Materials Science in Photocatalysis [1 ed.] 0128218592, 9780128218594

Table of contents :
Front Matter
Copyright
Contributors
Preface
Fundamentals of photocatalysis: The role of the photocatalysts in heterogeneous photo-assisted reactions
Introduction
Basic principles of photocatalysis
Conclusions
References
Preparation of photocatalysts by chemical methodologies
Introduction
Synthesis of photocatalysts as powders by chemical methodologies: An overview
Precipitation and co-precipitation
Sol-gel process
Hydrothermal/solvothermal syntheses
Microemulsion technique
Solid-state reactions
Citrate complex method
Solution combustion synthesis (SCS)
Conclusions
References
Preparation of photocatalysts by physical methodologies
Introduction
Bulk photocatalysts
Spray pyrolysis
Flame syntheses
Flame hydrolysis
Flame spray pyrolysis
Sonochemical method
Microwave irradiation
Mechanochemical synthesis
Preparation of supported photocatalysts as thin films and coatings
Wet-coating technologies: Dip-coating and spin-coating
Chemical vapor deposition
Physical vapor deposition
Vacuum deposition
Sputtering
Arc vapor deposition
Pulsed laser deposition (laser ablation)
Spray pyrolysis deposition
Electrodeposition
Conclusions
References
Flame-based synthesis of oxide nanoparticles for photocatalytic applications
Flame-based synthesis of oxide nanoparticles
General properties
Flame hydrolysis
Flame spray pyrolysis
Analogous synthesis methods
Other flame-based techniques
Application of flame-based synthesis to photocatalysis
Titania-based catalysts
Other oxides
Flame-based synthesis of materials with potential photocatalytic applications
Conclusions
References
Methodologies of synthesis of titania and titania-graphene photocatalysts
Introduction to synthesis methodologies
Gas phase methodologies for TiO2 production
Chemical vapor condensation (CVD)
Chemical vapor deposition (CVD) and physical vapor deposition (PVD) synthesis
Thermal plasma synthesis (TPS)
Atomic layer deposition (ALS)
Arc discharge synthesis (ADS)
Laser pyrolysis synthesis (LPS)
Liquid phase methodologies for TiO2 synthesis
Sol-gel methodologies
Microemulsion synthesis: Micelle and reverse micelle methodologies
Hydrothermal method
Solvothermal synthesis
Sonochemical synthesis
Co-precipitation synthesis
Other noncommon synthesis methodologies
Solid-phase production for TiO2 synthesis
Mechanical attrition
Mechanochemical synthesis
Removal of solvents: Washing step
Filtering and washing
Centrifugation and washing
Dialysis
Heat treatment step
Drying
Calcination
Calcination atmosphere
References
Experimental methods in thermodynamic and kinetic studies on photocatalytic materials
Introduction
Band gap energy determination
Band edges and Fermi level
Semiconductor/metal
Semiconductor/semiconductor
Influence of light on the Fermi level
Experimental methods
Semiconductor/electrolyte interface
Methods of the flat band potential determination
Density of states and intraband gap states
Kinetics of photoinduced electron transfer
Time-resolved techniques
Conclusions
References
Photoelectrochemical characterization of photocatalysts
Introduction
Fundamentals on photocurrent spectroscopy
Photoelectrochemical behavior of semiconductor/electrolyte junction
Energetics of semiconductor/electrolyte junction
Conclusions
References
Application of EPR techniques in the study of photocatalytic systems
Electron paramagnetic resonance: Short introduction
g-factor
Hyperfine interaction
EPR line shape and signal intensity
EPR studies of nanocrystalline photocatalysts
EPR studies of dispersed photocatalytic systems
References
Approaching photocatalysts characterization under real conditions: In situ and operando studies
Introduction. Promises and challenges of in situ and operando studies in Photocatalysis
Operando FTIR for the study of photocatalytic processes
Sampling techniques and experimental set-ups for FTIR
Molecular insights on photocatalytic reactions obtained by means of FTIR studies
Raman and other vibrational spectroscopies
X-ray absorption spectroscopy
XPS
Magnetic resonance techniques: NMR and ESR
Concluding remarks
References
Titanium(IV) oxide
Titanium(IV) oxide
Polymorphs of titanium(IV) oxide
Methods for preparation of polymorphs of titania
Photocatalytic activities of anatase and rutile
Mechanistic analysis of photocatalytic reactions by action spectra [13]
Heterogeneity in Evonik P25 [21]
Isolation of anatase and rutile from P25
Crystalline composition of P25 [21]
Synergistic effect of anatase and rutile in P25 [21]
Amorphous titania
Structure of amorphous titania
Photocatalytic activity of amorphous titania [33]
Brookite titania and TiO2(B) [36]
References
Development of hight active visible light-responsive TiO2 photocatalysts by applying ion engineering techniques
Introduction
Ion engineering techniques
Visible light responsive TiO2 photocatalysts
Modification of the electronic properties of TiO2 semiconductor photocatalysts by a metal ion implantation method [7 ...
Preparation of transparent TiO2 thin film photocatalysts and the Cr-implanted TiO2 thin films operating under visibl ...
One-step preparation of the visible light responsive TiO2 thin film photocatalysts by a RF-magnetron sputtering depo ...
Conclusions
References
Semiconductor sensitizer composites for enhanced photoinduced processes
Introduction
Semiconductors in photocatalysis
The TiO2 photocatalyst
Other semiconductors (ZnO, WO3, g-C3N4)
ZnO
WO3
g-C3N4
Porphyrin and phthalocyanine sensitizers
Porphyrins (general properties and synthesis)
Phthalocyanines (general structure and synthesis)
Structure-related issues
Electronic, spectrochemical, and photochemical properties
UV-vis absorption and emission spectra
Singlet oxygen generation
Advanced composite (hybrid) photocatalysts
The TiO2PS systems
The essentials of the photosensitized process
Methods of catalyst preparation
Physicochemical characteristics of hybrid catalysts
Application in photoinduced processes
Elimination of pollutants from water
Mixed semiconductor matrices TiO2/WO3
Photocatalyzed oxidation in the gas phase
Photocatalyzed reduction of CO2
Photocatalyzed reduction of Cr(VI)
Photochemical syntheses (g-C3Ns)
Hydrogen production
References
Nanostructured-based WO3 photocatalysts: Recent development, activity enhancement, perspectives and applicati ...
Introduction
Recent developments
Morphology effect on the photocatalytic properties of WO3
Precursor effect on the morphology of WO3
Morphology and its influence on the degradation of organic pollutants
Influence of crystalline structure on its morphology and photocatalytic behavior
Nanostructures of WO3
Heterostructures of WO3
Photocatalytic activity enhancement of WO3
Conclusive remarks and perspectives
References
Photoactive systems based on semiconducting metal oxides
Introduction
Photocatalytic phenomena explained via electron paramagnetic resonance technique
N-TiO2
Ce-ZrO2
CeO2-ZnO
Conclusions
References
Iron oxide-based magnetic photocatalysts: Recent developments, challenges, and environmental applications
Introduction
Iron oxide-metal oxide photocatalysts
Iron oxide-carbon containing photocatalysts
Iron oxide-polymer photocatalysts
Conclusion
References
Strategic design and evaluation of metal oxides for photocatalytic CO2 reduction
Introduction: Greenhouse gases and climate change
Photocatalysis and photocatalytic CO2 reduction
Strategic design and evaluation of metal oxides in photocatalytic CO2 reduction
Application requirements, desired material properties, and characterization of the photocatalysts
Light absorption from metal oxides
Thermodynamics of the carbon dioxide molecule
Mechanism behind photocatalytic CO2 reduction
Structural characteristics of a metal oxide
Strategic testing and evaluation
Identification of products and evaluation of photocatalysts in CO2 reduction
Conclusions
References
Nanostructured sulfide based photocatalysts using visible light for environmental and energy purposes
Introduction
Tri- and tetrahedrally coordinated sulfides
Sulfides having sixfold coordinated cations
In2S3
The spectral response and photo-corrosion resistance of In2S3
Research on the mechanism of degradation of the rhodamine B dye
Two-photon processes with V-substituted In2S3
Photocatalytic generation of H2 with a hydrogenase-In2S3 combination
Photo-electrochemical production of O2 by a laccase-In2S3 electrode
A system related to In2S3: ZnIn2S4
SnS2
The spectral response of SnS2
Two-photon processes with V-substituted SnS2
Photoelectrochemical production of O2 by a laccase-SnS2 electrode
Conclusions
References
Synthesis of MoS2 materials for photocatalysis applications and pollution abatement
Introduction
Structures and properties of molybdenum disulfide (MoS2)
Preparation of MoS2
Mechanical exfoliation and chemical exfoliation methods
Hydrothermal synthesis
Chemical vapor deposition
Modification of MoS2
Graphene doping
Metal ion doping
Application of MoS2
Photocatalytic water-splitting
Photocatalytic degradation
Photocatalytic CO2 reduction
Summary and perspective
References
Heterogeneous photocatalysts based on iso- and heteropolytungstates
Photocatalysis by polyoxotungstates
Decatungstate photocatalysis
Heterogeneous polyoxotungstates as photocatalysts
POTs heterogenized on photocatalytically inert supports
Impregnation
Partial oxidation of hydrocarbons
Partial oxidation of alcohols
Degradation reactions
Entrapment by sol-gel procedure
Partial oxidation of alcohols
Degradation reactions
Ionic exchange: Resins and modified silica supports
Partial oxidation of hydrocarbons
Oxidative bromination of hydrocarbons
Degradation reactions
Occlusion in polymeric membranes
Immobilization into metal organic frameworks
C-C bond formation
Partial oxidation
Degradation reactions
POTs heterogenized on photocatalytically active supports
Conclusions
References
Heterogeneous photo-assisted catalytic hydration/dehydration reactions based on Keggin and Wells-Dawson type ...
Introduction
HPAs based on heterogeneous photocatalysts
References
Niobate-based perovskites: Characterization, preparation, and photocatalytic properties
Aims and scope
Introduction
Perovskites oxides
Historical background
Structural and compositional flexibility
Photocatalytic applications
Niobates perovskites
Compositional variety
Preparation methods and morphology
Photocatalytic performance
Perspectives
References
Insights on the photocatalytic performances of LaFeO3 synthesized by solution combustion synthesis
Introduction
Synthetic strategy
Physical-chemical characterization of LaFeO3
Photocatalytic applications and performances of LaFeO3
Peculiar behavior related to pretreatment issues
References
TiO2-copper zinc tin sulfide (CZTS) photocatalytic thin films for up-scalable wastewater treatment
Introduction
TiO2-CZTS thin film deposition
TiO2-CZTS thin film characterization and testing
The photocatalytic removal of pollutants
Conclusions
References
Graphene and graphene-oxide for enhancing the photocatalytic properties of materials
Short general introduction
Graphene and GO, preparation, composition, structure, behaviors
Graphene and graphene oxide
Graphene and GO synthesis
Graphene and GO structure
Physico-chemical characterization
Applications in photocatalysis
H2O reduction
CO2 photoreduction for CO and fuel production
Other photocatalytic syntheses
GO and rGO in hybrid materials for water depollution and disinfection
Synthesis of hybrid materials
GO loading and degree of reduction
Oxide nanoparticles and organic substrates
Photocatalytic membranes
References
Nanostructured composites based on Bi and Ti mixed oxides for visible-light assisted heterogeneous photocatalysis
Introduction
Influence of Bi2O3 loading on the properties of TiO2+β-Bi2O3 composites
Influence of TiO2 support on the properties of TiO2+β-Bi2O3 composites
Concluding remarks and research perspectives
References
Composite materials in thermo-photo catalysis
Thermo-photocatalysis: A brief outlook
Environmental applications in thermo-photocatalysis
Energy-related applications of thermo-photocatalysis
Synthesis of chemicals with thermo-photocatalysis
Conclusions
References
Plasmonic photocatalysis
Introduction
Synthesis of plasmonic photocatalysts
Mechanism clarifications on plasmonic photocatalysts
Charge transfer (plasmon-assisted photocatalysis)
Energy transfer (plasmon-assisted photocatalysis)
Plasmonic heating (plasmon-assisted catalysis)
Property-governed mechanism and activity
Applications
Environmental purification-Degradation of organic compounds
Environmental purification-Inactivation of microorganisms
Solar energy conversion
Other applications of plasmonic photocatalysts
New trends for activity and stability enhancements
Nano-architecture design
Hybrid nanostructures
Phonic crystal-based plasmonic photocatalysts
Summary and conclusions
References
Graphitic carbon nitride-based metal-free photocatalyst
Introduction
Organic semiconductors: A focus on g-C3N4
Photophysical and electrochemical properties
Synthesis and modification of g-C3N4
g-C3N4-based heterojunction photocatalysts
g-C3N4 as photocatalyst: Applications
Water photosplitting
Artificial photosynthesis, photocatalytic CO2 reduction
g-C3N4 in advanced oxidation processes
g-C3N4-based photocatalytic disinfection
Synthesis of value-added compounds through g-C3N4-based photocatalytic processes
g-C3N4 beyond (photo)catalysis: Analytical applications
Conclusions
References
Two-dimensional layered double hydroxide based photocatalysts for environmental clean-up and renewable energy ...
Introduction
Characteristics of LDHs as photocatalysts
Compositional structures
Interlayer structures
Defective structures
Electronic properties
Applications of photocatalytic systems
Photocatalytic degradation of organic compounds
Photocatalytic organic synthesis
Photocatalytic water splitting
LDHs for photocatalytic oxygen generation
LDHs for photocatalytic hydrogen generation
Photocatalytic CO2 reduction
Photocatalytic N2 fixation
Photocatalytic elimination of NOx
Conclusion and perspective
References
Crystalline two-dimensional organic porous polymers (covalent organic frameworks) for photocatalysis
Introduction
Overview of photocatalysis
Overview of covalent organic frameworks (COFs)
Principles of photocatalytic water splitting reaction
COF based photocatalytic reactions
Hydrogen evolution reaction (HER)
Organic transformations
Reduction of CO2 to CO
Conclusions
References
Photocatalytic membranes and membrane reactors for CO2 valorization
Introduction
Membrane reactors
Membrane as separator or extractor
Membrane as distributor
Membrane as contactor
Membrane Reactor configurations
Photocatalytic membrane solutions
Photocatalytic membrane reactors for CO2 reduction
Photocatalytic CO2 reduction in continuous membrane reactors: Case studies
Photoelectrocatalytic membrane reactor
Perspectives and challenges
References
Photocatalysis by metal-organic frameworks
Introduction
MOFs as photocatalysts
General photocatalytic mechanism
Photocatalytic hydrogen generation
Photocatalytic CO2 reduction
Benzyl alcohol oxidations
Pollutant degradation
Photoredox reactions
Conclusions and future perspective
References
Ti-based metal-organic frameworks for visible light photocatalysis
Introduction
Visible light photocatalysis of Ti-MOFs for selective organic transformations
Visible light photocatalytic selective oxidation of amines
Photocatalytic selective oxidation of alcohols
Miscellaneous photocatalytic selective transformations
Concluding remarks
References
Homogeneous photocatalysts immobilized on polymeric supports: Environmental and chemical synthesis applications
Introduction
Homogeneous photocatalysts
Immobilization of homogeneous photocatalyst
Homogeneous photocatalysts immobilized on synthetic polymers
Homogeneous photocatalysts immobilized on biopolymers
Summary and future prospects
References
Soluble bioorganic substances from compost as photosensitizers for a sustainable homogeneous and heterogeneou ...
Introduction
Types of SBOs: From the original biowaste to extraction methods
Composition and general features of SBOs
Applications of SBOs
SBOs in homogeneous and heterogeneous catalysis
Characterization of SBO-containing photocatalytic materials
Performance of SBO-containing materials in photocatalysis
References
Biology-inspired photocatalysis: Recent advances in biomimetic photocatalytic nanosystems synthesis and appli ...
Overview of bioinspired photocatalysis
Biomaterial-assisted photocatalysts synthesis
Photocatalysts by plant extract-mediated synthesis
Biosynthesis and bioaugmentation of photocatalysts
Photocatalysts by biotemplated synthesis
Biostructure-inspired photocatalysts
Artificial leaf
Other natural hierarchical architecture inspirations
Enzymes and enzyme-like photocatalysts
Biohybrids in photocatalysis
Exploitation of biomolecules: Natural pigments and vitamins
Peptides and polymer photocatalysts
Melanin/polydopamine-based hybrids in photocatalysis
Bionic leaf
Concluding remarks and outlook
References
Fungal resistance on photocatalytic ceramic surfaces: The ultimate role of the metal in the AgTiO2 photocatal ...
Introduction
Experimental
Materials
TiO2
AgTiO2
Photocatalyst characterization
Tiles industrial manufacturing
Characterization of photocatalytic tiles
Fungi resistance tests
Algicidal activity
Results and discussion
Samples characterization
Fungal resistance
Literature overview
Antifungal activity of TiO2-based photocatalysts in light condition
Antifungal activity of TiO2-based photocatalysts in dark condition
AgNPsTiO2 based photocatalysts as antifungal materials
Results
Algae resistance results
Conclusions
References
Index
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Z

Citation preview

Materials Science in Photocatalysis

Materials Science in Photocatalysis

Edited by Elisa I. Garcı´a-Lo´pez Professor, Department of Biological Chemical and Pharmaceutical Sciences and Technologies (STEBICEF), University of Palermo, Palermo, Italy

Leonardo Palmisano “Schiavello-Grillone” Photocatalysis Group, Department of Engineering, University of Palermo, Palermo, Italy

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2021 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-821859-4 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Susan Dennis Editorial Project Manager: Aera F. Gariguez Production Project Manager: Bharatwaj Varatharajan Cover Designer: Victoria Pearson Typeset by SPi Global, India

Contributors Numbers in parentheses indicate the pages on which the authors’ contributions begin.

Zahra Abbasi (235), Department of Chemistry, Faculty of Science, Shahid Chamran University of Ahvaz, Ahvaz, Iran Sambandam Anandan (283), Department of Chemistry, National Institute of Technology, Trichy, India Masakazu Anpo (171), Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka, Japan

University, Center for Membrane Technology, Aalborg, Denmark Vlasta Brezova´ (125), Institute of Physical Chemistry and Chemical Physics, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Bratislava, Slovak Republic Adele Brunetti (523), Institute on Membrane Technology (ITM-CNR), National Research Council c/o The University of Calabria, Rende CS, Italy V. Capucci (649), IrisCeramica Group, Fiorano M.se, Italy

K Aswani Raj (505), Department of Chemistry, IIT Dharwad, Dharwad, Karnataka, India

Erik Cerrato (221), Department of Chemistry and NIS Centre, University of Torino, Torino, Italy

Ana Bahamonde (83), Environmental Catalystis Engineering Group, Institute of Catalysis and Petrochemistry, Madrid, Spain

G. Cerrato (649), Department of Chemistry, Universita` degli Studi di Torino, Torino, Italy

F. Baldassarre (603), Biological and Environmental Sciences Department & UdR INSTM of Lecce, University of Salento; Institute of Nanotechnology, CNR NANOTEC, National Research Council, Lecce, Italy Giuseppe Barbieri (523), Department of Environmental and Chemical Engineering, The University of Calabria, Rende CS, Italy Zuzana Barbierikova´ (125), Institute of Physical Chemistry and Chemical Physics, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Bratislava, Slovak Republic E. Bernardini (357), Chemistry Department, Torino University, Torino, Italy C.L. Bianchi (649), Department of Chemistry, Universita` degli Studi di Milano, Milano, Italy Vittorio Boffa (385), Aalborg University, Center for Membrane Technology, Aalborg, Denmark C. Bogatu (371), Center for Renewable Energy System and Recycling, Transilvania University of Brasov, Bras¸ ov, Romania Fabrı´cio Eduardo Bortot Coelho (385), Chemistry Department, Torino University, Torino, Italy; Aalborg

Federico Cesano (385), Chemistry Department; NIS Interdepartmental Centre, Torino University, Torino, Italy G. Ciccarella (603), Biological and Environmental Sciences Department & UdR INSTM of Lecce, University of Salento; Institute of Nanotechnology, CNR NANOTEC, National Research Council, Lecce, Italy Juan C. Colmenares (575), Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland Jose C. Conesa (267), Institute of Catalysis and Petrochemistry, CSIC, Madrid, Spain Juan M. Coronado (139), Institute of Catalysis and Petrochemistry, CSIC, Madrid, Spain M. Covei (371), Center for Renewable Energy System and Recycling, Transilvania University of Brasov, Bras¸ ov, Romania F. Deganello (357), Institute for the Study of Nanostructured Materials (ISMN)—Italian National Research Council (CNR), Palermo, Italy Francesco Di Franco (115), Dipartimento di Ingegneria, Universita` degli Studi di Palermo, Palermo, Italy R. Djellabi (649), Department of Chemistry, Universita` degli Studi di Milano, Milano, Italy

xv

xvi

Contributors

A. Duta (371), Center for Renewable Energy System and Recycling, Transilvania University of Brasov, Bras¸ ov, Romania Dana Dvoranova´ (125), Institute of Physical Chemistry and Chemical Physics, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Bratislava, Slovak Republic

Cd. Universitaria, San Nicola´s de los Garza, NL, Mexico Sam Hseien-Y. Hsu (575), School of Energy and Environment, Hong Kong, China Shin-Ting Hwang (283), Department of Environmental Engineering and Science, Feng Chia University, Taichung, Taiwan

Gabriela Dyrda (183), Institute of Chemistry, University of Opole, Opole, Poland

Ana Iglesias-Juez (139), Institute of Catalysis and Petrochemistry, CSIC, Madrid, Spain

Maya Endo-Kimura (421), Institute for Catalysis (ICAT), Hokkaido University, Sapporo, Japan

Lakshmanan Karuppasamy (283), Department of Environmental Engineering and Science, Feng Chia University, Taichung, Taiwan

D.

Fabbri (357), Chemistry University, Torino, Italy

Department,

Torino

Irina M. Factori (341), Human and Natural Sciences Center, Federal University of ABC—UFABC, Santo Andre, SP, Brazil Marisol Faraldos (83), Environmental Catalystis Engineering Group, Institute of Catalysis and Petrochemistry, Madrid, Spain Junting Feng (485), State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, China Pablo S. Ferna´ndez (341), Chemistry Institute, State University of Campinas—UNICAMP, Campinas, SP, Brazil

Marcin Kobielusz (95), Faculty of Chemistry, Jagiellonian University, Krako´w, Poland Ewa Kowalska (421), Institute for Catalysis (ICAT); Graduate School of Environmental Science, Hokkaido University, Sapporo, Japan Anna Kubacka (409), Institute of Catalysis and Petrochemistry, CSIC, Madrid, Spain Joanna Kuncewicz (95), Faculty of Chemistry, Jagiellonian University, Krako´w, Poland Maria Kuznetsova (341), Human and Natural Sciences Center, Federal University of ABC—UFABC, Santo Andre, SP, Brazil

Marcos Ferna´ndez-Garcı´a (409), Institute of Catalysis and Petrochemistry, CSIC, Madrid, Spain

Xianjun Lang (561), Sauvage Center for Molecular Sciences, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, China

Belen Ferrer (543), Chemistry Department, Universitat Polite`cnica de Vale`ncia, Valencia, Spain

Enzo Laurenti (589), Chemistry Department, Torino University, Torino, Italy

Paolo Fornasiero (485), Chemistry Department, INSTM and ICCOM-CNR Trieste Research Unit, University of Trieste, Trieste, Italy Fernando Fresno (139), Photoactivated Processes Unit, Institute IMDEA Energy, Madrid, Spain R. Galli (649), DiSTAS-Department for Sustainable Food Process, Universita` Cattolica del Sacro Cuore, Piacenza, Italy Hermenegildo Garcı´a (543), Instituto Universitario de Tecnologı´a Quı´mica, CSIC-UPV, Universitat Polite`cnica de Vale`ncia, Valencia, Spain Elisa I. Garcı´a-Lo´pez (3, 13, 37, 235, 319), Department of Biological, Chemical and Pharmaceutical Sciences and Technologies (STEBICEF), University of Palermo, Palermo, Italy Dimitrios A. Giannakoudakis (575), Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland D.B. Hernandez-Uresti (211), Autonomous University of Nuevo Leo´n, Physical-Mathematical Sciences Faculty,

Xia Li (561), Sauvage Center for Molecular Sciences, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, China Leonarda F. Liotta (13), Institute for the Study of Nanostructured Materials, Palermo, Italy Wojciech Macyk (95), Faculty of Chemistry, Jagiellonian University, Krako´w, Poland Giuliana Magnacca (357, 385, 589), Chemistry Department; NIS Interdepartmental Centre, Torino University, Torino, Italy Giuseppe Marcı` (13, 37, 319), Department of Engineering, University of Palermo, Palermo, Italy Michele Mazzanti (301), Department of Chemical, Pharmaceutical and Agricultural Sciences, University of Ferrara, Ferrara, Italy Giuseppe Mele (183), Department of Engineering for Innovation, University of Salento, Lecce, Italy Arianna Melillo (543), Chemistry Department, Universitat Polite`cnica de Vale`ncia, Valencia, Spain

Contributors

xvii

Marco Minella (449), Department of Chemistry and NIS—Nanostructured Interfaces and Surfaces Interdepartmental Centre, University of Turin, Turin, Italy

Ilenia Rossetti (63), Chemical Plants and Industrial Chemistry Group, Department of Chemistry, Universita` degli Studi di Milano and INSTM Unit Milano Universita`, Milan, Italy

Claudio Minero (449), Department of Chemistry and NIS—Nanostructured Interfaces and Surfaces Interdepartmental Centre, University of Turin, Turin, Italy

D. Sa´nchez-Martı´nez (211), Autonomous University of Nuevo Leo´n, Civil Engineering Faculty— Ecomaterials and Energy Department, Cd. Universitaria, San Nicola´s de los Garza, NL, Mexico

Alessandra Molinari (301), Department of Chemical, Pharmaceutical and Agricultural Sciences, University of Ferrara, Ferrara, Italy Nikolaos G. Moustakas (255), Leibinz Institute for Catalysis (LIKAT), Rostock, Germany Vaishakh Nair (575), National Institute of Technology Karnataka, Mangalore, India Sergio Navalo´n (543), Chemistry Department, Universitat Polite`cnica de Vale`ncia, Valencia, Spain Bunsho Ohtani (159), Institute for Catalysis, Hokkaido University, Sapporo, Japan Sibila A.A. Oliveira (341), Human and Natural Sciences Center, Federal University of ABC—UFABC, Santo Andre, SP, Brazil L. Operti (649), Department of Chemistry, Universita` degli Studi di Torino, Torino, Italy Maria Cristina Paganini (221), Department of Chemistry and NIS Centre, University of Torino, Torino, Italy L. Palmisano (3), Engineering Department, University of Palermo, Palermo, Italy D. Perniu (371), Center for Renewable Energy System and Recycling, Transilvania University of Brasov, Bras¸ ov, Romania Albin Pintar (397), Laboratory for Environmental Sciences and Engineering, Department of Inorganic Chemistry and Technology, National Institute of Chemistry, Ljubljana, Slovenia

Monica Santamaria (115), Dipartimento di Ingegneria, Universita` degli Studi di Palermo, Palermo, Italy Patricia V.B. Santiago (341), Chemistry Institute, State University of Campinas—UNICAMP, Campinas, SP, Brazil Rudolf Słota (183), Institute of Chemistry, University of Opole, Opole, Poland Fabrizio Sordello (449), Department of Chemistry and NIS—Nanostructured Interfaces and Surfaces Interdepartmental Centre, University of Turin, Turin, Italy Juliana S. Souza (341), Human and Natural Sciences Center, Federal University of ABC—UFABC, Santo Andre, SP, Brazil G. Spigno (649), DiSTAS-Department for Sustainable Food Process, Universita` Cattolica del Sacro Cuore, Piacenza, Italy Taymaz Tabari (95), Faculty of Chemistry, Jagiellonian University, Krako´w, Poland Mai Takashima (159), Institute for Catalysis, Hokkaido University, Sapporo, Japan Masato Takeuchi (171), Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka, Japan

Jovana R. Prekodravac (575), Vinca Institute of Nuclear Sciences—National Institute of the Republic of Serbia, University of Belgrade Belgrade, Belgrade, Serbia

Maria Luisa Testa (589), Institute for the Study of Nanostructured Materials, Italian National Research Council, Palermo, Italy ´ lvaro Tolosana-Moranchel (83), Nanotechnology and A Integrated BioEngineering Centre, School of Engineering, Ulster University, Belfast, Northern Ireland, United Kingdom

Alessandra Bianco Prevot (357, 589), Chemistry Department, Torino University, Torino, Italy

Mateusz Trochowski (95), Faculty of Chemistry, Jagiellonian University, Krako´w, Poland

Tharishinny Raja-Mogan (421), Institute for Catalysis (ICAT); Graduate School of Environmental Science, Hokkaido University, Sapporo, Japan

Maria Laura Tummino (357, 589), Chemistry Department, Torino University, Torino, Italy

M Rajeswara Rao (505), Department of Chemistry, IIT Dharwad, Dharwad, Karnataka, India

Kunlei Wang (421), Institute for Catalysis (ICAT), Hokkaido University, Sapporo, Japan; Northwest Research Institute, Co. Ltd. of C.R.E.C., Lanzhou, P.R. China

Ba´rbara S. Rodrigues (341), Human and Natural Sciences Center, Federal University of ABC—UFABC, Santo Andre, SP, Brazil

Qian Wang (485), State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, China; Chemistry Department,

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Contributors

INSTM and ICCOM-CNR Trieste Research Unit, University of Trieste, Trieste, Italy Zhishun Wei (421), School of Materials and Chemical Engineering, Hubei University of Technology, Wuhan, China Jerry J. Wu (283), Department of Environmental Engineering and Science, Feng Chia University, Taichung, Taiwan Andrea Zaffora (115), Dipartimento di Ingegneria, Universita` degli Studi di Palermo, Palermo, Italy

ˇ erjav (397), Laboratory for Environmental SciGregor Z ences and Engineering, Department of Inorganic Chemistry and Technology, National Institute of Chemistry, Ljubljana, Slovenia Shuaizhi Zheng (421), Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, School of Materials Science and Engineering, Xiangtan University, Xiangtan, China

Preface Materials science has become an extremely broad and multidisciplinary field of research and the emerging technologies where it is applied in the recent years have been astonishingly increasing. Photocatalysis, as a branch of catalysis, has been one of these technologic fields, where the study of the materials science resulted particularly important. This book attempts to cover the main aspects in the very broad field of materials science addressed to materials applied in heterogeneous photocatalytic process of various nature. A very wide class of photocatalytic materials have been treated here in an attempt of cover both the main basic aspects to approach the study of photocatalytic materials and also examples of solid photocatalysts, their features and applications, without, of course, being exhaustive due to the plethora of materials and aspects which would be treated in such an effort. This book reports information on several types of materials used in heterogeneous photocatalysis both from a theoretical and an applicative point of view. We hope that the 6 sections comprising 37 chapters will provide a fairly broad overview for readers interested in this fascinating field of research. In addition to very popular bare and mixed metal oxides and sulfides such as TiO2, ZnO, WO3, MoS2, other more complex and innovative materials are proposed such as metal-organic structures, graphene and graphene oxide, magnetic materials, plasmonic nanoparticles, and 2D materials among others. The reader will be able to evaluate the advantages and disadvantages of using the various materials, taking into account not only their cost but also their (photo)stability, toxicity, and ease of preparation. Moreover, readers can find in the book different types of examples of photocatalytic reactions of oxidation and reduction carried out in liquid–solid and in gas–solid systems where the different heterogeneous photocatalysts would be suitable. The various materials presented have been tested, among other reactions, for pollutants degradation, chemical transformations including CO2 activation, organic synthesis and applications of biomimetic photocatalytic systems, fungal abatement in photocatalytic surfaces. We would like to underline that the authors have tried to deal with the topics so that the reader who wishes to deepen them can find the necessary literature among the numerous references cited in each chapter. We believe that a book that summarizes concepts and topics from a certain field that can then be further explored in the scientific literature is particularly useful for young researchers and PhD students, as well as for experienced researchers who are about to start research in that field. Finally, we want to thank all the colleagues and friends who enthusiastically joined us in writing the book, and a last but not least thanks goes to Elsevier and the staff who accompanied us with great professionalism and patience during this rewarding adventure. Elisa I. Garcı´a-Lo´pez and Leonardo Palmisano Every great advance in science has issued from a new audacity of imagination John Dewey, 1929

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Chapter 1

Fundamentals of photocatalysis: The role of the photocatalysts in heterogeneous photo-assisted reactions Elisa I. Garcı´a-Lo´peza and L. Palmisanob a

Department of Biological, Chemical and Pharmaceutical Sciences and Technologies (STEBICEF), University of Palermo, Palermo, Italy, b Engineering

Department, University of Palermo, Palermo, Italy

1 Introduction The contemporary world is facing energy shortages and environmental problems, which hinder the evolution of human civilization giving rise to various types of crises. The huge consumption of energy and environmental pollution from traditional industrial processes, along with the increasing need for synthetic chemicals to satisfy current requirements, are urgent issues to be addressed for the development of new foundations for a sustainable future. These critical challenges need solutions that require an important upgrade in energy sources and the conception of new generation processes and manufacturing technologies. Researchers, over the past few decades, have been studying several methods to understand how to overcome these problems. In this context, heterogeneous photocatalysis appears as one of the useful technologies to help these urgent efforts, and it can contribute to managing the conversion of solar energy into chemical energy in a sustainable way. This introductory chapter discusses some essential characteristics of heterogeneous photocatalysis by making connections with thermal catalysis. We hope that their analysis together with the explanation of some key concepts leads to better understanding the contents of the chapters in this book. In particular, the chapter highlights the importance of the features and parameters of solid photocatalysts.

2 Basic principles of photocatalysis When the term photocatalysis first appeared in scientific literature, photochemists focused on the aspects related to specific processes accelerated by light. For their part, researchers active in catalysis highlighted the catalytic aspects, focusing their attention on the characterization techniques typical of thermal catalysis and on the reaction processes and mechanisms that can occur on the surface of solid particles, minimizing the importance of light for triggering and continuing the reaction. Both approaches should be combined to fully understand the photochemical and catalytic aspects of a heterogeneous photocatalytic reaction. There has always been complete agreement in the scientific community regarding the concepts and definitions of catalysis [1], whereas agreement on these same aspects of photocatalysis has been reached only in the last years [2–4]. The term photocatalysis first appeared in literature as early as 1911 [5], but only Baly et al., for the first time in 1921, studied the phenomenon in which light accelerated a catalytic reaction [6]. The pioneering papers in which photocatalysis was defined as the process in which the combination of light and a solid catalyst was able to influence a reaction were published in 1964 by Doerffler and Hauffe [7, 8]. From then on, sporadic examples of photocatalytic reactions have been reported up until the work of Honda et al. on the photoelectrolysis of water and the photoelectrocatalytic reduction of carbon dioxide using titanium dioxide (TiO2) [9]. An acceleration of research on heterogeneous photo-assisted catalysis occurred from 1972, when Fujishima and Honda published a work on UV light-induced water splitting using a TiO2 photoanode, although in principle this work fits better in the field of photo-electrocatalysis [10]. Since then, considerable efforts have been made not only to understand the fundamentals of heterogeneous photocatalysis, but also to apply this technology to solving several environmental problems, using both artificial energy and sunlight. The removal of viruses and bacteria, the elimination of polluting substances, the Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00004-0 Copyright © 2021 Elsevier Inc. All rights reserved.

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production of hydrogen, the performing of green organic syntheses, and the reduction of carbon dioxide are among some of the potential applications of heterogeneous photocatalysis. In the 1990s, heterogeneous photocatalysis was considered among the “advanced oxidation technologies” and was mainly applied to the degradation of very toxic and nonbiodegradable species both in the vapor and liquid phase [11–13]. However, more recently photocatalytic reactions have been directed toward the formation of compounds with high added value under mild experimental conditions, although in water, the green solvent par excellence, it is difficult to obtain high selectivity and the solubility of most organic molecules is low [14]. After the document by Fujishima and Honda, a great number of articles have been published concerning the heterogeneous field of photocatalysis. In this situation, the proposals of terms and definitions to correctly articulate the photocatalytic investigations seem to be important tools [15–18]. However, a definitive “Glossary of terms used in photocatalysis and radiation catalysis” (IUPAC recommendations) was published with a final agreement in 2011 [19], although it should be considered that the rapid evolution of science could suggest updates and changes to terms and definitions found in that document. The definition of photocatalysis was accepted as the “change in the rate of a chemical reaction or its initiation under the action of ultraviolet, visible, or infrared radiation in the presence of a substance, the photocatalyst, that absorbs light and is involved in the chemical transformation of the reaction partners.” The definition of photocatalyst was established as: “substance able to produce, by absorption of ultraviolet, visible, or infrared radiation, chemical transformations of the reaction partners, repeatedly coming with them into intermediate chemical interactions and regenerating its chemical composition after each cycle of such interactions.” This last definition was very much related to that of the catalyst. Heterogeneous catalysts are solids or mixtures of solids that cause or accelerate a chemical reaction without being modified. Heterogeneous catalysts can be metals, oxides, metal salts, sulfides, zeolites, and perovskites. Heterogeneous photocatalysts are solid semiconductors that act catalytically under light radiation. In a heterogeneous photocatalytic process, the solid photocatalyst is excited by a light radiation with suitable energy. In this process, an electron in the valence band (VB) of the semiconductor is excited by the photo-irradiation toward the empty conduction band (CB) of the solid, which is separated by a band gap from the VB. The electron in the VB, being promoted, leaves a positive hole in the VB. The photo-generated electrons and holes (e
/h+) are responsible for the reduction and oxidation reactions, respectively, occurring at the surface of the photocatalyst. Indeed, species adsorbed on the surface of a photocatalyst would be either reduced or oxidized. The material is, in practice, a heterogeneous catalyst, albeit it is active only when it absorbs photons with a suitable energy generating the (e
/h+) couple. The photocatalyst, like the catalyst, remains chemically unchanged during and after the reaction, and accelerates a chemical reaction participating in the mechanism of the reaction but not in the overall chemical process. A heterogeneous photocatalytic reaction can be followed, and its occurrence can be ascertained by (1) measuring the consumption of the starting substrates, (2) determining the reaction products once the irradiation has started, and (3) verifying the actual role of the solid as a catalyst by checking if its properties have been modified during the photoreaction. The relationship between photocatalysis and catalysis was addressed by Childs and Ollis, who questioned the actual nature of the photocatalyst and how to correctly compare the activity of the different photocatalytic materials [20]. A catalytic reaction can be divided into the following steps: (1) diffusion of the reactants from the fluid phase to the catalyst surface, (2) adsorption of the reagent(s), (3) reaction occurring in the adsorbed phase, (4) desorption of the product (s), and (5) its (their) transfer to the bulk of the fluid phase [21]. The steps that should be considered in a heterogeneous photocatalytic process are similar, but (2) and (3) seem quite different from those expected in thermal catalysis. This is because in a photocatalytic process the reactants are generally only physically adsorbed on the surface because the reaction is carried out at low temperature. After adsorption, several reactions and physical processes activated by light occur, including possible photo-adsorption and photo-desorption stages. On the contrary, in a catalytic process the adsorption can itself be considered a thermally activated chemical reaction, and it generally involves a chemisorption where at least one reactant is destabilized and gives rise to the desired product(s) [22]. Heating is not necessary for the photonic activation of a solid photocatalyst, but it is required in catalysis to activate the chemisorption of the reactant on the surface of the catalyst, its transformation, and its subsequent desorption. The main difference between a catalyst and a photocatalyst is that in thermal heterogeneous catalysis the solid is ready to perform its function, while in photocatalysis it must be “activated” by light generating hole-electron pairs before being considered a photocatalyst. Furthermore, the activity of the photocatalyst is maintained only by the continuous absorption of light. It is worth noting here that the photo-adsorption is a process due to the light absorption by the solid surface according to the following mechanism [23]: S + hn ! S∗

(1)

Fundamentals of photocatalysis Chapter

1

5

S∗ ! S

(2)

S∗ + R ! Rads

(3)

where S is the photo-adsorption center onto the photocatalytic surface, S* is the active state of the photo-adsorption center, R is the reactant molecule in the fluid phase, and Rads is the photo-adsorbed substrate onto the active site of the solid photocatalyst. The physical steps reported in (1)–(3) represent the initial steps in a catalytic photo-assisted reaction, that is, the radiation absorption by the solid photocatalyst (1) and (2) and the successive adsorption of the substrate (3). The irradiation of the solid surface modifies the properties of the photocatalyst surface [24, 25]. One can mention, for example, the hydrophilic/hydrophobic nature of the surface of the semiconductors, which become more hydrophilic (smaller contact angle) when irradiated and gradually go back to a more hydrophobic state when left in the dark or exposed to visible light [26]. In particular, modification of the properties of the surface is reversible and the surface returns to its original state in the absence of irradiation. After the activation of the photocatalyst due to absorption of light of suitable energy, the photocatalytic process takes place in some pure physical or chemical stages. In fact, as described previously for the catalytic cycle, the reactant(s) must reach the external surface of the photocatalyst, spread in the pores, photo-adsorb, and photoreact and therefore the product (s) must be desorbed. The main stages occurring in a photocatalytic process on the “activated-by-light” semiconductor particle surface are (1) the diffusion of reactant(s) to the photocatalyst surface through the boundary layer, (2) diffusion of reactant(s) from the reacting system into the pores of the photocatalyst, (3) photo-adsorption of reactant(s) on the pore surface, (4) photo-oxidation and photo-reduction reactions, (5) photo-desorption of the reaction product(s) from the pore surface, (6) diffusion of the product(s) out of the pores, and (7) diffusion of the products(s) away from the surface of the photocatalyst through the boundary layer. The diffusion processes may be easily affected by modifying the fluid dynamics of the system, while adsorption and desorption of the reagents inside of the pores and from the pores could be addressed by the textural properties of the solid photocatalyst, mainly the size and shape of the pores. The adsorption and desorption induced by light phenomena, whose extent depends on the type of species involved, are difficult to be quantified. The measurements of light absorption by the solid photocatalyst in dark conditions are only indicative of what would occur in the presence of light. This is an important issue for explaining the overall mechanism of a photocatalytic reaction. In addition, photo-adsorption and photo-desorption effects overlap with the disappearance or the appearance of the species due to the reaction steps, influencing their measured concentration values in the gaseous fluids or in the bulk of the liquid. Consequently, it is difficult to evaluate the extent of physical adsorption under irradiation due to the concurrent disappearance of reagent(s) during the photocatalytic reaction. As reported in Fig. 1, the photocatalyst, in this scheme exemplified by a TiO2 particle, absorbs a photon generating an electron/hole couple (step 1) that recombines (step 2) or gives rise to oxidation and reduction reactions (steps 3 and 4) on the surface or very close to it. The reagent(s) present on the fluid can adsorb more or less strongly depending on their chemicalphysical properties in relation to the surface of the catalyst. The impinging light produces electron-hole pairs if the thermodynamics constraints are fulfilled. Holes can be trapped by donor species as water molecules forming hydroxyl radicals (∙OH). Contemporaneously, electrons reduce acceptor molecules as O2, which also can produce hydroxyl or hydroperoxide radicals. These oxidant species formed in both oxidant and reductive paths attack generally unselectively the reagent(s)

Ox

6 e-

4

Ti

Red 2

1 Ox

Ti h+ HO

1

7

1

2

5

3 Red

CO2 + H2O

2

FIG. 1 Steps of heterogeneous photocatalysis in a TiO2 particle. (Adapted from C.B. Mendive, D. Hansmann, T. Bredow, D. Bahnemann, New Insights into the mechanism of TiO2 photocatalysis: thermal processes beyond the electron–hole creation, J. Phys. Chem. C 115 (2011), 19676, with permission of the American Chemical Society.)

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transforming it into intermediates and into the final products by means of various consecutive steps, which are resumed in step 5. The surface of the photocatalyst itself can trap electrons and holes, and Ti(IV) and/or Ti-OH sites, for instance in the case of TiO2 surface, can respectively play this role (steps 6 and 7). The features of a good photocatalyst include chemical and biological inertness, stability toward light irradiation (absence of photo-corrosion), easy and cheap production, and being harmless to the environment and living beings. The most attractive photocatalyst should be activated by natural solar light. Among all the semiconductors tested, the most popular has been TiO2. The majority of photocatalysts are inorganic solid materials, generally oxides that exist in various crystalline phases. Nevertheless, sulfides, metal salts, zeolites, and perovskites have also been used. More recently, some organic semiconductors such as boron nitride (BN) or carbon nitride solid (C3N4) have been studied as well. The subsequent chapters propose a wide range of examples. A heterogeneous photocatalyst should be a semiconductor possessing a crystalline structure. The absence of crystallinity prevents the solid to be used as photocatalyst. Indeed, only a crystalline semiconductor may absorb radiation of suitable energy (equal to or greater than the bandgap) generating efficient electron-hole pairs. The light absorption and the formation of photoproduced couples only occur in the spatially ordered atoms of a crystal, in which the radiation can enter in resonance. Amorphous phases absorb radiation, but the separation of electrons and holes that are responsible for surface redox processes is impossible. The crystalline morphology of the photocatalyst significantly influences the optical, chemical, and topographical characteristics of its surface, and consequently its activity. The ability to guarantee an optimization of these factors is the key to obtaining highly performing photocatalysts [27]. An interesting further aspect has been highlighted by some authors who report that 3D networks can be formed by some nanoparticles of metal oxides, which can work as photocatalysts. In the 3D network the particles are not ordered randomly, but their atomic planes are aligned in a precise way so that their contact can allow the transfer of the e
/h+ pairs, reducing greatly the interfacial trapping processes. This effect is called “antenna effect” [28]. Electrons and gaps can move to/on the surface and be trapped. The presence of a “cooperative network” in place of individual photocatalyst particles improves the photocatalytic activity of the entire system. Many reactions can be performed both catalytically and photocatalytically, but the two mechanisms are generally different [2, 13, 29]. The catalytic and photocatalytic processes can also occur simultaneously [30]. We have seen that the photocatalyst should (photo)adsorb two types of reactants, one to be oxidized and the other to be reduced, in order to assure the electroneutrality of the photocatalytic cycle. A photocatalytic reaction can produce partial and total oxidation reactions or give rise to a synthesis, depending on the negative or positive sign, respectively, of the variation of Gibbs free energy (DG) of the reaction. The photoinduced electron-hole carriers’ mobility, to react with the species adsorbed on the solid photocatalyst surface, mainly depends on two factors: the band energy positions of the semiconductor and the redox potential of the adsorbates. The higher energy level of the valence band (HOMO) determines the hole-oxidizing ability, whereas the lower energy level of the conduction band (LUMO) stands for the electron reduction potential. These two values measure the possibility of the photocatalyst to promote oxidations and reductions, respectively. An oxidation reaction is more favored the more positive the position of the VB, while a reduction reaction is more favored the more negative the position of the CB. In Fig. 2 the band positions for some semiconductors used often in heterogeneous photocatalysis are reported together with the reduction potentials of H2O to O2 and H+ to H2.

FIG. 2 Band positions and potential applications of some typical photocatalysts (at pH 7 in aqueous solutions).

No · OH or O2 -·

Photocatalysts possessing strong reduction abilities. Useful for CO2 reduction and/or H2 evolution

Potential vs. NHE [V]

ZnS SiC -3 TaON Cu2 O C3 N4 TiO2 SrTiO TiO2 CdS TaN5 Si Bi2 S3 3 ZnO -2 Anatase BiVO4 Rutile WO3 SnO2 -1 Fe2 O3 0 +1 +2 +3 Photocatalysts useful for Photocatalysts possessing strong overall water splitting at pH7 oxidation abilities. Useful to oxidise pollutants (by means of · OH)

2H+/H2 (-0.41V) H2 O/O2 (+0.82V)

Fundamentals of photocatalysis Chapter

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7

From a thermodynamic point of view, the reduction of the oxidized species of an adsorbed couple whose redox potential is more positive than the flat band potential of the CB is allowed, whereas the reduced species of a couple whose redox potential is more negative than the flat band potential of the VB can be oxidized. Fig. 2 depicts this situation, where three families of photocatalysts are illustrated in red, blue, and green. The red group includes those semiconductors possessing a very oxidant VB, and thus in the aqueous suspension they assure the formation of holes and therefore OH radicals (also O2
∙ and H2O2), which induce oxidation reactions. The formation of OH radicals occurs because the potential of the VB is more positive than that of the OH radical formation (see Fig. 2). The green group includes those photocatalysts with more negative CB positions; the photo-generated electrons produced under illumination of these materials have a strong reduction ability, and H2 evolution from H2O and/or CO2 reduction are possible. In other words, they can photo-reduce CO2 and give rise to water photo-splitting from a thermodynamic point of view. It should be remembered, however, that the actual occurrence of these mentioned processes is connected to many factors that influence the theoretical thermodynamic values shown in Fig. 2. For example, the presence of interband states between VBs and CBs hinders the photocatalytic reaction because these states favor electron-hole recombination; the lifetime of the photo-produced pairs decreases, preventing them from reaching the surface of the photocatalyst where the redox reaction takes place. From a kinetic point of view, the efficiency of the photocatalyst is maximum when the rate of the e
/h+ deactivation process (i.e., their recombination in the bulk or on the surface) is lower than that of these photo-produced pairs with the adsorbed substrates. The chemical reaction, involving the photo-adsorbed substrates is the most important step. As mentioned, heterogeneous photocatalysis can drive thermodynamically uphill reactions, for example, the splitting of water and reduction of CO2. Notably, differently to what occurs in catalysis, a photocatalytic process can come about also for reactions with DG > 0 that, under catalytic conditions, cannot occur to a significant extent unless energy is supplied. In the case of photocatalytic reactions, this energy is provided in the form of photons, as in the case of natural photosynthesis driven by sunlight. As mentioned, the efficiency of a heterogeneous photocatalytic reaction depends on many chemical and physical properties of the photocatalyst, for example, the presence of acid and basic sites, zero charge point, hydroxylation degree of the surface, specific surface area, shape and size of the particles, crystallinity, and porosity. An optimal combination of different properties such as activity, selectivity toward products with high added value, preparation with a nonpolluting method and ease of regeneration, long lifetime, nontoxicity, and low cost is also important. The relative weight of these properties is different, depending on the type of photocatalytic reaction to be performed and on the system (gas-solid or liquid-solid), photoreactor, and set-up chosen. Also, when designing a photocatalyst it may be necessary to find a compromise between the different properties. The different synthetic methods could concern some properties compared to others, thus favoring the required performances. It could be useful to remind here the most significant parameters used to assess the performance of a heterogeneous photocatalytic system, as defined by IUPAC [19]. All of them depend on the characteristics of the photocatalyst together with those of the set-up system used. Four parameters can be defined: quantum yield (F), photonic efficiency (x), turnover frequency (TOF), and turnover number (TON) [19]. The F is calculated as the number of product species formed in a photocatalytic process or, alternatively, the number of molecules of a reacted reagent (defined events, according to IUPAC), divided by the number of photons at a certain wavelength, absorbed by the system. This quantity has a well-quantified value only when the reaction conditions are also established, and it is necessary to know the quantity of photons absorbed by the solid photocatalyst. To simplify the quantification of the parameter, we can only consider the impinging radiation on the system before absorption. The x is commonly used in heterogeneous photocatalysis instead of F. Its definition is similar to that of F, but a specified wavelength range is considered instead of a certain wavelength. The TOF is the number of photoinduced transformations (product formed or reactant consumed), per catalytic site and per period of time. In heterogeneous photocatalysis, the number of active sites on the surface of the photocatalyst is often unknown, and the TOF value is determined by considering the surface area, although the comparison of the obtained values for photocatalysts prepared by using different methods is questionable. The TON is the number of times that the photochemical transformation goes through a photocatalytic cycle. This parameter is widely used in catalysis, but is quite descriptive for a photocatalytic process because the irradiation conditions influence the reaction rate and, for the reaction to occur, incident photons of adequate energy must be absorbed by the semiconductor, which usually consists of polycrystalline porous nanoparticles. Therefore, it is necessary to carefully measure the extent of the absorption of photons [31]. An open debate on the experimental methodologies that should be used to obtain these parameters is still ongoing [32]. Determination of the kinetics in photocatalytic reactions also requires careful determination of the radiation intensity field within the reactor. In fact, it has been experimentally confirmed that the intensity of light determines the reaction rate

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[33] because the absorption of the photons by the photocatalyst is, as previously mentioned, the main stage in a photocatalytic process. In this context, it can be emphasized that the results of photo-reactivity depend not only on the geometry and size of the reactor but also on some characteristics of the photocatalyst such as particle size, morphology, and illumination modes [34]. Since photocatalytic reactions occur on the surface [35], the driving forces are expressed on the basis of (1) the surface concentration of the charge carriers, which are a function of the absorbed light and (2) the surface concentration of the species to be oxidized and reduced. The dependence of the reaction rate, r, on the intensity of light can be represented with a power law. For low light intensity, the dependence of the rate on the photon flow is linear. This confirms the photo-induced nature of the activation of the catalytic process, with the participation of photo-induced electric e
/h+ charges in the reaction mechanism. However, above a certain value (ca. 250 W/m2) the reaction rate becomes proportional to the square root because the formation rate of the e/h+ pairs exceed the reaction rate, and their recombination is favored. The optimal use of light power corresponds to the domain in which r is proportional to the radiant flux [36]. Generally, the initial reaction rate, r0, increases with the concentration of the substrate and the kinetics can be considered pseudo-first order, tending to the limit value of order zero [36, 37]. This behavior can be interpreted by the LangmuirHinshelwood (LH) model considering that the species to be reduced and oxidized are adsorbed on the surface of the photocatalyst at different sites. Notably, other models in addition to the LH one have been used to study the kinetics of the photocatalytic reactions, but it is not aim of this chapter to present them. The LH model, according to which the reaction must involve surface sites induced by light absorption and by the presence of photo-adsorbed species, contemplates two essential parameters: the surface kinetic constant and the equilibrium adsorption constant, which refers to photo-adsorption/photo-desorption equilibrium phenomena. It is important to underline that both of these may be determined by the fitting applied to photo-reactivity data [38, 39] and depend on the local radiation intensity and on the characteristics of the photocatalyst. The high-performance applications of the photocatalytic reaction are linked to the possibility that the photocatalyst is made up of several crystalline phases of the same compound or that it is a composite made up of different materials whose contact creates a heterojunction, which plays a key role by improving the separation of the photoproduced pairs increasing their lifetime. In that case it is even more difficult than in the case of a single catalyst to rationalize the phenomena of photoadsorption/photo-desorption, due to the presence of different types of surfaces with different features. Notably, the size of the crystallites, the specific surface area, and the porosity are crucial parameters to guarantee good accessibility to the species to be adsorbed and reacted on the surface; in particular, the high-surface materials provide a great interface for any type of photocatalytic reaction with high-performance applications.

3

Conclusions

Heterogeneous photocatalysis has emerged as a technology that can help solve some urgent energy and environmental problems. This chapter discussed how the choice of materials generally used as catalysts can influence the efficiency of photocatalytic reactions; more details can be found by reading the various chapters of this book. Among the main advantages of a photocatalytic process are the mild experimental conditions in which the reactions are carried out, often at atmospheric pressure and at room temperature using cheap and nontoxic solid photocatalysts, and the potential exploitation of sunlight. The main parameters that influence the photocatalytic performance are in close relationship with the characteristics of the material used as a photocatalyst. An accessible interface with the appropriate functionality plays a vital role in the whole process. Many materials have traditionally been used as heterogeneous photocatalysts, such as bare and loaded/ doped inorganic semiconductors, glasses, fabrics, polymers, and various types of composites that often include or are supported on stoichiometric and nonstoichiometric oxides. In addition, many new innovative strategies have been designed in recent decades, as reported in the coming chapters of this book.

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[6] E.C.C. Baly, I.M. Heilbron, W.F. Barker, Photocatalysis. Part I. The synthesis of formaldehyde and carbohydrates from carbon dioxide and water, JCS Trans. 119 (1921) 1025–1035. [7] W. Doerffler, K. Hauffe, Heterogeneous photocatalysis I. The influence of oxidizing and reducing gases on the electrical conductivity of dark and illuminated zinc oxide surfaces, J. Catal. 3 (1964) 156–170. [8] W. Doerffler, K. Hauffe, Heterogeneous photocatalysis II. The mechanism of the carbon monoxide oxidation at dark and illuminated zinc oxide surfaces, J. Catal. 3 (1964) 171–178. [9] T. Inoue, A. Fujishima, S. Konishi, K. Honda, Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders, Nature 277 (5698) (1979) 637–638. [10] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [11] G. Marcı`, L. Palmisano (Eds.), Heterogeneous Photocatalysis, Relationships with Heterogeneous Catalysis and Perspectives, Elsevier, 2019, ISBN: 9780444640154. [12] J.C. Colmenares, Y.-J. Xu (Eds.), Heterogeneous Photocatalysis: From Fundamentals to Green Applications, Springer, 2016, ISBN: 978-3-66248719-8. [13] J. Chen, F. Qiu, W. Xu, S. Cao, H. Zhu, Recent progress in enhancing photocatalytic efficiency of TiO2-based materials, Appl. Catal. A. Gen. 495 (2015) 131. [14] F. Parrino, M. Bellardita, E.I. Garcı´a-Lo´pez, G. Marcı`, V. Loddo, L. Palmisano, Heterogeneous photocatalysis for selective formation of high-valueadded molecules: some chemical and engineering aspects, ACS Catal. 8 (2018) 11191–11225. [15] N. Serpone, A.V. Emeline, Suggested terms and definitions in photocatalysis and radiocatalysis, Int. J. Photoenergy 4 (2002) 92–131. [16] V.N. Parmon, Photocatalysis as a phenomenon: aspects of terminology, Catal. Today 39 (1997) 137–144. [17] S.E. Braslavsky, K.N. Houk, Glossary of terms used in photochemistry (IUPAC recommendations 1988), Pure Appl. Chem. 60 (1988) 1055–1106. [18] J.W. Verhoeven, Glossary of terms used in photochemistry (IUPAC recommendations 1996), Pure Appl. Chem. 68 (1996) 2223–2286. [19] S.E. Braslavsky, A.M. Braun, A.E. Cassano, A.V. Emeline, M.I. Litter, L. Palmisano, V.N. Parmon, N. Serpone, Glossary of terms used in photocatalysis and radiation catalysis (IUPAC recommendations 2011), Pure Appl. Chem. 83 (2011) 931–1014. [20] L.P. Childs, D.F. Ollis, Is photocatalysis catalytic? J. Catal. 66 (1980) 383–390. [21] M. Thomas, W.J. Thomas, Principles and Practice of Heterogeneous Catalysis, Wiley, 1996. [22] L. Palmisano, E.I. Garcı´a-Lo´pez, G. Marcı`, Inorganic materials acting as heterogeneous photocatalysts and catalysts in the same reactions, Dalton Trans. 45 (2016) 11596–11605. [23] N. Serpone, A.V. Emeline, Suggested terms and definitions in photocatalysis and radiocatalysis, Int. J. Photoenergy 4 (2002) 91–131. [24] G. Munuera, V. Rives Arnau, A. Sauceso, Photo-adsorption and photo-desorption of oxygen on highly hydroxylated TiO2 surfaces. 1. Role of hydroxyl-groups in photo-adsorption, J. Chem. Soc. Faraday Trans. 75 (1979) 736–747. [25] R.F. Howe, M. Gratzel, Electron-paramagnetic study of hydrated anatase under UV irradiation, J. Phys. Chem. 91 (1987) 3906–3909. [26] A. Fujishima, T.N. Rao, D.A. Tryk, Titanium dioxide photocatalysis, J. Photochem. Photobiol. C 1 (2000) 1–21. [27] C.B. Mendive, D. Hansmann, T. Bredow, D. Bahnemann, New insights into the mechanism of TiO2 photocatalysis: thermal processes beyond the electron–hole creation, J. Phys. Chem. C 115 (2011) 19676. [28] C.Y. Wang, C. Bttcher, D.W. Bahnemann, J.K. Dohrmann, A comparative study of nanometer sized Fe (III)-doped TiO2 photocatalysts: synthesis, characterization and activity, J. Mater. Chem. 13 (2003) 2322. [29] S. Dong, J. Feng, M. Fan, Y. Pi, L. Hu, X. Han, M. Liu, J. Sun, J. Sun, Recent developments in heterogeneous photocatalytic water treatment using visible light-responsive photocatalysts: a review, RSC Adv. 5 (2015) 14610. [30] E.I. Garcı´a-Lo´pez, G. Marcı`, F.R. Pomilla, L. Palmisano, Enhanced (photo)catalytic activity of Wells-Dawson (H6P2W18O62) in comparison to Keggin (H3PW12O40) heteropolyacids for 2-propanol dehydration in gas-solid regime, Appl. Catal. A. Gen. 528 (2016) 113–122. [31] M.D. Ballari, M.L. Satuf, O.M. Alfano, Photocatalytic reactor modeling: application to advanced oxidation processes for chemical pollution abatement, Top. Curr. Chem. (Cham.) 377 (2019) 22. [32] V. Augugliaro, M. Schiavello, L. Palmisano, Rate of photon absorption and turnover number: two parameters for the comparison of heterogeneous photocatalytic systems in a quantitative way, Coord. Chem. Rev. 125 (1993) 173–182. [33] C.S. Turchi, D.F. Ollis, Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack, J. Catal. 122 (1990) 178–192. [34] R.J. Brandi, C.A. Martin, O.M. Alfano, A.E. Cassano, A laboratory reactor for photocatalytic studies in slurry operation, J. Adv. Oxid. Technol. 5 (2002) 175–185. [35] C. Minero, F. Catozzo, E. Pelizzetti, Role of adsorption in photocatalyzed reactions of organic molecules in aqueous titania suspensions, Langmuir 8 (1992) 481–486. [36] J.M. Herrmann, Heterogeneous photocatalysis: an emerging discipline involving multiphase systems, Catal. Today 24 (1995) 157–164. [37] V. Augugliaro, E. Garcı´a-Lo´pez, V. Loddo, S. Malato-Rodrı´guez, I. Maldonado, G. Marcı`, R. Molinari, L. Palmisano, Degradation of lincomycin in aqueous medium: coupling of solar photocatalysis and membrane separation, Sol. Energy 79 (2005) 402–408. [38] M. Addamo, V. Augugliaro, A. Di Paola, E. Garcı´a-Lo´pez, V. Loddo, G. Marcı`, L. Palmisano, Removal of drugs in aqueous systems by photoassisted degradation, J. Appl. Electrochem. 35 (2005) 765–774. [39] M. Addamo, V. Augugliaro, E. Garcı´a-Lo´pez, V. Loddo, G. Marcı`, L. Palmisano, Oxidation of oxalate ion in aqueous suspensions of TiO2 by photocatalysis and ozonation, Catal. Today 107–108 (2005) 612–618.

Chapter 2

Preparation of photocatalysts by chemical methodologies Elisa I. Garcı´a-Lo´peza, Leonarda F. Liottab, and Giuseppe Marcı`c a

Department of Biological, Chemical and Pharmaceutical Sciences and Technologies (STEBICEF), University of Palermo, Palermo, Italy b Institute for

the Study of Nanostructured Materials, Palermo, Italy c Department of Engineering, University of Palermo, Palermo, Italy

1 Introduction The increasing energy demand caused by population growth, the extensive utilization of fossil fuels, and the fast development of industry have resulted in a serious energy crisis and have caused massive environmental pollution. Photocatalysis is considered one of the most potent techniques to solve energy and environmental issues by the use of abundant, omnipresent, and free solar irradiation. Heterogeneous photocatalysts allow light energy to be transformed into chemical energy useful for environmental remediation processes or energy production. The essence of the heterogeneous photocatalytic process is the solid photocatalyst. Many inorganic semiconductors have been widely investigated as photocatalysts. From already 40 years, by the understanding of the photocatalytic processes, many kinds of photocatalysts have been studied among them TiO2 soon emerged as one of the most interesting, along with other metal oxides and sulfides. Several alternative materials as nitrides, phosphates, non-metal and organic photocatalysts as this book will present in the following, have been developed. An enormous variety of chemicals have been used for the preparation of these and other (photo)catalysts and forsooth, photocatalytic materials can be prepared by many different routes, also because a plethora of solids can be conceived as photocatalysts, as exemplified in this text itself. Indeed, in the following chapters, a profusion of materials will be described as valid photocatalysts for a variety of applications. In the wide scenario of the preparation of (photo)catalysts, some general elementary steps should be always accomplished. According to Hutchings and Vedrine, these operations are based on two fundamentals: (a) a detailed knowledge of the scientific laws governing chemical and physical transformations, which are based on the fundamentals of inorganic and/or solid-state chemistry, and (b) the empirical observation related to the carefully preserved know-how [1]. Many industries are highly secretive concerning the preparation methodologies conceived for the materials to be used as (photo)catalysts because they can be subjected to patents and this could be one reason for a certain paucity in affording definitive and general studies in the field. This perception has been in part eradicated under the several scientific publications devoted to the field. Detailed preparation studies have been undertaken to establish general rules related to the (photo)catalytic reaction to be attained. Many preparation methods are disclosed in the patent literature, and also frequently are difficult to replicate for the lack of details in the methodologies or because of differences in the purity of reagents. The efficiency of heterogeneous photocatalysts is determined by many chemical and physical features: i.e., the nature of the material, shape of the particles, crystallinity degree, crystallite size point of zero charge, acidic and basic centers onto the surface, specific surface area, and porosity, the surface-to-volume ratio among others. Specifically, a photocatalyst is a semiconductor and among the above-mentioned parameters that influence semiconductor efficiency, in addition to the thermodynamic and kinetic constraints of the light-related processes, those playing a major role are crystallinity and surface area. Crystallinity determines the charge separation/recombination rate and charge mobility, while high surface area is necessary for efficient interfacial charge transfer processes beyond the particle domain. The surface area is an essential requirement as long as reactants should be accessible to a maximum number of active sites. However, it is worth mentioning that a very high SSA could lead to non-selective heterogeneous (photo)catalysts because, for instance, this material would be easily able to over-oxidize the desired product. On the contrary, in an alternative reaction, the optimum (photo)catalyst would require a high surface area. Consequently, we need to consider the optimum texture required for the solid by taking into account the reaction that will be carried out.

Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00006-4 Copyright © 2021 Elsevier Inc. All rights reserved.

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Moreover, other important features to consider in the designing of a (photo)catalyst is to possess the appropriate texture attrition resistance, adequate shape, and texture for the application and suitable pore structure. Generally speaking, all of the physico-chemical features of the photocatalytic material would be combined to obtain an efficient catalyst possessing the optimum combination to obtain the better activity and selectivity, to be easily regenerated, durable, nontoxic, economic, and sustainable in terms of preparation methodology and lifetime. Often all the requirements appear to contrast and the best compromise must be fulfilled during the photocatalyst conception and attainment. For this aim, different preparation methodologies and techniques allow addressing the required performances. There are many methods of nanostructures preparation, which can be divided into two approaches: “top-down” and “bottom-up” methods [2]. The first one is based on the splitting of the bulk, macroscopic materials to nanoparticles. Crumbling, ball milling, and different kinds of lithography are “top-down” methodologies. Much better results, giving materials with fewer defects and narrower size/shape distribution of grains provide the “bottom-up” methodologies, based on nanocrystallites growth by the formation of nanostructures “atom by atom.” This self-assembling process gives rise to the aggregation until the obtaining of nanoparticles. The “bottom-up” approaches are based mainly on chemical synthesis as, for example, precipitation, decomposition of organic precursors, synthesis under hydrothermal conditions, hydrolysis and subsequent condensation of reactants, surface redox exchange but also physical procedures as sputtering in a vacuum, condensation in liquid or gaseous phase among others. A scheme of “bottom-up” and “top-down” approaches to obtain photocatalytic nanoparticles is reported in Fig. 1. The development of new fabrication and processing technologies, along with a fundamental understanding of the relationship between the structure and properties of the powders continuously generates a variety of possibilities to produce photocatalysts not only as powders but also as films, composites, and coatings. The preparation methodologies of (photo)catalysts would be divided into two groups: (i) chemical and (ii) physical methods, as schematized in Fig. 1. The strategies of the physical methods are based on a top to down approach, starting from a bulk material that generates the final (photo)catalyst. Conversely, chemical methodologies are bottom-up approaches, aiming to assemble molecules to form a final solid, showing the advantage of tailoring the solid to address the suitable size, composition, and structure, appropriate for the target application. Moreover, chemical methods require low temperatures and less energy than the physical procedures resulting in more economical and sustainable [2]. In this chapter, we will discuss some general principles concerning the methods for the preparation of heterogeneous photocatalysts utilizing the chemical methodologies reported in Fig. 2. A survey of the main chemical approaches for the preparation of (photo)catalysts will be reported in the following paragraphs, along with some brief examples of photocatalysts prepared according to the described methodology.

Assembly from Atoms/Molecules

Size Reduction to nano-scale for Assembly

TOP-DOWN

BOTTOM-UP

FIG. 1 Scheme of two approaches to obtain nanoparticles that could be used as photocatalysts.

Preparation of photocatalysts by chemical methodologies Chapter

Preparation of bulk (photo)catalysts Physical procedures Pyrolysis Sputtering Ball-milling

2

15

FIG. 2 Chemical and physical methodologies for the preparation of bulk photocatalysts.

Chemical methodologies

Precipitation/co-precipitation Sol-gel

Laser ablation

Hydrothermal/Solvothermal

Electron-beam evaporation

Microemulsion

Electrospraying

Solid-state reactions

Electrochemical Microwaves/Electromagnetic field

Solution combustion synthesis

The main objective in the preparation of a photocatalyst is to produce (and be able to reproduce) a solid which can be used as a stable, active, and selective catalyst offering high surface area, good porosity, and suitable mechanical strength. The properties of a good catalyst, extended to the good photocatalysts is divided into two categories: (1) properties which determine directly catalytic activity and selectivity, such as bulk and surface chemical composition, local microstructure, and phase composition; and (2) properties which ensure their successful implementation in the catalytic process, as mechanical stability, porosity, shape, and size of catalyst particles. These needs generally require a compromise to produce a material that meets the contradictory demands imposed by industrial processes. We will focus in this chapter on the first category. Classically, three types of catalyst, and hence photocatalysts, can be distinguished: (i) bulk catalysts; (ii) impregnated catalysts; (iii) mixed-agglomerated powders. Bulk catalysts are the most classical solids used as photocatalysts; supports are prepared by similar procedures. Impregnated catalysts are usually obtained from preformed supports by impregnation with the active phase. Various substrates have been used as heterogeneous supports including oxides, glass, fabric, or polymers. The mixed-agglomerated catalysts comprise those materials obtained by mixing the active substances with powdered support or a support precursor and then agglomerating the mixture [3]. In this scenario, two general strategies will be discussed in this Chapter, i.e., precipitation and impregnation. These can be divided into various categories that will be described in the following. We will illustrate the main operations to obtain bulk solids to use as photocatalysts and also as supports, then we will consider the preparation of supported catalysts. The main unit operations applied for the preparation of (photo)catalysts involve the following procedures: (i) A precipitation step or an alternative process to obtain a solid (sol–gel, hydrothermal, solid–solid reaction, among others); (ii) filtration, (iii) washing; (iv) grinding and (v) calcination. The scheme reported in Fig. 3 represents the steps that could be followed. The preparation of a supporting material can foresee one or more further steps. Supported catalysts are prepared for a large variety of reasons such as obtaining bifunctional catalysts, improve some features as the dispersion of the active phase, mechanical resistance, or induced active phase-support interaction to further improve the (photo)catalyst activity.

2 Synthesis of photocatalysts as powders by chemical methodologies: An overview 2.1 Precipitation and co-precipitation Precipitation is a widely employed method to obtain either single or mixed heterogeneous photocatalysts [3–5]. The main advantage of the precipitation process is the possibility of creating pure and homogenous materials in one-pot, but the major disadvantages are the necessity to separate the solid phase after precipitation and the generation of large volumes of saltcontaining solutions. In this process, a solid phase is formed from a solution and this precipitated solid is the precursor of the final catalyst. The formation of the precipitate occurs by the addition of an agent which could change the pH, or by the modification of the temperature or evaporation of the solvent. The precipitation of the solid occurs by achieving the supersaturation conditions [5]. The parameters which turn the solution to the supersaturation conditions are concentration, temperature, and pH, as shown in Fig. 4. Commonly, one proceeds usually by precipitation from a solution by adding another solution to reach the necessary pH to obtain the solid. Generally, the supersaturation can be reached by: (i) increasing the concentration through solvent evaporation, (ii) lowering the temperature or (iii) by changing the pH. Achieving the

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Precursor Precipitation (Sol-gel, hydrothermal, etc…)

Precipitate Aging, wash, filtering Aging, sprydrying

Filtered cake

Dried solid precursor Shaping Solid precursor

Calcination Photocatalyst

Calcination

Shaping

Final solid Photocatalyst FIG. 3 Scheme of the steps in the preparation of a precipitated photocatalyst.

FIG. 4 Supersaturation dependence on concentration, T, and pH. Adapted with permission from C. Perego, P. Villa, Catalyst preparation methods, Catal. Today 34 (1997) 281–305. Copyright by Elsevier.

supersaturation, the system turns unstable and the precipitation of the solid occurs as a consequence of a small perturbation. The solid then is formed by the nucleation of small elementary particles and their further agglomeration. The formation of clusters, which spontaneously grow by the subsequent addition of monomers is the first stage of the nucleation process. The clusters reaching a critical size continue to grow. These primary particles begin to form a shared structure, the aggregates. The size of the crystals obtained depends on the ratio of the rate of nucleation to that of the crystal growth. The greater this ratio is, the smaller will be the crystallites and vice versa. If the rate of nucleation is faster than the rate of growth, numerous very small particles are formed and the obtained solid tend to be amorphous. The most common catalysts and photocatalysts prepared by precipitation are oxides and salts. The precursors are mainly metallic salts or hydroxides. The precursors are preferably chosen among salts possessing the metal cation accompanied by

Preparation of photocatalysts by chemical methodologies Chapter

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counter anions which would be easily decomposed by thermal treatments. This is because these anions could remain in the final solid photocatalyst as poisoning agents, playing a non-negligible role during the photocatalytic reactions. For instance, it is well known that chlorides or sulfides in an oxide semiconductor structure poison the photocatalyst. The solubility of inorganic salts and the environmental sustainability of the preparation foresee the preferable use of water as the solvent. The use of organic molecules as additives, for instance triblock copolymers (Pluronic), aid to control the pore structure of the precipitate and can be removed later during the calcination step by using a slow heating rate in the presence of oxygen. Other parameters, such as the concentration of the starting solutions, mixing procedure, stirring of the solution and aging, could influence the morphology, texture, and structure of the final catalyst. The experimental parameters should be well tuned during the precipitation because they influence the physico-chemical features of the final material as schematized in Fig. 5. Co-precipitation is the simultaneous precipitation in a solution of two or more solids. This method is often preferred to other processes because it involves simple steps and the composition is easy to control [6]. In the case of co-precipitation of mixed oxides, the crystals appear as soon as the solubility limit is reached. It is crucial the right choice of the precursor salts which should give rise to the precipitate in almost equal conditions. The pH should be carefully adjusted and preferably kept constant during precipitation. It is, therefore, considered preferable to add continuously the precursor salts to the precipitating agent solution rather than the reverse. The homogeneity of the co-precipitate depends on the differences in solubility between the components and the kinetics of precipitation. It is also necessary a vigorous stirring during the co-precipitation to obtain a homogeneous composition of the precipitate. If the precipitate of one component is much more soluble than the others, sequential precipitation can occur giving rise to gradients of concentration in the final solid and less intimate mixing of the components [6]. In the co-precipitation methodology, two techniques can be identified: (i) constant pH method and (ii) variable pH method in which the pH continuously changes during precipitation until the desired end-point is reached. The continued stirring of the precipitate in the precipitating solution (often called mother liquor) accounts for the change in the properties of the precipitate. This process is referred to as aging. For example, the co-precipitation method is often used to synthesize BiVO4 because of its simplicity, low cost and ability to be used for large-scale production [7]. The co-precipitation is carried out by dropwise addition of a Bi (NO3)3 solution to a vigorously stirred NH4VO3 one. When the pH is adjusted to 9 with NH4OH, the yellow precipitate obtained is eventually calcined at 200°C [8]. Also, the Ag3PO4 photocatalyst obtained by precipitation from AgNO3 and Na2HPO4 possessed very different chemical–physical features and photocatalytic activity than that obtained by an ion exchange among salts containing Ag(I) [9]. Some features as light absorbance and crystalline size were almost the same, albeit the specific surface area of the Ag3PO4 obtained by precipitation was nearly seven times larger than that through the ion-exchange. Also, the particle size of the precipitated was one order of magnitude smaller (0.3–0.4 mm) than that of the other (6–10 mm).

2.2 Sol–gel process A second methodology for the preparation of photocatalysis based on the sol–gel process. A sol is a suspension of colloidal particles whether amorphous or crystalline, whose sizes range from 1 nm to 1 mm; a gel is the coagulated form of sols, being TEXTURAL PROPERTIES

COMPOSITION PURITY

CRYSTALLINE PHASE

Precursor solution Composition Precipitating agent

SOLID PHOTOCATALYST

pH

FIG. 5 Parameters affecting the physico-chemical properties of the precipitate.

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colloidal solid particles encapsulating a liquid. Van der Waals and coulombic repulsive strengths among very small particles account for the stability of the sol. A further condensation of the sol into a three-dimensional network produces the gel which encloses molecules of solvent. The sol–gel preparation process usually employs metallic alkoxides M(OR)n as precursors, which are hydrolyzed to create polymeric oxide/hydroxide gels [10, 11]. A slow drying process gives rise to the final solid obtained by the removal of the solvent. Depending upon the drying process speed and conditions the final solid could present very different features. For example, if the evaporation of the solvent occurs under ambient pressure a xerogel is obtained; conversely, if supercritical conditions are used an extremely low-density material called aerogel is obtained [12–14]. Xerogel (the dense ceramic in Fig. 6) or aerogel necessitate a further thermal treatment to became ordered materials and being active (photo)catalysts. In general, alcogels are formed using alcohols as solvent whereas hydrogels are obtained in water. The gels obtained by hydrolysis of inorganic precursors generally consist of dense particles rather than polymeric clusters. The use of inorganic salts (such as nitrates, chlorides, acetates, carbonates, etc.) requires an additional removal of the inorganic anion [10]. The sol–gel method has demonstrated excellent control over the texture and surface properties of the final materials. The four key steps necessary to pass from the precursor to the solid formed by sol–gel are the following: (i) formation of a gel, (ii) aging of a gel, (iii) removal of the solvent, and (iv) heat treatment [15]. Fig. 6 schematized the overall procedure, including the applicability of this technology to form films or fibers. The sol–gel preparation of photocatalytic materials allows controlling the size and distribution of the pores of the material, allowing the introduction of several components in a single step and getting the solid at room conditions. Fig. 7 schematize the process. The versatility of the preparative lies in the number of parameters that can be manipulated in each of these steps [3].

FIG. 6 Scheme of the sol–gel process. Wikimedia Commons under the CC BY 4.0 license. https://en.wikipedia.org/wiki/File:SolGelTechnologyStages. svg.

Preparation of photocatalysts by chemical methodologies Chapter

Forma on

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19

FIG. 7 Schematic diagram showing the various steps of a sol–gel process.

Dissolve a precursor (metal salt or alkoxide) in a solvent

Commercial or preformed sol

Add water and acid or base for hydrolisis and condensa on

Destabiliza on of the sol (modify pH or concentra on)

Forma on of a gel

Formaon of a gel onto a solid support Aging of the gel

Aging

Evapora on Solvent removal Xerogel

Heat cancina on/sintering treatment

Drying

Supercri cal drying Aerogel cancina on/sintering

Final photocatalyst (powder, monolith, thin film, membrane)

The sol–gel process could take place in water or an alternative non-aqueous solvent. In an aqueous solution, the alkoxide reacts with water which plays both the role of solvent and reactant. In a nonaqueous solution, this double role is played usually by alcohol, but also an ether, ester, or even another alkoxide can be used. The first step corresponds to a partial hydrolysis with monomer formation (where M is a metal and R is an alkyl): M
ðORÞn + X H2 O ! M
ðOHÞx ðORÞn
x + x R
OH

(1)

The second step involves condensation, creating M-O-M bonds. A sol is formed by dehydration or de alcoholization reaction, being the olation process (Eq. 2) much faster than oxolation (Eqs. 3 and 4): M
OH + M
H2 O ! M
OH
M + H2 O ! Oxlation reaction

(2)


M
OH + HO
M
!
M
O
M
+ H2 O ! Oxolation reaction

(3)


M
OH + RO
M
!
M
O
M
+ ROH ! Oxolation reaction

(4)

The gel is formed as a result of the polymerization process which involves the formation of M-O-M bridges between the metallic atoms of the precursor molecules [10]. The polymerization is associated with the formation of a polydimensional network composed of (M–O–M) bonds accompanied by the production of H–O–H and R–O–H species. Polymerization builds larger and larger metal-containing molecules forming a kind of macromolecule containing thousands of monomers. The number of bonds that a monomer can form is called its functionality. Polymerization of titanium alkoxide, for instance, can lead to complex branching of the polymer, because a fully hydrolyzed monomer Ti(OH)4 is tetra functional. By modifying the conditions, for instance, under low water concentration, fewer than four of the OR or OH groups will be capable of condensation, hence, lower branching will occur. The mechanisms of hydrolysis and condensation, and the factors that drive the structure toward more or less branched structures, are the most critical issues in the sol–gel process. Experimentally, during the reaction, it is observed that the viscosity of the sol gradually increases with polymerization and cross-linking progress, up to the sol–gel transition point is reached. At this point, the viscosity abruptly increases and a gel formation occurs. Further increases in cross-linking are promoted by drying or other dehydration methods. The transition from the liquid solution to a cross-linked gel begins after a period of time known as gel time or gelation time. During the aging, the polycondensation reactions continue until the gel transforms into a solid mass, accompanied by contraction of the gel network and expulsion of solvent from the gel pores. Drying is the step during which water and other volatile liquids are removed from the gel network. The obtained precipitates are typically amorphous; hence, a heat treatment is necessary to obtain the final (photo)catalyst. During the heating treatment, the gel network collapse and the remaining organic species leave the solid so the physicochemical features of the final solid would be straightforwardly

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related to the annealing procedure. Submitting the solid to high or moderate temperatures could give rise to the sintering of the material and hence to a decrease in the surface area and porosity. Any parameters that affect either or both of these reactions are thus likely to impact on the properties of the final solid. Livage et al. [16] pointed out that the important variables are the relative rates of hydrolysis and condensation. The parameters to control during the procedure are: (i) the type of precursor, (ii) nature and amount of the solvent, (iii) pH, (iv) concentration, (v) temperature, and (vi) reaction time [17]. Both hydrolysis and condensation (Eqs. 1–4) are nucleophilic displacement reactions, so important parameters addressing the reaction are: (i) the steric hindrance of the alkoxy groups, (ii) the electronegativity of the metal atom and (iii) the ability of the central positively charged metal to increase its coordination number. The reactivity of the alkoxide increases as the electronegativity decreases and the size of the central atom increases. Usually, inorganic acids are added as they act as catalysts in the sol–gel reaction, protonating the negatively charged alkoxide ligands, thus increasing the rate by providing a better leaving group. Also, basic catalysts could provide improved nucleophilic attack and result in deprotonation and, therefore, enhance condensation. The reactivity of the transition metals alkoxides is very high. In general, their hydrolysis rate decreases by increasing the chain length of the alkyl group bonded to the M. Also, to prevent agglomeration of the particles formed, chelating agents could be used, e.g., acetylacetone, acetyl acetoacetate, glycol. Binary and ternary oxides have been also prepared by sol–gel process. For preparing mixed oxides or hydroxides, several possibilities exist: (i) co-precipitation by changing the pH of an aqueous solution; (ii) mixing sols of different hydroxides or oxides; (iii) mixing of sol and solution followed by gelation; (iv) preparing double alkoxides or alkoxy salts or (v) slow hydrolysis of two precursors [1]. The great difficulty that resides in the simultaneous hydrolysis of both the alkoxides is that the rates of hydrolysis are frequently different. If no care is taken one generates one oxide coating the other one. The homogeneity of the mixed oxide gels depends on the relative reactivities of the two alkoxides. For the preparation of double salts and alkoxides, the most popular method involves dissolving the two alkoxides in a mutual solvent, mixing the solutions, and refluxing at elevated temperatures. One can also prepare a double alkoxide by refluxing the oxide of the first element with the alkoxide of the second element in alcohol. To avoid the problem of heterogeneity it is also possible to prehydrolyze the more reactive alkoxide in moist air to passivate the hydrolysis process, or to hydrolyze first the less reactive alkoxide separately into long-chain polymers on which some hydroxyl groups would remain and to which the second alkoxide could be anchored. Recent developments in the synthesis of photocatalysts by sol–gel process have been reviewed [18]. Several works are specifically devoted to crystalline titania nanostructures [19]. Among other methodologies, the hydrolysis and condensation of titanium alkoxides or chlorides are of particular interest as long as they allow to finely address the physicochemical features of the final solid as specific surface area, nanoparticle size, degree of aggregation, among others [20]. The final product morphology is strongly influenced by the reactivity of the precursor, but also by the reaction conditions, such as the pH of the reaction medium, the water to precursor ratio, and the reaction temperature [21]. Titanium alkoxides easily react with water forming undefined titanium oxo/hydroxo species. The coordination of titanium with chemical additives, such as bidentate ligands, lowers the rate of hydrolysis and allows the formation of sols, particles, and gels which once dried are amorphous but commonly exhibit moderately high specific surface area and could be even mesoporous. An annealing step at temperatures higher than 300°C allows the transition from the amorphous to the crystalline phase producing the collapse of the pore system, decreasing the specific surface area, and increasing the particle size. It is uncommon to report the sol–gel synthesis of crystalline anatase titania at low temperatures with a high specific surface area or porosity [22]. It is possible to obtain crystalline titania polymorphs via sol–gel at a very low pH [23]. The synthesis of rutile without undergoing a thermal treatment is even more difficult than for anatase and necessitates pH values below zero [24]. TiO2 anatase and rutile nanoparticles are obtained by sol–gel at low temperatures in microemulsion systems in the presence of the Pluronic F127 template [25] or from titanium alkoxide solutions in the presence of Pluronic F123 [26]. Glycolates can be used as solvents in the sol–gel process in the so-called “polyol” process. Titanium glycolates, glycols, and polyols are formed and these “polyol” ligands control the hydrolysis rates of transition metal alkoxides [27]. When the reaction is carried out in the presence of an amine in an autoclave even single crystals can be obtained [28]. A further advantage of the glycolated precursors is their stability not only in alcohol but also in water. The glycolated titanium centers are very promising water-soluble precursors for the synthesis of titania. Saadoun et al. used three different water-soluble diolates of titanium to obtain anatase by microwave heating aqueous solutions containing these precursors [29]. Bis(2-hydroxyethyl) titanate was employed in a low-temperature sol–gel synthesis in the presence of a non-ionic surfactant to obtain mesoporous anatase TiO2 with 110 m2 g
1 with no heat treatment [30]. The simultaneous achievement of high bulk crystallinity and the formation of ordered mesoporous photocatalytic frameworks with high thermal stability is an interesting branch in the preparation of photocatalysts. Indeed, these both

Preparation of photocatalysts by chemical methodologies Chapter

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features are beneficial for promoting the diffusion of reactants and products, as well as for enhancing the photocatalytic activity by facilitating access to the reactive sites on the surface of photocatalyst. Among the sol–gel methodology, it should be considered also the nanocasting as a powerful approach for creating ordered mesoporous materials that are difficult to synthesize by conventional processes. The preparation begins with the introduction of the metallic photocatalyst precursors inside the nanospaces of a mesoporous hard template, which is used as a template to produce the mesoporous materials with controllable pore size, the morphology of the network. After fabrication of the photocatalyst inside the nanospaces, the template framework is selectively removed and the mesoporous ordered inorganic/organic material is obtained. Some mesoporous hard templates can be used as MCM-41, SBA-15, KIT-6, or mesoporous carbon [31]. The replication of an ordered mesoporous template gives rise to the mesoporous solid. In the first stage a template with ordered mesostructure, the nanosized “mold,” is prepared and then the template is infiltrated with precursors of desired metal oxides, and the pores are partially or fully filled after the evaporation of the solvent. The final step is the elimination of the mold and the obtaining of the final mesoporous photocatalyst. When the porous solid template is used (as activated carbons, silica gels, or mesostructured silica), the material obtained retains the 3D structure containing framework-confined pores [31]. The obtained solid possesses the inverse structure of the template itself and resulted to be a nanostructured material when a complete filling of the porosity of the template is reached by the precursor solution. When the porosity of the hard templates is made up of non-connected pores (as for instance in the MCM-41 silica) nanoparticulate materials are obtained. The preparation of both types of materials is illustrated in Fig. 8. The sol–gel route has been reported to obtain TiO2 with regular open-pore structures using a non-hydrolytic templatemediated strategy [32]. Organic solutions slow down the hydrolysis of Ti species, soft copolymer templates promote the formation of well-ordered octahedron titanium hydroxides, and post-thermal treatment is conducive to the removal of organic compounds. Samples obtained through the whole procedure usually possess pore structures within nanometer Inorganic precursor

Hard template

Nucleation and growing mechanism

Impregnation step

Calcination step

(crystalline material)

Calcination step

Initially the metallic salt is homogeneously distributed in the particle, During calcination, migration to the nucleation seeds takes place and formation of nanocrystals occurs (nanoparticles or nanostructures)

Template removal

If the concentration of the precursor solution and the porosity of the template are high and the latter is highly interconnected the material is a nanostructure

40 nm silica xerogel NANOSTRUCTURES If the concentration of the precursor solution is low or if the porosity of the template is formed by nonconnected pores, the material is formed by nanoparticles

Template removal

NANOPARTICLES 30 nm Fe2O3 20 nm Fe2O3

FIG. 8 Nanocasting routes for the preparation of nanoparticulate and nanostructured inorganic materials using silica as a template. Reproduced from T. Vald
es-Solı´s, A.B. Fuertes, Mater. Res. Bull. 2006, 41, 2187–2197 with Elsevier permission.

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Bu Ti

O

+

Ti

Cl

EtOH

Ti

O Ti

+

BuCl

P123

Ti-oxo-species

evaporat CH3

HO

photo- UV catalysis

CH2CH2O

III

CH2CHO

mesostructure

flow of phenol

CH2CH2O

H III

e– CB VB

h+ folding of the template

II

Rutile

Anatase

FIG. 9 Possible mechanism of the formation of mesoporous TiO2 with well-ordered pore morphology and high photoactivity [33].

scales. Chen et al. used TiCl4 and Ti(OBu)4 as the precursors and Pluronic P123 as the template, for producing well-ordered mesoporous TiO2, as shown in Fig. 9. The strategy was the non-hydrolytic evaporation-induced self-assembly (EISA) method giving rise to a three-dimensional mesostructured. The mechanism proposed by Chen et al. for the synthesis of the mesoporous TiO2 is schematized in Fig. 9. It is suggested that TiCl4 can react with EtOH to form Ti(Cl)4-x(OEt)x (x  2) species resulting in a highly acid mother solution (pH  1). The addition of Ti(OBu)4, acting both as Ti source and an extra oxygen donor, reduce the acidity of the solution. Meanwhile oligomers as TiXx(OH)yO2
(x + y)/2 (X ¼ OR or Cl; x  0.3–0.7; y  0–0.2) are formed. Ti-O-Ti bridges may result from the condensation between TidCl and TidOR. With the evaporation of ethanol and following thermospolymerization and successive calcinations to remove the template, TiO2 with a regular pore distribution is formed [33]. The molar ratio of TiCl4/Ti(OBu)4 is found to be of paramount importance for tuning the pore structure and the anatase and rutile phase composition of the final material. The final material showed excellent photocatalytic activity for the photodegradation of phenol.

2.3 Hydrothermal/solvothermal syntheses The hydrothermal synthesis procedure is rooted in the geological sciences. The British Geologist Sir Roderick Murchison in the mid-19th century used the term “hydrothermal” to describe the formation of minerals by hot water solutions rising from cooling magma. Since then, this methodology has been used for the synthesis of new materials and today the hydrothermal synthesis is known as the procedure aimed to obtain substances in a sealed and heated aqueous (hydro-) or organic (solvo-) solution at temperatures in the range 100–1000°C and pressures from 1 to 100 MPa [34]. In the catalysis field, these kinds of technology were primarily used for the preparation of zeolite materials. The most typical syntheses were those of the ZSM5 using tetra propyl ammonium hydroxide as templating agent, or of the mesoporous materials of controlled pore size and narrow size distribution (2–10 nm) such as MCM 41, initiated by the work of Mobil’s researchers in the early 1990s. This huge effort in molecular sieve materials has inspired several developments in the hydrothermal/solvothermal procedures for many kinds of solids with a large variety of monodisperse pore sizes either mono or tridimensional network possessing very different features. The hydrothermal/solvothermal conditions allow insoluble or difficult to dissolve precursors to solubilize as complexes due to the cooperative effect of temperature, pressure, and solvent. The properties of the solvent (dielectric constant, viscosity, density) and of the solution (solubility, stability, yield, and dissolution-precipitation reactions) strongly influence the final features of the obtained solid. Other properties of the solvent such as pH variation and expansion coefficient under hydrothermal conditions must be carefully evaluated for pressure and temperature since the solvent not only works as a suitable environment, but it can also act as a reactant. Hydrothermal synthesis is performed in sealed containers called high-pressure stainless-steel autoclaves, thick-walled steel cylinders with a hermetic seal that must withstand high temperatures and pressures for prolonged periods. These reactors are shown in Fig. 10. They are often provided of a covered batch vessel in Teflon that could be either stirred or not. The chemical inertness of Teflon prevents the inside of autoclave corrosion and hence the purity of the material during the preparation. Hydrothermal synthesis is generally defined as the process aimed to obtain crystal synthesis under high temperature and high-pressure conditions from substances that are insoluble at room conditions. The system could work under the subcritical or the supercritical conditions of the solvent. The vast majority of hydrothermal or solvothermal

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FIG. 10 (A) Typical components of an autoclave used for hydrothermal/solvothermal treatment and synthesis; (B) assembled autoclave; (C) commercially available stirred autoclave reactors with facilities to withdraw the fluids and externally pump the desired gas into the autoclave, magnetic stirrer, autoclave quenching facility. From K. Byrappa, T. Adschiri, Hydrothermal technology for nanotechnology, Prog. Cryst. Growth Charact. Mater. 53 (2007) 117–166 with Elsevier permission.

synthesis performed today in the synthesis of inorganic materials are conducted at temperatures below 300°C because the water ionic product (Kw) shows a maximum value at around 250–300°C. The critical temperature and pressure of water are 374°C and 22.1 MPa, respectively [35]. As already mentioned, the hydrothermal/solvothermal procedure can be classified into subcritical and supercritical depending upon the temperature. In the presence of water as a solvent, the range of 100–240°C is in the subcritical synthesis reaction, applicable to industrial and laboratory operations, for instance for the zeolites typical syntheses mentioned before. In the supercritical synthesis, the temperature could reach 1000°C and the pressure could reach 0.3 GPa. Considering water, the most common solvent, under high temperature and pressure, the density, surface tension, and viscosity will be lower, and the vapor pressure and ion product will be higher than at room conditions. According to the electronic theory, the reactions of organic compounds with polar bonds usually have some characteristics of ionic reaction. Therefore, when water is used as a medium and the reaction system is heated above its boiling point in a sealed reactor, the ionic reaction rate will be certainly accelerated, which is consistent with the Arrhenius equation, i.e., the reaction rate constant k will exponentially increase with the increase of reaction temperature. Because of the increased ionization constant of water caused by the increased temperature, the ionic or hydrolysis reaction for those insoluble inorganic materials at environment temperature or organic compounds can be promoted under the high temperature and pressure hydrothermal condition. The viscosity of water decreases with the increase in temperature. At temperature of 500°C and pressure of 0.1 G Pa, the viscosity of water only amounts to 10% of its value under normal conditions [34]. Hence, the mobility of molecules and ions in water under hydrothermal conditions is much higher than under normal conditions. A pressure–temperature diagram reported in Fig. 11 schematizes the conditions in which the hydrothermal/solvothermal reaction synthesizes are carried out in comparison with other preparation techniques. Noteworthy, this technology is not the most energy demanding and offers advantages due to the highly controlled diffusivity in a solvent

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FIG. 11 Pressure temperature map of materials processing techniques. From Elsevier with permission from K. Byrappa, T. Adschiri, Hydrothermal technology for nanotechnology, Prog. Cryst. Growth Charact. Mater. 53 (2007) 117–166 with Elsevier permission.

media in a closed system. The solids prepared by the hydrothermal/solvothermal method show typically a size reduced to the nanometer range, increased mechanical strength, enhanced diffusivity, higher specific heat, and electrical resistivity compared to their conventional coarse-grained counter-parts due to a quantization effect. Solvothermal method uses a non-aqueous solvent, and the reaction temperatures can be much higher than in the hydrothermal method since a variety of organic solvents with high boiling points can be chosen. Solvothermal processing allows many inorganic materials to be prepared at temperatures substantially below those required by traditional solid-state reactions. The products of solvothermal reactions are usually crystalline and do not require post-annealing treatments (Table 1). During the hydrothermal/solvothermal reaction, crystallization occurs in solutions by crystal nucleation and growth. The variables of the process are: (i) temperature, (ii) pH, (iii) reactant concentrations and (iv) additives, and they could be monitored to address the final solid to possess the suitable particle size and morphology. To do so the overall nucleation and growth rates, which depend on supersaturation, should be controlled [36]. Nucleation occurs when the solubility of the solute exceeds its limit in the solution (the solution becomes supersaturated); at that point, the solute precipitates into clusters of crystals that can grow to macroscopic size [37]. The solid continues to grow by the incorporation of units from the bulk solution into the crystal enhancing its size. We can consider that this process possesses four steps: (i) transport of

TABLE 1 Properties of commonly used hydrothermal/solvothermal solvents. Solvent

Critical Temperature [°C]

Critical pressure [MPa]

Water

374.1

22.1

Ethylenediamine

319.9

62.1

Methanol

239.2

8.1

Ethanol

241.1

6.1

Toluene

320.6

4.2

Ethanolamine

398.2

8

Preparation of photocatalysts by chemical methodologies Chapter

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units through a solution, (ii) attachment of units to the surface, (iii) movement of units on the surface, and (iv) attachment of units to growth sites. Supercritical water gives a favorable reaction field for particle formation, owing to the enhancement of the reaction rate and large supersaturation based on the nucleation theory, due to lowering the solubility. The solvent properties, such as dielectric constant and solubility, change dramatically under supercritical conditions. The dielectric constant of water decreases with increasing temperature and decreasing pressure. In particular, the dielectric constant is below 10 under supercritical conditions (78.5 at 25°C). The supercritical state of water allows to enhance the reaction rate and to favor the formation of the particles owing to the large supersaturation caused by the lowering of solubility [35] Hydrothermal synthesis in supercritical water enables the obtainment of ultrafine particles with a controlled morphology by varying the pressure and/or the temperature [38]. In the case of supercritical water technology, water is used as the solvent in the system, whereas supercritical fluid technology is a general term when solvents like CO2 and several other organic solvents are used, and because these solvents have lower critical temperature and pressure compared to water this greatly helps in processing the materials at much lower temperature and pressure conditions. From the perspective of the morphology of nanomaterials, the hydrothermal technique has been used to process nanomaterials with a variety of morphological features, such as a nanoparticle, nanosphere, nanotube, nanorod, nanowire, nanobelt, nanoplate, and so on. From the perspective of the composition of nanomaterials, the hydrothermal technique can be used to process almost all types of advanced materials such as metal, alloy, oxides, semiconductors, silicates, sulfides, hydroxides, tungstates, titanates, carbon, zeolites, ceramics, and a variety of composites, including nanohybrid and nanocomposite materials. Hydrothermal or solvothermal synthesis has been successful applied for the preparation of many kinds of solids, including photocatalysts. Compared with the hydrothermal process, the solvothermal ones could produce metal oxides that are smaller and have a narrower size distribution, and they could be produced at a lower temperature. Extensive work has been reported in the preparation of numerous nanostructured metal oxides through both hydrothermal and solvothermal processes [39]. For example, large research efforts were devoted to the synthesis of micrometer-sized single crystal TiO2 materials, e.g., anatase, with deliberately chosen exposed facets of the crystal, since it is assumed that the chemical reactivity of the material can be tailored in this manner. Typical synthesis pathways are hydrothermal approaches, e.g., in the presence of TiF4 or fluoride ions. [40]. In hydrothermal processes, the synthesis is typically carried out at rather high temperatures, resulting in agglomerated nanocrystals. Many different reaction conditions, precursors, catalysts, and even structuredirecting agents have been successfully employed to improve the properties of the final product. For instance, Cozzoli et al. demonstrated the controlled growth of anatase nanocrystals and high aspect ratio TiO2 anatase nanorods by hydrolysis of titanium alkoxides in the presence of oleic acid and amines as crystallization promoters in very mild hydrothermal conditions at 80–100°C [41]. Xu et al. [42] prepared single-crystalline TiO2 nanomaterial with controllable phase composition and morphology by hydrothermally treating suspensions of the H-titanate nanotubes at different pH and temperature. During hydrothermal treatment of H-titanate nanotubes dispersed in an acidic medium, the nanotubes transformed into tiny anatase nanocrystallites of about 3 nm as an intermediate of peptization. These intermediate nanocrystallites predominately transformed into anatase nanoparticles with the rhombic shape when the pH value was greater than or equal to 1.0, whereas primarily turned into rutile nanorods with two pyramidal ends at the pH value less than or equal to 0.5 thus forming TiO2 nanomaterials with controlled phase composition and morphology. The hydrothermal transformation of H-titanate nanotubes into single-crystalline TiO2 nanomaterials with different phase composition and morphology is shown in Fig. 12.

0 ≥1.

pH

H-titanate nanotube

tes alli yst iate) r c ed no Na term (in

FIG. 12 Schematic diagram for the mechanism of hydrothermal transformation from H-titanate nanotubes into single-crystalline TiO2 nanomaterial with different phase composition and morphology [42].

Rhombic Anatase crystal

pH£0 .5

Proto

nat

ion Nan oc (inte rystalli te rme diat s e)

(111)

(110) (001)

Rutile nanorod with two pyramidal ends

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B Fundamentals of preparation and characterization of photocatalytic materials

In the present context, it is worth mentioning here the ionothermal synthesis, an innovative method for preparing inorganic functional materials. These reactions are conducted in ionic liquids (IL) at high temperatures and in pressurized reactors, hence, ionothermal reactions are analogs of hydrothermal or solvothermal approaches by using IL as solvents. ILs are complex solvents with unique physicochemical properties, capable of all possible types of interactions with solutes. They can solubilize a variety of organic and inorganic compounds and can be designed to be immiscible or miscible with water and several organic solvents [43].

2.4 Microemulsion technique A microemulsion is a thermodynamically stable solution of two immiscible liquids forming microdomains stabilized by an interfacial film of surfactant. This micro-heterogeneous medium is usable for the generation of nanoparticles [44]. The microdomains may occur in: (i) oil in water (microemulsion) or (ii) water in oil (reverse microemulsion). The surfactant, possessing a polar (hydrophilic) head and an aliphatic (hydrophobic) chain tail, interacts at the oil/water interface reducing the interfacial tension. For example, water in oil microemulsions are aqueous phase microdroplets (10–25 nm in size) surrounded by a monolayer of surfactant molecules in a hydrocarbon solvent. These micro droplets can act as aqueous micro reactors for the synthesis of nanoparticles. The small dimension of these microreactors allows confining the nucleation of the particles, limiting their growth and agglomeration [45]. There exist two kinds of micelles, those formed in an aqueous medium, where the hydrophobic hydrocarbon chains of the surfactants are oriented toward the interior of the micelle, so are hydrophobic micro reactors, and the hydrophilic groups of the surfactants are in contact with the surrounding aqueous medium and a second type, the so-called “reverse micelles,” formed in non-aqueous medium where the hydrophilic head groups are directed toward the inside of the micelles, so they are hydrophilic microreactors, so presenting the hydrophobic groups directed outward the micelle [46]. The size of the particles prepared in microemulsions is related to the ratio of surfactant to water, being ca. equal to that of the water droplet in a reverse microemulsion [47]. Moreover, the composition of the particles is determined by the composition of the dispersed phase [48]. Often the hydrolysis of titanium alkoxides in microemulsion-based sol–gel methods resulted in an uncontrolled aggregation and flocculation except at very low concentrations. The reverse microemulsion-mediated technique (reverse micelle or water-in-oil microemulsion procedure) has been used successfully as microreactors to synthesize ultrafine particles with a narrow distribution of particle size by controlling the growth process [45]. Deorsola et al. [49] adopted water-in-oil microemulsions to produce ultrafine and nanometric particles. Indeed, the inverse microemulsion process has been the most popular technique for the preparation of TiO2 nanoparticles. The aqueous precursor solution can be ultrasonicated in an inert hydrocarbon forming the micelles in the presence of the surfactant to prevent coalescence. Additionally, an osmotic pressure agent is present in the droplets to avoid particle degradation by diffusion processes (Ostwald ripening). [19]. The synthesis of TiO2 nanoparticles has been carried out from a TiCl4 solution emulsified into the oil phase, as reported in Fig. 13A. The precipitation of spherical and ultrafine particles shown in Fig. 13B occurred due to the high instability of Ti precursor and the interaction among nanodroplets. The hydrolysis of the precursor and the condensation process, take place inside of the droplet microreactors. During the whole process, the size and number of the droplets, as long as the concentration inside of them remains constant controlling effectively the final particle size of the particles. Reverse micelle systems provide a micro-heterogeneous medium for the generation of nanoparticles. The approach proposed by Chhabra et al. consisted of a microemulsion-mediated process where the aqueous microdroplets are used as microreactors, as reported in Fig. 14. A dimeric micelle is generated in a short lifetime by collision, and a chemical reaction occurs by substance exchange during the collision. By repeating this collision, a further chemical reaction proceeds and nucleus generation occurs. Furthermore, the nucleus grows up to be a fine particle when the hydrolyzed species collide. As reported in Fig. 14, titanium(IV) hydroxide in microemulsion I precipitate by the collision and coalescence with microemulsion II, containing the basic NH4OH. The result is a precipitate precursor of the ultrafine TiO2 particles obtained after calcination [45]. Gao et al. reported in a reverse micro-emulsion system, the key steps for controlling the diameter of nano-TiO2 in a water–oil microemulsion splitting the nucleation stage from the crystal growth [50]. This method allows for the formation of stable reverse micellar nano titania gels, which is useful for the further preparation of uniform size TiO2 particles at the nanoscale. Also, a mixture of two polymorphs, both anatase and rutile TiO2 can be prepared by hydrothermal treatment of microemulsions [51]. Nitrogen-doped nanocrystalline TiO2 has been synthesized by a microemulsion-hydrothermal process in HNO3 or HCl [52]. In this procedure, no high calcination temperature is required for preventing the agglomeration and sintering of the final particles. Inaba et al. [53] prepared titanium dioxide nanoparticles in a reverse micelle

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FIG. 13 (A) Procedure scheme for the reactive microemulsion synthesis of TiO2 nanopowders and (B) SEM micrograph of TiO2 powders obtained through precipitation in microemulsion with 10 wt% emulsifier contents [49].

(A) TiCl4 2M solution

TiCl4 0.25M solution emulsifier

light mineral oil

stable emulsion

rotation/evaporation

precipitation under continuous stirring

centrifugation

triple washng with isopropanol

TiO2 nanospheres

(B)

system composed of water, Triton X-100 and isooctane. The TiO2 nanoparticles showed monodispersity, a large surface area and high degrees of crystallinity and thermostability. The particle size of TiO2 was controlled by changing the water content of the reverse micelle solution. The microemulsion technique has been also used for the preparation of nano-metallic particles on the oxide surface. For instance, to obtain Ag/TiO2, the precipitation technique is easy to apply, but the morphology and the size of the final nanoparticles are difficult to control, giving rise to unreliable properties. The water/oil microemulsion has been demonstrated to be an optimum microenvironment in which monodispersed, ultrafine Ag/TiO2 nanoparticles with a narrow size distribution can be obtained [54]. Zieli
nska obtained by a reverse micelle system narrow size distribution of Ag-TiO2 nanoparticles in the water in oil microemulsion by using water/AOT/cyclohexane system, where AOT stands for sodium bis-(2-ethylhexyl) sulfosuccinate, the most common surfactant used to form reverse micelles, as schematized in Fig. 15. The Ag-TiO2 nanoparticles were obtained with narrow particle size distribution regardless the Ag content in the reaction system. The effect of Ag content used for preparation on the photocatalyst structure, surface area, crystallinity, and efficiency of removal of model organic compound and model microorganisms from an aqueous phase.

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FIG. 14 Schematic diagram showing the preparation of TiO2 particles in microemulsions. Reproduced with permission from ACS V. Chhabra, V. Pillai, B.K. Mishra, A. Morrone, D.O. Shah, Synthesis, characterization, and properties of microemulsion-mediated nanophase TiO2 particles. Langmuir 11 (1995) 3307–3311.

Microemulsion I

Microemulsion II

Aqueous Phase TiCl4

Aqueous Phase NH4OH

Mix Microemulsions I and II

Collision and Coalescence of Droplets

Chemical Reaction Occurs (Titanium Hydroxide Precipitates)

FIG. 15 Schematic illustration of the Ag-TiO2 nanoparticles preparation method in AOT reverse micelles. From A. Zieli
nska, E. Kowalska, J.W. Sobczak, I. Ła˛cka, M. Gazda, B. Ohtani, J. Hupka, A. Zaleska, Silver-doped TiO2 prepared by microemulsion method: surface properties, bio- and photoactivity, Sep. Purif. Tech. 72 (2010) 309.

Precipitate

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2.5 Solid-state reactions The most common chemical procedures to prepare (photo)catalysts are carried out in wet-medium, albeit, also precursors in solid-phase can be used in a dry medium to obtain the final solid. These are solid-state reactions, namely solid-to-solid reactions in which both the starting precursor(s) and the ultimate material are solids. The final material is obtained in the absence of solvents but a significant amount of energy is generally required to reach the desired temperatures as long as the photocatalyst is formed at high temperatures. For instance, in a hypothetical solid-state reaction we can have: AX + BX ! ABX2

(5)

where A and B are cations and X is the counter anion. It is assumed that single solids AX and BX would react to form the ABX2 solid catalyst. The possibility that a solid-state reaction could occur depends on the structural properties of the precursors and the associated thermodynamic free energy change [55]. A reaction at the phase boundary of AX and BX by a transfer of matter from one phase to the other could take place with the formation of nuclei and the growth of the product. The most important factors that influence the rate of ABX2 formation are (i) the area of contact between the reacting solids, (ii) the rate of diffusion of the ions through the phases and especially through the product phase, (iii) the rate of nucleation of the product phase. The area of contact between the grains of the reacting solids depends on their total surface areas and it increases with the decreasing of the particles size. The diffusion of ions in a solid is very slow and practically does not occur at relatively low temperatures. The rate of diffusion is enhanced by increasing the temperature or introducing crystal defects (e.g., vacancies) in the reagents that could decompose before or during the reaction. The rate of nucleation forming the final solid can be maximized by using reagents with crystal structures similar to that of the product and consequently reducing the amount of the necessary structural reorganization. The solid-state reaction would be optimized by maximization of the surface contact area of the precursors. A very simple approach could be to physically ground by manual mixing with a mortar and pestle small quantities of the precursor solids in some cases along with a small amount of a volatile organic liquid to better homogenize the mixture. During the process of grinding and mixing, the organic liquid gradually volatilizes and evaporates completely after few minutes. An example of these kinds of methodology would be the preparation of iron niobate photocatalysts from Fe3O4 and Nb2O5. Two solids would be obtained, FeNb2O6 by calcination below 700°C while FeNbO4 at temperatures above 800°C. The FeNb2O6 showed an improved photocatalytic activity for the degradation of methyl orange than that of FeNbO4 [56]. Also, ZnTa2O6 nanopowders were prepared from Zn(NO3)26H2O and Ta2O5 at calcination temperatures from 800°C to 1100°C for 12 h. The photocatalysts were tested for the conversion of CO2 into CO by using H2O as the reductant [57]. It is possible to prepare binary materials efficient as photocatalysts just by a simple solid–solid reaction in a mortar. For instance, GaP/TiO2 composites exhibited a remarkable photocatalytic activity for CO2 reduction in the presence of water vapor producing methane. By decreasing the GaP:TiO2 mass ratio an increase in the photocatalytic activity of the composite was observed for up to a 1:10 mass ratio. The photocatalytic activity of the composite can be attributed to the band structures of the solids as well as to the efficient charge transfer between GaP and TiO2 heterojunction. The efficient heterojunction was formed by TiO2 Evonik P25 and GaP Aldrich powders mechanically milled with a mortar grinder provided with grinding tools in agate that mixed and triturated by pressure and friction with a speed of 100 rpm at room temperature [58]. For large quantities of reagents, a mechanical ball mill would be adopted and the process may take several hours. The homogenized mixture is at last heated at high temperatures. Recently the organic polymeric semiconductor g-C3N4 has been successfully used as a heterogeneous photocatalyst even by using solar irradiation [59–61]. This graphene-like layered material is prepared by bottom-up approaches and it is easily obtained by thermal polymerization of abundant nitrogen-rich precursors such dicyandiamide, cyanamide, melamine, urea, thiourea, or ammonium thiocyanate, giving rise to yellow powders showing specific surface areas (SSA’s) typically in the range 4–7 m2 g
1. A further solid transformation occurs favoring the thermal exfoliation of the 2-D material. This process is carried out by further thermal treatment of the g-C3N4, giving rise to a thermally etched material possessing enhanced SSA and better photocatalytic activity than the original graphitic solid [62].

2.6 Citrate complex method Among various methods applied for the synthesis of photocatalysts, it is worth to be mentioned the citrate complex method. Such technique has been successfully applied for the synthesis of a series of Aurivillius-type photocatalysts with the

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formula, ABi2Nb2O9 (A ¼ Sr, Ba) [63–65]. The authors employed a peroxo-citrate-niobium precursor and found that citrate complex method offers the advantage to be low-cost, environmentally friendly, and represents a good alternative to the conventional sol–gel technique. The authors found a positive effect of the crystallinity, specific surface area, and crystal structure of the so obtained materials on their photocatalytic activity. Based on such promising results, Wu et al. [66] synthesized photocatalytic materials with the same formula ABi2Nb2O9 (A ¼ Sr, Ba). They prepared the SrBi2Nb2O9 photocatalysts by mixing at 65°C for 1 h, homogeneously in stoichiometric ratio, peroxo-citrato-niobium precursor, and strontium–bismuth–citrate solution. As the water evaporated, the solution turned into a gel with a high viscosity. The gel was heated at 300°C for 2 h to remove organic compounds and then calcined at 650, 750, and 850°C for 4 h. The as-prepared samples were investigated in the photocatalytic redox reaction of methyl orange under UV-light irradiation, in an aqueous solution, for the first time. It was demonstrated that the photocatalysts were successfully synthesized by the citrate complex method at a relatively low temperature as compared to the solid-state reaction. Single-phase orthorhombic SrBi2Nb2O9 (SBN) was obtained after calcination at a temperature as low as 650°C, as compared with the solid-state method. The BaBi2Nb2O9 (BBN) crystallized in a tetragonal structure. The SBN samples showed higher photocatalytic activities than those of BBN samples, due to the dipole moment present in the SBN crystal. Indeed, the SBN crystallizing in a polar space group, permits a dipole moment along with perovskite layers (the c axis), due to the distortion of the framework of perovskite layers, whereas the BBN does not (see Fig. 16). ZnNb2O6 is an attractive microwave dielectric material in the wireless communication field due to its excellent dielectric properties, moreover, Wu et al. [67] successfully investigated the photocatalytic degradation of organic pollutants

FIG. 16 Schematic structures of ABi2Nb2O9 (A ¼ Sr, Ba). From W. Wu, S. Liang, X. Wang, J. Bi, P. Liu, L. Wu, J. Solid State Chem. 184 (2011) 81–88 with Elsevier permission.

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of ZnNb2O6 synthesized by the citrate complex method. They found that the desired crystalline phase can be obtained at relatively lower temperatures (650–850°C) as compared to the conventional solid-state reaction requiring temperatures as high as 1100°C. The sample calcined at 850°C showed the highest photocatalytic performance in the degradation of methyl orange thanks to the best compromise between crystallinity and surface area. In past decades, several niobates and tantalates photocatalysts have been investigated for the elimination of environmental pollution. The photocatalytic degradation of dyes was preliminarily investigated under UV irradiation with BiNbO4 and BiTaO4 powders [68]. A few years before, Zou et al. showed that BiTa1
xNbxO4 oxides are visible light-active photocatalysts [69, 70]. It is well known that the photocatalytic activity is closely related to the crystalline phase and structure of the photocatalysts. BiNbO4 and BiTaO4 have orthorhombic a and triclinic b phases. The a phase formed at 900°C is known to transform to the high-temperature b phase at 1020°C for BiNbO4 and at 1150°C for BiTaO4, respectively. Conventional solid-state reaction route has been applied to the synthesis of BiNbO4 and BiTaO4 but causes some problems, such as large grain growth, segregations of some components, and loss of stoichiometry due to the evaporation of Bi element at high temperature. So, wet-chemical methods have been developed and among them the citrate method has been reported as one of the most promising. Zhai et al. [71, 72] have prepared BiNbO4 and BiTaO4 powders by citrate method through water soluble Nb–citrate–peroxide (pero–Nb–CA) and Ta–citrate–peroxide (pero–Ta–CA) complexes. They were able to form BiNbO4 and BiTaO4 as pure triclinic phases at the low temperature of 700°C thanks to the advantage of the citrate method allowing the control of morphological and structural properties. Between the investigated catalysts the BiNbO4 calcined at 700°C showed the best photocatalytic efficiency in the methyl violet degradation under visible light and this was ascribed to the large surface area and more positive conduction band level. In addition to so far discussed Nb based materials, sodium niobate is an important n-type perovskite material possessing a wide range of applications and thanks to the structural flexibility, NaNbO3 shows polymorphism over a wide range of temperature and has been applied in vastly advanced technologies such as photocatalysis. A variety of methods like reverse micelle, sol–gel, solvothermal, hydrothermal, and polymeric citrate precursor methods have been explored to synthesize NaNbO3 as nano-sized material. Recently, the synthesis at a low temperature of high surface area NaNbO3 perovskite by polymeric citrate precursor (PCP) method has been described and the photocatalytic activity of the obtained nanoparticles was investigated by the degradation of (RB) as a probe reaction [73]. Enhanced photocatalytic degradation (86%) of the organic dye (RB) was found on the surface of the nanosized NaNbO3 in comparison with the bulk oxide. Besides the so far reported photocatalysts, the citrate complex method has been widely applied to other classes of photocatalytic materials, such WO3, La(Sr)Co(Fe)O3 perovskites, ZnO. WO3 is an n-type semiconductor, it is inexpensive material with high stability in an aqueous solution under acidic conditions and an attractive candidate for photocatalytic applications, being a visible-light responsive photocatalyst that absorbs radiation in the region up to 480 nm [74]. Among several reports, it is worthy to be mentioned the synthesis of WO3 nanoparticles by acid citric-assisted precipitation involving the complexation of the metal ion by the citric acid, its precipitation, and decomposition by thermal treatment. The photocatalytic activity was determined in the degradation reaction under UV and UV–vis irradiation of organic dyes with complex molecular structures, such as rhodamine B (rhB), indigo carmine (IC), methyl orange (MO) and Congo red (CR) [75]. All the samples prepared by acid citric-assisted precipitation showed better photocatalytic activity than the commercial WO3. Several perovskites such as SrTiO3 [76], BiVO4 [77], LaCoO3 [78] and Bi2WO6 [79] have been demonstrated as promising photocatalysts for oxidation reactions, mainly in liquid–solid regime, showing a strong visible light absorption and high quantum efficiency. Jiang and Wei [80] have prepared the LaCoO3 perovskite by sol–gel process and they have studied the photocatalytic activity for dyes bleaching. Based on their promising results, Garcı´a et al. [81] have successfully synthesized by citric method perovskite oxides based on LaCoO3 by substituting the La cation by Sr and the Co cation by Fe. The resulting lanthanum strontium cobalt ferrite (La1
xSrxCo1
yFeyO3
d) materials have been tested as photocatalysts in gas–solid regime by using natural solar light as the irradiation source. The sample with composition La0.6Sr0.4CoO3
d showed the best catalytic activity in 2-propanol degradation, likely due to the combination of several factors, such as the optimal crystallite particle sizes and grains dimension, high specific surface area, thanks to the proper citric method along with to the presence of oxygen vacancies and low band gap energy value. ZnO is one of the most important functional oxides, due to its low cost and availability it is highly suitable for industrial applications and it has been widely investigated in photochemical oxidative processes. Several authors have investigated the effect of synthesis parameters on the ZnO properties. Giraldi et al. [82] prepared ZnO samples by two different wet methods, precipitation, and citrate method, and studied the effect of the synthesis parameters on the photocatalytic activity in the Rhodamine B degradation. Recently Bao et al. [83] investigated the morphologies of ZnO with various microstructures synthesized by a facile solution-phase method using different molar ratios of zinc nitrate hexahydrate (Zn(NO3)2) to hexamethylenetetramine (HMTA) and trisodium citrate. In a typical procedure, zinc nitrate hexahydrate and

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hexamethylenetetramine were added into deionized water in the sealed beaker and stirred continuously for 30 min to form a clear solution at room temperature. Then, the sealed beaker was heated at 90°C and kept for 3.5 h in a thermostatic waterbath pot, and finally allowed to cool naturally at room temperature. The white precipitate was collected by centrifugation and washed with distilled water several times. The washed products were oven-dried at 125°C for 6 h and finally annealed in air at 350°C for 3 h. The molar ratios of Zn(NO3)2 to HMTA were 1:0.1, 1:0.2, 1:0.5, 1:1, 1:2, and 1:4. It was found that by varying the molar ratio of Zn(NO3)2 to HMTA the formation of hollow ZnO microstructures changed. Meanwhile, ZnO with rod-like, flower-like, or spherical-like microstructures can be tailored by varying the molar ratio of Zn(NO3)2 to HMTA and trisodium citrate during the growth process. The so prepared materials were applied in the photocatalytic decolorization of an aqueous solution of methyl orange under ultraviolet light irradiation. The results showed that hollow ZnO microstructures had extremely high photocatalytic activities compared with those of solid ZnO samples. This remarkable photolytic activity is mainly due to their unique hollow structures, which results in a larger surface area to volume ratio. Finally, we would like to mention very new results about the preparation of Sr-doped ZnO powders from a citrate-modified zinc precursor solution [84]. Very few papers have been published regarding the phase formation and photocatalytic properties of Sr-doped ZnO. Trisodium citrate dihydrate was chosen as a capping agent because citrate ions strongly interacted with metal ions and significantly influenced the crystal morphology. Sr ion was chosen as a dopant to change the stoichiometric metal ion content in the synthesized ZnO. It was demonstrated that the crystallite size and particle shape of undoped ZnO and Sr doped powders were strongly affected by the concentration of Na3C6H5O72H2O in the precursor solution. Moreover, citrate ions played an important role in controlling the morphology of the synthesized ZnO powders and the formation of the Sr-doped ZnO phase.

2.7 Solution combustion synthesis (SCS) The last methodology that will be discussed in this chapter concern the synthesis of materials by solution combustion. This technique allows the preparation of inorganic, ceramic, and composite materials with tunable properties. Materials prepared by SCS are often metal oxides and they can find use as catalysts or photocatalysts and for several other applications as in electrocatalysis, sensoristic, and electronics [85]. Solution combustion synthesis should be considered as a soft chemical method, and it is a great method in terms of simplicity, cost-efficiency, and powder quality of the product. SCS is a technique based on a fast and self-sustained redox reaction between a fuel and an oxidant in the presence of metal cations. Usually, oxidants are metal precursors themselves, like for example metal nitrates, and the fuel is an organic material, for example, citric acid, urea, or glycine, that can form complexes with the metal ions of interest. Interestingly, this technique allows to save not only time but also energy concerning other methodologies, like sol–gel, and moreover it is suitable for the preparation of a large variety of inorganic nanopowders with high reactivity and tailored defects [86]. Generally, the final products obtained by SCS present an excellent phase purity and high surface area, narrow particle size distribution, optimum agglomeration, and good sintering properties [87]. Moreover, it is noteworthy that SCS can be used with a variety of precursors, both soluble [88] and insoluble [89]. Due to the great versatility of this methodology, here it will be reported a general procedure to prepare a material by SCS. The solution combustion synthesis involves three main steps: (1) preparation of the combustion mixture (2) formation of the gel (3) combustion of the gel. As far as the step (1) is concerning, the desired metal salt precursors are mixed in a water solution with an organic fuel. By heating the mixture at moderate temperature (ca. 80°C), upon the dehydration of the solution, a gel network is formed (step 2). The final step (3) is the combustion process between the fuel and the oxidant (i.e., nitrates that are the counter anions of the metal and eventually an additional oxidant) intimately connected in the gel network and it is initiated by further heating of the system by a thermal or electric source. Generally, after few seconds, a fluffy powder, whose features are strictly connected with the parameters chosen for the synthesis, is obtained. In particular, the parameters that influence the characteristics of the final material are the kind of fuel used, the reducers-to-oxidizers ratio, and the fuel-to-metal cations ratio. The fuel that has often the triple function of reducer, complexing agent, and microstructural template is usually an organic compound and it shows a key role in the optimization of the properties of the material. Through the SCS is possible to prepare several single-phase compounds that otherwise would be difficult to synthesize by conventional methods. For example, the preparation of PrScO3, as a photo-catalyst, by a solid-state synthesis gave an impure product, but, on the contrary, Sayed et al., by using a glycine–nitrate combustion reaction in various oxidant-to-fuel ratios obtained in a single step a sample constituted of pure PrScO3 phase [90]. Stabilizing some of the metastable compounds in the phase relation is another aspect where combustion synthesis excels over conventional methods. For example, the stability of the fluorite structure in the Gd2
xCexZr2
xAlxO7 (0.0  x  2.0) system was attributed to the nonequilibrium nature of the solution combustion synthesis and the nanocrystalline nature of the compound [91]. Moreover, because the nano-powders obtained by combustion synthesis are highly fluffy and present a large number of active sites, they can be

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used as active catalyst support. For instance, a catalyst with 5% of palladium supported on CeZrO4-d in the fluorite structure was successful used in the heterogeneous Suzuki coupling in water, exhibiting 100% conversion of iodobenzene to biphenyl in 1 h [92]. Nair et al. report an example of photocatalyst preparation by SCS [93]. Authors, synthesized by combustion synthesis method a set of nano catalysts constituted of cerium doped titania in which the content of Ce was 0.5, 1 and 2 mol%. The combustion synthesis of pure and cerium doped TiO2 nanoparticles was carried out by using cerium nitrate and titanium isopropoxide as the source of the respective metals and glycine as the organic fuel. Titanium Isopropoxide and glycine in the 5:1 ratio were well mixed, transferred into a crucible, fired, and finally calcined at 500°C in a pre-heated muffle for 4 h. Final materials resulted in a mixture of anatase and rutile phases and, as reported by the authors, this fact positively affected the photocatalytic activity of these catalysts suggesting the existence of a synergistic effect between anatase and rutile powders under visible light. The photocatalytic activity was evaluated for the degradation of methylene blue (MB) under visible light irradiation. The degradation rates of MB on cerium doped TiO2 samples were higher than that of pure TiO2, probably because the introduction of structural defects (cationic ceria dopant) into the titania crystal lattice leads to the change of band gap energy. As a result, the excitation energy is expanded from UV light of anatase TiO2 to visible light for ceria doped titania. In particular, the sample containing 1% of Ce showed the highest photoactivity, probably because of the larger specific surface area showed by this catalyst and due to the higher separation efficiency of electron–hole pairs.

3 Conclusions Developing next-generation (photo)catalysts will require a fundamental understanding not only of the chemical–physical features of the material but also of the catalytic mechanisms and structure–property relationships. This chapter is an overview of some of the most used preparation processes to obtain bulk materials mainly by wet-chemical processes without using sophisticated devices. In particular, This chapter reports the chemical methodologies commonly adopted on laboratory scale, including precipitation/co-precipitation, sol–gel, hydrothermal/solvothermal procedures, microemulsion technique, solid-state reactions, and solution combustion synthesis, for the photocatalysts preparation. Moreover, for each photocatalyst prepared by the various methodology here reported, it is also described, at least, a photocatalytic application. In the next Chapter 3 some alternative methodologies will be described, namely those involving the use of tools more commonly reported for the preparation of photocatalysts.

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xNbxO4 (0< x< 1), Chem. Phys. Lett. 343 (2001) 303–308. [70] C.Y. Lee, R. Macquart, Q.D. Zhou, B.J. Kennedy, Structural and spectroscopic studies of BiTa1
xNbxO4, J. Solid State Chem. 174 (2003) 310–318. [71] H. Zhai, J. Kong, A. Wang, H. Li, T. Zhang, D. Wu, The polymerization effect on synthesis and visible-light photocatalytic properties of lowtemperature b-BiNbO4 using Nb-citrate precursor, Nanoscale Res. Lett. 10 (2015) 457. [72] H.F. Zhai, A.-D. Li, J.-Z. Kong, X.-F. Li, J. Zhao, B.-L. Guo, J. Yin, Z.-S. Li, D. Wu, Preparation and visible-light photocatalytic properties of BiNbO4 and BiTaO4 by a citrate method, J. Solid State Chem. 202 (2013) 6–14. [73] U. Farooq, R. Phul, S.M. Alshehri, J. Ahmed, T. Ahmad, Electrocatalytic and enhanced photocatalytic applications of sodium niobate nanoparticles developed by citrate precursor route, Sci. Rep. 9 (2019) 4488. [74] K. Sayama, H. Hayashi, T. Arai, M. Yanagida, T. Gunji, H. Sugihara, Highly active WO3 semiconductor photocatalyst prepared from amorphous peroxo-tungstic acid for the degradation of various organic compounds, Appl. Catal. B Environ. 94 (2010) 150–157. [75] D. Sa´nchez-Martı´nez, A. Martı´nez-de la Cruz, E. Lo´pez-Cu
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[81] E. Garcı´a-Lo´pez, G. Marcı`, F. Puleo, V. La Parola, L.F. Liotta, La1
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xCexScO3 (0.0 x 1.0) systems, Dalton Trans. 44 (2015) 16929–16936. [91] R. Shukla, F.N. Sayed, S. Phapale, R. Mishra, A.K. Tyagi, Evolution of different phases in metastable Gd2–xCexZr2–xAlxO7 (0.0  x  2.0) system: crucial role of reaction conditions, Inorg. Chem. 52 (2013) 7873–7880. [92] A.S. Burange, R. Shukla, A.K. Tyagi, C.S. Gopinath, Palladium supported on fluorite structured redox CeZrO4-d for heterogeneous suzuki coupling in water: a green protocol, Chem. Select 1 (2016) 2673–2681. [93] R.R. Nair, K.R. James Arulraj, S. Devi, Ceria doped titania nano particles: synthesis and photocatalytic activity, Mater. Today 3 (2016) 1643–1649.

Chapter 3

Preparation of photocatalysts by physical methodologies Elisa I. Garcı´a-Lo´peza and Giuseppe Marcı`b a

Department of Biological, Chemical and Pharmaceutical Sciences and Technologies (STEBICEF), University of Palermo, Palermo, Italy, b Department

of Engineering, University of Palermo, Palermo, Italy

1 Introduction As it has been mentioned in the two previous chapters, the efficiency of a heterogeneous photocatalysts is determined by its chemical and physical properties. The optimal photocatalyst would present a series of features including high activity and selectivity, easy regeneration, long lifetime, nontoxicity, and low cost, and its preparation would be easy and possibly obtained by an environmentally sustainable and green method. By using different preparation methodologies and by controlling the parameters during the synthesis, it is possible to address the process to obtain the searched properties thus favoring the required performances. The chemical methodologies, described in Chapter 2, used the classical approach to obtain photocatalysts for bulk and supported materials; however, the preparation of photocatalysts with the aid of physical methodologies and using more or less sophisticated apparatuses is constantly increasing in the scientific literature. The procedures of preparation of photocatalysts base on physical methodologies are generally considered as top-down approach. The physical methods comprise some techniques to obtain bulk materials but are mainly focused on the formation of supported materials or thin films. Some techniques, such as sputtering or evaporation, are commonly used, and other, more sophisticated, laser-assisted deposition resulted in diffusion. However, the deposition of many compounds can be a challenge, especially in achieving the desired size and morphology of the nanostructures. The most common methodologies based on physical procedures to obtain photocatalysts are summarized in Fig. 1. In the following paragraphs, a description of the various preparation methods, outlined in Fig. 1, is reported along with some examples of the photocatalytic materials obtained.

2 Bulk photocatalysts 2.1 Spray pyrolysis The spray pyrolysis procedure requires the acceleration of a liquid or a liquid-solid phase precursor from an atomizing nozzle (atomizer) to form a droplet of reagents that suffer a thermal reaction. The atomizers are divided into four classes: gas energy atomizers (pneumatic acceleration), mechanical, vibrational, and electrical energy; hence, spray pyrolysis is classified in three types: (i) air blast or pressurized spray pyrolysis, (ii) ultrasonic, or (iii) electrostatic spray pyrolysis [1]. The process basically consists on the thermal decomposition of droplets of a precursor in a hot wall reactor. A starting solution containing the precursor, usually a metal salt (nitrates, chlorides, and acetates), is dissolved in water or alcohol. Precursor droplets formed by atomization are transported by a gas carrier to a high temperature zone, where the droplets are pyrolyzed to form the final product. Solvent evaporation and solute precipitation take place within the droplet, followed by drying and thermolysis of the precipitate particle at high temperature to form a microporous particle and hence sintering to form a dense particle. As reported in Fig. 2A, the droplets, which are atomized from the starting solution, are introduced to the furnace. The evaporation of the solvent, drying, precipitation, and the reaction between the precursor and the surrounding gas, pyrolysis, or sintering occur inside the furnace to form the final (photo)catalyst. Alternatively, in the spray-drying method, colloidal particles or sols are used as precursors. Spray pyrolysis produce spherical uniform and dense particles sized from submicron to microns or even a nanostructured powder if the precursor is composed of colloidal nanoparticles. Multicomponent materials can be prepared in one-step if the precursor solution contains several metal compounds. Different morphologies of bulk materials can be obtained by modifying the parameters during the preparation, Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00007-6 Copyright © 2021 Elsevier Inc. All rights reserved.

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Physical methodologies for photocatalysts preparation Films/Coatings

Bulk (photo)catalysts

Dip-coating

Spray pyrolysis Wet-coating

Spin-coating

Flame hydrolisis Flame syntheses Flame spray pyrolisis

Chemical vapor deposition

Microwave/electromagnetic irradiation Physical vapor deposition Sonochemical irradiation

Vacuum deposition Sputtering Arc-vapor deposition Pulsed laser deposition

Spray pyrolisis deposition

Mecanochemistry

Electrochemical deposition FIG. 1 Physical methodologies for the preparation of bulk and film/coatings photocatalysts.

Starting solution / Precursor

Atomization Droplets

Gas-2 Gas-1 (carrier) Large droplets

Small droplets

On Line Solvent evaporation Solute diffusion

τsv ≈ τsl

Solid particle formation Dried particle

τsv s) pulses of light. Frequency-resolved techniques involve mainly (photo)electrochemical measurements, among which photoelectrochemical impedance spectroscopy (PEIS), intensity-modulated photocurrents spectroscopy (IMPS) and intensitymodulated photovoltage spectroscopy (IMVS) can be distinguished. These techniques are more challenging in understanding and require a more complex theory that should be adapted separately to each studied case.

5.1 Time-resolved techniques (a) Time-resolved UV-vis-NIR spectroscopic studies Time-resolved spectroscopic studies include mainly time-resolved luminescence and transient absorption measurements. Examination of the luminescence decay dynamics of the materials may give information about the charge carriers’ recombination rates. The energy of the emission bands carries also information about the energy levels from which the electrons recombine with the holes. However, it is difficult to draw conclusions about the trapping processes, trapped charge carriers’

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FIG. 10 Transient absorption spectrum of a TiO2 film in nitrogen-saturated D2O (full circles) composed of the absorption spectrum of electrons normalized in the near-IR wavelength range (solid line) and differential spectrum (empty circles) assigned to trapped holes. The spectrum of electrons is composed of a broadband with maximum at ca. 800 nm assigned to the trapped electrons and a monotonically increasing absorbance above 1000 nm characteristic for free electrons (A). Temporal profile of transient absorption observed for the selected wavelengths recorded for TiO2 film (B). (Reprinted with permission from T. Yoshihara, R. Katoh, A. Furube, Y. Tamaki, M. Murai, K. Hara, S. Murata, H. Arakawa, M. Tachiya, Identification of reactive species in photoexcited nanocrystalline TiO2 films by wide-wavelength-range (4002500 nm) transient absorption spectroscopy, J. Phys. Chem. B 108 (12) (2004) 3817. Copyright (2004) American Chemical Society.)

lifetimes, or mechanisms of interfacial charge transfer processes. Moreover, photocatalytic materials, typically semiconductors with indirect bandgap, are characterized by inefficient, or even scarce, luminescence. Thus, not all materials are suitable for luminescence studies. More informative time-resolved spectroscopic techniques are based on the transient absorption measurements (transient absorption spectroscopy, TAS) which are mainly carried out within the UV-vis-NIR range. The principle of these measurements uses the pump-probe approach. The studied sample is excited by a very short light pulse (usually from high energy lasers), followed by the detection of the changes in the absorption spectra of the excited sample using a probe white light pulses within the measurement time range [38]. The absorption changes are usually measured at the desired wavelength, successively selected for each pump-probe event. The observed signals represent exponential decays (or rises) to which the mono- or multiexponential function may be fitted to find the time constants of the observed processes (Fig. 10). For technological reasons, the UV-vis-NIR transient absorption measurements are usually performed in two temporal regimes: an ultrafast regime—covering processes lasting from several hundred femtoseconds to several nanoseconds, and a nano-tomillisecond time regime. In the case of semiconductors, any time-dependent change in the absorption spectra of the excited sample corresponds to the processes involving photogenerated charge carriers. Both free and trapped holes and electrons have specific absorption spectra (Fig. 10). Electrons in the conduction band are usually characterized by a very broad absorption band reaching the IR range. The trapped charge carriers are manifested in the spectrum by a set of narrower bands, which are characteristic of each material. In the case of TiO2, trapped holes and electrons are characterized by bands with maxima in the wavelength ranges between 350–630 and 600–800 nm, respectively [38, 39]. A precise assignment of the transient absorption bands to electrons and holes is sometimes difficult due to their overlap. Transient absorption measurements performed in the presence of different electrons or hole scavengers, under different atmospheres, allow not only to distinguish the absorption bands but also to investigate the surface trapping and interfacial charge transfer processes [40–42]. Also, an extension of the detection light range towards near-infrared regions allows deconvolution of the trapped and free electrons bands and helps in the interpretation of the observed changes [39, 43, 44]. Within this region, due to the lack of the interference of holes, mainly dynamics of the processes involving electrons may be followed. Moreover, the shape of the transient absorption bands in NIR is less sensitive to the surrounding conditions. A big advantage of these spectroscopic techniques is that they are not limited to a specific size of the particles of studied materials, which affects only the type of the used detection setup. It might be dedicated to the absorption measurements of transparent samples (colloidal solutions and films) or reflectance measurements of solid samples [45]. Nevertheless, the time-resolved reflectance spectroscopy is less sensitive compared to the absorption measurements due to the strong scattering of probe light and possible photoluminescence resulting from the recombination [39, 45]. The only caution which should be taken while performing TAS experiments is to pay attention on the possible structural changes, such as surface reorganization, which might be induced upon the sample illumination with the powerful pulsed laser light and may influence the absorption changes [46]. Transient absorption spectroscopy is a powerful tool not only in studying single-semiconductor systems but also in explaining the mechanisms of charge transfer processes taking place in

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heterostructures [47–49]. TAS measurements can be also combined with electrochemical methods, which allows studying charge carrier dynamics upon the applied bias, which is very useful in studies on materials for electrocatalysis [50,51]. (b) Time-resolved IR spectroscopy Time-resolved IR spectroscopy (TRIR) is another pump-probe technique operating in the time domain. What distinguishes TRIR from TAS, is the different sources of the pumping light (IR beam) and the detection system. In TRIR, two main approaches for probing the IR signal may be realized: the dispersive approach (including 2D IR spectroscopy) and the step-scan approach [52]. In TRIR, two main approaches for probing the IR signal may be realized: the dispersive approach (including 2D IR spectroscopy) and the step-scan approach [52]. Changes of the IR spectra of photoactive materials upon excitation can provide information on the temporal changes in the concentration of the trapped and mobile charge carriers [52–54]. The type of spectral changes and their interpretation depends mainly on the nature of the studied semiconductor. Some photocatalysts, such as TiO2, SrTiO3, or NaTaO3, show only nonspecific spectral changes, indicating that absorption of the probing IR photons leads to the charge carriers’ transition into a continuous band rather than to discrete energy levels (Fig. 11B) [53–56]. In such case, mainly the photoinduced electron transitions in the conduction band in an intraband manner or the transition from the shallow midgap trap-states to the conduction band are observed (Fig. 11A). However, for many intrinsic semiconductors (e.g., BiOCl, BiVO4, or g-C3N4), surface-modified or doped-materials and organic polymers, different spectra with well-defined and relatively narrow vibrational bands may be observed upon excitation (Fig. 11B) [52,57–59]. Interpretation of such spectral changes is challenging and using other techniques, such as ATR-IR or TAS spectroscopy, or performing TRIR experiments with various charge carrier scavengers, allowing differentiation between bulk and surface phenomena, are usually required for correct conclusions. The TRIR technique is not only useful in the evaluation of the charge carrier dynamics, but it also allows to monitor the very fast changes in the spectra of species adsorbed at the photocatalyst or created upon its excitation [52–54,59]. It enables tracking of short-lived intermediates at the surface of semiconductors in the first steps of photocatalytic processes. The TRIR technique together with the in situ ATR-IR measurements, which are sensitive to transformations of the adsorbed species at a temporal resolution larger than seconds, constitute a powerful tool for studying photocatalytic reaction mechanisms [58]. (c) Time-resolved microwave conductivity measurements Time-resolved microwave conductivity (TRMC) technique, also called transient photoconductivity, is a nondestructive and contactless method allowing in situ characterization of photosensitive powder materials [60–64]. It gives information on the excess charge carrier lifetimes and recombination and trapping dynamics usually within the 109–103 s time scale and effective mobility of the generated carriers [65]. The excitation of the powder sample leads to the generation of free charge carriers, which affects the conductivity of the materials. In turn, the change of the conductivity affects the probing

FIG. 11 Possible photoinduced transitions induced and observed within the IR region (A). The TRIR spectra recorded for TiO2 (B) [54] and BiOCl (C) [57]. ((B) From A. Yamakata, T.-a. Ishibashi, H. Onishi, Kinetics of the photocatalytic water-splitting reaction on TiO2 and Pt/TiO2 studied by time-resolved infrared absorption spectroscopy, J. Mol. Catal. A: Chem. 199 (1–2) (2003) 85. Copyrights: 2003 Elsevier; (C) From I. Benisti, Y. Paz, Transient FTIR measurements at nanoseconds resolution: correlating between faceting and photocatalytic activity in BiOCl, J. Electrochem. Soc. 166 (5) (2019) H3257. CC BY 4.0.)

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microwave field. The TRMC technique is based on time-resolved measurement of the changes of microwave power reflected by the sample caused by the generation and decay of charge carriers in the sample exposed to pulsed laser irradiation. Relative differences in the power of reflected microwave-induced by laser pulses are proportional to the differences in the conductivity according to the equation: X DPðtÞ ¼ ADsðtÞ ¼ Ae Dni ðtÞmi Pin i

(17)

where DP is a change of the reflected microwave power, Pin is an initial power, A is a sensitivity factor derived from the dielectric properties of the medium and the resonance characteristics of the cavity, Ds—induced conductivity, Dni(t)— change in the carrier concentration, mi—respective effective carrier mobility. To assure this linear dependency, a small perturbation approach should be preserved. The decay of the TRMC signal reflects not only the decay of free charge carriers but also their drift if they move between points of different electric field strength. In the TRMC technique, the changes caused by the deeply trapped charge carriers are usually neglected due to their small mobility. Therefore, obtained information applies only to mobile electrons and holes in the conduction and the valence band, respectively. In some cases, e.g., in TiO2, due to the much higher mobility of the electrons compared to the holes, the TRCM signal (microwave photoconductivity) may be attributed mainly to free electrons [64,66]. As with the transient absorption measurements, in the TRCM signal two time-ranges, can be distinguished: short-range and log-range decays. The first one, usually lasting up to tens of ns after the maximum of the excitation pulse reflects a fast electron-hole recombination process. The obtained signal is represented by the It/Imax ratio. The second range decay is related to slower processes involving trapped charge carriers, following trapping-detrapping events, including recombination or interfacial charge transfers, and the shape of the signal decay is characterized by several halftime lives associated with charge-carrier lifetimes (Fig. 12) [61]. Microwave photoconductivity measurements may be also performed in the presence of various electrons or holes scavengers giving information on dynamics of the interfacial charge transfer processes [62,66,67]. (d) Time-resolved photoelectrochemical methods. The time-resolved photoelectrochemistry constitutes a basis for alternative methods for following the fate of photogenerated charge carriers. Unlike the TAS or TRMC techniques, electrochemical methods require the use of a layer of the studied material deposited on a conductive substrate and a (liquid) electrolyte, which may influence the intrinsic properties of the semiconductor material. In the time-resolved photoelectrochemical techniques, semiconductors are usually excited by high power short laser pulses or chopped illumination [68]. Nonsteady-state photoresponse can be also measured after switching on and off continuous irradiation. The electrical response of the system perturbation caused by light can be

FIG. 12 Intensity-normalized TCRM signals induced by 300 nm laser pulses (3.2 ns FWHM) of an H2TPPC film alone (dashed trace), a nanocrystalline TiO2 film (nc-TiO2) alone (solid line) and nanocrystalline TiO2 film coated with H2TPPC (5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin) (dotted line). The arrow indicates the position of DGmax. (Adapted with permission from J.E. Kroeze, T.J. Savenije, J.M. Warman, Electrodeless determination of the trap density, decay kinetics, and charge separation efficiency of dye-sensitized nanocrystalline TiO2, J. Am. Chem. Soc. 126 (24) (2004) 7608. Copyright (2004) American Chemical Society.)

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FIG. 13 Processes occurring near the surface of the semiconductor upon excitation (SS, surface states) (A) [69]. The rate constants are defined in the text. Laser-induced photocurrent transient response recorded for hematite photoanode showing photocurrent overshoot (B) [50]. Theoretically calculated transient photocurrent response to chopped irradiation when no recombination (upper figure), almost complete recombination (in the middle), and partial recombination (bottom figure) occurs at the surface of n-type semiconductors (C) [68]. ((B) Reprinted with permission from F. Le Formal, S. R. Pendlebury, M. Cornuz, S.D. Tilley, M. Gratzel, € J.R. Durrant, Back electron–hole recombination in hematite photoanodes for water splitting, J. Am. Chem. Soc. 136 (6) (2014) 2564. . Copyright (1990) American Chemical Society. (C) Reprinted with permission from L.M. Peter, Dynamic aspects of semiconductor photoelectrochemistry, Chem. Rev. 90 (5) (1990) 753. Copyright (1990) American Chemical Society.)

followed at an open circuit, when photopotential is recorded, or under controlled-potential conditions, in which case the photocurrent is measured. In the latter one, the applied potential influences the potential drop in the space charge region near the semiconductor surface (i.e., the equilibrium density of electrons and holes at the surface) and allows to control the rates of generation and collection of charge carriers under excitation. The transient response of the illuminated semiconductor/electrolyte junction is a result of a number of processes (charge carriers trapping, recombination, interfacial electron transfer) taking place at different time scales (Fig. 13A) [69]. In laser-induced photocurrent transients, it is assumed that the light flash is sufficiently short to produce charge carriers fluxes. The minority charge carrier flux towards the surface causes charging of the space charge layer, changing its capacitance (CSC), which subsequently discharges [68]. The photocurrent decay will contain the contribution of the exponential discharge through the solution resistance with the RsolCSC time constant and the current induced by surface recombination (in n-type semiconductors it might be referred to back electron transfer). In the case of Faradaic processes occurring at the semiconductor surface, the photocurrent transients may contain also kinetic information of the charge transfer [68,70]. The transient photocurrent response to chopped light, especially observed at the applied potentials close to the flat band potential, usually does not follow the light intensity profile but shows additional characteristic spikes assigned to surface charging and recombination processes. They arise from the differences in the rates of those two processes. For n-type semiconductors, it is usually manifested by a very fast decay of photocurrent after turning on the light (charging process) and change of the sign of the current (the “overshoot” currents) after the light is off (Fig. 13C). The results of such photocurrents transient measurements can be used for qualitative characterization of the accumulated charges. (e) Frequency-resolved techniques Frequency-resolved methods have a great potential in studying dynamics of the photoinduced processes taking place on a time scale of microseconds to seconds. We can distinguish PEIS (photoelectrochemical impedance spectroscopy), IMPS (intensity-modulated photocurrent spectroscopy), and IMVS (intensity-modulated photovoltage spectroscopy) techniques. In general, in these methods, the sample is perturbed by periodically modulated stimulus (e.g., voltage, light) and the phase shift and magnitude changes of the output signal (e.g., current, voltage) are measured in relation to the input at various perturbation frequencies. The photoelectrode response recorded for each frequency depends on the rate constants of charge carriers trapping (ktr), recombination (krec), and charge transfer (through the surface states (kETtr) and through the valence band (kETfree)) processes (Fig. 13A). The main assumption of these techniques is that the low amplitude of the input signal

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–0.35 V –400

–3.0 V vs SCE –0.05

–800 0

400

800

1200

1600

2000

Re(Z(w))/W Im(Z(w))/W

0

–0.45 V –200

–0.10

–2.0

–0.15 –0.20 –0.25 –0.30

–1.0

–0.35 –0.40 –0.45

–400 0

(A)

Im(j(w)) / 10–7 A

Im(Z(w))/W

0

200

400

600

Re(Z(w))/W

800

1000

0.0 0.0

–0.50

–1.0

–2.0

–3.0

–4.0

–5.0

–6.0

–7

(B)

Re(j(w)) / 10 A

FIG. 14 Complex plots of impedance (PEIS) obtained for illuminated p-InP electrode measured at two different potentials (A). Complex plot of IMPS response of the p-InP electrode obtained at various potentials (B) [71]. (From E. Ponomarev, L. Peter, A comparison of intensity modulated photocurrent spectroscopy and photoelectrochemical impedance spectroscopy in a study of photoelectrochemical hydrogen evolution at p-InP, J. Electroanal. Chem. 397 (1–2) (1995) 45. Copyright 1995 Elsevier.)

assures a linear response of the system. Usually, the relationship between the perturbation (input, P(o)) and response (output, R(o)) is described by the transfer function, G(o): GðoÞ ¼

RðoÞ : PðoÞ

(18)

P(o) and R(o) signals may be represented by P0
eiot and R0
ei(ot+’), respectively, where P0 and R0 are magnitudes of the perturbation and response modulation, and f is a phase shift. The transfer function can be represented by a complex number (G(o) ¼ G0 cos(ot) + G0 i sin(ot)) and presented at the complex plot (Fig. 14). The PEIS refers to the electrochemical impedance spectroscopy performed under conditions of steady illumination. In PEIS technique, the current response of the sample (I(o)) on the applied voltage periodic perturbation (V(o)) is measured and used for the determination of the impedance of the measuring system (Z(o) ¼ V(o)/I(o)). Similar to EIS, the interpretation of the results may be based on the assumed electronic equivalent circuit of the measuring system, however, models based on kinetic equations allow to avoid difficulties related to the assignment of the physical meaning to the different electronic components. In this method, if the relatively low light intensity is used, illumination assures only charge carriers generation and it is the voltage perturbation, which affects the recombination rate constant [71,72]. For simplicity of the data analysis, it is usually assumed that the space charge capacitance is much smaller than the Helmholtz layer capacitance. In such a case, the potential perturbation is applied only across the depletion layer what influences the surface density of the majority carriers and results in changes in recombination rate constant. Unfortunately, for many materials (especially semiconductors with a high density of surface states) the potential perturbation influences not only the depletion layer but also Helmholtz layer capacitance (which are usually presented in equivalent circuits as connected in series) what leads to complex kinetic equations [73]. In the IMPS and IMVS techniques, the photocurrent or photovoltage, respectively, are measured in relation to the illumination perturbation [69,74]. The applied voltage in the case of IMPS is constant. The measured photoresponse value results from the competition between the trapping, recombination, and charge transfer processes occurring in the excited material (Fig. 13A). In this method, illumination assures charge carriers generation and the AC light perturbation affects the contribution of the basic processes in net photocurrent or photovoltage formation, depending on their rate constants. The main condition that must be assured is low light intensity and small amplitude of its perturbation, which assures insignificant changes of band banding, space charge capacitance, and total majority carriers concentration upon excitation, as well as the negligible influence of the AC illumination level perturbation on the diffusion coefficient of electrons [72]. The analysis of results is usually based on the set of kinetic equations describing fundamental processes (Fig. 14A) [72,74]. Analogously to the PEIS, the output signal (photocurrent and photovoltage or the quantum efficiency and impedance photovoltage transfer functions) obtained for various frequencies of the input perturbation is presented at the complex plot (Fig. 14B).

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Since in the IMPS and PEIS techniques the photoelectrode response (photocurrent) to illumination and potential changes will depend on the dynamics of the fundamental processes, both methods can be used for the determination of the surface recombination and charge transfer rates constants. The superiority of IMPS over PEIS, is that it allows for separation of the space charge and Helmholtz layers capacitances, which is rather difficult in the case of PEIS.

6 Conclusions Experimental methods used to elucidate thermodynamic properties of semiconductors, both as single materials or in combination with other components, have been presented. Starting from a simple and widely applied UV-vis spectroscopy, usually in the diffuse reflectance mode, one can determine the band gap energy of semiconductors. Although this method seems well known and easy to handle, particular attention has to be paid to analyze properly recorded spectra. Having Eg determined, it seems reasonable to determine energies (or potentials) of the conduction and valence band edges. Various approaches offer different complexity of measurements but also the different meaning of resulting values: the Fermi level, quasi-Fermi level, flat band potential, energy of various electronic states, etc. Depending on the material (single crystals, nanocrystalline materials, transparent or opaque films) but also expected information (redox properties of either nonexcited or excited semiconductors, potentials of band edges, work function) different methods can be applied. Analysis of the obtained data has to take into account physical and chemical processes taking place at interfaces and surfaces, like charging/discharging, band bending, splitting of Fermi level, trapping/detrapping, recombination, etc. To gain more complete information, a picture of the density of electronic states, including surface states, should be attained. In this way, one can recognize reactive states (their potentials) offering longer lifetimes of trapped charge carriers but still their high activity, or deep electronic states, usually acting as recombination centers. Several techniques offer experimental determination of DOS functions, at least close to the conduction band bottom. Among others, SE-DRS seems particularly useful, due to its simplicity (combination of UV-vis DRS and simple electrochemical techniques) and reliability. Finally, time- and frequency-resolved techniques support information on the kinetics of photoinduced charging/ trapping and discharging processes. In particular, the rates of charge carrier trapping, the lifetime of the trapped carriers, and the rates of recombination and interfacial charge transfer processes may be determined under the selected experimental conditions (e.g., the concentration of electrons and holes scavengers, spectral range, and intensity of incident light, pH, solvent type, applied potential in the case of photoelectrochemical techniques), what does not only inform about the material properties but also sheds light on mechanisms of photocatalytic or photosensitization processes, including charge transfer processes taking place at the interfaces in heterostructures. Moreover, it is possible to distinguish between processes involving electrons and holes. Since thermodynamics and kinetics of primary photophysical and photochemical processes govern the overall photoactivity of photocatalytic and photoelectrocatalytic systems, a proper determination of various thermodynamic and kinetic parameters, as well as their proper interpretation, are of paramount importance in understanding and designing photoactive systems.

Acknowledgment The work was supported by the Foundation for Polish Science (FNP) within the TEAM project (POIR.04.04.00-00-3D74/16).

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Chapter 7

Photoelectrochemical characterization of photocatalysts Francesco Di Franco, Andrea Zaffora, and Monica Santamaria Dipartimento di Ingegneria, Universita` degli Studi di Palermo, Palermo, Italy

1 Introduction Since the discovery, in 1972, of the water photosplitting by Fujishima and Honda [1], many efforts have been spent for the development of semiconductor photocatalysis. This is because photocatalysis is prone to inducing effective reactions at room temperature under sunlight irradiation, and it has the potential to establish ideal technologies which could efficiently convert clean, safe, and abundant solar energy into electrical and/or chemical energy [2, 3]. To prepare highly efficient and functional photocatalysts, tailoring of semiconducting materials, e.g., their chemical composition, physical properties, and band structure, is crucial for employing them for specific applications with desired reactivity and selectivity. In particular, the chemical composition includes elemental composition, chemical state, and structure whereas with physical properties, we can include physical structure, crystallographic properties, optical absorption, charge dynamics, defects, and colloidal and thermal stability [4]. Regarding materials’ band structure, it is very important in defining which reactions can be achieved with a specific photocatalyst. In fact, possible oxidation and reduction half-reactions that can be carried out on the surface of a semiconducting photocatalyst strongly depend on the energy position of semiconductor conduction and valence band [5–9], as well as on the reduction potentials of organic or inorganic compounds that are dependent on pH and temperature [10]. Among the other techniques that are useful to study the band structure of semiconducting photocatalysts, e.g., Density Functional Theory (DFT), Photoluminescence (PL), X-Ray Photoelectron Spectroscopy (XPS), the photoelectrochemical characterization carried out through Photocurrent Spectroscopy (PCS) technique can represent a practical, low-cost and reliable way in studying materials’ band structure since it is based on the analysis of the electrochemical response (photocurrent or photopotential) of a system under irradiation. In this way, we can get information about the energetics of the semiconductors (SC)/electrolyte (El) interface (flat band potential, conduction and valence band edges location, internal and/or external photoemission thresholds) [11]. The objective of this chapter is to review the theoretical background on the photoelectrochemical behavior of crystalline semiconductors and how it is possible to interpret PCS data to estimate the semiconductors bandgap, to locate energy levels of conduction and valence band as well as that of their Fermi level. This will be done to have information about the complete band structure of the semiconducting photocatalysts, relating it to their optical absorption and redox ability.

2 Fundamentals on photocurrent spectroscopy Photoelectrochemical characterization of semiconducting photocatalysts can be carried out through Photocurrent Spectroscopy (PCS) technique. PCS is currently widely employed for studying the solid-state properties of semiconducting and insulating materials [12–15] since the knowledge of their band gap is a prerequisite to any possible application in different fields such as solar energy conversion (photoelectrochemical and photovoltaic solar cells, photocatalysis) and microelectronics (high-k, high band-gap materials) [16–19]. As with other optical techniques, PCS is a non-destructive technique based on the analysis of the electrochemical response (in terms of photocurrent or photopotential) of the SC/El interface under irradiation with photons of suitable energy and intensity. Unlike other optical methods, PCS presents the characteristic that the photocurrent response of the material is directly related to the number of absorbed photons, i.e., the technique does not require a surface finishing so that it can be also used on rough or nanostructured surfaces [11].

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Despite PCS being able to provide information about the energetics of the photocatalyst/electrolyte interface (oxide flat band potential determination, conduction and valence band edges location, internal photoemission threshold), it has also some limitations owing to the following aspects: 1. only photoactive layers can be investigated; 2. surface layers having optical bandgap lower than 1 eV or larger than 5.5 eV could be characterized with a special setup or they are not experimentally accessible in aqueous solutions. A typical PCS setup is shown in Fig. 1. It is composed of a UV–vis xenon lamp coupled with a monochromator, which allows the irradiation of the specimen with a suitable wavelength (i.e., energy). The irradiation is interrupted with a known frequency (e.g., 13 Hz), usually with a mechanical chopper. It is coupled with a two-phase lock-in amplifier necessary to extract weak photocurrent signals in the presence of a large dark current. It is important to note that whenever the photocurrent is measured using a lock-in amplifier, the signal intensity depends on the ratio between the chopping angular frequency, oc, and the time constant of the electrical equivalent circuit of the junction, τ (with τ ¼ RtCt, where Rt and Ct representing the total resistance and capacitance of the junction) [20].

3

Photoelectrochemical behavior of semiconductor/electrolyte junction

To better understand the photoelectrochemical processes at the SC/El interface, it is useful describing the density of states (DOS) distribution in a crystalline as well as in an amorphous material. For a more complete description of the band model of solids, interested readers can refer to specialized literature [21–23]. Typically, amorphous materials maintain the same short-range order like their crystalline counterparts and the main differences come out from the absence of the long-range order, which is characteristic of crystalline phases [24–27]. Therefore, the density of electron states remains a valid concept for non-crystalline as for crystalline materials since the long-range disorder “perturbs but does not annihilate the band structure” [27]. The effect on the DOS distribution in crystalline as well as amorphous materials can be seen in Fig. 2. In Fig. 2A crystalline material band structure is shown. Band gap, Eg, is the energy distance between valence band edge, EV, and conduction band edge, EC. Fermi level, EF, is an important quantity since it refers to the free energy of electrons. In particular, Fermi level indicates the type and level of doping of the semiconducting material. The higher the n-doping, the closer EF is to the conduction band whilst, in the case of p-type semiconductors, electron acceptors in the crystal lattice accept electrons from the occupied valence band, creating holes [28]. Those materials, which have a “mid gap” Fermi level, are called insulators. For a typical crystalline material, there is no DOS distribution inside band gap. On the contrary, the absence of long-range order leads to the presence of a finite DOS within the so-called mobility gap, Em g , in the case of amorphous materials. According to the Cohen–Fritzsche–Ovshinsky (CFO) model, the non-crystalline structure would lead to overlapping band tails of localized states, as shown in Fig. 2B. Those derived from the conduction band would be neutral

FIG. 1 Experimental setup for PCS studies [11].

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FIG. 2 Model of the electronic structure for a crystalline semiconductor (A), amorphous semiconductor following the Cohen–Fritzsche–Ovshinsky (CFO) model (B), and amorphous semiconductor following the Mott–Davis model (C). Figure modified from F. Di Quarto, F. Di Franco, A. Zaffora, M. Santamaria, Photocurrent spectroscopy in passivity studies, in: K. Wandelt (Ed.), Encyclopedia of Interfacial Chemistry. Surface Science and Electrochemistry, 2018, pp. 361–371.

when empty and those from the valence band neutral when full. Anyway, the suggestion of overlapping tails is considered quite unlikely to apply to amorphous semiconducting and insulating materials that are transparent in the visible or infrared [27]. According to Mott–Davis model, the long-range lattice disorder leads to the DOS distribution shown in Fig. 2C. These differences between the distributions of electronic states in crystalline and amorphous materials influence the photoelectrochemical as well as the impedance behavior of the SC/El junction. The absorption process of incident light in the bulk of an SC is sketched in Fig. 3A: f0 (in cm2 s1) is the photon flux entering the semiconductor (corrected for the reflections losses at the SC/El interface), which is absorbed following the Lambert–Beer law and it is attenuated as the light enters the material.

FIG. 3 Schematic representation of an irradiated crystalline n-type SC/El interface under anodic polarization, showing the electron–hole pair generation (A) and the change of light intensity due to the absorption process within the semiconductor (B). Band-to-band direct transitions (C) and band-to-band indirect transitions (D). Fig modified from F. Di Quarto, F. Di Franco, A. Zaffora, M. Santamaria, Photocurrent spectroscopy in passivity studies, in: K. Wandelt (Ed.), Encyclopedia of Interfacial Chemistry. Surface Science and Electrochemistry, 2018, pp. 361–371.

118 SECTION B Fundamentals of preparation and characterization of photocatalytic materials

The rate of electron hole pairs generation (per unit of volume) at any distance from the SC surface, g(x), is given by: gðxÞ ¼ f0 a exp ðaxÞ

(1)

1

where a (in cm ), is the light absorption coefficient of the semiconductor, and it is a function of the impinging irradiation wavelength. In this discussion, it is assumed that each absorbed photon, having energy hn Eg generates a free electron hole couple. Optical transitions at energies near the band gap of a crystalline material may be direct or indirect. In the first case, no intervention of other particles is required, apart the incident photon and the electron of the valence band (see Fig. 3C); in the second case, the optical transition is assisted by the intervention of lattice vibrations (see Fig. 3D). Considering a polarization DFSC inside the SC, the width of the space-charge region, xSC, changes with the polarization according to the following equation:  

kB T 0:5 0

xSC ¼ xSC DFSC  (2) e where x0SC represents the space-charge width into the SC electrode at 1 V of band bending, and its value depends on the concentration of mobile carriers into the SC. In terms of electrode potential, UE, for not heavily doped SC and in the absence of an appreciable density of electronic surface states (SS), it is possible to write: DFSC ¼ UE  UFB ðref :Þ

(3)

where UFB(ref.) represents the flat band potential measured with respect to a reference electrode in the electrochemical scale. UFB is the electrode potential in which there is no excess charge on the semiconductor side of the junction. This means that, in an n-type semiconductor, the number of electrons is exactly matched by the number of ionized donor atoms (n-type doping is achieved by adding donor atoms that can ionize to supply electrons to the conduction band, leaving immobile donor ions D+ in the lattice). Because there is no excess charge in the semiconductor, there is also no electrical field. Whenever the electrode potential is more anodic than UFB, the semiconductor is in depletion condition, i.e., electrons are withdrawn from the electrode leaving a positive space charge region consisting of the ionized donor atoms [28]. It is important to note that acid/base equilibria for surface groups on (oxide) semiconductors give rise to a Nernstian pH dependence of the flat band potential, which shifts negative by 59 mV per pH unit at room temperature: UFB ¼ UFB ðpH ¼ 0Þ  0:059pH

(4)

The importance of flat band potential value is derived also by the relationship between UFB and EF: EF ¼ jejUFB + |e|Uref

(5)

where e is the electron charge and Uref is the potential of the reference electrode, used in the photoelectrochemical measurements, with respect to the vacuum scale [29, 30]. With these assumptions, it is possible to describe the response of the SC/El interface starting from the seminal paper published in 1959 by G€artner treating the behavior of illuminated solid-state Schottky barrier [31], coupled with the paper of Butler published in 1977 about the semiconducting properties of WO2 [32]. In the G€artner-Butler model, the total photocurrent, Iph, collected in the external circuit can be calculated as the sum of two terms: a migration term, Idrift, and a diffusion term, Idiff. The former considers the contribution of the minority carriers generated into the space-charge region whilst the latter takes into account the minority carriers entering the edge of the space-charge region from the bulk field-free region of SC. In this discussion, no light reflection is assumed, i.e., all the light is absorbed within the SC. Furthermore, it is assumed no recombination for minority carriers generated in the space-charge region of the SC, since the presence of an electric field is assumed to efficiently separate the photogenerated carriers. The same assumption is made for the minority carriers arriving at the depletion edge from the bulk region of the SC [20]. The total photocurrent in an n-type semiconductor can be expressed as follows: " # Dp eaxSC Iph ¼ Idrift + Idiff ¼ eF0 1  (6) + e p0 1 + aLp Lp where Dp and Lp are the hole diffusion coefficient and diffusion length respectively and p0 is the equilibrium concentration of holes in the bulk of the (not illuminated) SC. The same equation is valid for a p-type SC, with Dn and Ln (electron diffusion coefficient and diffusion length) instead of Dp and Lp and n0 (electrons equilibrium concentration) instead of p0.

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For wide band gap materials (i.e., Eg > 2 eV), where the bulk concentration of minority carriers is very small, Eq. (6) can be simplified by neglecting the diffusion term. In this case, if axSC < < 1 (slightly absorbed light) and aLp < < 1 (small diffusion length for minority carriers), the photocurrent flowing through the n-type SC/El interface can be expressed as:   kB T 0:5 0 Iph ¼ eF0 axSC UE  UFB  (7) e Therefore, a quadratic dependence of the photocurrent on the electrode potential is foreseen. The measurement of the photocurrent as a function of UE is called photocharacteristic, that can be used for getting out the flat band potential of the junction. In fact, by neglecting the term kBT/e in Eq. (7), a plot of (Iph)2 vs UE should intercept the potential axis at the flat band potential, UFB, regardless of the employed irradiating wavelength, l, as long as axSC gz.

1.2 Hyperfine interaction When the unpaired electron is in the vicinity of nuclei possessing nuclear magnetic moments, the electron energy levels are shifted due to the electron-nucleus spin interaction called nuclear hyperfine interaction, which often results in the multiline EPR spectra. The hyperfine structure of the EPR spectrum brings additional information on the paramagnetic systems. Analogously as was derived for the free electron, magnetic moment of a nucleus (mN) mN ¼ gN mN I

(5)

where gN is nuclear g-factor and mN nuclear magneton, is proportional to the nuclear spin angular momentum I vector, with modulus defined as pffiffiffiffiffiffiffiffiffiffiffiffiffiffi (6) j I j ¼ ħ I ð I + 1Þ where I is the nuclear spin quantum number and Iz ¼ –I, –I + 1, … +I is the nuclear spin angular momentum component along z direction (2  I + 1 values). The interaction of external magnetic field oriented along the z direction (B0) with the nuclear magnetic moment causes the changes in the energy of nuclear spin called nuclear Zeeman effect: E ¼ mN B ¼ gN mN B0 Iz

(7)

So if we have an electron spin interacting with nuclear spin, two terms should be added in Eq. (3), i.e., the nuclear Zeeman interaction (significantly smaller than the electron Zeeman interaction as mN ≪ mB) and a contribution derived from the hyperfine interaction representing the anisotropic dipole-dipole interaction averaged in isotropic systems (outside the nuclear volume) and the contact (Fermi) hyperfine energy contribution Ehf (inside the nuclear volume) [1]. Considering an electron spin and nuclear spin interaction in a rapid tumbling system (isotropic) with the external magnetic field B0, the total energy is defined as Etot ¼ g|mB |B0 Sz  gN mN B0 Iz + aSz Iz where a is the hyperfine coupling constant.

(8)

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B Fundamentals of preparation and characterization of photocatalytic materials

The selection rules for EPR phenomenon are: DSz ¼  1 and DIz ¼ 0 (the allowed transitions between energy levels characterized by different values Sz and the same Iz value). The simplest example represents the interaction of an unpaired electron with one I ¼ ½ nucleus which results in four energy levels and the allowed transitions are reflected by two lines in the EPR spectrum separated by a hyperfine coupling constant a [1, 2, 6]. Fig. 3 shows the calculated X-band EPR spectra demonstrating the hyperfine interaction of an unpaired electron (S ¼ ½) with the increasing number of equivalent I ¼ ½ or I ¼ 1 nuclei along with the Pascal triangles for the evaluation of line intensity ratio. Generally, when the unpaired electron interacts with many magnetic nuclei, each electron Zeeman level is separated into a manifold of several sublevels and the theoretical number of lines in the EPR spectrum can be evaluated as N ¼ ð2I1 + 1Þ ð2I2 + 1Þ…ð2In + 1Þ ¼

Y

ð2Ik + 1Þ

(9)

k

The magnitude of the hyperfine coupling is affected by the nuclear gyromagnetic ratio and by the degree of the interaction between the electron spin and the nuclear spin, which typically significantly decreases with the increasing number of bonds between the nucleus and the unpaired electron [4]. Fig. 4 shows a complex experimental EPR spectrum of methyl viologen radical cation (MV∙+) observed upon UV-light induced reduction of MV2+ in TiO2 suspension in deoxygenated dimethylsulfoxide (DMSO) along with its simulation calculated using the hyperfine coupling constants aN(2N) ¼ 0.425 mT, aH(6H) ¼ 0.410 mT, aH(4H) ¼ 0.164 mT and aH(4H) ¼ 0.131 mT.

I= ½

I= 1

Number of interacting equivalent nuclei, n

n=1 Number of equivalent nuclei, n I= ½ 0 1 2 3 4 5

n=2

Intensity ratio

1 1 1 1 2 1 1 3 3 1 1 4 6 4 1 1 5 10 10 5 1

n=3 Number of equivalent nuclei, n I= 1 0 1 2 3 4 5

n=4

n=5

345

346

347

348

349

Magnetic field, mT

350

345

346

347

348

349

Intensity ratio

1 1 1 1 1 2 3 2 1 1 3 6 7 6 3 1 1 4 10 16 19 16 10 4 1 1 5 15 20 45 51 45 20 15 5 1

350

Magnetic field, mT

FIG. 3 Theoretical X-band EPR spectra demonstrating the hyperfine interaction of unpaired electron (S ¼ ½) with the increasing number of equivalent I ¼ ½ and I ¼ 1 nuclei along with Pascal’s triangles for evaluation of line intensity ratio.

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FIG. 4 Experimental (1) and simulated (2) EPR spectrum of methyl viologen radical cation (MV∙+) obtained upon UVlight induced reduction of MV2+ in deoxygenated TiO2 suspension in DMSO.

2

334

335

336

337

338

339

Magnetic field, mT 1.3 EPR line shape and signal intensity The molecular motion in a sample with unpaired electrons affects the EPR line shape and/or line width. The EPR spectra in solutions are often characterized by narrow lines, as a result of motional averaging of the anisotropies [1, 5]. The dynamic processes such as hindered rotation, tumbling of molecule in a viscous media, interaction with other paramagnetic species or acid-base equilibrium, in and around the paramagnetic center, may induce the line shape effects [4]. The integrated intensity of an EPR signal is proportional to the concentration of unpaired electrons in the sample, providing further important information on the paramagnetic system, essential for the reaction kinetics and mechanism elucidation. The problems and prospects of quantitative EPR spectroscopy are analyzed in detail in Ref. [2]. The quantum chemistry approach is essential when interpreting EPR spectra by solving the spin-Hamiltonian equation and various programs can be applied for the EPR spectra manipulation and simulation. The WinSim program accessible at the homepage of EPR Center of the National Health Institute (https://www.niehs.nih.gov/research/resources/software/toxpharm/tools/index.cfm) is suitable for the simulation of multicomponent isotropic EPR spectra [7]. The program package EasySpin working on the MatLab platform represents a universal tool for analysis and simulation of a wide range of EPR spectra (http://www.easyspin.org/) [8]. The fundamentals and deeper insights in the EPR spectroscopy theory and applications are summarized in the referenced books.

2 EPR studies of nanocrystalline photocatalysts EPR spectroscopy is extensively employed in the exploration of the principal aspects of heterogeneous photocatalysis and surface chemistry [9–14]. Thanks to its high sensitivity it enables the monitoring and characterization of active sites present at low concentrations and intermediates crucial for the understanding of the complex reactions undergoing in the photocatalytic systems. EPR spectra measured in solid state especially at low temperatures bring unique insight into the geometric and electronic structure of the paramagnetic sites relevant within the (photo)catalytic processes, their interaction in the nearest environment, and the dynamics of the processes they are involved in. The primary step in the photoinduced reaction on the irradiated titanium dioxide photocatalysts involves the generation of electron-hole pairs which after an efficient separation migrate as e and h+ in the semiconductor crystal lattice, where they can be trapped, or to the surface where in addition to the generation of trapped sites they may interact with the adsorbed molecules/species (Fig. 5). The photogenerated electrons in the transition metal oxides tend to be localized on the metal ions [16]. In titanium dioxide, these electrons are stabilized in the d orbitals of Ti4+ cations and can be detected as Ti3+ ions by EPR at low

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FIG. 5 The general scheme of the primary processes undergoing upon TiO2 semiconductor photoexcitation depicting the generation of paramagnetic species traceable by EPR spectroscopy. The experimental (black line) and simulated (colored line) EPR spectra of Mg2+-doped anatase nanopowder with exposed {001} facets measured upon continuous UV irradiation (lmax ¼ 365 nm; irradiance 20 mW cm2) at 100 K (magnetic field sweep width, SW ¼ 30 mT). The spin-Hamiltonian parameters of denoted paramagnetic species are discussed in detail in Ref. [15].

temperatures [17, 18]. In the solid matrix, the ground state of the free ion is split by the effect of the crystal field, which together with the various distortions determine the principal values of the g-tensor depending on the energy splitting between various d orbitals [19]. In anatase allotrope, we recognize two main paramagnetic species attributed to the trapped photoelectrons, which exhibit significantly different EPR spectra. Bulk (interstitial) Ti3+ centers in the regular lattice sites of the anatase matrix are characterized with a rather narrow axially symmetric signal with g?  1.99 and gjj  1.96 [17]. Higher-field narrow-line EPR signals with the g-components in the region 1.973–1.968 coupled with less resolved g-component at 1.93 detected in the Q-band spectra for the specific 2D-TiO2 anatase nanosheets were attributed to the Ti3+ in the specific local symmetries [20]. On the other hand, a significantly broadened line centered at g ¼ 1.93 is assigned to the photoelectrons trapped in various Ti3+ centers located at the surface or in the subsurface regions [17]. The EPR signals of trapped electrons in different titanium dioxide polymorphs vary, even though the surrounding of the titanium ions remains octahedral. While the signals of Ti3+ in anatase and brookite allotropes are quite similar, in rutile, the corresponding signals are significantly different [18], which indicates that EPR spectroscopy is very sensitive to the structural parameters such as crystal field strength and related distortions. The photoelectrons localized in the surface trapping sites react with the adsorbed molecular oxygen resulting in the formation of another paramagnetic species, superoxide radical anion or hydridodioxygen radical (Fig. 5). Thus, the generation of O2∙–/HO2∙ upon irradiation of the titanium dioxide may serve as an evidence of the generation of reacting surface electrons. The shapes of the EPR spectra of the surface superoxide species depend significantly on the chemical nature and the nuclear spin of the atoms in their vicinity, as well as on the symmetry of the nearest surroundings [21]. The superoxide radical anion contains an unpaired electron in a 2p* antibonding orbital. The EPR spectrum of this species is typically characterized by a rhombic g-tensor, with the gzz component diagnostic of the surface site adsorbing the superoxide ions [16]. The photogenerated holes trapped rapidly after the excitation at the surface of the semiconductor are the key players in the primary oxidation process. Even though numerous studies have dealt with the identification of the trapped hole sites, the exact nature of these paramagnetic species remains under debate. Generally, the sites responsible for the stabilization of the holes in metal oxides are the lattice oxygen ions (O2) [16, 19]. The paramagnetic O∙ species generated upon the trapping

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are characterized with a variety of signals (g > 2) strongly dependent on the properties of the TiO2 crystal lattice and its localization within the nanocrystal. The holes tend to localize in the surface regions, which are greatly affected by the surface treatment and modification, which only adds up to the diversity of their EPR features [16, 22, 23]. The paramagnetic sites can be monitored in the TiO2 lattice even prior to the photoexcitation as a result of the synthetic procedure, postsynthetic treatment, or doping. High-temperature treatment under specific atmosphere often leads to the generation of point crystal defects which affect the optical and electrical properties of the material [12, 24]. These oxygen vacancies have been intensively studied theoretically and experimentally [25–27], and it was suggested that the unpaired electron is localized on one titanium ion adjacent to the oxygen vacancy, i.e., a singly ionized oxygen vacancy represents a Ti3+ ion next to the vacancy [28]. Differently located Ti3+ ions play a crucial role in the characterization of electron excess states in titania polymorphs [29]. For example, the EPR spectra in the reduced states in anatase matrix represent: (i) axial EPR signal with g? ¼ 1.992 and gjj ¼ 1.962 attributed to the Ti3+ ions located in the regular octahedrally coordinated site of anatase lattice; (ii) broad-line signal with gav  1.93 assigned to a collection of subsurface and surface reduced Ti3+ centers with large variation of local symmetries dominant at high annealing temperatures; (iii) paramagnetic species with g-components g ¼ 1.951 and g ¼ 1.973 observed upon progressive thermal treatment [17]. Another source of intrinsic paramagnetic sites in the titanium dioxide nanomaterials are coming from the different strategies employed to improve and/or shift the photocatalytic activity of the material, especially metal or non-metal doping. In addition to the formation of defects in the titania lattice due to the incorporation of aliovalent ions, the dopants themselves may be present in the form of paramagnetic species. The identification of these sites, potentially responsible for the tuned photoactivity in different light regions, represents a crucial point by the evaluation of the effect of the doping on the performance of the material. Consequently, the intensive EPR studies were performed on the TiO2 nanomaterials doped/modified with metal ions or metal oxides [30, 31]. The incorporation of nitrogen in titanium dioxide is widely performed employing various procedures with only slight variations in the material properties and chemical nature [32]. EPR spectroscopy enabled the identification of two types of paramagnetic species originating from nitrogen occurring in various N-doped TiO2 nanostructured materials. Molecular nitrogen oxide species (NO) as products of complex oxidation processes of ammonium salts occurring upon calcination of the solid, may be trapped in microvoids of the porous solid [33, 34]. More interesting paramagnetic species concerning the photocatalytic activity represent the monomeric nitrogen centers (Nb∙) incorporated in the bulk of N-doped TiO2 exhibiting a characteristic EPR signal with an orthorhombic g-tensor [33]. This species is stable even at a rather high temperature and is considered as the photoactive center responsible for the visible light activity of the material. Fig. 6 shows the effect of the annealing temperature applied by the postsynthetic treatment of the N-doped anatase nanostructures on the character of the paramagnetic centers present in the material. The X-band EPR spectra measured in dark at 100 K reveal dominant EPR signals of NO species (g-tensor components g1 ¼ 2.001, g2 ¼ 1.998,

(A)

(B)

FIG. 6 X-band EPR spectra of N-doped anatase nanostructures annealed at various temperatures (A) 350 °C and (B) 400 °C monitored upon the increasing temperature in the range of 100–298 K. The inset represents the EPR spectrum obtained after subsequent cooling of the sample back to 100 K.

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g3 ¼ 1.927 and hyperfine couplings from the nitrogen A1 < 0.1 mT, A2 ¼ 3.22 mT, A3 ¼ 0.96 mT) in the case of applied annealing higher than 400 °C, while in the N-doped anatase nanostructures annealed at 350 °C EPR signal of Nb∙ (g1 ¼ 2.005, g2 ¼ 2.004, g3 ¼ 2.003; A1 ¼ 0.23 mT, A2 ¼ 0.44 mT, A3 ¼ 3.23 mT) is present [35]. The role of EPR spectroscopy in the study of non-metal doped TiO2 nanostructures is summarized in Ref. [36]. Besides the intensively investigated metal oxide photocatalysts, recent attention is also drawn towards metal-free semiconducting materials like graphitic carbon nitride (g-C3N4) [37]. Despite g-C3N4 semiconductors possess high thermal stability, chemical resistance due to the strong covalent bond between C and N atoms, and the band-gap energy value suitable for the visible-light-induced activation (2.7 eV), their photocatalytic activity is negatively affected by the limited surface area and rapid recombination processes of photogenerated charge carriers [37]. However, the combination of g-C3N4 with TiO2 can merge the benefits of both materials obtaining a composite photocatalyst with suitable morphological properties and enhanced visible-light activity. The variations in synthesis significantly affect the structure and activity of the obtained semiconducting composite nanomaterial [38]. Also here, EPR spectroscopy represents a sensitive tool for the direct detection of paramagnetic defects in pristine g-C3N4 and composites solid matrices [39].

3

EPR studies of dispersed photocatalytic systems

The successful applications of photocatalytic systems in the purification/remediation of water and air are based on the effective production of reactive species, especially reactive oxygen species (ROS), with the capability to transform pollutants stepwise to CO2 and H2O [11]. The ROS represent radical or molecular species (e.g., O2∙, H2O2, HO∙) produced from molecular oxygen, through the four consecutive steps of one-electron reduction. The main player in photocatalytic degradations is hydroxyl radical, possessing very high reactivity towards the organic/inorganic molecules [40, 41]. In addition, HO∙-induced reactions (i.e., hydrogen abstraction, addition, electron transfer) often lead to the production of new free radicals with less or equal reactivity [40]. The dismutation reaction of O2∙ and its protonated form (HO2∙) in water results in the generation of hydrogen peroxide [42]. In water, O2∙/HO2∙ species along with H2O2 may be involved in the successive redox reactions producing hydroxyl or organic radicals. Contrary, O2∙ is characterized by excellent stability in aprotic media (e.g., DMSO), enabling the nucleophilic reactions coupled with the formation of peroxyl radicals [40, 42]. The detection of ROS and other radicals represents an important part in the evaluation of the photoactivity of potential photocatalytic materials. The specific adjustment of experimental conditions and the careful choice of the indirect EPR method facilitates the monitoring of various reactive radical intermediates, which can help us to gain a comprehensive picture of the photoinduced processes undergoing in the photocatalytic system. Why an indirect EPR technique should be applied in the reactive radical investigation? The short lifetime of transient paramagnetic species generated upon irradiation of dispersed photocatalysts limits their detection and identification by direct cw-EPR spectroscopy at room temperature. Various indirect EPR techniques were established for effective radical determination, and two main approaches will be shortly presented here—spin trapping and spin scavenging. The EPR spin trapping technique is based on the chemical reaction (trapping) of non-persistent free radicals (∙R) by a diamagnetic molecule called spin trap (ST) producing more stable free radical product—spin-adduct (Fig. 7). The most convenient spin trapping agents are nitrones, N-oxides, and nitroso compounds which produce rather stable nitroxide radical spin-adducts via the radical-addition reaction. An ideal spin trapping agent should meet the following requirements: (i) stability in the reaction system; (ii) dominance of the rapid genuine radical-trapping reaction with the negligible contribution of the spin-adduct “impostors” [43]; (iii) the formation of a rather uncomplicated EPR signal of the spin-adduct and at the same time bringing a large set of structural information on the radical trapped [44]. A variety of spin traps capable of simultaneous trapping of different free radicals and tracing them to their origin is available [44, 45]. However, it has to be understood clearly that after the radical is trapped, we no longer observe the EPR spectrum of the radical itself but the signal of the corresponding spin-adduct. Hence, the most important task is to unambiguously determine the chemical structures of the observed spin-adducts and by the analysis of the spin-Hamiltonian parameters, i.e., hyperfine coupling constants (hfcc) and g-value, to identify the trapped radical. An important advantage of the EPR spin trapping technique represents the capability to discriminate even more than three different radical species generated in one system simultaneously. The signals of the specific spin-adducts can be recognized by their distinctive spectral lines, and by performing the simulation analysis, all the signals comprising the EPR spectrum can be assigned to the individual spin-adducts. The proper choice of the spin trap is essential for the successful application of the EPR spin trapping method. The suitability of a certain spin trap is defined by: (i) its ability to exclusively trap the investigated free radical; (ii) the diversity of EPR spectra of different adducts so that trapped radicals can be easily distinguished; (iii) the stability of the spin trap and generated spin-adducts under applied experimental conditions. The sensitivity obviously depends on the concentration of the spin-adduct, which is directly dependent on the local

Application of EPR techniques in the study of photocatalytic systems Chapter

DMPO

PBN

DBNBS

8

133

DMPO-R

PBN-R

DBNBS-R

FIG. 7 Addition of free radical to N-oxide (DMPO), nitrone (PBN) and nitroso (DBNBS) spin trap forming the corresponding spin-adduct.

concentration of the spin trap, the number of transient radicals produced, the rate constants for the radical reaction with the spin trap, and the decay of the generated spin-adducts. The rate constants depend primarily on the nature of the transient radical and on the structure of the spin trap [44]. Consequently, the investigations employing the spin trapping technique in photocatalysis are rather complex and require detailed and very careful interpretation. Mistakes and misleading information can arise from an unwise choice of the spin trap. As many aspects have to be taken under consideration when choosing the spin trapping agent for a specific experiment, it mostly ends up in a compromise between comprehensive investigation and an ordinary quantification of radical production. Application of the spin trapping technique to follow the light-triggered processes provides extremely valuable information which enables us not only to assess the free radical generation in the studied system but also can shed more light on the reaction mechanisms. However, a very cautious approach is inevitable, since the formation of the radicals, which is determined by the behavior of the studied system, their trapping by the spin trapping agent, and the stability of so formed spin-adduct, is significantly affected by numerous external factors as well (solvent, presence of oxygen, irradiation source). Consequently, the experimental conditions have to be carefully considered in the interpretation of the obtained data. On the other hand, the specific choice of experimental system can expand the amount of information obtainable from the method, e.g., stabilization of selected radical intermediates in specific solvents [42, 46]. The valence- and conduction-band-edge position of photocatalysts affects the ROS generation via consecutive reactions with the photogenerated holes and electrons. The reduction and oxidation take place simultaneously upon exposure of photocatalyst dispersed in water, and ROS may be produced sequentially from O2 and H2O. Despite some experimental problems, the application of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trap enables the detection of both HO∙ and O2∙ radicals; however, EPR spectra monitored are strongly dependent on the reaction conditions. The UV exposure of aerated suspension of TiO2 in water led to an immediate appearance of the typical four-line signal of ∙DMPO-OH adduct (spin-Hamiltonian parameters aN ¼ 1.499 mT, abH ¼ 1.479 mT, g ¼ 2.0058) [46, 47], which represents genuine spin trapping (reaction of the photogenerated hydroxyl radicals with DMPO, rate constant  2.8–3.4  109 M1 s1 [44]). Nevertheless, the inverted spin trapping via DMPO∙+ formed by the spin trap oxidation by photoholes with the subsequent nucleophilic attack of water molecules, or the decomposition of unstable ∙DMPO-O2H spin-adduct (from the addition of photogenerated O2∙/HO2∙) cannot be excluded (scheme in Fig. 8). The addition of DMSO enables the corroboration of HO∙ formation in the aqueous photocatalyst suspensions, as rapid reaction of hydroxyl radicals with DMSO leads to the generation of methyl radicals [48], evidenced as the corresponding six-line EPR signal of DMPO-adduct with the carbon-centered radicals [46]. The detection of superoxide radical anion in an aqueous system is not straightforward due to: (i) rapid dismutation to hydrogen peroxide; (ii) low values of rate constants for the addition of O2∙/HO2∙ to DMPO (k ¼ 6.6  103 M1 s1 pH  5; k ¼ 1.0  10 M1 s1 pH 7.8 [44]) and (iii) low stability of ∙∙DMPO-O 2 /O2H spin-adduct (τ0.5 ¼ 59 s) and rapid ∙ ∙  interconversion of ∙DMPO-O /O H to DMPO-OH [44]. Despite that, the DMPO-O 2 2 2 /O2H spin-adduct can be monitored in the photocatalytic systems in which the conduction-band-edge potential is more negative than those of TiO2, revealing the stronger reduction ability of the photogenerated electrons in the reaction with molecular oxygen producing the superoxide radical anions more effectively. The polymeric carbon nitrides (g-C3N4) represent such photocatalysts [38], and upon in situ UV exposure of aqueous dispersions containing a composite of TiO2 P25/g-C3N4 (ratio 1:9 wt.) in the presence of

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DMPO DMPO HO O2 –/ HO2

(A)

+ ‒

HO H2O

h+

•DMPO-OH

•DMPO-O H 2

(B)

FIG. 8 The alternative reaction pathways of ∙DMPO-OH formation in the aerated aqueous suspension of photocatalysts. The experimental EPR spectra (black) along with their simulations (red (gray in print version; color figure can be viewed in the online version of this chapter); SW ¼ 7 mT) obtained upon irradiation of the aerated aqueous suspensions of (A) TiO2 P25 and (B) P25/g-C3N4(1:9 wt.) in the presence of DMPO spin trap (cphotocat ¼ 0.1 mg mL1, c0(DMPO) ¼ 0.04 mol L1, LED@365 nm dose 4.5 J cm2). b DMPO spin trap, the formation of two EPR signals corresponding to ∙DMPO-O 2 /O2H (aN ¼ 1.419 mT, aH ¼ 1.120 mT, ∙ g aH ¼ 0.124 mT; g ¼ 2.0058, rel. conc. 78%) and DMPO-OH (rel. conc. 22%) is observed (Fig. 8B). How can we enhance our chances to detect superoxide radical anions produced in the photocatalytic dispersions? For the effective detection of O2∙– new generation of N-oxide spin trapping agents were developed, e.g., 5-(ethoxycarbonyl)-5methyl-1-pyrroline N-oxide (EMPO), 5-(diisopropoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DIPPMPO) or 5-tertbutoxycarbonyl-5-methyl-1-pyrroline N-oxide (BMPO) with a significantly enhanced lifetime of the corresponding O2∙–/HO2∙ spin-adducts in water [44], but their complex EPR spectra representing the overlapped signals of diastereoisomeric spin-adducts partially hinder their large-scale applications in the photocatalytic systems. As O2∙– is stabilized in the aprotic solvents, the irradiation of photocatalysts dispersed in DMSO or acetonitrile (ACN) in the presence of suitable spin trap results in the generation of ∙ST-O 2 spin-adducts, which can be unambiguously identified by simulation analysis. However, two important points should be considered here: (i) the reactions of the solvent with the photogenerated ROS resulting in the generation of new free radicals, which are also trapped by ST; and (ii) the enhanced solubility of molecular oxygen in organic solvents [49] causing the line broadening [4], which eliminates the hyperfine structure in the EPR signals and thus hinders the detailed simulation analysis. This is more significant in acetonitrile, as the EPR spectrum measured upon in situ UV exposure of P25/DMPO/ACN/air system reveals four broad lines without hyperfine structure (Fig. 9A). Consequently, the EPR spectrum was monitored also after rapid post-radiation saturation by argon, leading to the significant decrease in the concentration of molecular oxygen in the system. The simulation analysis, ∙ in addition to the dominant ∙DMPO-O 2 spin-adduct, revealed the presence of other spin-adducts ( DMPO-OH, ∙ ∙∙ DMPO-OCH3, DMPOdegr) [46], with relative concentrations strongly dependent on the actual reaction conditions. The application of DMSO is more convenient due to the lower O2 solubility. The solvent interaction with reactive species results in the formation of several spin-adducts, nevertheless, their identification is easier than in ACN systems (Fig. 9B). The UV exposure of P25/DMPO/DMSO/air leads to the generation of four spin-adducts. Two major species represent spin∙– ∙ adducts ∙DMPO-O 2 , produced by the reaction of O2 with DMPO, and DMPO-OCH3 coming from the trapping of the radicals generated by ROS interactions with solvent [46]. The EPR spectra measured upon exposure of the aerated P25 suspensions in methanol or ethanol shown in Fig. 9C, D point again on the high oxygen solubility in both solvents and the necessity to analyze the EPR spectra monitored after lowering O2 concentration as was previously demonstrated in Ref. [46]. Some notes and recommendations for successful EPR spin trapping experiments in the photocatalytic systems: (i) careful choice of a spin trap with sufficient photostability and suitability for the detection of expected radicals; (ii) the application of various spin trapping agents (e.g., DMPO and DBNBS) may be inevitable to unambiguously identify the radicals produced; (iii) since the reactive radicals are detected as the corresponding spin-adducts, besides the genuine

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(C)

(B)

(D)

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FIG. 9 EPR spectra (SW ¼ 7 mT) measured at 293 K upon in situ irradiation of the aerated TiO2 P25 suspensions (blue, upper spectra in A, C, D) along with those found after rapid post-radiation saturation with argon (brown, lower spectra in A, C, D): (A) ACN; (B) DMSO; (C) methanol; (D) ethanol. (cphotocat ¼ 0.167 mg mL1, UVA c0(DMPO) ¼ 0.035 mol L1, (365 nm) dose of 6 J cm2).

spin trapping also other reaction pathways of the spin-adduct formation should not be overlooked; (iv) optimization of experimental conditions, as the photocatalyst’s loading, ST concentration, pH, solvent, atmosphere, presence of electron donors/acceptors in the system may significantly affect the generation and stability of spin-adducts; (v) careful simulation analysis of experimental EPR spectra allows the assignment of the free radicals trapped. The principle of spin scavenging lies in the reaction of stable or semistable radical, most frequently nitroxide radical (>NO∙), with the transient paramagnetic species (∙R) generated in the studied system, forming the corresponding EPR silent products (>NOR). The most common group of nitroxides are Tempo derivatives, e.g., 4-oxo-2,2,6,6-tetramethyl1-piperidine N-oxyl (Tempone) or 4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl (Tempol), which have the characteristic three-line EPR spectra originating from the dominant hyperfine interaction of the unpaired electron with the nitrogen nucleus [50]. The rate constants for the radical-radical reactions, i.e., studied unstable free radical with a nitroxide radical, often reach the diffusion-controlled values, and may be significantly higher than the rate constants of the radical-molecule reactions in spin trapping [44]. The application of spin scavenging technique, using Tempo derivatives at suitable concentration in the photocatalytic systems, can bring very important information on their total radical-generating capacity. Obviously, the identification of primary radicals causing the termination of the nitroxide group is not possible from the EPR measurement and requires the mass spectrometry analysis of products. The combination of EPR spin trapping and spin scavenging techniques may bring information not only on the character of radical(s) generated upon irradiation of photocatalyst under given experimental conditions, but allows also to estimate quantitatively the radical-producing ability of the system evaluated from the extent of eliminated >NO∙ groups. Fig. 10A represents the characteristic time-course of EPR spectra measured upon continuous LED@365 nm exposure of Tempol in aqueous dispersions of photocatalyst-free system (reference) and the presence of photocatalysts. No changes in the reference system monitored upon continuous exposure reflect sufficient photostability of Tempol. The UV exposure of the dispersed photocatalytic systems resulted in Tempol concentration decrease dependent on the type of photocatalysts. For TiO2 P25 within the defined time, the complete drop of the signal was observed evidencing the rapid elimination of Tempol (10 mmol L1), for the less-active g-C3N4 photocatalyst, only a small decrease was monitored. Using the relative concentration of Tempol, evaluated from the EPR spectra after double-integration [2] (Fig. 10B), we may calculate the exposure required to lower the initial Tempol concentration to one-half (apparent half-time, τ0.5,app); here under given experimental conditions, the τ0.5,app for P25 is 90 s and for g-C3N4 is approximately 10 times higher. The specific type of spin scavenging reaction represents the elimination of semistable radical cation of 2,20 -azino-bis(3ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS∙+) [15, 51]. The termination of the paramagnetic signal of ABTS∙+ is based on the photoinduced one-electron reduction of ABTS∙+ to its parent molecule, diamagnetic ABTS. The EPR spectrum of ABTS∙+ is very complex, consequently, for its quantitative evaluation in the photocatalytic systems, the

FIG. 10 (A) Set of individual EPR spectra (SW ¼ 6 mT) measured upon continuous LED@365 nm exposure of Tempol in water (reference) and in the aqueous photocatalytic dispersions (cphotocat ¼ 0.4 mg mL1, c0(Tempol) ¼ 10 mmol L1). (B) Dependence of the Tempol relative concentration on the irradiation time evaluated from the double-integrated EPR spectra measured under strictly analogous conditions.

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decline of an overmodulated EPR signal of ABTS∙+ (broad singlet at g ¼ 2.0036) is often monitored. It should be mentioned here that in aerated systems, superoxide radical, produced by the reaction of the photogenerated electron with molecular oxygen, is also involved in the reaction with ABTS∙+ forming EPR-silent ABTS molecule. The elimination of ABTS∙+ can be monitored also by UV/vis spectroscopy, since the radical cation ABTS∙+ in water has specific absorbance in the visible region (lmax ¼ 735 nm). Some notes and recommendations for successful EPR spin scavenging experiments in the photocatalytic systems: (i) careful choice of stable/semistable radical with sufficient photostability; (ii) optimization of experimental conditions (photocatalyst’s loading, stable/semi-stable radical concentration); (iii) EPR spectrometer settings and spectra acquisition allowing the precise double-integration [2]. EPR spectroscopy offers a wide spectrum of techniques enabling to obtain important information on the structure and reactivity of the photocatalytic materials. This chapter presents a short overview of the most common and accessible techniques and the deeper insights can be found in the sources referenced.

Acknowledgments The Scientific Grant Agency of the Slovak Republic (VEGA Project 1/0064/21) is acknowledged for the financial support. Vlasta Brezova´ and Dana Dvoranova´ thank Ministry of Education, Science, Research and Sport of the Slovak Republic for funding within the scheme “Excellent research teams.”

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Chapter 9

Approaching photocatalysts characterization under real conditions: In situ and operando studies Juan M. Coronadoa, Fernando Fresnob, and Ana Iglesias-Jueza a

Institute of Catalysis and Petrochemistry, CSIC, Madrid, Spain, b Photoactivated Processes Unit, Institute IMDEA Energy, Madrid, Spain

1 Introduction. Promises and challenges of in situ and operando studies in Photocatalysis Understanding the molecular machinery of catalytic processes is not only relevant from a purely academic point of view, but is also crucial to enhance the performance and maximize the economic impact of these technologies [1–3]. Acquiring an adequate knowledge of the key aspects of the catalytic transformations allows establishing structure-activity relationships, which can help redesign the physicochemical properties of the catalyst and thus, achieve better yield of the targeted products under milder conditions, or increase their durability. In the last decades, theoretical developments in computational chemistry, along with the availability of sensitive, reliable spectroscopic techniques with swifter response have prompted a significant change in the comprehension of mechanistic aspects of catalysis. In situ experiments, but especially the concept of operando studies, have prompted a change of paradigm by laying the foundations of the rational design in catalysis, in contrast to the traditional developments based on accumulated experience and educated guesses about what is required to prepare an optimal catalytic material. In situ experiments allow to investigate catalysts under conditions of concentration, pressure, temperature, and flow rates akin to those found in the real world, while operando (meaning “working” in Latin) measurements provide an additional level of information by simultaneously assessing the performance of the catalyst in relevant conditions [1–3]. In practice, this requires at least one analytical technique that is able to collect information about the working catalysts, while a second instrument monitors the evolution of the reaction, analyzing the outlet stream, as it is shown schematically in Fig. 1. However, it is frequently necessary to simultaneously combine two or more spectroscopic techniques to provide complementary information of the catalysts under an identical environment (e.g., temperature, composition, pressure) that can ensure the perfect comparability of results [4]. In this way, the obtained information will contribute to the understanding of the interaction between reactants and catalysts, to recognize the active sites, and to correlate the macroscopic metrics of catalyst performance, conversion, and selectivity, with the local structure and electronic properties of the photocatalyst. Time-resolved experiments are frequently used to follow the evolution of the catalyst state and the distribution of adsorbed species, or the mechanism of deactivation. The magnitude of the accessible time scales is usually determined by the instrument acquisition capacity and dictates the information that can be accessed. Therefore, particularly quick processes such as electronic transferences, or modulated kinetic experiments that allow discriminating between the so-called spectator or active species require high time resolution closed to turnover numbers (sub-millisecond or -nanosecond) and thus can be challenging to track. Besides, performing analysis with spatial resolution, either in 2D (in a layered configuration like electrodes) or 3D (e.g., honeycombs), is increasingly important [5]. This is because real catalytic bodies, despite comprising components in the micro-nano scale, have dimensions in the cm-mm range and present significant heterogeneities in composition and activity due to the presence of more or less inert additives such as binders [6]. Furthermore, realworld chemical reactors are intrinsically uneven, and large gradients of temperature, pressure, and composition are not rare, adding complexity to the study of real working systems. Therefore, a considerable research effort has been undertaken in the last years to develop new analytical set-ups in order to understand working catalysts at different scales of time and space. In addition, in situ investigations of catalysts in conditions not strictly equivalent to those prevalent in real operation, Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00030-1 Copyright © 2021 Elsevier Inc. All rights reserved.

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FIG. 1 Schematic diagram of an experimental setup for the spectroscopic study of photocatalysis under operando conditions.(© Reprinted with permission from M. El-Roz, M. Kus, P. Cool, F. Thibault-Starzyk, New operando IR technique to study the photocatalytic activity and selectivity of TiO2 nanotubes in air purification: influence of temperature, UV intensity, and VOC concentration, J. Phys. Chem. C 116(24) (2012) 13252–13263, https://doi.org/10.1021/jp3034819.)

can also provide valuable information to interpret the usually intricate results of operando experiments. Similarly, more specific tests of the type “start and stop” of a variable or with a modulated variation of a parameter such as pressure or reactant concentration, are useful to distinguish between the so-called spectator (inactive) and real intermediate species [7]. Besides the above-mentioned constrains which are common for all catalytic processes, in situ and operando investigation of working photocatalysis requires an additional degree of complexity since light is necessary to activate the process [8]. Therefore, the system must ensure the presence of the adequate optical windows for both driving the reaction and performing the analysis. In addition, to obtain meaningful results in operando conditions, the illuminated and the analytically sampled volume should be ideally coincident, to avoid misleading conclusions. In this regard, simulations studies have shown that a thickness of 1–3 mm of the benchmark photocatalyst, TiO2, absorbs all the UV radiation. So far, Fourier transform infrared spectroscopy (FTIR), using different configurations to adapt to the particularities of the process, has been the technique of choice for most of the operando studies of photocatalysis, as it will be discussed in detail in the following sections. Nevertheless, other techniques including X-ray absorption spectroscopies (XAS comprising XANES and EXAFS), Raman, XPS, NMR or EPR have been applied to ascertain the molecular scale mechanisms of photocatalytic reactions applied to a diversity of processes. In the following sections, the most relevant results obtained by each of the different techniques will be reviewed, paying attention to both the experimental set-ups employed and the molecular information obtained. Operando studies are prioritized but relevant in situ experiments are also described. Current limitations and future perspectives of this approach will be also discussed.

2

Operando FTIR for the study of photocatalytic processes

Infrared spectroscopy is a fundamental tool for the study of the mechanism of catalytic reactions. This technique detects vibrational transitions and can be applied to both surface and bulk analysis. However, in practice, structural vibrations of many photocatalysts are difficult to monitor because they appear at low wavenumbers, where the transmission of the usual optical windows (e.g., CaF2, or ZnSe) is extremely low. Some exceptions to this general observation are silicates, which are present, among other modes, a strong SidOdSi stretching feature at around 1070 cm
1 and more relevantly to photocatalytic studies, g-C3N4, which shows intense absorption in the 1100–1600 cm
1 range due to the stretching vibration of the aromatic CN heterocycle [9]. Crucially, FTIR is highly sensitive for the detection of the functional groups of organic molecules as well as for other relevant inorganic chemicals such as NOx, SOx, and COx, which are implied in a number of industrial and environmental relevant processes. Therefore, in general, monitoring the evolution of adsorbed species of those types is the main subject of operando FTIR studies because they can be readily detected, providing significant hints of the mechanism of photocatalytic reactions. Besides, illumination of the sample is relatively straightforward and does not interfere with infrared detection. Therefore, this technique lends itself to be applied in distinct microreactor configurations for in situ and operando studies as it will be discussed in this section. On the other hand, despite the clear advantages and popularity of FTIR, this technique is not devoid of some drawbacks. In this way, it is worth noting that the sampled fraction of photocatalyst can be significantly different from the irradiated

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volume, as the activating light (frequently UV) is usually fully absorbed by a thinner layer of solid than IR radiation. Accordingly, FTIR results could include information from both irradiated and photocatalytically inactive dark areas, complicating the analysis of the spectra. In addition, extensive overlapping of the bands of different surface complexes, often including the presence of intense features of spectator species, together with the lack of direct information about photoinduced electronic transferences, frequently make difficult the interpretation of the results. Nevertheless, adequate experimental design, together with the combination of FTIR with other techniques, can contribute to circumvent these hurdles. On the other hand, quantification of the spectroscopic results is limited by the fact that the molar absorption coefficients of surface complexes are generally unknown. Consequently, for many processes, only relative variations in the concentration can be determined in semiquantitative studies.

2.1 Sampling techniques and experimental set-ups for FTIR FTIR studies in liquid phase and, in particular, in aqueous solutions are approached using attenuated total reflection (ATR) cells, as it is shown in the scheme of Fig. 2 [10, 11]. Despite the high absorption of infrared radiation by water, which can turn this technique unfeasible for transmission experiments, the low penetration of the evanesced wave traveling through the internal reflection component into the solution (probing depths are in the 250–400 nm range in the region 2000– 1200 cm
1) allows avoiding the saturation of the signals. This configuration requires the deposition of the photocatalyst as a layer on the ATR optical element (frequently ZnSe, sometimes with other thin coatings for increased resistance), which according to the angle of incidence and the size of the component allows several rebounds within the sample, as it is shown in Fig. 2. Nevertheless, the contribution of the liquid is still significant, and more shallow penetration of the infrared radiation is desirable. This can be achieved with components with higher refractive index such as Ge, but this comes with higher cost of this element. Illumination to activate the process is provided through a quartz window on the top. In this way, the liquid flows in the free space sandwiched between the two optical components. However, genuine operando experiments are not usually possible in this configuration because simultaneous analysis of the liquid phase composition is seldom performed online by using HPLC or similar techniques, due to the need of conditioning first the samples (e.g., by filtering of solid particles). Furthermore, determination of the evolution of possible gas products can be also challenging. In addition, as it is mentioned below for the case of diffuse reflectance, real quantification with ATR is difficult. Transmission FTIR experiments using carefully designed cells, such as those shown in Fig. 3, are ideal in terms of spectra quality and efficient illumination of wafers, removing any possible contribution of dark areas of the photocatalysts to the spectra [12]. In this system, illumination can be performed in a direction parallel to that of the IR beam. However, pressing thin enough wafers of the photocatalysts is frequently time consuming, and it can be challenging with some materials. Furthermore, circulation of gases through the photocatalyst can require limited rates to avoid cracking. A similar alternative is the deposition of thin films of the photocatalyst in an IR transparent substrate such a Si wafer, as shown in Fig. 4 [13]. This configuration ensures full illumination of the sample, but signal intensity is reduced due to the little

FIG. 2 Scheme of the ATR-FTIR set-up for in situ photocatalytic studies in aqueous solutions. This system involves a UV fiber and a sapphire window for illumination, together with the internal reflection element. A peristaltic pump allows the continuous flow of the solution. (© Reprinted with permission from G.M. Haselmann, B. Baumgartner, J. Wang, K. Wieland, T. Gupta, C. Herzig, et al., In situ Pt photodeposition and methanol photooxidation on Pt/ TiO2: Pt-loading-dependent photocatalytic reaction pathways studied by liquid-phase infrared spectroscopy, ACS Catal. 10(5) (2020) 2964–2977, https:// doi.org/10.1021/acscatal.9b05588. Copyright (2020) American Chemical Society https://doi.org/10.1021/acscatal.9b05588.)

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FIG. 3 Scheme of transmission cell for photocatalytic studies. The component of this device is the following (l) adjusting nut for sealing the chamber, (2) IR, beam, (3) UY light guide, (4) Kalrer O-ring, (5) KBr windows,(6) spectrometer base plate, (7) IR cell support; (8) oven location, (9) sample wafer, (10) gas inlet, (11) external shell, (12) wafer holder, (13) thermocouple location, (14) air cooling outlet. (© Adapted with permission from M. El-Roz, M. Kus, P. Cool, F. Thibault-Starzyk, New operando IR technique to study the photocatalytic activity and selectivity of TiO2 nanotubes in air purification: influence of temperature, UV intensity, and VOC concentration, J. Phys. Chem. C 116(24) (2012) 13252–13263, https://doi.org/10.1021/jp3034819. Copyright (2020) American Chemical Society (N.d.). https://doi.org/10.1021/jp3034819.)

Casing Gas flow Si wafer Quartz windows BaF2 Windows Gas inlet and outlet Teflon gaskets

Sampling port

FTIR spectrometer Detector

IR source

Detector

IR source

FTIR gas cell FTIR spectrometer Pump Transmisión Cell for Photocatalytic Films

Vent

Dry air inlet

FIG. 4 Scheme of transmission cell for operando studies of photocatalytic thin films. In the upper panel, left side shows a cross-section view of the cell marking the path followed by the gas stream inside this device and the materials used for manufacturing the different components. In the down panel, right side displays a scheme of the experimental set-up, which allows the recirculation and the simultaneous analysis of the gas by a second FTIR instrument. Photograph in the left shows the arrangement of the lamps used for irradiation. (From M.D. Herna´ndez-Alonso, I. Tejedor-Tejedor, J.M. Coronado, M. A. Anderson, J. Soria, Operando FTIR study of the photocatalytic oxidation of acetone in air over TiO2-ZrO2 thin films, Catal. Today 143 (3–4) (2009) 364–373, https://doi.org/10.1016/j.cattod.2009.02.033.)

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mass of photocatalyst in the coating, in which, preparation requires specific routes that may not be easily applicable for all photocatalysts. In addition, efficient flow distribution to avoid the creation of preferential paths can be challenging. Diffuse reflectance FTIR spectroscopy (DRIFTS) is a widely used sampling technique for operando studies in catalysis. Commercial environmental chambers are flexible microreactors for solid-gas reactions and allow a fine control of flow rates and temperatures, as well as an efficient IR monitoring of the evolution of the catalyst. Besides, a third quartz window, originally intended for visual inspection, can be conveniently used for irradiation of the photocatalyst. Nevertheless, dead volume in the dome is significant and the contribution of the gas phase needs to be discounted from the spectra of the solids. This type of set-up can provide interesting results using a simple configuration that can be directly applied to powders, instead of wafers or coatings, and therefore, it can be quickly adapted to many processes in the gas phase. However, due to the mismatch between the volume of semiconductor illuminated by the excitation source and that probed by the IR beam, results should be carefully assessed to detect any possible contribution due to not-photoactivated processes. Failing to note that, performing specific blank experiments may lead to misinterpretations. In addition, quantification in the reflectance mode can be difficult if large intensity variations are observed.

2.2 Molecular insights on photocatalytic reactions obtained by means of FTIR studies As mentioned earlier, FTIR is especially suitable for monitoring the formation of organic and inorganic surface species during photocatalytic reactions. This allows the detection of adsorbed intermediates, which are seldom detected as products and provides important hints about the mechanism of the reaction. This has been fully exploited in a number of investigations, particularly for the oxidation of pollutants of different nature. Some representative examples, covering all sampling modes detailed before and illustrating the type of information that can be obtained from these studies, are discussed below. Operando DRIFTS was used to investigate the photocatalytic removal of low concentration of either ethanol or acetone vapors in air over TiO2 in a recirculation system, where gas phase composition is simultaneously monitored in transmission mode, using a multiple-reflection gas sampling cell (4.8-m of path length) [14]. Despite the above-mentioned limitations of DRIFTS, such configuration provided a proof of concept of how information about gas phase and surface composition can be extracted simultaneously in one experiment of photocatalytic oxidation. This work revealed how these oxygenated organics are molecularly adsorbed in the case of acetone, but they form ethoxide surface complexes in the case of ethanol. All these signals progressively decreased under irradiation in parallel to the removal of both acetone and ethanol in the gas phase. Although CO2 is the main product of the photodegradation of these molecules, the partial oxidation product acetaldehyde is also detected with significant concentration during ethanol elimination, both on the surface and the gas phase. This chemical is also found at low concentration in the case of acetone photooxidation, but only adsorbed on TiO2. DRIFT spectra reveal the buildup of surface acetate and formate species during illumination, which appear as the main intermediates for the oxidation of both acetone and ethanol. However, the persistence of these surface species may indicate incomplete illumination of the TiO2. In this respect, subsequent operando studies using photocatalytic coatings in transmission mode have confirmed that carboxylates can be in fact completely removed after completion of the acetone vapor photooxidation [13]. The effect of humidity of the photocatalytic degradation of formaldehyde, which is a widely distributed volatile organic compound (VOC), over TiO2 was also evaluated by DRIFTS [15]. This study indicates that formaldehyde is adsorbed by hydrogen bonding with surface hydroxyl groups of TiO2, and UV irradiation rapidly forms adsorbed formates which show bands at 1572 and 1361 cm
1 due to the asymmetric and symmetric n(OCO) stretching modes. In this work, the presence of humidity is found to be favorable for degradation of this pollutant because it promotes the photoinduced generation of hydroxyl radicals. Photocatalytic removal of other relevant VOCs such as toluene and cyclohexane has been studied in a recirculation system but using a specifically designed transmission cell for studying thin films. Photocatalytic elimination of toluene is particularly challenging because it induces a strong deactivation, leading to incomplete removal of this compound. FTIR is the technique of choice to determine how this phenomenon is related to the accumulation of surface intermediates. The comparison with the results obtained with methylcyclohexane, which has a similar molecular size and polarity but is more amenable to complete mineralization, can provide additional insight on the influence of aromaticity on these processes [16]. This work investigated the performance of TiO2, Ti1
xZrxO2 and ZrO2/TiO2 photocatalytic coatings on Si wafer for the oxidation of these selected pollutants. Gas phase results confirmed that methylcyclohexane (at a concentration of 7.13 mmol/L) is fully degraded with only traces of partial oxidation products in the gas phase and with highest rate measured for the ZrO2/TiO2 photocatalyst. The spectra of photocatalytic films present bands related to ketones such as methylcyclohexanone and carboxylate species, although more specific assignment is hindered by extensive overlapping of the vibration modes. The evolution of the lumped intensity of surface intermediates is plotted in Fig. 5 for TiO2 and

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1.0

1.0

0.8

0.8

Benz. OH+

0.4

0.2

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Normalized intensity

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FIG. 5 Time evolution of adsorbed methylcyclohexane, CO2 and surface intermediates during UV irradiation, over thin films of TiO2 (A) and ZrO2/ TiO2 (B) photocatalysts. In the case of Z/T, the evolution of the surface hydroxyl groups (3690 cm
1) during the reaction is also displayed. (From M. D. Herna´ndez-Alonso, I. Tejedor-Tejedor, J.M. Coronado, M.A. Anderson, Operando FTIR study of the photocatalytic oxidation of methylcyclohexane and toluene in air over TiO2-ZrO2 thin films: influence of the aromaticity of the target molecule on deactivation, Appl. Catal. B: Environ. 101 (3–4) (2011) 283–293, https://doi.org/10.1016/j.apcatb.2010.09.029.)

1419 1517

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ti

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FIG. 6 Time evolution of the relative concentration (right panel), absorbed toluene, benzoate, CO2 and surface hydroxyl groups during UV irradiation of ZrO2/TiO2 thin films and the corresponding spectra of the photocatalysts acquired in operando conditions (leftpanel). The spectra of absorbed benzoic acid is also included for comparison. (From M.D. Herna´ndez-Alonso, I. Tejedor-Tejedor, J.M. Coronado, M.A. Anderson, Operando FTIR study of the photocatalytic oxidation of methylcyclohexane and toluene in air over TiO2-ZrO2 thin films: influence of the aromaticity of the target molecule on deactivation, Appl. Catal. B: Environ. 101 (3–4) (2011) 283–293, https://doi.org/10.1016/j.apcatb.2010.09.029.)

ZrO2/TiO2, together with the variation of adsorbed methylcyclohexane and CO2 formation. These results unambiguously indicate that, although accumulation of surface species is significant for intermediate conversions of the pollutant, after complete removal of methylcyclohexane, the surface of the photocatalyst recovers its initial state, even it is partly hydroxylated due to the adsorption of water formed during the reaction. In contrast, toluene vapors (8.58 mmol/L) cannot be completely degraded, and the evolution of the surface composition with irradiation time, as it is displayed in Fig. 6, shows

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a swift depletion of the surface hydroxyls, and a simultaneous transformation of adsorbed toluene into benzoate complexes. These adsorbed species completely block the active surface of the photocatalytic films, and further degradation is extremely slow leading to the deactivation of the photocatalysts. This is likely because benzoate complexes are preferentially adsorbed in a perpendicular mode to the surface and; therefore, there is little interaction of the aromatic component with the active surface, impeding further oxidation. This example underlines the important variations of the efficiency of photocatalytic degradation of different substrates and how an operando approach can contribute to the understanding of the mechanism of deactivation. In the case of toluene and other pollutants, formation of surface complexes recalcitrant to further photo-oxidation illustrates the role that strong adsorption plays in determining the long-term stability of the photocatalyst. Operando approach using IR spectroscopy in transmission mode with self-supported wafers has been used in a number of studies of the photocatalytic elimination of pollutants such as methanol, phenol, CO or n-hexane [12, 17–19]. In the case of methanol photooxidation, these studies have revealed the higher proportion of reaction intermediates, such as formaldehyde, mono- and bidentate formate or methyl formate, are present on the surface of TiO2 when supported with relatively low loading (10 wt.%) on zeolite Beta, than on the pure semiconductor [17]. Despite the lower absolute activity of the TiO2 dispersed on the zeolite, the spectroscopic results stressed the important interactions between the semiconductor and the acidic solid that can promote the formation of methyl formate. On the other hand, the effect of the frequency of the irradiation source (visible or UV) on the selectivity is beautifully illustrated in an operando FTIR study of the photocatalytic oxidation of methanol on visible light using Au/CuOx/Nb2O5 as photocatalyst, using a transmission cell. As it can be observed in the spectra of Fig. 7, the intensity of the prominent bands at 1583 cm
1, which is associated with formate complexes generated during UV irradiation, remarkably decrease upon swapping to visible light illumination. That change in the spectra of the photocatalyst is simultaneous to the formation of CO2 in the gas phase and indicates that formate complexes are fully oxidized under visible light. This has been interpreted as a consequence of the specific activation of the CuO/Cu2O component rather than Nb2O5 under visible light illumination, leading to a sequential oxidation of methanol first by photogenerated superoxide species to yield formate and then by holes to produce CO2. On the other hand, the effect of acidity on the selectivity of the methanol photocatalytic oxidation over Au/Nb2O5 under UV irradiation has been also conveniently investigated by operando FTIR. Spectroscopic results revealed that methanol photooxidation over Au/ Nb2O5 photocatalyst with relatively low density of acidic sites of both Brønsted and Lewis type, takes place according to a mechanism with dioxomethylene as key intermediate. This species was identified by a band at 2876 cm
1 upon UV irradiation [19]. This surface species eventually leads to the formation of CO2 in the less acidic Au/Nb2O5. However, dioxomethylene is not detected on a more acidic photocatalyst of identical composition, which leads to the preferential formation of dimethoxymethane. This is proposed to occur via a hemiacetal intermediate, and although this last chemical is not detected due to its instability, other adsorbed species consistent with this route, such as methyl formate, are identified on the surface of the less acidic Au/Nb2O5. The study of the effect of humidity on the photocatalytic degradation of phenol vapors was also investigated by in situ FTIR in transmission mode [18]. These investigations revealed that in dry conditions phenol initially adsorbed forming phenate surface complexes on TiO2, but increasing the coverage leads to the formation of a multilayer of undissociated phenol molecules. Under high relative humidity, an apparent equilibrium between phenate and phenol species are

FIG. 7 FTIR transmission spectra of the adsorbed surface species on AuCu-Nb2O5 formed after UV (l 365 nm) irradiation and then during the irradiation with visible light (l > 390 nm). (© From https://doi.org/10.1016/j.apcatb.2019.117978.)

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established in the physisorbed water layer as indicated by the 1370 cm
1 band, associated with the d(OH) bending mode of molecular phenol. During UV irradiation, these surface species are converted to carboxylate, most likely oxalate according to the spectral comparison, and finally to carbonates. However, the presence of frequently observed intermediates of the photooxidation of phenol in aqueous solution, such as catechol and hydroquinone, was not be unambiguously detected due to the extensive overlapping with the bands of phenol. On the other hand, kinetic curves of the adsorbed phenol, taking the integrated intensity of the n(CO) band at 1265 cm
1, show a clear positive effect of the amount of adsorbed water on the elimination rate, which can be associated with the promotion by H2O of the formation of hydroxyl radicals. In this regard, the present study provides an interesting insight on how gas phase photoactivity can be compared to that in aqueous solutions, but also show the limitations imposed by the difficulty of isolating specific signals when dealing with complex and dynamic mixtures of closely related organic chemicals. In situ FTIR studies of gas phase pollutants have not been constrained to VOC, and, in fact, the number of reports on the degradation of inorganic chemicals such as NOx and, in lower extent, H2S is also significant [20–25]. Due to the promising results of photocatalysis for the removal of deleterious and prevalent NOx in urban environments and considering the suitability of infrared spectroscopy for the detection of nitrogenated surface complexes, several in situ studies of NOx elimination have been reported. In this case, the end-product of the reaction is NO3
as numerous FTIR analyses have confirmed, and once the saturation of the surface is achieved, the removal process loses efficiency. In order to enhance the performance, the incorporation of BaO as NOx trap component to the TiO2 has proved to be a successful way to extend the duration of the photocatalytic activity [24]. DRIFTS studies of these materials have shown the formation of NO2
and NO3
on the surface of the photocatalyst exposed to a NOx flow even in dark conditions (see Fig. 8). However, upon illumination, the formation rate of NO3
species with bridging (1603, 1249 cm
1), chelating (1582, 1302 cm
1), and monodentate (1496 cm
1) arrangements greatly increases. In parallel, a slight decrease in the NO2
species is evidenced under irradiation, suggesting that they are further photooxidized. However, the differences on the surface nitrates encountered between pure TiO2 and Ba-modified photocatalysts are limited to minor shifts in the position and small variations in the relative contribution of the surface complexes of different symmetry. These spectral changes have been interpreted as an indication of the presence of Ba-Ti mixed oxides moieties on the surface of these materials. Comparable results are also obtained with TiO2 modified with other alkaline-earth metals, while incorporation of alkaline metals results in the formation of bands at 1630–1690 cm
1 attributed to physisorbed NO2 [26]. On the other hand, a DRIFTS study of the photocatalytic oxidation of NO over Fe-doped TiO2 reveals the formation of a band at 1805 cm
1 under UV irradiation. That feature of the spectra of Fe-TiO2 has been attributed to dinitrosyl species coordinated to Fe2+ sites. These species are progressively depleted after turning off the irradiation, probably due to the re-oxidation of the surface site to Fe3+ and the subsequent desorption of NO. In a similar way, a sharp band of Pdd+-NO species at 1847 cm
1 was observed after NO adsorption on low metallic content Pd/TiO2 photocatalysts [20]. These samples have shown an improved performance for NOx removal even at low loading of the noble metal Pd, because the preparation method is tailored to significantly increase the dispersion of that expensive element. FIG. 8 DRIFT spectra of (A) TiO2 and (B) Ba/TiO2 (a) after the pretreatment at 773 K in air; (b) after addition of NO/ O2 gas for 30 min in the dark; (c) after irradiation for 3 h in NO/O2 gas. (Reproduced with permission from A. Yamamoto, Y. Mizuno, K. Teramura, S. Hosokawa, T. Tanaka, Surface Ba species effective for photoassisted NOx storage over Ba-modified TiO2 photocatalysts, Appl. Catal. B: Environ. 180 (2016) 283–290, https://doi.org/10.1016/j.apcatb.2015.06. 036, Elsevier. © From https://doi.org/10. 1016/j.apcatb.2015.06.036.)

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Although most investigations of NOx depletion are based on TiO2, the growing interest on developing of novel materials photoactive under visible light has expanded the chemical variety of the photocatalysts evaluated in last years. A promising photocatalyst that has been proposed for NOx removal under visible light illumination is BiOCl [23]. Using this material, in situ DRIFTS experiments have revealed the formation under visible light illumination of adsorbed N2O4 (NO2 dimers), nitrites (at 1174 and 1327 cm
1) and nitrates (at 809, 841, 1000, 1015, 1048, 1445 and 1482 cm
1). Similar DRIFT spectra have been acquired for more complex Bi2O2CO3/Bi4O5Br2 heterostructures [25]. In the case of the g-C3N4 photocatalysts, the overlapping with the structural vibrations of this material hurdles the identification of surface nitrogenated species, although vibrations observed below 1150 cm
1 suggest the formation upon visible light irradiation of adsorbed neutral NOx species, along with nitrites and nitrates [22, 27] In a completely different approach, the photoassisted selective catalytic reduction (SCR) of NOx with NH3 on Ag clusters supported on faujasite-type zeolite has been analyzed using transmission FTIR in operando conditions [21]. This study was performed combining visible light illumination with heating and with a simulated stream of exhaust gases containing not only NO traces, but also CO, CO2, H2O, NH3 and hydrocarbons. Spectra obtained under reaction conditions at 150°C show mainly bands of NH4 + (1400 cm
1) adsorbed on Brønsted sites, and NH3 overlapped by water adsorption (1642 cm
1). The intensity of these bands swiftly decreases upon illumination, while gas phase analysis shows a clear increase in NOx conversion. This behavior has been explained considering the SCR of NOx is promoted by local heating of the Ag clusters, which is induced by light absorption by plasmonic modes, and this facilitates the decomposition of NH2NO intermediates into N2 and H2O. On the other hand, in situ investigation of the photocatalytic oxidation of H2S has been approached cotemporally by DRIFT and transmission analysis, as to ascertain the relevant adsorbed species formed during this process. Photocatalysis is highly effective for removal of low concentration of this widespread toxic compound, which also contributes to the corrosion of equipment [28]. Similar to the previously discussed case of NOx removal, for photooxidation of H2S, the targeted product of the reaction is sulfate, which progressively accumulates on the surface, eventually leading to the deactivation of the photocatalyst. In this respect, a DRIFT study has confirmed that generation of sulfate on the TiO2 surface upon illumination is surprisingly quick for a process implying eight electrons per molecule. Transmission experiments, more reliable in terms of efficient illumination, were performed with photocatalytic coatings, and suggested that SO2
(characterized by bands at 1093 and 996 cm
1) is one of the initial intermediates of the photooxidation [29]. This early study stressed the relevance of applying complementary techniques to gather complete information about these processes. Selective photocatalytic oxidation processes for the generation of fine chemicals have been also approached by infrared spectroscopy under in situ conditions. In particular, gas phase selective conversion of propylene into propylene oxide has been investigated using isoreticular MOFs with different organic linkers as photocatalysts [30]. This work has shown that the oxidation mechanism is different on these hybrid compounds than on conventional semiconductor photocatalysts such as ZnO. Thus, formation of adsorbed acrylic acid and other carbonyl compounds suggested the presence of adsorbed acrolein and/or acetone on the MOF upon illumination in air/propylene mixtures. Processes of photocatalytic selective oxidation in aqueous solution have been also investigated by means of ATR over TiO2 in a continuous flow of the organic saturated with O2. For example, in the case of the photoinduced transformation of cyclohexane, the spectroscopic results showed a quick and selective formation of cyclohexanone on the photocatalysts surface after irradiation. However, desorption of this molecule, which is the targeted product, is hard to achieve, leading to further oxidation reactions that generates adsorbed carboxylate and carbonate-like compounds, which are unwanted products of the full mineralization of the organic substrate [11]. Photocatalytic oxidation of ethanol, acetaldehyde and acetic acid in aqueous solution, using both Pt-loaded and unmodified TiO2, has been also monitored in situ using ATR-IR [31]. These experiments show an important variation in the absorption baseline upon illumination of TiO2, which has been observed likewise in gas phase assays, that arises from the accumulation of photogenerated electrons [13]. This effect is much more moderated on Pt/TiO2 due to the more efficient management of these charge carriers on the metal-loaded photocatalyst, which results in better degradation efficiency. Formation of Pt0-CO species (with a sharp band at 2050 cm
1) during the reaction is observed and, eventually, this can be detrimental for the performance due to the well-known CO poisoning of this noble metal. Besides, in situ ATR results confirm that photooxidation of ethanol produces first acetaldehyde and, subsequently, acetic acid, which is harder to degrade and constitutes the rate limiting stage. In another recent report, the multivariate analysis of the evolution of the bands Pt0-CO was applied to monitor the metal deposition on TiO2 using methanol as a sacrificial agent, as displayed in Fig. 9 [11]. This study unveiled a dependence on Pt loading of the selectivity of methanol oxidation towards complete mineralization, which was ascribed to the availability of Pt sites and oxygen vacancies. The interest of photocatalytic reactions of small carboxylic acids and other oxygenated molecules such as formaldehyde to achieve the complete mineralization of pollutants and for being used as sacrificial agents for hydrogen production has

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FIG. 9 IR ATR spectra during UV illumination with 0.7 wt% Pt: of TiO2 (A) IR spectra obtained with the Si ATR element as the background spectrum (B) different spectra, where gray to black spectra correspond to the first 5 min of the reaction at 10 s intervals. Light shaded upper shifted spectra correspond to t ¼ 5–70 min acquired at 5 min and intervals. Assignment of most relevant bands are highlighted. Check original reference for color interpretation. (© Reprinted with permission from G.M. Haselmann, B. Baumgartner, J. Wang, K. Wieland, T. Gupta, C. Herzig, et al., In situ Pt photodeposition and methanol photooxidation on Pt/TiO2: Pt-loading-dependent photocatalytic reaction pathways studied by liquid-phase infrared spectroscopy, ACS Catal. 10(5) (2020) 2964–2977, https://doi.org/10.1021/acscatal.9b05588. Copyright (2020) American Chemical Society (N.d.). https://doi.org/10. 1021/acscatal.9b05588.)

prompted several ATR studies [32–34]. In this way, not only TiO2 but also other semiconductors such as lepidocrocite (g-FeOOH), which is an abundant mineral, have been investigated [34]. This material was selected because of its biogeochemical relevance, and the ATR experiments revealed that citric acid is protoxidized on lepidocrocite with remarkably higher rate at the most acidic conditions (pH 4). The product of the reaction is acetonedicarboxylic acid that can be subsequently transformed to acetoacetic acid but is progressively displaced from the surface of the solid in excess of citric acid. Interestingly, this study also shows that dissolution of lepidocrocite is a photoinduced process, likely due to ligand-to-metal charge transfer reactions of surface citrate complexes.

3

Raman and other vibrational spectroscopies

In contrast with infrared, Raman spectroscopy is frequently applied to the identification of solid phases present on heterogeneous catalysts, either dispersed in support or as a bulk component, because many of them are characterized by strong signals due to lattice vibrations [35]. Alternatively, surface-enhanced Raman spectroscopy (SERS) can be applied for tracking reactions of adsorbed molecules on noble metal substrates such as Au and Ag. However, some semiconductors like TiO2 have also proven to be active in SERS under certain specific conditions, providing an interesting tool for in situ and operando monitoring of photocatalytic reactions [36]. In addition, Raman presents the advantage of being insensitive to water molecules, which is frequently an important nuisance in infrared studies of photocatalysts under realistic conditions due to its strong absorption signals. SERS methodology has been applied to the study of the photocatalytic degradation of 4-aminothiophenol in inverse opal TiO2 [36]. This molecule is detected with low intensity on TiO2 P25 under red laser (l ¼ 785 nm) excitation, but the arrangement of this semiconductor as a photonic crystal enhances the signal of the adsorbed organic, which becomes clearly visible in the spectrum of inverse opal TiO2. Changing the excitation laser of the Raman to the green (l ¼ 532 nm) causes a

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clear modification to the spectra, which is attributed to the photoinduced dimerization of p-aminothiophenol to form 4,40 dimercaptoazobenzene. The presence of this compound is confirmed by HPLC and generates a new band in the Raman spectra due to N]N stretching (1437 cm
1). Thus, in this case, the green laser is playing two roles, as an analytical tool for Raman detection and as a photon source for activating the photochemical process. Continuous irradiation with green lights leads to the progressive decay of the intensity of the bands of adsorbed organics, due to the subsequent degradation of the photoproduced azo dimer. In contrast, red light is not significantly absorbed by the opal TiO2, and therefore, it can only induce some slight, thermally induced modifications of aminothiophenol at high power. On the other hand, as mentioned earlier, noble metal particles can induce SERS effect, facilitating the applications of this technique to metal-supported photocatalysts. Thus, in a similar study of the oxidation of p-aminothiophenol over Au/TiO2 using external UV illumination, the evolution of Raman spectra with irradiation shows the swift formation of p-nitrophenol [37]. The mechanism of this process has been proposed to imply oxygen activation on Au particles by capturing the photogenerated electrons. Operando SERS was also used to ascertain the mechanism of the photocatalytic CO2 reduction using a terpyridine nickel (II) complex, as a photocatalyst, and bipyridine Ru (II) complex, as a photosensitizer [38]. In these experiments, a flow of CO2 saturated with CH3CN/H2O vapors was introduced into the system. Upon UV light irradiation, bands at 1604, 1553, and 1485 cm
1 arose, which can be attributed to carboxylate species, COO
, bound to the metal. In addition, another feature centered at 2040 cm
1 is attributed to the C]O stretching vibration of an intermediate Ni complex. Sum frequency generation vibrational spectroscopy (SFG-VS) is a surface-specific technique, and it is progressively widening its scope of applications. In the area of photocatalysis, it has been applied for example to the study of photocatalytic oxidation of methanol on TiO2 (110) [39]. The results obtained upon UV irradiation allow identifying some photooxidation products such as formaldehyde and methyl formate by their n(CdH). However, these experiments were performed at subzero temperatures and reduced pressures, showing the current limitations of this technique for approaching realistic conditions.

4 X-ray absorption spectroscopy Synchrotron radiation facilities have been extensively used in the last years for operando and in situ studies of catalysts by means of time and space resolved X-ray absorption spectroscopy (XAS) [40]. In particular, X-ray absorption near edge structure (XANES) allows to monitor variations in the electronic state of the selected atomic components, while Extended X-ray absorption fine structure (EXAFS) can be applied to gauge the variations in the local symmetry, phase composition, and electronic properties of the targeted atom during catalytic processes. These spectroscopic tools are particularly suitable for following the evolution under reaction conditions of supported metals and, therefore, in the case of photocatalysts for unraveling the possible modification of co-catalysts under irradiation in kinetic experiments. Despite the growing interest in these techniques for elucidating electronic and structural modifications under operation conditions, investigations of photocatalysts using XAS are still relatively scarce, probably due to the additional difficulty of implementing an efficient illumination of the reaction set-up. An interesting operando XANES study allowed to characterize the atomic and electronic rearrangements of the photoinduced species and establish the mechanism of the photocatalytic Co@MOF(Ti) system used for hydrogen production [41]. These XAS measurements were performed in fluorescence mode using the experimental set-up shown in Fig. 10, which was provided with backward illumination. These assays were carried out in an anoxic suspension of the preequilibrated MOF in a CH3CN/H2O mixture, using triethylamine as a sacrificial agent, and applying on/off illumination cycles. Differential mode analysis of Co K-edge spectra (the color map of the XAS intensity evolution is displayed in Fig. 11) shows that visible light illumination induces several modifications in the spectra: a displacement of the XANES edge toward lower energies, together with a diminution of the white line intensity and the spectral feature around 7737 eV. These results indicate that visible light promotes the partial reduction of the Co2+, which takes place by direct charge transfer to the p and d (eg) states from Ti MOF framework. These partially reduced metal species are the active centers for hydrogen evolution. Spectral modifications became more prominent following prolonged illumination (up to 5 h). These changes are entirely reversible, so the spectra progressively reverted to the initial state in the dark and are fully recovered in a second illumination cycle (see Fig. 11). In addition, it was also possible to track the formation of Co-sites with a monomeric configuration that can connect to the Ti-oxo clusters of the MOF in the active state. Thus, XANES and EXAFS analyses unveiled the structure of the active metal sites, their electronic interaction with the photoactive MOF, as well as the charge localization process after photoexcitation with visible light. Gas-phase photocatalytic production of hydrogen using methanol as a sacrificial agent under continuous flow over Cu/TiO2 and NiCu/TiO2 photocatalysts was investigated by micro-XAS [42]. In this experimental set-up, on-top UV-irradiation was used for activating the photocatalyst, while perpendicular X-ray fluorescence emission was used

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FIG. 10 Photograph (left) of the set-up and schematic (right, up) and image (right, down) used for fluorescence XANES measurements photocatalysis under operando conditions. (© Reproduced from with permission from A. Iglesias-Juez, S. Castellanos, M. Manuel Monte, G. Agostini, D. Osadchi, M. A. Nasalevich, J.G. Santaclara, A.I. Olivos Suarez, S.L. Veber, M.V. Fedin, J. Gasco´n, Illuminating the nature and behavior of the active center: the key for photocatalytic H2 production in Co@NH2-MIL-125(Ti), J. Mater. Chem. A 6 (36) (2018) 17318–17322, https://doi.org/10.1039/c8ta05735d. The Royal Society of Chemistry (N.d.). https://doi.org/10.1039/c8ta05735d.)

FIG. 11 Intensity contour map of normalized differential Co K-edge XANES spectra during cycles of light switching for Co@MOF in CH3CN/TEA/ H2O solution. (© Reproduced from with permission from A. Iglesias-Juez, S. Castellanos, M. Manuel Monte, G. Agostini, D. Osadchi, M.A. Nasalevich, J. G. Santaclara, A.I. Olivos Suarez, S.L. Veber, M.V. Fedin, J. Gasco´n, Illuminating the nature and behavior of the active center: the key for photocatalytic H2 production in Co@NH2-MIL-125(Ti), J. Mater. Chem. A 6 (36) (2018) 17318–17322, https://doi.org/10.1039/c8ta05735d. The Royal Society of Chemistry (N.d.). https://doi.org/10.1039/C8TA05735D.)

for analysis. This arrangement allows to investigate the profile of in-depth variations in the photocatalyst bed due to the progressive attenuation of UV-light. Cu K-edge results indicate that only an external layer of the photocatalyst ( stand for the total density of electron traps (ETs) (mmol g
1). Specific surface area (m2 g
1) is shown in the third row. Abbreviations “A,” “R,” and “B” in the bottom row are anatase, rutile, and brookite, respectively, and “a” and “r” are anatase and rutile in minor compositions, respectively.

FIG. 2 Anatase contents of FP-6 (Showa Denko Ceramics) and TIO-13 (Catalysis Society of Japan) determined by the in situ temperature-controlled XRD analysis.

have been reported as good inhibitors of anatase-rutile transformation, and anatase-rutile transformation for TIO-13 at a higher temperature is, therefore, attributable to sulfate contaminants. On the other hand, since it is known that FP-6 does not contain sulfur as an impurity and is prepared from titanium(IV) chloride, it shows a lower transformation temperature, as has frequently been reported. Sulfate ions often remain even after repeated washing and are thought to stabilize the anatase surface. When calcined at a high temperature, ca. 800°C, well-crystallized anatase can be obtained. Nishimoto et al. showed by using infrared spectroscopy (IR) that sulfate ions cannot be removed completely by calcination up to about 1000°C [9]. The sulfate ions may also affect the photocatalytic activity. On the other hand, with chlorine ions (about 65% of the worldwide industrial production of titania is based on the chloride processes [10]), rutile is often obtained even if calcined at a low temperature; for example, fine rutile particles can be obtained when titanium(IV) chloride is hydrolyzed in an acidic hydrochloric acid solution [11]. However, there is no guarantee that the obtained rutile is “pure” rutile. In fact, in most commercially available titania products, even if the product is called “rutile” in the catalog, it gives an anatase peak in X-ray diffraction and it also has the possibility of containing amorphous titania that does not give an X-ray diffraction peak. In other words, it is challenging to obtain highly crystalline “pure” small-sized rutile particles as well as highly crystalline “pure” small-sized anatase particles as described earlier. Brookite is likely to be obtained in the presence of acetate ions. It has been reported that crystals of anatase, rutile, and brookite were formed by heating titanium trichloride solutions with various coexisting chemical substances under aerated conditions [12], but it is difficult to obtain a single phase of brookite (see Section 4).

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2 Photocatalytic activities of anatase and rutile 2.1 Mechanistic analysis of photocatalytic reactions by action spectra [13] Fig. 3 shows action spectra (wavelength dependence of photoinduced reaction rate) of three reaction systems with 12 commercially available titania (some of which were calcined in a laboratory): (a) methanol (50 vol%) dehydrogenation (2 wt% of platinum (Pt) was in situ deposited from chloroplatinic acid on titania) [H2 system], (b) oxygen (O2) generation from an aqueous silver sulfate solution (25 mmol L
1) [O2 system], and (c) acetic acid (5 vol%) oxidative decomposition [CO2 system] under monochromatic irradiation at wavelengths of 350, 370, 385, and 410 nm using a monochromator equipped with a xenon lamp. Even though only a few of the 12 titania are displayed in Fig. 3, it is clear that the action spectra are distributed evenly in the H2 system. On the other hand, they are remarkably skewed to one-side in the O2 system and the CO2 system, upper side except for one in the O2 system and lower side except for one in the CO2 system, even if the same samples are used. The exception samples in the O2 system and the CO2 system are those containing only anatase and rutile, respectively. In other words, for mixed TiO2 powders, the action spectra of the H2 system were intermediate between those of the pure anatase

FIG. 3 Representative action spectra of photocatalytic reactions of (A) H2 evolution, (B) Ag deposition, and (C) acetic acid decomposition. The TiO2 powders used were Merck (solid circle), Degussa P25 (open circle), Wako (anatase) + Ishihara CR-EL (open inverted triangle), JRC TIO-5 (open square), and Ishihara CR-EL calcined in air at 1200°C (solid square). (Reproduced by permission of the PCCP Owner Societies (Fig. 2 in T. Torimoto, N. Nakamura, S. Ikeda, B. Ohtani, Discrimination of active crystalline phases in anatase-rutile mixed titanium(IV) oxide photocatalysts through action spectrum analyses, Phys. Chem. Chem. Phys. 4 (2002) 5910–5914). Credit: Royal Society of Chemistry.)

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and rutile phases; however, the action spectra of the O2 system are similar to that of pure rutile powder, even though the mixture consisted predominantly of anatase, while the action spectra of the CO2 system are similar to (even blue-shifted from) that of pure anatase powder. Fig. 4 shows the relation between l1/2, wavelength giving normalized ’app of 0.5 to action spectra as shown in Fig. 3, and the anatase fraction obtained from the peak-height ratio of anatase and rutile in the X-ray diffraction pattern. In the H2 system, l1/2 decreases almost linearly as the anatase content increases, whereas the values of l1/2 are maintained at almost the same values as those in respective pure titania in the O2 system and the CO2 system even if the powder contains small amounts of rutile and anatase. Those results suggest that anatase and rutile show similar “relative” activities in the

FIG. 4 l1/2 values obtained for the photocatalytic reactions of H2 evolution (A), Ag deposition (B), and acetic acid decomposition (C) as functions of fanatase. The figures and symbols are the same as those in Fig. 1. (The TiO2 powders (BET surface area in the unit of m2 g
1) used were (1) Merck (13), (2) JRC TIO-2 (18), (3) Wako (anatase) (61), (4) Degussa P25 (50), (5) Merck + Ishihara CR-EL (11), (6) Wako (anatase) + Ishihara CR-EL (35), (7) Aldrich (rutile) (3.1), (8) JRC TIO-5 (2.6), (9) Wako (rutile) (8.1), (10) Ishihara CR-EL (8.2), (11) Ishihara CR-EL calcined in air at 1200°C (3.6), and (12) Degussa P25 calcined in air at 1200°C (2.1). Crystallite structures of the TiO2 powders were pure anatase (solid circle), pure rutile (solid square), and a mixture of both crystallites (open circle).) (Reproduced by permission of the PCCP Owner Societies (Fig. 3 in T. Torimoto, N. Nakamura, S. Ikeda, B. Ohtani, Discrimination of active crystalline phases in anatase-rutile mixed titanium(IV) oxide photocatalysts through action spectrum analyses, Phys. Chem. Chem. Phys. 4 (2002) 5910–5914). Credit: Royal Society of Chemistry.)

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H2 system but that only rutile and anatase show photocatalytic activity in the O2 system and the CO2 system, respectively. Furthermore, rutile absorbs light of a wavelength longer than that of anatase. When an anatase-rutile mixture sample is used in the CO2 system, in which rutile shows negligible activity, rutile absorbs light of the wavelength in which anatase is active, and then l1/2 shifts to a shorter wavelength than that in the case of pure anatase due to the “inner-filter effect.” Sample 4 in Fig. 4 is Evonik P25, the major composition of which is anatase and the minor composition of which is rutile. In the H2 system, it behaves like a mixture, and it behaves like rutile and anatase in the O2 system and the CO2 system, respectively. In the O2 system, only rutile activity has been observed even though the content of rutile is only a dozen percent in Evonik P25. (Note that this is a relative comparison of which phase is active in the mixture sample, not an absolute comparison of which phase shows higher photocatalytic activity between the pure anatase and the pure rutile in each reaction system.) Considering the O2 generation reaction associated with silver-metal deposition, the valence band position of titania, which is a simple metal oxide, is almost the same between anatase and rutile, as Scaife has suggested in [14], so it seems unusual that there are differences between the activities of anatase and rutile. However, further considerations using commercially available titania revealed that the photocatalytic activity in the O2 system can be determined only by the secondary particle size of the sample under conventional light irradiation such as irradiation from mercury or xenon lamps [15, 16]. Generally, rutile is often obtained by calcination at a high temperature for commercially available titania, and the secondary particle size usually tends to be larger. Therefore, it is thought that rutile (with a large particle size) shows higher activity in O2 evolution reactions. It has also been found that when titania is irradiated with high-intensity light, both anatase and rutile even with small particle sizes show sufficient activity [17]. On the other hand, regarding the O2 reduction reaction associated with decomposition of oxidative organics, the standard electrode potential (SEP) of the one-electron reduction reaction of O2 for superoxide anion radical generation (O2 + e
¼ O2
) is
0.28 V (vs standard hydrogen electrode (SHE)) and that for hydroperoxyl radical formation with protonation (O2 + H+ + e
¼ HO2) is
0.05 V, while that of the two-electron reduction reaction of O2 for hydrogen peroxide generation (O2 + 2H+ + 2e
¼ H2O2) is 0.70 V. The CBB of anatase is thought to be around
0.2 V [18], and it is therefore difficult to generate superoxide anion radicals, which have been reported to have a relatively long lifetime [19], regardless of the type of titania is used. In addition, in the case of rutile, with the conduction band located, ca. 0.2 V, on the anodic side, it is difficult to generate even hydroperoxyl radicals. Thus, it can be considered that anatase, in which excited electrons can reduce O2 by one electron, shows higher photocatalytic activity for acetic acid decomposition. Moreover, in the presence of O2, a mechanism of the radical chain reaction by which the reaction efficiency is significantly improved has been proposed [20]. For the case of H2 evolution, see Section 2.5. In any case, when a mixture is used as a photocatalyst, it has been thought that the higher the fraction is, the greater its contribution to photocatalytic activity is, but the results presented above suggest that this is a misperception.

2.2 Heterogeneity in Evonik P25 [21] What we cannot avoid in photocatalysis research is the use of Evonik (originally Degussa) P25. It is manufactured by Nippon Aerosil, and the codename is AEROXIDE TiO2 P 25. Regarding its properties, the crystalline composition (fraction) was often described as anatase and rutile content of “70:30” until the 1980s and later as a content of “80:20,” even though neither Evonik nor Nippon Aerosil officially reported its content. The specific surface area was officially reported to be “50  15 m2 g
1,” but a specific surface area of “50 m2 g
1” has been stated in many papers. Those “vague” physical properties of P25 originated from the manufacturing method. According to the manufacturer, P25 is manufactured by flowing titanium(IV) chloride vapor in an oxyhydrogen flame, in other words, a combustion method. Since the temperature of the oxyhydrogen flame varies greatly depending on the position, the product differs depending on where it passes in the flame, and fine particles generated in the low temperature part may pass through the high temperature part. Even if the operating conditions are kept constant, it is naturally expected that there is heterogeneity in the powder produced as a smoke. One of the keys to understand the structure of P25 is this heterogeneity. This heterogeneity was firstly reported in the following research by the authors [21], until then (or even now), it was not recognized.

2.3 Isolation of anatase and rutile from P25 When P25 is treated with a hydrogen peroxide solution containing ammonia (sodium hydroxide or sodium carbonate also being possible [22]) with an adjusted concentration, rutile is completely dissolved, leaving only anatase (yield of about 30%) [23]. The obtained gel by drying is then calcined at 400°C, and only the peak of anatase appears in the XRD pattern.

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On the other hand, only rutile can be isolated from P25 using hydrofluoric acid (selective dissolution of anatase) with reference to a previous study [24]. It is confirmed by the XRD analysis that only the rutile peak appears and that there is no anatase peak. When XRD patterns are standardized with the strongest diffraction peaks of anatase and rutile, respectively, it is found that both peak widths of isolated anatase and rutile are negligibly changed from the original XRD peaks with P25. This suggests that the crystallite sizes are not changed by those isolation processes. Generally, the smaller the particles are, the faster the dissolution rate is. Therefore, the above result suggests that the particle size distributions of anatase and rutile in original P25 are extremely narrow. It should be noted that both isolated anatase and isolated rutile include a small amount of noncrystallite (NC) components determined by the XRD analysis mixed with an internal standard such as nickel(II) oxide as described in the next section (the origin of that determined NC might be predominantly water, though). It has been suggested from transmission electron microscope (TEM) observations that P25 contains amorphous titania [25, 26]; however, a method for isolation of amorphous titania has not been established so far.

2.4 Crystalline composition of P25 [21] Spurr and Myers reported for the first time the relation between the weight fraction and XRD peak intensity ratios of artificial anatase-rutile mixtures [27], and the reported calibration curves have often been used to determine the anatase-rutile ratio in titania powders. However, the calibration curves neglect the possible presence of NC (or amorphous in some papers) content. There are other problems in the reported equation. One problem is that the equation was derived from the mixture of one kind of anatase and rutile, the physical properties of which were uncertain. Another problem is that the difference in X-ray absorption coefficients due to the difference in densities of anatase and rutile was not taken into consideration [28]. Therefore, we prepared calibration curves by using nickel(II) oxide (20 wt% NiO) as an internal standard (using an internal standard such as NiO enables determination of the absolute content of anatase or rutile if pure anatase or rutile powder is used) and we isolated anatase and rutile as standards since XRD peak intensity may depend on the particle size [9, 29] and anatase crystallites smaller than 30 nm might give a smaller peak intensity, it is necessary to make calibration curves by using pure anatase and rutile crystallites contained in their original mixture to determine crystalline compositions. Anatase, rutile, and NC contents in P25 were obtained as shown in Table 1. The typical crystalline composition of P25 was determined as 78% anatase and 14% rutile. The remaining was assumed as an NC phase, and the anatase-rutile-NC ratio was determined to be 78:14:8, but this result requires the following caution. It can be clearly seen that there is an appreciable fluctuation in the compositions even if the P25 powder was collected from the same package. In particular, the fluctuation in the NC composition seems large; sometimes there was no NC phase obtained even if the same P25 package was used. One of the reasons for the high fluctuation in the NC composition might be that it is calculated by subtraction. Datye et al. reported that they did not detect any amorphous phase in P25 [30], whereas Ohno et al. observed anatase, rutile, and amorphous domains by TEM with as-supplied P25 [26]. Such a discrepancy in the results of studies can be interpreted by the inhomogeneity of the crystalline composition of P25 originating from the production procedure (see Section 2.2). In addition, after suspension in water, ultrasonication, and then drying at 120°C to be recovered, P25 shows a smaller fluctuation in the crystalline composition and a significant decrease in photocatalytic activities (5%–30% depending on the kind of reaction). Note that these results were obtained by using one P25

TABLE 1 Crystalline composition of P25 collected from the same package (Table 1 in [21]). %Composition Entry

Anatase

Rutile

Amorphous

Anatase/rutile ratio

1

78

14

8

5.6

2

73

14

13

5.2

3

82

16

2

5.1

4

83

17

0

4.9

5

84

16

0

5.3

6

85

15

0

5.7

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FIG. 5 ERDT/CBB patterns of FP-6 collected from three almost same positions and a different position in the same package. Figures in denote the total density of ETs (mmol g
1).

package in the author’s laboratory, and the crystalline compositions for another P25 package (different batches of the production) or P25 stored in another laboratory is completely unknown. P25 often exhibits higher photocatalytic activity in various reaction systems, and it has therefore been used as a “defacto” standard; e.g., “our prepared photocatalyst shows twice the activity of P25.” However, using P25, which has high inhomogeneity as described above, as a standard sample seems to be rather problematic. In addition to the above-mentioned inhomogeneity in P25, Fig. 5 shows ERDT/CBB patterns of FP-6 powder. The three samples on the left were carefully collected from almost the same position, and the sample on the right was collected from a different position in the same package. It can be seen that ERDT patterns of the three samples on the left are similar and the ERDT pattern of the sample on the right is remarkably different from the others, while CBB positions, which reflect the bulk structure, of all of the samples are identical [6]. The differences in ERDT patterns may come from the differences in surface structures of the samples, and there is a possibility of high inhomogeneity in commercially available titania other than P25 even when it is collected from the same package.

2.5 Synergistic effect of anatase and rutile in P25 [21] It has been suggested in many papers that the reason why P25 exhibits relatively high photocatalytic activity is due to promotion of “charge separation” caused by contact of anatase with rutile (or amorphous [25]), which causes moving one of the electrons-holes generated in anatase or rutile particles to the other particle, for examples [31, 32]. The important point is whether the photocatalytic activity of P25 can be enhanced by the coexistence of anatase and rutile compared to the case of only anatase and rutile. As mentioned in Section 2.3, since amorphous titania cannot be extracted from P25, a titania mixture could be reconstructed by isolated anatase and rutile and by commercially available amorphous titania with an average composition ratio of 78:14:8. For amorphous titania, although sometimes water may be a possible origin of the so-called amorphous titania as described earlier, it has been reported that the photocatalytic activity of amorphous titania is negligible [33] and that the content is relatively small, and it is therefore considered that reconstruction using a commercially available product that was not originally included in P25 would be of no problem. The specific surface area obtained by the BET method, XRD pattern, and diffuse reflection spectrum of the reconstructed P25 was almost the same as that of the original P25. Fig. 6 shows the photocatalytic activities of the reconstructed P25 (R-P25) as well as the isolated anatase and rutile, commercial amorphous titania, and original P25 (treated by suspending in an aqueous solution and drying because the isolation process includes those procedures). The reactions are (a) oxidative decomposition of (5 vol%) acetic acid in an aerated aqueous solution, (b) oxidative decomposition of acetaldehyde (ca. 2200 ppm) in air, (c) dehydrogenation of methanol in a deaerated aqueous solution (50 vol%) containing chloroplatinic acid (hydrogen hexachloroplatinate(IV)) for in situ Pt deposition, and (d) O2 generation from silver sulfate in a deaerated aqueous solution (25 mmol L
1). The activity of the original P25 is standardized to 100%.

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FIG. 6 Normalized photocatalytic activities of (from left to right) original P25, isolated anatase and rutile, commercial amorphous titania and reconstructed P25 from isolated anatase and rutile with amorphous titania (R-P25) in the systems of (A) oxidative decomposition of acetic acid in an aerated aqueous solution, (B) oxidative decomposition of acetaldehyde in air, (C) dehydrogenation of methanol in a deaerated aqueous solution, and (D) oxygen liberation from a deaerated aqueous silver-sulfate solution. The figures above R-P25 are a weighted sum of photocatalytic activities calculated from those of isolated anatase, rutile, and amorphous particles.

First, R-P25 showed almost the same photocatalytic activities for all of the reaction systems as those of the original P25, and it can therefore be considered that P25 is composed of a simple mixture of anatase, rutile, and amorphous titania, indicating that the interaction and/or fusion of anatase and rutile particles in P25, if any, during its manufacturing process have no influence or negligible on the photocatalytic activity. A recent study has revealed that interparticle charge-transfer excitation occurs if anatase and rutile samples are thoroughly mixed [34]. In that case, we could consider some synergistic effects from the coexistence of anatase and rutile, such as P25. However, in fact, such effects might not lead to enhancement of photocatalytic activity as described later. Interestingly, for the oxidative decomposition of organic compounds in the presence of O2 in reactions (a) and (b), isolated anatase and rutile show higher and lower activities than those of the original P25 and R-P25, respectively. In other words, the activities for the original P25 and R-P25 are between those of isolated anatase and rutile. As described in Section 2.1, rutile shows lower activity in these reaction systems. Thus, it can be said that the activities were not enhanced by mixing with anatase and rutile. This was also supported by the fact that the sum (predicted value) of photocatalytic activities calculated from those of isolated anatase, rutile, and amorphous titania multiplied by the composition ratio (0.78, 0.14, and 0.08) becomes close to 100 (the number above the bar of R-P25). Furthermore, for acetic acid oxidative decomposition with samples mixed with isolated anatase and rutile with different mixing ratios without the addition of amorphous titania, the photocatalytic activities showed a linear relation with the mixing ratio and the photocatalytic activity of the original P25 was almost on the linear line [21], clearly indicating that there is no synergistic effect of the coexistence of anatase and rutile in P25 on the photocatalytic activity. On the other hand, in the O2 generation system (d), the activity of isolated rutile was higher and that of isolated anatase was lower. In this case, the predicted value is much smaller than 100, probably because the crystalline composition of active rutile is small, so the error must be larger, compared with the case of oxidative decomposition in systems (a) and (b), in which the rutile is inactive. From the results, it can be concluded that only anatase in P25 works in a reaction system in which anatase is active (systems (a) and (b)) and only rutile in P25 works in a reaction system in which rutile is active (system (d)), and in each system, isolated anatase and rutile show higher activity than that of the original P25 due to the absence of an inactive crystalline phase. Again, it can be concluded that there is no enhancement or synergistic effect due to the coexistence of anatase and rutile. For the methanol dehydrogenation reaction (c), photocatalytic activities of the original P25 and R-P25 are higher than those of only isolated anatase and rutile, and the predicted value is only 74, suggesting that there is enhancement of the activity due to the coexistence of anatase and rutile. However, a recent study revealed by action spectrum analysis that Pt is first deposited only on rutile particles when the amount of chloroplatinic acid (source of Pt) is very small and then Pt is also deposited on anatase particles when the amount of chloroplatinic acid is increased (unpublished data). In addition, it has been shown that when fine titania particles are aggregated or sintered by drying or calcination, electron transfer between those particles occurs to result in higher photocatalytic activity, i.e., only one Pt deposit is necessary to enhance photocatalytic activity for a particle aggregate [35]. From these facts, the photocatalytic activities of Pt-loaded isolated samples and those of Pt-loaded P25 samples cannot simply be compared.

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3 Amorphous titania 3.1 Structure of amorphous titania Even though amorphous titania is thought to be present, it is difficult to analyze. Strictly speaking, amorphous titania is an NC component for which the composition is titania (TiO2). In other words, the composition is the same as that of anatase and rutile, but it does not have a crystalline structure. It has a short-range order based on the TiO6 octahedron, not a longrange order. Since the structure of amorphous titania has not been defined, even if there are two samples with the same compositions and no peak in the XRD pattern, it is impossible to say that they have the same structures. A structure in which several layers on the surface of the titania crystal are amorphous is also a possibility, but its quantitative evaluation is practically impossible. As a result of the investigation of the crystalline structure by Rietveld analysis, it was found that the NC content in many commercially available titania powders is almost proportional to the specific surface area (unpublished data). This indicates that “disorder” part on the crystalline surface or surface-adsorbed water may be counted as an NC composition.

3.2 Photocatalytic activity of amorphous titania [33] Amorphous titania was commercialized by Wako Pure Chemical Industries, Ltd. (now Fujifilm Wako Pure Chemical Corp.). Fig. 7 shows the anatase content dependence of three types of photocatalytic activities using the amorphous titania. System A shows the amount of silver precipitation and acetone produced from an aqueous solution containing silver sulfate and 2-propanol, and systems B and C show the amounts of hydrogen and acetone produced from 2-propanol by using ex situ deposited and in situ photodeposited (from chloroplatinic acid) Pt-deposited titania, respectively. Except for the sample without amorphous titania (f(anatase) ¼ 1), the activity gradually decreased as the anatase content decreased. When extrapolated to zero anatase content (f(anatase) ¼ 0) by linear approximation, the activity is considered to be almost zero.

FIG. 7 Rate of photocatalytic reactions by anatase-amorphous mixture in their deaerated aqueous suspensions: (upper) system A with bare TiO2 powder, silver sulfate, and 2-propanol under acidified conditions; (middle) system B with ex situ platinized TiO2 powder and 2-propanol under acidified conditions, and (lower) system C with in situ platinized TiO2 and 2-propanol. Closed circles, open circles, and squares refer to H2, acetone, and Ag metal, respectively. (Reproduced from Fig. 8 in B. Ohtani, Y. Ogawa, S.-i. Nishimoto, Photocatalytic activity of amorphous-anatase mixture of titanium(IV) oxide particles suspended in aqueous solutions, J. Phys. Chem. B 101 (1997) 3746–3752, with permission from the American Chemical Society. Credit: ACS Publications.)

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It can be said, at least, that the amorphous titania in the anatase-amorphous mixture prepared by controlled calcination of the commercially available amorphous titania is inactive. The fact that amorphous titania is inactive in both reaction systems of silver ion reduction and hydrogen generation suggests that the cause is not electron and/or positive-hole transfer but rapid recombination of electrons and holes in amorphous titania.

4

Brookite titania and TiO2(B) [36]

There are few reports on the photocatalytic activity of brookite. This may be due to the lack of information on synthesis conditions under which only brookite can be obtained. To our knowledge, our report was the first report on the photocatalytic activity of brookite titania [37]. It was synthesized by a method reported previously [12]; however, calcination to improve the crystallinity led to transformation to rutile, and only brookite particles with sizes of 50–100 nm could be obtained. It should be noted that even if a pure brookite sample is obtained, it does not mean we can know “the photocatalytic activity of brookite” since the activity cannot be controlled only by the bulk crystal structure, as mentioned earlier. On the other hand, brookite prepared by the solvothermal method with ethylene glycol containing sodium acetate from an acetylacetone titanium complex [38] was thermally stable (there is a possibility that acetate ions contribute to the stabilization of the brookite crystalline structure) and rutile appeared only when calcination was performed at a temperature above 800°C [36]. Fig. 8 shows the dependence of photocatalytic activity on the calcination temperature. Up to 700°C (973 K), only brookite appears, but with calcination at a higher temperature, transformation to rutile occurs. The dependence of photocatalytic activity on calcination temperature was similar to that for anatase and rutile; however, there has been no comparison of the photocatalytic activity of brookite and that of anatase or rutile. Among the other polymorphs of titania, the photocatalytic activity of TiO2(B) has been reported [39]. TiO2(B) was prepared on the basis of a previous report [1]; however, the XRD pattern thought to be TiO2(B) was different from that reported and was similar to that of the intermediate material (potassium titanate). In order to confirm the composition of TiO2, the sample was dissolved in hot concentrated sulfuric acid, and the amounts of titanium and potassium were quantified by atomic absorption spectrometry. As a result, the titanium content was 60.2%, which was within 0.3% (same as the accuracy required for identification of organic compounds) compared with the calculated content of TiO2, 59.9%, and there was negligible occurrence of potassium. The photocatalytic activity was an order of magnitude lower than that of P25, while the ratios of hydrogen generation rates when methanol and 2-propanol were used as electron donors were 0.92–1.09 for P25 and slightly larger, 1.15–1.29, for TiO2(B), in which the crystals have a tunnel structure with diameters of about 0.4–0.7 nm. From this result, TiO2(B) is expected to show selectivity for substrate size in photocatalytic reactions owing to its unique crystalline structure.

FIG. 8 Effect of calcination temperature (Tc) on photocatalytic reaction rates for (A) CO2 evolution, (B) H2 evolution, and (C) O2 evolution from acetic acid, 2-propanol, and silver sulfate in aqueous suspensions of brookite TiO2, respectively. Details about the reaction conditions were described in the experimental section. (Reproduced from Fig. 4 in H. Kominami, Y. Ishii, M. Kohno, S. Konish, Y. Kera, B. Ohtani, Nanocrystalline brookite-type titanium(IV) oxide photocatalysts prepared by a solvothermal method: correlation between their physical properties and photocatalytic activities, Catal. Lett. 91(1–2) (2003) 41–47, with permission from Springer. Credit: Springer.)

Titanium(IV) oxide Chapter

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References [1] R. Marchand, L. Brohan, M. Tournoux, TiO2(B) a new form of titanium dioxide and the potassium octatitanate K2Ti8O17, Mater. Res. Bull. 15 (8) (1980) 1129–1133. [2] J.F. Banfield, D.R. Veblen, D.J. Smith, The identification of naturally occurring TiO2(B) by structure determination using high-resolution electron microscopy, image simulation, and distance-least-squares refinement, Am. Mineral. 76 (1991) 343–353. [3] P.Y. Simons, F. Dachille, The structure of TiO2 II, a high-pressure phase of TiO2, Acta Cryst. 23 (1967) 334–336. [4] H. Sato, S. Endo, M. Sugiyama, T. Kikegawa, O. Shimomura, K. Kusaba, Baddeleyite-type high-pressure phase of TiO2, Science 251 (4995) (1991) 786–788. [5] A. Nitta, M. Takase, M. Takashima, N. Murakami, B. Ohtani, A fingerprint of metal-oxide powders: energy-resolved distribution of electron traps, Chem. Commun. 52 (2016) 12096–12099. [6] A. Nitta, M. Takashima, M. Takase, B. Ohtani, Identification and characterization of titania photocatalyst powders using their energy-resolved distribution of electron traps as a fingerprint, Catal. Today 321–322 (2019) 2–8. [7] S.R. Yoganarasimhan, C.N.R. Rao, Mechanism of crystal structure transformations. Part 3—Factors affecting the anatase-rutile transformation, Trans. Faraday Soc. 58 (1962) 1579–1589. [8] J. Criado, C. Real, Mechanism of the inhibiting effect of phosphate on the anatase -> rutile transformation induced by thermal and mechanical treatment of TiO2, J. Chem. Soc. Faraday Trans. 1 79 (1983) 2765–2771. [9] S.-I. Nishimoto, B. Ohtani, A. Sakamoto, T. Kagiya, Photocatalytic activities of titanium(IV) oxide prepared from titanium(IV) sulfate, Nippon Kagaku Kaishi 1984 (2) (1984) 246–252. [10] The Essential Chemical Industry, Online, 2020. https://www.essentialchemicalindustry.org/chemicals/titanium-dioxide.html. (Accessed 15 October 2020). [11] H. Harada, T. Ueda, Photocatalytic activity of ultra-fine rutile in methanol-water solution and dependence of activity on particle size, Chem. Phys. Lett. 106 (3) (1984) 229–231. [12] M. Kiyama, T. Akita, Y. Tsutsumi, T. Takada, Formation of titanic oxides of anatase, brookite and rutile types by aerial oxidation of titanous solutions, Chem. Lett. 1 (1) (1972) 21–24. [13] T. Torimoto, N. Nakamura, S. Ikeda, B. Ohtani, Discrimination of active crystalline phases in anatase-rutile mixed titanium(IV) oxide photocatalysts through action spectrum analyses, Phys. Chem. Chem. Phys. 4 (2002) 5910–5914. [14] D.E. Scaife, Oxide semiconductors in photoelectrochemical conversion of solar energy, Sol. Energy 25 (1) (1980) 41–54. [15] O.-O. Prieto-Mahaney, N. Murakami, R. Abe, B. Ohtani, Correlation between photocatalytic activities and structural and physical properties of titanium(IV) oxide powders, Chem. Lett. 38 (2009) 238–239. [16] B. Ohtani, O.-O. Prieto-Mahaney, F. Amano, N. Murakami, R. Abe, What are titania photocatalysts?—An exploratory correlation of photocatalytic activity with structural and physical properties, J. Adv. Oxid. Technol. 13 (2010) 247–261. [17] S. Takeuchi, M. Takashima, M. Takase, B. Ohtani, Digitally controlled kinetics of titania-photocatalyzed oxygen evolution, Chem. Lett. 47 (2018) 373–376. [18] L. Kavan, M. Gr€atzel, S.E. Gilbert, C. Klemenz, H.J. Scheel, Electrochemical and photoelectrochemical investigation of single-crystal anatase, J. Am. Chem. Soc. 118 (1996) 6716–6723. [19] T. Daimon, T. Hirakawa, M. Kitazawa, J. Suetake, Y. Nosaka, Formation of singlet molecular oxygen associated with the formation of superoxide radicals in aqueous suspensions of TiO2 photocatalysts, Appl. Catal. A. Gen. 340 (2008) 169–175. [20] B. Ohtani, Y. Nohara, R. Abe, Role of molecular oxygen in photocatalytic oxidative decomposition of acetic acid by metal oxide particulate suspensions and thin film electrodes, Electrochemistry 76 (2008) 147–149. [21] B. Ohtani, O.-O. Prieto-Mahaney, D. Li, R. Abe, What is Degussa (Evonik) P25? Crystal composition analysis, reconstruction from isolated pure particles, and photocatalytic activity test, J. Photochem. Photobiol. A Chem. 216 (2010) 179–182. [22] T. Ohno, T. Nakai, Purification Method of Titanium Oxide, Japanese Patent; P 4628010, Daicel Corp, 2010. [23] B. Ohtani, Y. Azuma, D. Li, T. Ihara, R. Abe, Isolation of anatase crystallites from anatase-rutile mixed particles by dissolution with aqueous hydrogen peroxide and ammonia, Trans. Mater. Res. Soc. Jpn 32 (2) (2007) 401–404. [24] T. Ohno, K. Sarukawa, M. Matsumura, Photocatalytic activities of pure rutile particles isolated from TiO2 powder by dissolving the anatase component in HF solution, J. Phys. Chem. B 105 (12) (2001) 2417–2420. [25] R.I. Bickley, T. Gonzalez-Carreno, J.S. Lees, L. Palmisano, R.J.D. Tilley, A structural investigation of titanium dioxide photocatalysts, J. Solid State Chem. 92 (1) (1991) 178–190. [26] T. Ohno, K. Sarukawa, K. Tokieda, M. Matsumura, Morphology of a TiO2 photocatalyst (Degussa, P-25) consisting of anatase and rutile crystalline phases, J. Catal. 203 (1) (2001) 82–86. [27] R.A. Spurr, H. Myers, Quantitative analysis of anatase-rutile mixtures with an X-ray diffractometer, Anal. Chem. 29 (1957) 760. [28] B. Ohtani, Reminiscence in photocatalysis (5) anatase and rutile, Photocatalysis 65 (2020) 40–75. [29] S.-i. Nishimoto, B. Ohtani, H. Kajiwara, T. Kagiya, Correlation of the crystal structure of titanium dioxide prepared from titanium tetra-2-propoxide with the photocatalytic activity for redox reactions in aqueous propan-2-ol and silver salt solutions, J. Chem. Soc. Faraday Trans. 81 (1985) 61–68. [30] A.K. Datye, G. Riegel, J.R. Bolton, M. Huang, M.R. Prairie, Microstructural characterization of a fumed titanium dioxide photocatalyst, J. Solid State Chem. 115 (1995) 236–239. [31] T. Miyagi, M. Kamei, T. Mitsuhashi, T. Ishigaki, A. Yamazaki, Charge separation at the rutile/anatase interface: a dominant factor of photocatalytic activity, Chem. Phys. Lett. 390 (2004) 399–402.

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C Oxides and calcogenides

[32] Y. Mi, Y. Weng, Band alignment and controllable electron migration between rutile and anatase TiO2, Sci. Rep. 5 (2015) 11482. [33] B. Ohtani, Y. Ogawa, S.-i. Nishimoto, Photocatalytic activity of amorphous-anatase mixture of titanium(IV) oxide particles suspended in aqueous solutions, J. Phys. Chem. B 101 (1997) 3746–3752. [34] Y. Shen, A. Nitta, M. Takashima, B. Ohtani, Do particles interact electronically?—Proof of interparticle charge-transfer excitation between adjoined anatase and rutile particles, Chem. Lett. 50 (2021) 80–83. [35] K. Wang, Z. Wei, B. Ohtani, E. Kowalska, Interparticle electron transfer in methanol dehydrogenation on platinum-loaded titania particles prepared from P25, Catal. Today 303 (2018) 327–333. [36] H. Kominami, Y. Ishii, M. Kohno, S. Konish, Y. Kera, B. Ohtani, Nanocrystalline brookite-type titanium(IV) oxide photocatalysts prepared by a solvothermal method: correlation between their physical properties and photocatalytic activities, Catal. Lett. 91 (1–2) (2003) 41–47. [37] B. Ohtani, J.-i. Handa, S.-i. Nishimoto, T. Kagiya, Highly-active semiconductor photocatalyst: extra-fine crystallite of brookite TiO2 for redox reaction in aqueous propan-2-ol and/or silver sulfate solution, Chem. Phys. Lett. 120 (1985) 292–294. [38] H. Kominami, M. Kohno, Y. Kera, Synthesis of brookite-type titanium oxide nano-crystals in organic media, J. Mater. Chem. 10 (2000) 1151–1156. [39] B. Ohtani, K. Tennou, S.-i. Nishimoto, T. Inui, Photocatalytic activity of artificial titanium(IV) oxide—TiO2(B)—and Titanates suspended in aqueous solution of aliphatic alcohols, J. Photosci. 2 (1995) 7–11.

Chapter 11

Development of hight active visible light-responsive TiO2 photocatalysts by applying ion engineering techniques Masato Takeuchi and Masakazu Anpo Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka, Japan

1 Introduction Rapid economic growth in the 20th century has been achieved by consuming the earth’s limited energy resources. However, we are now faced with serious environmental problems, for example, the greenhouse effect caused by uncontrolled CO2 emissions, acid rain caused by air pollutants as well as polluted waterways. We will also be confronted with serious energy issues caused by the exhaustion of fossil fuels and their dramatic increase in price in the near future. Taking these multiple and intertwined issues into consideration, we need to urgently develop environmentally benign chemical processes to solve these issues for all living creatures. Photocatalysis has attracted much attention as “an environmentally harmonious catalyst” because photocatalysts possess the potential to reduce CO2, to decompose NOx, and to degrade various toxic organic compounds under light irradiation. In fact, the photocatalytic system is often recognized to be “an artificial photosynthesis” [1–9]. In recent years, transparent TiO2 thin films prepared on various substrates have been shown to be promising photo-functional materials for various applications. Although many products applying TiO2 photocatalysts have been commercialized, TiO2 as a wide-gap semiconductor (ca. 3.2 eV) necessitates the irradiation of UV light shorter than 388 nm in the wavelength in order to activate as a photocatalyst. To overcome the weakness of TiO2 semiconductor photocatalysts, many different approaches to develop TiO2 photocatalysts workable under sunlight irradiation have been intensively carried out but some breakthroughs have actually been reported before the 1990s. In 1991, Gr€atzel et al. reported an efficient energy conversion of sunlight into electricity by using TiO2 electrode and photo functional Ru-dye to absorb visible light [10]. This finding opened a new research field of dye-sensitized solar cells. However, there are still several issues that need to be resolved, such as the high enough stability of organic dyes and sealing techniques to prevent leakage of the liquid electrolyte. The development of visible light-responsive photocatalysts will, thus, give a clear solution for the purification of polluted environments as well as a critical breakthrough to convert solar energy into useful energy. In other words, the efficient utilization of abundant solar energy will be one of the biggest challenging goals for all chemists. In this chapter, we will deal with an innovative application of ion engineering techniques, such as metal ion implantation, ionized cluster beam (ICB) deposition, radio-frequency magnetron sputtering (RF-MS) deposition methods, to prepare well-defined TiO2 photocatalysts, especially the visible light-responsive TiO2 photocatalysts.

2 Ion engineering techniques Ion engineering techniques have been applied for surface modification of functional materials. Especially, ion implantation is an important technology to modify the electronic properties of silicon semiconductors. The schematic interactions between accelerated ions having different energies and solid surfaces are illustrated in Fig. 1. (A) When the ion beams having lower energies (less than hundreds eV) are irradiated onto solid surfaces, these ions are accumulated on the surfaces to form thin films like snow covering on the ground. (B) When the ion beams having middle energies (less than tens keV) are bombarded on solid surfaces, these ions sputter the atoms of solid surfaces as secondary ions. (C) When the ion beams are accelerated by higher energies of several tens keV, these ions are implanted within the deep bulk of solid surfaces without any significant damages (ion implantation). Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00033-7 Copyright © 2021 Elsevier Inc. All rights reserved.

171

172

SECTION C

Oxides and calcogenides

(A) Ions with low energy Formation of thin film + + +

+ +

(B) Ions with middle energy Sputtering + +

+ + +

(C) Ions with high energy Ion implantation +

+ +

+

+ +

+ +

FIG. 1 Schematic diagrams of the interaction between the accelerated ions having different energies and the solid surfaces.

3

Visible light responsive TiO2 photocatalysts

3.1 Modification of the electronic properties of TiO2 semiconductor photocatalysts by a metal ion implantation method [7–9, 11–22] TiO2 as a wide-bandgap semiconductor mainly absorbs UV light shorter than 388 nm in wavelength. From this viewpoint, TiO2 photocatalysts operating under not only UV but also visible light irradiations would be ideal for widespread commercialization. Since the 1970s, various approaches for the developments of visible light-responsive photocatalysts have been carried out by impregnation of metal oxide or metal ions onto TiO2 surfaces [23–27]. However, any remarkable results have not been obtained for several decades. These results have been explained by the reason that aggregated metal oxide or metal ions on the photocatalyst surfaces might work as a recombination center of the photo-formed electrons and holes. From this background, we directed our attention to the ion implantation method to modify the electronic property of TiO2 semiconductors. We have carried out implantation of various transition metal ions, such as V, Cr, and Fe., into the bulk of TiO2 and have successfully discovered smooth shifts of the absorption band of TiO2 semiconductor toward longer wavelength regions as shown in Fig. 2A [11–22]. The characteristic redshifts in the absorption spectra were also observed by implanting other transition metals, such as Mn, Ni, Co, and Cu. However, the implantation of Mg, Ti, and Ar ions was not effective to modify the electronic property of TiO2 semiconductors. These experimental results suggested that the smooth shift of the absorption band is attributed to a chemical interaction between TiO2 semiconductor and implanted transition metal ions. Furthermore, the smooth shift of the absorption band was observed only after calcinating the metal ion-implanted TiO2 samples under air conditions, indicating that the oxidation states of the implanted metal ions play an important role in the modification of TiO2 semiconductor. Fig. 2B shows the UV-vis absorption spectra of TiO2 doped with small amounts of Cr oxide by a simple impregnation method. Typical absorption band due to aggregated Cr oxide species (CrOx) was observed at around 420 nm but the absorption band due to TiO2 semiconductor at 390 nm did not change at all. These results suggest that the chemical doping of CrOx species modify the electronic property of TiO2 semiconductor in a

Visible light-responsive TiO2 photocatalysts Chapter

Solar spectrum

(d)

(a) 250

350

(c) (b) 450

(e’)

(d’) (c’)

173

FIG. 2 Diffuse reflectance UV-vis absorption spectra of (A) Cr ion-implanted TiO2 and (B) Cr ion-doped TiO2 as well as solar spectrum. Amounts of Cr ions implanted (mmol/g): (a) 0, (b) 0.22, (c) 0.66, (d) 1.3. Amounts of Cr ions doped (wt%): (a) 0, (b0 ) 0.01, (c0 ) 0.1, (d0 ) 0.5, (e0 ) 1 (0.1 wt% equals ca. 4.9 mmol/g-TiO2).

(B) K. M. function / a. u.

K. M. function / a. u.

(A)

11

(b’)

(a) 550

650

250

350

Wavelength / nm

450

550

650

Wavelength / nm

Yields of N2 formation / µmol•g-TiO2-1

different way from the metal implantation method. We have, thus, concluded that only metal ion-implantation could modify the TiO2 semiconductor to efficiently absorb visible light. In fact, we have confirmed that the metal ion-implanted TiO2 catalyzed various important reactions, such as decomposition of NOx, isomerization of cis-2-butene, and degradation of organic, under visible light irradiation. Fig. 3 shows the reaction time profiles for the photocatalytic decomposition of NO under visible light irradiation (l > 450 nm). The Cr ionimplanted TiO2 clearly showed an efficient photocatalytic reactivity for the decomposition of NO under visible light irradiation. In contrast, the Cr ion-doped TiO2 and the original TiO2 photocatalysts did not show any photocatalytic reactivity under the same irradiation condition. It should be emphasized that the photocatalytic reactivity of the metal ion-implanted TiO2 under UV light irradiation was almost similar to the unimplanted TiO2 catalyst. On the other hand, the metal iondoped TiO2 catalysts were confirmed to show much lower photocatalytic reactivity under UV light irradiation. These results suggested that the physically implanted metal ions may not work as a recombination center of electron-hole pairs. Furthermore, we have conducted some fieldwork experiments to investigate the visible light-responsive TiO2 photocatalysts under sunlight irradiation [16–19]. As shown in Fig. 4, the Cr and V ion-implanted TiO2 showed two to three times higher photocatalytic reactivity for decomposition of NO under sunlight irradiation as compared to the original TiO2. Thus, the advanced metal ion-implantation method has been successfully applied to modify the electronic property of the TiO2 semiconductor, operating as a photocatalyst under sunlight irradiation. These findings opened a new way to develop such visible light-responsive photocatalysts by applying the ion engineering technique.

1.5

off

on

off

on N2O

1.0 N2

Cr ionimplanted TiO2

0.5

original TiO2 0 -2

0

2

4

6

8

10

Time / h FIG. 3 Reaction time profiles for the photocatalytic decomposition of NO over the Cr ion-implanted TiO2 and the original TiO2 under visible light irradiation (l > 450 nm).

174

SECTION C

Oxides and calcogenides

x10-9

Yield of NO elimination / mol min-1

8

6

4

2

0 TiO2

Cr/TiO2

V/TiO2

Catalysts Solar beam intensity : 38.5 mW/cm2 Amount of catalyst : 3.6 g Flow rate : 18 L/min FIG. 4 Photocatalytic reactivity of the V ion, Cr ion-implanted TiO2 and the original TiO2 photocatalysts for the decomposition of NO under solar light irradiation.

3.2 Preparation of transparent TiO2 thin film photocatalysts and the Cr-implanted TiO2 thin films operating under visible light irradiation [20–22, 28–31] Photocatalytic systems are expected to be useful for the purification of polluted water and air. However, if powdered photocatalysts are used in aqueous solution systems, it is expensive to separate such fine powders from the suspension systems by a filtration after the photocatalytic reactions. This is the reason behind the immobilization of photocatalysts on various substrates that are necessary for their widespread applications with lower costs [28–30, 32–34]. In order to address the problem, various methods to immobilize the photocatalysts onto substrates, such as a sol-gel [35–37], a metal organic chemical vapor deposition (MOCVD) [38–41], and a direct deposition-technique with aqueous solutions [42–45] have been widely investigated. TiO2 powder is used as a white color pigment (titanium pigment) because of its high refractive index. However, uniformly coated TiO2 thin films show high transparency in visible light regions. Moreover, TiO2 surface has a unique property to show high wettability under UV light irradiation [46, 47]. Based on these backgrounds, TiO2 thin film photocatalysts have been applied to eliminate offensive odors or to decompose volatile organic compounds (VOCs) into harmless CO2 and H2O under UV light irradiation. TiO2 thin films are generally prepared by wet media, such as sol-gel, dip-coating, spincoating, and splay coating. However, the wet coating methods necessitate a post calcination process at relatively high temperatures after coating sol solutions. Thus, it is not easy to obtain transparent TiO2 thin films showing high photocatalytic reactivity on thermally unstable substrates, such as papers, fabrics, and plastics by the wet processes. In order to overcome the problem, the inorganic binders, which solidify at low temperatures, are used to immobilize TiO2 fine particles. However, the TiO2 particles buried in the binders do not show sufficient photocatalytic reactivity. On the other hand, we have applied the ion engineering techniques as a dry process to prepare the transparent TiO2 thin films on various substrates. Since the thin films are prepared in a high vacuum chamber, (i) contamination with some impurities can be avoided, (ii) organic solvents are not necessary as “environmentally-friendly processes,” (iii) thin films with high crystallinity can be prepared at relatively low temperatures. As a first step, we have applied an ionized cluster beam (ICB) deposition method to prepare transparent TiO2 thin films [20–22, 28–30]. Schematic diagram of the ICB deposition method was illustrated in Fig. 5. Titanium vapor obtained by heating Ti metal grains at ca. 2000°C was introduced into a high vacuum chamber, producing titanium clusters. The titanium clusters reacted with O2 (pressure: 2.7  105 kPa range) in the vacuum chamber and formed stoichiometric TiO2 clusters. The TiO2 clusters were then ionized by electron beam irradiation, accelerated by a

Visible light-responsive TiO2 photocatalysts Chapter

High vacuum chamber (ca. 10-8 kPa)

11

175

FIG. 5 Schematic diagrams of ionized cluster beam (ICB) deposition method.

Substrates (glass metal, etc.) TiO2 thin film

+

O2 atmosphere (2.7x10-5 kPa)

Electric field (0.5 – 1.0 kV)

+ +

e-

ee-

Ti metal

e-

Electron beam

crucible

Transmittance / a.u.

high voltage of 0.5 kV. Finally, the ionized clusters were finally bombarded onto the substrates, resulting in the formation of TiO2 thin films. The optical properties of the TiO2 thin films prepared by the ICB method were investigated by UV-vis absorption spectra, as shown in Fig. 6. Characteristic interference fringes due to transparent thin films were clearly observed in visible light regions. Furthermore, absorption edges of the TiO2 thin films shifted toward shorter wavelength regions as decreasing the film thicknesses. This phenomenon can be explained by quantum size effect of nano-sized TiO2 particles which compose of transparent thin films. In fact, the transparent TiO2 thin films showed even higher photocatalytic reactivity for decomposition of NO under UV light irradiation as compared with TiO2 thin films prepared by a sol-gel method. Furthermore, the correlations between the film thicknesses and the photocatalytic reactivity as well as BET surface areas (determined by N2 adsorption isotherms) and the wavelengths of absorption edges were summarized in Fig. 7. The TiO2 thin films having smaller film thickness absorbed UV light of shorter wavelength regions but showed higher photocatalytic reactivity. As the film thickness increased, the photocatalytic reactivity slightly decreased and leveled off. The behavior of photocatalytic reactivity was largely coincident with the BET surface areas and the wavelength of the absorption edges. That is, the photocatalytic properties of the TiO2 thin films are dependent on the film thicknesses. The TiO2 thin films prepared by the ICB method showed high enough photocatalytic reactivities under UV light irradiation. However, the thin films do not work as a photocatalyst under visible light irradiation. Thus, we applied the metal ion implantation method to modify the transparent TiO2 thin films. UV-vis transmittance spectra of the Cr ion-implanted

(a)

(b)

(c) 20 % (d)

200

400

600

800

Wavelength / nm FIG. 6 UV-vis absorption (transmittance) spectra of the TiO2 thin films prepared by the ICB deposition method. Film thickness: (a) 20, (b) 100, (c) 300, (d) 1000 nm.

Oxides and calcogenides

30

0.12

25 0.08 20 0.04

15

0

400

0

10 1200

800

280

300

320

340

Wavelength of absorption edge / nm

FIG. 7 Relationships between the photocatalytic reactivities for the decomposition of NO, the BET surface areas and the wavelengths of the absorption edges of the TiO2 thin films prepared by the ICB deposition method.

BET surface area (TiO2 side only) / m2

SECTION C

Yield of N2 formation / µmol

176

360

Film thickness / nm

TiO2 thin films were shown in Fig. 8. The absorption edges of the Cr ion-implanted TiO2 thin films slightly shifted toward visible light regions [20–22, 31]. In fact, the Cr ion-implanted TiO2 thin films looked like a transparent yellow color filter. Thus, we have successfully modified the electronic property of the transparent TiO2 thin films by the metal ion implantation method, as is the case with TiO2 powder system. Fig. 9 shows the reaction time profiles for decomposition of NO under visible light irradiation (l > 450 nm). The Crimplanted TiO2 thin film efficiently decomposed NO under visible light irradiation but the original TiO2 thin film did not show any photocatalytic reactivity. The reaction proceeded linearly against the light irradiation time. This means the Crimplanted TiO2 thin films adsorbed visible light and obviously operated as a photocatalyst. To elucidate the role of Cr ions implantation, the local structure of Cr ion implanted within TiO2 lattice was investigated by XAFS measurements [15–22]. As a result, we have found that octahedral CrO6 species are specifically incorporated within the TiO6 lattice positions at an atomic level. Furthermore, we have investigated the local structure of V ion implanted within the TiO2 lattice by electron spin resonance (ESR) measurements. As a result, unique reticular V4+ ions were observed for the V-implanted TiO2 catalyst

Transmittance / a.u.

FIG. 8 UV-vis absorption (transmittance) spectra of (a) TiO2 and (b, c) Cr ion-implanted TiO2 thin films. Amounts of Cr ions implanted (mmol/g): (a) 0, (b) 0.22, (c) 0.66.

200

(a)

(b)

(c)

300

400

500

Wavelength / nm

600

650

Visible light-responsive TiO2 photocatalysts Chapter

11

177

0.08

Yield of N2 formation / mmol

off

on

off

on

Cr-implanted TiO2 thin film 0.04

original TiO2 thin film 0

-2

0

2

4

6

8

10

Time / h FIG. 9 Reaction time profiles for the photocatalytic decomposition of NO over the Cr ion-implanted TiO2 thin film and the original TiO2 thin film under visible light irradiation (l > 450 nm).

after calcined in the O2 atmosphere at 450–550°C. However, such characteristic V4+ species having the same local structure were not observed for the TiO2 catalysts chemically doped with V ions. These findings suggested such unique local structures of the CrO6 species or the reticular V4+ species, which are achieved only by the metal ion implantation, are closely related to the design of TiO2 photocatalysts operating under visible light irradiation.

3.3 One-step preparation of the visible light responsive TiO2 thin film photocatalysts by a RF-magnetron sputtering deposition method [20–22, 48–57] We have succeeded to prepare the visible light-responsive TiO2 photocatalysts by applying the metal ion implantation method. Moreover, we have obtained valuable information that the low-valent transition metal ions incorporated within TiO2 lattice play an important role to modify the electronic property of TiO2 semiconductors and to realize the absorption in visible light regions [15–22]. However, less cumbersome preparation methods at lower cost have strongly been desired for widespread applications. Among various physical vapor deposition (PVD) methods, we have secondly chosen a radiofrequency magnetron sputtering (RF-MS) deposition method to prepare the visible light-responsive TiO2 thin film photocatalysts. A schematic diagram of the RF-MS deposition method was shown in Fig. 10. Oxide thin films are generally prepared by using a metal target and sputtering gas containing O2 as a reactive gas. However, in this study, we have used a stoichiometric TiO2 plate as a sputtering target and Ar gas as a sputtering gas without coexisting O2. When a magnetic field is vertically applied to an electric field in the presence of sputtering gas (in this case, Ar), ring-state Ar plasma is induced on

Heater Substrates TiO2 thin film

Gas plasma (Ar+) Source material (TiO2 plate) N

S

N

N

S

Magnet S

FIG. 10 Schematic diagram of radio-frequency magnetron sputtering (RF-MS) deposition method.

178

SECTION C

Oxides and calcogenides

FIG. 11 UV-vis absorption (transmittance) spectra of the TiO2 thin films prepared by the RF MS deposition method. Preparation temperatures (°C): (a) 100, (b) 200, (c) 400, (d) 600, (e) 700.

Transmittance / a. u.

the target material. Ar+ ions in the gas plasma sputter the TiO2 target surface to produce Ti4+ and O2 ions. These ions are uniformly accumulated onto the substrate surfaces to form TiO2 thin films. The photocatalytic properties of the TiO2 thin films are strongly affected by the preparation conditions such as the induced RF powers, substrate temperatures, distances between the target and substrates (DTS), and sputtering gas flow rates. Thus, we have fixed the induced RF power of 300 W and the DTS of 80 mm. And the substrate temperatures were changed from 100°C to 700°C. Since the thin films were deposited in a high vacuum chamber, contamination with various impurities could be avoided. Moreover, post calcination treatments were not carried out because TiO2 thin films having anatase structure were obtained even at 100°C. Film thicknesses were controlled to be 1 mm by changing the deposition time. The optical property is one of the most important factors determining photocatalytic performance. Fig. 11 shows the UV-vis transmittance spectra of the TiO2 thin films prepared at 100–700°C. The TiO2 thin films prepared at low temperatures (T < 200°C) showed a typical bandgap absorption at around 360 nm. These spectra were almost similar to those of the TiO2 thin films prepared by a sol-gel [35–37] or ICB method [28–30]. This result indicated that stoichiometric and transparent TiO2 thin films were obtained by the RF-MS method. On the other hand, as the preparation temperatures increased up to 600°C, the TiO2 thin films showed large absorption in visible light regions. The origin of the absorption in visible light regions could not be explained by contamination of impurities in the thin films because the impurity level of the target material was quite low (less than 0.1%). Moreover, the TiO2 thin films prepared at 600°C in the presence of O2 as a reactive gas did not show any significant absorption in visible light regions (data not shown). These results indicated that visible light-responsive TiO2 thin films could be successfully prepared only when the TiO2 target was sputtered with Ar (without coexistence of O2) at relatively high temperatures (T > 500°C). Photocatalytic reactivity of the TiO2 thin films prepared by the RF-MS method was evaluated by a NO decomposition reaction under UV light irradiation (l > 270 nm). Fig. 12A shows the reaction time profiles for the photocatalytic decomposition of NO over the TiO2 thin film prepared at 200°C and TiO2 powder (P25) under UV light irradiation. As in the case

(a) (b) (e)

(d) (c)

(Upper) Quartz, 373, 473, 573 K. (Lower) 673, 773, 873, 973 K. Film thickness: ~ 1.2 mm

20 %

200

400

600

800

Wavelength / nm

(A)

(B)

3

off

on

(a)

(b)

2 Light on

1

0

Yield of N2 formation / mmol m-2

3.0

Yield of N2 formation /mmol m-2

FIG. 12 (A) Reaction time profiles for the decomposition of NO under UV light irradiation (l > 270 nm) over the TiO2 thin film prepared at 200°C and (B) correlation between the photocatalytic reactivity (l > 270 nm) and the preparation temperatures.

-2

0

2

4

Time / h

6

8

10

l > 270 nm

2.0

1.0

0

100

300

500

700

Preparation temperatures / °C

Visible light-responsive TiO2 photocatalysts Chapter

11

179

of TiO2 thin films prepared by the ICB method, decomposition of NO linearly proceeded under UV light irradiation. Although the surface area of the thin film was essentially smaller as compared to the powder sample, the photocatalytic reactivity per surface area of the TiO2 films was almost comparable with the commercial TiO2 powder. The effect of the preparation temperatures on the photocatalytic reactivity under UV light irradiation was summarized in Fig. 12B. The TiO2 thin film prepared at 200°C, which consisted of an anatase phase, showed relatively higher photocatalytic reactivity among these samples. This phenomenon can be explained by an efficient irradiation effect of incident UV light because of high transparency of the TiO2 thin film prepared at 200°C. However, the TiO2 thin films prepared at higher than 400°C, which mainly consisted of a rutile phase, showed lower photocatalytic reactivity. It was notable that the TiO2 thin films having high photocatalytic reactivity could be prepared at low temperatures of ca. 200°C by applying the RF-MS deposition method. In fact, we have succeeded to prepare crystalline TiO2 thin films on thermally unstable polycarbonate substrates at 80°C [51]. TiO2 thin films prepared on the polycarbonate substrates showed a typical bandgap absorption at around 370 nm and interference fringes in visible light regions. Moreover, the TiO2 thin films hardly depressed the high transparency of the polycarbonate substrates. The crystalline TiO2 thin films on polycarbonates showed a high wettability under UV light irradiation (ca. 1 mW/cm2 at 360  20 nm). However, the high wettability slowly disappeared under dark conditions. When the UV light intensity was lowered to 0.05 mW/cm2, prolonged irradiation for at least 1 day was needed but the TiO2 films repeatedly showed high wettability. From these results, the RF-MS deposition method proved to be one of the most powerful techniques to prepare highly reactive TiO2 thin film photocatalysts even at low temperatures. Furthermore, we have focused on the photocatalytic reactivity under visible light irradiation of the yellow-colored TiO2 thin films prepared at higher than 500°C. Fig. 13A shows the reaction time profiles for the photocatalytic decomposition of NO over the TiO2 thin film prepared at 600°C and a commercial TiO2 powder (P25) under visible light irradiation. Although the TiO2 powder did not show any photocatalytic reactivity, the TiO2 thin film prepared at 600°C exhibited photocatalytic reactivity for the decomposition of NO. The photocatalytic reactivity under visible light irradiation as well as the relative intensities of absorption spectra at 450 nm was summarized in Fig. 13B. The transparent TiO2 thin films prepared at lower than 200°C hardly showed photocatalytic reactivity under visible light irradiation. While the yellowcolored TiO2 thin films prepared at higher than 500°C showed photocatalytic reactivity for NO decomposition under visible light irradiation. Moreover, the photocatalytic reactivity of the TiO2 thin films corresponded well with the relative intensities at 450 nm in the absorption spectra. These results clearly indicate that the TiO2 thin films prepared by the RF-MS method work as a visible light-responsive photocatalyst. Furthermore, the TiO2 thin films also showed efficient photocatalytic reactivity for degradation of various organic compounds (VOCs) as well as water splitting reaction into H2 and O2 under sunlight irradiation. The details should be referred to in our previous studies [52–57]. To clarify the mechanism of visible light absorption of the TiO2 thin films prepared by the RF-MS method, SEM observations were carried out. Fig. 14 shows the cross-sectional SEM images of the UV-type and visible-type TiO2 thin films. The UV-type TiO2 thin film prepared at 200°C consisted of TiO2 particles randomly sintered with each other. On the other hand, the visible-type TiO2 thin film prepared at 600°C was found to consist of orderly aligned columnar TiO2 crystals

(A)

(B) off

on

(a)

0.5 Light on

(b) 0

-2

0

2

4

Time / h

6

8

10

l > 450 nm

1.0

0.4

0.5

Relative Intensity at 450 nm of UV-VIS absorption spectra

0.8

Yield of N2 formation / mmol m-2

Yield of N2 formation / mmol m-2

1.0

0 0

100

300

500

700

Preparation temperatures / °C

FIG. 13 (A) Reaction time profiles for the decomposition of NO under visible light irradiation (l > 450 nm) over the TiO2 thin film prepared at 600°C and (B) correlation between the photocatalytic reactivity (l > 450 nm), the relative intensities at 450 nm in the UV-vis absorption spectra and the preparation temperatures.

180

SECTION C

Oxides and calcogenides

(A)

(B)

TiO2

Quartz

1 µm

1 µm

FIG. 14 Cross-sectional SEM images of (A) UV-type TiO2 thin film prepared at 200°C and (B) visible-type TiO2 thin film prepared at 600°C.

Values of O/(Ti + O)

(a) 0.666 0.65

2.000 1.933

(b)

0.6 0

50

100

150

Values of O/Ti

2.333

0.7

1.500 200

Depth / nm FIG. 15 Depth profiles of the Ti/O atomic ratios determined by AES measurements from the surface to bulk of the TiO2 thin films prepared at (a) 200°C and (b) 600°C.

(diameter; ca. 100 nm). Furthermore, the depth profile of O/Ti atomic ratios was investigated by AES measurements. As shown in Fig. 15, the O/Ti atomic ratio of the UV-type TiO2 thin films was almost constant of 2.0 from the surface to bulk, indicating that stoichiometric TiO2 film was formed on the glass substrate. On the other hand, the O/Ti atomic ratio of the visible-type TiO2 thin film, which showed yellow color, gradually decreased from the surface to bulk and reached a leveling-off at 1.933. Such a declined structure of the visible-type TiO2 thin films was confirmed to be stable even after heat treatment at 500°C. This result suggested the stoichiometric TiO2 layer at near surface of ca. 20–30 nm worked as a passive layer and protected the slightly reduced bulk part. Small amounts of oxygen vacancies in TiO2 lattice have been reported to distort the TiO6 octahedral unit and weaken the Ti–O bonds, resulting in the reduction of the splitting between bonding and nonbonding levels [58]. Taking these results into consideration, the unique declined structure of TiO2/TiOx thin films prepared by the RF-MS method was speculated to closely associate with the modification of electronic properties of TiO2 semiconductor to enable the visible light absorption.

4

Conclusions

In this chapter, we have reviewed the preparation of the well-defined TiO2 photocatalysts and visible light-responsive TiO2 photocatalysts by applying the advanced ion engineering techniques as well as various characterizations of the photocatalysts at a molecular level. From the viewpoint of the efficient utilization of solar light energy, various photocatalytic systems to purify the air and water polluted with harmful organic compounds have already been commercialized. However,

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further special attention has still been focused on the development of visible light-responsive photocatalysts by a simple process at lower costs. Although we have applied the sophisticated and expensive ion-engineering techniques, our pioneering findings proved that the developments of visible light-responsive photocatalysts was not impossible and might open a way for the second generation of photocatalysts. Moreover, we obtained various innovative insights for the preparation of TiO2 photocatalysts operating under visible light irradiation. These findings have been passed down to the present generation for further highly reactive photocatalysts as a promising candidate to efficiently utilize solar energy. Thus, these environmentally benign photocatalytic systems will provide new approaches for the “Sustainable Development Goals (SDG)” in our future.

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Oxides and calcogenides

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Fujishima, Preparation of transparent TiO2 thin film photocatalyst and its photocatalytic activity, Chem. Lett. 24 (1995) 841–842. [36] Y. Ohko, K. Hashimoto, A. Fujishima, Kinetics of photocatalytic reactions under extremely low-intensity UV illumination on titanium dioxide thin films, J. Phys. Chem. A 101 (1997) 8057–8062. [37] K. Negishi, T. Takeuchi, Ibusuki, Surface structure of the TiO2 thin film photocatalyst, J. Mater. Sci. 33 (1998) 5789–5794. [38] H.Y. Lee, Y.H. Park, K.H. Ko, Correlation between surface morphology and hydrophilic/hydrophobic conversion of MOCVD-TiO2 films, Langmuir 16 (18) (2000) 7289–7293. [39] D. Byun, Y. Jin, B. Kim, J.K. Lee, D. Park, Photocatalytic TiO2 deposition by chemical vapor deposition, J. Hazard. Mater. B73 (2000) 199–206. [40] P.-C. Chang, Z. Fan, D. Wang, W.-Y. Tseng, W.-A. Chiou, J. Hong, J.G. Lu, ZnO nanowires synthesized by vapor trapping CVD method, Chem. Mater. 16 (2004) 5133–5137. [41] C.-H. Lee, M.-S. Choi, Effects of the deposition condition on the microstructure and properties of ZnO thin films deposited by metal organic chemical vapor deposition with ultrasonic nebulization, Thin Solid Films 605 (2016) 157–162. [42] S. Deki, Y. Aoi, O. Hiroi, A. Kajinami, Titanium (IV) oxide thin films prepared from aqueous solution, Chem. Lett. 25 (1996) 433–434. [43] K. Shimizu, H. Imai, H. Hirashima, K. Tsukuma, Low-temperature synthesis of anatase thin films on glass and organic substrates by direct deposition from aqueous solutions, Thin Solid Films 351 (1999) 220–224. [44] N. Saito, H. Haneda, T. Sekiguchi, N. Ohashi, I. Sakaguchi, K. Koumoto, Low-temperature fabrication of light-emitting zinc oxide micropatterns using self-assembled monolayers, Adv. Mater. 14 (2002) 418–421. [45] L.E. Greene, M. Law, J. Goldberger, F. Kim, J.C. Johnson, Y. Zhang, R.J. Saykally, P. Yang, Low-temperature wafer-scale production of ZnO nanowire arrays, Angew. Chem. Int. Ed. 42 (2003) 3031–3034. [46] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Light-induced amphiphilic surfaces, Nature 388 (1997) 431–432. [47] R.-D. Sun, A. Nakajima, A. Fujishima, T. Watanabe, K. Hashimoto, Photoinduced surface wettability conversion of ZnO and TiO2 thin films, J. Phys. Chem. B 105 (2001) 1984–1990. [48] M. Takeuchi, M. Anpo, T. Hirao, N. Itoh, N. Iwamoto, “Preparation of TiO2 thin film photocatalysts working under visible light irradiation by applying a RF magnetron sputtering deposition method” (in Japanese), Surf. Sci. Jpn. 22 (2001) 561–565. [49] M. Kitano, M. Takeuchi, M. Matsuoka, J.M. Thomas, M. Anpo, Preparation of visible light-responsive TiO2 thin film photocatalysts by an RF magnetron sputtering deposition method and their photocatalytic reactivity, Chem. Lett. 34 (2005) 516–617. [50] M. Takeuchi, S. Sakai, M. Matsuoka, M. Anpo, Preparation of the visible light responsive TiO2 thin film photocatalysts by the RF magnetron sputtering deposition method, Res. Chem. Intermed. 35 (2009) 973–983. [51] M. Takeuchi, T. Yamasaki, K. Tsujimaru, M. Anpo, Preparation of crystalline TiO2 thin film photocatalysts on polycarbonate substrates by a RFmagnetron sputtering deposition method, Chem. Lett. 35 (2006) 904–905. [52] M. Matsuoka, M. Kitano, M. Takeuchi, M. Anpo, J.M. Thomas, Photocatalytic water splitting on visible light-responsive TiO2 thin films prepared by a RF magnetron sputtering deposition method, Top. Catal. 35 (2005) 305–310. [53] H. Kikuchi, M. Kitano, M. Takeuchi, M. Matsuoka, M. Anpo, P.V. Kamat, Extending the photoresponse of TiO2 to the visible light region: photoelectrochemical behavior of TiO2 thin films prepared by the radio frequency magnetron sputtering deposition method, J. Phys. Chem. B 110 (2006) 5537–5541. [54] M. Kitano, K. Iyatani, K. Tsujimaru, M. Matsuoka, M. Takeuchi, M. Ueshima, J.M. Thomas, M. Anpo, The effect of chemical etching by HF solution on the photocatalytic activity of visible light-responsive TiO2 thin films for solar water splitting, Top. Catal. 49 (2008) 24–31. [55] S. Fukumoto, M. Kitano, M. Takeuchi, M. Matsuoka, M. Anpo, Photocatalytic hydrogen production from aqueous solutions of alcohol as model compounds of biomass using visible light-responsive TiO2 thin films, Catal. Lett. 127 (2009) 39–43. [56] K. Iyatani, Y. Horiuchi, S. Fukumoto, M. Takeuchi, M. Anpo, M. Matsuoka, Separate-type Pt-free photofuel cell based on a visible light-responsive TiO2 photoanode: effect of hydrofluoric acid treatment of the photoanode, Appl. Catal. Gen. 458 (2013) 162–168. [57] S.A. Bilmes, P. Mandelbaum, F. Alvarez, N.M. Victoria, Surface and electronic structure of titanium dioxide photocatalysts, J. Phys. Chem. B 104 (2000) 9851–9858. [58] M. Matsuoka, A. Ebrahimi, M. Nakagawa, T.-H. Kim, M. Kitano, M. Takeuchi, M. Anpo, Separate evolution of H2 and O2 from H2O on visible lightresponsive TiO2 thin film photocatalysts prepared by an RF magnetron sputtering method—the effect of various calcination treatments on the photocatalytic reactivity, Res. Chem. Intermed. 35 (2009) 997.

Chapter 12

Semiconductor @ sensitizer composites for enhanced photoinduced processes Giuseppe Melea, Rudolf Słotab, and Gabriela Dyrdab a

Department of Engineering for Innovation, University of Salento, Lecce, Italy, b Institute of Chemistry, University of Opole, Opole, Poland

Abbreviations O2 or 1Dg O2 or 3S2 g 4-CP 4-NP CNSL DRS FDC g-C3N4 H2Pp HMF MB MPp Pc PMS POPs Pp RhB ROS TcPp

1 3

singlet molecular oxygen triplet molecular oxygen 4-chlorophenol 4-nitrophenol cashew nut shell liquid diffuse reflectance spectra 2,5-furandicarboxyaldehyde graphitic carbon nitride base porphyrin 5-hydroxymethyl-2-furfural methylene Blue metalloporphyrin phthalocyanine peroxymonosulfate persistent Organic Pollutants porphyrin Rhodamine B reactive oxygen species tetra (4-carboxyphenyl)porphyrin)

1 Introduction Research on photocatalytic materials has become an important part of contemporary science and technology. Great progress in this field supported by crucial discoveries has revealed the potential and versatility of the new generation molecular catalysts. Particularly, the search for semiconducting substrates demonstrating top optoelectronic properties resulted in the development of novel strategies in photocatalysis. In recent decades, about 200 different semiconductors have been assayed as suitable photocatalysts, including combinations between them or with other functional materials and/or their morphological modifications. Porphyrins and specifically the phthalocyanine derivatives have demonstrated a peculiar capacity to integrate with classical inorganic semiconductors, thus creating cutting-edge photosystems of enhanced catalytic activity. An enormous number of research reports devoted to such composite photocatalysts appeared hitherto in the scientific literature. This chapter features an attempt to assess the state of the art in photocatalysis with regard to materials and systems, considering the well-established results, but also the emerging ideas, and the envisaged new directions in this field in the near future. Our attention has been focused to show how the opportune combination semiconductor @ sensitizer produce targeted changes in the interfacial reactivity of the photocatalytic properties of the composites producing beneficial effects related with the better use of the visible/solar light for increasing the sustainability of processes such as, for example, the removal of pollutants, selective conversion of functional groups, and so on. Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00016-7 Copyright © 2021 Elsevier Inc. All rights reserved.

183

184

2

SECTION

C Oxides and calcogenides

Semiconductors in photocatalysis

Photocatalysis on semiconductors proved to open new attractive areas of application in terms of chemical syntheses [1–3] and essentially of environmental sustainability [4–6]. Principally, their effectiveness follows from the potential to produce diverse reactive species at the catalyst’s interface while irradiated with UV and/or visible light. Metal-oxide photocatalysts such as TiO2, ZnO, and WO3 or their composites are considered promising materials for the production of chemical fuels and for the degradation of organic pollutants. Titanium dioxide (TiO2) is by and large the most widely explored and employed photocatalyst because of its peculiar photochemical features, stability, and availability. On the other hand, the properties of ZnO have been less thoroughly investigated, mainly because of its photoinstability in an aqueous ambience [7, 8]. Tungsten trioxide (WO3) in turn, although capable of absorbing blue light [9], analogously to the other oxides also exhibits weak electron reduction ability due to the wide band gap, which limits its practical application. Graphitic carbon nitride (g-C3N4) is an interesting semiconductor and a promising candidate for photocatalytic systems. However, its main drawback follows from the same reason as in the case of metal oxides, i.e., low visible light absorption. Therefore, to improve the catalytic performance of these semiconductors, current research is primarily focused on the possibility of shifting the absorption edge into the visible region [10–12]. Dye sensitization of the semiconductor surface proved an effective solution [13]. Superficial molecular functionalization allows tailoring the semiconductors’ surface to introduce new active centers or modify the existing ones so as to meet specific needs, including the control of the interface. This survey comprises the recent knowledge and successful uses of semiconductor matrices impregnated by porphyrin (Pp) or phthalocyanine (Pc) derivatives. A selection of inorganic oxides, TiO2, ZnO, WO3, and g-C3N4 have been presented in unique composite systems combining the advantages of both the semiconductor and the sensitizer.

2.1 The TiO2 photocatalyst Among the three primary polymorphic forms of TiO2, rutile is the most stable one compared to the metastable anatase and brookite polymorphs, Fig. 1. Brookite is a rare TiO2 mineral and hence sporadically utilized. On the other hand, rutile and anatase, which are in focus of this review, are important commercial products, as well as one of the most explored inorganic oxides. Generally, a titanium dioxide crystal is an electron-rich material and belongs to the n-type semiconductors, the conductivity of which can be modified by doping the TiO2 lattice with other elements. By controlling the doping procedure, the n-type material can be transformed into p-type one. This is a crucial feature determining its optoelectronic and photophysical properties. Hence, TiO2 has been considered as an excellent candidate for supporting sensitizers or biomimetic molecules with the aim to improve its (photo)catalytic activity. Another widely used commercial TiO2 product is a blend composed of ca. 75% anatase and 25% rutile, known as Evonik P25 (formerly Degussa), which found multiple applications including the chemical industry, household products, food processing, pharmaceutics, and many more. It is also the most common photocatalyst available, characterized by a relatively large surface area (ca. 50 m2 g
1) [14–17]. This particular mixture of two different TiO2 polymorphs is known to display synergistic effects, and increased photocatalytic activity is observed compared to the pure phases [18]. The morphology of the pure microcrystalline anatase and P25 nano-material has been displayed in Fig. 2 along with the related powder diffraction patterns. Both rutile and anatase are non-toxic, chemically inert, amphoteric, biocompatible, and environmental-friendly. They are mechanically stable, show tolerance to harsh environmental conditions, and exhibit high photocatalytic efficiency. Each of these polymorphs reveals specific physical properties (Table 1) and consequently different photocatalytic performances. Anatase shows lower absorption of solar light than rutile due to the larger band gap than that of rutile [19]. Both of them are primarily active for UV radiation, nevertheless, the photocatalytic activity of anatase proved superior to that of rutile [20]. FIG. 1 Crystal structure packing of TiO2 polymorphs. Adapted from the American Mineralogist Crystal Structure Database (AMCSD) (rutile : P42/ mnm; anatase : 141/amd; brookite : Pbca)

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FIG. 2 SEM-FEG pictures showing the morphology of microcrystalline anatase, Tioxide Huntsman (left); and nanocrystalline Evonik P25 (right). XRD powder diffraction patterns; (A) anatase (JCPDS file 21-1272), (B) rutile (JCPDS file 21-1276), (C) Evonik P25.

TABLE 1 Selected physical properties of anatase and rutile. Property

Anatase

Rutile

Crystal structure

Tetragonal

Tetragonal

Lattice parameters (nm)

a ¼ 0.3785 c ¼ 0.9514

a ¼ 0.4594 c ¼ 0.29589

Unit cell volume (nm3)

0.1363

0.0624

3894

4250

3.2

3.0

Electrical conductivity (S m )

0.073

0.296

Refractive index

2.49

2.61

Solubility in HF

Soluble

Insoluble

Solubility in H2O

Insoluble

Insoluble

Hardness (Mohs)

5.5–6

6–6.5

-3

Density (kg m ) Band gap (eV)
1

Incidentally, also the different crystallographic orientations of the same polymorph may exhibit different photoactivity [21–26]. A choice of physical and structural properties of anatase and rutile has been reported in Table 1, [27]. Photoactivation of the pure TiO2 substrate proceeds via excitation of electrons (e
) at the surface layer with simultaneous creation of positively charged holes (h+), due to absorption of photons from the UV range (l < 400 nm). Thus, electron-hole pairs (charge carriers) have been produced which remain reactive only until separated. The better photocatalytic performance of anatase presumably follows from the fact, that this TiO2 variety seems to display greater surface adsorption capacity of hydroxyl groups, and a lesser recombination rate of charge carriers than rutile [28]. On the other hand, the inferior activity of rutile may also be related to its larger grain size [29, 30], lower specific surface area, and worse surface adsorption capacity [31, 32]. Moreover, the lifetime of photogenerated electrons and holes in anatase was found about an order of magnitude longer than in the case of rutile, which would greatly enhance their efficiency in chemical reactions occurring at the surface. Another important issue refers to the electronic structures of the photocatalysts and the effective mass of photogenerated charge carriers, which proved significantly influencing their transfer, as well as separation and mobility of photogenerated electron and hole pairs [33]. Effectively TiO2 demonstrates its photocatalytic performance only when utilizing ultraviolet light, however, due to the wide band gap it cannot make use of visible light or light of longer wavelength and so its overall solar activity is low. This is considered a disadvantage of TiO2 with respect to its potential application in visible light photocatalysis and photoelectrochemical devices, as well as photovoltaics and sensors. Also, the high overpotential, sluggish migration, and rapid recombination of photogenerated electron/hole pairs are crucial factors that restrict the further application of TiO2. Recently, a broad range of research efforts has been devoted to enhancing the optical and electronic properties of TiO2, resulting in improved photocatalytic activity [34].

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Based on the electronic band structure of solid matter, the mechanism of TiO2 activation by UV light involves three crucial steps, which determine its photocatalytic efficiency, resulting from the transfer of an electron from the TiO2 valence band (VB) into the conduction band (CB), denoted as e
CB. This implicates generation of an electron hole in the valence band +
(h+ VB) and consequently a reactive electron-hole pair (hVB/eCB), Eq. (1).
TiO2 + hn ! TiO2 hVB + =eCB
(1)
(2) TiO2 hVB + =eCB
! TiO2 + Heat; bulk or surface recombination
TiO2 hVB + =eCB
! TiO2 + hn; bulk or surface recombination (3) Eqs. (2) and (3) represent the probable deactivation of the excited system due to recombination of electrons (e
CB) and holes (h+ ) which may occur in bulk or at the surface of TiO . The recombination rate is usually very fast (on the nanosecond VB 2 scale) and may be accompanied by the release of extra energy in the form of a phonon (heat) or photon emission. As mentioned before, the separation of charge carriers is essential to keep the TiO2 matrix in an active state, and surface adsorbed oxygen molecules (O2(ad)) may help to prevent their recombination by trapping electrons from the conduction band. This would result in generating superoxide anion radicals(O∙
2 ), Eq. (4). These reactive oxygen species (ROS)may react with water molecules producing another highly reactive particle, the hydroxyl radical ðOH Þ, Eq. (5).
(4) eCB
+ O2ðadÞ ! O2 ∙
ðadÞ O2 ∙
ðadÞ + H2 O ! OH ðadÞ + OH
ðadÞ + O2ðadÞ

(5)

In general, oxygen molecules adsorbed on the TiO2 surface have been found to support the electron-transfer process and thus increasing the population of electron holes in the valence band. The redox potential of holes is high enough to oxidize water molecules or hydroxide ions, Eqs. (6) and (7), and almost any organic molecule [35]. ðhVB + Þ + H2 OðadÞ ! OH ðadÞ + H +

(6)

ðhVB + Þ + OH
! OH ðadÞ

(7)

The reactions are represented by Eqs. (4)–(7) are of fundamental importance in photocatalysis based on inorganic oxide semiconductors [36]. However, besides the charge carrier trapping, there are also other crucial factors, e.g., surface area, crystallinity, trap density, etc. which may affect the lifetime of the electron-hole pairs and hence the photocatalytic performance of the semiconductor [37]. More particulars have been provided in Section 4. The versatile physicochemical properties of TiO2 have made it to be widely used in metallurgy as the basic resource of titanium metal, as well as a component of many typical household items. The primary application of titanium dioxide is as a white pigment in paints, food coloring, cosmetics, toothpastes, polymers, and other products in which white coloration or UV filtration is desired [38]. In fact, TiO2-based materials had garnered extensive scientific interest since 1972 when Fujishima and Honda discovered photocatalytic splitting of water on a TiO2 electrode illuminated by UV light [39]. Semiconductor properties of TiO2 proved useful in many areas, such as sustainable energy generation and the removal of environmental pollutants. Over the past decades, TiO2 has found applications in many promising areas ranging from photovoltaics and photocatalysis to sensors [40–43]. It was demonstrated that TiO2 can improve the performance of composite catalyst systems [44–47] allowing to control the catalytic activity in many reactions, including photooxidation of various paraffins to aldehydes and ketones [48], photoreduction of CO2 [49], or mineralization of organic pollutant [50].

2.2 Other semiconductors (ZnO, WO3, g-C3N4) 2.2.1 ZnO Zinc oxide, ZnO is a wide-band gap semiconductor (3.37 eV) with a large exciton binding energy (60 meV) and near ultraviolet (NUV) absorption at room temperature, featuring excellent electrical, mechanical, and optical properties similar to TiO2 [51]. Furthermore, ZnO shows antifouling and antibacterial behavior and remarkable photocatalytic activity [52]. Besides, the production cost of ZnO is up to 75% lower than that of TiO2 and Al2O3 nanoparticles [53] and hence it has been suggested attractive for heterogeneous photocatalysis [54]. ZnO crystals occur in two polymorphic forms, adapted from wurzite (hexagonal) and the cubic zincblende (sphalerite) structure, Fig. 3. The photochemistry at the surface of ZnO grains generally relies on the creation of reactive OH and O2∙
radicals, generated by the h+/e
pairs, as well as H2O2 species. As for the other metal-oxide semiconductors, their

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FIG. 3 Crystal structure packing of ZnO polymorphs. Adapted from the American Mineralogist Crystal Structure Database (AMCSD) (wurzite : P63mc; zincblende (sphalerite) : F-43m)

photocatalytic activity definitely depends on the orientation of the crystal faces, which was proved in a methyl orange UV-photodegradation study [21]. This fact is believed to be related to the different polarities of the particular surfaces in a ZnO single crystal. The two polar ones are completely terminated by Zn2+ and/or O2
ions, while the nonpolar ZnO face shows mixed termination. The nonpolar face featured the highest photocatalytic activity presumably resulting from the considerably higher rate of radicals’ production at this crystallographic orientation. On the other hand, the lowest activity revealed at the O-terminated surface may follow from the inactivation of holes (h+) by the photolysis of ZnO at this crystal site (2h+ + ZnO ! Zn2+ + O), which would strongly inhibit the creation of OH radicals. Moreover, this reaction is responsible for the dissolution of ZnO observed in water, and for this reason, zinc oxide has not been considered an effective photocatalyst. These findings show that crystallographic orientations are of fundamental importance to control the photocatalytic activity, and this is also a great challenge for crystal growth engineering. Nevertheless, the recent progress in the preparation of nanocrystalline ZnO powders, including doping, heterojunction, and modification techniques allows one to assume that significant improvement of the photocatalytic performance of ZnO should become viable pretty soon. In this context, ZnO nanostructures have been shown as prominent photocatalyst candidates to be used, e.g., in solar photodegradation of "persistent organic pollutants" (POPs), owing to the fact that they are low-cost, non-toxic, and more efficient in the absorption across a larger range of the solar spectrum compared to TiO2 [54].

2.2.2 WO3 Tungsten trioxide (WO3) is a narrow band gap n-type semiconductor (2.4–2.8 eV) capable of absorbing blue light ( 12% solar light) and hence potentially attractive as a photocatalyst matrix [9]. Diffuse reflectance spectra (DRS) of pristine WO3 indicate absorption of light already around 480 nm and below, and display a fairly low reflectance in the UV range ( 4, due to chemical dissolution induced by OH
species (WO3ðsÞ + OH
! WO4 2
ðaqÞ + H + ). Incidentally, a similar photocorrosion effect was observed also at neutral and acidic pH, presumably induced by the photogenerated O2∙
radicals. Though, it was demonstrated that hydrogen treatment of nanostructured WO3 led to the creation of a nonstoichiometric WO3
x product featuring both enhanced photoactivity and photostability in water ambience. This finding may open up new opportunities for WO3 based photocatalytic systems [56].

2.2.3 g-C3N4 Graphitic carbon nitride (g-C3N4), with a moderate band gap (2.7 eV) and high chemical and thermal stability (>600°C), has been the hotspot in environmental photocatalysis. However, its performance is still unsatisfactory because of insufficient absorption of visible light, poor surface area, low electronic conductivity, and high recombination rate of photogenerated electron-hole pairs. Element doping or metal decorating is rendered to be a simple and effective strategy for improving its photocatalytic efficiency [60]. Its basic structure is composed of either heptazine or triazine units, which form layers similar to graphite Fig. 5. Hence g-C3N4is widely known as "graphitic carbon nitride", however, its structural characteristics are strikingly different from other carbon materials. The synthetic products are usually reported to be amorphous, nevertheless, crystalline g-C3N4 as well as individual spherical particles of g-C3N4 may also be produced, although perfectly crystallized particles (single crystal sheets) have never been obtained [59]. Interesting mesoporous g-C3N4 powders (amorphous) featured by BET surface area of 136–440 m2/g could be prepared as a synthetic mix with silica nanospheres. The optical absorption threshold for the crystalline product was found at ca. 460 nm, whereas for the amorphous one it was considerably shifted up to 680 nm [61]. Similarly, composites of amorphous g-C3N4 with carbon nanodots demonstrated much higher absorbance values in the range of 420-650 nm, compared to the base substrate [62]. Better photocatalytic performance can be achieved by coupling with various functional materials, including porphyrins and phthalocyanines [63, 64]. Possible applications of modified g-C3N4 photosystems are mainly focused on hydrogen evolution by water splitting, CO2 reduction, and pollutant degradation at ambient temperature. Another interesting use anticipated for metal-decorated g-C3N4 is hydrogen storage.

3

Porphyrin and phthalocyanine sensitizers

Photochemical activity of porphyrins and the chemically related phthalocyanines have been extensively explored for a pretty long time already. Nevertheless, these heteromacrocyclic compounds still have been ranked among the top attractive molecular materials offering versatile possible applications in many fields, including electronics, optoelectronics, catalysis, and medical therapies. Their unusual properties result from the specific chemistry revealed by this peculiar molecular arrangement basically comprised of four pyrrole units creating a stable macrocycle set-up. There is a vast literature devoted to these compounds, and hence only a brief introduction to this issue has been presented below, featuring the points considered most relevant to the chapter’s topic.

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FIG. 6 The porphine molecule (left), precursor of the porphyrin ligand family (right); X denotes peripheral (b) and R meso (m) substituents, respectively.

3.1 Porphyrins (general properties and synthesis) Porphyrins belong to a group of naturally occurring molecular systems (i.e., chlorophylls, heme), many of them showing biochemical activity crucial to life on Earth. General structure has been shown in Fig. 6. Over the last half century, porphyrins have been widely investigated for their activity toward natural proteins and in enzyme catalysis [65, 66]. Interestingly, the meso-substituted porphyrins do not occur in nature. For practical reasons, they may be synthesized according to a number of methods depending on the target product, the idea of which has been rendered in Fig. 7. These reactions are typically carried out in organic solvents at room temperature (under reflux) and most of them are based on the well-established Adler-Longo method [67]. The peculiar spectral and electronic properties justify the unabated interest in porphyrin-based compounds [68–70]. The porphyrin ligand (Pp) may coordinate different metal ions to form specific metalloporphyrin complexes (MPp). Besides, a wide range of possible molecular modifications, mainly based on versatile substitutions of the Pp ligand, allows to create diverse porphyrin systems featuring a pre-programed physicochemical character, Fig. 8.

3.2 Phthalocyanines (general structure and synthesis) Phthalocyanines represent a group of synthetic heteromacrocyclic compounds closely related to the porphyrin family. Unlike in porphyrin, the phthalocyanine macrocycle consists of four benzopyrrole units linked through N bridging atoms. This creates a very stable 16-membered inner aromatic p electronic system of coupled
C ¼ N
bonds (phthalocyanine core), Fig. 9. However, such arrangement precludes typical substitutions at the meso-N bridges, and only weak intermolecular interactions with electron-acceptor species may be effective at these sites of the macrocycle [71]. The phthalocyanine derivatives may be synthesized according to various methods depending on the desired product, the general idea of which has been displayed in Fig. 10. These reactions are carried out either in the molten solid phase or in organic media, and the choice is up to the substrates used and the target compound [72]. However, the most effective synthesis of typical metallophthalocyanines is essentially based on the method consisting of sintering of the solid phthalonitrile precursor and the appropriate metal or its salt (or nonmetal source) under anaerobic conditions [73]. The spectral and electronic properties of phthalocyanines are competitive against porphyrins. This has been supported by a great number of their derivatives synthesized so far, which include different metal cores and peripheral substitutions of the macrocycle, as well as diverse molecular set-ups, Fig.11. Consequently, to a large extent, their physicochemical features may be tailored according to the needs, with practical application in mind. Therefore, phthalocyanines have been classified among the most versatile and promising molecular materials [74, 75].

FIG. 7 General synthetic path for meso- and/or b-substituted porphyrins.

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FIG. 8 Selection of porphyrin derivatives and their structural modifications; (top row) base porphyrin H2Pp (tetraphenyl substituted); CuPp (tetraphenyl substituted); FePp (axially, b and m substituted); (lower row) homoleptic system with two M centers PpMn-O-MnPp; heteroleptic porphyrinphthalocyanine system with two different M centers PpMn-OH-CrPc; ZnPp based porphyrin cage structure. Where relevant the H atoms have been hidden for clarity. All structures adapted from the Cambridge Crystallographic Database deposits and their Ref codes are TPHPOR01, CUTPOR02, AGUYOQ, BAHZAL, CEPBAB, VIPKOW, respectively.

FIG. 9 Free-base phthalocyanine H2Pc and possible substitutions in metallophthalocyanines MPc; M is for the complex’s center (metal or nonmetal), L denotes axial ligands, and Y is for peripheral substituents.

FIG. 10 General scheme of the synthesis of phthalocyanine derivatives; the Y substituents may differ from each other and when all Y ¼ H unsubstituted MPc complexes are produced.

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FIG. 11 Selection of phthalocyanine derivatives and their structural modifications; (top row) CoPc; Rh(Me)Pc (axially and peripherally substituted); GdPc2 (sandwich complex); (lower row) homoleptic system with two directly bound M centers PcpyRh-RhpyPc; homoleptic system featuring two M centers coupled via an oxo-bridge PcAl-O-AlPc; triple-decker heteroleptic set-up involving two porphyrin ligands (peripherally substituted) and a phthalocyanine bridge PpNd-Pc-NdPp. Where relevant the H atoms have been omitted for clarity. All structures adapted from the Cambridge Crystallographic Database deposits and their Refcodes are COPTCY01, HASZEG, GAWBEL, NEXSIS, DAYYOR, UFADUA, respectively.

3.3 Structure-related issues Other than typical molecular systems like those presented in Figs. 8 and 11, both porphyrins and phthalocyanines are capable to create conjugates based on intermolecular interactions mainly with small electron acceptor species [76–82]. This is a major feature particularly of H2Pp, H2Pc, and MPc derivatives, which is considered a key issue in modulating their activity and optoelectronic properties, and hence critically important for application purposes. Effective conjugates (or adducts) are established due to direct interactions of the hosted species with the chromophore system (core) of the macrocycle. In the case of H2Pp derivatives, these are the two inner pyrrole nitrogen atoms, whereas in the case of phthalocyanines the reactive contact sites are the meso-N atoms of the ring, usually all of them equally as shown in Fig.12. Such conjugates may play a crucial role, e.g., in activating composite catalytic systems, i.a. due to promoting singlet oxygen (1O2) generation [71, 80, 83].

FIG. 12 Porphyrin and phthalocyanine conjugates featuring electron acceptors, BF3 and trifluoroacetic acid, TFA (DFT-derived models, courtesy of R. Słota and M.A. Broda, University of Opole). (Left) H2Pp⋯(BF3)2, the interacting atoms of B from BF3 (pink), and the porphyrin’s pyrrole N (blue) are highlighted; note the remarkable deformation of the Pp moiety. (Right) ZnPp⋯(TFA)4, the interacting meso-N atoms of the Pc core (blue) and H atoms of TFA are signaled. Other H atoms have been omitted for clarity.

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3.4 Electronic, spectrochemical, and photochemical properties 3.4.1 UV-vis absorption and emission spectra UV-vis electronic absorption spectra have played a key role in the research of porphyrins and phthalocyanines. The chemical and particularly photochemical activity of these compounds basically depends on the current state of the electrons involved in the conjugated p bonding system of their chromophores. This is clearly manifested in very strong absorption (molar absorptivity, e > 105 M
1 cm
1) of photonic energy from the UV and visible (Vis) spectral range, and in the position of absorption bands in the spectrum, Fig. 13. Typical porphyrins essentially show strong absorption in the near-UV range (strong B-band at ca. l400 nm) and reveal much less photoactivity in the visible range (a set of weak Q-bands ca. l500– 650 nm), whereas the phthalocyanines usually display relative intensive B-bands and especially strong Q-bands at the red edge of the spectrum (l > 600 nm). The position of absorption bands is sensitive to the polarity of the medium and electron acceptors (or donors) present in the system, as visualized in Fig. 13 by the altering spectra of LuPc2 following the solvent change (DMF ! benzene) and due to interaction with TFA species. Particularly the behavior of LuPc2, accompanied by distinct color changes (blue ! green ! orange) can be viewed as a unique demonstration of the strength and flexibility of the Pc’s molecular system. Similar feedback of the chromophore system in the sandwich LuPc2 and related lanthanide derivatives can be triggered off by changing the electric field potential (in an electrochemical cell), which defines the phenomenon of electrochromism [84, 85]. Although the porphyrins do not manifest such dramatic color changes, nevertheless free base H2Pp derivatives also may switch their tint, usually from purple into green, in the presence of electron-acceptor species. Excitation of the Pp and/or Pc chromophores results in the characteristic emission of fluorescence with lifetimes on the nanosecond scale (usually 420 nm) it was even better than featured by the TiO2-based composite [129]. Enhanced photocatalytic activity in the degradation of RhB under visible and solar light was observed also by some other composites based on nanostructural ZnO impregnated by porphyrins functionalized by cardanol-based substituents [146]. Another novel visible light photoactive composites have been developed by using the g-C3N4 substrate decorated by different metallo phthalocyanines. Zinc tetra carboxy phthalocyanine (ZnTcPc) proved to be a very effective sensitizer considerably extending the spectral response region of g-C3N4@ZnTcPc to l > 800 nm. This particular photocatalyst exhibited high efficiency in photodegradation of RhB and 4-CP. Interestingly, under alkaline conditions, the presence

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 of 1O2, O∙
2 , and OH reactive oxidizing species had been confirmed [147]. A recently synthesized Zn complex of phthalocyanine peripherally functionalized by four 4-carboxylphenoxy groups (a-ZnTcPc) was employed in a composite system based on the g-C3N4 matrix. It was successfully used in the decomposition of MB and tetracycline antibiotics under visible light irradiation, featuring high reaction yields. This study has provided some useful hints for designing other efficient g-C3N4-based photocatalysts functioning under solar light, which might have found application in eliminating recalcitrant organic pollutants [148]. Advanced oxidation processes using peroxymonosulfate (PMS) to neutralize water contaminants have received widespread attention. It was discovered that composite photocatalyst may improve the activity of PMS. Indeed, this was achieved by a system based on g-C3N4 impregnated by iron hexadecachlorophthalocyanine (FePcCl16) with the addition of imidazole (IMA) as a base ligand. Such photocatalyst was applied to activate PMS for carbamazepine, whose degradation resulted in 95% yield, which has been attributed to the synergistic effect of diverse active species, including superoxide radicals and singlet molecular oxygen [149].

4.2.2 Mixed semiconductor matrices TiO2/WO3 A quite novel nanohybrid system, including a mixed semiconductor matrix (TiO2/WO3) and a CuPp photosensitizer, revealing the potential of a high-performance photocatalyst for selective oxidation reactions under visible light irradiation has been developed. The base TiO2/WO3 nanocomposite was prepared according to a sonochemical-assisted hydrothermal method from nano-disk WO3 and TiCl4. It was impregnated by CoPp meso-substituted by four (4-carboxyphenyl) groups (CoTCPP). Both the TiO2/WO3 composite and TiO2/WO3@CoTCPP hybrid catalyst proved effective in photocatalytic oxidation of primary alcohols under aerobic conditions and irradiation by visible light (LED, 5 W). The photocatalytic efficiency of TiO2/WO3@CoTCPP reached 95%, compared with 70% featured by the TiO2/WO3 nanocomposite. The nanohybrid catalyst, TiO2/WO3@CoTCPP, was reused and recovered ten times without losing photocatalytic activity [150].

4.2.3 Photocatalyzed oxidation in the gas phase Heterogeneous photocatalysis under a gas–solid regime has become a major challenge for hybrid systems based on inorganic semiconductors and efficient light scavengers like porphyrin and phthalocyanine derivatives. Excellent properties revealed by the relevant composites in the water phase would imply similar performance at the gas–solid interface. According to the research reported thus far, these photocatalysts have potential for both industrial and environmental applications. 2-Propanol (2-PrOH) has been considered a major contaminant in indoor air and air streams. Successful attempts to oxidize 2-PrOH in the gaseous phase, at 25°C, were carried out by employing TiO2@LnPc2 hybrid systems, which demonstrated high activity toward photodegradation of 4-NP in water [111]. A continuous-flow tube photoreactor was used and the catalyst was in the form of a fixed solid bed placed on its bottom. The feeding gas was composed of oxygen, 2-PrOH, and water vapor, and the photoreactor was irradiated in a SOLARBOX unit with a 1500 W Xe lamp. Propanone and acetaldehyde intermediates were identified during the process, however, the final oxidation products were carbon dioxide and water. The highest photoactivity was shown by TiO2@HoPc2 [151]. A different innovative continuous-flow photoreactor system (tube-in-tube) including a TiO2@CuPc composite was applied to oxidize ethylene gas at room temperature, under irradiation of a 300 W tungsten lamp (Sanolux, with a solar light-similar spectrum). Commercial TiO2 (Aeroxide® P25, Evonik) was used and the CuPc sensitizer was a compound peripherally functionalized by four [4-(2,4-bis-(1,1-dimethylpropyl)phenoxy)] substituents. A thin layer of TiO2 was produced on the inside wall of the transparent inner tube (Pyrex glass) of the photoreactor, and it was then coated by a thin film of CuPc. Absorption of light from the visible range greatly enhanced the efficiency of the photocatalyst compared to not impregnated TiO2 film. It was found (GC analysis) that under the applied conditions ethylene was completely mineralized and the outlet gas contained only CO2 [152] (Fig. 19).

4.2.4 Photocatalyzed reduction of CO2 Heterogeneous catalysts dedicated to the reduction of CO2 have been investigated for many decades. The principal reasons for such intensive research in this field have been the constant depletion of fossil fuel reserves and the impact of anthropogenic CO2 on the environment. The economic aspect driving this challenge is to find an effective way to convert it into synthetic fuel. However, equally important is the search for smart photosystems capable of converting it in a mode similar to natural photosynthesis, and hence the hybrid photocatalysts relevant to the subject of this chapter may be considered a promising solution.

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UV

VIS CO2 H2O

C2H4

Quartz tube

TiO2 CuPc

FIG. 19 Schematic view of the continuous-flow tube photoreactor featuring an internal TiO2 thin film coating impregnated by CuPc. Adapted from A. Licciulli, A. De Riccardis, S. Pal, R. Nisi, G. Mele, D. Cannoletta. Ethylene photo-oxidation on copper phthalocyanine sensitized TiO2 films under solar radiation. J. Photochem. Photobiol. A: Chem, 346 (2017) 523–529.

Pioneering studies on the photoreduction of CO2 in aqueous solution, in the presence of photosensitive semiconductor powders suspended in the water phase, have proved the possibility of its conversion into organic compounds such as formic acid, formaldehyde, methyl alcohol, and methane [49]. These results prompted worldwide theoretical and experimental studies basically focused on the problem of efficient photocatalyst systems operating in aqueous environment [153–168]. Other solvents were also considered, and under anaerobic conditions, in DMF or acetonitrile the photocatalytic reduction of CO2 promoted by the reduced forms of FePc and CoPc produced CO and formate in DMF and acetonitrile solutions [169]. Formation of formic acid in aqueous solution using TiO2 impregnated with CoPc and ZnPc, under irradiation by visible light was also reported elsewhere [170, 171]. Photoreduction of CO2 into formic acid was found particularly effective in the presence of TiO2 (anatase) superficially impregnated by lipophilic H2Pc, CuPc or ZnPc of which the Pc macrocycles were peripherally functionalized by four highly branched [4-(2,4-bis-(1,1-dimethylpropyl)phenoxy)] substituents [172, 173]. Nanocomposites based on a hydrothermally pre-prepared porous WO3 and Z-scheme iron phthalocyanine, P-WO3@FePc, appeared very efficient photocatalysts in the reduction of CO2 into CO and CH4. The optimized P-WO3@FePc system allowed increasing the reaction yield 8-fold compared with the reference WO3 nanosheets. Moreover, it is feasible to replace FePc with other MPcs [130]. Ultrathin g-C3N4@MPc heterojunctions, constructed through a facile assembly process induced by surface OH anions, demonstrated exceptional photocatalytic performance in converting CO2 into CO and CH4. Among the MPc photosensitizers explored, CuPc proved the most effective one, featuring a 10-fold enhancement of photocatalytic activity under UV–vis irradiation of the optimized g-C3N4@CuPc compared with the base g-C3N4 [174]. Nanosheets of g-C3N4 impregnated by tetra(4-carboxylphenyl)porphyrin iron(III) chloride (FeClTCPP) were applied as photocatalyst in the reduction of CO2. This composite exhibited high activity in CO2 reduction under visible-light irradiation and a relatively good yield of CO produced.

4.2.5 Photocatalyzed reduction of Cr(VI) Hexavalent chromium compounds are toxic when inhaled or ingested and are considered highly hemotoxic, genotoxic, and carcinogenic [175]. On the other hand, trivalent chromium is a trace mineral that is essential to human nutrition. Therefore removal of Cr(VI) compounds from the environment has become an important issue, and specially designed composite photocatalysts are expected to help solve this problem. Promising results have been reported on efficient reduction of Cr(VI) ! Cr(III) compounds achieved by hybrid photocatalysts comprising peripherally substituted H2Pc (azomethine-bridged phenolic phthalocyanines) immobilized on TiO2 nanoparticles, H2Pc@TiO2. These photoactive nanocomposites were used under UV irradiation (l ¼ 365 nm) to reduce K2Cr2O7 in water solution at pH  2, in a batch reactor at room temperature. The photoconversion yield was assessed of 83.7%-99.8 %, compared with only 55.4% when bare TiO2 was tested. Such systems should be effective as catalysts in wastewater treatment [176]. Successful reduction of Cr(VI) species was carried out under visible light irradiation by employing a composite photocatalyst consisting of a hydroxoaluminium-tricarboxy-monoamide phthalocyanine adsorbed on a TiO2 nanopowder (Evonik P-25). To allow any progress of the reduction process it was found essential to introduce some amount

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of 4-chlorophenol (4-CP) into the reaction batch, which served as a sacrificial donor. In the absence of 4-CP, fast capture of TiO2 CB electrons by Cr(VI) species inhibits the formation of reactive oxygen molecules in the reductive pathway. Hence, the relative easy oxidation of 4-CP may render feasible the Cr(VI) ! Cr(III) conversion under visible light irradiation. Incidentally, the concentration of 4-CP had no effect on the decay rate of Cr(VI), and no 4-CP oxidation products had been observed [177]. Development of an effective photocatalytic converter of Cr (VI) ! Cr(III) prepared from porous TiO2 microspheres impregnated by a Cu(II) complex of protoporphyrin IX (TiO2@CuPP) was reported. Excellent photocatalytic activity of the composite was observed under visible light illumination (Hg-lamp, l  400 nm) of an aqueous solution of K2Cr2O7 at ambient temperature. Prompted by the promising results of the batch tests, a prototype of an active filter was constructed, by depositing the photocatalyst on a stainless-steel metal mesh. This device exhibited a significant practical potential for chemical filtering of toxic Cr(VI) ions (photoreduction to Cr(III)) along with physical cleaning of suspended particles present in water, and maybe a versatile solution for combating aqua-chromium hazards [178]. Another efficient photocatalyst was prepared from microcrystalline TiO2 (anatase form) impregnated with lipophilic porphyrins, H2Pp, and CuPp, meso-functionalized with [3-(pentadeca-8-enyl)-(phenol)] chains. Photoreduction of the toxic chromate Cr(VI) to the neutral chromium Cr(III) was performed in a water suspension of the particular composites, TiO2@H2Pp and TiO2@CuPp, under irradiation by UV-vis light (Osram Ultra Vitalux lamp). Both catalysts were found 3+ capable of converting Cr2O2
ions, and the synergy between porphyrins and titanium dioxide was apparently 7 into Cr manifested in this reaction. However, the TiO2@CuPp composite proved definitely more effective and demonstrated its practical value as a catalyst designed for reduction of Cr(VI) in polluted water resources [179].

4.2.6 Photochemical syntheses (g-C3Ns) Hybrid photocatalysts may also be used as a crucial component of diverse reaction systems designed to realize more or less complicated syntheses. Recently, the potential of novel composites based on graphitic carbon nitride, g-C3N4, and the thermo-exfoliated product, C3N4-TE, was demonstrated in a photocatalytic synthesis of 2,5-furandicarboxyaldehyde (FDC) through oxidation of 5-hydroxymethyl-2-furfural (HMF) under natural solar irradiation. The C3N4 matrices were impregnated by meso-tetra aryl substituted porphyrins, H2Pp, CuPp, and ZnPp, functionalized with lipophilic (3-n-pentadecylphenoxyethoxy) chains. The reaction was realized in a batch photoreactor in an aqueous suspension of the hybrid catalysts (liquid-solid regime). All of the tested composites effectively prompted the conversion of HMF, and in all cases, its partial oxidation product, FDC, was selectively synthesized. Although both carbon nitrate supports played a key role in this process, however, the porphyrins improved their visible light absorption, and hence the catalyst performance. Moreover, the pH of the batch during the syntheses was found to affect the photocatalytic activity of the composites. The best result was obtained by the C3N4-TE@ ZnPp system under neutral pH, featuring 73% conversion of HMF and a 36% selectivity to FDC. However, at pH 9 the most efficient photocatalyst proved C3N4-TE@ CuPp [63].

4.2.7 Hydrogen production Versatility of g-C3N4-based hybrid photocatalysts has been revealed also in the efficient, visible, light-driven production of hydrogen gas. Two versions of the graphitic support have been tested, a typical g-C3N4 substrate and a product containing a specified amount of co-catalyst Pt nanoparticles, Pt/g-C3N4, used to promote the photogenerated separation of charge carriers within the catalyst matrix. To improve their visible light response, the supports were impregnated by two different asymmetric meso-substituted Zn-porphyrins, containing one carboxyl group to secure efficient chemical binding with the support, which should have improved the electron transfer throughout the created photosystem. One of them was a zinc-5-(4-carboxyphenyl)-10,15,20-tri (3-pridyl)porphyrin, (ZnMT3PyP), involving one benzoic acid and three pyridine substituents, whereas the other one, zinc-5-(4-carboxyphenyl)-10,15,20-triphenylporphrin, (ZnMTPP) was substituted by one benzoic acid and three phenyl groups. The photoreaction was performed under anaerobic conditions in an aqueous suspension of the catalyst dispersed in a solution of ascorbic acid as an electron donor. The system was irradiated by visible light (l > 420 nm, cutoff filter) using a 300 W Xe-lamp. It was found that Pt-loading of the support was essential to enhance the production of H2, and only a few hydrogen gases could be generated when the composite without Pt-loading was used. Apparently, better catalytic activity was shown by the composites including pyridine substituents, and particularly Pt/g-C3N4@ ZnMT3PyP exhibited enhanced quantum yield of 25.1%, compared to 11.6% showed by Pt/g-C3N4@ZnMTPP. These results are very promising with regard to the application of porphyrin derivatives in photocatalytic systems employing solar energy conversion for generating H2 [180].

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Chapter 13

Nanostructured-based WO3 photocatalysts: Recent development, activity enhancement, perspectives and applications for wastewater treatment D. Sa´nchez-Martı´neza and D.B. Hernandez-Urestib a

Autonomous University of Nuevo Leo´n, Civil Engineering Faculty— Ecomaterials and Energy Department, Cd. Universitaria, San Nicola´s de los Garza,

NL, Mexico, b Autonomous University of Nuevo Leo´n, Physical-Mathematical Sciences Faculty, Cd. Universitaria, San Nicola´s de los Garza, NL, Mexico

1 Introduction Humankind is rapidly developing in both technological and industrial ways, which is causing mass amounts of pollutants to be released into the environment. These pollutants are threatening various ecosystems and may cause damage to human and animal health. This has provoked concern for environmental quality, in particular decontamination of water. All of this has led to the development of technologies with high efficiency as solutions to environmental pollution [1, 2]. In this sense, a variety of technologies has been developed for environmental remediation such as biological and physicochemical methods for photocatalysis and soil treatments for water remediation [2]. Photocatalysis is considered the most promising clean energy technology [1]. Since 1972, photocatalysis has attracted the interest of the scientific community because it has proved to be an important technology for controlling a variety of organic pollutants [3, 4]. A great number of semiconductor compounds have been validated on photocatalytic reactions, which work by oxidizing and decomposing organic pollutants with the help of solar energy, UV light, or simulated sunlight [3]. The photocatalytic reaction is an advanced redox process, which is mainly triggered by the action of light on a catalyst with semiconductor properties. The materials used in photocatalysis are commonly semiconductor solids (oxides or chalcogenides) known as photocatalysts, which form an infinite three-dimensional network. Titanium dioxide (TiO2) emerged as an ideal photocatalyst for degradation of organic pollutants due to its nontoxic nature, good chemical stability, and low cost [3]. However, this compound has some disadvantages, including low effect to visible light and poor quantum yield efficiency. In addition, photogenerated electrons and holes are recombined rapidly in TiO2 before they participate in the photocatalytic reaction [3]. In this sense, much research has been put into discovering novel materials with better photocatalytic performances, especially under visible light [4]. Among these materials, tungsten trioxide (WO3) has stood out. It is a semiconductor oxide greenish yellow in color with a band gap of 2.5–2.8 eV [5, 6]. This band gap property makes WO3 a promising material for applications in photocatalysis, since it can absorb the visible light region of the electromagnetic spectrum. However, pure WO3 undergoes fast recombination of photogenerated electron-hole pairs. To increase the photoactivity of WO3, different strategies such as heterojunction with other semiconductors, inclusion of a co-catalyst, metal doping, and a Z-scheme system have been explored [7, 8]. Despite its limitations, WO3 has been used in a wide variety of technological applications such as smart windows [9, 10], photochromic material [11, 12], catalysts [5, 13, 14], gas sensors [15, 16], and the photooxidation of water [6, 17]. Many studies have been published describing the preparation of WO3 in the form of nanoparticles, nanostructures, and thin films via various methods of soft chemistry such as sol-gel [18, 19], hydrothermal [20, 21], chemistry precipitation [22], microwave [23], ultrasound [24], electrospinning [25], and dip-coating methods [26], among others. WO3 exhibits a cubic perovskite structure based on the corner sharing of the regular octahedral with oxygen atoms at the corner and the tungsten atoms at the center of each octahedron [27], as shown in Fig. 1. It exhibits the polymorphism phenomenon and thus can be obtained in seven crystalline structures depending on their temperature: cubic, triclinic,

Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00008-8 Copyright © 2021 Elsevier Inc. All rights reserved.

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FIG. 1 Crystal structure of tungsten trioxide (WO3) [27].

monoclinic, orthorhombic, hexagonal, pyrochlorine, and tetragonal. The most representative crystalline structures are tetragonal (obtained at temperatures greater than 740°C), orthorhombic (obtained between 330°C and 740°C), triclinic (formed between 50°C and 17°C), and monoclinic (formed between 17°C and 330°C). The latter is the most stable crystalline structure of WO3 at room temperature [28]. This chapter discusses the most relevant photocatalytic applications of WO3 for water treatment. It examines recent scientific advances to increase photocatalytic activity and improve the application of this material in water treatment.

2

Recent developments

2.1 Morphology effect on the photocatalytic properties of WO3 Recent research has placed strong emphasis on controlling size and shape of particles in materials. This is due to the fact that these two variables play an important role in the physicochemical and photocatalytic properties of the materials. In this sense, many works have been carried out in WO3 preparation using different synthesis methods and under various experimental conditions that allow modification of its morphology to improve its potential for purifying contaminated water.

2.1.1 Precursor effect on the morphology of WO3 It is possible to prepare WO3 by three different synthesis methods starting from the same precursor, peroxotungstic acid. A common solution was prepared using peroxotungstic acid, which is made by adding 0.5 g tungsten powder (Wako, Japan) to 2.0 g of 30% hydrogen peroxide solution (Wako, Japan). After 2 h, some distilled water was added to make a 0.05 mL peroxotungstic acid solution. From this solution, WO3 was synthesized by three different methods. The first used a hydrothermal reaction heated in an autoclave at 180°C for 1.5 h. The precipitation of this reaction was removed, dried, and heated at 500°C for 2 h. The second synthesis method used the same hydrothermal procedure but with the addition of 0.2 mL acetyl acetone (Wako, Japan) and 0.1 mL ethanol (Wako, Japan) as surfactants to the peroxotungstic acid solution. The third method prepared for a simple thermal treatment of the starting precursor at 90°C for a period of 2 h. The precipitate obtained from the three methods underwent thermal treatment at 500°C for 2 h to obtain WO3. All the samples were crystallized in their monoclinic phase (m-WO3). Regarding morphology, the hydrothermal method results in hexagonal plates with an average diameter of 1.5 mm and thickness of 500 nm. A flower morphology with an average size of 3.5 mm was obtained by aqueous solution (acetyl acetone + ethanol) method and a morphology of nanosheets around 20 nm in size was obtained via a simple thermal treatment [29]. In addition, it is possible to obtain WO3 with morphology of nanosheets (see Fig. 2A) and nanorods (see Fig. 2B) using the same precursor (tungstic acid; H2WO4). These morphologies are obtained by adding CTAB and PEG, respectively, during preparation of tungstic acid solution. For this synthesis,

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FIG. 2 Transmission electron microscope (TEM) images of WO3: (A) nanosheets and (B) nanorods [30].

microwave equipment (2.45 GHz) was used to expose the solution for 5 min for the formation of WO3. The monoclinic phase (m-WO3) was obtained in both samples [30]. Another precursor used to obtain WO3 is hydrated ammonium paratungstate (H42N10O42W12  H2O, 99% purity; Sigma-Aldrich). This precursor was dissolved under continuous stirring in 50 mL of nitric acid solution (10% v/v, HNO3). Then, the pH of the solution was adjusted under stirring to 3 by the addition of ammonium hydroxide (NH4OH, 30% reagents; DUKSAN), and the stirring was maintained until a homogeneous solution was obtained. Then, the aqueous solution was transferred to a microwave-assisted hydrothermal reactor at 180°C, 200 W for 30 and 60 min [23]. Likewise, WO3 was synthesized by ultrasound method using the same precursor with CTAB (C19H42BrN; Sigma-Aldrich) at different molar rations into the solutions. This process resulted in an homogeneous solution of hydrated ammonium paratungstate and CTAB, which was exposed to ultrasound radiation for 5 h. [24]. After thermal treatment at 500°C for 2 h, WO3 was obtained. Synthesis of WO3 from hydrated ammonium paratungstate has also been reported. In this case, hydrated ammonium paratungstate was dissolved in 30 mL of ethanol and 3 mL of nitric acid was added to obtain an aqueous solution. The solution was dried at 70°C and the material obtained was heat treated at 500°C for 2 h to obtain WO3. In the microwave-assisted hydrothermal method, the hexagonal (h-WO3) and monoclinic (m-WO3) phases of WO3 were obtained with ovoid and irregular morphology approximately 50 nm in size under different experimental conditions, as shown in Fig. 3A. Using the ultrasound method, m-WO3 with a morphology of rectangular nanoplates with thickness of 30 nm and length between 100 and 200 nm was obtained, as shown in Fig. 3B. Using thermal treatment, m-WO3 was obtained with a morphology of nanospheres with diameter of 50–100 nm, as shown in Fig. 3C [23, 24, 31]. Using another precursor such as sodium tungstate (Na2WO4
2H2O), WO3 was prepared by heat treatment at 500°C for 2 h, and the m-WO3 phase with a nanoplate morphology was obtained, as shown in Fig. 4 [31]. Depending on the precursor and the synthesis method used, various morphologies of WO3 can be obtained, from simple shapes such as irregular nanoparticles, ovoid shapes, and nanospheres (1D) to nanosheets, nanoplates (2D), and even more complex nanoflowers (3D). The following section focuses only on morphologies that give rise to good photocatalytic performances in the treatment of water by degrading organic pollutants.

2.1.2 Morphology and its influence on the degradation of organic pollutants An important part of what will be addressed in this subtopic is how morphology can influence the degradation of organic pollutants for water treatment via photocatalysis. As mentioned in the previous section, WO3 in different morphologies can be obtained by various synthesis methods. The photocatalytic performance of WO3 obtained from peroxotungstic acid as precursor (hydrothermal, aqueous solution, and simple thermal treatment) in water treatment has been evaluated through the degradation of toxic organic pollutants such as isopropyl alcohol (IPA). WO3 with a hexagonal plate morphology presented a Brunauer-Emmett-Teller (BET) surface area around 7.6 m2 g 1, the sample with flowerlike particles showed a surface area of 12 m2 g 1, and the sample with a nanosheet morphology obtained a surface area of around 29.7 m2 g 1. It is important to highlight that depending on the synthesis method, it is not only possible to modify the morphology but also the physicochemical properties of the materials. These properties can also influence the photocatalytic activity, as in this case where the surface area was modified, obtaining the largest surface area for the sample with a nanosheet morphology. All the samples were used as photocatalysts, and they showed good results in the photocatalytic decomposition of IPA used to produce acetone. The sample with nanosheet morphology presented more edges and corners as well as larger BET surface area. Finally, the photocatalytic decomposition of IPA formed acetone as an intermediate, which was oxidized to the final products: CO2 and H2O [29].

214

SECTION

C Oxides and calcogenides

FIG. 3 Scanning electron microscope (SEM) images of WO3: (A) ovoid particles, (B) rectangular nanoplates, and (C) nanospheres [23, 24, 31].

FIG. 4 Scanning electron microscope (SEM) images of WO3 nanoplates [31].

WO3 has also been tested in the degradation of dyes such as rhodamine B (rhB) [23, 24, 31], indigo carmine (IC) [23, 24], and methyl orange (MO) [32], among others, the removal of drugs such as tetracycline (TC) [23] and ciprofloxacin (CP) [33], and the degradation of other water pollutants such as phenols [34]. WO3 synthesized via ultrasound from hydrated ammonium paratungstate (precursor) with a rectangular nanoplate morphology presented a better photocatalytic performance in rhB and IC degradation, whereas the same precursor with an ovoid nanoplate morphology obtained by microwave-assisted hydrothermal method was more efficient in TC degradation. For the degradation of MO dye, WO3 synthesized using hexagonal ammonium tungsten bronze (HATB) as a precursor by microwave-assisted

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hydrothermal method with a nanowire morphology showed the best photocatalytic activity. In the case of the antibiotic CP, WO3 with a nanoplate morphology showed the best photocatalytic behavior. Finally, for phenols, the morphology of disctype nanoplates was found to be the most efficient. Therefore, according to the previous discussion, morphology of WO3 in two dimensions (2D) has shown the best photocatalytic performance in the degradation of a wide variety of organic pollutants that cause water contamination. In this sense, the morphology with a lot of edges and corners favors the photocatalytic properties in the degradation of organic pollutants for water treatment [29].

2.2 Influence of crystalline structure on its morphology and photocatalytic behavior The crystalline structure most investigated due to its high efficiency in water treatment compared to that of h-WO3 is m-WO3. h-WO3 nanoparticles and nanowires have been prepared starting from a simple heating of hexagonal ammonium tungsten bronze (HATB) and (NH4)0.33 xWO3 (precursors) at 470°C in air. h-WO3 nanowires were obtained using the same precursors by the microwave-assisted hydrothermal reaction at 160°C. For comparative purposes, m-WO3 nanoparticles were prepared from (NH4)0.33 xWO3 annealed at 600°C in air [32]. Photocatalytic activity of the samples was evaluated in the degradation of MO and results showed that the morphology of nanowires was three times more efficient than that of nanoparticles in the h-WO3 samples. However, m-WO3 nanoparticles exhibited better photocatalytic behavior. Although h-WO3 nanowires have a behavior close to m-WO3nanoparticles, the latter are more effective. In this sense, the morphology can directly favor obtaining a material with high efficiency in water treatment through the decomposition of organic pollutants [32].

3 Nanostructures of WO3 WO3 has been investigated with the aim of improving its properties and photocatalytic performance in water treatment. For this reason, various methods have been sought to obtain nanoparticles, especially nanostructures. In this sense, various processes have been used to obtain WO3 as a nanostructure, such as hydrothermal synthesis via aging. Using this method, h-WO3 nanolamines with lengths between 30 and 200 nm and thicknesses between 20 and 70 nm were obtained [20]. Porous nanostructures of WO3 have also been developed in their orthorhombic structure with spherical morphology and flower shape by chemical method [20]. Moreover, nanostructures of nanorods, nanospheres, and nanoplates have been prepared via hydrothermal method by using sodium tungstate dihydrate (Na2WO4
2H2O) as the precursor [35]. Similarly, the hydrothermal method was used to obtain prismatic rod and brick nanostructures using Na2WO4
2H2O as the precursor. These nanostructures exhibited high efficiency in decomposition of rhB [36]. The most used method in the preparation of WO3 nanostructures is the hydrothermal one, but it is also possible to obtain this type of morphology via a simpler method such as chemical co-precipitation [37], acidic precipitation [38], and paper-assisted calcination [39], among others. The coprecipitation method was used to synthesize m-WO3 with a nanocube-like morphology with an average size of around 100 nm. This material showed good photocatalytic activity in the degradation of methylene blue (MB). In the same way, WO3 3D architectures of flowers and spheres were synthetized by acidic precipitation of sodium tungstate solution at 450°C for 2 h. The samples presented BET surface area around of 13.1 and 15.6 m2 g 1, respectively. To evaluate the photocatalytic activity, WO3 samples with flower and sphere morphologies were used as photocatalyst in the rhB degradation. The sample of WO3 with spherical morphology exhibited greater photocatalytic activity in degradation of rhB, due to fast diffusion of rhB molecules [37]. A simple synthesis method to obtain nanostructures of WO3 was developed using a paper-assisted calcination method that employed two filter papers and sodium tungstate dihydrate (Na2WO4
2H2O) as precursors. The obtained nanostructures showed good photocatalytic activity in the degradation of MB, which was attributed to BET surface area, band gap, and low PL [39]. Therefore, nanostructures provide materials with large surface area, thus increasing the active sites and making the material more efficient for photocatalytic application in water treatment [20]. In addition, this type of morphology can prevent the recombination of holes and electrons, which enhances photocatalytic performance [36]. In this sense, the photocatalytic properties shown in the degradation of organic pollutants using WO3 hierarchical nanostructures as photocatalysts make these structures potential candidates for use in water treatment [39].

4 Heterostructures of WO3 Great progress has been made in semiconductors for photocatalysis with improved photocatalytic performance, reduced combination processes, which is faster than the transport of charges, causing low photocatalytic efficiency, as well as

216

SECTION

C Oxides and calcogenides

Type I

e

-

Type II

BC e

BC Sem 1

BC Sem 2

BV +

h

-

Sem 1

BV BV

Type III

BC

BC Sem 2

e

-

BC BV +

h

Sem 1

Sem 2

BV +

h

BV FIG. 5 Types of heterostructures of semiconductor materials.

expanding the range of light absorption, particularly in the visible region to take advantage of solar radiation. Scientific advances such as the formation of heterostructures allow for the continual improvement of the photocatalytic efficiencies of materials [40]. Heterostructures are efficient for separating charges from semiconductors and improving their photocatalytic performance. Heterostructures consist of the strategic coupling of semiconductors with different bands. Depending on the bandgap energy and the electronic affinity of the semiconductors, they can be divided into three types, as shown in Fig. 5. In type I heterostructure, the valence band (VB) of semiconductor 1 is smaller than that of semiconductor 2, causing photogenerated charges (h+-e ) to transfer and accumulate in material 2. In the type II junction, the transfer of electrons (e ) occurs from semiconductor 2 to 1, due to the more negative conduction band (CB) position of 2. On the other hand, the holes (h+), can travel in the opposite direction to the more positive 1 to 2 VB, leading to efficient separation of photogenerated charges. Finally, type III is identical to type II, except for a more pronounced difference in the positions of the bands, requiring greater energy to promote the transfer of charges [40–42]. Therefore, type II heterostructure presents a more efficient configuration, favoring the separation of photogenerated charges, efficient use of solar energy, and improved photocatalytic activity in the degradation of organic pollutants for water treatment [40]. Type II heterostructure has attracted attention in the scientific community for the preparation of photocatalytic materials. In this sense, one of the heterostructures that has been made to a greater extent using WO3 is through its union with TiO2 [43–45]. It has been confirmed that a WO3/TiO2 heterostructure has better photocatalytic activity than either WO3 or TiO2 alone. Some heterostructures have been synthesized via precipitation method using different concentrations of WO3 and TiO2; optimal concentration was found to be 95% TiO2/5% WO3. The photocatalytic activity of the heterostructure was evaluated in the degradation of phenol and MB dye. Results showed that it degraded both organic pollutants under 50% visible light radiation, thus achieving a better photocatalytic performance than either of the materials separately [43]. Other TiO2/WO3 heterostructures have been prepared at a mole ratio of 1%, 3%, 5%, 7%, and 10% using a hydrothermal reactor at 180°C for 12 h. The morphology of the heterostructures was spherical and made up of nanoparticles sized 80 nm. The heterostructures were evaluated in the photocatalytic degradation of malachite green under visible light irradiation. The sample with a molar ratio of 3% exhibited the best photocatalytic activity, managing to degrade 99% of the malachite green pollutant in only 60 min. This corroborates the potential of the heterostructures to increase the photocatalytic activity of WO3 [45]. TiO2/WO3 heterostructures have also been synthesized in the form of thin films using the electrospinning technique. The samples were evaluated in the degradation of MO dye and showed good photocatalytic activity. It was possible to degrade 75% of the dye in 135 min [44]. Therefore, heterostructures can be obtained in powder form as well as in the form of thin films, achieving in all cases a better photocatalytic performance than using the separate materials. Not only have heterostructures of WO3 with TiO2 been synthesized to improve the photocatalytic performance of WO3. Several combinations with other compounds have also been reported such as the FeWO4/WO3 heterostructure. This heterostructure was synthesized by a molar ratio of FeWO4 and WO3 of 1%, 0.5%, and 0.25% via hydrothermal method at 180°C for 12 h. In this heterostructure, the FeWO4 particles tend to distribute on the WO3 surface. The heterostructure presented a better response to visible light than FeWO4 and WO3 separated, in the same way it was favored in a larger BET surface area. The 0.5FeWO4/WO3 sample exhibited the highest activity in the degradation of MB and TC, degrading

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both contaminants by 90% during 2 h [46]. Heterostructures of CuO-WO3 synthesized by a drop-casting method with different proportions of CuO particles have also been developed. The sample with a percentage of 0.75% by weight CuO presented the best photocatalytic activity in the degradation of MB, degrading 90% of the dye in 120 min compared to the sample of WO3 in a pure state, which only degraded 49.8% of MB [47].

5 Photocatalytic activity enhancement of WO3 With the main aim of increasing WO3 photocatalytic activity in wastewater treatment, composite materials and heterostructures have been reported in literature. We discuss these studies in this section. A carbon/WO3 nanotube composite was prepared by an in situ method via ultrasound and physical mixing. This process mechanically joins both compounds, and the resulting composite was successfully tested for the photocatalytic degradation of rhB. The carbon/ WO3 composite synthetized in situ showed good performance. This may be attributed to a synergistic effect, due to the increased surface area of the composite and the occurrence of electron transfer from WO3 to carbon nanotubes that reduced charge recombination [48]. Similarly, nanocomposites with WO3 nanorods onto a graphene surface (WO3/GR) were formed by using a hydrothermal method at 180°C for 24 h. It was observed that the graphene sheets inhibited the agglomeration of the WO3 nanorods, causing improvement in photocatalytic activity due to the increase of the specific surface area and the active sites for the photocatalytic process [49]. WO3-ZnO nanocomposites have also been prepared by an alternative ultrasound method that favored an increase in surface area due to the cavitation process, and therefore the photocatalytic activity increased [50]. Not only have composite materials that improve the photocatalytic properties of WO3 been prepared, but so have several heterostructures, as previously discussed. One of these is a heterostructure of WO3 with TiO2 (WO3/TiO2) obtained via solgel method [51]. The heterostructure was evaluated for the photocatalytic degradation of MB. Results showed that it degraded 100% of dye in 120 min. Some heterostructures even add a co-catalyst or are decorated with transition metals to enhance photocatalytic activity to a greater degree, as is the case of WO3/g-C3N4 heterostructures decorated with silver (Ag) nanoparticles [52]. These heterostructures were prepared by a simple method of evaporation and a subsequent heat treatment. Their photocatalytic activity was evaluated for the degradation of rhB and TC, and results showed that it achieved 100% degradation of these pollutants in only 40 and 140 min, respectively. Another type of study, which has been recently developed to improve heterostructures, is to design them under a Zscheme heterojunction. In comparison with the conventional heterostructure, the Z-scheme heterostructure has a higher redox capacity and consequently better efficiency for separating photogenerated charges (electron-hole pairs). WO3/g-C3N4 heterostructures represented by a Z-scheme heterojunction, as shown in Fig. 6, obtained a morphology of WO3 as flake-like particles deposited on irregular sheets of g-C3N4. The heterostructures of WO3/g-C3N4 were prepared via ultrasound at varying amounts of tungsten oxide (1, 3, 5, 7, and 10 wt%). The heterostructures were evaluated in the photodegradation of orange G dye and the antibiotic ciprofloxacin under solar-like irradiation conditions. The sample with 5 wt% WO3 exhibited greater photoactive behavior than the single WO3 and g-C3N4 materials [53]. In addition, an Agl/WO3 heterostructure acts as a Z-scheme photocatalyst obtained by a simple precipitation process. The heterostructure

FIG. 6 Z-scheme heterostructures of WO3/g-C3N4 [53].

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TABLE 1 WO3 composites, methods, and photocatalytic degradation. Heterostructures

Method

Photocatalytic degradation

Reference

WO3/g-C3N4

Thermal treatment

Tetracycline

[55]

WO3-CuS

Hydrothermal

Methyl Blue and Cr(VI)

[2]

Bi2O3-WO3

Hydrothermal

Rhodamine B and tetracycline

[8]

BiVO4/WO3

Hydrothermal

Sulfasalazine

[56]

Cu2O/BiVO4/WO3

Hydrothermal

Sulfasalazine

[57]

NiO-WO3

Hydrothermal

Eosin Yellow

[58]

WO3/g-C3N4/Bi2O3

Thermal treatment

Tetracycline

[59]

GO/WO3 QDs/TiO2

Sonochemical method

Rhodamine B

[60]

WO3/g-C3N4

Hydrothermal

Acid Orange 7

[61]

WO3/MoS2

Hydrothermal

Rhodamine B

[62]

WO3@g-C3N4

Hydrothermal/reflux

Rhodamine B

[63]

WO3/SiO2

Thermal treatment

Methyl Blue and Indigo Carmine

[64]

WO3/SnNb2O6

Hydrothermal

Rhodamine B

[65]

WS2/WO3

Solvothermal

Rhodamine B

[66]

presented a nanoflower morphology and was evaluated for the degradation of TC. The Z-scheme heterostructure degrades 100% of the organic pollutant in 50 min under irradiation of visible light [54]. As such, the Z-scheme heterostructure allows an efficient separation of the photogenerated charge carriers, a decrease in their recombination, and enhanced photocatalytic performance compare to conventional heterostructures.

6

Conclusive remarks and perspectives

In summary, the development of composite materials and heterostructures enhances to a great extent the photocatalytic properties of WO3, making it a promising candidate for water treatment. The high photocatalytic performance shown by the heterostructures is due to the fact that this type of configuration decreases the recombination of photogenerated hole-electron pairs (h+-e ) thus increasing the formation of reactive oxygen species [53]. Furthermore, other factors that can influence the good photocatalytic performance shown by heterostructures is the synergistic effect of both materials and the increase in the surface area of the composite material [48]. Table 1 lists heterostructures and composite materials using WO3 along with the publications in which they are described. This table details the method of obtaining the heterostructure, its application, and photocatalytic efficiency in water treatment. These studies allow for a broader panorama of the potential use of WO3 in ridding water of toxic organic species that damage the planet’s ecosystem.

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Nur Koyuncua, I. capacity/performance from wastewater by adsorption/photocatalysis, J. Water Process Eng. 36 (2020) 101390. [48] T.A. Saleh, V.K. Gupta, Functionalization of tungsten oxide into MWCNT and its application for sunlight-induced degradation of rhodamine B, J. Colloid Interface Sci. 362 (2011) 337–344. [49] X. Hu, P. Xu, H. Gong, G. Yin, Synthesis and characterization of WO3/graphene nanocomposites for enhanced photocatalytic activities by one-step in-situ hydrothermal reaction, Materials 11 (2018) 147. [50] Y.M. Hunge, A.A. Yadav, V.L. Mathe, Ultrasound assisted synthesis of WO3-ZnO nanocomposites for brilliant blue dye degradation, Ultrason. Sonochem. 45 (2018) 116–122. [51] G. Haia, H. Zhang, The photocatalytic applications of TiO2-WO3 heterostructure in methylene blue, Am. Sci. Res. J. Eng. Technol. Sci. 61 (2019) 135–142. [52] J. Chen, X. Xiao, Y. Wang, Z. Ye, Ag nanoparticles decorated WO3/g-C3N4 2D/2D heterostructure with enhanced photocatalytic activity for organic pollutants degradation, Appl. Surf. Sci. 467 (2019) 1000–1010. [53] A.I. Navarro-Aguilar, S. Obregon, D. Sanchez-Martinez, D.B. Hernandez-Uresti, An efficient and stable WO3/g-C3N4 photocatalyst for ciprofloxacin and orange G degradation, J. Photochem. Photobiol. A: Chem. 384 (2019) 112010. [54] L. Zhi, S. Zhang, Y. Xu, J. Tu, M. Li, D. Hu, J. Liu, Controlled growth of AgI nanoparticles on hollow WO3 hierarchical structures to act as Z-scheme photocatalyst for visible-light photocatalysis, J. Colloid Interface Sci. 579 (2020) 754–765. [55] T. Pan, D. Chen, W. Xu, J. Fang, S. Wu, Z. Liu, K. Wu, Z. Fang, Anionic polyacrylamide-assisted construction of thin 2D-2D WO3/g-C3N4 stepscheme heterojunction for enhanced tetracycline degradation under visible light irradiation, J. Hazard. Mater. 393 (2020) 122366. [56] N. Omrania, A. Nezamzadeh-Ejhieh, A comprehensive study on the mechanism pathways and scavenging agents in the photocatalytic activity of BiVO4/WO3 nano-composite, J. Water Process Eng. 33 (2020) 101094. [57] N. Omrani, A. Nezamzadeh-Ejhieh, A comprehensive study on the enhanced photocatalytic activity of Cu2O/BiVO4/WO3 nanoparticles, J. Photochem. Photobiol. A: Chem. 389 (2019) 112223. [58] D.R. Rosaline, S.S.R. Inbanathan, A. Suganthi, M. Rajarajan, G. Kavitha, R. Srinivasan, H.H. Hegazy, A. Umar, H. Algarni, E. Manikandan, Visiblelight driven photocatalytic degradation of eosin yellow (EY) dye based on NiO-WO3 nanoparticles, J. Nanosci. Nanotechnol. 20 (2020) 924–933. [59] L. Jiang, X. Yuan, G. Zeng, J. Liang, X. Chen, H. Yu, H. Wang, Z. Wu, J. Zhang, T. Xiong, In-situ synthesis of direct solid-state dual Z-scheme WO3/ g-C3N4/Bi2O3 photocatalyst for the degradation of refractory pollutant, Appl. Catal. B: Environ. 5 (2018) 376–385. [60] C. Wanga, S. Luoa, C. Liub, C. Chen, WO3 quantum dots enhanced the photocatalytic performances of graphene oxide/TiO2 films under flowing dye solution, Inorg. Chem. Commun. 115 (2020) 107875. [61] A. Priya, R.A. Senthil, A. Selvi, P. Arunachalam, C.K. Senthil, J. Madhavan, R. Boddula, R. Pothu, A. Al-Mayouf, A study of photocatalytic and photoelectrochemical activity of as-synthesized WO3/g-C3N4 composite photocatalysts for AO7 degradation, Mater. Sci. Energy Technol. 3 (2020) 43–50. [62] G. Li, J. Hou, W. Zhang, P. Li, G. Liu, Y. Wang, K. Wang, Graphene-bridged WO3/MoS2 Z-scheme photocatalyst for enhanced photodegradation under visible light irradiation, Mater. Chem. Phys. 246 (2020) 122827. [63] S. Borthakur, P. Basyach, L. Kalita, K. Sonowal, A. Tiwari, P. Chetia, L. Saikia, Sun light assisted degradation of pollutant dye in water by WO3@gC3N4 nanocomposite catalyst, New J. Chem. 44 (2020) 2947–2960. [64] S. Sharma, S. Basu, Highly reusable visible light active hierarchical porous WO3/SiO2 monolith in centimeter length scale for enhanced photocatalytic degradation of toxic pollutants, Sep. Purif. Technol. 231 (2020) 115916. [65] X. Ma, W. Ma, D. Jiang, D. Li, S. Meng, M. Chen, Construction of novel WO3/SnNb2O6 hybrid nanosheet heterojunctions as efficient Z-scheme photocatalysts for pollutant degradation, J. Colloid Interface Sci. 506 (2017) 93–101. [66] G. Li, Y. Wang, J. Bi, X. Huang, Y. Mao, L. Luo, H. Hao, Partial oxidation strategy to synthesize WS2/WO3 heterostructure with enhanced adsorption performance for organic dyes: synthesis, modelling and mechanism, Nanomaterials 10 (2020) 278.

Chapter 14

Photoactive systems based on semiconducting metal oxides Maria Cristina Paganini and Erik Cerrato Department of Chemistry and NIS Centre, University of Torino, Torino, Italy

1 Introduction Among the materials employed in photocatalytic applications, transition metal-based semiconducting oxides play a paramount role. These materials are characterized by a band structure in which the valence band (VB), made up by the overlap of 2p orbitals of O2
, is fully occupied and the empty conduction band (CB), based on the d and s states of the metal ion, are energetically separated by a forbidden gap of some electron volts also called as “band gap” [1]. The systems employed in photocatalysis are usually n-type semiconductors. This physical property is due to the presence of intrinsic defects (such as vacancies) or chemical impurities that create shallow donor states whose electrons are easily thermalized in the conduction band. The well-established mechanism of heterogeneous photocatalysis depends on the electronic structure of the material and is based on the promotion of electrons from the VB to the CB occurring when photons of suitable energy impinge the solid. This phenomenon of charge separation implies the formation of mobile holes (h+) in the valence band and mobile electrons (e
) in the conduction band. The charge carriers that, moving in the crystal, reach the surface escaping recombination are potentially capable to entail reductive (e
) and oxidative (h+) redox processes, respectively. The occurrence of a surface redox process depends, as a first approximation, on the interplay between the (electro) chemical potentials of the carriers in the bands and those of the redox pairs in the fluid phase in contact with the solid. Typical reductive processes are, for instance, the reduction of hydrogen ions to H2 or that of carbon dioxide, both included in the challenging area of artificial photosynthesis. At variance, for reactions in the field of environmental photocatalysis, the main reduction process concern O2 that forms superoxide (O2
), a typical reactive oxygen species, or ROS, while the holes in the valence band oxidize water molecules forming OH radicals, another oxidative ROS, able to mineralize organic molecules present in various systems such as, for instance, wastewater [2–4], as sketched in Fig. 1. The main factors limiting the performance of a heterogeneous photocatalyst based on a semiconducting oxide are two. The former is the recombination of charge carriers that limits the quantum yield of the process reducing the fraction of photonic energy converted into chemical energy. The second is related to the band gap amplitude of the solid. The most common semiconducting oxides for photocatalysis have, in fact, a band gap value corresponding to photons in the range of the ultraviolet radiation making necessary the use of these high-energy photons in order to perform charge separation. This causes a severe problem for large-scale applications aiming to use sunlight in photocatalysis since the fraction of UV photons in solar radiation at the earth’s surface is 5% only [5]. The obvious expedient of selecting photocatalysts with band gap corresponding to visible frequencies implicate, as a consequence, a net variation of the electrochemical potential of the bands compromising the redox ability of the system. Different strategies have been developed to overcome this important shortcoming. The first one consists of engineering the electronic and optical properties of the oxide by doping, i.e., by the deliberate insertion of a foreign atom into the lattice, with the formation of novel states inside the band gap, allowing charge separation with energy lower than the pristine material band gap. Fig. 2 depicts how the doping procedure can modify the energy gap for n-doped and p-doped semiconductors, referred to as the energy Fermi level (EF, defined as the level at highest energy occupied at T ¼ 0 K). Another approach consists of coupling two distinct semiconductors with the formation of active interfaces or heterojunctions. A heterojunction is defined as the surface of contact between two semiconductors resulting in a particular band alignment [6, 7] that depends on the band potentials and bandwidths of each semiconductor. Fig. 3 reports three distinct cases of band alignment. The best performances have been observed in the case of type-II heterojunction, in which the 



Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00018-0 Copyright © 2021 Elsevier Inc. All rights reserved.

221

222 SECTION C Oxides and calcogenides

FIG. 1 Schematic representation of charge carriers separation in a semiconductor and the possible photocatalytic reaction entailed.

FIG. 2 Schematic representation of the energy diagram change due to different doping procedures.

FIG. 3 Schematic representation of the possible bands’ diagram occurring at semiconductor interfaces [6].

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FIG. 4 Interface charge carriers’ migration. Comparison between direct Z-scheme or S-scheme and type-II heterojunction.

band gaps of the two semiconductors are staggered. With this approach the charge carrier’s separation is improved since electrons and holes are stabilized on distinct materials, thus preventing direct recombination. It has to be noticed, however, that in type-II heterojunctions (that is inspired to the Z-scheme of natural photosynthesis) the prospective redox ability of photogenerated carriers is attenuated since, after excitation, electrons move to a less negative potential and holes to a highest positive one (Fig. 3, center). Recently, heterojunctions with a new type of scheme involving an alternative flow of the carriers at the interface have been developed, indicated as “direct Z-scheme or Sscheme” (Fig. 4). In this particular case, the bands are staggered as in the case of scheme II but a particular work function (indicated as W1 and W2 in Fig. 4) of the two materials (indicated as PC-I and PC-II in Fig. 4) creates a peculiar electric field (displayed as E in Fig. 4) at the interface causing the selective recombination of electrons and holes shown in Fig. 4. The spatial separation of the two photo-induced carriers is maintained but, differently from the case of type-II heterojunction, in this case, the most favorable redox potentials present in the system are exploited [8, 9]. All the efforts to obtain novel and more efficient photocatalytic systems have to be accompanied by a deeper insight into the mechanisms of photocatalytic processes. To this aim the systematic use of advanced physical techniques of characterization is mandatory. In this chapter, we will illustrate the application of electron paramagnetic resonance (EPR) in investigating the properties of photocatalytic materials active under irradiation of visible light. EPR is a particularly efficient technique in the characterization of heterogeneous photocatalysts for more than one reason. First, EPR is a quite sensitive technique allowing the detection of small amounts of paramagnetic entities. In photocatalytic phenomena, based on the separation and reactivity of single charge carriers, such species bearing unpaired electrons are extremely common. Last but not least EPR has an additional feature consisting of the possibility of recording spectra in the dark or under illumination with frequencies typical of photocatalytic reactions (UV-vis), since there is no interference between the latter and the microwaves inducing magnetic resonance. This opportunity allows to investigate either the ground state or the excited states of the systems and it also makes possible to perform a sort of “operando” spectroscopy monitoring the state of the system during a photocatalytic process.

2 Photocatalytic phenomena explained via electron paramagnetic resonance technique When a metal oxide is irradiated with light having suitable energy, charge separation occurs in the solid, according to the following Eq. (1) MeOX + hu ! e
ðCBÞ + h + ðVBÞ

(1)

224 SECTION C Oxides and calcogenides

When the irradiation is performed under vacuum the photoinduced charge carriers migrate in the crystal and a fraction of them can be stabilized at specific sites in the solid provided, that the temperature is low enough to prevent electron-hole recombination. In such a case it is often possible to monitor by EPR, the paramagnetic entities resulting from the described process. Photo-excited electrons are generally stabilized on metal cations, Eq. (2), forming, in most cases, shallow donor energetic levels just below the conduction band edge, Men + + e
! Men
1

(2)




On the other hand, the photoinduced holes are stabilized at the oxygen ions of the lattice (O2 ), producing the paramagnetic species O
, Eq. (3): 

O2
+ h + ðVBÞ ! O




(3)

The g tensor of the O
ion (2px 2 ,2py 2 , 2pz 1 ), has been reported by Brailsford. It shows, in general, a rhombic symmetry that, neglecting second-order terms, becomes Eq. (4): gzz  ge ;gxx ¼ ge + 2l=DE1, gyy ¼ ge + 2l=DE2

(4)

where ge is the free spin value (2.0023), l is the spin-orbit coupling constant, and DE1 and DE2 are the energy differences relating to the separation between 2pz and the other two p-orbitals caused by the crystal field effect as shown in the scheme of Fig. 5 [10–13]. The EPR signals of trapped electrons and trapped holes generally fall in distinct regions of the EPR spectrum, making easier the analysis of their stabilization [14]. A further aspect of the EPR approach to the oxide photochemistry is the investigation of the interfacial reactivity of photogenerated charge carriers. In these cases, irradiation is performed in the presence of a fluid phase (gas or liquid) in contact with the solid. In such a way, the photo-induced charge carrier may react at the surface with molecules acting as scavengers of electrons and holes, respectively. In general, if a surface electron transfer occurs, this leads to new paramagnetic entities that are monitored by EPR. The occurrence of this surface reactivity is, as a first approximation, the fingerprint of mobility capable to lead the reactive charge at the surface, a preliminary condition for the occurrence of photocatalytic reactions. To evaluate the presence of photogenerated electrons at the surface, irradiation in the O2 atmosphere is performed leading to the formation of adsorbed superoxide radical anions (O2
)according to the following Eq. (5): 

O2ðgasÞ + e
ðCBÞ ! O2
ðadsÞ 

(5)

∗

The O2
radical is characterized by an unpaired electron in a 2pp antibonding orbital, giving rise to a rhombic EPR signal with the following components in Eqs. (6)–(8) [15, 16]: 

gxx ¼ ge

(6)

gyy ¼ ge + 2l/E (7) gzz ¼ ge + 2l=D

FIG. 5 Crystal field effects on a: (A) O
radical ion, (B) O2
radical ion. 



(8)

Photoactive systems based on semiconducting metal oxides Chapter

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225

where D is the separation between the two 2pp* orbitals of the absorbed superoxide species due to the electric field of the adsorption cationic site and E is the separation between the highest of the two 2pp∗ orbitals and the 2ps one. Considering that D is much smaller than E, the value of gzz results to be very sensitive to the cation crystal field and is therefore diagnostic of the nature of the surface site absorbing the O2
ions [15]. The photoinduced holes can be stabilized at the solid surface by the oxide ions of the solid producing O
ions (Eq. 9) 



h + ðVBÞ + O2
ðsurf Þ ! O
ðsurf Þ 



(9)

The formation of surface holes can be verified by irradiation in presence of H2. This molecule reacts with O
undergoing a homolytic scission and generating reactive hydrogen atoms (Eq. 10) capable of injecting electrons into the material (Eq. 11) that are usually stabilized at cationic sites in the lattice [17]. 

O
ðsurf Þ + H2 ! OH
ðsurf Þ + H 



(10)

H + O2
ðsurf Þ ! OH
ðsurf Þ + e




(11)

In the case of heterogeneous photocatalytic reactions in aqueous media, ROS are produced upon irradiation at the liquidsolid interface. The most important of these reactive species is the hydroxyl radical (OH ) produced by photo-induced holes (Eq. 9) reacting with H2O, Eq. (12), if the valence band (electro) chemical potential is more positive than that of water oxidation. 

h + ðVBÞ + H2 O ! OH + H + 

(12)

Usually, the lifetime of OH radicals is too small to allow their direct detection by EPR. This issue can be bypassed using the spin trapping technique based on the reaction of the radical with a diamagnetic molecule (the trap) producing a stable paramagnetic adduct easily detected by EPR [18]. The most common spin trapping agents are aliphatic or cyclic nitrones such as the 5.5-dimethyl-1-pyrroline N-oxide, also called DMPO, that reacting with OH radicals, forms the stable paramagnetic adduct shown in Eq. (13). 

ð13Þ



The DMPO-OH paramagnetic adduct is characterized by the well-known EPR spectrum reported in Fig. 6, whose structure is due to the hyperfine interaction of the unpaired electron with both 14N (I ¼ 1) and 1H (I ¼ 1/2) nuclei. To conclude, the EPR technique represents an efficient and valuable tool for the characterization of heterogeneous photocatalysts and, in particular, to perform preliminary screenings of the potentiality of new materials. In particular, EPR provides information on the basic photo-physical and photochemical properties of a given material. In the following of this chapter, we will illustrate the role of EPR to characterize visible light active (VLA) photocatalytic systems made up either by doped semiconducting oxides or by interfaced solid-solid systems.

330

331

332

333

334

B[mT] 

FIG. 6 EPR spectrum of DMPO-OH adduct.

335

336

337

226 SECTION C Oxides and calcogenides

2.1 N-TiO2 Titanium dioxide, despite a band gap value corresponding to photons of the UV region, has been for many years the benchmark material for photocatalytic applications [19]. To extend its adsorption to the visible frequencies, much more abundant in the solar radiation, strategies based on doping the material have been proposed using either transition metal ions or, more recently, nonmetallic elements such as carbon, sulfur, and, with particular success, nitrogen. The first paper reporting the introduction of nitrogen species in TiO2 was published in 1986 [20] and deals with the improved adsorption in the visible of N-doped TiO2 but only after 15 years, it was released that this yellowish solid is active in photocatalytic reactions under visible light [20]. N-doped titania is usually prepared by sol-gel synthesis or by other wet-chemistry methods introducing a nitrogen-containing substance in the reactant’s mixture. The onset of photocatalytic activity under visible light was explained in terms of the presence of some nitrogencontaining species in the bulk or at the surface of titania. An intense debate on the nature and structure of these centers started after the publications of the mentioned report characterized, [21] inter alia, by substantial contributions of EPR results, often coupled with DFT computational calculations. The optical absorption of a typical N-TiO2 system is shown in Fig. 6, panel A, which compares the UV-vis diffuse reflectance spectroscopy (DRS) spectra of pristine and of nitrogen-doped TiO2 (anatase polymorph). Bare TiO2 is characterized by the typical band gap transition at wavelengths lower than 400 nm while the nitrogen modified material shows an additional shoulder in the visible region, centered around 440 nm that explains its color. In parallel to this additional optical absorption, it was found that doped TiO2, reported in panel B of Fig. 7, is also characterized by an EPR signal due to a previously unreported species containing nitrogen. The signal (Fig. 7, panel B) is based on a hyperfine triplet indicating that the unpaired electron interacts with a single nucleus of 14N (nuclear spin I ¼ 1) and the corresponding species is photoactive since its intensity varies under irradiation. In particular, this occurs under the blue light of about 440 nm corresponding to the maximum absorption in the visible. These EPR results, in strict connection with DFT calculations, have been crucial to understand the nature and the features of the nitrogen defect. Actually, two types of photoactive centers have been described for N-TiO2. The first one is based on substitutional nitrogen (N substitutes O in some lattice sites) and the second, more interesting from a photocatalysis standpoint, is preferentially formed using wet-chemistry methods. It is based on an N atom positioned in an interstitial site of the structure and linked to a lattice O2
thus forming a charged NO group in the bulk of the solid (formally NO 2
). Besides this paramagnetic species, a diamagnetic one with two electrons in the HOMO is also present, which can be formally written as NO3
. Both these chemical species correspond to intraband gap centers located few tenths of eV above the valence band edge that, as already mentioned, are responsible for the optical absorption in the visible of the doped system [22–24]. Fig. 8 illustrates the photoactivity of the system under irradiation with monochromatic visible light (l ¼ 440 nm, blue) in oxygen atmosphere. In such conditions, the EPR spectrum of a superoxide species absorbed at the surface on Ti4+ (gzz ¼ 2.028-2.023, gyy ¼ 2.009 and gxx ¼ 2.003) shows up (Fig. 8B) overlapping the signal of NO 2
(Fig. 8A) [22]. The formation of O2
can be ascribed to the photo-excitation of electrons from the diamagnetic NO3
centers to the TiO2 conduction band (Eq. 14) with successive electron transfer to O2 (Eq. 6). The additional NO 2
species generated by irradiation can be interpreted as a hole trapped at NO3
sites 







FIG. 7 Panel A: UV-vis DRS spectroscopy reflectance spectra of TiO2 (dash-dot line) and N-TiO2 (solid line). Panel B: EPR spectrum of N-TiO2 before and after irradiation with monochromatic light at l ¼ 440 nm in vacuum.

Photoactive systems based on semiconducting metal oxides Chapter

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227

(NO2-).

a)

b)

(NO2-)+ O -2

340

345

350

355

360

B [mT] FIG. 8 EPR spectra (77 K) of N-TiO2 (a) in the dark; (b) after 1 h of monochromatic visible irradiation (l ¼ 440 nm) in O2. (Adopted from G. Barolo, S. Livraghi, M. Chiesa, M.C. Paganini, E. Giamello, Mechanism of the photoactivity under visible light of N-doped titanium dioxide. Charge carriers migration in irradiated N-TiO2 investigated by electron paramagnetic resonance, J. Phys. Chem. C 116 (2012) 20887–20894.)

NO3
+ huðl¼437 nmÞ ! NO 2
+ e
ðCBÞ 

(14)

The formation of superoxide does not occur by irradiating pristine TiO2 under O2 with the same type of light. Fig. 9 displays the intensity decrease of the NO 2
signal upon NIR irradiation confirming occurrence of the photoexcitation from VB to the intraband gap NO 2
that is converted in the diamagnetic species NO3
. The following Eq. (15) summarizes the phenomenon [25]: 



e
ðVBÞ + huðl¼1550 nmÞ + NO 2
ðparamagneticÞ ! h + ðVBÞ + NO3
ðdiamagneticÞ 

(15)

The effect of the processes in Eqs. (14), (15) explains a possible mechanism of electron excitation from the VB to the CB by the combined effect of both NIR and visible photons. This mechanism, confirms that N-TiO2 is a true VLA system, has been unraveled by the joint use of EPR and specific monochromatic irradiations (Fig. 9 right side).

2.2 Ce-ZrO2 The case of cerium doped zirconium dioxide represents an attempt to use a large band gap semiconductor (ZrO2, Eg ¼ 5 eV, Fig. 10A) in photocatalysis in order to exploit its favorable potentials for reductive and oxidative processes, using however low energy photons in the process. This has been possible to introduce Ce ions in the structure whose 4f empty levels act as a 2-

.

(NO )

a

b

330

335

340

345

B [mT] FIG. 9 Left: EPR spectra (77 K) of N-TiO2: (a) in the dark; (b) during monochromatic IR light (l ¼ 1550 nm) in vacuum. Right: Schematic representation of the photoexcitation mechanisms observed in N-TiO2 upon visible (437 nm) and IR (1550 nm) light irradiation. (Adopted from N. Serpone, A.V. Emeline, Semiconductor photocatalysis—past, present, and future outlook, J. Phys. Chem. Lett.3 (2012) 673–677.)

228 SECTION C Oxides and calcogenides

FIG. 10 Relationship between the band structure of ZrO2 and TiO2 and the redox potential of water splitting.

bridge to convey electrons from the VB to the CB using visible light. The resulting scheme reproduces a forecast by Emeline and Serpone who proposed a “third-generation” photocatalysts with this structure (Fig. 10B) [26, 27]. The addition of 0.5% moles of cerium ions (Ce4+) to ZrO2 is capable to completely overturn the optical and electronic properties of the bare oxide, converting it from a purely UV light-sensitive material into a VLA system. The choice of rareearth elements as doping source provides a unique opportunity, since their chemistry differs from that of the d-block metals. In particular, the 4f orbitals, shielded by the more external 5s2 and 5p6 orbitals, don’t directly take part in the bonding and form rather localized electronic states (see below). The UV-vis spectra of Ce-doped and bare zirconia are compared in Fig. 11. While the band gap transition of the pure oxide falls at 244 nm (5.08 eV), in the case of the doped material the absorption experiences a substantial red-shift with a weak absorption tail in the visible region. To better understand the effects of such weak visible absorption, EPR spectra have been recorded under irradiation with visible light having l 420 nm at 77 K and keeping the material under vacuum (Fig. 12) [28]. The sensitivity to visible light of the system shows up in terms of the formation of both trapped electrons and trapped holes EPR signals in such conditions. Electrons are trapped by Zr4+ ions that are reduced to Zr3+ (axial signal at high field with gk ¼ 1.977 and g┴ ¼ 1.959) while trapped holes show the typical O
signal at low field (Fig. 12) [29, 30]. This phenomenon has been interpreted, with the support of DFT calculations, in terms of the formation of mid-band gap Ce4+ 4f empty states. These permit first step in the excitation conveying electrons from the valence band to the 4f states with low energy photons (about 2.5 eV). These excited electrons are, most likely, further promoted by a second photon to the conduction band of the system and stabilized at suitable Zr4+ sites [28]. 

1 ZrO2 Ce-ZrO2

K.M. F(R)

FIG. 11 Normalized UV-vis absorption spectra of ZrO2 and Ce-ZrO2, 0.5% by moles.

300

400

500

0 200

250

300

350

400

λ [nm]

450

500

550

600

Photoactive systems based on semiconducting metal oxides Chapter

14

229

3+

Zr a)

O

b)

310

320

-

330

340

350

360

B [mT] FIG. 12 EPR spectra of Ce-ZrO2 (0.5% cerium by moles) recorded at 77 K under vacuum: (a) dark (the material already contains trace of Zr3+ defects); (b) irradiation having with polychromatic light l 420 nm.

Like in the case of N-TiO2, to verify the presence of reactive photo-excited carriers at the surface visible irradiations have been performed in the presence of either hole or of electron scavengers (H2 and O2, respectively) according to the scheme illustrated in Section 3. As shown in Fig. 13, visible light irradiation in oxygen atmosphere causes the formation of adsorbed superoxide according to Eq. (5) (Section 3). The same experiment performed in hydrogen led to the formation of a further amount of Zr3+ ions (traces of Zr3+ defects are always present in the starting material) according to the complex process described in Eqs. (10), (11) ending up with the injection of an electron in the solid which is trapped by a Zr4+ ion. In short, the experiments illustrated in Fig. 13 firmly point to the migration of reactive electrons and holes at the surface of Ce-ZrO2 irradiated with polychromatic visible light. Despite the relatively weak concentration of photogenerated surface charge carriers, which could actually limit the photocatalytic activity of the system, this result demonstrates that the insertion of Ce ions in ZrO2 generates a VLA system which has shown appreciable photocatalytic activity in some practical applications of photocatalysis performed under visible irradiation [31–33]. DFT calculations corroborated these experimental results clarifying that the visible photo-sensibility of Ce-doped ZrO2 is due to the incorporation of isovalent Ce4+ in the oxide lattice producing a series of unoccupied highly-localized 4f levels in the middle of ZrO2 band gap. As stated, before these very levels are the bridge that allows the double electron transition described in Fig. 14 and reproducing the scheme proposed by Emeline and Serpone for photocatalysts of the third generation.

Zr

3+

a)

x2

b)

-

O2 c)

325

330

335

340

345

350

355

B [mT] FIG. 13 EPR spectra of Ce-ZrO2 upon irradiation at 77 K with light having l > 420 nm: (a) background in dark, (b) in O2 atmosphere, (c) in H2 atmosphere.

230 SECTION C Oxides and calcogenides

FIG. 14 Two-step visible photons absorption mechanism of Ce-ZrO2.

2.3 CeO2-ZnO ZnO is a semiconducting oxide often considered in the past for photocatalytic applications despite the fact that its band gap value (3.4 eV) requires the use of a UV light source for irradiation. At variance with the previous case, there is no affinity between ZnO and CeO2 that differ in terms of structure (wurtzite for ZnO and fluorite for CeO2), ionic charge, and size. The result of the combination of these two oxides is the formation of a heterogeneous system with the dispersion of ceria nanocrystals on ZnO and the formation of a solid-solid heterojunction between the two components. Samples with ceria loading ranging between 1% and 10% have been prepared and tested by the described EPR-based procedure to preliminary investigate their photochemical behavior [34–36]. TEM images, in Fig. 15 show the presence of clustered colonies of ceria nanoparticles anchored at ZnO surfaces, confirming the presence of a biphasic system rather than a doped one. EPR spectroscopy has been crucial in unraveling the VLA properties of the CeO2/ZnO interface. Fig. 16 shows the spectrum of this hybrid system (Ce loading 1% by moles) recorded in the dark and showing the trace of tiny amounts of defects already present in the as-prepared material. These are due to trapped-electron defects (g ¼ 1.96, high field) corresponding to shallow electron donors always present in ZnO and likely generated by interstitial hydrogen impurities [37, 38] and to trapped holes (O
ions) visible in the low field portion of the EPR spectrum (Fig. 16A), respectively [10]. In spectrum (B), recorded upon irradiation with polychromatic light having l 420 nm under vacuum, both the trapped electron and trapped hole signals already present in the background markedly increase their intensity showing that charge separation occurs in the system by the action of visible photons. The intensity increase of the trapped hole signal (Fig. 16B) is however higher than that of trapped electrons with g ¼ 1.96. This is caused by the fact that a fraction of the photoexcited electrons have been trapped by the cerium dioxide component forming reduced Ce3+ ions. These are paramagnetic ions ([Xe]4f1 configuration) however EPR silent since, due to the large value of the spin-orbit coupling constant of the Ce atom, they are not visible in the conditions of the reported experiment (CW-EPR, 77 K). A fraction of the photoformed electrons, therefore, escapes detection so explaining the described intensity imbalance between the two signals in Fig. 16 [36]. Coupling EPR data and DFT calculation, it has been possible to establish that the CeO2-ZnO interface is a true visible-light-active heterojunction whose presence explains the reported results. Cerium dioxide is 

FIG. 15 TEM image of CeO2-ZnO.

Photoactive systems based on semiconducting metal oxides Chapter

O

14

231

-

a)

b)

g = 1.96 trapped electrons

330

335

340 B [mT]

345

350

FIG. 16 EPR spectra at 77 K of CeO2-ZnO in vacuum (a) dark and (b) after irradiation with l > 420 nm.

formally classified as insulator since the gap between the valence band and the conduction band is about 6 eV; however empty, localized, 4f levels of cerium are found almost 2.9 eV above the VB edge [39]. Theoretical calculations have shown that the VB of the two oxides have a very similar flat band potential (Fig. 17). The 4f levels of cerium, in the CeO2-ZnO heterojunction, are thus located few tenths of eV below the ZnO conduction band limit. This allows an easy electronic transition from the conduction band of ZnO to the empty, localized, 4f states of Ce4+, forming partially reduced, EPR silent Ce3+ ions. The partial migration of photoexcited electrons on Ceria particles improves the spatial separation of the carriers thus favoring the photocatalytic effects. As in the previously reported case, the surface activity of photo-promoted electrons and holes has been evaluated performing the visible irradiation under molecular oxygen and hydrogen atmospheres, respectively, with the results reported in Fig. 18, panel A and B. The formation of O2
species absorbed on Zn2+ (gzz ¼ 2.049-2.055, gyy ¼ 2.009 and gxx ¼ 2.003 [15, 40, 41] and on Ce4+ (g ¼ 2.035 and gk ¼ 2.011 [42], confirms that the electrons, photo-excited by visible photons are able to migrate at the surface and to interact with both components of the hybrid system. Also, in the case of irradiation under hydrogen, the result is extremely clear-cut since practically no trace of the trapped hole observed after irradiation under vacuum (Fig. 17) are now detected and only the signal at g ¼ 1.96 growths in intensity to indicate the high efficiency of the surface reactivity of the photoformed holes according to the process described in Eqs. (10), (11). The reactivity and the photo-oxidative ability of holes generated by visible light have been monitored also at the solidliquid interface using the spin trapping technique, i.e., irradiating a CeO2-ZnO aqueous suspension in presence of DMPO (see Eq. 12). Fig. 19 reports the EPR trace of the DMPO-OH adduct obtained by irradiation showing the propensity of the CeO2-ZnO mixed material to form OH radicals by the action of light having l 420 nm. The comparison with the same experiment performed using bare ZnO unambiguously clarifies this effect. In conclusion, also for ZnO, the addition of a small amount of cerium gives rise to the formation of a photo-sensible material, where the intimate contact between the phases CeO2 and ZnO is the cause of the visible light activity. The 





FIG. 17 Schematic diagram of the final CeO2-ZnO band alignment and the possible photoexcitation of the composite system. Values are reported in eV.

232 SECTION C Oxides and calcogenides

A

a)

. O on Zn -

gzz = 2,055

B

a)

2+

2

b)

b)

gzz = 2,032

c) O

325

2

.- on Ce4+

330

335

340

B[mT]

345

350

330

335

340 B [mT]

345

350

FIG. 18 Panel A: EPR spectra at 77 K of CeO2-ZnO: (a) background, spectrum in dark, (b) irradiation with visible light (l > 420 nm) in the presence of molecular oxygen, (c) computer simulation of the whole spectrum (b). Panel B: EPR spectra at 77 K of CeO2-ZnO: (a) background, spectrum in dark, (b) irradiation with visible light (l > 420 nm) in the presence of molecular hydrogen.



FIG. 19 EPR spectra of the DMPO/OH adduct produced by irradiation of an aqueous suspension of the CeO2-ZnO and bare ZnO materials upon visible light (l 420 nm).

presence of CeO2 and especially of the empty, localized 4f states allows a beneficial charge carrier separation, with a fraction of photo-excited electrons stabilized on the defective cerium states. The described system, after the promising preliminary tests described above, was employed with good results in photocatalytic reactions for the abatement of emerging pollutants such as acesulfame-K, iopamidol, and bisphenol-A [43–45].

3

Conclusions

The present chapter has been devoted to describe the role played by electron paramagnetic resonance to understand the photochemical and photophysical phenomena related to photocatalysis. This magnetic technique is extremely useful in performing preliminary tests aimed to verify the potentiality of newly produced photoactive systems. This is done by monitoring the photoinduced separation of charge carriers and their migration and reactivity at the surface in various conditions of irradiation. Particular attention has been paid to systems based on semiconducting oxides modified by doping in order to obtain visible light active (VLA) systems. The modifications considered consists of the insertion of heteroatoms in the oxide lattice-like in the case of the well-known nitrogen-doped titania (N-TiO2) or in that of Ce-ZrO2 forming intraband gap states that play a key role in promoting excited electrons to the conduction band. It has also been shown that similar concepts apply in the case of a hybrid system made up of immiscible oxide like CeO2 and ZnO that, however, form a particular solid-solid interface, or heterojunction, showing photoactivity under visible light similar to that observed in the case of doped materials.

Photoactive systems based on semiconducting metal oxides Chapter

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233

Acknowledgments Financial support from the Italian MIUR through the PRIN Project 20179337R7, MULTI-e “Multielectron transfer for the conversion of small molecules: an enabling technology for the chemical use of renewable energy” and the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant Agreement No. 765860 (AQUAlity) are gratefully acknowledged.

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234 SECTION C Oxides and calcogenides

[31] C. Gionco, M.C. Paganini, E. Giamello, O. Sacco, V. Vaiano, D. Sannino, Rare earth oxides in zirconium dioxide: how to turn a wide band gap metal oxide into a visible light active photocatalyst, J. Energy Chem. 26 (2016) 270–276. [32] C. Gionco, S. Herna´ndez, M. Castellino, T.A. Gadhi, J.A. Mun˜oz-Tabares, E. Cerrato, A. Tagliaferro, N. Russo, M.C. Paganini, Synthesis and characterization of Ce and Er doped ZrO2 nanoparticles as solar light driven photocatalysts, J. Alloys Compd. 775 (2019) 896–904. [33] F.E. Bortot Coelho, C. Gionco, M.C. Paganini, P. Calza, G. Magnacca, Control of membrane fouling in organics filtration using Ce-doped zirconia and visible light, Nanomaterials 9 (2019). [34] E. Cerrato, C. Gionco, M.C. Paganini, E. Giamello, Photoactivity properties of ZnO doped with cerium ions: an EPR study, J. Phys. Condens. Matter 29 (2017) 1–7. [35] E. Cerrato, C. Gionco, I. Berruti, F. Sordello, P. Calza, M.C. Paganini, Rare earth ions doped ZnO: synthesis, characterization and preliminary photoactivity assessment, J. Solid State Chem. 264 (2018) 42–47. [36] E. Cerrato, C. Gionco, M.C. Paganini, E. Giamello, E. Albanese, G. Pacchioni, Origin of visible light photoactivity of the CeO2/ZnO heterojunction, ACS Appl. Energy Mater. 1 (2018) 4247–4260. [37] E. Cerrato, M.C. Paganini, E. Giamello, Photoactivity under visible light of defective ZnO investigated by EPR spectroscopy and photoluminescence, J. Photochem. Photobiol. A 397 (2020) 112531. [38] A. Janotti, C.G. Van de Walle, New insights into the role of native point defects in ZnO, J. Cryst. Growth 287 (2006) 58–65. [39] E. Wuilloud, B. Delley, D. Schneider, Y. Baer, Spectroscopic evidence for localized and extended f-symmetry states in CeO2, Phys. Rev. Lett. 53 (1984) 202–205. [40] M. Iwamoto, Y. Yoda, N. Yamazoe, T. Seiyama, Study of metal oxide catalysts by temperature programmed desorption. Oxygen adsorption on various metal oxides, J. Phys. Chem. 82 (1978) 2564–2570. [41] R.D. Iyengar, V.V.S. Rao, A.C. Zettlemoyer, ESR studies of the interaction of O2, NO2, N2O, NO and Cl2 with zinc oxide, Surf. Sci. 13 (1969) 251–262. [42] M. Che, J.F.J. Kibblewhite, A.J. Tench, M. Dufaux, C. Naccache, Oxygen species adsorbed on CeO2/SiO2 supported catalysts, J. Chem. Soc. Faraday Trans. 69 (1973) 857–863. [43] P. Calza, C. Gionco, M. Giletta, M. Kalaboka, V.A. Sakkas, T. Albanis, M.C. Paganini, Assessment of the abatement of acelsulfame K using cerium doped ZnO as photocatalyst, J. Hazard. Mater. 323 (2016) 471–477. [44] M.C. Paganini, D. Dalmasso, C. Gionco, V. Polliotto, L. Mantilleri, P. Calza, Beyond TiO2: cerium-doped zinc oxide as a new photocatalyst for the photodegradation of persistent pollutants, Chem. Select 1 (2016) 3377–3383. [45] O. Bechambi, L. Jlaiel, W. Najjar, S. Sayadi, Photocatalytic degradation of bisphenol A in the presence of Ce-ZnO: evolution of kinetics, toxicity and photodegradation mechanism, Mater. Chem. Phys. 173 (2016) 95–105.

Chapter 15

Iron oxide-based magnetic photocatalysts: Recent developments, challenges, and environmental applications Zahra Abbasia and Elisa I. Garcı´a-Lo´pezb a

Department of Chemistry, Faculty of Science, Shahid Chamran University of Ahvaz, Ahvaz, Iran, b Department of Biological, Chemical and

Pharmaceutical Sciences and Technologies (STEBICEF), University of Palermo, Palermo, Italy

1 Introduction The design of photocatalysts with high activity in the visible region and the ability to be easily separated and recycled from the reacting solution has focused the attention of many researchers. Conventional methods of photocatalysts separation from solution, such as centrifugation or filtration, decrease the photocatalysts amount and performance, moreover, some separation methods may lead to secondary contamination [1]. Effective separation of photocatalytic nanoparticles is obtained by depositing these particles onto the surface of solid supports such as ceramics, glass, polymers, zeolites, etc. In this manner, the separation of nanoparticles is simplified, but the photocatalytic activity is reduced due to the limited exposure of the photocatalyst to light, reduced surface area, and lower interaction with the reaction medium. Therefore, the use of materials easily separable such as magnetic photocatalysts would be preferred [2]. Magnetic nanoparticles have been synthesized in different structures and phases such as pure metals (i.e., Fe, Co, Ni), compounds (i.e., CoPt3, CoPt, NiPd), metal oxides, or spinel ferrites with the general formula of MFe2O4, where M is a divalent metal cation (CuFe2O4, NiFe2O4, CoFe2O4, ZnFe2O4, MnFe2O4, MgFe2O4). Although pure metals such as Fe, Co, Ni possess the highest magnetization saturation, they receive small interest due to their high natural toxicity and sensitivity to oxidation [3, 4]. In contrast, iron oxides are less sensitive to oxidation and can provide a steady magnetic response. Among the magnetic nanoparticles, hematite (a-Fe2O3), magnetite (Fe3O4), maghemite (g-Fe2O3), and ferrite (MFe2O4), are the most widely used due to their non-toxicity, relatively low cost, high specific surface area, good adsorption abilities, and biocompatibility [5–7]. The most important factor in the use of magnetic materials is that the photocatalyst should remain as a powder after being magnetized by nanoparticles and their surface area should not decrease. Also, they can be reversed using a magnet during the catalytic reaction in an organic suspension [8]. Magnetic photocatalysts are revised in this chapter. We will discuss the challenges faced by the researchers to improve the efficiency and recovery of magnetic photocatalysts, along with the advancements made in this field.

2 Iron oxide-metal oxide photocatalysts Many photocatalysts used for chemical and environmental depollution possess an oxide as the active photocatalyst or support. The shape and structure of photocatalysts, the effect of pH, and the presence of layers around the magnetic core strongly influence the efficiency and recovery of magnetic photocatalysts. Iron oxides with a saturation magnetization of more than 1 emu
g1 can be easily separated using an external magnetic field [9]. Iron oxide (Fe2O3), including a-, b-, e-, and g-Fe2O3, has a narrow band gap (1.9–2.2 eV). In a-Fe2O3, oxygen atoms are in a hexagonal close-packed arrangement with Fe3+ ions occupying two of every three octahedral sites, presenting no periodic vacancies [10–12]. Maghemite (g-Fe2O3), a typical ferromagnetic mineral, is thermally unstable, and it is transformed to hematite at higher temperatures [12]. b-Fe2O3 is a rare kind of iron oxide that exhibits a body-centered cubic structure where Fe+3 ions occupy two nonequivalent octahedral crystallographic sites; it is the only iron oxide showing a paramagnetic behavior at room temperature. As a thermodynamically unstable polymorph, it is transformed into either a-Fe2O3 or g-Fe2O3 when heated. The e-Fe2O3 presents an orthorhombic crystal structure derived from the close-packing of four layers of oxygen. Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00009-X Copyright © 2021 Elsevier Inc. All rights reserved.

235

236

SECTION

C Oxides and calcogenides

FIG. 1 Scheme of the structures of (A) Fe2O3: a-Fe2O3, b-Fe2O3, g-Fe2O3, e-Fe2O3. The colored spheres at the centers of polyhedrons and white spheres on the edges represent the iron and oxygen atoms, respectively; (B) Fe3O4. Panel (A) reproduced from S. Sakurai, A. Namai, K. Hashimoto, S.-I. Ohkoshi, First observation of phase transformation of all four Fe2O3 phases (g !e!b !a-phase) J. Am. Chem. Soc. 131 (2009) 18299–18303, with ACS permission. Panel (B) reproduced from B. Arndt, R. Bliem, O. Gamba, J.E. van der Hoeven, H. Noei, U. Diebold, G.S. Parkinson, A. Stierle, Atomic structure and stability of magnetite Fe3O4 (001): an X-ray view, Surf. Sci. 653 (2016) 76–81, with Elsevier permission.

e-Fe2O3 can be regarded as an intermediate polymorph presenting similarity to both g-Fe2O3 and a-Fe2O3 [13–15]. The crystal structures of a-, b-, e-, and g-Fe2O3 are shown in Fig. 1. In Fe3O4, oxygen atoms are in a cubic close-packed arrangement with a chemical formula of (Fe3+)tetrahedral 2+ [Fe Fe3+]octahedralO4 and have an inverse spinel structure. The cubic inverse spinel Fe3O4 is ferromagnetic at temperatures below 858 K [16, 17]. Considering Fe3O4 as a semiconductor photocatalyst, it can be both n- and p-type. Fermi level of both n- and p-type is in a low-mobility spin-polarized 3d band. However, Fe3O4 possesses low band gap energy (1 eV) and presents the lowest resistivity of any metal oxide attributable due to the rapid exchange of electrons between Fe2+ and Fe3+ in octahedral sites [18]. The morphology of a-Fe2O3 strongly influences its photocatalytic ability. Different morphologies of a-Fe2O3 have been investigated as photocatalyst for dibutyl phthalate (DBP) degradation. The DBP conversion efficiency for hollow, burger-like, shuttle-like, flake, and walnut was 94%, 80%, 73%, 65%, and 57%, respectively. The a-Fe2O3 hollow morphology showed the highest activity. The dispersion of a-Fe2O3 hollow nanoparticles and higher specific surface area as well as higher ability to form hydroxyl radicals (%OH) have been introduced as possible factors in the high activity of this morphology [19]. The a-Fe2O3 hierarchically nanostructured hollow structures exhibit a better photocatalytic property than ring-like nanoparticles [20]. Hierarchically nanostructured hollow spheres consisting of organized a-Fe2O3 nanosheets with small pores are reported in Fig. 2. The magnetic properties of magnetite depend on their morphologies. Magnetite has been obtained from polyols by adding KOH. The concentration of KOH influences the nucleation rate, while the hydroxyl number of the polyols controls

Iron oxide-based magnetic photocatalysts Chapter

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FIG. 2 (A–D) SEM and (E and F) TEM micrographs of hierarchically nanostructured a-Fe2O3. Reproduced from S.-W. Cao, Y.-J. Zhu, Hierarchically nanostructured a-Fe2O3 hollow spheres: preparation, growth mechanism, photocatalytic property, and application in water treatment, J. Phys. Chem. C 112 (2008) 6253–6257, with American Chemical Society permission.

the growth rate. The BET-specific surface areas for the cubes, spheres, and octahedral resulted in 292, 319, and 390 m2 g1, respectively [21]. Some of the morphologies obtained are reported in Fig. 3. a-Fe2O3/g-Fe2O3 nanorods have been prepared by a thermal decomposition/redox methodology [22]. a-Fe2O3/ g-Fe2O3 nanorods (100 nm length and 20 nm diameter) showed a heterojunction between a-Fe2O3 and g-Fe2O3. The magnetizations values at 1104 Oe are about 0.29, 68.8, and 26.8 emu g1 for a-Fe2O3, g-Fe2O3, and a-Fe2O3/g-Fe2O3. The presence of g-Fe2O3 in the structure expedites the possible application of photocatalyst [22]. The morphology of magnetic photocatalysts has been engineered by preparing g-Fe2O3/TiO2 composites by an electrospray method. The structure of the obtained g-Fe2O3/TiO2 is the so-called Janus hollow bowls (JHBs). The g-Fe2O3/ TiO2 JHBs possess a transition layer of Fe3+-doped-TiO2 between TiO2 and g-Fe2O3 and a high photocatalytic activity under visible light. g-Fe2O3 is a separable and acceptable photocatalyst due to the narrow band (2.2 eV) and sensitive magnetic response. The presence of a transition layer of Fe3+-doped-TiO2 between the phases; g-Fe2O3 and TiO2 increases the production of the pair electron–hole, which in turn increases its efficiency as photocatalyst for rhodamine B (RhB) dye degradation [23]. Photocatalysts composed of a magnetic core Fe3O4 and TiO2 shell were used for the first time by Beydoun et al. [24] The activity of the titania-coated magnetite was lower than that of pure TiO2 and has been found to decrease with an increase in heat treatment during the preparation of the solid. Flower-like core-shell magnetic photocatalysts Fe2O3/ TiO2 have been synthesized for the decomposition of paracetamol. The degradation efficiency increases by increasing the TiO2 content in the Fe2O3/TiO2 core-shell [25]. XRD and Raman spectra have shown that Fe2O3 contains two phases hematite (a-Fe2O3) and maghemite (g-Fe2O3) whereas TiO2 resulted in anatase. The average crystallite size remained constant after the TiO2 coating [25]. The photocatalytic degradation of paracetamol increased from 52.5% to 98% by increasing the TiO2 content by 50% by weight. This increase can be attributed to the separation of the effect of the electron–hole pairs with a narrow gap Fe2O3 with a wide band gap TiO2. Both TiO2 and Fe2O3 are photoexcited under UV–visible irradiation producing electron–hole pairs, as shown in Fig. 4. The electron–hole separation occurs because of the photogenerated electrons on the conduction band (CB) of Fe2O3 move to the lower potential CB of TiO2. These electrons could be used by the % adsorbed oxygen to generate the reactive superoxide radical anion (O 2 ). On the other hand, the holes on the valence band (VB) of TiO2 shift to the VB of Fe2O3 possessing higher potential. The holes could be trapped by H2O molecules to form a strong oxidizing hydroxyl radical (%OH) [26]. The a-Fe2O3 nanocrystals have been synthesized by adding Cu2+, Zn2+, and Al3+ ions. Morphologies changed with the addition of different ions, as shown in Fig. 5. Three-dimension flower-like a-Fe2O3@TiO2 core-shell nanocrystals with

238

SECTION

C Oxides and calcogenides

FIG. 3 The Fe3O4 Crystals with various morphologies a Cube, truncated octahedron, octahedron (B–D), sphere, truncated cube, the equilateral octahedron (E–G). (A) Ideal cube structure; (H) ideal octahedral structure. Reproduced from L. Zhao, H. Zhang, Y. Xing, S. Song, S. Yu, W. Shi, X. Guo, J. Yang, Y. Lei, F. Cao, Morphology-controlled synthesis of magnetites with nanoporous structures and excellent magnetic properties, Chem. Mater. 20 (2008) 198–204, with American Chemical Society permission.

FIG. 4 Schematic diagram of charge transfer in the photoexcited TiO2/Fe2O3 core-shell photocatalyst. From reference A.-M. Abdel-Wahab, A.-S. Al-Shirbini, O. Mohamed, O. Nasr, Photocatalytic degradation of paracetamol over magnetic flower-like TiO2/Fe2O3 core-shell nanostructures, J. Photochem. Photobiol. A Chem. 347 (2017) 186–198, with Elsevier permission.

orthorhombic, cubic, or discal morphologies were synthesized. The relative amount of TiO2 coating was different in the samples. The order of atomic ratio was a-Fe2O3 (Al)@TiO2 (15%) > a-Fe2O3 (Cu)@TiO2 (5%) > a-Fe2O3(Zn)@ TiO2 (3%), which reflects that the shape of a-Fe2O3 was an influential factor to develop the amount of TiO2 coating [26]. The best photocatalytic results were obtained in the presence of the discal a-Fe2O3@TiO2 nanocrystals. A 1% wt of Fe3O4 in a TiO2 structure has been used to bleach about 40% of the methyl orange (10 ppm) dye in 60 min by using UV irradiation at room temperature. However, due to the low percentage of iron oxide, the material showed low magnetic strength. The replacing of Fe3O4 with Fe2O3 significantly reduced methyl orange (MO) discoloration [27]. A Fe2O3 core in the TiO2/Fe2O3 structure reduces the photocatalytic activity of TiO2 under UV–vis irradiation. Adding a SiO2 layer between Fe2O3 and TiO2 weakens the effect of the Fe2O3 core but leads to an active and magnetically separable photocatalyst. Excessive light absorption by the Fe2O3 core under ultraviolet radiation or visible radiation is one of the

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FIG. 5 Scheme of the formation of a-Fe2O3 and a-Fe2O3@TiO2 with thorhombic (A), cubic (B), and discal (C) shapes. Reproduced from J. Liu, S. Yang, W. Wu, Q. Tian, S. Cui, Z. Dai, F. Ren, X. Xiao, C. Jiang, 3D flowerlike a-Fe2O3@ TiO2 core–shell nanostructures: general synthesis and enhanced photocatalytic performance, ACS Sustain. Chem. Eng. 3 (2015) 2975–2984, with ACS permission.

reasons for the decrease in the catalytic activity of TiO2 [28]. In the structure of N-TiO2/Fe3O4/SiO2, SiO2 is present as a shell around Fe3O4 to prevent both optical decomposition of Fe3O4 and chemical interactions between Fe3O4 and N-TiO2. In this photocatalyst, N-TiO2 acts as a photocatalyst under solar light whereas iron oxide possesses magnetic properties [29]. In the core-shell structures, g-Fe2O3 @SiO2@TiO2, the SiO2 shell was precipitated by the sol–gel method onto the Fe3O4 microspheres. By sequential coating of Fe3O4@SiO2 with TiO2, a corn-like structure is obtained. After heating to 350 °C, TiO2 crystallizes to increase photocatalytic activity, while Fe3O4 is converted to Fe2O3. In this structure, Fe2O3 acts as a visible photocatalyst due to its narrow band (2.2 eV) [30]. Direct contact between the magnetic photocatalysts g-Fe2O3 and TiO2 usually results in increased recombination of e/h+ couples. Therefore, a SiO2 layer plays an important role in photocatalytic performance [31]. In the Fe3O4@SiO2@CeO2 structure, it has been observed that direct contact of CeO2 with the surface of the Fe3O4 magnetic nanoparticles reduced the photocatalytic efficiency. This is because Fe3O4 can act as an e/h+ recombination center. The presence of a SiO2 layer between the two oxides is beneficial [32]. A three-dimensional flower-like core-shell photocatalyst of Fe3O4@Bi2O3 showed photocatalytic activity in the presence of visible light [33]. This structure presented a flower-like shape and possesses super magnetic properties. Its magnetization is approximately 41 emu
g1. The Bi2O3 nanoparticles were obtained by hydrolyzing Bi2(OCH2CH2O)3 in the surface of Fe3O4, then the nanoparticles have grown in two dimensions leading to the formation of nanofilms on the spherical surface of the iron oxide. The photocatalytic activity of Fe3O4@Bi2O3 has approximately 7–10 order higher than that of pure Bi2O3 [33]. Bi2O3 has a narrow band-gap (2.8 eV) [34]. An important strategy to improve these deficiencies is creating a composite structure to enhance the separation of photogenerated electron–hole pairs. One alternative possibility appears to use alternative semiconductors as layered structures as gC3N4 where flower-like Fe3O4@Bi2O3 structures can be placed on its surface. In this manner, the specific surface area of the photocatalyst increases and the electron–hole recombination reduced, thus increasing the degradation efficiency [35]. Experiments carried out in the presence of radical scavengers show that the electron generated in the CV of Bi2O3 moved to the surface of Fe3O4 and hence it reacts with the positive hole of the g-C3N4 valence band (VB), as schematized in Fig. 6. The +  % % electron on the g-C3N4 surface reacting with O2 produces O 2 . The h of Bi2O3 in turn reacts with OH to produce OH % %% + (see Fig. 6). This research concluded that the O2 , OH, and h have the main active species in the photocatalytic process. The g-C3N4 semiconductor possesses a suitable gap band (2.7 eV) and also a surface area to favorably adsorb pollutants to be degraded in a photocatalytic process. The g-C3N4 nanosheets effectively reduce aggregation of magnetic iron oxide nanoparticles, keeping the magnetic properties of the oxide. The high performance of the g-C3N4-Fe3O4 as photocatalyst is due to a synergistic effect due to the great surface-exposure area, high visible light absorption efficiency, and enhanced e/h+ separation properties [36]. The combination of g-C3N4/Fe3O4 with other semiconductor materials still increases the photocatalytic performance. BiOCl/g-C3N4/Cu2O/Fe3O4 (BGC-F) photocatalysts are a good example. BiOCl with an indirect band gap 3.2–3.6 eV has been investigated by many researchers owing to its great biocompatibility, but with wide energy, bandgap limits its use in a wider range as it acts in the UV region [37]. Cu2O is a p-type semiconductor with excellent visible light response owing to its direct band gap 2.0–2.4 eV, natural abundance, and non-toxic nature that makes it a favorable material for photocatalysis [38]. BGC–F shows excellent optical activity owing to its p-n-p combination, suitable band gap, developed spectral response and higher charge separation. The photoluminescence and electrochemical

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H+

O2 e–

Potential (V) vs. NHE

·O2–

Fe3O4

·OH

e–

e– h+ g – C3N4 h+

·OH

Bi2O3 H2O

FIG. 6 Schematic diagram of charge transfer in the photoexcited Fe3O4@Bi2O3/g-C3N4 photocatalyst.

studies have demonstrated a high separation of the charge carriers due to an essential electric field (n-p-n junction) and migration of the carriers [39]. ZnO has been extensively used as photocatalysts resulting a good candidate to form a magnetic composite in the presence of Fe3O4. Both the pH of the solution and the amount of the photocatalyst are important parameters influencing the structure and performance of the magnetic photocatalyst. By increasing the pH during the synthesis of iron oxide nanoparticles, the FeO content increases, which in turn increases the magnetic properties of the material. At pH 6, iron oxide resulted in Fe2O3, which influences the photocatalytic properties of ZnO due to its band gap. When the preparation was carried out at pH 10, the iron oxide obtained was the Fe3O4 crystalline phase. Fe3O4, conversely to Fe2O3, did not affect the photocatalytic activity of ZnO because it is an insulator. Increasing the contribution of FeO:Fe2O3 improved the photocatalytic properties of ZnO, which is due to the porous magnetic structure of Fe2O3, increasing the adsorption efficiency and thus affecting the efficiency. In contrast, the presence of a magnetic phase significantly affects the magnetic properties of the material and thus the possibility of composite recovery after optical decomposition processes [40]. The presence of ZnO with Fe2O3 has shown synergism between both, resulting in more active Fe2O3/ZnO and Fe3O4/ZnO photocatalyst than bare ZnO or Fe2O3 for the degradation of formaldehyde [41]. The Fe3O4/ZnO photocatalyst, due to the nature of the paramagnetic nanoparticles, tends to accumulate, and a stable suspension may not be formed even with stirring. The use of a high amount of photocatalysts in the reaction suspension leads to the accumulation of nanoparticles and reduces the efficiency of the photocatalyst. Hence, it seems essential to determine the optimal amount of magnetic photocatalyst to be used. Regarding the Fe3O4/ZnO structure after eight cycles, the photocatalytic performance for the degradation of four antibiotics (Sulfamethoxazole, Trimethoprim, Erythromycin, and Roxithromycin) has not shown a notable decrease (Fig. 7). The possible genotoxicity has been estimated for the safety of deliberations of Fe3O4/ZnO. To this aim, the DNA degradation of in vitro assay through electrophoresis has been conducted. The results of DNA degradation after incubation in the presence of Fe3O4/ZnO nanocomposite (0.0125–1 g L1) resulted in a negligible DNA degradation (1.6%). In this study, a magnetic photocatalytic reactor under optimal conditions has been used [42]. Yang et al. located ZnO nanorods on the surface of a Fe3O4@SiO2 core-shell structure. Also, CdS nanoparticles were eventually deposited on Fe3O4@SiO2/ZnO. Fe3O4@SiO2 has been used for easy separation of the photocatalyst, but the photocatalytic degradation is due to the composite ZnO/CdS. The CdS coating increases the specific surface area, the adsorption ability of the pollutants on the photocatalyst surface and extends the absorption spectrum to the visible region [43].

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1. Synthesis of non-toxic photocatalyst

4. High yields of antibiotic degradation

100 100

Sulfa methoxazole

Removal (%)

DNA degradation (%)

80 60 40 50 nm

20

Fe3O4/ZnO

80

Trimethoprim

60

Erythromycin

40

Roxithromycin

20 0 0.0 –20

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Fe3O4/ZnO concentration (g L–1)

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4

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8

Cycle 3. Recovery of Fe3O4/ZnO with efficient magnetic separarion unit

2. Operation of photocatalytic reactor

1.13 0.5

55

0.45

45 0.4

40 35

0.35

30

0.3

25

0.25

20

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FIG. 7 Fe3O4/ZnO composite for the degradation of antibiotics: (1) TEM image and DNA degradation percentage; (2) photocatalytic reactor; (3) recovery photocatalyst; (4) yelds of antibiotic degradation. Reported from L. Ferna´ndez, M. Gamallo, M. Gonza´lez-Go´mez, C. Va´zquezVa´zquez, J. Rivas, M. Pintado, M. Moreira, Insight into antibiotics removal: exploring the photocatalytic performance of a Fe3O4/ ZnO nanocomposite in a novel magnetic sequential batch reactor, J. Environ. Manage. 237 (2019) 595–608, with Elsevier permission.

Magnetic field strength, B (T)

50

15

0.05

0 –5 –10

0

10

20

30

40

50

0 –1 2.66x10

Both ZnO/Fe2O3 and TiO2/Fe2O3 photocatalysts have been deposited on the Clinoptilolite zeolite and used as photocatalysts for the degradation of diphenhydramine (DPH) [44]. The presence of the zeolite increased the specific surface area of the photocatalysts that were 219 and 112 m2.g1 for ZnO/Fe2O3/Zeolite and TiO2/Fe2O3/Zeolite, respectively. The degradation of DPH with ZnO/Fe2O3/Zeolite reached its maximum at alkaline pH. At a neutral pH, due to the electrostatic repulsion between ZnO/Fe2O3/Zeolite and DPH, the degradation efficiency decreased. At acidic pHs, ZnO leaches Zn2+ in solution, and ZnO eventually loses its photocatalytic properties. Optimal performance of the TiO2/Fe2O3/Zeolite was observed at acidic pH due to photocorrosion. According to the results, maximum DPH degradation efficiency has been achieved by TiO2/Fe2O3/Zeolite at acidic pH and ZnO/Fe2O3/Zeolite at alkaline pH. The efficiency of photocatalytic degradation as long as the solid structure is schematized in Fig. 8. Tungstophosphoric acid (H3PW12O40), a Keggin type heteropolyacid (HPA), (see Chapters 18 and 19) has been used in the synthesis of Bi2WO6 to form a three-dimensional needle-shaped structure of irregular size randomly dispersed on Fe3O4. Ibuprofen (IBP) has been used to evaluate the photocatalytic efficacy of the composite. The Fe3O4 nanospheres are obtained by a solvothermal method in ethylene glycol which generates the hydrophilic surface of the Fe3O4 possessing hydroxyl groups on their surfaces. Bi2WO6 nanocrystals are formed by a subsequent hydrothermal process. Fe3O4 nanospheres with an average diameter of 84 nm were prepared and some needle-like Bi2WO6 nanoparticles were attached to the surface of magnetite nanoparticles, eventually, the HPA was added [45]. In this photocatalyst, PW12O403 anion, (POM in Fig. 9), acts as an electron transporter and prevents e/h+ recombination. The adsorbed O2 and H2O2 can easily trap an electron in the LUMO of the POM anion to yield the oxidizing species, followed by the attack of radicals to organic molecules. Therefore, POM improves photodegradation efficiency. At low pH, the carboxylic group on the IBP molecule is shifted to the acidic form, so an important factor in the degradation of IBP is its weak acid strength (pKa ¼ 4.9). The use of the Keggin POM as an acidic agent in Fe3O4/Bi2WO6 nanohybrids furnishes active sites to the photocatalyst. Fe2O3/SrCO3 nanohybrids were synthesized and their surface modified by the Keggin heteropolyanion (HPA) PW12O403 [46]. The HPA-Fe2O3/SrCO3 photocatalyst was used for the degradation of ibuprofen (IBP) under sunlight

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UV

UV

Diphenhydramine

(DPH)

H2O2

H2O 100

TiO2

ZnO

HNO3

CO2 TiO2

ZnO

TiO2

ZnO

Max. DPH degradation efficiency by TiO2/Fe2O3/Zeolite = 80%

DPH degrdation efficiency (%)

90

[DPH]=1 mg/L [DPH]=10 mg/L [DPH]=100 mg/L

80 70

Max. DPH degradation efficiency by ZnO/Fe2O3/Zeolite = 95%

60 50 40 30 20 10 0 0.5

1 Photocatalyst concentraion(g/l)

2

FIG. 8 Composite for the degradation of DPH. Reported from reference N. Davari, M. Farhadian, A.R.S. Nazar, M. Homayoonfal, Degradation of diphenhydramine by the photocatalysts of ZnO/Fe2O3 and TiO2/Fe2O3 based on clinoptilolite: structural and operational comparison, J. Environ. Chem. Eng. 5 (2017) 5707–5720, with Elsevier permission.

FIG. 9 Schematic diagram of the photocatalytic reaction of IBP on Fe3O4/Bi2WO6 nanohybrid from [45] with Elsevier permission. POM stands for Polyoxometallate which is the general family where heteropolyacids can be included (See Chapters 18 and 19), and it refers to the tungstophosphoric acid (H3PW12O40).

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irradiation. The increase in activity and efficiency of HPA–Fe2O3/SrCO3 can be attributed to the more efficient and effective separation of electron–hole pairs due to the transfer of excited electrons HPA–Fe2O3/SrCO3 [46].

3 Iron oxide-carbon containing photocatalysts An extensive assortment of adsorbents available to be coupled with semiconductor materials for the photocatalytic air and wastewater treatment, such as activated carbon, zeolite, graphene oxide, silica oxide, zinc oxide, carbon nanotubes (CNT), CuO, polymeric adsorbents, and many others, have been studied [47]. The main objective to integrate photocatalyst with adsorbents are to enhance its photocatalytic activity. Among the various adsorbents in the structure of photocatalytic materials, the use of carbonic materials such as activated carbon [48] graphene, graphene oxide [49], biochar [50] has been extensively considered. These materials which can be produced at low cost possess also interesting features as wide surface area, pore volume, and the possibility to be modified on their surface to improve their benefits as good absorbers to remove contaminants. Anatase titania-coated magnetic activated carbon prepared by a sol–gel gel process was used for the degradation of phenol under UV irradiation. The photocatalyst was assisted by magnetic core (Fe2O3) and the activated carbon, to introduce magnetic separability and increase the adsorption activity, respectively. The photocatalytic activity of the composite was higher than that of the bare TiO2 or TiO2-activated carbon composite [51]. Also, carbon nanofibers have been deposited on TiO2 to enhance its absorption ability. A composite with Fe3O4 nanospheres gave rise to a magnetic photocatalyst labeled as TiO2@Fe3O4@C-NF. This was obtained by the mixture of home-prepared spherical magnetic Fe3O4 nanoparticles from FeCl3 by a hydrothermal process with commercial TiO2 nanoparticles and carbon nanofibers (C-NF). The mixture, in different proportions, undergoes a hydrothermal treatment. The magnetic nanoparticles possess Fe3O4 (magnetite) and Fe2O3 (hematite) structures, whereas TiO2 was anatase, and the carbon nanofibers, possessing hexagonal graphite structures. The obtained photocatalyst resulted in active degradation of some antibiotics, nonsteroidal antiinflammatory, and the methylene blue dye [52]. It is worth reminding here that the use of dyes as model molecules is often reported to test the photocatalytic activity of heterogeneous photocatalysts; however, the use of alternative and more challenging molecules is a must to fully understand the mechanistic aspects of the photocatalytic heterogeneous process. Indeed, as reviewed by Paz et al., the use of dyes as model compounds does not fulfill the required parameters to be a suitable model contaminant to be used as a model molecule for testing photocatalysts [53]. This is due to a variety of factors mainly related to the presence of a second mechanism, sensitization, among others. In photocatalytic reactions, carbon provides enhanced absorption of pollutants by increasing the active and available surface. The authors investigate some mechanistic aspects of the degradation of paracetamol by this TiO2@Fe3O4@C-NF photocatalyst. Considering that paracetamol possesses a hydroxyl group and an amide group at the para-position at the aromatic ring, the contribution of both the electron OH donor group, and the NO2 electron group is considered. Indeed, a charge attraction occurs between the negatively charged paracetamol species (weak acid with phenol functional group; pKa ¼ 9.5) and the positively charged TiO2 nanoparticles surface. According to the authors, the semiconductor surface improves the electrostatic bonding of paracetamol molecules, which in turn accelerates the speed of light transmission and decay of the photocatalytic free radical, the photoreduction, and the photocatalytic free-radical bombarding and cleavage [52]. Bi2O3, a semiconductor with a narrow band gap, is widely used in photocatalysis for environmental remediation by its nontoxic and good visible photocatalytic performance [54]. Corn cobs can be used as biomass precursors to obtain carbon. They obtained the composite by mixing up the carbon with the Fe2O4 precursor obtaining C/Fe3O4, as illustrated in Fig. 10. The bismuth salt, solved in etyleneglycol (EG, in Fig. 10), was mixed and the mixture suffered a solvothermal treatment giving rise to the final powder. The C/Fe3O4/Bi2O3 composite photocatalyst exhibited photoabsorption in the visible radiation. The C/Fe3O4 in the C/Fe3O4/Bi2O3 composite provides magnetic features to photocatalyst and it serves as electron transfer and storage center to transfer electrons from Bi2O3 so reducing electron–hole recombination. Tetracycline has been oxidized by 91% in 90 min in the presence of a 10% wt of C/Fe3O4 in the composite. These results were obtained despite the low surface area of the photocatalyst, 24 and 11 m2
g1, for 10%w C/Fe3O4 and bare Bi2O3, respectively. A microwave-deposition method was used to obtain a composite where Fe3O4 and Bi2O3 re supported on activated carbon. Active carbon, obtained from biomass (withered petals), plays a double role, i.e., it was used to support the semiconductor both allowing a good dispersion and avoiding agglomeration of Fe3O4 and Bi2O3 particles but also acts as an adsorbent for the pollutants. It is worth mentioning that, any time that carbon is used to increase the surface area of a photocatalyst, also in the presence of iron oxide quantum dots, the amount of these particles must be balanced avoiding covering the semiconductor surface and hence reducing the photocatalytic efficiency [55].

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FIG. 10 Scheme of the synthetic steps to obtain the C/Fe3O4/Bi2O3 composite photocatalyst. From reference N. Gao, Z. Lu, X. Zhao, Z. Zhu, Y. Wang, D. Wang, Z. Hua, C. Li, P. Huo, M. Song, Enhanced photocatalytic activity of a double conductive C/Fe3O4/Bi2O3 composite photocatalyst based on biomass, Chem. Eng. J. 304 (2016) 351–361, with Elsevier permission.

In addition to the mentioned influence of carbon-magnetic structures on the photocatalyst surface, carbon-based adsorbents are also effective in separating the magnetic species in the powder so preventing the magnetic materials to become patterned. This effect was studied by investigating the effect of Biochar (BC) in the composite Bi2WO6/Fe3O4/BC, where BC would prevent the accumulation of Bi2WO6 and Fe3O4. The Bi2WO6 semiconductor (band gap of ca. 2.6–2.7 eV) is a solid of the Aurivillius family, composed of [Bi2O2]2+ and [WO4]2 stacked layers, and the interlayer oxygen atoms are shared to form the cationic and anionic stacks. Moreover, it has low toxicity and chemical stability, and its performance as a photocatalyst is limited by a low separation efficiency of carriers [56]. The biochar-based photocatalyst assembled with flower-like microspheres Bi2WO6 and Fe3O4 nanoparticles, schematized in Fig. 11, was prepared by a hydrothermal method, and it exhibited a better photocatalytic activity for ofloxacin (OFL) and ciprofloxacin (CIP) degradation than the bare materials. Both pollutants were almost completely degraded within 30 min, and the total organic carbon (TOC) removal of OFL and CIP was of ca. 83% and 91%, respectively, after 60 min of visible LED irradiation [57].

FIG. 11 Scheme of the Bi2WO6/Fe3O4/Biochar photocatalyst structure along with the reactive oxygen species (ROS) is active during the photocatalytic activation for the degradation of drugs. Reported from Z. Wang, X. Cai, X. Xie, S. Li, X. Zhang, Z. Wang, Visible-LED-light-driven photocatalytic degradation of ofloxacin and ciprofloxacin by magnetic biochar modified flower-like Bi2WO6: the synergistic effects, mechanism insights and degradation pathways, Sci. Total Environ. (2020) 142879, with Elsevier permission.

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FIG. 12 Schematized procedure for the preparation of the Fe3O4@C@Cu2O core/shell composite used in [58].

Cuprous oxide (Cu2O) as a p-type semiconductor (band gap 2.17 eV) used as photocatalyst and a core-shell bean-like structure of Fe3O4@C@Cu2O material was synthesized by a self-assembly approach, as schematized in Fig. 12, including solvothermal and hydrothermal reactions followed by the Cu (II) chemical reduction. The mean diameter of the Fe3O4@C microspheres resulted in ca. 50 nm (ca. 40 nm Fe3O4 and 10 nm thick the C deposit). The particles were connected and arranged into a chain-like structure (length 50–200 nm). The final composite shows Cu2O nanoparticles of about 8 nm in size randomly deposited on the surface of the magnetic spheres. The hydrophilic groups of the carbon layer act as binders stabilizing the interaction between Fe3O4 and Cu2O [58]. The ferromagnetic behavior of Fe3O4@C@Cu2O allows the fine dispersion of the photocatalyst in an aqueous solution. Two-dimensional layer structure carbon-based materials, as graphene, reduced graphene oxide or C3N4 were used to obtain magnetic highly adsorbent composites. In addition to the physical properties of the absorbents, as the distribution of cavities and high surface area, the adsorption capacity is also related, for the 2-D materials, to the chemical structure constituted by aromatic rings. These sheets often possess a series of defects and inconsistencies along with a series of edges. The unsaturated carbon atoms at these edges have high energy potential and are highly reactive due to their unpaired electrons. Reduced graphene oxide (rGO) can be used as a source of carbon and resulted effective to obtain highly active magnetic photocatalysts. It should be considered that excessively reduced graphene oxide will reduce the efficiency of the photocatalyst because a large amount will cause graphene oxide layer aggregation. This insight will prevent the optimum achievement of the light source irradiance and hence the formation of active species on the photocatalyst surface. An optimal amount of graphene oxide in the photocatalytic structure is of great importance. Reduced graphene oxide rGO-loaded-magnetite composites prepared by co-precipitation have been used to photocatalytically oxidize the carbamazepine (CBZ) drug by using visible light irradiation. Parameters as rGO loading, catalyst dose, pH, and different ions influenced the photocatalytic degradation process. The composites were easily magnetically separable, and they were recycled without losing their photocatalytic ability. A reduced graphene oxide mass ratio of 10% in the photocatalytic structure has been reported as the optimal. Fig. 13 schematizes the photocatalysis mechanism for the degradation of CBZ in the presence of the rGO–Fe3O4. The incident light energy allows electrons and holes in the conduction band (CB) and valence band (VB) of the Fe3O4. When bare Fe3O4 is used, the photogenerated electrons and holes recombine immediately after their generation. The presence of r-GO allows the efficient separation of e/h+ couples giving rise to the oxidative radicals. The electrons are transferred to the rGO surface and scavenged by O2 molecules producing superoxide radicals (O∙ 2 ) which can also give rise to the very %OH radical species. The valence band protons can also be scavenged by water molecules and produce
OH radicals that oxidize CBZ [59]. An important feature of graphene oxide (GO) is that due to its unique two-dimensional properties, it can provide a large surface area for the photocatalyst. Graphene oxide can play a significant role in electron transfer in photocatalytic composites. Because GO is electrically conductive, it accelerates e/h+ separation and reduces recombination. Magnetic graphene oxide-loaded Ce-doped titania (MGO–Ce–TiO2) was prepared. GO was synthesized from graphite powder by a modified Hummers method, and then a certain amount of GO was dispersed in water and mixed with a homeprepared Fe3O4 suspension, so obtaining a Fe3O4–GO in ratio 1:1, labeled as MGO. A Ce-doped TiO2 was coated on MGO to obtain the composite (MGO–Ce–TiO2). The adsorption of tetracycline molecules on the composite increases by increasing the MGO content, achieving the maximum value when the MGO is 20%. The Ce–TiO2 semiconductor is excited under visible light, and the formed radicals are rapidly transferred to the tetracycline pollutant molecules adsorbed to be oxidized [60]. The ability to absorb light, particularly in the visible range of the spectrum, is an important factor among photocatalytic materials, also, for this reason, GO is often used as a shell for iron oxide in magnetic photocatalysts as shown in Fig. 14 [61]. Fe3O4 spheres have been synthesized with a solvothermal method (heated at 200 °C for about 10 h). Further treatment with 3-Aminopropyltrimethoxysilane (APTMS) was utilized to positively charge the surface of the synthesized iron oxide spheres, hence the APTMS–Fe3O4 possesses a positive surface. Considering that GO has a negative surface, hence, GO was deposited onto the APTMS–Fe3O4 spheres by an electrostatic layer-by-layer method. Eventually, the

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FIG. 13 Schematic mechanism for the generation of oxidative species during the photo-degradation process. Reported from M. Moztahida, J. Jang, M. Nawaz, S.-R. Lim, D.S. Lee, Effect of rGO loading on Fe3O4: a visible light assisted catalyst material for carbamazepine degradation, Sci. Total Environ. 667 (2019) 741–750, with Elsevier permission.

FIG. 14 Scheme of the synthesis steps to obtain the Fe3O4@rGO@TiO2 composite, where Fe3O4 modified by 3Aminopropyltrimethoxysilane (APTMS) to give APTMS–Fe3O4, is then wrapped by GO by electrostatic interactions and finally gives Fe3O4@rGO@TiO2 by a hydrothermal treatment. Reproduced with permission from X. Yang, W. Chen, J. Huang, Y. Zhou, Y. Zhu, C. Li, Rapid degradation of methylene blue in a novel heterogeneous Fe3O4@rGO@TiO2-catalyzed photoFenton system, Sci. Rep. 5 (2015) 10632.

TiO2 nanoparticles were attached to cover the Fe3O4@GO, and GO was incompletely reduced to rGO during the hydrothermal reaction concurrently (see Fig. 14). As shown in Fig. 15, the presence of graphene oxide as a shell around the magnetic nucleus causes the transfer of electrons from the outer semiconductor to the nucleus and facilitates the reduction of Fe (III) to Fe (II), which is essentially a heterogeneous photo-Fenton reaction. The rate of oxidation/reduction is due to the exchange of electrons between the two states Fe (III) to Fe (II). Magnetite is the main core of the Fe3O4@rGO@TiO2 photocatalyst, which is used as a photocatalyst to obtain H2O2 and hence HO% radicals for the methylene blue dye bleaching. The presence of graphene oxide not only increases the dye adsorption but also reduces the TiO2 band gap from 3.2 to 2.8 eV. TiO2 [61]. Some authors report that rGO can act also as a semiconductor due to the remaining oxygen present for the incomplete reduction of GO to rGO and the existence of sp2 and sp3 carbons on its surface. The sp2-carbon-containing regions behave like a conducting band due to the free mobility of the electrons, while the sp3carbon-containing regions behave like a valence band due to the electrons being held tightly. The number of such sp2 and sp3 domains in rGO is lower than those present in GO. A core-shell structure of Fe3O4@rGO, on which ZnO nanosheets were deposited and Ag2CO3 nanoparticles

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FIG. 15 Scheme of (A) heterogeneous photo-Fenton degradation mechanism of methylene blue dye in the presence of Fe3O4@rGO@TiO2; (B) electron transfer from the TiO2 conduction band to magnetite and (C) degradation of the dye in the presence of different photocatalysts. Reproduced with permission from X. Yang, W. Chen, J. Huang, Y. Zhou, Y. Zhu, C. Li, Rapid degradation of methylene blue in a novel heterogeneous Fe3O4@rGO@TiO2-catalyzed photo-Fenton system, Sci. Rep. 5 (2015) 10632.

loaded, was used photocatalytically for a dye bleaching. The reduced graphene oxide acted as a barrier to electron transfer for magnetite to capture electron transitions and holes between the ZnO and Ag2CO3 semiconductors and increased the dye rate degradation [62].

4 Iron oxide-polymer photocatalysts Magnetic photocatalysts possess, as above reported, appropriate optical and electronic properties along with good dispersibility to be used as photocatalysts in liquid–solid regime. Nevertheless, this kind of photocatalytic particle has a tendency to aggregate due to the large surface-to-volume ratio, so reducing their efficiency. Metal oxide nanoparticles are generally present on hydrophilic surfaces and are finely dispersed in polar environments such as water, alcohols, and polar polymers. Therefore, to disperse these particles in hydrophobic environments, such as non-polar solvents or hydrophobic polymers, the use of polymers is a good strategy to improve their effectiveness. Conductive polymers, such as polyaniline, polypyrrole, and polythiophene, are widely used due to their unique electrical and optical properties including high absorption coefficient, good electron transfer, high electron mobility, and excellent stability. TiO2 coated on magnetic poly (methyl methacrylate), labeled as TiO2/mPMMA microspheres have been prepared with the aim of using this material for the photocatalytic degradation of p-phenylenediamine (PPD) [63]. TiO2 nanoparticles possess photocatalytic activity, whereas the polymer encapsulated magnetite nanoparticles confer the material an easy extraction, recovery, and reuse. Chen et al. prepared Fe3O4 magnetic nanoparticles of ca. 8 nm size. They added oleic acid into the precursor FeCl2 and FeCl3 containing solution forming oleic acid-coated magnetite (Fe3O4) nanoparticles (OMP).

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FIG. 16 (A) SEM image of TiO2/mPMMA microspheres; (B) TEM image of microtomed TiO2/mPMMA microspheres. Reproduced with Elsevier permission from Y.-H. Chen, Y.-Y. Liu, R.-H. Lin, F.-S. Yen, Photocatalytic degradation of p-phenylenediamine with TiO2-coated magnetic PMMA microspheres in an aqueous solution, J. Hazard. Mater. 163 (2009) 973–981.

They also obtained poly (methyl methacrylate) by polymerization, and they introduced the OMP into the polymerization suspension to encapsulate the magnetite in the polymer particles. Then, the mPMMA microspheres were mixed with commercial TiO2 at 220 °C, close to the glass transition temperature of the mPMMA microspheres, for the better adherence of TiO2 on the mPMMA surface. A micro-sized TiO2/mPMMA photocatalysts were obtained (4–8 mm size) as shown in Fig. 16A. The magnetite nanoparticles resulted well encapsulated and dispersed inside of the PMMA microspheres, as shown in Fig. 16B. The titania nanoparticles coated on the mPMMA microspheres form a coating with a 100–250 nm thickness. The specific surface area of the final photocatalyst is ca. 2 m2 g1, and it showed certain activity for the PPD under 254 nm irradiation. Magnetic TiO2/Polystyrene composites with a multilayer of titania nanoparticles deposited on the surface of a polymer matrix enclosing magnetite nanoparticles were also prepared by an emulsion polymerization method, and methylene blue bleaching was carried out to evaluate the photocatalytic activity and reusing ability [63, 64]. Polyaniline (PANI) is a conductive polymer that can be used as a semiconductor in photocatalysis. It possesses a narrow band gap (2.8 eV), high absorption coefficients in the visible light range, and high mobility of charge carriers. Chen et al. coupled PANI with the MnFe2O4 spinel to obtain a magnetically recoverable photocatalyst [65]. The polyaniline/ MnFe2O4 composite shows good stability and higher photocatalytic activity for rhodamine B bleaching than bare polyaniline or MnFe2O4 nanoparticles under visible light irradiation. The results show a synergistic effect between both PANI and MnFe2O4. The lowest unoccupied molecular orbital (LUMO) excited electrons from polyaniline can easily migrate to the conduction band MnFe2O4, while the MnFe2O4 valence band holes migrate to the polyaniline orbital, as schematized in Fig. 17. MnFe2O4 can be activated by visible light irradiation generating photoproduced e/h+ couples. The absorption of photons by PANI gave rise to transitions p ! p*, and successively electron transfer p* ! orbital state of the excited electrons occurs. For the PANI/MnFe2O4, the close combination of both materials in the composite allows the excited electrons in the LUMO of polyaniline to easily migrate to the MnFe2O4 conduction band. The holes then migrate to p ! orbital polyaniline in the VB MnFe2O4 valance band at the same time, because here the electrons and holes produced by the light move in opposite directions, reducing the recombination and giving rise to efficient separation of the photoproduced couples so increasing the photocatalytic ability of the material. The N–K2Ti4O9/MnFe2O4/PANI composite photocatalyst was obtained by an in situ oxidative polymerization method with various mass ratios of N–K2Ti4O9 to MnFe2O4 [66]. When PANI was added, the photocatalytic activity of N–K2Ti4O9/MnFe2O4 increased. The PANI was a boundary between the magnetic core MnFe2O4 and N–K2Ti4O9. A lower adsorption amount of dye was observed in the samples non containing PANI. Different ratios of MnFe2O4 and N–K2Ti4O9 were uninfluential for the rhodamine B (RhB) adsorption rate; however, the RhB degradation achieved the maximum in the presence of the N–K2Ti4O9/MnFe2O4/PANI (7:3) photocatalyst. The best result has been explained taking into account the high adsorption capacity, the high utilization of the visible light irradiation, and the enhanced charge carrier transfer of PANI in the composite. A possible photocatalytic degradation mechanism of RhB in the presence of N–K2Ti4O9/ % % % MnFe2O4/PANI composites is reported in Fig. 18. The reactive oxygen species (ROS) O 2 , HOO and OH radicals have been suggested to be responsible for the RhB bleaching. Polyaniline was added to FeO/ZnO showing band gap values of 3.5 and 1.8 eV for FeO/ZnO and FeO/ZnO/polyaniline respectively. The addition of polyaniline results in the encapsulation of FeO/ZnO, which prevents shrinkage and increases

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O2•–/OH+ O2/H2O2

RhB Degradation

– – π* e e

hv

e–e–

hv

PANI π Degradation

h+h+

MnFe2 O4 h+h+

RhB

•OH

OH–

FIG. 17 Separation of the photoproduced couples e/h+ and transfer of the photo-generated charge carriers in the MnFe2O4/PANI photocatalyst under visible light irradiation.

FIG. 18 Scheme of the proposed photocatalytic mechanism for the RhD dye bleaching in the presence of N–K2Ti4O9/MnFe2O4/PANI composites. Reproduced with Elsevier permission from Q. Chen, Q. He, M. Lv, X. Liu, J. Wang, J. Lv, The vital role of PANI for the enhanced photocatalytic activity of magnetically recyclable N–K2Ti4O9/MnFe2O4/PANI composites, Appl. Surf. Sci. 311 (2014) 230–238.

contact surface area. Encapsulation of metal oxides in the matrices leads to increased thermal and photocatalytic properties as well as enhances mechanical strength. The FeO/ZnO/polyaniline showed high adsorption ability, and the removal of 3-aminophenol was complete in dark conditions [67]. It must be said that the limited dispersion of conjugated polymer photocatalysts in solvents limits the options for different photocatalytic reactions. One of the design criteria for photocatalysts containing polymers is solvent compatibility. Good dispersion of polymeric photocatalysts causes mass transfer between reactants and catalytic surfaces and accelerates their interaction, so improving the photocatalytic activity. The dispersion of polymeric photocatalysts in solvents depends on their intrinsic surface properties, and photocatalysts are only compatible with certain types of solvents, i.e., organic or aqueous solvents. Magnetically bound polymer nanoparticles synthesized from conjugated polyelectrolytes have been reported to demonstrate wettability in solvents. The dispersion of photocatalysts can be changed by the exchange of anions on

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nanoparticles. This strategy allows the composites to operate in a wide range of solvents. The optimal wettability of photocatalysts in the desired solvent has been investigated by two model reactions under visible light irradiation, namely the photocatalytic degradation of tetracycline in water and the photooxidative coupling of benzylamine in acetonitrile [68]. The polymeric nanoparticles attached to magnetic particles can be easily recovered by magnetic separation from reaction solvents. Recent reports have used Fe3O4/ZnO polymer composites to selectively remove doxycycline antibiotics. Methylmethacrylate (MMA) has been used as a functional monomer because its acidic group can form strong hydrogen bonds with hydroxyl, amide, and doxycycline groups. Trimethylolpropane trimethaacrylate (TRIM) as a cross-linker was used, and 2,20 -azobis-izobutyronitrile (AIBN) was applied as initiator. The Fe3O4/ZnO polymer composite removed 92.7% of doxycycline in 6 h, while in equal amounts of ZnO and polymer nanoparticles removed 35% and 20% of doxycycline in an analogous experiment, respectively. The comparison shows an increase of about 15% in the removal efficiency of Fe3O4/ZnO magnetic polymer composite [68]. Studies to investigate the degradation of the methylene blue dye in the presence of PVP–bipyridine have shown higher degradation compared to Fe3O4@PVP–bipyridine. Pure PVP–bipyridine appears to be more active than Fe3O4@PVP–bipyridine in producing reactive oxygen species (ROS) under the same conditions under visible light. However, the difference in efficiency is cost-effective due to the efficiency of the magnetic composite in its recovery and easy recycling in real-world wastewater treatment [69]. Research has addressed the effect of different monomers on the structure of polymer photocatalysts and changes in their efficiency. In TiO2@SiO2@Fe3O4 photocatalyst, a magnetic photocatalyst with excellent transparency was prepared by using MMA. For instance, the effect of methyl methacrylate (MMA) concentration on the photocatalytic efficiency of the composite has been investigated. The nature and concentration of monomers are one of the important parameters in the efficiency of this type of magnetic TiO2 based polymeric photocatalysts. The concentration should be such that the three-dimensional cavities in the polymerization process are formed optimally, and on the other hand, if the concentration is higher than the optimal limit, it will saturate the cavities and reduce the specific surface area available to the photocatalyst. Because the polymer layer is placed on the surface, it covers the active photocatalytic sites of TiO2 and prevents light absorption, which greatly reduces photocatalytic activity. Different monomers have been tested to investigate the differences in the effect of the monomers on the photocatalytic efficiency of the composite [70].

5

Conclusion

The development of novel photocatalysts seems to be necessary to improve efficiency and solve the current drawbacks of the existing photocatalysts. The use of photocatalytic magnetic materials allows the easy recovery of the solid from the reacting medium. Iron oxide nanoparticles are a suitable option for this purpose due to their magnetic properties, low-cost synthesis, and high specific surface area. In addition to the advantages of iron oxide-based magnetic photocatalysts (IOMPs) as photocatalyst recovery, electron exchange and reduction of electron–hole recombination at the photocatalyst surface are photocatalysts that can be activated by the visible part of the electromagnetic spectrum particularly when coupled with other semiconductors. The magnetic photocatalyst composites usually possess a core/shell structure in which iron oxide acts as the core, the shell plays a pivotal role in the efficiency and performance of the iron oxide in the photocatalytic activity.

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[65] S. Zeng, J. Yang, X. Qiu, Z. Liang, Y. Zhang, Magnetically recyclable MnFe2O4/polyaniline composite with enhanced visible light photocatalytic activity for rhodamine B degradation, J. Cerma. Soc. Jpn. 124 (2016) 1152–1156. [66] Q. Chen, Q. He, M. Lv, X. Liu, J. Wang, J. Lv, The vital role of PANI for the enhanced photocatalytic activity of magnetically recyclable N–K2Ti4O9/ MnFe2O4/PANI composites, Appl. Surf. Sci. 311 (2014) 230–238. [67] S. Sharma, G. Sharma, A. Kumar, M. Naushad, G.T. Mola, A. Kumar, F.A. Al-Misned, H.A. El-Serehy, F.J. Stadler, Visibly active FeO/ZnO@PANI magnetic nano-photocatalyst for the degradation of 3-aminophenol, Top. Catal. 63 (2020) 1302–1313. [68] A. Mohammadi, S. Pourmoslemi, Enhanced photocatalytic degradation of doxycycline using a magnetic polymer-ZnO composite, Water Sci. Technol. 2017 (2018) 791–801. [69] R. Djellabi, J. Ali, B. Yang, M.R. Haider, P. Su, C.L. Bianchi, X. Zhao, Synthesis of magnetic recoverable electron-rich TCTA@ PVP based conjugated polymer for photocatalytic water remediation and disinfection, Sep. Purif. Technol. (2020) 116954. [70] Z. Lu, F. Chen, M. He, M. Song, Z. Ma, W. Shi, Y. Yan, J. Lan, F. Li, P. Xiao, Microwave synthesis of a novel magnetic imprinted TiO2 photocatalyst with excellent transparency for selective photodegradation of enrofloxacin hydrochloride residues solution, Chem. Eng. J. 249 (2014) 15–26.

Chapter 16

Strategic design and evaluation of metal oxides for photocatalytic CO2 reduction Nikolaos G. Moustakas Leibinz Institute for Catalysis (LIKAT), Rostock, Germany

1 Introduction: Greenhouse gases and climate change According to NASA, since 2005 the average atmospheric concentration of carbon dioxide (CO2) has risen from 380 (July 2005) to 416 ppm (May 2021) surpassing the 400 ppm line on March 2015 [1]. Each of the past three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850. An approximate increase in global mean temperature of 1°C above preindustrial levels can be attributed to human activities and especially to the excessive utilization of fossil fuels in industry and transportation. Projections have been made of the global warming reaching 1.5°C between 2030 and 2050, if the concentrations of greenhouse gases (GHGs) continue to increase at the current rate [2]. As CO2 is one of the most important GHGs, the increase of its concentration is linked to the increase of the planet’s average temperature which in turn leads to climate change and extreme weather phenomena. The effects of the climate change are expected to heavily affect humanity in the next decades if measures to reduce the concentration of CO2 are not taken as soon as possible. A natural fluctuation in the average global CO2 concentration and temperature can be considered normal and, in many cases, anticipated (for example after volcanic eruptions). The scientific community though has reached a consensus that these major increases observed in the past centuries are unnatural and can only be explained when considering the energy-intensive human activities starting from the industrial revolution in the mid-1700s [2]. Only by reducing these anthropogenic CO2 emissions through the implementation of carbon neutral, or even better of carbon negative, processes it is possible to mitigate the harsh outcomes of the irrationally high consumption of fossil fuels. The broad use of fossil fuels is the direct outcome of their many intrinsic advantages: they possess a high energy density [3], they can be used efficiently especially with modern combustion engines, are relatively inexpensive, and they are safe to use and transport. Ever since suitable methods were developed and processes invented to exploit the aforesaid advantages, fossil fuels played a critical role in the blooming of human civilization. But fossil fuels also come with some major disadvantages: their being finite and spatially unevenly distributed can lead to conflicts between countries, they are hazardous to produce and highly polluting to water bodies if spilled and to air when burned, hugely contributing to the greenhouse phenomenon. A literature search from 2000 to 2019 was attempted using the Web of Science literature search platform (https://apps.webofknowledge.com/). The search was conducted using the “TOPIC” search function, and the terms “Green Energy,” “Sustainability,” “Environment,” and “Climate Change” were used as individual search terms. As it can be seen from Fig. 1, it is evident that the scientific literature output has drastically increased especially after 2005 for all the searched terms mirroring the increased interest of the society and the research groups in topics dealing with green energy and environmental protection. Technologies that harvest renewable sources of energy have been developed over the years, with wind generators and photovoltaics finding their way into the energy mix of many countries worldwide. The main disadvantage of wind and solar energy is that they are both intermittent, that is, their availability fluctuates. But they share a common disadvantage with fossil fuels: their distribution is not spatially uniform. All the above explain why it is not possible to build an electric power system relying exclusively on photovoltaics and wind generators without employing some storage media like batteries which however store energy at considerably lower energy densities per volume compared to fossil fuels [3].

2 Photocatalysis and photocatalytic CO2 reduction The first study on photocatalysis is attributed to A. Fujishima and K. Honda in 1979 [4]. The term photocatalysis originates from the combination of two Greek words: ’oB (phos) meaning light and kaτάlusiB (catalysis) meaning to break apart. Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00019-2 Copyright © 2021 Elsevier Inc. All rights reserved.

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256 SECTION C Oxides and calcogenides

30000

Number of publications

25000

Green Energy Sustainability Environment / 5 Climate Change

20000

15000

10000

5000

20 00 20 01 20 02 20 03 20 04 20 05 20 06 20 07 20 08 20 09 20 10 20 11 20 12 20 13 20 14 20 15 20 16 20 17 20 18 20 19 20 20

0

Year FIG. 1 Scientific literature output on environment-related search topics.

When referring to the International Union of Pure and Applied Chemistry (IUPAC), the definition of photocatalysis is a: “Change in the rate of a chemical reaction or its initiation under the action of ultraviolet, visible or infrared radiation in the presence of a substance—the photocatalyst—that absorbs light and is involved in the chemical transformation of the reaction partners” [5]. Photocatalysis is a process widely known and used in many applications including (among others) the degradation of pollutants like dyes and pharmaceuticals found in aqueous solutions. Compared to classical catalysis, photocatalysis can drive both thermodynamically spontaneous reactions (negative Gibbs energy, DG < 0) and energystoring reactions (positive Gibbs energy, DG > 0). The reason behind this characteristic of photocatalysis is that the oxidation and reduction steps involved in the process are spatially and chemically separated. The photogenerated charge carriers become separated with electrons (e
) and holes (h+) driving in parallel the reduction and oxidation reactions, respectively. A photocatalyst should play a double role when participating in a light-driven reaction: (i) it should provide active sites for the reactants to adsorb so that the reaction can take place (catalytic role) and (ii) it should be able to absorb incoming photons of sufficient energy producing photogenerated positive (h+) and negative (e
) charge carriers (photoabsorbing role) [6]. Its role as a catalyst connotates that the photocatalyst is not consumed, regenerates after each reaction cycle and remains structurally intact during and after the reaction under photo-irradiation. The pioneering publications of Halmann et al. [7] and of Inoue et al. [8] suggested the formation of hydrocarbons from CO2 using light and thus they are considered as the founding studies of the photocatalytic CO2 reduction field. Though there are also different approaches for the conversion of CO2, this chapter focuses only on the light-driven (photocatalytic) CO2 conversion process [9]. Photocatalytic CO2 conversion can be considered as a more “elegant” process as the only inputs involved are the reactants CO2 and H2O, and light. By coupling this process with a renewable energy source (e.g., to power the irradiation source), it is possible to perform the reaction in a carbon neutral way. The design and synthesis of materials able to photocatalytically convert CO2 to hydrocarbons efficiently is a crucial step for the successful industrial implementation of this process. If CO2 photoreduction succeeds in being realized in an industrially appealing way with high efficiency and low associated costs, it can significantly contribute to the simultaneous combat against the climate change (by reducing the atmospheric concentration of CO2) and the energy crisis (by producing fuels from the otherwise useless CO2). In a similar to Fig. 1 literature search, it can be seen (Fig. 2) that both the research fields of “photocatalysis” and “photocatalytic CO2 reduction” draw attention increasingly (especially since 2010) with 50% of the publications on photocatalysis and the 60% on photocatalytic CO2 reduction been published in the past 4 and 3 years, respectively (till 2019). The photocatalytic conversion of CO2 is often referenced as “artificial photosynthesis” as it tries to mimic the -highly intricate and optimized- natural photosynthesis process using the same inputs as plants do to produce sugars: CO2, H2O, and solar light participating in highly complicated reactions on the chloroplasts.

Strategic design and evaluation of metal oxides in photocatalytic CO2 reduction Chapter

6000 5500

approx. 50% Photocatalytic CO2 reduction

60%

Photocatalysis

16

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FIG. 2 Scientific literature output on the fields of photocatalysis and photocatalytic CO2 reduction.

Number of publications

5000 4500 4000 3500 3000 2500 2000 1500 1000 500

20 00 20 01 20 02 20 03 20 04 20 05 20 06 20 07 20 08 20 09 20 10 20 11 20 12 20 13 20 14 20 15 20 16 20 17 20 18 20 19 20 20

0

Year

The reduction (either as the opposite of increase or as the opposite of oxidation) of CO2 is performed on “artificial chloroplasts” usually in the form of a nanostructured semiconducting metal oxide, the photocatalyst. The products of the reaction (often called “solar fuels” as they store solar energy into chemical bonds) depending on the experimental parameters comprise of a mixture of hydrocarbons, alcohols, CO, and H2 containing no other element besides carbon, hydrogen, and oxygen (Table 1). Photocatalytic CO2 reduction has a high potential to act as an important green energy source but unfortunately even after 40 years of research reaction efficiencies are still quite low [10]. While the efficiency
1 remains in the low range of a few mmol g
1 cat h , the process cannot be considered attractive for an upscaled industrial application. The main hurdles of photocatalytic CO2 reduction responsible for keeping the efficiencies low are as follows: (i) the mechanism behind the conversion of CO2 to hydrocarbons being still not clear, (ii) the high thermodynamic stability of the CO2 and H2O molecules, (iii) the effect that oxygen (as a product of the reaction or present in the reaction chamber because of system leak points) has on the reaction when present during the reaction, (iv) the design and synthesis of efficient photocatalysts with high selectivity toward specific hydrocarbons of choice (e.g., CH4), and (v) the difficulty of obtaining reliable and comparable experimental results to provide a basis for the extraction of general trends. These challenges will be addressed in more detail in the rest of this chapter.

TABLE 1 Electrochemical redox potentials for CO2 reduction to useful chemicals. Eoredox V vs NHE

Equation


0.41

1

H2O → ½ O2 + 2H+ + 2e


0.82

2

CO2 + e
→CO2



1.90

3

CO2 + H + + 2e
→HCO2



0.49

4

CO2 + 2H+ + 2e
→ CO + H2O


0.53

5

CO2 + 4H+ + 4e
→ HCHO + H2O


0.48

6

CO2 + 6H+ + 6e
→ CH3OH + H2O


0.38

7


0.24

8

Reaction 2H2O+ + 2e
→ H2





CO2 + 8H + 8e → CH4 + 2H2O +

258 SECTION C Oxides and calcogenides

3

Strategic design and evaluation of metal oxides in photocatalytic CO2 reduction

The synthesis of photocatalysts for the reduction of CO2 can be considered as one of the five steps of a strategic design scheme (Fig. 3) proposed by the author. In the first step the application requirements and the corresponding desired material properties are pinpointed. This step includes the identification of the limitations posed by the thermodynamics of the reaction and the experimental setup and conditions. A photocatalyst with well-defined properties should be able to meet those set limitations. When the limitations have been identified and the desired material properties have been selected, the best-fitting synthesis technique should be selected (step 2). In cases where more than one synthesis processes provide materials with a similar set of properties, the most environment friendly one should be selected (e.g., the one using fewer organic solvents or other harmful substances). In the third step, the characterization of the material is performed with the aim of confirming that the synthesized material(s) possess(es) the targeted properties. At this point, it is reasonable to employ only the characterization techniques that study the critical properties for the material’s performance in the reaction. As a way to reduce time and costs, only a small carefully selected subset of materials should be studied possessing strategically chosen ranges of properties (e.g., metal loadings). At a later stage, when there is evidence that the material can efficiently drive the photoreaction, then a more in-depth study of the properties-of-interest can be performed to identify the best performing material. If the characterization results suggest that the materials do not possess the required properties, an alteration of the synthesis protocol (or of the synthesis procedure itself) should be made, creating a feedback loop between synthesis and characterization. After the successful completion of steps 2 and 3, the testing of the selected materials can be performed (step 4). The evaluation of the collected data takes place in step 5 and the results can be compared either with benchmark materials such as unmodified or commercially available photocatalysts and/or to already published results, but always considering the parameters under which the experiments were conducted. If needed, a different set of experimental parameters can be examined creating a feedback loop between steps 4 and 5. If after the evaluation step the efficiency of the materials is (relatively) low, then either a different synthesis process should be followed, or the desired material’s properties and application limitations should be revised. The goal of this chapter is to discuss in more detail the foregoing five steps toward a strategic design of metal-oxide-based materials for the photocatalytic reduction of CO2. The discussion will be split into two parts: (i) application limitations, desired material properties, and characterization of the photocatalysts (steps 1–3) and (ii) testing and evaluation (steps 4 and 5). While many excellent review articles have been published on the first part [9–14], the pitfalls of testing and evaluation are rarely being considered and for this reason special emphasis will be given here on providing a discussion of important factors that need to be reckoned while designing a photocatalytic reactor, setting up experimental processes, testing and evaluating photocatalysts so that reliable results can be collected. In this chapter, the author aims to share some information and tips collected from the scientific literature and personal experience in the hunt of unlocking the safe that is photocatalytic CO2 reduction, not with brute force but by identifying the critical gears behind the underlying locking mechanism.

FIG. 3 Strategic design scheme for photocatalyst in CO2 photoreduction.

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3.1 Application requirements, desired material properties, and characterization of the photocatalysts The photoreduction of CO2 poses many serious challenges which prevent its more efficient implementation. In general, the reaction follows six steps: (i) the adsorption of the reactants onto the surface of the photocatalyst, (ii) the absorption of light from the photocatalyst leading to the generation of charge carriers (e
and h+) which activate the adsorbed reactants, (iii) the formation of surface-bound reaction intermediates, (iv) their transformation to products, (v) the desorption of the products from the surface, and (vi) the regeneration of the photocatalyst. Each one of those steps significantly affects the overall efficiency and selectivity achieved by the photocatalyst but in turn these steps get affected by the selected experimental parameters (e.g., elevated temperature and/or pressure). Metal oxides have been widely used in many photocatalytic applications also including the photoreduction of CO2. The most commonly used metal oxide, either as the main studied material (bare or modified) or as a reference, is titanium dioxide (TiO2) as it has a low cost, it is nontoxic and is stable toward photocorrosion [15]. Titanium dioxide has been widely characterized and tested in many photocatalytic applications [16]. As expected, in many publications studying the reaction mechanism and kinetics, TiO2 is the material of choice [6] but the main conclusions can be adapted to other metal oxides as well. In the following paragraphs, the most important challenges of CO2 photoreduction and a photocatalyst’s desired set of properties will be discussed.

3.1.1 Light absorption from metal oxides The energy difference between the conduction band (CB) bottom and valence band (VB) top edges is called energy band gap (Eg), and it determines the range of wavelengths of solar light that a photocatalyst can absorb. A photocatalyst can have either a direct (e.g., TiO2 rutile) or an indirect (e.g., TiO2 anatase) Eg. Their difference is that in the case of the indirect Eg both a photon and a phonon need to be involved as a change in the momentum is required for the optical transition. Most common metal oxides have a wide Eg which translates in light absorption in the ultraviolet region (UV) taking up only 3%– 5% of the solar light spectrum [13]. When photons of sufficient energy reach a semiconducting metal oxide, electrons get excited (at a femtosecond scale [17]) from the VB to the CB where they can perform a reduction of surface adsorbed molecules (e.g., CO2 reduction) while holes remain in the VB taking part in the respective oxidation reactions (e.g., H2O oxidation). This charge separation is often hindered by e
and h+ recombination phenomena occurring at a microsecond time scale before the charge carriers can participate in their respective reactions. An e
can recombine with a h+ either with the release of heat (nonirradiative pathway) or light (irradiative pathway). The majority of the photogenerated charge carriers (90% [18]) recombine rapidly (at a microsecond time scale [19]) after excitation. This recombination occurs either in the bulk or on the surface of the metal oxide, and it is one of the main causes (but not the only one) for the decrease in the efficiency of a photocatalytic CO2 reduction reaction. In the picosecond to nanosecond time range a competition between the trapping and recombination of the photogenerated charge carriers occurs while in the millisecond to microsecond range the interfacial charge transfer competes with the recombination of the trapped species [20]. By suppressing the charge recombination an improvement of the photocatalytic performance could be achieved. The intensity of the incident light is a factor known to affect the photocatalytic process [21]. At low light intensities (I) the photocatalytic rate (r) has a first-order dependence on the light irradiation (r  I). At higher intensities as the influence of the recombination process becomes higher, this changes from a first order to a square root dependence of the rate on the light intensity (r  √ I). Finally, at very high light intensities the photocatalytic rate remains constant (r  I0). There are many methodologies employed to expand the light absorption capabilities of metal oxides and to induce visible light activity. These methodologies include the incorporation of metal atoms (Au, Ag, Pt, Cu, Fe, etc.) or ions in the crystalline lattice, non-metals (N, S, P, C, etc.), surface modification to introduce defects (e.g., oxygen vacancies—Vos), surface decoration, Z-schemes with other semiconducting metal oxides, photosensitization with (visible) light absorbing compounds and dyes, etc. [14, 18]. Apart from increasing the range of the absorbed photon wavelengths, many of the aforesaid modification techniques also act as charge carrier traps through the formation of defects or interfaces reducing in that way the recombination rate. A high concentration of the so-created defects or interfaces though can potentially act as recombination centers, counterbalancing in a way the improved light absorption capability of the semiconductor. By doping the photocatalyst with foreign atoms, intragap energy states are formed directly below the CB for e
—accepting atoms and directly over the VB for e
— donating atoms. In both cases a narrowing of the Eg is succeeded leading to the absorbance of photons with longer wavelengths. During the modification of a metal oxide with other metal atoms, special care should be taken on the selected metal loading [22]. While the addition of a metal cocatalyst generally increases the photocatalytic efficiency, this stands true only up to a certain metal loading. After that limit, a further increase in loading can prove detrimental to the efficiency due to coverage of the catalyst’s surface active sites, blockage of the incident light (shading effects), the creation of more recombination sites and the formation of aggregates leading to a nonuniform distribution of the metal atoms. The addition of a metal on a metal oxide can lead to the formation of a Schottky barrier at the interface which can improve the charge

260 SECTION C Oxides and calcogenides

separation and as a result the photocatalytic activity of the system [23]. The Schottky barrier prevents the transfer of electrons from the deposited metal back to the metal oxide, trapping them in the metal species. The localized surface plasmon resonance (LSPR) effect can be induced in the metal/semiconductor interface due to the synchronization of the oscillation frequency of the magnetic component of the incident irradiation with that of the electrons trapped in the interface leading to more energetic charge carriers and in turn to better photocatalytic efficiencies [24]. This LSPR effect can also extend the light absorption edge of the photocatalyst into the visible light range. The preexistence and/or the formation of Vos is discussed to increase the light absorption capabilities of a metal oxide and subsequently to improve its efficiency in photocatalytically producing hydrocarbons from CO2. Titania is a nonstoichiometric n-type semiconductor with native Vos existing in its crystal lattice. As Vos are formed, two electrons are transferred to the neighboring Ti4+ cations reducing them to Ti3+. Oxygen vacancies are considered as defects and e
traps improving the separation of the charge carriers which in turn can lead to increased efficiencies in CO2 photoreduction. The formation of Vos leads to intragap energy states directly below the CB of TiO2 acting as e
traps. Common methods to form Vos include light irradiation (a color change from white to light blue is often observed in TiO2 as a result), and thermal treatment under a reducing environment. As will be discussed in more detail in the next paragraphs of this chapter, Vos play also a very important role in the adsorption of CO2 on the surface of the catalyst, as well as in the critical, for the initiation of the photoreduction process, one-e
transfer from the photocatalyst’s CB to the bent adsorbed CO2. The formation of a Z-scheme through the combination of two semiconductors of different Eg values (usually one with a wide and one with a narrow Eg) offers a double advantage: the increased light absorption range through the incorporation of the narrow Eg semiconductor and the configuration of a highly oxidative VB and a highly reductive CB. The way a Z-scheme works is the following: the photogenerated e
of the CB of the wide band gap photocatalysts (e.g., TiO2) recombine with the positive h+ left on the VB of the narrow band gap material (e.g., C3N4). In this configuration, the highly oxidative holes of the VB of the wide Eg photocatalyst and the highly reductive electrons of the CB of the narrow Eg photocatalyst are available to drive the respective redox reactions. The modification of common metal oxides with narrow Eg quantum dots, and their photosensitization with visible-light harvesting complexes and dye molecules have been referenced in the literature [25]. In the case of dye sensitization, the e
are injected from the dye’s excited LUMO to the CB of the metal oxide located in a more positive potential. The stability of the dyes under UV irradiation and their interaction with H2O are challenges that need special consideration.

3.1.2 Thermodynamics of the carbon dioxide molecule Carbon dioxide is a linear molecule (D∞h geometry) with its two oxygen atoms connected to the carbon atom with two double covalent bonds (O ¼ C ¼ O) at an 180o angle. These double bonds in combination with the fact that CO2 is the end-product of combustion reactions make it thermodynamically very stable. This is also illustrated by its highly negative standard Gibbs free energy of formation (DGf°CO2 ¼
394 kJ mol
1). Photocatalytic CO2 reduction is a multistep consecutive process in which the transport of multiple e
and protons takes place as seen in Table 1. For the photocatalytic CO2 reduction reaction to take place, the first step is that the reactant molecules (in the simplest implementation of the reaction just CO2 and H2O) should be strongly adsorbed on the surface of the photocatalyst. Many studies have been performed describing how CO2 absorbs on the surface of a metal oxide. The orientation of a CO2 molecule adsorbed on a metal oxide can possibly be different than the linear structure of the nonadsorbed molecule. As an example, in TiO2 the possible orientations can be grouped in the following four categories [26]: (i) linear structures including parallel and perpendicular orientations of the CO2 molecule regarding to the photocatalyst’s surface, (ii) monodentate carbonates, (iii) bidentate carbonates including bridging and chelating carbonates, and (iv) monodentate and bidentate bicarbonates. The different binding orientations of CO2 to the photocatalyst’s surface and the different surficial atom(s) that either the carbon or oxygen atom of CO2 binds with, affects the charge carriers’ transfer from the CB of the semiconductor to CO2. More strongly (loosely) bound CO2 can mean that the formed intermediates and products will be desorbed slower (faster) from the surface of the photocatalyst leading to its less (more) efficient regeneration. In ambient conditions, it is known that the surface of metal oxides is covered (but not uniformly [27]) by H2O layers [28]. This means that CO2 can adsorb either directly on the surface site (e.g., on a Vo) or on the hydroxylated layer and then the interaction between CO2 and H2O should be further studied [29]. In the photocatalytic CO2 reduction process, the double C]O bonds (exhibiting a high dissociation energy of  750 kJ mol
1) of the adsorbed CO2 molecules must be ruptured and H2O must be photocatalytically split (Eqs. 1 and 2, Table 1) for C1 or C1+ hydrocarbons to be formed. It is a thermodynamic requirement for a metal oxide active in photocatalytic CO2 reduction to possess a VB top more positive (more anodic) than the H2O splitting electrochemical potential (1.23 V) and a more negative (more cathodic) CB bottom than the desired reduction potential of the studied CO2 reaction. In simpler words, the photocatalyst’s VB and CB band edge potentials should straddle the redox potentials of the reaction. In reality, there is a significant activation barrier that requires an overpotential

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exceeding the reduction and oxidation potentials of CO2 and H2O, respectively, to efficiently drive the photoreduction [30]. Unfortunately, CO2 possesses a highly negative LUMO energy meaning that the one e
transfer to it, requiring a reduction potential of E0redox ¼
1.9 eV (Eq. 3, Table 1) cannot be performed by most common metal oxides [31]. The adsorption of CO2 to the surface of the photocatalyst can lead to CO2 being bent because of the repulsive forces developed between the acquired e
(located at the electrophilic C atom) and the free e
pairs on the O2 atoms of the metal oxide’s surface [32]. This bend results in a decrease in the reduction potential of CO2/CO
2 which is considered to be the enabling step for the initiation of the CO2 photoreduction process [33]. The adsorption of CO2 in Vos can lead to more stable formations where the etransfer is more efficiently possible than in other nondeficient surfaces [34, 35].

3.1.3 Mechanism behind photocatalytic CO2 reduction One additional major challenge in the field of CO2 photoreduction is that the underlying mechanism has not been clearly identified yet. Most studies on the mechanism have been conducted using TiO2 as the material in focus, with the pathways identified being (in general) applicable to other semiconducting metal oxides as well. In literature three main reaction pathways have been proposed for the production of CH4 from CO2 named after the main intermediates in each: the formaldehyde, the glyoxal, and the carbene pathway (Fig. 4) [12]. One step that all the three mechanisms have in common is the one-electron activation of CO2 to produce the CO
2 radical (uphill reaction) as explained in previous paragraphs. Out of the three proposed pathways, the glyoxal one proposes that C2 molecules should be formed. While all three pathways seem possible, the predicted intermediates have not been experimentally identified. For a more comprehensive discussion of the mechanism of CO2 photoreduction the reader is encouraged to refer to the work of S. N. Habisreutinger and co-workers [12]. It is clear that the development of highly efficient photocatalysts passes through the unraveling of the CO2 photoreduction mechanism.

3.1.4 Structural characteristics of a metal oxide Apart from the thermodynamic limitations set by the photoreaction, the size of the metal oxide’s nanoparticles, its specific surface area and porosity are factors which can heavily contribute to the photocatalyst’s efficiency. Smaller-sized nanoparticles present the advantage that the photogenerated charge carriers need a shorter migration distance to reach the surface of the photocatalyst and participate in the respective redox reactions. In addition, a larger concentration of trapping sites is present at lower nanoparticle sizes thus facilitating a better separation of the charge carriers. The specific area and the porosity are factors that can be critical when it comes to the adsorption of the reactants. Especially as for smaller nanoparticles the surface-to-volume ratio is higher, more reactants can potentially adsorb on the active sites on the surface of the catalyst. A higher porosity can lead to similar benefits as more reactants can reach the active sites. While having more reactants adsorbed on a photocatalyst is advantageous, the bonding strength of those on the active sites should be also discussed. If the reactants bind too strongly on the photocatalyst, then it might be possible that the surface is not regenerated sufficiently blocking the active sites and effectively poisoning the material. A more detailed discussion about poisoning and deactivation of a photocatalyst is presented later in this chapter. Crystalline materials with a specific geometry (e.g., nanotubular structure) exhibit better performance in CO2 photoreduction when compared with nanoparticulated metal oxides as the former provide an easier migration of the photogenerated charge carriers and an improved adsorption of the reactants [36].

FIG. 4 Possible reaction pathways in photocatalytic CO2 reduction.

262 SECTION C Oxides and calcogenides

3.2 Strategic testing and evaluation Going toward more sustainable processes, the cost and harmfulness to the environment of the photocatalysts and chemicals used for their synthesis should be also considered. The modification of a metal oxide with one or more noble metals like Au, Ag, or Pt can produce photocatalysts with higher efficiency, but this should be weighed against the cost to synthesize such a material. Alternatives to those precious metals (e.g., Cu or non-metal atoms like N, C, or S) should be always examined as prospective cocatalysts especially when an efficient (both in terms of product yields and cost) industrial scale application is the ultimate goal. The reusability of a photocatalyst is another critical parameter in the development of a sustainable process. Ideally, a photocatalyst performs optimally for many reaction cycles without a loss in efficiency or its complete deactivation while remaining structurally intact when introduced to (photo-)catalyze the reaction. The accumulation of reaction intermediates on the surface of the catalyst blocking its available active sites is one of the main reasons for the deactivation of a photocatalyst. This poisoning of the photocatalyst requires a subsequent (heat-) treatment process where all adsorbed intermediates and products are removed from its surface rejuvenating it to participate again in the reaction. However, these additional steps result in downtime and increased costs when considering an industrial application. When modifying photocatalysts in the context of a sustainable process, friendly to environment modifiers should be used if the reaction requirements allow. For instance, cadmium sulfide (CdS) is thermodynamically sensible to be used in a Z-scheme as its CB edge potential is more negative than most commercially available metal oxides, but its use should be avoided due to presence of Cd, and alternative solutions should be considered. Similar efforts should be made to use environment friendly solvents and when possible nonorganic solvents. Nonorganic solvents also help avoiding leftover Cimpurities on the photocatalyst particularly when a high calcination temperature is not an option. As previously pointed out, the modification of a pure metal oxide with an increasing percentage of foreign metal atoms does not correlate linearly with the photocatalyst’s ability to photocatalytically produce hydrocarbons from CO2. This translates into using metals (or precursors of metals) wisely to retain a balance between the yield of the produced hydrocarbons and the quantity of metal modifier used.

3.2.1 Identification of products and evaluation of photocatalysts in CO2 reduction There are many parameters that affect the qualitative and quantitative characteristics of the products’ mix. In such a multiparametrical process, a direct comparison between different experimental setups and parameters becomes rather challenging and thus conclusions can be more safely reached, when comparing sets of materials in the same reactor and conditions. As there is no predetermined set of experimental parameters or a universal reactor employed by all research groups working in the field, the actual product distribution and concentrations of products for a specific photocatalyst might be different depending on the environment selected. Thus it might be beneficial to discuss about trends and relative yields or photonic efficiencies. Τhe identification and origin verification of the products of the reaction are crucial steps for the correct evaluation of the photoreaction. By evaluating the efficiency of a photocatalyst accurately, an optimization feedback loop can be employed and useful trends regarding the underlying CO2 conversion mechanism can be extracted. As CO2 is thermodynamically very stable, any other carbon-containing impurity will react faster in the reaction chamber under irradiation. These impurities can come from many sources: (i) Synthesis and calcination of materials: It is usual at the end of the synthesis of a photocatalyst that a calcination step is employed to produce the desired structure of the material and to remove leftover (organic) solvents. Depending on the synthesis protocol the chosen calcination temperature might not be sufficiently high to completely remove all C-containing substances and applying a higher temperature might structurally damage the photocatalyst. When performing calcination under a specific gaseous environment (e.g., synthetic air, H2, or an inert gas) a continuous gas flow should be applied to hinder the re-deposition of impurities on the heat-treated samples. The purity of the applied gases should be checked to avoid deposition of C-impurities, especially in cases where the supply is provided by (oil) pumps (e.g., building infrastructures). (ii) Gas carrying pipelines and sealing materials: In a gas-phase photocatalytic reactor, the reactant gases are introduced to the reaction chamber using gas carrying pipes usually made of stainless steel. These gas lines should be sufficiently purged before being used as accumulated impurities might be present. Connection joints are weak points where leaks could occur leading to air coming inside the lines. Leak detection should be carried out periodically to ensure that the purity of the reactant gases remains as intended without any back-diffusion of impurities. The presence of the air’s O2 can also lead to a decrease in the efficiency of the reactor as reverse (backward) reactions are possible because of the oxidation of products or of intermediates. It is probable that a low observed production of hydrocarbons may originate from an unidentified air leak and not from the material not being active. It is advisable that custom-made or commercially available stainless steel or aluminum reactors receive a heat-treatment to remove Cimpurities introduced mostly during the processing of the raw metal used to construct the reactor or during welding. (iii) Metallic sealing rings: The sealing of the reaction chamber should be preferably performed using metallic sealing rings

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or gaskets. This is particularly important if these sealing materials come in contact with the reactant gases and light. As the light intensity used in CO2 reduction experiments can be quite high, by irradiating nonmetallic sealing materials (e.g., elastomeric O-rings) C-containing products might be formed which remain in the reaction chamber or get adsorbed on the photocatalyst. Sealing rings should be changed as often as possible for a perfect sealing so as not to let air come in during the reaction. If using metallic sealing rings is not an option (e.g., when a quartz window comes in tight contact with the reaction chamber), the design of the reactor should be such that no light reaches the nonmetallic sealing rings and/or that UV-resistant elastomeric rings are employed. (iv) Purity of gaseous reactants and pressure regulators: When trying to identify how efficient a studied material performs in CO2 photoreduction or when performing mechanistic studies, gases of the highest purity should be used. The use of high-purity gasses coupled with the appropriate pressure regulators minimizes the amount of C-impurities introduced in the reactor. (v) Proper handling of materials and reactors: It is important that gloves are being worn when handling the reactor or any special sample holder inserted in the reaction chamber. Oily residues from fingerprints can produce hydrocarbons under irradiation leading to the overestimation of the material’s performance. (vi) Vacuum pumps: If the use of a vacuum pump is required to evacuate the reaction chamber or the gas carrying pipelines, an oil trap should be attached to prevent oil (or oil fumes) from polluting the experimental setup. As the observed concentrations of the photocatalytic CO2 reduction products are quite low, it is important that sensitive identification methods are used. Gas chromatography (GC) is being widely used for the identification of the reaction products using a combination of FID (flame ionization), TCD (thermal conductivity), and BID (barrier discharge ionization) detectors. Modern GCs can accurately identify products at concentrations as low as 1 ppm for most targeted molecules. With this high sensitivity a convenient identification of impurities is possible. For this reason, it is highly advisable that blank experiments are performed before each set of measurements. These blank experiments should contain: (i) Measurements without a photocatalyst but with an inert gas atmosphere under illumination to identify C-impurities coming from the reactor itself (e.g., reactant gases, sealing rings, leaks). If impurities are identified, their source should be spotted before initiating any further measurements. (ii) Measurements with the photocatalyst present under inert gas atmosphere and illumination: These measurements will act both as a cleaning step and as a set of blank measurements. To collect reliable results, a cleaning process is critical before measuring a sample in CO2 photoreduction. The cleaning process should comprise of two steps: (i) calcination of the sample at a temperature high enough to remove any loosely bound impurities but not so high that it damages the material or changes its structure and (ii) cleaning with light under an inert gas (e.g., Ar or He) atmosphere. The second cleaning step, which takes place with the photocatalyst inside the reaction chamber, allows C-containing species (and adsorbed CO2) to react under light producing hydrocarbons that are removed from the reactor. Overtime, the concentration of these hydrocarbons gets lower finally reaching a sufficiently low and steady concentration (or impurities get completely removed). In this state, most (or all) of the impurities on the surface of the material are removed and the produced hydrocarbons would not be added up to those produced by the actual CO2 reduction process. The said measurements should be the minimum blank measurements performed before a sample is being tested. The combination of a wellcalibrated mass spectrometer (MS) and 13C labeling (in the form of 13CO2) can provide a definite proof of the origin of the products in CO2 photoreduction. Unfortunately, this combination is quite expensive and not easily available for groups to perform extensive measurements on samples. For this reason, the simple blank experiments proposed above can be sufficient when an MS is not available. As studied in a previous publication of our group [36], in a large percentage of the literature about TiO2 and photocatalytic CO2 reduction no blank experiments were performed (or if performed were not mentioned in the publications) and thus the origin of the hydrocarbon products was not clear. These uncertain results impede researchers from deriving reliable conclusions and should not be considered when strategically designing a material. Special attention should be paid about the use of C-containing samples, like g-C3N4 or graphene-based photocatalysts, as they can possibly act as a constant carbon pool [37].

4 Conclusions Photocatalytic CO2 reduction is a multi-parameter reaction and while the reaction is very interesting and its potential of becoming an alternative “green source” of energy is very high, the challenges needed to be overcome are still significant. As the mechanism behind the photoreduction of CO2 is still not clear despite the extensive efforts of many research groups globally, the synthesis of optimally performing photocatalysts becomes even harder. As the problems that CO2 photoreduction is called to solve become continuously more ominous, a strategic design, testing, and evaluation system should be set. This system is based on defining as many of the thermodynamic requirements set by the reaction, selecting the most desirable material properties overcoming the thermodynamic limitations and setting up measurement protocols and high-purity experimental setups able to provide reliable and reproducible data without over- or underestimating the efficiency of the tested materials. Metal oxides have been extensively used in the field of photocatalysis

264 SECTION C Oxides and calcogenides

as they have many desirable intrinsic properties. The high thermodynamic stability of the CO2 molecule asks for photocatalysts able to straddle both the CO2 reduction potential as well as the H2O oxidation one. As most commonly available metal oxides do not possess the required straddling combination of potentials, it is necessary that modification techniques get involved. These techniques, preferably cost effective and sustainable toward the environment, should lead to a photocatalyst possessing a very unique set of properties: offering surface-located active sites for the reactants to bind, having the required reduction and oxidation potentials, inhibiting the recombination of charge carriers and also exhibiting an as wide light absorption range as possible to harvest more incoming photons. Such a unique combination of properties is understandably not easy to accomplish but a careful design of the catalyst is overall more logical than an extensive trial-and-error approach. As research time is valuable and there is only a finite number of combinations of materials and experimental parameters that can be measured, it is critical that the results obtained are reliable. The sources of C-impurities and O2 leaks in a CO2 photoreduction experiment can be many, but if identified they can (in the most cases) be effectively eliminated. Blank experiments should be the minimum requirement in CO2 reduction in an attempt to identify the true origin of the observed products.

Acknowledgments The author thank the German Ministry of Education and Research (Bundesministerium f€ ur Bildung und Forschung, BMBF; F€ ordermaßnahme CO2WIN, F€orderkennzeichen 033RC024A, PRODIGY) for financing this work and Prof. Dr. Jennifer Strunk (LIKAT, Rostock) for the fruitful discussions and her constructive comments during the preparation of this chapter.

Dedications This chapter is dedicated to the loving memory of Maria Moustaka and Alexandra Loupasi.

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[17] R. Qiana, H. Zong, J. Schneider, G. Zhou, T. Zhao, Y. Li, J. Yang, D.W. Bahnemann, J. Hong Pan, Charge carrier trapping, recombination and transfer during TiO2 photocatalysis: an overview, Catal. Today 335 (2019) 78–90. [18] J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo, D.W. Bahnemann, Understanding TiO2 photocatalysis: mechanisms and materials, Chem. Rev. 114 (2014) 9919–9986. [19] H.H. Mohamed, D.W. Bahnemann, The role of electron transfer in photocatalysis: fact and fictions, Appl. Catal. B Environ. 128 (2012) 91–104. [20] V. Etacheri, C. Di Valentin, J. Schneider, D. Bahnemann, S.C. Pillai, Visible-light activation of TiO2 photocatalysts: advances in theory and experiments, J. Photochem. Photobiol. C: Photochem. Rev. 25 (2015) 1–29. [21] D.F. Ollis, E. Pelizzetti, N. Serpone, Photocatalyzed destruction of water contaminants, Environ. Sci. Technol. 25 (1991) 1522–1529. [22] X. Li, J. Yu, M. Jaroniec, X. Chen, Cocatalysts for selective photoreduction of CO2 into solar fuels, Chem. Rev. 119 (2019) 3962–4179. [23] A.L. Linsebigler, G. Lu, T. John, J. Yates, Photocatalysis on TiO2 surfaces: principles, mechanisms and selected results, Chem. Rev. 95 (1995) 735–758.  [24] K. Koc´ı, K. Mateˇju˚, L. Obalova´, S. Krejc´ıkova´, Z. Lacny´, D. Placha´, L. Capek, A. Hospodkova´, O. Sˇolcova´, Effect of silver doping in the TiO2 for photocatalytic reduction of CO2, Appl. Catal. B Environ. 96 (2010) 239–244. [25] H.-L. Wu, X.-B. Li, C.-H. Tung, L.-Z. Wu, Semiconductor quantum dots: an emerging candidate for CO2 Photoreduction, Adv. Mater. 31 (2019) 1900709. [26] L. Mino, G. Spoto, A.M. Ferrari, CO2 capture by TiO2 anatase surfaces: a combined DFT and FTIR study, J. Phys. Chem. C 118 (2014) 25016–25026. [27] J. Baltrusaitis, J. Schuttlefied, E. Zeitler, V.H. Grassian, Carbon dioxide adsorption on oxide nanoparticle surfaces, Chem. Eng. J. 170 (2011) 471–481. [28] G. Ketteler, S. Yamamoto, H. Bluhm, K. Andersson, D.E. Starr, D.F. Ogletree, H. Ogasawara, A. Nilsson, M. Salmeron, The nature of water nucleation sites on TiO2 (110) surfaces revealed by ambient pressure X-ray photoelectron spectroscopy, J. Phys. Chem. C 111 (2007) 8282–8287. [29] F. Parrino, C. De Pasquale, L. Palmisano, Influence of surface related phenomena on mechanism, selectivity, and conversion of TiO2-induced photocatalytic reactions, ChemSusChem 12 (2019) 589–602. [30] F. Fresno, I.J. Villar-Garcı´a, L. Collado, E. Alfonso-Gonza´lez, P. Ren˜ones, M. Barawi, V.A. de la Pen˜a O’Shea, Mechanistic view of the main current issues in photocatalytic CO2 reduction, J. Phys. Chem. Lett. 9 (2018) 7192–7204. [31] X. Chang, T. Wang, J. Gong, CO2 photo-reduction: insights into CO2 activation and reaction on surfaces of photocatalysts, Energy Environ. Sci. 9 (2016) 2177–2196. [32] C. Peng, G. Reid, H. Wang, P. Hu, Perspective: Photocatalytic reduction of CO2 to solar fuels over semiconductors, J. Chem. Phys. 147 (2017), 030901. ´ lvarez, M. Borges, J.J. Corral-Perez, J.G. Olcina, L. Hu, D. Cornu, R. Huang, D. Stoian, A. Urakawa, CO2 activation over catalytic surfaces, [33] A. A ChemPhysChem 18 (2017) 3135–3141. [34] H. Zhao, F. Pan, Y. Li, A review on the effects of TiO2 surface point defects on CO2 photoreduction with H2O, J. Mater. 3 (2017) 17–32. [35] V.P. Indrakanti, H.H. Schobert, J.D. Kubicki, Quantum mechanical modeling of CO2 interactions with irradiated stoichiometric and oxygen deficient anatase TiO2 surfaces: implications for the photocatalytic reduction of CO2, Energy Fuel 23 (2009) 5247–5256. [36] N.G. Moustakas, J. Strunk, Photocatalytic CO2 reduction on TiO2-based materials under controlled reaction conditions: systematic insights from a literature study, Chem. Eur. J. 24 (2018) 12739–12746. [37] F.R. Pomilla, M.A.L.R.M. Cortes, J.W.J. Hamilton, R. Molinari, G. Barbieri, G. Marcı`, L. Palmisano, P.K. Sharma, A. Brown, J.A. Byrne, An investigation into the stability of graphitic C3N4 as a photocatalyst for CO2 reduction, J. Phys. Chem. C 122 (2018) 28727–28738.

Chapter 17

Nanostructured sulfide based photocatalysts using visible light for environmental and energy purposes Jose C. Conesa Institute of Catalysis and Petrochemistry, CSIC, Madrid, Spain

1 Introduction Photocatalysis is gaining a relevant role in a wide number of fields: environment protection, energy-related applications, fine chemicals synthesis, or detection of specific chemicals. Photocatalysis is, indeed, known for over one century; to this author’s knowledge, the first paper on heterogeneous photocatalysis (the homogeneous variant is older) was published in 1913 by B. Moore and T.A. Webster [1], describing the photoreduction of CO2 to formaldehyde on colloids of uranium or iron oxides using visible light. Because of the current urgent need to limit the increase of CO2 in the atmosphere, this is certainly an inspiring work. This article was published shortly after the premonitory one of G. Ciamician [2] whose last sentences claimed “So far, human civilization has made use almost exclusively of fossil solar energy. Would it not be advantageous to make better use of radiant energy?” Only almost six decades later could an article by Fujishima and Honda appear in 1971 [3] (thus, 1 year before the famous one in Nature) proposing a practical way to photo-dissociate water into O2 and H2 using photo-electrochemistry. Fujishima and Honda used in their work a single crystal of rutile TiO2. This material, due to its band gap Eg ¼ 3.0 eV, absorbs light only in the near UV range, making it inadequate to convert wide ranges of the solar spectrum. Other oxides like anatase TiO2, ZnO, or SrTiO3, with band gaps above 3.0 eV, have the same limitation, although for environmental protection and some fine chemicals syntheses the anatase-type TiO2 photocatalyst remains unsurpassed. Thus, many other materials have been studied trying to enlarge the solar spectrum utilizable. This includes doping anatase with cations or anions, or developing completely different oxides like the relatively efficient BiVO4 (with Eg ¼ 2.4 eV); also (oxy)sulfides, (oxy)nitrides, selenides, doped carbons (in several varieties), and other materials have been proposed to widen the solar spectrum range used. Of course, many of these materials can be used as well in photo-electrochemical (PEC) systems; one example is Fe2O3 in the hematite polymorph, which has rather a small mobility of the photo-induced charge carriers leading to a high recombination rate unless ultra-small thicknesses are used, but it is actively studied for PEC applications due to its abundance and convenient band gap ( 1.9 eV). Mixing of different phases to facilitate the separation of photoinduced electrons and holes is also a much-investigated topic. Besides, for energy-related applications in most cases, a co-catalyst is required to facilitate the evolution of O2 or H2, the reduction of CO2, or the conversion of biomass-derived substances. This chapter focuses on the use of sulfides as photocatalysts; these can in most cases use visible light, in some cases even infrared light. Extensive reviews on photocatalysis using sulfides for environmental protection [4] and organic molecule transformations [5] have appeared recently, as well as others, dealing with more general photocatalysts (including sulfides), devoted to CO2 reduction [6] and H2 generation [7, 8]. Also, sulfides containing several cations have been examined for photocatalysis or energy harvesting purposes [9, 10]. Besides, the mixing of sulfides with other materials has been investigated extensively to better separate the photogenerated electrons and holes (the so-called Z scheme when both semiconductors absorb light) [11]. It is known that sulfides are prone to photo-corrosion, especially under oxidizing conditions; therefore, efforts are directed to minimizing or avoiding this process [12]. Because of this problem, many sulfide photocatalysts are used mainly for photoreduction processes, as is the case of CO2 reduction or H2 generation. In many cases, this requires using a sacrificial agent, e.g., sulfide or sulfite ions, which makes the process valid for basic studies but not for practical use unless the sacrificial agent is the one derived e.g., from biomass.

Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00012-X Copyright © 2021 Elsevier Inc. All rights reserved.

267

268 SECTION C Oxides and calcogenides

2

Tri- and tetrahedrally coordinated sulfides

One tetrahedrally coordinated sulfide that has been considered for photocatalysis is ZnS. It has the drawback of having a rather large band gap: 3.4 eV [13], so that it absorbs only UV light. Still, it has found use in photocatalysis for both organic pollutants removal and H2 production [14]. Interestingly, this material can be obtained in very different shapes, which may influence its photophysical and photocatalytic properties [15]. Maybe, the sulfide most studied for photocatalysis is CdS, also tetrahedrally coordinated. Due to the fast mobility of its photo-excited charge carriers (electrons and holes) and its sizeable capability of absorbing visible light, its band gap is 2.48 eV [13] and implies that only the spectral range 8 wt%) is doped on the CdS surface, the rate of photocatalytic hydrogen production will be reduced due to the possibility that the BP-MoS2 nanosheets may inhibit photogenerated electrons. Besides, the lower BP-MoS2 loading also showed a reduced hydrogen production rate which is recognized to fewer active sites in the nanohybrid. Huang et al. prepared 2D ZnIn2S4/MoS2 composites for photocatalytic water-splitting [64]. The possible reaction mechanism is shown in Fig. 9. According to the results, the superior photocatalytic performance of 2D ZnIn2S4/MoS2 nanohybrid influence the following issues, one is the quantum confinement effect of ultrathin ZnIn2S4 and effective charge transfer to MoS2 nanosheets, second is the huge and tight surface interaction between ZnIn2S4 and MoS2 nanosheets accelerates the separation of photoinduced holes and electron.

5.2 Photocatalytic degradation With the acceleration of global industrialization, organic pollution of the water environment has become one of the global environmental protection issues. In particular, large-scale discharge of high-concentration organic wastewater poses an important hazard to the environment and organisms owing to its durability and poisonousness. Upto now, photocatalysis

FIG. 8 (A) Photocatalytic hydrogen generation for different MoS2/TiO2 composites vs different annealing temperatures. (B) Photocatalytic mechanism diagram for hydrogen evaluation reaction with MoS2/TiO2 photocatalysts. Reprinted with permission from F.J. Zhang, X. Li, X.Y. Sun, C. Kong, W.J. Xie, Z. Li, J. Liu, Surface partilly oxidized MoS2 nanosheets as a higher efficient cocatalyst for photocatalytic hydrogen production, Appl. Surf. Sci., 487 (2019) 734–742. Copyright (2019) Elsevier.

292

SECTION

C Oxides and calcogenides

FIG. 9 (A, C) Photocatalytic hydrogen evaluation mechanism with pure ZnIn2S4 and (B, D) hybrid 2D ZIS/M. Reprinted with permission from L. Huang, B. Han, X. Huang, S. Liang, Z. Deng, W. Chen, … H. Deng, Ultrathin 2D/2D ZnIn2S4/MoS2 hybrids for boosted photocatalytic hydrogen evolution under visible light, J. Alloys Compd. 798 (2019) 553–559. Copyright (2019) Elsevier.

is considered to be the most cost-effective and ecologically friendly technique. The basic principle of photocatalysis is to convert solar energy into chemical energy and produce associated free radical groups with redox ability. Recently, the researchers work to develop photocatalysts with great photo responsiveness, such as BiVO4, Bi2WO6, TiO2, and MoS2. Among these photocatalysts, MoS2 has attracted people’s attention due to its excellent adsorption capability and adjustable energy band structure. Various photocatalysts using composite catalysts composed of molybdenum disulfide have shown excellent photocatalytic performance as summarized in Table. 2. Recently, bismuth-based photocatalysts have received a remarkable concern for their outstanding optical properties, such as BiFeWO6, Bi2S3, and BiOI. To further increase its photocatalytic performance, Senthil et al. synthesized the BiFeWO6/MoS2 composite materials by hydrothermal method and then degraded pollutants under visible light irradiation [65]. The results show that 5 mg BiFeWO6 doping can significantly improve the degradation effect and the 5 mg BiFeWO6/ MoS2 composite material exposed the highest photocatalytic performance and removing about 100% of RhB under 75 min of irradiation. Pure BiFeWO6, pure MoS2, and composite BiFeWO6/MoS2 materials have Eg values of 2.03, 1.91, and 1.81 eV, respectively. The decrease in the Eg value of BiFeWO6/MoS2 composite materials proves that their visible light absorption capacity is improved, which in turn reduces electron–hole re-entry combined to improve the ability of dye degradation. The proposed mechanism for the photocatalytic dye degradation by BiFeWO6/MoS2 composite is shown in Fig. 10A. The Z migration process significantly reduces the recombination rate, improves the separation efficiency of photo-excited electron–hole pairs, and increases the photocatalytic performance of BiFeWO6/MoS2 composites. Drmosh et al. synthesized Bi2S3/MoS2/TiO2 ternary photocatalyst by the microwave-assisted hydrothermal method [66]. The result showed that photocatalyst exhibits an enhanced ability of photocatalytic activity. After 4 min of sunlight exposure, 99% MB removal was achieved. The proposed mechanism for photocatalytic dye degradation on BiFeWO6/MoS2 composite is shown in Fig. 10B. The photodegradation of MB can be endorsed to the introduction of multiple ways of electron transfer, effectively inhibiting the photoelectron-hole recombination in the ternary heterostructure nanocomposite system. The combination of Bi2S3 nanorods, MoS2 nanosheets, and TiO2 nanotubes increases the surface area and expands the light collection ability, enhancing the photocatalytic activity. Based on the above, introducing MoS2 into a Bi-based material to

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TABLE 2 Photocatalytic degradation of MoS2-based hybrid nanomaterials. Early concentration

Reaction time (min)

Degradation efficiency

References

Photocatalysts

Source of light

Pollutants

BiFeWO6/MoS2

Xe lamp with 500 W (l > 420 nm)

RhB

75 mL 25 mg/L

75

100%

[65]

Bi2S3/MoS2 /TiO2

250 W Xe lamp

MB

200 mL 10 mg/L

4

99%

[66]

MoS2/TiO2

White-light LED lamp

RhB

30 mL 30 mg/L

120

95%

[67]

B/ZnO/MoS2

90 W white light emitting CFL lamps

MB

50 mL 1  10
5 M

180

95%

[68]

MoS2/ZnO

Natural solar light

MB

100 mL 10 mg/L

20

97%

[69]

Z-scheme MoS2/ g-C3N4

Xe lamp with 500 W

BPA

40 mL 10 mg/L

150

96%

[70]

MoS2-GO

Xe lamp with 500 W

MB

50 mL 10 mg/L

60

99%

[71]

Cu/RGO/MoS2

Xe lamp with 300 W

RhB

100 mL 20 mg/L

30

100%

[72]

SnO2-MoS2

85 W CFL

MB

50 mL 100 mg/L

120

58.5%

[73]

2,4-DCP

80 mL 10 mg/L

20

100%

MB

80 mL 40 mg/L

12

100%

14

100%

22

100%

MR SnO2/Ag/MoS2

300 W Xe lamp

RhB MO

94% [74]

BPA, bisphenol A; 2,4-DCP, 2,4-diclorophenol; MB, methylene blue; MO, methyl orange; MR, methylene red; RhB, rhodamine B.

form a heterostructure can significantly increase the surface area and active sites of the catalyst. Moreover, the combination of MoS2 into a Bi-based material can efficiently suppress the quick recombination of photo electron–hole (e/h) pair and extend the life of carriers. Zinc dioxide (ZnO) and titanium oxide (TiO2) are well-known photocatalysts, which have the advantages of chemical stability, nontoxicity, and earth-abundant. However, they have a wider bandgap and only absorb ultraviolet light. To increase their absorption spectral range, one of the best approaches is to couple it with a small bandgap photocatalyst. Su et al. synthesized several layers of MoS2@TiO2 hollow sphere heterostructure by the hydrothermal method [67]. A small amount of MoS2 layer has a high full-spectrum absorption capability, and thereby enhancing the light absorption capability of the composite material. Also, MoS2 rises the number of active sites and additionally reduces the recombination rate of photo electron–hole pairs. Therefore, compared by the way of pure MoS2 and TiO2 hollow spheres, MoS2@TiO2 composite has higher photocatalytic activity. After 120 min of exposure to visible light, 95% of rhodamine B (RhB) was degraded. Kaur et al. dispersed MoS2/ZnO in 100 mL of MB aqueous solution (10 mg/L) [68, 69]. After 20 min of natural light exposure, 97% MB was decomposed. Compared with pure ZnO, the PL (Photoluminescence) intensity of the MoS2/ ZnO heterostructure is strongly quenched, and thereby improving the photochemical quantum efficiency of the heterostructure. It has been reported that compounding with conductive materials, such as carbon-based nanomaterials, can support separate solar light-driven electron–hole pairs, and thus enhancing the photocatalytic activity of semiconductors. Liu et al. synthesized a Z-scheme type photocatalyst (MoS2/g-C3N4) via a facile ultrasonic dispersion and annealing process [70]. Bisphenol A was used as the target contaminant to estimate the photocatalytic activity of MoS2/ g-C3N4 composite. 0.5% MoS2/g-C3N4 composite showed superior photodegradation activity. During the visible light irradiation at 150 min, the photodegradation amount of MO reached 96%. The close Z scheme between the heterostructure of

294 SECTION C Oxides and calcogenides

FIG. 10 A The mechanism of Z-scheme BiFeWO6/MoS2 composite for photodegradation of RhB under visible-light irradiation. (B) A plausible mechanism showing heterostructure and double Zscheme e/h pair transfer occurring onto the surface of ternary Bi2S3/MoS2/TiO2 under UV–vis radiation. Panel (A) Reprinted with permission from R.A. Senthil, S. Osman, J. Pan, Y. Sun, T.R. Kumar, A. Manikandan, A facile hydrothermal synthesis of visible-light responsive BiFeWO6/MoS2 composite as superior photocatalyst for degradation of organic pollutants, Ceram. Int. 45(15) (2019) 18683–18690, Copyright (2019) Elsevier.Panel (B) Reprinted with permission from Q.A. Drmosh, A. Hezam, A.H.Y. Hendi, M. Qamar, Z.H. Yamani, K. Byrappa, Ternary Bi2S3/MoS2/TiO2 with double Z-scheme configuration as high performance photocatalyst, Appl. Surf. Sci. 499 (2020) 143938, Copyright (2020) Elsevier.

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MoS2/g-C3N4 enhances the photocatalytic activity, thereby improving the light collection ability, promoting rapid charge separation, and creating highly active sites. Ding et al. synthesized MoS2-GO nanocomposites by a hydrothermal gel technique and studied their photocatalytic activity for the degradation of MB [71]. The results showed that 10% of MoS2-GO exhibited the highest degradation rate of MB after 60 min of solar light irradiation and 99% MB was degraded. The introduction of MoS2 substantially enhances the light absorption capacity of the material and decreases the recombination rate of the photogenerated electron–hole, thereby improving the photocatalytic performances. Moreover, SnO2 is a wide bandgap with an n-type photocatalyst. The SnO2-modified composite material not only has a larger specific surface area and porosity [75] but also increases the bandgap of MoS2, where MoS2 can exhibit a quantum confinement effect which brings MoS2 redox potential changes. Khan et al. synthesized a MoS2 with flower-like nanostructure by hydrothermal method and at the same time modified with Ag and combined with SnO2 to reduce the charge recombination rate, thus improving the photocatalytic activity of visible light [74]. The results show that the absorption of visible light enormously increases due to the addition of both Ag and SnO2, whereas the SnO2/Ag/MoS2 hybrid material gave the highest photocatalytic performance because it can degrade 100% of 2,4-dichlorophenol (2,4-DCP) in 20 min. Besides, the degradation of MO, RhB, and MB by SnO2/Ag/MoS2 also showed high photocatalytic activity compared with pristine MoS2 as shown in Fig. 11. The photocatalytic degradation of MB and RhB exposed complete removal within 12 and 14 min, and for the degradation of MO, about 88% was degraded within 16 min. Therefore, the incorporation of different materials into MoS2 can encourage the light absorption capacity of the material, thereby increasing the photocatalytic activity.

5.3 Photocatalytic CO2 reduction The population expansion and worldwide unavoidable industrialization alterations to the earth’s atmosphere by releasing reactive greenhouse gas, such as carbon dioxide (CO2), has produced various environmental disasters. Therefore unique approaches have been required to decrease the emission of CO2, including the CO2 conversion and capture [76].

FIG. 11 (A–D) The photocatalytic performance of MoS2, Ag/MoS2, and x-SnO2/Ag/MoS2 nanocomposites under the visible light irradiation. Reprinted with permission from B. Khan, F. Raziq, M.B. Faheem, M.U. Farooq, S. Hussain, F. Ali, … H. Tian, Electronic and nanostructure engineering of bifunctional MoS2 towards exceptional visible-light photocatalytic CO2 reduction and pollutant degradation, J. Hazard. Mater. 381 (2020) 120972. Copyright (2020) Elsevier.

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The CO2 conversion into syngas fuels through a photocatalysis technique is being received potential attention for mitigation of CO2, in which the application of solar energy is an auspicious approach owing to its properties of inexhaustibility and green energy sources [77]. The reduction of CO2 by a photocatalytic method is generally considered as a proton-assisted multiple-electron reduction reaction, which starts with the O ¼ C ¼ O bond cleavage to produce fresh carbon bonds and includes various intermediate steps as seen in Table 3. Jia et al. recently prepared heterostructure composite MoS2/TiO2 by a hydrothermal method and the prepared nanocomposites with small bandgap enhanced photocatalytic CO2 reduction into a hydrocarbon fuel using visible light irradiation [79]. Fig. 12A and B illustrate the yields of CH4 and CO between the catalysts versus light irradiation time. The higher amounts of the products of CO (268.97 mmol/g.cat) and CH4 (49.93 mmol/g.cat) were produced by the 10% MoS2/ TiO2 heterostructures, which enhanced photocatalytic activity than pure MoS2 and pure TiO2 (P-25). Besides, the selectivity of the CO2 reduction into a generated product was also measured. Inspired by this, the establishments of CO and

TABLE 3 Mechanism of CO2 photocatalytic reduction major yields with their associated redox potential values (pH 7) [78]. Reaction CO2 + 8H+ + 4e
! CH4 + 2H2O


CO2 + 6H + 6e ! CH3OH + H2O +


0.24 V
0.38 V

Formaldehyde


0.48 V




Formic acid


0.61 V




Carbon monoxide


0.53 V

CO2 + 2H + 2e ! CO + H2O +

Methane Methanol

CO2 + 2H + 2e ! HCOOH +

E0Redox




CO2 + 4H + 4e ! HCHO +

Product

300

60

(A) 200 150 100 50 0

40 30 20 10 0

0

1

2

3

4

6

5

1

0

Irradiation time (h)

2

3

4

5

6

Irradiation time (h) 300

300

268.97

200

100

50.51

(D) th

CO CH4

49.93

2th

1

250

Products(mmol/g.cat)

(C) Product(Pmol/g.cat)

TiO2 5% MoS2/TiO2 10% MoS2/TiO2 15% MoS2/TiO2 MoS2

50 Yield of CH4 (mmol/g.cat)

Yield of CO (Pmol/g.cat)

(B)

TiO2 5% MoS2/TiO2 10% MoS2/TiO2 15% MoS2/TiO2 MoS2

250

3th

4th

200 150 100 50

23.02 0

6.02

3.07 TiO2

MoS2/TiO2-10% Photocatalysts

MoS2

0 0

3

6

9

12

15

18

21

24

27

Time (h)

FIG. 12 Photocatalytic CO2 reduction products of (A) CH4, (B) CO, (C) CH4, and CO versus visible light irradiation time for 6 h. Reprinted with permission from P.Y. Jia, R.T. Guo, W.G. Pan, C.Y. Huang, J.Y. Tang, X.Y. Liu, … Q.Y. Xu, The MoS2/TiO2 heterojunction composites with enhanced activity for CO2 photocatalytic reduction under visible light irradiation. Colloids Surf. A Physicochem, Eng. Asp. 570 (2019) 306–316. Copyright (2019) Elsevier.

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CH4 need two and eight electrons in CO2 photoreduction. According to Fig. 12C, the yield of CH4 is lower than that of CO, which is defined by two important reasons. The active sites of TiO2 are possibly enclosed by CO and C product during the CO2 photocatalytic reduction, which is lacking to produce more numbers of CH4 [80–82]. Furthermore, the presence of an excessive amount of oxygen-containing groups on the MoS2 surface hampers the absorption of H2O molecules, noticeable to a deficient resource of protons, which also results in the desired manufacture of CO [83]. The stability of the heterojunction nanomaterials was also studied for photocatalytic CO2 reduction. To evaluate the samples’ stability, the 10% heterojunction nanomaterials of MoS2/TiO2 were investigated for four rounds (6-h each cycle). The results are presented in Fig. 12D and the photocatalytic performances of the heterojunction composites slightly altered after visible light irradiation. Besides, with the increase of the visible light irradiation time, the yield of the CO and CH4 was also extended, where overall results proved that the 10% MoS2/TiO2 heterojunction materials are relatively steady during visible light irradiation. The photocatalytic CO2 reduction mechanism is simply understood and shown in Fig. 13. This figure showed that the conduction band of MoS2 edge potential (
0.93 V) is more negative than that of TiO2 (
0.55 V). Hence, the MoS2 sheets can easily transfer the photo-induced electrons to TiO2 via their heterojunction interface. Likewise, the huge variance between the MoS2 and TiO2 valance band edge potential could favor the transfer of photo-generated holes from TiO2 to MoS2. Also, the inner electric field could put away the preference of electron–hole recombination and produce a huge amount of holes on the MoS2 surface and a great number of electrons on the TiO2 surface [80]. Therefore the holes on the MoS2 valance band are surrounded by the water to form H+ and O2, respectively. Simultaneously the electrons on the CB of TiO2 can be trapped by CO2 to produce CO and CH4. In conclusion, the synergistic effect between the heterostructured of both MoS2 and TiO2 prevents the recombination of hole pair electrons, which results in an improvement on the photocatalytic CO2 reduction performances of heterojunction by MoS2/TiO2 nanomaterials.

6 Summary and perspective This article has briefly reviewed the fundamental characteristics and preparation methods of MoS2 and related applications. In the recent years, many catalysts based on MoS2 have been developed for many applications. In addition, this article also introduces how to improve the catalytic performance of MoS2. Recently, a large number of reports indicate that MoS2 can Pontential (V vs. NHE) pH=7

–0.93

CH4/CO – CB e

CH2/H2O

CB e

-

–0.55

CO2/CO–0.52V hn

CO2/CH4–0.24V

0 hn H2O/O2+0.82V

+0.95

VB h+

+2.49

MoS2 VB

+

h

H2O O2/H+

TiO2

E1

e–

Contact electric field

EB

h+

E2

e– h+

Potential barrier

TiO2

Interface

MoS2

FIG. 13 Possible photocatalytic reduction mechanism of CO2 over MoS2/TiO2 heterojunction nanocomposites, where electric field E1 and E2 are initiated by the redistribution of interface structured TiO2 and MoS2 composite. Reprinted with permission from P.Y. Jia, R.T. Guo, W.G. Pan, C.Y. Huang, J.Y. Tang, X.Y. Liu, … Q.Y. Xu, The MoS2/TiO2 heterojunction composites with enhanced activity for CO2 photocatalytic reduction under visible light irradiation. Colloids Surf. A Physicochem, Eng. Asp. 570 (2019) 306–316, Copyright (2019) Elsevier.

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replace Pt as the most effective cocatalyst, indicating MoS2 can further promote its applications. However, the quick recombination of photo-induced electron–hole pairs, controlled active edge sites, and problems in recovering photocatalysts are the key obstacles which delay the commercialization of MoS2 nanomaterials. Therefore, much efforts should be paid to overcome these shortcomings in the future and to prepare highly stable and effective MoS2 photocatalysts, making them as more advantage in photocatalytic applications.

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Kim, Few layered black phosphorus/MoS2 nanohybrid: a promising co-catalyst for solar-driven hydrogen evolution, Appl. Catal. B Environ. 241 (2019) 491–498. [64] L. Huang, B. Han, X. Huang, S. Liang, Z. Deng, W. Chen, H. Deng, Ultrathin 2D/2D ZnIn2S4/MoS2 hybrids for boosted photocatalytic hydrogen evolution under visible light, J. Alloys Compd. 798 (2019) 553–559. [65] R.A. Senthil, S. Osman, J. Pan, Y. Sun, T.R. Kumar, A. Manikandan, A facile hydrothermal synthesis of visible-light responsive BiFeWO6/ MoS2 composite as superior photocatalyst for degradation of organic pollutants, Ceram. Int. 45 (15) (2019) 18683–18690. [66] Q.A. Drmosh, A. Hezam, A.H.Y. Hendi, M. Qamar, Z.H. Yamani, K. Byrappa, Ternary Bi2S3/MoS2/TiO2 with double Z-scheme configuration as high performance photocatalyst, Appl. Surf. Sci. 499 (2020) 143938. [67] J. Su, S. Yu, M. Xu, Y. Guo, X. Sun, Y. Fan, W. Zhao, Enhanced visible light photocatalytic performances of few-layer MoS2@TiO2 hollow spheres heterostructures, Mater. Res. Bull. 130 (2020) 110936. [68] S. Kumar, V. Maivizhikannan, J. Drews, V. Krishnan, Fabrication of nanoheterostructures of boron doped ZnO-MoS2 with enhanced photostability and photocatalytic activity for environmental remediation applications, Vacuum 163 (2019) 88–98. [69] M. Kaur, A. Umar, S.K. Mehta, S. Singh, S.K. Kansal, H. Fouad, O.Y. Alothman, Rapid solar-light driven superior photocatalytic degradation of methylene blue using MoS2-ZnO heterostructure nanorods photocatalyst, Materials 11 (11) (2018) 2254. [70] H. Liu, J. Liang, J. Du, Q. Gao, S. Fu, L. Li, J. Zhou, Promoting charge separation in dual defect mediated Z-scheme MoS2/g-C3N4 photocatalysts for enhanced photocatalytic degradation activity: synergistic effect insight, Colloids Surf. A Physicochem. Eng. Asp. 594 (2020) 124668. [71] Y. Ding, Y. Zhou, W. Nie, P. Chen, MoS2-GO nanocomposites synthesized via a hydrothermal hydrogel method for solar light photocatalytic degradation of methylene blue, Appl. Surf. Sci. 357 (2015) 1606–1612. [72] H. Liang, P. Hua, Y. Zhou, Z. Fu, J. Tang, J. Niu, Fabrication of Cu/RGO/MoS2 nanohybrid with energetic visible-light response for degradation of rhodamine B, Chin. Chem. Lett. 30 (12) (2019) 2245–2248. [73] A. Rani, K. Singh, A.S. Patel, A. Chakraborti, S. Kumar, K. Ghosh, P. Sharma, Visible light driven photocatalysis of organic dyes using SnO2 decorated MoS2 nanocomposites, Chem. Phys. Lett. 738 (2020) 136874. [74] B. Khan, F. Raziq, M.B. Faheem, M.U. Farooq, S. Hussain, F. Ali, H. Tian, Electronic and nanostructure engineering of bifunctional MoS2 towards exceptional visible-light photocatalytic CO2 reduction and pollutant degradation, J. Hazard. Mater. 381 (2020) 120972. [75] S.P. Vattikuti, C. Byon, C.V. Reddy, R.V.S.S.N. Ravikumar, Improved photocatalytic activity of MoS2 nanosheets decorated with SnO2 nanoparticles, RSC Adv. 5 (105) (2015) 86675–86684. [76] E.D. Bates, R.D. Mayton, I. Ntai, J.H. Davis, CO2 capture by a task-specific ionic liquid, J. Am. Chem. Soc. 124 (6) (2002) 926–927. [77] L. Cao, S. Sahu, P. Anilkumar, C.E. Bunker, J. Xu, K.S. Fernando, Y.P. Sun, Carbon nanoparticles as visible-light photocatalysts for efficient CO2 conversion and beyond, J. Am. Chem. Soc. 133 (13) (2011) 4754–4757. [78] C. Wang, X. Zhang, Y. Liu, Promotion of multi-electron transfer for enhanced photocatalysis: a review focused on oxygen reduction reaction, Appl. Surf. Sci. 358 (2015) 28–45. [79] P.Y. Jia, R.T. Guo, W.G. Pan, C.Y. Huang, J.Y. Tang, X.Y. Liu, Q.Y. Xu, The MoS2/TiO2 heterojunction composites with enhanced activity for CO2 photocatalytic reduction under visible light irradiation, Colloids Surf. A Physicochem. Eng. Asp. 570 (2019) 306–316. [80] J.Y. Tang, R.T. Guo, W.G. Zhou, C.Y. Huang, W.G. Pan, Ball-flower like NiO/g-C3N4 heterojunction for efficient visible-light photocatalytic CO2 reduction, Appl. Catal. B Environ. 237 (2018) 802–810. [81] M. Tahir, B. Tahir, Dynamic photocatalytic reduction of CO2 to CO in a honeycomb monolith reactor loaded with Cu and N doped TiO2 nanocatalysts, Appl. Surf. Sci. 377 (2016) 244–252. [82] I.H. Tseng, W.C. Chang, J.C. Wu, Photoreduction of CO2 using sol-gel derived titania and titania-supported copper catalysts, Appl. Catal. B Environ. 37 (1) (2002) 37–48. [83] Z. Xiong, Y. Luo, Y. Zhao, J. Zhang, C. Zheng, J.C. Wu, Synthesis, characterization and enhanced photocatalytic CO2 reduction activity of graphene supported TiO2 nanocrystals with coexposed {001} and {101} facets, Phys. Chem. Chem. Phys. 18 (19) (2016) 13186–13195.

Chapter 19

Heterogeneous photocatalysts based on iso- and heteropolytungstates Alessandra Molinari and Michele Mazzanti Department of Chemical, Pharmaceutical and Agricultural Sciences, University of Ferrara, Ferrara, Italy

1 Photocatalysis by polyoxotungstates Polyoxotungstates (POTs) are tungsten oxygen anion clusters of nanometric size characterized by a wide variety of structures [1]. Generally speaking, POTs can be divided into two main groups: isopolytungstates, constituted only by W and O atoms, and heteropolytungstates where a heteroatom (X ¼ Si, P, S, Ge, etc.) different from W and O is also present. The most important isopolytungstate is decatungstate anion that is viewed as the union of two W5O18 units, bonded through shared oxygen in vertices with internal empty space. Each subunit is made up of WO6 octahedra that has sides and oxygen atoms in common. Among heteropolytungstates, Keggin ([XM12O40]n]) and Dawson ([X2M18O62]n]) types represent the most common structures: a central tetrahedric XO4 unit is surrounded by a definite number of WO6 octahedra. POTs exhibit very peculiar structure-dependent chemical and physical properties (acid-base and redox properties) which are thermally and oxidatively stable and undergo photoinduced multielectron transfers without changing their structures. These features make them attractive materials for applications as redox catalysts. Moreover, irradiation with UV and/ or near UV light in the ligand to metal charge transfer band causes an intramolecular charge transfer from O2-based highest occupied molecular orbital (HOMO) to the W6+-based lowest unoccupied molecular orbital (LUMO) leading to the formation of a photoexcited state. The resulting photoexicted POT is highly reactive both in oxidation and reduction reactions and can trigger chemical transformations on organic and inorganic molecules that are transparent in the wavelength range employed. The entire process is truly photocatalytic when the photoexcited POT is regenerated in its initial state at the end of the reaction cycle (as it happens for a thermal catalyst), while the light is a stoichiometric reagent. This is the base of the utilization of POTs as photocatalysts. Also, POTs have been applied to a wide range of new applications [2–5], and most of them have been recently reviewed [6].

1.1 Decatungstate photocatalysis Considering photoredox applications in the liquid phase, POTs have common photochemical processes. However, decatungstate anion is the polyoxotungstate that has the absorption band at the highest wavelengths of all. In fact, [O2-W6+] LMCT absorption band is in the UV region (lmax ¼ 324 nm, e ¼ 14,100 M1 cm1) with a tail that extends until 380 nm, partially overlapping to the UV solar emission spectrum. This renders the isopolyoxoanion, the king photocatalyst of the family, principally because UV light excites exclusively the decatungstate anion, and it is not absorbed by the organic substrates to be transformed that remain always in their ground state [7]. Moreover, the absorption spectrum of decatungstate anion in solution overlaps that of TiO2, the benchmark material in photocatalysis, so that the polyoxoanion is considered the soluble model of the semiconductor (SC) [8, 9]. Proper counter cation warrants solubility in various media: for example (n-Bu4N)4W10O32 dissolves in CH3CN and Na4W10O32 is soluble in water. The cascade of events that take place after light absorption by decatungstate anion has been deeply studied (Fig. 1): the 4∗ photoactivation of W10O4 32 leads to a LMCT excited state (W10O32 ), in analogy to the photoinduced charge separation processes which occur in TiO2. This state decays in few picoseconds to a very reactive transient, named wO, with an oxy radical character. The quantum yield for the generation of this species is around 0.5–0.6. The high reactivity of wO has been recently quantified reporting that its redox potential is +2.44 V vs SCE [10, 11]. Thus, the transient wO can give rise to different processes depending on the experimental conditions. In organic solvent and in aerated conditions (Fig. 1 path [1]), wO can initiate the oxidation of several organic substrates (RH) through Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00035-0 Copyright © 2021 Elsevier Inc. All rights reserved.

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SECTION

C Oxides and calcogenides

FIG. 1 Photoactivation of decatungstate anion with UV light and subsequent possible reactive pathways.

hydrogen atom abstraction or electron transfer mechanism, depending on the actual properties of the chosen substrate RH. Both mechanisms lead to the reduced form of decatungstate (W10O325 or HW10O324) and the substrate-derived radical (R). Oxidation of W10O325 by O2 restores starting W10O324, closing the photocatalytic cycle. O2 is reductively activated to peroxy species [12]. In aqueous solutions and aerated conditions (Fig. 1 path [2]), the direct reaction of water with wO leads to the formation of OH radicals. The occurrence of this reaction has been the object of debate in literature for a long time until some researchers demonstrate with independent techniques the formation of OH radicals [13]. Also, it has been demonstrated that hydroxyl radicals originate from both H2O oxidation and H2O2 reduction. The two pathways just described above opened to the possibility of photocatalyse oxidation reactions of organic molecules. In detail, reactions of path [1] can be controlled and partial oxidation is achievable with the conversion of starting molecules into high-added value intermediates. Conversely, in aqueous environment (path [2]), the formation of OH radicals, which are very reactive and unselective oxidant species, is suitable for photocatalytic degradative reactions where the main purpose is depollution [14]. Another possibility occurring in organic solvent and de-aerated conditions should be considered (Fig. 1 path [3]): W10O325 (recognizable because of its blue color) can be accumulated in the reaction vessel together with the substratederived radical (R). These species could start a very large variety of reactions of interest in organic chemistry which results in the formation of CdC, CdN, CdSi, CdF bonds, opening to new and mild synthetic applications [15–17].

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1.2 Heterogeneous polyoxotungstates as photocatalysts The main shortcoming of homogeneous catalytic processes is the cumbersome separation of the catalyst (often expensive) from reaction products and its quantitative recovery in an active form. The growing need for the development of environmentally benign, recoverable catalysts which could replace the current homogeneous chemical procedures pushed the research towards the heterogenization of the catalysts on a solid carrier. In this framework, the use of heteropolyanions in their solid state led to the development of practical significance [18]. However, since their specific surface area was very low (1  10 m2 g1), immobilization of polyoxotungstates onto a large surface area of a solid carrier was preferred [19]. In principle, dispersion of catalytic clusters on supports with high surface area should increase the accessibility of the catalytic sites for the molecule to be transformed so increasing the catalytic performance. Moreover, as most of the POTs are very soluble in water or organic solvents, their heterogenization should provide more freedom in the choice of the dispersing medium. Easiness of recovery and the possibility of recycling the heterogenized catalyst are the other important advantages. Related to this aspect, when a heterogeneous catalyst is considered, one has to evaluate if there is a certain degree of release of the photocatalytic cluster in the solution or not. This factor is particularly important in the case of immobilized polyoxotungstates because they are also active as homogeneous photocatalysts. In fact, leaching of the photocatalyst during the reaction time inhibits the researcher to be sure that the process is truly heterogeneous. Also, when leaching occurs in some extent, the immobilized photocatalyst recovered at the end of the reaction contains a lower number of photoactive species with respect to the beginning and consequently the loss in efficiency during a consecutive run is obvious. Another fundamental parameter to be considered in a (photo)catalytic process aimed at the production of functionalized intermediates or complete mineralization of the substrate is selectivity [20]. Heterogeneous system represents a suitable means to tailor selectivity through the control of the microscopic environment surrounding the photoactive center, that is immobilized on the surface or inside pores of the support. In detail, it is believed that the support should affect the physical and chemical properties of the (photo)catalytic system through specific and projected textural characteristics. Well-defined structures at the nanometer scale afford a good design of active sites for obtaining selective (photo)catalytic processes [21]. The ongoing interest in POTs heterogeneous photocatalysis has triggered the development of a variety of synthetic procedures. In the following, we provide an overview of the most common preparation procedures for the immobilization of POTs both on inactive and photocatalytically active oxides, focusing on those characteristics, emerging from the structural and morphological investigation, useful in a photocatalytic process. For the various supported POTs, we will report the most significant photocatalytic applications, to elucidate their potentiality and possible future developments. In this regard, a special emphasis is devoted to those materials where a proper design allows a peculiar size selectivity or leads to a light absorption into the visible region.

2 POTs heterogenized on photocatalytically inert supports 2.1 Impregnation The oldest attempt to immobilize POTs on solid support used the “impregnation” procedure. Typically, it consists of suspending a weighted amount of solid support in a POT solution with a known concentration. After stirring for a desired period, the solvent is slowly evaporated, thanks to mild heating. The obtained heterogeneous photocatalyst is usually dried at 100°C for several hours and then used. The impregnated POTs on silica or alumina as support investigated in the photooxidation of hydrocarbons and alcohols have been recently reviewed [22, 23].

2.1.1 Partial oxidation of hydrocarbons Immobilization of H3PW12O40 on colloidal silica by impregnation to obtain H3PW12O40/SiO2 is one of the first examples in literature [24]. It was proposed that the acidic aqueous environment containing dissolved H3PW12O40 caused the protonation of surface silanol groups (SiOH+2 ) that could, in turn, fix the polyoxoanion through electrostatic interactions. The illumination of this heterogeneous photocatalyst (l ¼ 254 nm, low pressure Hg lamp) dispersed in neat cyclohexane led to its partial oxidation to the corresponding alcohol and ketone, without the formation of CO2. Interestingly enough, the one/ol product ratio decreased from 2.3 to 1.1 when a more polar reaction mixture was used, indicating that the polarity of the solvent could affect selectivity. At a pH close to neutrality, organic ammonium cations and inorganic sodium cations were adsorbed on the negatively charged silica surface (SiO) through electrostatic interactions. The cations acted as a bridge between the negative surface

304

SECTION

C Oxides and calcogenides

and decatungstate anion according to the pattern SiO/counterion+/W10O4 32 . Likely, this kind of interaction was responsible for the anchoring of some polyoxotungstates on silica, such as (nBu4N)4W10O32, (Et3NH)4W10O32 and Na4W10O32 [25]. DR-UV spectra of the three heterogeneous photocatalysts showed that W10O4 32 clusters were present on the surface of silica without any appreciable modification (absorption maximum at 325 nm). Also, the IR band characteristic of the decatungstate structure and CdH stretching of the organic cations were present on immobilized samples, indicating that the polyoxoanion structure was preserved and supported the proposed interaction between POT and silica surface. Table 1 reports that the decrease in the surface area, attributable to the fixation of the decatungstate salt on the silica surface, was related to the size of counter cations. Moreover, surface polarity measurements carried out with Reichardt’s dye + (EN T ), point out that nBu4N created a less polar and more hydrophobic environment around the photoactive species on + the surface than Et3NH and Na+ did. This result determined an increase in the cyclohexanone/cyclohexanol ratio by decreasing the hydrophobic character of the surface. Authors proposed that the alcohol remained adsorbed on the polar surface near to the photoactive species, so favoring its oxidation to ketone. This modest but reproducible difference in reactivity suggests a possible approach to be followed if high chemoselectivity has to be obtained. Impregnation of increasing amounts of (nBu4N)4W10O32 on mesoporous MCM-41 (surface area of 1240 m2/g and total pore volume of 1.130 cm3/g, with around 70% of pores 2–4 nm in size) led to new heterogeneous systems (MCM-41/W10% and MCM-41/W30%) employed in the partial oxidation of cycloalkanes [26]. Spectroscopic measurements evidenced that the W10O4 32 unit was present in the solid support without modification, and morphological investigation showed that increasing amount of (nBu4N)4W10O32 loaded on MCM-41 determined a decrease in both the surface area and in the total pore volume of the obtained systems. However, analysis of pore distribution showed that (nBu4N)4W10O32 filled first the largest pores and only when more decatungstate was added, the pores of sizes of less than 4 nm were also filled. The photocatalytic activity of these heterogeneous systems had been assessed in the O2-oxidation of cyclohexane and cyclododecane to the corresponding alcohols and ketones. The most relevant result of this study was the ability of MCM-41 to disperse higher amounts of decatungstate with respect to amorphous silica, thus supporting a greater number of photocatalytic sites without forming aggregates. Indeed, an increase in decatungstate loading from 10% to 30% resulted in higher photochemical efficiency. On the contrary, when silica of a surface area of about 100 m2/g was used as support, a significant decrease in activity was observed, probably due to aggregation phenomena. Moreover, MCM-41/W10% was stable and can be employed at least three times without loss of activity. Concerning the product selectivity, the maximum value of ketone/alcohol ratio was 2.6. Authors proposed that the extended and polar surface of MCM-41 facilitated the accumulation of the alcohol at the interfaces, thus favoring its subsequent oxidation. The photocatalytic activity of (nBu4N)4W10O32 impregnated on amorphous silica (SiO2/W10%) in the oxidation of cycloalkenes (such as cyclohexene and cyclooctene) could be controlled in some way by the presence of a co-catalyst [27, 28]. For example, an iron porphyrin complex dissolved in solution affected product selectivity in favor of the allylic alcohol. In fact, the complex-mediated decomposition of allylic hydroperoxides to give the corresponding alcohol became important, and the ketone to alcohol ratio decreased from 3.4 to 1.8. Moreover, residual catalytic activity of the iron porphyrin complex after interruption of irradiation further diminished the said ratio to 1.45. When the SiO2/W10% system dispersed in CH3CN was used in the oxidation of cyclohexene, the obtained oxidized products were the hydroperoxide (1), allylic ketone (2) and alcohol (3), and epoxide (4) Fig. 2 [28]. The product distribution is shown in Table 2 (Entry 1).

TABLE 1 Surface area, polarity measurements, and cyclohexanone/cyclohexanol ratio of unmodified SiO2 and of (nBu4N)4W10O32/SiO2, (Et3NH)4W10O32/SiO2 and Na4W10O32/SiO2. Material

SSA (m2/g)

ETNa

Cyclohexanone/cyclohexanolb

SiO2

95  2

0.93

0

Na4W10O32/SiO2

83  2

0.83

2.9

(Et3NH)4W10O32/SiO2

83  2

0.80

1.3

(nBu4N)4W10O32/SiO2

54  2

0.51

a

1.0 4

The indicated materials were separately put in contact with a solution of Reichardt’s dye in CH2Cl2 (1  10 M) at room temperature. The fast adsorption of the dye caused different coloring of the solids. After removal of the solvent, DR-UV-vis spectra were recorded and from lmax ETN has been calculated. b The ratio was determined from alcohol and ketone concentration obtained after 4 h irradiation (l > 300 nm) of suspensions of one of the indicated material (15 g/L) in neat cyclohexane, in the presence of O2 (1 atm).

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FIG. 2 Oxidation products from oxygenated organic solutions containing cyclohexene obtained after irradiation of SiO2/W10%.

TABLE 2 Photocatalytic properties of SiO2/W10% in the oxygenation of cyclohexene. Products distribution (%) Entry

Dispersing medium

(1)

(2)

(3)

(4)

(5)

Others

Chemoselectivity [(4) + (5)]/ [(1) + (2) + (3)]

1

CH3CN/C6H10 (0.1 M)

82.5

10

6

1.5

–

–

0.015

2

CH2Cl2/C6H10 (0.1 M)

24.5

16

8.5

1.5

42.5

7.0

0.9

3

CH2Cl2/C6H10 (0.01 M)

30

6.5

0.5

1.5

54

7.5

1.5

Interestingly, the substitution of CH3CN with CH2Cl2, significantly changed the product distribution (Table 2, entry 2): in particular, the main product became the chlorohydrin (5). Considering that halohydrins can undergo facile conversion to the corresponding epoxides under slightly alkaline conditions, it was observed that the chemoselectivity of the process was completely different in the presence of the halogenated solvent. The ratio (4) + (5)/(1) + (2) + (3) passed from 0.015 (Entry 1) to 0.9 (Entry 2) and to 1.5 when a lower starting concentration of cyclohexene was employed (Entry 3). NaY zeolite effects on the photocatalytic properties of H2NaPW12O40, H4SiW12O40 and H3PMo12O40 had been investigated by employing 1,2-dichlorobenzene (DCB) as a kinetic probe [29]. The observed significant rate enhancement in DCB conversion was attributed to the constrained environment typical of the zeolite that increased encounter probability for co-adsorbed DCB and POM, showed high O2 binding ability and suppressed back electron transfer reaction.

2.1.2 Partial oxidation of alcohols (n-Bu4N)4W10O32 was deposited on the surface of g-alumina and of silica by wet impregnation at different starting pH values (respectively below and above the zero-point charge of the support, pzc) [30]. Characterization of the prepared samples (Al2O3/W10 and of SiO2/W10), carried out by BET, XRD, UV-vis DR, XPS measurements, showed that higher dispersion of W(VI) oxo-species was obtained in SiO2-supported catalysts. Within the same support, the dispersion was higher when the impregnation pH is lower than pzc of the support. The photocatalytic activity of Al2O3/W10 and of SiO2/W10 systems had been assessed in the oxidation of secondary and primary benzyl alcohols. In particular, a series of p-alkyl substituted benzyl alcohols was included to evaluate the chemo-selectivity of these photocatalytic systems. In fact, these alcohols bear two distinguishable benzylic hydrogen atoms, one on the alcohol carbon and one on the p-alkyl substituent, that both potentially can be cleaved under photooxidation conditions (Fig. 3). Photooxidation of primary benzyl alcohols with Al2O3/W10 and of SiO2/W10 resulted in the corresponding p-substituted carboxylic acids. This result was not in agreement with that reported in a quite simultaneous work (see later) [31], where decatungstate entrapped in a silica matrix by sol-gel procedure photocatalyzed the oxidation of primary benzyl alcohols to the corresponding aldehydes. Concerning stability, Al2O3/W10 and of SiO2/W10 systems undergo low leaching of decatungstate (90%). All these findings indicated that heterogenization had an important effect in decreasing the oxidizing ability of sodium decatungstate in water favoring the accumulation of carbonylic products. Adsorption phenomena of glycerol on silica can not only enhance its local concentration in the proximity of decatungstate but also prevent the subsequent oxidation of the formed carbonylic compounds. In line with results reported for POTs entrapped into silica matrices via sol-gel technique, zirconia supported Na4W10O32 used in the photooxidation of primary and secondary benzylic alcohols with O2 led to aldehydes and ketones with very good yields [38]. The photocatalyst was recyclable and reusable several times without loss of activity.

FIG. 4 Reaction sequence between photogenerated OH radicals and glycerol.

TABLE 3 Photocatalytic oxidation of glycerol by Na4W10O32 and Na4W10O32/SiO2.a Selectivity (%) of detected productsc

Photocatalytic system

Converted glycerol (mol × 1026)

Sum of detected products (mmol)

Mass balanceb (%)

Na4W10O32

19.2

9.1

47.4

32.8

2.8

9.4

2.3

Na4W10O32/ SiO2

9.6

6.8

70.8

59.4

5.6

5.3

0.3

GAD

DHA

GA

CO2

GAD, glyceraldehyde; DHA, dihydroxyacetone; GA, glycolic acid. a Na4W10O32 (4  104 M) or Na4W10O32/SiO2 (8 g/L) in aqueous solution (3 mL) containing glycerol (1  102 M) irradiated 1200 at l > 290 nm at 298 K and 760 Torr of O2. The reported values are the mean of three repeated experiments. b Mass balance is the ratio (%) between the sum of detected products and the converted glycerol. c Selectivity (%) is expressed as mmol of product divided by mmol of converted glycerol. The remaining 100% is due to other oxidized derivatives that have not been identified.

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2.2.2 Degradation reactions A new photocatalyst (Na4W10O32/SiO2/BTESE) had been prepared by simultaneous hydrolysis of TEOS and 1,2-bis (triethoxysilyl)ethane (BTESE) in the presence of dissolved Na4W10O32 [39]. This material was able to scavenge and accumulate significant amounts of toluene from saturated water solutions, due to the large surface area, porosity, and above all the high hydrophobicity (pointed out by FTIR investigations and microgravimetric analysis using toluene in the vapor phase as adsorptive). Photoexcitation of this system suspended in water samples saturated with toluene led to the formation of OH radicals that caused almost complete mineralization of toluene to CO2. Hu and coworkers synthesized microporous Keggin type polyoxometalate materials by incorporating H3PW12O40 and H4SiW12O40 into a silica matrix [40]. The obtained POT/SiO2 were porous materials with uniform micropores (0.58 nm) and high specific surface areas (400  500 m2/g). Their photocatalytic activity had been assessed in the degradation of aqueous traces of organochlorine pesticides. In particular, hexachlorocyclohexane and pentachloronitrobenzene were oxidized by irradiating a POT/SiO2 slurry (lmax of heterogeneous POT is 266 nm). Studies on the reaction mechanisms indicated that OH radicals were responsible for the oxidation. Also, results from GC-MS and IC techniques used to monitor and follow product intermediates, allowed the researchers to show that the pesticides were completely degraded to CO2 and HCl. The same group entrapped Na4W10O32 and (nBu4N)4W10O32 inside a silica network [41]. UV-DR, FTIR, ICP-AES, SEM investigation showed the structural integrity of the anionic cluster upon heterogenization. The obtained microporous materials were compared with the two silica-supported decatungstates prepared by impregnation in the photocatalytic degradation of trichorofon, an organophosphorus pesticide. It was found that Na4W10O32/SiO2 prepared by sol-gel method had the best performance, whereas the heterogeneous systems containing (nBu4N)4W10O32 suffered an important leakage of the catalyst from the support. This instability was ascribed to the weak interaction between (nBu4N)4W10O32 and ≡ SiOH groups. In a third study of the group H3PW12O40, H4SiW12O40 and Na4W10O32 were heterogenized on SiO2 and tested in the photocatalytic degradation of hydroxy-butanedioic acid (malic acid, MA), a model molecule for partially oxidized products found in either biomass or in various degradative processes [42]. Kinetic studies showed that the rate of disappearance of MA in the presence of included Keggin units was similar but lower than that observed in the case of Na4W10O32/SiO2. Concerning the reaction mechanism, it was suggested that photo-oxidation proceeded through a combination of two pathways: a direct oxidation of MA and an oxidative process with the involvement of OH radicals. Besides plenary iso- and heteropolytungstates, monovacant lacunary derivatives of the Keggin anion of general formula “XW”11 (X ¼ P, Si, Ge, B) were prepared by removal of a tungsten-oxygen octahedral moiety from a saturated polyoxotungstate framework [43]. Interestingly, the resulting lacunary anion was highly nucleophilic and reacted easily with electrophilic groups. Hydrolysis of TEOS in the presence of the lacunary anions led to a homogeneous silica sol of XW11-SiO2 hybrid materials formed through the grafting of silanol groups onto the surface of XW11. The sol was then added dropwise to polystyrene (PS) spheres, and condensation around them occurred. Finally, PS template removal from the material led to white XW11-SiO2 powders with peculiar characteristics. UV, IR, and elemental analysis gave evidence that the primary XW11 structures remained intact after the formation of silica hybrid materials. Moreover, TEM images showed that XW11-SiO2 formed after removal of PS template were macroporous (medium diameter was 300 nm) with a wall thickness ranging from 55 to 100 nm. Suspensions of these XW11-SiO2 in aqueous solutions containing malic acid (100 ppm) and irradiation with an internal high-pressure mercury lamp (l ¼ 312 nm) under aerated conditions led to almost complete disappearance of malic acid after 180 min irradiation. However, the main oxidation products (acetic and formic acid) were no more detected by IC only after 500 min UV irradiation, indicating a slow degradation process. Interestingly, ICP-AES analysis showed that leaching of W from the silica matrix was negligible.

2.3 Ionic exchange: Resins and modified silica supports Immobilization of POT onto an ion exchange resin is usually achieved by dissolving the desired polyoxoanion in a proper solvent and suspending within the solution an amount of the solid resin [44, 45]. Then, mixtures are gently stirred for some hours at ambient temperature. UV-vis spectrophotometry is used to evaluate the extent of immobilization. The exchanged resin is filtered off, washed, and sometimes dried under vacuum overnight. As an alternative to ionic exchange resins, silica can be functionalized on the surface with alkyl ammonium cations covalently bound on the solid support [46]. The preparation of anchored alkyl ammonium salts is performed by reacting silica and the suitable (alkylaminopropyl) trialkoxysilane in toluene at reflux. Then, immobilization of polyoxoanion is carried out as described earlier.

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2.3.1 Partial oxidation of hydrocarbons Na4W10O32 was immobilized on powders of organic ion exchange resins and of carbon material. The heterogenized photocatalysts were active in the oxidation of cyclohexane in acetonitrile [44]. Observed differences in performances between the heterogeneous materials were attributed to the kind of support. In particular, the polymers had a crucial role since the nature and strength of interaction/bond between decatungstate and support determined catalyst stability, prevented leaching of decatungstate, provided adequate adsorption/desorption condition for reagents and products, whereas when carbon material was used as support, an important release of decatungstate was observed. Since the surface of the mesoporous silica was hydrophilic and the photoactive sites may not have been easily accessible to hydrocarbon molecules, it was reasonable that the introduction of further organic fragments on the silica surface offered the unique opportunity to fabricate new hydrophobic heterogeneous W10O4 32 systems. For this reason, the organo-modified SBA-15 was prepared by an initial insertion of alkyl groups via refluxing with Cn-Si(OEt)3, followed by grafting 3aminopropyl groups (AP) on the parent support [47]. Then, immobilization of W10O4 32 on hydrophobically modified SBA-15 was carried out. The resultant material was denoted as W10/Cn-AP-SBA, where Cn is the alkyl group (n is the number of the carbon atoms of the alkyl group). Authors first examined the photooxygenation of ethylbenzene, chosen as a probe, using O2 as the oxidant at ambient temperature and found that among all catalysts tested, the octyl-grafted catalyst showed the highest reactivity toward the conversion of ethylbenzene to acetophenone. The reason for such alkyl length-dependence behavior was ascribed to the similar dimensions of the octyl groups relative to that of the decatungstate W10O4 32 anions (ca. 1 nm), which could afford more suitable conditions for the highly active photooxygenation. Also, W10/ C8-AP-SBA had been proved to be highly efficient and chemoselective in the photooxygenation of a range of aryl alkanes to corresponding ketones under mild conditions. The efficiency and stability of the catalyst had also been demonstrated convincingly by conducting six successive runs without an appreciable drop in the reaction rate.

2.3.2 Oxidative bromination of hydrocarbons Photochemical excitation of (nBu4N)4W10O32 heterogenized on commercial resin Amberlite IRA-900 in chloride form and dispersed in a CH3CN/H2O mixture containing an alkene caused the reductive activation of O2 to form alkyl peroxides [45]. Interestingly, when sodium bromide was also dissolved in the solution, the anionic exchange resin was fundamental in fostering the enrichment of bromide anions to the surface. Therefore, the photoinduced formation of peroxides close to the bromide anions favored the formation of brominating species (“Br+”) in the proximity of the solid surface. As a consequence, an increase in the yield of epoxides and bromohydrins was observed upon heterogenization. Moreover, the solid matrix caused the conversion of activated arenes such as phenol and anisole to monobrominated derivatives. This result was interesting since, with classical organic methods, it is quite difficult to selectively stop the functionalization of activated arenes to the monobromination step. Starting from these findings, Symeonidis et al. immobilized (nBu4N)4W10O32 onto a polymer that involves poly-(isopropylacrylamide-alkylammonium) organic component [48]. This new catalyst had been employed in the aerobic epoxidation of substituted cycloalkenes and styrenes, with moderate yields and with a better performance with respect to homogeneous conditions. Also, when a chlorinated solvent was employed, an important formation of 1,2-chlorohydrins was obtained. Concerning the stability, the polymeric photocatalyst was reused two times without loss of activity, while in the third run a lower yield was obtained. Silica modified as described above was used for heterogenization of W10O4 32 , which was firmly held on the support by ionic bond with tetra-alkyl, or trialkyl, or monalkyl ammonium cations [46]. These materials had been employed as photocatalysts for the O2-assisted oxidation of 1,3-butanediol and 1,4-pentanediol. As the alkyl chains of ammonium cations were substituted by hydrogen atoms, the polarity of the environment surrounding the photoactive species increased, favoring the preferential adsorption of the primary OH group of the more hydrophilic head of diol molecule with respect to the secondary OH group placed in the hydrophobic tail. Thus, the ratio between aldehyde to ketone passed from 0.06 to 0.63 for 1,3-butanediol and from 3.0 to 7.5 for 1,4-pentanediol.

2.3.3 Degradation reactions The heterogeneous photocatalyst (silica-NH+3 /Na3W10O 32) had been prepared from commercial silica gel functionalized with 3-aminopropyl groups and Na4W10O32. The system was tested in the photocatalytic removal of contaminants of emerging concern (i.e., atenolol, levofloxacin, trimethoprim, ATN, LEVO, TMP respectively, C0 ¼ 10 ppm, Fig. 5) from water at room temperature, atmospheric pressure, and at pH values similar to that of natural waters [49]. The heterogeneous photocatalyst could be recycled several times without loss of activity. Concerning the degradation mechanism, ESR spin trapping technique demonstrated that drug degradation was mediated by OH radicals as it occurred in homogeneous conditions [13]. In fact, HPLC-MS analysis pointed out the formation of hydroxylated derivative compounds as first

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FIG. 5 Drugs structures and relative values of pKa.

intermediates in the overall degradation pathway. Considering pKa values of the investigated drugs (Fig. 5), it was then proposed that easy reaction between OH radicals formed on the surface and drug molecules could be due to their protonated or zwitterionic form at the working pH of 6, which allowed the approach of the positively charged drug to the photoactive anion which covered the surface. The same researchers extended their study to other two drugs having different structures and physical and chemical properties (Fig. 5), such as the antidepressant carbamazepine (CBZ), a very recalcitrant molecule, and the antibiotic sulfamethoxazole (SMX) [50]. Contrarily to what was observed with ATN, TMP, and LEVO irradiation of silica-NH+3 / Na3W10O 32 system did not cause any appreciable photocatalytic degradation for these two drugs despite the demonstrated ability of the photoexcited heterogeneous system to produce OH radicals (Table 4). This behavior had been ascribed to the fact that silica particles covered by decatungstate anions presented a negatively charged shell on the surface, and at the working pH of 6, CBZ was in its neutral form and SMX presented an equilibrium between neutral and even negative form. Thus, a difficult contact between photogenerated OH radicals and drug molecules was in agreement with the absence of its degradation. One of the main advantages of heterogeneous photocatalysis was the flexibility in choosing the support with proper characteristics. Thus, based on previous results [39], it was believed that the system SiO2/BTESE/Na4W10O32 obtained by direct hydrolysis of TEOS and BTESE (see Section 2.2.2) should be hydrophobic enough to favor the approach of organic drugs dissolved in an aqueous matrix. Importantly, XRD analysis showed that decatungstate anion was unmodified after encapsulation inside the mesoporous network. FTIR spectra gave evidence that the organic fragments coming from BTESE were distributed within the framework, and drug molecular dimensions were suitable with the pore sizes of the support. As a confirmation, irradiation of SiO2/BTESE/Na4W10O32 aqueous suspensions containing CBZ or SMX for the desired time led to their degradation. In detail, after 4 h illumination (l > 300 nm), more than 80% of the two drugs

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TABLE 4 Kinetic constants values determined by fitting the drug concentration decrease with a first-order kinetic equation (C/C0 5 exp(2kt)) upon photoexcitation of silica-NH+3 /Na3W10O2 32 and of SiO2/BTESE/Na4W10O32. Photocatalyst

Drug

k (h21)

Silica-NH+3 /Na3W10O 32

ATN

0.408

LEVO

0.444

TMP

0.336

CBZ

0.024

SMX

0.021

CBZ

0.97

SMX

0.48

SiO2/BTESE/Na4W10O32

disappeared from the aqueous solution. In Table 4, pseudo first-order kinetic constants for the two heterogeneous systems clearly showed that the charge taken by the drug molecules at pH 6 was an obstacle in the case of silica-NH+3 /Na3W10O 32, whereas an increase in hydrophobicity and placement of decatungstate anion inside pores (as in SiO2/BTESE/Na4W10O32) allowed also the photodegradation of neutral and negatively charged molecules. H3PW12P40 was successfully anchored to the surface of amino-functionalized magnetic mesoporous silica microspheres to lead Fe3O4@SiO2@meso-SiO2-NH2-PW [51]. The prepared microspheres had large SSA (135 m2/g) and open mesopores (5 nm). Their photocatalytic activity had been assessed in the degradation of rhodamine B dissolved in aqueous solution. Although it has not been investigated if the observed decrease of dye concentration corresponded to mineralization or simply to conversion into noncolored fragments, it was noticeable that the system was recyclable several times and that recovery of the microspheres at the end of the reaction was very easy, thanks to high magnetization. Hu and coworkers prepared a series of solid POT photocatalysts by using 3-aminopropyltriethoxy silane functionalized porous silica materials (APS-SiO2) as support and transition metal substituted polyoxometalate (TMSP) clusters as photoactive sites, such as (K6[Ni(H2O)SiW11O39], abbreviated as (SiW11Ni)) [52]. In the TMSP, the transition metal coordinated five O2 ligands, while the last coordinating site on the metal was occupied by an aqua ligand. This ligand could be displaced by the nitrogen of modified silica. In fact, spectroscopic and elemental analysis data showed that the SiW11Ni clusters were attacked to the silica support via NidN dative bonding and that the Keggin unit structure remained intact upon heterogenization. The covalent bond allows a chemical attachment of the cluster onto the support, so preventing any leaching. The porous SiW11Ni-APS-SiO2 composites were tested in the photocatalytic degradation of aqueous rhodamine B. It was reported that their activity was higher than that of sole homogeneous SiW11Ni clusters evidencing that the porous structure of the heterogeneous photocatalysts plays a fundamental role in concentrating inside pores, the reactants (rhodamine B and O2) which are very close to OH radicals produced by the photocatalytic cluster. Analysis of intermediates and final products showed that the dye was completely mineralized after some hours of irradiation.

2.4 Occlusion in polymeric membranes Heterogenization of (nBu4N)4W10O32 in different polymeric membranes had been performed using the two following procedures [53, 54]. In the first one, the polymer was dissolved in the proper solvent, stirred at room temperature and then the decatungstate salt was added. After 24 h stirring, the solution was put on a glass plate, exposed to air, and finally immersed for 2 h in a coagulation bath containing deionized water at 20°C. Then, the membrane was removed, washed, and dried at 60°C under vacuum. In the second method, a prepolymerization step carried out in the presence of a cross-linker additive was followed by the addition of decatungstate. The polymeric membranes used were: polyether-ether ketone (PEEK-WC), polysulfone (PSf), polyvinylidene fluoride (PVDF), polydimethylsiloxane (PDMS). FT-IR spectra confirmed that W10O4 32 was included within the membrane and that its structure was basically unaltered. Energy dispersive X-ray spectrometry (EDX) gave information about the catalyst distribution in the polymeric film. Then, the photocatalytic performance of membrane/W10 systems had been studied in the oxidation of several alcohols soluble in water. Among the systems stable under irradiation, it was observed that recycling performance was often critical: the decrease in activity had been mainly attributed to the incomplete extraction of residual products that prevented an efficient supply of fresh reagents in subsequent photocatalytic runs. However, the alcohols underwent oxidation to carbonylic products. Perfluoropolymers

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possessed excellent oxidative resistance due to which CdF bonds showed dioxygen preferential permeability. Thus, a fluorous-tagged decatungstate (RfN)4W10O32 (RfN ¼ [CF3(CF2)7(CH2)3]3CH3N+) had been embedded within fluoropolymeric films (Hyflon) [55]. The resulting hybrid materials exhibited remarkable activity in the solvent-free oxygenation of benzylic hydrocarbons (such as ethylbenzene, indane, tetralin). Moreover, compared to polyvinylidene fluoride/W10 system, Hyflon/W10 showed a remarkable improvement in morphology and photocatalytic performance. All the described studies helped the development of the research, leading to the consciousness that solid support must have an active role in determining the efficiency and selectivity of a photocatalytic process. For example, when the reaction medium is water, the choice of appropriate hydrophobic support is crucial because it induces the sorption of an organic substrate in close proximity to the active site, so favoring the selectivity tuning [56].

2.5 Immobilization into metal organic frameworks Metal organic frameworks (MOFs) are a subclass of the broader coordination polymers family. We can describe them by an “empirical formula” expressing the metal(s), the ligand(s), and their stoichiometry in the repetitive unit. MOFs can complement the inorganic molecular sieves and can be used as host matrices with suitable characteristics [57, 58]. In fact, their pore size, shape, and dimensionality can be finely controlled by the judicious selection of the building blocks (metal and organic linker) and by their connections. Thus, immobilization of a POT within such a solid host matrix can be considered a convenient approach to develop heterogeneous photocatalysts with suitable characteristics.

2.5.1 C–C bond formation A novel MOF/decatungstate photocatalyst was synthesized by a solvothermal reaction between (nBu4N)4W10O32, Cu (ClO4)26H2O, and the ligand 4,40 -bipyridine (BPY) at pH 2.3 [59]. The photoactive W10O4 32 was embedded in the pores ˚  6.8 A ˚ channel. The pores were able to of Cu-BPY via electrostatic attraction to generate a composite system with a 13.3 A absorb various aliphatic nitriles (such as isovaleronitrile), which were the possible substrates for the alkylation by acrylonitrile and by other alkenes bearing electron-withdrawing groups. Authors hypothesized that the substrates could enter the pores of the MOF and that the spatial proximity between the adsorbed substrate and the photoactive center facilitated the in situ generation of the alkyl radicals from the aliphatic nitriles. Solvothermal reaction of 4,40 -bipyridine (BPY), Cu(NO3)23H2O and K5[SiW11O39Ru(H2O)]10H2O gave the first example of MOF containing a Ru-substituted polyoxometalate [60]. Moreover, the direct connection of Cu(II) ions [SiW11O39Ru(H2O)]5 provided the possibility of synergistic catalysis between a photocatalyst and a metal catalyst. This heterogeneous system was investigated in the CdC bond formation between several acetophenones and N-phenyltetrahydroisoquinoline under visible light photoexcitation of the POT. Interestingly, it was proved that the use of bulky acetophenones corresponded to a significant decrease in conversion, so confirming that reactants diffused into the cages of MOFs and that the reaction between them took place within pores. Moreover, the proximity between the intermediates avoided unwanted side reactions, enhancing selectivity.

2.5.2 Partial oxidation The reaction of (nBu4N)4W10O32, Cu(ClO4)26H2O, and 3-amino-4,40 -bipyridine by a diffusion method led to a novel 3D ˚  8.2 A ˚ [61]. decatungstate-based MOF (DT-NPY), with an 1D hydrophilic/hydrophobic channel with dimensions of 8.4 A UV-vis absorption spectrum of DT-NPY in the solid state revealed a red shift at wavelength around 400 nm compared to the free decatungstate anion. In addition, a new absorption at 626 nm, ascribable to coordinated copper (II) cations, was observed. Thus, photocatalytic properties of DT-NPY (l > 400 nm) were examined for cyclohexane oxidation in the presence of O2 under mild conditions. Efficiency was comparable to that obtained with other heterogeneous decatungstate catalysts with the peculiar advantage of using visible light as a renewable energy source.

2.5.3 Degradation reactions Only a few papers reported the photocatalytic activity of POT/MOF systems in the bleaching of dyes. No deep investigation on the mechanism was given, so here, we report briefly the composition of the photocatalyst and the main results obtained. Decatungstate anion was incorporated by electrostatic interaction into a one-dimensional ladder MOF, whose starting components were Co(NO3)26H2O and 4,40 -bis(pyridine-N-oxide) [62]. Since the obtained framework exhibited light absorption properties in the 230–650 nm range, its photocatalytic properties were investigated in the degradation of several dyes by monitoring their decrease of absorbance. Almost all the colored dyes (9.6 mg/L1) disappeared in 9 h irradiation by using a Xe lamp.

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A novel compound namely [(H2toym)2(SiW12O40)]6H2O in which “toym” is 2,4,6-tris[1-(4-oxidroxypyridinium)ylmethyl]-mesitylene), had been hydrothermally synthesized [63]. Its structure had been determined by XRD, IR, elemental analyses, TG analyses, and UV-vis techniques. The “host” supramolecular network had large pores and consisted of partly protonated toym ligands, whereas HPA “guest” was located in the pores and dispersed between two organic layers in the complex. The compound exhibited photocatalytic activity for methylene blue degradation under UV. Several poly(ionic liquid)s (PILs) and multielectronic-process sustaining polyoxometalate (POM, [H7P8W48O184]33) were assembled into new green and water-insoluble nanomaterials (POM@PILs) [64]. They were visible light photosensitive (l > 400 nm), whereas the two starting components were not. Thus, a synergic effect was invoked for the complete photodegradation of acid orange 7 (AO7) in aerobic media. The photocatalysts were recoverable and recyclable. Unlike many adsorbents of anionic dyes, the positive charges in the PILs were not pH-dependent.

3 POTs heterogenized on photocatalytically active supports The first example of immobilization of a POT on a photocatalytically active support was provided by Lykakis and coworkers who immobilized decatungstate anion on high-surface-area mesoporous TiO2 nanoparticles [30]. They used this new catalyst for the photooxidation of selected aromatic alcohols to the corresponding carbonyl products (l > 320 nm) [65]. The obtained system was selective but less active in comparison to the homogeneous decatungstate. Keggin-type heteropolyoxotungstates anions (HPA) had been heterogenized on TiO2 as the most used photocatalytically active support. A synergic effect was often observed with respect to TiO2 alone since UV illumination causes both separation charge in the SC and excitation of HPA to give HPA∗ (Fig. 6). In addition, it was stated that photogenerated electrons in TiO2 directly transfer from the conduction band to HPA∗, which is a better electron acceptor than O2 from a thermodynamic perspective [66, 67]. The result was a delay in the recombination between holes and electrons, and a longer lifetime of the separated charges allowed an enhanced degradation of organic molecules [68]. Moreover, HPA reoxidation by O2 leads to dioxygen-activated species (such as OH radicals) that contribute to the oxidation process. 4 On this basis, in aerated conditions, the addition of PW12O3 40 and of SiW12O40 to TiO2 suspensions resulted in a rate enhancement of 1,2-dichlorobenzene oxidation [69] and of 2,4-dichlorophenol degradation [70]. It was reported that loading the PW12O3 40 species on the surface of TiO2 by impregnation enhanced charge separation, thereby accelerating the hydroxylation of initial substrate but not its mineralization. In fact, aromatic intermediates, which are more toxic than the starting one, were formed. These studies infer that enhancement of charge separation in TiO2 photocatalysis does not always result in improvement of the efficiency of mineralization of organic substrates. Incorporation of H3PW12O40 into a polyvinyl alcohol TiO2 colloidal suspension [71] or its entrapment by the titania network during the hydrolysis of Ti(IV) tetraisopropoxide [72] gave new heterogeneous photocatalysts where HPA entered

FIG. 6 Synergy between HPA and titanium dioxide under irradiation with UV light.

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the pores to interact with internal surface hydroxyl groups of Ti resulting in (≡ Ti-OH+2 )(H2PW12O 40) composite. These systems had been studied in the photodegradation of several dye molecules by monitoring the decrease of absorbance. DR-UV-vis and XRD analysis agreed on the fact that the introduction of HPA in TiO2 framework resulted in changes in the coordination environment that caused a red shift (about 15 nm) in the absorption edge. Thus, thanks to intimate contact between the two components, HPA/TiO2 showed a 96% conversion of methyl orange after 600 visible light irradiation (l > 420 nm). Moreover, a role of the porous structure of HPA/TiO2 was recognized, since it enhanced mass transport for molecules in and out of the pore structure and increased adsorption of the reactants [72]. A general survey on the literature concerning POT/TiO2 composites led to the common statement that the enhancement of photocatalytic activity with respect to bare TiO2 is strictly related to several factors such as a good dispersion of POT on TiO2 surface, a strong connection among POT and TiO2 that determines a decrease of band gap energy and an active role of pores in governing adsorption-desorption equilibria of reactants and products. However, some other examples deserve to be cited because of their peculiar characteristics: firstly, the use of lacunary HPA instead of saturated Keggin unit opens the possibility to covalently link HPA to the support. For example, the lacunary K7PW11O39/TiO2 composite showed a covalent bond between the terminal nucleophilic oxygen atoms of K7PW11O39 and the electrophilic titanium atoms in the SC [73]. This system had solar simulated photocatalytic activity in the degradation of rhodamine B and diethyl phthalate and presented some important benefits, such as stability, negligible leaching, better dispersion of HPA on the surface. Secondly, multiple cobalt or nickel substituted HPA in association with colloidal TiO2 absorbed visible light and underwent multielectron reduction steps [74]. They were used in the degradation of acid orange 7 (AO7) in a deaerated aqueous solution containing 2-propanol. Interestingly, visible irradiation caused total bleaching of the dye, then a blue color arising from the reduced form of HPA was observed. This result was in agreement with the reductive cleavage of the azo bond of AO7 by photoreduced HPA through a 4e/4H+ process, where 2-propanol was the sacrificial reagent. As it could be expected, in the presence of O2, the activity was reduced owing to the competition between dye and dioxygen for the reoxidation of the reduced HPA. ZrO2 has been seldom used as a photocatalyst owing to its wide band gap energy of 5.0 eV (ca. 250 nm). However, entrapment of H3PW12O40 into zirconia matrix by sol-gel technique led to an efficient heterogeneous photocatalyst for the aerobic oxidation of alcohol to the corresponding carbonylic compounds [75]. Interestingly, it was shown by DR-UV-vis spectra that after formation of the POM/ZrO2 nanocomposite, the band relative to the starting POM (270 nm, O ! W CT) and the one relative to ZrO2 (245 nm, O ! Zr CT) both disappeared, whereas, a broad absorption band appeared with a significant red shift. This band tailed into the visible region until 410 nm. The outstanding photocatalytic activity of this nanocomposite was attributed to the existence of this broad band. Guo and coworkers reported the preparation of H3PW12O40/ZrO2 composites with different H3PW12O40 loading levels [76]. They used a wet impregnation method followed by final calcination. The increase of calcination temperature caused the change in zirconia phase, from amorphous, to tetragonal and monoclinic as evidenced by XRD analysis. FT-IR and Raman scattering spectroscopies showed the integrity of the Keggin structure after impregnation. Its immobilization determined a decrease in the specific surface area of pristine ZrO2, and some of the pores were blocked up after grafting of the POT, as evidenced by BJH pore size distribution curves. UV-vis DRS results indicated that impregnation of the Keggin unit on ZrO2 framework red shifted the band edge absorption threshold, so giving rise to a narrowing of the band gap. In fact, the photoactivity of sole ZrO2 in the degradation of 4-nitrophenol (4-NP) and methylene blue dye (MB) was enhanced when the composite H3PW12O40/ZrO2 systems were used. From 4-NP and MB concentration profiles vs irradiation time, it was evident that homogeneous dispersion of the Keggin unit on the ZrO2 framework played an important role to improve the photocatalytic activity of the composites. However, HPA loading levels higher than 20% caused partial segregation and accordingly, the photocatalytic activity decreased. ICP-AES measurements carried out on centrifuged irradiated solutions showed that the leakage of the Keggin unit was negligible and photoactivity of recycled material remained almost unchanged for three consecutive runs. Zirconia-supported Ti-substituted Keggin-type polyoxometalates Li5PW11TiO40/ZrO2 (PW11Ti/ZrO2) and K7PW10Ti2O40/ZrO2 (PW10Ti2/ZrO2) were prepared by incorporating PW11Ti and PW10Ti2 clusters into a zirconia matrix via a sol-gel technique that used hydrolysis of zirconium n-butoxide in the presence of Li5PW11TiO40 or K7PW10Ti2O40 clusters [77]. Characterization (DR-UV, FT-IR, 31P MAS NMR, ICP-AES, N2 adsorption) of the materials indicated that the clusters were chemically attached to the zirconia supports and that Keggin structure remained intact. The heterogeneous photocatalytic activities were tested by studying the degradation of aqueous dye naphthol blue black. The dye was min2 eralized into the inorganic products (CO2, NH+4 , NO 3 , and SO4 ions) by irradiating slurries of the materials, which exhibited UV absorption maxima around 250–260 nm requiring a high-pressure mercury lamp as the light source. Inclusion of Keggin or Dawson units into the Ta2O5 framework led to H3PW12O40/Ta2O5 and H6P2W18O62/ Ta2O5 composite systems [78]. The structure of POT remained intact after immobilization within Ta2O5 structure. The

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as-prepared composites were successfully used in the photocatalytic degradation of salicylic acid and rhodamine B. The observed activity was ascribed to the properties of the materials, such as porous structure, small particle size, homogeneous dispersion of the POM unit within the Ta2O5 framework. Likely, the most important factor responsible for the enhancement of the photocatalytic activity of H3PW12O40/Ta2O5 and H6P2W18O62/Ta2O5, relative to that of bare HPA units and Ta2O5 alone, was due to their optical absorption properties. In fact, it was established that their absorption threshold onset continuously extended to the visible region (l > 420 nm), corresponding to decreased bandgap energy. In addition, it was claimed that the strong interaction between POM and Ta2O5 ensured little deactivation of the catalysts after three catalytic recycling runs.

4 Conclusions Appropriate heterogenization of polyoxotungstates is of great interest in research to provide photocatalytic systems able to perform the organic transformation and pollutant abatement. Heterogeneous photocatalysts based on POTs can be easily removed from the reaction vessel and often re-used without loss of activity when their stability is enhanced. In addition, we have demonstrated that well-defined textural characteristics and adsorption properties of the support represent a suitable means to tailor the selectivity of the process under investigation. The preparation of POTs both in inactive and photocatalytically active oxides has been carried out by several described methods. Characterization of the materials supports the statement that POT integrity is maintained after heterogenization. At first, the bond between the support and the POT was mainly electrostatic but strong enough to render leaching negligible. Nevertheless, we have reported some examples of POTs covalently bonded to the support. An alternative approach, which is still in its infancy, is represented by host-guest materials. Here, the guest POT is incorporated into an inorganic/organic supramolecular host and photoactivity of the guest can be combined with the catalytic activity of the metal ion present in the framework. Also, control of pores size and shape opens to the possibility of realizing processes with high selectivity. The use of photocatalytically active support causes an increase in the performance that is sometimes due to a synergistic effect between the two components. In some cases, a decrease in band gap is observed, and a red shift of the absorption threshold in the visible region is achieved.

Acknowledgments This work is based on the results of several years of work under the guidance of my teachers and colleagues Prof. Andrea Maldotti and Dr. Rossano Amadelli, who recently retired. My gratitude goes to them for personal scientific growth.

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Chapter 20

Heterogeneous photo-assisted catalytic hydration/dehydration reactions based on Keggin and Wells–Dawson type heteropolytungstates Elisa I. Garcı´a-Lo´peza and Giuseppe Marcı`b a

Department of Biological, Chemical and Pharmaceutical Sciences and Technologies (STEBICEF), University of Palermo, Palermo, Italy, b Department

of Engineering, University of Palermo, Palermo, Italy

1 Introduction Since Fujishima and Honda studied the UV light-induced water cleavage by a TiO2 photoanode [1], photocatalysis has attracted significant attention for the degradation of organic and inorganic pollutants and recently also selective partial oxidation [2] or synthetic chemistry [3]. TiO2 and other metal oxides, traditionally used as photocatalysts, have evidenced several drawbacks, as the small number of photons absorbed in the visible region, the high recombination rate for the photoproduced electron–hole pairs, the difficulty to enhance the activity by loading or doping, and the deactivation in the absence of water vapor, among others [4]. Consequently, strong efforts have been addressed to develop alternative materials [5]. In this context, relatively low attention has been focused on the use of polyoxometalates as heterogeneous photocatalysts. Polyoxometalates (POMs) are a wide class of discrete nanosized transition metal-oxygen clusters presenting intriguing architectures. These clusters exhibit interesting physicochemical properties allowing their wide use in catalysis, materials science, analytical chemistry, surface and interface science, medicine and life science, electro-, photo- and magnetic chemistry [6]. The large number of structural types of POMs can be divided into three classes: heteropolyanions (HPAs), isopolyanions (IPAs) and Mo-blue, and Mo-brown reduced POM centers [7]. Focusing our attention on HPAs, several types are known, including acids and their salts. According to Pope and Muller [8], it is convenient to classify them starting from the symmetrical ‘parent’ polyanion; i.e., Keggin, Wells–Dawson, Anderson–Evans, or Dexter–Silverton among other structures, as exemplified in Fig. 1. These polyanions are discrete anionic metal oxides of groups 5 and 6 assembled by the condensation of metal oxide polyhedra (MOx), where M can be W(VI), Mo(VI), V(V), Nb(V), Ta(V) [8, 9]. The metal atoms are called addenda atoms, and they can change their coordination with oxygen from 4 to 6 because the MOx polyhedra condense in solution upon acidification. Although oxygen is the main ligand that coordinates with the addenda atoms, other atoms/groups such as sulfur, bromine, nitrosyl, and alkoxy are also reported in POM clusters [10]. When the POM frameworks exclusively contain the addenda metals from groups 5 and/or 6 and oxygen, the clusters are called isopolymetalates or isopolyanions (IPAs), such as the Lindqvist type anion [M6O19]2
, which is also reported in Fig. 1. When the POMs include additional elements along with the addenda metals and oxygen, they are known as heteropoly complexes or heteropolyanions (HPAs) which can be formed via condensation of MOx polyhedra around the central heteroatom. Many different elements can act as heteroatoms in HPAs, presenting various coordination numbers as the 4-coordinate (tetrahedral) heteroatom in the Keggin and Wells–Dawson structures (e.g., PO43
, SiO44
, and AsO43
); 6-coordinate (octahedral) in the Anderson–Evans structures (e.g., Al(OH)63
and TeO66
); or 12-coordinate (Silverton) in [(UO12)Mo12O30]8
. In this chapter, we will focus our attention on the Keggin and Wells–Dawson structures. The Keggin anion was the first cluster to be characterized and the most used in heterogeneous (photo)catalysis. Both Keggin {XM12O40}
y and 3
Wells–Dawson {X2M18O62}y
anions present the heteroatom X in an XO
and SiO43
and the 4 coordination, as PO4 so-called addenda atom is whether commonly W or Mo. The HPAs tungstophosphates are the largest POM subclass. The Keggin structure in the polyanion [PW12O40]3
can be described as an assembly of a central tetrahedron of phosphorus Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00024-6 Copyright © 2021 Elsevier Inc. All rights reserved.

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FIG. 1 Polyhedral representation of some HPA structures (A) Lindqvist (M6O192
), (B) Anderson–Evans (XM6O24n
), (C) Keggin (XM12O40n
), (D) Wells–Dawson (X2M18O62n
) and (E) Preyssler (NaP5W30O11014
). From https://commons.wikimedia.org/wiki/File:POMs.png .

coordinated to oxygen. This is the center of the HPA, which is coordinated to four peripheral [W3O13] blocks that complete 12 octahedra in total, whose centers are occupied by tungsten. In each [W3O13] block, three octahedra are connected by an edge of oxygen and each block shares with oxygen each of the other blocks as well as the central tetrahedron. This cluster, shown in Fig. 1, possesses a diameter of ca. 1.2 nm [11–13]. It is possible to find the HPA lacking one (monovacant) or more addenda atoms, which result in the formation of the ‘lacunary’ HPA, in contrast with the ‘plenary’ one. Some scientific current research is focused on the HPAs functionalization to covalently attach these clusters to organic or inorganic species. For that aim, the first step is the removal of one or several metal ‘octahedra’ from the original HPA to generate the lacunary species. We have to consider that the HPAs are negatively charged clusters, whose electroneutrality was satisfied by the presence of a counter cation. In non-lacunary (plenary) HPAs, the negative charge is delocalized over the entire cluster structure whereas in the lacunary HPAs, the oxygen atoms are more nucleophilic and hence more reactive toward electrophilic organic and inorganic groups to form covalent bonds, as we will discuss later. The lacunary HPAs can also assemble to form new species. The Wells–Dawson anion may be considered derived from the Keggin one. Two Keggin clusters are needed to obtain a Wells–Dawson cluster. The removal of one group of three octahedra, a [M3O9] block, from each Keggin anion, and the successive linking of the two obtained lacunary clusters results in the nearly ellipsoidal Wells–Dawson anion cluster [14] (see Fig. 1). The phosphotungstic Wells–Dawson HPA synthesis was described for the first time by Kehrmann in 1892. Dawson detailed its crystallographic structure in 1953. The structure, known as a isomer, possesses two identical “half units” of the central atom surrounded by nine octahedral units PM9O31 linked through oxygen atoms (Fig. 1D). An isomeric structure b is generated by the rotation of one-half unit p/3 around the P–P-axis [14]. Most POMs are highly soluble in a variety of polar and polar-organic solvents. This is due to the ability of POMs to interact with the solvent via electrostatic forces, hydrogen bonding, and covalent and non-covalent interactions. In general, HPAs are prepared in an aqueous medium by acidification of a solution containing the parent species, but the preparation conditions can give rise to a large family of compounds. The variables are: (i) concentration/type of metal oxide anions, (ii) pH and type of acid, (iii) type and concentration of electrolyte, (iv) heteroatom concentration, (v) possibility to introduce additional ligands, (vi) reducing agent, (vii) temperature and solvent [7]. As an example, the Keggin cluster can be obtained by a simple polycondensation reaction in an acidic medium between the phosphate (or silicate) and tungstate ions: PO4 3
+ 12 WO4 2
+ 24 H + ! PW12 O40 3
+ 12 H2 O

(1)

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The Wells–Dawson heteropoly acid, H6P2W18O62 can be obtained, for instance, through the synthesis of the K6P2W18O62 salt [15]. In this synthesis, H3PO4 is added to an aqueous solution of Na2WO4 and the solution is refluxed for 8 h. The salt is precipitated by adding KCl and purified by recrystallization at 5°C. The final product, filtered, washed, and then vacuumdried for 8 h is a salt that can be transformed in the phosphotungstic Wells–Dawson acid cluster, H6P2W18O62, by the “etherate method” [16]. This process implies the addition of ether and concentrated HCl to the aqueous solution of the a/b mixture of the K6P2W18O62 salt. H6P2W18O62 formed a compound with ether, which can be separated by extraction. The ether solution containing the HPA is eventually dried, for instance, in a vacuum desiccator until crystallization of the HPA as a polycrystalline solid. HPAs show hierarchical structures of paramount importance to understand their role as catalysts. The heteropolyanion itself forms the so-called primary structure corresponding to the clusters shown in Fig. 1 and also in Fig. 2A. The primary structure corresponds to the metal oxide cluster heteropolyanion itself. The secondary structure is a three-dimensional arrangement including counter cations and water molecules (Fig. 2B). Indeed, as previously mentioned, in the real solid material along with the anionic cluster, countercation is also present, accompanied by some polar molecules, such as water molecules or even alcohols. Hence, the secondary structure is formed by the combination of several primary units. The assembly is neutralized by countercations (H+ or monovalent cations such as Na+, NH4+, Cs+). Primary structures form secondary structures which are rather mobile assemblies, where the anions are interacting with polar molecules as water which are present in the bulk of crystallites giving rise to the protonated clusters (see Fig. 2B). Primary structures of ˚  11.7A ˚ and 11.2 A ˚  14.4 A ˚ , respectively [17]. H3PW12O40 and H6P2W18O62 present molecular dimensions of ca. 11.7A The secondary structures condense in small particles of different sizes that can exceed several tens of nanometers. The countercation H+ appears as a protonated dimer of water in the form of dioxonium H5O+2 . Each H5O+2 links four [PW12O40]3
anions, forming with them hydrogen bonds involving terminal O atoms [18], as reported in Fig. 2B. Dioxonium is removed at ca. 150–200°C [18]. HPAs are thermally unstable and, in particular, the thermal stability of the Keggin structure decreases in the following order: H3[PW12O40] > H3[SiW12O40] > H3[PMo12O40] > H3[SiMo12O40], but it can be enhanced by the formation of appropriate salts [19]. The presence of water in the secondary structure is important, for instance considering the use of HPAs as catalysts, because it influences the acidity and the adsorption properties of HPAs and consequently their catalytic activity [11–13]. HPAs contain up to 30 H2O molecules of crystallization per anion, that desorb progressively by increasing the temperature. The solid becomes completely dehydrated at ca. 350–500°C. The amount of water of crystallization in the sample dramatically changes with the experimental conditions. A sample of Wells–Dawson type HPA kept at room temperature (it should be kept over saturated Mg(NO3)2 solution) contained around 29 H2O molecules per mole of HPA. Similarly, the formula of Keggin is H3PW12O4020 H2O. The strong acidity of the HPAs, both in solid-state and in solution, can be attributed to the large dimension of the polyanion which favors the delocalization of protons in the structure. HPAs such as H3PW12O40, H4SiW12O40, H3PMo12O40,

FIG. 2 Structural hierarchy of heteropoly compounds: (A) Keggin cluster (primary particle), (B) secondary structure, and (C) tertiary structure (texture: particle size, porosity, surface area, etc.). Reproduced from G. Marcı`, E.I. Garcı´a-Lo´pez, L. Palmisano, Heteropoly acid-based materials as heterogeneous photocatalysts, Eur. J. Inorg. Chem. 2014 (2014) 21–35, with Wiley permission.

322

SECTION

C Oxides and calcogenides

H4SiMo12O40, or H6P2W18O62 possess stronger Brønsted acidity than conventional inorganic acids and consequently, the corresponding anions have weak Brønsted basicity, softer than that of nitrate and sulfate anions [20], which makes these molecules easy to handle without hazardous corrosive problems, unlike mineral acids [21]. The order of acidity for the Keggin structures is the following: H3[PW12O40] > H3[SiW12O40] H3[PMo12O40] > H3[SiMo12O40] [22, 23] and the acidity strength in the available surface of the solid HPAs depends on the nature of the countercation. When it is small (for instance Na+), the behavior of the HPA is very similar to that of the acidic form and the species is highly soluble in water and polar solvents; conversely, when it is voluminous (K+, Cs+, NH4+), the solubility of the HPAs in water decreases. The usable acidity for a (photo)catalytic reaction depends on the size of the particles (tertiary structure, Fig. 2C). In most cases, the reagent molecules cannot penetrate the tertiary structure, and only a small fraction of the total acidity is accessible. However, due to the ability of HPAs to solve polar molecules, the latter can react more easily (reaction in “pseudo-liquid phase”) [11]. Misono et al. have demonstrated that a solid heteropoly compound can act as a catalyst in a gas phase reaction in three different ways, i.e., as (i) surface-type catalyst, (ii) pseudo-liquid bulk-type, and (iii) bulk-type catalyst [11]. The surfacetype catalysis is the ordinary heterogeneous catalysis that takes place bi-dimensionally on the solid surface, in contrast with the bulk-type catalysis. When the diffusion of reactant molecules in the lattice rather than pores of the solid is faster than the reaction, a pseudo-liquid phase is formed in the bulk where the catalytic reaction can proceed. The catalytic behavior of HPAs is strongly influenced by the modifications that may occur in the secondary structure where the Br€ onsted acidity is located and which would be altered by the number of water molecules involved. The structural modifications that may occur both in the secondary and tertiary structures of the HPAs influence their hydration state determining the number and strength of the surface acid sites along with the accessibility of the molecules. The acid sites in the secondary structure are H5O2+ species. Baronetti et al. reported the importance of the pseudo-liquid phase formation in Wells–Dawson acid during methyl tertbutyl ether (MTBE) synthesis and methanol dehydration to dimethyl ether in the gas phase at 100°C [16]. The authors observed that the loss of catalytic activity of Wells–Dawson acid with the increase of the pre-treatment temperature was due to the loss of water molecules in the secondary structure. The important role of water has been also described for the catalytic behavior of the Keggin HPA [18]. Among the HPAs, those having Keggin-type structures are currently used in industrial catalytic processes [19], while the Dawson-type HPAs have been used for the selective catalytic oxidative dehydrogenation of isobutene to 2-methyl-propene [24] or for MTBE production [25]. Another important feature of many HPAs and particularly of the Keggin and Wells–Dawson is that they are easily reduced. The addenda metal atoms are mostly in their highest oxidation states (d0) and thus these clusters exhibit fast reversible redox transformations under mild conditions. This property is very useful in the application of POMs as photocatalysts in pollutants photo-degradation. When they accept one or more electrons, mixed-valence species are formed, and the so-called heteropoly blues retain the structure of the parent oxidized anions [22]. Further details of the chemistry of POMs have been summarized in various books, reviews, and thematic issues [8, 26– 28]. As mentioned above, POMs have been used in the design of various multifunctional materials with different techniques, and they are the base of multifunctional materials used to solve different emerging issues. Omwoma et al. have revised the abilities of POM-based molecular compounds and functionalized POM-containing composites in providing effective solutions to various environmental problems [29]. By keeping in mind that photocatalysis is a branch of catalysis, it is worth highlighting the important role of HPA-based materials in catalysis both in solution and in a solid state. The long list of review articles and special issues devoted to this field published in the last decades [30–34], along with several patents and the commercialization of several catalytic processes involving HPAs, is a clear indication of their practical significance. Looking for strong acid catalysts, heteropoly acids such as H3PW12O40 are able to catalyze a wide range of homogeneous catalytic processes at low temperatures [11–13] such as esterification, transesterification, hydrolysis, Friedel-Crafts alkylation and acylation, and Beckmann rearrangement [35, 36]. Interestingly, some special POMs also possess basic properties and can be used in base-catalytic reactions [37]. From the practical perspective, heterogeneous catalysis is preferred because of the advantages of the facile catalyst separation from the reaction products. The POMs solubility in water and polar solvents causes difficulties in the recovery, separation, and recycling of the catalysts, which affects their use in systems that require environmentally friendly efficient transformations. Therefore, an imperative is to develop easily recoverable and recyclable HPAs based catalysts for practical industrial applications. The heterogenization of HPA clusters has been an important issue because of the low specific surface area of unsupported HPAs (1/10 m2 g
1). HPA dispersion on supports with high surface area increases the accessibility to their acidic sites and consequently increases their catalytic activity. The classical strategy to heterogenize HPA clusters involves supporting them on an oxide. Alternatively, the cesium or potassium salts (Cs2.5H0.5PW12O40 or K2.5H0.5 PW12O40) are

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insoluble solids with micro and mesoporosity [38]; however, the sizes of their particles are very small and problems can arise during the separation procedure. Recently, heterogenized HPAs have been prepared associated with inorganic, organic, or organometallic moieties. The supported HPAs evidenced important changes in structure, acid strength, and redox properties with respect to the unsupported material. It is important to remind that the photocatalytic reaction occurs on the solid surface, and the substrates need to diffuse into the active sites to be chemically adsorbed while the products need to be desorbed from the solid through a reverse process. Therefore, the pore structures, as well as the hydrophilichydrophobic properties, significantly affect both activity and selectivity. Consequently, porous material support is generally preferred to uniformly distribute the active sites and to decrease the mass transfer resistance. Several oxides have been used as HPAs support for (photo)catalytic purposes, for instance SiO2 [39–42], Al2O3 [43], ZrO2 [44], Ta2O5 [45] or carbon [46]. Silica interacts weakly with the Keggin and Wells–Dawson anions preserving their structure. Conversely, interaction with basic solids, as MgO, ZnO, or Al2O3, induces their decomposition, resulting in a decline of acidity [47]. Metal–organic frameworks (MOFs), mesoporous polymers, magnetic nanoparticles, porous carbons, zeolites, have been also used. Parallel studies have been extensively devoted to the use of HPAs as photocatalysts in homogeneous systems [48]. In fact, absorption of light by the ground electronic state of the solubilized HPA produces an excited state HPA*. The light absorption gives rise to an O ! M ligand to metal charge transfer (LMCT) in the HPA cluster, for instance, in the PW12O403
Keggin structure, from an O2
to a W6+ at the W–O–W bonds. An electron is promoted from a spin-paired, doubly occupied bonding orbital (HOMO) to an empty, antibonding orbital (LUMO), resulting in the generation of a species with a hole center (O
) and a trapped electron center (W5+). This charge transfer observed at 260 nm, corresponding to 4.8 eV, for the plenary Keggin H3[PW12O40] is qualitatively analogous to the bandgap of a solid semiconductor metal oxide that also generates an electron–hole pair under irradiation [49]. The proposed mechanism of the HPA-based photocatalysis involves several steps [48]. Firstly, the pre-association between the HPA and the substrate (S), followed by absorption of light by the formed complex obtaining the excited state species (HPA*) with a high reduction potential. The latter in the presence of an electron donor (substrate S) gives rise to the heteropoly blue reduced form (HPA
) that absorbs at 650 nm (reactions 2–4). The further step is the re-oxidation of HPA
upon exposure to an oxidant, for instance, O2 (reaction 5): HPA + S ! ðHPA
SÞ

(2)

ðHPA
SÞ ðl < 400 nmÞ ! ðHPA∗
SÞ


ðHPA∗
SÞ ! HPA + S 

+

HPA
+ O2 ! HPA + O2 


(3) (4) (5)

The formation of highly reactive OH radicals has been proposed to enhance the photooxidation performance according to reactions (6)–(8): O2 
+ H2 O ! HO2  + OH


(6)

HO2 ! O2 + H2 O2

(7)

O2 
+ H2 O2 !  OH + OH
+ O2

(8)

Hydroxyl radical (%OH) oxidizing species have been experimentally detected in the presence of HPAs by photolysis or EPR experiments. The reoxidation of the HPA catalyst to its original oxidation state (reaction 5) is reported to be the ratedetermining step in the HPAs photocatalytic cycle [50], and it is performed by an electron acceptor, such as O2 dissolved in the suspension or, in the absence of O2, by other electrophilic species [51]. It is necessary to mention also that heterogeneous POM-based catalysts can entail some disadvantages, such as leaching of the active sites. Moreover, heterogeneous POM-based catalysts usually exhibit inferior catalytic performance than their homogeneous counterparts, mainly due to the mass transfer resistance and the diffusion limitation on the active sites. To overcome the above limitations, many approaches have been proposed to improve the stability and catalytic performance. Wang’s workgroup reviewed the strategies to obtain POM-based heterogeneous catalysts and concluded that mainly two pathways can be undertaken: “immobilization” or “solidification” [23]. The different strategies to “heterogenize” the HPA cluster are summarized in Fig. 3. Based on the different host-guest interactions, POM-based active sites can be immobilized through adsorption, ion exchange, covalent linkage, encapsulation, and substitution, etc. The porous support will not only provide a larger surface to highly distribute the active sites but will also greatly influence the activity and selectivity because the host-guest interaction can affect the physicochemical features of the HPA active center. To design an efficient-supported heterogeneous POM-based material, it is necessary to choose an appropriate heterogenization methodology.

324

SECTION

C Oxides and calcogenides

FIG. 3 Strategies for preparing heterogeneous POM-based materials for catalytic/ photocatalytic purposes.

In the present chapter, we can classify the heterogenized HPAs into three main groups: (i) HPA immobilized by a photocatalytic inactive support, (ii) HPA immobilized onto a photocatalytic active material (generally a semiconductor oxide) and (iii) HPA heterogenized by immobilization in a host–guest insoluble composite.

2

HPAs based on heterogeneous photocatalysts

HPAs can be supported on a photocatalytic inactive support or a photocatalytic active support. In the latter composites, the concomitant excitation of both HPA and support, generally a semiconductor oxide, synergistically enhances the photoactivity of the whole powder. As far as the HPAs supported on photocatalytic inactive materials are concerned, the mechanism of the photocatalytic process is explained in the same manner as described in the previous section for the homogeneous systems. In these composites, the support plays a double role, i.e., modifying some of the physicochemical features of the HPA and also improving the adsorption ability of the substrates to react with the active species. H3PW12O40/SiO2 and H4SiW12O40/SiO2 prepared by incorporating the HPAs into the silica matrix via a sol–gel technique were tested as heterogeneous photocatalysts for the degradation of malic acid, that was totally mineralized in ca. 2 h of UV irradiation [31, 52, 53]. Intermediate products identified by using either these supported HPAs or bare TiO2 were the same, indicating that in both systems OH radical was the main oxidant species. H3PW12O40 was also anchored to amino-functionalized Fe3O4/SiO2/meso-SiO2 microspheres using chemical bonds to aminosilane groups. The resultant microspheres contain an HPA loading of ca. 17% wt. Rhodamine B dye was bleached under UV irradiation more quickly than by using the solubilized HPA, with the further advantage of the magnetically recovered solid photocatalyst [54]. Authors point out that the HPA interacts with the silica support through an acid–base reaction, hydrogen, and covalent bonds, between -NH2 and H3PW12O40; however, they admit that some leaching of the HPA unit occurs. H3PW12O40 immobilized on NH4Y and NH4ZSM5 zeolites have been prepared by wet impregnation [55]. The acidic properties of both series of composites are rather similar, despite the higher acid strength of NH4ZSM5. The samples with higher HPA content showed band gap energy values similar to those reported for TiO2. These materials were active for the photocatalytic degradation of 4-chlorophenol in liquid–solid regime. Enhanced photocatalytic activity in the presence of the HPA was observed. As far as the HPAs supported on photocatalytic active semiconductors are concerned, the activity of heterogenized HPAs is further enhanced. The synergistic effect between the HPA and the semiconductor, particularly when TiO2 is used as support, has been extensively reported. The Keggin type HPA supported on TiO2 has been the most studied heterogeneous photocatalyst. The mechanism of the photocatalytic reaction for this kind of composite is reported in Fig. 4 where it is depicted how the semiconductor oxide

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FIG. 4 Scheme of the mechanism acting in synergism between HPA and a photocatalytic active semiconductor under irradiation. Reprinted with permission from G. Marcı`, E.I. Garcı´a-Lo´pez, L. Palmisano, Heteropoly acid-based materials as heterogeneous photocatalysts, Eur. J. Inorg. Chem. 2014 (2014) 21–35. Copyright 2014, Wiley.

directly transfers the photo-generated electrons from the conduction band to the HPA* formed by excitation of the HPA (see reaction 3). The Keggin anion [PW12O40]3
∗, by a thermodynamic perspective, is a better electron acceptor than O2 (+0.22 V (pH independent) and
0.33 V (at pH 0), vs NHE, respectively) [56]. Notably, the reduction potential of [PW12O40]3
∗ is more positive than the flat-band potential of the conduction band of TiO2 particles (ca.
0.19 V vs NHE (at pH 0)) [57]. In this way, the electron transfer from the conduction band of TiO2 to [PW12O40]3
∗ is thermodynamically favorable to yield [PW12O40]4
species. The enhanced degradation of organic compounds by using HPAs on semiconductor oxides in the presence of UV light has been on this basis thoroughly explained in the literature. The first attempt to understand the previous mechanism has been done by Yoon et al. who prepared an HPA/TiO2 by incorporating H3PW12O40 into a polyvinyl alcohol TiO2 colloidal suspension. The system was illuminated with UV light (300–375 nm) for the photodegradation of a dye (methyl orange) [58]. HPA was simply added (not immobilized) to the TiO2 suspension, and the extent of the photoinduced reaction depends not only on the HPA/TiO2 ratios but also on the irradiation wavelength and intensity. The photoinduced charge-carrier generated at the heterojunction HPA-TiO2 was very efficient. The authors evidenced much stronger hydrogen bonds, between the oxygen atoms of Keggin anion and the hydroxyl groups of the TiO2, in the solution phase than in the dried state. An acidic-base interaction between the HPA unit and the TiO2 matrix exists. The (TiOH+2 ) and (H2PW12O
40) species are responsible for that interaction [19]. Further studies by Majima’s group demonstrated that the initial reduction rate of HPA is greatly enhanced in the presence of TiO2 upon UV irradiation [59]. The reaction scheme occurring in the presence of this composite, with a double electron-transfer mechanism, resembles the “Z-scheme” mechanism invoked in the photosynthesis process, as reported in Fig. 5. This mechanism implies that an interfacial photoinduced electron transfer takes place from the conduction band of TiO2 to HPA, resulting in a reduction of PW12O403
to PW12O404
, the heteropoly blue species. PW12O404
can be oxidized back to PW12O403
through hydrogen atom abstraction from polyvinyl alcohol (PVA) [58]. In Fig. 5, PW12O403
is denoted as HPA and PW12O404
as heteropoly blue (HPB), and it is produced by direct electron transfer from the TiO2 conduction band to the ground-state HPA in addition to photoreduction through the excited state of HPA. The published results clearly indicate that the electron injection from PW12O404
* to the conduction band of TiO2 is the main reason for the significant enhancement in the one-electron photocatalytic oxidation reactions studied. The efficiency of electron transfer increases in the order H2W12O406
< SiW12O404
< PW12O403
, depending on the reduction potential of the HPA [56]. The use of HPAs immobilized on TiO2 has been extensively studied in the liquid–solid regime for several photocatalytic model reactions. In general, the primary Keggin structure remained unmodified after the immobilization of TiO2. The increasing activity of the composite material with respect to the homogeneous HPA or bare semiconductor is explained by considering the mechanism in Figs. 5 and 6, i.e., an interfacial electron transfer from the conduction band of TiO2 to the incorporated HPA occurs, hence a reduction of the HPA after its activation by UV light takes place and eventually oxidation of the reduced HPA species gave rise to %OH radicals, the ultimate species responsible for the oxidation process. Ozer and Ferry added PW12O403
and SiW12O404
to a TiO2 suspension resulting in a significant rate enhancement for 1,2-dichlorobenzene oxidation [60]. The reduction potentials of the HPA used by these authors are +0.219 V vs NHE for H2NaPW12O40 [61] and + 0.055 V vs NHE for H4SiW12O40 [62]. Mixtures of H3PW12O40 with commercial TiO2 Degussa

326

SECTION

C Oxides and calcogenides

HPVA

ADP + P1 HPA* P430

HPB*

Cyt. b563

CB

Fd

Q(C560)

visible light HPB

e–

Plasto quinonc

NADP+

hn2 P700

PVA

ATP

ADP + P1

Cyt. f

e–

H2O

HPVA VB

S

ATP P680

PVA

O2

e– Methyl + 2H+ Orange Hydrazine

HPA hn1

TiO2

hn1

(A)

hn2

(B)

FIG. 5 Energy diagram: (A) Photosynthesis in green plants; (B) Photoinduced electron transfer at the heterojunction of HPA/TiO2 colloids in the presence of 0.1% PVA as an electron donor. Reprinted with permission from M. Yoon, J.A. Chang, Y. Kim, J.R. Choi, K. Kim, S.J. Lee, Heteropoly acid-incorporated TiO2 colloids as novel Photocatalytic systems resembling the photosynthetic reaction Center, J. Phys. Chem. B 105 (2001) 2539– 2545. Copyright 2001, American Chemical Society.

HO

H+

HO

W P O Ti

(A)

(B)

FIG. 6 The proposed structures of the composites formed between (A) “lacunary” HPA and TiO2 (K7PW11O39/ TiO2) and (B) “plenary” HPA and TiO2 (H3PW12O40/ TiO2). Reprinted with permission from F. Ma, T. Shi, J. Gao, L. Chen, W. Guo, Y. Guo, S. Wang, Comparison and understanding of the different simulated sunlight photocatalytic activity between the saturated and monovacant Keggin unit functionalized titania materials. Colloids Surf. A 401 (2012) 116–125. Copyright 2012, Elsevier.

P25 were used for 2,4-dichlorophenol photocatalytic degradation in aqueous media [63]. H3PW12O40 species on the surface of TiO2 accelerated the hydroxylation of the substrate but not its mineralization, which was somewhat suppressed in the presence of the HPA. An increase in the HPA loading increased the concentration of the toxic intermediates. Guo’s group mixed up titanium isopropoxide and H3PW12O40 and the material obtained was successively autoclaved and calcined. The obtained H3PW12O40/TiO2 photocatalytic bleached ten organic dyes in aqueous systems under visible light irradiation (l > 420 nm) [64]. An electrostatic interaction between HPA and TiO2 occurred during the hydrolysis of titanium tetraisopropoxide in the presence of the Keggin ion that remained entrapped by the protonated hydroxyl groups on the TiO2 surface (≡Ti-OH), resulting in interactions between (≡TiOH2+) and (H2PW12O40
). Hydrogen bonds could be formed between the oxygen atoms of Keggin ion and the hydroxyl groups of the TiO2 surface (WO⋯ HO-Ti). The authors attributed the photocatalytic activity not only to the synergistic effect of the HPA on the semiconductor oxide (Fig. 5) but also to the porous structure of the nanocomposite that enhanced mass transport and increased adsorption of the reactants. A loading on TiO2 was carried out also with a series of Keggin HPAs possessing different heteroatoms, i.e., [Xn+ W12O40](8
n)
, where Xn+ ¼ P5+, Si4+, Ge4+ [65]. These materials showed higher photocatalytic activity than both bare TiO2 and HPAs for the dye reactive brilliant red X-3B degradation under visible light. Two variables influenced the bleaching rate: HPA loading and the presence of different heteroatoms in the HPA (P> Si > Ge). The same authors used the CsxH3-xPW12O40/TiO2 (x ¼ 0.5 to 3.0)

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327

photocatalyst as well for the photocatalytic degradation of 4-nitrophenol, methyl orange, and rhodamine B under UV irradiation [66]. These composites were more effective than the bare HPA or the titania alone. The photoactivity was attributed to the higher surface acidity, the mesoporosity, and the synergistic effect between HPA and the semiconductor. H3PW12O40 was also deposited on mesoporous TiO2. The preparation included both sol–gel (in the presence of a template, Pluronic 123) and hydrothermal treatments, and it was used to obtain a well-distributed 3D interconnected mesopore structure partially covered by the HPA [67]. The solid was used for the photocatalytic mineralization of some dyes under UV. Also, H3PW12O40/TiO2 was active in the mineralization of aqueous phthalate esters as di-n-butyl phthalate (DBP), diethyl phthalate (DEP), and dimethyl phthalate (DMP) under simulated sunlight irradiation [68]. The excellent photocatalytic performance of the composite was again attributed to the synergism among HPA and titania, not only by considering the presence of %OH, h+, and O%2
formed under irradiation, but also to the porous morphology of the solid that favored the substrate adsorption. The successful use of simulated sunlight irradiation depended on the kind of composite HPA/TiO2, but it was not satisfactorily clarified. These results, obtained by using dyes as substrates, should be taken with prudence, in fact, the absorption of light and the photoreactivity could be compromised by the type of molecule used. The formation of surface complexes activated by visible light when dyes are used makes dyes unsuitable for these studies [69, 70]. Both saturated and monovacant Keggin unit functionalized TiO2 materials, H3PW12O40/TiO2 and K7PW11O39/ TiO2 can be prepared by sol–gel followed by solvothermal treatment. Diethyl phthalate and rhodamine B were photocatalytically degraded under simulated solar irradiation [71]. The different photocatalytic activity between the saturated and monovacant Keggin functionalized TiO2 was attributed to their different electron-accepting ability. Fig. 6 shows the bonds between the HPA and the titania support in both composites. Usually, the preparation of HPA supported on TiO2 gives rise to a composite where an acid–base interaction and hydrogen bonds between the HPA and the semiconductor occur. In the K7PW11O39/TiO2, a covalent bond between the terminal nucleophilic oxygen atoms of the HPA and the electrophilic titanium atoms in Ti-OH groups is present. This approach opens the possibility to use functionalized HPAs that can be covalently linked to the support avoiding the problematic leaching of HPA to the liquid reaction medium. The attractiveness of the covalent approach has been stressed by Proust [72], who indicates as benefits (i) the enhanced stability despite external variation of pH (in the stability range of the HPA) or ionic strength, (ii) the enhanced control on the number and relative orientation of the components, and (iii) the better dispersion in a polymer or on a surface. We have seen up to here the use of HPAs as acidic catalysts as well as photo-catalysts. Now, we afford the use of these materials to take advantage of their acidic and redox abilities contemporaneously. Indeed, an increase in the catalytic performance of these materials has been observed when the UV irradiation is additionally supplied to the heated catalytic system. 2-Propanol dehydration [15, 73, 74] and propene hydration [50, 75] have been carried out by using some of these clusters to compare their performance in both catalytic and catalytic photo-assisted conditions. The hydration of propene at ambient conditions is a reaction of great interest. The industrial propene catalytic hydration to 2- propanol is carried out at moderate temperatures (ca. 150–200°C). and pressure (2 MPa) in the presence of an acid catalyst as phosphoric acid supported on silica, strong acidic resins [76], beta zeolite [77], and other acidic zeolites [78], whereas the reverse reaction, i.e., the dehydration of 2-propanol to form propene is considered a reaction used to characterize the acidity of a catalyst. In a gas–solid regime, propene and/or propanone are formed at ambient pressure and moderate temperature (in the range 140–325°C) and the selectivity to these products depends on the acidity-basicity of the catalyst. The more propene is obtained the more acidic is the solid catalyst [79]. The propene hydration is not an easy reaction to be carried out because it is thermodynamically limited by the mentioned reverse reaction at high temperature. The industrial hydration of propene to obtain 2-propanol has been carried out by using aqueous solutions of H3PW12O40. This catalytic reaction performed in gas–solid regime has been the object of several patents [80, 81]. The 2-propanol dehydration reaction by using a Keggin heteropoly acid H3PW12O40 (PW12) supported on commercial and home-prepared SiO2 and TiO2 samples and a comparison between catalytic and photocatalytic performances have been studied [73]. A continuous (photo)reactor working in gas–solid regime was used at atmospheric pressure and temperatures in the range of 60–120°C. The photocatalytic experiments were performed furtherly illuminating the heated system with UV LEDs. The presence of light significantly increased the dehydration rate. The Keggin heteropoly acid played the key role both for the catalytic and the photocatalytic reactions, in fact, the acidity of the cluster accounted for the catalytic role, whereas both the acidity and the oxidant ability of PW12 were responsible for the increase in the reaction rate of the photoassisted reaction. In addition, important differences were observed for the various supported materials because the nature and the strength of interaction of HPA-support influenced their acidity and therefore the (photo)activity. This important point has been confirmed recently by using alternative support for the HPA as C3N4 and BN [74].

328

SECTION

C Oxides and calcogenides

Propene formation rate is reported in Fig. 7 both for experiments carried out under catalytic and photocatalytic conditions. The PW12 was deposited onto home-prepared SiO2 and TiO2 (those with the suffix exA) and commercial ones (with the suffix A). CNT stands for carbon nanotubes which were also used as support [73]. An increase in the reaction rate under photo-assisted condition appears clear in all of the cases. In addition, there is an increase in the catalytic activity of the binary materials with respect to the bare HPA. The good dispersion of the HPA on the support surface, which increased the contact between PW12 and 2-propanol along with differences in the number and strength of the PW12 Brønsted acid sites account for the improvement of the activity of the binary system with respect to the bare PW12 sample. The dehydration of 2-propanol occurred, as described in Fig. 8A, using an acid–base mechanism (elimination E1) involving the dioxonium ions placed between PW12 anions. As demonstrated by Okuhara et al., heteropoly acids possess a discrete and mobile ionic structure that can retain in the bulk a large number of polar water molecules with very high proton mobility [12]. Proton on the PW12 surface is coordinated by two water molecules forming the dioxonium species, H5O2+, which bridges Keggin ions units. The H5O2+ sites contribute to stabilizing the polar water molecules with the formation of the “pseudo-liquid” phase. A direct correlation between the reactivity and the acidity of the surface in terms of the amount of H5O+2 is the key factor to obtain the maximum catalytic activity. This insight was reported previously by Ivanov et al. who compared the catalytic activity of acidic zeolite HZSM-5 with non-supported and supported H3PW12O40 for the propene hydration reaction to obtain 2-propanol [82]. Supported HPA resulted much more active than the bare corresponding sample and the HZSM-5, substantially due to their stronger acidity. The HPA-based materials showed a significant activity only from 100°C. The maximum activity in hydration was measured at 130°C. Ivanov studied also the mechanism of the reaction using FTIR concluding the important role of the dioxonium species, H5O+2 . During the catalytic and catalytic photo-assisted experiments, the formation of diisopropyl ether occurred along with propene formation and the mechanism proposed can justify its formation as well (see Fig. 8B). PW12 heteropoly acid can also act as a photocatalyst, and the addenda atoms in HPAs (W(VI) in the H3PW12O40) are typically fully oxidized (d0 electron configuration) and; indeed, light absorption causes the promotion of an electron from a spin-paired, doubly occupied bonding orbital (HOMO), to an empty, antibonding orbital (LUMO), resulting in the generation of an oxo-centered radical. The photoexcited cluster species (indicated as PW∗12) is highly reactive. Consequently, during the catalytic photo-assisted reaction, in addition to the previously mentioned acid–base mechanism, photosensitized PW12* can trap an electron from 2-propanol giving rise to the blue species (PW
12) (see Fig. 9). Notably, all the materials containing PW12 become strongly blue colored under irradiation only in the presence of electron donors as 2-propanol. The photocatalytic dehydration of 2-propanol forming propene in the presence of PW12 is reported in Fig. 9. Moreover, to explain the higher activity of the PW12 under irradiation conditions, a change in their acidity can be also invoked, due to their reduction to PW
12. Notably, the photoactivity observed under the same experimental conditions when HPA was supported on semiconductors was higher than when it was supported on insulators. From the perusal of Fig. 7 and

Rate of propene formation [mmol·h-1·g-1PW12]

14.0 Dark

Pi = 0.28 W

Pi = 0.50 W

12.0 10.0 8.0 6.0 4.0 2.0 0.0

PW12/SiO2 A

PW12 PW12/TiO2 A PW12/SiO2 ex A PW12/TiO2 exA

PW12/CNT-1 PW12/CNT-0.5

FIG. 7 Propene formation rate per gram of PW12 for the catalytic (dark gray) and photocatalytic 2-propanol dehydration reaction by using two different irradiation conditions (0.28 W and 50 W (gray and light gray, respectively). t ¼ 80°C, [2-propanol] ¼ 0.5 mM. Figure reported from E.I. Garcı´a-Lo´pez, G. Marcı`, F.R. Pomilla, A. Kirpsza, A. Micek-Ilnicka, L. Palmisano, Supported H3PW12O40 for 2-propanol (photo-assisted) catalytic dehydration in gas-solid regime: the role of the support and of the pseudo-liquid phase in the (photo)activity, Applied. Catal. B 189 (2016) 252–265. with Elsevier permission.

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FIG. 8 Proposed reaction pathway for the catalytic 2-propanol dehydration to obtain (A) propene and (B) diisopropyl ether, in the presence of the Br€ onsted acidic species on the heteropoly acid catalysts. Figure reported from E.I. Garcı´a-Lo´pez, G. Marcı`, F.R. Pomilla, A. Kirpsza, A. Micek-Ilnicka, L. Palmisano, Supported H3PW12O40 for 2-propanol (photo-assisted) catalytic dehydration in gas-solid regime: the role of the support and of the pseudoliquid phase in the (photo)activity, Applied. Catal. B 189 (2016) 252–265. with Elsevier permission.

by comparing the PW12/SiO2 A and PW12/TiO2 A samples (prepared by the same method), an enhancement of the reactivity is evident in the presence of light when the support was TiO2. The beneficial influence of the semiconductor material has been extensively reported before [15, 50, 73–75]. Fig. 10 represents the hypothesized reaction pathway for 2-propanol dehydration. In the absence of O2, the holes formed under irradiation on the TiO2 valence band can react with supported PW
12 species which transfer electrons to restore PW12 (see the last step of Fig. 9). Contemporaneously, the dehydration can proceed with the obtainment of propene by the reducing counterpart. We have demonstrated through EPR spectroscopy that the increase in reactivity under irradiation could be attributed to the ability of photoexcited PW12 to trap electrons, particularly if the PW12 is supported [83]. EPR spectroscopy indicated the likely occurrence of electron/hole transfer between the heteropoly acid and the oxide before and during the UV irradiation. Catalytic and catalytic photo-assisted tests for 2-propanol dehydration to propene were successfully carried out by using not only the supported H3PW12O40 Keggin HPA (PW12) but also the presence of the H6P2W18O62 Wells–Dawson structure (P2W18). Before going into detail with the comparison in the activity of both HPAs, it is worth remaining here that the P2W18 is a home-prepared HPA, hence before using it as (photo)catalyst, it is necessary to evaluate the actual presence of its cluster structure. FTIR is a useful technique to confirm the P2W18 cluster structure after the synthesis but also the retention of both Keggin and Wells–Dawson cluster structures after their dispersion into the different supports. For instance, taking into consideration the commercial SiO2 and TiO2 materials as support, the FTIR spectrum of both PW12 and P2W18 based materials showed a series of specific transitions as reported by Bielanski [84]. Remarkably, all of the supporting materials showed no important differences in the FTIR spectrum before and after the catalytic or the photocatalytic experiments [15]. The spectra of the catalytically or photocatalytically used PW12 and P2W18 supported on TiO2, showed neither shifts nor differences in relative intensity of the bands or additional bands with respect to the freshly prepared samples. With this respect, it has been reported that the modification of the secondary structure of the

330

SECTION

C Oxides and calcogenides

VI

W

V

W• O •

O

Photoexited state

Ground state

(PW12)

(PW12*)

(PW12-)

(PW12*) H H C H H 3C C O H H

H H 3C C

H H 3C C

H H C•

H H3C C

H H C•

H H C• H O+ H

(PW12-)

(PW12) H

+

H O+ H

C

C H

CH3

H O

H

H

FIG. 9 Proposed reaction pathway for the catalytic photo-assisted 2-propanol dehydration to propene occurring together with the catalytic reaction (Fig. 8) in the presence of the materials containing PW12. Figure reported from E.I. Garcı´a-Lo´pez, G. Marcı`, F.R. Pomilla, A. Kirpsza, A. Micek-Ilnicka, L. Palmisano, Supported H3PW12O40 for 2-propanol (photo-assisted) catalytic dehydration in gas-solid regime: the role of the support and of the pseudoliquid phase in the (photo)activity, Applied. Catal. B 189 (2016) 252–265. with Elsevier permission.

H H

C H

C CH3

H

H H C•

e-

H3C C +

TiO2 Band Gap

(PW12)

h+ (PW12-) FIG. 10 Synergistic effect between the PW12 cluster and TiO2 in the binary composite for the catalytic photo-assisted 2-propanol dehydration to form propene. Figure reported from E.I. Garcı´a-Lo´pez, G. Marcı`, F.R. Pomilla, A. Kirpsza, A. Micek-Ilnicka, L. Palmisano, Supported H3PW12O40 for 2propanol (photo-assisted) catalytic dehydration in gas-solid regime: the role of the support and of the pseudo-liquid phase in the (photo)activity, Applied. Catal. B 189 (2016) 252–265. with Elsevier permission.

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HPAs, for instance by a partial loss of water, increases the disorder in the secondary structure, hence inducing some vibrational transitions to become broader and with lower intensities. The primary structure remains stable although the distance between the primary units can change. This behavior was not observed as both HPAs maintained their secondary structure after catalytic and photocatalytic reactions even after 120°C. This finding can be explained by taking into account that the dehydration reaction produced water molecules that maintained an extent of hydration sufficient for the Br€onsted sites to be still active. The acidic properties of HPAs are key for the understanding of the (photo)catalytic activity of these materials. The acidity of the bare and supported HPAs was evaluated by NH3-TPD experiments (ammonia Thermal Programmed Desorption) carried out from room temperature up to 600°C. Indeed, it is known that on strong acidic sites, NH3 desorbs above 550°C. The NH3-TPD technique provides information on the total acidity of the catalysts without distinguishing between Br€ onsted acid sites (typical of HPAs and SiO2) and Lewis acid sites, typical of the defective TiO2 used as support. The amount of NH3 desorbed (in ppm/g HPAs) is a quantitative evaluation of the number of active sites, while the temperature of desorption indicates the strength of the acidic sites. As reported in Fig. 11, no NH3 desorption occurs at temperatures below 200°C, suggesting the absence of weak acid sites. Fig. 11A shows that for the bare and SiO2 supported P2W18, the main desorption occurs with a broad and intense peak centered at 495°C. NH3 desorption in the range of 200–550°C is attributed to medium strength acid sites and a pronounced shoulder at around 300°C was also detected for P2W18/SiO2. By comparison with the profile of bare P2W18 and that of bare SiO2 support, it can be concluded that such low-temperature feature is due to the contribution of SiO2 medium acid sites. It is hence concluded the presence of some strong acid sites both in P2W18 and P2W18/ SiO2 samples. Conversely, in the case of bare and SiO2 supported PW12, the main desorption peak of ammonia was detected at 600°C confirming that the acid sites in Keggin-type powders are stronger than that of Wells–Dawson ones.

FIG. 11 NH3-TPD profiles vs time and temperature for SiO2 supported HPAs (A), for TiO2 supported HPAs (B), and for bare SiO2 and TiO2 (C). Bare P2W18 and PW12, are also reported for the sake of comparison. Reproduced with Elsevier permission from F.R. Pomilla, F. Fazlali, E.I. Garcı´a-Lo´pez, G. Marcı`, A.R. Mahjoub, I. Kritsov, L.F. Liotta, L. Palmisano, Keggin heteropoly acid supported on BN and C3N4: comparison between catalytic and photocatalytic alcohol dehydration, Mater. Sci. Semicond. Process. 112 (2020) 104987..

332

SECTION

C Oxides and calcogenides

Rate of propene formation [mmol·h-1·g-1]

Fig. 11B reports the NH3-TPD curves of TiO2 supported HPAs. A broad peak centered at around 450°C was observed for both samples. Along with such features, attributed to the Lewis acid sites typical of pristine TiO2 (see Fig. 11C), the peaks corresponding to the Br€ onsted acid sites of the HPAs were clearly detected at 495 for P2W18 /TiO2 and at 600°C for PW12/TiO2. From these experiments, we have concluded that PW12 presents stronger acid sites with respect to P2W18; however, the average amount of acid sites in P2W18 is significantly higher than that in PW12. Consequently, the activity of the samples should be related not only to the strength of the acid sites but also to their amount. The catalytic and photocatalytic propene formation rates in the presence of SiO2 and TiO2 supported HPAs vs the concentration of 2-propanol in the feeding stream are reported in Fig. 12. The overall catalytic and photocatalytic behaviors of the bare and supported PW12 and P2W18 are shown at different concentrations of the substrate (2-propanol). The supported Wells–Dawson was more active than the Keggin HPA for the catalytic process, although at the lowest 2-propanol concentration, the activities of the supported samples were very similar. The different catalytic activity of PW12 and P2W18-supported samples could be related, as for the bare HPAs, to the higher amounts of acid sites present on the P2W18 supported catalysts. Propene formation rate increased under UV irradiation for all of the solids and the supported P2W18 samples were always the most active ones. The rate of propene formation reported in Fig. 12 showed the same trend for all of the experiments, i.e., firstly increased and then decreased by increasing 2-propanol concentration. There is a maximum concentration of 2-propanol for a maximum of the reaction rate, a further increase in the substrate concentration a dramatic decrease of reactivity occurs.

3.0

5.0

2.5

4.0

2.0 3.0 1.5 2.0

1.0

1.0

0.5 0.0

0.5 1.0 1.5 2.0 2.5 2-propanol concentration [mM]

Rate of propene formation [mmol·h-1·g-1]

(A)

0.0

3.0

0.0

(B)

0.0

0.5 1.0 1.5 2.0 2.5 2-propanol concentration [mM]

3.0

10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0

(C)

0.0

0.5 1.0 1.5 2.0 2.5 2-propanol concentration [mM]

3.0

FIG. 12 Reaction rate of propene formation per gram of HPA vs 2-propanol concentration for runs carried out by using PW12 (black) and P2W18 (gray) based (photo)catalysts. The dotted lines are referred to catalytic reactions, whereas the full lines to photo-assisted catalytic processes. (A) pristine PW12 and P2W18; (B) PW12/SiO2 (black) and P2W18/SiO2 (gray); (C) PW12/TiO2 (black) and P2W18/TiO2 (gray). Flow rate of the feeding gas equal to 100 mL min
1, temperature 80°C and 0.5 g of catalyst.

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This phenomenon is due to the uncommon ability of the HPAs to absorb polar substrates in their bulk (secondary structure) giving rise to catalytic reactions in the “pseudo-liquid” phase, as well described by Misono [11–13, 35]. Therefore, the solubility of 2-propanol in the water molecules of the secondary structure of the HPAs should be invoked to explain their reactivity. This phenomenon has been also observed for the catalytic formation of tertiary ethers in the gas phase by the addition of polar alcohols (methanol or ethanol) to isobutene for experiments carried out with H4SiW12O40 and H6P2W18O62 [85] or in the presence of PW12 and P2W18 [25, 86]. The Wells–Dawson presented both higher catalytic and photocatalytic activities than the Keggin HPA, particularly when supported. In both cases, UV irradiation increased the activity. The acidity of the cluster accounts for the catalytic role, and it was observed that the P2W18 showed higher activity with respect to PW12 due to the presence of higher number of acidic sites. The higher catalytic activity of the Wells–Dawson with respect to the Keggin one, P2W18 vs PW12, was reported by several authors [84]. Bielanski et al. highlights that this result would contrast with the fact that the order of acidities for the two pristine HPAs is reversed. Indeed, according to Shikata et al., the differential absorption heat of ammonia is equal to 150 and 190 kJ/mol for P2W18 and PW12, respectively, and the value of the Hammett Function H0–2.9 and
3.6 for P2W18 and PW12, respectively [87]. Shikata et al. explains the above inconsistency between the catalytic activity of the two HPAs, which should be higher with higher acidity, by considering the differences in the secondary structure of PW12 and P2W18. These secondary structures are very much related to the experimental conditions. For instance, preheating temperatures, which are called in literature as activation temperatures for catalytic runs in the presence of HPAs, in the range of 150–250°C, gave rise to crystalline secondary structures for PW12 but leave amorphous the P2W18. The elliptical shape of the Wells–Dawson anion could not be suitable for the formation of a stable crystalline secondary structure. Conversely, an amorphous and flexible structure would be present and in this, the absorption-desorption of the reactant molecule would be easier and hence the catalytic activity higher with respect to PW12. Indeed, the spherical shape of the PW12 Keggin anion would favor a crystalline cubic structure, where absorption-desorption would be slower and hence catalytic activity lower. Baronetti and co-workers reported analogous results [16]. The apparent activation energy of 2-propanol dehydration reaction to propene was estimated by applying the Arrhenius equation. In particular, by carrying out the experiments at an increasing temperature at a concentration of 2-propanol equal to 3 mM, as shown in Fig. 12, the reaction rate of propene formation reaches a plateau in any case and 2-propanol conversion is less than 10%. This finding indicates that 2-propanol completely covered the surface of the catalysts for a concentration equal to 3 mM giving rise to a paramount absorption in the pseudo-liquid phase and consequently, the reaction rate becomes of pseudo-zero order [88]. In this condition, the rate of propene formation can be written as: r ¼ k0 ,

(9)

in which “k0” is the pseudo-zero order rate constant of the reaction. The apparent activation energies Ea and the preexponential factors A were determined from the Arrhenius equation in the form: ln r ¼ ln k0 ¼ ln A


Ea RT

(10)

Eq. (10) has been applied for the runs carried out both in the presence and in the absence of UV light by using both the bare and the supported HPA materials in the catalytic or the photocatalytic reaction. The Arrhenius plots for the catalytic and catalytic photo-assisted reactions in the presence of PW12 and P2W18 supported on SiO2 and TiO2 are reported in Fig. 13. The values of Ea and ln A estimated using these plots also for runs in the presence of the bare HPAs are reported in Table 1. The values of Ea estimated in the presence of UV light (catalytic-photo assisted) were always lower than those under dark conditions (catalytic), justifying the higher reactivity observed under UV irradiation. Interestingly, as reported in Table 1, the value of Ea in the presence of P2W18/SiO2 under catalytic photo-assisted conditions decreased more significantly ( 14%) than in the presence of P2W18/TiO2 ( 6%), although the photoreactivity for the last sample appeared still higher. This apparent contradictory result can be explained by taking into account the estimated value of the pre-exponential factor, A, that is related to physical–chemical factors as adsorption/desorption of reagent and products. Indeed, the A factor percentage decrease under irradiation conditions is higher in the presence of P2W18/ SiO2 ( 10%) than with P2W18/TiO2 ( 3%). Notably, this factor influences the kinetic constant in the opposite way to Ea as reported before by Bond et al. [88].

334

SECTION

C Oxides and calcogenides

6

8

PW12/SiO2

4

2

2

0

0

-2

-2

-4

-4

-6

-6 0.0025

0.0027

0.0029

P2W18/SiO2

4

ln r

ln r

6

0.0031

-8 0.0025

0.0027

6

0.0031

5

PW12/TiO2

5

P2W18/TiO2

4

4

3

3

2

2

ln r

ln r

0.0029 1/T [K-1]

1/T [K-1]

1

1 0

0

-1

-1

-2

-2 -3 0.0025

0.0027

0.0029

0.0031

-3 0.0025

0.0027

1/T [K-1]

0.0029

0.0031

1/T [K-1]

FIG. 13 Plots of “ln r” vs “1/T” were used to calculate “Ea” and “ln A”. Experiments carried out using the SiO2 and TiO supported HPAs as catalysts (full line) or photocatalysts (dotted line). The units of “r” are mmol h
1 g
1 HPA.

TABLE 1 Apparent activation Energy (Ea) and logarithm of the pre-exponential factor (ln A) for catalytic and catalyticphoto assisted 2-propanol dehydration. ln A

Ea (kJ/mol) Catalyst

Catalytic

Catalytic-photo assisted

Catalytic

Catalytic-photo assisted

PW12

186

157

56

50

PW12/SiO2

151

144

51

50

PW12/TiO2

101

96

35

34

P2W18

175

152

56

51

P2W18/SiO2

161

139

52

47

P2W18/TiO2

92

86

32

31

References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [2] F. Parrino, et al., Heterogeneous photocatalysis for selective formation of high-value-added molecules: some chemical and engineering aspects, ACS Catal. 8 (2018) 11191–11225. [3] G. Palmisano, E.I. Garcı´a-Lo´pez, G. Marcı`, V. Loddo, S. Yurdakal, V. Augugliaro, L. Palmisano, Advances in selective conversions by heterogeneous photocatalysis, Chem. Commun. 46 (2010) 7074–7089.

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Chapter 21

Niobate-based perovskites: Characterization, preparation, and photocatalytic properties Ba´rbara S. Rodriguesa, Maria Kuznetsovaa, Sibila A.A. Oliveiraa, Irina M. Factoria, Patricia V.B. Santiagob, Pablo S. Ferna´ndezb, and Juliana S. Souzaa a

Human and Natural Sciences Center, Federal University of ABC—UFABC, Santo Andr
e, SP, Brazil, b Chemistry Institute, State University of

Campinas—UNICAMP, Campinas, SP, Brazil

1 Aims and scope Perovskite oxides have been extensively used as photocatalyst for several chemical reactions. This class of compounds exhibits the general formula ABO3, where the A site is occupied by a larger cation, and the B site is occupied by a smaller cation. The attractiveness of these materials is related to their enormous compositional and structural variety, which allows the production of catalysts with unique physical-chemical properties. Niobates perovskites is a subclass of perovskite oxides in which the B-site is occupied by Nb5+ species. The structural and chemical properties of these materials have been studied for 50 years; however, their application in photocatalytic processes is very recent and still understudied.

2 Introduction According to the IRENA (International Renewable Energy Agency), which is an intergovernmental organization that supports countries in their transition to a sustainable energy future; the raising in the global population and industry development causes several problems for the environment [1]. First, it promotes the growth of energy demand and, consequently, the pollution caused by the production and use of fossil energy. Second, it increases the environmental pollution caused by the dumping of organic matter in rivers and the emission of toxic gases into the air. All these issues have stimulated the development of sustainable and clean energy production, conversion, and energy storage systems [2–6] as well as strategies for environmental depolluting [7–9]. Photocatalysis has long been studied for clean energy and environmental applications. Therefore the number of applications based on photocatalysis has increased; as a consequence, the range of materials systems used as catalysts has been developed [10–15]. Among the several photocatalytic applications, the production of hydrogen from water has been of particular interest [4, 6, 15, 16]. Conversion of CO2 to hydrocarbons is also attracting much attention, mainly because it can also promote a reduction of CO2 emission [17–19]. Photocatalysis has been applied as well for the degradation of organic compounds for water treatment [8–10, 14], de-coloration of industrial dyes [20, 21], antimicrobial action, nitrogen fixation [22–24], and removal of air pollutants [25–28]. TiO2-based materials are the most investigated for photocatalytic applications [29–32], followed by other metal oxides, due to their stability and oxidation-resistance [12, 33, 34]. Among the several classes of materials investigated, perovskite oxides and their derivatives have been gaining much attention due to their versatile properties [35–37]. In contrast to halide perovskites, that have attracted global interest recently, perovskite oxides exhibit broader applications due to their stability in moisture, in high temperatures, in the environment, which makes its practical use possible [38, 39]. Perovskite oxides are a class of compounds that exhibit the general formula ABO3, where the A site is occupied by a larger cation, and the B site is occupied by a smaller cation. ABO3 perovskite-type structure results in innumerous combinations of compounds with unique physical-chemical properties. Perovskite oxides also exhibit advantages over the corresponding binary oxides, including favorable band edge potentials allowing various photoinduced reactions, broader scope to design and alter the band structure, and other photophysical properties. Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00032-5 Copyright © 2021 Elsevier Inc. All rights reserved.

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Niobates perovskites are a substantial class of materials [40, 41]. However, its applications for photocatalytic purposes are still understudied. This chapter focuses on reviewing the niobates-based perovskite photocatalysts. The compositional and structural diversity of these materials is discussed and correlated to the morphology and photocatalytic activity. Finally, the perspectives for future development are suggested.

2.1 Perovskites oxides 2.1.1 Historical background The history of perovskites started in 1839 when the mineralogist Gustav Rose (1798–1873) examined a mineral with formula CaTiO3 found in the Ural Mountains. The name “perovskite” was given as a tribute to another Russian mineralogist, Count Lev Alekseyevich von Perovski. After this, several other minerals with the same crystalline structure were found, and “perovskite” became the nomenclature of minerals with the standard composition ABX3, where X is not necessarily oxygen. This class of materials represents the main component of Earth’s interior. About 38% of its composition is Bridgmanite (Fe, Mg)SiO3, a perovskite that is only stable at high pressure and temperature, explaining why it is not found at Earth’s crust [42]. The interest in studying perovskites started in 1940 when the dielectric and ferroelectric properties of BaTiO3 were discovered [42]. These properties quickly encouraged the use of this material in applications such as capacitors and transducers. Thus, in 1945, BaTiO3 was used worldwide to substitute muscovite mica [KMg3(AlSi3)O10(OH)2] as an insulator in capacitors due to its high dielectric constant [43]. Since then, the interest in studying perovskites has increased over the years. Fig. 1 shows results for searches in Web of Science using several terms related to this work. Using the word “Perovskite,” we observed a fast increase in the number of publications from the end of the 1980s until 2010, when the number of publications exploded, until today. For the oxides, the trend is similar but the increase in the publications from 2010 was a lot more modest than for perovskites. The difference between them is due to the huge interest in the past decade in studying halide perovskites, promoted by the discovery of the first visible-light sensitizer for photovoltaic cells based on a hybrid organic-inorganic perovskite [44, 45]. Finally, Perovskite niobates accounts for a low fraction of the papers published until now. Among the events cited before, there is other important point in the history of the developments of Perovskites oxides, worth to be mentioned here. In the 1970s, the catalytic ability of perovskites begins to be evaluated intensively. For example, LaCoO3 showed exceptional performance as automotive exhaust catalyst [46]. Also, manganate and cobaltate perovskites demonstrated comparable results with noble metals for CO and NO oxidations [47, 48]. These results stimulated the study of perovskite as catalyst for other important reactions. In the 1980s, the perovskites oxides gained more attention by the superconductivity property of perovskites as YBa2Cu3O7
d.

FIG. 1 Number of publications over the years for the searches: (i) Perovskite + niobates (the results were multiplied by 20), (ii) perovskites + solar, (iii) perovskites + oxides, and (iv) perovskites. The graphic to the right does not show the results for the search “iv” to permit a better comparison of the other searches. The number of publications was divided by 1000.

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Niobate perovskites reports are dated from the early 1970s, where crystalline phase transitions were studied and correlated to ferroelectric properties [49, 50]. For the next 20 years, very few authors published on these materials, and the publications were focused on their ferroelectric, piezoelectric, and dielectric properties [51–53]. Although they exhibit suitable properties, only in 1993, niobate perovskites were applied as photocatalysts [16, 54–56]. This brief historical revision shows that the interest in studying perovskites has increased over the years together with the discovering of new properties, and consequently, applications. Depending on the composition/structure, these materials can present different properties like ferroelectricity, magnetoresistance, high superconductivity, (electro) catalytic activity for oxidation and reduction reactions, photo sensibilization and lithium mobility, among others. All these properties can be tuned by the rational design of perovskites due to its structural and compositional flexibility [57]. Thus, in the next section, we describe the structure of perovskites and some relationships between composition, structure, and properties.

2.1.2 Structural and compositional flexibility For the conventional structure ABX3, a large-size cation is commonly at position A, a medium-size cation at position B and an anion at X (Fig. 2). The relation between the size of the ions is a crucial factor for perovskites that determine if a given combination of ions will form a perovskite or not and, if it is the case, which will be the crystalline structure. The importance of ions radii generated the Goldschmidt tolerance factor (t), t ¼ ððrA + rBÞÞ=√ ð2ðrB + rXÞÞ where rA, rB, and rX are the radii of the cation A, cation B, and anion X, respectively. For a perfectly cubic perovskite structure, the tolerance factor (t) is equal to 1, although stable cubic structures have been reported for values between 0.8 and 1.0. If t < 0.75, the perovskite adopts a hexagonal ilmenite structure (FeTiO3). If 1.0 < t < 1.13, it will present a hexagonal symmetry [47]. As showed before, perovskites are formed by ions, then, besides attending the radii requirements, it is also necessary to keep the structure charge neutrality. Taken these two constraints into account, the possibilities of single perovskite composition is reduced to: A1+ B5+ O3, A2+ B4+ O3, A3+ B3+ O3, A4+ B2+ O3, and A5+ B1+ O3. Therefore, FIG. 2 The figure show the high flexibility of perovskites structures. Perfect cubic crystal lattice (center), showing the cation A at the corner positions of the cubic unit cell, the cation X is in the center and the anions B at the corner of the central octahedron. Rock-salt, randon and layered structures are typically adopted by double perovskites (top). These structures exhibit B cations at alternating octahedral. P21/n, I4/m, and R-3 are the corresponding space group symmetries of three of the many different tilted structures (left bottom). Finally, when an octahedral tilt is very accented, some B–O bonds can be broken and the 3D characteristic of a traditional perovskite structure can be converted into nanowires (1D perovskite) or layered perovskites (2D perovskite). (© From https://doi.org/10.1039/ C8EE01574K.)

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almost 90% of the periodic table elements can be considered in principle as suitable candidates to take part in a perovskites structure. Ideal single perovskite (ABX3) crystallizes in a cubic structure (Pm3m). For example, this structure is observed for SrTiO3. In this arrangement, Ti4+ is in an octahedral site, coordinated by 6 O2
while Sr2+ is in a cuboctahedral site coordinated by 12 O2
. As showed before, by changing the ions, it is possible to obtain perovskite-type oxides with different crystalline structures. A way of looking at these structural changes is to see changes in the octahedra formed by the anions. Thus the most common distortions, that can occur independently or in combination, are as follow: (1) B-cation displacement: it usually occurs when a very small cation is positioned at B site (t ≪ 1). The cation slightly displaces from the center of BX6 without altering its octahedral arrangement. This change leads to a structure transformation to tetragonal, trigonal, or orthorhombic depending on the direction of the shift. (2) Octahedral tilt or rotation: this modification is normally related to modification at A-cation. When it is too small, it promotes a BX6 torsion to reduce the dimensions of the cavity and stabilize the structure with a t smaller than 1. (3) BX6 distortion: in this case, the repulsive interaction between the electrons of the cation and anions, leads to an elongation or flattening of the BX6 octahedra to promote the stabilization of the structure (Jahn-Teller effect). This effect can be induced by a change in the charge of the cation due to the corresponding change in the filling of its d orbital. The most common elements combination for single perovskites is as follows: (i) the site A is occupied by a lanthanide, alkaline, or alkaline-earth, (ii) the position B by a 3d, 4d, or 5d transition metal [47], and (iii) an oxide, carbide, nitride, halide, and hydride occupy the X site [58]. However, some organic groups as NH+4 , (CH3NH3)+, [(CH3)4N]+, and (NH2] CHNH2)+ can also be considered at position A. The addition of these organic ions gives rise to hybrid organic-inorganic perovskites, materials of great interest in solar cells [59]. For the case of niobates, the X site is occupied by O, the site B for Nb and the site A can be occupied by several different atoms as discussed later. Besides the possibility of a total substitution at position A or B, which generate different materials with different properties, it is also possible to promote a partial substitution at one or both sites generating double-perovskites [60]. Thus this strategy has been widely explored in the literature to change the properties of a given material or to finely tune the characteristics of a perovskite to improve a particular property. The niobates have not been an exception and both, partial and total substitution by several elements has been done. Most of the investigated photocatalysts based on niobates perovskites exhibit the layered perovskite-related structures known as Dione-Jacobson type AnBnO3n+2. The Dione-Jacobson phases are represented by A0 Ak
1BkO3k+1 and AnBnO3n+2 materials, where B is Nb5+. A0 Ak
1BkO3k+10 usually contain an alkali metal at the A0 site; also, they show high photocatalytic activity under UV light irradiation. This group also exhibits superconductivity, dielectric behavior, good electron conductivity, photoluminescence, and easy ion-exchange [40, 41, 55]. This layered structure consists of negatively charged niobium oxides sheets, composed by corner-sharing NbO6 octahedra with A-cation in the gaps between them. The octahedral layers can vary along the c-axis. Positively charged A0 - cations are located in the interlayers space (Fig. 3A). The layered structure promotes the separations of the photogenerated charges, increasing the photocatalytic activity; it also allows higher efficiency in the doping process and other kinds of modifications when compared with nonlayered materials. These materials can also be easily exfoliated (as will be described later), leading to the production of nanosheets [40, 41, 55]. FIG. 3 Representations of ideal distorted DioneJacobson type (A) an AnBnO3n+2 (B) structures.

Niobate-based perovskites Chapter

21

345

AnBnO3n+2 are composed by layers of corner-shared NbO6 octahedra zig-zag along b-axis and chain-like along a-axis, and A cations at the interlayers (Fig. 3B); the tick of the layers can vary [40]. Besides the advantages ascribed to the layered structure, the AnBnO3n+2 also exhibit good ferroelectricity, which has been reported to increase the photocatalytic activity [40]. It was found that spontaneous polarization in the ferroelectric domain can induce an internal polar electrical field, which can promote the separation of photogenerated electrons and holes and accelerate the movement of electrons and holes in opposite directions [61]. Table 1 summarizes some of the niobate perovskites where the A and/or A0 sites have been replaced by several different cations.

2.2 Photocatalytic applications Heterogeneous photocatalytic processes use the incident radiation as the driven energy to activate the catalytic material and perform or accelerate chemical reaction at its surface [89, 90]. These reactions would usually require intense energy inputs (high temperature or pressure) to be carried out without the use of such photocatalysts. The use of perovskite oxides as photocatalysts has been studied for numerous chemical reactions [35–37] and understanding their mechanisms is fundamental to design and develop new materials. The photoexcitation of perovskite oxides—and metal semiconductors in general—with photons at suitable wavelength, promotes electrons to the conduction band (CB), leaving holes at the valence band (VB). Most photocatalytic reactions involve reduction and/or oxidation reactions using the photogenerated charges. Thus, thermodynamically, the main requirement for the photocatalyst being used in a particular chemical reaction is the adequate potential of the VB and CB. Fig. 4 shows the standard potentials of some common reactions and the energy of the conduction and valence band of several semiconductors, including niobates perovskites [89, 90]. Water splitting into hydrogen and oxygen is one of the most studied photocatalytic reactions. In this reaction, the photogenerated holes in the VB oxidize water molecules to oxygen. The electrons that were excited to the conduction band CB reduce protons to hydrogen. Fig. 4 shows the band alignment of several catalysts. Those catalysts with VB below 1.23 eV and CB above 0.00 eV will have holes able to oxidize water and electrons able to reduce protons [89, 90]. For example, NaNbO3 can be used for water splitting. GaAs has the CB in a suitable position, but the VB is too high, not permitting to generate holes with enough energy to oxidize water. On the other hand, KCa2NbO10 has both bands at lower energies than the reactions, then the holes will be able to oxidize water, but the electrons will have too low energy to reduce the protons. H2 O + 2h + ! 1=2O2 + 2H +

(1)

2H + + 2e
! H2

(2)

To speed up the photoelectrochemical reactions and/or to circumvent the problems of band alignments, photoelectrochemical devices can be used [91, 92]. To study this kind of device is not the aim of this review, and detailed information can be found elsewhere [93]. However, it is important to highlight that the advantage of the photoelectrochemical process is that the external energy input can be used to set the energy of the electrons (and holes), permitting to attain the energy values needed to perform the desired reaction. Degradation of organic molecules and dyes for water treatment is also an important application of perovskite oxides. For this application, the photogenerated electrons reduce adsorbed oxygen molecules to superoxides, and the holes oxidize hydroxyl ions to hydroxyl radicals. Fig. 4 shows that this reaction needs much more energy than the oxidation of water. The produced radicals can degrade organic molecules [94]. It is interesting to note in Fig. 4 that the standard potential for the equilibrium between CO2 and several organic molecules is around 0 V, that is, at much higher energies than the oxidation of water. In other words, this means that from a strictly thermodynamic point of view, it is much easier to oxidize the organics to CO2 than water to O2. In fact, all the materials in Fig. 4 generate holes with enough energy to convert the organics in CO2. CO2 is a greenhouse gas that can be captured when generated in several industries or from the atmosphere and converted to several liquid fuels (alcohols, aldehydes, acids, hydrocarbons, etc.) using (photo)(electro)chemical processes [38]. Fig. 4 shows that the potential of these reactions is close that for the hydrogen evolution reaction. Thus materials with CB with enough energy to reduce water to H2 have suitable energies also to reduce CO2 to a myriad of organic molecules. As previously described, the physical-chemical properties of perovskite oxides can be controlled by changing the chemical composition. In the following section, some of these properties are described for niobates.

346

SECTION

D Composites and heterojunctions. Tertiary materials

TABLE 1 Band-gap energy of selected niobate perovskites. Catalyst

EBG (eV)

References

1

LiNb0.5Ta0.5O3

4.30

[62]

2

Ca2Nb2O7

4.1

[63]

3

HCa2Nb1.5Ta1.5O10

3.80

[64]

4

NaNbO3

3.4–3.8

[61, 62, 65]

5

HCa2Nb2TaO10

3.76

[64]

6

RbCa2Pb0Nb3O10

3.7

[66]

7

HCa2Nb2.7Ta0.3O10

3.65

[64]

8

H1.78Sr0.78Bi0.22Nb2O7

3.62

[67]

9

RbCa2Nb3O10

3.6

[55]

10

CsCa2Nb3O10

3.6

[55]

11

KCa2Nb3O10

3.6

[55, 68–71]

12

KNb3O8

3.06–3.6

[61, 72]

13

HCa2Nb3O10

3.4–3.59

[64, 73]

14

HCa1.5Sr0.5Nb3O10

3.55

[64]

15

NaNb0.5Ta0.5O3

3.53

[62]

16

RbCa2Nb3O10

3.50

[74]

17

HCaSrNb3O10

3.50

[64]

a

[75]

Entry

18

RuOx-Ca2Nb3O10

3.5

19

KNb0.5Ta0.5O3

3.48

[62]

20

HCa0.5Sr1.5Nb3O10

3.46

[64]

21

HSr2Nb3O10

3.40

[64]

+

Ca2Nb3O
10

22

K

3.38

[76, 77]

23

RbLaNb2O7

3.30

[74]

24

N
Ca2Nb3O10

3.26

[71]

25

c-KNbO3

3.14–3.24

[78, 79]

26

o-KNbO3

3.2

[78, 79]

27

CsSr2Nb3O10

3.2

[55]

28

RbSr2Nb3O10

3.2

[55]

29

KSr2Nb3O10

3.2

[55, 70]

30

RbCa1.5Pb0.5Nb3O10

3.1

[66]

31

RbSr2Nb3O10

3.10

[74]

32

Ni-CH3CH2NH2/H1.78Sr0.78Bi0.22Nb2O7

3.09

[67]

33

t-KNbO3

3.08

[79]

34

K6Nb10.8O30

3.03

[72]

35

RbCaPbNb3O1

3.0

[66]

36

Ca0Pb2Nb3O1

3.0

[66]

37

CsBa2Nb3O10

3.0

[55]

38

RbCa0.5Pb1.5Nb3O10

2.9

[66]

39

AgLaNb2O7

2.85

[74]

Niobate-based perovskites Chapter

21

347

TABLE 1 Band-gap energy of selected niobate perovskites—cont’d Entry

Catalyst

EBG (eV)

References

40

Ag0.5La0.5Nb2O6

2.85

[80]

41

AgNbO3

2.81

[81]

42

AgNb13O33

2.76

[82]

43

AgNb7O18

2.76

[83]

44

AgSr2Nb3O10

2.66

[74]

45

AgCa2Nb3O10

2.65

[74]

46

(TBA/H)Pb2Nb3O10

2.63

[84]

47

NaNbO3
xNx

2.6

[85]

48

HPb2Nb3O10

2.58

[84]

49

HPb2Nb3O10

2.58

[86]

50

NaNb1
xRuxO3

2.3

[87]

51

H1
2xPb2Nb3
xCrxO10

2.29

[86]




52

N

/Nb+4 -Ca2Nb3O10

2.47

[71]

53

N/Nb+4 -[Ca2NaNb4O13]

1.5

[88]

a

Visible absorption band centered at 546 nm ascribed to the ruthenium complex.

FIG. 4 Standard potentials for several electrochemical reactions of importance for this review. We also plot the energy of the valence and conduction hands of several common semiconductors and niobates perovskites. The scale of energies is in eV and vs the NHE.

3 Niobates perovskites 3.1 Compositional variety A large number of metals have been investigated to produce photocatalysts based on niobates perovskites. Table 1 summarizes some of the reported materials and their band-gap energy values. The niobates composed of metals from groups 1 and 2 exhibit high band-gap energy, which typically varies from 3.0 to 4.5 eV (see compounds 1–38 in Table 1), being activated by UV-light [62–70, 72–74, 78, 79]. For most of these materials, the valence band is formed by hybridization

348

SECTION

D Composites and heterojunctions. Tertiary materials

of the Nb 4d-5s-5p and O 2s-2p states of NbO6 octahedral units, and the conduction band consists of a contribution by overlapping of Nb 4d orbitals [55, 87]. According to Kulischowa and co-workers, increasing the radius of the interlayer cations promotes a narrowing of the band-gap energy [55]. Decreasing the band-gap energy of niobates perovskites can be done by doping it with metals like ruthenium [75, 87] and chromium [86]. The metal replaces some of the octahedral NbO6 forming RuO6 sites. The introduction of Ru induces a slight expansion in the unit cell, and, depending on the amount of ruthenium, changes in the bond angles of the octahedral host lattice [87]. Alternatively, metal complexes of ruthenium [75] or nickel [67], can be intercalated into the interlayers of niobates perovskites, generating hybrid materials with a broader visible light response. Cation-anion codoping is also a promising strategy to decrease the band-gap values [71, 85, 88]. Also, according to Wang and co-workers, doping the niobates perovskites with single or double anions should increase the visible light harvest of the resultant materials [95]. However, these two strategies have not been extensively studied yet. Lead [84, 86] and silver [74, 80, 82, 83] niobates perovskites are usually sensitive to visible light (see compounds 39–46, 48, 49, and 51 in Table 1). The low band-gap energy is associated with the highest-energy valence band states composed of Pb 6s and O 2p or Ag 4d and O 2p orbital contributions within the interlayer spacings [80, 83, 84].

3.2 Preparation methods and morphology Nanostructured photocatalysts usually exhibit enhanced photocatalytic activity, when compared with bulk materials due to their higher available area. Moreover, the morphology plays a critical role in the efficiency of the catalysts as it is directly related to the exposed crystalline planes. Niobates perovskites exhibit a rich polymorphism, such as nanocubes, nanorods, nanowires, nanospheres, nanoplates, nanosheets, flake-like, quadrate column, and nanoflowers (Fig. 5). These morphologies are controlled by the synthesis conditions used in solid-state methodologies [61, 62, 69, 71, 73–77, 85, 86, 94], sol-gel techniques [61, 80, 82, 83], molten salt synthesis [55], or hydrothermal methods [63, 65, 72, 78, 79, 87]. The use of specific reactants that induce regular shapes is also common. NaNbO3 nanowires, for instance, can be obtained under hydrothermal conditions starting from an oxalate niobium complex and trioctylamine (TOA) in alkaline media. The TOA promotes self-assembly that induces the directional growth [78]. Nanosheets can be obtained using tetrabutylammonium hydroxide (TBAOH) or ethylamine aqueous solution that induces delamination of the layered materials [64, 67–71, 73, 75, 77, 84]. In the following step, the layered nanosheets are restacked by adding a suitable cation (Fig. 6). However, well-defined shapes can also be obtained without the use of such reactants. In these cases, A and A0 cations (Fig. 3) play a crucial role in the resultant morphology. RbSr2Nb3O10 nanoplatelets, for instance [63], were obtained through solid-state synthesis. Paul and Choo reported the synthesis of NaNb1
xRuxO3 under alkaline hydrothermal conditions using Nb2O5 as the reactant. The resultant materials undergo from irregular to well-defined cube, and nanowires when the content of ruthenium vary from 0% to 5% [87]. Hu and co-workers prepared H1
2xPb2Nb3
xCrxO10 through solidstate synthesis. They showed that the structure change from irregular layers to squares as the chromium content increase [86]. Bulk niobates perovskites also can be used for photocatalytic applications, although they are usually less efficient than their nanostructures analogous [63]. FIG. 5 Electron microscopies images of niobates perovskites with different morphologies.

Niobate-based perovskites Chapter

21

349

FIG. 6 Schematic representation of exfoliation and restacking process using TBAOH. (© From https://doi.org/10.1016/j.cattod.2018.03.037.)

3.3 Photocatalytic performance Niobates perovskites have been explored in photocatalytic water splitting (see compounds 1–35 in Table 2), dye photodegradation (see compounds 36–41 in Table 2), and photodegradation of organic compounds (see compounds 42–45 in Table 2). The photocatalytic activity of these materials is enhanced through several strategies such as increasing the surface area, extending the spectral absorption, and decreasing the recombination rates. Nanostructured photocatalysts show higher surface area (m2 g
1) for the chemical reactions than the corresponding bulk materials, which usually result in materials with enhanced photocatalytic activity than bulk ones [64, 65, 75, 78, 80, 83, 86, 87]. Also, controlling the crystalline structure of these nanomaterials is also very important to improve the charge transport [78, 79]. For instance, the higher photoreactivity of cubic NaNbO3 (c-NaNbO3) is due to the higher symmetry of its crystal structure [79, 96]. The conduction band edge of c-NaNbO3 possesses a delocalized orbital covering Nb and O atoms; thus the photogenerated electrons could transfer along the x, y, and z directions isotropically, which is beneficial for the transfer of photogenerated electrons [96]. In the same manner, Zhang et al. [79], ascribed the higher photocatalytic activity of cubic KNbO3 to its symmetry. The authors also suggest that orthorhombic KNbO3 show better photoactivity due to the higher preference of the photogenerated electrons to stay at the surface, compared with that of the tetragonal KNbO3 (that show higher symmetry than orthorhombic KNbO3). Converting solar energy to electricity or storing it in chemical fuels represents the forefront of renewable energy research. Since the main portion of the sunlight fallout is on the visible region of the spectra, it becomes fundamental to make the catalysts sensitive to visible light. As previously described, the main strategies to achieve this goal are using suitable metals to produce low band-gap niobates oxides [74, 84, 86], intercalation visible-light sensitive complexes (Fig. 7A) [67, 75], or doping with metals [75, 86, 87] or nonmetals (Fig. 7B) [71, 85, 88]. The presence of metal dopants also helps to decrease the recombination rates by acting as hole traps, leading to an increase in the photogenerated charges lifetimes [75, 86] and increasing the photogenerated electron mobility in the sample [72]. The formation of heterojunctions is another strategy to increase the lifetimes of the photogenerated charges. In this case, the correct band alignment causes a charge separation, decreasing the recombination rates (Fig. 7C) [72, 73, 76, 77, 94]. The use of metal co-catalysts, like platinum, is also widely used to improve the charge separation (see photocatalysts in Table 2). The electron is transferred from the conduction band of the niobate to the Pt. Thus, the resultant Pt- induces reduction reactions (Fig. 7A) [64, 87]. Other co-catalysts have been evaluated, but platinum is the one that results in higher efficiencies [55, 70].

4 Perspectives As shown in this chapter, the number of publications about perovskite oxides in the past 50 years has been continuously increasing. The knowledge accumulated during these decades allowed researchers to finely tune the composition/structure, size, and shape of perovskite oxides by using a huge variety of synthetic approaches. In consequence, we know today how to obtain materials with a broad spectrum of physicochemical properties.

350

SECTION

D Composites and heterojunctions. Tertiary materials

TABLE 2 Compilations of selected niobate perovskites used as catalysts for water photosplitting, dye and organic molecules photodegradation. Entry

Catalyst

Application

Irradiation

Activity
1

Cocatalyst

References

Pt

[74]

1

RbLaNb2O7

Photocatalytic water splitting

UV light

331 mmol h

2

AgLaNb2O7

Photocatalytic water splitting

UV light

1145 mmol h
1 of H2

Pt

[74]

3

RbCa2Nb3O10

Photocatalytic water splitting

UV light

1418 mmol h
1 of H2

Pt

[74]

4

AgCa2Nb3O10

Photocatalytic water splitting

UV light

13,616 mmol h
1 of H2

Pt

[74]

5

RbSr2Nb3O10

Photocatalytic water splitting

UV light

1929 mmol h
1 of H2

Pt

[74]

6

AgSr2Nb3O10

Photocatalytic water splitting

UV light

3265 mmol h
1 of H2

Pt

[74]

7

o-KNbO3

Photocatalytic water splitting

UV light

234 mmol h
1 of H2

Pt

[79]

8

c-KNbO3

Photocatalytic water splitting

UV light

174 mmol h
1 of H2

Pt

[79]

9

t-KNbO3

Photocatalytic water splitting

UV light

318 mmol h
1 of H2

Pt

[79]

10

Ca2Nb2O7

Photocatalytic water splitting

UV light

0.6 mmol h
1 of H2

11

CsCa2Nb3O10

Photocatalytic water splitting

UV light

1800 mmol h
1 of H2

Rh

[55]

12

CsSr2Nb3O10

Photocatalytic water splitting

UV light

1100 mmol h
1 of H2

Rh

[55]

13

CsBa2Nb3O10

Photocatalytic water splitting

UV light

500 mmol h
1 of H2

Rh

[55]

14

RbCa2Nb3O10

Photocatalytic water splitting

UV light

1300 mmol h
1 of H2

Rh

[55]

15

RbSr2Nb3O10

Photocatalytic water splitting

UV light

1600 mmol h
1 of H2

Rh

[55]

16

KCa2Nb3O10

Photocatalytic water splitting

UV light

1600 mmol h
1 of H2

Rh

[55]

17

KSr2Nb3O10

Photocatalytic water splitting

UV light

1650 mmol h
1 of H2

Rh

[55]

18

NaNb0.5Ta0.5O3

Photocatalytic water splitting

UV light

80 mmol h
1 of H2

Pt

[62]

19

LiNb0.5Ta0.5O3

Photocatalytic water splitting

UV light

8.3 mmol h
1 of H2

Pt

[62]

20

KNb0.5Ta0.5O3

Photocatalytic water splitting

UV light

25 mmol h
1 of H2

Pt

[62]

21

KCa2Nb3O10

Photocatalytic water splitting

UV light

27 mmol h
1 of H2

Pt

[69]

of H2

[63]

Niobate-based perovskites Chapter

21

351

TABLE 2 Compilations of selected niobate perovskites used as catalysts for water photosplitting, dye and organic molecules photodegradation—cont’d Entry

Catalyst

Application

Irradiation

Cocatalyst

Activity
1

References

22

KCa2Nb3O10

Photocatalytic water splitting

UV light

0.8 mmol h of H2 0.2 mmol h
1 of H2 0.5 mmol h
1 of H2 1.9 mmol h
1 of H2 5.5 mmol h
1 of H2 27.3 mmol h
1 of H2

None Mn Co Rh Ir Ru

[70]

23

NaNbO3 (nanowires)

Photocatalytic water splitting

UV light l > 300 nm

110 mmol h
1 of H2

Pt

[65]

24

NaNbO3 (bulk)

Photocatalytic water splitting

UV light l > 300 nm

40 mmol h
1 of H2

Pt

[65]

25

N
Ca2Nb3O10

Photocatalytic water splitting

UV-vis light

190 mmol h
1 of H2

Pt

[71]

26

N/Nb+4 -Ca2Nb3O10

Photocatalytic water splitting

UV-vis light

429 mmol h
1 of H2

Pt

[71]

27

HPb2Nb3O10

Photocatalytic water splitting

UV-vis light

50 mmol h
1 of H2

Pt

[84]

28

(TBA/H)Pb2Nb3O10

Photocatalytic water splitting

UV-vis light

25 mmol h
1 of H2

Pt

[84]

29

Ni-CH3CH2NH2/ H1.78Sr0.78Bi0.22Nb2O7

Photocatalytic water splitting

UV-vis light

372.67 mmol h
1 of H2

[67]

30

RuOx-Ca2Nb3O10

Photocatalytic water splitting

UV-vis light

118 mmol h
1 of H2

[75]

31

N/Nb+4 -[Ca2NaNb4O13]

Photocatalytic water splitting

UV-vis light

973 mmol h
1 of H2

Pt

[88]

32

o-KNbO3

Photocatalytic water splitting

UV-vis light l > 300 nm

141 mmol h
1 of H2

Pt

[78]

33

c-KNbO3

Photocatalytic water splitting

UV-vis light l > 300 nm

318 mmol h
1 of H2

Pt

[78]

34

H1
2xPb2Nb3
xCrxO10

Photocatalytic water splitting

Visible light

25 mmol h
1 of H2

[86]

35

CdS/HCa2Nb3O10

Photocatalytic water splitting

Visible light

450 0.6 mmol h
1 of H2

[73]

36

KNb3O8

Dye photodegradation

UV light

63%

[72]

37

Cu-KNb3O8

Dye photodegradation

UV light

93%

[72]

38

NaNbO3-Ru composite

Dye photodegradation

Visible light

95%

[94]

39

Ag0.5La0.5Nb2O6

Dye photodegradation

Visible light

95%

[80]

40

AgNb7O18

Dye photodegradation

Visible light

95%

[83]

41

AgNb13O33

Dye photodegradation

Visible light

90%

[82]

42

BiOCl/K+ Ca2Nb3O
10

Photocatalysis of tetracycline

UV-vis

94.5%

[76] Continued

352

SECTION

D Composites and heterojunctions. Tertiary materials

TABLE 2 Compilations of selected niobate perovskites used as catalysts for water photosplitting, dye and organic molecules photodegradation—cont’d Entry

Catalyst

Application

Irradiation

Activity

Cocatalyst

References

43

NaNb1
xRuxO3 (Nanocubes)

Photocatalysis of phenol

Visible light

55% of degradation

Pt

[87]

44

NaNbO3
xNx

Photocatalysis of 2-propanol

Visible light

39% degradation

Pt

[85]

45

WO3/K+ Ca2Nb3O
10

Photocatalysis of tetracycline hydrochloride

Simulated sunlight

85.8%

FIG. 7 (A) Schematic illustration of H2 evolution from water using a wide-gap oxide semiconductor, a Ru(II) complex photosensitizer and Pt as co-catalyst under visible light. (B) Band structure of N-doped NaNbO3. (C) Schematic representation of CdS HCa2Nb3O10 heterojunction. (© From https:// doi.org/10.1016/j.apcatb.2014.03.036.)

[77]

Visible light

4 H+

–

e

H2

D Ru(ll) complex

D•

2

e–

N 2p Metal oxide semiconductor

Pt nanoparticle

0 LUMO e–

H+

–2

C.B. H2 D D•

HOMO

ª

–4

V.B.

(A)

(B)

G

F Q

Z G

Potential/eV vs. NHE

CdS Visible Light

HCa2Nb3O10

CB e–

–

e

UV Light

1.2

h+ 3

(C)

H2

CB

–0.95 –0.5 0

e–

H+ H+/H2

h+ VB

Hole Scavenger VB

The development of the synthetic methods started well-before than the improvement of the computational methods for the simulation of (photo)catalytic systems [97]. The computers only appeared after several decades of research with perovskites. As a consequence, a revision of the literature shows that the most common approach has been the nonrational strategy for the design of new materials, that is, researchers prepare a series of perovskites oxides using one or more elements they are interested in or modify an existing material that has previously shown some success for the desired application to improve its performance.

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In our opinion, we are now ready to go a step further and move to the rational design of materials. To do it more effectively, we need to establish a more efficient feedback loop between the groups that work with (photo)(electro)catalysis and those performing computational experiments (mainly DFT). Today, groups working with DFT can simulate structures (and calculate their properties) and the interaction with molecules, in some cases, with amazing precision [97]. There are even groups using conventional DFT tools together with machine learning [98] to screen suitable candidates for a given application. However, to do it efficiently, the researchers need to work together. For example, researchers must fully characterize the materials they have created; otherwise, the DFT experiment will be performed with a structure that could not be the structure used as a catalyst. It is desirable to perform characterizations in situ because it makes no sense to characterize and simulate the structure of the synthesized material if it changes its structure in the conditions where the reaction is carried out. After knowing the structure, computational experiments will permit us to extract valuable information, for example, the electronic structure of the material. However, it is not enough if we want to understand the performance of a material in terms of the binding energy of the reaction intermediates. Thus, FTIR and Raman in situ experiments could help to detect reaction intermediates, in which adsorption energies could determine the performance of the perovskite oxide for a given reaction. Summarizing, groups that work in catalysis should fully characterize a series of materials applied to a reaction to provide to computational chemistry groups with as detailed information as possible (activity and structure of each material + intermediates and products). The best scenario would be that where the computational experiments found a descriptor (e.g., the binding energy of one intermediate) that permits to construct a volcano plot and, consequently, predict the activity of the material. With this knowledge, it will be possible to run DFT experiments with different perovskites and look at the activity predicted, considering the descriptor previously found. Even more interesting would be to used machine learning and ask the computer to randomly permute the sites A, B, and X to find suitable materials for the reaction. Thus it is essential to prepare materials systematically, that is, changing a few parameters progressively to be able to really connect results of (photo)(electro)catalytic activities or reaction selectivity with physicochemical properties of the materials. As shown in this review, researchers have synthesized many niobates perovskites with different properties. However, only a fraction of them has been used as photocatalyst. Thus there is a big room for discovering in this field. We would like to call attention to the lack of papers using niobates perovskites in the field of photoelectrochemistry, although several materials have suitable band gaps.

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Chapter 22

Insights on the photocatalytic performances of LaFeO3 synthesized by solution combustion synthesis Alessandra Bianco Prevota, D. Fabbria, E. Bernardinia, F. Deganellob, Maria Laura Tumminoa,*, and Giuliana Magnaccaa a

Chemistry Department, Torino University, Torino, Italy, b Institute for the Study of Nanostructured Materials (ISMN)—Italian National Research Council

(CNR), Palermo, Italy

1 Introduction In the past 30 years, a great attention has been devoted to advanced oxidation processes (AOPs) as innovative approaches for environmental remediation, particularly for wastewaters treatment. AOPs exploit the formation of reactive species, in particular %OH radicals, able to degrade organic pollutants [1–6]. Among AOPs, the heterogeneous photocatalysis induced by UV–vis light in the presence of semiconductor oxides proved to be very efficient [2]. However, one of the main drawbacks of the most used photocatalyst, TiO2, is the need of ultraviolet light for its photoactivation, which limits the use of the solar light to a reduced portion of wavelengths [7–9]. This limitation has shifted the attention of the scientific community toward other oxides, for example perovskites. Among them, LaFeO3 (LF) is a mixed oxide with perovskite-type structure [10–13] (Fig. 1), which has gained increasing interest as a photocatalyst active under visible light, due its small bandgap energy, generally ranging from 1.86 to 2.36 eV. Therefore it has been already tested for the degradation of various pollutants present in aqueous solution [14–20]. In a basic ABO3 perovskite structure type (Fig. 1), A-site cations have a large ionic radius, whereas B-site cations have a small one [21]. Depending on the oxidation state of the B-site cations, oxygen vacancies are produced (d) [22]. Both A- and B-sites can be replaced by dopant elements, changing the physical–chemical properties and performance of the original perovskite compound [21, 23]. Furthermore, oxygen sites can be also replaced with other anions, as for example Cl [24], further extending the potentiality of this class of compounds. LaFeO3 and LaFeO3-based materials have been already considered for application as sensors, oxidation catalysts for air pollutants, fuel cell electrodes, and oxygen permeation membranes [25–27]. In addition, LaFeO3 is considered as a representative model of ABO3-type perovskite structure for fundamental studies, both experimental [28–31] and theoretical [32]. The results obtained for LaFeO3 can be then extended to LaFeO3-based materials, doped-LaFeO3, or even other ABO3-type perovskite composites, where the perovskite is the main active phase. Several synthetic approaches have been attempted for the preparation of LaFeO3 and LaFeO3-based materials, such as solid-state synthesis [33, 34], solvothermal synthesis [35], sol–gel techniques [36–38], co-precipitation [39, 40], hydrothermal synthesis [41, 42], microwave synthesis [43, 44], and solution combustion synthesis [28, 29, 45–48]. Solution combustion synthesis (SCS) is a fast, efficient, and versatile methodology for the preparation of multicomponent mixed oxides [49–52] (Fig. 2). SCS has been successfully employed for perovskite-type compounds with photocatalytic activity using various templating agents, aiming at obtaining a controlled microstructure and other physical–chemical properties of the materials. In this chapter, the main features and performances of LaFeO3 photocatalysts prepared by solution combustion synthesis are discussed, with a particular focus on the effect of the organic matter used as a fuel. Some odd outcomes in terms of performances and data reproducibility are evidenced, and useful insights on the experimental procedures are given. In particular, the influence of a sonication pretreatment and aging on the LaFeO3 suspension behavior is deepened. The * Current Address: Institute of Intelligent Industrial Technologies and Systems for Advanced Manufacturing, Italian National Research Council, Biella, Italy. Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00023-4 Copyright © 2021 Elsevier Inc. All rights reserved.

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FIG. 1 Pictorial description of the perovskite-type ABO3-d structure.

FIG. 2 Main features of the solution combustion synthesis.

eventual concurrence of other degradation mechanisms related to chemicals released in solution by LaFeO3 is investigated, as well. Finally, a possible correlation of the physical and chemical stability of LaFeO3 with the degradation performance of pollutants is proposed.

2

Synthetic strategy

Most of the important properties for the photocatalytic performances, such as oxygen availability/defectivity, texture, morphology, surface composition, and band gap can be tuned by selecting the synthesis methodology and/or the synthesis parameters. Among the various preparation methodologies, solution combustion synthesis (SCS) allows tuning a large

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number of parameters that can lead to a remarkable control on the properties of the final materials, if the synthesisstructure–properties relationships are sufficiently known [49–51]. Solution combustion synthesis is based on both the concepts of propellant and sol–gel chemistry. The starting point is the combustion mixture, which contains metal precursors (oxidants), fuel (reducing agent), water, and additives. In most cases, the combustion mixture is a solution, although insoluble components can be also present. The combustion mixture is subjected to dehydration up to formation of a gel network, which has a fundamental role in ensuring high interaction among the various components. Activation of the combustion process (i.e., by increasing the temperature over 200°C) causes a thermal decomposition of the gel, involving a fast and self-sustaining exothermic reaction leading to the evolution of large amount of gases and, finally, to the formation of a solid product [50]. Any organic compound able to react with an oxidant, starting a self-sustaining combustion process, can be considered as a fuel. During the combustion, the fuel is oxidized by the oxidants to gaseous products (Fig. 2), whereas metal cations take the oxygen from the oxidants and are converted to oxides. Citric acid is the most common fuel for the synthesis of LaFeO3 and LaFeO3-based materials because it contains three carboxylic functional groups and one hydroxyl group and, in its dissociated form, is able to form complexes with most of the metal cations of the periodic table. After evaporation of water from the solution, the formation of the gel occurs by the link of metal cations to the functional groups of dissociated citric acid. A plausible structure of the La3+-Fe3+-citrate complex is shown in Fig. 3 [53]. The role of the gel network is to keep the metal cations at the right distance so that, after the fast combustion process, they are in the right position to form the mixed perovskite oxide. The fuel-type is very important for a control of the perovskite properties, although other fuel-related parameters need as well to be selected to tailor the final material, namely the fuel-to-metal cations ratio (F/M) and the reducers-to-oxidizers ratio (ɸ) [50, 54]. These two fuel-related parameters are able to control the structural, textural, microstructural, redox properties of perovskite-type materials, and thus the choice of the fuel(s) type and amount need to be properly modulated. Beside citric acid, other fuels/fuel mixtures have been also suggested for the synthesis of LaFeO3 powders, such for example glycine [55], various carbohydrates [45, 48] and glucosecitric acid mixtures [56]. An advantage of SCS is the possibility to use waste precursors, both soluble and insoluble, increasing the level of sustainability of the synthetic process. For example, a LaFeO3 was prepared by SCS from citric acid starting from lanthanum nitrate and rust waste as sustainable iron source and tested as heterogeneous catalyst for the abatement of propylene in gas phase [30]. Despite itsinsoluble nature, rustwas able to interact with the other components of the combustion mixture forming a LaFeO3 catalyst with the same catalytic activity of the reference sample prepared from iron nitrate. Varying the amount of rust waste, binary systems LaFeO3-La2O3 or LaFeO3-Fe2O3 were also obtained, although the iron-rich binary system was more active than the other one. In another work, bio-organic soluble substances (SBO) derived from composted green waste were used as ecofriendly fuels for the preparation of a LaFeO3 active in the photodegradation of organic pollutants [28]. SBO are macromolecules with high molecular weight polydispersity, whose hypothesized structure is shown in Fig. 4. Since SBO possess bio-surfactant features, they were able to confer peculiar properties to the LaFeO3 final material. Using a soft-hard templating approach, high surface area LaFeO3 perovskite-containing materials can be also prepared in a one-pot strategy by SCS using silica with high porosity as hard template [29, 57]. The main advantage is the possibility to control porosity, surface area, and surface properties based on the porosity and texture of the original silica used. In addition, the chemical stability of the perovskite increased, because the LaFeO3 nanoparticles are strongly connected to the amorphous silicates formed during the combustion process [29]. Another strategy introduces the variable of the semi-reductive environment (nitrogen) that allows the production of LaFeO3/carbon binary systems with higher surface area defectivity and peculiar features, depending on the fuel used [48].

FIG. 3 A plausible structure of the La3 + -Fe3 + citrate complex inspired by Khalil et al.

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FIG. 4 Virtual molecular fragment of the bio-organic soluble molecule (SBO) inspired by Deganello et al. [28].

3

Physical–chemical characterization of LaFeO3

The classical physical–chemical characterization methods for solid materials apply as well to perovskite oxides: X-ray diffraction analysis, N2 adsorption at 77 K, electron microscopies, among others, allows determining structure and morphology of LaFeO3. Powder X-ray diffraction (XRD) is the first choice to obtain information on the phase composition of LaFeO3 and LaFeO3-based powders. This information helps to find correlations between synthesis, processing, structure and performance of the perovskite-type material. The amount and quality of information obtained from XRD characterization is higher for higher resolution of the XRD patterns and increases when Rietveld analysis is applied. Rietveld analysis is a methodology for the refinement of X-ray and neutron diffraction patterns, working with a statistical approach to refine a theoretical diffraction pattern that matches with the experimental one [58–60]. In Fig. 5, the graphical Rietveld refinement of a LaFeO3 powder prepared by SCS from citric acid and calcined at 700°C for 5 h is represented. The LaFeO3 formation temperature is a crucial parameter, which can be determined using XRD. Indeed, following a general rule, the higher is temperature required for LaFeO3 formation, the lower is the obtained surface area.

FIG. 5 Rietveld refinement of the XRD data for a single-phase LaFeO3 powder prepared by SCS from citric acid and calcined at 700°C per 5 h. The most meaningful reliability factors of the fitting (Rwp, R2F, and X2) are also reported in the figure. Taken from Magnacca et al., G. Magnacca, G. Spezzati, F. Deganello, M.L. Testa, A new in situ methodology for the quantification of the oxygen storage potential in perovskite-type materials, RSC Adv. 3 (2013) 26352–26360. https://doi.org/10.1039/c3ra44930k.

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LaFeO3 is a very stable perovskite with orthorhombic Pnma structure, which starts forming between 400°C and 700°C, as observed in [9, 38, 61], although Hao et al. [47] reported the formation of LaFeO3 already at 200°C. In general, perovskite formation temperature depends on the synthesis methodology and processing parameters [62]. In particular, formation temperature of LaFeO3 prepared by solution combustion synthesis is affected by the earlier mentioned F/M value [38]. When the temperature is too low, amorphous La and Fe oxides/carbonates/hydroxides are formed [61]. Doped LaFeO3 may have higher formation temperature and higher percentage of secondary phases, especially when alkaline earth metals are present at the A-site [63]. In fact, the high tendency of alkaline earth metals to form carbonates and the stability of alkaline earth carbonates cause a shift of the formation temperature of a pure single-phase perovskite to much higher temperatures [63]. An accurate analysis of XRD patterns gives also information on the cell parameters, size of crystallites, microstrain, and structure defectivity. For example, XRD results can be correlated with those obtained from complementary techniques, as N2-adsorption, scanning and transmission electron microscopies, to know the agglomeration degree of crystals and the eventual relationships between surface area and crystal size or specific textural properties of the powder [29]. Perovskite materials are typically nonporous solids with low surface area [64], although solution combustion synthesis allows obtaining micrometrical macroporous particles with higher surface area than materials obtained with other synthesis procedures [56, 65]. The timescale of the synthesis reaction, in fact, is so short to block the coalescence and sintering of the formed particles [50, 51]. For similar reasons, the particles often entrap unreacted precursors, which remain in closed void spaces of the solid and isolated from the outside [31, 66]. This may be an issue in photocatalytic application, if the solid is submitted to ultrasonication procedure for increasing the dispersion: the ultrasounds are capable of breaking the thin walls of the pores, allowing the unreacted precursors to be released in the suspension media, affecting the photocatalytic activity of the perovskite [28, 66]. N2 adsorption is a suitable technique to evidence these behaviors, as reported in Deganello et al. [28]. In that paper, the sonication procedure was carried out in a controlled way, preparing a suspension of the solid into ultrapure water, submitting it to several half-a-hour sonication cycles to avoid undesired activation of the material due to an excessive heating. The suspending water was removed every 2 h after simple decantation of the powder, the organic and metal content quantified via total organic carbon (TOC) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) analyses, and a new dose of fresh and clean water was added to the powder for the following 2 h. The release of organics, Fe3+ and La3+ in solution was observed during the tests, and the implications correlated to the photocatalytic activity will be discussed in the following paragraphs. At the same time, the powder before and after the sonication treatment, properly recovered and dried, was characterized through N2 gas-volumetric adsorption, revealing an increase of the specific surface area and total pore volume, probably caused by the opening of closed porosity of the material during the sonication treatment with the consequent release of entrapped unreacted synthesis precursors. As perovskite materials behave as oxygen storage agents, several methods [67–70] are applied to evaluate their redox capacity, typically temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), temperatureprogrammed desorption (TPD), and O2-pulse measurements. These methods quantify the oxygen availability but sometimes do not reveal in situ reactive oxygen, which is the oxygen, not releasable but reactive toward substrates. In addition, X-ray Photoelectron Spectroscopy, Secondary Ion Mass Spectrometry, and Near Edge X-ray Absorption Fine Structure are widely used to characterize the redox properties of perovskites. Recently, Tummino et al. [71] discussed the electrochemical behavior of doped strontium ferrate perovskites in relation to the redox capacities, which were evaluated through TPR as well as thermogravimetric (TGA) measurements carried out in inert atmosphere with a preliminary reduction/oxidation of the material. The procedure allowed assessing not only the release of mobile and lattice oxygen, quantified as weight loss observed in the thermal profile, but also the related thermal stability of materials overcoming frequent oxidation/reduction cycles. In all cases, the methods deal with the evaluation of the redox capacity of perovskite B-sites under different experimental conditions. Besides the aforementioned techniques, a less common method which allows evaluating the redox capacity of the perovskite material is described in the paper by Magnacca et al. [31], dealing with the use of a reactive probe-molecule, namely CO, whose interaction with LaFeO3 was detected by in situ FTIR spectroscopy and complementary microgravimetry. Gaseous CO can interact with the solid surfaces during the adsorption process changing its vibrational feature. Therefore the analysis of the vibrational spectra allows identifying the types of molecule–solid interaction. In this particular contest, CO molecules can interact with available oxygen atoms of the perovskite to form carbonate-like groups at the material surface, characterized by IR absorption bands in the range 1850–800 cm
1 (Fig. 6). The extent of the interaction can be quantified performing the same experiment in a microgravimetric apparatus. The procedure comprehends: (1) a preliminary step carried out at different temperatures (from 25°C to 700°C) in the presence of gaseous oxygen to oxidize the material (maximum oxygen availability), or an alternative pretreatment in vacuum to reduce the material (minimum oxygen availability); (2) the contact with 100 mbar of gaseous CO; and (3) registration of FTIR spectra and mass evolution in time.

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FIG. 6 Differential spectra of LaFeO3 in the carbonate-like vibration region. Admission of 100 mbar CO on sample pretreated at 400°C (left section) and 700°C (right section), under oxidative (O) and reductive (R) conditions. The broken-line curves correspond to a brief CO contact (5 min), the solid-line curves correspond to a prolonged contact (16 h). CO contact on material pretreated at 25°C did not cause any modification in the spectra which were not reported for the sake of brevity. The experiments were carried out at the temperature of the IR beam (about 60°C).

The data indicate the higher the oxygen availability in the material, the higher the amount of CO interacting with the material. This reflects in higher complexity and intensity of the IR bands formed on CO contact and higher CO interacting amounts. As expected, the amount of CO interacting with a sample pretreated at 25°C is very limited (no modification in the FTIR spectra was observed), as the availability of perovskite oxygen atoms is very limited, whereas the higher amount of CO entrapped was observed after an oxidizing pretreatment at the highest temperature, as reported in Table 1. LaFeO3 are p-type semiconducting materials [20], with a band gap lower than 3 eV. According to a general mechanism, when the irradiating light possesses an equal or greater energy than LaFeO3 band gap energy, the absorption of a photon promotes an electron from its valence band to its conduction band, producing a positively charged hole in the valence band and an electron in the conduction band. If their recombination rate is such as to allow the migration to the surface, they can give rise to a series of redox processes with donor or acceptor molecules adsorbed or located near the surface [72]. Standing these conditions, the exact determination of the band gap value allows assessing the photophysical and photochemical properties of the materials. The traditional method to determine the band gap is based on the elaboration of UV–vis diffuse reflectance spectrum through a Tauc plot. Tauc proposed this method in 1966 [73], which was further developed by Davis and Mott [74]. The Tauc method correlates the band gap energy with the X-axis intersection point of the linear fit of the steep, linear increase of light absorption with increasing energy typical of semiconductors. It is applicable to all semiconductors showing negligible absorbance below the band gap, typically due to presence of defects, doping and bulk or surface modification procedures. When the absorbance below the band gap becomes nonnegligible, an optimization of the analytical procedures is necessary to contain/overcome these limitations [75].

4

Photocatalytic applications and performances of LaFeO3

LaFeO3 is a semiconductor with advantageous band gap that permits its use also on visible light: when irradiated, it is able to produce a positively charged hole (h +), an electron (e-), and a chain reaction leading to the formation of reactive oxygen species (ROS) as %OH, O2%2, 1O2. in solution [19]. Thanks to these abilities, the lanthanum ferrites have been exploited in different applications, i.e., wastewater remediation, H2 production from water, removal of toxic gases [43, 47, 76–78]. Constant efforts to improve their performance have been made by tuning several parameters, from the doping elements to the synthesis methods [20, 61, 79–82]. In this context, the SCS allows to tune LaFeO3 features and reactivity. In a TABLE 1 Amount of CO interacting with reduced (R) and oxidized (O) materials. Sample

R/O 25°C

O 400°C

O 700°C

R 400°C

R 700°C

% mass increase

0.01

0.216

0.374

0.147

0.311

The number indicates the temperature of the preliminary pretreatment.

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previous work, Deganello et al. [28] produced different LaFeO3 by SCS and compared their performance as photocatalysts in aqueous pollutants degradation: a pure and a 20% Ca-doped LaFeO3 synthesized by citric acid (labeled as LF and LF-Ca, respectively) and a LaFeO3 prepared using biowaste-derived substances SBO (LF-B). The reason of the calcium addition in the citric acid-derived sample was connected with the presence of calcium residues inside the SBO that remained in the perovskite structure of LF-B after the combustion. In this way, it was possible to better compare samples with similar composition (LF-Ca and LF-B), but prepared using different combustion fuels. Such perovskite-type oxides were tested subjecting their suspensions to a simulated solar light, for the degradation of two substrates: 4-methylphenol (4MP) and the dye Crystal Violet (CV), which were chosen as model pollutants, able to show different charges depending on the adopted experimental conditions and, thus, behave differently. In particular, the 4MP substrate presents an equilibrium with its negatively ionized form, whereas CV is a cationic dye that brings a positive charge in a wide range of pH. Regarding the degradation of 4MP, all the three samples brought to a progressive disappearance of 4MP throughout the whole duration of the experiment, but with different rates. After 6 h of irradiation, LF and LF-Ca performed significantly better than LF-B: the abatement reached were 89% for LF, 72% for LF-Ca, and 38% for LF-B (Fig. 7). To explain this discrepancy in the efficiency, the eventual contribution of calcium (or other impurities) was not considered as a crucial factor; rather, the physical characteristics of the oxides and the operational pH of the suspensions containing 4MP were taken into account. The pH was 8.0 in the presence of LF and 10.0 in the presence of LF-B photocatalyst. Thus 4MP, which have a dissociation pK of about 10, was assumed to be in its nondissociated form for more than 90% in the presence of LF, whereas about 50% was present in the anionic form when using LF-B. Such information was correlated with the surface charge of LaFeO3 at the operating pHs: LF possessed a positive zeta potential that could favor the interaction between the photocatalyst surfaces with the delocalized 4MP electrons of the aromatic system. On the contrary, LF-B surface resulted negatively charged, due to the complex organic moieties and functionalities constituting the SBO precursor, which left its imprinting on the oxide. Therefore a possible repulsion between the negatively charged surface of LF-B particles and the dissociated (negative) form of 4MP as well as the aromatic electrons was hypothesized to be the cause of the reduced efficiency observed for LF-B. LF and LF-B catalysts demonstrated an opposite behavior in the presence of CV solution. First, it was evidenced a strong coloration of the LF-B powder (noted during the step of separation of the catalyst from the substrate solution), index of the dye adsorption occurred on the perovskite surface. Such behavior was consistent with the negative zeta potential of the LF-B surface and the cationic nature of CV, creating a strong affinity driven by electrostatic interactions. Indeed, after 6 h of irradiation, the complete color abatement was observed in the presence of LF-B, whereas LF brought to a slight bleaching of the CV solution, amounted to 30%, due to the repulsion of cationic dye molecules and LF positively charged surface. Some preliminary outcomes were obtained by the same research group, who studied the influence of pre-treated SBO as a fuel in SCS for LaFeO3 preparation. The SBO pretreatment consisted of an acid washing [83], causing several

FIG. 7 Photodegradation of 4MP in the presence of LF, LF-Ca, and LF-B. Taken from Deganello et al. F. Deganello, M.L. Tummino, C. Calabrese, M.L. Testa, P. Avetta, D. Fabbri, A.B. Prevot, E. Montoneri, G. Magnacca, A new, sustainable LaFeO3 material prepared from biowaste-sourced soluble substances, New J. Chem. 39 (2015) 877–885. https://doi.org/10.1039/c4nj01279h.F. Deganello, M.L. Tummino, C. Calabrese, M.L. Testa, P. Avetta, D. Fabbri, A.B. Prevot, E. Montoneri, G. Magnacca, A new, sustainable LaFeO3 material prepared from biowaste-sourced soluble substances, New J. Chem. 39 (2015) 877–885. https://doi.org/10.1039/c4nj01279h.

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modifications in the supramolecular structure of SBO, which were reflected in the final LaFeO3 (LF-P). For instance, both the oxide surface area and porosity decreased, lessening the availability of catalytically active sites. LF-P was employed in 4MP degradation in the same test conditions described earlier, achieving an abatement of 8% after 6 h that is much lower than the values obtained using LF, LF-Ca, and LF-B (Fig. 7 for comparison). Yahya et al. [56] evaluated the effect of the double combustion fuel (glucose-citrate) adopted during the SCS of lanthanum ferrite. The basic idea was the improvement of fuel chelating action toward the metal precursors by the addition of citric acid as a coadjutant of glucose, which, alone, possesses weak electron-donating groups. Therefore two specimens of LaFeO3 were prepared from glucose (here, LF-G) and glucose-citric acid mixture (here, LF-GC). The mixed-fuel approach led to a decrease of particle agglomeration degree and enhanced the surface area of the final perovskite powder, but caused also an increase of the band gap value (1.70 eV for LF-G vs 2.02 eV for LF-GC). Both LF-G and LF-GC were employed as photocatalysts activated by visible light for the abatement of aqueous solutions of the anionic azo dye Reactive Black 5. The initial adsorption–desorption equilibrium evidenced that—being equal the positive zeta potentials of the two oxide surfaces—the amount of adsorbed dye was higher in the case of LF-GC, due to the favorable morphological features. Moreover, the bleaching of the azo dye after 80 min was 70% in the presence of LF-G, whereas the complete abatement was reached by LF-GC, despite the wider band gap. This latter result was attributed to the synergistic effect of both adsorptive and photodegradative phenomena. The influence of the annealing temperature on photocatalytic properties of LF-GC was also considered in a following study of the same research group [84]. Among different LaFeO3 nanoparticles calcined at 400°C, 500°C, and 600°C, those prepared at the lowest calcination temperature resulted the most promising photocatalysts for the degradation of humic acid, due to their amorphous nature and the presence of surface defects (which allowed the photocatalyst to harvest more visible light compared with crystalline phase), in addition to the highest surface area [84]. Similarly, Hao et al. [47] lowered the calcination temperature of LaFeO3 samples synthesized by gel-combustion method (using citric acid), to maintain their porous structure, as much as possible, avoiding the sintering that generally occurs at high temperatures. Three samples treated at 200°C, 300°C, and 400°C were developed and are herein indicated as LF-200, LF-300, and LF-400. Their band gap values were estimated to be 1.71, 1.94, and 2.00 eV, respectively. Such oxides were employed for the photocatalytic reduction of Cr(VI) to Cr(III): this reaction is highly desirable since Cr(VI)contaminated water is significantly toxic. All the samples, tested under visible light for 160 min, reached a good degree of conversion, in the order LF-200 > LF-300 > LF-400, which was clearly related to the calcination temperatures and their effects on the physical–chemical characteristics. A further strategy to optimize the efficiency of lanthanum ferrites were put in place by Shi et al. [20]: they fabricated hydrogenated LaFeO3 photocatalysts (LF-H) through the ultrasonic assisted self-combustion sol–gel method, coupled with the hydrogen annealing treatment. The hydrogen annealing was conducted to raise the presence of oxygen vacancies, which are common defects inside the perovskite lattice, able to allow the hopping mechanism of charges [20, 85]. The photocatalytic ability of such perovskites was investigated by the degradation of the dye rhodamine B, using visible light irradiation. The study deepened the charge-transfer mechanism and predicted the formation of the ROS involved. It was found that the hydrogenation step enhanced the efficiency of LaFeO3 in light harvesting and reduced the photogenerated carriers’ recombination, due to their favorite transfer provided by the presence of surface oxygen vacancies. For the sake of completeness, it is worth to mention other applications of SCS-derived lanthanum ferrite photocatalysts, beyond the aqueous pollutant degradation. The work of Parida et al. [86] evidenced the water splitting efficiency of thermally activated nanoLaFeO3 photocatalysts irradiated by visible light, attributing a significant role to the surface area and particle size properties. The group of Iervolino et al. studied different systems based on LaFeO3 for hydrogen production, driven by the photodegradation of glucose in water [87–89]. The samples were prepared by citric acid-assisted SCS and their features were modulated by Ruthenium doping [88] and, additionally, by coupling the Ru-LaFeO3 with magnetic maghemite [89]. An interesting aspect of such binary systems was the possibility to join the photocatalytic process used for the production of hydrogen with a heterogeneous photo-Fenton process to mineralize the unconverted organics in the wastewater coming from the photoreactor. Finally, co-doped LaFeO3 samples (developed by citric acid-assisted SCS with variable amount of Cobalt at B-site) were adopted as catalysts under photothermal conditions, namely 350°C with a visible light irradiation, for the CO2 reduction to CH4 and CH3OH using water as a hydrogen source [90]. In this case, a remarkable impact of the cobalt-doping fraction was assessed.

5

Peculiar behavior related to pretreatment issues

Hereinafter original data are presented concerning a sample of LaFeO3 synthesized by solution combustion synthesis starting from citric acid (LF). Its chemical–physical characteristics and photocatalytic performance are assessed to have

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a reference material when compared with products obtained using different fuels. To evaluate the photocatalytic performance of LF, a group of compounds of environmental interest were chosen, i.e., 2,4,5-trichlorophenol (2,4,5TCP), 4-methylphenol (4MP), 4-ethylphenol (4EP), and 4-tert-butylphenol (4-tert-BP). The photocatalytic tests were conducted by irradiating solutions with a substrate concentration of 10 mg L
1. The concentration of LF was optimized for each substrate, and was equal to 500 mg L
1 for 2,4,5 TCP and 1200 mg L
1 for alkylphenols. The samples were irradiated with a Xe lamp equipped with a 340 nm filter to simulate solar radiation. Preliminary irradiation tests were performed in the absence of LF, to assess the photostability of the pollutants: the photolysis of the TCP was about 40% after 6 h of irradiation, while the photolysis of alkylphenols was negligible. The kinetic of the photocatalytic process was compared with the decrease in dissolved organic carbon. In the presence of LF, all substrates were degraded with different kinetic, in particular 2,4,5-TCP featured the slowest one. In all cases, it was observed that the mineralization of organic carbon was slower than substrate degradation, reasonably due to the formation of transient transformation products. After making replicates of the substrate degradation to verify the reproducibility of the system, a systematic difference of results was observed between tests run with LF suspended by stirring and suspended by sonication. In particular the efficiency of the catalyst seemed to increase strongly after sonication, as it clearly appears in Fig. 8. To shed light on this aspect, the stability of the catalyst was studied. N2 adsorption measurements carried out on the powder as it is and on the powder recovered after sonication showed an increase of surface area and pore volume for the treated powders, as mentioned in the previous paragraph. Moreover, analysis carried out on the aqueous phase, after the separation from LF powder, showed a progressive release in solution of Fe (III) and La (III) ions and organic matter. The presence of the latter appeared rather strange, considering that LF was calcined at 700°C in air. Based on these observations, it was hypothesized that LF retained part of the precursors used for its synthesis within closed porosities, which cannot be reached by air during calcination. The sonication treatment could have caused the opening of the closed pores, the increase of the surface area and the release in solution of the unreacted precursors, all factors that could lead to a greater catalytic activity. In fact, analysis carried out on the organic matter released in solution highlighted the presence of oxalate, acetate and formate ions, at concentration of 0.3  0.05 mg L
1, 2.5  01 mg L
1, and, 1.6  0.2 mg L
1, respectively, which could form photoactive complexes with the Fe(III) ions released in solution. Therefore the occurrence of a photo Fenton-like photocatalytic process could be hypothesized, in accordance with the faster abatement of the substrates. Indeed, the literature reports on photo Fenton-like processes performed in the presence of Fe(III)-carboxylate complexes, which are able to form of Fe(II) and hydroxyl radicals (%OH) on the photoexcitation through a ligand-to-metal charge transfer process [91, 92]. Fig. 9 displays an example of the reactions that can occur for irradiated Fe(III)-oxalate complex. To define an experimental protocol for obtaining reproducible results, LF photocatalytic features were further investigated, focusing on the degradation of 4-methylphenol (4MP). Two different LF samples were considered: PSK-C, prepared as described earlier (calcination included) and PSK-NC, not calcined at the end of the synthesis. Three samples

FIG. 8 Photodegradation of 2,4,5-TCP in presence of LF suspended by stirring and LF suspended sonication.

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FIG. 9 Representation of the reactions sequence hypothesized for irradiated Fe(III)–oxalate complex Inspired by Zuo et al. [92].

FIG. 10 Degradation of 4MP vs irradiation time for sonicated PSK-C.

suspensions of PSK-C underwent sonication for different time before irradiation in the presence of 4MP. Fig. 10 reports the profiles of 4MP disappearance, including the one obtained with PSK-C suspension without sonication. The sonication enhances the degradation process, independently from the duration, although the surface area only slightly increases (from 15 m2 g
1 without sonication to 18 m2 g
1 after 27 h of sonication). The role of released Fe (III), La(III), and organics on the photocatalytic process was deepened, employing a PSK-NC after sonication and washing pretreatment. Fe and La were detected in solution at trace level, whereas the organic carbon released from PSK-NC suspension in solution after 6 h of sonication was approximately 5 mg L
1. These results confirm the hypothesis that the pores can trap a certain amount of organic matter coming from the synthesis reagents, in agreement with Deganello et al. [28] that used soluble bio-based substances SBO as a fuel for the synthesis of LaFeO3 for photocatalytic applications. Fig. 11 displays the comparison among the photocatalytic performances of three different LF suspensions toward 4MP: (i) PSK-NC as

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FIG. 11 Effect of the treatments of PKS-NC suspensions on the degradation kinetic of 4MP.

it is, (ii) PSK-NC after sonication (24 h), washing, and re-suspension, (iii) PSK-NC after sonication, washing, calcinations, and re-suspension. No relevant differences in the surface area have been evidenced among the three different powders after the treatments. From the kinetics, it is evident that, when inorganic and organic impurities are washed out, the 4MP degradation is slowed down, thus confirming the role of a homogeneous process occurring in solution. The concomitant occurrence of a homogeneous photo Fenton-like process in the case of both Fe and carboxylate ions released from the LF enhances the degradation efficiency, but lacks of reproducibility. Based on these results, it clearly appears the need of a proper pretreatment to “purify” the materials to assess their activity as heterogeneous photocatalysts, allowing a correct comparison among different materials obtained with the same synthetic approach.

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Thongtem, Characterization of perovskite LaFeO3 synthesized by microwave plasma method for photocatalytic applications, Ceram. Int. 45 (2019) 4802–4809, https://doi.org/10.1016/j. ceramint.2018.11.175. [83] S. Tabasso, S. Berto, R. Rosato, J.A.T. Marinos, M. Ginepro, V. Zelano, P.G. Daniele, E. Montoneri, Chemical modeling of acid-base properties of soluble biopolymers derived from municipal waste treatment materials, Int. J. Mol. Sci. 16 (2015) 3405–3418, https://doi.org/10.3390/ijms16023405. [84] N. Yahya, F. Aziz, J. Jaafar, W.J. Lau, N. Yusof, W.N.W. Salleh, A.F. Ismail, M. Aziz, Impacts of annealing temperature on morphological, optical and photocatalytic properties of gel-combustion-derived LaFeO3 nanoparticles, Arab. J. Sci. Eng. (2020), https://doi.org/10.1007/s13369-02004874-z. [85] M.L. Tummino, E. Laurenti, F. Deganello, A. Bianco Prevot, G. Magnacca, Revisiting the catalytic activity of a doped SrFeO3 for water pollutants removal: effect of light and temperature, Appl. Catal. B Environ. 207 (2017), https://doi.org/10.1016/j.apcatb.2017.02.007. [86] K.M. Parida, K.H. Reddy, S. Martha, D.P. Das, N. Biswal, Fabrication of nanocrystalline LaFeO3: an efficient sol-gel auto-combustion assisted visible light responsive photocatalyst for water decomposition, Int. J. Hydrog. Energy 35 (2010) 12161–12168, https://doi.org/10.1016/j. ijhydene.2010.08.029. [87] G. Iervolino, V. Vaiano, D. Sannino, L. Rizzo, P. Ciambelli, Production of hydrogen from glucose by LaFeO3 based photocatalytic process during water treatment, Int. J. Hydrog. Energy 41 (2016) 959–966, https://doi.org/10.1016/j.ijhydene.2015.10.085. [88] G. Iervolino, V. Vaiano, D. Sannino, L. Rizzo, V. Palma, Enhanced photocatalytic hydrogen production from glucose aqueous matrices on Ru-doped LaFeO3, Appl. Catal. B Environ. 207 (2017) 182–194, https://doi.org/10.1016/j.apcatb.2017.02.008. [89] G. Iervolino, V. Vaiano, D. Sannino, L. Rizzo, A. Galluzzi, M. Polichetti, G. Pepe, P. Campiglia, Hydrogen production from glucose degradation in water and wastewater treated by Ru-LaFeO3/Fe2O3 magnetic particles photocatalysis and heterogeneous photo-Fenton, Int. J. Hydrog. Energy 43 (2018) 2184–2196, https://doi.org/10.1016/j.ijhydene.2017.12.071. [90] L. Xu, M.N. Ha, Q. Guo, L. Wang, Y. Ren, N. Sha, Z. Zhao, Photothermal catalytic activity of combustion synthesized LaCo:XFe1-xO3 (0  x  1) perovskite for CO2 reduction with H2O to CH4 and CH3OH, RSC Adv. 7 (2017) 45949–45959, https://doi.org/10.1039/c7ra04879c. [91] D. Nansheng, W. Feng, L. Fan, X. Mei, Ferric citrate-induced photodegradation of dyes in aqueous solutions, Chemosphere 36 (1998) 3101–3112,https://doi.org/10.1016/S0045-6535(98)00014-9. [92] Y. Zuo, J. Hoigne, Formation of hydrogen peroxide and depletion of oxalic acid in atmospheric water by photolysis of iron(III)-oxalato complexes, Environ. Sci. Technol. 26 (1992) 1014–1022, https://doi.org/10.1021/es00029a022.

Chapter 23

TiO2-copper zinc tin sulfide (CZTS) photocatalytic thin films for up-scalable wastewater treatment A. Duta, M. Covei, C. Bogatu, and D. Perniu Center for Renewable Energy System and Recycling, Transilvania University of Brasov, Brașov, Romania

1 Introduction The world population significantly increased during the past 70 years and so did the share of urban inhabitants, from about 30% in 1950 to 56% in 2020 [1]. As the data in Fig. 1 shows, the trend is still set toward an increase, with obvious consequences on the food, water, and energy as primary needs that have to be satisfied for meeting the demands of this population. Moreover, the economic development also has a continuously increasing trend that is based on raw materials and energy consumption. During the past decades, these trends were analyzed and were found to pose a real threat to humankind, thus the concept of sustainable development was officially formulated in 1987, and started to be seriously considered, with specific actions targeting various areas, including raw materials protection and wastes recycling, integrating environment policies, and development strategies, [2]. An important chapter in the sustainable development strategy is related to the quality improvement or at least preservation of the natural waters, currently endangered by the improper treatment of the discharged wastewater or by the discharge limits that, although low, are not zero. Thus, pollutants are continuously sent into the environment and loading it with nonnatural species. The European strategy contains specific objectives on the reassessment of the discharge limits of the high toxicity pollutants and on updating the list of priority pollutants along with establishing the watch list of substances for Union-wide monitoring in the field of water policy (EU Decision 2018/840). This watch list was extended with 10 groups among which there are the neonicotinoid pesticides, e.g., imidacloprid. The current wastewater treatment processes are not fully effective in removing these pollutants at low concentrations, and these discharged pollutants are stepwise degrading the quality of the natural waters. Thus, the treated wastewater reuse represents a target for the (near) future, as it does not involve its discharge in the natural water bodies but its use in the same processes or other different ways, as in farming processes. The European Green Deal (COM(2019) 640 final) launched a concerted strategy for a climate-neutral, resource-efficient, and competitive economy, and the New Circular Economy Action Plan (EU, 2020) has specific tasks oriented, e.g., on wastes reduction by monitoring the implementation of the Drinking Water Directive for reducing the need for bottled water thus reducing the plastic wastes. The wastewater reuse asks for processes that are efficient and effective in the removal of low concentrated pollutants of concern, particularly organic pollutants. Membrane processes (e.g., reverse osmosis) can be highly efficient in the micropollutants removal, preparing the water for reuse but are energy-intensive and raise additional issues related to the concentrated pollutants that result in the process, thus being difficult to integrate into the traditional wastewater treatment flow. For up-scalable advanced wastewater treatment targeting the water reuse a specific prerequisite is to find solutions that are not raising cost issues for neutralizing the by-products, thus the pollutants’ decomposition, up to mineralization, represents the main target. The advanced oxidation processes (AOPs) can well meet this prerequisite when removing organic or biorecalcitrant pollutants at low concentrations (ppm or ppb); if well designed, the AOPs end up with gaseous mineralization products (usually CO2, NOx, etc.) that can be easily removed from the treated water, preparing it for reuse, e.g., for washing/cleaning or plants watering. During the past four decades, the AOPs designed based on heterogeneous photocatalysis were developed and optimized. However, despite the huge effort invested, there are only a few advanced wastewater treatment plants based on photocatalysis and most of them are only at the pilot level, e.g., at Plataforma Solar de Almerıa (CIEMAT) in Spain. Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00029-5 Copyright © 2021 Elsevier Inc. All rights reserved.

371

372 SECTION D Composites and heterojunctions. Tertiary materials

FIG. 1 Dynamic of the world population and the urban population during 1950–2020.

The main cause of this situation is related to the photocatalyst that has to be aqueously stable, thus it usually is a wide bandgap semiconductor that can be activated only by UV radiation and this represents a significant process limitation because of the cost. The first investigated [3, 4] and mostly reported photocatalyst is TiO2 that is aqueously stable over a broad pH range. However, it has a wide bandgap, of 3.2 eV for the anatase polymorph and 3 eV for rutile, thus can only be activated by UV or radiation close to the UV spectral range (with the wavelength l < 387 nm for the anatase polymorph or l < 413 nm for rutile). Various methods for extending the activation range toward a larger share of Vis are reported, such as anion and/or cation doping, metal loading on the surface, or developing heterostructures with narrow bandgap semiconductors [5]. The last alternative represents a viable option if the proper conduction and valence band alignment is reached, as these structures well resemble a photovoltaic cell (n-p junction). Such examples are the TiO2/CuxS diode type structure, [5] and the SnO2/CuInS2/TiO2, [6, 7] multilayered structure with very good photocatalytic and mineralization efficiency, up to 90%, in the methylene blue (MB) degradation under simulated solar radiation at low irradiance values (G ¼ 34 W/m2). Despite the very good performance of the structure containing CuInS2, its large-scale use may be limited because indium (In) is a scarce element and is already employed in manufacturing solid-state solar cells (CIS-cells). Following the indium scarcity, different p-type semiconductors were investigated; one alternative was to replace it with more abundant elements, such as zinc (Zn) and tin (Sn), as in the Cu2ZnSnS4 (CZTS) quaternary compound [8, 9]. The heterogeneous AOPs can use either photocatalytic powders or thin films. Obviously, the photocatalytic powders are more efficient, as they can expose a larger surface to the pollutant(s) adsorption and decomposition; however, a large specific surface is mainly a micropowders property and their (re)use requires advanced and costly separation steps in the photocatalytic process. Thus, the use of thin-film photocatalyst(s) deposited on various substrates represents a viable option when considering upscaling. Photocatalytic heterostructures of TiO2–CZTS type were developed at laboratory scale (on 1.5  1.5 cm2 samples) to test their photocatalytic efficiency, [10, 11] and stability, [12] under various irradiation sources (UV, Vis, or their mix as simulated solar radiation) at low irradiance values. This work proved that the multistructures deposited by Spray Pyrolysis Deposition are Vis-active and aqueously stable, being recommended for upscaling. The results recorded on larger thin films are discussed in this chapter, considering the photocatalytic removal of the standard dye, methylene blue (MB), according to the international standard ISO 10678:2010, Fine ceramics (advanced ceramics, advanced technical ceramics)—Determination of the photocatalytic activity of surfaces in an aqueous medium by degradation of methylene blue. Further on, parallel tests were developed to observe the influence of the organic pollutant type, with a specific reference to the imidacloprid pesticide and phenol.

2

TiO2-CZTS thin film deposition

The method used when depositing the thin, small-sized film of TiO2-CZTS that proved the highest photocatalytic efficiency was that using an outer TiO2 layer with 20 deposition sequences (CT20) [11]. This method was further considered to obtain the larger-sized thin films, deposited on glass with 4 mm thickness. A two-layered structure was considered, with the following components:

TiO2-CZTS photocatalytic thin films Chapter

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Ø Layer 1 (CZTS), deposited by Spray Pyrolysis (SPD) on the glass substrate. The deposition used a precursor system with a water–ethanol solvent mixture where there was dissolved copper chloride, zinc chloride, tin tetrachloride, and thiourea in a Cu:Zn:Sn:S atomic ratio of 1.8:1.2:1:10. This Cu-poor and Zn-rich composition promotes a good charge carriers’ flow in the material thus avoiding recombination. The water ratio in the solvent was varied (100%, 90%, 75%, and 50%) to identify the composition that corresponds to the most homogeneous layers. The large thiourea excess was selected to avoid the formation of the Cu2-xS component and previous results outlined that for x ¼ 1.2 a pure CZTS phase could be deposited, [13]. Moreover, the sulfur loss during the deposition and annealing is expected to be compensated by the thiourea excess and this choice is also confirmed by other groups, [14]. The deposition temperature was optimized at 300 °C to limit the sulfur loss from the layer, in good agreement with the results presented by [15], where the optimal temperature of the SPD deposited CZTS thin layer was found to be 250–350 °C, resulting in a bandgap value of 1.45–1.55 eV. The number of deposition sequences was optimized at 15, with a 60s break between two consecutive sequences, allowing to reach a porous and homogeneous surface structure. This layer was subject to annealing, at 400 °C for 1 h, [10], covered by a ceramic bowl (to limit the sulfur loss). Ø Layer 2 (TiO2), obtained by SPD using a precursor system containing TiCl4 dissolved in ethanol, in a 0.05 M concentration. The choice of this precursor is motivated by the dense morphology of the TiO2 layer as compared to the more porous aspect of the layers deposited using titanium alkoxides (e.g., titanium tetraisopropoxide, TTIP). The film has to be thin to allow a good Vis-transmittance, without scattering, allowing the (simulated) solar radiation to reach the CZTS layer, but large enough to protect the CZTS layer against hydrolysis. Moreover, a thicker TiO2 outer layer outlined a better crystallinity degree, [10], thus 20 deposition sequences were used, with a 60s break between two consecutive spraying sequences; the deposition temperature was 350 °C. After deposition, the sample was subject to thermal treatment for 1 h, at 400 °C, covered by a ceramic bowl (to limit the sulfur loss and prevent the oxidation of the CZTS layer, to which the oxygen access is possible through the rather rough titania layer).

3 TiO2-CZTS thin film characterization and testing ˚, The crystallinity of the films was investigated using a Bruker D8 Discover X-ray Diffractometer (XRD, CuKa1 ¼ 1.5406 A step size 0.02, scan speed 3 s/step, from 20 to 70°). Scanning electron microscopy (SEM) used a Hitachi SEM S-3400 N type 121 II apparatus coupled with a Thermo Scientific UltraDry energy dispersive X-ray spectrometer (EDX). Atomic force microscopy (AFM) analysis used an NT-MDT model NTGRA PRIMA EC microscope while the Raman analyses employed a LabRAM HR800 Horiba device. The photocatalyst overall area considered for testing was 20  30 cm2. This area was obtained by depositing five square plates of 10  10 cm2, one plate of 8  10 cm2, and five square plates of 2  2 cm2, as shown in Fig. 2. This structure of the OUTLET n

lll

INLET n FIG. 2 The geometrical components of the photocatalytic surface.

l

374 SECTION D Composites and heterojunctions. Tertiary materials

photocatalytic area was selected to allow the full characterization of the stability of the thin film samples, considering the limited available area in the characterization devices (SEM, EDX, AFM, and XRD). The upper side of the photoreactor is covered with a quartz plate, with a good UV and Vis transmittance, higher than 90%. The thickness of the aqueous solution layer over the photocatalytic surface in the photoreactor is 2 cm, allowing a good radiation penetration to the photocatalytic thin film, with reduced scattering. The photocatalytic decomposition of pollutants was investigated in a continuous flow regime, in an upper flow, as outlined in Fig. 2. The methylene blue, MB (99.8%, Merck), phenol Ph (99.5%, Scharlau), and imidacloprid, IMD (99.8%, Dr. Ehrenstorfer GmbH) pollutants were selected to maintain consistency with previous tests, using small-sized photocatalytic films (1.5  1.5 cm2), where the same species were used [10, 11]. The selection of Ph and IMD is motivated by their stability, being thus difficult to degrade and by their different molecular structures, while MB is subject to the standardized testing procedure (ISO 10678:2010). Moreover, IMD is highly water-soluble (0.58 g/L) and toxic, being included in the emergent pollutant category and has been banned in the EU since April 2018. The solution volume subject of testing was 4 L/cycle, with the initial pollutant concentration of 10 ppm. First, the pollutant solution with a corrected pH (pH ¼ 8) was circulated for 1 h in dark, to reach the adsorption–desorption equilibrium. Afterwards, the photoreactor was irradiated for 8 h with simulated solar radiation or 6 h with natural solar radiation. Three cycles were run, using one single set of photocatalytic plates; after each cycle, a volume of 4 L of distilled water was circulated through the photocatalytic setup (the overall volume of the photocatalytic reactor is 1.5 L) to regenerate the plates, by removing the potentially adsorbed photocatalytic by-products. After 30 min, the photoreactor was emptied and the regeneration step was restarted. This regeneration procedure with two washing steps was applied when the MB solution and the Ph solution were tested. When IMD was subject to decomposition, the washing step was done 3 times, to remove as much as possible the various potential by-products. During the tests there were modified the irradiation conditions, as follows: Ø Cycle 1 used simulated solar radiation, with the irradiance values of GVis ¼ 650–800 W/m2 and GUV ¼ 20 W/m2, thus a total irradiance ranging in the interval 670–820 W/m2 was employed. Ø Cycle 2 used, after 1 h in dark, for 3 h the same irradiation conditions as in Cycle 1 and afterward, for 5 h a reduced irradiance value of the Vis component (GVis ¼ 480–560 W/m2), thus a total irradiance value during this part of the experiment was 500–580 W/m2. Ø Cycle 3 was run in identical conditions as Cycle 1, to test the stability of the photocatalytic coating and the effect of the potentially strongly adsorbed by-products that were not removed during the substrate regeneration with water. The irradiance was measured in five points over the photocatalytic surface, using a Delta-T Devices, type BF3 pyranometer and outlined that the highest irradiance values correspond to the lower part of the photocatalytic surface. The liquid flow through the photoreactor was optimized to be as uniform as possible, thus a laminar regime was selected (up to Re ¼ 2063) and a liquid flow of 0.5–1.5 L/min. The initial pollutant concentration was set at 10 ppm, for all three pollutants; this is a low concentration but still higher than the discharge limits or the recommended values (lower than 1 ppm). The photodegradation efficiency, Z, was calculated based on the initial absorbance of the pollutant solution (A0) and the absorbance after 1, 2, 4, 6, and 8 h of irradiation (A), recorded at the maximum absorbance wavelength for the pollutant (lMB ¼ 664 nm, lIMD ¼ 269 nm, lPh ¼ 270 nm), using a UV–Vis–NIR spectrophotometer, Perkin Elmer Lambda 950 and applying Eq. (1): Z ¼ 100  ðA0  AÞ=A0

(1)

To assess the contribution of adsorption to the process efficiency, similar experiments were run in dark, using the same continuous flow photoreactor and parameters as in the photocatalytic experiments, measuring the solution(s) absorbance corresponding to contact times equal to those used in the photocatalytic experiments.

4

The photocatalytic removal of pollutants

The photocatalytic activation of the n-p diode structure when irradiated with simulated solar radiation (UV + Vis) runs as follows, [10]: Ø The Vis radiation passes through the upper, TiO2 transparent thin film and is absorbed by the narrow band gap CZTS thin film, at the bottom of the heterostructure;

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Ø The electrons are activated from the CZTS valence band (VB, positioned at 0.72 eV) to the conduction band (CB at 0.74 eV), leaving behind holes; Ø The electrons promoted in the CB of the CZTS can be transferred to the CB of the TiO2 where they act as reductants and, if O2 is adsorbed at the surface, they can form the highly reactive superoxide (O 2 ) species; Ø The UV radiation is absorbed by the TiO2 film, where electron–hole pairs will form in the CB (0.33 eV) and in the VB (3.0 eV) respectively, similar to the CZTS layer; Ø Holes can diffuse from the TiO2 VB to the CZTS VB, closing the electrical circuit; Ø The holes generated in the VB of TiO2 act as oxidants and may react with adsorbed hydroxide anions leading to the formation of hydroxyl radicals, used in the degradation of the pollutant. The CZTS and the TiO2 band gap alignment well supports the charge flow, significantly avoiding the electron–hole recombination, as the VB and CB in the narrow band-gap semiconductor (CZTS) are higher than the VB and CB in the wide bandgap semiconductor (TiO2) as presented in Fig. 3A. However, potential by-products resulted when depositing the CZTS layer (e.g., Cu2SnS3) may hinder this flow, as their energy bands may no longer respect the adequate alignment, [10], as shown in Fig. 3B. Thus, various interfaces can be expected in the generically called TiO2/CZTS composite assembly and not all of them are VIS active, depending on the position of the valence and conduction bands of the possible interim products. Obviously, the oxidation species (the hydroxyl radical and the superoxide anion) are effective if they meet the pollutant molecules before their deactivation; thus, the adsorption of the pollutant molecules on the photocatalytic surface represents a prerequisite, and the photocatalytic mechanism starts with the pollutant molecules adsorption, followed by the activation of the heterostructures under irradiation. This is why a contact time of 1 h was designed before starting the irradiation, and this is why the pH was adjusted at 8. Higher pH values would preserve the negative surface charge of the photocatalyst but, considering the pKa values of IMD (11.12, [16]) and Ph (9.98, [17]), this value cannot be too high, to support the

E [eV] –2

e– e– e–

–1

CB

0 1

VB

Radia on

TiO2

CZTS –0.74 eV CB Eg=1.46 eV

e– e– e– e– –0.41 eV

0.72 eV h+ h+ h +

Eg=3.45 eV

2

3.04 eV

3

VB

4

h+ h+ h+

h+

(A) E [eV]

Cu2ZnSnS4

Cu2SnS3

–2 –1

CB

0 1

e–

e– e– e–

VB

CB

–0.74 eV Eg=1.46 eV h+ h + h +

CB VB

h+ h+ h+

e– e– e–

e– –1.06 eV

–0.41 eV

Eg=3.93 eV

1.38 eV

Eg=3.345 eV 2.87 eV

VB

h+

Radia on

e– e– e– CB

0.08 eV

Eg=1.3 eV

0.72 eV

2 3

e– e– e–

TiO2

ZnS

3.04 eV VB

h+ h+ h+

h+ h+ h+

4 h+

(B)

h+

FIG. 3 Energy bands alignment (A) in the glass/CZTS/TiO2 heterostructure and (B) in the glass/CZTS/Cu2SnS3/ZnS/TiO2 heterostructure.

376 SECTION D Composites and heterojunctions. Tertiary materials

Intensity [a.u.]

electrostatic attractions promoting the adsorption. The point of zero charge (PZC) of the photocatalytic surface was evaluated by potentiometric titration and showed multiple points in the pH range of 4.9–7.2 [11] that correspond to various sulfide and titania phases. Thus, a pH of 8 will support a negative surface charge, well suited to promote the adsorption of the pollutant molecules and to protect the photocatalytic layer against dissolution. The optimal duration of the adsorption process was investigated in the dark when it could be observed that, after 1 h, the amount of the adsorbed pollutant molecules did no longer vary; thus, it was concluded that 1 h represents the required duration to reach the adsorption–desorption equilibrium, [10]. The adsorption efficiencies recorded after 1 h in dark were, for a 4-ppm pollutant concentration, of 8.55% for methylene blue, 2.14% for phenol, and 0.52% for IMD, when using the small-sized samples. These results outline that the structure of the pollutant molecule is important as small molecules with rather rigid structures as that corresponding to phenol are better adsorbed than the large and rather flexible structures, as those in IMD. The photocatalytic efficiencies recorded when using these substrates reached values following the same trend, as 35.27% for MB, 13.73% for Ph, and 10.53% for IMD, outlining that adsorption represents a critical step in the photodegradation mechanism, [11]. Following these results, tests developed on largely sized photocatalytic plates were run aiming to outline the importance of the crystallinity and surface properties (roughness, elemental composition, and surface charge) on the oxidation of the pollutant in a process run in continuous flow. The XRD results on the first deposited layer confirm the CZTS deposition. However, the XRD data cannot well distinguish between CZTS and another ternary (Cu2SnS3) or binary (ZnS) compounds that may result during the deposition as the major peaks of these are overlapping, as outlined in Fig. 4. These by-products may lead to multiple bandgaps in the thin film (of 1.55, 1.81, and 1.84 eV), corresponding most likely to nonstoichiometric copper sulfide(s), as the Tauc plot in Fig. 5 shows [18]. The confirmation of the CZTS deposition was done using the Raman spectra that outline the characteristic peak (s) at 331–333 cm1. The XRD results collected on the CZTS-TiO2 thin films outline that anatase is the only crystalline polymorph of the outer TiO2 layer. The overall crystallinity degree of the thin films was 40%–42%, and this average value represents the result of a good compromise between the deposition and annealing temperatures and the highly limited decomposition of the quaternary compound, CZTS. The layers deposited using a 1:1 water: ethanol solvent mixture have a smooth surface as the image in Fig. 6 shows, thus recommend these photocatalytic plates for the continuous flow demonstrator photoreactor. Photocatalytic tests were first developed using MB solutions. In the beginning, the experiments targeted to outline the influence of the irradiance value and of the pollutant concentration on the photocatalytic efficiency. Two groups of tests were done as a first step, on small-sized photocatalytic plates (1.5  1.5 cm2) irradiated with UV–Vis radiation with a low irradiance value (34 W/m2) and with a high irradiance value (650–850 W/m2) delivered by the solar simulator, in the laboratory photoreactor (a quartz beaker, in static regime). During each test, before irradiation, the samples were kept 1 h in dark to reach the adsorption–desorption equilibrium. The experimental results are inserted in Table 1.

100% H 2 O 90% H 2 O 75% H 2 O 50% H 2 O 20

30

40

50

60

70

80

o

2 [ ] FIG. 4 XRD diffraction spectra of the CZTS thin films deposited using precursor systems with various water content.

TiO2-CZTS photocatalytic thin films Chapter

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377

16

15

(aE)2

E=1.81eV 14

13 1.0

E=1.84eV

E=1.55eV 1.2

1.4

1.6

1.8

2.0

2.2

2.4

E [eV] FIG. 5 Tauc plot of the thin film obtained using a precursor system with 1:1 water: ethanol ratio (50% water).

FIG. 6 SEM and AFM images of the CZTS thin film obtained using a 1:1 water: ethanol solvent.

These results confirm that the irradiance significantly influences the process efficiency in the experimental conditions (static regime, without any flow). However, a 20 times (or more) increase in the irradiance value leads to only doubling the overall efficiency (after 1 h of adsorption in dark followed by an irradiation duration 3 times lower) proving that at higher irradiance values the process is under diffusion control. Thus, the process efficiency is mainly controlled by the amount of pollutant molecules adsorbed on the photocatalytic surface. The results also confirm a higher efficiency when using lower concentrated solutions, regardless of the irradiation values. For these solutions, the efficiency is about 1.36 times higher when using high irradiance and 1.48 times higher when using low irradiance values. These results outline that the adsorption rate is not directly responsible for the process efficiency, as the adsorption rate would increase with concentration; most likely, the photocatalytic surface is clogged during the process with decomposition by-products that desorb slower and the amount of such by-products is lower when using less concentrated solutions. Even during the first step, in dark, the adsorption efficiency is almost constant, as a possible consequence of the main process that influences the adsorption at such low concentrations, the diffusion rate of the pollutant to the photocatalytic surface.

378 SECTION D Composites and heterojunctions. Tertiary materials

TABLE 1 Influence of the irradiance value on the photocatalytic decomposition of MB. Irradiance [W/m2]

MB concentration [ppm]

Irradiation duration [min]

Adsorption efficiency [%]

Overall efficiency [%]

34

4

480

9.98

40.27

34

10

480

10.83

27.04

650–850

4

180

10.16

75.23

650–850

10

180

10.36

55.57

Furthermore, the photocatalytic removal efficiency of MB was investigated using the MB solution with the standard concentration of 10 ppm, in the demonstrator photoreactor with the photocatalytic surface of 600 cm2 (20  30 cm2) in a continuous flow regime, under UV–Vis radiation, with an irradiance value of 650–850 W/m2. The experimental results show an adsorption efficiency of 3.85% (recorded in dark, with a covered surface of the quartz upper plate of the photoreactor); after starting the solar simulator, for about 1 h, the MB removal efficiency remains almost unchanged (3.15%–3.92%). Then, the removal efficiency steadily increases and reaches 32.34% after 540 min contact time with the photocatalytic plate, out of which 480 min of irradiation of the MB solution. This efficiency is significantly lower than that recorded in the static regime, and the reasons cannot be related to the ratio value between the solution volume and the photocatalytic area during the experiments as this ratio was almost similar in the demonstrator (5000 mL/ 600 cm2 ¼ 8.33 mL/cm2) and in the quartz beaker (20 mL/2.25 cm2 ¼ 8.88 mL/cm2). Thus, the decrease in the efficiency value may be related to the flow rate that was 1.5 L/min in the demonstrator reactor and no flow (0 L/min) in the quartz beaker, where the small-sized photocatalytic plates were inserted. The liquid flow may have a “wash-out” effect, partially removing the weakly adsorbed pollutant molecules, thus leading to lower adsorption efficiencies (3%–4%), with consequences on the decomposition process that will have a less amount of MB molecules available for the oxidation reactions with the HO% radicals, thus a lower overall removal efficiency of MB. The next group of tests was developed using the demonstrator photoreactor, in a continuous flow regime, using the MB solution in three consecutive photocatalysis cycles, as described in Section 3; the variation of the photocatalytic efficiency during these cycles is presented in Fig. 7. The results outline that the photocatalytic decomposition of MB depends on the irradiance value, as expected. However, there is a time delay of 1–2 h between the moment when the irradiance is decreased (after 240 min from starting the experiment) and that when the efficiency is slowly increasing as a result of the surface on which pollutant molecules are adsorbed along with decomposition products or with by-products. It is also to notice that the third cycle, run in identical conditions with the first cycle, has a similar variation of the efficiency values during the first 6 h, and then a slight decrease can be observed as a possible result of partial surface clogging. The efficiency decrease can be the result of more extended adsorption of the reaction products on the photocatalytic surface, leading to its clogging, and this can be the result of the surface modification(s). To check these assumptions on the surface modifications/stability, SEM and AFM results were recorded for the thin films that passed through the first two cycles and the results are included in Fig. 8. Moreover, the variation in the elemental surface composition investigated using EDX, is outlined by the results in Table 2, for the samples I and III, positioned as described in Fig. 2. As the results in Table 2 show, the photocatalytic surface is covered with very thin films as the substrate elements (from glass: Si, Na, Ca, Mg) are observed in a rather consistent percentage (mostly due to the silicon dioxide content). Moreover, the chlorine content is low outlining that the precursor system was almost fully processed to reaction products. The results also show that, during functioning, the density of the photocatalytic layers increases, as the concentration of the substrate elements decreases when running the third cycle as compared to the values recorded in the beginning and after each of the other two cycles. Additionally, the results show a rather uneven clogging and densification, more intense close to the edge of the photoreactor (Sample I). The AFM results show that the stress on the thin films varies according to their position on the photocatalytic plate. Closer to the surface edge (Sample I) the thin films roughness variation is higher (265.15%, after the first cycle) than the values recorded for the thin film closer to the central part of the photoreactor (18.38% for sample III), in Fig. 2, as

TiO2-CZTS photocatalytic thin films Chapter

N

S+

CH3

Cl–

379

FIG. 7 (A) Methylene blue molecular structure; (B) Variation of the MB removal efficiency during the three photocatalytic cycles.

N H3C

23

CH3 N CH3

(A) 35,00

Irradiance: G = 800 W/m2 Cycle1: UV + 100% VIS Cycle2: UV + 100% VIS Cycle3: UV + 100% VIS

30,00

Efficiency [%]

25,00 20,00 15,00 10,00

Irradiance: Cycle1: UV+100% VIS (G = 800 W/m2) Cycle2: UV+ 50% VIS (G = 500 W/m2) Cycle3: UV+ 100%VIS (G = 800 W/m2)

5,00 0,00

(B)

0

60

120

180

240

300

360

420

480

540

Time [min]

a result of the carefully designed distribution of the liquid flow inside the reactor, over the photocatalytic plate. After the second cycle, the average roughness variation is significantly higher (560% for sample I and 217.64% for sample III) so that the overall surface roughness ends up to very close values (of about 43 nm) for both samples. It is also worth noticing that the aggregates on the surface, mainly consisting of TiO2 as the EDX analyses showed on those particular points, are well attached and are not (completely) removed during the processes. However, the SEM image of sample III after the first cycle shows the partial peeling of the upper layer, close to the TiO2 aggregates on the surface, indicating a stress area on this surface. Similar investigations were run using pollutant solutions with imidacloprid (10 ppm) and phenol (10 ppm). The results included in Fig. 9 (imidacloprid, IMD) and in Fig. 10 (phenol, Ph) outline that the processes have certain particularities depending on the molecular structure of the compound subject of decomposition/oxidation. The photocatalytic removal of imidacloprid runs with higher efficiencies than those corresponding to MB: 59.75% after the first cycle, 60.57% when running according to the irradiation pattern of the second cycle, and 58.05% after the third cycle that used only UV radiation after 240 min of the process. The efficiency values are similar, regardless of the radiation type, the irradiance value, or the flowrate (0.5, 1.0, 1.5 L/min). During the second cycle, using a lower irradiance value (G ¼ 500 W/m2) due to a lower Vis share, there was an identical efficiency variation as in the first cycle (G ¼ 800 W/m2). Thus, the third cycle was designed using only UV radiation, having an overall lower irradiance value (G ¼ 200 W/m2) and the IMD removal efficiency variation was the same. These results may be correlated with the imidacloprid structure. The pesticide has a large but rather flexible molecule with polar bonds almost all over the structure, Fig 9A, thus it can be adsorbed in any position, and it can react with any of these polar bonds with the oxidizing species. The results also outline that the oxidizing species produced during the photocatalytic process are in a large enough amount even when using the lower irradiance source and/or these species are effectively used before their de-activation. As the irradiation pattern in Cycle 2 was similar for MB and IMD, it may be concluded that the IMD molecules did react in a larger number with the hydroxyl radicals than MB did, confirming that the amount of available reaction sites for the IMD molecules was larger and/or the adsorption degree of IMD was higher on the photocatalytic substrate.

FIG. 8 SEM and AFM images of the thin photocatalytic films in the beginning, after the first cycle, and after the second cycle.

TABLE 2 Variation in the elemental surface composition before and after the three photocatalytic (PC) cycles. Sample I before PC

Sample III before PC

Sample I after Cycle 1

Sample III after Cycle 1

Sample I after Cycle 2

Sample III after Cycle 2

Sample I after Cycle 3

Sample III after Cycle 3

Cu

10.62

9.62

7.74

8.50

9.19

4.24

13.49

11.05

Zn

7.63

6.45

4.77

5.85

6.40

2.82

12.03

7.51

Sn

7.11

6.50

7.04

5.91

6.99

5.33

8.46

4.85

S

22.00

18.84

22.09

15.08

21.41

13.62

22.63

16.06

Ti

4.36

4.74

4.83

4.25

4.18

4.47

8.31

12.59

O

38.93

43.26

46.50

42.77

38.20

50.62

33.98

43.46

Cl

1.75

1.35

0.78

0.28

0.83

0.46

–

–

C

0.0

0.0

0.0

10.62

9.19

15.16

–

–

Substrate elements

7.61

9.24

6.25

6.74

7.85

3.27

–

4.49

Sample/ element [at.%]

TiO2-CZTS photocatalytic thin films Chapter

(a) 80

70

60

Irradia on: G=800 W/m2 □ Cycle1: UV+100% VIS ◊ Cycle2: UV+100% VIS ○ Cycle3: UV+100% VIS

Efficiency [%]

50

40

30

Irradia on: □ Cycle1: UV+100% VIS (G=800 W/m2) ◊ Cycle2: UV+ 50% VIS (G=500 W/m2) ○ Cycle3: UV+ 0% VIS (G=200 W/m2)

20

10

0 0

60

120

180

240

300

Time [min]

360

420

480

540

(b)

FIG. 9 (A) Molecular structure of IMD and (B) Variation of the imidacloprid removal efficiency during the three photocatalytic cycles.

FIG. 10 Variation of the phenol removal efficiency during two photocatalytic cycles.

23

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382 SECTION D Composites and heterojunctions. Tertiary materials

FIG. 11 Transmittance variation of the samples I and III during the photocatalytic cycles using the MB solution.

The photocatalytic tests also allowed evaluating the mineralization efficiency of the imidacloprid, and values were recorded when running Cycle 1. The total organic carbon (TOC) removal efficiency was 25.78%, while the total nitrogen removal efficiency was 6.06%. These results outline that the imidacloprid decomposition up to mineralization runs less efficient (as expected) and the decomposition of the nitrogen-containing intermediates runs following the slowest mechanism(s). The photocatalytic processes were investigated also using solar radiation, in infield conditions. The solar irradiance varied between 110 and 610 W/m2, and photocatalytic removal efficiency of up to 20% was reached after 8 h, regardless of the flowrate (0.5 or 1 L/min). These results show that an optimization step may be required when implementing the photocatalytic processes for advanced wastewater treatment according to the irradiance values of the solar radiation in the implementation location. The phenol decomposition runs with low efficiency (up to 17% after 8 h of irradiation) as the results in Fig. 10 show, following the much lower size of the phenol molecule that allows higher mobility, thus a limited adsorption capacity. The mineralization efficiency of phenol is 4% after 8 h of irradiation, well confirming the stability of this pollutant molecule. A final group of tests was done to evaluate the stability of the samples, by observing the transmittance variation. The results in Fig. 11 outline a decrease in the transmittance values for both samples (I and III) when passing through the three experimental cycles. The transmittance variation is rather different during the first two cycles for the analyzed samples: for the edge sample (Sample I), there is a consistent decrease in the transmittance after Cycle 1, as a possible consequence of surface clogging with by-products or unreacted MB molecules. This decrease is followed by a transmittance increase after Cycle 2, indicating that the clogging effect is reduced. For the centrally positioned sample (Sample III), the transmittance registers a slight increase (as a possible result of the surface “wash-out” effect) after the first cycle, being almost unmodified after the second cycle. After the third cycle, the transmittance variation is almost similar for both samples, in good agreement with the roughness values previously discussed, and maybe the result of the adsorbed molecules on the surfaces with similar RMS values. These results confirm that possible surface changes may occur at the beginning of the use of the thin films but end up in similar final structures after several photocatalysis/regeneration cycles.

5

Conclusions

Two-layered thin film photocatalyst were deposited using an up-scalable technique, spray pyrolysis deposition, on substrates with an overall area of 100 cm2 and were tested in processes under simulated solar radiation with irradiance values and spectral composition close to the infield values. The photocatalytic processes were run in a continuous flow, using a demonstrator photoreactor with an overall surface of the photocatalytic plate of 600 cm2. The results outlined that the first step of the photocatalytic mechanism, the pollutant adsorption on the photocatalyst surface, plays an important role in the process of overall efficiency and strongly depends on the pollutant molecular structure, thus on its removal efficiency.

TiO2-CZTS photocatalytic thin films Chapter

23

383

The best removal efficiency, of about 60%, was recorded for the imidacloprid pesticide. The efficiency remains almost similar regardless of the irradiance value or the spectral composition of the radiation used during the 480 min of irradiation. The lowest efficiency recorded was close to 17% and corresponds to phenol, while in the tests using the MB solution, the efficiency ranged between 22% and 28% depending on the irradiance value of the UV–Vis radiation, after 420 min of irradiation. Based on the surface analysis results, the stability of the photocatalytic plates can be considered good; however, a densification process was observed for the layers subject to more photocatalytic cycles and the plates positioned close to the photoreactor central axis are subject to larger stress during the liquid flow, thus there might be considered a different lifetime for the plates positioned close to the edge and for those close to the central axis of the photocatalytic overall plate. The results outline that the CZTS/TiO2 photocatalytic plates represent promising candidates for advanced solar-active photocatalytic wastewater treatment, if the process is well optimized, considering the aqueous flow and the irradiance value of the radiation. Moreover, these solar-active photocatalytic layers also represent potential candidates for solar-active self-cleaning surfaces, to be employed, e.g., on the opaque parts of the building walls.

Acknowledgments This work was supported by a grant from the Romanian Authority for Scientific Research and Innovation, CNCS/CCDI-UEFISCDI, project number PNIII-P2-2.1-PED-2016-0514, within PNCDI III.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

. https://www.worldometers.info/world-population/world-population-by-year/. World Commission of Environment and Development, Our Common Future, Oxford University Press, 1987. A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (5358) (1972) 37–38. B. Ohtani, Photocatalysis A to Z—what we know and what we do not know in a scientific sense, J. Photochem. Photobiol. Photochem. Rev. 11 (2010) 157–178. L. Andronic, L. Isac, S. Miralles-Cuevas, M. Visa, I. Oller, A. Duta, S. Malato, Pilot-plant evaluation of TiO2 and TiO2-based hybrid photocatalysts for solar treatment of polluted water, J. Hazard. Mater. 320 (2016) 469–478. A. Enesca, M. Baneto, D. Perniu, L. Isac, C. Bogatu, A. Duta, Solar-activated tandem thin films based on CuInS2, TiO2 and SnO2 in optimized wastewater treatment processes, Appl. Catal. B Environ. 186 (2016) 69–76. A. Duta, L. Andronic, A. Enesca, The influence of low irradiance and electrolytes on the mineralization efficiency of organic pollutants using the Visactive photocatalytic tandem CuInS2/TiO2/SnO2, Catal. Today 300 (2018) 18–27. K. Kim, A. Razzaq, S. Sorcar, Y. Park, C.A. Grimes, S.I. In, Hybrid mesoporous Cu2ZnSnS4 (CZTS)–TiO2 photocatalyst for efficient photocatalytic conversion of CO2 into CH4 under solar irradiation, RSC Adv. 6 (2016) 38964–38971. M.P. Suryawanshi, G.L. Agawane, S.M. Bhosale, S.W. Shin, P.S. Patil, J.H. Kim, V. Moholkar, CZTS based thin film solar cells: a status review, Mater. Technol. Adv. Perform. Mater. 28 (1) (2013) 98–109. M. Covei, D. Perniu, C. Bogatu, A. Duta, CZTS-TiO2 thin film heterostructures for advanced photocatalytic wastewater treatment, Catal. Today 321322 (2019) 172–177. M. Covei, C. Bogatu, D. Perniu, I. Tismanar, A. Duta, Comparative study on the photodegradation efficiency of organic pollutants using n-p multijunction thin films, Catal. Today 328 (2019) 57–64. C. Bogatu, M. Covei, D. Perniu, I. Tismanar, A. Duta, Stability of the Cu⁠2ZnSnS⁠4/TiO⁠2 photocatalytic thin films active under visible light irradiation, Catal. Today 328 (2019) 79–84. S. Kermadi, S. Sali, A.F. Ait, L. Zougar, M. Boumaour, A. Toumiat, N.N. Melnik, D.W. Hewak, A. Duta, Effect of copper content and sulfurization process on optical, structural and electrical properties of ultrasonic spray pyrolysed Cu2ZnSnS4 thin films, Mater. Chem. Phys. 169 (2016) 96–104. N. Sebaa, M. Adnane, A. Djelloul, A. Abderrahmane, T. Sahraoui, Effect of increasing concentrations on sprayed Cu2ZnSnS4 thin films, J. Nano Electron. Phys. 11 (2019), 05009. S. Thiruvenkadam, D. Jovina, R.A. Leo, The influence of deposition temperature in the photovoltaic properties of spray deposited CZTS thin films, Sol. Energy 106 (2014) 166–170. R. Zabar, T. Komel, J. Fabjan, M.B. Kralj, P. Trebse, Photocatalytic degradation with immobilised TiO2 of three selected neonicotinoid insecticides: imidacloprid, thiamethoxam and clothianidin, Chemosphere 89 (2012) 293–301. M.D. Liptak, K.C. Gross, P.G. Seybold, S. Feldgus, G.C. Shields, Absolute pKa determinations for substituted phenols, J. Am. Chem. Soc. 124 (2002) 6421–6427. M. Covei, C. Bogatu, D. Perniu, S. Cisse, A. Duta, Comparative study of the electrical properties of CZTS-TiO2 and CZTS-ZnO heterojunctions for PV applications, in: International Semiconductor Conference (CAS), INSPEC: 18280361, 2018.

Chapter 24

Graphene and graphene-oxide for enhancing the photocatalytic properties of materials Federico Cesanoa,b, Vittorio Boffac, Fabrı´cio Eduardo Bortot Coelhoa,c, and Giuliana Magnaccaa,b a

Chemistry Department, Torino University, Torino, Italy, b NIS Interdepartmental Centre, Torino University, Torino, Italy, c Aalborg University,

Center for Membrane Technology, Aalborg, Denmark

1 Short general introduction An atomic or a quasiatomic layer of carbon atoms is the main structure of the so-called 2D carbon materials. Among them, graphene, graphene allotropes and derivatives (i.e., hydrogenated graphene), graphyne (i.e., planar structure of intermittent sp1- and sp2-hybridized carbon atoms), fluorographene (i.e., fluorinated graphene), graphene oxide (i.e., oxidized graphene), reduced graphene oxide, and graphyne and graphdiyne (i.e., graphene introduced by acetylenic chains), are some of the last discovered 2D-based materials [1, 2]. In general terms, the limitation of “one-atomic-thick layer” does not matter when new properties in relation to 3D counterparts (i.e., graphite, graphite oxide) are taken into consideration. Therefore, when a carbon material as thin as about a single atomic carbon layer or just beyond, reveals some unique properties and it could be still considered a 2D carbon material, albeit it is made of one/two/three or more layers [3]. In such cases, these carbon materials have the potential to revolutionize fundamental concepts and make new technologies feasible, including photochemical/catalytic syntheses. Due to the small bandwidth, carbon materials, including graphene, are not suitable for use as a self-directed photocatalyst, but they could work as co-catalysts or as photosensitizers for a photocatalyst [4].

2 Graphene and GO, preparation, composition, structure, behaviors 2.1 Graphene and graphene oxide Graphene is constituted by a monoatomic layer of sp2 hybridized C atoms conceptually obtained from graphite after separation of a single layer, even if, in practice, graphene is always constituted by several layers interacting with each other ˚ [5]. Given the absence of defects, the material is typically through the p-system with a typical interlayer distance of 3.5 A hydrophobic and in dry form tends to form translucent black flakes and/or membranes by sheet agglomeration. Graphene oxide (GO) is a compound of carbon, oxygen, and hydrogen whose relative ratios can be modified mainly through synthesis conditions. It can be imagined considering a graphene sheet where some C atoms are oxidized when oxygen-containing groups are introduced in the structure. These functionalities, typically carboxylic, phenolic, and epoxidic groups, enlarge the separation between two stacked layers and make the material hydrophilic. It typically appears as a yellow to brown material when dried. Given the presence of the functional groups dispersed along the sheets, GO, more than graphene, is prone to functionalization and interaction with other materials for the synthesis of organic-inorganic hybrids. [5].

2.2 Graphene and GO synthesis The synthesis of graphene can be pursued following two different approaches: top-down and bottom-up [6]. In the first approach, graphite is used as a precursor and submitted to mechanical or chemical exfoliation, a procedure in which strong interaction between the two adjacent layers (quantified as interlayer bond energy of 2 eV/nm2) is destroyed Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00015-5 Copyright © 2021 Elsevier Inc. All rights reserved.

385

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D Composites and heterojunctions. Tertiary materials

[7]. As an alternative, GO can be reduced by thermal annealing or chemical procedures: in this case, the material is called reduced-GO (rGO). In the second approach, namely bottom-up, the material is built layer-by-layer. This can be achieved through pyrolysis, epitaxial growth or chemical vapor deposition [8] with carbonaceous gases generating graphene layers, supplying a material with some impurities derived from the used precursors. In these cases, the graphene layer is typically obtained on support and needs to be detached to be used in nanosheets [5]. In the most commonly used top-down approach, GO is usually made by reacting graphite powder with strong oxidants. Various methods have been developed from the first Brodie’s synthesis in 1858 [9], as often the used reactants produce toxic gases. This is also the case of Staudenmaier’s method [10], which was revised and improved to arrive at the modified Hummers’ [11] and improved Hummers’ procedure [12] in 2010. In all cases, the use of permanganate can introduce Mn(II) impurities in the synthesized GO, therefore Peng et al. [13] proposed a low-cost, safe, simple, and environmentally friendly method for large-scale production of GO avoiding permanganate use. The synthesis method affects the structure and physico-chemical properties of GO, as well as the dosage of oxidant and the quality of graphite used as a precursor.

2.3 Graphene and GO structure Graphene and GO preserve the chemical structure of parent graphite and should be composed of only one layer of C atoms. This is the theoretical structure of the materials, but usually few layers are assembled together, and an interlayer distance ˚ as reported above, can be observed. The value observed for graphene corresponds to the value expected for graphite, 3.5 A ˚ whereas GO shows a higher interlayer spacing (even larger than 7 A) due to the sterical hindrance of the oxygenated groups, depending on the oxidation degree [14]. For describing the structure of GO, several structural models were considered, the most widespread use was proposed by Lerf-Klinowski [15]. It provides a hybrid electronic structure containing both the conducting p states from sp2 carbon domains (a poorly functionalized zone) and also the s states from the sp3 carbon domains (highly functionalized region).

2.4 Physico-chemical characterization Graphene and GO can be characterized in terms of structure and chemical composition by means of several techniques. X-Ray Diffraction (XRD) allows to determine the sheet separation and, as a consequence, the oxidation degree; Raman Spectroscopy determines the order/disorder degree observing the C hybridization; X-Ray Photoelectron Spectroscopy (XPS), FTIR spectroscopy, and 13C and 1H Nuclear Magnetic Resonance (NMR) allows to recognize the presence of carboxylic, epoxidic, and phenolic groups; z-potential measurements allow evaluating the surface charge modified by the functionalization degree [14]. Transmission Electron Microscopy (TEM) can supply a direct image of morphology and sheet dispersion and a measurement of the interlayer distance at a nanometric level. The results of the most used characterization techniques can be summarized in Fig. 1 showing the principal features of graphite, graphene (or rGO), and GO. ˚ , correThe ordered structure of graphite can be easily recognized by XRD measurements. A typical distance of 3.5 A ˚ sponding to the (002) plane, produces a sharp signal at 2y
25 degrees, whereas the (004) signal, d ¼ 1.9 A, falls at 2y
55 degrees. Indeed, the introduction of oxygenated groups in the structure causes the enlargement of the interplanar distances ˚ ) and 2y
42 degrees ((100) plane, d ¼ 2.13 A ˚ ) [16]. and the appearance of a signal at 2y
10 degrees ((001) plane, d ¼ 8.9 A ˚ All signals are intended as measured with Cu Ka source (l ¼ 1.54060 A). The Raman spectrum shows two typical signals at 1350 and 1580 cm1, namely D (from Diamond) and G (from Graphite) bands. They are related to A1g breathing mode, typical of a network of sp3 hybridized C atoms, and E2g in-plane mode related to a framework of sp2 hybridized C atoms. The relative intensity value of the two signals is a measure of the order/disorder degree of the structure: a high value indicates a large presence of defects, i.e., oxygenated functional groups [17]. Analogously to Raman, also UV-Vis spectroscopy can supply interesting features about the presence of oxygenated groups in the graphene structure: a signal at 231 nm is typical of p-p* electronic transition of sp2 CdC bond, whereas a shoulder at 303 nm is related to n-p* transitions of nonbonding electrons of unsaturated oxygenated groups [17].

3

Applications in photocatalysis

In this chapter, it is described the topic of photocatalytic applications dedicated to selected synthetic routes, of inorganic and organic compounds (i.e., CO2 reduction, H2O reduction, and other organic fine chemical synthesis under UV-Vis

Graphene and graphene-oxide for enhancing the photocatalytic properties Chapter

24

387

FIG. 1 Sect. A: graphite (solid line), graphene (or rGO) (dotted line), and GO (broken line) diffractograms; Sect. B: GO Raman spectra with an indication of D and G bands; Sects. C and D: TEM images at low and high magnification of graphene sheets on a typical TEM sample holder (in Sect. D the overlapping of several graphene sheets is observable).

radiation), not easily prepared through other preparation methods. It will be shown that the topic is multidisciplinary, ranging from inorganic to organometallic and polymer chemistry.

3.1 H2O reduction The energy of light effectively used to decompose water by using a photocatalyst (TiO2) was described for the first time by Fujishima and Honda [18] as schematized in the following reaction scheme: TiO2 + 2hv ! 2e + 2p + ðexcitation of TiO2 by lightÞ 2p + + H2 O ! ½ O2 + 2H + ðreaction occurring at the TiO2 electrodeÞ 2e + 2H + ! + H2 ðreaction occurring at the Pt electrodeÞ H2 O + 2hv ! + ½ O2 + H2 ðoverall reactionÞ Since that, a great research effort has been undertaken, by using metal oxides consisting of d0 cations (i.e., Ti4+, Zr4+, Ce4+, Nb5+, Ta5+, W6+) or d10 cations (i.e., Zn2+, Ga3+, In3+, Ge4+, Sn4+, Sb5+, Mo6+, etc.) [19]. Along with these library materials, TiO2 is by far the most reported active photocatalyst. As UV light is only ca. 4% of the solar energy spectrum, wide band semiconductors, including TiO2, should be engineered in the bandgap through metal/nonmetal ion doping to enhance photon absorption of the photocatalyst. Together with noble metals, used as sensitizers or co-catalyst, lower cost and higher availability compounds have been proposed, for instance, graphene and GO layers, which are flexible enough to adapt to curved surfaces like TiO2 nanoparticle ones [20]. Lv et al. [21] developed graphene composite materials without the presence of noble metal, demonstrating that graphene, attached to a semiconductor surface (i.e., CdS, TiO2), behaves as platinum in receiving and relocating electrons, thus acting as an effective hydrogen evolution promoter. Afterward, several studies on graphene-based photocatalysts have been developed, based on the concept that a heterojunction

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D Composites and heterojunctions. Tertiary materials

formation can improve the charge separation efficiency and reduce the recombination of the photogenerated electron-hole pairs [22]. In this regard, carbon-based systems, including fullerenes, carbon nanotubes (CNTs), graphene, and its derivatives have been widely used in different heterojunctions as electron mediators or sensitizers to increase the conductivity or the visible light adsorption, respectively [23–25]. The so-called “heterojunction strategy” is well-reviewed by Afroz et al. [26]. Other recent studies, relevant for the formation of sensitized TiO2 for photocatalytic applications, provide evidence of the in-situ formation from acetylene of polycyclic aromatic hydrocarbons (PAHs) and more extended domains at the surface of TiO2 [27, 28] and MoS2/TiO2 hybrids [29, 30]. Contrariwise, graphene can be turned into a semiconductor by doping (e.g., N, P, etc.) or introducing functionalities. For example, two C sublattices can be substituted by O in graphene with the formation of CO covalent bonds, thus impairing the original orbitals and forming a bandgap. Introducing additional O atoms, the band gap is gradually enlarged, the maximum of the valence band slowly shifts from the graphene p orbital to the O 2p orbital, while p* orbital remains at the minimum of the conduction band [31]. Contrary to graphene, GO possesses many O surface functionalities and it works as a perfect material to promote photogenerated charges in the water decomposition process [31]. GO-TiO2 electron transfer mechanism is shown in Fig. 2A. The (hetero)junction obtained between GO (p-type) and TiO2 (n-type) works as a separator of the photogenerated electron-hole pairs. Likewise, a p-n heterostructure for solar light absorption can be formed between ZnO and GO. Usually, GO works as an electron trap for enabling excitonic separation and storing the separated electrons (Fig. 2B) [31]. Karim et al. [32] showed the formation of linear and branched alkylamine between rGO layers. The authors have found that the alkylamine-rGO hybrid serves as a dual proton-electron conductor and works as a pure-organic-water-splitting photocatalyst, with higher photoactivity than TiO2, which was used as a reference material (Fig. 3D). The high photoactivity was ascribed to the N doping of the rGO layers together with the bandgap opening by the longer alkylamine moieties serving as intercalation molecules (Fig. 3A–C).

FIG. 2 Scheme of the p-n heterojunction formation between GO and TiO2 (A); GO layers acting as electron probes for enabling the excitonic separation and storing of the separated electrons (B). (Adapted with permission from T.F. Yeh, J. Cihl
r, C.Y. Chang, C. Cheng, H. Teng, Roles of graphene oxide in photocatalytic water splitting, Mater. Today 16 (3) (2013) 78–84.)

FIG. 3 Representations of the structure of alkylamine-rGO hybrids (A), N-doped rGO scheme highlighting pyridinic, graphitic, pyrrolic N atoms, and N oxidative terminations (B), the opening of the bandgap of alkylamine-rGO hybrids from rGO (zero band gap, C); and photocatalytic H2 generation efficiency by alkylamine-rGO hybrids as compared to bare TiO2 used as a reference material (D). (Adapted with permission from M.R. Karim, M.M. Rahman, A.M. Asiri, S. Hayami, Branched alkylamine-reduced graphene oxide hybrids as a dual proton-electron conductor and organic-only water-splitting photocatalyst, ACS Appl. Mater. Interfaces 12 (2020) 10829–10838.)

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FIG. 4 Hybrids of Py2Mo- (A) and Im2Mo-grafted onto graphene sheets (B); scheme representing the processes of the photoexcited electron transfer and H2 evolution occurring over the Py2Mo/graphene photocatalyst in the presence of TEA under UV-Vis light (C). Reaction yield enhancement is due to the presence of graphene acting both as a photosensitizer and a H2 catalyst. (Adapted with permission from M. Feliz, P. Atienzar, M. Amela-Corts, N. Dumait, P. Lemoine, Y. Molard, S. Cordier, Supramolecular anchoring of octahedral molybdenum clusters onto graphene and their synergies in photocatalytic water reduction, Inorg. Chem. 58 (2019) 15443–15454.)

Feliz et al. [33] have recently reported the preparation of supramolecular adducts with reduced GO. The organic countercations, consisting of octahedral Mo clusters, which are connected with linkers consisting in an imidazolium head and a long alkyl chain or a pyrenic moiety, were grafted on rGO, playing both as a photosensitizer and as a catalyst for H2 production (Fig. 4).

3.2 CO2 photoreduction for CO and fuel production The carbon dioxide, a linear molecule with O]C]O structure containing characteristic s- and p-bonds, is chemically inert and thermodynamically stable. For this reason, the reduction of CO2 is extremely endothermic and energetically difficult. In detail, all standard Gibbs free energies of reactions for the reduction of CO2 to H2O, CO, CH3OH, CH4, HCOOH, and HCHO are very positive (see for example: [34]) and a large quantity of external energy is required to exceed energetic barriers for breaking the C]O bond and then forming the CdH and CdC bonds [35]. Solar light-induced CO2 reduction with H2O using photocatalytic processes is possible. Usually, upon the absorption of photons with energies as high as the energy bandgap (Eg) of the semiconductor, the photoactive material is excited and electron-hole pairs (charge carriers) are generated [36] (Scheme 1). Later, such isolated charge carriers can shift at the active sites of the catalyst (routes 1 and 2 in Scheme 1) and subsequently be transferred to the adsorbed molecules (CO2 and H2O). Water is oxidized to O2 by the hole with the release of H+, while CO2 is reduced by electrons with the assistance of H+ and entails a series of reactions to form CO and/or fuels (mainly CH3OH and CH4) (proton-assisted-multi-electron reduction processes) [37]. Nevertheless, the charge carriers could recombine in the bulk (route 3 in Scheme 1) or on the surface (route 4 in Scheme 1) of the semiconductor with the release of heat or photons. The latter processes, known as surface recombination or bulk recombination processes are both disadvantageous to the photocatalyst efficiency. The addition of graphene to the photoactive material can enhance the separation of charge carriers and inhibit the recombination paths. Furthermore, graphene may work as an electron acceptor, thus inducing the electron transfer from the photoactive material and guiding an additional path, nonradiative, for the spatially separated charge carriers and their shift (route 5 in Scheme 1). It is worthy of attention the fact that the high surface area of graphene and the strong p-p conjugation with CO2 favor the CO2 adsorption.

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SCHEME 1 Representation of photocatalytic CO2 reduction to CO and fine chemicals over graphene-based systems in the presence of water. With photoexcitation, electron-hole pairs are spatially separated and free to move. 1, 2, 3, 4, and 5 refer to different pathways of charge carriers: 1 and 2 are related to transfer to the adsorbed molecules, 3 and 4 to bulk and surface recombinations, and 5 to graphene working as an electron acceptor. VB and CB refer to the valence and conduction bands, respectively. (Inspired to M.Q. Yang, Y.J Xu, Photocatalytic conversion of CO2 over graphene-based composites: current status and future perspective, Nanoscale Horiz. 1 (2016) 185–200.)

Yang et al. [35] and Chen et al. [38] have recently reviewed 2D nanomaterials, including graphene-based ones, for CO2 reduction. In another recent paper, Wang et al. [39] obtained a high CO2 uptake in a porous-hypercrosslinked polymer/TiO2/graphene composite (Fig. 5A) with a relatively high surface area (988 m2/g). The composite exhibited high photocatalytic performance, especially for CH4 production (27.62 mmolg1 h1). The authors attributed such high photoactivity to the combination of photocatalytic activity ascribed to TiO2-graphene coupled system and to the sorption properties of the porous hyperbranched-polymer structure, respectively (Fig. 5B and C). GO is commonly used in photocatalytic reactions as a co-catalyst, but limited data are available about the intrinsic ability of GO for the photocatalytic reduction of CO2 free of other photocatalysts. Kuang et al. [40] have recently shown the ability of GO in the activation of light irradiation and photocatalytic reduction of CO2. The authors illustrated two important roles played by light irradiation: (i) helping in the CO release through photolysis and (ii) formation of surface defects and restoring the large p-conjugated network (Fig. 5D and E).

3.3 Other photocatalytic syntheses The photocatalytic activation of CO2 described in the previous paragraph leads to the formation of a restricted number of fine chemicals obtained by photocatalytic processes. In a very recent paper, Radhika et al. [41] reviewed some other synthetic processes triggered by light for the synthesis of inorganic and organic compounds, including those involving graphene and GO. In another paper, Yang et al. [42] reported the selective photocatalytic oxidation of various substituted benzyl alcohols to the corresponding aldehydes or ketones over a Fe-based metal-organic framework (MIL53-Fe)/graphene hybrid catalyst under visible light irradiation and ambient conditions. The authors reported selectivity of 99% for aldehydes or ketones and attributed the enhancement of the photoactivity to the decrease of the electron-hole recombination rate due to the presence of graphene into the MIL-53(Fe) structure. Roy et al. [43] reported the photocatalytic reduction of nitrobenzene to form aniline upon UV light irradiation (l < 420 nm) over a graphene-ZnO-Au photocatalyst. The high photoactivity and yield were ascribed to the simultaneous presence of graphene and Au nanoparticles with ZnO. Graphene acted as an inhibitor toward the electron-hole recombination and electron transport properties, while the Au nanorods assisted in efficient charge separation. TEM, XRD, and Raman analysis of the hybrid material revealed that the small nanoparticles and nanosheets work in a synergistic manner: e are rapidly transferred from ZnO to graphene, minimizing the charge carrier recombination, bringing about a rapid reduction of nitrobenzene in methanol, which acted as a hole trap to avoid the recombination of the electron-hole pairs. As a matter of fact, it was hypothesized a more energetically favorable path of the photogenerated electrons, which are transferred from the semiconductor conduction band toward graphene and then to the Fermi energy level of Au nanoparticles, rather than through a direct path from the semiconductor to Au [41].

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FIG. 5 Porous polymer/TiO2/graphene: representation of the composite structure (A), volumetric CO2 adsorption/desorption isotherms at 273 K (B), and CO2 reduction photocatalytic performance as compared to standard materials (C). CO2 reduction over photoactivated GO (D and E): release of CO through photolysis after irradiation and formation of surface defects and large p-conjugated network on GO by irradiation. (Panel (A–C) adapted from S. Wang, M. Xu, T. Peng, C. Zhang, T. Li, I. Hussain, J. Wang, B. Tan, Porous hypercrosslinked polymer-TiO2-graphene composite photocatalysts for visible-lightdriven CO2 conversion, Nat. Commun. 10 (676) (2019) 1–10 licensed under CC BY 4.0; Panel (D and E). adapted with permission from Y. Kuang, J. Shang, T. Zhu, Photoactivated graphene oxide to enhance photocatalytic reduction of CO2, ACS Appl. Mater. Interfaces 12 (3) (2020) 3580–3591.)

4 GO and rGO in hybrid materials for water depollution and disinfection Photocatalytic materials can achieve cost-effective mineralization of water organic pollutants and deactivation of pathogens without the addition of chemicals. In reason of that, over the past years, they have been proposed for remediation and treatment of a large variety of water and wastewater streams. TiO2 [44], ZnO [45], ZrO2 [46], and WO3 [47, 48] are among the semiconductor materials used for this application. However, when used alone, these materials often show important drawbacks, which prevent their application on a commercial scale. For instance, TiO2 is the most widely used material for the photocatalytic depollution of water, in reason of its high photocatalytic activity under UV light, easy synthesis, low cost, and stability. Nevertheless, the different crystal structures of TiO2 have a band gap too wide for exploiting the visible radiation of the solar spectrum, making them scarcely active when used in combination with sunlight. In this context, the synergistic combination of GO and rGO with semiconductors allows for photocatalytic composite materials with enhanced properties. According to Elimelech et al. [49], graphene-based materials can enhance the photocatalytic activity of TiO2 for the degradation of pollutants at different stages of the oxidation process. First, attachment on rGO may create a heterojunction, allowing for better exploitation of the light energy (Fig. 2A). Once the electron-hole pair is generated, the electron can be quickly transferred to the graphene-based material through the sp2-hybridized network [50], thus hindering electron-hole recombination and facilitating the generation of oxidative species (Fig. 2B). The last stage consists of the adsorption of the pollutant on the graphene-based material. In the case of aromatic molecules, this interaction can occur through p-p stacking. However, residual functional groups on rGO can interact with polar or charged moieties as well. Upon adsorption, oxygen reactive species generated at the oxide surface can efficiently degrade the pollutant (Fig. 6). Here we should stress that the synergistic interaction between the graphene material and the semiconductor is strongly dependent on the material composition, synthesis conditions, and substrate to be degraded. Hence, we give a short review of the conditions that are most appropriate for the synthesis and the application of these composite materials.

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FIG. 6 Representation of the abatement of an organic pollutant molecule adsorbed by TiO2 nanoparticle-anchored rGO, close to the position where reactive oxygen species (ROS) are generated. rGO ribbon and TiO2 sections are here TEM imaged.

4.1 Synthesis of hybrid materials Graphene-based photocatalysts can be prepared from preformed semiconductor particles or by in situ growth and crystallization from precursors (e.g., titanium tetrachloride [51] or titanium (IV) isopropoxide [52]). The first approach has the advantage that the photocatalyst can be synthesized in conditions, which allow for controlling its particles size, shape, and crystal phase(s) and therefore to optimize its morphology for the intended application. For example, one of the most common routes for the preparation of this nanocomposite is mixing a dispersion of graphene oxide (GO) with the commercial titania powder P25 (Degussa Evonik), which is the benchmark photocatalyst for the abatement of water pollutants [53]. GO surface is negatively charged even at acidic pH, therefore electrostatic attraction between GO and the positive charge semiconductor nanoparticles (e.g., TiO2 [54] and BiVO4 [55]) assures good interaction between the two materials during synthesis. In reason of that, with this method, it is possible to achieve a homogenous dispersion of the semiconductor nanoparticles over GO sheets, which can be subsequently reduced to rGO by thermal or hydrothermal treatment. The second approach consists of the in-situ nucleation and growth of the photocatalytic nanoparticles in GO dispersions by sol-gel or hydrothermal methods. In this way, the oxygen functional groups on GO can coordinate with the metal oxide precursors thus assuring a strong interaction and interphase between photocatalyst nanoparticles and GO in the consolidated final material. Moreover, Naknikham et al. [56] observed that the presence of GO during synthesis can reduce particle sized and agglomeration, by providing a large number of nucleation sites, but does not influence the type of crystal phase. In addition, calorimetric studies show that the strong chemical bonding between TiO2 and GO retards the thermal transitions from GO to rGO (e.g., from 149°C to 198°C) and from amorphous TiO2 to anatase phase (e.g., from 470°C to 480°C [56]).

4.2 GO loading and degree of reduction The amount of GO loaded in the nanocomposite is paramount for its photocatalytic performances. Indeed, if on the one hand a wide interface is needed for the synergistic interaction between GO and the ceramic semiconductor; on the other hand, high concentrations of GO prevent the light to excite the photocatalytic centers, thus decreasing the efficiency of the catalyst [50, 57–59]. Nevertheless, a broad range of optimum GO loading can be found in the literature. For instance, Naknikham et al. studied photodegradation of phenol by TiO2 with GO loading ranging from 0 to 1 wt% [60] The sample with 0.05 wt% GO loading showed the highest degradation rate under simulated solar light, while samples with GO loading 0.5 wt% had lower activity than the pure TiO2 reference. On the contrary, Calza et al. [61] reported enhanced catalytic activity for composites P25 catalysts containing 20 wt% of reduced GO (rGO) in the degradation of an antipsychotic compound, namely risperidone, both under simulated solar light and visible light. These differences are not surprising, considering that the term “graphene oxide” refers to a large class of graphene-based materials with different degrees of oxidation. The concentration of oxide functions has a strong impact on the electronic properties of GO and therefore on its ability to interact with light and ceramic photocatalytic nanoparticles. Moreover, the more reduced is GO, the higher is the tendency for restacking, and the lower is its dispersibility during the synthesis of the nanocomposite, which implies lower interaction with the photocatalyst. Nevertheless, both GO and rGO have an intrinsic 2D structure and therefore they can offer higher specific interphase than the ceramic semiconductor nanoparticles. For this reason, optimum photocatalytic performances are typically reported for low GO loadings. As discussed above, GO is characterized by a high concentration of oxygen-containing functional groups, a considerable fraction of which can be removed by heating at mild temperature (T > 200°C) or by chemical reaction with a reducing agent, e.g., hydrazine [62]. Therefore, the photocatalytic activity of GO-TiO2 nanocomposites can be enhanced by a postsynthesis treatment designed to partially reduce GO and thus enhance its electron carrier and light absorption

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abilities. Moreover, exposure to UV-Vis light can also induce the partial reduction of GO, i.e., during photocatalysis. This process is undesired because it can affect the performances of the photocatalyst and at the same time the degradation of GO can contaminate water with side-products, which are potentially toxic. However, it has been shown that these side-products can be mineralized by the photocatalyst after prolonged irradiation times [63].

4.3 Oxide nanoparticles and organic substrates Over the past years, a large research effort has been addressed in the optimization of rGO-TiO2 photocatalysts [8, 64]. Nevertheless, synergisms with rGO in the degradation of organic pollutants have been reported also for other semiconductor nanoparticles, such as tin oxide (SnO2) [65], zinc oxide (ZnO) [66], tungsten oxide (WO3) [67] silver orthophosphate (Ag3PO4) [68], bismuth vanadate (BiVO4) [55], cadmium sulfide (CdS) [69], and zinc sulfide (ZnS) [70]. rGO-TiO2 and the other rGO-photocatalyst materials were successfully applied to the degradation of a broad spectrum of water contaminants of emerging concern. Among them, we can find pharmaceuticals (e.g., acetaminophen [71], carbamazepine [57], diclofenac [63], diphenhydramine [72], ibuprofen [73], and risperidone [61]), pesticides (e.g., diuron, alachlor, isoproturon, and atrazine [74]), and industrial additives (e.g., bisphenol A [75]). Despite that, the ability of graphene-based materials to improve degradation kinetics for all substrates is still under debate. Indeed, as shown in Fig. 6 the photocatalytic activity of these nanocomposites depends also on the interaction of the pollutant molecules with the carbon matrix. Amal and Ng [76] studied the enhancement of the photocatalytic performance of TiO2 by adding rGO for various types of organic pollutants. They observed that the length of the alkyl chain in alcohols and carboxylic acids has a negligible influence on the overall photodegradation activity of the nanocomposite. On the contrary, the density of hydroxyl groups and the presence of aromatic rings in the organic pollutants can promote their interaction with rGO (i.e., their adsorption in the proximity of the TiO2 photocatalytic particles), thus enhancing the photodegradation activity of the nanocomposite.

4.4 Photocatalytic membranes Among the promising applications of GO, there is fabrication of molecular selective membranes [77–80]. In this context, a few GO-TiO2 photocatalytic membrane concepts have been proposed in the recent years with the aim to exploit the filtering properties of GO layers and the synergistic interaction of GO with TiO2 for the photocatalytic abatement of the organic pollutants, for the inactivation of pathogens and for keeping the surface of the membrane clean during filtration. Such type of membranes was prepared according to the procedure, which is sketched in Fig. 7A, both as free-standing films [81] or

FIG. 7 (A) Steps for the fabrication of a rGO-TiO2 membrane and example for its application in dead-end filtration module irradiated by UV LEDs: (B) open module and (C) close unit. (Panel (B and C) are reprinted from N.W. Jiang, D. Wang, Y. Liu, W. Nie, J. Li, F. Wu, P. Zhang, P. Biswas, J.D. Fortner, Engineered crumpled graphene oxide nanocomposite membrane assemblies for advanced water treatment processes, Environ. Sci. Technol. 49 (2015) 6846–6854 (https://pubs.acs.org/doi/10.1021/acs.est.5b00904, further permission related to the material excerpted should be directed to the ACS).)

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deposited on porous supports, e.g., cellulose acetate (pore size: 0.45 mm) [82], poly(ether sulfone) (pore size: 20 nm) or ultrafiltration ceramic monoliths (pore size: 10–50 nm) [83]. Up to now, these devices have been tested in dead-end filtration modules, as the one depicted in Fig. 7B and C, that is, in experimental conditions that are still far from the full-scale cross-flow filtration systems. Nevertheless, GO-TiO2 membranes present several advantages over pure TiO2 photocatalytic layers, including outstanding water permeability, high rejection of organic pollutants and to be flexible and easy-to-coat on a large variety of supports. GO-TiO2 membranes have been shown to be effective in the retention and abatement of various model pollutants, such as Acid Orange 7, Methylene Blue, and Methyl Orange. Moreover, they can reject and partially degrade model fouling agents, such as humic acid [84], thus preserving the permeate flux during filtration.

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Vardhaman, D. Barpuzary, C.V. Sastri, M. Qureshi, Visible light assisted photocatalytic hydrogen generation and organic dye degradation by CdS–metal oxide hybrids in presence of graphene oxide, RSC Adv. 2 (2012) 12122–12128. [70] Y. Zhang, N. Zhang, Z.-R. Tang, Y.-J. Xu, Graphene transforms wide band gap ZnS to a visible light photocatalyst, the new role of graphene as a macromolecular photosensitizer, ACS Nano 6 (11) (2012) 9777–9789. [71] H. Tao, X. Liang, Q. Zhang, C.-T. Chang, Enhanced photoactivity of graphene/titanium dioxide nanotubes for removal of acetaminophen, Appl. Surf. Sci. 324 (2015) 258–264. [72] S. Morales-Torres, L.M. Pastrana-Martı´nez, J.L. Figueiredo, J.L. Faria, A.M.T. Silva, Graphene oxide-P25 photocatalysts for degradation of diphenhydramine pharmaceutical and methyl orange dye, Appl. Surf. Sci. 275 (2013) 361–368. [73] L. Lin, H. Wang, P. 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Yue, Carbon-based building blocks for alcohol dehydration membranes with disorder-enhanced water permeability, Carbon 118 (2017) 458–466. [78] R.R. Nair, H.A. Wu, P.N. Jayaram, I.V. Grigorieva, A.K. Geim, Unimpeded permeation of water through helium-leak-tight graphene-based membranes, Science 335 (2012) 442–444. [79] J.R. Werber, C.O. Osuji, M. Elimelech, Materials for next-generation desalination and water purification membranes, Nat. Rev. Mater. 1 (2016) 16018. [80] H. Huang, Y. Ying, X. Peng, Graphene oxide nanolayer: an emerging star material for novel separation membranes, J. Mater. Chem. A 2 (2014) 13772–13782. [81] L. Zhu, L. Gu, Y. Zhou, S. Cao, Direct production of a free-standing titanate and titania nanofiber membrane with selective permeability and cleaning performance, J. Mater. Chem. 21 (2011) 12503–12510. [82] P. Gao, Z. Zhaoyang, M. Tai, D.D. Sun, W. Ng, Multifunctional graphene oxide-TiO2 microsphere hierarchical membrane for clean water production, Appl. Catal. 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Chapter 25

Nanostructured composites based on Bi and Ti mixed oxides for visible-light assisted heterogeneous photocatalysis Gregor Zˇerjav and Albin Pintar Laboratory for Environmental Sciences and Engineering, Department of Inorganic Chemistry and Technology, National Institute of Chemistry, Ljubljana, Slovenia

1 Introduction Heterogeneous photocatalysis is one of the advanced oxidation processes (AOPs) which are commonly used for wastewater treatment [1, 2]. Common to all AOPs is the generation of reactive oxygen species (ROS) and employing them to oxidize water-dissolved organic compounds into CO2 and water and associated inorganic species in the final process called mineralization. In heterogeneous photocatalysis, a catalyst (semiconductors (SC)) upon illumination with light (energy of the light must be same or greater than the band gap (BG) energy of SC) produces electrons (e
). Electrons are moved from the valence band (VB) of the SC into the conduction band (CB) and leave holes (h+) in the VB. Generated charge carriers can then diffuse to the catalyst surface and participate in reduction/oxidation reactions to produce ROS. On the other hand, if the charge carriers are not used for generating ROS the e
and h+ can recombine and produce for example heat. Consequently, the selection of an appropriate SC is of great importance to deal with a successful heterogeneous photocatalytic system. The other two important elements are the configuration of the reactor system and the light source. One of the most used materials as a photocatalyst, for the process of heterogeneous photocatalytic wastewater treatment, is TiO2 [3–5]. The advantages of TiO2 are photo- and chemical stability, low cost, water insolubility under most reaction conditions, and non-toxicity. The major drawbacks of TiO2 are its high BG energy which allows us to use only UV light illumination to trigger its catalytic activity, and fast recombination of e
-h+ pairs. One of the approaches to overcome the drawbacks of TiO2 would be to combine it with another semiconductor with lower BG energy and to use this semiconductor at the same time as a visible-light photosensitizer [6–9]. In general, the drawback of semiconductors with low BG energy is that due to the narrow BG the recombination of visible-light generated charge carriers is favored. However, the combination of such materials with TiO2 would enable us to slow down the charge carrier recombination as TiO2 would act as a sink for visible-light charge carriers generated by the low BG semiconductor. An appropriate visible-light photosensitizer candidate would be the low BG energy semiconductor Bi2O3. The BG energy of Bi2O3 is, depending on the Bi2O3 polymorph, in the range between 2.4 eV for tetragonal b-Bi2O3 and 2.8 eV for monoclinic a-Bi2O3 [10–14]. The combustion synthesis procedure was used to synthesize Bi2O3 as it offers several advantages over other synthesis procedures that were used in the past [15–19]: the synthesis is reproducible and can be performed at low temperatures, the synthesis time is short, the handling is user friendly and the crystallinity and purity of the synthesized material is high. Bismuth nitrate (Bi(NO3)3  5H2O) was used as the starting material for Bi2O3 and dissolved in HNO3, where also citric acid (C6H8O7  H2O) was present as a fuel. During the synthesis, bismuth nitrate is transformed into bismuth carbonate ((BiO)2CO3), which upon thermal decomposition forms different Bi2O3 polymorphs: (BiO)2CO3 ! b-Bi2O3/ (BiO)2CO3 ! b-Bi2O3 ! b-Bi2O3/a-Bi2O3 ! a-Bi2O3 (decomposition temperature from 250°C to 500°C) [[20, 21]. In our case, we utilized the decomposition temperature of 300°C, because utilizing higher temperatures would result in the formation of a-Bi2O3 polymorph with BG energy of 2.8 eV [20]. In the case of TiO2 + Bi2O3 composite synthesis, TiO2 was added to the HNO3 solution containing bismuth nitrate and citric acid; the resulting material was calcined at 300°C, too. More details about the synthesis procedure of concerned materials can be found in [22–24]. Further, we will discuss the importance of an optimal loading of b-Bi2O3 to obtain a TiO2 + b-Bi2O3 composite with maximal photocatalytic

Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00020-9 Copyright © 2021 Elsevier Inc. All rights reserved.

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activity, and describe how the properties of TiO2 + b-Bi2O3 composites are influenced by the morphology and crystallinity of used TiO2.

2

Influence of Bi2O3 loading on the properties of TiO2 + b-Bi2O3 composites

To investigate the influence of Bi2O3 loading on the properties of TiO2 + b-Bi2O3 composites, we synthesized materials where we used commercially available TiO2 from company ChristalACTiV (DT-51, assigned as TiO2-P) as TiO2 support and varied the ratio between Ti and Bi from 1:0.2 to 1:0.8. The results of scanning electron microscopy (SEM), transmission electron microscopy (TEM) (see Fig. 1) and N2 adsorption/desorption measurements reveal that the TiO2-P solid is present in the form of ellipsoid shaped nanoparticles with a length of around 30 nm and a diameter

FIG. 1 SEM images of TiO2-P and TiO2-P + Bi2O3 composites with different ratios between Ti and Bi. The images below SEM micrographs show the experimental selected area electron diffraction (SAED) patterns of the prepared samples and comparison with simulation patterns of b-Bi2O3 and anatase TiO2. Details about the SEM and TEM measurements can be found in our previous publication [23].

Nanostructured composites based on Bi and Ti mixed oxides Chapter

25

399

of around 20 nm, and the specific surface area equals to 86 m2/g. On the other hand, the synthesized pure b-Bi2O3 is shaped as nanoplates/flakes with a thickness of around 300; the resulting specific surface area of only 4.7 m2/g was observed. Also, a difference in the total pore volume of b-Bi2O3 (0.02 cm3/g) and TiO2-P (0.29 cm3/g) is significant. The morphology of TiO2-P + Bi2O3 composites is similar to the morphology of pure TiO2-P with decreasing the specific surface area from 79 to 60 m2/g with an increasing amount of b-Bi2O3 in the composite. Also, the total pore volume of TiO2-P + Bi2O3 composites decreased in the same trend from 0.26 to 0.20 cm3/g. Selected area electron diffraction (SAED) patterns of TiO2-P + Bi2O3 composites show that only anatase TiO2 and b-Bi2O3 are present in the synthesized composites (Fig. 1). These results are in good agreement with the findings of XRD examination (Fig. 2), where only diffraction peaks belonging to anatase TiO2 and b-Bi2O3 are present. The intensity of b-Bi2O3 diffraction lines in the XRD diffractograms of TiO2-P + Bi2O3 composites was increasing with the increasing amount of b-Bi2O3, which is logical since also the SEM–EDX elemental analysis revealed that the actual ratio between Ti and Bi was the same as the nominal ratio. Furthermore, XRD results also showed that the average crystallite size of b-Bi2O3 in the composites increased with the increasing amount of b-Bi2O3 and was in the composite with the highest amount of Bi2O3 (31 nm) almost the same as for pure b-Bi2O3 (33 nm). This indicates that if the amount of b-Bi2O3 in the composite is too high, it might start to agglomerate and form a separate b-Bi2O3 phase which might not be in appropriate contact with TiO2. The ability of synthesized TiO2-P + Bi2O3 composites to absorb light in the visible range was confirmed using UV–vis DR measurements. The obtained results are illustrated in Fig. 3. In the range between 400 and 550 nm, we can observe a strong absorption, which is increasing with the increasing amount of b-Bi2O3. The influence of TiO2-P onto the acquired UV–vis DR spectra of the composites is expressed in the range below 400 nm and corresponds to the absorption of UV light. Electrochemical impedance spectroscopy (EIS) analysis was used to test the ability of prepared materials to generate charge carriers upon visible-light illumination on one hand and on the other to test the separation of generated e
-h+ pairs (Fig. 4). The results show that the composites can generate charge carriers under visible-light illumination as the diameters of semicircles in the Nyquist plots of the composites are smaller in comparison to the diameter of the semicircle belonging to TiO2-P. The results further reveal that there is a junction between TiO2-P and b-Bi2O3 and that TiO2-P is acting as a sink for visible-light generated carriers from b-Bi2O3 as the diameter of the semicircle in the b-Bi2O3 Nyquist plot is larger than that of the composites. Under visible-light illumination, the composites have more charge carriers available to participate in generating ROS than pure TiO2-P and b-Bi2O3. One can also clearly see that if the amount of b-Bi2O3 in the composites is too high (the ratio between Ti:Bi exceeds 1:0.4), the efficiency for separation of charge carriers drops. Other authors b-Bi2O3

TiO2-P TiO2-P+b -Bi2O3=1:0.2 TiO2-P+b -Bi2O3=1:0.4

Intensity, a.u.

TiO2-P+b -Bi2O3=1:0.8 a-TiO2-R a-TiO2-R+Bi2O3 TiO2-R TiO2-R+Bi2O3

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

2 Theta, degree FIG. 2 XRD patterns of pure TiO2 support and pure b-Bi2O3 as well as their composites (green vertical lines (light gray in print version) represent bBi2O3, blue lines (dark gray in print version) (BiO)2CO3, and red lines (dark gray in print version) anatase TiO2). Details about the experimental conditions of the XRD measurements are provided in our previous publications [22, 23].

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attributed these observations to the phenomena that the excess amount of b-Bi2O3 might act as a recombination center for generated charge carrier pairs [25]. The ability of TiO2-P + Bi2O3 composites to generate more charge carriers under visible-light illumination compared to pure TiO2-P and b-Bi2O3 solids was also well expressed in the experiments of visible-light assisted degradation of waterdissolved endocrine-disrupting compound bisphenol A (BPA) depicted in Fig. 5. These results also demonstrate that due to the junction between TiO2-P and b-Bi2O3 the photocatalytic efficiency of TiO2-P + Bi2O3 composites under visible-light illumination is improved. However, if the amount of b-Bi2O3 present in the composites is too high, the catalytic activity drops. Other researchers also noticed that there existed an optimal amount of added Bi2O3 to obtain TiO2 + Bi2O3 composites with the highest photocatalytic activity [11, 12, 14]. The decline in photocatalytic activity with the increasing amount of b-Bi2O3 can be attributed to several phenomena: (a) the junction between TiO2 and agglomerated Bi2O3 is not appropriate; (b) the visible-light photosensitizing effect is reduced as the excess of Bi2O3 agglomerates and is therefore not well dispersed in the composite, and (c) with the increasing amount of Bi2O3 the specific surface area of composites decreases. The results of total organic carbon (TOC) content measurements of initial and end-product BPA solutions in the combination with the elemental CHNS analysis of fresh and spent catalyst samples importantly reveal that concerning TiO2-P + Bi2O3 composites, the main pathway of BPA removal from the aqueous solution under visible-light illumination was in all cases mineralization of BPA and/or BPA degradation products into water and CO2 (TOCM), and to a lesser extent adsorption of organic species onto the catalyst surface (TOCA, Table 1). Based on the above presented results of performed measurements, we can provide a plausible charge carrier migration cascade in TiO2-P + Bi2O3 composites under illumination with visible light (Fig. 6). To calculate the valence band edge (EVB) of TiO2 and b-Bi2O3, the Mulliken electronegativity theory [26, 27], expressed by Eq. (1), was used: EVB ¼ X
Ee + 0:5∗EBG ,

(1)

where the VB edge potential is EVB, the electronegativity of the semiconductor is X and the energy of free electrons on the hydrogen scale is Ee (about 4.5 eV). From the UV–vis DR measurements (Fig. 3) we can obtain the band-gap energies (EBG) of the semiconductors. The measured EBG values are 3.2 eV for TiO2 and 2.45 eV for b-Bi2O3. Based on the literature data [27], their corresponding electronegativities equal to 5.81 and 6.21 eV, respectively. The conduction band edge (ECB) was calculated using Eq. (2): ECB ¼ EVB
EBG

(2)

The calculated values of EVB and ECB for TiO2-P are 2.91 and
0.29 eV, and for b-Bi2O3 they equal to 2.93 and 0.48 eV. One of the reasons for the poor photocatalytic activity of b-Bi2O3 as observed in Fig. 5 is that the ECB of b-Bi2O3 is too low (i.e., 0.48 eV) to provide the sufficiently negative potential for the visible-light generated e
to oxidize the surface adsorbed

TABLE 1 The removal of total organic carbon (TOC) content from aqueous BPA solutions at the end of the degradation run and the amount of real TOC mineralization (TOCM) and TOC accumulation (TOCA). Further information about the experimental conditions of obtaining TOC results is provided in our publications [22, 23]. TOC

TOCM

TOCA

Catalyst

%

b-Bi2O3

5

1

4

TiO2-P

0

0

0

TiO2-P + Bi2O3 ¼ 1:0.2

14

11

3

TiO2-P + Bi2O3 ¼ 1:0.4

23

20

3

TiO2-P + Bi2O3 ¼ 1:0.8

18

16

2

a-TiO2-R

0

0

0

a-TiO2-R + Bi2O3

21

16

5

TiO2-R

11

4

7

TiO2-R + Bi2O3

51

43

8

Nanostructured composites based on Bi and Ti mixed oxides Chapter

25

401

1.8

b-Bi2O3 TiO2-P TiO2-P+Bi2O3=1:0.2 TiO2-P+Bi2O3=1:0.4 TiO2-P+Bi2O3=1:0.8 a-TiO2-R a-TiO2-R+Bi2O3 TiO2-R TiO2-R+Bi2O3

1.6

Absorbance, a.u.

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 250

300

350

400

450

500

550

600

650

700

Wavelength, nm FIG. 3 UV–Vis DR spectra of prepared materials. The experimental conditions of the UV–Vis DR measurements can be found in our previous publications [22, 23].

20000

b-Bi2O3

TiO2-P TiO2-P+Bi2O3=1:0.2 TiO2-P+Bi2O3=1:0.4 TiO2-P+Bi2O3=1:0.8

17500 15000

FIG. 4 Nyquist plots of pure TiO2-R and b-Bi2O3 and their composites under visible-light illumination. The experimental conditions of the electrochemical impedance measurements can be found in our previous publication [23].

-Z'', ohm

12500 10000 7500 5000 2500 0 0

5000

10000

15000

20000

25000

30000

35000

40000

45000

Z', ohm

O2 into O2
(E (O2/O–2) ¼
0.33 V vs NHE and E (O2/O2H) ¼
0.05 V vs NHE) through a fast single-electron reaction and so the charge carrier recombination is favored [28, 29]. Results of UV–vis DR measurements show that in TiO2 + Bi2O3 composites visible-light generated h+ in the b-Bi2O3 VB can be transferred to the upper lying TiO2 VB via a heterojunction [30] and produce hydroxyl radicals (Eq. 3) OH
+ h + ! OH∙

(3)

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D Composites and heterojunctions. Tertiary materials

1.0 0.9 0.8 0.7

C/C0, /

0.6 0.5

b-Bi2O3 TiO2-P TiO2-P+Bi2O3=1:0.2 TiO2-P+Bi2O3=1:0.4 TiO2-P+Bi2O3=1:0.8 a-TiO2-R a-TiO2-R+Bi2O3 TiO2-R TiO2-R+Bi2O3

0.4 0.3 0.2 0.1 0.0 0

15

30

45

60

75

90

105

120

Time, min FIG. 5 Photocatalytic degradation of water-dissolved bisphenol A (BPA, c0 ¼ 10 mg/L) under the visible-light irradiation (halogen lamp, 150 W, lmax ¼ 520 nm, UV cut off the filter at 410 nm) in the presence of the prepared catalysts (125 mg/L). More details about the experimental conditions of BPA degradation can be found in our previous publications [22, 23].

The e
in the b-Bi2O3 CB can participate in the reactions shown in Eqs. (4) and (5) (depending on the pH value) [31–33]: O2 + 2H + + 2e
! H2 O2

EO ¼ + 0:682 V ðvs NHEÞ

(4)

O2 + 4H + + 4e
! 2H2 O

EO ¼ + 1:23 V ðvs NHEÞ

(5)

A p-n junction can be established between the p-type semiconductor b-Bi2O3 and the n-type semiconductor TiO2-P if there is a tight chemical bonding between them [10]. After a p-n junction is formed, an equilibrium state of the semiconductors’ Fermi levels (EF) is established meaning that the Fermi level of TiO2-P is moved down and the Fermi level of b-Bi2O3 is moved up. This in turn results in the formation of an inner electric field at the surfaces of the semiconductors [34]. The formation of a p-n junction allows the transfer of visible-light generated e
in the b-Bi2O3 CB to TiO2 CB. As illustrated in Fig. 6, we can distinguish between the roles of each component in the TiO2-P + Bi2O3 composites. The role of b-Bi2O3 is to act as a visible-light photosensitizer and the role of TiO2-P is to be the sink for visible-light generated b-Bi2O3 charge carriers, as the BG energy of TiO2-P is too high to make it photocatalytically active under visible-light illumination.

3

Influence of TiO2 support on the properties of TiO2 + b-Bi2O3 composites

Two major factors are influencing the overall kinetics of photocatalytic reactions. First is the adsorption of substrates onto the catalyst surface and their reduction/oxidation by charge carriers. The second is the rate of recombination of charge carriers. Several studies report that in comparison to crystalline materials amorphous solids exhibit lower or negligible photocatalytic activity [35–40] due to the recombination of e
-h+ pairs at defects in the bulk. Contrary to this studies are showing that high specific surface area amorphous/disordered materials display higher catalytic activity vs their crystalline counterparts with a lower specific surface area [41–48]. This is ascribed to the fact that charge carriers have only to diffuse a short distance, due to smaller bulk, to reach the catalyst surface and that due to higher specific surface more substrates are adsorbed and available to react with charge carriers. So ideal TiO2 support for the TiO2 + b-Bi2O3 composite should exhibit: (i) high crystallinity to hinder the e
-h+ recombination, and (ii) large specific surface area to better adsorb substrates. To identify how structural parameters (high specific surface area vs crystallinity) of TiO2 support impact the photocatalytic activity of TiO2 + b-Bi2O3 composites, we synthesized composites with high specific surface area

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FIG. 6 Plausible charge carrier migration cascade in TiO2 + b-Bi2O3 composites under visible-light illumination. Illustration (A) shows the heterojunction between TiO2-P or TiO2-R and b-Bi2O3. In (B) the formation of the p-n junction between TiO2 and b-Bi2O3 is presented. The p-n junction between b-Bi2O3 and (BiO)2CO3 in TiO2-R + Bi2O3 and a-TiO2-R + Bi2O3 composites is illustrated in (C).

amorphous TiO2 (a-TiO2) and anatase TiO2 nanorods (TiO2-R) with the Ti:Bi ratio of 1:0.4. To examine only the influence of specific surface area of the TiO2 support on the properties of the composite, the TiO2-P + Bi2O3 ¼ 1:0.4 composite was also included in the comparison. The details about the synthesis of TiO2-R and a-TiO2-R samples can be found in our previous publication [22]. The procedure to synthesize TiO2-R + Bi2O3 and a-TiO2 + Bi2O3 composite was the same as for the TiO2-P + Bi2O3 ¼ 1:0.4 composite. The SEM images of the TiO2 supports reveal that we deal with the rod-like shape of a-TiO2-R and TiO2-R samples on one side and the nanoparticle-like shape of the TiO2-P solid on the other (see Figs. 1 and 7). The different morphologies were also reflected in the specific surface areas of the TiO2 supports, where the highest specific surface area was obtained in the case of amorphous a-TiO2-R with 278 m2/g followed by TiO2-R with 105 m2/g and TiO2-P with 86 m2/g. The morphology of prepared composites is dominated by the morphology of the TiO2 supports, which did not change during the synthesis procedure (Figs. 1 and 7). The specific surface area of TiO2-R + Bi2O3, a-TiO2-R + Bi2O3, and TiO2-P + Bi2O3 ¼ 1:0.4 composited dropped in comparison to pure TiO2 supports to 81, 217, and 70 m2/g, respectively. The XRD patterns (Fig. 2) of the composites revealed that in the case of TiO2-R + Bi2O3 and TiO2-P + Bi2O3 ¼ 1:0.4 samples TiO2 is present in anatase form and amorphous in the case of a-TiO2-R + Bi2O3 solid. In XRD diffractograms of TiO2-R + Bi2O3 and a-TiO2-R + Bi2O3 composites not only peaks belonging to b-Bi2O3 are found, but also peaks belonging to bismuth carbonate ((BiO)2CO3). As already described above, (BiO)2CO3 is formed from bismuth nitrate during the synthesis procedure, and depending on the applied decomposition temperature it can be transformed to different polymorphs of Bi2O3. During the synthesis of TiO2-R + Bi2O3 and a-TiO2-R + Bi2O3 composites, the supplied combustion heat energy was not sufficient to completely transform (BiO)2CO3 into b-Bi2O3; consequently, three phases (TiO2, b-Bi2O3, and (BiO)2CO3) are present in the obtained materials. The results of UV–vis DR measurements (Fig. 3) reveal that the BG energy of amorphous a-TiO2-R (3.4 eV) is wider than the BG energies of anatase TiO2-R (3.28 eV) and TiO2-P (3.24 eV). The UV–vis DR spectra of the composites indicate that the prepared solids exhibit a visible-light response as they show strong absorption of light in the 375–550 nm range. This can be ascribed to the presence of b-Bi2O3 phase, whereas absorption in the range between 250 and 375 nm is influenced by the TiO2 phase. The third component present in ternary TiO2-R + Bi and a-TiO2-R + Bi composites is the wide BG semiconductor (BiO)2CO3 with the BG energy between 3.1 and 3.2 eV [21]. This means that it can be photocatalytically activated only by UV light with l < 400 nm and so it can influence the UV–vis DR spectra of TiO2-R + Bi and a-TiO2-R + Bi samples only in the region below 400 nm. Therefore, (BiO)2CO3 does not influence the ability of the investigated composites to absorb visible light. This implies that the presence of b-Bi2O3 in these solids is of crucial significance to enable them to be photocatalytically active under visible-light illumination.

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FIG. 7 SEM images of a-TiO2-R, TiO2-R, a-TiO2-R + Bi2O3, and TiO2-R + Bi2O3 samples.

Electrochemical measurements (Fig. 8) show that all TiO2 + b-Bi2O3 composites are capable to generate charge carriers under illumination with visible light, which is due to the visible-light photosensitizing effect of the b-Bi2O3 phase. As expected, the results also clearly show that pure TiO2 supports are not capable to generate charge carriers under visible-light illumination due to their wide BG. Pure b-Bi2O3, on the other hand, is capable to generate charge carriers under visible-light illumination, but due to low BG energy of this material, they quickly recombine. The recombination of b-Bi2O3 visible-light generated charge carriers are hindered in the TiO2 + b-Bi2O3 composites due to the junction with TiO2 and in the cases of a-TiO2-R + Bi and TiO2-R + Bi samples also due to the junction with (BiO)2CO3. If we compare only composites, the a-TiO2-R + Bi sample has generated the lowest number of charge carriers among all tested composites, which might be ascribed to the presence of amorphous TiO2. Heterogeneous photocatalytic experiments of oxidative degradation of water-dissolved BPA conducted in the presence of synthesized materials under visible-light illumination show that all composites are catalytically active (Fig. 5). The anatase TiO2 containing composites have exhibited better catalytic activity than the a-TiO2-R + Bi2O3 composite containing amorphous TiO2. This indicates that the overall kinetics of photocatalytic BPA degradation is more influenced by the hindered recombination of charge carriers due to the crystallinity of TiO2 than by the adsorption of BPA/BPA degradation products on the catalyst surface affected by high specific surface area. Higher photocatalytic activity of TiO2-R + Bi2O3 composite in comparison to TiO2-P + Bi2O3 composite can be ascribed to the fact that besides TiO2, (BiO)2CO3 can also act as another sink for b-Bi2O3 charge carriers generated by illumination with visible light, thus improving the extent of their utilization. The highest number of carbon-containing species accumulated on the surface of composites (TOCA) during BPA degradation runs was observed for TiO2-R and a-TiO2-R based samples (Table 1). This is not surprising if we consider the fact that they exhibit high specific surface area, which promotes adsorption of BPA/BPA degradation products on the catalyst surface. As can be seen in Table 1, adsorption of BPA/BPA degradation products was not the main reaction pathway of the visible-light assisted BPA degradation, but mineralization to CO2 and H2O as reflected by TOCM values. To summarize, in the overall kinetics of visible-light triggered photocatalytic BPA degradation the hindered recombination of charge carriers originating from the crystallinity of TiO2 is a more important factor than the high specific surface area of amorphous TiO2 and subsequent reduction/oxidation steps of oxygen and BPA/BPA degradation products.

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light on

90

light on

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E-Bi2O3

light on

TiO2-P TiO2-P+Bi2O3=1:0.4 a-TiO2-R a-TiO2-R+Bi2O3 TiO2-R TiO2-R+Bi2O3

80

Current density, P A/cm2

25

70 60 50 40 30 20 10 0 0

200

400

600

800

1000

1200

Time, min FIG. 8 Measurements of the photocurrent densities at the surfaces of prepared catalysts under visible-light illumination. Detailed experimental conditions of the electrochemical measurements can be found elsewhere [22].

Further, we will discuss the charge carrier migration cascade in TiO2-R + Bi2O3 and a-TiO2-R + Bi2O3 composites as the charge carrier migration cascade in TiO2-P + Bi2O3 composite was already described above (Fig. 6). The obtained EBG energies for TiO2-R and a-TiO2-R samples from the UV–Vis DR measurements are 3.28 and 3.4 eV, respectively. The calculated EVB values (Eq. 2) of TiO2-R and a-TiO2-R solids are 2.99 and 3.11 eV as the ECB for both equals
0.29 eV. The literature reports [21] that the BG energy of (BiO)2CO3 is 3.1 eV, so the calculated ECB and EVB values of (BiO)2CO3 are 3.32 and 0.16 eV. In the a-TiO2-R + Bi2O3 composite, the a-TiO2-R VB edge is positioned lower than the VB edge of b-Bi2O3, which means that h+ generated in the b-Bi2O3 VB cannot be transferred to the TiO2 VB upon a heterojunction, but a p-n junction is needed. In the case of TiO2-R + Bi2O3 composite, the charge carrier migration cascade between TiO2-R and b-Bi2O3 is the same as for the previously discussed TiO2-P + Bi2O3 composite. In a-TiO2-R + Bi2O3 and TiO2-R + Bi2O3 composites (BiO)2CO3 is also present. This means that between the n-type semiconductor (BiO)2CO3 and p-type semiconductor b-Bi2O3 a p-n junction can be established [49–51]. This in turn results in an equilibrium of (BiO)2CO3 and b-Bi2O3 Fermi levels (EF) and the formation of an inner electric field at the interface between the phases (Fig. 6C) that enables a transfer of visible-light generated b-Bi2O3 electrons to the (BiO)2CO3 CB thus prolonging their lifetime.

4 Concluding remarks and research perspectives Using solution combustion synthesis, we can successfully form a junction between the wide BG semiconductor TiO2 and low BG semiconductor b-Bi2O3 regardless of the TiO2 polymorph used. The junction enables us to transfer visible-light generated charge carriers in b-Bi2O3 to TiO2 and so prolong their lifetime. Based on the tightness of the contact between TiO2 and b-Bi2O3 and the BG of TiO2 support, a hetero- or a p-n junction can be formed between the oxides. The type of the junction determines the type of transferring charge carriers. The heterojunction enables the transfer of b-Bi2O3 holes and the p-n junction the transfer of b-Bi2O3 electrons. The right amount of added b-Bi2O3 is of crucial importance to obtain TiO2 + b-Bi2O3 composites with high photocatalytic activity. If the amount of added b-Bi2O3 exceeds the optimal loading, it can agglomerate and form a segregated b-Bi2O3 phase which acts as a recombination center for the charge carriers. Depending on the TiO2 polymorph, a ternary composite composed of TiO2, b-Bi2O3, and (BiO)2CO3 can be formed when the supplied heat energy during the synthesis is not sufficient to completely transform (BiO)2CO3 into b-Bi2O3. (BiO)2CO3 can form a p-n junction with b-Bi2O3 upon which visible-light generated b-Bi2O3 electrons can be transferred to (BiO)2CO3 that consequently supports TiO2 in its role as the sink for b-Bi2O3 visible-light generated charge carriers.

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Nowadays powdered photocatalysts have been mostly investigated in batch slurry reactors. This makes it obligatory to apply stirring due to the sedimentation of photocatalyst particles, and subsequent sedimentation, centrifugation, and filtration steps after each run to recover the dispersed photocatalyst from treated water. These procedures are obviously time consuming and economically inefficient. In the future, we must pay attention to the development/improvement of reactor systems to enhance the industrial utilization of heterogeneous photocatalytic liquid-phase reactions. Correspondingly, we have to find solutions for the efficient immobilization of photocatalysts on solid support as bound particles or a thin solid film, and at the same time achieve overall photocatalytic performance not being influenced by mass-transfer resistances in the reactor system.

Acknowledgments The authors acknowledge the financial support from the Slovenian Research Agency (research core funding No. P2–0150).

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Chapter 26

Composite materials in thermo-photo catalysis Anna Kubacka and Marcos Ferna´ndez-Garcı´a Institute of Catalysis and Petrochemistry, CSIC, Madrid, Spain

1 Thermo-photocatalysis: A brief outlook Thermo-photocatalysis is an exciting new field of research that attempts to combine thermal and light energy sources to carry out chemical processes of industrial interest. Thermal (or conventional) catalysis is a mature research field presenting success in facilitating numerous environmental, energy, and chemical synthesis applications. Photocatalysis is also an intensively investigated research field but with significantly less industrial development. A key point for the development of the thermo-photo research field is the fact that a significant number of reactions have been successfully explored from both sides, thermal and light-induced processes [1]. Thus, in this context, the braiding of the two concepts may be possible and can find industrial application. However, to find such utility, a set of conditions needs to be fulfilled. A primary condition concerns the energy balance of the process. A practical (industrially oriented) implementation of a thermo-photo process requires synergistic action, rendering higher (photo-thermal) activity than the sum of the photoactivity at room temperature and the corresponding thermal activity along with the corresponding temperature of operation. Measuring the three reaction rates (r) mentioned and making a simple analysis (rT-P
(rP + rT); where T and P stand for, respectively, thermo and photo) could be straightforward yet poorly informative. The concept of synergy is obviously complex in a thermo-photocatalytic process and primarily demands a joule-to-joule analysis of the overall efficiency of the catalytic process. Even more complex is the setting up of a framework for understanding the basic ground of the energy balance. It is therefore important to understand not only the overall energy conversion (joule-to-joule analysis) but also the efficiency in using each photon at the corresponding process. This last point is well defined in photocatalysis and in turn demands analysis of the quantum efficiency of the reaction [2]. The rigorous calculation of the quantum efficiency requires solving the radiative transfer equation, with explicit consideration of thermal effects for the case we are dealing with. Only by carrying out the aforementioned tasks are we able to make an initial assessment of the potential interest of a thermophotocatalytic process and determine the usefulness of the combination of energy sources in a particular chemical reaction. An additional condition is the development of all reactor and process engineering. The inclusion of light on wellestablished thermal catalytic processes is not an easy task and critically depends on the process itself (gas or liquid phase, etc.), and more importantly for the purposes of this chapter, on the catalytic material and the exact physico-chemical phenomena involved in the thermo-photocatalytic process. As a simple, initial classification of the materials, we can consider those that can couple conventional thermal and photo sources. Thermal (hereafter referred to as conventional) catalytic processes transfer energy to the catalysts mostly by conduction and convection, and this can be combined with a photon source without a primary thermal input. Another option is the use of radiation, mostly, although not exclusively in the IR range, to be combined with more energetic wavelengths (UV and visible) to provide a dual thermo-photo energy source. This last case has obvious consequences in opening the sun as a renewable and cost-free energy source of the reaction. Although we need to obtain different materials for each case, the rather complex physical origin of the thermo-photo process makes not obvious how to analyze materials using the combination of the mentioned energy sources. The materials used are always complex in nature (mostly composite materials) and their exact way of working in the thermo-photo research field is an open question, as the field is in its infancy. Therefore, we focus on the analysis of such complex materials mostly based in the process carried out. A simple classification comes for the main driver of the chemical process, related to the application field. Mostly environmental and energy-related chemical processes have been studied in thermo-photocatalysis, although some works were also devoted to analyzing the synthesis of high-added-value chemical compounds [3]. We will review the most recent and appealing

Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00031-3 Copyright © 2021 Elsevier Inc. All rights reserved.

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environmental, energy, and “synthetic” applications with emphasis on catalytic materials and interpretation of the activity based on the main physico-chemical phenomena taking place under reaction conditions, that is, under the combined effect of thermal and photonic energies. In each case we will attempt to detail the way light and heat are combined and identify the most significant physico-chemical characteristics of the solids playing a role in the catalytic process.

2

Environmental applications in thermo-photocatalysis

Environmental application of thermo- and photocatalysis is a field subject to intensive research, which has been summarized in a significant number of reviews [4]. It is also an important topic in thermo-photocatalysis and, particularly, for the elimination of pollutants thorough oxidation reactions. Elimination of gas-phase pollutants and organic molecules, the so-called volatile organic compounds (VOCs), has been extensively analyzed. Several systems analyzed the combination of conventional thermo- and photocatalysis. Some systems concern oxides such as SrTiO3 [5], or FeOx [6], but most work has been presented using composite systems based on cerium oxide. The Ag and F modification of SrTiO3 was used to eliminate hydrocarbons (toluene, benzene, xylene) under visible light illumination (>420 nm) from room temperature to 90°C. The synergistic effect between energy sources increases with temperature and requires the cooperative work of Ag, through a plasmonic effect, and the improvement in radical species handling observed in presence of F doping [5]. In the FeOx case, the catalyst was promoted with Pd and operates in the 60–140°C range using visible light (420–760 nm). The performance in the CO-NO reaction was scrutinized, and results showed that optimum activity was achieved at 100°C. This optimum concerns not only the elimination of both molecules but also selectivity to nitrogen in the reduction of nitric oxide [6]. Nevertheless, ceria-based systems are certainly the most studied systems for thermo-photoelimination of VOCs. As analyzed in bare ceria, the presence of vacancies is intrinsic to thermo-photoactivity of this oxide [7]. This is commonly referred as a Ce2O3/CeO2 composite, suggesting somehow an independent-like (isolated) behavior of defect centers with respect to the ceria structure. In any case, the unique combination of the n-type semiconductor and oxygen ion conductor properties of the cerium oxide provokes that absorbed photons (exciting electron from the valence band (O2p) to an empty conduction band (Ce4f) with the formation of a Ce4+(e
)O2
(h+) pair while electrons photogenerated remain localized) lead to charge species having a large lifetime (i.e., favoring charge carrier separation). At the same time, the oxygen defects can reach the surface (an essentially thermal effect) facilitating the activation of the VOCs. Such a general scheme has been applied for composite samples presenting a combination of this oxide material with titania [8] or more complex formulations that combine these oxides with Mn oxide and Pt [9]. The binary CeO2-TiO2 system was essayed in 2-propanol oxidation in the 120–360°C range under UV (365 nm) illumination [8]. A kinetic formalism with explicit treatment of light was utilized to interpret the physico-chemical phenomena taking place under thermo-photoactivation. Fig. 1A presents partial results concerning the oxidation of 2-propanol for temperatures from 250°C to 270°C. Following the analysis made in the work, presence of titania would define the right photo-handling properties, while ceria triggered a profitable new thermo-photo process. Synergistic effects can be observed (depending on the temperature range) in the single oxides but are maximized in the composite sample. The kinetic analysis was based on the well-established initial steps of all thermo-photocatalytic processes and included explicitly the light/ thermal-matter interaction (see kinetic equation in Fig. 1B). This allowed to determine the true kinetic constants providing general information (i.e., eliminating the influence of the experimental conditions and thus providing general validity to the results). To highlight differences between the composite and the photoactive TiO2 reference, the ratio of the kinetic parameters of the two samples was obtained; it is graphically presented in Fig. 1B. The analysis of kinetic constants allowed quantitative measurement of the differential performance and indicated that the active centers are characteristic of the composite system. The full analysis defined a thermo-photo synergistic effect based on the simplified scheme presented in Fig. 1C. The scheme highlights the decisive role of the interface between the two oxides. The ceria-titania interface has a two-fold role; it plays an important role in charge handling but also is an “active” center in the transformation of the pollutant. The enhancement of activity could thus have a main root in a more effective interaction of the oxidant species with the pollutant, achieved under both the thermal- and photo-triggered steps of the mechanism and considering the joint use of both energy sources. As another example of useful combination of energy sources, we highlight the more complex PtTiO2/Ce-MnOx composite material in benzene oxidation from 120°C to 180°C using a light source with a central wavelength of 254 nm. The synergistic effect was discussed considering that (1) TiO2 was used as a photocatalyst, (2) Pt was an excellent thermal catalyst, and (3) the Ce-MnOx oxides play two important roles. The two last oxides primarily act as thermal catalytic components activating the reactant molecules and, key for the thermo-photo process, also supply lattice oxygen and surface-active oxygen to promote the oxidation of benzene [9]. Cerium oxide is also the most common component in systems using mostly the “full” (UV-vis-IR) radiation range to eliminate VOCs within thermo-photocatalytic processes. The previously discussed combination of Ce and Mn oxides was

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FIG. 1 (A) Temperature influence in thermo and thermo-photocatalytic oxidation of 2-propanol. TiO2 (top; gray bars), TiO2-CeO2 (middle; blue bars), and CeO2 (bottom; red bars). T, thermal and UV + T, thermo-photo experiments. (B) Ratio between kinetic parameters for TiO2-CeO2 sample and TiO2 reference. (C) Schematic representation of thermo-photo synergistic effect in TiO2-CeO2 catalyst. (From M.J. Mun˜oz-Batista, A.M. Eslava-Castillo, A. Kubacka, M. Ferna´ndez-Garcı´a, Thermo-photo degradation of 2-propanol using a composite ceria-titania catalyst: physico-chemical interpretation from a kinetic model, Appl. Catal. B: Environ. 225 (2018) 298–306, https://doi.org/10.1016/j.apcatb.2017.11.073. Copyright 2018, Elsevier.)

used to eliminate acetaldehyde using the full spectrum of the Xenon (Xe) lamp [10]. In this case, the authors showed an increasing promoting effect with temperature (directly related to an irradiance intensity increase) in the 25–75°C range. This is suggested to be connected with the thermo-photo effect in the coverage of reaction intermediates poisoning the surface, taking as reference the situation at room temperature. Similarly, both (Ce and Mn) oxides were supported in titania and assayed in the elimination of benzene using excitation wavelengths in the 420–830 nm range that heat the catalysts over 185°C [11]. A rather parallel work (for benzene oxidation) used OMS-2 (KMn8O16) instead of titania for supporting Ce/Mn oxides. A clear synergistic effect between the two (heat and light) energy sources was achieved although, in this case, the physico-chemical origin was not fully elucidated [12]. This support (OMS-2) was previously used with Mg- and/or Cedoping. In a series of works, authors reported such composite as an active thermo-photocatalyst for benzene degradation under full solar spectrum (UV-vis-IR) irradiation, simulated using a CHF-XM500 Xe lamp. No significant photocatalytic conversion together with remarkable thermo-catalytic activity indicates that the high conversion exhibited by the so-called Mg-OMS-2-B sample (B denote specific Mg doping amounts Mg/Mn (ICP) ¼ 0.0036) under UV-vis-IR illumination would have complex physico-chemical roots to be explored. The material also showed a rather slow deactivation in long, time-on-stream tests for VOC elimination processes [13, 14]. The LaMnO3-CeO2 composite system (with nearly equal quantity of both components) was tested in the degradation of toluene using IR light (>800 nm). With temperatures in

412 SECTION D Composites and heterojunctions. Tertiary materials

the range of 180–300°C range, the authors pay attention to the activation of the pollutant molecule in presence or absence of oxygen and show that oxygen interaction with the molecule and availability at the catalyst surface play important roles in the reaction [15]. Other systems using radiation as dual light and heat source for eliminating VOCs use alternative oxides. The usage of defect-abundant Co3O4 nanorods is a representative example. This material was used in the elimination of benzene under UV-vis-IR irradiation. The presence of Co(II) vacancy defects was shown to be critical using a comparison with a commercial Co3O4 oxide. Again, and as occurs in ceria, defect-related effects and the influence in specific reduction/ oxidation steps of the Co cation (in this case, the reduction step is significantly favored) was shown to facilitate the activation and evolution of the VOCs and presumed to be responsible for the synergistic effect of heat and light. This is schematically presented in Fig. 2. The system also showed stability over a long time in stream tests [16]. Finally, we can mention the Pt/BiVO4/TiO2 composite system, which was also used for benzene elimination in temperature range of 30–100°C range. Results demonstrated that the composite sample with a Pt loading amount of 1.0 wt% presented the best activity for a reaction temperature of 80°C. An electron paramagnetic resonance analysis of radical formation showed the enhanced radical formation of both hole- and electron-related species and that the synergistic effect relays in the contact of Pt with the support and subsequent effects in charge handling [17]. Liquid-phase elimination of pollutants was also explored using the entire UV-vis-IR spectral range. Cerium oxide within the so-called Ce2O3/CeO2 composite showed activity in the elimination of methyl orange. The ceria sample showed 1.5 and 2.7 times increase in activity under UV and visible light when temperature increased from 8°C to 65°C, respectively. The authors explained the result using the same physical phenomena described previously for ceria-based samples in VOC elimination. Essentially, efficient charge photo-handling in the regions of high defect density at high temperature. In detail, electrons can be captured by surface oxygen to form superoxide and holes by H2O (or OHd) to produce hydroxyl radicals, which are responsible for methyl orange degradation [18]. An additional case showing synergy was achieved using a Mn3O4/MnCO3 composite material. The synergistic effect was explained by the fact that Mn3O4 can be excited by the light source and generate electrons and holes, following the main step(s) of a pure photocatalytic process. Then, the electric conductivity of lattice oxygen can be increased as the temperature is raised, which causes more holes to transfer to the surface thereby increasing the activity of the process [19]. The Er3+-BiVO4/TiO2 complex heterostructure [20], and Er3+, Yb3+, or Er3+/Yb3+ cation-doped TiO2 [21] correspond to composite samples tested using phenol as the pollutant molecule target. The results obtained suggest that the concurrence of the TiO2 and Er3+-BiVO4 composite heterostructure has improved performance due to an optimized charge separation process. Finally, thermo-photocatalysis also found utility

FIG. 2 Schematic representation of solar-light-driven thermo-catalysis and photoactivation on Co3O4 with Co2+ vacancy defects. (From L. Lan, Z. Shi, Q. Zhang, Y. Li, Y. Yang, S. Wu, X. Zhao, Defects lead to a massive enhancement in the UV-vis-IR driven thermocatalytic activity of Co3O4 mesoporous nanorods, J. Mater. Chem. A 6(16) (2018) 7194–7205. https://doi.org/10.1039/c8ta01362d. Copyright 2018 Royal Society of Chemistry.)

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in disinfection utilizing the whole UV-vis-IR range. More concretely, a Er-W containing TiO2 showed high disinfection activity against Gram-negative (Escherichia coli) and a Gram-positive (Staphylococcus aureus) bacterium using wavelengths from 365 to 960 nm. In this study, heat and light effects appeared to promote activity for illumination wavelengths greater than roughly 550 nm, whereas plasmonic-type effects related to surface WOx (probably doped with Er) species appeared to control activity and provide a wide range of wavelengths where the solids can profit from solar light [22]. Note that Er and similar cations (Yb, Ho, etc.) used in these investigations can be involved in upconversion processes to generate UV from IR photons together with local heating. The radiative and nonradiative phenomena directly connected with nearinfrared (NIR) photon de-excitation occur in the lanthanide-containing materials, thus their respective effect(s) in photoactivity cannot be easily decoupled.

3 Energy-related applications of thermo-photocatalysis Most of the thermo-photocatalytic works available in the energy-related literature concern the production of fuels. More concretely, literature reports focus on the production of hydrogen from renewable sources like alcohols potentially coming from bio-resources as well as the synthesis of hydrocarbons, usually methane, and/or methanol from reduction of carbon oxides. The first reaction is linked with the water-gas shift reaction as optimum catalysts for hydrogen production are usually highly active in both reactions. The second general reaction is the reduction of CO2 as well as the transformation of synthesis gas (CO/CO2/H2) mixtures. Considering hydrogen production from reforming of organic molecules and/or water splitting, the effect of temperature in photocatalysis has been assessed in several studies. For liquid-phase reactions and temperatures below 60–70°C, photocatalysts such as Pt/TiO2, Ga2O2
xNx, and TiO2
xNx were tested under visible light irradiation, and the results indicated that pressure had a large effect on the photocatalytic hydrogen evolution, whereas reaction temperature had little effect on hydrogen production [3]. Contrarily, titanium disilicide (TiSi2) [23] showed increasing H2 production efficiencies with temperature. Another study produced H2 over a Pt-TiO2 catalyst using methanol as a sacrificial reagent and a 300 W high-pressure Xe lamp at room temperature with Ar as a shielding gas. Liquid-phase H2 production was reported to be 13.0 mmol g
1 under UV-vis light irradiation, whereas H2 production reached 27.4 mmol g
1 under the full UV-vis-IR full spectrum irradiation. However, under exclusively IR light irradiation, there is no hydrogen production, indicating no excitation of Pt/TiO2 under IR light. The UV light excites TiO2 to generate electrons. The electrons generated with UV light are transferred to the surface of the Pt nanoparticles to react with H+ and to generate hydrogen. The vis-IR light provided heat to enhance the oxidation of CH3OH to produce H+. Thus, this work suggested that the IR light is just replacing the effect of an external heating source to promote the solar catalytic hydrogen production process [24]. Contrarily to the titania-based material just mentioned, the use of tungsten-based oxides with well-known plasmonic properties in the NIR range allowed to profit from this electromagnetic region. Specifically, a composite based on Ag supported by W18O49 nanorods was shown to obtain hydrogen from an aqueous solution of NH3BH3 subjected to illumination with wavelength greater than 750 nm. This set-up leads to catalyst temperatures around 100°C. The study provides evidence that hydrogen yield is closely connected with the generation of hot electrons at the oxide component as well as fine control of hot electron relaxation (increasing lifetime) through interfacial charge between components [25]. For other Pt/TiO2 catalyst and testing the gas phase reforming of methanol positive effects were observed between 100°C and 140°C using conventional heating and UV excitation. An IR study suggested this was directly connected with a decrease of poisoning effects in turn connected with the adsorption of CO (and maybe other intermediates) over the Pt surface. CO can be generated by several reactions concerning degradation of methanol, formaldehyde of formic acid moieties generated by hole attack to the surface carbon-containing species [26]. The origin of the heat-light synergy was further analyzed in a Ru/TiO2 catalyst utilized in the reforming of methanol. In this case, the co-catalyst is itself a composite with a core-shell structure, having a ruthenium oxide external layer and a metallic core (see panels A and B of Fig. 3). The larger the Ru loading, the greater the thickness of the oxide shell structure. This allows joining of the plasmonic properties of the oxide to the metallic core behavior as an electron sink. The first extends the (useful) utilization of sunlight or photons from other sources from UV to vis-NIR and the second facilitates charge separation. Optimum thermo-photoactivity was obtained for all samples at 240°C, with a strong synergy between light and heat as measured by the excess rate (difference between the thermo-photo rate and the sum of the photo and thermo rates) presented in Fig. 3. A mechanistic study using in situ IR was utilized to interpret the catalytic behavior. The marked dependence of activity on ruthenium loading was ascribed to the different Ru surface layer response to the reaction atmosphere and the different interaction with reactant and intermediates molecules. Particularly important is how the carbon-containing entities originated by hole attack to methanol (specifically, formaldehyde and formic type species) interact with the surface(s) and how these molecules evolved in order to maximize the hydrogen yield. This evolution considers their decomposition forming carbon monoxide

414 SECTION D Composites and heterojunctions. Tertiary materials

FIG. 3 TEM micrographs illustrating details of the ruthenium-containing core-shell structures presented in Ru/TiO2 samples: 3 wt% Ru/TiO2 (A) and 5 wt% Ru/TiO2 (B). Reaction (C) and excess (D) rates values obtained in the thermo-photo production of H2. (Reprinted with permission from U. CaudilloFlores, G. Agostini, C. Marini, A. Kubacka, M. Ferna´ndez-Garcı´a, Hydrogen thermo-photo production using Ru/TiO2: heat and light synergistic effects, Appl. Catal. B: Environ. 256 (2019), https://doi.org/10.1016/j.apcatb.2019.117790. Copyright 2019, Elsevier.)

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and hydrogen always coupled with the water gas shift reaction [27]. So, in this case, a combination of optoelectronic properties, based in the “ternary” ruthenium oxide-ruthenium-titania specific morphology, with chemical properties, primarily associated with the ruthenium external layers, appear at the core of the synergy effect. The other type of reaction capturing interest in the field of thermo-photo processes concerns the transformation of carbon oxides. Transformation of CO2 into fuels is an important reaction. Most catalysts only display generation of CO and methane. Defective semiconductors are used, as occurs in many of the previous reactions, to accomplish this task. Specifically, the generation of CO and CH4 was reported over a layered BiOI-type material under NIR light irradiation (700 nm). It was stated that the few-layered BiOI powder displayed higher conduction band minimum (CBM) position with respect to bulk-type samples, which led to greater reduction ability of the catalyst. Additionally, the existence of oxygen vacancies was proposed to influence the photocatalytic reduction activity and reaction product chemical nature. The oxygen vacancies extend the range of light absorption, leading to NIR light-responsive CO2 photo-reduction in this material, although authors did not discuss the associated thermal effects [28]. In another work, Bi/Bi4O5I2 composites were tested in the same context. The dramatically enhanced thermo-photocatalysis for CO and CH4 production can be attributed to thermo-photo effect(s) connected somehow with Bi nanoparticles. Bi is able to absorb IR light and simultaneously heats the sample, contributing also to the chemistry of the CO2 transformation, likely taking place at the interface between components [29]. Finally, a defective Mo(VI) oxide was also texted in carbon dioxide reduction and rendered the mentioned CO/CH4 products. Again, comparison with a bulk-type sample demonstrates the need of defects and the associate plasmonic effect to achieve efficient use of the UV-vis-IR electromagnetic range. In addition, the authors pay attention to the heating of the catalysts under UV-vis-IR illumination, showing that UV-vis heats the defective oxide up to 105°C, while IR heats it to 120°C. Using the full illumination, the temperature reaches 160°C. The analysis of the reaction using spectroscopy and theoretical calculations showed that defects decrease the recombination of charge carrier species. In addition, they render beneficial kinetic effects related to the modulation of the activation energy of the hydrogenation process, considering this issue the initial COOH∗ intermediate as well as subsequent ones in the way to generate methane [30]. Supported metals were also used in this reaction. Profiting from the whole 300–2500 nm electromagnetic range of the sun was attempted with metals supported on alumina and titania. Using hydrogen as a reductant, metals supported in the first oxide achieved high selectivity to methane (>95%) in the case of Ru, Rh, Ni, Co, and Pd, while CO was the dominant product for Pt and Fe. These materials reached different temperatures during reaction, between 275°C and 350°C for Ru, Rh, Ni, and Co, and around 125°C for Pd, Pt, Ir, and Fe. Using water and the titania support, the formation of methane reaches 100% selectivity for Pt and Fe. The effective transformation of carbon dioxide was thus demonstrated over a whole UV-vis-IR illumination, although the physical interpretation of the process was not analyzed [31]. Conventional heating was also explored using titania-based catalysts. For Cu-promoted titania, the production of methane was studied in presence of carbon quantum dots. The study was able to show that thermo-photo conditions maximize activity at 250°C. Synergy between heat and light appears as a combination of a light-triggered mechanism with the reactive and intermediate molecules activated by Cu(I) species with a thermally related effect promoting the reduction of Cu(II) species and subsequent production of active Cu species. The latter seems to benefit significantly by presence of the carbon component, which acts an electron reservoir during reaction [32]. In another work, the presence of oxidic-type Co species supported on titania nanotubes was analyzed under UV irradiation at 120°C. Activity for CO/CH4 production was significantly boosted under the combined used of both energy sources. Authors analyzed the catalytic effects of different calcination treatments and showed that small defective Co oxide particles provided greater activity and significant quantities of both products, while larger particles displayed decreasing activity and preferential formation of CO. As shown in Fig. 4, this is based on a more efficient handling of charge, facilitating hole trapping by small Co entities and generating more protons, favoring the generation of methane (requiring more electrons and holes, 8 vs 2, per product molecule) and increasing activity simultaneously [33]. Finally, methanol synthesis from syngas (CO/CO2/H2) mixtures was attempted with CuO/ZnOx/Al2O3 composites using conventional heating combined with strong light irradiation conditions corresponding to 6–16 suns. The conventional heating (called reactor temperature in the work) temperature utilized was 350°C. When the catalysts were illuminated, the surface of the materials reached the same temperature, although the bulk was at an inferior temperature. Therefore, under illumination the reactor as well as the syngas mixture were set to (different) temperatures, which were allowed to reach the desired 350°C at the catalyst surface in all cases. A summary of catalytic experimental details and output is presented in Table 1. Optimum activity achieved under specific combination of light and heat (process TPC2 in Table 1) was primarily ascribed to the effects of light in charge separation between the CuO and ZnO components. This places holes preferentially in the CuO surface, which subsequently oxidized H2 and helped to stabilize Cu(II) species, while electrons appear dominantly in the ZnO surface and facilitate CO activation. Concerning Cu chemical state, light plays a similar role to

416 SECTION D Composites and heterojunctions. Tertiary materials

FIG. 4 Scheme of photothermocatalytic reaction over three typical samples: (A) TiO2
x; (B) big CoOx clusters modified TiO2
x; and (C) small CoOx clusters modified TiO2
x. (Reprinted with permission from Y. Li, C. Wang, M. Song, D. Li, X. Zhang, Y. Liu, TiO2-x/CoOx photocatalyst sparkles in photothermocatalytic reduction of CO2 with H2O steam. Appl. Catal. B: Environ. 243 (2019) 760–770. :https://doi.org/10.1016/j.apcatb.2018.11.022. Copyright 2019, Elsevier.)

TABLE 1 Temperatures and catalytic performances in TC (thermo-catalytic), TPC1 (thermo-photocatalytic 16 suns), and TPC2 (6 suns) processes.a Conversion rate 21 (mmol g21 ) cata h

Yield 21 (mmol g21 ) cata h

CO

MeOH

MeOH

DME

reactor

syngas

12

3

36

60

350

350

TPC1

2

2

98

1

80

93

TPC2

11

9

6

250

260

Catalytic process TC

8.5

Selectivity (%)

Temperature (°C)

a Reaction condition: H2/CO ¼ 2:1, P ¼ 3 MPa, GHSV ¼ 20Lh
1 gcata
1 , reaction time ¼ 20 h. Reprinted with permission from X. Wu, J. Lang, Y. Jiang, Y. Lin, Y.H. Hu, Thermo-photo catalysis for methanol synthesis from syngas, ACS Sustain. Chem. Eng. 7 (23) (2019) 19277–19285, https://doi.org/10.1021/acssuschemeng.9b05657. Copyright 2019 American Chemical Society.

CO2 (when included in the syngas mixture) and favors selectivity to methanol against other products (dimethoxyethane and others). The authors showed that the energy balance favors the thermo-photo process against the conventional thermal process [34].

4

Synthesis of chemicals with thermo-photocatalysis

Synthesis of fine or high-added-value chemical compounds using thermo-photocatalysis has been analyzed in several cases. A topic recently addressed within this field corresponds to hydrogenation reactions. In a recent work, a representative molecular organic framework (MOF), the so-called Zn(2-methylimidazole)2 (ZIF-8), was grown on Pd NCs to obtain Pd NCs@ZIF-8 with a core-shell structure. This is schematically shown in Fig. 5. This composite structure displays a plasmonic band covering the UV-vis spectral range that, upon light absorption, can generate high temperature to drive the hydrogenation reaction(s) by thermo-photoconversion. The catalytic performance of the Pd NCs@ZIF-8 composite was used to carry out thermo-photo-based hydrogenation of 1-hexene under irradiation with different light intensities (Fig. 5A). The hydrogenation product was formed in only 37% after 30 min at room temperature in the dark. Strikingly, the performance of the composite catalyst was significantly improved by full-spectrum irradiation with a 100 mW cm
2 Xe lamp, and 66% yield of desired product was formed at 50°C. Under visible light irradiation (100 mW cm
2) at room temperature, the Pd NCs@ZIF-8 catalyst afforded the desired product a yield of 52%. Furthermore, the nanocomposite catalyst

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FIG. 5 Mechanism for thermo-photocatalysis-based organic compound synthesis using a ZIF-8-based nanocatalyst. (Reprinted with permission from Q. Yang, Q. Xu, S.H. Yu, H.L. Jiang, Pd nanocubes@ZIF-8: integration of plasmon-driven photothermal conversion with a metal-organic framework for efficient and selective catalysis, Angew. Chem. Int. Ed. 55(11) (2016) 3685–3689. https://doi.org/10.1002/anie.201510655. Copyright 2016 WileyVCH Verlag GmbH&Co. KGaA, Weinheim. Reprinted with permission from F. Wang, Y. Huang, Z. Chai, M. Zeng, Q. Li, Y. Wang, D. Xu, Photothermal-enhanced catalysis in core-shell plasmonic hierarchical Cu7S4 microsphere@zeolitic imidazole framework-8, Chem. Sci. 7(12) (2016) 6887–6893. https://doi.org/10.1039/c6sc03239g. Published by The Royal Society of Chemistry.)

showed excellent recyclability and selectivity because large olefins could not reach the Pd active sites [35]. Similarly, a core-shell Cu7S4@ZIF-8 nanostructure displayed thermo-photocatalytic functionality. The hierarchical Cu7S4 hollow microsphere core acts as a plasmonic nanoheater with a thermo-phototransduction efficiency of 31.1% and the surface temperature increased to 94°C under the irradiation of a 1450 nm laser. As depicted in Fig. 5B, the localized heat converted from light energy directly acts on the surrounding ZIF-8 shells with synergistic acid-base catalytic sites (the properly positioned Zn(II) and imidazolate functional groups), and leads to improved catalytic activity for valuable cyclocondensation reactions [36].

418 SECTION D Composites and heterojunctions. Tertiary materials

Another type of reaction profiting from thermo-photoexcitation corresponds to partial oxidation of organic compounds. The activity of WO3-type oxide nanosheets in contact with N-doped carbon quantum dots presented outstanding activity for cyclohexene transformation under illumination of a 300 W Xe lamp leading to a temperature between 100°C and 140°C. Optimum catalytic output is achieved at 120°C. The efficient contact between components and subsequent charge separation is combined with the adequate activation of redox properties taking place mainly as a thermal effect. Both phenomena facilitate the activation and transformation of the organic molecule and an important selectivity to partial oxidation products, cyclohexanone and cyclohexanol [37]. In another contribution, a novel Zn3In2S6@ZnO composite was utilized for the selective oxidation of aromatic alcohols to corresponding aldehydes. The material possesses both photocatalytic and thermo-catalytic activities when prepared by a hydrothermal method and a solvent-assisted interfacial reaction. A mixture of alcohol and photocatalyst was dispersed in benzotrifluoride (BTF) in a Teflon-lined stainless steel autoclave illuminated with a 300 W Xe lamp equipped with a 420 nm cutoff filter (l > 420 nm). It has been reported that ZnO is a promising thermo-catalyst for dehydrogenation of alcohols, while Zn-In-S (e.g., ZnIn2S4 and Zn3In2S6) is an important ternary semiconductor system, with a direct band gap (2.4–2.8 eV for bulk materials) and outstanding optical and electrical properties. Under visible light irradiation, the Zn3In2S6 is selectively excited in the composite material. The photoexcited electrons were captured by the adsorbed oxygen to generate  O2
. It is known that  O2
and h+ are the significant reactive species for the photocatalytic selective oxidation of aromatic alcohols to corresponding aldehydes by deprotonation processes. Meanwhile, the surface oxygen atom (O) and zinc atom (Zn) of ZnO interact with the hydrogen (H) and the oxygen of the hydroxy group of benzyl alcohol, respectively. As a result, the benzyloxy group (PhCH2O
) and a proton are formed, which weakens the CdH and the OdH bonds. This results in an easier pathway for the reactive species ( O2
and h+) to produce benzaldehyde and H2O. Thermal energy promotes the activity of some of oxygen-related radical species. The catalyst’s reusability in repeated experiments under the same conditions revealed that the conversion of benzyl alcohol as well as the yield of benzaldehyde remained constant [38].

5

Conclusions

In this work we reviewed the most interesting and/or recent contributions to the field of thermo-photocatalytic reactions. The use of dual energy sources in catalysis is a novel and actively investigated research field. It can find application in the promoting of classical chemical reactions with the inclusion of light into chemical processes well established at industrial level or to progress in the use of sunlight in chemical industry by achieving a complete profit of the solar light photon range, from UV to IR. For all cases, the application demands novel catalytic formulations, typically consisting of composite materials. These materials have found utility in several chemical reactions related to the production of energy vectors, control and elimination of pollutants, and transformation and valorization of chemical molecules. In this chapter we presented select examples of solids performing the aforementioned chemical processes and analyzed the physico-chemical background of the thermo-photoactivity. This survey of the thermo-photocatalytic process shows a rather broad combination of the multiple physico-chemical phenomena sustaining the activity of such materials. Most, although not all, of the physico-chemical phenomena analyzed are related with light absorption by semiconductors and/or metals that can simultaneously trigger light to charge carrier conversion and generate local heating spots. In addition, combination of nonradiative forms of nonlocal heating with light has been also utilized. Both types of physico-chemical phenomena are opening new catalytic pathways closely connected with novel mechanisms that are in turn linked with new (from both sides, thermo or photo alone) active species and/or the alteration of energy exchange and handling in catalytic solids. All these phenomena are not well understood and therefore require further investigation.

Acknowledgments Authors thank MINECO (Spain; PID2019-105490RB-C31 Grant) and CSIC for financial support. M.F.G. is fully indebted to Prof. F. Ferna´ndezMartı´n for general discussions.

References [1] L. Palmisano, E.I. Garcı´a-Lo´pez, G. Marcı`, Inorganic materials acting as heterogeneous photocatalysts and catalysts in the same reactions, Dalton Trans. 45 (29) (2016) 11596–11605, https://doi.org/10.1039/c6dt02059c. [2] S.E. Braslavsky, A.M. Braun, A.E. Cassano, A.V. Emeline, M.I. Litter, L. Palmisano, N. Serpone, Glossary of terms used in photocatalysis and radiation catalysis (IUPAC recommendations 2011), Pure Appl. Chem. 83 (4) (2011) 931–1014, https://doi.org/10.1351/PAC-REC-09-09-36.

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Chapter 27

Plasmonic photocatalysis Maya Endo-Kimuraa, Shuaizhi Zhengb, Tharishinny Raja-Mogana,c, Zhishun Weid, Kunlei Wanga,e, and Ewa Kowalskaa,c a

Institute for Catalysis (ICAT), Hokkaido University, Sapporo, Japan, b Key Laboratory of Low Dimensional Materials and Application Technology of

Ministry of Education, School of Materials Science and Engineering, Xiangtan University, Xiangtan, China, c Graduate School of Environmental Science, Hokkaido University, Sapporo, Japan, d School of Materials and Chemical Engineering, Hubei University of Technology, Wuhan, China, e Northwest Research Institute, Co. Ltd. of C.R.E.C., Lanzhou, P.R. China

List of abbreviations AO7 AOPs AS CB DAPs DRS DSSCs ETs IO IPCE LEMF LSPR MB MLCT MTBT NM NPs NRs OAPs OCs ORR PBG PCs POM PRET RET RhB ROS SCs VB

acid orange 7 advanced oxidation processes action spectrum conduction band decahedral anatase particles diffuse reflectance spectrum dye-sensitized solar cells electron traps inverse opal incident photon-to-current efficiency local electromagnetic field localized surface plasmon resonance methylene blue metal-to-ligand charge transfer methyl tert-butyl ether noble metal nanoparticles nanorods octahedral anatase particles organic compounds oxygen reduction reaction photonic bandgap photonic crystals polyoxometalate plasmon resonance energy transfer resonant energy transfer Rhodamine B reactive oxygen species semiconductors valence band

1 Introduction The continuous increase in consumption of fossil fuels as the consequences of the fast development of industry, the world population growth, and the shortage of renewable energy resources as well as the increase in environmental pollutions has become the major global problems. Therefore, the development of environment-friendly, clean, safe, and sustainable energy technologies is one of the biggest challenges in the recent times. Among all the available options, solar energy has been considered as one of the most promising, clean, and renewable energy sources on the planet. Accordingly, the study on the environment-friendly processes using solar energy has been intensively investigated. For example, Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00036-2 Copyright © 2021 Elsevier Inc. All rights reserved.

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photocatalysis under solar radiation (solar photocatalysis) has been proposed as one of the reliable solutions for three main crises of modern times: environment, water, and energy (critical for all humanity problems, as pointed out by Smalley [1]). The solar energy in the presence of photocatalyst might efficiently decompose chemical and microbiological pollutants and thus purify the environment, including water, air, and surfaces, as well as be converted into electricity and useful fuels [2–4]. Although there are various types of photocatalysts, probably, wide-bandgap semiconductors (SCs) are still the most intensively studied despite their low harvesting efficiency of sunlight. It must be reminded that though the wide bandgap results in the low light harvesting (due to absorption edge at ca. 400 nm), it also causes high reactivity for redox reactions under UV, since valence band (VB) and conduction band (CB) are separated by broad energy barrier (usually >3.0 eV) and thus possessing high oxidation and reduction power, respectively [5–7]. However, in the case of all SCs, the recombination of charge carriers (bulk and surface [8]) is the main reason for low quantum yields of photocatalytic reactions. Accordingly, SCs have been doped, surface-modified, and coupled with various elements/compounds, which results in successful enhancement of photocatalytic performance [9–13]. Among various modifiers, probably, noble metals (NMs) have been the most intensively used for the inhibition of charge carriers’ recombination, as NMs are excellent scavengers of electrons because of higher work function of NMs than the electron affinity of SCs, which results in the formation of Schottky barrier. Kraeutler and Bard were the first proving this almost a half-century ago [14]. Since then, various reports on NM-modified SCs have been published including the optimization of the synthesis methods to get the best properties (resulting in the highest photocatalytic activities) and the clarifications of the mechanism for different photocatalytic reactions [15–18]. Moreover, some clusters and complexes of NMs with visible light (vis) absorption abilities have been used for surface modifications of SCs to obtain vis-responsive materials, for example, PtCl4 [19] and [Pt3(CO)6]2
6 [20]. Additionally, another property of NMs has been used to activate wide-bandgap SCs under vis irradiation, i.e., localized surface plasmon resonance (LSPR) [21]. Accordingly, during the last decade, many studies have been presented for NM-modified SCs with the ability of working under vis irradiation. The obtained photocatalysts (NM/SCs) have been named as plasmonic photocatalysts, which is quite convenient to distinguish their well-known improved activity under UV irradiation from that under vis (plasmonic excitation). It should be underlined that the activity under UV comes from SC excitation with electron transfer from VB to CB, whereas under vis irradiation, photocatalyst is active due to LSPR of NMs (as discussed in the next sections). LSPR results from the confinement of a surface plasmon in a nanoparticle (NP) smaller than the wavelength of light exciting the plasmon, i.e., when an NP is irradiated, the oscillating electric field causes the coherent oscillation of conduction electrons. Although the plasmonic properties of NMs were discovered more than 100 years ago and already commercialized in various fields, including SERS, medicine, and optical data storage, their photocatalytic applications are quite new. The first application of LSPR in photocatalysis started at the beginning of this century, but it was used only for the characterization of gold NPs, i.e., their formation, their properties, and the stability under UV irradiation [22]. Later, Au/TiO2 photocatalyst was shown to be photocatalytically active under vis irradiation for photocurrent generation [21] and oxidative degradation of methyl tert-butyl ether (MTBT) [23] and 2-propanol [24]. The direct proof presenting that vis activity is caused by plasmon resonance has been obtained by action spectrum (AS) analyses, where AS resembles respective absorption spectrum, reaching the maximal activity (e.g., quantum yield) under irradiation at the wavelengths with the strongest LSPR, as shown in Fig. 1 [21, 24]. The photoabsorption properties of plasmonic photocatalysts depend on the kind of NM, the properties of NM deposits (size and shape), and the environment (refractive index of medium). Gold and silver have been mainly applied for plasmonic photocatalysis, but also other NMs such as palladium [25], platinum [26], and copper [27] have already been proposed. For example, LSPR for spherical Au and Cu NPs is detected at 520–580 nm [28, 29] and at 410–430 nm for spherical Ag NPs [30, 31]. Two absorption peaks are observed for rod-like nanostructures with transverse and longitudinal LSPR at shorter and longer wavelengths, respectively [32]. Accordingly, it has been proposed that the broader LSPR peak, for example, due to polydispersity in NMNPs, results in higher overall activity under vis irradiation due to efficient light harvesting [24, 32]. Although the research on plasmonic photocatalysis is quite new, many reports on the synthesis, properties, applications, and mechanisms, including some review papers, book chapters, and journal special issues [33–39], have already been published. Accordingly, this chapter summarizes these reports, discussing positive and negative aspects of plasmonic photocatalysts, and gives examples of the most promising applications.

2

Synthesis of plasmonic photocatalysts

The properties of both NMs and wide-bandgap SCs strongly influence the properties of the resultant plasmonic photocatalysts, such as specific surface area, crystallinity, crystallite size, photoabsorption properties, and thus resultant

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FIG. 1 Absorption and action spectra of (left) short-circuit photocurrent for Au
TiO2 photoanode and (right) 2-propanol oxidation on bare (□) and gold(▪) modified rutile titania; Adapted with permission from Y. Tian, T. Tatsuma, Mechanisms and applications of plasmon-induced charge separation at TiO2 films loaded with gold nanoparticles, J. Am. Chem. Soc. 127 (2005) 7632–7637.; E. Kowalska, R. Abe, B. Ohtani, Visible light-induced photocatalytic reaction of gold-modified titanium(IV) oxide particles: action spectrum analysis, Chem. Commun. (2009) 241–243, respectively. Copyrights (2005) ACS and (2009) RSC.

photocatalytic activities. Therefore, various studies on the morphology design and optimization of the properties have been conducted, as shortly presented in this section. Titania (titanium(IV) oxide, titanium dioxide) is probably the most widely investigated wide-bandgap SC due to high activity, stability, low price, and negligible toxicity [3, 7, 40–42]. Accordingly, titania has been mainly applied as SC for plasmonic photocatalysts. Besides, other supports have also been used, including other SCs: ZrO2 [43], ZnO [44], CeO2 [45], CdS [46], reduced graphene oxide (rGO), Fe2O3 [43], KNbO3 [47], g-C3N4 [48], AgCl [49], Ag2MoO4/AgBr [50], and ZrO2@CoFe2O4 [51] and even insulators, i.e., SiO2 (in the case of plasmonic-assisted catalysis, due to plasmonic heating) [43]. Both crystalline (anatase [52], rutile [53], and brookite [54]) and amorphous [55] forms of titania have been used, including the mixed-phase materials, such as famous P25 (anatase, rutile, and amorphous phase) [56] and P90 (anatase and rutile) [57]. There are different strategies for the preparation of plasmonic photocatalysis. For example, either NMs are synthesized and then deposited on the titania surface [58], or titania is synthesized on the NM nanostructures [59]. Usually, commercial titania samples are used as a support for NPs of NMs. The most common method is the direct reduction of NM precursor (NM cations) on titania by chemical [60], photochemical [61], thermal [43], and sonochemical [62] methods. Some studies propose simultaneous synthesis of both NM deposits and SCs during the hydrothermal process [63], laser pyrolysis [64], and hydrolysis of titanium precursor (tetrabutoxide [65] and titanium isopropoxide [66]) in the presence of NM precursor, followed by titania crystallization during heating, which causes the simultaneous formation of NM deposits (thermal reduction). Various shapes and structures of NM deposits have been prepared, such as NP (most typical) [67], clusters [56], nanoplates (hexagonal and triangular) [68], nanospheres [69], nanocages [70], nanorods (NRs) [71], and nanofilms [72]. Besides simple NM deposits on titania, other complex nanostructures have also been prepared, such as three-dimensional (3D) network of titania aerogels with incorporated gold NPs inside the structure [73], titania mesocrystals with gold NPs deposited on basal or lateral surfaces [74], core-shell, and Janus structures [59]. Besides titania NPs, various other structures have been proposed, including titania films, titania/silica films, mesoporous titania, titania inverse opal (IO), 3D titania aerogels, titania nanotubes (TNTs), titania mesocrystals, nanofibrous titania mats, and ZnO NRs. Moreover, titania might be itself supported on other materials for various purposes, such as (i) to increase specific surface area (mesoporous silica [61], alumina [23], and zeolite [65]), (ii) for easy recovery of photocatalyst after reaction, for example, deposition on either larger objects, such as silica glass slides, glass helix, and Rachig ring [75], or preparation of magnetically separable photocatalysts (magnetic material as a core, silica as an interlayer, titania as a shell, and NM on the surface [76, 77]), (iii) to control the size and shape of NM deposits (mesoporous silica [61]), and (iv) to enhance the photocatalytic activity, for example, by separation of charge carriers (fluorine-doped tin oxide (FTO) film [78], FTO with CuO/SnO2 [79], and zeolite [65]). It should be pointed out that both the morphology of the support (usually titania) and the method of NM deposition influence the resultant properties of plasmonic photocatalysts. For example, larger gold NPs are formed on larger NPs

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of titania (photodeposition method), probably due to the lower content of surface defects (electron traps, ETs) working as active sites for NMs’ deposition, i.e., the less the defect number is, the larger the NMNPs are due to aggregation of NPs on well-crystalline titania surface [32, 67]. Similarly, larger gold NPs have been formed on pure anatase than on anatase-rutile mixture [72]. Additionally, it has been found that the size of pores in titania film limits the size of deposited gold NPs [78]. The method of deposition is also crucial for the size of NM deposits, for example, the largest NPs are formed by photodeposition than the sol-immobilization method, and the smallest one is obtained by the deposition–precipitation method [58]. Moreover, the thermal treatment has been applied for (i) titania crystallization, (ii) reduction of NM precursor, (iii) removal of impurities (e.g., Cl
from NM precursor and capping/stabilizing agents of gold colloids), (iv) enhancement of the interactions between NMs and SCs, and (v) gold aggregation. It should be pointed out that the conditions during photocatalyst testing might change the properties, for example, NM reshaping has been caused by an increase in temperature [80] or irradiation [81]. Similarly, the co-deposition of other components might change the properties of plasmonic metal. For example, an increase and a decrease in the size of gold NPs have been observed after pre- and post-adsorption of ruthenium dye on the titania surface, respectively [82]. In summary, it should be pointed out that many different nanostructures of plasmonic photocatalysts have been proposed, characterized, and used as exemplified in Fig. 2. Generally, it is believed that the morphology governs activity, but it is also proposed that some types of morphology might be limited to specific applications, for example, the co-catalyst might only be necessary for efficient hydrogen evolution. Some examples on morphology-governed activity and the study on the use of the morphology for mechanism clarifications are discussed in the next sections.

3

Mechanism clarifications on plasmonic photocatalysts

Although many reports on plasmonic photocatalysis have been published, including the mechanism discussion, the final mechanism has not been decided yet. The reason could originate from a huge variety of photocatalyst structures, reaction conditions, and tested systems. For example, some experiments are performed under UV/vis irradiation, and thus, both SCs and NMs are excited, and since titania activity under UV is usually much higher than that of plasmonic photocatalysts under vis, it might be expected that titania excitation with an electron transfer to NM would be the preliminary pathway. Although other experiments are performed under vis irradiation, sometimes, bare SCs exhibit some activity under vis (e.g., due to impurities), and thus, much complex mechanism might be expected. Additionally, some experiments have been performed with dyes as testing molecules (though not recommended [41, 84]), which sensitize SCs. It should be reminded that under UV irradiation, SCs are excited, and charge carriers are formed, i.e., electrons and holes in CB and VB, respectively. Next, they might either migrate to the surface of the photocatalyst or recombine into the bulk. On the surface, two possibilities are also considered, i.e., surface recombination or redox reactions with adsorbents. Obviously, charge carriers’ recombination is not wanted and results in much lower quantum yields of photocatalytic reactions than expected (100%). In contrast, charge carriers on the photocatalyst surface in the presence of oxygen and water might FIG. 2 Exemplary structures of plasmonic photocatalysts: (A) NMNPs deposited on [83] facets of decahedral anatase particles (DAPs), (B) NMNPs deposited on octahedral anatase particles (OAPs), (C) core/shell NPs (both configurations could be applied, i.e., NM as a core or as a shell), (D) tri-layer NRs (with two kinds of NMs), (E) NM deposited on SC flakes, (F–G) SC mesocrystals with NM deposited on basal (F) and lateral (G) surfaces, and (H) Janus structure.

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form reactive oxygen species (ROS), including the most powerful hydroxyl radicals. Accordingly, photocatalysis has been considered as one of the advanced oxidation processes (AOPs), which might be used for the mineralization of all pollutants. After SC modification with NMs, much higher activity is usually observed, as recombination is highly suppressed [16, 22, 85–88]. In the case of some reactions, NMs are even necessary since bare SCs are inactive. For example, hydrogen evolution takes place on the surface of NMs, whereas bare titania is hardly active without NM deposits [18, 32, 89]. However, in the case of some reactions, a decrease in activity has been noticed after NM deposition, especially for reactions occurring on the titania surface, as a result of (i) decreasing number of available electrons (due to their transfer from SC to NM) [90], (ii) “inner filter effect” as NM blocks SC surface against efficient photons’ absorption [91], and (iii) a decrease in reagents’ adsorption on the SC surface [91]. Summarizing, under UV irradiation, SCs are photoexcited and NMs adsorbed on the surface prevent charge carriers’ recombination by scavenging of photo-generated electrons, which usually results in much higher photocatalytic activity. However, under vis irradiation, wide-bandgap SCs could not be excited, and thus, incident photons must be absorbed by NMs via LSPR. After plasmonic excitation, three main mechanisms of plasmonic photocatalysis have been proposed, as follows: i. Charge transfer ii. Energy transfer iii. Plasmonic heating It might be proposed that (i) charge and (ii) energy transfer might be considered as plasmon-assisted photocatalysis, where photo-generated charge carriers participate in the reactions, whereas (iii) plasmonic heating should be considered as plasmon-assisted catalysis since only thermal effect (similar to “dark” catalysis) initiates chemical reactions. All proposed mechanisms are shortly discussed below.

3.1 Charge transfer (plasmon-assisted photocatalysis) In the case of plasmonic photocatalysis, the charge transfer usually means the transfer of “hot” electrons (after plasmonic excitation). However, the transfer of “hot” holes has been also proposed [78]. Probably, Tian and Tatsuma were the first to propose the transfer of electrons from Au to CB of titania in 2005 during photocurrent generation [21]. In 2007, Orlov et al. suggested the decomposition of organic compounds (OCs) by electron transfer from Au NPs to TiO2, but for gold nanoclusters (100 nm]) on large rutile particles because of excellent lightharvesting abilities (broad LSPR peak), as compared with other Au/TiO2 with monodispersed gold NPs deposited on different commercial titania samples [24, 32, 67]. The mechanism of plasmonic photocatalysis might also depend on the properties and kind (as already discussed for Au/ ZnO and Au/TiO2) of SCs. An interesting study has been shown for the mechanism dependence on the thickness of the titania layer in the case of tri-layered gold(core)/silver/titania(shell) [111]. An electron transfer has been suggested for thin titania layers ( TiO2), whereas, under vis irradiation, only Au/TiO2 is active due to plasmonic excitation of gold.

4.2 Environmental purification—Inactivation of microorganisms Environmental purification does not mean only the degradation of chemical pollutants but also mean the inactivation of microorganisms. In 1985, the first report on the antimicrobial effect of photocatalyst was reported by Matsunaga et al., claiming that titania and Pt-loaded titania under UV irradiation could sterilize algae, yeast, Gram-positive, and Gramnegative bacteria via coenzyme A (CoA) oxidation [123] [61]. Markowska-Szczupak et al. comprehensively summarized the bacterial killing mechanisms of titania photocatalysts [4], pointing the three main mechanisms, i.e., peroxidation of cell membrane phospholipids [65, 66], direct DNA damage [63], and the oxidation of CoA [61]. These mechanisms are considered to be initiated by the ROS formation, and among ROS, hydroxyl radicals (OH) are the most active against bacteria [124, 125] despite their short half-lifetime. Moreover, it is proposed that other ROS (e.g., O2
and H2O2) and surface charge carriers (e
/h+) influence the bacterial inactivation [126, 127]. Accordingly, the plasmonic photocatalysts are surely promising candidates as antimicrobial agents, especially after consideration of their main advantageous: (i) intrinsic properties of NMs, (ii) charge carriers separation (inhibition of e
/h+ recombination) under UV (not plasmonic effect), and (iii) vis-response (plasmonic photocatalysis). Although the effects (i) and (ii) could not be considered as plasmonic photocatalysis, they are quite important for the overall activity, and thus, the exemplary details of effect (i)—intrinsic property of NMs—are shortly described below. The antimicrobial properties of silver and copper have been known since ancient times, and probably silver exhibits the best antimicrobial activities. Additionally, silver is safe for humans, and thus, it has been widely applied in cosmetics, shoes, clothes, detergents, face masks, and even for the purification of cutlery and teeth. Silver is proposed to be more effective for Gram-negative bacteria than Gram-positive ones, probably because of the differences in their biological structures. Up to now, different mechanisms of bactericidal action of Ag NPs have been proposed in the literatures, as follows:

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(1) Adsorption of Ag+ on the bacterial cell wall (negatively charged), causing its disintegration with a release of bacterial content outside the cell, resulting in the cell lysis and finally in death [128, 129]; (2) Deenergenization of bacteria and destabilization of the plasma membrane, causing a massive leakage of protons and a collapse of membrane potential [130]; (3) Interaction between silver and thiol groups of enzymes (respiratory and transport; vital for the cells’ function), causing the uncoupling of respiration from ATP synthesis [131]. Moreover, Ag NPs also exhibits the activity for inactivation of viruses, fungi, and algae. In the case of antivirus action, the mechanism considering the interaction with viral surface glycoproteins, resulting in the inhibition of (i) virus binding to target cells and (ii) virus replication (in the case of penetration of virus cells by Ag NPs) [132, 133], has been proposed. Similar bactericidal mechanisms to those of Ag have been proposed for copper, in which Cu ions might adsorb on the surface of bacteria and then cause the denaturation of surface proteins [134] and oxidative stress in the bactericidal process [135]. The accumulation of Cu ions inside bacteria has been reported as the main mechanism of inactivation of bacteria by Cu-modified blotting paper [136]. Cu2O has higher activity in the inactivation of bacteria than CuO and Ag [134], but Cu NPs are less active than Ag NPs against some species of fungi [137]. Although the bactericidal action of Au NPs has already been proposed, contradictory results have been reported. For example, the negligible activity of Au NPs for both Gram-negative and positive bacteria [138–140] and high activity of Au NPs [140–143]. The bactericidal mechanism of gold is focused on the ability of Au NPs to change the membrane potential and inhibit (i) activities of ATP synthase, decreasing ATP level, and (ii) the subunit of ribosome for tRNA binding [143]. Moreover, antifungal activity depending on the size of Au NPs has been reported [144] as well as antiviral effect by the inhibition of viral attachment, entry, and cell-to-cell spread [145]. Accordingly, the high antimicrobial activity of NMNPs has caused huge interest in NM-modified titania, resulting in many published reports on Ag/TiO2 [89, 118, 120, 127, 146–154], Cu/TiO2 (also copper oxide) [89, 120, 155–161], and Au/ TiO2 [147, 148, 162], including even the review paper on plasmonic photocatalysis for the inactivation of microorganisms [38]. Among these systems, the high activity of silver-modified titania is well known due to the intrinsic activity of silver in the dark and enhanced activity of titania under irradiation. The concentration and release of Ag+, its adsorption (or Ag/ TiO2) onto bacteria surface, and ROS generation are ought to be important for bactericidal action. The significant role of ROS in the decomposition of bacterial cells has been proposed in many studies. For example, Ag/AgBr/ TiO2 photocatalyst has caused oxidative attack from the exterior to the interior of Escherichia coli by OH, O2
and holes under vis, resulting in the death of bacterial cells [163]. It should be pointed out that not only destruction of bacteria cells (e.g., protoplast formation, as shown in Fig. 3 [147]) but also their complete degradation (mineralization) estimated by continuous CO2 evolution (Fig. 4; [146]) has been achieved on Ag/TiO2 photocatalysts. Additionally, it should be pointed out the antibacterial activity of Ag/TiO2 under vis irradiation is usually much higher than that in the dark, resulting in complete decomposition of bacteria cells [146, 148]. The properties of photocatalysts are crucial for the overall antimicrobial effect, and usually, a decrease in the size of Ag NPs results in an increase in activity as a result of large specific surface area. Interestingly, it has been even shown that the activity of Ag/TiO2 is higher in vis than that in UV (Fig. 5) [89], probably resulting from partial oxidation of silver (indirect proof for an electron transfer mechanism under plasmonic excitation), which is known as more active (against bacteria) than zero-valent one. In contrast, under UV, electron-rich silver, because of the opposite direction of an electron transfer, i.e., from TiO2 to Ag, might result in even repulsion between Ag surface and bacteria. Considering the silver release during irradiation, the contradictory data have been published, suggesting that: (i) besides ROS, the released silver ions (Ag+) are highly active antimicrobial agents [164] and (ii) high stability of Ag/TiON (on polyester) without Ag leaching [165]. Van Grieken et al. have presented that the release of Ag+ happens in the dark (enhanced bactericidal effect), whereas UV irradiation stabilized Ag deposits [150]. Similarly, enhanced antivirus effect has been caused by effective adsorption of bacteriophage MS2 on the silver sites, whereas released silver contributes to the overall antiviral activity [151]. Interesting data have been found for titania modified with bi-metal (Au/Ag) NPs with high stability under vis irradiation, probably resulting from stabilization of silver by gold (electron transfer from gold to electrondeficient silver) [166, 167]. Recently, plasmonic photocatalysts have also been successfully applied as antifungal agents [89, 118, 120, 148, 168], despite more complex structure of fungi (than bacteria and viruses), and thus, their high resistance against various antimicrobial materials/methods. For example, Ag-modified faceted titania (octahedral anatase particles; OAPs) is more active than OAPs modified with Cu, Ag, and Pt both in the dark and under vis irradiation (about 50% higher activity under vis than that in the dark during the first hour) [89]. It has been suggested that antifungal properties depend on the fungal strain, NM

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FIG. 3 SEM images of the decomposition of Escherichia coli under vis (l > 420 nm) irradiation of Ag/TiO2 photocatalyst: (A) healthy bacteria cells, (B) bacteria covered with titania before photocatalytic reaction, (C–F) destroyed bacteria cells after (C) 1 h, (D–E) 3 h, and (F) 24 h of irradiation. Adapted with permission from M. Endo, Z.S. Wei, K.L. Wang, B. Karabiyik, K. Yoshiiri, P. Rokicka, B. Ohtani, A. Markowska-Szczupak, E. Kowalska, Noble metalmodified titania with visible-light activity for the decomposition of microorganisms, Beilstein J. Nanotechnol. 9 (2018) 829–841. Copyright 2018, Creative Commons Attribution.

FIG. 4 E. coli decomposition presented as a decrease in bacteria number (closed symbols) and CO2 evolution (open symbols), tested in the suspension of titania under vis irradiation (l > 450 nm) and the dark. Adapted with permission (after formatting) from E. Kowalska, Z. Wei, B. Karabiyik, A. Herissan, M. Janczarek, M. Endo, A. MarkowskaSzczupak, H. Remita, B. Ohtani, Silver-modified titania with enhanced photocatalytic and antimicrobial properties under UV and visible light irradiation, Catal. Today 252 (2015) 136–142. Copyright 2015 Elsevier.

content, and the morphology of photocatalyst [118, 120, 168, 169]. The synergism between two different NMs has been shown for Ag and Cu (zero-valent and CuO) in respect to mono-modified titania [120, 169]. In the case of copper, despite deposition of zero-valent copper (e.g., under anaerobic conditions) on wide-bandgap SCs, usually, mixed-oxidation states of Cu have been obtained in Cu/TiO2 photocatalysts, i.e., Cu(0), Cu+, and Cu2+, because of easily oxidation of copper in air [148, 170]. It has been found that Cu2O/TiO2 is more active than titania modified with zerovalent copper and with CuO and that the optimal ratio of Cu(I) to Cu(II) in CuxO/TiO2 photocatalyst is important for high antimicrobial properties [156]. Moreover, similar to silver, it has been proposed that copper ions are responsible for antibacterial properties [159]. In the case of two kinds of faceted anatase particles (OAPs and decahedral anatase particles [DAPs]), it has been found that only titania modified with Ag and Cu possesses higher activity under vis irradiation than bare samples (negligible activity), whereas the samples modified with Au and Pt do not [147]. Accordingly, it has been

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FIG. 5 Antibacterial activity of OAP samples modified with NMNPs under UV/vis irradiation (left) and vis irradiation (right). Adapted with permission (after formatting) from Z. Wei, M. Endo, K. Wang, E. Charbit, A. Markowska-Szczupak, B. Ohtani, E. Kowalska, Noble metal-modified octahedral anatase titania particles with enhanced activity for decomposition of chemical and microbiological pollutants, Chem. Eng. J. 318 (2017) 121–134. Copyright 2017 Creative Commons Attribution.

suggested that the oxidation state of NMs might be crucial for the overall activity, for example, the surface oxidation state of less NMs is mainly positive (+1), whereas Au and Pt are zero-valent. The vis-irradiation results in the formation of a larger ratio of +1 charge than 0, thus allowing easier adsorption of NMs on negatively charged bacteria surface. Moreover, this behavior indirectly supports the mechanism of charge transfer under plasmonic excitation. Although Ag-modified titania photocatalysts usually exhibit the highest antibacterial activity, Au-modified ones show to be the most active against mold fungi, especially for efficient inhibition of their sporulation, as shown in Fig. 6 [148].

4.3 Solar energy conversion In the case of photocurrent generation, contradictory results have been published considering the effect of NMs on the activity of wide-bandgap SCs, mainly depending on irradiation wavelengths. Although this chapter does not discuss the activity under UV (too many reports and not “plasmonic” effect), the comparison between performance under UV and vis is shortly shown here as it is quite important for the overall activity and mechanism clarifications. For example, the photoanode of WO3 modified with gold-polyoxometalate (Au/POM) exhibits enhanced IPCE (incident photon-tocurrent efficiency) mainly under UV (till 470 nm) and inactivity at LSPR range, indicating only catalytic effect of Au/ POM [171]. Similarly, modification of CuWO4 with Au NPs and 2-nm TiO2 layer results in photocurrent enhancement only at wavelengths shorter than LSPR with a maximum at 390 nm [101]. On the contrary, an increase and a decrease of photocurrent have been observed under vis and UV irradiation, respectively, for porous titania modified with Au NPs [78]. The effect has been explained by electron transfer mechanism, i.e., under vis irradiation, “hot” electron transfer from Au into TiO2 and then to FTO, whereas under UV, the opposite direction of charge transfer, that is, from TiO2 to Au, instead of FTO, results in activity decrease. In another study, a five-fold increase of photocurrent under vis irradiation has been observed after modification of C-doped TiO2 with gold-nanocages [70]. The enhanced activity under UV/vis range of irradiation has also been reported for TiO2 aerogels modified with Au NPs [112] and hematite nanoflakes decorated with Au NPs [172]. It has been proposed that the large interface between SCs and NMs, the uniform distribution of NMs, and the morphology of photocatalysts are important for efficient transfer of charge carriers. For example, 55- and 23-fold increase in IPCE has been achieved after titania modification with 8 wt% of gold incorporated inside and deposited on the surface, respectively, of the titania aerogel network [112]. Additionally, Au NPs have been used to enhance the performance of dye-sensitized solar cells (DSSCs) because of efficient light scattering [173]. The first use of Au in DSSCs was proposed by McFarland and Tang in 2003 for solar cells composed of photoreceptor (merbromin dye) adsorbed on Au/TiO2/Ti [174]. Four-step photon-to-electron conversion has been proposed, such as (1) light absorption by dye, (2) transfer of electrons from excited dye to Au, (3) transfer of electrons above Schottky barrier to CB of TiO2, and (4) electron collection at back ohmic contact (Ti). The research on the fuel generation by plasmonic photocatalysts mainly focuses on half-reaction of water splitting, i.e., water reduction in the presence or absence of sacrificial hole scavengers (mainly alcohols). A huge number of studies have been published for NM-modified wide-bandgap SCs under UV irradiation with high photocatalytic performance. Then,

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FIG. 6 (A–F) Representative photographs of P. chrysogenum cultivated for 4 days under fluorescent light on Au/TiO2 (A,D), bare TiO2 (B,E), and without photocatalyst-reference sample (C,F); (G,H) Number of spores after 5 days of growth under vis irradiation for P. chrysogenum (G) and A. melleus; ST01, TIO12, and STF10—different samples of commercial titania. Adapted with permission from M. Endo, Z.S. Wei, K.L. Wang, B. Karabiyik, K. Yoshiiri, P. Rokicka, B. Ohtani, A. Markowska-Szczupak, E. Kowalska, Noble metal-modified titania with visible-light activity for the decomposition of microorganisms, Beilstein J. Nanotechnol. 9 (2018) 829–841. Copyright 2018 Creative Commons Attributions.

biomass conversion under conditions resembling solar radiation has been reported (Ne lamp with mainly vis emittance), showing that wastewater from sugar industry might be an efficient electron donor for hydrogen evolution with four times higher activity after titania (supported on silica) modification with 0.5 wt% of gold [175]. Although many reports show high activity of NMs/SCs under UV, the large majority of studies indicate their inactivity under vis, which is quite reasonable considering an electron transfer mechanism and negligible activity of bare SCs, that is, opposite electron transfer under vis than that under UV, and thus electron-deficient metal cannot be the reduction site for hydrogen evolution. To overcome this problem, the use of additional co-catalyst has been proposed, such as bi-metal modified titania. For example, Bian et al. have modified titania mesocrystals with NPs of Pt and Au, where Pt was deposited on lateral surfaces, whereas Au on basal ones, resulting in enhanced separation of charge carriers, that is, electron transfer from Au NPs via titania network to Pt NPs [74]. Other examples without the use of second co-catalyst have been proposed for morphology-controlled and mixed-phase photocatalysts. For example, Janus nanostructures of amorphous TiO2 and Au NPs (with enhanced LSPR at the interface

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between Au and TiO2) are much more active than core(Au)/shell(TiO2), bare TiO2, and Au NPs samples for hydrogen evolution under vis irradiation (58%, 1%, and ≪ 1%, respectively) [59]. In another study, Au-modified mixed-phase titania (anatase/rutile) has been recommended for efficient hydrogen evolution, whereas Au-modified rutile has shown low activity, and Au-modified anatase and Au-modified brookite have been inactive, suggesting the charge separation by additional electron transfer between two titania polymorphs, that is, “hot” electron transfer from Au via rutile to anatase. Moreover, the scavenging of “hot” electrons from Au NPs by lattice Ti4+ with subsequent reduction of protons by formed Ti3+ has been proposed as the main reason of vis-activity for hydrogen evolution. Similarly, hydrogen has been evolved on mixed-phase titania (anatase/rutile; P25), but only very little activity has been observed under vis (470 nm) than that at shorter irradiation wavelength (400 nm) [56].

4.4 Other applications of plasmonic photocatalysts Plasmonic photocatalysts have also been proposed for other applications, including synthesis of OCs and biomedical applications, as shortly presented here. Although the application of titania and NM-modified titania for the synthesis of OCs has been intensively investigated, the use of plasmonic photocatalysts has started recently. For example, the reduction of nitrobenzene (in methanol solution) to nitrosobenzene and aniline has been proposed on Au/LaTiO2 photocatalyst under vis irradiation with the highest activity at 3 wt% of Au loading [176]. Selective oxidation of various alcohols to ketones and aldehydes by plasmonic excitation of Au NPs deposited on titania (P25) has also been reported with high conversion (99%) of 1-phenylethanol to acetophenone [108]. With an increase in the interest of plasmonic photocatalysis and the ability to absorb photons at the therapeutic window (650–1350 nm; light with a maximum depth of penetration in tissue) by morphology design, the plasmonic materials have been used for medical purposes. One of the novel applications of titania-based photocatalysts is the cancer therapy. For example, bare titania [177] and modified titania, including hydrogenated black titania [178, 179], titania modified with NMs [180–182], and titania modified with NPs possessing up-conversion properties [183], have shown anticancer potential. Among them, NM-modified titania samples have been considered as the most promising due to high photocatalytic activity and efficient light harvesting at broad irradiation ranges (UV/vis/NIR). For example, the application of Au NRs allows the shift of LSPR from vis into NIR, enabling in vivo imaging and therapy via selective photothermal effect (plasmonic heating) of cancer cells [184]. Xu et al. have shown that Au/TiO2 kills the carcinoma cells efficiently, depending on the content of gold, and 2 wt% shows the highest activity [185]. An interesting study has been performed by Seo et al., where Ag/AgBr/TiO2 kills the mammalian cancer cells in vitro under vis irradiation and also reduces the tumor volume in vivo, as shown in Fig. 7 [182]. Although plasmonic photocatalyst might be used as an antitumor agent, the lack of cell specificity limits their commercialization. Accordingly, titania has been modified with some components to selectively bind to cancer cells, for example, epidermal growth factor (EGF)-epidermal growth factor receptor (EGFR) [186] and folic acid-folate receptor [187]. The bound titania to receptor might be uptaken by endocytosis and then incorporated into cytoplasm and nucleus, and the generated ROS under irradiation might attack lipids, proteins, nucleic acid, causing cell death. Moreover, a decrease in the necessary content of gold has been achieved by the design of nuclear-targeted Au NPs, inhibiting the migration of cancer cells [181, 188]. Additionally, plasmonic materials might be applied for drug delivery. For example, titania modified with Au NRs has been successfully used for gambogic acid delivery (GA; efficient anticancer drug), allowing its stable dispersion [180]. FIG. 7 Photographs of A431-heterograft tumor on mice in the absence of Ag/ AgBr/TiO2 and under irradiation for 10 min (left), in the presence of Ag/AgBr/ TiO2, but without irradiation (center), and in the presence of both Ag/AgBr/TiO2 and irradiation for 10 min (right). Reprinted with permission J.H. Seo, W.I. Jeon, U. Dembereldorj, S.Y. Lee, S.W. Joo, Cytotoxicity of serum protein-adsorbed visiblelight photocatalytic Ag/AgBr/TiO2 nanoparticles, J. Hazard. Mater. 198 (2011) 347–355. Copyright (2011) Elsevier.

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Interestingly, the TiO2/Au NRs material possesses high photothermal conversion efficiency (plasmonic heating) and thus causing additional anticancer effect when irradiated with low-intense laser at 808 nm.

5

New trends for activity and stability enhancements

As mentioned above, plasmonic photocatalysts have been intensively studied for various purposes, such as environmental purification (water purification, wastewater treatment, air purification, and surface cleaning) and solar energy conversion (in electricity and fuels). Various compounds and reactions have already been tested, and usually, vis activity has been confirmed. However, the efficiency is much lower than that under UV irradiation, even for bare titania samples. The charge carriers’ recombination, that is, back electron transfer (NM ! SC ! NM), has been suggested as the main reason for this shortcoming. Accordingly, various novel nanostructures have been proposed to inhibit this recombination. Moreover, an increase in light harvesting has been suggested as an efficient method for the improvement of the overall photocatalytic performance. Various strategies have been proposed for the improvement of performance (photoactivity and stability) of plasmonic photocatalysts such as (i) modification of properties of NMs, supports and their interfaces, (ii) coupling of plasmonic photocatalysts with other materials (SCs, metals, insulators, molecular photocatalysts, and dark catalysts), and (iii) preparation of novel nanostructures with enhanced separation of charge carriers and/or improved efficiency of light harvesting, as shortly presented in this section.

5.1 Nano-architecture design At present, improvement of photocatalytic activity mainly focuses on efficient light harvesting, which means the ability to use overall solar spectrum (a vast range of photoabsorption). Therefore, modification of titania with NM deposits possessing various sizes and shapes has been proposed. The first report discussing that higher activity is caused by broader absorption has been presented for large particles of rutile modified with gold NPs and NRs of various sizes [32]. In those samples, LSPR possesses two main absorption peaks at shorter (ca. 520 nm) and longer (ca. 600 nm) wavelengths due to the presence of gold NRs (transverse and longitudinal LSPR). Since then, NM NRs have been extensively studied for plasmon-assisted photocatalysis. For example, gold NRs have been used as a core for trilayered Au/Ag/TiO2 [111]. NRs are proposed as more efficient nanostructures than NPs, mainly due to the ability of light photoabsorption at broader wavelengths, that is, 520–700 nm for Au NPs and 520–1000 nm for NRs [61]. Furthermore, an increase in aspect ratio of Au NRs deposited on titania increases the resultant activity due to the broadening of LSPR [115]. To improve the photocatalytic performance of plasmonic photocatalysts, various modifications of support properties have also been proposed, mainly to hinder back electron transfer from CB of titania to gold NPs. For example, Bian et al. have proposed an “advanced superstructure system” [74]. This structure is based on titania mesocrystals in which electrons from gold NPs migrated through the titania nanocrystal networks from the basal surfaces to the edges of the plate-like mesocrystals, where they are temporarily stored for further reactions. This structure results in enhanced photocatalytic activity by more than one order of magnitude, as compared with that of conventional particulate Au/TiO2 system (Au NPs on TiO2 NPs), due to this anisotropic electron flow-hindering recombination of electrons and holes in the gold NPs. The interface between NMs and SCs is also crucial, and thus, removal of molecules present on the surface of NM or between NM and SCs might cause an increase in the photocatalytic activity. For example, the removal of stabilizing agents from nanoporous titania film results in significant enhancement of generated photocurrent [78]. The enhancement of the contact (interface) between NM and SC is also important. For example, the thermal treatment (annealing) might cause: (i) NM and SC fusion with partial embedding of NMNPs into the SC structure [78] and (ii) SC aggregation, which might result in a larger number of SC particles/aggregates with deposited NMNPs (usually some NPs of SCs are unmodified) [189]. Localization of gold NPs on the support surface is also crucial [74]. In the case of titania mesocrystals (described above), deposition of Au NPs on the basal surface, but not on the lateral one results in an efficient transfer of electrons from Au NPs to the lateral titania surfaces through nanocrystals networks (the charge carriers recombination in the case of Au NPs on the lateral surface: Au ! TiO2 ! Au) and thus prolonging the electron lifetime. Similarly, negligible vis activity has been observed for one of the most active titania photocatalysts, that is, DAP, when Au NPs have been deposited on [83] facets, resulting probably from the fast charge carriers’ recombination (electron transfer: Au ! TiO2 ! Au), due to an intrinsic property of DAP, that is, electron and hole opposite migration to [83] and [001] facets, respectively [190], as shown in Fig. 8 [191]. In contrast, OAP with only one type of facets [83] exhibits the highest photocatalytic activity after

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FIG. 8 The proposed mechanism of “hot” electron migration in Au/OAP (right) and charge carriers’ recombination in Au/DAP (left). Reprinted with permission Z. Wei, M. Janczarek, M. Endo, K.L. Wang, A. Balcytis, A. Nitta, M.G. Mendez-Medrano, C. Colbeau-Justin, S. Juodkazis, B. Ohtani, E. Kowalska, Noble metal-modified faceted anatase titania photocatalysts: octahedron versus decahedron, Appl. Catal. B Environ. 237 (2018) 574– 587. Copyright (2018) Creative Commons Attribution.

modification with Au NPs, despite one of the worst photoabsorption properties (narrow LSPR), among 15 commercial and DAP samples modified with Au NPs in the same procedure, probably due to fast transfer of electrons in shallow ETs [192], as shown in Fig. 8.

5.2 Hybrid nanostructures Advanced nanostructures have been proposed for performance improvement of plasmonic photocatalysts by their coupling with other photocatalysts, catalysts, and insulators for more efficient electron/energy transfer and improvement of photoabsorption and adsorption properties. In this section, several examples of novel nanostructures are presented below. Three and/or four-component nanocomposite materials with a magnetic core (Fe3O4), SiO2 interlayer (in some cases), TiO2 shell, and NM deposits on the surface [55] have been proposed for various applications, such as biomedical applications as magnetic resonance imaging (MRI) contrast agents, magnetic drug delivery, and for easier recovery of photocatalyst (with vis activity due to plasmonic photocatalysis) after use [76, 77]. WO3 modified with gold-POM NPs shows an enhanced vis activity because of the presence of both Au (energy transfer and catalytic effect in the dark) and POMs (inhibition of charge carriers’ recombination) [142]. Interesting hybrid nanostructure (bi-overlayer plasmonic photocatalyst), composed of two photocatalysts: Au/TiO2 and Cu/SnO2, deposited on the opposite sides of FTO, exhibits simultaneous activity, i.e., degradation of OCs on Au/TiO2 surface and oxygen reduction reaction (ORR) on the Cu/SnO2 side [79]. It has been proposed that the high activity of this photocatalyst (four times higher than Au/TiO2) originates from the efficient separation of charge carriers due to an electron transfer from Au to CuO (Au ! TiO2 ! FTO ! SnO2 ! CuO). Another example of hybrid nanostructure has been proposed for CO oxidation on Au-loaded hedgehog-shaped TiO2 nanospheres additionally modified with CuO [193]. Numerous advantages of this system have been suggested: (i) CuO promotes the dispersion of Au NPs on the TiO2 surface, (ii) CuO reinforces the electron interaction between Au NPs and TiO2 (efficient charge mediator because of vis absorption), (iii) CuO promotes an efficient electron transfer from Au NPs to TiO2, and (iv) CuO allows efficient oxygen adsorption and activation. Additionally, other compounds than plasmonic metal NPs are also adopted as sensitizers to broaden the vis absorption of wide-bandgap SCs, for example, transition metal complexes. In particular, ruthenium(II) complexes are commonly used since they exhibit suitable photophysical and photochemical properties, especially a desirable vis absorption of metal-toligand charge transfer (MLCT). In addition, the strong electric field (E-field) provided by plasmonic NPs might significantly enhance the absorption of molecules localized in their evanescent field [194]. Therefore, the combination of transitional metal complexes with NMs might induce a synergetic effect and enhance the photocatalytic activities under vis [195]. Zheng et al. have conducted the systematic studies on hybrid photocatalysts composed of TiO2 modified with NMNPs and ruthenium(II) complexes for decomposition of OCs [82, 195, 196]. The UV/vis diffuse reflectance spectrum (DRS) has confirmed that both modifiers contribute to the visible light response, attributing to LSPR and MLCT. In addition, due to the presence of both LSPR and MLCT absorbance of the hybrids, enhancement of photocatalytic activity would be expected under vis irradiation. However, different results have been obtained, that is, a decrease in the activity for bi-modified samples than mono-modified ones. Accordingly, it has been suggested that charge carriers have recombined in the second modifiers instead of being transferred to the surface of titania. Similar findings have been obtained for bi-metalmodified titania (Ag/Au/TiO2), where only samples with NMNPs deposited separately on titania support, i.e., Ag NPs and Au NPs show enhanced activity, whereas core/shell NPs of NMs have caused a decrease in the activity by possible charge carriers’ recombination (Au ⇆ Ag) [197]. Besides the role of photosensitizer, ruthenium(II) complex itself might also serve as a photocatalytic unit in a hybrid system. Maeda group takes advantage of a ruthenium(II) dinuclear complex RuBLRu’ in combination with various metalloaded SCs to construct hybrid materials for photocatalytic reactions, as shown in Fig. 9 [198–202]. RuBLRu’ has been

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FIG. 9 Hybrid photocatalysts of RuBLRu’/Ag/semiconductor (self-drawn based on Maeda research [194–198]).

adsorbed on Ag-loaded TaON to form RuBLRu’/Ag/TaON for the visible light-driven photocatalytic reduction of CO2 [199, 202], and the UV/vis DRS confirms broad LSPR by Ag in the vis region. Importantly, Ag NPs loaded on TaON have enhanced the photocatalytic activity of the hybrid, though there is no clear experimental evidence for the participation of LSPR of Ag in the photocatalytic reaction. Later, ATaO2N (A ¼ Ca, Sr, Ba) perovskites have been used as SCs [201]. Among three perovskites, CaTaO2N shows reasonable performance, and therefore, it has been utilized as the building block toward the hybrid of RuBLRu’/Ag/CaTaO2N photocatalyst that forming HCOOH via CO2 reduction with high selectivity (>99%) under visible light (l > 400 nm). In this work, it has been proposed that Ag NPs do not work as a co-catalyst, but rather as a promoter for interfacial electron transfer and RuBLRu’ might exist in close vicinity of the Ag/CaTaO2N interface. Further on, metal-free organic SC of graphitic carbon nitride (C3N4) has been used to construct a RuBLRu’/ Ag/C3N4 photocatalyst [200]. This hybrid exhibits high selectivity for CO2 to HCOOH conversion (87%–99%) with the mechanism based on the Z-scheme principle, and thus, Ag is considered as a conductor but not as a photosensitizer, i.e., Ag NPs collect electrons (lifetimes of several milliseconds) from the CB of C3N4, which are further transferred to the excited state of RuBLRu’, thereby promoting photocatalytic CO2 reduction. Moreover, this work has been extended to a series of hybrids based on GaN:ZnO solid solutions, TaON, and Ta/N-codoped TiO2 SCs, resulting in RuBLRu’/Ag/GaN: ZnO, RuBLRu’/Ag/TaON, and RuBLRu’/Ag/TiO2:Ta/N [198]. Similar to the previous studies, Ag NPs have not been considered as a photosensitizer but as a conductor (not plasmonic photocatalysis). Despite the aforementioned work from Maeda group, LSPR might be used to enhance the photoinduced oxidation activity of dye concerning the synthesis of chemical compounds under vis irradiation. Excitation of the MLCT band in ruthenium(II) complex generates an excited state that is both a good oxidant and a good reductant [194]. Mori et al. have designed and prepared a hybrid nanosized photocatalyst composed of core/shell Ag@SiO2 NPs with an anchored [Ru (bpy)3]2+ dye, as shown in Fig. 10 [203]. It has been verified that the enhanced LEMF near Ag NPs might boost the

FIG. 10 Hybrid nanosized photocatalyst composed of core/shell Ag@SiO2 NPs with an anchored [Ru(bpy)3]2+ dye (self-drawn based on [203]).

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excitation rate and quantum efficiency of [Ru(bpy)3]2+ and thus increase the energy and/or electron transfer to O2, enhancing the photooxidation activity. Moreover, by comparing with Au NPs, the match of plasmon resonance frequency and excitation frequency of the dye is also important for the enhancement of photocatalysis reaction. In consistence, An et al. have shown that the spectral overlap enables a substantial resonant plasmonic enhancement of MLCT and photoreactivity of [Ru(bpy)3]2+ [194]. In fact, earlier work has demonstrated that photocurrent generation based on photoexcitation of Ru(bpy)2+ 3 is efficiently enhanced by LSPR from the Ag electrode [204]. It is worth mentioning that the distance between the plasmonic metal and ruthenium(II) complex is critical as well, the preferential localization of dye in a direct vicinity of NMNPs is ideal for achieving an E-field-enhanced photoexcitation of ruthenium(II) complex, as the plasmonic E-field intensity decays with an increase in a distance from NM [194]. However, quenching of the photoexcited ruthenium(II) complex by the nearby plasmonic metal might also occur. For instance, controlling the distance between the Ru(bpy)2+ 3 and Ag using the Langmuir–Blodgett technique leads to efficient photocurrent generation [204]. Ferroelectrics have also been applied for photocatalysis reactions since the polarization of ferroelectrics might improve the charge carriers’ separation and thus the photocatalytic activities [205]. Accordingly, a range of ferroelectrics has attracted interest in photocatalysis, whereas their photocatalytic performance still needs to be improved. Ferroelectrics usually have a large bandgap, a low absorption coefficient, and a low-photon conversion efficiency when used alone. Therefore, various methods have been tried to enhance their photocatalytic efficiency, among which the modification with NMNPs turns out to be efficient. For example, Su et al. have enhanced the activity of ferroelectric BaTiO3 after its modification with Ag [206]. Besides the ferroelectricity and specific charge transfer kinetics in the Ag/BaTiO3 hybrid, LSPR of Ag is suggested to contribute to the enhancement as well. Wu et al. have prepared a ternary Z-scheme heterojunction photocatalyst of BaTiO3/Au/g-C3N4, in which Au NPs besides, acting as an electron mediator, could absorb vis by LSPR effect to inject “hot” electrons into CB of g-C3N4 to participate in the photocatalytic reactions [207]. Recently, a dual-modified photocatalyst of BaTiO3, that is, Ag/BaTiO3/MnOx, where LSPR of Ag NPs on BaTiO3 has been noticed, but its influence on the photocatalytic reactions has not been discussed [208]. Lan et al. have observed an enhanced photocatalytic activity in one-dimensional KNbO3 nanowires modified with Au NPs due to LSPR and interband transitions on Au NPs, depending on the size of Au NPs, that is, an increase in the activity with an increase in the size of Au NPs from 5 to 10 nm [47] (which might suggest more important LSPR effect for the overall activity). In another study, it has been proposed that the improved photocatalytic activity and stability of Ag/AgNbO3 is caused by the LSPR effect, ferroelectric polarization, specific exposed facets, and high crystallinity at the heterogeneous interface [209]. Meanwhile, BiFeO3 (one of the most promising multiferroic materials) decorated with NPs of Au and Ag has proven to be active under visible light irradiation due to strong field enhancement, especially near Ag NPs [210]. Similarly, as mentioned in the former paragraph (dye/NM hybrids), the presence of LSPR does not guarantee its participation in promoting photocatalytic activities. For example, Cui et al. have coated BaTiO3 with nanostructured Ag, resulting in vis absorption due to SPR, but photo-decolorization of RhB (under both UV- and visible-light-blocking filters) indicates that “hot” electrons do not contribute to the photocatalytic activity [211]. Besides, it has been proposed that the geometry of metal deposits on ferroelectric substrate plays an important role in enlarging or inhibiting the polarization effect, and thus, the metal deposits should not be too thick to block the effect from polarization [212]. Chao et al. have indicated that in the case of Au/BaTiO3, the “hot” electrons (LSPR of Au) have only little effect on the improvement of visible-light photocatalytic performance [213]. In summary, as pointed by Kumar et al., it is important to realize that the integration of plasmonic metals with ferroelectrics might be beneficial for two aspects: (1) the injected charge carriers are effectively driven to spatially separate over the ferroelectric particles and (2) tuning the Schottky barrier height, for example, by changing the chemical composition of NM/SC, thus providing an interfacial contact that favors the hot charge injection [214]. Both effects can prolong the lifetimes of both the excited electrons and holes. Therefore, for effective utilization of LSPR in ferroelectric-based photocatalysts, more efforts are needed to reveal the critical parameters and achieve higher photocatalytic efficiency.

5.3 Phonic crystal-based plasmonic photocatalysts As presented in the previous sections, both efficient light harvesting and charge separation are crucial for the overall photocatalytic performance of plasmonic photocatalysts. Therefore, photonic crystals (PCs) with slow photon effect and photonic bandgap (PBG) have been proposed for the efficient harvesting of incident photons in plasmonic photocatalysis. The proposed studies involve mainly titania in the form of IO modified with deposits on NMs. Although the study on PCs-based plasmonic photocatalysts has just started, and several reports have been published (including the first review paper [39]), it is obvious that the perfect matching between red-/blue-edges of PBG and LSPR of NMs by tuning of the void diameter of IO and the size of NMs, respectively, results in significant enhancement of photocatalytic activity. PCs modified with NM have recently been proposed for different photocatalytic reactions, such as decomposition of OCs [215], water splitting [216], and solar cells [64]. It has been proposed that the extended path of light (length and

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duration) resulting from the PCs feature, that is, a multi-scattering phenomenon, should be responsible for the enhanced quantum efficiency of photocatalytic reactions. It should be mentioned that probably, the most difficult is to fabricate PCsbased plasmonic photocatalysts to avoid the destruction of Bragg’s diffraction during NMs loading. NMs are usually deposited on TiO2 either before or after its infiltration inside opal (IO are prepared from opal template by TiO2 infiltration and subsequent removal of opal template) by thermal, chemical, and photochemical (photodeposition) methods. NM-modified PCs have been applied in photocatalysis because of several advantages, i.e., (i) to allow vis absorption via LSPR, (ii) slow photon effect resulting in plasmonic field enhancement, and (iii) inhibition of charge carriers’ recombination (not plasmonic photocatalysis). It should be pointed out that plasmonic field enhancement is only possible when PBG corresponds to LSPR wavelengths. In some cases, mismatch in PBG and LSPR can even result in a decrease in activity, as reported for Au and Ag NPs deposited on TiO2-coated nanoporous alumina IO with PBG of 800 nm and LSPR in the range of 400–550 nm [217]. Indeed, significant enhancement of activity (ca. 2.3 in comparison to Au NPs on nanoparticulate TiO2) has been observed when Au NPs are deposited on TiO2 PCs when LSPR overlaps with the blue-edge of PBG [218]. Two other studies have also confirmed higher activity in photoelectrochemical water splitting when red-edge PBG of PCs (TiO2 NRs PCs and Mo:BiVO4) matches with LSPR of Au NPs [219, 220]. In the case of PCs-based plasmonic photocatalysts (and any other PCs-based photocatalysts), the integrity of IO structure is very important since any cracks and disorders influence the photoabsorption properties. At the same time, experiments with original and crushed structures show that uncrushed photocatalysts possess much higher activity [221, 222], confirming the importance of perfect IO morphology. Although a decrease in activity has been observed for partially crushed samples, it must be mentioned that the activity of these PCs is much higher than that by particulate photocatalysts. The co-participation of two functions of NMs, that is a plasmonic photosensitizer (under vis) and scavenger of electrons (usually under UV) that has been used by Rahul et al. when TiO2-IO has been co-modified with NPs of Au and Pt [223]. It has been proposed that Au excitation by LSPR results in “hot” electron transfer via TiO2 to Pt on which H2 evolution takes place. Similar to all other NM-modified SCs, the amount of NMs, their morphology, and localization on PCs are crucial for the overall activity, as already discussed in the review paper [39]. For example, it has been suggested that the deposition of NM layer on PCs is not recommended since it might work as a filter avoiding light to reach IO. Accordingly, it has been proposed that the preparation of IO with incorporated NM inside voids would be the most recommended, as confirmed by a preliminary study [224].

6

Summary and conclusions

In this chapter, plasmonic photocatalysis mainly refers to photocatalysts of NMs-modified SCs, for which the LSPR of NM carries a significant benefit. Plasmonic photocatalysis holds the promise as one of the reliable solutions toward the crises of environment, water, and energy, due to their ability to work under vis irradiation. Therefore, understanding the mechanism of plasmonic photocatalysis is essential. Up to now, three mechanisms of charge transfer, energy transfer, and plasmonic heating have been proposed. However, because of the diversity in photocatalyst structures, reaction conditions, and tested systems, the mechanisms might be more complex and thus are still worth further exploration. Promisingly, the discovery of plasmonic photocatalysis has led to increasing applications that involving environmental purification and solar energy conversion, despite the short history of this topic. As an ongoing development, new applications in other areas like organic synthesis and biomedicine appear as well. In general, the photocatalytic performance of plasmonic photocatalysts strongly depends on various factors regarding the properties of both NMs and SCs, interaction between them, etc. However, vis harvesting efficiency and charge carrier recombination remain as the key factors hindering the further application. Till now, various attempts have been put to improve the overall efficiency, and new trends to utilize special functionalized SCs such as ferroelectrics and PCs are put forward in the hybrid structures. Therefore, it is likely that more advanced hybrids remain to be discovered and investigated in the nearest future.

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[207] M. Wu, T. Ding, Y. Wang, W. Zhao, H. Xian, Y. Tian, T. Zhang, X. Li, Rational construction of plasmon au assisted ferroelectric-BaTiO3/Au/gC3N4 Z-scheme system for efficient photocatalysis, Catal. Today (2019), https://doi.org/10.1016/j.cattod.2019.1004.1061. in press. [208] Y.F. Cui, H.H. Sun, G.D. Shen, P.P. Jing, Y.P. Pu, Effect of dual-cocatalyst surface modification on photodegradation activity, pathway, and mechanisms with highly efficient Ag/BaTiO3/MnOx, Langmuir 36 (2020) 498–509. [209] Z.Z. Yu, B.W. Zhan, B.H. Ge, Y.T. Zhu, Y. Dai, G.J. Zhou, F.P. Yu, P. Wang, B.B. Huang, J. Zhan, Synthesis of high efficient and stable plasmonic photocatalyst Ag/AgNbO3 with specific exposed crystal-facets and intimate heterogeneous interface via combustion route, Appl. Surf. Sci. 488 (2019) 485–493. [210] X.G. Zhang, B. Wang, X.Z. Wang, X.H. Xiao, Z.G. Dai, W. Wu, J.F. Zheng, F. Ren, C.Z. Jiang, Preparation of M@BiFeO3 nanocomposites (M¼Ag, Au) bowl arrays with enhanced visible light photocatalytic activity, J. Am. Ceram. Soc. 98 (2015) 2255–2263. [211] Y.F. Cui, J. Briscoe, S. Dunn, Effect of ferroelectricity on solar-light-driven photocatalytic activity of BaTiO3-influence on the carrier separation and stern layer formation, Chem. Mater. 25 (2013) 4215–4223. [212] Y.F. Cui, S.M. Goldup, S. Dunn, Photodegradation of rhodamine B over ag modified ferroelectric BaTiO3 under simulated solar light: pathways and mechanism, RSC Adv. 5 (2015) 30372–30379. [213] C.Y. Chao, Y.P. Zhou, T.L. Han, Y.Y. Yang, J.Y. Wei, H. Li, W.W. He, Ferroelectric polarization-enhanced photocatalytic properties and photoinduced charge carrier behavior of Au/BaTiO3, J. Alloys Compd. 825 (2020), 154060. [214] V. Kumar, S. O’Donnell, B. Zoellner, J. Martinez, G. Wang, P.A. Maggard, Interfacing plasmonic nanoparticles with ferroelectrics for hot-carrierdriven photocatalysis: impact of Schottky barrier height, ACS Appl. Energy Mat. 2 (2019) 7690–7699. [215] M.O.A. Erola, A. Philip, T. Ahmed, S. Suvanto, T.T. Pakkanen, Fabrication of Au- and Ag-SiO2 inverse opals having both localized surface plasmon resonance and Bragg diffraction, J. Solid State Chem. 230 (2015) 209–217. [216] H. Zhao, Z.Y. Hu, J. Liu, Y. Li, M. Wu, G. Van Tendeloo, B.L. Su, Blue-edge slow photons promoting visible-light hydrogen production on gradient ternary 3DOM TiO2-Au-CdS photonic crystals, Nano Energy 47 (2018) 266–274. [217] S.Y. Lim, C.S. Law, L. Liu, M. Markovis, A.D. Abell, A. Santos, Integrating surface plasmon resonance and slow photon effects in nanoporous anodis alumina photonic crystals for photocatalysis, Cat. Sci. Technol. 9 (2019) 3158–3176. [218] Y. Lu, H.T. Yu, S. Chen, X. Quan, H.M. Zhao, Integrating plasmonic nanoparticles with TiO2 photonic crystal for enhancement of visible-lightdriven photocatalysis, Environ. Sci. Technol. 46 (2012) 1724–1730. [219] X. Zhang, Y. Liu, S.T. Lee, S.H. Yang, Z.H. Kang, Coupling surface plasmon resonance of gold nanoparticles with slow-photon-effect of TiO2 photonic crystals for synergistically enhanced photoelectrochemical water splitting, Energ. Environ. Sci. 7 (2014) 1409–1419. [220] L.W. Zhang, C.Y. Lin, V.K. Valev, E. Reisner, U. Steiner, J.J. Baumberg, Plasmonic enhancement in BiVO4 photonic crystals for efficient water splitting, Small 10 (2014) 3970–3978. [221] Y.F. Wang, D.B. Xiong, W. Zhang, H.L. Su, Q.L. Liu, J.J. Gu, S.M. Zhu, D. Zhang, Surface plasmon resonance of gold nanocrystals coupled with slow-photon-effect of biomorphic TiO2 photonic crystals for enhanced photocatalysis under visible-light, Catal. Today 274 (2016) 15–21. [222] C.T. Dinh, H. Yen, F. Kleitz, T.O. Do, Three-dimensional ordered assembly of thin-shell au/TiO2 hollow nanospheres for enhanced visible-lightdriven photocatalysis, Angew. Chem. Int. Ed. 53 (2014) 6618–6623. [223] T.K. Rahul, N. Sandhyarani, Plasmonic and photonic effects on hydrogen evolution over chemically modified titania inverse opals, ChemNanoMat 4 (2018) 642–648. [224] T. Raja-Mogan, A. Lehoux, M. Takashima, E. Kowalska, B. Ohtani, Bright-Part Design for Efficient Photocatalysis, under Preparation, 2020.

Chapter 28

Graphitic carbon nitride-based metal-free photocatalyst Marco Minella, Fabrizio Sordello, and Claudio Minero Department of Chemistry and NIS—Nanostructured Interfaces and Surfaces Interdepartmental Centre, University of Turin, Turin, Italy

1 Introduction In 2019, the articles published and indexed with the term “g-C3N4” on the Scopus database were more than 1500; of these, 41 are reviews and 1079 papers are dedicated to the photocatalytic properties of this material (indexed as “g-C3N4 and photocat*”). The aim of this chapter is not to review all the scientific literature regarding the photocatalytic and semiconducting properties of g-C3N4, for which some excellent reviews reported and commented the numerous works that are published every year on these topics [1–5]. The term photocatalysis was largely used since the early 1980s to indicate the photoactivation of a chemical reaction through the absorption of a photon from a species that is unchanged at the end of the process, that is, the photocatalyst [6]. Even if the first studies on photocatalytic process were carried out in the beginning of the 20th century (with the works by Plotnikow in 1910 [7] and Landau in 1912–1913 [8, 9]), only from the late 1970s with the studies regarding the photoassisted electrosplitting of water on titanium dioxide electrodes by Fujishima and Honda [10], the photocatalyzed reduction of carbon dioxide on irradiated semiconductor [11], and the use of irradiated semiconductors to degrade environmental pollutants [12–14] (with the seminal contributions – among others – of the research group of the University of Turin headed by E. Pelizzetti [15–18]), the investigation of the photocatalytic process started to have the actual profile and weight. On the basis of the rough, but incisive division proposed by Serpone and Emeline [19], the studies regarding the photocatalytic processes on irradiated semiconductors can be divided into three sequential generations. The first generation of photocatalysts were pristine inorganic semiconductors (in the beginning, mainly ZnO and TiO2, then largely TiO2 only). These materials – irradiated under UV light – promote the production of reactive species whose nature, stability, and reactivity were in-depth investigated through rigorous, innovative, and effective experimental approaches able to (i) clarify the most important features of the photocatalytic process and (ii) be seminal for the explosion of the scientific interest on these topics. In some cases, these basic studies seem overlooked from the authors of the thousands of articles published every year in photocatalysis. The second generation of photocatalysts tried to bypass an intrinsic drawback of the pristine medium bang gap (Eg) semiconductors, that is, their limited ability to absorb the UV portion of the solar spectrum due to their medium/high band-gap. To push the onset of absorption toward longer wavelengths, the insertion of atoms (non-metals or metals) in the crystalline structure of the semiconductor has been deeply explored, starting from the Asahi et al.’s article [20] on the visible-light photocatalytic activity of nitrogen-doped TiO2. The real nature of the color centers (oxygen vacancies with the production of Ti+3 centers vs localized states) and the positioning of the intra-band-gap states causing the visible sensitization were strongly debated topics. The third generation is based on the production of heterostructures formed by two or more inorganic semiconductors activated by a multiphoton absorption. The coupling of two semiconductors (in-depth debated below) generates an interface that can or cannot operate as a recombination space depending on the band structures of the coupled semiconductors. The idea was not fully new, as it was first explored in 1980s by Serpone et al. [21]. It is in some way the consequence of the comprehension of the operational mechanism of charge transfer in semiconductors that later gave the conception of the so-called dye-sensitized cells, in which the electron transfer from a molecular dye (that works as an antenna for visible photons) to a semiconductor occurs [22]. Some of the later-proposed heterostructures show a quite efficient conversion of the visible fraction of the solar spectrum reaching the Earth’s surface. After the discovery of the structure of the fully aromatic C60 fullerene by Kroto et al. [23] and that of carbon nanotubes by Iijima [24], and especially after the seminal work regarding the properties and the structure of graphene by Novoselov

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et al. [25], the new synthetic carbon allotropes were diffusely applied in almost all the fields of science. Their most interesting characteristics are mainly related to their peculiar architectures (from the 0 dimension of the carbon quantum dots and fullerene to the bidimensionality of the graphene sheets passing through the 1D single or multiwall carbon nanotubes) and to their easy functionalization by the common procedures typical of the organic chemistry, which allow an almost infinite possibilities of chemical modifications. Furthermore, their semiconductor properties and, in some case, their impressive electronic conductivity (e.g., the ballistic conductivity – negligible resistivity – of perfect graphene or CNTs) attracted massively the attention of the scientists operating in the field of photocatalysis. These materials are good candidates to solve environmental drawbacks such as element availability, disposal problems of the devices at the end of life, and significant toxicity. Furthermore, their high chemical flexibility allows an easy and effective coupling with inorganic phases to give hybrid systems. These innovative organic materials find application in electronics, for example, oLED, for solar energy conversion in photovoltaic devices or for the CO2 direct conversion to fuels. Applications are also emerging in the field of decontamination of environmental compartments. From the in-depth analysis of their photocatalytic properties, their weakest points emerged, namely scarce stability, low conductivity of non-perfect structures, difficulty of a massive synthesis without defects, and scarce charge-transfer kinetics. Among the most innovative metal-free and visibly active photocatalyst, there is the so-called graphitic carbon nitride (g-C3N4), a conjugated polymer with lamellar structure similar to graphite and with the same ability to be exfoliated in monolayers (2D structures) or structures composed of few layers. Graphitic carbon nitride is in this light a new hot spot for the research in the field of photocatalysis. The question if these metal-free photocatalysts are the fourth generation of photocatalysts is already open. If we pay close attention to the transition from inorganic semiconductors to completely organic or hybrid organic/inorganic materials, these photocatalysts seem to be something completely new. On the contrary, if we stress on their semiconductor and photocatalytic properties, it is manifest that it is not essential to introduce new paradigms for describing their photocatalytic behavior, and consequently, we are not facing with a really new generation of photocatalytic materials. This chapter critically evaluates the works on g-C3N4 emphasizing not only the consolidated conclusions about the photoactivity of this material, but also the contradictions regarding its properties. The reasons of the conflicting conclusions on the photocatalytic properties of g-C3N4 are often related to the lack of a detailed description of the synthesis – frequently carried out in uncontrolled or only partially controlled conditions – with the results that a plethora of different materials are grouped under the name g-C3N4. Furthermore, the photochemical experiments are often done in non-standardized conditions. This makes the comparison difficult among photochemical results obtained in different laboratories. In other cases, the experimental evidences are often interpreted in the light of band structures and junction properties often not demonstrated, but only hypothesized. By considering this and the impressive efforts of the scientific community on this topic, it is clear that it is quite difficult to extract decisive and robust conclusions pointing the attention to all the singular details. In this chapter, we will try to give an overall vision of the topic and draw some general conclusions.

2

Organic semiconductors: A focus on g-C3N4

Among organic semiconductors, (semi)conductive polymers are materials with suitable properties to fabricate electrodes because of the possibility to form films and to functionalize their surface with suitable co-catalysts through covalent bonds, ensuring fast electron transfer to the catalytic moiety. Moreover, their structure can be easily modified, e.g., with modification of the monomers through molecular engineering, with a consequent change in the material properties. The organic semiconductors properties like the energy gap, CB and VB potentials, can thus be tuned, according to the application for which they are designed [26, 27]. Recent reports claimed large photocurrents, also in aqueous environments [28, 29], encouraging photon efficiency for polyanilines, polythiophenes, and polypyrroles and sketched solutions for the main drawbacks of this class of materials, namely, the low-charge-carrier mobility and the photo- and the electrostability [30, 31]. In this context, g-C3N4 offers wide perspectives for improvement because it shares with other organic semiconductors the tunability of its properties and the facility of functionalization, with unparalleled physico-chemical features, such as its extraordinary thermal stability. Thermogravimetric analysis shows limited weight losses until 600 °C. Sublimation of g-C3N4 begins at 450 °C with a slow rate, even though only at 600–650 °C, the sublimation rate increases, whereas decomposition occurs at 750 °C [32, 33]. Experimental TGA results may vary depending on the particular synthesis conditions and the degree of polymerization achieved in the particular material under study. Also, the chemical stability of g-C3N4 is outstanding. In fact, g-C3N4 is stable in water, alcohol, DMF, THF, diethyl ether, and toluene, even though its solubility is very limited [32]. The only exception is represented by the irreversible hydrolysis in molten alkali metal hydroxides. Conversely, the reversible exfoliation in concentrated acids, which is fully reversible upon neutralization [33], could be a positive feature.

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2.1 Photophysical and electrochemical properties g-C3N4 absorption edge falls at 450–470 nm, corresponding to 2.6–2.7 eV. Even though g-C3N4 can still be viewed as a wide band-gap semiconductor, its absorption in the visible range is not negligible, giving it some advantages compared with materials like ZnO or TiO2. However, its optical absorption varies markedly with synthesis conditions, porosity, and doping. g-C3N4 has a direct band-gap, and therefore, the recombination between electron and hole is radiative. The photoluminescence at 470 nm can be used to evaluate not only the magnitude of the band-gap, but also the charge-carrier lifetime. The potential of the valence and conduction bands determines the behavior of a semiconductor and the reactions that can be promoted under irradiation. Such potentials heavily depend on the physico-chemical properties of g-C3N4, such as porosity, degree of polymerization, and doping. Compared with ZnO or TiO2, g-C3N4 has a significantly more negative conduction band potential, implying that it would be more suitable than these two materials for the photocatalytic reduction of CO2. On the other hand, g-C3N4 valence band potential is less positive, penalizing it in advanced oxidation processes compared with the above-mentioned oxides. In analogy with the metal oxides, also in the case of g-C3N4, the orbitals of the least electronegative atom (in this case C) give a larger contribution to the density of states of the conduction band, while the orbitals of the most electronegative atom (in this case N) predominantly constitute the valence band states (Fig. 1; [34]). Despite its wide band-gap and its outstanding thermal and chemical stability, which associate g-C3N4 to metal oxide semiconductors, g-C3N4 shares an important property with other organic materials. In metal oxides, charge carriers are usually treated as belonging to extended states, with limited references to localization and to the effects of charge localization on the distortion of the lattice; in the case of organic semiconductors, the charge localization and its coupling with molecular adjustments necessary to accommodate the modified electron density are referred as polaron. The formation of the polaron stabilizes the charge carrier, and sometimes, a second charge carrier can be accommodated, with a further energy gain, and the formation of a bipolaron. In organic semiconductors like polythiophenes, polypyrroles, and polyaniline, their oxidation causes a local structure modification from an aromatic-like to a quinoid-like structure (see Fig. 2F and G [35]). Lattice distortions were proposed by Meek and coworkers, who, in their computational study [34], showed how position 1 N atoms belonging to neighboring heptazine units were significantly less distanced in cationic structure compared with their neutral homologues, and how the planarity of the structure was affected by the removal of an electron (Fig. 2A–E). Pristine g-C3N4 is usually an n-type semiconductor as witnessed by anodic photocurrents [36] and negative photopotentials. Nonetheless, doping and functionalization can induce a p-type behavior, with opposite sign in photocurrent and photopotential (vide infra [37, 38]).

5 4 3

N (S) N (V) C (S) C (V)

Energy (eV)

2 1 0 –1 –2 –3 –4 –5

Y

*

X

FIG. 1 Atomic character resolved Kohn
Sham band structures of the polymeric C3N4 periodic system. (Reprinted with permission from G.A. Meek, A. D. Baczewski, D.J. Little, B.G. Levine, Polaronic relaxation by three-electron bond formation in graphitic carbon nitrides. J. Phys. Chem. C 118 (8) (2014) 4023–4032. Copyright (2014) American Chemical Society.)

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FIG. 2 (A–E): minimum energy structures for the neutral (left panels) and cationic (right panels) melam (A), melem dimer (B), polymeric melem trimer (C), 2-D melem trimer (D), and 2-D melem hexamer (E). The distances between position 1 nitrogen atoms are evidenced on each of the 10 structures, as their variation upon electron abstraction. (F, G: polaron and bipolaron formation in polythiophene and polyaniline, evidencing the associated structure changes. Reprinted with permission from G.A. Meek, A.D. Baczewski, D.J. Little, B.G. Levine, Polaronic relaxation by three-electron bond formation in graphitic carbon nitrides. J. Phys. Chem. C 118 (8) (2014) 4023–4032. Copyright (2014) American Chemical Society.)

3

Synthesis and modification of g-C3N4

The first synthesis of carbon nitride was reported in 1834 by Berzelius and Liebig through the thermolysis of mercury(II) thiocynate. In this light, C3N4 is one the oldest synthetic polymer reported. In particular, Liebig discovered melamine and its dimer (melam), trimer (melem), and uncondensed polymer (denoted by Liebig as melon) recognizing them as heptazine

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and triazine-based molecular compounds. Liebig carried out the pyrolysis of ammonium chloride and potassium thiocynate that gave a yellow compound named as melon, which is insoluble in water [39]. A year later (1835), Gmelin synthesized potassium derivate of the melon by heating potassium ferricynide and sulfur in a crucible obtaining potassium hydromelonate [40]. Further advances in the comprehension of the chemical structure of this synthetic polymer were achieved in 1922 by Franklin, who reported that carbon nitride (with the general formula (C3N4)x) could be the final product of the polymerization and condensation of melon [41], and in 1937 by Pauling and Sturdivant who proposed for the first time a coplanar s-triazine unit as the elementary structure of this polymer [42]. The high insolubility of this compound and its quite high chemical inertness moved out the attention of the scientific community. Only in the 1990s, the attention for this polymeric compound rose again as a consequence of the theoretical study by Teter and Hemley who investigated the relative stability, structure, and physical properties of carbon nitride polymorphs by first-principle calculations. They predicted that the b-C3N4 phase (the dense phase with a totally sp3-hybridized structure) could potentially be synthesized at high pressure and would have bulk hardness and module values equal or higher than those of diamond [43]. Unluckily, this phase is quite unstable compared with the high thermodynamic stability of the graphitic C3N4. The growth of the interest of the scientific community for g-C3N4 is related to the discovery of its catalytic properties – as metal-free catalyst for Friedel–Crafts reactions [44] – and especially as photocatalyst for the visible light-induced water photosplitting [45]. These seminal works opened the route not only for the investigation of the (photo)catalytic properties of this material but also for the discovery of innovative methods of synthesis. The dominant method for the synthesis of g-C3N4 is the solid-state thermal condensation of nitrogen-rich organic precursors. In the seminal work on water photosplitting, Wang et al. proposed the use of cyanamide as the precursor of g-C3N4. At roughly 550 °C, cyanamide starts its condensation to give dicyandiamide, melamine, melem, and finally, the polymeric g-C3N4. The change in the calculated energy diagram for all the intermediates in the synthesis of carbon nitride is reported in Fig. 3 [46] as computed by Thomas et al. Theoretically, after the formation of melamine, further condensations can proceed via the triazine route to C3N4, or alternatively, melamine can form melem and then follow the tri-s-triazine route to give C6N8. Usually, the g-C3N4 precursor is simply placed in a covered crucible and heated in air or in controlled atmosphere (e.g., Ar, N2, He, NH3, and H2) at different temperatures, in a range that spans from 450 °C to 650 °C. The most used precursors are cyanamide, dicyanamide, melamine, urea, thiourea, guanidine, thiocyanate, and guanidine hydrochloride. For an overall evaluation of the precursors proposed and the range of experimental conditions adopted in the thermal condensation synthesis of g-C3N4, please refer to Refs. [46–48]. Even if it is quite complex to recognize definitive roles, it is undeniable that the nature of the precursor and the experimental conditions of the synthesis (e.g., the final temperature of the condensation, but also the adopted ramp) affect abruptly the final properties of the synthesized carbon nitride, especially in terms of C/N ratio, specific surface ratio (due to the different thermal exfoliation obtained), porosity, and band edge absorption (directly related to the value of energy gap). As an example, Yan et al. synthesized g-C3N4 through condensation of melamine at different heating temperatures, observing that moving the final temperature from 500 °C to 580 °C, the C/N ratio changed from 0.721 to 0.742 and the energy gap from 2.8 to 2.75 eV [49]. Mo et al. reported that only at calcination temperature higher than 500 °C, a fully g-C3N4 structure can be formed. The red shift of the absorption edge in the reflectance spectra of g-C3N4 has been usually ascribed to the increase of the degree of conjugation in the synthesized structure as a consequence of a higher polymerization degree [50]. In this light, it is also manifest that the identification of carbon nitride as “g-C3N4” is in some way more a convention than a real stoichiometric identification (vide infra for further comments regarding the stoichiometry and the intrinsic doping of these materials). –400

Ecoh (kJ/mol)

–425 cyan dicyandiamide –450 –475

amide

melamine melam melem

melamine chain

–500 –525 –550 –575

dimelem melon

melom sheet C3N4 C6N8

FIG. 3 The change in the calculated energy diagram for all the intermediates in the synthesis of carbon nitride. (Reprinted with permission from € € J.M. Carlsson, Graphitic carbon nitride materials: variation of structure A. Thomas, A. Fischer, F. Goettmann, M. Antonietti, J.-O. Muller, R. Schlogl, and morphology and their use as metal-free catalysts. J. Mater. Chem., 18(41) (2008) 4893–4908. https://doi.org/10.1039/B800274F)

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Peculiar g-C3N4 architectures were produced through template-assisted methods. Here, the polycondensation of nitrogen- and carbon-rich precursor is carried out in the presence of an organic or inorganic template such as mesoporous silica or block polymers. Silica nanoparticles were used by Wang et al. to carry out the thermal cyanamide condensation to obtain a mesoporous graphitic carbon nitride [36]. On the other hand, 1-butyl-3-methylimidazolium tetrafluoroborate was used as soft organic template for the synthesis of B and F-doped mesoporous graphitic carbon nitride with a quite high surface area (444 m2 g
1) [51]. In addition to the methods of synthesis based on the solid-state thermal condensation of precursors, which are the most applied methods for g-C3N4 production mainly for their simplicity, other procedures have been proposed (see Fig. 4 for a general scheme of the most applied methods for the g-C3N4 synthesis). Solvothermal methods have been proposed for the condensation of the usual precursors in the presence of solvent and under high temperature and pressure. Montigaud et al. reported the condensation of melamine and cyanuric chloride with triethylamine as solvent in supercritical conditions (at P ¼ 130 MPa and T ¼ 250 °C). In the same work, they also proposed the pyrolysis of melamine (P ¼ 2.5 GPa, T ¼ 800 °C) in the presence of hydrazine as an effective route for the synthesis of carbon nitride [52]. The solvothermal procedure has also been applied in microwave-assisted synthesis. Hu et al. carried out the synthesis of g-C3N4 both in solvothermal conditions – cyanuric chloride and sodium azide were solvothermally treated with acetonitrile in autoclave at 220 °C for 15 h – and in a microwave oven by using the same reagents, but with a heating program of only 120 min (saving more than 13 h for carrying out the same solvothermal synthesis; [53]). A further alternative is the ionothermal method in which the condensation of carbon- and nitrogen-rich compounds is carried out in the presence of a mixture of inorganic salts with a melting point lower than the temperature of the polycondensation of s-heptazine. The molten salt mixture commonly used for the synthesis of g-C3N4 is the eutectic lithium chloride/potassium chloride (45:55 wt%, Tm ¼ 352 °C; [54]). At T > Tm, this LiCl–KCl mixture shows excellent and peculiar solvent properties owing to its high-temperature stability and non-corrosive properties. Furthermore, this eutectic has good solvation ability toward small polar molecules and, vice versa, good ability to aggregate higher molecular weight molecules, facilitating the condensation of the carbon nitride network. Bojdys et al. [55] investigated the formation of highly crystalline graphitic carbon nitride by condensation of dicyandiamide in the 45:55 wt%, molten lithium chloride: potassium chloride mixture. The FTIR analysis of the materials indicated the presence of a structure with few defects and few unreacted end groups and, therefore, an extensively condensed framework. With the same approach, Zou et al. used dicyandiamide and barbituric acid as co-precursors in lithium chloride and potassium chloride molten mixture with the aim to restrain the agglomeration, allowing the formation of a layered g-C3N4 [56]. A complete different approach for the synthesis of g-C3N4 is the electropolymerization. Halevy et al. reported the coatings with graphitic carbon nitride (g-C3N4) of the surface of a Ti/TiO2 nanotubes (NTs) electrode through melamine electropolymerization followed by heat treatment. The electropolymerization is a good tool to immobilize polymers on the surface of electrodes, especially for its high reproducibility, the homogeneity of the obtained thin coatings, and the possibility to easily and carefully control film thickness by adjusting the electrochemical parameters (potential and current density). Halevy et al. synthesized a polymelamine coating on TiO2 NTs grown on the surface of a metal titanium electrode. A simple potential cyclic scanning allowed the formation on the electrode surface of a polymelamine (polyM) coating. To convert the polyM film in a g-C3N4 coating, a treatment at 550 °C for 4 h was proposed [57].

FIG. 4 General scheme of the most applied methods for the g-C3N4 synthesis.

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Other proposed approaches for the synthesis of g-C3N4 coatings are the chemical vapor deposition (CVD) and the physical vapor deposition (PVD). As an example, the former was used by Zhang et al. for producing a quite interesting g-C3N4 with crystalline/amorphous lateral-like homostructures. The authors used a multistep procedure of deposition and polymerization of melamine to create g-C3N4 amorphous/crystalline homostructures. The control of the crystalline properties of the deposited g-C3N4 was carried out working at two different temperatures: at 650 °C for depositing the amorphous phase and at 750 °C for the production of the crystalline phase [58]. On the contrary, the PVD approach allows the deposition of preformed g-C3N4 on a substrate. The most important limitation of this approach is associated to the thermal instability of g-C3N4, which can decompose at 550 °C releasing many relatively high-mass gaseous derivatives. Sun et al. successfully implemented PVD of bulk g-C3N4 in N2 atmosphere, which protects partially the g-C3N4 structure. g-C3N4 sheets (CNS) that maintained semiconducting properties with 2.61 eV energy gap were obtained. During the PVD process, CNS condensed on the cold substrate with the common tri-s-triazine-based structure, but with a high degree of defectivity [59]. Once synthesized, especially in the case of syntheses based on the thermal condensation at the solid state of nitrogenrich precursor, g-C3N4 shows bulky properties and often scarce surface area due to the staking of the polymeric layers. As for the graphite/graphene couple, the exfoliation of the bulk g-C3N4 into monolayers or structures composed of few layers of g-C3N4 is a quite energetic process because of the strong interactions between the p-delocalized orbitals of each 2D g-C3N4 layer. The principal methods for obtaining the exfoliation of bulk g-C3N4 are (i) the sonochemical exfoliation and (ii) the thermal exfoliation. The sonication-assisted liquid exfoliation uses both organic solvents: isopropyl alcohol [60, 61], methanol [62], ethanol + water [63], formamide and tetrahydrofuran [64], and strong acid/base solutions. H2SO4 is able to intercalate into the interlayers of the bulk g-C3N4, successfully generating nanosheets with the thickness of a single atomic layer [65]. The same exfoliation ability was observed during the hydrothermal treatment of melamine-derived g-C3N4 in concentrated NaOH solution [66]. On the contrary, the thermal exfoliation consists of a quick thermal treatment of g-C3N4 able to generate single nanosheets. The exfoliation can be facilitated through the intercalation of NH4Cl into the interlayer of the carbon nitride structure. The thermal decomposition of the salt favors the exfoliation of the bulky structure [67]. Last, the sequential application of a thermal and a sonochemical step has also been reported as an efficient method for the production of single atomic layer g-C3N4 nanosheets with a thickness of 0.4–0.5 nm [61, 62]. Metals and non-metals of all the groups of the periodic table have been used as dopants in g-C3N4 [2]. For metal doping, the most common strategy is introducing the desired element mixing one of its salts together with the g-C3N4 precursor(s) prior the thermal treatment. To improve the dispersion of the metal with the precursor(s), the salt can be dissolved in an appropriate solvent, which is then removed during the thermal treatment, possibly in a preliminary low temperature step. In the final g-C3N4 structure, metals can be complexed in the cavities among heptazine units, where six N atoms can coordinate the metal cations. The cation size can influence the coordination of the metal. Although Na+ can be coordinated in-plane in the above-mentioned cavity, K+ cannot be accommodated in the plane, resulting in bridges between two g-C3N4 layers that contribute to delocalize and separate photogenerated charge carriers [68]. Incorporation of metals in the g-C3N4 cavities can thus ease its exfoliation. Thanks to their coordination, metals can then extend the g-C3N4 visible absorption. Moreover, when used during the synthesis, metal chlorides increase the C:N ratio in the resulting g-C3N4 to values closer to 0.75, indicating that their addition facilitates the deamination in the self-polymerization of dicyandiamide [69]. K-doped materials, obtained introducing a KOH solution during the synthesis with dicyandiamide, were characterized by larger specific surface area [2], less recombination, slightly improved visible absorption, and significantly more positive band potentials compared with pristine g-C3N4 (Fig. 5). Heavy doping makes the conduction band potential too positive for the reduction of O2, with repercussions on Rhodamine B degradation, which is better on slightly doped materials compared with undoped g-C3N4 [70]. Doping with other metals leads to similar results. As reported by Jiang et al. [2], doping with Fe, Cu, Ce, Co, Eu, Mo, W, Y, and Zr significantly increased the specific surface area and the rate of photocatalytic reactions. The band-gap was reduced in a limited extent 1 eV, the band-gap was reduced by 0.3 eV or more [72]. Also, non-metal elements such as sulfur, carbon, phosphorus, boron, fluorine, bromine, and iodine have been employed as dopants for g-C3N4. Their inclusion in the g-C3N4 framework can extend the visible light absorption, introducing states within the band-gap. Such states cannot only accelerate photogenerated electron–hole separation and transfer, but also improve their recombination, as in the case of metal doping [2, 73]. B and F can be incorporated simultaneously, employing ionic liquids with BF
4 as the anion [74]. Amino borane can be employed as an alternative to replace C in the g-C3N4 structure with B, a stronger Lewis acid, making B-doped g-C3N4 – with its imminic N – even a more effective bifunctional catalyst [75].

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FIG. 5 Band-gap structures of pristine g-C3N4 and K(x)–CN, where x is the K concentration employed during the synthesis. (Reprinted with permission from S. Hu, F. Li, Z. Fan, F. Wang, Y. Zhao, Z. Lv, Band gap-tunable potassium doped graphitic carbon nitride with enhanced mineralization ability. Dalton Trans., 44(3) (2015) 1084–1092. https://doi.org/10.1039/C4DT02658F)

Similarly, P can be introduced with the same approach with PF
6 [76]. Because the source of P influences the doping site [76], the other precursors such as diammonium hydrogen phosphate [77], 2-aminoethylphosphonic acid [78, 79], (hydroxyethylidene)diphosphonic acid [80], hexachlorocyclotriphosphazene [81, 82], ammonium hexafluorophosphate [76], and phosphorous acid [83] have also been employed. Depending on the phosphorus source, doping can be interstitial into the g-C3N4 lattice to form PdN bonds or substitutional at corner carbon and bay-carbon sites. DFT calculations suggested that P preferentially substitutes C1 atoms compared with C2 positions in the heptazine ring, introducing intra band-gap states with energy just above the valence band [2]. Conversely, S-doped g-C3N4 is characterized by an increased VB width in combination with a more energetic CB minimum. This electronic structure is the consequence of the substitution of sulfur for lattice nitrogen and a simultaneous quantum confinement effect, leading to a material with good activity in both proton reduction and organic compound oxidation [2]. Oxygen-doped g-C3N4 was synthesized employing a hydrothermal approach in the presence of H2O2 [84]. XPS measures evidenced that O was introduced into the lattice forming CO and NCO functions, witnessing that O atoms can directly be bound to sp2 carbon atoms. Following O-doping, the energy of the CB minimum was reduced by 0.21 eV, while the VB maximum was not affected, resulting in an extension of the visible light response. NH4F was employed to introduce F in the g-C3N4 lattice. Due to its electronegativity, F binds preferentially to C, possibly changing its hybridization from sp2 to sp3 [85]. Likewise, NH4I and NH4Br can be used to promote I and Br doping. I can substitute sp2 N atoms increasing the energy of both valence and conduction bands with a slightly reduced band-gap, while Br-doping leads to a minor, though measurable, band-gap shrinking [86–88]. Computations showed that C doping would result in an improved conductivity and narrower band-gap [89]. First attempts were just made adding C-containing compounds during the polymerization, like ethanol [89], glucose [90], or melamine resin foam [91]. Even though such methods allowed the authors to obtain materials with increased C content, they also evidenced the limits in directing the C atom insertion into a desired position. The resulting materials displayed properties more dependent on the synthesis procedure than on the C content [92]. The molecular doping can be more effective in controlling where the dopant atoms will be positioned in the polyheptazine network. To direct the insertion of C atoms, different N-containing monomers were proposed for molecular doping of g-C3N4, with the possibility to modulate conductivity, band energy, polymerization degree, cavity size, and porosity [37, 47, 93]. For example, barbituric acid was introduced in the g-C3N4 network through a copolymerizing process with dicyandiamide. The barbituric acid-doped carbon nitride showed improved light absorption, photocurrents, and photocatalytic degradation of aniline [94]. The copolymerization lowered both the CB and VB positions, but slightly widened the band-gap. Other compounds that anchored to g-C3N4 to promote a narrower band-gap and an enhanced light absorption are p-nitrobenzoic acid [95], phenylurea [96], 4-aminobenzencarbonitrile [97], 2-aminothiophenecarbonitrile [98], and polyacrylonitrile [99].

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The case of triaminopyrimidine is of particular interest because of its structural similarity to melamine. Triaminopyrimidine can be copolymerized with melamine in molar ratio from 0 to 1, leading to C-doped carbon nitride with maximized structural similarity with its pristine homologue [37, 93]. Capilli et al. showed that a triaminopyrimidine:melamine ratio of 3:1 led to a material with improved visible absorption, which was translated into improved 2-fluorophenol degradation under visible-only irradiation. The doped specimen also showed a reversal of its semiconducting behavior, from n-type to p-type, with positive photopotential. The band potentials were also affected: the valence band became slightly more negative, while the conduction band was significantly shifted to a more positive potential. Other organic compounds, even without N atoms, were proposed as additives to modify the g-C3N4 post synthesis. As an example, different aldehydes – also heteroaromatic – increased the conjugation of carbon nitride through a post synthesis treatment at 250 °C with the formation of Schiff bases. The modified material displayed increased H2 production, reduced defect density with reduced H content, and an increased C:N ratio closer to the expected value of 0.75 [92]. The post-synthesis treatment led to better results than the direct copolymerization of the aldehyde with the g-C3N4 precursor. A similar approach was carried out with pyromellitic anhydride. Also, in this case, the post-synthesis treatment yielded better results than the copolymerization of melem with pyromellitic dianhydride [100–102]. Other post-functionalization strategies involve the protonation at room temperature, which does not alter the CdN network, but allows the tuning of the band-gap and improves the possibility of dispersion in water [103]. Additionally, S-doping can be induced with a post-synthetic treatment in H2S atmosphere at 450 °C [104]. In summary, molecular doping is a distinctive way for modifying g-C3N4, as it is usually not available for inorganic semiconductors. Copolymerization or anchoring small amounts of structure-matching organic groups at the edges of g-C3N4 nanosheets can also significantly modify its band-gap and light absorption properties.

4 g-C3N4-based heterojunction photocatalysts The band structure of the pristine g-C3N4 has been described in-depth above. The band-gap of 2.7 eV allows the promotion of electrons from its valence band to the conduction band through the absorption of visible photons. The positioning of the g-C3N4 conduction band (roughly
1.5 V vs NHE) is quite high with respect to the other medium-band-gap semiconductors (slightly lower than that of silicon carbide that is one of the semiconductors with the highest in energy and lowest in the scale of potentials – CB), and consequently, the electrons in its CB are good reductants. On the contrary, its VB is located at lower potentials (roughly 1.5/1.4 V vs NHE), and so the photoproduced holes in this band are low oxidants. As a consequence, the direct oxidation of water to give %OH is hindered from the thermodynamic point of view despite what is usually reported for other semiconductors (e.g., TiO2, ZnO, SnO2, and Ta2O5) which have the VB located at potentials higher than the redox potential for the couples %OH/OH
and %OH/H2O (the real role in the oxidation of recalcitrant substrate of the free or surface bound %OH with respect to the direct transfer of the hole at the solid/electrolyte interface is a not totally clarified aspect) [105, 106]. This is in agreement with the scarce ability of the irradiated pristine g-C3N4 to promote the oxidation of recalcitrant species. In this light, the photocatalytic activity of g-C3N4 has been mainly exploited not as it is, but through the coupling of this organic semiconductor with other semiconductors (inorganic and/or organic) or cocatalysts (both for the reductive and oxidative site). So, before moving to the specific heterostructures proposed in the literature with g-C3N4, a brief and general presentation of the photochemical properties of the main semiconductor-based heterostructures is reported here. In the case of the binary catalyst–semiconductor or semiconductor–semiconductor heterostructure (but the same is true also for hybrid systems composed by more than two materials), the reactive sites can or cannot be located on the material that absorbs the photons and in which the electron–hole pair is first generated. As a consequence of the electronic structure of the materials composing the hybrid and of the environment in which it works, the e
cb and the h+vb can follow different pathways. The possible activation of chemical useful processes (vide infra for the main applications of the g-C3N4) is always limited by the deactivation process in which the e
cb/h+vb couple recombines (in the bulk, at the surface or through adsorbed substrates which can act as recombination centers) dissipating the energy of the photon absorbed as heat or in a radiative way (fluorescence emission). The dissipation of energy through fluorescence is a strong drawback when the main goal of the photocatalytic process is the promotion of a chemical process (e.g., production of high-value species or the degradation/deactivation of harmful molecules/pathogens), while it is an essential process when g-C3N4-based materials are used as sensing (vide infra). The coupling of two materials generates an interface that has often peculiar properties strongly related to the chemical, electronic, and semiconducting features of the coupled materials. Although on the basis of the features of the materials alone, it is possible to infer the overall properties of the heterostructure, too often in our opinion, the properties of the interface are only supposed, and no experimental evidences are reported to support the proposed structure hypothesis. The coupling of two solids implies the equilibration of their Fermi levels (i.e., the redox

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FIG. 6 Scheme of the possible heterostructures formed by coupling two semiconductors and charge carriers fate in the case both semiconductors are irradiated with energy higher than their energy gap (hn Eg): (A) Type I, (B) Type II, and (C) Type III heterojunction. (Adapted with permission from W.-J. Ong, L.-L. Tan, Y.H. Ng, S.-T. Yong, S.-P. Chai, Graphitic carbon nitride (g-C3N4)-based photocatalysts for artificial photosynthesis and environmental remediation: are we a step closer to achieving sustainability? Chem. Rev., 116(12) (2016) 7159–7329. https://doi.org/10.1021/acs.chemrev.6b00075)

potential of the solid) in the overall heterostructure and the bending of the bands at the interface. The coupling of two semiconductors at equilibrium can give three types of different heterojunctions only. In the so-called Type I (see Fig. 6A), the CB and VB of the lower band-gap semiconductor are confined between the bands of the other semiconductor (straddling gap). The energetics of the bands involves that the e
cb of the higher band-gap semiconductor (Fig. 6A) migrates toward the CB of the other material, and here, all the e
cb, both those produced in the higher Eg semiconductor and those produced in the lower Eg one, react at the surface with an oxidant, for example, oxygen adsorbed at the surface or a substrate with a redox couple potential higher (lower in energy) than that of the CB. Similar is the dynamics of the h+vb. The hole in the VB of the higher Eg semiconductor can migrate in the VB of the other semiconductor and there react with reductant species at the surface (e.g., substrates to be oxidized or surface hydroxyl group or adsorbed water that give high reactive bound- or free-hydroxyl radicals). In this case, the reactive sites (reductive and oxidative) are at the surface of the same material, and in principle, this architecture can increase the probability of dissipative phenomena (recombination of the e
/h+ couple) at the surface or mediated by adsorbed substrate. The Type II heterojunction (staggered gap) can be obtained when the CB and the VB of the first semiconductor are higher in energy with respect to the CB and VB of the other one (the one on the right in Fig. 6B), respectively. The electron can migrate from the higher to the lower conduction band and at the surface of the second semiconductor reacts, while the hole produced in the semiconductor with the lower VB can migrate in the semiconductor with the higher VB and react at the surface with the substrate with a redox couple characterized by lower redox potentials. In the Type II heterostructure, the reactive sites are spatially separated, and the recombination at the surface is in principle limited. The last option (Type 3) is that the CB of the semiconductor 1 is lower in energy than the VB of the semiconductor 2. In this case, no transfer of charge is operational between the two materials, and the two phases work as two independent photocatalysts (see Fig. 6C). The band alignment, present in the Type II heterojunctions, is similar to the so-called Z-scheme. However, in a Z-scheme system, the e
cb in semiconductor 2 (those characterized by the CB lower in energy) can recombine with the h+vb photogenerated in semiconductor 1. The Z-scheme solves in principle a theoretical drawback of the Type II heterostructure that is the decreased reduction and oxidation power of the charge carriers with respect to the maximum achievable (the most reducing species is the e
cb in semiconductor 1, and the most oxidizing species is the h+vb in semiconductor 2, see Fig. 6B). At the same time and not considering recombination, in Type II heterojunctions, the absorption of two photons (one from semiconductor 1 and the other from semiconductor 2) gives rise to the transfer of two electrons to the oxidant species in solution and two holes to the reductant (with a theoretical maximum quantum yield of 100%). The Z-scheme can be obtained coupling two semiconductors in the absence (Fig. 7A) or in the presence (Fig. 7B) of a conductor and/or an electron mediator at the interface. The Z-scheme is at the basis of the photosynthetic process in the green vegetables, where the transfer of electrons from the excited photosystem II (PS2) to the photosystem I (PS1) is mediated by a complex biochemical electron transport chain. Numerous examples are reported in which a metal (e.g., Ag, Au, and Cu) or a semiconducting organic phase (e.g., graphene) is placed at the interface between the two

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FIG. 7 Scheme of heterostructures in which Z-scheme is operational: (A) in the absence of electron mediator, (B) in the presence of an electron conductor and/or mediator, and (C) scheme of a Schottky junction between an irradiated semiconductor and a metallic reduction catalyst. In all the schemes, the semiconductor is irradiated with photons with energy higher than their energy gap (hn Eg). (Adapted with permission from W.-J. Ong, L.-L. Tan, Y.H. Ng, S.-T. Yong, S.-P. Chai, Graphitic carbon nitride (g-C3N4)-based photocatalysts for artificial photosynthesis and environmental remediation: are we a step closer to achieving sustainability? Chem. Rev., 116(12) (2016) 7159–7329. https://doi.org/10.1021/acs.chemrev.6b00075)

semiconductors with the aim to extract the excited electrons from the CB of the semiconductor 1 and transfer to the semiconductor 2. A Z-scheme allows (i) a high charge separation efficiency and a spatial separation between the reductive and oxidative sites that are located on the surface of different semiconductors; (ii) the maximum exploitation of the redox potential of the system being the reductive species the e
cb located in the CB with the lowest redox potential (i.e., that of Semiconductor 1 in Fig. 7) and the oxidant the h+vb in the VB with the highest potential (i.e., that of Semiconductor 2 in Fig. 7). On the contrary to Type II heterojunction, a Z-scheme requires two photons for the production of one reactive couple e
cb/h+vb because one of the two charged couples generated recombines. In the absence of recombination, the maximum quantum yield is consequently 50%. The coupling of the semiconductor with co-catalysts is an alternative strategy to increase the efficiency of the photocatalytic process through the reduction of the overpotentials that characterize the electron and hole transfer at the surface. Islands of noble metals (e.g., Au, Pt, Pd, and Ag) on the semiconductor surface of the semiconductor (see Fig. 7C) have two different and concomitant roles: (i) the equilibration of the Fermi level between the two solids creates a Schottky junction with the development of a depletion layer that generally favorites the transfer of e
cb from the irradiated semiconductor to the metal and consequently depress the recombination processes in the bulk of the semiconductor; (ii) noble metals usually act as efficient catalyst for the reduction (e.g., the reduction of H+ to give gaseous H2 is usually quite efficient on noble metal such as Pt; [107–110]). Equally, on the semiconductor surface, it is possible to deposit oxidative co-catalysts able to favor the injection of holes into the solution promoting, as an example, the quite complex multielectronic oxidation of water to oxygen [111, 112]. Furthermore, the presence of nanometric noble metals islands at the surface can generate visibleactivated surface plasmon resonance (SPR), where the role of the visible light to activate the photocatalytic process has not been totally clarified yet. Finally, the semiconductor-based heterostructures used in photocatalytic applications can be created coupling a semiconductor with carbonaceous nanomaterials (CNMs, e.g., graphene, single or multiwall carbon nanotubes, reduced graphene oxide, and carbon quantum dots) usually characterized by high/very high conductivity due to their delocalized p–p network. Also, in this case, the carbonaceous phase can play a double role: (i) enhance the separation of the e
cb/ h+vb promoting the monodirectional migration of the electron toward the carbonaceous phase and (ii) act as excellent reduction catalyst especially if decorated with nanometric noble metal islands [113]. In these cases, the dynamics of the photogenerated charges can be quite complex because the CNMs also show semiconductor properties and usually visible absorption. Then, the real fate of the charges photogenerated both in the semiconductor and in the CNMs can be quite complex to be a priori predicted. Furthermore, the high-oxidant h+vb formed in the semiconductor can oxidize the carbonaceous scaffold, with the consequence of a scarce stability of the hybrid catalyst [114]. Both heterojunction and Z-scheme devices exploit the incoming electromagnetic energy to generate charge carriers. However, once generated, electrons and holes are employed in remarkably different ways in the two kinds of device. It is frequently stated that Z-schemes allow better exploitation of the excitation energy, whereas considerable amount of energy is wasted in heterojunctions. Is it always the case? At first, Z-schemes could appear more efficient because they

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FIG. 8 Energy diagrams of devices working as Z-schemes (A and B) or heterojunctions (C and D). Redox acceptor and donor are denoted as A and D, respectively. Double-ended arrows represent the losses of the device, and colored arrows represent the incident irradiation. Optimized devices (B and D) display smaller losses compared with sub-optimal architectures (A and C).

preserve the energy of the most reactive carriers. Nevertheless, to achieve efficient charge-carrier separation, the least reactive electrons and the least reactive holes are lost by recombination. In fact, a perfectly working Z-schemes loses half of its charge carriers through recombination (Fig. 8A and B). From this reasoning, we can infer two important consequences: 1. In the Z-scheme, two photons are needed to generate an active electron–hole pair. Conversely, in a heterojunction, every photon can generate an electron–hole pair, which is then separated by the built-in potential. Heterojunction could therefore reach a quantum yield of 1, at least in principle. Energy loss in a heterojunction – in the absence of recombination – follows a very different mechanism, vide infra. 2. From these considerations, we can understand the criteria that should be followed to build efficient Z-schemes and heterojunctions. To fully exploit the Z-scheme strengths, the two semiconductors involved should have a very limited overlap between the band-gaps (Fig. 8B). It is then possible to minimize the losses, maintaining the same energy of the transferred carriers. Moreover, with this strategy, the energy requirements for the incoming irradiation are less stringent, as less energetic photons can still promote the same redox reaction. This strategy is adopted by the natural photosystems I and II to promote reactions with very demanding energetic requirements, such as water oxidation and CO2 fixation, with visible – even red – light. It must be noted that in a Z-scheme device, the charge-carrier separation is entirely based on kinetics of recombination of the first e
cb/h+vb couple, as there are no electric fields (a junction between the semiconductors involved in a Z-scheme would drive the charge carrier in the opposite direction). Z-schemes can therefore maximize the reactivity of minority carriers at both sides of the device with visible or NIR excitation. In the case of heterojunctions, charge-carrier generation is followed by their separation through the built-in potential. This separation is dissipative, lowering the energy of the carriers and consequently their reactivity, that is, the chemical work they can perform. This separation can be seen as a further thermalization of the charge carriers. Because the energy loss mechanism is radically different compared with Z-schemes, the

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optimal heterojunction architecture will be completely different from the optimized Z-scheme device (Fig. 8C and D). In this case, the losses are minimized when the band-gaps are significantly overlapping (Fig. 8D), even though such an overlap will reduce the entity of the electric field. Limited overlap between semiconductors could lead to large fields, which could be useful in photovoltaic applications. However, in the field of photocatalysis or solar fuels, such an arrangement will lead to large losses of the electromagnetic energy. Moreover, when the junction is formed between a p-type and an n-type semiconductor, the electric field will work toward the accumulation of the majority carriers at both sides at the heterojunction, whereas larger reactivity is usually associated with the minority carriers. In this case, a Z-scheme would perform better. In the light of these considerations, we can sketch the best options for g-C3N4. Due to its moderate band-gap and quite energetic valence and conduction bands, a Z-scheme with g-C3N4 would involve large losses. The only strategy to limit such losses would be the coupling with semiconductors with very large electron affinity (i.e., very positive CB), which could improve the hole transfer of the device. Nevertheless, such a semiconductor could suffer from limited stability if electrons are not efficiently transferred to g-C3N4. Conversely, heterojunctions involving g-C3N4 could be useful to increase the charge separation. The heterojunction should be built-in order to accumulate holes in the g-C3N4. The opposite would require a semiconductor with less-positive VB and more-negative CB. Such a semiconductor would have problems of stability because g-C3N4 VB has already limited overpotential for the oxidation of water. The coupled semiconductor could be oxidized and dissolved in water. Conversely, g-C3N4 CB is already very negative, and the coupling with another semiconductor could still lead to a device capable of performing a broad spectrum of reduction reactions. Taking into account all these constraints, the best strategy to improve charge-transfer separation and to increase the kinetics of the charge transfer would be the formation of a heterojunction with a semiconductor with slightly more positive band potentials. Going beyond the proof-of-concept stage and also considering the efficiency figures, the best strategy with g-C3N4 is to drop the coupling with another semiconductor and to couple it with co-catalysts specific for the reactions of interest. With modified carbon nitride, functionalized or heavily doped, BG energy and band potentials can change substantially [37], and the previous considerations have to be modified accordingly. For example, if the band-gap is considerably narrowed, new possibilities for the formation of Z-schemes will arise. Conversely, if the band-gap is not significantly narrowed, but the band potentials are shifted, new effective heterojunctions will be envisaged. After this overview, it emerges that, very frequently, the examples of heterojunctions proposed in the literature are justified only making comparisons with the pristine semiconductors, discarding energy efficiency considerations for their design. Only seldom are apparent quantum yields computed, and even more rarely, charge-transfer dynamics are effectively measured and proven. One of the very few examples is the use of photoacoustic spectroscopy in WO3/C3N4 heterostructures, where Z-scheme was demonstrated only when the two materials were thoroughly mixed in a planetary mill [115].

5 g-C3N4 as photocatalyst: Applications 5.1 Water photosplitting While the reduction potential of the proton to give H2 is quite accessible, the anodic reaction, the oxidation of water to give O2, is more complex from a thermodynamic and kinetic point view, due to the four electrons and four protons transfer needed together with the formation of an OdO bond. The band potentials of g-C3N4 are suitable for both reactions, and suitable catalysts can improve the kinetics of the charge-carrier transfers. Following the previous considerations, heterojunctions and Z-schemes could improve the charge-carrier lifetime, but paying a price to the overall energy efficiency. The addition of a co-catalyst could improve the charge-transfer kinetics and charge-carrier separation without compromising the apparent quantum yield and without worrying about band alignment as in heterojunctions, where improved separation could lead to less-reactive charge carriers. Even though co-catalysts usually contain metals, metalfree approaches are currently being developed owing to their advantages from cost and sustainability point of view. Platinum is by large the most employed co-catalyst. Because of its widespread use with other semiconductors, the facility of its deposition, and the excellent kinetics in hydrogen evolution reaction, Pt is the ideal co-catalyst to test different materials. Pt has been loaded to g-C3N4 obtained from urea, dicyandiamide, thiourea [116], or doped adding phenylurea [96], 4-aminobenzencarbonitrile [97], 2-aminothiophenecarbonitrile [98], barbituric acid [117], polyacrylonitrile [99] during the synthesis, or coupled with polypyrrole [118], poly-3-hexylthiophene [119], or MoFs like UiO-66 [120] after the synthesis. Other metals employed to increase the kinetics of H2 evolution are Au [121], Ni as Ni/NiO [122, 123], and Zn [124], while Co is employed to improve the water oxidation reaction ([125] in the presence of Ag+ as electron scavenger) or also in the form of Co-Pi [126].

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As an alternative to metals, sulfides have also been coupled with g-C3N4. Among these, there are MoS2 [127], WS2 [128], NiS [129–132], NiS2 [133], and CoS [134]. Almost always, a hole scavenger is employed to minimize the electron recombination. Even though H2 gas evolution is observed, also at very good quantum yields [135], the process involving a scavenger cannot be labeled as water photosplitting, but as the photoreforming of the scavenger employed, usually TEOA or methanol. Due to the paramount role played by the scavenger in this process, one must be very careful when comparing quantum yields of radically different processes like water photosplitting and the reforming of reducing organic compounds. In their monumental review, Ong and coauthors [4] report only one example of WPS performed without scavenger [118] among over than 80 reports of WPS. The authors used a composite of g-C3N4 and polypyrrole and proposed a hybrid mechanism between the Z-scheme and heterojunction. Indeed, polypyrrole injects electrons into g-C3N4 CB as in heterojunction, but the photoproduced holes in g-C3N4 VB react with water to give H2O2 as in a Z-scheme. The authors do not propose a sink for polypyrrole VB holes, which could hardly be transferred to solution species, especially in the absence of scavengers due to their limited oxidation potential. A possible explanation for the fate of polypyrrole holes is the self-oxidation of the polymer. In that case, the polypyrrole itself would also serve as sacrificial agent. The combination of g-C3N4 with carbon quantum dots (CDs) reported by Liu and coworkers [136], in which O2 and H2 – in the correct stoichiometric ratio – evolved without scavengers and with visible irradiation, represents one of the few proper and genuine examples of water photosplitting. Even if the quantum efficiency (Q.E.) decreases with the wavelength, 16% Q.E. can be reached, and even though comparable quantum yields were reported in the literature, including reports of platinized TiO2, in this case, the result is of particular significance because it was obtained in the absence of hole scavengers, and hole transfer to water was demonstrated and measured. To further highlight the importance of the result, we can compare the Liu’s reports with the experiment performed by Hu and coworkers. They reported a nanocomposite heterojunction composed of g-C3N4/InVO4. The material demonstrated good charge-separation properties and an apparent quantum yield of 4.9% at 420 nm [137]. Nevertheless, in this case, a methanol solution was employed as hole scavenger, and no O2 evolution was observed during the experiment. The mechanism of water oxidation does not involve a concerted four-electron mechanism, but it is a stepwise process with two two-electron steps yielding H2O2 that is then decomposed to give O2 and H2O. The rate increases with CDs loading because CDs catalyze H2O2 decomposition, which is the rate limiting step. This approach is most convenient with C3N4 in a reaction like WPS, coherently with the above discussion, that is, g-C3N4 is suitable for WPS because of its bandgap and band energy, and the most efficient way to improve its activity is loading with co-catalysts.

5.2 Artificial photosynthesis, photocatalytic CO2 reduction Artificial photosynthesis presents several similarities with WPS. Both processes convert electromagnetic energy into chemical energy, both processes are composed by two semireactions, and the oxidation semireaction is the same, that is, water oxidation to yield O2. The main difference is the reduction semireaction, which involves CO2. Besides the starting substrate, the two reduction semireactions have striking differences: 1. Although proton reduction can only lead to one product, that is, H2, CO2 reduction can lead to a vast pool of products, with different oxidation states and also with more than one-carbon atom in the structure; 2. Although hydrogen reduction involves two electrons in a two-stage mechanism [138, 139], there are several possible reactions in the case of CO2 reduction involving different numbers of electrons transferred (even >8), elementary steps, and formed or broken bonds. 3. It is impossible to define an unambiguous reduction potential for CO2 reduction. For CO2 reduction, the product of the reduction and the number of electrons transferred must always be considered to properly define a reduction potential. 4. The optimal design of a material for CO2 reduction requires the definition of the desired reduction product. Once the potential of the reduction reaction is identified, the conduction band potential requirements will be known as well. The selectivity can change varying the CB potential. Due to the stability of CO2 and to the considerable rearrangements needed to convert it to reduced species, one could conclude that CO2 reduction potential will be significantly more negative than the proton reduction potential. Although this is indubitably true for CO2 monoelectronic reduction, requiring
1.9 V vs NHE [140], the reduction reactions involving the transfer of 2, 4, 6, or 8 electrons and the formation of closed-shell products have the most accessible reduction potentials ([141], Table 1). Nonetheless, a device capable of reducing CO2 is also thermodynamically able to evolve H2 when water is present. Although the conduction band potential of g-C3N4 is sufficiently negative to promote such reactions, the

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TABLE 1 Reduction potentials for CO2 vs NHE [141]. Reaction

E° (V) vs NHE




CO2 + e →

CO2%



1.9




CO2 + 2H + 2 e → HCOOH +


0.61




CO2 + 2H + 2 e → CO + H2O +


0.53




2 CO2 + 2H + 2 e → H2C2O4 +


0.49





0.48





0.38




CO2 + 8H + 8 e → CH4 + 2 H2O


0.24





0.20

CO2 + 4H + 4 e → HCHO + H2O +

CO2 + 6H + 6 e → CH3OH + H2O + +

CO2 + 4H + 4 e → C + 2 H2O +

reorganization required to obtain the products from CO2 makes even more compelling the coupling with an appropriate cocatalyst, which will determine the selectivity toward reduction products. Due to the molecular reorganization needed to promote CO2 reduction and to the quite negative reduction potentials required, charge transfer can be sluggish, leading to large losses due to recombination. To increase the charge-carrier separation and to improve apparent quantum yield (AQY), there are three main strategies: 1. Introducing a bias, chemical or electrical; 2. Improving charge separation, thanks to a heterojunction, with the drawback of dissipating energy, as outlined before; 3. Improving charge-transfer kinetics building a Z scheme, reducing energy efficiency, or coupling the semiconductor with co-catalysts, trying to preserve the energy of the carriers as much as possible. All the approaches have been tested, sometimes also combining different methods, for example, employing a co-catalyst and an external bias (see Table 2). Concerning the co-catalysts, the best catalysts employed for H2 evolution will not be

TABLE 2 Performance of g-C3N4-based photocatalysts toward CO2 photoreduction. Material

Dopant/ cocatalyst

Products

Light source

Apparent quantum yield

References

g-C3N4 (urea precursor)

None

CH3OH, CH3CH2OH

Vis

0.18%

[142]

g-C3N4

Pt

CH4, CH3OH, HCHO

UV–Vis

N/A

[143], [72]

g-C3N4

Pd

H2, CO, CH4

Vis

99%, the amount of hydrogen evolved closely matched the conversion data. Selectivity was always between 50% and 90%, without appreciable correlation with conversion data [217]. Benzaldehyde was also obtained by Liu and coworkers starting from benzyl bromide. They were able to achieve selectivity >99% for benzaldehyde with conversion 99% when g-C3N4-modified post-synthesis with NaOH was irradiated in the visible in the presence of O2 [218]. Similarly, the alcoholic function in 5-hydroxymethyl-2-furfural was oxidized with selectivity up to 53% and large conversion >99% to 2,5-furandicarboxaldehyde. The authors demonstrated that exfoliated samples performed better and that O2 %- was the species responsible for the substrate oxidation [219]. On the same substrate, Wu and coworkers also observed the importance of the type of solvent to improve conversion and selectivity. In particular, the mixture of CH3CN and PhCF3 gave the best results. The authors ascribed this effect to the good solvent properties toward O2, which is the precursor of the effective oxidizing agent [220]. Besides C-H and O-H oxidation, N-H oxidation is even more interesting because many biologically active compounds contain N atoms. The oxidation of amines can be promoted under visible light irradiation on mesoporous g-C3N4 to give imines, which, in turn, may undergo further consecutive reactions. Su et al. were able to optimize reaction conditions to convert benzylamine into N-(benzylidene)benzylamine. The same strategy could be extended to heterocyclic amines containing nitrogen and sulfur atoms to obtain benzoxazoles, benzimidazoles, and benzothiazoles [221–223]. g-C3N4 can also work in the dark. Goettmann and coworkers employed it as a catalyst for Friedel–Crafts reactions starting from benzene with the direct use of carboxylic acids, alcohols, ammonium salts, and even urea to obtain toluene, p-xylene, mesitylene, cumene, p-diethylbenzene, and benzonitrile [44]. Chen and coworkers reported the synthesis of aryl ketones starting from the aromatic parent compound and a substituted iodobenzene (yields with bromo- and chloro-derivatives were significantly lower) with visible light and oxygen. Nevertheless, in this case the authors had to optimize the catalysts, because with pristine g-C3N4 yields were below 3%. The synthesis of g-C3N4 had to be optimized and g-C3N4 decorated with co-catalysts like Ag and Co oxide. Nevertheless, neither oxidants, besides O2, nor other reactants, nor noble metals were required [224]. g-C3N4 was also employed to activate N-hydroxy compounds with visible light to perform selective allylic oxidation under mild conditions, avoiding the employment of any metal derivative or organic oxidizing agents. Conversions up to 44% and selectivity up to 99% were reported [225]. g-C3N4 was also used as a support for Au or Pd nanoparticles for C2H2 and other alkynes hydrochlorination and semihydrogenation. Nitrile hydrogenation was performed to obtain secondary and tertiary amines, whereas CO2 hydrogenation over g-C3N4-supported Pd at 150 °C yielded HCOOH [226, 227]. Zhang and coworkers proposed the oxidation of sulfides to sulfoxides with isobutyraldehyde-modified g-C3N4. By modulation of the isobutyraldehyde (IBA) concentration in solution, which acted as electron and oxygen atom shuttle, they were able to control the products, conversion, and selectivity. In the absence of IBA, the selectivity for the sulfoxide was >99%, but conversion was 90% [228]. A summary of the main value-added compounds obtained through (photo)catalysis with g-C3N4-based materials is reported in Table 5.

6 g-C3N4 beyond (photo)catalysis: Analytical applications Fluorescence is the most exploited property of g-C3N4 in sensing. Fluorescent emission is the other side of the coin. As g-C3N4 has a direct band-gap, the recombination between electron and hole is radiative. The pholuminescence at 470 nm indicates charge-carrier recombination. Then, the scavenging of a charge carrier should reduce the fluorescence emission. Metals can be probed, thanks to fluorescence attenuation, as in the case of Cu. Cu2+ can be reduced to Cu+ with a CB electron, with the consequent suppression of photoluminescence. The same principle was extended to Fe3+, heparin, Hg2+, and Cr(VI) [229–231].

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TABLE 5 Syntheses of value-added compounds obtained through (photo)catalysis with g-C3N4-based materials. Material

Dopant/ cocatalyst

Reactants

Products

g-C3N4

None or Au

Alkanes, O2

Alcohols, aldehydes, ketones

Vis

[212, 213, 74, 214]

g-C3N4

None or P

Ar-CH2-OH, O2

Ar-CHO

UV or Vis

[215, 219, 220]

g-C3N4

ZnO, Fe2O3

Ph-CH2-OH, O2

Ph-CHO

UV

[216]

g-C3N4

Pt, [Ru(tpa) H2O)2]2+

Ar-CH2-OH

Ar-CHO, H2

Vis

[217]

Ph-CH2-Br, O2

Ph-CHO

Vis

[218]

Vis

[221, 222, 223]

g-C3N4

Light source

(∗)

References

Mesoporous g-C3N4

Ar-CH2-NH2, O2

Ar-HC¼NH

Mesoporous g-C3N4

Ph-H, CH3OH

Ph-CH3, p-xylene, mesytilene

Dark

[44]

Mesoporous g-C3N4

Ph-H, HCOOH

Ph-CHO

Dark

[44]

Mesoporous g-C3N4

Ph-H, N(CH3)4Br

Ph-CH3

Dark

[44]

Mesoporous g-C3N4

Ph-H, urea

Ph-CN

Dark

[44]

Ar-H, Ph-I, O2

Ar-CO-Ph

Vis

[224]

Cholesteryl acetate, HBT(∗∗), O2

7-Keto-cholesteryl acetate

Vis

[225]

R-CC-R0 , H2 R-CC-R0 , HCl CO2, H2 R-CN, H2

R-HC¼CH-R0 R-HC¼CR0 -Cl HCOOH (R-CH2)3N

Dark

[226, 227]

R-S-R0 , IBA, O2

R-SO-R0 , R-SO2-R’

Vis

[228]

g-C3N4

Ag, co-oxides

g-C3N4 g-C3N4

Mesoporous g-C3N4

Au, Pd

Ph ¼ phenyl. Ar ¼ aryl. tpa ¼ tris(2-pyridylmethyl)amine. (*) reaction can proceed further to give cyclizations. (**) HBT ¼ 1-hydroxybenzotriazole (HBT), other N-hydroxy compounds were employed such as N-hydroxysuccinimide and N-hydroxyphthalimide. IBA ¼ isobutyraldehyde.

The coupling of g-C3N4 to a fluorescence quencher allowed the dosing of reducing species like glutathione down to 0.2 mM or ascorbic acid, thanks to fluorescence restoration [230, 232]. Electrochemical cathodic luminescence in the presence of persulfate has been proposed for the detection of Cu down to 0.9 nM and dopamine [233, 234]. Electrochemical sensing can also be performed without irradiation. g-C3N4 nanosheets intercalated with Li+ ions were employed in a fast-response and fast-recovery electrochemical sensor for humidity. The impedance of the device varied as a function of the moisture content, and the authors attributed the responsiveness of the sensor to the proton conductivity within the first layer of adsorbed water [235]. Conversely, for oxide sensors, the proton conductivity is triggered only when there are at least three adsorbed water layers, due to the surface hydroxyl groups that hinder proton diffusivity [236]. This different mechanism makes g-C3N4-based devices more performing than ceramic-based sensors because the latter display reduced responsiveness when moisture content rapidly decreases. Since Zhang et al. demonstrated that g-C3N4 exfoliated nanosheets can be biocompatible, several applications in biosensing and bioimaging emerged. g-C3N4 proved to be particularly suitable as biomarker for the labeling of cell membranes. In that context, the thin nanosheets could also be used as two-photon absorption probes. The technique

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revealed that g-C3N4 nanosheets can penetrate the nuclei and can be used to obtain their positions and other morphological details [237]. Thanks to its stability and photoactivity, g-C3N4 nanosheets can be used for in-situ ROS generation for cancer therapy, whereas, thanks to its morphology, g-C3N4 can also be used as high-capacity carrier for drug release. pH changes associated with cancer cell metabolism trigger the drug release, which could also be monitored through imaging [238]. Imaging of biothiols such as cysteine, homocysteine, and glutathione was demonstrated in vitro in the presence of Ag+, which quenches the persistent g-C3N4 luminescence. When present, thiols can complex Ag+, restoring the fluorescence [239]. g-C3N4 luminescence can also be exploited for DNA sensing. g-C3N4 has a very large affinity for single-stranded DNA, while double-stranded DNA is not bound. The fluorescence of suitably marked-DNA is quenched in the presence of g-C3N4 and in the absence of the target DNA, while fluorescence is progressively restored with increasing target DNA concentration. Other analytes interacting with single-stranded DNA can be dosed as well, such as Hg2+. Sensitivity and reliability of the method was improved, thanks to the design of a ratiometric fluorescence method employing two probes with two different emission wavelengths [240]. The fluorescence of N-doped carbon quantum dots has been exploited for the detection of the enzyme b-glucuronidase at very low concentration, thanks to an inner-filter effect method. b-Glucuronidase has been associated with cancer cell activity in numerous kinds of tumors, and its detection could allow early diagnosis [241]. Capilli et al. demonstrated that the synthesis of g-C3N4 in the presence of N-rich compounds, such as lysine, allowed its subsequent functionalization with immunoglobulin G [242]. The protein-g-C3N4 conjugate was employed as a fluorescent probe in a Forster resonant energy transfer (FRET) quenching assay, which is very sensitive toward human immunoglobulin G. Gold nanoparticles were attached to Staphylococcal protein A and employed to quench the carbon nitride nanoparticles fluorescence by FRET. g-C3N4 also found application for glucose sensing both photoelectrochemical [243] and electrochemical [244]. g-C3N4 has been also used in analytical separation science, thanks to its peculiar polarity, hydrophilic/hydrophobic features, and high specific surface area when exfoliated. Thanks to the ability of the surface of g-C3N4 to strongly interact with different species, this material is a promising sorbent to be applied in analytical chemistry with similar/complementary properties of graphene and graphene-like phases. As a consequence of the nature and chemical reactivity of the g-C3N4 surface, its interaction with analytes happens through complex mechanisms based on complexion, hydrogen bond, redox reaction, p–p conjugation, hydrophobic effect, acid–base reaction, and electrostatic interaction. This amplifies the potential applications of g-C3N4 as sorbent in analytical chemistry. In particular, g-C3N4 has been tested for the sample pretreatment (clean-up of the sample and analyte preconcentration) in different extraction modes, such as solid-phase extraction (SPE), magnetic solid-phase extraction (MSPE), solid-phase microextraction (SPME), and dispersive solid-phase extraction (DSPE). Kang and coworkers [245] investigated the g-C3N4 interlayer distance as a function of the potassium-doping level. The interlayer distance was modulated, and this modified the sorption ability of the g-C3N4-K toward Ba2+. The authors developed an ICP–OES method based on a preliminary step of preconcentration of barium on this carbon nitride phase. The developed method was used to determine Ba2+ concentration in water samples from Yellow River and samples of sea fish. There are numerous examples of analysis of organic compounds based on the use of SPE cartridge packed with g-C3N4. Speltini and coworkers [246] developed a low-cost silica-supported graphitic carbon nitride phase applicable for the extraction and successive analysis of fluoroquinolone drugs in environmental waters. Guan et al. [247] used graphitic carbon nitride as solid-phase adsorbent for determining benzoylurea pesticides in juice samples through HPLC equipped with UV detector. g-C3N4 phases have also been used as dispersive phase for the analysis of both inorganic and organic species. In this case, the coupling of the g-C3N4 with a magnetic material can give a rapid and quantitative recovery of the sorbent. As an example, a magnetic graphitic carbon nitride, coupling with g-C3N4 and the magnetic phase SnFe2O4, was surfacemodified with N-[3-trimethoxysilyl)propyl]ethylenediamine (TPED) to increase the complexing ability of the solid phase. This material was used as nano-adsorbent for the ultrasound-assisted magnetic-dispersive microsolid-phase extraction and successive analysis of Pb2+ and Cd2+ in foodstuff (vegetable, fish meat, and water) [248]. The same analytical approach was used by Zhao et al. [249] who hydrothermally synthesized a ternary nanocomposite, g-C3N4/Fe3O4/MoS2, that was successively used as an adsorbent in MSPE for the separation, clean-up, and preconcentration of trace fluoroquinolones in chicken and eggs. DSPE was also used by Wang et al. [250] for the quantification of phenoxy carboxylic acids by using nonmagnetic g-C3N4. This phase was easily recovered after the selective adsorption

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FIG. 11 Summary of the main analytical applications of g-C3N4.

of the analytes through a 0.45 mm filter, washed with methanol and the analytes quantified by direct analysis in real timemass spectrometry (DART-MS). A similar approach was used by Rajabi et al. [251] who proposed an air-assisted dispersive micro-SPE method (AA-dm-SPE) based on the use of g-C3N4/Fe3O4 hybrid for the extraction and successive analysis of polycyclic aromatic hydrocarbons (PAHs). In AA-dm-SPE, the sorbent phase is dispersed into aqueous sample by air bubbles with the aim to promote the phase transfer. The magnetic material is then separated by using a magnetic field, the PAH extracted with a proper solvent and then analyzed (GC-FID). The possibility to cover a fiber for SPME with g-C3N4 phases has been employed to analyze trace species by gas chromatography. Pang and coworkers [252] coated a stainless steel wire with a nitrogen-doped metal organic framework-based porous carbon (synthesized for carbonization of graphitic carbon nitride in the presence of a MOF (NH2-MIL-125) as template). The coated fiber was used for the micro-extraction of 14 organophosphorus pesticides from different fruit and vegetable samples and then analyzed by GC–MS. A similar approach was proposed by Feng et al. [253] who coated an SPME fiber with a graphitic carbon nitride derivative (nanosheets having a large surface area and large mesopores) that was used for the determination of PAHs in water. The good extraction ability observed was related to multiple interactions of PAHs with the g-C3N4 phase (both hydrophobic and strong p–p interactions). Further examples of the use of g-C3N4-based material as sorbent in SPE can be found in the following work [254]. Finally, the first examples of g-C3N4 as stationary phase in capillary GC-column have been reported by Zheng et al. [255, 256]. These authors investigated the separation ability of statically coated g-C3N4 column observing a quite high efficiency (3700–4700 plates/m for n-dodecane) and a general weak polarity. In particular, the capillary columns with g-C3N4 stationary phase showed superior separation performance for some critical analytes (e.g., preferential retention for aromatic compounds), good repeatability, and thermal stability up to 280 °C. A schematic representation of the main analytical applications of g-C3N4 is reported in Fig. 11.

7

Conclusions

g-C3N4 is certainly a hot topic in photocatalysis as witnessed by the growing trend of the related publications, which generated great expectations about this type of materials. As outlined in this chapter, g-C3N4 potentiality arises mainly from the quite negative valence band potential, while its less-positive valence band allows its use in organic synthesis and controlled oxidations. These properties make g-C3N4 complementary with the most studied photocatalytic oxides, such as TiO2 and ZnO. This caused the blooming of a plethora of heterostructures, hybrids, and composites with several different mechanisms of charge-carrier separation and transfer, often only proposed and very seldom demonstrated. The great amount of research effort spent in such direction is similar to the situation already observed for graphene derivatives and nanocarbons, which were largely employed in photocatalysis in the latest years. Also, in that case, the

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charge-transfer mechanism was lengthy debated, and again, the synthetic procedures could significantly modify the materials, leading to different properties and behaviors and making comparisons difficult when not impossible [158]. One possible path for improvement is a more rigorous investigation of g-C3N4 semiconducting and electronic properties to improve fundamental knowledge, as it has been the case with TiO2 and other oxides. Because the mere abatement of organic compounds, or merely of dyes, cannot give an answer to the fundamental understanding, the study must be performed with suitable experimental techniques, with the employment of suitable scavengers, with the identification of as many as possible reaction products, and the kinetics under different experimental conditions (system concentrations, chemical, or electrical bias). Compared with metal oxides, g-C3N4 has unique properties, such as being metal-free and with large flexibility due to the endless possibilities arising from molecular doping and functionalization. Researchers have begun to exploit these features, as witnessed by the collection of reports here exposed. As in the case of the other above-mentioned classes of materials, namely, nanocarbons and oxides, applications in organic synthesis, sensing, biosensing, and imaging could gain further importance in the future and possibly lead to devices with a technology readiness level high enough to find widespread application, with tangible effects not limited to academic papers.

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[241] S. Lu, G. Li, Z. Lv, N. Qiu, W. Kong, P. Gong, Y. Wu, Facile and ultrasensitive fluorescence sensor platform for tumor invasive biomaker b-glucuronidase detection and inhibitor evaluation with carbon quantum dots based on inner-filter effect, Biosens. Bioelectron. 85 (2016) 358–362, https://doi.org/10.1016/j.bios.2016.05.021. [242] G. Capilli, S. Cavalera, L. Anfossi, C. Giovannoli, M. Minella, C. Baggiani, C. Minero, Amine-rich carbon nitride nanoparticles: synthesis, covalent functionalization with proteins and application in a fluorescence quenching assay, Nano Res. 12 (8) (2019) 1862–1870, https://doi.org/10.1007/ s12274-019-2449-x. [243] X.Y. Zhang, S.G. Liu, W.J. Zhang, X.H. Wang, L. Han, Y. Ling, ⋯, H.Q. Luo, Photoelectrochemical platform for glucose sensing based on g-C3N4/ ZnIn2S4 composites coupled with bi-enzyme cascade catalytic in-situ precipitation, Sens. Actuators B 297 (2019) 126818, https://doi.org/10.1016/j. snb.2019.126818. [244] K. Tian, H. Liu, Y. Dong, X. Chu, S. Wang, Amperometric detection of glucose based on immobilizing glucose oxidase on g-C3N4 nanosheets, Colloids Surf. A Physicochem. Eng. Asp. 581 (2019) 123808, https://doi.org/10.1016/j.colsurfa.2019.123808. [245] J.-Y. Kang, W. Ha, H.-X. Zhang, Y.-P. Shi, Sandwich-like, potassium(I) doped g-C3N4 with tunable interlayer distance as a high selective extractant for the determination of Ba(II), Talanta 215 (2020) 120916, https://doi.org/10.1016/j.talanta.2020.120916. [246] A. Speltini, F. Maraschi, R. Govoni, C. Milanese, A. Profumo, L. Malavasi, M. Sturini, Facile and fast preparation of low-cost silica-supported graphitic carbon nitride for solid-phase extraction of fluoroquinolone drugs from environmental waters, J. Chromatogr. A 1489 (2017) 9–17, https://doi.org/10.1016/j.chroma.2017.02.002. [247] W. Guan, Z. Long, J. Liu, Y. Hua, Y. Ma, H. Zhang, Unique graphitic carbon nitride nanovessels as recyclable adsorbent for solid phase extraction of benzoylurea pesticides in juices samples, Food Anal. Methods 8 (9) (2015) 2202–2210, https://doi.org/10.1007/s12161-015-0116-8. [248] B. Fahimirad, A. Asghari, M. Rajabi, Magnetic graphitic carbon nitride nanoparticles covalently modified with an ethylenediamine for dispersive solid-phase extraction of lead(II) and cadmium(II) prior to their quantitation by FAAS, Microchim. Acta 184 (8) (2017) 3027–3035, https://doi.org/ 10.1007/s00604-017-2273-5. [249] B. Zhao, H. Wu, Y. Liu, X. Tian, Y. Huo, S. Guan, Magnetic solid-phase extraction based on g-C3N4/Fe3O4/MoS2 as a magnetic adsorbent for HPLC-UV determination of fluoroquinolones in chicken and eggs, Anal. Methods 11 (11) (2019) 1491–1499, https://doi.org/10.1039/ C9AY00208A. [250] J. Wang, J. Zhu, L. Si, Q. Du, H. Li, W. Bi, D.D.Y. Chen, High throughput screening of phenoxy carboxylic acids with dispersive solid phase extraction followed by direct analysis in real time mass spectrometry, Anal. Chim. Acta 996 (2017) 20–28, https://doi.org/10.1016/j. aca.2017.10.007. [251] M. Rajabi, A.G. Moghadam, B. Barfi, A. Asghari, Air-assisted dispersive micro-solid phase extraction of polycyclic aromatic hydrocarbons using a magnetic graphitic carbon nitride nanocomposite, Microchim. Acta 183 (4) (2016) 1449–1458, https://doi.org/10.1007/s00604-016-1780-0. [252] Y. Pang, X. Zang, H. Li, J. Liu, Q. Chang, S. Zhang, Z. Wang, Solid-phase microextraction of organophosphorous pesticides from food samples with a nitrogen-doped porous carbon derived from g-C3N4 templated MOF as the fiber coating, J. Hazard. Mater. 384 (2020) 121430, https://doi.org/ 10.1016/j.jhazmat.2019.121430. [253] Z. Feng, C. Huang, Y. Guo, W. Liu, L. Zhang, Graphitic carbon nitride derivative with large mesopores as sorbent for solid-phase microextraction of polycyclic aromatic hydrocarbons, Talanta 209 (2020) 120541, https://doi.org/10.1016/j.talanta.2019.120541. [254] Y. Sun, W. Ha, J. Chen, H. Qi, Y. Shi, Advances and applications of graphitic carbon nitride as sorbent in analytical chemistry for sample pretreatment: a review, in: Recent Advances and Trends in Analytical Nanoscience and Nanotechnology, vol. 84, 2016, pp. 12–21, https://doi.org/ 10.1016/j.trac.2016.03.002. [255] Y. Zheng, Q. Han, M. Qi, L. Qu, Graphitic carbon nitride nanofibers in seaweed-like architecture for gas chromatographic separations, J. Chromatogr. A 1496 (2017) 133–140, https://doi.org/10.1016/j.chroma.2017.03.060. [256] Y. Zheng, M. Qi, R. Fu, Graphitic carbon nitride as high-resolution stationary phase for gas chromatographic separations, J. Chromatogr. A 1454 (2016) 107–113, https://doi.org/10.1016/j.chroma.2016.05.073.

Chapter 29

Two-dimensional layered double hydroxide based photocatalysts for environmental clean-up and renewable energy production Qian Wanga,b, Junting Fenga, and Paolo Fornasierob a

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, China, b Chemistry Department, INSTM

and ICCOM-CNR Trieste Research Unit, University of Trieste, Trieste, Italy

1 Introduction The natural mineral hydrotalcite with a formula of [Mg6Al2(OH)16]CO3
4H2O was found in 1842 and first synthesized in 1942 by Feitknecht [1]. In contrast to cationic clays, the hydrotalcite is fairly rare in nature [2, 3]. In the late 1960s, Taylor synthesized MgFe-containing hydrotalcite-like compounds and found that the structural characteristics were quite similar to that of MgAl-containing hydrotalcite [4]. Since then, compounds with hydrotalcite-like structure were known as layered double hydroxides (LDHs). x+ 3+ The synthetic LDH materials are a broader family than hydrotalcites, with the general formula of [M2+ 1-x Mx (OH)2] 2+ 3+ n [Ax/n]n
mH2O, where M represents a divalent cation, M represents a trivalent cation, and A represents the anions needed to obtain the electroneutrality of the compound [2, 5–7]. As shown in Fig. 1, the divalent and trivalent cations octahedrally coordinate with hydroxyl groups to form positively charged brucite-like layers, charge-compensating anions exist in the interlayer galleries together with water molecules. The mole ratio of M2+/(M2++M3+) generally falls within the composition range of 0.2  x  0.33. Furthermore the composition of cations and anions is flexibly exchangeable. In some cases, monovalent and tetravalent cations, typically Li+ [8, 9], Ti4+ [10], and Mn4+ [11], can occupy the position of divalent and trivalent cations. LDHs represent some of the most technologically promising materials due to their simplicity of synthesis from cheap reagents, involvement of non-harmful precursors, low toxicity of their possibly produced decomposition products, and no need of expensive rare elements [2, 12]. LDHs and their calcined or reduced derivatives have found many applications as traditional heterogeneous catalysts or catalyst supports for various significant reactions in both industry and academy [6, 13]. Except for the intrinsic activity of components, the advantages of LDH-based catalysts come from the atomic dispersion of metal cations, which suppresses agglomeration of the active sites, and the novel micro/mesoscopic or hierarchical structures assembled by LDHs. Therefore, stable and recyclable LDH-based catalysts have potential as sustainable nanomaterials in the development of the chemical industry. The twenty-first century has experienced many great advancements in all fields of society, improving every aspect of human lives. However, the rapid consumption of fossil fuels for energy and environmental deterioration are two important issues. Along with the increasing concerns for sustainable development, the research to define new trajectories for clean energy and consequent pollution abatement are central priorities for today’s society [14, 15]. New scientific breakthroughs are expected to deeply consider the sustainability issue, which means design processes and materials that use less and renewable energy and that are based on materials and chemical reagents that are abundant, renewable, inexpensive, or that can be recycled or reused. In this regard, photocatalysis is an intrinsic sustainable process as, ideally, it is based on the use of solar light. Photocatalyst design must consider the nature, abundance, and toxicity of the chemical elements that will compose it, as well strategies for an efficient exploitation of sunlight, and in particular of the visible part of the spectrum (43%) with respect to the small fraction of ultraviolet (only 5%) that is used by today’s titanium dioxide (TiO2) benchmark system [16]. To optimize photocatalytic performances, efforts have to be made not only to improve visible-light-driven Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00028-3 Copyright © 2021 Elsevier Inc. All rights reserved.

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FIG. 1 The idealized structure of carbonate-intercalated LDHs with different M2+/M3+ molar ratios showing the metal hydroxide octahedral stacked along the crystallographic c-axis, as well as water and anions present in the interlayer region. Adapted from G. Fan, F. Li, D.G. Evans, X. Duan. Catalytic applications of layered double hydroxides: recent advances and perspectives. Chem. Soc. Rev. 43 (20) (2014) 7040–7066 with permission. Copyright 2014, The Royal Society of Chemistry.

semiconductor materials, but also to reduce recombination of photogenerated charge carriers and obtain the correct energy level of valence and conduction bands able to selectively promote the desired redox processes. Inspired by the prosperous application of LDHs in traditional catalysis, researchers are exploring their potential as photocatalysts. Over the past decade, LDHs have emerged as promising photocatalyst candidates in the fields of environmental clean-up and renewable energy production thanks to the following positive aspects: 1. The incorporation of photoactive metal cations into the structure of LDHs can tune the visible-light-responsive band gaps from 2.0 to 3.4 eV. [16] 2. A fine control of particle size and surface defects can be easily achieved altering the LDHs electronic structure and greatly enhancing the efficiency of photogenerated charge separation, leading to enhanced photocatalytic reactions rates. 3. The unique 2D structure of LDHs makes it easy to form composites with other photocatalysts. 4. LDHs can be easily synthesized at low cost and on large scale, as most of them are composed of abundant elements. These attractive features make LDHs significant structural platforms for the development of novel semiconductor photocatalysts. This chapter provides an overview of the main characteristics of LDH materials relevant for photocatalytic applications along with a selection of applications of LDH-based photocatalytic systems in environmental clean-up and renewable energy production, including degradation of organic pollutants, organic synthesis, H2/O2 production from water splitting, CO2 photoreduction, and N2 fixation. Particular emphasis is dedicated to understanding the relationships between structural characteristics of LDHs and light-harvesting capability. It should be noted that making objective comparisons about the photocatalytic performance of any photocatalyst is a hard task, because benchmarking photocatalysts is quite complex [17]. Actually, as many as possible performance parameters are required to evaluate photocatalytic activity, which would give different information: 1. Quantum yield (QY) or apparent quantum yield (AQY) provides the fraction of light that is effectively used by the catalyst. 2. Quantum efficiency (QE) and photonic efficiency (PE) refer to the rate of photochemical events divided by the absorbed photon flux, the latter to the ratio of the photoreaction rate measured for a specified time interval to the rate of incident photons. 3. Reporting rates of product formation (both per surface area and per mass of catalyst) over extended time provides indication of possible different mechanisms, costs/processability, and stability of the catalyst. Comparison based on multiple descriptors is essential for photocatalyst screening as well for industrialization. It is incomplete and potentially misleading to make comparisons based on only one descriptor. Unfortunately, many studies still discuss material performances in an incomplete manner, limiting the real possibility of exhaustive and critical comparison. Therefore, to avoid making misleading comments, we will try to minimize the one-sided performance comparison among different publications, and we encourage more detailed photocatalyst benchmarking in the future.

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2 Characteristics of LDHs as photocatalysts As introduced, LDHs are periodical supermolecules with unique structural properties. In one way, the super-molecular interaction between brucite layers and interlaminar anions is induced from (1) electrostatic attraction between a positively charged host layer and negatively charged guest anions and (2) hydrogen bonding between the OH groups on the surface of brucite-like layers and the oxygen atoms of anions in the interlayer [2]. In the other way, in the brucite layers, the metal cations are dispersed with a uniform manner in an octahedral environment [18] and within a oxo-bridged M2+–O–M3+/4+ matrix [19], which would show quite different natures than the single-component oxides [20]. By strategically engineering its structural characteristics, efficient LDH-based photocatalysts can be obtained for particular applications.

2.1 Compositional structures One of the attractive features is that the metal cations in host layers of LDHs are flexibly tunable in a wide range. Previous empirical rules for the synthesis of LDHs have been drawn that a metal cation with an ionic radius not too different from that of Mg2+ is promising to form brucite-like LDH layers [3, 9, 12]. After abundant experimental work, many photoactive transition metal cations (for example, Zn2+, Cr3+, Ti4+, Co2+, 2+ Cu , Ni2+, and Fe3+) have been successfully incorporated into LDHs, which enables flexible combination of two or three different metals in the layers. It has been summarized from numerous studies that the band gap of LDH materials can be adjusted from 2.0 to 3.4 eV, which benefits the photoresponse of these materials in the visible-light region [6, 16]. Garcia et al. [21] synthesized a ZnCr-LDH as a visible-light-responsive material and regarded Cr as a dopant for zinc oxides. The composition of ZnCr-LDH is analogous to Cr-doped ZnO, but with the advantage that the dopant metal presents in welldefined structural positions and with high amount. Moreover, many studies [22–24] revealed the charge transfer between the ions at octahedral sites in the brucite-like units was partly responsible for the enhanced photocatalytic activity under visible light. In addition, LDHs can be prepared on a large scale to realize reproducibility. Due to these reasons, LDHs are regarded as attractive materials for serving as doped semiconductors.

2.2 Interlayer structures As the super-molecule bonding (including Van der Waals’ forces and electrostatic attraction) between the host layers and guest anions is relatively weak, LDHs provide great possibilities for structural regulation on the vertical scale. The most common way of utilizing this feature is to exfoliate bulk layers into positively charged single-layer or few-layer nanosheets and build various special catalytic heterostructures with these nanosheets and other materials. Because of intimate contact between the two semiconductors, the transfer distance of photoinduced charge carriers within the materials can be substantially shortened, which benefits electron–hole separation efficiency as compared to a single material [13, 25–28]. Moreover, exfoliating bulk-layered materials into nanosheets would maximize the exposed surface and offer plentiful active sites, and further increase the light-harvesting ability compared with raw LDHs [24, 29, 30]. Hwang et al. prepared a layer-by-layer ordered nanohybrid through self-assembly of ZnCr-LDH with layered titanate nanosheets [30]. The layer-by-layer ordered structure enhanced the absorption of visible light and depressed the photoluminescence signal, and thus showed an effective electronic coupling effect. In addition, the nanohybrid exposed much more external surface due to the porous stacking structure than the pristine ZnCr-LDH, thereby providing more active sites and facilitating the diffusion of reactants. Because of the co-contribution of the effective electronic coupling and porous structures, this nanohybrid material greatly enhanced the photocatalytic O2 evolution activity compared to pristine ZnCr-LDH. Similarly, Wei et al. [31] fabricated a visible-light-responsive photocatalyst by anchoring NiTi-LDH nanosheets to the surface of reduced graphene oxide (RGO) sheets, which displayed excellent photocatalytic activity and a quantum efficiency as high as 61.2% at 500 nm. Recently, the heterojunction of LDH with g-C3N4 has drawn a lot of attention [27]. Although g-C3N4 was regarded as a superior photocatalytic material, it still suffered from low visible-light-absorption efficiencies and high recombination rate of photogenerated carriers [32, 33]. However, the heterojunction of LDH with g-C3N4 is found to be an effective solution [27]. For example, a series of NiFe-LDH/g-C3N4 composites were synthesized via face-to-face self-assembly of exfoliated LDH nanosheets with g-C3N4 [34]. The band gap energies for pure LDH and pure g-C3N4 are 2.2 and 2.7 eV, respectively, thus the band gap values for these hybrid materials with different LDH/g-C3N4 weight ratios are tunable in the range of 2.2 to 2.7 eV. It was found that the average lifetime of the photo-generated charges in the optimal NiFe-LDH/g-C3N4 (9:1 weight proportion) composite is 1.6 and 1.5 times longer than that of g-C3N4 and LDH, which is due to the coupling

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of two semiconductors. Correspondingly, the excited electrons or holes with a longer lifetime on the surface are beneficial for photocatalytic H2 and O2 evolution. Except for the layer-by-layer heterostructure, g-C3N4 and NiFe-LDH nanosheets were assembled in situ into a 3D N-doped graphene framework architecture using a facile hydrothermal method [35]. In the ternary g-C3N4/Graphene/NiFe-LDH system, 3D N-doped graphene framework architecture performed as an electron mediator between the LDH and g-C3N4, resulting in several advantageous features, including effective light-trapping, multidimensional electron transport pathways, short charge transport time and distance, strong coupling effect, and improved surface reaction kinetics. In some photocatalytic systems, the interlaminar anions and water molecules also played important roles. Parida et al. [36] investigated photocatalytic performance of carbonate-intercalated ZnCr-LDH and found the carbonate would be oxidized to carbonate radicals by holes, thereby inhibiting the rapid recombination of electrons and holes as well as suppressing the backward reaction to some extent. Subsequently, they found the molybdate-intercalated ZnY-LDHs showed better photodegradation of organic pollutants than nitrate and tungstate-intercalated LDHs and clarified the performance enhancement mechanism from the view of molecular orbital energy level. As for molybdate, its highest occupied molecular orbital (HOMO) does not bond with a fully filled O 2p orbital, while its lowest unoccupied molecular orbital (LUMO) contributes to Mo 4d orbital, hence the molybdate anion easily gets photoexcited to form e-h+ pairs [37]. In addition, the interlayer water molecules were also reported to have an important effect on the photocatalytic charge separation process [20].

2.3 Defective structures In general, when exfoliating or controlling two-dimensional materials to atomically thick scale, brand-new structural and electronic properties can emerge [38]. Decreasing the thickness of bulk materials to an atomically thick scale usually results in increasing surface free energy, which directly results in dislocation or removal of surface atoms. Tavares et al. [39] investigated the exfoliation of ZnAl-LDH and MgAl-LDH with different anions using DFT calculations with periodic boundary conditions and found the exfoliated single layers were ion-defective and dehydrated, which exhibited altered electronic structure and band gaps. For all exfoliated materials, the band gap values decreased when the exfoliated sheets were dehydrated, and the band gap values increased when defects induced in relation to the pristine structure. In addition to ion defect, oxygen vacancy (VO) is very easy to induce structural distortions in octahedral MO6 formation units (Fig. 2) [40]. When acting as photocatalysts, the defects and structural distortions modulate the electronic structure, and even influence the chemisorption behavior of reactants [16]. Zhao et al. [20] investigated the defective structure of Ti-containing LDHs and found that the defects can be divided into three categories (metal/oxygen monovacancies, oxygen-vacancy clusters and micropores), which can be distinguished by positron annihilation spectra (PAS) via analyzing positron lifetimes (τ) and intensities (I). The ratio of the three kinds of defects provided by PAS results show that the twodimensional LDH nanosheets possessed more oxygen-vacancy clusters than the layered oxides and hydroxides with similar composition. The surface defects could serve as trapping sites for electrons and thus suppress electron–hole recombination FIG. 2 Schematic polyhedral representation of the ultrathin LDH structure with defective MO6 octahedra at the nanosheet edge or surface. (B) Biaxial strain for MO6 octahedra in LDH nanosheets. (C and D) Undistorted MO6 octahedron (C) and the corresponding strained MO6 octahedron (D). (E) 2D structural model for an LDH monolayer from DFT calculations viewed from above (VO marked by the yellow dot; light grey dot in print version). Adapted from Y. Zhao, Y. Zhao, G.I. Waterhouse, L. Zheng, X. Cao, F. Teng, et al., Layered-double-hydroxide nanosheets as efficient visible-light–driven photocatalysts for dinitrogen fixation. Adv. Mater. 2017, 29 (42), 1703828 with permission. Copyright 2017, Weily.

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for photocatalytic reactions. Furthermore, they varied the lateral size of ZnAl-LDH sheets from 5 mm to 30 nm and then reduced the nanoplatelet thickness to a few layers. X-ray absorption fine structure (XAFS) experiments revealed the unsaturated local atomic arrangements of ZnO6-x octahedron in the ultrathin LDH. ESR and PAS studies provided evidence for the appearance of Zn+–Vo defective complexes. In the CO2 photocatalytic reduction, Zn+–Vo was found to serve as adsorption sites for CO2 and trapping sites to facilitate electron transfer to the reactant, thus enhancing photocatalytic CO2 reduction rates [41]. In addition, narrowing size dispersion of LDH in lateral dimensions can also induce surface defects.

2.4 Electronic properties The uniqueness of the structural properties of LDHs has an important influence on their electronic structure and therefore on photoactivity. First, in view of the formation of the layered structure in LDHs, the divalent and trivalent cations octahedrally coordinate with hydroxyl groups to form positively charged brucite-like layers. As such, the polymerization of three-centered bridging OH groups (n(O3-H)) helps stabilize the LDHs, and this cationic structure is long-range ordered [42, 43]. On the basis of structural properties, Sato et al. carried out DFT calculation to study the detailed electronic structure of LDHs [42]. In their model, each bridging oxygen atom in metal–oxygen–metal (M–O–M and Al–O–Al) is set to be bonded to one hydrogen atom to form an OH group, and the terminal oxygen atoms are set to be bonded to the two hydrogen atoms of the OH2 group, as shown in Fig. 3. One part of the model including the linkage around the n(O3-H) is extracted for the calculation, as shown in Fig. 4. DFT calculations indicate that the electronic structure of the divalent cations, including valence electronic configuration, natural bond orbitals, natural charge transfer, and bond order, remarkably influence the geometries and the n(O3-H) frequency for the calculated clusters. Due to structural characteristics, the oxo-bridged linkages of bi-metals become a visible-light-induced redox center in LDH-based photocatalysts, which means the electronic excitation from one metal to another [44]. In fact, while pure titanium oxides are not capable of efficiently utilizing the solar spectrum due to the limitation of wide band gap [45], by synthesizing NiTi-LDH, CuTi-LDH, or ZnTi-LDH, the absorption bands can be extended to the visible-light region [20, 30]. For a CoFe-LDH photocatalytic system, the Co–O–Fe oxo-bridges was found to be able to enhance light absorbance and lower charge carrier recombination, which makes it a promising visible-light-responsive photocatalyst for water oxidation [46]. Yan et al. preformed systematic computational investigation on the electronic structures of fourteen kinds of transition metal-containing LDHs [47]. As shown in Fig. 4, band gaps of these transition metal-containing LDHs range from 1.34 eV (NiFe-LDH) to 3.002 eV (ZnTi-LDH), indicating that all these LDHs allow easy electron–hole separation under visible light. The DOS results exhibit that O-2p orbital mainly contributes to the valence band maximum (VBM), so that the photoinduced hole tends to be localized in the O atom of the hydroxyl group, and these holes directly facilitate the oxidation of the hydrogen-bonded water. Besides, the Ni-3d, Cu-3d, and Zn-3d orbitals also contribute to VBM, while Ti-3d does not show a strong influence on the VBM of Ti-based LDHs. As for the conduction band minimum (CBM), d-orbitals of the transition metals are the main components. The preceding results provide a theoretical basis about why the electronic structure of LDHs can be modified by changing the metal cations in the LDH matrix.

FIG. 3 (A) Computational model of [M2Al(OH2)9(OH)4]3+ (M ¼ divalent metal cation) clusters. (B) One part of the cluster model including the linkage around the three-centered bridging OH group, which is used to analyze the calculation data. Adapted from H. Sato, A. Morita, K. Ono, H. Nakano, N. Wakabayashi, A. Yamagishi, Templating effects on the mineralization of layered inorganic compounds: (1) density functional calculations of the formation of single-layered magnesium hydroxide as a brucite model. Langmuir 19 (17) (2003) 7120–7126 with permission. Copyright 2003, American Chemical Society.

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FIG. 4 Total density of states (TDOS) and partial density of states (PDOS) for the MIInMIII/IV-A-LDHs (MII ¼ Mg, Co, Ni, Cu, Zn; MIII ¼Cr, Fe; MIV ¼ Ti; n ¼ 2, 3, 4; A ¼ Cl, NO3, CO3). The Fermi level is displayed with a dashed red line (dark grey line in print version). Adapted from S.M. Xu, H. Yan, M. Wei, Band structure engineering of transition-metal-based layered double hydroxides toward photocatalytic oxygen evolution from water: a theoretical–experimental combination study. J. Phys. Chem. C 121 (5) (2017) 2683–2695 with permission. Copyright 2017, American Chemical Society.

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3 Applications of photocatalytic systems For a long period, LDHs have been used as solid base catalysts or supports/precursors for supported metal catalysts in traditional catalysis. Nowadays, LDHs have found applications in photocatalysis, including degradation of organic pollutants, organic synthesis, H2/O2 production from water splitting, CO2 photoreduction, and N2 fixation.

3.1 Photocatalytic degradation of organic compounds Generally, the photocatalytic degradation process involves the generation of photo-excited electron–hole pairs and subsequent formation of oxidizing radicals, including hydroxyl radicals (OH▪), superoxide radicals (O2▪), and hydroperoxyl radical (HO2▪), as shown in Fig. 5 [48]. Recently, LDHs have been widely applied in visible-light photocatalytic degradation of organic pollutants such as dyes, chlorinated aromatics, and carboxylic acids into harmless CO2, H2O, and simple inorganic acids. As mentioned, many LDH materials show absorbance of visible light due to narrow band gaps. Furthermore, LDHs are well known for their high adsorption properties of anionic species because of high specific surface area and hierarchical structure. ZnO-based materials showed good activity for photocatalytic degradation of organic compounds, thus researchers are investigating the photocatalytic properties of calcined Zn-LDHs. Furthermore, calcination of LDHs leads to the formation of layered double oxides (LDOs) or mixed metal oxides (MMOs), which form a ZnO-based heterojunction structure and further improve the electron-transfer properties of semiconductors. Prevot et al. [23] noted that calcination temperature had great impact on the LDH-derivates. They studied the evolution of the physico-chemical features and photocatalytic properties of ZnCr–CO3 phase calcined at different temperatures. The uncalcined ZnCr–CO3 showed high levels of LDH crystallinity, while the beginning of the dehydroxylation and collapse of interlaminar carbonate occurred during thermal treatment above 150 °C and led to an amorphous phase. Until above 600 °C, the high crystallinity of ZnO and ZnCr2O4 appeared in XRD. With the evolution of structure, a huge change in the adsorption behaviors of Orange II (OII) and photocatalytic properties were observed. The pristine LDH displayed greater adsorption capacity than all the calcined samples, and the OII molecules were adsorbed at the surface but not intercalated into the interlaminar space.

FIG. 5 Schematic illustration of an LDH-based photocatalyst for degradation of organic pollutants and the generation of active radicals. Adapted from G. Zhang, X. Zhang, Y. Meng, G. Pan, Z. Ni, S. Xia, Layered double hydroxides-based photocatalysts and visible-light driven photodegradation of organic pollutants: a review. Chem. Eng. J. 392 (2020) 123684 with permission. Copyright 2019, Elsevier.

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Further study revealed that the photogenerated HO radicals upon ZnCr-600 were around twenty-three times greater than the ZnCr–CO3 LDH at 365 nm. The photodegradation rates were in the sequence of ZnCr-600  ZnCr-800 > ZnCr500 > ZnCr-500 > ZnCr–CO3 LDH > ZnCr-400 > ZnCr-300, and corresponding degradation percentage after 5 h reached 34% and 62% over ZnCr–CO3 and ZnCr-600, respectively. It thus can be inferred that the adsorption capacity and photo-oxidation ability are two main factors; the great adsorption capacity of the ZnCr–CO3 LDH makes up for its shortcoming of lower oxidation ability. Similarly, Zhang et al. [49] prepared ZnAl-MMO by calcining LDH precursors ranging from 400 °C to 800 °C. The ZnAl-600 exhibited the best catalytic performance toward OII photodegradation, which was an outcome of multiple continuous changing factors including band gap, surface area, and exposed catalyst sites. When the ZnAl-LDH precursor was calcined at 800 °C, the structure revealed more detail [50]. As confirmed by TEM, the ZnAl2O4 spinel nanoparticles were homogeneously dispersed in a continuous ZnO network. The direct contact of ZnAl2O4 with ZnO formed a heterojunction nanostructure, which showed a stronger coupling interaction compared with the similar ZnO/ZnAl2O4 prepared by chemical coprecipitation or physical mixing method. For ZnO and ZnAl2O4, the CBM/VBM stood at 4.19/7.39 eV, and  3.36/7.16 eV, respectively, so the electrons at the CBM of ZnAl2O4 would transfer to that of ZnO, and holes from the VBM of ZnO would transfer to that of ZnAl2O4. Therefore, the coupling of ZnAl2O4 and phases could lead to efficient separation of photogenerated e and h+ pairs, thus enhancing photodegradation. Through reconstruction of ZnAl-LDH in an aqueous solution containing TiO2 nanoparticles and Ce4+, a Ti-Ce/LDH heterostructure was constructed [51]. Subsequently, it was calcined at 650 °C and transformed into TiO2–CeO2/ ZnAl-MMO. The TiO2–CeO2/ZnAl-MMO exhibited enhanced photocatalytic efficiency for phenol degradation compared with the original Ti–Ce/LDH, due to the effective interfacial electron transfer between ternary semiconductors of TiO2, CeO2, and ZnO. Except for the Zn-containing LDH derivate, a MgO–MgFe2O4 derived from MgFe-LDH was reported as highly efficient at producing superoxide radicals or other oxidizing species of oxygen, and exhibited greater photodegradation activity than P25 under the same condition [52]. LDHs without calcination were also investigated for photocatalytic degradation. ZnTi-LDH was reported to be more active than pure ZnO and TiO2 for the degradation of methylene blue [53]. The band gap of ZnTi-LDH was determined to be 3.06 eV, smaller than that of P25 (3.19 eV), pure ZnO (3.21 eV), and TiO2 (3.22 eV). In addition, the LDH exhibited a hierarchical microsphere structure and high specific surface (92–108 m2
g1), which benefited the diffusion and adsorption of methylene blue reactants. Due to the low band gap, hierarchical microsphere structure, and high specific surface, the photocatalytic performance was significantly enhanced in comparison to that of ZnO and TiO2. Moreover, other reports proposed that OH groups of LDHs could serve as adsorption sites for organic compounds. Based on this speculation, Valente et al. [54] simply loaded CeO2 nanoparticles on the surface of MgAl-LDHs for photodegradation of phenolic compounds. The MgAl-LDH was calcined at 500 °C and then suspended in bidistilled water to be reconstructed, causing an increase in the number of surface hydroxyl groups. The MgAl-LDH component is not semiconductor material, but indeed enhances the activity compared to CeO2. Through structure-performance studies, it was proposed that the formed intermediate complex during reaction seems locally formed on the vacancy site at the LDH/ CeO2 heterojunction, and the base sites of LDHs (hydroxyl groups as Br€onsted base sites and lattice oxygen as Lewis basic sites) strongly adsorbed the organic compounds. Then, CeO2 was used for accepting the electron and transferring it to O2 molecule. Boudjemaa et al. [55] also reported that d-Fe2O3/MgAl-LDH could improve the photocatalytic activity of d-Fe2O3 under visible light. They proposed a degradation mechanism that the photodegradation of methylene blue first took place in the photogenerated holes at the surface of LDHs. In addition, it is proposed that the abundant OH groups in the surface of host layers can be oxidized by holes to generate OH▪, which makes LDH-based photocatalysts show high oxidation potential. Taking dimethylpyridine N-oxide (DMPO) as the radical scavenger, the ESR experiments found the active radicals formed in the catalytic system. The signals of DMPO-OH▪ and DMPO- O2▪ were observed in the Cr-LDH catalytic system under visible light irradiation [56]. It was ascribed to that OH▪ generated from the surface OH groups after accepting photoexcited hole. Because of the large amount of OH groups on the LDH surface, MCr-LDH exhibited superior photocatalytic activity for degradation of 2,4,6-trichlorophenol (2,4,6-TCP) compared with P25. Heterojunction with other semiconductor materials is also a common method. Carja et al. [57] prepared a calcined Ag/ ZnAl-LDH for decomposition of 4-NPh. The band gap was calculated to be 3.1 eV, revealing that the calcined Ag/LDH heterostructure can harvest solar light. Kim et al. [58] constructed D-Au/TiO2/LDH composite material to enhance decontamination efficiency toward methyl orange. The decontamination cycle could be divided into two steps. First, TiO2/LDH composite material adsorbed the pollutant in contaminated water. Once the material reached its maximum adsorption capacity (1400–1459 mg/g), the second step of sunlight illumination was carried out to regenerate catalysts back to pristine situation. During the second step, Au acted as light-active plasmonic nanoparticles, increasing the regeneration performance by strong photothermal and localized heating effects. Therefore, the composite material retained 97% of its original

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adsorption capacity even after five cycles of reuse. These results demonstrated the great potential of LDH composite materials as multi-functional photocatalysts for organic pollutant degradation. In addition, the non-noble metal Mn was found to be a useful dopant for adjusting the light-absorption capacity of ZnAl-LDH in the UV region [59]. The preceding examples are only a small part of the reported application of LDH-based materials for photodegradation. They were selected as representative cases to illustrate the advantages of LDHs. In summary, the narrow band gap of LDHs contribute to the good absorbance of visible light; the formation of heterojunctions at the interface of LDH and other constituents benefits the separation of photogenerated e and h+ pairs; hierarchical structure and high specific surface provide enough space for the diffusion of organic molecules while the abundant hydroxyl groups and lattice oxygen strongly adsorb reactants; and the surface hydroxyl groups can react with h+ to generate reactive OH▪ for oxidation.

3.2 Photocatalytic organic synthesis In the previous section, we discussed the oxidation process to achieve dyes and organic pollutant degradation by means of photoactivated reactive oxygen species. Photocatalytic selective organic transformation is a less investigated photoredox process but is also potentially more relevant and challenging, as it has promising potential for triggering reactions under mild conditions. As such, LDH-based semiconductors are beginning to be applied in this field. At first, LDHs attracted interest for photocatalytic organic synthesis due to their surface acidic or basic sites, which may chemisorb and then activate reactant molecules. Wu et al. [60] chose a non-semiconductor MgAl-LDH to support Au-Pd alloy nanoparticles and used this material for photocatalytic selective oxidation of benzyl alcohol with oxygen as the oxidant under visible light. The Au9–Pd1/LDH showed efficient photocatalytic activity with 91.1% conversion and 99% selectivity, which was four times greater than that for Au9–Pd1/TiO2 under the same conditions. Moreover, Au/ LDH and Pd/LDH only achieved conversion of 3.6% and 1.5%, respectively. Further studies show that the localized surface plasmon resonance of the Au-Pd alloy contributes to the formation of hot electrons upon irradiation, then the electrons transfer from Au-Pd nanoparticles to the surface adsorbed molecular oxygen to form superoxide radicals (O2▪), which are the main reactive species detected. The LDH provides surface base sites (hydroxyl groups) adsorbing benzyl alcohol to form a metal-alkoxide intermediate. Thus, the intermediate could be more easily oxidized by O2▪ to form benzaldehyde. To reduce costs, they developed photo-responsive ZnxTi-LDH (x represents the Zn2+/Ti4+ molar ratio) nanosheets directly as photocatalysts [61]. The Zn2Ti-LDH displayed 61.0% benzyl alcohol conversion and 77% benzaldehyde selectivity under 4 h full spectrum of light irradiation. It also achieved 38.8% conversion and >99% benzaldehyde selectivity under visible light irradiation. BET and FT-IR analysis indicate that the Zn2Ti-LDH exposes more OH groups than the other LDHs, thus providing more activation sites for benzyl alcohol. Under visible light irradiation, the benzyl alcohol molecules would be exited and deprotonated, forming metal-alkoxide coordination species. Then the surface coordination species help to induce oxygen vacancies, which promote the separation of electrons from the photoexcited surface coordination species and provide active sites for enhanced O2 adsorption and activation, thus generating O2 ▪ radicals via the trapped photoelectrons. Subsequently, the metal-alkoxide intermediate is selectively oxidized by the O2 ▪ radicals to form benzaldehyde, as shown in Fig. 6. In addition, Song et al. [62] achieved efficient photocatalytic oxidation of benzene to phenol with a selectivity of 87.2% also using ZnTi-LDH. Similarly, the abundance of oxygen vacancies can enhance electron–hole separation efficiency and contribute to O2 ▪ formation. Optical and electrochemical characterization demonstrated that the ZnTi-LDH showed matched band structure with the potential of benzene oxidation to phenol, leading to excellent catalytic performance under light irradiation. In comparison with the oxidation mechanism for degradation, it seems the formation of O2 ▪ radicals selectively oxidize organic compounds, while the OH▪ radicals lead to non-selective degradation to small molecules. Oxidative desulfurization of diesel is a promising process for preventing SOx pollution. Essentially, dibenzo thiophene (DBT) is transformed into biphenyl and SO2 during this process, removing sulfur from gasoil. Hosseini et al. [63] studied three types of photocatalysts (Ni-Co2, Ni-Fe2, and Co-Fe2 LDH/Fe3O4) in the desulfurization of the gasoil model. The highest DBT removal yield was predicted to be 76% by response surface methodology (RSM), while the experimental result showed 74.6% removal yield under optimum conditions over Ni-Co2 LDH/Fe3O4. Among three samples, Ni-Co2 LDH/Fe3O4 possessed the largest specific surface area (125.68 m2/g) and smallest band gap (2.73 eV). For photocatalytic mechanism, the surface hydroxyl group of LDH materials is indispensable. After the generation of holes and electrons during irradiation, surface adsorbed oxygen and surface hydroxyl groups of LDH produce O2  and H+, respectively. Consequently, O2  reacts with H+ twice to generate adsorbed H2O2. Both electrons and holes react with H2O2 to generate OH for the following reaction with DBT molecules. This magnetic nanocomposite constructs a new inexpensive platform for design and easily separative photocatalysts for desulfurization reaction.

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FIG. 6 Possible mechanism for selective oxidation of BA to BAD over ZnTi-LDH under visible light. (dotted circle: oxygen vacancy). Adapted from Zou J., Wang Z., Guo W., Guo B., Yu Y., Wu L., Photocatalytic selective oxidation of benzyl alcohol over ZnTi-LDH: the effect of surface OH groups. Appl. Catal. B Environ. 2020, 260, 118185 with permission. Copyright 2020, Elsevier.

Parida et al. recently reported two green photocatalytic routes for robust Suzuki coupling reaction [64] and one-pot imine tandem synthesis [65]. In the iodobenzene and phenylboronic acid Suzuki coupling reaction, a GO/ZnCrLDH@AuPd nanocomposite exhibited biphenyl yield of 99.5% after 2 h of visible light illumination, which is 6.1 times and 2.8 times more than the yield of GO/LDH@Au and GO/LDH@Pd. After evaluating the roles of Au-Pd alloy and supports, it was revealed that the visible-light responsive support could increase the electron density over Pd atoms to activate iodobenzene, while the remaining holes in the LDH stimulate the coupling partner phenylboronic acid for the Suzuki coupling reaction. Moreover, they placed particular emphasis on investigating the influence of Au-Pd alloy on an amine functionalized ZnCr-LDH/MCM-41 nanocomposite, for the one-pot imine synthesis through photoalkylation of benzyl alcohol with nitrobenzene. The alloy nanoparticles more efficiently harvest light than the single Pd and Au nanoparticles. In addition, the charge heterogeneity formed on the alloyed structures enhances the interaction between metal with substrates, promoting the coupling between photogenerated benzaldehyde with aniline in the tandem reactions. Therefore, in terms of photocatalytic organic synthesis, the formation of particular reactive oxygen species and activation of reactants are as important as the properties of the semiconductor, such as the separation of photoelectron–hole pairs or regulating the band structures.

3.3 Photocatalytic water splitting Splitting water into H2 and O2 using solar energy is an important area of research and the search for efficient and stable semiconductor photocatalysts for this reaction has garnered a lot of interest. The photocatalytic route to acquire oxygen and hydrogen from water splitting requires consumption of four holes and electrons, then formation of O–O/H–H bonds. We discuss the performance of LDH photocatalysts for water splitting in this section.

3.3.1 LDHs for photocatalytic oxygen generation Since ZnO-based photocatalysts have shown good activity for oxygen evolution, Garcia et al. [21] incorporated Zn2+ ions with Cr3+, Ti4+, and Ce4+ ions into LDH layers. Among these LDHs, the ZnCr-LDH was the most active for the water splitting into oxygen under visible light illumination; the oxygen formation efficiency increased with the Cr content. In addition, the quantum efficiency of ZnCr-LDH was 1.6 times greater than that of WO3 under the same conditions. TiO2 has proven to be an effective photocatalyst for water splitting, but its UV response severely limits its application. To overcome this drawback, Ti cations have been incorporated into LDHs together with other metal cations. With this approach, a large and easily controllable amount of Ni/Cu was introduced into a titanium compound by Kang et al [66]. Results showed an efficient metal-to-metal charge transfer between the Ni/Cu–O–Ti oxo-bridged linkages. Thus, the new energy levels introduced by Ni or Cu can extend the photoresponse region to the visible range. Although the construction of LDH is a way to improve the photoresponse properties of TiO2, and the as-synthesized bulk NiTi-LDH is indeed more active for producing oxygen than conventional TiO2 under the same conditions, the photocatalytic efficiency

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of the general LDHs is still much lower than expected. In this regard, some effective strategies for improving the photocatalytic performance of LDHs are under development. Zhao et al. [67] proposed to decrease the horizontal and vertical size of LDH plates to enhance the exposure of the surface-active sites and shorten the photogenerated charge diffusion paths. In 2014, they tested the visible light O2 generation activity of NiTi-LDH with a thickness of 2.0 nm and a lateral dimension of 30 nm, showing a dramatic enhancement in the O2 evolution rate (12.71 mmol
m2
h1) with respect to the bulk NiTi-LDH with a lateral dimension of 2 mm (2.44 mmol
m2
h1) [67]. TiO2 only achieved a rate of 0.2 mmol
m2
h1 under the same conditions, as reported by other groups [66]. Moreover, a high apparent QY of 65.0% and 20.0% was obtained for the NiTiLDH nanosheets under monochromatic irradiation at 400 and 650 nm, respectively. Structural studies revealed the Ti3+-oxygen vacancies complex contained in ultrathin LDH nanosheets acts as a self-doping in the band gap of LDH. Specifically, the photoexcited electron is trapped by the oxygen vacancy, and its interaction with the AgNO3 sacrificial agent is facilitated, further improving electron–hole separation efficiency. The hole left in the valence band accumulates on the surface Ti3+ defects, accounting for the enhanced efficiency of water splitting into O2. The formation of heterojunctions with other functional semiconductor materials is also used to improve the photocatalytic performance for O2 evolution. As mentioned, the LDH-based nanohybrids, like ZnCr-LDH/Titanate [30], NiTiLDH/rGO [31], and ZnCr-LDH/polyoxometalate [68], are fabricated by an electrostatic self-assembly method. There is no doubt that the photoactivity of these nanohybrids is far superior to the corresponding LDH and titanate, rGO, or polyoxometalate precursors, as well as the physical mixture of two components. The commonalities of these materials for improving activity are creating more surface catalytic sites by decreasing the plate sizes of LDH, extending the lightabsorption range, and improving electron–hole separation via heterojunction effect between the two components. Amani-Ghadim et al. [69] prepared a kind of heterostructure nanocomposite consisting of MZnAl-LDH/ZnS (M ¼ Co or Mn) quantum dots (QDs) through the self-assembly of electropositive LDH nanosheets and electronegative ZnS QDs. Although only partial ZnS QDs were intercalated into the interlaminar gallery of LDHs, these nanocomposites showed unexpectedly improved visible light response compared to that of ZnS QDs and LDHs. As calculated from the Tauc and Mott–Schottky plots, the VBM energy level of ZnS QDs was determined to be lower by 0.44 and 0.22 V vs. normal hydrogen electrode (NHE) than the CBM level of the CoZnAl and MnZnAl LDHs, respectively. Moreover, the conduction band of ZnS was larger than that of the CoZnAl and MnZnAl LDHs (shown in Fig. 7). The photogenerated electrons and holes tended to localize in the MZnAl-LDH when irradiated by visible light. Accordingly, the transport of charge carriers from the MZnAl-LDH to the ZnS QDs was observed to be difficult. In addition, effective electronic coupling between ZnS QDs and MZnAl-LDH greatly improved electron–hole separation efficiency and UV–Vis light-harvesting ability. In comparison to pristine LDHs and ZnS QDs, the obtained heterostructures exhibited superior photocatalytic activity of oxygen evolution and the degradation of Acid Red 14 under visible light irradiation.

FIG. 7 Proposed band structure alignments of (A) Co/ZnS and (B) Mn/ZnS heterostructures. Adapted from A.R. Amani-Ghadim, F. Khodam, M.S.S. Dorraji, ZnS quantum dot intercalated layered double hydroxide semiconductors for solar water splitting and organic pollutant degradation. J. Mater. Chem. A. 7 (18) (2019) 11408–11422 with permission. Copyright 2020, The Royal Society of Chemistry.

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3.3.2 LDHs for photocatalytic hydrogen generation After applying ZnCr-LDH for oxygen generation, its activity for hydrogen evolution from water splitting was also examined. Results showed that ZnCr catalysis systems have great potential [44]. Then the Ni2+ was incorporated into ZnCr-LDH with varied proportions. The oxo-bridged linkage of Zn and Cr contributed to metal-to-metal charge transfer under photoexcitation, causing the transformation from ZnII/NiII–O–CrIII to ZnI/NiI–O–CrIV, which acted as a photoinduced center for hydrogen evolution. In addition, Fe-containing LDHs have been screened [70]. In a FeMgAl-LDH, the FeO6 octahedron enables the conduction band energy to be more negative (versus NHE) than that of the H2O/ H2 redox couple, thus contributing to H2 generation. In addition to Cr and Fe, the Ti-containing LDHs are also used for photocatalytic H2 evolution. The TiO6 units are orderly dispersed and form oxo-bridging linkages with other metal octahedrals (Ni, Zn, Mg). The covalent interaction between TiO6 and NiO6 shows the most obvious effects on modifying the electronic structure and the separation of the photoexcited e–h pairs. The best H2 production rate achieved by Ni/Zn/ MgAlTi-LDHs is approximately eighteen times greater than that of the K2Ti4O9 reference sample. There are other applications of calcined LDH photocatalysts for H2 generation besides water splitting. Mantilla et al. [71] studied the relationship between calcination temperature and charge transfer process. The heterojunction structure of ZnO/Zn6Al2O9 (< 700 °C) or ZnO/ZnAl2O4 (> 700 °C) can be prepared from ZnAl-LDH precursors. The H2 evolution rate showed a volcanic curve trend while the calcination temperature increased, but the sample treated under 600 °C exhibited the maximum value. According to analysis, even though the band gap values decreased with the increase of the calcination temperature, the ZnO/Zn6Al2O9 structure showed a lower recombination rate of carriers due to a smaller energy barrier in the valence band. Deposition of Au and Ag nanoparticles on LDH surfaces as cocatalysts is a common strategy to enhance H2 production yield. Garcia et al. [72] prepared Au/ZnAl-LDH, Au/ZnCeAl-LDH, and their corresponding derived MMOs for H2 generation under solar irradiation. Based on their results, the activities of MMO samples were lower than those of the LDH samples, which was due to the increase of Au particle sizes after calcination. The formation of larger Au nanoparticles decreased the number of joint active sites between gold and the support, leading to lower performances. Furthermore, the introduction of Ce into the matrix of the LDH promoted the oxidation of the deposited Au particles during the reacting process. XPS study of the fresh and irradiated catalysts indicates that during reaction Au0 species were oxidized to Au+ or Au3+ species in both Au/ZnAl-LDH and Au/ZnCeAl-LDH. However, a larger population of oxidized Au atoms on the surface was detected in the Au/ZnAlCe-LDH, and its efficiency for H2 production was improved. The authors proposed light absorption by Au surface plasmon resonance band and subsequent electrons injection into the conduction band of the LDH. Methanol used as a sacrificial agent replenishes electrons to the Au nanoparticles. With the photocatalytic process cycling, the two kinds of Au species in neutral and positive states reached steady states. In addition, strong electronic coupling between LDH nanosheets and other semiconductors is also a common strategy to promote H2 evolution from water splitting. For example, a MoS2/NiFe-LDH was synthesized and the resulting p–n heterojunction modified the optical and electronic properties of the composite [57]. This nanocomposite showed superior photocatalytic activities in degradation of rhodamine B with H2 evolution. Kim et al. [73] constructed a novel photocatalyst of 2D-NiFe-LDH nanosheets anchored on 1D CdS nanorods to construct a heterojunction structure. By introducing 42 wt% of NiFe LDH to CdS nanorods, the hydrogen evolution rate of the obtained nanocomposite achieved thirty times that of the bare CdS nanorods and 800 times that of the NiFe LDH. The surprisingly enhanced activity was dependent on the heterojunction structure. As a combined study of light-harvesting capability and band edge potentials exhibited, the interfacial contact of heterostructures allowed efficient carrier transport so that the photogenerated electrons in CdS can be easily transferred to the NiFe LDH, and therefore the recombination of carriers can be greatly suppressed. Simultaneously, the thin and flexible nature and high specific surface area of NiFe LDH provided a significant number of catalytically active sites for the hydrogen evolution reaction.

3.4 Photocatalytic CO2 reduction LDH materials possess intrinsic basic sites, the hydroxyl groups as Br€onsted base sites and lattice oxygen as Lewis basic sites, which show strong chemisorption capacity for acidic CO2 [74]. Therefore, LDH-based materials are gradually emerging as effective photocatalysts for CO2 reduction. Izumi et al. [75] reported that ZnAl-LDH was active for CO2 photoreduction to CO, because its band gap (3.5–5.7 eV) was sufficient to proceed with the reaction steps (CO2 + 6H+ + 6e ! CH3OH + H2O; CO2 + 2H+ + 2 e ! CO + H2O) at negative reaction potentials. Meanwhile, ZnCuGa-LDH was active for CO2 photoreduction to methanol with the assistance of H2 as a reductant and under UV–visible light. They discovered that specific interaction of Cu sites with CO2 was

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spectroscopically suggested to enable coupling with protons and photogenerated electrons to form methanol. Teramura et al. [76] investigated a series of LDHs (Ni, Mg, and Zn as divalent metals, and Al, Ga, and In as trivalent metals) for converting CO2 to CO and O2 in water. Interestingly, the NiIn-LDH was most active to form CO2, while MgIn-LDH was most active to form O2. In addition, it was also noted that Cl can improve the activity and selectivity of the LDH by acting as an effective hole scavenger in Ni-based LDHs. Furthermore, they screened sixteen different kinds of transition metals containing M2+–M3+ LDHs (M2+ ¼ Co2+, Ni2+, Cu2+, and Zn2+; M3+ ¼ V3+, Cr3+, Mn3+, and Fe3+) for the photocatalytic conversion of CO2 [77]. Among these LDHs, Ni2V-LDH showed the highest activity for CO evolution due to the existence of V3+ and its large specific surface area. The authors proposed that the combination of V3+ and Cl played significant roles like a redox couple in the consumption of photogenerated holes during the photocatalytic conversion of CO2 into CO. However, the Ni2V-LDH was easily leached in the reaction solution containing NaCl.. Based on the previous study, surface decoration on LDHs with a cocatalyst has been widely investigated. Similar to photocatalytic systems, various LDHs with different compositions are loaded with noble metals (Pt, Au, Ag) as cocatalysts for the photoreduction of CO2 [78–80]. However, their CO formation rates are still limited and would increase the cost greatly. Therefore, research is developing cost-effective and easily synthesized cocatalysts [16]. Katsumata et al. [81] loaded Cu2O nanoparticles on ZnCr-LDH via a soft-chemical in situ reduction process. The [email protected] exhibited superior activity for the conversion of CO2 into CO compared to CuZnCr-LDH and pristine ZnCr-LDH in pure water. During the photocatalytic reaction, the Cu2O undergoes a self-reduction under UV irradiation resulting in the Fermi level shifting in the positive direction and the formation of unoccupied Cu-3d orbitals below the conduction band of Cu2O. Then, the electron could transfer from the conduction band of Zn-Cr LDHs to the unoccupied Cu-3d orbitals of the surfaceloaded Cu2O nanoparticles, which enables the Cu2O tto become highly active reaction sites for CO evolution from CO2 in water. Therefore, the light-harvesting material is still ZnCr-LDH and the cocatalyst functioning as effective electron traps is Cu2O. However, this material also generates a large proportion of H2 together with CO (H2:CO ¼ 0.62:1), which is a waste of the reducing ability. It should be noted that the CO2 photoreduction efficiency of bulk LDH-based photocatalysts is still quite low except under strong UV light. Lee et at [82]. synthesized a hierarchical P25@LDH nanocomposite for aqueous CO2 photoreduction to CO, in which the P25 nanoparticles were encapsulated within the microporous CoAl-LDH matrix. The photoexcited electron transferred from the CoAl-LDH to P25 and the hole transferred from P25 to the CoAl-LDH, and thus the photoinduced charge carrier lifetimes were effectively prolonged by the spatial separation. These low cost P25@CoAl-LDH nanocomposites also exhibited a good apparent quantum efficiency greater than that of pure CoAl-LDH. Recently, Zhao et al. [83] used monolayer NiAl-LDH nanosheets (m-NiAl-LDH) and bulk NiAl-LDH (b-NiAl-LDH) for CO2 photoreduction into CO and CH4, which significantly enhanced the CO2 photoreduction efficiency. Under visible light irradiation, the formation rates of CH4 and CO for m-NiAl-LDH are enhanced 542 and 4.5 times compared with that of b-NiAl-LDH. Notably, the selectivity of CH4 can be improved from 0.11% for b-NiAl-LDH to 16.54% for m-NiAlLDH, while the selectivity of byproduct H2 can be suppressed from 43.84% for b-NiAl-LDH to 13.26% for m-NiAl-LDH. Under irradiation greater than 600 nm, the selectivity of CH4 increased to 70.3%. Meanwhile, the H2 evolution can be totally suppressed over m-NiAl-LDH. In addition, the formation rate of CH4 is still 8.9 times greater than that of b-NiAl-LDH. To provide a deep insight into the advantages of m-NiAl-LDH, a detailed structural investigation was performed by a combination of XAFS experiments and DFT calculation. It was found that the monolayer nanosheets exposed plentiful surface with coordinately unsaturated metal defects (denoted as VM) and hydroxyl defects (denoted as VOH), which caused defect state in the forbidden zone of monolayer NiAl-LDH. As shown in Fig. 8, under irradiation greater than 600 nm, the photogenerated electrons matching the energy levels of the photosensitizer and m-NiAl-LDH only localized at the defect state, providing a driving force of 0.313 eV to overcome the Gibbs free energy barrier of CO2 reduction to CH4 (0.127 eV), rather than that for H2 evolution (0.425 eV). Therefore, the defective m-NiAl-LDH exhibited superior selectivity and activity to CH4 formation.

3.5 Photocatalytic N2 fixation The photocatalytic process of transforming N2 to NH3 with sunlight at ambient temperature is a promising and attractive synthetic route for the Earth’s nitrogen cycle. However, the activation of relatively inert N2 is incredibly challenging. In addition, the amount of ammonia produced is usually so small that it cannot be firmly attributed to N2 fixation rather than contamination from ammonia that is either present in air, human breath, or ion-conducting membranes, or generated from labile nitrogen-containing compounds (for example, nitrates, amines, nitrites, and nitrogen oxides) that are typically present in the nitrogen gas stream, the atmosphere, or even the catalyst itself [84]. Therefore, benchmarking protocols for the reaction and a standardized set of control experiments designed to identify and then eliminate or quantify the sources of contamination required to be developed. In this regard, Chorkendorff et al. [84] proposed an isotope measurement

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FIG. 8 (A) The Gibbs free energy diagrams for photocatalytic CO2 reduction to CH4 on various sites, without driving force. Blue (dark grey in print version) number here denotes the Gibbs free energy barrier for CO2 reduction to CH4 with the unit of eV. (B) The side view of each CO2 reduction intermediate on the defects of m-NiAl-LDH. (C) Gibbs free energy diagram for H2 evolution on defects of m-NiAl-LDH, with Gibbs free energy barrier labeled. (D) The projected density of states with the defect state and conduction band minimum (CBM) labeled. (E) The energy levels for the singlet and triplet excited states of photosensitizer Ru(bpy)3Cl2, together with the band edge placements for the CBM, defect state, and valence band minimum (VBM) of mNiAl-LDH versus the normal hydrogen electrode (NHE). Adapted from L. Tan, S.M. Xu, Z. Wang, Y. Xu, X. Wang, X. Hao, et al., Highly selective photoreduction of CO2 with suppressing H2 evolution over monolayer layered double hydroxide under irradiation above 600 nm. Angew. Chem. 131 (34) (2019) 11986–11993 with permission. Copyright 2019, Weily.

by using 15N2 as a reactant, which enables reliable detection and quantification of the production of ammonia. Otherwise, it is necessary to use the same batch of catalysts and reagents for the blank control test (without filling N2 or reaction under darkness) every single time. Recently, LDH-based semiconductors have begun to be applied in this reaction. Zhang et al. [40] first examined the photocatalytic activity of N2 fixation over several ultrathin LDH nanosheets (NS). The studied divalent metals included Mg, Zn, Ni, and Cu, and the trivalent metals included Al and Cr. Under **UV–Vis irradiation and in the presence of water, almost all the mentioned candidates were active to form NH3, with an order of CuCr-NS (184.8 mmol
L1) > NiCr-NS (56.3 mmol
L1) > ZnCr-NS (31.2 mmol
L1) > ZnAl-NS (38.2 mmol
L1) > NiAl-NS (22.3 mmol
L1). The wide band gap ( 5.0 eV) of the MgAl-NS determined it would be inactive. Specifically, Cr3+ showed huge advantages when changing UV–Vis to the visible-light region because of its strong absorption in the visible-light range. Among the three Cr-containing LDH NS, CuCr-NS was most active for NH3 evolution, only accompanied by O2 evolution and no N2H4 or H2. The NH3 evolution rate varied upon changing pH of the reaction medium, which was 35.7 mmol
L1 at pH 8.0, and gradually

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increased to 142.9 mmol
L1 at pH 6.9, 175.3 mmol
L1 at pH 5.8, and finally 220.9 mmol
L1 at pH 5.0. The bulk CuCrLDH with reference only gave trace amounts of NH3, once again showing the advantages of ultra-thin materials. The origin of superior activity upon CuCr-NS was mainly related to the defective structure (oxygen vacancy, structural distortions, and compressive strain), which was imparted by the strong Jahn–Teller effect of Cu2+. Structural studies displayed that the oxygen vacancy-rich CuCr-NS contributed to a more negative conduction band and more efficient charge transfer than bulk CuCr-LDH. Furthermore, DFT calculation studied the band structure for CuCr-LDH with and without VO defects. As shown in Fig. 9, the VO doping induced a new defect level shown in the middle of the band gap compared to that of CuCr-Pure, mainly consisting of contributions from the unoccupied Cr 3d orbitals in LDH. After adding compressive strain in CuCr-VO, even stronger defects were formed, which was also due to unoccupied Cr 3d orbitals. These additional energy states were speculated to serve as electron-trapping sites that enhance electron transfer from LDH to N2. Accordingly, the adsorption energy calculation declared that N2 adsorption was favored on CuCr-VO and CuCr-VO-Strain than on defect-free CuCr-Pure. The introduction of VO and strain effect further lengthened the N-N distance and weakened the N2 bond. Compared with the N-N distances in N2H2 and N2H4, it suggested that N2 was adsorbed molecularly on

FIG. 9 Band structure for CuCr-LDH with and without VO defects: (A) CuCr-Pure, (B) CuCr-VO, and (C) CuCr-VO-Strain. (D) Adsorption energies of N2 on CuCr-Pure, CuCr-VO, and CuCr-VO-Strain. (E) N-N distance of free N2, N2 on CuCr-VO, N2 on CuCr-VO-Strain, N2H2, and N2H4. (F) Charge density distribution for the VBM of CuCr-VO-Strain. Adapted from Y. Zhao, Y. Zhao, G.I. Waterhouse, L. Zheng, X. Cao, F. Teng, et al., Layereddouble-hydroxide nanosheets as efficient visible-light-driven photocatalysts for dinitrogen fixation. Adv. Mater. 29 (42) (2017) 1703828 with permission. Copyright 2017, Weily.

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CuCr-VO and CuCr-VO-Strain via coordinately unsaturated VO sites. In summary, the defective structure led to increased interaction between the LDH and N2, thus enhancing NH3 evolution efficiency.

3.6 Photocatalytic elimination of NOx Apart from the N2 fixation process, the elimination of toxic nitrogen oxide gases (NOx) from the atmosphere is another crucial process within the Earth’s nitrogen cycle. Pavlovic et al. [85] reported the photocatalyst of ZnAl-LDH with high photocatalytic activities in degrading NO gases. They compared samples with different crystallization. While the pore microstructures and band gap values of different samples were similar, the greatest NO removal efficiency of around 55% was found for the photocatalyst with poorer crystallization. Specifically, the surface area determined by BET method is an important characteristic for regulating NO removal efficiency. Moreover, LDH catalyst exhibited about 6% of NO2 emissions, which was even lower than commercial TiO2 materials. Thus, the easily prepared and inexpensive ZnAl-LDH photocatalyst opens a promising pathway for NOx photocatalytic transformation.

4

Conclusion and perspective

Over the past decades, LDH-based materials have displayed great potential as semiconductors for photocatalysis, thanks to their compositional flexibility and tunable particle size, structure, surface defects, and electronic characteristics. In addition, industrial mass production of LDHs has been achieved at low cost, making them even more attractive. This chapter highlights the characteristics and selected applications of LDH-based photocatalysts in the fields of environmental clean-up and renewable energy production, including degradation of organic pollutants, organic synthesis, H2/O2 production from water splitting, CO2 reduction, and N2 fixation. The application range of photocatalytic reactions is continuously expanding, and new concerns are being raised. Despite the reported improvements, we hope that LDHbased materials can make progress in the following aspects: 1. For the popular and environmental-relevant photocatalytic degradation of contaminants (such as organic dyes, organochlorine pesticides, or pharmaceuticals), most reported studies merely perform investigation of a single organic compound, which is misleading for assessing photoactivity. As Mul et al. [63] commented, there are multiple components present in practical wastewater, so evaluation of the mutual effects of these contaminants on their photocatalytic degradation rates is necessary to evaluate the feasibility of practical application. Besides, the molecular structure of the studied organic compound would affect the results, so the catalyst may not be universal for a variety of pollutants. Hence, studies must be carried out in mixtures of organic contaminants. 2. Generation of H2/O2 from water splitting is an attractive approach to transform solar energy in chemical forms. However, most of the studies are carried out with sacrificial agents to minimize the recombination of excited electrons and holes, which is a waste of the oxidizing or reducing power. In this regard, the coupling of water splitting with other reactions, for example, organic transformations [86] or dye degradation, is very desirable to utilize wasted excited holes to boost H2 production and oxidation simultaneously. 3. For CO2 photoreduction, labile 13CO2 is often needed to confirm mechanism and track original carbon source. Particularly for LDH, whether the interlayer carbonate anions could participate in the reaction is a concerning problem. 4. Photocatalysis for organic synthesis has been realized in many reactions, such as oxidation, cycloadditions, crosscoupling reactions, amino functionalization, ATRA reactions, and fluorinations [87]. Since LDHs and their derivatives are efficient to chemisorb and convert various organic compounds in traditional catalysis, these LDHs with semiconductor properties are also promising for photocatalytic organic synthesis.

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Chapter 30

Crystalline two-dimensional organic porous polymers (covalent organic frameworks) for photocatalysis K Aswani Raj and M Rajeswara Rao Department of Chemistry, IIT Dharwad, Dharwad, Karnataka, India

1 Introduction 1.1 Overview of photocatalysis Catalysis is a process in which a substance accelerates a chemical reaction without being consumed or altered in the process [1]. Such substances that can perform this remarkable feat are coined as catalysts. Enzymes are the most common and efficient catalysts found in nature which expedite the biochemical reactions necessary for life. For example, the enzyme ptyalin present in saliva assist in speeding up the conversion of starch to glucose and complete the process in minutes what would otherwise take weeks. Inspired by nature’s strategies to enhance the rate of thermodynamic reactions, scientists have continually designed and developed various compounds by as embling diverse building blocks and successfully employed them as catalysts. These catalysts play a key role in catalyzing various chemical reactions in the pharma, chemical, and petrochemical industries [1, 2]. Natural photosynthesis utilizes solar energy (catalyst) to activate the photosystems (II and I) to extract electrons from water (water splitting) followed by reduction of NADP+ to NADPH by adding a pair of electrons and hydrogen ions. This photocatalytic process involves multiple complex steps however, the photocatalyst (chlorophyll) in the presence of sunlight successfully catalyzes the entire process. Inspired by nature, light-assisted photocatalyzed processes (photocatalysis) that utilize light energy to drive chemical reactions or convert light energy to chemical energy have gained prime importance owing to the availability of abundant renewable solar energy. According to IUPAC, photocatalysis is the “change in the rate of reaction or its initiation under the action of light in the presence of a substrate” and the photocatalyst is the “one that absorbs light and is involved in the chemical transformation of the reaction partners”. Photocatalysis has been regarded as one of the clean technologies owing to its sustainability and environmental friendliness and has been employed in medical and environmental applications etc. [3]. To drive a photocatalytic reaction, an efficient photoactive material (catalyst) that absorbs the light and executes the reaction is highly important. Thus, significant efforts have been employed in developing efficient photocatalysts. In this line, several metal oxides (TiO2, ZnO, ZnS, and WO3) [4, 5, 6, 7], and chalcogenide semiconductors (CdSe) [8] and some modified materials of these metal oxides via incorporation of dopants and assembling composites, etc. were developed and tested as photocatalysis. Despite the high photocatalytic activity and chemical stability, limited structural variations, as well as resistance to active-site engineering limit the importance of these inorganic materials. On the other hand, homogenous photocatalysts based on Ru and Ir-based metal complexes [9] or organic dyes (rose Bengal and fluorescein) [10, 11], that allow structural refinement to fine-tune electronic properties have been explored. But the compounds suffer from low stability, toxicity, and non-recyclability and most importantly from inefficient light absorption in the visible region. Later, organic polymer-based photocatalysts [12] came to the spotlight owing to their unique advantages of efficient light absorption, variable optical bandgaps, long-lived excited states, and also high charge carrier mobilities. Furthermore, these organic polymeric photocatalysts are susceptible to structural tunability that in turn allows the fine-tuning of electronic properties as desirable to a specific photocatalytic process. Organic porous (amorphous [13, 14], and crystalline [15, 16, 17]) polymers and Metal–Organic Frameworks (MOFs) [18, 19], have been extensively explored for the various photocatalytic process. Though MOFs offer integrated optoelectronic properties of both organic (high light-harvesting ability and tunable bandgaps) and inorganic (defined catalytic sites), their chemical

Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00011-8 Copyright © 2021 Elsevier Inc. All rights reserved.

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instability and low efficiency in exciton migration and charge separation severely limits their application in photocatalysis. On the other hand, the high chemical and hydrolytic stability of 2D p-conjugated polymers in conjunction with the large surface area, well accessible pores make these polymers perfect candidates for photocatalysis. Among the known 2Dporous polymers, carbon nitrides [20] suffer chemical diversity while the amorphous porous organic polymers lack crystallinity i.e., long-range order. On the other hand, the crystalline 2D-organic polymers famously called Covalent Organic Frameworks (COFs) possess high-quality features such as long-range order (facilitate the high-efficiency exciton migration), 2D p-delocalization (enable the visible absorption), and structural tunability that is desirable for the photocatalysis [15,16,17,18]. Besides, COFs are stable in acids, bases, and aqueous solutions. Thus, COFs have been recognized as special materials for photocatalysis and being actively explored for various applications. Before going into the details of COF catalyzed photocatalytic processes, a brief general view of COFs will be presented below.

1.2 Overview of covalent organic frameworks (COFs) COFs are an emerging class of porous crystalline materials derived purely from organic components in which organic building blocks linked through covalent bonds [21, 22, 23, 24], Ever since the first report of Yaghi and co-workers [25] on boroxine and boronate-based crystalline COFs, the field has been rapidly developing because of their attractive features such as well-defined porosity (Fig. 1A), well defined molecular arrangement (Fig. 1C), and ease of functional tunability, etc. These materials have been envisaged to have widespread applications in the area of gas storage, optoelectronics, and catalysis, etc. [22,23,24,25]. To obtain well-defined and ordered crystalline COFs, dynamic covalent chemical reactions have been employed for the synthesis (Scheme 1A). These reversible reactions allow the polymer to attain thermodynamic minima via breaking improper linkages between the building blocks and relinking them. This is called the error-correction mechanism. Following such strategy, a wide variety of linkages related to BdO, CdC, CdN, dCOdC ¼ CdNHd etc. have been successfully introduced by condensing the appropriate functionalized ((OH)2, dB(OH)2, dNH2, dCHO) building blocks. Another critical point to take note of in achieving the well-defined crystalline COFs is to employ rigid, planar, and highly p-conjugated building blocks (pyrene, anthracene, perylene, porphyrin, phthalocyanine, etc.) (Scheme 1B). The large p-conjugated building blocks also stabilize the COF framework and prevents it from collapsing. Besides the crystallinity, building blocks also govern the COF topology. For instance, a combination of C2-symmetric building blocks with C3-symmetric building blocks leads to hexagonal pore topology, while the same C2 with C4 renders square topology (Fig. 2). Various combinations of building blocks open up limitless possibilities. The typical synthesis of COFs involves the condensation of building blocks in a mixture of solvents (for example mesitylene-dioxane-AcOH) at 100°C for a few days [22,23,24,25]. However, to obtain a crystalline material, the reaction conditions such as solvent ratios, reactant concentrations, temperature, and time have to be optimized for a mixture of solvents. On the other hand, thin films of 2D polymers ranging from a single layer to nanometer thick are also be grown on substrates like single-layer graphene, indium-doped tin oxide (ITO), etc. COFs possessing imine (dC ¼ Nd), alkene (dC ¼ Cd), and ketoenamine (dCOdC ¼ CdNHd) hydrazine, azine, etc. linkages promote p-electron communication (though the presence of heteroatoms induces high bond polarization which restricts the extended p-delocalization) between the integrated building blocks thereby establishes intra-sheet pconjugation (in-plane). It renders a new set of emerging electronic properties for the COF material compared to the corresponding building blocks [24]. On the other hand, no such electronic interactions are possible for the COFs developed using boroxine-linked COFs due to inefficient BdO [26] bond. In addition to the linkers, nodes will also play a critical role in propagating p-conjugation in COFs [22,23,24,25]. The presence of tri-fold symmetric (meta-connected) building blocks

FIG. 1 Diagrammatic illustration of (A) eclipsed (top view); (B) columnar (side view) stacking modes of a COF and (C) typical powder XRD of a COF. Adapted with permission from N. Huang, L. Zhai, H. Xu, D. Jiang, Stable covalent organic frameworks for exceptional mercury removal from aqueous solutions. J. Am. Chem. Soc. 139 (2017) 2428–2434. Copyright 2017 American Chemical Society.

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SCHEME 1 Representative examples of linkages (A), building blocks (B) which are employed in the synthesis of COFs.

C4-sym

Square Topology N

N

OHC

Hexagonal Topology

CHO

O B O

CN

NC

O B O

O B O

OHC

O OB

CHO CN

CN

NC

N N

CN

NC

C2-sym

OB O

BO O

O OB

O BO

N N NC

HO B HO HO

CN NC

N

OH B OH

C2-sym

OH BO O

N

HO HO

O B O

OH OH

OB O

C3-sym

O B O

FIG. 2 Reticular synthesis and building block derived topological design of COFs.

introduces cross-conjugation into COFs which limits the p-delocalization, while tetra-fold or hexafold symmetry nodes support direct conjugation. COFs consisting of the later type of nodes possess unhindered p-delocalization pathways but such building blocks are scarce [24]. Another key feature that COFs possess is p-electronic interactions in an outof-plane direction. As in the case of graphite, the 2D-COF sheets stack together (Fig. 1B) [26] to result in highly ordered stacked material in which the individual 2D sheets couple electronically to develop out-of-plane p-delocalization. Thus, the electronic properties of COFs are governed by in-plane and out-of-plane p-interactions depending on the nature of the linkages presented between the building blocks. It has been observed that the ordered columnar p interactions between COF-2D sheets result in stronger electronic communication compared to the cross-conjugated in-plane interactions. As a result, more efficient charge transport (carrier mobility) has been observed in the out-of-plane direction. Besides, p-p

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stacking interactions of 2D-COF sheets also develop a conduction path that will promote charge carrier transportation and exciton migration across the framework. By an appropriate selection of building blocks, semiconducting COFs (strong visible light absorption, high electrical conductivity, high mobility, etc.) [24, 27, 28, 29], that are suitable for photovoltaic and solar cell applications can be realized. On the other hand, luminescent COFs are advantageous for sensory applications [30]. So far, a plethora of strategies via various combinations of building blocks, functional groups, and linkers, etc. have been utilized for the development of a wide variety of p-conjugated COFs [22,23,24,25]. This will allow fine-tuning of electronic and optical properties of the polymers to the finest degree as desirable for a specific application. In particular, the strong propensity of COFs toward exhibiting narrow band gaps and strong visible to near infra-red (NIR) absorption along with long-range crystallinity and high surface area, makes these materials suitable for photocatalysis [15,16,17,18].

1.3 Principles of photocatalytic water splitting reaction As the conventional energy resources of the world such as coal and petroleum products are rapidly depleting, there is a need for alternative fuels for energy production. Further, to mitigate global warming, alternative energy sources are required to be clean, renewable, and affordable. Among the various renewable energy sources available so far, hydrogen (H2) has been identified as one of the most efficient energy vectors owing to its clean energy-carrying ability and high calorific value. Thus, it is envisaged that hydrogen as a fuel has the potential to provide a durable solution for the energy crisis and related environmental issues, hence, it kindled a special surge of interest for developing sustainable methods for hydrogen production [31, 32]. So far, most of the industrial hydrogen production is dependent on fossil materials such as steam methane reforming and petroleum products [33]. It involves a steam treatment of methane over Ni-based catalysts under high temperature (>700°C) and high pressure (>15 bar). This process suffers from the limitation of producing carbon dioxide as a by-product. On the other hand, electrolysis of water (famously called water splitting) has been discovered to be a promising alternative for the generation of hydrogen, owing to its zero greenhouse gas emissions. In this technique, the water will be split into hydrogen and oxygen using electricity. However, the pathways employed for the generation of electricity and resultant emissions must be examined to classify it as a clean energy process. Recently, hydrogen production from photocatalytic water splitting using sunlight gained prominence due to the availability of solar energy and water in abundance. In general, the water-splitting reaction (WSR) involves two basic reactions; hydrogen evolution reactions (HER) and oxygen evolution reaction (OER) [34]. 2H + + 2e ! H2 E0 ¼ 0 V ðHERÞ 2H2 O ! O2 + 4H + + 4e E0 ¼ + 1:23 V ðOERÞ H2 O ! H2 + 1=2O2 DE0 ¼ 1:23 V and DG0 ¼ 237:2 kJ=mol ðoverall reactionÞ From the above equations, it can be understood that the WSR is an energy raising process with a net Gibbs free energy of 237.2 kJ/mol and also involves a high activation barrier. It necessitates the need for a catalyst to promote the water-splitting reaction. As a result, various catalysts promoted solar energy-based techniques such as Photothermochemical- [35], Photobiological- [36], Photoelectrochemical- (PEC), Photocatalytic water splitting methods have been developed for hydrogen production. In this chapter, we restrict the discussion to only PEC and photocatalytic water splitting as the COF materials exhibit water splitting based on these two techniques. Photoelectrochemical (PEC) water splitting [37]: In this process, a photoelectrochemical (semiconducting) material utilizes sunlight to produce hydrogen from water. The PEC method was the first concept to be used to demonstrate water splitting in 1972 by Fujishima and Honda [38]. Similar to a typical electrochemical set-up, the PEC cell is also comprised of an anode (photoanode) and a cathode (photocathode) that is immersed in an electrolyte solution, and the other end is connected via an external wire to complete the circuit. Oxygen evolution takes place at photoanode while hydrogen evolution is produced at photocathode. Several 2D nanostructures were studied for electrochemical hydrogen generation [7, 39]. However, the main drawback of the PEC is its poor stability and low efficiency. PEC water splitting process (Fig. 3), typically involves several steps [40]; (1) the generation of photo-excited electron–hole (e-h+) pairs via absorption of solar energy by photocatalyst having suitable band-gap; (2) separation of photo-excited e-h+ pairs followed by oxidation of water by photoexcited holes at the photoanode electrode to produce oxygen and H+; (3) the transfer of photo-excited electrons to cathode and reduction of protons to produce hydrogen gas.

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FIG. 3 Diagrammatic illustration of photocatalytic hydrogen generation (left) and PEC (right) from water splitting.

Photocatalytic water splitting [40]: In this process, water is dissociated to hydrogen and oxygen using a semiconducting material (photocatalyst) which absorbs the sunlight as in the PEC water splitting process. However, the photocatalyst is suspended in water under light irradiation for hydrogen production. The photocatalytic water splitting process also follows a similar mechanistic path as the PEC cell such as light absorption, charge separation, charge migration, and redox (water oxidation and water reduction) reactions (Fig. 3). However, unlike the PEC where separation followed by migration of photo-excited e-h+ pairs to cathode and anode is observed, the generated charges populate at the semiconductor/ electrolyte interface and participate in WSR to generate hydrogen. However, it is important to note that not all the photo-generated charge carriers will contribute to H2 generation, a portion of them will undergo recombination to dissipate the energy into thermal energy at the photocatalyst surface. Another fundamental requirement [40] for the photocatalyst is to have aligned electronic levels with water oxidation and water reduction process. That means, the valence band (VB) edge of the photocatalyst should be more positive than the oxidation potential of H2O to O2 (1.23 V vs NHE) and the conduction band (CB) of the photocatalyst must be more negative than the reduction potential of H+ to H2 (0.00 V vs NHE). Thus, the photocatalyst must have a bandgap over 1.23 eV; however, taking into account the thermal losses, the materials possessing the band gap between 1.5 and 2.5 eV are desirable. Besides, the ideal photocatalyst should possess the qualities of optimal band structure for an efficient light harvest, minimal recombination of photo-generated electron-holes, and resistance to photo corrosion. In a photocatalytic water splitting reaction, a photocatalyst, co-catalyst, and sacrificial donor are also used to enhance the efficiency of the reaction. The co-catalysts play a key role in providing more active sites, reducing the potentials of hydrogen reduction, and also promoting the photoexcited e-h+ pairs dissociation on the photocatalytic surface. Transition metals such as Pt, Ni, Ru, Ag, and Au have been widely used as co-catalysts [41]. On the other hand, the sacrificial agents are critical in forbidding the charge recombination via capturing the photoexcited electrons and holes. Typically, methanol, sodium ascorbate, triethanolamine (TEOA) and lactic acid, etc. are employed as sacrificial agents of holes in HER reactions, and silver nitrate (AgNO3) and sodium iodate (NaI) are employed as sacrificial agents of electrons in OER reactions.

2 COF based photocatalytic reactions 2.1 Hydrogen evolution reaction (HER) In this section, we will present the details of various COF photocatalysts developed for hydrogen production through a water-splitting reaction. As discussed in the earlier section the novelty of the COF photocatalyst lies in its long-range ordered molecular arrangement. Such highly defined molecular structures of crystalline materials promote the transport of photogenerated charge carriers and also play a critical role in the efficient separation of photoexcited e-h+ pairs which are key in the efficient water oxidation/reduction reactions. To establish the fact, Cooper et al [42] investigated the photocatalytic behavior of structurally relevant but geometrically (crystalline and amorphous) different benzo-bis (benzothiophene sulfone) COFs (CP1 crystalline and amorphous; Fig. 4). Extended p-delocalization supported by the ketoenamine linker leads the COFs to exhibit narrow bandgap properties (optical band gap of 1.85 eV) suitable for visible light photocatalysis; however, both crystalline and amorphous COFs showcase identical electronic properties owing to their identical structural features. On the other hand, the photocatalytic hydrogen evolution reactions (HER) with these COFs using ascorbic acid as a sacrificial electron donor and Pt as a co-catalyst exhibited phenomenal variability. The hydrogen evolution for crystalline COF (CP1) (10.1 mmol g1 h1) is 10-fold higher than that of the amorphous analog (1.1 mmol g1 h1), indicates the importance of the long-range order in the material. In another study [43], the crystalline

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FIG. 4 Photocatalytic COFs for HER (CP1–CP8).

sp 2-carbon linked triazine-based COFs (CP2; Fig. 4) also proved to exhibit superior photocatalytic activity over its amorphous congener. The average hydrogen evolution with photocatalytic crysitalline COF was achieved as 14.6 mmol h1 which is almost two times higher than that of the amorphous material (9.5 mmol h1). Interestingly, the photocatalytic activity of the crystalline COF is also highly influenced by the microsize morphology. The H2 production rate of crystalline CP2 has been dropped from 14.6 to 7.5 mmol h1 when the morphology is modified from fibrillar morphology to changed cracked rods and particles. Morphology of the material plays a critical role in the light-harvesting ability and the dispersity of the material (Fig. 5). Besides the crystallinity, COF photocatalysts must have an appropriate electronic band structure to match up with the redox potentials of the water to achieve enhanced hydrogen evolution from photocatalytic water splitting. To achieve this, fine-tuning of the electronic properties of COFs via structural modifications has been employed as one of the successful formulae. In this direction, Lotsch and co-workers [44] have studied the effect of structural tweaking on the HER behavior of four COF analogs (CP3–6; Fig. 4). Based on the parent COF (CP3) consisting of skeletal 1,3,5-triphenylbenzene, the structural modification has been carried out by replacing –CH groups with nitrogen atoms in the central benzene moiety. As the number of nitrogen atoms per benzene ring increases from zero to three, the dihedral angle reduces from 38.7o to 0o which amplifies enhanced p-delocalization within the COF framework which in turn directly influences the electronic properties of the COFs. Moreover, the nitrogen-rich compounds also facilitate the fast transportation of photo-induced

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FIG. 5 (A) Schematic representation of increased planarity via incorporating nitrogen atoms in the central phenyl ring; (B) Comparison of photocatalytic HER activity of COF photocatalysts (CP3-CP6). CP3-CP6, adapted with permission from V. S. Vyas, F. Haase, L. Stegbauer, G. Savasci, F. Podjaski, C. Ochsenfeld, B. V. Lotsch, A tunable azine covalent organic framework platform for visible light-induced hydrogen generation. Nat. Commun. 6 (2015) 8508. Copyright 2015 Springer Nature.

charges and thus reduces the e-h+ recombination leading to enhanced photocatalytic activity. The HER using these COF photocatalysts in the presence of sodium ascorbate and Pt showed a four-fold increase in the hydrogen production upon replacing each CH with N in the central benzene ring, i.e., CP3 to CP6 generate 23, 90, 438, 1703 mmol h1 g1 of hydrogen respectively (Fig. 3C). Interestingly, the photocatalytic activity of CP6 surpassed the performance of benchmark carbon nitride-based photo-catalysts [45]. Similarly, to understand the effect of COF functionalization on HER, ketoenamine COF (CP7; Fig. 4) was substituted with different functional groups having different electronic effects ((CH3)2, dCH3, dH and -NO2) have been studied [46] for the photocatalysis. Interestingly, the COF with –(CH3)2 (CP7(CH3)2) substituent displayed a superior photocatalytic activity with hydrogen evolution of 8.33 mmol h1 g1 (for 30 h) compared to other COFs (3.07 mmol h1 g1 for CP7-CH3, 1.56 mmol h1 g1 for CP7-H, and 0.22 mmol h1 g1 for CP7-NO2). The study revealed that the electron-donating substituents influence the photocatalytic activity of the COFs positively via improving the charge separation efficiency. A hydrazone-based photo functional p-conjugated COF [47] was also developed for HER reaction (CP8; Fig. 4). Akin to CP6, the central planar triphenylazine unit in the framework facilitates the formation of planar 2D sheets and narrows the optical band gap to 2.8 eV, making it suitable for water splitting reaction. Under visible light irradiation, the HER reaction with CP8 photocatalyst in the presence of the sacrificial donor (10 vol% aqueous triethanolamine) showed stable and continuous hydrogen gas evolution of 1970 mmol h1 g1 which is interesting, much superior to Pt-modified amorphous melon, g-C3N4 [48] and crystalline poly (triazine imide) [46]. A strong visible light absorbing diacetylene [49] (CP9, optical bandgap 2.31 eV) and acetylene (CP10, optical bandgap 2.34 eV) linked ketoenamine COFs have been used as photocatalysts for HER (Fig. 6). The diacetylene moiety exhibited a profound influence on photocatalytic activity with hydrogen gas evolution of 324 mmol h1 g1 compared to the acetylene unit (30 mmol h1 g1). The better performance of CP9 is associated with the high charge carrier mobility supported by enhanced p-delocalization within the framework. It will lead to rapid migration of photo-excited e-h+ pair to photocatalyst surface and reduces charge recombination and thus enhances photocatalytic activity. Interestingly, Li et al [50] have developed a COF (CP11) that will catalyze the seawater splitting to produce hydrogen (Fig. 7A). It is important to note that, unlike pure water, the major challenge in the seawater splitting reactions will be overcoming the undesirable competitive adsorption of alkaline/alkaline-earth metal ions over co-catalyst. Thus, the COF (CP11) is judiciously functionalized with thioether groups to specifically bind to Au co-catalyst and suppress the side reactions. The visible-light photocatalytic HER of seawater with the COF using Au and TEOA produced 581 mmol g1 for 4 h. Though the COF exhibited an appreciable H2 evolution from seawater splitting which is, however, relatively low compared to that of pure water (1720 mmol g1 for 4 h). The control tests followed by the analysis indicated that Mg2+ ions present in the seawater chelates to the nitrogen atoms of the COF and affect the electronic structure of the photocatalyst and decrease the catalytic activity. Owing to the outstanding activity of metallic Pt and the ability to reduce the over-potential of H2 reduction reaction, most photocatalytic reactions involve Pt metal as the co-catalyst. However, it is a noble metal and extremely rare and expensive; thus, it is highly desirable to be replaced with non-precious metal-based co-catalysts. In this direction, Lotsch et al [51] have

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FIG. 6 Structures of photocatalytic COFs for HER CP9 and CP10. Adapted with permission from P. Pachfule, A. Acharjya, J. Roeser, T. Langenhahn, M. Schwarze, R. Schomacker, € A. Thomas, J. Schmidt, Diacetylene functionalized covalent organic framework (COF) for photocatalytic hydrogen generation. J. Am. Chem. Soc. 140 (2018) 1423–1427. Copyright 2018 American Chemical Society.

demonstrated an azine-linked COF (CP12) for photocatalytic hydrogen evolution by replacing Pt with the molecular catalyst [chloro(pyridine)cobaloxime] (Fig. 7B). The COF photocatalyst using cobaloxime as co-catalyst and sacrificial donor of TEOA achieved an H2 evolution rate of 782 mmol h1 g1 with a turnover number of 54.4 at 20 h. Interestingly, the performance of this modified photocatalytic system is on par with Pt co-catalyzed COF systems [45] and also Pt-modified amorphous melon (720 mmol h1 g1) [46] and g-C3N4 (840 mmol h1 g1) [49]. Later the group [52] has achieved even better performance when thiazolo[5,4-d] thiazole-linked COF photocatalyst (CP13) and nickel-thiolate hexameric cluster (NiME) co-catalyst has been employed for the photocatalysis. These materials under standard conditions enabled the production of hydrogen at a maximum rate of 941 mmol h1 g1 with turnover number TONNi > 103 (Fig. 7C). Generally, the COF photocatalysts possess the band edges that are suitable either for hydrogen reduction or oxygen reduction which is only half-reaction in the photocatalytic water splitting reaction. Very recently, to address this issue and to realize a single material photocatalyst for overall water splitting reaction, Wan et al [53] came up with a strategy of assembling various linkages and building blocks that are catalytically active for HER and OER respectively. Generally, nitrogen atoms provide catalytic active sites for HER while aromatic benzene-like structures serve as active sites for OER. Following this strategy, four aromatic building blocks have been introduced into the COF framework (CP14) via imine, azine, and azo linkages (Fig. 8A). The analysis of the bandgaps of the resultant COFs revealed that nine out of twelve COFs possess a suitable band structure that matches well with the chemical reduction potential of HER and OER reactions. In particular, the COFs that possess triazine in the framework are perfect candidates for the overall water splitting reaction (Fig. 8B). A COF is also tested as a photoelectrode in a PEC water splitting [54]. The photoelectrochemical performance of thin films (100 nm thickness) of imine p-conjugated 2D COF (CP15) (linear sweep voltammetry in aq. 0.1 M Na2SO4 under AM 1.5 illuminations through the substrate in the potential range between 1.1 and 0.2 V vs RHE) show current up to 1.5 mA cm2 which increases to 4.3 mA cm2 for 500 nm thick films. The COF is highly stable and indicated no sign of decomposition in the long-term photo-enduring studies.

2.2 Organic transformations In organic chemistry, the synthetic methods which are mild, efficient, and convenient for the transformation of CdH into CdC bonds have gained a great impetus owing to their key importance [13]. In this line, the utilization of light to drive organic reactions offers sustainable and environmentally friendly routes for the synthesis of fine chemicals which are otherwise having complex synthetic procedures. A wide variety of light-absorbing materials such as inorganic semiconducting

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FIG. 7 (A) Photocatalytic H2 evolution experiments from seawater using CP11; Schematic representation of photocatalytic HER with CP12 (B) and CP13 (C) in Co and Ni-based molecular co-catalysts, respectively.

materials, transition metal complexes, metal oxides, organic dyes, mesoporous silica, g-C3N4, MOFs, etc. [55, 56], have been studied and proved to be highly efficient for the photocatalytic transformations. However, some of these catalysts suffer from the limitations of low-recyclability, high price, and tedious separation while others are immune to structural modifications. The key features of 2D-COFs such as high crystallinity, excellent porosity, and hydrolytic- and chemical stability is in sync with the prerequisites of a photocatalyst for the use in organic transformations. Liu et al group [57] developed a triazine incorporated 2D COF (CP16; Fig. 9A) for photocatalytic crossdehydrogenative coupling (CDC) reactions. The triazine unit at the node plays a critical role both in imparting the planarity for the 2D sheet leading to enhanced intrasheet p-delocalization and enabling donor-acceptor interactions with the electronrich dimethoxy aryl moiety. These interactions, in turn, enhance the crystallinity of CP16 and facilitates strong visible absorption (lmax ¼ 470 nm). The photoactive CP16 successfully photocatalyzes the oxygen-mediated aerobic CDC reaction of N-phenyltetrahydroisoquinolines (THIQ) with various nucleophiles (Fig. 9A). Interestingly, it exhibited a broad substrate adaptability including the synthesis of a-amino dialkyl phosphonates, substituted dialkyl malonates, substituted nitromethane derivatives, b-amino-ketones in good yields. The photocatalytic CDC reactions have also been performed with a visible light absorbing hydrazone COF [58] (450 nm) (CP17) (Fig. 9B). The CDC reactions between tetrahydroisoquinolines and various nucleophiles (nitromethane, acetone, and phenylethyl ketone) performed using CP17

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FIG. 8 (A) Photocatalytic COFs (CP14) for HER and OER and (B) their calculated energy levels. Horizontal dashed lines present the redox potentials of the water. Adapted with permission from ref. Y. Wan, L. Wang, H. Xu, X. Wu, J. Yang, A simple molecular design strategy for two-dimensional covalent organic framework capable of visible-light-driven water splitting, J. Am. Chem. Soc. 142 (2020) 4508–4516, Copyright 2016 American Chemical Society.

photocatalyst (visible light, O2) produced corresponding substituted nitromethane derivatives in quantitative yields (b-amino ketone, and b-amino aryl ketone). Furthermore, COFs are highly stable against the reaction conditions and retain their photocatalytic activity even after a few cycles. In these catalytic reactions, singlet oxygen plays a critical role. Upon visible light illumination, as a first step, COF undergoes photoexcitation and oxidizes the substrate, and turns to anion radical (COF–). The reduced COF transfer an electron to oxygen to produce O– 2 and returns to its neutral state. Later, the oxidized substrate interacts with O– 2 and undergoes a series of intermediate steps (radical state and iminium ion) before participating in the nucleophilic addition with nucleophiles to deliver the products [58] (Fig. 9). Photocatalytic sulfoxidation of thioethers has been carried out with an imine-based COF (CP18; Fig. 9D) [59]. The COF was crystalline and stable and visible absorbing (450 nm) with a bandgap of 2.5 eV. As noticed for the water-splitting reaction of CP1 and CP2, the oxidation reaction is facile with the crystalline COF material compared to its amorphous congener. It reinforces the importance of the long-range order of the COF. Moreover, CP18 is also tested for the oxidative hydroxylation of phenylboronic acid into phenol in moderate yields (50%). However, the same oxidative hydroxylation reaction in the presence of benzoxazole COF (CP19) [60] delivers an excellent yield of up to 99% (Fig. 9E). Also, a broad substrate scope has been realized including electron-donating and electron-withdrawing substituted aryl boronic acids. Wong et al [61] have used a fully p-conjugated COF for aerobic oxidation of amines into imines (CP20; Fig. 10A). The COF has been built with C]C linkages which render extraordinary stability even under harsh conditions such as 9 M HCl and 9 M NaOH. Also, it will also enable more effective p-delocalization leading to enhanced electron delocalization. The optical band gap for CP20 was estimated to be as low as 1.75 eV. Interestingly, benzothiadiazole COF [62] (CP21; Fig. 10B) was also utilized for the photoreduction of Cr (IV) species (K2Cr2O7). Under visible light irradiation, the COF reduced Cr (IV) to Cr (III) with 99% efficiency without a need for a sacrificial agent or pH adjustments. The

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FIG. 9 Photocatalytic COFs (CP16-CP19) for organic transformations. The inset presents a plausible mechanism of the CDC reaction by CP16. CP16 adapted with permission from Y. Zhi, Z. Li, X. Feng, H. Xia, Y. Zhang, S. Zhan, Y. Mu, X. Liu, Covalent organic frameworks as metal-free heterogeneous photocatalysts for organic tranformations. J. Mater. Chem. A 5 (2017) 22933–22,938. Copyright 2017 Royal Society of Chemistry.

FIG. 10 Photocatalytic COFs (CP20-CP21) for organic transformations.

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outstanding performance of CP21 was attributed to the more negative conduction band of COF (0.30 V) compared to the redox reaction potential of Cr (VI) to Cr (III) (1.33 V vs NHE). Furthermore, the benzothiadiazole unit is found to improve the photocatalytic activity of the COF via facilitating the photo-generated charge carrier separation and migration.

2.3 Reduction of CO2 to CO The current energy production via the combustion of conventional fossil fuels produces large amounts of CO2 which is a greenhouse gas and contributes heavily to global warming. The conversion of CO2 into valuable fuels such as carbon monoxide (CO), methane (CH4), and methanol (CH3OH), etc. using solar energy will solve not only environmental issues but also the energy crisis [63]. Thus, several molecular- (homogenous) and inorganic semiconductors- (TiO2, ZrO2, In2O3, CdS, ZnGa2O4, ZnGe2O4) based photocatalysts have been studied for CO2 reduction [64, 65, 66]. Unfortunately, these compounds suffer limitations of poorly visible light-harvesting, poor CO2 adsorption, low selectivity, unsuitably aligned conduction/valence band positions, and rapid charge recombination. On the other hand, MOFs exhibit outstanding photocatalytic reduction of CO2 as the functionalized channels can effectively absorb CO2 while the open metal sites in parallel activate CO2 [67]. However, the poor hydrolytic stability coupled with inferior electronic conductivity makes the MOFs unsuitable for this application. By sharp contrast, chemical and hydrolytic stability of COFs coupled with a large surface area and tunable pore sizes serve as perfect candidates for the photoreduction reaction of CO2. A specific approach of coupling a molecular catalyst within the p-conjugated COF framework has been employed to achieve both efficient charge separation and to introduce active sites for CO2 reduction. Following this strategy, Yang et al [68] have integrated a photoactive triazine 2D COF (CP22; Fig. 11) with a Re-complex [Re(bpy)CO3Cl] by utilizing the coordination sites of bipyridyl unit. The material (CP22) was tested for the selective photoreduction of CO2 to CO in water. Re-CP22 photocatalyst with Xe lamp as a light source (cut-off wavelength ¼ 420 nm) using TEOA sacrificial donor produced 98% of CO (15,000 mmol g1 in 20 h reaction) with a 98% selectivity over H2 evolution. In general, the photoreduction of CO2 in an aqueous solution involves multi-electron reduction, hence promotes the production of H2, CH4, and HCOOH along with the targeted product of CO. Interestingly, a better performance was witnessed for CP22 (TON-48) compared to its corresponding homogeneous photocatalyst [Re(bpy)(CO)3Cl] (TON-22), suggests that COF pores provide a special environment toward the stability of a catalyst that leads to improved activity. In a typical mechanism (Fig. 11), photoexcited CP22 undergoes intramolecular charge transfer (ICT) from COF unit to Re-complex unit that will be promptly reduced by

FIG. 11 Photocatalytic COF CP22 and a plausible mechanistic path of CO2 reduction to CO by CP22. Adapted with permission from ref. S. Yang, W. Hu, X. Zhang, P. He, B. Pattengale, C. Liu, M. Cendejas, I. Hermans, X. Zhang, J. Zhang, J. Huang, 2D covalent organic frameworks as intrinsic photocatalysts for visible light-driven CO2 reduction, J. Am. Chem. Soc. 140 (2018) 14614–14,618. Copyright 2018 American Chemical Society.

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TEOA leading to a charge separation state of TEOA+-(COF-Re). It will then capture the CO2 to form COF-CO2 adducts by dissociation of Cl ions from the Re-complex {TEOA+-(COF-Re [CO2]) or/and TEOA+-(COF-Re [CO2H])} which will eventually decompose to deliver CO and the original Re-COF (Re-CP22). Similarly, ketoenamine COF loaded with Ni [69] (CP23) also exhibited a superior photoreduction of CO2 to CO. The system generated 4057 mmol g1 of CO in a 5 h reaction with a 96% selectivity over H2 evolution. The special microenvironment coupled with the keto nodes present in the COF pores stabilize the Ni catalyst and also facilitate the activation and conversion of CO2 reduction (Fig. 12). Similar studies are also conducted on Co, Ni, and Zn loaded anthraquinone integrated ketoenamine COF [70] (CP24; Fig. 12). Among the three, Co-COF (Co-CP24) exhibited high photocatalytic CO2 reduction to CO (1020 mmol h1 g1) while Zn-COF (Zn-CP24) produced a major product of HCOOH (152.5 mmol h1 g1) with 90% selectivity over CO evolution. It has been observed that the quinone oxygen of the anthraquinone unit in COF plays a critical role in stabilizing the metal centers which results in better performance in the photoreduction of CO2 because a model COF functionalized with anthracene units showed a poor performance. The COF photocatalysts discussed so far have been fabricated via highly polarizable linkers (imine, and b-ketoenamine) which limits p-delocalization in the 2D framework. This will further lead to blue-shifted absorption and less effective visible light absorption. Aiming at the increased conjugation length in the COF framework, Cooper and co-workers [71] have developed a COF (CP25; Fig. 13) with olefin linkers via Knoevenagel condensation that possesses strong visible light absorption. The COF (CP25) was further modified with [Re (CO)5Cl] by utilizing the coordinating sites of bipyridyl moieties (Re-CP25). Re-CP25 under light irradiation for 17.5 h produced CO at a rate of 1040 mmol g1 h1 with 81% selectively over H2, (TON-18.7) [under 1 atm CO2 in a mixture of MeCN and triethanolamine (TEOA)]. As observed in the earlier examples, the molecular Re-catalyst is unstable under these reaction conditions and decomposed within 3 h (TON-10.3). Besides the strategy of metal center impregnation into photoactive COFs, metal-free COFs have also been developed for the photocatalytic reduction of CO2. However, to perform the reaction, the optical band gap of photoactive COFs must be more negative than the redox potential of CO2 to reduce to CO (CO2/CO, 0.53 V vs NHE) and the valence band maximum must be more positive to oxidize water (O2/H2O, 0.82 V vs NHE). In this line, carbazole-triazine-based donor-acceptor (D-A) COF [72] (CP26; Fig. 13) having suitable energy band structure and strong visible light absorbing capability has been developed for metal-free photocatalytic reduction of CO2 to CO in water. CP26 uses the nitrogen sites in the triazine moiety to effectively adsorb the CO2 via dipole and quadrupole interactions while water molecules through the hydrogen interactions. Further, strong D-A interactions in the COF facilitate narrow bands and effective exciton dissociation and enhanced charge transport which promotes the reduction without co-catalyst. The COF as a photocatalyst produced CO at a rate of 102.7 mmol g1 h1 with 98% selectively and also the stoichiometric amount of

FIG. 12 A diagrammatic representation of the photoreduction of CO2 to CO by CP23 (left). Structure of COF photocatalyst (CP24) (right). CP23, adapted with permission from W. Zhong, R. Sa, L. Li, Y. He, L. Li, J. Bi, Z. Zhuang, Y. Yu, Z. Zou, A covalent organic framework bearing single Ni sites as a synergistic photocatalyst for selective photoreduction of CO2 to CO, J. Am. Chem. Soc. 141 (2019) 7615–7621. Copyright 2019.

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FIG. 13 Photocatalytic COFs (CP25 and CP26) for the reduction of CO2 to CO.

O2 (51.3 mmol g1 h1). The crystallinity of the COF was also found to be vital for exhibiting efficient CO2 reduction. As the amorphous material (CP) shows a drastic drop in the performance by lowering CO evolution to 43.4 mmol g1 h1. This could be attributed to the reduced recombination of photogenerated carriers and also minimized charge trapping defects offered by the crystalline COF.

3

Conclusions

COFs are an emerging class of crystalline porous organic polymers derived purely from covalent bonds. By their long-range order (crystallinity), permanent porosity, high surface area, COFs find applications in the areas of gas storage, optoelectronics and energy applications, etc. In particular, 2D-COFs, owing to their tunable bandgaps, extended p-delocalization, and strong visible to near-infrared (NIR) light absorption, have empowered them as photoactive materials for photocatalysis. Considerable progress has been made in the photocatalytic COF design and their applications for diverse reactions (photocatalytic hydrogen evolution, photocatalytic organic transformations, and photoreduction of CO2, etc.). In this chapter, we have presented a comprehensive summary of the recent developments of photocatalytic COFs. However, despite significant developments, the field is still in its infancy and the COF materials suffer from a few limitations such as (a) lack of long-term stability under harsh conditions; (b) lack of consistency in obtaining high degrees of crystallinity; (c) lack of efficient in-plane p-delocalization due to cross-conjugation and polarizable linkers, etc., which needs to be addressed to realize their full potential. Overall, the studies on COFs based photocatalysts have manifested that these materials possess great potential to provide the solution for the ever-increasing world energy demand and environmental issues by efficiently using solar energy.

Acknowledgments M.R.R. and A.R.J. thank the Science and Engineering Research Board (SERB), for the financial support through the Early Career Research Award (ECR/2018/002285).

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Chapter 31

Photocatalytic membranes and membrane reactors for CO2 valorization Adele Brunettia and Giuseppe Barbierib a

Institute on Membrane Technology (ITM-CNR), National Research Council c/o The University of Calabria, Rende CS, Italy, b Department of

Environmental and Chemical Engineering, The University of Calabria, Rende CS, Italy

1 Introduction With 36 Gton of CO2 emitted per year from burning of fossil fuels like coal, oil, and natural gas, as well as cement production, deforestation, and more, greenhouse gas emissions are the main contributors to climate change. Even though important progress has been made in the development of efficient capture processes that assure a high level of purity, the main issue of CO2 final destination remains. If new types of storage are the most likely option, the identification of new sustainable methodologies for reusing CO2 producing new energy sources is a key challenge. As such, various technologies have been developed to convert CO2 into valuable biofuels or chemical compounds. Converting CO2 into valuable products requires hydrogen donors as co-reactants. The direct oxidative reactions lead to the production of short-chain olefins (C2H2, C3H6), oxygenated products (CH3OH, CH2O, HCOOH), and hydrocarbons. The indirect oxidative route yields syngas, which can be further converted into other fuels or chemicals. Methanol is recognized as the most appropriate candidate for essential solar fuel for the anthropogenic cycle of carbon [1]. Photocatalytic CO2 reduction with water to fuels represents a greener process and an attractive route from economic and environmental points of view [2]. CO2 can be converted by irradiating it with UV light at room temperature and ambient pressure and thus solar energy can be directly transformed and stored as chemical energy. Choosing a good photocatalyst is fundamental to obtain a successful rate of the processes. One should also consider that the photocatalytic conversion of CO2 is a surface reaction involving two important stages: (1) CO2 adsorption to the catalyst surface and (2) CO2 decomposition under UV irradiation in the presence of reductants. Therefore, the mass transfer rate of CO2 and the catalyst surface area are two other important parameters to control to improve photocatalytic efficiency. Consequently, catalyst configuration during the photochemical reaction is of great importance for increasing product yield. One option is to use a membrane reactor for this purpose. In membrane reactors, the membranes are coupled to a chemical reaction carried out in the same unit. The membranes have the sole function of separating one of the end products, shifting the reaction toward further conversion or toward a catalytic function. In most applications, the result is an improvement in productivity. Depending on the type of reaction, the membrane can be polymeric or inorganic. Polymer membranes have a quite limited range of applications mainly consistent in some area of homogeneous catalysis or in bioreactors. In the photocatalytic conversion of CO2, membrane reactor use offers a series of advantages such as better exposition of catalyst to UV light to carry out the reaction, tailoring of reactants and catalyst contact, reduction of catalyst aggregates formation, easier recovery of the catalyst, which can be simply reused, and better control of fluid dynamics. Moreover, polymeric membranes are easily handled and cost less compared to other inorganic supports. This chapter describes the main applications of membrane reactors for photocatalytic conversion of CO2 along with current research trends, focusing on the impact that membrane technology can have on the photocatalysis. It also describes photocatalytic reduction of CO2 in a continuous membrane reactor where the catalyst is embedded in a Nafion membrane to illustrate the potentialities of membrane technology for this application.

2 Membrane reactors The functions of a membrane in a membrane reactor are as follows: l

Extractor or separator: for selectively removing products from the reaction mixture

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Distributor: for controlling the addition of reactants to the reactor Contactor: for optimizing the contact between reactants and catalyst or the contact between two phases

2.1 Membrane as separator or extractor In the most common class of membrane reactors, the membrane has the role of separator or “extractor.” Here, one or more of the products generated by the chemical reaction is continuously removed through the membrane and recovered in the permeate (Fig. 1A–B). The selective removal of reaction products from the reaction volume (e.g., “D” in Fig. 1A) allows the equilibrium conversion to be shifted toward further products production, enhancing the yield of the reaction and, in the meantime limiting the undesired side reactions involving the targeted reaction product.

2.2 Membrane as distributor The membrane used as distributor has the main function of controlling the addition of reactants to a reaction mixture through the membrane itself. The membrane can operate for distributing the limiting reactant along the reactor to prevent side or secondary reactions, or selectively dosing one component from a mixture to another that is retained in the other side of the membrane (Fig. 1C–D). These two functions can be combined.

2.3 Membrane as contactor The membrane contactor principle is based on the use of membranes that are not necessarily selective but are in some cases catalytic. The membrane divides the membrane reactor in two zones, in most cases containing reactants in different phases such as liquid–liquid or gas–liquid. The role of the membrane is to act as a support for providing a good contact area between these two phases (Fig. 1E–F). In the case of liquid–liquid reactions, the membrane material can be affine to one phase and not to the other to keep the two phases separated, allowing contact only on the membrane interface. For gas–liquid reactions, the membrane must be

FIG. 1 (A–B) Membrane operating as selective separator or extractor. (C–D) Membrane operating as selective distributor. (E–F) Membrane operating as contactor.

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non-wettable to ensure that the pores are free of liquid even at relatively high liquid pressure and, thus, the operation is stable with a high overall mass transfer coefficient (Fig. 3). A configuration of membrane contactor that has found large application is the “flow-through catalytic membrane reactor.” The function of the membrane is to provide a reaction volume with short and controlled residence time and high catalytic activity. The catalyst placed inside the membrane pores can be better exploited with respect to the conventional fixed bed where the channeling phenomenon occurs. This results in an intensive contact between reactants and catalyst, implying high catalytic activity.

3 Membrane Reactor configurations The different types of membrane reactor configurations are classified according to the relative placement of the two most important elements of this technology: the membrane and the catalyst. The main configurations are: l

l l

The catalyst is physically separated from the membrane and is either packed or finely dispersed on one side of the membrane itself The catalyst is dispersed in the membrane The membrane is inherently catalytic

The first configuration is often called an “inert” membrane reactor in opposition to the two other configurations, which are “catalytic” membrane reactors. The membrane can have a cylindrical or flat sheet shape. Cylindrical membranes are subdivided according to their dimensions: “tubular membranes” with a diameter greater than 10 mm, “hollow fibers” with a diameter of a few hundred microns, and “capillary membranes” with intermediate sizes (>1 mm). Tubular membranes are placed inside a pressureresistant tube. The capillary and hollow fiber membranes are assembled in a module with the free ends of the fibers potted with, for example, epoxy resins or silicone rubber. The flat sheet membranes are usually assembled in a spiral wound or plate-and-frame configuration, where sets of two membranes are placed in a sandwich-like fashion with their feed sides facing each other and separated by spacers. When such a plate-and-frame module is wrapped around a central collection pipe, a spiral wound module is obtained. In most cases, when the membrane acts as separator with or without catalytic function the membrane module has a tube-in-tube configuration with, in the former case, the catalytic bed placed in the annulus or in the core. The same configuration can be assumed if the membrane is catalytic. All these systems can be operated in continuous mode, with co-current or counter-current configuration. Obviously, in addition to the catalyst/membrane arrangement, the reactor module configuration depends also on the reaction phases and on the function of the membrane as well as on economic considerations, with the correct engineering parameters being employed to achieve this.

4 Photocatalytic membrane solutions Immobilization of the catalyst in or on membranes has recently found application in photocatalytic conversions. Membrane reactor use has several advantages such as better exposition of catalyst to UV light to carry out the reaction, tailoring of reactants and catalyst contact, reduction of catalyst aggregates formation, easier recovery of the catalyst, which can be simply reused, and better control of fluid dynamics. There are various possibilities of photocatalyst immobilization in membranes [3]: photocatalyst coated on the membrane, photocatalysis blended into the membrane matrix, and membrane manufactured with a pure photocatalyst. The coating of photocatalyst can be performed using different methods [4] such as dip-coating, electro-spraying particles, magnetron sputtering, and deposition of gas-phase nanoparticles. This approach can reduce membrane permeability to the leaching of the photocatalyst during experiments [5]. The technique of blending a photocatalyst into the membrane matrix foresees that the photocatalyst is entrapped in the polymeric matrix during its formation. This limits the leaching of the photocatalyst, but during the preparation catalyst particles can form aggregates or can segregate close to one membrane face. The use of a flocculant or the appropriate handling of preparation parameters can limit these aspects. A membrane can be manufactured with a pure photocatalyst so that the immobilization of the photocatalyst in/on the membrane support is unnecessary. A crucial step in designing a photocatalytic membrane is choosing the right material, which needs to exhibit good mechanical, thermal, and chemical stability under UV irradiations. When the photocatalyst is embedded in the membrane, photostability and transparency are additional aspects to be addressed. Moreover, the membrane transport properties, like permeability and selectivity, need to be considered and can be tailored by an appropriate selection of the membrane

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structure and composition. The incorporation of the photocatalyst in the membrane adds a certain level of complexity to the membrane preparation phase, owing to the necessity to firmly entrap the catalyst and finely distribute, preserving, in the meantime, the catalyst structure and activity. Polymeric, inorganic, and metallic materials are usually used for photocatalytic membrane preparation. Typical polymers include polyamide [6], polyvinylidene fluoride (PVDF), polyethersulfone (PES) [7, 8], polyurethane (PU) [9], polyethylene terephthalate (PET) [10], polyacrylonitrile (PAN) [11], and polytetrafluoroethylene (PTFE) [12]. Direct contact with the reaction environment exposes the membrane to possible damage induced by the contemporary presence of irradiation and oxidizing species. This is the reason why inorganic ceramic membranes found some application in photocatalysis, thanks to their excellent thermal, chemical, and mechanical stability. However, the high manufacturing costs are the main limitation to ceramic membrane utilization. The membrane preparation technique and conditions depend on the membrane material and the desired structure and morphology. The most used techniques for preparing catalytic membranes are phase inversion, coating, interfacial polymerization, sintering, stretching, and track-etching [13]. The phase inversion technique is currently the most used for polymeric membranes.

5

Photocatalytic membrane reactors for CO2 reduction

The use of photocatalytic membranes for CO2 hydrogenation is relatively recent and is based on the principle of using the membrane as a support for the photocatalyst, which can be deposited on one side or embedded in the membrane matrix. CO2 and water are fed into the membrane reactor, fully exposed to solar light and, depending on the reactor operating mode, they can be forced to pass through the membrane or can lap the membrane surface where the catalyst is deposited. The outlet stream will be mainly constituted by MeOH, EtOH, HCOOH, HCHO, acetone, and so on together with unconverted water and CO2 (Fig. 2). The produced species are biofuels, since the reactants are fully biological and largely available in nature, in addition to the fact that CO2 is an environmental issue. Usually, conventional photoreactors are limited by a small surface-area-to-volume ratio, a non-uniform distribution of light, and, as a consequence, poor light utilization efficiency [14, 15]. The immobilization of the catalyst into polymeric membrane supports has been demonstrated as a valid solution to reduce these problems, offering better catalyst exposition to UV light, the possibility of tuning the contact between reactants and catalyst, reduction of catalyst aggregates, and better control of fluid dynamics. There are various studies in the literature reporting on titanium dioxide (TiO2) deposited on or entrapped in photocatalytic membranes [16–22] for the purification of water or wastewater treatment, as well as degradation of phenol and other organics [23–25]. TiO2 is the most widely studied photocatalyst, thanks to its wide availability, cheap cost, nontoxicity, long stability, and other desirable properties [26]. It can be used in pure form or doped with different metals and non-metals, or it can be highly dispersed in other materials to enhance its own photoactivity. Perfluorinated membranes are the most suitable materials for supporting TiO2, owing to their superior chemical stability and transparency, which assures a good distribution of the light through the entire membrane thickness. Nafion is a perfluorinated material most used as a support for semiconductor particles and a stabilizing agent for semiconductor microcrystalline colloids [27]. It is constituted by a highly hydrophobic perfluorinated hydrocarbon backbone and many side chains with fixed sulfonic end groups capable of interacting with polar species. Studies on the use of membranes with embedded photocatalysts for CO2 conversion can be traced back to 1997, when Premkumar and Ramaraj [28] prepared metal porphyrin and phthalocyanine-adsorbed Nafion membranes for

FIG. 2 Photocatalytic CO2 hydrogenation. Scheme of photocatalytic CO2 hydrogenation in membrane reactor. © No permission Required.

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photocatalytic reduction of CO2 to formic acid. Pathak et al. [29] soaked TiO2 nanoparticles in the porous cavities of commercial Nafion membranes and found an improved CO2 conversion, which they attributed to the homogeneous dispersion of the photocatalyst in Nafion thin films. In another work [29], the same authors carried out other catalytic tests using TiO2-loaded Nafion membranes coated with silver metal via photolysis, obtaining mainly methanol. A big hurdle affecting the scaling up of this reaction is related to the low conversion and low catalytic efficiency of the TiO2 photocatalyst. This latter aspect is ascribable to the fast recombination of the electron–hole pairs photogenerated across the microstructure of TiO2 photocatalysts, as well as their low adsorption capacity. Consequently, there is a strong effort to develop alternative catalytic materials [30, 31] with improved properties for enhancing photocatalytic hydrogenation of CO2. Graphite carbon nitride (g-C3N4) is a photocatalyst that is attracting more and more attention. It has a 2D nanosheet structure consisting of tri-s-triazine units connected with tertiary amino groups, which owns regularly distributed triangular water-selective permeation nanopores throughout the entire laminar structure. [32, 33]. The presence of the spacers between the g-C3N4 nanosheets also creates nanochannels that allow water transport, retaining bigger molecules [34, 35]. g-C3N4 has applications in many fields, such as membrane separation [36, 37], photocatalysis [38–40], and electronic devices [41]. In addition, the presence of N basic sites promotes the adsorption of CO2. This photocatalyst is active under UV–Vis light with a band gap energy of about 2.7 eV and absorption of light at about 455 nm [42–44]. Because of its characteristics, various researchers have studied the photocatalytic adoption of graphitic C3N4 for CO2 reduction [45–53], [54] and CO2 capture [55], as it all ows creation of charge carriers for the reduction of CO2 pre-adsorbed/activated molecules by surface basic sites on nitrogen-rich carbon nitride frameworks. Recently, researchers have investigated the synthesis of composite hybrid materials for improving photocatalytic activity, exploiting the advantages of inorganic and organic photocatalysts [56]. g-C3N4/ TiO2 composites were used for various applications including O2 photoreduction [57–65]. For the first time Petit et al. [56] used TiO2/C3N4 nanosheet nanocomposites for the reduction of CO2 under UV–Vis irradiation using H2 as a sacrificial agent and demonstrated how composite materials increase CO2 adsorption capacity and can suppress electron–hole recombination by facilitating charge transfer, improving the photocatalytic reduction of CO2. The greater band gap of TiO2 (ca. 3.0 eV) [62] promotes a hard water oxidation semi-reaction (+1.23 eV vs SHE) even though TiO2 has a less negative conduction band (
0.3 eV) [62] and thus less tendency to reduce CO2. By a favored H2O reaction with photogenerated h+, the free H+ ions required for CO2 reduction semi-reaction are available. The coupling of these two catalysts promotes CO2 conversion, enhancing the composite capability in terms of H2O oxidation and CO2 reduction. Using photocatalytic membrane reactors for CO2 reduction is a new application that developed only over the last 5 years. In 2016, Sellaro et al. [66] proposed the first membrane reactor operated in continuous mode where TiO2 photocatalysts were embedded in a Nafion matrix. Successively, Pomilla et al. [67] entrapped a C3N4 photocatalyst in a Nafion matrix. A further advancement of the work was done by Brunetti et al. [68], who used a continuous photocatalytic reactor where the membrane was constituted by composite catalyst C3N4/TiO2 embedded in a Nafion matrix. We provide more details on the results obtained by these membrane reactors in the next section. Table 1 compares the performance of some photocatalytic membrane reactors with other results available for C3N4 and TiO2 in the literature. The photocatalytic membrane reactor with a C3N4 catalyst embedded in a Nafion membrane allowed to obtain an MeOH flow rate/catalyst weight significantly greater than that obtained with other reaction systems. In comparison with the membrane reactor [66] where a TiO2-based catalyst was embedded in a Nafion matrix, there was a wider product distribution, but a comparable MeOH production. The photocatalytic membrane reactors operated in continuous mode exhibited better performance than batch systems, with an MeOH production rate greater than most of the ones obtained by using only C3N4 as the catalyst. They also exhibited better or comparable performance when C3N4 was used as a co-catalyst in batch conditions. The absence of CH4 and CO in membrane reactors confirmed the advantage of the continuous operating mode, which allows the substrate to undergo a lower degree of reduction, as fresh CO2 is continuously fed into the system and the produced species are continuously removed from the catalytic sites, reducing the possibility of over-oxidation [90]. Another strategy for CO2 reduction in photocatalytic membrane reactors was introduced by Maina et al. [31] who integrated inorganic semiconductor nanoparticles across metal organic framework materials, realizing a new hybrid material that combines the high CO2 adsorption capacity of metal–organic frameworks (MOFs) and the ability of semiconductor nanoparticles to generate photoexcited electrons. With the encapsulation of TiO2 and Cu–TiO2 nanoparticles within zeolitic imidazolate framework (ZIF-8) membranes (Fig. 3), the authors obtained only MeOH and CO as reaction products with a photocatalytic performance of 7 mg of the semiconductor nanoparticle, which enhanced the pristine ZIF-8 membrane yield by 233% and 70% for CO and MeOH, respectively.

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TABLE 1 Comparison with results reported in literature. –1 Flow rate/catalyst weight, mmol g–1 catalyst h

Reactor configuration

MeOH

EtOH

HCHO

Acetone

HCOOH

CH4

C3N4/TiO2 catalyst embedded in a Nafion membrane

Continuous

1.7–44.7

0.1– 4.1

0-26.8

0.2–6

0

C3N4 catalyst embedded in a Nafion membrane

Continuous

0.9–17.9

0.3– 14.9

0–27

0–1.8

TiO2 catalyst embedded in a Nafion membrane

Continuous

12.6–45

0

0

0

TiO2 nanoparticles in porous cavities of commercial Nafion membranes

Continuous

56 (supercritical CO2 as feed)

Catalyst

C3N4

Batch

C3N4

Batch

C3N4 catalyst placed in glass fiber

0

[70]

0

0

0

[69]

Traces

0

0

[68]

[29]

[73] 0.5

Batch

C3N4

Batch

0.26

C3N4

Batch

10

C3N4

Continuous

Traces

3MEA-C3N4

Batch

0.28

AgBr/C3N4

Batch

C3N4 3%Cu/TiO2

Batch

[72]

1250

Continuous

C3N4

CdIn2S4/C3N4 20% wt.

0

38

25

Batch

C3N4/ZIF 8 (nanocluster on C3N4 nanotubes)

Ref.

5

C3N4 nanotubes

2Au-C3N4 catalyst placed in glass fiber

CO

5.2

0.28

[74] [75]

0.62–1

[76]

traces

[77]

12

[78] [79]

2 102

0.34

[78]

21

[73]

1118

Continuous

[47] 4.3

6.5

[48]

Batch

0.75

[49]

Continuous

42.7

[53]

6SO/0.12B0.2P-C3N4

Batch

Ag-N doped C3N4-G-50

Batch

20

Membrane microreactor

Continuous

111

TiO2/Y-zeolite anchored on Vycor glass

Batch

5

8

[81]

Ti-mesoporous zeolite

Batch

3.5

7.5

[82]

Ti-b(OH) and Ti-b(F)

Batch

5.9

1

[83]

6.25–7.5 50

[77] [52] [80]

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TABLE 1 Comparison with results reported in literature—cont’d –1 Flow rate/catalyst weight, mmol g–1 catalyst h

Reactor configuration

MeOH

Batch

0.78

[84]

Cu/TiO2 film supported on optical-fiber

Continuous

0.45

[85]

Cu/TiO2 and Ag/ TiO2 film on opticalfiber

Continuous

4.12

[86]

Batch

0.075

Catalyst TiO2 and 2% Cu/ TiO2 in NaOH solution

TiO2 anatase

EtOH

HCHO

Acetone

HCOOH

CH4

CO

Ref.

0.40

[87]

2.13

[88]

TiO2 polymorphs

Continuous

3.15

TiO2 polymorphs

Continuous

2.1

[89]

Batch

26

[90]

phtalocyanines/ TiO2

Reprinted with permission from F.R. Pomilla, A. Brunetti, G. Marcı`, E.I. Garcia-Lopez, E. Fontananova, L. Palmisano, G. Barbieri, CO2 to liquid fuels: photocatalytic conversion in a continuous membrane reactor, ACS Sustain. Chem. Eng. 6 (7) (2018) 8743–8753, https://doi.org/10.1021/acssuschemeng. 8b01073, Copyright {2018} American Chemical Society.

FIG. 3 Hybrid inorganic membrane. Scheme of hybrid inorganic membrane. Reprinted with permission from J.W. € L. Grundy, E. Des Ligneris, Z. Yi, Maina, J.A. Schutz, L. Kong, et al., Inorganic nanoparticles/metal organic framework hybrid membrane reactors for efficient Photocatalytic conversion of CO2, ACS Appl. Mater. Interfaces 9(40) (2017) 35010–35017, https://doi.org/10.1021/acsami. 7b11150, copyright (2017) American Chemical Society. © From https://doi.org/10.1021/acsami.7b11150

UV

CO2

CO, Methanol

Semiconductor nanoparticles

MOF membrane

The use of optofluidics in microreactor design was recently proposed as a way to enhance mass and photon transport, spatial illumination homogeneity, and light penetration through the entire reactor. It was used in various photocatalytic processes such as water purification [74], water-splitting [91] and photocatalytic fuel cells [92]. In 2016, Cheng et al. [14] proposed a novel optofluidic membrane microreactor with high surface-area-to-volume ratio, enhanced photon and mass transport, and uniform light distribution for the photocatalytic reduction of CO2 with liquid water (Fig. 4). A TiO2 catalyst was coated on a carbon paper followed by hydrophobic treatment by PTFE for the separation of gas/liquid
1 phases. The maximum MeOH yield was 111.0 mmol g
1 catalyst h , which is among the best in the literature.

6 Photocatalytic CO2 reduction in continuous membrane reactors: Case studies In the last 5 years, we focused on the developing and analyzing photocatalytic membrane reactors for CO2 valorization. The photocatalytic membranes used consisted of a Nafion matrix with photocatalysts embedded into it, placed in a reactor

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FIG. 4 Optofluidic membrane microreactor. Schematic of the optofluidic membrane microreactor for photocatalytic reduction of CO2. Reprinted with permission from X. Cheng, R. Chen, X. Zhu, Q. Liao, X. He, S. Li, L. Li, Optofluidic membrane microreactor for photocatalytic reduction of CO2, Int. J. Hydrog. Energy 41(4) (2016) 2457–2465. https://doi.org/10.1016/j.ijhydene.2015.12.066, copyright (2016) Elsevier. © From https://doi.org/10.1016/j.ijhydene. 2015.12.006 .

UV light

TiO2 Liquid water

Products

CO2

CO2 Carbon paper

operating in continuous mode. In this section, we describe/summarize the main results obtained by using three different photocatalytic membranes: 1. TiO2 in Nafion 2. C3N4 in Nafion 3. C3N4/TiO2 in Nafion In all cases, we evaluated the performance of the photocatalytic membrane reactor as a function of the contact time (Eq. 1). Since the conversion is low (less than 0.1%), CO2 conversion does not represent a parameter reliable to be experimentally evaluated. Instead, the membrane reactor performance was evaluated through the produced species flow rate/catalyst weight (Eq. 2), the total converted carbon/catalyst weight ratio (Eq. 3), and reaction selectivity (Eq. 4). Contact time ¼

Catalyst weight embedded in the membrane ,S CO2 food flow rateðSTPÞ

flow rate of the Produced species compound flow rate mmol ¼ ¼ Catalyst weight Amount of catalyst dipsersed in the membrane h gCatalyst X Carbon flow rate for anyproduced species Converted carbon mmol ¼ ¼ Catalyst weight h gCatalyst Amount of catalyst dipsersed in membrane i
species flow rate Reaction selectivity ¼ X ,
i
species flow rate i

(1) (2)

(3) (4)

All the three catalytic membranes underwent a cleaning procedure before starting photocatalytic reaction measurements to remove any residual solvent and other low-molecular-weight organics possibly present in the polymer solution that, otherwise, could be released during the reaction measurements, thus invalidating the results. More details on this procedure can be found in [66–68]. The first membrane used consisted of a TiO2 catalyst embedded in a Nafion matrix. [68]. The exploration of various casting conditions revealed the importance of the solvent used and the catalyst amount for obtaining good distribution of the catalyst in the membrane matrix and, consequently, good performance. Three membranes were prepared (Fig. 5): the first two contained 1.2%wt. of TiO2 with respect to the weight of the membrane and differed from each other owing to the casting solvent, which was a solution of MeOH:H2O (50:50%wt.) for the first membrane and a solution of EtOH:H2O (50:50%wt.) for the other membrane. This latter solvent was used for the third membrane, where the photocatalyst content corresponded to 5%wt. with respect to the weight of the membrane. At equal TiO2 amount (1.2%wt. in the membrane), the membrane prepared using EtOH:H2O (50:50%wt.) as solvent (labeled in Fig. 6 as “well dispersed”) allowed to obtain a better distribution of the catalyst in the Nafion matrix (Fig. 6), which reflected in a greater MeOH flow rate/TiO2 weight with respect to Membrane 1, where we observed a partial catalyst deposition (Fig. 5). A further increase of the catalyst amount to 5%wt. produced a partial segregation and resulted in a significant decrease of the photocatalytic performances of the membrane. Both membranes (Membranes 1 and 3) showing catalyst segregation were less transparent. From this observation, we deduced that partial catalyst segregation and formation of aggregates are crucial parameters for membrane performance. Some strategies to reduce membrane formation time are accelerating solvent evaporation (e.g., higher temperature or gas sweeping), functionalizing TiO2 with surfactants, or using different polymeric materials.

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FIG. 5 Photocatalytic membranes. SEM images of the three photocatalytic membranes. Reproduced from M. Sellaro, M. Bellardita, A. Brunetti, E. Fontananova, L. Palmisano, E. Drioli, G. BarbieriCO2 conversion in a photocatalytic continuous membrane reactor. RSC Adv. 6(71) (2016) 67418–67427. https://doi. org/10.1039/c6ra06777h, with permission of The Royal Society of Chemistry. © From https:// doi.org/10.1039/co´ra06777h

FIG. 6 MeOH production by means of a photocatalytic membrane. MeOH flow rate/TiO2 weight as a function of catalyst content percentage in the polymeric membrane. Reprinted with permission from M. Sellaro, M. Bellardita, A. Brunetti, E. Fontananova, L. Palmisano, E. Drioli, G. BarbieriCO2 conversion in a photocatalytic continuous membrane reactor. RSC Adv. 6(71) (2016) 67418–67427. https://doi.org/10.1039/ c6ra06777h, Copyright (2019) Elsevier © From https://doi.org/10.1039/co´ra06777h

Using the membrane with the better TiO2 distribution, no CH4 or CO were observed, whereas a MeOH production flow rate/TiO2 weight of 45 mmol gcatalyst
1 h
1 was obtained under mild experimental conditions, which was among the highest values in literature up to that date. Inspired by these findings, we carried out reduction of CO2 to fuels in a continuous photocatalytic membrane reactor equipped with a Nafion membrane with embedded exfoliated C3N4. The membrane was dense on the air-facing surface (Fig. 7A), whereas some pores were detected on the casting plate-facing surface together with a partial segregation of the catalyst (Fig. 7A). Scanning electron microscope (SEM) images of the cross-section confirmed the dense nature of the membrane and revealed the presence of catalyst micro-aggregates, mainly in the bottom part (Fig. 7C). As mentioned, the reaction performance is significantly affected by the feed molar ratio and contact time. At 2 s, (Fig. 8), MeOH and EtOH flow rate increased with the H2O/CO2 feed molar ratio reaching 17.9 and 14.9 mmol gcatalyst
1 h
1, respectively, at a feed molar ratio of 5, whereas flow rate reduced at 4 and 1.7 mmol gcatalyst
1 h
1 when the feed molar ratio was set at 0.5, where a water defect induced a promotion of HCHO formation that reached 27 mmol gcatalyst
1 h
1. Contrarily, MeOH and EtOH production were promoted by an excess of water. At a low contact time MeOH and EtOH were the favored species, whose productivity decreased as contact time increased. This is in contrast to HCHO and acetone, in which productivity increased as a result of oxidation and secondary reaction between intermediates, respectively (Fig. 9). The total converted carbon/catalyst weight (Eq. 3) provides an indication of the capability of a system to convert (Fig. 10). At H2O/CO2 feed molar ratio of 5 and contact time of 2 s, 47.6 mmol gcatalyst
1 h
1 were converted, which is eight times greater than that measured in the worst conditions (H2O/CO2 feed molar ratio ¼ 5 and contact time ¼ 18.7 s).

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FIG. 7 C3N4-loaded Nafion membrane. SEM of (A) air-facing magnification 5kX; (B) casting plate-facing surfaces (5 kX; (C) cross-section (3 kX) of C3N4-loaded Nafion membrane. Reprinted with permission from A. Brunetti, F.R. Pomilla, G. Marcı`, E.I. Garcia-Lopez, E. Fontananova, L. Palmisano, G. Barbieri, CO2 reduction by C3N4-TiO2 Nafion photocatalytic membrane reactor as a promising environmental pathway to solar fuels, Appl. Catal. B Environ. 255 (2019) https://doi.org/10.1016/j.apcatb.2019.117779, Copyright (2018) American Chemical Society. © From https://doi.org/10.1016/j. apcatb.2019.117779

FIG. 8 Flow rate of reaction products. Flow rate/catalyst weight of the various species formed as a function of H2O/CO2 feed molar ratio at a contact time of 2 s. © No permission Requied.

The use of composite photocatalysts is a promising strategy to exploit the advantages offered by two different catalysts. In this perspective, we developed a photocatalytic membrane where the embedded catalyst was a composite of g-C3N4 and TiO2 to enhance the catalytic membrane capability in terms of H2O oxidation and CO2 reduction [57, 59, 65]. As in the other cases, this membrane was also dense with some catalyst micro-aggregates present in the whole cross-section, but more concentrated in the bottom part (Fig. 11). As already observed for the other photocatalytic membranes, MeOH formation was promoted at a high H2O/CO2 feed molar ratio, reaching 44.7 mmol gcatalyst
1 h
1. A H2O/CO2 feed molar ratio of 0.5 corresponded to a significantly lower

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FIG. 9 Flow rate of reaction products. Flow rate/catalyst weight of the various species formed as a function of C3N4 weight/CO2 feed flow rate at H2O/CO2 feed molar ratio equal to 5. © No permission Requied.

FIG. 10 Total converted carbon. Total Converted carbon as function of contact time and H2O/CO2 fixed molar ratio. © No permission Requied.

FIG. 11 C3N4-TiO2-loaded Nafion membrane. SEM of (A) air-facing (magnification 10 kX), (B) cross-section (magnification 1.5 kX), (C) casting plate-facing surfaces (10 kx); (D) casting plate-facing surfaces—BSE (10 kx) of C3N4-TiO2-loaded Nafion membrane. Reprinted with permission from A. Brunetti, F.R. Pomilla, G. Marcı`, E.I. Garcia-Lopez, E. Fontananova, L. Palmisano, G. Barbieri, CO2 reduction by C3N4-TiO2 Nafion photocatalytic membrane reactor as a promising environmental pathway to solar fuels, Appl. Catal. B Environ. 255 (2019) https://doi.org/10. 1016/j.apcatb.2019.117779, Copyright (2019) Elsevier. © From https://doi.org/ 10.1016/j.apcatb.2019.117779

MeOH production rate (1.7 mmol gcatalyst
1 h
1) and a significant drop in terms of total converted carbon/catalyst weight to less than 10 mmol gcatalyst
1 h
1. In other words, a high feed molar ratio promotes both methanol selectivity production and total conversion (Fig. 12). Similar trends were obtained at a contact time of 3.5 s (Fig. 13) where MeOH was still the favored product at a high H2O/CO2 feed molar ratio. However, the total converted carbon was less than half of that obtained at a contact time of 2 s.

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FIG. 12 Flow rate of reaction product. Species flow rate/catalyst weight and reaction selectivity of the reaction products as a function of H2O/ CO2 feed molar ratio at a contact time of 2 s. Reprinted with permission from A. Brunetti, F.R. Pomilla, G. Marcı`, E.I. Garcia-Lopez, E. Fontananova, L. Palmisano, G. Barbieri, CO2 reduction by C3N4-TiO2 Nafion photocatalytic membrane reactor as a promising environmental pathway to solar fuels, Appl. Catal. B Environ. 255 (2019) https://doi.org/10.1016/j.apcatb.2019. 117779, Copyright (2019) Elsevier. © From https://doi.org/10.1016/j.apcatb.2019.117779

FIG. 13 Reaction products distribution. Species flow rate/catalyst weight of the products as a function of the contact time at H2O/CO2 feed molar ratio of 2 and 5. Reprinted with permission from A. Brunetti, F. R. Pomilla, G. Marcı`, E.I. Garcia-Lopez, E. Fontananova, L. Palmisano, G. Barbieri, CO2 reduction by C3N4-TiO2 Nafion photocatalytic membrane reactor as a promising environmental pathway to solar fuels, Appl. Catal. B Environ. 255 (2019) https://doi.org/10.1016/j.apcatb.2019.117779, Copyright (2019) Elsevier. © From https://doi.org/10. 1016/j.apcatb.2019.117779

The photocatalytic membrane reactor is thus highly productive at a high feed molar ratio and a low contact time, reaching the highest result with 58.2 mmol gcatalyst
1 h
1 of converted CO2. Reaction pressure is another parameter, which most often affects the performance of a membrane reactor through reactant adsorption and product desorption. In this view, we explored reactor performance at 5 bar of reaction pressure, fixing the feed molar ratio and contact time that provided the best performance at 3 bar (Fig. 14). At these conditions, MeOH and HCHO were the sole products with a lower amount of MeOH and a greater amount of HCHO with respect to the results obtained at 3 bar. This was explained considering that a higher reaction pressure can induce a hindered desorption, which leads to partial oxidation reactions and thus to the formation of HCHO to the detriment of MeOH. Total converted carbon resulted close to that obtained at 3 bar, indicating that the system capability of converting carbon does not change significantly at 5 bar. Even though the reaction is thermodynamically favored by pressure owing to a reduction in mole number, this positive effect is counterbalanced by difficulties in desorption, which leads to a reduced advantage on total conversion. Comparison of the three photocatalytic membrane reactors (Fig. 15) clearly shows that the reactor with a C3N4/Nafion membrane exhibited prevalent formation of alcohols (MeOH and EtOH) with a total production of 32.8 mmol gcatalyst
1 h
1 and 47.6 mmol gcatalyst
1 h
1 of total converted carbon at feed molar ratio equal to 5. The C3N4-TiO2/Nafion membrane reactor, instead, resulted in greater production of alcohols (48.8 mmol gcatalyst
1 h
1) with a marked methanol formation (44.7 mmol gcatalyst
1 h
1) with respect to EtOH, under the same conditions. The system was globally more productive than the previous one with a carbon converted/catalyst weight of 58.2 mmol gcatalyst
1 h
1 with more selectivity toward MeOH, obtaining 83.2 versus 53.6 of the C3N4/Nafion membrane reactor. The reaction selectivity increased as the TiO2 content increased, confirming that the presence of TiO2 leads to a process much more selective toward methanol but less performant in terms of productivity, which instead resulted promoted by the presence of C3N4.

7

Photoelectrocatalytic membrane reactor

There is growing concern for the development of efficient photocatalytic membrane reactors using renewable and sustainable energy sources, such as solar energy [93]. Moreover, several studies investigated the possibility of physically separating the

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FIG. 14 Converted carbon. Species flow rata/and converted carbon/catalyst weight at 3 and 5 bar. Reprinted with permission from A. Brunetti, F.R. Pomilla, G. Marcı`, E.I. Garcia-Lopez, E. Fontananova, L. Palmisano, G. Barbieri, CO2 reduction by C3N4-TiO2 Nafion photocatalytic membrane reactor as a promising environmental pathway to solar fuels, Appl. Catal. B Environ. 255 (2019) https://doi.org/10.1016/j.apcatb.2019.117779. Copyright (2019) Elsevier. © From https://doi.org/10.1016/j.apcatb.2019.117779

FIG. 15 Reaction selectivity. MeOH flow rate/catalyst weight (filled in blue triangle (dark gray in print version) over blue left axis) and reaction selectivity toward MeOH (filled in red circle (dark gray in print version) over red right axis) and converted carbon (green bars (light gray in print version)) for C3N4-TiO2-Nafion (blue bars (light gray in print version)) as a function of the TiO2 percentage in the catalyst embedded in the photocatalytic membrane. Reaction pressures 3 bar, H2O/CO2 feed molar ratio ¼ 5 and contact time ¼ 2.0 s. Reprinted with permission from A. Brunetti, F.R. Pomilla, G. Marcı`, E.I. Garcia-Lopez, E. Fontananova, L. Palmisano, G. Barbieri, CO2 reduction by C3N4-TiO2 Nafion photocatalytic membrane reactor as a promising environmental pathway to solar fuels, Appl. Catal. B Environ. 255 (2019) https://doi.org/10.1016/j.apcatb.2019.117779, Copyright (2019) Elsevier. © From https://doi.org/10.1016/j.apcatb.2019.117779

oxidation and reduction reaction by using a photoelectrochemical device instead of direct photoreduction to minimize the competing formation of H2 and H2O2, which consume H+ and electrons, thus reducing the process efficiency [94, 95]. The solar-driven reduction of CO2 to synthesis gas (syngas) was developed using nanosized earth-abundant catalysts and silicon photovoltaics under combined photocatalysis and electrolysis conditions at ambient conditions ([71, 96]. Syngas is a gas mixture consisting primarily of hydrogen and carbon monoxide, from which liquid transportation hydrocarbon fuels and other value-added chemical products can be produced. The prototype reactor consists of three main components: (1) a CudZn cathode made of copper foam coated with low-cost nanosized zinc flakes as a catalyst for reducing CO2 to syngas; (2) a photoanode formed by an adapted silicon heterojunction solar-cell structure with a nickel foam catalyst for the oxygen evolution reaction; and (3) a bipolar membrane separating the catholyte and anolyte compartments. The membrane photoelectrolysis reactor exhibited a stable and bias-free operation with a solar-to-syngas conversion efficiency of 4.3%. Stable and tunable H2:CO ratios were obtained along with high CO Faradaic efficiencies of up to 85% and a CO current density of 39.4 mA cm
2 [71, 96]. Another interesting photoelectrocatalytic reactor was developed for converting CO2 to long-chain hydrocarbons (C5) at room temperature and atmospheric pressure, using Pt nanoparticles on carbonbased electrodes, solar energy, and environmental conditions (22°C and atmospheric pressure) [71, 96]. The reactor operated in a continuous flow with the working electrode directly in contact with CO2 in the gas phase, whereas the membrane separating the anode and cathode compartment was a Nafion117 cation-exchange membrane.

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Perspectives and challenges

The conversion of CO2 into valuable fuels and chemicals is a promising approach to combating both fuel shortages and climate change. CO2 photocatalytic conversion is an emerging route with many potentialities but also many challenges owing to a complex reduction mechanism and low productivity. The selection of a proper photocatalyst is thus essential for obtaining good performance, but equally important is catalyst configuration during the reaction. Issues related to the small surface area-to-volume ratio, non-uniform light distribution, and poor light utilization efficiency are usually encountered in existing photoreactors, limiting product yield. Catalyst immobilization into the membrane matrix and thus the use of a membrane reactor for CO2 photoreduction is an interesting and valid potential solution to these problems. Its use offers several advantages such as a better catalyst exposition to UV light to carry out the reaction, tuning of contact between reactants and catalyst, reduction of catalyst aggregate formation, easier recovery of the catalyst, which can be simply reused, and better control of fluid dynamics In this chapter, we highlighted the assets offered by the continuous operating mode of a membrane reactor coupled with the use of an enhanced photocatalyst directly embedded in a dense polymer matrix. However, a lot of work still needs to be done since catalytic performance is strictly related to membrane preparation. The formation of aggregates and the segregation of catalysts can reduce the productivity and activity of the reaction system. Therefore, improved membrane preparation techniques that assure a uniform distribution of the catalyst into the membrane together with an appropriate design of the reaction and understanding of the operating conditions’ effect on the reaction itself are essential aspects that require further investigation.

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Chapter 32

Photocatalysis by metal-organic frameworks Arianna Melilloa, Sergio Navalo´na, Bel en Ferrera, and Hermenegildo Garcı´ab a

Chemistry Department, Universitat Polite`cnica de Vale`ncia, Valencia, Spain b Instituto Universitario de Tecnologı´a Quı´mica, CSIC-UPV, Universitat Polite`cnica de Vale`ncia, Valencia, Spain

1 Introduction Compared to conventional catalysts in which an active site is used to overcome the energy barrier from reagents and substrates to products [1, 2], a photocatalyst is a species able to use the energy of light to promote the transformation of substrates into products [3, 4]. The energy of a photon is related to its wavelength by the relation E ¼ hc/l, which indicates that the energy of ultraviolet (UV) and visible light is in the order of magnitude of covalent bonds, therefore, being able to promote the transformation of a substrate in contact with the photochemically generated transient state into products [4]. In heterogeneous photocatalysis, the catalyst is a solid or material that converts, upon light irradiation, substrate into products [3]. Most of the solid photocatalysts are metal oxides, such as TiO2 in various crystallographic phases, titanates, ZnO, CeO2, and oxyanions like BiVO3 [5, 6]. However, the oxides of high lattice energy make these materials more difficult to tune, modify, or alter to adapt their photochemical properties such as the band gap and energy band in alignment with the desired values [5, 6]. In most cases, metals oxides and oxyanions have a band gap over 3 eV, meaning that these solids can only undergo excitation by photons of the UV region [5, 6]. Since natural solar light only contains about 4% of UV light, with wavelength lower than 380 nm, and about 48% of visible light, it could be of interest to expand the photoresponse of the photocatalysts into the visible region [5]. The common strategies that have achieved a limited success in decreasing the band gap of inorganic semiconductors include metal or nonmetal doping, amorphization of the outer most external layer of the crystals, or deposition of organic or inorganic light harvesters that can excited the metal oxide [5, 7–9]. Therefore, a continuous challenge in photocatalysis has been the search for other type of solid photocatalysts, including metal sulfides [10], chalcogens [11], oxyhalides [12], and metal-free materials such as carbon nitride (C3N4) [13] or defective graphene [14]. However, each class of these materials also presents different limitations, such as lack of stability in the case of metal sulfides or low efficiency due to the transparency in the case of two-dimensional materials. In this context, “metal-organic frameworks (MOFs)” have become among the best-studied solid photocatalysts due to their high efficiency in different reactions including solar fuels production, photooxidations, pollutant photodegradation, or photoredox processes. Fig. 1 summarizes the main opportunities that MOFs offer as photocatalysts. This chapter focus on the photocatalytic activity of MOFs. In the next section, we describe the structure of MOFs and summarize the main features that are responsible for their photocatalytic efficiency as well as the prevalent mechanism of the photocatalytic reactions. In the subsequent sections, we briefly discuss the main reaction types for which MOFs appear to be the materials of choice as photocatalysts. In the last section, we summarize the current state of the art and give our views of future of MOFs as photocatalysts.

2 MOFs as photocatalysts MOFs are crystalline porous materials whose lattice is constituted by nodes of one or a few metals ions that are connected and hold in place by rigid organic linkers, typically aromatic polycarboxylates ([15–17]). The interaction between the metal nodes and the organic linkers is attractive Coulombic forces and metal-ligand coordination bonds. Besides aromatic polycarboxylic acids, nitrogenated heterocycles such as 2-methylimidazole and benzoimidazolates are also frequently used as Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00026-X Copyright © 2021 Elsevier Inc. All rights reserved.

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FIG. 1 Possible applications of MOFs as photocatalysts.

linkers [18]. The interaction between the metal nodes and the organic linkers defines open and highly porous materials. MOFs rank among the solids with the highest surface area and pore volume. Fig. 2 illustrates the structural properties of MOFs. In addition, MOFs exhibit very low framework density, meaning that the mass enclosed in a certain volume in MOFs are among the lowest mass for any currently known material. Therefore MOFs have very open crystal lattice, particularly compared to related porous aluminosilicates, like zeolites and mesoporous silicates [19]. The main feature that can serve to understand why MOFs have attracted so much interest as gas adsorbents [20, 21] and solid catalysts [22] are the large diversity of transition metals, metal clusters, and organic linkers that can be employed in the preparation of MOFs [23]. There are by now over 70,000 MOFs reported of virtually all the transition metals. Also, regarding the linker, bi-, tri-, and tetrapodal rigid linkers have been used [16] [24, 25]. The linker may also contain additional substituents besides the coordination center. Besides phenylene derivatives, linkers of naphthalene, anthracene, perylene and other condensed polycyclic aromatics, as well as five- and six-member ring heterocycles have been used [18]. Another feature of MOFs is the possibility to be designed to meet special needs for specifically application. In the synthesis of MOFs, there is the possibility to predict beforehand the geometry of the pores and their dimensions based on the structure of the nodes and the coordination geometry of the organic linker [26]. Thus replacement of one organic linker by other with the same coordination directionality, while keeping the same metal precursors, leads generally to isomorphic MOFs with the same geometry and with surface area and pore dimensions related to the relative size of the linkers [26]. Besides the intrinsic properties of MOFs, they can also act as platforms to anchor or support active species [27, 28]. These active species can be present in the synthesis of the MOFs “de novo synthesis” or can be introduced after the synthesis of the material by means of “postsynthetic” treatments [27, 28]. A particular example of this role of MOFs as platform is the anchoring of photocatalytically active polypyridyl complexes of transition metals and the covalent grafting

Crystalline porous solid Chemical and termal stability

Open porosity with the minor intensity of net

MOFs’ properties Superficial area of 5000 m2/g

Flexible structure Possibility of introduce guest in the porosity FIG. 2 The structural properties of MOFs.

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= Zr oxo-cluster

b)

= organic linker

NH2

FIG. 3 Molecular structure of (A) UiO-66(Zr)-H and (B) UiO-66(Zr)-NH2.

of dyes and chromophores [29]. Also, this chapter is related to the encapsulation of metal nanoparticles (MNPs) inside the intracrystalline void space of MOFs [30]. Examples are presented in the following sections of modified MOFs, in which the photocatalytic activity derives from, or is increased by, the presence of these active centers. Another example of versatility of MOFs is related with the flexibility of some MOFs that contain in the node more than one metal [23, 31]. These materials are generally named as “mixed-metal MOFs.” In other cases, MOFs can contain on the same structure more than one linker with the same binding geometry but differing in the presence of substituents or in the presence of heteroatoms at the linker. These materials are referred to as “mixed-linker MOFs” [23, 31]. In relation to what has been previously reported, mixed-metal MOFs can exhibit much enhanced photocatalytic activity in comparison with mono-metallic MOFs due to the more appropriate energy alignment and overlapping of the atomic orbitals with those of the linkers [23, 31]. Also, mixed-linker MOFs make possible to introduce auxochromic and bathochromic substituents that increase the molar absorptivity of the organic chromophores and shift the maximum wavelength of the absorption band toward the red [23, 31, 32]. Fig. 3 shows some of the possible structural modifications that are possible in MOFs to increase their photocatalytic activity (Fig. 3). All these properties are very interesting from the photocatalytic point of view and the simplicity in which they can be implemented in the MOF structure in contrast with the previously commented difficulty to modify metal oxides. Although there is a large number of MOFs already reported in the literature and this number grows continuously, most of the reports dealing with MOFs as photocatalysts are limited to only a small number of MOFs [23, 31, 32]. One of the main reasons for the limited use of large variety of MOFs is the notorious lack of stability of many of them that cannot stand water and other solvents for long periods of time. In this regard, it has to be commented that MOFs have to be continuously surveyed regarding their structural stability under reaction conditions. This is particularly important in the case of MOFs based on divalent cations, for which due to the weak Coulombic attraction and very large porosity, and large pore dimension structural stability is at risk. However, even though not all MOFs are stable, there are by now convincing evidence of the stability for certain MOFs due to the robustness of the lattice [23]. The structural stability is one of the reasons why materials based on zirconium nodes are among the mostly used MOFs as photocatalysts [33]. One of these cases of highly stable MOFs is the UiO-66(Zr) family that stand up to 450°C without undergoing structural collapse [33]. The UiO-66 solids can also stand boiling water for prolonged periods of time maintaining its structure. The main reason of this structural stability of UiO-66 is the tetravalent character of Zr4+ ions, the stability of Zr6O4OH4 nodal cluster that interacts strongly with terephthalate linkers. A similar case is the MIL-100(Cr or Fe) family that exhibits very high thermal and chemical stability [11, 15, 24]. Fig. 4 summarizes some of the MOFs most widely used as photocatalysts.

3 General photocatalytic mechanism Organic photochemistry was developed in the 1980s by studying in solution the behavior of probe molecules and determining their light absorbance and the fate of the electronic excited states by means of fluorescence and time-resolved absorption spectroscopy as well as analysis of the products under steady-state irradiation and quenching studies [4, 31]. A large number of organic probes with different structures and functionalities as well as transition metal complexes were the subject of intensive study to understand the elementary processes triggered by photon absorption. One particular case is the photochemical behavior of ruthenium tris(bipyridine) that exhibits in solution an absorption band at lmax of 460 nm with a shoulder in the blue side that is responsible for its visual orange color. Upon photoexcitation, an intramolecular electron transfer from the metal ion as donor to the organic linker as electron acceptor takes place giving rise to a long-lived triplet excited state that decays in the microsecond time scale via “intersystem crossing” from triplet to the singlet ground state. This intersystem crossing decay occurs with emission of phosphorescence characterized by a relatively

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FIG. 4 Metal nodes/clusters and primary structures of UiO-66(Zr) (A and B) and MIL-125(Ti) (C and D) solids.

room temperature high quantum energy and lem of 540 nm. Besides molecular oxygen as triplet quencher, this phosphorescence is also inhibited by the presence of electron acceptors or electron donors, such as methyl viologen and tertiary amines, respectively. Similar to the case of ruthenium tris(bipyridine)2+, several other transition metal complexes such as metal porphyrins, phthalocyanines, or bis(2,20 -bipyridine)bis(isothiocyanate)ruthenium (II), among others, undergo analogous processes via an intramolecular electron transfer metal-to-ligand or ligand-to-metal that is responsible for the appearance of an intramolecular charge transfer transient state. This photochemical behavior is relevant to understand the general photochemical mechanism undergoes by MOFs upon excitation either to the organic linker or the metal node. In a certain way, the photocatalytic mechanism of MOFs can be understood as being similar to that of transition metal complexes considering MOFs as a polymeric example of these coordination polymers that are not in solution but in the solid state [23, 31, 32]. Fig. 5A illustrates the general photochemical elementary steps occurring after light absorption in MOFs. As it is indicated there, light absorption in MOFs results in an electron transfer from the electron-rich organic linker to the electron poor metallic node [23, 31, 34, 35]. Depending on the mobility of the charge carrier, this geminated charge separation can migrate by electron jump or by delocalization through a continuous valence or conduction band through all the crystal. In the last case, this electron-hole migration event is analog to the charge delocalization occurring in metal oxide semiconductors, such as TiO2, in which absorption of one photon produces an electron jump from the oxygen lone electron pair orbital to the empty 3d Ti4+ orbital [36]. This event causes a charge separation event and subsequent fast delocalization

FIG. 5 (A) General photocatalytic event occurring upon excitation of the organic linker resulting in a single electron transfer to the metal node of the MOF. (B) Diffuse reflectance UV-vis of MIL-l25(Ti) (b0 ) and MIL-125(Ti)-NH2 (b00 ).

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of electron through the conduction band, constituted mainly by the overlapping of the empty 3d orbitals of Ti4+. After the initial charge separation, additional migration of the charge carrier can be also possible if MOFs incorporate within the internal pores metal NPs such as Pd, Ag, or Pt or another guest such as metal complexes [31]. In addition, the presence of substituents in the ligand can contribute to facilitate charge transfer to the metal node by acting as electron-donor substituent. The case of amino groups is particularly notable since these groups introduce a visible band at about 400 nm due to the nitrogen lone pair electron that give electron density to the p cloud of the aromatic ring [31, 32]. Therefore amino groups introduce visible light response in some MOFs (Fig. 5B) while they act as electron donor in the charge separation event. It should be commented that the presence of a second amino group on the aromatic ring further shifts the absorption onset to the red in the visible region. In addition, the use of naphthalene or anthracene as chromophores in the linker can also expand the range of wavelength that can be used to promote photoexcitation in MOFs. One related case of special interest is the use of tetraphenylcarboxylic porphyrin and metal porphyrins as linkers [19]. Porphyrins have a strong band in the visible region, so these units are suitable ligands for the synthesis of a variety of photoresponsive MOFs. Regarding the metal node and considering that most of the transition metals absorb visible light, another possibility is to excite these metal clusters using an appropriate wavelength [32, 34]. However, it should be noted that most of the visible absorption bands in transition metals are due to the split of the d orbitals, and these absorptions are related to d-d electron transitions located at the metal ions. Photon absorption and d-d excitation generally results in a fast decay to the ground state, without causing electron transfer between the nodes and linkers. In this regard, there are some exceptions, particularly those related to d10 transition metals. In these cases, excitation of the metal node could result in an effective electron transfer to the metal node as electron acceptor from the organic linker in the ground state. Transient absorption spectroscopy is a powerful technique to detect photochemically generated transient states as well as monitoring their kinetics [4, 23]. In these studies, a short energy laser pulse, as for instance of a few nanosecond duration, can generate a high concentration of transient states in the micromolar range sufficient to be detected by the change in the absorption spectrum [37, 38]. Furthermore, by monitoring a specific wavelength, the temporal profile of the signal can be fitted to a particular kinetic, thus providing information on the lifetime of the photogenerated transient. As an example, Fig. 6 shows the transient absorption spectra recorded for UiO-66(Zr)-NH2 upon 355 nm later excitation as well as the temporal profile of the signals. In those cases, where the transient spectrum corresponds to a single species, the temporal profile monitored at different wavelengths should coincide. However, if two or more transient species, as it is usually the case in MOFs, are responsible for the transient signals, differences in the temporal profile monitored at different wavelengths can be monitored. It is a common observation that the transient signals, recorded for MOFs upon laser absorption, decay in the millisecond timescale, and they are orders of magnitude longer lived than the common lifetime of intramolecular charge transfer state observed for metal complexes in solution.

a)

b) 300nm 420nm 500nm 550nm 600nm

6000 0.15 4000

0.10

2000

0.05 0.00 Intensity

Intensity

0 –2000 –4000

–0.05 –0.10 –0.15 –0.20 –0.25

–6000

–0.30

–8000

–0.35

–10000

–0.40 300

400

500 600 Wavelength (nm)

700

800

0

50

100 150 Time (ms)

200

250

FIG. 6 (A) Diffuse reflectance transient absorption spectra of UiO-66(Zr)-NH2 MOF recorded upon 355 nm laser excitation under argon atmosphere after 2.80 ms (square), 11.20 ms (circle), 35.20 ms (up triangle), and 199 ms (down triangle). (B) Temporal profile of the transient signal upon 355 nm laser excitation under argon atmosphere monitored at different wavelengths for UiO-66(Zr)-NH2.

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The assignment of the transient signals recorded in the laser flash measurements can be inferred by their behavior in the presence of quenchers [37, 39]. Typical quenchers that can be used to determine the nature of the transient signals are electron donors, such as methanol and amines, or electron acceptors such as oxygen and dichloromethane, among others. Thus for instance, if the signals in the absorption spectrum correspond to hole generated by electron removal, their lifetime should be shorter when electron donor quenchers, such as methanol, are present in the sample during the measurement. A parallel behavior should occur when the signals are due to photogenerated electrons, and the measurement is carried out in the presence of electron acceptor quenchers. Typical quenchers in these time-resolved measurements are gasses or liquids of low boiling point. In this way, the quencher can diffuse easily through the solid during the lifetime of the photogenerated transient signal. These types of measurements are the base of the proposed general mechanism of ligand-to-metal photoinduced electron transfer. However, in some cases, the detected transient absorption spectra correspond to electrons, holes, or even the combination of both. Steady-state measurements using electron donors and acceptors can also provide evidence supporting the occurrence of charge separation upon photoexcitation [23]. Particularly, convincing photochemical tests are those that allow visual observation of the generation of radical cations after excitation, either by electron abstraction from the MOFs or electron donation [37]. Typical probes in this steady-state irradiation experiments are viologens and/or the use of 2,20 -bipyridiniums such as paraquat. These heterocyclic cationic molecules are strong electron acceptors, and the resulting radical cations after gaining one electron exhibit intense green or blue colors. Moreover, these radical cations having a strong coloration are considerably stable in the absence of oxygen and metals and, therefore, can be seen by naked eyes. Colorimetric titration knowing the molar absorptivity of the radical cation bands can also be used to quantify the concentration of these species, and in this way, the absorption intensity can be used to estimate the charge separation quantum efficiency. In addition, since the viologen and paraquat can have a range of reduction potentials, by employing a series of these probes, it is possible to determine the reduction potential of the photogenerated electrons. Fig. 7 illustrates the main features of viologen that are of relevance with regard to the use as visual probe to evidence charge separation in MOFs. Similar to the case of viologen probes, a series of aromatic and heterocyclic electron donors can be used to demonstrate the generation of positive holes upon MOFs excitation [37]. One of these probes commonly used are phenylenediamines. Substitution of nitrogen atoms with methyl and alkyl groups provides stability to the radical cations generated when phenylenediamine donates one electron to a photogenerated hole. This stability is enough to allow visual detection of these radical cations. Again, the oxidation potential of the hole can be estimated by the use of a series of phenylenediamine electron donors having a range of oxidation potentials. Also, colorimetry can be used to determine quantitatively the concentration of these species. Fig. 7 shows the case of N,N,N0 ,N0 -tetramethyl-p-phenylenediamine, an electron-rich aromatic compound that becomes blue after single-electron oxidation. The stability of this radical cation from N,N,N0 , N0 -tetramethyl-p-phenylenediamine is so remarkable that was the first-ever radical cation detected in the 19th century by Wurst [40]. Therefore the fundamental photochemical study both with transient spectroscopy or steady-state techniques supports the photogeneration of electrons, at the metal node, and holes, at the organic linkers, as the most general photogenerated

FIG. 7 UV-vis absorption spectra of an acetonitrile suspension of the UiO-66(Zr/Ce) MOF in the presence of Left: methyl viologen (MV2+), as electron acceptor, before (A) and after (B) 10 min Xenon lamp irradiation. Right N,N,N0 ,N0 -tetramethyl-p-phenylenediamine (TMPDA) as electron donor before (C) and after (D) 5 min Xenon lamp irradiation. The photographs of the insets show the visual appearance of the suspensions under each condition.

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transients in MOFs that are responsible for their photocatalytic activity [23, 34, 35]. If one of the charge carriers, either electron of holes, undergoes fast delocalization through the lattice, then the behavior of this MOFs is similar to that of typical n- or p-type inorganic semiconductors with the advantage of high flexibility in the design, convenient synthesis, and offering a large number of possibilities for tuning and structural modification. It is known that this state of charge separation can be reached directly or through the intermediacy of localized excitons that are the precursors of the electron migration. In this way, it can be proposed that triplet excited state localized at the organic linker “exciton” is the first species that after its formation evolves subsequently donating electrons to the metal node. Evidence for this main event requires fluorescence or ultrafast transient absorption measurements detecting photogenerated transition species in the picosecond or nanosecond timescale. Comparing the behavior of the linker in solution in the presence or absence of metals is also considered as an evidence to understand the generation of the charge-separated state [39]. After having discussed the basic principles on the use of MOFs as photocatalysts and the occurrence of charge separation either localized due to slow charge migration or delocalized if charge carriers move fast, the next sections describe succinctly the main uses of MOFs as photocatalysts.

4 Photocatalytic hydrogen generation In the context of the ongoing shift from fossil fuels to renewable energy sources, one possibility that has attracted considerable attention has been the production of hydrogen from water using solar light as primary energy source by employing a suitable photocatalyst [41]. The use of solar light in combination with a photocatalyst for the production of reduced chemical species such as hydrogen, methane, or formate is a field broadly denoted as “solar fuels” production. In the field of MOFs, Garcia and co-workers were the first to report the photocatalytic hydrogen generation using UiO-66(Zr)NH2 solid in the presence of a sacrificial electron donor [42]. Fig. 8 shows the elementary reaction triggered by light that results in the one-electron reduction of proton of water into hydrogen. The process is analog to those intensely studied using metal oxide semiconductors as photocatalysts. After this initial report, many other studies have shown the feasibility of the photocatalytic hydrogen generation using MOFs. The target in this area has been to increase the efficiency of the process, particularly under irradiation with visible light, by exploiting the flexibility that MOFs offer in the use of any transition metal and specific linkers more suited as light harvesters [31]. Despite the considerable progress made on the photocatalytic hydrogen generation, the fact that most of the reports still use a sacrificial electron donor, such as tertiary amines and alcohols, makes the process unpractical [31]. To obtain hydrogen from solar light and water using MOFs as photocatalysts, these materials should also be able to promote simultaneously oxygen evolution by water oxidation (Fig. 8, Eq. 3). This oxidation semireaction is considerably more challenging and kinetically more demanding than hydrogen evolution [31]. Firstly, from the thermodynamic point of view, the electrochemical potential required for water oxidation is high, i.e., 1.23 V under standard conditions with respect to standard hydrogen electrode. In addition, kinetically the process requires four electrons and the removal of four protons in comparison with the simplicity of hydrogen generation. For these thermodynamic and kinetic reasons, photocatalytic overall water splitting consisting in the simultaneous evolution of H2 and O2 in the stoichiometric proportions present in water has been much less documented compared to hydrogen evolution [31]. Fortunately, recent studies have shown that some MOFs such as MIL-125(Ti)-NH2 [43, 44] and MIL-101(Al) [43, 45] are stable photocatalysts that are able to promote the overall water splitting. As in the case of inorganic metal oxides, in photocatalytic water splitting, the presence of a co-catalyst incorporated either on the external surface of the crystal or, preferably, within the pores increases one order of magnitude or more the efficiency of the process [44]. In the case of MOFs and due to their large porosity, it is possible to incorporate this co-catalyst as nanoparticles (NPs) inside the pores. The promotional effect of co-catalysts strongly depends on many factors, including preparation procedure, particle size, location, and the interfacial contact, among others. It has been generally observed that the internal location of

FIG. 8 Photocatalytic overall water splitting.

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FIG. 9 Photodeposition of metal NPs on the catalyst surface through reduction (A) or (B) oxidation of the metal salt precursors. Methanol and Ce4+ are frequently employed as sacrificial electron and hole donors, respectively.

the co-catalyst NPs within the MOF pores resulted in a more efficient activity than when the co-catalyst particle is located exclusively on the external surface [46]. The preparation of these co-catalysts can be done by the so-called photodeposition method that is a green procedure and also shows evidence of the photogeneration of electrons and holes upon irradiation of MOFs. In the photodeposition method, metal or metal oxide NPs are precipitated on the photocatalyst by irradiation of the MOF in the presence of a solution that contains the metal salt precursor in an oxidation state that is soluble in the medium [44]. Fig. 9 illustrate the photodeposition process for generation of metal or metal oxide NPs. Thus, for instance, one particular case is the use of Pt NPs as co-catalyst for H2 evolution. These Pt NPs can be obtained by reduction of H2PtCl6 using methanol as the source of electrons and a suitable MOF as photocatalyst. In this particular case, photogenerated electrons produce the reduction of Pt4+ to Pt0 that precipitates forming NPs at the locations where the electrons are present inside MOFs pores. In an analogous way, but now promoting oxidation, photodeposition of Ru4+ entities inside the MOFs can be achieved starting from Ru3+ ions and promoting its oxidation to Ru4+ oxidation state by photoirradiation. Since in aqueous solution Ru4+ forms insoluble ruthenium oxide, the deposition of RuO2 NPs takes place in the location in which the holes are located. As commented earlier, the photocatalytic overall water splitting can be performed efficiently using the MIL-125(Ti)NH2 MOF as photocatalyst containing both Pt and RuO2 NPs as co-catalysts [44]. It should be commented that a recent study has shown that long-term (weeks) irradiation of MOFs containing carboxylate ligand results into partial MOF decarboxylation reaching values as high as 30% of the total carboxylic groups present in the material. Therefore to develop viable MOFs as photocatalysts, the durability of these materials under photoirradiation periods of months must be convincingly proved.

5

Photocatalytic CO2 reduction

Due to the interest in the increasing atmospheric CO2 emissions to minimize global warming and climate change, some reactions involving the use of CO2 as starting material have gained much current importance [47]. Among them, one possibility is to mimic natural photosynthesis where CO2 is reduced to glucose by electrons from water in the photosynthetic center of green plants and algae. In the so-called artificial photosynthesis method, a photocatalyst promotes the CO2 reduction upon light excitation in the presence of water [48, 49]. This photocatalytic reaction using water as electron source is considerably more endergonic (Eq. 4, Fig. 10) and kinetically more demanding than the overall water splitting. For thermodynamic and kinetic reasons, the current efficiencies for CO2 reduction are smaller than the values achieved for H2 and O2 generation from water. Moreover, although MOFs have been widely reported as efficient photocatalyst for CO2 reduction, the concept of artificial photosynthesis using water as electron source has still to be proved [31]. In the studies reported, sacrificial electron donors typically triethanolamine or tertiary amines have always been used [29, 31]. So water is a poor electron donor in most of the photocatalytic processes and, particularly, in the artificial

6 (CO2 + H2O)

hn+ Chlorophyll

FIG. 10 CO2 reduction upon light excitation in the presence of water.

C6H12O6 + 6 O2

(Eq. 4)

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photosynthesis. It is clear that the quantum efficiency and productivity so far reported would be necessary lower when water is used as electron source. In most of the cases reported on photocatalytic CO2 reduction by MOFs, the reactions have been performed with suspended powders in acetonitrile as solvent under saturated CO2 conditions in the presence of tertiary amines at room temperature [29, 31]. These conditions are highly favorable since the solubility of CO2 in acetonitrile is orders of magnitude higher than the micromolar solubility of CO2 in water at neutral or acidic pH values. Under these conditions, it can be proved that MOFs exhibit photocatalytic activity that is comparable to that achieved using inorganic semiconductors, particularly under visible light irradiation [31]. The concept previously commented of introducing amino groups at the linker to extend the absorption onset into the visible region and the use of mixed metals also applies to the photocatalytic CO2 reduction, enhancing the activity in comparison with parent MOFs. In addition, the presence of basic -NH2 increases the adsorption of acidic CO2 inside the MOF, and this effect is a positive factor contributing to enhance the photocatalytic activity. Again, similar to the case of photocatalytic overall water splitting, the most widely studied MOFs are those that exhibit high structural robustness such as the Zr-containing MOFs and MIL family, particularly the UiO-66 and MIL-125 series. In one of the examples reported in the literature, it has been demonstrated that the partial substitution of Zr4+ with Ti4+ ions increases the photocatalytic activity of UiO-66(Zr)-NH2 for CO2 reduction [50]. Theoretical calculations suggest that the photocatalytic activity of UiO-66(Zr) is hampered by the fact that both frontier orbitals, the highest occupied crystal orbital and the lowest unoccupied crystal orbitals, are localized on the organic linker, and they do not overlap with those of the [Zr6O4(OH)4]12+ nodes [38]. As consequence, not only the lifetime of the excited state is shorter, in the submicrosecond timescale, due to the easy decay to the ground state of the organic ligand exciton, but also the electron transfer from the terephthalic linker to the [Zr6O4(OH)4]12+ node is highly inefficient. To overcome this limitation, it was proposed that the partial replacement of a certain percentage of Zr4+ by Ti4+ should be favorable for the photocatalytic activity, since Ti4+ would introduce different atomic orbitals with better energetic alignment and special location to accommodate electrons from the organic linker [39, 50]. In this way, Ti4+ ions act as mediator favoring the transfer of electrons from the organic linker to the metal node. Fig. 11 illustrates the role of Ti4+ as electron relay in the photoinduced electron transfer from the organic terephthalate to the metal node. Transient absorption spectroscopy studies of UiO-66(Zr)-NH2 and UiO-66(Zr/Ti)-NH2 provide some spectroscopic evidence supporting the proposed role of Ti4+ in the decay of the photogenerated excited state [39]. As consequence of the more efficient ligand-to-node electron migration, mixed-metal UiO-66(Zr/Ti)-NH2 exhibits an improved photocatalytic activity for CO2 reduction to formic acid [50]. In fact, the reaction product widely reported in the photocatalytic CO2 reduction by MOFs is formic acid or formates, sometimes accompanied by CO. Regarding product selectivity in the photocatalytic CO2 reduction, it should be mentioned that several different products have been reported depending on the photocatalyst and the conditions of the reaction [51]. The most wanted products due to their liquid state under ambient conditions and its high combustion energy are methanol or alcohols. However, due to the instability of alcohols under photocatalytic conditions, methane is frequently the most abundant products, particularly when noble metal co-catalysts are introduced in the system. Methane has a higher energy content, and therefore its formation is most wanted for the use as fuel than formate that has lower energy content. As it can be seen in Eq. 5, Fig. 12, methane is an eight-electrons reduction product, while formate is only a two-electrons reduction product, and as the energy content

FIG. 11 The role of Ti4+ in mixed-metal UiO-66(ZrxTiy)-NH2 enhances charge migration from the organic terephthalate to the hexa metallic node in the photocatalytic process.

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FIG. 12 Reduction of CO2 to formate.

increases with the number of electrons in the reduction step, formate is a product with lower value as energy vector. Further improvement of the reaction should be the control of the product selectivity toward other products, the increase of the photocatalytic efficiency and to prove the durability of the photocatalyst in extended irradiation periods. Photocatalytic CO2 reduction using H2O as electron donor would be a milestone in this area.

6

Benzyl alcohol oxidations

In the previous photocatalytic reactions, the main target was the generation of chemicals by reduction of a substrate with the purpose of using the product as fuel by storing solar energy. Alternatively, photocatalysis can also be used to produce commodity and fine chemical compounds of interest for the chemical industry [3, 31]. One of these examples is the photocatalytic aerobic oxidation of benzylic alcohols. Oxidation of benzylic alcohols to carboxylic acids or carboxylic products of high added value is also a reaction of industrial interest. The process can be performed catalytically by using noble metals such as Au, Pt, or Pd NPs supported on metal oxides or carbon materials. However, there is an interest in finding alternative catalysts and processes based on more abundant base metals. In this context, photocatalysis is a well-known general methodology to generate reactive oxygen species from both molecular oxygen and from water that can subsequently react with organic compounds [52]. Fig. 13 illustrates the general photocatalytic pathway resulting in the generation of reactive oxygen species. These reactive oxygen species generally exhibit low selectivity in the oxidation of organic compounds [52]. Therefore they are mostly used for the degradation of organic pollutants in aqueous or gas phase. However, considering the high reactivity of benzylic positions in benzylic alcohols, the photocatalytic process can be controlled to afford selectively products that arise from the alcohol oxidation. Oxidation of benzylic alcohols by MIL-125 was among the first reports on the use of MOFs as photocatalysts [53]. In a further application of benzylic alcohol oxidation, the possibility to perform cascade reactions has also been reported (Fig. 13) [54]. A cascade or tandem reaction includes the combination of two or more elementary reactions in a single process. The cascade reaction represents an example of process intensification since the combination in one of two or more reactions without compromising the selectivity and yield of the final products minimizes the need of tedious intermediate workup and isolation of intermediate compounds. Fig. 14 illustrates some of the examples of cascade reactions where one of the steps involved in a photocatalytic reaction.

FIG. 13 (A) Photochemical generation of reactive oxygen species. (B) Cascade reaction leading to 2-substituted benzothiazoles including a step consisting of a photocatalytic oxidation of alcohols and subsequent condensation with ortho-aminothiophenol.

FIG. 14 Example of cascade reaction involving a photocatalyst.

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As it can be seen in this scheme, the final products are frequently heterocyclic compounds, and the photocatalytic oxidation of benzylic positions is combined with subsequent cyclic condensation of the carboxylic compound without the need of isolation from the reaction mixture. The common crucial key of cascade reaction is the combination of process that required different reagents and conditions. In the present example, the photocatalytic transformation occurs selectively for one of the reagents without the interference of other reagents that are also present during the photocatalytic transformation but undergo reaction with the photogenerated product.

7 Pollutant degradation As previously commented, the generation of reactive oxygen species is a general way to promote the degradation of organic compounds in water [55]. Biological wastewater treatment relies on the action of microorganisms present in the activated sludge for fermenting and decomposing organic compounds in water. However, while biological treatments are efficient for the treatment of urban wastewaters, in the case of industrial waters that can contain nonbiodegradable compounds or even toxic compounds, alternative methodologies and, in particular, photocatalytic advanced oxidation processes are more advantageous. In this context, to compare the efficiency of different photocatalysts, the International Union of Pure and Applied Chemistry has proposed the use of phenol as probe molecule to test the photocatalytic activity in bio-reluctant wastewaters containing aromatic compounds. Phenol is an example of a compound that can neutralize the microorganisms generally employed in biological wastewater treatments. In one of the studies, the photocatalytic degradation of phenol and its tert-butyl derivatives in water was studied using MOFs as photocatalysts [56]. It was observed that phenol becomes more easily degraded than 2,4-di-tert-butylphenol, despite that the butylated phenol should be more reactive considering the influence of alkyl substituent on the reactivity of aromatic compounds. Based on the comparative efficiency in the degradation of these two phenolic compounds, it was proposed that the photocatalytic degradation activity in the case of MOFs being a porous material requires the diffusion of the pollutant inside the internal cavities of the photocatalyst, where degradation would take place. In this way for pollutants with large molecular dimensions, only the external surface of MOFs particles would be accessible, and their photocatalytic degradation would be slower compared to analogous compounds whose molecular dimensions allowed their diffusion inside the pores where the photocatalytic degradation process would predominantly occur. Accordingly, in MOFs, the activity of the external surface would be minor vs the activity of the internal voids. This would indirectly indicate that the reactive oxygen species, supposedly responsible for the photocatalytic degradation, would be present mainly in the internal pores rather than as free radicals in the solution. In porous solids, it has been frequently reported that only a small proportion of the generated radical species can reach the state of free radicals. Most of these radicals are generated in the internal cavities, and only a fraction of them are able to diffuse as free species to the solution. Accordingly, those pollutants that can access to the interior of the porous photocatalyst will be degraded at much higher rate than those others that can only become attacked by free species in solution. This concept of preferential internal degradation will be unique for porous catalysts and can be used for the selective degradation of some pollutants in the presence of other chemicals. It has to be mentioned that photocatalytic advanced oxidation technique lacks in general of any selectivity, and therefore there is an opportunity to introduce selectivity by using MOFs of suitable pore dimension [55]. Fig. 15 illustrates the relative reactivity toward photocatalytic degradation inside the MOF pores vs the exterior and the proposed reason for this reactivity order.

FIG. 15 General scheme that illustrates the between MOFs (cubic structure) with pollutants of different sized and free radical species.

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The development of MOFs as photocatalysts for pollutant degradation requires the design of robust structures stable in aqueous media. These MOFs should stand the presence of naturally occurring ions in water, such as carbonates and bicarbonates, that may compromise their stability as well as the presence of natural organic matter that may block the MOF pores and then decrease their activity. It is also important to evaluate the long-term stability of MOFs as photocatalysts (months) with the generation of highly reactive oxygen species such as hydroxyl radicals that can also promote self-degradation of MOFs.

8

Photoredox reactions

Since the seminal study of Macmillan developing light activated organocatalysis based on the generation of radical anions, it has been a continuous interest in exploring and developing further this area, generally denoted as photoredox reactions [57]. In this type of reactions, light excitation of transition metal complexes such as iridium or ruthenium polypyridyl results in a long-lived triplet excited state that is able to abstract one electron from electron donors such as tertiary amines resulting in the photochemical generation of the reduced form of the metal complex that is able to initiate an organic reaction. Fig. 16 illustrates the general operation mechanism in photoredox reactions. In most of the cases, photoredox reactions are triggered by soluble transition metal complexes, frequently involving noble metal or costly metals such as iridium and ruthenium [57]. One common strategy in heterogeneous catalysis is to attach successful homogeneous catalyst onto an insoluble solid that allows the easy separation of the valuable homogeneous catalyst and its reuse [31]. In the particular case of photoredox processes, it would be of interest to apply a similar strategy. Considering that UiO-67 solid contains biphenyl-4,40 -dicarboxylic acid as a linker and the reported synthesis of mixedligand UiO-67 with both biphenyl-4,40 -dicarboxylic acid and some 2,20 -bipyridinedicarboxylic acid, it is possible to construct several polypyridyl metal complexes on the MOF structure [58, 59]. It can be anticipated that this type of mixed-linked MOFs can be used as heterogeneous photocatalyst to promote photoredox reactions. Fig. 17 illustrates the preparation of a mixed-ligand UiO-67. According to this route and considering the stability of the asymmetrically substituted ruthenium tris-bipyridyl having two carboxylate groups, a synthesis of UiO-67 was performed de novo using Zr4+ salt, biphenyl-4,40 -dicarboxylic acid, and a certain percentage of ruthenium tris(bipyridyl) complex.

O Br

Ph

Photoredox catalysis

O Ph

e–

CO2– N N

N Ru N

N N CO2–

TEOA h+ TEOA+ FIG. 16 Heterogeneous photoredox debromination promoted by Ru(bpy)3-UiO-67(Zr) MOF using triethanolamine (TEOA) as electron donor.

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N

N Cl Ru Cl

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2+

CO2–

CO2H

32

N

N

EtOH/H2O (1:1)

N

N

N

Ar (g), 95 °C

N

N Ru

+

N N

N CO2–

CO2H

Ru-bipy

1 CO2– ZrCl4

1

+ DMF, 120 °C, 48h N

CO2–

N

N

Ru N

N N

FIG. 17 De novo synthesis on Ru(bpy)3-UiO-67(Zr).

Characterization of the resulting material provides support to the anchoring of ruthenium tris(bipyridyl) to the lattice of UiO-67. The resulting Ru(bpy)3-UiO-67 exhibits photocatalytic activity for the dehalogenation of the a-haloacetophenone in acetonitrile solution containing TEOA [59]. As expected based on the stronger bonding of chloride respect to bromide, the photo-dehalogenation reaction is faster and occurs with higher yields for a-bromoacetophenone compared to the a-chlorinated analogs. Available experimental data suggest that the mechanism involves the photocatalytic reduction of the carbon-halogen bond forming the halide anion and an aromatic ketyl radical. This ketyl radical behaves as an electrophilic species attacking electron-rich carbon double bonds. Therefore the photoredox process can be used to promote the photocatalytic addition of a-haloacetophenones to styrene forming a-phenyl butyrophenone as final product in good yield. As expected, the Ru(bpy)3-UiO-67 catalyst can be separated by filtration and reused as photocatalyst in a subsequent run. This study illustrates the advantage of using a highly active photocatalyst in comparison to the soluble analog. Transient absorption spectroscopy has allowed detection of different intermediates generated by light absorption. Quenching evidence suggests a mechanism as that indicated in Fig. 18. As it can be seen, the process is triggered by the strong visible light absorption of the Ru polypyridyl complex and subsequent electron transfer from the ligand to the metal resulting a long-lived charge-separated state. In a similar manner, the MOF MIL-101(Cr)-NH2 has been reported as scaffold of a subphthalocyanine as active unit to perform the catalytic hydrogenative debromination of a-bromoacetophenone to acetophenone under simulated sunlight irradiation (Fig. 19) [60].

9 Conclusions and future perspective The purpose of this chapter is to illustrate the flexibility in the design and preparation of diverse MOFs materials and their use as photocatalysts. Modification of the metal node or organic linker and incorporation of active guests or cocatalysts are strategies that in a predictable way can lead to materials exhibiting photocatalytic activity. In some cases, the efficiency of MOFs as photocatalysts is among the best achieved so far. This chapter briefly summarizes the performance of MOFs for different photocatalytic reactions including solar fuel production, photooxidations, pollutant degradation, and photoredox processes. It is clear that the field will continue progressing in the near future. The use of two-dimensional MOFs with special morphology and small thickness can serve to prepare films with considerable interest for the glass coating in the preparation of photoreactors. These MOF films also open the area of photoelectrocatalysis, the combination of light and electrochemical potential, that still has to develop for MOFs.

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FIG. 18 (i) Photoinduced electron transfer upon 532 nm excitation of Ru(bpy)3-UiO-67 MOF; (ii) photoinduced charge-separated state involving Ru species; (iii) electron migration; (iv) very long-lived charge-separated state (longer than 10 ms), no Ru species involved.

FIG. 19 (A) The structure of SubPc-Cl and its covalent linkage with the -NH2 groups present in the terephthalate organic ligand of MIL-101(Cr)-NH2. (B) The time-conversion plot of the hydrogenative debromination of a-bromoacetophenone to acetophenone using MIL-101(Cr)-NH2, SubPc@MIL-101 (Cr)-NH2, and without any catalyst. Reaction conditions: a-bromoacetophenone (1 mmol), TEOA (10 mL), CH3CN (3 mL), and MOF (10 mg, 5.1  104 mmol SubPc).

Besides two-dimensional MOFs, heterojunction of MOFs with other materials such as inorganic semiconductors or graphene will also lead to improved photocatalytic materials. Special structures of MOFs with core-shell morphology or with hierarchical structure and ordered inverse opal or regular arrays could also increase the photocatalytic efficiency by better light harvesting and efficient vectoral electron transport from one side of a membrane to the other. This type of structuring will also be useful for the construction of photovoltaic devices such as solar cell and light-emitting diodes where photoinduced charge separation like photocatalyst is the key elementary event in the process.

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Thus it can be concluded that even though MOFs have been nowadays consolidated as efficient photocatalyst, the combination of inorganic clusters and organic functionalities together with porosity characteristic of MOFs will be further exploited in the future for the development of novel generations of photocatalysts.

Acknowledgments S.N. thanks the Fundacio´n Ramo´n Areces (XVIII Concurso Nacional para la Adjudicacio´n de Ayudas a la Investigacio´n en Ciencias de la Vida y de la Materia, 2016), Ministerio de Ciencia, Innovacio´n y Universidades RTI 2018-099482-A-I00 project, and Generalitat Valenciana grupos de investigacio´n consolidables 2019 (ref: AICO/2019/214) project for their financial support. Also, the financial support by the Spanish Ministry of Economy and Competitiveness (Severo Ochoa and RTI2018-098237-B-C21) and Generalitat Valenciana (Prometeo 2017-083) is also gratefully acknowledged.

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Chapter 33

Ti-based metal–organic frameworks for visible light photocatalysis Xia Li and Xianjun Lang Sauvage Center for Molecular Sciences, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, China

1 Introduction Nowadays, the shortage of sustainable energy and the demand for a clean environment have stimulated a lot of explorations on the utilization of solar energy. Distinctly, photocatalysis, presenting a green alternative for converting solar energy into chemical energy, can synchronously resolve these two major problems faced by human beings worldwide [1, 2]. Ever since the discovery of the splitting of water under the irradiation of ultraviolet (UV) light into fuels such as H2 and O2 using the TiO2 electrode in 1972 [3], many endeavors have been dedicated to metal oxide semiconductor photocatalysts like TiO2. Due to its high chemical durability, economic viability, and excellent redox ability, TiO2 has stood as the main facet of semiconductor photocatalysis for decades in diverse redox reactions [4–7]. Nevertheless, the wide bandgap of TiO2 (3.2 eV) prompts the inefficient absorption of visible light, instigating undesirable photocatalytic behavior under the irradiation of solar light. Various attempts including doping with metal or nonmetal [8–12], combining with various narrow bandgap semiconductors [13, 14], interfacing with surface complexes [15, 16], etc., have been deliberated to increase the ability of trapping visible light so that one can better the photocatalytic efficiency of TiO2 under the irradiation of solar light. Among them, using organic dyes as ligands to directly harvest visible light or binding with Ti4+ located at the surface of TiO2 via ligand-to-metal charge transfer (LMCT) to extend the light absorption range is one of the most promising approaches, forming inorganic–organic hybrid materials suitable for visible light photocatalysis. Consequently, the optical property of TiO2 was enhanced, which is better for the solar energy conversion via photocatalytic process. By this way, a variety of surface complexes have been fabricated to embark a variety of applications in photocatalysis [17–20]. Thus great attention has shifted toward inorganic–organic hybrid materials. As a newly appeared category of inorganic–organic hybrid functional materials [21], Ti-based metal–organic frameworks (Ti-MOFs) constitutes a significant breakthrough in the development of Ti-containing photocatalysts. Ti-MOFs are intriguing crystalline micro-/mesoporous materials assembled by organic linkers and Ti–oxo clusters via the rigid covalent bonds, which have received significant attention because of their unique structural characteristics of highly ordered porous networks as well as large specific surface areas. In addition, their design flexibility and diversified post-modification strategies make these materials suitable for a variety of potential applications [22–26]. Interestingly, similar to TiO2-based inorganic–organic hybrid materials, the organic linkers in Ti-MOFs are regarded as the antenna to harvest solar light to generate charge-separated states, whereas the metal nodes in Ti-MOFs act as isolated semiconductor quantum dots. Therefore, upon light irradiation, Ti-MOFs can be excited directly or activated by the organic linkers via the linker-toTi cluster charge transfer, which in turn endows them with high potential in photocatalysis. In the past decades, tremendous undertakings have been devoted to Ti-MOFs [26]. Due to structural diversity and tailorability, Ti-MOFs have some exceptional advantages in comparison with traditional semiconductor photocatalysts like TiO2. The discovery of a photoactive Ti-based MOF, namely MIL-125(Ti), was pioneered by Dan-Hardi et al. [27], which implies great capacity of Ti-MOFs as photocatalysts for selective organic transformations. Since then, the expansion of TiMOFs as photocatalysts for organic transformations induced by visible light has attracted increasing attention. Several kinds of Ti-MOFs have been created as visible photocatalysts and the strategies to optimize their visible light-induced activity such as the optical bandgap, charge carrier mobility, and redox ability have also been intensively investigated. However, the development of Ti-MOFs as visible light photocatalysts is still in its infancy as compared with the semiconductor-based ones. Only scant Ti-MOFs have been reported to be applied for selective organic transformations as visible light photocatalysts so far.

Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00001-5 Copyright © 2021 Elsevier Inc. All rights reserved.

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In this context, this chapter focuses on the employments of Ti-MOFs as photocatalysts for organic transformations induced by visible light. Moreover, the corresponding mechanisms and strategies to elevate the visible light absorption and facilitate the charge separation are also discussed to cater to the development of highly adept Ti-MOFs photocatalysts. Finally, the perspective on the challenges and possible direction of travell of Ti-MOFs visible light photocatalysis for selective organic transformations are given out. The summary in this chapter would lay the groundwork for more breakthroughs of Ti-MOFs in visible light photocatalysis to obtain value-added organic compounds under benign conditions.

2

Visible light photocatalysis of Ti-MOFs for selective organic transformations

As the most abundant energy available on the earth, sunlight is currently thought to be the most ideal source of energy for sustainable chemistry. Thus, selective organic transformations by photocatalysis, which depends on sunlight as the energy source to drive organic conversions under much mild reaction conditions, are highly desirable. Hitherto, numerous photocatalysts have been developed with molecular oxygen (O2) as an oxidant. In recent years, Ti-MOFs, novel promising heterogeneous photocatalysts, have brought new incentives. The photoactive Ti–oxo clusters as essential inorganic components in the frameworks. Ti-MOFs show intriguing optical response or photoactivity and have been exclusively studied as fascinating photocatalysts.

2.1 Visible light photocatalytic selective oxidation of amines Imines are key intermediates in the synthesis of pharmaceuticals or fine chemicals. The selective oxidation of amines is the preferable approach to synthesize various imine compounds and has therefore been the subject of continuous research efforts throughout the world. Ti-MOFs are proven to be a class of effective visible light photocatalysts for the selective oxidation of amines. In 2012, by introducing NH2 groups into the organic linker, i.e., 2-aminobenzene-1,4-dicarboxylic acid (BDC-NH2), of MIL-125(Ti), Li and coworkers realized the reduction of CO2 by NH2-MIL-125(Ti) photocatalysis under visible light irradiation [28], the seminal work to initiate visible light photocatalysis of Ti-MOFs. To expand the utilization of NH2-MIL-125(Ti), the same group found that it also applicable for photocatalytic selective aerobic oxidation of amines to imines [29]. A broad scope of amines including benzylamines and heterocyclic amines can be effectively converted to the corresponding imines using O2 over NH2-MIL-125(Ti) under the irradiation of visible light. Furthermore, the NH2-MIL-125(Ti) is quite robust during the photocatalytic process as validated from the similar patterns of X-ray diffraction and the results of the recyclability test. It should be noted that no oxidation ensues for amines that lack Ca–H, manifesting the participation of Ca–H in the imine generation. Mechanistic studies unveil that photogenerated Ti3+ in NH2-MIL-125(Ti) through a pathway of LMCT in which the electron from the visible light excited organic ligand BDC-NH2 travels to the Ti-oxo clusters that can reduce O2 to form superoxide anion (O
2). And then, the O
2  attacks the carbon-centered radical emerged from the amines to yield the corresponding aldehydes. Subsequently, the interaction between aldehydes and the unreacted amines creates the final imine products via dehydration (Fig. 1). This work offers a green and sustainable alternative for the conversions of amines. Interestingly, a later combined experimental and computational study that clarifies the exact effect of the functionality of amine on the Ti-MOF’s optical properties was reported by Walsh and coworkers [30].

FIG. 1 The proposed mechanism of the photocatalytic selective oxidation of amines over NH2-MIL-125(Ti). © From https://doi.org/10.1016/j.apcatb. 2014.09.054

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FIG. 2 The proposed mechanism for the photocatalytic selective oxidative coupling of benzylamines over Ru(bpy)3@MIL125(Ti). © From https://doi.org/10.1016/ j.jcat.2019.08.038

For MIL-125(Ti), the emergence of -NH2 gives rise to the upshift of highest occupied molecular orbital (HOMO), whereas the lowest unoccupied molecular orbital (LUMO) keeps unchanged. When compared the substituents including –NH2, –OH, –CH3, and –Cl with diverse electron-donating ability, it suggests that the bandgaps can be narrowed to a large degree via strong electron-donating ability of the substituents, providing further insights for rational design and construction of efficient Ti-MOFs visible light photocatalysts. In addition, the Ru(II) polypyridyl complex (Ru(bpy)3)2+ could be incorporated into the MIL-125(Ti) for light-harvesting that can capture the wider range of visible light [31]. Due to the improved absorption of visible light and the efficient separation of charge carriers, the resulting photocatalyst Ru(bpy)3@MIL-125(Ti) shows adequate activity and selectivity toward oxidation of amines to imines with aerial O2.This scheme combines the merits of [Ru(bpy)3]Cl2 and MIL-125(Ti), avoiding their shortcomings. Mechanistic studies reveal that, during the photocatalytic process, O
2 and e are the major reactive species (Fig. 2). In this scheme, [Ru(bpy)3]Cl2 worked as a sensitizer to capture visible light and was stimulated to generate Ru2+*. With aerial O2 as an oxidant, via a novel single electron transfer process, Ru2+ was regenerated from Ru+. The O
2 was produced from O2 through photogenerated electron transfer. Consequently, in the reductive quenching cycle, the as-formed Ti3+ was transformed to Ti4+. Meanwhile, benzylamine was oxidized into PhCH2NH
2 +. The generated benzylamine radical cation PhCH2NH
2+ then reacted with O
2 to produce an imine intermediate via a dehydrogen process. The efficient energy transfer to O2 is conductive to the photooxidative coupling of benzylamines into imines after the removal of the side product ammonia. The proposed mechanism was confirmed with an additional proof that the benzaldehyde generated during the photocatalytic process has been promptly proved by gas chromatography–mass spectrum. During the reaction, the interaction with substrate could be facilitated by pores of MIL-125(Ti). Recently, Pd/NH2-MIL-125(Ti) nanoparticles (NPs) were integrated by fixing ultrasmall Pd nanocrystals (420nm) O2 O2·–

e– LUMO

e–

e– –0.52ev

–0.43ev

CdS MIL-125(Ti) O2·– HOMO

h+

R-CH2OH•

R-CHO

h+ h+ R-CH2OH FIG. 10 The possible mechanism of the photocatalytic selective oxidation of benzyl alcohol to benzaldehyde over CdS/MIL-125(Ti). © From https://doi. org/10.1039/C7PP00073A

CdS can produce charge carriers upon visible light irradiation. Due to the close interfacial contact of MIL-125(Ti) and CdS, as well as matched band positions, the photogenerated electrons of CdS can efficiently transfer to MIL-125(Ti) and then reduce O2 to generate O
2 . The benzyl alcohol was oxidized by the hole to produce the related carbocation radical. Subsequently, the O
2  reacts with the carbocation radical to generate benzaldehyde. Very recently, by a novel and facile seed growth method, a sequence of covalent-connected Ti-MOF/covalent organic framework (COF), NH2-MIL-125(Ti)@TAPB-PDA [1,3,5-tris(4-aminophenyl) benzene-terephthaldehyde], with diverse shell thickness of TAPB-PDA have been synthesized and systematically investigated [40]. The introduction of suitable amount of COF could adjust the inherent optical and electronic properties, therefore improving the photocatalytic performance distinctly. The NH2-MIL-125(Ti)@TAPB-PDA-3 showed about 2.5 times of conversion of benzyl alcohol than that of NH2-MIL-125(Ti) with excellent cycling durability. The enhanced photocatalytic activity was explained by the enhanced transfer of photogenerated charge carriers between the COF and NH2-MIL-125(Ti) through the covalent bond (Fig. 11). To be specific, COF was stimulated by visible light irradiation to produce electrons and holes because of its innate small bandgap. Then part of photogenerated electrons of COF flow to the NH2-MIL-125(Ti) for its lower LUMO than that of COF, which then reduce O2 to form O
2 . While the holes accumulated in the COF could oxidize benzyl alcohol to carbon-centered radicals. Eventually, the target benzaldehyde was obtained by the interaction between the carbon-centered benzyl radical and O
2 . Besides, the combination of carbon materials with NH2-MIL-125 may also be adopted to enhance their activities for visible light photocatalytic selective oxidation of alcohols [41]. It was found that a sandwich-like hierarchical nanoarchitecture constituted of photoactive MOF materials and reduced graphene oxide (rGO) exhibited excellent performance for the selective oxidation of benzyl alcohol. The mechanistic studies reveal that introducing rGO enhances the photocatalytic activity of NH2-MIL-125(Ti) regardless of the groups on the phenyl ring. Promoted by extraordinary carrier mobility of graphene, the separation of charge carriers was better. Therefore, under visible light motivation, photoinduced electrons rapidly transfer to the graphene layer and then reduce O2 to form O
2. At the same time, aromatic alcohols are turned into radical carbonium cations by the way of deprotonation, which then interact with O
2  to generate aromatic aldehydes correspondingly. Although many efforts concentrate on the decoration of existing Ti-MOFs to enhance their photocatalytic performance for organic transformations, more endeavors have also been dedicated to the development of new photocatalysts

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FIG. 11 The proposed mechanism of the photocatalytic selective oxidation of benzyl alcohol over NH2-MIL-125@TAPB-PDA-3. © From https://doi.org/10.1016/j.jechem.2019.07.014

of Ti-MOFs. Among them, PCN-22 (PCN ¼ porous coordination network) was fabricated with demonstrable activity for the photocatalytic oxidation of alcohol with a wide visible light response [42]. To mimic porphyrin-sensitized TiO2 in a MOF matrix, PCN-22 was prepared from porphyrinic ligands tetrakis(4-carboxyphenyl)porphyrin (TCPP) and preformed Ti–oxo carboxylate clusters. In the PCN-22, the photoactive sites are the Ti–oxo clusters, whereas the ligand TCPP extends the light absorption range. In the end, a PCN-22/TEMPO (TEMPO ¼ 2,2,6,6-tetramethylpiperidinyloxy) system for photocatalytic oxidation of alcohol was designed, mimicking dye/TiO2/TEMPO system. Under visible light irradiation for 2 h, the conversion of benzyl alcohol reached 28% with high selectivity nearly 100% to benzaldehyde. Furthermore, PCN-22 can be easily recovered by centrifugation and reused three times with no obvious loss in activity and selectivity. A possible photocatalytic mechanism was proposed in Fig. 12. Stimulated by the visible light, the TCPP linkers in PCN-22 donate electrons to Ti7O6 clusters, furnishing [TCPP]+. Next, [TCPP]+ oxidize TEMPO to TEMPO+ which, through a pathway of two-electron transfer, transforms alcohol into aldehyde. To date, PCN-22 is the first single crystalline Ti-MOF constructed from a tetracarboxylate linker, a significant breakthrough in the designed synthesis of highly crystalline Ti-MOFs. FIG. 12 The proposed mechanism for the PCN-22/ TEMPO system. © From https://doi.org/10.1039/ c5sc00916b

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2.3 Miscellaneous photocatalytic selective transformations Apart from the above mentioned organic transformations, many other types of transformations can also be achieved by photocatalysis of Ti-MOFs. Recently, NH2-MIL-125(Ti)/TiO2 with different molar ratios were synthesized by a onepot solvothermal approach for visible light-induced selective oxidation of cyclohexane [43]. The NH2-MIL-125(Ti)/ TiO2 showed three times higher photocatalytic performance than that of NH2-MIL-125(Ti). The superior photocatalytic activity is attributed to the matched energy band structure and close interfacial contact between TiO2 and NH2-MIL125(Ti), which can promote the transfer of photogenerated carriers and reduce the recombination of electron–hole pairs. The mechanistic studies reveal that the photogenerated holes are greatly involved in the oxidation process, and the NH2-MIL-125(Ti) can be regarded as a sensitizer for Aeroxide P25 TiO2 (Fig. 13). The organic linker BDC-NH2 in NH2-MIL-125(Ti) works as an antenna to harvest visible light, leading to the appearance of the electron and hole. The photoinduced holes then oxidize cyclohexane (C6H12) to generate cyclohexyl radical (C6H11%) while the photoelectrons transfer to the surface of Aeroxide P25 TiO2 from NH2-MIL-125(Ti) and then were trapped by the dissolved O2 to form O
2 . Hydrogen peroxide (H2O2) can further accept electron to produce hydroxyl radical (%OH). The O
2  and %OH are the major reactive oxygen species for the transformation of cyclohexane, which can react with cyclohexyl radical to produce alcohol and ketone via an intermediate of peroxyl radical (C6H10OO% ). Furthermore, the reusability of the NH2-MIL-125 (Ti)/TiO2 was demonstrated by the consecutive four-time recycled experiments. More interestingly, it was found that NH2-MIL-125(Ti) can also be applied for visible light photocatalytic reduction reactions. When triethanolamine (TEOA) was used as the hole scavenger, photocatalytic debromination of decabromodiphenyl ether (BDE 209) was successfully observed under visible light irradiation [44]. Further low-temperature ESR experiments indicated that TEOA engages in the evolution of Ti3+, which shuttle the electron to BDE 209, rendering the products of reductive debromination. Additionally, a MOF@MOF [ZIF-67@NH2-MIL-125 (ZIF ¼ zeolitic imidazolate framework)] prepared via the precipitation approach using 2-methylimidazole–MeOH (MeOH ¼ methanol) solution was proven to be a robust visible light photocatalyst for the efficient reduction of 4-nitrophenol and can be reused without loss its activity, showing superior activity and selectivity than Aeroxide P25 TiO2 [45]. In addition, MIL-125(Ti)/Ag/g-C3N4 heterostructure was promptly fabricated via a facile photodeposition and benign chemical process, which can concurrently execute the photocatalytic reduction of nitro compounds and the oxidation of alcohols (Fig. 14; [46]). The Ag NPs was evenly photodeposited on the surface of g-C3N4 and MIL-125(Ti), improving the absorption of visible light by virtue of the surface plasmon resonance. Moreover, the Ag NPs play a key role as a bridge of electron conduction between g-C3N4 and MIL-125(Ti)/Ag, resulting in efficient separation of photoinduced charge

FIG. 13 The possible mechanism of photocatalytic selective oxidation of cyclohexane over NH2-MIL-125/P25 under visible light irradiation (l 420 nm). © From https://doi.org/10.1016/j.mcat.2018.04.004

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FIG. 14 The proposed mechanism for the photocatalytic process by MIL-125/Ag/g-C3N4. © From https://doi.org/ 10.1016/j.apcatb.2016.12.012

carriers. Therefore the MIL-125/Ag/g-C3N4 photocatalyst displayed the notably enhanced photocatalytic performance compared with the g-C3N4, parent MIL-125(Ti), MIL-125(Ti)/Ag, as well as MIL-125(Ti)/g-C3N4. Furthermore, the MIL-125(Ti)/Ag/g-C3N4 photocatalyst exhibited excellent reusability upon visible light irradiation. Recently, Ti-MOFs could act as efficient photocatalysts in polymerization reactions. Two-dimensional (2D) MOF-901 was synthesized by integrating the chemistry of MOFs and COFs [47]. This method contains in situ formation of an aminefunctionalized Ti–oxo cluster, Ti6O6(OCH3)6(AB)6 (AB ¼ 4-aminobenzoate), which was linked with benzene1,4-dialdehyde via imine condensation reactions of dynamic organic covalent bond formation, typical chemistry of COFs. The MOF-901 is constituted with hexagonal porous layers that are likely stacked in staggered conformation (hxl topology). The incorporation of Ti4 + engenders MOF-901 effective photocatalyst in the polymerization of methyl methacrylate (MMA). The resulting poly-MMA was of a high number-average molar mass as well as low polydispersity index. The MOF-901 is a better photocatalyst than Aeroxide P25 TiO2. Additionally, the MOF-901 photocatalyst can be recycled without obvious decrease in activity. Subsequently, a waterstable and chemically durable Ti-MOF, MOF-902 was fabricated using the in situ cluster formation technique to further engineering the optical absorption [48]. MOF-902, with larger conjugation linking blocks, is an isoreticular structure to MOF-901. MOF-902 displayed greatly better photocatalytic activity than MOF-901 in the polymerization reaction. The photocatalytic property of MOF-902 also outdo Aeroxide P25 TiO2 and the other MOFs possessing similar bandgaps.

3 Concluding remarks In this chapter, the recent advances on photocatalytic organic transformations over Ti-MOFs under visible light irradiation have been summarized. Moreover, the mechanisms and strategies to enhance the absorption of visible light and boost the separation of charge are also discussed to shed light on the development of highly efficient photocatalysts of Ti-MOFs. From the above discussions, one can conclude that Ti-based MOFs are promising photocatalysts for visible light-induced organic transformations. However, most investigations were mainly concentrated on the modification of the two accessible Ti-MOFs, namely MIL-125(Ti) and NH2-MIL-125(Ti), for enhancing the photocatalytic performance. Therefore, it is of great challenge and significance to synthesize novel Ti-MOFs with photoactive organic ligands for visible light-induced photocatalytic selective organic transformations. Moreover, in consideration of 2D ultrathin materials have more exposed active sites, short transport path of the charge carriers, and enlarged light penetration, it is conceivable that 2D Ti-MOFs should be prepared for photocatalytic organic transformations. In a nutshell, this work would pave the way for more breakthroughs to obtain value-added organic compounds by visible light photocatalysis of Ti-MOFs.

Acknowledgments Financial support from the National Natural Science Foundation of China (Grant numbers 21773173 and 22072108) and the start-up fund of Wuhan University is gratefully acknowledged.

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Chapter 34

Homogeneous photocatalysts immobilized on polymeric supports: Environmental and chemical synthesis applications Jovana R. Prekodravaca, Vaishakh Nairb, Dimitrios A. Giannakoudakisc, Sam Hseien-Y. Hsud, and Juan C. Colmenaresc a


Institute of Nuclear Sciences—National Institute of the Republic of Serbia, University of Belgrade Belgrade, Belgrade, Serbia, Vinca

b

National Institute of Technology Karnataka, Mangalore, India, c Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland,

d

School of Energy and Environment, Hong Kong, China

1 Introduction Photocatalysis has become a substantial field of research in the past 20 years, dealing with different chemical transformation reactions, such as degradation of harmful pollutants [1–5] and synthesis of a wide range of organic chemicals [6–9], following the principles of green chemistry for synthetic chemistry. Though the significant results and research outcomes related to photocatalysis, the commercialization of this technique is still making a slow progress. The challenges in application of heterogeneous photocatalyst, which can perform chemical reactions under visible light [10], are recognized as the dominant reason to retard the upscaling of photocatalytic processes. Many research groups in the area of photocatalysis have been focusing on the development of adequate homogeneous photocatalyst with upscaling properties, including visible light-driven systems relating photoredox processes to mimic the natural photosynthesis [11]. However, a number of disadvantages associated with homogeneous catalytic systems still affect the long-term usage [12, 13]. Some of the most important advantages and challenges regarding homogeneous and heterogeneous photocatalysts promoting or restricting their wider application are presented in the Table 1. Consequently, it is beneficial to develop a new class of homogeneous photocatalyst with enhanced stability and reusability [11]. Clearly, a straightforward solution for this challenge is the immobilization of homogeneous photocatalyst onto stable solid-state materials, resulting in heterogeneous-supported photocatalyst. This immobilized homogeneous photocatalyst features both advantages of homogeneous (high activity and selectivity) and heterogeneous catalysts (stability, ease separation from the reaction medium, and the possibility of reuse in subsequent reaction cycle). In this regard, several types of material such as silica, polymers, metal-organic framework, as well as some carbon-based supports have been used for this purpose [14–16]. Polymer-based supports can be further classified as synthetic polymer and biopolymer supports, which found to be stable and easy to modification for immobilizing the homogeneous photocatalyst. Synthetic polymers such as polypropylene (PP), polystyrene (PS), poly(methyl methacrylate) (PMMA), and polydimethylsiloxane, among others evolved due to many advantages from the economical point of view, high stability, and wider range of availability, aiming to improve the photocatalytic efficiency [17]. Therefore developing a system of synthetic polymer support for immobilization of homogeneous photocatalyst is the next level of research for carrying out heterogeneous visible light photocatalysis for selected chemical reactions [18–22]. In the past several years, the immobilization of homogeneous photocatalyst on biopolymer supports is progressing [23, 24]. Besides the biodegradability and nontoxicity, biopolymers provide functional groups helpful in attaching pollutants onto the photocatalyst surface selectively and very effectively, helping in photocatalyst dispersibility and recovery [25]. Common biopolymers such as cellulose, chitosan, and lignin have been already applied in different photocatalytic applications using semiconductor-based composite; however, its application as substrates for immobilization of homogeneous photocatalyst is still restricted and not fully explored. Materials Science in Photocatalysis. https://doi.org/10.1016/B978-0-12-821859-4.00002-7 Copyright © 2021 Elsevier Inc. All rights reserved.

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TABLE 1 Homogeneous versus heterogeneous photocatalysts. Homogeneous photocatalysts

Heterogeneous photocatalysts

Same phase as reactants and products (liquid phase)

Different phase as reactants and products (liquid/gas/solid)

Require lower temperature operations

Reactions possible under high temperature

Higher activity

Lower activity

Higher selectivity

Lower selectivity

Easy heat transfer

Heat transfer can be an issue

Difficult catalyst separation

Easy catalyst separation

Expensive recycling

Simple recycling

Well-understood reaction mechanism

Poorly understood reaction mechanism

The aim of this chapter is to introduce to the readers the novel approach toward developing a heterogeneous photocatalytic system based on the immobilization of well-known homogeneous photocatalyst such as organometallic complexes, organic dyes, and polyoxometalates (POMs) on synthetic and biopolymer-based supports. The classification of various immobilization techniques based on the support type, method for preparation, and the photocatalytic application is provided. Finally, the prospect and limitation for the wider range of applications of immobilized homogeneous photocatalyst in chemical transformations and environmental remediation are provided.

2

Homogeneous photocatalysts

In general, industrial application of homogeneous catalytic process is minimal in comparison to heterogeneous catalytic process; however, its superior catalytic activity has resulted in significant interest among researchers working in the field of catalysis. Homogeneous photocatalyst such as organometallic complexes, POMs, and organic dyes, have been extensively studied in photochemistry since these catalysts can absorb visible light, allowing the reaction to be carried out under a much milder conditions compared with the ultraviolet photochemistry. Moreover, in such a catalytic process, there is a limited scattering of the incident light thereby simplifying the photon transport phenomena. The homogeneous photocatalytic process represents one of the possible pathways for photochemical degradation of emerging contaminants [26–31] in natural waters as well as organic synthesis reactions [32–34]. The organometallic (coordination) complexes consisted of central transition metal ions such as ruthenium (Ru), iridium (Ir), palladium (Pd), iron (Fe), rhenium (Re), rhodium (Rh), cobalt (Co), nickel (Ni), manganese (Mn), copper (Cu), or chromium (Cr) bonded to organic ligands play an important role in homogeneous catalysis [34]. Based on ligand type, different complexes can be produced using the same transition metal center as their reactivity is significantly influenced by the nature of surrounding ligands. The photo reactivity of several organometallic complexes of Fe, Cu, and Cr showed vital environmental significance, subsequently they can generate reactive oxygen species necessary for different redox reactions in pollution remediation [35–40]. In recent years, several works reported employing eosin Y in photoredox catalysis to accelerate the development of new strategies enabling the formation of carbon-carbon and carbon-heteroatom bonds [41]. With this in mind, eosin Y was employed for direct carbon-hydrogen (CdH) arylation of heteroarenes with aryl diazonium salts as well as carbon-carbon (CdC) and carbon-phosphor (CdP) bond formation [42]. On the other hand, rose bengal (RB) was used for oxyamination, coupling, and acylnitroso ene reactions. Methylene blue (MB) is another organic photosensitizer widely used in both biology and chemistry, most notably for its ability to generate singlet oxygen (1O2) [43, 44]. Now, POM as photocatalyst has several advantages comparing to other photoredox catalysts including organometallic complexes and organic dyes. POMs as homogeneous photocatalyst have attracted significant attention due to their photoinduced properties such as charge transfer, redox, and acid based, as they evoke selective transformations of different organic functional groups. POM-based photocatalyst has been applied to extensive range of reactions such as hydrogen (H2) and oxygen (O2) evolution, reduction of carbon dioxide (CO2) and metals, and the degradation of organic pollutants and dyes. Photocatalytic aerobic oxidation of organic substrates, bond formation carbon-nitrogen (CdN), carbon-fluorine (CdF), and CdC bonds, as well as dehydrogenation reaction of alkenes are only some of potential applications of POM photocatalyst in organic synthesis [26, 28, 33, 45].

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TABLE 2 Synthetic versus biopolymers as supports for photocatalysts immobilization. Synthetic polymers

Biopolymers

Low production costs

High production costs

Decreasing availability

Increasing availability

High flexibility, strength, and resistivity

Low physicochemical resistance

High thermal stability

Not stable under higher temperatures

Nondegradable

Degradable

Mostly nonrenewable

Mostly renewable

Low sustainability

High sustainability

Alongside with many impressive application possibilities for all of the mentioned homogeneous photocatalysts, several drawbacks are still limiting their wider application. The most important challenge is the recovery of photocatalyst after reaction, which could lead to prevention of leakage and loss of photocatalyst into the environment. On the other hand, one of the biggest issues from the economical point of view is the reusability of a photocatalyst in several reaction cycles. Among various explored approaches by the research community for solving these issues, the immobilization of a homogeneous photocatalyst on solid supports was found to be a promising solution. Heterogenization of homogeneous photocatalyst through immobilization on solid supports has been carried out on synthetic polymeric materials due to their extraordinary chemical, physical, and mechanical properties. However, the usage of synthetic polymeric materials as supports in photocatalysis makes a disbalance from the environmental point of view. From one side, these polymeric composite materials have slow decomposition and nonrenewable properties that leads to the accumulation of waste materials in the environment, but from the other side, this slow degradation can help in making a more stable composite catalyst that can be reused in several reaction cycles, and additionally, some of those composites can be prepared by recycling plastic wastes. The increase in environmental pollution on global scale has drawn attention of researchers toward developing safe and clean technologies and economical processes that can be used for sustainable energy and environmental applications. Recently, renewable raw materials such as biopolymeric materials have shown to be a possible replacement for synthetic polymer-based supports for immobilization of homogeneous photocatalyst due to their availability and ease of modification. In addition to the role of catalyst support, these natural materials have shown to enhance the photocatalytic reaction, especially by providing higher surface area and active sites for adsorption of reactants. There are however still some limitations on this topic that need further studies (Table 2).

3 Immobilization of homogeneous photocatalyst The immobilization of homogeneous photocatalyst on solid supports leads to heterogenization, which is in contradiction to the nature of homogeneous catalysis characterized by a molecular dispersion within the reaction mixture. However, advantages from the immobilization of homogeneous photocatalyst (better separation of the catalysts from the reaction medium and reusage of the same material) are beneficial in the synthesis of chemical/pharmaceutical products as well as in the remediation of different environmental contaminations. The topic of immobilization has been the subject of several hundreds of publications, where some publications are more general, while others focused on the use of a certain type of support as well as connection between photocatalyst and supporting material [46, 47]. The immobilization techniques are of great importance since the connection between homogeneous photocatalyst and substrate affects not only the activity of the photocatalyst but also the reusability ability. Yang et al. [48] reported the concept related to a reversible catalyst by supporting catalyst on polymeric support like PS through hydrogen bonding-mediated self-assembly in polymerization reaction of methyl methacrylate (MMA). The PS support could release the catalyst/ligand complex as free molecules at raised temperatures (60°C) but readsorb the catalyst after the MMA polymerization for separation. Covalent immobilization is the most widely used method for heterogenization (immobilization) of homogeneous photocatalyst [49, 50]. One of the examples is covalently attached Re-bipyridine complex [Re(bpy) (CO)3Cl] on solid-state surface through simple organic linkages that has excellent stability under visible light and high activity toward CO2 reduction. Comparing the same complex [Re(bpy)(CO)3Cl] immobilized only by physisorption, lower

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stability and photocatalytic activity of the system were observed [51]. Due to versatility of the immobilized catalyst (metalloporphyrin) found by Yang et al. [48], the photocatalyst immobilized on cellulose support could be easily recovered from the mixture during catalytic reaction after controlled radical polymerization of diverse monomers under visible light. Reusage for three reaction cycles without sacrificing catalytic activity was observed, which could make it potentially applicable to other functionalized catalysts and hydroxyl (OH) group abundant supports for diverse applications [52]. Noncovalent immobilization in the form of electrostatic interactions, adsorption, or trapping of the catalyst inside the support pores proved to be one more interesting way for immobilization since it does not require preliminary modifications of a catalyst surface prior to immobilization. For this purpose, several different studies were performed to immobilize Pd, Rh, Ru, and other transition metal complexes containing aromatic ligands on active carbon support [53]. The catalytic activity of a given homogeneous catalyst should not be affected by immobilization. Moreover, the immobilization should allow the diffusion of reaction mixture components in and out while catalyst remains confined yet able to perform translation and rotation motions at least on a local scale.

3.1 Homogeneous photocatalysts immobilized on synthetic polymers Selection of the suitable polymer support for photocatalyst immobilization depends on the polymer stability and chemical inertness to the reaction conditions as well as low price and accessibility [54]. Organometallic complexes have been immobilized on different synthetic polymers for various applications. Examples are complexes of Ru, Pd, and Rh immobilized on polymer supports like PS, PP, and others [55–57]. These modified photocatalysts have been successfully used in urea synthesis through reductive carbonylation, and it was observed that the type of substrate used in the immobilization method can influence the catalytic activity [55]. From the aspects of alleviation of climate change, photoreduction transformation of CO2 into valuable chemicals has received considerable attention. Among many photocatalytic complexes such as Ir(III), Re(I), Ru(II), and Co(II), the Re complexes enable highly efficient and selective production of carbon monoxide (CO). However, these complexes exhibit some drawbacks regarding absorption in the visible range and catalyst stability that could be solved by heterogenization of Re complexes on porous organic polymer (POP) [58]. The CO2 conversion into chemical feedstocks is an opportunity to produce greener materials and reduce CO2 from the atmosphere. Several reports from the literature highlighted the usage of [Re(bpy)(CO)3Cl] as a photocatalyst for reduction of CO2 to CO. Liang et al. [58] synthesized an [(a-diimine)-Re(CO)3Cl] photocatalyst (Fig. 1) and immobilized it over POP. Photocatalytic CO2 reduction under visible light irradiation, in the presence of obtained composite material, led to generation of CO and H2 (from the adventitious water present). The slow formation of CO at the beginning led to stable production rate after 100 min of irradiation. Another research group reported CO2 photoreduction conversion to CO under visible light by immobilization of Re(I) complex on PP-based porous polycarbazoles. The CO production rate of 623 mmol/g/h was achieved [57]. The PS-supported Schiff base complexes of Co(II), Cu(II), Fe(III), Ni(II), and Mn(II) as heterogeneous catalyst were reported in several papers as appropriate reusable and green catalyst for oxidation of alcohols, olefins, and cyclohexene by using hydrogen peroxide (H2O2) or tert-butyl hydroperoxide as oxidants [56, 59, 60]. Today visible light photocatalysis is mostly focused on photoredox reactions involving alcohol oxidation, CdH activation, H2 evolution, and others [61–63], with very few reports on CdC or carbon-heteroatom bond forming reactions (cross-coupling reactions). Among various cross-coupling reactions, Suzuki-Miyaura and Mizoroki-Heck reactions are the most widely used reactions in organic chemistry for CdC bond formation due to the ease in performance, mild reaction conditions, and high reaction yield. These reactions are usually performed using homogeneous photocatalyst based on Pd complexes [64]. As stated before, the challenge faced in the recovery of the homogeneous catalyst leads in this case to leakage of costly and toxic Pd catalyst into the environment. Immobilization and stabilization of Pd catalyst on substrates, such as conjugated microporous polymer (CMP), lead to formation of efficient visible light photocatalyst for Suzuki coupling at room temperature. In the work carried out by Wang et al. [65], polymer backbone structure of poly(benzoxadiazole) network (B-BO3) was used as a semiconducting polymer support for Pd nanoparticle immobilization [Pd@B-BO3] to perform the coupling reaction of various aryl halides with phenylboronic acid under visible light. After 2 h of irradiation, the reaction of iodobenzene with phenylboronic acid produced a quantitative yield of 98% of the desired product (biphenyl), while reactions in dark conditions or without the presence of Pd led only to the trace yield of desired product. Comparing yield to other aryl halides, Wang et al. [65] concluded that the iodides convert more readily comparing to bromide, while the presence as well as the position of electron donating or withdrawing groups affect the conversion rate. The proposed reaction mechanism presented in Fig. 2 shows the formation of electron (e–) and hole (h+) on the semiconducting B-BO3. The generated e
migrates toward Pd active center and attacks the carbon-iodine (CdI) bond in iodobenzene forming aryl complex with Pd. Meanwhile, generated h+ activates the carbon-boron (CdB) bond of

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FIG. 1 The synthesis of [(a-diimine)Re(CO)3Cl] photocatalyst (1 POPs; 1-Re POP/[Re(CO)5Cl]). (© From Site isolation leads to stable photocatalytic reduction of CO2 over a rhenium-based catalyst. Chem. Europ. J. 12(51) (2015) 18576–18579. https://doi.org/10.1002/chem.201502796.)

phenylboronic acid forming another aryl-Pd complex. In the final step, formed aryl-Pd complexes progress to the formation of desired product, biphenyl. In another work, Jiang et al. [66] found that integration of organic dye, as a homogeneous photocatalyst, into the skeleton of CMP resulted in the formation of highly porous polymer photocatalyst thereby enhancing the performance of many photocatalytic organic reactions due to high surface area of >830 m2/g and many useful chemical and physical properties. RB dye as a homogeneous photocatalyst of low cost and low toxicity was incorporated into the skeleton of CMP by Pd-catalyzed Sonogashira-Hagihara cross-coupling polycondensation reaction which led to the formation of polymer photocatalysts RB-CMP (Fig. 3), with high activity in heterogeneous photocatalytic aza-Henry reactions (>97%). The recyclability and photocatalytic stability of the polymer photocatalyst were examined for at least 10 reaction cycles and