids are activated through their fundamental absorption band [16].

10.2 Hydrogen economy and photocatalytic splitting of water Hydrogen is one of the most important clean, renewable, and sustainable energy sources and currently is produced by steam reforming. Moreover, photocatalytic water splitting into oxygen and hydrogen is the most effective way of H2 production, an important step in realizing a H2 economy. As discussed in Section 10.1.3, a semiconductor that absorbs in the visible range is desired, but most semiconductors have band gaps ranging up to the near-UV absorption range. Because of its nontoxicity, abundance, and low cost, TiO2 has long been a promising material for water splitting, as first reported by Fujishima and Honda in 1972 under UV light [18,19]. However, TiO2 has performed poorly because of its fast carrier recombination and absorption of solar radiation in the ultraviolet region because of a large band gap. Efforts to overcome such issues by doping/forming composites are now employed. In order to warrant a good electron-hole transfer, the bottom of the conduction band and the top of the valence band of the semiconductor must have higher energy than those of TiO2. Single-component semiconductors have shown poor photocatalytic activity because of their fast recombination kinetics. Co-catalysts have been used in addition to semiconductors to enhance the conversion efficiency by improving the charge transfer through forming heterojunction interfaces [20]. Fig. 10.8 shows co-catalysts attached to a semiconducting nanoparticle utilized for hydrogen and oxygen evolution [21]. Polymers with extended π-conjugated electron systems have attracted considerable attention recently because of their absorption coefficients in the visible region and high conductivity, allowing high mobility in charge carriers. Among conductive polymers, polyaniline (PANI) has been widely used to improve electronic conductivity as well as solar energy transfer and photocatalytic activity of TiO2, because of PANI’s easiness of preparation, comparatively low cost, and excellent environmental stability. There are some examples of PANI combined with other semiconductors for H2 generation, such as the case of PANI-CdS composite nanoparticles synthesized by He et al. [22] for direct H2 evolution in the presence of an SO32 - / S2 - sacrificial reactant. A twostep reaction came up where, first, a nitrogen atom of PANI formed an initial complex with Cd(CH3COO)2 through an interaction of the π-conjugated chain. Second, the PANI-Cd(CH3COO)2 intermediate reacted with thioacetamide to form CdS. These hybrid photocatalysts were used to generate hydrogen with a 5 h experiment of the as-prepared photocatalysts, as shown in Fig. 10.9A. As the diagram shows, the activity of hydrogen production of the PANI-CdS composite catalyst gradually decreased as the PANI content increased. PANI-CdS-1 showed the highest activity for hydrogen

Photocatalysts for hydrogen generation and organic contaminants degradation225 Potential (V vs.NHE) pH = 0 H2-evolution cocatalyst (ii) +

(H /H2)

0

(iii)

H2



e

H+

e–

CB (i)

(O2/H2O)

+1.23

H 2O h+

VB h+

O2

(iii)

(ii)

O2-evolution cocatalyst

Semiconductor photocatalyst

Fig. 10.8  Schematic illustration of photocatalytic water splitting over a semiconductor photocatalyst loaded with H2- and O2-evolution co-catalysts [21]. Reproduced from J. Ran, J. Zhang, J. Yu, M. Jaroniec, S.Z. Qiao, Earth-abundant cocatalysts for semiconductor-based photocatalytic water splitting. Chem. Soc. Rev. 43 (2014) 7787–7812, with permission of the Royal Society of Chemistry. Flush with nitrogen 300

PANI-CdS-1 PANI-CdS-2

250

Amount of hydrogen (µmol)

Amount of hydrogen (µmol)

300

PANI-CdS-3 PANI-CdS-4

200

PANI-CdS-5 CdS

150 100 50 0

(A)

250 200 150 100 50 0

0

1

2

3 Time (h)

4

0

5

(B)

5

10

15

20

Time (h)

Fig. 10.9  (A) Activity for hydrogen evolution by different PANI-CdS catalysts (molar ratio of PANI and CdS from 1 to 5: 0.5, 0.7, 1, 1.5, 2) and CdS under visible part of Xe lamp (400 W) irradiation in a Pyrex glass with 180 mL 0.25 M Na2SO3/0.35 M Na2S solution. (B) Hydrogen evolution over PANI-CdS-1 under visible part of Xe lamp (400 W) irradiation in a Pyrex glass with 180 mL 0.25 M Na2SO3/0.35 M Na2S solution for 20 h [22]. Reprinted from K. He, M. Li, L. Guo, Preparation and photocatalytic activity of PANI-CdS composites for hydrogen evolution. Int. J. Hydrog. Energy 37 (2012) 755–759. Copyright (2011), with permission from Elsevier.

production. Fig. 10.9B shows a 20 h H2 evolution under visible light irradiation from an aqueous solution containing the sacrificial reagents SO32 - / S2 - over PANI-CdS-1 without of a Pt co-catalyst. It can be seen that the photocatalyst was stable enough during the reaction. Very recently, conjugated microporous poly(benzothiadiazole, BBT)/TiO2 heterojunction-based materials were reported for visible-light-driven H2

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production [23]. According to this result, this heterojunction enhanced the separation efficiency for electrons and holes generated. These results showed that 6.7 wt.% BBT/ TiO2/Pt exhibited a robust H2 production ability in a 7 h test, and up to ∼1246 μmol H2 was detected, which corresponds to a H2 production rate of ∼5933 μmol/h/g, ∼18.0 times higher than that of pure BBT. Their results indicate that the composite of BBT and Pt/TiO2 as well as the tight interaction between BBT and Pt/TiO2 are important in enhancing the photogenerated carriers’ separation and then for photocatalytic H2 production. Another approach in enhancing the lifetime of charge carriers and suppressing the electron-hole recombination is to use a p-type semiconductor along with TiO2, i.e., an n-type semiconductor. For example, the p-n junction of NiO/TiO2 exhibited higher photocatalytic reduction activity than pristine TiO2 but lower oxidation activity [24]. Porphyrins, which are a group of heterocyclic organic compounds, are good absorbers of visible light [25]. The excited states formed with the help of porphyrin aid in charge transfer. Porphyrin-based dyes have several intrinsic advantages, such as their rigid molecular structures with large absorption coefficients in the visible region and many reaction sites, i.e., four meso and eight β positions that are readily available for functionalization, which enables fine-tuning of the optical and physical properties of porphyrins. Porphyrins exhibit excellent photophysical, photochemical, ­ electrochemical, and structural properties [26]. Many metalated porphyrins have also been studied. Photocatalytic properties of porphyrins can be controlled by inserting metal cations into the porphyrin ring. Al- and Zn-metalated mesotetrakis(4-­carboxylphenyl)porphyrin H2TCPP metal organic frameworks (MOFs) were synthesized by Fateeva et al. for hydrogen generation [27]. The BET surface area of the aluminum-porphyrin MOF was observed to be 1400 m2 g−1, which presented a large surface area for reaction sites. The metalated porphyrins were found to generate H2 that could be enhanced by using platinum [27]. Note that the addition of Pt aids in easier hydrogen dissociation and has been investigated in detail [28–30]. The role of porphyrin-modified TiO2 in the degradation of 4-nitrophenol using visible ultraviolet light was investigated by Duan et  al., who found that the photoactivities of ­porphyrin-based TiO2 structures were greater than that of pristine TiO2 [31]. Dichloro and dihydroxo tin porphyrins were investigated, and it was established that a meso-site peripheral substituent influenced the photodegradation activity. MOFs have also been employed for photocatalysis owing to the former’s porous coordination networks. MOFs are a class of advanced materials comprising metal ions/clusters linked with organic molecules that provide additional support for photocatalytic materials because of MOFs’ high surface area, structural versatility, and chemical stability. These MOF materials are constructed by joining metal or metal oxides (secondary building units, SBU) with organic linkers using strong bonds to create an open crystalline framework with permanent porosity—for example, MOF-5 with inorganic [Zn4O]6+ groups connected to an octahedral array of [O2C– C6H4–CO2]2 (1,4-benzenedicarboxylate) groups. Many different kinds of MOFs can be prepared by changing the linker and the inorganic part, and MOFs exist in both semiconducting and conducting states [32]. Direct use of metal organic frameworks for photocatalytic hydrogen production was first reported by Silva et al. They found

Photocatalysts for hydrogen generation and organic contaminants degradation227

that Zr-containing metal organic frameworks formed by terephthalate (UiO-66) and 2-­aminoterephthalate ligands [UiO-66(NH2)] were two notably water-resistant MOFs exhibiting photocatalytic activity for hydrogen generation in methanol or water/methanol upon irradiation at a wavelength longer than 300 nm [33]. Maximum amounts of 2.4 and 2.8 mL H2 have been obtained after 3 h of irradiation over UiO-66 and UiO-66(NH2). According to another report, the organic linker absorbed visible light and electrons transferred from an excited state to the conduction band of a photoactive oxo cluster. Pt/Ti-MOF-NH2 exhibited efficient photocatalytic activity for the HER under visible light irradiation, and a H2 evolution rate of ≈367 μmol h−1 g−1 could be achieved [34]. It was also confirmed that the catalyst could be reused at least three times without significant loss of its catalytic activity. The incorporation of CdS and UiO-66 enhanced the photocatalytic activity of both parts of the reaction, and the optimum HER activity was obtained at a UiO-66-to-CdS weight ratio of 1:1. A high H2 evolution rate of 25,770 μmol h−1 g−1 was obtained in the presence of 1 wt.% MoS2, approximately two-fold higher than that of 1 wt.% Pt/UiO-66-CdS. The H2 evolution rate of MoS2/UiO66-CdS reached a maximum when the amount of MoS2 in the composite was 1.5 wt.% (32,500 μmol h−1 g−1). The advantage of using MoS2 as a highly active co-catalyst to replace Pt in MOF-based photocatalysts to enhance the HER activity was demonstrated by Shen et al. [35]

10.3 Photocatalytic degradation of organic contaminants Oxidation of organic impurities in water by photocatalysis can be achieved by using the right type of band gap semiconductors [36,37]. Organic contaminants in water and in solid chemical waste have become a serious problem and have grown exponentially since the beginning of the industrial revolution. Manufacturing sectors across the globe continue to push the level of these pollutants, which is causing several carciogenic diseases and threatening sustainable living. This section deals with the photocatalytic degradation of organic pollutants, a topic that is gaining popularity because solar radiation is used as the source of energy and atmospheric dioxygen as the oxidant; both of which are readily and abundantly available [38]. Conjugated polymer/semiconductor nanocrystal nanocomposites are of special interest in this chapter because of their synergistic effect in enhancing optoelectronic properties and their fast reaction kinetics in degrading pollutants. Several conjugated polymers such as PANI, polypyrrole (PPy), P3HT, PEDOT, and Polythiophene, along with some of the semiconductor nanocrystals such as TiO2, SnO2, ZnO, WO3, and Bi2MoO6, have been used in the preparation of conjugated nanocrystal nanocomposites [17]. The latest developments in photocatalysts based on conjugated polymer nanocrystal nanocomposites and their working mechanism with band alignments behind organic pollutants degradation are discussed here, as well as aspects of material engineering to achieve photocatalysts for effective degradation of organic pollutants. PANI has been the most studied conjugated polymer among conducting polymers because of its excellent chemical stability, high conductivity, corrosion protection, low cost of synthesis, and interesting redox properties. PANI plays a significant role

228

Multifunctional Photocatalytic Materials for Energy 1.0

0.8

0.8

0.6

0.6

0.4

blank P/T-100°C TiO2xNx

0.2

P/T-150°C P/T-200°C

0

1

1st run 2nd run

3rd run 4th run

6

18

5th run

0.4 0.2

TiO2-200°C

0.0

(A)

C/C0

C/C0

1.0

0.0 2

3

4

Illumination time (h)

5

6

0

(B)

12

24

30

Illumination time (h)

Fig. 10.10  (A) Liquid-phase photocatalytic degradation of MO over the PANI/TiO2 nanocomposite marked as P/T, TiO2, and TiO2−xNx catalysts. (B) Photodegradation stability of MO over the P/T-200 C nanocomposites under visible light (420 nm 99 >99 >99

Photochemical reaction conditions: CO2 bubbling for 20 min before irradiation using a high-pressure Hg lamp with a cutoff filter (λ 90%) in a DMA solution, giving a TON of 27 [23].

12.3 Photoelectrochemical CO2 reduction using molecular-based photocathode coupled with a semiconductor photoanode At present, photocatalytic CO2 reduction coupled with water oxidation in a powderbased suspension system remains a challenge. An alternative way to achieve CO2 reduction using water as the electron source is to construct a photoelectrochemical

298

Multifunctional Photocatalytic Materials for Energy

Fig. 12.5  A schematic illustration of a hybrid photoelectrochemical cell with Z-scheme configuration. From G. Sahara, H. Kumagai, K. Maeda, N. Kaeffer, V. Artero, M. Higashi, R. Abe, O. Ishitani, J. Am. Chem. Soc. 138 (2016) 14152 (http://pubs.acs.org/doi/full/10.1021/jacs.6b09212).

cell, where the oxidation and reduction sites are physically separated and an external bias can be applied to complete the reaction. A schematic illustration of this hybrid Z-scheme system consisting of a CoOx/TaON photoanode and a RuRe/NiO cathode is shown in Fig.  12.5 [24]. This tandem cell configuration would help to suppress backward reactions (e.g., oxidation of CO2 reduction products and reduction of O2). It is also possible to develop CO2 reduction and water oxidation systems individually in order to optimize the whole system. Inoue et  al. devised a prototype of such a ­molecular-based cathode material using a dyad with a Zn(II) porphyrin as a photosensitizer and a Re(I) complex as a catalyst for CO2 reduction, which was immobilized on a p-type NiO electrode [25]. This system demonstrated a reduction of CO2 to CO under 436 nm visible light; however, the TONCO and Faradaic efficiency were both low (TONCO = 10, Faradaic efficiency =6.2%). Supramolecular photocatalysts based on RuRu′ and RuRe appear to be suitable as molecular photocathodes for CO2 reduction because of their excellent CO2 reduction performance, as previously described. Ishitani et  al. examined a photocathode consisting of a RuRe supramolecular photocatalyst and a p-type NiO semiconductor as a potential photocathode for CO2 reduction [26]. The electrode was easily prepared by immersing it overnight in a porous NiO electrode in an MeCN solution containing a metal complex. The RuRe/NiO electrode was subject to irradiation (>460 nm) using a 300 W xenon lamp in a DMF/TEOA (5:1, v/v) mixed solution containing Et4NBF4 (0.1 M) as a supporting electrolyte under a CO2 atmosphere. Table 12.3 summarizes the results of the photoelectrochemical CO2 reduction. The as-prepared RuRe/NiO electrode was found to produce CO under an external bias of −1.2 V versus Ag/AgNO3, with TONCO of 32 and high Faradaic efficiency (~98%)

Photoelectrochemical CO2 reduction using a RuRe/NiO cathodea Products

Entry

Electrode

Metal complex/nmol

Potential/V vs. Ag/AgNO3

hv

CO2

CO/nmol (TONCO)

H2/nmol

1

RuRe/NiO

7.9

−1.2





255 (32)

n.d.

2 3 4 5 6 7 8

RuRe/NiO RuRe/NiO RuRe/NiO RuRe/NiO NiO Ru(PS)/NiO Re/NiO

7.1 5.9 4.8 6.5 – 16.3 3.8

−1.0 −0.8 −1.2 −1.2 −1.0 −1.0 −1.0

○ ○ ×b ○ ○ ○ ○

○ ○ ○ ×c ○ ○ ○

115 (16) n.d. n.d. n.d. n.d.d n.d.d n.d.d

n.d. n.d. n.d. n.d. n.d.d n.d.d n.d.d

Irradiated at λ > 460 nm using a 300 W Xe lamp for 5 h. In the dark. c Under an Ar atmosphere. d 22 h irradiation. Reproduced with permission from G. Sahara, R. Abe, M. Higashi, T. Morikawa, K. Maeda, Y. Ueda, O. Ishitani, Chem. Commun. 51 (2015) 10722. a b

Faradaic efficiency/% 62 (0−3 h) 98 (3−5 h) 41 0 0 0 0 0 0

Hybrid Z-scheme nanocomposites for photocatalysis299

Table 12.3 

300

Multifunctional Photocatalytic Materials for Energy

(entry 1). Neither H2 nor HCOOH formed. Tracer experiments using 13C-enriched CO2 revealed that the produced CO originated solely from CO2. With increasing the applied potential upon anodic polarization, the performance was declined (entries 2, 3). This strongly suggests that the electrons used for CO2 reduction came from the NiO electrode. Control experiments showed that no CO formation was observed in the absence of either light, CO2, or RuRe (entries 4–6). Modification of a NiO electrode with a model complex of RuRe (i.e., Ru(PS) or [Re(P2-dmb)(CO)3Br], abbreviated as Re) also did not yield CO production (entries 7, 8). Therefore both the NiO electrode and RuRe are required for photoelectrochemical CO2 reduction. Thus it was shown that the visible light irradiation of the immobilized RuRe resulted in highly selective CO2 reduction in a nonaqueous condition using electrons supplied from an external electric circuit through the NiO electrode. The RuRe/NiO photocathode was then coupled with a semiconductor CoOx/TaON photoanode, which was reported to be active for water oxidation to O2 under visible light [27]. First, photoelectrochemical measurements using the RuRe/NiO photocathode were conducted to examine its ability to reduce CO2 in aqueous solution, and the results are summarized in Table 12.4. Although the selectivity to CO formation was not as high as that in a nonaqueous condition because of the competitive H2 evolution, the RuRe/NiO photocathode worked in a CO2-purged aqueous solution containing 50 mM NaHCO3 (pH = 6.6) under visible light (λ > 460 nm), with a clear cathodic ­photo-response starting at approximately −0.1 V versus Ag/AgCl. Again, it was found that CO formation required RuRe, visible light, and CO2. CoOx/TaON has been reported to work as a highly efficient, stable water-oxidation photoanode under visible light [27]. Although an electrode consisting of only TaON was not very stable under the working conditions because of the self-oxidative decomposition by photogenerated holes, modification of the CoOx nanoparticles, which served as a water-oxidation promoter, enabled the stable performance. The CoOx/ TaON photoanode functioned even in a NaHCO3 aqueous solution under an Ar atmosphere, producing O2 with 89% Faradaic efficiency. Combining these two electrodes, CO2 reduction and water oxidation were attempted in a two-electrode configuration, where the photoanode was separated from the photocathode using a Nafion membrane. During the photoelectrolysis, in the gas phase of the photocathode chamber, both CO and H2 were detected by a gas chromatograph. In the liquid phase of the photoanode chamber, evolved O2 was detected using an O2 sensor. Visible light irradiation (λ > 400 nm) was conducted from the back side of the RuRe/NiO photocathode, and the transmitted portion of light penetrated through the RuRe/NiO, whereas the Nafion membrane was absorbed by the CoOx/TaON photoanode. An electrochemical bias of −0.3 V versus CoOx/TaON was applied to RuRe/NiO with a potentiostat used to facilitate the whole reaction. Because of the pH difference between the two electrodes, there was also a chemical bias (0.10 V), which accelerated the reaction. After 60 min of irradiation, O2 (77 nmol) evolved in the anode chamber and CO (79 nmol, TON = 17) with a small amount of H2 (6 nmol) in the cathode chamber, corresponding to 68% and 37% Faradaic efficiencies. The low Faradaic efficiency in the cathodic reaction was attributed to a partial reduction of Ni3+ species existing in the NiO electrode, as is observed in similar photoelectrochemical reactions in both

Photoelectrochemical CO2 reduction using various electrodesa

Entry

Sample

Metal complex/nmol

Potential/Vb

CO/nmol (TONCO)

H2/nmol

Faradaic efficiency/%

1

RuRe/NiO

11.2

−0.7

2 3 4 5 6d 7e

RuRe/NiO RuRe/NiO Ru(PS)/NiO Re/NiO NiO-RuRe RuRe/NiO

10.3 9.6 11.5 15.9 10.2 9.4

−0.3 0 −0.7 −0.7 −0.7 −0.7

241 (22) 361 (32)c 111 (11) n.d. n.d. n.d. n.d. n.d.

13 36c 10 n.d. 21 n.d. n.d. 11

59 64c 45 21

56

Electrode was irradiated at λ > 460 nm for 5 h in 50 mM NaHCO3 aq solution under a CO2 atmosphere. Versus Ag/AgCl (sat. KCl). c 12 h irradiation. d In the dark. e Under an Ar atmosphere. From G. Sahara, H. Kumagai, K. Maeda, N. Kaeffer, V. Artero, M. Higashi, R. Abe, O. Ishitani, J. Am. Chem. Soc. 138 (2016) 14152 (http://pubs.acs.org/doi/full/10.1021/jacs.6b09212). a b

Hybrid Z-scheme nanocomposites for photocatalysis301

Table 12.4 

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Multifunctional Photocatalytic Materials for Energy

organic and aqueous solutions [26,28]. Without RuRe, no CO2 reduction product was observed in the cathode chamber, with a small amount of O2 (