Photocatalytic Functional Materials for Environmental Remediation E-Book
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- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
A comprehensive volume on photocatalytic functional materials for environmental remediation As the need for removing large amounts of pollution and contamination in air, soil, and water grows, emerging technologies in the field of environmental remediation are of increasing importance. The use of photocatalysis--a green technology with enormous potential to resolve the issues related to environmental pollution--breaks down toxic organic compounds to mineralized products such as carbon dioxide and water. Due to their high performance, ease of fabrication, long-term stability, and low manufacturing costs, photofunctional materials constructed from nanocomposite materials hold great potential for environmental remediation. Photocatalytic Functional Materials for Environmental Remediation examines the development of high performance photofunctional materials for the treatment of environmental pollutants. This timely volume assembles and reviews a broad range of ideas from leading experts in fields of chemistry, physics, nanotechnology, materials science, and engineering. Precise, up-to-date chapters cover both the fundamentals and applications of photocatalytic functional materials. Semiconductor-metal nanocomposites, layered double hydroxides, metal-organic frameworks, polymer nanocomposites, and other photofunctional materials are examined in applications such as carbon dioxide reduction and organic pollutant degradation. Providing interdisciplinary focus to green technology materials for the treatment of environmental pollutants, this important work: * Provides comprehensive coverage of various photocatalytic materials for environmental remediation useful for researchers and developers * Encompasses both fundamental concepts and applied technology in the field * Focuses on novel design and application of photocatalytic materials used for the removal of environmental contaminates and pollution * Offers in-depth examination of highly topical green-technology solutions * Presents an interdisciplinary approach to environmental remediation Photocatalytic Functional Materials for Environmental Remediation is a vital resource for researchers, engineers, and graduate students in the multi-disciplinary areas of chemistry, physics, nanotechnology, environmental science, materials science, and engineering related to photocatalytic environmental remediation.
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Table of Contents
Cover
List of Contributors
Preface
1 Titanium Dioxide and Carbon Nanomaterials for the Photocatalytic Degradation of Organic Dyes
1.1 Introduction
1.2 Principles and Mechanism of Photocatalysis
1.3 Importance of Titanium Dioxide
1.4 Titanium Dioxide for the Photocatalytic Degradation of Organic Dyes
1.5 Carbon Nanomaterials for the Photocatalytic Degradation of Organic Dyes
1.6 Conclusion and Trends
References
2 Visible Light Photocatalytic Degradation of Environmental Pollutants Using Metal Oxide Semiconductors
2.1 Introduction
2.2 Photocatalysis
2.3 Mechanism and Fundamentals of Photocatalytic Reactions
2.4 Synthesis of Different Photocatalysts
2.5 Factors Affecting Photocatalytic Degradation
2.6 Metal Oxide Semiconductors
2.7 Ternary/Quaternary Oxides
2.8 Composites Semiconductors
2.9 Sensitization
2.10 Conclusions
References
3 Contemporary Achievements of Visible Light‐Driven Nanocatalysts for the Environmental Applications
3.1 Introduction
3.2 Types of Photocatalytic Reactor Models
3.3 Modification of Semiconductor Nanoparticles
3.4 Emerging Photocatalysts
3.5 Mechanisms of Photocatalysis
3.6 Conclusion
References
4 Application of Nanocomposites for Photocatalytic Removal of Dye Contaminants
4.1 Nanocomposites and Applications
4.2 Dyes: Introduction, Classification, and Impacts on the Environment
4.3 Strategies of Dye Contaminant Removal
4.4 Photodegradation and the Removal of Dyes Using Nanocomposites
4.5 Photocatalytic Reactors for Dye Degradation
4.6 Summary
References
5 Photocatalytic Active Silver Phosphate for Photoremediation of Organic Pollutants
5.1 Introduction
5.2 Properties of Ag
3
PO
4
5.3 Photoremediation of Organic Pollutants
5.4 Conclusions and Future Prospects
Acknowledgments
References
6 Plasmonic Ag‐ZnO: Charge Carrier Mechanisms and Photocatalytic Applications*
6.1 ZnO‐Based Photocatalysis
6.2 Why Deposit Silver on ZnO Surface?
6.3 Methods to Decorate Silver NPs on the Surface of ZnO
6.4 Mechanism of Charge Carrier Transfer Dynamics in Ag‐ZnO
6.5 Influence of Silver Content on Optimizing the Photocatalytic Activity
6.6 Structure–Morphology Relationship on Photocatalytic Activity
6.7 Co‐modification of Ag‐ZnO for Photocatalysis
6.8 Conclusion and Future Prospects
References
7 Multifunctional Hybrid Materials Based on Layered Double Hydroxide towards Photocatalysis
7.1 Introduction
7.2 Hybrid LDHs from LDH Precursors
7.3 Photocatalytic Applications of Different LDH‐Based Hybrid Materials
7.4 Conclusions
References
8 Magnetically Separable Iron Oxide‐Based Nanocomposite Photocatalytic Materials for Environmental Remediation
8.1 Introduction
8.2 Synthesis Techniques for Magnetic Nanophotocatalyst Composites
8.3 Three Types of Semiconductor Magnetic‐Based Nanocomposites
8.4 Graphene‐Based Magnetically Separable Composites
8.5 The Effect of Iron Oxide‐Based Photocatalysts on Pollutants
8.6 Summary
References
9 Photo Functional Materials for Environmental Remediation
9.1 Introduction
9.2 Photoelectric Effect
9.3 Photo Functional Materials (Photocatalysts)
9.4 Photodegradation of Textile Dyes
9.5 Semiconductor‐Based Photocatalysts
9.6 Carbon Nanotubes (CNTs)
9.7 Photo Functional Semiconductors on CNT Hybrid Materials for Tunable Optoelectronic Devices
9.8 Fabrication of CdS Quantum Dot Sensitized Solar Cells Using Nitrogen‐Functionalized CNTs/TiO
2
Nanocomposites
9.9 Graphene Sheet
9.10 CdS/G Nanocomposites for Efficient Visible Light Driven Photocatalysis
9.11 Graphitic Carbon Nitride (g‐C
3
N
4
)
9.12 Conclusions
References
10 Graphitic Carbon Nitride‐Based Nanostructured Materials for Photocatalytic Applications
10.1 Introduction
10.2 General Mechanism: Reaction Pathway
10.3 g‐C
3
N
4
and Composites in Photocatalytic Degradation
10.4 Conclusions and Future Directions
Acknowledgments
References
11 Metal–Organic Frameworks for Photocatalytic Environmental Remediation
11.1 Introduction
11.2 Structural Features of MOFs
11.3 Synthesis of MOFs
11.4 Photocatalytic MOFs by Design
11.5 Photocatalytic Applications of MOFs
11.6 Conclusions and Future Prospects
Acknowledgment
References
12 Active Materials for Photocatalytic Reduction of Carbon Dioxide
12.1 Introduction
12.2 CO
2
Photoreduction – Essentials
12.3 Heterogeneous Photocatalytic Reduction of Carbon Dioxide with Water
12.4 Nanomaterials and New Combinations of Materials for Carbon Dioxide Reduction
12.5 Selection of Materials
12.6 Material Modifications for Improving Efficiency
12.7 Perspectives in the Photocatalytic Reduction of Carbon Dioxide
Acknowledgement
References
Index
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Classification, characteristics, and applications of dyes.
Chapter 3
Table 3.1 Selected applications of photocatalysis.
Table 3.2 Various types of photocatalytic reactors [33].
Table 3.3 List of various perovskite oxide photocatalysts investigated.
Table 3.4 Summary of g‐C
3
N
4
supported nanophotocatalysts and its photocatalytic e...
Table 3.5 Proposed mechanism for the heterogeneously coupled photocatalysts and ...
Table 3.6 Proposed mechanism for p‐n heterojunction photocatalysts and their app...
Chapter 4
Table 4.1 A summary of polymer‐based nanocomposites (PNCs) used for photocatalyt...
Chapter 7
Table 7.1 Summary of reported LDH‐based hybrid materials for organic dye degrada...
Chapter 8
Table 8.1 Summary of organic dye photocatalytic degradation by magnetic nanocomp...
Chapter 11
Table 11.1 Some of the MOF structures explored for visible light‐driven photocat...
Table 11.2 MOFs explored for photocatalytic reduction of greenhouse gas CO
2
unde...
Table 11.3 MOFs explored for the photocatalytic reduction of Cr(IV) under visibl...
Chapter 12
Table 12.1 The processes employed to reduce carbon dioxide to useful chemicals [...
Table 12.2 Reduction potential values for carbon dioxide. The value for hydrogen...
Table 12.3 Typical semiconductors used for the photoreduction of carbon dioxide.
Table 12.4 List of semiconductors with data on bandgap, valence band maximum, an...
List of Illustrations
Chapter 1
Chart 1.1 Classification of dyes.
Scheme 1.1 Schematic illustration for the generation of oxidative species ...
Figure 1.1 Crystalline structures of TiO
2
: (a) anatase, (b) rutile, and (c...
Figure 1.2 (a) and (b) Photocatalytic performances of L:TiO
2
and TiO
2
nano...
Figure 1.3 Schematic representation for the nanoparticles (NPs) and nanocl...
Figure 1.4 (a) SEM image, (b) FE‐TEM image, (c) STEM image, and (d–f) EDS ...
Figure 1.5 Synergistic mechanism between AC‐TiO
2
composites.
Figure 1.6 The reaction mechanism for photocatalytic degradation of methyl...
Figure 1.7 Relationship between methylene blue concentration and reaction ...
Figure 1.8 (a) AFM image, (b) TEM image, and (c) HR‐TEM image of GQDs. (d)...
Chapter 2
Figure 2.1 (a) Energy level diagram of TiO
2
/CdS. (b) Absorbance spectra. (...
Figure 2.2 (a, b) Morphological images, (c) concentration versus time curv...
Figure 2.3 (a, b) Morphological images, (c) concentration versus irradiati...
Figure 2.4 (a, b) High resolution transmission electron microscope images,...
Figure 2.5 (a) Energy level diagram of Bi
2
O
3
/Si‐TiO
2
, (b) transmission ele...
Chapter 3
Figure 3.1 Summary of various catalytic processes utilized in the area of ...
Figure 3.2 (a) A classical description of localized surface plasmon resona...
Figure 3.3 (a) UV‐vis spectra of Au
3+
to Au
0
conversion by 42 kHz ultrasou...
Figure 3.4 Schematic representation of Kohn–Sham one‐electron states and s...
Figure 3.5 Crystal structures of selected perovskite oxides.
Chapter 4
Figure 4.1 Classification of dyes based on their origin and their chemical...
Figure 4.2 Schematic representation of photocatalytic dye degradation mech...
Figure 4.3 A plausible degradation mechanism of eosin‐Y using hesperidin‐T...
Figure 4.4 Possible mechanisms for the photodegradation of eosin‐Y fragmen...
Figure 4.5 (a) UV‐vis spectral changes of FG using TiO
2
‐Ag
3
VO
4
during phot...
Figure 4.6 Magnetization curves of: (a) MnFe
2
O
4
, (b) MnFe
2
O
4
/g‐C
3
N
4
, and (...
Figure 4.7 (a) UV‐vis spectral changes of MO (10 ppm) using MnFe
2
O
4
/g‐C
3
N
4
Figure 4.8 (a) UV‐vis spectral changes of eosin‐Y (0.025 mM) in the presen...
Figure 4.9 A scheme showing an electron–hole transfer mechanism in CdS‐CuW...
Figure 4.10 (a) Photocatalytic activity of Ag
2
WO
4
, g‐C
3
N
4
, and Ag
2
WO
4
@g‐C
3
Figure 4.11 (a) UV‐vis spectral changes of MV dye using 3% BiOI‐ZrO
2
. (b) ...
Figure 4.12 (a) TEM image of YVO
4
. (b) TEM image of AgI. (c) The plausible...
Figure 4.13 (a) TEM image of ZnO nano‐triangles @ g‐C
3
N
4
nanofoils. (b) Re...
Figure 4.14 (a) and (b) HR‐TEM images of Fe
3
O
4
‐Ag
2
WO
4
. (c) The plausible e...
Figure 4.15 Photographs and diagram of an immersion‐type photoreactor.
Chapter 5
Figure 5.1 (A) Band gap excitation of the semiconductor under UV light ill...
Figure 5.2 Unit‐cell structure of cubic Ag
3
PO
4
, showing (a) ball and stick...
Figure 5.3 Schematic illustration of the possible formation mechanism of A...
Figure 5.4 (a) TEM image of Ag
3
PO
4
nanocrystals. Inset in (a) is the photo...
Figure 5.5 Scanning electron microscopy (SEM) micrographs of the (a) three...
Figure 5.6 (a) The crystal structure of Ag
3
PO
4
. Relaxed geometries for the...
Figure 5.7 (a) Schematic representation of the electron/hole (e
−
to ...
Figure 5.8 (a) Schematics of growth process of Ag‐P/m‐Ti‐X composites sphe...
Figure 5.9 Schematic representation of growth process of APO/NFO composite...
Figure 5.10 Schematic representation for the mechanism for the instantaneo...
Chapter 6
Figure 6.1 Change in the morphology of Ag nanoparticles in different reduc...
Figure 6.2 Combined piezo/solar‐photocatalytic process of Ag‐ZnO nanotetra...
Figure 6.3 Charge carrier dynamics in ZnO‐CdS‐Ag ternary systems.
Figure 6.4 Charge carrier dynamics in Ag‐ZnO@carbon sphere (CS) heterostru...
Chapter 7
Figure 7.1 Schematic illustration of a semiconductor particle‐based photoc...
Figure 7.2 Photocatalytic mechanism of BiOCl‐NiFe LDH composite under visi...
Figure 7.3 Mechanism using for adsorption‐assisted photodegradation of MO ...
Figure 7.4 Representation of detailed the interaction of dye with LDH and ...
Chapter 8
Figure 8.1 Cycling tests of photocatalytic degradation of MB using NiFe
2
O
4
Figure 8.2 Schematic representations of three different semiconductor magn...
Figure 8.3 (a) SEM image of NiAl LDH/Fe
3
O
4
‐RGO
25
. TEM images of (b) NiAl L...
Figure 8.4 (a) Reproducibility of NiAl LDH/Fe
3
O
4
‐RGO
25
in the photodegrada...
Figure 8.5 (a) Recycle experiments of degrading RhB (30 mg l
−1
) on F...
Figure 8.6 Regeneration studies of γ‐Fe
2
O
3
–TiO
2
nanoparticles after five c...
Chapter 9
Figure 9.1 The photoelectric effect.
Figure 9.2 Synthetic dyes and their classification.
Figure 9.3 Energy band gaps for various semiconductors.
Figure 9.4 Mechanism of photodegradation.
Figure 9.5 Illustrations of the different carbon nanotube structures (CNTs...
Figure 9.6 Noncovalent assembly of CdS‐SWNT hybrid nanomaterials with cont...
Figure 9.7 Characterization of the CdS/SWNT hybrids with controlled densit...
Figure 9.8 Photoresponses of the hybrid film. (a) Reversible change of I–V...
Figure 9.9 (a) The incident photon to current conversion efficiency (IPCE)...
Figure 9.10
C
/
C
0
versus time (in hours) plot for the photodegradation/de...
Figure 9.11 Schematic presentation of the charge separation and transfer o...
Chapter 10
Figure 10.1 Schematic overview of photogeneration of charge carrier electr...
Figure 10.2 (a, b) Typical FE–SEM images of g‐C
3
N
4
with different dimensio...
Figure 10.3 Schematic view of the detailed mechanism of pollutant degradat...
Figure 10.4 (a) XRD patterns of MnO
2
, g‐C
3
N
4
, and g‐C
3
N
4
/MnO
2
, (b) SEM ima...
Figure 10.5 (a) Comparison of the photocatalytic activities of TS, CN, and...
Figure 10.6 Schematic illustration of the charge carrier separation and tr...
Figure 10.7 The detailed mechanism of photodegradation of MB over the SnO
2
Chapter 11
Figure 11.1 (a) Organic‐containing and (b) metal‐containing secondary buil...
Figure 11.2 Some of the secondary building units (SBUs) and topologies rep...
Figure 11.3 (a) Structure of various MOFs [17] and (b) schematic illustrat...
Figure 11.4 Photocatalytic mechanisms in (a) NH
2
‐MIL‐125(Ti) [64] and (b) ...
Figure 11.5 (a) Phenol degradation by MOF‐5 (the
y
‐axis shows moles of phe...
Figure 11.6 (a) Photocatalytic mechanism in MIL‐53(Fe) [79] and (b) estima...
Figure 11.7 (a) Photographic images of (i) fresh NH
2
‐MIL‐125(Ti), (ii) TEO...
Figure 11.8 Photocatalytic mechanism of NH
2
‐UiO‐66(Zr) toward CO
2
reductio...
Figure 11.9 (a) Photocatalytic reduction of Cr(VI) over NH
2
‐MIL‐88B(Fe). (...
Figure 11.10 (a) Photocatalytic degradation profile of TC and (b) mechanis...
Chapter 12
Figure 12.1 Schematic diagram of photoexcitation and electron‐transfer pro...
Figure 12.2 One adsorption geometry for carbon dioxide that facilitates it...
Figure 12.3 Energy level diagram of layers and oxygen‐deficient WO
3
atomic...
Figure 12.4 Pd/TiO
2
on Nafion catalyst system for the photochemical reduct...
Figure 12.5 Conduction band potentials (open squares) and valence band pot...
Figure 12.6 Possible adsorption modes (physisorbed, chemisorbed with C or ...
Figure 12.7 Qualitative molecular orbital diagram for carbon dioxide.
Figure 12.8 Flow through nanotube array loaded with co‐catalyst for photoc...
Guide
Cover
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Photocatalytic Functional Materials for Environmental Remediation
Edited by
Alagarsamy Pandikumar
Functional Materials Division CSIR-Central Electrochemical Research Institute Karaikudi, Tamil Nadu, India
Kandasamy Jothivenkatachalam
Department of Chemistry, Bharathidasan Institute of Technology(UCE - BIT Campus)Anna University Tiruchirappalli, Tamil Nadu, India
Copyright
This edition first published 2019
© 2019 John Wiley & Sons, Inc.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions.
The right of Alagarsamy Pandikumar and Kandasamy Jothivenkatachalam to be identified as the authors of the editorial material in this work has been asserted in accordance with law.
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Library of Congress Cataloging‐in‐Publication Data
Names: Pandikumar, Alagarsamy, editor. | Jothivenkatachalam, Kandasamy, 1973‐ editor.Title: Photocatalytic functional materials for environmental remediation / edited by Alagarsamy Pandikumar (Functional Materials Division, CSIR‐Central Electrochemical Research Institute, Karaikudi, Tamil Nadu, India), Kandasamy Jothivenkatachalam (Department of Chemistry, Bharathidasan Institute of Technology (UCE ‐ BIT campus), Anna University, Tiruchirappalli, Tamil Nadu, India).
Description: First edition. | Hoboken, New Jersey : John Wiley & Sons, Inc., 2019. | Includes bibliographical references and index. | Identifiers: LCCN 2019008193 (print) | LCCN 2019021592 (ebook) | ISBN 9781119529910 (Adobe PDF) | ISBN 9781119529897 (ePub) | ISBN 9781119529842 (hardcover)
Subjects: LCSH: Photocatalysis. | Nanocomposites (Materials)–Environmental aspects. | Nanostructured materials–Environmental aspects. | Carbon dioxide mitigation.
Classification: LCC QD716.P45 (ebook) | LCC QD716.P45 P56 2019 (print) | DDC 628.5028/4–dc23
LC record available at https://lccn.loc.gov/2019008193
Cover Design: Wiley
Cover Images: © R.Tsubin / Getty Images, © Geir Pettersen / Getty Images,© Jacky Parker Photography / Getty Images,© Valentyn Volkov / Shutterstock, © djgis / Shutterstock
List of Contributors
Sambandam Anandan
Nanomaterials and Solar Energy Conversion Lab, Department of Chemistry
National Institute of Technology
Trichy, India
David Contreras
Centre of Biotechnology
University of Concepcion
Concepcion, Chile
Pazhanivel Devendran
Department of PhysicsInternational Research Centre
Kalasalingam Academy of Research and Education,
Krishnankoil, India
Trong‐On Do
Department of Chemical Engineering
Laval University
Quebec, Canada
Kumaraguru Duraimurugan
Solar Energy Lab, Department of Chemistry
Thiruvalluvar University
Vellore, India
Kiros Guesh
Department of Chemistry
Aksum University
Axum, Ethiopia
Raghavachari Kavitha
Department of Chemistry
Vijaya College (Affiliated to Bangalore University)
Basavanagudi, Bengaluru Karnataka, India
Hyun‐Seok Kim
Division of Electronics and Electrical Engineering
Dongguk University‐Seoul
Seoul, South Korea
Shivashankar Girish Kumar
Department of Chemistry, School of Engineering and Technology
CMR University
Bengaluru, Karnataka, India
Vignesh Kumaravel
Department of Environmental Sciences
School of Science, Institute of Technology Sligo
Sligo, Ireland
King‐Chuen Lin
Department of Chemistry
National Taiwan University Taipei, TaiwanandInstitute of Atomic and Molecular SciencesAcademia Sinica
Taipei, Taiwan
Jagannathan Madhavan
Solar Energy Lab, Department of Chemistry
Thiruvalluvar University, Vellore India
Ramalinga V. Mangalaraja
Advanced Ceramics and Nanotechnology LaboratoryDepartment of Materials Engineering
University of Concepcion
Concepcion, Chile
Lagnamayee Mohapatra
Energy Storage Department
Qatar Environment and Energy Institute
Doha, Qatar
S. Thangaraj Nishanthi
Electrochemical Power Sources Division
CSIR: Central Electrochemical Research Institute
Karaikudi, Tamil Nadu, India
Sachin V. Otari
Department of Chemical Engineering
Konkuk University
Seoul, South Korea
Dhananjaya Patra
Department of Chemistry
Texas A & M University
Doha, Qatar
Nalenthiran Pugazhenthiran
Advanced Ceramics and Nanotechnology Laboratory
Department of Materials Engineering
University of Concepcion
Concepcion, Chile
Nivea Raghavan
Department of Nanosciences and Technology
Karunya Institute of Technology and Sciences
Coimbatore, Tamil Nadu, India
Mohan Sakar
Centre for Nano and Material Sciences
Jain University
Bengaluru, India
Panneerselvam Sathishkumar
Department of Chemistry
Aksum University
Axum, Ethiopia
Sivaraman Somasundaram
Department of Chemistry
Kongju National University
Kongju, Republic of Korea
Channe Gowda Sushma
Department of Chemistry, School of Engineering and Technology
CMR University
Bengaluru, Karnataka, India
Meenakshisundaram Swaminathan
Department of ChemistryInternational Research Centre
Kalasalingam Academy of Research and Education
Krishnankoil, India
Sakthivel Thangavel
Key Lab of Advanced Transducers and Intelligent Control SystemMinistry of Education and Shanxi Province, College of Physics and Optoelectronics
Taiyuan University of Technology
Taiyuan, Republic of China
Jayaraman Theerthagiri
Centre of Excellence for Energy Research
Sathyabama Institute of Science and Technology
Chennai, India
Nagamalai Vasimalai
Department of Chemistry
B.S. Abdur Rahman Crescent Institute of Science & Technology
Vandalur, Chennai, India
Pitchaimani Veerakumar
Department of Chemistry
National Taiwan UniversityTaipei, Taiwanand Institute of Atomic and Molecular SciencesAcademia Sinica
Taipei, Taiwan
Gunasekaran Venugopal
Department of Materials Science, School of Technology
Central University of Tamil Nadu
Thiruvarur, Tamil Nadu, India
Balasubramanian Viswanathan
National Centre for Catalysis Research
Indian Institute of Technology‐Madras
Chennai, India
Hemraj M. Yadav
Department of Energy and Materials Engineering
Dongguk University
Seoul, South Korea
Preface
Increasing environmental concerns are driving a growing need for clean and renewable energy sources. Photocatalysis driven by visible light is a promising strategy which can be used in many applications, such as the removal of organic pollutants, hydrogen production, air purification, and biological studies. Photocatalysis has been considered to be one of the most promising technologies for the production of solar fuel as well as the mineralization of pollutants. As photocatalysts use photon energy, either from sunlight or from simulated illumination, they are relatively inexpensive, non‐toxic and ecofriendly. Upon illumination, the semiconductor photocatalysts undergo charge separation. Holes are produced in the valance band and electrons are promoted to the conduction band. These electrons and holes are then involved in redox reactions with adsorbed species.
Multifunctional photocatalytic materials are of interest in designing and constructing advanced light energy harvesting assemblies for both energy production and environmental remediation. In this book you will find recent developments in multifunctional photocatalytic materials, such as semiconductors, nanocomposites, quantum dots, carbon nanotubes, and graphitic materials, along with novel synthetic strategies and details of their physicochemical properties. These materials are suitable for the photocatalytic conversion of CO2 into solar fuels and value‐added products. Also, photocatalysts are used to generate H2 via the water splitting reaction and are used to remove contamination. The elaboration of molecular systems and interfaces for the conversion of CO2 into energy‐rich molecules is an important technical and environmental challenge, because the abundance of CO2 in the atmosphere contributes significantly to the greenhouse effect. In this perspective, important research activities are directed toward the preparation of photocatalysts containing: (i) a photosensitizer unit, which initiates photochemical one‐electron transfer events, and (ii) a catalyst, which stores reducing equivalents to achieve multi‐electron reduction of CO2 and produce fuels.
This book presents a collection of twelve chapters written by researchers who are the leading experts in their fields of study. In their chapters they explain the strategies to overcome the challenges in photocatalytic functional materials for environmental remediation. The first chapter of this book is a succinct summary of the state‐of‐the‐art of titanium dioxide and carbon‐based nanomaterials for the photocatalytic degradation of organic dyes in wastewater. Chapters 2 and 3 focus more on the aspects of visible light driven photocatalysts and their impact on energy and environmental applications. Chapters 4, 5, and 6 explore the plasmonic effect of the nanocomposites. Chapters 7, 8, and 9 discuss the details of the multifunctional hybrid materials and their applications. Chapters 10 and 11 address the key challenges in the fabrication of photocatalysts and give possible strategies to improve the efficiency of the photocatalysts. Chapter 12 describes the reduction of CO2 using functionally active materials.
Finally, we would like to express our sincere thanks to all the authors for sharing their knowledge on photocatalytic functional materials for environmental remediation. They have made it possible to produce this book for the benefit of those interested in visible light harvesting by functional materials and applications of this process. We are very grateful to all the authors whose chapters make this a valuable book.
Dr. Alagarsamy Pandikumar (Editor)
Scientist
Functional Materials Division
CSIR‐Central Electrochemical Research Institute
Karaikudi‐630 003, Tamil Nadu, India
Dr. Kandasamy Jothivenkatachalam (Editor)
Professor
Department of Chemistry
Bharathidasan Institute of Technology
Anna University (UCE – BIT Campus)
Tiruchirappalli‐620 024, Tamil Nadu, India
1Titanium Dioxide and Carbon Nanomaterials for the Photocatalytic Degradation of Organic Dyes
Nagamalai Vasimalai
Department of Chemistry, B.S. Abdur Rahman Crescent Institute of Science & Technology, Vandalur, Chennai, , India
Abbreviations
A
absorbance
AC
activated carbon
Ads
adsorption
AOP
advanced oxidation process
BC
brilliant blue dye
BG
brilliant green dye
C
dye concentration
CB
carbon black
cb
conduction band
CDs
carbon dots
CNFs
carbon nanofibers
CNTs
carbon nanotubes
CQDs
carbon quantum dots
DSAC
date stone‐activated carbon
ETAD
Ecological and Toxicological Association of Dyestuffs and Manufacturing Industry
eV
electron volt
GAs
graphene aerogels
GO
graphene oxide
GrF
graphite felt
h
Plank's constant
k
a
the equilibrium constant of the reactant
k
r
the specific reaction rate constant
L:TiO
2
lysozyme‐coated TiO
2
nanoparticles
LD
lethal dose
MB
methylene blue
MO
methyl orange
N‐doped CDs
nitrogen doped carbon dots
Ox
oxidation
PCNFs
porous carbon nanofibers
PHF
polyhydroxy fullerenes
PVA
polyvinyl alcohol
r
reaction rate for the oxidation of reactant
Red
reduction
RhB
rhodamine B
TiO
2
@C
TiO
2
@activated carbon nanocomposite
TiO
2
@CF
carbon felt supported TiO
2
vb
valence band
XRD
X‐ray powder diffraction
α
alpha
β
beta
λ
em
emission wavelength
λ
ex
excitation wavelength
1.1 Introduction
The global environment is being polluted by many toxins. In particularly, water pollution is a major concern, because water is very important to all living beings and accounts for around 70–90% of their body weight. Hence, the quality of water resources will directly affect the life of humans and other living beings. Industrial development is persistently connected with the water pollution. The World Bank estimates that 17–20% of water pollution is caused by the dyeing and textile industries. India is the second largest manufacturer of dyestuffs. Globally, ~106 tons of synthetic dyes are produced yearly [1]. It is estimated that worldwide annually 280 000 tons of textile dyes are disposed in the effluent of textile industries [2].
In ancient days, dyes were acquired from natural sources. For example, during the Roman empire, only ministers and kings wore purple dyed fabrics. During the middle ages, ruby‐red fabrics were reserved for the most important clergy. Most of the natural coloring agents are of inorganic origin (semiprecious stones, malachite, clays, minerals, and metal salts) or are organic dyes. Organic dyes include those of animal origin and those of plant origin.
There is a broad spectrum of different organic compounds with different physical and chemical properties. Among the different organic dyes, anthraquinone (e.g. madder root) is of special interest. Madder is a bright red‐colored traditional dyestuff. In eighteenth and nineteenth century, the red pants of Napoleon's army and British soldiers were dyed with madder [3]. Alizarin, lucidin, and other compounds are present in madder root extract, but lucidin has a mutagenic nature, which severely restricts the use of madder root [3].
Recently, the synthetic dyes have replaced the traditional natural dyes, due to their low cost and the vast range of new colors offered. In 1856, William Henry Perkin accidentally discovered the world's first synthetic dye. In the nineteenth century, 10 000 new synthetic dyes were developed [4].
Generally, the light‐absorbing functional groups in the dye molecules are called “chromophores.” Chromophores contain the hetero atoms N, O, and S, which contain non‐bonding electrons. The followings are the examples of chromophores: –N=N–, C=NH, –C=C–C=C–, =C=O, –NO2, CH=N–, N–OH, C=S, and NO–OH groups. The electron‐acceptor groups are called “auxochromes,” and which are generally present on the opposite side of the electron‐donor molecules and their important function is to increase the color. Indeed, the common meaning of the word auxochrome is color enhancer. Some auxochromes are –NH2, –HSO3, –OH, and –COOH. These groups can give a higher affinity to the fibers. The “chromogen” is a part of the chromophore structure, along with auxochrome, and aromatic structure (normally benzene, anthracene, or naphthalene rings). Generally, synthetic dyes exhibit different chemical and physical properties, which are dependent on their structural diversity.
Synthetic dyes can be classified as follows: acidic, basic, direct, metallic, dispersed, pigment, mordant, reactive, sulfur, solvent, and vat dyes (Chart 1.1) [4]. The synthetic dyes are classified based on their physical, chemical properties, and their structural functionalities (Table 1.1). The usage of synthetic dyes is based on their compatibility with the type of textile substrates that are being processed. Generally, acid, direct, and reactive dyes are anionic aqueous soluble dyes; basic dyes are cationic dyes; and disperse, solvent, and pigment dyes are non‐ionic dyes. More details of dyes and their applications are given in Table 1.1 [3].
Chart 1.1 Classification of dyes.
Table 1.1 Classification, characteristics, and applications of dyes.
Group of dyes
Characteristics and applications
Ref.
Acid dyes
Acid dyes are water‐soluble anionic dyes and are used for the dyeing of synthetic fibers, silk, wool, nylon, leather, modified acrylics, ink‐jet printing, paper, food, and cosmetics.
[
5
,
6
]
Basic dyes
Basic dyes are water‐soluble cationic dyes that can apply directly to cellulosic with no mordants. The positive charge is localized with an ammonium group and applied for dyeing of silk, wool, polyacrylonitrile, cotton, modified polyester, modified nylons, and tannin‐mordanted cotton.
[
7
,
8
]
Vat dyes
Vat dyes are water‐insoluble dyes. They are generally applied as soluble leuco salt, after reduction in an alkaline solution with hydrogen sulfide. The leuco is formed by the reoxidation to the insoluble keto structure. Vat dyes are used to dye cotton, linen, soap, and rayon.
[
9
,
10
]
Reactive dyes
Reactive dyes can directly react with the fiber molecules to form chemical bonds. They have simple chemical structures and are extensively used for the dyeing of cellulosic fabric and fibers.
[
11
,
12
]
Organic pigments
Organic pigments are used in cotton, cellulosic, paper, and blended fabrics. They are negatively charged compounds, made from rocks, minerals, plants, and animals.
[
13
,
14
]
Direct dyes
Direct dyes are water‐soluble anionic dyes that have a high affinity with cellulose fibers. These dyes are used to dye cotton, regenerated cellulose, leather, paper, nylon, and blends.
[
15
,
16
]
Dispersed dyes
Dispersed dyes are water‐insoluble, non‐ionic dyes. They require additional factors such as dye carrier, pressure, and heat to penetrate synthetic dyes. They are easily disperse in aqueous media wherever the dye is dissolved into fibers. Commonly used for the dyeing of synthetic fibers.
[
17
,
18
]
Azonic dyes
Azonic dyes contain one or more azo groups. These dyes are used in printing inks and pigments.
[
19
,
20
]
Solvent dyes
These dyes are water insoluble. Carboxylic acid, sulfonic acid, or quaternary ammonium groups can be present in their structure. Solvent dyes are used in solvent inks, wood staining, waxes, plastics, coloring oils, and gasoline.
[
21
,
22
]
Oxidation dyes
Oxidation dyes belong to three major chemical families – diamines, aminophenols, and phenols or naphthols. These dyes are used as colorant materials for hair dyeing.
[
23
,
24
]
Developed dyes
This type of dye can be diazotized and coupled on the fiber after applying to the fibers. They form shades that are faster to washing. The developed dyes are used to dye cellulosic fibers and fabrics.
[
25
,
26
]
Sulfur dyes
Sulfur dyes are made by heating heterocyclic aromatic compounds with species that release sulfur. Sulfur dyes are classified as sulfur bake, polysulfide bake, and polysulfide melt dyes. Disulfide bridges can be formed during the oxidation, because monomeric molecules become cross‐linked into large molecules forming disulfide bridges. Sulfur dyes have a good affinity with cellulose materials such as cotton, viscose, jute, and flex.
[
27
]
Indigoid dyes
This are expensive dyes and made from Tyrian purple. During the oxidation process of indigoid, phenylacetic acid is formed. Indigoid dyes are used in textiles, linen, cotton, and wool and dyeing of denim jeans jackets.
[
28
]
1.1.1 Impact of Dye Effluents on the Environment and Health
During the dyeing process around 10–50% of dyes stuffs are released into wastewater [19, 29]. Textile, paper, leather, cosmetics, food processing, drug, paint, printing, pigments, rubber and plastic industries [30] are the major source of harmful dyes and dyestuffs disposal. The Ecological and Toxicological Association of Dyestuffs and Manufacturing Industry (ETAD) has been estimated that out of 4000 screened dyes, 90% dyes that have lethal dose (LD50) values greater than the World Health Organization recommended level. Among the tested dyes, direct, basic, and di‐azo dyes show the higher toxicity rates [31]. Therefore, the disposal of untreated dyes and dye effluents discharged into the environmental water bodies will highly detrimental to water quality. The dyestuffs can interfere with the penetration of sunlight (visible light) into water, resulting in a hindrance of photosynthesis and a decrease in the dissolved oxygen level. The dye‐contaminated water also increases the biological oxygen demand. On the other hand, most of the synthetic dyes are soluble in organic solvents and these are harmful to the living organisms.
Further, synthetic dyes that include aromatic rings in their structure are regarded as toxic, xenobiotic, and carcinogenic [29,32–34]. Besides, this type of dye can transfer their toxicity into aquatic organisms and cause intense injuries to human beings, including allergy, dermatitis, cancer, skin irritation, and can affect the reproductive system and kidneys, brain, liver, and central nervous system [35]. Therefore, the degradation of toxic dyes in environmental and wastewaters is now very important to protect the living beings from their hazard [36].
Recently, several techniques have been used for the removal/degradation of dyes from industrial wastewater and environmental water, including chemical precipitation, reverse osmosis, conventional coagulation, electrodialysis, electrolysis, adsorption, ion‐exchange, and photocatalytic degradation [36]. Among the used different techniques, the photocatalytic method is the most efficient method to solve the environmental problem [31].
1.2 Principles and Mechanism of Photocatalysis
Photocatalytic degradation technique is an attractive choice for the degradation of organic dyes from wastewater. There are several methods available to produce hydroxyl radicals, e.g. Fenton‐based processes [37], ozone‐based processes [38, 39], and photocatalytic processes [40–43]. Among them, the photocatalytic process is an environmentally friendly process with significant advantages over other existing methods. The photocatalytic degradation method has been reported to be effective for the degradation of organic dyes from wastewaters and soils [44–49].
The photocatalytic decoloration of dyes is believed to take place according to the following mechanistic pathways. During the UV light irradiation on a catalyst, electrons are promoted to the conduction band (cb) from the valence band (vb). As a result, an electron–hole pair is produced (Scheme 1.1) [44].
Scheme 1.1 Schematic illustration for the generation of oxidative species in a photocatalyst.
where, e−cb and h+vb are the electrons in the conduction band and the electron vacancy in the valence band, respectively.
These entities could migrate to the catalyst surface, where they enter in a redox reaction with other species on the surface. Generally h+vb can react with surface‐bound H2O and produce ˙OH radicals. On the other hand, superoxide radical anions of oxygen are produced through the reaction of e−cb and O2 [50].
These reactions prevents the recombination of electron and hole that were produced in the first step. The ˙OH and O2˙ are produced in a similar manner, and then react with dye to cause the decoloration of dye.
The mechanism for the generation of oxidative species in a photocatalytic study is shown in Scheme 1.1.
Generally, the photocatalytic mechanism of dye degradation is of two types – the direct photocatalytic pathway and the indirect photocatalytic pathway – and these will be discussed next.
1.2.1 Direct Photocatalytic Pathways
The best example of direct photocatalytic mechanism is the Langmuir–Hinshel wood process.
1.2.1.1 The Langmuir–Hinshel Wood Process
This process is applicable for heterogeneous photocatalysis. The hole is trapped by the adsorbed dye molecule and forms the reactive radicals, which can decay as a result of recombination of electrons. The Langmuir–Hinshel wood expression is given below [51]:
where r is the reaction rate for the oxidation of reactant (mg/l min), ka is the equilibrium constant of the reactant (l/mg), kr is the specific reaction rate constant for the oxidation of the reactant (mg/l min), and C is the dye concentration.
1.2.1.2 The Eley–Rideal Process
In this process, photofragmentation and subsequently trapping of the holes was obtained due to the surface defects of the free carriers. The surface active centers (S) can react with dye (chemisorption) to form an adduct species (S–dye)+, which will be further decomposed or could be the recombined with electrons.
The reaction scheme is given below [52]:
Photogeneration of free carriers
Hole trapping by surface defects
Physical decay of active centers
Chemisorption
1.2.2 Indirect Photocatalytic Mechanisms
In this process, the photogeneration of electron–hole pairs will occur on the surface of the catalyst. Then, the hole is trapped by H2O molecules and leads to form HO˙ and H+ and the electrons are allowed to form H2O2, with further decomposition of more OH− radicals by their reaction with oxygen, which is supplied by the medium. Finally, the obtained HO˙ radicals are responsible for the oxidation of the dye molecules, which produce the intermediates CO2 and H2O [53, 54]. The detailed mechanism is given below:
Generally, the efficiency of the degradation of organic dyes can be measured by their absorption maximum value from UV‐vis spectrum, because organic dyes have a good light absorption properties. For example, methyl orange dye shows an absorption maximum at 463 nm. Therefore, the photocatalytic degradation process of methyl orange was monitored at 463 nm by UV‐vis spectrophotometry.
The photocatalytic degradation efficiency of organic dyes have been calculated by the following equation:
where A0 is the equilibrium absorption value of the organic dye solution and A is the absorption value of the organic dyes solution at a specific irradiation time.
Several nanomaterials are used for the photodegradation of organic dyes, including ZnO, Fe3O4, TiO2, ZnS, CdS, WO3, Fe2O3, Bi2WO4, and carbon nanomaterials [55, 56] etc., Among them, TiO2 and carbon nanomaterials have received much attention because of their fascinating physical and chemical properties.
1.3 Importance of Titanium Dioxide
Titanium dioxide (TiO2) is an interesting material for photocatalytic applications because of its photostability, ease of availability, biologically inertness, low energy consumption, high photocatalytic activity, low operating temperature, relatively high chemical stability, suitable flat band potential, etc. [57–60]. The photocatalytic activity of TiO2 is highly dependent on its surface and other physicochemical properties, such as crystal composition, particle size distribution, surface area, band gap, porosity, and surface hydroxyl density. Particle size is important for the photocatalytic studies, as the catalyst efficiency is related to the surface area. Lower particle size materials will have a greater surface coverage and increased number of active surface sites, which will enhance the expected activity [61, 62].
There are three different crystalline forms of TiO2 (anatase, rutile, and brookite) and show different physical and chemical properties. Detailed information about TiO2 is given next.
1.3.1 Rutile
Rutile is a commonly available crystalline structure of titanium dioxide (titania). The name “rutile” was given by Abraham Gottlob Werner (1800), and it means “reddish color mineral” in Latin [61, 63]. Rutile has lower molecular volume than the other two forms. Most TiO2 that comes from metamorphic and igneous rocks has the rutile crystal structure (Figure 1.1). The unit cell of rutile is a tetragonal cell and its optical band gap is ~3.0 eV [61, 64]. Rutile is used as a white pigment in paints, polymers, and paper. Rutile is the most commonly used form of TiO2 in the world [65–67].
Figure 1.1 Crystalline structures of TiO2: (a) anatase, (b) rutile, and (c) brookite.
Source: Reproduced from Ref. [58].
1.3.2 Anatase
Anatase is the second natural form of TiO2 (Figure 1.1). Anatase is named from the Greek Anatasis, which means “elongation.” Anatase is metastable at atmospheric temperature and pressure. Anatase can transform irreversibly to rutile at high temperature [68, 69]. Generally, anatase can be prepared by chemical method [70, 71]. The anatase band gap was reported to be ~3.1 to 3.4 eV [72]. This band gap difference is ascribed to the change in the particle size and semiconductor density carriers [73].
1.3.3 Brookite
Brookite is the third natural form of TiO2. Crystallographer and mineralogist Henry James Brooke (1771–1857) has been honored as the basis for the name brookite. Brookite has a orthorhombic (instead of tetragonal) structure and its band gap has been estimated to be ~3.3 eV [74–78] (Figure 1.1).
Anatase shows a higher photocatalytic activity than rutile [66, 67]. Possible explanations are as follows: the larger band gap of anatase reduces the light penetration that can be absorbed and increases electron transfer from the anatase to adsorbed molecules [64, 79]. Surface properties play an important role in the adsorption of molecules and consequent charge transfer to the molecule.
Surface properties can be subdivided as follows [61]:
chemical nature
electronic structure
interactions with molecules and surface defects
surface potential differences that can affect the charge transfer from the photocatalyst to the molecules [
80
].
1.4 Titanium Dioxide for the Photocatalytic Degradation of Organic Dyes
In recent years, TiO2 has been extensively used as a photocatalyst, due to its non‐toxicity, strong oxidation potential, and high photostability, etc. [81].
The rutile phase exhibits higher photocatalytic efficiency than the anatase phase. For example, Shiga et al. used TiO2‐modified electrodes to monitor the photoelectrochemical activities which showed that the anatase phase had a higher photoactivity than the rutile phase [82]. The observed high photocatalytic activities of the anatase phase are due to the greater hole trapping ability, which shows a lower recombination rate compared to the rutile phase [83, 84]. Therefore, anatase is considered as the most photochemically active phase of TiO2, due to the above‐mentioned effects combined [85].
These days, researchers are using various forms of TiO2, including TiO2 powder, metal doped TiO2, multi‐atom doped TiO2, carbon nanomaterials combined TiO2, etc., for the photocatalytic degradation of organic dyes [68, 86].
For example, Mulmi et al., have developed lysozyme‐coated TiO2 nanoparticles (L:TiO2) and confirmed the phase is anatase L:TiO2 by X‐ray powder diffraction (XRD). The size of the nanoparticles was reduced from 30 to 17 nm, and the band gap energy also was decreased from 3.3 to 3.1 eV when they used lysozyme. The synthesized L:TiO2 nanoparticles exhibit the better photocatalytic degradation performance of methylene blue (MB) and methyl orange (MO) under ultraviolet (UV) irradiation. Both the photocatalytic degradation reactions follow pseudo‐first‐order reaction kinetics (Figure 1.2) [87].
Figure 1.2 (a) and (b) Photocatalytic performances of L:TiO2 and TiO2 nanoparticles toward the photodegradation of methylene blue and methyl orange dye solutions, respectively. (c) and (d) Absorbance variations of methylene blue and methyl orange dye solution utilizing L:TiO2 nanoparticles. The insets show the color changes of the corresponding dye solution at different irradiation times.
Source: Reproduced from Ref. [87].
1.4.1 Approaches Enhance the Photocatalytic Activity of TiO2
There are some common limitations to the various semiconductors used in the photocatalytic degradation of dyes. Examples are low photostability and the irradiation of the catalysts in aqueous media leads to photocorrosion, subsequent elution of metal ions into the water, and, finally, the complete dissolution of the solid photocatalysts. The next limitation is the tendency to agglomerate the nanoparticles [88, 89].
TiO2 has many merits, such as high activity under UV irradiation, photostability, biological inertness, relative chemical stability, low operational temperature, low energy consumption, water insolubility under typical environmental conditions, disinclination to photocorrosion, and natural abundance (which makes it cheaper than other semiconductor photocatalysts) [90]. However, the application of TiO2 for the treatment of wastewater in industrially viable scale continues to face a series of technical challenges.
One of the fundamental problems that exists with the utilization of TiO2, in as far as decontamination of environmental pollutants and many other applications are concerned, is the fact that it has a relatively wide band gap (3.2 eV), which means it only displays photoactivity under ultraviolet irradiation at a wavelength of 387 nm. Only 4% of the solar energy incident on Earth can be used when TiO2 is used in wastewater treatment [91, 92]. The photocatalytic activity is determined by the transport property of photo‐excited carriers from the interior to the surface of photocatalysts, by the high rate of electron–hole recombination, and by low interfacial charge‐transfer rates of the photo‐generated carriers in TiO2 particles and results in a lowered quantum yield and inefficient photocatalysis [93, 94]. Various time‐resolved techniques are employed to evaluate the photo‐excited carrier dynamics of TiO2; the carrier lifetimes determined are extremely fast, ranging from picoseconds to microseconds [94]. Another issue that hinders the application of TiO2 on an industrial scale is the fine particle size of TiO2, combined with its large surface area‐to‐volume ratio and high surface energy, which makes it inclined to agglomeration and difficult to decant post‐treatment [89]. Therefore, it is important to enhance the photocatalytic efficiency of TiO2.
The approaches used to enhance the photocatalytic activity of TiO2 can be generalized as either chemical modifications or morphological (physical) modifications. Within the chemical and physical modifications, three strategies can be listed as:
(a) doping;
(b) surface modification;
(c) sensitization.
The modifications would ideally be achieved in the following ways:
(i) tuning the band gap;
(ii) reducing the rate of charge carrier recombination;
(iii) promoting the target reaction and increasing the surface‐active sites.
1.4.2 Metal and Multi‐Atom Doped TiO2
The photocatalytic activities of TiO2 have been improved after the surface modification of TiO2 with noble metals to reduce the electron–hole recombination [68,95–100]. Further, the scientists have proved that noble metal‐doped TiO2 shows high reproducibility and excellent stability. For example, Gupta et al. prepared Ag‐doped TiO2 and used it as a photocatalyst for the photodegradation of two different dyes – methyl red and violet 3. The Ag‐doped TiO2 decolorized >99% of violet 3 and 86% of methyl red [101].
Seery et al. also decolorized the rhodamine 6G dye using Ag‐modified TiO2 under visible light [102]. The authors confirmed that the observed highly efficient decolorization is due to the increase of visible light absorption in silver nanoparticles. Further, Gunawan et al. have observed the reversible photoswitching of silver nanoparticles on TiO2 surfaces [103]. Metallic silver nanoparticle‐modified TiO2 formed the Ag+ state during the exposure of visible light. These optical properties of silver nanoparticle‐modified TiO2 could be attractive for further tailoring of the band gap in TiO2 nanomaterials [104].
Many studies have revealed that co‐doping of TiO2 with metals and non‐metals can reduce the electron–hole recombination, which will effectively improve the photocatalytic activity of TiO2 [105]. Xing et al. have prepared the carbon and lanthanum co‐doped TiO2, using the hydrothermal method, and reported photocatalytic effects under UV and visible light irradiation [106]. Yan et al. have investigated the higher photocatalytic activity from TiO2–SiO2–NiFe2O4 for the degradation of methyl orange [107]. The observed higher photocatalytic activity is suspected to be due to the role of hydroxyl radical and electron–hole recombination of TiO2–SiO2–NiFe2O4.
Yang et al. synthesized Mo and C co‐doped TiO2 nanoparticles. They have reported that 1% of Mo–C4/TiO2 exhibited an excellent photocatalytic degradation of rhodamine B dye under visible light [108]. The synthesized Mo and C co‐doped TiO2 nanoparticles show higher photocatalytic activity than TiO2 and mono‐doped catalysts. The obtained higher photocatalytic activity is due to the synergistic effect between Mo and C, which helps to increase the absorption of visible light and affects the photo‐induced electron–hole separation.
The shape of TiO2 can also play an important role to enhance its photocatalytic activity. For example, nanocages, nanorods, nanotubes, nanosheets, nanobowls, nanobels, and nanosprings have been applied [109]. The higher photocatalytic activity observed in the case of TiO2 nanotubes is because of their tube diameter and thickness [110, 111]. However, much longer tubes cause a decrease in the photocatalytic degradation rate, whereas the higher thickness of the tube increased the photocatalytic activities. The obtained higher photocatalytic activities are attributed to the effective separation of electron–hole pairs and higher surface area of the nanotube [68,112–114].
A second method is the doping of dye sensitizer molecules on the TiO2 surface. Ikeda et al. reported TiO2‐coated phenolic compounds and used them as a photocatalyst under visible light [115]. Zhang et al. also modified TiO2 nanoparticles with catechol (4.0 wt% catechol/TiO2) without affecting the crystalline nature of TiO2 [116]. The catechol‐modified TiO2 enhanced the photocatalytic efficiency to degrade acid orange 7 dye, due to the surface complexation of catechol. Zhang et al. recently reported the selective oxidation of alcohols in the presence of anthraquinonic dye by TiO2 nanoparticles [117]. The same authors have proposed the mechanism for the formation of a dye radical cation, which was oxidized by the nitroxyl radical. Further, poly(aniline) [118] and poly(thiophene) [119] have also been used as TiO2 dopants and used for the degradation of the dyes.
Raliya et al. have synthesized TiO2, ZnO, and graphene oxide (GO), TiO2/ZnO, TiO2/GO, ZnO/GO, and TiO2/ZnO/GO nanomaterials. Then, they have applied them for the photocatalytic degradation of methyl orange (MO) dye [120]. The scheme for the degradation of MO is given in Figure 1.3. Initially, TiO2, ZnO, and GO are used for the degradation of MO. Among them, higher photocatalytic degradation was observed from ZnO nanoparticles, followed by TiO2 then GO. Then, TiO2/ZnO/GO, TiO2/ZnO, TiO2/GO, and ZnO/GO nanocomposites were used for the photocatalytic degradation of MO. TiO2/ZnO/GO showed a higher photocatalytic efficiency for the degradation of methyl orange dye. The photocatalytic degradation efficiency was further enhanced when the concentration of ZnO and GO were increased.
Figure 1.3 Schematic representation for the nanoparticles (NPs) and nanoclusters (NCs) mixed with methyl orange.
1.5 Carbon Nanomaterials for the Photocatalytic Degradation of Organic Dyes
Carbon nanomaterials have been extensively used in the field of catalysis because of their fascinating physical and chemical properties, such as chemical inertness, stability, adequate mechanical properties, excellent electron mobility, and high porosity [121–123]. Carbon‐based nanomaterials can enhance the catalytic activity, which can be much better than polymers. For example, carbon nanomaterials, including activated carbon, graphite, graphitized carbon, carbon blacks, nanotubes, fullerenes, and nanofibers, nanohorns, nanowalls, and mesoporous carbon [88,124–127], have been used for the photocatalytic degradation of organic dyes.
Many studies are available in the literature to confirm that the good photocatalytic efficiency of TiO2 and applied for different environmental applications [128, 129]. However, the photocatalytic efficiency of TiO2 is still not up to the mark to achieve very high levels of photocatalytic efficiency. Therefore, researchers are using different technologies to modify the band gap of TiO2 through creating oxygen vacancies, chemical doping, and surface modification, etc. [88,130–137].
They have been taken many efforts to improve the photocatalytic efficiency of TiO2. TiO2
