Nanophotocatalysis and Environmental Applications: Materials and Technology [1st ed.] 978-3-030-10608-9, 978-3-030-10609-6

Table of contents :
Front Matter ....Pages i-xiii
Nanostructured Imprinted Supported Photocatalysts: Organic and Inorganic Matrixes (Cícero Coelho de Escobar, João Henrique Z. dos Santos)....Pages 1-48
Supporting Materials for Immobilisation of Nano-photocatalysts (R. Goutham, R. Badri Narayan, B. Srikanth, K. P. Gopinath)....Pages 49-82
Non-metal (Oxygen, Sulphur, Nitrogen, Boron and Phosphorus)-Doped Metal Oxide Hybrid Nanostructures as Highly Efficient Photocatalysts for Water Treatment and Hydrogen Generation (M. S. Jyothi, Vignesh Nayak, Kakarla Raghava Reddy, S. Naveen, A. V. Raghu)....Pages 83-105
Challenges of Synthesis and Environmental Applications of Metal-Free Nano-heterojunctions (Vagner R. de Mendonça, Osmando F. Lopes, André E. Nogueira, Gelson T. S. T. da Silva, Caue Ribeiro)....Pages 107-138
Perovskite-Based Materials for Photocatalytic Environmental Remediation (Ashish Kumar, Suneel Kumar, Venkata Krishnan)....Pages 139-165
Carbon Nitride: A Wonder Photocatalyst (Biswajit Choudhury)....Pages 167-209
Graphene and Allies as a Part of Metallic Photocatalysts (Annelise Kopp Alves)....Pages 211-220
Silver-Based Photocatalysts: A Special Class (Vicente Rodríguez-González, Agileo Hernández-Gordillo)....Pages 221-239
Green Synthesis of Novel Photocatalysts (Shubhrajit Sarkar, Santanu Sarkar, Chiranjib Bhattacharjee)....Pages 241-261
Electrodeposition of Composite Coatings as a Method for Immobilizing TiO2 Photocatalyst (V. S. Protsenko, A. A. Kityk, E. A. Vasil’eva, A. V. Tsurkan, F. I. Danilov)....Pages 263-301
Spinning Disk Reactor Technology in Photocatalysis: Nanostructured Catalysts Intensified Production and Applications (Javier Miguel Ochando-Pulido, Marco Stoller, Luca Di Palma, A. Martínez-Férez, Giorgio Vilardi)....Pages 303-333
Back Matter ....Pages 335-336

Citation preview

Environmental Chemistry for a Sustainable World

Inamuddin Gaurav Sharma Amit Kumar Eric Lichtfouse Abdullah M. Asiri Editors

Nanophotocatalysis and Environmental Applications Materials and Technology

Environmental Chemistry for a Sustainable World Volume 29

Series editors Eric Lichtfouse, Aix-Marseille University, CEREGE, CNRS, IRD, INRA, Coll France, Aix-en-Provence, France Jan Schwarzbauer, RWTH Aachen University, Aachen, Germany Didier Robert, CNRS, European Laboratory for Catalysis and Surface Sciences, Saint-Avold, France

Other Publications by the Editors

Books Environmental Chemistry http://www.springer.com/978-3-540-22860-8 Organic Contaminants in Riverine and Groundwater Systems http://www.springer.com/978-3-540-31169-0 Sustainable Agriculture Volume 1: http://www.springer.com/978-90-481-2665-1 Volume 2: http://www.springer.com/978-94-007-0393-3 Book series Environmental Chemistry for a Sustainable World http://www.springer.com/series/11480 Sustainable Agriculture Reviews http://www.springer.com/series/8380 Journals Environmental Chemistry Letters http://www.springer.com/10311

More information about this series at http://www.springer.com/series/11480

Inamuddin • Gaurav Sharma • Amit Kumar Eric Lichtfouse • Abdullah M. Asiri Editors

Nanophotocatalysis and Environmental Applications Materials and Technology

Editors Inamuddin Chemistry Department, Faculty of Science King Abdulaziz University Jeddah, Saudi Arabia

Gaurav Sharma School of Chemistry Shoolini University Solan, Himachal Pradesh, India

Amit Kumar School of Chemistry Shoolini University Solan, Himachal Pradesh, India

Eric Lichtfouse CEREGE, CNRS, IRD, INRA, Coll France Aix-Marseille University Aix-en-Provence, France

Abdullah M. Asiri Chemistry Department, Faculty of Science King Abdulaziz University Jeddah, Saudi Arabia

ISSN 2213-7114 ISSN 2213-7122 (electronic) Environmental Chemistry for a Sustainable World ISBN 978-3-030-10608-9 ISBN 978-3-030-10609-6 (eBook) https://doi.org/10.1007/978-3-030-10609-6 Library of Congress Control Number: 2019933568 © Springer Nature Switzerland AG 2019 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG. The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

Photocatalysis is promoted and utilized as the most preferred and versatile technology for environmental pollution remediation, fuel production, organic catalysis, clean energy production, etc. The utilization of light energy to remove lowest levels of persistent and emerging pollutants as well as hydrogen production makes this process green and sustainable. The technology of photodegradation of pollutants is widely used and commercialized. For the practical and commercial applications, a photocatalyst should be easy to fabricate, cost-effective, solar-active, stable, highly v

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Preface

efficient and reusable. The first ‘star’ photocatalysts, ZnO and TiO2, have lost their lustre because of the numerous disadvantages, and various novel photocatalysts have been into play. In addition to metal oxides, sulphides, nitrides, oxynitrides, halides, oxyhalides, phosphates, phosphides and so on have also been exploited. Single photocatalyst has some of its own disadvantages that led to the design of new composite catalysts utilizing carbon-based materials, polymers, biological macromolecules, inorganic polymers and various supports for higher efficiency and stability. It seems that the composite materials have been and will be an inexorable trend in designing of photocatalysts. The crux of fabrication lies in developing a particular interface with higher synergic interactions. With an increasing number of pollutants in the environment and limited literature available, a continuous research on designing of the photocatalysts is required for their mineralization. One such development was the formation of heterojunctions with inorganic and organic semiconductors. The intimately interfaced semiconductors with different band positions and electron structures promise higher quantum yield and large charge flow. Some researchers also work on utilizing agriculture and industrial waste to prepare supports for the photocatalysts. In recent years, catalysts such as graphitic carbon nitride, carbon, graphene, graphene oxide and conducting polymers are used to form parts of heterojunctions that increase the ‘bio’ quotient of materials. In addition to the development of new photocatalytic materials, technological advancements have also played an immensely important role in designing, fabrication and final utilization. These include electrodeposition immobilization techniques, chemical vapour deposition, sensitization, molecular imprinting, electrospraying, plasma cleaning, microwave, ultrasonic synthesis, electrospinning, etc. These have enabled the researchers to prepare ‘engineered’ photocatalysts such as layered materials, polymer-imprinted materials, 2D materials, catalysts with switchable counterparts and many other novel materials. Photocatalytic reactors also play an important role to achieve the maximum potential of the catalysts as well as the light source. An effective reactor can only be achieved by proper optimization of parameters, process, catalyst, instrumentation and external factors. The reactors are also combined with flocculation, coagulation, adsorption and disinfection along with photocatalytic technology for complete water treatment. Various types of photocatalytic reactors are fabricated such as slurry reactor, spinning disk reactor, microfluidic reactor, membrane photoreactor, fluidized bed reactor, planar falling film reactor, cyclone reactor and sometimes combinations of these reactors. In addition, hybrid reactors are also designed for coupling degradation of pollutants with simultaneous energy production. Nanophotocatalysis and Environmental Applications: Materials and Technology is focussed on designing and development of various novel photocatalysts as well as new technological advancements for synthesis as well as photocatalysis process. Various classes of photocatalysts, including metal-based, metal-free, carbon nitride, silver-based, graphene and its derivatives based, perovskite and magnetic photocatalysts, are discussed in detail. The photocatalytic technology is discussed in terms of the joint application of morphology, energy band, structural, interfacial

Preface

vii

and technology engineering. Based on thematic topics, the book edition contains the following 11 chapters: Chapter 1 presents the survey related to the most recent works in the area of the molecularly imprinted photocatalyst. The primary work of this chapter is discussing the most recent investigations related to the organic and inorganic matrices as photocatalysts. The characterization methods applied to the molecularly imprinted photocatalyst are also reviewed. Chapter 2 provides a critical review of the application of various supporting materials used for immobilization of photocatalysts. Various immobilization aids, such as zeolites, clay and ceramics, carbonaceous materials, glass, cellulosic materials, polymers and metallic agents along with techniques used for immobilization such as cold plasma discharge, RF magnetron sputtering, dip coating, polymerassisted hydrothermal decomposition, solvent casting, photo etching, spray pyrolysis and electrophoretic, have been reviewed in detail. Finally, certain usual techniques employed for the characterization of the catalyst particles and their applications are also discussed. Chapter 3 examines the different strategies utilized for the synthesis of non-metaldoped hybrid nanostructured metal oxides, their properties, photocatalytic mechanism and the parameters required to judge the photocatalytic performance execution amid wastewater treatment and hydrogen generation. Chapter 4 discusses imaginative methodologies utilized for the synthesis of heterostructures and their applications beyond degradation of contaminants in water via heterogeneous photocatalysis, for example, photoreduction/oxidation of metallic particles and gas-stage responses, showing the versatility of such materials. Chapter 5 is dealing with the strategies involved in the design and development of perovskite-based photocatalysts for organic pollutant degradation and CO2 reduction applications. A short presentation of perovskite materials, principles and mechanism of photocatalysis involved in organic pollutant degradation and CO2 reduction processes is talked about in detail. Chapter 6 discusses the different experimental tools adopted to design and modify carbon nitride (g-C3N4) as wonder photocatalyst for clean, green and sustainable energy generation. Chapter 7 gives a brief overview of the methods used to synthesize graphene, graphene oxide and reduced graphene oxide, alongside their use in photocatalytic degradation of pollutants. Chapter 8 reviews about the use of silver nanoparticles functionalized on nanostructured materials intended for photocatalytic applications. Activity insights into the plasmonic effect, electron-hole mediator and bacteriostatic inhibition of pathogenic colonies at room temperature are also provided. Chapter 9 discusses greener synthesis approaches employed for the synthesis of novel photocatalysts. Chapter 10 deals with the electrodeposition of Fe/TiO2 composite coatings using environmentally friendly aqueous methanesulphonate iron plating baths containing colloidal TiO2 particles.

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Preface

Chapter 11 deals with the production of nanostructured catalysts in a continuous spinning disk reactor. The main features of the spinning disk reactor technology are reported and analysed. Spinning disk reactor technology is compared to conventional technologies, and the current application of this technology to selected nanoparticle (Titania, magnetite, MgO and hydroxyapatite) synthesis is discussed. This book is the consequence of the commendable cooperation of authors from various interdisciplinary fields of science. It thoroughly examines the most generous, starts to finish, and forefront research and reviews. We are thankful to all the contributing authors and their co-authors for their regarded commitment. We may, moreover, need to thank all copyright holders, authors and others individuals who agreed to use their figures, tables and schemes. Though every effort has been made to secure the copyright approvals from the individual proprietors to consolidate reference to the imitated materials, we should need to offer our sincere proclamations of disappointment to any copyright holder if unintentionally their benefit is being infringed. Jeddah, Saudi Arabia Solan, India Solan, India Aix-en-Provence, France Jeddah, Saudi Arabia

Inamuddin Gaurav Sharma Amit Kumar Eric Lichtfouse Abdullah M. Asiri

Contents

1

Nanostructured Imprinted Supported Photocatalysts: Organic and Inorganic Matrixes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cícero Coelho de Escobar and João Henrique Z. dos Santos

2

Supporting Materials for Immobilisation of Nano-photocatalysts . . R. Goutham, R. Badri Narayan, B. Srikanth, and K. P. Gopinath

3

Non-metal (Oxygen, Sulphur, Nitrogen, Boron and Phosphorus)Doped Metal Oxide Hybrid Nanostructures as Highly Efficient Photocatalysts for Water Treatment and Hydrogen Generation . . . M. S. Jyothi, Vignesh Nayak, Kakarla Raghava Reddy, S. Naveen, and A. V. Raghu

1 49

83

4

Challenges of Synthesis and Environmental Applications of Metal-Free Nano-heterojunctions . . . . . . . . . . . . . . . . . . . . . . . . 107 Vagner R. de Mendonça, Osmando F. Lopes, André E. Nogueira, Gelson T. S. T. da Silva, and Caue Ribeiro

5

Perovskite-Based Materials for Photocatalytic Environmental Remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 Ashish Kumar, Suneel Kumar, and Venkata Krishnan

6

Carbon Nitride: A Wonder Photocatalyst . . . . . . . . . . . . . . . . . . . . 167 Biswajit Choudhury

7

Graphene and Allies as a Part of Metallic Photocatalysts . . . . . . . . 211 Annelise Kopp Alves

8

Silver-Based Photocatalysts: A Special Class . . . . . . . . . . . . . . . . . . 221 Vicente Rodríguez-González and Agileo Hernández-Gordillo

9

Green Synthesis of Novel Photocatalysts . . . . . . . . . . . . . . . . . . . . . 241 Shubhrajit Sarkar, Santanu Sarkar, and Chiranjib Bhattacharjee

ix

x

Contents

10

Electrodeposition of Composite Coatings as a Method for Immobilizing TiO2 Photocatalyst . . . . . . . . . . . . . . . . . . . . . . . . 263 V. S. Protsenko, A. A. Kityk, E. A. Vasil’eva, A. V. Tsurkan, and F. I. Danilov

11

Spinning Disk Reactor Technology in Photocatalysis: Nanostructured Catalysts Intensified Production and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 Javier Miguel Ochando-Pulido, Marco Stoller, Luca Di Palma, A. Martínez-Férez, and Giorgio Vilardi

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335

Contributors

Annelise Kopp Alves Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil R. Badri Narayan Department of Chemical Engineering, SSN College of Engineering, Chennai, India Chiranjib Bhattacharjee Department of Chemical Engineering, Jadavpur University, Kolkata, India Biswajit Choudhury Physical Sciences Division, Institute of Advanced Study in Science and Technology (An Autonomous Institute Under DST, Government of India), Guwahati, Assam, India F. I. Danilov Ukrainian State University of Chemical Technology, Dnipro, Ukraine Gelson T. S. T. da Silva Department of Chemistry, Federal University of São Carlos, São Carlos-SP, Brazil Institute of Energy and Climate Research (IEK-3), Forschungszentrum Jülich GmbH, Jülich, Germany Cícero Coelho de Escobar Departamento de Engenharia Química, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil Vagner R. de Mendonça Federal Institute of Education, Science and Technology of São Paulo, Itapetininga-SP, Brazil Luca Di Palma Department of Chemical Engineering Materials Environment, Sapienza University of Rome, Rome, Italy João Henrique Z. dos Santos Instituto de Química, Universidade Federal do Rio Grande do Sul, Porto Alegre, CEP, Brazil K. P. Gopinath Department of Chemical Engineering, SSN College of Engineering, Chennai, India xi

xii

Contributors

R. Goutham Department of Chemical Engineering, SSN College of Engineering, Chennai, India Agileo Hernández-Gordillo Instituto de Investigaciones en Universidad Nacional Autónoma de México, Coyoacán, México

Materiales,

M. S. Jyothi Department of Chemical Technology, Faculty of Sciences, Chulalongkorn University, Bangkok, Thailand A. A. Kityk Ukrainian State University of Chemical Technology, Dnipro, Ukraine Venkata Krishnan School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Mandi, Himachal Pradesh, India Ashish Kumar School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Mandi, Himachal Pradesh, India Suneel Kumar School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Mandi, Himachal Pradesh, India Osmando F. Lopes Institute of Chemistry, Federal University of Uberlândia, Uberlândia-MG, Brazil A. Martínez-Férez Department of Chemical Engineering, University of Granada, Granada, Spain S. Naveen Department of Basic Sciences, School of Engineering and Technology, CET, Jain University, Bangalore, India Vignesh Nayak Center for Nano and Material sciences, Jain University, Bangalore, India André E. Nogueira Department of Chemistry, Institute of Exact and Biological Sciences, Federal University of Ouro Preto, Ouro Preto-MG, Brazil Javier Miguel Ochando-Pulido Department of Chemical Engineering, University of Granada, Granada, Spain V. S. Protsenko Ukrainian State University of Chemical Technology, Dnipro, Ukraine A. V. Raghu Department of Basic Sciences, School of Engineering and Technology, CET, Jain University, Bangalore, India Kakarla Raghava Reddy School of Chemical & Biomolecular Engineering, The University of Sydney, Sydney, NSW, Australia Caue Ribeiro Institute of Energy and Climate Forschungszentrum Jülich GmbH, Jülich, Germany Embrapa Instrumentation, São Carlos-SP, Brazil

Research

(IEK-3),

Contributors

xiii

Vicente Rodríguez-González División de Materiales Avanzados, IPICYT, Instituto Potosino de Investigación Científica y Tecnológica, San Luis Potosí, México Santanu Sarkar Environment Research Group, R&D, Tata Steel Ltd., Jamshedpur, India Shubhrajit Sarkar Department of Chemical Engineering, Jadavpur University, Kolkata, India Embrapa Instrumentation, São Carlos-SP, Brazil B. Srikanth Department of Chemical Engineering, SSN College of Engineering, Chennai, India Marco Stoller Department of Chemical Engineering Materials Environment, Sapienza University of Rome, Rome, Italy A. V. Tsurkan Ukrainian State University of Chemical Technology, Dnipro, Ukraine E. A. Vasil’eva Ukrainian State University of Chemical Technology, Dnipro, Ukraine Giorgio Vilardi Department of Chemical Engineering Materials Environment, Sapienza University of Rome, Rome, Italy

Chapter 1

Nanostructured Imprinted Supported Photocatalysts: Organic and Inorganic Matrixes Cícero Coelho de Escobar and João Henrique Z. dos Santos

Contents 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2 Fundamental Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2.1 Polymerization with Organic and Inorganic Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2.2 Estimation of Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3 General Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.4 Parameters for Selectivity Essays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.5 Parameters for Competitiveness Essays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.5.1 Molecularly Imprinted Photocatalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.5.2 Characterization of Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 1.6 Final Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Abstract A serious shortcoming in heterogeneous photocatalytic oxidation is its low selectivity to the target contaminants. In this sense, preferential photodegradation by means of molecularly imprinted has been explored in the literature. This chapter aims to review some of latest works in respect of combination of molecularly imprinted with photocatalysis, covering the years from 2013 to 2018. The main findings of some of the latest studied are discussed in terms of organic and inorganic matrixes. The characterization methods applied to the molecularly imprinted photocatalyst (MIP) are also reviewed. Our conclusion is that the use of MIP based either on organic or inorganic matrixes is a promising way to enhance selectivity in the photocatalysis.

C. C. de Escobar (*) Departamento de Engenharia Química, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil J. H. Z. dos Santos Instituto de Química, Universidade Federal do Rio Grande do Sul, Porto Alegre, CEP, Brazil e-mail: [email protected] © Springer Nature Switzerland AG 2019 Inamuddin et al. (eds.), Nanophotocatalysis and Environmental Applications, Environmental Chemistry for a Sustainable World 29, https://doi.org/10.1007/978-3-030-10609-6_1

1

2

C. C. de Escobar and J. H. Z. dos Santos

Keywords Molecularly imprinted · Photocatalyst · Titanium dioxide · Selectivity photocatalysis · Preferential photodegradation · Imprinting factor · Textural characterization · Structural characterization · Template · Extraction

1.1

Introduction

Water resources, such as rivers and lakes, may be contaminated with a variety of organic, inorganic, and microbial pollutants. For instance, it has been showed the presence of pharmaceuticals and personal care products, pesticides, and dyes. It turns out that conventional water treatment methods do not guarantee the complete removal of these compounds. In this sense, heterogeneous photocatalysis has emerged as a promising water and wastewater treatment technology. However, a serious shortcoming in heterogeneous photocatalytic is its low selectivity to the target contaminants. In order to overcome this drawback, recently research has focus attention to enhance the selectivity of the photocatalysis. Recently, several reviews have been published in the field of photocatalysis, encompassing some aspects such as photodecomposition of organic compounds (Szczepanik 2017; Khaki et al. 2017), visible light photocatalysis (Etacheri et al. 2015; Bora and Mewada 2017), clay–TiO2 nanocomposites (Szczepanik 2017), disinfection/inactivation of harmful pathogens (Laxma et al. 2017), air purification (Boyjoo et al. 2017) and hydrogen production (Acar et al. 2014). Preferential photodegradation has been discussed elsewhere (Paz 2006; Ghosh-Mukerji et al. 2001; Shen 2016; Lazar and Daoud 2013). Among the approaches for obtaining preferential photodegradation, it included pH changing and doping (Paz 2006). Molecularly imprinted photocatalyst as a means to achieve selectivity has recently been discussed (Shen 2016). However, there is still a lack of a review focusing on the latest works on molecularly imprinted photocatalysis, in terms of methods of synthesis, characterization of the resulting materials, and efficiency in selectivity. This chapter aims to review some of latest works in respect of combination of molecularly imprinted with photocatalysis, covering the years from 2013 to 2018. Although we do not cover all of the published works, it will be enough to give a clear perspective of the field. We begin with fundamental concepts and a brief discussion about the definition of the parameters used to estimate selectivity. In the following section, we present the discussion about several aspects such as the nature of the imprinted matrix, photocatalyst precursors, extraction method, and photocatalytic process (competitiveness versus selectivity). Then, the most used characterization methods of the prepared materials are discussed in Sect. 1.5.2.

1 Nanostructured Imprinted Supported Photocatalysts: Organic and. . .

1.2

3

Fundamental Concepts

Molecular imprinting is defined as a technique to create template-shaped cavities in polymer matrices having the ability of molecular recognition in the spaces vacated by the templating species (Whitcombe et al. 2014). The concept of molecular imprinting is based on the assembly of a cross-linked polymer matrix around a template. After that template is removed, recognition sites are created. Figure 1.1 shows a schematic representation of the principle of molecular imprinting. Typically, there are three main steps involved in the production of molecularly imprinted polymers: (i) the assembling of the functional monomers around the template, (ii) polymerization in the presence of cross-linker, and (iii) an extraction process to remove the template (Chen et al. 2016).

1.2.1

Polymerization with Organic and Inorganic Matrix

According to recent reviews (Lofgreen and Ozin 2014; Chen et al. 2016), it has been reported that there are at least five main methods of molecular imprinting: (i) noncovalent, (ii) electrostatic/ionic, (iii) covalent, (iv) semi-covalent, and (v) metal center coordination. All of them have been explored using a variety of templates (e.g., food ingredients, peptides, drugs, proteins, pollutants). Regarding the chemical nature of the polymer, the matrix could be either organic or inorganic. For the organic matrix, the choice of the monomer depends on the method used to achieve molecular imprinting. For instance, in the case of covalent molecular imprinting, the templates are covalently bounded to monomers such as phenyl-α-D-mannopyranoside and 4-vinylbenzeneboronic acid (Li et al. 2016). In the case of noncovalent approach, methacrylic acid as functional monomer and trimethylolpropane trimethacrylate as cross-linker are typically used.

Fig. 1.1 Schematic representation of the principle of molecular imprinting

4

C. C. de Escobar and J. H. Z. dos Santos

For the inorganic matrix, sol–gel method is the most widely investigated approach. It uses metal alkoxide molecular precursors to produce a metal oxide. First, the precursors form a colloidal solution (a sol), which gradually develops into an integrated network (or gel) of amorphous material. For instance, tetraalkoxysilanes such as tetraethoxysilane or tetramethoxysilane can be used aiming for the formation of silica (Lofgreen and Ozin 2014). If an appropriate interaction between a template and alkoxide precursors is carried out, imprinted materials based on the inorganic matrix can be produced. The use of organic or inorganic matrices has its own advantages and disadvantages (Wu et al. 2016; Bagheri and Piri-Moghadam 2012; Deng et al. 2014; Whitcombe et al. 2014; Lofgreen and Ozin 2014); Farrington and Regan 2009). Table 1.1 summarizes some main features of each approach. One has to keep in mind that there are some characters that may be present for both matrices, such as nonspecific interactions between template molecules and the polymer that could reduce the ability for molecular recognition. A necessary condition for complete removal of contaminant is its mineralization, which can be achieved by using advanced oxidation processes (AOPs). Considering emerging contaminants, the heterogeneous photocatalysis is one of the more promising strategies among AOPs. According to the International Union of Pure and Applied Chemistry (IUPAC 1997), photocatalysis is the change in the rate of a chemical reaction or its initiation under the action of ultraviolet, visible, or infrared radiation in the presence of a substance—the photocatalyst—that absorbs light and is involved in the chemical transformation of the reaction partners. The use of photocatalyst as pollutant remediation has several advantages, such as low cost and low toxicity (Nakata and Fujishima 2012). In the degradation of organic pollutants, the generated radicals (e.g., •OH and •O2) are the main oxidants. It turns out that the molecules that are preferentially adsorbed Table 1.1 Advantages and disadvantages for an inorganic and organic matrix for molecularly imprinted materials Advantages

Inorganic High thermal and chemical stability

Ease of preparation

Disadvantages

Gelation at ambient temperature Higher porosity and surface area Good optical properties Stability under ultraviolet light Presence of structural/surface defects Presence of inactive surface Cracking and shrinking problems during drying steps Necessary attention to exert control over the diffusion between the template and imprinted silica

Organic The relative simplicity of the process (in the case of noncovalent approach) A varied range of chemical functionalities that can be targeted Better controllability during synthesis

Time and solvent consuming Swelling of sorbents in the presence of water and solvents Possible degradation of organic moieties upon extended ultraviolet illumination

1 Nanostructured Imprinted Supported Photocatalysts: Organic and. . .

5

on the surface of the photocatalyst will be decomposed first. In other words, a serious shortcoming in heterogeneous photocatalytic oxidation is its low selectivity to the target contaminants, which is translated by the fact that photocatalysts cannot differentiate highly toxic target pollutants from other organic compounds of low toxicity (Paz 2006; Sharabi and Paz 2010). This is especially a great concern for real samples, where effluent streams may contain highly toxic organic pollutants (normally non-biodegradable) that coexist with less toxic and biodegradable molecules. It turns out that usually the former is present at a lower concentration, and thus it is desirable preferential degradation toward the most toxic substance. To overcome the problem of the coexisting nontarget molecules, the molecular imprinting technique can be exploited to both increase the selectivity and serve as a support for photocatalytic degradations.

1.2.2

Estimation of Parameters

Selective photocatalysis is divided into two categories (Lazar and Daoud 2013): (i) selective formation, when a specific product is desired, or (ii) selective degradation, when one or more components are a must, preferably to the detriment of others. In this review, we are only interested in the latter case. In addition, the studies are represented by mixed and/or non-mixed molecules. Herein, we have labeled the studies containing non-mixed molecules as selectivity ones, whereas the studies containing mixed molecules as competitiveness ones. For the estimation of the selectivity and competitiveness photodegradation, the authors have employed several parameters. Although there is some divergence between the studies, the most commonly used parameters are defined below following Eqs. 1.1–1.7. It must be kept in mind that not necessarily the studies have used these equations, but we have, always when possible, calculate the values based on the information provided by the authors.

1.3

General Parameters

Imprinting factor (IF) Degradation or adsorption ðtarget moleculeÞ using MIP IF ¼ Degradation or adsorption ðtarget moleculeÞ using NIP

ð1:1Þ

P25 Factor (FP25) FP25 ¼

Degradation ðtarget moleculeÞ using MIP Degradation ðtarget moleculeÞ using P25

ð1:2Þ

6

C. C. de Escobar and J. H. Z. dos Santos

1.4

Parameters for Selectivity Essays

Selectivity factor (SF) SF ¼

Degradation ðtarget moleculeÞ using MIP ðnon  target moleculeÞ using MIP

ð1:3Þ

Selectivity factor based on P25 SFP25 ¼

1.5

Degradation ðtarget moleculeÞ using MIP Degradation ðnon  target moleculeÞ using P25

ð1:4Þ

Parameters for Competitiveness Essays

Competitiveness factor (CF) CF ¼

Degradation ðtarget moleculeÞ using MIP Degradationðnon  target moleculeÞ using MIP

ð1:5Þ

Competitiveness factor based on P25 (CFP25) CFP25 ¼

Degradation ðtarget moleculeÞ using MIP Degradation ðnon  target moleculeÞ using P25

ð1:6Þ

P25 Factor for competitiveness essays (FP25 competitiveness) FP25 competitiveness ¼

Degradation ðtarget moleculeÞ using MIP Degradation ðtarget moleculeÞ using P25

ð1:7Þ

Obs. 1: Herein, MIP refers to molecularly imprinted photocatalyst, and NIP refers to non-imprinted photocatalyst. Obs. 2: Another reference instead of P25 may be used, so the parameter will be calculated accordingly. Obs. 3: All the above parameter can also be estimated using the constant kinetic instead of degradation values.

1 Nanostructured Imprinted Supported Photocatalysts: Organic and. . .

1.5.1

Molecularly Imprinted Photocatalyst

1.5.1.1

Supported Photocatalysts: Imprinted Matrixes

7

Table 1.2 summarizes the main findings of some of the latest studies involving molecularly imprinted photocatalyst. As it can be seen, organic matrixes have been mostly used in the development of imprinted materials. For instance, Lu et al. (2014) prepared a magnetic molecularly imprinted photocatalyst using methyl methacrylate as the functional monomer and enrofloxacin hydrochloride as a template molecule. Briefly, TiO2@SiO2@Fe3O4 (TSF) previously prepared was dissolved together with polyethylene glycol 4000 in distilled water. Then, a certain amount of template and methyl methacrylate was put together into the microwave reactor. After magnetic stirring, this mixed solution was placed for 12 h in the dark under a nitrogen atmosphere. Afterward, trimethylolpropane trimethacrylate as a cross-linker, azobisisobutyronitrile as a radical initiator, and above TSF were added into this microwave reactor, and the polymerization was carried out in the microwave synthesizer. The extraction of the template proceeds by adding under the ultraviolet light irradiation for 5 h. The whole scheme of the synthesis is shown in Fig. 1.2. In the case of an inorganic matrix, metal alkoxides can be used as both matrix and photocatalyst precursors. In the development of doped photocatalyst, Wu et al. (2016) used NH4F and nitrophenol as doped agent and template molecule, respectively. Tetrabutyl orthotitanate was mixed together with the above suspension. Then, acetic acid was added dropwise. The photocatalyst was obtained after collected from an autoclave. Finally, the material was calcined at 450  C for 3 h to remove template molecule. A schematic diagram of the synthesis is shown in Fig. 1.3. For the inorganic matrix, due to the negligible swelling, it is crucial to exert control over the diffusion between the template and imprinted silica, which can be done by controlling the shape of particles or incorporating different kinds of porogenic materials (Lofgreen and Ozin 2014). In addition, one cannot neglect cracking and shrinking problems during drying steps associated with hydrolytic sol–gel technology (Si and Zhou 2011). In this context, de Escobar et al. (2015) have prepared non-hydrolytic sol–gel photocatalyst using rhodamine B as the template. Nevertheless, the degradation efficiency was twofold lower than that of the material produced by acid-catalyzed sol–gel hydrolytic route. As a general problem with the above methods shown in Table 1.2, one can cite the high cost required for post-processes, which include separation and recycle (Zhao et al. 2014). To overcome these problems, membranes and films have been explored (Zhao et al. 2014; Xu et al. 2014). In the case of the latter, the rationale is that films could act as both photocatalyst and an imprinted recognition layer, and thus selective photocatalytic degradation ability would be greatly increased (Xu et al. 2014).

Photocatalyst precursor

TiO2 (P25)

TNBT

Imprinted matrix (O, organic; I, inorganic)

O

I

T: 4-nitrophenol C: 2-nitrophenol

T: tetrazine

Template molecule (T), competitive molecule (C), selective molecule (S), others (O)

Table 1.2 Photodegradation studies based on MIP

Washed with hot distilled water/ammonia solution (8:2 v/v) and followed by extraction using hot water.

Methanol/ ammonia solution (1:1v/v) in a Soxhlet extraction system.

Extraction method

Competitiveness for photodegradation. Standard solution.

There is no competitiveness or selectivity tests. Standard solution.

Photocatalytic process

300 W xenon lamp Ccatal. ¼ 1500 mg/L Csubstrate ¼ 10 mg/L pH ¼ 3.0–9.0

UV-C radiation Ccatal. ¼ 100 mg/L Csubstrate ¼ 16 mg/L pH ¼ 2.0

Reaction parameters Precipitation polymerization was carried out in an aqueous medium. Acrylamide was used as functional monomer and N, N0 -methylenebisacrylamide as cross-linking agent. The results showed that the MIP nanocomposite has improved the photodegradation of tartrazine by 100% compared to NIP nanocomposite and pure nanophotocatalyst which yielded at 63% and 56%. Inorganic framework molecularly imprinted TiO2/ SiO2 nanocomposite was prepared by sol-hydrothermal method. Tests were done under simulated solar light irradiation. MIP showed excellent reusability.

IF3 degr. ¼ ca. 3.0 CF3degr. ¼ ca. 2.0

Highlights on synthesis and comments on performance

IF2degr. ¼ 1.5 F2P25 ¼ 1.77

SI or IF or CF

References

Deng et al. (2014)

Arabzadeh et al. (2016)

8 C. C. de Escobar and J. H. Z. dos Santos

T: 2,4-dinitrophenol O: phenol, 4-chlorophenol, 2,4,6trichlorophenol, pentachlorophenol (PCP)

T: ciprofloxacin hydrochloride (CH) S: ciprofloxacin hydrochloride (CP), tetracycline (TC), and amelia card star (ACS)

P25

NaCl/TiO2 (P25)

O

O

Soxhlet with a mixed solution of methanol/ glacial acetic acid (9:1v/v) for 48 h.

Na2CO3 solution (0.13 g/L).

Selectivity for photodegradation. Standard solution.

There is no competitiveness or selectivity tests reuse and recovery tests. Standard solution. Tap water. River water.

150 W xenon lamp (simulated solar light irradiation) Ccatal. ¼ 1000 mg/L Csubstrate ¼ 10 mg/L

20 W UV light Ccatal. ¼ 2000 mg/L Csubstrate ¼ 2 mg/L

2,4-dinitrophenol was used as an analog template for preparation of the MIP-coated photocatalysts to selectively remove a group of chlorophenols (phenol, 4-chlorophenol, 2,4,6-trichlorophenol and pentachlorophenol) in water In a mixture system, the MIP-coated P25 markedly enhanced the photodegradation of the 2,4,6-trichlorophenol. The MIP system showed an apparent rate constant of 1.74-fold higher compared to the P25. There is no tests of photodegradation of the template molecule (2,4-initrophenol). The modified NaCl/TiO2 photocatalyst was synthesized by surface molecularly imprinted technology. The acrylamide (AM), trimethylolpropane acrylate (TMPTA), and 2,2-azobisisobutyronitrile (AIBN) were considered to be functional monomer, cross-linking, and initiate

n.d.

IF2 degr. ¼ 1.4 SF2degr. ¼ 1.5 (for ACS)

(continued)

Liu et al. (2014)

Huang et al. (2013)

1 Nanostructured Imprinted Supported Photocatalysts: Organic and. . . 9

Photocatalyst precursor

TNBT

Imprinted matrix (O, organic; I, inorganic)

O

Table 1.2 (continued)

T: tetracycline C: oxytetracycline (OTC) and ciprofloxacin (CIP)

Template molecule (T), competitive molecule (C), selective molecule (S), others (O)

Washed with deionized and UV light irradiation under magnetic agitation.

Extraction method

Competitiveness for photodegradation. Standard solution.

Photocatalytic process

150 W tungsten lamp (visible light) Ccatal. ¼ 2000 mg/L Csubstrate ¼ 20 mg/L

Reaction parameters

IF2degr. ¼ 1.35 CF3degr. ¼ 1.67 (for OTC) CF3degr. ¼ 1.25 (for CIP)

SI or IF or CF

Molecularly imprinted photocatalyst was prepared by using the surface imprinting technique and the photo-induced method The optimal photo-induced polymerization time was 2 h, and the optimal functional monomer was MAA (methacrylic acid) (compared to acrylamide, methyl methacrylate, and 4-vinylpyridine). The photodegradation rate for degradation of TC with MIP reached nearly 77% in 90 min under the visible light irradiation.

reagents, respectively. The degradation ratio of template molecule could reach 70.90% in 60 min under visible irradiation For MIP photocatalyst, the first kinetic of photocatalytic degradation was lower for the template compared to the other molecules.

Highlights on synthesis and comments on performance

Lu et al. (2013)

References

10 C. C. de Escobar and J. H. Z. dos Santos

TNBT (N–F codoped)

P25

TNBT

I

O

O

T:4-nitrophenol

T: methyl orange (MO) C: sunset yellow (SY) and rhodamine B (RB)

T: 2-nitrophenol (2-NP) and 4-nitrophenol (4-NP) C: 2-nitrophenol and 4-nitrophenol

100  C for 12 h and then calcined at 500  C for 3 h.

Microparticles were stirred with NaOH to desorb the template molecules.

Calcined at 450  C for 3 h.

There is no competitiveness or selectivity tests. Comparison with P25.

Selectivity for photodegradation. Competitiveness for photodegradation. Standard solution. Reusability.

Competitiveness for adsorption. Standard solution. Competitiveness for photodegradation. Reusability.

300 W xenon (visible light) Ccatal. ¼ 2000 mg/L Csubstrate ¼ 10 mg/L

Ultraviolet irradiation (40 W) Ccatal. ¼ 1875 mg/L Csubstrate ¼ 9.8 mg/L

400 W metal halide lamp (visible light) Ccatal. ¼ 1000 mg/L Csubstrate ¼ 10 mg/L

Molecularly imprinted TiO2/WO3-coated magnetic Fe3O4@SiO2 nanocomposite was developed for photocatalytic degradation. The imprinted

A core–shell organic–inorganic hybrid material of surface-imprinted chitosan –TiO2 composite was prepared with methyl orange as the template. The material could be reused directly without further desorption and regeneration for 10 cycles with preserving 60%.

CF2degr. ¼ 10.28 (MO over RB) IF2degr. ¼ ca. 1.7

IF1ads. ¼ ca. 2.1 IF2degr. ¼ 2.38 F2P25 ¼ 7.1

N–F-codoped and molecularly imprinted TiO2 were prepared by simple ethanol– water solvothermal method using 2-nitrophenol (2NP) and 4-nitrophenol (4NP) as template molecules.

CF1ads. ¼ ca. 2.5 (2-NP over 4-NP) CF3degr. ¼ 1.93 (2-NP over 4-NP) IF3degr. ¼ 1.54

(continued)

Wei et al. (2015)

Xiao et al. (2015)

Wu et al. (2016)

1 Nanostructured Imprinted Supported Photocatalysts: Organic and. . . 11

Photocatalyst precursor

TNBT

TNBT

Imprinted matrix (O, organic; I, inorganic)

O

O

Table 1.2 (continued)

T: methylimidazole-2thiol S: florfenicol

T: ciprofloxacin (CP) S: danofloxacin mesylate (SM) and tetracycline (TC)

Template molecule (T), competitive molecule (C), selective molecule (S), others (O)

Rinsed with deionized water, methanol, and ethanol each for several times until there

Exposition of 500 W tungsten lamp for 3 h under magnetic agitation in air.

Extraction method

Selectivity for photodegradation. Standard solution. Reusability.

Selectivity for adsorption. Selectivity for photodegradation. Standard solution. Reusability.

Standard solution. Reusability.

Photocatalytic process

300 W mercury lamp (UV – light) Ccatal. ¼ 2000 mg/L Csubstrate ¼ 10 mg/L

500 W tungsten lamp (visible light) Ccatal. ¼ 1000 mg/L Csubstrate ¼ 10 mg/L

Reaction parameters

Co2+ ion-doped Ti(OH)4 xerogel supported by magnetic fly-ash prepared. Ti (OH)4 used as precursor was obtained from hydrolysis reaction of TNBT. The as-prepared material exhibited a superior-specific oriented recognition capability for selectively degrading ciprofloxacin molecules by comparing with danofloxacin mesylate and tetracycline. Magnetic and dual conductive imprinted photocatalysts were synthesized through the suspension polymerization method. Polypyrrole was

IF3degr. ¼1.70 SF3degr. ¼ 6.42

catalyst could degrade 4-nitrophenol with a firstorder reaction rate of 0.1039 h-1.

Highlights on synthesis and comments on performance

IF1ads. ¼2.85 IF2 degr. ¼ 1.57 SF2degr. ¼ 1.57 (CP over TC)

SI or IF or CF

Luo et al. (2014)

Lu et al. (2016a)

References

12 C. C. de Escobar and J. H. Z. dos Santos

O

Co(NO3)26H2O

T: 2-mercaptobenzothiazole S: 1-methylimidazole-2thiol

UV light irradiation for 3 h with the magnetic agitation under an air atmosphere.

was little template molecule in the solution.

Selectivity for photodegradation. Standard solution. Resusability.

500 W xenon lamp (visible light) Ccatal. ¼ 2000 mg/L Csubstrate ¼ 10 mg/L

IF1ads. ¼ 1.72 IF2degr. ¼ 1.53 SF2degr. ¼ 2.35

An imprinted CoFe2O4/ multi-walled carbon nanotubes photocatalyst (MWCNTs) was prepared through the combination of a hydrothermal method and suspension polymerization using pyrrole as the imprinted functional monomer and conductive polymerizable monomer, and CoFe2O4/MWCNTs as the matrix material. Compared with other photocatalysts, the imprinted one not only had

used in combination with multi-walled carbon nanotubes (MWCNTs) When pyrrole was used as the function monomer, polymerization time was set as 24 h, and the adding dose of pyrrole was chosen as 8 mmol, MIPs exhibited the highest photocatalytic activity. The results showed that the removal efficiency was approximately half of that of the first circulation operation after four consecutive cycles.

(continued)

Lu et al. (2015)

1 Nanostructured Imprinted Supported Photocatalysts: Organic and. . . 13

Photocatalyst precursor

TNBT

Imprinted matrix (O, organic; I, inorganic)

O

Table 1.2 (continued)

T: rhodamine B S and C: rhodamine 6G

Template molecule (T), competitive molecule (C), selective molecule (S), others (O)

Washed by Na2CO3 solution and deionized water.

Extraction method

Selectivity for adsorption. Competitiveness for adsorption. Selectivity for photodegradation. Standard solution. Reusability.

Photocatalytic process

400 W metal halide lamp with an UV filter (visible light) Ccatal. ¼ 800 mg/L Csubstrate ¼ 10 mg/L

Reaction parameters

IF1ads. ¼ 1.72 SF1ads. ¼ ca. 1.37 CF1ads. ¼ca. 2.1 IF3degr. ¼ 2.15 SF3degr. ¼ 1.15

SI or IF or CF

Molecularly imprinted polymer-coated codoped TiO2 nanocomposites were synthesized by a surface molecular imprinting technique using p-phenylenediamine as the functional monomer. The MIP/Co–TiO2 nanocomposites showed higher adsorption capacity for the target contaminants than the Co–TiO2 nanoparticles and NIP/Co– TiO2 nanocomposites, which could be attributed to larger surface area and surface-imprinted cavities of MIP/Co–TiO2 nanocomposites. Moreover, the MIP/Co–TiO2

high photocatalytic efficiency but also possessed the strong ability to selective recognition and photodegradation of template.

Highlights on synthesis and comments on performance

Liu et al. (2016)

References

14 C. C. de Escobar and J. H. Z. dos Santos

Fe2O3

TNBT

I

O

T: bisphenol A C: phenol

T: methylene blue S: p-nitrophenol

Washed with a Na2CO3 solution (0.2 g/L) for five times

Washed with DI water and anhydrous ethanol for several times and then dried at 80  C for 6 h followed by calcination carried out at 450  C for 3 h.

Selectivity for adsorption. Competitiveness for adsorption. Standard solution.

Selectivity for photodegradation. Standard solution.

300 W xenon lamp (visible light) Ccatal. ¼ 1000 mg/L Csubstrate ¼ 8 mg/L

500 W high pressure mercury light (UV light) Ccatal. ¼ 133.33 mg/ L Csubstrate ¼ 10 mg/L

Methylene blue was used as a structure-directing agent to synthesize molecularly imprinted a-Fe2O3 via hydrothermal synthesis. It was demonstrated that the structure-directing agent may be utilized to effectively control and adjust the properties of a-Fe2O3 including crystallinity, morphology, microstructure, the distribution of the pore sizes, specific surface area, and surface site density. Molecularly imprinted TiO2/graphene photocatalyst was prepared with bisphenol A (BPA) as the template molecule (target pollutant) and o-phenylenediamine (OPDA) as functional monomers by the surface molecular imprinting method. The combination between BPA and OPDA led to the formation of the precursor, and the

IF2degr. ¼ 1.69

IF1ads. ¼ ca. 1.92 CF1ads. ¼ 10.9 IF3degr. ¼ 1.37

nanocomposites exhibited its high reusability and stability.

(continued)

Lai et al. (2016)

Fang et al. (2016)

1 Nanostructured Imprinted Supported Photocatalysts: Organic and. . . 15

Photocatalyst precursor

TNBT

Imprinted matrix (O, organic; I, inorganic)

O

Table 1.2 (continued)

T: enrofloxacin hydrochloride (ENHR) C: tetracycline (TC)

Template molecule (T), competitive molecule (C), selective molecule (S), others (O)

Aqueous solution under UV light irradiation for 5 h with the magnetic agitation under an air atmosphere.

Extraction method

Competitiveness for photodegradation. Comparison with P25. Standard solution. Reusability.

Photocatalytic process

300 W tungsten lamp (visible light) Ccatal. ¼ 1666.6 mg/ L Csubstrate ¼ 20 mg/L

Reaction parameters

IF3degr. ¼ 1.13 CF2degr. ¼ 1.3 F3P25 ¼ 1.03

SI or IF or CF

Magnetic imprinted TiO2 photocatalyst was prepared via a microwave heating method based on enrofloxacin hydrochloride as the template molecule, methyl methacrylate as the functional monomer, and TiO2@SiO2@Fe2O4 as the matrix material TSF was synthesized by a mild sol–gel method. The results indicated that MITP possessed

subsequent polymerization of OPDA initiated by ultraviolet radiation ensured the realization of MIP-TiO2/ graphene. The MIP photocatalyst exhibited fast dynamics, high adsorbing capacity, and satisfactory selectivity by the affinity to the template molecules as well higher photocatalytic activity compared to non-imprinted systems.

Highlights on synthesis and comments on performance

Lu et al. (2014)

References

16 C. C. de Escobar and J. H. Z. dos Santos

Titanium tetrachloride

P25

O

O

T: 2-nitrophenol (2NP) and 4-nitrophenol (4NP) C: bisphenol-A

T: salicylic acid

Washed with Na2CO3 solution (0.13 g/L) for 5 times.

Washed using 500 mg/L Na2CO2 solution. Metal halide lamp (visible light) Ccatal. ¼ 400 mg/L Csubstrate ¼ 20 mg/L

Philips 9W UV lamp Ccatal. ¼ 100 mg/L Csubstr. ¼ 50 mmol/ L

There is no competitiveness or selectivity tests.

Competitiveness for photodegradation. Comparison with P25. Standard solution.

Molecularly imprinted polymer-coated S-doped TiO2 nanocomposites were synthesized by a surface molecular imprinting technique using p-phenylenediamine as the functional monomer; its photocatalytic activity was enhanced over the range of visible region. By using a suitable transition state analog (i.e., 2-NP, 4-NP) as the template, the prepared MIP (o-phenylenediamine was used as a monomer) coated TiO2 photocatalysts (i.e., 2NP-P25, 4NP-P25) reduced the apparent activation energy and hence enhanced the photocatalytic degradation of the target molecule (nitrobenzene) in both the absence and presence of nontarget pollutants Moreover, the special molecular recognition inhibited the accumulation of unwanted intermediates.

IF1ads. ¼ ca.1.31 IF3degr. ¼ ca.1.26

IF2degr. ¼ 1.41 CF3degr. ¼ 0.78 (2NP as template) F3P25 ¼ 2.4 F3P25 competitiveness ¼ 4.4 CFP25 ¼ 0,68

hierarchical spherical structure and good monodispersity. MIP photocatalyst proved to exhibit an excellent photochemical stability and a higher photocatalytic efficiency than other photocatalysts.

(continued)

Shen et al. (2014)

Wang et al. (2014)

1 Nanostructured Imprinted Supported Photocatalysts: Organic and. . . 17

Photocatalyst precursor

TNBT

Imprinted matrix (O, organic; I, inorganic)

O

Table 1.2 (continued)

T: 2-nitrophenol (2NP) and 4-nitrophenol (4NP) S: 2-nitrophenol (2NP)

Template molecule (T), competitive molecule (C), selective molecule (S), others (O) Calcined at 500  C for 3 h in the furnace in air.

Extraction method

Reaction parameters 300 W Xeon lamp (visible light) Ccatal. ¼ 2000 mg/ L Csubstrate ¼ 10 mg/ L

Photocatalytic process Selectivity for adsorption. Competitiveness for adsorption. Selectivity for photodegradation. Standard solution. Reusability.

IF1ads. ¼ 1.16 SF1ads. ¼ 15.40 CF1ads. ¼ >6.0 IF3degr. ¼ 12 SF3degr. ¼ 2.47 (all parameters estimated considering 2NP as target molecule)

SI or IF or CF Inorganic framework molecularly imprinted TiO2/ WO3 nanocomposites with molecular recognitive photocatalytic activity were prepared by a facile one-step sol–gel method using tetrabutyl orthotitanate as titanium source as well as the precursor of functional monomer which could complex with template molecules. Compared with non-imprinted TiO2, the MI photocatalysts not only exhibit higher adsorption capacity and selectivity for the target contaminant but also show enhanced photocatalytic activity in degrading the target contaminant. The prepared MI samples were stable for up to four photocatalytic cycles without obvious decrease in the photocatalytic degradation of target pollutants.

Highlights on synthesis and comments on performance

References Luo et al. (2013)

18 C. C. de Escobar and J. H. Z. dos Santos

P25

P25

I (M)

I (F)

T: estrone C: bisphenol A (BPA) and phenol (PH)

T: methyl orange C: methyl red

Calcined at 400  C.

Washed with distilled water. 1000 W UV light at 365 nm emission peak Cmembrane ¼ 1.5 g/ L Csubstrate ¼ 1 mM

20 W UV light with an emission peak at 254 nm Ccatal. ¼ 2500 mg/ L Csubstrate ¼ 10 mg/ L

Competitiveness for photodegradation. Comparison with P25. Standard solution. Reusability.

Selectivity for photodegradation. Competitiveness for photodegradation. Comparison with P25. Standard solution. Tap water.

Molecularly imprinted TiO2/calcium alginate (T/CA) membrane was synthesized by using T/CA as supporting matrix and ethylenetri(betamethoxy) ethoxysilane, and aminopropyltriethoxysilane as functional monomers. Methyl orange was adsorbed more and faster by the molecularly imprinted membrane than the non-imprinted membrane. MIP showed a good selectivity for the photodegradation of target molecule when choosing methyl red as competitive molecules. The degradation rates of methyl orange were almost unchanged with the cycle times. A MI TiO2 hybridized magnetic ferroferric oxide (Fe3O4) nanoparticles through a semi-covalent approach by a liquid phase deposition method was prepared. The obtained Fe3O4@SiO2@imprinted TiO2 (tetraethoxysilane as

IF1ads. ¼ 2.27 IF1ads. ¼ 1.36 IF2degr. ¼ 1.05 SF2degr. ¼ 1.1 F2P25 ¼ 3.8 IF1ads. ¼ 2.27 IF1ads. ¼ 1.36 IF2degr. ¼ 1.05 SF2degr. ¼ 1.1 F2P25 ¼ 3.8

IF3degr. ¼ 3.63 SF3degr. ¼ 3.28 (BPA) and 4 (PH) CF3degr. ¼ 5.5 (BPA) and 4.5 (PH) (standard solution) CF3degr. ¼ 6.3

(continued)

Xu et al. (2014)

Zhao et al. (2014)

1 Nanostructured Imprinted Supported Photocatalysts: Organic and. . . 19

Photocatalyst precursor

CdS

Imprinted matrix (O, organic; I, inorganic)

O

Table 1.2 (continued)

T: tetracycline C: salicylic acid (SA), oxytetracycline HCl (OTC-HCl) and ciprofloxacin (CIP)

Template molecule (T), competitive molecule (C), selective molecule (S), others (O)

Washed with a mixture of methanol– acetic acid (95:5v/v) using Soxhlet extraction.

Extraction method

Selectivity for photodegradation. Standard solution. Reusability.

Ground water. River water. Reusability.

Photocatalytic process

500 W Xe lamp (visible light) Ccatal. ¼ 1000 mg/ L

Reaction parameters silica source) displayed high adsorption capacity, fast kinetics, and high selectivity. In the presence of ten times of coexisting nontarget compounds the apparent rate constant for photodegradation of target estrone over the hybridized nanoparticles was about six times of that over net TiO2. A thermal-responsive surface molecularly imprinted photocatalyst of poly (N-isopropylacrylamide) (PNIPAm)-modified CdS/halloysite nanotubes (HNTs) was prepared using surface molecular imprinting technology and evaluated as a potential effective photocatalyst to selectively remove tetracycline (TC) existing in aquatic environments. Free radical polymerization of TC,

IF1degr. ¼ 1.29 SF3degr. ¼ 1.96 (CIP), 6.39 (SA), 2.34 (OTC-HCl)

Highlights on synthesis and comments on performance

(BPA) and 4.8 (PH) (river water) F3P25 ¼ 2.38 P25 competitiveness ¼ 3153 CFP25 ¼ 2,9 (BPA) CFP25 ¼ 2,7 (PH)

SI or IF or CF

Xing et al. (2013)

References

20 C. C. de Escobar and J. H. Z. dos Santos

O

ZnFe2O4ZnFe2O4

T: ciprofloxacin S: enrofloxacin and 5-sulfosalicylic acid

Under UV light irradiation.

Selectivity for photodegradation. Standard solution. Reusability.

200 W lamp (visible light) Ccatal. ¼ 1000 mg/ L Csubstrate ¼ 20 mg/ L

IF3degr. ¼ 2.0 SF3degr. ¼ 1.3 (ENR), 1.7 (SSA)

A magnetic imprinted photocatalyst with the antigen/antibody-like function was obtained by coupling the imprinted polymer onto ZnFe2O4 nanocrystals. Compared with non-imprinted ZnFe2O4/ PPy composite, imprinted

NIPAm, N,N0methylenebisacrylamide (MBA), and 2,20azobisisobutyronitrile (AIBN) was used as a molecular template, functional monomer, crosslinking agent, and initiator, respectively. Selectivity is dependent mainly on the difference in the molecular size and shape between the target and nontarget pollutants. The results showed that the photocatalytic activity of surface-imprinted photocatalysts could be tunable by changing the environmental temperature. Furthermore, the photocatalysts retained their activity after being used for three times.

(continued)

Wang et al. (2016)

1 Nanostructured Imprinted Supported Photocatalysts: Organic and. . . 21

Photocatalyst precursor

TiCl4 and P25

Imprinted matrix (O, organic; I, inorganic)

I

Table 1.2 (continued)

T: rhodamine B S and C: rhodamine 6G (Rh6G) and methylene blue (MB)

Template molecule (T), competitive molecule (C), selective molecule (S), others (O)

Ultrasoundassisted using methanol as the solvent (approximately 70 mL) Ultrasoundassisted using methanol as the solvent (approximately 70 mL).

Extraction method

Selectivity for adsorption. Competitiveness for adsorption. Selectivity for photodegradation. Competitiveness for photodegradation. Standard solution.

Photocatalytic process

125 W Hg vapor lamp (UV light) Ccatal. ¼ 550 mg/L Csubstrate ¼ 20 mg/ L

Reaction parameters

IF3degr. ¼ from 1.0 to 4.0 SF2P25degr. ¼ from 1.5 to 2.8 (Rh6G) and from 0.6 to 1.9 (MB) CF2degr. ¼ 0.8 (Rh6G) and 1.7 (MB) F2P25 ¼ from 0.24 to 1.15 CF2P25 ¼ ca. 1.5

SI or IF or CF

A series of MI photocatalysts were prepared from four different routes, namely, acid, basiccatalyzed, two-step, and non-hydrolytic sol–gel routes. Compared to the commercial photocatalyst (P25), an increase in selectivity (up to 180%) and competitiveness (up to 290%) was obtained. Regarding the acid route, the absence of HCl during the catalyst synthesis resulted in a loss of degradation compared to P25 and

ZnFe2O4/PPy composite not only had high photocatalytic efficiency but also possessed the strong selective ability to specifically recognize and preferentially degrade ciprofloxacin. The imprinted composite possesses excellent separation ability and satisfactory photocatalytic stability.

Highlights on synthesis and comments on performance

de Escobar et al. (2015)

References

22 C. C. de Escobar and J. H. Z. dos Santos

P25

P25

I

O

T: 2, 4-dinitrophenol S and C: 4-nitrophenol

T: atorvastatin, diclofenac, ibuprofen, tioconazole, valsartan, ketoconazole, and gentamicin

Washed sequentially with Na2CO3 solution (0.4 M) and water.

Heating (1  C/ min) 125 W Hg vapor lamp (UV light) Ccatal. ¼ 660 mg/L Csubstr. ¼ 20 mg/L

300 W xenon lamp (UV light) Ccatal. ¼ 1000 mg/ L Csubstrate ¼ 15 mg/ L

There is no competitiveness or selectivity tests. Comparison with P25. Standard solution.

Competitiveness for adsorption. Selectivity for photodegradation. Competitiveness for photodegradation. Reusability.

MI photocatalyst containing low TiO2 loading (7.00–16.60 mg/ L of TiO2) was prepared via an acidcatalyzed sol–gel route using different classes of pharmaceutical compounds as the template. In comparison to the commercial sample (P25), the authors have shown that degradation performances were enhanced from 48 from 5 to 427%. A water-compatible surface molecularly imprinted thiolfunctionalized titanium dioxide (TiO2) material was prepared in water using o-phenylenediamine (OPDA) as both functional monomer and cross-linker. Faster photodegradation activity for 2, 4-DNP compared with the non-imprinted samples.

IF2degr. ¼ from 1.3 to 12.7 F2P25 ¼ from 0.2 to 4.0

IF1ads. ¼ 4.5 CF1ads. ¼ 2.8 IF2degr. ¼ 1.5 IFdegr.3 ¼ 1.9 F2P25 ¼ 1.4 F3P25 ¼ 2.5

a threefold lower factor of competitiveness compared to the system in which HCl was used. In the regeneration tests, degradation was maintained at 80% of that of initial tests for up to three cycles.

(continued)

Zhou et al. (2018)

de Escobar et al. (2016)

1 Nanostructured Imprinted Supported Photocatalysts: Organic and. . . 23

Photocatalyst precursor

ZnFe2O4

P25

Imprinted matrix (O, organic; I, inorganic)

O

O (M)

Table 1.2 (continued)

T: methylene blue S: methylene orange

T: tetracycline S and C: enrofloxacin hydrochloride

Template molecule (T), competitive molecule (C), selective molecule (S), others (O)

Washed with a mixture of methanol, acetic acid, and distilled water

Deionized water into simulated sunlight irradiation (250 W xenon lamp)

Extraction method

Selectivity for photodegradation. Standard solution. Reusability.

Selectivity for photodegradation. Competitiveness for photodegradation. Reusability.

Photocatalytic process

10 W UV-C light

250 W xenon lamp (simulated sunlight irradiation) Ccatal. ¼ 200 mg/L Csubstrate ¼ 20 mg/ L

Reaction parameters Imprinted ZnFe2O4/Ag/ poly-3,4-ethylenedioxythiophene (PEDOT) was synthesized by the microwave polymerization method and surface imprinting technique. Due to the introduction of Ag/PEDOT into the surfaceimprinted layer, MI was able to enhance the photocatalytic activity based on Ag which acted as the mediator; meanwhile, owing to the existence of the imprinted cavity in the surface-imprinted layer, the selectivity had been significantly improved. MIP-TiO2 was synthesized by using a mixture of tetrahydrofuran as porogen, methacrylic acid as monomer, and N,N0 -methylenebis (acrylamide) as cross-linker.

IF2degr. ¼ 15 (powder form) SF2degr. ¼ 2.0 (powder form) F2P25 ¼ 0.6 (powder form)

Highlights on synthesis and comments on performance

IF2degr. ¼ 1.3 (selectivity test) SF2degr. ¼ 1.4 IF2degr. ¼ 1.3 (competitiveness test) CF2degr. ¼ 1.35 F2ZnFe2O4 ¼ 4.7 SF2ZnFe2O4 ¼ 3.6 ZnF2O4 competitiveness ¼ 5,0 CF2ZnF2O4 ¼ 3.7

SI or IF or CF

References

Melvin Ng et al. (2017)

Lu et al. (2017a)

24 C. C. de Escobar and J. H. Z. dos Santos

T: ciprofloxacin S: tetracycline

T: diclofenac (DIC) S: fluoxetine (FLU) and paracetamol (PARA)

CoFe2O4

P25 and Cu2Odoped P25

O

O

Washed with methanol containing 10% v/v acetic acid.

Under simulated sunlight irradiation (250 W xenon lamp) for 3 h with deionized water.

with a volume ratio of 8:1:1.

Selectivity for photodegradation. Standard solution.

Selectivity for photodegradation. Standard solution. Reusability.

Ultraviolet lamp (366 nm) Ccatal. ¼ 330 mg/L Csubtrate ¼ 18.3 mg/L

250 W xenon lamp (simulated sunlight irradiation) Ccatal. ¼ 1 mg/L Csubstrate ¼ 10 mg/ L

Then, the sample was immobilized in polysulfone (PSf) ultrafiltration membrane. It was able to enhance the separation capability of PSf membrane and create the selectively photocatalytic property on PSf membrane. Both powder and membrane with imprinted TiO2 were investigated. A stable core–shell imprinted Ag-(poly-ophenylenediamine)/ CoFe2O4 (imprinted Ag-POPD/CoFe2O4) was synthesized via the surface imprinting technique. POPD is a conductive polymer used as the functional monomer. MIP exhibited the superior specific recognition capability for selective photodegradation of ciprofloxacin. MIP based on precipitation method (methacrylic acid as monomer) was prepared with and without being doped with Cu2O. In contrast to the nonselective

IF2degr. ¼ 1.03 (MIP immobilized in membrane) SF2degr. ¼ 2.8 (MIP immobilized in membrane)

IF1ads. ¼ 1.43 IF2degr. ¼1.3 SF2degr. ¼ 1.5 F2CoF2O4 ¼ 5.4 SF2CoF2O4 ¼ 4.3

IF1ads.¼ 1.59 IF2degr. ¼1.38 (non-doped) SF2degr. ¼ 1.55 (FLU, non-doped photocatalyst)

(continued)

de Escobar et al. (2018)

Lu et al. (2017b)

1 Nanostructured Imprinted Supported Photocatalysts: Organic and. . . 25

Photocatalyst precursor

ZnO

Imprinted matrix (O, organic; I, inorganic)

I

Table 1.2 (continued)

T: danofloxacin mesylate S: ciprofloxacin and tetracycline

Template molecule (T), competitive molecule (C), selective molecule (S), others (O)

Washed with deionized water under a simulated sunlight

Extraction method

Selectivity for photodegradation. Standard solution.

Photocatalytic process

250 W xenon lamp simulated sunlight Ccatal. ¼ 200 mg/L

Reaction parameters commercial sample of TiO2, the average value of selectivity of the imprinted catalysts for photocatalytic degradation of DIC was estimated to be 2.8, which suggests that the specific binding sites created by the molecular imprinting are essential for gaining high catalytic selectivity and efficiency. After six cycles of testing under UV light, the imprinted catalysts are maintained.

A hollow capsule-like recyclable functional MI ZnO/C/ Fe3O4 was synthesized by employing the convenient

IF1ads. ¼ 1.35 IF2degr. ¼ 1.2 F2ZnO ¼1.4 SF2degr. ¼ 1.15

Highlights on synthesis and comments on performance

SF2degr. ¼ 2.84 (PARA, non-doped photocatalyst) F2P25 ¼ 1.54 (non-doped photocatalyst) IF2degr. ¼ 1.23 (FLU, doped photocatalyst) SF2degr. ¼ 7.96 (PARA, doped photocatalyst) F2P25 ¼ 2.6 (doped photocatalyst) SF2P25 ¼ 1.41 (FLU, non-doped photocatalyst) SF2P25 ¼ 0,63 (PARA, non-doped photocatalyst)

SI or IF or CF

Lu et al. (2016b)

References

26 C. C. de Escobar and J. H. Z. dos Santos

2

Measured from adsorption capacity, Q (mg/g) Measured from photodegradation degradation at final time of reaction 3 Measured from kinetic experiments TNBT tetrabutyl titanate

1

irradiation with the magnetic agitation under an air atmosphere

Csubstate ¼ 20 mg/ L (ciprofloxacin) SF2degr.¼ 1.18 (tetracycline) and mild solgel method with the combination of the surface imprinting technique, which possessed magnetic separation properties. An ultrathin C layer provided transfer of the photo-generated electrons coated on Fe3O4 and then to ensure that ZnO can uniformly grow on the surface.

1 Nanostructured Imprinted Supported Photocatalysts: Organic and. . . 27

28

C. C. de Escobar and J. H. Z. dos Santos

Fig. 1.2 Schematic diagram of the synthesis of the magnetic imprinted TiO2 photocatalyst (Reprinted from Lu et al. 2014. Copyright 2018 Elsevier)

Although several methods for molecularly imprinted matrixes have been investigated, it seems that there is a lack of study regarding the parameters that could positively affect the photocatalytic activity. For instance, for a specific template or a class of template (e.g., dyes or pharmaceuticals compounds), it is not clear yet which matrix would be better for preferential degradation. One interesting question is: if all process variables were kept the same (pH, temperature, substrate concentration), which kind of matrix would achieve better results? In this kind of studies, other factors may also need to be considered, such as cost and time for each choice. Another problem intrinsically to the choice of the matrix is the comparison with the commercial photocatalyst. One has to keep in mind that one disadvantage of the use of photocatalysts semiconductors (such TiO2 and ZnO) is their low selectivity. Thus—whenever it is feasible—it is necessary to estimate the amount of

1 Nanostructured Imprinted Supported Photocatalysts: Organic and. . .

29

Fig. 1.3 Schematic diagram of the synthesis of N–F codoped molecularly imprinted TiO2 (Reprinted from Wu et al. 2016. Copyright 2018 Elsevier)

photocatalyst precursor used in the prepared material during each photocatalytic test and compare with the commercial sample in terms of selectivity or competitiveness.

1.5.1.2

Photocatalyst Precursors

Most of the research done in the field of imprinted photocatalysis has used TiO2 as the photocatalyst. Typical precursors used are a commercial sample of TiO2 (such as DegussaP25) and tetrabutyl titanate. One reason for that is due to the decades of research showing that TiO2 is one of the most promising materials aiming at the degradation of pollutants (Schneider et al. 2014). However, some authors have explored other catalysts, such as CdS (Xing et al. 2013), ZnFe2O4 (Lu et al. 2017a), Co (Lu et al. 2016a), Fe2O3 (Fang et al. 2016), and ZnO (Lu et al. 2016a). In addition, molecularly imprinted combined with doped materials for applications in visible light have also been studied (Lu et al. 2013, 2014, 2016a; Wu et al. 2016; Wei et al. 2015; Liu et al. 2016; Lai et al. 2016; Wang et al. 2014; Luo et al. 2013; Xing et al. 2013). As commented above, molecularly imprinted inorganic framework such as TiO2/ SiO2 composites have been studied (Deng et al. 2014; de Escobar et al. 2015). For instance, Deng et al. (2014) prepared catalyst by a sol-hydrothermal method using tetrabutyl orthotitanate and tetraethyl orthosilicate as the precursor of functional monomer. Some drawbacks are associated with these approaches. Since the silica surface is quite inert, it is very difficult to synthesize highly dispersed metal oxides on the silica surface. In addition, the rate of hydrolysis of titanium alkoxide is usually faster than those of silicon alkoxides (Sen et al. 1999), thus inducing the precipitation

30

C. C. de Escobar and J. H. Z. dos Santos

of TiO2 and framework TiO2 that is formed if the titanium source is not carefully hydrolyzed. As an alternative to this drawback, highly dispersed surface TiO2 crystallites could also be achieved from inorganic titanium sources, such as TiCl4 (Gao and Wachs 1999). de Escobar et al. (2015) compared the use of P25 with TiCl4 as the precursor of TiO2 in a silica matrix. Selective degradation toward rhodamine B reached better values when using TiCl4, but P25 showed the best results for competitive photocatalysis.

1.5.1.3

Extraction Method

Several methods have been used for the extraction of the template in materials produced by the molecular imprinting concept. The most common one is washing the sample with a solvent which is chosen based on the molecule (solubility) to be extracted. In addition, ultraviolet light exposition has also been used. However, one concern regarding the latter is the possibility of polymer degradation, especially for organic matrices. Calcination is another possibility of extraction but is rather limited to temperature resistant materials, such as imprinted photocatalyst based on silica network. In this sense, structural and morphological characterization is necessary to prove the polymer integrity. Moreover, even for silica-imprinted materials, some concerns regarding heating conditions must be have taken into account in order to avoid the collapse of silica network and losses of surface area (de Escobar et al. 2015). During the extraction, some methods have been employed to show the extraction template. Common ways are using UV-Vis spectroscopy or high-performance liquid chromatography for molecule detection (Deng et al. 2014; Wu et al. 2016). After the extractions, CHN analysis has been used to show the removal of the template and surface area or pore measurements as indirect evidence of pore formations (Wang et al. 2016; Liu et al. 2016; Lu et al. 2016a).

1.5.1.4

Photocatalytic Process: Competitiveness Versus Selectivity

According to Table 1.2, either selectivity and/or competitiveness photocatalytic process have been explored. In the first case, the degradation is carried out with only one molecule, and normally the template and non-template molecule is evaluated (Xiao et al. 2015; Luo et al. 2014; Fang et al. 2016; Luo et al. 2013; Xu et al. 2014; Xing et al. 2013; Wang et al. 2016; de Escobar et al. 2015). According to Paz (2006), although limited, this kind of test is still valuable, especially for qualitative guidelines. In the case of competitiveness, there is a mixture (two or more molecules), and normally the template molecule is one of the chosen (Deng et al. 2014; Lu et al. 2013, 2014; Xiao et al. 2015; Shen et al. 2014; Zhao et al. 2014; Xu et al. 2014; de Escobar et al. 2015). Another approach is the evaluation of competitiveness for adsorption and then selectivity for photodegradation (Liu et al. 2016). In some cases, there are no competitiveness or selectivity tests (Arabzadeh et al. 2016; Huang et al. 2013; Wei et al. 2015; Wang et al. 2014). However, essays comparing bare

1 Nanostructured Imprinted Supported Photocatalysts: Organic and. . .

31

photocatalysts (e.g., P25) or non-imprinted were conducted in order to evaluated FP25 and IF. The majority of works have only tested samples with synthetic solutions. A minority have studied real samples (top or river water) (Huang et al. 2013; Xu et al. 2014). In this sense, more effort is necessary to simulate real conditions in which several compounds are present. One interesting investigation would be samples derived from secondary treatment to test the viability of selective degradation in real scenarios. The neat form of P25 is rather difficult to recycle. However, the use of molecularly imprinted photocatalyst provides a possibility to reuse the material, which is of great importance for its application in environmental technology. According to the recent literature, there have been explored two different ways to recycle, i.e., using cycles of regeneration (extraction of remaining template molecule and degradation products) (de Escobar et al. 2015; Wu et al. 2016, Wei et al. 2015, Luo et al. 2014, Lu et al. 2016a, Liu et al. 2016, Luo et al. 2014, Luo et al. 2013, Xu et al. 2014) or without regeneration or desorption processes (de Escobar et al. 2016; Huang et al. 2013).

1.5.1.5

Reaction Parameters

In recent years, there has been a growing concern to exploit the applications of photocatalysis into the visible region, thus allowing the use of solar energy (ShahamWaldmann and Paz 2016; Etacheri et al. 2015). In this respect, imprinted photocatalyst in principle could enhance the adsorption of the desired molecule and further be degraded pollutants using visible light sources. Fortunately, several works have been in regard to this approach (Liu et al. 2014; Lu et al. 2013, 2014, 2016a; Wu et al. 2016; Wei et al. 2015; Lai et al. 2016; Wang et al. 2014, 2016; Luo et al. 2013; Xing et al. 2013). Typically, the power of lamp used varied from 150 to 500 W with xenon, and tungsten lamp is the most common. It is well known that subtract and catalyst concentration is among the main physical parameters that influence reaction rate (r) (Herrmann 2010). For instance, the reaction rate is proportional to the mass of catalyst until it reaches a plateau due to the full absorption of photons. This is true not matter either the regime or design of the photoreactor. It turns out that molecularly imprinted photocatalyst can be optimized for the concentration of both catalyst and subtract. However, although there are some exceptions (Arabzadeh et al. 2016; Shen et al. 2014; Xing et al. 2013), most works have used fixed concentrations, and thus there is a lack of optimization that may impose difficulties for practical applications. For imprinted materials, the effect of molar ratio between template and monomers on the photocatalytic activity could be valuable information to design better photocatalyst for selectivity degradation. In addition, the effect of monomer during the protocol synthesis could influence the photocatalytic activity. Considering the results shown in Table 1.2, only a few works have studied those parameters (Lu et al. 2013, 2014, 2016a, Luo et al. 2014; Wang et al. 2016; Xing et al. 2013). In addition, some authors have explored the influence of different adding doses of functional

32

C. C. de Escobar and J. H. Z. dos Santos

monomer together with polymerization times on the photocatalytic activity (Lu et al. 2017b). Another issue related to the synthesized materials is the comparison with the photocatalyst precursor. One has to keep in mind that the imprinted photocatalyst contains both the precursor and the matrix. Typically, in cases in which P25 is chosen, a fixed mass of precursor is used. During the photocatalytic essays, the authors normally use the same mass of molecularly imprinted photocatalyst and for the bare P25. If the mass of molecularly imprinted photocatalyst is the same as P25 (used as a control), a direct comparison may be compromised once the first is composed by P25 and matrix. In this sense, a more reliable and honest comparison could be done if the percentage of the precursor in the imprinted photocatalyst is estimated. For instance, a test using 100 mg of a supported catalyst, bearing 10% Ti-wt., has to be compared to a test with P25 using an equivalent amount of Ti, i.e., 16,7 mg of commercial TiO2. For this purpose, characterization techniques such as thermogravimetric analysis, scanning electron microscopy with energy-dispersive X-ray spectroscopy, and atomic absorption spectrometry may be helpful. For instance, for samples containing organic matrices and P25, a rough estimation using thermogravimetric analysis could be easily done, once the reminiscent mass may be attributed to the P25. In cases of the inorganic matrix, a rough estimation is possible if one assumes that the prepared material contains homogeneous and welldistributed particles of TiO2 derived from P25 (de Escobar et al. 2016). The parameter IF is one important way to measure the extension in which the imprinted samples achieve higher results of degradation due to the presence of specific pores for a molecule in comparison with non-imprinted materials. Due to the nature of the interaction between the molecules and the pores, the authors also quantify IF from adsorption essays that can be evaluated both together or independently of photocatalytic essays. In the former case, some authors have evaluated IF from the so-called dark stage, which is characterized by solution in contact with the photocatalyst before the lamp be turned on. In this sense, de Escobar et al. (2016) have estimated an IFads. from 1.3 to 12.7 for silica-based molecularly imprinted photocatalyst designed for removal of pharmaceuticals compounds. The most common procedure to estimate IF is to carry out adsorption test before the photocatalytic essays. In doing so, it is possible to estimate values of maximum adsorption that is normally expressed in terms of the equilibrium concentration of a solute on the surface of an adsorbent (qe). This information can be used to calculate IF according to Eq. 1.1. According to Table 1.2, the authors have reported IFads. ranging from 1.16 (Luo et al. 2013) for TiO2/WO3 to 4.5 (Zhou et al. 2018) watercompatible molecularly imprinted photocatalyst. In the case of a photocatalytic reaction, IFdegr. can be either estimated from selectivity or competitiveness essays. For instance, from the study of Lu et al. (2017), it is possible to calculate that IFdegr. for both cases are equal to 1.3. In the investigation of molecularly imprinted photocatalyst for diclofenac degradation (de Escobar et al. 2018), values of IF of 1.38 and 1.23 for non-doped and doped P25, respectively, in selectivity photodegradation were calculated. In the evaluation of molecularly imprinted photocatalyst for diclofenac degradation (de Escobar et al.

1 Nanostructured Imprinted Supported Photocatalysts: Organic and. . .

33

2018), both IFads. and IFdegr. were higher than unity during and after six cycles of reuse, indicating that the imprinted sites in molecularly imprinted photocatalyst were reserved and capable of molecular recognition after each regeneration. As a general trend, according to Table 1.2, the majority of works herein discussed have shown IFdegr. higher than 1.0, irrespectively of selectivity or competitiveness essays. Considering only the degradation stage of the photocatalytic reaction, the average IF from the results of Table 1.2 were estimated in ca. of 1.9. In a similar fashion as IF, it can be seen that the parameter CF for both selectivity and competitiveness is higher than unity. Taking together, these results suggest that the presence of imprinted sites in the molecularly imprinted photocatalyst could enhance the ability of recognition and degradation toward the desired molecule. One way to evaluate comparisons with commercial samples is to estimate FP25. Values higher than unity suggest a positive effect of imprinted sites present in the photocatalyst. For instance, Xu et al. (2014) compared the molecularly imprinted photocatalyst based on P25 in a single component system for the degradation of estrone (template molecule) showing that FP25 ¼ 2.38. Shen et al. (2014) calculated FP25 ¼ 2.4. One must keep in mind that in calculating FP25, the authors are following Eq. 1.2. Although limited, photocatalytic essays comparing FP25 could be useful for preliminary information about the viability of photocatalyst combined with molecularly imprinted. In this sense, another way to compare the results with P25 (or another reference sample) is to use Eq. 1.4, in which the degradation of the nontarget is considered. In this sense, de Escobar et al. (2018) have estimated SFP25 equal to 1.41 (in comparison with nontarget fluoxetine). Using CoF2O4 as a reference photocatalyst, Lu et al. (2017b) have estimated SFCoF2O4 higher than 4.0. In addition, using ZnFe2O4 (Lu et al. 2017a) as a reference, it was found that SFZnFe2O4 was higher than 3.5. One of the main objectives to design molecularly imprinted photocatalysts is to increase selectivity toward a desirable molecule. Considering that TiO2 is one of the most studied photocatalysts, it is natural that many authors have focused attention to improve selectivity by combining Ti moieties with imprinted cavities. It may be argued that the competitive photocatalytic essays using those new materials synthesized by MI techniques are not enough to prove to enhance selectivity when compared to the commercial samples of TiO2. It turns out that bare TiO2 itself should also be evaluated in multicomponent tests. Thus, compared to the molecularly imprinted photocatalysts, bare TiO2 samples (or some other corresponding catalyst precursors) should present lower degradation values. For these calculations, both Eqs. 1.6 or 1.7 may be used. According to Table 1.2, only a minority of authors have carried out such comparisons (Shen et al. 2014; Xu et al. 2014; de Escobar et al. 2015; Lu et al. 2017a). For instance, from the results of Shen et al. (2014) of a study for degradation of nitrophenol, it was possible to estimate that the molecularly imprinted photocatalyst could enhance the degradation of the target molecule 4.4fold in comparisons of bare P25. In the competitiveness photodegradation of dyes (de Escobar et al. 2015), a CFP25 was estimated to be 1.5. As discussed above, P25 is not the only photocatalyst precursor used to develop molecularly imprinted photocatalyst. In this sense, Lu et al. (2017a) compared the performance of both

34

C. C. de Escobar and J. H. Z. dos Santos

bare ZnFe2O4 and molecularly imprinted photocatalyst based on ZnFe2O4 in competitiveness essays. It was shown that the molecularly imprinted photocatalyst can enhance five times the photodegradation.

1.5.2

Characterization of Photocatalysts

Molecularly imprinted photocatalyst can be characterized by several methods that are normally employed in catalyst field. As shown in Fig. 1.4, the methods can be classified into four classes: elemental, textural, structural, and morphological.

Fig. 1.4 Methods employed for characterization of molecularly imprinted photocatalyst

1 Nanostructured Imprinted Supported Photocatalysts: Organic and. . .

1.5.2.1

35

Elemental Characterization: Inductively Coupled Plasma Optical Emission Spectrometry and CHN Analysis

A few authors have used coupled plasma optical emission spectrometry for the characterization of molecularly imprinted photocatalyst. This determination could be important to evaluate the metal site content in the catalyst and subsequently to calculate the catalytic activity. For instance, for the preparation of magnetic imprinted TiO2 photocatalyst, Lu et al. (2014) have demonstrated that, in comparison with the non-imprinted photocatalyst, the content of Fe3O4, SiO2, and TiO2 of the imprinted photocatalyst had been remarkably reduced after coating the surfaceimprinted layer. In the synthesis of molecularly imprinted photocatalyst for the photocatalytic degrading of ciprofloxacin, coupled plasma optical emission spectrometry was used to demonstrate that the Co2+ was doped in the Ti(OH)4 xerogel layer surface (Lu et al. 2016a). Elemental CHN analysis has been investigated for some authors. In the study described above (magnetic imprinted TiO2) (Lu et al. 2014), the authors have shown the presence of nitrogen in the imprinted photocatalyst, thus suggesting the presence of residue of the template during the synthesis. Huang et al. (2013) used CHN analysis to study the loading of the polymer for both molecularly imprinted photocatalyst and non-imprinted photocatalyst. From the results, it was calculated that the loading of active photocatalytic precursor (P25) in both imprinted and non-imprinted photocatalysts was the same during the photodegradation.

1.5.2.2

Textural Characterization: Specific Area, Pore Volume, Pore Diameter, and Small-Angle X-Ray Scattering

The specific area is one of the most important textural characterizations. In the field of photocatalysis, it is normally assumed that surface area is connected with adsorption and photocatalytic abilities of photocatalysts (Liu et al. 2016). In the literature, the most common method used is the Brunauer–Emmett–Teller (BET) method. Together with specific area, pore volume and pore diameter are often estimated. As can be seen from Table 1.3, most of the authors have related higher values of specific area and pore volume for molecularly imprinted photocatalyst in comparison with non-imprinted photocatalyst (Liu et al. 2016; Wang et al. 2016; Deng et al. 2014; Fang et al. 2016; Lu et al. 2013, 2016a, 2017b; Luo et al. 2013; Wang et al. 2016; Wu et al. 2016; de Escobar et al. 2015; Melvin Ng et al. 2017). The increase of area observed for molecularly imprinted photocatalyst varies from a small such 9% (Liu et al. 2016) to a great one such as 805% (Lu et al. 2016a). This trend is noted either for the inorganic or organic matrix (Deng et al. 2014; Wang et al. 2016; de Escobar et al. 2015). The authors have suggested that this higher value of specific area probably results from the presence of specific recognition sites after the template molecules have been removed from the polymer, i.e., the generation of unique three-dimensional imprinted cavities. An exception is for

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C. C. de Escobar and J. H. Z. dos Santos

Table 1.3 Specific area, pore volume, and pore diameter for the studied molecularly imprinted photocatalyst (MIP) and non-imprinted photocatalyst (NIP)

MIP MIP/Co–TiO2 nanocomposites ZnFe2O4/polypyrrole composite TiO2/SiO2 nanocomposite Fe2O3 composite TiO2/graphene Codoped TiO2 nanocomposites Photocatalysts based on fly-ash cenospheres Co2+ ion-doped Ti(OH)4 xerogel supported by magnetic fly-ash TiO2/WO3 S-doped TiO2 ZnFe2O4/polypyrrole composite N-F codoped TiO2 Silica-based photocatalyst Water-compatible photocatalyst Ag-(poly-o-phenylenediamine)/ CoFe2O4

MIP NIP MIP NIP MIP NIP MIP NIP MIP NIP MIP NIP MIP NIP MIP NIP MIP NIP MIP NIP MIP NIP MIP NIP MIP NIP MIP NIP MIP NIP

Specific area (m2/g) 51.7 47.4 173.9 141.6 186.4 141.6 11.6 5.2 124 122 51.7 47.4 22.5 17.5 199.0 22.3 101.0 71.0 58.0 60.0 173.9 141.6 41.9 36.4 634.5 463.4 40.1 39.4 199.0 9.4

Pore volume (cm3/g)

Pore diameter (nm)

References Liu et al. (2016) Wang et al. (2016)

0.36 33 0.129 0.081

7.2 7.2 48.0 36.9

Deng et al. (2014) Fang et al. (2016) Lai et al. (2016) Liu et al. (2016)

0.03 0.02 0.13 0.07

4.9 3.8 2.5 13.8

Lu et al. (2013) Lu et al. (2016a) Luo et al. (2013) Wang et al. (2014) Wang et al. (2016) Wu et al. (2016)

0.55 0.29

30.9 28.9

de Escobar et al. (2015) Zhou et al. (2018)

2.6 28.9

Lu et al. (2017b)

S-doped TiO2, in which the authors noted that the specific areas for molecularly imprinted photocatalyst were a little smaller than that non-imprinted nanocomposite (Wang et al. 2016). In another case, the authors have not found any difference between the values of imprinted and non-imprinted samples (Zhou et al. 2018). An increase of specific area has also been observed for molecularly imprinted material design for adsorption. For instance, silica-based materials were prepared by the acid-catalyzed sol–gel method using different pharmaceuticals (fluoxetine, gentamicin, lidocaine, morphine, nifedipine, paracetamol, and tetracycline) as a template. The authors measured the area for the encapsulated material and compared the

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change of area after the extraction process. After solvent extraction, the specific area of all the systems increased between 120% and 270%, thus indicating the formation of cavities that produce an imprinted material (Morais et al. 2012). Similar results were reported for silica imprinted with rhodamine B used as a template for five different sol–gel routes (de Coelho Escobar and dos Santos 2014). Regarding pore diameter, the authors have reported conflicting results. For instance, the estimated pore diameter for Co+2-doped photocatalyst was ca. of fivefold higher than non-imprinted photocatalyst (Lu et al. 2015), and the estimated diameter for molecularly imprinted Fe2O3 composite was 1.3 higher than the respective non-imprinted. However, the results obtained from other synthesis protocols seem to suggest that the pore diameter is not affected by the presence of the template, i.e., the values of pore diameter of molecularly imprinted and non-imprinted photocatalysts are quite the same (Deng et al. 2017; de Escobar et al. 2015). Although scarce, small-angle X-ray scattering has been used for characterization molecularly imprinted photocatalyst. Small-angle X-ray scattering can provide information about sample geometry and fractal nature by utilizing the structure formed by the organizational levels composed of a Guinier region and a Potency Law. The former provides an estimation of the Guinier radius of gyration, while the latter provides details about the system organization (P values). For instance, molecularly imprinted photocatalyst based on silica matrix has been investigated by small-angle X-ray scattering, showing that formation of pores in silica-based materials is related to the aggregation of primary particles (de Escobar et al. 2015). Comparing the imprinted systems to the non-imprinted systems, the former showed higher P values (de Escobar et al. 2016). According to the authors, this result indicates that the presence of template increases the material roughness. In addition, it was found that radius of gyration for imprinted systems was higher than for non-imprinted systems and that radius of gyration was statistically correlated with adsorption and degradation. This suggests that the presence of molecular recognition sites could be linked to higher radius of gyration values, which is supported by a lack of correlation with non-imprinted systems (de Escobar et al. 2015).

1.5.2.3

Structural Characterization: Fourier-Transform Infrared Spectroscopy, Zeta Potential, Differential Reflectance Spectroscopy, X-Ray Diffraction, and X-Ray Photoelectron Spectroscopy

Fourier-transform infrared spectroscopy has been widely used to primarily to prove the interaction of the matrix with the template or the photocatalyst precursor. In a preparation of imprinted TiO2/WO3-coated magnetic Fe3O4@SiO2 nanocomposites (Wei et al. 2015), the authors have characterized the material indicating an occurrence of a SiO2 layer on the surface of the Fe3O4 particles. For the synthesis of thermal-responsive surface imprinted of poly(N-isopropylacrylamide) (PNIPAm)modified CdS/halloysite nanotubes (HNTs) (Xing et al. 2013), the presence of deformation of Al–O–Si, Si–O–Si, and O–H groups of the inner hydroxyl groups

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(at 536 cm1, 1031 cm1, and 909 cm1, respectively), as well the peaks at 1650 cm1, 1528 cm1 (secondary amide C¼O stretching), and 1390 cm1 (deformation of methyl groups on –C(CH3)2), was shown that was attributed to the characteristic peaks of poly(N-isopropylacrylamide). For water-compatible molecularly imprinted (Zhou et al. 2018), TiO2 materials have been functionalized by 3-mercaptopropyltrimethoxysilane (Zhou et al. 2018). In this sense, Fourier-transform infrared spectroscopy was used to show both the presence Ti-O skeletal vibration and S-H bond stretching vibration, at 661 cm1 and 2561 cm1, respectively, thus showing that SH group was successfully grafted onto the surface of TiO2. Results also indicated that molecularly imprinted photocatalyst layer had a structure of the polymer o-phenylenediamine (used as functional monomer). In order to prove that acrylamide and trimethylolpropane acrylate were filled in the surface of NaCl/TiO2, Fourier-transform infrared spectroscopy was conducted, showing a stronger band at 3371 cm1, which belonged to the peaks of OH stretching vibrations and NH2 stretching vibrations. In the synthesis of TiO2/fly-ash cenospheres (Lu et al. 2013), it was shown that the peaks near of 500–600 cm1 may be due to the inorganic oxide TiO2 absorption. In the light of other peaks, it was argued that it is an indication that the imprinted polymer was successfully coated on the surface of POPD/TiO2/fly-ash cenospheres. In a similar way, synthesized magnetic nanoparticles showed a strong peak at 1089 cm1 of Si-O-Si in the Fe3O4@SiO2@TiO2/WO3 and Fe3O4@SiO2 samples, indicating the occurrence of a SiO2 layer on the surface of the Fe3O4 particles (Wei et al. 2015). In the syntheses of Ag/poly-3,4-ethylenedioxythiophene/ZnFe2O4, the results revealed absorption peaks that suggest the surface-imprinted layer was successfully formed (Lu et al. 2017a). Similarly, in the preparation of molecularly imprinted photocatalyst, TiO2 (Melvin Ng et al. 2017), the presence of peaks ascribed to C¼O stretch, C-H stretches, and N-H vibrations was able to show that N,N0 -methylenebis (acrylamide) (cross-linker) and methacrylic acid (functional monomer) had successfully grafted on TiO2 nanoparticles. In the preparation of molecularly imprinted TiO2 photocatalysts for degradation of diclofenac in water based on an organic polymer network (de Escobar et al. 2018), it was shown that the Fourier-transform infrared spectroscopy spectra of the materials after six cycles of use were nearly identical to that of the fresh sample. This indicates that the structure of the polymers, and presumably the imprinted sites, was not destroyed over several cycles of reaction under the action of ultraviolet light. Zeta potential is a parameter used for quantifying the electrical potential of the solid particle surface and is normally investigated in different pH, and the isoelectric point—the pH value at which the zeta potential is approximately zero—can be estimated. The surface is positively charged when the pH medium is lower than isoelectric point (suggesting a higher affinity for anions) and is negatively charged when pH is higher than isoelectric point. Typically, a surface charge may be one of the reasons for the different adsorption capacities. For instance, adsorption tests were carried out at pH ¼ 6.0 using a surface molecularly imprinted TiO2/graphene

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photocatalyst (Lai et al. 2016). In this study, the target molecule (bisphenol A) has negative charge in the studied pH; therefore it can be more easily attracted to the positively charged surface of molecularly imprinted photocatalyst that has shown an isoelectric point of 7.55, while non-imprinted photocatalyst the isoelectric point was 3.55. Similar characterization was investigated using a sol-hydrothermal synthesis of inorganic framework molecularly imprinted TiO2/SiO2 nanocomposite (Deng et al. 2014) and for water compatible imprinted for degradation of 2,4-dinitrophenol (Zhou et al. 2018). Another approach is to measure the zeta potential in the same liquid medium in which photocatalysis is carried out and then analyze possible relationships between other parameters. In this sense, in a study using the sol–gel route for the preparation of molecularly imprinted silica-based materials, a strong Spearman correlation between photocatalytic degradation and zeta potential for non-imprinted systems was found (de Escobar et al. 2015). Considering that this correlation was not observed for the imprinted systems, the authors concluded that surface charge played a more important role in the photodegradation of materials without molecular imprinting. For imprinted materials, other characteristics, such as the presence of pore volume, may have been more influential on the higher observed degradation results for molecularly imprinted photocatalyst. Differential reflectance spectroscopy is usually carried out for the study of light absorption. In the selective degradation of ciprofloxacin by NaCl/TiO2 imprinted photocatalyst (Liu et al. 2014), a redshift in the absorption edge when compared with non-imprinted photocatalyst was observed. According to the, this result due to the process of molecularly imprinted, which introduced some organic substances and thus reducing the band gap. In other studies, imprinted samples have shown a redshift, thus indicating that the organic polymer produced via the polymerization of phenylenediamine under ultraviolet irradiation can effectively enhance the absorbance of light. The redshift of molecularly imprinted photocatalyst indicates that the organic polymer with a polyaniline-like structure produced via the polymerization of o-phenylenediamine (functional monomer) under ultraviolet irradiation can effectively enhance the absorbency of light. In addition, the redshift of absorption edge means the decrease of the band gap of the photocatalyst. According to the authors, this information helps to explain the higher photocatalytic activity observed for imprinted than the non-imprinted photocatalyst. Similar results were found for several other authors (Liu et al. 2016; Xing et al. 2013; Zhou et al. 2018). Comparing differential reflectance spectroscopy results of N–F codoped, imprinted photocatalyst, non-imprinted photocatalyst, and TiO2 (Wu et al. 2016), it was shown that, although TiO2 did not show absorption in the visible region, the results of non-imprinted and imprinted were quite similar, because both contain doped F that narrow the band gap of TiO2 and induce the enhancement of the visible light absorption. However, the results revealed that the addition of template molecules had no obvious effect on light absorption. In the case of doped molecularly imprinted photocatalyst, a shift is also expected to the longer wavelength. In the synthesis of imprinted/Co–TiO2, it was shown an absorption in the visible region by imprinted in comparison with TiO2 nanoparticles.

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As discussed by the authors, the doping of Co element could decrease the band gap of the photocatalysts and improve the amount of photoelectron (Liu et al. 2016). Several other studies have used differential reflectance spectroscopy characterization to provide similar results (Lu et al. 2016a; Wang et al. 2014; Luo et al. 2014). The band gap is usually estimated by differential reflectance spectroscopy (Luo et al. 2014). Normally, the narrower band gap is, the higher photocatalytic activity of the imprinted photocatalyst is. For instance, Lu et al. (2013) have calculated that band gap based on fly-ash cenosphere was 2.16 eV, lower than non-imprinted ones. It was attributed due to the modification of o-phenylenediamine on the surface of TiO2/fly-ash cenospheres. Similar calculations were done in other studies (Xing et al. 2013; Lu et al. 2014; Wang et al. 2014). In the investigation of magnetic TiO2-molecularly imprinted photocatalyst based on enrofloxacin hydrochloride as the template molecule and methyl methacrylate as the functional monomer, a matrix TiO2@SiO2@FeO3 (TSF) was prepared and compared with non-imprinted samples. Compared with P25, the absorption intensity of TSF was significantly increased in the ultraviolet and visible light range, indicating that the absorption of TSF could be effectively modified and extended by the introduction of Fe3O4 and SiO2. Moreover, the absorption intensity of molecularly imprinted photocatalyst was lower than that of TSF. As commented by the authors, the surface-imprinted layer may have covered the active sites of TiO2, which made the light could not completely reach the surface of the TiO2. In addition, although there was a decrease in the absorption intensity for imprinted in comparison with TSF, this decrease was not great due to the presence of methyl methacrylate (functional monomer) in the imprinted. X-ray diffraction analysis is mostly used to show the presence of crystalline structures of the photocatalyst precursor. Once that the majority of studies is still limited to TiO2, three main crystalline structures are investigated: rutile, brookite, and anatase, and the latter shows the high photocatalytic activity in the photodegradation of most pollutants due to its low recombination rate of photogenerated electrons and holes (Sajjad et al. 2009; Li et al. 2008). For instance, in the sol-hydrothermal synthesis of inorganic framework MI SiO2/SiO2 nanocomposite (Deng et al. 2014), the X-ray diffraction matched well with the standard anatase pattern, which were represented by the peaks at 2θ ¼ 25.488 , 38.556 , 48.135 , 54.095 , 55.17 , and 62.71 that are ascribed to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), and (2 0 4) reflections, respectively. The authors have demonstrated that the crystallinity of molecular imprinted was better than that of non-imprinted, suggesting that the addition of the template resulted in enhancement of TiO2 crystallinity. In other studies, it was shown that no remarkable differences among imprinted and non-imprinted were found, thus suggesting that the presence of molecularly imprinted polymers had no effect to crystal structure (Liu et al. 2016; Wu et al. 2016). To show the effect of calcination on the formation of crystalline phase, Xu et al. (2014) demonstrated that the deposited TiO2 did not show anatase crystal form during the absence of heating for the preparation of imprinted-TiO2 hybridized magnetic Fe3O4 nanoparticles.

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Several other authors have employed X-ray diffraction analysis for similar investigations discussed above (Lai et al. 2016; Liu et al. 2014; Luo et al. 2013; Wang et al. 2014; Xu et al. 2014; Zhao et al. 2014; de Escobar et al. 2016, 2018; Zhou et al. 2018). Molecularly imprinted photocatalyst containing other materials than TiO2 precursor has also been investigated by X-ray diffraction. In this sense, the presence of peaks attributed to Fe3O4 was observed in the synthesis of imprinted TiO2/WO3-coated magnetic Fe3O4@SiO2 nanocomposite. In the preparation of imprinted nano α-Fe2O3 (Fang et al. 2016), it was found that diffraction peaks were identical to those in the standard X-ray diffraction spectrum of α-Fe2O3 and that the material was not contaminated during the synthesis process, thus showing were high purity. In the use of CoFe2O4 as a photocatalyst precursor, it was showed diffraction peaks attributed to cubic CoFe2O4 with a spinel structure (Lu et al. 2017b). The presence of cuprous oxide was detected by X-ray diffraction in the Cu2Odoped TiO2 imprinted (de Escobar et al. 2018). By using X-ray diffraction analysis, it was possible to demonstrate that Ag had been successfully loaded on poly-3,4-ethylenedioxythiophene (PEDOT) (functional monomer) in the synthesis of imprinted ZnFe2O4/ Ag/PEDOT. A similar conclusion was obtained in which Ag had successfully loaded on poly-o-phenylenediamine (functional monomer) (Lu et al. 2017b). The average crystallite sizes of anatase can be calculated by applying the Debye– Scherrer formula. This information provides the authors with some useful conclusions. For instance, Wu et al. (2016) have shown that the crystalline size of the samples slightly decreased with the involvement of template molecules. Some authors have reported that average crystallite was consistent with the scanning electron microscope and transmission electron microscopy (images (Wang et al. 2014; Xing et al. 2013). In addition, smaller crystalline size could be favorable for the improvement of photocatalytic activity (Bhosale et al. 2014). X-ray photoelectron spectroscopy has been used as a complementary surface characterization. In the synthesis of molecularly imprinted polymer-coated codoped TiO2 nanocomposites (Liu et al. 2016), it was shown that X-ray photoelectron spectroscopy spectrum presented peaks attributed to Ti 2p3/2, Ti 2p1/2, and a binding energy located at 781 eV assigned to the Co 2p, thus indicating that Co element was present as Co2+ in the imprinted samples. A similar investigation was carried out in the synthesis of Co2+ ion doped onto the surface of Ti(OH)4 xerogel supported by magnetic fly-ash (Lu et al. 2016a). X-ray photoelectron spectroscopy was also used to conclude that Fe3O4 and ZnO were both successfully formed in the functional imprinted ZnO/C/Fe3O4 for selectively photodegrading danofloxacin mesylate (Lu et al. 2016b). Similarly, the presence of Ag in the preparation of Ag/poly-3,4ethylenedioxythiophene/ZnFe2O4 photocatalyst was showed from the existence of peaks attributed to Ag 3d5/2 and Ag 3d3/2 (Lu et al. 2017a). In the study of molecularly imprinted photocatalyst TiO2/WO3-coated magnetic nanocomposite for photocatalytic degradation of 4-nitrophenol (Wei et al. 2015), it was revealed that the outermost layer of the final composite consists mainly of TiO2 and that X-ray photoelectron spectroscopy results were used to suggest that a threelayer core-shell composite was successfully prepared. Fang et al. (2016) studied molecularly imprinted photocatalyst containing α-Fe2O3 by using X-ray photoelectron spectroscopy analysis. They showed that

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the binding energy of Fe 2p decreased 0.1 eV after the photocatalysis essays, thus indicating an increase in electron density. Also, the authors noted that the binding energy of O 1 s increased by 0.1 eV, indicating that the electron density of O 1s falls after photocatalysis. Taking into account these findings, it was suggested that the interaction between methylene blue (template molecule) and α -Fe2O3 was from a complexation adsorption. In the preparation of N–F codoped molecularly imprinted photocatalyst, Wu et al. (2016) have used X-ray photoelectron spectroscopy analysis for qualitative and quantitative analysis. It was shown that the peaks of binding energy at around 399.3 and 401.4 eV mainly resulted from nitrogen as O–Ti–N and Ti–N–O in the TiO2 lattice, respectively. Then, it was estimated that N and F atomic contents from the X-ray photoelectron spectroscopy were 0.65% and 2.49%, respectively, thus suggesting the presence of band gap of TiO2 that induced the enhancement of the visible light absorption. These results supported the findings of the degradation of the target molecule-simulated solar light.

1.5.2.4

Morphology: Scanning Electron Microscope, Transmission Electron Microscopy, Field Emission Scanning Electron Microscopy, and High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy

Molecularly imprinted photocatalyst and non-imprinted photocatalyst have been compared by using scanning electron microscope analysis. For instance, in the investigation of imprinted based on fly-ash cenospheres (Lu et al. 2013), it was demonstrated that after coated TiO2 and imprinted layer, the shape of imprinted samples is still kept spherical in comparison with non-imprinted samples. In the synthesis of imprinted/Co–TiO2, the sizes of molecular imprinted photocatalyst and non-imprinted nanocomposites, it was shown that larger particles were obtained if compared to the control system of Co–TiO2 nanoparticles, probably because of the polymer on the surface of imprinted photocatalyst. In the synthesis of Ag/poly-3,4ethylenedioxythiophene/ZnFe2O4 molecular imprinted, after coating the surfaceimprinted layer, an additional inhomogeneous covering layer was clearly observed on the surface of ZnFe2O4, thus indicating the presence of surface-imprinted layer (Lu et al. 2017a). In the selective degradation of ciprofloxacin with modified NaCl/TiO2 (Liu et al. 2014), scanning electron microscope images revealed that the obtained molecularly imprinted photocatalyst was spherical in shape. Using transmission electron microscopy analysis, it was further revealed that molecularly imprinted photocatalyst showed relatively rough and well-clustered micro-islands, suggesting the formation of imprinting layer. A similar conclusion was drawn from Wang et al. (2016), in which with the assistance of energy-dispersive X-ray spectroscopy mapping results it was confirmed that the imprinted ZnFe2O4/polypyrrole composite was successfully obtained. In developing materials for selective mineralization of low-level chlorophenols, molecularly imprinted photocatalyst was prepared using substrate

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analog as a template (Huang et al. 2013). By using transmission electron microscopy analysis, it was shown that molecularly imprinted photocatalyst particles had a coreshell structure. In the synthesis of surface molecularly imprinted TiO2/graphene, energydispersive X-ray spectroscopy analysis was used to confirm the existence of o-phenylenediamine (used as functional monomers) in the molecularly imprinted photocatalyst. According to the authors, the energy-dispersive X-ray spectroscopy analysis results conformed to the scanning electron microscope images (Lai et al. 2016). Energy-dispersive X-ray spectroscopy was used to show the presence of Ti atoms from a TiO2 precursor in the preparation of molecularly imprinted TiO2 photocatalysts for degradation of (de Escobar et al. 2018). The mutual combination of these techniques can also be found in several other works (Xing et al. 2013; Xiao et al. 2015; Melvin et al. 2017; de Escobar et al. 2018). In the study of water-compatible molecularly imprinted, TiO2 materials have been functionalized by 3-mercaptopropyltrimethoxysilane (Zhou et al. 2018). In this sense, energy-dispersive X-ray spectroscopy was used to show the existence of a sulfur element, thus indicating the successful preparation of TiO2-SH. In the synthesis of imprinted Fe2O3, Fang et al. (2016) have shown by using transmission electron microscopy images that the pores rise along with increasing the molar ratio of the template (methylene blue). In other studies, transmission electron microscopy and high-resolution transmission electron microscopy images have been used to show that the mean diameter obtained is consistent with other techniques, such X-ray diffraction (Deng et al. 2014; Lu et al. 2016b). However, some authors (Liu et al. 2016; Wu et al. 2016) have reported that particle sizes in the scanning electron microscope are larger than that in the computed results of X-ray diffraction, and that could be attributed to aggregates of the particles. In a similar way to Fourier-transform infrared spectroscopy analysis discussed before, scanning electron microscope has been used to prove information about the stability of the photocatalyst under several cycles of reaction (de Escobar et al. 2018). For instance, in the evaluation of molecularly imprinted photocatalyst for selective degrading of ciprofloxacin, it was demonstrated that after five cycles, the morphology of imprinted samples is nearly the same as that of the original sample (Lu et al. 2016a). In the preparation of core-shell bio-affinity chitosan–TiO2 composite, atomic force microscopy was employed for the surface structure and roughness study (Xiao et al. 2015). It showed an increase of roughness for molecular imprinted photocatalyst samples in comparison with blank samples devoid of TiO2, thus illustrating that TiO2 was successfully coated on the surface of chitosan particles. In a recent study (Zhou et al. 2018), the morphology of molecularly imprinted photocatalyst was analyzed in comparison with non-imprinted by using field emission scanning electron microscopy. It was shown that both were very similar to each other, and it was attributed mainly due to the fact that the coating on TiO2 was too thin. In the characterization of hollow capsule-like recyclable functional ZnO/C/Fe3O4 (Lu et al. 2016b), high-angle annular dark-field scanning transmission electron

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microscopy was employed. It showed the presence of the elements of C, O, Fe, Zn, F, and N. According to the authors, the presence of F and N was caused by danofloxacin mesylate (template molecule) that also existed in the imprinted ZnO/C/ Fe3O4 without elution (non-extracted sample).

1.6

Final Remarks

In conclusion, based on some of the latest, the development of molecularly imprinted photocatalyst is a useful approach to overcome the lack of selectivity in the degradation of pollutants. Excepted for the intrinsic limitation, the use of either organic or inorganic is useful, and more research is a need in order to compare, in the same conditions, which one would be better. Moreover, further works should investigate with more frequency comparison between bare TiO2 and molecularly imprinted photocatalyst conducted in multicomponent tests. In addition, real samples need to be explored in the context of preferential photodegradation. Furthermore, the use of another photocatalyst instead of TiO2 precursor has been explored by some authors, but there still are more possibilities to be studied. The synthesized molecularly imprinted photocatalyst has been characterized by several methods based on the elemental, textural, structural, and morphological analysis. Elemental analysis seems to have been used in little frequency but is one useful way to evaluate the metal site content in the catalyst and subsequently to calculate the catalytic activity. Together with X-ray diffraction, the specific area is one of the most techniques used. The latter has been used to show that, compared with non-imprinted photocatalyst, the increase of area in the molecularly imprinted photocatalyst is due to the extraction of the template molecule. As structural technique, X-ray diffraction has been useful to provide an easy way to show the presence of both photocatalyst precursor and doping material in molecularly imprinted.

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Chapter 2

Supporting Materials for Immobilisation of Nano-photocatalysts R. Goutham, R. Badri Narayan, B. Srikanth, and K. P. Gopinath

Contents 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Challenges in Developing Photocatalytic Water Treatment Systems and Need for Immobilisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Matrices for Immobilisation of Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Carbon Nanotubes and Graphene Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.4 Clay and Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.5 Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.6 Other Uncommonly Used Supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Common Methods of Immobilisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1 Dip Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Cold Plasma Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3 Polymer-Assisted Hydrothermal Decomposition (PAHD) . . . . . . . . . . . . . . . . . . . . . . . . 2.4.4 RF Magnetron Sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.5 Photo-Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.6 Solvent Casting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.7 Electrophoretic Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.8 Spray Pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.9 Sol-Gel Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract This paper provides a critical review on the application of various supporting media for immobilising commonly used photocatalysts for degrading organic pollutants. The immobilisation of photocatalysts can exclude expensive and infeasible post-treatment recovery of spent photocatalysts at a large scale. Certain usually implemented immobilisation aids such as zeolites, clay and ceramics,

R. Goutham · R. Badri Narayan · B. Srikanth · K. P. Gopinath (*) Department of Chemical Engineering, SSN College of Engineering, Chennai, India e-mail: [email protected] © Springer Nature Switzerland AG 2019 Inamuddin et al. (eds.), Nanophotocatalysis and Environmental Applications, Environmental Chemistry for a Sustainable World 29, https://doi.org/10.1007/978-3-030-10609-6_2

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carbonaceous materials, glass, cellulosic materials, polymers and metallic agents which have been already studied by a lot of researchers have been reviewed. The study justified that factors like low density, ease of availability, high durability, mechanical stability and chemical inertness are important factors required for the selection of suitable supports for catalysts. Common techniques for immobilisation such as cold plasma discharge, RF magnetron sputtering, dip coating, polymerassisted hydrothermal decomposition, solvent casting, photo-etching, spray pyrolysis and electrophoretic deposition have been discussed in depth. Finally, certain usual techniques employed for the characterisation of the catalyst particles and their applications are also discussed. Keywords Photocatalysis · Immobilisation · Electrodeposition · Thermal treatment · Sol-gel method · Sold plasma discharge · RF magnetron sputtering · Photo-etching · Electrophoretic deposition · Polymer-assisted hydrothermal decomposition

2.1

Introduction

Nowadays, photocatalysis is being broadly used for the treatment of pesticides (Cruz et al. 2015; Gar Alalm et al. 2015; Quiñones et al. 2015; Radwan et al. 2016), aquatic pollutants like dyes (Wang et al. 2015b; Babu et al. 2016; Zhu et al. 2016) and various pharmaceuticals (Ahern et al. 2015; Fathinia et al. 2015; Hernandez-Gordillo et al. 2015; Maeng et al. 2015; He et al. 2016; Naraginti et al. 2016). Therefore, researchers are concentrating on synthesising non-toxic, stable, economically viable and photo-corrosion-resistant catalysts (El-Roz et al. 2013). In spite of massive concern over such pollutants, treatment using archetypical techniques has not been effective. Advanced oxidation processes (AOPs) prove to be highly fruitful in degrading such harmful pollutants. AOPs are optically driven processes, which deals with the treatment of pollutants by employing hydroxyl radicals (OH•). As shown in Fig. 2.1, among all AOPs, photocatalysis is of high significance due to the relative flexibility of the process. The boon of this process is that it can thoroughly eliminate recalcitrant compounds into benign and simpler compounds which can be converted into harmless compounds by natural mechanisms. Moreover, this process does not merely remove the pollutant from the system as in certain conventional

AOPs

Ozonisation

UV photolysis

Ultrasonication

Persulphate oxidation

Fig. 2.1 Various commonly used advanced oxidation processes

Microwave assisted oxidation

Fenton process

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Applications of AOPs

Air and water treatment

H2 production

Sterlisation

self-cleaning surfaces

CO2 reduction to fuels

Decomposing PAHs

Fig. 2.2 Various commonplace applications of AOPs

treatment techniques like adsorption, but it eliminates the pollutant molecules completely. The various applications of photocatalysis apart from pollution abatement have been shown in Fig. 2.2. Normally semiconducting transition metal oxides like ZnO, TiO2, NiO and WO3 (Doerffler and Hauffe 1964; Muller and Steinbach 1970; Asahi et al. 2001; Justicia et al. 2002; Khan et al. 2002); sulphides like ZnS, MoS2, CdS and In2S3 (Matsumura et al. 1985; Zhang and Li 2004; Zhao et al. 2007); halides such as AgCl and BiOI (Wang et al. 2009a, b); etc. are employed as photocatalysts for environmental pollution abatement. The recovery and recycling of these nanoparticles from the effluent stream on a larger scale are very tough and an expensive process. Hence, the photocatalytic degradation process is pretty much a challenging task at the industrial level. In addition to it, TiO2 and ZnO nanoparticles present in the effluent stream can result in grave cytotoxicity and genotoxicity to human and aquatic lives (Kim et al. 2010; Vevers and Jha 2008; Song et al. 2010; Premanathan et al. 2011). In this regard, techniques which assist in reusing the spent catalyst will make the photocatalysis process eco-friendly. A lot of researchers have discussed the feasibility of employing immobilised photocatalyst for the photocatalytic degradation of organics (Zeng et al. 2010; Razak et al. 2014; Veréb et al. 2014; Yadini et al. 2014; Dong et al. 2014; Marothu et al. 2014; Miranda-García et al. 2014; Sabri et al. 2015; Wang et al. 2015a; Li et al. 2015; Akerdi et al. 2016; Barrocas et al. 2016; Mohite et al. 2016; Nadarajan et al. 2016; Ramasundaram et al. 2016; Ray and Lalman 2016; Sobhana et al. 2016; Ghoreishian et al. 2016; Jansson et al. 2016; Lin et al. 2016; Lee et al. 2017). Different methodologies have been successfully applied to immobilise the catalyst on various supporting materials like metallic supports, glass substrates, zeolites, photoelectrodes, etc. Additionally, immobilisation has also been successfully done on natural media like chitosan (Nadarajan et al. 2016), cellulose (Zeng et al. 2010), Luffa cylindrica fibres (El-Roz et al. 2013), etc. Table 2.2 provides a non-exhaustive list of such immobilised catalysts. Natural fibres are reported to be highly used for immobilising enzymes and catalysts. The important reason behind this is their cost, biodegradability, environmentally benign nature, renewability and easy availability. Such fibres lend a threedimensional rigid support for the photocatalyst. However, photocatalysis takes place by the irradiation of the catalyst surface with light of proper wavelength. Therefore, such heterogeneous immobilised photocatalytic systems exhibit mass transfer limitations, because of the reduction in the effective surface area for reaction and irradiation when compared to homogeneous catalytic systems (Dijkstra et al. 2001;

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Parra et al. 2004). In this article, we mainly emphasise on the preparation and the application of immobilised supported semiconducting photocatalysts for the treatment of wastewater.

2.2

Challenges in Developing Photocatalytic Water Treatment Systems and Need for Immobilisation

With the fact that TiO2 remains an archetypical, benchmark photocatalyst being used at large for wastewater treatment applications, most of the works reported in the literature on photocatalytic wastewater treatment that uses TiO2 nanoparticles have employed these nanoparticles in slurry/free particle form. This can be explained due to the high volumetric reactive oxidative species generation, proportional to the number of active sites on the surface of the photocatalyst when TiO2 nanoparticles remain in the suspension (Serpone 1997). Nevertheless, the entrapment of these photocatalyst nanoparticles on an inert substrate provides an inherent disadvantage of losing a number of surface active sites, which in turn reduces the activity of photocatalysts and enlarges the mass transfer limitations. Additionally, immobilisation of the catalysts results in an increased operational difficulty as light will not be able to effectively reach the surface active sites of the catalyst, thereby reducing photonic activation (Chong et al. 2010). All these factors support the application of slurry-type photocatalyst reaction systems. But, in slurry photocatalyst system, an expensive additional post-treatment catalyst recovery step is often needed to reduce the overall operational cost of industrial photocatalytic wastewater treatment systems. This separation process is important to prevent the loss of catalyst particles and addition of photocatalyst itself as a new pollutant in the treated water. There are a number of physical treatment strategies such as hybridisation with conventional sedimentation (Liriano-Jorge et al. 2014), crossflow filtration (Doll and Frimmel 2005), membrane filtrations (Sopajaree et al. 1999), etc. that are capable of recovering the spent catalysts from the treated wastewater streams. However, there are significant shortcomings to employ membrane-based separation processes for nano-photocatalyst recovery. This includes membrane fouling, usage of membranes with appropriate pore size, pore clogging, regeneration, backwashing, etc. (Chong et al. 2010). Nonetheless, a number of recent studies have focussed their attention on immobilising nanophotocatalysts on immobilisers that can improve the surface contact with the target pollutant/wastewater. Some common examples of such immobilising agents include glass, activated carbon, silica, polymers, pumice-like minerals, quartz, cellulose, plant fibres, metals, reactor walls, fly ash, Vycor glass, optical fibres, Raschig rings, etc. (Srikanth et al. 2017). Table 2.1 gives the advantages and disadvantages of the suspended (slurry)-type and immobilised photocatalytic reactors (De Lasa et al. 2005).

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Table 2.1 Some advantages and disadvantages of slurry-type and immobilised photoreactors (Srikanth et al. 2017) Slurry-type photoreactors Advantages There is a uniform dispersion of catalysts in the reactor when catalysts are in the form of suspension; this facilitates better mixing and hence better operating efficiency A significant increase in the ratio of illuminated photocatalyst surface area to reactor volume No significant catalyst fouling as spent catalysts is frequently replaced or discharged Suspended catalyst reactors offer lower pressure drops as there is no significant frictional drag exerted on catalyst particles by the flowing effluent Disadvantages Requires tedious and expensive post-treatment separation processes for the recovery and reuse of photocatalyst from the treated effluent streams Aggregation of catalytic particles especially at higher concentrations

2.3 2.3.1

Immobilised photoreactors Advantages Can provide continuous operation of the reactor

Improvement in the removal of organic material from aqueous phase while using immobilising agents with adsorptive properties Separation of the catalyst from the final treated effluent stream is extremely easy

Disadvantages Possible catalyst deactivation and catalyst washout

Lower catalyst accessibility to photons Significant external mass transfer resistance at low flow rates of the pollutant to be treated. This is because of an increase in diffusion path length of the reactant from the bulk to the catalyst surface Undesired thermal gradients may exist within the reactor as immobilised reactors offer poor temperature control Channelling (formation of stagnant dead zones) in the reactor will result in increased flow rates along walls of reactor leading to lower conversions

Matrices for Immobilisation of Photocatalysts Glass

Despite many recent evolutions in the field of photocatalysis, glass still constitutes to be an interesting medium of entrapment for most of the photocatalytic applications. Glass is used as a substrate in a number of physical forms such as plates (Mascolo et al. 2007; Kushwaha et al. 2015; Maleki et al. 2016), beads (Manassero et al. 2016; Zifang et al. 2016), etc. Moreover, instead of using external components like beads or rings as anchoring agents, many authors have also reported the use of glass for construction of the photocatalytic reactor and then coating the catalyst as a uniform

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layer over the reactor walls (Alrousan et al. 2012; Keane et al. 2014; Marugán et al. 2015; Shen et al. 2015). Owing to its inertness, coupled with the ability to withstand high calcination temperatures, favourable optical properties and cost, glass is still being preferred for the photocatalytic studies (Endres et al. 1999; Bansal and Doremus 2013). Previously researchers have investigated the degradation of HCHO, a refractory pollutant molecule using ZnO nanoparticles immobilised on glass plates, in a benchscale photocatalytic reactor, fitted with a peristaltic pump (for mixing of the reactant mixture). The authors had carried out the pretreatment of glass slides in concentrated NaOH solution, by immersing the glass slides in NaOH for 24 h, followed by the coating of nanoparticles on the prepared glass slides, and then drying the prepared slides at 350
C for 3 h. A strong adherence of the ZnO catalyst after calcination over the -OH functionalised glass plates had been observed, primarily due to intermolecular dehydration between the -OH groups on the surface of ZnO particles and those on the glass slides. The experimental studies suggest that, by irradiation under UVC lamps at a peak intensity of 254 nm for 120 min, an apparent reaction rate of kapp ¼ 0.0265 L/min with about 96.08% removal of HCHO was obtained, indicating the successful utilisation of the ZnO photocatalysts immobilised on functionalised glass plates for the mineralisation of HCHO (Darvishi Cheshmeh Soltani et al. 2015).

2.3.2

Carbon Nanotubes and Graphene Oxides

Carbonaceous materials such as those made of CNT and rGO are increasingly being employed as supports to simplify the post-application separation of photocatalyst from the aqueous reaction media. Additionally, these carbonaceous materials are capable of producing high efficiency with photocatalyst particles owing to their high specific areas and excellent electronic, adsorption, thermal and mechanical properties in addition to chemical inertness and stability (Pant et al. 2015; Petronella et al. 2015). CNTs possess a unique hollow and layered structure that aids the formation of electron-hole pairs which can move freely along the cylindrical nanostructure, which would sharply decrease the recombination rate of the photo-generated electron-hole pairs (Chowdhury and Balasubramanian 2014; Petronella et al. 2015). Therefore, an increased efficiency in the photocatalytic activity of photocatalysts can be achieved by a synergistic combination of photocatalysts with carbon nanotubes, due to the migration of electron-hole pairs and effective charge separation. Murgolo et al. (2015) reported a novel nanosized anatase phase titania crystals supported over SWCNTs for the photodegradation of 22 contaminants of emerging concern under UV and simulated solar irradiation. Synthesis of TiO2 was reported to have been performed by hydrolysis of titanium(IV) isopropoxide (TTIP) catalysed by trimethylamine N-oxide (TMAO) dihydrate in the presence of oleic acid (OLEA) at 100
C, under standard air-free conditions. About 2.3 g of SWCNT was dispersed in 70 g of OLEA using sonication for about 15 min. The obtained suspension was

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stirred vigorously under vacuum at 100
C for 15 min. Under N2 flow, 16 mmol TTIP was added followed by the addition of 5 mL of a 1 M aqueous TMAO dihydrate to the reaction mixture. The resultant solution was kept closed at 100
C and mixed vigorously with mild reflux with water for over 5 days to promote hydrolysis and crystallisation of product particles. After this time period, the reaction was stopped by removing heat source, and the grey powder of SWCNT/TiO2 NR heterostructure was collected, dispersed in a suitable organic solvent and used in the study. The degradation under UV irradiation was performed by using a low-pressure 17 W Hg arc lamp, and a similar degradation under solar light was performed taking a 1500 W Xe arc lamp for simulated solar light. The authors report that for the nanosized TiO2 supported on SWCNTs, in case of terbutaline, 2-phenyl-5-benzimidazole, BP-1, triclosan and metoprolol, a photosensitising effect of the wastewater was observed to produce a rise in degradation rate between 14% and 17% times. For the remaining pollutants, the reduction of the degradation rates was reported in the range of 11–98%. Also, Pant et al. (2013) employed electrospinning technique for the immobilisation of TiO2 nanofibres on rGO. This study reports a simple two-step electrospinning process to synthesise TiO2 precursor solution containing polymer nanofibres that are anchored on GO sheets, followed by reduction using calcination. The entire electrospinning process was performed at room temperature under an applied voltage of 18 kV, tip-to-collector distance of 15 cm and solution feed rate of 1 mL/h. Characterisation studies revealed that the GO sheets were micro-sized and were highly interconnected with the TiO2 nanofibres. The graphene sheets were found to contain corrugated surface with cavities, which were decorated with TiO2 nanofibres. The effectiveness of the prepared GO/TiO2 catalysts in photocatalytic degradation application was evaluated using rhodamine B as a model pollutant under a mild UV irradiation (2.2 W/cm2 UV radiation). The study concludes that the per cent removal of RhB with the rGO/TiO2 catalyst as 97.4% for a reaction time of up to 180 min and the process at same conditions gave removal of 64.1% ca. using bare TiO2, thus indicating the successful utilisation of rGO-immobilised TiO2 as photocatalysts. The increased photocatalytic activity of the prepared rGO/TiO2 photocatalysts could be because of the facts that (1) the reduced graphene oxide material consists of numerous pores that provide enough surface area for effective adherence of TiO2 nanofibres. Also, during electrospinning, the TiO2-polymer fibres and rGO sheets provide a well-packed layered graphene oxide sheets. Notably, the calcination also leads to the production of some surface deformities in the catalyst material (in the form of corrugated structure cavities) that are again occupied by the TiO2 nanofibres. (2) Additionally, graphene being a very good electron acceptor, it can act as a sink for the electrons produced from the TiO2 nanofibres due to its 2-D, conjugated planar π  π structure. This way, the electrons that are photoexcited in TiO2 NFs are quickly transported from the CB of the nanofibre into rGO. This process effectively eliminated the possibility of charge recombination, thereby increasing the rate of e-h+ pair-induced photocatalytic reactions.

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Zeolites

Zeolites are a specific group of aluminosilicate and microporous materials that have phenomenal adsorbing properties. There are numerous alkali and alkaline earth ions like Mg2+, Ca2+, K+, Al3+ and Na+. Some of the basic zeolites include stilbite, natrolite, phillipsite, etc. (Mariner and Surdam 1970). Zeolites are mostly found in those places where volcanic ash reacts with water. The zeolites having numerous pores with varying pore diameter, it can adsorb the substances separating based on their molecular size and thus acting as molecular sieves (Rollmann et al. 2007). This very attribute of zeolites qualifies it as a reliable, cost-effective and eco-friendly medium for binding the catalyst with its substrate (Huang et al. 2008; Mohamed and Mohamed 2008). In photocatalysis, colloidal TiO2 is dominantly used as a catalyst, but the in-treatment aggregation and post-treatment operations were the major challenges that were faced. These limitations were overcome (Kovacic et al. 2016) by using TiO2 (Aeroxide P25) which was embedded on iron-exchanged zeolites (FeZ). The SEM/EDAX studies revealed that the orientation of titania particles on the zeolite was non-homogeneous. A 450 W Xe arc lamp was used for UV light irradiation to carry out the photodegradation of diclofenac (DCF) with TiO2-FeZ. The studies were carried out in the reactor of volume 0.09 L at 25
C with DCF solutions of up to 0.1 mM concentration. The efficiency of both pure TiO2 and immobilised TiO2-FeZ photocatalyst in the removal of DCF was measured, and the degradation of DCF using immobilised catalyst was more effective at low pH. However, pure TiO2 showed higher efficiency at neutral pH. It was also reported that even after a catalyst coating of up to five layers of TiO2-FeZ, the DCF degradation was as high as 92% when the reaction was carried out for 1 h. In addition to providing ease of separation, the catalyst was also advantageous in maintaining long-term catalytic activity and in the utilisation of low-cost substrate. This immobilisation also reduces the band gap energy by the formation of TiO-O-Fe bond along with a rise in the effective surface area and porosity of resultant thin film as a result of which an increase in the photocatalytic activity was observed.

2.3.4

Clay and Ceramics

Two of the important attributes for any substrate to be a reliable immobilising agent are its chemical stability and a relatively large surface area. Having primarily equipped with these qualities clays, ceramics and other siliceous materials are treated as promising embedding medium (Xuzhuang et al. 2009; Guan et al. 2015). Paul et al. (2012) have analysed the degradation of alachlor, sulfosulfuron, imazaquin, chlorotoluron and bromacil with nano-anatase immobilised on laponite. A combination of six tubular 20 W Hg lamps with a maximum wavelength of 365 nm was used for photodegradation studies. The alachlor and bromacil initial concentration was 10 ppm, whereas chlorotoluron, imazaquin and sulfosulfuron

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were taken at 5 ppm. The authors observed excellent degradation of the impurities with immobilised catalysts under UV light; however in the absence of irradiation, the degradation was negligible. When the experiment was carried out for 1 h, the observations showed 80% of chlorotoluron and bromacil degradation efficiency. Likewise, the degradation efficiency with imazaquin and alachlor varied between 60% and 100% for the same experimental conditions. The authors had also reported a good degradation efficiency when the experiment was carried out with catalysts having 15 mmol of Ti/g clay material at 150
C and 200
C. The experiment also further explained the relationship between degradation rates with pore volume and crystallinity of each photocatalyst. In order to establish this, the authors used two different catalysts of the same loading but with different pore volume; it was concluded that the catalyst with larger pore structure showed more efficiency. The reason for that was due to the higher adsorption of the impurity; in addition to this, it is also stated that when the pore volume goes down below a critical value, the adsorption of the pollutants becomes negligible and the degradation efficiency sharply decreases.

2.3.5

Polymers

Polymers are employed as entrapment medium mainly because of their ease in manufacturing. This also paves way for effective usage of scarp polymers whose effect on the environment is negative (Singh et al. 2013). Lei et al. (2012) synthesised TiO2 entrapped in polyvinyl alcohol (PVA) polymer for photocatalysis of methyl orange (MO) under solar light. MO is one of the most common azo dyes which is carcinogenic and highly photostable. PVA was specifically chosen in this case because of it being a hydrophilic polymer with excellent film-forming ability. Adding to that the swelling ability of cross-linked PVA is obtained by physical or chemical cross-linking methods. This can ensure that the entrapped titania has a higher residence time with the pollutants which enhances the catalytic activity. The photodegradation studies were carried out with the as-prepared TiO2/PVA under light radiation of wavelength 300 nm. The primary advantage of using an immobilised catalyst was also tested by reusing the catalyst for about 25 cycles. The authors had reported that after every cycle the decrease in efficiency was found to be in a small scale suggesting that very negligible loss of catalyst has taken place after every batch. This is attributed to the formation of Ti-O-C covalent bonds between TiO2 particles and PVA matrix as a result of which the titania was firmly embedded in PVA matrix. After analysing several varying conditions, the studies revealed that hybrid TiO2/PVA film with 10 wt% titania, treated at 140
C for a period of 120 min, exhibited optimum efficiency. The investigation was further carried out by varying amount of catalyst over the film, and surprisingly this change in concentration had little effect on the efficiency of degradation. This phenomenon can be explained due to the agglomeration of titania nanoparticles in high

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concentrations, due to which the free surface active sites gets decreased, thereby inhibiting the MO molecules from the catalyst. In another work presented by Magalhães et al. (2011), the synthesis of novel buoyant TiO2/LDPE with different titania concentrations (32, 68, 82 wt.%) was shown. The catalyst was used for the degradation studies of methylene blue dye under UV and visible light radiation. In order to maximise the light utilisation efficiency, improve the use of buoyant photocatalyst and avoid the oxygenation of solution, constrained reaction conditions such as nonstirred and non-oxygenated were employed. The authors had also reported that the catalysts had buoyant properties as their densities were laid in the order of about 0.55–0.75 (0.04 g/ cm3). Maintaining the constrained conditions along with surface illumination, the prepared photocatalysts showed extraordinary degradation efficiency. However, this was compared with the effect of pure titania, and it showed lower photocatalytic activity due to its settling nature. This also leads to the improper illumination of the catalyst and becomes another reason for the lower photocatalytic activity. The catalysts were recovered by using mechanical sieving, and it has been reused for eight times. The authors have found out that after first three batches almost complete decolouration of the dye and a constant degradation rate was obtained. This indicates that it is feasible to reuse the catalyst without any appreciable loss in efficiency. The photodegradation of the polymer due to the free radicals produced in the intermediate stages of the process was also studied. This was analysed using infrared studies, and it was observed that even after prolonged reaction periods of 15 h and 30 h under UV light, the degradation of LDPE molecules by the free radicals, especially •OH, was considerably less under the same conditions which clearly indicates the durability of the polymeric substrate (Magalhães et al. 2011).

2.3.6

Other Uncommonly Used Supports

Apart from these conventional supports, many other uncommon supports were also used as effective immobilising medium, such as those based on metals (Guo et al. 2010; Bu et al. 2016; Murgolo et al. 2016), fibres (Ghoreishian et al. 2016), alginate beads (Wong et al. 2015; Sarkar et al. 2015; Majidnia and Idris 2016), etc. which have been used for the entrapment of the photocatalysts. These materials are being advantageous in the ways of procurement in large scale and also economical. They are also easy to separate from the treated effluent streams. Murgolo et al. (2016) showed a cost-effective method for preparing photocatalytic stainless steel sheet (SS) embedded with TiO2 powder (commercially available Degussa P25). Here, titania has been coated over stainless steel plate by means of metal organic chemical vapour deposition (MOCVD) method. The MOCVD was done in a horizontal hot-walled reactor heated over 400
C. Titania films were then deposited on SS meshes having a pore diameter of 35 μm size with the help of 8  20 cm rectangular mesh substrate for each of the deposition (showed as TiO2-SS). The so-prepared nano-anatase films possessed an average thickness of

2 Supporting Materials for Immobilisation of Nano-photocatalysts

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1500 nm. The photocatalytic studies using the TiO2-SS sheets were performed by the decolourisation of common antibiotics such as solutions of metoprolol, carbamazepine, gemfibrozil, warfarin and trimethoprim under UV radiation. The degradation was performed with a 40 W low-pressure mercury vapour lamp. Calculation of the electrical energy per magnitude of removal (EE/O) evaluated by the following Eq. (2.1) validated the higher performance of the new catalyst with respect to Degussa P25. Electrical energy per order magnitude ¼

3:84  UV power ðkWÞ
V ðLÞ  k min1

   EE kWh O m3 ð2:1Þ

where k is the first-order rate constant for the degradation of the pollutants. The nanoTiO2-SS possessed a slightly lower energy requirement for the removal of all impurities tested in the degradation (24.3–31.8 kWh m3 as opposed to 32.8–39.3 kWh m3 for bare titania at 100 mg L1 concentration). The results demonstrated that, under these given conditions, the nanoTiO2-SS catalyst showed higher efficiency in the degradation when compared to that of the conventional pristine catalyst. One more novel, biodegradable, natural entrapment medium is showed in the work by El-Roz et al. (2013), where a normal hydrothermal sol-gel method was used to immobilise TiO2 onto Luffa cylindrica fibres for the degradation of methylene blue. The study was performed with an expectation of higher degradation efficiency because of the following properties of an ideal immobilising agent: (1) Luffa fibres are very benign, yet chemically very stable. They can withstand temperatures of up to 170
C and the alkalinity of the reaction media. (2) The struts of the fibres have microchannels, which allow a good adsorption of the catalyst onto the surface of the fibre. (3) The extensive network of Luffa fibres provides a complex threedimensional network which can provide excellent anchorage for the catalysts, without affecting the incidence of the supplied light onto the catalyst surface. Characterisation of the prepared photocatalyst composites exhibited specific surface area of the titania/Luffa composites as about 16 m2 g1, while that of titania powder synthesised under the same condition is 235 m2 g1. As mentioned before, the photocatalytic efficiency of the synthesised catalysts was found to be more than three times higher than the case of self-supported P25-TiO2 taken as reference. This was mainly due to the high dispersion of the nanoparticles on the entrapment medium. Off late, several published works in immobilised photocatalysts for pollution remediation have changed their aim on the direct immobilisation of nanophotocatalyst on some viable biodegradable polymer. Biodegradable polymers have proved to be an effective replacement over non-biodegradable organic/inorganic supports for environmental sustainability reasons. Biodegradable polymers are a given set of polymers that can be decomposed by an enzymatic method to form favourable products, which on discharging into the environment does not cause any

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negative effects. A similar immobilising medium is the alginate beads. The authors, Sarkar et al. (2015), had embedded titania nano-photocatalyst onto alginate beads and employed them in the study of the degradation of emerging pharmaceutical pollutants such as chlorhexidine (CHD), ibuprofen (IBP) and atenolol (ATL). The titania-embedded beads were prepared by a conventional solvent casting process where 100 mL of casting solution has a homogeneous 4.0 g (4%) of sodium alginate powder and 1.0–4.0 g of titania nanoparticle in ultrapure water. Initially, nano-titania was added to 100 ml water and stirred for 30 min to form homogeneous suspension followed by which sodium alginate was added to that solution, thereby giving a mixture of TiO2 and sodium alginate. Approximately 100 mL of the mixture was mixed in a dropwise manner into a 0.5 M 400 mL CaCl2 solution by means of a 10 mL syringe with a needle diameter of 0.8 mm and length of 38 mm; this led to the formation of titania-impregnated alginate beads (TIAB). The TIAB thus formed was allowed to stay overnight along with the CaCl2 at 25
C for curing. It was then rinsed with ultrapure water for several times. A batch study having the pollutants suspended was made which showed that maximum percentage of CHD that can be removed at a substrate to catalyst ratio (S/C) of 2.5, pH 10.5 and temperature (30
C). Having the same parametric conditions, during the photodegradation in PBPR, the degradation of CHD was measured to evaluate its degradation percentage with time. First, about 99% removal has been observed for a 1-h residence time inside PBPR. IBP, CBZ and ATL can be removed up to 99%, 99% and 85% after 20 min, 40 min and 60 min, respectively, in suspension mode of operation. In all the above cases, the initial concentration was maintained at 10 mg/L. These results suggest a promising utilisation of alginate beads as an effective entrapment medium for the nanophotocatalysts.

2.4

Common Methods of Immobilisation

Apart from the fact that the immobilising medium itself plays a crucial role in determining the photoactivity of the catalyst particles, the immobilising technique adopted for entrapment of the photocatalyst also plays a crucial role in deciding the activity of photocatalyst. Thus, the technique chosen for photocatalyst immobilisation must be such that it does not degrade the photocatalyst or the intended supporting medium. Entrenchment of nano-photocatalysts onto the suitable support has been reported in literature via a number of techniques such as dip coating (Molina et al. 2012), sol-gel synthesis (Boiarkina et al. 2013), thermal treatment (Byrne et al. 1998), MOCVD (Yang et al. 2004), electrodeposition, spray pyrolysis and hydrothermal treatment (Srikanth et al. 2017). This section lays a thorough insight into such commonly reported immobilisation strategies for anchoring photocatalyst on supporting media. Table 2.2 provides a non-exhaustive list of such immobilisation techniques and immobilisation agents recently reported in the literature.

Phenol, halophenol and bisphenol A Benzoic acid, methylene blue

Carbendazim

2018

2017

2017

2017

2018

2016

2018

2018

2018

2017

Hadjltaief et al.

Murgolo et al.

Zhang et al.

Vaiano et al.

O’Neal Tugaoen et al.

M.E. Melo Jorge et al.

Norman et al.

Suryavanshi et al. Kaur et al.

Zhou et al.

Rhodamine B

Rhodamine B

p-Chlorobenzoic acid (pCBA)

Terephthalic acid

Warfarin, trimethoprim, metoprolol, carbamazepine, gemfibrozil Methylene blue

Malachite green, Congo red

Real industrial wastewater

2017

Pajootan et al.

Pollutant Diethyl phthalate

Year 2017

Researcher Hung et al.

Stainless steel mesh

Al2O3 based ceramic paper Polystyrene

TiO2 film

TiO2 nanorod arrays ZnO

TiO2 and Fe-doped TiO2 g-C3N4/BiOI heterojunction

Silica supports

Ca1xHoxMnO3 and Ca1xSmxMnO3 Iron(III) phthalocyanine ZnO

Electrospun PAN nanofibres

Clay beads

On photoanode

Spongin

Quartz optical fibres coupled to LEDs

TiO2

ZnO

CNTs deposited on carbon plates Tunisian clay

Substrate Glass plate

TiO2

Catalyst PANi/CNT/TiO2

Table 2.2 A non-exhaustive list of recently reported immobilisation agents and methods

Dip coating

Heat attachment

Electrodeposition

Dip coating

RF magnetron sputtering

Dip coating

Metal organic chemical vapour deposition (MOCVD) Two-step hydrothermal method Solvent casting

Sol-gel

Method of immobilisation 1. Hydrothermal method 2. Sol-gel Solvent evaporation

(continued)

Norman et al. (2018) Suryavanshi et al. (2018) Kaur et al. (2018) Zhou et al. (2018)

Zhang et al. (2017) Vaiano et al. (2017) O’Neal Tugaoen et al. (2018) Barrocas et al. (2016)

References Hung et al. (2017) Pajootan et al. (2017) Bel Hadjltaief et al. (2018) Murgolo et al. (2017)

2 Supporting Materials for Immobilisation of Nano-photocatalysts 61

Methylene blue

Acid orange 7

Orange II

Methyl orange

2017

2016

2017

2017

2017

2016

2018

I. Jansson et al.

Gadhi et al.

Tasbihi et al.

Hatat-Fraile et al. Liji Sobhana S. S et al. Znad et al.

Gaseous toluene

Acetic acid, acetaldehyde and trichloroethylene Indigo carmine

Pollutant Reactive Orange 16

Year 2016

Researcher Seyed Majid Ghoreishian et al. Baba et al.

Table 2.2 (continued)

Quartz fibre membrane

Au-Pd nanoparticles TiO2

TiO2

Co-precipitation, sol-immobilisation Direct templating technique

Dip coating

Fibreglass cloth

Hydrocalcites containing Ni(II) and Fe(III) Multilamellar vesicles (MLVs) mesoporous ZSM-5

Sol-suspension method

Corning glass, silica discs

β-Bi2O3 films TiO2

Zeolite

WO3-Pt

Polymer optic fibre

TiO2 thin films

Method of immobilisation Cold plasma discharge

Atmospheric pressure plasma Photodeposition and lyophilisation Spray pyrolysis

Substrate Spacer fabrics

Catalyst TiO2:ZnO

Baba et al. (2017) Jansson et al. (2016) Gadhi et al. (2017) Tasbihi et al. (2017) Hatat-Fraile et al. (2017) S.S et al. (2016) Znad et al. (2018)

References Ghoreishian et al. (2016)

62 R. Goutham et al.

2 Supporting Materials for Immobilisation of Nano-photocatalysts

2.4.1

63

Dip Coating

The dip coating technique is carried out with the aid of a dip coating apparatus comprising of an adjustable motor used to control the pullout rate, which in turn determines the thickness of the film coated over the substrate. The process is initiated by immersing the support into a precursor solution of known composition and viscosity followed by the removal of support plate from the dip at a steady and a slow speed. This technique could be employed for the production of both a thin-film and a thick-film coat of nanoparticles over the substrate (Sonawane et al. 2003). Prior to deposition, the supports are degreased by cleaning and then drying the wet sample. Generally, the anchorage medium is sonicated at very high frequencies for a period of about 1 h to improve its adhesive tendency towards the catalyst. But, with an exception in some cases, it is avoided to prevent the agglomeration of particles on the catalyst surface. After dipping the substrate in a suitable precursor sol, it is pulled out at a constant velocity of up to 1 mm/s. A thin film of TiO2 is formed over the surface, which is primarily air dried at room temperature followed by drying at 100
C for 2 h and then sometimes by using a hot air oven based on the nature of the material. Finally, the films are dried using an electric furnace at 400
C for an hour. This method is chiefly employed for coating uniform films of photocatalyst over supporting materials like glass plates, stainless steel plates, quartz plates, etc. Lastly, the entire coated support is washed using deionised water to recover the unbound catalysts by recycling. However, when supports with highly intricate morphologies like glass beads, Raschig rings, etc. are to be coated, the drying period is generally extended for about a week. A typical schematic representation of this process is shown in Fig. 2.3. The application of nanoscale, N-doped TiO2 for the photocatalytic removal of dyes like methylene blue and eriochrome black T were investigated by Vaiano et al. (2015). The sol-gel method was used for immobilising the photocatalyst over glass spheres with the help of a Pyrex cylindrical photochemical reactor equipped with a

Fig. 2.3 A schematic representation of dip coating of photocatalysts on inert supports. Initially, the supports are cleaned thoroughly and the coating precursor solution is prepared (a). The cleaned supports are then immersed in the precursor solution of known viscosity and composition (b). By using a motor, the support is gradually pulled out of the precursor at a very slow velocity (c). The coated support is then dried suitably to remove moisture and excess precursor solution (d). (Reprinted from Srikanth et al. 2017)

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peristaltic pump. The pumping action was necessary for the continuous mixing of aqueous solutions, and the degradation studies were carried out under both UV light and visible light. The studies were carried out with beads that were coated for up to six times. And it was observed that, at constant N-doping, the glass beads dip-coated for four times exhibited the maximum degradation efficiency. Similar results were achieved from beads that were dip-coated six times. This implies that proper irradiation of the photocatalytic surface is obligatory for effective degradation to occur. That is, as the number of the coating increases, it also increases the overall amount of N-doped TiO2 present on the surface; hence, irradiation in the case where a large number of coatings are given tends to be unsatisfactory. Molina et al. (2012) studied the effect of using goethite (a-FeOOH) particles immobilised on a glass tube prepared by the method of dip coating as photocatalysts. The glass tube was sonicated at 30 kHz for half an hour using an ultrasonicator. After which it was pretreated by immersing in a KOH/isopropanol bath (with a KOH concentration of 200 ppm) for 24 h. Then the tube was submerged in a suspension created using goethite powder for a minute. Once the coating process was completed, the glass tube was slowly removed from the suspension and was air dried at 110
C followed by calcination. Finally, the sample was treated with distilled water to remove any unbound particles. The so-prepared photocatalyst was used for the mineralisation of organic pollutants such as nicotine, ranitidine, etc.

2.4.2

Cold Plasma Discharge

CPD is an eco-friendly and an advanced technique by which new polar functional groups could be introduced to the surface of immobilising substrate, which enhances the anchoring of photocatalysts to the substrate without altering the natural properties of the bulk material (Ghoreishian et al. 2016). Latterly, this method is being employed as a pretreatment process to promote the adhesive and hydrophilic characteristics of the immobilising matrix. CPD is preferred over other conventional pretreatment techniques for the reason that plasma-based processes do not require the use of water or any other allied reagents for surface modification, hence resulting in a more economical approach. The major advantage of CPD includes the reduction in pollutant concentrations along with the reduction of the cost involved for the effluent treatment (Carneiro et al. 2001; Parvinzadeh and Ebrahimi 2011). Cold plasma is nothing but a partially ionised gas whose electron temperatures are way much higher than their respective ion temperatures. Therefore the excited electronic species, as well as the low-energy molecular species, are thus capable to initiate a sequence of reactions in plasma volume with the elimination of excessive heat generation that leads to substrate degeneration. Since most of the textile raw materials are known to be heat-sensitive polymers, this technique is prominently applied in fabric industries. Few merits of this technique include adhesion of coatings, improved hydrophilicity, printability, changing physical and electrical properties, cleaning or disinfection of fibre surfaces,

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65

induced hydro- and oleophobic properties, etc. Besides, nonthermal plasma surface modifications could be achieved over large textile areas (Bogaerts et al. 2002). CPD method generally involves the application of external energy to generate and maintain the plasma; this energy is commonly supplied by external sources like light, heat, electric, magnetic, etc. among which magnetic energy is predominantly used with the help of a magnetron sputtering device. The external energy supplied creates a temporary vacuum inside the vessel chamber, across which a constant voltage is passed. And then the pressure inside the chamber is slowly increased by passing clean dry air through it at a constant rate, which initiates the ionisation process resulting in the discharge of cold plasma (Hoffmann et al. 2013).

2.4.3

Polymer-Assisted Hydrothermal Decomposition (PAHD)

Polymer-assisted hydrothermal decomposition is one of the frequently opted techniques for immobilisation process; it is merely the combination of polymer-assisted deposition and hydrothermal methods. Since PAHD have the ability to control viscosity as well as the bonding with metal ions, they play an indispensable role in case of water-soluble polymers converting them hydrophobic. Therefore, PAHD eliminates the presence of erratic morphology and broad distribution of particle size, along with the benefit of forming films that have a thickness as large as tens of microns. Lin et al. (2016) have worked on the employment of this technique to create TiO2/ rGO nanocomposite films over SOFs. PAHD is comparatively an easy and a cheap process than other immobilisation techniques (Singh and Kaur 2010) which enables the formation of a range of good-quality materials by means of an accurate control over the stoichiometric ratio of precursor solutions, polymers and dopants, for multiphase materials. It has been found that the durability and stability of Side Glowing Optical Fibres (SOFs) have been enhanced by using PAHD, thereby making them suitable for coating with photocatalysts both in air and water (Lin et al. 2015). Whereas, the hydrothermal method when used lonely requires a very low deposition temperature in the range of 180–200
C, for coating catalysts over SOFs mainly due to degradation of silicone rubber coating of SOFs at temperatures above 250
C. The photodegradation of emerging pollutants such as sulfamethoxazole, ibuprofen and carbamazepine was studied using both visible and UV light radiation, at a pH of six and room temperature. Thereby the following results have been observed: I. The increase in degradation of sulfamethoxazole was found to reach a maximum of 35%, when irradiated under UV light for 3 h, compared to the results obtained under direct photolysis. II. An increase in rGO content improved the activity of the catalyst predominantly in the range of 0–2.7%. III. The TiO2/rGO catalyst sample containing 2.7% rGO exhibited the maximum degradation efficiency of 81% for ibuprofen, 54% for carbamazepine and 92%

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for sulfamethoxazole after irradiating for 3 h. But, a very high degradation of up to 83% was achieved for ibuprofen when the catalyst sample was containing 4% rGO. IV. The authors have also reported that a degradation rate of 0.539 h1, 0.199 h1 and 0.080 h1 was observed when samples were irradiated using high-pressure UV, low-pressure UV and visible light, respectively. V. The durability of the catalyst was tested by recycling the catalyst for about 15 times and by studying its effectiveness in each run. The surface area and adsorption sites of the catalyst were found to increase during their usage owing to the fact that as the process continues, the polymeric substances start to detach from the surface of the catalyst, thereby causing fluctuations in the percentage degradation of ibuprofen. Yet still, the overall efficiency in each run was found to be above 80%, and the efficiency was observed to be slightly higher than that observed during the initial 20-h time period.

2.4.4

RF Magnetron Sputtering

Due to its inherent versatility and homogenous surface coverage property even at low temperatures under controlled conditions (Kuo et al. 2012) has made it be the most effective method in existence for preparing thin films of immobilised photocatalysts. The RF magnetron sputtering is a similar technique to that of cold plasma discharge. The unique advantage of RF sputtering is that it provides flexibility to its user over designing the shape and size of the metallic nanoparticles synthesised, along with their distribution on oxide films evenly with suitable techniques (Zuo 2010). A schematic arrangement of the set-up commonly used for RF magnetron sputtering is shown in Fig. 2.4.

SUBSTRATE

RF POWER SUPPLY (13.6 MHZ) TARGET

MAGNET

ENTRANCE OF GAS

VACUUM SYSTEM

Fig. 2.4 Schematic representation of RF Magnetron sputtering set-up. (Reprinted from Srikanth et al. 2017)

2 Supporting Materials for Immobilisation of Nano-photocatalysts

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In the work carried out by Cao et al. (2014) on “highly antibacterial activity of N doped TiO2 thin films coated on stainless steel brackets under visible light irradiation”, this technique was used for the synthesis of thin films of titania made up of 99.99% of ceramic TiO2. The RF was kept at 200 W, the temperature was maintained at 573 K and the pressure was brought down to 0.001 Pa with the help of a turbo molecular pump. After this, the substrate was cooled and annealed to nitrogen at various temperatures, and the photocatalytic activity was started. It was clear from the results that after an hour, the degradation efficiency increases in proportion to the sputtering time. It was also reported that a maximum degradation of 40% of the pollutants was achieved in 60 min when the sputtering operation was carried out for 3 h. It was also found out that the thickness of the film exhibited a direct effect over the sputtering time because when a thin film was used, the source light provided was found to pass through it, decreasing the light efficiency, hence resulting in an overall decrease in the photocatalytic activity. Succeeding this Rashid et al. (2015) conducted a work on “ZnO nanoparticles thin films synthesized by RF sputtering for photocatalytic degradation of 2-chlorophenol in synthetic wastewater”. The authors synthesised thin films of zinc nano-oxide particles employing RF magnetron sputtering at 100 W. The substance that had to be scaled down to nanosize was placed in the chamber, the pressure in the base of the chamber was maintained at 106 torr and, during this state, argon gas was passed into the chamber until its pressure rose to 5  103 torr. Under such conditions of increased pressure with simultaneous power application, plasma is formed which results in the production of nanoparticles. It is also easy to vary the thickness of the material using this technique just by altering the time of deposition.

2.4.5

Photo-Etching

To understand the photo-etching process, it is essential to understand the fundamental concepts of lithography. Lithography is commonly employed for the microfabrication of thin parts or even bulk supports made of any substance. This phenomenon involves the transferring of a geometric pattern from a photomask onto a photoresist on the substrate, simply by means of using light radiation. The most widely recognised methods for photo-etching or also known as photolithography are to coat the substrate with a thin, uniform film of a light-sensitive photoresist over the substrate. A mask for the planned opening pattern is then set over the photoresist. At that point, UV light is illuminated over this arrangement, which initiates the polymerisation of the photoresist. Once the expected structure has been acquired on the substrate, the mask is removed, and un-polymerised portions are removed by dissolving in trichloroethylene solution. Any unwanted excess parts of the substrate not covered by the mask can be effortlessly removed by using HCl solution (Das and Das 2009).

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Nawi et al. (2012) in their work successfully immobilised commercially procured P25 TiO2 blended with epoxidised natural rubber (ENR50) and polyvinyl chloride (PVC) on a glass plate by means of photo-etching. ENR is a highly degradable polymeric material and could thus be photocatalytically photo-etched and could function as the precursor for the formation of pores and also once completely removed from the immobilised photocatalyst could increase the surface area and pore volume of the immobilised P-25 TiO2 particles. The degradation study was made using the degradation of methylene blue under visible light radiation obtained from a 45 W fluorescent lamp. The authors suggest that the P25TiO2/ENR/PVC nanocomposites immobilised using photo-etching showed a much higher photocatalytic activity than their non-immobilised and powder phase TiO2 counterparts. This can be inferred from the fact that the TiO2/ENR/PVC composites immobilised by photo-etching provided a 99% removal efficiency within 90-min reaction time, while only a 94% removal was obtained under similar conditions using non-immobilised catalysts. This can be attributed to the oxidation of PVC and formation of polyenes that consist of conjugated double bonds, which are capable to function as photosensitisers. The increase in catalytic activity of the photo-etched photocatalyst samples could be attributed to the formation of macropores on the surface of the catalyst. The effect of recycling the catalyst was studied by reusing the photo-etched catalysts for up to ten times in the degradation of methylene blue over a reaction time of 90 min successively, under a 45 W compact fluorescent lamp. The average k value throughout the ten cycles was determined to be 0.054  0.003 min1, suggesting that photo-etched immobilised TiO2/ENR/PVC composites possess good sustainability and reusability upon its recycled applications. Here, it is to be noted that the fabricated photocatalyst plate remained intact even until the tenth cycle. This indicates the successful utilisation of photo-etching as a promising technique of immobilising the photocatalysts.

2.4.6

Solvent Casting

To produce thin polymeric films, melt pressing is commonly used. But to cast heatsensitive polymers as a thin film, one may make use of solvent casting. In this method, the polymer is taken in the form of a solution in a suitable solvent (e.g. CCl4, THF, methylene chloride, etc.) and is spread on a Teflon surface or a flat glass plate, and the solvent is gradually evaporated by means of using a steady N2 stream. After the solvent is evaporated completely, one may dry the casted polymer by using an IR lamp (Scheirs 2000). Teixeira et al. (2016) reported the use of poly(vinylidene difluoride)-co trifluoroethylene nanocomposites containing varying TiO2 loads such as 5, 10 and 15 wt. % and ZnO nanoparticles 15 wt. %. These nanocomposites were produced by solvent casting, and their efficiency in degrading methylene blue was studied. All of the synthesised nanocomposite samples were recycled for up to three times so as to determine their reusability. It was observed that an increase in the photocatalyst

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concentration causes a rise in photocatalytic activity. The degradation rates of 15% of TiO2 and ZnO were found to be similar, while a decrease in photoactivity in the order of 6%, 16%, 13% and 11% after three utilisations for 5%, 10% and 15% TiO2 and 15% ZnO was observed, respectively. These results help to conclude that the photocatalytic activity of the synthesised nanocomposites is largely unaffected even upon recycling, thereby making the nanocomposite prepared by the solvent casting, an effective reusable, photocatalyst candidate. In late 2014, Martins et al. (2014) synthesised Er- and Pr-doped TiO2 for the photodegradation of MB. Pristine Er- and Er-Pr-co-doped TiO2 nanoparticles were prepared by a microwave-assisted method. Titanium(IV) isopropoxide and absolute ethanol were mixed under magnetic stirring, and to this mixture, acetic acid was added. Varying amounts of erbium(III) nitrate pentahydrate and praseodymium(III) nitrate hexahydrate were then added to the prepared mixture to obtain Er/TiO2 and Pr/TiO2 catalysts, respectively, of varying atomic ratios. Following a 5 min of magnetic stirring, deionised water was added, and the mixture was heated to 120
C in a Teflon-lined steel autoclave. The synthesised nanoparticle suspension was centrifuged, and the settled nanoparticles were resuspended in ethanol under sonication. These steps were followed by addition of 1 g of the copolymer to the solution, producing a concentration of 10 wt% polymer, and kept under magnetic stirring until complete dissolution. Finally, the resultant solution was poured into a petri dish, and the DMF solvent was evaporated at room temperature. The synthesised catalysts were found to have a large surface area (273 m2/g) and a low band-gap energy (2.3 eV). Additionally, the prepared catalysts were found to have about 75% microporous structure, which effectively increased the photocatalytic performance.

2.4.7

Electrophoretic Deposition

Electrophoretic deposition (EPD) provides the advantage of depositing a thin singlelayer film or multilayer films of varying thickness over a target substrate. An enhanced substrate formation having complex geometry can be easily performed suing EPD (Djošić et al. 2006). This is advantageous as nanoparticle immobilisation by dip coating often produces a non-uniform surface morphology, thereby necessitating repeated coating so as to get the required coat thickness. Such repeated dip coating becomes not only cumbersome in industrial operations but also makes the process uneconomical on large scale (Byrne et al. 1998). EPD is useful under such applications as it can deposit well-mixed nanoparticle suspensions onto fine featured conductive supports/templates (Sullivan et al. 2012). Electrophoretic deposition works by transportation of electrical charges between two electrodes (an anode and a cathode) immersed in a liquid medium (taken in the form of an electrolyte). The primary difference between an electrophoretic deposition cell and electroplating cell lies in the type of electrolyte employed. While conductive electrolytes are used in electroplating, EPD cells use dispersions prepared in

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Fig. 2.5 A conventional electrophoretic deposition set-up. The catalyst to be coated is dispersed in a suitable dielectric solvent, and a steady DC voltage (25–400 V depending on deposit thickness required) is applied across the electrodes. The ions produced by the electrolysis migrate towards an oppositely charged electrode by electrophoretic motion, and in this way the catalyst is deposited over the support (corresponding electrode). (Reprinted from Srikanth et al. 2017)

dielectric media. Thus, in a conventional EPD cell, the coating is produced by the deposition of relatively larger powder particles (nanoparticles) that may be polymeric, ceramics or other non-metallic materials on the target substrate. A typical EPD process involved the dispersion of catalyst particles in a suitable solvent, in the form of a stable colloidal dispersion. Owing to electrostatic interaction with solvent molecules, these dispersed colloidal particles gain a surface electric charge. When an external electric field is applied across such a charged, dispersed particles, each of these particles tends to move towards an electrode that possesses an opposite electric charge. This phenomenon is what is commonly known as “electrophoresis”. Upon reaching the oppositely charged electrode, the electrical charges annihilate each other, and the particles become electrically neutral. During this process, these particles get deposited on the work part (corresponding electrode) (Fig. 2.5). Under such an applied electric field, the electrophoretic motion of the charged particles will occur. The particles thus get deposited first at those regions that are maintained at a higher electric potential. Once the first layer of particles gets deposited on the work part, the electric potential of coated regions decreases, and the electric potential distribution over the entire plate becomes uniform after a period of time. This way, a uniform and even deposit will be produced over the entire work part even in the presence of surface of cavities, cracks and crevices. Dunlop et al. (2008) reported the photocatalytic inactivation of Clostridium perfringens spores on TiO2 electrodes that were fabricated by EPD method. In this work, Degussa P25 TiO2 particles were suspended in methanol and coated onto the substrates by the applying an electric voltage. A typical set-up used for electrophoretic deposition is shown in Fig. 2.6. The coated supports were then annealed at 500
C in the air for 1 h. The Degussa-Ti alloy electrode provided a maximum

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Fig. 2.6 A typical bench scale sprays pyrolysis chamber. The substance to be coated over the immobiliser is taken in the form of a precursor solution filled in the storage vessel. The solution is then allowed to pass through a spray gun under pressure using compressed carrier gas and is sprayed onto the supporting media, maintained at a high temperature. The sprayed medium is then cooled and annealed before further usage. (Reprinted from Srikanth et al. 2017)

percentage of spore inactivation (about 99.7% of the initial spore count) after irradiation for 120 min. The porous nature of the TiO2 film was engendered using electrophoretic immobilisation of TiO2 powder. Ultimately, a large surface area to geometric area ratio was obtained, which is very effective in photocatalysis especially where small molecules can diffuse into the pores to reach the photoactivated surface. These fractures and pores are formed by thermal contraction and surface stress induced on the film upon drying the electrodes after EPD. Additionally, some more fractures may also be produced during the annealing process largely due to the difference in thermal expansion coefficients of the deposited photocatalyst layer and the support.

2.4.8

Spray Pyrolysis

Spray pyrolysis is used commonly to prepare thin photocatalyst films economically, in an eco-friendly way. Other advantages of this strategy include ease in the synthesis of layered films and adherent and large area deposition capabilities (Mahadik et al. 2014). Spray pyrolysis works by passing a dispersion of precursors

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through an aerosol generator to produce droplets that are suspended in an inert carrying gas (commonly N2). The droplets are then heated by passing the aerosol through a tube placed inside a furnace (de la Garza et al. 2010). This method has the advantage of producing narrow drop size distributions and, in turn, narrow particle size distributions (Tsai et al. 2004). Spray pyrolysis is an economical method that does not need sophisticated equipment and can be easily implemented in laboratories and industries alike. Non-volatile precursor molecules such as metal nitrates, chlorides and fluorides can also be used to prepare the aerosol, avoiding the need for volatile carbon-containing precursors. Additionally, dye doping into the spray pyrolysis films can be performed easily by dissolving the dye solutions in the starting precursor solutions. However, in most cases, the spray pyrolysis-coated films were often used as electrodes for the photoelectrocatalytic degradation of pollutant molecules. A simple diagrammatic representation of a spray pyrolysis apparatus is described in Fig. 2.6. Often the coating of catalysts over inorganic supports, like ceramics, glass or organic polymeric supports, may lead to the dislodging, leaching and dissolution of the catalyst from the substrates. To address this problem, Li and Zhao (2010) used for spray pyrolysis technique to deposit thin films of TiO2 on conducting glass plates (spray-deposited fluorine-doped tin oxide on glass (FTO)). The glass supports were initially cleaned using ultrasound in an organic solvent such as trichloroethylene, acetone, ethanol, etc. and then finally in double-distilled water. Firstly, the fluorinedoped tin oxide (FTO) conducting coat on glass were prepared by spray pyrolysis of tin chloride pentahydrate (SnCl4.5H2O) and ammonium fluoride (NH4F) as precursor salts. This FTO-coated glass was then used as a substrate for deposition of TiO2 films using spray pyrolysis. The precursor solution for spray pyrolysis consisted of titanyl acetylacetonate dissolved in methanol. The precursors were sprayed using a pneumatic glass nozzle with compressed air as a carrier gas onto FTO supports maintained at a constant temperature. The rate of spraying was fixed at 4.5 ml/min, and the volume of spraying solution was varied between 30 and 210 ml to obtain films of varying thickness. The spray-deposited TiO2 thin films thus produced were reported to exhibit remarkable photoactivity under UV illumination in the presence of 0.1 N NaOH. Under depletion conditions, incident photon to current conversion efficiency (IPCE) of 0.8 was observed for a deposited film thickness of 330 nm at UV irradiation wavelength of 313 nm. Similarly, an IPCE value of 0.7 was obtained for 600-nm-thick film at 365 nm illumination. These values represent an excellent possibility of utilising the spray-coated TiO2 nanofilms for the photodegradation of organic pollutants.

2.4.9

Sol-Gel Process

The sol-gel process is a very simple, pervasive technique that is used to fabricate metal oxides using the corresponding metal salt solutions as precursors. These precursors play a crucial role in deciding the surface morphology of the deposited

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photocatalysts. These precursors may lead to the production of an extensively integrated (gel-like) network of discrete particles or polymers. The sol-gel approach is a low-cost, low-temperature technique that allows better control over the chemical composition of the end product. Even trace quantities of dopants, like organic dyes (in case of producing dye-sensitised photocatalysts) or rare earth, can be introduced in the coating sol, and they will eventually end up uniformly dispersed in the final product. On a general note, metal alkoxides and metal chlorides are the ubiquitously used precursors for the sol-gel process. These compounds undergo numerous hydrolysis and condensation reactions to produce interlinked networks of metal centres (M) that are connected by oxo (M-O-M) or hydroxy (M-OH-M) bonds. Excess solvent, if any present, is fairly easily removed by a drying process, which ultimately leads to volume shrinkage and densification of the photocatalyst film. The drying can also be followed by thermal treatment, which favours the further polycondensation and enhances the mechanical properties and structural integrity of the films via sintering, densification and grain growth. The inherent advantages of this method include (Srikanth et al. 2017): 1. 2. 3. 4. 5.

The lower temperature of preparation. The better purity of deposits from a given raw material. Better homogeneity from a given raw material. Effective control over the particle size, shape, distribution and properties. The possibility of tuning the material structure by changing the solvent used or by using other supports for coating. 6. Provides better mixing for multicomponent mixtures. 7. New non-crystalline solids can be produced even outside the range of normal glass formation. Expósito et al. (2017) reported the designing of a rotating disc reactor (RDR) consisting of TiO2 nanoparticles entrenched on a glass plate by using a facile sol-gel coating process to study the degradation of antipyrine. The TiO2 sol was prepared 1:8:3:1.1:0.05 molar ratios of titanium isopropoxide, isopropanol, acetylacetone, water and acetic acid, respectively. The glass discs were then withdrawn from the sol at a constant rate of 1 mm/s, air dried for 5 min in fume hood and annealed in an oven at 100
C for 30 min. After coating, the glass discs were further calcined in a furnace at 500
C for 1 h. The calcination process helped to produce photocatalytically active anatase sites on the film. In order to ensure that cracking is minimum, the temperature output from the furnace was spiked up at a uniform rate of 2
C/min. This process was repeated for up to four times to obtain a thick coat of TiO2 films on the glass discs.

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Conclusion

Photocatalysis is an efficient and green choice for removing hazardous, recalcitrant organic pollutants from air and water resources. There are a lot of factors that remain as hindrances for the implementation of photocatalysis for water treatment at large scale. Out of all the above-mentioned reasons, the growing need for novel alternatives to TiO2 and the economic factor is the foremost reasons. Quite recently, in order to make the process more sustainable, it is observed that a lot of researchers have started to implement the photocatalysts on support media. In this work, we have shown a list of recent immobilised photocatalytic models (both TiO2 and non-TiO2) which can be considered as newer alternatives for further study and discussion. Other important properties of photocatalysts immobilised on supporting materials and their respective efficiency in removing various pesticides, dyes herbicides and pharmaceuticals have been studied thoroughly in this review. A fascinating research area of employing biodegradable materials as support media has also been discussed. Eventually, it was found that more research could be carried out for synthesising novel supporting materials that can withstand the inherent mass transfer drawbacks of immobilised catalysis. Scaling up of the models of immobilised photocatalysts needs a detailed study. A comprehensive priority must be laid on recycling the immobilised photocatalysts without prominent loss in its photocatalytic activity. Apparently, it is pleasing to realise that a large scale employment of these types of immobilised catalysts can aid in mitigating a great environmental stress caused due to dangerous industrial effluents, toxic organic compounds, domestic sewage, pharmaceuticals, etc.

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Chapter 3

Non-metal (Oxygen, Sulphur, Nitrogen, Boron and Phosphorus)-Doped Metal Oxide Hybrid Nanostructures as Highly Efficient Photocatalysts for Water Treatment and Hydrogen Generation M. S. Jyothi, Vignesh Nayak, Kakarla Raghava Reddy, S. Naveen, and A. V. Raghu

Contents 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11

Catalytic TiO2/Its Hybrids Doped with Non-metals (Oxygen, Sulphur, Nitrogen, Boron and Phosphorus) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation Methods of Non-metal-Doped TiO2 Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Dopant Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Photocatalyst Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Co-doping (e.g. O and S) and Tri-doping (N, O and S) of TiO2 . . . . . . . . . . . . . . . . . . . . . . . . Preparation Methods and Effect of Co- and Tri-doping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic Doped TiO2 for Applications in Water Treatment and Hydrogen Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytic ZnO/Its Hybrids Doped with Non-metals (Oxygen, Sulphur, Nitrogen, Boron and Phosphorus) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation Methods of Non-metal-Doped ZnO Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Dopant Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Photocatalyst Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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M. S. Jyothi Department of Chemical Technology, Faculty of Sciences, Chulalongkorn University, Bangkok, Thailand V. Nayak Center for Nano and Material sciences, Jain University, Bangalore, India K. R. Reddy (*) School of Chemical & Biomolecular Engineering, The University of Sydney, Sydney, NSW, Australia S. Naveen · A. V. Raghu (*) Department of Basic Sciences, School of Engineering and Technology, CET, Jain University, Bangalore, India © Springer Nature Switzerland AG 2019 Inamuddin et al. (eds.), Nanophotocatalysis and Environmental Applications, Environmental Chemistry for a Sustainable World 29, https://doi.org/10.1007/978-3-030-10609-6_3

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3.12 3.13

Co-doping (e.g. O and S) and Tri-doping (N, O and S) of TiO2 . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic Doped ZnO for Applications in Water Treatment and Hydrogen Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.14 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract Inorganic metal oxide semiconductor-based photocatalyst plays key role in the photocatalytic process for applications such as environmental pollution (air and water) and hydrogen generation, due to their physico-chemical and photocatalytic properties. However, they are only active under ultraviolet irradiation, and it is a major drawback of oxide-based photocatalysts. The designing of visiblelight-driven photocatalysts based on metal oxides is very important for the highly efficient photocatalytic process. Doping of metal oxides (e.g. TiO2, ZnO, ZrO2) with non-metals such as oxygen, sulphur, nitrogen, boron and phosphorus elements enhances their photocatalytic efficiency under visible-light irradiation due to the strong oxidizing ability of non-metals. In this chapter, we discussed recent advances in various methodologies for the synthesis of series of above non-metal-doped hybrid nanostructured metal oxides, their properties, photocatalytic mechanism and the parameters (e.g. dopant concentration, photocatalyst content, morphological structures and band gap characteristics) that decide the photocatalytic performance for photocatalytic applications such as wastewater treatment and hydrogen generation. This chapter will provide novel ideas for the synthesis strategies of metal-free efficient photocatalysts with superior visible-light response for applications in the fields of the environment and energy. Keywords Metal oxide (TiO2, ZnO) semiconductors · Non-metal (oxygen, sulphur, nitrogen, boron and phosphorus) dopants · Non-metal-doped metal oxide hybrids · Photocatalysis · Visible-light-driven photocatalysts · Band gap properties · Wastewater treatment · Hydrogen generation

3.1

Catalytic TiO2/Its Hybrids Doped with Non-metals (Oxygen, Sulphur, Nitrogen, Boron and Phosphorus)

Titanium dioxide has been long known and used by mankind in various fields of application, namely, pigment in paints, coatings, sunscreens, ointments and toothpaste, aviation, aerospace and production industries (Alireza Khataee 2012; Jahedi et al. 2009). Titanium often finds itself counted among one of the most widely used materials as photocatalyst owing to its superior properties illustrated as multiple potential utilization, resistance towards bacteria, super hydrophilicity, chemical stability and cost-effective synthesis (Spanos 1995; Ghasemi et al. 2009; Johnston and Small 2011). And, it also has been the focal point in vivid fields of research areas

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such as wastewater treatment, solar cells, hydrogen production and membrane technology. TiO2 has proven its eminence and its optical and electronic properties could be supervised by diverse methods such as doping, thermal treatment and giving assistance (Rengaraj et al. 2007). Literally, Titania was first industrially manufactured from Norway, the United States and Germany in 1918 (Alireza Khataee 2012). TiO2 crystals generally exist in three distinct forms, rutile, anatase and brukite (Sen et al. 2005), of which rutile phase is considered to be the most stable phase (Kumar and Devi 2011), whereas anatase has withdrawn most of the attention owing to its superior photocatalytic property (Rengifo-Herrera et al. 2008). TiO2 is itself a very significant material, and further research in hybrids has even elevated its applications. TiO2 has been extensively researched for the preparation of its hybrids in order to tune their properties and reduce the band gap by doping method. Doping generally constitutes the lattice substitution at either Ti4+ or O2
sites and occupation at the interstitial sites of the bulk lattice (Sushma and Kumar 2017). Elemental doping gives the key benefits of a decrease in the band gap, the formation of new energy levels and the creation of oxygen vacancies, and it also subdues charge recombination (Hamadanian et al. 2017). The purpose of this report is to present and discuss the preparation methods, change in properties and modification and applications of nanostructured titanium dioxide/its hybrids. Doping of titania with non-metals is studied to be advantageous in shifting the ultraviolet active region of titania to visible-light region for aiding photocatalysis in comparison to metal doping (Zhang et al. 2015; Jing et al. 2013). Some of the most notable elements doped with TiO2 are nitrogen, sulphur, oxygen, boron, phosphorus, carbon, etc. This report will deal with the likes of TiO2-doped non-metal hybrids using oxygen (O), sulphur (S), nitrogen (N), boron (B) and phosphorus (P). Numerous articles have been reported on the catalytic behaviour of TiO2-doped hybrids, and some of which are elaborated below on the changes in the performance and utility; doping of TiO2 with mesoporous carbon containing oxygen vacancy leads to the increased sensitivity of the surface catalyst towards visible light and prevents the electron-hole pairs owing to the oxygen vacancy. The mesoporous surface encouraged adsorption giving degradation of rhodamine B with 97.3% removal efficiency in 9 min under UV light (Lu et al. 2017). Reduced graphene oxide coated with nitrogen-doped TiO2 showed improved photocatalytic efficiency and degradation rate for the removal of rhodamine B. It was observed that with a nitrogen-doped TiO2 coating on the surface of rGO, the photocatalytic activity was enhanced by 17.8 times in comparison to that of the virgin TiO2. This phenomenon was due to the effective charge separation and increase in active catalytic sites owed to the doped nitrogen and rGO. The composites also exhibited enhanced properties of adsorption, electron-hole pair lifetime and the absorbance of visible light (Zhang et al. 2018). Similarly, TiO2 doped with carbon has shown improved photocatalytic activity. Carbon source was varied from furfural, chitosan and saccharose to understand the effects of a precursor which lead to the changed surface area and XRD pattern. The photocatalytic activity shown by the hybrid was up to 4.4 times greater than the

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e¯ OH¯

O2

¯.

O2

e¯ e¯ e¯ e¯

Vis

P,I,S,B dopant levels Pollutant

Mineralization

UV

Carbon Sensitizer

Vis N,C,S,B dopant levels

OH¯

H2O/OH¯

h+

h+ h+

h+

F dopant levels

Non metal doped Titania for water punification Fig. 3.1 Schematic representation of non-metal-doped photocatalysis for water purification. (Reprinted from Devi and Kavitha 2013 with permission from Elsevier)

commercial TiO2 (Matos et al. 2016). Overall graphical representation for the photocatalysis mechanism in non-metal-doped titania (Fig. 3.1) was given by Gomati and Kavitha (2013). Different hybrid TiO2 photocatalysts are well reported prepared by using vivid experimental procedures, each of them having their characteristic feature. Some of the well-known procedures are discussed in the preceding section.

3.2

Preparation Methods of Non-metal-Doped TiO2 Photocatalysts

There are different ways of doping, which can be carried out using two different paths either by following a single-step process or two-step processes. The former consists of doping of the non-metal directly into the TiO2 lattice at the time of TiO2 synthesis, which will require calcination at high temperatures. The latter process deals with the initial synthesis of TiO2 followed by the incorporation of the doping material (Zhang et al. 2010). Some of the well-recognized and formally used methods for preparation of doped TiO2 catalyst are, namely, sol-gel, hydrothermal, solvothermal, co-precipitation and sonication. Sol-gel process is known since the mid-1800s and is considered the most common and preferred process due to its advantages such as reliability, reproducibility, low cost and controllability (Hamadanian et al. 2017). The schematic representation of sol-gel preparation method is shown in Fig. 3.2. Sol-gel process is mainly used in the synthesis of thin films and powder catalyst and has the advantage of controlling its processing parameters giving chemical homogeneity (Wang and

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Fig. 3.2 Schematic representation of the sol-gel synthesis of non-metal-doped TiO2. (Reprinted from Nasirian et al. 2017 with permission from Springer)

Ying 1999). This also allows having the control over its surface properties and its nanostructure (Malengreaux et al. 2014; Grätzel 2001), thus preparation of composites with desired properties. It is a simple yet effective technique, which involves the formation of a metal-oxo-polymer network at low temperatures. Common precursors such as metal alkoxides or metal salts are employed in the process (Judeinstein and Sanchez 1996). For example, one-step synthesis of N-doped TiO2 was performed by M. J. Powell et al. via sol-gel method using N,N,N0 ,N0 -tetramethylethane-1,2diamine and titanium n-butoxide as the precursors for photocatalysis (Powell et al. 2014). The hydrothermal process is a generally preferred method for obtaining more crystalline products and is energy efficient (Zhang et al. 2006). It has the flexibility

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of playing with its operating conditions such as pH, temperature, pressure, etc., which allows controlling its crystalline phase, grain size and morphology (Wang and Ying 1999). However, it is considered to be a tedious process to handle owing to the use of additives and surface treatments (Byrappa 2001). Appavu’s research group reported the simple one-step hydrothermal synthesis of N, S co-doped TiO2/rGO composites by using thiourea and titanium tetraisopropoxide as the starting material. The composites showed improved photocatalytic activity and proved beneficial in the degradation of dyes in the visible region (Brindha and Sivakumar 2017). Solvothermal synthesis is yet another well-known technique for the doping purposes, which is similar to the hydrothermal process, but uses a solvent instead of water as the reaction medium (Huang et al. 2006). The solvothermal route also has the advantages of controlling particle morphology, crystalline nature and high surface area (Inoue et al. 1997; Kang 2003). For instance, G. Yang et al. reported the doping of TiO2 with N by using amine as the nitrogen precursor. The synthesis was carried out in the ethanol-water reaction medium (Yang et al. 2010). The above three are the most utilized methods; however, electrospinning is a noteworthy process used for obtaining doped nanofibres. For example, N-doped TiO2 nanofibres were prepared using polyacrylonitrile as the support material and cyanide as the nitrogen source (Hassen et al. 2016). However, electrospinning is considered as a tedious process, and the fibres tend to show low efficiency, hence less preferred for the preparation of photocatalysts (Zhou et al. 2009a). For the further investigation in the search for different synthetic approaches, pyrolysis is gaining steady momentum, for example, doping of sulphur into TiO2 by flame spray pyrolysis technique. It used the approach of initial formation of miscible precursor solution containing titania as well as sulphur. Titanium tetraisopropoxide and sulphuric acid were used as the antecedent for Ti and S, respectively, with ethanol as the fuel. Then the method concludes with flame spray pyrolysis (Boningari et al. 2018). This illustrates a simple one-step approach for doping. Similarly, the laser was used instead of flame by M. Scarisoreanu and group for the doping of S into TiO2 lattice followed by C surface coating. The pyrolysis kick starts with help of gaseous/liquid starting materials; TiCl4 contributes for Ti and C2H4 for C and S2(CH3)2 towards S. The advantage is that the whole process takes place in a single step and is successful in decreasing the band gap compared to that of the commercially available TiO2 (Scarisoreanu et al. 2014). In addition, the combination of two or more synthetic procedures has demonstrated in successful doping of TiO2. Processes like the microwave are coupled with a solvothermal route to successfully fabricate carbon-doped titania, where the precursors (titanium(IV) isopropoxide and saccharose) were initially treated with microwave radiation using ethanol as the reaction medium followed by calcination (Rangel-Mendez et al. 2018). Apart from the discussed techniques to improve TiO2 properties, co-doping and tri-doping are also studied to further enhance the properties of TiO2 as a catalyst, which will be covered in the forthcoming sections.

3 Non-metal (Oxygen, Sulphur, Nitrogen, Boron and Phosphorus)-Doped. . .

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Effect of Dopant Concentration

With different approaches followed for doping, there are different effects on the performance as well structure of the doped composites based on the nature of dopant, dopant concentration and preparation method (Hernández et al. 2017). These effects not only can contribute to the better efficiency of the catalyst but could also alter the properties to negative impact. The most observed phenomenon is due to non-metal doping enhancement in photocatalytic efficiency, changes in morphological structure (Daghrir et al. 2013) and increase in oxygen vacancies (Emeline et al. 2007) and can also lead to semiconductor electronic structure (Ferrari-Lima et al. 2015). The doping of anions, namely, N, S and C, leads to the overlap of its p orbital with the 2p orbital of O, changing the optical and conducting property of the catalyst (Rattanakam and Supothina 2009). The decrease in dopant concentration can actually be beneficial as it can aid in minimizing the effect of band gap narrowing and promote electron-hole pair separations (Sun et al. 2008). Whereas, increase in the dopant concentration can lead to change in the crystal structure of the photocatalyst. This can lead to decrease in its efficiency, as noticed by R. Asahi et al. on increasing the concentration for N in TiO2 (Asahi et al. 2001). In addition, H. Irie et al. noticed that the dopant can act as recombination spots at higher concentrations, leading to reduced quantum yield during the decomposition of gaseous 2-propanol under UV source (Irie et al. 2003). Hence, optimum concentration of the dopant becomes a very significant element during synthesis. Among all the non-metals used as dopant materials, N and O have been declared as the foremost in giving the superior photocatalytic properties’ invisible region (Chen et al. 2007).

3.4

Effect of Photocatalyst Concentration

Photocatalyst concentration during doping plays a very crucial role in maintaining its cost and energy (Saggioro et al. 2011). It has been seen that the photocatalytic efficiency of the material can be enhanced by maintaining the concentration as noticed by Kaur and group and increasing the concentration of photocatalyst the number of pollutants adsorbed on the surface during the photocatalytic reaction was enhanced. This is owed because of the enhancement of absorbed photons on the surface of the photocatalyst, which increases the number of electron-hole pairs and hydroxyl radicals (Kaur et al. 2016). This, in turn, increases the photocatalytic efficiency.

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Co-doping (e.g. O and S) and Tri-doping (N, O and S) of TiO2

In the run to improve the photocatalytic activity of TiO2 to further extent, co- and tri-doping were put forth. Co-doping is nothing but the doping of the photocatalyst with two dopants instead of a single dopant, and similarly, tri-doping is doping the photocatalyst with three dopant elements. These concepts have shown to give positive effects with synergetic properties achieved by the photocatalyst owing to the dopants and have also lead to reduced electron-hole recombination’s (Huo et al. 2016). This can be done in different ways such as metal, non-metal and metal-nonmetal co-doping. This section aims in dealing only with the non-metal co-doping. The most frequently found co- and tri-dopes are of N, S and C anions. Some of the examples for anion co- and tri-doping have been deliberated in Table 3.1.

3.6

Preparation Methods and Effect of Co- and Tri-doping

The preparation methods followed for co- as well as tri-doping are similar to those followed for doping as discussed in the previous section (Sect. 3.2), namely, sol-gel, hydrothermal, solvothermal, etc. with the necessary changes applicable such as

Table 3.1 Examples of co- and tri-doped TiO2 photocatalyst Dopants N-C co-doped N-C co-doped N-S co-doped B-doped B-N co-doped N-F-B tri-doped C–N–S tri-doped C–N–S tri-doped C,N,S tri-doped C,N,S tri-doped

Preparation method Sol-gel

Contaminant Methylene blue

Reference Chen et al. (2007)

Hydrothermal

Phenol

Dolat et al. (2012)

Hydrothermal

Methyl orange

Wei et al. (2008)

Sol-gel

Cr(VI) and benzoic acid

Giannakas et al. (2016)

Sol-gel

Methyl orange

Hydrothermal

Nitrogen oxide

Hamadanian et al. (2017) Wang et al. (2009)

Modified sol-gel Hydrothermal

X-3B (reactive brilliant red dye, C.I. reactive red 2) Rhodamine B

Lin et al. (2013) Wang et al. (2017)

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Table 3.2 Examples of different methods used for co- and tri-doping of TiO2 Dopants N-S co-doped C-N-S tri-doped S–N–C tri-doped C-N co-doped N-S co-doped

Preparation method Sol-gel Hydrothermal and solid-state chemical reduction method Sonochemical Microwave irradiation method Thermal annealing

Contaminant Methyl orange Methyl orange Diclofenac Methyl orange E. coli inactivation

References Gao et al. (2011) Yan et al. (2017) Ramandi et al. (2017) Li et al. (2017) Rengifo-Herrera et al. (2010)

incorporation of two or more dopants at the time of synthesis. Some of the examples are presented in Table 3.2. As seen in the impacts of doping on the photocatalyst in the previous section, coand tri-doping also give some good characteristics to the photocatalyst. It is described that co- and tri-doping enhance the photocatalytic efficacy of TiO2 to a better extent in comparison to single-doped TiO2 photocatalyst (Lin et al. 2013; Cheng et al. 2012). One of the most important features is the synergetic property acquired by the photocatalyst from the dopants. Dopants will reduce the band gap of TiO2 and make visible light active and will aid in reducing recombination (Gaikwad et al. 2016). This phenomenon is observed as the dopants form a separate band near to the valence band (Irie et al. 2003) or get combined with the valence bad decreasing the band gap (Asahi et al. 2001). Co-doping can also surge the number of hydroxyl groups on the surface of the photocatalyst as noticed by Yu et al. (2012a). Thus, all these factors can increase the photocatalytic efficiency of the photocatalyst. Doping of TiO2 with elements such as N and S also prevents crystallite agglomeration, and aid in the formation of large pores and high surface is the catalyst (Yang et al. 2009a). However, it is also observed that high concentrations of dopants can have negative effects on the efficiency, for example, S. A. Bakar and group observed that on increasing the concentration of N and S, the crystallinity and pore size of TiO2 decrease. Also, the excess defect sites act as recombination centres which lower its photocatalytic efficiency (Bakar and Ribeiro 2016). Hence, an optimum concentration has to be maintained to achieve the desired properties to the photocatalysts.

3.7

Photocatalytic Doped TiO2 for Applications in Water Treatment and Hydrogen Generation

TiO2 is considered an excellent photocatalyst for the degradation of organic compounds from aqueous water owing to its low cost, simple preparation and relative nontoxicity (Palaniandy et al. 2015). Doped TiO2 has further improved its properties

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Table 3.3 Examples of a TiO2-doped photocatalyst for water treatment Photocatalyst N-TiO2

Contaminant Methylene blue

Feed concentration 6.7  10
5 M

Removal efficiency 100%

TiO2 N-TiO2

Rhodamine B

1.0  10
5 M

N-/F-/S-doped TiO2 thin films S, N co-doped TiO2

Azo dyes

25 mg/l

TiO2
6% N– TiO2
64% N–TiO2/C – 100% 80–100%

Methyl orange

7 mg/L

95%

C-TiO2

Rhodamine B

6–60 mgL
1

–

C-TiO2

2-Sec-butyl-4,6dinitrophenol Eosin B, rhodamine 6G, rhodamine B

20 mg L
1

100%

37.6 μM

–

N-TiO2/C

S- and N-doped TiO2

Reference Huang et al. (2017) Jia et al. (2018)

Behpour et al. (2017) Khalilian et al. (2015) Cinelli et al. (2017) Wei et al. (2017) Asiri et al. (2014)

Table 3.4 Examples of a TiO2-doped photocatalyst for hydrogen generation Photocatalyst C,N co-doped TiO2 C-doped titania nanotube array Platinum-loaded N-TiO2 S22
-doped TiO2 Carbonate-doped TiO2 Gadolinium and nitrogen co-doped TiO2

Hydrogen generation rate 81.8 mmol g
1 h
1 150 mmol cm
2 h
1 30 mmol h
1 g
1 9610 μmol h
1 g
1 6108 μmol h
1 g
1 10,764 μmol g
1

Reference Liu and Syu (2013) Dubey et al. (2017) Preethi et al. (2016) Sun et al. (2017) Wang et al. (2018) Mandari et al. (2018)

and is showing promise in water treatment and hydrogen generation. The different non-metal dopants give different efficiency and durability. Some of the well-known examples are N, S co-doped TiO2 for the degradation of ibuprofen and naproxen from the pharmaceutical wastewater. The photocatalyst was able to degrade ibuprofen and naproxen to 85% and 99.3%, respectively (Eslami et al. 2016). In single-doped photocatalyst, S-doped TiO2 was able to degrade hepatotoxin microcystin-LR to 76% under visible light (Han et al. 2014). On the other hand, hydrogen generation is also reported by the use of C-doped TiO2 from organic fatty acids under visible light with a production rate of 373.8 mmol∙g
1∙h
1 (Li et al. 2018). Different water purification- and hydrogen generation-based photocatalyst have been studied, some of which are tabularized in Tables 3.3 and 3.4, respectively.

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Catalytic ZnO/Its Hybrids Doped with Non-metals (Oxygen, Sulphur, Nitrogen, Boron and Phosphorus)

Zinc oxide (ZnO) is one of the successful photocatalysts and emerged as efficient and promising material in green environmental management. ZnO possesses some unique characters like direct and wide band gaps near UV region, strong oxidation ability and larger free excitation binding energy (Anderson and Chris 2009; Peng et al. 2006). ZnO is odourless, bitter in taste and water-insoluble and generally occurs as white hexagonal crystal, and it also occurs as large bulk single crystals (Reynolds et al. 1996). It is a characteristic n-type semiconductor having a wide band gap of 3.37 eV with the high excitation binding energy of 60 meV (Huang et al. 2015). ZnO is environmental friendly, has good compatibility with living organisms and offers a wide range of applications (Dubbaka 2011). ZnO is found to be economic with greater performance (Dindar and Içli 2001; Akyol et al. 2004) for the degradation of certain compounds including azo dyes. It is widely used as a catalyst in ceramic bodies, fertilizers, rubber and paint industries and cosmetics (Porter 1991). ZnO mainly exists in three structural forms, rock salt, wurtzite and cubic zincblende (Fig. 3.3). Among them, wurtzite structure possesses highest thermodynamic stability; rock salt structure will be yielded under higher pressure conditions (Lu et al. 2006). Most stable structure wurtzite crystal has two lattice parameters a and c with values 0.3296 nm and 0.52065 nm, respectively, at ambient temperature and pressure. A prefect wurtzite crystal of ZnO with four atoms per unit cell corresponds to one longitudinal-acoustic (LA), two transverse-acoustic (TA), three longitudinal-optical (LO) and six transverse-optical (TO) branches (Coleman and Jagadish 2006). ZnO is piezoelectric and pyroelectric because of its non-centrosymmetric structure of P63mc space group (Moore and Wang 2006). At different indentations, ZnO

Fig. 3.3 Different structural forms of ZnO (Reproduced from Özgür et al. 2005)

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hardness varies from 4 to 5 GPa, which is relatively low and are considered during processing and fabrication of ZnO-based devices (Fang et al. 2007; Zhang et al. 2014). This non-centrosymmetric structure offers displacement of positive and negative charges due to external pressure-induced lattice distortion which results in local dipole moment. Due to tetrahedrons stacking along a particular direction and because of spontaneous polarization, ZnO surface will be charged. Further to attain stability, charged surface results in nano ring or coil structure (Behera 2011). This piezoelectricity of ZnO offers various applications in the field of acoustic wave resonator and optic modulator and force sensing. ZnO has a band gap energy almost same as TiO2, and hence its photocatalytic activity is predicted to be similar, and also, ZnO is relatively cheaper (Daneshvar et al. 2004). As in ideal photocatalytic reactions, ZnO particle absorbs photons of energy greater than its band gap energy under illumination. These photo-induced electrons initiate the formation of an electron-hole pair, and the oxidation takes place in the presence of oxidizing species. However, more concern is required to avoid the recombination of a photo-induced electron to achieve better efficiency. Doping is one of the methods to overcome this disadvantage. Though doping methods, doping concentrations and contribution of dopants in avoiding electron-hole recombination must be scrutinized to obtain highly efficient ZnO. In addition, charge separation will also greatly influence a generation of hydroxyl radicals and active oxygen species (Kato et al. 2005). Non-metal-doped ZnO exhibits enhanced photocatalytic activity than pristine ZnO. Doping with nitrogen offers N 2p states above the VB maximum of ZnO and hence increases the visible-light absorption. In such case, photo-generated electron-hole pairs exist between N 2p states and Zn 3d CB and also result in narrowed band gap (Batzill et al. 2006). Nitrogen doping offers genuine p-type ZnO, where insertion of nitrogen in the place of oxygen, without strain, is feasible for wurtzite lattice due to similar radii of both of them (Stavale et al. 2014). Photoexcited electrons in CB could react with surface-adsorbed oxygen producing superoxide radical anions which eventually converted into hydroxyl radicals (Nagaveni et al. 2004). In carbon-doped ZnO, obviously, in water treatment and adsorption of the pollutant on the surface of the photocatalyst, C-ZnO will show stronger UV absorption, and also surface oxygen vacancies of ZnO nanostructure induce new energy levels below CB of ZnO (Guo et al. 2009; Lai et al. 2011). Likewise, in sulphur-doped ZnO, oxygen vacancies and surface defects become photo-generated electron trapping centres (Chen et al. 2008; Patil et al. 2010). In the case of boron-doped ZnO, ionic bonding between boron and oxygen is stronger than zinc and oxygen, because of the smaller ionic radius of boron than zinc (Lokhande et al. 2001). On a broad spectrum, doping methods and concentration and nature of dopants significantly affect the overall efficiency of doped photocatalysts.

3 Non-metal (Oxygen, Sulphur, Nitrogen, Boron and Phosphorus)-Doped. . .

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Preparation Methods of Non-metal-Doped ZnO Photocatalysts

Several methods are available to synthesize non-metal-doped ZnO nanoparticles. Annealing ZnO with required non-metal precursor is one of the most widely followed methods. In this case, firstly ZnO nanoparticles will be prepared by any of the methods followed for usual photocatalyst preparation methods. Later, prepared ZnO will be annealed with dopant source. Xunyu Yang et al. used ammonia solution as a dopant source; ZnO nanowires were first prepared by the hydrothermal method, followed by annealing in ammonia to incorporate N as a dopant (Yang et al. 2009b). At first, ZnO was annealed with ammonia for 30 min at 530  C, and then annealing was continued for 30 min under nitrogen atmosphere. Two-zone tube furnace was used to prepare nitrogen-doped ZnO, where ZnO nanowires were kept in the first zone and the temperature was set to 530  C and the temperature of zone 2 was set to be 900  C. The UV-visible absorption onset of doped ZnO exhibited redshift towards visible region. Nitrogen doping was also implemented via thermal nitration in ammonia gas flow (Rajbongshi et al. 2014). Photoelectrochemical water splitting was improved by two strategies, secondary branching and nitrogen doping. Light harvesting was enhanced by increasing roughness factor and band gap narrowing by nitrogen doping. The mechanochemical method is another best method to induce non-metal into oxides (Yin et al. 2003). In this method generally, ZnO and a source of the corresponding dopant will be subjected to high stress. When high-energy ball mill is applied to solid with another solid material, the solid-state reaction will occur between those two solids, and the yield depends on milling time (Yadav and Yadav 2014). The average particle size will be increased by the cold welding process. Whereas, the fragmentation process leads to breakage of composite particles. Mechanical stressing induces surface defects on ZnO nanoparticles, and these temporally evolved defects purely depend on the amount of stress induced (Kwade 1999). Chemical spray technique and spray pyrolysis are the commonly used method of preparation of ZnO thin films as well as doped ZnO thin films (Pawar et al. 2005, 2009; Wenas et al. 1991). The method is known for its high simplicity, ease of incorporation of impurities and possible large area commercial deposition (Bian et al. 2004). ZnO nanoparticles of different sizes and shapes are prepared by the hydrothermal method by varying the additives (Fig. 3.4). For nitrogen doping, hydrothermally prepared ZnO will be annealed with nitrogen source like ammonia and urea (Yang et al. 2009b; Qiu et al. 2012). Doped ZnO is also prepared by microemulsion method where surfactant, cosurfactant and oil phase as a continuous phase and the tetrabutyl titanate dissolved in nitric acid as the aqueous phase. When the transparent solution is obtained, dopant source is added; once the homogeneous solution is obtained, it is transferred to Teflon-lined autoclave and then heated under particular temperature

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Fig. 3.4 Formation of different shapes of ZnO via hydrothermal synthesis. (Reproduced from Zhou et al. 2013)

(Cong et al. 2007). Microwave irradiation was employed to synthesize N-doped ZnO with controlled size and doping level (Herring et al. 2014). Chemical vapour deposition (CVD) and electrochemical deposition (ECD) were also employed for non-metal doping onto ZnO (Bae et al. 2004; Wang et al. 2007). In classic CVD synthesis substrate is exposed to volatile precursor, and it reacts or decomposes on substrate producing ZnO at different temperatures. In ECD, electric field-assisted electrochemical deposition takes place in porous anodized oxide templates. Both methods are preferentially used to prepare ZnO nanowire or films. To conclude, selection of the method of preparation of ZnO purely depends on doping level, doping concentration as well as structure and morphology of ZnO.

3.10

Effect of Dopant Concentration

Like doping methods, the concentration of dopants also contributes to determining the potentiality of photocatalyst with respect to change in morphology, size and formation of defect centres. Increase in dopant concentration allows greater visiblelight absorption and shifts Eg values to lower side. This is due to the introduction of surface structural defects and lattice alteration during doping. Change in dopant concentration induces change is morphology as well. However, the introduction of dopant may not change the crystalline structure of pristine photocatalyst, but a change in size is very common (Macías-Sánchez et al. 2015). When urea was used as a nitrogen source, the crystallite size of pristine ZnO was more. During preparation of the catalyst, urea acts as a surfactant and inhibits the growth of ZnO crystals, and hence, size of photocatalyst is reduced (Qin et al. 2011). And hence optimizing

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the concentration of dopant is more important to obtain a photocatalyst with better efficiency.

3.11

Effect of Photocatalyst Concentration

The use of optimal concentration of photocatalyst in any photocatalysis is more significant with regard to the cost-effectiveness and lowering energy consumption. It is generally considered that increase in photocatalyst concentration increases the number of adsorbed photons for photoreactions subsequently increasing the overall efficiency of the reaction. However, increased absorbed photons enhance a number of electron-hole pairs and credits for higher photocatalytic degradation (Kaur et al. 2016). Many reports are available (Zhu et al. 2014; Jianfeng et al. 2010; Yu et al. 2016) to prove the same.

3.12

Co-doping (e.g. O and S) and Tri-doping (N, O and S) of TiO2

Co-doping and tri-doping are the methods to obtain higher efficient photocatalyst with respect to change in morphology and crystal structure. Doping of two or more metals increases the surface hydroxyl groups or shifts the wavelength of absorption to the visible-light region from UV region or affects the crystalline structure and morphology (Zhiyong et al. 2007; Zhang 2016; Yu et al. 2012b).

3.13

Photocatalytic Doped ZnO for Applications in Water Treatment and Hydrogen Generation

Heterogeneous photocatalysis is promising technology for organic dye removal from aqueous solutions as well as for the production of hydrogen from non-carbonaceous sources like water using solar light. Doping of non-metals to ZnO allows higher absorption of light and decrease in band gap energy, which further improves the photoelectrochemical performance of ZnO. Some of the non-metals doped and co-doped ZnO materials for water treatment are listed in Table 3.5.

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Table 3.5 List of non-metal-doped ZnO materials used for water treatment Photocatalyst B-ZnO N-ZnO C, N-ZnO

N-ZnO

Application Bisphenol A Bisphenol A Methylene blue Rhodamine B Methylene blue Rhodamine B Formaldehyde Rhodamine B Rhodamine 6G Methylene blue Amaranth

N-ZnO

Cr2O7
2

N-ZnO

Methylene blue Phenol

N-ZnO C-ZnO S, N-ZnO C-ZnO N-ZnO N-ZnO N-ZnO

N-ZnO

3.14

Feed concentration 100 ppm 100 ppm 10 ppm

Removal efficiency 77% 60% 100%

Reference Patil et al. (2011) Patil et al. (2011) Liang et al. (2016)

5 ppm 10 ppm

78% 99.7%

Oliveira et al. (2018) Xue et al. (2015)

5 ppm As such 0.02 nM 10 ppm

82.4% – 57% 94.3%

10 ppm

89.3%

10 ppm

88.5%

2.9  10
4 mol/ L –

73.5%

Cai et al. (2017) Zhou et al. (2009b) Zhang et al. (2013) Sudrajat and Babel (2016) Sudrajat and Babel (2017) Sudrajat and Babel (2017) Shifu et al. (2009)

60%

–

9.6%

Rajbongshi et al. (2014) Rajbongshi et al. (2014)

Conclusions

A systematic view on non-metals like carbon-, oxygen-, nitrogen-, sulphur- and fluorine-doped photocatalysts is presented. Non-metal doping allows formation of localized states in the band gaps of semiconductor photocatalysts owing to visible range electronic transitions. But ease of availability of photo-induced hole in such states is found to be difficult. Nitrogen doping results in a band just above the valence band, whereas fluorine allows formation of Ti3+ ions due to its electronegativity and charge compensation effect. When donor character of the element is considered, instead on donating valance electron, nitrogen will share them with an oxygen lattice in pi-bonding and forms species of NO. However, boron and carbon donates three and two valence electrons, respectively. It is also evident that co-doping of non-metals decreases the number of intrinsic defects, which reduces the electron-hole recombination. The present chapter also concludes that doping methods, dopant concentration, photocatalyst concentration and parameters like temperature and pH play a major role to achieve an efficient photocatalyst as well as process.

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Zhang D et al (2013) A facile method for synthesis of N-doped ZnO mesoporous nanospheres and enhanced photocatalytic activity. Dalton Trans 42(47):16556–16561. https://doi.org/10.1039/ C3DT52039K Zhang X et al (2014) Effect of aspect ratio and surface defects on the photocatalytic activity of ZnO nanorods. Sci Rep 4:4596. https://doi.org/10.1038/srep04596 Zhang Y et al (2015) C-doped hollow TiO2 spheres: in situ synthesis, controlled shell thickness, and superior visible-light photocatalytic activity. Appl Catal B Environ 165:715–722 Zhang Y, Yang HM, Park S-J (2018) Synthesis and characterization of nitrogen-doped TiO 2 coatings on reduced graphene oxide for enhancing the visible light photocatalytic activity. Curr Appl Phys 18(2):163–169 Zhiyong Y et al (2007) ZnSO4–TiO2 doped catalyst with higher activity in photocatalytic processes. Appl Catal B Environ 76(1):185–195. https://doi.org/10.1016/j.apcatb.2007.05.025 Zhou FL, Gong RH, Porat I (2009a) Polymeric nanofibers via flat spinneret electrospinning. Polym Eng Sci 49(12):2475–2481 Zhou X et al (2009b) Synthesis, characterization and its visible-light-induced photocatalytic property of carbon doped ZnO. Mater Lett 63(20):1747–1749. https://doi.org/10.1016/j. matlet.2009.05.018 Zhou Q et al (2013) Hydrothermal synthesis of various hierarchical ZnO nanostructures and their methane sensing properties. Sensors 13(5):6171 Zhu Y-P et al (2014) Carbon-doped ZnO hybridized homogeneously with graphitic carbon nitride nanocomposites for photocatalysis. J Phys Chem C 118(20):10963–10971. https://doi.org/10. 1021/jp502677h

Chapter 4

Challenges of Synthesis and Environmental Applications of Metal-Free Nano-heterojunctions Vagner R. de Mendonça, Osmando F. Lopes, André E. Nogueira, Gelson T. S. T. da Silva, and Caue Ribeiro

Contents 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Semiconductors in Heterogeneous Photocatalysis: Overview and Basic Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Synthesis and Processing of Heterostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Challenges in Synthesis and Processing of Heterostructures . . . . . . . . . . . . . . . . . . . . 4.2.2 Obtaining of Heterostructures by Simultaneous Phase Growth . . . . . . . . . . . . . . . . . . 4.2.3 Obtaining of Heterostructures by Phase Growth onto Preformed Supports . . . . . 4.2.4 Obtaining of Heterostructures Using Preformed Particles . . . . . . . . . . . . . . . . . . . . . . . 4.3 Applications of Metal-Free Nano-heterojunctions for Environmental Protection . . . . . . . 4.3.1 Water Splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Photoreduction of Carbon Dioxide (CO2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3 Photocatalytic Processes Applied to Heavy Metal-Contaminated Water Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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V. R. de Mendonça (*) Federal Institute of Education, Science and Technology of São Paulo, Itapetininga-SP, Brazil e-mail: [email protected] O. F. Lopes Institute of Chemistry, Federal University of Uberlândia, Uberlândia-MG, Brazil A. E. Nogueira Department of Chemistry, Institute of Exact and Biological Sciences, Federal University of Ouro Preto, Ouro Preto-MG, Brazil G. T. S. T. da Silva Department of Chemistry, Federal University of São Carlos, São Carlos-SP, Brazil Institute of Energy and Climate Research (IEK-3), Forschungszentrum Jülich GmbH, Jülich, Germany C. Ribeiro Institute of Energy and Climate Research (IEK-3), Forschungszentrum Jülich GmbH, Jülich, Germany Embrapa Instrumentation, São Carlos-SP, Brazil e-mail: [email protected] © Springer Nature Switzerland AG 2019 Inamuddin et al. (eds.), Nanophotocatalysis and Environmental Applications, Environmental Chemistry for a Sustainable World 29, https://doi.org/10.1007/978-3-030-10609-6_4

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4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

Abstract The rapid and unceasing population growth, concomitant with the need for economic development, has led to numerous environmental problems. The most remarkable problems are the release of pesticides in ground water or carbon dioxide in the atmosphere. In this context, nanomaterials are becoming increasingly vital for environmental protection due to their versatile compositions and means of application. It is well established that most of research devoted to environmental applications of heterostructures, materials made up of semiconductors that share a common interface, have addressed the degradation of organic contaminants in water. However, there are several emerging uses of heterostructures, for example, in gaseous systems as chemical reaction promoters and gas sensors. Since the properties presented by nanomaterials are strictly related to the morphology of the solid, the development of controllable and reproducible synthesis methods are one of the major focus of research in materials science. Currently, there is intense research towards synthesis methods able to produce heterostructures with controlled morphology and structural/surface properties useful for environmental applications. In this chapter, we discuss innovative approaches for synthesis of heterostructures, giving examples of several different systems, and applications beyond degradation of contaminants in water via heterogeneous photocatalysis, such as photoreduction/oxidation of metallic ions and gas-phase reactions, showing the versatility of such materials. Keywords Photocatalysis · Photodegradation · Photoreduction · Nanomaterials · Environmental · Wastewater treatment · Carbon dioxide photoreduction · Heavy metal abatement · Heterostructures · Semiconductors

4.1

Introduction

Life-threatening environmental issues, such as potable water contamination by organic compounds from agricultural or industrial wastewater, have been attributable to the rapid global population growth. This fact is automatically linked to the need for economic development (Nakata and Fujishima 2012). In view of this, water use management has been the focus of research in several countries, considering that 70% of the worldwide water consumption are credited to the agricultural and/or industrial activities. In developing countries, fertilizers and pesticides can be pointed out as major water contaminants. Several methods of treating water are known, such as coagulation, filtration, sedimentation, and adsorption. However, these techniques merely entail pollutant

4 Challenges of Synthesis and Environmental Applications of Metal-Free. . . Table 4.1 Reduction potential of chemical species versus normal hydrogen electrode

Specie Fluorine Hydroxyl radical (•OH) Oxygen Ozone Hydrogen peroxide Permanganate Chlorine Iodine

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Reduction potential (V) 3.03 2.80 2.42 2.07 1.78 1.68 1.36 0.54

Fig. 4.1 Examples of homogeneous and heterogeneous processes applied in hydroxyl radicals’ generation. As can be seen, the hydroxyl radical can be generated by action of ultraviolet (UV) irradiation, combined or not with semiconductor (SC), ultrasound (US) and numerous chemicals

phase changes, thus representing non-destructive methods. The management of the solid phase separated from the aqueous phase remains a challenge. Among the recognized destructive methods for contaminant-dissolved water treatment, such as organic effluents, the Advanced Oxidation Processes stand out mainly due to their simplicity and efficiency of contaminants degradation (Andreozzi 1999). Advanced Oxidation Processes are based on the formation of radicals with high oxidizing effect, for instance, hydroxyl radicals (•OH) and superoxide anion radical (O2•
). As denoted in Table 4.1, these radicals have the ability to promote degradation of several distinct organic compounds (Chen et al. 2010). In addition to suitable reduction potential values, the feasibility of hydroxyl and superoxide anion radicals being simply obtained in an aqueous medium and resulting in water-soluble oxidation products configures significant advantages to Advanced Oxidation Processes. Numerous methods have been applied for hydroxyl radical (•OH) formation as exposed in Fig. 4.1. They present specific advantages, and the application of a specific method must be related to the available conditions. Despite the efficiency, homogeneous processes exhibit certain drawbacks compared to heterogeneous techniques, such as the final solution pH, which normally

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requires correction before disposal and reagents consumption, resulting in more expansive methods. Therefore, methods based on heterogeneous systems are interesting both from environmental and economic perspectives due to the possibility of recycling the material that promotes radical formation. A variant of these processes is the hydroxyl radical generation by photochemical activation of nanostructured semiconductors in a process entitled heterogeneous photocatalysis (Linsebigler et al. 1995). Researches on heterogeneous photocatalysis displayed vertiginous growth after the work of Fujishima and Honda published in 1972 (Fujishima and Honda 1972). Interestingly, notwithstanding the importance of this report, the term heterogeneous photocatalysis, or even just photocatalysis, was not utilized in their text. Nonetheless, the authors detected water oxidation and the evolution of hydrogen (H2) and oxygen (O2) gases when a TiO2 suspension was irradiated in an electrochemical cell, launching an innovative area of photocatalysis research (Teoh et al. 2012). The relevance of the Fujishima and Honda work is evident because it reached more than 21,000 citations just over 45 years, which corresponds to an average of approximately 470 citations per year (more than one per day). This research was the main responsible for what can be called “modern heterogeneous photocatalysis”. Moreover, among several possibilities, the anatase TiO2 polymorph has become the “battle horse” of this research field (Serpone and Emeline 2012). Evidently, the same scientific principle applied in electrochemical cell presented by Fujishima and Honda can be idealized for organic contaminant oxidation in an aqueous medium. Thenceforth, several semiconductors have been tested as photocatalysts (Fan et al. 2017). To the best of our knowledge, the oldest research on photocatalytic properties of semiconductors, namely, ZnO, Sb2O3 and TiO2, carried out by Markham, dates back to 1955 (Markham 1955). In this work, various types of chemical reactions that these oxides could promote were described, among them the oxidation of organic compounds under ultraviolet radiation. Subsequently and especially after the aforementioned work performed by Fujishima and Honda, several types of research were devoted to understanding photocatalytic processes involving oxidation and degradation of organic compounds promoted by an ample variety of semiconductors.

4.1.1

Semiconductors in Heterogeneous Photocatalysis: Overview and Basic Concepts

A semiconductor band structure can be simply characterized in its fundamental state by a filled valence band and an empty conduction band. The energy difference between these two levels is called band gap energy. If the material absorbs energy superior or equal to its band gap energy, one electron can be promoted from valence to conduction band, leaving behind a positive hole in the valence band. This chemical event induces the heterogeneous

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Fig. 4.2 Simplified mechanisms of photocatalysis on a semiconductor surface: (1) charge photogeneration by absorption of photons with energy higher than semiconductor band gap (Ebg), electrons (negative charge – e
) and holes (positive charges – h+); (2) recombination of photogenerated charges with release of absorbed energy as photons or phonons; (3) reduction process by transfer of electron of conduction band to electron acceptors adsorbed on the solid surface (usually O2); (4) formation of hydroxyl radical (•OH) by electrons transfer from groups adsorbed on the solid surface, such as H2O or OH
, to pervade positive holes in valence band; (5) oxidation and posterior degradation of contaminants by means of hydroxyl radicals (•OH)

photocatalysis process, as exemplified in Fig. 4.2, which also illustrates events that lead to the formation of hydroxyl radical, the most important agent in organic compounds degradation (Gaya and Abdullah 2008). Valence band holes are powerful oxidants with a potential reduction between +1.0 and +3.5 V vs normal hydrogen electrode, while conduction band electrons are excellent reducing agents (+ 0.5 to
1.5 V vs normal hydrogen electrode). These values are dependent on the semiconductor and certain medium conditions, such as pH (Grätzel 2001). Both hydroxyl groups and water adsorbed on semiconductor surface can be oxidized to hydroxyl radicals, which contain sufficient potential to oxidize various organic compounds (surface-bonded OH/•OH ¼ 1.6 V vs normal hydrogen electrode) (Tojo et al. 2004). Contaminants could also be directly oxidized by valence band holes from a prior stage to these impurities’ adsorption which occurs on particle surface (information not shown in Fig. 4.2). Several factors influence semiconductors’ photoactivity, including specific surface area (Goesmann and Feldmann 2010), crystallinity, nature and/or quantity of groups/species present onto semiconductor surface, preferential exposure of a more reactive crystalline plane and the redox potential of both valence and conduction band (De Mendonça and Ribeiro 2011).

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Fig. 4.3 Calculated energy positions of conduction band edges and the valence band of selected semiconductors. (Adapted from Li et al. 2016a with permission from The Royal Society of Chemistry)

Figure 4.3 portrays the energy values (relative to vacuum) and reduction potential (corresponding to normal hydrogen electrode) of several semiconductors. According to this figure, it is possible to verify the applicability of a given semiconductor in the photocatalysis process involving the formation of superoxide and hydroxyl radicals. The spontaneity condition, although under standard conditions, is presented in the equations below for radical hydroxyl (∙OH) formation by the action of valence band hole and the establishment of superoxide anion (O2•
) radical through photoexcited electrons (e
) action in the conduction band. Initially, the hydroxyl radical formation should be analysed. For a spontaneous reaction, the positive holes in valence band (SCVB(h+)) must present a reduction potential higher than 1.6 V vs normal hydrogen electrode, since this is the value for the reduction potential of adsorbed hydroxyl groups (
OHads).


SCVB(h ) + e ! SCVB OHads + e
!
OHads SCVB(h+) + OHads!SCVB + •OHads +

•

E Reduction (V) X 1.6 (X-1.6)

Eq. 4.1 Eq. 4.2 Eq. 4.3

Analyzing the formation of the superoxide anionic radical by molecular oxygen and excited electrons (e) in the semiconductor conduction band (SCCB), it can be observed that the reaction is only spontaneous when the reduction potential of the conduction band in this excited state is lower than 0.33 V.





SCCB + e ! SCCB(e ) O2 +
! O2•
SCCB(e
) + O2! SCCB + O2•


E Reduction (V) X
0.33 (
3.33-Y)

Eq. 4.4 Eq. 4.5 Eq. 4.6

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With basis on these data and Fig. 4.3, it is achievable to select at least the possible candidates to be applied as photocatalysts in a heterogeneous photocatalysis process conducted in an aqueous medium. However, there are both scientific and technological obstacles for the application of semiconductors in effluents treatments that still need further studies. Among the main limiting factor, the need for ultraviolet radiation (wavelength lower than 400 nm) to activate most semiconductors, such as TiO2 and ZnO (band gap energy of 3.2 and 3.4 eV), can be underlined, which turns solar light use complicated. This is an aspect of extreme importance since the inexhaustible solar energy is one of, whether not the more, important renewable energy resources. Nevertheless, solar radiation contains approximately 3% of ultraviolet, 44% of visible light and 53% of infrared radiation, which considerably limits, in economic terms, the reaction performance under solar radiation (Chatterjee and Dasgupta 2005; Madhusudan et al. 2013; Schultz and Yoon 2014). In this context, semiconductors that could be activated under visible radiation attract much interest (Chen et al. 2010). Interesting examples are ternary bismuth oxides, especially bismuth vanadate (BiVO4), an n-type semiconductor that can exist in three different polymorphs: tetragonal structures (zirconium-type or scheelite-type) and monoclinic structure (scheelite-type). Among these three polymorphs, reports show that the monolithic phase (band gap energy of 2.4 eV) presents enhanced photocatalytic performance under visible radiation, principally due to its band gap energy (Li et al. 2013; Zhou et al. 2009; Zhang et al. 2006, 2008; Obregón et al. 2012), which is resulted from the hybridization of the 2s orbital of bismuth with the 2p orbital of oxygen. This causes a shift in the compound valence band, reducing the energetic difference between valence and conduction band (Park et al. 2013; Yang et al. 2013). Another possibility for contaminants degradation using visible radiation and consequently solar radiation is through the photosensitization mechanism. This occurs when a molecule that absorbs visible light is adsorbed on the semiconductor surface and is excited from its fundamental state to the excited state (Henderson 2011). The molecule in the excited state can, spontaneously, inject electrons into the semiconductor conduction band. Therefore, this will be oxidized, resulting in the first stage of the molecule degradation. An example of this mechanism, as shown in Fig. 4.4, is the degradation of Rhodamine B dye by niobium pentoxide (Nb2O5) (Lopes et al. 2014). Niobium pentoxide displays a band gap of about 3.1 eV; thus, it cannot be activated under visible radiation. Its non-photoactivity in the visible region, at least by the mechanism discussed in Fig. 4.4, can be confirmed by assaying with molecules that present none radiation absorption in the visible region, such as atrazine pesticide. However, when carrying out tests under equal conditions with Rhodamine B dye, which is susceptible to adsorption on the semiconductor surface, degradation can be observed. This fact points out that photosensitization plays a fundamental role in dye photodegradation. Moreover, this statement is best understood when checking the compound redox potential both before and after the excitation by visible radiation. When irradiated,

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Fig. 4.4 Degradation mechanism of Rhodamine B dye (RhB) through the niobium pentoxide surface sensitization process. In this mechanism, Rhodamine B dye adsorbs onto Nb2O5 surface, and then this is excited from its fundamental state to the excited state by visible irradiation absorption. Rhodamine B in the excited state can, spontaneously, inject electrons into the Nb2O5 conduction band. Finally, this molecule will be oxidized

Rhodamine B (0.95 V vs normal hydrogen electrode) is excited to Rhodamine B* (
1.42 V vs normal hydrogen electrode). Accordingly, the electron can spontaneously be transferred to the conduction band of Nb2O5 (
0.9 V vs normal hydrogen electrode) in the dye excited state. Also, this electron in Nb2O5 conduction band could spontaneously reduce the molecular oxygen (O2) to the superoxide radical (O2•
) (
0.33 V vs NHE), in accordance with Eq. 4.5 and data shown in Fig. 4.3. Recombination of electron-hole pair formed in the semiconductor, as shown in process 2, Fig. 4.2, occurs within a few nanoseconds, impeding charge migration to semiconductor surface and posterior formation of radicals. This negatively reflects on the photocatalyst performance for organic pollutant degradation (Teoh et al. 2012). The importance of O2 during the photocatalytic process becomes thus highlighted since oxygen is reducible by capturing the excited electron in semiconductor conduction band and slowing down the recombination process presented in Fig. 4.2. The conduction band containing the electron must have a reduction potential sufficiently negative to reduce the molecular oxygen (O2/O2•
¼
0.33 V vs normal hydrogen electrode), as portrayed in the set of Eqs. 4.4, 4.5 and 4.6 (Jang et al. 2012). In order to overcome this issue of fast recombination of electron-hole pairs, certain strategies have been adopted to enhance charge lifetime after photon adsorption (Jang et al. 2012). Composites of the active phase containing metals, such as silver (Ag+/Ag 0.799 V vs normal hydrogen electrode), gold (Au3+/ Au 1.5 V vs normal hydrogen electrode) and platinum (Pt2+/Pt 1.2 V vs normal

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Fig. 4.5 (a) Different types of heterojunctions between semiconductors. In this image, Evac represents the reference point of energy (vacuum); Ec is the conduction band energy; Ev is the valence band energy; Ef denotes Fermi energy; φ is the work function and χ portrays electroaffinity; (b) schematic representation of one heterostructure (type 2) composed of two distinct semiconductors and charge migration responsible for the increase in the photogenerated charge lifetime

hydrogen electrode), are promising since these metals can spontaneously capture the photogenerated electrons (Heiligtag et al. 2014). Another important strategy to diminish the pair recombination rate is the formation of heterostructures (Jang et al. 2012; Chandrasekharan and Kamat 2000; Li et al. 2014), systems containing more than one material, usually semiconductors, in a single particle, e.g. structures formed of gold nanoparticles over titanium dioxide (TiO2/Au), molybdenum disulphide and carbon nitride (MoS2/g-C3N4) and tin

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Table 4.2 Examples of the different type of heterostructures applied as photocatalysts Heterostructure F3O4/ZnO

Type I

Application MB dye oxidation

Bi2S3/CdS Bi2S3/CdS BiOCl/BiOBr MoS2/ZnInS4 g-C3N4/Nb2O5 BiVO4/g-C3N4 Ag3PO4/g-C3N4 TiO2/SnO2 TiO2/SnO2 TiO2 –nanoparticle/ nanorod TiO2/doped vacancy BiOI – nanosheets/ microplates CeO2 – facet-based g-C3N4 – bulk/ nanosheets

I I II II II II II II II Homojunction

RhB dye oxidation MR dye oxidation MO dye oxidation H2 evolution RhB and AML oxidation RhB and p-NP oxidation MB dye oxidation RhB dye oxidation RhB dye oxidation Benzene oxidation

Homojunction Homojunction

Water splitting RhB and BPA oxidation, H2 evolution CO2 reduction MB dye oxidation

Homojunction Homojunction

Reference Karunakaran et al. (2014) Shi et al. (2014) Fang et al. (2011) Jia et al. (2017) Li et al. (2017) da Silva et al. (2017) Zhao et al. (2016) Liu et al. (2016a) Yuan et al. (2014) Liu et al. (2007) Wang et al. (2016) Liu et al. (2017) Huang et al. (2017) Li et al. (2015) Choudhury and Giri (2016)

In the table, MB is methylene blue, RhB is Rhodamine B, MR is methyl red, MO is methyl orange, AML is amiloride, p-NP is p-nitrophenol, and BPA is bisphenol-A

dioxide over titanium dioxide (TiO2/SnO2). In this context, the junctions between materials are called heterojunctions. Heterostructures are classified into three different types, depending on the relation between energy bands of the constituent materials. The possibilities are illustrated in Fig. 4.5a. Each type of heterostructure is considered more appropriate for specific applications, depending on their electronic properties resulting from the creation of a defined interface between materials. The suitability of heterostructure for a given process is directly related to the charge migration that takes places in the connection interface between the two materials. This charge migration is due to differences in the chemical potentials of electrons in the crystalline structure, represented by the Fermi level (De Mendonça et al. 2014). Charge movement direction is ruled by the work function of each material and will occur until the required thermodynamic equilibrium is established. Even though the application of type 1 heterostructures is common in several systems, the most suitable heterojunction is the type 2 in terms of photocatalysis application. In type 2 model, photogenerated charge migration arises in opposite directions due to the relation between semiconductor bands and mainly due to their Fermi levels. This relocation prevents further electron-hole pair recombination. A semiconductor with appropriate band positions would act as O2 (capturing electrons) in a photocatalytic system (Jang et al. 2012). Table 4.2 exhibits several

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cases of heterostructures applied to degradation of organic contaminants found in aquatic environments. It is possible to verify a vast scope of possibilities in the assembly of heterostructures containing semiconductors. Also, as presented in Table 4.2, materials presenting same composition and a crystalline phase, but different morphologies, can act as heterostructures (forming homojunction), since different crystallographic directions may present different electronic properties, mainly charge mobility. Serpone et al. published in 1984 one of the pioneer studies about charge transfer between semiconductors (Serpone et al. 1984). The authors have succeeded in demonstrating that coupling between TiO2 and CdS leads to a noticeable separation of charges, which prevents recombination. In this case, electrons photogenerated in conduction band of CdS after energy absorption were transferred to conduction band of TiO2, while positive holes remained in the electronic CdS structure. Thenceforth, various combinations of semiconductors have been developed and studied. An interesting example is the case of TiO2 Degussa P25, which presents the anatase and rutile phases in its structure and has been a topic of discussion in literature for several years. TiO2 is studied due to several reasons, such as commercial availability, stability, low cost, non-toxicity, adequate values of electron reduction potential and photogenerated holes in the conduction and valence bands, respectively. Nevertheless, there are also historical reasons for the outstanding attractiveness of TiO2, more specifically after the seminal work of Fujishima and Honda. During the mid-1990s, researchers attempted to explain the elevated photoactivity of TiO2 mixing phases when compared to isolated phases. The hypothesis of electron transfer from anatase to electron traps at less energetic sites of rutile phase, leading to a longer lifespan of photogenerated charges, was accepted until the beginning of the twenty-first century (Hurum et al. 2003). However, several other hypotheses on this thrilling topic emerged to date. For instance, an article published in 2017 demonstrated that photocatalytic production of hydrogen gas indicated that the synergism between anatase and rutile phases, frequently attributed to the lifespan increase of photogenerated charges, may be connected to an expansion of anatase phase network, which would in turn affect the material electronic/ catalytic properties (Wahab et al. 2017). In contrast, another work also published in 2017 has directly shown, by Kelvin probe force microscopy (KPFM) and home-built spatially resolved surface photovoltage spectroscopy (SRSPS), the charge separation at the interface of TiO2 phase junction (Gao et al. 2017). These reports reveal how explanations are still divergent, even with the increasing quantity of publications on heterostructures application in photocatalytic processes. In this chapter, different strategies for synthesis and processing to obtain heterostructures presenting interface that enables charge migration will be presented. Afterwards, the most diverse environmental application of these heterostructured nanomaterials will be discussed. Moreover, the organic contaminant degradation will also be addressed with focus on water-splitting processes, artificial photosynthesis, photoreduction and heavy metal photooxidation.

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Synthesis and Processing of Heterostructures Challenges in Synthesis and Processing of Heterostructures

The foremost challenge in developing efficient heterostructures for distinct applications is the definition of which semiconductors can be combined in order to exhibit a maximized electron-hole pair lifespan and simultaneously present appropriate positions of valence and conduction bands to conduct photooxidative reactions. Nonetheless, the need to create interfaces/junctions between two semiconductors that often present none crystallographic similarities (i.e. comparable values of interplanar distances) generates an additional challenge, which is the synthesis and/or processing to obtain the final heterostructure. Numerous synthesis methods have been applied to obtain heterostructures from different semiconductors, such as sol-gel, polymer precursors and hydrothermal method. However, the major variations in obtaining heterostructures lie in the utilized strategy rather than in the method itself, i.e. simultaneous growth of two phases in the equal reaction medium, phase growth over a preformed second phase or heterostructure formation applying preformed particles as building blocks. Prior to any discussion regarding strategies that can be used to obtain heterostructures, understanding the mechanisms involved in crystal growth is paramount, since this description can be extended to the comprehension of heterostructure formation. During the particle formation process within a reaction system, two stages can be emphasized, nucleation and growth. In certain systems, these processes occur simultaneously and are difficult to distinguish. Nonetheless, there is a sequence of events similar to the mechanism described in Fig. 4.6 that denote the free energy variation of a nucleus as a function of its radius, obtained by Eq. 4.7, considering spherical particles (Leite and Ribeiro 2012). ΔGðr Þ ¼ 4=3πr 3 ΔGV þ 4πr 2 γ

ð4:7Þ

Intending to enable the growth process, the nucleus must overcome the energy barrier. The nucleus-free energy equation ΔG(r) contains a term referring to free energy of solid formation per volume unit ΔGV and another related to its surface energy (γ). Since the ratio between the atoms’ quantity on the surface and on its interior is inversely proportional to the particle radius (r), the minor the particle, the greater the term contribution referring to surface energy in Eq. 4.7. Thus, below a determined critical radius r*, which stages the typical functions of atoms composing the nucleus, this is unstable and can be dissolved. Whether this critical radius value is reached, the bonding energy among atoms composing the crystal becomes sufficiently elevated and the particle tends to expand until chemical equilibrium is achieved (Leite and Ribeiro 2012). Several studies have demonstrated that the mechanism described is not solely responsible for crystal growth, especially in nanometric systems. Oriented

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Fig. 4.6 Free energy variation of a nucleus during the particle growth. Below a critical radius, a reversible process occurs; for radius higher than the critical radius, the process becomes irreversible. The processes are schematized as chemical reactions at the bottom part (Dalmaschio et al. 2010)

coalescence was proposed as another significant process for nanometric structures growth, being also noticed in micrometric scales (Huang et al. 2003; Penn and Banfield 1998a, b; Ribeiro et al. 2005). Various works have used this mechanism to explain the anisotropic growth of different materials, such as SnO2 (Lee et al. 2005; Stroppa et al. 2011), TiO2 (Ribeiro et al. 2009; Da Silva et al. 2011), ZnS (Yu et al. 2005; Zhang and Zhang 2013) and BiVO4 (Ai and Lee 2013) among others. Nanocrystals can expand through crystallographic alignment and coalescence of neighbouring particles by common interface elimination. Considering the localized nature of the oriented coalescence mechanism, the process often leads to the formation of nanoparticles or nanostructures with irregular morphologies (anisotropic), which are not expected in classical mechanisms. Studies have elucidated that this mechanism is significant at the early growth stages and may be involved in the formation of anisotropic particles in suspension, such as nanorods and nanowires, through particle consumption as building blocks (Lee et al. 2005; Stroppa et al. 2011; Polleux et al. 2005; Ribeiro et al. 2006).

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For the synthesis of heterostructures, both the classical (Ostwald maturation) (Mokari et al. 2005; Chen et al. 2012; Hu et al. 2013) and nonclassical (oriented coalescence) (Talapin et al. 2007; Zhang et al. 2009; Ribeiro et al. 2007; Yu et al. 2011), growth particle mechanisms at equilibrium can be explored. A noteworthy challenge in obtaining heterostructures is the control of morphology and compound surface properties that are suitable for the specific application. In addition, it is fundamental that interfaces between different phases are achieved and that phase segregation is avoided. In next sessions, these characteristics will be discussed in terms of each chosen strategy.

4.2.2

Obtaining of Heterostructures by Simultaneous Phase Growth

Scheduling feasibility for viable industrial plants from a synthetic route involving merely a single stage is a strong argument in favour of heterostructure formation by simultaneous phase growth (Alves de Castro et al. 2015). On the other hand, the simultaneous crystallization process may lead to phase segregation or undesired doping. This implies that detailed studies are still needed to propose reliable methods, aiming to ensure the desirable oxide proportion that can enable scaled-up production (de Mendonça et al. 2015, 2017). One possible route for obtaining heterostructures via simultaneous crystallization is the use of polymer precursors, which was originally described in 1967 and further became popular as one of the most versatile methods for the obtaining of several compounds (Pechini 1967). One a particular case is the system made up of TiO2 and WO3. The three-dimensional polymer network can sustain the cations homogeneously dispersed throughout its structure if it is properly obtained, which can prevent oxide/hydroxide precipitation and phase segregation over the calcination step. This synthesis method was effective in preparing TiO2/WO3 heterostructures with photocatalytic activity on Rhodamine B degradation superior to those of the single TiO2 and WO3 phases or their physical mixture (Alves de Castro et al. 2015). Mourão et al. (2012) evaluated the formation of TiO2/SnO2 heterostructures in highly alkaline solutions with further simultaneous crystallization under hydrothermal conditions. The authors observed that the heterostructure exhibited higher photocatalytic performance in comparison to the single phases due to the reduction of the electron-hole pair recombination velocity. Junctions formed between oxide polymorphic phases, which are called homojunction, can also be obtained via simultaneous crystallization, as demonstrated by the synthesis of heterostructures from the rutile and anatase TiO2 phases by simultaneous crystallization using hydrothermal process (Libanori et al. 2012). The anisotropy that could expose specific crystallographic planes and the synergy arising from the coexistence of the anatase and rutile phases improved the photocatalytic activity in comparison with the single anatase and rutile phases.

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Based on the titanium dioxide P25 (Degussa) heterostructure, which is made up of anatase and rutile phases, it is also possible to obtain several other homojunction with elevated photoactivity. In this context, the synthesis of a heterostructure from monoclinic and tetragonal BiVO4 phases was proposed using a simultaneous phase growth strategy by hydrothermal crystallization (Lopes et al. 2016). The BiVO4 monoclinic/tetragonal homojunctions were proven to be controlled by the reaction parameters. The role played by the heterostructures on the catalytic oxidation of organic pollutants was also verified. Therefore, the so-called soft-chemical methods could enable the synthesis of specific heterostructures using the one-step simultaneous crystallization approach. Nevertheless, a clear drawback of such approach is the impossibility of producing heterostructures with controlled morphology and composition.

4.2.3

Obtaining of Heterostructures by Phase Growth onto Preformed Supports

Phase crystallization onto a pre-existing surface is a useful strategy for the synthesis of heterostructures with minimal doping effect. The structural interfaces and in some cases the morphology all phases contained in the heterostructure could be tailored effectively. This approach was successfully applied to growth of SnO2 onto preformed TiO2, with deposition carried out using not only the hydrolytic sol-gel method but also the polymer precursor method (De Mendonça et al. 2014). An effective morphological control was demonstrated, and that the formed interfaces increased the photocatalytic activity in comparison with the pure SnO2 and TiO2 phases. Also importantly, the polymer precursor method attained high dispersion of SnO2 phase onto TiO2 due to slow formation of the former over the calcination step, thereby leading to heterostructures with higher photoactivity in relation to the hydrolytic sol-gel method. Another interesting example is the use of hydrothermal methods to precipitate phases onto a preformed supporting material, as shown for new g-C3N4/Nb2O5 heterostructures synthesized from hydrothermal Nb2O5 particles growth on preformed g-C3N4 with morphology and phase proportion control (Carvalho et al. 2016). The g-C3N4/Nb2O5 heterostructures exhibited homogeneous dispersion of the Nb2O5 particles onto the g-C3N4 surface, which resulted in superior photocatalytic activity in comparison to the pure g-C3N4 and Nb2O5 phases or their physical mixture. A similar strategy was reported for the synthesis of heterostructures based on CuO growth onto preformed Nb2O5, which also resulted in enhanced photocatalytic activity for chromium (VI) reduction. Recently, a modified surface-supported phase crystallization strategy was reported, which consisted in using a preformed phase containing the same metallic element of the phase to be crystallized, thus forcing interfacial assembly between semiconductors because the metallic ions present at the preformed particle surface

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act as a reactant for the second phase growth and due to the solubility difference between the two phases. The suitability of this modified strategy relies on the phases to be formed. For instance, BiVO4 nanoparticles can be grown onto a sacrificial preformed Bi2O3 surface, using the principle that BiVO4 is less soluble than Bi2O3. This strategy is also useful for exploiting the preformed phase as a template for the heterostructure morphology. For the BiVO4/Bi2O3 system, the number of heterojunctions can be tuned by the reaction parameters (temperature, vanadium concentration, etc.) and by the preformed Bi2O3 particle size. The BiVO4/Bi2O3 heterostructures exhibited an enhanced photocatalytic activity in relation to the pure BiVO4 and Bi2O3 phases, which was particularly attributed to the increase of spatial separation in the photogenerated electron-hole pairs due to type 2 heterostructure assembly (Lopes et al. 2017).

4.2.4

Obtaining of Heterostructures Using Preformed Particles

An innovative heterostructure synthesis method is the use of two preformed phases. The main advantage of this method is the easier morphological and structural control involved in the synthesis of free particles, which can be further used as building blocks to form heterostructures. This process occurs via orientated coalescence by particle collision, where interfaces are created by crystallographic alignment and coalescence of neighbouring particles with the elimination of common interfaces. Such mechanism requires that both faces have similar crystallographic planes. A typical case is TiO2/SnO2 heterostructure (de Mendonça et al. 2015, 2017), which can be assembled by collision-induced heteroaggregation of preformed TiO2 and SnO2 phases with subsequent orientated coalescence under hydrothermal conditions. The formation of heterojunctions is confirmed by the increase of photogenerated charge lifespan that is correlated indirectly with the hydroxyl radical formation rate and by the increased photocatalytic activity on Rhodamine B oxidation. The use of two preformed phases to synthesize heterostructures has also been demonstrated for TiO2 and WO3 (de Castro et al. 2014). The authors confirmed that the interfaces between the TiO2 and WO3 phases were created from an orientated coalescence mechanism and that these interfaces increased the photocatalytic activity on Rhodamine B degradation. Moreover, the synthesis of g-C3N4/Nb2O5 heterostructures starting from preformed g-C3N4 e Nb2O5 phases comprising different surface charges was reported (da Silva et al. 2017), although the interface creation was induced by sonochemical treatment. The formation of heterostructures was demonstrated by the increased charge carrier lifespan as well as the enhanced photocatalytic activity for organic pollutants degradation.

4 Challenges of Synthesis and Environmental Applications of Metal-Free. . .

4.3 4.3.1

123

Applications of Metal-Free Nano-heterojunctions for Environmental Protection Water Splitting

Hydrogen is considered as the ideal combustible for the future, once the nonrenewable resources commonly used for energy production will be eventually depleted (Zhang et al. 2017). Hydrogen is also considered as a “green” fuel because its final combustion product is water. Hydrogen fuel is also advantageous due to its energy conversion efficiency, which is 2.5-fold larger than most conventional energy sources (Tee et al. 2017; Christopher and Dimitrios 2012). However, the successful use of hydrogen as a fuel, considering both environmental and economic scenarios, requires the development of effective hydrogen generation processes. Nowadays, 95% of the commercial hydrogen production is based on fossil sources, while renewable sources correspond to only 5% (De Oliveira Melo and Silva 2011). The efficient hydrogen production from renewable sources is still a topical challenge. Hydrogen generation processes based on photoactive systems have attracted much attention, particularly due to the possibility of using water and solar radiation as carbon-free renewable inputs (Tentu and Basu 2017; Maeda 2011; Nguyen and Wu 2018). Water photolysis requires semiconductors having conduction bands with more negative potential in relation the potential for reducing H+ to H2 (E ¼ 0V vs normal hydrogen electrode at pH ¼ 0), whereas their valence bands must display a more positive potential than the potential necessary for oxidizing H2O to O2 (E ¼ 1.23 V vs normal hydrogen electrode) (MA et al. 2013). The global reaction is not thermodynamically favourable with free Gibbs energy of ΔG ¼ 238 kJ mol
1 and cell potential of ΔE ¼ 1.23 V, but it could occur if electromagnetic radiation with wavelength lower than 1000 nm (1.23 eV, which correspond to photons with energy in the near IR range) is applied (MA et al. 2013; Kudo 2003). Photo-induced electrons are transported towards the semiconductor surface where they participate in the reduction/oxidation reaction of the adsorbed water molecules. Each step comprising the photocatalytic water molecule breakage, including radiation absorption, charge separation, charge transport and surface adsorption capacity, plays a vital role on the semiconductor efficiency for hydrogen production (Kudo 2003; Maeda and Domen 2010). In this context, developing type and architectures of semiconductors with desired properties have been the current scientific focus, mainly for the better use of solar radiation. There is a wide variety of semiconductors, such as oxides, sulphides and nitrites, able to promote water molecule breakdown for hydrogen fuel production (Chen et al. 2016; Balogun et al. 2017). Synthesis of non-metallic photocatalysts (g-C3N4, graphene and carbon nanotubes) from abundant precursors has been recently investigated as an attempt to reduce the hydrogen production costs (Ong et al. 2016; Cao and Yu 2016). In particular, carbon nanotubes, graphene and g-C3N4 have attracted much interest as metal-free semiconductors for photocatalytic application. g-C3N4 shows a band gap

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Fig. 4.7 Positions of valence and conduction bands of TiO2/g-C3N4 heterostructure, showing a type 2 heterostructure, and redox potentials vs normal hydrogen electrode of H+/H2 and
OH/O2 at pH ¼ 7. In such structure, excited electrons tends to accumulate in TiO2 conduction band, while positive holes accumulate in g-C3N4 valence band Fig. 4.8 Possible mechanism of photogenerated charge carrier transfer in a heterostructures formed by g-C3N4 nanosheets, carbon nanotubes and Bi2WO6 under visible light exposure. In the picture, CNT means carbon nanotubes and BWO is the bismuth tungstate Bi2WO6

with good response at the visible range and conduction and valence band potentials,
1.3 eV and 1.4 eV, respectively, in relation to the normal hydrogen electrode, ideal for the water molecule breakage process (Figs. 4.7 and 4.8) (Liu et al. 2016b; Zhou et al. 2017). The adequate band potentials of g-C3N4 alongside its good thermal and chemical stability, easy preparation and low cost make g-C3N4 a good semiconductor for photocatalytic hydrogen production (Srinivasu et al. 2014).

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Table 4.3 Photocatalytic activity of different g-C3N4-based heterostructures Material CuS-ZnS1-xOx/gC3N4 g-C3N4/CoO

Radiation Ultraviolet

Lamp potency 300 W

Evolution of H2 (μmol. g
1.h
1) 12.20

Visible

300 W

651.3

MoS2/g-C3N4/GO

Visible

300 W

165.00

CaIn2S4/g-C3N4

Ultraviolet

500 W

102.00

CoTiO3/g-C3N4 CoP-CdS/g-C3N4

Visible Visible

300 W 300 W

858.00 23,536.00

Reference Chang et al. (2017) Mao et al. (2017) Wang et al. (2017a) Jiang et al. (2015) Ye et al. (2016) Wang et al. (2017b)

The photocatalytic efficiency of g-C3N4 is limited by its low surface area, fast recombination of electron-hole pairs and slow charge transfer. Strategies have been adopted to minimize these effects and increase the photocatalytic efficiency of g-C3N4. Template-based synthesis has been widely used to improve the photocatalytic properties of g-C3N4 (Zhou et al. 2017; Ke et al. 2017). The morphological changes induced by the templates increase the number of pores and consequently increase surface area and improve heterojunction formation, which are vital structural aspects to increase the charge separation efficiency of g-C3N4. An interesting example of such strategy is the synthesis of heterostructures composed by g-C3N4 and reduced graphene, represented by RGO. Several reports show that the photocatalytic activity of g-C3N4 is enhanced after formation of heterostructures, Table 4.3 (Li et al. 2016b; Nikokavoura and Trapalis 2018).

4.3.2

Photoreduction of Carbon Dioxide (CO2)

Photocatalytic CO2 reduction, also referred to as artificial photosynthesis, is one promising long-term solution to transform radically the current fossil fuel-based economy into a sustainable “photon” economy (Mao et al. 2017; Lim et al. 2015; Mondal et al. 2015; Li et al. 2016c). In recent years, the significant progress on understanding the CO2 reduction mechanism has attracted the attention of many researchers, also because the CO2 atmospheric abundance makes it a highly attractive feedstock (Morris et al. 2009). Indirect solar energy storage is another advantage of the photocatalytic CO2 reduction. The main products and the reduction potentials, represented by E Reduction, of the photocatalytic CO2 reduction are described below (Li et al. 2016c; Habisreutinger et al. 2013):

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0.53
0.61
0.48
0.38
0.24

CO2 + 2e
+ 2H+ ! CO + H2O CO2 + 2e
+ 2H+ ! HCOOH CO2 + 4e
+ 4H+ ! HCOH + H2O CO2 + 6e
+ 6H+ ! CH3OH + H2O CO2 + 8e
+ 8H+ ! CH4 + 2H2O

Eq. 4.8 Eq. 4.9 Eq. 4.10 Eq. 4.11 Eq. 4.12

The CO2 photoreduction occurs according to different steps that involve adsorption, activation and dissociation of the carbon and oxygen bond. The activation step is especially challenging because of carbon dioxide highly stable and inert (D’Alessandro et al. 2010). Competition with other reduction reactions such as H2 evolution from H2O photolysis and O2 to O2
reduction significantly reduces the CO2 photoreduction efficiency (Ma et al. 2014). A possible alternative would be identifying oxides with high surface affinity to CO2, and simultaneously, with electronic properties suitable to reduce the gas via a photo-activated mechanism. Photoreducing CO2 is complex and more difficult to be performed than the H2 evolution, mainly due to the high CO2 stability, related to its enthalpy value of 750 kJ mol
1, and the assortment of products formed in the CO2 photoreduction process (Chang et al. 2016). In the first step, the CO2 molecule is adsorbed on the photocatalyst surface and reacts with electrons to produce carbon dioxide radicals (CO2•
), which further reacted with H+ ions to form surface •CH3 radicals, ultimately producing CH4 (Lingampalli et al. 2017): •H

•
H þ þe
þ H þ e
CH3 CO2 e
e
CO2

þ þe
Hþ þ e
CH4

ð4:13Þ

Generally, H2O is an ideal electron donor and hydrogen source for the photocatalytic CO2 reduction. Nevertheless, one problem of using H2O as a H+ ion generator is the competition with the H2 formation process (Zhang et al. 2014). The photoreduction of CO2 to CH4 requires eight electrons (Eq. 4.14), while the H2 generation requires only two electrons (Eq. 4.15). Additionally, the reaction mechanisms are more complex due to several steps in which electrons and protons must be transferred to CO2 molecules consecutively and simultaneously, making the photoreduction both thermodynamically and kinetically difficult (Han et al. 2017). CO2 + 8e
+ 8H+ ! CH4 + 2H2O H+ + 2e
! H2


0.61
0.48

Eq. 4.14 Eq. 4.15

In order to solve the kinetic and thermodynamic limitations of the CO2 photoreduction, the semiconductors must exhibit an efficient charge separation and good affinity to CO2 so that H2 formation is prevented, while the charges are transferred to CO2. As described earlier, the charge separation challenge has been overcome through the synthesis of heterojunctions with suitable properties. For example, Wang and collaborators demonstrated that when the CO2 photoreduction is carried out in the

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presence of the CdSe/Pt/TiO2 heterostructures, CH4 is formed in a suitable manner of 48 ppm g
1 h
1, with traces quantities of H2 among other products. When Pt is replaced by Fe, the photoreduction is directed towards H2 generation, reaching values higher than 55 ppm g
1 h
1, thus showing that the type of semiconductor heterostructure is fundamental for the effective CO2 photoreduction (Wang et al. 2010). Carbon nanotube composites also attract considerable interest due to their intrinsic properties, such as remarkable mechanical strength, large surface area and good electrical conductivity. They can assume semiconductor, semimetallic or metallic electrical behaviour depending on their helicity and tube diameter. Furthermore, the presence of π-electrons at the carbon nanotube surface enables fixation of atoms and molecules that are essential for the creation of junctions between two phases. Thus, carbon nanotubes are good candidates for the development of heterostructures (Yan et al. 2015; Price et al. 2016). Metallic oxide/carbon nanotube heterostructures were recently reported as effective catalysts for H2 generation and CO2 photoreduction from water splitting and for degradation of organic pollutants (Lashgari et al. 2017; Sampaio et al. 2018; Lv et al. 2017; Chung et al. 2017). Jiang et al. described the synthesis of ternary heterostructures, namely, Z-scheme, based on g-C3N4 nanosheets, carbon nanotubes and Bi2WO6 nanosheets, in which the carbon nanotubes were employed as an electron mediator between the two nanosheets (Fig. 4.9). They observed that the ternary heterostructures exhibited a better performance on the tetracycline chlorinates degradation when compared to the pure materials (Jiang et al. 2018). Sun et al. reported the synthesis of heterostructures formed by metal-free compounds for photocatalytic applications as an alternative to the conventional semiconductor oxides. They described the formation of heterostructures between C3N4 sheets and carbon nanospheres, in which the junctions resulted in a fivefold increase of photoactivity in relation to pure C3N4. This expands the concept of designing Fig. 4.9 Schematic of photocatalytic oxidation of As(III) to As(V), showing the formation of radicals that react with the metal

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nanomaterials for applications that require elevated photoactivity performance (Sun et al. 2014).

4.3.3

Photocatalytic Processes Applied to Heavy MetalContaminated Water Treatment

Heavy metal ions including chromium (Cr), mercury (Hg), lead (Pb), cadmium (Cd), chromium (Cr) and arsenic (As) can be found as contaminants in residual waters. The high toxicity and environmental impacts caused by heavy metals have motivated the development of technological processes for removing these contaminants from wastewaters, in particular, semiconductor-based photocatalysis. Photoreduction using semiconductors is also widely used for the treatment of effluents contaminated with hexavalent chromium (Cr(VI)), which is an awfully toxic species that possesses mutagenic and even carcinogenic effect on living organisms (Pradhan et al. 2017). On the other hand, trivalent chromium (Cr(III)) is an essential nutrient for the human body, being used at tiny amounts in metabolism processes (Anderson 1997). Cr(VI) is widely used in the various industrial process, such as electroplating and leather tanning, which are known for generating large amounts of effluents. Cr(VI) is typically found as anionic species Cr2O72
or CrO42
, which do not precipitate with raising pH (Nogueira et al. 2016). In this case, one step to treat Cr(VI)-containing effluents is to reduce chromium (IV) to chromium(III) so that chromium can be precipitated in further steps (Gao and Liu 2017). Photoreduction is as an alternative to conventional chromium treatment processes, for example, the use of reducing agents (H2S and ascorbic acid) in acidic medium because it does not generate secondary residues and uses solar radiation as an energy source, which makes the process more economically feasible (Vinuth et al. 2015). The chromium (VI) photoreduction in semiconductors can be summarized by the following equations, when TiO2 is applied in the process (Djellabi et al. 2016): TiO2 þ hv $ e
CB þ hþ VB 2H2 O þ 4h

þ

VB

$ O2 þ 4H

þ

HCrO4
þ 7Hþ þ 3e
$ Cr3þ þ 4H2 O

ð4:16Þ ð4:17Þ ð4:18Þ

After the absorption of energy higher than its band gap energy and the formation of electron in the conduction band (e
CB) and holes in the valence band (h+VB), the photogenerated charges act oxidizing water and reducing the anion HCrO4
. The process depends on the initial Cr(VI) concentration in aqueous solution and follows a different mechanism depending on pH. Studies have shown that Cr (VI) photoreduction at pH above 4 follows the first-order kinetics, whereas the

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reaction kinetics assumes zero order when the photoreduction is conducted at pH between 1 and 3 (Wang et al. 2004). 4HCrO4
þ 16Hþ ! 4Cr3þ þ 10H2 þ 3O2

ð4:19Þ

2Cr2 O7 2
þ 16Hþ ! 4Cr3þ þ 8H2 O þ 3O2

ð4:20Þ

4CrO4 2
þ 10H2 O ! 4CrðOHÞ3 þ 8OH
þ 3O2

ð4:21Þ

The Nb2O5/CuO heterostructure was proved to be highly efficient for Cr (VI) photoreduction. This was because of the more intense visible light absorption and the fact that the formed junctions facilitated the charge transfer, reducing the electron-hole pair recombination. The Nb2O5/CuO heterostructure exhibited photoactivity 20-fold higher than those of the pure semiconductors (Nogueira et al. 2017). Different from metallic semiconductors used for Cr(VI) photoreduction, g-C3N4 exhibits conduction band potential of
1.3 V versus normal hydrogen electrode, which is lower than that of Cr(VI)/Cr(III) reduction, enabling g-C3N4 to reduce Cr (VI) to Cr(III) when excited by UV and visible radiation. However, the fast electronhole pair recombination of g-C3N4 decreases its photocatalytic activity on Cr (VI) reduction (Dong and Zhang 2013). Besides g-C3N4, other carbonaceous materials, such as carbon nanotubes, activated carbon, graphene and so on, can be applied for producing heterojunctions and consequently enhance the photocatalytic activity of semiconductors, because they not only improve the semiconductor response at the visible light range but also promote an effective electron-hole pair separation (Li et al. 2018; Woan et al. 2009; Tsai et al. 2011). Another important environmental problem is the presence of arsenic in wastewater, which is a major public health concern worldwide. Similarly, to chromium, some reports suggest that arsenic is an essential nutrient at low concentrations; however, its prolonged ingestion at high levels can provoke health problems such as cancers and cardiovascular and neurological diseases. In this sense, the development of technologies for arsenic removal from wastewaters in which the metal concentration exceeds limits determined by the World Health Organization is currently an important claim (Guan et al. 2012). Inorganic arsenic is present in an aqueous medium as As(III) and As(V), the first occurring in reducing environments, while the second occurs in oxidizing environments. The As(III) species occurs generally in groundwater and As(V) in surface waters where the above conditions are observed. As(III) is more toxic than As (V) and more difficult to be removed from water by most treatment methods (Guan et al. 2012; Shafiee and Jafari 2017). As(III) is typically oxidized to As(V), which is further removed by adsorption, precipitation, ionic exchange and other techniques. The oxidation of As(III) to As(V) can be accomplished by the use of a semiconductor, in which the hydroxyl radicals formed in photocatalytic activation process reduces the As3+ ions to As5+ ions, as represented in the following diagram.

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Arabnezhad et al. synthesized TiO2/ZnO heterostructures with different Ti/Zn ratios and applied them to As(III) photooxidation. The TiO2/ZnO heterostructures presented excellent performance both under visible and ultraviolet radiation exposure. The results showed that the heterojunctions formed between the oxides were determined the increase of photocatalytic activity of the heterostructure, where the photocatalysis process was mediated mainly by the presence of hydroxyl radicals (Shafiee and Jafari 2017). These outcomes denote the always higher photoactivity of heterostructures in relation to pure non-metallic semiconductors. It is worth pointing out that researches on heterostructures for photocatalytic applications are very recent. Most reports are still dedicated to using TiO2 assembled with metallic promoters, such as Pt, Ag, Rh, Rd and Cu, and metallic oxides CuO, WO3 and Nb2O5, among others.

4.4

Conclusion

In conclusion, it is demonstrated that nanomaterials are becoming increasingly vital for environmental protection due to their versatile compositions and means of application. It is well established that most of research devoted to environmental applications of heterostructures, materials made up of semiconductors that share a common interface, have addressed the degradation of organic contaminants in water. Additionally, there is intense research towards synthesis methods able to produce heterostructures with controlled morphology and structural/surface properties useful for environmental applications. As discussed in this chapter, there are several emerging uses of heterostructures. These structures have been successfully applied in different systems for energy and environmental requirements, such as carbon dioxide photoreduction to hydrocarbon fuels, hydrogen evolution and heavy metal abatement. Finally, it has been demonstrated that the right material combination presenting suitable electronic properties, linked to a synthesis method that allows a control of morphology and interface, makes possible the obtainment of heterostructures that exhibit the required properties lead to different reactions of environmental interest.

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Chapter 5

Perovskite-Based Materials for Photocatalytic Environmental Remediation Ashish Kumar, Suneel Kumar, and Venkata Krishnan

Contents 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Basic Principles of Photocatalytic Environmental Remediation . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Fundamentals of Photocatalytic Pollutant Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Fundamentals of Photocatalytic CO2 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Perovskite-Based Materials in Photocatalytic Environmental Remediation . . . . . . . . . . . . . 5.4 Photocatalytic Pollutant Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Photocatalytic CO2 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract In recent years, although several inorganic photocatalysts, including metal oxides, sulphides and nitrides, have been explored for environmental remediation applications, perovskite oxides (ABO3) have gained much attention due to their low cost, excellent stability and structural tunability. This book chapter discusses the design and development of perovskite-based photocatalysts for organic pollutant degradation and CO2 reduction applications along with recent findings and advances in this category of materials. After a brief introduction on the general structure of perovskite materials, the description of basic principles of photocatalysis and mechanisms involved in organic pollutant degradation and CO2 reduction processes have been discussed in detail. The focus is mainly on the strategies involved in the design of perovskite photocatalysts with enhanced photocatalytic activity. Subsequently, some recent reports on diverse organic pollutant degradation and CO2 reduction using perovskite-based photocatalysts are discussed and summarized in tables. Finally, a summary is provided in order to comment on the recent progress and development of perovskite photocatalysts for the utilization of solar energy, and a perspective on the future research in this field is discussed. A. Kumar · S. Kumar · V. Krishnan (*) School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Mandi, Himachal Pradesh, India e-mail: [email protected] © Springer Nature Switzerland AG 2019 Inamuddin et al. (eds.), Nanophotocatalysis and Environmental Applications, Environmental Chemistry for a Sustainable World 29, https://doi.org/10.1007/978-3-030-10609-6_5

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Keywords Perovskite oxides · Layered perovskites · Photocatalysis · Pollutant degradation · CO2 reduction · Solar energy harvesting

5.1

Introduction

Among the large range of available photocatalyst materials, perovskite materials constitute a huge family of promising semiconductor photocatalysts and have been extensively studied for photocatalysis applications because of their costeffectiveness, excellent stability, structural tunability, and proficient photocatalytic performance. The term ‘perovskite’ was initially referred to the mineral form of calcium titanate (CaTiO3), discovered by a German mineralogist Gustav Rose in 1839, and he named it in the honour of a Russian mineralogist Count Lev Aleksevich von Perovski (Attfield et al. 2015). Later, this term was generalized to the compounds having formula ABO3, which are known as perovskite materials. In the ABO3 crystal, A and B are two cations of different sizes (typically A > B), and oxygen exists as an anion which forms a bond with both A and B site cations. These sites provide room for most of the elements of the periodic table, which leads to the formation of a large family of such materials by rational accommodations of different elements at A, B and O sites (Pena and Fierro 2001). The availability of different sites for alteration draws the huge attention of researchers to explore the possibilities of development of efficient photocatalytic materials. The ideal perovskite-type ABO3 material possesses cubic symmetry with space group Pm3m, where the larger A cation is 12-fold coordinated and relatively smaller B cation is sixfold coordinated in the octahedron environment of the O anions as shown in Fig. 5.1. Eight BO6 octahedra constitute the three-dimensional framework Fig. 5.1 The ideal structure of cubic ABO3 perovskite showing BO6 octahedral units (O at corners and B at the centre of octahedra, and A at the centre of the cube). Reproduced with permission from (Shi and Guo 2012) copyright 2012, Elsevier Publishers

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of the cubic structure with shared corners along with A site cation lying at its centre (Shi and Guo 2012). As different cations have different ionic radii, most of the perovskite materials undergo lattice distortions, leading to the lowering of symmetry to other phases, i.e. orthogonal, tetragonal, monoclinic, rhombohedral and triclinic phases. These lattice distortions change the electronic band structure and dipole moment of the perovskites as a result of altered crystal field effects. Consequently, the behaviour of photoinduced charge carriers, including excitation and transfer, also get influenced which has a huge impact on the photocatalytic properties of such materials (Shi and Guo 2012; Hu et al. 2011; Li et al. 2004). The deviation from the ideal perovskite structure was first given by Goldschmidt in terms of tolerance factor (t) given by the following equation (Goldschmidt 1927): pffiffiffi t ¼ ðr A þ r O Þ= 2ðr B þ r O Þ Here rA, rB and rO are the ionic radii of A, B, and O elements, respectively. An ideal cubic perovskite crystal has t-value equal to unity, although the stable perovskites with t-value lying between 0.76 and 1.13 are also known (Labhasetwar et al. 2015; Nagai et al. 2007). For lower values of tolerance factor, i.e. t < 1, the perfect cubic structure can be lowered to orthorhombic, rhombohedral or other lower symmetry. The perovskites with t > 1 possess hexagonal structure because ionic radii of A > B (e.g. BaNiO3; t ¼ 1.13), which are stabilized by the formation of metalmetal bonds between B cations of BO6 octahedra and are less common than the cubic perovskites (Labhasetwar et al. 2015). The rapid depletion of fossil fuels, industrialization and human activities are the root cause of environmental pollution. Untreated industrial wastes and improper burning of fuels are responsible for the pathetic condition of water reservoirs and atmosphere around us. Currently, it is a great challenge and demand of the world to attain environmental sustainability for human survival in the upcoming century. Solar energy is one of the attractive alternatives for addressing these issues particularly due to its cleanness, easy availability, and accessibility. Natural sunlight consists of three major parts, viz. ultraviolet, visible, and near-infrared radiations (NIR), which can be utilized for specific applications. Fortunately, the photocatalysis process enables us to utilize some part of this solar energy, as a most promising method for addressing the environmental issues. Depending on the optical properties of different materials, specific energy radiations can be used for various photocatalytic applications, like water splitting for hydrogen generation, organic pollutant degradation, organic transformations, CO2 reduction, etc. Also, the development of efficient photocatalytic materials, which can harvest a wider region of the solar spectrum, is of huge significance for photocatalysis concern. The utilization of solar energy by means of such efficient photocatalytic materials can be useful for increasing the world’s energy security by decreasing the dependency on non-renewable energy resources and by combating environmental pollution to restrict its impact on climate change.

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In this book chapter, we have discussed the development of perovskite-based photocatalysts for environmental remediation applications, including organic pollutant degradation and CO2 reduction along with recent findings and advances on these materials. The reports on different perovskite materials, viz. oxides, mixed oxides, and layered perovskites, have been included. We anticipate that this monograph provides a comprehensive account of the utilization of perovskite-based materials for photocatalytic environmental remediation applications along with the recent advancements in this field.

5.2

Basic Principles of Photocatalytic Environmental Remediation

Semiconductor photocatalysis has recently drawn the huge attention of researchers around the world, and the activities are rising exponentially because of its widespread applications in the field of energy generation, environmental remediation and various industrial processes (Kumar et al. 2017a). As the energy consumptionrelated processes rely on the utilization of renewable and pollution-free solar energy, semiconductor photocatalysis is most advantageous and offers a cost-effective solution for energy and environment-related problems. Moreover, in comparison to the conventional pollutant treatment methods, which involves the transfer of pollutants from one medium to another, semiconductor photocatalysis leads to the mineralization of organic pollutants to non-detrimental products, such as CO2 and H2O, and do not produce any secondary waste in most of the cases. Other advantages of semiconductor photocatalysis are the requirements of mild reaction conditions, generally short reaction time and can be carried out in aqueous, gaseous and solid phase media (Kabra et al. 2004; Chen and Mao 2007). Therefore, semiconductor photocatalysts provide a promising route for environmental remediation with absolute reliability and minimum operational costs.

5.2.1

Fundamentals of Photocatalytic Pollutant Degradation

Currently, the world has a great challenge to compete with the shortage and demand for clean water as increasing population and industrialization process has led to the contamination of the water resources. For addressing this issue, the most effective and possible solution is the recycling of wastewater. Although, to yield more water resources, various strategies like water harvesting from rain and fog are effective in this domain. But the presence of toxic organic pollutants limits the use of such water again. The textile industries dispose of a huge amount of waste which contains organic dyes, pharmaceutical waste from drug industries and pesticides/herbicides used by human for the protection and production of crops, which all go in the water

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reservoirs directly or indirectly causing serious environmental hazards (Fu et al. 2013; Köck-Schulmeyer et al. 2013; Kumar et al. 2017b). For the removal and degradation of such organic pollutants, various techniques such as ultrafiltration, chemical coagulation, adsorptive ion-exchange and adsorption on activated charcoal, silica gel and metal-organic frameworks, etc. have been widely studied in literature (Konstantinou and Albanis 2004; Dawood and Sen 2013; Ali 2012). However, these techniques are effective only in migrating the organic pollutants from aqueous phase to another phase which leads to a secondary pollution. Recycling of materials from above-mentioned processes involves non-greener and expensive treatments which limit their use. Also, it has been found that conventional wastewater treatment technologies are unproductive for management of dye wastewater because these pollutants possess high chemical stability (Forgacs et al. 2004). In this regard, heterogeneous photocatalysis is an environmentally benign method that offers a complete solution for wastewater treatment as it produces biodegradable and less toxic products. Several efforts have been made by researchers to degrade the wastewater pollutants by using photocatalysis process (Zhang et al. 2016; Kumar et al. 2017c; Wu et al. 2018; Hailili et al. 2018; Wan et al. 2018; Qu et al. 2018; Hailili et al. 2017). A photocatalyst is a semiconductor material which possesses some band gap (Eg) value and absorbs ultraviolet or visible light depending on it. When light falls on such materials, the absorption of photons takes place, provided the energy of photons should be greater than or equal to the band gap of the material. This process leads to the excitation of an electron (e) from the valence band (leaving behind a hole (h+)) in the conduction band of the material. These photogenerated charges migrate to the catalyst surface, where they undergo recombination to produce heat or carry out the redox reactions with the adsorbed pollutant molecules directly or indirectly in solution or gas phase. Figure 5.2 shows the photoexcitation and charge separation process in a photocatalytic material and their subsequent utilization for degradation process of organic pollutants (Wang et al. 2015a). The photogenerated e and h+ are powerful reducing and oxidizing agents, respectively. The h+ oxidizes the pollutant molecules directly or reacts with hydroxyl anion (OH) to generate OH* radicals, which subsequently degrades the pollutant molecules. Similarly, e in the conduction band of the material reduces the oxygen adsorbed on its surface to generate highly reactive superoxide radicals (O2*). Chong et al. have explained this process in various steps as follows (Chong et al. 2010): I. Absorption of light and excitation of photocatalyst: photocatalyst þ hϑ ! e ðConduction bandÞ þ hþ ðValence bandÞ II. Charge-carrier trapping of e:

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O2 Organic pollutants n igratio

Exeitation

e¯

s

n oto

Ph

M

e¯ O2¯

CB

Semiconductors VB h+

Migra

tion

Eg

h+

OH

>

hv

CO2 + H2O H2O/OH¯

Fig. 5.2 The general mechanism of photocatalytic organic pollutant degradation over semiconductor materials. Reproduced with permission from (Wang et al. 2015a) copyright 2015, Elsevier Publishers

e ðConduction bandÞ ! e ðTrappedÞ III. Charge-carrier trapping of h+: hþ ðValence bandÞ ! hþ ðTRÞ IV. Recombination of photogenerated charges: e ðTrappedÞ þ hþ ðValence bandÞ or hþ ðTrappedÞ ! e ðConduction bandÞ þ Heat V. Scavenging of photoexcited e: O2 ðadsorbedÞ þ e ! O2 ∗ VI. Oxidation of hydroxyl ion to hydroxyl radical: OH þ hþ ! OH∗

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VII. Photodegradation of organic pollutants by OH*: Pollutants þ OH∗ ! Intermediates=CO2 þ H2 O VIII. Direct photoholes: Pollutants þ hþ ! Intermediates=CO2 þ H2 O IX. Protonation of superoxides: O2 ∗ þ OH∗ ! HOO∗ X. Co-scavenging of e: HOO∗ þ e ! HO2 XI. Formation of H2O2: HO2  þ Hþ ! H2 O2 Step (I) involves the absorption of photons of sufficient energy, which results in the generation of photogenerated e and h+ in the conduction band and valence band, respectively, and steps (II) and (III) represent the surface-trapped e and h+, respectively. In step (IV), the excited e recombines with the h+ (valence band) in nanoseconds to generate heat in absence of any e scavenger. The recombination of photogenerated charges is a serious concern in the photocatalysis process because it curtails the photocatalytic performance of the materials. The photogenerated charge carriers should react with some species in the mixture so that they can perform the redox reaction. Therefore, the prolongation of recombination time is very much required for the successful operation of photocatalysis process which can be achieved by using e scavengers. Also, h+ can react with OH to produce OH* radicals and e combine with adsorbed O2 to generate O2* radical which prevents the recombination of photogenerated charges as shown in steps (V) and (VI). The OH* radicals being very reactive, h+ species as well react with the organic pollutants to degrade them and results in the formation of non-toxic products as shown in steps (VII) and (VIII). Further, O2* radicals can undergo protonation to form the hydroperoxyl radical (HO2*) and subsequently H2O2 as presented in steps (IX) to (XI). As HO2* radical also exhibits scavenging properties, therefore co-existence of these reactive species can prolong the recombination time of h+ (trapped) in the photocatalysis process. However, the existence of all these species entirely depends

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on the presence of aqueous medium and dissolved oxygen only. Nonaqueous reaction system does not allow the formation of these species and retards or restricts the photocatalytic degradation of organic pollutants. Thus, for the success of photocatalytic pollutant degradation reactions, the reaction media should be aqueous, and recombination of photogenerated charge carriers should be prevented.

5.2.2

Fundamentals of Photocatalytic CO2 Reduction

Due to limitations in the availability of fossil fuels, the search for a new source of energy which is abundant, cheap and maintainable has been increased in the past two decades (Yu et al. 2014). The excessive consumption of fossil fuels leads to the high emission of carbon dioxide (CO2), a greenhouse gas, and its level rapidly increased in the atmosphere (Yu et al. 2014). It is well-known that the increasing level of CO2 in the environment has raised serious concerns like global warming, acid rain and increase in the sea level. The correlation of global temperature with increasing CO2 level has been shown in Fig. 5.3. It is clear from this correlation that with an increase in the concentration of CO2 in the atmosphere, the temperature has also increased with every passing decade. It has been predicted by the International Panel on Climate Change that there will be a 1.9  C increase in the global temperature by

Fig. 5.3 Correlation showing the increase in global temperature with increasing CO2 concentration over decades. Reproduced with permission from (Zeng et al. 2018) copyright 2018, IOP Science Publishers

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the end of the twenty-first century if the concentration of CO2 rises up to 590 ppm, which will pose serious climate changes (Solomon 2007; Zeng et al. 2018). Therefore, it is very important to develop techniques which can be utilized to alleviate this issue and also get an alternative for non-renewable energy sources to meet global energy needs. In this regard, various strategies to convert CO2 into other chemicals using processes, like photocatalytic reduction (Bonin et al. 2014; Laitar et al. 2005; Cokoja et al. 2011; Aurian-Blajeni et al. 1983; Morris et al. 2009), electrochemical reduction (Hori et al. 1985; Le et al. 2011), photoelectrochemical reduction and thermochemical reduction (Ashley et al. 2009; Rodriguez et al. 2015; Jadhav et al. 2014), have been employed by researchers. Fortunately, semiconductor-based photocatalysis has emerged as one of the most promising technologies utilized at an unprecedented rate after the pioneering works in the 1970s to fulfil the requirement of environmentally benign energy source and solve the energy crisis (Habisreutinger et al. 2013). In this regard, the development of pathways such as sunlight-driven CO2 reduction to various common chemical fuels, such as carbon monoxide (CO), methane (CH4), methanol (CH3OH), etc., has been extensively developed and reported (Wang et al. 2015b; Dhakshinamoorthy et al. 2012; Dai et al. 2015; Kwak and Kang 2015). Moreover, the band gap engineering, optimization of crystal structure and morphology control have been carried out to enhance the photocatalytic efficiency (Ola and Maroto-Valer 2015). Therefore, the investigations on the crystal structure and electronic structure are important to understand the entire photocatalytic process by studying the photoinduced charge carrier generation and their transfer across the interface. Hence, it can be inferred that the development of materials for photocatalytic CO2 reduction can be considered Holy Grail to alleviate the effect of increasing CO2 concentration on the environment. Perovskite materials are good candidates for the photocatalytic reduction of CO2 and are widely explored in this context (Hou et al. 2017; Xu et al. 2017). The various steps involved in the mechanism (Fig. 5.4) of photocatalytic reduction CO2 are explained below (Sun et al. 2018; Kanhere and Chen 2014): Fig. 5.4 Schematic representation of the photocatalytic reduction of CO2 into various possible products. Reproduced with permission from (Kanhere and Chen 2014) copyright 2014, MDPI Publishers

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1. In the first step, the adsorption of CO2 on the photocatalyst surface along with the reductant takes place. 2. Then, the absorption of light of a suitable energy (hϑ  Eg ) by a perovskite photocatalyst results in the separation of electrons and holes. 3. Further, the transfer of the photogenerated charges to the surface of the catalyst. 4. Photogenerated charges react with the species adsorbed on the catalyst surface. 5. Finally, desorption of products formed takes place, and reactants get re-adsorbed again to continue the reaction. It is noteworthy to mention here that for the photocatalytic reduction of CO2 in the presence of water, the valence band maximum of the photocatalytic materials should be less than the reduction potential of water, and the position of conduction band minimum should be higher than the reduction potential of the CO2 (Xie et al. 2011; Zhou et al. 2013). The suitable alignment of band positions in the operating photocatalytic material leads to the oxidization of water molecules at its valence band and reduces the CO2 at its conduction band by utilizing the photogenerated charges. Also, the photogenerated charges have great tendency to undergo recombination; therefore, the efficient transfer of photogenerated charges is highly required in order to get good photocatalytic performance. Thus, the above-mentioned second and third processes are the crucial steps in the photocatalytic CO2 reduction process. The presence of a good reductant which can provide protons in the reaction by consuming h+ species generated at the valence band of the photocatalyst also boosts up the photocatalytic CO2 reduction process (Shi et al. 2017).

5.3

Perovskite-Based Materials in Photocatalytic Environmental Remediation

The main obstacles that limit the photocatalytic performance of a material are the low absorption of light and photogenerated electron-hole pair recombination. Most of the perovskite materials have wide band gap, and they are capable of harvesting only ultraviolet region of the solar spectrum. As ultraviolet region contribution to the solar spectrum is minimal (about 5%), therefore, it is enviable to develop visible light active materials, as visible light constitutes a larger portion (about 42%) of the solar spectrum. In this regard, band gap engineering enables the researchers to tune the band gap of perovskites. As perovskite materials offer three different sites for alteration (A, B and O sites), as discussed earlier, doping of different elements can be done in order to enhance their photocatalytic performance. Generally, transition metal cations of second and third transition series act as donor species due to their electron-rich d-orbitals. Also, the high atomic energy of 4d and 5d orbitals does not alter the conduction band minimum of the host materials and maintains the high electron mobility. On the other hand, doping at the O site by the anions containing 2p or 3p orbitals with high atomic energies than that of oxygen results in the increase of the valance band maximum and hence decreases the overall band gap of the

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materials. When dopants (e.g., nitrogen) containing 2p atomic orbitals (potential energy higher than O 2p atomic orbitals) are introduced, the formation of new energy bands takes place above the valence band maximum without affecting the conduction band level and results in the decrease in band gap. This process leads to the formation of a visible light active photocatalyst, which can be utilized for environmental remediation applications (Wang et al. 2015b). Shi et al. have prepared N-doped NaNbO3 (NaNbO3-xNx) photocatalysts and observed that N-doped samples exhibit two absorption edges at 365 nm and 480 nm corresponding to pristine NaNbO3 and newly formed N 2p bands which lie above the valence band comprising of O 2p bands (Shi et al. 2009). This means N-doping results in enhanced absorption in the visible region of light. In another study, Meng and co-workers have reported the synthesis of N-doped La2Ti2O7 nanosheets and found a significant decrease in the band gap from 3.28 to 2.51 eV. They attributed this band gap decrease to the hybridization of N 2p orbitals of the dopant with the O 2p orbital of the valence band of La2Ti2O7 which results in shifting of the valence band maximum upwards (Meng et al. 2012). Furthermore, the morphology and structure of the perovskite materials can also alter their band gap (Li et al. 2012; Lin et al. 2007; Zhang et al. 2010). The other serious concern which curtails the photocatalytic activity of a perovskite photocatalyst is the fast recombination of electron-hole pairs, which are generated by the absorption of photons from the incident light. Therefore, it is necessary to suppress the recombination rate of photogenerated charge carriers in order to boost the photocatalytic performance. In this regard, various strategies can be used for suppressing the electron-hole recombination, which involves the decrease in the diffusion length by reducing the size and preparing different nanostructures (Kato et al. 2003; Thirumalairajan et al. 2013), coupling with conducting material like graphene (Kumar et al. 2017c; Hu et al. 2014a, b) and coupling with other semiconductor materials with suitable band position for efficient charge transfer (Huang et al. 2014). The conducting materials like graphene provide support for catalyst particles and act as an electron acceptor provided conduction band of the photocatalyst should be more negative than the graphene band (Jie et al. 2014; Wang et al. 2017a). Yang et al. have prepared three-dimensionally ordered Bi2WO6-RGO hydrogel composites and reported their high photocatalytic performance towards methylene blue removal from wastewater (Yang et al. 2017). They attributed this enhanced activity to the adsorptive degradation and efficient charge transfer. Thus, the advanced synthesis methods and coupling of photocatalysts with graphene and other semiconductor materials can help in the suppression of recombination rate of photogenerated charges and result in the enhancement of overall photocatalytic performance of the photocatalysts.

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Photocatalytic Pollutant Degradation

As discussed earlier, the capability of semiconductor materials of absorbing solar light irradiations in the aqueous medium enables the charge separation in them and results in the formation of electrons and holes, which can be utilized for the mineralization of organic pollutants into smaller fragments, such as carbon dioxide and water. The strategies involved in the design and development of perovskitebased semiconductor photocatalysts have been already discussed in details in Sect. 5.3. In this section, the recent reports on perovskite-based materials for the photocatalytic degradation of various organic pollutants such as different dyes, pharmaceutical drugs, etc. have been discussed and summarized in Table 5.1. Various perovskites including titanates, tantalates, niobates, ferrites, etc. have been utilized by researchers for organic pollutant degradation. CaTiO3 is an alkali metal titanate and comprises of earth-abundant, non-toxic elements which respond to ultraviolet light only due to its wide band gap (3.5 eV) (Huang et al. 2015). Xian et al. reported the synthesis of CaTiO3-graphene composite by a facile mixing followed by thermal drying (Xian et al. 2014). It was found that CaTiO3-graphene composite exhibits enhanced photocatalytic activity towards the degradation of MO as compared to bare CaTiO3 under ultraviolet light irradiation. Our group also reported N-doped CaTiO3 (NCT) coupled with RGO (RGO-NCT bifunctional composites) by adopting the hydrothermal method (Kumar et al. 2017c). The as-prepared composites were examined for the photocatalytic removal of MB and TBZ under visible light irradiation. In comparison to the bare CaTiO3, NCT shows significant visible light activity, which was further enhanced by coupling RGO with it. NCTG20 composite with 20 wt% RGO content was most efficient in the adsorptive photocatalytic degradation of MB and TBZ. The enhanced photoactivity was attributed to the suppressed electron-hole pair recombination and adsorption properties. In another work, we coupled CaTiO3 with g-C3N4 to form a g-C3N4-CaTiO3 (1:1 ratio) heterojunction photocatalyst for the degradation of RhB under ultraviolet, visible and sunlight irradiations (Kumar et al. 2018). The as-prepared heterojunction photocatalyst showed highest photocatalytic activity under sunlight irradiation and was also effective for the degradation of non-photosensitizing pollutant BPA. The enhanced photocatalytic performance of g-C3N4-CaTiO3 heterojunction was ascribed to the appropriate band positions, close interfacial contact between both counterparts and extended light absorption, which ultimately results in transfer of photogenerated charges across the heterojunction and suppresses their fast recombination. The mechanism involved is depicted in Fig. 5.5. SrTiO3 has also been widely explored for the photocatalytic degradation of organic pollutants. Xie et al. synthesized Fe-doped SrTiO3 and examined the photocatalytic degradation of RhB under visible light irradiation (Xie et al. 2008). The metal-to-metal charge-transfer excitation of Ti(IV)-O-Fe(II) linkage formed in the Fe-doped SrTiO3 was the key point for the enhanced photocatalytic performance of Fe-doped SrTiO3. NaTaO3 perovskite possesses a band gap of 4.0 eV which enables this material to absorb ultraviolet light radiations only. Various modifications have been made by

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Table 5.1 Summary of different perovskite materials for photocatalytic pollutant degradation Sl. No. 1

Photocatalyst CaTiO3-gC3N4

Light source Ultraviolet, visible and sunlight

2

N- CaTiO3RGO

Visible

3

g-C3N4-NSrTiO3

Visible

4

In2S3/NaTaO3

5

AgTaO3/AgBr

Simulated solar light Visible

6

Ag2O-NaNbO3

Visible

7

Ag@AgCl/ AgNbO3 BiFeO3-Ndoped graphene LaFeO3/TiO2

Visible

Visible

10

LnFeO3 (Ln¼Pr, Y)

Visible

11

La0.7Sr0.3MnO3

Sunlight

12

Simulated sunlight Visible

14

Ag-loaded Bi5O7I/Sr2TiO4 g-C3N4/ Bi2WO6 CQD/Bi2WO6

15

La2Ti2O7

Ultraviolet

8

9

13

Visible

Visible

Pollutant (concentration) RhB (1  105 M) BPA (5  105 M) MB (4  105 M) TBZ (2  105 M) RhB (5 mg L1) 4-CP (5 mg L1) TC (10 mg L1) MO (2  105 M) RhB (10 mg L1) MB (10 mg L1) Congo red (10 mg L1)

Reaction time [min] 120

Reference Kumar et al. (2018)

180

Kumar et al. (2017c)

60

Kumar et al. (2014)

120 180 30

Xu et al. (2018)

120

Wang et al. (2017b) Zhang et al. (2017) Yang et al. (2015)

180

Li et al. (2017)

MB

120

RhB (105 mol L1) MO (13 ppm)

120

Xiang et al. (2017) Li et al. (2014)

MO (105 M)

180

Ibuprofen (500 μM) RhB (10 mg L1) CIP (10 mg L1) BPA (10 mg L1) TC (10 mg L1) MB (25 mg L1)

60

80

25

120

40

Ghiasi and Malekzadeh (2014) Hu et al. (2017) Wang et al. (2017c) Di et al. (2015)

RahimiNasrabadi et al. (2017)

RhB rhodamine B, MB methylene blue, BPA bisphenol A, TBZ thiabendazole, MO methyl orange, 4-CP 4-chlorophenol, TC tetracycline, CIP ciprofloxacin

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Fig. 5.5 Mechanism of degradation of RhB under sunlight irradiation using the g-C3N4-CaTiO3 heterojunction photocatalyst. Reproduced with permission from (Kumar et al. 2018) copyright 2018, Beilstein-Institut Publishers

researchers to enhance the photocatalytic activity of this material. Xu et al. reported the synthesis of plasmonic Ag/AgCl/NaTaO3 photocatalysts by adopting the precipitation and photoreduction method (Xu et al. 2016). The photocatalytic performance of as-prepared photocatalysts was examined by studying the degradation of MB, RhB and phenol under visible light irradiation. The TEM image of 30% Ag/AgCl/ NaTaO3 photocatalyst and the mechanism involved in the degradation process of pollutants is depicted in Fig. 5.6. The effective separation of photogenerated charges and the surface plasmon resonance effect with strong visible light absorbance were the main reasons for the enhanced photocatalytic activity. Li and co-workers successfully doped sulphur in NaTaO3 by hydrothermal reaction route, and it was found that S-doped NaTaO3 showed excellent visible light activity towards the photocatalytic degradation of MO and phenol (Li et al. 2015). The reason of enhanced visible light absorption and a decrease in the band gap of S-doped NaTaO3 was due to the overlapping of Ta 5d and S 3p orbitals which results in the upward shifting of valence band maximum in the resulting photocatalyst. In niobates, Lan et al. reported the synthesis of KNbO3 nanowires anchored with Au nanoparticles and their enhanced photocatalytic activity towards the degradation of RhB under ultraviolet and visible light irradiation as compared to the bare KNbO3 (Lan et al. 2011). The size of the anchored Au nanoparticles was 5 nm and 10 nm, and the activity of KNbO3 decorated with 10 nm Au nanoparticles was found to be

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Fig. 5.6 (a) TEM image of 30% Ag/AgCl/NaTaO3 photocatalyst showing the presence of different constituents and (b) mechanism of photocatalytic reduction of organic pollutant degradation under visible light irradiation. Reproduced with permission from (Xu et al. 2016) copyright 2016, Elsevier Publishers

the highest. The superior photocatalytic activity was mainly ascribed to the surface plasmon resonance effect along with the interband transitions on Au nanoparticles. Yan et al. reported the degradation of Au-supported KNbO3 nanocubes for the degradation of MB (Yan et al. 2014). Jiang et al. prepared various nanostructures (nanowires, nano towers, nanocubes and nanorods) of KNbO3 and studied their photocatalytic performance for the degradation of RhB under ultraviolet-visible light irradiation. The photocatalytic activity follows the order nanocubes > nanowires > nanorods > nano towers, suggesting the active role of microstructures involved in the degradation process (Jiang et al. 2013). BiFeO3 is a ferrite perovskite and is of particular interest owing to its magnetic nature and interesting optical properties. Deng et al. reported the g-C3N4/BiFeO3 heterojunction photocatalyst for the photocatalytic removal of RhB under visible light irradiation (Deng et al. 2018). They found that at optimum composition of 10 wt.% BiFeO3, the g-C3N4/BiFeO3 heterojunction exhibits 1.4 times higher photocatalytic activity as compared to pure g-C3N4. Moreover, mechanical pressing and electrical poling improved the photocatalytic performance of g-C3N4/BiFeO3 heterojunction by 1.3 times and 1.8 times, respectively. The enhanced photocatalytic activity of g-C3N4/BiFeO3 heterojunction was attributed to the easy migration, supressed recombination rate of the photogenerated charges via Z-scheme charge transfer model and ferroelectric polarization-induced inner field. This is a good example of utilization of ferroelectric properties to enhance the performance of visible light active photocatalysts. Li et al. coupled the BiFeO3 with the N-doped graphene sheets to form an efficient composite material for the degradation of CR under visible light irradiation (Li et al. 2017). The lowering of the band gap and efficient charge transfer of photogenerated electrons from BiFeO3 to N-doped graphene, which suppress the electron-hole pair recombination, were the reasons for enhanced photocatalytic performance.

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Layered perovskite materials are also studied for photocatalytic pollutant degradation applications in addition to the ideal ABO3 perovskite oxides. Layered perovskites (Ruddlesden-Popper (RP) phase, Aurivillius (AL) phase and Dion-Jacobsen (DJ) phase) can consist of an infinite slab of ABO3 structures separated by some motif. H2CaTa2O7 (Eg ¼ 3.9 eV) (Wang et al. 2014) and Li2CaTa2O7 (Eg ¼ 4.4 eV) (Liang et al. 2008) are the RP phase double perovskites which possess a wide band gap and have been reported for the photocatalytic degradation of RhB under ultraviolet light irradiation. Kumar et al. have made Sn2+ and N3 substitutions in RP phase K2La2Ti3O10 for the visible light degradation study of RhB (Kumar and Uma 2011). They observed a large red shift in the absorption of as-prepared material, and the decrease in the band gap was attributed to the overlapping of the O 2p orbitals and 5s orbitals of Sn. Additionally, urea was used as a nitriding agent which not only introduced nitrogen in the final sample, but the leftover carbon nitride polymer was also helpful in the overall enhancement of photocatalytic performance. Hu et al. synthesized a visible light active Ag-loaded Bi5O7I/Sr2TiO4 heterojunction for the photocatalytic degradation of MO under visible light (Hu et al. 2017). In another recent study, Luo et al. reported a Z-scheme hybrid La2NiO4/g-C3N4 composite for the visible light degradation of MO (Luo et al. 2018). The AL phase is another type of perovskite in which Bi2WO6 is the most studied material for the photocatalysis applications. Bi2WO6 is a narrow band gap (Eg ¼ 2.8 eV) material and is capable of harvesting visible light. Li et al. studied the effect of phosphate doping in Bi2WO6 for the enhanced removal of various pollutants (Cr (VI), RhB and antibiotics) in the presence of visible light irradiation. The enhanced photocatalytic degradation activity was ascribed to the PO4 doping which alters the conduction band and valance band positions of Bi2WO6 sample, and hence increases the redox ability. Also, the separation of photogenerated charge carriers was facilitated in the PO4-doped Bi2WO6, which results in the enhancement of photocatalytic performance (Li et al. 2016a). In another interesting study, Tang et al. used a non-ionic surfactant Triton-X 100 for the adsorption and photodegradation of norfloxacin in the presence of Bi2WO6 (Tang et al. 2016). Norfloxacin is a pharmaceutical pollutant which has lower solubility in water. The role of Triton-X 100 is described as a solubility enhancer for norfloxacin in water, and it also helps in the adsorption of norfloxacin on the catalyst surface. The Triton X-100 monomer gets adsorbed on the Bi2WO6 surface and acts as nonaqueous cage for the norfloxacin, which results in enhanced degradation. This study provides an attractive pathway for the degradation of such pollutants which are partially soluble in aqueous medium. The various reports based on perovskite materials for photocatalytic pollutant degradation have been summarized in Table 5.1.

5 Perovskite-Based Materials for Photocatalytic Environmental Remediation

5.5

155

Photocatalytic CO2 Reduction

The process of photosynthesis in green plants utilizes the sunlight to convert CO2 and H2O into O2 and carbohydrates under ambient conditions. For the photocatalytic CO2 reduction or artificial photosynthesis, the reduction of CO2 and oxidation of H2O take place simultaneously to complete a neutral carbon cycle. Inspired by the natural photosynthesis process, the photocatalytic CO2 reduction in the energy-rich products provides an alternative path to address the energy-related issues and diminish the greenhouse gas effects. Although perovskite materials have been explored for photocatalytic CO2 reduction, research scopes are still open in this field. In this particular section, the recent reports on perovskite-based materials for photocatalytic CO2 reduction have been discussed and tabulated in Table 5.2. The role of titanate perovskites for photocatalytic reduction of CO2 to methane (CH4) was investigated in 1978 by Hemminger and co-workers (1978). In this study, gas-phase water and CO2 were adsorbed on the surface of reduced SrTiO3 with (111) crystal phase in contact with Pt foils. The ultraviolet light from a high-pressure mercury lamp, corresponding to the band gap energy of SrTiO3, was used as irradiation source and formed CH4 as photocatalysis product which was detected using an ultrahigh vacuum chamber under high pressure. This entire process occurs without any applied potential between SrTiO3 and Pt foils and in the absence of liquid electrolytes. In addition, a thermal reaction, leading to the formation of CH4 on SrTiO3-Pt, has also been studied. Ye et al. reported leaf-architectured 3D hierarchical artificial photosynthetic system of perovskite titanates for photocatalytic Table 5.2 Different perovskite materials for photocatalytic CO2 reduction Sl. No. 1

Photocatalyst CaTiO3-basalt

2

Rh/Au- SrTiO3

3

PbTiO3-basalt

4

Ca-NaTaO3

5

LiTaO3

6 7 8 9

NaNbO3 and KNbO3 NaNbO3 Bi2WO6-CQDs MnCo2O4

10

BaZrO3

Reaction conditions LP (CO2, H2O)

Major product CO

Reference Im et al. (2017)

GP (CO2, H2O gas) GP (CO2, H2O)

CO

Li et al. (2016b)

CH4

Do et al. (2016)

LP (CO2, H2O)

CO

GP (CO2, H2)

CO

GP(CO2, H2O)

CH4

Visible Visible Visible

GP(CO2, H2O) GP (CO2, H2O) GP (CO2, H2O)

CH4 CH4 H2, CO

Ultraviolet

GP (CO2, H2O)

CH4

Nakanishi et al. (2017) Teramura et al. (2010) Shi and Zou (2012) Shi et al. (2011) Kong et al. (2017) Wang et al. (2015c) Chen et al. (2015)

Incident light Ultravioletvisible Visible Ultravioletvisible Ultravioletvisible Ultravioletvisible Ultraviolet

GP gas phase, LP liquid phase

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reduction of CO2 into hydrothermal fuels such as CO and CH4 (Zhou et al. 2013). They investigated the role of morphology mainly three-dimensional structures of titanates, such as SrTiO3, CaTiO3 and PbTiO3, for photocatalytic reaction with enhanced light-harvesting capability. Such morphology exhibits high-specific surface area and excellent photostability under longer irradiation. Furthermore, Kang et al. (Im et al. 2017) reported core-shell structures of CaTiO3 on basalt fibre for photocatalytic CO2 reduction reaction. The Ca/Ti ratio was varied over basalt fibre, and photocatalytic conversion of CO2 achieved was CH4  17.8 μmol g1 and CO  73.1 μmol g1. The other perovskites, tantalates, niobates and ferrites are also used for photocatalytic CO2 conversion by the researchers. Teramura et al. (2010) reported their investigations on ATaO3 (A¼ Li, Na, K) for photocatalytic CO2 conversion using H2 as a reductant. The effect of band gap potentials of respective tantalates has been studied and found to exhibit the photocatalytic activity in order LiTaO3 > NaTaO3 > KTaO3 corresponding to their band gap values of 4.9 eV, 4.1 eV and 3.7 eV (versus NHE) for LiTaO3, NaTaO3 and KTaO3, respectively. No product was detected in the absence of photocatalysts, and the amount of CO evolved in LiTaO3 (0.42 mol g1) was found to be eight times higher than KTaO3 under 24 h irradiation. In addition, it has also been inferred that, with increasing calcination temperature of the photocatalyst, the activity for CO2 conversion increases, which indicates the improved charge separation and chemisorption of CO2 on the catalyst surface. Recently, Ye et al. (Zhou et al. 2015) also reported alkaline titanates ATaO3 (A¼ Li, Na, K) from biomass (activated carbonized wood) in the presence of Au as a cocatalyst and demonstrated it as efficient photocatalysts for CO2 reduction. The activated carbonized biomass which was used as precursor material provides the large specific surface area and high hierarchical pore volume, which eventually enhance the photocatalytic activity. Hou et al. (2016) demonstrated a pathway to enhance the visible light absorption tendency of sodium tantalate by introducing nitrogen and oxygen vacancies in sodium tantalate nanocubes (VO-NaTaON) and coupling this with nitrogen-doped graphene quantum dots (N-GQD) by adopting solution-etching-induced phase-transition and in situ reduction methods. The N and O vacancies help to tune the band gap of this material which has been confirmed by the ultraviolet-visible diffuse reflectance spectra and electron paramagnetic resonance measurements. The introduction of vacancies results in the enhanced absorption by photocatalysts for ultraviolet region to visible region and exhibits superior photocatalytic CO2 conversion efficiency to hydrocarbon fuels. Additionally, N-GQD/VO-NaTaON catalysts also show enhanced light absorption in the visible region and superior photocatalytic activity. In niobates, Shi et al. (Shi and Zou 2012) prepared NaNbO3 and KNbO3 by adopting conventional solid-state reaction method. The band gap calculated by diffuse reflectance spectroscopy was 3.4 eV and 3.1 eV for NaNbO3 and KNbO3, respectively. These prepared catalysts were utilized for the photocatalytic reduction of CO2 to CH4. It was found that high photoinduced charge mobility and adsorption of reactant in KNbO3 as compared to NaNbO3 result in enhanced photocatalytic activity. As mentioned above, the design and fabrication of visible light-responsive

5 Perovskite-Based Materials for Photocatalytic Environmental Remediation

CH4

e¯ e¯

e¯ CB hv

e¯ Pt

157

CB

CO2

VB h+

h+

VB C3N4 NaNbO3

Fig. 5.7 Photocatalytic reduction of CO2 over g-C3N4/NaNbO3 heterojunction photocatalyst. Reproduced with permission from (Shi et al. 2014) copyright 2014, American Chemical Society Publishers

niobate perovskites are highly anticipated and growing research field in the last decade. In this regard, Shi et al. (2014) have utilized the graphitic carbon nitride (g-C3N4), a metal-free, n-type organic polymeric material with excellent thermal stability and visible light-harvesting capability because of its narrow band gap (~2.7 eV). The g-C3N4 was coupled with NaNbO3 nanowires to form visible lightresponsive heterojunction photocatalyst with intimate interfacial contact which is highly significant for charge transfer to reaction sites over catalyst surface. The prepared g-C3N4/NaNbO3 heterojunction photocatalyst exhibits superior photocatalytic activity for CO2 reduction under visible light irradiation which was several times higher than bare catalyst (g-C3N4). This remarkably improved photocatalytic activity has been attributed to the excellent heterojunction formation between g-C3N4 and NaNbO3, proper band alignment with resonant band structures with facilitated charge transfer as presented in Fig. 5.7. The transferred photoinduced charges result in the reduction of CO2 to CH4. Jing et al. studied the effect of N-doping in porous perovskite-type LaFeO3 to harvest visible light efficiently, coupled with TiO2 to form nanocomposite photocatalysts (Humayun et al. 2016). The photocatalytic activity of these nanocomposites has been evaluated for CO2 conversion reaction and found to exhibit enhanced photocatalytic activity as compared to porous LaFeO3. Hence, it is clear that the coupling of LaFeO3 with TiO2 results in improved photocatalytic activity with the increased specific surface area, enhanced visible light harvesting due to N-doping and efficient charge transfer across LaFeO3-TiO2 interface as presented in Fig. 5.8. The same photocatalyst has also been found to be effective for 2,4-dichlorophenol decomposition and hence act as a multifunctional catalyst.

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CO 2

Highest level -0.9 eV

e¯

OH/Org+

LFO-AC 2.0 eV h+ h+ h+

H2O/Org

-1

-0.3 0.2

2.02 eV O2p+N2p N-LFO O2

e¯ e¯

N-LFO-AC 1.82 eV

534nm

e¯

-0.3 eV Fe3d is 0.2 eV V 680nm

620nm

496nm

ΔE (3.1 eV) 400nm

Ti3d

O2p LFO

E° vs NHE

O2 -0.9 eV

Fe3d

2.2 eV

CH 4 CO 2 O2

0 1

TiO2 Eg = 3.2 eV 2.02 2.2

2

2.9

3 4

Fig. 5.8 Photocatalytic activity LaFeO3-TiO2 nanocomposite showing interfacial charge transfer. Reproduced with permission from (Humayun et al. 2016) copyright 2016, American Chemical Society Publishers

Therefore, such studies have opened a new window for functional perovskite-type ferrites materials for various applications. Layered perovskites are an important class of materials owing to their unique structural properties. Various layered perovskites have been utilized by the researchers for CO2 reduction reactions. Zhou et al. (2011) synthesized very interesting ultrathin and uniform Bi2WO6 square nanoplates of specific thickness by hydrothermal synthesis method and investigated its photocatalytic activity for CO2 reduction by harvesting visible light photons. The exposed (001) facet of Bi2WO6 has been found to be active for photocatalytic activity. It has also been demonstrated that ultrathin morphology helps to promote the transfer of photoinduced charge carriers to the reaction sites over the catalyst surface, wherein they participate in photoreduction reaction. Hence, the recombination of photoinduced charge carriers has been suppressed effectively, which facilitates the photocatalytic reduction of CO2 into hydrocarbon fuels. It is worth to mention here that the high specific surface area of photocatalysts significantly boosts the photocatalytic activity by providing abundant reaction sites as compared to their bulk counterparts. The numerous active sites on catalyst surface effectively adsorb the reactant molecules and hence enhance the photocatalytic activity. Very recently Kang et al. (Kwak et al. 2017) have fabricated layered perovskite, Sr2TiO4 photocatalyst by sol-gel method and demonstrated its activity for CO2 conversion reaction. The surface of Sr2TiO4 photocatalyst was modified via hydrogen treatment (H-Sr2TiO4) and exfoliation (E-Sr2TiO4) to increase its surface area. It is well reported that exfoliation leads to the increase in surface area by the formation of smaller particles. Herein, the E-Sr2TiO4 exhibits superior photocatalytic activity for CO2 photoreduction as compared to Sr2TiO4 and H-Sr2TiO4. The enhanced photocatalytic activity of E-Sr2TiO4 photocatalyst has

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been attributed to two factors: (i) the high specific surface area due to exfoliation treatment which provides abundant exposed reaction sites to adsorb large amount of CO2 and H2O and (ii) narrow band gap which enables catalyst to utilize more visible light photons and generate larger number of electrons and holes in conduction band and valence band, respectively. Iizuka et al. (2011) tuned the band gap of layered perovskite ALa4Ti4O15 (A ¼ Ca, Sr and Ba), wherein Ag has been used as a cocatalyst for photocatalytic reduction by bubbling CO2 into an aqueous suspension of the photocatalyst. The Ag nanoparticles prepared by the liquid-phase chemical reduction method were used as cocatalyst with BaLa4Ti4O15 photocatalyst during the CO2 reduction. The main reduction product was CO in aqueous medium on the optimized Ag/BaLa4Ti4O15 photocatalyst with no H2 evolution. The evolution of O2 in a stoichiometric ratio showed that water acts as a reducing reagent for the CO2 reduction. The various reports based on perovskite materials for photocatalytic CO2 reduction to value-added chemicals have been summarized in Table 5.2.

5.6

Summary

The photocatalysis process is an outstanding solution for energy and environmental problems, and it is imperative to develop novel photocatalytic materials with superior feature and functions to surpass the existing limitations of this process. In this book chapter, recent progress on ABO3 perovskite photocatalysts and the alterations to make them more efficient for the environmental remediation applications have been discussed in detail. The unique structure of perovskite materials offers a huge platform for the design of new materials by means of alteration at A, B and O sites. The availability of different sites can be utilized by researchers to explore the possibilities to develop more efficient photocatalytic materials. In addition to doping for tuning the band gap in the visible light region, the suppression of photogenerated charges is equally important in order to achieve maximum photocatalytic activity. In this regard, the coupling of photocatalysts with other materials, such as semiconductors, graphene, etc., to prepare composites and heterojunctions is well studied. The appropriate positions of bands in such composites or heterojunctions allow the transfer of photogenerated charges and suppress their recombination, eventually boosting the photocatalytic performance. The combination of all these different perovskite modification techniques can pave the path to the future research in this area and can result in a deeper understanding of the role of these techniques, which can in turn help in the development of advanced photocatalysts for environmental remediation applications, which includes organic pollutant degradation and CO2 reduction. Thus, this research area will continue to emerge in the upcoming years, and perovskite-based photocatalysts are needed to be explored more deeply.

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Chapter 6

Carbon Nitride: A Wonder Photocatalyst Biswajit Choudhury

Contents 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Photocatalytic Attributes of the Different Forms of g-C3N4 Nanostructures . . . . . . . . . . . . 6.2.1 Bulk and Nanosheets of g-C3N4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Shape-Tailored g-C3N4 Nanosheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3 Surface Modification by Defect Engineering and Doping . . . . . . . . . . . . . . . . . . . . . . . 6.2.4 Carbonaceous Materials-C3N4 Nanocomposite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.5 Metal Oxide-g-C3N4 Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.6 Metal Sulphides-C3N4 Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract The growing demand of energy and industrial advancements for a better livelihood is also bringing several environmental challenges, which mostly includes global warming, change in weather pattern, and air and water quality variation. The present energy needs are derived from fossil fuels such as coal and natural gas and petroleum. Fossil fuels are responsible for the emissions of greenhouse gases such as CO2 and NO. Similarly, many industrial and medical wastes released into the soil are contaminating surface and ground water quality. Many of these problems can be tackled with by the aid of photocatalysis. Water splitting using solar energy can generate renewable hydrogen (H2) energy which can be an alternative to the fossil fuels for solving energy-related problems. Similarly, utilization of solar energy can also improve water and air quality to a larger extent. The solving of these problems requires the design of a suitable photocatalyst that can effectively utilize ultraviolet (UV) and visible (Vis) light for performing photocatalytic reactions. Graphitic carbon nitride (g-C3N4) is an emerging 2D layered carbonaceous material with enormous photocatalytic applications in the field of water splitting, water detoxification, CO2 reduction, NO removal, etc. The widespread use in the B. Choudhury (*) Physical Sciences Division, Institute of Advanced Study in Science and Technology (An Autonomous Institute Under DST, Government of India), Guwahati, Assam, India e-mail: [email protected] © Springer Nature Switzerland AG 2019 Inamuddin et al. (eds.), Nanophotocatalysis and Environmental Applications, Environmental Chemistry for a Sustainable World 29, https://doi.org/10.1007/978-3-030-10609-6_6

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area of photocatalysis is because of their visible light absorbing band gap, nanoporous nature, good thermal stability, better separation of charge carriers, and appropriate band structure for coupling with different types of photocatalytic active semiconductor material. This chapter discusses the different experimental tools adopted to design and modify g-C3N4 for the desired photocatalytic applications. The different methods which are considered are thermal and chemical exfoliation, shape tailoring, surface modification by defect and doping engineering, nanocomposites with carbonaceous materials, semiconductor metal oxides, metal sulphides, etc. The modified g-C3N4 can efficiently harvest the entire solar light spectrum from visible to NIR and show largely available photogenerated electrons and holes for participation in photocatalysis. This photocatalyst can be effectively used for water purification, H2 generation, O2 evolution, CO2 reduction, NO removal, etc. All these issues have been discussed in this chapter with an idea to broaden the futuristic applications of g-C3N4 as wonder photocatalyst for clean, green, and sustainable energy generation. Keywords Graphitic carbon nitride · Semiconductor metal oxides · Defects · Doping · Absorption · Charge carriers · Nanocomposite · Water splitting · Photocatalytic degradation · CO2 removal

6.1

Introduction

Energy demand has been increasing for every day-to-day activity of our lives including from electricity consumption and transportation to the ever-growing industrialization. Reports say that the energy demand will increase to 50–60% by 2030 (Pickrell 2006). A larger portion of energy is derived from fossil fuels such as coal, natural gas, and petroleum. Even though these energy sources are fulfilling the energy demands, the emissions of hazardous CO2 and NO in the atmosphere from the burning of these fossil fuels are creating havoc for every livelihood present on earth. In Paris agreement in December 2015, several countries showed their concern regarding the CO2 greenhouse gas emissions and targeted to keep global warming below 2
C (Boettcher et al. 2016). Scientists are showing serious concern about an alternative source of energy which can reduce the energy crisis and is environmental friendly at the same time. The ultimate source of all the energy sources is sun. The earth’s surface is receiving solar energy in every minute of the day and continuously throughout the year. Renewable energy source can be generated by harnessing the solar energy. It is expected that the use of energy efficient renewable sources can reduce the CO2 emissions to 75% by 2050 (Johansson et al. 1992). One such clean and renewable energy source is H2 which can be generated by water splitting under solar irradiation. Soil and ground water pollution is another serious issue which is also increasing with the urbanization and technological and industrial advancements. The major sources of water pollution are agricultural wastes, oil wastes, medical wastes, organic wastes, domestic sewage, and radioactive wastes. Photocatalysis is a

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phenomenon in which solar energy can be efficiently utilized to generate chemical energy and drive photochemical reactions. Water is highly abundant on the earth’s crust, and by photocatalysis water can be dissociated to H2 and O2 which can act as a clean and renewable energy source. Photocatalysis is also efficient in reducing CO2 and NO emission and in cleaning of water contaminated with organic compounds and metal ions. Design of a suitable photocatalyst which can harness the complete solar energy spectrum is the foremost requirement for triggering efficient photocatalytic reactions. Heterogenous photocatalysis utilizes a semiconductor for efficient generation of H2 and water and air pollution degradation. The well-known and most abundantly used semiconductor photocatalyst is TiO2. TiO2 is a stable photocatalyst and can show high photocatalytic efficiency under UV light. The main drawback of this photocatalyst is their very short carrier lifetime and inefficiency to perform photocatalysis in the entire visible light spectrum. Here in, we discuss the photocatalytic attributes of a two-dimensional carbon-based photocatalyst, graphitic carbon nitride, or g-C3N4. A decade ago many theoretical and experimental results predicted a new solid material made of C and N atoms with hardness comparable to that of diamond. Among its different phases, hexagonal β-C3N4 was considered to have the highest bulk moduli followed by cubic C3N4. Synthesis of polycrystalline β-C3N4 was first reported by the application of laser beam ablation on graphite sheet with the assistance of ion/atom beam bombardment (Niu et al. 1993). Other preparation techniques of g-C3N4 include plasma deposition or sputtering and ion beam deposition that resulted in the amorphous or crystalline forms of these phases (Chubaci et al. 1993; Han and Feldman 1988; Corkill and Cohen 1993). The structure of βC3N4 consisted of CN4 tetrahedra with C atoms forming regular tetrahedron with four nearest N atoms, whereas each N atom forms a 120
bond angles with three nearest-neighbour C atoms (Corkill and Cohen 1993). Another stable form of carbon nitride with thermal stability up to 550–600
C under ambient condition is g-C3N4. The discovery of metal-free g-C3N4 as a visible light active photocatalyst is dated back to 2006 when a group of scientist reported superior H2 energy evolution from two-dimensional layered structures of g-C3N4 (Wang et al. 2009). g-C3N4 has alternately bonded C and N atoms in an aromatic s-triazine (C3N3) or tri-s-triazine or s-heptazine ring (C6N7). If we search the history of the origin of this form of carbon nitride, we need to consider the initial work on “melon” by the then famous chemists Berzelius and Liebig in nearly 1830 (Lotsch and Schnick 2006; Wang et al. 2012). Secondary nitrogen with interconnected tri-s-triazines constitutes melon strand, whereas tertiary amines interconnecting tri-s-triazines constitute C3N4 (Zheng et al. 2012). Formation of carbon nitride from different precursors involves intermediate having s-triazine or tri-s-triazine ring system. Some of the sources having s-triazine ring are melamine, cyanuric acid, and cyanuric chloride, whereas tri-s-triazine ring prevails in melem, cyameluric acid, melonates, 2,5,8-triazido-sheptazine, etc. (Schwarzer et al. 2012; Miller et al. 2007; Holst and Gillan 2008). Theoretical study has shown that tri-s-triazine is energetically more preferable over s-triazine-based systems (Kroke et al. 2002). The basic difference in the chemical properties between benzene, tri-s-triazine, and s-triazine is that the former is more

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electron-rich than the latter two (Schwarzer et al. 2012). The precursors for g-C3N4 are low cost and widely abundant, such as urea, thiourea, and aminodichlorotriazine (Gibson et al. 2008). The starting precursor influences the reactivity, surface area, and optoelectronic properties of g-C3N4 (Dong et al. 2013). g-C3N4 is considered as a wonder photocatalyst because of its environment-friendly nature, strong chemical stability, and large surface area in its ultrathin form. Unlike widely explored photocatalyst TiO2 which is a UV active with a band gap of 3.2 eV, bulk g-C3N4 possesses a band gap of 2.7 eV with ability to absorb visible light. In this chapter, we will discuss the photocatalytic attributes of g-C3N4 in its pristine and modified forms. We will provide a detailed study on how the structurally and optically tuned g-C3N4 can be efficiently used for a diverse of photocatalytic applications, such as dye degradation, water splitting, and removal of environmental pollutants such as CO2 and NO.

6.2 6.2.1

Photocatalytic Attributes of the Different Forms of g-C3N4 Nanostructures Bulk and Nanosheets of g-C3N4

g-C3N4 can be easily synthesized from low-cost precursors such as urea (Choudhury and Giri 2016), thiourea (Zhao et al. 2015a), melamine (Yan et al. 2009), dicyandiamide (Xu et al. 2014), and thiourea dioxide (Yuan et al. 2017). Thermal condensation of these precursors in the temperature range of 500–600
C in a covered crucible results in the formation of g-C3N4 (Fig. 6.1). Most of these preparation methods generate bulk g-C3N4. The unique features of bulk g-C3N4 are its low surface area, large recombination of photoexcited carriers over its surface, and a band gap of ~2.7 eV (Wang et al. 2009; Choudhury and Giri 2016). A fully condensed g-C3N4 exhibits an intense diffraction peak at 2θ ¼ 27.4
corresponding to (002) plane with an interlayer d-spacing of 0.326 nm equivalent to that of graphite. Minor diffraction peak at 2θ ¼ 13.0
attributes to (100) plane with an intralayer d-spacing of 0.680 nm (Fina et al. 2015). Selection of starting precursors and processing temperature strongly affects the physiochemical and optoelectronic properties of g-C3N4 (Dong et al. 2013; Yan et al. 2009; Cao et al. 2015; Zhang et al. 2012). A decrease in the optical band gap from 2.8 to 2.75 eV is reported in melamine-derived g-C3N4 (surface area, 8 m2/g) on an increase in the calcination temperature from 500 to 580
C (Yan et al. 2009). Thermal condensation of thiourea in the temperature range of 450–600
C results in the growth of a g-C3N4 with a surface area of 52 m2/g and a band gap variation from 2.71 to 2.76 eV (Zhang et al. 2012). The large stacking of layers in bulk g-C3N4 is predicted to result in its low surface area. Thermal oxidation of bulk g-C3N4 in air and layer cleavage by chemical exfoliation are strategies adopted to produce ultrathin, large surface area g-C3N4.

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Fig. 6.1 Precursors for the preparation of tri-s-triazine ring-based graphitic carbon nitride (central ring)

Thermal oxidation of g-C3N4 for a longer time period results in layer thinning with concomitant increase in the specific surface area from 22 to 245 m2/g (Ren et al. 2017). Similarly, oxidative etching of bulk C3N4 in a static air at 500
C for 2 h can synthesize g-C3N4 nanosheets (Fig. 6.2). The specific surface area increases from 50 m2/g (bulk) to 306 m2/g (nanosheets) (Niu et al. 2012). The lifetime of photogenerated charge carriers increases with a decrease in layer thickness, as revealed by the results of fluorescence lifetime measurements. The nanosheets show H2 production rate of 170 μmolh1 under UV-visible light, which is more than five times higher than that of bulk g-C3N4. Under only visible light, the H2 production rate for nanosheets is three times higher than that of bulk counterpart. By varying the calcination time period (0–240 min), the thickness of g-C3N4 can be successfully reduced with simultaneous increase in the surface area from 31 to 288 m2/g (Dong et al. 2013). Similarly, the RhB degradation increases from 40.9% (0 min) to 100% (240 min) under visible light. Liquid-phase ultrasonic exfoliation is another suitable method for the production of large volume of g-C3N4 nanosheets. Ultrasonic treatment of bulk g-C3N4 in water can exfoliate the nanosheets (Fig. 6.3). The prepared nanosheets have a diameter of 70–160 nm with thickness of ~2.5 nm containing seven C-N layers (Zhang et al. 2013a). Exfoliation of g-C3N4 layers in H2SO4 can reduce the average layer thickness to 0.4 nm with an enlarged

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Fig. 6.2 (a) Schematic of the bulk g-C3N4 and the g-C3N4 nanosheet structures after thermal exfoliation. (b) A volume comparison of 50 mg powder of bulk g-C3N4 and g-C3N4 nanosheets. (Reproduced with permission from Niu et al. 2012. Copyright 2012 John Willey and Sons)

Fig. 6.3 (a) Schematic showing liquid exfoliation from bulk g-C3N4 to ultrathin nanosheets. (b) Photograph of bulk g-C3N4 powder and aqueous suspension of ultrathin g-C3N4 nanosheets. (c) Theoretically generated perfect crystal structure of the g-C3N4 projected along the z-axis. (Reprinted with permission from Zhang et al. 2013a. Copyright 2013 American Chemical Society)

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(a)

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Fig. 6.4 Schematic of synthetic strategy. (a) Intercalation of H2SO4 (98 wt%) in the interplanar space of g-C3N4. (b) Exfoliation of g-C3N4 into a single layer due to the rapid heating effect of H2SO4 (98 wt%) mixing with water. (c) Photocatalytic H2 production in the presence of bulk and monolayer-C3N4. (d) Photocatalytic decomposition of phenol under visible light irradiation (λ > 420 nm). (e) Stability measurement for fifth cyclic run. (Reproduced with permission from Xu et al. 2013. Copyright 2013 Royal Society of Chemistry)

surface area of 205.8 m2/g (Xu et al. 2013) (Fig. 6.4a, b). The nanosheets show H2 production rate of 230 μmolg1h1, while only 90 μmolg1h1 H2 production is recorded with bulk g-C3N4 (Fig. 6.4c). As compared to bulk g-C3N4, the nanosheets are highly effective in the faster decomposition of phenol (Fig. 6.4d) with photocyclic stability for consecutive 5th run (Fig. 6.4e). A series of experiments for g-C3N4 layer exfoliation has been conducted in different solvents, such as isopropyl alcohol, N-methyl pyrrolidone, water, ethanol, and acetone (Yang et al. 2013). Stable dispersion of g-C3N4 nanosheets with high specific surface area of 384 m2/g is achieved in isopropyl alcohol. The bulk and nanosheets show H2 evolution rate of 93 μmolh1 and 10 μmolh1, respectively. The main requirement for stable liquid-phase exfoliation is to match the surface energies of solute and solvent with a minimized enthalpy of mixing. g-C3N4 exfoliation in highly basic condition with 8 M NaOH can result in nanosheets of thickness ~1.6 nm as revealed by atomic force microscope (AFM) (Tian et al. 2015). A solvothermal method has been adopted to synthesize atomically thin g-C3N4

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Fig. 6.5 (a) Photocatalytic activities of the (i) monolayer mesoporous g-C3N4 nanomesh, (ii) the bulk g-C3N4, and (iii) the traditional g-C3N4 bulk and stable hydrogen evolution by the monolayer mesoporous g-C3N4 nanomesh under irradiation with visible light (λ > 420 nm). The photocatalytic reaction was tested for 30 h, with evacuation every 5 h (dashed line). (b) H2 evolution rate on the monolayer mesoporous g-C3N4 nanomesh as a function of wavelength in the visible light spectrum. The inset in (b) shows the H2 evolution under 550 nm light irradiation. (Reprinted with permission from Han et al. 2016. Copyright 2013 American Chemical Society)

nanosheets in isopropanol-water mixture (v:v ¼ 3:2 Han et al. 2016). The mesoporous atomic-layered nanosheets of g-C3N4 show remarkably high H2 evolution rate of 8510 μmolh1g1 with an apparent quantum efficiency of 5.1% at 420 nm (Fig. 6.5a, b).

6.2.2

Shape-Tailored g-C3N4 Nanosheets

Two-dimensional (2D) layered structures of g-C3N4 can be tuned into structures with different shape and morphology, such as tubes, rings, rods, mesoporous structures, and hollow spheres, by adopting different synthetic pathways. These shape-tailored g-C3N4 with modified structural and optoelectronic properties show efficient results in visible light photocatalytic activity as compared to the 2D bulk or nanosheets. Tunable porous structure of g-C3N4 can be synthesized by adopting softand hard-templating method. The high surface area and the charge-transporting channels in the mesoporous structure can be suitable for designing efficient photocatalytic systems. Hexagonally arranged ordered mesoporous g-C3N4 (OMPG) is synthesized using cyanamide as precursors and SBA-15 as template (Chen et al. 2009). The porous g-C3N4 has a pore volume of 0.34 cm3g1 and a pore size of 5.3 nm. The OMPG shows H2 evolution rate of 85 μmolh1 in the presence of Pt as a cocatalyst and triethanolamine as a sacrificial reagent. The efficiency is five times higher than that of bulk g-C3N4. An improved method for the preparation of OMPG involves HCl treatment of SBA-15 silica and mixing with cyanamide (Zhang et al. 2013b). Vacuum sonication and heat treatment at 55
C incorporate cyanamide inside the

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silica pores. Finally silica is etched with NH4HF2, and the product is calcined at 550
C to collect the high surface area (517 m2/g) OMPG with high porosity (0.49 cm3g1). The mesoporous structure shows H2 evolution activity of 1450 μmol over an irradiation period of 4 h. The apparent quantum yield (AQY) is 6.77% at 455 nm. The enhancement is attributed to the large surface area and hexagonal channel for charge transport. Cosolvent-free nano-casting route synthesis of OMPG is highly efficient in the degradation of methyl orange. The activity is 30 times higher than that of bulk g-C3N4 (Gao et al. 2015). Ordered mesopores with high surface area provide maximum sites for dye adsorption. The nanochannels in the porous structure provide easy pathways for charge diffusion from bulk to the surface, thus effectively suppressing carrier recombination. g-C3N4 in porous SiO2 beads was synthesized by heating dicyanamide precursors in the beads at different temperatures under N2 flow (Li et al. 2017a). Porous g-C3N4 was collected by etching SiO2 in 15% NH4HF2 (Fig. 6.6a). H2 evolution rate with mesoporous structure increases eight times (53 μmolh1) than that of bulk C3N4 (7 μmolh1) (Fig. 6.6b). Rhodamine B degradation in the bulk and mesoporous g-C3N4 was 23.2% and 97.4% (Fig. 6.6c). Facile charge transport in the 3D interconnected porous network could lead to electron-hole separation and enhancement in the photocatalytic activity. Mesoporous g-C3N4 can also be synthesized using soft template, such as pluronic-P123 (Yan 2012). Using this soft template and melamine as precursors, mesoporous g-C3N4 are synthesized with high surface area of 90 m2/g and light absorption range extending to near-infrared region (800 nm). Without the use of template, g-C3N4 shows only 60.5 μmolh1 H2 evolution rate under visible light. However, with 10.8 wt % P123, the H2 production rate reaches 148.2 μmolh1. The mesoporous structure shows superior photocatalytic activity above 700 nm and records H2 evolution of 3.6 μmolh1. In another soft template-mediated growth process, different nonionic surfactants are considered, such as Pluronic F-127, Pluronic P-123, and Triton X-100, and different sources for g-C3N4 are melamine and cyanuric acid (Peer et al. 2017). The 3D ordered mesoporous graphitic carbon nitride developed in this method has a high surface area of 270 m2/g and a porosity of 0.8 cm3/g. The system shows efficient degradation of rhodamine B under the visible light. The catalytic efficiency is attributed to its high surface area and strong utilization of visible light. Hollow spheres of g-C3N4 are equally effective for visible light photocatalytic applications. Hollow spheres can harvest sufficient amount of visible light by multiple reflections of incident light within the hollow spheres and can induce generation of sufficient density of photo carriers. A core-shell structure of silica nanoparticles as core and thin mesoporous silica as shell has been used as hard template for cyanamide insertion (Sun et al. 2012). Thermal condensation and removal of silica template generate hollow spheres of g-C3N4. Photocatalytic H2 production for the hollow spheres is 224 μmolh1 which is 25 times higher than that of bulk g-C3N4 (9 μmolh1) with impressively high apparent quantum yield of 7.5%. Hollow mesopores of g-C3N4 can be synthesized by the molecular cooperative assembly of triazine (Jun et al. 2013a). Melamine and cyanuric acid in equimolar

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Fig. 6.6 (a) Schematic of the preparation of porous silica beads, macropores and 3D interconnected porous silica beads (MCN), and reverse porous silica beads. (b) Rate of photocatalytic H2 evolution and (c) photodegradation of rhodamine B and over (I) SiO2 beads, (II) the as-synthesized SiO2/MCN-560 composite, (III) PCN-560 and (IV) MCN-560. (Reprinted with permission from Li et al. 2017a. Copyright 2017 Royal Society of Chemistry)

quantities in dimethyl sulphoxide solvent can form hydrogen-bonded cyanuric acidmelamine (CM) supramolecular aggregates. Thermal condensation at 550
C under N2 flow produces hollow mesoporous spheres of g-C3N4. Photocatalytic results show 60% rhodamine B degradation within 10 min of irradiation, and the entire decomposition is achieved within 1 h of light exposure. In the molecular assembly approach, a change in solvent, precursors, and temperatures can significantly improve the three-dimensional (3D) network assembly, textural and morphology properties of the synthesized mesoporous C3N4 structure (spheres, rods, needle) (Jun et al. 2013b; Shalom et al. 2013). By adopting this approach, four to nine times enhancement in H2 production is achieved with a quantum efficiency of 2.3%. Similarly, 50% rhodamine B degradation can be achieved within 15 min of irradiation using the hollow structures. g-C3N4 hollow spheres can be grown from H-bonded complex of cyanuric acid-melamine in the presence of dimethyl sulphoxide as a solvent and ionic liquid as a cosolvent (Zhao

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et al. 2018). The ionic liquid influences the growth of H-bonds and changes the texture of the assembly. H2 production rate with the hollow spheres is 157 μmolh1 which is 30 times higher than that of the value of 4.7 μmolh1 recorded in bulk C3N4. Copolymerization reaction of hollow spheres of g-C3N4 with aromatic monomer such as 2-aminothiophene-3-carbonitrile (ATCN) forms a complex carbon nitride sphere thiophene (CNST) with different ATCN content. This complex has an extended π network framework and enhances the visible light utilization efficiency of the hollow spheres (Fig. 6.7a) (Zheng et al. 2015). The H2 production rate

Fig. 6.7 (a) Synthetic strategy for the hollow nanospheres via shell engineering. (b) Wavelength dependence of hydrogen evolution rate on CNST0.05 and inset shows stability test of Pt/CNST0.05 photocatalyst with prolonged visible light irradiation (λ > 455 nm). (Reproduced with permission from Zheng et al. 2015. Copyright 2016 Royal Society of Chemistry)

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Fig. 6.8 (a) Illustration showing g-C3N4 nanorods (CNRs) in anodic aluminium oxide (AAO) template via three steps: (1) filling up of the template (grey) with monomer cyanamide (white), (2) heating the filled templates at 600
C under N2 flow for 4 h, and (3) removal of template by etching and collecting CNRs (yellow). The top right of (a) shows the CNRs in AAO before template removing. Bottom right shows the proposed reaction mechanism for water oxidation and reduction. (b) H2 and O2 evolution reaction from water by CNRs (Reprinted with permission from Li et al. 2011. Copyright 2011 American Chemical Society). (c) Apparent rate constants for methylene blue (MB) photodegradation over blank, g-C3N4 nanoplates and g-C3N4 nanorods under visible light (λ > 420 nm, [MB] ¼ 0.03 mM). (Reprinted with permission from Bai et al. 2013. Copyright 2013 American Chemical Society)

is 79 μmolh1, which is higher than that of pure g-C3N4 at wavelength longer than 550 nm (Fig. 6.7b). Nanosheets of g-C3N4 can be rolled up to fabricate electronically tuned structure such as nanorods, nanofibres, nanowires, etc. The tuned structure can influence the optical absorption, charge carrier separation, etc. Condensed graphitic carbon nitride nanorods can be synthesized by thermal condensation of cyanamide at 600
C filled in the nanochannels of anodic aluminium oxide (AAO) template (Li et al. 2011). AAO template is removed by etching with 1 M HCl (Fig. 6.8a). Higher in-plane carrier mobility and more positive valenceband position enhance the oxidation-reduction power for H2 and O2 evolution (Fig. 6.8b). Rolling up of nanoplate to nanorod can be controlled by refluxing bulk g-C3N4 in a water-methanol mixture solvent for the time duration of 1–6 h (Bai et al. 2013). The refluxing time of 3 h can result in nanorods having a diameter of 100–150 nm and length 0.5–3 μm. The lamellar to nanorod conversion increases the numbers of catalytically active sites and facilitates in-plane charge carrier migration for the suppression of their recombination. These nanorods show efficient result in the removal of methylene blue from water (Fig. 6.8c). Hard-template (SBA-15)-mediated growth of g-C3N4 nanorods can acquire a diameter of 80–120 nm and a length of 650 nm (Li et al. 2012a). The nanorods

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have a surface area of 110–200 m2/g and show excellent efficiency in the photoreduction of 96% 4-nitrophenol with metal nanoparticles as a cocatalyst. Low-cost, template-free growth of g-C3N4 nanotubes can be achieved by compact packing of melamine in a crucible using a vibrator followed by pyrolysis at 500
C (Wang et al. 2014). The designed nanotube shows a higher rate of photodegradation of methylene blue than that of bulk g-C3N4. Pre-treatment of melamine with HNO3 in ethylene glycol as solvent followed by pyrolysis at 450
C forms tubular g-C3N4 (Tahir et al. 2013). With a surface area of 182.6 m2g1, the nanotube displays remarkable photocatalytic activity in the decomposition of methyl orange and methylene blue. The concentration of HNO3, calcination temperature, and solvent can change the morphology of the systems from nanotube and microstrings to nanofibres (Tahir et al. 2014a, b). These one-dimensional (1D) forms of g-C3N4 are quite efficient in the removal of rhodamine B, methyl orange, and methylene blue from contaminated water under the illumination of visible light.

6.2.3

Surface Modification by Defect Engineering and Doping

Surface modification of g-C3N4 nanosheets can be achieved by surface defects’ introduction or by doping and codoping with metal and nonmetal ions. The surfacemodified g-C3N4 display remarkably enhanced photocatalytic activity in water splitting, water purification, and air pollutant degradation. One of the studies has shown that g-C3N4 with confined carbon defects can efficiently decompose 96.2% rhodamine B within 120 min of visible light irradiation, while only 11.8% rhodamine B decomposition is recorded with bulk g-C3N4 (Di et al. 2016). The photocatalytic efficiency of the ultrathin defective g-C3N4 is due to its large surface area, easy migration of carriers from bulk to surface, and suppression of carrier recombination due to the trapping of photogenerated carriers on carbon defects. Ultrathin g-C3N4 with carbon vacancies show positive effect in reducing NO to N2 under visible light (Dong et al. 2017a). While the ultrathin structure favours strong chemisorptions of NO and easy migration of charge carriers from bulk to surface, the carbon defects act as active trap centres and suppress charge carrier recombination. NO with oxygen-bearing sites attach with carbon vacancies (Cv) forming Cv-O-N. The photogenerated electrons dissociate this bond releasing N2 and O2 (Dong et al. 2017a). Holey nanosheets of g-C3N4 with carbon vacancies are synthesized by the thermal treatment of bulk g-C3N4 under NH3. Accruing a surface area of 196 m2/g, the nanosheets show superior efficiency in H2 production rate of 89.2 μmolh1 which is 20 times faster than that of 4.2 μmolh1 recorded in bulk g-C3N (Liang et al. 2015). Structural disorder in g-C3N4 induced by hydrogenation is an effective strategy for better absorption of visible light, larger carrier separation, and an increase in donor-density electrons (Tay et al. 2015; Tu et al. 2017; Zhou et al. 2018). Nitrogen-related vacancies are predominantly formed during hydrogenation.

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In order to understand the impact of hydrogenation on their visible light photocatalytic performance, bulk g-C3N4 is treated under different atmosphere, such as air, N2, forming gas (5% Ar + 95% Air), and pure H2 (Tay et al. 2015). g-C3N4 treated with pure H2 shows absorption spectrum extended from visible up to 700 nm with a band gap reduction to 2.0 eV. For different samples, the H2 evolution reaction rate follows the increasing order: C3N4 (air), C3N4 (N2), C3N4 (forming gas), and C3N4 (H2). Hydrogenation of g-C3N4 at different temperature also changes the defect level distribution in the band gap (Tu et al. 2017). Nitrogen vacancyrelated defect states are present near the conduction band. As the density of nitrogen vacancies increases, the distributions of trap levels extend from near the conduction band edge to the deep in the band gap. The high density of nitrogen vacancies concomitantly increases visible light absorption and shows improved photocatalytic activity in H2 evolution reaction and CO2 reduction. The donor density electrons after hydrogenation can be increased from 5.85  1021 to 7.87  1021 cm3 in g-C3N4 (Zhou et al. 2018). The hydrogen evolution rate for hydrogenated g-C3N4 is 758.9 μmolg1 h1, whereas 524.6 μmolg1 h1 H2 is evolved in the presence of bulk g-C3N4. Similarly, the rate of methylene blue degradation using hydrogenatedg-C3N4 is 0.11 min1, whereas the rate constant value is only 0.015 min1 in bulk g-C3N4 (Zhou et al. 2018). Hydrogenation of a pre-oxygenated g-C3N4 has a synergetic influence in the improvement of the photocatalytic performance (Sun et al. 2017). Initially, oxygenated functional groups such as C¼O, C-O are introduced into g-C3N4 by hydrothermal treatment. Hydrogenation calcination of this oxidized g-C3N4 generates nitrogen vacancies into the system. The valence and conduction band position of g-C3N4 are effectively modified by oxygenation-hydrogenation approach. The effective band gap reduces to 2.3 eV with an enhancement in the visible light absorption. The oxygenated functional groups and the nitrogen vacancies efficiently trap the photogenerated carriers and suppress their recombination. This modified g-C3N4 shows AO7 dye degradation within 180 min of irradiation with an apparent rate constant of 5.56  103 min1, which is nine times higher than that of bulk g-C3N4 (Sun et al. 2017). One of the studies reports the negative attributes of the nitrogen vacancies on the photocatalytic activities of rod-like g-C3N4 with fewer defects, synthesized via ultrasound-assisted molecular arrangement strategy (Huang et al. 2018) (Fig. 6.9a). Reduction in defects improves the visible light absorption, enhances carrier separation, and accelerates rhodamine B degradation at a rate constant of 0.035 min1, which is 16.7 times higher than the corresponding bulk counterpart. Similarly, the hydrogen evolution rate is found to be 246 μmolh1, which is 8.7-fold higher than that of bulk (Fig. 6.9b). One of the synthetic strategies to incorporate nitrogen vacancy in g-C3N4 involves copolymerization of melamine with monomer 4-diamino-6-phenyl-1,3,5triazine (DPT) at an elevated temperature. This nitrogen-deficient g-C3N4 has a high surface area and narrow band gap and shows improved charge carrier separation (Zhang et al. 2017). Holey nanosheets of g-C3N4 with nitrogen vacancies display higher photocatalytic activity for H2O2 production as compared to that of bulk g-C3N4 (Shi et al. 2018a). The nanosheets are synthesized by photo-assisted etching

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Fig. 6.9 (a) Schematic showing synthesis of rod-like g-C3N4 (RCN). (b) Evaluation of the photocatalytic activity of RCN in (b) rhodamine B degradation and (b) H2 evolution. (Reprinted with permission from Huang et al. 2018. Copyright 2018 American Chemical Society)

of bulk g-C3N4 in a hydrazine solution. The mixture was irradiated with a Xenon lamp (300 W) for different time periods with light intensity controlled by current output between 7 and 21 A. The etching creates abundant holes, generates exposed catalytic sites, and produces diffusion channels for reactants and charge carrier diffusion to and from the bulk to the surface. The holey nanosheets with narrow band gap and available free charge carriers show H2O2 evolution at a rate of 4.8 μmolh1, which is nearly ten times faster than that of bulk g-C3N4 (Shi et al. 2018a). Uncondensed g-C3N4 contains catalytically relevant functional groups such as oxygen-bearing groups and C and N containing cyanamide moiety. The H2 evolution rate in the presence of cyanamide defects is 24.7 μmolh1 (Lau et al. 2016). The BET surface area of cyanamide containing g-C3N4 is more than three times higher than g-C3N4 without cyanamide. The H2 evolution activity normalized with BET surface area is 22.2 μmolh1m2 versus 7.5 μmolh1m2 for amorphous g-C3N4. Similarly, cyano-functional groups over a three-dimensional (3D) graphitic carbon nitride are synthesized by freeze-drying and calcination of a mixture of thiourea and NaCl (Chen et al. 2018). Because of the multiple reflections of incident light within the open 3D porous structure, the visible light utilization efficiency is enhanced. The cyano group acts as an active trap centres and prohibits carrier recombination. The recorded H2 production ability of the porous structure is 1590 μmolh1g1, which is many times higher than that shown by bulk g-C3N4 (240 μmolh1g1). Introduction

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of cyano defects and phosphorous doping in g-C3N4 improves the H2 evolution rate by 6.7 times and CO2 reduction by 1.58 times as compared to that of pristine g-C3N4 (Liu et al. 2018). The electron-withdrawing nature of cyano groups inhibits carrier separation and improves visible light absorption, whereas P dopant narrows the band gap and acts as trap centres for photogenerated carriers. Thus, defects and dopants have synergetic influence in the overall improvement of the photocatalytic performance of the system. Carbon self-doping on bridging N site in g-C3N4 could extend the π-delocalization favouring electron transfer among the g-C3N4 network. This strategy is helpful in the removal of rhodamine B, Cr(VI) to Cr(III) conversion, and efficient H2 evolution reaction (Dong et al. 2012). Oxygen doping in g-C3N4 can tune the optical properties of the latter by shifting the optical absorption towards higher wavelength and inhibit carrier recombination (Li et al. 2012b). Oxygendoped g-C3N4 shows 84% methylene blue degradation, whereas only 24% dye decomposition is recorded in with bulk g-C3N4. Similarly, H2 evolution rate becomes doubled when g-C3N4 lattice is doped with oxygen. Oxygen doping by plasma treatment is an all-new strategy to improve the visible light photocatalytic activity of g-C3N4 (Qu et al. 2018). Oxidation of g-C3N4 partially breaks the tertiary amine groups and gives rise to structures having secondary amines and hydroxyl groups. Rhodamine B degradation over the oxidized g-C3N4 is substantially improved since oxygen doping enhances the hydrophilicity of the catalyst and thereby favours better adsorption of the dye over g-C3N4 surface. Oxygen doping can generate C-O-C bond formation and changes the band structure. A change in the band structure can change the optical absorption, carrier lifetimes, surface area, etc. The rate of rhodamine B degradation and H2 evolution with oxygen-doped g-C3N4 is 0.249 min1 and 3174 μmolg1h1, respectively. The rate kinetics for dye degradation and H2 evolution is much slower with bulk g-C3N4 with the measured values 0.007 min1 and 846 μmolh1g1, respectively (Wei et al. 2018). Another strategy for surface modification is doping and codoping of g-C3N4 with metal/nonmetal ions. Codoping serves to be beneficial for the generation of catalytically active reaction sites, enhancement of visible light absorption, and efficient charge carrier separation. A theoretical study on boron and oxygen codoped with g-C3N4 layer shows that the band gap narrowing is much pronounced when boron and oxygen are located on two different s-heptazine units in g-C3N4 (Feng et al. 2018). Codoping in the same s-heptazine unit has a negligible influence on the band gap modification. Furthermore, there is an effective charge transfer from boron containing s-heptazine to oxygen containing ring. In this way the electron-hole separation is maintained in the system with superior H2 production in the oxygen-doped unit and O2 production in the boron-doped unit (Feng et al. 2018). Iodide ions such as I3 and I5 modified g-C3N4 nanosheets are reported to produce 104.3 μmol H2 under visible light exposure (Li et al. 2017b). This remarkable result is due to the participation of polyiodides p-orbitals which act as a bridge for electron tunnelling and hopping between far-placed carbon atoms in g-C3N4 network. In this way, the lifetime of charge carriers is prolonged because of faster charge transfer performance. Synergetic effect of potassium and iodine doping on the improved photocatalytic H2 production was studied (Guo et al. 2016). While iodine doping improved visible

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Fig. 6.10 Intercalation of metal ions between interlayers of g-C3N4 (Reprinted with permission from Xiong et al. 2016. Copyright©2016 American Chemical Society)

light absorption, potassium doping facilitates charge transportation in the g-C3N4 network. Intercalation of potassium in the interlayer spacing develops a bridge between stacked layers and thereby extends the π-conjugation. In this way a better electron delocalization and charge transportation can be achieved in the system (Fig. 6.10). While potassium doping has positive impact on the photocatalytic performance of g-C3N4, sodium doping is shown to have a negative impact on the overall photocatalytic activity (Xiong et al. 2016). Sodium doping increases the in-planar electron density and reduces the effective band gap. The drawback is that the doping accelerates carrier recombination. Incorporation of cationic potassium ions and anionic nitrate ions into g-C3N4 introduces a bioriented channel between interlayers (Cui et al. 2017). This process is expected to reduce the energy barrier for charge transportation. The bioriented channel provides easy charge flow in opposite direction between interlayers, thus favouring charge separation and transportation. By this process effective oxidation of NO has been achieved. Phosphorous and sulphur codoped with g-C3N4 could be synthesized by the thermal oxidation of a mixture of thiourea with different percentage of hexachlorotriphosphazene (HCCP) gas (Jiang et al. 2017). P and S codoping could accelerate the degradation of tetracycline and methyl orange at a higher rate than that of bulk g-C3N4. This is achieved due to the increased surface area by codoping induced crystal growth inhibition, charge carrier separation, and visible light absorption. Mesoporous g-C3N4 with surface modification by sulphur and oxygen codoping could be synthesized by the polymerization of melamine and trithiocyanuric acid (You et al. 2017). An improved sixfold enhancement in rhodamine B degradation is encountered on codoped g-C3N4 over undoped g-C3N4. Sulphur-modified g-C3N4 nanosheets display a specific surface area of 60 m2g1, whereas the bulk g-C3N4 possesses a surface area of only 10 m2/g. The system shows O2 evolution rate of 20.1 μmolh1, which is four times higher than that of bulk g-C3N4 (Zhang et al. 2011). Interestingly, an addition of a small amount of

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Co3O4 (3 wt %) to the sulphur-modified g-C3N4 could lead to an enhancement in the O2 evolution rate to 25.1 μmolh1. The enhancement is due to the better charge separation whereby the photogenerated holes are transferred to Co3O4. DFT study considers individual and codoping of boron/fluorine into g-C3N4 (Ding et al. 2016). By adopting codoping strategy, the water oxidation-reduction and the band positions of codoped system are quite matched for constructing photoelectrochemical cell. The position and location of doping alter the electronic properties of g-C3N4 and influence the photoelectrochemical properties of the system. When g-C3N4 is doped with sulphur and phosphorus, sulphur prefers a substitutional site, and phosphorous prefers the interstitial site. The interstitial phosphorous creates a new channel for carrier migration leading to a higher carrier mobility with a larger carrier separation (Ma et al. 2012). In the case of metal-doped g-C3N4 system, the sp2 nitrogen atom of heptazine provides metal coordination site. Metal stabilizes the hybridization between conduction band and valence band but does not create any electronic states in the band gap. The photocatalytic activity is enhanced since dopant acts as trap centres and separates carrier recombination. For iron-doped g-C3N4 system, strong hybridization is established between dopant orbital and delocalized planar electrons of g-C3N4. The system demonstrates H2 generation rate of 16 mmolg1h1 (Gao et al. 2016). Ruthenium-C3N4 complex is a good example of single atom catalyst with g-C3N4 base structure for H2 evolution reaction (Peng et al. 2017). Ruthenium ions show strong affinity towards pyridinic nitrogen of heptazine ring forming strong ruthenium-C3N4 complex. Charge transfer occurs from C3N4 to ruthenium centres making the metal centres as the sites for hydrogen adsorption.

6.2.4

Carbonaceous Materials-C3N4 Nanocomposite

g-C3N4 forms heterostructures with similar type of carbon nitrides with different band structures and with other carbonaceous materials, e.g. fullerenes, carbon nanotube, mesoporous carbon spheres, graphene, carbon-based aromatic molecules, and so on (Ong et al. 2016; Zhao et al. 2015b). The idea is to form a metal-free carbon-based nanocomposites for designing a superior photocatalytic system. Triazine-/heptazinebased hybrid heterostructure of g-C3N4 can be synthesized via one-pot microwaveassisted molten-salt (KCl/LiCl) process with single melamine precursors at 550
C (Liu et al. 2017). The work describes different polymerization methods adopted to synthesize g-C3N4, such as electric-resistance heating (er), microwave heating (mw), and electric-resistance molten-salt (er-ms) method. As compared to thermally prepared g-C3N4, the heterojunction display remarkably enhanced photocatalytic activity. The hydrogen evolution rate of microwave-molten-salt heterostructure g-C3N4 is about 1480 μmolg1h1 and 300 μmolg1h1 for electric-resistance molten-salt composite of g-C3N4. However, individual g-C3N4 systems fabricated from electric-resistance heating and microwave heating show H2 production rate of 95 μmolg1h1 and 63 μmolg1h1, respectively. The apparent quantum yield for microwave-moltensalt composite of -g-C3N4 is 10.7% at 420 nm incident light. The superior

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photocatalytic activity is attributed to the combination of triazine/heptazine ring that provides photogenerated electron-hole separation and charge migration through the interface. Hydrothermal treatment of a mixture of protonated g-C3N4 derived from thiourea and urea precursors could result in the formation of isotype heterostructures (Meng et al. 2017). The hydrogen evolution rate as shown by the composite derived from thiourea and urea is 96.8 μmolh1. The individual system of g-C3N4 obtained from urea and thiourea shows H2 evolution rate of 42.4 μmolh1 and 50.8 μmolh1, respectively. Protonation-assisted enhancement of surface area and interface-mediated charge separation promote the superior photocatalytic activity in the sample. Isotype heterostructure derived from the calcination of a mixture of thiourea and melamine exhibits sulphur-doping that induced narrowing of the band gap and effective separation of electrons and holes (Shi et al. 2015). The heterostructure displays rhodamine B degradation of 90% after 1 h irradiation, which is higher than that of bulk g-C3N4. An ultrasonic dispersion-assisted electrostatic self-assembly between acidified g-C3N4 nanosheets (ACNS) with bulk g-C3N4 could lead to the development of an isotype heterojunction displaying remarkable activity in methyl orange removal (Yang et al. 2016). With 30% acidified g-C3N4 in ACNS/g-C3N4, 96.2% degradation of methyl orange is achieved. ACNS and g-C3N4 show only 53.7% and 48.9% methyl orange degradation under visible light. Heterojunction prepared from melamine with urea as modifier shows rhodamine B and methyl orange degradation, which is 7.2 and 3.7 times faster than that of bulk g-C3N4 (Wang et al. 2016). The high rate of H2 evolution 498.9 μmolh1 is another fascinating photocatalytic attribute of the heterostructure. An increased surface area, narrow band gap, and photogenerated electron-hole separation through the interface promote the enhancement in the photocatalytic activity. A metal-free heterojunction of g-C3N4 having type I and type II structure was constructed from different starting precursors (Dong et al. 2015) (Fig. 6.11). The heterostructure derived from dicyandiamide and urea (type II) displays electron transfer from the conduction band of dicyandiamide to urea. In type I derived from melamine and urea, the electron transfer occurs from urea to melamine. Type I structures show 41.3% NO removal activity, whereas type II shows 39.3% removal NO under the visible light.

Fig. 6.11 Illustration of (a) type II and (b) type I g-C3N4/g-C3N4 heterostructures working under visible light irradiation. (Reproduced with permission from Dong et al. 2015. Copyright 2015 Royal Society of Chemistry)

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Addition of a small carbon ring to g-C3N4 could modify the electronic structure of the composite system and thus influences the photocatalytic performance. Incorporation of C4N2 from 2,4,6-triaminopyrimidine into the ring network of g-C3N4 can induce the rolling up of the planar g-C3N4 into tubular g-C3N4 (Ho et al. 2015). The composite has better π-electron conjugation and narrow band gap and improved photogenerated electron-hole separation. The total concentration of NO was decreased from 605 to 365 ppb in the presence of a pure g-C3N4 under the illumination of visible LED light. NO concentration was further decreased up to 245 ppb in the presence of the conjugate system. Cring-C3N4-based heterostructure with strong π-π interaction can be synthesized by the thermal conjugation of a glucose molecule and melem (Che et al. 2017). There is strong in-plane sp2 hybridization between the Cring and C-N bonds of melem. Photocarrier diffusion length is significantly elongated across the interface favouring photocarrier separation and transfer across the interface. The Cring-C3N4 shows H2 production rate of 371 μmolh1g1 with an average quantum efficiency of 5% at 430 nm. Fullerene is a unique carbon allotrope containing 60 π electrons and is an excellent electron acceptor. g-C3N4 could be a good host material for the decoration of C60. A study has shown that addition of 1 wt% C60 to C3N4 could accelerate rhodamine B degradation from 54% for bulk to 97%. The high photocatalytic activity is ascribed to the facile electron-hole separation across the interface (Chai et al. 2014). It was further established that the van der Waals heterostructure of C3N4 and C60 could be a promising catalyst for the H2 production under visible light (Li et al. 2016). The hybrid has valence band maximum constituted of unsaturated nitrogen atoms of C3N4 and conduction band minimum of C60. The unsaturated nitrogen coordinated to two carbon atoms in the ring promotes H2 production under visible light. The bridging and the three coordinated nitrogen lying at the interface weaken the interaction between C3N4 and C60. Carbon@C3N4 core-shell structure with g-C3N4 layer covering up the carbon spheres is a promising catalyst for H2 evolution (Ma et al. 2017). The core-shell structure is constructed by the columbic and π-π interaction between negatively charged carbon spheres and protonated g-C3N4 (positively charged). The core-shell structure has a surface area of 126.2 cm2g1, whereas pure g-C3N4 has a surface area of only 12.8 cm2g1. Carbon spheres act as electron sinker and promote carrier separation across the interface. The core-shell structure and the bulk g-C3N4 display H2 production rate of 129 μmolh1 and 16 μmolh1, respectively. Carbonconjugated C3N4 in the presence of cobalt as catalyst promoter display remarkable electron mobility with electrocatalytic oxygen evolution and oxygen reduction reaction (Niu et al. 2018). The system has a high electron mobility, and the nitrogen-coordinated cobalt sites are responsible for efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Integration of g-C3N4 nanosheets with carbon quantum dots shows absorption extending from visible to the near-infrared region (Xia et al. 2015). On near-infrared excitation at 808 nm, the composite shows H2 production rate of 6.76 μmoh1g1 (Fig. 6.12a). The rate increases to 50.5 μmolh1g1 and 219.5 μmoh1g1 under the UV-visible and visible light, respectively (Fig. 6.12c, d).

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Fig. 6.12 Photocatalytic H2 evolution rates under (a) 808 nm laser. (c) UV-Vis and (d) visible light irradiation. (b) Photocurrent density of g-C3N4 nanosheets (black curve) and the composite of g-C3N4 nanosheets with 10 wt % carbon quantum dots (red curve) under 808 nm laser biased at 0.4 V. (Reproduced with permission from Xia et al. 2015. Copyright 2015 Royal Society of Chemistry)

The composite of ordered mesoporous carbon with g-C3N4 exhibits superior photocatalytic activity in rhodamine B and 2,4-dichlorophenol degradation (Shi et al. 2014). There are two benefits of adding ordered mesoporous carbon to g-C3N4. The mesoporous carbon is an excellent electron acceptor thus favouring efficient electron-hole separation. The nanocomposite of ordered mesoporous carbon-g-C3N4 has a high surface area and thus promotes better rhodamine B adsorption. Nanocomposite of carbon nanotube-g-C3N4 containing 0.2% carbon nanotube displays nearly twofold enhancement in water splitting over pure g-C3N4 (Chen et al. 2014). The nanocomposites form strong p-π conjugation with carbon nanotube acting as electron sinker. Furthermore, the composite shows a broad absorption band extending up to 800 nm beneficial for the enhancement of photocatalysis. Multiwall carbon nanotube-g-C3N4 composite displays monotonously increasing broad absorption band extended towards higher wavelength (Ge and Han 2012). While pure g-C3N4 shows H2 evolution rate of 2.03 μmolh1, the rate increases to 7.58 μmolh1 after an addition of 2% multiwall carbon nanotube. This carbon

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nanotube has high electron storage capacity and acts as an effective electron transfer channel stimulating electron transfer from g-C3N4 to the nanotube. Graphene is a zero band gap and highly electron conducting semimetal with high surface area. Graphene-based materials which are used in the composites with g-C3N4 are graphene oxide, reduced graphene oxide, and graphene quantum dots (Han et al. 2017; Rajender et al. 2017). 2D-2D composite between graphene and g-C3N4 can develop a composite with better interfacial contact and facile charge injection from one material to another based on their band positions. Graphene/gC3N4 composite with 5% graphene oxide displays nearly 100% methylene blue degradation under 410 nm LED light irradiation (Dai et al. 2014). Nearly half of methylene blue decomposition is recorded in the presence of pure g-C3N4. Interfacemediated charge carrier separation and transfer are much facilitated with graphene oxide acting as electron transporter, thus favouring the superior photocatalytic activity. A composite of g-C3N4/reduced graphene oxide shows superior photocatalytic performance in rhodamine B degradation under the visible light. With 1% reduced graphene oxide content in g-C3N4, the rate of dye degradation increases four times than that of pure g-C3N4. The charge separation mediated by electron transfer from g-C3N4 to reduced graphene oxide could be responsible for the higher photocatalytic activity (Zhao et al. 2016). Three-dimensional porous g-C3N4 with graphene oxide aerogel is an efficient visible light active photocatalyst. The 3D porous network provides pathway for rapid mass transport, strong adsorption sites, and better light-harvesting ability due to multiple reflections of incident light. The interface provides carrier separation. The composite shows methyl orange removal of 92%. The bulk g-C3N4 and 2D-2D hybrid show only 12% and 30% methyl orange degradation (Tong et al. 2015). The H2 production rate is tremendously enhanced on adding graphene to g-C3N4. Without graphene g-C3N4 shows H2 production of 147 μmolh1g1, whereas with 1 wt % graphene, the H2 production rate increases to 4451 μmolh1g1. Further increase in graphene content increases opacity and light scattering leading to a decrease in H2 evolution rate. Graphene acts as electron sinker, and the photoexcited electrons transferred from g-C3N4 to graphene are accumulated over Pt nanoparticles and participate in H2 production (Xiang et al. 2011). Another theoretical work reports that the oxygen atom can significantly change the band gap of reduced graphene oxide/g-C3N4 composite. The oxygen atom in the reduced graphene oxide is an electron-rich site, and g-C3N4 has higher hole content. The negatively charged oxygen atoms are the driving sites for the efficient production of H2 under visible light (Xu et al. 2015). The hybrid of mesoporous g-C3N4/graphene and mesoporous g-C3N4/graphene oxide shows remarkable visible light photocatalytic activity in NO removal (Li et al. 2014). The composites are packed by the H-bonding and via π-π interaction. The composite has three times higher surface area and pore volume over bulk g-C3N4. NO removal shown by bulk g-C3N4 is 16.8%, whereas mesoporous composite shows 50.3% efficiency. The high photocatalytic activity can be ascribed to the large visible light absorption, high surface area, and pore volume in the composite providing catalytically active sites for the separation of photogenerated electrons and

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Fig. 6.13 (a) Three-dimensional structure and (b) electronic band structure of g-C3N4/51% pyromellitic diimide unit; (c) mechanism for photocatalytic H2O2 evolution. (Reprinted with permission from Kofuji et al. 2016. Copyright 2016 American Chemical Society)

holes. A hybrid photocatalyst of carbon nitride-aromatic diimide-graphene nanohybrid displays solar to H2O2 energy conversion with 0.2% efficiency (Fig. 6.13a–c). The solar to H2O2 production follows the steps: H2 O þ 2hþ ! 1=2O2 þ 2Hþ ð1:23 V vs:NHEÞ O2 þ 2Hþ þ 2e ! H2 O2 ð0:68 V vs:NHEÞ
H2 O þ 1=2 O2  H2 O2 ΔG ¼ 117 Kj mol1 The photogenerated electron transfers from C3N4/diimide to the reduced graphene oxide. The free electrons are involved in the overall O2 reduction, whereas holes on C3N4/diimide oxidize water (Kofuji et al. 2016). The 2D-2D hybrid nanostructure of protonated g-C3N4/reduced graphene oxide displays remarkable enhancement in CO2 to CH4 conversion under daylight lamp (Ong et al. 2015). With an optimum loading of 15 wt% rGO in g-C3N4, the CH4 evolution is 13.93 mmolg1 with a photocurrent quantum yield of 0.56%. Face-to-face contact area is enhanced in the 2D-2D stacking providing facile charge carrier separation transfer and recombination. Reduced graphene oxide provides better pi-pi interaction with CO2 and active CO•- radicals production (Ong et al. 2015): rGO=pCN ! rGO ðe Þ þ pCN ðhþ VBÞ pCN ð2hþ VBÞ þ H2 O ! pCN þ 2hþ þ 1=2O2 rGO ð8e Þ þ CO2 þ 8Hþ ! rGO þ CH4 þ 2H2 O Graphene quantum dots-g-C3N4 is another composite which is gaining enormous attention for water splitting. A theoretical study has shown that a change in the size of graphene quantum dots can tailor the effective band gap of graphene quantum dots/graphitic carbon nitride to maximize the visible light absorption. A molecular model of C24H12/C3N4 can obtain a theoretical band gap of 1.92 eV, suitable for

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solar-to-chemical energy conversion. The composite forms type II heterostructure with facile interfacial electron transfer leading to an electron-rich g-C3N4 and holerich graphene quantum dots. Water splitting is much favoured with g-C3N4 providing active sites for H+ adsorption and conversion to H2 (Ma et al. 2016).

6.2.5

Metal Oxide-g-C3N4 Nanocomposites

From its inception in 1972, TiO2 has been considered the most widely explored photocatalytic material. However, its wide band gap (3–3.2 eV) limits its photocatalytic activity under UV light. Many recent studies have shown that TiO2 in conjunction with g-C3N4 could be an active visible light photocatalyst. A DFT+U calculations has reported that C3N4/TiO2 is a van der Waals heterostructure with a type II band alignment (Lin et al. 2017). The heterostructure shows good response to visible light absorption as the layer numbers increase in g-C3N4 and the separation distance decreases between TiO2 and g-C3N4. The decrease in the separation distance suppresses the carrier recombination because of the faster photogenerated electron transfer from g-C3N4 to TiO2 in close contact. The TiO2 nanoparticles trapped in the 3D meso-/macroporous g-C3N4 have an enhanced visible light absorption at the pore walls, followed by exciton dissociation and carrier separation (Wu et al. 2018). Simulation study shows that the light propagation and scattering occur inside the mesoporous channels and the trapped TiO2 concentrate the light intensity at the edge of the pore walls. This heterostructure is quite efficient in removing rhodamine B from water. g-C3N4 grown over TiO2 mesospheres with partially exposed TiO2 surface is quite efficient as a photocatalyst (Chen et al. 2016). The composite has an extended visible light absorption spectrum due to possible nitrogen doping from g-C3N4 precursors during calcination. The as-grown heterostructure shows high efficiency in the degradation of rhodamine B and phenol over an irradiation time of 140 min under visible light. Mesoporous TiO2 macrospheres/mesoporous gC3N4 heterostructure is constructed by infiltrating cyanamide solution inside the mesoporous channel of anatase TiO2 microspheres (Wei et al. 2017) (Fig. 6.14a). Calcination at 550
C crystallizes g-C3N4 within the porous structure. The interface of the heterostructure is stabilized through Ti-N bond resulting from the N doping on oxygen site of TiO2 during calcination. The heterostructure is highly efficient in removing phenol within 60 min of irradiation time with apparent rate constant eight times higher than that of bulk g-C3N4 (Fig. 6.14b). Recycling test confirms high stability of the hybrid structure (Fig. 6.14c). g-C3N4 nanosheets decorated over TiO2 nanotube arrays display several strong absorption peaks in the visible region with intensity of absorption increasing towards near-infrared region (Sun et al. 2016). The composite is a stable photoelectrode under UV and visible light and shows an enhanced photocurrent density and photoelectrochemical performance. The photoanode constructed of this heterostructure shows H2 production rate of 19.1 μmolh1 under visible light irradiation (Yang et al. 2015). Decoration of Co-Pi particles

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Fig. 6.14 (a) Schematic of the nanocoating strategy for the preparation of TiO2/g-C3N4 microspheres (TOCN-x) with x denoting different volumes of cyanamide. (b) Photocatalytic decomposition of phenol over TiO2 microspheres, TOCN-0.1, TOCN-0.5, TOCN-1, and TOCN-2, and g-C3N4 (CN) under visible light irradiation. (c) Recycling test of the TOCN-1 photocatalyst. The system was kept in the dark prior to testing. (Reprinted with permission from Wei et al. 2017. Copyright 2017 American Chemical Society)

over the g-C3N4-TiO2 nanotube arrays induces better charge carrier separation and reduces the activation energy barrier for water oxidation. The photocatalyst is quite effective for seawater splitting (Li et al. 2015a). TiO2 with exposed (001) and (101) facets are photocatalytically reactive sites for hybridization with C3N4 (Wang et al. 2017a). A core-shell structure of (001) TiO2@C3N4 has better contact interface, containing more reactive sites, with abundant surface adsorbed –OH groups. This core-shell structure is quite effective in removing 2 mg tetracycline antibiotic within 9 min of light exposure. TiO2 hollow nano-box has reactive (001) and (101) facets. These facets form a heterojunction inside TiO2 with photogenerated electrons accumulated over (101) facet and holes over (001) (Huang et al. 2015) (Fig. 6.15). g-C3N4, however, can form a stable heterostructure with (101) facet of TiO2. TiO2(101)/C3N4 can efficiently degrade X3B dye. Facets present on different polymorphs of TiO2 show different reactivity in its pristine as well as in heterostructure forms. Photoactivity of anatase TiO2 nanofibres is considered to be higher than TiO2 (B) nanofibre. However, using near coincidence site lattice theory, it is established that the HS of g-C3N4/TiO2 (B) with exposed (001) facets is more reactive than C3N4/TiO2 (A) with (100) facet (Zhang et al. 2014). g-C3N4 decorated over {001} facet of anatase TiO2 is highly effective in the degradation of rhodamine B, methyl orange, and methylene blue under UV and visible light. Rhodamine B shows the highest photocatalytic activity followed by methylene blue and methyl orange (Liu et al. 2016a). Nitrogenated oxide removal with 10 wt % C3N4 over {001} TiO2 is 59.4%, whereas it is only 23.9% and 15.4% for {001} TiO2 and C3N4 (Song et al. 2016). High photocatalytic efficiency is recorded with an optimum level of g-C3N4 over TiO2. At higher loading, g-C3N4 shields the visible light to reach on the TiO2 surface and reduces photocatalytic efficiency of the system. In the heterostructure of C3N4/ TiO2, g-C3N4 absorbs visible light and transfers electrons to TiO2, thus promoting carrier separation. TiO2 itself is inefficient to utilize visible light in photocatalysis. Introduction of intermediate defect states can favour visible light absorption in TiO2. Self-doping of TiO2 with Ti3+ is one such favourable approach to improve the

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Fig. 6.15 Direct Z-scheme g-C3N4-/TiO2-hybridized photocatalyst shows superior photocatalytic activity by contacting g-C3N4 with exposed (1 0 1) facets due to efficient removal of the photogenerated electrons accumulated on (1 0 1) facets of high-energy TiO2. (Reproduced with permission from Huang et al. 2015. Copyright 2015 Elsevier)

photocatalytic efficiency. Ti3+-self-doped TiO2 decorated over mesoporous g-C3N4 nanosheets (Ti3+-TiO2/C3N4) show a narrow band gap of 2.21 eV (Tan et al. 2018). This heterostructure shows H2 evolution rate of 290.2 μmolh1g1, which is 4 and 14 times higher than that of pristine g-C3N4 and TiO2. Similarly, this self-doped heterostructure removes methylene blue with an apparent rate constant of 0.038 min1, which is 26.7 and 7.6 times higher than pure TiO2 and g-C3N4 over an irradiation time of 100 min (Li et al. 2015b). The interface of Ti3+-TiO2/C3N4 can be stabilized by oxygen doping in g-C3N4 (Li et al. 2017c). At the interface of the hybridized structure (Ti3+-TiO2/oxygenated-C3N4), the interaction occurs between Ti3+ and –OH groups on oxygenated-C3N4. The electron transfer rate increases from C3N4 to TiO2 by this hybridization process. The composite with 1:2 mass ratio of Ti3 + -TiO2 to oxygenated C3N4 shows three times higher rhodamine B degradation efficiency than that of pristine Ti3+-TiO2 or g-C3N4 (Hu et al. 2017). Because of the mesoporous structure, the composite shows strong visible light-harvesting ability and better mass transfer; the high surface area favours enhanced pollutant adsorption. Ti3+ self-doping reduces the effective band gap, and the heterojunction effectively suppresses carrier recombination. The composite shows 98.5% phenol degradation, reduces Cr3+ (97%), and displays 572.6 μmolh1g1 H2 evolution reaction rate. Similar to TiO2, ZnO is also a wide band gap semiconductor with a band gap of 3.0 eV. ZnO is inefficient under visible light, and under UV light the catalyst suffers from severe photocorrosion if irradiated for sufficiently longer period (Wang et al. 2011). Persistent photocatalytic reaction of ZnO for 96 h for methylene blue

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degradation shows that the rate constant of the reaction decreases from 0.0107 min1 within 2 h to 0.0015 min1 after 96 h with hardly any methylene blue degradation after the long exposure time. In g-C3N4/ZnO composite, the initial rate of methylene blue degradation is 0.0305 min1, which becomes 0.0277 min1 after 96 h of irradiation. Therefore, photocorrosion of ZnO is substantially suppressed on adding g-C3N4 to the system. ZnO is unaffected to visible light irradiation, but C3N4 absorbs visible light. Since g-C3N4 absorbs visible light, the electron and hole separation is achieved by favouring electron transfer from g-C3N4 to ZnO followed by radical formation for taking part in photocatalysis. The visible light photocatalytic activity of g-C3N4/ZnO is tremendously enhanced if g-C3N4 is decorated over oxygen-deficient ZnO (Wang et al. 2017b). ZnO with less defect content has a reduced visible light absorption efficiency. Thus, the visible light photocatalytic activity is attributed to the photosensitising effect of g-C3N4, since it absorbs in the visible region. The situation changes if ZnO contains sufficient oxygen vacancies. The presence of the intermediate defect states makes ZnO an efficient absorber of visible light. Both zero-dimensional ZnO (0D-ZnO) and g-C3N4 absorb visible light in the composite. A Z-scheme photocatalytic mechanism could be applicable for the enhanced photocatalytic activity. The electrons trapped at the defect levels recombine with the holes in the valence band of g-C3N4 at the interface. The recombination leaves electrons in the conduction band of C3N4 and holes in the valence band in ZnO. Thus, the recombination probability is largely suppressed in the heterostructure. The electrons and holes localized in the two semiconductors participate in photocatalysis. A recorded 95% 4-chlorophenol degradation is achieved within 60 min of irradiation. The C3N4/0D-ZnO is also efficient in the H2 evolution reaction with a reaction rate of 322 μmolh1g1. One of the main advantages of heterostructure is the substantial suppression of carrier recombination by the interfacial charge transfer. It is reported that the presence of silicate linkage at the interface between ZnO and g-C3N4 is favourable to increase the charge transfer efficiency (Liu et al. 2015). Silicate acts as a bridge for the easy transportation of charge carriers from g-C3N4 to ZnO. Thus, the composite is quite efficient for the degradation of phenol and increases the production rate of H2. g-C3N4-coated ZnO hollow tubes are synthesized via a chemical deposition method (Lv et al. 2015). g-C3N4 to ZnO weight ratio has significant influence on their visible light photocatalytic activity. An optimum 10 wt % of g-C3N4 over ZnO display enhanced photocatalytic activity with 94% rhodamine B degradation within 2 h irradiation under visible light. Increasing g-C3N4 content could reduce the charge transfer from g-C3N4 to ZnO with a decrease in the photocatalytic activity. Shapetailored dumbbell and cone-type ZnO nanostructures are synthesized over g-C3N4 nanosheets in an in situ growth process (Fageria et al. 2015). The composite with dumbbell-shaped nanostructures displays higher rate of methylene blue and phenol degradation over cone structures; pure shows the least activity. The composite with dumbbell-shaped ZnO has a surface area of 45.3 m2/g, and the cone-shaped ZnO-gC3N4 displays a surface area of 23.6 m2/g. The large surface area in the dumbbell ZnO nanocomposites provides higher adsorption sites for dye molecule. Furthermore, the enhanced interfacial charge separation provides benefit for the superior

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Fig. 6.16 (a) Energy band alignments of isolated WO3 and g-C3N4, (b) energy band bending after forming g-C3N4/WO3heterostructures, (c) the resulting interfacial charge transfer behaviours, and (d) schematic illustration of the Z-scheme-dominated photocatalytic mechanism over g-C3N4/WO3 heterostructures. (Reproduced with permission from Liang et al. 2017. Copyright 2017 Royal Society of Chemistry)

photocatalytic activity in the dumbbell-shaped ZnO. A core-shell structure of ZnO nano-triangles with C3N4 nanofoil is a quite efficient photocatalyst and degrades 100% rhodamine B within 60 min of visible light irradiation (Vignesh et al. 2015). Unlike TiO2 and ZnO which are wide band gap semiconductors, WO3 has a band gap absorption (2.4–2.8 eV) in the visible region (Dong et al. 2017a, b). WO3 with different shapes of nanowires, nanorods, and nanoflowers has been synthesized via the hydrothermal method in different acidic conditions. g-C3N4 wrapped around these shape-tailored WO3 show remarkable visible light activity in the degradation of rhodamine B phenol, salicylic acid, and photoreduction of Cr (VI) (Liang et al. 2017) (Fig. 6.16). The evaluation of the band structure of the composite clearly reveals that the conduction band of WO3 is more positive than the standard redox potential of O2/ ∙O2 (0.28 eV vs. NHE) (Zhu et al. 2017a). Therefore, the electrons in the conduction band of WO3 cannot reduce O2 to O2•-. Similarly, the valence band of g-C3N4 is less positive than the standard reduction potential ∙OH/OH (+2.4 eV vs. NHE). Thus, a Z-scheme photocatalytic mechanism is proposed for the system in which the conduction band electrons in WO3 recombine with holes in

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the valence band of g-C3N4 (Fig. 6.16d). This process leaves g-C3N4 with electron accumulation sites and WO3 with hole-residing sites. These electrons and holes in the two semiconductors perform the photocatalytic reaction. WO3-g-C3N4 is equally effective in the photodegradation of pharmaceuticals pollutant in water, e.g. sulfamethoxazole (SMX) antibiotic (Zhu et al. 2017a). A 1D/2D heterostructure of WO3-x/g-C3N4 is constructed by a solvothermal method. Electron transfer occurs from g-C3N4 to WO3-x (Lou and Xue 2016). Oxygen-deficient WO3 is a stronger absorber of entire visible light and acts as electron donor site for participation in superoxide radical formation. Electrons are transferred from g-C3N4 to WO3 and then transported along nanowire direction to suppress carrier recombination. More than 90% methyl orange degradation has been achieved within 35 min of visible light irradiation. WO3/g-C3N4 nanocomposite displays 15 times higher H2 evolution rate (400 μ mol h1g cat1) than that of pure g-C3N4 under visible light exposure (Cheng et al. 2017). A hollow microsphere of WO3-g-C3N4 is constructed with welloptimized ratio of the precursors for WO3 and g-C3N4 (Xiao et al. 2018). The shell of the microspheres contains well-distributed WO3 and g-C3N4 nanoparticles. Multiple interface structure is expected to be formed between WO3 and g-C3N4 with improved separation of photogenerated electrons and holes. The composite is quite effective in the photodegradation of antibiotics such as ceftiofur and sodium tetracycline hydrochloride under visible light. One of the studies has shown that the hostguest architecture can profoundly influence the photocatalytic activity of WO3/C3N4 composite. Atomic-scale imaging has revealed that the interface is built by the vertical standing of C3N4 over the flat facets of WO3 nanocuboids (Yu et al. 2017). The interface is stabilized by well-defined W-O-N-(C)2 covalent bonds. The composite structure shows H2 production rate of 3.12 mmolh1g1 which is seven times higher than that of pure g-C3N4. There is a direct electron transfer from WO3 to 2D nanosheets of g-C3N4. The electrons reaching the nanosheets can easily transport along the in-plane direction and reduce the carrier recombination possibility.

6.2.6

Metal Sulphides-C3N4 Nanocomposites

Metal sulphides are also considered as strong candidate for photocatalytic dye degradation, water splitting, etc. (Zhang and Guo 2013). However, the corrosive nature of the semiconductor, because of the presence of sulphur, strongly reduces its stability for the aforementioned applications (Weide et al. 2016). g-C3N4 has a wellmatched band structure with the metal sulphides. Therefore, in the heterostructures of g-C3N4-metal sulphides, the photogenerated holes from the valence band of CdS can readily transfer to the g-C3N4 and suppress the carrier recombination and photocorrosion (Xu et al. 2017). Complete covering up of CdS nanowires with the g-C3N4 nanosheet could reduce the photocorrosion of CdS, fascilating hole migration from CdS to g-C3N4 (Zhang et al. 2013). In this way, the photostability of CdS is enhanced, and the heterostructure shows H2 production rate of 4152 μmolh1g1 at

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an optimum g-C3N4 amount of 2 wt %. The photocatalytic activity of CdS/g-C3N4 is studied by the monitoring of the oxidation of p-methoxyphenyl sulphide (Xu et al. 2017). Catalytic activity shows the best results when the mass ratio of g-C3N4/CdS is 0.3 and the preparation temperature is 300
C. A higher mass ratio and preparation temperature can reduce the light penetration depth at the interface and lead to the partial decomposition of the CdS. The presence of methoxy groups at ortho, para, and meta positions can significantly influence the photooxidation of the sulphide compounds. The highest photocatalytic activity is recorded when the methoxy group is at the para position. CdS quantum dots decorated C3N4 nanosheets exhibit excellent photocatalytic activity in the H2 production and dye degradation (Fan et al. 2016). Because of the small size of the quantum dots, they can be very easily occupied on the rolled and curled edges of the nanosheets and exhibit better interfacial contact. Rhodamine B degradation is 88.2% which is two times higher than the recorded results in their pristine forms. Similarly, the H2 production rate with the composite is 4.967 mmolh1g1. This is nearly 50 times and 60 times higher than that of CdS showing 0.997 mmolh1g1 and g-C3N4 displaying 0.083 mmolh1g1 H2 production rate (Zhang et al. 2018). Unlike CdS, ZnS is a wide band gap semiconductor with a band gap of 3.91 eV (Torabi and Staroverov 2015). g-C3N4-ZnS forms a staggered type II heterostructure with uninterrupted photogenerated electron and holes separation and transfer across the interface. More than 90% nitrophenol reduction and 4 mmolg1h1 H2 production are achieved by the heterostructure within a reaction time of 240 min (Suyana et al. 2016). A rod-like composite has been constructed of C3N4-ZnS via a hydrothermal condition (Wang et al. 2015). With 10 wt % C3N4/ZnS, the heterostructure shows 93% decomposition of methyl orange within 100 min of irradiation. ZnS quantum dots are decorated over g-C3N4 nanosheets by atomic layer deposition (Kim et al. 2017). The numbers of cycles during atomic layer deposition strongly influence the photocatalytic activity of the heterostructure with more than 90% methylene blue degradation with five numbers of atomic layer deposition cycles. ZnO nanospheres/g-C3N4 nanosheet with large dye adsorption and light-harvesting capacity shows high efficiency in the degradation of methyl orange and tetracycline antibiotic under the visible light (Yan et al. 2017). With a 50% C3N4/ZnS, the methyl orange degradation increases by more than 3 times and 12 times than that of pure ZnS and g-C3N4, respectively. Zero-dimensional/two-dimensional van der Waals heterostructures of g-C3N4SnS2 nanosheets can be prepared via an ion-exchange process with SnCl2 and Na2S as the precursors for SnS2 (Liu et al. 2016b). SnS2 nanoparticles of 5–10 nm sizes are well dispersed over g-C3N4 nanosheets of 2.8 nm thickness. This composite can effectively degrade 95.3% methyl orange dye over an irradiation time of 25 min. The mechanism for the photocatalysis is proposed to be Z-scheme in which the conduction band electrons of SnS2 directly recombine with the valence-band holes leaving free electrons and holes left behind in the conduction band of g-C3N4 and valence band of SnS2, respectively. Altogether a different mechanism is proposed for the photocatalytic enhancement of a SnS2/C3N4 heterojunction synthesized by solvothermal method (Zhu et al. 2017b) (Fig. 6.17).

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Fig. 6.17 The change in absorption of rhodamine B in the presence of (a) pure g-C3N4 and (b) SnS2/g-C3N4 heterojunction. (c) The photocatalytic activities of different catalysts and (d) rate kinetic determination from the kinetic plots of various catalysts in rhodamine B solution. (Reproduced from Zhu et al. 2017. Copyright 2017 Royal Society of Chemistry)

The interface of SnS2 and g-C3N4 is stabilized through C-S bonding, and this bond acts as a channel for charge transport from g-C3N4 to SnS2. The electrons are accumulated on the conduction band of SnS2, and the holes are left behind in g-C3N4. The photogenerated electrons and holes recombination are thus prohibited. The heterostructure shows rhodamine B degradation of 96.8% within 105 min of irradiation. This heterostructure is also effective in the reduction of CO2 to hydrocarbons. A hydrothermal synthesis procedure was adopted with Sn4+ precursors and L-cysteine as sulphur precursors (Di et al. 2017). L-cysteine provides sulphur for Sn4 + and anchors amine groups over the g-C3N4 surface. These amine groups enhance the adsorption of CO2 over the g-C3N4 surface. CO2 to CH3OH conversion gives a yield of 1.2 μmolg1 and 0.8 μmolg1. On the other hand, CH3OH increases to 2.3 μmolg1 when the heterostructure is the catalyst. In recent years, MoS2 has been evolved as an emerging material for use in electronics, catalysis, energy-related fields, etc. (Wang and Mi 2017). MoS2 is

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2D-layered materials with interlayer spacing of 0.62 nm. Bulk MoS2 is an indirect band gap semiconductor (Eg~1.29 eV). Indirect to direct band gap transition occurs with reducing layer numbers achieving a band gap of 1.9 eV in case of monolayer (Splendiani et al. 2010). The heterostructures of MoS2 with g-C3N4 are found to have emerging applications in photocatalysis. MoS2 are decorated over g-C3N4 nanosheets in the form of nanodots, two-dimensional layers. An understanding of the charge carrier dynamics in the heterostructures can help in the understanding of the efficiency of the photocatalyst. Using time-resolved transient absorption spectroscopy, the electron injection rate and charge transfer efficiency in g-C3N4/MoS2 nanodot are found to be 5.96  109 s1 and 73.3%, respectively, which is 4.8 times faster and 2 times higher than that of the corresponding values in g-C3N4/MoS2 monolayer hybrid (Shi et al. 2018b). The presence of unsaturated terminal sulphur atom and strong junction contact of MoS2 dot with g-C3N4 make g-C3N4/MoS2 nanodot a better photocatalyst than g-C3N4/MoS2 monolayer. The H2 evolution rate in the nanodot hybrid is 660 μmolg1h1, whereas the monolayer hybrid shows H2 production rate of 83.8 μmolg1h1. The percentage of MoS2 over g-C3N4 nanosheets can significantly influence the photocatalytic performance of the hybrid. In one of the studies, the percentage of MoS2 was varied from 0.1% to 2% (Yan et al. 2016). The highest photodegradation of methyl orange is achieved with 0.1 wt% MoS2/g-C3N4. With the increase in the percentage of MoS2 over g-C3N4 nanosheets, there is a consecutive decrease in the visible light utilization efficiency of g-C3N4. Moreover, more numbers of active surface sites of g-C3N4 are covered up by MoS2, thus restricting the facile electron transfer from g-C3N4 to MoS2. Similarly, optimum 1 wt % MoS2 quantum dots/C3N4 nanosheet shows H2 production of 19.66 μmolg1h1 (Jin et al. 2016). In order to improve the photocatalytic performance of the MoS2/C3N4 heterostructure, different synthesis methods are adopted to decorate layered MoS2, quantum dots, and 3D porous MoS2 over g-C3N4 nanosheets. Few such synthesis methods involve an in situ light-induced deposition method (Zhao et al. 2015c), freeze-drying approach (Cao et al. 2017a), ion-exchange hydrothermal method (Wang et al. 2018), cross layering (Cao et al. 2017b), solid-state mixing (Bian et al. 2018), etc. In in situ visible light-induced method, [MoS4]2 ions are well dispersed over g-C3N4 nanosheets, and the system is irradiated with visible light. Photogenerated electrons in g-C3N4 reduce [MoS4]2 ions to MoS2, forming g-C3N4/MoS2 hybrid. The composite with 2.89 wt % MoS2/C3N4 shows H2 production rate of 252 μmolg1h1 over 6 h irradiation. The unsaturated sulphur atom on the exposed edges strongly binds H+ in solution. After light exposure, the electron transferred from g-C3N4 to MoS2 reduces H+ to H2. A three-dimensional porous MoS2/g-C3N4 is synthesized by freeze-drying of MoS2 and g-C3N4 precursors, followed by calcination at high temperature. Because of the strong ability of the three-dimensional heterostructure to maximize visible light utilization efficiency, the H2 production rate of 1640 μmolg1h1 can be achieved. Ion-exchange hydrothermal method is adopted to deposit MoS2 quantum dots over protonated and unprotonated g-C3N4 nanosheets. The H2 production rate over protonated heterostructure is 1.42 mmolh1g1. Unprotonated heterostructure shows only

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0.45 mmolh1g1 H2 production. A cross layered heterostructure of MoS2/C3N4 is fabricated by adding hydrothermally synthesized MoS2 suspension in aqueous dicyandiamide solution followed by drying and calcination at 550
C under Argon flow. The photocatalytic attribute of the heterostructure is due to the fast electron transfer from g-C3N4 to MoS2 due to cross layering, which favourably shortens the electron transport distance. Vertically aligned MoS2 over g-C3N4 was synthesized by the solid-state mixing of MoO3 and g-C3N4 followed by calcination under N2 flow. The heterostructure is stabilized by interfacial N-Mo-S bond. This interfacial region is an active site for electron accumulation and hydrogen generation.

6.3

Summary

In this chapter it is discussed that g-C3N4 can be exploited as a wonder 2D photocatalyst by adopting different strategies for the modification of the structural and optical properties. These include thermal or chemical exfoliation, defect engineering, doping, shape tailoring, and composite with different semiconducting materials. Layer thinning by exfoliation can enhance the surface area and accelerates carrier separation by easy bulk to surface migration. Defect engineering can enable g-C3N4 with numerous photocatalytic active sites, enhance visible light absorption, and suppress carrier recombination. Doping with metal and nonmetal ions can provide active interlayer channel for charge separation and migration, band gap modification, and suppression of carrier recombination. Shape tailoring in the form of hollow spheres, tubes, rods, and strings can induce the system with better visible light absorption, surface area enhancement, carrier separation, etc. The composite with semiconductor materials can help build up interfacial contact for facile charge separation. The ultimate aim of all the different types of modification on g-C3N4 is to make them an excellent photocatalyst for effective H2 generation, pollutant degradation, NO removal, CO2 reduction, etc.

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Zhao H, Dong Y, Jiang P, Miao H, Wang G, Zhang J (2015c) In situ light-assisted preparation of MoS2 on graphitic C3N4 nanosheets for enhanced photocatalytic H2 production from water. J Mater Chem A 3:7375–7381. https://doi.org/10.1039/C5TA00402K Zhao H, Chen S, Quan X, Yu H, Zhao H (2016) Integration of microfiltration and visible light driven photocatalysis on g-C3N4 nanosheet/reduced graphene oxide membrane water treatment. Appl Catal B 194:134–140. https://doi.org/10.1016/j.apcatb.2016.04.042 Zhao S, Zhang Y, Zhou Y, Wang Y, Qiu K, Zhang C, Fang J, Sheng X (2018) Facile one step synthesis of hollow mesopores g-C3N4 spheres with ultrathin nanosheets for photoredox water splitting. Carbon 126:247–256. https://doi.org/10.1016/j.carbon.2017.10.033 Zheng Y, Liu J, Liang J, Jaroniec M, Qiao SZ (2012) Graphitic carbon nitride materials: controllable synthesis and applications in fuel cells and photocatalysis. Energy Environ Sci 5:6717–6731. https://doi.org/10.1039/C2EE03479D Zheng D, Pang C, Liu Y, Wang X (2015) Shell engineering of hollows C3N4 nanospheres via copolymerisation for photocatalytic H2 evolution. Chem Commun 51:9706–9709. https://doi. org/10.1039/C5CC03143E Zhou M, Hou Z, Chen X (2018) The effects of hydrogenation on graphitic C3N4 nanosheets for enhanced photocatalytic activity. Part Part Syst Charact 35:1700038. https://doi.org/10.1002/ ppsc.201700038 Zhu W, Sun F, Goei R, Zhou Y (2017a) Construction of WO3-g-C3N4 composites as efficient photocatalysts for pharmaceutical degradation under visible light. Cat Sci Technol 7:2591–2600. https://doi.org/10.1039/C7CY00529F Zhu A, Qiao L, Jia Z, Tan P, Liu Y, Ma Y, Pan J (2017b) C – S bond induced ultrafine SnS2 dot/porous g-C3N4 sheet 0D/2D heterojunction: synthesis and photocatalytic mechanism investigation. Dalton Trans 46:17032–17040. https://doi.org/10.1039/C7DT03894A

Chapter 7

Graphene and Allies as a Part of Metallic Photocatalysts Annelise Kopp Alves

Contents 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Synthesis of Graphene, Graphene Oxide and Reduced Graphene Oxide . . . . . . . . . . . . . . . . 7.3 Application of Graphene and Derivatives as Metallic Photocatalysts . . . . . . . . . . . . . . . . . . . 7.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract Metallic photocatalysts are materials that present a metallic behaviour relating to the mobility of the electron in their energy band. Conduction and valence bands in metallic conductors are very close or overlap; thus these materials have a small or absent energy gap separating the occupied and empty energy levels. Graphene, for its turn, is an atom-thick sheet of sp2-hybridized carbons that is considered a zero bandgap semimetal material. It has an electrical band structure that permits a very rapid conduction, i.e. electrons have high mobility with little scattering, thus acting like an electron pool, promoting charge separation and rapid transfer in photocatalytic applications. In this chapter a brief review of the main methods to obtain graphene and derivative graphene oxide and reduced graphene oxide is presented, alongside with examples of their use in photocatalysis. Keywords Graphene · Reduced graphene oxide · Photocatalysis · Metallic photocatalysts

A. K. Alves (*) Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil e-mail: [email protected] © Springer Nature Switzerland AG 2019 Inamuddin et al. (eds.), Nanophotocatalysis and Environmental Applications, Environmental Chemistry for a Sustainable World 29, https://doi.org/10.1007/978-3-030-10609-6_7

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Introduction

Metallic photocatalysts are materials that present a metallic behaviour relating to the mobility of the electron in their energy band structures. Considering a traditional semiconductor photocatalysis process, it is well understood that there is an electron transition, when the electron is properly excited, from an occupied valence band to an empty conduction band. In the case of a material that is a metallic conductor, where these bands are very close or superposed, the clear concept of valence and the conductive band no longer exists. Thus, metallic conductors have a small or absence of energy gap separating the occupied and empty energy levels. In this case, it is possible to define the electron transition to a partially occupied band (conduction band) from the highest fully occupied energy band (valence band) (Xu et al. 2012). When this electron transfer occurs, either in semiconductor or in metallic photocatalysts, an electron/hole pair is generated. The recombination of these charges is very probable unless there is a separation mechanism such as an applied potential, or, as in the case of the metallic photocatalyst, its intrinsic characteristic of high conductivity implies a rapid charge transfer (Xu et al. 2012). In the case of these materials, electrons can be excited using the visible light spectrum, since metals can absorb visible wavelengths, which is one of the most appealing characteristics of this type of photocatalysts. Since photocatalysis is considered a phenomenon that depends on the formation of an electron/hole pair, different strategies have been done to improve the performance of this reaction, which includes the design of a specific structure and the doping of semiconductors with metals or noble metals. More recently, another alternative has been receiving attention: the use of carbonaceous materials such as carbon nanotubes, fullerenes, graphene and its derivatives: graphene oxide and reduced graphene oxide. These materials can act as photocatalysts due to the fact that these structures are delocalized conjugated systems that can act as an electron pool promoting charge separation and a rapid transfer (Putri et al. 2015). The ideal graphene sheet has an electrical band structure that permits a very rapid conduction, i.e. electrons have high mobility (15,000 cm2/Vs) with little scattering (Han et al. 2016). Graphene is an atom-thick sheet of sp2-hybridized carbons that is considered a zero bandgap semimetal material (Zhang et al. 2015) where its antibonding orbitals (acting as conduction band) and bonding orbitals (acting as valence band) degenerate and touch at Brillouin zone corners. This zero bandgap is attributed to the identical environment of the two sublattices of carbon atoms in the graphene unit cell (Han et al. 2016). There are basically four typical structures of graphene materials used in photocatalysis (Han et al. 2016): nanoscaled zero-dimensional graphene quantum dots, one-dimensional graphene nanoribbons, two-dimensional graphene nanosheets and three-dimensional graphene frameworks (Fig. 7.1). Each structure has a specific function and property that affect the photoreaction.

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Fig. 7.1 Structures and properties of different graphene materials and their multifunctionality in heterogeneous photocatalysis. (Reprinted with permission of Han et al. 2016)

To tailor the electrical properties of graphene photocatalysts, researches have been made to achieve an ideal doping structure. There are two possibilities (Han et al. 2016): (a) Surface transfer doping: the doping group added into the graphene sheet can donate (type n conductor) or remove electrons (type p conductors) from graphene. (b) Substitutional doping: when carbon atoms of the graphene lattice are substituted by atoms with a different number of valence electrons. The formation of n-type conductors is achieved when the substituting atom has more valence electrons, p-type conductors are formed when the foreign atom has fewer valence electrons, and also, additional states in the graphene density of states are formed due to the additional free charge carriers introduced by the dopant. Both alternatives also open up the bandgap of graphene, turning it into a semiconductor. The doped heteroatom also promotes the formation of active sites for catalysis, breaking the electrical neutrality of graphene (Han et al. 2016).

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Synthesis of Graphene, Graphene Oxide and Reduced Graphene Oxide

The first report of the isolation of a graphene single layer was made by the Nobel Prize winners Andre Geim and Konstantin Novoselov (Novoselov et al. 2004). They have obtained graphene films using the micromechanical exfoliation method, i.e. using an adhesive tape to repeatedly peel off small layers of graphene from highly oriented pyrolytic graphite. Their method produced sheets up to 10 μm in size and thickness as small as 3 nm. Nowadays, this method has been improved and is capable to produce high-quality layers of sizes limited by the single crystal grains of the original graphite (Bonaccorso et al. 2012). In general, for any method of obtention, the number of layers of graphene is determined by light scattering; Raman spectroscopy, which also detects disorders, defects and doping; transmission electron microscopy at the edges of the layers; and atomic force microscopy. Since that first discovery, different methods have been developed to produce high-quality graphene layers (Fig. 7.2).

Fig. 7.2 Schematic illustration of the main graphene production techniques. (a) Micromechanical cleavage (mechanical exfoliation). (b) Anodic bonding. (c) Photoexfoliation. (d) Liquid-phase exfoliation. (e) Growth on SiC. (f) Segregation/precipitation from the carbon-containing metal substrate. (g) Chemical vapour deposition. (h) Molecular beam epitaxy. (i) Chemical synthesis using benzene as a building block. (Reprinted with permission of Bonaccorso et al. 2012)

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In the anodic bonding technique, graphite is pressed against a glass sheet, and a high voltage (0.5–2 kV) is applied, and the glass is heated at around 200  C. The applied voltage and the temperature are parameters used to control the number of layers of the obtained graphene over the glass substrate (Moldt et al. 2011). For graphene layers to be obtained using the photoexfoliation method, the technique of laser ablation is used. Laser pulses of a specific energy density, related to the number of layer one likes to exfoliate, are bombarded over graphite flakes under vacuum (Dhar et al. 2011). Liquid exfoliation of graphite flakes is a method that requires that graphite is firstly dispersed in a specific liquid (water, alcohols, N-methylpyrrolidone, among others) in the presence of a surfactant (e.g. sodium dodecylbenzene sulfonate), following an ultra-sonification process to exfoliate the original material (Hadi et al. 2018). After exfoliation, a suspension is formed, and usually, ultracentrifugation is used to separate the different layers’ thickness, i.e. few layers are located at the supernatant, and multilayers and non-exfoliated material are in the bottom of the centrifuge tube. After this process, it is necessary to separate the graphene layers from the solvent and surfactant. This is done by washing the obtained layers with a solvent that removes the surfactant and applying heat to remove the solvents. The liquid exfoliation is cheap and easily scalable and is an ideal method to produce inks, thin films and composites (Bonaccorso et al. 2012). It is also possible to obtain graphene layers using a technique that involves the evaporation of silicon atoms from SiC. This method uses temperatures higher than 1000  C and vacuum or low-pressure argon chambers (Emtsev et al. 2009). Basically, Si- or C-oriented surfaces of SiC wafers are heated until the silicon evaporates, leaving behind carbon atoms that start to organize themselves in the honeycomb layers of graphene. When carbon is deposited over a metal such as Cu, Ni, Au, Pt and Ir that are non-forming carbides metals, a preferential growth of graphene/graphite layer is possible over these metal surfaces. In this process, a source of carbon (pure carbon, hydrocarbons, polymers, etc.) deposited atoms over the surface of the substrate of interest, at temperatures high enough to promote the precipitation of graphene under cooling (Li et al. 2009). The diffusion of carbon into the inner layers of the metals should be avoided, so the operational conditions, time and temperature, should be carefully observed. This process of precipitation and growth can also be achieved using the chemical vapour deposition method. In fact, the first uniform, large-area graphene layer grown in copper foil that has a low-carbon solubility used the chemical vapour deposition technique and methane as the carbon precursor (Li et al. 2009). An affordable way to have graphene-based single layers in reasonable quantities is the chemical conversion of graphite to graphene oxide. Usually graphene oxide is produced using Hummers’ method, which is based on the oxidation of graphite using strong acids and permanganate (Hummers and Offeman 1958). Graphene oxide has on its basal plane oxygenated bearing hydroxyl and epoxy groups, in addition to carbonyl and carboxyl groups located at the sheet edges (Fig. 7.3). This configuration gives graphene oxide hydrophilicity and rapid water exfoliation, yielding dispersions of single-layered sheets (Singh et al. 2011).

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Fig. 7.3 Oxidation of graphite to graphene oxide and reduction to reduced graphene oxide. (Reprinted with permission of Singh et al. 2011)

After exfoliation of graphene oxide and obtention of single layers, it is possible to try to reduce them to graphene, but the majority of reported methods yield the formation of a structure called reduced graphene oxide (Fig. 7.3). There are many ways that this reduction can be achieved, usually based on chemical, thermal or electrochemical methods (Pei and Cheng 2012). The challenge is to control the reaction conditions to produce high-quality reduced graphene oxide, similar to pristine graphene. Defects and residual functional groups, however, alter the structure of the carbon plane, and then it is expected that the reduced form has different properties as graphene (Pei and Cheng 2012).

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Application of Graphene and Derivatives as Metallic Photocatalysts

It is well known that the key parameters for a good photocatalyst are its ability to separate and transfer the photo-generated electron/hole pairs, as well as as active in the visible light as possible. With this in mind, researches have been exploring the synergy between photocatalysts based on metal particles (that absorb the visible spectrum) and graphene and derivatives (that have the excellent mobility of charge carriers and optical transmittance, practically transparent to visible light). In the work of Oliva et al. (2018), flexible graphene – Al2O3: Eu3+ and SrAl2O3: 3+ Bi composites were obtained using a modified casting method. A flexible sheet of the composite, with dimensions of 3.5  3.5 cm  1 mm, was used to evaluate the photocatalysis of methylene blue solution. It was observed that the composites absorb light through all the ultraviolet, visible and near-infrared ranges (250–1000 nm). The synthesized materials, besides promoting photocatalysis, also physically absorb the dye. The composite with Sr removed 44% of the dye by physical adsorption and 66% by photocatalytic degradation, while the Al composite removed 21% and 79% of the dye by adsorption and photocatalysis, respectively. Besides been an excellent electron acceptor and transporter, graphene oxide has in its structure oxygen-containing functional groups that promote its hydrophilicity and facilitate the anchoring of semiconductors, metallic nanoparticles and organic molecules (e.g. dyes) (Wang et al. 2017; Wang and Astruc 2018). There are different approaches to produce metal nanoparticle/graphene photocatalysts. The most significant are in situ methods (chemical reduction, hydrothermal and electrochemical techniques) and ex situ methods (physical mixing of nanoparticles with graphene and derivatives) (Wang and Astruc 2018). The simultaneous reduction of a metal precursor and graphene oxide is the most frequently used method to produce metallic/graphene and derivative composites for photocatalysis. Usually, a chloride metal precursor is mixed with graphene oxide dispersed in water, and a reductant, such as NaBH4, is added (Wang and Astruc 2018). In the work of Wang et al. (2017), bismuth/graphene oxide composites were obtained using a solvothermal technique, in which Bi(NO3)3 was dissolved in HNO3 solution, in the presence of ethylene glycol containing dispersed graphene oxide and polyvinylpyrrolidone. This mixture was treated in an autoclave at 160  C for 12 h. The obtained material, composed of nanospheres of bismuth with around 200 nm and graphene oxide layers, was filtrated, rinsed and dried (Fig. 7.4). The Bi/graphene oxide composite was tested for the photocatalytic removal of 660 ppb of NO underflow into a water dispersion of the photocatalyst, with a radiation of 280 nm. The authors observe a reduction of about 50% of NO after approximately 20 min of irradiation in comparison to just 35% for the same radiation period of Bi nanometric spheres.

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Fig. 7.4 Bi/graphene oxide composite particles: (a, b) are scanning electron microscopy images; (c) is a transmission electron microscopy image; and (d) a high-resolution transmission electron microscopy image. (Reprinted with permission of Wang et al. 2017)

Nanoparticles of gold-doped reduced graphene oxide are reported by Kumar et al. (2016). The electrostatic interactions between positively charged Au nanoparticles and negatively charged graphene oxide were used to produce graphene oxide-coated Au nanoparticles. A solution of the metal nanoparticles was dropped into graphene oxide solution followed by stirring and centrifugation. To produce reduced graphene oxide, the precipitate was dispersed in distilled water in the presence of hydrazine at 95–100  C and then washed and centrifuged. The colour change of the solution from pale yellow-brown to dark brown indicates the conversion of graphene oxide into reduced graphene oxide. An excellent visible light-responsive photocatalyst for the photoconversion of CO2 into formic acid under visible light irradiation was observed. The results are attributed to the high selectivity (>90%) and higher conversion efficiency of the catalyst and the presence and reduced state of graphene acting as an electron acceptor and transporter.

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Conclusion

In summary, metallic photocatalysts present a metallic behaviour relating to the mobility of the electron in their energy band. In the case of graphene and its derivatives, electrons have high mobility with little scattering; this fact in terms of photocatalysis promotes the fundamental charge separation and rapid transfer that can promote high photocatalytic efficiency. In general, graphene and allies doped with metallic particles appear to be promising photocatalysts. The metallic particle composition, size, morphology and defects present on the graphene and derivatives are key parameters for the determination and enhancement of their functionality and potential applications in photocatalysts.

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Chapter 8

Silver-Based Photocatalysts: A Special Class Vicente Rodríguez-González and Agileo Hernández-Gordillo

Contents 8.1 Silver and Its Enhanced Properties as Photoactive Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . 8.2 Photocatalytic Applications of Silver-Based Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Photocatalytic Applications of Silver-Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1 Oxidation Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.2 Reduction Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.3 New Renewable Energy Source Reactions for CO2 Reduction . . . . . . . . . . . . . . . . . . 8.3.4 Antibacterial Properties Enhanced with Photoactive Silver-Nanoparticles . . . . . . 8.4 Practical Applications of Silver-Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5 Conclusion Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract Silver-nanoparticles are more and more used not only in traditional antimicrobial applications but also in other interesting nanotechnology areas such as innovative nanostructures for green chemistry, disinfection of pathogenic microorganisms in agriculture fields and hospitals, nanomedicine and renewable clean energy processes like hydrogen production. The low-cost and practical synthesis methods of silver-nanoparticles, which are in contrast with those using noble and transition metals, are indeed a tangible alternative. These silver-nanoparticles cocatalysts are easy to prepare and inexpensive, and with mild condition methods, zero dimension (0D) to third dimension (3D) nanostructured particles can be obtained, whose size and shape can be modulated according to specific applications. In this chapter, a practical review about the use of silver-nanoparticles in mesoporous and graphene-based materials and in coatings intended for photocatalytic applications is reported. Activity insights into the plasmonic effect, electron-hole mediator and V. Rodríguez-González (*) División de Materiales Avanzados, IPICYT, Instituto Potosino de Investigación Científica y Tecnológica, San Luis Potosí, México e-mail: [email protected] A. Hernández-Gordillo Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Coyoacán, México © Springer Nature Switzerland AG 2019 Inamuddin et al. (eds.), Nanophotocatalysis and Environmental Applications, Environmental Chemistry for a Sustainable World 29, https://doi.org/10.1007/978-3-030-10609-6_8

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bacteriostatic inhibition of pathogenic colonies at room temperature are provided. The oxidation state is also a significant factor for understanding the role played by silver-nanoparticles in the enhancement of nanotechnology and environmental photocatalytic applications. The discussion featured in this chapter is based on suitable examples from the literature concerning photoactive silver-nanoparticles functionalized on nanostructured materials. Keywords Silver-nanoparticles materials · Nanocatalysts · Antibacterial photocatalysis · Plasmon resonance · Photoreduction

8.1

Silver and Its Enhanced Properties as Photoactive Nanoparticles

Silver is considered as a noble and transition metal located in the 5th period and group 11 of the periodic table, being 47 its atomic number. Silver is close to metals such as Rh, Ru, Pt, Pd and Au, which are excellent cocatalysts for environmental and heterogonous catalysis applications. Since the coordination chemistry of silver is low, it provides stability in aqueous media with its colloidal size; in addition, as silver oxidation numbers are + or 2+, it can work with d10 or d9 orbitals (Xiu et al. 2011). Surface reactions can be driven by silver-nanoparticles due to their lower AgO degree. Silver as nanoparticles is known to be a powerful bacteriostatic metal. The reduction of silver ions requires neither high thermal energy nor strong irradiation or reductive gas environments (Xiu et al. 2011, 2012; Rodriguez-Gonzalez et al. 2016). Its nonbonded atomic and covalent radii of 2.11 and 1.36 Å, respectively, limit the possible substitutional doping into the framework of metallic oxide materials. A strong interaction with photons from UV, visible and natural solar light provokes excitation known as surface plasmon resonance on silver-nanoparticles, which depends on the shape and size of the nanoparticles. This resonance lightsilver-nanoparticles interaction results in strong electric fields on the surface of the nanoparticles, which can induce photochemical transformations either through localized heating of the nanostructure as plasmonic functions or by dosing the energetic charge carriers to the reactants adsorbed on the surface of silver-based photocatalysts, working as Schottky barrier agents (Xiu et al. 2011, 2012; Rodriguez-Gonzalez et al. 2016).

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Photocatalytic Applications of Silver-Based Photocatalysts

Chemical properties have to be considered in order to understand the enhanced photocatalytic activity, where the nanometric size of silver-nanoparticles is a key feature for d-block chemistry. Firstly, the size plays an important role in controlling electronic transfers. One of the backward reactions in the photocatalytic aqueous degradation of contaminants is the recombination of photoexcited electrons. Normally, with the suitable light radiation UV-vis, in semiconductors like TiO2, ZnO, CeO2, etc., an electron from the valence band can be promoted to the conduction band, leaving an electron, deficiency known as hole in the valence band, and causing an excess of negative charge in the conduction band, which are oxidizing and reducing species, respectively. These generated species can participate in surface redox reactions and generate secondary reactive oxygen species. However, the excited electron reactive is unstable, and they can go back to the valence band to be stable again, provoking a recombination that is a backward reaction in Fig. 8.1 (Obregón et al. 2016). It is here where the size of the silver-nanoparticles plays a crucial role, attracting excited electron species to a semiconductor surface. If the surface of the silver-nanoparticles is below 5 nm, the trapped electron of the silvernanoparticles can be easily transferred to an adsorbed contaminant molecule and enhance the electronic transfer so that the oxidation or reduction reactions may also generate secondary species such as hydroxyl radicals and superoxide oxygen. If the particles are above 5 nm, other phenomena can be favoured due to the accumulation of excited electron which produces the surface plasmon resonance effect; in this case, the bigger the silver-nanoparticles, the more intense and complex the resonance effects, which are due to dipole to multipole plasmon resonances. In addition, recombination can be favoured if the electron transfer rate is closer to the recombination rate. With nanoparticles bigger than 20 nm, the possibility of core or surface silver oxide species (AgO, Ag2O) is favoured, where also the shape has to be considered. If the silver-nanoparticles are smaller, e.g. less than 2 nm, the possibility

Fig. 8.1 Photoactive mediator as the main factor enhancing reactivity. Excited electron (ecb
*) and hole (hvb+) on silver-nanoparticles, AgNPs

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TiO2 3,2 eV Ag°NPs/TiO2 < 3,2 eV

Fig. 8.2 Schematic representation of the photocatalytic process on a boosted silver-based photocatalyst. Silver-nanoparticles, AgNPS, excited hole-electron (hvb+- ecb
*)

of presenting positive and negative partial prompt charges has to be considered, Agnδ+ or Agnδ
. Figure 8.1 represents the possible roles played by silvernanoparticles in the enhancement of the photoactivity of a photocatalyst (Linic et al. 2015). Silver-nanoparticles have to be highly dispersed on a semiconductor surface to boost its photoactive properties. Normally, the plasmon resonance of silvernanoparticles supported on a metal oxide is observed in the visible region at around 400 nm as a broad absorption band. The band gap energy should be controlled to absorb sunlight in the visible range to be more efficient and also satisfy the minimum energy requirement for the hole-electron excitation. In Fig. 8.2, concise representative band theory conditions are outlined in order to understand the way silvernanoparticles can boost and/or mediate the semiconductor photoactivity according to the energy required to lead the reaction to photooxidation, photoreduction, etc. The virtual scheme shows the potential energy surface required for different reactions: H+/H2 reduction, O2/H2O oxidation, CO2 reduction, etc. Likewise, the external stimulus (light energy) that has to be used to excite the hole-electron infers the prevailing reaction according to the band position and characteristics of the silver-nanoparticles (size and shape) (Xiu et al. 2012; Obregón et al. 2016). Practical photocatalytic aqueous reactions may oxidize the silver-nanoparticles, so their incorporation method has to guarantee a negligible lixiviation in the aqueous milieu and the reduction/stabilization of the silver-nanoparticles after use. The excitation can be produced according to the radiation wavelength from the UV to the visible range using artificial powerful UV light or xenon lamps for visible radiation, solar simulators, light-emitting diode (LED) devices (blue, red, green and UV) and natural solar light. These effects can produce different reactions that control the enhanced silver photoactive process; so, the following applications will illustrate the silver-based photocatalytic function (Fig. 8.3).

8 Silver-Based Photocatalysts: A Special Class

225

Fig. 8.3 Photocatalytic applications of supported silver-nanoparticles

8.3

Photocatalytic Applications of Silver-Nanocomposites

Water is the matrix used to study the photocatalytic properties of silvernanocomposites that photoinduced reactions like oxidation which allows the discolouration of contaminant organic dyes in aqueous medium or degradation of recalcitrant organic pollutants. The reduction reaction is also important for organic photochemical reactions, CO2 conversion and hydrogen production, which is known as water splitting. On the other hand, day by day, another important application is the photocatalytic disinfection of pathogenic microorganisms, which has become very important.

8.3.1

Oxidation Reactions

Normally, the oxidation of organic contaminants can be possible by using strong chemical oxidants, via biological ways, using thermal process and just silvernanoparticles (Bogireddy et al. 2016); however, this chapter has been focused on heterogeneous photocatalytic processes involving oxidation by the generation of reactive oxygen species like the soft reactions that take place in wastewater remediation, which also provide a good part of mineralization. In this sense, dyes, pesticides and solvents have been the most studied contaminant models using functionalized silver in different semiconductor materials. Recently, some drugs, additives, pharmaceuticals, personal care products and endocrine-disrupting compounds have also been widely studied, and most of them are examples of emerging contaminants. Metal oxide semiconductors have been typically used for the functionalization of silver-nanoparticles, where TiO2 has been one of the most common semiconductors; however, this nanocomposite type is activated under UV light (Rostami-Vartooni et al. 2016), but it can also work in visible light (Rostami-

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Oxidation Potential (ENH)

Ag-Nanoparticles EBC – -1 0

O2 e-

e-

e-

-•

H+

→ •OH

e-

O2

SPR 3.2 eV 2-3 eV

1

OHh+

2 3 + EBV

ZnO

h+

•OH

+ Dye = CO2 + H2O

Semiconductors - Ag2SO4, - Ag2WO4 - Cu2O, - Carbon-Fibers

Fig. 8.4 Construction of ternary Ag-ZnO composites with effective charge separation for the degradation of contaminant dyes. (Adapted from Cao et al. 2015)

Vartooni et al. 2016; Yu et al. 2017; Wang et al. 2017a), even in natural sunlight (Geetha et al. 2015; Gong et al. 2012). In this case, silver-nanoparticles deposited or impregnated on TiO2 can act as electron traps, avoiding the recombination of electron-hole pairs and improving the charge transfer process. The photocatalysis efficiency of silver-nanoparticles functionalized on TiO2 has been used in the discolouration of dyes, mainly methylene blue or other azo dyes in aqueous solution. By using SnO, Cu2O, BiOCOOH and ZnO, the photocatalytic activity of noble metal-semiconductor heterostructures is enhanced, which is attributed to the fact that silver-nanoparticles strongly excite the surrounding semiconductor, producing a high amount of photogenerated charge carriers (Lei et al. 2015, 2017; Tao et al. 2017; Sharma et al. 2018), although Santos Patil attributes the improved charge carrier separation to their hierarchical nanostructure forming a closely packed interpenetrating network (Patil et al. 2016; Rafaie et al. 2017). Even by the construction of ternary Ag-ZnO composites with carbon fibres (Pant et al. 2016), Cu2O (Chiang and Lin 2015), Ag2SO4 (Cao et al. 2015) or Ag2WO4 (Pirhashemi and Habibi-Yangjeh 2017), the photocatalytic activity is improved by an effective charge separation (Fig. 8.4). Other semiconductor materials have been used in the functionalization with silvernanoparticles including reduced GO (Borthakur et al. 2017; Jeyapragasam 2016), g-C3N4 (Nagajyothi et al. 2017; Ye et al. 2018; Faisal et al. 2016), CdO (Saravanakumar et al. 2018), Bi2WO6 (Wu et al. 2015) and NiAl-LDH (Tonda and Jo 2017). Most of the silvernanoparticles nanocomposites mentioned above have been successfully effective in the discolouration-degradation of contaminant dyes, mainly reporting on methylene blue, methyl orange, rhodamine B and indigo carmine dye (Scheme 8.1), where the main reactive oxygen species are the superoxide and hydroxyl radicals. In the example, better

8 Silver-Based Photocatalysts: A Special Class

227

Scheme 8.1 Schematic representation of the possible photooxidation way of rhodamine B and indigo carmine dyes over Ag/rGO nanocomposites. (Adapted with permission from MartinezOrozco et al. 2013)

photoactivity of silver-nanoparticles/graphene nanocomposites due to a fast photogenerated charge separation caused by a superficial Schottky junction on Ag/rGO and high adsorption of aromatic dye molecules on graphene is illustrated (Wen et al. 2011; Martinez-Orozco et al. 2013). A variety of silver-nanoparticles binary and ternary nanocomposites exhibiting high photocatalytic properties using mainly visible light has been extensively reported. X. Xiao (Xiao et al. 2015) reported the preparation of silver-nanoparticles by the photoreduction process, depositing on an AgBr cubic cage or hollow porous tetrahedron. The photoactivity of the stable nanocomposite was attributed to the sensitivity of AgBr to visible light and the silver plasma resonance effect of silver-nanoparticles that generates the superoxide radical and Br0 species, which are the reactive species responsible for the degradation of contaminant dyes. Even Y. Pang reported that the Ag-AgBr nanocomposite is capable of generating the powerful hydroxyl radical (OH) necessary for the contaminant degradation (Pang et al. 2017). By constructing an Ag/AgCl heterojunction on titanium phosphate nanoplates or CdWO4, the photoactivity is tremendously enhanced. In this case, the silver plasmon-induced holes lead to the formation of Cl0 species, which together with the superoxide radical possess excellent oxidation capability to oxidize the organic contaminant dye (Ao et al. 2017); even the composite can degrade phenolic compounds due to high energy electrons through the silver plasma resonance effect (Wen et al. 2017). Similar strategies have been used to construct heterojunctions based on Ag-AgX (X¼Br, Cl) by different synthesis methods (Cai et al. 2016) or functionalized on different supports like PVA porous spheres (Chen et al. 2016), on metal-organic frameworks like MIL-101 (Gao et al. 2016), on reduced GO (Yang et al. 2018) and on N/rGO (Wang et al. 2017b); all these compounds are being used in the degradation of contaminant dyes (Fig. 8.5).

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V. Rodríguez-González and A. Hernández-Gordillo Pollutants

Ag-Nanoparcles e-

Semiconductor - rGO - N-rGO - MIL-101 - PVA

e-

SPR

e-

O2- •

Products

O2 2-3 eV

OH- + Cl- or Br-

h+

h+

AgX (X=Br, Cl)

•OH

+ Cl0 or Br0

Pollutants

Products

Fig. 8.5 Silver-nanoparticles nanocomposites based on AgX (X¼Br
, Cl
) for the degradation of contaminant dyes. (Adapted from Wang et al. 2017b)

Among the silver-nanoparticles nanocomposites mentioned above, few of them are used in the degradation of recalcitrant compounds under visible light due to the fact that powerful oxidizing reactive oxygen species are required. For example, the bifunctional Ag-AgBr/MCM41 was used in the degradation of aniline (Guan et al. 2017), while Ag-AgBr combined with a solid solution of BiOBr and BHO (BiO (OH)0.06Br0.94) was used in the degradation of chlorophenol compounds (Ji et al. 2016a). An interesting material can also be found for the degradation of chlorophenol, dichlorophenol and trichlorophenol, which are silver-nanoparticles deposited on Ag2O that lead to the desired mineralization (Hu et al. 2015). In all the composites, the formation of the superoxide and hydroxyl radicals was demonstrated by electron paramagnetic resonance (EPR) techniques. On the other hand, silver-nanoparticles nanocomposites have been tested for the degradation of pharmaceutical compounds, for example, silver-nanoparticles on WO3 were prepared for the degradation of sulfanilamide (Zhu et al. 2016), while silver-nanoparticles on ZnO were used in the degradation of the ofloxacin drug (Kaur et al. 2017; Zhao et al. 2017) by means of solar natural light. Notwithstanding, total mineralization is difficult to be achieved due to the formation of stable secondary products. By constructing a ternary heterostructure of Ag-ZnO with encapsulated carbon spheres, the degradation of metronidazole can also be carried out under sunlight. In addition, heterojunctions based on Ag-AgX (X¼Br, Cl) were also tested for the degradation of acetaminophen or tetracycline (Ma et al. 2017; Shi et al. 2017); however, the role played by halogen (X0) species was not involved in the degradation mechanism, because the superoxide and hydroxyl radicals were the main powerful oxidizing reagents.

8 Silver-Based Photocatalysts: A Special Class

229

Table 8.1 Summary of silver-nanoparticles nanocomposites with semiconductors for the degradation of diverse contaminants under visible light or sunlight Sample Ag/MO (M¼ Ti, W, Bi, Cd, Zn, W) Ag-AgX (X¼Cl, Br) Ag-AgX/MO (X¼Cl, Br)

Contaminant Dyes (RhB, IC, MO, MB) Dyes, phenol, drugs Dyes, phenol, drugs

Reactive oxygen species O22
, OH O22
, X0, OH O22
, X0, OH

According to the above-reviewed literature, photooxidation reactions with pharmaceutical compounds, several intermediary molecules or by-products are produced, where some are toxic and restrict the mineralization. Mainly for oxidation reactions, induced activation by UV or visible radiation allows the silvernanoparticles to take the function of the Schottky junction in the Ag/semiconductor interface, controlling the charge separation and inhibiting the recombination of the hole-electron pair, where the oxidation process promotes the generation of many by-products that affect the mineralization of the contaminants (Table 8.1).

8.3.2

Reduction Reactions

Normally, the reduction reaction is carried out for the chemical transformation of organic compounds, mainly from nitro-compounds to amino compounds by the hydrogenation/electron transfer process. This reduction reaction can be possible by using strong chemical reducing agents by a thermal or photocatalytic process, which is dealt with in this chapter. S. Giri reported the preparation of silver-nanoparticles deposited on the PPy-MAA support, which is effective for the chemical transformation of diverse nitroaromatic compounds, including a variety of substituents on the aryl ring (-CO2Me, -CO2H and -Cl) with high selectivity for the reduction of the nitro group in the presence of NaBH4 (Giri et al. 2017). In this case, the borohydride ions (BH4
) release hydrogen, which is adsorbed onto the silver-nanoparticles surface, generating a silver hydride complex (Ag-H), which acts as the main active reducing agent. Different strategies are used for the immobilization of silver-nanoparticles on different supports like husk-SiO2-aminopropylsilane (Davarpanah and Kiasat 2013), palm shell agro-waste-derived carbon supports (Sudhakar and Soni 2018), porous carbon from plant biomass (Ji et al. 2016b), oxygenated mesoporous carbon from cotton fabric (Ji et al. 2016c), mesoporous γ-Al2O3 (Naik et al. 2012), fibrous nanosilica (KCC-1) (Dong et al. 2014), polyaminocyclodextrin (Russo et al. 2015) and MnFe2O4@SiO2 (Kurtan et al. 2016), where the support is mainly used to prevent silver particle aggregation. Other strategies are aimed at obtaining silvernanoparticles by different techniques, for example, from bionanocomposites from fungal mycelia (Narayanan and Sakthivel 2011), which were also tested in this reduction reaction. The effectivity of these silver-nanoparticles nanocomposites to carry out such chemical transformation strongly depends on the silver particle size,

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Reductant Agent: - NaBH4, - NH2NH2, - LiAlH4 e- + H + Ag

25 – 100 °C Reflux Ag

Ag

Ag

Support -

γ-Al2 O3 MnFe2O4 @SiO2 CA/Fe2 O3 Fe2 O3 -rGO

-

Carbon mesoporous SiO2-aminopropylsilane, nano-silica (KCC-1), Polyaminocyclodextrin.

Fig. 8.6 Silver-nanoparticles supported on materials for the thermal reduction of nitroaromatics using reducing agents. (Adapted from Sudhakar and Soni 2018)

the used amount of NaBH4 and the reaction temperature, which varies from 25 to 100  C (Fig. 8.6). By constructing ternary silver-nanoparticles nanocomposites using cellulose acetate-ferric oxide nanocomposites (CA/Fe2O3) or Fe2O3-rGO, the chemical transformation of nitroaromatics can be improved at room temperature by using NaBH4 or hydrazine, respectively (Paul et al. 2016; Bakhsh et al. 2018). Silver-nanoparticles nanocomposites using semiconductor materials have been also reported for chemical transformation, which is photoinduced under visible light by the semiconductor material and that mainly depends on the charge carrier separation. In this case, silver-nanoparticles are reported to be effective catching electron species that can greatly suppress the electron-hole recombination. For example, silver-nanoparticles on TiO2 can be used for quinaldine production from nitrobenzene under photoirradiation conditions, although it does not require the use of acids or reducers (Selvam and Swaminathan 2011). Silver-nanoparticles on TiO2 can be also used for the chemical transformation of nitroaromatic compounds to amino compounds under UV light by using hydrazine as a sacrificial electron donor, which is also known as reducing agent (Hernandez-Gordillo et al. 2014). Nevertheless, the electron transfer for this chemical reaction can be improved when silvernanoparticles are deposited on doped TiO2-Cu (Fig. 8.7), where the high dispersion of silver-nanoparticles is the key factor to achieve high photocatalytic activity (Hernández-Gordillo and González 2015). The construction of heterostructured silver-nanoparticles materials with MIL-125/g-C3N4 can efficiently improve the bifunctional visible light photoactivity for the above chemical transformation (Yang et al. 2017). In this case, the photoinduced reaction is conducted in isopropyl alcohol under N2 at atmospheric pressure to generate anaerobic conditions and carry out the electron transfer process.

8 Silver-Based Photocatalysts: A Special Class

231 UV light Ag

Ag Cu+

NH2 NH2 NH2 NH2

Cu+

Ag

Cu+

TiO2 Cu+

Ag

Cu+

Cu0 Cu0

h+

Ag

(0.5 M pH>9)

Ag

0

Cu

TiO2

NH2 NH*

Cu0

Ag

e-

Cu0

Ag

Fig. 8.7 Silver-nanoparticles on doped TiO2-Cu with high photocatalytic activity for the photoreduction of nitroaromatics using hydrazine. (Adapted with permission from Hernández-Gordillo and González 2015)

8.3.3

New Renewable Energy Source Reactions for CO2 Reduction

After Fujishima reported on hydrogen production by means of TiO2, numerous studies using photocatalysts with the help of a scavenging agent to favour H2 selectivity were published. This photocatalytic process to produce hydrogen from water is a promising way to transform solar energy into clean and renewable hydrogen fuel energy. In addition, the photoreduction of carbon dioxide (CO2) in the presence of water to generate CO, methane (CH4), methanol (CH3OH), etc. is another promising solar fuel technology. Despite this CO2 photoreduction reaction is theoretically feasible on TiO2, it still has very low efficiency. For this reason, TiO2 and diverse semiconductors involving the immobilization of silver-nanoparticles have been extensively reported not only for H2 generation but also for the photoreduction of CO2, separated and simultaneously. Silver-nanoparticles have been commonly used as cocatalysts to improve the photoactivity of the TiO2 semiconductor due to the fact that the Ag valence affects the catalytic properties. Generally, Ag(0) nanoparticles work as electron sinks to accept electrons photogenerated from the TiO2 semiconductor. The presence of Ag (0) nanoparticles results in a surface plasma resonance effect which improves the photocatalytic performance for CO2 photoreduction. Ag(I) has been also reported to act as an electron trap to prevent the recombination of carrier charges. For example, silver-nanoparticles with controllable Ag valence deposited on porous TiO2 microspheres with different crystal phases have been prepared through a sequential hydrothermal method, ultrasonic spray pyrolysis and in situ photoreduction process (Liu et al. 2013). It was found that Ag(0) deposited on TiO2 with a mixture of the anatase/brookite phases was the most photoactive catalyst for the photoreduction of CO2, using a mixed gas of CO2/H2O/CH3OH under solar irradiation simulation. In this case, methanol works as a hole scavenger. Other studies have demonstrated that the synthesis method is an important parameter to incorporate silver-nanoparticles into TiO2. By using the ultrasonic spray pyrolysis method, smaller silver-

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nanoparticles with better dispersion on TiO2 and a higher fraction of Ag(0) species can be obtained, which facilitates the charge transfer and improves the photocatalytic activity (Zhao et al. 2012). In spite of the fact that the H2 production and CO2 photoreduction are competitive processes, the H2/CO product selectivity ratio varies depending on the physicochemical properties of TiO2, even the formation of methane (CH4) can be observed in some cases (Cheng et al. 2017; Tahir et al. 2017). By changing the TiO2 semiconductor to ZnO, the size of the silver-nanoparticles is larger than that on TiO2, affecting the photocatalytic properties and exhibiting low CO2 conversions, with the formation of CH4 (Collado et al. 2013). Carbon fibres decorated with silver-nanoparticles, with 20–50 nm diameters, have also been successfully synthesized, where the enhanced selectivity to transform CO2 into CH3OH was observed. Similarly, the good dispersion of silvernanoparticles enhances the charge separation and the good adsorption of CO2 as well as the CO2 conversion (Ding et al. 2017). The use of graphitic carbon nitride (g-C3N4) decorated with silver-nanoparticles can effectively induce the photo-driven H2 production separately. The silver-nanoparticles/CdS material was also effective as a photocatalyst for the CO2 photoreduction reaction, where CO was the main product. In this case, the reaction was conducted in the presence of triethanolamine instead of methanol (Zhu et al. 2017). It means that depending on the type of semiconductor support and the type of hole scavenger, the plasmonic silver-nanoparticles play the dual role of not only assisting the separation of photogenerated electron-hole pairs from the recombination in the semiconductor (TiO2, ZnO, carbon fibres) but also the absorption of visible light energy required to achieve the enhanced photocatalytic activity (Fig. 8.8).

Support: - TiO2 , - Carbon CFs - CdS

UV light H2 O e-

H+ e-

Ag

Ag

Hole scavenger: Methanol, Triethanolamine

eOH + H+

Support

CH4 +CH3 OH CO

H2 eAg

CO2

Fig. 8.8 Silver-nanoparticles deposited on semiconductors with the photocatalytic response for the photoreduction of CO2 to CO, CH4 and CH3OH with simultaneous H2 production. (Cheng et al. 2017)

8 Silver-Based Photocatalysts: A Special Class

8.3.4

233

Antibacterial Properties Enhanced with Photoactive Silver-Nanoparticles

Silver is largely known as a universal antimicrobial agent in the colloidal state and has been used for more than one century; it is also stable in aqueous medium, which allows the disinfection of human pathogens like Escherichia Coli, salmonella and some phytopathogenic fungi (Wilke et al. 2018). Silver-nanoparticles chemical interactions may cause toxic stress to microorganisms and the inhibition of the reproduction of colonies by low coordination of components of the cellular wall and biopolymers segregated by microorganism as a defence. The first studies using copper or silver-nanoparticles on TiO2 describe the fragmentation of the cell wall by means of the production of reactive oxygen species during the radiation of TiO2 using UV light. The pathophysiological properties of titania seem to be a good option to rapidly deactivate bacteria, Cu and TiO2 based on the destructive generation of reactive oxygen species. Nanoparticles can cause the rupture of bacterium membrane cells due to lipid peroxidation resulting from the generation of reactive oxygen species under visible and mainly UV light, and also some cell wall permittivity has been suggested (Chen et al. 2014; Kubacka et al. 2013). Silver species are capable of oxidizing cell constituents such as proteins, lipids and nucleic acids. During the photokilling process of harmful bloom algae, the microorganism is first covered until it is immobilized and starts to be damaged by reactive oxygen species generated by the semiconductor. After 5 min, the algae body is deformed and starts to fragment until it is totally destroyed. The role of silver particles consists of capturing electrons and inhibiting electron recombination. In addition, the biocide properties of silver enhance the fatal damage to the algae using solgel TiO2 and cuboids of calcium titanate functionalized with silver-nanoparticles (Fig. 8.9) (Lee et al. 2013; Rodríguez-González et al. 2010). For phytopathogenic microorganism disinfection, the high dispersion effect of surface silver-nanoparticles species (Ag+, Ag0 and Ag-O) all over the titanate nanotubes enhances the charge separation for generating reactive oxygen species that stress the fungus. Silver species boost the biocide effect on the fungus Botrytis cinerea by inhibiting its reproduction and the sharp nanotube morphology can penetrate the cell, accelerating the vacuolation and invagination that produce death in 20 min (Fig. 8.10). A representative scheme was proposed based on scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations for different inactivation times. Similar studies have been reported for E. coli, a pathogenic human bacterium, where also the oxidative stress caused by the photoactive nanocomposite photokilled the bacteria in 45 min; also the cell wall remained with some possible permeability, and the oval body size increased due to the segregation of cellular content. In conclusion, silver-nanoparticles (80%, 1 h) against rose bengal dye. They also found that the kinetics of the rose bengal dye degradation followed the LangmuirHinshelwood mechanism (Vidya et al. 2016).

9.3

Limitation of Green Synthesis

In spite of the numerous advantages of green synthesis elaborated already in this chapter, a few restrictions of this route are still present today. The synthesis of nanoparticles by green route has significant potential and various benefits over the conventional chemical and physical routes. However, green synthesis of nanoparticles has been applied for small-scale production only. It is difficult to control the size and morphology of nanoparticles at the time of production. Recently numerous attempts are being initiated to overcome these restrictions and scale up the nanoparticles production by greener synthesis route.

9.4

Conclusions: Future Scope

Nowadays, there is a requirement to develop a feasible and eco-friendly process for the preparation of metallic nanoparticles, which minimize or entirely eliminate the use of hazardous chemicals. Worldwide researchers are already focused on alternative ways to synthesize metallic nanoparticles as elaborated in this chapter. Accordingly, the greener synthesis of metallic nanoparticles is getting an advantage over the other synthesis route. This chapter also provides knowledge about the utilization of several organisms, like plants, bacteria, fungi, and algae for the eco-friendly synthesis of metallic

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nanoparticles, which also act as a photocatalyst. Despite lots of advancement, there are still some serious issues that ought to be resolved in the future: (1) prepare novel photocatalysts which have higher efficiency, and utilize sunlight as energy source, (2) make the green synthesis route as an energy-efficient technique by eliminating energy-intensive steps, (3) identify the risks related with biological and human exposure to metallic nanoparticles, and (4) develop some techniques by which the size and morphology of nanoparticles can be regulated at the time of production. Hence, further investigations are required for the betterment of current processes and methods, which will provide benefits and challenges for the research community and general public in the future. Acknowledgments Shubhrajit Sarkar acknowledges financial support from the University Grants Commission (UGC), India, and Department of Science and Technology (DST), India. The authors would also like to thank Jadavpur University, India, for support.

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Chapter 10

Electrodeposition of Composite Coatings as a Method for Immobilizing TiO2 Photocatalyst V. S. Protsenko, A. A. Kityk, E. A. Vasil’eva, A. V. Tsurkan, and F. I. Danilov

Contents 10.1 10.2

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic Fe/TiO2 Composites Electrodeposited from Methanesulfonate Aqueous Plating Bath . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.1 Electrodeposition of Iron-Titania Composites Using Methanesulfonate Electrolyte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2 Surface Morphology, Microstructure, and Microhardness of Iron-Titania Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.3 Photocatalytic Performance of Iron-Titania Composites . . . . . . . . . . . . . . . . . . . . . . 10.2.4 Improving the Corrosion Resistance of Photocatalytic Fe/TiO2 Composite Coatings by Electrodeposition of Protective Ceria Layer . . . . . . . . . . . . . . . . . . . . . 10.3 Photocatalytic Ni/TiO2 Composites Electrodeposited from Electrolyte Based on a Deep Eutectic Solvent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.1 Electrodeposition of Nickel-Titania Composites Using an Electrolyte Based on Deep Eutectic Solvent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.2 Surface Morphology and Microstructure of Nickel-Titania Composites . . . . . . 10.3.3 Photocatalytic Performance of Nickel-Titania Composites . . . . . . . . . . . . . . . . . . . . 10.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract In order to immobilize the TiO2 photocatalyst particles, various kinds of supports can be used. One of these involves the electrodeposition of metal matrices with entrapped titania particles. This work focuses on the electrodeposition and characterization of two new types of photocatalytic composite coatings. The first part of the chapter deals with the electrodeposition of Fe/TiO2 composite coatings using environmentally friendly aqueous methanesulfonate iron plating baths containing colloidal TiO2 particles. The effects of bath composition and electrolysis conditions on the content of titania particles in coatings were investigated; the surface morphology, microstructure, and microhardness of coatings were characterized. The V. S. Protsenko (*) · A. A. Kityk · E. A. Vasil’eva · A. V. Tsurkan · F. I. Danilov Ukrainian State University of Chemical Technology, Dnipro, Ukraine © Springer Nature Switzerland AG 2019 Inamuddin et al. (eds.), Nanophotocatalysis and Environmental Applications, Environmental Chemistry for a Sustainable World 29, https://doi.org/10.1007/978-3-030-10609-6_10

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photocatalytic performance of Fe/TiO2 electrodeposited coatings was evaluated in the reactions of decomposition of methyl orange and methylene blue dyes in water under the action of ultraviolet radiation. Although iron electrodeposited matrix is cheap, nontoxic, and easily repairable support for TiO2 photocatalysts, it is not corrosion-resistant enough. Therefore, special attention is needed to improve the corrosion resistance of photocatalytic Fe/TiO2 composite coatings via the electrodeposition of a protective ceria layer on their surface. The second part of this chapter is devoted to the electrodeposition of photocatalytic Ni/TiO2 composite coatings from colloidal electrolyte based on deep eutectic solvents which are now considered as promising analogues of room temperature ionic liquids. The fabrication of Ni/TiO2 composites from choline chloride based plating bath is reported. The Ni/TiO2 composite coatings manifest a photocatalytic activity toward the reaction of photochemical degradation of methylene blue organic dye in water solution. Keywords Immobilized TiO2 · Photocatalyst · Composite · Coatings · Electrodeposition · Fe/TiO2 · Ni/TiO2 · Methanesulfonate electrolyte · Deep eutectic solvents

10.1

Introduction

Advanced oxidation processes belonging to high-performance, environmentalfriendly, and available methods of wastewater treatment have ensured very effective removal of dangerous organic pollutants (Oturan and Aaron 2014). Advanced oxidation processes have been defined as water treatment processes which are based on the in situ generation of a powerful oxidizing agent, such as hydroxyl radicals (•OH), obtained at a sufficient concentration by means of different chemical, photochemical, electrochemical, and sonochemical reactions to effectively decontaminate waters (Glaze et al. 1987). There is a wide range of different kinds of advanced oxidation processes including Fenton process, ozonation, peroxonation, photolysis of H2O2, photolysis of O3, photo-Fenton, electro-Fenton, and heterogeneous photocatalysis (TiO2/ultraviolet) (Oturan and Aaron 2014; Fujishima et al. 2000; Pelaez et al. 2012). Among the advanced oxidation processes, heterogeneous photocatalysis involving TiO2 photocatalyst seems to be very promising as it completely mineralizes the contaminations existing in either a gas phase or a liquid phase (Ahmad et al. 2016). It is known that the photocatalytic properties of TiO2 are associated with the formation of holes and electrons under irradiation by ultraviolet light forming reactive species, such as •OH and O
 2 (Oturan and Aaron 2014; Ahmad et al. 2016). Thus, titania is considered a material “close to being a practically ideal photocatalyst in several important aspects” (Oturan and Aaron 2014), and it is of great interest and value for developing new systems and technologies based on advanced oxidation processes. The heterogeneous TiO2 photocatalysis has been

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widely applied in recent years, because it is very efficient for treating a substantial range of organic pollutants. Heterogeneous titania photocatalysts can be used either in dispersed form (as powders or aqueous suspensions) or in thin film form (fixed immobilized TiO2 catalytic layers) (Oturan and Aaron 2014; Ahmad et al. 2016). Slurry systems manifest a number of advantages: these are easy to prepare and use, provide a very high specific surface, and can be easily aerated to prevent the recombining of electron-hole pairs and, therefore, increase catalytic efficiency (Oturan and Aaron 2014). However, the suspensions of fine powdered TiO2 are unstable and trend toward particles coarsening and sedimentation; as a result their photocatalytic performance is reduced. In addition, slurry systems require time-consuming and costly processes of titania posttreatment. However, there is no need to filtrate and separate catalytic particles in the case of immobilized TiO2 catalytic layers; such systems can be very stable and show relatively high photocatalytic activity for a long time (Shan et al. 2010). In order to anchor the TiO2 photocatalyst particles, a great number of different kinds of supports have been developed: glasses, silica materials, activated carbon, polymers, zeolites, stainless steel, alumina clays, fibers, and others (Shan et al. 2010; Pozzo et al. 1997). In such photocatalysts, the TiO2 particles are uniformly dispersed in a continuous matrix or are deposited on the surface of a support. In this work, we intend to focus upon electrodeposited metallic matrices as carriers of titania photocatalytic particles. This is because the electrodeposition of composites containing TiO2 particles in a metallic matrix seems to be a practically feasible way to design new photocatalysts with immobilized TiO2 dispersed phase (Rajeshwar et al. 2001) and has some advantages of other methods of TiO2 immobilization. The electrochemical embedding of finely dispersed nonmetallic phase in metallic matrices consists in electrolyzing a solution containing the dissolved metallic salt and the particles in suspension. Figure 10.1 illustrates the main physicochemical processes involved in the electroplating of particles into a growing metallic matrix. The incorporation of particles into metallic coatings includes convection and/or diffusion of the particles toward the cathode surface, migration driven by the potential gradient across an electrical double layer, particle adsorption on the surface, and entrapment of particles into the growing metal matrix (Low et al. 2006; Walsh and Ponce de Leon 2014). It is known that the electrodeposited composite coatings have gained considerable attention due to their enhanced physicochemical and service properties as compared with “pure” metal coatings (Low et al. 2006; Walsh and Ponce de Leon 2014). The electrodeposition technique enables the controlled and flexible fabrication of composites with predictable composition, structure, and properties. Electroplating procedure is available, relatively simple, and not expensive. In addition, the electrodeposition technique allows easily restoring worn layers. In most cases, the studies on composite electrodeposition were aimed to produce wear- and/or corrosion-resistant protective coatings, self-lubricating layers, and dispersion-strengthened coatings. However, a very important and interesting task

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Fig. 10.1 The processes involved in co-electrodeposition of insoluble particles into a growing metal matrix to form a composite metal coating. (Reprinted from Walsh and Ponce de Leon 2014 by permission of Taylor & Francis Ltd)

is to obtain electrodeposits with some catalytic and photocatalytic properties (Musiani 2000). In this connection, electrochemical deposition of composites containing dispersed particles of titania may be a very interesting approach to the development of new potential supports for TiO2, and there are a number of publications dealing with this research issue. Spanou et al. (2013) have reported the immobilization of titanium dioxide nanoparticles in coatings with nickel matrix using a direct current electrodeposition method. The photocatalytic activity of the obtained composite coatings was studied under ultraviolet illumination. Fine-tuning of electrodeposition parameters was shown to facilitate the adjustment of the nickel matrix structural and morphological characteristics and the titania nanoparticle encapsulation on the electrodeposit surface. In this way, the photo-induced self-cleaning properties of the composite coatings (photocatalysis and hydrophilicity) can be optimized. Ni-TiO2/TiO2 multilayers were electrodeposited from a nickel plating bath containing TiO2 sol by means of a pulse plating technique (Mohajeri et al. 2017), and then the obtained coatings were heat-treated at different temperatures. The photocatalytic efficiency of the multilayers was analyzed by the degradation of methyl orange dye. The multilayers heated at 450  C exhibited the highest photocatalytic activity. This was explained on the basis of the highest percentage of the anatase phase present in the coating annealed at that temperature. The fabrication of Ni/TiO2 composite coatings by the electrolysis of colloidal aqueous nickel plating bath was recently investigated (Sknar et al. 2017). An increase in the titania content in the deposits resulted in an increase in the photocatalytic activity of the composites. It was shown that the photocatalytic performance of Ni/TiO2 composite coatings is associated with the degree of coverage of their surface with TiO2 particles.

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Nanocomposite films consisting of TiO2 dispersed particles and continuous zinc matrix were formed on steel plates by an occlusion electrodeposition method (Ito et al., 1999). The electrodeposited composites were highly active in the reaction of the photocatalytic oxidation of acetic aldehyde. The study (Luan and Wang 2014) reported the electrochemical synthesis of Ag/ bamboo-type TiO2 nanotube composites. The authors showed an improved activity toward the photocatalytic degradation of methylene blue dye in aqueous solution. Composite materials consisting of silver, titanium dioxide, and bamboo charcoal have been prepared via stepwise methods including sol-gel synthesis, wet impregnation, and electrochemical deposition (Laohhasurayotin and Pookboonmee 2013). The photocatalytic performance of these materials in reaction of photocatalytic methylene blue dye removal was investigated. Even such a brief survey shows a lively interest in the electrodeposition of metalbased composite coatings to anchor the TiO2 photocatalyst particles. In most cases, the immobilization of titania particles is performed using Ni matrix electrodeposited from common aqueous solutions (Spanou et al. 2013; Mohajeri et al. 2017; Sknar et al. 2017). Evidently, the search for novel types of electrochemical plating systems can provide a further improvement in photocatalytic performance of immobilized TiO2 photocatalyst. This work is aimed to present the main results of our recent investigations dealing with electrochemical deposition and characterization of new types of photocatalytic TiO2-containing composites. In the first part of the chapter, the fabrication of photocatalytic Fe/TiO2 composites is presented by using methanesulfonate aqueous electrolytes (Protsenko et al. 2014, 2017, 2018; Danilov et al. 2017; Vasil’eva et al. 2016). In the second part of the chapter, the characterization aspects of photocatalytic Ni/TiO2 composites prepared from electrolyte based on a deep eutectic solvent are discussed.

10.2

Photocatalytic Fe/TiO2 Composites Electrodeposited from Methanesulfonate Aqueous Plating Bath

As stated above, Ni matrix is mostly used for composite electrodeposition. We think that the replacement of the nickel electrodeposited matrix by an iron one can provide a number of advantages. Indeed, nickel and its salts are highly expensive and toxic. A skin contact with metallic nickel leads to a contact dermatitis. Ferrous electrochemical systems are free from those defects. From this point of view, the development of electrodeposited composite Fe/TiO2 coatings as versatile photocatalysts is very promising because iron and its salts are relatively cheap, available, and low toxic (Protsenko et al. 2014, 2017, 2018; Danilov et al. 2017; Vasil’eva et al. 2016). To electrodeposit Fe coatings, different types of plating baths can be used. Recently, we suggested a methanesulfonate plating bath for iron matrix electrodeposition (Vasil’eva et al. 2013; Protsenko et al. 2015a).

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Methanesulfonate electroplating baths are based on methanesulfonic acid CH3SO3H. Methanesulfonic acid is considered as a “green acid” due to its environmental advantages (Gernon et al. 1999; Walsh and Ponce de León 2014). It is easily biodegradable. Methanesulfonates of various metals are highly soluble in water and the electrical conductivity of their aqueous solutions is high. Because of these advantages, electrochemical systems of methanesulfonic acid and its salts are very promising for electroplating of different metals, alloys, and composites (Walsh and Ponce de León 2014). Thus, the electrodeposition of iron-titania layers using plating baths based on methanesulfonic acid satisfies the requirements of the concept of “green chemistry.”

10.2.1 Electrodeposition of Iron-Titania Composites Using Methanesulfonate Electrolyte Fe/TiO2 composite coatings with photocatalytic activity were electrochemically synthesized in this work using two different kinds of TiO2 particulates. In one case, Degussa (Evonik) P25 TiO2 nano-powder was used. Degussa P25 is a commercially available TiO2 powder sample which is widely applied as an efficient photocatalyst in various kinds of photocatalytic reaction systems (Ohtani et al. 2010; Chen and Mao 2007). Degussa P25 is composed of anatase and rutile crystallites (in the ratio of about 80:20, respectively) with an average particle size of 25–30 nm. To prepare composites electroplating baths, nano-powder of Degussa P25 TiO2 without any pretreatment was introduced into electrolyte. The composition of methanesulfonate electroplating bath and electrodeposition parameters are summarized in Table 10.1. However, there is another, alternative way to obtain colloidal solution for composite electrodeposition. It involves the formation of TiO2 sol by chemical precipitation method via hydrolysis of titanium alkoxides solutions (Mital Gupta and Tripathi 2012) and further addition of obtained TiO2 hydrosol to the electroplating electrolyte (Chen et al. 2010a; Chen and Gao 2010). Generally, the synthesis of

Table 10.1 Composition of plating bath and electrodeposition parameters (Protsenko et al. 2018) Bath components and electrodeposition conditions Fe(CH3SO3)2 TiO2 particles content pH Temperature Current density Stirring Cathode Anode

Values 1.25 mol dm3 0–12 g dm3 1.3 25  C 5–20 A dm2 Magnetic stirrer, ca. 60 rev min1 Mild steel or copper Mild steel

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inorganic dispersed particles from solution state is considered a promising approach to fabricate the nanoparticles with unique and tunable properties (Wu et al. 2016). To prepare TiO2 sol, the following procedure was used. 5.3 mL of tetrabutyl orthotitanate (Ti(OBu)4) was added dropwise under continuous magnetic stirring to the 0.1 M aqueous solution of methanesulfonic acid at the temperature of 80  C to achieve the concentration of 25 g L1 (referred to the metal titanium content). A white sediment of TiO2 is formed instantly due to Ti(IV) hydrolysis. Then, hot suspension was stirred by means of magnetic stirrer for 8 h to obtain a transparent opalescent TiO2 sol. To fabricate the composite electrodeposition plating bath, a desired value of titania sol was introduced into the methanesulfonate iron (II) electrolyte (Table 10.1) and then the electroplating was conducted immediately. The size distributions of TiO2 particles in the synthesized sol and in the methanesulfonate iron plating electrolyte were estimated from dynamic light scattering measurements (Zetasizer Nano ZS (Malvern Instruments Ltd., Great Britain) equipped with the He-Ne laser (633 nm)). As can be seen from Fig. 10.2, the initial TiO2 sol mainly contains nano-sized particles with a diameter of about 48 nm. The measured zeta potential of the particle surface is about +26 mV; a positive value of zeta potential is natural in acid medium since the isoelectric point of titania is known to be about pH 5.8 (Thiemig and Bund 2008). The synthesized sol seems to be rather stable and no visible aggregation was observed during 4 weeks of testing. After the introduction of titania sol into the iron electroplating bath, the particles aggregation occurs and the colloidal system becomes polydisperse (Fig. 10.2b). There are two main fractions with the particles diameter of ca. 70 nm and ca. 887 nm, their volume percentages being practically equal. Figure 10.2c displays the size distribution of dispersed particles in the methanesulfonate iron electrolyte containing Degussa P25 TiO2 powder. It is seen that there is no fraction of nanosized particles and the average diameter of aggregated particles is close to 1 μm. This result is in good agreement with the data obtained by sedimentation analysis (Vasil’eva et al. 2016). The coarsening of the colloidal particles is explained by the DLVO (Derjaguin, Landau, Verwey, Overbeek) theory. According to this concept, an increase in ionic strength leads to the compression of the diffuse part of double electrical layer resulting in easy aggregation of particles. The partial coarsening of the particles in iron plating electrolyte with a relatively high electrolytes concentration can be seen even by the naked eye as cloudiness of solution. The aggregation of the particles can cause their sedimentation and subsequent phase separation. This is why the electrochemical synthesis of the Fe/TiO2 composite was carried out under the conditions of a continuous bath agitation with a magnetic stirrer. The content of inert dispersed phase in a metallic matrix of composite electrodeposits depends on various factors, the main being the concentration of dispersed phase in colloidal electrolyte and current density of composite electrodeposition. Figure 10.3 shows the content of TiO2 in the composite coatings as a function of titania mass concentration in plating bath and applied current density. As can be seen, the content of titania in composites increases with increasing TiO2 content in the plating bath and decreases with increasing cathodic current density. This

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Fig. 10.2 Size distribution of TiO2 particles: (a) in synthesized titania sol, (b) in colloidal methanesulfonate iron plating electrolyte containing synthesized titania sol, and (c) in colloidal methanesulfonate iron plating electrolyte containing suspended Degussa P25 TiO2 particles. (Reproduced from Protsenko et al. 2018)

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Fig. 10.3 Dependences of TiO2 content in composite coatings on current density and titania concentration in electrolyte. (Reproduced from Protsenko et al. 2018)

behavior is typical of the processes of composites electrodeposition (Low et al. 2006; Walsh and Ponce de Leon 2014). It is important that the content of TiO2 embedded in an iron matrix is larger in the case of the introduction of homemade TiO2 sol into the plating electrolyte than in the case of the addition of Degussa P25 TiO2 particles, other conditions being equal. This is probably due to the differences in sizes of colloidal particles. According to previous report (Low et al. 2006), as the size of nanoparticles becomes smaller, more can be incorporated into a metal deposit per unit volume. When Degussa P25 TiO2 particles are introduced into the methanesulfonate iron electroplating bath, the most probable radius of aggregates existing in electrolyte and being embedded in metallic matrix is about 1 μm, which is greater than in the colloidal electrolyte containing homemade TiO2 sol. The embedding of titania in composite can be associated with the adsorption of colloidal particles on the electrode surface, as suggested by Guglielmi’s two-step adsorption model (Guglielmi 1972). This kinetic model is based on the assumption that dispersed phase particles are incorporated into metallic matrix as a result of their adsorption on the electrode. It is proposed that adsorption of colloidal particles on the electrode surface can be described by the following Langmuir-type equation (Guglielmi 1972; Bahadormanesh and Dolati 2010):

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σ¼

kC ð1  θ Þ 1 þ kC

ð10:1Þ

where σ is the surface coverage due to weak adsorption, θ is the surface coverage due to strong adsorption, C is the volume fraction (percentage) of particles in suspension, and k is the adsorption coefficient. Here, adsorption of dispersed phase particles on the surface of the deposited metal occurs in two stages. The first stage, denoted by Guglielmi as “weak adsorption,” has a physical nature; particles fixed on the electrode surface largely remain coated by adsorption-solvate shells. At the second stage of the so-called strong adsorption, dispersed phase particles lose these shells and are firmly fixed on the metal surface. It is supposed that only those dispersed phase particles which are strongly adsorbed on the metal surface can be ultimately included into the structure of the forming composite electrodeposited coatings. The main equation describing the kinetics of the co-deposition of the dispersion phase and metallic matrix has the following form (Guglielmi 1972):   C Mi0 ðABÞη 1 ¼ þC e α nFρm υ0 k

ð10:2Þ

where α is the volume fraction of particles in the composite coating, M is the atomic mass of the metal, i0 is the exchange current of the metal deposition process, n is the number of electrons in the equation of the electrochemical reaction, F is the Faraday’s number, ρm is the metal density, η is the overpotential of the electrochemical reaction of metal deposition, A is a constant in the kinetic equation of the electrochemical reaction i ¼ i0eAη, and B and υ0 are the constants describing the kinetics of the process of inclusion of nonmetal phase particles into the composite coating and similar to the constants A and i0 for the electrochemical process. In our papers (Protsenko et al. 2018; Vasil’eva et al. 2016), we have reported that the kinetics of co-deposition of iron and titanium dioxide particles (both Degussa P25 and synthesized sol) adequately obeys the Guglielmi’s model (Guglielmi 1972) with some modifications as suggested (Bahadormanesh and Dolati 2010). It is important to note that the calculated adsorption coefficient for the particles of the homemade TiO2 sol is k ¼ 13.05. This is remarkably higher than the value obtained for the adsorption coefficient of Degussa P25 TiO2 particles (k ¼ 2.8). A higher adsorption coefficient of the particles in the synthesized sol indicates stronger adsorption of these particulates on the growing iron matrix surface than in the case of entrapping Degussa P25 TiO2 particles. This phenomenon can be associated with various values of particles size as well as with some differences in the structure and composition of the TiO2 particles. While in electrolyte containing Degussa P25 TiO2, the average particles diameter is about 1 μm and the percentage of smaller fraction is low, the colloidal electrolyte contains a large amount of nano-sized TiO2 particles. Lower size means a larger excess of surface energy, which should provide increased inclination of colloidal particles to adsorption. Enhanced adsorption of

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nano-sized TiO2 particles ensures their advanced introduction into metallic matrix, which explains a high TiO2 content in composite compared with the case of Degussa P25 TiO2 (Fig. 10.3).

10.2.2 Surface Morphology, Microstructure, and Microhardness of Iron-Titania Composites The typical surface morphology of Fe/TiO2 composites obtained from methanesulfonate electrolyte with the addition of TiO2 particulates is shown in the scanning electron microscopy images (Fig. 10.4). Nodular structure with a great number of protrusions was observed on the surface of composites deposited from the plating bath containing TiO2 sol (Fig. 10.4a). When compared to the surface of coatings obtained from electrolyte with added Degussa P25 TiO2 particles (Fig. 10.4b), the application of synthesized TiO2 sol yielded more uniform surface population by smaller titania aggregates. In addition, there were no visible cracks on the surface when the composite layer was fabricated from iron plating bath containing TiO2 sol. Figure 10.5 represents the results of energy-dispersive X-ray analysis performed on the coatings surface shown in Fig. 10.4a. The analysis indicates only the presence of Fe, O, Ti, and C on the surface. The detection of oxygen is associated both with oxygen originated from titania particles and with oxygen from oxide film forming on fresh deposited iron surface. The carbon spectrum seems to be originated from butyl radical which may adsorb on TiO2 surface in the course of the preparation of titania sol. The energy-dispersive X-ray spectrum of the surface of Fe/TiO2 composite coatings deposited from the bath containing aggregated Degussa P25 TiO2 particles is quite similar to that presented in Fig. 10.5 with one predictable exception: it does not display the presence of carbon. The X-ray diffraction spectra revealed the presence of crystallized α-Fe with the body-centered cubic (bcc) lattice (Fig. 10.6). The reflections of the Cu foil substrate were also detectable. No TiO2 peaks were seen from the composite coatings, probably due to relatively low quantity and highly dispersive distribution of the nano-sized titania particles (Protsenko et al. 2018). It should be observed that the X-ray diffraction analysis of TiO2 powder, obtained by the procedure developed in this work and dried at 80  C, showed poorly crystallized anatase (Fig. 10.7). Similar results were reported earlier for a hydrated titanium (IV) oxide precipitated from an aqueous solution (Baura-Peña et al. 1991). Figure 10.8 demonstrates that the Fe/TiO2 electrodeposited composites are considerably harder than pure iron coating. The microhardness of deposits obtained from electrolyte with the addition of nano-sized titania sol exceeds the value observed for coatings produced from the bath containing aggregated Degussa P25 TiO2 particles.

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Fig. 10.4 Scanning electron microscopy images of Fe/TiO2 composites obtained (a) from electrolyte containing synthesized titania sol and (b) from electrolyte containing suspended TiO2 Degussa P25 particles. The composites contain 5 wt.% TiO2 (Reproduced from Protsenko et al. 2018)

The strengthening of composite electrodeposits is attributed to dispersion strengthening via Orowan mechanism (Protsenko et al. 2018). This dispersion hardening effect (i.e., the Orowan mechanism) is associated with a dispersion of fine particles, which impede the motion of dislocations in the metallic matrix resulting in an increase in the material hardness (Chen et al. 2010b). It is known that closely spaced dispersed particles ensure more pronounced dispersion

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Fig. 10.5 Energy-dispersive X-ray spectrum of the surface of Fe/TiO2 composites obtained from electrolyte containing synthesized titania sol (Reproduced from Protsenko et al. 2018)

Fig. 10.6 X-ray diffractogram of Fe/TiO2 composite coating deposited on Cu substrate from electrolyte containing synthesized titania sol. The composite contains 5 wt.% TiO2. (Reproduced from Protsenko et al. 2018)

strengthening. Then, the introduction of nano-sized particles into the iron matrix in the case of electrolyte containing TiO2 sol results in formation of composites with high density and fine size of titania particles as compared with the composites obtained from electrolyte with aggregated Degussa P25 TiO2 particles. Therefore, the coatings, deposited by the electroplating procedure involving the application of TiO2 sol instead of TiO2 powder, possess enhanced microhardness. An improved microhardness of Fe/TiO2 composite coatings is important and advantageous in terms of their possible practical application.

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Fig. 10.7 X-ray diffractogram of dried TiO2 powder obtained by the procedure developed in this work. (Reproduced from Protsenko et al. 2018)

Fig. 10.8 Microhardness of Fe and Fe/TiO2 composites obtained from methanesulfonate electrolytes containing synthesized titania sol and suspended TiO2 Degussa P25 particles. The composites contained 5 wt.% TiO2. (Reproduced from Protsenko et al. 2018)

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10.2.3 Photocatalytic Performance of Iron-Titania Composites The photocatalytic activity of TiO2 particles immobilized on Fe/TiO2 composite electrodeposits was tested in the reaction of photochemical degradation of methyl orange dye in 0.1 M NaOH solution under the action of ultraviolet radiation. The decolorization kinetics was studied at the temperature of 298 K. The photocatalytic performance was evaluated using an in-house fabricated reactor (Fig. 10.9). A DKB 9 UV lamp with an effective spectral range of 180–275 nm was used as irradiation source. The intensity of ultraviolet irradiation was 3 mW cm2. The lamp was arranged over the dye solution at a distance of 10 cm from its surface. In the course of photocatalytic decomposition, the dye solution with working volume of 20 mL was continuously stirred with a magnetic agitator. Steel plate with the electrodeposited composite catalyst (3 cm2) was placed at a depth of 2 mm in the solution parallel to its surface (i.e., at the angle of 90 to the ultraviolet radiation beam). The content of a dye in the solution was determined as a function of irradiation time from the absorbance change. The kinetic curves of the decolorization process are shown in Fig. 10.10. The decay in dye concentration during their photodegradation often obeys the pseudofirst-order kinetics (Lachheb et al. 2002). To ascertain the kinetic model describing the reaction of methyl orange destruction, the plots following the first-order model were obtained according to Eq. (10.3): ln C ¼ ln C0  kτ

Fig. 10.9 Schematic diagram of reactor for photochemical decomposition of dyes. (Reproduced from Protsenko et al. 2017)

ð10:3Þ

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Fig. 10.10 Kinetic curves of the degradation of methyl orange dye: (1) in the dark, (2) under ultraviolet irradiation without photocatalyst, (3) under ultraviolet irradiation with Fe/TiO2 photocatalyst (electrolyte contained synthesized titania sol), and (4) under ultraviolet irradiation with Fe/TiO2 photocatalyst (electrolyte contained suspended TiO2 Degussa P25 particles). All solutions contained 0.1 M NaOH. The composites contained 5 wt.% TiO2. In the insert: firstorder linear transforms logCMO vs. τ. (Reproduced from Protsenko et al. 2018) Table 10.2 Calculated values of apparent rate constant for the reaction of methyl orange dye degradation under ultraviolet irradiation in 0.1 M NaOH (Protsenko et al. 2018) System Without photocatalyst (photolysis) Fe/TiO2 photocatalyst (synthesized sol) Fe/TiO2 photocatalyst (Degussa P25)

k (min1) 0.0054 0.0064 0.0135

Here C is the concentration of dye at time τ; C0 is the initial concentration of dye (at τ ¼ 0), and k is the apparent rate constant. Kinetic curves linearized in the coordinates of “logarithm of the methyl orange concentration versus time” (see the insert in Fig. 10.10) do indicate the pseudo-first reaction order. The slopes of these straight lines give the apparent rate constants of decolorization process (Table 10.2). The obtained results show that methyl orange dye does not undergo spontaneous decomposition in the dark in the presence or absence of iron-titania composite coatings. The photochemical destruction of methyl orange (i.e., photolysis) proceeds in solutions exposed to ultraviolet radiation even without any photocatalyst. The decolorization process appreciably accelerated in the presence of the Fe/TiO2 composite electrodeposits, which definitely suggests their photocatalytic properties. It is seen from the experimental data that the apparent rate constant for the Fe/TiO2 composite electrodeposits obtained from TiO2 sol-based electrolyte is

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ca. two times lower than that for the composite deposited from electrolyte containing suspended Degussa P25 TiO2 particles. It means that the photocatalytic activity of coatings fabricated by using the method described above is relatively low. Earlier, the photocatalytic activities of amorphous TiO2 and crystallized anatase TiO2 powder were compared (Ohtani et al. 1997). The photocatalytic activity of amorphous TiO2 was revealed to be negligible, while anatase crystallites showed appreciable photocatalytic performance. The negligible activity of amorphous TiO2 was attributed to recombination of photoexcited electron and positive hole at defects located on the surface and in the bulk of particles. Probably, similar interpretation is applicable to explain the results of our work. Indeed, TiO2 particles embedded in iron matrix from the electrolyte containing synthesized titania sol are composed of poorly crystallized anatase phase; evidently, the particles include a considerable part of amorphous hydrated TiO2 as the procedure of composites preparation does not involve their sintering. Amorphous TiO2 must contain many imperfections (e.g., impurities, dangling bonds, or microvoids) which cause electronic states in the band gap; these imperfections may behave as recombination centers for electrons and positive holes (Ohtani et al. 1997). As a result, the photocatalytic activity of the Fe/TiO2 composite coatings fabricated from TiO2 sol-based electrolyte is relatively low, although not negligible.

10.2.4 Improving the Corrosion Resistance of Photocatalytic Fe/TiO2 Composite Coatings by Electrodeposition of Protective Ceria Layer All data on photocatalytic activity of Fe/TiO2 composite coatings given in previous subsection were obtained for a relatively concentrated alkaline solution (0.1 M NaOH) that is far from the compositions of real wastewaters. The choice of an alkaline solution was due to a fast corrosion of iron matrix in weak acid or neutral aqueous solutions. Hence, it is necessary to reliably protect iron matrix in Fe/TiO2 composites against corrosion damage before using them as photocatalysts for wastewater treatment. In order to improve the corrosion resistance of Fe matrix, cerium oxide layers can be electrodeposited on the metal surface. The ceria films have been successfully used for the corrosion protection of iron and steel (Stoychev 2013; Yang et al. 2011; Creus et al. 2006; Hamlaoui et al. 2009, 2010) and considered as a very promising alternative to the chromate coatings (Stoychev 2013). Ceria layers can be fabricated from various types of electrolytes: chloride, nitrate, acetate, etc. The application of methanesulfonate systems for ceria electrodeposition was reported for the first time in our work (Protsenko et al. 2017); thereby entirely environmentally friendly corrosion inhibitors for iron and its composites have been proposed. An important task was to estimate the effects of protective ceria layers on the photocatalytic performance of immobilized TiO2 particles.

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Ceria layers were deposited on the surface of as-deposited Fe and Fe/TiO2 coatings prepared in methanesulfonate plating bath described above. Degussa P25 TiO2 particles were used in these experiments. The electrolyte for ceria cathodic synthesis contained 0.5 M Ce(CH3SO3)3; its pH value was adjusted to 1.3 (Protsenko et al. 2017). The electrodeposition was carried out in a galvanostatic mode at the temperature of 25  C. After ceria layer electrodeposition, the samples were rinsed in double distilled water and dried in a hot air jet (ca. 80  C) for 3 min. Close inspection of the variation of electrode potential with deposition time and the analysis of literature data allowed us to reveal the following electrochemical and chemical reactions occurring on the electrode surface during ceria layers electrodeposition (Protsenko et al. 2017). First of all, hydrogen evolution reaction and electroreduction of oxygen dissolved in solution take place on the electrode in an aqueous medium (Hamlaoui et al. 2009): 2H 2 O þ 2e ! H 2 þ 2OH  þ



ð10:4Þ

2H 3 O þ 2e ! H 2 þ 2H 2 O

ð10:5Þ

O2 þ 2H 2 O þ 4e ! 4OH 

ð10:6Þ





O2 þ 2H 2 O þ 2e ! 2OH þ H 2 O2

ð10:7Þ

All these reactions are accompanied by an increase in pH value in the near electrode solution layer because methanesulfonic acid is a very strong acid and its solutions do not show any pronounced buffering effect. As a result, the following reactions of Ce(III) hydroxides and hydroxo-complexes formation can occur on the electrode (Hamlaoui et al.2009): Ce3þ þ 3OH  ! CeðOH Þ3

ð10:8Þ

4Ce3þ þ O2 þ 4OH  þ 2H 2 O ! 4½CeðOH Þ2þ 2

ð10:9Þ

In addition, the further oxidation of Ce(III) with the formation of Ce(IV) complex ions and cerium (IV) oxide layer becomes possible: 2Ce3þ þ 2OH  þ H 2 O2 ! 2½CeðOH Þ2þ 2

ð10:10Þ

CeðOH Þ3 þ O2 ! 4CeO2 þ 6H 2 O

ð10:11Þ

½CeðOH Þ2þ 2 ! CeO2 þ 2H 2 O

ð10:12Þ

If the current density exceeded the value of about 3 mA cm2, the electrodeposition experiments showed the formation of a brownish spongy deposit of CeO2 practically without any adhesion to the substrate. Therefore, the electrochemical synthesis of ceria layer was performed in all further experiments at current densities less than 3 mA cm2.

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The cathodic treatment of Fe/TiO2 coatings in methanesulfonate electrolyte leads to the change in their visible surface appearance. If before treating coatings are light gray, they become grayish-blue with a sheen resembling temper colors on oxidized steel surface after the formation of ceria films. At the same time, the scanning electron microscopy images of Fe and Fe/TiO2 electrodeposits presented in Fig. 10.11 do not show any noticeable differences between electrodeposits with and without ceria films. Indeed, similarly to the scanning electron microscopy images given above (see Fig. 10.4b), there is a superficial network of cracks on the surface of pure iron (Fig. 10.11a). In the case of Fe/TiO2 electroplating, the scanning electron microscopy analysis reveals a number of agglomerated flakelike TiO2 particles entrapped by iron matrix (Fig. 10.11b). Thus, no visible specific features of surface morphology appear in scanning electron microscopy images after the cathodic treatment of samples with Fe and Fe/TiO2 coatings in methanesulfonate electrolytes. This behavior may be connected with a very small thickness of the obtained ceria layer. The gravimetric measurements do not show any weight gain of the samples even after 30 min of electrochemical treatment in methanesulfonate solutions. If we take into account the sensitivity of the utilized analytical balance (Δm ¼ 5
105 g), the density of CeO2 (ρ ¼ 7.65 g cm3), and the surface area of the sample (S ¼ 1 cm2), then we can easily estimate the upper limit of the thickness of cerium oxide layer (δ) by means of the following formulae: δ¼

Δm ρ
S

ð10:13Þ

The calculated value of the thickness proved to be close to 60–70 nm. Although the scanning electron microscopy images did not show any changes in the surface morphology of Fe and Fe/TiO2 coatings after cathodic treatment in methanesulfonate electrolyte, the energy-dispersive X-ray investigations clearly revealed the presence of cerium oxides on the surface. Table 10.3 summarizes the typical results of local energy-dispersive X-ray analysis performed at several points over the surface which are marked in Fig. 10.11. The energy-dispersive X-ray analysis indicated the presence of Fe, O, Ti (only for Fe/TiO2 composites), and Ce (only for the coatings treated in Ce(CH3SO3)3 solution). No other elements were detected. As stated above, the flakelike particles with an average size of about several micrometers that can be clearly seen on the surface in scanning electron microscopy images are associated with agglomerated TiO2. According to energy-dispersive X-ray analysis data, the surface in these regions consists essentially of titanium, oxygen, and iron. The surface of Fe/TiO2 electrodeposits in the regions which are free from the flakelike agglomerated particles mainly consists of iron and oxygen and also contains a small amount of titanium. The electrodeposited cerium (in the form of its oxide) is for the most part concentrated in the zones without TiO2 agglomerates (i.e., on the segmental surfaces

282 Fig. 10.11 Scanning electron microscopy images of (a) Fe/TiO2 composite coating, (b) Fe coating with electrodeposited ceria layer, (c) Fe/TiO2 composite coating with electrodeposited ceria layer (0.25 mA cm2, 1800 s), (d) Fe/TiO2 composite coating with electrodeposited ceria layer (1 mA cm2, 1800 s). The composites contain 5 wt.% TiO2 (Degussa P25) (Reproduced from Protsenko et al. 2017)

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Table 10.3 Chemical composition at points marked in Fig. 10.11 (Protsenko et al. 2017) Elements O Ti Fe Ce

Content (at. %) 1 2 64.92 11.77 6.04 0.54 29.04 87.69 – –

3 16.64 – 82.79 0.57

4 61.16 12.87 25.33 0.64

5 14.04 1.52 83.14 1.30

6 65.51 7.55 26.32 0.62

7 12.20 1.01 85.53 1.26

Fig. 10.12 Variation of open circuit potential with time for Fe coating and Fe/TiO2 composite coatings without and with electrodeposited ceria films in 3% NaCl solution at 298 K. Cathodic treatment in 0.5 M Ce(CH3SO3)3 was performed at 1 mA cm2 during 1800 s. The composites contain 5 wt.% TiO2 (Degussa P25) (Reproduced from Protsenko et al. 2017)

corresponding to the “free” iron matrix). The variation of current density has no considerable effect on the cerium content on the surface. Despite a relative small thickness of the synthesized cerium oxide layers, their adhesion to the substrate seems to be satisfactory. At least, according to the obtained energy-dispersive X-ray analysis data, these ceria films cannot be removed from the surface after rinsing by water. The open circuit potential vs. time curves were recorded for Fe coating and Fe/TiO2 composite coatings without and with electrodeposited ceria films in an aggressive environment containing 3% NaCl (Fig. 10.12). The obtained results show a decrease of open circuit potential during a short transition period; then the open circuit potential value reaches practically a steady-state condition. The introduction of titania particles into the iron matrix (i.e., electrodeposition of composite coatings) results in an increase of open circuit potential indicating an enhancement of corrosion stability. An increase in open circuit potential means that the improvement in corrosion resistance is provided not only by the formation of a protective physical barrier, which partially blocks the electrode surface and is composed of “inert” TiO2 particles, but also by the formation of corrosion microcells in which TiO2 acts as cathode and Fe matrix as anode.

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A further shift of corrosion potentials to nobler values is observed when cerium oxide layers have been deposited on the composite surface. An increase of open circuit potential of the samples, modified by ceria film, is attributed to the occurrence of another cathodic process in addition to the oxygen and Ti(IV) electroreduction reactions and the hydrogen evolution reaction. Indeed, the reduction of CeO2 as a very effective depolarization process can occur at the corrosion potentials according to the following reactions (Stoychev 2013): CeO2 þ 2H þ ! CeðOH Þ2þ 2

ð10:14Þ

 þ 2CeðOH Þ2þ 2 þ 2e ! Ce2 O3 þ H 2 O þ 2H

ð10:15Þ

The released Ce(III) compounds can be again oxidized to Ce(IV) which provides the so-called self-healing properties of ceria thin films (Guergova et al. 2015). To obtain comprehensive data for the kinetics of corrosion process, the electrochemical impedance spectroscopy method was used. Nyquist plots (imaginary values of electrode impedance, Zimag, versus real values of electrode impedance, Zreal) were obtained at open circuit potential for the “pure” Fe coatings and Fe/TiO2 electrodeposited composites with and without ceria layer on their surface (Fig. 10.13). Nyquist plots of as-deposited iron and iron/titania composite coatings without CeO2 film look like conventional depressed semicircles which indicate that

Fig. 10.13 Nyquist plots of Fe and Fe/TiO2 composite coatings with and without modification by ceria layer in 3% NaCl solution at open circuit potential and temperature of 298 K. Cathodic treatment in 0.5 M Ce(CH3SO3)3 was performed at 1 mA cm2 during 1800 s. The composites contain 5 wt. % TiO2 (Degussa P25). The symbols denote the measured (experimental) impedance spectra and solid lines represent fitted results (Reproduced from Protsenko et al. 2017)

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Fig. 10.14 Electrical equivalent circuits modeling the solid electrode/solution interface: (a) for as-deposited coatings without surface ceria modification and (b) for coatings modified by ceria layer (Reproduced from Protsenko et al. 2017)

the electrochemical reaction was controlled by the charge transfer step and occurred on inhomogeneous surface. Nyquist plots recorded for iron and iron/titania composite coatings with electrodeposited film of cerium oxide are more complicated. In addition to capacitive loop at higher frequencies, the electrochemical impedance spectra show diffusion behavior at lower frequencies and indicate Warburg impedance (Zhou and Shen 2013). Therefore, it can be concluded that the impedance values measured at the high frequencies range only responded to the surface characteristics of the corroded Fe matrix, whereas the precipitation of CeO2 films forms a thin passive barrier layer which is responsible for diffusion behavior which resulted in Warburg resistance at lower frequencies (Zhou and Shen 2013). Taking into account the above arguments, two typical electrical equivalent circuits were used to fit experimental data as shown in Fig. 10.14. The first equivalent circuit (Fig. 10.14a) contains polarization resistance of the electrochemical reaction (Rct), constant phase element (CPE), and ohmic resistance of solution (Rsol). This electrical equivalent circuit was used for the description of the corrosion behavior of as-deposited coatings without surface ceria modification. The second equivalent circuit (Fig. 10.14b), used for the modeling of electrochemical impedance spectra recorded for the corrosion of the coatings with CeO2 layer, additionally contains the so-called finite Warburg impedance (Danilov et al. 2014). The experimental data in Fig. 10.13 are presented as symbols and the continuous lines are obtained by curves fitting using the electrical equivalent circuits shown in Fig. 10.14. Some of the calculated kinetic parameters are summarized in Table 10.4. In the theory of electrochemical impedance spectroscopy, the introduction of constant phase elements into the electrical equivalent circuits is related to the energy and geometric heterogeneity of the electrode surface. The higher the value of n, the

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Table 10.4 Calculated electrochemical impedance parameters (Protsenko et al. 2017) Electrode Fe coating Fe coating modified by CeO2 Fe/TiO2 coating Fe/TiO2 coating modified by CeO2

Rsol (Ω) 9.74 8.01 5.43 6.31

Rct (Ω cm2) 354.1 587.9 443.6 611.6

n 0.603 0.583 0.569 0.605

Fig. 10.15 Kinetic curves of the degradation of methyl orange dye in aqueous solution: (1) in the dark, (2) under ultraviolet irradiation without photocatalyst, (3) under ultraviolet irradiation with Fe/TiO2 modified by ceria layer, and (4) under ultraviolet irradiation with Fe/TiO2 (not modified). The composites contained 5 wt. % TiO2. In the insert: first-order linear transforms logC ¼ f(τ). (Reproduced from Protsenko et al. 2017)

smaller is the extent of fractal roughness of the electrode (Mulder and Sluyters 1988). As can be seen in Table 10.4, the values of parameter n are appreciably less than unity. This means a relatively high extent of fractal roughness of the electrode and, in the general case, could be favorable to catalytic performance. In the adopted electrical equivalent circuits, the value of Rct can be considered as a parameter which defines the corrosion resistance of a coating in a general way. It is clear that the introduction of TiO2 particles into the iron matrix results in some improvement in the corrosion stability. However, the electrodeposition of ceria layers has a far greater influence on the values of Rct both for “pure” iron coatings and for Fe/TiO2 ones. All these results indicate that the modification of the deposits surface by cerium oxide films considerably enhances the corrosion stability. It is important to ascertain the effect of cerium oxide films upon the photocatalytic activity of composite electrodeposits. The kinetic curves for the process of methyl

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orange decolorization are shown in Fig. 10.15. These data were obtained in aqueous solutions without the addition of NaOH (in contrast to those presented in Fig. 10.10). Similarly to the behavior observed in solutions of 0.1 M NaOH, methyl orange molecules do not undergo spontaneous decomposition in the dark (without ultraviolet illumination) both in the presence of the Fe/TiO2 composite immersed in the dye solution and in its absence. In addition, photocatalytic properties of Fe coatings modified by ceria layers were not observed under the condition of ultraviolet radiation. The destruction of methyl orange molecules occurs in solutions exposed to ultraviolet radiation and this process considerably accelerates in the presence of the Fe/TiO2 composite catalyst (see Fig. 10.15). The reduction in concentration of dyes during their photodegradation often obeys the pseudo-first-order kinetics as kinetic curves 2 and 3 are well linearized in the coordinates of Eq. (10.3) (see the insert in Fig. 10.15). The slopes of these lines give the values of apparent rate constants which are equal to 0.0011 and 0.0018 min1 for the dye destruction without catalyst (i.e., photolysis) and in the presence of the Fe/TiO2 photocatalyst, respectively. A kinetic curve was recorded that characterizes the photocatalytic destruction of methyl orange dye in the presence of Fe/TiO2 composite catalyst without its cathode treatment in cerium (III)-based electrolyte (Fig. 10.15, curve 4). It turned out that the obtained kinetic curve has a minimum, the occurrence of which is somewhat unexpected. In order to elucidate this behavior, we measured the time dependences of absorbance of water and methyl orange dye solutions contacting with Fe/TiO2 coatings in the dark (Fig. 10.16). As can be seen, absorbance does not change with time in the case of Fe/TiO2 composites modified by CeO2 both in water and in a neutral solution of methyl orange dye. However, as-deposited Fe/TiO2 coating without cathode treatment shows a growth in measured absorbance about 15 min after the beginning of experiment. This phenomenon is due to a rapid corrosion damage of unprotected iron matrix in water. The corrosion products in the form of yellowish iron hydroxide sol are accumulated in liquid which are also easily visible. Evidently, the absorbance associated with the presence of iron hydroxide sol distorts the results of the spectral determination of dye content and causes an observed increase in light absorption. Taking into account these observations, one can conclude that the appearance of an ascending segment of curve 4 in Fig. 10.15 (after reaching a minimum) is caused by the accumulation of Fe matrix corrosion products in a solution rather than by the actual changes in dye concentration. In addition, the data presented in Fig. 10.16 confirm that the modification of Fe/TiO2 composites surface by ceria layer provides a good protection against corrosion, whereas composites without protective films rapidly corrode in aqueous medium even at pH close to neutral value. It should be observed that the kinetic curves recorded for the Fe/TiO2 composite photocatalysts, which were modified by ceria layers electrodeposited at different current densities (0.25 and 1 mA cm2), coincide with each other. If we compare the initial segments in curves 2 and 4 (Fig. 10.15) where corrosion products of unmodified Fe/TiO2 surface do not strongly distort the results of the

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Fig. 10.16 Variation of absorbance with time for (1), (2) pure water and (3), (4) for aqueous solution of methyl orange dye in which Fe/TiO2 coatings (2), (4) without any surface modification and (1), (3) modified by CeO2 were immersed. Wavelength λ ¼ 490 nm, absorption layer thickness l ¼ 30 mm. (Reproduced from Protsenko et al. 2017)

spectral measurements, then we can easily see that these segments of the curves practically coincide. This means that the cathode treatment in cerium (III) methanesulfonate solutions does not affect the photocatalytic activity. It was interesting to establish the effect of the modification by ceria layer upon the photocatalytic activity of Fe/TiO2 composites in an alkaline solution where iron does not corrode and, hence, the corrosion products will not interfere with spectral measurements. Figure 10.17 presents the kinetic curves of the degradation of methyl orange dye under ultraviolet irradiation in 0.1 M NaOH solution in the presence of the Fe/TiO2 photocatalysts with and without modification by CeO2 film. The lines are given in the coordinates of log C vs. time. As can be seen, these lines practically coincide. The corresponding apparent rate constant was calculated to be 0.0128 min1. The results for the photolysis of methyl orange dye (i.e., in the absence of Fe/TiO2 composite) are shown for comparison. The apparent rate constant of photolysis is equal to 0.0054 min1. Two conclusions can be drawn on the basis of these findings. First, the rates of photolysis and photocatalytic destruction increase with higher pH’s under other identical conditions. According to earlier report (Guillard et al. 2005), an enhanced photocatalytic efficiency of TiO2 at basic pH is mainly attributed to an increase in the surface density of TiO adsorption sites rather than to an increase in the generation of more •OH radicals formed by the reaction of OH with positive holes in semiconductor. Second, the electrodeposition of a cerium oxide film on the surface

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Fig. 10.17 Kinetic curves of the degradation of methyl orange dye in 0.1 M NaOH: (1) under ultraviolet irradiation without photocatalyst, (2) under ultraviolet irradiation with Fe/TiO2 (not modified), and (3) under ultraviolet irradiation with Fe/TiO2 modified by ceria layer. The data are presented as first-order linear transforms logC ¼ f(τ). (Reproduced from Protsenko et al. 2017)

of Fe/TiO2 composites has no effect on the photocatalytic performance both in alkaline solutions and in solutions with pH’s close to neutral values. At the same time, it is well-known that the efficiency of TiO2 photocatalytic materials can be improved by their doping with other elements, including cerium (Chen and Mao,2007). However, neither the occurrence of photocatalytic activity in the visible light region nor the improvement of ultraviolet light-induced photocatalytic properties after electrochemical treating the composites in electrolytes containing Ce(III) ions have been detected. Probably, the adopted procedure of cathodic treatment does not lead to the doping of TiO2 because the electrodeposition of ceria films is performed after the synthesis of titania particles and, therefore, cannot change the chemical composition, crystal, and electronic structures which are crucial issues in TiO2 doping (Chen and Mao 2007). The photocatalytic activity of TiO2 particles immobilized on composite electrodeposits modified by protective ceria layer was evaluated not only in the reactions of photochemical degradation of methyl orange but also toward the photochemical destruction of methylene blue dye in water. Methylene blue dye is often used in testing of the performance of various types of photocatalysts (Lachheb et al. 2002). The plot of change in the concentration of methylene blue dye with the time of irradiation is shown in Fig. 10.18. The kinetics of methylene blue dye photodegradation followed the pseudo-first reaction order. The calculated value of the apparent rate constant was stated to be 0.0056 min1 and independent of the initial concentration of the dye that is typical of the first-order reaction.

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Fig. 10.18 Kinetic curves of the photocatalytic degradation at various initial concentrations of methylene blue dye in solution. In the insert: first-order linear transforms logC ¼ f(τ). (Reproduced from Protsenko et al. 2017) Table 10.5 Calculated values of apparent constant and half-life for the reaction of methyl orange and methylene blue dyes degradation on Fe/TiO2 photocatalyst under UV irradiation (Protsenko et al. 2017) System Methyl orange dye in aqueous solution (at natural pH) Methyl orange dye in 0.1 M NaOH aqueous solution Methylene blue dye in aqueous solution (at natural pH)

k (min1) 0.0018 0.0128 0.0056

τ1/2 (min) 385.1 54.2 123.8

In order to contextualize the obtained data on methyl orange and methylene blue dyes photodegradation by using the composite Fe/TiO2 photocatalyst, the calculated values of apparent rate constant and half-life of organic dyes are summarized in Table 10.5. The half-life, τ1/2, for the processes of the pseudo-first reaction order was evaluated by the following expression: τ1=2 ¼

ln 2 k

ð10:16Þ

As follows from the obtained data, Fe/TiO2 composite coatings can be considered as a relatively efficient photocatalyst with respect to the reaction of destruction of the methyl orange and methylene blue dyes in aqueous solutions under exposure to ultraviolet radiation. As concerns the mechanism of heterogeneous photocatalysis taking place at the surface of TiO2 particles immobilized on iron matrix, it evidently satisfies the

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recognized basic mechanism which involves the steps of the absorption of a photon, the excitation of an electron with simultaneous generation of a positive hole, the further oxidation of water molecules with the formation of •OH radicals, and the reduction of molecular oxygen with the formation of superoxide radical anions, hydroperoxyl radicals, and hydrogen peroxide (Pelaez et al. 2012; Chen and Mao 2007). All these reactive oxygen-containing species can contribute to the oxidative pathways such as the degradation of organic pollutants. It is known that TiO2-/ ultraviolet-based photocatalysis provides practically total mineralization of organic dyes (such as methyl orange and methylene blue) and their final oxidation into CO2, SO42, NO3, and H2O (Lachheb et al. 2002; Sonawane et al. 2004; Houas et al. 2001).

10.3

Photocatalytic Ni/TiO2 Composites Electrodeposited from Electrolyte Based on a Deep Eutectic Solvent

Composite coatings with immobilized TiO2 particles can be synthesized from different types of plating baths. Commonly, the electrodeposition of composite materials is performed using aqueous electrochemical systems (Low et al. 2006; Walsh and Ponce de Leon 2014). Although aqueous colloidal plating baths are relatively simple, cheap, and available, they have a number of drawbacks. For instance, the processes of aggregation of dispersed phase (coagulation and flocculation) can rapidly occur in water solutions (Vasil’eva et al. 2016; Protsenko et al. 2015a; Danilov et al. 2016) which results in phase separation (sedimentation) and deterioration of the properties of the obtained composites. Recently, an alternative to “usual” aqueous plating baths has been developed; it is based on deep eutectic solvents (Abbott and McKenzie 2006; Abbott et al. 2008, 2013; Smith et al. 2014). Deep eutectic solvents consist of a eutectic mixture of organic and inorganic compounds having a melting point significantly lower than that of either individual component (Smith et al. 2014). Deep eutectic solvents are now considered as a new and promising generation of room temperature ionic liquids; they can be used in various fields of application. Deep eutectic solvents are distinguished by a great number of excellent properties, such as wide electrochemical potential windows, high solubility of metal salts, relatively high conductivity, negligible vapor pressure, easy accessibility, and environmental safety (Smith et al. 2014). Owing to these important advantages, deep eutectic solvents attract considerable attention in many fields of research and industry, especially in electrochemistry and electroplating. To date, a number of works have been published in which different aspects of electrodeposition of various metals and alloys were reported (Ghosh and Roy 2014, Ghosh and Roy 2015; Costovici et al. 2016; Abbott et al. 2015; Kityk et al. 2017; Protsenko et al. 2015b; Bobrova et al. 2016). There are also several published works devoted to composites electrodeposition from deep eutectic

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solvent-based plating baths (Abbott et al. 2009, 2012; Li et al. 2015, 2016; Pereira et al. 2017). To the best of our knowledge, the preparation and characterization of Ni/TiO2 electrodeposited composites from deep eutectic solvent-based electrolytes have not been reported yet. At the same time, the use of these electrochemical systems can be a very promising approach to synthesize Ni/TiO2 electrodeposited composites with photocatalytic activity. The main results of our preliminary unpublished study on fabrication of photocatalytic Ni/TiO2 composites deposited from deep eutectic solvent-based bath are presented below.

10.3.1 Electrodeposition of Nickel-Titania Composites Using an Electrolyte Based on Deep Eutectic Solvent A nickel electroplating bath based on a deep eutectic solvent which is commercially known as ethaline (Abbott et al. 2008, 2013, 2015; Kityk et al. 2017) was used where deep eutectic solvent was composed of a eutectic mixture of choline chloride and ethylene glycol in a molar ratio of 1:2, respectively. The preparation of the nickel plating bath has been described in detail (Kityk et al. 2017). The content of Ni(II) in electrolyte was 1 mol dm3. TiO2 nano-powder (Degussa P 25, Evonik) was used without any pretreatment. To obtain colloidal electrolyte for composite electrodeposition, a weighed portion of TiO2 Degussa P 25 nano-powder was introduced directly into the electroplating bath maintaining the concentration level of TiO2 in the bath as 1, 2, 5, 10, or 15 g dm3. Immediately after that, the electrolyte was stirred for 1 h by mechanical agitator and then ultrasonically treated with an ultrasonic disperser for 1 h (22.4 kHz, 340 W dm3) to reach uniform distribution of colloidal particles in the plating bath. Figure 10.19 shows the effect of the concentration of TiO2 nano-powder in the plating bath on the titania content (determined by X-ray fluorescence analysis) in the electrodeposited composites. A gradual increase in the content of titania particles in coatings was observed with increasing the concentration of TiO2 in colloidal electrolyte. This behavior resembles to that observed for “usual” aqueous plating baths (Spanou et al. 2013; Sknar et al. 2017) and is well within the framework of Guglielmi’s model of composites electrodeposition (Guglielmi 1972). The deep eutectic solvent containing plating bath under study exhibited excellent dispersion stability. Coagulation and sedimentation even after ceasing electrolyte stirring (at least, during 1 week of observations) were not noticed.

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Fig. 10.19 The effect of titania concentration in the plating bath on TiO2 content in composites electrodeposited at stirring rate of 500 rpm and current density of 10 mA cm2

10.3.2 Surface Morphology and Microstructure of NickelTitania Composites Scanning electron microscopy images of pure Ni coatings exhibited a relatively even surface with a nodular structure where the average diameter of nodules was about 0.5–2 μm (Fig. 10.20a). There was a small amount of defects (small protrusions) on the surface, however, cracks were not observed. The energy-dispersive X-ray spectroscopy revealed that the surface of pure nickel coatings is chiefly made of Ni, and the presence of small amount of such elements as O, C, and N was detected too. When TiO2 nano-powder was added into the electrolyte, a number of spherical nodules appeared on the surface (Fig. 10.20b). A typical energy-dispersive X-ray spectrum of Ni/TiO2 composite coating is shown in Fig. 10.21. The presence of Ni, Ti, O, C, and Cl on the surface was observed. Trace amounts of oxygen, carbon, and chlorine can be associated with the inclusion of some organic components of electrolyte which were adsorbed on the electrode surface. The typical X-ray diffraction pattern of Ni/TiO2 composite coating deposited from the electrolyte containing deep eutectic solvent is shown in Fig. 10.22. The reflections of face centered cubic nickel and anatase TiO2 planes were detected. Due to a small amount of TiO2 in coatings, the corresponding reflections from anatase phase are weak; they could be seen quite well on X-ray diffraction patterns only for

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Fig. 10.20 Scanning electron microscopy images of (a) pure nickel and (b) Ni/TiO2 composite coatings electrodeposited at TiO2 nano-powder concentration of 10 g dm3. All coatings were deposited at stirring rate of 500 rpm and current density of 10 mA cm2

samples obtained from the plating baths with the highest content of TiO2 particles (10 and 15 g dm3). The broad half-width values of X-ray diffraction profile of Ni indicate the nanocrystalline structure of the metallic matrix. The average crystalline sizes of the deposits were calculated based on the full width at half maximum of X-ray peaks of diffraction using Scherrer’s equation. The calculated values of crystalline sizes were close to ca. 10 nm.

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Fig. 10.21 Typical energy-dispersive X-ray spectrum of Ni/TiO2 composite coating electrodeposited at stirring rate of 500 rpm, current density of 10 mA cm2, and TiO2 nanopowder concentration of 10 g dm3

Fig. 10.22 Typical X-ray diffractogram of Ni/TiO2 composite coating electrodeposited from deep eutectic solvent-based electrolyte at stirring rate of 500 rpm, current density of 10 mA cm2, and TiO2 nano-powder concentration of 15 g dm3

10.3.3 Photocatalytic Performance of Nickel-Titania Composites The photocatalytic properties of TiO2 particles immobilized on Ni/TiO2 composite electrodeposits were tested in the reaction of photochemical degradation of methylene blue dye in water under the action of ultraviolet radiation. Figure 10.23 represents the kinetic curves of decolorization process which follow the pseudo-first-order

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Fig. 10.23 Kinetic curves of methylene blue dye degradation in water under ultraviolet irradiation in the absence and in the presence of Ni/TiO2 photocatalyst. The data are presented as first-order linear transforms log C vs. time. Coatings were electrodeposited at stirring rate of 500 rpm and current density of 10 mA cm2 Table 10.6 Calculated values of apparent rate constant for the reaction of methylene blue dye degradation under ultraviolet irradiation. The coatings were electrodeposited at stirring rate of 500 rpm and current density of 10 mA cm2 Content of TiO2 nano-powder in the plating bath (g dm3) 0 (without photocatalyst, i.e., photolysis) 10

k (min1) 0.0023 0.0111

kinetics. The slopes of the straight lines plotted in the coordinates of logarithm of the methylene blue dye concentration vs. time allow calculating the apparent rate constants of decolorization process (Table 10.6). The determined value, 0.0111 min1, is comparable to those obtained in the case of Fe/TiO2 photocatalysts (see Table 10.5).

10.4

Conclusion

The electrodeposition of composite coatings with titania particles embedded in a metallic matrix is a promising, effective, and available way to immobilize TiO2 heterogeneous photocatalyst. The electrodeposition technique enables easy and flexible control of the physicochemical properties of deposited coatings. In the general case, electroplating can provide a surface with high microhardness and

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corrosion resistance as well as good bonding between the metal matrix and TiO2 particles. Composite coatings can be easily repaired after possible mechanical damage, wear, erosion, and corrosion. During the gradual degradation of such composite heterogeneous photocatalysts, still newer layers appearing from the bulk will be exposed on the surface, thereby maintaining the photocatalytic activity. This work showed that iron electrodeposited matrix is a cheap, available, and environmentally safe anchor for TiO2 particles; additionally, it possesses satisfactory mechanical properties. The electrochemical synthesis of Fe/TiO2 composite coatings is reported using environment-friendly methanesulfonate electrolytes. The fabricated Fe/TiO2 composites manifested photocatalytic activity toward the reaction of photochemical destruction of methyl orange and methylene blue dyes in aqueous solutions under exposure to ultraviolet radiation. The electrodeposition of thin ceria film on the surface of Fe matrix was found to improve the corrosion resistance. This creates favorable conditions for the practical application of such photocatalysts for the degradation of organic pollutants in aqueous media with natural pH (i.e., close to neutral values). Without protective cerium oxide films, the composite Fe/TiO2 coatings undergo fast corrosion damage. It is important that the electrodeposited layers of cerium oxides do not reduce the photocatalytic activity of TiO2 aggregates embedded in iron matrix. This chapter also reports the preparation of photocatalytic Ni/TiO2 composite coatings using an electrochemical system on the basis of deep eutectic solvents, a new generation of room temperature ionic liquids. In the past two decades, deep eutectic solvents have been considered as promising alternatives to conventional aqueous systems and organic solvents and to their predecessor “usual” ionic liquids in numerous and diverse fields of application. Our preliminary results indicate that the content of titania in coatings electrodeposited from the choline chloride-based plating bath can reach ca. 2.35 wt. %. The Ni/TiO2 composite coatings were shown to manifest an appreciable photocatalytic activity toward the reaction of photochemical degradation of methylene blue organic dye in water solution. Deep eutectic solvent-based plating bath exhibited excellent dispersion stability without any stabilizing additives, and no signs of aggregation and sedimentation were observed. Thus, the plating bath on the basis of deep eutectic solvent has a definite advantage in this respect over “common” aqueous systems.

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Chapter 11

Spinning Disk Reactor Technology in Photocatalysis: Nanostructured Catalysts Intensified Production and Applications Javier Miguel Ochando-Pulido, Marco Stoller, Luca Di Palma, A. Martínez-Férez, and Giorgio Vilardi

Contents 11.1 11.2 11.3

Introduction: Process Intensification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spinning Disk Reactor Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nano-photocatalyst Production by Spinning Disk Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.1 TiO2 Production and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.2 Magnetite Production and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.3 MgO Production and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.4 Hydroxyapatite Production and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4 About the Use of Photocatalytic Spinning Disk Reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract The use of photocatalysis in environmental remediation processes has become more important in the last decade, mainly due to the notable efforts made by researchers in this field. The photocatalytic process requires a semiconductor material (photocatalyst), usually a metal oxide, which can be activated through the energy transported by ultraviolet light or visible light waves. The activated photocatalyst generates active compounds, such as hydroxyl radicals and superoxide ion, able to degrade very recalcitrant and non-biodegradable compounds present on the catalyst surface or in the liquid medium. The efficiency of the pollutant removal process is affected by various factors related to the employed photocatalyst, such as mean dimension, size distribution, physical structure and energy required for the activation. The photocatalyst characteristics are strongly dependent on the production process, and several researchers have developed new intensified production J. M. Ochando-Pulido (*) · A. Martínez-Férez Department of Chemical Engineering, University of Granada, Granada, Spain e-mail: [email protected] M. Stoller · L. Di Palma · G. Vilardi Department of Chemical Engineering Materials Environment, Sapienza University of Rome, Rome, Italy © Springer Nature Switzerland AG 2019 Inamuddin et al. (eds.), Nanophotocatalysis and Environmental Applications, Environmental Chemistry for a Sustainable World 29, https://doi.org/10.1007/978-3-030-10609-6_11

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processes that require particular equipment. In the present chapter, the production of nanostructured catalysts in a continuous Spinning Disk Reactor is discussed. The main features of Spinning Disk Reactor technology are reported and analysed, i.e. rotational velocity, disk diameter, disk surface material and roughness, focusing on the production of nanoparticles to be used in the photocatalytic application, in view of the process intensification of photocatalysis application in the field of environmental remediation. A general overview about process intensification and its application to chemical engineering is presented, and the advantages offered by Spinning Disk Reactor technology, in terms of an increase of process efficiency due to the misinformation of operative conditions in reactors, are illustrated. Basing on the Spinning Disk Reactor characteristics and operative conditions, nanoparticle production by Spinning Disk Reactor compared to conventional technologies and the current application of this technology to selected nanoparticles (titania, magnetite, MgO and hydroxyapatite), synthesis is discussed. Spinning Disk Reactor technology allows to produce active semiconductor particles, characterized by a mean size significantly below 100 nm and with a narrow unimodal distribution, improving the quality of these products in comparison with those produced through conventional processes and equipment. Finally, the application of vertical and horizontal Spinning Disk Reactor configuration to the degradation of refractory compounds by photocatalysis is reviewed, aiming at evaluating process efficiency and the produced nanoparticle characteristics, to assess the key parameters and the limiting factors of the technology. Keywords Process intensification · Spinning disk reactor · Intensified production · Titania · Iron oxide · Magnesium oxide · Environmental remediation · Recalcitrant pollutants · Innovative equipment · Intensified photocatalysis

11.1

Introduction: Process Intensification

The significant chemical process industry evolution that occurred in the last 30 years was strictly related to the development of new intensified processes, characterized by remarkable efficiency and innovative equipment (Becht et al. 2009). Process intensification is considered as one of the most valuable development pathways for the chemical process industry leading, generally, to an efficiency increase and plant size decrease (Gogate 2008). Actually, the process intensification term itself underwent to the same above-mentioned evolution of chemical process industry: this was due mainly to the increasing world demand for clean green energy and products, space and safety. At the beginning, the term process intensification was related to “devising exceedingly compact plant which reduces both the ‘main plant item’ and the installations costs”, focusing only on the development of innovative processes which could led to the reduction of plant size, installation costs and to the process efficiency increase (Ramshaw 1983). Subsequently, various different process intensification definitions were proposed in the literature, and, in 2009, Van Gerven and

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Stankiewicz reported six of them, underlining the evolution of the concept itself (Van Gerven and Stankiewicz 2009). In particular, one of the most complete definition has been proposed by the European Roadmap for Process Intensification (ERPI): “process intensification provides radically innovative principles in process and equipment design which can benefit (often with more than a factor two) process and chain efficiency, capital and operating expenses, quality, wastes, process safety and more”. European Roadmap for Process Intensification definition considered the “benefits”, achievable from both producers (cost, efficiency), consumers (quality, cost), environment (wastes, plant size) and operatives (process safety). All these aspects should be taken into account to well analyse the several process intensification definitions reported in the literature. In fact, depending on the author’s experience and knowledge, some of the above-mentioned process intensification features and consequences were stressed. In particular, processes functional integration is the base of some of the previous reported definitions (Dautzenberg and Mukherjee 2001). The process integration is possible by means of multifunctional reactors, particular devices that are designed to let several functions (reactions and separation processes) occur in a simultaneous way; the main purpose is the optimum integration of mass, heat and momentum transfer within a single reactor vessel (Stitt 2004). For others, the size reduction of devices represents the process intensification fundament, considering that all those operations driven by viscous force, molecular diffusion, surface tension and conduction heat transfer can be remarkably enhanced by conducting them in micro-spaces (Agar 1999; Mae 2007). Looking at these worthwhile definitions, it can be assumed that all of them contribute to defining process intensification concept. Van Gerven and Stankiewicz, to clarify it, proposed four fundamental approaches of process intensification in four domains: spatial, thermodynamic, functional and temporal (Van Gerven and Stankiewicz 2009). The first one concerns the effectiveness maximization of intraand intermolecular events (Bakker 2004), led by the control of reactor geometry, energy, the frequency of molecule collisions and their mutual orientation (Janiak and Kofinas 2007). The second concerns the uniformation of operative conditions for all molecules inside the reactor vessel, implying that the same residence time, temperature gradient and meso-micromixing conditions can lead to obtain uniform products (Wu et al. 2007). The third deals with the optimization of driven forces and the specific surface area to which these forces are applied, meaning that the transport rates across interfaces must be enhanced to intensify a process. The latter one regards the synergistic effects of partial processes maximization. All these intensified approaches may play a fundamental role in the production of nano-photocatalysts and in the development of new photoreactors (Stoller et al. 2011; Vaiano et al. 2014). The manufactured materials at the nanoscale (mean size 1–100 nm) are characterized by peculiar physicochemical characteristics, due to their dimension. In this dimensional range, the material properties such as melting point, fluorescence, electrical conductivity, magnetic permeability and chemical/photochemical reactivity are size-dependent (Rao et al. 2002). This phenomenon is known as “quantum size effect” (QSE) and is due to the decreased volume of the material cluster at the nanoscale that leads to a remarkable or negligible discretization of the quantum

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levels in function of the cluster dimension (Halperin 1986). A profitability result deriving from quantum size effect is the possibility to tune/customize the nanomaterial properties selecting the cluster to mean dimension optimizing the production process. In general, this requires bottom-up production approaches by chemical synthesis, and procedures are developed mostly at lab scale batch-wise and with a very low yield, thus yet with a limited industrial interest. The use of a Spinning Disk Reactor for the production of nano-photocatalysts has already demonstrated to be a suitable instrument to cover all the four process intensification fundaments, leading to the synthesis of nanomaterials with improved photoactivity (Ruzmanova et al. 2013) and chemical reactivity (Vilardi et al. 2017). In addition, in the last decade, researchers have investigated the combination of photoreactor with Spinning Disk Reactor technology, leading to a significant intensification of photocatalysis efficiency and rate of recalcitrant compounds abatement, such as azo dyes (Zhang et al. 2011). In this chapter, the main characteristics of Spinning Disk Reactor technology are analysed and discussed, focusing on the application of these particular reactors for photocatalyst synthesis and on photocatalytic process intensification, in particular, to solve environmental issues.

11.2

Spinning Disk Reactor Technology

In the beginning, the Spinning Disk Reactor has commercially developed for the intensification of gas-liquid reactions (Jachuck et al. 1997), based on a patent from 1925 (Buhtz 1925). Nowadays, Spinning Disk Reactor represents one of the most promising process intensified technologies for the heat and mass transfer rate enhancement, obtained through centrifugal fields generated in the thin liquid film on the reactor disk by the rotation action (Boodhoo and Jachuck 2000). In this reactor, the reactive phase is fed on the rotating disk from where it flows radially outwards as a thin film over the disk (Fig. 11.1). The high centrifugal fields provide several benefits, such as those reported below: • The extremely high-gravity fields generated can produce a thin film (0.05–0.5 mm), where heat transfer, mass transfer and mixing rates are greatly intensified. The short path lengths and the high surface area per unit volume provide the opportunity for rapid molecular diffusion and enhanced heat transfer, even on scale-up (Fig. 11.2). • Rotating equipment can itself be considered as “self-cleaning equipment” towards fouling phenomena, guaranteeing at the same time maximum exposed area during the process. This is particularly important in those processes where the solid content of process fluid may pose fouling issues. • The very short and controllable residence times achieved under the centrifugal action ensure a non-significative degradation risk for heat-sensitive materials employed.

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Fig. 11.1 Scheme of a Spinning Disk Reactor (Ramshaw and Cook 2005). The reactor may present one or more feed point for liquid and gas phases, the disk could be provided of an internal heat exchanger fluid circulation, and usually the external walls are temperature controlled. The reactive phase moves on the disk rotating surface generating a thin liquid film where the reactions occur

Fig. 11.2 Liquid film in a Spinning Disk Reactor (Boodhoo and Harvey 2013). The thin liquid film allows to enhance the mass transfer rate among the different phases present in the specific process. The centrifugal force tends to throw out the liquid from the disk centre, where the film thickness is higher

The Spinning Disk Reactor configuration and characteristics vary in function of the required process yield and the type of reaction to be developed. Usually disk diameter varies in a range from 0.1 to 1 m and the rotational speed from 100 to 4000–5000 rpm (higher disk

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diameter lower the maximum rotational speed); the disk surface can be smooth, grooved and meshed, depending on the specific application, as well as the disk material that generally is stainless steel but can be glass, brass (to enhance heat transfer) or Teflon (to reduce fouling issues). Typical operating temperature and pressure lay in the ranges
20 to 300  C and 1–10 bar, respectively, whereas feed flowrate is strictly dependent on disk characteristics and speed (i.e. optimum film thickness and film breakdown) (Boodhoo and Harvey 2013). Extensive research has been done with respect to the gas-liquid mass transfer rate (Aoune and Ramshaw 1999; Sisoev et al. 2008), liquid-solid mass transfer rate (Burns and Jachuck 2005; Peev et al. 2006) and the heat transfer rates (Harmand et al. 2013). In reactive precipitation reactions, Spinning Disk Reactor processing has already been shown to improve precipitation methods of organic and inorganic nanomaterials (Van Gerven and Stankiewicz 2009). For instance, in the gas-liquid precipitation of calcium carbonate (Hetherington 2006) and liquid/liquid precipitation of barium sulphate (Cafiero et al. 2002), significantly smaller crystals with a narrower size distribution than the conventional stirred tank technique have been shown to be feasible. Among the suitable Spinning Disk Reactor characteristics for nanoparticle production by precipitation methodology, the uniform and rapid micromixing environment generation on the rotating disk represents one of the most fundamental; the micromixing conditions provide better control on formation and local distribution of supersaturation in the liquid film, influencing nanoparticle nucleation and crystal growth kinetics (which is function of molecular diffusion phenomena to the growing crystals) (Gunn and Murthy 1972).

11.3

Nano-photocatalyst Production by Spinning Disk Reactor

The practice of producing nanoparticles by Spinning Disk Reactor was nowadays tested on a wide range of different materials and offers many advantages compared to other techniques such as: • • • • • • • •

The requirement of small-medium volume and mild T process Limited equipment cost Nanoparticle aggregation minimized by the use of additives High controllability of modal size of the product Production of tailored and/or doped nanoparticles Scale-up possibilities due to the modularity of the equipment Continuous mode operation High purity achieved by a bottom-up approach

The only limit to the technology is the existence of a suitable chemical synthesis route completely in the liquid phase to achieve the chemical precipitation of the product.

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In the next sections, some examples of nanoparticles successfully produced by means of a Spinning Disk Reactor are reported.

11.3.1 TiO2 Production and Applications Titanium dioxide (TiO2) is a fine white powder and finds a market in a wide range of industrial applications comprising electronics and photocatalytic applications, in paint, food and cosmetics industries (Weir et al. 2012). It can offer very high whiteness and opacity in paper, paints and plastic applications. TiO2 provides with a considerable hiding capacity, that is, with a superior capability to cover up certain compounds more efficiently than any other white pigment. In fact, currently, TiO2 is certainly the most widely used material in paints and plastic industrial sectors for these purposes (Gázquez et al. 2014; Middlemas et al. 2013; RSC – Advancing the Chemical Sciences 2016). TiO2 singular characteristics rely on its refractive index, which indicates the capability for light scattering and bending. In this regard, the TiO2 refractive index is beyond that of diamond (Fattakhova-Rohlfing et al. 2014). TiO2 is a powerful photocatalyst as well, as it can degrade the vast majority of organic pollutants upon exposure to sunlight, and as a result, it is being industrially exploited for a large variety of finalities, such as self-cleaning materials and ceramic blocks and bricks, air and heater treatment and tumour abatement. Among the most interesting researches, it is worth highlighting the use TiO2 catalytic characteristics for removal of nitrogen oxides from the air, cleaving it into compounds more easily washable by rainfall (Hassan et al. 2010). On another hand, TiO2 is also used for removal of ripening hormone ethylene in storage places of perishable fruits, vegetables and cut flowers, as well as for degradation of toxins produced by blue-green algae. Titania is also used for stripping of organic pollutants from water and wastewater, for instance, of trichloroethylene and methyl tert-butyl ether, although sometimes non-desirable intermediate compounds are formed. TiO2 is an effective photocatalyst for water and air treatment, and it is also used in the catalytic production of gases (Sakthivel and Kisch 2003). The cleavage of organic compounds is triggered with TiO2 excitation by supra-bandgap photons, and this thereafter leads to redox reactions in which OH radicals, formed onto the photocatalyst surface, take the lead in the abatement processes. TiO2 also finds application in electronics, given that it is a semiconductor material, as part of the photochemical device that transforms light into energy. Titania is an electron-hole cycle intermediary (Carp et al. 2004; Chen and Mao 2007). Titanium dioxide is a non-toxic compound that can be used with an ought contraindication in a wide range of cosmetic and skin care brands including lipsticks, body powder, soap, toothpaste, sunscreens and ultraviolet light filters, white pigment and food colouring, among others. It is also incorporated in the wrapping of food products as well as to tobacco cigars (Chen and Mao 2007; Liu et al. 2014).

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TiO2 can exist in three different crystalline forms, that is, anatase, rutile or brookite. The first two are industrially the most interesting due to their chemical stability and photocatalytic activity. Rutile is thermodynamically more stable and thus the simpler to synthesize, and owed to this, it is commonly used in the paint industry (Bourikas et al. 2014). Anatase is more difficult to produce, whereas, on the other hand, its photoactivity is higher. Otherwise, brookite tends to transform into rutile at low temperatures (Bourikas et al. 2014). TiO2 is not available in nature in a readily usable form and hence needs refinement into a fine and uniform particle size to exploit its characteristics efficiently. TiO2 is commonly associated with Fe in the shape of ilmenite or in pure form as rutile beach sand. An alternative is the obtention of titania nanosol or nanopowder by chemical synthesis. Among others, widely used methods for titania production are sol-gel and wet chemical precipitation processes (Chen and Mao 2006). Among procedures to obtain TiO2 nanoparticles, we should pinpoint chemical vapour titania deposition, oxidation of titanium tetrachloride and flame synthesis. Industrial production of TiO2 relies commonly on either the chloride or the sulphate proceedings, which involve rutile or ilmenite ores (Chernet 1999; Krchma and Schaumann 1951; Sasikumar et al. 2004). Physical operational parameters comprising mainly the rotational speed, disk surface texture, location of feed inlet points, water to precursor proportion and flowrates have been highlighted to have important effects on the particle size, particle size distribution (PSD) and particle yield of TiO2. Adequate mixing is crucial for control of the TiO2 particles size and size distribution, as smaller particles are obtained with more intense stirring. The most common procedure used currently for production of titania nanoparticles is plasma spray gas synthesis (Du et al. 2015; Gardon and Guilemany 2014), which ensures major yields even though nanoparticles can contain impurities. Furthermore, nanoparticles’ crystal phase, shape and size distribution very highly with the used feedstream. Otherwise, wet chemical synthesis based on the reactionprecipitation process is the most suitable procedure to obtain high-purity titania nanoparticles. Moreover, this will produce a sol-gel material useful for a variety of coating finalities (Devi et al. 2014; Su et al. 2004). In case of titania synthesis in Spinning Disk Reactor, the most suitable appears the one by means of titanium alkoxides as a precursor. The reaction may be conducted with titanium tetraisopropoxide and 0.1 M H solution at room temperature. Rapid precipitation of slightly water-soluble titania occurs, following the next reaction schedule (Ruzmanova et al. 2013, 2015; Stoller et al. 2011, 2017): Hydrolysis : TiðOC3 H7 Þ4 þ 4 H2 O ! TiðOHÞ4 þ 4 C3 H7 OH Polycondensation : TiðOHÞ4 ! TiO2ðagglomeratedÞ þ 2 H2 O Disaggregation : TiO2ðagglomeratedÞ ! TiO2 ;

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The volumetric ratio between the two reagent solutions is maintained equal to 2/15. Rotational disk velocity can be controlled, but a maximum value of 1500 rpm was recommended. In these conditions, titania nanoparticles having a modal diameter of about 30 nm were obtained. The final titania crystal phase depends on postprocessing processes. In this framework, better yields of particles of diameters in the nanosize range and narrower pore size distributions are usually attained upon increased disk stirring and flowrates, as well as onto carved disk surfaces. These conditions ensure the optimal hydrodynamic conditions for intensive micromixing, leading to the almost ideal plug flow regime of the fluid film moving along the disk surface (Sasikumar et al. 2004). Production of titania in a conventional stirred tank reactor by reactive precipitation has been demonstrated to be below that of Spinning Disk Reactor performance. The latter has been reported to be capable to provide better performance in terms of better characteristics of the obtained particles and major performance efficiency of titania production rate. The latter can be explained by the more homogeneous and intensive mixing conditions in a lower volume of Spinning Disk Reactor in contrast with stirred tank reactor (Mohammadi et al. 2014). The former can ensure the production of nanoparticles exhibiting pore size distributions much narrower than stirred tank reactors without major specific energy costs (Mohammadi et al. 2014). The control of the particle size and pore size distribution of TiO2 is possible by fine setting the operating parameters of the Spinning Disk Reactor. The interaction between the disk stirring velocity and reagent inlet flowrate has been underlined to have significant effects on the particle size, pinpointing the benefits of working upon major disk speeds. Relying on the latter (~1200 rpm), increased flowrates and high water/titanium tetraisopropoxide ratio ensure the production of smaller particles with narrower distributions (Sasikumar et al. 2004). On another hand, grooved disks have also been reported to produce smaller nanoparticles of smaller average size for different operating conditions. This can be explained by the changing hydrodynamic conditions, which provide film rupture minimization (Sasikumar et al. 2004). Furthermore, by feeding the inlet titanium tetraisopropoxide at a certain distance from the disk centre, the mean properties of the particles are promoted. This can be explained by the better micromixing of titanium tetraisopropoxide into the water, which is improved upon the major shear created, leading to a thinner water film (Mohammadi et al. 2014; Stoller et al. 2009, 2011; Vaiano et al. 2014). Hence, the hydrolysis reaction rate is enhanced, promoting nucleation vs. growth. The generation of smaller and more uniformly sized particles, as a result, is also owed to the major dissipation of energy upon this enhanced micromixing upon those operating conditions. Because of the large surface/volume ratio as well as due to strong interactions with metal catalysts, mesoporous titania nanoparticles are also widely employed in metal catalysed oxidation (Ho and Yeung 2006; Nie et al. 2013; Grünert et al. 2014), hydrogenation (Torres et al. 2013) and wastewater reclamation (Ochando-Pulido et al. 2013; Ruzmanova et al. 2013; Stoller et al. 2014).

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Stoller et al. (2014) and Ochando-Pulido et al. (2013) used ultraviolet light/TiO2 photocatalysis with self lab-made ferromagnetic-core nanoparticles with the aim also to reduce membranes fouling in the purification of three-phase and two-phase olive mill wastewater, respectively (Figs. 11.3 and 11.4). Ruzmanova and co-workers (2013) addressed the reclamation of 3-phase olive mill wastewater by means of a photocatalytic treatment with N-doped TiO2 sol-gel under visible light. Better performances were demonstrated in contrast with undoped nanoparticles, ensuring around 60% chemical oxygen demand (COD) abatement efficiencies. Moreover, the prepared photocatalyst could be used for several batches without losing efficiency. The proposed process can constitute a certainly promising solution for the reclamation of this type of effluent and similar ones rich in a high concentration of organic pollutants (Fig. 11.5).

11.3.2 Magnetite Production and Applications The use of magnetite nanoparticles is increasing of interest due to their magnetic properties and the possibility to use them as nanocores for the subsequent production of more complex core-shell nanostructures (Harris et al. 2003; Sun and Zeng 2002). Magnetic nanocore-based particles widely find use in water treatment processes and medical applications. In the first case, the nanocores are coated with silica and a catalyst, such as titania, or an adsorbent, in order to eliminate pollutants. After use, by a magnetic trap, it is possible to recover almost all material from the water stream. In medical applications, the nanocores are employed for localized, internal thermal treatment (e.g. cancer ablation), allowing the addressing of the nanocores inside the body with high precision to the treatment point and avoiding dispersion (Mayo et al. 2007; Wei and Viadero 2007). Nowadays, among the most studied magnetic nanomaterials, we should highlight ferrite or iron oxide nanoparticles (maghemite or magnetite crystal structure). As soon as the particles become smaller than 120 nm, self-agglomeration is prevented since they become superparamagnetic (that is, their magnetic property emerges only by applying an external magnetic field) (Abbas et al. 2013; Wei et al. 2011). It is possible to sensibly increase their magnetic behaviour by controlled clustering of single nanoparticles forming magnetic nanobeads. In case of core-shell magnetite nanoparticle production, unfortunately, their surface is certainly inert, and strong covalent bonds with functionalization molecules are not easily permitted. For this reason, a solution is to coat a silica shell onto their surface and can perform by many available techniques, where the Stroeber one is nowadays the most used (Bruce et al. 2004; Dang et al. 2010). The silica layer can thereafter be functionalized by means of covalent bonds such as organo-silane and fluorescent dye molecules (Wan et al. 2010). Ferrite nanoparticle clusters with narrow particle size distribution consisting of superparamagnetic oxide nanoparticles (approx. 80 magnetite nanoparticles

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Fig. 11.3 Typical titania product obtained by means of Spinning Disk Reactor (transmission electron microscope (TEM) image, nanosizer measurement, X-ray diffraction spectrum). Note as both transmission electron microscope and Nanosizer results show the presence of particles with a mean dimension of 10 nm and characterized by a narrow size distribution. The X-ray diffraction spectrum shows clearly the typical peaks of anatase near 25 and 45

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Fig. 11.4 Ferromagnetic core catalyst with γ-Fe2O3/SiO2/TiO2 layer-structure (Ochando-Pulido et al. 2013). Note the typical core-shell structure of the catalyst that allows to obtain an active photocatalyst simple to be recovered through magnetic separation process

Fig. 11.5 Transmission electron microscope (TEM) photograph (right caption) of the final ferromagnetic catalyst nanoparticles (Stoller et al. 2014). The particle mean size below 50 nm and the particle aggregates were characterized by a maximum size of 200 nm

particles/bead) coated with a silica layer present a range of advantages if compared to metallic nanoparticles (Zaitsev et al. 1999): • Major chemical stability and narrow pore size distribution (key for biomedical applications) • Improved colloidal stability, as magnetic agglomeration is impeded • Tuneable magnetic moment upon nanoparticle cluster size and superparamagnetic characteristics (disregarding the nanoparticle cluster size) • Enhancement of straight covalent functionalization Magnetite is a common magnetic ferrous-ferric iron oxide in which electrons can hop between Fe2+ and Fe3+ ions in the octahedral sites at ambient temperature conditions, which renders magnetite a very appealing half-metallic material.

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Commonly, it is produced by the combination of the adequate quantities of Fe (II) and Fe (III) salts in basic solution, which leads to precipitation of the mixed valence oxide Fe3O4 as follows (Wu et al. 2001): 2FeCl3 þ FeCl2 þ 8NaOH ! Fe3 O4 þ 8NaCl þ 4H2 O The production of magnetite nanoparticles with determined size control has been a technological concern in the last decades. In fact, synthesis of magnetic particles exhibiting a target size and adequate pore size distribution has been a challenge for scientists and engineers. Especially, avoiding particle aggregation has consistently been a problem (Moharir et al. 2012; Sun and Zeng 2002). In this context, studies have been carried out on the synthesis of magnetite in Spinning Disk Reactor trying to focus on the obtention of uniform particle size distributions together with maximum conversion, in comparison with batch and semi-batch mechanically stirred tank reactor (Moharir et al. 2012). With the aim to optimize the efficacy of Spinning Disk Reactor technology in terms of uniform and narrow pore size distribution as well as conversion maximization, the impacts of different operating conditions in Spinning Disk Reactors have been well examined. In this regard, lower conversion levels have been reported in contrast with the conventional approach, but particle size distribution is significantly improved in Spinning Disk Reactors. Even though the overall productivity has been reported to be lower in Spinning Disk Reactor if compared to conventional stirred tank reactor, the obtained results in terms of particle size distribution pinpoint for the success of Spinning Disk Reactor approach to produce magnetite. A deeper focus on the reported results permits highlighting the considerable much better particle size distribution at the lower power consumption of Spinning Disk Reactor in contrast with conventional stirred tank reactor. This aspect is key with a view on the further use of the obtained magnetite particles (Moharir et al. 2012). Moharir et al. (2012) recently reported a research study on the optimization of Spinning Disk Reactor operating conditions in which it was underlined that the flowrate of the reagents fed, the disk stirring seed and mean diameter, as well as its surface type have a determinant impact on the attained conversion efficiency. An operating range for the main variables comprising the flowrate and rotation velocity depending on the disk diameter was well determined by the authors. Scale-up guidelines relying on the residence time and film thickness on the basis on the available scientific literature can be followed reliable for magnetite production, validated relying on experimental tests under various operation scales (Moharir et al. 2012; Stoller et al. 2011). Magnetite nanoparticles find an important role in clinical medicine, particularly the ones coated, in the fields of resonance imaging, for instance, in the diagnosis and tumour therapies. For these purposes, magnetite nanoparticles are usually dispersed in water to form water-based suspensions, having the ability to interact with an external magnetic field in order to be oriented towards a concrete area, enabling these

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users (Moharir et al. 2012). Superparamagnetic nanoparticles presenting uniform physical and chemical properties, that is, with sizes below 20 nm as well as narrow particle size distributions, are required for technological and medical uses (Moharir et al. 2012). In the case of Spinning Disk Reactor production, Fe3O4 nanoparticles can be successfully produced at high yield. The reaction allowing to obtain this product is as follows (Chiavola et al. 2016): 2FeCl3 þ 2 H2 O þ 2 Na2 SO3 ! 2 FeSO4 þ 4NaCl þ 2Hþ þ 2HCl FeSO4 þ 2NH4 OH ! FeðOHÞ2 þ ðNH4 Þ2 SO4 3FeðOHÞ2 ! Fe3 O4 þ 2H2 O þ H2 Fe3 O4 ! FeO  Fe2 O3 ðMagnetiteÞ The rotational speed of the disk was operated at the maximum value of 1500 rpm, and the reactants were injected at 3 cm from the disk centre. In these conditions, nano-mag particles exhibit a modal diameter of about 18 nm (Fig 11.6).

11.3.3 MgO Production and Applications Magnesium oxide is a mineral characterized by remarkable physical and chemical stability at high temperatures, and therefore it is considered an exceptional refractory material (Roggenbuck and Tiemann 2005; Shih et al. 2007). For this reason, it has been used in several industrial applications and in various wastewater, air and soil treatment processes for its acid buffering capacity and, as a consequence, noticeable effectiveness in stabilizing soluble heavy metal species (Minami et al. 2012; Sharma and Jeevanandam 2011; Srivastava et al. 2015). These peculiar characteristics are of great interest when the material is synthesized as core-shell particles, using magnetite or another magnetic core to allow a rapid and total recovery of the composite after its employment in situ applications. Magnesium oxide is still the most employed ceramic material, characterized by a high melting temperature (about 2800  C), added in fire-resisting bricks or crucibles. Moreover, it is added also in superconductor products and used in catalysis and in painting industry (Booster et al. 2003; Donnary 1972). The decomposition of magnesium hydroxide or magnesium carbonate still represents the most used production pathway (Ardizzone et al. 1998; Choudhary et al. 1994; Jost et al. 1997; Utamapanya et al. 1991). A range of methods for Mg(OH)2 nanoparticles production have been reported, among which we can remark sol-gel methods (Utamapanya et al. 1991; Wang et al. 1998a, b), hydrothermal processes (Ding et al. 2001; Yu et al. 2004) and precipitation (Lv et al. 2004). However, most of the above-mentioned production processes cannot be successfully scaled-up, mainly due to high costs (Tai et al. 2007).

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Fig. 11.6 The typical magnetite nanoparticle products obtained by means of the Spinning Disk Reactor (scanning electron microscope image, Nanosizer measurement, X-ray diffraction spectrum). Note as both scanning electron microscope and Nanosizer results show the presence of particles with a mean dimension of almost 10 nm and characterized by a quite narrow size distribution. The X-ray diffraction spectrum shows clearly the typical peaks of magnetite

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The chemical reactions involved in the formation of magnesium hydroxide and oxide are as follows: MgCl2ðaq:Þ þ 2NaOHðaq:Þ ! MgOH2ðsÞ # þ 2NaClðaq:Þ
 Δ MgOH2ðsÞ ! MgOðsÞ þ H2 OðgÞ " > 350 C The production of magnesium hydroxide as a precursor for the synthesis of MgO is the less expensive and requires only an alkali solution (NaOH or KOH) and magnesium chloride salt, which can be successfully recovered from seawater or from natural brines (Phillips et al. 1977). However, the notable difficulty that characterized the MgO synthesis procedure is represented by the calcination process that must be carefully controlled for the production of nanosized MgO crystals. While the crystalline structure of Mg(OH)2 is hexagonal, the MgO crystalline structure is a periclase one, analogous to the atomic arrangement of NaCl crystals (Booster et al. 2003). Because of this particular arrangement, the MgO morphology may be very different in shape (cubic, octahedral and polyhedral) (Booster et al. 2003; Donnary 1972). Utamapanya and co-workers (1991) synthesized MgOH2 through a sol-gel method. The gel was obtained by stirring Mg(OCH3)2 with water in a methanoltoluene solvent mixture. Subsequently, a hypercritical drying procedure was followed to obtain from the gel nanosized MgOH2 crystals, characterized by a surface area of 1000 m2/g and 3.5 nm average size. Finally, the obtained nanoparticles were calcined at 500  C to yield MgO nanoparticles with a mean size of 8 nm in size and 300–400 m2/g surface area. The main drawback of the procedure was represented by the protocol to produce and handle the hazardous Mg precursor that resulted not convenient (Tai et al. 2007). Rod-like, lamellar-like and needle-like morphologies characterized the Mg(OH)2 nanoparticles synthesized by Ding and co-workers (2001) through a hydrothermal methodology. The particles presented a wide range of sizes (2–200 nm). The authors used various magnesium precursors, including magnesium powder and magnesium sulphate, and different solvents, such as ethylenediamine and NaOH solutions. The residence time in the reactor was set from 2 up to 20 h, varying the operative temperatures in the range 80–110  C. Subsequently, the nanoparticles were calcinated up to 450  C, to yield MgO nanoparticles characterized by lamellar-like and needle-like morphologies and a mean size of 20 nm. Song and co-authors (2002) synthesized lamellar-like MgOH2 nanoparticles (20–80 nm) with a different synthesis path and process intensification equipment: MgO nanoparticles were produced through the reaction of a magnesium chloride solution with an ammonium hydroxide solution/gaseous ammonia in a high-gravity rotating packed-bed reactor at 60–160  C. Another fundamental equipment employed to produce round- and disk-shaped Mg(OH)2 nanoparticles presenting 50–80-nm-diameter and 10-nm-thick and 77 m2/g Brunauer, Emmett and Teller (BET) surface has been successfully produced using a Spinning Disk Reactor (Tai et al. 2007). Various operating variables that affected the Mg(OH)2 particle size were

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examined by Tai and co-workers (2007), including reactant concentration, spinning disk rotation speed, the concentration ratio of magnesium salt to sodium hydroxide and the flowrates of the reactant solutions. Among the four above-mentioned variables, the last two ones most influenced the mean size and shape of the nanoparticles: the mean size increased from 47.5 to 143.6 nm when the flowrate of the reactant solution increased from 0.28 to 0.75 L/min. MgO nanoparticles (30–70 nm), characterized by polyhedral shape, were obtain through the calcination of the 47.5 nm Mg(OH)2 powder of lamellar shape up to 600  C. The produced Mg(OH)2 nanoparticles were found to present interesting advantages applied as a flame retardant. The calcination process, as mentioned previously, strongly influenced the shape, reactivity and mean dimension of MgO crystals. For instance, calcining at 1500–2000  C leads to the production of a MgO unreactive form (dead burnt magnesia), that can be successfully used as a refractory material, is characterized also by a negligible surface area. Hard-burned magnesia can be obtained calcining at 1000–1500  C which is characterized by limited surface area and reactivity, whereas light-burned magnesia can be produced through the calcination at 700–1000  C that is characterized by an appreciable reactivity (the well-known caustic calcined magnesia). Although some decomposition of the carbonate to oxide occurs at temperatures below 700  C, the resulting materials appear to reabsorb carbon dioxide from the air (Meshkani and Rezaei 2009; Patil and Bhanage 2013; Sharma and Jeevanandam 2011; Yu et al. 2004).

11.3.4 Hydroxyapatite Production and Applications Hydroxyapatite is the analogous Ca/P-based compound that constitutes the human hard tissues. The morphology and composition of the lab-made material are very similar to the biological material, considering the stoichiometric Ca/P molar ratio of 1.67 and the hexagonal structure (Fathi et al. 2008; Ma and Liu 2009). A peculiar feature of hydroxyapatite is its remarkable physical and chemical stability, with respect to other calcium phosphates. In particular, hydroxyapatite shows notable thermodynamic stability under physiological conditions as temperature, pH and composition of the body fluids (Dudek and Adamczyk 2013; Fathi et al. 2008; Mucalo 2015). The development of nanotechnology has led to an increasing interest in the production of biocompatible nanomaterials, such as hydroxyapatite. Nano-hydroxyapatite is attracting interest as a biocompatible material, because of its analogous features with human hard tissue, particularly for its use in prosthetic applications, considering that bone and teeth enamel is largely composed of a form of this mineral (Zakaria et al. 2013). The main interesting properties of hydroxyapatite are (i) biocompatibility, (ii) bioactivity, (iii) osteoconductivity, (iv) non-toxicity and (v) noninflammatory nature. Because of these peculiar characteristics, nano-hydroxyapatite bioceramic

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has got a variety of applications that include (Ignjatovic and Uskokovic 2004; Mistry et al. 2011; Mucalo 2015; Song et al. 2008; Sundar Raj et al. 2013; Yusuf et al. 2009): orthopaedic and dental implant coating (hydroxyapatite usually covers titanium and stainless steel implants in order to reduce the risk related to implant rejection), bone tissue engineering and void fillers for orthopaedic, restoration of periodontal defects, traumatology, spine, repair of mechanical furcation perforations and apical barrier formation maxillofacial and dental surgery, re-mineralizing agent in toothpastes, edentulous ridge augmentation, drug and gene delivery, desensitizing agent in post teeth bleaching and in early carious lesions treatment. Hydroxyapatite can also be produced as powders, beads or directly as blocks in order to be placed where there are bone voids or defects (Deville et al. 2006; Mucalo 2015; Roveri and Iafisco 2010). In fact, because of its biocompatibility and characteristics, hydroxyapatite may improve the bone re-growth and the defect restoration. Another important result of this process is the remarkable kinetic bone growth and shorter time healing, with respect to those that characterize classical techniques that do not use hydroxyapatite (i.e. allogenic and xenogeneic bone grafts). Enamel composition is. In dentin, the nano-hydroxyapatite represents 70 wt.% (Arnold et al. 2001; Besinis et al. 2014, 2012). As nano-hydroxyapatite is the main component of enamel (97 wt.% nanohydroxyapatite and 3 wt.% organic material and water), its use leads to achieve several important results, such as an appearance of bright white and the removal of the diffuse surface reflectivity of light by closing the small pores. Enamel apatite and natural dentinal compounds can be successfully substituted with synthetic nanohydroxyapatite that is able to mimic both their size and structure. To this reason, toothpaste and mouth-rinsing solutions are filled with nano-hydroxyapatite particles that promote the restoration of demineralized enamel or dentin surfaces by their incorporation in the defects (Huang et al. 2011; Lelli et al. 2014; Li et al. 2008). Another important feature of nano-hydroxyapatite is related to its surface activity and sorption capacity: experimental evidence has demonstrated that the absorption and decomposition of CO in nano-composite air filters can be effectively improved through the addition of nano-hydroxyapatite in the composite body; this application promotes the nano-hydroxyapatite use for the reduction of automotive exhaust pollutants (Mahabole et al. 2005; Wang et al. 1998a, b). In 2014, an alginate/ nano-hydroxyapatite composite was produced and tested as an adsorbent for fluoride removal (i.e. ion exchanger) in situ applications (Medellin-Castillo et al. 2014). Other important industrial applications of nano-hydroxyapatite can be found in the process industry; in fact, as a consequence of hydroxyapatite noticeable surface activity, recently this nanomaterial has been used in industrial catalysis and protein separation, suggesting that many innovative applications for nano-hydroxyapatite and analogous compounds have not been developed yet (Shen et al. 2008; Wei and Ma 2004). As regards the hydroxyapatite synthesis, the sol-gel method still represents the most employed process, because of its simplicity and low cost. On Spinning Disk Reactor, the reaction for nano- hydroxyapatite production purposes takes place between calcium chloride and ammonium phosphate, in presence of ammonium hydroxide, according to the stoichiometry (Parisi et al. 2011):

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10CaCl2 þ 6ðNH4 Þ2 HPO4 þ 8NH4 OH ! Ca10 ðPO4 Þ6 ðOHÞ2 þ 20ðNH4 ÞCl þ 6H2 O Ammonium hydroxide is used to maintain a pH value near to 10 in order to maximize the product yield. Therefore, three solutions were fed to the Spinning Disk Reactor: a 10% aqueous solution of NH4OH at a flowrate of 80 mL/min was fed at the centre of the disk as a bulk stream. Parallel to this, two aqueous streams having both a flowrate of 100 mL/min and a solute mass fraction of 5.6% of CaCl2 and 3.5% of (NH4)2HPO4, respectively, were fed at 3 cm from the disk centre. The calcium/ phosphate (Ca/P) ratio of 1.67, corresponding to stoichiometric hydroxyapatite, was maintained, leading to the production of nanoparticles of hydroxyapatite with a high purity (Parisi et al. 2011). By using the maximal rotational velocity of the available Spinning Disk Reactor, equal to 147 rad/s, nanoparticles of the model diameter of 78 nm were obtained (Parisi et al. 2011) (Fig. 11.7).

11.4

About the Use of Photocatalytic Spinning Disk Reactors

Since photocatalytic processes require the presence of an activated semiconductor able to accelerate a photoinduced reaction, the two main well-known limitations to be handled, in order to achieve an industrial scale-up of photocatalysis, are photon transfer and mass transfer limitations (considering liquid phase reactions) (Van Gerven et al. 2007). Regarding the latter limitations, Spinning Disk Reactor, slurry reactor, monolithic and micro-reactors have already demonstrated their suitability and noticeable performances to enhance mass transfer. This issue is of minor importance in gas/solid systems, but the majority of photoinduced reactions are carried out in liquid media (Brosillon et al. 2008). Thus, the contact maximization between catalyst and reagents in the process fluid still remains an important research field in photocatalysis application. Among the various reactor configurations designed to maximize the reagent mass transfer to the catalyst active surface, Spinning Disk Reactor and micro-reactors represent those characterized by the highest catalyst-coated surface per reaction liquid volume (A, m2 m
3). For instance, Gorges and co-workers reported a catalyst-coated surface per reaction liquid volume value of 12,000 m2 m
3 for their micro-reactor system, whereas various researchers estimated, for the Spinning Disk Reactor system, a larger catalyst-coated surface per reaction liquid volume value, in the range 20,000–66,000 m2 m
3 (Van Gerven et al. 2007). These values are significantly higher with respect to those reported for other reactor configurations, such as annular/immersion reactor (catalyst-coated surface per reaction liquid volume A ¼ 27–2667 (Mukherjee and Ray 1999; Ray and Beenackers 1998)) and optical fibre/hollow tube reactor (catalyst-coated surface per reaction liquid volume A ¼ 46–2000 (Mukherjee and Ray 1999; Wang and Ku 2003; Wu et al. 2005)).

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Fig. 11.7 Typical hydroxyapatite product obtained by means of the Spinning Disk Reactor (scanning electron microscope image, Nanosizer measurement, X-ray diffraction spectrum). Note as both scanning electron microscope and Nanosizer results show the presence of particles with a mean dimension of almost 100 nm and characterized by a quite narrow size distribution. The X-ray diffraction spectrum shows clearly the typical peaks of hydroxyapatite

As already discussed, Spinning Disk Reactor shows important features, as the reduced mass transfer limitations, a consequence of the thin film and turbulent flow combination, which leads to the maximization of mass transfer coefficient (Vilardi et al. 2017), reliable product quality and larger conversion (Boodhoo and Harvey 2013). Therefore, in the last two decades, several studies focused on the development of Photo-Spinning Disk Reactor that investigated different configurations and the variation of most influencing operative parameters (inlet flowrate, disk velocity,

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Fig. 11.8 Photo-Spinning Disk Reactor used by Dionysiou and co-workers (2000). The equipment takes advantage from the enhanced mass transfer rate typical of rotating thin liquid film to intensify the photocatalytic process. The ultraviolet light lamps are positioned on both sides of the disk, and the disk surface is coated with the photocatalyst (titania) material

medium pH and temperature) on the efficiency of photocatalytic reactions, in particular in environmental applications (Boiarkina et al. 2013a; Dionysiou et al. 2000; Zhang et al. 2011). Two main different Spinning Disk Reactor configurations were investigated in literature: vertical and horizontal disk. The former configuration implies that half disk is immersed in the liquid reagent and the top half entraining the liquid and is exposed to the light source. Dionysiou and co-workers analysed the removal efficiency of 4-chlorobenzoic acid (0.288 mM) in vertical Photo-Spinning Disk Reactor (vessel volume of 3.5 L), fixing a rotational velocity to 4 rpm, initial pH ¼ 3 and with ultraviolet light-activated TiO2 immobilized on disk (Fig. 11.8 displays the reactor scheme) (Dionysiou et al. 2000). The employed disk was made of 304 stainless steel, with a diameter of 49.5 cm and a thickness of 0.32 cm. Ultraviolet light source was positioned at 10 cm from the disk and emitted ultraviolet light sources in the range 300–400 nm, with a peak at 365 nm. Authors obtained a target compound removal efficiency larger than 50% in 120 min and almost a total removal in 360 min; the maximum total organic carbon (TOC) removal efficiency obtained in this study was close to 60% in 360 min. The authors estimated also an apparent average photonic efficiency (ζ av) value equal to 2.7% for the experiments carried out at 4 rpm (Eq. 1 defines ζ av). ζ av


ntarget pollutant mol min
1 transformed
¼ nphotons einstein min
1 incident on the rotating disk

A similar Photo-Spinning Disk Reactor has been employed by Hamill and co-authors (Hamill et al. 2001) to mineralize 3,4-dichlorobut-1-ene, phenols and a dye in aqueous solution. The reactor configuration is reported in Fig. 11.9.

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Fig. 11.9 Series of Photo-Spinning Disk Reactor used by Hamill and co-workers (2001). The authors employed four disks in series to treat high amounts of various organic pollutants, i.e. phenol, 3,4-dichlorobut-1-ene and a dye

In this study, four glass disks were coated with TiO2 (catalyst loading equal to 0.3 mg cm
2), and the photodegradation of 0.2 mM 3,4-dichlorobut-1-ene was performed using eight Phillips TL 8 W/05 ultraviolet light lamps (1 W and λmax ¼ 365 nm). The kinetic tests were conducted at 136 rpm for 60 min, achieving more than 70% 3,4-dichlorobut-1-ene removal. The experimental data were fitted by first-order kinetics, estimating a pseudo-first-order constant k equal to 0.0242 min
1. Regarding phenol (0.2 mM) and DB (0.3 mM), k values of 0.0085 and 0.0055 min
1 were obtained, respectively. The vertical configuration has been selected also by Zhang and co-authors (Zhang et al. 2011) for the treatment of rhodamine B. In their work, the authors electrochemically coated the disk (8.5 cm of diameter and 1.5 cm of thickness, see Fig. 11.10) with titania nanotubes with a mean size of 25 nm and a Brunauer, Emmett and Teller (BET) surface area of 55 m2 g
1. The vessel volume was 39 mL and made of quartz, to let the selected incident ultraviolet light wavelength (λ ¼ 254 nm) pass through the vessel and activate the catalytic surface. The rotational velocity influence was investigated on the rhodamine B degradation efficiency (initial rhodamine B concentration fixed to 20 mg L
1 and reaction time set at 180 min) in the range 5–60 rpm, establishing the optimal value to 30 rpm, for which a degradation efficiency close to 90% was obtained. The same treatment efficiency was achieved by varying the initial concentration to 10 and 50 mg L
1, whereas it decreased to about 50% when the rhodamine B concentration was increased to 100 mg L
1. An interesting study where a vertical Photo-Spinning Disk Reactor has been recently conducted by Li and co-workers, regarding the post-treatment of actual textile wastewater (Li et al. 2015). The authors obtained a COD (chemical oxygen demand, mg L
1) removal of about 72% in 180 min of photocatalytic treatment, at pH ¼ 6 and disk velocity set equal to 20 rpm, employing a wedge-structured titanium disk of 7.5 cm of diameter and two 11 W mercury lamps (Philips, main emission wavelength 254 nm) as ultraviolet light source was used and placed 3 cm away from the disk. The disk was coated with colloidal TiO2 suspension, and the wedged structure was fabricated by an electrical discharge linear cutting machine (DK7732, Jiangsu Taizhou Computer Numerical Control Co. Ltd., China),

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Fig. 11.10 Scheme of Photo-Spinning Disk Reactor used by Zhang et al. (2011). The ultraviolet light source is positioned out of the reactor that is provided of a quartz wall to let the ultraviolet light waves reach the effluent inside the reactor. The disk surface is coated by titania nanotubes with a mean size of 25 nm

generating wedges of width equal to 2 mm and four different heights (2, 4, 6 and 8 mm, corresponding to a width/height ratio of 1/1, 1/2, 1/3 and 1/4, and surface area of 96 cm2, 179 cm2, 261 cm2 and 355 cm2, respectively). On the other hand, numerous studies reported the use of Spinning Disk Reactor according to the horizontal configuration (Li et al. 2015). For instance, Boiarkina and co-authors worked on methylene blue degradation comparing the removal efficiency and performances of a Photo-Spinning Disk Reactor and an annular photoreactor (Boiarkina et al. 2013a). Both glass disk (20 cm of diameter) and quartz sleeve were coated using a TiO2 colloidal suspension, after a calcination at 500  C for 60 min. The ultraviolet light source was a 20 W low-pressure mercury lamp (monochromatic, peak wavelength at λ ¼ 254 nm ultraviolet light, Steriflow), and the reactant mixture was maintained at 26–27  C during the experiments. The ultraviolet light irradiance (W m
2) was measured along the disk surface, determining a peak of 2.7  2.9 W m
2 occurs at the centre axis aligned with the lamp; the annular reactor showed a peak of 150 W m
2 (the higher value is due to the different configurations of the two reactors, since the latter has the catalyst surface directly in contact with the ultraviolet light lamp). The methylene blue removal efficiency was described in function of volumetric (RV) and surface (RS) rates of photocatalytic reaction; in both cases Photo-Spinning Disk Reactor outperformed the annular reactor, returning average RV ¼ 3.6  1.5 E-4 mol m
3 s
1 and RS ¼ 0.76  0.31 mol m
2 s
1, that were significantly larger with respect to those obtained through the annular reactor (equal to 0.099  0.018 mol m
3 s
1 and to 1.29  0.24 mol m
2 s
1, respectively).

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The same authors monitored the photocatalytic degradation of dehydroabietic acid using the same Photo-Spinning Disk Reactor configuration, in another work (Boiarkina et al. 2013b), focusing on the flow structure influence on the dehydroabietic acid degradation kinetics. The authors found that the maximum value of reaction rate (close to 1 mol m
2 s
1) occurred at an intermediate flowrate of 15 mL/s and rotational speeds of 100 and 200 rpm, where the reaction kinetics switched from first order to second order with a change in the flow structure.

11.5

Conclusions

Nanoparticle production resulted successfully improved through the Spinning Disk Reactor technology. The higher production rate and a more controlled size distribution can be achieved, through the homogenization of mixing and operative conditions in the reactor, with respect to the conventional synthesis technologies. In the view of process intensification of the photocatalytic process, nanocatalysts produced by Spinning Disk Reactor may enhance the degradation rate of biorefractory compounds, including phenols and dyes. As a result of the quantum size effect occurring at the nanoscale, such positive properties can be addressed to the higher chemical reactivity and photoactivity of the nanoparticles produced by Spinning Disk Reactor. Similar results have been widely demonstrated in the past couple of decades applying Spinning Disk Reactor technology to the production of several nanomaterials used for environmental remediation, including titania, magnetite, MgO and hydroxyapatite. Since Spinning Disk Reactor configuration and characteristics vary in function of the required process yield and the type of reaction to be developed, a wide range of operating conditions have been pointed out in literature, covering the conditions of productions of several nanoparticles. An optimization study is always required before industrial application. As a matter of fact, Spinning Disk Reactor technology accomplishes with all the requirements of process intensification: the only limit to this technology is the need of a liquid phase to achieve the chemical precipitation of the product. Such features, in addition to an easy scale-up from lab testing to the industrial application, favour Spinning Disk Reactor technology implementation for the production of a wide range of nanoparticles.

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Index

A Absorption, 31, 32, 38–40, 42, 94–97, 111, 113, 114, 117, 123, 128, 129, 143, 145, 148–150, 152, 154, 156, 175, 178–180, 182, 183, 186–191, 193, 194, 198, 199, 232, 243, 245, 251, 287, 288, 291, 320 Antibacterial photocatalysis, 243, 253, 255

B Band gap properties, 84 Biosynthesis, 250–252

C Carbon dioxide (CO2) photoreduction, 125–128, 130, 158, 231, 232 Charge carriers, 171, 178–180, 182, 183, 188, 189, 193, 198 Coatings, 35, 53, 84, 235, 265, 310 Cold plasma discharge (CPD), vii, 62, 64–66 Composites, vii, 29, 59, 85, 114, 149, 176, 215, 226, 265, 316 CO2 reduction, vii, 116, 125, 141, 142, 146–148, 155–159, 180, 182, 199, 224, 231, 232 CO2 removal, 170, 199

D Deep eutectic solvents (DESs), 267, 293, 297 Defects, 4, 91, 94–96, 98, 179–184, 191, 193, 199, 214, 216, 219, 267, 279, 293, 320 Doping, 2, 64, 85, 120, 148, 179, 212, 222, 289

E Electrodeposition, vi, vii, 60, 61, 264–297 Electrophoretic deposition (EPD), 70 Environmental, v, vi, 31, 51, 93, 141, 168, 222, 242, 268, 306 Environmental remediation, 140–159, 242, 243, 326 Extraction, 2, 3, 7, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 30, 31, 37, 44

F Fe/TiO2, vii, 267, 283, 296, 297

G Graphene, vii, 38, 55, 123, 149, 184, 212, 227 Graphitic carbon nitride (g-C3N4), vii, 61, 115, 150, 169, 226 Green synthesis, 242–256

H Heavy metals abatement, 130 Heterogeneous photocatalysis, vii, 4, 97, 110–117, 143, 213, 243, 246, 264, 290 Heterostructures, vii, 55, 115, 184, 226 Hydrogen generation, vii, 84–98, 123, 141, 199, 255

I Immobilization, vii, 50, 229, 231, 235, 236, 265–267

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336 Immobilized TiO2, 265, 267, 279, 291 Imprinting factor (IF), 5 Innovative equipment, 304 Intensified photocatalysis, 305 Intensified production, 317 Iron oxide, 312, 314

L Layered perovskites, 142, 154, 158

M Magnesium oxide, 316 Metallic photocatalysts, 212–219 Metal oxides (TiO2 ZnO) semiconductors, 224, 225, 247 Methanesulfonate electrolyte, 268, 273, 276, 281, 297 Molecularly imprinted, vii, 2–4, 7–40, 42–44

N Nanocatalysts, 242, 326 Nanocomposites, 2, 36, 37, 39–42, 65, 68, 158, 184–199, 225–236, 253, 267 Nanomaterials, 117, 127, 130, 242, 249, 306, 308, 312, 319, 326 Ni/TiO2, 266, 267, 295, 297 Non-metal (oxygen sulfur nitrogen boron and phosphorus) dopants, vii, 70, 84–98, 179, 182, 199 Non-metals doped metal oxide hybrids, 84–98 Novel photocatalysts, vi, vii, 242–256

P Perovskite oxides (ABO3), 140, 154, 159 Photocatalysis, v, 2, 50, 85, 110, 140, 168, 212, 226, 242, 264, 306 Photocatalysts, v, 2, 51, 84, 110, 140, 169, 212, 222, 242, 264, 306 Photocatalytic degradation, vii, 5, 7, 39, 41, 51, 55, 67, 97, 146, 150, 152, 154, 217, 246, 254, 267, 290, 326 Photodegradation, v, 2, 5, 8, 30, 32, 33, 35, 39, 40, 44, 54, 56–58, 60, 69, 72, 113, 145, 154, 176, 178, 179, 195, 198, 251, 253, 255, 277, 287, 289, 290, 324 Photoetching, 68 Photon energy (hυ), 243, 245, 247 Photoreduction, vii, 117, 125–130, 152, 158, 179, 194, 224, 227, 231, 232 Phytosynthesis, 250, 252–255

Index Plasmon resonance, 152, 153, 222–224 Pollutant degradation, vii, 141–146, 150–154, 179, 199 Polymer assisted hydrothermal decomposition (PAHD), 65, 66 Preferential photodegradation, 2, 44 Process intensification, 304–306, 318, 326

Q Quantum size (Q-size), 248, 306, 326

R Reactive oxygen species (ROS), 223, 225, 226, 228, 229, 233, 234, 242, 253 Recalcitrant pollutants, 74, 225 Reduced graphene oxide, vii, 55, 85, 188, 189, 212, 216, 218 RF magnetron sputtering, vii, 61, 66, 67

S Selectivity photocatalysis, 2, 5, 30–32 Semiconductor-metal oxides, 224, 225, 247 Semiconductors, 28, 89, 110, 140, 212, 223, 243, 288, 309 Silver-nanoparticles materials, vii, 230, 232 Solar energy harvesting, 140 Sol-gel method, 4, 36, 59, 61, 87, 90, 118, 121, 158, 316, 318, 320 Spinning disk reactor, 304–326 Structural characterization, 30, 34, 37–42, 44

T Template, 3, 4, 7, 28, 30, 31, 33, 35–37, 39–44, 69, 96, 122, 125, 174, 175, 178, 249 Textural characterization, 34–37, 44 Thermal treatment, 60, 73, 85, 179, 312 Titania, 54, 85, 233, 264 Titanium dioxide (TiO2), vi, vii, 2, 51, 85, 110, 157, 169, 223, 243, 264, 309

V Visible-light-driven photocatalysts, 84

W Wastewater treatment, vii, 2, 52, 85, 143, 264, 279 Water splitting, 95, 116, 117, 127, 141, 168, 170, 179, 187, 189, 190, 195, 225, 243