Photocatalysis for Environmental Remediation and Energy Production: Recent Advances and Applications 9783031277061

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Green Chemistry and Sustainable Technology

Seema Garg Amrish Chandra Editors

Photocatalysis for Environmental Remediation and Energy Production Recent Advances and Applications

Green Chemistry and Sustainable Technology Series Editors Liang-Nian He State Key Lab of Elemento-Organic Chemistry, Nankai University, Tianjin, China Pietro Tundo Department of Environmental Sciences, Informatics and Statistics, Ca’ Foscari University of Venice, Venice, Italy Z. Conrad Zhang Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China

Aims and Scope The series Green Chemistry and Sustainable Technology aims to present cutting-edge research and important advances in green chemistry, green chemical engineering and sustainable industrial technology. The scope of coverage includes (but is not limited to): – Environmentally benign chemical synthesis and processes (green catalysis, green solvents and reagents, atom-economy synthetic methods etc.) – Green chemicals and energy produced from renewable resources (biomass, carbon dioxide etc.) – Novel materials and technologies for energy production and storage (bio-fuels and bioenergies, hydrogen, fuel cells, solar cells, lithium-ion batteries etc.) – Green chemical engineering processes (process integration, materials diversity, energy saving, waste minimization, efficient separation processes etc.) – Green technologies for environmental sustainability (carbon dioxide capture, waste and harmful chemicals treatment, pollution prevention, environmental redemption etc.) The series Green Chemistry and Sustainable Technology is intended to provide an accessible reference resource for postgraduate students, academic researchers and industrial professionals who are interested in green chemistry and technologies for sustainable development.

Seema Garg · Amrish Chandra Editors

Photocatalysis for Environmental Remediation and Energy Production Recent Advances and Applications

Editors Seema Garg Amity Institute of Applied Sciences Amity University Noida, Uttar Pradesh, India

Amrish Chandra Amity Institute of Pharmacy Amity University Noida, Uttar Pradesh, India

ISSN 2196-6982 ISSN 2196-6990 (electronic) Green Chemistry and Sustainable Technology ISBN 978-3-031-27706-1 ISBN 978-3-031-27707-8 (eBook) https://doi.org/10.1007/978-3-031-27707-8 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed 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, expressed 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

Photocatalytic material and its advancement have received increasing demands due to their great potential in various fields such as degrading recalcitrant organic pollutants and airborne pathogens, in biomedical and medical field, and for CO2 reduction and H2 production. As more work is going on in this area so large number of articles are published every year. Still, there is need to develop advanced material for enhancing efficiency and detailed study of the material. This book provides modification of photocatalytic material for enhancing the activity and its applications in various fields and theoretical studies and computational perspectives. This book comprises a detailed emphasis on synthesis of advanced photocatalytic materials and their combination with membrane technologies and photoelectrocatalysis. Photocatalytic activity enhancement has been discussed by suitable reactor design and operating parameters and by click chemistry. The book further emphasizes the applications of photocatalysts towards degradation of organic pollutants and airborne pathogens in air, for improving air quality, their biomedical applications and role of photocatalysts in green organic synthesis. Theoretical studies of photocatalytic material have been detailed via strain engineering, theory, modelling and computational aspects and conversion of CO2 at interfaces. CO2 reduction and H2 production have also been covered in the book using bismuth-based material, nanocomposites and MOFs materials. This book brings to light much of the advance research in the field of photocatalysis for environmental remediation and energy production. The book will thus be of relevance to researchers in the field of material science, environmental science and technology, photocatalytic applications, newer methods of energy generation and conversion and industrial applications. The book has structured into five parts for ease of comprehension: 1. Synthesis of Photocatalyst by Various Methods part comprises of the first four chapters: Chapter 1 of the book describes the synthesis of new nanoscale complexes of detonation diamonds and fullerenols containing gadolinium atoms. It has been shown that the formation of complexes is mainly due to electrostatic attraction between diamond particles carrying a positive charge and

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fullerenol molecules, which became electronegative by partial splitting of protons from hydroxyls in aqueous media. Chapter 2 starts by discussing photocatalysts and the principles and mechanism of the photocatalytic degradation process. Subsequently, alloy-based photocatalysts’ preparation, properties and degradation performance have been elaborated. Finally, challenges, conclusions and outlooks in the studies of alloy-based photocatalysts have been summarized. Chapter 3 discusses the current research targeting the fundamentals of photocatalytic reactions and the role of nanomaterials in the formation of thin-film coatings. Light on the organic and inorganic methods for forming photocatalytic active thin-film coatings and their anti-microbial properties have been shown. Chapter 4 reports the production of 3D printed titanium dioxide (TiO2 ) macroarchitectures to be employed as photocatalysts for water purification under solar radiation. The approach developed is an effective alternative for the production routes of the TiO2 photocatalysts used nowadays, since 3D printing is a highly cost-effective technique, simple, fast and easily scalable which make these materials capable to be used in industrial environment. 2. Photocatalytic Activity Enhancement part carries Chaps. 5–7: Chapter 5 introduces the recent developments on photocatalytic reactors and the commonly used ones are illustrated. The reaction mechanisms and stoichiometry of the reactants within the reactor are presented in detail. This chapter provides an overview to the parameters (pH, temperature, pollutant concentration, photocatalyst dosage, oxidants and coexisting inorganic ions) and presents a perspective to the photoreactor concept and process parameters. Chapter 6 deliberates an inquisitive flush of exercise in the field of visible light arbitrated click chemistry for diversified desired synthesis. This chapter highlights the recent prosperity in the synthesis of organic structures through click reaction with the help of visible light radiation. Sometimes, the insertion of the new cyclic ring or functional group within the structure may lead to enhance bioactivity of the synthesized organic molecule. Different pathways of transformation of molecules in the presence of visible light using variegated methods have been demonstrated here. Chapter 7 shows the synthesis of complexes of detonation nanodiamonds with metal-organic scintillators, which are activated by X-rays, creating a secondary emission of light to excite a photosensitizer additionally attached to the diamond platform. Structures of this kind can serve to deliver a photosensitizer (Radachlorine) into living tissues, when nanodiamond-scintillator complexes play the role of converters of penetrating radiation into the optical range, which is important for expanding the therapeutic possibilities of PDT. 3. Applications of Photocatalysts comprises Chaps. 8–11: Chapter 8 presents current knowledge of human health concerns caused by volatile organic compounds (VOCs) and biological contaminants. These contaminants contribute to air pollutants that impair all environmental elements. Heterogeneous photocatalytic processes using semiconductor photocatalyst would serve as a promising technology and an efficient approach for removing VOCs and airborne pathogens. Chapter 9 discusses the qualities of the ideal photocatalysts, as well as current photocatalytic materials for making improvement of air quality and public health.

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Nowadays, photocatalysis has been used to remove significant pollutants from the atmosphere by purifying water and oxidizing a variety of organic compounds. Chapter 10 explores metal oxide photocatalytic nanomaterial and their composite by different modifications. In this chapter, definition of metal oxides nanoparticles as photocatalysts and their synthesis methodology are discussed in detail. Chapter 11 discusses the recent advances in semiconductor-based photocatalytic organic reactions including, selective oxidation reactions (alcohols and amines), reduction reaction (nitro compounds) and hydrocarbylation of benzene compounds. A variety of semiconductor materials involved in organic synthesis along with their representative photocatalytic mechanism have also been given. 4. Theoretical Studies of Photocatalytic Material part covers seven chapters (Chaps. 12–14): Chapter 12 discusses the recent advances regarding strain engineering for tuning the photocatalytic activity of MOFs. Moreover, a concise summary of the present challenges and an outlook for the designing of photocatalytic process selectivity of MOFs have also been provided. Chapter 13 describes materials properties, including bulk and surface characteristics, are the key to efficient photocatalysis. Thus, development of new theoretical models of photocatalyst materials and interfaces is critical to the design and engineering of new semiconductor photocatalyst systems. With rapid advances in new algorithms and computational techniques, it is now possible to simulate interacting systems of many electrons and nuclei as in condensed matter and molecules. Chapter 14 explores the production of functional materials for specific gas adsorption techniques that is expanding quickly due to their potential use in processes including carbon capture and sequestration (CCS) and CO2 conversion. For screening the series of ionic liquids (ILs), DFT studies have been carried out to identify the appropriate combination for the CO2 conversion applications. Interfacial catalytic material models can serve as a guide for the design of innovative electrocatalyst for the conversion of carbon dioxide into value-added products. 5. Advances in Photocatalytic Material for CO2 Reduction and H2 Production part covers five chapters (Chaps. 15–19): Chapter 15 summarizes structural and functional modifications of bismuth-based catalyst and recent progress of rational approach for optimizing catalysts loading, including component regulation, morphology design, doping, facet engineering and defects engineering on the single bismuth-based photocatalytic system, co-catalyst loading, heterojunction construction, localized surface plasmon resonance and polarization. Finally, perspectives and opportunities are presented for future trends of photocatalytic H2 production and CO2 conversion. Chapter 16 elaborates on various types of semiconducting materials and co-catalyst such as carbon-based materials, metal complex, perovskites and nanocomposites. The mechanism of electronhole separation and CO2 conversion is discussed in detail. Chapter 17 discusses nanostructured conjugated polymers and their fascinating new properties such as versatility and conductance, which reduces the electrode-to-electrolyte interface resistance. Pseudo-capacitors and asymmetrical supercapacitors are being created as a result of ongoing research in electrochemical supercapacitors technology. The power storage properties of photocatalytic conducting polymers and

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the venture into the energy resource management sector have been discussed in this chapter. Chapter 18 summarizes the mechanism, various preparation methods and the recent developments in MOFs photocatalysts for the application of photocatalytic H2 production. In addition, the challenges and the different strategies adapted to improve the solar light absorption and to reduce the excitons recombination in achieving the efficient MOF materials for the photocatalytic H2 production have been discussed. Chapter 19 reports recent advancements in transition-metal-based semiconductor photocatalysts and their hybrids with carbon materials for photocatalytic water remediation and solar water-splitting reaction. In this chapter, the modification of the surface morphology and the design of the interface between the various components of the photocatalysts have been discussed. The book is useful for university students, researchers and engineers who wish to initiate research in photocatalysis or to enhance know how of the advanced synthesis and various applications of photocatalysts with its theoretical studies. Noida, India

Seema Garg Amrish Chandra

Contents

Part I 1

2

Synthesis of Photocatalyst by Various Methods

Modification of Detonation Nanodiamonds with Endofullerenols to Obtain Magnetic Photosensitive Structures for Theranostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vasily T. Lebedev, Yuri V. Kulvelis, Alexander Ya. Vul, Georgy S. Peters, Mikhail A. Vovk, Vera A. Orlova, Timur V. Tropin, Maria V. Popova, Olga I. Bolshakova, and Eduard V. Fomin Preparation of Alloy and the Application for Photocatalytic Degradation Under Solar/UV and Visible Light Irradiation . . . . . . . Saifullahi Shehu Imam, Noor Haida Mohd Kaus, Mohd Amirul Ramlan, and Usman Saidu

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Photocatalytically Active Thin-Film Coatings . . . . . . . . . . . . . . . . . . . . Ishika Aggarwal, Anubhav Jain, Tejendra K. Gupta, Sucheta Sengupta, and Manoj Raula

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Photocatalytic Activity of 3D Printed TiO2 Architectures Under Solar Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R. Xue, M. L. Matias, A. Pimentel, J. V. Pinto, E. Fortunato, R. Martins, and D. Nunes

Part II

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Photocatalytic Activity Enhancement

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Photocatalytic Reactors Design and Operating Parameters on the Wastewater Organic Pollutants Removal . . . . . . . . . . . . . . . . . . 103 Gizem Saygı, Özlem Kap, Fehime Çakıcıo˘glu Özkan, and Canan Varlikli

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Visible Light Mediated Click Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . 153 Lalan Chandra Mandal and Bidyut Saha

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Contents

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Effective X-ray Luminescent Hybrid Structures of Nanodiamonds Associated with Metal–organic Scintillators . . . . . 167 Yuri V. Kulvelis, Natalia P. Yevlampieva, Daniil S. Cherechukin, Vasily T. Lebedev, Timur V. Tropin, Eduard V. Fomin, Vladimir G. Zinovyev, and Alexander Ya. Vul

Part III Applications of Photocatalysts 8

Photocatalytic Degradation of Organic Pollutants and Airborne Pathogen in Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Pankaj Chowdhury, Noshin Hashim, and Ajay K. Ray

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Application of Photocatalysts to Improve Indoor Air Quality and Health: A Sustainable Environmental Approach . . . . . . . . . . . . . 235 Deep Chakraborty and Krishnendu Mukhopadhyay

10 Recent Progress in Biomedical Applications of Metal Oxide Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 Aditi Saxena, Parul Khurana, and Sheenam Thatai 11 Role of Heterogeneous Semiconductor Photocatalysts in Green Organic Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 Umair Alam Part IV Theoretical Studies of Photocatalytic Material 12 Strain Engineering for Tuning the Photocatalytic Activity of Metal–Organic Frameworks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 Monika Dubey, Subhasha Nigam, and Monika Joshi 13 Theory, Modeling and Computational Aspects Regarding the Mechanisms of Activation of Photocatalysts . . . . . . . . . . . . . . . . . . 305 Chinmay Rakesh Shukla, Deepak Singh Rajawat, and Sumant Upadhyay 14 Electrocatalytic Activation and Conversion of CO2 at Solid–Liquid Model Interfaces: Computational Perspectives . . . . 329 Shanmugasundaram Kamalakannan, Kandhan Palanisamy, Muthuramalingam Prakash, and Majdi Hochlaf Part V

Advances in Photocatalytic Material for CO2 Reduction and H2 Production

15 Bismuth-Based Photocatalytic Material for Clean Energy Production and CO2 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363 Isha Arora, Harshita Chawla, Amrish Chandra, Suresh Sagadevan, and Seema Garg

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16 Efficient Photoactive Materials for CO2 Conversion into Valuable Products Using Organic and Inorganic-Based Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395 Marimuthu Senthilkumaran, Venkatesan Sethuraman, and Paulpandian Muthu Mareeswaran 17 Conducting Polymer Hybrid Nanocomposites-Based Photocatalytic Material for Energy Applications . . . . . . . . . . . . . . . . . 417 S. Uday, Harshita Chawla, Amrish Chandra, and Seema Garg 18 Recent Developments in MOFs Materials for the Photocatalytic H2 Production by Water Splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439 N. Subha, Malathi Arumugam, and M. Mahalakshmi 19 Interface Engineering of Nano-Photocatalysts for Hydrogen Evolution Reaction and Degradation of Organic Pollutants . . . . . . . . 449 Kommula Bramhaiah and Santanu Bhattacharyya

Contributors

Ishika Aggarwal Amity Institute of Applied Sciences, Amity University, Noida, India Umair Alam Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur, India Isha Arora Department of Chemistry, Amity Institute of Applied Sciences, Amity University, Noida, Uttar Pradesh, India Malathi Arumugam Center of Excellence On Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, Thailand Santanu Bhattacharyya Department of Chemical Sciences, Indian Institute of Science Education and Research, Berhampur, Odisha, India Olga I. Bolshakova B.P.Konstantinov Petersburg Nuclear Physics Institute, NRC “Kurchatov Institute”, Gatchina, Leningrad Distr., Russia Kommula Bramhaiah Department of Chemical Sciences, Indian Institute of Science Education and Research, Berhampur, Odisha, India Deep Chakraborty Department of Environmental Health Engineering, Sri Ramachandra Faculty of Public Health, Sri Ramachandra Institute of Higher Education and Research, Chennai, Tamilnadu, India Amrish Chandra Amity Institute of Pharmacy, Amity University, Noida, Uttar Pradesh, India Harshita Chawla Department of Chemistry, Amity Institute of Applied Sciences, Amity University, Noida, Uttar Pradesh, India Daniil S. Cherechukin Saint Petersburg State University, St. Petersburg, Russia

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Pankaj Chowdhury Department of Chemical and Biochemical Engineering, University of Western Ontario, London, ON, Canada; Trojan Technologies, London, ON, Canada Monika Dubey Department of Applied Science and Humanities, IIMT Engineering College, Greater Noida, Uttar Pradesh, India Eduard V. Fomin B.P.Konstantinov Petersburg Nuclear Physics Institute, NRC “Kurchatov Institute”, Gatchina, Leningrad Distr., Russia E. Fortunato CENIMAT|i3N, Department of Materials Science, School of Science and Technology, NOVA University of Lisbon and CEMOP/UNINOVA, Caparica, Portugal Seema Garg Department of Chemistry, Amity Institute of Applied Sciences, Amity University, Noida, Uttar Pradesh, India Tejendra K. Gupta Amity Institute of Applied Sciences, Amity University, Noida, India Noshin Hashim Department of Chemical and Biochemical Engineering, University of Western Ontario, London, ON, Canada Majdi Hochlaf Université Gustave Eiffel, COSYS/IMSE, Champs Sur Marne, France Saifullahi Shehu Imam Department of Pure and Industrial Chemistry, Bayero University, Kano, Nigeria Anubhav Jain Amity Institute of Applied Sciences, Amity University, Noida, India Monika Joshi Amity Institute of Nanotechnology, Amity University, Noida, Uttar Pradesh, India Shanmugasundaram Kamalakannan Department of Chemistry, Faculty of Engineering and Technology, SRM Institute of Science and Technology, Kattankulathur, Chengalpattu, Tamil Nadu, India Özlem Kap Physics of Complex Fluids, Faculty of Science and Technology, MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands Noor Haida Mohd Kaus School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia Parul Khurana G.N.Khalsa College, University of Mumbai, Mumbai, India Yuri V. Kulvelis B.P.Konstantinov Petersburg Nuclear Physics Institute, NRC “Kurchatov Institute”, Gatchina, Leningrad Distr., Russia Vasily T. Lebedev B.P.Konstantinov Petersburg Nuclear Physics Institute, NRC “Kurchatov Institute”, Gatchina, Leningrad Distr., Russia

Contributors

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M. Mahalakshmi Department of Chemistry, Sri Sivasubramaniya Nadar College of Engineering, Kalavakkam, India Lalan Chandra Mandal Department of Chemistry, Krishna Chandra College, Hetampur, Birbhum, West Bengal, India R. Martins CENIMAT|i3N, Department of Materials Science, School of Science and Technology, NOVA University of Lisbon and CEMOP/UNINOVA, Caparica, Portugal M. L. Matias CENIMAT|i3N, Department of Materials Science, School of Science and Technology, NOVA University of Lisbon and CEMOP/UNINOVA, Caparica, Portugal Krishnendu Mukhopadhyay Department of Environmental Health Engineering, Sri Ramachandra Faculty of Public Health, Sri Ramachandra Institute of Higher Education and Research, Chennai, Tamilnadu, India Paulpandian Muthu Mareeswaran Department of Industrial Chemistry, Alagappa University, Karaikudi, Tamilnadu, India; Department of Oceanography and Coastal Area Studies, Alagappa University, Karaikudi, Tamilnadu, India Subhasha Nigam Amity Institute of Biotechnology, Amity University, Noida, Uttar Pradesh, India D. Nunes CENIMAT|i3N, Department of Materials Science, School of Science and Technology, NOVA University of Lisbon and CEMOP/UNINOVA, Caparica, Portugal Vera A. Orlova V.G.Khlopin Radium Institute, St.Petersburg, Russia Fehime Çakıcıo˘glu Özkan Department of Chemical Engineering, Faculty of Engineering, Izmir Institute of Technology, Urla, ˙Izmir, Turkey Kandhan Palanisamy Department of Chemistry, Faculty of Engineering and Technology, SRM Institute of Science and Technology, Kattankulathur, Chengalpattu, Tamil Nadu, India Georgy S. Peters NRC “Kurchatov Institute”, Moscow, Russia A. Pimentel CENIMAT|i3N, Department of Materials Science, School of Science and Technology, NOVA University of Lisbon and CEMOP/UNINOVA, Caparica, Portugal J. V. Pinto CENIMAT|i3N, Department of Materials Science, School of Science and Technology, NOVA University of Lisbon and CEMOP/UNINOVA, Caparica, Portugal Maria V. Popova B.P.Konstantinov Petersburg Nuclear Physics Institute, NRC “Kurchatov Institute”, Gatchina, Leningrad Distr., Russia

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Contributors

Muthuramalingam Prakash Department of Chemistry, Faculty of Engineering and Technology, SRM Institute of Science and Technology, Kattankulathur, Chengalpattu, Tamil Nadu, India Deepak Singh Rajawat Department of Chemistry, IIS (Deemed to be University), Jaipur, India Mohd Amirul Ramlan Program and Institutional Planning Division, Department of Polytechnic & Community Colleges Education, Ministry of Higher Education, Persiaran Perdana, Putrajaya, Malaysia Manoj Raula Amity Institute of Applied Sciences, Amity University, Noida, India Ajay K. Ray Department of Chemical and Biochemical Engineering, University of Western Ontario, London, ON, Canada Suresh Sagadevan Nanotechnology and Catalysis Research Centre, University of Malaya, Kuala Lumpur, Malaysia Bidyut Saha Homogeneous Catalysis Laboratory, Department of Chemistry, The University of Burdwan, Burdwan, West Bengal, India Usman Saidu Department of Chemistry, Sule Lamido University, Jigawa State, Kafin Hausa, Nigeria Aditi Saxena Department of Chemistry, Amity Institute of Applied Sciences, Amity University, Noida, Uttar Pradesh, India Gizem Saygı Department of Chemical Engineering, Faculty of Engineering, Izmir Institute of Technology, Urla, ˙Izmir, Turkey Sucheta Sengupta Amity Institute of Advanced Research and Studies (Materials and Devices), Amity University, Noida, India Marimuthu Senthilkumaran Department of Industrial Chemistry, Alagappa University, Karaikudi, Tamilnadu, India; Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Pashan Road, Pune, Maharastra, India Venkatesan Sethuraman Research and development, New Energy Storage Technology, Lithium-Ion Battery Division, Amara Raja Batteries Ltd, Karakambadi, Andhra Pradesh, India Chinmay Rakesh Shukla Amity Institute of Nanotechnology, Amity University, Uttar Pradesh, Noida, India N. Subha Department of Chemistry, Sri Sivasubramaniya Nadar College of Engineering, Kalavakkam, India Sheenam Thatai Department of Chemistry, Amity Institute of Applied Sciences, Amity University, Noida, Uttar Pradesh, India

Contributors

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Timur V. Tropin Joint Institute for Nuclear Researches, Dubna, Moscow Dist., Russia S. Uday Department of Chemistry, Amity Institute of Applied Sciences, Amity University, Uttar Pradesh, Noida, India Sumant Upadhyay Amity Institute of Nanotechnology, Amity University, Uttar Pradesh, Noida, India Canan Varlikli Department of Photonics, ˙Izmir Institute of Technology, Urla, ˙Izmir, Turkey Mikhail A. Vovk St. Petersburg State University, St. Petersburg, Russia Alexander Ya. Vul Ioffe Institute, St. Petersburg, Russia R. Xue CENIMAT|i3N, Department of Materials Science, School of Science and Technology, NOVA University of Lisbon and CEMOP/UNINOVA, Caparica, Portugal Natalia P. Yevlampieva Saint Petersburg State University, St. Petersburg, Russia Vladimir G. Zinovyev B.P. Konstantinov Petersburg Nuclear Physics Institute, NRC “Kurchatov Institute”, Gatchina, Russia

Part I

Synthesis of Photocatalyst by Various Methods

Chapter 1

Modification of Detonation Nanodiamonds with Endofullerenols to Obtain Magnetic Photosensitive Structures for Theranostics Vasily T. Lebedev, Yuri V. Kulvelis, Alexander Ya. Vul, Georgy S. Peters, Mikhail A. Vovk, Vera A. Orlova, Timur V. Tropin, Maria V. Popova, Olga I. Bolshakova, and Eduard V. Fomin Abstract Progress in nanodiamond technologies allows regulate diamond surface properties, amounts of grafted functional groups, and positive (negative) potential of particles in aqueous media. It stimulates the applications of diamonds as V. T. Lebedev (B) · Y. V. Kulvelis · M. V. Popova · O. I. Bolshakova · E. V. Fomin B.P.Konstantinov Petersburg Nuclear Physics Institute, NRC “Kurchatov Institute”, Gatchina, Leningrad Distr., Russia e-mail: [email protected] Y. V. Kulvelis e-mail: [email protected] M. V. Popova e-mail: [email protected] O. I. Bolshakova e-mail: [email protected] E. V. Fomin e-mail: [email protected] A. Ya. Vul Ioffe Institute, St.Petersburg, Russia e-mail: [email protected] G. S. Peters NRC “Kurchatov Institute”, Moscow, Russia M. A. Vovk St. Petersburg State University, St. Petersburg, Russia e-mail: [email protected] V. A. Orlova V.G.Khlopin Radium Institute, St.Petersburg, Russia T. V. Tropin Joint Institute for Nuclear Researches, Dubna, Moscow Dist., Russia e-mail: [email protected]

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Garg and A. Chandra (eds.), Photocatalysis for Environmental Remediation and Energy Production, Green Chemistry and Sustainable Technology, https://doi.org/10.1007/978-3-031-27707-8_1

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advanced nanoplatforms with chemical and radiation resistivity (laser light, UV, X- and γ-rays) and luminescent properties useful for catalysis and biomedicine. The modification of diamonds with fullerenes C60 , C70 , and endofullerenes M@C2n (n ≥ 30) with captured magnetic metal atoms (M) of 4f and 3d elements is a prospective way to create medical preparations based on carbon and metal–carbon structures as active scavengers of free radicals, photosensitizers, minimally toxic, and effective contrasting agents for MRI diagnostics owing to magnetic atoms encapsulated inside firm carbon cages. Authors have developed the synthesis of fullerenes and endofullerenes with 4f, 3d elements and found new ways to transform pristine carbon structures to water-soluble fullerenols by two-stage hydroxylation. The studies of fullerenols and endofullerenols by optical absorption, Raman spectroscopy, small-angle neutron and synchrotron radiation scattering, and other methods have confirmed their expected structure, the coordination of atoms, and showed fractal molecular ordering in aqueous media. Further, taking various proportions of components, the authors prepared the complexes by the association of electronegative Gadolinium fullerenols with diamonds carrying positive charges. The stability of such structures was proved during cyclic temperature variation (25–70–25 °C) when their ordering in solutions was detected at nanoscales by X-ray scattering. Following NMR measurements (25 °C) on protons in these aqueous systems allowed to find longitudinal (T 1 ) and transversal (T 2 ) relaxation times: T 1 < T 2 in pure diamond dispersion, T 1 ≤ T 2 in fullerenol solution, but T 1 « T 2 in the dispersion of complexes. Thus, by complexing there were prepared so-called negative contrast agents very needed in MRI practice. Final biological tests on cell cultures showed low toxicity of complexes that is desirable for the implementation in theranostics. Keywords Endofullerene · Diamond · Complex · Magnetic · Theranostics · Radiation · Biology

1.1 Introduction The development of biomedicine in the areas of photodynamic therapy (PDT), magnetic resonance and computed X-ray tomography (MRI, CT), and diagnostics using luminescent labels is currently largely determined by the development of nanostructures that together have photocatalytic, luminescent, and magnetic properties [1– 11]. A wide field of photochemotherapy includes PDT as a method, the principle of which is that the injected drug, a photosensitizer (PS), under the action of laser light, enters an excited state in order to transfer energy to molecular oxygen in tissues. As a result, oxygen passes from the basic triplet state to the reactive singlet state and predominantly destroys tumors, in the cells of which PS is accumulated mainly on membranes and mitochondria [12–14]. It is very important to combine PDT and fluorescent diagnostics. This is possible due to the choice of PS, for example, among many derivatives of hematoporphyrin, chlorins, benzoporphyrins, pheophorbides, porphycenes, phthalocyanines, and naphthalocyanines [12]. It is known [15] that

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water-soluble phthalocyanines photoinactivate bacteria. Molecules of lutetium diphthalocyanine (LuPc2 ) are efficient generators of singlet oxygen, while other representatives of the lanthanide diphthalocyanines (LnPc2 ) series acquire this ability as a result of protonation with acids [16]. The required combination of functional characteristics in preparations for theranostics can be achieved by attaching PS to nanoparticles capable of being photoactive nanoplatforms with pronounced luminescent properties [17–29]. Of considerable importance is the targeted delivery of drugs (PS), in particular, through their adsorption on magnetic particles while maintaining the photodynamic activity of the PS [30]. The authors [30] combined hydrophobic bacteriochlorin (PC) with magnetite particles for targeted delivery and concentration of the drug in the affected organ during MRI. Thus, it was possible to control the accumulation of PS in the lesion region, reducing the time of surgical intervention. Achievements in the field of medical applications of magnetic nanoparticles are discussed in review [31]. In addition to porphyrins and derivatives traditionally used as PSs, fullerenes (C60 , C70 ) and their endohedral complexes with metals are of interest. In particular, complexes with metal nitrides, M3 N@C80 , showed a strong photodynamic effect in killing gram-positive and gram-negative bacteria [32]. Endofullerenes Dy@C82 , Gd@C82 , and La@C82 generate singlet oxygen well, which has been proven by the example of olefin oxidation [33, 34]. Li@C60 complexes are superior in this respect to fullerene C60 [35]. For medical applications of endofullerenes, it is necessary to solve the problems of transferring these hydrophobic molecules into aqueous media. This is possible by grafting hydrophilic adducts to them. No less attractive is the binding of fullerenes and endofullerenes with hydrophilic nanoplatforms (watersoluble polymers, diamonds, and other particles) [36, 37]. For example, C60 fullerenes were grafted with functional groups through which they were bound to gold particles [36]. The advantage of fullerenes over other molecular objects was that, together with high photodynamic activity, they were resistant to the action of singlet oxygen in the ground energy state. Oxidation of C60 and C70 occurred only during the interaction of singlet oxygen with triplet-excited molecules [38]. Molecules C60 and C70 were coupled with hydrophilic polymers (γcyclodextrin, polyvinylpyrrolidone), providing solubility in water while maintaining catalytic properties [39]. In many cases, the modification of the fullerene surface occurs through the addition of light atoms (C, O, H). This leads to the appearance of sp3 hybridization of carbon orbitals at the sites of grafting among the sp2 carbon of the fullerene, which affects the photoactivity of the fullerene [40]. The authors [40] studied the photophysical properties of aqueous solutions of nanoparticles based on fullerene and derivatives, C60(OH)x(ONa)y (x + y = 24, y = 6–10), under UV (350 nm) excitation, detecting quantum yields for singlet oxygen (1 O2 ) by luminescence, 15– 20% and 6%, respectively. Although the fluorescent quantum yield for fullerenols was only 0.3%, the Gaussian fluorescence spectrum in the wavelength band λ ~ 500– 750 nm had a maximum at λ ~ 600 nm. The spectrum included the absorption band of the medical PS, Radachlorin (RC, 662 nm). So fullerenols, even with a low quantum yield for singlet oxygen, can serve as converters of UV and X-ray quanta to the

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visible range for RC activation in PDT. Along with this, a significant fact is that the photochemical action of fullerenes and derivatives is not limited by the generation of singlet oxygen. This follows from the results [41]. In this study, the phototoxicity of [(γCyD)2 /C60 ] complexes with fullerene in the cavity between γ-cyclodextrin molecules for human keratinocytes in comparison with a similar characteristic of C60 (OH)24 fullerenols was determined [41]. Under UV excitation of complexes and fullerenols in D2 O, singlet oxygen was detected with quantum yields that differed by an order of magnitude (0.76 and 0.08). At the same time, confocal fluorescence microscopy showed a higher level of hydrogen peroxide production in cells incubated with fullerenol under illumination. Hence, by means of C60 (OH)24 , mainly superoxide O2 ·− was generated. The phototoxicity of fullerenol was due to free radical reactions rather than the formation of singlet oxygen [41]. As a result, cell viability data showed that the (γ-CyD)2 /C60 complex is ~60 times more toxic than C60 (OH)24 fullerenol [41]. It is clear from the analysis performed that the C60 fullerenes are favorable for the generation of singlet oxygen (96% quantum yield). However, the functionalization of fullerenes sharply reduces the quantum yield with respect to singlet oxygen, and for C60 (OH)24 fullerenols, the effect is lower by an order of magnitude [39]. The factors that determine the catalytic activity of fullerene derivatives are discussed in review [42] in connection with the development of composite photocatalysts with fullerenes (problems of organic synthesis and hydrogen production, decomposition of pollutants, and disinfection of water with catalytic antibacterial agents). Along with the creation of such PDT preparations, it is necessary to search for nanoplatforms (polymers, metal oxides, diamonds) capable of maintaining and enhancing the photodynamic activity of the delivered preparations. There are reasons to consider detonation nanodiamonds (DND) as such, which have a special set of physicochemical and photocatalytic properties to serve for the delivery of functional molecules to biological media as antibacterial agents, contrast agents, phosphors and PS for MRI and PDT, and luminescent labels for control of the distribution of the administered drug in the organs. DND-based platforms are attractive as radiation converters for PS excitation in X-ray PDT (X-PDT). As a rule, diamonds exhibit pronounced luminescent properties in the fields of ionizing radiation (electrons, UV, X-ray quanta), while remaining resistant to the action of radiation and chemical factors (oxidizing agents, free radicals) [43, 44]. At the same time, DND particles have a large specific surface area. It can be grafted with various functional groups (hydroxyls, carboxyls, etc.), molecules, drugs, metal ions, for example, lanthanides [45–48], which can ensure the conversion of X-ray radiation to the visible range for PS excitation. It is quite possible to create luminescent complexes that include PS and medical polymers on diamond platforms with lanthanide ions both on the surface and inside the diamond lattice [44, 49–54]. In this work, the conditions for obtaining and synthesizing molecular complexes of Gd@C82 (OH)X (X ~ 30) endofullerenols with hydrophilic DND particles were found, and the physicochemical and structural properties of these new objects were studied. In the final experiments, they were tested for toxicity against various cell cultures. As

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the basis of molecular complexes, selected DND crystals with useful physicochemical and photocatalytic properties can serve to deliver various functional molecules to biological media (antibacterial agents, contrast agents, phosphors and PS for MRI and PDT, and luminescent labels to control the distribution of the administered drug in organs). In this case, water-soluble endofullerenols Gd@C82 (OH)X (X ~ 30) were chosen as such functional molecules as highly pronounced antioxidants and PS resistant to UV, X-ray, gamma, and neutron radiation [8, 10, 39–41, 55–57]. In the Gd@C82 (OH)X molecule, the ion Gd3+ is a strong phosphor in blue spectral region (~400 nm) [16], which corresponds to the wavelengths in the Soret absorption band of commercially used PSs (RC). In addition, the Gd atom has a magnetic moment. Therefore, endofullerenol molecules are capable of accelerating the transverse spin relaxation of protons in a biological environment, creating a contrast in MRI diagnostics. In addition, the binding of endofullerenols to DND makes it possible to obtain an MRI preparation with magnetic molecules on the surface of a diamond carrier in aqueous media. This ensures maximum interaction of functional molecules with the environment to enhance contrasting while achieving both luminescent and photodynamic effects. DND complexes with endofullerenols were analyzed in aqueous solutions by optical absorption spectroscopy and small-angle X-ray scattering, which made it possible to study the nature of the ordering of diamonds and the molecular component in the complexes depending on their composition, total concentration, and temperature on the scales of ~100 –103 nm.

1.2 Experimental 1.2.1 Samples and Methods The resulting binary complexes were based on detonation nanodiamonds that underwent several stages of technological processing [45, 58]. It involved first grinding and etching in acids of initial powders composed of diamond aggregates in order to obtain disaggregated diamonds of a certain size (4–5 nm) with a high surface quality, free from amorphous carbon (graphene fragments that initially covered diamond crystals). Further annealing in a hydrogen flow (500 °C) made it possible to obtain DNDZ+ diamond crystals with hydrogen atoms and hydroxyls grafted to the surface, which ensured a positive potential of particles in aqueous media (30–70 mV) [45]. Using a similar heat treatment in air flow (450 °C), carboxyl groups were attached to the surface of the particles, which served to create a negative potential of DNDZparticles in water [45]. Giving a charge to diamond particles guaranteed for many months the stability of their aqueous colloids with diamond content of up to ~3 wt%. Concentration of

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systems up to 5–7 wt% led to their transformation into thixotropic hydrogels. It was a consequence of the aggregation of particles into linear chains with their following binding. It led to the formation of diamond network according to neutron scattering and rheology data [46, 59]. Water-soluble derivatives of endofullerenes [60, 61], endofullerenols Gd@C82 (OH)X (X ~ 30) with gadolinium atoms inside C82 cages, contain a number of OH groups grafted as a result of hydroxylation in two stages [62]. This provided the required degree of solubility of Gd@C82 (OH)X molecules in water [63–65]. To obtain endofullerenes, the electric arc method was used to evaporate tubular graphite electrodes filled with Gd2 O3 powder (1 wt%). The resulting carbon soot was subjected to extraction in xylene to remove some nonpolar fractions such as empty fullerenes (C60 , C70 , and higher homologues). Then the residue was dissolved in dimethylformamide to isolate polar Gd@C82 molecules. The resulting samples of Gd@C82 and Gd@C82 (OH)X derivatives were certified in terms of chemical composition by X-ray fluorescence analysis [66]. The endohedral structure of the obtained metal–carbon molecules, the localization of the metal atom in the cavity of the carbon cage, and its coordination with carbon atoms within the first and second coordination spheres are confirmed by the EXAFS method, similarly to experiments on the endofullerenes with iron atoms [67]. Previously, such kind endofullerenes were tested by Mössbauer spectroscopy that testified the endohedral structure of Dy@Cn (n = 80, 82, 84) [68]. For the preparation of complexes (DF1), the proportion between the mass concentrations of fullerenols and diamonds C Ful :C DND = 1:10 was initially chosen (Table 1.1). Sample DF1 with DNDZ+ diamonds had the concentration of C DND = 1.29 wt% and contained an order of magnitude less fullerenols, C Ful = 0.129 wt%. Nevertheless, such a proportion ensured that the surface of diamonds was coated with fullerenols, according to estimates of the number of their molecules and the total surface area of diamond particles. The component mixtures were exposed to ultrasound at room temperature, which led to the formation of a homogeneous colloid. The resulting complex showed long-term stability at room temperature as observed over the course of a month. In further studies of the DF1 complex, its physicochemical and structural properties were compared with those for the corresponding DF4 diamond dispersion with the same diamond content as in the complex (Table 1.1). Another reference sample was the aqueous solution of fullerenols with the concentration C Ful = 0.129 wt% (DF7, Table 1.1). Then diluted aqueous colloids of the complex, diamonds, and fullerenols (DF1, DF4, DF7) were tested in optical density measurements D(λ) in the wavelength range λ = 190–1100 nm (Fig. 1.1). In these systems, an increase in light absorption at low wavelengths was observed. It is mainly explained by scattering on colloidal particles (molecular aggregates). For fullerenol, the characteristic absorption peak was detected with the maximum at λ = 199 nm. DNDZ+ diamonds showed the absorption peak in ultraviolet region at shorter wavelength λ = 192 nm. Both peaks are visible for the complex while shifted toward longer wavelengths (206 and 197 nm) that testified the binding of the components (Fig. 1.2).

1 Modification of Detonation Nanodiamonds with Endofullerenols … Table 1.1 Composition of the samples of aqueous dispersions of nanodiamonds (DNDZ+, DNDZ−), endofullerenols with gadolinium, and complexes of the components

Fig. 1.1 Optical density of the dilute aqueous solution of DF1 complex (concentration 0.0070 wt%), DNDZ+ diamond dispersion (0.0064 wt%) and fullerenol solution (0.0031 wt%) (1–3)

Fig. 1.2 Optical density data in the short-wavelength region for DF1 complex, DNDZ+ diamond dispersion, and fullerenol solution (1–3) at the same concentrations as in Fig. 1.1. The positions of absorption maxima are marked

9

Sample No.

C DND , wt%

C Ful , wt%

C Ful /C DND

DF1

1.29 (DNDZ+)

0.129

1/10

DF2

0.022 (DNDZ+)

0.011

1/2

DF3

0.30 (DNDZ+)

0.15

1/2

DF4

1.29 (DNDZ+)

0

0

DF5

0.022 (DNDZ+)

0

0

DF6

0.30 (DNDZ+)

0

0

DF7

0

0.129

–

DF8

0

0.011

–

DF9

0

0.15

–

DF10

0.32 (DNDZ−)

0

0

DF11

0.32 (DNDZ−)

0.15

1/2

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In the following experiments, DF2 complexes were prepared under conditions of higher enrichment in fullerenols (mass ratio for components C Ful :C DND = 1:2), but at low concentrations of diamond DNDZ+ (C DND = 0.022 wt%) and fullerenol (C Ful = 0.011 wt%), so that ensure better dispersion of those and other particles in an aqueous solution (Table 1.1). Reference samples were DF5 and DF8. To find out how much the structure of the fullerenol-enriched complex depends on its content in solution, we prepared more concentrated colloid DF3 with amounts of diamonds and fullerenols, C DND = 0.3 wt%, C Ful = 0.15 wt%, as well as one-component samples DF6 and DF9 (Table 1.1). A similar procedure for mixing negatively charged DNDZ- diamonds with fullerenols did not allow to prepare a stable colloid. In binary system DF11 with the concentrations C DND = 0.3 wt% and C Ful = 0.15 wt%, we observed a separation of components, since their particles had charges of the same sign (negative) and repel each other in solution. In further experiments with the DF11 system, colloids DF9 and DF10 served as reference samples. The samples were studied by small-angle scattering of synchrotron radiation under conditions of temperature variation (25–70 °C) in a heating–cooling cycle to determine the degree of thermal stability of colloids (SAXS station STM, Synchrotron of Kuchatov Institute, Moscow). The samples were poured into thin-walled glass capillaries and placed in a temperature-controlled unit for measuring synchrotron radiation scattering. Background measurements were carried out using a capillary filled with water. The scattering intensities from the sample I(q) = I S (q) – I W (q) depending on the modulus of the scattering vector (q) were found as the difference between the data for the sample and water. Additionally, the samples were studied by NMR on protons to evaluate the magnetic relaxation characteristics in relation to the effect of magnetic gadolinium atoms on spin relaxation of protons in the surrounding water. At 25 °C for the complexes of DNDZ+ (0.3 wt%) and Gd@C82 (OH)X (mass ratio fullerenol: diamond = 1:2) and for the samples of diamonds, fullerenos, and pure water, the proton NMR measurements were carried out (Bruker Avance III 500 MHz, Resource Center “Magnetic Resonance Research Methods”, St-Petersburg State University). As a result, the longitudinal and transverse relaxation times (T 1 , T 2 ) were found for samples DF3, DF6, and DF9. This made it possible to estimate the relaxivities of solutes, rl1,2 = [1/T 1,2 −1/T 1,2w ]/CS , from the relaxation times for the samples (T 1,2 ) and water (T 1,2w ) after a normalization of the differences of reciprocal relaxation times to the total concentration (C S ) of dissolved substances (mg/ml). Finally, the samples were tested for toxicity to biological cell lines. We have chosen the complexes DNDZ+ (0.67 mg/mL) + Gd@C82 (OH)x (0.33 mg/mL) with the mass ratio fullerenol: diamond = 1:2 and determined their toxicity on two types of biological cells. We used non-tumor ECV cells (human umbilical vein endothelial cells) and HeLa tumor cells (human cervical carcinoma cells) (Institute of Cytology of RAS, St.Petersburg, Russia). Cells were cultured under standard conditions (DMEM medium containing glutamine (Capricorn Scientific, Germany), antibiotics (penicillin and streptomycin, Biolot, Russia), and BS 10% (Biolot, Russia). The incubation at 37 °C

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in the atmosphere with 5% CO2 (24 h) after seeding in the plates was performed (96 wells, 10,000 cells per well). Then, the complexes were added to the cells at different concentrations (1; 10; 100 µg/mL). The culture without complexes was used as a control. After incubation (24 or 48 h), 10 µl of MTT working solution (5 mg/ml) were added to the wells of the plate (except of wells serving for comparison) and the incubation continued for another 3 h. Then the medium was removed from all wells, and 100 µl of DMSO (dimethyl sulfoxide, Serva) to dissolve the formazan crystals was added. After 30 min incubation at room temperature until complete dissolution of the formazan crystals, the optical density (D) of the contents of the wells was measured (spectrophotometer Multiscan FC, Thermo Scientific, wavelength 540 nm). Cell viability in the samples was determined as a percentage relative to the control sample by the formula [D – Do ]/[Dcont – Do ], where Dcont and Do are the data for the control sample and the sample without MTT. The difference between experimental and control data was determined using the Tukyer-Kramer test of the Kyplot program. Differences were considered statistically significant when p < 0.05 (probability of error in rejecting the null hypothesis). Possible morphological changes in cells due to the action of the complexes were determined visually.

1.2.2 Formation of Complexes in Aqueous Solutions: Synchrotron Scattering Data The electrostatic attraction between positively charged diamonds and fullerenols, which have acquired a negative charge due to the partial splitting of protons, leads to the formation of binary complexes. This is facilitated by the hydrophobic interactions of the components, since the grafted hydrogen atoms and hydroxyls can be nonuniform located on the faces of DNDZ+ diamonds, as well as on the surface of fullerenols according to the data of quantum chemical modeling and neutron studies of fullerenols in solutions [69, 70]. As a result of the mixing of the components with the subsequent exposure of the solution to ultrasound, the initial structures of the components are destroyed and stable binary aggregates of diamonds and fullerenols are formed. This is directly indicated by the data of the initial and final experiments at 25 °C in the temperature cycle with the samples DF1, DF4, and DF7 (Fig. 1.3). In these experiments, scattering by dense diamond particles much exceeded the effect from fullerenols (by two orders of magnitude). The data for them are shown in Fig. 1.3 with a factor of 10 for the convenience of comparing the results. Due to the introduction of small amount of fullerenols (10 wt%) into the ensemble of diamond particles, a qualitative change in the behavior of the scattering intensity occurred (Fig. 1.3). In the initial DF4 dispersion with DNDZ+ diamonds, the intensity profile had a maximum at the modulus of scattering vector qmax = 0.0613 nm−1 .

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Fig. 1.3 Synchrotron radiation scattering intensities I(q) at 25 °C for the samples in the initial state, DF1 complex (1), dispersion of diamonds DF4 (2), fullerenols DF7 (3), and the data (4–6) for the same samples (25 °C) after thermal cycle versus modulus of scattering vector (q). The data for sample DF7 are plotted with a factor of 10 for ease of comparison

This indicated a short-range order in the arrangement of diamond aggregates at the characteristic distance L 1 ~ 2π /qmax ~ 100 nm. However, in the binary system, a monotonic increase in intensity at low scattering vectors was detected. This indicated a scattering on individual aggregates not ordered in solution. Along with this, in both systems, the nanostructures showed a good stability in the heating–cooling cycle. At 25 °C at the beginning and at the end of the cycle, the structural data differed little (Fig. 1.3) and were close to the results at maximum temperature (70 °C) (Fig. 1.4). In the dispersion of DNDZ+ diamonds, when heated to 70 °C, no shift of the peak of scattering curve indicated the temperature stability of the sample structure. The solutions of the complexes were stable also when heated, as well as the system with fullerenols (Fig. 1.4). Further, similar stability was observed for the complexes enriched with fullerenols (C Ful :C DND = 1:2) and their components (samples DF3, DF6, DF9) (Fig. 1.5). Fig. 1.4 Scattering intensities, I(q), at 70 °C for the solutions of the DF1 complex (1), diamonds DF4 (2), and fullerenols DF7 (3, data multiplicated to a factor of 10 for ease of comparison)

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Fig. 1.5 Scattering intensities for DF3 complexes (1) (mass ratio fullerenol: diamond = 1:2) in aqueous solution at initial temperature (25 °C). Similar data for diamond dispersion DF6 and fullerenol solution DF6 (2, 3). The curves (4–6) are plotted for the same samples at the end of thermal test (25 °C)

In solutions of DF3 complexes (Fig. 1.5), the concentration of diamonds (C DND = 0.3 wt%) was four times lower than in the colloid of DF1 complex (Fig. 1.3), while the amounts of fullerenols were the same in both cases (Table 1.1). In Fig. 1.5, the data for the DF6 diamond dispersion show a peak of intensity with a maximum at qmax = 0.0493 nm−1 . It corresponds to the spacing between aggregates, L 2 ~ 2π /qmax ~ 130 nm, higher by 30% than that in the DF4 sample with diamond share four times more. The appearance of diffraction maximum can be explained by the contacts of aggregates in solution at a distance of their diameter. The size of an aggregate is related to its mass by the relation L 2 ~ M β where the index β = 1/2 and 1/3 in the cases of chain (Gaussian) and globular formations. From the ratio of distances between aggregates in the samples DF6 and DF4, it follows that in the dilute system, the aggregate mass M 2 = M 1 (L 2 /L 1 )1/β is approximately two times bigger than that in concentrated dispersion. The difference between the systems is also manifested in the fact that the structure of the sample with a reduced amount of diamonds changes to a greater extent in heating–cooling cycle (Fig. 1.5). This difference is also expressed for complexes DF1 and DF3. At a lower concentration of diamonds, they show a greater relative increase in scattering intensity by thermal cycling (Figs. 1.3 and 1.5) which caused a change in the mass of aggregates. At the same time, at the maximum temperature (70 °C), DF3 complex demonstrates a higher increase in the scattering intensity (Fig. 1.6) and the growth of the mass of aggregates comparative to this one for DF1 sample (Fig. 1.4). The influence of diamond share on the structure of solutions of complexes (25 °C) is seen in Fig. 1.7, where presented scattering intensities are normalized to diamond concentrations. The data in Fig. 1.7 show that the variation of diamond concentration (0.022–1.29 wt%) and the amount of fullerenols (10–50 wt%) do not alter significantly main features of complexes structuring. In diamond dispersions, a similar normalization of the data has shown really good stability of diamond structures with a variation of concentration and the sign of charge of particles (Fig. 1.8).

14 Fig. 1.6 Scattering intensities, I(q) versus modulus of scattering vector (q), at 70 °C for solutions of the DF3 complex (1), diamonds DF6 (2), and fullerenols DF9 (3, data are given with a factor of 10 for ease of comparison)

Fig. 1.7 Scattering intensities I N (q) versus modulus of scattering vector (q), normalized to the concentration of diamond component (25 °C) for initial solutions of complexes DF1, DF3, DF2 (1–3) versus the modulus of scattering vector

Fig. 1.8 Scattering intensities I N (q) versus modulus of scattering vector (q), normalized to the concentration of the diamond component, for initial diamond dispersions DF4, DF6, DF10 (1–3) depending on the modulus of the scattering vector at a temperature of 25 °C

V. T. Lebedev et al.

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Scattering data in double logarithmic coordinates make it possible to judge the forms of self-organization in dispersions and complexes. The intensity profiles (Figs. 1.7 and 1.8) have linear sections, on which the behavior of the intensities approximately follows the power dependences I(q) ~ 1/q2 and I(q) ~ 1/q4 . Consequently, on scales ~2π /q ~ 10−1 nm, the diamond structures of the type of linear Gaussian chains dominated in such aqueous systems. It is noteworthy that the formation of complexes does not disturb the chain ordering of diamonds in aqueous media. The ordering manifests itself approximately the same for DNDZ+ and DNDZ− diamonds with positive and negative potentials (Figs. 1.7 and 1.8). For diamond dispersions, experiments with transmission electron microscopy (TEM) have shown that chain structures (linear, branched) of diamond particles are preserved even when water is removed from the dispersion layer (Fig. 1.9). The morphology of the dried dispersion of DNDZ+ diamonds is represented by linear fragments and branched fractal aggregates on a scale of ~100 nm. Aggregates of this size overlap (Fig. 1.9). This agrees with the data of synchrotron radiation scattering (Figs. 1.3, 1.4, 1.5 and 1.6). On the scattering curves (Figs. 1.3, 1.4, 1.5 and 1.6), the interference maximum corresponded to the distance between the contacting aggregates ~100 nm. The linear segments of the scattering curves in the range q ~ 0.07–0.7 nm−1 for solutions of complexes and diamond dispersions were approximated by power functions I(q) = AF /qDf . Finally, the fractal dimensions Df of the chain fragments that make up the aggregates and the amplitude factors Af were found. The temperature dependences of the parameters for the solution of complexes DF1 and the dispersion of diamonds DF4- are shown in Figs. 1.10 and 1.11. Fig. 1.9 Morphology of the dried layer of DNDZ+ aqueous dispersion of diamonds

16 Fig. 1.10 Amplitude factor and fractal dimension of diamond aggregates in aqueous solution of DF1 complexes versus temperature (a, b)

Fig. 1.11 Amplitude factor and fractal dimension of diamond aggregates in DF4 dispersion versus temperature (a, b)

V. T. Lebedev et al.

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In the complexes, during the temperature cycle, a non-monotonic behaviors of the parameters Af , Df are observed within small limits with a return almost to the initial values. It should be concluded that approximately equilibrium diamond structures with a fractal dimension Df ~ 2.3 are formed in solutions of the complexes that resembles some aggregates observed earlier in dispersions of detonation diamonds [46, 47, 59]. In the DF4 dispersion, diamonds are associated into chain structures with a fractal dimension of Df ~ 2.3. It differs little from that for the complexes in the initial state (25 °C) and when heated to 70 °C with subsequent cooling (Fig. 1.11b). At the same time, with almost the same fractal indices of structures in the DF1, DF4 systems, the scattering ability of the aggregates of the complexes is ~40% lower than that of similar structures in diamond dispersion. This means that during the preparation of the complexes in the binary mixture, the binding of diamonds to fullerenols took place, and this process competed with the association of diamond particles. As a result, the aggregation number for the complexes decreased in proportion to the ratio of the parameters AfCOM /AfDND ~ 0.6. The fractal description of the structuring of diamond particles in an aqueous dispersion and solution of complexes referred to the scales of ~101 –102 nm, much larger than the particle size. At the same time, for a general understanding of the structure of these systems, the information about the characteristics of particles (size, surface quality) and their spatial coordination at the primary level is important, at distances comparable to the particle diameter d P ~4.5 nm (data of dynamic light scattering in water diamond dispersion). The data in the range of moduli of the scattering vectors q ~ 0.7–1.1 nm–1 makes it possible to obtain such information when comparing the behavior of the scattering intensities with the Porod law for particles with a sharp boundary, I(q) = AP /q4 . Verification of the implementation of this law for the diamonds used is important to prove a high quality of their surface, free from amorphous carbon (fragments of graphene), which, as a rule, covers detonation diamonds. To clarify this fact, the data for the DF4 diamond dispersion and the DF1 complex were presented as q4 I(q) versus q4 argument and compared with the linear function. q 4 I (q) = A P + B · q 4 ,

(1.1)

where the constant AP = K N (ΔK)2 ϕ2π S t includes the calibration factor K N , the contrast factor of diamond particles ΔK against solvent, their volume fraction ϕ, and total surface area S t . The modified scattering intensities q4 I(q) obey a linear dependence (1.1) at 25–70 °C in the measurement cycle on the diamond dispersion DF4 (Fig. 1.12). Similar behavior was observed for q4 I(q) in the case of DF1 complexes (Fig. 1.13). For DF1 complexes, the parameter AP , which is proportional to the total particle surface area S t , is only ~60% of the value for the DF4 diamond dispersion. Consequently, the formation of complexes leads to a deficit in the area of the free surface of diamonds. This indicates additional binding of particles along diamond facets when

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Fig. 1.12 Modified scattering intensities q4 I(q) for DF4 diamond dispersion versus q4 . Data at 25 °C (initial sample) and 70 °C (1, 2). Lines are approximation functions (1.1)

Fig. 1.13 Modified scattering intensities q4 I(q) versus q4 for DF1 complex in aqueous solution. Data at 25 °C (initial sample) and 70 °C (1, 2). Lines are approximation functions (1.1)

their surface no longer contributes to scattering. Such an effect is observed in the entire temperature cycle of scattering measurements for these samples (Fig. 1.14). Diamonds in DF4 dispersion are characterized by a closed cycle (Fig. 1.14). In such a cycle, when the sample is heated, the parameter AP and the total free surface area S t of the particles first decrease that reflects the appearance of additional particle contacts. However, this trend reverses when the sample is heated to 70 °C, when the AP parameter and the total free surface area of the particles reach a maximum, which indicates a weakening of the particle contacts as a result of intense thermal motion of water molecules and particles. These structural changes are reversible. Cooling the system to 25 °C leads to the restoration of the initial structure of aggregates in the dispersion, when the total free surface area of the particles returns to the initial value, which can be considered equilibrium (Fig. 1.14). A different character of behavior was observed for complexes which showed open temperature cycle for the changes of the parameter AP (Fig. 1.14). As a result of thermal cycling, the AP value increased by ~20%, as did a free area of diamond

1 Modification of Detonation Nanodiamonds with Endofullerenols …

19

Fig. 1.14 Parameter AP , proportional to the total surface area of particles S t in diamond dispersion DF4 (1) and solution of complexes DF1 (2), versus temperature in the cycle of scattering measurements

particles in the complex. Consequently, the binding capacity of fullerenols decreased by heat treatment, possibly due to partial segregation of fullerenols during the rearrangement of the structure of diamond aggregates upon heating. It should be taken into account that the fraction of the free surface of diamonds depends on the manner of their binding at the primary level of particle coordination with the nearest neighboring particles. Information about this was obtained from data processing using the Debye scattering function, [ ]2 I (q) = Id / 1 + (qrC )2

(1.2)

Here, the first parameter I d is the intensity in the limit of small scattering vectors (forward scattering intensity), and the second one, r C ≤ d P , characterizes the scale of correlations within the first coordination sphere around a particle with a diameter d P . Accordingly, function (1.2) can be applied to the description of the data in the range of magnitudes of the scattering vector q ≥ 2π/(3d P ) ~ 0.5 nm−1 where primary particle groups are detected. As the analysis of the data showed, the function (1.2) satisfactorily approximates the experimental curves in the specified range of scattering√vectors for the DF4 dispersion and the DF1 complex at√25–70 °C. The values 1/ I(q) versus q2 , indeed, follow the linear dependences 1/ I(q) = I d −1/2 [1 + (q·r C )2 ] (Figs. 1.15 and 1.16). For DF4 dispersion and DF1 complexes, the fitting parameters I d , r C were found as the functions of temperature (Figs. 1.17 and 1.18). Although the temperature dependences of the parameters I d (T ), r C (T ) are completely different for the diamond dispersion and the solution of the complexes (Figs. 1.17 and 1.18), in both cases the corresponding profiles of the I d (T ) and r C (T ) curves are similar. Hence, these parameters are correlated. In a pure diamond dispersion, with an increase in temperature from 37 to 50 °C, an abrupt increment in the scattering intensity and the correlation radius of primary groups occurs, followed by a moderate growth in the parameters when the samples are heated to 70 °C and further upon cooling to 25 °C (Fig. 1.17).

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Fig. √ 1.15 Data 1/ I(q) versus q2 for DF4 dispersion of diamonds at the initial temperature (25 °C). The line shows the approximation of the data by function (1.2)

Fig. √ 1.16 Data 1/ I(q) versus q2 for the solution of DF1 complexes at the initial temperature of 25 °C. The line shows the approximation of the data by function (1.2)

Consequently, in the temperature range of 37–50 °C, the association of particles became stronger. After the thermal cycle, the size and mass of primary groups got greater magnitudes. With the size ratio r Cmax /r Cmin = 1.25 ± 0.01, the volume occupied by the group increases in the proportion (r Cmax /r Cmin )3 ≈ 1.95, but its mass increases more strongly as the intensity ratio I dmax /I dmin = 2.28 ± 0.04. Consequently, the volume fraction of particles in such primary formation increases by a noticeable amount, (I dmax /I dmin )/(r Cmax /r Cmin )3 −1 ≈ 17%, in the heating–cooling cycle. However, in the solution of complexes, an opposite temperature effect was observed (Fig. 1.18). By heating to 70 °C, the intensity I d and the correlation radius r C decreased, as did upon further cooling to 25 °C. In the groups, heat treatment caused a threefold decrease in the aggregation number according to the ratio I dmax /I dmin = 3.17 ± 0.04 with a one and a half decrease in their size, r Cmax /r Cmin

1 Modification of Detonation Nanodiamonds with Endofullerenols … Fig. 1.17 Fitting parameters versus temperature for scattering data approximation with function (1.2) for diamond dispersion DF4: a forward intensity I d , b correlation radius r C

Fig. 1.18 Fitting parameters versus temperature for scattering data approximation with function (1.2) for aqueous solution of DF1 complexes: a forward intensity I d , b correlation radius r C

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Fig. 1.19 Scattering intensity I d versus r C 3 for the solution of DF1 complexes and diamond dispersion DF4 (1, 2)

= 1.45 ± 0.01, and a slight increase in the proportion of particles in their volume, (I dmax /I dmin )/(r Cmax /r Cmin )3 −1 ≈ 4%. The analysis of the correlations between the parameters I d and r C for the samples showed (Fig. 1.19) that the scattering intensity in both cases rises approximately in proportion to the volume of the group, I d = α·r C 3 , with coefficients α d = 1.17 ± 0.03 arb. un. and α c = 0.72 ± 0.01 arb. un. for systems with diamonds and complexes, respectively. Normalization of the coefficient for complexes to the corresponding value for diamonds, α c /α d = 0.62 ± 0.02 = (ΔK c /ΔK d )2 gives the ratio of contrast factors for groups, ΔK c /ΔK d = 0.78 ± 0.01. In both cases, such a factor, ΔK c,d ≈ (ΔK DND ·vP ·n1c,1d )/(8π r C 3 ), is proportional to this one for diamond (ΔK DND ), particle volume (vP ), and aggregation number (n1c,1d ) with a normalization to the total volume of a group (8πr C 3 ). The ΔK c,d values are lower than the ΔK DND in proportion to the ratio of the particle-filled volume vP ·n1c,1d to the total group volume, 8π r C 3 . The relationship ΔK 1 /ΔK 4 < 1 means that the diamonds modified with fullerenols form groups with lower aggregation numbers, n1c /n1d = ΔK c /ΔK d = 0.78 ± 0.01. With the same type of particle packing in a group, this may be due to the fact that the diamonds with adsorbed fullerenols become larger by a doubled diameter of fullerenol, d F ≈ 1.3 nm. Due to the shell of fullerenols, the effective volume of such a particle increases relative to its initial value with the coefficient (1 + d F /d P )3 ≈ 2.1. For the probability of diamond binding ξ with contacts via fullerenols and opposite probability of direct contacts of diamonds (1–ξ), the value ξ is determined by the equation ξ = (n1d /n1c −1)/[(1 + d F /d P )3 –1] ≈ 0.26 ± 0.02. Thus, in the case of complexes, ~26% of the diamonds in the groups are associated through fullerenols and the primary groups are less dense than those in the diamond dispersion. Reducing the data for the scattering intensity to absolute units, using the known surface area of the particles, corrected for the fraction of closed surface (~50%), made it possible to find the aggregation numbers n1 (T ) in the diamond dispersion

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Fig. 1.20 Aggregation numbers n1 (T ) a for primary groups in the solution of DF1 complexes (1) and diamond dispersion DF4 (2) and the fraction of space filled with particles V F (T ) b in the volume occupied by the group in these samples (1, 2) versus temperature

and the solution of complexes versus temperature, and to estimate the fraction of the space (V F ) filled with particles in the volume of groups (Fig. 1.20). In the solution of DF1 complexes, primary groups are formed of modified diamonds. Such a group include n1c (T ) ~ 4–12 particles, which occupied the share of V Fc ~ 40% in the total volume of group. A purely diamond dispersion is characterized by stronger aggregation with numbers n1d (T ) ~ 9–19 which are one and a half times higher along with the increased volume fraction of particles V Fd ~ 60–70% in groups. Such local formations, comparable in size to individual particles, build large fractal aggregates in diamond dispersions and solutions of complexes. To obtain detailed information about the structure of the solutions of complexes, diamond dispersions, and the solutions of fullerenols, we reconstructed the correlation functions from the scattering data using the ATSAS software package [71, 72].

1.2.3 Analysis of Spatial Correlations Between Scattering Centers in Aqueous Systems of Complexes, Diamonds, and Fullerenols The indirect Fourier transform of the data for the scattering intensities I(q) made it possible to obtain pair spatial correlation functions γ (R) for scattering centers

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located at different distances R ~ 100 –102 nm. Centers are understood as individual atoms in diamonds and fullerenol molecules, whole molecules and diamonds, as well as aggregates of these objects. In isotropic samples, to describe correlations, it is convenient to use the distribution function P(R) = R2 γ (R) of distances between scattering centers. The P(R) spectrum for the solution of DF1 complexes, shown in Fig. 1.21, at the temperatures 25–70 °C demonstrates a broad peak in the range of radii R = 0–150 nm that testifies the aggregates at these scales. The P(R) distribution maximum maintains a stable position, Rmax ~ 40 nm, with temperature variation in the heating–cooling cycle. The peak amplitude remains approximately constant when the sample is heated to 70 °C, but subsequent cooling enhances the structuring of the complexes, as evidenced by the increase in amplitude (Fig. 1.21, data 2). From the spectra obtained at each temperature, the integral parameters of the aggregates, the intensity I o in the limit of small scattering vectors, and the gyration radii of the aggregates RG were found (Fig. 1.22). As it turned out, the heating of DF1 sample leads to a slight decrease in the scattering intensity I o with a gradual decrease in the size RG of the aggregates of complexes. Subsequent cooling leads to the recovery of the intensity with an increase of ~20%, while the radius of gyration of these structures showed a smaller growth by ~6%. As a result of the passage of the thermal cycle, the structures of the complexes have approached the equilibrium state. Finally, their characteristics, intensity, and radius have increased by 14 and 3%, respectively (Fig. 1.22). As we found, such massive aggregate includes more than two hundred particles (nA ) (Fig. 1.23a). For the aggregation numbers, nA ~ 220–270, it was estimated a volume fraction of diamonds (V AF ) in the region occupied by an aggregate with a total volume vA = (4π /3)3/2 RG 3 . The fraction V AF = (nA vP )/vA ~ 0.017–0.018 is almost five times

Fig. 1.21 Correlation spectra for the solution of DF1 complexes at the beginning and end of the cycle (25 °C) (1, 2) and at 37; 50; 70 °C (3–5)

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Fig. 1.22 Integral parameters of complexes DF1, intensities I o , and gyration radii RG , versus temperature (a, b)

Fig. 1.23 Aggregation numbers nA a and volume fractions of diamond particles V AF b in the volume occupied by an aggregate of DF1 complex in aqueous solution versus temperature. The horizontal line (b) corresponds to the volume fraction of diamonds in solution

higher than the volume content of diamonds in the solution of complexes, ϕ = 0.0037 (Fig. 1.23b). Such aggregates consist of 10–20 primary groups according to the obtained data (Figs. 1.20 and 1.23). Relatively slight structural changes in the solutions of the complexes during the passage of the thermal cycle (Fig. 1.23) indicate a stability of aggregates. As a result of temperature cycling, the aggregates have increased their mass, size, and density within small limits. Binding of

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fullerenols with diamonds stimulated their ordering into the structures with the diameter DAG ~ 2RG ~ 80–90 nm. This is smaller than a spacing between aggregates, L AG ~ N AG −1/3 ~ 150 nm, with numerical concentration N AG = N P /nA ~ 3.2·1014 cm−3 , where N P ~ 7.7·1016 cm−3 is the number of diamonds at the content of 1.29 wt%. At given DAG and L AG , the aggregates occupy only a small part of the sample volume, ϕ AG ~ (π /6)DAG 3 N AG ~ 10%. Therefore, their contacts are rare, and corresponding spatial correlations are not observed (Fig. 1.21). At the same time, in diamond dispersion DF4 there were detected qualitatively different patterns of structuring (Fig. 1.24). Compared to the solution of complexes, the correlations of diamond particles in the dispersion are characterized by the presence of two spectral peaks. The first of them corresponds to the aggregates with the radius R1max ~ 20 nm (peak maximum position), which is half the size of the structures of complexes. The position and amplitude of the first peak remain stable at temperature variation. Hence, the size and the mass of diamond aggregates depend weakly on temperature. Along with this, such aggregates are mutually coordinated in space at the characteristic distance R2max ~ 100–110 nm, as evidenced by the position of the second peak. The areas under the curves of the first and second peaks are comparable. Hence, on average, near each aggregate a similar object is localized. The results are consistent with the scattering data (Figs. 1.3 and 1.4) that showed a diffraction maximum corresponding to the evaluated distance between the aggregates. It should be noted that adjacent aggregates generally do not overlap. This is indicated by the minimum on the P(R) curves at R1min ~ 70 nm, especially deep at 70 °C, when the P(R) values are negative. This effect is due to the excluded volume factor, when the mutual repulsion of the aggregates dominates. However, this tendency is weakened by performed heat

Fig. 1.24 Correlation spectra for the diamond dispersion DF4 at the beginning and the end of the cycle (25 °C) (1.2) and at 37; 50; 70 °C (3–5)

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treatment. It caused a partial overlap of aggregates created clusters separated from each other, as indicated by the minimum on the P(R) profile at R2min ~ 160 nm (Fig. 1.24). The integral parameters of the aggregates were gotten from the data. The spectrum (25 °C) measured at the end of the temperature cycle was integrated within the∫first peak. In this way, the forward scattering intensity was determined, I o = 4π P(R)dR ≈ 340 arb. un. The issued aggregation number nA ≈ 100 in diamond dispersion became almost three times lower than a similar parameter nA ≈ 270 for the complexes under the same conditions (Fig. 1.23a). Then the gyration radius of the aggregates dispersion was calculated from the ratio of integrals, RGA 2 ∫ 2 in diamond ∫ = (1/2) R P(R)dR/ P(R)dR, within the same limits. The radius RGA ≈ 19.6 nm is twice as low as the RGA magnitudes for the complexes (Fig. 1.22b). Meanwhile, with the found structural parameters (nA , RGA ), filling the volume of the diamond aggregate vA = (4π /3)3/2 RG 3 with particles (nA vP )/vA ≈ 0.07 turned out to be four times higher than a similar value for the complexes (Fig. 1.23b). Thus, it has been established that a modification of diamonds with small amounts of fullerenols (weight ratio of 1:10) causes a transformation of pristine diamond aggregates into large but less dense structures. The regularities found were confirmed with an increase in the proportion of fullerenols in the composition of the complexes. It can be seen from a comparison of the correlation spectra for DF3 complexes (diamond:fullerenol = 1:2) and diamond dispersion DF6 (Figs. 1.25 and 1.26). In these samples, the diamond content (0.3 wt%) was four times lower than in the first series of experiments (DF1, DF4) in order to weaken the tendencies of aggregation of complexes with a high proportion of fullerenols. As the data showed (Fig. 1.25), heating to 70 °C induces a noticeable increase in the peak amplitude while maintaining its shape and the position of the maximum Rmax ≈ 40–50 nm, which is approximately the same as for DF1 complexes with a low fraction of fullerenols (Fig. 1.21). Enrichment of the complex with fullerenols does not cause a significant increase in the size of aggregates. Upon thermal treatment of enriched complexes, structural changes are completely reversible. The initial and final P(R) spectra coincide practically (Fig. 1.25). Thus, at higher proportion of fullerenols, it is possible to stabilize better the formations of complexes. In the DF1 sample diluted with fullerenol, the effect of temperature was expressed in a subsequent increase in the aggregation number in the final state of the system (25 °C) (Fig. 1.23). At high content of fullerenols, the DF3 complexes showed good temperature stability in solutions and reversible structural changes. On the contrary, a diluted DF6 diamond dispersion showed quite large qualitative and quantitative changes in the correlation spectrum as a result of thermal cycling (Fig. 1.26). In the initial state, the diamond aggregates exhibited some tendency to overlap. As a result of the heating–cooling cycle, the interactions of the aggregates became so much stronger that provoked their significant overlap. Massive clusters of 6–7 aggregates were formed. It follows from the ratio of spectral integrals, Int /In1 ~ 6.5, in the region of the first peak (In1 ) and in the entire range of radii (Int ) (Fig. 1.26). It is important to note that the aggregate radii, judging by the stable

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Fig. 1.25 Correlation spectra for DF3 complexes at the beginning and end of the cycle (25 °C) (1, 2) and at 37; 50; 70 °C (3–5)

Fig. 1.26 Correlation spectra for the dispersion of DF6 diamonds at the beginning and end of the cycle (25 °C) (1, 2) and at 37; 50; 70 °C (3–5)

position of the first peak Rmax1 ≈ 20 nm, did not change significantly with temperature variation. The aggregate diameter ~2Rmax1 ≈ 40 nm was noticeably smaller than a characteristic distance between them Rmax2 ≈ 120–130 nm in the initial state. The effect of temperature disturbed the coordination and short-range order in the arrangement of aggregates. In the final state of the system, clusters with a wide

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distribution of distances between aggregates dominated, as can be seen from the continuous spectrum of correlations (Fig. 1.26). As it turned out, a strong dilution of a solution of the DF3 complexes by an order of magnitude, up to a concentration of 0.022 wt%, did not lead to the destruction of nanoscale aggregates of complexes (Fig. 1.27). Contrariwise, the aggregate size increased to 50–60 nm, and these structures became more thermally sensitive. When heated from the initial temperature to 70 °C, the peak amplitude increased threefold, but these changes turned out to be reversible, and upon cooling to 25 °C, the spectrum has restored to the original profile. Further, we compared the data obtained for systems with positively charged DNDZ+ diamonds with the results for solutions and dispersions based on DNDZ− diamonds with a negative potential (Figs. 1.28 and 1.29). As it was established above, when trying to prepare complexes of diamonds DNDZ− and fullerenols both having negative charges, segregation of the components occurred in such binary systems. This is revealed also in the spectra (Figs. 1.28 and 1.29). The mutual repulsion of diamonds and fullerenols did not allow a formation of large composite aggregates, as it is possible when the components have different charge signs. In binary system with DNDZ− and fullerenols, the aggregates (radius of ~20 nm) of approximately the same size as in the DNDZ− dispersion were found. A main difference between the patterns of correlations in both systems was that initially diamonds created aggregates mutually coordinated at characteristic distance of ~130 nm, while in the presence of fullerenols this kind of ordering almost disappeared (Figs. 1.28 and 1.29). Thus, fullerenols violated an initial short-range order of DNDZ− diamonds. In the space between diamond aggregates, there could be

Fig. 1.27 Correlation spectra for the DF2 sample (diluted solution of DF1 complex). Data at the beginning and end of the cycle (25 °C) (1, 2) and at 37; 50; 70 °C (3–5)

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Fig. 1.28 Correlation spectra for binary system DF11 with diamonds DNDZ− and fullerenols (mass ratio 1:2). Data at the beginning and end of the cycle (25 °C) (1, 2) and at 37; 50; 70 °C

Fig. 1.29 Correlation spectra for DF10 dispersion with DNDZ− diamonds. Data at the beginning and end of the cycle (25 °C) (1, 2) and at 37; 50; 70 °C

some molecules and clusters of fullerenols, which, due to the repulsive forces from diamonds, destroy a coordination of diamond aggregates. In this regard, we considered how fullerenols are ordered in solutions. Data for fullerenols with a concentration in the aquatic environment of 0.129 wt% are shown in Fig. 1.30. The molecules form aggregates, the size of which Rmax ~ 40–80 nm progressively increases upon heating and further upon subsequent cooling of the

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solution. When reconstructing the correlation functions from the scattering data, the integral parameters of fullerenol structures, the forward scattering intensities I o , and the gyration radii RG (Fig. 1.31) were obtained. During the temperature cycle, both parameters increased, the radius of gyration doubled, and the intensity seven times. The aggregation number nAF (T ) showed growth approximately in proportion to the cube of radius (Fig. 1.32a). From the data (Fig. 1.32b) for the volume content of fullerenols vf AF (T ) inside the aggregates, it can be seen that the proportion of fullerenols in the volume of aggregates is 3–7 times higher than their average content in solution. However, such formations of fullerenols are rather rarefied and can mix with diamond aggregates. Thus, fullerenols by the formation of complexes can really contribute to the enhancement of diamond aggregation, since the fullerenols themselves actively demonstrate such a tendency. On the whole, this explains the regularities in the structuring of complexes in solutions. The information on the ordering of complexes in solutions at different concentrations and temperatures is important in the perspective of their biomedical applications, in particular, as contrast agents in MRI diagnostics, when the aggregation degree of complexes and their stability in solutions are essential. This directly affects their magnetic relaxation properties. To determine the magnetic relaxation characteristics of the complexes, NMR experiments were carried out for them.

Fig. 1.30 Spectra of molecular correlations of fullerenols in aqueous solution (sample DF10, concentration 0.129 wt%). Initial and final data (25 °C) (1, 2), spectra at 37; 50 and 70 °C (3–5)

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Fig. 1.31 Integral parameters of structures in fullerenol solutions versus temperature: a forward scattering intensity I o , b gyration radius RG

Fig. 1.32 Aggregation numbers nAF (T ) a for fullerenols and the volume fractions of fullerenols vf AF (T ) b inside aggregates in the sample DF7 with low total molecular volume fraction (line) versus temperature

1.3 Magnetic Relaxation Properties of Complexes and Tests on Biological Cells By Using Complexes For the sample of the DF3 complex enriched with fullerenol, we performed the proton NMR measurements (25 °C) to determine the longitudinal and transverse proton spin relaxation times (T 1 , T 2 ). Similar data were obtained separately for the components, diamond dispersion DF6 and fullerenol solution DF9, as well as for

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pure water (H2 O) (Fig. 1.33). The data in Fig. 1.33 showed a transverse relaxation time of proton spins being lower in all the samples, T 2 < T 1 . The reciprocal relaxation times (1/T 1 , 1/T 2 ) were found, corrected for the corresponding parameter for water (1/T 1,2w ), and normalized to the concentrations of the solutions (C S ). As a result, we calculated the relaxivities rl1 = (1/T 1 −1/T 1w )/C S and rl 2 = (1/T 2 −1/T 2w )/C S (Fig. 1.34). As far as in all cases, rl2 > rl 1 , these substances belong to the agents which can create negative contrast in MRI procedures. According to the data in Fig. 1.34, the values of rl1 and rl2 differ greatly for diamonds and complexes, while for fullerenols they are comparable. To ensure good contrast and resolution in MRI images, it is necessary to get a high difference between the parameters, (rl2 − rl 1 ). In this sense, the complexes are more effective comparative to the components. Both fullerenols and diamonds showed lower differential effects (Fig. 1.34). Fig. 1.33 Longitudinal and transverse relaxation times (T 1 , T 2 ) (1, 2) of proton spins in NMR experiments (25 °C) on aqueous systems of complexes, diamonds, and fullerenols (samples DF3, DF6, DF9)

Fig. 1.34 Relaxivities of dissolved substances (diamonds, fullerenols, complexes)

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To compare more accurately complexes and fullerenols, we found also both relaxation rates, rl1Gd = (1/T 1 −1/T 1w )/C Gd and rl 2Gd = (1/T 2 −1/T 2w )/C Gd , normalized to the concentration of gadolinium (C Gd ) in the solutions of these substances. The differences, rl2Gd − rl1Gd , enabled us to estimate finally the effectiveness of these objects as contrast agents for MRI (Fig. 1.35). In both cases, the gadolinium magnetic atoms accelerate more strongly a transverse spin relaxation of protons than their longitudinal relaxation (Fig. 1.35). The ratio of the parameters Δr 21com /Δr 21ful ≈ 1.7 for the complex and fullerenol has indicated that the formation of complexes allowed realize better useful magnetic properties of fullerenols. Their localization on the surface of diamonds makes more the effect of Gd atoms on the protons in surrounding water. Obviously, the large surface area of diamonds is of decisive importance here. Being on it, fullerenols are in more intensive contact with aqueous environment than when they are mainly inside aggregates. In the future, the creation of complexes in which diamonds are a platform for magnetic fullerenols will make it possible to achieve a greater effect of contrasting in MRI than when fullerenols are used in pure form. In this way, it seems possible to make the transport of active molecules to the desired organs and tissues more manageable in theranostics, taking into account the useful properties of fullerenols as generators of reactive oxygen species when excited by light, ultraviolet, and x-rays. Samples of DNDZ+(0.67 mg/mL) + Gd@C82(OH)x (0.33 mg/mL) complexes with the mass ratio fullerenol:diamond = 1:2 were tested for toxicity to non-tumor ECV cells (human umbilical vein endothelial cells) and HeLa tumor cells (human cervical carcinoma cells). Possible morphological changes in cells due to the action of the complexes were determined visually. It was found that in these experiments, the cell morphology did not change as a result of the addition of complexes to the cell medium (concentrations 1; 10; 100 µg/ml). The viability of both cells at the minimum dose of the drug (1 µg/ml) was of 80–90%. With an increase in its concentration to 10 and 100 µg/ml, this parameter decreased to 70–80% and 40–70%, respectively. As a result, it was shown that doses in the range of 1–10 µg/ml do not produce a significant toxic effect. Thus, the complexes can be recommended to be used in the indicated amounts for biomedical applications. Fig. 1.35 Differences of reciprocal proton spin relaxation times normalized to gadolinium concentration in the samples containing fullerenols and complexes

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1.4 Summary As a result of the researches, new nanoscale complexes of detonation diamonds and fullerenols containing gadolinium atoms were synthesized for the first time. It was shown that the formation of complexes is mainly due to electrostatic attraction between diamond particles carrying a positive charge and fullerenol molecules, which became electronegative by partial splitting of protons from hydroxyls in aqueous media. The formation of complexes is also promoted by hydrophobic interactions of the components, since the distribution of functional groups (H, OH) on diamond facets could be irregular, and the grafting of hydroxyls to fullerene surfaces is nonuniform also according to quantum chemical calculations [69]. Experiments of smallangle scattering of synchrotron radiation made it possible to reveal the ordering mechanisms in the ensembles of diamond particles and complexes in aqueous media and demonstrated the temperature stability of the formed nanosized aggregates in both cases in the heating–cooling cycle (25–70 °C). As it is shown in synchrotron experiments and confirmed by electron microscopy data, diamonds themselves tend to form chain structures when particles approach each other, the faces of which can carry charges of different signs despite the common positive potential of the particles. Modification of diamonds with fullerenols leads to the transformation of the mechanism of diamond binding, when fullerenols are able to play the role of crosslinks between diamond particles and their chain fragments, which ultimately results in a twofold increase in the scale of the formed structures, the gyration radius of which is an order of magnitude greater than the diameter of a diamond particle. The complexes of diamonds and fullerenols are self-organized in nanoscale aggregates which mainly retain the initial structure in aqueous media, both with strong dilution of solutions and with temperature exposure when heated to 70 °C. According to NMR data on protons, the magnetic relaxation characteristics of the complexes exceed those for fullerenols by ~70%. Thus, the developed complexes can serve as effective contrast agents for MRI. In biological tests, it was found that the morphology of cells did not change as a result of the addition of complexes to the cell medium (concentrations of 1– 100 µg/ml). It has been shown that doses of 1–10 µg/ml do not produce a significant toxic effect. Hence, the complexes in the indicated amounts can be proposed to be used in biomedicine. Such new complexes combine the useful properties of diamonds, as a platform inert to chemical attacks and ionizing radiation, and magnetic fullerenols, which can serve as antioxidants, photosensitizers, and magnetic relaxation contrast agents in theranostics. Acknowledgements The work was supported by Russian Fund for Basic Researches (gr. No 1829-19008 mk). For Lebedev V. T., the work was supported by a personal grant from Governor of Leningrad district of Russia. Authors thank engineers I. N. Ivanova and L. I. Lisovskaya for technical assistance.

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Chapter 2

Preparation of Alloy and the Application for Photocatalytic Degradation Under Solar/UV and Visible Light Irradiation Saifullahi Shehu Imam, Noor Haida Mohd Kaus, Mohd Amirul Ramlan, and Usman Saidu Abstract Heterogeneous photocatalysis has remained an effective and promising approach for addressing the plethora of environmental challenges. Sequel to that, significant efforts have been made in recent years to understand the core challenges and develop efficient photocatalysts. To date, numerous photocatalysts have been created, including alloy-based materials such as Fe-based alloys, Mg-based alloys, and so on, and their photocatalytic activity studied. The current chapter starts by discussing photocatalysts and the principles and mechanism of the photocatalytic degradation process. Subsequently, alloy-based photocatalysts’ preparation, properties, and degradation performance were elaborated. Finally, challenges, conclusions, and outlooks in the studies of alloy-based photocatalysts have been summarized. Keywords Photocatalysis · Photofunctional materials · Photoactive materials · Alloy-based photocatalyst

2.1 Introduction Environmental protection and remediation are currently the two worldwide issues of essential concern, owing to rapid industrialization, environmental pollution, and abnormal climate changes [1]. Even though water covers 70% of the planet, only S. S. Imam Department of Pure and Industrial Chemistry, Bayero University, P.M.B 3011, Kano, Nigeria e-mail: [email protected] N. H. M. Kaus (B) School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia e-mail: [email protected] M. A. Ramlan Program and Institutional Planning Division, Department of Polytechnic & Community Colleges Education, Ministry of Higher Education, Persiaran Perdana, Presint 4, 62100 Putrajaya, Malaysia e-mail: [email protected] U. Saidu Department of Chemistry, Sule Lamido University, Jigawa State, P.M.B 048, Kafin Hausa, Nigeria e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Garg and A. Chandra (eds.), Photocatalysis for Environmental Remediation and Energy Production, Green Chemistry and Sustainable Technology, https://doi.org/10.1007/978-3-031-27707-8_2

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2.5% is deemed safe for industrial and residential use [2]. This is due to various industrial, agricultural, and domestic wastes (more than 50% are organic pollutants) discharged into water (some sources of water pollution shown in Fig. 2.1), resulting in water contamination [3]. Such contaminants can cause mutagenic or carcinogenic effects on aquatic, human, and other living beings or adverse environmental threats [4, 5]. In fact, according to a WHO report, half of the world’s inhabitants would be residing in a water-stressed environment by 2025 [6]. Thus, the elimination of pollutants from wastewater is very necessary. Various treatment techniques such as conventional (e.g. coagulation, filtration, precipitation, biodegradation, etc.), established recovery processes (e.g. evaporation, solvent extraction, membrane separation, ion-exchange, incineration, etc.), and emerging treatment methods (e.g. advanced oxidation, biomass, nanofiltration, biosorption, etc.) are used in wastewater treatment, as depicted in Fig. 2.2 [8]. However, the numerous weaknesses of the conventional methods, including high cost, large production of sludge, undesired formation of secondary products, and pollutant chemicals, have a high proclivity for being transferred from one phase to another, etc., and have resulted in the rapid alternative techniques development [8]. Advanced oxidation processes (AOPs), which could be homogeneous or heterogeneous, are now among the most promising approaches for eliminating organic pollutants from wastewater and can be used even on a vast scale [9]. The main goal of AOPs is to reduce the concentration of harmful and toxic substances to a safe level before it is released into the water stream [8]. Various types of AOPs are shown in Fig. 2.3. The AOPs involve in situ generations of high potential chemical oxidants assisted by ozone (O3 ), UV light, Fenton’s reagent, or a catalyst [11]. However, among the AOPs, photocatalysis is considered promising, cheap, green, sustainable, and environmentally friendly in solving energy and environmental challenges [3, 8, 12]. For such reason, efforts have now been devoted to excavating novel photocatalysts that are visible light active [13].

Fig. 2.1 Some sources of water pollution. Reproduced with permission from Ref. [7]

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Fig. 2.2 Wastewater treatment technologies. Reproduced with permission from Ref. [8]

Fig. 2.3 Schematic classification of AOPs [10]

Nowadays, amorphous and nanocrystalline alloys are common functional materials having superior properties [14]. Amorphous alloys (also referred to as metallic glasses) are advanced materials with disordered packing structures and superior catalytic capabilities, which have received significant attention [15, 16]. They are made by processing the intrinsic crystalline structure into long-range disordered and short-range ordered atomic structures [17]. Amorphous alloys are metastable, have high strength, and have soft magnetic properties [18]. Furthermore, they resist ionic corrosion due to their homogeneous microstructure and absence of defects such as grain boundaries [19]. Amorphous alloys have demonstrated ultrafast efficiency in degrading and mineralizing dyes [20]. They were found to be more effective than

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their crystalline counterparts and are now regarded as ideal prospects for remediation of environmental wastewater [21].

2.2 Photocatalysts The emergence of photocatalyst began in 1972 following the discovery of titanium dioxide (TiO2 ) by Fujishima and Honda [22]. A photocatalyst is a material capable of accelerating photo-reactions [23]. To initiate the process, the action of ultraviolet (UV), visible (VIS), or infrared radiation (IR) is essential [24]. Furthermore, for a material to become a photocatalyst, it should (i) not be consumed or participate directly in reaction and (ii) provide alternative mechanism routes from accelerated reaction mechanism and existing photo-reactions [23]. One of the early studies involving photocatalysis for wastewater treatment was conducted by Bahnemann et al. [25]. Developing a photocatalyst with high efficiency for water splitting under solar irradiation necessitates the following: (1) good solar light-harvesting properties, (2) bandgap of appropriate energy having valence and conduction bands positioned appropriately for the desired reactions, and (3) a high level of stability is expected of the photocatalyst under experimental conditions [26].

2.3 Principle and Mechanism of Photocatalytic Degradation Process Photocatalysis is a photoinduced process [27]. It combines photochemistry and catalysis [28]. Light and catalyst (semiconductor) are required to initiate or facilitate chemical transformations during the process [29]. The entire process has been established to occur on the catalyst’s surface [1]. The catalyst (semiconductor molecule) has a valence band (VB) occupied with stable energy electrons and a higher energy conduction band (CB) which is entirely vacant [23]. The photocatalytic degradation process (Fig. 2.4) begins with the photocatalyst being stimulated with light having photon energy greater than or equal to the bandgap of the semiconductor [30]. This leads to the in situ generation of electron and hole (e− /h+ ) pairs, possibly within femtoseconds (fs) [31]. Subsequently, within picoseconds (ps) or nanoseconds (ns), photogenerated charge carriers can be trapped, while electron and hole in a few tens of nanoseconds recombine [30]. However, such recombination should be avoided in photogenerated couples to induce photocatalytic reactions [24]. Previous researchers reported species such as hydroxyl radicals (· OH) and · −have  superoxide radicals O2 as the major active species in photocatalytic reaction [32]. These species can oxidize, decompose, or mineralize organic pollutants [33]. The main reactions during photocatalysis are expressed in Eqs. (2.1)–(2.4) below [1].

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Fig. 2.4 Basic principle of photocatalytic oxidation process for the degradation of organic pollutants. Reproduced with permission from Ref. [30] hυ − photocatalysts → eCB + h+ CB

(2.1)

·− e− CB + O2 → O2

(2.2)

+ · h+ VB + H2 O → H + OH

(2.3)

· pollutants + O·− 2 + OH → Degradation products

(2.4)

The relative energy positions of the catalyst and the substrate determine the ability of a catalyst to carry a specific reaction [24]. The adsorbed molecule can be oxidized if its potential is below that of the photo-hole or reduced if its reduction potential is above that of the photoelectrons. Other factors, including charge carrier mobility, light absorption, redox potential, etc., promote photoactivity [34].

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2.4 Preparation and Properties of Alloy-Based Photocatalysts Over the years, various methods have been reported to synthesize alloy-based photocatalysts. In general, it has been established that the physicochemical properties of materials such as photocatalysts are connected with methods employed during synthesis [35]. The methods employed for the synthesis of alloy-based photocatalysts include the following.

2.4.1 Mechanical Alloying Method This method is commonly employed to synthesize fine-grained and homogeneous alloys in solid state [36]. For instance, Eskalen et al. [37] synthesized Co60 Fe18 Ti18 Nb4 alloy in a planetary ball mill with 125 mL stainless steel at a rotational speed of 300 rpm, using a ball to powder ratio of 20:1. During the synthesis, the milling time interval was varied. It was found that both the structure and crystalline intensity of the powder varied with milling intervals, an effect attributed to fracturing and cold welding. Kursun et al. [14] also synthesized Mg65 Ni20 Y15-x Lax via mechanical alloying at room temperature under argon atmosphere. The crystalline and particle size of Mg65 Ni20 Y15-x Lax (x = 1, 2, 3) decreased by varying the reaction time from 5 to 75 h. Furthermore, after 75 h of milling, three different intermetallic phases, including Mg24 Y5 , Mg17 La2 , and Mg2 Ni, have been formed. Another alloy synthesized via mechanical alloying is Cu3 SnS4 [38]. During the synthesis, Cu, Sn, and S metallic powders were mixed in the ratio 3:1:4, with milling time varied from 20, 30, 40, 50, and 60 h, at a rotation speed of 800 rpm. Their study found that CuS and SnS phase appeared at milling time between 20 and 50 h but disappeared after milling time of 60 h. They thus concluded that the reaction time of the mechanochemical alloying process has a great role in cultivating the crystallization of Cu3 SnS4 .

2.4.2 Solvothermal Method This is a wet chemical method that occurs at 1 atm pressure in a closed system and above the boiling temperature of the mineralizer, which could either be water or various organic solvents such as ethylene glycol, methanol, and ethanol [39]. The process is cheap, efficient, environmentally friendly, with a high product yield having good crystallinity [39, 40]. For example, Kar et al. [41] synthesized Cdx Zn1-x S nanorod alloys using the solvothermal technique. Initially, specific amounts of zinc acetate dihydride, cadmium acetate dihydrate, and thiourea were dissolved in a mixed solution of double distilled water and ethylenediamine. After vigorous stirring for

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about 15 min, the solution was then transferred into a Teflon-lined stainless steel autoclave and maintained at 170 °C for 8 h in a preheated oven. Using such method, a product with perfect crystalline hexagonal wurtzite structure was obtained. Other product features include interplanar spacing of 0.34 nm and photoluminescence (PL) emission peaks at 459 and 535 nm. The one-step hydrothermal process has been used by Wei et al. [42] to produce Au–Ag alloys with adjustable size and composition. The major chemicals employed during the synthesis are gold (III) chloride trihydrate (HAuCl4 ·3H2 O), silver nitrate (AgNO3 ), and 3-aminopropyltriethoxysilane (APTES). The experimental parameters adjusted the size and composition of the Au–Ag alloys. Although the synthesized Au–Ag alloys had sphere-like morphology (Fig. 5a–c), however by varying the molar ratio of Au3+ /Ag+ employed during synthesis from 3:1, 3:3 and 3:5, the average sizes of the alloys were found to be 7.93 ± 0.60 nm, 8.29 ± 0.74 nm, and 6.43 ± 1.26 nm. On the other hand, the surface plasmon resonance (SPR) wavelength blue-shifted with an increase in Ag content in the alloy (Fig. 5d).

Fig. 2.5 TEM images of the Au–Ag alloys synthesized at Au3+ /Ag+ molar ratio of a 3:1, b 3:3, c 3:5, and d UV–vis spectra of Au–Ag alloys, respectively. Reproduced with permission from Ref. [42]

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Fig. 2.6 TEM images of the Au–Ag alloys synthesized using a 0.5 mL, b 1 mL, c 2 mL of APTES, and d UV–vis spectra of Au–Ag alloys, respectively. Reproduced with permission from Ref. [42]

Also, by varying the volume of APTES introduced during the synthesis from 0.5 mL, 1 mL, and 2 mL, the sizes of the alloys increased from 8.29 ± 0.74 to 8.66 ± 1.26 and then to 12.03 ± 1.59 nm (Fig. 6a–c). Similarly, there was a red shift in the SPR wavelength of the Au–Ag alloys (Fig. 6d).

2.4.3 Co-Reduction Method Another method commonly employed in the synthesis of alloys is the co-reduction method. Yue et al. [43] employed a fast etch and slow fill strategy to transform Ag nanoparticles (Ag NPs) into Au–Ag alloy nanoparticles. The process involved a coreduction reaction and galvanic replacement. Sodium citrate was used as a reducing agent and stabilizer. The reaction time was varied from 0 s, 5 s, 1 min, 3 min, 5 min to 9 min. Initially, the Ag NPs had a solid structure with a quasi-spherical shape (Fig. 7a). After subjecting the Ag NPs to 20 µmol HAuCl4 for various reaction times, the pores in the outer shell narrowed with time until they vanished. The Au–Ag alloy nanoparticles formed a complete shell (Fig. 7c, d).

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Fig. 2.7 TEM images of the Ag sacrificial templates and reaction intermediates collected at a 0 s, b 5 s, c 1 min, d 3 min, e 5 min, and f 9 min, respectively. Reproduced with permission from Ref. [43]

2.4.4 Green Synthesis Another common approach used in the production of alloys is green synthesis. The process discourages using toxic chemicals as reducing agents by employing safer, cost-effective, biocompatible replacements, e.g. monosaccharides, disaccharides, polysaccharides, etc. For instance, Kushwah et al. [44] reported the biosynthesis of Ag@Cu alloy NPs using Aegle marmelos and Citrus limetta fruit peel extract as double reducing agents. The phenolic compounds present in the extract will oxidize during the production, and the metallic ions (Ag and Cu ions) will reduce, giving the zerovalent Ag@Cu alloy. The phenolic compounds also serve as stabilizing/capping agents. The Ag@Cu alloy synthesized using this method had a crystalline FCC structure, spherical shape, and 5–7 nm particle size. Sun et al. [45] synthesized Au–Ag alloy nanoparticles via co-reduction of HAuCl4 and AgNO3 in one pot, using soluble starch as a capping and reducing agent. The nanoparticles had a face-centred cubic polycrystalline structure and were quasispherical, with an average size varying from 16.5 to 24.8 nm as the synthesis time increased from 10 to 50 min. Following the synergistic effect between the Ag and Au elements and the abundance hydroxyl groups of the soluble starch, the synthesized Au–Ag alloy nanoparticles exhibited excellent photocatalytic performance towards the degradation of 4-nitrophenol. In another study, Sun et al. [46] again synthesized Au–Ag alloy via a green co-reduction process using Chinese wolfberry fruit extract,

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which served as a surfactant and a reductant. The process was rapid and involved three stages of hydrolysis, nucleation, and growth. The synthesized Au–Ag alloy nanoparticles had an average particle size below 15 nm, uniform alloy microstructure, and homogeneous composition.

2.4.5 Other Methods Other approaches have been employed to synthesize alloy photocatalysts, apart from the above-reported methods. For instance, novel Zr2 Ni1 Cu7 trimetallic nano-alloy (TNA) has been synthesized as well by Sharma et al. [47] via a stepwise microwave reduction method, using trisodium citrate as a reducing agent. The Zr2 Ni1 Cu7 was highly agglomerated (Fig. 8a, b). Also, the lattice fringes arrangement in the HRTEM results in Fig. 8c shows the d-spacing of the constituent metals, thus confirming the successful fabrication of Zr2 Ni1 Cu7 trimetallic nano-alloy. The technique of sonochemical exfoliation has also been used by Patel et al. [48] for the synthesis of Cux Sn1-x Se (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) ternary alloy nanocrystal. The nanocrystals were crystalline with dimensions of 5–20 nm and bandgap in the range of 1.751–1.801 eV. Although a gradual decrease in methylene

Fig. 2.8 TEM images (a, b,c) of Zr2 Ni1 Cu7 . Reproduced with permission from Ref. [47]

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blue (MB) and crystal violet (CV) absorbance peaks was observed with increased Cu content in Cux Sn1-x Se ternary alloy, SnSe exhibited higher photocatalytic activity. An ultrasonic surfactant-assisted self-assembly method was used in another study to produce Znx Cd1-x S (x = 0.00, 0.05, 0.2, 0.28, 0.78 and 0.9) alloy by Wang et al. [49]. During the synthesis, a change in the colour of the suspension from jacinth to yellowy signalled its completion. The XRD characterization result (Fig. 9a) revealed that the diffraction peaks shifted gradually to larger angles with increased Zn content, but the absorption edge became blue-shifted (Fig. 9b). Moreover, the broad diffraction peaks in the case of the Znx Cd1-x S imply the alloy’s small crystal size. Fig. 2.9 a Powder XRD patterns and b UV–vis absorption spectra with band gap (inset) of Znx Cd1-x S alloy. Reproduced with permission from Ref. [49]

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2.5 Degradation Performance of Alloy-Based Photocatalysts Various studies have reported on the use of different alloys as photocatalysts for the degradation of a variety of organic contaminants. For instance, Sharma et al. [47] reported the use of novel Zr2 Ni1 Cu7 trimetallic nano-alloy (TNA) as a photocatalyst for the degradation of methylene blue (MB) dye in the presence and absence of H2 O2 . During their study, H2 O2 acted as a degradation rate enhancing agent (Fig. 2.10) by generating more hydroxyl radicals. The rate constant was 0.002 min−1 in the absence of H2 O2 , but escalated to 0.006 min−1 in the presence of H2 O2 . Kar et al. [41] synthesized a series of Cdx Zn1-x S nanorod alloys by changing the Cd/Zn ratio. Due to better crystallinity, the nanorod morphology has excellent photoinduced charge separation efficiency. The degradation efficiency of Cdx Zn1-x S nanorod alloys towards norfloxacin antibiotic was found to vary with the ratio of Cd and Zn in the alloy (Fig. 2.11). Best performance (71%) was recorded using Cd0.8 Zn0.2 S catalyst among all the alloys under identical experimental conditions. The use of UV–Vis and FTIR spectra towards studying the photocatalytic degradation of methylene blue using Mg65 Ni20 Y15-x Lax (x = 1, 2, 3) alloy has been reported by Kursun et al. [14]. The photocatalytic performance of the Mg65 Ni20 Y15-x Lax (x = 1, 2, 3) alloys remained efficient in the absence of heat, light, or any oxidant. Methylene blue solution became colourless after 30 min using Mg65 Ni20 Y15-x Lax (x = 1, 2, 3) alloys, and the Mg65 Ni20 Y13 La2 alloy with the smallest crystalline size displayed the best photocatalytic performance. In a different study by Eskalen et al. [37], Co60 Fe18 Ti18 Nb4 quaternary alloy was employed as a photocatalyst to degrade methyl blue dye under UV (xenon light) irradiation. After 60 min, the colour of the methyl blue solution transformed from blue to nearly colourless, and 98.3% degradation efficiency was recorded. Worth Fig. 2.10 Pseudo-first-order kinetics for the photocatalytic degradation of MB dye in the presence and absence of H2 O2 . Reproduced with permission from Ref. [47]

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Fig. 2.11 Comparison of the photocatalytic activity towards norfloxacin degradation under solar irradiation over ZnS, Cd0.2 Zn0.8 S, Cd0.4 Zn0.6 S, Cd0.6 Zn0.4 S, Cd0.8 Zn0.2 S, and CdS. Reproduced with permission from Ref. [41]

mentioning, Ramya et al. [50] reported that Co-based metallic glasses have excellent stability with minor mass loss compared to Fe-based and Mg-based metallic glasses. The influence of Zn on photocatalytic activity of Znx Cd1-x S alloy towards the degradation of rhodamine B dye has been studied by Ramya et al. [49]. The alloy’s photocatalytic performance depends on the zinc content, as high efficiency was recorded using the alloy with low amount of zinc (Fig. 2.12). The remarkable blueshift in bandgap of Znx Cd1-x S with an increase in Zn content results in a more negative conduction band, leading to a decrease in visible light adsorption. This will reduce the number of photogenerated charge carriers.

Fig. 2.12 Photocatalytic degradation of RhB over Znx Cd1-x S alloy with different Zn content. (–•–, RhB; –z–, rhodamine species). Reproduced with permission from Ref. [49]

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2.6 Challenges Although various alloys have been synthesized and their photocatalytic performance evaluated, few challenges remain persistent based on the data available in the literature. Firstly, it is established that the photocatalytic performance of alloys depends on their physicochemical properties, which are connected with methods employed during synthesis. Although alloys with appreciable photocatalytic degradation performance have been reported, much effort is still needed in synthesizing more high-performing alloy photocatalysts capable of dealing with the rising pollution levels and a constantly changing environment. Secondly, proxy studies conducted usually target one specific pollutant. However, real wastewater is complex and contains numerous impurities. Thus, it is vital to consider degrading real wastewater using alloy photocatalysts to promote their real-life application. Finally, researchers focus on using powdered alloy photocatalysts in treating synthetic wastewater. This approach might not be applied easily in large-scale wastewater treatment plants due to recovery difficulties and leaching possibilities. Thus, it is recommended to consider using immobilized alloy photocatalysts in degrading organic pollutants in wastewater to make the process facile, convenient, applicable at a large scale, and minimizes leaching possibilities.

2.7 Conclusions and Outlooks The present chapter reviews various studies regarding alloy photocatalysts’ synthesis, properties, and performance. Although methods including mechanical alloying, coreduction, and solvothermal have been employed to synthesize numerous alloys, interestingly, the feasibility of synthesizing alloys photocatalysts using the safer and less toxic green method has also been reported. During the synthesis, it was observed that variation of parameters, including composition, reaction time, etc., directly impacted the physicochemical properties of the alloys photocatalysts. Thus, it could be concluded that optimization is necessary to produce high-performing alloy photocatalysts. Currently, the application of alloys as photocatalysts to degrade organic pollutants has been reported with appreciable results. However, results in the literature revealed that the performance of the alloys photocatalysts during degradation could be further enhanced by optimizing the molar ratio of the metallic cations employed during synthesis or by introducing oxidants such as H2 O2 during degradation. It must be emphasized that photocatalytic degradation of organic pollutants using alloys is still at a primitive stage, and therefore, much is needed to be explored.

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Chapter 3

Photocatalytically Active Thin-Film Coatings Ishika Aggarwal, Anubhav Jain, Tejendra K. Gupta, Sucheta Sengupta, and Manoj Raula

Abstract Photocatalytically active thin-film coatings are of primary interest for various industrial processes and applications. Thin films on a nanoscale enable extraordinary electrical, chemical, mechanical, and thermal properties with a large surface area, photocatalytic, antibacterial, antiviral, antifouling, and self-cleaning properties. Surface functionalization is essential to improve the performance and wide use of large-area thin-film coatings for environmental remediation. Nanomaterials play a crucial role in developing innovative methods for making functional thin-film coatings with improved performance. Several coating approaches such as inorganic, polymeric, spray, chemical vapor deposition (CVD), and anodization for the protection against corrosion have been adopted. Photocatalytically active thin films are known for their extensive applications for organic pollutant and antimicrobial activities. In this chapter, we discuss about the current research targeting the fundamentals of photocatalytic reactions and the role of nanomaterials in the formation of thin-film coatings. We shed light on the organic and inorganic methods for forming photocatalytic active thin-film coatings and their antimicrobial properties. Keywords Thin films · Coatings · Corrosion protection · Photocatalytic activity · Nanomaterials

I. Aggarwal · A. Jain · T. K. Gupta (B) · M. Raula Amity Institute of Applied Sciences, Amity University, Sector-125, Noida 201313, India e-mail: [email protected] M. Raula e-mail: [email protected] S. Sengupta Amity Institute of Advanced Research and Studies (Materials and Devices), Amity University, Sector-125, Noida 201313, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Garg and A. Chandra (eds.), Photocatalysis for Environmental Remediation and Energy Production, Green Chemistry and Sustainable Technology, https://doi.org/10.1007/978-3-031-27707-8_3

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3.1 Introduction to Thin-Film Technology For many years, several hazardous wastes generally the by-products of the manufacturing processes are coming from the industries into the water streams. These wastes are generally known as toxic organic dyes that are harmful to humans and aquatic life [1, 2]. Also, to deal with climate change, the ejection of CO2 should be reduced [3]. Few treatment methods are proposed to reduce the contamination caused by these toxic organic wastes [4]. Marine biofouling is the accumulation of undesirable substances on the surfaces that are submerged in water such as ship hulls, which result in many difficulties in water transportation like slow speed, consumption of more fuel, and deterioration of mechanical parts [5]. So, to deal with such problems, nanoscale thin films are a viable solution. These films enhance physical, mechanical, and chemical properties, including antibacterial, non-toxic, larger surface area, self-cleaning, hydrophobic, antifouling, photocatalytic, and oleophobic properties [6–8]. In the advancement of the modern technology, the thin films and coatings are of great aspect as it is the need of the hour and has a vast application in each and every field from medicinal to industrial ones [9]. Different types of technologies and tech devices have been made by the thin films and their coatings which leads to easier and simple lifestyle. The thin films are the building block of the nanoparticles [10]. A vast improvement in the preparation and development of thin films can be witnessed within few years [11]. For understanding and better functioning of any device, a proper knowledge of thin film is required [12]. The coated films mainly enhance the visible region absorption [13]. Since last two decades, photocatalytic reactions have been increased in the presence of nanoparticles. Many metals oxide-based nanostructured semiconductor materials have been used as a photocatalysts for numerous applications [8]. Large-area photocatalytic thin films show great probability for green technological applications in reduction of global warming, pollutions, and save energy. These films can incorporate many surface properties such as abrasion resistant, self-cleaning antifouling, hydrophobic, antibacterial by addition of nanoparticles, and/or hybrid structures and/or surface modifications [8]. In this chapter, we mainly focus on the photocatalytically active thin films that have a great potential to solve industrial as well as environmental areas of concern. Photocatalysis based on semiconductors is a non-polluting and environment-friendly technique that is a well-known method for resolving energy crisis, alleviating environmental pollution, and dye pollutant degradation [14]. In photocatalytic reactions, the bandgap of a semiconductor determines the chemical potentials of the photogenerated electrons and holes participating in a redox reaction which is shown in schematic diagram in Fig. 3.1 [15]. This will be significantly important for all the scientists and researchers who are entering in the field of thin-film coating-based technology.

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Fig. 3.1 Schematic of photocatalytic electron–hole pair generation reaction

3.2 Deposition Methods of Thin Films As shown in Fig. 3.2, several techniques such as vacuum based and non-vacuum based were used by many researchers for thin-film deposition on solid substrate for various applications such as antibacterial coating, super hydrophobic coating, water splitting, oxidation of organic pollutants [8].

Fig. 3.2 Various vacuum-based and non-vacuum-based methods for thin-film deposition

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3.2.1 Vacuum-Based Methods Vacuum-based methods such as evaporation, pulsed laser deposition, and sputtering were used to synthesize high quality thin film on solid substrate. These techniques are discussed below.

3.2.1.1

Evaporation

Evaporation is the simplest and most effective technique for deposition of thin film. In this process as shown in Fig. 3.3, the material is heated at in a high vacuum chamber at low pressure. This process involves several openings from which a small amount of liquid is continuously fed into the top of still. Due to the gravitational force, the liquid flows down slowly through the evaporator wall, and then with the help of mechanical wipers, a thin film is formed covering the evaporating surface [16]. The BiOCl thin films were firstly prepared by the process of thermal evaporation. The degradation reaction of rhodamine B in aqueous solution and in gaseous phase the photocatalytic oxidation of nitric oxide showed the photocatalytic effect of thin film. It was found that the surface of BiOCl thin films becomes active when they are activated by sun like irradiations [17]. Similarly, the thin films of BiOBr were also formed by thermal evaporation deposition. They maintained their photocatalytic activity high even after four cycles of use [18]. Using the method of thermal evaporation under vacuum, the thin films of molybdenum trioxide and tungsten trioxide were made to deposited on glass substrates which showed a good photocatalytic activity in the degradation reaction of wastewater under UV–visible light irradiation [19]. With use of different method of deposition, variation in nucleation and growth mechanism can be observed as in the case of titania thin films [20].

Fig. 3.3 Schematic diagram of evaporation method of thin-film deposition

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Fig. 3.4 Schematic diagram of sputtering method of thin-film deposition

3.2.1.2

Sputtering

In sputtering process as shown in Fig. 3.4, coating material is in form of atoms, and molecules are ejected and deposited on with bombardment of energetic particles. Using pulsed DC magnetron sputtering, the photoactive anatase-TiO2 films were deposited on expanded polystyrene surface whose photocatalytic activity was measured by bleaching the aqueous solution of methylene blue under UV–A, UV–B, and sunlight irradiation [21]. Further, the ZnO–SnO2 thin films can be synthesized at room temperature by the use of RF magnetron sputtering method whose photocatalytic activity was further investigated by treating it with drug and the degradation of drug confirmed the photocatalytic activity of ZnO–SnO2 thin films [22]. With the help of DC magnetron sputtering, the thin films of Cu2 O were deposited. The thin films are even reusable after the third cycle usage as well. The antibacterial activity of sputtered coated Cu2 O thin films has a wide application in technological fields as well including the touch screen appliances [23]. The thin films of nickel were deposited on the alumina foil by the magnetron sputter. The sputtered nickel films help in preventing the pore blocking in the foams of alumina, and these films increases the electron density on the surface of the catalyst [24].

3.2.1.3

Pulsed Layer Deposition

Pulsed layer deposition is a vacuum-based deposition method which provides some advantages for the thin-film growth in comparison to the traditional technologies used. The main advantage is that it can deposit the thin films on different substrate regardless of their electrical, chemical, or mechanical properties [25]. By using the pulsed electrophoretic deposition technique (Fig. 3.5), preparation of thin films of titanium dioxide immobilized over treated stainless steel was done [26]. The TiO2 thin films are deposited on the silica substrates by pulsed PECVD which are grown at low temperature, and these films showed high photocatalytic activity due to presence

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Fig. 3.5 Schematic diagram of pulsed laser deposition method for thin-film deposition

of anatase crystalline nanocolumns at the surface [27]. Epitaxial rutile-TiO2 (011) and anatase-TiO2 (001) films have been grown by pulsed laser deposition on Al2 O3 (1102) and LaAlO3 (100), respectively [28]. Baddeleyite NbON thin films were epitaxially grown on yittria-stabilized zirconia (YSZ) (100) substrates by using nitrogen-plasmaassisted pulsed laser deposition [29].

3.2.2 Non-Vacuum-Based Methods Non-vacuum-based methods are low-cost methods for thin-film coating on solid substrate. Sol–gel method, nanoparticle-based method, and electrodeposition method are the best selected methods for thin-film coatings.

3.2.2.1

Electrodeposition

Electrodeposition is a facile, cost-effective, simple, and environmentally friendly method [30]. In this method without any chemical reaction, the reduction of cationic ion takes place onto the substrate resulting in the deposition of thin metallic films. The synthesis of Cu2 ZnSnS4 on titanium oxide for photocatalytic application is done by the electrodeposition technique, and its photocatalytic effect is studied by degradation rate of methylene blue dye solution [31]. Another application involves the synthesis of Se-rich films by electrodeposition method. The parameter that highly affects the photocatalytic process is the thickness of the film. Sb2 Se3 is presented as a promising light absorber material of low cost and good photoelectrocatalytic activity [32]. The ZnSe thin films were also prepared by the electrochemical deposition method. They are an excellent superconductor material, and their photocatalytic property is studied by degradation of methyl orange, and surprisingly within 5 h, the complete methyl orange was degraded [33]. The Hematite–Titania thin films were prepared by the cathodic and anodic electrodeposition of amorphous Titanium oxyhydroxide gel and

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iron oxyhydroxide and their photocatalytic activity was evaluated in presence of cobalt phosphate catalyst [34].

3.2.2.2

Sol–gel Method

The TiO2 thin films were prepared by the sol–gel method. In order to increase the photocatalytic activity of the titanium dioxide films, an addition of graphene oxide was done [35]. Titanium dioxide absorbs high energy as it is a large band gap semiconductor. The facile and simplest way to synthesize it is sol–gel chemical route [36]. The titanium dioxide films modified with the graphene oxide were prepared on a glass substrate by a sol–gel method [35]. The photoinduced and photocatalytic studies show that a graphene-TiO2 composite thin sheet efficiently generates photocurrents and removes contaminants [36]. The titanium dioxide thin films can be synthesized by the green sol–gel route as well which is an eco-friendly approach as here the sol formulation does not require the solvent [37]. Another such use of sol–gel method is in the preparation of undoped zinc oxide and magnesium-doped zinc oxide which are used for photocatalytic activity as they result in degradation of methylene blue dye under the ultraviolet irradiation [38]. Even the zirconia thin films can also be synthesized with the help of sol–gel method, and its photocatalytic activity can be verified by using methyl red which is decomposed under ultraviolet C irradiation [39].

3.2.2.3

Nanoparticle-Based Method

In the preparation of the thin films, the modification of the surface can be done by the nanoparticles. With the help of the sol–gel method, Sr-doped ZnO thin films were prepared, and then, the CuO nanoparticles were deposited on the porous, thin film of ZnO which leads to the formation of CuO–ZnO heterojunction. Then its photocatalytic activity was studied where the photocatalytic degradation of methylene blue was observed [40]. Another modification in the surface of the ZnO thin films can be done with the help of Ag nanoparticles which enhances the photocatalytic property [41]. By a three-step method, WO3 thin films which were loaded with Pt–Ag bimetallic alloy nanoparticles were prepared, and the result was highly effective. The result of the photocatalytic degradation of the methylene blue was very high when compared to the pure thin films of the same materials [42]. The thin films of positively charged TiO2 nanoparticles can be fabricated by the help of the layer-by-layer selfassembling method whose photocatalytic property was verified by the oxidation of iodide and the decomposition of the methyl orange dye [43]. The photocatalytic property of TiO2 can be improved by using the conjugated polymer together with TiO2 nanoparticles which results in the enhancement of the photocatalytic degradation [44].

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3.3 Roles of Nanomaterials in Catalytic Thin-Film Coating The silver nanoparticles decorated on the surface of TiO2 thin films via electrostatic force of attraction, then Ag/TiO2 composite film shows a synergistic antifungal mechanism under dark and light conditions [45]. The thin films made of cobalt ferrite nanoparticles embedded in silica/zirconia sol–gel matrix results in high features [46]. Further, an important application is the doping of magnetic nanoparticles to peptide coatings maintains all the benefits of nonmagnetic coatings, and simultaneously imparts superparamagnetic properties to the coating, which could greatly expand the potential uses of bioelectronics [47]. We know that powder coating is of great help and advantageous in our daily life as it has a vast number of applications. A thin-film layer is created on the surface which can work aa a protective layer. Coating properties are improved by adding nanoparticles such as clay, SiO2 , TiO2 , and Al2 O3 to the polymer composition. These coatings are used for reducing the gas permeability, thermal stability, optical clarity, and increasing the mechanical and corrosion properties of coatings, and also these coatings have high mechanical properties that can be pointed to its high abrasion resistance [48] (Table 3.1).

3.4 Factors Affecting the Performance of Catalytic Thin Films 3.4.1 Effect of Thickness The basic properties and efficiency of a catalytically active thin films depend upon some deposition parameters like film thickness, size of the grain, and quality of the substrate. As thin films are widely used for semiconductor fabrications, TiO2 semiconductor which has numerous properties like high catalytic activity, non-toxicity, low cost, and long-term stability has been synthesized using several methods like chemical vapor deposition (CVD), hydrothermal, plasma, and sol–gel [55–57]. Various Cu–TiO2 films of different thickness were fabricated by sol–gel dipping method with thickness in the range of 1.5–2.5 µm. The various properties of these thin films such as surface morphology and thickness were measured and studied by atomic force microscope (AFM) and surface profilometer, respectively. Photocatalytic activity of these thin films was examined via measuring the degradation of aqueous solution of methylene blue (MB) under visible light irradiation using 500 W halogen lamp [58]. As more layers of Cu–TiO2 were deposited on the substrate; there was a reduction in full width and half maximum (FWHM), which shows that with increase in thickness of film, the crystallization becomes more perfect [59]. With increase in the number of coatings, the stacking fault is also reduced because of the liberation of stress which resulted in depression in micro stain (µ) and dislocation density ‘δ’ [60].

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Table 3.1 Nanomaterials used in photocatalytic thin-film coatings on solid substrate S. Nanomaterial Preparation Experimental No. technique route

Transmittance Uses

1

CZO

Spin coting technique was used for deposition of CZO transparent conductive thin film on glass substrate

Around 75% in visible range

These films can Rani et al. be developed [49] and utilized in many optoelectrical devices

2

TiO2 –GO

Spin coating technique is used for deposition of thin layer on glass substrate

Around 85% ranging from visible to near infrared region

TiO2 –GO thin films can be used as transparent conducting oxide for solar cells

Belhadj et al. [50]

3

SiO2

Sol–gel Method

Liquid phase deposition technique is used for deposition of thin film on glass substrate

Around 91.5% in UV–visible range

SiO2 films are preferred for dye sensitized solar cells

Ahmed et al. [51], Chen et al. [52]

4

Au–ZrO2

Sol–gel Method

For the Around 75% creation of in UV–visible transparent range nanocrystalline zirconium thin films, inorganic precursor route was taken. Zirconium oxychloride octahydrate was used for fabrication of precursor solution

These films have good thermodynamic stability and can be used in thermal barrier coating

Berlin and Joy [53], Hojabri et al. [54]

Sol–gel method

References

Since, the increase in layers results in more crystalline structure, the absorption edge of the layers shifts toward the visible region up to 440 nm. This also happens when changes happen in the thickness of the films [61]. The shift of the absorption edge toward longer wavelength indicates that the bandgap of the semiconductor also decreased to around 0.35 eV [62]. This can be concluded that as the thickness or the layers of thin films increases, there is an alteration in the optical properties

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or responses of a substrate which is decreased bandgap and absorption at higher wavelength in visible region, and these changes can be used for its photocatalytic applications. To study the effect of thickness on the catalytic performance, all the three thin films of different thickness were dipped into methylene blue (MB) and kept under irradiation for 3 h. It was found that in determining the catalytic efficiency, the reduced band gap and increased surface roughness plays an important role [63, 64]. All films were showing degradation, but the film with 1.89 µm shows more enhanced catalytic activity compared to other films because the number of photogenerated electron–hole pairs reaching the surface were decreasing as the thickness was increased. So, it was found that the 5 times dip Cu–TiO2 film is concluded to be having better photocatalytic efficiency [58].

3.4.2 Effect of Substrate Titanium dioxide (TiO2 ), due to its excellent physical and chemical properties, is used as semiconductor material. According to some researchers, when they used stainless steel as a substrate instead of glass for TiO2 thin-film coating, better photocatalytic performance was observed [65]. This suggests that substrate plays an important role in altering the photocatalytic properties of a thin film. The TiO2 films were prepared on four different substrates (glass, SiO2 , Si and Pt) using magnetron sputtering system. XRD study shows that the TiO2 films on different substrates show preferential orientation along (110) plane and contain mixed phases. Thin film synthesized on Si substrate shows better orientation than others which indicates that substrate influences TiO2 phase structure [66]. From photoluminescence spectroscopy, the samples were characterized for studying the effect of substrate as if the intrinsic emission peak is higher, the crystallization is better with fewer defects whereas emission peak for defects shows opposite results. The thin film of TiO2 deposited on Pt and Si substrate shows better crystallinity than glass substrate which exhibits poor crystallinity. Especially, TiO2 films deposited on Si substrate by magnetron sputtering show better crystallinity as elevated intrinsic emission band is observed [67]. From Raman spectroscopy, due to increased force constants, the Raman bands shift toward higher wavenumber which is the fundamental source of film stress. So, it can be seen that TiO2 thin film on glass substrate has highest force constant among all the substrates. The films deposited on Si and Pt substrate show better crystallization. In Si, there is uniform grain size while in Pt substrate, the surface is rough and inhomogeneous, and the grain size is bigger than Si substrate. From all the experimental data, it can be concluded that Si substrate is better choice for TiO2 film preparation of high performance than other substrates [68].

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3.5 Growth of Large-Area Catalytic Thin Films Growth methods are crucial for figuring out the components of a thin film on which these particles or enzymes are embedded, as well as the size, orientation, and form of catalytic material systems. Catalytic reactions in thin films are often impacted by surface and crystallinity characteristics. In order to confine the crystal, surface, and electronic structure of materials, various techniques are used to produce thin films and photocatalytic metal oxides. Since they start forming structures in aqueous solutions, permit the formation of homogenous films, and allow the management of growth parameters and film properties, liquid-based techniques are expected to be effective. To fully utilize the photocatalytic effect, thin-film formation must be carried out while photocatalytic activity is maintained at the highest level. Till date, considerable effort has been put into achieving high photocatalytic efficiency over a broad area. Photocatalytic particles are decreased using the hydrothermal method using organic solvents [69, 70] under circumstances of high temperature and vapor pressure. By adjusting the synthesis conditions, it is possible to preserve the crystalline phase, shape, and grain size [71, 72]. TiO2 can be produced using TiSO4 , Ti powder, TiCl4 , H2 , and TiO(C2 O4 ) [73], where inorganic materials are recognized at low temperatures [74]. Meanwhile, this method does not require any extra thermal processing. In order to acquire photocatalytic particles, chemical solution decomposition [75], two-step wet chemical method [76], and chemical vapor decomposition [77] techniques have been operated. The sol–gel approach has many advantages, such as homogeneous coatings forming at low temperatures and controlling the structure of a nanosized material starting from the early steps of the process. This illustrates the potential for coating large and complex surfaces with multifunctionality at a reasonable cost [78]. Typically, the films show mesoporous structures created by neutral and ionic surfactants [79]. When a solid thin film is produced using only alkoxide or non-alkoxide processes, depending on the physical and chemical characteristics of the final products, hydrolysis and polycondensation take place. The starting materials for the alkaoxide method are metal alkoxides, while the materials for the non-alkaoxide route are inorganic salts. There are other benefits of sol–gel methods where various metals are added to form films for increasing photocatalytic effect such as Ca2+ , Pb2+ , Ag+ , Fe3+ , V5+ , Co2+ , Au3+ , and La3+ . Chemical vapor deposition (CVD), with its adaptability and quick processing capabilities that allow for large-area processing, is helpful in many situations. Compounds are produced through chemical reactions or through the decomposition of gas-phase precursors depending on the activation method, pressure, and precursors. Instead of using chemicals, physical vapor deposition (PVD) gathers components from the gas phase to coat the substrate. The most often used method in PVD systems is thermal evaporation, which requires heating to 900 °C in an ambient hydrogen environment to produce TiO2 that is sufficiently conductive [80]. Spray pyrolysis utilizes aerosol precursor solution and runs this aerosol to the target, dissenting to chemical vapor deposition, needing diffusion. Although this

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approach may quickly coat huge areas with relatively acceptable structural characteristics at a low cost and with high reproducibility, uniformity is a challenge. Different methods exist for creating metal oxide particles, but it has not been demonstrated that any of them can be used to produce low-cost, large-area surfaces. Although early experimental works indicated that microemulsions were promising, the synthesis of controlled titania with microemulsions has not yet been simple [208]. Operating electrochemical synthesis can manage advanced film qualities of structures including nanoporosity, epitaxial layers, superlattices, and quantum dot structures. When an acidic and oxygen-free environment is required for electrolysis to succeed, the parameters temperature, potential value, pH, and present density are modified to achieve the desired qualities. It has been shown that non-aqueous solutions can be used to confound this issue. In order to create large-area films that can be shaped with the desired qualities, magnetron sputtering and atomic layer deposition are also taken into consideration. Architecture, the automobile sector, and environmental remediation are just a few of the industries where large-area catalytic coatings will find their place in the world of green applications. Over the past few decades, significant advancements have been noted, providing explanations and raising expectations for the future.

3.6 Super Hydrophobic Coatings From past few years, researchers are focusing on the synthesis of super hydrophobic coatings on different types of substrates as its distinct properties like water repellency, anticorrosive, self-cleaning, antiicing, antifogging and antifouling are proven to be useful in automotive, marine, and medical applications. Also, wash-free clothing is also an upcoming development to fabric industry which researchers are looking forward. Researchers are exploring both organic [81, 82] and inorganic ways [83] to develop these fabrics.

3.6.1 Super Hydrophobic Polymer-Based Coatings Isotactic propylene (i-pp) granules weighing 5 g were introduced to a combination of p-xylene (60% volume) and ethyl methyl ketone (40% volume). At room temperature, the granules were fully insoluble in the mixture. However, the granules were miscible at a temperature of 130 °C, resulting in the development of a clear and translucent sol. The vessel was covered with a lid to lessen evaporation loss. As a substrate, a bleached cotton cloth with 100% pure yarn was used. All organic impurities from the substrate were removed by repeatedly washing in deionized water and an ethanol-acetone solution. To get rid of any remaining ethanol-acetone mixture, the cloth is then dried overnight at room temperature. The dried cloth was then cut to the necessary sizes and immersed for 15 min at 130 ± 5 °C in the translucent

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sol of i-pp, p-xylene, and ethyl methyl ketone. Following room temperature drying, the cloth is put through a series of characterization tests to determine the durability and quality of the ultra-hydrophobic polymer coating. Here, the precipitator is ethyl methyl ketone, and the solvent is p-xylene [84]. The water contact level is 154° for this superhydrophobic polymer-based coating. The i-pp coating showed stability in both strong alkaline as well as acidic environment. The roughness after coating was found to be increased than that of the cloth without coating. The super hydrophobicity of the i-pp coating was stable and also retained even after frequent washing with detergents. It also showed super oleophilic behavior in presence of oils [85].

3.6.2 Super Hydrophobic Polymer Nanocomposite-Based Coating Sodium silicate was dissolved in Millipore water, followed by addition of ammonium chloride solution. This mixture was then sonicated for 30 min with 90% amplitude. The white product was then filtered and washed with deionized water and alcohol until chlorine cannot be detected. Then, the final product was filtered and dried at room temperature in vacuum for 24 h to get SiO2 powder. Then, the polystyrene encapsulated SiO2 was prepared by mini-emulsion polymerization technique. Styrene Monomer and Hexadecane as hydrophobic agent were mixed with sodium lauryl sulfate solution in Millipore water in protected multi-necks flask. The mixture was degassed and was stirred for 45 min/800 rpm. By using ultra-sonification for 10 min, mini-emulsion was prepared. Stream of nitrogen was applied, and the mixture is then cooled with ice water. Dispersed SiO2 in water at different concentrations using sonicator for 5 min was added to mixture. Then the temperature was raised to 72 °C. An aqueous solution of AIBN as an initiator was added after being degassed. Then the reaction mixture is cooled to 25 °C. The solution is immersed in ice bath and then precipitated in MeOH, and this precipitation was done by dropping the latex solution to the reaction mixture drop wise. Polystyrene was then filtered and dried overnight in vacuum. When studied from size exclusion chromatography (SEC), relatively low polydispersities were found to be with unimodal chromatograph. A system that has positively or negatively charged surface densities may be desirable for different charged surfaces [86]. There is positive charge on the SiO2 nanoparticles surface that proves to be the reason for them to become an excellent coating material on negatively charged surfaces. The nanocomposites polymers prepared have smooth surface, and they have equal size of particles (70 nm). The particle distribution size was low because of the particle size and zeta potential. The wettability or the hydrophobicity of the coating depends largely on the charge of zeta potential [87].

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3.7 Conclusion Till date, catalytic thin films have been used by several researchers as a surface coating for wide range of applications such as photocatalytic activity, electrical, mechanical, optoelectronic, thermal, magnetic, and biomedical applications. Photocatalytic thin film produced by sol–gel method shows antireflective, antifogging, water repellent, and photocatalytically active properties. The significant potential of catalytic coatings to fight environmental issues with widespread use is quite important. On a global scale, greenhouse gas emissions are continuous and increasing. As a result, adopting smart and functional coatings is a viable option to this major problem of green gas emissions and will contribute to the reduction of pollution. Nanotechnology can improve the performance of coatings and reduces the greenhouse gas emissions through huge environmental remediation. However, due to industrial challenges such as catalytic efficiencies in the visible spectral region and cost-effective production processes for ecological and sustainable coatings with the desired functionalities, commercialization of such goods is still problematic. Acknowledgements Authors extend their thanks and appreciation to Amity Institute of Applied Sciences, Amity University, Uttar Pradesh, Noida, India, for their constant support and encouragement in this COVID-19 pandemic.

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Chapter 4

Photocatalytic Activity of 3D Printed TiO2 Architectures Under Solar Radiation R. Xue, M. L. Matias, A. Pimentel, J. V. Pinto, E. Fortunato, R. Martins, and D. Nunes Abstract The present study reports the production of 3D printed titanium dioxide (TiO2 ) macro-architectures to be employed as photocatalysts for water purification under solar radiation. These architectures, designed as a block material, were printed using a stereolithography 3D printer, and the TiO2 nanopowder produced under microwave irradiation was further impregnated to the printed block before the UV curing of the resin. Microwave synthesis was also used to produce TiO2 thin films directly on the macro-architectures without any seed layer. Printed blocks with different sizes and cavities within its structure, forming crossed channeled structures, were investigated. In fact, the characteristics of the 3D printed blocks, including their size and number of holes/channels for reaction, played a crucial role on the photocatalytic behavior of the materials. These 3D printed TiO2 architectures had their photocatalytic activity assessed from rhodamine B (RhB) degradation under solar radiation, reaching 72% of RhB degradation in 360 min. The approach developed is an effective alternative for the production routes of the TiO2 photocatalysts used nowadays, since 3D printing is a highly cost-effective technique, simple, fast, and easily scalable which make these materials capable to be used in industrial environment. Keywords Macro-architectures · 3D printing · TiO2 nanopowder · Pollutant degradation · Solar radiation

4.1 Introduction Photocatalysis can be referred as synergistic chemical phenomenon of photons and catalyst where the light and a photoactive material are used to accelerate a chemical reaction [1, 2]. Generally, the reaction happens due to interfacial redox reactions R. Xue · M. L. Matias · A. Pimentel · J. V. Pinto · E. Fortunato · R. Martins (B) · D. Nunes (B) CENIMAT|i3N, Department of Materials Science, School of Science and Technology, NOVA University of Lisbon and CEMOP/UNINOVA, Caparica, Portugal e-mail: [email protected] D. Nunes e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Garg and A. Chandra (eds.), Photocatalysis for Environmental Remediation and Energy Production, Green Chemistry and Sustainable Technology, https://doi.org/10.1007/978-3-031-27707-8_4

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of electron–holes pairs generated at the surface of the semiconductor. When the semiconductor is exposed to photonic energy higher than its band gap, photoelectrons in the valence band (VB) are excited to the conduction band (CB), leaving holes (h+ ) behind (Fig. 4.1). These h+ diffuse to the surface oxidizing adsorbed water molecules forming hydroxyl radicals (•OH− ) which are responsible for the decomposition of organic and inorganic compounds [1–3]. Simultaneously, electrons (e− ) in the CB react with molecular oxygen (O2 ) in the air, generating superoxide anions radicals (O2 •− ). This superoxide ion takes part in the oxidation reaction within the TiO2 molecule and prevents the electron–hole recombination [4, 5]. The organic pollutants will be decomposed into carbon dioxide (CO2 ), water (H2 O) and mineral acids in the presence of the semiconductor photocatalyst and reactive oxidizing species [6]. In terms of photocatalysts, several materials have been reported [5, 7–10], but the mostly commonly used material is TiO2 [11]. Nevertheless, the main drawbacks of TiO2 as photocatalysts are related to rapid recombination of the photogenerated electron–hole pairs and poor affinity toward hydrophobic organic pollutants [12]. TiO2 is a well-known n-type semiconductor that has been extensively employed as photocatalyst for water and air purification [6, 13–18], fuel generation, and synthetic chemistry applications [9, 19], but it can also be employed in other applications such as in solar cells [20–22], sensors [23], electrochromic devices [24, 25], and so on. TiO2 occurs in nature, appearing in three crystalline phases: the tetragonal phases, anatase and rutile, and an orthorhombic phase, brookite [6, 26]. Rutile is the most stable TiO2 phase, and both metastable anatase and brookite phases transform into rutile when heated [27, 28]. Rutile and anatase TiO2 phases have optical band gaps of 3.0 and 3.2 eV, respectively [29], while for brookite, its optical band gap in literature varies from 3.13 to 3.40 eV [27, 29]. These values classify TiO2 as a wide band gap material, and since wider bandgap requires higher energy such as ultraviolet (UV) light irradiation to activate the semiconductor, this can be a limitation when it comes to photoactivity under sunlight [6]. The photocatalytic activity of the TiO2 phases is controversial in literature; however, TiO2 anatase phase is normally employed as photocatalyst due to its

Fig. 4.1 Mechanism of semiconductor photocatalysis. Adapted from Ref. [6]

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higher photocatalytic activity, followed by rutile, that is less photoactive [12, 30– 33]. Recently, it has been found that the mixture of both phases displayed higher photocatalytic activity than pure phases [34]. Brookite, in contrast, is not largely explored as a photocatalyst; however, the interest on this material has been growing lately [27, 35–37]. In the case of anatase, its better photocatalytic activity has been reported to be due to several factors, including its larger band gap when compared to rutile, which raises the conduction band maximum to higher energy levels relative to redox potentials of the adsorbed molecules, thus influencing the reductive power of electrons [33]; also related to the longer lifetime of photoexcited electrons and holes on anatase [33]; and the concentration of oxygen vacancies is higher for anatase nanomaterials, which leads to a greater charge separation efficiency [28]. The exposure of high active facets has also been reported as a predominant effect to the higher photocatalytic activity of anatase. The adsorption and desorption of molecules and the charge transfer between photoexcited electrons and reactant molecules depend on the surface atomic arrangement, which changes with different orientations of crystal facets [4]. The higher photocatalytic activity for anatase was reported as {111} > {001} > {100} > {101} [38]. Several production techniques have been reported for TiO2 nanostructures or thin films including wet-chemical techniques [39, 40], sol–gel method [41, 42], thermal evaporation [43], magnetron sputtering [44, 45], atomic layer deposition [46], anodization [47, 48], electrodeposition [49, 50], hydrothermal and solvothermal syntheses [51–54], microwave irradiation [13, 24, 37, 55], among others. However, the latter has several advantages over the conventional hydrothermal method, since microwave irradiation is a fast, inexpensive, and simple synthesis method [56–59], that relies on the efficient heating of solvents and/or reagents [56], thus providing accurate temperature control and uniformity of the produced materials [60]. Another advantage of microwave synthesis is the possibility of growing TiO2 nanostructures in diverse substrates, from rigid to flexible, such as glass [13], cellulose [6, 14, 16], and polymeric ones [37]. The seek for growing nanostructures or thin films on substrates is related to the recovery issues, with poor separation of the nanoparticles after the treatment and low re-utilization rate [31, 61]. Membrane technology applied in water purification has also been widely studied due its to advantages, such as small installation size, low chemical consumption, low environmental impacts, and high quality of water purification [61, 62]. However, to successfully produce enhanced photocatalyticmembranes, highly efficient photocatalysts, such as TiO2 , must be incorporated in the membrane matrix. Thus, new ideas of immobilizing TiO2 nanoparticles on substrates while preserving the photocatalytic activity are being thought. One technology that has surfaced recently in photocatalysis is the integration of additive manufacturing (AM) techniques. AM is a production method that has several advantages, including its operation simplicity, precision, low-cost, capability to produce complex structures, and is considered a low waste production alternative [63, 64].

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With the advance of AM techniques, alternative methods such as stereolithography (SLA) 3D printing for photocatalytic purposes have emerged. SLA printing can be an added improvement to photoactivity performance, since it allows the possibility of creating 3D interconnected architectures. Moreover, 3D interconnected structures covered by a catalyst can lead to large surface-to-volume ratios and uniform irradiance onto the photocatalyst surface [65]. Scaffolds and honeycombs are examples of 3D hierarchical structures that provide large surface area, high pore volume, and high specific stiffness and strength contributing to an efficient diffusion pathway for pollutants and consequently a better photocatalysis performance [66, 67]. For those 3D hierarchical structures, it is imperative to tailor the design parameters, such as channel geometry, diameter, porosity, and surface-to-volume ratio to maximize photocatalytic efficiency [65]. The integration of TiO2 on 3D printed materials for wastewater treatment has also been growing lately. Bergamonti et al. [68] reported TiO2 chitosan scaffolds prepared by 3D printing using commercial P25-TiO2 with photocatalytic activity for wastewater remediation, i.e., amoxicillin photodegradation under UV/Vis irradiation. Sopha et al. [69] produced large 3D Ti meshes fabricated by direct ink writing with TiO2 nanotube (TNT) layers produced using bipolar electrochemistry. The TNTlayer-modified 3D Ti meshes showed a superior performance for the photocatalytic degradation of methylene blue in comparison to TiO2 -nanoparticle-decorated and non-anodized Ti meshes (with a thermal oxide layer), resulting in multiple increases in the dye degradation rate. In another study, it was reported 3D printed hierarchically porous TiO2 scaffolds decorated with Pd nanoparticles for the efficient reduction of highly concentrated 4-nitrophenol wastewater [70]. The present work reports the production and photocatalytic activity of 3D printed TiO2 macro-architectures under solar radiation. TiO2 nanostructures in the form of powder were synthesized under microwave irradiation and further impregnated on the printed blocks produced with the SLA 3D printing technique. Two different sizes of the printed blocks were investigated, with different numbers of channels available for reaction. Microwave synthesis was also used to fabricate TiO2 thin films directly on the surface of the printed blocks without any seed layer. The 3D materials were designed for increasing the reaction/exposed area, with strong adsorption capacity and high catalytic activity for pollutant degradation under solar radiation. The structural characterization of the nanopowders and 3D printed materials after impregnation or microwave synthesis have been carried out by X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with X-ray energy dispersive spectroscopy (EDS). The optical properties were assessed for the TiO2 nanostructures through diffuse reflectance spectroscopic studies, and the photocatalytic activities of the 3D printed architectures have been evaluated from the evolution of rhodamine B degradation under solar radiation. The reusability characteristics of the best photocatalyst have been investigated. Moreover, to the best of the author’s knowledge, the approach described in this study has never been reported by direct impregnation of microwave synthesized TiO2 nanopowders into a printed block before UV curing, without any preliminary or post-processes, together with the synthesis of TiO2 thin films covering the 3D printed macro-architectures with no seed layer.

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4.2 Experimental Procedure 4.2.1 Synthesis of the TiO2 Nanostructures TiO2 nanostructures (powder material) were synthesized using solvothermal method assisted by microwave irradiation. To prepare the solution, 2.5 mL of a 1 M solution of anhydrous oxalic acid (Sigma-Aldrich; CAS: 114-62-7) was added to 57.5 mL of absolute ethanol (Sigma-Aldrich; CAS: 64-17-5) and left to stir in the magnetic plate for 3 min. Afterward, 2 mL of titanium (IV) isopropoxide (TTIP) (Sigma-Aldrich; CAS: 546-68-9) was added to the solution and stirred for 1 h. For the microwave synthesis, 20 mL of the prepared solution was poured into a capped vessel and put into the microwave (CEM Focused Microwave Synthesis System Discover SP). The optimized parameters set for the TiO2 nanostructures’ synthesis were 90 °C, 2 h, 10 bars, and 100 W. After the microwave synthesis, the nanopowder was filtered using the centrifuge (Neya 16 Remi Centrifuge). In total, 3 cycles of Millipore water and 1 cycle of isopropyl alcohol (IPA) under the same condition of 4000 rpm for 5 min were done to wash the nanostructures. The washed nanopowder was placed into a desiccator at 60ºC under vacuum and left to dry for 3 h. Finally, the powder was grinded and stored in a recipient for use.

4.2.2 Design and Printing of the 3D Macro-Architectures 3D macro-architectures were designed on Onshape [71], a computer-aided design (CAD) software system (Cambridge, Massachusetts, USA). Two models with different sizes designated as: Large structure (Fig. 4.2a) and thin structure (Fig. 4.2b). The dimensions of both large and thin printed blocks are indicated on Fig. 4.1. All the structures were printed on a Form-2 stereolithography printer using liquid resin (Formlabs High Temp Resin for Heat Resistance).

Fig. 4.2 Design and dimensions of the 3D printed TiO2 architectures indicated as large structure in (a) and thin structure in (b). The real photograph of the 3D printed blocks is presented in (c)

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The structures were removed from the printer and washed with IPA to remove the residual uncured liquid resin. The structures were submersed in IPA bath for 6 min [72]. Then, the uncured architectures were dried and were exposed to UV irradiation (PSD Pro Series Digital UV Ozone System NOVASCAN) for 30 min with a temperature of 60 °C to complete the polymerization. Both structures (large and thin) are composed by cavities with centered crossed channels along x-, y-, and z-axes. The large structure is constituted by 5 × 5 × 11 holes, having a calculated surface/exposed area of 20,217 mm2 , whereas the thin structure is composed by 2 × 5 × 11 holes with a calculated surface area of 8084 mm2 .

4.2.3 Incorporation of the TiO2 Nanostructures in the 3D Printed Architectures Prior to UV curing, all the 3D printed architectures (large and thin structures) were impregnated with the TiO2 nanostructures produced under microwave irradiation (Sect. 2.1). For the impregnation, 0.1 g of the TiO2 nanopowder was poured into a bottle with the 3D printed block inside (Fig. 4.3). The nanostructures were dispersed on the surface of the block by manually shaking the bottle for 5 min. Then, the printed block was placed for UV post-curing process. Afterwards, the impregnated printed blocks were washed on Sonorex ultrasonic baths by Bandelin in Millipore water for 10 min at room temperature, to remove the nanostructures that did not adhere on the surface. The materials with just the impregnation of TiO2 nanostructures will be hereafter called: L_NS (for large structure with nanostructures) and T_NS (for thin structure with nanostructures). In order to improve the photocatalytic activity of the 3D printed architectures, the printed blocks after impregnation and UV curing were subjected to microwave synthesis. For the microwave (MW) synthesis method, it involved the placement of the 3D printed block inside the MW vessel and the addition of the solution for the synthesis of TiO2 nanostructures described in Sect. 2.1. The parameters set for the

Fig. 4.3 Schematic of the production of the 3D printed architectures with the impregnation of the TiO2 nanostructures and after microwave synthesis

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microwave synthesis were: 90 °C, 2 h, 10 bars, and 100 W. This method enables the direct growth of TiO2 nanostructures on the surface and inner walls of the 3D printed architectures. This approach will be hereafter called: L_NSMW (for large structure with nanostructures after microwave synthesis) and T_NSMW (for thin structure with nanostructures after microwave synthesis).

4.2.4 TiO2 Nanostructures and 3D Printed Architectures Characterization XRD experiments were performed using a PANalytical’s X’Pert PRO MPD diffractometer equipped with a X’Celerator 1D detector and using CuKα radiation. The XRD data were acquired in the 20–70o 2θ range with a step size of 0.0334 º. For comparison, powder diffractograms of rutile, anatase, brookite have been simulated with PowderCell [73] using crystallographic data from reference [74]. SEM observations were carried out using a Regulus 8220 Scanning Electron Microscope and a Carl Zeiss AURIGA CrossBeam FIB-SEM workstation equipped for EDS measurements. The dimensions of individual TiO2 nanostructures have been determined from SEM micrographs using the ImageJ software [75]. Room temperature reflectance measurements were performed in the 250–800 nm range with a PerkinElmer lambda 950 UV/VIS/NIR spectrophotometer equipped with a 150 mm diameter integrating sphere. The calibration of the system was achieved by using a standard reflector sample (reflectance, R = 1.00 from Spectralon disk). The band gap of the TiO2 films was estimated from reflectance spectra using the Tauc plot method [76–78].

4.2.5 Photocatalytic Activity The photocatalytic activity of the 3D printed architectures was evaluated at room temperature from the degradation of Rhodamine B from Sigma Aldrich under a solar light simulating source. A LED solar simulator was used—LSH-7320 set at 1 SUN intensity at room temperature. For each experiment, the 3D printed block was placed on the bottom of the reaction recipient, and 50 mL of the rhodamine B solution (5 mg/L) was stirred for 30 min in the dark to establish absorption– desorption equilibrium. The absorbance spectra were registered every 30 min using a PerkinElmer lambda 950 UV/VIS/NIR spectrophotometer to a total of 360 min. Reusability tests were performed on the 3D printed architectures that had the best performance. In these tests, the best photocatalyst was dried after the first exposure, at 50 °C for 1 h, and the liquid was discarded. The reusability tests were carried out by the repeated solar radiation exposure of the same sample in fresh solutions.

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4.3 Results and Discussion TiO2 nanostructures were successfully synthesized under microwave irradiation. 3D printed macro-architectures with two dimensions, different numbers of holes, and crossed channels, were produced using the SLA 3D printing technique, and the microwave synthesized TiO2 nanopowder was further impregnated using a simple mixing approach. Microwave synthesis was also used to grow TiO2 thin films on the 3D printed blocks after impregnation and without any seed layer. The 3D printed macro-architectures were systematically investigated and correlated to their final photocatalytic behavior under solar radiation. Rhodamine B degradation was used as model dye.

4.3.1 TiO2 Nanostructures Figure 4.4a shows the SEM image of the TiO2 nanostructures synthesized under microwave irradiation. The microwave synthesis resulted in very fine particles, appearing as nanospheres. The average sphere diameter calculated was 17.05 ± 3.45 nm. The nanopowder produced was also analyzed by X-Ray diffraction, and the results are presented in Fig. 4.4b. The TiO2 crystalline phase present in the nanopowder is anatase with tetragonal crystallographic structure, and no other secondary phases were detected. The characteristic peaks of the TiO2 anatase phase positioned at 2θ = 25.25°, 37.71° and 48.01° are assigned to (101), (004), and (200), respectively [79]. Moreover, no other peaks associated to impurities, such as Ti(OH)4 were found. The mean particle size calculated from Scherrer’s equation was 10 nm [80]. Diffuse reflectance spectroscopic studies were carried out for the TiO2 nanopowder. Its optical band gap energy was determined through the Tauc method. The Tauc Eq. (4.1) is described as follows [81]:

Fig. 4.4 SEM image of the TiO2 nanostructures (a) together with its XRD diffractogram (b), where the simulated rutile, brookite and anatase diffractograms are presented for comparison

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(αhν) = A(hν − E g )m

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(4.1)

where α is the absorption coefficient, h is the Planck constant, n is the photon’s frequency, A is an energy independent constant, E g is the energy band gap, and m is a constant which depends on the nature of the electronic transition and is equal to ½ for allowed direct transitions and 2 for allowed indirect transitions. Additionally, the corresponding absorption spectra α can be substituted by the measured reflectance spectra, R, through the application of the Kubelka − Munk function (F(R), Eq. (4.2)) [82, 83]: F(R) =

k (1 − R)2 = 2R s

(4.2)

where R represents the absolute reflectance of the nanostructures, k is the absorption coefficient, and s is the scattering coefficient. Therefore, to calculate the energy band gap of the nanostructures, Eqs. 4.1 and 4.2 were integrated, and (F(R)hν)1/m versus (hν) was plotted by using the measured reflectance, and then, the linear region was extrapolated (Fig. 4.5) [84]. This approach can be applied for all semiconducting materials that do not absorb light of the sub-band gap energy (or show a negligible absorbance) [85]. The direct band gap value estimated was 3.22 eV, within the reported values for the TiO2 anatase phase [29, 86]. Moreover, it has been considered a direct band gap, since it was previously reported that for very fine TiO2 anatase nanoparticles, the direct transition is more favorable [87]. Fig. 4.5 Optical band gaps (F(R)hν)1/m versus (hν) plots for the TiO2 nanostructures

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4.3.2 3D Printed Macro-Architectures Figure 4.6 shows the SEM images of the 3D printed architectures after the TiO2 nanostructures impregnation and after microwave synthesis. As both large and thin structures were produced with the same conditions and parameters, both materials presented identical characteristics, and so, only the results for the large structure are presented in this section. From Fig. 4.6a, it is clear that the microwave irradiation does not destroy the printed block structure or its cavities. Figure 4.6b, c reveal that the impregnation of the TiO2 nanopowder on the surface of 3D printed blocks was successfully achieved. From Fig. 4.6b, it can be observed the presence of TiO2 agglomerates in the micrometer range size that are distributed throughout the resin matrix. Nevertheless, using the mixing approach, the TiO2 nanostructures remain as larger agglomerates, without a uniform covering of the printed blocks surface. The SLA 3D printing techniques requires an UV post-curing process after coming out of the printer, to convert monomers into highly crosslinked polymer networks [88]. During the curing process, the raw printed blocks suffer thermal expansion due to UV light and temperature exposure [89]. Thus, the final block tends to shrink, reducing its size at room temperature, with an enhance of the mechanical properties compared to the raw printed material before UV curing [90, 91]. Hence, by impregnating the TiO2 nanostructures on the surface of the raw uncured printed block, these nanostructures are slowly embedded in the resin during the UV curing procedure and stay attached to the surface after the curing and cooling processes. The 3D printed architectures after impregnation were then exposed to microwave irradiation, and from Fig. 4.6c, it can be observed a full covering of the printed block, forming a continuous and uniform TiO2 thin film composed by TiO2 nanospheres. This continuous film successfully covered the exterior of the printed blocks; however, microwave synthesis also guaranteed the covering of the interior walls of both structures. As observed for other rough substrates, the growth of TiO2 thin films without any seed layer or chemical treatment for adhesion is expected to be due to substrate roughness facilitating nucleation and fixation of the TiO2 structures [14, 16]. Moreover, the thickness of this film could not be inferred precisely due to the roughness of the surface and heterogeneities observed, since the micrometer sized TiO2 agglomerates of the previous impregnation are still discernible after microwave synthesis. EDS analyses were also carried out and are presented in Fig. 4.7. It can be observed the presence of C, O, and Ti for both conditions investigated. The strong presence of C is related to the resin used for 3D printing. As expected from the SEM images in Fig. 4.6b, c, Ti appears as micrometer sized agglomerates in the L_NS printed block. On the other hand, the L_NSMW material shows a more uniform distribution of Ti, with a higher signal throughout the material, proving the homogeneous covering of its surface. No impurities were detected.

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Fig. 4.6 SEM images of a 3D printed macro-architecture (a) together with the surface of the printed block after TiO2 nanostructures impregnation (L_NS) in (b), and after TiO2 impregnation and microwave synthesis (L_NPMW) in (c). The insets show the TiO2 agglomerates in (b) and the formation of a TiO2 thin film in (c)

4.3.3 Photocatalytic Behavior of the 3D Printed Macro-Architectures The photocatalytic activity of the 3D printed TiO2 macro-architectures was evaluated through the degradation of RhB under solar radiation using a sun simulator. Both large and thin structures were investigated, considering the TiO2 impregnation and microwave synthesis. The contribution to the RhB degradation from the 3D printed block without TiO2 nanostructures was also studied. Figure 4.8a–d show the RhB absorbance spectra at different solar exposure times with the different 3D printed

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Fig. 4.7 SEM and EDS analyses on the 3D printed TiO2 macro-architectures after TiO2 nanostructures impregnation in (a) and after impregnation and microwave synthesis in (e). The corresponding EDS maps of C (b and f), O (c and g), and Ti (d and h)

TiO2 architectures acting as photocatalytic agents. After photocatalytic experiments, it can be observed a pinkish coloration of the 3D printed blocks. Figure 4.9 presents the degradation ratio (C/C0 ) versus exposure time, where C is the concentration of the pollutant at each exposure time and C0 is the initial solution concentration [92]. The gradual RhB degradation under solar radiation could be inferred for all conditions up to 360 min. A 3D printed block without TiO2 nanostructures was also measured during the solar radiation exposure experiments, and no significant RhB photodegradation has been observed over time (Fig. 4.9a). The limit of 360 min was imposed due to the first appearance of a hypsochromic shift. This shift is a consequence of self-photosensitization of the dye when exposed under visible light. It is a common occurrence when running visible light assisted photocatalysis using dye [93, 94]. Apart from the decomposition of the dye through the destruction of the chromophore structure in photocatalytic processes, RhB molecules can shift to an excited state under visible light. During their excited state, they transfer electrons to the CB of the photocatalyst and RhB cation radicals are formed, which are responsible for the successive N-de-ethylation reactions [5, 95, 96]. N-de-ethylated products are the reason for the gradual hypsochromic shift in the absorbance peak since the N-de-ethylation of RhB is known to be a stepwise process [97, 98]. Hence, when hypsochromic shift starts to appear, it means that the degradation of RhB is no longer predominantly due to the action of the photocatalyst. From Figs. 4.8 and 4.9, it can be observed that the large structure condition (L) had a better performance when compared to the thin structure (T ). This behavior was expected due to the higher number of cavities (more channels for reaction) and larger reaction/exposed area. Moreover, the design of crossed channels is expected to maximize the contact between the solution and photocatalyst, increasing photocatalytic efficiency of the printed blocks. Both L_NS and L_NSMW materials (Fig. 4.8a, c) reached RhB degradation values of 72%. Nevertheless, it is evident that the L_NSMW

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Fig. 4.8 RhB absorbance spectra under solar radiation for the L_NS material (a), T_NS material (b), L_NSMW material (c) and T_NSMW material (d) at room temperature. The slight differences in the initial values (0 min) are related to different RhB solutions used for the photocatalytic measurements. The insets show the photographic images of the 3D printed blocks after photocatalysis

Fig. 4.9 RhB degradation ratio (C/C0 ) versus exposure time for the 3D printed TiO2 architectures under solar radiation in (a) together with the corresponding photocatalytic reaction rates with the respective fitting curves in (b)

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material was more effective regarding the RhB photodegradation under solar radiation, reaching the threshold/ hypsochromic shift earlier than the L_NS. The T_NS material reached 31% of RhB degradation, and the T_NSMW degraded 44% for the same time exposure. For both large and thin structures, microwave synthesis expressively enhanced the photocatalytic activity of the 3D printed architectures (28% more efficient for the larger structure). To enable quantitative comparison between the 3D printed TiO2 architectures, the photocatalytic reaction rates were determined (Fig. 4.9b). The calculated reaction rates were: 0.0038 min−1 for the L_NSMW, 0.0034 min−1 for the L_NS, 0.0015 min−1 for the T_NSMW, 0.0011 min−1 for the L_NS and 0.000283 min−1 for the 3D printed block without TiO2 nanostructures. The pseudo-first-order rate constant was calculated from the slope of ln (C o /C) versus the solar radiation exposure time [76]. The photocatalytic reaction rate was higher for the L_NSMW attesting its superior photocatalytic activity under solar radiation. The photocatalytic activity relies on several factors including the band gap energy, crystallite size, degree of crystallinity, specific surface area, active facets, among others [99, 100]. In this work, the impregnated TiO2 nanostructures were investigated, revealing that microwave synthesis resulted in very fine spherical particles (~17 nm as calculated by SEM images) with the TiO2 anatase phase, and band gap value within the values reported for anatase phase [29, 86]. The nano-sized particles are known to have enhanced redox ability, with the migration of electrons and holes to their surface being facilitated, and the electron–hole recombination reduced, increasing thus the photocatalytic performance [101]. Thus, for all conditions investigated, a significant contribution from the TiO2 nanostructures is expected, especially due to the higher specific surface area usually observed for the smaller particles [102]. Nevertheless, as all the 3D printed TiO2 macro-architectures had the TiO2 nanostructures impregnated with the same approach, a similar contribution from these particles is expected for all the materials. In terms of the printed design, the size, number of holes, and crossed channels on the 3D printed macro-architectures played a significant impact on the overall photocatalytic activity. The larger structure had a calculated surface/exposed area 2.5 times higher than the Thin structure, with a greater number of holes/channels available for reaction. Thus, the enhanced photocatalytic efficiency is attributed to the design of the 3D printed block. The relation between the design of the 3D printed materials and their photocatalytic activity has been previously demonstrated. Elkero et al. [65] reported the production of 3D printed Au/TiO2 monoliths for photogenerating hydrogen from water/ethanol gaseous mixtures under dynamic conditions and UV radiation. The influence of the diameter of microfilaments on the photocatalytic activity has been demonstrated. In fact, diameter of the microfilaments was a critical design parameter for the efficient photoproduction of hydrogen, because it determined the total geometric exposed area. It was shown 3D printed monoliths with filaments of 580, 410 and 200 µm in diameter, and the 200 µm microfilament was more efficient with more channels for reaction. It can also be confirmed that by using microwave synthesis for producing a TiO2 thin film after the TiO2 nanostructures’ impregnation, it contributed to increase the

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Fig. 4.10 a Different cycles of RhB degradation ratio (C/C0 ) versus exposure time for L_NSMW material under solar radiation. b Photocatalytic reaction rates with the respective fitting curves

photocatalytic activity of the large structure materials. The complete and uniform covering of the printed block surface (exterior and interior walls) satisfactorily accelerated the RhB photodegradation under solar radiation, reaching the threshold/ hypsochromic shift earlier when compared to the L_NS material (compare Fig. 4.8a, c). Moreover, as observed in Fig. 4.6, the thin film is composed by nanospheres. Thus, a significant contribution to the enhanced photocatalytic efficiency of the L_NSMW can also be associated to the presence of the TiO2 nanostructures composing the thin film [16, 103]. This thin film associated with the impregned TiO2 agglomerates enhanced the overall photocatalytic performance of the large structure material. Regarding the photocatalytic stability of the 3D printed macro-architectures, cycling tests were carried out for the best photocatalyst, i.e., L_NSMW, demonstrating reusability characteristics over time (Fig. 4.10a, b). From Fig. 4.10a, it can be observed that the 3D printed block can be reutilized several times without any significant loss of its photocatalytic efficiency. The reaction rates of the 3 cycles tested were (Fig. 4.10b): 0.0038, 0.0037, and 0.0035 min−1 , attesting the consistent RhB degradation under solar radiation after several cycling tests. The photocatalytic experiments revealed that the use of 3D printing associated to the simple mixing of microwave synthesized TiO2 nanostructures with subsequent microwave synthesis to deposit uniform TiO2 thin films enabled the fabrication of enhanced photocatalysts with great stability and superior reusability properties under solar radiation. The produced materials also avoid the recovery of powder, especially nanopowder, that represents a huge drawback in wastewater cleaning treatments.

4.4 Conclusions 3D printed macro-architectures were successfully produced using the SLA 3D printing technique by considering two different dimensions, different numbers of

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holes and crossed channels. A novel approach was demonstrated with the impregnation of the photocatalyst on the printed block, in which microwave synthesized TiO2 nanostructures were incorporated on the resin before UV curing, by mixing the powder without any pre- or post-processes, reducing additional costs, and avoiding time-consuming techniques. To increase the photocatalytic activity of the printed blocks, microwave synthesis was used to produce TiO2 thin films, without any seed layer, after the TiO2 nanostructures impregnation. TiO2 nanostructures were produced under microwave irradiation, and very fine TiO2 anatase particles appearing as spheres were observed, as well as in the thin film. The photocatalytic activity of the 3D printed architectures was evaluated with the rhodamine B degradation under solar radiation, with the best photocatalyst reaching values of 72%. The large structure had comparatively much higher degradation rates than the thin structure due to the higher number of holes/channels for reaction and larger reaction/exposed area. Therefore, the design of the architectures played a critical role on the final photocatalytic behavior. Moreover, the presence of the microwave synthesized TiO2 thin film associated with the TiO2 impregnated agglomerates justified the enhanced photocatalytic performance among the materials studied. The best photocatalyst also demonstrated stability and reusability characteristics over time. The 3D printed blocks developed in this study associated with the microwave synthesized TiO2 nanostructures can be an alternative for the materials used in water purification, resulting in stable and reusable photocatalysts, without the powder recovery requirement, and produced through simple, fast, and cost-effective techniques that can be easily adapted to industrial scale. Acknowledgements This work was financed by national funds from FCT—Fundação para a Ciência e a Tecnologia, I.P., in the scope of the Projects LA/P/0037/2020, UIDP/50025/2020, UIDB/50025/2020, and UI/BD/151292/2021 of the Associate Laboratory Institute of Nanostructures, Nanomodelling and Nanofabrication—i3N. The authors also acknowledge Fundação para a Ciência e a Tecnologia for funding the Project ICARUS under the reference PTDC/EAMAMB/30989/2017. The work was also partially funded by the Nanomark collaborative project between INCM (Imprensa Nacional—Casa da Moeda) and CENIMAT/i3N. The acknowledgments also go to EC project SYNERGY H2020-WIDESPREAD-2020-5, CSA, proposal n° 952169, and for the European Community’s H2020 program under grant agreement No. 787410 (ERC-2018AdG DIGISMART). The authors also thank PhD student Sara Silvestre for producing the 3D printed materials. This work is part of the Master of Science thesis of Rita Xue titled “3D printed TiO2 nanostructures for water purification using sunlight,” carried out at NOVA University of Lisbon, Portugal.

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Part II

Photocatalytic Activity Enhancement

Chapter 5

Photocatalytic Reactors Design and Operating Parameters on the Wastewater Organic Pollutants Removal Gizem Saygı, Özlem Kap, Fehime Çakıcıo˘glu Özkan, and Canan Varlikli Abstract Industrial and municipal wastewater include various organic pollutants such as dyes, pesticides, pharmaceutical products, and aromatic compounds. Due to their health and environmental risks, the removal of organic pollutants from wastewater is a significant subject. Photocatalytic technology has become one of the most efficiently used application for treating wastewater due to its simple operation, high economic benefits and potential in eliminating secondary pollution. The photocatalytic activity depends on the morphology, surface area and band gap energy of the photocatalysts. Ion doping and immobilization on the support surface are the efficient ways to enhance the photocatalytic degradation efficiency. The photocatalytic reactors design certainly plays a critical role in the photocatalytic applications. In this chapter, the recent developments on photocatalytic reactors are introduced, and the commonly used ones are illustrated. The reaction mechanisms and stoichiometry of the reactants within the reactor is presented in detail. The efficiency of photocatalytic process is mainly governed by the operating parameters of pH, temperature, pollutant concentration, photocatalyst dosage, oxidants and coexisting inorganic ions. This chapter provides an overview to these parameters and presents a perspective to the photoreactor concept and process parameters.

G. Saygı · F. Ç. Özkan (B) Department of Chemical Engineering, Faculty of Engineering, Izmir Institute of Technology, Gulbahce Koyu, 35430 Urla, ˙Izmir, Turkey e-mail: [email protected] G. Saygı e-mail: [email protected] Ö. Kap Physics of Complex Fluids, Faculty of Science and Technology, MESA+ Institute for Nanotechnology, University of Twente, Enschede 7500AE, The Netherlands e-mail: [email protected] C. Varlikli Department of Photonics, ˙Izmir Institute of Technology, Urla, ˙Izmir 35430, Turkey e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Garg and A. Chandra (eds.), Photocatalysis for Environmental Remediation and Energy Production, Green Chemistry and Sustainable Technology, https://doi.org/10.1007/978-3-031-27707-8_5

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Keywords Photocatalytic reactors · Photodegradation · Design of photoreactor · Wastewater · Organic pollutants · Space–time yield · Reactor efficiency · Langmuir–Hinshelwood kinetics

5.1 Introduction Wastewater organic pollutants caused by industrialization, economic growth and urbanization have been one of the most severe threats to environment and human beings [1]. In recent years, the various synthetic organic materials such as dyes, pesticides flame retardants and antibiotics have been extensively used [2, 3]. Modern people generally need these organic chemicals, and yet the discharge of them to the aquatic environment causes water pollution. Due to the resistance of those pollutants to natural degradation and their high stability under sunlight, the conventional wastewater treatment plants cannot provide sufficient cleaning of the wastewater [3]. Therefore, wastewater treatment technologies for the organic pollutant have gained much attention among the scientists, stakeholders in plants and engineers [4]; numerous organic pollutant treatment methods have been studied including biological treatment [5, 6] physical removal [7, 8] and chemical degradation [9, 10]. Biological treatment carries the potential negative effects on ecological balance. Physical removal processes like filtration, adsorption, flocculation and sedimentation cannot degrade the organic pollutants and most of the times, only useful as pretreatment techniques [3]. Therefore, chemical degradation methods have gained much attention. The photocatalytic degradation process is one of the most promising method for eliminating organic pollutants from wastewater [11]. The photocatalytic degradation has various advantages over other treatment methods, such as the operation at room temperature, the ability of effective mineralization of most organic compounds, cost effectiveness and the capability of forming a hybrid system by assembling with the other conventional methods [12–16]. The photocatalytic degradation method converts organic pollutants to non-toxic or less toxic components with the photocatalyst [17], which are semiconductors [18, 19]. The morphology, surface area and band gap energy of the photocatalysts significantly influence the photocatalytic activity [20, 21]. In general, to enhance the photocatalytic performance, the modification of photocatalysts is needed [22]; doping [23] and immobilization onto the support surface [24] are the main techniques for the improvement of photocatalysts capacity. Doping photocatalysts by using metal/nonmetal elements can enhance its light absorption range and narrow the band gap energy of the photocatalyst with the formation of new energy levels [25]. For the immobilization techniques, support materials (zeolites, clays, graphenes, etc.) are utilized to obtain the composite catalyst which have lower recombination rate and higher degradation capacity of organic pollutants [26]. This chapter aims to provide specific guidelines on photoreactors design and operating parameters for organic pollutant removal from wastewater. Its summarizes stoichiometric and kinetics effects of types of organic contaminants, properties

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of photocatalysts, photocatalytic reactor geometries, reaction mechanisms, irradiation sources and reactor operating parameters (pH, temperature, pollutant concentration, photocatalyst dosage, oxidants and coexisting inorganic ions) affecting the photocatalytic degradation process.

5.2 Organic Pollutants in Wastewater Water pollution is a common problem facing people around the world due to the industry, human activities, and agriculture. The industrial and domestic wastewater are generally environmentally persistent to the conventional treatment systems [27]. The large amounts of these pollutants are produced and released to the water bodies. Therefore, these organic contaminants have been threatened the human and animal health [27, 28]. The water bodies are seriously polluted by four main classes of organic pollutants including dyes, pesticides, pharmaceuticals and personal care products and aromatic compounds. The chemical structures of commonly used organic pollutants are illustrated in Fig. 5.1. Organic Dyes Methylene Blue:

Eriochrome:

Rhodamine B:

Indigo Carmine:

Dichlorodiphenyl trichloroethane (DDT):

Parathion:

Malathion:

Pharmaceuticals and Personal Care Products Doxycycline: Sulfamethoxazole:

Caffeine:

Salicylic acid:

Aromatic Compounds Toluene:

Naphthalene:

Fluorene:

Pesticides Atrazine:

Phenanthrene:

Fig. 5.1 Chemical structures of common organic pollutants

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5.2.1 Organic Dyes Organic dyes are used to give color to fabrics and other materials. The worldwide usage of organic dyes causes a serious environmental contamination. The textile industry is the main source for this pollution [29]. According to the World Bank, about 17–20% of water contamination caused by textile and dyeing industries [30]. Current studies demonstrated that 10–12% of total consumption of dyes such as methylene blue, Indigo Red, Rhodamine B, Eriochrome, Black-T, thymol blue and Carmine, Red 120 are used in textile industries, and approximately 20% of the used dye are lost during processes and create wastewater [30, 31]. The release of these reactive, direct, disperse, acidic or basic dyes may have carcinogenic effects and may cause rhinitis, asthma, allergic reactions and dermatitis [32]. Therefore, the removal of organic dyes from aquatic environments is essential.

5.2.2 Pesticides Pesticides are considered as one of the deadly contaminants. Among the twelve most hazardous chemicals around the world, nine of them are determined as pesticides and the intermediate compounds of pesticides [33]. Pesticides are the synthetic chemicals used for destroying pests and providing food security to meet the global population demand [34]. Pesticides contamination caused by different sources such as industrial effluents, agricultural runoff and chemical spills grows via bio-magnification [35]. They can have direct or indirect routes of entering the environment. The classification of pesticides is created considering the target organisms such as herbicides (kill weeds), algaecide (kill algae), bactericide (kill bacteria), insecticide (kill insects), fungicide (kill fungi) and rodenticide (kill insects) [33]. Another classification is carried out by considering the chemical groups such as organochlorine, triazines, substituted urea, organohalogen, organophosphorus, synthetic pyrethroid, carbamates and phenol derivatives [33]. The structure of pesticides comprises complex groups, and their intermediate products could be more lethal than precursor compound. Many studies reported that approximately 98% of insecticides cannot achieve quick degradation and enter the environment [36]. The potential health and environmental risks of pesticides is a great concern all over the world.

5.2.3 Pharmaceuticals and Personal Care Products Pharmaceuticals and personal care products (PPCPs) are common organic pollutants produced over one million tons worldwide [37]. The municipal wastewater could enter water, soil and air by different stages, which is the major pathway of PPCPs contaminant [38]. PPCPs are encountered in groundwater, rivers, reservoirs and

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even in oceans. Urban areas are the main zones where PPCPs release through large amounts of industrial and domestic effluents [39]. Pharmaceuticals such as antibiotics, analgesics, anti-inflammatories, hypertensive blockers, antipyretics, antimicrobials, hypertensive blockers, adrenergic agonists, endocrine disrupting compounds and psycholeptics are nonbiodegradable and persistent products with significant risks on ecology and human health [40]. The long-term exposure of PPCPs cause genotoxicity, endocrine disruption, fetal development and carcinogenicity [37, 41].

5.2.4 Aromatic Compounds Aromatic compounds are one of the most persistent and recalcitrant among environmental organic contaminants. Aromatic compounds are classified as polycyclic aromatic hydrocarbons (PAHs), heterocyclic compounds including one or more S, O or N atoms in the aromatic ring, alkylated PAHs, etc. [42, 43]. Some of them is considered as highly toxic and carcinogenic. It is critical to detect these compounds to protect human health and ecology [42, 44]. PAHs are the significant aromatic compounds caused by incomplete combustion of fossil fuels and/or fuel spills. PAHs interfere the water bodies directly or indirectly and cause increasing concerns due to the largely unknown synergic/agonistic and individual effects on the environment [45]. The organic pollutants in wastewater and their toxic concentrations for living species are summarized in Table 5.1. Due to the toxic and persistent nature of these organic pollutants, they cannot be completely removed by using conventional biological, physical and chemical water treatment processes such as flocculation, adsorption, filtration, chlorination, precipitation, bioremediation and coagulation [40]. The innovative methods for removing organic compounds from wastewater have attracted much attention currently. Photocatalytic degradation process has been most promising solution due to low cost and high efficiency [40].

5.3 Photocatalytic Degradation of Organic Pollutants The photocatalytic degradation process is an eco-friendly method used for degradation of various organic pollutants such as dyes, pesticides, PPCPs and aromatic compounds into non-toxic products like CO2 , H2 O and other harmless compounds [33].

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Table 5.1 Organic pollutants in wastewater (Data was collected from Ref. [46]) Pollutant

Toxic concentrations to human health and aquatic species

Removal challenges

Organic dyes (anionic and cationic dyes)

>1 ppm

Nonbiodegradability and stability

Pesticides

>0.3 ppm

Phosphorus and nitrogen containing pesticides released from industry cause a significantly high chemical oxygen demand (COD) value Many different types of pesticides with different structures and various pH values of the waste water (range 0.5–14)

PPCPs

It is reported in ng/l–µg/l in wastewater

Hydrophilicity and persistence

Aromatic compounds

Benzene > 0.01 ppm Xylene > 0.5 ppm Toluene > 0.7 ppm

Difficulty for oxidizing and stability

PAHs > 0.0007 ppm

Nonbiodegradability, low recovery while treating through adsorption and membrane filtration

5.3.1 Photocatalytic Degradation Process The reactions of the photocatalytic degradation process initiate with the absorption of a photon that has the energy (hv) equal or higher than the band gap energy (E bg ) of the photocatalyst [47]. The absorption of photon causes the charge separation; the electron (e− ) at the valence band (VB) of the semiconductor is promoted to the conduction band (CB) and leaves a hole (h+ ) behind (Fig. 5.2). The recombination of the hole and the electron has to be prevented for achieving high degradation efficiency. The reactions for the photocatalytic degradation of organic pollutant on the semiconductor (such as TiO2 ) surface are defined in Eqs. 5.1–5.8 [29, 48]. The activated electrons react with the oxidant to form the oxidized product. The O2 which is adsorbed on the surface of semiconductor or dissolves in water can react with generated electrons and produce superoxide radical anion (O2 −• ). The generated electrons can reduce the organic pollutant, and generated holes could oxidize the pollutant or react with H2 O or OH− to form OH• radicals. The OH• radicals have the redox potential of +2.8 V, and they are strong oxidizing agents. These radicals and the other oxidant species such as peroxide radicals provide the photocatalytic degradation of organic pollutants [48]. + TiO2 + hν → TiO2 (e− CB + hVB )

(5.1)

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+ • TiO2 (h+ VB ) + H2 O → TiO2 + H + OH

(5.2)

− • TiO2 (h+ VB ) + OH → TiO2 + OH

(5.3)

−• TiO2 (e− CB ) + O2 → TiO2 + O2

(5.4)

+ • O−• 2 + H → HO2

(5.5)

Organic Pollutant + OH• → Degradation Products

(5.6)

Organic Pollutant + h+ VB → Oxidation Products

(5.7)

Organic Pollutant + e− CB → Reduction Products

(5.8)

The photocatalytic degradation processes on the semiconductor carry out in the following main steps [33]: Transfer of reactants to the semiconductor surface; (i) (ii) (iii) (iv)

Adsorption of the reactants. Photocatalytic reactions in the adsorbed phase. Desorption of the generated products. Diffusion of the products from the semiconductor surface.

Fig. 5.2 Mechanism of photocatalytic degradation of organic pollutants (Reprinted from Ref. [46] with permission from Elsevier)

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5.3.2 Photocatalyst Photocatalytic degradation of organic pollutants is performed with the aid of a catalyst. The catalyst used in this process is entitled as “photocatalyst,” generally metal oxides semiconductors such as TiO2 , ZnO, SnO2 , WO3 , CuO, LaCoO3 , Fe2 O3 , MoO3 , V2 O5 , SrTiO3 , CdS and SnO2 [20, 49]. The photocatalyst should be nontoxic, inexpensive, chemically/biologically inert, stable and be active for the light in visible or/and UV region [49]. As mentioned above, in order to achieve an efficient photocatalytic degradation of organic pollutants, there are some properties to be considered with respect to photocatalysts such as morphology, surface area and band gap energy [20, 21]. The semiconductors can be used as pristine, doped or as a composite by immobilization on support material for the photocatalytic degradation of organic pollutants [50].

5.3.2.1

Photocatalyst Morphology

The photocatalyst morphology is significant in photocatalytic processes. The change in morphology could affect crystallinity, crystallite size, structural defects, band gap, pore volume and surface area of the photocatalyst [20, 21, 51, 52]. The slight altering in morphology can affect photocatalytic activity significantly. The higher crystallinity is preferable for photocatalyst due to the improved charge collection efficiency and carrier mobility [53]. The defect in crystal can trap the photogenerated species and decrease the photocatalytic activity [54]. The crystalline structure improves electronic conductivity, which provides higher charge generation to increase the degradation capacity [51]. Phan et al. (2011) tuned the morphology of the TiO2 nanostructure by hydrothermal preparation using hydrochloric acid. They indicated that the morphology of the TiO2 affects photocatalytic degradation activity significantly. The flower-like and prisms morphology of TiO2 showed the highest photocatalytic degradation of methylene blue [12]. Fan et al. (2016) tuned the TiO2 nanostructure by solvothermal reaction at different temperatures. The modified structures such as rose-like, sea-urchin-like and chrysanthemum-like exhibited higher degradation of Rhodamine B (>96.5%) compared to commercial TiO2 (59.5%) [55]. Rong and Wang (2021) monitored morphology through time-dependent hydrothermal growth and studied the effect of crystallinity of tungsten oxide (WO3 ) on the degradation performance of tetracycline (Fig. 5.3); 3 and 9 min of heating produced the irregular oval precipitate and the elliptical crystals, respectively, 2, 4 and 6 h of heating increased the thickness of the sheets, when the heating duration was extended to 8 h, hollow cavity was formed and at 10 h perfect hollow nest-like WO3 micro/nanostructure was obtained and further increment of the heating duration resulted in structural cracks. The degradation efficiency is reported to be higher for the heating times of 4, 7, and 10 h due to the cavity structure and the crystallinity of the photocatalyst [22].

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Fig. 5.3 SEM images of the WO3 structure after different treatment times: a 3 min; b 9 min; c 15 min; d 2 h; e 4 h; f 6 h; g 8 h; h 10 h and i 16 h; j formation process (Reprinted Ref. from [22] with permission from Elsevier)

Zheng et al. (2019) have reported that the methylene blue degradation performance of flower-like MgO was higher than spherical, rod-like, trapezoidal and nestlike morphologies of MgO. Furthermore, the flower-like MgO performed the higher photocatalytic activity compared to conventional photocatalysts such as Degussa P25 TiO2 , α-Fe2 O3 , N-doped TiO2 , WO3 , g-C3 N4 , ZnO and BiVO4 [56]. 5.3.2.2

Surface Area of Photocatalyst

The surface area is one of the main concerns while designing the photocatalyst with high photocatalytic activity. The higher surface area is necessary due to more reacting sites and fast generation of reacting species [20]. Commercial P25 TiO2 has low surface area (50 m2 g−1 ) due to agglomerated, randomly oriented structures and recombination tendency [57]. To overcome this limitation of TiO2 , Naik et al. (2020) carried out innovative process to synthesize

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Fig. 5.4 Process to synthesize super porous TiO2 (Reprinted from Ref. [57] with permission from Elsevier)

TiO2 with the surface area of 238 m2 g−1 . The reflux and sol–gel methods were applied to obtain super porous structures (Fig. 5.4). The TiO2 synthesized from butanetetracarboxylic acid (or tricarballylic acid) and urea gel mixtures improved the surface area of TiO2 due to increase of carboxylic groups and presence of urea. The synthesized super porous catalyst with mesoporous structure exhibited higher degradation activity for Amaranth dye due to the fast diffusion kinetics and high adsorption capacity [57]. The degradation of ceftiofur sodium by Ag–TiO2 (469 m2 g−1 ) was remarkably higher than the Degussa P25 TiO2 in terms of improved surface area [58]. The photoluminescence intensity of TiO2 nanotubes was lower than commercial nanoparticles that reduced the charge recombination rate and consequently increased the charge separation [58].

5.3.2.3

Band Gap Energy of Photocatalyst

The photocatalytic reaction is preferred to be performed with minimum energy. The selected photocatalyst should facilitate the reaction under wide electromagnetic radiation spectrum range, especially visible light. The band gap of the photocatalyst has to be narrow, less than 3 eV, for initiating the reaction by visible light photons [51]. Band gap energies of several photocatalysts and corresponded wavelengths are illustrated in Table 5.2. TiO2 is the most commonly used semiconductor photocatalyst that have the band gap energy of 3.0 eV for the rutile form, 3.2 eV for the anatase and brookite forms [50, 59]. Although anatase has a higher band gap energy than rutile, it has higher photocatalytic performance due to band structural properties [60]. The indirect band gap of anatase provides longer lifetime for carriers than the direct band gap of other forms of TiO2 . The mixing of rutile and anatase could perform higher degradation efficiency compared to individual form. The mixture with 80:20 anatase to rutile indicates better degradation performance than other ratios [61]. ZnO is another widely used photocatalyst with the band gap energy of 3.2 eV that is similar to TiO2 . Iron oxide exists in different crystal phases such as maghemite (ν-Fe2 O3 ), hematite (αFe2 O3 ), wustite (FeO) and magnetite (Fe3 O4 ) [50]. The band gap energies of iron oxides are between 2.0 and 2.3 eV. Tungsten oxide (WO3 ) is the semiconductor which

5 Photocatalytic Reactors Design and Operating Parameters … Table 5.2 Band gap energies of photocatalysts and corresponded wavelengths [63]

113

Semiconductor

Band gap energy (eV)

Wavelength (nm)

TiO2 (rutile)

3.0

413

TiO2 (anatase)

3.2

388

ZnO

3.2

388

Fe2 O3

2.3

539

WO3

2.8

443

ZnS

3.6

335

CdS

2.4

516

α-Bi2O3

2.9

428

has different structures like cubic, monoclinic, triclinic, hexagonal, tetragonal and orthorhombic. It is the striking candidate for photocatalytic degradation of organic pollutants with the band gap energy of 2.8 eV [50, 62]. The band gap energy of bismuth oxide (2.86 eV) has been decreased by doping with Mn, V, Nb and Gd ions, and the band gap energies were determined as 2.71, 2.77, 2.81 and 2.83, respectively. The degradation efficiency of Mn-doped Bi2 O3 (lowest band gap) was the highest. The visible light photodegradation capacity of Bi2 O3 improved by doping Mn and V ions in terms of new electronic states [64]. 5.3.2.4

Doping of Photocatalyst

Heterogeneous photocatalysts have become the most promising technology to improve the photocatalytic degradation of organic pollutants [65]. Metal ion doping, non-metal doping or co-doping of metals are applied to decrease charge carrier recombination. Metals dopants like Pt, Ag, Pd, V, Au, Th, Mo, etc., and non-metal dopants like C, F, N, S, etc., are used to increase photocatalytic performance of photocatalysts [48, 65]. Dopants enhance the photocatalytic activity of photocatalyst in a number of ways such as band gap narrowing, specific surface area for adsorption, oxygen vacancies, electron trapping and impurity energy level formation [30]. Due to the narrow band gap of photocatalyst, more electron–hole pairs are formed, and photocatalytic activity is increased [66]. Impurity energy levels, band gap narrowing, and oxygen vacancies enhance the photocatalytic performance even at visible light [30, 67]. Doping of the photocatalyst prohibits the recombination of the holes and electrons, which improves the photocatalytic activity [68]. Under the visible light region, electronic properties of semiconductor could be modified by doping of metal to the crystal lattice of photocatalyst [30]. Nevertheless, the increment of photocatalytic activity could decrease beyond the optimum amount of dopant. Fan et al. (2020) have reported that Bi-doped titanate nanobulks had higher surface area, smaller band gap, more crystallinity and better photocatalytic degradation of naproxen compared to un-doped titanate [69]. Villanueva et al. (2017) synthesized

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the Ge, Zn and Ga-doped TiO2 by colloidal route to use in degradation of 2,4dichlorophenoxyacetic acid. The order of photocatalytic activity was observed as Ge-TiO2 > Ga-TiO2 > Zn-TiO2 > TiO2 . The enhancement of activity was explained by the energy level reduction of the 2p1/2 and 2p3/2 electrons with doping compared to pristine TiO2 [70]. In Yakout et al. (2020) work, Mn-doped α-Bi2 O3 was used in photocatalytic degradation of Congo Red, methylene blue and 4-nitrophenol under solar light. Three-dimensional structures of pristine and Mn-doped Bi2 O3 with high crystallinity were illustrated in Fig. 5.5. Doping of Mn2+/3+ ions inspired the new electronic states and red shifts in the band gap. Mn-doped Bi2 O3 exhibited excellent photocatalytic activity due to the increasing light absorption and prevention of electron–hole pairs with trapping center [64]. Liu et al. (2005) show that the doping method and dopant concentration are extremely important considerations. The highest degradation of Rhodamine B was determined by doping of 0.5% Zn2+ (mole) in TiO2 particles with solid phase reaction method [71]. Lee et al. (2021) have reported that the band gap energy of TiO2 was decreased from 3.25 eV to 3.01 eV after La3+ ion doping. This enhanced the response to the visible light and photocatalytic degradation increased to 1.5 times under sunlight [72].

Fig. 5.5 Three-dimensional structures of a pristine Bi2 O3 and b Mn-doped Bi2 O3 (Reprinted from Ref. [64] with permission from Elsevier)

5 Photocatalytic Reactors Design and Operating Parameters …

5.3.2.5

115

Photocatalyst Immobilized on the Support

Most of the photocatalysts are utilized in powder form and have some shortcomings during photocatalytic degradation process [73]: (i) Powder catalysts can be easily agglomerated at the preparation step [74]; (ii) powder catalysts are separated from water phase difficultly; therefore, treatment costs could be increased [75]; (iii) powder catalysts can behave as short circuit microelectrode in water, which increase the recombination of electrons and holes [76]. The immobilization of photocatalysts on the support surfaces is a remarkable solution to overcome these shortcomings [73]. The assembly of photocatalyst into the photocatalytic reactors can be easily accomplished by the selection of suitable support material. The widely used support materials are glasses, zeolites, clays, silicon-rich ceramics and graphene [77–79]. The support materials could be organic or inorganic, and each one has some advantages and disadvantages. Therefore, while the choosing of support material, some significant points must be considered such as specific surface area, types of reactor design, price, reusability, characteristics of the pollutants, catalytic activity, degradation resistance, adhesion for photocatalyst, light transmittance and operating conditions [73]. The preferable support material should have high surface area, easy separation from liquid, well light transmittance and strong adhesion to the photocatalyst [73, 80]. The immobilization of photocatalyst on the selected support material can be performed by using various methods such as chemical vapor deposition, coupling method, seeding method, sol–gel method, liquid phase deposition and impregnation method [81–83]. The suitable immobilization method can be selected by considering the types, properties and structure of support, the type of photocatalyst and the application conditions [73]. So far, TiO2 have been immobilized on the support surfaces such as graphene [84], reduced graphene oxide [85], polyaniline [86], activated carbon [87], polyvinyl alcohol [88], carbon nanotubes [89] and polyester [90] to facilitate the photocatalytic degradation process. Feng et al. (2021) reported that TiO2 particles were immobilized on the gC3 N4 surface closely. This composite catalyst had lower recombination rate, wider light absorption ranges and superior degradation of methylene blue and Rhodamine B dyes [26]. Montanez et al. (2015) immobilized the TiO2 on HZSM-11 zeolite and observed the complete degradation of herbicide dicamba [91]. The TiO2 was immobilized on the glass plate to degrade the imidacloprid pesticide. Under UV light, this supported photocatalyst were very effective to degrade the organic pollutant [92]. Lei et al. (2018) combined the titanate nanotubes with graphite oxide to enhance the degradation capacity of methylene blue. The degradation mechanism was illustrated in Fig. 5.6. The titanate with graphite oxide had higher BET surface area (139.9 m2 /g) compared to pristine titanate (94.4 m2 /g). Reducing recombination rate and increasing surface area of composite photocatalyst improves the photocatalytic performance [93]. Vaiano et al. (2018) immobilized the ZnO on glass spheres by using the dip coating method. The highest mineralization efficiency for acid blue 7 was observed using only one dip coating step [94]. Tran et al. (2020) indicated that the aggregation of TiO2 nanoparticles reduced by combining TiO2 and graphene oxide. This composite

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Fig. 5.6 Photocatalytic degradation mechanism of organic pollutant by graphite oxide grafted titanate nanotubes (Reprinted from Ref. [93] with permission from Elsevier)

catalyst exhibited high degradation efficiency for phenol with a lower photocatalyst dose [95]. The examples of photocatalysts used for photocatalytic degradation of organic pollutants were illustrated in Table 5.3.

5.4 Design of the Photocatalytic Reactors for Organic Pollutants In this part, photocatalytic reaction mechanisms and the photon effects on the mole balance, the effect of the reactor design and photoreactor types were discussed.

5.4.1 The Mole Balance of the Organic Pollutants Organic pollutant (Org) can be degraded by using the photocatalytic reactors. The mole balance is given for different reactor, assuming that the reactor is isothermal and diffusion is eliminated [104]. (Forg )o − (Forg ) + ∫ G org =

dNorg , dt

(5.9)

where (F org )0 and F org are the flowrates (mol/time) of the organic component in and flow out to the reactor, respectively. Norg is the mole of organic component (mol) in the system. Gorg is the rate of the generation. ' G org = Rorg V,

(5.10)

Target pollutant

Sulfadiazine Sulfamethoxazole

Rhodamine B

2-chlorophenol

Temephos

Tetracycline

Methylene blue

Atrazine

4-nitrophenol Congo Red Methylene blue

Triclosan

Naproxen

Rhodamine B

2,4-dichlorophenoxyacetic acid

Photocatalyst

TiO2

TiO2

TiO2

ZnO

WO3

MgO

Fe3 O4 -TiO2 /rGO

Mn-doped α-Bi2 O3

Bi7 O9 I3 /Bi

Bi-doped titanate

La-doped TiO2 @ halloysites

Zn, Ga and Ge-doped TiO2

124–140

82.84

39.14

–

–

–

31.8–142.9

18.07

–

50

35.8

50.17

Surface area, m2 g−1

3.18–3.23

3.01

2.19

2.29

2.71

Fe3 O4 :1.4 TiO2 : 3.2

4.99–5.23

–

3.2

–

3.09

3.03

Band gap, eV

Table 5.3 Photocatalytic degradation of organic pollutants using various photocatalyst

UVP lamp (2000

W/cm2 )

UV lamp (250 W)

Metal Halogen lamp (500 W)

Xe lamp (500 W)

50

99.36

99.9

85.2

94 97 99

98

Sunlight (11–4 pm, August)

99.9

Sunlight (800–850 W/m2 ), Visible light UV-light

94.3

50

98

97.8

99 99

Effic., %

Mercury lamp (power: 400 W)

Xe lamp (300W)

Low power sunlight simulator lamp (150 W, with irradiance of 360 W/m2 )

Medium pressure Mercury arc lamps (250 W)

UV

Xenon lamp (300 W) with the filters

Light (Intensity)

(continued)

[70]

[72]

[69]

[100]

[64]

[99]

[56]

[22]

[98]

[97]

[55]

[96]

References

5 Photocatalytic Reactors Design and Operating Parameters … 117

Target pollutant

Methylene blue

Rhodamine B Methylene blue

Ceftiofur sodium

Bisphenol A Phenol

Naphthalene, Phenanthrene Pyrene

Sulfasalazine

Photocatalyst

Titanate @GO

TiO2 /g-C3 N4

Ag-TiO2

Ag2 O/Bi5 O7 I

Graphene Oxide/Ag3 O4

Cu2 O-BiVO4 -WO3

Table 5.3 (continued)

–

–

27.23

469

99.06

139.9

Surface area, m2 g−1

2.46

2.3

2.88

3.0

2.48

–

Band gap, eV

Tungsten Lamp (100W)

Xe lamp (500 W)

White LED light (5W)

Simulated sunlight

Xenon lamp (300 W)

Mercury lamp (175 W, 1.40–1.45 mW/cm2 )

Light (Intensity)

85.7

82.1 100 100

99.9 61.4

87

97 100

97.5

Effic., %

[103]

[102]

[101]

[58]

[26]

[93]

References

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5 Photocatalytic Reactors Design and Operating Parameters …

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' where Rorg is the rate of reaction and depends on the reactor type and V is the reactor volume. The reaction rate should be considered with the integrations of the generation. For the batch reactor, the mole balance is

dNorg ' = ∫ Rorg dV. dt

(5.11)

The mole balance for the continuous flow reactor (CSTR): (Forg )o − (Forg ) +

∮

V ' Rorg dV =

0

dNorg . dt

(5.12)

At the steady state condition and the homogeneous mixing, (Forg )o − (Forg ) +

∮

V 0

' dV = 0. Rorg

(5.13)

' Rorg dV = 0.

(5.14)

The design equation will be (Forg )o − (Forg ) +

∮

V 0

(Forg )o − (Forg ) ' = −Rorg . V

(5.15)

In the Packed bed reactor, we eliminate the radial direction, and the mole balance equation can be written as Forg|w − Forg|w+Δw + Rorg dW = 0.

(5.16)

In the liquid phase reactor, we can neglect the pressure in the reactor, dForg = Rorg , dW

(5.17)

where W is the photocatalyst amount. Photocatalytic reaction is the heterogenous system, in this case seven steps of the reaction is very important [104]: 1. 2. 3. 4. 5. 6.

Mole transfer of the organic pollutant to external surface of the photocatalyst Diffusion of the organic pollutant from the internal pore of the photocatalyst Adsorption of the organic pollutant onto photocatalytic surface (RAds ) The photocatalytic reaction in the photocatalyst (Rs ) Desorption of the product from the photocatalytic surface (Rd ) Diffusion of the products from the photocatalytic surface

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7. Mass transfer of the products from the external surface to the bulk fluid. For the liquid phase photocatalytic reaction mole transfer on the external surface (step 1), adsorption (step 3) and reaction on the surface (step 4) are important. Diffusion of the reactant and product in the pore can be eliminated (step 2 and 6). If there is well mixing in that case external mass transfer was eliminated (step 7). So that adsorption of reactant on the photocatalyst surface and the desorption of the product (Rs ) from the source of photocatalyst is very important for heterogeneous reaction. In the heterogenous reactions, there will be adsorption (Rad ), surface reaction (Rs ) and the desorption (Rd ) on the catalyst surface [104]. In the photocatalytic system, the current I (coulombs/s) is related with Eq. 5.18; I =

E E = , Rt (RAds + Rs + Rd )

(5.18)

where Rt is the total resistance and E is the voltage, I is the current (coulombs/s).

5.4.2 Reaction Rate The Langmuir–Hinshelwood approach is used for photocatalytic systems. Because the system is heterogeneous; the adsorption on the catalyst is considered [105]: Rorg = −

dCorg k K Corg , = dt 1 + K Corg

(5.19)

where k is the reaction rate constant on catalyst surface (mol/cm3 sec), and K is the adsorption Langmuir equilibrium constant (cm3 /mol) C org is the concentration of the organic pollutant (mol/cm3 ), and t is time (sec). At low concentration of organic pollutant (10–3 < KC org ) [105], −

dCorg = k K Corg = k ' Corg . dt

(5.20)

At the high concentration of organic pollutant (KC org ≫ 1) [105], −

k K Corg dCorg = =k dt K Corg

(5.21)

In this case, zero order reaction kinetics is effective on the reactions. Comparing the space–time yield (STY) can be calculated for more effective reactor (Eq. 5.22); STYCSTR =

V , τ

(5.22)

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where τ is the space time (sec). τ=

V ϑ

(5.23)

where V is the volume of the reactor (m3 ) and υ is the volumetric flow rate (m3 /sec). The mole balance of the CSTR reactor at steady state condition is ( ) ( ) ϑ Corg,0 − Corg (Forg,0 ) − Forg = = −Rorg V V

(5.24)

depending on the concentration of organic pollutant space time and the space time yield (STY) are given below. For low concentration, τ=

Corg,0 − Corg V = . ϑ k ' Corg

STYCSTR =

V = τ

V Corg,0 −Corg k ' Corg

(5.25) .

(5.26)

For high concentration, τ=

Corg,0 − Corg V = . ϑ k

STYCSTR =

V = τ

V Corg,0 −Corg k

(5.27) .

(5.28)

For plug flow reactor (PFR) at state conditions, ∫ dForg = ∫ −Rorg dV .

(5.29)

The space–time and space–time yield of PFR is V = τ= ϑ

∮

Corg

Corg,0

dCorg . Rorg

(5.30)

For the low concentration, STYPFR = For high concentration,

V V = Corg, −Corg . τ − kCorg

(5.31)

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STYPFR =

V V ). = ( Corg τ k ln Corg,0

(5.32)

The standardized lamp power (LP) is measured for energy density in the reactor area [106]: LP =

P ×1 V

(5.33)

where P is the lamp power (W) and V is the reactor volume (m3 ). 1 is the normalized the unit of the reaction volume (m3 ). The photocatalytic space–time yield (PSTY) can be used as (Eq. 5.33): PSTY =

STY LP

(5.34)

to decide which reactor is the best reactor for the organic pollutant [107]. In general form, the mole balance of the plug flow reactor can be expressed as follow: ) 1 ∂ Norg, ∂ Norg,z ∂Corg 1 ∂ ( r Norg,r + + + Rorg = . r ∂r r ∂ ∂z ∂t

(5.35)

Flux of organic pollutant reactant is given in Eq. 5.35. Norg = −cDorg,B ∇ y A + Corg ϑ

(5.36)

where Norg are molar flux relative to stationary axes (mol/s.m2 ). Dorg,B is diffusion coefficient for organic pollutant through component B (m2 /s), C org is the concentration of organic pollutant in equilibrium with the bulk composition of the gas phase (mol/m3 ), c is the total molar concentration (mol/m3 ), υ is velocity of organic pollutant solution (m/s). Assuming that the system is at the steady state condition, there is the onedimensional direction (z) of wastewater, and the diffusion is eliminated. ( ) ∂ Corg ϑ + Rorg = 0 ∂z

(5.37)

When assume that the τ is the retention time of the organic pollutant, these reactions can be written as ( ) ∂ Corg = −Rorg . (5.38) ∂τ ∮ Corg dCorg τ =− . (5.39) Corg,0 Rorg

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In the photocatalytic reactor H2 O2 , H2 O and gas component (C) are used to increase the reaction efficiency. The reaction rate can be expressed with these equations [108]: p −Rorg = k I m C n Corg

(5.40)

where I is the light intensity (W/m2 ). The annular photocatalytic reactor, radiation of the energy of UV light can be expressed with the Lambert’s law of absorption, I = Io

r1 −E(r −ri ) e r

(5.41)

where E is the monochromatic absorbance of organic wastewater. The combination of the equations (5.40) and (5.41) the photocatalytic rate is ( r )m 1 p −Rorg = k Io e−E(r −ri ) C n Corg r

(5.42)

where m, n and p are the superscript of light intensity, C component and organic pollutants, respectively. Depending on the reaction rate, the adsorption equilibrium (internal and/or external) on catalysis is very effective for photocatalytic reactions. Photocatalysis, reaction and the irradiation of light are very important for the selection of the best reactor.

5.4.3 Photoreactor Types Due to the comprehensive design of photoreactors, it is hard to classify them as the conventional reactor types. Lamp position, photoreactor geometry and configuration, photocatalyst arrangement and flow types are some of the main factors that should be considered while designing a photoreactor [109]. In the liquid phase photoreaction systems, batch, semi-batch and continuous stirred reactors are generally used. Nowadays, microfluidic, solar reactors, bed reactor, thin-film reactor, annular reactor, flow reactor, immersion well reactor and multiamp reactor are new reactors for the photocatalytic reactor. Batch and continuous photocatalytic reactors are illustrated in Fig. 5.7. Selection of the photon source may change by factors such as the reactor scale and photocatalyst type. Most common photon sources used in the photoreactors can be divided into two as natural and artificial. Sun is the natural type of irradiation choice. For solar lighted photoreactors, solar concentrators can be used to enhance the light emittance and hence the reactor efficiency [112]. Artificial irradiation options are usually low-pressure mercury lamps, middle-pressure mercury lamps, high-pressure mercury lamps and light-emitting diodes (LED). Medium pressure mercury lamps

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Fig. 5.7 Schematic representation of a batch (Reprinted from Ref. [110]) and b continuous (Reprinted from Ref. [111]) photocatalytic reactors

are most commonly used photon source between the three other mercury lamps. It operates in pressures between 1 and 10 atm and temperatures between 600 °C and 800 °C [113]. To avoid the thermal effects, it is used with a water-cooled immersion jacket. Lamp systems can be added to the reactor system in two ways, internally or externally (Fig. 5.8). Externally placed lamps can be perpendicular or parallel to the reactor, and multiple lamps can be used. Internally placed lamps are immersed in the reaction medium usually with a quartz jacket. Because of their operation temperatures, they are also equipped with a cooling system. There are three ways to apply a cooling system to these lamps, a cooling jacket with circulating cool water, a cooling jacket that is used for the reactor system itself, or a water bath that carries the whole reactor system [109]. Photoreactor types are usually divided by their deployed state of photocatalysts, such as photoreactors with suspended or immobilized photocatalysts. In suspension photoreactor models, photocatalysts stay deployed in the reaction media, whereas in the immobilized models, photocatalysts are immobilized onto a catalyst support. Both have their own advantages and disadvantages [112]. One of the common photoreactor types is tubular photoreactor. It is suspension reactor with a recirculating setting that passes the contaminated water and photocatalyst through its glass walls for efficient irradiation using either solar or artificial light sources. Flat plate photoreactor (Fig. 5.9a) type can also be given an example for suspension photoreactors. It is aimed to enhance the solar irradiation with this

Fig. 5.8 Internally UV photoreactor; schematic representation of a the spiral reactor and b an annular photocatalytic benchmark reactor and c photograph of the actual spiral reactor (Reprinted from Ref. [114] with permission from Elsevier)

5 Photocatalytic Reactors Design and Operating Parameters …

125

type of configuration [112]. Slurry photoreactors (Fig. 5.9b) have a crucial advantage of high surface area of photocatalyst per unit volume and high mass transfer rates, which enable a more efficient reaction. On the other hand, an important disadvantage of the slurry photoreactors are the need of post-treatment for catalyst recovery which would not be feasible on a large-scale application [115]. Packed bed photoreactor type can be an example of immobilized photoreactor model. It has a setting of supported photocatalysts that are added to the reactor bed which aims the increasing contact area. It is important to consider the depth of the reactor due to the photon transmittance [112]. Batch type reactors are another great example for photoreactors; it is working either as a suspension or immobilized photoreactor. It operates throughout the reaction by no adding or extracting. Batch reactor type can be extensively encountered in wastewater treatment. There is also batch recirculation photoreactor model where the water media recirculates continuously passing through the light source [113]. In photocatalytic membrane reactor (Fig. 5.9c) configuration photocatalysts can be used either suspended or immobilized onto the membrane surface. Both examples have good effects on water

Fig. 5.9 Schematic representation of a flat plate (reprinted Ref. from [116] with permission from Elsevier), b slurry (reprinted Ref. from [117]) and c membrane photoreactors (reprinted Ref. from [118])

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purification considering the membrane can also be an additional barrier. However, it is also predicted that the immobilized photocatalyst system can shorten the life of membrane [115]. To conclude, there is still a need to research about photocatalytic processes to bring the photoreactors into industrial scale [109]. To overcome the insufficiency in this field could promise greener solutions for the wastewater organic pollutant treatment.

5.4.4 Selection of Irradiation Source Light irradiation is a critical operational parameter for designing a photocatalytic process to remove organic pollutants. The generation of electron–hole pairs, which cause the degradation of organic pollutants, can be achieved using different light sources at a specific wavelength and intensity for various photocatalytic materials. The following section summarizes how the photocatalytic conversion efficiency of the materials has been affected by changing the parameters mentioned above in the reactors.

5.4.4.1

Types of Light Sources

The light sources play a critical role in providing high photocatalytic activity via the generation of the electron–hole pairs. Therefore, selecting the appropriate light source would affect the efficiency of the photocatalytic conversion. The photocatalyst materials have a wide band gap (E g ) and the chosen lamp source spectrum should line up with the band gap of the material. Photocatalytic conversion of pollutants is mainly achieved through the application of ultraviolet lamps (UV) covering the wavelength range from 10 to 400 nm, which categorized into three regions as UVA (315–400 nm), UVB (280–315 nm) and UVC (100–280 nm). The conventional UV lamps are made of heavy metals, mainly mercury vapor lamps. They have some drawbacks such as high energy consumption, short lifetime, operational difficulties and both environmental and health issues. Contrary to these disadvantages, environmentally friendly, low-cost, more compact, longer lifetime, energy-efficient lamps; lightemitting diodes (UV-LED) were developed in connection with the semiconductor industry evolution. LED lamps do not have any geometrical constrictions; therefore, they have an advantage over conventional UV lamps in reactor design. Although LED lamps are promising, there are studies in the literature in which higher photodegradation is obtained for conventional lamps [119, 120]. However, this was due to the inhomogeneous light distribution in the reactor. Therefore, LED lamps, which can be positioned very flexible due to the point light characteristic, should be compatible with the reactor design.

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The maximum illumination of the light source, which can be tuned with the lamp positions, on the catalysis surface affects the kinetic rate of the photocatalytic reaction. According to the place of the lamp sources, the reactors are divided into three types: immersion, external and distributor. If the light source is inside the chamber, the reactor is called an immersion type, and if it is outside, it is called an external type. The light source is radiated on the catalysis with a reflector or optical fiber for the distributed reactor. UV-LED sources are suitable for use in these three types [121]. The correctly positioned light source in a reactor would allow the light distribution via absorbing and scattering processes from liquid to the catalyst [122]. Reducing energy consumption is the ideal option for photocatalytic reactions. For this purpose, many studies are carried out to obtain optimum efficiency and stability for photocatalytic materials tuning the chemical structures and physical properties by band gap engineering [123–125]. TiO2 is most commonly used due to its specific properties such as stability, low cost and optical properties [126]. The electronic band gap structure of the TiO2 is 3.2 eV, and it absorbs the near-UV light (λ ≤ 387.5 nm). Therefore, TiO2 can initiate the electron–hole pairs from the solar light irradiation for photocatalytic reactions [127]. The solar light is composed of 5% UV light [128], and this natural source reduces the cost of the process. On the contrary, the scale of the experimental setup needs to be enlarged, which increases the cost. The photodegradation of three pharmaceutical chemicals of carbamazepine, diclofenac and ibuprofen were conducted in the controlled batch mode photoreactors [129]. Figure 5.10 shows the artificial light photoreactor with an LED-UV light source on the left and the natural sunlight reactor on the right. The LED-UV source has the same surface area as the tank to provide uniform irradiance. The natural sunlight and UV light intensities were controllable in the range of 0–60 and 5–85 W/m2 , respectively. While the tank volume was 2 L for the artificial lamp source reactor, the natural sunlight cylindrical tank was 300 L. The higher degradation was obtained for natural sunlight reactors. This result was explained for the following reasons: A broader UV spectrum that solar light has boost photocatalytic degradation. In addition, air bubbles caused by flowing in the artificial reactor increased oxygenation. It has been reported that oxygenation negatively affects the photodegradation of carbamazepine and ibuprofen [130].

Fig. 5.10 Experimental photoreactor setup for artificial light (left) and natural sunlight (right) photoreactor (Reprinted from Ref. [129] with permission from Elsevier)

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5.4.4.2

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Wavelength of Source

The decreasing wavelength of the LEDs increases forward voltage, and generated photons are more energetic. Thus, the experimental results showed that the smaller wavelength for light sources performs better for photocatalytic degradation [131]. The absorption spectrum of photocatalytic materials is a criterion for choosing an efficient wavelength range. Therefore, the maximum wavelength of the lamp source should be lower or equal to the critical wavelength of the photocatalytic material. Moreover, the absorption wavelength of the pollutant should also be considered. The wavelength of the light source should not be within the maximum absorption range of the pollutant [132]. Two different photoreactors were designed with the wavelength of 365 nm and 254 nm as high-power light-emitting diodes (HP-LEDs) reactor and a traditional UV-lamp photoreactor, respectively. Figure 5.11 shows the HP-LED setup with the lamp sources and the reactor. The degradation of reactive blue dye was conducted using ZnO nanoparticles as catalysts. The reaction rate was two times greater under the UV-LEDs light than the traditional UV lamps. Moreover, the energy consumption was five times, and the photoreactor setup cost was four times higher for traditional UV lamps than that of the HPLEDs [133].

Fig. 5.11 a Scheme of the HP-LED photoreactor, b Homemade HP-LED photoreactor, c all HPLEDs off, d all HP-LEDs on (Reprinted from Ref. [133])

5 Photocatalytic Reactors Design and Operating Parameters …

129

High power LED365 nm and commercial 398 nm lamps were used to remove Coumarin photocatalytic removal using ZnO and TiO2 as photocatalysts [134]. Excitation efficiency was taken into account to determine the absorption properties and the band gap of the photocatalyst. The LED365nm source was absorbed 100% by ZnO, however 80% by TiO2 due to its optical properties. On the other hand, the LED398nm source was absorbed only 20% by ZnO and TiO2 because 80% of light reflection were observed at 398 nm and generates less electron–hole pair than 365 nm photons [135].

5.4.4.3

Intensity of Source

The optimum photocatalytic conversion efficiency relies on the uniformity of the light intensity in the reactor space. In addition to being directly related to the lamp source used, the light intensity is also associated with the lamp geometry, the distance between the lamp and the catalysis, attenuation coefficient of the lamp, optical properties of the reactor walls and ambient [136, 137]. The presence of sufficient light intensity within the reactor space is essential. In this case, the photocatalytic reaction proceeds independent of the light intensity, and the electron in the valence band can jump to the conduction band by generating an electron–hole pair. In the case of the middle light intensity, the reaction rate decreases because the electron–hole pair separation and recombination processes compete with each other. Therefore, the reaction rate is proportional to the square root of the light intensity [118]. However, the higher light intensity inputs do not increase the photocatalytic performance for each reaction, which causes energy consumption [138]. Figure 5.12 shows the removal efficiency of the methyl orange (MO) and direct red 16 (DR16) dyes depending on the light intensity. In this study, the LED lamps source with a wavelength of 405 nm was used, and its effect on photocatalytic removal efficiency was determined for various light intensities changing from 15 to 25 W/cm2 . As a result, the conversion efficiency was enhanced to 80% and 90% in 25 W/cm2 for DR16 and MO, respectively. Furthermore, it was determined that the light penetration increased with increasing intensity, and that the irradiation time was the minor factor influencing the removal efficiency [139]. However, optimizing the morphological characteristics of the photocatalytic materials and increasing the irradiation time can improve the degradation of the methylene blue [140]. The light intensities, reactor types, light source and power, catalyst used for optimum efficiency in removing different types of organic pollutants are summarized in Table 5.4 from the literature studies conducted in recent years.

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Fig. 5.12 Light intensity effect on the removal efficiency for different catalyst concentration at 10 mg/L dyes concentration. (Reprinted from Ref. [139] with permission from Elsevier)

5.5 Operating Parameters The photocatalytic degradation of organic pollutants is affected by various significant parameters including temperature, pH, pollutant concentration, catalyst dosage, oxidants and coexisting inorganic anions (Fig. 5.13). The efficiency of a photocatalytic reaction can be enhanced by using the optimum reaction parameters [51]. Photocatalytic efficiencies of various photocatalysts and optimum operating parameters of the studies in literature are summarized in Table 5.5.

5.5.1 pH For the photocatalytic degradation of organic pollutants, pH is one of the most significant parameter that affects the effectiveness in numerous ways [30]. The optimum pH for a photocatalytic reaction is associated with organic pollutant, nature and type of the catalyst and electrostatic forces between them [65]. The effect of pH on the photocatalytic process is generally complicated and depends on (i) the agglomeration of semiconductor; (ii) the ionization state of semiconductor surface; (iii) the location of CB and VB of the catalyst; (iv) hydroxyl radicals in solution [165]. The adjustment of the solution pH can change the surface charge or the isoelectric point. The pH effect on the photocatalytic degradation could be determined by using the point of zero charge (PZC) of the semiconductor. The positively charged organic pollutants are adsorbed at pH > PZC and the negatively charged contaminants are adsorbed at

Indigo Carmine and Bisphenol-A mixture

Oxytetracycline (OTC) UV-LED/365

Methyl orange

Coliform bacteria

Methylene blue

Methyl ethyl ketone

Rhodamine B

Spiramycin/tylosin

Methylene blue

Atenolol

Cephalexin

Clofibric acid

Semi-batch

Spiral microchannel reactor

Batch reactor

Sunlight reactor

Built-in water reactor

Simulated bench-scale reactor

Continuous-flow microreactor

Slurry reactor

Flow-membrane reactor

Slurry reactor

Flow reactor

Batch reactor

UV-A/365

Xenon lamp/540

UV/ > 300

UV/365

UV

UV-LED/365

UV/254

UV-A/365

UV/300–450

Solar light

Solar light

UV/230–280

Tetracycline hydrochloride (TCH)

Intimately coupled photocatalysis and biodegradation (ICPB) reactor

Light source/wavelength (nm)

Pollutants

Type of reactor

TiO2 TiO2 TiO2 TiO2 TiO2 TiO2 TiO2

3.42 W/m2 6.2 mW/cm2 90 W/m2 1.5 mW/cm2 100 W/m2 2 mW/cm2 W/m2

CuWO4 / Bi2 S3 ZnO

400 W/m2 8.1770 mW/cm2

160

TiO2 , Bi2 O3 , WO3 , Cu2 O

TiO2

150 W

20W

Bi2 O3

PVB/ TiO2

15W/6000 µw/cm2

94,500 lx

Catalyst

Light power/Intensity

Table 5.4 Comparison of the photocatalytic reaction parameters for various types of photoreactors

93

81.7

85

50

90

71

–

93

51

–

99.33

70

97

Degradation (%)

[150]

[149]

[84]

[122]

[148]

[147]

[137]

[146]

[145]

[144]

[143]

[142]

[141]

Ref.

5 Photocatalytic Reactors Design and Operating Parameters … 131

132

G. Saygı et al.

Fig. 5.13 Operating parameters affected to photocatalytic activity

pH Coexisting inorganic anions

Temperature

Operating Parameters Pollutant concentration

Oxidants

Photocatalyst dosage

pH < PZC [165]. Photocatalytic degradation process starts with the adsorption of organic pollutants on the photocatalyst surface [65]. The adsorption of the neutral molecules is also affected by the pH due to the dissociation of them to the charged species. The PZC of the TiO2 is between the pH of 6–7.5. The amphoteric properties of the titanol groups (Ti–OH) define the surface charge of the TiO2 [166, 167]. Therefore, the TiO2 surface has positive charge at acidic pH and negative charge at basic pH (Eqs. 5.43, 5.44) [165, 166]. In acidic conditions, the TiO2 nanoparticles can be agglomerated, and the surface area could be decreased. In alkaline conditions, the formation of OH• increases the photocatalytic activity [165]. TiOH + H+ → TiOH+ 2 (pH < PZC)

(5.43)

TiOH + OH− → TiO− + H2 O (pH > PZC)

(5.44)

When the pH was higher than 6, the TiO2 surface had negative charges and Eosin B were repelled. Under acidic conditions, the higher adsorption of Eosin B enhanced the photodegradation capacity [168]. Gong et al. (2021) reported that ibuprofen degradation with the CoFe2 O4 /TiO2 catalyst was favorable under acidic conditions. Ibuprofen has the pKa value of 4.91, and it is neutral and negatively charged under and above this pH. At alkaline conditions, HCO3 − formed in the solution and competed with ibuprofen. Besides, there was a repulsion between the photocatalyst and target pollutant at alkaline conditions. The surface area was also decreased near the PZC of catalyst. All these reasons cause to increasing degradation efficiency under acidic

Ammonia phenol

Diclofenac sodium

Ibuprofen

Oxytetracycline

Ethylparaben

Diazinon

N-doped ZnO

g-C3 N4 /BiVO4

CoFe2 O4 /TiO2

AgI decorated ZnSn(OH)6

rGO/TiO2

Cu-doped ZnO

2,4,6-trichlorophenol

RhB-sensitized BiOClBr

Orange G

Reactive Orange 16

TiO2

Methyl orange

Catechol

ZnO

Pt-doped TiO2

Resorcinol

ZnO

Sepiolite-TiO2

Target pollutant

Catalyst

20 mg/L

50 mg/L

10 mg/L

100 µM

10 mg/L

50 mg/L 10 mg/L

20 mg/L

10 mg/L

10 mg/L

50 mg/L

74 mg/L

100 mg/L

Pollutant conc.

0.2

0.7

0.5

0.5

–

1

3

0.8

0.4

0.4

0.24

2.5

Catalyst dose (g/L)

7

3

>5

7.03

3.17

6.5

6.2

3

4.2

< 6.8

3

6.8

ph

H2 O2

–

–

H2 O2 Ferrate Peroxymonosulfate Persulfate

H2 O2

H2 O2

H2 O2

–

–

–

–

–

Oxidant

Table 5.5 Photocatalytic efficiencies of various photocatalysts and optimum operating parameters

−

–

HCO3 − Cl−

Na2 SO4 , NaCl, Na2 CO3

HCO3 Cl− F− Br− SO4 2− NO3 −

–

–

–

–

–

NaNO3 Na2 SO4 NaHCO3 Na2 CO3 NaCl

–

–

Coexis. ion

99.9

99.3

(continued)

[161]

[160]

[159]

[18]

56

93.4

[158]

[157]

[156]

[155]

[154]

[153]

[152]

[151]

Ref.

68.9

93.2 93.9

86.2

98.8

92.3

78

69.8

100

Effic. (%)

5 Photocatalytic Reactors Design and Operating Parameters … 133

Target pollutant

Rhodamine B

Atrazine

Doxycycline

Catalyst

Au-TiO2/SiO2

In,S-TiO2 @rGO

BiVO4

Table 5.5 (continued)

200 mg/L

20 mg/L

100 mg/L

Pollutant conc.

0.018

1

0.3

Catalyst dose (g/L)

9.5

5.4

12

ph

H2 O2

Persulfate, p-benzoquinone, Salicylic acid Oxalate

–

Oxidant

98

100

Cl− SO4 2− NO3 − PO4 3− Br O3 −

95.2

PO4 3−

Effic. (%)

–

Coexis. ion

[164]

[163]

[162]

Ref.

134 G. Saygı et al.

5 Photocatalytic Reactors Design and Operating Parameters …

135

conditions [18]. In the study of Tong et al. (2021), it was observed that the degradation of doxycycline by BiVO4 was increased with the increasing pH from 3.5 to 9.5. At the further increase of the pH from 9.5 to 11.5, the degradation rate was not increased due to the lower potential of hydroxyl radicals at higher alkalinity [164]. Goulart et al. (2021) performed the Levofloxacin degradation by using the new Ti/Ruthenium-Titanium /ZnO composite. The characteristics of Levofloxacin was significantly changing with pH, which illustrated in Fig. 5.14 Levofloxacin had the zwitterion and neutral forms between the pH of 6.0 and 8.0. At the pH of 6.02 (pKa1 ), the protonated piperazinyl groups formed and the carboxylic acid groups were in dissociation balance, which affected the photocatalytic degradation. At the pH close to 8.15 (pKa2 ), the degradation efficiency was decreased due to the deprotonation of the piperazinyl groups [19]. Baran et al. (2008) showed that the degradation efficiency of a TiO2 for Bromocresol Purple was increased sixfold when the pH was decreased from 8.0 to 4.5. Such an increase was explained by changes in both the charge of dye and the TiO2 surface [169]. The pH effect for the degradation of oxytetracycline by Silver iodide decorated ZnSn(OH)6 was studied [159]. The PZC of oxytetracycline was 5.4, when the pH was higher than the PZC, the degradation efficiency was enhanced due to the nature of the antibiotic [159].

Fig. 5.14 Characteristics of Levofloxacin and chlorine species at various pHs (Reprinted from Ref. [19] with permission from Elsevier)

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5.5.2 Temperature The temperature of photocatalytic reaction affects the catalyst performance and overall efficiency of the system [51]. Photocatalytic degradation of organic pollutants is generally carried out at room temperature. The photocatalytic reactions could be performed between 20 and 80 °C. Between the temperature of 20 and 60 °C, the photode-composition rate is increased [165]. The formation of bubbles and generation of free radicals is observed with the increase of temperature. The temperature increase would also reduce the electron–hole recombination [30]. Molecular collisions are increased with the higher kinetic energy at higher temperatures. When the temperature is below 20 °C, the photocatalyst activity reduces and the desorption of the final products could be the rate limiting step. At the temperature around 0 °C, the activation energy is high [166]. Conversely, at temperature above 80 °C, the exothermic adsorption of the organic pollutant becomes difficult, which would be the rate limiting step [165, 170]. Above 80 °C, the temperature of the system is close to the boiling point of water, and this inhibits the surface interactions [51]. Mozia et al. (2009) indicated that the degradation of Acid Red 18 by using anatase TiO2 was enhanced with increase of the temperature from 313 to 335 K. At lower temperatures, the desorption rate of final products inhibited the reaction [171]. Tong et al. (2021) prepared the small-sized particles of BiVO4 for degradation of doxycycline. When the reaction temperature was increased from 5 °C to 25 °C, the photocatalytic degradation capacity was enhanced [164]. Malik et al. (2016) studied the effect of temperature on organic dye degradation. The photocatalytic degradation of Rhodamine B by using Au-TiO2 /SiO2 catalyst was performed between the temperatures of 30 °C and 80 °C. The degradation efficiency was decreased with increasing temperature [162]. In contrary, Meng et al. (2018) observed that the increasing temperature had a positive effect on the photocatalytic degradation of Congo Red by using the three different catalyst such as TiO2 , ZnO and g-C3 N4 . The degradation of Congo Red by ZnO at the temperatures of 4, 25 and 45 °C is given in Fig. 5.15. At the higher temperature (45 °C), the free radicals formed quickly that increased the photocatalytic activity dramatically. This severe increment of activities for all the catalysts was based on the new mechanisms of photothermocatalytic oxidation [172].

5.5.3 Pollutant Concentration The initial organic pollutant concentration is a crucial factor that must be considered. The photocatalytic degradation process depends on the adsorption of organic pollutant on the photocatalyst surface. Only the adsorbed amount of pollutant can undergo the photocatalytic reaction, not the amount in bulk of the pollutant solution [30]. By using the fixed photocatalyst dosage, the degradation percentage reduces

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137

Fig. 5.15 Degradation of Congo Red by ZnO at the temperatures of 4, 25 and 45 °C. (Reprinted from Ref. [172] with permission from Elsevier)

with increasing initial concentration of pollutant [173]. As the amount of pollutant concentration increases, more pollutant is absorbed on the catalyst surface, and less photons could reach the catalyst surface. The excessive pollutants cause to the blocking effect, therefore, there would be less OH• ions generation and photocatalytic activity reduces [30]. The excessive pollutant also hinders the direct contact between photogenerated radicals or holes and the organic pollutant in bulk [51]. The studies have reported that the efficiency of organic pollutant degradation reduced with the increase of the initial pollutant concentration [153, 160, 161]. Mahvi et al. (2009) presumed the reasons for the decrease of the efficiency with increase of the dye concentration. The dye ions could cover the active sites of the catalyst surface and the formation of OH• would be decreased. The other possible reason is absorption of the considerable amount of UV by the dye molecules due to the reduced concentrations of OH• and OH2 • . The formation of by-products could interfere to the degradation process [153]. Siboni et al. (2017) showed that the initial diazinon concentration significantly affected to the degradation efficiency of Cudoped ZnO nanorods. When the diazinon concentration was increased from 10 to 50 ppm, the degradation efficiency was decreased dramatically from 99.96 to 21.91% [161]. Pardeshi and Patil et al. (2009) achieved the complete degradation of 100 ppm Resorcinol by using the optimum amount of ZnO catalyst. After 100 ppm of solution, degradation efficiency was decreased [151]. Alvarez et al. (2021) reported that the degradation rate constant of ethylparaben with GO/TiO2 reduced with rising initial pollutant concentration. The reason was that the formation of intermediate compounds competed with the ethylparaben for the reactive species [160].

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5.5.4 Photocatalyst Dosage Photocatalyst dosage is a significant parameter affecting the photocatalytic performance. Regarding economic feasibility of a process, using a minimum catalyst amount is preferable to obtain maximum efficiency. There is a positive correlation between the photocatalyst dosage and reaction rate until adding an optimum dosage [162]. Using photocatalyst above the optimum dosage causes the negative effects. The excessive photocatalyst dosage leads to the blocking effect, reducing luminous transmission, decreasing light penetration depth and increasing turbidity [51]. Light hindering causes the dead zones that not be activated by light [174]. Due to excessive catalyst amount, the catalyst may tend to agglomerate with high surface energy. The diffusion path length rises and the surface area reduces due to the agglomeration of catalyst [51]. Furthermore, the photocatalyst amount is directly relevant to the capital expenses, and the lower cost of catalyst materials is preferable for water purification [174]. Ahmadi et al. (2021) studied the catalyst dosage parameter to determine the optimal activated carbon–ZnO nanocomposite catalyst. The degradation efficiency of Acid Blue 25 increased with increasing nanocomposite dosage due to the producing of sufficient hydroxyl radicals [175]. Zhou et al. (2018) used the sepiolite–TiO2 catalyst for degradation of Orange G and illustrated that using high photocatalyst amount caused the increasing particle–particle interactions, agglomeration and increased diffusion path length [155]. Zheng et al. (2021) stated that the excessive amount of BiOClBr catalyst lead to the aggregation, scattering of photons, shielding effect, covering of the photosensitive surfaces and hence, obtained lower photocatalytic degradation efficiency for 2,4,6-trichlorophenol [154]. Bazrafshan et al. (2019) observed that the increasing ZnO dosage enhanced the catechol degradation. The reason was explained with the increased reaction sites or surface area, absorbed more UV radiation and more hydroxyl radicals. Absorption of more UV radiation by ZnO lead to generation of more free electrons and more small holes on the catalyst surface. Generated electrons and holes enhanced the photocatalytic performance [152].

5.5.5 Oxidants The electron–hole recombination of the photocatalytic process is the main limitation. Using the appropriate donor or electron acceptor reduces the recombination rate and rises the quantum yield. The addition of external electron acceptors/oxidants to the solution could enhance the photocatalytic efficiency significantly [65]. Molecular oxygen is the commonly used electron acceptor. Recently, many studies focused to increase the photocatalytic degradation of organic pollutants by the addition of

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139

oxidants such as H2 O2 , (NH4 )2 S2 O8 and KBrO3 [18, 157, 158, 163, 164]. The addition of oxidants prevents the e− /h+ pair recombination and increases the generation of hydroxyl radicals [65]. H2 O2 is the most commonly used oxidant that produces strong hydroxyl radicals by peroxide bond cleavage and generated electron trapping [176]. Mandor et al. (2021) explained the direct photolysis of H2 O2 to form hydroxyl radicals under UV radiation for enhancing the photocatalytic degradation of phenol [157]. Huang et al. (2008) determined the optimum H2 O2 concentration to obtain maximum methyl orange degradation by using Pt–TiO2 –natural zeolite composite as photocatalyst [156]. Sun et al. (2018) were also interested in the concentration of oxidant to increase the degradation capacity of gC3 N4 /BiVO4 for diclofenac sodium [158]. The degradation rate was increased with increasing H2 O2 concentration until reaching the optimum concentration. Exceeding the optimum amount caused to decrease in the degradation efficiency. The sufficient concentration of H2 O2 provides to the generation of hydroxyl radicals by the Eqs. 5.45–5.47 [156]. H2 O2 + e− → OH− + OH•

(5.45)

− • H2 O2 + O−• 2 → O2 + OH + OH

(5.46)

H2 O2 + hν → 2OH•

(5.47)

The hydroxyl radicals are the dominant and strong oxidization species in photocatalytic reactions. The organic pollutants can be completely oxidized by the hydroxyl radicals on the semiconductor surface under light irradiation. Although the addition of H2 O2 increases the generation of hydroxyl radicals, the excessive concentration of H2 O2 reduces the degradation efficiency by the following reactions (Eq. 5.48–5.50) [156]. H2 O2 + OH• → H2 O + HO•2

(5.48)

HO•2 + OH• → H2 O + O2

(5.49)

H2 O2 + 2h+ → O2 + 2H+

(5.50)

Selvam et al. (2007) studied the effect of oxidants such as H2 O2 , IO4 − , S2 O8 2− , ClO3 − and BrO3 − on the degradation of 4-fluorophenol under UV light. These oxidants act as both electron scavengers and strong oxidant themselves. The addition of oxidant enhances the photocatalytic activity of TiO2 with the order of IO4 − > BrO3 − > S2 O8 2− > H2 O2 > ClO3 − [177]. The addition of IO4 − oxidant leads to the generation of IO3 • , OH• and IO4 • radicals that are strong reactive intermediate species (Eqs. 5.51–5.53). These species also promote the efficiency by the free radical pathways [177].

140

G. Saygı et al. • −• IO− 4 + hν → IO3 + O

(5.51)

O−• + H+ → OH•

(5.52)

− • OH• + IO− 4 → OH + IO4

(5.53)

The addition of BrO3 − oxidant enhances the photocatalytic activity with the reaction between the conduction band electron and BrO3 − [178]. This reaction (Eq. 5.54) reduces the e− /h+ pair recombination [177]. − + − BrO− 3 + 6eCB + 6H → Br + 3H2 O

(5.54)

The S2 O8 2− oxidant could react to form SO4 •− anion in solution (Eq. 5.55). This radical anion can react with H2 O to generate hydroxyl radicals (Eq. 5.56) [177]. •− S2 O2− 8 → 2SO4

(5.55)

2− • + SO•− 4 + H2 O → OH + SO4 + H

(5.56)

5.5.6 Coexisting Inorganic Anions The coexisting inorganic anions such as Cl− , SO4 2− , CO3 2− , HCO3 − , HPO4 2− and NO3 − significantly affect the photocatalytic activity. The impact of coexisting species has been studied so far [18, 153, 159, 160, 163, 164]. The influence of the inorganic anions is generally complicated [166]. These species compete with the organic pollutant and cover the active reaction surfaces, therefore, decrease the surface activity. They also lead to the scavenging of generated charge carriers [51]. Rizal et al. (2021) reported the inhibition of methylene blue degradation in presence of inorganic anions such as SO4 2− , CO3 − , Cl− , NO3 − and H2 PO4 − . These ions generally exist in industrial wastewaters and affect the photocatalytic activity significantly. The inhibition effect of anions was observed in the order of H2 PO4 − > CO3 2− > SO4 2− > Cl− > NO3 − [179]. The existing carbonate salt undergoes the ionization reactions and there would be the equilibrium of CO3 2− , H2 CO3 − and HCO3 − ions. The photocatalytic degradation reactions at the existence of carbonate salt can be represented by the Eqs. 5.57–5.62. [179, 180] •− • − CO2− 3 + OH ↔ CO3 + OH

(5.57)

•− + CO2− 3 + h → CO3

(5.58)

5 Photocatalytic Reactors Design and Operating Parameters …

141

•− • HCO− 3 + OH ↔ CO3 + H2 O

(5.59)

•− + HCO− 3 + h ↔ HCO3

(5.60)

CO32− + H+ ↔ HCO− 3

(5.61)

HCO32− + H+ ↔ H2 CO− 3

(5.62)

In the presence of H2 PO4 − ions, the photocatalytic degradation reactions can be represented by the Eqs. 5.63–5.68. [179, 181, 182]. + • H2 PO− 4 + h → H2 PO4

(5.63)

− • • H2 PO− 4 + OH → H2 PO4 + OH

(5.64)

2− − H2 PO− 4 + OH → HPO4 + H2 O

(5.65)

3− − HPO2− 4 + OH → PO4 + H2 O

(5.66)

+ H2 PO− 4 + H → H3 PO4

(5.67)

H3 PO4 + H2 O ↔ H3 O+ + H2 PO− 4

(5.68)

Chloride is the other common ion in industrial wastewater. It affects the photocatalytic degradation of organic pollutants by the following reaction series (Eqs. 5.69–5.72) [179, 183–185]. Cl− + OH• → ClOH•−

(5.69)

Cl− + h+ → Cl•

(5.70)

Cl− + Cl• ↔ Cl•− 2

(5.71)

Cl• + Cl• ↔ Cl2

(5.72)

Furthermore, in the presence of NO3 − and SO4 2− anions, the photocatalytic degradation reactions can be given by the Eqs. 5.73–5.80 [179, 183–186]. SO42− + h+ → SO•− 4

(5.73)

142

G. Saygı et al. •− • − SO2− 4 + OH ↔ SO4 + OH

(5.74)

SO42− + H+− → HSO− 4

(5.75)

•− • HSO− 4 + OH ↔ SO4 + H2 O

(5.76)

− 2− SO•− 4 + eCB → SO4

(5.77)

•− NO− + NO•2 3 + hν → O

(5.78)

+ • NO− 3 + h → NO3

(5.79)

• − • NO− 3 + OH → OH + NO3

(5.80)

The CO3 2− , H2 PO4 − , NO3 − and Cl− anions act as a hole and OH• scavenger; however, only the SO4 2− anion shows different influence on the photocatalytic degradation mechanism. The SO4 2− anions produce the plenty of SO4 •− ions that inhibit electrons [179]. Chang et al. (2021) studied the effects of coexisting anions such as SO4 −2 , HCO3 − , Cl− and NO3 − on the photocatalytic degradation of Triclosan by Bi7 O9 I3 /Bi. The photocatalyst surface was positively charged when the pH was lower than the PZC value of the catalyst, and anions could be adsorbed on the active sites of the photocatalyst surface, which hinder the degradation reaction. Sulfate anions inhibited the photocatalytic activity more than chloride ions due to the carrying of higher charges compared to chloride ion [100].

5.6 Conclusion Over the last two decade, the photocatalytic technology has grown significantly for environmental remediation. Clean water is one of the main strained resources. Therefore, in the near future, photocatalytic degradation process will have been the part of widely used treatment technologies. Photocatalytic degradation is a green technology for the treatment of wastewater to use the water in agricultural, municipal or drinking purposes. Photocatalytic reactors and photocatalysts have been developed recently. As reviewed in this chapter, the morphology, surface area and band gap energy of the photocatalysts have been considered by many researchers to achieve complete mineralization of various organic pollutants. The paper outlines the modifications of the photocatalysts such as doping and immobilization onto support surface. Reactor design has many significant considerations such as the mole balances for different photocatalytic reactors (Batch, CSTR, Packed bed, Annular, microreactor, membrane

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reactor), reaction mechanism and reaction rates, which were discussed in detailed. Various operating parameters influence the photocatalytic process and degradation efficiency of the photocatalysts. For instance, some of the organic pollutants can be degraded at lower pH, whereas others at higher pH. Therefore, the photocatalytic reaction should be carried out at the suitable pH. Operating temperature, organic pollutant concentration, catalyst loadings, oxidant agents and coexisting inorganic ions perform their individual effects on the photocatalytic degradation of organic pollutants. Therefore, all these parameters should be considered to achieve effective degradation of any organic pollutant. Acknowledgements Fehime Çakıcıo˘glu-Özkan is thankful to Bilge Su Erdo˘gan for her contribution of this chapter.

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Chapter 6

Visible Light Mediated Click Chemistry Lalan Chandra Mandal and Bidyut Saha

Abstract Since the beginning human civilization, human beings have been searching for hidden mysteries of nature. Nature by natural process synthesizes millions of compounds within itself with the help of easily available reaction conditions. These compounds are proved to have the potentiality to serve human societies in various ways. Synthetic organic chemists also have been investigating various methodologies to synthesize bio-active molecules and designing of efficient compounds. Click chemistry can be considered as one of the important workbench which is directly or indirectly connected with many branches of science like physics, chemistry, biology, nano and material sciences. It has proved to have remarkable application in the field of synthesis of diversified molecular architecture, biosensors, preparation of nanoparticle, preparation of new polymers and so many relevant field. The foundation of click chemistry also fulfils the aspect of the green approach of a reaction. Various literature survey reports have revealed that visible light mediated organic conversions have received an extensive response for designing eco-friendly chemical synthesis for furnishing the desired products from the small molecules through activation. Nowadays, accessible synthetic pathway and productive visible light sources has become most relevant in the modern chemistry of synthetic wings. In this Chapter, we will deliberate an inquisitive flush of exercise in the field of visible light arbitrated click chemistry for diversified desired synthesis. Our appreciation will highlight the recent prosperity in the synthesis of organic structures through click reaction with the help of visible light radiation. Sometimes the insertion of the new cyclic ring or functional group within the structure may lead to enhance bioactivity of the synthesized organic molecule. Different pathways of transformation of molecules in the presence of visible light using variegated methods will be demonstrated in this Chapter.

L. C. Mandal Department of Chemistry, Krishna Chandra College, Hetampur, Birbhum, West Bengal 731124, India B. Saha (B) Homogeneous Catalysis Laboratory, Department of Chemistry, The University of Burdwan, Burdwan, West Bengal 713104, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Garg and A. Chandra (eds.), Photocatalysis for Environmental Remediation and Energy Production, Green Chemistry and Sustainable Technology, https://doi.org/10.1007/978-3-031-27707-8_6

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Keywords Click chemistry · Visible light mediated reactions · Organic synthesis · Eco-friendly chemical synthesis · Enhance bioactivity

6.1 Introduction To invent new chemical architecture, novel reactions and assist the prosperity of synthetic organic chemistry is an inherent and time honoured affection of chemists. Now a days, click chemistry established itself as utmost potent paradigms for synthesizing unique organic molecules, material chemistry as well as and bio-oriented science, drug chemistry. This field enticed deep interest among the scientist over the last decades [1]. The proposal of new term “click” chemistry by a Nobel Prize winner K. Barry Sharpless in the year 2001 caused a stir by publishing a landmark review which included the a new strategy for organic synthetic chemistry [2]. A cluster of mighty linking chemical reactions are included by click chemistry that are facile to perform, high yielding, wide in scope, require no or minimal purification, create only inoffensive by-products that can be removed without chromatography, having high yields, and adaptable in the unification of diversified chemical structures considering zero requirement of prior condition of protection steps. To be good candidate of click reaction, process should have to be featured in such a way that should have high efficiency, pretty atom economy, mild reaction conditions, easily available starting materials and simple purification or isolation [3]. Diversified Molecular architectures, modularization and productivity are indispensable in organic synthesis which are used to prepare diverse complexes and compounds. It is needless to mention that natural products are resulted due to joining small molecular units through anabolism and photosynthesis path [4]. Click chemistry gives a trackway for synthesizing of divers heterocyclic compounds, peptides, triazolefused heterocycles, amino acids and chromophores, etc. [5, 6]. Knowingly or unknowingly, light drives numerous significant operation in nature. Synthesis of glucose via photosynthesis in plant leaves and production of ozone layer in the atmosphere are important natural processes drives by light [7]. In light mediated chemical reactions, highly reactive molecules are formed due to the absorption of light, which consequently undergo in chemical transformation. Various chemical reactions like unimolecular reactions [8], such reactions as dissociation, association, isomerization, as well as reactions involving bimolecular framework [9]. Similarly, intermolecular electron transfer, and cycloaddition reaction can also be activated by light source [10]. Light mediated chemical reactions can be controlled temporally or spatially by emphasizing the photon focussing onto a desired target region and by differing time of exposure, intensity as well as wavelength as per requirement. In conventional thermal reactions such control is not possible [11]. Classical click reactions can be

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availed advantages by blending the light mediated click reactions with the help of convenience of photochemical process. During the past decade, visible light has been recognized as an important source for driving eco-friendly green chemical synthesis. Visible light gets more advantages in comparison to relatively short-wavelength ultraviolet light due to some important reasons, viz highly abundance of solar spectrum, easy accessibility of visible light, lesser chance of by-product formation. Notwithstanding, excellent number of simple organic compounds are found to have transparent to visible light, an extensive numbers of transition metal complexes as well as several organic dyes are adequately utilized as proficient photocatalysts for synthesizing the manifold organic skeleton. With enhancement of perception for the performance of visible light driven chemical transformation by photoclick process, further exploitation of the visible light assisted click reactions is needed are bloom.

6.2 Classification Click Reactions It is very difficult to classify specifically the click reactions. The main requirement for the click reactions to have the characteristics of the chemical reactions that occur in nature. Most of the organic click reactions carried out in laboratory are the mimic of the reaction that take place in nature. It is the desire of the organic chemist to find out the reasonable procedure of that click reactions. There are four prime classifications of the click reactions have known to be established [12, 13]. These are as follows: Cycloaddition: These reactions include 1,3, dipolar cycloaddition reactions and hetero-Diels Alder reactions. Nucleophilic addition reaction: These refers to the carbonyl group reactions which included the hydrazine hydrazones, thiourease formation, formation of amides, oxime, aromatic heterocycles from carbonyl group reactions are also of same types reactions. Nucleophilic ring-opening reactions: Heterocyclic compounds having strained unbolt its structure in this categories. Many reactions like opening of epoxide ring, aziridinium ions, etc. are of this classification. Additions to carbon–carbon multiple bonds: This classification belongs to epoxidation formation, dihydroxylation reactions, aziridination addition reactions, nitrosyl halide additions. Some Michael reactions are considered the following type of reactions. Click reaction motivated synthetic approach now a days are in captivating emotion to the chemists. Multi-component reactions in green way have achieved its prosperous achievement for so many reactions including aldol condensation accompanied by Michael addition, Michael addition/Mannich Reaction and Ugi reaction with Diels Alder Reactions, etc. [14] Reaction of an alkyne with an azide is the most well-known and prime click reaction where neither alkyne nor the azide remain active under physiological conditions but reaction takes place in certain temperature. Whereas

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reactions with non-catalysed condition are non reio-selective and reaction rate is very slow. Besides that, the greatly enhancement of the 1,4-regioselectivity have been observed due to introduction of electron-deficient alkynes at terminal. The mentioned factors keep down the utilization of non-catalysed Huisgen cycloaddition as an effective conjugation path [15].

6.3 Visible Light Mediated Reactions Giacomo Ciamician, pioneering chemist discussed the bright and broad prospect of photochemistry in 1900 [16]. The organic chemistry is an ever growing field and organic photochemistry carries its unique journey through its intense developments, and now a days, it has become a dominant synthetic key. The extensive research in the field of photochemistry revealed that the field of visible light guided organic synthesis have contributed a notable position during the past decade. There are some important advantages of visible light in comparison with ultraviolet light. The advantages in this regards are (i) greater availability of the solar light, (ii) Effortlessly attainable equipment and (iii) lesser by-product formation. Various research literature have shown to have transparency to visible light for most of the simple molecules but a broad range of transition metal complexes and for synthesizing some complex organic dye, yield became very low. Photocatalysts also accelerates the productivity of the organic reaction with the help of visible light (Fig. 6.1). It has been observing since the last few years, organic chemists have been trying to develop updated and puissant methodology in regard to visible light facilitate reactions. More than 70 reviews are focussed on visible light mediated organic reactions. These works have been published in many reputed journals like Accounts of Chemical Research [17] and Chemical Reviews [18], etc. In the journey of modern organic synthetic chemistry, designing of well efficacious reactions is major exposure for the prosperity of unique technologies. Some specific possessions like atom efficiency, environmentally benign conditions and

Fig. 6.1 General reaction scheme for the catalyst-free azirine-based visible light induced cycloaddition reaction

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sustainability are highly desired in this regards [19]. Particularly, irradiation arbitrated processes offer a justifiable approach for effective ligation reactions and their remarkable significance is affirmed by the contemporary publications [20]. Hence, a greater number of diversified photolysis approach trust on unfriendly UV irradiation and have been accepted that merge the auspicious characteristics of cycloaddition reactions with the convenience of light mediated chemistry [21], viz temporal and spatial control. Ii is needless to mention that UV-operated ligation systems have various superiority over traditional cycloaddition systems. But, the universally accessible energy of natural light (390–700 nm, i.e. range of visible light) performs a key role of recent trends in synthesis guided photolysis reactions [22]. The blooming of process using light in the visible range have been reached at the glorious position through photoredox catalysis, which can be considered as a gifted drive in synthetic organic chemistry [23]. Furthermore, effective cycloaddition reactions of dipolar categories employ with metal as well as without metal photoredox catalysts with the assistance of visible light [24]. It is also true that an approach for without catalyst ligation and with assistance of visible light which are communicated in the contemporary studies and expected to be explore in future also. Two important points should be mentioned to have photoreactive compounds are: (i) The moiety should have capability for absorption of light in the region of the deserved wavelength range and (ii) The generation of species having reactivity should be efficient. Mueller et al. [25] combined these characteristics to form a solution with the help of reactive photochemical azirine species and capability of absorption of pyrene (Chromophore). In their works, they observed for azirine, the expanded excitation wavelength in the region of the visible light. In such manner, using quick and effective cycloaddition reaction, they synthesized utilizing unique and powerful synthetic method (Fig. 6.1). For the consideration of designing photoreactive compounds (az-py) in terms of its wavelength, should be designed in judicious manner. Although it is true that to absorb light in visible range is essential but suppression for decomposition in the natural light is also needful. Hence, Mueller et al. [25] designed target photoreactive entity in such a way so that absorption occurs at the light of visible range to access the logical activation with minimum energy providing light source of visible region for being able to have efficient activation with low-energy visible light sources to avoid operation problem in the time of sample preparation or carrying on synthesis. Besides, For consideration of light absorption, pyrene moiety play its decisive role and incorporation of carbon framework in cycloadducts also occurs. In this way it is providing additional wishful features such as an anchor for π–π stacking or built-in fluorescent marker [26]. To unravel the diversity of reactions with the help of light in the visible range, cycloaddition reaction among az-py was carried on for synthesizing the reaction. Simultaneously same species was used to carry on with polymeric species (Fig. 6.2). For being confirmed the fruitful cycloaddition reactions and to justify for the attainability of different dipolarophiles, cycloaddition reactions of molecules having small size and traditionally involve functional moiety were directed.

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Fig. 6.2 An overview of occurrence of the reactions with az-py (middle), which was irradiated with a visible light LED setup (l = 410–420 nm) in the presence of polymeric species (left) and small molecules (right)

Synthesizing of various chemical architecture using light in the visible range with environmentally safe perspective have received enormous feedback from different corner of the scientists. To develop the target oriented products from activated molecules having small size and utilizing easily accessible natural light in the visible region now a days have occupied a space in the synthetic organic chemistry [27–29]. By the assistance of light in the visible range, occurrence of thiol-yne type click reactions have been observed. And in this perspective, Burykina et al. [30] worked for synthesizing various types of vinyl sulphides (Fig. 6.3, Method 1) with high productivity and selectivity. The schematic route refers to the without transition metal and delivered Markonvikov-kind outcome via a radical photoredox path (Fig. 6.3 Method 1). Nowadays, as per research finding of Wu et al. [31], triazole related products were synthesized (Fig. 6.3, Method 2) via the mechanism of photoredox via transfer of electron. They observed the reaction between benzyl azide and phenylacetylene utilizing diversified photocatalysts with the help of natural reaction environment like with assistance of light in the visible range. Formation of triazole derivatives are formed with the help of catalyst (piq)2 Ir(acac) or TPPT-Cl. The schematic pathway is solar catalysis pathway which is high yielding, region-selective and prosperous atom economic (Fig. 6.3 Method 2).

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Fig. 6.3 Visible light mediated synthesis of vinyl sulphides and triazole analogs

To conduct the fulfilment of light irradiation with lower wavelength, Mishiro et al. noted photoclick reaction of cyclopropenones indirectly to produce the cyclooctyne. Earlier reports had pointed out that the cyclopropenone takes part in decarboxylation under the assistance of visible light from the excited singlet state, and hence, photosensitization through transfer of energy from a low energy, UV absorbing chromophore from visible light sensitizer would be impossible. Therefore, the photolysis would in this way indirectly, then, constrainedly go ahead through a completely varied decarbonylation process. The contributors revealed the abstraction of electron of cyclopropenone, generate radical cation having unstability, due to opening of the structure having ring that would withdraw an electron from photocatalyst to regenerate into carbon dioxide and alkyne via decomposition [32]. The choice of substituents is the important factor for being tuneful the wavelengths at which the reaction will be carried out. The reaction carry on from 300 to 420 nm with excitation of single photon and at 700 nm by excitation of multiphoton [33]. The enhancement of absorbance of visible spectrum was observed due to replacement of phenyl ring substituent on any side of tetrazole moiety with oligothiophene substituents and thus bathochromic shifting that allowed to accomplish 405 nm exposure (Fig. 6.4) [34]. Lederhose et al. observed a identical shifting in absorbance through the synthesis of aryl tetrazole having a pyrene-substituted, approving reaction with exposure of light at 410–420 nm wave length [35]. Besides, the azirine having pyrene-bound structure exhibited significant bathochromic shift absorbance with the light of visible range at wavelengths 400– 410 nm (Fig. 6.5) [25]. This reaction can be considered as the link of pyrene to various small molecules and alkene terminal polymers with 50% to quantitative yields. A especially interesting accomplishment of the radicals are differing in productivity produced by various photo initiators. It has been observed that hexaarylbiimidazoles (HABI) can be considered as weak initiators for reaction of (meth) acrylate chain addition but efficient towards reaction with thiol-ene [36].

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Fig. 6.4 Different photoabsorptions changing of substituents on tetrazole moiety

Fig. 6.5 Pyrene azirine through visible light activation and conjugation to electron-deficient alkenes. EWG = electron withdrawing group. R=H, COOEt, PEG [25]

Besides, in comparison to other radicals produced via photodecomposition, it has been found that HABI-based radicals are reasonably stable. It is remarkable to mention that HABI is found to absorb at the light of higher wavelength accommodating initiation of 469 nm. It has been observed that primarily absorption of maximal type I initiators found to have in UV region, Even though photoinitiators like phosphine oxide[37], acyl germane [38] and acylstannane [39] have shown absorption spectra as extended form and cleavage kind radical initiators and gradually deeper in visible region of spectrum around past 500 nm. Notwithstanding mentioned advancement, utilization of photoinitiators like acylstannane and acylgermane are remains as exceptional. In recent years, several recent research studies have revealed that visible light induced photoredox catalysis is being considered as one of the major hot topics for synthesizing of valuable chemical candidate owing to inherent green characteristics of light, good functional group tolerance and high reactivity. A numbers of recent works have shown that the some compounds having alkene, imine and alkyne moiety afford their respective radical and its cation and anion form through a reductive or oxidative quenching cycle of the photocatalyst (PC) in the exited state via a single electron transfer mechanism [40]. This idea inspired to Zheng-Guang Wu et al. to speculate that reaction of azide–alkyne cycloaddition (AAC) would be executed through a photoredox and electron-transfer radical mechanism in lieu

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of coordination of alkyne with metal-catalysed. The description of the projected mechanism shown in Fig. 6.1c. The generation of radical cation via single electronoxidation of alkyne occurs in the presence of the excited PC [41]. Combination of azide with intermediate A forms the intermediate B as a result of which intermediate C produces via cycloaddition reaction. Finally, owing to a single electron reduction of C by photocatalyst take steps the expected 1,4- disubstituted 1,2,3-triazole with generation photocatalyst for the further cycle. In this way, a unique route using AAC reaction with the assistance of visible light through radical mechanism is arrived. In this reaction Zheng-Guang Wu et al. introduced a visible light assisted click reaction of alkyne and azide: formation of 1,4-disubstituted 1,2,3-triazole via photocatalyzed AAC (PcAAC) reaction through photoredox and strategy of electron-transfer having regioselectivity and attractive proficiency, that was really a significant expletive and advancement of click chemistry (Fig. 6.6). Liu et al. [42] reported that click reaction with the help of light of visible region and release hinge between phenoxylfumarates and monoarylsydnone (MASyd). They have observed that the production of pyrazoline via cycloaddition reaction which go through a photoaromatization to constitute a pyrazole structure. In the meantime, the

Fig. 6.6 Photocatalysed Azide Alkyne cyclisation (PCAAC)

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Fig. 6.7 Fluorogenic photo-click and release

Fig. 6.8 Screening of photoclick reactions between the selected Syds and DEFA. Experiments were carried out in CH3 CN: H2 O = 1: 1, irradiated with the indicated light sources. [Syd] = 30 mM, [dipolarophile] = 150 mM. a Reaction scheme

photoaromatization also acts as the incentive for going out fluorophores which are quenched in the shape of phenoxylfumarates (Figs. 6.7 and 6.8).

6.4 Conclusion In this chapter, we deliberated a broad spectrum of visible light mediated click chemistry for diversified chemical synthesis. There are so many chemical reactions take place in the presence visible light with the help catalyst and other required criteria among them click reaction under the assistance of visible light have some specialty. Our study have highlighted the recent prosperity in the synthesis of organic structures through click reaction with the help of visible light radiation. Sometimes insertion of new cyclic ring or functional group within the structure may lead to enhance bioactivity of the synthesized organic molecule. Different pathways of transformation of molecules in presence of visible light using variegated methods has been demonstrated in this Chapter. Nature is the best laboratory to evoke knowledge in our heart. In recent times, the searching of hidden mystery of different chemical

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reaction under the exposure of visible light is the most interesting field among the scientists. Our work must boost to the researcher of this field to find out new strategies for investigating in depth.

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15. Huisgen R (1963) 1,3-Dipolar cycloadditions. Past and future. Angew Chem Int Ed Engl 2:565–598 16. Ciamician G (1912) The photochemistry of the future. Science 36:385–394 17. Special issue: photoredox catalysis in organic chemistry. Acc Chem Res 49 (2016) 18. Special issue: photochemistry in organic synthesis. Chem Rev 116:9629–10342 (2016) 19. (a) Kolb HC, Finn MG, Sharpless KB (2001) Angew Chem Int Ed 40:2004; Angew Chem 113:2056; (b) BarnerKowollik C, Du Prez FE, Espeel P, Hawker CJ, Junkers T, Schlaad H, Van Camp W (2011) Angew Chem Int Ed 50:60; Angew Chem 123:61 20. Tasdelen MA, Yagci Y (2013) Angew Chem Int Ed 52:5930; Angew Chem 125:6044 21. (a) Liu Z, Liu T, Lin Q, Bao C, Zhu L (2015) Angew Chem Int Ed 54:174; Angew Chem 127:176; (b) Liu Z, Lin Q, Sun Y, Liu T, Bao C, Li F, Zhu L (2014) Adv Mater 26:3912; (c) Yu Z, Ohulchanskyy TY, An P, Prasad PN, Lin Q (2013) J Am Chem Soc 135:16766; (d) Orski SV, Poloukhtine AA, Arumugam S, Mao L, Popik VV, Locklin J (2010) J Am Chem Soc 132:11024; (e) Pauloehrl T, Delaittre G, Bruns M, Meißler M, Bçrner HG, Bastmeyer M, Barner-Kowollik C (2012) Angew Chem Int Ed 51:9181; Angew Chem (2012) 124:9316; (f) Oehlenschlaeger KK, Mueller JO, Heine NB, Glassner M, Guimard NK, Delaittre G, Schmidt FG, BarnerKowollik C (2013) Angew Chem Int Ed 52:762; Angew Chem 125:791 22. Yoon TP, Ischay MA, Du J (2010) Nat Chem 2:527 23. (a) Xuan J, Xiao W-J (2012) Angew Chem Int Ed 51:6828; Angew Chem 124:6934; (b) Prier CK, Rankic DA, MacMillan DWC (2013) Chem Rev 113:5322 24. (a) Zou Y-Q, Lu L-Q, Fu L, Chang N-J, Rong J, Chen J-R, Xiao W-J (2011) Angew Chem Int Ed 50:7171; Angew Chem 123:7309; (b) Rueping M, Leonori D, Poisson T (2011) Chem Commun 47:9615; (c) Maity S, Zhu M, Shinabery RS, Zheng N (2012) Angew Chem Int Ed 51:222; Angew Chem 124:226; (d) Xuan J, Xia X-D, Zeng T-T, Feng Z-J, Chen J-R, Lu L-Q, Xiao W-J (2014) Angew Chem Int Ed 53:5653; Angew Chem 126:5759 25. Mueller JO, Schmidt FG, Blinco JP, Barner-Kowollik C (2015) Visible-light-induced click chemistry. Angew Chem Int Ed 54:10284–10288 26. (a) Lee OP, Yiu AT, Beaujuge PM, Woo CH, Holcombe TW, Millstone JE, Douglas JD, Chen MS, Fr¦chet JMJ (2011) Adv Mater 23:5359; (b) Burattini S, Greenland BW, Merino DH, Weng W, Seppala J, Colquhoun HM, Hayes W, Mackay ME, Hamley IW, Rowan SJ (2010) J Am Chem Soc 132:12051; (c) Xu Z, Singh NJ, Lim J, Pan J, Kim HN, Park S, Kim KS, Yoon J (2009) J Am Chem Soc 131:15528 27. Liu Q, Wu L-Z (2017) Recent advances in visible-light-driven organic reactions. Natl Sci Rev 4(3):359–380. https://doi.org/10.1093/nsr/nwx039 28. Yu X-Y, Chen J-R, Xiao W-J (2020) Visible light-driven radical-mediated c–c bond cleavage/functionalization in organic synthesis. Chem Rev. https://doi.org/10.1021/acs.che mrev.0c00030 29. Jagan MR, Narayanama, Stephenson CRJ (2011) Visible light photoredox catalysis: applications in organic synthesis. Chem Soc Rev 40:102–113 30. Burykina JV, Shlapakov NS, Gordeev EG, BurkhardK¨onig, Ananikov VP (2020) Selectivity control in thiol–yne click reactions via visible light induced associative electron upconversion. Chem Sci 11:10061–10070 31. Wu Z-G, Liao X-J, Yuan L, Wang Y, Zheng Y-X, Zuo J-L, Pan Y (2020) Visiblelightmediated click chemistry for highly regioselective azide-alkyne cycloaddition via photoredox electrontransfer strategy. Chem Eur J 26(25):5694–5700. https://doi.org/10.1002/chem.202 000252 32. Mishiro K, Kimura T, Furuyama T, Kunishima M (2019) Phototriggered active alkyne generation from cyclopropenones with visible light-responsive photocatalysts. Org Lett 21:4101–4105 33. Ramil CP, Lin Q (2014) Photoclick chemistry: a fluorogenic light-triggered in vivo ligation reaction. Curr Opin Chem Biol 21:89–95 34. An P, Yu Z, Lin Q (2013) Design of oligothiophene-based tetrazoles for laser-triggered photoclick chemistry in living cells. Chem Commun 49:9920–9922

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35. Lederhose P, Wüst KNR, Barner-Kowollik C, Blinco JP (2016) Catalyst free visible light induced cycloaddition as an avenue for polymer ligation. Chem Commun 52:5928–5931 36. Ahn D, Sathe SS, Clarkson BH, Scott TF (2015) Hexaarylbiimidazoles as visible light thiol-ene photoinitiators. Dent Mater 31:1075–1089 37. Kolczak U, Rist G, Dietliker K, Wirz J (1996) Reaction mechanism of monoacyl- and bisacylphosphine oxide photoinitiators studied by 31p-, 13c-, and 1h-Cidnp and Esr. J Am Chem Soc 118:6477–6489 38. Ganster B, Fischer UK, Moszner N, Liska R (2008) New photocleavable structures. Diacylgermane-based photoinitiators for visible light curing. Macromolecules 41:2394−2400 39. Mitterbauer M, Knaack P, Naumov S, Markovic M, Ovsianikov A, Moszner N, Liska R (2018) Acylstannanes: cleavable and highly reactive photoinitiators for radical photopolymerization at wavelengths above 500 Nm with excellent photo bleaching behavior. Angew Chem Int Ed 57:12146−12150 40. (a) Xie J, Jin HM, Hashmi ASK (2017) Chem Soc Rev 46:5193; (b) Jagan MRN, Corey RJS (2011) Chem Soc Rev 40:102–113; (c) Prier CK, Rankic DA, MacMillan DWC (2013) Chem Rev 113:5322–5363; (d) Ravelli D, Fagnoni M, Albini A (2013) Chem Soc Rev 42:97; (e) Chen JR, Hu XQ, Lu LQ, Xiao WJ (2016) Chem Soc Rev 45:2044 41. (a) Mattes SL, Farid (1980) JCS Chem Comm 126–128; (b) Wang K, Meng LG, Wang L (2017) Org Lett 19:1958–1961 42. Liu H, Zheng T, Zheng Y, Li B, Xie X, Shen X, Zhao X, Yu Z (2021) Visible-light induced photoclick and release strategy between monoarylsydnone and phenoxylfumarate. Chem Commun 57:8135–8138. https://doi.org/10.1039/d1cc02841c

Chapter 7

Effective X-ray Luminescent Hybrid Structures of Nanodiamonds Associated with Metal–organic Scintillators Yuri V. Kulvelis, Natalia P. Yevlampieva, Daniil S. Cherechukin, Vasily T. Lebedev, Timur V. Tropin, Eduard V. Fomin, Vladimir G. Zinovyev, and Alexander Ya. Vul Abstract Effective X-ray-excited metal–organic scintillators were used to create complexes with nanodiamonds in order to obtain optically active nanoplatforms capable of delivering photosensitizer molecules to living tissues in photodynamic therapy procedures. Hydrophilic detonation nanodiamonds with specially modified surface that had a positive potential in aqueous media due to saturation with grafted hydrogen atoms were associated with hydrophobic phosphors based on linear alkylbenzenes with organic modifiers carrying gadolinium atoms controlled by X-ray fluorescence spectrometry. This made it possible to convert X-rays into photons in the wavelength range of 350–550 nm, including the Soret absorption

Y. V. Kulvelis (B) · V. T. Lebedev · E. V. Fomin · V. G. Zinovyev B.P. Konstantinov Petersburg Nuclear Physics Institute, NRC “Kurchatov Institute”, Gatchina, Russia e-mail: [email protected] V. T. Lebedev e-mail: [email protected] E. V. Fomin e-mail: [email protected] V. G. Zinovyev e-mail: [email protected] N. P. Yevlampieva · D. S. Cherechukin Saint Petersburg State University, St. Petersburg, Russia e-mail: [email protected] D. S. Cherechukin e-mail: [email protected] T. V. Tropin Joint Institute for Nuclear Researches, Dubna, Russia e-mail: [email protected] A. Ya. Vul Ioffe Institute, St. Petersburg, Russia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Garg and A. Chandra (eds.), Photocatalysis for Environmental Remediation and Energy Production, Green Chemistry and Sustainable Technology, https://doi.org/10.1007/978-3-031-27707-8_7

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band of the Radachlorin® photosensitizer. Binary and ternary formations, diamondscintillator and diamond-scintillator-Radachlorin®, were additionally stabilized with polyvinylpyrrolidone. As a result, functional nanostructures were obtained that are stable in aqueous media in the temperature range of 20–50 °C according to smallangle neutron scattering data and optical absorption measurements. As shown by neutron experiments, ensembles of diamond particles in combination with the indicated modifiers form chain-like fractal structures on scales of tens of nanometers. These structures retain the photoluminescent properties of the scintillator and photosensitizer that is confirmed by measurements of the luminescence in prepared colloids upon UV excitation. The colloids exhibited intense secondary radiation in visible and near-IR ranges. The developed functional materials are being tested on biological cells and animals for subsequent applications in X-ray photodynamic therapy as combined converter-photosensitizers. Keywords Radiation · Scintillator · Luminescence · Phosphor · Nanodiamond · Complex

7.1 Introduction Actual tasks of biomedicine stimulate to synthesize various functional nanostructures with a lot of functional properties (magnetic, luminescent, photocatalytic) for desirable applications in photodynamic therapy (PDT), magnetic resonance imaging (MRI), computer tomography scan (CT) and X-ray tomography, diagnostics with luminescent labels [1–13]. The PDT uses the activation of a photosensitizer (PS) injected into a tumor by a laser beam so that its excited molecules transfer energy to molecular oxygen in the tissues where oxygen molecules pass from the basic triplet state to the excited chemically reactive singlet state and destroy tumors efficiently. Meanwhile, presently the PDT is not sufficiently implemented in clinical practice due to high absorption of light in living tissues that restricts strongly the available depth at which this therapy is effective. For X-rays, most tissues and organs are translucent and this radiation really has great advantages in diagnostics and radiation therapy [14–17]. X-rays can first activate scintillators when exciting PS in the deeply located tumors that can also reduce normally used radiation dose in X-ray therapy (RT) to prevent unwanted damages in healthy tissues. This makes very attractive the X-ray photodynamic therapy (X-PDT) by means of luminescent nanoparticles to be excited by X-rays [15–17]. The action of X-rays on nanosensitizers [2, 18–20] should be considered assuming that RT uses high energy quanta (102 –103 keV) to be converted into visible light to activate photosensitizers for PDT for singlet oxygen (1 O2 ) generation. As the converters of X-rays, various fluorides or oxides (CeF3 , TiO2 , etc.) of heavy metals served (for example, Tb3+ ions producing a green glow), and in other cases, the hydrophilic micelles of GdEuC12 chelates with the built-in hypericin as PS were used [2]. Additionally, in X-PDT these heavy elements well absorbing X-rays are

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useful to visualize the tumors [2]. Meanwhile, heavy metals are highly toxic, that is required to limit their amounts in preparations. Therefore, it seems prospective to use nanodiamonds as non-toxic converters and nanoplatforms for PS delivery since these particles are luminescent under ionizing radiations (X-rays, electrons, UV) [21, 22]. Nanodiamonds are chemically inert and resistant to ionizing radiation and have a large surface for grafting functional groups [23, 24], molecules, drugs, rare earth ions [25, 26] appropriate for X-ray conversion and PS excitation that stimulated a design of luminescent complexes of diamonds with PS and medical polymers [22, 27–32]. When developing PS, it is important to impart luminescent properties to them [33–41], and to provide targeted delivery methods, for example, by binding to magnetic particles, while maintaining the photodynamic activity of the PS [42]. For example, a hydrophobic PS (bacteriochlorin) has been combined with magnetite particles to deliver drugs and create contrast in MRI with controlling PS accumulation in the affected organ to reduce surgery time [42]. Such applications of magnetic nanoparticles are discussed in review [43]. Among recently developed PS, the carbon structures, fullerenes and endofullerenes, are considered also as effective substances to be applied in PDT because of their antimicrobial activity and good ability to generate singlet oxygen [44–47]. For this purpose, can be used also water-soluble phthalocyanine derivatives, which inactivate bacteria during PDT [48]. Lanthanide diphthalocyanines are able to generate singlet oxygen mainly as a result of protonation with acids [49]. In the cases of fullerenes and diphthalocyanines there is a common problem to transfer these highly hydrophobic substances to aqueous media that can be achieved by means of nanoplatforms (polymers, nanoparticles) [50] saving photodynamic properties of grafted PS molecules [51–53]. In general, the most attractive for applications in medicine (PDT, MRI, CT) are organometallic compounds that provide the overall effect of diagnostics and therapy (theranostics) due to the presence of rare earth elements. Depending on the tasks in the series of these elements, it is possible to select atoms with high magnetic moments and the desired absorption bands of light and ionizing radiation (UV, X-ray). The combination of several compounds with different dominant properties (magnetic, luminescent) in the preparation can give the desired effect of enhancing its functional properties. These types of mixtures are known as liquid scintillators (LSc) [54, 55], which functional properties are regulated by the choice of the combination of components and characteristics of the components. In the present work, a scintillator [54, 55], which served as an X-ray converter to optical range, was combined for the first time with a medical PS, Radachlorin®, on a common nanoplatform to enhance the effects of luminescence and PS excitation. Nanodiamonds were chosen as a nanoplatforms being the particles with a special set of useful properties for biomedicine. The aim of the work was to synthesize and study the physico-chemical and structural properties of molecular complexes of organic scintillator (Sc) which contained gadolinium atoms and is associated with hydrophilic detonation nanodiamonds (DND). DND particles were considered as active platforms providing a reliable delivery of functional molecules into biological media (antibacterial agents, luminescent labels, contrast agents for MRI, phosphors and PS for PDT).

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7.2 Experimental 7.2.1 Samples On the basis of aqueous dispersion of detonation diamonds with a positive (negative) surface potential (DNDZ+, DNDZ-) [23, 24], there were prepared the complexes with a liquid scintillator (SC, Gd-LS, density 0.86 g/cm3 , boiling point 280–311 °C) described earlier [54, 55]. The SC is composed on linear alkylbenzenes (LAB) in the form of a mixture of monoalkyl derivatives of benzene (C16 H26 , C17 H28 , C18 H30 , C19 H32 ) and special additives. The LAB molecule is a phenyl with an attached side chain (10–13 carbon atoms). LAB is a low toxic substance used for the production of biodegradable detergents. The SC has a high optical transparency, and exhibits luminescence (band 325–450 nm, maximum at 342 nm) when excited by UV radiation (wavelength 275 nm) [54, 55]. To increase the yield of luminescence, a primary additive, 2,5diphenyloxazole (DPO, 0.5 wt.%), was introduced into the LAB. The secondary additive, 1,4-bis(2-methylstyryl) benzene (bis-MSB, 0.0025 wt.%), allowed the shift of the luminescence band [55]. The SC contained gadolinium (0.39 wt.%) as part of a complex obtained by means a reaction of gadolinium chloride (GdCl3 ) with 3,5,5trimethylhexanoic acid (TMHA), which is a stable ligand for dissolving gadolinium in LAB. To obtain a stable complex of SC with hydrophilic detonation diamonds, we linked the SC (practically insoluble in water) with hydrophilic polymer, poly(vinylpyrrolidone) (PVP). The latter is capable to form molecular complexes via charge transfer and hydrogen bonds [27, 56]. Both, the SC and PVP (K-17, molecular weight 9.5 × 103 ) we dissolved in isopropyl alcohol (IPA). At first, the liquid SC was mixed with IPA under ultrasonic homogenization. PVP being highly soluble in IPA, was added to this solution. Thus, a solvation shell of SC and IPA molecules were formed around the PVP chains, and binding of the polymer with the scintillator occurred. Namely this procedure for obtaining the SC-PVP complexes was used. In the opposite case, when SC was added to the PVP solution, the association of the components was hindered. The PVP chains in solution were surrounded by a solvate shell of IPS molecules, and a direct interaction of SC with PVP was blocked. At the next stage, IPA was evaporated from the samples at 60 °C, below its boiling point (82.4 °C). After removal of the solvent, the samples contained SCPVP complexes and unreacted liquid SC (high boiling point of 300 °C). The samples were added to the aqueous dispersion of diamonds (DNDZ+ or DNDZ-), and residual liquid SC was removed from its surface. In this way, the aqueous colloids of SCPVP-DNDZ+ and SC-PVP-DNDZ+ complexes (samples no 1 and 2, Table 7.1) were obtained, which remained stable for a month. The content of the components in the complexes specified in the process of their synthesis is given in Table 7.1. To test the ability of diamonds to bind directly the SC, the SC-DNDZ+ complexes (sample no 3) were prepared without the polymer. The surface of DNDZ+ particles

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Table 7.1 Complexes composition Sample no

PVP, mg/mL

DND, mg/mL

SC, given; measured, mg/mL

RC, mg/mL

Gd, mg/mL

54; 56 ± 2

–

0.22 ± 0.01

54; 23 ± 2

–

1

7.0 (K-17)

7.0 (Z+)

2

7.0 (K-17)

7.0 (Z-)

3

–

7.0 (Z+)

4

–

7.0 (Z+)

5

7.0 (K-17)

7.0 (Z+)

6

7.0 (K-17)

7.0 (Z+)

7

3.5 (K-17)

7.0 (Z+)

8

14.0 (K-17)

9

54; 43 ± 2

–

–

–

28; 23 ± 2

–

100;75 ± 2

–

55

–

7.0 (Z+)

55

–

3.5 (K-17)

3.5 (Z+)

–

10

7.5 (K-30)

3.5 (Z+)

27; 28 ± 2 10.75

–

11

7.5 (K-30)

3.37 (Z-)

10.75

–

12

1.75 (K-17)

1.75 (Z+)

0.573

0.035

13

1.75 (K-17)

1.75 (Z-)

0.573

0.035

14

3.5 (K-17)

3.5 (Z+)

0.717

–

15

3.5 (K-17)

3.5 (Z+)

0.717

0.07

0.17 ± 0.01 0.09 ± 0.01 –

0.11 ± 0.01 0.29 ± 0.01

0.11 ± 0.01

was saturated with hydrogen, while at the surface there were hydrophobic areas which could adsorb SC molecules. Initially, the SC was dissolved in IPA. The solution was combined with the aqueous dispersion of DNDZ+, sonicated, kept for a day at 20 °C. Then unreacted SC was removed from the surface of the solution, and IPA and water were evaporated. The solid residue was dissolved in water, and finally there was obtained a colloid of SC-DNDZ+ (sample no 3, Table 7.1). For comparison, the dispersion of pure diamonds (sample no 4, Table 7.1) was taken. In the samples no 5–9 (Table 7.1), the DNDZ+ diamonds with a protonated surface were used [23]. They are able to form hydrogen bonds with PVP and SC that contribute to the stability of the obtained colloids. Their structure and properties were studied depending on the ratio of the components. Samples no 5 and 6 contained different amounts of SC (28 and 100 mg/mL, Table 7.1) at a fixed ratio between PVP and DND to determine how the SC modulates the structure and physico-chemical properties of complexes. In the samples no 7 and 8, the content of PVP was varied (3.5 and 14.0 mg/mL, Table 7.1) at a constant relation to diamonds and SC to reveal the stabilizing effect of PVP for the complex. To assess the effect of the total concentration of components on the ternary complex in aqueous environment, the sample no 9 was prepared by diluting twice the sample no 1 (Table 7.1). In ternary complexes, the PVP of low molecular weight (K-17, MW ~9500) was predominantly used. To understand how the length of the PVP chain affects the physico-chemical properties of ternary complexes, the samples no 10 and 11 were prepared with the polymer of larger molecular weight (K-30, MW ~25,000).

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At the final stage, four-component complexes were produced by adding the medical PS Radachlorin® (RC) as the fourth component (samples no 12, 13 and 15 in Table 7.1). The synthesis procedure included a dissolution of PVP in dimethylformamide (DMF) with the addition of a small fraction of RC aqueous solution (0.35 wt. %). The mixture was combined with aqueous dispersions of DNDZ+ or DNDZdiamonds, and then the SC was added. The compositions were dried, and the residue was dissolved in water to obtain the colloids of the samples no 12, 13, 15. The content of gadolinium (Gd) in the obtained complexes was determined by X-ray fluorescence analysis using the gamma source of 109 Cd for external excitation. The data were compared with a standard having known Gd content. According to the data in Table 7.1, the concentrations of gadolinium in the colloids of complexes lie in the range of 0.1 ≤ CGd ≤ 0.3 mg/mL for given amounts of SC, CSC = 28–100 mg/mL. Based on the found contents of Gd, the experimental SC concentrations CSCE = 23–75 mg/mL were calculated for the samples with diamonds DNDZ+ (samples no 1, 5, 6 in Table 7.1). The CSCE values (Table 7.1) correspond approximately to the concentrations specified during synthesis, while experimental values are lower, CSCE < CSC , since the SC was partially removed from the samples during the synthesis. In the case of DNDZ- with negatively charged surface (sample no 2, Table 7.1), the estimated amount of SC, CSCE = 43 ± 2 mg/mL, is less than in the similar sample no 1 with positive DNDZ+ particles, CSC = 56 ± 2 mg/mL. This indicates a lower binding ability for SC molecules and diamonds DNDZ- at the same amounts of PVP. Without the polymer, a direct binding of DNDZ+ and SC molecules (sample no 3, Table 7.1) is even less effective, CSC = 23 ± 2 mg/mL at the same given content of SC (54 mg/mL).

7.2.2 Methods Aqueous colloids of the complexes were tested by measuring the optical density in the UV and visible ranges (wavelengths 190–1100 nm) by diluting and taking into account the absorption and scattering of light by colloidal particles. To evaluate the effectiveness of the complexes as luminophores, they were exposed by X-rays (wavelength 0.154 nm), and the luminescence of the samples was measured versus their composition, type of diamonds (DNDZ+, DNDZ-) and preparation method. Spectral data on the luminescence of samples upon excitation with UV radiation (wavelength 260 nm) were obtained at the Resource Center “Optical and laser methods for studying substances” of Saint Petersburg State University (spectrofluorimeter Fluorolog-3, Horiba). Small volumes of samples no 1–9 (amounts of 50 µL was added to 3 mL of water, diluted 61 times) were prepared for the spectral measurements (1 cm quartz cell). Samples no 10–14 were diluted by the following way: 0.3 mL of initial colloid was added to 3 mL of water for spectral studies (11-fold

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dilution). For comparison, the luminescence spectra of the initial SC dissolved in IPA (5% volume concentration) were measured. In a series of small-angle neutron scattering experiments (Dubna, JINR, IBR-2 reactor, YuMO spectrometer), the colloidal samples 1–9 were studied at 20 and 50 °C.

7.3 Results and Discussion 7.3.1 Optical Absorption The optical absorption spectrum in the UV, visible and near-IR areas was measured first for pure SC in IPA (Fig. 7.1). A dilute solution of SC (0.042 mg/mL) showed three absorption bands in the UV region (213; 261; 304 nm), which correspond to the data [1, 2]. The SC molecules in complexes in aqueous media retain mainly their original electronic properties according to experiment data. This is confirmed by absorption peaks detected at positions corresponding to the pure SC in the IPA, as shown by the data for the complex SC-PVP-DNDZ+ (sample no 1, Table 7.1) (Fig. 7.2). For the complex in the aqueous medium, the same characteristic absorption bands are visible in the range λ ~ 210–350 nm, as for the initial SC (Fig. 7.1). There is also an absorption peak associated with diamonds and polymer (~ 200 nm), and in the range λ > 400, the magnitudes of D(λ) monotonically decrease due to light scattering on diamond and polymer particles (aggregates) (Fig. 7.2). Unlike complexes based on DNDZ+, the systems with the DNDZ- showed spectra on which the absorption bands at 210–350 nm related to SC molecules are almost not displayed (except of peak at 222 nm) (Fig. 7.3). A low absorption observed in Fig. 7.1 Optical density of SC in IPA (0.042 mg/mL) versus light wavelength λ = 190–1100 nm

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Fig. 7.2 Optical density D(λ) of aqueous colloids of complexes SC-PVP-DNDZ+ (diluted sample no 1) with the concentrations 0.35 and 0.18 mg/mL (data 1, 2) and dispersion of DNDZ+ (diluted sample no 4; 0.017 mg/mL) (data 3) versus light wavelength

Fig. 7.3 Optical density D(λ) spectra for aqueous colloids of complexes SC-PVP-DNDZ- with concentrations 0.28 and 0.14 mg/mL (data 1, 2) (diluted sample no 2, Table 7.1) versus light wavelength

the range of λ = 400–1000 nm indicates a weak aggregation of such complexes (Fig. 7.3). These differences indicate a lower ability of DNDZ- diamonds to form complexes with SC molecules which was confirmed in subsequent X-ray experiments showed a weaker luminescence of exposed SC-PVP-DNDZ- complexes comparative to SCPVP-DNDZ+ (Figs. 7.4 and 7.5).

7.3.2 X-ray Luminescence In these experiments, the data for the complexes were compared with the luminescence spectrum of pure SC. When it was excited by X-ray radiation (CuKα line,

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Fig. 7.4 X-ray luminescence spectra for the complex SC-PVP-DNDZ+ (sample no 1) (data 1) and for pure SC (data 2), normalized by a factor of 60 to compare with the sample no 1

Fig. 7.5 Luminescence spectra of the sample no 1 (data 1) and sample no 2 (data 2) containing the diamonds DNDZ+ and DNDZ-, respectively

wavelength λ = 0.154 nm), overlapping spectral bands of luminescence were detected at characteristic wavelengths of ~400; 420; 450; 480 nm (Fig. 7.4). In the colloid of SC-PVP-DNDZ+ (sample no 1) there were detected the same bands as in solution of pure SC while more overlapped in corresponding positions (Fig. 7.4). For such a complex, the absorption is stronger at smaller wavelengths, and its maximum is at λ ~ 400 nm, in contrast to the data for SC with peak position at 420 nm (Fig. 7.4). During subsequent measurements, it was found that a substitution of DNDZ+ diamonds having a hydrogen-saturated surface to DNDZ- with a negative the ζ-potential due to grafted carboxyl (acid) groups strongly affects the luminescent properties of the complexes. In the case of DNDZ- particles, the luminescence is decreased by an order of magnitude (Fig. 7.5). When the surface of diamonds is saturated with hydrogen (DNDZ+), the effect is of ~16 times higher as compared to the data for DNDZ- carriyng carboxyls on

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the surface. Probably, the carboxyls somewhat oxidized SC that led to luminescence quenching. Thus, by adjusting the properties of the surface of diamonds (type and number of grafted groups, electric potential), it is possible to regulate a luminescence in a wide range, that is important for prospective medical applications of complexes.

7.3.3 Luminescence Under UV and Visible Radiation The luminescence of diamonds, SC and complexes was studied by UV and visible light irradiation of the samples using the wavelengths λ = 260; 405; 532; 540; 662 nm. Two of them corresponded to Soret absorption band (405 nm) and the Qband (662 nm) of RC. So, we monitored a dependence of luminescence on the energy of exciting radiation that is important for prospective applications of such objects in PDT. Initially, the aqueous dispersion of DNDZ+ (sample no 4) was tested by diluting the sample to a concentration of 0.115 mg/mL (Fig. 7.6). UV radiation (260 nm) has induced the emission in the band λ = 300–500 nm (Fig. 7.6). So, in this way it is possible to excite the PS (Radachlorin®, Soret band, 405 nm). A glow in the specified and wider spectral ranges is a specific of diamonds due to the presence of nitrogen-vacancy lattice defects (charged, neutral) [21, 22]. Similar measurements were carried out for SC and complexes under UV radiation in diluted colloids. Relative to the data for pure IPA, the SC dissolved in it demonstrated a luminescence of ~200 times higher, taking into account the volume concentrations of these substances (Fig. 7.7). The SC spectrum is similar to that in the case of X-ray luminescence from undiluted liquid SC. In comparison with X-ray data, in the case of UV activation of the SC solution, emission with a shift of ~20 nm to the short-wave region is observed. At the same time, the width of the spectrum almost does not change, it includes the same four Fig. 7.6 Luminescence spectrum of aqueous dispersion of DNDZ+ (0.115 mg/mL) by UV excitation (260 nm)

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Fig. 7.7 Luminescence spectrum of SC (5% vol. in IPA) (data 1) and pure solvent (data 2) irradiated by UV (260 nm). For comparison, X-ray luminescence data (data 3) for undiluted SC are presented

bands with some redistribution of intensities (Fig. 7.7). Hence, we can conclude that luminescence mechanisms are similar for both radiations. The performed analysis of the properties of diamonds and SC, as basic components, allowed us to proceed to the study of luminescent properties of the samples no 1–9 with different proportions of components (Table 7.1). The spectra of the samples no 1–9 containing SC are similar to each other and have a main maximum at λmax ~ 380 nm (Fig. 7.8) while the diamonds (sample no 4) showed a weak peak (Fig. 7.8) with the maximum at λmax ~ 440 nm (Fig. 7.6). Hence, this strong emission is caused mainly by the inclusion of SC in complexes. We analyzed the spectral amplitude Imax at λmax ~ 380 nm as depending on the proportion of PVP at a fixed amount of SC (54 mg/mL, Table 7.1). In the systems with both types of diamonds (DNDZ+, DNDZ-), the PVP addition caused a linear gain in the amplitude (Fig. 7.9). For the complexes with both diamonds and polymer Fig. 7.8 Luminescence spectra of the samples no 1–9 (data 1–9) exposed by UV (260 nm)

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at their fixed amounts (7 mg/mL), the proportionality of the amplitude I max to the SC amount by synthesis is also found (Fig. 7.10). For the samples with other fractions of diamonds and the polymer, the I max values were reduced to the specified fixed fractions of diamonds and PVP (7 mg/mL), assuming the proportionalities I max ~ C PVP , I max ~ C DND . Eventually, the modified I max data obeys a linear dependence versus CSC (Fig. 7.10). The data analysis has shown that the diamonds, in comparison with SC molecules, give two orders in magnitude less luminescence effect and mainly serve as the platforms for transferring hydrophobic SC to aqueous media with preservation of luminescent properties. These properties are needed for a generation of reactive oxygen species in the environment of complexes studied further. Fig. 7.9 Maximum intensity of luminescence versus PVP concentration in the samples at fixed amount of SC specified during synthesis (54 mg/mL, Table 7.1). Data for the samples with DNDZ+ (data 1) and DNDZ(data 2) are plotted. Data linear fit is shown

Fig. 7.10 Luminescence intensities at the maximum of the spectrum (λmax ~ 380 nm) versus content of SC in the samples with fixed amounts of PVP (7 mg/mL), and DNDZ+ (7 mg/mL) (data 1) or DNDZ(7 mg/mL) (data 2) (Table 7.1). Data for other concentrations of DND or PVP are reduced to base fractions of components proportional to actual amounts (data 3). Data linear fit is shown

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7.3.4 Singlet Oxygen Generation in Aqueous Colloids of Complexes Luminescence characteristic of singlet oxygen in the 1260–1280 nm band (maximum at a wavelength of ~ 1270 nm) was detected in the aqueous colloids of the samples [29]. As it turned out, binary complexes of SC-DNDZ+ (sample no 3) are able to generate singlet oxygen under UV irradiation, whereas pure diamonds (sample no 4) showed no effect (Fig. 7.11). For complex of SC molecules with diamonds, a peak of luminescence is observed in the band 1260–1280 nm (Fig. 7.11) that confirms a presence of singlet oxygen in aqueous colloid of SC-DNDZ+ excited by UV radiation. An order of magnitude greater effect was recorded by UV excitation of SC-PVP-DNDZ+ complexes, which also demonstrated a peak of luminescence of singlet oxygen (maximum at λ ~ 1273 nm) (Fig. 7.12). However, for the complex SC-PVP-DNDZ- with negatively charged diamonds, a singlet oxygen was not detected (Fig. 7.12), although this complex showed luminescence at λ = 350–450 nm (Fig. 7.8). Consequently, diamonds with a positive surface potential forming stable complexes of SC-PVP-DNDZ+ are suitable for generating singlet oxygen. The luminescence intensity associated with singlet oxygen increased approximately in proportion to the fraction of SC set by synthesis (data for the samples no 5, 6 in Fig. 7.13). Based on the results of X-ray and UV experiments with SC-DNDZ+, SC-PVPDNDZ+ and SC-PVP-DNDZ- complexes, we concluded that the DNDZ+ diamonds with a surface saturated with hydrogen are suitable for generating singlet oxygen with these excitation methods, whereas DNDZ- diamonds with acidic carboxyl groups do not provide photocatalytic activity. Direct association of SC with DNDZ+ gives smaller effect than using a combined PVP-DNDZ+ platform, which binds SC hydrophobic molecules better. Fig. 7.11 Luminescence intensity I(λ) in near-IR range versus wavelength upon excitation of complex SC-DNDZ+ (data 1) and pure diamonds DNDZ+ (data 2) in aqueous media with UV radiation (260 nm). Spline functions for the data are shown

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Fig. 7.12 Near-IR spectra for aqueous colloids of SC-PVP-DNDZ+ (data 1) and SC-PVP-DNDZ(data 2) in the range of wavelengths of singlet oxygen characteristic emission (samples exposed by UV, 260 nm)

Fig. 7.13 Near-IR spectra for SC-PVP-DNDZ+ colloids in the λ-range including the emission band of singlet oxygen. Contents of SC in the samples no 5 and 6: 28 and 100 mg/mL (data 1, 2)

Such complexes are stable in an aqueous environment and have the increased ability to generate singlet oxygen. At the same time, it is important to understand how a luminescence depends on the wavelength of exciting radiation to find the best ways to stimulate optical emission for pumping photosensitizer (PS), Radachlorin® (RC), having two main absorption bands (Soret, Q-band) at wavelengths of 405 and 662 nm. The luminescent properties of complexes (samples no 10, 11, 14, 15) with diamonds SC-PVP-DNDZ+ and SC-PVP-DNDZ- were studied under UV excitation at wavelengths of 260, 405 and 662 nm. Absorbing quanta with a wavelength of 260 nm, both complexes demonstrate emission with approximately the same spectra in the wavelength interval of 330–450 nm, including the Soret band. So, to combine the spectra with the maximum of this band, excitation at the wavelength of ~300 nm is required (Fig. 7.14a). When these complexes are activated at the wavelength of

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Fig. 7.14 Luminescence spectra of SC-PVP-DNDZ+ (data 1) and SC-PVP-DNDZcomplexes (data 2) in aqueous colloids (samples no 10, 11, 14, 15) upon excitation at wavelengths of 260 nm (a), 405 nm (b) and 662 nm (c). The optical density profile (data 3) with Soret band (a, b) and Q-band (c) for aqueous solution of RC (concentration 0.017 mg/mL) is shown

405 nm, both secondary radiation spectra generally go beyond the limits of the Soret band (Fig. 7.14b), and the mode is unacceptable to activate PS. When irradiating the samples with the light wavelength of 662 nm (maximum Q-band of RC), the emission spectra are detected at the wavelengths of 700–850 nm above the Q-band (Fig. 7.14c). Hence, there is no need to use such type of activation. The luminescent properties of complexes under UV irradiation do not depend much on the type of diamonds included. The nature of the diamond surface (charge and grafted groups) has a weak effect on the electronic properties of phosphor molecules, demonstrating similar spectral patterns of luminescence in samples with DNDZ+ and DNDZ-. However, the ability of complexes to induce singlet oxygen generation depends on the type of diamonds (Fig. 7.15), which was noted above for samples no 1 and 2 with amount of SC of 54 mg/mL (Fig. 7.12). In this case, the quantity of SC in the sample no 10 with positively charged diamonds was 5 times less that led to a decrease in the amplitude of luminescence peak for singlet oxygen (maximum at λ ~ 1270 nm). The change of DNDZ+ to DNDZ- diamonds caused threefold decrease in peak intensity (maximum at λ = 1274 nm, Fig. 7.15).

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Fig. 7.15 Luminescence of singlet oxygen in the dispersion of SC-PVP-DNDZ+ (sample no 10) (data 1) in comparison with the effect for SC-PVP-DNDZ- complex (data 2)

These triple complexes with positively charged diamonds possess wellpronounced luminescent properties and provide a higher conversion of UV quanta into optical range. Hence, they can serve as photo-active platforms for PS to enhance a photodynamic effect. The useful properties were improved by linking PS molecules to triple complexes. Even at low fractions of SC and RC the SC-PVP(DNDZ+)-RC complex showed a significant effect of singlet oxygen generation comparative to SC-PVP-(DNDZ-)-RC system (Fig. 7.16). Substantial differences were recorded also in the luminescence for both complexes under UV irradiation (260; 662 nm, Fig. 7.17a,c). UV radiation (405 nm) induced the emission from SC-PVP-(DNDZ+)-RC and SC-PVP-(DNDZ-) complexes showed similar profiles (Fig. 7.17b). However, a stronger peak of luminescence for SC-PVP-(DNDZ+)-RC complex (λmax = 670 nm) is closer to the maximum of the Q-band for RC (λmax = 651 nm) than a similar peak for the SC-PVP-(DND Z-)-RC complex (λmax = 676 nm). Consequently, the SC-PVP-(DNDZ+)-RC complex provides better conditions for PS activation than the complex with DNDZ-. The data of optical spectrometry, luminescent X-ray and UV spectroscopy for complexes we compared further with the results of their structural studies by neutron scattering.

7.3.5 Structure of DND Containing Complexes To understand the structure of molecular complexes, we studied first the aqueous diamond dispersions. In water DNDZ+ particles acquire a positive charge, since the electrons from their surface transfer to the hydrate shell. At the same time, the diamonds with a positive potential (30–70 mV), usually have faces with different signs of charge [23]. The electrostatic attraction between such faces of neighboring

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Fig. 7.16 Luminescence spectra of aqueous colloids of quaternary complexes SC-PVP-(DNDZ+)-RC (data 1) and SC-PVP-(DNDZ-)-RC (data 2) in the near-IR range

Fig. 7.17 Luminescence spectra of SC-PVP(DNDZ+)-RC (data 1) and SC-PVP-(DNDZ-)-RC complexes (data 2) in water upon excitation by UV radiation (260; 405 and 662 nm) (a, b, c). Optical density profile for aqueous solution of RC (0.017 mg/mL) is shown in arbitrary units (b, data 3)

particles leads to the formation of chain (branched) fractal structures at mass concentrations of diamonds below ~ 5 wt.% and hydrogels above this threshold [24]. This ordering is preserved when drying a highly dilute dispersion on a carbon grid in transmission electron microscopy (TEM) experiments showed diamonds with a diameter dP ~ 5 nm, associated into clusters (size ~ 50–100 nm) (Fig. 7.18). In neutron scattering experiments on the samples no 1–9 this type ordering of diamonds was detected.

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Fig. 7.18 Diamond aggregates observed after drying of DNDZ+ aqueous dispersion (TEM data)

From neutron scattering data for DNDZ+ dispersion (sample no 4), the volume fractions F(d) of particles (aggregates) versus diameter in spherical approximation were found (Fig. 7.19) using the ATSAS package [57, 58]. The spectra at 20 and 50 °C showed approximately the same main peaks related to individual particles and their pairs (Fig. 7.19). The F(d) data (20 °C) in the interval d = 0–12 nm were fitted by the sum of Gaussian functions, [ ] [ ] ϕ(d) = f 1 · exp −(d − d1 )2 /2α12 + f 2 · exp −(d − d2 )2 /2α22

(7.1)

Fig. 7.19 Volume fractions F(d) of particles (aggregates) versus diameters in aqueous dispersion of DNDZ+ at 20 and 50 °C (data 1, 2). The thick line (1) shows the fitting of the main peak by the sum of functions (Eq. 7.1) for individual and aggregated diamonds (curves 2, 3). The inset shows aggregates (~15–25 nm) within the first and second coordination spheres around particles

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where f 1 = 5.6 ± 0.6 nm−1 and f 2 = 3.4 ± 0.4 nm−1 are the amplitude factors for particles and their pairs with the most probable diameters d1,2 and size dispersions α 1,2 . The particles have a characteristic size d 1 = 4.7 ± 0.6 nm (α 1 = 3.1 ± 0.9 nm) in accordance with TEM data (Fig. 7.18). The size of pairs d2 = 8.4 ± 5.5 nm (α 2 = 4.6 ± 3.2 nm) is found less than twice particle diameter, d 2 /d 1 ≈ 1.8, due to shape anisotropy of pairs, for which a spherical approximation is too rough. Then the integrals (I 1 , I 2 ) of partial functions in Eq. (7.1) over diameters were calculated. The ratio (I 2 /I 1 ) = 2P2 /(1-2P2 ) ≈ 0.90 gave the probability of pairs formation P2 ≈ 0.24 when their number N 2 = P2 N P is proportional to the total number of particles N P . Thus, the fractions of associated and free particles, 2P2 and (1-2P2 ), are approximately the same (~ 50%) that confirms a strong aggregation in DNDZ+ dispersion. The introduction of modifiers (SC, PVP) into the system can affect diamond ordering assuming its strengthening or weakening due to hydrophobic SC and hydrophilic polymer. However, in all the cases the neutron data demonstrate mainly the scattering from the diamonds having a much higher contrast in watet than that for SC and PVP. Modification of DNDZ+ with SC has little effect on diamond structuring. The scattering cross sections of the modified and original samples (no 3, 4) differ slightly, mainly at the low qs (Fig. 7.20). The SC in the diamond system caused some scattering amplification at q ≤ 0.1 nm−1 only. The difference of cross sections, Δσ (q) = σ 3 (q) − σ 4 (q), indicates some globular structures that have arisen by partial capturing of hydrophobic SC molecules inside diamond cages (Fig. 7.21). Such structures have a fractal surface with the dimension DS = 6 − Ddif ~ 2.4. The index Ddif = 3.57 ± 0.31 was found from the data approximation by the function Δσ (q) ~ 1/qDdif (Fig. 7.21, inset). Polymer introduction into the complex, when the compound SC-PVP-DNDZ+ was obtained, did not change a behavior of diamond ensemble fractal ordering (Fig. 7.22). In the interval 0.1 ≤ q ≤ 1 nm−1 , the behavior of the cross section, Fig. 7.20 Neutron scattering cross sections for aqueous colloids (20 °C) of SC-DNDZ+ complex (data 1) and DNDZ+ dispersion (data 2) (samples no 3, 4) versus momentum transfer. Lines show the characteristic behaviors (q−2 ; q−4 ) of cross sections for diamond chains and individual particles

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Fig. 7.21 Difference of cross sections Δσ (q) = σ 3 (q) − σ 4 (q) of the samples no 3 and 4 versus momentum transfer (q). In double logarithmic scales it is shown data fitting by fractal scattering fubction ~ 1/qDdif (inset)

ln(σ ) = A − Df · ln(q), (Fig. 7.22 is defined by the parameters A = −1.30 ± 0.02, Df = 2.29 ± 0.02 which are close to similar values for the pure diamond dispersion (A = −1.28 ± 0.02, Df = 2.28 ± 0.02) and the binary system SC-DNDZ+ (A = − 1.36 ± 0.02, Df = 2.32 ± 0.02). In these systems, the fractal index Df = 2.28–2.32 corresponds to linear and branched chains of particles and does not differ much from the Df = 2 for statistical Gaussian chains. This is consistent with the TEM data for dried DNDZ+ dispersion, where chain aggregates of diamonds have been preserved (Fig. 7.18). However, the Fig. 7.22 Cross sections for SC-PVP-DNDZ+ (data 1) and SC-PVP-DNDZ(data 2) colloids (samples no 1, 2) versus momentum transfer. The lines show data fitting by the function σ ~ 1/qDf

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use of diamonds DNDZ- with negative charge intensified the ordering in SC-PVPDNDZ- system, where the index, Df = 2.48 ± 0.01 and the parameter A = −0.74 ± 0.01 have increased that indicated more branched structures (Fig. 7.22). Neutron data confirmed a stability of diamond structures formed both in pure dispersions and with polymer and molecular additives. In SC-PVP-DNDZ+ complexes, the increase of SC fraction, C SC = 28– 100 mg/mL, at fixed polymer amount (7 mg/mL) did not produce noticeable structural changes (samples no 1, 5, 6) (Fig. 7.23). A variation in polymer fraction (C PVP = 3.5–14 mg/mL) in SC-PVP-DNDZ+ complex with a fixed content of SC (C SC = 54 mg/mL) did not lead also to remarkable structural changes (Fig. 7.24). The SC-PVP-DNDZ+ complexes of with low and high PVP concentrations (CPVP = 3.5 and 14 mg/mL) and the same DND content (7 mg/mL) showed minor difference in data (Fig. 7.24). Fig. 7.23 Cross sections of SC-PVP-DNDZ+ colloids (28 and 100 mg/mL of SC, samples no 5, 6) (data 1, 2) versus momentum transfer. Concentrations of PVP in the samples are equal (7 mg/mL)

Fig. 7.24 Cross sections vs momentum transfer for SC-PVP-DNDZ+ systems with PVP concentration of 3.5 mg/mL (data 1) and 14 mg/mL (data 2) (samples no 7, 8) and a constant SC amount (54 mg/mL)

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Thus, the main factor regulating the structuring of diamond systems with molecular and polymer components is the state of the diamond surface. In the presence of carboxyl groups on DNDZ- diamonds (sample no 2), the aggregation was noticeably increased relative to the data for the DND Z+ complex (sample no 1) due to the active formation of hydrogen bonds between polymers and diamonds, as it was observed in PVP-DNDZ- colloids [28]. Another factor affecting the ordering of colloids is the temperature which effect was studied for the samples heated from 20 to 50 °C. As it turned out, the SC-PVPDNDZ+ complexes are thermostable and do not show large structural changes when heated (Fig. 7.25). To identify subtle temperature effects, the data at 50 °C were normalized to the cross sections measured at 20 °C. The ratio ρ = σ (q, T = 50 °C)/σ (q, T = 20 °C) was analyzed as depending to spatial scale R = 2π /q. The functions ρ(R) for DNDZ+ dispersion and other systems, SC-DNDZ+, SC-PVP-DNDZ+, SC-PVP-DNDZ- (samples no 1, 2, 4, 3), were compared. By heating pure DNDZ+ dispersion, short-range correlations of particles at the distance R1 ~ 5.0 nm comparable to their diameter (d P ) became revealed (Fig. 7.26). Fig. 7.25 Cross sections for SC-PVP-DNDZ+ colloid (sample no 1) at 20 and 50 °C (data 1, 2) versus momentum transfer

Fig. 7.26 Ratio of cross sections ρ = σ (R, T = 50 °C)/σ (R, T = 20 °C) for the DNDZ+ dispersion (sample no 4) versus radius R = 2π /q. Line is a spline function. Maxima Positions (R1 –R5 ) for different correlations are marked

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Fig. 7.27 The data ρ = σ (R, T = 50 °C)/σ (R, T = 20 °C) for the SC-DNDZ+ system versus radius R = 2π /q. Line shows a spline function. Correlation peaks are marked (R1 -R5 )

Indeed, enhanced temperature has intensified the contacts of particles and their aggregation as evidenced by the maxima of the ρ(R) at R2 ~ 7.4 nm, R3 ~ 14.7 nm, R4 ~ 23.4 nm, R5 ~ 48.7 nm. The first three maxima reflect stronger arrangement of particles in chain fragments, other peaks at R ~ R4 and R ~ R5 indicate closer overlap of chain fragments and branched clusters of connected fragments, and the radii R4 and R5 characterize the diameters of both structures. The presence of hydrophobic SC in DNDZ+ system (sample no 3) stimulated temperature effect of diamond aggregation, that is evident by the increase in amplitude and the shift of the second maximum to smaller radii (R2 ~ 6.5 nm) in comparison with the peak for pure DNDZ+ dispersion (R2 ~ 7.4 nm, Fig. 7.27). A decrease in the R2 distance for SC-DNDZ+ complex means a tighter transverse contact of chain diamond fragments. Along them, the spacing between neighboring particles became shorter by heating, R1 ~ 4.4 nm. More intense aggregation of heated diamonds led to the growth of chain fragments, R3 ~ 18.1 nm and R4 ~ 26.7 nm, by 10– 20% with a slight compression of clusters (R5 = 46.4 nm) by ~4%. The temperature effects in the initial and modified dispersion differ qualitatively. It demonstrates the role of SC as a structuring agent in diamond ensemble. Polymer addition and the formation of SC-PVP-DNDZ+ and SC-PVP-DNDZcomplexes (samples no 1, 2) significantly changed the temperature effects (Fig. 7.28). In the SC-PVP-DNDZ+ system, more power diamond association is expressed in the fusion of the first peak (R1 , longitudinal correlations of particles in chains) with the second maximum (R2 , transverse contacts of chain fragments) (Fig. 7.28a). The peaks R3 , R4 became overlapped and formed a broad hump (R ~ 20 nm, contacts between chain units). At last, heating stimulated clustering at the scale of R5 ~ 50 nm (Fig. 7.28a). The temperature induced ordering in SC-PVP-DNDZ+ system is qualitatively different from that for the DNDZ+ dispersion due to the interactions of diamonds with SC and polymer. Structural changes are pronounced also in the hot colloid of SC-PVP-DNDZcomplexes (Fig. 7.28b) where the diamonds via carboxyls form hydrogen bonds with the polymer integrating and stabilizing diamond structures. Meanwhile, the second peak (R2 ) has been reduced, discrete peaks at the scale of chain aggregates,

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Fig. 7.28 Cross section ratios ρ = σ (R, T = 50 °C)/σ (R, T = 20 °C) for SC-PVP-DNDZ+ (a) and SC-PVP-DNDZ- colloids (b) (samples no 1, 2) versus radius R = 2π /q. The R2 , R5 maxima indicate pair correlations of diamond chain fragments and their aggregates

R ~ 20–40 nm, have almost disappeared, but the correlations between clusters of aggregates at the distances R5 ~ 50 nm have remained (Fig. 7.28b). Next, we analyzed the temperature behavior of SC-PVP-DNDZ+ complexes as dependent on SC and PVP amounts. With the increase in the proportion of SC (C SC = 28–100 mg/mL) at fixed polymer content (7 mg/mL) the pattern of correlations changed markedly (Fig. 7.29). At small amount of SC (28 mg/mL) in SC-PVP-DNDZ+ complexes, the temperature effect was revealed by the same set of peaks (Fig. 7.29a) as in pure DNDZ+ dispersion (Fig. 7.26). Larger SC amount (54 mg/mL) caused mainly the overlap and merging of peaks R2 , R3 , R4 (Fig. 7.29b). But at high SC concentration (100 mg/mL) the growth of the first peak (R1 ) was detected (Fig. 7.30c). Hence, SC molecules at higher temperature stimulated chain binding of diamonds and the contacts of chains (Fig. 7.29c). The introduction of various amounts of polymer (3.5–14 mg/mL) into the SCDNDZ+ systems at fixed SC fraction (54 mg/mL) led to better integration of diamonds into chain and branched cluster structures, but without a significant change in the correlation pattern (Fig. 7.30). Hydrophilic polymer, unlike hydrophobic SC, did not disturb markedly the association mode of diamonds, but promotes the ordering of chain aggregates and their clusters when heated (Fig. 7.30). Further, it was important to find out the role of the total concentration of the substance in the diamond colloid in connection with temperature effects. Therefore,

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Fig. 7.29 Cross-section ratios ρ = σ (R, T = 50 °C)/σ (R, T = 20 °C) for SC-PVP-DNDZ+ systems with different amounts of SC (28; 54; 100 mg/mL) (a, b, c) (samples no 5, 1, 6) at a constant concentration of PVP (7 mg/mL) versus radius R

the sample no 9 was prepared, by diluting the sample no 1 twice. The resulting SC-PVP-DNDZ+ system contained low amounts of SC (27 mg/mL), diamonds (3.5 mg/mL) and PVP (3.5 mg/mL). The sample no 9 showed the temperature effects similar to the data for the sample no 5 with the same fraction of SC, but doubled diamond content (Fig. 7.31). However, in a dilute system (sample no 9), the peak of transverse correlations (R2 ~ 6.5 nm) was pronounced and indicated more contacts of diamond chain fragments. Hence, a diluted system was more susceptible to thermal fluctuations. The analysis of the behaviors of aqueous systems with diamonds and modifiers at room and enhanced temperatures concerned more qualitative aspects of the ordering of diamonds in the initial and modified states. Subsequently, we performed a quantitative analysis of the structuring of complexes in aqueous media.

7.3.6 Spatial Correlations of Diamonds in Aqueous Medium To obtain more structural information, we reconstructed the functions γ (R) of pair correlations between scattering centers in the samples at the scales R ~ 100 –102 nm by scattering data treatment (ATSAS software) [57, 58].

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Fig. 7.30 Ratios ρ = σ (R, T = 50 °C)/σ (R, T = 20 °C) for SC-PVP-DNDZ+ systems with different PVP fractions (3.5; 7; 14 mg/mL) (samples no 7, 1, 8) (a, b, c) and fixed SC amount (54 mg/mL) versus radius R

Fig. 7.31 Comparison of the functions ρ = σ (R, T = 50 °C)/σ (R, T = 20 °C) for SC-PVP-DNDZ+ systems with different diamond and polymer contents – 3.5 mg/mL, 3.5 mg/mL (data 1) against 7 mg/mL, 7 mg/mL (data 2) at the same SC amount (27 mg/mL) (samples no 9 and 5)

In isotropic colloids, the function G(R) = R2 γ (R) at R ≥ d P characterize the sample-averaged probabilities of detecting a second particle in a spherical layer at a distance R from the first particle at the origin of coordinate system. The G(R) function is the distributions of distances between particles in the system. Analysis of the data for the dispersion of DNDZ+ and the SC-DNDZ+ complex (samples no

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Fig. 7.32 Correlation spectra of the initial DNDZ+ dispersion (a) and SC-DNDZ+ complex (b) at 20 and 50 °C (data 1, 3 and 2, 4). Solid curves represent fitting functions (7.2)

3, 4) showed similar behaviors of correlations depending on the radius both at room and elevated temperatures (Fig. 7.32). Modification of diamonds with SC molecules intensified aggregation revealed in the growth of peaks by ~10%. At the same time, the size of aggregates still remains practically unchanged, as it can be seen from a stable position of a spectral maximum at 20 and 50 °C. Consequently, the SC caused some compaction of aggregates (Fig. 7.32). A detailed analysis of the behaviors of correlations showed that the data conform the function G(R) = g1 R 2 · exp(−R/RC1 ) + g2 R 2 · exp(−R/RC2 ) [ ] + g3 R 2 · exp −(R − L)2 /δ 2

(7.2)

Here, the first term describes the correlations mainly within individual particles, the second one corresponds to the aggregates, and the third one characterizes some contacts between them. The terms with the amplitudes g1,2,3 include the radii RC1

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Fig. 7.33 Model of diamond ordering in aqueous media

and RC2 , within which there are corresponding correlations. The length L denotes a spacing between aggregates with a dispersion δ. The model describes the particles with the nearest environment (small groups), chain aggregates assembled from groups and creating clusters contacting in colloid (Fig. 7.33). The model (Fig. 7.33) describes three-level diamond ordering at the scales R ≤ 100 nm in the samples no 3, 4 and in all the systems (samples no 1–9) with various compositions at different temperatures. In this model the interactions of diamonds with modifiers are evaluated via fitting parameters in function (7.2) (Figs. 7.34 and 7.35). For DNDZ+ dispersion and SC-DNDZ+ complexes, the initial part of the G(R) curves (Fig. 7.33) in the range 0 < R ≤ 5 nm demonstrates atomic correlations within diamonds in small groups having short correlation radii, RC1 = 2.04 ± 0.05 nm in diamond dispersion and RC1 = 1.93 ± 0.05 nm in binary system (20 °C). The amplitudes of correlators, g1 = 0.0169 ± 0.0003 cm−1 nm−3 , g1 = 0.0168 ± 0.0003 cm−1 nm−3 , are the same in basic√and modified systems. This implies the gyration radius of primary groups, r G1 = 6·RC1 ~ 5 nm ~ d P . It is close to particle diameter as far as it reflects the contacts of particles within the first coordination sphere. The number of particles in groups n1 = σ o1 /σ i we estimated for diamond dispersion (sample no 4) by comparing the measured forward cross section σ o1 = 8πg1 RC1 3 with calculated parameter for independent scattering from the particles, σ i = (ΔK 2 )ϕvP ≈ 1.43 cm−1 , using their contrast factor against water, ΔK = 12.2 · 1010 cm−2 , volume fraction, ϕ = 0.002 and the volume of particle vP = πdP 3 /6 = 48 nm3 . At 20 °C we found low aggregation numbers for the group, n1 = 2.5 ± 0.2 in DNDZ+ dispersion and n1 = 2.1 ± 0.2 in SC-DNDZ+ colloid. Hence, each diamond contacts with 1–2 neighboring particles (chain association). By heating the diamond dispersion to 50 °C, we detected a larger aggregation number, n1 = 2.8 ± 0.2, which indicates some branching of diamond chains. In binary system, due to SC, the integration of diamonds into aggregates became stronger when at both temperatures

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Fig. 7.34 Approximation parameters in correlation function (7.2) for the samples no 1–9 at 20 and 50 °C (data 1, 2): a, c - amplitudes of correlators g1 , g2 ; b, d - correlation radii RC1 , RC2

the peak has increased by ~ 10% keeping a stable position of maximum, RM = 2RC2 ~ 13–15 nm, corresponding to doubled radius RC2 ~ 7 nm (Figs. 7.32 and 7.34d). With a transition from diamonds to complex (20 °C), the aggregate radius showed a weak increase from RC2 = 6.71 ± 0.06 nm to RC2 = 7.07 ± 0.06 nm, while correlator amplitude remained quite the same, g2 = 0.0085 ± 0.0002 cm−1 nm−3 and g2 = 0.0087 ± 0.0002 cm−1 nm−3 (Fig. 7.34c, d). The aggregation numbers, n2 = 45.2 ± 1.5, n2 = 53.8 ± 1.6, for DNDZ+ and SC-DNDZ+ showed enhanced diamond integration (~20%) due to their modification by SC. Heating the diamond dispersion and the colloid of complexes (50 °C) induced only a weak growth of correlation radii, RC2 = 7.21 ± 0.06 nm and RC2 = 7.35 ± 0.12 nm, and a gain in the amplitudes of correlators, g2 = 0.00825 ± 0.00016 cm−1 nm−3 , g2 = 0.00933 ± 0.00032 cm−1 nm−3 . The aggregation numbers, n2 = 54.5 ± 1.7, n2 = 65.3 ± 3.9, in DNDZ+ and SC-DNDZ+ systems became ~20% larger. In the original and modified systems (20 °C), the numbers of groups in an aggregate did not differ much, ν = n2 /n1 = (18.1 ± 1.6), ν = (25.6 ± 2.6). At 50 °C, this number in diamond dispersion gained slightly, ν = (19.5 ± 1.5), while modified diamonds showed higher effect, ν = (34.4 ± 5.8). From the number ν ~ 20–30 and group size ~ 2r G1 ~ 10 nm follows the value of gyration radius of aggregates in Gaussian chain approximation, RGGA = (2r G1 )(ν/6)½ ~ 20 nm, according to the data on the formation of chain aggregates from diamonds

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Fig. 7.35 Fitting parameters in correlation function (7.2) for the samples no 1–9 at 20 and 50 °C (data 1, 2): a - amplitudes of correlator g3 ; b, c - distance L between aggregates with dispersion δ

(Fig. 7.20). Independently, the √ values of RC2 ~ 7 nm give practically the same aggregate gyration radius, RGA = 6·RC2 ~ 17 nm ~ RGGA . Hence, such structures can be considered as Gaussian chains or their conglomerates. The diameter of aggregates in spherical approximation, L A = 2(5/3)1/2 ·RGA ~ 44 nm, is consistent with their spacing in dispersion, L ~ 40–50 nm ~ L A (spacing spread δ ~ 10 nm ~ RC2 ) (Fig. 7.35b, c). This indicates some contacts and overlapping of aggregates. In DNDZ+ dispersion, the aggregates contact at 20 and 50 °C while this is true in SC-DNDZ+ system only at room temperature. Upon heating the parameter g3 disappeared because of repulsion of aggregates (Fig. 7.35a). This confirmed that diamonds shield hydrophobic SC molecules when hydrophobic fragments of diamond surface are turned inside the aggregates, while hydrophilic fragments are turned outward. It reinforces a mutual repulsion of the aggregates. In contrast to DNDZ+ dispersion, the SC-DNDZ+ complexes change the structure upon heating, and their additional stabilization in aqueous media is required. Therefore, we used hydrophilic polymer PVP with a high ability to form complexes with organic molecules and nanoparticles [28]. This enabled us to regulate the diamond ordering depending on their surface charge that led to very different structuring in SC-PVP-DNDZ+ and SC-PVP-DNDZ- systems (Fig. 7.36). More intense structuring of SC-PVP-DNDZ- complex (sample no 2) with negatively charged diamonds was observed relative to SC-PVP-DNDZ+ system

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Fig. 7.36 Spectra G(R) for aqueous colloids of complexes SC-PVP-DNDZ+ (sample no 1) (data 1, 2) and SC-PVP-DNDZ- (sample no 2) (data 3, 4) at 20 and 50 °C (data 1, 3; 2, 4). Curves are fitting functions (Eq. 7.2)

(sample no 1) where diamonds have had a positive potential (Fig. 7.36). These samples differed in the parameters (g1 , g2 , RC1 , RC2 ) (Fig. 7.34). At the first level, there was an increase in amplitudes, g1 = 0.0164 ± 0.0003 cm−1 nm−3 , g1 = 0.0280 ± 0.0029 cm−1 nm−3 (20 °C, samples no 1, 2). g1 = 0.0158 ± 0.0002 cm−1 nm−3 , g1 = 0.0252 ± 0.0006 cm−1 nm−3 (50 °C, samples no 1, 2). correlation radii, RC1 = 2.12 ± 0.05 nm, RC1 = 2.49 ± 0.09 nm (20 °C, samples no 1, 2). RC1 = 2.00 ± 0.05 nm, RC1 = 2.87 ± 0.20 nm (50 °C, samples no 1, 2). and aggregation numbers, n1 = 2.75 ± 0.21, n1 = 7.61 ± 1.16 (20 °C, samples no 1, 2). n1 = 2.23 ± 0.17, n1 = 10.48 ± 2.36 (50 °C, samples no 1, 2). When DNDZ+ diamonds are replaced by DNDZ-, the ternary complexes qualitatively change their structure. At the level of primary groups there is a transformation of chain fragments into globules, when the aggregation numbers n1 increase by 3–5 times, and gyration radius of globules becomes larger than diamond diameter, r G1 = √ 6·RC1 ~ 6–7 nm > d P . Similar tendencies are revealed at the second level also. At 20 °C, as a result of replacing DNDZ+ with DNDZ-, the correlation amplitude almost doubles, g2 = 0.0089 ± 0.0002 cm−1 nm−3 , g2 = 0.0162 ± 0.0028 cm−1 nm−3 , correlation radii increase also, RC2 = 6.51 ± 0.05 nm, RC2 = 7.59 ± 0.94 nm, as well aggregation numbers, n2 = 43.1 ± 1.3, n2 = 124 ± 57.

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At 50 °C the growth in amplitude is even more significant, g2 = 0.0087 ± 0.0002 cm−1 nm−3 , g2 = 0.0203 ± 0.0007 cm−1 nm−3 , although the radius rises slightly. RC1 = 7.35 ± 0.08 nm, RC1 = 7.59 ± 0.10 nm. As a result, the aggregation numbers increase markedly, n2 = 61.2 ± 2.4 and n2 = 157 ± 8. The heating stimulates aggregation more strongly in the system with DNDZ+, where the increase in n2 reaches ~ 42%, against ~ 26% in the colloid with DNDZ-. At the same time, at 20 °C, the numbers of primary groups ν = n2 /n1 = (15.7 ± 1.3) and n2 /n1 = (16.3 ± 7.9) differ little in ternary complexes with DNDZ+ and DNDZ-. However, at 50 °C these values diverge. In the case of positively charged diamonds, the number of groups in the aggregate doubles, ν = (27.4 ± 2.4), but in the case of negative charge of diamonds, the degree of integration decreases, ν = (15.0 ± 3.5). Along with this, the ternary complex with DNDZ+ demonstrates approximately the same temperature change in ν values as the SC-DNDZ+ system. Interaction and coordination of clusters of aggregates in systems with diamonds of different charges also differ greatly. In dispersion of ternary complexes with DNDZ+, interactions of aggregates are observed that are inherent in diamond dispersion and in system with binary complex (samples no 3, 4). Initially (20 °C), the aggregates are in contact at a distance of diameter L ~ 40–50 nm, but at 50 °C, the contacts weaken (the amplitude of the g3 correlator decreases by a factor of three, Fig. 7.35a). In the case of the DNDZ- complex, the trend is reversed. At 20 °C, clusters of aggregates unite into massive structures of ~ 2δ ~ 60 nm in size (L = 0), when the amplitude of the correlator (g3 ) increases by two orders of magnitude. But when heated (50 °C), such super-clusters are destroyed (Fig. 7.35a). Such a strong structural difference between SC-PVP-DNDZ- and SC-PVP-DNDZ+ complexes can be explained by the appearance of many hydrogen bonds between PVP and DNDZbearing carboxyl groups at the surface. In this respect, DNDZ+ particles are more inert. In this regard, it was important to elucidate the effect of the amount of SC (specified during the synthesis) on the structure of SC-PVP-DNDZ+ complexes. Initially, at 20 °C under conditions of varying SC concentration (28–100 mg/mL) at fixed contents of diamond (7 mg/mL) and PVP (7 mg/mL), the correlation spectra depend significantly on the amount of SC (Fig. 7.37). Main peak of G(R) spectrum with the maximum position Rmax ~ 16 nm is most intense at the minimum proportion of SC (28 mg/mL) (Fig. 7.37a). The addition of SC (54 mg/mL) caused not an increase, but a decrease in the peak amplitude by ~ 20% with a shift to smaller radii, Rmax ~ 12 nm. This indicates lower weight and size of aggregates (Fig. 7.37a). Enrichment of the sample with a modifier (100 mg/mL) is accompanied by a slight strengthening in correlations (~ 7%) with a gain in aggregate size, Rmax ~ 14 nm (Fig. 7.37a). So, we concluded that SC concentration, C SC ≤ 54 mg/mL, is sufficient to obtain ternary complexes.

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Fig. 7.37 Spectra G(R) for aqueous colloid of SC-PVP-DNDZ+ complexes at 20 °C (a) and 50 °C (b) with different amount of SC: 28 mg/mL (data 1, 4); 54 mg/mL (data 2, 5); 100 mg/mL (data 3, 6). Solid curves correspond to fitting function (7.2)

Heating the samples (50 °C) led to a disappearance of spectral differences. When varying the amount of SC, the spectra differ little and are close to the data at 20 °C for a sample with a minimum content of SC (Fig. 7.37b). Aggregates with an intermediate and high proportion of SC are initially metastable and, under the influence of temperature, transform into a stable form inherent in the complex with a small addition of SC (Fig. 7.37b). These conclusions are confirmed by the analysis of the parameters of diamond groups, aggregates and their clusters (Figs. 7.34 and 7.35). At 20 °C at the first structural level, a decrease in the amplitude and radius of correlations was observed, g1 = 0.0171 ± 0.0002 cm−1 nm−3 (C SC = 28 mg/mL), g1 = 0.0164 ± 0.0003 cm−1 nm−3 (C SC = 54 mg/mL), g1 = 0.0161 ± 0.0002 cm−1 nm−3 (C SC = 100 mg/mL), and correlation radii, RC1 = 2.19 ± 0.04 nm (C SC = 28 mg/mL),

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RC1 = 2.00 ± 0.05 nm (C SC = 54 mg/mL), RC1 = 2.04 ± 0.05 nm (C SC = 100 mg/mL), that led to a decrease in aggregation numbers as the content of SC increased (28– 100 mg/mL), n1 = 3.2 ± 0.2, n1 = 2.3 ± 0.2, n1 = 2.4 ± 0.2. At the first level, heating (50 °C) reduces the difference in parameters. At different fractions of SC, the amplitude is almost constant g1 ≈ 0.016 cm−1 nm−3 , the correlation radius changes slightly, RC1 = 1.98–2.12 nm. Aggregation numbers become similar for all scintillator concentrations (28; 54; 100 mg/mL): n1 = 2.2 ± 0.2, n1 = 2.2 ± 0.2, n1 = 2.4 ± 0.2. At the second structural level (20 °C), with an increase in SC fraction, a decrease and subsequent recovery of the numbers, n2 = 66.8 ± 2.3, n2 = 43.1 ± 0.2, n2 = 64.3 ± 2.2, was observed. This is mainly due to a variation in the size of aggregates (~ 20%) at a lower spread of amplitudes g2 (~ 10%). The radius RC2 ≈ 7.8 nm, after decreasing to RC2 ≈ 6.5 nm, reached a larger value, RC2 ≈ 7.5 nm. Then the values of n2 we normalized to the number of particles in groups n1 to determine how many groups ν = n2 /n1 make up an aggregate. At the share of SC equal to 100 mg/mL, the parameter ν = n2 /n1 = 26.8 ± 2.4 was the highest. Aggregates of complexes with an intermediate and minimal content of SC (28; 54 mg/mL) had ~ 20–30% less number of groups, ν = 19.6 ± 1.8, ν = 20.9 ± 1.5. At 50 °C, at the second level, the differences between samples with different amount of SC are little pronounced. The amplitudes of the correlators have the values with a little spread (2–6%) from the average magnitude, g2 = 0.0095 ± 0.0002 cm−1 nm−3 (C SC = 28 mg/mL), g2 = 0.0087 ± 0.0002 cm−1 nm−3 (C SC = 54 mg/mL), g1 = 0.0088 ± 0.0002 cm−1 nm−3 (C SC = 100 mg/mL). correlation radii, RC2 = 6.93 ± 0.06 nm (C SC = 28 mg/mL), RC2 = 7.35 ± 0.08 nm (C SC = 54 mg/mL), RC2 = 7.46 ± 0.07 nm (C SC = 100 mg/mL), showed small deviations (1–4%) from the average value in the series. As a result, the aggregation numbers did not differ much from each other for the indicated SC fractions, n2 = 55.7 ± 1.9, n2 = 61.2 √ ± 2.4, n2 = 64.3 ± 2.3. From the RC2 values, the gyration radii, RG2 = 6·RC2 ~ 16–19 nm, and the diameters of aggregates ~ 2 RG2 ~ 30–40 nm were estimated. The latter also corresponds to the size of clusters of chain aggregates capable of mutually interpenetrate. Hence, the coordination of clusters at a distance of L ~ 40–50 nm (Fig. 7.35b) reflects their contacts.

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At 20 °C, for complexes with SC fraction of 28 mg/mL, the correlations on the scales R ~ 2RG2 are not expressed (g3 = 0) (Fig. 7.36a), and repulsion between clusters prevails. The temperature (50 °C) stimulates the contacts of the aggregates when they approach each other at a distance of 2 RG2 ~ L ~ 50 nm. When the content of SC becomes 54 mg/mL, the aggregates are ordered both at initial and hither temperature, but an increase in the content of SC (100 mg/mL) leads to the opposite effect. In this case the coordination of aggregates is observed only at room temperature, being destroyed when heated (Fig. 7.35). As noted above, the temperature (50 °C) levels out the structural differences in complexes with different SC amounts. They pass into a more equilibrium state, which is noticeable at three structural levels (Figs. 7.34 and 7.35). As follows from the data analysis, to create a three-component complex, it is sufficient to use the concentration of C SC = 28 mg/mL, additional amounts of SC have little effect on the structuring of diamond particles. At the same time, it was important to elucidate the role of the polymer in the formation and structuring of complexes. The amount of PVP in SC-PVP-DNDZ+ complexes varied within C PVP = 3.5– 14 mg/mL, fixing SC (55 mg/mL) and diamond (7 mg/mL) contents (samples no 8, 1, 7). Correlation spectra (Fig. 7.38), as well as above (Fig. 7.37), showed that due to heating, the spectral curves of the samples converge, i.e., more equilibrium structures of a general type are formed. The spectra include intense peaks with maxima positions Rmax ~ 13–15 nm and weakly pronounced broad humps in the interval R ~ 40–70 nm. This reflects the presence of small groups of diamonds bound into aggregates and clusters (Fig. 7.38). From the parameters of groups and aggregates (Fig. 7.34) the aggregation numbers (n1 , n2 ) and the quantities of groups in the aggregates ν = n2 /n1 were determined. They are given depending on the PVP content together with the correlation radii RC1 , RC2 in Fig. 7.39. At 20 °C, the addition of PVP to the complexes stimulated aggregation with the increase in radii, RC1 ~ 1.9–2.1 nm, and numbers, n1 ~ 2.1–2.6, of primary groups. Heating of the samples (50 °C) promoted diamond association by their enrichment with PVP, when the parameters reached the values RC1 ~ 3 nm, n1 ~ 3 (Fig. 7.39a, b). The temperature relatively weakly affected on the aggregates, the radius RC1 increased by ~ 6%, the number n2 and quantity of groups in the aggregates increased by ~ 20% (Fig. 7.39). The PVP influenced more on primary association, but not on a large-scale aggregation. Structuring in the first coordination sphere around particles modified by SC molecules is important for the stability of complexes in aqueous media. The stability of colloids depends also on the overall concentration of dissolved substances. In final experiments, the diluted twice sample no 1 was tested (Fig. 7.40) to find out how the total content of components influences on the stability of the system that is important in biomedical applications of nanostructures (PDT, MRI). At 20 °C, the main correlation peak for a diluted twice sample demonstrated exceeding (~10%) relative to the data for sample no 1 (Fig. 7.40a). This indicated an enhanced integration of particles at the scales of R ≤ 40 nm due to a decrease in the complex concentration. But this difference practically disappeared when the samples are heated, demonstrating similar structures (first, second levels) in them.

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Fig. 7.38 Spectra G(R) for aqueous colloids of SC-PVP-DNDZ+ complexes at 20 (a) and 50 °C (b) with different specified PVP amounts (3.5; 7; 14 mg/mL) (data 1–3; 4–6). Solid curves are the fitting functions (7.2)

At the same time, with a reduced concentration of the complex, contacts between units in the range of radii R ~ 40–70 nm (Fig. 7.40b) are more pronounced. The differences between the initial and diluted systems at two temperatures are manifested in aggregation numbers. At the first level, the diluted sample is structured more than the original, according to obtained aggregation numbers n1 = 2.6 ± 0.2, n1 = 2.9 ± 0.3 (sample no 9) and n1 = 2.8 ± 0.2, n1 = 2.2 ± 0.3 (sample no 1) at 20 and 50 °C. At the second level, the differences observed at 20 °C were aligned during heating, as shown by the values n2 = 53.2 ± 1.9, n2 = 65.5 ± 3.5 (sample no 9) and n2 = 43.1 ± 1.3, n2 = 61.2 ± 2.4 (sample no 1) at 20 °C and 50 °C. A small difference in aggregation numbers n1 , n2 in both colloids allowed us conclude that the concentration variation does not cause significant changes in the structure of triple systems of SC-PVP-DNDZ+ in aqueous colloids. The structural analysis at characteristic scales (particles, aggregates, clusters) was supplemented with integral parameters of the samples. The scattering cross sections were approximated by Guinier function σ (q) = σ o ·exp[−(qRGB )2 /3] at low

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Fig. 7.39 Structural parameters of the samples at 20 °C (data 1, 3, 5, 7, 9) and 50 °C (data 2, 4, 6, 8, 10) versus amount of PVP in complexes: a - correlation radii RC1 , RC2 (data 1, 2; 3, 4); b aggregation numbers n1 , n2 (data 5, 6; 7, 8); c - number of groups in aggregates ν (data 9, 10) at initial and higher temperatures

momentum transfers to find forward cross sections σo , gyration radii RGB and aggregation numbers nAB for the structures on a large scale available within the resolution of the experiment (Fig. 7.41). In the diagrams (Fig. 7.41), the gyration radii and aggregation numbers determined are mostly consistent with similar parameters from the previous analysis of correlations at the level of aggregates and their clusters, which indicates the reliability of the structural data obtained.

7.4 Summary For the first time, complexes of detonation nanodiamonds with metal–organic scintillators are synthesized, which are activated by X-rays, creating a secondary emission of light to excite a photosensitizer additionally attached to the diamond platform. Structures of this kind can serve to deliver a photosensitizer (Radachlorin®) into living tissues, when nanodiamond-scintillator complexes play the role of converters of penetrating radiation into the optical range, which is important for expanding the therapeutic possibilities of PDT.

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Fig. 7.40 Spectra G(R) for SC-PVP-DNDZ+ (data 1) (sample no 9) and calculated for the sample no 1 with a factor of 0.5 (data 2) at 20 °C (a); similar data at 50 °C (data 3, 4) (b). Solid lines are fitting functions (7.2)

It is shown that diamonds with a hydrogen-saturated surface, which acquires a positive potential in an aqueous (biological) medium, are effective in these tasks. In the course of the research, ways were found to stabilize the diamondscintillator and diamond-scintillator-Radachlorin® complexes using a medical polymer (polyvinylpyrrolidone) at ambient and elevated temperatures (20–50 °C). Together with X-ray luminescent properties, such structures have the ability to serve as magnetic contrast agents in MRI diagnostics, since scintillator molecules include magnetic gadolinium atoms. These composite structures, studied in neutron scattering experiments, demonstrated fractal properties with respect to the binding of diamonds into linear and branched chain aggregates, which, when modified with a scintillator, polymer, and photosensitizer, retain the photocatalytic properties inherent in the components. The resulting nanocomposite structures have real prospects for implementation in biomedical practice.

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Fig. 7.41 Aggregation numbers nAB (a) and the radii of gyration RGB (b) at 20 °C (data 1) and 50 °C (data 2) for the observed structures in the samples found by Guinier approximation of cross sections

Acknowledgements The work was supported by Russian Foundation for Basic Researches (grant No 18-29-19008 mk). Authors thanks to engineers I.N. Ivanova and L.I. Lisovskaya for technical assistance.

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Part III

Applications of Photocatalysts

Chapter 8

Photocatalytic Degradation of Organic Pollutants and Airborne Pathogen in Air Pankaj Chowdhury, Noshin Hashim, and Ajay K. Ray

Abstract According to World Health Organization, air pollution kills millions of people worldwide every year. In addition, several epidemiological findings have uncovered the impacts of air pollution on respiratory and cardiovascular systems. This chapter presents current knowledge of human health concerns caused by volatile organic compounds (VOCs) and biological contaminants. These contaminants contribute to air pollutants that impair all environmental elements. Heterogeneous photocatalytic processes using semiconductor photocatalyst would serve as a promising technology and an efficient approach for removing VOCs and airborne pathogens. Considering the potentially toxic effect of these air pollutants, emerging mitigation approaches such as the photocatalysis process are explained elaborately in this chapter, including fundamental principles of photocatalysis, reaction mechanism, reaction kinetics, and photoreactor designs suitable for air purification. Furthermore, the photocatalytic process as a paradigm explores existing techniques utilized in research and commercial applications. Significant efforts have been made to include information from worldwide sources for this investigation. Keywords Airborne pathogens · Air pollution · Hydroxyl radical · Photocatalysis · Photoreactor · PM · ROS · TiO2 · VOCs

8.1 Introduction We start this chapter by addressing a few questions about air quality and its impact on the global population. We will also talk about the existing and potential control strategies here. Let us start with understanding the importance of air quality for the survival of living beings. Air pollution is, as defined by World Health Organization P. Chowdhury (B) · N. Hashim · A. K. Ray Department of Chemical and Biochemical Engineering, University of Western Ontario, London, ON, Canada e-mail: [email protected] P. Chowdhury Trojan Technologies, 3020 Gore Road, London, ON, Canada © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Garg and A. Chandra (eds.), Photocatalysis for Environmental Remediation and Energy Production, Green Chemistry and Sustainable Technology, https://doi.org/10.1007/978-3-031-27707-8_8

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(WHO), the contamination of the indoor or outdoor environment by any chemical, physical, or biological agent that modifies the natural characteristics of the atmosphere. Outdoor and indoor air pollution causes respiratory and other diseases and is a significant cause of morbidity and mortality [1]. Further, WHO data shows that majority of the global population breathes air that exceeds WHO guideline limits containing high levels of pollutants. Ambient air pollution accounts for an estimated 4.2 million deaths yearly due to stroke, heart disease, lung cancer, and acute and chronic respiratory diseases. Burning fuels produce a variety of health-damaging pollutants, including particulate matter (PM), methane, carbon monoxide, polyaromatic hydrocarbons (PAHs), and volatile organic compounds (VOCs). Many studies have demonstrated a direct relationship between exposure to PM and negative health impacts. In addition, smaller-diameter particles are generally more harmful, and ultrafine particles can penetrate tissues and organs, posing severe health hazards. Therefore, several environmental organizations have accentuated the importance of air quality management [1]. Therefore, the next relevant question here is to identify the major air pollutants that are harmful to humans and animals and recognize the sources of those pollutants. WHO identified six major air pollutants: PM (including bacteria, viruses, fungi, mold, and bacterial spores), ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead. Other significant pollutants include VOCs, PAHs, and dioxins. Air pollution can affect all environmental elements, including groundwater, soil, and air. Additionally, it causes a severe threat to living organisms [1, 2]. Therefore, this chapter discusses two major air pollutants: VOCs and biological contaminants for indoor air. WHO defined VOCs as organic compounds having a boiling point range of 50–260 °C, excluding pesticides. In indoor air, the most occurring VOCs include aromatics, aldehydes, and halocarbons. Indoor air usually consists of a more significant number of VOCs at a higher concentration than outdoor air. VOC discharge from building materials is the largest indoor air pollutants source. According to Brown et al. [3], the average concentration of each VOC in established buildings usually is below 50 µg/m3 but higher than 5 µg/m3 . In addition, household cleaning products substantially contribute to the air pollutants in an indoor environment [4]. VOCs are accountable for indoor air smells. Short-term exposure affects irritation of the eyes, nose, throat, and mucosal membranes, while those of long-duration exposure incorporate toxic reactions. In particular, VOCs, such as toluene, benzene, ethylbenzene, and xylene, cause cancer in humans [2, 5, 6]. Most PM is produced in the atmosphere by chemical reactions between pollutants such as sulfur dioxide and nitrogen oxides from automobiles, power plants, and other industries. PM10 are inhalable particles with diameters ≤ 10 µm. PM2.5 are fine inhalable particles with diameters ≤ 2.5 µm. PM contain microscopic solids or liquid droplets that are so small that they can be inhaled, triggering severe health problems. Some particles < 10 µm in diameter can get deep into our lungs, and some may even get into the bloodstream. Smaller particles (2.5 µm in diameter) pose the most significant health risk [7]. Again, the half-lives of PM2.5 and PM10 particles

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in the atmosphere are long because of their small sizes, which permits their longstanding suspension in the atmosphere and spread to distant locations where people and the environment may be exposed to the same magnitude of pollution [2, 8]. When a group of airborne microorganisms of various sizes and types is present in the air, associated with PMs, it is called airborne particles or bioaerosols. Bioaerosols range from 0.001 to 100 µm and combine solid and semi-solid materials with biotic and abiotic pieces [9, 10]. Bioaerosols may include 15–25% of PM (wt%) combined with living or dead bacteria, fungi, viruses, secondary metabolites, pollens, and dust and could cause infectious diseases, respiratory and chronic health problems [11– 13]. Under current circumstances caused by the COVID-19 pandemic, bioaerosols could have led to the transmission of SARS-COV-2 [14]. Data suggests that the airborne microbial portion of the PM has aggravated public health crises. Airborne aerosols consist of large amounts of bacteria induced by the atmospheric and local terrestrial environment. Furthermore, the air quality index (AQI) is linked to cultured bacteria and fungi concentrations. AQI < 200 increases the concentration of airborne fungi, while AQI > 200 increases the concentration of airborne bacteria. Chemical and microbiological compositions can detect connections between PM and AQI and, consequently, the microorganisms that belong to them [15–18]. Finally, let us discuss the most critical aspect of this introduction: the mitigation strategies to control VOCs and biological contaminants. Conventional ventilation systems designed for indoor air pollution are insufficient for removing VOCs [4, 19–21]. The established technologies for VOC removal incorporate adsorption, AOP, membrane separation, liquid absorption, and catalytic combustion [22–24]. These processes often require post-process treatment [20]. In addition to toxic chemicals (such as formaldehyde) being utilized in some of these processes, the energy consumption cost is also high due to the processes’ demand for high operating temperature [25, 26]. There are four major techniques to eliminate airborne pathogens in indoor air. These are ventilation, air distribution, air filtration, and air disinfection [27, 28]. This chapter will mainly consider the disinfection methods (e.g., UV-C, O3 , photocatalysis). Heating, ventilation, and air-conditioning (HVAC) system with high-efficiency particulate air (HEPA) filter is prevalent for indoor air purification but cannot remove microorganisms from the air. The advanced oxidation process (AOP) produces active radicals that play a primary role in degrading air-containing microorganisms. Among the AOPs, the photocatalysis process is extremely promising in removing air pollutants. In this perspective, adding a photocatalyst to the HVAC system would facilitate the microbial decontamination of air [29]. Photocatalytic oxidation is a widely studied process for eliminating VOCs and airborne pathogens from the air. It is one of the most popular and promising methods due to its high efficiency, cost-effectiveness, and environmental benefits. Several different photocatalysts are used in the photocatalytic oxidation process. TiO2 shows the highest efficacy in eliminating harmful compounds [21–23, 26, 30–33]. TiO2 is commercially available in different crystalline forms with a broad range of particle characteristics. It is photochemically stable, non-toxic, and can be reused for a prolonged time. TiO2 needs only low-energy photons (UVA) for its activation process.

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Besides, TiO2 photocatalyst can be modified by different means to make the catalyst active under lower-energy photons (such as visible wavelength ranges from solar light) [22, 23]. TiO2 band gap is 3.2 eV which requires λ < 380 nm for its electron excitation. Therefore, as per Planck’s law (E = hc/λ), with the use of visible light (λ > 400 nm), the energy associated with a larger wavelength of light is not adequate to overcome the TiO2 band gap. Hence, the creation of electron–hole (e−/h+) pairs might not occur unless sufficient energy is provided. Several approaches are available to modify the semiconductor photocatalyst to utilize the entire solar spectra, especially visible ones. Some of the well-known methods used to narrow down the band gap of a photocatalyst and achieve a visible light activity in a conventional photocatalyst are metal/nonmetal doping, valance band-controlled photocatalysts, solid solution photocatalyst, composite semiconductors, metal ion implantation, and dye-sensitization [34–36]. The following sections describe photocatalysis fundamentals, reaction mechanism, reaction kinetics, and current research advancements on photocatalytic systems, focusing on photocatalytic decontamination/disinfection of VOCs and airborne pathogens. We also report different photocatalytic reactor configurations currently in use for air treatments.

8.2 Basic Principle of Heterogeneous Photocatalysis Photocatalysis relates to two underlying topics: catalysis and photochemistry because of the requirement for photons (light) and catalysts (semiconductors) to start the chemical reaction. The photon could be obtained from either visible or UV irradiation, depending on the semiconductor materials. The semiconductor materials would have a filled valance band (VB) and empty conduction band (CB). Photons could trigger the VB electron with adequate energy equal to or greater than the bandgap energy (E g ) between CB and VB. Upon excitation, the electron moves from VB to CB, leaving a positive charge in VB, known as a hole (h+ ). This phenomenon is known as charge separation, the initial step of a photocatalytic reaction (Fig. 8.1). The electron–hole pairs generated by photocatalysis could encompass several reactions such as (i) recombination of electrons and holes, (ii) trapping of electrons and holes in metastable surface states, and (iii) reaction with electron donors or electron acceptors adsorbed on the semiconductor surface or within the surrounding electrical double layer of the charged particles commencing oxidation/reduction processes [37–39]. After the electronic excitation, the photocatalysis process produces e− /h+ and finally produces hydroxyl radicals (HO·). Hydroxyl radical is the second-highest oxidizing species (oxidation potential 3.03 V) after fluorine (oxidation potential 2.8 V), which reacts non-selectively with most of the organic pollutants in wastewater/air and produces carbon dioxide, water, and mineral acid [40].

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Fig. 8.1 Overall photocatalytic process (Reprinted with permission of Chen et al. [37])

8.3 Photocatalysis—Reaction Kinetics Several kinetic models could explain the heterogeneous photocatalysis process. In a photocatalytic reaction, the interaction between the adsorbed pollutant and the surface of the photocatalyst plays a significant role. Langmuir–Hinshelwood’s (L– H) model assumes that the adsorption of the reactants happens on the catalyst surface. For example, in a bimolecular reaction with reactants M and N, the reaction rate is directly proportional to the surface coverage (ϕ) of the reactant [41]: r = kϕ M ϕ N

(8.1)

where ϕi varies as ϕi =

Ki Xi 1 + Ki Xi

(8.2)

where K i = adsorption constant, X i = partial pressure in the gas phase or concentration in the liquid phase. Therefore, Eq. 8.2 develops

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r=

kKM KN XM XN (1 + K M X M )(1 + K N X N )

(8.3)

where k = true rate constant. Typically, the concentration of M would be higher than N. Hence, the change in concentration of M stays constant since its small utilization makes a trivial change in concentration. Thus, Eq. 8.3 became r=

k' K N X N (1 + K N X N )

(8.4)

where k ' = pseudo true rate constant. The L–H model contemplates reactant adsorption/desorption kinetics under steady illumination on the surface of the photocatalyst. However, the L–H model cannot determine a meaningful correlation between the incoming radiant flux and the reactant concentration [41]. The Direct–Indirect (D–I) model shows the correlation between the rate constant, pollutant concentration, and photon flux [42]. The model offers key theories on direct, indirect, inelastic interfacial, and adiabatic charge transfer, confirming a physical sense of the kinetic constraints involved. The concentration of reactant adsorbed on catalyst surface is expressed as follows: [X ] S =

{

1 + a [X ]aq +

((

ab[X ]aq ))(

d kox k−1

[ ko ] k1 O S2−

d [X ] + kox S

)}

(8.5)

where a = the adsorption constant, b = desorption constant, [X]s = reactant concentration on the surface of photocatalyst, [X]aq = equilibrium concentration of reactant, k 1 , and k −1 = adsorptions, and the desorption constant, O S2− = intrinsic surface states, d kox = hole transfer rate constant [41].

8.4 Photocatalytic Degradation of Volatile Organic Compounds The efficiency of photocatalytic oxidation of VOCs has been well studied and documented. Wang et al. [4] reported an in-depth review of and effectiveness of the photocatalytic system under UV and visible light. Sharmin and Ray [43] discuss the significance of ultraviolet light-emitting diode (UVLED) as a new concept in photocatalytic treatment. Yao et al. [44] investigated TiO2 as a photocatalyst by incorporating it into a metal–organic framework (MOF), allowing photocatalytic activity to occur under natural visible light. Furthermore, a fundamental review on photocatalytic progress in the degradation of the organic compound was reported by

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Gaya and Abdullah [45]. The team also explored the effects of operation parameters in a photocatalytic process and the organic pollutant’s ecotoxicity level at different treatment steps. A group of French scientists tested photocatalytic activity for aircraft cabins, and they successfully demonstrated the ability of the process to eliminate VOCs and microorganisms [46]. Furthermore, Daikoku et al. [47], Japanese researchers, studied the influenza virus and reported five minutes of inactivation via a photocatalytic air cleaner. Birnie et al. [48] recently studied the commercial applicability of photocatalytic reactors in terms of design and effectiveness in removing VOCs. In another study, Degefu and Liao [49] investigated p-type and n-type nanocomposite ability as photocatalysts. It showed an increase in photocatalytic activity of the air decontamination system on the laboratory scale. Kinetic modeling of VOC degradation under photocatalytic treatment was also studied and reported by Assadi et al. [50]. The photocatalytic process of removing VOCs is still in the early stage. Some foreseen trends for the development would include natural light-activated photocatalyst material with or without competence in selective photodegradation. Research studies indicate that removing VOCs at high levels is possible, but not enough data for environmental and ideal applications worldwide are reported. More work needs to be performed for catalysts re-activation to allow continuous VOC degradation. This will eliminate the early discharge of intermediates produced during the process. These compounds are often more harmful than parent pollutants, which can critically be dangerous if discharged into the indoor air.

8.4.1 Reaction Mechanism and Kinetics for the Photodegradation of VOC The energy required to activate a photocatalyst depends on its band gap energy [33]. TiO2 photocatalyst can degrade the pollutants under UV light (λ < 380 nm) in the presence of air or oxygen. The activation of TiO2 by UV light can be inscribed as T i O2 + hv → e− + h + .

(8.6)

The generation of e− /h+ pairs is the main step in the photodegradation process, as mentioned in Eq. 8.6. It is important to note that the creation of e− /h+ pairs requires sufficient energy to conquer the band gap between VB and CB of the semiconductor photocatalyst. Therefore, this energy must be greater than the bandgap of the TiO2 for e− /h+ to be generated. Thus, oxidation–reduction reactions take place on the semiconductor surface as shown in Eq. 8.7 (oxidative reaction) and Eq. 8.8 (reductive reaction): OH− + h+ → OH·

(8.7)

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These e− /h+ pairs mainly recombine to produce heat. Nevertheless, in the presence of air/oxygen, the photo-generated electrons are trapped and produce superoxide ions − ) as presented in Eq. 8.8: (O2ads O2ads + e− → O− 2ads .

(8.8)

In the process of converting organic compounds, oxygen from air prevents the recombination of electron–hole pairs, and organic matter (such as formaldehyde: HCHO) can be converted to carbon dioxide and water vapor, as follows [22, 23]: TiO2 + hv → e− + h+ .

(8.9)

HCHO + H2 O + 2h+ → HCOOH + 2H+

(8.10)

HCOOH + 2h+ → CO2 + 2H+

(8.11)

O2 + 4e− + 4H+ → H2 O

(8.12)

Oxidation:

Reduction:

As per Eqs. 8.9–8.12, formaldehyde forms an intermediate of HCOOH (formic acid), which eventually is oxidized to CO2 and water vapor. Understanding the reaction kinetics of pollutant decomposition is required for practical applications, such as full-scale photocatalytic reactor design. Therefore, researchers have investigated the dependency of different experimental parameters such as light intensity, oxygen concentration, pollutant concentration, pH, and temperature on photocatalysis reaction rate [4, 22, 23]. In photocatalytic reaction, the reaction kinetics follows one or several of the following five steps: (i) mass transfer of reactants from bulk to photocatalyst surface, (ii) adsorption of reactant to photocatalyst surface, (iii) photocatalytic reaction forming intermediates and products, (iv) desorption of reactant and products from the photocatalyst surface, and (v) mass transfer of products from the photocatalyst surface to bulk [33]:. The L–H model is generally applied in quantifying and analyzing the photocatalytic reaction rate. There were two main assumptions in the L–H model: (i) Each molecule would have equal heat of adsorption, and (ii) each molecule will return to the gas phase after colliding with another adsorbed molecule [33]. The adsorption constant is typically achieved from the dark adsorption reaction of the photocatalyst. However, several studies indicated the inaccuracy in those values from kinetic data [30, 33]. Therefore, the reaction rate constant and adsorption constants are determined from experimentally measured kinetic data from the photocatalytic reaction. Extended L–H model reaction can determine the photocatalytic reaction rate of VOC

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mixtures using rate law [33]. ri = (∅i K i Ci )/(1 + K 1 C1 + K 2 C2 + . . .)

(8.13)

where ∅i = rate constant for component i, K i and Ci = adsorption and concentration of i component in the mixture, respectively. Since reaction kinetics an important for the design of photocatalytic reactors, researchers widely studied it [22, 23, 26, 30, 31, 33]. These experiments showed that the L–H kinetic model provides sufficient laboratory data for the design estimation of photocatalytic reactors. Furthermore, experimentally determined kinetic parameters can also model and predict the required size in larger-scale reactors in many studies [44]. Additionally, extensive models were developed by researchers to optimize operating conditions for a system to obtain specific or desired outcomes (i.e., cost, shorter time) [30]. Many intermediates are formed during the photocatalyst reaction. A few of these intermediates will poison some active sites and reduce the efficacy of the photocatalyst. Moreover, the intermediates can be more toxic and should be removed or oxidized to carbon dioxide. Therefore, research should emphasize intermediate detection and their re-adsorption and oxidation. Researchers could also work on modeling photocatalytic reactor performance to be used on a pilot scale.

8.4.2 Photocatalytic Reactors for the Treatment of VOCs This section reviews the current photocatalytic reactors used in research and industrial applications. Reactor design aims to attain a high reaction yield by removing as many pollutants as possible while consuming the minimum energy. Therefore, the reactor has to provide sufficient residence time among catalysts, reactants, and photons [48]. The necessary criteria of an effective photocatalyst are high catalyst surface area to allow contact with as large a volume of reactants as possible. The powder form of TiO2 could provide a vast surface area. However, the immobilization process onto a substrate results in a large surface area reduction of TiO2 . Therefore, practical design is crucial to increase the surface area while using immobilized TiO2 and confirm it is adequately irradiated. The immobilization method also requires careful consideration to coat the substrate surface adequately. In the case of commercial application, one should consider coating thickness, coverage, robustness, simplicity, cost, and repeatability [43, 48]. The light source would be the most expensive part of the photoreactor unless we use any renewable light source such as sunlight. As the incident light (i.e., photons) contributes significantly to operating costs, it is necessary to use them effectively and ensure that few photons are wasted that do not contact the photocatalyst and start oxidation. Moreover, the reactor should be designed efficiently so that reactor is fully illuminated from the light source, and there should not be any unilluminated photocatalyst [48, 51]. Photocatalytic reactors could be categorized according to their

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configurations. Photoreactor design and operations depend on gas flow rate, gas flow behavior (laminar or turbulent), gas residence time, and the distance between the light source and target compounds. Some major photoreactors are flat plate, honeycomb monolith, membrane or fiber reactors, and fluidized-bed reactors [52].

8.4.2.1

Flat Plate Reactor

The flat plate consists of two flat glass plates with a gap between the plates. The photocatalyst is usually coated onto the interior surface of each plate, and an external light source (usually UV lamps with reflectors) will be used as a source of energy to activate the catalyst. The photocatalyst is immobilized as a thin layer on the surface to allow the entire catalyst to absorb the light. The fluid passes through the gap between the plates where it is treated. The flat plate reactor can obtain a significant convective mass transfer rate and reaction rate. Flat plate reactors are continuous flow reactors that usually offer low catalyst surface area and thus poorly illuminated catalysts [48, 53]. Details of the experimental photoreactor are shown in Fig. 8.2. Radiation intensities at the irradiated reactor wall (x–z plane at y = 0). Radiation intensities at the opposite reactor wall (x–z plane at y = H R ), and at the two lateral walls corresponding to x–y planes at z = 0 and z = W R .

8.4.2.2

Honeycomb Monolith Reactor

This configuration contains several channels with internal dimensions of around 1 mm (Fig. 8.3). A tiny layer of photocatalyst is coated onto the walls of the channels. The channels can have various monolith formations (square channeled monoliths of different dimensions, porous cylindrical ceramic monoliths, etc.). The UV light is parallel to the reaction area, which causes a low reaction rate even if the mass transfer and reaction area is high. This type of design benefits a low-pressure drop and a high surface area to volume ratio [31, 48].

8.4.2.3

Fluidized-Bed Reactor

A fluidized-bed reactor (FBR) can be used to perform many forms of chemical reactions (Fig. 8.4). For example, in an FBR reactor, a fluid (liquid or gas) is passed through a solid granular (usually catalyst or coated with catalysts) at high speed to suspend the solids, which causes it to behave like a fluid. FBR has a uniform distribution of fluid in the bed that many inlet nozzles can supply at the base of the reactor. Furthermore, a light source could be placed at the center of the catalyst bed. If a protective surface is created around the reactor, this setup will minimize energy loss to the surrounding [33]. One of the main benefits of FBRs is that the system can be operated continuously. Additionally, the unit has a low-pressure drop, high

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Fig. 8.2 Flat plate photoreactor: a front view; b side view. (1) Reaction space, (2) fluid in, (3) fluid out, (4) plate of radiation entrance, (5) UV lamp, and (6) parabolic reflector (Reprint with permission from Brandi et al. [53])

Fig. 8.3 Honeycomb monolith photoreactor [54]

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Fig. 8.4 Fluidized-bed photoreactor (Reprint with permission from Zhao and Yang [33])

throughput, and high catalyst surface area, providing an efficient reactant and catalyst contact time.

8.4.2.4

Membrane or Fiber Reactors

These reactors usually consist of a UV lamp surrounded by several layers of glass fiber coated with TiO2 . A polymer membrane (with different polymer material, pore sizes, and thickness) or stainless-steel mesh can also be used instead of glass fiber in such reactors. However, immobilized TiO2 onto fiberglass cloth was more stable than immobilized TiO2 onto stainless steel or polymer membranes [48].

8.5 Photocatalytic Disinfection of Different Airborne Pathogens Matsunaga et al. [55] first showed the antimicrobial activity of UVA-activated TiO2 . Later, Sjogren and Sierka [56] demonstrated the viral inactivation capability of TiO2 photocatalyst, which widely recognized heterogeneous photocatalysis as a superior technology in viral disinfection. The structure of microorganisms is complex and very different from the organic molecules. Usually, microbes are protected by

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membranes or layers made of organic molecules of variable thickness. Moreover, microbes possess protection and recovery mechanisms that help them withstand the oxidative stress caused by photocatalysts [57, 58]. Photocatalysis could destroy many microorganisms, including Gram-negative and Gram-positive bacteria, fungi, algae, protozoa, and viruses [59]. Gram-negative bacteria have a triple-layer cell wall with an inner membrane, a thin peptidoglycan layer, and an outer membrane. In contrast, Gram-positive bacteria have a thicker peptidoglycan layer and no outer membrane. Gram-positive bacteria were more resistant to photocatalytic disinfection than Gram-negative bacteria [60, 61]. Photocatalyst could disinfect viruses, whereas Fungi, algae, and protozoa are susceptible to photocatalytic disinfection [62]. Escherichia coli IM303 and K12 ATCC10798 organisms could be disinfected by TiO2 coated air filter and Degussa P25 coated glass fiber air filter, respectively [63, 64]. Gram-negative bacteria such as Legionella pneumophila CCRC 16,084 can be disinfected by UV-C light in a TiO2 air filter [62, 65]. Lin and Li [66] reported the disinfection of a fungi, Penicillium citrinum, with TiO2 coated air filter. Nakano et al. [67] used UVA irradiation in the presence of TiO2 thin film to disinfect the influenza virus. Under partial UVA intensity, a 4-log reduction of the influenza virus was observed. Furthermore, the author showed that the TiO2 thin film could be applied to disinfect the influenza virus in the air. Therefore, TiO2 could be an efficient photocatalyst for the disinfection of other airborne viruses, and it can be used to impede viral transition through the air [67, 68]. Choi and Cho [69] investigated metal (Mg, Fe, and Mn) deposited TiO2 photocatalyst for the disinfection of influenza virus H1N1 under visible light (λ > 410 nm) and achieved 2-log reduction within 30 min of irradiation [68].

8.5.1 Reaction Mechanism and Kinetics for Airborne Pathogen Disinfection 8.5.1.1

Reaction Mechanism

Different researchers explain the photocatalytic inactivation mechanism of microorganisms (Fig. 8.5). There are three possible photocatalytic viral disinfection mechanisms: (i) physical damage of viruses, (ii) metal ion toxicity obtained from metalincluding photocatalysts, and (iii) chemical oxidation by reactive oxygen species (ROS) generated over the photocatalysts [68, 70]. Chemical oxidation initiated by ROS is the principal mechanism of virus disinfection by photocatalysts. The disinfection mechanism includes the decomposition of the cell wall and the cytoplasmic membrane because of ROS generation such as ·OH and H2 O2 [68, 71]. Direct oxidation of cell components can happen when cells directly contact the photocatalyst. Hydroxyl radicals and H2 O2 are engaged close to and distant from the catalyst. Furthermore, ·OH can be generated from the reduction of metal ions by H2 O2 [62, 73]. Matsunaga et al. [55] proposed that ROS generated in the light-induced

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Fig. 8.5 Photocatalytic inactivation mechanism of microorganisms (Reprinted with permission of Podporska-Carroll et al. [72])

inactivation process degrades Coenzyme A, which is the mechanism for bactericidal activity. Saito et al. [74] used transmission electron microscopy (TEM) to establish that cell demise was led by damage to the cell wall membrane. Manes et al. [75] observed that the peroxidation of the phospholipid component of the cellular wall due to the potential ROS attack leads to its disruption [41]. Most researchers have established that cell membrane damage is responsible for disinfection [62, 76]. Almost 99% of the cells are made of organic molecules (DNA, RNA, amino acids, lipids, protein, etc.) and could be oxidized with photocatalysis [77]. ROS, such as superoxide radical, hydroperoxide radical, and hydroxyl radical, disintegrate the microbial cell wall [72, 78]. ROS could damage both extracellular and intracellular target sites. ROS mainly attacks the sulfhydryl bonds and nucleotides of cell membranes, lipids, and DNA [79]. The hydroxyl radicals readily attack lipids and initiate a catalytic chain reaction to damage the pathogen’s unsaturated fatty acid and cell membrane. However, with a change in fatty acid composition and a change in the structural arrangements of lipid bilayer or cross-linked peptide chains, the ROS’s effectiveness differs [77, 80]. Again, photocatalyst particles larger than 300 nm impede the diffusion of ROS within the cell [70, 81]. Cell damage caused by ROS through oxidative stress can generate a repair response in microbial cells. In such a process, microbes restore stability within their cells by passing electrons from one species to another [72]. Adequate ROS can break down polypeptide chains by changing the molecular structure of amino acids through modification of their charge. Furthermore, ROS, including ·OH, can modify microbial morphology [82]. Humidity and morphology have crucial roles in the disinfection of viruses. AOP significantly damages enveloped viruses in humid conditions. However, non-enveloped viruses are more resistant to damage under humid conditions compared with a dry environment [57, 79]. The antibacterial mechanism of the Ag–ZnO composite was studied by Matai et al. [83]. They also outlined the plausible routes of inactivation, as mentioned in Fig. 8.6.

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Fig. 8.6 Antibacterial mechanism of Ag–ZnO composite (Reprinted with permission of Matai et al. [83])

The first route illustrated the direct interaction of the composite with the bacterial cell. Surface oxidation could be the reason behind such a process that would dissolve the Zn and Ag ions. Additionally, electrostatic interaction may come in direct contact with the bacterial cell wall. Secondly, the bacterial cell wall rupture could occur by the reaction with ROS or ions. Inhibition or alteration of the DNA replication by interacting ROS or ions with sugar-phosphate groups affecting gene alteration is the third approach to inactivation. This process alters the protein expression that is accountable for cellular operation. Moreover, the membrane disruption releases intracellular materials that eventually start the cell lysis [41, 83].

8.5.1.2

Reaction Kinetics

The photocatalytic disinfection kinetics of microorganisms is governed by several factors such as (i) various microorganism strains, (ii) different target organs inside their body, and (iii) mineralization of the target component. Moreover, the disinfection process incorporates a few other factors such as different photocatalyst dosage, irradiation intensity, solution pH, turbidity, temperature, and the structure of the microorganism [84–87]. To effectively design a photocatalytic disinfection system, accurate kinetic information is necessary. Incorrect performance evaluation of the reactor may result in inadequate disinfection and could not achieve the regulatory limit. Therefore, a robust mechanistic model is required to establish the most efficient contact time, catalyst dosage, and irradiation time [77]. There are four common kinetic models such as (1) Chick’s model, (2) Chick Watson model, (3) Delayed Chick-Watson model, and (4) Hom’s model. The general expression of the differential rate law used by these kinetic models is expressed as follows:

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dN = K m N x C n t m−1 dt

(8.14)

is the rate of inactivation, N is the number of survivors at contact time t, where dN dt K is the rate constant, C is disinfectant concentration, and m, n, and x are empirical constants [41]. However, these models are not applicable for heterogeneous photocatalysis as those were developed based on a homogenous chemical reaction. Therefore, the different constants and fitting parameters developed often have no real meaning in the actual processes being modeled [77]. Instead, mechanistic models are often favored over empirical models, although they are more cumbersome from a computational point of view. Some mechanistic models are the microbe–catalyst interaction, adsorption, collision, lipid peroxidation, and series-event models. They are supposed to be more robust than empirical models. They are well fitted for application to complex situations such as changes in radiation source (UV lamps or solar light), the addition of other chemicals, and variation in water quality or source [77]. Integrating biological parameters into the model might be beneficial in differentiating the sensitivity of different organisms. Knowledge of the fundamental reactions during disinfection will be crucial in advancing and modeling the process [41, 77].

8.5.2 Photocatalytic Reactors for the Treatment of Airborne Pathogens Photocatalytic reactor systems can be categorized according to their design. The most popular reactor types used in air purification systems are fluidized-bed reactors, annular, plate, and honeycomb monolith reactors, as described in Sect. 8.4.2. This section will present a few nonconventional photoreactors used to inactivate microorganisms. Pal et al. [63] studied TiO2 -mediated inactivation of E. coli K-12 (ATCC 10,798) (Gram-negative bacteria) in a continuous annular reactor. The inactivation rate of E. coli K-12 improved with increased UV intensity, TiO2 loading, and relative humidity. A UV-A dose of 30–204 mJ/cm2 at an average UV-A intensity of 0.5–3.4 mW/cm2 , at a residence time of 1.1 min, was adequate for complete inactivation of E. coli K-12 continuously passing through the reactor. Lu et al. [88] developed a continuous flow-through reactor loaded with the photocatalyst coated onto polyurethane foam to inactivate airborne bacteria. The photoreactor is a cylinder-shaped double-layer quartz casing with dimensions of 7 cm diameter × 35 cm length. They conduct experiments under polyurethane foam-only and UV (254 nm)-only conditions to test the filtration of polyurethane foam and the inactivation of UV irradiation. The bioaerosols containing bacteria went through the photoreactor from bottom to top and were then collected and quantified. The inactivation efficiency of airborne E. coli by UV-only treatment was 2.5 lg order. At the

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same time, the photocatalytic process accomplished 3.4 lg order under UV irradiation nm under the same operating condition (95% relative humidity and retention time of 4.27 s) [88]. Shiraki et al. [89] built a UV-LED-based photocatalytic air cleaner with a TiO2 coated aluminum plate system as a photocatalyst for the inactivation of aerosolassociated influenza virus for indoor air. Their photocatalytic system successfully disinfected the aerosol-associated influenza virus, demonstrating that it could detoxify and clean the air in a confined area [68]. Doss et al. [90] designed a LED (392 nm) flow-through structured photocatalytic system consisting of TiO2 /β-SiC solid alveolar foams for air purification treating airborne viruses such as T2 bacteriophage. The photoreactor was engineered to include 40 or 56 LEDs and a 65 × 40 mm (d × h) disk-shaped TiO2 /β-SiC foam photocatalyst with a mean cell size of 5440 µm. The photocatalytic reactor also consists of an inner fan functioning at 10 m3 /h and the printed circuit boards for LEDs. The TiO2 /β-SiC foam photocatalyst disk occupied a total volume of 133 cm3 , so the tests were performed with a residence time of 0.048 s. TiO2 /b-SiC solid alveolar foams showed photocatalytic activity and filtration effect in deactivating this virus. However, only the filtration effect corresponded to a 1 log reduction, with an apparent time constant of 43.1 min. In contrast, the photocatalytic filtration process showed (with 56 LEDs, 60 min of run time) a 3-log reduction with an apparent time constant of 11.0 min after correcting for the natural decay of the bioaerosol. They also demonstrated the positive effect of LED intensity by increasing the number of LEDs (40 to 56 LEDs) on the logarithmic reduction. These results proved the potential application of LEDs and solid alveolar foam as an impressive and energy-saving technique to decontaminate viruses through photocatalysis [68, 90]. Kim and Jang [91] investigated the disinfection of airborne virus MS2 by vacuumUV (VUV, λ ≤ 200 nm) within a limited irradiation time (0.004–0.125 s). The photoreactor consists of a hollow cylinder made of stainless steel with an inner diameter of 28 mm and a height of 210 mm. A sheet of either TiO2 or Pd-deposited TiO2 , with a surface area of 150 mm × 90 mm, was fixed to the inner wall of the cylinder. A low-pressure VUV mercury lamp with a significant emission at 254 nm and a minor emission at 185 nm was placed along the center of the reactor. The flowthrough air disinfection system used Pd-TiO2 photocatalyst with VUV to achieve MS2 disinfection (1-log reduction) and photogenerated ozone elimination. The gasphase reaction in VUV light includes light of 254 nm wavelength, the light of 185 nm wavelength, ozone oxidation, water ionization, and successive attack of reactive oxidant species [22, 23]. Compared with UV photolysis, ozone-only treatment, and ozone is combined with UV photolysis, VUV photolysis treatment demonstrated the greatest deactivation efficiencies for MS2 viruses. This VUV photocatalysis system can provide superior capacity bioaerosol inactivation in HVAC systems, irrespective of the weather condition [68].

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8.6 Reactors Used in Commercial Applications Several photoreactors are described in Sects. 8.4.2 and 8.5.2 to treat VOCs and airborne microorganisms. The flat plate reactor and membrane fiber configurations combined with other air purifying technologies (HEPA filters or ionizers) are primarily found in commercial applications [48]. The main advantage of photocatalytic reactors is converting air pollutants using non-hazardous and environmentally safe materials. Significant research and development are in progress to develop visible light-activated photocatalyst material and improve photocatalyst supports with higher surface area. Therefore, the commercial applicability of photocatalytic air treatment systems would most certainly be a success if the above milestones are achieved. A photocatalytic reactor may be combined with an HVAC system for better indoor air purification performance. A Health-Related Index (HRI) is typically used to assess the performance of photocatalytic air cleaners [52]. It is defined as follows: HRIi =

Ci RELi

(8.15)

where RELi is the recommended exposure limit of the air pollutants i. HRI is the sum of all HRI. ∑ HRI = HRI (8.16) i

Table 8.1 mentions a list of photocatalytic air purification systems used in industrial and domestic applications.

8.7 Conclusion Air purification for indoor air has become an extreme concern recently because of the increased risk to human health due to high levels of VOCs and biological contaminants. The average concentration of VOC in conventional buildings is usually 5–50 µg/m3 . VOCs, such as toluene, benzene, ethylbenzene, and xylene, are carcinogenic. The airborne microbial fraction of the particulate matter has also worsened the public health crises. Heterogeneous photocatalysis is a promising technology that has been applied to remove VOCs and airborne pathogens. In the photocatalytic reaction, hydroxyl radical is the active species, the second-highest oxidizing species after fluorine. The Langmuir–Hinshelwood model best describes VOC degradation kinetics. In comparison, photocatalytic disinfection kinetics of microorganisms is mainly based on the homogeneous chemical reaction. VOCs are mineralized through photocatalytic oxidation and produce carbon dioxide and water vapor. The disinfection mechanism of microorganisms involves the disintegration of the cell wall because of reactive oxygen species generation (superoxide radical, hydroperoxide radical, and

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Table 8.1 Photocatalytic air purification systems Manufacturer

Product name

Product details

References

AirOasis

iAdaptAir® HEPA UV Air Purifier

Photocatalytic air purifier uses a technology called AHPCO, or advanced hydrated photocatalytic oxidation

[92]

GENESIS AIR

GENESIS AIR Photocatalysis GAPTM

UV + TiO2 -based photocatalysis; the photocatalytic oxidation reaction occurs at the coated surfaces within the air purifier

[93]

Platinum Air Care

RPS 600S 6-Stage Portable Pre-filter + electrostatic [94] Air Purification System precipitator collection Cell + VOC filter + UV + photocatalytic filter + negative ion generator

CASPR Group

CASPR 1000

Natural catalytic converter [95] technology consists of a special UV light and photocatalyst, which utilizes the environment to create an advanced oxidation process naturally

AirPura

AirPura P600-C

Activated carbon, [96] medical-grade HEPA filter, photocatalytic oxidizer for enhanced chemical filtration, UV-C germicidal light for mold, viruses, and bacteria

PureAir air purification PCO3-14-16

Uses photocatalytic [97] oxidation technology to reduce levels of airborne volatile organic compounds, cooking odors, common household odors, airborne dust particles, mold spores, and pollen

Airocide

TiO2 /UVC technology of airocide reduces microorganisms

APS-300

[98]

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hydroxyl radical). The reactive oxygen species mainly attacks the sulfhydryl bonds and nucleotides of cell membranes, lipids, and DNA. The most common reactor types used in air purification systems are fluidized-bed reactors, annular, plate, and honeycomb monolith reactors. The flat plate reactor and membrane fiber configurations combined with other air purifying technologies are also found in commercial applications. UV-A, UV-C, VUV, and UV-LED-based photocatalytic air cleaners with TiO2 -based photocatalyst have demonstrated significant deactivation efficiencies for airborne microorganisms. A lot of impediments need to be addressed in the field of photoreactor development to make the process sustainable and economically viable. Considerably more research in sun light-driven photocatalysis and reactor modeling would be valuable for economical reactor design for indoor air purification.

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Chapter 9

Application of Photocatalysts to Improve Indoor Air Quality and Health: A Sustainable Environmental Approach Deep Chakraborty and Krishnendu Mukhopadhyay

Abstract Almost half of the world’s population is still exposed to indoor air pollution (IAP), caused by solid fuel combustion. Millions of mortality continues to occur as a result of unhealthy indoor air, one of the major causes of mortality as indicated in the current estimates of the global disease burden. Indoor air pollution remediation may be accomplished in two ways: prevention and removal. Various prevention efforts have been made with the steps like changes in cooking fuels, installing improved cookstove (ICS), etc. in household settings, though significant results of remediation are still awaited. ICSs include the use of air cleaning technologies which not only improves combustion efficiency but also saves fuel energy. In recent years, the following air cleaning approaches have been employed: filtering and adsorption, electrostatic air purifying, purifiers and gas adsorption filtration, ozonation, nonthermal plasma and photocatalysis. In addition, the Pradhan Mantri Ujjwala Yojana (PMUY, India) started taking the lead from intervention approaches. Photocatalysis is one among the fastest emerging technology for pollutant treatment, leveraging the mechanism of reactions with photoemissions. Its characteristics, such as low cost and great efficiency, have drawn the attention of researchers all over the world, resulting in many industrial uses and extensive research and development. Now-a-days photocatalysis has been used to remove significant pollutants from the atmosphere by purifying water, and oxidizing a variety of organic compounds. This chapter discusses the qualities of the ideal photocatalysts, as well as current photocatalytic materials for making improvement of air quality and public health. Keywords Photocatalyst · Indoor air pollution · Photocatalytic oxidation · Sustainable public health

D. Chakraborty (B) · K. Mukhopadhyay Department of Environmental Health Engineering, Sri Ramachandra Faculty of Public Health, Sri Ramachandra Institute of Higher Education and Research, Chennai, Tamilnadu 600116, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Garg and A. Chandra (eds.), Photocatalysis for Environmental Remediation and Energy Production, Green Chemistry and Sustainable Technology, https://doi.org/10.1007/978-3-031-27707-8_9

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9.1 Introduction According to the 2015 Global Burden of Disease Report, the projected loss of 2.9 million deaths and 85.6 million lifespans adjusted for the impairment was household air pollution (HAP), making it the eighth-most possible risk factor in the global disease burden. Still, in rural India, more than 75% of women are habituated to cook with biomass fuels for their daily cooking activities. Among both the women and children who have the most intensive exposure shown in previous studies, the association between HAP and respiratory performance is highest. Still IAP is a significant contributor to the worldwide health burden [1, 2]. Despite the fact that smoke exposure from unprocessed biomass (wood, animal dung, agricultural wastes, and grasses) and coal can exert a wide variety of negative health impacts, a large portion of the world’s population continues to use them as their primary cooking fuels. IAP has been an ignored research domain until lately. However, in recent years, there has been a considerable rise in public awareness about IAP, and it has been discovered as a crucial causative agents in a variety of diseases. Volatile organic compounds (VOCs), respirable particulate matter (PM), carbon monoxide (CO), polycyclic aromatic hydrocarbons (PAHs), sulphur dioxides (SOx ), nitrogen oxides (NOx ), and variety of other poisonous inorganic and organic chemicals are among the hazardous air pollutants emitted by biomass burning [3]. Many study have found that due to the exposure of these toxic pollutants health implications like cardiovascular disease, respiratory diseases, birth defects, among women and growth retardation among children are very prominent. Methods of remediation of biomass smoke from the kitchen room have adopted by many researchers like change of cooking fuels, installation of improved cookstove, and development of ventilation index. But using photocatalytic approaches needs to be focussed more to improve indoor air quality. In 1972, Fujishima and Honda [4], first showed interest on photocatalysis. The researchers sought to reproduce photoinduced redox processes by oxidizing water and lowering carbon dioxide using a semiconductor exposed to UV light [5]. Photocatalysis, which uses the mechanism of reaction with the assistance of light, is among the fastest emerging techniques for the management of air pollution (photo emissions). Still, wide array of spaces are there for do more research in this fields. Hence this chapter will portrait about the indoor air qualities, agents of indoor air pollution, and different potentials of photocatalytic procedures to improve air quality and health.

9.2 What is Photocatalyst Photocatalysis can be divided in two parts; photo, means “light” and Catalysis refers to a substance that alters the pace of a chemical process without being affected in the end. Photocatalysis was described by Fujishima et al. [6] as “the catalysis of a photochemical process over a hard surface.” Photoreaction, from the other hand, is often

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regarded as a photo-induced or photo-activated action. Photocatalysis is the study of photoreactions that take place on the surface of a catalyst. A “catalysed photoreaction” begins once the initial photo-excitation event takes place inside an adsorbate molecule that subsequently reacts with the catalyst substrate’s ground state. The photo-excited catalyst, on the other hand, interacts with the ground state adsorbate molecule after the first photo-excitation occurs in the catalytic substrate. The method is known as a “sensitized photoreaction.” In most circumstances, photocatalysis or photoreactions are used interchangeably. As a result, advanced oxidation processes have been used to signify photocatalysis systems based on catalysis and photochemistry. Those that create hydroxyl radicals have demonstrated increasing effectiveness among these methods. UV light may be exploited in a variety of ways, however photolysis can happen when the incident light would be well absorbed by the chemical compounds. These active radicals can also be prepared using a semiconductor which helps to absorbs light ranges in UV as and when it comes into contact with water [7].

9.3 Photocatalytic Materials Used in Air Pollution Research Along with TiO2 , and ZnO the photocatalytic activities are also dominated by compounds like SnO2 , Fe2 O3 , CdS, WO3 , [8]. Among all compounds TiO2 stands supreme when compared to ZnO, CdS, WO3 those having drawbacks likes weak at low pH, disintegration, and less efficient but in case of TiO2 these properties showed positive scenario along with nontoxicity [9]. Ability to oxidize and longstanding photo-stability makes TiO2 a well versatile photo-induce catalyst and now-a-days it often used to oxidize compounds like organic and inorganic in air and water. TiO2 is also a widely used material and both inexpensive and non-toxic [10]. It comes in three different crystalline forms: anatase, rutile, and brookite. With bandgap energies of 3.2 and 3.0 eV, anatase and rutile belong to the big bandgap semiconductors. Anatase is the polymorph phase of an enzyme that has been broadly utilized in photocatalytic depollution techniques. However, combinations of anatase and rutile have been shown to be more photocatalytically active than the pure compounds.

9.4 Pollutants in Indoor Air 9.4.1 Biological Pollutant Indoor dampness has been linked to several respiratory health issues, and microorganisms have been postulated as a possible cause. However, little is known about the exact functions of infectious and noninfectious microorganisms, as well as their components, in illnesses affecting indoor settings. The lack of understanding of the role of

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microorganisms in the development and exacerbation of those diseases is largely due to a lack of reliable quantitative exposure evaluation methods and an understanding of which specific microbial agents are most likely to be responsible for the suspected health effects. The majority of research use questionnaires to estimate exposure, and just a handful have attempted to measure microbe exposure. Fungi are exceedingly common and are thought to be a biological pollutant. High levels of fungus exposure have been linked to a variety of health issues, including eye irritation, skin infections, and lung infections. Pathogenicity is caused by the production of volatile organic compounds by some fungus. Indoor airborne fungus are roughly 20 times more prevalent in the summer than in the winter. The airborne viable spores is influenced by climatological factors like as temperature and relative humidity. Among all the species like Aspergillus sp., Penicillium sp., and Cladosporium sp. are the mostly found inside the building in all seasons. In temperate areas the Cladosporium sp. is the most common fungus followed by Penicillium sp., Alternaria sp., and yeast. Watery wall or wet spots act as a microorganism source because microorganism can strive for their development on it [11].

9.4.2 Chemical Pollutant IAP has been linked to hazardous emissions such as, cigarette smoke compound, pesticides residues, cleaning chemicals, particles, house dust, cotton fibres, and allergens. Human-created environments support millions of fungus, pollen, spores, bacteria, viruses, and insects. Burning emissions and culinary operations emit CO2 , SO2 , CO, NO2 , and PM into indoor air settings. Additionally, office equipment such as monitors, fax machines, printers, and other workplace devices emits O3 and organic substances. Floor mat, plywood, laminate, carpet, paint, varnish, and ply board are examples of common building materials that might emit hazardous chemicals [12].

9.5 Technology Adopted for Remediation of Indoor Air Pollution 9.5.1 Improved Cookstove Distribution and installation of improved cookstove (ICS) for biomass fuel uses have been conducted by many researches [13, 14]. Improve cookstove can be best fitted for the rural villagers who are habituated to continue cooking with traditional biomass fuels. Improve cookstove can reduce the fuel consumption, smoke emissions, and improve the indoor air quality. ICS can reduce more efficiently the particulate emissions than traditional cookstove (TCS). In a long term pilot study, it has been found that ICS has performed better to reduce the all the pollutant namely, CO, CO2 , O3 ,

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NO2 , SO2 , PM2.5, and PM10. Not only it reduced the pollutant concentrations but also the health conditions of the women participation also improved. But maintenance of TCS have some limitations in a long term basis. Furthermore, emission reduction in TCS for organic pollutants are limited. Rather, photocatalytic compounds can easily degrade the organics [13].

9.5.2 Improved Cooking Fuels Improved cooking fuels like Liquid Petroleum Gas (LPG), Biogas, and electricity are one of the best choices that any households can adopt for a sustainable indoor air quality. Emissions form these pollutants upon burning in stoves emits very less concentrations of pollutants which have been demonstrated by many researchers [3, 15, 16]. But, these fuels are economically high in prices and sometimes it is not possible to use by the population who are still belongs to economically weaker section. Combinations of ICS and improve cooking fuels may be one of the best options to take the indoor air quality under control.

9.5.3 Modifications of Ventilation Pattern While considering the impact of indoor pollution, adequate indoor ventilation plays a significant role in reducing exposure, and health effects. The primary target in indoor pollution abatement process is to preserve 21% of oxygen content in terms of air volume followed by checking the removal of carbon monoxide, maintaining acceptable temperature and humidity, and minimizing other recognized hazards in terms of particulates and gases. Therefore, the comfort index relies upon the perfect ventilation with required air volume, temperature, relative humidity, air velocity, air exchange rate, etc. Households’ room index, cooking fuel, cooking behaviour, and other hazardous sources are also playing important role to assess indoor air quality. Based on literature data published recently, it has been observed that the modification of ventilation pattern would be an amicable solution at workplaces as well as in domestic indoor environments [17–20]. Building ventilation has three basic elements like ventilation rate, air-flow direction and air distribution pattern and to calculate the air change per hour (ACH) of a building, it is recommend to the standard set by ASHRAE minimum limit of 0.35 ACH. This limit determine safety and acceptable indoor infrastructure. However, the existence of fresh air matters at last. In this continuum, another technology called photocatalytic oxidation (PCO) is proposed to be a model air purification technique which may have potential to remediate toxic air components into a benign form. A comprehensive strategy is necessary to build photocatalytic air filtration systems to integrate the system with design, structure, morphology, and other requisite conditions and technologies [21].

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It is evidenced that the use of photocatalysts has opened a new dimension in mitigating hazardous pollutants in the area of exposure risk and health. In recent past, a method was reported on the development and performance evaluation of active and passive photocatalytic oxidation (PCO) systems under real circumstances using multi-compound volatile organics (VOC) at less than 10 ppb concentration and in a short contact times (https://indoor.lbl.gov/ventilation-and-air-cleaning). The use of PCO technology is a safe and effective way to improve indoor air quality. For an example, air purifiers with PRO-Cell convert ozone into safe, healthy oxygen when a number of other market available air purification systems produce harmful ozone.

9.5.4 Ozonation Ozone is a known powerful oxidizing agent and is being used extensively in waste water purification system. Ozonation is a form of chemical oxidation process that produces highly reactive oxygen species capable of attacking a broad array of chemical substances as well as all microorganisms. In such a pre-treatment procedure, ozonation can really be adjusted in a range of methods to transform persistent and bioresistant organic molecules into bioavailable precursors, which are then processed by biological treatment methods. The combination of ozonation with photocatalytic oxidation, and that is the introduction of irradiation to a semiconductor throughout the aerobic conditions or air, is known as photocatalytic induce ozonation. The first published work on the removal of pollutants from water began this procedure in 1978. Photocatalytic ozonation remains one of the most costly treatment processes, and its application for removing biodegradable contaminants from water is not cost-effective. This oxidation approach is very useful for eliminating weakly biodegradable organic substances or enhancing the biodegradation of waste waters carrying these chemicals [22]. However beneficial implications in the process of ozonation of volatile organic compounds (VOCs) in indoor air environment are still limited. Moreover, ozone itself is a potential health hazard which may deteriorate human lung function and respiratory health. Therefore, it is necessary to conduct a detailed and comprehensive study on the performance of ozonation air cleaner system. A study conducted by Chen et al. [23] indicated that ozone generators showed limited control potentials for the application in occupied spaces when high ozone concentrations exceeded the public health standards set by OSHA/NIOSH (100 ppb). Another study was also conducted by producing innovative ozonation duct system with ozone lamp as a source to evaluate the ozonation performance under the desired conditions [24].

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9.5.5 Adsorption The degradation of gaseous pollutants consists of two steps in respect of photodegradation, the first, adsorption onto the catalysts’ surfaces; and the second photodegradation by electron–hole pair’s radicals (· OH and O−2 ) under the irradiation of UV/visible light. Both adsorption capacity and photocatalytic activity are critical in indoor air filtration. The relative importance of these two components are heavily influenced by actual situations such as concentrations of pollutants, characterization, and retention period. A short retention time is an important design parameter for organic pollutant elimination in catalyst-based indoor air filtration systems. A research found that the concentration and residency duration for indoor air purification corresponded to realistic circumstances. Because the C=O group of carbonyls has a strong attraction for TiO2 , they are quickly degraded by breaking the feeble C–C bond. Henry’s law constant is typically used to measure the percentage removal of indoor pollutants above a catalyst. As a result, the produced microporous TiO2 may function well in the elimination of carbonyls containing Henry’s law constants equivalent to or greater than cyclohexanone [25]. Pollutant adsorption on the surface of the photocatalyst is indeed a necessary stage that considerably influences the reaction rate and removing effectiveness. Adsorption of challenge compounds on photocatalytic activity improves interaction among photocatalyst and reactant molecules, resulting in a faster oxidation rate. Because of the struggle between VOC and water molecules for adsorption sites, the importance of the adsorption stage is especially crucial when handling with gas streams having high moisture levels (common in house mechanical hvac systems).

9.5.6 Filtration Adequate ventilation, air cleaning, and filtration would help people as we spend a lot of time indoors. Air cleaning devices are installed to remove air pollutants from indoor air environment. Ethically, nothing should be vented with mechanical device but needs to incorporate filters in the process of exhausting the air out. Periodic change and disposing filters is another compliance that needs to be maintained. By passing through filters, air pollutants are trapped in the filter medium. Carbon and fibreglass are two examples of common filters. However, high efficiency particulate air filter (HEPA), minimum efficiency reporting value (MERV) rated filters, fibre glass, etc. are proven to be highly effective in different working environments. HEPA filters are highly effective in removing animal hair, but to remove air-mould or bacteria HEPA combined with ultra violet light are observed to be more effective. A HEPA-activated carbon combination is better for asthma patients as it removes allergen or odour from air and removes smokes efficiently. However, substantial pressure dips in HEPA filtration systems can lead to high energy demand

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and environmental noise, necessitating the extensive usage of pumping devices [26]. To address issues such as pressure drop and noise, another class of TiO2 -treated UV LED activated charcoal filters was evolved, in which antibacterial property on bacteria was discovered to be mechanically trapped on the filters and demonstrated efficient reduction of total suspended particulates and VOCs [27].

9.5.7 Photocatalytic Oxidation and Removal of Organic Compounds Photocatalytic oxidation (PCO) is a very powerful air cleaning technology that removes gaseous pollutants as low as ppb level concentrations. It can be operated in room temperature, active towards a range of air contaminants and releases benign final products like carbon dioxide and water. When HEPA filters are able to filter particles minimum upto 0.3 micron size, PCO process shows higher efficiency, and can destroy particles as small as 0.001 microns. To address issues such as pressure drop and noise, another class of TiO2 -treated UV LED activated charcoal filters was evolved, in which antibacterial property on bacteria was discovered to be mechanically trapped on the filters and demonstrated efficient reduction of total suspended particulates and VOCs (https://indoor.lbl.gov/ventilation-and-air-cleaning). Among a number of available photocatalysts like CeO2 , ZnO, TiO2 , ZrO2 , SnO2 , etc., TiO2 has been proved to be most efficient in respect of photocatalysis, stability, and all other suitable properties [28]. For treating large volumes of gas there can be problems of loss of TiO2 while used as a thin supported layer, but the use of composite incorporated bodies with major TiO2 contribution strength may be increased with abrasion resistance. The degradation of alkane begins with proton removal to generate alkyl radical that subsequently interacts with oxygen to make alkyl proxy radical. This alkyl surrogate radical could create carboxylic acid during interactions with alkane or HO· 2 [29]. Alcohols and natural acids, according to Nimlos et al. [30], can sometimes be adsorbed across the TiO2 surface through both dissociative adsorption at oxygen connecting sites and hydrogen bonding to the Hydroxyl group, however aldehydes will only be adsorbed via hydrogen bonding to the surface of Hydroxyl group. Using an FTIR in situ reactor, Hauchecorne et al. [31] developed a very difficult chemical route for PCO of acetaldehyde against TiO2 . Acetaldehyde is adsorbed on the surface of TiO2 , where it is subjected to aldol condensing (the production of 3-hydroxybutanal and crotonaldehyde) and oxidation processes. After additional illumination, those originally generated species are transformed into various intermediary such as acetic acid, formic acid, and formaldehyde. Majority of the VOCs are toxic in nature and pose threat to the human. To remove these compounds, various methods like filtration, scrubbing, adsorption, and absorption are used. However, efficient removal from air matrices is a big challenge. Photocatalytic oxidation

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method emerged as an efficient VOC removal system in air matrices. Contaminants are eliminated by a reaction catalysed that convert contaminants into CO2 and H2 O with the assistance of such a suitable photocatalyst and light source in the photocatalytic-induced oxidation process [32].

9.6 Photocatalysis with Ozone Since it absorbs ultraviolet (UV) radiation and blocks off all other damaging wavelengths, O3 in the ozonosphere aids the ecosystem for protection. A high proportion of ozone near the earth’s surface, on the other hand, does have an influence on public health and it may lead to financial damages, including such degradation to rubber and plastic items. When ozone concentrations surpass 0.1 ppm, this could induce migraines, throat irritation, and pulmonary injury in people. Scanners and laser printers generate ozone in controlled office areas, with maximum concentrations approaching 2000 ppm. As a result, safeguarding global health against rising ozone exposure levels is a top priority. To create composite materials, noble and transition metal oxides could be based on MnO2 , Fe2 O3 , Ag2 O, NiO, and other p-type oxide semiconductors. Those compound catalysts destroy gaseous ozone efficiently. At room temperature, activated carbon fibres (ACFs), and activated carbon granules are promising tools for decomposing ozone. However, given the high oxidation potential of ozone, that irreversibly damages the properties of activated carbon, the prospect of utilized activated carbon is low. Although frequent replacement of activated carbon can assure lengthy ozone dissolution rate, the everyday maintenance required is cumbersome. Ozone degradation efficiency can be improved by treating the surface with silver (Ag) and platinum, as was recently revealed for the photocatalytic activity of TiO2 (Pt). According to a number of research, Pt/TiO2 has superior photocatalytic activity than Ag/TiO2 . However, Ag is more cost-effective than the other noble metals, therefore using it attached to TiO2 for improving photocatalysis has this benefit. Ohtani et al. investigated the ozone degrading catalytic and photocatalytic activities of Ag attached onto various kinds of TiO2 . According to their findings, ozone degradation was more effective when more Ag was placed onto the TiO2 . However, a lack of bioactive substances might lead to a false assessment of the photocatalytic performance. On the other hand, a substantial increase in Ag causes Ag agglomerations to develop on the TiO2 surface. The Ag nanoparticles’ capacity for nanocatalysis is reduced by these aggregated Ag particles. The effectiveness of nano-Ag/TiO2 in eliminating pollutants by catalysis and photocatalysis is inhibited by both of the above-mentioned events. The effort involved the synthesis of a nano-Ag/TiO2 catalyst for ozone degradation that contained silver nanoparticles with controllable sizes. The amount of reactant utilized in a photoreduction approach was used to regulate the diffusion and diameters of the active silver nanoparticles. Additionally, all dark and UVA (365 nm) light

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irradiation have been used to examine the process and effectiveness of the nanoAg/TiO2 catalyst for gaseous ozone destruction. The nano-Ag/TiO2 catalyst shown high performance for ozone destruction in the influence of ultraviolet (UV) excitation, at 98–99% for all studies. However, after 160 min, the effectiveness of pure TiO2 for ozone degradation significantly decreased and was no longer effective. This is due to the fact that ozone damaged TiO2 ’s surface and created an oxidizing layer; as a result, the photocatalytic activity of the reoxidized TiO2 for ozone breakdown was significantly diminished. The outcomes also showed that even without UV irradiation, the nano-Ag/TiO2 catalyst was capable of degrading ozone. By lowering the sizes and successfully spreading the silver nanoparticles, the photocatalytic activity of nano-Ag/TiO2 for ozone breakdown was improved [33].

9.7 Photocatalysis with ZnO Because of its broad bandgap, ZnO has possibilities as a photocatalyst element. It’s also significant due to its UV absorption and strong photosensitivity that triggers the breakdown of numerous contaminants. According to Shakti and Gupta [34], zinc oxide is an N-type semiconductor with a significant charge carrier binding energy of 60 meV and a broad bandgap of 3.37 eV. Because ZnO nanostructures are inexpensive, non-toxic, and more effective at absorbing a significant portion of the solar radiation than TiO2 , they have been demonstrated to be notable photocatalyst prospects for application in photocatalytic degradation. An alternate photocatalyst to TiO2 has been found to be ZnO, an ntype semiconductor oxide. ZnO does have the same bandgap energy as TiO2 but demonstrates greater absorptivity throughout a significant portion of the spectral region [35]. Associated with a higher level light harvesting effectiveness, Yun et al. [36] demonstrated that greater loading of organic molecules could be immobilized on Al-doped ZnO comparing to pure ZnO. According to reports, bandgap energy is a key aspect in deciding how photoactive ZnO will be in a specific application. It may be projected that ZnO’s performance as a photocatalyst in photocatalytic degradation applications may be increased by methods such metal/non-metal doping, linking ZnO with other semiconductors, and linking with nanocarbon. By changing the bandgap, reducing the rate of electron–hole pair conjugation, enhancing charge separation efficiency, enhancing the productivity of hydroxyl radicals, generating tiny particles with such a high surface areas, and facilitates distribution in a medium, such methods will improve their performance.

9.8 Conclusion It is well evident from the above discussion that photocatalytic remediation approach may be adopted in future to get better indoor air quality. A vast array of pollutant can

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be easily remediate by various photo activation processes. Apart from the installation of ICS and high price cooking fuels it can be a better alternative for rural as well as urban residents to achieve a sustainable health. But adaptation such techniques needs more stringent research and collaborations of various stakeholders. Moreover, long term pilot scale study needed to establish its potentials in various sectors. Acknowledgment Authors are thankful to the Indian Council of Medical Research (ICMR, Govt. of India) for the award of Research Associate Fellowship (Award No. 3/1/2(3)/Env./2020-NCD-II, dated 18.01.2021) to Dr. Deep Chakraborty for conducting this research.

References 1. Chakraborty D, Mukhopadhyay K, Mitra P, Mondal NK (2022) Optimization of household ventilation with improved cookstove: an amicable approach to strengthen indoor air quality and public health. IOS Press. https://doi.org/10.3233/AISE220008 2. Mukhopadhyay K, Chakraborty D, Natarajan S, Sambandam S, Balakrishnan K (2022) Monitoring of polycyclic aromatic hydrocarbons emitted from kerosene fuel burning and assessment of health risks among women in selected rural and urban households of South India. Environ Geochem Health. https://doi.org/10.1007/s10653-022-01276-y 3. Chakraborty D, Mondal NK, Datta JK (2014) Indoor pollution from solid biomass fuel and rural health damage: a micro-environmental study in rural area of Burdwan, West Bengal. Int J Sustain Built Environ 3:262–271 4. Fujishima A, Honda K (1972) Electrochemical photolysis of water at a semiconductor electrode. Nature 238:37–38 5. Fujishima A, Zhang XT (2006) Titanium dioxide photocatalysis: present situation and future approaches. Build Environ 9(5–6):750–760 6. Fujishima A, Zhang XT, Tryk DA (2008) TiO2 photocatalysis and related surface phenomena. Surf Sci Rep 63(12):515–582 7. Ranjit KN, Zain MFM, Jamil M (2016) An environment-friendly solution for indoor air purification by using renewable photocatalysts in concrete: a review. Renew Sustain Energy Rev 62:1184–1194 8. Vinu R, Madras G (2010) Environmental remediation by photocatalysis. J Indian Inst Sci 90(2):189–230 9. Litter MI (1999) Heterogeneous photocatalysis: transition metal ions in photocatalytic systems. Appl Catal B Environ 23(2–3):89–114 10. Sundar et al (2020) Chapter 9: Photocatalysts for indoor air pollution: a brief review. In: Green photocatalysts for energy and environmental process, environmental chemistry for a sustainable world, vol 36. https://doi.org/10.1007/978-3-030-17638-9_9 11. Chaudhuri et al (2020) Development of health risk rating scale for indoor airborne fungal exposure. Arch Environ Occu Health. https://doi.org/10.1080/19338244.2019.1676187 12. Tran VV, Park D, Lee YC (2020) Indoor air pollution, related human diseases, and recent trends in the control and improvement of indoor air quality. Int J Environ Res Public Health 17(8):2927 13. Chakraborty D, Mondal NK (2021) Reduction in household air pollution and associated health risk: a pilot study with an improved cookstove in rural households. Clean Techn Environ Policy 23:1993–2009 14. Chakraborty D, Mondal NK (2021) Estimation of nitrogen dioxide (NO2 ) due to burning of household biomass fuel and assessment of health risk among women in rural West Bengal. Curr World Environ 45–52. https://doi.org/10.12944/CWE.16.SpecialIssue1.04 15. Chakraborty D, Mondal NK (2017) Assessment of health risk of children from traditional biomass burning in rural households. Expo Health 10:15–26

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16. Chakraborty D, Mondal NK (2018) Hypertensive and toxicological health risk among women exposed to biomass smoke: a rural Indian scenario. Ecotoxicol Environ Saf 161:706–714 17. Chakraborty D, Mondal NK (2022) Optimization of rural indoor kitchen structure and minimizing the pollution load: a sustainable environmental modeling approach. In: Cognitive data models for sustainable environment, pp 181–202. https://doi.org/10.1016/B978-0-12-8240380.00011-0. ISBN: 978-0-12-824038-0 18. Mukhopadhyay K, Ramasamy R, Mukhopadhyay B, Ghosh S (2014) Use of ventilation-index in the development of exposure model for indoor air pollution—a review. Open J Air Pollu 3(2). https://doi.org/10.4236/ojap.2014.32004 19. Ramasamy R, Mukhopadhyay K (2021) Developing empirical formula of ventilation index for assessing PM2.5 exposure in biomass-fuel using households. Curr World Environ 16(1):158– 162 20. Ramasamy R, Mukhopadhyay K (2022) Implementing a ventilation index for assessing indoor air PM2.5 concentrations in biomass-using households. Environ Monit Asses 194(2):1–11 21. Weon S, He F, Choi W (2019) Status and challenges in photocatalytic nanotechnology for cleaning air polluted with volatile organic compounds: visible light utilization and catalyst deactivation. Environ Sci Nano 6:3185–3214 22. Rajeswari R, Kanmani S (2009) A study on synergistic effect of photocatalytic ozonation for carbaryl degradation. Desalination 242(2009):277–285 23. Chen W, Zhang JS, Zhang Z (2005) Performance of air cleaners for removing multiple volatile organic compounds in indoor air. ASHRAE Trans 111(1):1101–1114 24. Zhong L, Haghighat F (2014) Ozonation air purification technology in HVAC applications. ASHRAE Trans 120:1–8 25. Lv J, Zhu L (2013) Highly efficient indoor air purification using adsorption-enhancedphotocatalysis-based microporous TiO2 at short residence time. Environ Technol 34(11):1447– 1454 26. Liu C, Hsu PC, Lee HW et al (2015) Transparent air filter for high-efficiency PM2.5 capture. Nat Commun 6:6205 27. Curto BL, Tarsini P, Cigada A (2016) Development of a photocatalytic filter to control indoor air quality. J Appl Biomater Funct Mater 14(4):496–501 28. Mo J, Zhang Y, Xu Q, Zhu Y et al (2009) Determination and risk assessment of by-products resulting from photocatalytic oxidation of toluene. Appl Catal B: Environ 89:570–576 29. Yu KP et al (2006) The correlation between photocatalytic oxidation performance and chemical/physical properties of indoor volatile organic compounds. Atmos Environ 40:375–385 30. Nimlos MR, Wolfrum EJ et al (1996) Gas-phase heterogeneous photocatalytic oxidation of ethanol: pathways and kinetic modelling. Environ Sci Technol 30:3102–3110 31. Hauchecorne B, Terrens D et al (2011) Elucidating the photocatalytic degradation pathway of acetaldehyde: an FTIR in situ study under atmospheric conditions. Appl Catal B: Environ 106:630–638 32. Ayturan ZC, Dursursun S (2018) Usage of photocatalytic oxidation for the removal of air pollutants. Int J Ecosys Ecol Sci 8(4):711–716 33. Lin YC, Lin CH (2008) Catalytic and photocatalytic degradation of ozone via utilization of controllable nano-Ag modified on TiO2 . Environ Prog:496–502 34. Shakti N, Gupta PS (2010) Structural and optical properties of sol-gel prepared ZnO thin film. Appl Phys Res. https://doi.org/10.5539/apr.v2n1p19 35. Qiu R, Zhang D et al (2008) Photocatalytic activity of polymer-modified ZnO under visible light irradiation. J Hazard Mater 156:80–85 36. Yun S, Lee J, Chung J, Lim S (2010) Improvement of ZnO nanorod-based dye-sensitized. J Phys Chem Solids 71:1724–1731

Chapter 10

Recent Progress in Biomedical Applications of Metal Oxide Photocatalysts Aditi Saxena, Parul Khurana, and Sheenam Thatai

Abstract Along with urbanisation and industrialisation, environmental remediation is a global concept to maintain sustainable environment. Photocatalysis is an efficient technique to find best solution of environmental problems. Development of photocatalytic techniques is viable solution to combat various infections, removal of organic pollutants in gaseous phase to purify indoor air, for water and wastewater treatment. Advances in photocatalysis show that current life science problems can be improved and solved by using photocatalytic nanomaterial. Nanosized metal oxides play an essential role in photocatalytic system due to light absorption properties and charge transport characteristics. Metal oxide photocatalytic nanomaterial and their composite by different modifications have been explored in this chapter. In this chapter, definition of metal oxides nanoparticles as photocatalysts and their synthesis methodology are discussed in detail. Keywords Metal oxides nanoparticles · Environmental problems · Photocatalysis · Substrate · Thin films

10.1 Introduction Photocatalysis is a science that involves using a catalyst to speed up chemical reactions that involves light absorption phenomena. A photocatalyst is a material that can absorb light and produce electron–hole pairs. This allows chemical changes of reaction participants and regenerates the chemical composition of the material after each cycle of such interactions [1]. Metal oxide is activated with ultraviolet and also visible light. Photoexcited electrons are promoted from the valence band to the conduction band and generate an A. Saxena · S. Thatai (B) Department of Chemistry, Amity Institute of Applied Sciences, Amity University, Sector-125, Noida, Uttar Pradesh 201313, India e-mail: [email protected] P. Khurana G.N.Khalsa College, University of Mumbai, Mumbai 400019, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Garg and A. Chandra (eds.), Photocatalysis for Environmental Remediation and Energy Production, Green Chemistry and Sustainable Technology, https://doi.org/10.1007/978-3-031-27707-8_10

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electron/hole pair (e− /h+ ). A pollutant adsorbed on the photocatalyst surface can be reduced or oxidised by the photogenerated pair (e− /h+ ). Metal oxide’s photocatalytic activity is derived from two sources: (i) OH radicals are generated by oxidation of OH− ions (ii) O2 radicals are generated by reduction of O2 . Pollutants can be degraded or transformed into less toxic by-products by both radicals and anions reacting with them. Metal oxides for example TiO2 , ZnO, SnO2 , and CeO2 which are abundant in nature have been widely used as heterogeneous photocatalysts. This is due to their band gap, high photocatalytic activity, biocompatibility, remarkable stability, and capacity to create charge carriers when stimulated with the light energy. Metal oxides can be used as photocatalysts because of their favourable combination of electronic structure, light absorption qualities, charge transport characteristics, and excited lifetimes. Metal oxides can be utilised as a photocatalyst to break down hazardous chemical compounds, photovoltaics, and even split water into hydrogen and oxygen [2]. A large number of binary composites such as V2 O5 –TiO2 , V2 O5 –ZnO, and Au– Fe2 O3 have been designed and synthesised where semiconductor particles play a significant role in exclusion of ecological pollutants and cations present like V, Au, Pd, and Pt increases the photocatalytic activity [3]. Heterogeneous photocatalysis using metal oxides such as TiO2 , ZnO, SnO2 , and CeO2 has proven to be effective in degrading a variety of contaminants into biodegradable chemicals and then mineralising them to harmless carbon dioxide and water. The nanoparticles of transition metal oxides are used for adsorption process due to active sites and high surface area. These metal oxides are efficient, cost-effective, and environment friendly materials for environmental remediation.

10.2 Properties of Metal Oxide Catalysis Bulk materials have been used for many years but recently, nanosized form of oxides is being used in many products. Nanotechnology research has gained attraction, resulting in novel solutions in the fields of health care, materials science, optics, and electronics. Nanoparticles are essentially different forms of basic elements that are created by altering the atomic and molecular characteristics of the requisite elements. Metal oxides are used as supports of active phases like SiO2 , Al2 O3 , ZnO, mesoporous oxides, etc. It may influence catalytic properties due to electrical and thermal conductivity which begins from metal oxide interactions. TiO2 and ZnO are widely used due to their photocatalytic properties [4]. TiO2 and ZnO are finding extensive applications in sunscreens, cosmetics, bottle coatings, etc. The electrical characteristics of ZnO are difficult to quantify due to the wide range of sample quality. The background carrier concentration varies greatly depending on the layer quality which is found to be 1016 cm3 . The biggest documented n-type doping is 1020 electrons cm3 , while the largest reported p-type doping is 1019 holes

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cm3 . At 300 K, the exciton binding energy of ZnO is 60 meV, which is one of the reasons for its popularity in optoelectronic device applications [3]. ZnO also has a broad defect-related peak ranging from 1.9 to 2.8 eV, which is a common optical characteristic. The cause of the green band’s luminescence is still unknown, and it has been attributed to a range of various impurities and faults in the past. Purity of 99% ZnO nanoparticles with wurtzite crystalline structure and physical appearance as defined in the rod-like, star-like, and isometric clusters. The ZnO particle number size distribution’s median diameter (D50: 50% of the number below this diameter) is between 30 and 55 nm, while the (D1: 1% below this size) is greater than 20 nm. When considering high-power/high-temperature devices, a semiconductor’s thermal conductivity is a crucial parameter to consider. In a pure crystal, it is primarily limited by phonon–phonon scattering, which is governed by the vibrational, rotational, and electronic degrees of freedom. Point defects abound in ZnO, as they do in most other semiconductors, and these have a substantial impact on heat conductivity. The highest recorded thermal conductivity values come from a study of vapour-phase grown ZnO samples that assessed conductivity on the polar sides [5]. There are four atoms per unit cell in single crystal wurtzite ZnO, resulting in 12 phonon modes. The following modes are critical for understanding the crystal’s thermal, electrical, and optical properties: One branch is longitudinal acoustic (LA), two are transverse acoustic (TA), three are longitudinal optical (LO), and six are transverse optical (TO). The Raman and infrared active A1 and E1 branches, while the non-polar E2 branches are just Raman active. Akira Fujishima developed and published the photocatalytic characteristics of nanosized TiO2 in 1972. The Honda-Fujishima effect was named after the process that occurred on the surface of TiO2 . In thin film and nanoparticle form, it has the potential to be used in energy production since it can break water into hydrogen and oxygen as a photocatalyst [5, 6]. The hydrogen gathered could be used as a source of energy. Efficiency of this process can be increased by doping the oxide with carbon. By introducing disorder to the lattice structure of the surface layer of TiO2 nanocrystals, infrared absorption was enabled which increased efficiency and durability. For photocatalytic applications, visible-light-active nanosized anatase and rutile TiO2 have been created. Due to a number of variables, TiO2 has a lot of potential as an industrial solution for wastewater purification or remediation. The process is conducted in the presence of natural oxygen and sunshine and hence takes place in natural settings. It is wavelength selective and is enhanced by UV radiation. The photocatalyst is low-cost, easy to find, non-toxic, chemically and mechanically stable and has a high turnover rate. Unlike direct photolysis procedures, no photocyclised intermediate compounds are formed. The substrates have been completely oxidised to CO2 . Thin coatings of TiO2 can be supported on suitable reactor substrates and easily removed from treated water [7]. Photocatalytic antimicrobial coatings, which are typically thin films placed to furniture in hospitals and other surfaces susceptible to infection, make use of the photocatalytic destruction of organic matter [8]. The pyroelectric effect is greater in ferroelectric materials, which display high hysteretic electrical polarisation below a

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threshold temperature, known as the Curie temperature, which may be reversed by an applied electric field. The primary contribution to pyroelectricity is often greater than the secondary impact, especially in ferroelectrics. The thermal factors are generally minimal in non-ferroelectric or weakly ferroelectric materials like ZnO, and they correspond very well for the heat capacity, which is indeed induced by thermal movements. The tetrahedral coordination is characteristic of sp3 covalent bonding, but these substances also have a considerable ionic character, which tends to raise the band gap beyond what would be predicted from covalent bonding.

10.3 Synthesis Method of Nanoparticles (i) ZnO NPs Without additional refinement, zinc nitrate, hypochlorite, and ethanol were procured and utilised. Nanomaterials were made by employing zinc acetate and sodium hydroxide as precursors in a wet chemical process. 0.5 M ethanol solvent solutions of zinc nitrate (Zn (NO3 )2 ·24H2 O) were kept under continual stirring for one hour using a magnetic stirrer to thoroughly dissolve the zinc nitrate, and a 0.9 M ethanol solvent solution of sodium hydroxide (NaOH) was made in the same way with one hour of stirring. Following complete solubility of metal salts, 0.9 M NaOH water solution was subjected dropwise (slowly for 45 min) to the vessel walls under high-speed steady stirring. After adding all of the sodium hydroxide, the process was permitted to continue for 2 h. For 2 h, the glassware remained sealed in this state. After the reaction was completed, the liquid was allowed to rest overnight before the resultant solution was carefully separated. The deposit was eliminated after centrifuging the residual solution for 10 min. Precipitated ZnO NPs were washed 3 times with distilled water and acetone to eliminate by-products attached to the nanoparticles and then dried at 60 °C in an air environment. Zn(OH)2 is entirely transformed to ZnO during drying. The optical and nanostructural characteristics of the produced ZnO NPs were investigated. An X-ray diffraction pattern was used to record the X-ray diffraction grating for ZnO NPs using Cu K radiation with a wavelength of 0.1541 nm. The microstructure of the specimen was examined using electron microscopy [TEM, SEM with EDAX] which was also utilised to analyse the composition of the generated ZnO NPs. A UV–VIS spectrophotometer was used to record the optically transmitting spectra of ZnO distributed in water. A spectrofluorometer was used to study the luminescence properties (PL) distribution of ZnO NPs dispersed in water [9]. The sol–gel process was used to create a ZnO nanostructure. A weighing scale was used to weigh 2 g of Zn(CH3 COO)2 and 8 g of NaOH to make a solution. Then, using a measuring cylinder, 10 and 25 ml deionised water were measured. Then, 2 g Na2 Cr2 O7 was dissolved in 15 ml of water. The mixtures were agitated. For around 5 min, mix a standard solution Zn (CH3 COO)2 using a magnetic stirrer. The solution

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comprising both salt solution and zinc acetate was then titrated dropwise using a burette packed with 100 ml ethanol. A white precipitate was generated as a result of the reaction [10]. (ii) TiO2 NPs Chemours, Cristal Global, Venator, Kronos, and Tronox are the top five TiO2 pigment processors in 2019. Akzo Nobel, PPG Industries, Sherwin Williams, BASF, Kansai Paints, and Valspar are among the major pigment grade titanium dioxide end consumers in the paint and coatings industry. In 2010, worldwide TiO2 material need was 5.3 Mt, with yearly increase of 3–4%. The manner of production is determined on the feedstock. Other feedstocks include improved slag, in addition to ores. The TiO2 NPs pigment is produced in the hematite crystalline structure by both the sulphate and chloride processes, although the sulphate course may be altered to generate the anatase form. Anatase is utilised in fibre and paper applications because it is softer. The sulphate processing is a batch process, while the salt process is an ongoing cycle. Using 0.5 mmol Ti(NO3 )4 , TiO2 NPs can be synthesised. A little amount of urea (CO(NH2 )2 ) was dissolved in 70 ml of sterile distilled water, and the solution was constantly stirred. After that, the uniform solution was then poured into a Teflon-lined 100 mL conical flask that was autoclaved for 12 h at 180 °C as shown in Fig. 10.1 [11]. After that, it was brought to ambient temperature and left to cool. After then, a white precipitate developed. The condensate was thoroughly cleaned with enough distilled water and 100% ethanol, after which they were dried overnight 353 °F. The precipitate formed was eventually washed away. Then, it was dried at 450 °F in the open air for 15 h at 35 °C [12].

Fig. 10.1 Sol–gel preparation of TiO2

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10.4 Mechanism for Photocatalysts In recent years, antibacterial nanomaterials cantered on titanium dioxide (TiO2 ) have received a lot of attention. When the inorganic phase dispersion and biodynamic interlayer connection are properly established, the biocidal action is a consequence of the regulation of charge (electron–hole) transporters at the interfaces of the exposed surface of the specimen, giving potent and long-lasting capabilities [13]. First, TiO2 NPs exhibit a broad range of antimicrobial action against pathogens, encompassing Staphylococci and Gram-positive bacteria, as well as fungus, which is particularly important for drug-resistant strains. Second, and perhaps more crucially, TiO2 -polymer nanocomposites are innately green and have a non-contact biocidal effect. As a result, no potentially hazardous NPs (with unknown impacts on human health) must be released into the media to accomplish disinfection. Among all inorganic photocatalyst, ZnO has the highest photocatalytic effectiveness and is much more approachable than TiO2 . ZnO can absorb a lot of UV radiation and has a greater sensitivity to it [13], so its conductivity increases greatly, and this property triggers the connection of ZnO with microorganisms. Its photoconductivity lasts long after the UV light is turned off, and it’s thought to be due to a surface electron depletion area closely linked to negatively oxygen species deposited on the surface. The loosely attached oxygen on the surface is rapidly desorbed when exposed to UV light. Photocatalysis is defined as a photoinduced electrochemical reaction that can harm or kill organisms. Under UV light, ZnO NPs in aqueous medium have a phototoxic effect, producing reactive oxygen species (ROS) including such HCl and oxidative ions. The active species produced have the capacity to penetrate cells and hence inhibit or kill bacteria. The photocatalytic efficiency of ZnO NPs has been used in bio-nanotechnology and bio-nanomedicine for several antibacterial applications because of this procedure. Consequently, of the generated free radicals, the increased bioactivity of ZnO was thought to be due to the absorption of UV radiation by ZnO. The capacity of a material to transport photoinduced electrons to deposited molecules on its sample is determined by the placements of the CB and VB edges, as well as the adsorbate’s redox potential. The acceptor molecules’ or species’ potential level must be thermodynamically set beneath (i.e. high favourable than) the lower portion of the material CB [14].

10.5 Various Fields of Application (i) Water Treatment Organic and inorganic contaminants that are dissolved or suspended in water can directly interact with gaps or ions created during semiconductor excitation. Dehydrogenation, proton transfer, O2 18 and O2 16 and dinitrogen isotopic

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exchange, metallic coating, and liquid purification are all likely alternatives of photocatalytic reactions in the aqueous phase. The following factors influence the efficiency of pollutant degradation: • • • •

Varieties of photocatalysts and their qualities Nature and concentration of pollutant pH temperature illumination range and strength photocatalyst loading Presence of extra chemicals dissolved oxygen concentration.

Heavy metals such as lead, arsenic, cadmium, chromite, and nickel are removed using photocatalytic techniques in water treatment. Photocatalysis’ photoreduction ability has also been utilised to recover valuable metals like gold, platinum, and argent from industrial effluent. According to the redox process, heavy metals can be detoxified as minute crystallites formed on the photocatalyst surface [15]. Toxic inorganics can be removed using TiO2 -activated photosystems as O2 + 2CN− → 2OCN− 5O2 + 4H+ + 4CN → 2H2 O + 4CO2 + 2N2 5O2 + 6NH3 → 2N2 + N2 O + 9H2 O. (ii) Hydrogen Production Electrocatalytic water and helium synthesis from hydrogen production is an upward reaction, with a substantial contribution in Gibbs free energy as the reaction progresses G = H − T S = 273 kJ/mol To perform photoelectrochemical water breaking, the metal oxide semiconductors’ energy gap must be greater than 1.23 eV (1000 nm). Besides the energy band gap matching, the voltage of the VB and CB of semiconductors is another essential property. As a result, the bottom portion of the VB must be lower than the H+ /H2 reduction potential, while the upper section must be larger than the O2 /H2 O photocatalytic activity shown in Table 10.1. The efficiency of hydrogen generation via photocatalytic water splitting is also affected by other factors such as charge separation, mobility, the lifespan of photoinduced electron–hole pairs, and overpotentials. Because the splitting of water generating oxygen and hydrogen is an energy-consuming process, the oxidation process of oxygen and hydrogen to water may continue quickly [16]. This process comprises of some essential steps: • Striking of light source • Release and absorption of light particles

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Table 10.1 Photocatalytic production of H2 and O2 in the presence of additives

• • • •

Type of additive

H2 (µmol/h)

None

O2 (µmol/h)

72

36

Sodium hydroxide

242

129

Sodium phosphate

228

113

Sodium carbonate

378

190

Sodium metaborate

164

84

Disodium phosphate

129

65

Sodium hydrogen carbonate

607

319

Sodium sulphate

112

56

Sodium chloride

91

48

Hydrogen chloride

46

19

Phosphoric acid

65

33

Sulphuric acid

85

39

New bond formation between the excited particles Old bond breaking of the excited particles Dislocation of the charged particles Surface reaction phenomenon of water molecules with these particles.

(iii) Air Pollution Removal Combustion processes, adhesive and construction material outputs, acids, antiseptics, adhesive, personal care products, oils, leatherette, tobacco, chlorinated fluids, and combustible material are all sources of indoor pollutants. VOCs such as xylene, benzene, and toluene can be produced by flooring, and considerable levels of formaldehyde are emitted by building interiors and furniture, as well as heat treatment and burning. This latter substance is a hazardous VOC as well as a recognised carcinogen as shown in Table 10.2 [17]. Biological pollutants such as mould and pollen from air conditioning units, cooking, and building materials (glue, paint, and solvents) are other sources of indoor Table 10.2 Examples of photocatalyst-pollutant interaction

Pollutant

Photocatalyst

Efficiency

Light source

Acetylene

TiO2

85% mineralisation

UV at 10 mW/cm

Cyclohexane

TiO2

100% conversion

UV LEDs at 90 mW/cm2

Formaldehyde

ZnO

25% conversion

UV at 3.6 mW/cm2

NOx

TiO2

16% conversion

UV 365 at 1.0 mW/cm2

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pollution. Finally, VOCs produced outside may pollute the inside atmosphere through air exchange, and vice versa [18]. The mechanism followed is as follows: NO + HO → HNO2 HNO2 + OH → NO2 + H2 O NO2 + OH → HNO3 − NO + O− 2 → NO3

HNO2 → H+ + NO− 2 − 2NO + O− 2 + 3e → 2NO2

HNO3 → H+ + NO− 3 3NO2 + 2OH− → 2NO3 + NO + H2 O However, little is known about deactivation processes under various operating settings, regeneration strategies, the creation of intermediate chemicals that pose health hazards, and the protracted functioning of photo electrocatalytic systems in real-world scenarios.

10.6 Biomedical Application of Metal Oxide Photocatalysis (i) Anti-cancer Activity ZnO NPs have anticancer action via causing the production of reactive oxygen species (ROS) as well as promoting apoptosis. In addition, the electrostatic characteristics of ZnO NPs, which have been used for anticancer activity. Because of the neutral hydrophilic groups accommodate on their surface, ZnO NPs have a unique surface charge behaviour. In an aqueous solution with a high pH, protons (H+ ) flow away from the particle surface, exposing a negative charge exterior with partly bound oxygen atoms (ZnO). Charged particles out from surroundings are transported to the pore walls at lower pH

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levels, producing a positive charge surface. ZnO NPs have an isoelectric point of 9– 10, which means they have a significant positive surface charge under physiological circumstances [19]. Cancer cells, on the other hand, exhibit substantial negative membrane potentials and an increased concentrations of protonated phospholipid bilayer (phosphatidylserine) along the surface membrane. Electrostatic attraction causes cancer cells’ interactions with ionised ZnO NPs, boosting cellular absorption, internalisation, and lethality of all nanostructures. The above-mentioned features of ZnO make them a good choice for use as single agents or in combination with another chemotherapeutics. Furthermore, ZnO NPs have been investigated for drug loading to improve cellular absorption and synergistic action. Treatment methods of ZnO NPs have indeed been carried out to boost its longevity and expand their cell selectivity [20]. Researchers have used ZnO NPs for the packaging of active medicines because they have intrinsic anticancer characteristics. It was expected that loading medicines onto ZnO NPs will boost the therapeutic action of chemotherapeutic treatments. The exterior of ZnO NPs has really been embellished with designed to target moieties, notably vitamin B12 and mucin 1 oligonucleotides, to hit folic sensors and mucin 1 upregulated on tumour cellular membrane, to boost the targetability and selectivity for cancer cells. (ii) Bio-imaging While ZnO broad band gap semiconductor capabilities are excellent for killing cells by creating reactive oxygen species (ROS), their intrinsic photoluminescence features are useful for biosensing. Wang and colleagues developed a ZnO gated system that was developed for diagnostic imaging bioimaging and pHtriggered on-demand medication release. In vitro and in vivo, ZnO NPs have also been used as contrast media for response to these stimuli imaging, giving comprehensive information for tumour diagnosis. Jiang and colleagues also looked into using ZnO nanosheets to image grown cells. They used ZnO nanosheets to treat model leukaemia K562 cells, and under UV illumination, they saw yellow-orange light emission around or inside the cells, indicating that the ZnO nanostructures had successfully penetrated the cells [21]. (iii) Anti-bacterial Activity The capacity of ZnO NPs to cause oxidative stress also contributes to their antibacterial activity. The substituent of pulmonary enzymes interacts with Zn+ ions produced by ZnO, limiting their function. ZnO NPs have been shown to influence the biological membranes and cause the generation of reactive oxygen species (ROS). When bacteria encounter ZnO NPs, they absorb Zn+ , which inhibits the function of respiratory enzymes, creates reactive oxygen species (ROS), and produces free radicals, resulting in oxidative stress. Microbial walls, chromosomes, and organelles are permanently damaged by ROS, resulting in bacterial cell death.

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Ghasemi studied the effect of ZnO nanoparticles on the efficacy of the traditional medicines like ciprofloxacin and ceftazidime, as well as their modes of action, versus susceptible Acinetobacter baumannii, an opportunistic bacterium that causes pneumonia and meningitis. The bioactivity of both medicines enhanced in the context of a minimum inhibitory concentration of ZnO NPs, according to the findings. Combining antibiotics with ZnO NPs improved antibiotic absorption and transformed bacterial cells from rods to cocci. Malformed organisational integrity, increased flexibility and ionisation of living cells, and protein leakage have all been linked to ZnO NPs. The formation of reactive oxygen species (ROS) and mitochondrial dysfunction were also reported. These findings point to zinc oxide and medicines working together as an alternate therapy for bacterial illness. NPs also have been discovered to enhance the antimicrobial effect of crystal violet, a chromophore. The author also created magnesium (10%)-doped ZnO NPs and utilised them to lower E. coli numbers without using a photosensitizer or white-light activation. Zhang and co. investigated both needs to be continuously given and time-monitored phenotypic microbial sensitivities to ZnO NPs using a robust edge Raman spectrometry method. Their findings revealed spectral change patterns that were both obvious and informative. Significant changes were seen in reduced levels rather than increased ranges, demonstrating a decrease in ZnO NP bioavailability when dosages were increased [22]. (iv) Wound Healing Because of its strong antibacterial capabilities and zinc’s epithelialisationstimulating impact, ZnO NPs have also been effectively employed in wound dressings. ZnO NPs-loaded-sodium nanocomposite acacia hydrogels were recently developed and found to have a positive impact on sheep progenitor cells at low ZnO NPs doses. The cells were cytotoxic to high concentrations of ZnO NPs, whereas SAGAZnO NPs hydrogels greatly decreased the toxicity while preserving the positive antibacterial and healing activities. Thomas et al. developed chitin gel cream and ZnO hybrid patches with improved swelling ratios, as well as blood clotting and antimicrobial activities. In Sprague– Dawley rats, in vivo tests demonstrated improved wound healing, as well as quicker re-epithelialisation and protein formation [23]. (v) Anti-fungal Activity ZnO NPs are also excellent antifungal agents in complement to their antimicrobial activities. Surendra and co-workers synthesised ZnO NPs from M. oleifera, which were harmful to two plant disease strains, Alternaria solani and Sclerotium rolfsii. Gunalan et al. investigated the antimicrobial activity and stability NPs in several fungal strains and discovered that they were capable of damaging and food diseases in the following order: Rhizopus stolonifera > Fusarium flavus > A. niger nidulans > T. harzianum.

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As a result, the scientists speculated that ZnO NPs may be useful in the food and farming industries. ZnO NPs have a concentration-dependent influence on Candida albicans viability, according to Lipovsky et al. C. albicans vitality was reduced by more than 95% when ZnO NPs were used. Exciting ZnO NPs with visible-light boosted yeast cell death even more. Because of their intrinsic propensity to generate ROS production and trigger apoptosis, ZnO NPs, like other metallic NPs, offer great medicinal potential. ZnO NPs are useful as antitumor, antimicrobial, and fungicide agents because of their properties. When loaded and delivered with other medicinal treatments, ZnO NPs were shown to have stimulatory activity. Despite ZnO is a fairly benign metal oxide that has been authorised for aesthetic treatments by the FDA, the toxicity of ZnO NPs is still a worry due to its big surface region and ferrous character. Although the biological possibilities of ZnO are presently being investigated, further research is needed to determine their toxicity profile to get the most advantages from this intriguing oxide [24]. (vi) Toxicity of Metal Oxides Because ZnO NPs have a greater solubility than TiO2 NPs, several investigations have found that ZnO NPs are more poisonous than TiO2 NPs. The creation of ROS via particles appears to be the major feature of toxicity, which really is vital for cell damage caused by oxidative stress. Researchers found that when NP concentration and size declined, ROS production from NPs increased. Several studies have found a link between NP toxicity and lower cell viability as NPs concentrations rise, with postulated causes including physical contact with microalgae and NPs, ROS production, and the shadowing effect (aggregation). By going through or connecting to cells, NPs can harm or infiltrate the cell wall. Electrostatic force can increase particle adhesion on cytoplasmic membrane as surface area rises, producing cellular injury to membranes. Because the shadowing influence limits the amount of light available to algae for photosynthesis, NP aggregation can influence the shading by obstructing or sticking towards the surface of epithelial membrane. Because of the surface area and sensitivity of NPs, the degree of accumulation can be a critical determinant in defining the action of NPs on cells. The growth inhibition index is calculated as Growth Inhibition (% ) = (1 − (UVabs at 6 hrs)/(UVabs control)) × 100 Electron microscopy (TEM) was used to characterise ZnO NPs, and the average size was determined to be between 25 and 40 nm in diameter. MTT and neutral red absorption tests were used to determine cell viability after HEK cells were exposed to ZnO NPs for 3, 24, and 48 h. The findings clearly demonstrated that ZnO NPs generated dosage and time dependent toxicity, with toxicity occurring within 3 h after exposure and intensifying after 24 h. Giemsa staining was used to examine cellular

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morphology, and after 24 h, significant alterations were seen at 75 g/ml concentration [25]. According to studies, the hydrophilic nature of ZnO NPs in the extracellular area produces a rise in intracellular Zn2+ , which alters the activity of Zn-dependent proteins and transposable elements. Lysosomal instability occurs when there is a high quantity of Zn2+ ions present. When ZnO NPs interact with cells, they produce reactive oxygen species (ROS), which impair mitochondrial and lysosomal functions, resulting in protein catabolism. ROS-dependent mortality is also triggered by bone morphogenetic breakdown caused by nano ZnO. Chromatin condensation caused by nanoparticle nuclear interactions is also thought to be an indication of cellular death. Finally, precautions should be made while using such potentially dangerous nanomaterials in order to avoid harmful deleterious effects [26]. Because of their solubility, ZnO NPs have a hazardous impact. Nano particles disintegrate in the periplasm, raising intracellular [Zn2+ ] levels. The mechanism behind the enhanced internal [Zn2+ ] content and the disintegration of ZnO NPs in the environment is currently unknown. Necroptosis, raised ROS quantities, impaired mitochondrial functioning, and the creation of tubular internal constructions have all been documented when skin cells are subjected to ZnO NPs. Nanoparticles influence neutrophils, macrophages, and myeloid. Thus, the synthesis of nanostructures must be done by accounting for the toxicity media for better photocatalytic activity [27].

10.7 Limitations of Photocatalysts Although much research has been done on enhancing the photocatalytic activity of TiO2 surfaces by poisoning or surface modification, there has been little work on improving the photocatalytic performance of ferromagnetic TiO2 NPs under visible light by poisoning or surface functionalisation. Furthermore, the physicochemical resilience and long-term viability of non-metal dyed or edge TiO2 NPs have not been investigated for repeated usage. Designing a multifunctional photocatalyst that combines outstanding visible-light catalytic performance, adsorption ability, excellent durability, and magnetism distinctiveness is required. It is still challenging to obtain full photodegradation of certain POPs using photocatalysts. More research should be conducted with the goal of improving photocatalyst reactivity. In order to broaden the spectrum of application, it is also required to investigate the possibilities of combining TiO2 hybrid innovations with several other methods (e.g. biological means and electrodynamics). Furthermore, the end or transitional components of photodegradation may not be harmless [28]. The degradation products may provide a greater risk than the parent chemical. By-products that are harmful to the environment might cause a reduction in rate of the reaction and subsequent contamination. The sensitivity of the nano catalyst or the whole electrocatalytic process, particularly for modified photocatalysts, is

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little known. Identification and characterisation of responsiveness, cytotoxicity and outcome on a fundamental level. The oxidations mediated by TiO2 are non-selective. The dissolution rate of a wide range of compounds is expected to be comparable because they are driven by an oxygen radical’s mechanism. On one side, this lack of consideration may be helpful, but poor specificity means the catalysis cannot identify between very dangerous chemicals and pollutants with minimal toxicity [29]. Many of the low-toxicity pollutants can be easily defeated by cellular mechanisms, however, many of the extremely harmful compounds are not. As a result, a photocatalytic activity system that can preferentially breakdown impurities using optical and/or sunlight irradiation, in addition to genetic approaches, is required. • Light absorption characteristic can be achieved by using suitable band gap. It will, in turn, enhance the photoactivity. • Surface area: Volume ratio can be achieved by decreasing the size of nanoparticle. It will increase the light absorption rate. • Charge separation can also be achieved by introducing co-catalysts which will also help in enhancing the photocatalytic activity. The following are the primary constraints of transition metal oxide photosensitizers for large-scale practical applications: • For binary metal oxide photocatalysts, a high charge recombination of electronic– hole couples culminate in a low charge density [30]. • The gathering of light waves is restricted. As photocatalysts, broad band gap semiconductor oxides are commonly used. For example, the energy band gap frequency of TiO2 is 3.2 eV, and the matching absorption spectrum is 387.5 nm, leading to low ultraviolet region light absorption. Nevertheless, light source accounts for just 5–7% of the spectral region, while light in the visible and thermal radiation account for 46 and 47% of the continuum, respectively. • Organic pollutant selective sorption from the aqueous phase is poor [31]. • The photocatalytic treatment of industrial waste with a high number of organic impurities poisons the photocatalytic activity, causing it to breakdown. • Photo catalytic isolation and recovery from the reaction medium is difficult. Because of the nano-size of the photocatalysts granules, traditional techniques such as filtering and centrifugation are ineffective. The economics and long-term viability of heterogeneous photoanode for water purification are challenged by this constraint [32]. • Non-mobilisation of photocatalysts also reduce their effectiveness rate on the surface.

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References 1. Lima MJ, Pastrana-Martínez LM, Sampaio MJ et al (2018) Selective production of benzaldehyde using metal-free reduced graphene oxide/carbon nitride hybrid photocatalysts. ChemistrySelect 3(28):8070–8081. https://doi.org/10.1002/slct.201800962 2. Zhang YX, Tang S, De ZW, Yu YX (2019) Noble metal-free photocatalysts consisting of graphitic carbon nitride, nickel complex, and nickel oxide nanoparticles for efficient hydrogen generation. ACS Appl Mater Interfaces 11(16):14986–14996. https://doi.org/10.1021/acsami. 9b01704 3. Xie L, Liu P, Zheng Z, Weng S, Huang J (2016) Morphology engineering of V2 O5 /TiO2 nanocomposites with enhanced visible light-driven photofunctions for arsenic removal. Appl Catal B 184:347–354. https://doi.org/10.1016/j.apcatb.2015.11.014 4. Marinella F, Dami B (2012) Introduction to the analysis and risk of nanomaterials in environmental and food samples. Compr Anal Chem 59:1–32. https://doi.org/10.1016/B978-0-44456328-6.00001-3 5. Zong X, Li C (2018) Photocatalytic water splitting on metal oxide-based semiconductor photocatalysts, vol 1. Elsevier Inc. https://doi.org/10.1016/B978-0-12-811631-9.00007-7 6. Iwashina K, Iwase A, Ng YH, Amal R, Kudo A (2015) Z-schematic water splitting into H2 and O2 using metal sulfide as a hydrogen-evolving photocatalyst and reduced graphene oxide as a solid-state electron mediator. J Am Chem Soc 137(2):604–607. https://doi.org/10.1021/ ja511615s 7. Strunk J (2018) Requirements for efficient metal oxide photocatalysts for CO2 reduction. Elsevier Inc. https://doi.org/10.1016/b978-0-12-811167-3.00010-9 8. Grabowska E, Marchelek M, Paszkiewicz-Gawron M, Zaleska-Medynska A (2018) Metal oxide photocatalysts. Elsevier Inc. https://doi.org/10.1016/B978-0-12-811634-0.00003-2 9. Martha S, Chandra Sahoo P, Parida KM (2015) An overview on visible light responsive metal oxide based photocatalysts for hydrogen energy production. RSC Adv 5(76):61535–61553. https://doi.org/10.1039/c5ra11682a 10. Gunjakar JL, Kim TW, Kim HN, Kim IY, Hwang SJ (2011) Mesoporous layer-by-layer ordered nanohybrids of layered double hydroxide and layered metal oxide: highly active visible light photocatalysts with improved chemical stability. J Am Chem Soc 133(38):14998–15007. https://doi.org/10.1021/ja203388r 11. Chan SHS, Wu TY, Juan JC, Teh CY (2011) Recent developments of metal oxide semiconductors as photocatalysts in advanced oxidation processes (AOPs) for treatment of dye waste-water. J Chem Technol Biotechnol 86(9):1130–1158. https://doi.org/10.1002/jctb.2636 12. Tana T, Guo XW, Xiao Q et al (2016) Non-plasmonic metal nanoparticles as visible light photocatalysts for the selective oxidation of aliphatic alcohols with molecular oxygen at near ambient conditions. Chem Commun 52(77):11567–11570. https://doi.org/10.1039/c6cc05186c 13. Cheng J, Deming TJ (2011) Synthesis of polypeptides by ROP of NCAs. Pept Mater 2011(310):1–26. https://doi.org/10.1007/128 14. Pany S, Nashim A, Parida K (2017) Titanium-based mixed metal oxide nanocomposites for visible light-induced photocatalysis. https://doi.org/10.1007/978-3-319-62446-4_10. 15. Halder A, Zhang M, Chi Q (2016) Electrocatalytic applications of graphene–metal oxide nanohybrid materials. In: Advanced catalytic materials— photocatalysis and other current trends. Published online 2016. https://doi.org/10.5772/61808 16. Zhang W, Naidu BS, Ou JZ et al (2015) Liquid metal/metal oxide frameworks with incorporated Ga2 O3 for photocatalysis. ACS Appl Mater Interfaces 7(3):1943–1948. https://doi.org/ 10.1021/am5077364 17. Aratani Y, Suenobu T, Ohkubo K, Yamada Y, Fukuzumi S (2017) Dual function photocatalysis of cyano-bridged heteronuclear metal complexes for water oxidation and two-electron reduction of dioxygen to produce hydrogen peroxide as a solar fuel. Chem Commun 53(24):3473–3476. https://doi.org/10.1039/C7CC00621G 18. ChemInform (1988) 19, No. 17, Abstract 028

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19. Pawar SG (2011) Effect of annealing on structure. J Nano- Electron Phys 3:185–192 20. Xiao J, Xie Y, Cao H, Nawaz F, Zhang S, Wang Y (2016) Disparate roles of doped metal ions in promoting surface oxidation of TiO2 photocatalysis. J Photochem Photobiol A Chem 315:59–66. https://doi.org/10.1016/j.jphotochem.2015.09.013 21. Anpo M, Kawamura T, Kodama S, Maruya K, Onishi T (1988) Photocatalysis on Ti–Al binary metal oxides: enhancement of the photocatalytic activity of TiO2 species. J Phys Chem 92(2):438–440. https://doi.org/10.1021/j100313a039 22. Kamat P V, Lightcap I, Murphy S, Krishnamurthy S (2013) (Invited) Semiconductor-metal nanoparticles anchored on graphene oxide: photocatalysis and SERS detection. In: ECS meeting abstract 2013, MA2013-01(30), pp 1082–1082. https://doi.org/10.1149/ma2013-01/ 30/1082 23. Hanley C, Layne J, Punnoose A (2008) Preferential killing of cancer cells and activated human T cells using ZnO nanoparticles. Nanotechnology 19(29). https://doi.org/10.1088/0957-4484/ 19/29/295103 24. Fusion CdM (2018) Accepte us. Published online 2018 25. Sudha KG, Ali S, Karunakaran G, Kowsalya M, Kolesnikov E, Rajeshkumar MP (2020) Ecofriendly synthesis of ZnO nanorods using Cycas pschannae plant extract with excellent photocatalytic, antioxidant, and anticancer nanomedicine for lung cancer treatment. Appl Organomet Chem 34(4):1–20. https://doi.org/10.1002/aoc.5511 26. Pairoj S, Damrongsak P, Damrongsak B et al (2019) Antiradical properties of chemo drug, carboplatin, in cooperation with ZnO nanoparticles under UV irradiation in putative model of cancer cells. Biocybern Biomed Eng 39(3):893–901. https://doi.org/10.1016/j.bbe.2019. 08.004 27. Anitha J, Selvakumar R, Murugan K (2019) Chitosan capped ZnO nanoparticles with cell specific apoptosis induction through P53 activation and G2/M arrest in breast cancer cells— in vitro approaches. Int J Biol Macromol 136:686–696. https://doi.org/10.1016/j.ijbiomac. 2019.05.217 28. Roshini A, Jagadeesan S, Cho YJ, Lim JH, Choi KH (2017) Synthesis and evaluation of the cytotoxic and anti-proliferative properties of ZnO quantum dots against MCF-7 and MDAMB-231 human breast cancer cells, vol 81. Elsevier B.V. https://doi.org/10.1016/j.msec.2017. 08.014 29. Istirokhatun T, Yuni U, Andarani P, Susanto H (2018) Do ZnO and Al2 O3 nanoparticles improve the anti-bacterial properties of cellulose acetate-chitosan membrane? In: MATEC web conference 2018, vol 156, pp 0–4. https://doi.org/10.1051/matecconf/201815608009 30. Preedia Babu E, Subastri A, Suyavaran A et al (2018) Anticancer potential of ZnO nanoparticleferulic acid conjugate on Huh-7 and HepG2 cells and diethyl nitrosamine induced hepatocellular cancer on Wistar albino rat. Nanomed Nanotechnol Biol Med 14(2):415–428. https://doi. org/10.1016/j.nano.2017.11.003 31. Anandan M, Dinesh S, Krishnakumar N, Balamurugan K (2016) Improved photocatalytic properties and anti-bacterial activity of size reduced ZnO nanoparticles via PEG-assisted precipitation route. J Mater Sci Mater Electron 27(12):12517–12526. https://doi.org/10.1007/s10854016-5764-y 32. Nair S, Sasidharan A, Divya Rani VV et al (2009) Role of size scale of ZnO nanoparticles and microparticles on toxicity toward bacteria and osteoblast cancer cells. J Mater Sci Mater Med 20(SUPPL. 1):235–241. https://doi.org/10.1007/s10856-008-3548-5

Chapter 11

Role of Heterogeneous Semiconductor Photocatalysts in Green Organic Synthesis Umair Alam

Abstract The transition of the industrial approach towards sustainable chemical manufacturing requires the development of highly efficient, non-toxic heterogeneous photocatalysts. Recently, semiconductor-mediated heterogeneous photocatalysts utilising sunlight as an energy source have drawn much attention in the field of organic synthesis due to their nature of easy recovery and simple chemical work up. This process of chemical synthesis is termed as green and sustainable approach as it requires the photocatalyst particles and light as the energy source for the chemical reaction. In this chapter, the recent advances in semiconductor-based photocatalytic organic reactions including, selective oxidation reactions (alcohols and amines), reduction reaction (nitro compounds), and hydrocarbylation of benzene compounds are discussed. A variety of semiconductor materials involved in organic synthesis, along with their representative photocatalytic mechanism, have also been given. Understanding the possible mechanism of the different chemical reactions may help the people working in the area of photocatalysis in developing new and efficient visible light sensitive photocatalysts for green organic synthesis. Keywords Semiconductors · Photocatalysis · Visible light · Heterogeneous photocatalysis · Organic synthesis

11.1 Introduction The manufacturing of chemical products such as food additives, pharmaceuticals, perfumes, and pesticides usually requires organic compounds. The traditional synthesis of organic compounds by chemical transformation not only operates under high temperature and pressure but also produces toxic by-products. The complex conventional synthesis process is avoided by employing photocatalysis technology, as photocatalysis for organic transformation can readily occur under ambient conditions

U. Alam (B) Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Garg and A. Chandra (eds.), Photocatalysis for Environmental Remediation and Energy Production, Green Chemistry and Sustainable Technology, https://doi.org/10.1007/978-3-031-27707-8_11

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while utilising light as an energy source. As a result, the light-mediated photocatalytic process is considered an eco-friendly and sustainable route. Among the broad spectrum of solar energy, visible light is abundant in nature and accounts for 43% of solar light [1]. The main problem is that the reactant molecules cannot directly absorb the visible light to proceed the chemical reaction. Therefore, utilising a visible light photocatalyst that acts as a bridge for the energy transfer between visible light and substrates will be of special importance [2]. Five categories of photocatalysts have been reported, and they are represented as inorganic complexes (Ru and Ir metals-based complexes), dyes (homogeneous organic photocatalysts), semiconductors, nanoparticles of noble metals, and other catalytic materials (heterogeneous photocatalysts) [3]. The easy recovery of heterogeneous photocatalysts from the reaction products provides a green way for organic chemical reactions. The low energy gap semiconductors like g-C3 N4 , WO3 , and BiVO4 also follow the same principal photocatalytic mechanism [4]. When these semiconductors are exposed to light with a higher energy than their energy gap, electrons and hole pairs are produced. The produced photo excitons are separated and moved to the catalyst surface to drive the chemical reactions [5]. Different widths and band positions are found in different semiconductor materials. So, the separated electrons and holes have different oxidation and reduction potentials. Consequently, the separated photoexcited excitons in the conduction and valence bands generate a few reactive oxygen species. The reactive oxygen species generated over the catalyst surface are ˙O2 − , ˙HO2 , ˙OH, and 1 O2 , which are key species in most chemical processes [6]. As is well known, during the photocatalytic degradation of organic contaminants, free radical species and holes are assumed to be the most active species. These species degrade the organic substances to CO2 and H2 O molecules as the final products. The substantial generation of reactive species, and particularly the production of ˙OH in an aqueous medium, stimulates the subsequent oxidation of organic compounds, which reduces the selectivity and yield of the desired chemical molecules. Therefore, the production of free radical species should be controlled during the photochemical reaction. As reaction media, researchers mostly used benzotrifluoride (BTF) or acetonitrile (CH3 CN) as organic solvents to control the overproduction of free radical species. The importance of the above solvent is that it prevents the production of ˙OH radicals and favors the high selectivity. It is important to mention here that the polarity of solvents influences product selectivity [7]. In this regard, Biswas et al. studied the influence of solvents in photocatalytic cyclohexanol oxidation over TiO2 surfaces [8]. They added that the cyclohexanol preferentially adsorbs over the catalyst surface in non-polar solvents, resulting in low selectivity and overoxidation. However, a weak adsorption of cyclohexanol over the surface of TiO2 was seen when polar solvents were used as the reaction media. Researchers always prefer visible lamp over UV for the chemical reactions because UV light causes undesirable mineralisation caused by high-energy photoinduced holes. The inherent properties of photocatalysts play a significant role in the production and separation of photoinduced charge carriers. The category of reactive species generation and their amount can also be controlled by changing the intrinsic characteristics, which may, in turn, control the efficiency of photocatalytic reactions.

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The best example is represented by Zhang et al. using Bi2 WO6 as a photocatalyst [9]. The insufficient VB potential of Bi2 WO6 suppresses the formation of highly reactive ˙OH radicals from water, thereby suppressing further hydroxylation or mineralisation of benzaldehyde. Therefore, by determining the essential effects like the change in composition, modification in crystalline structure, and electronic structure, we can develop highly efficient photocatalysts for selective organic synthesis. The chapter focuses on the role of semiconductor photocatalysts (heterogeneous systems) in the synthesis of organic chemicals at ambient conditions. We start by describing the reaction mechanism and the recent developments in the field of chemical transformation. The chemical conversion reactions such as oxidation reactions (alcohols and amines), reductions of nitro compounds, and hydrocarbylation of benzene compounds are examples of organic synthesis. The inherent properties of photocatalytic materials augmenting the catalytic efficiency are explored in order to comprehend the modification strategies, and a direction for future investigations is also provided based on these advancements.

11.2 Selective Oxidation Reactions The carbonyl compounds, including aldehyde and ketone, were synthesised using a conventional approach, where strong oxidising agents (permanganate, hypervalent iodine, and chromate) are used to convert carbonyl compounds to aldehydes or ketones. In contrast, the semiconductor-mediated oxidation reactions (oxidation of alcohols) using only oxygen as the oxidant do not produce toxic products [10– 12]. The photocatalytic approach is very simple and eco-friendly, and it receives significant interest from the scientific community.

11.2.1 Aldehydes/Ketones Formation via Oxidation Reactions Aldehydes and ketones are reported to be crucial organic molecules for producing medicines and other chemicals, and they are produced by selectively oxidising primary or secondary alcohols.

11.2.1.1

Reaction Mechanisms

Due to their enormous potential in both research and industry, numerous studies have been conducted to develop highly effective photocatalysts and examine their reaction mechanisms. In this section, we have taken the example of alcohols oxidation over g-C3 N4 and discussed its reaction mechanism in detail. Utilising g-C3 N4 as a catalyst for the oxidation reaction, which is initiated by superoxide radicals, is a typical reaction [13]. The negative value of its conduction band is the reason for the

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Fig. 11.1 Plausible mechanism for selective oxidation reaction (alcohols to their corresponding compounds) over mpg-C3 N4 (a) and in the presence of Bi12 O17 Cl2 nanobelts (b)

formation of superoxide radicals as the primary species. If the reaction is carried out under visible light, the carbonyl group of alcohols is oxidised to aldehydes or ketones by the mesoporous (mpg-C3 N4 ) in a variety of organic solvents. The surface basicity of mesoporous g-C3 N4 is the reason for the high conversion efficiency, which helps in the deprotonation of alcohols. A mechanism for oxidising alcohols selectively using g-C3 N4 as a photocatalyst is proposed and given in Fig. 11.1a. The detailed mechanism of direct participation of hole is proposed by Xiao et al. [14]. They investigated the benzyl alcohol oxidation mechanism over Bi12 O17 Cl2 nanobelts in an acetonitrile solvent, as shown in Fig. 11.1b. It is pertinent to mention that the transformation of alcohols can also take place in the absence of oxygen, and the reason for providing the molecular oxygen is to separate the charge carriers from recombination, as oxygen serves as an electron acceptor during the reaction. The preceding procedure creates a hole in the VB and can be used in the deprotonation of ionised benzene anions (BA− ) and their related intermediates. Furthermore, the redox potentials of benzyl alcohol and its derived compounds (benzaldehyde) were calculated with the help of cyclic voltammetry measurements and found to be 1.98 V and 2.5 V (vs. NHE), respectively. Therefore, a correlation can be made between the selectivity of the oxidation of benzyl alcohols and the potential of the valence band. The correlation has been confirmed by studying the photocatalytic activity of Degussa P-TiO2 , graphitic carbon nitride (g-CN), In(OH)x Sy , compounds of bismuth oxobromides, and copper(I) oxide. The direct involvement of Bi4 O5 Br2 nanoflakes hole into oxidation of benzyl alcohol was verified by Zheng et al. [15]. They further suggested that the alkoxide anions (BA− ) are the priorities for adsorption and oxidation by Bi4 O5 Br2 , even when the number of holes is present in low amount than the neutral molecule of benzyl alcohols. Migani et al. examined a reaction mechanism wherein the reaction was mediated by the holes to oxidise methanol to formaldehyde over the surface of TiO2 using theoretical calculation [16], as shown in Fig. 11.2. Contrary to the widely known

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Fig. 11.2 Suggested route for the photocatalytic CH3 OH oxidation. Reused with the permission from Blancafort et al. Copyright (2016) American Chemical Society

paradigm, where charge carriers are separated before taking part in redox reactions. An excitonic interfacial proton-coupled electron transfer (PCET) process occurs when the O−H bond is dissociated during the reaction. The exciton states bind the photoinduced excitons (h+ −e− ) pair, and then, the excitons are transported with the help of the facet of TiO2 (1 1 0) to the CH3 OH adsorbate via a coordinated energy transfer (in the form of a photon) to produce chemical energy. The h–e pair separates post breaking of O–H bond. In the above reaction, the methoxy radical (excited state radical) transfers a proton during the dissociation reaction to form an adsorbed formaldehyde radical anion, which subsequently undergoes another photochemical step to form formaldehyde. Their findings demonstrated that adsorbed CH3 OH is an additional active species in photocatalytic CH2 O oxidation. The role of excitons in the formation of formaldehyde is also addressed in the same study. Gu et al. proposed that holes and hydroxyl radicals were the most important active species when alcohols were oxidised in a TiO2 suspension [17]. Furthermore, DFT calculations (Fig. 11.3) in combination with experimental evidence were used to propose a transformation pathway. The finding suggested that ˙O2 − radicals contribute little to the oxidation reaction because they are unstable in water and easily transformed into hydrogen peroxide.

11.2.2 Strategies to Modify Heterogeneous Photocatalysts In the previous section, we addressed the significance of reactive species in transformation reactions and proposed many typical reaction mechanisms for various catalytic systems. The methodologies used to modify heterogeneous photocatalysts

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Fig. 11.3 Plausible mechanism for the oxidation reaction (alcohols to aldehydes) in aqueous TiO2 . Reused with permission from Zhao et al. Copyright (2016) Elsevier

in selective oxidation reactions will be briefly discussed in this section. To better comprehend and gain insight into the modification strategies, a relation between the basic properties of catalysts and their photocatalytic performance considering the points of detailed reaction processes. The reactions processes, such as the light harvesting ability of the catalysts, the photoinduced generation of excitons and their interaction in the reaction medium to generate reactive species, and the adsorption of molecules in any phase (liquid or gas) are discussed.

11.2.3 Effect of Metals Loading The loading of metal ions, especially the noble metals, on the surface of metal oxide photocatalysts (ZrO2 , Al2 O3 and SiO2 ) promotes the organic chemical transformation reactions. The noble metals play the role of cocatalysts when they are deposited over the surfaces of TiO2 , ZnO, CeO2 , C3 N4 , and Bi2 MoO6 [18–26]. The charge transfer in noble metals deposited photocatalysts is termed as interfacial charge transfer and their mechanisms can be named the plasmonic effect and the Schottky junction.

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As we are aware that the surface plasmon resonance (SPR) phenomenon is produced, when the energy of incident light is matched with the frequency of surface electrons of the noble metals. The collective oscillation of free electrons in noble metals causes the SPR effect. The broad absorption appears in the case of noble metals deposited photocatalysts is due to the SPR effect [22, 27–29]. The absorption band appears at 530, 400, and 580 nm are due to the Au, Ag, and Cu NPs, respectively. An example of Au NPs deposited CeO2 was illustrated by Tanaka et al. They discovered that several p-substituted benzyl alcohols were converted to their corresponding compounds when exposed to visible light in aqueous suspension [25]. A strong absorption band at 550 nm was appeared after the loading of Au NPs over the surface photocatalyst. The electrons at the CB of CeO2 were transferred by the Au NPs to activate the O2 . The electron transferred by Au NPs may create a vacancy that can oxidise the adsorbed benzyl alcohol molecules, which may provide an electron to Au to restore its original metallic state. Importantly, the size of the Au particles may have an impact on the SPR effect of photoabsorption intensity. The case of Au acting as a cocatalyst on the surface of oxygen vacancy created BiOCl is reported by Li et al. [30]. They found that the Au nanoparticles working as a cocatalyst enhanced the overall efficiency and selectivity under aerobic conditions using visible light as an energy source. Moreover, the change in the reaction pathway could be seen and presented in Fig. 11.4. The electron generated at the surfaces of oxygen vacancy created BiOCl by the SPR effect of Au could reduce the oxygen molecules to produce superoxide radicals, whereas the hole created by the transfer of electrons could abstract the α-H atom of the reactant molecule (benzyl alcohol) to generate carbon-based radicals. The above pathway provides an oxygen bridged structure post the combination of carbon-centred radicals with superoxide radicals. The final product (benzaldehyde) containing oxygen atom exchanged from O2 is obtained after the cleavage of carbon–oxygen and oxygen–oxygen bonds. When it comes to the oxygen vacancy bismuth oxychloride (BiOCl-OVs), the only ˙HO2 radical that causes the creation of carbon-based radicals, the resulting benzaldehyde does not have any swapped oxygen atoms. In addition to that, the role of hole participation is negligible. The use of noble metals other than Au, Ag, and Cu over the surface of photocatalysts is well demonstrated in the photocatalytic oxidation reactions. Pt and Pd are the other examples discussed in this section. These metals also act as cocatalyst at the surface of semiconductors and facilitate charge separation, as mentioned in the case of other noble metals. The Fermi levels the Pt and Pd are reported to be lower than those of most of the semiconductors. The transfer mechanism in these metals modified semiconductors is slightly different than that of the other noble metals as these metals act as pools to trap the photogenerated electrons during the process. The electrons captured by these metals from semiconductors promote the multielectron reduction reaction [31–34]. Tomita et al. reported excellent selectivity (97.6%) but low conversion efficiency (11.3%) during the partial oxidation of 2-propanol to acetone [35]. The reason for the low efficiency is that WO3 has low affinity towards acetone molecules, which can rule out the possibility of further decomposition of products. The conversion

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Fig. 11.4 Oxidation mechanism of benzyl alcohol. a m/z ratio of benzaldehyde in 18 O2 atmosphere using modified BiOCl. b Increased concentration of 18 O-labelled benzaldehyde in Au-BiOCl-OV. c FTIR spectra of modified BiOCl and benzaldehyde before and post adsorption. d A proposed reaction mechanism of oxidation reaction over Au-BiOCl-OV. Reused after getting permission from Li et al. Copyright (2017) American Chemical Society

efficiency drastically increased to 96.0 and 99.3% after loading 1 wt% of Pt and PdOx , respectively, over the surface of WO3 . The increase in conversion efficiency post modifications also leads to the formation of abundant H2 O2 . The high production of holes may attenuate the selectivity of acetone. Also, the generated H2 O2 may also act as a hole scavenger in the reaction of Pd-deposited WO3 , which decreased the selectivity to 79.4%. In contrast, a sudden decrease in the selectivity of Pt-deposited WO3 can be observed as H2 O2 played the role of electron scavengers. Moreover, the generated hydroxyl radicals during the reaction were also involved in the oxidation reaction. Plasmonic bands have also been identified in Pt and Pd, and their bands are found in the UV region if the particle size is small. This band can be shifted towards the visible spectrum if particle sizes are increased. The value of the working function, which is an important part of charge carriers at the metal/semiconductor surface, is also related to the size of the particles [36]. For instance, an array of Pt/TiO2 was prepared by Shiraishi et al. and applied them for the oxidation (alcohol) reaction using visible light [24]. By changing the particle size of Pt from 3 to 4 nm, a comparatively low Schottky barrier and a reasonably large number of surface deposited Pt atoms were obtained. The small particle size and uninform deposited Pt nanoparticles lead to high photocatalytic activity. In the composite, a plasmon induced electron transfer

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takes place from Pt nanoparticles to the conduction band of TiO2 , which in turn, participates in the photocatalytic reaction.

11.2.4 Non-metal Cocatalysts Loading Impact When semiconductor materials are mixed with non-noble metal cocatalysts like dyes, carbonaceous species, or semiconductor oxides, the properties of the different materials can be used in a single composite system. The benefit of this composite is that the compounded materials can change the properties of the final materials by increasing the light harvesting abilities, reactant adsorption, and activation properties. If the band structure of compounded materials is appropriately matched, the internal electric field generated by the multiple band structures will promote charge carrier transmission. Shiraishi et al. reported the preparation of WO3 -coated TiO2 for improving the selectivity towards single-phase TiO2 [37]. The inefficient conduction band of WO3 is the reason for the absence of superoxide radicals and holes. It is significant to note that while the internal electric field drives the charge transfer, the catalyst is inactive for the aldehyde breakdown. One important advantage is that WO3 support over TiO2 increases the adsorption affinity towards the benzaldehyde. A further advantage of WO3 support is that it reduces the aldehyde overoxidation site given by the exposed TiO2 surface, hence increasing aldehyde selectivity. Graphene-based materials have drawn much attention and are one of the best materials in the carbon family. Three types of graphene-based materials are reported so far including graphene, graphene oxide, and reduced graphene oxide. Two-dimensional structures, tuneable electronic structures, superior electron conductivity, and stability are only a few of their extraordinary qualities [38–40]. These materials help in the separation of photoinduced excitons and provide reactive sites when they are mixed with semiconductor photocatalysts [39]. For example, rGO promotes the conversion reaction and maintains high selectivity in the composite of C3 N3 S3 polymer with rGO [41]. The excited electron of C3 N3 S3 can be trapped by the rGO moieties to initiate the reaction of oxygen reduction as they play the role of cocatalyst. Furthermore, the rGO/C3 N3 S3 hybrids with layered sandwich structures facilitate the separation of photoinduced excitons. Meng et al. reported using a BN/In2 S3 composite to convert aromatic alcohols into aldehydes while exposed to visible light [42]. The composite material was found to show better activity than that of the single component system. The existence of surface nitrogen vacancies in BN creates a negative charge in the system, which attracts and transfers the photogenerated holes, resulting in the separation of charge carriers and an improvement in photocatalytic activity.

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11.2.5 Tuning of Electronic Structure The overall photocatalytic reactions are divided into three major steps. These steps start with the creation of photoinduced excitons, their migration to the surface, and end with their involvement in the reaction. The electronic properties associated with the intrinsic characteristics of the semiconductor photocatalysts play an important role in driving the catalytic process. For example, the introduction of energy level between the VB and CB by doping with foreign elements, surface defect engineering, and complex formation on the surface elevates the catalytic performance of photocatalysts. In the large bandgap semiconductor materials (TiO2 , Nb2 O5 ), the response to visible light is determined by the complex formation on the surface of catalysts during the transformation reaction, as the surface complexes change the electronic properties of the materials [43, 44]. As depicted in Fig. 11.5, the substrates having electron-populated atoms (X = O, S, and N) are adhered to the catalysts through weak bond interaction. The interaction through the surface creates impurity states above the VB, which are known to donate electrons during the photocatalytic reaction. The electron transfer by this medium is termed as ligand-to-metal charge transfers (LMCT), which is started by the absorption of visible light. After absorption of visible light, the metal oxides transfer electrons to the CB and leave a hole in the VB for the reduction and oxidation reactions. The Zavahir group reported on a system of this kind and asserted that the V6 O13 -alkoxide complex possesses broad visible light absorption [45]. They added that the selective oxidation of alcohols is catalysed by mixed-valence V6 O13 on a variety of supports, and that the described photocatalysts have exhibited remarkable selectivity towards aldehydes and ketones. A complex formation in the form of V6 O13 -alkoxide started the photocatalytic reaction by absorbing the visible light over a broad range. Then, an α-H atom is released with the assistance of O2 during the excitation reaction of the V6 O13 -alkoxide complex, which in turn, produces corresponding carbonyl compounds. Additionally, it was discovered that the grafted Fig. 11.5 Surface complexation heteroatom modified metal oxides (metals = Ti, Zn or Nb) for green light driven photocatalytic reaction

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photocatalysts were an effective catalyst for breaking and activating the C–H bonds of saturated aromatic hydrocarbons, resulting in large yields of aldehydes, ketones, alcohols, acids, and esters. Recently, the photocatalytic performance of surface-modified TiO2 was investigated by Higashimoto et al. [46]. They found that Fe(III)-ions modified TiO2 has dramatically increased photocatalytic activity in the selective oxidation of alcohols. The grafting of Fe(III) ions generated a change in the electronic structure of TiO2 , which has two aspects: (1) hybridisation of electron filled Fe 3d orbitals with alcoholate species O 2p; (2) formation of acceptor levels by unoccupied Fe 3d orbitals. Excitation from the hybridisation levels to the CB of TiO2 occurs when exposed to visible light. Furthermore, the excited electron could be confined by the acceptor levels formed by Fe(III) ions, speeding up the charge separation and electron reduction reaction processes.

11.2.6 Effects of Surface Modification The performance of a photocatalyst mainly depends on the surface catalytic properties of the semiconductors. This is because the adsorption, activation, and desorption of reactants all happen on the surface of the catalysts. The surface properties of catalysts that matter the most include porosity, adsorption and desorption capacity, surface area, surface defects such as hydrophilicity and hydrophobicity, acidity and basicity, and organic groups. By modifying the surface, we can change all the above properties. For example, Zhao et al. found that treatment of TiO2 with SiO2 and a protonation treatment (Bronsted acid) accelerates the dissolution of surface Ti from the Ti-peroxide, which aids the selective oxidation of alcohols to aldehydes [47]. Also, Zhang et al. reported that acid treatment of g-C3 N4 improves its catalytic properties toward the selective oxidation reaction [48]. They found the effective role of sulphuric acid treatment in the enhanced reaction and explained the reason in many aspects discussed here. The acid treatment increases the surface area and forms a porous structure. The bandgap energy is increased by the quantum confinement effect, and more significantly, additional acid sites are created over the surface, which makes the process of b-hydride elimination easier. Furthermore, the effect of defects (surface or bulk) density ratio over the surface of TiO2 catalysts was demonstrated by Yan et al. by calcining anatase or rutile TiO2 [49]. Further, use of the calcined TiO2 was made in the oxidation reaction of phenylethanol to acetophenone. When compared to bulk defects TiO2 , the surface defects TiO2 (particularly oxygen vacancies) was shown to exhibit the highest activity since the bulk defects catalysts frequently serve as recombination sites, whereas the surface defects catalysts can capture the charge carriers and thereby improving the separation efficiency of charge carriers. The other advantage of an oxygen defects catalyst is that it can promote the adsorption of reactant and help in absorbing a broad range of visible light.

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11.3 Selective Conversion of Amines to Imines The oxidation of amines to imines is one of the most important chemical transformations because it is a key step in the reaction of the drug and pesticide intermediates [50]. The traditional method requires a strong oxidising agent such as 2-iodylbenzoic acid or N-tert-butylbenzenesulfinimidoyl chloride for the oxidation reactions, which are toxic in nature and cause the production of wastewater [51]. In the conventional route, the condensation of amines and carbonyl compounds (aldehydes) takes place at the same time. However, the toxic by-products are the reason to avoid the conventional method. Recently, semiconductor-based heterogeneous photocatalysts have been at the forefront in the oxidation of amines, as this method provides a safe and green way for the oxidation reactions using visible light as an energy source to initiate the reaction and O2 as an oxidant. Three novel routes for the synthesis of imines have been explored by Chen et al. [52]. The first pathway involves the cross-coupling of alcohols and amines, whereas the second step follows the self-coupling of primary amines. The final and third steps include the oxidative dehydrogenation of secondary amine. A new pathway using oxygen as an oxidant has occurred when both thermal energy and sunlight are used as the energy source. However, self-coupling of primary amines and oxidative dehydrogenation of secondary amines are the techniques that have garnered the most interest from the scientific community because amine as the organic substrate guarantees great selectivity towards the desired imine.

11.3.1 Reaction Mechanism The existing mechanism for the self-coupling reaction of primary amines, as described in the literature (Fig. 11.6), has been concisely outlined by Park et al. [53]. The process begins with the abstraction of protons from amine molecules to activate them. The resulting nitrogen-centered cation radicals then react with superoxide to form the corresponding imines. In method A, the imine was obtained by a dehydration condensation process of an amine molecule via an aldehyde intermediate. An imine intermediate is produced without generating any other by-product except the H2 O2 in paths B, C, and D. The generated H2 O2 also promotes the reaction by serving as an oxidant during the formation of the imine intermediate, which may, in turn, afford the final imine product by following paths C or D. The reaction mechanism of amines oxidation in the presence of g-C3 N4 is demonstrated by Wang et al. and presented in Fig. 11.7 [54]. The process begins with the formation of charge carriers caused by irradiating mpg-C3 N4 with an energy equal to or greater than the bandgap energy. After the excitation, electrons and holes are free to participate in the reaction. The oxidative coupling reaction discussed here is the important reaction and proceeds in several steps. Initially, the carbocationradical type intermediate is formed after the loss of an electron from amine. The

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Fig. 11.6 Suggested reaction pathway for the coupling reactions of primary amines to imines. Reused with permission from Park et al. Copyright (2012) American Chemical Society

superoxide radicals would then extract a proton and a hydrogen atom to generate the corresponding imine. The abstraction of proton and hydrogen are rate-determining steps in the oxidation reaction. The imine and a water molecule are produced by the two-electron cycle on mpg-C3 N4 . Simultaneously, the positively charged hole is free to associate with the imine, making the imine more susceptible to nucleophilic attack by the amine to produce the aminal. Finally, the aminal group would go through a hole assisted elimination reaction to remove ammonia, resulting in the final coupled product [55].

11.3.2 Modifications of Semiconductor-Based Photocatalysts 11.3.2.1

Regulating the Crystal Structure

As explained in the preceding section, the light harvesting, reactant adsorption, and activation capabilities of the identical semiconductors are determined by their electrical structure and surface structure. Dai et al. provide the clearest explanation

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Fig. 11.7 Proposed mechanism for photo-oxidation of amine catalysed by semiconductor

of the idea by creating a mixed-crystal phase catalyst that is composed of 35% anatase form of TiO2 [54]. They then used the above mixed-phase photocatalysts for aerobic benzylamine-to-imine coupling under visible light. The composite material was found to show better activity than that of single component photocatalysts, as the composite increases the adsorption capacity of TiO2 towards the benzylamine and also helps in maximising the redox ability of the materials. Benzyl amines incorporation with TiO2 results in the development of many surface complexes that aid in the extensive absorption of visible light. The formation of heterojunction between both phases also minimises the back electron transfer and elevates the charge separation efficiency. Like this, it has been observed that Nb2 O5 microfibers in their crystalline and amorphous forms have higher activity for oxidising benzylamine compounds to imine [56].

11.3.2.2

Effect of Doping

The doping of N and oxygen vacancies (OV) in TiNb2 O7 created a new energy level in the material and altered the physico-chemical properties of TiNb2 O7 . Another important role of doping is that it lowers the bandgap energy to get more absorption of visible light [57]. Under green light irradiation, benzylamine conversion was greatly enhanced, but untreated or mono-modified catalysts failed to meet the requirement. As is well known, a rising star in the family of graphene is the graphene quantum dots (GQDs) as they have broad scopes in catalysis. Kim et al. examined the structure– activity interactions of GQDs doped with heteroatoms [58]. They found out that the

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composition and quantity of heteroatoms influence photoactivity in the coupling reaction of benzylamine. The nitrogen- and sulphur-codoped GQDs were found to show the better catalytic activity and have a high yield (98%) for N-benzylbenzaldimine under the conditions of light and oxygen. The yield obtained by the nitrogen- and sulphur-codoped GQDs catalysts is higher than that of the mono-doped GQDs. The excellent photocatalytic activity of codoped GQDs could be described by the following three factors; (1) an electron-rich environment was created by the presence of N and O atoms and other conjugated C atoms with the sp2 hybridised C, which helps in increasing the Fermi level and broad absorption of visible light, (2) the fewer structure defects in codoped GQDs promote the longer excited singlet state lifetime, favouring in the production of superoxide, (3) the codoped GQDs have a higher number of pyridinic N atoms on the surface of the catalyst compared to the pyrrolic version. The pyridinic N atoms helps in enhancing the oxygen reduction reaction processes. Furthermore, the same group created photocatalysts using MoSx -doped hollow carbon dots for imine production [59]. When compared to undoped quantum dots, a similar effect was seen due to the pyridinic N atoms that help in improving the performance. Additionally, the inclusion of MoSx particles promotes the reaction utilising broad visible light and an O2 reduction reaction.

11.3.2.3

Effects of Heterojunction Composites

A literate is available wherein the Cu nanoparticle (surface plasmon) supported over graphene acts as a photocatalyst for the conversion of benzylamine. The selectivity and conversion towards the N-benzylbenzald-imine were 93 and 99%, respectively [60]. Graphene sheets sustained the Cu nanoparticles in their metallic state rather than any other oxide forms (Cu2 O or CuO) during the catalytic reaction. As a result, it was found that the reported catalyst had good potential for reuse in organic synthesis. The heterojunction photocatalysts have also been tested for the selective conversion of amine to imine using visible light source. Some recent developments are discussed here. Recently, Dong et al. created the g-C3 N4 /Bi2 WO6 catalyst to synthesise imines from amines under visible light using O2 as an oxidant [61]. They concluded that the holes and ˙−O2 radicals are the main species responsible for the formation of imines. Fu et al. also studied the synthesis of imines over CdS/g-C3 N4 heterojunction using O2 as an oxidant in visible light [62]. They have also found that the hole and ˙−O2 radicals were the sole contributors in the oxidation reaction, as confirmed by the ESR analysis and scavenger experiments. Very recently, Dhakshinamoorthy et al. have synthesised a metal organic framework named UiO-66(Ce) via a solvothermal method and investigated the oxidation reaction under aerobic condition using visible light as a source [63]. The reaction mechanism of all the examples above follows the same pathway as discussed in the case of mpg-C3 N4 photocatalyst. During photocatalysis, the band edges of the photocatalytic materials play an important role in generating reactive oxygen species (ROS) and directing the oxidation of reactants. When the conversion process is inefficient, charged species with

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significant redox abilities are especially important. The redox ability of the photogenerated excitons can be increased by designing a photocatalyst with a Z-scheme charge transfer system. The main advantage of Z-scheme-based photocatalysts is to minimise the back electron transfer processes while simultaneously retaining strong redox abilities. The Z-scheme-based reaction mechanism in the oxidation reaction is explained by taking the example of AgI/Ag/AgVO3 nanocomposites [64]. The reaction proceeds by combining the electrons with holes through a metallic mediator (Ag0 ). The remaining electrons and holes after their separation were present in the distinct sites for the reduction of O2 and oxidation of amines, respectively. The single component photocatalysts such as AgI and AgVO3 evinced very low selectivity and conversion efficiency for benzylamine compared with a 20% AgI/Ag/AgVO3 photocatalyst.

11.3.2.4

Effects of Surface Modifications

The redox abilities of semiconductors can be increased by developing a nanoscale morphology because the energy gap can be enlarged due to the quantum confinement effect. The effect of quantum confinement is well explained by Raza et al. by comparing the photocatalytic activity of bulk and exfoliated WS2 nanosheets [62]. According to their findings, bulk WS2 with layered multilayers was inactive because of its limited bandgap energy (1.3 eV). On the other hand, an exfoliated monolayer WS2 nanosheet has shown excellent activity due to its large bandgap energy (1.9 eV), as this bandgap fulfills the reduction of O2 and oxidation of benzylamine. If the single component photocatalyst has different facets, such as multifaceted TiO2 , Cu2 O, BiVO4 , and BiOCl, spontaneous charge separation happens in singlephase semiconductors as well [62, 65, 66]. Yuan et al. synthesised BiVO4 with a 1.17 ratio of (0 4 0) and (1 1 0) facets and investigated its activity for the aerobic oxidation of different modified amines (N-t-butyl and dibenzyl amines) using a visible light source [65]. Upon photoexcitation, the photogenerated electrons and holes are assembled on the (0 4 0) and (1 1 0) facets of BiVO4 , respectively, leading to efficient separation of charge carriers. BiOCl ultrathin nanosheets (BiOCl C-UTNSs) were prepared by Wu et al. [67] for the synthesis of secondary amines in acetonitrile. The hydrophobic surface, along with abundant oxygen vacancies on the (0 0 1) facets contributes to the excellent activity of BiOCl C-UTNSs. The excellent activity with 78% conversion and 94% selectivity was reported on the hydrophobic surface of BiOCl under irradiation for 1 h. A very weak photocatalytic activity with 15% conversion was observed using BiOCl as a hydrophilic nanosheets under the same reaction conditions.

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11.4 Reduction of Nitro Compounds The reduction of nitroaromatic to anilines is regarded as an efficient method since anilines are essential industrial chemicals used in the production of dyes, agrochemicals, medicines, and resins. The general chemical reduction strategy often necessitates high pressure and temperature while using molecular hydrogen (H2 ), which may have an impact on other functions such as carbonyl and vinyl groups. The photocatalytic reduction of nitro compounds is a green approach, as it avoids the use of molecular H2 , high pressure and temperature. Recently, a series of visible light active semiconductors have been utilised for the reduction of nitroaromatics to their corresponding anilines using an oxygen-free solution.

11.4.1 Reaction Mechanism The six-electron reduction process is used in the photocatalytic reduction process. The process either starts with the intermediates (nitroso and hydroxylamine) or a condensation route. The condensation routes process through azoxybenzene, azobenzene, and hydrazobenzene [68]. The photocatalytic reaction system requires the addition of alcohols, acids, or NaBH4 . The addition of alcohols traps the photogenerated holes, acids act as hydrogen source; and NaBH4 in the reaction medium plays the role of a reductant [68]. The different photocatalytic reduction pathways are proposed in Fig. 11.8.

11.4.2 Engineering in Heterogeneous Photocatalysts 11.4.2.1

Crystalline Structure Controlling Effect

The low valence states in the metal oxide catalysts serve as efficient binding and activation sites and are responsible for reducing electron-deficient groups like (– NO2 ). The presence of Ti3+ in rutile TiO2 has been found to show excellent activity for the hydrogenation of nitrobenzene in alcohol. The rutile phase TiO2 with Ti3+ was reported to perform better than that of anatase TiO2 due to the large amount of Ti3+ on the surface of TiO2 . The interfacial charge transfer takes place between VB of TiO2 and nitrobenzene due to the interaction between Ti3+ and nitrobenzene, which helps in weakening the N–O bonds and promoting the transformation of nitrobenzene to aminobenzene. Aside from the surface structure, the electronic properties of semiconductors (crystal forms and facets) also influence the catalytic activity and facilitate the hydrogenation of nitrobenzene [69].

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Fig. 11.8 Possible reduction mechanism to produce anilines

11.4.2.2

Effects of Doping

The reduction of nitroaromatics has also been tested using heteroatoms-doped materials, as the heteroatom doping is known to tailor the electronic properties of the materials, which may in turn enhance the photocatalytic activity. Yang et al. reported nitrogen-doped graphene and used a small amount of NaBH4 as a reductant. The photocatalytic performance of the materials was tested for the reduction of nitroarenes in an aqueous suspension [70]. They also concluded that the graphite N form is critical for accelerating the reaction between the various doped nitrogen species. The graphite N form type doped materials help in the activation of adjacent carbon atoms. The developed materials had the lowest adsorption energy for nitroarenes, allowing for bulk production of the corresponding anilines.

11.4.2.3

Creation of Multicomponent Composites

The creation of multicomponent composites is the most ubiquitous way to promote the reduction of nitroarenes because multicomponent composites improve the life time of charge carriers during the catalytic reaction. Dai et al. reported the hydrogenation of nitrobenzene to aniline using CdS/g-C3 N4 composites as photocatalysts [71]. The reaction is an example of the reduction of nitrobenzene coupled with the oxidation of alcohols under anaerobic conditions. The oxidation took place at the VB of g-C3 N4 through the photogenerated holes present in the VB of g-C3 N4 .

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At the same time, the electrons available in the CB of CdS take part in the reduction of nitrobenzene. An example of a binary composite (ZnO/CdS) with enhanced performance for the reduction of nitroarenes is presented, with the results indicating that the ZnO/CdS photocatalyst performed better than ZnO alone under simulated light irradiation [72]. The enhanced photocatalytic performance for the reduction reaction is explained by the Z-scheme charge transfer mechanism. However, a competition between type II and Z-scheme mechanisms was noticed in the case of the ZnO/CdS photocatalyst. The introduction of Au nanoparticles between ZnO and CdS resulted in the formation of a Z-scheme heterostructure, which was found to have double the activity of a ZnO/CdS photocatalyst. The vectorial Z-scheme was only allowed among the ternary systems, where the electrons in the CB of ZnO were transferred to the VB of CdS through the Au nanoparticles that bridged ZnO and CdS. The above systems improved charge separation ability and, more crucially, avoided photo-corrosion of CdS as holes generated during the reaction are consumed by ZnO. The role of carbon-supported heterogeneous photocatalysts was also investigated for the reduction of nitro-organics under visible light irradiation. In this regard, a series of carbon-based materials such as CdS decorated over graphene, In2 S3 over carbon nanotubes (CNTs), Pt and TiO2 nanoparticles over the reduced graphene oxide, and TiO2 and g-C3 N4 over the sheets of graphene have been reported for the reduction of nitroarenes [73]. The detrimental recombination of the photoinduced electron hole pair was hindered due to the presence of conjugated sp2 C atoms in the carbon-based materials. Furthermore, the role of CNTs was explored, since the inclusion of CNTs may result in substrate adsorption over the catalysts, which may boost the performance in the reduction reactions. Recently, Li et al. investigated the photocatalytic synthesis of anilines using BiVO4 /Fe2 O3 heterojunction under visible light irradiation [74]. They proposed that the nitrosobenzene intermediate is the intermediate that directs the synthesis of anilines. In the same paper, they discussed the mechanism on the surface of the catalyst and proposed that the catalyst follows the Z-scheme pathway. Under visible light irradiation, both semiconductors get energy and excite their electrons from the valence band of both semiconductors to their conduction bands. After excitation, the electrons from the CB of Fe2 O3 transferred to the VB of BiVO4 , thus minimising the back electron transfer process and ultimately enhancing the life time of the photogenerated excitons. After the above charge transfer process, electrons accumulate at the CB of BiVO4 and holes are gathered at the VB of Fe2 O3 . The holes present in the VB get oxidised the hydrazine hydrate into 6H+ and 6e− to fulfill the electrons deficiency. These protons and electrons participate in the nitro group reduction. At the same time, the electrons collected in the CB of BiVO4 promote the reduction reactions with the help of electrons and protons produced by the oxidation of hydrazine hydrate. The above processes increase the selectivity as well as the conversion yield of the nitro compound reduction. In the same year, the selective hydrogenation of nitrobenzene to different derivatives of anilines in the presence of CQDs/ZnIn2 S4 nanocomposites was studied by Wang et al. [75]. The electron-rich CQDs can act as frustrated Lewis acid–base pairs (FLPs) to encourage the hydrogenation of nitrobenzene to

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nitrosobenzene (NBS), phenylhydroxylamine (NPH), and then aniline with the help of triethanolamine (TEOA) as a hydrogen source. This process results in a higher yield of aniline over CQDs/ZnIn2 S4 nanocomposite than over bare ZnIn2 S4 . The reduction of nitroaromatic compounds was investigated by Lei et al. under visible light irradiation using Ce doped UiO-66/graphene composite [76]. The group studied the role of Ce, graphene, and Zr in the advancement of reduction of nitroaromatic compounds and discussed the mechanism of reduction over the catalyst surface. The role of graphene in the composite material is that the graphene helps to increase the adsorption of nitrobenzene and the separation of charge carriers. The doping of Ce acts as a mediator and can switch over Ce3+ and Ce2+ after oxidation and reduction reactions. The separated electrons participate in the reduction reaction of nitroaromatic compounds, whereas the holes are trapped by the RCH2 OH, which in turn provides H+ as the reducing agent.

11.5 Benzene Compounds Hydrocarbylation Benzene hydrocarbylation gives phenol which is widely used in both industry and society at large. The most common methods used for the synthesis of phenol are sulfonation, toluene-benzoic acid, chlorination, or cumene processes, which are traditional approaches and require harsh reaction conditions and toxic chemicals [77]. Researchers have recently focused on the direct hydroxylation of benzene to phenol using catalysts and light because it is a green and environmentally acceptable approach.

11.5.1 Mechanisms for the Synthesis of Phenol The direct hydroxylation of benzene is first started by the activation of the unsaturated sp2 C–H bond, which is initiated by the attack of holes or hydroxyl radicals having strong oxidation ability. The different active oxygen species produced throughout the reaction (mostly ˙O2 − , ˙HO2 and ˙OH) combine with the already established cationic radical species of benzene to yield phenol as a result. Thuan et al. explained the reaction mechanism of phenol synthesis over the surface of the anatase and rutile phases of TiO2 [78]. The ˙OH and h+ were the principal active species for hydroxylation reactions, where oxygen is transported from H2 O to phenol, as illustrated in Fig. 11.9a. In rutile phase TiO2 , the organic peroxide radicals or ions are formed by the attack of O2 or ˙O2 − on the activated benzene Fig. 11.9b. At last, the phenol is formed, and the oxygen is originated from water molecule oxygen atom. The fundamental distinction between the two TiO2 phases is the difference in energy levels and/or atomic arrangement on the surface. The oxidation potential of benzene in an acetonitrile solvent was reported by Markel et al. [79]. They have discussed that the required potential for the oxidation

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Fig. 11.9 Proposed mechanism for the synthesis of phenol via benzene a reaction is proceeded in the presence of anatase TiO2 and H2 O is used as the source of oxygen, and b TiO2 in rutile form and pure O2 is supplied during the reaction. Reused after permission from Matsumura et al. Copyright (2010) American Chemical Society

of benzene is 2.48 ± 0.03 V (vs. SCE), which further suggests that wide bandgap semiconductors are necessary for the oxidation of benzene. Zheng et al. provided an explanation for the usage of the non-plasmonic metal cocatalysts over the surface of TiO2 for the oxidation of benzene [80]. The synthesis steps for the formation of phenol are given in Fig. 11.10. Pristine TiO2 was unsuitable for the activation of benzene due to its large bandgap energy, whereas Au@TiO2 was found to show better activity with 63% yield and 91% selectivity for phenol within 3 h of light irradiation. The surface plasmon resonance (Au NPs) would transfer the photoexcited electrons to the CB of TiO2 , where reduction of oxygen takes place. At the same time, the oxidation of adsorbed phenoxy anions happens at the electrondeficient Au nanoparticles, resulting in the formation of phenol. The electronegativity of the metal influenced the oxidation of phenoxy anions, which is considered a major step in the formation of phenol during the reaction. Ag@TiO2 was found to show negligible activity, and the reason is that Ag is least electronegative of the noble metals. The order of electronegativity of noble metals is given as (Au > Pt > Ag). The photocatalytic activity of Pt@TiO2 was found in the order of its electronegativity, indicating a catalytic efficiency in the middle of Au and Ag-deposited TiO2 .

11.5.2 Engineering in Semiconductor Photocatalysts Devaraji et al. investigated the oxidation of benzene in water using V-doped TiO2 loaded with Au nanoparticles [81]. An enhanced activity was observed when the loading content of vanadium in TiO2 –Au was (2 atom %). The conversion efficiency

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Fig. 11.10 Plausible mechanism for the formation of phenol using TiO2 as the catalyst

was 18% with the modified material, whereas pure TiO2 photocatalyst was found to be inefficient in the conversion reaction. The reaction is preceded by the holes in TiO2 as Au/TV2 cannot generate the charge carriers under the illumination of visible light. The SPR-induced holes do not have sufficient oxidation potential to oxidise the organic substrate. The Au and V elements operate as electron sinks/traps to eliminate the recombination of carriers, potentially increasing the utilisation of holes in TiO2 . The presence of H2 O2 and oxygen, which aid in the generation of hydroxyl radicals by consuming electrons, contributes to the high catalytic efficiency. The hydrocarbylation of benzene over WO3 photocatalysts can be increased by depositing Pt nanoparticles on the surface of WO3 [82]. The Pt-loaded nanoparticle over WO3 has shown some interesting improvement in the conversion of benzene, which has been found to increase the conversion of benzene from 16.4 to 69.4% with high selectivity for phenol (74%) under visible light irradiation. However, a large amount of CO2 is generated as a by-product by both TiO2 and Pt/TiO2 during the hydrocarbylation reaction. The different electronic structures and adsorption capabilities of WO3 and TiO2 may account for the variation in selectivity between these materials for organic molecules. Because WO3 has a lower adsorption affinity for benzene, the holes it produces prefer to mix with water instead of benzene. The hydroxyl radicals produced by the reaction of VB holes with WO3 are responsible for the production of phenol. Hence, the oxygen atom added to phenol is derived from the aqueous water molecule during the photocatalytic reaction of WO3 . The Pt-deposited materials promote the production of H2 O2 , but the produced H2 O2 contributes little to the decomposition of phenol. The direct oxidation of benzene molecules is possible with a portion of photogenerated holes of Pt/TiO2 but the final product (phenol) is obtained by the synergistic attack of O2 or ˙−O2 . Phenol containing O atoms produced in the Pt/TiO2 is originated from both dissolved molecular O2 and

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H2 O. When a significant contact between TiO2 and organic molecules takes place, phenol is overoxidised into CO2 and H2 O. Choi et al. developed hydrophobically modified mesocellular siliceous foam for entrapping TiO2 nanoparticles to enable benzene hydrocarbylation, which is a common technique to get hydrophilic products from hydrophobic reactants [83]. After entrapping TiO2 nanoparticles in hydrophobically modified mesocellular siliceous foam, the selectivity and yield of the generated phenol increased significantly. Further degradation of phenol was avoided due to the simultaneous adsorption and desorption of benzene over the surface of TiO2 .

11.6 Conclusion and Future Prospects The use of heterogeneous photocatalysts in organic synthesis is demonstrated to be a green and environmentally friendly approach, but the process still faces some drawbacks due to unsatisfactory conversion and selectivity. The fundamental cause of the low conversion efficiency is the insignificant presence of certain active species due to thermodynamic or kinetic restrictions. Additionally, the efficiency of target compounds being produced during organic oxidation processes tends to decrease owing to the excessive generation of reactive oxygen species, particularly strongly oxidising species like the nonselective hydroxide radical and the oxidisable holes produced in the deep VB of wideband gap semiconductors. Controlling the level of participation of different active species by changing heterogeneous photocatalysts is crucial for achieving effective selective organic synthesis. The recent discoveries on modifying the heterogenous photocatalysts and their use in organic synthesis reactions, as well as their reaction mechanism, have been elaborated in the chapter. The role of intrinsic properties of the catalysts in light absorption, improving the life time of charge carriers, adsorption or desorption of molecules, and the participation of reactive oxygen species is explored. This chapter offers readers an idea of modifying the heterogeneous photocatalysts in order to apply them for various organic reactions. The present emphasis is on combining inorganic transformation with selective organic synthesis on a single catalytic device. For example, additional benefit will be obtained if the added organic hole scavengers convert into a valuable substance instead of CO2 during the hydrogen production reaction. The rate of hydrogen evolution is higher in this case than overall water splitting. Likewise, nitrogen fixation reactions can also be possible. However, more theories and techniques are still needed for further completion to extend the study application to other organic synthesis reactions. The selective photoreduction reactions are restricted to the nitro compounds only, and further exploration is still needed. The studies are very scant for alkenes, aldehydes, ketones, etc. More research is also needed on cross-coupling reactions such as C–C, C–N, C–O, and C–S coupling. The precise roles and detailed kinetic steps of numerous active species involved in photocatalytic transformation events must be determined using improved characterisation techniques and reliable theoretical simulations.

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Part IV

Theoretical Studies of Photocatalytic Material

Chapter 12

Strain Engineering for Tuning the Photocatalytic Activity of Metal–Organic Frameworks Monika Dubey, Subhasha Nigam, and Monika Joshi

Abstract Recently, metal–organic frameworks (MOFs) are considered as an innovative class of 3D materials with potential applications in water treatment, energy storage, and sensing. MOFs exhibit unique properties toward their structures including inorganic nodes in connection with organic linkers. MOFs have many energy-induced applications like gas storage, hydrogen production, sensing, and catalysts, but they also have become preferred option for photocatalytic activity due to their large surface area, flexibility, porosity, and band gap tuning. The advanced design and strain engineering for tuning the photocatalytic activity of MOFs are very crucial. The substitution of metal zones in inorganic nodes and their connection with organic linkers decides the electronics, optical properties, and photocatalytic process selectivity of designed MOFs. Herein, we have elaborately discussed the recent advances regarding strain engineering for tuning the photocatalytic activity of MOFs. Moreover, a concise summary of the present challenges and an outlook for the designing of photocatalytic process selectivity of MOFs have also been provided. Keywords Photocatalysts · Metal–organic frameworks (MOFs) · Strain engineering · Band gap modulation · Tuning selectivity

12.1 Introduction Catalysis processes are used to enhance the rate of reaction via the substances which are called catalysts [1]. Catalysts are not utilized in the process, they react to form

M. Dubey Department of Applied Science and Humanities, IIMT Engineering College, Greater Noida, Uttar Pradesh 201310, India S. Nigam Amity Institute of Biotechnology, Amity University, Noida, Uttar Pradesh 201313, India M. Joshi (B) Amity Institute of Nanotechnology, Amity University, Noida, Uttar Pradesh 201313, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Garg and A. Chandra (eds.), Photocatalysis for Environmental Remediation and Energy Production, Green Chemistry and Sustainable Technology, https://doi.org/10.1007/978-3-031-27707-8_12

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intermediates and remain unchanged after reaction. Highly efficient catalytic materials with an exciting and fast-paced process are a hot subject to be researched [2]. Advanced catalytic processes [3] are used to make more than 95% (by volume) of today’s chemical products. Photocatalysis is one of the widely used advanced catalytic processes. It is a prominent technique to convert the light energy into chemical energy [4]. Photocatalysts get excited via light harvesting and produce electron and hole pairs. This electron and hole execute chemical oxidation or reduction because of their activated states as shown in Fig. 12.1. Photocatalytic removal of organic and inorganic pollutants, water splitting, reduction of CO2 along with other gases, and detection of compounds are eco-friendly and sustainable approach of advanced oxidation process [4]. Metal–organic frameworks (MOFs) provide excellent potential for the rational design of novel photocatalytic materials in above-mentioned areas. MOFs are inorganic–organic mixed crystalline porous substances which include a systematic array of charged metal ions encircled by organic linking molecules [5]. The metal ions show nodes that connect the sides of linkers simultaneously making of a persistent, aviary-like formation. MOFs have a remarkable high internal surface area because of exhibiting hollow structure. They have been proven ideal for a variety of energy-related applications, including gas storage, hydrogen production, heterogeneous catalysis, drug delivery as well as photocatalysis due to their porosity and flexibility [6]. They are good photocatalysts if they have band gap range between 1.5 and 3.5 eV. Minimum required band gap for photocatalysis is almost 1.23 eV and for an effective photocatalysis in visible light band gap should be in range of 2–3 eV. If band gap energy is more than 3 eV (ranges 3–3.5 eV), effective photocatalysis occurs under UV irradiation [7]. Their derivatives have unique properties, allowing them to be used as a very adaptable platform for photocatalysis, which is a fast growing interdisciplinary study area [8]. Figure 12.2 shows the different photocatalytic applications of MOFs. Fig. 12.1 Schematic representation of photocatalytic process in semiconducting nanoparticles

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Fig. 12.2 Various applications of MOF nanoparticles

Researchers are now experimenting with a variety of functionalization methods for MOFs, including post-synthetic modification [9]. These alterations can be accomplished by using functional ligands to build metal clusters or organic ligands and embedding useful molecules such as organometallic, metal nanoparticles, enzymes, heteropoly acids, and other molecules within the pores [10]. MOFs’ materials provide significant benefits as catalysts over other commonly used photocatalytic materials such as clays, zeolites, and mesoporous silica [11]. Among the many prominent multifunctional catalytic properties of MOFs, the following are worth mentioning: (1) structural tuning, (2) component diversity, such as linkers, nodes, and holes (3), (4) highly tunable and uniform porous environments, (5) recognition and transport of products and substrates, (6) the well-defined and adjustable framework allows to comprehend the catalytic mechanism, (7) structure–function relationship at the molecular level, and (8) rigorous catalysis in a wide range of catalytic reactions [12]. MOFs belong to the soft materials’ class because their structures are highly open, allowing them to withstand minor mechanical strains without irreversible structural changes. Their adaptability has lately been put to use in the construction of strain-based sensors with previously unheard-of performance [13]. Strain engineering in MOFs can be an effective way to change the electrical structure and band gap, as well as the optical and photocatalytic capabilities [12]. Figure 12.3 shows the different approaches to enhance the photocatalytic ability of designed MOF. In this chapter, we introduce photocatalytic behavior of MOFs, their electrical conductivity, the charge transfer phenomenon over the light-excited material, and different strategies to amplify the photocatalytic activity of MOFs [14]. Due to their porosity, flexibility, and high surface-to-volume ratio, MOFs can not only be superior materials for the gas adsorption, but they are also a good substance to intern

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Fig. 12.3 Different approaches to enhance the photocatalytic ability of designed MOF

the size of the loaded catalyst in nanometer range. In MOFs’ structure, the metal nodes and organic linkers both can be simply tuned for particular functionality [15]. Therefore, MOFs exhibited more promising photocatalytic performance compared to other semiconductor materials. However, stability, reusability, band gap tuning, and poor electronic conductivity are main issue with MOFs. So, modification of MOFs via organic linkers, doping of metal ions, or mixing of other materials can overcome such limitations [16]. Herein, the advanced design and strain engineering for tuning the photocatalytic activity of MOFs are discussed.

12.2 Strain Engineering for Tuning Photocatalytic Activities 12.2.1 Electrical Conductivity Tuning Electrical conductivity is an intrinsic feature of the material [17]. In case of semiconductor materials, conductivity depends on charge carrier transportation [18]. Most of MOFs have a drawback of poor conductivity, which depends on the charge transfer between organic linker and metal cluster [19]. Previous studies suggested that doping can largely boost the electrical conductivity of MOFs based on the polarons/bipolarons and solitons mechanism and

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1D/2D MOFs exhibited higher electrical conductivity in contrast to 3D framework. 7,7,8,8-tetracyanoquinodimethane (TCNQ) is a good electron acceptor molecule. Some studies reported that the conductivity of MOFs can be tuned via doping of TCNQ [14]. Since TCNQ exhibits high electron affinity and the charge transfer within framework. Huang et al. effectively enhanced the electronic conductivity of a porous structure 3D-MOF Cu-TATAB (where TATAB = 4,4' ,4'' -((1,3,5-Triazine2,4,6)tris(azanediyl))tribenzoic acid) by > 4 order of magnitude via doping of TCNQ into its framework [20]. Initially, Cu-TATAB has conductivity 9.75 × 10−12 S/cm and shows insulating behavior at room temperature, and after doping of TCNQ into its framework, the conductivity of TCNQ@Cu-TATAB was reported as 2.67 × 10−7 S/cm showing semiconducting behavior at room temperature. Similarly, Sengupta et al. tuned the electrical conductivity of 2D framework of [Cu(TPyP)Cu2 (O2 CCH3 4] and 3D framework of [Cu(TPyP)·CuCl2 ]2 ·5TCE·7H2 O] (where TPyP = 5,10,15,20tetra-4-pyridyl-21H,23H-porphine/porphyrin and TCE = tetrachloroethane/CHCl3 ) via doping of TCNQ by > 3 order of magnitude [21]. According to BET analysis, the high surface area of photocatalysts is a significant factor in photocatalytic activity. MOFs have high surface areas of several hundred m2 /g evaluated by the BET technique, depicting the possibility of application for efficient photocatalytic materials. Some studies reported synthesis of conducting frameworks via iodine doping. Lu et al. prepared an iodine-doped 3D microporous MOF exhibiting conductive polymer (porous polyaniline network) with well-defined ordered micropores of 0.84 nm diameter, BET surface area of 986 m2 /g, and a high electric conductivity of 0.125 S/cm [22]. Hence, it is expected that further enhanced electronic conductivity and framework topology control of MOFs will provide more prominent photocatalysis in the lightto-chemical energy conversion.

12.2.2 Band Gap Tuning At nanoscale, the width of valence band (VB) and conduction bands (CB) is low, and the band gap between VB and CB is more in nanoscale materials than bulk [23]. A large band gap resists the mobility of electrons and confines the quantum size effect. Band gaps are fundamental keys of tuning the electrical and optical properties of MOFs. Hence, it is important to study the band gap tuning of MOFs to understand the better way of their photocatalytic properties [17]. The band structures of designed MOFs can be computed applying density functional theory (DFT) [9]. The computed band structures represent MOF as an insulator with band gap > 4 eV, which means a very low population of the conduction band. A semiconductor MOF has an intermediate band gap (< 4 eV) that permits transportation of free charge carriers from valence band to the conduction band by optical or thermal excitation [10]. A metallic MOF exhibits at least one band overlapping the Fermi level, allowing continuum of electronic states that presents high conductivities. Many of reported

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Fig. 12.4 Construction of heterostructured ZnIn2 S4 @NH2 -MIL-125(Ti) nanocomposites for visible-light-driven H2 production [26] (copy right permission)

MOFs show insulating behavior because of large energy band gaps [24]. For example MOF-5 or Zn4 O(BDC)3, where BDC = benzene-1,4-dicarboxylate, shows a band gap of 4.6 eV. The MOF Zn2 (TTFTB), where TTFTB = tetrathiafulvalene tetrabenzoate, shows a band gap of 1.75 eV, as a semiconductor. The MOF Ni3 (HIB)2 , where HIB = hexaiminobenzene, represents a metallic behavior via band overlapping the Fermi level [7]. Figure 12.4 shows the construction of heterostructured via making a nanocomposite of ZnIn2 S4 @NH2 -MIL-125(Ti) for visible-light-driven H2 production. Therefore, it is expected that further electronic band modulation and framework topology control of MOFs will bring us more promising photocatalytic properties in the solar-to-energy conversion [25].

12.2.3 Morphology and Topography Tuning Surface morphology, size, dimensions, structure, and topography of metal–organic materials represent the surface area, flexibility, porosity, band gap tuning, nodes, and linking. MOFs are characterized according to different aspects of their surface morphologies which affect their photocatalytic activities [7]. Hence, information regarding morphology and topography of MOFs is important for their photocatalytic applications in different sectors such as storing, sensing, splitting, and others [20]. Morphology and topography tuning via surface modification affects the optical, electrical, and structural features of MOFs. Tominaka et al. reported the topochemical conversion of crystalline dense insulated MOF [CuI Cl(ttcH3 ], (ttcH3 = trithiocyanuric acid), into an amorphous, semiconducting MOF [CuI 1.8 (ttcH1.2 )] [23]. According to literature, the coordination metal–organic materials of low dimensionality exhibit electrical conductivity. But, some 3D structures like Cu–TCNQ and Ag–TCNQ also represented high conductivity at room temperature [27]. Rarely, porous MOFs exhibit intrinsic electrical conductivity. Nevertheless, Cu[Cu(PDT)2 ]

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and Cu[Ni(PDT)2 ] (PDT = 2,3-pyrazinedithiolato) are porous MOFs having conductivity of 6 × 10–4 S cm−1 and 1.0 × 10–8 S cm−1 , respectively, at ambient conditions. Talin et al. reported tuning conductivity in MOFs via infiltration of nanopores with redox-active and conjugated guest molecules. They demonstrated Cu3 (BTC)2 a tunable, air-stable electrical conductive (7 S/m) infiltrated with TCNQ as guest molecule [28]. MOFs’ photocatalyst surface modification enhances the charge separation, transportation, and lifetime of positive ions for reduction of the recombination to increase the photocatalytic ability.

12.2.4 Linking Tuning The substitution of metal zones in inorganic nodes and their connection with organic linkers decides the electronics, optical properties, and photocatalytic process selectivity of designed MOFs [29]. Hendon et al. reported tuning of linker in MOF-125 (the highly studied MOF as photocatalyst) via animating linker BDC-(NH2 )2 and other functional groups (−OH, −CH3, −Cl) [30]. MOFs having metal ions such as Fe− , Co− , Cd− , and Ni− were investigated to exhibit visible-light harvesting photocatalytic capability. This photocatalytic efficiency has been attributed to the corresponding metal ions [13].

12.2.5 Stability Tuning However, the stability of MOFs during the photocatalytic reaction is an important issue needed to be further considered [27]. MOFs exist far from the equilibrium state (stability) because of their high surface energy. Hence, MOFs are highly unstable. In some cases, MOFs are easily modified or react with active materials to reach a relatively stable state. This causes desired or undesired phenomenon to photocatalytic ability. So, MOFs exhibit dual nature of high reactivity and a poor stability. Such properties of MOFs show positive and negative both effects for photocatalytic process [7]. However, most of studies focus on the high reactivity of MOFs, while their poor stability has been ignored. Some photo-induced phenomenon affects the framework of MOFs. For instance, the incidental pyrolyzation will destroy the framework of MOFs under high-intensity light irradiation (a photocatalytic process) because of the thermal effect of photons. Some studies reported stable structure of MOFs via doping. For example, Zeng et al. constructed a highly stable framework exhibiting robust metal–organic pillars (Zn-lactate) conjugated with organic groups with double walls [31]. The square-shaped channels are connected to the DMF or methanol. The activated empty phase provides excellent iodine affinity and a stable MOF structure. Figure 12.5 represents a visible-light-assisted indium-based highly stable

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Fig. 12.5 Visible-light-assisted indium-based highly stable MIL-68-NH2 /graphene oxide composite photocatalyst structure with enhanced photocatalytic activity [16] (copy right permission)

MIL-68-MOF/graphene oxide nanocomposite photocatalyst structure with enhanced photocatalytic activity.

12.3 Present Challenges with MOF Tuning The overall photocatalytic behavior of MOFs depends on light harvesting, photons’ generation charge carriers (electrons and holes) separation and transportation, and surface reaction. The light absorption of the MOFs directly belongs to the band gap and surface area. Simultaneously, photogenerated charge carriers which get transferred to the surface can only perform photocatalytic process, and rest charge carriers are recombined. Additionally, some photogenerated charge carriers may dissolve in the photocatalyst rather than performing the target application [11]. Most of MOFs are composed of organic molecules linking metal centers with very low chagre transport facility. Hence, to enhance the photocatalytic capability of MOF, introduction of metal ion, tuning of linking or node, doping, implantation of guest molecules etc. approaches are used. MOFs have porosity, and due to porous structure, MOFs are not highly stable. Tuning of MOF with other molecules may cover the porous zone or active sites of MOF. Tuning of MOFs may affect the basic features of MOFs like flexibility and porosity. Although many of research in laboratory has been done on basic photocatalytic features of MOFs, but for industrial purpose or on field applications, there are still some challenges. Laboratory studies only verify the feasibility and mechanism of photocatalytic behavior of MOFs. To amplify the industrial photocatalytic applications of MOFs, there are numerous strands in the real on-field production methods and conditions of the MOFs which are uncontrollable and not secure as in the laboratory. Hence, the development of low-cost, feasible, eco-friendly, stable, reusable,

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and large-scale production processes is needed to realize in case of photocatalytic industrial application of MOFs.

12.4 Future Perspectives Generally, the thermal stability of MOFs is determined by the coordination number and local coordination environment, thus searching that MOFs with optimized coordination between metal cluster and organic linker for high thermal stability are of great importance. On the other hand, controlling the pyrolysis of MOFs in certain condition to expand their micropores into mesopores or promote their electrical conductivity while maintaining the open diffusion channels and ensuring the monodispersion of metal centers is believed to open up a new way to achieve higher photocatalytic efficiency with stability of MOFs. To improve the MOFs’ photocatalytic efficiency structure, band gap and base materials can be tuned via doping, implantation of guest molecules. It is important to optimize the design and parameters of MOF to attain the favorable photocatalytic efficiency. Tuning of parameters plays an important role to achieve optimal photocatalytic efficiency. A well-designed MOF as a photocatalyst can not only justify the reaction efficiency, but also minimize the waste of energy and materials and exhibit the economic benefit.

12.5 Conclusions In summary, MOFs as photocatalyst are hot area of research. MOFs have high porosity, flexibility, and large surface area. The advanced design and strain engineering for conductivity, band gap, surface morphology, and stability enhance the photocatalytic performance of MOFs. The strain engineering tuning of MOFs is very important for more harvesting of solar energy and conversion of photo energy to chemical energy at industrial scale. The introduction of guest material, doping, implantation, composite, nodes/linker modification is used to tune photocatalytic properties of MOFs. Modification using electron acceptor materials may be an effective way to improve the photocatalytic activity of MOFs. Moreover, some other factors like pH of solution, amount of catalysts also affect the photocatalytic process of the MOFs. MOFs have instability issues. The reusability, reproducibility, and stability are significant parameters for MOFs on field applications. So, it is very significant to optimize the design and parameters of MOF to obtain the favorable photocatalytic capability. Strain engineering tuning is important to achieve optimal photocatalytic performance of MOFs. A well-designed MOF as a photocatalyst verify the reaction effectiveness with economic benefits.

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Chapter 13

Theory, Modeling and Computational Aspects Regarding the Mechanisms of Activation of Photocatalysts Chinmay Rakesh Shukla, Deepak Singh Rajawat, and Sumant Upadhyay

Abstract The success of any photocatalysis process relies on meeting the significant technical challenges in developing improved photocatalyst systems. Materials properties, including bulk and surface characteristics, are the key to efficient photocatalysis. Thus, development of new theoretical models of photocatalyst materials and interfaces are critical to the design and engineering of new semiconductor photocatalyst systems. With rapid advances in new algorithms and computational techniques, it is now possible to simulate interacting systems of many electrons and nuclei as in condensed matter and molecules. Naturally, being in the domain bridging between molecules and condensed matter systems, nanostructures can be ideally explored using first-principles electronic structures’ calculations. Through such calculations, it is quite possible to understand atomic and electronic structure and dependent properties of nanostructures and access related information at sub-nanometer level. Thus, first-principles calculations on high-performance computers provide a costeffective (in comparison to experimental efforts) virtual laboratory for elucidating the fascinating interplay between physical properties (e.g., atomic structures, defects, interfaces) and the electronic structure of materials and testing new ideas for possible new and efficient photocatalyst materials and devices. Keywords Density functional theory · Modeling · Photocatalysis · Computational techniques · First principles

13.1 Introduction Photocatalysis is the process undergone by a material in presence of a photocatalyst. ‘Photocatalyst’ [1] is a term comprising two ideas, namely photo meaning light or light derived, and catalyst is any material that can alter or change the rate of C. R. Shukla · S. Upadhyay (B) Amity Institute of Nanotechnology, Amity University, Uttar Pradesh, Noida, India e-mail: [email protected] D. S. Rajawat Department of Chemistry, IIS (Deemed to be University), Jaipur, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Garg and A. Chandra (eds.), Photocatalysis for Environmental Remediation and Energy Production, Green Chemistry and Sustainable Technology, https://doi.org/10.1007/978-3-031-27707-8_13

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reaction within the presence of a photon, in this case. The general way, in which photocatalysis is studied, also employs the reactions undergone by a material or number of materials in presence of light with a semiconductor. A photocatalyst can also be addressed as a substrate that absorbs light and brings about catalytic activity for chemical reactions. In a more generalized way, it can be said that all photocatalysts are of the semiconducting nature. The phenomena of photocatalysis are the outcome of the generation of an exciton by the effect of light exposure on its surface. The photocatalytic reactions can be broadly characterized into two different categories on the fact, the phase of reactant and photocatalyst: • Homogeneous photocatalysis: This type of reaction is occurring when both the reactant and catalyst are in the same, i.e., solid state, liquid state or gaseous state. • Heterogeneous photocatalysis: This is the kind of reaction occurring in the case when both are in different phases, i.e., gas–solid, liquid–solid and so on. The property of semiconductors to show electrical conduction under light at room temperatures a photocatalyst. Under the presence of light, the semiconductors generate excitons which are basically an electron and a hole pair. This electron and hole pair is then separated in conduction and valance bands, respectively. The exciton generation leads to a photoexcited state. Here, thus produced excited electron in the conduction band is utilized by an acceptor, which gets reduced, and the hole in the valance band is taken up by the donor molecule, rendering it oxidized. The nature of producing simultaneously electron and hole pair is the reason for the high importance of photocatalysis. The end state of the excited electron and hole pair depends on the place, at which the edges of conduction and valance bands are situated with respect to the redox positions of the reactants. The reactions between the substrate and the semiconductor can be categorized into four segments depending on the band edge positions of conduction and valance band with respect to redox band edges of reactants. 1. If the redox level of reactant is lower than that of the catalyst’s conduction band, then the reactant(s) reduces. 2. When the redox level of reactant is higher than the valance band of the catalyst, then the reactant(s) is oxidized. 3. The redox reactions are not undergone by the reactant if its redox bands are higher than the conduction band and lower than the valance band of the catalyst. 4. The redox reactions are undergone by the reactant when it satisfies the positioning of its redox bands lower to the conduction and higher to the valance band of the catalyst.

13.1.1 Need for Theoretical Models The questions around the catalysis which can [2] be expectedly addressed by quantization of chemical bonding and liable to guide the creation of materials with efficient catalytic activity, are:

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1. How does the periodic trend affect activity of the catalyst or in a particular reaction the exposed element as the surface site will it act like the most active catalyst site? 2. Is there any kind of preference given to ‘alloyed sites’ over pure sites? Or is it that the ‘chemical interpolations’ effective and possible across the periodic table? 3. What is the way in which the rate limiting step’s activation energy lowered by the catalyst’s surface atomic sites such that an increase in the rate is observed in accordance with difference in energetically limited reaction rates involving the formation of intermediates followed by a pathway to attain products linked to reactants or simply the detailed mechanism? 4. What is the variation in the rates for different operating conditions for a specific catalytic system with increased rates of different reactions? Before embarking in a study of heterogeneous catalysis with the methods of theoretical chemistry, one must carefully consider its inherent requirements: The inherent requirements must be fully understood before applying the methods of theoretical chemistry for the study of heterogeneous catalysis, i.e., 1. Idealizing models belonging to whether the catalyst in the solid state has ionic, covalent or metallic character. 2. The fact that how stable the catalyst’s morphology remains with the reaction conditions along with the surface anisotropy and surface reconstructions’ effects in conjunction to the solid’s surfaces and thus the unsaturated atoms’ co-ordination at the outermost layers on the catalytic event. 3. There must be a consideration for the anomalies associated with heavy transition atoms as catalyst’s active site containing 5d orbitals and preparation for f-shell electrons forthcoming as an effective inclusion of actinide and lanthanide atoms, as used mostly. 4. The difference in the level of theory used for the representation of solid surface and the molecular reactants should be least. The work done by heterogeneous catalysis is not only seen by the motive of reducing activation energy but also optimizes the probability of coming in contacts with the reactants, i.e., simultaneously being able to be in close vicinity, prefectorially showing its effect on the rate equation. The challenge to overcome with respect to realistically describing the surface speciation to the operating conditions. The operations of catalytic surfaces are dynamic, i.e., the physisorption or chemisorption of the reactant takes place, dissociating or not, attaining the surface partnering, moving generally by the diffusions over surfaces, coming down to reacting together as surface products, diffusing onto the surfaces themselves and eventually ending up desorbed. Models of microkinetic nature try to analyze and view the array of these complex events into somewhat simplified equations, that work reasonably well in many situations. Diffusion and reaction barriers act as parameters in these equations, and adsorption free energies are described by the surface free energy. Thus, to visualize the real-time experimental reactivity data using microkinetic systems, a firstprinciples study for the catalytically obtained reaction system can provide the inputs

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for microkinetic systems. To abstain from any assumptions, one can alternatively take up the molecular dynamics which can show the film of events on the microscopic level. Molecular dynamics (MD) simulations allow the reconstruction of temporal trajectories followed by molecular species by integration of Newton’s dynamic laws. According to Hellmann–Feynman theorem, the expectation values from first derivatives of Hamiltonian in accordance with nuclei coordinates in spatial configuration can be applied to wave function, in these components, to compute forces using first principles (Ab initio MD, or AIMD). From [3] being able to compute each intrinsic rate, to starting from the current configurations, first-principles kinetic Monte Carlo simulations (FP-kMCs) consider all possible microscopic events at each step, to be used as an alternative to AIMD. To update the events list as the rate determining step based on the activation barrier, slow computation part of the code, for the calculation of all possible rates at each given point of time and step, the inputs are in the form of barriers, stored in the datasets. Pre-computed barriers from first principles correspond to the FP-kMC. The so-called supercell method was the base from which the physicists and chemists could speak about the crystalline nature which could then be transposed successfully into computational surface science and heterogeneous catalysis. The crystal symmetry including the translational symmetries as defined by the Bravais unit cell is observed by constraining the wave function itself, and the boundary conditions now imposed to the wave function are now periodic along the three directions of space. The enlightenment is obtained on the catalytical theatrics of molecular species as the players of it, and it is observed by sliding in a vacuum in place of atomic layers, in order to mimic interfaces. The problem regarding the rise in dangling bonds due to the cluster models has been eliminated with the help of well-defined Miller indices which are introduced and used as to the normal unit vector. Again, the supercell calculations were the pivot in calculating and describing function of prescribed chemical potentials by surface phase diagrams of gas–solid interface surface speciation. Prediction of interfacial free energies and thus the solid crystallites’ equilibrium morphologies by ‘Ab Initio Atomistic Thermodynamics’. The attempt to model catalytic surfaces at reaction conditions as recognized as the starting point for establishing the reference equilibrium surface states.

13.1.2 Theoretical Models Used The whole purpose and need for the understanding and visualizing in the form of models have been explained. But, what kind of models to use and what are the models available in visualizing need discussion.

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Density Functional Theory (DFT)

The geometric structure and electronic [4] structure, general quantum mechanical (QM) derived properties are necessary to predict the reactivity of catalytic systems which is a non-trivial task. When the case of surface reactions is taken up, there are many phases being involved, and solving the Schrodinger’s [5] equation is already a difficult task for small system, let alone phases. As mentioned, the importance of theoretical calculations in reactions to follow through with chemical reactions and get an insight into them, it always is proven useful. To better understand the chemical reactions, one has to apply QM to the observed system. The QM calculations solve the Schrodinger’s equations, which in its purest form provide with the energy of electron and nuclei-induced configuration. There are two general approaches to do this. One way to solve this by using non-classical wave function, to obtain energy, using approximation of Schrodinger’s equations. DFT or density functional theory, which uses the electronic density to evaluate energy, is the other. Both have their own equal merits in providing a non-thermodynamical energy representation for a system of electrons. DFT is a system which has been refined to such an extent by using a whole level of approximations, and to overcome the gaps by the approximations, different functionals and their values are added to create a general design with or without special uses. This fact is seen clearly with the type of exchange–correlation functionals available, and the extent of accuracy of these calculations, as stated by Jacob’s ladder, is very much viable. The list starts from basic DFT calculations of LSDA which is less time consuming but more of a throw to the exact energy estimation, and this grows onto the exact correlational and exact exchange, where the chance of error is as low as higher limit of 0.1 eV. These are done on the basis of application and time with resources. The role of theoretical models in photocatalytic mechanism is show in Fig. 13.1. All of these come down to the calculations that can be achieved by the theories with accuracies that fall to available time and resources. Geometry Optimizations These kinds of calculations are sewing in the atomistic properties into molecular configurations coupled with the Born–Oppenheimer approximations on potential energy surface, meaning optimizing geometry of any surface or system having individual atomic optimizations and thus whole molecule’s geometric optimizations. The higher and more powerful Hesse calculations give the best optimized geometries but again take a whole lot of time, whereas the gradient calculations or in comparison will take a route where they do not have to go on calculating force every time using Hessian calculations.

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Fig. 13.1 Showing design and development of photocatalytic systems by combining advanced characterization methods with theoretical models

Transition-State (TS) Optimizations The transition-state optimization or TS optimizations are a follow through of Geometry Optimizations, wherein first-order saddle points except one internal coordinate all are at a minimum, to find transition states. This is very necessary to model in surface reactions and quantum chemistry to identify the transition states. But, there is again no guarantee that one will be able to obtain these, and it is highly possible that the automated approaches do not find them. The one thing thus incorporated in the QM method is by using reactant and product coordinates originally or by assuming them to create and see a small 2–3 bond making breaking calculation, but for a bigger system, it fails. The other bend around these is using nudge elastic band method or NEB which can break out from the expensive Hessian calculator, by the means of not considering the whole topology of a potential energy surface, thus revealing the TS as the maximum energy point. There are other models as well which use coupling of different geometries but not as efficient as NEB and also provide the visualization in a bulk system. Vibrational Frequencies These are calculated on the bases of experimental IR spectrum-evolved vibrational frequencies for the equilibrium structure with the help of harmonic oscillator’s approximation for potential. The calculated vibrational frequencies are analyzed to understand after the geometric optimizations whether there is a stable intermediate in a reaction or TS formation. It is recommended to calculate vibrational frequencies

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at the end of each and every optimization for the very reason mentioned here, but there are cons to it as there is no approximation of anharmonicities of vibrational modes which is present experimentally.

13.1.2.2

Molecular Dynamics (MD)

For a thorough understanding, dynamic simulations are useful as in experiments, the only calculated parts are reaction rates and probabilities. The time evaluation offered by the dynamic simulations is very helpful in comparing with the [6] experimental reactions as the probabilistic derivations of adsorption or reactions can be modeled back. This interdependent approach of modeling PES or potential energy surface using dynamic simulations based on the PES is very useful. This can be seen in the catalysis perspectives where the molecular configurations and components show the lower activation barriers with adsorption than without, thus providing a detour in multidimensional configuration with low energy barrier traversing. The fact that not all reactions can be followed by TS monitoring wherein molecular and atomic adsorption takes place and the kinetic energy relaxation of these impinging molecules needs to be observed which can be done by kinetics only. Ab Initio MD To theoretically calculate chemical reactions, the combination [7] of molecular dynamics with force, which is calculated straight from the electronic structures, is utilized in Ab initio MD. The bond breaking and formation work hand in hand because it operates on solving the electronic structure directly from each step. In complex systems, the solving of Schrodinger’s equation becomes impossible which is attainable for nucleus and electron. For this reason as discussed in the DFT case, several approximations are required to solve them. One of the greatest approximations are of the Born–Oppenheimer [5] approximations which states the motion of nucleus as stationary with respect to electrons due to the much greater mass. Above all, there are furthermore approximations used, namely the Hartree–Fock molecular dynamics [8], Car–Parrinello [9, 10] molecular dynamics, Kohn–Sham molecular [11] dynamics and path integral molecular [12] dynamics. Reactive Force Fields’ MD AIMD-based MD calculations do report high level of accuracy but are restricted to the size of the molecules that can be calculated for it. For the case of polymers or big chain compounds, the use of AIMD is not possible as it cannot show the many available number of active sites, and thus, a different approach of reactive force fields is used or ReaxFF. The way in which AIMD calculated electronic structure for each step, here reactive force fields are introduced which are basically bond

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order-dependent energies. E system = E bond + E over + E under + E val + E pan + E tors + E conj + E vdw + E coulomb, where each of the terms is corrections for different anomalies that occur in results and can be used for a huge variety now of systems where the calculations are made in such a way that they model the pi-bond, sigma-bond and second pi-bond.

13.1.2.3

QM/MM

This kind of system is designed [13] for specific applications in systems wherein some few 100 atoms’ system inside a tens of thousands of atoms’ system is considered, where QM is applied to the first system and MM or molecular mechanics to the other. The biggest issue with this kind of method is the conflict of boundary application between the two subsystems which affects the accuracy of the calculations. The only way out of it is to carefully figure out how the QM side dangling bond which will be at the boundary and MM side dangling bond are addressed, wherein the QM side is dealt with H-atoms to cap it, whereas for the MM atoms, it needs to be such that no-physical bonds, but also electrostatic influences are on the QM system. A force field is being used for the calculations of MM system, but this is also a problem when it comes to the QM system, which again is affecting accuracy. This force field for the MM system is a generically parameterized energy function with analytical potential, sometimes system specifically parameterized as well. The boundary function once used appropriately is a smooth sailing boat but is highly sensitive to where it is being put, i.e., in biological nodes, nucleic acid and polymers are 1D in nature and are not that much specific, but for a system like MOF or metal organic frameworks, this is highly specific where the boundary is being kept. In case of MOFs, the most important part when studying them is to cover multiple nearby bonds in 3D high connectivity networks, which is the biggest failure in QM-MM boundary method treatment, which is the point of failure. The fact that metal oxide nodes are present they contribute to partial atomic charges and polarity of bonds in QM/MM methods. There are two proposals two treat these using first F* multilink for boundary operando in MOFs and bond-tuned link atom.

13.1.3 Theoretical Models for Metal Oxide Catalysts [14] 13.1.3.1

Yttrium Oxides

Yttrium not a less known material and its oxide in bulk phase is used in ceramics, catalysis, etc. The modeling of yttrium-based oxides by DFT has been studied. There

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was a [15] report on (Y2 O3 )n clusters with investigations on its structural and electronic properties, where n = 1–10. The use of generalized gradient approximations using the PBE XC and PAW pseudopotential was done. Bulk-phase yttria periodic calculations with same factors were also done. The bulk phase of yttrium oxide has the coordination numbers of 6 and 4, respectively, for Y and O. The derivation of lowest energy clusters was also done from the bulk structure. There was no variation in HOMO–LUMO gap observed for the change in size of clusters. When the clusters were neutral in nature, the HOMO–LUMO gap varied from 2.04 to 2.88 eV, and for charged clusters, it was 0–0.5 eV.

13.1.3.2

Titanium Oxide

This is one of the most successful and widely studied semiconducting metal oxides, and thus, there are numerous DFT-based calculations around it. A defect free [16] with semi-ionic bonding nanocrystals in nanocrystalline materials was observed using the predictions of general bonding principles. Anatase form was used for all the studies. With the help of large core potential, the effect of structural relaxation was seen for larger cluster of Ti16 O32 with PW86x-PW91c. The Ti–O bond ranges were found to be 1.7–2.3 Å; even though there were large local relaxations, the charge distribution was balanced throughout the cluster. The LANL2DZ basis set and B3LYP hybrid method were used for calculating the density of states for these clusters. A basis set of single zeta (SZ) was used for large clusters’ impact on structural relaxations and electronic properties. The relaxed structures showed a significant shift in adsorption thresholds toward lower wavelengths. The atomic hydrogen-based Ti8 O16 cluster [17] interactions with resulting oxygen molecule adsorption defect species, showing reduction of the cluster, were studied using 6-31G for oxygen, 6-31++G** for hydrogen and LANL2DZ for titanium. This shows the formation of a stable OH species without a significant activation barrier as the calculations were done for reaction of surface oxygen and atomic hydrogen resulted in. Ti+3 oxidation is seen to be achieved due to reduction. The catalytic effect of Ti atoms is seen to rejuvenate with the help of molecular oxygen reduction to O2− and oxidizes Ti back to +4 state. The shifting and disappearing of single occupied Ti-3d tend to disappear from bandgap contribution with O2− specie formation. Amorphous TiO2 has been also seen for H-production via [18] photocatalysis by DFT. It was said that the efficiency was a trade-off between its abundance and cost in comparison to its crystalline part, via electronic analysis. To make this trade-off lesser, doping was analyzed in the structure to make sure that the charge generation increases via narrowing of bandgap to visible light region and reduced recombination by the distance between catalytic core and semiconductor surface.

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Zirconia Oxide

The zirconia-based Cu atoms’ impact on reactivity [19] was studied using LANL2DZ pseudopotentials and B3LYP/6-31G** theory. For n = 5 in (ZrO2 )n , they observed a reasonably constant reactivity, reporting the strong dependence of IP and EA on number of building blocks. The experimental bulk values were matched for an infinite number of atoms taken for bandgap calculation suggesting zirconia being unable to conduct. The addition of Cu atom saw the changes in bandgap which had reduced and implementing a better charge transfer by zirconia. The flow of charges was observed from copper to zirconia as well. The observations were further seen with Zr acting as an electron acceptor when situated besides Cu, and Cu being the donor also has dual active sites. The zirconia cluster models of up to Zr5 O24 H28 [20] were studied for CO2 adsorption. The calculation was done by the means of TZVP basis set in combination with TPSS XC functional in the meta-GGA. Grimme’s empirical dispersion model was also employed to visualize and correct the adsorbate and surfaces’ weak interaction. The initial geometry of the structure determined the fate of adjacently adsorbed molecules whether they would dissociate or remain adsorbed. Both the processes were found to release high amounts of energy. An intermediate adsorption energy was found when the CO2 adsorbed on a single unsaturated Zr in an apical manner.

13.1.3.4

Iron Oxide

The kinetics of high-temperature water–gas shift reaction were [21] demonstrated into a single two-step regenerative model. Different surfaces of Fe3 O4 were studied using DFT with B3LYP/TZV** theory level in cluster model use. Multi-units of almost sic were utilized for calculation of energies of oxygen adatom localization on octahedral sites of cation. It was found that the missing oxygen anions from the octahedral cation adsorption sites coordinated to be proportional to the computed energies of localization. The active site being at {111} surface was understood from the fact that experimental enthalpy of localization was found closed to {111} surface.

13.1.3.5

Tungsten Oxide

The results in bulk case of WO3 have not been so good for observed water splitting but still have a manageable bandgap of 2.8 eV which is tunable and has been done via means of doping and co-doping for changing its electronic and structural properties. This kind of variation was seen with the help of Mo-doped [22, 23] WO3 nanowires which showed enhanced photoactivity in visible light with the bandgap alterations at 0.48 eV when Mo was varied from 0 to 0.75. These transition metal-induced effects were further observed at Ni-doped when it showed highest H2 production under UV light [24].

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Metal Oxide Composites

A study on Tiy Oz and Lix Tiy Oz was reported [25] using the B3LYp/BPW91-631G*. The interpretation of stable geometry modeled experimental Raman spectra was done for nanocrystalline Li–Ti–O of Ti3 O7 , Ti4 O8 and Li2 Ti7 O14 2+ were used. Calculated geometries were used to predict the DOS diagrams and vibrational spectra with harmonic approximation. The DOS diagrams and vibrational modes were obtained from calculated geometries with harmonic approximation. The Lix Tiy Oz was calculated, and real structures had no satisfactory results due to smaller size. The variations in experimentally calculated cubic Li–Ti–O oxides’ bandgap were observed as being overestimated with simulations. Once the size of cluster is increased, the calculated values fall in line with the experimental values. Raman spectra calculated under the harmonic approximation for the optimized structure reproduced all important features of experimental spectra. All the important features of experimental Raman spectra were highlighted with added harmonic approximations in calculated results. The hybrid DFT method-based [26] Z-scheme of g-C3 N4 /TiO2 heterojunctions was visualized for photocatalysis. The results fell in favor of the heterojunction as the bandgap had lowered overall compared to pristine state, thus effective photocatalysis. It was found that even after the heterojunction formation, the bulk structure remained intact along with the effective charge distribution at the interface. Figure 13.2 shows the electron transfer which was noted from g-C3 N4 to TiO2 . The improvement in the photocatalytic activity was seen when the system reached equilibrium, i.e., when there was net charge accumulation, it gave rise to a built-in electric field at the interface; this aided the effective transfer of charges from g-C3 N4 to TiO2 showing a charge separation.

Fig. 13.2 Direct Z-scheme photocatalytic mechanism for g-C3N4/TiO2 heterostructure

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A three-component composite system [27] has also been observed which shows TiO2 /graphene/MoS2 with 0.25 wt% graphene. The results were supposedly unique as the overall H-production was found to be at 165.3 µmol/h, with graphene acting as electron reservoir and MoS2 aiding efficient adsorption site, in comparison to TiO2 /MoS2 which was 17 times lesser. A recent advance of theoretical models used in metal oxide [28, 29] is given in Table 13.1. Table 13.1 . Metal oxides/composites

Study/variations done

Results/differences observed

Reference

2D TiO2 from rutile-phase monolayer

Reduced from bulk to 2D form and studied using DFT+U

Observed a bandgap of 2.1 eV which is smaller than that of anatase and rutile phases

[30]

2H MoO2 and 1T NiO2

Used G0 W0 method of DFT calculations on lowered dimensioned transition metal oxides (TMOs)

Total energy-based stability prediction as monolayers with bandgap identification at 2.20 and 2.15 eV, respectively

[31]

Fe3+ -doped TiO2

Doping of Fe(3) to Ti (4) studied using BLYP functional and LANL2DZ double-zeta basis set

Changes in the band [32] structure owing to the extra states due to Fe doping owing to red shift in bandgap

Mo-doped n-type TiO2

Doping of Mo atom in the matrix and analyzed using PBE functional ultra-soft pseudopotential with BFGS optimizations

Induction of Mo level [33] below CB of TiO2 showing efficient conduction of charges and increase in excitations

Mo-doped mBiVO4

Doping of Mo atom along with HSE06 functional with ultra-soft pseudopotentials

Effective charge separation [34] of electron–hole pair was observed but with no changes to bandgap, by means of reducing total effective mass of hole

NaTaO3 –Mo, N co-doped Mo and N are co-doped into NaTa.O3

A continuum in band [35] structure is observed which increases charge mobility. Along with this band edge positions, favor thermodynamics as H2 and O2 band edge positions (continued)

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Table 13.1 (continued) Metal oxides/composites

Study/variations done

Results/differences observed

Reference

WO3 /BiOCl Creating a heterojunction heterojunction composite using conjugated gradient approach and calculated using PAW method with PBE functional in GGA DFT

A PDOS analysis revealed [36] that the heterojunction had a suitable band alignment by the interfacial structures and also allowing the separation of charge carriers due to interfacial gap states absence

GaO3 /MgO-modified TiO2

The change in bandgap was a red shift and revealed a localization of TiO2 surface for charge separation

Modification of TiO2 with MgO and then creating a heterostructure and visualizing with DFT+U with Blöchl’s PAW approximately and PW91 XC functional

[37]

13.1.4 Theoretical Model for Carbon-Based Catalysts 13.1.4.1

MOFs

The IRMOF-1 or the MOF-5 is built on the structure of Zn4 O(BDC)3 , is the linker molecule and also is the first isoreticular MOF. The electronic [38] structure for the MOF-5 was given by LSDA framework. It was found to have a bandgap of 3.5 eV which matches too well with the experimental value and also is seen to be dominated by O 2p states of zirconia cluster and C and O of linker as well in the HOCO and the LUCO is dominated by p states of C and O in linker. The changes in bandgap are seen when O-central in zirconia cluster gets substituted by Te, Se or S which greatly tunes it (Fig. 13.3). The [39] enhancement in the bandgap followed the path where Te showed the lowest followed by Se and S with the end at 2.5 eV wherein metal substitutions’ effect is also tuned up. The other kind of framework with similar oxide and linker is called UiO-66(Zr) whose structure is Zr6 O4 (OH)4 (BDC)6 being utilized for being very stable in photocatalysis. To achieve visible light absorption and photocatalysis for a more reactive nature, there have been studies seen with respect to metal doping, like Ti on to the nodes, or linker functionalization, with functional groups like I, NO2 and NH2 , to tune the electronic structure for the required [40–47] applications. Apart from these, a decrease in bandgap is also noted gradually from UiO-66(Zr) toward UiO-67(Zr) downward UiO-68(Zr) [48]. The other kind of study of similar UiO-66(Zr) with Ti doping was done to analyze the photoexcitation. The photoexcitation was observed when a node-specific peak was found which arisen due to local excitation due to the Ti doping, which when

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Fig. 13.3 Shows the substantial tunability of the bandgap (Eg) of paradigm MOF-5 via the substitution of Zn4O with X4Y in the node and its corresponding absorption region in the visible light spectra

further functionalized with amino groups showed LMCT excitations which had meant that a small effect of charge separation on the excitation spectrum [44]. The other kind of MOFs [49] investigated is MIL-125(Ti) which has the formula Ti8 O8 (OH)4 (BDC)6 . The density of states (DOS) study found that the HOCO has high influence from C and O 2p states, whereas the LUCO was influenced majorly by the hybridizations of Ti–O 3d-2p orbitals in the octameric titania clusters (Fig. 13.4). These results [50] meant that the charge separation could be realized where electrons would be easily localized on the metals and holes on the linkers, which was also observed for the UiO-66(Ce) case.

13.1.4.2

Graphene

Graphene’s excellent electronic properties [51] come into employing it and replacing the present noble metal contacts and co-catalyst which can drive solar fuel generation by the redox reactions. The other way graphene and its analogues like oxides with optimized level of oxidations, to creating them as candidates for non-metal semiconductor derived photocatalysts in the form of aromatic macromolecule photoabsorbers. The electronic properties [52] can be further tuned by the ways of functionalizing graphene using bonding (covalent), interactions (adsorption, pi-pi staking) and lattice incorporation making it either possess a n- or p-type characteristics (Fig. 13.5).

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Fig. 13.4 Showing the frontier electron density of unsubstituted MIL-125: a the valence band is composed of the C-2p orbitals making these favorable for linker-based bandgap modifications; and b the conduction band is composed of O 2p orbitals and Ti-3d orbitals

Fig. 13.5 Schematic illustration of different types of interactions involved in the adsorption of targeted adsorbates on graphene-based composite photocatalysts

The very first investigations using first [53]-principles calculations were performed on g-C3 N4 / graphene hybrid. The study was able to show the enhancement in bandgaps toward visible light absorption of the composite where it was also observed a 2D–2D charge transfer via graphene to g-C3 N4 which was free from dangling bonds as well. There was also a gap opening of 70 meV reported between the hybrids which resulted from the strong electronic coupling by charge transfers. A very similar study was also performed on the same composite to evaluate co-relations between electronic structures and properties. There was a revelation that charge distribution can achieve interlayer charge movement between graphene and g-C3 N4 . With [54] the application of an external electric field, an important

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phenomenon of tuning of bandgap could be effectively achieved. This work conclusively showed the tunability of bandgap of the bilayer with high mobility of carriers as a very transposable candidate nanomaterial for the various photocatalysis. Hybrid DFT calculations were performed in the case [55] of studying gC3 N4 /RGO with varying concentrations of O-atom and investigate strong interactions between them. The PDOS was evaluated for the four composites made of g-C3 N4 /RGO which was used to reveal the interactions acting between them. When compared to g-C3 N4 monolayer in the previous composite studies, it gave the idea of VBM contributions by C and N1 atoms, specifically for the case of g-C3 N4 /RGO-2. In the similar way except for the case of g-C3 N4 /RGO-2, the N2 atoms contributed very well to the CBM. It also specifically showed the lowering of CB and VB of g-C3 N4 sheet in comparison to the composite-4’s VBM, especially. It was also seen that the crucial role played by the O-atom has its influence on the interactions between the g-C3 N4 and RGO sheets. In another result case, it was seen that appropriate O-atom concentration can alter the bandgap from direct to indirect. The 3 and 4 composite types in a heterojunction format showed the formation of a type-2 heterojunction favorable in photocatalysis and the O-atom being more dominant in positively shifting the CB and the VB of the composites in comparison to g-C3 N4 (Fig. 13.6). Apart from the 2D models’ composite systems, it was also investigated for TiO2 /graphene where anatase phase of titania and graphene was analyzed to show notable charge transfer in the electronic ground state, with a greater density of

Fig. 13.6 a Structure of g-C3 N4 b RGO c band structure and DOS of g-C3 N4 d band structure of RGO e composite of RGO/g-C3 N4 f HOMO–LUMO sharing and distributions of composite g band structure and TDOS of composite [55]

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holes arising in graphene. An [56] investigation of simultaneous analysis of titania– graphene and nanoribbon-functionalized graphene–titania using DFT calculations was done. It was also [57] seen that the interactions of different phases of titania with graphene aided efficient chemisorption for rutile and anatase phases of TiO2 nanostructures equally for graphene sheet, but physisorption of rutile phase of TiO2 . From yet another report on these composites, it was confirmed that the formation of heterojunction created [58] increased visible light excitation by the Ti-3d being above C-2p and thus having enhanced charge separation and photocatalytic activity. A review on carbon [28]-based materials in given in Table 13.2. Table 13.2 . Material/composite

Method/variations done

Results/observed changes

Reference

g-C3 N4 (001)/BiVO4 (010)

A nanocomposite based on the heterojunction of individual planes is created with Van der Waals correction using Grimme DFT-D3 in GGA+U approach in PAW and GGA-PBE for the exchange

Exhibition of type-2 favorable semiconductor heterojunction was observed along with an effective space-charge region creation suitable

[59]

Ag-ZnO/CNT

A composite was studied for a doped ZnO with Ag and carbon nanotube using GGA+U approaches with double numerical plus d-functions basis set and effective core potentials

The enhanced [60] absorption intensity of Ag-doped ZnO nanotubes was attributed to the increase concentration of Ag. Efficient charge separation and visible light photoactivity across the interface of CNT/TiO2 hybrid materials

TiO2 (anatase)/fullerene

A composite was created between C60 and TiO2 which was observed within PAW method and GGA functional of the PW91

They showed [61] enhanced visible light absorption as fullerene created additional states between VBM and CBM along with acting as a sensitizer and nanostructures bandgap tuner (continued)

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Table 13.2 (continued) Material/composite

Method/variations done

Results/observed changes

Reference

TiO2 /graphene/MoS2 composite

The ternary nanocomposites were explored via conjugated gradient minimization technique with double-zeta plus polarization basis sets and norm-conserving pseudopotential

There was enhanced [62] electron–hole separation in the TiO2 clusters which was due to the accumulation of the excited electron in the MoS2 film and graphene

Graphene/Bi2 WO6 (001) composite

They were visualized using first-principles DFT calculation. The Bader charge and charge distribution analysis were done as well

They showed a [63] narrowing in the bandgap along with enhancement of the visible light absorption which was attributed to the differences in their work functions

13.1.5 Recent Progress in Theory and Modeling on Photocatalysis DFT acts like a typical research method in quantum [64] mechanics but brings the powerful evidence of convenient methods for modeling specific catalytic reductions and reactions. The present use of DFT has brought about the ideas on active site distributions and mapping them; formulating the adsorption energies from calculations for the reactions and by the means of formulating NH3 and NOx adsorption mechanism on the O-vacancies; calculating for the elementary reactions the activation energies for the determination of rate control step of the whole reaction; can use the transition state energy and intermediate specie calculation the path of the chemical reaction undertaken; can also explain specific mechanism of chemical reactions of different catalysts; can study the aging mechanism of the catalyst undertaken by the hydrothermal route and the poisoning mechanism and study the modifications for the new designs of proposed catalysts. The experimental phenomena can be observed by several characterization techniques for band structure, bandgap, adsorption, modification, structures, etc., but DFT can be singularly used to model each and every aspect before going in for the experiment, thus reducing the countless and aimless optimizations steps. They also help in going into atomic scale phenomena and understanding the cause of a certain type of forthcoming. If modeling was not effective, DFT can also do more with Geometry Optimizations, single-point energy-based calculations, reaction paths and predict activation barriers, illustrate the characteristics of the bond and do the spectral analysis including Raman. The major discrepancy in catalyst cost

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generation comes from the very specific parameters required, but with DFT, this can be modeled to give an optimum and mid-way solution with least compromise of properties.

13.2 Conclusion DFT still suffers with the fact that it needs to use H-atoms at the end sites and boundaries which influence the results obtained and cause deviations from actual surfaces. The accuracy of the one method of calculations ONIOM is very much dependent on the use of only high precision layers, and for the larger unit cells, the periodic calculations are not suitable. There are still the variations of the accuracy caused by the huge computation costs and time between hybrid and plane wave functions; with the gradient approximations, there are still so much approximations needed to add that the result verification is also a task, and with more complex geometries and reaction sites, the whole calculation is more complex and expensive. Still, we find that results obtained from calculation results in special systems require very special functions only. The end goal of this continuous research is to obtain results that are consistent matchable and have greater accuracy to being able to model wholly the experimental viewpoint of each reaction system, in which case, we receive lots of new calculating methods and models. These methods and models and software for calculations have also endless developments. The fact that we are using evolved tools and software for calculations we can find at times that situations lead us back to the problems of low accuracy. Difficulties and opportunities coexist. It is well known that with problems come the opportunities as well wherein new methods get developed tested and compared based on existing theories. DFT will itself be improved and developed to be the guide in synthesizing of catalysts and predict very own reaction mechanism.

References 1. Ameta R, Solanki MS, Benjamin S, Ameta SC (2018) Photocatalysis. In: Advanced oxidation processes for wastewater treatment: emerging green chemical technology. pp 135–175. https:// doi.org/10.1016/B978-0-12-810499-6.00006-1 2. Toulhoat H (2010) Heterogeneous catalysis: use of density functional theory. In: Encyclopedia of materials: science and technology. pp 1–7. https://doi.org/10.1016/B978-008043152-9.022 43-0 3. Reuter K, Scheffler M (2003) First-principles atomistic thermodynamics for oxidation catalysis: surface phase diagrams and catalytically interesting regions. Phys Rev Lett 90(4):4. https://doi. org/10.1103/PHYSREVLETT.90.046103/FIGURES/2/MEDIUM 4. Keith JA, Anton J, Kaghazchi P, Jacob T (2011) Modeling catalytic reactions on surfaces with density functional theory. In: Modeling and simulation of heterogeneous catalytic reactions. pp 1–38. https://doi.org/10.1002/9783527639878.CH1

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Chapter 14

Electrocatalytic Activation and Conversion of CO2 at Solid–Liquid Model Interfaces: Computational Perspectives Shanmugasundaram Kamalakannan, Kandhan Palanisamy, Muthuramalingam Prakash, and Majdi Hochlaf Abstract The exploration and production of functional materials for specific gas adsorption techniques are a field that is expanding quickly due to their potential use in processes including carbon capture and sequestration (CCS) and CO2 conversion. For screening the series of ionic liquids (ILs), we have used DFT studies to identify the appropriate combination for the CO2 conversion applications. As liquid–solid model interface, we used ILs adsorbed on gold surfaces (i.e., ILs@Au). Initially, we found the suitable IL-decorated gold surface. In addition, we investigated the effect of alkyl and also the anionic moieties at the gold electrode surface. From these studies, we identified the efficient composite material for the CO2 conversion application. Our all computations show that the IL–Au(111) interfacial interactions can tune the CO2 activation/conversion. This results in the formation of reduced or activated state of CO2 molecule. Further, a lower energy barrier was attained for the selected combination that is used to activate and convert CO2 molecule. In conclusion, our interfacial catalytic material models can serve as a guide for the design of innovative electrocatalyst for the conversion of carbon dioxide into value-added products. Keywords CCS · First-principle calculations · DFT+D3 ionic liquids · Gold surface · Solid–liquid interface · Electrocatalyst · CO2 activation and conversion

14.1 Introduction Capturing and reducing CO2 emissions from industry, power plants, and other sources are an active field in fundamental and applicative research. Oil refineries and power S. Kamalakannan · K. Palanisamy · M. Prakash (B) Department of Chemistry, Faculty of Engineering and Technology, SRM Institute of Science and Technology, Kattankulathur, Chengalpattu, Tamil Nadu 603 203, India e-mail: [email protected] M. Hochlaf Université Gustave Eiffel, COSYS/IMSE, 5 Bd Descartes, 77454 Champs Sur Marne, France e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Garg and A. Chandra (eds.), Photocatalysis for Environmental Remediation and Energy Production, Green Chemistry and Sustainable Technology, https://doi.org/10.1007/978-3-031-27707-8_14

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plants, in particular, are responsible for around 25–30% of total CO2 emissions in the environment [1]. These industries produce flue gas that contains a mixture of hazardous gases with moisture. It also emits 15 and 80% of CO2 . This leads to environmental drawbacks such as ecosystem loss, global warming, ocean acidification, and greenhouse gas emission [2–5]. Nowadays, there are various techniques dedicated to the capture and conversion of carbon gases. They are electrochemical, thermal, solar, and photocatalytic reductions [6, 7]. For instance, CO2 is converted as a value-added fine chemical like formic acid, methanol, etc. [8, 9]. To achieve this, the development of practical technologies for CO2 capture, activation, and functionalization using electrochemical reduction, artificial photosynthesis, biochemical, and hydrogenation approaches is actively carried out. However, the inertness and the high stability of CO2 molecules restrict the effective reduction. This instigates the design and development of an appropriate catalyst for CO2 reduction. A significant scientific effort needs to develop catalysts for CO2 transformations by natural principles. In the last few decades, ardent efforts have been made to elucidate suitable catalysts. The green solvent, ionic liquids (ILs) are emerging catalysts for CO2 adsorption and conversion processes. Indeed, the room-temperature ionic liquids (RTILs) are used to capture and convert CO2 into value-added products (VAPs) [10, 11]. Mainly, ILs are composed of an organic or inorganic cation/anion pairs. Also, ILs possess unique properties such as high heat-resisting capacity, good electrical conductivity, and wide electrochemical window [12]. These make them a unique and potential effectual co-catalyst or electrolyte for energy storage and CO2 capture applications [13, 14]. There are several advantages for RTILs when compared with the aqueous amine electrolytes. Therefore, most of the earlier reports utilized RTILs’ electrolytes for various applications such as CO2 capture [15], metal-ion extraction [16], energy storage [17]. Nonetheless, among these applications, none of them is used as an effective catalyst for CO2 reduction [18]. The attractions between the ILs and CO2 can effectively modify the geometry of the CO2 molecule inducing linear to bent modifications. Furthermore, the nature of ILs can be altered by several parameters such as functional groups, temperature, pressure, nature, and size of ions. These parameters change the conductivity, melting point, and viscosity of the ILs [19]. Thus, many authors paid attention to design task-specific ILs with the use of RTILs [20]. The ILs successfully lower the overpotential of CO2 electrochemical reduction (CO2 ER) processes at the liquid medium, which indicates the effectiveness of the IL as co-catalyst [18]. Recent experimental work reported that the decrease in the CO2 ER occurs through the formation of [CO2 -IMz] intermediate complexes [21]. However, it has also been proposed that this co-catalyst effect is not only limited to [IMz]+ type ILs, but it concerns different cationic structures such as pyridinium, ammonium, and phosphonium cationic groups. Besides, this cation-modified cocatalyst concept was recently applied to chloride-based deep eutectic solvents as well [22]. However, there are many characteristics, i.e., self-diffusion, large viscosity, and external electrical conductivity of isolated ILs that should inhibit the CO2 capture and reduction behaviors. This could be resolved with the use of an assisted cathode/ILs interfacial system, i.e., heterogeneous catalyst.

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In the literature, there are various types of cationic and anionic ILs structures reported, where Imidazolium (IMz), Pyridinium (Pyr), Ammonium (Am), Guanidinium (Gua) are the cations and Chlorine [Cl]− , Tetrafluoroborate [BF4 ]− , Hexafluorophosphate [PF6 ]− , bis(trifluoromethylsulfonyl)amide [TFSA]− are the anions [23]. Among these cations, most of the research attention was focused on the [IMz]+ -based ILs since the [IMz]+ cation minimizes the energy barrier for the CO2 reduction by complexing the CO2 molecule [24]. In the last decade, we focused our investigations to study the model of solid–liquid interface formed by these ILs adsorbed on gold surface and their subsequent interaction with CO2 . We established a strategy for a systematic characterization of the interfacial effects occurring at the CO2 @ILs@Au model interface, where we determine the stable structures, the energetics, and the reactivity of CO2 at this model interface. Our work shows that this model interface is efficient for CO2 capture and activation and conversion to VAPs. In this chapter, we will present these used methodologies, the main achievements, and results.

14.1.1 Heterogeneous Catalyst for CO2 Reduction The heterogeneous catalyst can significantly reduce CO2 into value-added products when compared with homogeneous catalysts. This is due to their specific electrochemical systems overpotential, Faradaic efficiency (FE), and current density [25– 28]. Although the energy requirement for CO2 reduction by aqueous and metal catalysts is a foremost problem, the heterogeneous catalyst plays significant role in the capture and conversion of CO2 molecules. In addition to experimental research, theoretical investigations have revealed atomic-level mechanistic insights. To accomplish the successful heterogeneous catalyst for effective CO2 reduction, we believe that computational studies can provide valuable information. Indeed, quantum chemical calculations could provide deep insights into mechanistic aspects, thus expanding the knowledge of catalytic processes. They allow to identify the adsorption sites, charge transfer, and catalytic conversion mechanism of gas molecule at the microscopic level. During the chemical transformation, the change in the electronic structure of the system gives better understanding about the mechanisms in action. Indeed, some pioneering theoretical papers have successfully provided new concepts for the development of homogeneous and heterogeneous catalysts for CO2 reduction. Besides its capture, the CO2 conversion to advantageous compounds will pave the way for novel domains for industrial developments.

14.1.2 CO2 Activation and Conversion At the metal surface, the CO2 molecule could be reduced to other chemicals such as methanol (CH3 OH) and formic acid (HCOOH) [29]. There are different metal catalysts that are commercially available for CO2 ER applications [30]. Thus, the

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catalytic surfaces such as Cu, Ag, Au, and Pt act as a potential cathode material for reduction [31]. The Au nanoparticles (AuNPs) proposed as a promising catalyst for electrochemical CO2 reduction (ECR) [32]. The vicinity of the catalytic site and their local chemical environments significantly influence the interfacial CO2 ER. Also, they found that catalytic activity of the heterogeneous system is directly tuned by noncovalent, stabilization/destabilization of liquids or ligands on a substrate (cathode and anode) [33–35]. The electrode/electrolyte medium can be used to remove the CO2 gas from the feed gas produced from the industries. The up-to-date achievements of computational techniques are of great help to identify the structure, stability, and interfacial physical chemical phenomena occurring, at the atomic level, in the vicinity of these composite materials [36, 37]. Potential catalyst developments based on molecule/surface interactions appeared in recent years, because of the efficient and simple way to tune the electrode/electrolyte for energy storage and CO2 reduction applications. In this context, our model solid–liquid interface material was proved as a good candidate for these purposes through the electrocatalytic transformation of CO2 into the CO2 −* radical anion. When compared with the earlier reports of isolated traditional amine-based aqueous solvents, this approach is very promising [38–41]. To study the electronic structure and stability of ILs, CO2 @IL, and CO2 @ILs-CO2 interfacial systems, we have employed density functional theory (DFT) calculations using Gaussian16 [42], CP2K [43], and VASP [44, 45] software’s. Computations are done using PBE and GGA method [46] to describe the exchange−correlation term [47] where we also consider dispersion corrections in our calculations. This is essential to explain how carbon-based composite materials and organic molecules interact on metal surfaces [48, 49]. We have primarily conducted two different types of calculations: (i) is “gas phase type computations,” which treat bare ILs, and (ii) “solid–liquid periodic computations,” which represent the ILs/Au(111) interface and when CO2 is interacting with this interface. Evaluating the CO2 activation as well as the energetic and structural impacts of the ILs caused by the adsorption on the gold surface should be aided by the comparison of the two sets of data [50].

14.2 Characterization of the Ionic Liquids Recent reports dealing with the CO2 adsorption on various [BMIm]+ containing ILs established the following binding capacity order for the anion partner: [Cl]− < Dicyanamide [DCA]− 2 > 4 > 6 > 8 > 10 with a small difference for n ≥ 2. Remarkably, the rich and poor electron density can be used to contact with gold surface to minimize the system. Recently, Prakash and his co-workers reported that greater positive electron density localized on the cationic of MIm+ ring, whereas the negative electrostatic potentials situated in the anionic moieties [59]. The positive potentials in the cationic portion decrease as the alkyl chain lengthens.

14.3 Characterization of the Ionic Liquids@Au(111) Surface The design of heterogeneous catalyst material is a growing field that effectively converts the CO2 molecule into value-added products. The electrical double layer and interfacial interactions of liquid co-catalyst could provide a new strategy to activate the CO2 molecule. Still, there are major challenges that remain for CO2 reduction, which are hydrogen evolution reaction (HER) and high overpotential for CO2 reduction. These two common characteristics suppress the desired product formation during CO2 RR and the high energy requirement for regeneration. Currently, numerous researches focus on find out efficient electrocatalytic material for CO2 reduction. In this context, electrocatalyst (i.e., solid–liquid interface) is a versatile material, and it can be used to reduce the CO2 concentration from the atmosphere. The enhancement of catalytic reactivity for ECR at low overpotentials could be achieved by the functionalization of the metal catalyst with suitable ILs, organic molecule [60], and ligands (phthalocyanine complexes) [61]. Generally, a cathode material

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Fig. 14.3 Cation–anion distances and the optimized geometries of the hydrophobic ILs (Ref. [59])

can be used as an efficient material to reduce the CO2 molecule. The ILs attached to gold surface can serve as a potential electrocatalytic material for these purposes.

14.3.1 Effect of Hydrophilic Ionic Liquids at the Gold Surface In order to investigate the interaction between the ILs and gold surface, we have employed first principle computations. Initially, we start with the ILs at gas phase. The computed gas-phase structures of [BMIm]+ [DCA]− , [Cl]− , and [HCOO]− ILs were used to study interfacial interaction between IL and Au(111) surface. In

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Fig. 14.4 Minimized structures of ILs at gold surface and lowest interaction distances depicted (in Å) between ILs and metal surface. We are also giving different interaction sites of various ILs over gold surface (Ref. [48])

order to study the liquid and metal interaction, we have incorporated dispersion terms. The optimized equilibrium structures of ILs close to the bulk phase Au(111) are shown in Fig. 14.4. The adsorption distances of Im+ ring at the Au(111) surface range between ~ 3.577 and 3.723 Å. These distances imply that vdWs and π stacking interactions between the hydrophilic ILs and gold surface. Furthermore, we note the anchor-assisted H-bonds (AAHBs) between Calkyl- H· · · Au(s) as those observed for Im+ contact with Aun clusters [62]. The ion-pair interactions of ILs can be tuned after interacting with electrode surface. This is due to strong covalent and weak vdWs interactions between the ILs and electrode material. Mainly, the N atom of DCA− anion interacted with top site of the Au, whereas Cl− strongly adsorbs on bridge site of the gold surface. For the HCOO− and DCA− , anions the computed ion-pair H-bond distances (of 1.759 Å and 1.946 Å, respectively) demonstrate a significant interaction at the interface; however, the ion-pair distance of [BMIm]+ [Cl]− at interface is comparatively larger (2.562 Å); this is due to the formation of Au–Cl bond. The large deviation between the cation– anion pair is due to Au–Cl and Au–N bonding. The strength of interaction is obtained from E ads . Using the periodic PBE(+D3)/TZVP DFT computations, the adsorption energies of ILs@Au(111) complexes are shown in Table 14.2. As can be observed, where ILs are minimized at the appropriate method and basis set, there are significant differences in the E ads optimized by the inclusion and without inclusion of dispersion corrections. As a result, the dispersion interaction is important for the stabilization of ILs on the gold surface. The PBE+D3 E ads is undoubtedly considered to be more accurate because of the vdWs kind of interactions included with this adsorptions’ process. Hereafter, we will refer to this set of data. The other Au–N distances are 2.237 Å i.e. slightly longer, with the exception of [BMIm]+ [DCA]− @Au(111). These hydrophilic ILs have comparable E ads of − 81.78 kcal/mol for Cl and − 78.06 kcal/mol for DCA, respectively (Table 14.2). Using TZVP optimized geometry, the predicted E ads at TZV2P for the same ILs is − 71.23 and − 70.62 kcal/mol or around 10 kcal/mol less. We observed that the energies of HCOO– with BMIm+ system have lower. At the PBE+D3/TZV2P technique, the comparable BE is in fact − 22 kcal/mol. This proves that the kind of anion and the

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Table 14.2 Calculated adsorption energies (E ads , in kcal/mol) and distances (in Å) between the cation and the anion of [BMIm]+ [X]− @Au(111) using PBE and PBE + D3 methods (Ref. [48]) [BMIm]+ [X]− where PBE/TZVP PBE+D3/TZVP PBE+D3/TZV2Pa PBE+D3/6-311++G** X= E ads E ads E ads Dads Dgp Cl

− 23.99

− 81.78

− 71.23

2.56

1.94

DCA = (CN)2 N

− 13.08

− 78.06

− 70.62

1.95

2.27

5.04

− 19.26

− 22.38

1.76

1.57

BF4

14.42

− 41.49

− 24.40

2.20

2.10

PF6

7.11

− 46.57

− 27.82

2.01

2.16

CH3 SO3

2.87

− 8.41

− 17.32

1.92

1.93

OTF = CF3 SO3

6.60

− 40.98

− 27.38

2.27

2.03

133.13

− 28.58

− 38.38

2.11

1.97

HCOO

TFSA = (CF3 SO2 )2 N aE

ads calculated from PBE+D3/TZVP geometries for TZV2P basis set (consider only for IL)

binding method of ILs affect the binding strength of ILs (either dispersive or via covalent bond).

14.3.2 Effect of Hydrophobic Anions and BMIm+ Cation at the Au(111) Surface Figure 14.5 depicts the optimized geometries of hydrophobic ILs on gold surface along with lowest interaction distances bewteen ILs and Au(111) surface. We took into account [BMIm]+ [X]− ILs with X equal to BF4 , PF6 , CH3 SO3 , OTF, and TFSA. With the exception of [BMIm]+ [TFSA]− , this figure demonstrates that [BMIm]+ is stabilized by stacking interactions and typically lies parallel to the Au(111) surface. In fact, [BMIm]+ [TFSA]− shows a substantial H-bond interaction between the TFSA anion and the –C2 group of [BMIm]+ cation. Additionally, our simulation results show that the alkyl groups were strongly anchored with the gold surface except [PF6 ]− and [TFSA]− -based ILs. The core of the [BMIm]+ cation and the surface of Au(111) are separated by estimated distances that fall between ~ 3.56 Å and 3.80 Å. The stability of the hydrophobic ILs at the surface of Au(111) is also aided by anchor-assisted H-bonds in addition to the interactions already stated. Depending on the anion, the estimated distances between Calkyl and HAu range from 2.8 to 3.2 Å. In contrast to hydrophilic anions, the other IL systems consisting anions are interacting with the gold surface by weak vdWs interactions. Indeed, these anions are only coordinated with aromatic IMz+ cation and they are located above this cationic ring. The ion-pair distances of these ILs vary from ~ 1.759 to 2.267 Å after adsorption at gold surface. In these hydrophobic ILs, a single cation is taken into account, but various E ads are computed. For [BMIm]+ [X]− , where X = PF6 , BF4 , CH3 SO3 , OTF = CF3 SO3 , and TFSA = (CF3 SO3 )2 N, we estimate E ads of − 46.57, − 41.49,

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Fig. 14.5 Minimum energy geometries of hydrophobic ILs at gold surface by using PBE+D3/TZVP method (Ref. [48])

− 8.41, − 40.98, and − 28.58 kcal/mol at the DFT+D3 level (Table 14.2). These energies are 50% lower than hydrophilic ILs. We list in Table 14.3 the [BMIm]+ cations’ binding energies with regard to the anion in order to clarify the effect of the anion with gold electrode and BMIm+ cation. Using the formula below, these adsorption energies are calculated: E ads (cation) = E BMIm@Au(111) − (E Au(111) + E BMIm ),

(14.1)

Table 14.3 Calculated Lowdin charges (in a.u.) of ILs in isolated phase and at the interface (Ref. [48]) [BMIm]+ [X− ] Where X =

ILs@Au(111) surface

Cl

1.38

DCA HCOO

[BMIm]+

[X− ]

Total charge of IL

Total charge of the Au(111) surface

0.73

2.11

− 2.11

1.47

0.35

1.82

− 1.82

0.57

− 0.41

0.16

− 0.16

BF4

1.23

− 0.67

0.56

− 0.56

PF6

1.09

− 0.57

0.52

− 0.52

CH3 SO3

0.92

− 0.42

0.50

− 0.50

OTF

1.22

− 0.56

0.66

− 0.66

TFSA

0.90

− 0.55

− 0.34

− 0.34

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where E BMIm@Au(111) , E Au(111) , and E BMIm energies are deduced from SCF calculations. The computed E ads of electrolyte at gold surface states that the adsorption of cation dominates over the anion, and the energies are changed from − 94.0 to − 106.0 kcal/mol, i.e., greater (in absolute value) than the hydrophobic ILs by at least two times. In addition, the alkyl group contribution is strong enough to change the stability of interfacial systems (i.e., electrolytes@gold surface). This is based on the number of –CH2 subunits that adsorb to the surface of Au(111). The cathode surfaces show suitable interaction sites as a result of the knowledge of the specific IL adsorption sites at gold surfaces.

14.3.2.1

Site Selective Binding of [BMIm]+ [X]− ILs on Gold Surface

Figure 14.5 depicts the cation/selective anion adsorption sites. In order to forecast how the anion will affect the binding character, we also simulated energetics for [BMIm]+ at Au(111) surface for a variety of anions that were optimized according to Eq. (14.1). We discovered that anions are essential for site selectivity, selective adsorption, and surface reactivity. While, the IL cationic part and alkyl groups interact with the gold electrode via vdWs and AAHBs interactions, while the interaction between anions relies on whether they are hydrophilic or hydrophobic. With the exception of [BMIm]+ interacting with [BF4 ]− in the top position and [BMIm]+ interacting with [HCOO]− in the hcp(h) position, the cation is usually attracted by bridge sites. In spite of this, the anions are attracted to various sites according to their size. For instance, hydrophilic anions are bonded at the b (Cl− ) and t (DCA− ) sites. According to our study, the calculated E ads for [BMIm]+ at the bridge site are lower than the E ads for the top and hcp adsorption sites. It is interesting to notice that in contrast to the other ILs, [BMIm]+ interacting with HCOO and BF4 anions is significantly adsorbed. This demonstrates that the top/hcp spots of the gold surface are where the N-based moiety adsorbs. Alkyl chain adsorption on the Au surface is one of the various forms of interaction at the contact. Three different types of interactions exist, including C–H· · · Au, CH2 · · · Au, and CH3 ·· · · Au. All of these interactions result in an increase in E ads and have an impact on the local charge density. It improves the mechanisms involved in charge transfer at electrode– electrolyte interfaces.

14.3.2.2

Interfacial Charge Transfer

In the discussion that follows, Lowdin charges will be used (in Table 14.3). These reveal the charge of ILs at the interface which is significantly increased when compared with gas-phase systems. The computed charges of hydrophilic ILs at interface are three times higher than isolated phase with exclusion of HCOO− . The calculated charges for hydrophobic ILs range between 1.5 and 2 times. These modifications affect the surface reactivity at the microscopic scale and are of great significance.

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Compared to the anion, the adsorbed cation can lead to a greater binding with electrode surface. It should be noted that AFM experiments on ILs at the surface of Au(111) demonstrated a related result with the creation of an EDL at the IL and gold electrode. The order of magnitude of the charge distribution seen at the gold surface for [BMIm]+ [X]− @Au(111) is X = Cl > DCA > OTF > BF4 > PF6 > CH3 SO3 > TFSA > HCOO. Consequently, the charge transfer process has the ability to direct and impact how the IL interacts with the gold electrode material. Our research should therefore be useful in helping to choose the best composite materials for electrochemical and energy storage applications.

14.3.2.3

Density Surface Analysis

We have reported EDA by using the PBE method as developed in the C2PK code [43]. Numerous research works have demonstrated that the transport of charge at interfacial systems is greatly influenced by the charge accumulation and depletion process [56– 59]. The transport of charge through the electrochemical window depends on by the type of ILs [63, 64]. For instance, most of the previous studies showed that the alignment of the ILs at charged surfaces in STM and AFM, followed by the development of EDL of ILs adsorbed at solid surfaces, is caused by the changing of the potential ranges [65]. The counter anion BF4 − has a synergistic action that lowers the cohesive energy between metals [66]. Currently, we find reasonably strong interactions between the surface of Au(111) and [BMIm]+ . As a result, the charge transfer phenomenon may enhance the electrostatic surface defects, which then easily attract hydrophilic anions. Recent experimental studies of ILs at gold surface demonstrate that ILs do in fact form an adlayer on the gold electrode [67].

14.3.3 Impact of Alkyl Groups of the (Cn MIm+ ) at the Au(111) Surface The further stability of the alkyl groups by AAHBs on the Au surface is depicted in Fig. 14.6. Adsorption energies are hence relatively high in absolute values. We calculated adsorption energy of − 110.3 kcal/mol, for instance, for [MIm]+ cation adsorbed on gold electrode. The lowest interaction distance between aromatic IMz+ rings is 2.6 and 3.6 Å. The predicted interaction range between the surface and [MIm]+ steadily decreases as the –CH2 subunit increases. This is because numerous C–H· · · Au, H-interactions with the surface of gold electrode are formed by the alkyl chains –CH2 group(s). Strong variations in the predicted E ads , which vary from − 110 to − 183 kcal/mol as the alkyl chain is prolonged, are observed. The [HMIm]+ cation is strongly bounded with gold electrode compared to shorted –CH2 groups (i.e., [BMIm]+ ).

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Fig. 14.6 Computed structures of different alkyl groups at gold surface by using PBE+D3/TZVP method (Ref. [59])

The corresponding alkyl end groups for –C4 and –C6 protrude from the surface. However, the –[C6 MIm]+ cationic group having –CH2 subunits is parallelly binding with the electrode. The alkyl groups adsorbs at the electrode at top and bridge sites. This results in larger (in absolute value) of more E ads value for [C10 MIm]+ |E ads | (183.2 kcal/mol) compared with other groups. In any case, the longer alkyl chains become even more stable due to the longer chain length of alkyl groups on the gold electrode.

14.3.3.1

Effect of Alkyl and Anions at Interface

For these investigations, we have chosen the hydrophilic anions [Cl] and the hydrophobic anions [PF6 ] and [TFSA]. Figure 14.7 shows the [Cn MIm]+ [X]− @Au(111) optimized geometries’ (where n = C0 , C2 , C4 , C6 , C8 , C10 ; X = Cl, PF6 , TFSA) ILs. The alkyl groups of ILs cationic moiety interacted with the electrode surface by AAHBs’ stacking interaction. This agrees with earlier SERS analyses and calculations of the interactions between methylimidazole-containing molecules and metallic Au(111) surfaces [68–72]. We discovered that the length of the alkyl chain affects the adsorption sites for [Cl]− anions. Both [C1 MIm]+ [Cl]− and [C2 MIm]+ [Cl]− at electrode surface exhibit substantial adsorption or chemisorption of Cl− anions. The Cl− moieties in [C4 MIm]+ and [C6 MIm]+ chemisorbed at bridge sites. The Cl− anion in [C8 MIm]+ [Cl]− and [C10 MIm]+ [Cl]− at electrode surface was bonded with the hcp sites. This change in adsorption sites is due to the tuning of local modification of the Au(111) surface upon adsorption of the ILs. The distance between the central rings of basic [C1 MIm]+ unit for [Cn MIm]+ at electrode surface changes from 3.5 to 3.7 Å. In present study, the

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Fig. 14.7 Computed structures of fluorinated ILs at gold surface done at the PBE + D3/TZVP level of theory (Ref. [59])

E ads of hydrophilic ILs varies between − 107 and − 50.0 kcal/mol. These values are 50% decreased compared to bare unit at electrode surface (Table 14.4). It is observed that there is the very less E ads for shorter alkyl chain than the longer groups; this is due to involvement of (Alkyl) C–H· · · Au interaction between alkyl groups and gold surface. In addition to this, different behaviors are observed for hydrophobic ILs when compared with hydrophilic-based ILs systems. Indeed the [Cn MIm]+ cation with hydrophobic anions are poorly interacting with electrode

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Table 14.4 Calculated BEs (in kcal/mol) of alkyl groups and alkyl chains with anions of ILs at the interface (Ref. [59]) Methods

ILs@Au(111)

Adsorption energies (E ads ) n = 0–10 0

MIm]+

PBE+D3 [Cn /TZVP [Cn MIm]+ [Cl]− [Cn MIm]+ [PF6 ]− [Cn MIm]+ [TFSA]−

2

4

6

8

10

− 110.3 − 109.0 − 114.0 − 125.9 − 132.2 − 183.2 − 50.4

− 72.1

− 81.8

− 94.3

− 96.0 − 107.3

− 32.9

− 31.4

− 46.6

− 42.6

− 24.8

− 31.7

− 24.2

− 29.8

− 28.6

− 42.7

− 50.0

− 53.8

surface. In fact, there is no direct interaction between the anions and the surface. They instead contact with the cation that has been adsorbed to the metallic surface [59]. Instead of the several H-bonds mentioned above for isolated ILs, the anions are currently attached to the IMz+ core through π-stacking. Heinz et al. calculated that the adsorption energy for [C2 MIm]+ [ES]− ethyl sulfate adsorbed at gold surface is about to − 30 to − 40 kcal/mol [72]. The earlier report is close to the adsorption energies of [C2 MIm]+ [PF6 ]− and [C2 MIm]+ [TFSA]− ILs with gold surface. However, the computed E ads is less than that for Cl− (in absolute value). In contrast, the MIm+ ring is slightly aligned toward the surface [TFSA]-based system at interface. However, in [PF6 ]-based complex system, the [MIm]+ cation is adsorbed in a tilting parallel stacking with the gold surface. This tilting effect is due to H-bonding between the electronegative group (–O) of anion and –C2 –H of cation. As a result, the [Cn MIm]+ [PF6 ]− and electrode vary from 3.5 to 3.7 Å, whereas a noticeably lower (of 2.4–3.0 Å) is calculated between the [Cn MIm]+ with [TFSA]− ILs and electrode surface. The ILs E ads at the Au(111) surface is greatly impacted by these structural effects. Furthermore, compared to the [Cl]− anion-based ILs, the hydrophobic anions reduce E ads energies by 30%. The ILs, E ads at the gold surface are greatly impacted by these structural effects. Furthermore, compared to the [Cl]− anion-based ILs, the hydrophobic anions reduce E ads energies by 30%.

14.3.3.2

Epitaxial Contacts of ILs at Gold Surface

Earlier studies demonstrated the presence of ILs’ soft epitaxial contacts at the Au surfaces [72, 73]. The –N atoms of the Im+ ring strongly interact on the first layer of the Au surface in the case of hydrophilic case. The epitaxial sites of the gold atoms in the second layer are in contact with the nitrogen atoms of [Im]+ in the case of [PF6 ]− and [TFSA]− .

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Table 14.5 Löwdin charge transfer analysis of [Cn MIm]+ at the gold electrode (where Cn = C0 , C1 , C2 , C4 , C6 , C8 , and C10 ) (Ref. [59]) [Cn MIm]+ @Au(111) surface

Gas phase n

[Cn MIm]+

[Cn MIm]+

Charge of the Au(111) surface

0

1.00

1.24

− 0.24

2

1.00

1.32

− 0.32

4

1.00

1.35

− 0.35

6

1.00

1.72

− 0.72

8

1.00

1.91

− 0.91

10

1.00

2.12

− 1.12

14.3.3.3

Charge Population Analysis

The current research on biomolecular adsorption at metal surface focuses on the significance of induced charges [69]. We observe a similar impact in this investigation, which is much in line with the charge transfer analyses of the ILs@Au(111) complexes. For instance, the Löwdin population analysis is used to quantitatively identify the accumulated charges of each atoms. The quantity of the charge accumulation of Au surface of the [Cn MIm]+ [X]− is determined by the Löwdin charge transfer analysis. Following adsorption, there is a negative charge which is transferred to the gold surface from the [Cn MIm]+ groups (shown in Table 14.5). The amount of electronic charge transfer to the gold surface in the case of [HMIm]+ [PF6 ]− @Au(111) is − 0.75 a.u. and [DMIm]+ [TFSA]− @Au(111) is − 0.98 a.u., respectively. The less and high charge accumulations are obtained for [Cn MIm]+ [PF6 ]− @Au(111) when n = 0–6.

14.3.3.4

Charge and Electron Density

Our earlier report states that the dispersion of positive and negative electron densities accumulates over the alkyl groups, when the alkyl length increases the positive and neutral electron densities to locate/disperse at various positions [59]. For the cationic constituents of [Cn MIm]+ [Cl]− @Au, similar results are observed. The charge density for [Cn MIm]+ [PF6 ]− at the electrode surface is primarily spread between the [Im]+ ring and Au surface. The oxygen atoms in the alkyl groups or the Im+ ring of [Cn MIm]+ [TFSA]− @Au(111) play critical roles in maintaining electrostatic behavior throughout the molecular structure. These changes have a substantial impact on the microscopic interfacial structure and the interfacial interactions between ILs and Au surface. Similar to this, earlier research on ILs at the graphite surface demonstrated that charge transfer phenomena can occur at carbon-based

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interfacial structures [74]. This analysis is helpful to find the effective interactions between the metal and ILs, which will induce CO2 reduction and conversion.

14.4 Electrocatalysis of CO2 Reduction CO2 could be reduced to other chemicals such as methyl alcohol (CH3 OH) and formic acid (HCOOH) at the cathode surface [29]. There are different metal cathodes (metal catalysts such as Au, Ag, Pt, and Cu) that are commercially available for the CO2 ER application [30, 31]. For instance, gold is viewed as a promising cathode material for the reduction of CO2 into CO and other products [75]. In particular, the Au nanoparticles served as a promising catalyst for electrochemical reduction (ECR) of CO2 [32]. Nevertheless, the gold nanoparticles are unstable, which lead to irreversible aggregation resulting in restraining the catalytic performances. These issues can be circumvented by the functionalization of amine solvents or by the use of suitable liquid co-catalyst (ILs) adsorbed to the Au surface [60].

14.4.1 Interaction of CO2 with Ionic Liquids We have chosen common hydrophilic ILs with changing the alkyl chain lengths from n = 0 and 6, composed of a [Cl]− anion and a [Cn MIm]+ cation. The [Cl]− atom is strongly bounded with the top site of Au atom; this is due to the electropositive nature of gold surface which attracts the electronegative Cl− anion. We have used PBE+D3/6-311++G** level of theory to optimize the CO2 @ILs complexes system. We observed the strong electrostatic interaction between the C2 –H and [Cl]− atom of the ILs. This strong electrostatic interaction is responsible for single site adsorption and limits the other site adsorption at Au(111) surface. The proton transfer from [MIm]+ moiety to [Cl]− that resulted from the CO2 @[C0 MIm]+ [Cl]− optimizations produced a CO2 @MIm+ @HCl cluster. In fact, we see that a –H+ is transferred from the N1 –H group of [MIm]+ [C1 MIm]+ , which bonds to the nearby [Cl]− atom. There is no –H+ transfer seen between the [MIm]+ moiety and [Cl]− moieties in the complexes of CO2 @[Cn MIm]+ [Cl]− (n = 1–6). Instead, a pair of [Cn MIm]+ [Cl]− ions is created. In this case, the nucleophilic [Cl]− ion interacts with the electrophilic CO2 carbon atom as well as the MIm+ cation via anion-induced dipole-quadrupole interactions (acid–base Lewis interactions). Alkyl groups further partially interact with CO2 through C–H· · · O H-bonded interactions. Certainly, this happens between the –CH3 (C–H) group and CO2 (O). The MIm+ ring of C2 –H component serves as an acceptor of electrons, and the spherical Cl− ion acts as an donor of electrons [24]. The computed BE energies range between − 28.4 and − 29.0 kJ/mol, with only a very minor enhancement as the alkyl chain is lengthened from 1 to 6 (Table 14.6). Thus, the only major factor affecting the interaction of CO2 molecules with

14 Electrocatalytic Activation and Conversion of CO2 at Solid–Liquid … Table 14.6 BSSE-corrected BE in kJ/mol of CO2 @[Cn MIm]+ [Cl]− (n = 0–6) in isolated system (Ref. [50])

n

347

CO2 @[Cn MIm]+ [Cl]− Gas Phasea BE b

0

–

1

− 28.40

2

− 28.53

3

− 28.91

4

− 28.95

5

− 28.99

6

− 28.99

a PBE+D3/6-311++G** b Proton

level

transfer occurs

bare ILs is the length of the alkyl chain. The closely computed ESP images for CO2 @[Cn MIm]+ [Cl]− (n = 1–6) complexes serve as evidence for this (Fig. 14.8). By reducing its respective activation barriers, these structural alterations to the CO2 molecule can contribute to the into its activation [76]. However, according to our theoretical research, CO2 maintains its linearity when interact with bare gold surfaces because CO2 stays comparatively far away from the gold surface [77].

Fig. 14.8 Electrostatic potential plots (isovalue 0.02 a.u) of CO2 @[Cn MIm]+ [Cl]− (Ref. [50])

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14.4.2 Adsorption of CO2 at Hydrophilic ILs-Decorated Gold Surface Based on the E ads , density accumulation/depletion, and charge transfer analyses, the CO2 interaction at the different alkyl chains [Cn MIm]+ [Cl]– @gold surface is investigated. We looked into the adsorption/activation of CO2 at the [Cn MIm]+ [Cl]– @Au(111) (n = 0–6) liquid–solid interface since the IL attached to Au(111) surface may cause CO2 structural and electronic perturbations through several types of non-covalent interactions. The PBE+D3/TZVP level was used for these bulk periodic calculations. Calculations demonstrate that, as was previously discussed for [Cn MIm]+ [Cl]– @Au, the MIm+ ring of the cationic component of the IL lies parallel to the gold surface. The CO2 @[C0 MIm]+ [Cl]− @Au(111) does not exhibit a H– migrated from aromatic IMz+ ring to Cl− atom, contrary to the isolated phase of CO2 @[C0 MIm]+ [Cl]− clusters. In all our complex system, the CO2 approaches to the MIm+ ring. At these metal ILs, CO2 can interact in two different ways: Mode 1, where CO2 is adsorbed parallel to the gold surface, and Mode 2, where CO2 is orthogonal to this gold surface. This shows that the angle at which CO2 lands at the solid–liquid interface determines the adsorption mechanism for CO2 . Figure 14.9 illustrates Mode 1 and 2 adsorptions of CO2 @interface. The MIm+ cation is bound to CO2 in Mode 1 through π-stacking interaction. The aromatic ring of MIm+ is thus parallel to gold surface. The –O atom of CO2 and the H atom of the –CH3 group in MIm+ ((CO2 ) O· · · H(CH3 )) have weak long-range interactions, with the –O atom being located between ~ 2.9 and 3.4 Å from the H atom. In addition, Fig. 14.10 demonstrates that CO2 maintains its linear form after adsorption. However, an increase of the CO bond is predicted, where the calculated CO bond lengths range from 1.172 to 1.175 Å.

Fig. 14.9 Representation of different adsorption mechanisms of CO2 at electrode–electrolyte medium. Mode 1 and Mode 2 correspond to the adsorption of CO2 parallel and perpendicular to the metal surface, respectively

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Fig. 14.10 CO2 @[Cn MIm]+ [Cl]− @Au-optimized geometries by PBE+D3/TZVP at gold electrode surface (Ref. [50])

Table 14.7, which clears the E ads of CO2 at the [Cn MIm]+ [Cl]− @Au(111) interface, also demonstrates that the interaction between the two entities reduces from n = 0 to n = 3 and subsequently exhibits the reverse behavior while adding −CH2 subunits to [Cn MIm]+ is increased. The resultant E ads changes accordingly from − 48.70 to − 22.13 kJ/mol for 0 {101} > {001}. The {100} facets have stronger adsorption sites and more negative photo-generated electrons for CO2 photoreduction reaction compared to other facets. The {100} facets have superior electronic feature and adsorption sites, enhancing the CO2 reduction efficiency (Fig. 16.11) [67]. Gao et al. have reported catalytic CO2 reduction to CH4 and CO under visible light using hybrid material (TiO2 nanosheets (NSs)/tetra(4-carboxyphenyl) porphyrin (TCPP) [33]. The hybrid material TiO2 NSs/11.5% TCPP shows a maximum yield of CO2 reduction product compared to pristine TiO2 material under visible light. Li et al. reported the CO2 conversion (under visible light) to CO/CH4 using the asymmetric structure of zinc porphyrin (ZnPy)-sensitized nanosized TiO2 [68]. The

Fig. 16.11 {001} and {101} surface heterojunction (a) (Reproduced with permission Ref. [65] Copyrights 2014, American Chemical Society), band positions of TC and TW samples. CB, conduction band; VB, valence band (b). The absolute potential of the standard hydrogen electrode is − 4.44 eV (Reproduced with permission Ref. [66] Copyrights 2015, Royal Society of Chemistry)

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Fig. 16.12 Time-yield plot of CO and CH4 over TiO2 NSs/11.5% TCPP (a) (Reproduced with permission Ref. [33] Copyrights 2019, Elsevier), the possible mechanism of CO2 photoreduction to CO/CH4 generation over the ZnPy-sensitized TiO2 (b) (Reproduced with permission Ref. [68] Copyrights 2017, Royal Society of Chemistry)

absorption peak of the ZnPy is 426 nm, and the weak triplet peaks are between 530 and 630 nm (Q-band). The electron transfer ZnPy to TiO2 is confirmed by increasing the Q-band absorption with an increase in the % weight of ZnPy/TiO2 . The TCCP acts as a photosensitizer for solar light harvesting, transfers the photo-induced electron to TiO2, and inhibits the electron–hole pair recombination. The TCPP provides an electron transfer channel and a more negative LUMO than TiO2 (CB) (Fig. 16.12). The porphyrin skeleton and carboxylic group benefit electron transfer from TCPP to TiO2 under a visible-light region [33, 69, 70]. The porphyrin molecules are enhancing the result of photocatalytic CO2 reduction performance under visible light. There are various strategies, such as (1) crystal phase engineering and phase heterojunction, (2) crystal facet engineering and surface heterojunction, and (3) surface engineering which are employed to improve the photocatalytic activities of TiO2 [12, 33, 63, 67, 71, 72].

16.4 Amine Group Functionalized Metal–Organic Frameworks (NH2 -MOFs) Metal–organic frameworks (MOFs) are a class of the crystalline porous network, and the networks are formed by means of linking organic molecules (linkers) and metal ions/clusters [73–75]. The porous nature, high surface area, and tuneable pore size are unique features of MOFs. The MOF materials are used in various types of applications such as gas separation, environmental remediation, drug delivery, sensors, energy storage, and conversions [76, 77]. MOFs materials are widely explored in CO2 capture and conversion applications because of their high surface area and porosity nature. The high surface area and porosity of the MOFs enhance the active sites on the surface and channels for reactant adsorption, improving conversion efficiencies

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Fig. 16.13 Schematic representation of photocatalytic CO2 reduction process over MOFs and the redox potentials for the reduction of CO2 into various products (Reproduced with permission Ref. [82] Copyrights 2020, Elsevier)

[78–80]. The functionalization of heteroatoms on organic linkers also increases the CO2 adsorption on the surface with ionic interactions, enhancing the CO2 reduction reaction efficiency and higher-order hydrocarbon products [75, 81]. The MOFs materials act as semiconductors, organic linkers, and metal center which serve as photo excitation medium and transport medium. The MOF material observes light energy to excite electrons from the highest occupied molecular orbitals (HOMO) to the lowest unoccupied molecular orbitals (LUMO). The photo-generated electrons on the HOMO diffuse to the catalytic centers to react with adsorbed CO2 , initiating the formation of intermediates. For choosing the MOF materials for the CO2 reduction, the LUMO should be above the reduction potential of the various products (Fig. 16.13). The HOMO and LUMO gap are depended on the organic ligand and metal ions [73, 82–84]. The easily tuneable structure of MOFs enhances the electronic and optical properties. The various types of MOFs materials and bandgap profiles are given in Fig. 16.14 [84]. Amine groups containing MOF materials have provided an additional pathway for conversion. TiO2 /MOF (MOF=NH2 -UiO-66) composites (Type II heterojunction system) for CO2 to CO reported by Crake et al.: The composite material depicts 9 times CO2 production than the TiO2 (Fig. 16.15a) [85]. The NH2 -UiO-66 has large surface area and pores, enhancing the CO2 uptake capacities, and the pore space needed to promote access to the catalytic sites. The NH2 -UiO-66 also acts as a charge separator, and the amine group provides an additional pathway of the electron transfer. NH2 -UiO-66 composite with poly (triphenylamine) [79] and graphene [86] materials also reported, in the both composite materials, the light energy is observed, and electron is ejected from organic ligand to metal center (Zr3+ ). The poly-triphenylamine and graphene materials are used to inhibit the recombination of electron–hole pairs during the photocatalytic process. Wang et al. reported CO2 reduction to formate using Fe-based MOF ((NH2 -MIL-101(Fe), NH2 -MIL-53(Fe), NH2 - MIL-88B (Fe)) with dual excitation path way mechanism (Fig. 16.15b) [87].

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Fig. 16.14 Band structures and positions on the basis of redox potentials at pH 7 for metal–organic framework photocatalysts (Reproduced with permission Ref. [84] Copyrights 2021, Elsevier)

Fig. 16.15 Schematic diagram of TiO2 /NH2 -UiO-66 for CO2 conversion mechanism (a) (Reproduced with permission Ref. [85] Copyrights 2017, Elsevier), and the amino functionalized Fe-based MOF dual excitation mechanism (b) (Reproduced with permission Ref. [87] Copyrights 2014, American Chemical Society)

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When the MOF observed the light energy, the electron excited from ligand to Fe3+ . Another pathway is an excitation of the amine group and the electron transfer from the amine functional group to the metal center. Other MOF with an amine group and its composite such as NH2 -MIL-125(Ti) [78] and NH2 -UiO-66/SiC [88] materials shows the high conversion rate compared to parent MOF and its composite materials. The post-modification of alkylamine on MOF (MIL-101-Cr) materials for CO2 to CO is reported by Xie et al. [80]. The functionalization of MOF with alkylamine (ethylenediamine, diethylenetriamine, triethylenetetramine) shows a high CO2 to CO conversion rate. The alkylamines modified MOFs have high CO2 uptake because the amine group (base nature) easily attracts the CO2 molecule (acid nature). The alkylamine functionalization promotes the charge carrier and the rate of electron migration, as a result enhancing the CO2 reduction rate. The three-component composite Z-Scheme materials such as ZnO/r-GO (GO)/NH2 -UiO-66 are reported by Meng et al. [89]. The oxygen-deficient ZnO ejected electron under light irradiation, the electron transfer through the reduced GO to MOF. The r-GO and MOF trap the electron as result and inhibit the charge carrier recombination process. The amine functional group MOF materials and its composite materials show the higher CO2 uptake capacity and the additional path way of electron transfer, enhancing the CO2 conversion rate.

16.5 Conclusion The photo-induced charge separation and inhibition of electron–hole pair recombination are the key steps to achieve efficient CO2 conversion. In this chapter, various types of photo active materials (g-C3 N4 , perovskite, TiO2, and MOFs) and its composite material are discussed. The metal doping in g-C3 N3 materials enhancing the electron transfer and inhibiting the recombination process. In TiO2 -based materials as well as in MOFs, the fabrication of composite materials inhibits the recombination process and also improves the light harvesting efficiency. The amine and other hetero group containing semiconducting materials show the high conversion capacity. Still, the research focused on the bond edge near the reduction potential of CO2 and visible active semiconducting materials are progressing. This chapter gives an overview of mechanism of materials toward CO2 conversion.

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Chapter 17

Conducting Polymer Hybrid Nanocomposites-Based Photocatalytic Material for Energy Applications S. Uday, Harshita Chawla, Amrish Chandra, and Seema Garg

Abstract Rising energy needs, environmental degradation, and natural resource depletion have prompted academics to examine the creation of clean energy systems for energy conversion and storage technologies. Electrochemical energy generation is another alternative energy source, and in an attempt to address these energy needs and advancement issues, photocatalytic conducting polymer-based nanocomposites are implemented for energy and environmental applications, challenges, electrochemical energy storage technologies, such as batteries and electrochemical supercapacitors, which have become popular in recent years. Nano-sized conducting polymerfabricated hybrid nanocomposites-based photocatalytic materials are implemented as electrodes or active materials for energy-related devices due to the perceived low density, superior efficiency, environmental stewardship, potential relatively inexpensive quality, and electrochemical sustainability. Their bigger surface zones, higher conductivities, and greater flexibility make them a vital advancement for these energy sectors. Furthermore, nanostructured conjugated polymers have fascinating new properties such as versatility and conductance, which reduces the electrode-toelectrolyte interface resistance. Pseudo-capacitors and asymmetrical supercapacitors are being created as a result of ongoing research in electrochemical supercapacitors technology. The power storage properties of photocatalytic conducting polymers and the venture into the energy resource management sector have been discussed in this chapter. S. Uday · H. Chawla · S. Garg (B) Department of Chemistry, Amity Institute of Applied Sciences, Amity University, Uttar Pradesh, Sector-125, Noida 201313, India e-mail: [email protected] A. Chandra Amity Institute of Pharmacy, Amity University, Uttar Pradesh, Sector-125, Noida 201313, India Amity Institute of Public Health, Amity University, Uttar Pradesh, Sector-125, Noida 201313, India

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Garg and A. Chandra (eds.), Photocatalysis for Environmental Remediation and Energy Production, Green Chemistry and Sustainable Technology, https://doi.org/10.1007/978-3-031-27707-8_17

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Graphical Abstract

Keywords Conducting polymers · Hydrogen generation · Supercapacitors · Li-ion batteries · Metal–organic framework-MOF · Solar cells

17.1 Introduction As new views and difficulties emerge, the energy sector’s thirst for innovative functional materials continues to expand, particularly in domains where environmental stewardship meets vigorous progression [1–3]. Because of their socioeconomic relevance, high environmental resilience, and electrical conductance, as well as their beneficial mechanical, optical, and electronic characteristics, conducting polymers have gotten a lot of attention. Conducting polymers are implemented in the fabrication of electrostatic materials, conducting adhesives, electromagnetic shielding against electromagnetic interference [4]. Conjugated polymers, which offer potential benefits over tiny molecules and inorganic materials, are one of the most promising types of green contenders for meeting the ever-increasing needs for the future generation of sustainable and adaptable energy-related technologies. Modification of the geometry and insertion of multiple operational moieties can be used to modify the characteristics of conjugated polymers. Furthermore, superior performance can be obtained as a result of the benefits of nanostructures, such as their huge surface areas and reduced charge transfer paths. As a result, nanostructured conjugated polymers with diverse characteristics may be synthesized and used in a variety of energy-related organic devices [5]. Because of its ramifications for hydrogen generation and the urgent requirement to minimize fossil fuel reliance, light-sourced photocatalysis might alleviate several global energy challenges. To reduce our environmental effects, we are always looking

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for novel, greener sustainable chemical solutions. Remediation via water and photocatalysis, in particular, has received considerable attention in the modern days which can be aided by conducting polymer hybrid nanocomposites [6–8]. From the core, the nanoscale aids applications such as nano/molecular electronic development, and thus the special characteristics induced are conducting framework, electrical characteristics, reversible doping procedure, controllable chemical and electrochemical properties, and easy handling in the nanocomposites like polyaniline, polypyrrole, poly-formaldehyde resin, and more. These are used in field-effect transistors, field dispersion and electrochromic devices, supercapacitors, and actuators, among other things [9, 10]. The amenities are appreciably available in the nanoscale due to the optimized versatility linked to better amenities of the strain inside of electrodes, the elevated area of contact among the active electrode substance, and the electrolyte. This presents reduced polarization, diminishment during cycling, a slightly shorter diffusion length for electrons and ions to commute via the electrodes [5]. A photoactive substance absorbs a light energy photon that stimulates an electron in the valence band (VB) and moves it to the conduction band (CB) when subjected to the legitimate range of wavelengths of light. As a result, a positively charged hole will be left in the VB, and this hole will engage with the migrating electron through coulombic interactions. The excitation potential required to make an electron–hole pair establish a bound steady state is independent of the parent atom (ion) and can be transmitted from one to the next. This produces excitons, which are “excitation waves forms” that travel down the crystal [11]. Table 17.1 aided [12] from the data published by the United States—Department of energy and the United States council for automotive research can give a bright outlook on the economic revolution the venture of the photocatalytic energy industry is targeted to bring on. The applications of the conducting polymer hybrid nanocomposites-based photocatalytic materials in energy resource management or lamely in the energy sector are vast and profound. For the amicability of the chapter, we will focus on one important vector in each important dimension of energy resource managing in this chapter.

17.2 Energy Harnessing 17.2.1 Case Study of Solar Energy Solar energy capturing using nanostructures is a cost-effective solution to address environmental and energy challenges [19]. Samim et al. [20] demonstrated that simple adsorption of ZnO nanoparticles on poly(diphenylbutadiyne) nanofiber surface contributes to successful sensitization of semiconductor nanocrystals for natural sunlight harvesting, to provide simple and cost-effective ways to design

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Table 17.1 Targeted economic strategies that are based on the energy application of photocatalytic conducting polymers Energy application venture

Cost details in USD

Challenges

Further references

CH4 storage

$10/Kg

− 40–85 °C temperature range maintenance up to min 100 cycles

[13]

H2 synthesis

$3.10/Kg–$3.70/Kg

Diminishing capital investments and harnessing feedstock

[14]

H2 storage

$10/kWh

High-density H2 storing

[15]

PV-cells

$0.06/kWh—utility

Diminish the build and hence the retail cost despite quality raw materials

[16]

EV batteries

$125kwh

Range, charge time, and battery heftiness

[17]

Fuel cells

$40/kw (stability cycle-5000)

Power density enhancement is too slow

[18]

hybrid nano-heterojunctions with multiple features and functionality suitable for solar cells, chemical sensors, and optoelectronic-associated devices (Fig. 17.1). When coupled with metal oxide (e.g., TiO2 ), conducting polymers exhibit unique qualities of light absorption and hole transport, which may lead to improved photovoltaic efficiencies via enhanced electron migration [4, 21, 22]. Metal–organic frameworks (MOFs) have been a boon since their discovery and throughout their development journey due to their unique properties such as storing gas molecules, improving gas infiltration, aiding mass, electron, and charge mass transit, threshing exoteric energy, promoting reactant activation, and improving conductivity and durability [12, 23] In case of the spectra of solar energy conversion, we will have a special case study on metal–organic frameworks in each section as conjugated nanocomposites.

17.2.1.1

Photocatalysis-Based Hydrogen Synthesis

The electrolysis of water that yielded hydrogen (H2 ) and the possible catalysts to rapidize the reaction is the need for research advancement as H2 is such vital energy. Rubidium-based MOF {Ru2 (1,4-BDC)2 }n asserted by the virtue of visible light was first employed to get 4.82% of quantum yield which has led to the foundation of the research in this spectrum. MOFs guided with the Nobel-core nanoparticles have been the new venture here; C3 N4 , metal oxides/complexes, and poly-oxometallates have been employed to integrate to MOFs to get a yield around four times higher. WDPOM clusters were integrated with pores of SMOF-1 aiding mass and e− transfer.

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Fig. 17.1 Photocatalytic efficiency of solo PDPB fibers (red), ZnO NPs (green), and the amalgamated PDPB-ZnO (blue) with their light harnessing nano-heterojunction. a UV light. b Visible-light (1 sun) irradiation. c Effect of EDTA, Cu2+ , and TBA on its photocatalytic activity. d Photocurrent responses without bias voltage republished with permission from [20]

The photocatalytic activity of the novel Co@MOF composite is significantly higher than that of the homogeneous Co-based molecular catalyst and the pristine NH2 -MIL-125(Ti) catalyst. Furthermore, Co@MOF has maintained a consistent TOF of 0.8 for 65 h, demonstrating its great resilience under the current conditions. Diffusion restrictions are not detected in this artificial Co@MOF catalysis system, and charge density is nevertheless effective even at large molecular catalyst workloads, which accounts for the high photocatalytic performance [12]. The monometallic (Pt, Ni–NiO) and bimetallic (Pt–Ni nanoparticles) in their colloidal precipitation form were obtained by radiolytic reduction on the surface of Polypyrrole (PPyNSs), and nanostructures predicated on PPyNSc loaded with mono- and bimetal nanoparticles have strong photocatalytic properties to produce hydrogen. This study also shows that photocatalytic activity is highly dependent on metal loading. When compared to the other loading rate percentages, PPyNSs loaded with 0.2% Pt showed the most activity. For the production of green hydrogen, modification of nickel-based nanoparticles has already shown promising results. After the formation of a heterojunction between PPy and NiO–Ni nanoparticles, the photocatalytic activity of 5%Ni-PPyNSs was considerably enhanced [24].

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Photocatalysis-Based CO2 Reduction

To utilize the gases such as CO, HCOOH, MeOH, and so on which are the carriers of the energy, CO2 being a major pollutant needs to be mutated into feasible energy on an industrial scale. For this venture, MOFs were used initially from [ReI(CO)3 (dcbpy)Cl]; it was increased paramount by UiO-67 doping to the same. Under identical circumstances, a comparison of the photocatalytic activity of NH2 -MIL-125(Ti) and the parent MIL-125(Ti) confirms that the current visiblelight-driven photocatalysis is linked to amino derivatization. A porphyrin-based MOF, PCN-222 catalyst shows great activity in visible-light-aided CO2 reduction (Fig. 17.2). The MOF-based photocatalysts have been reported to show steady CO2 reduction even at lower quantities of CO2 (ex. 5%). This is an appealing attribute for a practically applicable and scalable photocatalyst [12, 25–27].

Fig. 17.2 a 3D depiction of PCN-22 b PCN-22 gas uptake at a specified temperature (isotherm) c 2D representation on PCN-22 with N-bonding and chelating ring formation depiction d quantity of COOH– versus time plot. (a, b, d) are readapted from [12] (order number: 5277660599753)

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In Solar Cells

Another exciting use of MOFs is solar cells, also called photovoltaic cells, which are electronic devices for the direct conversion of sunlight to electrical power. The performance of a solar cell is determined by the ordered array of photoactive molecules. On fluorine-doped TiO2 , a porphyrin-based MOF thin film of [Zn-SURMOF2] has been produced that is extremely porous, crystalline, and monolithic fluorine-based tin oxide (FTO). For the solar cell design, (I– ) and (I3 − ) electrolytes in acetonitrile were used to coat the MOF film. The efficiency of the completed solar cell is 0.2%. The photophysical performance of the enhanced solar cell was greatly enhanced to h = 0.45% after immobilization of Pd+2 ions in the porphyrin cores of [Zn-SURMOF2]. As a result, the [Zn-SURMOF-2] acts as an indirect bandgap semiconductor that prevents direct electron–hole interaction, resulting in improved photophysical characteristics. The weak semiconductive characteristics of MOFs have hampered their use in photovoltaic solar cells. MOFs, in particular, offer an intriguing potential in solar cells. For thin film deposition, MOF-525 nanocrystals with a diameter of 140 nm were introduced to a CH3 NH3 PbI3 -xClx perovskite prelude mixture. The photovoltaic performance of the [MOF@perovskite] solar cell is higher than that of the pure perovskite slender film cell, which can be ascribed to the improved morphology or crystallinity of the perovskite phase generated by the MOF addition [12, 28, 29]. Other than this, MOFs have a great deal of impact in the modern energy resource management research in the arcs of fuel cells—PEMFCs, protonic conduction [at reduced temperatures (< 100 °C) and extensive temperature (> 100 °C) setups], O2 reduction, supercapacitors, Li-based batteries, and water splitting which can be detailly accessed through [12].

17.3 Energy Transmission 17.3.1 Nanodevices Because of their superior electrical conductivity, mechanical flexibility, and low cost, most conducting polymers are well suited for the creation of electronic devices. Integrating metals, semiconductors, carbon nanomaterials, and thermal insulation polymers into conducting polymers to generate nanocomposites may modify conductance, which might be useful in light emission diodes, transistors, and photovoltaic devices [4]. The number of unfavorable flaws, which act as trapping and recombination sites, can be reduced or eliminated by improving the crystalline quality of polymer particles. The carrier migration is further aided by the good stacking of polymer chains with high electrical conductivity. Moreover, the smaller particle size

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Fig. 17.3 a Potential versus time: galvanostatic charge–discharge plots for PPy-TB4 and b Potential versus time galvanostatic charge/discharge profiles at [1A/g:PPy] for sole PPy, PANI-TB2, and PPy-TB4 reprinted with permission from [30] Copyright {2022} American Chemical Society

breaks down barriers between the reaction sites on the surface, lowering the possibility of fusion. In charge transfer with water, the smaller polymer nanoparticles provide greater surface surfaces for more active reaction sites [5] Table 17.2.

17.3.2 Conductive Polymers Using extremely crystalline and conjugated trypan blue (TB) molecules as both dopant and gelator, a molecular self-assembly technique for the creation of nanostructured conductive polymer networks by in situ polymerization was developed. Ionically bonding the protonated TB aligns the free sulfonic acid functional groups in a specific region. The steric and ionic associations between the TB and PPy backbones drove the self-assembly mechanism, which resulted in interlinked TB-doped PPy nanofibers. The TB concentration was crucial in controlling the shape of the PPy hydrogels. These conductive polymer gels showed conductivity (3.3 S/cm) and capacitance (647 F/g at 1 A/g). It might be due to the linked conductive matrix, which allows for uniform charge carrier distribution over the electrode [30] (Fig. 17.3).

17.4 Energy Storage 17.4.1 Electrochemical Energy Systems Electrochemical energy mechanisms, particularly microbial fuel cells and rechargeable lithium batteries, are gaining traction and attracting a lot of attention, not only because they can create power directly from renewable energy sources or provide

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electrical backups, and because they can contribute to sustainable development by removing pollutants while producing energy. The exploration and development of innovative design techniques to acquire high-performance and long-term stability materials are critical to consider improving the performance of these devices and meeting the ever-growing demands for varied applications [5]. Due to the extensive p-conjugation and remarkable electronical conductance of conjugated nanostructures, nanostructured polymer nanocomposites offer significant promise as a major resource for the future generation of electrochemical energy harnessing systems [31].

17.4.1.1

Li-ion Batteries

The capacity and endurance of traditional rechargeable nickel–cadmium or nickelmetal hydride batteries are restricted. Lithium-ion batteries, on the other hand, are the most exciting and practical rechargeable batteries since they are lighter and have a much bigger capacity, meeting the needs of portable gadgets, electric cars, and largescale grid infrastructure. Because of their substantial power density and strong cycle performance, the fabricated nanomaterials are suitable choices for Li-ion battery electrodes. One of the most efficient ways to enhance the potential retention of cathode materials is to use composites fabricated conductive polymers in lithium– sulfur batteries, which combines the conductive framework of conjugated polymers with the large capacity of elemental sulfur [32–34] (Fig. 17.4). For this application, the synergistic and intrinsic characteristics of the conductive polymer nanocomposite are critical. Because silicon has the largest capacity

Fig. 17.4 Illustration of discharge and charge in in situ vulcanized Li–S readapted with permission from [35]

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of all the anode materials studied, silicon conducting composite materials should blend silicon’s high capacity with conductive polymer’s superior electronic conduction. In situ polymerization forms a 3D porous nanostructured hydrogel that bonds to the silicon substrate and acts as a persistent conductivity channel for electrical conduction, according to a novel suggested approach for creating high-performance composite electrodes. In situ polymerization has been used to create an upgraded nanostructured (Silicon nanoparticles along with PANI composite) electrode. The composite Si nanoparticles/PANI showed good electronic transmittance. Even after a thousand cycles, the discharge and charge data demonstrate a very substantial bidirectional capacity of about 1600 mAhg−1 , which is significantly superior to any other metal oxide anode material examined. Even over anode graphite, it has noteworthy capacity at high current densities [5].

17.4.1.2

Fuel Cells

Electrochemical processes turn chemical energy directly into electricity in fuel cells. Fuel cells have gained popularity in recent decades because of their potential use in electric cars. Direct methanol fuel cells (DMFCs) have been a study focused on the field of energy applications due to their high energy conversion efficiency, fuel mobility, and environmental stewardship. Electrocatalyst effects on DMFC performance have been thoroughly researched, and fabricated conducting polymers have emerged as promising electrocatalyst supports [36, 37]. The extraction of energy from organic wastes is the strategy’s game-changer. Biodegradable compounds are decomposed into tiny molecules by electrochemically active bacteria, which then pass the electrons created throughout the metabolic activities to the anode. The electrons then proceed to the cathode, where they decrease the electron acceptors, resulting in the creation of electrical power and the elimination of organic waste at the same time. Because microorganisms may self-replicate and the catalyst supports for organic-matter oxidation are self-sustaining, methanol fuel cells are a green treatment technique [5, 38, 39]. When contrasted to the efficiency of batteries, conjugated polymers such as PANI and PPy with strong electrical conductivity can enhance the power and current efficiency of MFCs to some extent, but the performance is still far from being employed in actual applications.

17.4.2 Supercapacitors Supercapacitors are among the most intriguing energy storage technologies, with possibilities in electric cars, instantaneous power sources, and other areas. Supercapacitors have a greater specific time in comparison to lithium-ion batteries. Carbon, metal oxides, and conducting polymers are the three most common types of electrode materials used in supercapacitors. Although conducting polymers have an enhanced

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specific capacitance, they have low cycle stability. Fabrication of conducting polymer nanocomposites has solved this disadvantage [40, 41]. Energy is often stored in dielectric materials between two electrically conducting electrodes in traditional capacitors. When a voltage difference between both the electrodes is induced, energy is stored on the opposing plates, which can collect an equal quantity of positive and negative charges. Capacitance is a property of a material or a device agglomerated with such units that specifically relate to power storage [42–44] (Fig. 17.5). Electrochemical redox substances could be mixed with conducting activated carbon to increase the capacitance of supercapacitors’ electrode materials. When compared to purely carbon-based electrochemical double-layer capacitors, the energy storage strategy at the electrode–electrolyte interface is not by a physical way; instead, a few quick reversible redox processes take occur to achieve larger capacitance. Pseudo-capacitors are a form of electrochemical capacitors. Ruthenium oxide is among the most often utilized pseudo-capacitor electrode materials. Pseudo-capacitors energy storage in three different ways: (i) surface adsorption of electrolyte ions, (ii) redox reactions involving ions as from electrolyte mixture, and (iii) recoverable doping of integral conducting polymeric composites. Asymmetrical supercapacitors are another form of supercapacitor that stores energy using different electrodes. The electrodes are constituted of a power-driven battery kind of faradic electrode and even a capacitive carbon-based electrode like that of graphene and more. This unique method allows for the adjustment of the operating voltage window while also increasing the energy density due to electrochemical redox behaviors at the faradic electrode [42, 45–47].

Fig. 17.5 A pictorial representation of incorporated photocatalytic conducting polymer into a supercapacitor setup aided by faradic and carbon-based electrodes

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17.4.3 Thermoelectric Generators They are systems that use the Seebeck effect to transform heat flow into electrical energy and are regarded as a sustainable alternative for generating power from heat losses and natural heat sources. Organic thermoelectrics has their very own uses in low-end waste-heat extraction and on-demand cooling, despite their inorganic cousins’ superior thermoelectric efficiency at medium and high temperatures. Polymer nanostructures have emerged as an important research topic for photocatalytic implementations due to their non-toxic property, ease of handling, wide availability, and ability to have controllable bandgaps through chemical functionalization, making them a viable alternative to metals, metal oxides, or metal chalcogenides. Polymer nanostructures can replace inorganic materials in solar energy conversion beyond solar cells due to their light-gathering and hydrogen evolution capabilities. However, research on chemically stable and thermally noncorrosive polymer photocatalysts for visible-light hydrogen evolution is underway [5, 48–51].

17.4.4 Case Study on Polydopamine Fabricated Photocatalytic Nanocomposite Polydopamine (PDA) is a widely used mussel sealant with a wide range of applications in biomedical and drug transfer disciplines, as well as catalysis and photocatalysis. Because of its huge number of functional units and ease of polymerization on practically any surface, PDA is a popular choice for catalysis and conductance. It contains a lot of dopamine and lysine ends [8, 52, 53]. It is frequently used in the production of Li-ion cells, antimicrobial substance fabrication, biosensors design, molecular imbuing, tissue engineering, and bioimaging, but it is less well-known in band structure construction, and electron transmission operations are also vast. There exists a discrepancy in the structure of PDA based on studies concentrated on H-bonding, p–p stacking, p–cation attraction and charge transmission, and the aryl-aryl linkages leading to the covalent coupling of the oxidized and cyclized dopamine monomeric units. Due to recent research, the present accepted structure has oligomers, with the occurrence of indole units and open-chain dopamine units. The whole hefty research can be analyzed through [54]. It can be synthesized by the techniques like oxidative direct polymerization of dopamine hydrochloride induced by pH > 7 or by spin solvent casting procedures [54, 55]. PDA acts as a remarkable catalyst for energy transfer reactions. The efficiency issues of carrier charge separation rate and stability in PDA can be addressed by surface modification. This will result in enhanced photocatalytic activity via TiO2 , ZnO, ZnS, Fe3 O4 . This will yield higher electrical energy transfer rate and enhanced electron transmission conductance [56]. And with metal oxide-based fabrication, nitrogenic ends thus obtained can help to achieve vast electroconductivity

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Fig. 17.6 Depiction of the electron transfer property enhancement via ZnS/PDA hybrid with various PDA layer thicknesses, namely (PDA1, PDA2, PDA3), and comparison over sole properties of ZnS and ZnO were a energy plot showing diminished bandgap b plot of potential versus NHE showing accumulation of photo-generated carriers c thickness versus H2 evolved plot showing durability d hydrogen evolution statistics with time and e depicts the current density of photocurrent with and without illumination obtained and f impedance spectra. Readapted from [58] (order number 5277670091249)

and oxygen reduction reaction activity with stability and cost-efficiency for cell manufacturing [57]. Hydrogen manufacturing via PDA and zinc sulfide is a key for energy production and is commendable due to its high stability after irradiation for a day and still being photoactive by over 78% is desirable [8] (Fig. 17.6; Table 17.2).

17.4.5 Case Study of Graphene Graphene is a two-dimensional (2D) carbon nanomaterial with a large specific high surface area of about 2600 m2 g−1 (as well as excellent electrical and thermal characteristics). Graphene has long been thought of as a kind of adaptable and oneof-a-kind building block for useful materials. Cross-linking graphene or graphene oxide or its reduced form with various inorganic or organic species, such as inorganic nanoparticles, polymeric materials, multifaceted organic substances, and metal ions/complexes, has been used to create graphene-based hybrid materials that help in energy transmission and retention [90, 91]. Graphene is often fabricated with PANI, polypyrrole, and poly(ethylenedioxythiophene) PEDOT kind of moieties so that their specific capacitance is the sole driving factor for the energy storage, would be enhanced up to 300–1500 F/g from just 100–264 F/g [92]. Inorganically, metals oxides are used for the fabrication like that of MnO2 , RuO2 , Ni(OH)2, and more which can be studied in Table 17.3 [90].

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Table 17.2 Conductive photocatalytic polymers with their fabrication moieties and the respective energy sector application where it is employed Conductive photocatalytic polymer

Fabricating moieties

Energy sector application

Reference

PANI

Metal based

Electrocatalysis, asymmetric supercapacitor device, dielectrics

[59–62]

Electrochromics, visible-light photocatalytics, pseudo-capacitors, and microbial fuel cells

[62–65]

Au, Fe

Inorganic TiO2 , ZnO, RuO2 , MnFe2 O4

Carbon based/organic

PPy

CNT, 3D-rGO, graphene, CNT-COOH

Flexible supercapacitors [66–71] and their electrodes with high performance

Metal based

–

–

Li battery (anodes) and rechargeables, asymmetric supercapacitors

[72–75]

Flexible solid-state supercapacitors

[76–79]

rGO, graphene Metal based

Fuel cells

[80]

Photovoltaic cells and Li-ion batteries

[81, 82]

Conductive transparent electrodes, electrochemical capacitors, fiber supercapacitors

[83–85]

–

–

– Inorganic CuO, CoO, Fe2 O3 , ZnCo2 O4 , LiV3 O8 Carbon based/organic PEDOT: PSS

Au Inorganic V2 O5, Mn2 O3 Carbon based/organic MWCNT, rGO, SWCNT

PEDOT

Metal based – Inorganic V2 O5 , Fe2 O3 , NiO/Ni(OH)2

Perovskite and dye solar [86–88] cells, asymmetric supercapacitors

Carbon based/organic

Supercapacitor

Carbon

[89]

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Graphene quantum dots, another essential graphene structure, can be used in biosensing devices, fuel cells, electrochemical immunosensors for biomedical applications, supercapacitors, and solar cells with efficiency [93].

17.5 Conclusion and Future Prospects Due to the complicated microstructures of conducting polymers, there are still issues with realizing their full potential in nanodevices, such as repeatability and maneuverability of monomer unit’s nanotubes/wires, doping level consistency, and improving the processability of conducting polymer nanomaterials. Supercapacitors based on Table 17.3 Applications of graphene and its modifies form in the energy sector Graphene application

Modifying moiety

Achieved output

Reference

Supercapacitor electrode materials

PANI

Stability and specific capacitance (SC) of 210 F/g

[94]

Graphene oxide + PANI

SC = 555 F/g, discharge current density = 0.2 A/g

[95]

MnO2 , RuO2

Vast cyclability; SC = 300 F/g

[96, 97]

Ni(OH)2

SC = 1335 F/g

[98]

Carbon nanotubes

Capacity = 730 mAh/g

[99]

C60 fullerenes

Capacity = 784 mAh/g

SnO2

Capacity = 810 mAh/g

[100]

PEDOT + MnO2

Capacity = 1835 mAh/g

[101]

TiO2 (In anode)

Excellent charge transport rate and efficiency of photochemical conversion

[102]

Vertically aligned carbon nanotubes

Conversion η = 83%

[103]

POM

Increased H2 uptake by 0.5%

[104]

Porous graphene oxide frameworks

H2 uptake 1.2 wt%

[105]

Platinum nanoparticles

Higher catalytic activity, stability and CO tolerance max power density = 260 mW/m2

[106]

Li-ion batteries’ electrode material

Solar cells

H2 storage

Fuel cells

Graphene oxide + Nafion Peak power density = 0.042 W/cm2

[107]

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conducting polymers have to garner attention in the field of energy applications due to their high specific capacitance. Their stability, on the other hand, is not particularly excellent. The addition of additional nanocomponents, such as a scaffolding of conducting polymers, is predicted to improve the stability of the supercapacitor device. Furthermore, making use of nanostructures to optimize the efficiency of energyrelated systems is crucial. The fast development of nanotechnology offers potential solutions to present energy-related systems’ fundamental problems, such as nonappreciable efficiency, expensive demand, and big size. The study of hot electrons or piezoelectrically produced carriers on PDA-based composites might lead to new piezoelectric catalysis and PDA-related energy functions similarly in various other prevalent conjugated conducting nanopolymers like PPy, polyurethane, poly-formaldehyde resin, and more. The development and research in MOFs, as well as the logical combination and manufacturing of MOF precursors, as well as the appropriate calcination method, all work together to exploit the potential in the performance of MOF derivatives. Although pivotal development has been achieved in nanopolymers-based devices, the electrochemical performance, which includes energy density, specific capacitance, specific capacity, cycle life, and stability, is still insufficient to make them commercially viable. Although there were numerous technological hurdles in miniaturizing methanol fuel cells, it is still one of the most efficient approaches to creating reduced cost and limited mass methanol fuel cells. Another option is to create nanostructured composite materials with higher power densities than sheer nanostructured conjugated polymers. Furthermore, good performance might be attained by modifying the composition (e.g., pore size, macroporosity) and surface (e.g., roughness, biocompatibility) of bioelectrodes. Through this detailed chapter, we analyze the research statistics that a lot of output has been achieved on utilizing the fabricated conducting polymer in energy applications; furthermore, advancement is needed in the scale as well as the dimension for it to replace the conventional and prevalent inorganic moieties.

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Chapter 18

Recent Developments in MOFs Materials for the Photocatalytic H2 Production by Water Splitting N. Subha, Malathi Arumugam, and M. Mahalakshmi

Abstract Metal–organic frameworks (MOFs) are three-dimensional crystalline materials made up of metallic nodes and organic polytopic linkers. The choice of the possibility of functionalization widens the application of MOF in various fields. The photosensitizing capability under solar light and catalytic activity with high surface area of MOF offers it as a better material for the photocatalytic H2 production. This chapter summarizes the mechanism, various preparation methods, and the recent developments in MOFs photocatalysts for the application of photocatalytic H2 production. In addition, the challenges and the different strategies adapted to improve the solar light absorption and to reduce the excitons recombination in achieving the efficient MOF materials for the photocatalytic H2 production have been discussed. Keywords Metal–organic frameworks · H2 production · Aqueous medium · Solar energy

18.1 Introduction The intensification of fossil fuel consumption owing to the expansion of people’s population increased the release of harmful pollutants into the earth’s atmosphere. To overcome this issue, the fossil fuel must be replaced by an alternate clean and sustainable energy fuel. Hydrogen is the best alternate fuel, since it burns with high calorific value, without the emission of hazardous substances [1, 2]. Hydrogen can be produced by various methods like coal gasification, steam reforming, electro/photocatalytic water splitting, and cryogenic distillation [3]. The photocatalytic water splitting considered as the best route to produce hydrogen among them, since water is as raw material and the sun as a driving force [4]. Fujishima and Honda have first discovered N. Subha · M. Mahalakshmi (B) Department of Chemistry, Sri Sivasubramaniya Nadar College of Engineering, Kalavakkam 603110, India e-mail: [email protected] M. Arumugam Center of Excellence On Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Garg and A. Chandra (eds.), Photocatalysis for Environmental Remediation and Energy Production, Green Chemistry and Sustainable Technology, https://doi.org/10.1007/978-3-031-27707-8_18

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the water splitting into hydrogen and oxygen by electrochemical photolysis of water in the year 1972 using TiO2 semiconductor as a photoanode [5]. The photocatalytic water splitting process involves three steps (see Fig. 18.1): (i) Absorption of light by photocatalyst exceeds its band gap energy and excite photogenerated electrons (e– ) to the conduction band (CB) and leaves hole (h+ ) in the valance band (VB), (ii) the photogenerated e– and h+ pairs diffuse to the photocatalysts surface or recombine, (iii) on the photocatalyst surface, h+ oxidizes water to O2 and proton (H+ ) and the excited e– reduces H+ to H2 . + Semiconductor + hν → e− CB + hVB

(18.1)

H2 O + 2h+ → 1/2O2 + 2H+

(18.2)

2H+ + 2e− → H2

(18.3)

The photocatalytic water splitting is thermodynamically achievable when the CB of photocatalyst is more negative than redox potential of H+ /H2 (0 eV vs. normal hydrogen electrode (NHE)) and VB is more positive than redox potential of H+ /H2 (+ 1.23 eV vs. NHE) [6, 7]. To improve hydrogen production, three important factors need to be considered and those are efficiency of light absorption by photocatalyst, band structure, and efficient utilization of e– and h+ for redox reactions. The various inorganic semiconductor such as TiO2 , Ta2 O5 , Cu2 O, ZnO, and some metal sulfides, metal selenides, and oxynitrides have been verified for the photocatalytic hydrogen [8]. Nevertheless, the rate of hydrogen production of the inorganic semiconductors Fig. 18.1 Photocatalytic water splitting mechanism

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was low. In order to solve this problem, researcher around the world has tried to made heterojunction semiconductor photocatalysts, impregnation of carbon-based materials, co-catalysts with semiconductor photocatalysts. Although, the efficiency was still low for the practical purpose. In the recent years, metal–organic frameworks (MOFs) have attracted many scientists due to its versatile properties which has been discussed in this chapter.

18.2 Metal–Organic Frameworks MOFs are solid crystalline porous material consists of metal ions and organic linkers. They are versatile in diverse field applications due to high porosity, structural tenability, and high surface area. The recent studies show that MOFs are excellent precursors for the synthesis of heterojunction-based photocatalysts for hydrogen production. The functionality of the frame works can be modified by changing the metals and organic linkers in the MOFs. The solar spectrum constitutes ~5% UV, ~45% visible light, and ~50% near IR light. The solar light responsive photocatalysts are cost-efficient, since utilize only solar radiation from UV to NIR. The organic linkers in MOFs are working as an antenna to harvest light from the source. Under light irradiation, the organic linkers absorb light energy and excite the e– from highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) and then the redox reaction occurs on the MOFs surface. It is also like a “skeleton” to link catalytic metal active nodes to form highly porous structured MOFs. The semiconducting and charge transfer behavior of MOFs were confirmed by Feng et al. [9]. In their study, the MOFs taken were Zirconium-Metalloporphyrin PCN-222, where they observed charge transfer from porphyrin ligand to Zr− oxo clusters, i.e., ligand to cluster charge transfer (LCCT) occurs under visible light irradiation.

18.3 Metal–Organic Frameworks for Photocatalytic Hydrogen Production Owing to its light absorption property, MOFs are used for photocatalytic hydrogen production. The photoactive MOFs absorb light energy by the help of organic linkers and the photogenerated e– and h+ can be utilized for the further chemical reactions. [Ru2 (p-BDC)2 ]n (p-BDC = 1,4-benzenedi-carboxylate) is the first MOF material which was used as photocatalyst for hydrogen production [10]. Around the world, many scientists have been working on various methodologies to improve hydrogen production using MOFs. Lanthanide metal ions have high coordination number, and it can form many structures with MOF materials. It is mainly due to the 4f inner electronic structure

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of the lanthanide metal ions. Especially, Ce-based MOFs have owned much attention due to its Lewis acid sites, switching tendency between the Ce4+ /Ce3+ redox couple, and it can facilitate ligand-to-metal charge transfer (LMCT) from the lowlying empty 4f orbitals of Ce4+ [11–13]. Wang et al. synthesized Ce-based organic framework materials [UIO-66(Ce)] and UIO-66(Ce)/ZnCdS composite synthesized by microwave irradiation and their rate of hydrogen production were evaluated [14]. The rate of hydrogen production of ZnCdS nanoparticle decorated UIO-66(Ce) was ~2 times higher than that of ZnCdS. Since ZnCdS nanoparticle decorated UIO-66(Ce) showed stronger light absorption and reduced charge recombination. The mechanism of charge transfer process for ZnCdS nanoparticle decorated UIO-66(Ce) is given in Fig. 18.2. The CB potential of ZnCdS is lower than that of UIO-66(Ce); hence, photo-excited e– transfer takes place from CB of ZnCdS to the unoccupied molecular orbital of UIO-66(Ce), and the h+ in the VB is consumed by sacrificial agents and thereby limiting the recombination of photo-excited electron hole. The sacrificial agent used in this work was Na2 S.9H2 O and Na2 SO3 . Ti-based MOFs have created much attention among the scientists due to its advantages such as Ti4+ with smaller ionic radius forms stronger Ti–O bonding, different structure of metal-oxo clusters can be formed and the reversible redox transition between Ti4+ /Ti3+ helps to store charges effectively in Ti-based MOFs [15]. Wang et al. [16] synthesized isostructural MIL-125-Ti with different terephthalic: amino-terephthalic acid ratios and their photocatalytic performance were evaluated in triethanolamine and methanol. MIL-125-Ti in methanol showed ~45 times better photocatalytic activity than NH2 -MIL-125-Ti under broad UV irradiation. They discussed two pathways such as LMCT and O-Ti for the charge generation and separation process in NH2 -MIL-125-Ti and MIL-125-Ti as shown in Fig. 18.3. Mott–Schottky analysis confirmed that the LUCO is dominated by Ti(3d), upon excitation the e– will be localized in LUCO level Ti(3d) and reduce Ti4+ to Ti3+ species.

Fig. 18.2 Photocatalytic pathway mechanism over UIO-66(Ce)/ZnCdS composites [14]. Reprinted with permission from Ref. [14]. Copyright (2021) Elsevier

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Fig. 18.3 a and b Illustration of proposed LMCT mechanism and direct O–Ti transition [16]. Reprinted with permission from Ref. [16]. Copyright (2020) Elsevier

The addition of Pt co-catalyst in MIL-125-Ti helped to free up the photo-excited e– to reduce H+ to H2 and for regeneration of Ti4+ . Hence, the hydrogen production was increased on MIL-125-Ti than NH2 -MIL-125-Ti. Lv et al. [17] through simple high temperature (450 °C) annealing of Ti-based MOFs (MIL-125), they have obtained mesopores TiO2 with large surface area and oxygen vacancies. Mesoporous TiO2 with high surface area (176.9 m2 g−1 ) and oxygen vacancies improved the light absorption and photocatalytic hydrogen production of about 3.59 mmol g−1 h−1 . Copper is a non-noble and earth-abundant metal. The divalent copper ions form tetra or hexa coordinated structure. The various oxidation states of copper were involved in variety of redox reactions. Zang et al. designed first Cu-MOF for visible light-driven hydrogen production. Cu-MOF can effectively photocatalyzed water with any co-catalyst [18]. Li et al. [19] synthesized CuII -MOF nanoribbons decorated with Pt nanoparticles (Pt/CuII -MOF) for photocatalytic hydrogen production of about 2.51 mmol g−1 h−1 which is ~5 and ~2 times higher than those of the referential Pt nanoparticles and CuII -MOF. The nanoribbons are constructed from CuII -MOF. The photocatalytic hydrogen production of crystalline Pt/CuII -MOF nanoribbons was investigated in the presence of fluorescein sodium (FI) as photosensitizer and triethylamine (TEA) as a sacrificial e– donor. Based on the results obtained, they have proposed a mechanism for enhanced hydrogen production. Cao et al. [20] observed improved photocatalytic hydrogen production of about 547.5 µmol after being loaded MnCdS with Cu-MOFs (MCF). Due to combined effect of MnCdS (MCS) and Cu-MOFs, MCF could be able to achieve effective charge separation, high surface area, and visible activity. The proposed S-scheme heterojunction charge transfers pathway for improved photocatalytic H2 production is given in Fig. 18.4. The high charge transfer efficiency was observed only in coupled MnCdS and Cu-MOFs. The MCS appears positive charged by losing electrons, while Cu-MOFs are negative charged by gaining electrons at the interface. Thus, the internal electric field is formed at the interface. Transfer and accumulation of electrons cause the band edge bends. Under light irradiation, the e– from the CB of Cu-MOFs will recombine with the h+ from the VB of the MCS by the Coulomb interaction. The e– from the CB of MCS will be utilized for H2 evolution. The h+ from the VB of the Cu-MOFs is scavenged by Na2 S/Na2 SO3 (h+ scavenger). In the recent years, transition metal phosphides (TMP) have attracted many scientists in the field of photocatalytic hydrogen production. Liu et al. [21] first proposed

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Fig. 18.4 Photocatalytic H2 production mechanism over MCF [20]. (Reprinted with permission from Ref. [20]. Copyright (2020) Elsevier)

that Ni–P co-catalyst for hydrogen production. The negatively charged non-metallic atoms and isolated metal atom in Ni–P, respectively, act as proton binding and hydride binding sites, since Ni–P has poor conductivity. Ran et al. [22] synthesized carbon-supported Ni-based metal–organic frameworks (Ni–P@C) and they were coupled with S-vacancy ZnIn2 S4 (Vs-ZIS) for improving photocatalytic H2 production. The Ni–P@C/Vs-ZIS-2 photocatalyst showed H2 production of about 11,064 µmol g−1 h−1 with quantum efficiency 12.4%. Teng Li et al. [23] synthesized Ni-MOF-74/Ni2 P/MoSx solution-based mixing method and obtained photocatalysts with high dispersity. This ternary heterojunction reduced charge recombination and improved visible light absorption. The amount of hydrogen produced by Ni-MOF74/Ni2 P/MoSx is 286.16 µmol g−1 after 5 h of reaction. Ni2 P in the composite enhanced the visible light absorption and MOF acted as a e– trap. Yang et al. [24] prepared Co clusters-based MOF (Co3 -XL) by the ligand N,N ' -bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxdiimide bi(1,2,4-triazole), containing abundant carbonyl oxygen atoms. Co3 -XL has a narrow band gap of 1.82 eV and has high photocurrent response. The efficient light absorption and charge separation efficiency of Co3 -XL showed the hydrogen production of about 23.05 µmolg−1 h−1 in the presence of Pt co-catalyst and RhB as photosensitizer. Based on the results obtained, they have proposed photocatalytic water splitting for Co3 XL and is shown in Fig. 18.5. Under irradiation, the photoelectrons in RhB further transferred from the ligand XL to the cobalt oxygen clusters. The photoelectrons on the surface of Co3 -XL will be captured by Pt co-catalyst and the reduction of H+ to H2 occurs. The change in morphology may also influence the rate of photocatalytic H2 production which was witnessed by Yu et al. [25] and they utilized aluminum-based porphyrinic metal–organic frameworks (Al-TCPP) as a photocatalyst. The exfoliated MOFs of Al-TCPP (TCPP = tetrakis(4-carboxylphenyl) porphyrin) showed 21-fold higher rate of H2 production (1.32 × 104 µmol h−1 g−1 ) than bulk catalyst due to efficient charge separation, extended surface sites, and morphology. Nanosheets were obtained after the exfoliation process. The morphology has also influenced the rate of recombination, and this was confirmed by the photoluminescence spectra (PL) of

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Fig. 18.5 Photocatalytic H2 production mechanism of Co3 -XL for H2 evolution [24]. Reprinted with permission from Ref. [24]. Copyright (2021) American Chemical Society

exfoliated MOFs of Al-TCPP (Al-TCPP-MOL) in Fig. 18.6 which showed less intensity peak than other photocatalyst, i.e., Al-TCPP-Bulk and Al-TCPP-carambola-like structured photocatalysts. Li et al. [26] showed that with the dye sensitization, the photocatalytic H2 production over Fe-based MOFs (MIL 53(Fe)) is possible even if Fe-based metal–organic frameworks have inappropriate CB position and large overpotential. Pt nanoparticles on Fe-MOFs have further enhanced the H2 production by lowering the overpotential for H2 evolution. Eosin Y (EY) dye was used for the dye sensitization of MIL 53(Fe). The flat band potential of bare (MIL 53(Fe)) and dye sensitized (EY-MIL 53(Fe)) were measured by Mott–Schottky plot, as shown in Fig. 18.7. The measured flat band potential of EY-MIL 53(Fe) (− 0.39 v vs. RHE) negatively shifted CB position of Fig. 18.6 PL emission spectra of Al-TCPP photocatalysts [25]. Reprinted with permission from Ref. [25]. Copyright (2021) Elsevier

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Fig. 18.7 Mott–Schottky plots of EY-sensitized MIL-53(Fe) before and after EY sensitization under light irradiation [26]. Reprinted with permission from Ref. [26]. Copyright (2021) Elsevier

MIL 53-Fe by the deposition of EY and improved H2 production of EY-MIL 53(Fe) (315 µmol g−1 h−1 ) than MIL 53(Fe) (9.95 µmol g−1 h−1 ). NH2 -MIL-53(Fe) and AgSCN composite synthesized by Li et al. [26] showed the improved H2 production (4742 µmol g−1 h−1 ) than bare material due to the separation and transmission efficiency of the photon-generated carriers by the combination of NH2 -MIL-53(Fe) and AgSCN. Kampour et al. [27] constructed a novel MOF-MOF heterojunction for photocatalytic hydrogen production. The heterojunction between MIL-167 and MIL125-NH2 improved optoelectronic properties and charge separation. The amount of hydrogen produced by MIL-167/MIL-125-NH2 is 455 µmol g−1 h−1 which is higher than bare MIL-167 (0.8 µmol g−1 h−1 ) and MIL-125-NH2 (51.2 µmol g−1 h−1 ). In recyclability study, MIL-167/MIL-125-NH2 heterojunction showed consistent rate of H2 production which confirms the stability of the material.

18.4 Conclusion In this chapter, the recent development in MOF and MOF-based photocatalysts for photocatalytic hydrogen production was discussed. The mechanistic pathways for the MOF-based photocatalysts have also been discussed. MOFs materials are made up of metal ions and organic linkers and have high porosity, structural tunability, and high specific surface area. The reports reveal that the synthesis of heterojunction photocatalysts with MOFs materials shows excellent efficiency in hydrogen production under visible light. In addition, the synthesis of transition metal ions-based MOF materials shows a higher hydrogen production rate under visible light. The cheap and harmless nature of MOFs materials and their composites are the best candidates for hydrogen production. However, laboratory research of the MOFs materials further improves in large-scale commercial energy production applications using sunlight.

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Chapter 19

Interface Engineering of Nano-Photocatalysts for Hydrogen Evolution Reaction and Degradation of Organic Pollutants Kommula Bramhaiah and Santanu Bhattacharyya Abstract The global energy crisis and the gradual depletion of freshwater resources are rising problems across the world. To address these issues, various research groups are working effectively to develop alternative approaches to the replacement of fossil fuels and to attain pure and clean water. Both of these problems can be addressed simultaneously by developing efficient photocatalysts. In photocatalysis, the proper use of sunlight in terms of the solar-driven photocatalytic process is found to be an effective and green approach for both water purification and hydrogen fuel generation in terms of energetically uphill water-splitting reactions. In this chapter, we report on recent advancements in transition-metal-based semiconductor photocatalysts and their hybrids with carbon materials for photocatalytic water remediation and solar water-splitting reaction. In this chapter, we critically discuss the modification of the surface morphology and the design of the interface between the various components of the photocatalysts. Interface engineering can improve the physicochemical properties and charge-carrier dynamics, which eventually improve the required overall photocatalysis process for both wastewater treatment and solar fuel generation. Specifically, it highlights the metal-free carbon-based nanomaterials such as carbon dots/graphene quantum dots, graphene oxides, carbon nitride, etc. As noteworthy alternatives, these carbon materials can be used as a solo-photocatalyst for hydrogen generation through solar water splitting reaction and wastewater remediation by generating various reactive species. Keywords Semiconductors · Photocatalysis · H2 generation · Remediation of water pollutants · Carbon nanomaterials

K. Bramhaiah (B) · S. Bhattacharyya Department of Chemical Sciences, Indian Institute of Science Education and Research, Government ITI Building (Transit Campus), Engineering School Road, Berhampur, Odisha 760010, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Garg and A. Chandra (eds.), Photocatalysis for Environmental Remediation and Energy Production, Green Chemistry and Sustainable Technology, https://doi.org/10.1007/978-3-031-27707-8_19

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19.1 Introduction Architecting innovative photocatalytic nanomaterials for enhanced solar-based energy harvesting and wastewater treatment is an important approach to alleviating environmental issues and energy crises due to the enormous growth of population and urbanization. Lately, remarkable efforts are made by several researchers in the invention/architecture of numerous categories of photocatalysts for a wider range of photocatalytic applications [1–5]. Although a huge number of photocatalytic nanomaterials have been developed with various defined structures and morphologies; their overall photocatalytic activities are quite limited, because the kinetics of the photo-induced charges are not considerably improved. Therefore, one should look for innovative new photocatalytic materials. Generally, the photocatalytic process primarily includes the following three key steps: (i) formation of photo-induced charge carriers under photoexcitation, (ii) separation and transportation of charge carriers to the photocatalyst surface, and (iii) utilization of these free carriers for redox reactions on the surface [6–8]. Therefore, to achieve enhanced photocatalytic performance, each step (the fundamental steps discussed above) has to be precisely controlled. However, research focusing on the optimization of surface and interface features could also be significant pathways. Generally, surface parameters include surface composition, along with phase, facets, areas, pores, vacancies, surface states, and band bending, which can critically control the overall photocatalytic performance [1, 9, 10]. Additionally, selectivity can also be obtained by achieving proper surface adsorption and activation abilities. However, the interface between two counterparts of the hybrids is essential, as charge separation occurs mainly at the interface [1, 11]. Modification of the interface can improve charge separation by minimizing competitive recombination processes. Furthermore, due to the establishment of an internal electric field at the interface, the charge carriers (electron and holes) are spatially separated from different components, which facilitates oxidative and reductive photochemical reactions, i.e., the overall photocatalytic efficiency increases [1, 12]. Therefore, to improve the charge separation and enhance the photocatalytic activity, constructing a heterojunction is a promising approach. However, because of the diversity of synthesis approaches, band energy, morphology, and structures, heterojunction photocatalysts have dissimilar transfer mechanisms for photogenerated carriers. Overall, these are typically categorized into four types, which are as follows: (i) semiconductor–semiconductor (S–S) heterojunction, (ii) semiconductor– metal (S–M) heterojunction, (iii) semiconductor–carbon (S–C), such as activated carbon, CNTs, and graphene, heterojunction, and (iv) multicomponent heterojunction [11–16]. The S–S heterojunction is further divided into the following types [17]. A p–n heterojunction is an efficient way for charge collection and separation. Generally, p- and n-type materials contact each other and form p–n-type composite, resulting in electric potential, which can direct the electrons and holes in the opposite direction [1, 10, 16, 18]. The schematic illustration of the band diagram is shown in Fig. 19.1a. The p–n heterojunction has several advantages including huge charge separation, longer lifetimes of charge carriers, faster diffusion to the active sites,

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and different redox sites. Non-p–n junction: The semiconductors with matching band potentials form an efficient heterojunction (Fig. 19.1b). Usually, the electron transfer happening under the light illumination will be as per their band alignment: if the CB level of semiconductor B is lower than the CB of A, the electrons will transfer from semiconductor A to B. While the B level is higher than the A, then the electrons will transfer from B to A [14, 16] which results in, effective charge separation and migration. Another approach is the Schottky junction: This is a form of a semiconductor and noble metal cocatalysts. Under the light illumination, the photogenerated electrons transfer from metal to semiconductor (Fig. 19.1c). For example, Li et al. developed a Au/TiO2 mesoporous composite, where the enhanced photocatalytic activity for phenol oxidation and chromium reduction might be due to the enhanced light absorption and improved quantum efficiency [19]. Another efficient heterostructure is the semiconductor-carbon heterostructure, where various carbon materials and semiconductors are used. In particular, a great deal of graphene-based semiconductor heterojunction has been reported. Graphene is an atomic thin layer with a 2D structure along with various significant properties, including superior conductivity, greater electron mobility, and extremely huge specific surface area, the production cost being very low. Therefore, it has arisen as a significant host/guest material for constructing various novel functional materials. It can induce charge separation and suppress photogenerated charge-carrier recombination; furthermore, it can provide huge surface area for various surface redox reactions (Fig. 19.1d) [6, 7]. However, the heterojunction mentioned above is limited in some way or another by a few disadvantages, such as the poor visible light absorption, photoresponse, and charge transfer and recombination. To overcome these issues, developing multicomponent heterojunctions, where two or more active materials for the entire solar light absorption along with effective electron transfer components. Schematic illustration of the multicomponent heterojunction is shown in Fig. 19.1e. In these types of system, semiconductor A and semiconductor B can be excited with full-range light illumination as a result of their different photo-absorption ranges. Therefore, multicomponent heterojunction systems could be an efficient sytems compared with the other heterojunction to attain superior photocatalytic activites [3, 9, 18, 20, 21].

19.2 Fundamental Principles/Thermodynamics of Semiconductor Photocatalysts When light irradiation is conducted with an energy equal to or greater than their band gap, the electron present in the VB of the semiconductor is promoted to the CB, parting a hole behind. Later, these photo-induced electrons and holes migrate to the surface of the semiconductor photocatalyst mainly through the diffusion process and are involved in the various redox reactions. A typical photocatalytic mechanism is illustrated in Fig. 19.2. Usually, photocatalytic processes involve the following consecutive processes; (i) light-harvesting; (ii) charge excitation/separation; (iii)

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Fig. 19.1 Schematic illustration of various heterojunctions a p–n heterojunction, b non-p–n heterojunction, c Schottky junction, d carbon-semiconductor heterojunction, and e multicomponent heterojunction

Fig. 19.2 Schematic illustration of important parameters in surface/interface engineering and mechanisms for redox processes in photocatalysts

charge migration/transport to the active site; (iv) interfacial charge transfer; (v) finally charge utilization for redox reactions [3, 6, 7, 18]. Similarly, the overall efficiency of photocatalysis broadly depends on all of these ongoing processes during photocatalysis. To achieve a better enhanced photocatalytic performance, one should optimize these processes. However, unwanted charge recombination that occurred in bulk as well as on the surface of the photocatalyst should be minimized to achieve more free carriers accessible for photocatalysis. In particular, hindering the charge recombinations in the bulk and surface of the photocatalysts has been established to be the most

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significant issue in enhancing the overall photocatalytic efficiency of semiconductor photocatalytic materials [6, 7]. For the bare semiconductor, photogenerated electrons and holes can recombine in just several nanoseconds, whereas redox reactions start happening at nanoseconds to microseconds. Therefore, to boost the photocatalytic activity, the tactic that has typically been applied is to form various heterojunctions by coupling semiconductors with secondary substances such as noble metals and other semiconducting nanostructures. As a result, free carriers can achieve enhanced lifetimes (up to microseconds) before recombination, which eventually enhances the possibility of photogenerated charge carriers participating in redox reactions. Numerous approaches have been exploited to efficiently separate the photogenerated charge carriers such as electron–hole pairs in semiconductor photocatalysts, such as doping, metal loading, and/or introduction of a heterojunction. Among these approaches, engineering heterojunction in photocatalysts has been demonstrated to be one of the most significant routes to fabricating advanced photocatalysts. Moreover, the production of photocatalysts with high-energy crystal facets along with heterojunction interfaces can also encourage charge-carrier separation and faster reaction kinetics. However, the basic principle for the overall water-splitting reaction on the semiconductor photocatalyst is as follows. Under light illumination, electrons get excited to the CB and leave the holes in the VB. These excited electrons can participate in the reduction of H+ ions to generate H2 ; on the other hand, holes cause an oxidation reaction of H2 O to form O2 [4, 6, 7, 9, 16]. Additionally, to attain superior photocatalytic performances, the photocatalyst should fulfill all the parameters, which are shown in Fig. 19.2. Furthermore, the water-splitting reaction is an uphill process [6, 7]. The sluggish reaction kinetics on the photocatalyst surface could result in the accumulation of photogenerated electron holes, which leads to the hastening of the undesired photocorrosion processes, thus significantly diminishing their photocatalytic activity. To improve the photocatalytic activity, the following modification has to be done, including loading cocatalysts, reactive facets, defects, and heteroatom doping, coupled with light harvesters, etc.

19.3 Engineering Interfacial Parameters of Semiconductor Nanostructures Interface engineering is recognized as a noteworthy strategy for the construction of hybrid nanocomposites with high-performance photocatalytic activity, which is aided by efficient interfacial electron transfer. However, before the interface is engineering, one should understand the significant role of the interface and the design of effective structures as per the charge kinetic models. In addition, one should keep in mind that the performed interfacial defects can be a recombination center for photogenerated electron holes. So, to get better photocatalytic activities, these should be

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Fig. 19.3 Schematic illustration of interfacial parameter engineering

minimized [1, 10, 13, 16, 20]. However, when we know the work function of the materials, then it is possible to make an efficient interface between the components. In general, when the two materials come into getting contact, the work function of the components which are used for accepting the electrons/providing the hole should be higher than the other component (providing electrons/accepting holes) presenting another side of the interface [1]. However, during light illumination, the direction of charge transfer can depend on the specific charge kinetics. Furthermore, to obtain better photocatalytic performance, different interfacial parameters must be considered such as interfacial composition, various defects, vacancies, electronic coupling, facets, and band bending and should be engineered [1, 8, 18, 20, 22]. The schematic representation of various interfacial engineering is shown in Fig. 19.3. In the interfacial engineering of photocatalysts, the first and foremost important parameter is the interfacial composition. In this process, a new component is introduced into the hybrid structure to form interfaces. The new component may not participate in light absorption and redox reactions, but it can help in the charge transfer. For example, for better interfacial contact between the hydrophilic CoOx (oxidation cocatalyst) and hydrophobic Ta3 N5 (semiconductor) components, a thin layer of magnesia has been coated over a Ta3 N5 to form a hydrophilic surface. This coating helps to improve interfacial contact and minimize defect density, which results in higher photocatalytic activity. Another important parameter is the interfacial area. Generally, a larger interfacial area means more channels for the charge transfer, especially in the 2D materials with the greater interfacial area (as the top and bottom surface), usually promoting better charge transfer and possessing very short distances from the interface to the surface are an advantage to get better redox reactions [23]. Defects across the interface may act as a recombination center for charge carriers, which should be minimized.

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The formation of a single-crystalline interface is an effective approach to minimize defects. In this context, Bai et al. developed a single-crystalline Pd-Cu2 O core–shell material with an ultimate interface. Generally, widely used Pd/Cu2 O structures may have a larger number of defects at the interface and minimize the charge transfer, and the holes, which are trapped on the Pd may not be available for the oxidation reaction. To overcome this problem, they made single-crystalline core–shell structures, where you can effectively utilize both holes and electrons for the redox reactions [24]. However, some positive roles for interfacial defects have also been reported. Interfacial vacancies serve as electron donors and promote charge transfer [25]. The interfacial charge separation may also depend on the facets of the components. Dong et al. made tunable TiO2 crystal facets such as (100), (101), and (001) facets on graphene using an anion-assisted approach. They observed an enhanced photocatalytic activity for the TiO2 (100) facets, which might be due to the formation of the Ti–C bond. However, in other cases, they found a Ti–O–C bond, which hinders the charge transfer process [15]. In another example, the facet dependent charage separation has been observed in BiOI/g-C3 N4 compound [10]. The bend bending can be seen in which two components with various Fermi levels are coupled. To get the equilibrium, the Fermi level band bending occurs at the interface. For example, in the g-C3 N4 /BiPO4 compound, the work function of BiPO4 is higher than that of g-C3 N4 ; therefore, the photogenerated electrons move from g-C3 N4 to BiPO4 through the interface, until equilibrium is achieved. As a consequence, the energy level of g-C3 N4 displays a negative shift, and an opposite shift for BiPO4 [26]. As discussed earlier, several vital factors determine the photocatalytic activity of a given semiconductor, which are potentially improved when carbon materials including graphene, CNTs, carbon nitrides, etc., are incorporated with the photocatalysts [6, 7]. Graphene, an atomic thin layer, 2D material, has attracted much attention from the scientific community since its discovery in 2004 [27]. Its exceptional properties include a huge specific surface area, excellent electrical conductivity, high flexibility, and significant optical, mechanical, and thermal properties. Therefore, graphene and its derivatives have been widely used as a component in photocatalysis [6, 7]. The functionalization of graphene can also be done and used as a photocatalyst for the solar water-splitting reaction. However, the production rates are very limited, which can be further improved by coupling with the other components [3].

19.4 Characterization of Interfaces in Semiconductor Photocatalysts An effective interface formed between the two components consistently governs the activity and stability of the hybrid photocatalysts. Therefore, it is essential to examine the quality of the interface in a hybrid photocatalyst. In this context, to analyze and understand the structural, elemental, and morphological characteristics at the

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interface of photocatalysts, various advanced characterization techniques have been developed, including high-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), conductive atomic force microscopy (CAFM), and kelvin probe force microscopy (KPFM) [8]. TEM is an advantageous technique that can be used to visualize the size, details of crystalline nature, orientation, and phase, along with exposed crystal facets of photocatalytic materials. Usually, the lateral size and approximate thickness of the materials can be visualized using low-resolution TEM images as it is well known that the various crystalline materials possess different crystal orientations and exposed facets along with lattice distances, which can be visualized using HRTEM images. However, for the amorphous materials, no lattice spacings can be observed due to the random arrangement of atoms. For example, the BiOI/La2 Ti2 O7 heterojunction is made using a simple wet chemical approach such as hydrothermal and solution phase heating approaches. The low-resolution TEM image of BiOI showed a smaller sheet morphology and is randomly decorated/distributed on the surface of larger La2 Ti2 O7 sheets (Fig. 19.4a). However, from the HRTEM imaging, the lattice spacing of both systems has been observed, which are around 0.276 nm (002 planes of La2 Ti2 O7 ) and 0.302 nm (012 planes of BiOI) along with the clear interface between them (Fig. 19.4b) [28]. Furthermore, the detailed surface and local atomic structures along with vacancies, defects, dopants, and structural distortions can be analyzed (higher atomic number atoms display more contrast) using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). For instance, Wu et al. fabricated defective BiOCl nanosheet structures employing ethylene glycol treatment. Furthermore, the detailed atomic features and (001) and (100) facets along with the thickness (< 20 nm) of the BiOCl nanosheet structures have been studied using HAADF-STEM images (Fig. 19.5a, e) [29]. The uneven surface at the outer edges and coarse distribution of Bi atoms on the facets (001), (Fig. 19.5a, b). Figure 19.5c, d show significant relocation and surface reorganization of Bi. However, in the case of black BiOCl facets (100), sharp surface edges are present (Fig. 19.5e, f). Furthermore, the relocation of Bi atoms and defects formation can be seen in Fig. 19.5g. In addition to the characterization of surface atomic arrangements in the materials, the heterostructure and interface are also recorded using the HAADF-STEM imaging technique. But, bulk defects cannot be incarnated by HAADF-STEM. These can be analyzed using positron annihilation techniques. This is a remarkable technique for determining defects/vacancies and their relative concentrations by measuring the lifetime of the positron. X-ray photoelectron spectroscopy (XPS) is a unique technique for the examination of the elemental composition and the surface chemical structure at the interface of composite photocatalysts. For example, the deconvoluted C1s spectra of pristine g-C3 N4 display two peaks around 284.6 eV (C–C bond (sp2 )) and 288.0 eV (N– C=N (sp2 hybridized) bonds). The deconvoluted N1s spectra display three peaks around 398.5 eV (C–N=C bonds in triazine rings), 399.6 eV (the N–(C) 3 group), and 400.7 eV (surface C–N–H amino) [30]. However, the surface/chemical species of g-C3 N4 depend on the synthesis approach, which can be easy to understand.

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Fig. 19.4 Morphology and interface study of the BiOI/La2 Ti2 O7 heterojunction using TEM imaging a TEM images and b HRTEM images [28]

Fig. 19.5 HAADF-STEM images of BiOCl a (001) facets, b the outer layer, and c the grain interior and d schematic of BiOCl crystal structure. HAADF-STEM images of e (100), f the outer layer, and g the grain interior [29]

If you coupling the g-C3 N4 with the other components, the peak positions are changed compared to the bare g-C3 N4 , suggesting the possible transport of electrons through the interface interactions. For example, Wen et al. developed black phosphorus (BP)/g-C3 N4 composite on a large scale using a ball milling approach. They observed that the N1s XPS spectra display more negative binding energy peaks

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compared with bare g-C3 N4 . Furthermore, the HR-XPS spectra of P2p displayed a new peak around 133.4 eV, which corresponds to the formation of the p–n bond at the interface [31]. Furthermore, to understand the formation of the interface and confirm the peak shifts, Shi et al. fabricated a bare g-C3 N4 , a physical mixture of g-C3 N4 /MoS2 , and MoS2 /g-C3 N4 composite. The peaks C1 and N1 of the MoS2 /gC3 N4 composite show a lower shift of binding energy around ~0.6 eV compared with the bare g-C3 N4 , while the physical mixture did not exhibit a peak shift, confirming that the electronic effects can be seen in the heterointerface but not in simple physical mixing [32]. Henceforth, certainly, XPS measurements help in understanding the various interfaces and their roles in the photocatalytic mechanisms. The electronic structures of semiconductor photocatalysts directly regulate intrinsic optical and electronic properties. In addition, these factors are associated with the band gap and their positions, as well as charge-carrier behavior. All of these processes affect the overall photoexcitation pathways and photocatalytic behaviors. Therefore, investigation of the electronic structure of the photocatalyst materials is vital to gaining more understanding of the photocatalytic mechanism and can further help to optimize the photocatalytic process. Typically, an atomic force microscope (AFM) is utilized for thickness analysis measurements of 2D materials, which can provide topographic information [33]. However, this is not useful for understanding the interface between various components. For a better understanding of the interface, a conductive atomic force microscope (CAFM) is an effective tool to study the conductivity of 2D materials. The interface between the different components can be visualized. For example, the electrical conductivity of various phases, such as the 2H and 1T phases of MoS2 flakes, has been measured using CAFM. Mostly, a T-like phase of MoS2 was observed in the basal plane, and less conductive 2H phases were observed at the edges of the MoS2 sheets [22]. Madhuri et al. fabricated ZnO and rGO/ZnO heterostructure films and studied the photocurrent distribution employing a CAFM. Wherever the ZnO aggregate is present in the nanoscale regions, they display a Schottky barrier and a very small amount of currents. However, when the rGO/ZnO films display good currents due to the intimate contact and interface between them [34]. Furthermore, an effective interface in the photocatalyst can alter the surface potentials, which can be studied by Kelvin probe force microscopy (KPFM) [8, 35].

19.5 Photocatalytic Water Splitting for Hydrogen Generation Photocatalytic hydrogen production via the water-splitting reaction is considered an ideal pathway for future energy demands because of its economic and environmental superiority. Therefore, the development and exploration of suitable photocatalysts with exceptional performance are essential. Therefore, numerous semiconductors have been investigated in the literature. In this regard, carbon-based semiconductor

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photocatalyst materials are economically feasible photocatalysts for practical applications. Usually, two types of interface effects can be seen in the carbon-based materials, which include (i) the Schottky junction; formed between the metallic carbon and semiconductors and (ii) heterojunctions; including the p–n junction and the Zscheme; formed between semiconducting carbon materials and traditional semiconductors [6, 7, 18, 23, 36]. Typically, photo-induced charge transfer will occur across the contact interface. Henceforth, making a better and large contact interface between the components could be an efficient approach to achieving better-photo-induced charge transfer and separation. Accordingly, constructing 2D–2D layered junctions between the carbon and semiconductor materials are an efficient way to obtain abundant surface active sites and huge interfacial charge transfer. In this context, Xiang et al. fabricated graphene-modified TiO2 (001 exposed facets) nanosheets using a microwave hydrothermal treatment in an ethanol–water solvent medium, which displayed excellent photocatalytic evolution activity of H2 even without Pt content. The higher efficiency is due to the 2D–2D layered junction (Fig. 19.6a) and the lower absolute potential of graphene, which facilitates the electron transfer followed by the reduction of H+ [37]. In another example, g-C3 N4 /rGO composite (2D-2D junction), during the photo illumination, the electrons can effectively be transferred to rGO layers from the g-C3 N4 because of the lower Fermi of rGO [8], leading to a reduction in electron–hole recombination (Fig. 19.6b). Furthermore, the intimate interfacial contact between these 2D layers can efficiently induce charge transfer, resulting in an improved electron density in the rGO layers. The hydrogen production rates for the 2D/2D g-C3 N4 /rGO compound and the bare g-C3 N4 are as follows 557 µmol g−1 h−1 and 158 µmol g−1 h−1 , respectively [38]. Furthermore, the CB potentials of the semiconductors can be tuned/changed by coupling with the graphene sheets. Sun et al. fabricated the highly active photocatalyst by attaching Bi2 WO6 nanocrystals to graphene nanosheets. They found that the CB potential of the Bi2 WO6 nanocrystals is shifted to a more negative potential (decreased from + 0.09 to − 0.3 V), which is due to the chemical interaction between graphene and the Bi2 WO6 nanocrystals [8, 39]. Integration of 2D carbon materials with various semiconductor nanostructures to form a Type 1 system is an alternative approach for developing efficient photocatalysts by improving light absorption and charge separation. For example, Yang et al. fabricated 2D/2D nanosheet g-C3 N4 @ZnInS4 nano leaf structures and 2D/0D g-C3 N4 nanosheet@ZnInS4 microspheres using surfactant-assisted solvothermal approach (Fig. 19.7). However, the 2D/0D nanosheet g-C3 N4 based on ZnInS4 NP composite (Fig. 19.7a) suffered from the aggregate structure and poor interface between ZnInS4 and g-C3 N4 due to point-to-face contact. However, the 2D/2D nanosheet structures of the nano leaf g-C3 N4 @ZnInS4 (Fig. 19.7b) showed larger contact interface areas along with much faster charge transfer rates, resulting in enhanced H2 production under visible light without Pt (Fig. 19.7c). The 2D/2D gC3 N4 nanosheets in the ZnInS4 nano leaf showed (HER = 2.78 mmol g−1 h−1 ) nearly 69.5, 15.4, 8.2, and 1.9 times higher production than bare g-C3 N4 nanosheet, the bare ZnInS4 microspheres, the 2D/0D g-C3 N4 nanosheet in the ZnInS4 microsphere compound and the bare ZnInS4 nano leaf, respectively [40].

460 Fig. 19.6 a TEM image of the 2D/2D compound g-C3 N4 /rGO, b schematic diagram to illustrate the photocatalytic mechanism [38]

Fig. 19.7 Schematic illustration of various interfaces for a 2D/0D heterojunction, b 2D/2D heterojunction, and c mechanism of photocatalytic hydrogen evolution over nanoleaf structures of g-C3 N4 @ZnInS4 under visible light irradiation (λ ≥ 420 nm) [8, 40]

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The introduction of plasmonic noble metal-based nanostructures could significantly improve light absorption, as well as charge carriers in the adjoining photocatalysts due to their SPR property. Usually, a Schottky junction is formed due to the close contact between the plasmon metal and semiconductor, which results in the transport of electrons taking place either from plasmon metal or semiconductor to the other components until their Fermi levels attain equilibrium, and the direction of electron transfer depends on the nature of the semiconductor [41]. For example, Khalid et al. fabricated a nanocomposite of Ag–TiO2 /graphene using a hydrothermal method. They observed enhanced photocatalysis, which is due to the lower Fermi level of Ag than the CB of TiO2 . As Ag displays SPR, upon visible light illumination, the photoexcited electrons from the Ag can pass through the Schottky barriers via TiO2 /graphene interface to the CB of TiO2 . Finally, some of these energetic electrons participate in the reduction of protons. Meanwhile, other energetic electrons in the TiO2 CB are transferred to the graphene layers. These are also further participating in the reduction of a proton because the potential of graphene is lower than the CB of TiO2 and slightly higher than the potential of H+ /H2 [42]. However, making stable cocatalysts over the 2D–2D heterojunction is tedious, which limits their long-term photocatalytic activity and stability. To overcome these problems, Li et al. constructed a 3D porous g-C3 N4 /GO framework (PCN/GO) followed by AgBr NP decoration. Due to its porous structure, the water molecules easily reach the Ag/AgBr active sites (Fig. 19.8), resulting in an excellent hydrogen evolution rate of ~3.69 mmoles g−1 h−1 , without the Pt cocatalysts, [43]. Recently, spatial depositing of dual cocatalysts on semiconductor photocatalysts is an efficient approach for attaining enhanced photocatalytic activities, since complete charge separation can be achieved by loading reductive and oxidative cocatalysts simultaneously. For example, Qin et al. fabricated a composite photocatalyst of Pt/TiO2 nanotube/CoOx , where the Pt nanoclusters deposited on the inner surface of the TiO2 , which can act as electron acceptors as well as active centers for proton reduction. On the other hand, CoOx nanoclusters on the outer surface of TiO2 nanotube Fig. 19.8 Photocatalytic water-splitting reaction mechanism for hydrogen production on the 3D g-C3 N4 /GO-Ag/AgBr photocatalyst under visible light illumination [43]

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Fig. 19.9 TEM image of a porous CoOx /TiO2 /Pt photocatalyst with spatially separated Pt and CoOx dual cocatalysts along with a schematic illustration of the photocatalytic mechanism of water into hydrogen [44]

can accept holes and participate in the oxidation reaction. An ultra-low concentration of the cocatalysts achieved remarkable photocatalytic activity [44] (Fig. 19.9). Chemical doping through various heteroatoms is a significant way to change the optoelectronic features of graphene and its derivatives. Heteroatom doping into the lattice surface of graphene sheets can simultaneously open up the band gap and catalytic active sites, then it can obtain the ability of intrinsic sole photocatalytic activity [6, 7]. Usually, two types of doping can be seen in graphene, including (i surface doping: creating additional functional groups over the graphene sheets, depending on the tendency to donate or withdraw ability, the graphene could be an n- or p-type material, and (ii substitutional doping: substitution of carbon atoms by other atoms with a different number of valence electrons; this type of doping introduces additional states in the graphene [5, 45]. Atoms with additional valence electrons lead to an n-type conductivity, and atoms with fewer valence electrons lead to p-type conductivity in graphene. For instance, Levarato et al. fabricated the nitrogen-doped graphene employing the high-temperature pyrolysis approach using chitosan as a precursor and showed it as a visible light photocatalyst without using platinum cocatalysts for hydrogen production via water splitting. Interestingly, with increasing the pyrolysis temperature, the nitrogen amounts decrease (as the temperature variation from 200 to 900 °C; the nitrogen content decreased from 16.2 to 5.4%, respectively), and photocatalytic activity mostly depends on the crystallinity and pyrolysis temperature [46]. In another example, Yeh et al. fabricated the N-GO QDs and utilized them as solo visible light active photocatalysts to achieve a complete water splitting. N-GO QDs were manufactured using GO along with NH3 at a higher temperature followed by oxidation treatment to transform GO sheets into GO QDs (Fig. 19.10). The developed N-GO QD photocatalyst consisted of nitrogen doping along with various oxygen functional groups on its surface, resulting in the formation of a p–n-type electrochemical diode structure, (Fig. 19.10b) [47].

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Fig. 19.10 a Complete water splitting for the production hydrogen and oxygen by nitrogen-doped graphene oxide quantum dots as visible light photocatalyst and b schematic illustration of intrinsic hybrid structures along with the photocatalytic mechanism [47]

Recently, metal halide perovskite gained much attention because of its outstanding optoelectronic feature, such as tunable band gap, especially to utilize full solar light, huge carrier mobility, and long diffusion lengths. For example, Nam et al. developed a stable MAPbI3 in water medium by regulating I and H+ concentrations and demonstrated as a photocatalyst for H2 production. The powder of MAPbI3 powder in the aqueous HI solution could effectively split the HI into H2 and I− 3 under visible light illumination [48]. Accordingly, upon consideration of all these outstanding optoelectronic features of perovskite materials, it is predicted that, to attain efficient photocatalytic activity, they could be integrated with a appropriate semiconductor to construct a Z-scheme heterojunction. Kuang et al. manufactured a α-Fe2 O3 /AmineRGO/CsPbBr3 Z-scheme heterostructure for efficient photocatalytic reduction of CO2 and oxidation of water (Fig. 19.11). In this, delicately controlled interfacial interaction, an efficient electron transfers from the α-Fe2 O3 to CsPbBr3 , which leads to improved charge separation and extended lifetime. Furthermore, the matched energy band structure allows the formation of a direct Z-scheme, and charge transfer from the α-Fe2 O3 to CsPbBr3 , resulting in enhanced photo-induced charge-carrier separation. Furthermore, presence of amino-functionalized rGO helps regulate the interfacial interaction of α-Fe2 O3 and CsPbBr3 [4].

19.6 Photocatalytic Degradation of Organic Pollutants Earth’s water is one of the abundant natural resources, but only 1% of the resources are available for human consumption. This freshwater is contaminated by various organic

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Fig. 19.11 Schematic illustration of the solid-state Z-scheme a-Fe2 O3 /Amine-RGO/CsPbBr3 photocatalyst and the photocatalytic mechanism [49]

and inorganic pollutants. Heterogeneous photocatalysis is an efficient approach to degrading both aquatic and environmental organic contaminants. The application uses naturally available abundant sunlight in the presence of a semiconductor photocatalyst to accelerate the degradation of water contaminants and the destruction of highly toxic molecules. Carbon-based materials can be used in several ways for environmental remediation and pollutant removal [50]. They can be used as an adsorbent to reduce the concentration of pollutants, decompose pollutants into less toxic molecules, and reduce the number of low-valence species from high-valence species (generally high valance species are toxic). Mostly, the degradation of dyes over graphene nanocomposite photocatalysts has three sequential steps including dye adsorption, photoactivation of semiconductors, and generation of reactive oxygen species (ROS). A schematic illustration of various stages of dye degradation in graphene nanocomposites is shown in Fig. 19.12. Initially, the dye molecules are adsorbed on the surface of the graphene sheets through the π–π* stacking since the dye molecules and graphene both have aromatic structures (Fig. 19.12). Therefore, the adsorption capacity of graphene semiconductor nanocomposites for organic dye molecules can be higher than that of bare semiconductor photocatalysts. These adsorbed dye molecules on the surface of graphene can readily be available for the oxidative species generated over the photocatalyst (semiconductor), making it a more effective and efficient degradation process. In the second step, the nanocomposite absorbs the irradiated light. When the photocatalyst is integrated with the graphene sheets, the light absorption range shifts to a broad range. In the case of the rGO-P25 nanocomposite (TiO2 ), the absorption band is redshifted approximately 30–40 nm compared to the bare TiO2 nanoparticles, leading to the narrowing of the band gap of

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TiO2 particles. In some cases, the redshift is greater than 50 nm. For example, in the TiO2 nanorods/graphene nanocomposite system, there is a redshift in light absorption from 325 to 400 nm [51]. In the third step, the generation of reactive oxygen species occurs by photo-induced electrons, which are responsible for the degradation of dye molecules. It is well known that the charge-carrier recombination is quite high in bare semiconductor systems, and once the graphene sheets are integrated with the semiconductors, the electron–hole pair recombination can be suppressed, and the electron transport will be improved. Upon light irradiation, the VB electrons are excited to the CB of a semiconductor, and these electrons can travel to the graphene through the sp2 carbon network. Photo-induced electrons are transferred to O2 and generate reactive oxygen species, which further take part in the degradation of the dye to simple molecules [11]. The overall photocatalytic mechanism for the degradation of pollutants over the semiconductor-based nanocomposites can be summarized as follows: Semiconductor nanocomposite + hν → h+ + e− O2 + e− → O−∗ 2 + ∗ O−∗ 2 + H → HOO

HOO∗ → O2 + H2 O2

Fig. 19.12 Schematic illustration of dye degradation on the surface of graphene hybrid nanocomposite photocatalysts [11]

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H2 O2 + hν + e− → 2∗ OH h+ + H2 O → HO∗ + H+ ∗ Pollutant + h+ + O−∗ 2 + HO → Degradation products

In addition to electron–hole recombination, carbon materials also prevent corrosion and leaching of the semiconductor material in water, thereby enhancing the long life of photocatalysis. Over the past few years, graphene is well established as an efficient electron acceptor as well as a transporter, resulting in a prolonged lifetime of photogenerated charge carriers. The fabrication of graphene-based semiconductors is of great interest due to the diversity of availability of functional nanomaterials including TiO2 , Fe2 O3 , ZnO, CuO, ZrO2 , NiFe2 O4 , MnFe2 O4 , CdO, and CdS. Among various semiconductor photocatalysts, metal oxides, such as ZnO and TiO2 , are essential and widely used as photocatalysts for the degradation of organic contaminants in water due to their excellent properties, including low toxicity, cheap, abundant on earth, easy to synthesize, environmentally friendly, and extreme photocatalytic activity [5]. Tien et al. reported rGO/ZnO spheres composite and synthesized via a fast and straightforward microwave solvothermal reaction. Under visible light irradiation, the prepared rGO–ZnO photocatalyst exhibited photocatalytic activity for the degradation of MB dye 10 times higher than that for the bare ZnO spheres and rGO alone. This enhanced photocatalytic activity due to an increase in visible light absorption, a decrease in the band gap, and effective charge separation [52]. Furthermore, the size of the semiconductor particles on the graphene sheets plays a crucial role in photocatalytic activity. ZnO NPs with smaller sizes (20–100 nm) on the rGO displayed an enhanced activity compared with the ZnO particles with 50–500 nm sizes. They also observed efficient inhibition of photocorrosion along with good synergic interaction between rGO and ZnO for rGO–ZnO (20–100 nm) [53]. Furthermore, exploring the effect of various types of graphene sheets including thermally expanded, and chemically oxidized GO, and morphology of ZnO nanostructures on the photocatalytic activity [54, 55]. Generally, in semiconductor nanostructures, the photocatalytic efficiencies typically depend on their morphology and oxygen defects. Usually, GO can act as a surfactant to tune the morphologies of the semiconductor nanostructures as well as defect densities. For instance, by changing the amount of GO, the morphology of ZnO can be facilely transformed from a 1D rod with a pointed tip to a hexagonal tube architecture. A schematic illustration of the evolution of ZnO morphologies is shown in Fig. 19.13a. The surface functional groups on the GO act as nucleation sites and strongly interact with the metal ions. The growth rate along with the (0001) c-axis is highest for the Wurtzite ZnO. Therefore, a pointed tip at the end of the c-axis for the bare ZnO NRs has been observed. However, in the presence of GO, the growth of the ZnO was restricted along the c-axis, leading to flat-tipped ZnO NRs. GO with abundant hydroxyl groups on the basal plane can bind to the ZnO and change the

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kinetics of the nucleation and growth process. At a lower concentration of GO (~1 wt%), the dissolution of ZnO has not occurred. As the centration of GO increased to ~3 wt%, the ZnO hollow tubes have appeared, which is due to the erosion of polar ZnO. Furthermore, by increasing the GO concentration to ~10 wt%, the rGO sheets warped to the polar facets of ZnO and stabilized ZnO from dissolution. The oxygen vacancies were studied by ESR measurements and shown in Fig. 19.13b. As the concentration of GO increases, the oxygen vacancies increase, as shown in Fig. 19.13b. For the rGO and bare ZnO NRs, no apparent signal is detected under ESR because of the unavailability of the paramagnetic species. In this way tuning the morphology and defects densities, one can tune the light absorption properties from UV light (bare ZnO) to the visible region [21]. The photocatalytic performances of the rGO–ZnO nanocomposites with various amounts of GO are shown in Fig. 19.13c, d. When the amount of GO is increased, the electron–hole recombination decreases but the active sites on the photocatalysts were shielded, which leads to a decrease in the overall photocatalytic activity (Fig. 19.13d). However, with an optimum amount of GO along with the semiconductor photocatalysts, one can achieve the enhanced photocatalytic performances. Generally, the dye molecules, especially, the MB dye molecule, which is in the solution can be transferred to the catalyst surface and adsorbed via π–π conjugation between the MB molecules and aromatic regions of the graphene. Moreover, graphene can extend the light absorption region and inhibit the recombination of charge carriers and helps in transportation. Zhang et al. observed that the TiO2 exhibited a higher photocatalytic activity with the graphene than the carbon nanotubes, which might be due to the enormous 2D planar structure, which facilitates a better platform for dye adsorption and photo-induced charge separation [56]. In some cases, composites of graphene-based semiconductors other than TiO2 exhibited higher photocatalytic performance. Zhang et al. synthesized graphene-based SnO2 and TiO2 nanocomposites using a simple solution phase approach followed by calcination and evaluated their photocatalytic activity for the degradation of RhB in water. Interestingly, they reported that the graphene-SnO2 nanocomposite showed higher photocatalytic performance than the graphene-TiO2 composite, which is mainly due to thermodynamically favorable electron transfer (significant potential difference) from RhB* to SnO2 [44]. The morphology and surface defects of the photocatalyst are also vital factors in deciding the photocatalytic performance of the semiconductor. An et al. manufactured a graphene-based one-dimensional WO3 nanorods composite and observed a 53-fold enhancement in the photocatalytic activity for the graphene-WO3 nanorods composite compared to commercial WO3 [57]. Different synthesis methods can also significantly affect the morphology and defect states of semiconductors, resulting in changes in their emission characteristics and photocatalytic performances. Recently, our group reported the fabrication of binary and ternary ZnO nanocomposite systems possessing NPs and NRs morphology over the rGO and rGO–Au systems using wet chemical approaches including solution phase and hydrothermal methods, respectively. We observed a variation in the morphologies as well as defect states of ZnO by varying synthesis methods. The ternary nanocomposite of rGO–Au–ZnO NPs prepared by the solution

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Fig. 19.13 a Proposed schematic reaction mechanism for the formation of the rGO–ZnO nanocomposite; b oxygen vacancies (determined by ESR measurements); c kinetics of photocatalytic reduction of Cr(iv) over the composites; d degradation performances of various composites [21]

phase approach contains various oxygen defect states that exhibited faster kinetics toward RhB degradation (Fig. 19.14a–c) under UV light than the bare morphologies of ZnO and rGO–ZnO NPs and NRs. Further, PL measurements reveal that the quenching in the band edge and defect states band emission due to the effective excited-state electron transfer from the ZnO CB to both rGO and Au levels, resulting in efficient charge separation; the lifetimes of electrons and holes are altered (Fig. 19.14e). Further, these charge carriers are accessible for generation of reactive species, which are responsible for oxidation followed by degradation of dye molecules. Based on the PL studies and degradation kinetics, it has been concluded that the photocatalytic degradation rates are more dependent on the suppression of the charge carrier’s recombinations than the higher surface coverage. The effective charge separation is provided by rGO in the binary system and by rGO and Au in the ternary system (Fig. 19.14e). The intimate physical interface formed between ZnO, rGO, and Au in the ternary nanocomposite by the in situ preparation method facilitates effective charge transfer across the components. We also studied the degradation response with respect to the intensity of incident light. Degradation rates increased with increasing light intensity. At higher intensities, the rate varies with the square root of light intensity (fig. 19.14d) [58].

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Fig. 19.14 a Time evolution absorption spectra of the RhB solution in the presence of the rGO–Au– ZnO NP composite under UV light, b kinetics of the degradation of the RhB dye in the presence and absence of various ZnO systems, c degradation efficiency of various photocatalysts for the RhB dye, d kinetics of the degradation of the RhB dye at various light intensities, and e schematic representation of the possible charge transfer mechanism in the rGO–Au–ZnO NP composite [58]

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The introduction of plasmonic metal nanostructures over semiconductors is an efficient way to get efficient visible light photocatalysis. Plasmonic behavior can be tuned by altering the morphology, size, and composition. As we discussed earlier, noble metals such as Ag, Au, and Cu exhibit SPR absorption in the visible-to-nearinfrared (NIR) region and act as a light harvester. Furthermore, the noble metal could act as a conductor for the efficient separation of photo-induced charge carriers that are generated on the semiconductors. For example, Song et al. fabricated a plasmonic Ag/Ag2 CO3 –rGO nanocomposite by taking advantage of the inherent chemical activity of rGO that arises due to the introduction of defect sites. Defects in sp2 carbon materials can reduce some of the metal ions. Compared to pure Ag2 CO3 and Ag2 CO3 /GO, the Ag/Ag2 CO3 –rGO nanocomposites exhibited enhanced photocatalytic activity (Fig. 19.15). The possible mechanism for the enhanced photocatalytic activity of the Ag/Ag2 CO3 –rGO nanocomposite is shown in Fig. 19.15. Due to the narrow band gap, Ag2 CO3 can be easily excited under visible light to generate photo-induced charge carriers such as electron–hole pairs. Due to the ultrahigh charge-carrier mobility of graphene, the excited electrons transfer from the Ag2 CO3 conduction band to the graphene layer, resulting in suppression of electron–hole recombination. The holes will remain in the valence band of Ag2 CO3 and can directly oxidize pollutants. At the same time, photogenerated electrons transferred to the surface of rGO can reduce adsorbed O2 to H2 O2 , followed by the formation of effective hydroxyl radicles. Furthermore, Ag NPs formed can be excited by visible light irradiation due to SPR absorption and induce an enhanced electric field near the semiconductor-noble metal interface, resulting in more electrons being transferred to the rGO layer [59]. Most of the literatures have focused on powdered heterostructures. However, these powdered systems display some drawbacks, such as aggregation, poor durability, and recyclability, and in some cases, the need for a filtration process to recover the photocatalysts. To solve these issues, Yang et al. grew vertically aligned ZnO NR arrays over the substrates coated with rGO without aggregation, followed by the chemical bonding of high-density unaggregated plasma-reactive particles.

Fig. 19.15 Schematic illustration of the photocatalytic mechanism of the Ag/Ag2 CO3 –rGO nanocomposite upon visible light irradiation and the kinetic graph of various photocatalysts [59]

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Under solar light irradiation, Ag NPs-ZnO NRs-graphene exhibited 13.8 times enhanced photocatalytic activity than the bare ZnO NRs, which is mainly due to the synergic effect of plasmonic and electronic coupling. The vertically aligned ZnO NRs display greater advantages such as a well-oriented, ordered, and single-crystalline phase which has significantly enhanced the transport and collection charge carriers. In the case of particle-based ZnO nanostructures synthesized on the solid substrate, a variety of grain boundaries, and un-proper interfaces formed in-between the ZnO NPs could hamper the migration of charge carriers to the active sites, resulting in poor photocatalytic activity. Moreover, the ZnO NRs without any aggregation exhibit a high surface area for the attaching of Ag NPs. The interaction of Ag NPs with ZnO NRs effectively enhances the visible light absorption by SPR, acting as a sink for the electrons. Furthermore, graphene accepts photo-induced excited electrons from Ag NPs and ZnO NRs through a continuous pathway provided by ZnO NRs [60]. Recovery and reusability of the photocatalyst materials after the photocatalysis process are of significant importance concerning sustainable management. In this context, separation of suspended photocatalyst materials using a magnet is an appropriate approach to re-collect the photocatalysts. It is well known that magnetic iron oxide and spinel ferrite materials are best due to their exceptional magnetic properties, biocompatibility, chemical, and thermal constancy. Especially, the magnetic photocatalyst materials can be easily removed from the reaction mixture using external magnets. Therefore, these materials are certainly useful in large-scale industrial applications. Magnetic iron oxide and spinel ferrite exhibit small band gaps (~2 eV), which makes them active under visible light irradiation. Gao et al. prepared a homogeneous compound of GO-Fe3O4 using a facile low-temperature solution approach. The tight attachment between the graphene and Fe3 O4 nanoparticles was due to the metal-carbonyl coordination, which is confirmed by the FTIR spectra. The catalyst displayed high catalytic activity for RhB degradation in an aqueous solution with H2 O2 with a wide pH range of 2.09 to 10.09 under visible light illumination [61]. Fu et al. fabricated a magnetically separable graphene-ZnFe2 O4 nanocomposite (Fig. 19.16a, b) employing a simple hydrothermal approach and used it for visible light photodegradation of MB in water along with H2 O2 . The suspension of nanocomposites can be easily separated after the photocatalytic cycle (Fig. 19.16e) using an external magnet by utilizing the magnetic nature of ZnFe2 O4 . Compared to pure ZnFe2 O4 , the graphene-ZnFe2 O4 nanocomposite system exhibited a significant enhancement in adsorption and photocatalytic activity (Fig. 19.16c,d). The enhanced adsorption of MB dye on graphene mainly due to the π-π interaction between MB and π-conjugated areas of the graphene sheets. The graphene-ZnFe2 O4 nanocomposite exhibits a dual function under visible light; as a photocatalyst for MB dye degradation and generator of hydroxyl radicles from the decomposition of H2 O2 [62]. Furthermore, they prepare MnFe2 O4 -graphene and CoFe2 O4 -graphene nanocomposite using the facile solution phase and the hydrothermal method, respectively. Interestingly, bare MnFe2 O4 and CoFe2 O4 display no photocatalytic activity under visible light. However, when it is combined with graphene, it displays an excellent photocatalytic activity [62, 63].

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Fig. 19.16 a, b Pictures of ZnFe2 O4 -Graphene suspension with and without magnet, c absorption spectra of the MB solution in the presence of the ZnFe2 O4 -Graphene nanocomposite, d bar diagram of the remaining MB after equilibrium, and e graph of recyclability of the ZnFe2 O4 -Graphene nanocomposite for degradation of MB [62]

Fu et al. prepared leaf-like BiVO4 crystals on graphene sheets using a hydrothermal approach and demonstrated its ability to degrade the dyes. They observed enhanced photocatalysis upon visible light illumination because of their combined effects [64]. Later, Chen et al. studied interface interactions, charge separation, and their transfer, further their impact on the photocatalytic activity of the graphene-BiVO4 (001) compound by first-principle calculations. It was found that after the equilibration of the graphene/BiVO4 interface, the photogenerated electrons from the BiVO4 continuously flow to the C pz orbital of the graphene under visible light and cannot migrate back to the BiVO4 , leading to effective charge separation. Furthermore, it also indicated that the interaction between graphene and BiVO4 induces an increase in optical absorption in the region of visible light [65]. Gao et al. fabricated the graphene-Bi2 WO6 and demonstrated their photocatalytic activity. The strong electronic interaction along with charge equilibration inbetween the graphene–Bi2 WO6 leads to negative shifts in the Fermi level followed by high migration of photo-induced electrons, resulting in a delay of electron–hole recombination [66]. Gawande et al. fabricated an rGO–BaCrO4 composite using GO, potassium chromate, and barium carbonate as starting materials using a sol–gel chemical deposition approach. The chemical interaction (Ba–C) between graphene and BaCrO4 leads to the formation of homogeneous composites. Graphene accepts photoexcited electrons from BaCrO4 and increases the lifetime of charge carriers

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[67]. The materials mentioned above exhibit limited visible light absorption or instability. Therefore, there is great demand for a visible light photocatalyst. Yan et al. had done pioneer work on the Ag3 PO4 photocatalyst, which is a promising material for visible light photocatalysis [68]. However, the pure and bare Ag3 PO4 photocatalyst is prone to photocorrosion and decomposition due to low active Ag during photoreaction, which could greatly worsen its photocatalytic activity. To overcome these drawbacks, GO is an ideal supporting material for the preparation of GO– Ag3 PO4 composites. Liu et al. manufactured a GO enwrapped Ag3 PO4 composite using a simple ion-exchange process by taking CH3 COOAg and Na2 HPO4 as the initial reactants along with GO. The composite exhibited improved photocatalytic performance along with great stability toward acid orange 7 degradation in water [69]. Ye et al. reported a synthesis of CdS–graphene nanocomposites employing hydrothermal methods and explained its visible light activity for methyl orange (MO) degradation. Interestingly, the GO loading decreases the band gap of CDs, due to the strong electronic interaction between the CdS and graphene components. Further, transient photocurrent studies reveal the formation of heterojunction followed by excellent charge separation, which leads to > 95% of MO degradation within 60 min light irradiation [70]. Additionally, in replacement to conventional semiconductors, it is desired to develop inimitable and efficient hybridizers for full solar light, ultrastable, and recyclable graphene-based semiconductor photocatalysts for sustainable purposes. In this connection, visible light-triggered plasmonic photocatalysts are a significant alternative to conventional photocatalysts. In the literature, silver/silver halide (Ag/AgX, X = Br, Cl) is an alternative plasmonic photocatalyst for the degradation of pollutants under visible light. Their remarkable photoresponse is due to the presence of metallic Ag nanoparticles and gives rise to visible plasmonic resonance, resulting in enhanced photocatalytic activity and efficient charge separation [71]. Usually, AgX particles are unstable, upon photo illumination, these AgX particles are partially reduced to an Ag and form stable Ag/AgX composites. However, the intrinsic solubility of Ag/AgX in the aqueous medium greatly reduces its long-term stability. Subsequently, the combination of these Ag/AgX materials with graphene may increase the stability and photocatalytic performance by a synergistic effect. Recently, Zhu et al. reported the GO-enraged Ag/AgX (X = Br, Cl) nanocomposite, which is facilely assembled in the water–oil system at room temperature. The experimental results revealed that the photogenerated electrons migrate from the Ag/AGX to the GO sheets through percolation process [71] (Fig. 19.17). Gao et al. recently reported the effect of the extent of GO reduction on photocatalytic activity of graphene-based semiconductors. As we know that GO contains numerous oxygen functional groups over the basal plane and edges of the sheets and exhibits poor electron conductivity. Therefore, researchers are introducing rGO into Ag/AgX systems to improve performance. The work function of the GO depends mainly on the oxygen functional groups. Theoretical and experimental results suggest that with an increasing degree of reduction, work function can also increase. Therefore, the reduction extent of the GO can impact the photocatalytic activity of the Ag/AgX–rGO photocatalyst. Gao et al. fabricated an Ag/AgX (Cl and Br)-partially

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Fig. 19.17 Comparison of the photocatalytic reaction constant and apparent quantum efficiency (AQE) of a Ag/AgCl–PrGO, b Ag/AgBr–PrGO nanocomposites, and c schematic of a photocatalytic reaction mechanism of Ag/AgX (Cl and Br)–PrGO nanocomposites under visible light LED irradiation [14]

reduced graphene oxide (PrGO) system employing a simple and effective roomtemperature photoreduction approach. SPR generated electrons in Ag NPs transferred to AgX followed by reducing dissolved O2 to O−· 2 and the holes left in the Ag NPs oxidize X − to X, which further facilitates RhB dye degradation. However, when the excessive photoreduction of GO leads to an increase in the work function of graphene above the CB of AgX, resulting in difficulty of charge transfer followed by a decrease in photocatalytic activity. Therefore, the photocatalytic activity greatly depends on the alignment of the band positions in the components [14].

19.7 Summary and Future Perspectives Significant challenges persist in the progress of affordable, efficient, and easy synthesis approaches for the fabrication of superior heterojunction photocatalysts on a large scale for real applications. Moreover, there are more advancements in the control of the morphology, interface engineering, crystallization, etc., needed. Moreover, systematic studies are also desirable for a better understanding of the migration pathways of the photogenerated charge carriers in photocatalysts. However, to date, there has been no direct evidence of seeing the actual migration pathway of the photogenerated charge carriers at the heterojunction interface. Therefore, to investigate this, more powerful characterization tools are indeed needed. Furthermore, for a better understanding of the interface charge transport pathways and their mechanisms, more theoretical calculations and approaches are highly desirable. In addition,

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the fabrication of a new variety of photocatalytic materials is supposed to be one of the key research goals to develop efficient photocatalysts. As it is well understood that the existing photocatalytic materials possess many drawbacks including high price, huge band gaps, low surface area, high charge recombination, etc. Therefore, it is essential to find cheap, facile, and advanced materials for photocatalytic applications. In this connection, carbon dots (CD)/graphene quantum dots (GQD) have emerged as alternative promising photocatalytic materials, without the use of any noble metals as cocatalysts [6, 7, 72–74]. CDs/GQDs are 0D quasispherical nanoparticles and were first discovered in 2004. Chemically, CDs/GQDs are comprised of sp2 /sp3 hybridized carbon atoms and display unique photophysical and chemical properties including a full solar light absorption from ultraviolet to the near-infrared range, bright photoluminescence, electron mediation, excellent photostability, good solubility on polar solvents, chemical stability, easy functionalization, and bio-compatible due to their nonmetallic nature. Furthermore, a plethora of inexpensive precursors and a wide variety of synthesis approaches make them explosive to use as photocatalysts. But, the practical utilization of these CDs/GQDs as a solo-photocatalyst, without the use of any noble metal cocatalyst, is lacking. In addition to this, very few studies are available in the literature on understanding the fundamental structure-property correlation. Therefore, more research is indeed needed in this direction. In our view, the overall photophysical properties of CDs/GQDs depended mainly on inherent structural features such as aromatic, amorphous domains, heteroatom functionalities, distribution of defects, and vacancies. By critically controlling the ratiometric proportions of all of these states, we can manipulate the photophysical properties and photocatalytic performances. Nevertheless, the photocatalytic activity of CDs is very deprived compared with the commercial and traditional metal-based photocatalysts, which might be due to the poor visible light absorption ability and competitive recombination processes. Henceforth, it is highly needed further research in this direction, especially to improve visible light absorption, charge separation, and improving active sites.

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