Titanium Dioxide (TiO2) and Its Applications 0128199601, 9780128199602

Table of contents :
Title-page_2021_Titanium-Dioxide--Tio---and-Its-Applications
Titanium Dioxide (TiO2) and Its Applications
Copyright_2021_Titanium-Dioxide--Tio---and-Its-Applications
Copyright
Contents_2021_Titanium-Dioxide--Tio---and-Its-Applications
Contents
List-of-contributors_2021_Titanium-Dioxide--Tio---and-Its-Applications
List of contributors
About-the-series-editor_2021_Titanium-Dioxide--Tio---and-Its-Applications
About the series editor
About-the-editors_2021_Titanium-Dioxide--Tio---and-Its-Applications
About the editors
Preface-to-the-series_2021_Titanium-Dioxide--Tio---and-Its-Applications
Preface to the series
Preface-to-the-volume_2021_Titanium-Dioxide--Tio---and-Its-Applications
Preface to the volume
1---Introduction_2021_Titanium-Dioxide--Tio---and-Its-Applications
1 Introduction
1.1 Economic aspects
1.2 Summary of the book
References
2---Properties-of-titanium-diox_2021_Titanium-Dioxide--Tio---and-Its-Applica
2 Properties of titanium dioxide
2.1 Introduction
2.2 Structural properties
2.2.1 Structures of TiO2
2.2.2 Main techniques used for TiO2 structural analysis
2.3 Structure and defects
2.3.1 Defectivity
2.3.1.1 Point defects
2.3.1.2 Line defects
2.3.1.3 Interfacial defects
2.3.1.4 Bulk defects
2.3.2 Surface defectivity
2.3.2.1 O vacancies
2.3.2.2 Ti defects
2.3.2.3 H defects
2.3.3 Surface and lattice distortion
2.4 TiO2 morphologies
2.5 Thermodynamic properties
2.6 Electronic properties
2.7 Electrical properties
2.8 Optical properties
2.9 Photon-induced behavior
2.10 Mechanical and rheological properties
2.10.1 Mechanical properties
2.10.2 Rheological properties
References
3---Structural-and-electronic-properties-of-Ti_2021_Titanium-Dioxide--Tio---
3 Structural and electronic properties of TiO2 from first principles calculations
3.1 Introduction
3.2 Electronic structure calculations on TiO2: methodological aspects
3.2.1 The bandgap issue
3.2.2 Excess electrons (and holes) in TiO2: the localization problem
3.2.3 Oxygen vacancies
3.2.4 Interstitial Ti species
3.2.5 Photoexcited carriers
3.3 Titania heterojunctions and nanoparticles: computational modeling of cutting-edge materials
3.3.1 Separation of photoexcited charge carriers in titania nanocomposites
3.3.2 Computational modeling of titania nanoparticles
3.4 Conclusions
Acknowledgments
References
4---Synthesis-and-characterization-of-titanium-d_2021_Titanium-Dioxide--Tio-
4 Synthesis and characterization of titanium dioxide and titanium dioxide–based materials
4.1 Introduction
4.2 Preparation methods
4.2.1 Preparation methods of powdered TiO2-based materials
4.2.1.1 Sol–gel
4.2.1.2 Precipitation and coprecipitation
4.2.1.3 Hydrothermal and solvothermal syntheses
4.2.1.4 Sonochemical method
4.2.1.5 Microwave irradiation
4.2.1.6 Spray pyrolysis
4.2.1.7 Impregnation
4.2.1.8 Deposition–precipitation
4.2.2 Preparation methods of TiO2 film
4.2.2.1 Dip-coating
4.2.2.2 Spin–coating
4.2.2.3 Chemical vapor deposition
4.2.2.4 Physical vapor deposition
4.3 Characterization techniques of TiO2
4.3.1 X-ray diffraction
4.3.2 Scanning electron microscopy
4.3.3 Transmission electron microscopy
4.3.4 Brunauer–Emmett–Teller-specific surface area determination
4.3.4.1 Adsorption–desorption phenomena
4.3.4.2 Brunauer–Emmett–Teller isotherm
4.3.4.3 Brunauer–Emmett–Teller surface area determination
4.3.4.4 The preparation of TiO2 samples
4.3.4.5 Used gases for Brunauer–Emmett–Teller analysis
4.3.4.6 Brunauer–Emmett–Teller instrument and its working principle
4.3.5 Diffuse reflectance spectroscopy
4.3.6 Photoluminescence spectroscopy
4.3.7 X-ray photoelectron spectroscopy
4.3.8 Thermal gravimetric analysis
References
5---Synthetic--natural-and-bioinspired-dyes-as-T_2021_Titanium-Dioxide--Tio-
5 Synthetic, natural and bioinspired dyes as TiO2 sensitizers in sustainable solar cells
5.1 Introduction
5.1.1 Photovoltaic technology
5.1.2 Dye-sensitized solar cells
5.2 Semiconductors
5.2.1 Bands formation
5.2.2 The occupation of the orbitals
5.2.3 Titanium dioxide
5.3 Dyes
5.3.1 Synthetic dyes
5.3.2 Natural dyes
5.3.2.1 Anthocyanins
5.3.2.2 Betalains
5.3.2.3 Chlorophylls
5.3.2.4 Other vegetable dyes
5.3.3 Computational details
5.3.4 Bioinspired
5.4 Other functional materials
5.4.1 Characteristics and performance of CEs
5.4.2 Characteristics and performance of electrolytes
5.5 Assembly and characterizations for DSSCs
5.5.1 Development of photoanodes and cathodes
5.5.2 Spectroscopic techniques
5.5.2.1 Raman spectroscopy: TiO2
5.5.2.2 UV-vis and TiO2 emission spectroscopy
5.5.3 Cyclic voltammetry
5.5.4 Roughness and desorption factor
5.5.5 Characteristic I-V curves
5.5.6 Quantum efficiency: IPCE, APCE, and LHE
5.5.7 Electrochemical impedance spectroscopy
5.5.8 Tafel electroanalysis
5.6 Conclusions
References
6---TiO2-based-materials-for-photocatalyti_2021_Titanium-Dioxide--Tio---and-
6 TiO2-based materials for photocatalytic hydrogen production
6.1 Introduction
6.2 Photocatalytic water splitting with TiO2
6.3 Development of sensitive TiO2-based photocatalysts for H2 generation
6.3.1 Bandgap engineering
6.3.2 Surface TiO2 sensitization
6.4 Separation of photogenerated charges in TiO2-based photocatalysts for H2 generation
6.4.1 Charge separation in TiO2 phase junctions
6.4.2 Charge separation in shape-controlled anatase TiO2
6.4.3 Noble metal nanoparticles deposition and Schottky junction fabrication
6.4.4 Fabrication of heterojunctions
6.4.5 Loading cocatalysts on TiO2
6.5 Sacrificial agents in photocatalytic hydrogen production: from overall water splitting to biomass reforming
6.6 Conclusion and perspectives
References
7---TiO2-based-devices-for-energy-relat_2021_Titanium-Dioxide--Tio---and-Its
7 TiO2-based devices for energy-related applications
7.1 Introduction
7.1.1 Titanium dioxide for energy harvesting
7.1.2 Titanium dioxide for energy storage
7.2 Energy storage applications
7.2.1 Supercapacitors
7.2.1.1 Significance of TiO2 polymorph (brookite, anatase, rutile)
7.2.1.2 Significance of nanostructures (nanotubes, nanorods, nanowires)
7.2.2 Batteries
7.2.2.1 Significance of TiO2 polymorph (brookite, anatase, rutile)
7.2.2.2 Significance of nanostructures (nanotubes, nanorods, nanowires)
7.2.3 Hydrogen production and storage
7.2.4 Others
7.3 Conclusion and outlook
References
8---Heat-transfer-by-using-TiO2-nan_2021_Titanium-Dioxide--Tio---and-Its-App
8 Heat transfer by using TiO2 nanofluids
List of abbreviations
8.1 Introduction
8.2 Preparation and characterization of TiO2 nanofluids
8.2.1 Nanoparticles preparation
8.2.2 Preparation of nanofluids
8.2.3 Parameters influencing the aggregation and stability of TiO2 nanofluids
8.2.4 Nanoparticle size measurements
8.2.5 Z-potential measurements
8.2.6 pH measurements
8.3 Heat conduction in TiO2 nanofluids
8.3.1 Influence of particle load
8.3.2 Influence of temperature
8.3.3 Influence of thermal conductivity of the base fluid
8.3.4 Influence of particle cluster size and shape on thermal conductivity
8.3.5 Influence of surfactant
8.3.6 Influence of ultrasonic treatment
8.4 Heat convection in TiO2 nanofluids
8.4.1 Forced convection
8.4.1.1 Influence of particle loading and Re
8.4.1.2 Influence of particle size
8.4.1.3 Influence of temperature
8.4.1.4 Influence of geometry of flow channel
8.4.2 Natural convection
8.4.2.1 Factors influencing natural convection heat transfer of TiO2 nanofluids
8.4.2.2 Influence of nanoparticle type and load
8.5 Boiling heat transfer of TiO2 nanofluids
8.5.1 Influence of nanoparticle type
8.5.2 Influence of particle loading
8.5.3 Influence of surface roughness
8.5.4 Influence of the heater material
8.5.5 Influence of ionic additive
8.6 Applications of TiO2 nanofluids
8.7 Future investigations
References
9---TiO2-as-white-pigment-and-valorization-of-_2021_Titanium-Dioxide--Tio---
9 TiO2 as white pigment and valorization of the waste coming from its production
9.1 Introduction
9.1.1 Titanium minerals
9.1.2 Titanium ore purification
9.2 Routes for the manufacture of titanium dioxide pigments (Pigment White 6)
9.2.1 The chloride process
9.2.2 Sulfate process
9.3 Properties and applications of Pigment White 6
9.3.1 Properties
9.3.2 Applications
9.3.2.1 Coatings, plastics, and paints
9.3.2.2 Printing inks and paper
9.3.2.3 Pharmaceutical and cosmetic industries
9.3.2.4 Textiles
9.3.2.5 Food industry
9.4 Valorization of coproducts and wastes generated
9.4.1 Main wastes generated in the sulfate process
9.4.2 Main wastes generated in the chloride process
References
10---Titanium-dioxide-based-nanomaterials--appli_2021_Titanium-Dioxide--Tio-
10 Titanium dioxide–based nanomaterials: application of their smart properties in biomedicine
10.1 Introduction
10.2 Smart properties of titanium dioxide–based nanomaterials
10.2.1 Advanced photodynamic therapy approached based on hybrid titanium dioxide–based nanomaterials
10.2.2 Advanced sonodynamic therapy approached based on hybrid titanium dioxide–based nanomaterials
10.3 Tissue engineering
10.4 Drug delivery
10.5 Other applications
10.6 Conclusion and perspectives
References
11---TiO2-in-the-food-industry-and-_2021_Titanium-Dioxide--Tio---and-Its-App
11 TiO2 in the food industry and cosmetics
11.1 Introduction
11.2 Titanium dioxide as food additive
11.2.1 Titanium dioxide in food
11.2.2 Influence of titanium dioxide on human health
11.3 Titanium dioxide for food preservation
11.3.1 Antibacterial effects
11.3.2 Ethylene degradation
11.3.3 Active packaging
11.4 Titanium dioxide in cosmetics and personal care products
11.4.1 Regulations
11.4.2 Safety of sunscreens
11.5 Conclusion
References
12---Titanium-dioxide--antimicrobial-surfac_2021_Titanium-Dioxide--Tio---and
12 Titanium dioxide: antimicrobial surfaces and toxicity assessment
12.1 Introduction
12.2 Antibacterial and antimicotic properties
12.2.1 Adverse effect of TiO2 on bacteria
12.2.2 Adverse effects of TiO2 on fungi
12.3 Toxicity assessment on TiO2 NPs
12.3.1 Regulations
12.3.2 Exposure route and biodistrubution
12.3.2.1 Inhalation
12.3.2.2 Ingestion
12.3.2.3 Skin penetration
12.4 Antimicrobial surfaces
12.5 Conclusion
Conflicts of interest
Acknowledgments
References
13---Functionalization-of-glass-by-TiO2-bas_2021_Titanium-Dioxide--Tio---and
13 Functionalization of glass by TiO2-based self-cleaning coatings
13.1 Introduction
13.2 Main principle behind self-cleaning behavior
13.3 Applications of self-cleaning glass and main commercial products
13.3.1 Commercial self-cleaning glasses
13.3.1.1 Pilkington Activ Clear/Blue/Neutral by Pilkington Group Limited
13.3.1.2 Neat Glass produced by Cardinal Glass Industries
13.3.1.3 Self-cleaning glass by Fuyao Glass Industry Group Co. Ltd. UV
13.3.1.4 SunClean by PPG residential Glass
13.3.1.5 BIOCLEAN by Saint-Gobain Glass UK Ltd
13.3.1.6 Renew by Viridian Glass
13.4 Doped TiO2–based coatings for improved self-cleaning ability
13.4.1 Mechanism of doped-TiO2 coatings for glass
13.4.2 Synthesis strategies
13.4.2.1 Wet-deposition methods
13.4.2.2 Dry-deposition methods
13.5 Future tendencies: multilayer coatings for multifunctional glass
13.5.1 Multilayer structures for improved self-cleaning and antireflective ability
13.5.2 Self-cleaning and energy-saving multilayer structures
13.6 Conclusion
References
14---TiO2-as-a-source-of-titan_2021_Titanium-Dioxide--Tio---and-Its-Applicat
14 TiO2 as a source of titanium
14.1 TiO2 production from titanium minerals
14.1.1 Production of titanium-rich slag from titanium minerals
14.1.2 Production of TiO2 from titanium-rich slag
14.2 The Kroll process from TiO2 to Ti
14.3 Electrolytic production of Ti from TiO2 in high-temperature molten salts
14.4 Electrodeposition of Ti in low-temperature liquid salts
Acknowledgments
References
15---TiO2-in-the-building-sect_2021_Titanium-Dioxide--Tio---and-Its-Applicat
15 TiO2 in the building sector
15.1 Introduction
15.2 TiO2 in cement-based materials
15.2.1 General goals of the use of TiO2 in cement-based materials
15.2.2 Use of TiO2 for functional cement-based materials
15.2.2.1 Air-purifying cement-based materials
Role of the climatic conditions
Interactions with the cement matrix
Durability of the photocatalytic activity
Improvement strategies, by TiO2 doping and modifications
15.2.2.2 Water-purifying cement-based materials
15.2.2.3 Self-cleaning cement-based materials
15.2.2.4 Antimicrobial cement-based materials
15.2.2.5 Final remarks
15.2.3 Use of TiO2 for structural cement-based materials
(a) Nano-TiO2 modifies the properties of concrete at the fresh state (rheology and hydration speed)
(b) Nano-TiO2 modifies the properties of hardened concrete (mechanical strength and durability)
15.2.4 Patents on cement-based materials with TiO2
15.3 TiO2 in geopolymers
15.4 TiO2 in ceramic tiles
15.4.1 Ceramic tiles production
15.4.2 Exploitation of TiO2 in ceramic tiles
15.4.3 International patents on photocatalytic ceramic tiles
15.4.4 Standards
15.5 TiO2 in cultural heritage conservation
15.6 Environmental and health concerns in the use of TiO2 in building materials
15.7 Conclusion and perspectives
References
16---TiO2-oxides-for-chromogenic-devices-a_2021_Titanium-Dioxide--Tio---and-
16 TiO2 oxides for chromogenic devices and dielectric mirrors
16.1 TiO2 in electrochromic devices
16.1.1 Deposition techniques
16.2 TiO2 in photo-electrochromic devices
16.3 TiO2 optical properties
16.4 Modeling distributed Bragg reflectors
16.5 Bloch surface waves and microcavity modes
16.6 Conclusion
References
17---TiO2-in-memristors-and-resistive-rand_2021_Titanium-Dioxide--Tio---and-
17 TiO2 in memristors and resistive random access memory devices
17.1 Introduction
17.2 Fundamentals on resistive switching
17.2.1 Electrochemical metallization memories
17.2.2 Valence change memories
17.3 TiO2 in memristors and resistive random access memories: fabrication methods and performances
17.3.1 Anodizing
17.3.2 Atomic layer deposition
17.3.3 Sputtering
17.4 Conclusions and perspectives
References
18---Applications-of-TiO2-in-sensor_2021_Titanium-Dioxide--Tio---and-Its-App
18 Applications of TiO2 in sensor devices
List of abbreviations
18.1 Introduction
18.2 Titanium dioxide in sensor field: principles and mechanisms of action
18.2.1 Mechanism of sensing
18.2.1.1 Resistive-type gas sensors (chemiresistors)
18.2.1.2 Optical sensing
18.2.1.3 Photoconductive devices
18.2.1.4 Photoelectrochemical sensing
18.3 Gas sensors
18.3.1 H2O (humidity)
18.3.2 Dihydrogen (H2)
18.3.3 Dioxygen (O2)
18.3.4 CO2
18.3.5 NH3
18.3.6 CO
18.3.7 NO2
18.3.8 Volatile organic compounds
18.3.8.1 Ethanol
18.3.8.2 Acetone
18.3.8.3 Formaldehyde
18.3.8.4 Trimethylamine
18.3.8.5 Toluene
Conclusion
18.4 Biosensors
18.4.1 Glucose
18.4.2 DNA and biomarkers
18.4.3 Pesticides
18.4.4 Cholesterol derivatives
18.4.5 H2O2
18.4.6 Urea
18.4.7 Glutamate
18.4.8 Bacteria (Escherichia coli, etc.)
18.4.9 Other analytes
18.5 Sensors for environmental applications
18.5.1 Detection of organic pollutants
18.5.2 Detection of dyes
18.5.3 TiO2 in molecular imprinting technology
18.5.4 Metal ions detection
18.6 Fabrication of nanoscale sensors and future prospects
18.7 Conclusion
References
19---TiO2-photocatalysis-for-environme_2021_Titanium-Dioxide--Tio---and-Its-
19 TiO2 photocatalysis for environmental purposes
19.1 General overview on air and water pollution
19.2 General remarks on advanced oxidation processes
19.3 TiO2 photocatalysis for the removal of volatile organic compounds from gaseous stream
19.4 TiO2 photocatalysis for indoor air purification
19.4.1 TiO2 photocatalysis with forced air
19.4.2 TiO2 photocatalysis in indoor environments
19.5 TiO2 photocatalysis for the removal of organic pollutants from water and wastewater
19.6 Conclusion and future perspectives
References
20---Fine-chemistry-by-TiO2-heterogeneou_2021_Titanium-Dioxide--Tio---and-It
20 Fine chemistry by TiO2 heterogeneous photocatalysis
20.1 Introduction
20.2 Reactions of partial oxidation
20.2.1 Oxidation of alcohols to aldehydes
20.2.2 Hydroxylation of aromatic compounds
20.2.3 Epoxidation of alkenes
20.3 Reactions of partial reduction
20.3.1 Hydrogenation of double and triple carbon–carbon bonds
20.3.2 Reduction of carbonyls
20.3.3 Reduction of N-containing functional groups
20.4 Reactions of alkylation
20.4.1 Reactions of addition
20.4.2 Substitution reactions in aromatic compounds
20.4.3 Reactions of carbonyl alkylation
20.5 Conclusion
References
21---Catalytic-applications-of-T_2021_Titanium-Dioxide--Tio---and-Its-Applic
21 Catalytic applications of TiO2
21.1 Introduction
21.2 Titania as catalytic support: role of the strong metal–support interaction
21.3 The role of defects on catalytic performances
21.4 Main reactions involving titania-based catalyst
21.4.1 NOx removal
21.4.1.1 Selective catalytic reduction of NOx
21.4.1.2 Catalytic oxidation of NOx
21.4.2 Deacon process
21.4.3 Reactions with sulfur-rich compounds
21.4.3.1 Hydrodesulfurization processes
21.4.3.2 Claus process
21.4.4 Direct synthesis of hydrogen peroxide
21.4.5 Fischer–Tropsch synthesis
21.4.6 Water–gas shift reaction
21.4.7 CO2 methanation
21.4.8 Biofuels production
21.4.8.1 Transesterification of triglycerides
21.4.8.2 Upgrading of pyrolysis oils
21.4.9 Dehydrogenations, selective oxidations, and hydrogenations
21.4.9.1 (Oxy)dehydrogenations
21.4.9.2 Selective oxidations of alkanes, alcohols, and aromatics
21.4.9.3 Selective oxidations of heteroaromatic compounds
21.4.9.4 Olefin epoxidation
21.4.9.5 Selective oxidation of ammonia to nitrogen
21.4.9.6 Hydrogenations
21.5 Conclusion and outlooks
References
Index_2021_Titanium-Dioxide--Tio---and-Its-Applications
Index

Citation preview

Titanium Dioxide (TiO2) and Its Applications

The Metal Oxides Series Edited by Ghenadii Korotcenkov Forthcoming Titles Metal Oxides: Powder Technologies, Yarub Al-Douri (ed.), 9780128175057 Palladium Oxides Material Properties, Synthesis and Processing Methods, and Applications, Alexander M. Samoylov, Vasily N. Popov, 9780128192238 Metal Oxides for Non-volatile Memory, Panagiotis Dimitrakis, Ilia Valov, 9780128146293 Metal Oxide Nanostructured Phosphors, H. Nagabhushana, Daruka Prasad, S.C. Sharma, 9780128118528 Nanostructured Zinc Oxide, Kamlendra Awasthi, 9780128189009 Metal Oxide-Based Nanostructured Electrocatalysts for Fuel Cells, Electrolyzers, and Metal-Air Batteries, Teko Napporn, Yaovi Holade, 9780128184967 Multifunctional Piezoelectric Oxide Nanostructures, Sang-Jae Kim, Nagamalleswara Rao Alluri, Yuvasree Purusothaman, 9780128193327 Titanium Dioxide (TiO2) and Its Applications, Francesco Parrino, Leonardo Palmisano, 9780128199602 Transparent Conductive Oxides, Mirela Petruta Suchea, Petronela Pascariu, Emmanouel Koudoumas, 9780128206317 Metal Oxide-Based Nanofibers and Their Applications, Vincenzo Esposito, Debora Marani, 9780128206294 Metal Oxides in Nanocomposite-Based Electrochemical Sensors for Toxic Chemicals, Alagarsamy Pandikumar, Perumal Rameshkumar, 9780128207277 Metal-Oxides for Biomedical and Biosensor Applications, Kunal Mondal, 9780128230336 Metal Oxide-Carbon Hybrid Materials, Muhammad Akram, Rafaqat Hussain, Faheem K Butt, 9780128226940 G

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Published Titles Colloidal Metal Oxide Nanoparticles, Sabu Thomas, Anu Tresa Sunny, Prajitha V, 9780128133576 Cerium Oxide, Salvatore Scire, Leonardo Palmisano, 9780128156612 Tin Oxide Materials, Marcelo Ornaghi Orlandi, 9780128159248 Metal Oxide Glass Nanocomposites, Sanjib Bhattacharya, 9780128174586 Gas Sensors Based on Conducting Metal Oxides, Nicolae Barsan, Klaus Schierbaum, 9780128112243 Metal Oxides in Energy Technologies, Yuping Wu, 9780128111673 Metal Oxide Nanostructures, Daniela Nunes, Lidia Santos, Ana Pimentel, Pedro Barquinha, Luis Pereira, Elvira Fortunato, Rodrigo Martins, 9780128115121 Gallium Oxide, Stephen Pearton, Fan Ren, Michael Mastro, 9780128145210 Metal Oxide-Based Photocatalysis, Adriana Zaleska-Medynska, 9780128116340 Metal Oxides in Heterogeneous Catalysis, Jacques C. Vedrine, 9780128116319 Magnetic, Ferroelectric, and Multiferroic Metal Oxides, Biljana Stojanovic, 9780128111802 Iron Oxide Nanoparticles for Biomedical Applications, Sophie Laurent, Morteza Mahmoudi, 9780081019252 The Future of Semiconductor Oxides in Next-Generation Solar Cells, Monica Lira-Cantu, 9780128111659 Metal Oxide-Based Thin Film Structures, Nini Pryds, Vincenzo Esposito, 9780128111666 Metal Oxides in Supercapacitors, Deepak Dubal, Pedro Gomez-Romero, 9780128111697 Solution Processed Metal Oxide Thin Films for Electronic Applications, Zheng Cui, 9780128149300 Transition Metal Oxide Thin Film-Based Chromogenics and Devices, Pandurang Ashrit, 9780081018996 G

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Metal Oxides Series

Titanium Dioxide (TiO2) and Its Applications Edited by

Francesco Parrino Department of Industrial Engineering, University of Trento, Trento, Italy

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

Series Editor

Ghenadii Korotcenkov Department of Theoretical Physics, Moldova State University, Chisinau, Moldova

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. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-819960-2 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

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Contents

List of contributors About the series editor About the editors Preface to the series Preface to the volume

xv xix xxi xxiii xxvii

Section 1 Titanium dioxide: synthesis and characterization

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Introduction Francesco Parrino and Leonardo Palmisano 1.1 Economic aspects 1.2 Summary of the book References Properties of titanium dioxide Francesco Parrino, Francesca Rita Pomilla, Giovanni Camera-Roda, Vittorio Loddo and Leonardo Palmisano 2.1 Introduction 2.2 Structural properties 2.2.1 Structures of TiO2 2.2.2 Main techniques used for TiO2 structural analysis 2.3 Structure and defects 2.3.1 Defectivity 2.3.2 Surface defectivity 2.3.3 Surface and lattice distortion 2.4 TiO2 morphologies 2.5 Thermodynamic properties 2.6 Electronic properties 2.7 Electrical properties 2.8 Optical properties 2.9 Photon-induced behavior 2.10 Mechanical and rheological properties 2.10.1 Mechanical properties 2.10.2 Rheological properties References

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13 14 14 16 18 19 22 26 26 29 32 33 35 39 42 42 46 46

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Contents

Structural and electronic properties of TiO2 from first principles calculations Sergio Tosoni, Giovanni Di Liberto and Gianfranco Pacchioni 3.1 Introduction 3.2 Electronic structure calculations on TiO2: methodological aspects 3.2.1 The bandgap issue 3.2.2 Excess electrons (and holes) in TiO2: the localization problem 3.2.3 Oxygen vacancies 3.2.4 Interstitial Ti species 3.2.5 Photoexcited carriers 3.3 Titania heterojunctions and nanoparticles: computational modeling of cutting-edge materials 3.3.1 Separation of photoexcited charge carriers in titania nanocomposites 3.3.2 Computational modeling of titania nanoparticles 3.4 Conclusions Acknowledgments References Synthesis and characterization of titanium dioxide and titanium dioxide based materials Marianna Bellardita, Sedat Yurdakal and Leonardo Palmisano 4.1 Introduction 4.2 Preparation methods 4.2.1 Preparation methods of powdered TiO2-based materials 4.2.2 Preparation methods of TiO2 film 4.3 Characterization techniques of TiO2 4.3.1 X-ray diffraction 4.3.2 Scanning electron microscopy 4.3.3 Transmission electron microscopy 4.3.4 Brunauer Emmett Teller-specific surface area determination 4.3.5 Diffuse reflectance spectroscopy 4.3.6 Photoluminescence spectroscopy 4.3.7 X-ray photoelectron spectroscopy 4.3.8 Thermal gravimetric analysis References

Section 2 5

Energy applications

Synthetic, natural and bioinspired dyes as TiO2 sensitizers in sustainable solar cells Nunzio Genitori and Gaetano Di Marco 5.1 Introduction

67 67 68 68 70 72 74 75 76 76 78 81 81 81 87 87 88 88 104 109 109 115 121 127 135 141 144 147 152

167 169 169

Contents

6

vii

5.1.1 Photovoltaic technology 5.1.2 Dye-sensitized solar cells 5.2 Semiconductors 5.2.1 Bands formation 5.2.2 The occupation of the orbitals 5.2.3 Titanium dioxide 5.3 Dyes 5.3.1 Synthetic dyes 5.3.2 Natural dyes 5.3.3 Computational details 5.3.4 Bioinspired 5.4 Other functional materials 5.4.1 Characteristics and performance of CEs 5.4.2 Characteristics and performance of electrolytes 5.5 Assembly and characterizations for DSSCs 5.5.1 Development of photoanodes and cathodes 5.5.2 Spectroscopic techniques 5.5.3 Cyclic voltammetry 5.5.4 Roughness and desorption factor 5.5.5 Characteristic I-V curves 5.5.6 Quantum efficiency: IPCE, APCE, and LHE 5.5.7 Electrochemical impedance spectroscopy 5.5.8 Tafel electroanalysis 5.6 Conclusions References

169 171 176 176 176 177 180 180 182 189 191 191 191 192 193 193 193 194 196 197 201 203 204 206 207

TiO2-based materials for photocatalytic hydrogen production Maria Vittoria Dozzi and Elena Selli 6.1 Introduction 6.2 Photocatalytic water splitting with TiO2 6.3 Development of sensitive TiO2-based photocatalysts for H2 generation 6.3.1 Bandgap engineering 6.3.2 Surface TiO2 sensitization 6.4 Separation of photogenerated charges in TiO2-based photocatalysts for H2 generation 6.4.1 Charge separation in TiO2 phase junctions 6.4.2 Charge separation in shape-controlled anatase TiO2 6.4.3 Noble metal nanoparticles deposition and Schottky junction fabrication 6.4.4 Fabrication of heterojunctions 6.4.5 Loading cocatalysts on TiO2 6.5 Sacrificial agents in photocatalytic hydrogen production: from overall water splitting to biomass reforming 6.6 Conclusion and perspectives References

211 211 212 214 214 217 218 218 218 220 222 225 228 230 230

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7

8

Contents

TiO2-based devices for energy-related applications C.G. Jothi Prakash and R. Prasanth 7.1 Introduction 7.1.1 Titanium dioxide for energy harvesting 7.1.2 Titanium dioxide for energy storage 7.2 Energy storage applications 7.2.1 Supercapacitors 7.2.2 Batteries 7.2.3 Hydrogen production and storage 7.2.4 Others 7.3 Conclusion and outlook References

241

Heat transfer by using TiO2 nanofluids Vittorio Loddo and Giovanni Camera Roda List of abbreviations 8.1 Introduction 8.2 Preparation and characterization of TiO2 nanofluids 8.2.1 Nanoparticles preparation 8.2.2 Preparation of nanofluids 8.2.3 Parameters influencing the aggregation and stability of TiO2 nanofluids 8.2.4 Nanoparticle size measurements 8.2.5 Z-potential measurements 8.2.6 pH measurements 8.3 Heat conduction in TiO2 nanofluids 8.3.1 Influence of particle load 8.3.2 Influence of temperature 8.3.3 Influence of thermal conductivity of the base fluid 8.3.4 Influence of particle cluster size and shape on thermal conductivity 8.3.5 Influence of surfactant 8.3.6 Influence of ultrasonic treatment 8.4 Heat convection in TiO2 nanofluids 8.4.1 Forced convection 8.4.2 Natural convection 8.5 Boiling heat transfer of TiO2 nanofluids 8.5.1 Influence of nanoparticle type 8.5.2 Influence of particle loading 8.5.3 Influence of surface roughness 8.5.4 Influence of the heater material 8.5.5 Influence of ionic additive 8.6 Applications of TiO2 nanofluids 8.7 Future investigations References

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241 242 242 243 243 250 254 258 260 260

267 267 270 270 272 274 274 275 275 276 278 278 279 279 285 285 285 287 293 295 296 296 297 298 298 298 299 300

Contents

Section 3 9

10

11

ix

TiO2 in our life

TiO2 as white pigment and valorization of the waste coming from its production Manuel Jesu´s Ga´zquez, Silvia Marı´a Pe´rez Moreno and Juan Pedro Bolı´var 9.1 Introduction 9.1.1 Titanium minerals 9.1.2 Titanium ore purification 9.2 Routes for the manufacture of titanium dioxide pigments (Pigment White 6) 9.2.1 The chloride process 9.2.2 Sulfate process 9.3 Properties and applications of Pigment White 6 9.3.1 Properties 9.3.2 Applications 9.4 Valorization of coproducts and wastes generated 9.4.1 Main wastes generated in the sulfate process 9.4.2 Main wastes generated in the chloride process References Titanium dioxide based nanomaterials: application of their smart properties in biomedicine Giada Graziana Genchi 10.1 Introduction 10.2 Smart properties of titanium dioxide based nanomaterials 10.2.1 Advanced photodynamic therapy approached based on hybrid titanium dioxide based nanomaterials 10.2.2 Advanced sonodynamic therapy approached based on hybrid titanium dioxide based nanomaterials 10.3 Tissue engineering 10.4 Drug delivery 10.5 Other applications 10.6 Conclusion and perspectives References TiO2 in the food industry and cosmetics Annachiara Berardinelli and Filippo Parisi 11.1 Introduction 11.2 Titanium dioxide as food additive 11.2.1 Titanium dioxide in food 11.2.2 Influence of titanium dioxide on human health 11.3 Titanium dioxide for food preservation 11.3.1 Antibacterial effects 11.3.2 Ethylene degradation 11.3.3 Active packaging

309 311

311 312 313 315 315 317 319 319 321 327 328 329 330 337 337 338 339 342 343 345 349 350 351 353 353 354 354 355 357 357 360 361

x

12

13

14

Contents

11.4

Titanium dioxide in cosmetics and personal care products 11.4.1 Regulations 11.4.2 Safety of sunscreens 11.5 Conclusion References

362 364 364 365 366

Titanium dioxide: antimicrobial surfaces and toxicity assessment Valeria De Matteis, Mariafrancesca Cascione and Rosaria Rinaldi 12.1 Introduction 12.2 Antibacterial and antimicotic properties 12.2.1 Adverse effect of TiO2 on bacteria 12.2.2 Adverse effects of TiO2 on fungi 12.3 Toxicity assessment on TiO2 NPs 12.3.1 Regulations 12.3.2 Exposure route and biodistrubution 12.4 Antimicrobial surfaces 12.5 Conclusion Conflicts of interest Acknowledgments References

373

Functionalization of glass by TiO2-based self-cleaning coatings Corrado Garlisi, Gabriele Scandura, Ahmed Yusuf and Samar Al Jitan 13.1 Introduction 13.2 Main principle behind self-cleaning behavior 13.3 Applications of self-cleaning glass and main commercial products 13.3.1 Commercial self-cleaning glasses 13.4 Doped TiO2 based coatings for improved self-cleaning ability 13.4.1 Mechanism of doped-TiO2 coatings for glass 13.4.2 Synthesis strategies 13.5 Future tendencies: multilayer coatings for multifunctional glass 13.5.1 Multilayer structures for improved self-cleaning and antireflective ability 13.5.2 Self-cleaning and energy-saving multilayer structures 13.6 Conclusion References

395

TiO2 as a source of titanium Xingli Zou, Zhongya Pang, Li Ji and Xionggang Lu 14.1 TiO2 production from titanium minerals 14.1.1 Production of titanium-rich slag from titanium minerals 14.1.2 Production of TiO2 from titanium-rich slag 14.2 The Kroll process from TiO2 to Ti

429

373 375 375 378 378 378 379 383 386 386 386 386

395 396 402 405 408 408 412 414 414 417 420 421

429 429 431 433

Contents

xi

14.3

Electrolytic production of Ti from TiO2 in high-temperature molten salts 14.4 Electrodeposition of Ti in low-temperature liquid salts Acknowledgments References

15

TiO2 in the building sector Elisa Franzoni, Maria Chiara Bignozzi and Elisa Rambaldi 15.1 Introduction 15.2 TiO2 in cement-based materials 15.2.1 General goals of the use of TiO2 in cement-based materials 15.2.2 Use of TiO2 for functional cement-based materials 15.2.3 Use of TiO2 for structural cement-based materials 15.2.4 Patents on cement-based materials with TiO2 15.3 TiO2 in geopolymers 15.4 TiO2 in ceramic tiles 15.4.1 Ceramic tiles production 15.4.2 Exploitation of TiO2 in ceramic tiles 15.4.3 International patents on photocatalytic ceramic tiles 15.4.4 Standards 15.5 TiO2 in cultural heritage conservation 15.6 Environmental and health concerns in the use of TiO2 in building materials 15.7 Conclusion and perspectives References

Section 4 16

17

438 445 446 447 449 449 449 449 451 458 461 461 462 462 464 467 468 468 470 473 474

TiO2 devices and their applications

481

TiO2 oxides for chromogenic devices and dielectric mirrors Alessandro Cannavale and Giovanni Lerario 16.1 TiO2 in electrochromic devices 16.1.1 Deposition techniques 16.2 TiO2 in photo-electrochromic devices 16.3 TiO2 optical properties 16.4 Modeling distributed Bragg reflectors 16.5 Bloch surface waves and microcavity modes 16.6 Conclusion References

483

TiO2 in memristors and resistive random access memory devices Andrea Zaffora, Francesco Di Franco, Roberto Macaluso and Monica Santamaria 17.1 Introduction

507

483 484 488 492 493 498 501 501

507

xii

Contents

17.2

18

Fundamentals on resistive switching 17.2.1 Electrochemical metallization memories 17.2.2 Valence change memories 17.3 TiO2 in memristors and resistive random access memories: fabrication methods and performances 17.3.1 Anodizing 17.3.2 Atomic layer deposition 17.3.3 Sputtering 17.4 Conclusions and perspectives References

508 508 510

Applications of TiO2 in sensor devices Giuseppe Mele, Roberta Del Sole and Xiangfei Lu¨ List of abbreviations 18.1 Introduction 18.2 Titanium dioxide in sensor field: principles and mechanisms of action 18.2.1 Mechanism of sensing 18.3 Gas sensors 18.3.1 H2O (humidity) 18.3.2 Dihydrogen (H2) 18.3.3 Dioxygen (O2) 18.3.4 CO2 18.3.5 NH3 18.3.6 CO 18.3.7 NO2 18.3.8 Volatile organic compounds 18.4 Biosensors 18.4.1 Glucose 18.4.2 DNA and biomarkers 18.4.3 Pesticides 18.4.4 Cholesterol derivatives 18.4.5 H2O2 18.4.6 Urea 18.4.7 Glutamate 18.4.8 Bacteria (Escherichia coli, etc.) 18.4.9 Other analytes 18.5 Sensors for environmental applications 18.5.1 Detection of organic pollutants 18.5.2 Detection of dyes 18.5.3 TiO2 in molecular imprinting technology 18.5.4 Metal ions detection 18.6 Fabrication of nanoscale sensors and future prospects 18.7 Conclusion References

527

512 512 516 519 521 522

527 528 530 531 534 536 538 539 541 542 543 544 545 552 554 555 556 558 558 559 560 560 561 562 563 565 567 568 569 571 572

Contents

19

20

21

TiO2 photocatalysis for environmental purposes Olga Sacco, Vincenzo Vaiano and Diana Sannino 19.1 General overview on air and water pollution 19.2 General remarks on advanced oxidation processes 19.3 TiO2 photocatalysis for the removal of volatile organic compounds from gaseous stream 19.4 TiO2 photocatalysis for indoor air purification 19.4.1 TiO2 photocatalysis with forced air 19.4.2 TiO2 photocatalysis in indoor environments 19.5 TiO2 photocatalysis for the removal of organic pollutants from water and wastewater 19.6 Conclusion and future perspectives References Fine chemistry by TiO2 heterogeneous photocatalysis Giuseppe Marcı`, Elisa I. Garcı´a-Lo´pez and Leonardo Palmisano 20.1 Introduction 20.2 Reactions of partial oxidation 20.2.1 Oxidation of alcohols to aldehydes 20.2.2 Hydroxylation of aromatic compounds 20.2.3 Epoxidation of alkenes 20.3 Reactions of partial reduction 20.3.1 Hydrogenation of double and triple carbon carbon bonds 20.3.2 Reduction of carbonyls 20.3.3 Reduction of N-containing functional groups 20.4 Reactions of alkylation 20.4.1 Reactions of addition 20.4.2 Substitution reactions in aromatic compounds 20.4.3 Reactions of carbonyl alkylation 20.5 Conclusion References Catalytic applications of TiO2 Salvatore Scire`, Roberto Fiorenza, Marianna Bellardita and Leonardo Palmisano 21.1 Introduction 21.2 Titania as catalytic support: role of the strong metal support interaction 21.3 The role of defects on catalytic performances 21.4 Main reactions involving titania-based catalyst 21.4.1 NOx removal 21.4.2 Deacon process 21.4.3 Reactions with sulfur-rich compounds 21.4.4 Direct synthesis of hydrogen peroxide

xiii

583 583 586 589 591 592 596 600 602 603 609 609 610 610 612 614 615 615 617 619 626 626 627 629 631 631 637

637 638 640 641 641 644 646 649

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Contents

21.4.5 21.4.6 21.4.7 21.4.8 21.4.9

Fischer Tropsch synthesis Water gas shift reaction CO2 methanation Biofuels production Dehydrogenations, selective oxidations, and hydrogenations 21.5 Conclusion and outlooks References Index

651 653 655 656 658 666 667 681

List of contributors

Samar Al Jitan Department of Chemical Engineering, Khalifa University, Abu Dhabi, United Arab Emirates; Research and Innovation Center on CO2 and H2 (RICH Center), Khalifa University, Abu Dhabi, United Arab Emirates Marianna Bellardita Department of Engineering, University of Palermo, Palermo, Italy Annachiara Berardinelli Department of Industrial Engineering, University of Trento, Trento, Italy; Center Agriculture Food Environment, University of Trento, Trento, Italy Maria Chiara Bignozzi Department of Civil, Chemical, Environmental and Materials Engineering, Alma Mater Studiorum University of Bologna, Bologna, Italy Juan Pedro Bolı´var Department of Integrated Sciences, Faculty of Experimental Sciences, University of Huelva, Huelva, Spain; Research Centre of Natural Resources, Health and the Environment (RENSMA), University of Huelva, Huelva, Spain Giovanni Camera-Roda Department of Civil, Chemical, Environmental, and Materials Engineering, University of Bologna, Bologna, Italy Alessandro Cannavale Department of Sciences in Civil Engineering and Architecture, Polytechnic University of Bari, Bari, Italy Mariafrancesca Cascione Department of Mathematics and Physics “Ennio De Giorgi”, University of Salento, Lecce, Italy Valeria De Matteis Department of Mathematics and Physics “Ennio De Giorgi”, University of Salento, Lecce, Italy Roberta Del Sole Department of Engineering for Innovation, University of Salento, Lecce, Italy Francesco Di Franco Department of Engineering, University of Palermo, Palermo, Italy

xvi

List of contributors

Giovanni Di Liberto Department of Materials Science, University of MilanoBicocca, Milan, Italy Gaetano Di Marco Institute for Chemical and Physical Processes (IPCF), National Research Council, Messina, Italy Maria Vittoria Dozzi Department of Chemistry, University of Milan, Milan, Italy Roberto Fiorenza Department of Chemical Sciences, University of Catania, Catania, Italy Elisa Franzoni Department of Civil, Chemical, Environmental and Materials Engineering, Alma Mater Studiorum University of Bologna, Bologna, Italy Elisa I. Garcı´a-Lo´pez Department of Biological, Chemical and Pharmaceutical Sciences and Technologies (STEBICEF), University of Palermo, Palermo, Italy Corrado Garlisi Department of Chemical Engineering, Khalifa University, Abu Dhabi, United Arab Emirates; Research and Innovation Center on CO2 and H2 (RICH Center), Khalifa University, Abu Dhabi, United Arab Emirates Manuel Jesu´s Ga´zquez Department of Applied Physics, Marine Research Institute (INMAR), University of Cadiz, Ca´diz, Spain Giada Graziana Genchi Smart Bio-Interfaces, Istituto Italiano di Tecnologia, Pontedera, Italy Nunzio Genitori Institute for Chemical and Physical Processes (IPCF), National Research Council, Messina, Italy Li Ji State Key Laboratory of ASIC and System, School of Microelectronics, Fudan University, Shanghai, P.R. China C.G. Jothi Prakash Madanjeet School of Green Energy Technology, Pondicherry Central University, Pondicherry, India Giovanni Lerario CNR Nanotec, Institute of Nanotechnology, Lecce, Italy Vittorio Loddo Department of Engineering, University of Palermo, Palermo, Italy Xiangfei Lu¨ School of Water and Environment, Chang’An University, Xi’an, P. R. China Xionggang Lu State Key Laboratory of Advanced Special Steel, School of Materials Science and Engineering, Shanghai University, Shanghai, P.R. China

List of contributors

xvii

Roberto Macaluso Department of Engineering, University of Palermo, Palermo, Italy Giuseppe Marcı` “Schiavello-Grillone” Photocatalysis Group, Department of Engineering (DI), University of Palermo, Palermo, Italy Giuseppe Mele Department of Engineering for Innovation, University of Salento, Lecce, Italy Silvia Marı´a Pe´rez Moreno Department of Integrated Sciences, Faculty of Experimental Sciences, University of Huelva, Huelva, Spain; Research Centre of Natural Resources, Health and the Environment (RENSMA), University of Huelva, Huelva, Spain Gianfranco Pacchioni Department of Materials Science, University of MilanoBicocca, Milan, Italy Leonardo Palmisano “Schiavello-Grillone” Photocatalysis Group, Department of Engineering (DI), University of Palermo, Palermo, Italy; Department of Engineering, University of Palermo, Palermo, Italy Zhongya Pang State Key Laboratory of Advanced Special Steel, School of Materials Science and Engineering, Shanghai University, Shanghai, P.R. China Filippo Parisi Department of Physics and Chemistry, University of Palermo, Palermo, Italy Francesco Parrino Department of Industrial Engineering, University of Trento, Trento, Italy Francesca Rita Pomilla Department of Materials Science, University of MilanoBicocca, Milano, Italy R. Prasanth Madanjeet School of Green Energy Technology, Pondicherry Central University, Pondicherry, India Elisa Rambaldi Centro Ceramico, Bologna, Italy Rosaria Rinaldi Department of Mathematics and Physics “Ennio De Giorgi”, University of Salento, Lecce, Italy Olga Sacco Department of Chemistry and Biology “A. Zambelli”, University of Salerno, Fisciano, Italy

xviii

List of contributors

Diana Sannino Department of Industrial Engineering, University of Salerno, Fisciano, Italy Monica Santamaria Department of Engineering, University of Palermo, Palermo, Italy Gabriele Scandura Department of Chemical Engineering, Khalifa University, Abu Dhabi, United Arab Emirates; Research and Innovation Center on CO2 and H2 (RICH Center), Khalifa University, Abu Dhabi, United Arab Emirates Salvatore Scire` Department of Chemical Sciences, University of Catania, Catania, Italy Elena Selli Department of Chemistry, University of Milan, Milan, Italy Sergio Tosoni Department of Materials Science, University of Milano-Bicocca, Milan, Italy Vincenzo Vaiano Department of Industrial Engineering, University of Salerno, Fisciano, Italy Sedat Yurdakal Department of Chemistry, University of Afyon Kocatepe, Afyonkarahisar, Turkey Ahmed Yusuf Department of Chemical Engineering, Khalifa University, Abu Dhabi, United Arab Emirates; Research and Innovation Center on CO2 and H2 (RICH Center), Khalifa University, Abu Dhabi, United Arab Emirates Andrea Zaffora Department of Engineering, University of Palermo, Palermo, Italy Xingli Zou State Key Laboratory of Advanced Special Steel, School of Materials Science and Engineering, Shanghai University, Shanghai, P.R. China

About the series editor

Ghenadii Korotcenkov earned his PhD in material sciences in 1976 and his doctor of science degree (doctor habilitate) in physics in 1990. He has more than 45 years of experience as a scientific researcher. For a long time, he has been the leader of the gas sensor group and manager of various national and international scientific and engineering projects carried out in the Laboratory of Microand Optoelectronics, Technical University of Moldova and supported from international foundations and programs such as the CRDF, the MRDA, the ICTP, the INTAS, the INCO-COPERNICUS, the COST, and the NATO. From 2007 to 2008, he was an invited scientist in the Korean Institute of Energy Research, Daejeon, South Korea. Then, until the end of 2017, he was a research professor at the School of Materials Science and Engineering at the Gwangju Institute of Science and Technology, Gwangju, South Korea. Currently, he is the chief scientific researcher (research professor) at the Department of Physics and Engineering at the Moldova State University, Chisinau, the Republic of Moldova. Dr. G. Korotcenkov is either the author or editor of 39 books, including the 11volume Chemical Sensors series published by the Momentum Press (United States), 15-volume Chemical Sensors series published by Harbin Institute of Technology Press (China), 3-volume Porous Silicon: From Formation to Application published by CRC Press (United States), 2-volume Handbook of Gas Sensor Materials published by Springer (United States), and 3-volume Handbook of Humidity Measurements published by CRC Press (United States). In addition, at present, he is a series’ editor of Metal Oxides series, which is being published by Elsevier. Starting from 2017, already 18 volumes have been published within the framework of that series. Dr. G. Korotcenkov is the author and coauthor of more than 650 scientific publications, including 30 review papers, 38 book chapters, and more than 200 articles published in peer-reviewed scientific journals [h-factor 5 42 (Scopus) and hfactor 5 53 (Google Scholar citation)]. In the majority of publications, he is the first author. Besides, he is the holder of 18 patents and has presented more than 250 reports at national and international conferences, including 17 invited talks. He was a coorganizer of more than 20 international scientific conferences. Research activities of Dr. G. Korotcenkov are honored by the Honorary Diploma of the Government of the Republic of Moldova (2020), the Prize of the Academy of Sciences of Moldova (2019), an award of the Supreme Council of Science and

xx

About the series editor

Advanced Technology of the Republic of Moldova (2004); Prize of the Presidents of the Ukrainian, Belarus, and Moldovan Academies of Sciences (2003); and National Youth Prize of the Republic of Moldova in the field of science and technology (1980), among others. He also received a fellowship from the International Research Exchange Board (IREX, United States, 1998), Brain Korea 21 Program (2008 12), and Brainpool Program (Korea, 2007 08 and 2015 17).

About the editors

Francesco Parrino is an Assistant Professor of Chemistry at the University of Trento, Italy. He graduated cum laude in Chemical Engineering from the University of Palermo in 2005 and got a PhD in Inorganic Chemistry from the Friedrich Alexander University of Erlangen Nu¨rnberg, Germany, in 2009. His research activity deals with the preparation and characterization of photocatalysts for degradation of pollutants and for green synthesis of organic molecules. His scientific production conjugates mechanistic and fundamental aspects of heterogeneous photocatalysis with engineering issues for industrial applications. He has authored approximately 80 joint papers in collaboration with scientists from all over the world and several communications for international conferences on these topics. Leonardo Palmisano is a Professor of Chemistry at the Department of Engineering of the University of Palermo, Italy. His research has focused on the field of heterogeneous photocatalysis and various topics concerning preparation, characterization with many bulk and surface techniques, and testing of various types of photocatalysts. He has coordinated many national and international research projects (bilateral projects with Spain, India, and Egypt) and has obtained financial support by the European Union to carry out experiments with the solar photoreactors at Plataforma Solar de Almerı´a (Spain). He has collaborated with many scientists from all over the world to publish more than 300 joint papers in international journals, books, and conference proceedings. He also holds five patents.

Preface to the series

The field of synthesis, study, and application of metal oxides is one of the most rapidly progressing areas of science and technology. Metal oxides are one of the most ubiquitous compound groups on earth, which has large variety of chemical compositions, atomic structures, and crystalline shapes. In addition, metal oxides are known to possess unique functionalities that are either absent or insignificantly present in other solid materials. In particular, metal oxides represent an assorted and appealing class of materials, properties of which exhibit a full spectrum of electronic properties—from insulating to semiconducting, metallic, and superconducting. Moreover, almost all the known effects, including superconductivity, thermoelectric effects, photoelectrical effects, luminescence, and magnetism, can be observed in metal oxides. Therefore metal oxides have emerged as an important class of multifunctional materials with a rich collection of properties, which have great potential for numerous device applications. Specific properties of the metal oxides—such as the wide variety of materials with different electrophysical, optical, and chemical characteristics; their high thermal and temporal stability; and their ability to function in harsh environments—make metal oxides very suitable for designing transparent electrodes, high-mobility transistors, gas sensors, actuators, acoustical transducers, photovoltaic and photonic devices, photo- and heterogeneous catalysts, solid-state coolers, high-frequency and micromechanical devices, energy harvesting and storage devices, nonvolatile memories, and many others in the electronics, energy, and health sectors. In these devices, metal oxides can be successfully used as sensing or active layers, substrates, electrodes, promoters, structure modifiers, membranes, and fibers, that is, these can be used as both active and passive components. Among other advantages of metal oxides are the low fabrication cost and robustness in practical applications. Furthermore, the metal oxides can be prepared in various forms such as ceramics, thick films, and thin film. At that for thin film deposition can be used deposition techniques that are compatible with standard microelectronic technology. The last factor is very important for large-scale production, because the microelectronic approach promotes low cost for mass production, offers the possibility of manufacturing devices on a chip, and guarantees good reproducibility. Various metal oxides nanostructures, including nanowires, nanotubes, nanofibers, core-shell structures, and hollow nanostructures, can also be synthesized. As it is known, the field of metal oxide–nanostructured morphologies (e.g., nanowires, nanorods, and nanotubes) has become one of the most active research areas within the nanoscience community.

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Preface to the series

The ability to create a variety of metal oxide based composites and to synthesize various multicomponent compounds significantly expand the range of properties that metal oxide based materials can have, making these a truly versatile multifunctional material for widespread use. As it is known, small changes in their chemical composition and atomic structure can be accompanied by the spectacular variation in properties and behavior of metal oxides. Even now, advances in synthesizing and characterizing techniques are revealing numerous new functions of metal oxides. Taking into account the importance of metal oxides for progress in microelectronics, optoelectronics, photonics, energy conversion, sensor, and catalysis, various books devoted to this class of materials have been published. However, one should note that some books from this list are too general, some are collections of various original works without any generalizations, and others were published many years ago. But, during past decades, great progress has been made on the synthesis as well as the structural, physical, and chemical characterization and application of metal oxides in various devices, and a large number of papers have been published on metal oxides. In addition, till now, many important topics related to metal oxides study and application have not been discussed. To remedy the situation in this area, we decided to generalize and systematize the results of research in this direction and to publish a series of books devoted to metal oxides. One should note that the proposed book series Metal Oxides is the first one, devoted only to metal oxides. We believe that combining books on metal oxides in a series could help readers in searching required information on the subject. In particular, we plan that the books from our series, which have a clear specialization by its content, will provide interdisciplinary discussion for various oxide materials with a wide range of topics, from material synthesis and deposition to characterizations, processing, and then to device fabrications and applications. This book series is prepared by a team of highly qualified experts, which guarantees its high quality. I hope that our books will be useful and comfortable in use. I would also like to hope that readers will consider this Metal Oxides book series like an encyclopedia of metal oxides which enables to understand their present status, to estimate the role of multifunctional metal oxides in design of advanced devices, and then based on observed knowledge to formulate new goals for further research. The intended audience of present book series is scientists and researchers, working or planning to work in the field of materials related to metal oxides, that is, scientists and researchers whose activities are related to electronics, optoelectronics, energy, catalysis, sensors, electrical engineering, ceramics, biomedical designs, etc. I believe that this Metal Oxides book series will also be interesting for practicing engineers or project managers in industries and national laboratories, which would like to design metal oxide based devices, but do not know how to do it and how to select optimal metal oxide for specific applications. With many references to the vast resource of recently published literature on the subject, this book series will be serving as a significant and insightful source of valuable information, providing scientists and engineers with new insights for understanding and improving existing

Preface to the series

xxv

metal oxide based devices and for designing new metal oxide based materials with new and unexpected properties. I also believe that this Metal Oxides book series would be very helpful for university students, postdocs, and professors. The structure of these books offers a basis for courses in the field of material sciences, chemical engineering, electronics, electrical engineering, optoelectronics, energy technologies, environmental control, and many others. Graduate students could also find the book series to be very useful in their research and understanding features of metal oxides synthesis, study, and application of this multifunctional material in various devices. We are sure that all of them will find the information to be very useful for their activity. Finally, I thank all contributing authors and book editors who have been involved in the creation of these books. I am thankful that they agreed to participate in this project and for their efforts in the preparation of these books. Without their participation, this project would have not been possible. I also express my gratitude to Elsevier for giving us the opportunity to publish this series. I especially thank all teams of editorial office at Elsevier for their patience during the development of this project and for encouraging us during the various stages of preparation. Ghenadii Korotcenkov

Preface to the volume

The chapters of this book report information on titanium dioxide both from a theoretical and applicative point of view. The importance of this material is due to its peculiar structural, physicochemical, and intrinsic electronic properties. Although concerns have been raised in recent years about the use of TiO2 on the skin and as an additive in some foods or its presence in the environment as nanoparticles, its annual production in the form of a powder is between about 5 and 10 million t. In fact, this oxide has proven to be very useful in many applications as can be understood by reading this book. In particular, corrosion resistance, brilliance, and electronic properties make it an essential constituent for paints, inks, and other types of materials and devices. Its photocatalytic activity, directed toward the oxidation of harmful substances both in liquid solid and in gas solid systems, and more recently toward selective photosynthesis, is intensively studied with the aim of finding convenient applications from an industrial point of view. The book is certainly not exhaustive but attempts to give an overview as complete as possible for the reader who may be interested in one or more fields in which TiO2 plays an important role. We also want to emphasize that the authors have set the topics covered in the various chapters in such a way that readers who wish to have a deeper understanding can easily find the necessary literature among the references cited. Among metal oxides, titanium dioxide is one of the most versatile, and the differences between the properties of the three most important polymorphic forms, namely, anatase, brookite, and rutile, in our opinion, have not yet been sufficiently studied. We believe that the simultaneous presence of different TiO2 phases in particular ratios and/or the formation of composites also with other species can continue to give encouraging results in all fields of application, as well as being a stimulus for a thorough basic structural and surface research. In the introductory chapter, additional information on the structure of the book will be reported, along with a more detailed description of the role played by TiO2 in various fields. We wish to thank all the authors very much for their enthusiastic contribution in the preparation of this book which we hope will be useful to students, researchers, and workers involved in activities where TiO2 is an essential material. Finally, special thanks go to Elsevier and to the staff who accompanied us with great professionalism in this challenging, yet rewarding adventure. Francesco Parrino and Leonardo Palmisano

Introduction

1

Francesco Parrino1 and Leonardo Palmisano2 1 Department of Industrial Engineering, University of Trento, Trento, Italy, 2 Department of Engineering, University of Palermo, Palermo, Italy

1.1

Economic aspects

The industrial success of TiO2 consists mainly in its unique properties as a pigment [1]. The covering ability of titanium dioxide is even not comparable with that provided by possible alternatives. Paint formulations containing other white pigments would require higher amounts and many more layers of paint to achieve the same covering effect. This is mainly due to the high refractive index (RI) of TiO2, which can reach values up to 2.73 for the rutile phase [2]. In fact, the overall path length of light through a film possessing a high RI is shorter than that through a film with lower RI. Therefore the higher the RI, the lower the thickness below which the film appears white and opaque. Size and distribution of TiO2 particles must be controlled during the production process, as they also affect final paint properties such as gloss, dispersion, and hiding. For instance, light scattering is optimized for welldispersed TiO2 particles of size less than 0.5 µm, while gloss and dispersion of the paint are negatively affected for larger particles [3]. Other great benefits offered by TiO2 with respect to other white pigments are its high resistance to corrosion and photocorrosion and its unique optical properties. Generally, TiO2 particles in paints are covered with alumina, silica, and organic coatings to promote dispersibility, hiding, durability, and photostability [4]. In coating applications, surface modification is performed also to improve chalk resistance and gloss retention in outdoor applications, and to favor physical spacing between the particles in order to lead to superior hiding effect. TiO2 efficiently scatters visible and infrared radiation while absorbing UV light. Therefore objects containing TiO2 last longer especially when exposed to weathering, resist to heat and light, thus finally resulting in less waste production. Also, the mechanical stability of the coatings is improved in the presence of TiO2. For instance, TiO2-containing paints used in the vehicles industry materials increase stability, persistence, and resistance to scratches [5]. Moreover, high brilliance, color strength, and opacity targets can be obtained with lower amounts of resources. In this sense, TiO2 contributes to circular economy and environment care, improving efficiency and optimizing the use of resources for many products, which are maintained in use for as long as possible, thus reducing generation of wastes. TiO2 enables reuse and recycle of objects at the end of their life, enhancing the longevity of the products. As an example, Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00018-3 © 2021 Elsevier Inc. All rights reserved.

4

Titanium Dioxide (TiO2) and Its Applications

TiO2-containing coatings are nowadays used for recycling of wood and furniture, which is limited by the uneven color and texture of the used products. Although the use of TiO2 in paints is very important as it covers approximately 70% of the total volume of pigments, applications in polymeric materials, inks, chemical fibers, cosmetics, food, and paper are expanding more and more and covering an important part of the market. Titanium dioxide is produced starting from minerals containing titanium such as rutile and ilmenite. The titanium dioxide content in rutile is of c. 95%, while in ilmenite ranges between 50% and 60%, thus making rutile a high desirable feedstock [6]. The TiO2 market faced substantial issues between 2014 and 2015, due to the introduction into the paints and pigment market of cheaper TiO2 by asian producers. The infrastructure sector suffered mostly the situation, but paint and coatings continued to be the largest outlet. The increased production resulted in destocking of inventories which continued until 2016, when consumption picked up again and triggered the economic increase. Europe, for instance, witnessed a capacity reduction of c. 6.5% in those years and the production facility in Calais (Venator) closed in 2015 [7]. Nowadays, more than 50 companies are producing TiO2 in China, while c. 20 companies are located elsewhere around the world. However, only seven companies account for more than half of the global production. The major producers are the Chemours Company, Cristal, Venator Materials Corporation, Lomon Billions, Kronos, and Tronox Limited. They account for more than 60% of the global production, as it is apparent from Fig. 1.1 [8]. While paints and pigments represent the major application field of TiO2, the ceramic industry is expected to trigger the market growth in the near future. The market size in 2018 was calculated at 15.76 billion dollars, and it can be predicted a CAGR (compound annual growth rate) of over 8% in the period between 2019 and 2025. The largest market is Asia, being China the leading country. The

Figure 1.1 Global TiO2 capacity [8].

Introduction

5

situation is expected to remain unchanged, as the growing building sector and the investments in infrastructures in Asia will drive the market in the forthcoming years. North American countries accounted for c. 16% of the market volume in 2018, and the sector of infrastructures is expected to spur the market in the next years. The rising of photovoltaic market in Central and South America will encourage the demand for TiO2 that is used for the manufacture of photovoltaic modules. Similarly, also Europe will witness an economic growth in the TiO2 market, due to the presence of car manufacturers in countries such as Germany, Italy, and France, which are investing in the production of lightweight electric vehicles. In this field, TiO2 is widely used as it confers to paints resilience and brightness. Twenty percent of the world TiO2 production occurs in Europe with a market of c. 3 billion euros. Only 30% of the titanium dioxide produced in Europe is exported abroad, while c. 70% is consumed in Europe especially for coatings, paper, paints, and inks. The world consumption of TiO2 in 2017 is summarized in Fig. 1.2 [9]. The economic trend of the TiO2 market is strictly bound to that of the sectors which mostly consume it, that is, paints, coatings, plastics, and paper. TiO2 with small particles size (below 150 nm) is used in catalysis, sunscreens, and electroceramics. These fields, although of scientific relevance, do not significantly affect the TiO2 market, being relatively small. The consumption of TiO2 in Japan and Europe is forecasted to remain virtually constant, while the real driver is and will be China, which continues to rapidly expand in sectors such as plastics and coatings. By taking into account the demographic differences, the potential growth of

Figure 1.2 World consumption of TiO2 in 2017 [9].

6

Titanium Dioxide (TiO2) and Its Applications

Figure 1.3 Extent (in billion US dollars) of TiO2 market in the United States by application from 2014 to 2025 [10].

the Chinese market is high. In fact, while in Europe and the United States 2.7 kg of TiO2 is consumed in a year per capita, in China, this value is now only 1.1 kg and has therefore broad margins for improvement. For instance, China consumed c. 32% of the total TiO2 production between 2014 and 2017, while this value is forecast to increase at an annual growth rate of c. 3.5% over the next 5 years. The Chinese market will boost also the overall market growth, especially in the sector of construction due to the high demand for anticorrosive coatings in architecture. Fig. 1.3 summarizes the extent of the TiO2 market size in the United States from 2014 to 2025 by application [10]. Paints and coatings are key sectors, but the contribution of the plastic market will be higher during the next few years. Moreover, natural disasters in Florida and other countries will boost the construction market in the United States. If the demand forecasts will develop according to expectations (4% 5% annual), the production capacity must increase up to 300,000 cubic tons every year. Also in Europe, the most relevant application sectors of TiO2 are paints and coatings, and plastics, which account for a market of 650 billion euros. The paper industry is less significant. However, TiO2 is increasingly used as filler by papermakers. For instance, the amount of TiO2 used in wallpaper can reach values of 10%, while in de´cor paper, it ranges between 20% and 40% by weight.

1.2

Summary of the book

TiO2 applications for paints, coatings, and plastics absorb most of the TiO2 production and justify the industrial importance of TiO2 and its pervasive presence in our daily life. However, the unique properties of TiO2 enable its use in advanced technological sectors [11 13]. The possibility to provide charge separation can be used for the transformation of solar energy into electricity, for hydrogen generation, for

Introduction

7

electronic applications such as in memristive devices, and for batteries and energy devices. The ability of irradiated TiO2 to produce highly oxidizing species has been exploited in the field of biomedicine, photodynamic therapy, environmental remediation, in the building sector, and for chemical syntheses. However, the real implementation and commercialization of these applications is not straightforward and depends on factors that are not only merely economic in many cases, but also of socio-political and cultural nature. In this book, particular attention is dedicated to these applications and to novel and sustainable technologies. The book is divided into five parts. The first section, containing Chapters 2 4, deals with fundamental properties of titanium dioxide. Chapter 2, Properties of titanium dioxide, presents a survey of the main physicochemical properties of TiO2 upon which rely all of the applications of this material. Structures and morphologies of titanium dioxide have been taken into account, also in correlation with thermodynamic properties. Bulk and surface defectivity has been described in detail, because of its relevant consequences in terms of catalytic activity. The basic mechanisms of interaction between photons and TiO2 have been discussed on the basis of the optoelectronic features of the semiconductor. Finally, electrical, mechanical, and rheological properties of TiO2 have been presented. Chapter 3, Structural and electronic properties of TiO2 from first principle calculations, reports a computational description of structural and electronic properties of titanium dioxide by means of density functional theory calculations. In particular, the methodological approaches allowing to prevent mismatches between experimental and theoretical results have been described. Computational results on pristine and reduced titanium dioxide have been reviewed with respect to the nature of the main intrinsic defects, i.e. oxygen vacancies and interstitial Ti. Finally, theoretical results on the photodynamic behavior in titanium dioxide nanocomposites and on large and complex nanoparticles have been provided. Chapter 4, Synthesis and characterization of titanium dioxide and titanium dioxide based materials, describes preparation methods and characterization techniques for bare and coupled TiO2 systems. Particular attention has been paid both to the fundamental aspects and practical applications of the different preparation and characterization methods surveyed, by providing several examples in different research areas. The second section, containing Chapters 5 8, deals with energy applications of titanium dioxide. Chapter 5, Synthetic natural and bioinspired dyes as TiO2 sensitizers in sustainable solar cells, deals with dye-sensitized solar cells (DSSC) for the transformation of solar energy in electric current. This chapter reviews the fundamental and mechanistic aspects of DSSC and highlights the advantages of DSSC over traditional silicon-based photovoltaics. In fact, even if the efficiencies are lower (c. 10%) with respect to silicon cells, DSSC have significantly lower production costs and more sustainable features. Particular attention has been paid to the possibility of implementing bioinspired solar cells in the framework of a greener approach. Chapter 6, TiO2-based materials for photocatalytic hydrogen production, deals with hydrogen generation in the presence of TiO2. In fact, the pioneering work of

8

Titanium Dioxide (TiO2) and Its Applications

Fujishima and Honda [14] opened the route to plenty of applications where variously modified TiO2 has been used for the production of hydrogen as the green fuel of the future. Chapter 7, TiO2-based devices for energy-related applications, describes the most important TiO2-based nanomaterials for energy harvesting and storage. In particular, it has been provided a detailed summary of energy storage applications using TiO2-based nanomaterials in supercapacitors and batteries. Moreover, some issues concerning hydrogen storage and other forms of energy storage using TiO2 nanomaterials have been discussed. Chapter 8, Heat transfer by using TiO2 nanofluids, deals with heat transfer characteristics of TiO2 nanofluids, which nowadays appear to be very close to practical applications. The preparation and characterization of TiO2 nanofluids and the influence of several parameters on heat transfer have been reviewed. Finally, challenges and future perspectives in the field have been discussed. The third section, containing Chapters 9 15, reports the TiO2 applications related to our daily life. Some of them are consolidated and widely commercialized, but attention is paid to sustainability and innovative aspects. Chapter 9, TiO2 as white pigment and valorization of the waste coming from its production, reports on the widely recognized use of TiO2 as a pigment, justified mainly by its purity, high RI, and easily tunable properties. The two main industrial production processes of titanium dioxide pigments, that is, the sulfate and the chloride processes, have been described. Finally, the relevant aspects related to valorization of wastes and coproducts generated from industrial processes operating since decades ago have been analyzed in the framework of a circular economy approach. Chapter 10, Titanium dioxide based nanomaterials: Application of their smart properties in biomedicine, deals with the challenges and perspectives related to biomedical applications of TiO2-based materials, that is, in the field of tissue engineering, drug delivery, and cancer therapy. The most recent findings in these fields have been presented by focusing particularly on the properties tunable by exposure to a contactless source of stimulation, which actually makes titanium dioxide a “smart” material. Chapter 11, TiO2 in the food industry and cosmetics, reports on the use of TiO2 in food and cosmetic industry, by specifically focusing on the related regulatory issues. Both the uses of titanium dioxide as food additive and as active component of smart packaging to prolong the shelf life of food products have been reviewed. Moreover, technical features of TiO2 particles when used in cosmetic and personal care products have been described. Chapter 12, Titanium dioxide: Antimicrobial surfaces and toxicity assessment, analyzes the antimicrobial properties of TiO2 and the main applications in commercial surfaces and products. The cytotoxic mechanisms at cellular level and the consequent impact on living organisms have been reported. Finally, it is highlighted that the large use of TiO2 in commercial products does not match a comprehensive understanding of their potential harmfulness. Therefore a detailed analysis of aspects related to toxicity assessment has been presented.

Introduction

9

Chapter 13, Functionalization of glass by TiO2-based self-cleaning coatings, covers the main applications of TiO2 as self-cleaning material for functionalized glasses. The chapter starts by describing the basic principles of self-cleaning, that is, the ability of TiO2-based coatings to display photoinduced hydrophilicity and to mineralize absorbed organic compounds. Thereafter, features and advantages of several commercially available self-cleaning glasses have been presented, along with the strategies used to improve the performances and to extend the range of activity toward the visible radiation. Finally, the chapter reviews promising researches focusing on the combination of TiO2 with other materials to form layered structures, which provides multifunctional properties such as simultaneous self-cleaning, antireflective, and energy-saving capability. Chapter 14, TiO2 as a source of titanium, describes techniques and issues of converting TiO2 in metallic titanium. In fact, titanium is one of the most important metallic materials with several unique technological properties, which is mainly produced from titanium minerals through the “Kroll” process. This chapter reviews the currently used industrial process, and starting from its high processing costs, reviews methods and techniques to produce titanium from TiO2, valorizing this material at the end of its life cycle. Therefore along with the conventional Kroll process, the chapter also reviews research works related to the electrolytic production of titanium from titanium dioxide in liquid salts. Chapter 15, TiO2 in building sector, reports TiO2 applications in the building sector, with particular reference to cement-based materials and tiles. It is highlighted that the introduction of TiO2 in these materials influences both functional (antipollution, self-cleaning, and antimicrobial) and structural (mainly mechanical strength) properties of the composite. Moreover, advanced applications in geopolymers and in the field of cultural heritage have been considered in detail. The fourth section, containing Chapters 16 18, deals with several TiO2-based devices for optical, electronic, and sensor applications. Chapter 16, TiO2 oxides for chromogenic devices and dielectric mirrors, reviews the applications of TiO2 in optical applications. In particular, in the field of chromogenics, both the applications of TiO2 as cathodic electrochromic material in electrochromic devices and as photo-electrode in photo-electrochromic devices, aiming at a responsive shielding of solar radiation, have been considered. Moreover, the uses of titanium dioxide as a transparent n-type conductor in novel photovoltaic cells, with widespread applications for solar energy utilization, especially toward building integration, have been reported. Finally, the possibility to use TiO2 to design Bragg reflectors and optical devices for light confinement has been described on the basis of the high RI and transparency in the visible range of this material. Chapter 17, TiO2 in memristors and resistive random access memory devices, covers one of the most recent applications of TiO2 thin films in memristors, considered the most promising nonvolatile memories and building units for neuromorphic computing. The chapter aims to describe fabrication methods of TiO2 thin films for electronic devices and to highlight the relationship between method and layer properties. Fundamentals on mechanisms and physicochemical processes of memristors, that is, resistive switching in oxide thin films, will be provided, and the different

10

Titanium Dioxide (TiO2) and Its Applications

types of devices will be described. Finally, the chapter focuses on the correlation between the oxide characteristics and the resistive switching phenomenon, that is, the performances of memristors. Chapter 18, Applications of TiO2 in sensor devices, provides an overview on the TiO2-based sensor devices for the detection of a large variety of chemical species in different environments, starting from pioneering studies to recent innovations. The detection of small molecules in gaseous or vapor phases, as well as the recognition of more complex species in aqueous or biological matrices, has been examined and rationalized in the light of basic working principles. Special attention has been paid to material modification and optimization for the development of innovative sensors for different applications. Finally, the fifth section, comprising Chapters 19 21, deals with catalytic applications of TiO2. Chapter 19, TiO2 photocatalysis for environmental purposes, starts with a brief introduction on the main polluting substances for air and water and focuses on the possibility to degrade them by decontaminating the polluted streams by TiO2 photocatalysis as an advanced oxidation process. The advantages and limitations of this technology with respect to traditional methods for the treatment of gaseous and water pollutants will be highlighted. Basic mechanisms of environmental photocatalysis will be presented along with several case studies on the use of TiO2 in the photocatalytic removal of volatile organic compounds from gaseous stream, for photocatalytic indoor air purification, and in the decontamination of aqueous effluents. Chapter 20, Fine chemistry by TiO2 heterogeneous photocatalysis, reports on the applications of TiO2 as photocatalyst for the production of fine chemicals. Oxidation, reduction, and coupling reactions, together with fuels production, for example, have been reported under mild experimental conditions, using both artificial and sunlight, in organic solvents and water. Particular attention is paid on the possibility to address the selectivity of the synthetic processes by tuning crystallinity and surface physicochemical properties of the photocatalyst. Moreover, the influence of parameters such as the setup configuration and the reactor geometry and the benefits deriving from coupling photocatalysis with a membrane separation unit will be discussed. Various examples of photocatalytic reactions in the presence of TiO2 have been reported, such as oxidation of alcohols to aldehydes, hydroxylation of aromatic compounds, epoxidation of alkenes, hydrogenation of carbon carbon multiple bonds, and some types of alkylation reactions. Chapter 21, Catalytic applications of TiO2, aims to discuss the main applications of TiO2, in its different forms, as effective support or catalyst for thermocatalytic and photo-thermocatalytic reactions, focusing on the role of its structural and physicochemical properties on the catalytic performances. In particular, the use of TiO2 as heterogeneous catalyst and/or active support for metals or other oxides has been reported. In fact, many reactions of industrial relevance involve titania-based catalysts, for example, NOx abatement, Fischer Tropsch and water gas shift, Deacon and Claus processes, hydrodesulfurizations and hydrogenations, selective reductions, and selective oxidations.

Introduction

11

References [1] J.H. Braun, A. Baidins, R.E. Marganski, TiO2 pigment technology: a review, Prog. Org. Coat. 20 (1992) 105 138. [2] K. Mo¨ls, L. Aarik, H. M¨andar, A. Kasikov, A. Niilisk, R. Rammula, et al., Influence of phase composition on optical properties of TiO2: dependence of refractive index and band gap on formation of TiO2-II phase in thin films, Opt. Mater. 96 (2019) 109335. [3] M. Diebold, Application of Light Scattering to Coatings. A User’s Guide, Springer International, 2014. ISBN: 978-3-319-12014-0. [4] M.C.F. Karlsson, Z. Abbas, R. Bordes, Y. Cao, A. Larsson, A. Rolland, et al., Surface properties of recycled titanium oxide recovered from paint waste, Prog. Org. Coat. 125 (2018) 279 286. [5] H.-J. Streitberger, K.-F. Dossel, Automotive Paints and Coatings, John Wiley & Sons, Weinheim, 2008. [6] M. Achimoviˇcova´, C. Vonderstein, B. Friedrich, Mechanically activated rutile and ilmenite as the starting materials for process of titanium alloys production, in: M. Janus (Ed.), Titanium Dioxide, IntechOpen, 2017. [7] Huntsman plans to close remaining TiO2 operations at Calais; provides Pori update, Addit. Polym. 2017 (2017) 7 8. ,https://www.prnewswire.com/news-releases/huntsman-announces-intention-to-close-remaining-operations-at-its-titanium-dioxide-facilityin-calais-france-300425205.html.. [8] J. Chang, TiO2 Players in Major Asset Shuffle, ICIS, 2018. ,https://www.icis.com/ explore/resources/news/2018/07/19/10243097/tio2-players-in-major-asset-shuffle/.. [9] I.H.S. Markit, Chemical Economics Handbook, 2018. ,https://ihsmarkit.com/products/ titanium-dioxide-chemical-economics-handbook.html.. [10] Titanium Dioxide (TiO2) Market Size, Share & Trends Analysis Report by Application (Paints & Coatings, Plastics, Pulp & Paper, Cosmetics), by Region (North America, Europe, APAC, MEA, CSA), and Segment Forecasts, 2019 2025, Report ID: 978-168038-705-6, 2019. ,https://www.grandviewresearch.com/industry-analysis/titaniumdioxide-industry.. [11] J.A. Haider, Z.N. Jameel, H.M. Al-Hussaini, Review on: titanium dioxide applications, Energy Procedia 157 (2019) 17 29. [12] X. Wang, M. Anpo, X. Fu, Current Developments in Photocatalysis and Photocatalytic Materials: New Horizons in Photocatalysis, first ed., Elsevier, 2019, Paperback ISBN: 9780128190005, eBook ISBN: 9780128190036. [13] M. Janus, Application of Titanium Dioxide, IntechOpen, 2017, ISBN: 978-953-513430-5. [14] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37 38.

Properties of titanium dioxide

2

Francesco Parrino1, Francesca Rita Pomilla2, Giovanni Camera-Roda3, Vittorio Loddo4 and Leonardo Palmisano4 1 Department of Industrial Engineering, University of Trento, Trento, Italy, 2 Department of Materials Science, University of Milano-Bicocca, Milano, Italy, 3 Department of Civil, Chemical, Environmental, and Materials Engineering, University of Bologna, Bologna, Italy, 4 Department of Engineering, University of Palermo, Palermo, Italy

2.1

Introduction

Titanium dioxide (TiO2) was discovered in 1791 by the clergyman and mineralogist William Gregor who produced a white metal oxide by calcining black magnetic sands from Menachan in Cornwall (England) [1]. TiO2 naturally occurs mainly as anatase and rutile phases, and rarely as brookite. Usually, natural rutile crystals are impure and, therefore, the first research was limited to ceramic samples, but later (around the year 1950), a single colorless, large synthetic rutile crystal was obtained with the Boule technique [2]. TiO2 is a nontoxic, biocompatible, and inexpensive material with very high dielectric constant and chemical stability. It is a semiconductor with a bandgap ranging from 3.0 to 3.2 eV corresponding to a light absorption edge of c. 387 nm. Depending on its chemical composition, this oxide could present various values of electrical conductivity mainly due to the presence of oxygen defects [3], while the contribution of intrinsic free carriers is negligible even at high temperatures. As a UV light absorber, TiO2 is a white pigment widely used in paints since the 1920s, when it replaced the most important white pigment known until then, namely lead white. Moreover, the high stability to corrosion, the low-cost production, and the nontoxicity make it a good candidate as food additive. Due to its electronic properties, TiO2 has been largely studied and employed as photocatalyst for environmental and green-synthesis purposes. In this contest, it can be used as an additive in building surfaces because of its air detoxification and selfcleaning features. In the form of nanoparticles, TiO2 has unique electronic properties and is a good candidate for use in dye-sensitized solar cells, a photovoltaic technology that converts sunlight into electricity [4,5]. In addition, by taking into account all of the abovementioned properties and outstanding mechanical and rheological behavior, TiO2 is up today one of the most important components of cosmetic products such as UV sunscreen. Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00001-8 © 2021 Elsevier Inc. All rights reserved.

14

Titanium Dioxide (TiO2) and Its Applications

A large number of efforts have been made to synthesize TiO2 materials with different methods to tune its physicochemical properties and to extend its use. In this chapter, we summarize the most important properties of TiO2 underlying the wide range of applications of this material. However, a brief summary of the various chapters of this book in which most of the possible applications are listed is available in Chapter 1.

2.2

Structural properties

2.2.1 Structures of TiO2 TiO2 exists at least in 11 crystalline forms that are illustrated in Fig. 2.1, and whose structural features are summarized in Table 2.1. They are rutile, anatase, brookite, TiO2(B), hollandite-like TiO2(H), ramsdellite-like TiO2(R), columbite-like TiO2(II), baddeleyite-like, TiO2(OI), cotunnite-like TiO2(OII), and fluorite-like cubic phases

(A)

(B)

(C)

(D)

(E)

(F)

(G)

(H)

(I)

(J)

(K)

Figure 2.1 Structures of 11 TiO2 phases rendered in the polyhedron models: (A) rutile, (B) anatase, (C) brookite, (D) TiO2(B), (E) hollandite-like TiO2(H), (F) ramsdellite-like TiO2(R), (G) columbite-like TiO2(II), (H) baddeleyite-like, (I) TiO2(OI), (J) cotunnite-like TiO2(OII), and (K) fluorite-like cubic phases. Unit cells are outlined by using thin lines.

Table 2.1 Structural data of 11 crystalline TiO2 phases taken from cited references. TiO2 phases Structural data

Rutile

Anatase

Brookite

TiO2(B)

TiO2(H)

TiO2(R)

TiO2-(II)

Baddeleyitelike

TiO2(OI)

TiO2(OII)

Cubic

Crystal structure Space group Group # a (A) b (B) c (C) a (
) b (
) γ (
)  Density (g/ cm3) Polyhedron type

Tetragonal

Tetragonal

Orthorhombic

Monoclinic

Tetragonal

Orthorhombic

Orthorhombic

Monoclinic

Orthorhombic

Orthorhombic

Monoclinic

P42/mnm 136 4.5941 4.5941 2.9589 90 90 90 4.248

I1/amd 141 3.7842 3.7842 9.5146 90 90 90 3.895

Pbca 61 9.184 5.447 5.145 90 90 90 4.123

C2/m 12 12.1787 3.7412 6.5249 90 107.054 90 3.734

I4/m 87 10.161 10.161 2.970 90 90 90 3.461

Pbmn 62 4.9022 9.4590 2.9585 90 90 90 3.868

Pbcn 60 4.515 5.497 4.939 90 90 90 4.329

P21/c 14 4.589 4.849 4.736 90 90 90 5.092

Pbca 61 9.052 4.836 4.647 90 90 90 5.251

Pnma 62 5.163 2.999 5.966 90 90 90 5.763

Fm 3m 225 4.516 4.516 4.516 90 90 90 5.761

Octahedron

Octahedron

Octahedron

Octahedron

Octahedron

Octahedron

Octahedron

Augmented triangular prism

Triaugmented triangular prism

Cube

Polyhedra per cell unit  Edge sharing CN(Ti)  Lattice energy (kJ/mol)

2

4

6

8

8

4

4

4

Distorted augmented triangular prism 8

4

4

2

4

3

6 0

6 24.75

6 18.53

6 49.16

6 73.05

6 68.49

6 8.86

7 155.5

7 141.97

9 141.97

8 147.98



Calculated,   per polyhedron, and    relative to rutile calculated at 0K and 0 GPa.

16

Titanium Dioxide (TiO2) and Its Applications

[615]. Moreover, one low-density and two high-density noncrystalline TiO2 types have been reported [1618]. The first six phases (depicted in orange in Fig. 2.1) are stable at atmospheric or low pressure, and their densities are between c. 3.5 for TiO2(H) and c. 4.2 g/cm3 for rutile. The last five phases are high-pressure phases (in green in Fig. 2.1) and have higher densities ranging from c. 4.3 for TiO2(II) to c. 5.8 g/cm3 for the cubic phase. The six TiO2 phases stable at atmospheric pressure comprise TiO octahedra variously sharing corners, edges, and/or faces. On the other hand, the high-pressure phases are octahedra, augmented and triaugmented triangular prisms, or cubes, as a consequence of the changes in the coordination number (from 6 to 9) caused by the increasing pressure. Four phases, that is, rutile, brookite, anatase, and TiO2(B), can be found in nature. The high-pressure phases are commonly obtained in laboratory under controlled pressure conditions. Anatase and TiO2(B) contain chains of edge-sharing octahedra in a single orientation, whereas this occurs in two orientations for rutile, brookite, and TiO2(H). Similar considerations allowed to justify phase transition results in the relevant literature [1922]. Moreover, the number of shared edges enables to roughly predict the sequence of structure energies, being the TiTi distance (and the structure energy) lower with increasing the number of shared edges. For instance, the phase stability of the three most used TiO2 phases is rutile . brookite . anatase, being the number of shared edges per octahedron 2, 3, and 4, respectively. Similar results have been obtained by lattice energy calculation [23] for all of the six phases which are stable at atmospheric pressure and whose stability is in the following order: rutile . brookite . anatase . TiO2(B) . TiO2(R) . TiO2(H) [24]. More in detail, four of the eight neighbors of each octahedra share edges in anatase, while the other ones share corners. In rutile the octahedral structure shares two edges and eight corners. The corner-sharing is along the [1 1 0] direction and stacks with their long axis alternation by 90 degrees [25]. The TiO bond lengths are ˚ for anatase and 1.946 and 1.983 A ˚ for rutile in the equatorial 1.937 and 1.966 A and axial directions, respectively [25]. In brookite, both corners and edges are connected [26].

2.2.2 Main techniques used for TiO2 structural analysis X-ray diffraction (XRD) is a powerful tool to identify the phases of polycrystalline TiO2 materials. The theoretical X-ray patterns of the 11 TiO2 crystalline phases are shown in Fig. 2.2. Patterns are generally quite separated so that the phases can be easily identified. This technique enables also to estimate the crystallinity and the crystal grain size (D), for instance by using the Scherrer equation (Eq. 2.1): D5

Kλ βcosθ

(2.1)

Properties of titanium dioxide

17

Figure 2.2 X-ray patterns of TiO2 crystalline phases. Source: Reproduced by permission from ACS Publications (H. Zhang, J.F. Banfield, Structural characteristics and mechanical and thermodynamic properties of nanocrystalline TiO2, Chem. Rev. 114 (2014) 96139644).

where K is a dimensionless constant, λ is the wavelength of the X-ray radiation, β is the full width at half-maximum, and 2θ is the diffraction angle of the diffraction peak [27]. However, the patterns can be quite broad and signals may overlap for very small TiO2 nanoparticles or for poorly crystalline samples. In these cases, Rietveld analyses [28] can be used to identify the various polymorphs, especially if present in multiple-phase TiO2 samples. Alternatively, atomic pair-distribution function analysis [29] performed by using synchrotron wide-angle X-ray scattering (WAXS) enables to distinguish between different phases in the same sample due to the very high signal-to-noise ratios and the high quality of the X-ray scattering images. Raman spectroscopy is a simple alternative to identify TiO2 nanophases in solid mixtures [30,31]. Fig. 2.3 shows the Raman vibrations signals of rutile, anatase, and brookite.

18

Titanium Dioxide (TiO2) and Its Applications

640

Raman intensity (a.u.)

Anatase

398

517

324

243 Rutile

638 515

397

247

637 395

Brookite

200

300

612 446 514

400

500

600

700

800

Wavelength (nm)

Figure 2.3 Raman vibrations of rutile, anatase, and brookite TiO2 [32].

Anatase and rutile present six Raman-active fundamentals in the vibrational spectrum: three Eg modes at c. 144, 197, and 640 cm21, two B1g modes at 399 and 519 cm21, and an A1g mode at 513 cm21 [33]. The Eg peak derives from symmetric stretching, the B1g peak is related to symmetric bending, and the A1g peak to antisymmetric bending vibrations of OTiO. Brookite TiO2 presents a peculiar Raman behavior, due to the presence of two signals at 446 and 612 cm21 which are not present in the spectra of the other two polymorphs [3437]. Increasing the particle size of TiO2 results in broadening and red or blue shifts of the different vibration frequencies [33]. Choi et al. [37] suggested that both broadening and shifts of the Raman bands with decreasing particle diameter depend on a volume contraction observed in anatase TiO2 nanoparticles due to increasing radial pressure.

2.3

Structure and defects

Defects in TiO2 play a role of paramount importance in almost all of the applications as they often determine or influence its physicochemical properties [3842]. For instance, defects are responsible for the resistive switching properties of TiO2, greatly influence the mechanical properties, induce modifications that address surface interactions in (photo)catalytic applications [43], induce electronic changes, and enable tuning of the optical properties. On the basis of the dimension (D) of the defective structure it is possible to distinguish (1) point defects (0D), (2) line defects (1D), (3) interfacial defects (2D), and (4) bulk defects (3D). Fig. 2.4 shows the dimension scale of the four classes of defects [44].

Properties of titanium dioxide

19

Figure 2.4 Dimension scale of defects. Source: Adapted from C.R.N. Barrett, W.D. Tetelman, S. Alan (Ed.), The Principles of Engineering Materials, Prentice-Hall, Englewood Cliffs, NJ, 1973, p. 554.

2.3.1 Defectivity 2.3.1.1 Point defects Point defects in TiO2 have been widely studied, as they are relatively abundant in nanocrystalline structures and are in the size range of single atomic sites. The various point defects in TiO2 are summarized in Fig. 2.5. Interstitial defects, also known as Frenkel defects, occur when a site, generally not occupied in the perfect lattice (Fig. 2.5A), is occupied by an atom (grey point in Fig. 2.5B). In TiO2 the extra atom can be a foreign one (impurity interstitial), titanium (interstitial Ti), or oxygen (interstitial O). Ti interstitials are common defects in TiO2 and greatly influence surface reactions. On the contrary, only recently O interstitials have been experimentally detected [46], even if their presence has been earlier theoretically predicted [47,48]. In fact, even if the charge of interstitials O (22) is lower than that of interstitials Ti (14), the latter are favored (especially in rutile) due to the close-packed structure of the oxygen lattice sites. The lattice distortion deriving from interstitial defects produces a high energy configuration that enhances the local molecular interactions with adsorbed molecules. Vacancy defects are generated when oxygen (Fig. 2.5C, O vacancies) or titanium (Fig. 2.5D, Ti vacancies) atoms normally occupying a lattice site are missing. O vacancies, abundantly present in reduced TiO2 samples, are responsible for the n-type character of TiO2, due to the two free electrons generated during their formation [41,49]. Obviously, the presence of oxygen vacancies is associated with Ti interstitials. Ti vacancies have been detected in an oxidized p-type TiO2 single crystal at temperatures higher than 800
C [50]. In samples prepared by solgel methods under oxygen-rich conditions, the amount of Ti vacancies is higher when the crystallite size decreases [51,52]. When an electrically neutral TiO2 unit leaves the lattice and migrates to the surface, the bulk vacancy (one Ti and two O vacancies) thereby generated is named Schottky defectivity. Under low pressure of oxygen, the loss of

20

Titanium Dioxide (TiO2) and Its Applications

Figure 2.5 (A) Perfect lattice, (B) oxygen vacancy, (C) Ti vacancy, (D) interstitial defect, (E) substitution of O by a foreign atom, and (F) substitution of Ti by a foreign atom [45].

elemental oxygen from TiO2 is possible with the concomitant formation of oxygen vacancies and/or Ti interstitials. The four electrons available for each released O2 molecule usually create Ti31 centers or go into the conduction band. Substitution defects are generated when a foreign atom substitutes oxygen (Fig. 2.5E) or titanium (Fig. 2.5F) sites in the lattice of TiO2. Positive ions (P [5355], S [56], Fe [57,58], V [59,60], Cr [61], Ni [62], and Pt [63]) generally substitute Ti sites, while negative ions (N [6467], F [68,69], C [70,71], S [72,73], B [74], and P [55]) generally substitute O sites.

Properties of titanium dioxide

21

2.3.1.2 Line defects Line defects, also known as dislocations, can be edge-type (Fig. 2.6AD), screwtype (Fig. 2.6EH) [75,76], or a mixture of them. Edge-type dislocations are generated when an extra plane of atoms is inserted into the lattice (Fig. 2.6AC), while screw dislocations are formed through shearing of the left part of the crystal with respect to the right part (Fig. 2.6EG). Dislocations cause a significant distortion of the chemical bonds along the surface lines which often extends through the bulk. They can be generated in TiO2 through (1) mechanical methods, for instance by treating TiO2 at high temperature and pressure [77]; (2) imperfect oriented attack, when two perfect crystals attach in a disoriented way [7880], especially during their preparation; (3) quenching, due to heterogeneous temperature distribution [81]; (4) film formation, as a result of the mismatch between the film and the support [76,82]. Even if with some exception, small nanoparticles virtually do not present dislocations due to the high energy configuration generated when two dislocations are closely packed [77,83]. Recently, dislocations generated via imperfect oriented attachment have been observed in TiO2(B) nanowires [79]. Dislocations, especially if localized at the grain boundaries of TiO2 nanoparticles [84], cause remarkable variations of the mechanical properties.

2.3.1.3 Interfacial defects The most common 2D defects in TiO2 are grain boundaries, that is, the interfaces between crystals, grains, different phases, different orientations, or different materials

Figure 2.6 Formation of edge-type (AC) and screw-type (EG) defects in TiO2. High Resolution Transmission Electron Microscopy (HRTEM) image of an edge-type defect (D) and a Scanning Tunneling Microscopy (STM) image of a screw-type defect (H). Source: Reproduced by permission from Springer Nature (Z. Zhang, J.T. Yates, Defects on TiO2—key pathways to important surface processes, in: J. Jupille, G. Thornton (Eds.), Defects at Oxide Surfaces, Springer International Publishing, Cham, 2015, pp. 81121).

22

Titanium Dioxide (TiO2) and Its Applications

Figure 2.7 Interfacial defects in TiO2: stacking faults [(A) scheme, (B) TEM image] and grain boundaries [(C) scheme, (D) TEM image]. Source: Reproduced by permission from Springer Nature (Z. Zhang, J.T. Yates, Defects on TiO2—key pathways to important surface processes, in: J. Jupille, G. Thornton (Eds.), Defects at Oxide Surfaces, Springer International Publishing, Cham, 2015, pp. 81121).

[8587]. Interfacial defects can be also generated when an interruption of the stacking sequence of atom planes occurs. These defects are generally named stacking faults [84,88,89]. The red lines in Fig. 2.7 indicate stacking faults (Fig. 2.7A) and grain boundaries (Fig. 2.7C). Fig. 2.7B and D show a stacking fault in anatase TiO2 [88] and grain boundaries between anatase and rutile in commercial TiO2 P25 Evonik [87], respectively.

2.3.1.4 Bulk defects Bulk defects are conventionally identified with large (c. 100 μm) voids or foreign inclusions in the bulk. For instance, the voids within TiO2 nanotubes have been considered ordered void defects [90], while metal cores embedded in a TiO2 shell can be considered inclusions defects [91].

2.3.2 Surface defectivity The surface of TiO2 can be considered an extended 2D defect. The broken symmetry of the surface generates atoms with lower coordination compared to those in the bulk and, in general, different configurations and new structures strongly interacting with external molecules. Fig. 2.8 shows that the surface of TiO2 presents aggregated clusters or voids (3D defects), terraces (2D defects), steps and kinks (1D defects), adatoms, and vacancies (0D defects). Some relevant information on the most important surface defects are reported in the following subsections.

Properties of titanium dioxide

23

Figure 2.8 Scheme of the most common surface defects (A) and STM image of rutile TiO2(1 1 0) surface (B). STM, Scanning tunneling microscopy. Source: Reproduced by permission from Springer Nature (Z. Zhang, J.T. Yates, Defects on TiO2—key pathways to important surface processes, in: J. Jupille, G. Thornton (Eds.), Defects at Oxide Surfaces, Springer International Publishing, Cham, 2015, pp. 81121).

2.3.2.1 O vacancies An oxygen vacancy can exist either in the bulk or on the surface of TiO2. On the rutile TiO2(1 1 0) surface an oxygen vacancy is more stable on the surface than in the bulk [9295]. The atomic structure of the rutile TiO2(1 1 0) surface with some defects is shown in Fig. 2.9 [25]. The surface of TiO2(1 1 0) consists of rows of titanium and oxygen atoms along the [0 0 1] direction. Titanium atoms at the surface can be fivefold coordinated with one dangling bond, or sixfold coordinated as in the bulk. Oxygen atoms are present in plane and in bridging position. In plane, oxygen atoms lie within the main surface plane and are threefold coordinated as in the bulk, while bridge bonded oxygen (BBO) atoms are twofold coordinated. Due to their undersaturation, BBO atoms can be easily removed upon TiO2 calcination giving rise to BBO vacancies (BBOVs) that are commonly considered the most reactive site on TiO2(1 1 0) [96]. BBOV can be detected by scanning tunneling microscopy images that evidence BBOVs as bright spots, which can be statistically counted. Each oxygen vacancy leaves an excess of two valence electrons that partially occupy empty Ti 3d orbitals and determine (at least partially) the formation of an energy state at c. 0.8 eV below the Fermi level. This result has been confirmed by resonant photoemission spectroscopy [9799] by Ultraviolet Photoelectron Spectroscopy [97,100], and Electron Energy Loss Spectroscopy [101] on TiO2(1 1 0) and by theoretical approaches [96,102,103]. The defect electrons reduce Ti41 to Ti31, especially on the subsurface second layer Ti atoms [104,105]. The charge localization along with the lattice distortion thereby generated forms a so-called polaron. Due to the high mobility of polarons that move with low activation energy, defect electrons behave as they were delocalized near to the surface [106108]. Unlike in rutile TiO2(1 1 0), oxygen vacancies are unstable on the surface of anatase TiO2(1 0 1) and preferentially occupy subsurface sites [95,109].

24

Titanium Dioxide (TiO2) and Its Applications

Figure 2.9 Ball and stick model of rutile TiO2(1 1 0) surface. O atoms: gray spheres, Ti atoms: black spheres. Bridging bond oxygen atoms, in-plane oxygen atoms, Ti interstitials, bulk, and surface oxygen vacancies (Ovac) are also indicated. Source: Reproduced by permission from Springer Nature (U. Diebold, Structure and properties of TiO2 surfaces: a brief review, Appl. Phys. A 76 (2003) 681687).

Various experimental techniques are known to induce oxygen vacancies in TiO2 samples: thermal annealing [110112], electron bombardment [113,114], UV irradiation [115,116], doping [117,118], and specific reactions (Marsvan Krevelen mechanism) [119,120].

2.3.2.2 Ti defects Both Ti interstitials and vacancies can be present in TiO2. Even if Ti vacancies have been often detected [5052,121], interstitial Ti defects are believed to play an essential role in the surface chemistry of TiO2 [122136]. Ti interstitials are usually Ti31 ions [137141] and can be formed upon TiO2 sputtering or annealing and in slightly reduced TiO22x (0.0001 , x , 0.0004) samples [41,142]. The electron excess generated by the presence of interstitials Ti can migrate to the surface, thus facilitating surface interaction with O2 [122,123,143] for catalytic applications of TiO2.

2.3.2.3 H defects The presence of H both on the surface and the bulk of TiO2 derives from interaction of TiO2 with hydrogen-containing molecules (e.g., water or elemental hydrogen) and it has remarkable effects in the field of catalysis and photocatalysis [144146]. Hydrogen can be found on oxygen sites as terminal or bridge OH groups at the

Properties of titanium dioxide

25

TiO2 (1 1 0)

(A)

BBO

OH/TiO2 (1 1 0)

+ H2O, 300K

BBOH

BBOV

(B)

H

H O

H O

Bridge

H

O

+ – O – Ti – O –

H

– O – Ti – O –

Terminal

H

– O – Ti – O –

Figure 2.10 Adsorptive water dissociation and formation of surface OH groups at defective TiO2 surface (A) and formation of bridge and terminal OH groups (B). Source: Adapted from Z. Zhang, J.T. Yates, Defects on TiO2—key pathways to important surface processes, in: J. Jupille, G. Thornton (Eds.), Defects at Oxide Surfaces, Springer International Publishing, Cham, 2015, pp. 81121; 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) 589602 [163].

surface of TiO2 [147152], while on Ti sites or on oxygen vacancies as TiH species [149,151,153155]. It can also penetrate into the lattice of TiO2 due to its small size as described in several papers [41,96,147,148,155159]. Formation of OH groups on the surface is related to dissociative adsorption of water at the BBOV sites on the rutile TiO2(1 1 0) surface, giving rise to one OH (named BBOH) at the vacancy and the other OH at a vicinal BBO site, as described in Fig. 2.10A [160162]. Fig. 2.10B describes more generally the formation of bridge and terminal OH groups. Surface OH groups can undergo protonation or deprotonation and are responsible for the acid and basic properties of the surface, along with nonhydrated Lewis acid (Ti41) and basic (O22) sites. Moreover, OH groups determine the hydrophilicity and the charge of the TiO2 surface, are mainly responsible for chemical adsorption of molecules [164], and influence the dynamic of the photogenerated charges mediating water oxidation under irradiation [165]. Other H defects are not stable at room temperature and atmospheric pressure, as they present a high reactivity and mobility. Such adsorbed H species have been detected at a TiO2(0 0 1) crystal cathode [166].

26

Titanium Dioxide (TiO2) and Its Applications

2.3.3 Surface and lattice distortion Molecular dynamics predicted that an isolated 5-nm-sized anatase TiO2 nanoparticle in vacuum is characterized by a distorted and amorphous surface layer, while the bulk counterpart maintains the anatase crystal structure [167169]. This configuration has been confirmed by synchrotron WAXS measurements and X-ray absorption spectroscopy, and it is known as coreshell structure. The coreshell structure, shown in Fig. 2.11, is considered responsible for the mechanical behavior of TiO2. Notably, in aqueous environment the surface dangling bonds are partially saturated by water molecules thus increasing the crystallinity of the structure. For small nanoparticles, surface stresses cause lattice contraction or expansion, depending on the direction of the resultant force [168]. These effects can be detected by XRD analysis. The nature and extent of surface stress depend on morphology, crystalline structure, particle size, surface environment, and chosen synthesis procedure. For these reasons, both lattice expansion and contraction have been observed in TiO2 nanomaterials [170180].

2.4

TiO2 morphologies

This section aims at summarizing the main morphologies in which TiO2 can be synthetized. The different morphologies present different surface-to-volume ratio and different orientations, which afford greatly variating physicochemical properties for various applications. The interested reader is referred to the references below for more details on the different morphologies hereby listed.

Figure 2.11 Molecular dynamics simulation prediction of the structure (cross section) of a B5 nm anatase particle in vacuum. The surface layer (A) and the near-surface layer (B) are highly distorted, but the interior of the nanoparticle (C) retains typical anatase bulk structure. Region (C) disappears as particle diameter decreases toward B2 nm. Source: Reproduced by permission from ACS Publications (H. Zhang, J.F. Banfield, Structural characteristics and mechanical and thermodynamic properties of nanocrystalline TiO2, Chem. Rev. 114 (2014) 96139644).

Properties of titanium dioxide

27

The morphologies of TiO2 nanoparticles of size ranging between 1 and 100 nm can be categorized into three groups: spherical, elongated, and planar. Among the spherical nanoparticles, nanospheres [181], nanogranules [182], nanoplatelets [183], and nanopores [184] have been reported. The elongated nanoparticles comprise nanorods [185], nanotubes [186,187], nanowires [188], nanofibers [189,190], nanoneedles [191], nanopetals [192], nanowhiskers [193,194], nanotrees [195], and nanofilaments [196,197]. Planar nanoparticles are nanoplates [198,199], nanosheets [200], nanofilms [201], and nanocoatings [202,203]. Very often the morphology addresses the field of application of TiO2. For instance, spherical nanomaterials are preferred in electronics [204], photodynamic therapy [205,206], sensors [207], probes [208], catalysis [209,210], and antimicrobial applications [211213]. Elongated nanomaterials have often been used for display technologies [214,215], microelectromechanical systems [216], optical sensors [203] and biological sensing [217,218], imaging [219,220], and drug delivery. Finally, planar TiO2 nanomaterials are utilized for packaging purposes, coatings [221226], and in surgical procedures [227], for wound dressing materials and for tissue engineering [228,229], and as cellular scaffolds [230]. Figs. 2.122.14 summarize the main morphologies of the amorphous, anatase, brookite, and rutile phases, respectively.

Figure 2.12 SEM images of TiO2 anatase: (a1 and a2) nanotubes, (a3 and a4) nanofibers, (a5) nanorods, (a6) nanorods with nanoflakes, (b1) nanorod arrays, (b2) nanoribbons, (b3) nanowires, (b4b6) nanosheets, (c1) 3D network, (c2) nanospheres, (c3) nanocups, (c4 and c5) nanospheres, and (c6) nanoparticles. Insets (a1, a4, a6, b5, and c1) provide the corresponding higher magnification images. Source: Reproduced by permission from The Royal Society of Chemistry (R. Verma, J. Gangwar, A.K. Srivastava, Multiphase TiO2 nanostructures: a review of efficient synthesis, growth mechanism, probing capabilities, and applications in bio-safety and health, RSC Adv. 7 (2017) 4419944224 [231]).

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Titanium Dioxide (TiO2) and Its Applications

Figure 2.13 SEM images of TiO2 brookite: (a1 and a2) nanorices, (a3) nanoparticles, (a4) nanobullets, (b1) nanorices, (b2) eggshell, (b3) nanotubes, (b4) nanoflowers, (c1 and c2) nanorods, (c3) nanotubes, (c4) nanorods. Inset (a4) illustrates the corresponding tilt-view of nanobullets (a1, a2, and b1). Source: Reproduced by permission from The Royal Society of Chemistry (R. Verma, J. Gangwar, A.K. Srivastava, Multiphase TiO2 nanostructures: a review of efficient synthesis, growth mechanism, probing capabilities, and applications in bio-safety and health, RSC Adv. 7 (2017) 4419944224).

Figure 2.14 SEM images of TiO2 rutile: (a1) nanorods, (a2a5) nanorod arrays, (a6) nanorod spheres, (b1, b3, and b4) nanorod bundles, (b2) flower-like super architectures, (b5) urchinlike, (b6) nanospheres, (b7) nanotrees, (b8) a single nanoparticle, (c1) self-assembled nanowires, (c2) microspheres, (c3) nanorod arrays, (c4) quasi spheres, (c5) microspheres, and (c6) nanoparticles. Insets: (lower inset in a1) nanorods at higher magnification, (upper inset in c2) top view of the nanorods with square-shaped ends. Source: Reproduced by permission from The Royal Society of Chemistry (R. Verma, J. Gangwar, A.K. Srivastava, Multiphase TiO2 nanostructures: a review of efficient synthesis, growth mechanism, probing capabilities, and applications in bio-safety and health, RSC Adv. 7 (2017) 4419944224).

Properties of titanium dioxide

2.5

29

Thermodynamic properties

At atmospheric pressure and room temperature, rutile is the most stable phase [232]. However, as a matter of fact, hydrolysis methods and other wet chemistry syntheses often afford TiO2 anatase [233]. These experimental findings suggested the existence of a size-dependent stability region of the TiO2 phases. In particular, anatase could be more stable than rutile soon after its nucleation from solution. For instance, Hwu et al. [234] found that small anatase nanoparticles, with size up to 50 nm, were more stable than rutile and that phase transitions occurred at temperatures higher than 973 K. Various phase transitions have been experimentally observed upon calcination. Even if the final phase was rutile starting from brookite or anatase, both anatase to brookite and brookite to anatase phase transitions were observed prior to the final conversion to rutile. The occurrence of those transitions suggest close energetics, and in particular that the surface enthalpies of the three main phases are strongly affected by experimental conditions and particle size [235,236]. In fact, the entropic term for the three main polymorphs is virtually the same, and consequently, the values of enthalpy can be used instead of those of free energy, although free Gibbs energy thermodynamically governs the phase stability (ΔG 5 ΔH 2 TΔS). The pioneering work of Banfield and coworkers tried to clarify the sizedependent stability of the three main TiO2 polymorphs [235,237]. A stability crossover size of 14 nm was found: below this size the anatase phase was the most stable one, while rutile was stable above this value. This finding was explained by considering the higher surface energy of rutile with respect to anatase. Contradictory experimental results have been reported as far as the brookiteanatase transition is concerned. For instance, Ye et al. observed that brookite did not turn directly into rutile after calcination, but the transition proceeded after obtaining the anatase phase [238]. On the other hand, Kominami et al. reported that the transition of brookite into rutile occurred directly above 973 K [239]. Zhang and Banfield clarify the size-dependent transition sequences of anatase, brookite, and rutile during isothermal and isochronal transformations [236]. Rutile and anatase appeared to be thermodynamically stable when the particles’ size was bigger and smaller than 35 and 11 nm, respectively, brookite when the size ranged between 11 and 35 nm. The surface energies of different surfaces of rutile and anatase were calculated by atomistic simulations [240]. The surface energies of rutile were 1.85, 1.78, 2.08, and 2.02 J/m2 for the [0 1 1], [1 1 0], [1 0 0], and [2 2 1] surfaces, while for anatase [0 1 1] and [0 0 1] the values were 1.40 and 1.28 J/m2, respectively. These values give rise to crystal morphologies consistent with those experimentally observed [240]. Moreover, by considering also the surface stresses of these morphologies in dry conditions, a crossover phase stability of c. 15 nm has been calculated according to the experimental results of Gribb and Banfield [237]. Notably, the presence of water at the interface reduces the surface free energies with respect to the dry conditions both in air and in vacuum. The calculated crossover size in this case is c. 11 nm, in agreement with experimental results [241].

30

Titanium Dioxide (TiO2) and Its Applications

Figure 2.15 Enthalpy of nanocrystalline TiO2. The dark solid line represents the points of lowest enthalpy for the three phases at different surface areas. Source: Reproduced by permission from ACS Publications (X. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications, Chem. Rev. 107 (2007) 28912959).

On the other hand, high-temperature oxide melt drop solution calorimetry results provided the stability ranges of the three phases as illustrated in Fig. 2.15 [242]. Anatase was thermodynamically stable for surface areas higher than c. 3200 m2/ mol, that is, particle size lower than c. 40 nm. Brookite was stable for surface areas ranging between c. 590 and 3200 m2/mol, that is, for particle sizes between 40 and 200 nm. Rutile was stable for surface area values lower than 590 m2/mol, that is, for particle sizes higher than 200 nm. Barnard and coworkers performed a series of theoretical studies on the phase stability of TiO2 nanoparticles in different environments by a thermodynamic model [243247]. The results highlight the importance of the nanoparticle surface state (determined by the surface environment) on the phase stability of TiO2. In fact, the morphology of TiO2 nanoparticles can be modified, thus influencing the average surface energies and stresses. The authors demonstrated that, in different pH conditions and surface environments, the size of the phase transition of a TiO2 nanoparticle varied from 2.6 to 23 nm and was accompanied by its shape changes, as shown in Fig. 2.16 [243247]. However, according to their calculations, the surface energies of surfaces in vacuum are lower than those of hydrated surfaces. This finding contradicted the results of the calorimetry experiments because the calculations considered oxygenated surfaces and not the hydroxylated ones, as is generally expected. Moreover, the contribution of the electrostatic potential of the surface and its dependency on the pH (i.e., the zero-point charge of TiO2) were not considered. By taking into account these two parameters, Barnard and Xu derived the diagram in Fig. 2.17 by means of DFT calculations [248]. It is evident that the solution conditions (mainly the pH)

Properties of titanium dioxide

31

Figure 2.16 Morphology predicted for anatase (top) with (A) hydrogenated surfaces, (B) with hydrogen-rich surface adsorbates, (C) hydrated surfaces, (D) hydrogen-poor adsorbates, and (E) oxygenated surfaces, and rutile (bottom) with (F) hydrogenated surfaces, (G) with hydrogen-rich surface adsorbates, (H) hydrated surfaces, (I) hydrogen-poor adsorbates, and (J) oxygenated surfaces. Source: Reproduced by permission from ACS Publications (H. Zhang, J.F. Banfield, Structural characteristics and mechanical and thermodynamic properties of nanocrystalline TiO2, Chem. Rev. 114 (2014) 96139644).

Figure 2.17 Phase existence of nanostructured TiO2 with various surface terminations (hydrated, hydroxylated, or a mixture of the two) as a function of temperature and particle diameter. Source: Reproduced by permission from ACS Publications (H. Zhang, J.F. Banfield, Structural characteristics and mechanical and thermodynamic properties of nanocrystalline TiO2, Chem. Rev. 114 (2014) 96139644).

32

Titanium Dioxide (TiO2) and Its Applications

modify the extent of the stability region. For instance, the stability region of anatase at neutral pH values is smaller than in basic conditions, in accord with the experimental observation that anatase is more favored in solutions at basic pH.

2.6

Electronic properties

X-ray photoelectron and X-ray absorption and emission spectroscopies [234,245, 249255] have been generally used to determine the electronic structure of TiO2. The electronic map of TiO2 derives from the combination of Ti eg, Ti t2g (dyz, dzx, and dxy), O pσ, and O pπ orbitals. In Fig. 2.18 the molecular orbital structure for TiO2 anatase is illustrated. The valence band is composed by (1) the σ bonding obtained by hybridization of O pσ and Ti d states, (2) the π bonding in the middle energy part deriving from O p

Figure 2.18 Electronic structure of TiO2 anatase. Source: Adapted from X. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications, Chem. Rev. 107 (2007) 28912959.

Properties of titanium dioxide

33

and Ti d orbitals, and (3) the O pπ states in which the contribution of Ti d states is virtually negligible. The bottom part of the conduction bands is mainly composed of dxy states, while at higher energies, Ti eg and t2g bands are antibonding with p states. Therefore, in anatase, nonbonding states are localized close to the bandgap. TiO2 rutile presents a similar situation, even if the nonbonding character close to the bandgap is less pronounced [256]. This is due to the fact that each octahedron in rutile shares eight corners and two edges with neighbors, while in anatase, each octahedron shares ˚ ) is four edges and four corners. This implies that TiTi distance in rutile (2.96 A ˚ ), where the Ti dxy orbitals at the bottom of the much shorter than in anatase (5.35 A conduction band remain quite isolated. However, the relative position of the conduction band edge in anatase and in rutile is still an object of debate. Generally, the conduction band edge in anatase is reported to lie at higher energy (of c. 0.2 eV) with respect to rutile [257,258], but recent calculations [259] report the opposite situation, that is, with the conduction band edge of rutile lying at higher energy than in anatase. These different situations give rise to different mechanisms of interface electron transfer in mixed anataserutile TiO2 samples, as in the commercial P25 Evonik. Electron transfer from rutile to interband surface states of anatase is supported by Electron Paramagnetic Resonance (EPR) experiments [260,261]. Electron transfer in the opposite direction has been also reported in agreement with the relative energy difference between the two phases [262,263]. In both cases, however, the interface electron transfer is responsible for the known higher photocatalytic activity of anataserutile mixtures with respect to the single components, due to better spatial separation and to the consequent higher lifetime of the photogenerated charges. The described electronic structure changes considerably for small TiO2 nanoparticles due to the so-called quantum size effect that eventually results in bandgap widening [264266]. In fact, as the size of the nanoparticle is similar to the de Broglie wavelength of the charge carriers, their quantum mechanical behavior in a confined space results in the discretization of the energy states which finally widens up the bandgap [267]. Even if the existence of the quantum size effect in TiO2 is generally accepted, the critical size below which it becomes apparent is still under debate, as it depends on the effective mass of the charge carriers [268]. Some authors observed the blue shift (of c. 0.2 eV) of the bandgap for TiO2 nanoparticles sized up to 2 nm [269,270]. On the other hand, Monticone et al. did not observe quantum size effect but only a size-dependent variation of the first allowed direct transition [271].

2.7

Electrical properties

The electrical properties of TiO2 nanostructures are dependent on the type of TiO2 polymorph, crystallographic direction, and defectivity. Table 2.2 summarizes some electrical properties of rutile and anatase phases along two directions.

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Titanium Dioxide (TiO2) and Its Applications

Table 2.2 Structural, optical, and electrical properties of rutile and anatase. Polymorphs

Rutile

Anatase

!c Most stable state Bandgap at 10 K Nature gap Static dielectric constant (ε0 in MHz range) High-frequency dielectric constant εN (λ 5 600) Refractive index (at λ 5 600) Room temperature mobility in crystal

Room temperature mobility in polycrystalline thin film

|c (1 1 0) [272]

!c

|c (1 0 1) [273]

3.051 [274,275] Indirect [274,275] 173 [277]

3.035 [274,275] Direct [274,275] 89 [277]

3.46 [276]

8.35 [280]

6.76 [280]

6.25 [281]

2.89 [280]

2.60 [280]

2.50 [281]

0.11 cm2/V s [30,282], 0.010.1 cm2/V s (high impurity concentration) [282], 0.61.5 cm2/V s [283] 0.1 cm2/ V s [282,285]

Indirect [276] 48 [278]

3.42 [276] Direct [276] 31 [279] 6.50 [281] 2.55 [281]

15 cm2/V s [276,284]

0.14 0.1 cm2/V s [276]

TiO2 crystals have high resistivity (B1015 Ω cm) [286]. Oxygen vacancies, titanium interstitials, and reduced crystal surfaces introduce shallow electron donor energy states that enhance the electric conductivity of TiO2 [286,287] and well explain the n-type character of TiO2. In particular, the extent of TiO2 reduction in terms of oxygen loss within the lattice has been correlated with the n-type conductivity enhancement. Oxygen vacancies and titanium interstitials affect conductivity and ionization energy [288]. In rutile, titanium interstitials present ionization energies of c. 0.0070.08 eV [289], while oxygen vacancies result in a shallow (0200 meV) and a deep donor level (600750 meV) contributing to electrical conductivity [282,286,290]. This scenario is less understood in TiO2 anatase [41,284,286,291]. Generally, oxygen vacancies are the dominant phenomenon below 870 K under weakly reducing conditions, while the effect of titanium interstitials prevails at higher temperatures and under strongly reducing conditions [30,286]. However, the recent discovery of the amphoteric features of TiO2, which expresses both n- and p-type characters depending on the oxygen activity [292], highlights the key role of titanium vacancies in addition to oxygen vacancies and titanium interstitials. Therefore, it has been proposed that titanium vacancies may be responsible for the retrieved p-type conductivity at high oxygen activities [50]. Replacement of oxygen with water vapor gives rise to a significant enhancement of the electrical conductivity of TiO2 films [289].

Properties of titanium dioxide

35

Carrier mobility is lower in rutile than in anatase. As a consequence, in rutile the conduction mechanism mainly relies either on small polaron hopping mechanism [293] or phonon scattering [293]. The dependence of the mobility on the temperature for rutile allowed also to hypothesize peculiar charge transport models such as the multiband conduction model [294]. Unlike in rutile, anatase shows an Arrhenius-type thermally activated conduction [295]. In both cases, however, high concentration of lattice impurities above a critical value strongly affects the conductivity [296]. For instance, Nb and Ta atoms increase the conductivity, acting as electron donors [297], while Cr, Mn, and Fe decrease the conductivity acting as electron acceptors [298]. The possibility to opportunely tune the resistivity and to change it upon application of an electric field is the basis of the microelectronic applications of TiO2. It is worth to remind that the resistive switching features (i.e., the possibility to cycle many times TiO2 between two or more resistance states) offered by TiO2 enabled its use in nonvolatile memory technologies.

2.8

Optical properties

Fig. 2.19 shows the main electronic transitions in TiO2. Light absorption in TiO2 mainly relies on direct band-to-band transitions (Fig. 2.19a) when the photon energy is equal to the bandgap energy. Electrons excited to higher energy states dissipate nonradiatively the energy surplus and decade to the lower edge of the conduction band (Fig. 2.19b). In the presence of impurity levels close to the conduction or valence band, intrabandgap electronic transitions are also possible (Fig. 2.19c and d). Absorption of low energy photons has been also observed as intraband transitions (Fig. 2.19e). Recently, the generation of excitons, a strongly electronhole bound state, has been demonstrated to occur in anatase TiO2 [299]. These quasi one particles, held together by Coulomb interactions, lie energetically close to the conduction band and can split into independent holes and electrons.

Figure 2.19 Excitation mechanisms in TiO2.

36

Titanium Dioxide (TiO2) and Its Applications

Absorpon coefficient (a.u.)

The photogenerated charges can radiatively or nonradiatively recombine. Recombination mainly occurs when intermediate energy levels are present, playing the role of traps. TiO2 is an indirect semiconductor. In fact, the minimum of the conduction band is shifted with respect to the maximum of the valence band in the k-space (energy vs momentum) and the optical transition is indirect, as the change of both momentum and energy is required. Absorption of both a photon and a phonon corresponds to a quantum of lattice vibration. Indirect transitions typically lead to lower absorption coefficients than the direct ones, being the involved two-step process less probable than the single step occurring in direct semiconductors. However, for TiO2 (and other indirect semiconductors), indirect transitions are allowed due to a large dipole matrix element and a large density of states for the electron in the valence band [300]. The absorption spectrum calculated by means of the imaginary part of the dielectric function for the three main TiO2 polymorphs is illustrated in Fig. 2.20. The absorption onsets are red shifted with respect to the experimental observations because theoretical methods generally significantly underestimate the optical properties of semiconductors. However, the differences in trend among the three phases are usually valid. The absorption edge of rutile is at the lowest energy, followed by anatase and brookite, which possess a wider bandgap. Anatase has a relatively low absorption coefficient between 500 and 750 nm and, in particular, the absorption coefficient of brookite is even higher than that of anatase at c. 500 nm. Experimental results are

R B A

300

400

500

600

700

800

900

1000

Wavelength (nm) Figure 2.20 Wavelength dependence of the absorption coefficient obtained by ab initio calculations for (A) anatase, (B) brookite, and (R) rutile TiO2 phases. Source: Adapted from D. Reyes-Coronado, G. Rodrı´guez-Gattorno, M.E. Espinosa-Pesqueira, C. Cab, R. de Coss, G. Oskam, Phase-pure TiO2 nanoparticles: anatase, brookite and rutile, Nanotechnology 19 (2008) 145605 [301].

Properties of titanium dioxide

37

qualitatively comparable with the theoretical ones, although conflicting results have been also reported. For instance, the bandgap of brookite, which behaves as an anisotropic direct semiconductor, has been reported being larger [302] or smaller [303,304] than that of anatase. The most straightforward method to measure the bandgap of TiO2 is to record the diffuse reflectance R0N spectrum and calculate the KubelkaMunk function (F(R0N ), Eq. 2.2), which can be considered proportional to the absorption coefficient, by assuming that scattering is independent from wavelength [305,306].
ð12R0N Þ2 F R0N 5 2R0N

(2.2)

A plot of (F(R0N ) hν)1/2 versus the incident photon energy (hν) and a linear extrapolation of the curve to the x axis allows to find the bandgap energy, as shown in Fig. 2.21 for anatase, brookite, and rutile TiO2 phases. The reader is referred to Ref. [308] for some experimental guidelines in this regard. The Fermi level describes the occupation of energy levels in a semiconductor at thermodynamic equilibrium. Generally, in the case of TiO2 the difference between the conduction band edge and the Fermi level can be considered negligible. This assumption justifies many experimental techniques used to obtain the quasi-Fermi level of TiO2 samples, which have been summarized by Beranek [309]. The reported optical properties of TiO2 may greatly differ due to the different techniques used or to the different preparation methods and properties of the samples. Fig. 2.22 reports the conduction band edge and the bandgap experimentally determined for pure samples of anatase, rutile, and brookite prepared by thermohydrolysis of TiCl4 in water at 100
C [307]. Theoretical methods allow to calculate the density of states in semiconductors. The resulting electronic map is far more detailed than the simplistic model illustrated in Fig. 2.22. Notably, two experimental methods have been independently

Figure 2.21 Tauc plot of (A) anatase, (B) brookite, and (R) rutile TiO2 phases. Source: Adapted from A. Di Paola, M. Bellardita, R. Ceccato, L. Palmisano, F. Parrino, Highly active photocatalytic TiO2 powders obtained by thermohydrolysis of TiCl4 in water, J. Phys. Chem. C 113 (2009) 1516615174 [307].

38

Titanium Dioxide (TiO2) and Its Applications

Figure 2.22 Experimentally determined bandgaps and conduction band edges of anatase, brookite, and rutile TiO2 phases. Source: Reproduced by permission from ACS Publications (A. Di Paola, M. Bellardita, R. Ceccato, L. Palmisano, F. Parrino, Highly active photocatalytic TiO2 powders obtained by thermohydrolysis of TiCl4 in water, J. Phys. Chem. C 113 (2009) 1516615174).

developed to estimate the electronic population associated with the different energy levels in semiconductors, and in particular TiO2 [310,311]. Both methods, by means of different spectroscopic techniques, provide a “digital print” of the semiconductor, which allows to distinguish univocally between different samples that have similar physicochemical properties. The bandgap of several TiO2 polymorphs has been predicted through “ab initio calculations” [312315], and in particular for rutile (3.23 eV), brookite (3.86 eV), anatase (3.57 eV), columbite-like (4.09 eV), baddeleyite-like (3.51 eV), cotunnitelike (3.22 eV), pyrite-like (2.69 eV), fluorite-like (2.39 eV), and tridymite-like (5.67 eV) phases. Brookite and tridymite-like TiO2 have a direct bandgap, while the direct bandgap energies are very close to the indirect ones for rutile and columbitelike TiO2, indicating a quasidirect bandgap character. Conflicting results have been reported as far as the influence of morphology on the optical properties of TiO2 is concerned. According to Sato and Sakai, the bandgap is wider for nanosheets than for bulk TiO2 [316,317]. On the contrary, Bavykin et al. provided evidences that different TiO2 nanotubes with different internal diameters presented similar optical properties [318]. Analogously, Enyashin and Seifert reported that the density of states of anatase nanotubes, nanostrips, and nanorolls was similar to the corresponding bulk materials [319]. It is worth to mention that for photocatalytic applications in which the reaction takes place in TiO2 suspensions, the knowledge of the optical properties of the reacting medium is essential to correctly determine the reaction kinetic and design a suitable photoreactor. In fact, photons, which can be considered immaterial reagents,

Properties of titanium dioxide

39

cannot be mixed as a conventional compound, but their concentration varies through suspension resulting in an intrinsically irregular radiant field. The reader is referred to the relevant literature to know more on this complex issue that is out of the aim of this chapter [320].

2.9

Photon-induced behavior

Absorption of photons of suitable energy results in excitation of TiO2, that is, typically in generation of excited electrons in the conduction band and positive holes in the valence band. As previously mentioned, these charges can recombine or migrate and get trapped at the surface where they can undergo interfacial electron transfer. In photovoltaic applications, electrons flow through an external circuit thus generating electricity. In photocatalytic applications, charges induce redox reactions at the TiO2 interface which constitute the mechanistic basis of many light-based applications of TiO2 such as photocatalytic remediation, photocatalytic syntheses, photodynamic therapy, and superhydrophilic surfaces. In this section only, some basic aspects of the main events deriving from the interaction of light and TiO2 are presented. The reader is referred to the chapters dedicated to the corresponding applications for details. Time-resolved absorption spectroscopy is usually employed to determine the time scale and the dynamics of the photogenerated charges. Moreover, EPR spectroscopy allows to detect holes and electrons localized at oxygen anions (O2) and unsaturated cations (Ti31), while EPR silent electrons in the conduction band may be traced by their IR absorption determined by their excitation within the conduction band (see Fig. 2.19e). Eq. (2.3) describes the light absorption step with the concomitant generation of electrons in the conduction band (e2) and holes in the valence band (h1), which typically occurs in the time scale of femtoseconds.
(2.3) TiO2 1 hν ! TiO2 e2 ; h1 The photogenerated charges can be trapped at shallow or deep traps. The surface trapping of holes and electrons occurs between 50 and 170, and 100 and 260 fs, respectively. Generally, OH groups act as surface traps according to the following equations: h1 1 TiðIVÞ 2 OH ! TiðIVÞ 2 O  1 H1

(2.4)

e2 1 TiðIVÞ 2 OH ! TiðIIIÞ 2 OH2

(2.5)

Trapping of holes and electrons at bulk sites occurs between 200 fs and 50 ps, respectively. Typically, electrons are trapped at Ti sites producing bulk Ti31 species, while holes react with lattice or surface oxygen lattice sites [321323], finally producing oxygen vacancies according to the following equation: h1 1

1 22 1 O lattice ! O2 1 vacancy 2 4

(2.6)

40

Titanium Dioxide (TiO2) and Its Applications

Notably, Eq. (2.6) can be read also from the right to left indicating the high tendency of oxygen vacancies to adsorb molecular oxygen. Recombination of the holes and electrons at the surface occurs in 110 ps and at times above 20 ns, respectively, and can be described according to the following equations: h1 1 TiðIIIÞ 2 OH2 ! TiðIVÞ 2 OH

(2.7)

e2 1 TiðIVÞ 2 O  1 H1 ! TiðIVÞ 2 OH

(2.8)

Deeply trapped holes are rather long-lived and quite unreactive with respect to shallow or free ones [324]. The O2 molecules effectively trap electrons at the interface, giving rise to highly oxidizing hydroxyl and superoxide radicals by means of the reductive path shown in the following equations: O2 1 e2 ! O2 

(2.9)

O2  1 H1 ! HO2 

(2.10)

HO2  1 HO2  ! H2 O2 1 O2

(2.11)

H2 O2 1 e ! OH 1 OH

(2.12)

Notably, reduction of oxygen by trapped electrons occurs at time scales shorter than 100 ns, while conduction band electrons reduce oxygen in time scale ranging between 10 and 100 μs. Hydroxyl radicals can be also formed through water oxidation according to the following equation: H2 O 1 h1 !  OH 1 H1

(2.13)

The mechanism of water oxidation has been reported as a concerted electron proton transfer, in which hydrogen bonding of water molecules and the surface of TiO2 play a key role. The effect is that the mobility of the charges is improved together with the lifetime of the holes on the surface up to the order of magnitude of the minutes, thus suppressing recombination [325]. Singlet oxygen is another highly oxidizing species that can be formed at the surface of TiO2 mainly through energy transferbased mechanisms [326]. Hydroxyl and superoxide radicals, together with singlet oxygen and photogenerated holes, are the main actors in oxidative degradative and synthetic photocatalytic applications. The superhydrophilicity of TiO2 surfaces is a photoinduced phenomenon that is difficult to interpret. It has been demonstrated that this phenomenon is not directly related to the photocatalytic activity of TiO2 [327]. It has been proposed that UV irradiation enhances the hydroxylation degree at the surface of TiO2 induced by UV

Properties of titanium dioxide

41

Figure 2.23 Band flattening mechanism of TiO2 under UV irradiation: (A) charge separation in the space charge layer; (B) band flattening determined by hole accumulation at the surface; (C) band flattening determined by electron accumulation in the bulk region. Source: Reproduced by permission from ACS Publications (J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo, et al., Understanding TiO2 photocatalysis: mechanisms and materials, Chem. Rev. 114 (2014) 99199986).

light [328330]. More recently, this phenomenon has been linked to tensile stress induced by dissociative adsorption of water and the concomitant generation of a metastable surface [331]. When TiO2 is in contact with an electrolyte under dark conditions, the so-called band bending occurs as shown in Fig. 2.23. In fact, to establish thermodynamic equilibrium at both sides of the interface [332335], electrons transfer across the interface from the semiconductors to the electrolyte which results in formation of a space charge depletion layer, because the region close to the surface is “depleted” of its majority charge carriers. The accumulation of positive charges at the semiconductor side and the induced negative charges on the solution side hinder further electron transfer. The equilibrium is established and the bands are bent upward close to the interface [333]. Under irradiation, charges are generated and localized at the outer surface region (Fig. 2.23A) because the UV light penetration depth is c. 160 nm [336]. The recombination of the charges is delayed due to the bending of the bands. In fact, the electric field that is produced upon band bending directs the electrons toward the bulk and the holes toward the surface. The presence of the latter ones reduces the negative charge of the solvent side layer and, as a result, band flattening is obtained [337,338] (Fig. 2.23B). However, band flattening can be also justified by a band shift toward negative potentials of the Fermi level in the bulk region (where electrons are transferred during irradiation) as shown in Fig. 2.23C. The abovementioned considerations are valid only if TiO2 consists of perfect crystals as the possible presence of the various types of defects that are present in real solids is not taken into account in the schematic above. Titanium dioxide does not absorb visible light, and solar applications are limited by the fact that the UV light which can be absorbed constitutes only about 4% of the solar emission. Various techniques can be used to extend the absorption range of TiO2 toward visible light, as (1) bulk doping of TiO2, (2) coupling of TiO2 with visible

42

Titanium Dioxide (TiO2) and Its Applications

lightresponsive semiconductors, (3) preparation of metalTiO2 composites, and (4) modification of TiO2 surface. Bulk doping typically introduces intermediate energy levels by enabling excitation in the visible range. Coupling TiO2 with visible light active semiconductors is beneficial only if spatial charge separation is allowed through the opportune relative position of valence and conduction band edges of the counterparts. The most commonly used technique is the photosensitization of TiO2 through surface modification. If the modifying agent (D) absorbs visible light by itself, such as in the case of dyes or metal nanoparticles, its excited state (D ) injects an electron into the conduction band of TiO2, while its oxidized form (D1 ) behaves as the photogenerated hole. This mechanism, known as photoinduced electron transfer, is summarized by the following equations: G

D 1 hν ! D

(2.14)

D 1 TiO2 ! TiO2 ðe2 Þ 1 D1

(2.15)

If the modifying agent itself does not absorb visible light but creates a charge transfer complex with TiO2 which can be excited by visible light, the resulting mechanism is known as optical electron transfer and it can be summarized by the following equations: TiO2 1 D ! ½TiO2 2 D

(2.16)

  ½TiO2 -D 1 hν ! TiO2 ðe2 Þ-D1

(2.17)

Small organics as diols or inorganic molecules such as SO2 typically present visible lightinduced optical electron transfer when adsorbed on the surface of TiO2. The reader is referred to Ref. [339] for details on the photosensitization techniques of TiO2.

2.10

Mechanical and rheological properties

2.10.1 Mechanical properties The elasticity of materials is expressed by their bulk modulus. Synchrotron highpressure XRD and diamond anvil cells techniques [340] have been widely used to investigate the bulk modulus of TiO2. Table 2.3 summarizes bulk moduli and several other properties of nanostructured TiO2 materials. Morphology, crystalline structure, particle size, pressure environment, and other features of TiO2 greatly affect the bulk modulus of TiO2. The results in Fig. 2.24 show that the bulk modulus of TiO2 presents a nonmonotonic trend with the particle size [344]. The maximum values of the bulk modulus (c. 250 GPa) are evident for a particle size of c. 15 nm. This behavior has been satisfactorily modeled on the basis of the

Table 2.3 Bulk modulus of several nanocrystalline and bulk TiO2 phases. Phase

Synthesis method

NP shape

NP size (nm)

Bulk modulus (GPa)

Pressure range (GPa)

Pressure medium

Reference/note

Anatase Anatase

Commercial Commercial

NA NA

Bulk 3040

178 243

08 016

NaCl None

Anatase

Nano samples: hightemperature colloidal method

Elongated rod

3.5 3 5.0

204

010

MEW 16:3:1

[341] [342]/ Nonhydrostatic [343]/Rather scattered data

Bulk sample: commercial Nanosample: crystallization from amorphous nanopowders

Rice shaped Near spherical

3.8 3 5.0 Bulk

319

Anatase

112

ME 4:1

[344]

Bulk: commercial

4.0

6.5 7.2 9.0 13.5 15.0 21.3 24.0 30.1 45.0 Bulk (Continued)

Table 2.3 (Continued) Phase

Synthesis method

NP shape

NP size (nm)

Bulk modulus (GPa)

Pressure range (GPa)

Pressure medium

Reference/note

Anatase

Commercial

NA

20

169

011

MEW 13:3:1

[345]

Anatase Baddeleyite

Elongated rod

40 50200 Bulk

198 176 303

010 1740

ME 4:1 Ar

[346] [347]

Elongated rod

2535

298

1546

Ar

[8]

Rutile Rutile

Hydrothermal Via compression of commercial anatase Via heated compression of commercial 32-nm anatase NIST standard Milling of bulk rutile

ME 4:1 MEW

[348] [349]

Hydrothermal

230 211 210 234

08 020

Rutile

Bulk 10 Bulk 15

018

NA

Rutile

Ethylene glycolmediated synthesis

25

240

09

ME 4:1

[17]/Possibly nonhydrostatic [350]

Baddeleyite

Near spherical

NP means nanoparticle; NA means not available; M, E, and W mean methanol, ethanol, and water, respectively, in volume ratio.

Properties of titanium dioxide

45

Figure 2.24 Dependence of TiO2 bulk modulus on particle size. Source: Reprinted from H. Zhang, J.F. Banfield, Structural characteristics and mechanical and thermodynamic properties of nanocrystalline TiO2, Chem. Rev. 114 (2014) 96139644.

coreshell structure model. For small-sized TiO2 nanoparticles the bulk modulus increases with the particle size because of the (1) enhanced overlap of dislocation strain fields with respect to larger particles, (2) absence of interlocking dislocation networks, and (3) the statistical importance of the more compressible surface layer with respect to the bulk in small particles [344]. These effects dominate for particle sizes lower than 15 nm, thus explaining the trend of the bulk modulus as shown in Fig. 2.24. Morphology, however, affects the bulk modulus of TiO2. For instance, short rice-shaped TiO2 nanoparticles have a bulk modulus maximum (319 GPa) higher than the one of longer rod-shaped particles (204 GPa) [343]. The bulk moduli of various TiO2 polymorphs have been calculated by lattice dynamic and ab initio simulations. Simulations indicated the TiO2(OII) phase as the one with the highest bulk modulus (c. 380386 GPa). Experimental results provided a slightly higher value of the bulk modulus at room temperature (431 GPa) and a hardness of c. 38 GPa at 155160 K, which makes TiO2(OII) one of the hardest oxide material, even if some consisting results have been reported in this regard [15,351]. Nano-indentation and compression/bending tests are used to measure the bulk modulus of TiO2 films and nanowires under microscopic observation [352,353]. For instance, a 140190 nm TiO2 anatase film deposited on AISI 316L stainless steel and Ti6Al4V alloy substrates had a hardness of c. 57 GPa and Young modulus between 228 and 116 GPa, respectively. Notably, being the Young modulus of the film similar to that of the support, the mechanical stability to deformation of the whole integrated system is remarkable. Similar values were obtained for TiO2 films deposited on AISI 1018 low-carbon steel by means of various techniques [354].

46

Titanium Dioxide (TiO2) and Its Applications

2.10.2 Rheological properties The term “nanofluid” refers to colloidal nanoparticle suspensions in various solvents. Nanofluids find applications in fields such as advanced heat transfer, solar collectors, microfluidics, and drug delivery [355358]. Addition of TiO2 nanoparticles to liquid paraffin reduces wear for tribological applications [359]. The size of the nanoparticles remarkably affects the lubricant properties of TiO2 [360362]. Viscosity is probably the most crucial feature of nanofluids, as it influences not only mechanical properties but also the convective heat transfer coefficient and implies significant variations in terms of pumping power costs [363,364]. Both theoretical and experimental approaches have been performed to investigate the viscosity of nanofluids [365367]. The results show that nanofluids have viscosity values higher than those predicted by the Einstein model due to aggregation of the nanoparticles. In particular, it was found that the enhancement of viscosity of nanofluids with respect to the base fluids does not follow the heat transfer performances which are even worse than those of the base fluids. Adjusting the pH values can reduce the abovementioned increase of viscosity. Other parameters affecting viscosity are temperature, aggregation degree, volume fraction, and aggregation of nanoparticles. For instance, the absolute viscosity of aqueous TiO2 nanofluids decreases with increasing temperature [368], while the ratio between the viscosities of the nanofluid and water does not depend on temperature. Khedkar et al. [369] demonstrated that the viscosity of TiO2 suspensions in ethylene glycol nanofluids linearly increases with the concentration of nanoparticles. In brief, it is generally believed that the low viscosity is more preferable for nanoparticle suspensions in practical applications considering the pumping power and pressure drop. The properties of TiO2 allow to use irradiation or an external electric field to tune the viscosity of nanofluids [370,371]. An external electric field induces linear aggregates of nanoparticles parallel to the direction of the field, thus increasing the viscosity of the nanofluid [372,373]. The influence of UV light irradiation on the viscosity was recently studied by Smith et al. [374] both in water (conductive) and silicon oil (insulating). The viscosity increases upon UV irradiation (during 30 s) of c. 2.5% and c. 12.3% in water and silicon oil, respectively. Particle aggregation and electron accumulation have been identified as possible key parameters that control the phenomenon. The observed viscosity increment is irreversible until a certain critical time (unless the nanofluid is treated with ultrasounds), after which further viscosity increases are reversible. The observed photo-rheological phenomenon has been explained by invoking the effects of aggregation and hydrophilicity changes induced by UV irradiation.

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Properties of titanium dioxide

47

[3] A. Yildiz, S.B. Lisesivdin, M. Kasap, D. Mardare, Electrical properties of TiO2 thin films, J. Non-Cryst. Solids 354 (45) (2008) 49444947. [4] P. Bonhˆote, J.E. Moser, N. Vlachopoulos, L. Walder, S.M. Zakeeruddin, R. HumphryBaker, et al., Photoinduced electron transfer and redox-type photochromism of a TiO2anchored molecular diad, Chem. Commun. 10 (1996) 11631164. [5] M. Gr¨atzel, K. Kalyanasundaram, Artificial photosynthesis: efficient dye-sensitized photoelectrochemical cells for direct conversion of visible light to electricity, Curr. Sci. 66 (10) (1994) 706714. [6] W.H. Baur, A.A. Khan, Rutile-type compounds. IV. SiO2, GeO2 and a comparison with other rutile-type structures, Acta Crystallogr. B 27 (11) (1971) 21332139. [7] M. Horn, F. Schwebdtfeger C, P. Meagher E, Refinement of the structure of anatase at several temperatures, Z. Kristallogr. Cryst. Mater. 136 (1972) 273281. [8] W. Baur, Atomabstande und Bindungswinkel im Brookit, TiO2, Acta Crystallogr. 14 (3) (1961) 214216. [9] T.P. Feist, P.K. Davies, The soft chemical synthesis of TiO2 (B) from layered titanates, J. Solid State Chem. 101 (2) (1992). [10] M. Latroche, L. Brohan, R. Marchand, M. Tournoux, New hollandite oxides: TiO2(H) and K0.06TiO2, J. Solid State Chem. 81 (1) (1989) 7882. [11] J. Akimoto, Y. Gotoh, Y. Oosawa, N. Nonose, T. Kumagai, K. Aoki, et al., Topotactic oxidation of ramsdellite-type Li0.5TiO2, a new polymorph of titanium dioxide: TiO2(R), J. Solid State Chem. 113 (1) (1994) 2736. [12] P.Y. Simons, F. Dachille, The structure of TiO2II, a high-pressure phase of TiO2, Acta Crystallogr. 23 (2) (1967) 334336. [13] L.S. Dubrovinsky, N.A. Dubrovinskaia, V. Swamy, J. Muscat, N.M. Harrison, R. Ahuja, et al., The hardest known oxide, Nature 410 (2001) 653654. [14] M. Mattesini, J.S. de Almeida, L. Dubrovinsky, N. Dubrovinskaia, B. Johansson, R. Ahuja, High-pressure and high-temperature synthesis of the cubic TiO2 polymorph, Phys. Rev. B 70 (2004) 212101. [15] N.A. Dubrovinskaia, L.S. Dubrovinsky, R. Ahuja, V.B. Prokopenko, V. Dmitriev, H.-P. Weber, et al., Experimental and theoretical identification of a new high-pressure TiO2 polymorph, Phys. Rev. Lett. 87 (2001) 275501. [16] V. Swamy, A. Kuznetsov, L.S. Dubrovinsky, et al., Size-dependent pressure-induced amorphization in nanoscale TiO2, Phys. Rev. Lett. 96 (2006) 135702. [17] V. Swamy, E. Holbig, L.S. Dubrovinsky, V. Prakapenka, B.C. Muddle, Mechanical properties of bulk and nanoscale TiO2 phases, J. Phys. Chem. Solids 69 (2008) 23322335. [18] D. Machon, M. Daniel, V. Pischedda, S. Daniele, P. Bouvier, S. Le Floch, Pressureinduced polyamorphism in TiO2 nanoparticles, Phys. Rev. B 82 (2010) 140102. [19] J.F. Banfield, D.R. Veblen, Conversion of perovskite to anatase and TiO2 (B): a TEM study and the use of fundamental building blocks for understanding relationships among the TiO2 minerals, Am. Mineral. 77 (1992) 545557. [20] R.L. Penn, F. Banfield Jillian, Oriented attachment and growth, twinning, polytypism, and formation of metastable phases: insights from nanocrystalline TiO2, Am. Mineral. 83 (1998) 10771082. [21] R.L. Penn, F. Banfield Jillian, Formation of rutile nuclei at anatase {112} twin interfaces and the phase transformation mechanism in nanocrystalline titania, Am. Mineral. 84 (1999) 871876. [22] 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) 343353.

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Structural and electronic properties of TiO2 from first principles calculations

3

Sergio Tosoni, Giovanni Di Liberto and Gianfranco Pacchioni Department of Materials Science, University of Milano-Bicocca, Milan, Italy

3.1

Introduction

Titanium dioxide (TiO2) has attracted the attention of the computational chemistry community since many years, due to its interesting properties for catalytic and photocatalytic applications, its low cost, and abundance [1
10]. In particular, a lot of effort has been dedicated to the biggest issue harming the application of titania-based materials in photocatalytic devices exploiting visible light, namely, its large bandgap (3.0
3.2 eV, depending on the phase), which limits the absorption of sunlight by titania to a very small portion of the spectra (B5%). Beyond photocatalysis, titania is widely studied as a support in heterogeneous catalysis or sensing material. The resulting intense computational activity has spanned over bulk titania (from the widely studied rutile and anatase phases to the less common polymorphs) [11], as well as titania surfaces [12,13] as adsorbant for molecules [14] or support for metal single atoms [15,16] and clusters [17
19], or dye sensitizers [20,21]. Point defects in titania have also been thoroughly investigated by means of calculations [22]. The key aspect of such simulations, as often, is the balance between efficiency and accuracy. On the one hand, theoretical simulations of catalysts and photocatalysts must account for large and complex systems with a reasonable computational effort. Structures bearing point defects can be as well computationally demanding if one wants to keep the defects’ concentration at a realistic scale, that is, at low concentration. On the other hand, some quantities such as the bandgap and the ionization potential of titania need to be accurately calculated, in order to provide a solid estimate on charge transfer phenomena at the interface between titania and supported clusters or molecules, as well as on the electronic excitation at the base of any photocatalytic process. In spite of its apparent chemical simplicity, however, accurate calculations of titania’s electronic structure still represent a challenge. The aim of this chapter is to account for these methodological aspects in the frame of the recent specialistic literature (Section 2). We will then focus on the nature and computational description of key point defects (Section 3) and we conclude with an excursus in a relatively new and hot topic, namely, the computational description of TiO2 nanoparticles and heterojunctions. Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00019-5 © 2021 Elsevier Inc. All rights reserved.

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3.2

Titanium Dioxide (TiO2) and Its Applications

Electronic structure calculations on TiO2: methodological aspects

3.2.1 The bandgap issue Achieving an accurate theoretical description of the TiO2 bandgap would allow us to quantitatively predict the energies of defect-related trap states in the gap, with a positive outcome toward the design of titania-based materials with improved optical properties. Unfortunately, the description of the bandgap of semiconductors and insulators is problematic for first principles electronic structure calculations, especially for transition metal oxides like TiO2, where correlation effects on Ti(3d) orbitals are important [23,24]. TiO2 is a wide bandgap semiconductor, whose valence band is predominantly composed of O(2p) orbitals, while the conduction band displays mainly Ti(3d) character. Early calculations of the anatase band structure based on the Hartree
Fock theory yielded a minimum energy gap larger than 10 eV [25], with a substantial overestimation of the experimental value of B3.2
3.4 eV given by optical absorption measurements [26,27]. Density functional theory (DFT) does not provide the desired result either, since both the local density approximation (LDA) [28,29] and the generalized gradient approximation (GGA) [30,31] strongly underestimate the bandgap of titania. The very popular Perdew
Burke
Ernzerhof (PBE) [32] functional, for instance, yields for anatase bandgap values in the range of 2.08 [31], 2.19 [30], 2.20 [33], and 2.36 eV [31]. This fact has been explained as a consequence of the self-interaction error present in standard DFT functionals [34]. In order to partly solve these issues, various strategies have been implemented, such as the use of a “U” correction that can be added to the local and semilocal density functionals (LDA 1 U and GGA 1 U) [35
38]. The role of the U parameter is to treat the strong on-site Coulomb interaction of localized electrons by adding to the Hamiltonian a Hubbard-like term. This provides a simplified description of strongly correlated electronic states (Ti(d) orbitals in the case of titania), while the rest of the valence electrons is treated by the usual DFT approximations. Another approach consists in the introduction of a fraction of exact exchange, as in hybrid functionals [39]. Hybrid functionals were introduced in quantum chemistry with the aim to correctly describe molecular properties such as the atomization energy [40,41]. The exact Fock exchange energy Ex is added to GGA type functionals (whose exchange and correlation contributions are denoted Ex GGA and Ec GGA , respectively); the general form of the exchange-correlation energy for one-parameter hybrid functionals is Exc 5 αEx 1 ð1 2 αÞEx GGA 1 Ec GGA :

(3.1)

The fraction of exact exchange, α in Eq. (3.1), may vary from 0 (original GGA functional) to 1 (Hartree
Fock exact exchange potential) [42]. Several hybrid functionals have been designed, differing by the specific choice of the GGA functional approximating the semilocal part and the value of α [34]. Among all, the PBE0 [43] and B3LYP [44] functionals are perhaps the most popular. B3LYP does not

Structural and electronic properties of TiO2 from first principles calculations

69

follow Eq. (3.1) but is based on a different three-parameter formula fitted to reproduce a set of experimental data (20% of exact exchange, or α 5 0.20). Instead, PBE0 builds on the original Becke’s one-parameter hybrid with α 5 0.25. Hybrid functionals have been widely applied to solid-state as well, showing often a significant improvement over GGA in the description of bandgaps, as well as of other properties [45
47]. Some hybrid functionals specifically designed for solid systems have also been developed. In particular, range-separated hybrids were proposed, displaying a spatial partition where the Coulomb kernel is split in short- and long-range contributions. A range-separated hybrid where the long-range part of the Coulomb interaction is neglected (short-range hybrids) is the Heyd
Scuseria
Ernzerhof (HSE) functional [48,49] where the same amount of exact exchange as in PBE0 (α 5 0.25) is adopted in the short-range. Screened exchange functionals have been successfully applied to the calculation of the properties of solids [50
54]. In the case of titania, standard hybrid functionals, such as B3LYP [25] and PBE0 [24,26], overestimate the bandgap. With B3LYP, which has a 20% fraction of exact exchange, bandgap values of 3.9 [11] and 3.98 eV [21] have been reported, while an even larger value, 4.50 eV [21], is obtained with PBE0, which has a larger fraction (25%) of exact exchange. The screened hybrid functional HSE06 [28] yields a bandgap in better agreement with the experiment (3.58 eV) [27]. A drawback of hybrid functional calculations is that α is determined empirically, often by best fitting some properties over databases of molecular systems. There is thus no warranty that the same level of accuracy is kept when dealing with different properties of different systems. An advance, in this sense, is represented by a scheme where the fraction of exact exchange in the hybrid functional is set equal to the inverse of the optical dielectric function, 1/εN. The approach is known as dielectric-dependent DFT. One can eliminate any residual empiricism by iteratively determining α and εN until convergency is reached [55
57]. Even though dielectric-dependent functionals did not prove to be systematically superior to standard hybrids over a wide range of oxides, good estimates (as good as HSE) of the bandgap of TiO2 are provided with this approach [58]. When discussing methodological aspects of the bandgap calculation in oxides and, more in general, in semiconductors and insulators, a remark needs to be done: DFT is a ground-state theory, and the energy difference between the valence and conduction band edges (the so-called eigenvalue gap) is not strictly comparable to optical absorption or emission measurements. A more fundamental, though computationally demanding, approach to the study of the electronic properties and the determination of the bandgap is offered by the many-body perturbation theory as formulated in the Green function (G)-screened dielectric tensor (W) GW approximation [59]. During the last few years, there have been several investigations of the TiO2 band structure based on this approach [60
62]. These studies have also stressed the conceptually important difference between the one-particle gap, as inferred from photoemission and inverse photoemission measurements, and the optical gap, as obtained from optical absorption measurements, where also the effects of the electron
hole interaction contribute. For the minimum indirect one-particle gap in anatase, the results of GW calculations from

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Titanium Dioxide (TiO2) and Its Applications

different groups range from 3.56 [61] to 3.79 [60] to 3.83 eV [62], that is, still about 0.2
0.6 eV larger than the measured optical gap of 3.2
3.4 eV. This has raised the question of the role of excitonic effects in the experimental determination of the bandgap of TiO2. Excitonic effects can be effectively accounted for by using the GW eigenvalues and dielectric terms as input in the Bethe
Salpeter equation. In the case of TiO2, the excitonic effects on the bandgap are very small (i.e., the exciton binding energy is negligible), but the shape and the intensity of the simulated absorption spectra are significantly affected by electron
hole interactions [61
63].

3.2.2 Excess electrons (and holes) in TiO2: the localization problem Titania samples in the form of powders, single crystals, or thin films are often reduced and electronically conducting [64,65]. The reduction is associated with the population of the Ti(3d) orbitals composing the oxide’s conduction band, with a change in the formal Ti oxidation state from Ti41 in pristine TiO2 to Ti31 in the reduced one [33]. The presence of Ti31 species is at the basis of many applications related to electronic conductivity such as photocatalysis [4,66], photochemical water splitting [3], or dye-sensitized solar cells [1]. The presence of Ti31 species in reduced titania is related to several experimental features observed upon reduction. The color of the samples, for instance, turns bluish, and its intensity correlates with the degree of reduction, a fact attributed to d-d electronic transitions [67,68]. Photoelectron measurements [69
72] and energy loss spectra [73] reveal the presence of a gap state located 2 eV above the valence band edge and conversely 1 eV below the conduction band minimum. Ti31 ions display a paramagnetic 3d1 electronic configuration and are thus active in the Electron Paramagnetic Resonance (EPR) [74
76]. Also, surface science techniques such as scanning tunneling microscopy were recently able to map the contribution from excess electrons in reduced titania [77]. If one can thus assume the presence of reduced Ti31 centers as proven, the debate on their origin and nature is still ongoing. First, reduced Ti31 centers may originate from interstitial Ti species or from the removal of neutral oxygen atoms from the lattice. The reduction of TiO2 samples can be obtained by preparation under nonstoichiometric conditions or upon annealing in vacuum. In both cases, the material loses oxygen. Depending on the method used, a different degree of nonstoichiometry is achieved with an excess of Ti ions with respect to O ions. Both TiO2-x and Ti11xO2 stoichiometries are consistent with this picture. In the first case, a number of oxygen ions are lacking from the lattice positions, in the second case, excess Ti atoms are present in interstitial sites of the lattice. A mild thermal treatment of a stoichiometric sample generates initially O vacancies; longer treatments or annealing at higher temperatures may result in an oxygen depletion from the external layers of the material and to the consequent diffusion of Ti species into the bulk with formation of

Structural and electronic properties of TiO2 from first principles calculations

71

interstitials. Most likely, both species are simultaneously present, although their concentration can be different and depend on the history of the sample [78
80]. Moreover, it is not fully understood if the excess electron fully localizes on a single Ti ion, or it is delocalized over several sites. From the point of view of the electronic structure, the localized solution corresponds to an energy state in the oxide’s bandgap, while the delocalized solution implies that the conduction band is populated to some extent [81]. The localized or delocalized nature of the excess electron influences transport phenomena [3,82]. This complexity reflects into challenging aspects for the theoretical description of reduced titania [33]. The self-interaction energy problem inherent to LDA and GGA functionals, for instance, does not affect only the bandgap, as previously described, but results also in an overestimation of the electron delocalization [34]. Also in this respect, DFT 1 U or hybrid functionals permit to observe localized solutions as well. When this localization occurs, a polaron forms, that is, a local distortion around the Ti ion that traps the excess electron (see below). The formation of polarons also has important consequences on the mobility of electrons and holes in the material. However, this matter remains to some extent elusive, given the strong dependence of the results on the adopted level of calculation. Beyond the case of reduced titania, the localized or delocalized nature of electrons hosted in Ti(3d) states is crucial also for photocatalytic applications of titania. As we mentioned above, photocatalysis is based on the absorption of light with consequent electron excitation and creation of holes in the VB (h1) and electrons in the CB or in localized orbitals of the system (e2). The distinction between localized or delocalized electrons and holes has a direct effect on the photoconductivity, which is enhanced if the charge carriers are mobile and delocalized. The localization of electrons and holes leads to the formation of polarons [83,84]. Polarons are local lattice distortion induced by the localization of a charge carrier. The proper description of polarons in electronic structure calculations presents some problems. In first principles calculations, the formation of polarons can be achieved by adding or removing one electron to the supercell. It is thus necessary to permit a full relaxation of the ionic coordinates during the simulation in order to see the formation of a polaron, with the complication related to the existence of several local minima. In Figs. 3.1 and 3.2 examples of fully delocalized and fully localized electrons in CB and holes in VB are reported for rutile TiO2 from the literature [85]. The localization of the charge carriers is related to a key quantity, the selftrapping energy, defined as the energy gain of the localized solution with respect to the delocalized one. The calculated self-trapping energy has no direct experimental counterpart, but the comparison to EPR data [86,87] is instructive, since this technique permits to estimate the level of electron localization from the hyperfine coupling constant [88]. We will now discuss in more detail the computational description of intrinsic defects in titania, by keeping in mind the crucial aspect of the localization.

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Figure 3.1 Plot of charge density associated with (A) a free (delocalized) electron and (B) a (localized) self-trapped electron polaron in TiO2. Ti atoms: blue (large) circles; O atoms: red circles. (C) The configuration coordinate diagram shows energy as a function of the lattice distortion for a free electron and a localized electron. Source: Reproduced with permission from A. Schleife, J.B. Varley, A. Janotti, C.G. Van de Walle, Conductivity and Transparency of TiO2 from First Principles, Proceedings SPIE 8822, Solar Hydrogen and Nanotechnology VIII, 2013, p. 882205 (Accessed 16 September 2013). https://doi.org/10.1117/12.2024566.

Figure 3.2 Plot of charge density associated with (A) a free (delocalized) hole and (B) a (localized) self-trapped hole polaron in TiO2. Ti atoms: blue (large) circles; O atoms: red circles. (C) The schematic configuration coordinate diagram shows energy as a function of the lattice distortion for a free hole and a localized hole. Source: Reproduced with permission from A. Schleife, J.B. Varley, A. Janotti, C.G. Van de Walle, Conductivity and Transparency of TiO2 from First Principles, Proceedings SPIE 8822, Solar Hydrogen and Nanotechnology VIII, 2013, p. 882205 (Accessed 16 September 2013). https://doi.org/10.1117/12.2024566.

3.2.3 Oxygen vacancies The removal of one neutral lattice oxygen atom leaves two extra electrons in the lattice (compared to the formal 22 oxidation state of O ions in TiO2). In ionic solids, such as MgO, the strong long-range electrostatic Madelung potential stabilizes the trailing electrons in the cavity formed upon the oxygen removal. In TiO2, where the ionic character is only partial and the energy position of the empty states (conduction band) is much lower, the two electrons tend to fill the empty Ti(3d) orbitals, resulting in Ti31 species [89]. The two electrons associated with the defect may pair up on the same site giving a singlet closed-shell state or form open-shell singlet or triplet states. Moreover, as

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mentioned above, the two electrons may be delocalized over several Ti ions or localized on single Ti ions. One cannot exclude a situation where one electron is localized and the other is delocalized onto several sites [90,91]. Note that some of these solutions imply the breaking of the lattice symmetry. As shown in a methodologic computational study of defect states in bulk anatase [33], the electronic structure features of the oxygen vacancy strongly depend on the adopted functional. With the PBE GGA functional, a completely delocalized solution is found, where the two electrons are shared on all Ti ions, singlet and triplet states are almost degenerate, and the lattice distortion induced by the vacancy formation is small. The energy levels related to the excess electrons appear at the edge of the conduction band (Fig. 3.3). Clearly, such a picture is hardly concealable with the experimental evidence discussed above, and in particular with the presence of a gap state at about 1 eV below the conduction band.

Figure 3.3 Total (DOS) and projected density of states (PDOS) for the oxygen vacancy in anatase TiO2 obtained with B3LYP (A, partially localized solution; B, fully localized solution; C, delocalized solution), PBE 1 U (A, partly localized solution; B, fully localized solution), and PBE methods. The PDOS is on the Ti31 atoms; the Ti31 atoms of the localized solution are considered also for the PDOS of the delocalized solution. The energy zero is set at the top of the valence band. The dotted line indicates the Fermi energy. Insets: Spin density plots. Blue and red shaded areas indicate the PDOS projected onto the sixfold and fivefold coordinated Ti atoms, respectively. Reprinted with permission from C. Di Valentin, G. Pacchioni, A. Selloni, J. Phys. Chem. C 113 (2009) 20543
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The hybrid functional B3LYP gives a completely different solution. The ground state is a triplet, and the extra electrons localize on different Ti ions (not necessarily among the first neighbors of the removed O ion). Three solutions were obtained (Fig. 3.3): (A) one electron is localized on a Ti5c atom while the second one is delocalized on two layers of lattice Ti6c ions; (B) one electron is localized on a Ti5c atom and the other one is localized on an adjacent Ti6c ion; (C) both electrons are delocalized as in the PBE solution. The energy difference between (A) and (B) solutions is less than 0.01 eV, while solution (C) is 0.17 eV higher in energy. Localization, thus, is a delicate problem, given the existence of several localized solutions almost degenerate in energy. The delocalized solution is slightly higher in energy, but the difference is small enough to speculate that thermal effects will play an important role in promoting electron transport in anatase. The density of states (DOS) plots in Fig. 3.3 show for solutions (A) and (B) gap states whose energy is, with respect to the conduction band edge, compatible with photoemission measurements. Like in B3LYP, GGA 1 U (U 5 3 eV) yields almost the same energy for (A) and (B) localized solutions. Also, in this case, gap states are associated with the oxygen vacancy (Fig. 3.3). The correction of the self-interaction error is, thus, crucial in order to reproduce the experimental evidence on Ti31 sites in reduced titania.

3.2.4 Interstitial Ti species As mentioned above, upon high-temperature treatments Ti species leave their lattice positions and occupy interstitial sites [78]. However, it is hard to determine the charge state of these species experimentally. A simple computational model to study this problem consists in the introduction of a neutral Ti atom in the anatase or rutile bulk lattice, followed by a structure relaxation [92]. The Ti atom is spontaneously oxidized, releasing extra electrons to the titania framework, in analogy to the case of the oxygen vacancy. The Ti interstitial assumes a distorted pyramid coordination with five oxygens (Fig. 3.4). The corresponding electronic structure

Figure 3.4 Structure of an interstitial Ti31 ion in bulk anatase. The four optimized distances are ˚ . For comparison, the B3LYP Ti-O distances for d1 5 1.939, d2 5 1.934, d3 5 1.997, d4 5 3.460 A ˚ ˚ (Ti-Oax), respectively. stoichiometric bulk anatase are 1.940 A (Ti-Oeq) and 2.020 A Adapted with permission from E. Finazzi, C. Di Valentin, G. Pacchioni, J. Phys. Chem. C 113 (2009) 3382
3385. Copyright (2009) American Chemical Society.

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is characterized by several defect states in the bandgap; these states are localized and involve the Ti interstitial as well as other reduced Ti6c lattice sites. Using the B3LYP functional, a solution where the interstitial Ti has a 13 charge state is more stable than any Ti21 or Ti1 solution, thus assessing that only Ti31 species are present as interstitial cations. The spin density is mostly localized on the interstitial Ti.

3.2.5 Photoexcited carriers Upon UV irradiation, titania absorbs photons generating holes in the valence band and excited electrons in the conduction band. This process has been studied by several experimental techniques, such as EPR [93
97], photoluminescence [26], O2 photodesorption [98], and picosecond X-ray spectroscopy [99], but a full characterization of the nature of these states has not been reached yet. Spin-polarized B3LYP calculations for the excess hole, electron, and hole
electron pairs in bulk anatase were performed to shed some light on the structure and energetics of these excited species (structures and spin density contour plots are reported in Fig. 3.5) [12]. No localization is found for the excess electron, neither for the excess hole, unless the ionic structure is fully relaxed, allowing for the formation of a small polaron. Then, the extra electron is localized on a Ti lattice site (the fraction of electron density trapped on the site is 80%), which changes its formal oxidation state from Ti41 to Ti31, with a self-trapping energy of 0.23 eV. The Ti-O bonds ˚ ). around the reduced center undergo a remarkable elongation (0.05 A In the case of the hole, the localization on an O22 site, which assumes O2 character, implies a larger trapping energy (0.74 eV) compared to the electron. The ˚ ). polaronic elongation of the O-Ti bonds is very large (0.1
0.2 A

Figure 3.5 Spin density plot of an electron
hole pair (A), one extra electron (B) and one extra hole (C) in bulk anatase TiO2. Adapted with permission from C. Di Valentin, A. Selloni, J. Phys. Chem. Lett. 2 (2011) 2223
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The photoexcited electron
hole couple can be simulated by forcing the system’s spin state to a triplet configuration, which implies the promotion of an electron from the valence band to the conduction band, mimicking the photoexcitation process. After full geometry relaxation, the self-trapping energy of the electron/hole pair is 0.58 eV, less than the sum of the self-trapping energies of isolated hole and electron, indicating some destabilizing interaction between the respective polarons. This relaxed excited state configuration lies 3.5 eV above the singlet ground state. Both the electron and the hole are strongly localized on a Ti and O site, respectively.

3.3

Titania heterojunctions and nanoparticles: computational modeling of cutting-edge materials

3.3.1 Separation of photoexcited charge carriers in titania nanocomposites The efficiency of photocatalytic devices is often hindered by the very fast recombination of the photoexcited charge carriers: the photogenerated electron and hole may recombine, bringing the system back into its ground state, before any desired chemical reaction can actually take place on the surface of the nanoparticle. A strategy to partly overcome this problem relies on composite materials, where two components are interfaced to form a so-called type-II (staggered gap) heterojunction (Fig. 3.6). Here, the favorable alignment of the band edges drives the photoexcited electrons toward the CB edge lower in energy (Component II in Fig. 3.6) and the holes toward the VB edge located at higher energy (Component I in Fig. 3.6). In this way, a spatial separation of the charge carriers is ensured, which prolongs their lifetime and increases the photoactivity. Titania, a key material in photocatalysis, has also been widely applied in heterojunctions for photocatalytic purposes. For instance, we can find structures made by TiO2 joined with a metal [100
102], graphene [103
105], or with other oxides [106,107]. There were also instances where composites interfacing two components of the same oxide, namely titania, were prepared. To name one, the famous P25 photocatalyst

Figure 3.6 Schematic representations of three different types of heterojunction, classified according to the reciprocal alignment of valence and conduction band edges.

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consists of a mixture of anatase (75%) and rutile (25%) [108]. Recently, a heterostructure was proposed where both components are actually anatase TiO2, and the heterojunction consists of an interface between the (101) and (001) faces [109,110]. The strong photoactivity detected experimentally was attributed to junction’s effects related to the staggered-type band alignment of the two coexposed surfaces. In an attempt to rationalize this finding, we performed hybrid functional calculations of the coexposed (101) and (001) faces [111], confirming that indeed a type-II heterojunction is formed, where the valence band edge of TiO2 (101) lies below the (001) edge. Similarly, the conduction band edge of the (101) surface is lower in energy than the (001) surface. The photoexcited electrons are thus driven to the (101) moiety, while the holes are favorably hosted on the (001) component. By forcing the system in a triplet spin state, as previously described for anatase bulk, we simulated the absorption of one photon and the generation of a photoexcited electron and a hole in the system. As shown in Fig. 3.7A, as long as the ionic structure is kept fixed at the ground state geometry, a vertical excitation is obtained. The electron is fully delocalized in the conduction band of the (101) moiety, as evidenced by the absence of spin density, while the hole is delocalized over several O sites on the (001)

Figure 3.7 Coexposed (101)/(001) anatase surfaces. Different triplet states configurations, with their corresponding energy with respect to ground state configuration. In yellow are reported spin density iso-surfaces (0.005 |e|/(Bohr3)); in pink and red are shown Ti and O atoms. Orange and blue circles indicate hole and electron polarons, respectively. (A) Vertical transition, that is, triplet charge distribution at the ground state geometry. (B, C) Two different configurations characterized by electron and hole in the same and in separated surfaces, respectively. Reprinted with permission from G. Di Liberto, S. Tosoni, G. Pacchioni, J. Phys. Chem. Lett. 10 (2019) 2372
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component of the heterojunction. The energy cost related to the vertical excitation is 3.40 eV. Upon structural relaxation and induction of polaronic distortion, first the hole (Fig. 3.7B) and then the electron (Fig. 3.7C) become trapped and localized. Their spatial separation on the two opposite sides of the composite can be well appreciated. The self-trapping energy gain is 2.03 eV for the hole and 0.49 eV for the electron.

3.3.2 Computational modeling of titania nanoparticles Titania nanoparticles smaller than 10 nm are nowadays considered as essential components for materials with a wide range of applications, spanning from water remediation (exploiting their photocatalytic activities) [112] to medicine, where they have shown strong theranostic potential [113
115]. Nanoparticles, however, intrinsically differ from their bulk counterparts under many aspects: surface area, quantum confinement effects influencing their bandgap, complex morphology influencing their chemical reactivity, and so on. From the computational point of view, this fact represents a challenging problem: on the one hand, as discussed in the previous sections, high-quality DFT methods such as hybrid functionals are necessary in order to correctly describe band structure and physical properties of titania, on the other hand, a realistic model of a nanoparticle as large as few nanometers in diameter implies several hundreds of TiO2 formula units, which puts the feasibility of the calculations severely at stake. This fact stimulated an intense investigation along two main lines: (1) a methodological approach aiming at finding cheaper methods accurate enough to provide useful hints on electronic and structural properties of titania nanoparticles and (2) an approach focused on structural aspects, aiming at studying dependence and scalability of key properties such as the bandgap on nanoparticle size. We start this brief overview from a paper where excitation energies of small (TiO2)n clusters (n 5 1
13) have been calculated with various time-dependent (TD) hybrid DFT approaches and benchmarked against high-quality coupled-cluster methods [116]. The results show a general robustness with respect to the choice of the functional and a general good performance of TD-DFT with respect to the high-quality benchmark. In particular, the robustness of the TD-DFT approach is enhanced if one considers the nanoparticle in an aqueous environment, as it is the case in both environmental and biomedical applications. More critical is the case of the polaron formation related to the localization of the excitons: here, B3LYP showed to behave less accurately compared to range-separated hybrid functionals such as CAM-B3LYP or BHLYP [117]. The life path of charge carriers and excitons of nanoparticles in the size range of 2
3 nm has been studied by means of B3LYP hybrid functional calculations [118]. Localization was found to take place on differently coordinated sites with very similar energies. The shape effect (cuboctahedral vs spherical) and the surface hydroxylation were considered too. However, one still needs computationally cheaper methods than hybrid functionals to treat particles in the nanometric regime. For this purpose, the simplified tight-binding density functional theory method (DFTB) has been benchmarked against standard hybrid DFT for the case of titania nanoparticles [119,120]. The main approximation of DFTB compared to DFT consists of replacing the twoelectron repulsive terms with parameterized pairwise terms depending on the atom’s

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charge [121]. The usage of DFTB successfully permitted to scale-up the nanoparticle’s size from few hundred atoms up to several thousands of atoms, recurring also to demanding global optimization techniques to determine their global minimum energy structure. The interaction of TiO2 with the aqueous environment was also studied by means of molecular dynamics techniques [119]. An alternative approach has been recently applied to the study of titania’s nanoparticles, where very large structures are scanned with semiempirical force field techniques, and then the more promising ones are refined with single-point calculations at the hybrid DFT level exploiting supercomputing resources. In this way, it has been possible to study very large structures, aiming at checking the convergence of some relevant properties of the nanoparticles with respect to their size. The reciprocal band alignment of anatase and rutile, perhaps the key factor determining the photoactivity of the P25 catalyst [122], was shown to converge to the bulk values only for particles as large as 15 nm [123]. A proper bulk-like behavior of anatase nanoparticles is reached at an even larger size (20 nm), regardless of the shape and truncation criteria, octahedral or cuboctahedral, of the particles (Fig. 3.8) [124].

Figure 3.8 Octahedral (top) and cuboctahedral (bottom) (TiO2)N nanoparticles. Reprinted with permission from O. Lamiel-Garcia, K.C. Ko, J.Y. Lee, S.T. Bromley, F. Illas, J. Chem. Theory Comput. 13 (2017) 1785
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Another key aspect of the application of TiO2 nanoparticles is their surface functionalization. In the case of photocatalysis, the nanoparticle’s surfaces are often decorated with small-gap molecules such as the dye sensitizers, in order to increase their light absorption capability in the visible region of the spectrum. The photons captured by the dye sensitizer generate charge carriers that are then injected to the TiO2 nanoparticle, preventing recombination and increasing the photocatalytic activity [125,126]. Even more critical is the case of the functionalization of titania nanoparticles for in vivo usage in nanomedicine: here, the surface decoration has the key role of increasing the nanoparticle’s biocompatibility, prevent toxic effect and ensure that the nanoparticles are correctly delivered through the human body. To this aim, extensive computational studies of titania nanoparticles decorated with catechol or dopamine have been performed (Fig. 3.9) [127].

Figure 3.9 Adsorption configurations (side views) and energies per molecule in electronvolt for one dopamine molecule on the surface of the spherical TiO2 nanoparticles as obtained by DFT (DFTB) calculations. Relevant distances are reported in Angstrom. Source: Reproduced from C. Ronchi, M. Datteo, M. Kaviani, D. Selli, C. Di Valentin, J. Phys. Chem. C 123 (2019) 10130
10144 with permission.

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Conclusions

The literature on the theoretical aspects of titania is very abundant and grows steadily. In this chapter, we briefly discussed some aspects related to the computational modeling of this important oxide. The key message is that it is necessary to adopt methods that tackle the self-interaction error effectively, allowing for a good description of the titania’s electronic structure, as well as the localization of excess electrons and holes related to intrinsic defects or the photoexcitation process. While it is not mandatory to quantitatively reproduce the bandgap of TiO2, the account of its trapping capability is a key aspect to contribute with insightful calculations to the quest for new photocatalytic or bioactive titania-based materials. Thanks to accurate DFT approaches, it has been possible in the past two decades to increase considerably our knowledge of the fundamental properties of titania in anatase or rutile forms, the nature of the various surfaces of these two phases, the role and electronic nature of the most common defects, such as the oxygen vacancies and the titanium interstitials. It has also been possible to describe the process of light absorption, electron
hole formation, in a quite precise way. More recently, the use of DFT approaches has been extended to the study of oxide heterojunctions, with particular attention to the proper alignment of valence and conduction band edges, an essential prerequisite to properly predict the occurrence of charge separation at the interface between different oxides or different facets of the same material. The challenge for the computational modeling becomes even stronger if one wants to deal with large and complicated objects, such as realistic models of titania nanoparticles. Here it is necessary to couple computationally cheap techniques with high-quality hybrid functional calculations in a careful process of benchmarking. Overall, electronic structure theory has considerably contributed over the past few years to better understand the pros and cons associated with the use of TiO2 in photocatalytic processes, the role of nanostructuring, creation of defects, formation of composite phases, and in the design of new efficient photocatalytic materials.

Acknowledgments Financial support from the Italian Ministry of University and Research (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 grant Dipartimenti di Eccellenza—2017 “Materials For Energy” are gratefully acknowledged.

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10144.

Synthesis and characterization of titanium dioxide and titanium dioxide
based materials

4

Marianna Bellardita1, Sedat Yurdakal2 and Leonardo Palmisano1 1 Department of Engineering, University of Palermo, Palermo, Italy, 2 Department of Chemistry, University of Afyon Kocatepe, Afyonkarahisar, Turkey

4.1

Introduction

In recent years the scientific community has shown considerable interest in titanium dioxide (TiO2) due to its use as photocatalyst, support in catalysis, white pigment, additive in sunscreens and cosmetics, and component in advanced devices as the photovoltaic cells and the sensors. Different physicochemical and structural features of bare TiO2 and TiO2-based materials can be tailored and exploited by means of distinct preparation methods depending on the application, as, for instance, the biomedical and (photo)catalytic ones. Consequently, a great interest has been devoted to the preparation and characterization of titanium dioxide and titanium dioxide
based materials, modifying the different synthesis parameters with the aim to address particular properties. The methods described for the obtainment of TiO2, can be also used for other (photo)catalysts both in the form of powders and films supported on various materials [1
5]. Moreover, the same TiO2 samples prepared in a specific way can be used for different applications. On the other hand, some particular features of the samples are requested, as their characteristic growth dimensions from 0 to 3, where 0 is considered the number of dimension for the nearly spherical nanoparticles (in the nanometer range in all three directions); 1 for nanowires, nanorods, nanobelts, and nanotubes; 2 for nanosheets; and 3 for porous nanostructures. For example, nanoparticles and porous nanostructure can be mainly used as (photo)catalysts, while nanowires, nanorods, and nanobelts as components in photovoltaic and electrical energy storage applications, and nanosheets in the dielectric and biomedical ones. Other very important parameters are the shape and the size of TiO2 nanoparticles because they influence the chemical and electronic properties. In particular, the particle dimension is related to the crystallinity degree, the charge separation, the type of exposed crystalline planes, and the nature of localized charge carriers that, in turn, regulate the redox properties of the nanoparticles. In this chapter a brief survey of the most common preparation methods used for the synthesis of TiO2 materials employed in various fields (catalysis, photocatalysis, optics, electronics, energy storage and production, ceramics, pigments, cosmetics, sensors) is reported along with some examples of applications. The synthesis Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00021-3 © 2021 Elsevier Inc. All rights reserved.

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methods described later can be used for the preparation of bare TiO2 in the form of powder or film, anion- and cation-doped TiO2, and TiO2 coupled with other materials, nanocomposites. The bulk, surface, and morphological characterization techniques of bare TiO2 and TiO2-based materials are very important to determine properties that are often related to the method of preparation and on which the behavior of the material in the various applications described in this book may depend. In this chapter, only some fundamental characterization techniques are briefly described and summarized along with few examples from the existent literature. The crystalline phases, the size of the primary particles, and the degree of crystallinity of the TiO2 samples can be determined by the X-ray diffraction (XRD) technique with different methods, some of which will be described in the following. In order to determine the specific surface area of the material and the size of the pores with the distribution of their volume, the BET method is one of the most used. The morphological properties can be examined by electron microscopy techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM, in particular, is useful for determining structural properties such as defects in the lattice and the presence of heterojunction(s) in composite materials. Bandgap energy of TiO2-based materials can be determined by diffuse reflectance spectroscopy (DRS) technique. Qualitative information on the recombination of the photogenerated charges of TiO2-based materials can be obtained by means of photoluminescence (PL) spectroscopy. The amount of physically adsorbed water molecules and OH groups on the TiO2 surface can be evaluated by thermal gravimetric analysis (TGA). X-ray photoelectron spectroscopy (XPS), finally, is a powerful technique that provides qualitative and quantitative information on the type and oxidation state of the atoms that make up the TiO2-based materials by studying their binding energy.

4.2

Preparation methods

4.2.1 Preparation methods of powdered TiO2-based materials 4.2.1.1 Sol
gel The sol
gel process is a synthesis wet-chemical method used from the mid-1800s [6,7] for the production of materials to be used in (photo)catalysis, ceramic and electronic industry, for chemical sensors, membranes and solid-state electrochemical device’s fabrication, as fibers, and for photochromic applications. The method involves hydrolysis of organometallic or inorganic precursors in the presence of alcohol and small amounts of water with the formation of a colloidal suspension (sol), which undergo various forms of hydrolysis and polycondensation reactions until the formation of a highly viscous material (gel) [6
8]. The gel drying allows the removal of the solvent with the formation of a solid phase. By changing the evaporation method of the gel, it is possible to prepare a variety of materials.

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If the gel is dried by evaporation, the capillary forces will result in shrinkage, the gel net will collapse and a xerogel will form. If drying is performed under supercritical conditions, the network structure is retained and the liquid component of the gel is replaced with a gas. The result is an aerogel with an extremely low density. The aerogel maintains high porosity and has very high pore volume. Xerogels or aerogels are converted into a useful form by additional thermal treatments, and the physicochemical properties of the final product depend on temperature, heating rate, and time. In fact, being the process carried out at room temperature or close to it, the resulting materials are amorphous or partially crystalline and a thermal treatment is generally needed to transform the dried gel into the desired solid. From the abovementioned considerations, it can be understood why samples prepared by the sol
gel method show a multiplicity of properties, and the sol
gel method is one of the most used to prepare TiO2 and TiO2-based materials [9
15]. In particular, it is very useful and convenient to obtain powdered TiO2. Notably, the specific surface area along with the shape and the size of the particles, and the pores volume and their distribution can be tuned depending on the intended application. Low pressures and temperatures are used to prepare both pure and composite samples with a stoichiometry control (due to the possibility of contemporary multicomponent adding). The textural and surface properties of the final materials depend, of course, on the different parameters chosen as type and concentration of the precursor, solvent, precursor/solvent ratio, pH, additive presence, reaction temperature. Li et al. [10] succeeded in the TiO2 size and polymorph type control by carrying out a sol
gel synthesis at low temperature (,100 C). They started from tetrabutyl titanate as the precursor, ethanol as the solvent, and HCl as the catalyst and used different molar ratios, r, between water and Ti(O
Bu)4 (in the range 1
8). The gelation time decreased by increasing the water concentration and the transition temperature anatase!rutile decreased by increasing r (Fig. 4.1). Moreover, the particle size and the rutile amount were controlled by adjusting the aging time of the dry gel. Rutile is generally more used as pigment and support in catalysis, while anatase is more popular as heterogeneous photocatalyst because it showed often to be the most oxidant polymorphic form. It has been reported that the photoefficiency of the less common brookite phase in some photoreactions is intermediate to that of the anatase and the rutile polymorphs, and that the contemporary presence of two or three phases can give rise to very active samples [16]. As far as the biomedical applications are concerned, it has been noticed that the phototoxicity of rutile particles is less significant than that of the anatase ones. Therefore by adjusting the synthesis conditions, it is possible to prepare TiO2 containing different amounts of anatase and rutile, depending on the intended application. The hydrolysis of titanium ethoxide in the presence of different acids and bases as catalysts allowed to prepare TiO2 powders with different crystalline phases (anatase, brookite, and rutile) at 70 C [17]. In particular, an anatase
brookite mixture was obtained in the absence of catalyst, all the three crystalline phases were observed in the presence of HCl, and anatase poorly crystallized was formed in the presence of oxalic acid or ammonium hydroxide. By calcination of the samples the size of the crystallites increased and conversion into rutile occurred.

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Titanium Dioxide (TiO2) and Its Applications

Figure 4.1 Temperature dependence of percentage and particle size of rutile in the samples prepared with different water concentrations. Source: Reprinted from Y. Li, T.J. White, S.H. Lim, Low-temperature synthesis and microstructural control of titania nano-particles, J. Solid State Chem. 177 (2004) 1372
1381 with permission, ©2004 Elsevier Publishing.

Another important controlling parameter is the pH of the system that affects the relative rate of hydrolysis and condensation and then the relative amount of TiO2 polymorphs [18]. Velardi et al. [19] prepared TiO2 powders from tetraisopropoxide (TTIP) by varying the pH from 1 to 10 with HNO3 or NaOH. In strong acidic medium a mixture of rutile and anatase with a dominance of the former was obtained while anatase was formed in basic conditions. Two disadvantages of TiO2 utilization are the inability of activation by visible light and the fast recombination of the electron/hole pairs. A way to overcome these limits is the doping/loading with metal/nonmetal species. Sol
gel is a very suitable method used to prepare doped catalysts as the different components can be added during the hydrolysis step [20
25]. Wiranwetchayan et al. [25] investigated the effects of three different polymeric precursors (PVP, PEG, and Tween 60) on the features of anatase TiO2 films. In all cases, spherical particles with different particle sizes were obtained, and then the precursor did not influence the shape of the particle, as can be seen in Fig. 4.2. The sample with the smallest crystallite size was the most efficient, probably due to the high specific surface area. TiO2 films consisting of all the three most common polymorphs, that is, anatase, rutile, and brookite prepared by sol
gel method have been used for photovoltaic applications, including dye-sensitized solar cells, polymer-inorganic hybrid solar cells, inorganic solid-state solar cells, and perovskite solar cells [26]. Nanostructured TiO2 samples obtained by sol
gel proved to be very suitable for the production of different types of sensors due to its large surface area and good electron transition [27].

4.2.1.2 Precipitation and coprecipitation Precipitation is a preparation method in which a solid phase is formed from a solution when the concentration of the compound exceeds its solubility. The

Synthesis and characterization of titanium dioxide and titanium dioxide
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91

Figure 4.2 SEM images, particle size diameter, and rhodamine B abatement in the presence of TiO2 films synthesized in the presence of different polymeric precursors. SEM, Scanning electron microscopy. Source: Reprinted from O. Wiranwetchayan, S. Promnopat, T. Thongtem, A. Chaipanich, S. Thongtem, Effect of polymeric precursors on the properties of TiO2 films prepared by sol-gel method, Mater. Chem. Phys. 240 (2020) 122219 with permission, ©2020 Elsevier Publishing.

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Titanium Dioxide (TiO2) and Its Applications

precipitated solid can be recovered and properly treated for the intended applications. Both single-component and mixed/coupled (photo)catalysts can be prepared also as supported materials to be used in different applications [1,28,29]. In particular, multicomponent materials can be obtained by coprecipitation, that is, by the simultaneous precipitation of more solids from the solution [28,30]. Precipitation/ coprecipitation represents a versatile preparation method because the composition and the properties of the final materials are easily tuned, and many components can be simultaneously added to obtain differently functionalized samples [31]. A precipitate is formed from a supersaturated liquid solution by varying some parameters such as temperature, pH, addition of precipitating agents, solvent amount by means of its evaporation or crystallization, and mixing of two solutions or liquids that react chemically [32,33]. Precipitation starts with nucleation on impurity particles, then the nuclei grow into observable crystallites. Sometimes the growth is accompanied by the formation of secondary nuclei, so crystallites with different sizes can be formed. Notably, nucleation and growth can occur simultaneously or sequentially. Nucleation is a tricky process, in particular when more than one component is added to the starting solution. It begins with the formation of clusters capable of spontaneously growth until a critical radius is reached. This corresponds to the minimum size at which a particle can survive in solution. Clusters smaller than this size tend to redissolve, while the bigger ones can continue to grow as particles from the solution are attached to (Fig. 4.3). The nucleation rate depends on concentration, temperature, and two energetic factors: the activation energy barrier that needs to be overcome to produce a critical-size nucleus and the activation energy for an atom to migrate across the interface separating the nucleus and matrix (for the theoretical aspects of the nucleation rate and thermodynamic aspects see Refs. [28,30,34,35]). If the rate of nucleation is faster than the rate of growth, small particles form and an amorphous solid is generally obtained. The nucleation is usually induced by fast decomposition of the precursor or by a rapid variation in the concentration of the species involved in the chemical mechanism of particle formation. The further step is the aggregation that is very important in determining the final features of the materials. Clusters of nanoscale primary particles aggregate into micrometer-scale secondary particles that are held together by physical or chemical forces, such as crystal bridges. Aggregation leads to fewer and

Figure 4.3 Schematic representation of the nucleation
growth mechanism.

Synthesis and characterization of titanium dioxide and titanium dioxide
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93

larger, but yet porous particles. Porosity is determined by how the primary particles are stacked, and it can be tuned by influencing the stacking. Finally, to obtain a solid, filtration, washing, drying, calcination, and sometimes activation steps are necessary. The growth proceeds through two main mechanisms: monomer addition from solution or Ostwald ripening. In the first case the adsorption of a monomer onto the formed nuclei and its decomposition induces the growth. In the second case the high surface energy of small nanocrystals allows for them to redissolve (in the presence of ligands/adsorbed species), releasing new monomers into solution that can then redeposit onto the surface of the larger particles. By controlling at the atomiclevel nucleation and growth processes, it is possible to obtain shape-controlled nanocrystals with well-defined surfaces and morphologies [36]. Precursor materials are generally metal salts, such as chlorides, sulfates, and nitrates, while precipitating agents are mainly hydroxides, carbonates, and hydroxocarbonates. Water is the most common solvent because organic solvents are much more expensive and environmentally hazardous. Moreover, the solubility of many metal salts is lower in organic solvents. Additive not strictly necessary for the precipitation process is often present in the starting mixture to give certain properties to the prepared materials. For example, organic molecules, successively removed by calcination, can be used to control the pore structure of the precipitate, and metal salts can be added to dope TiO2 with the double role of shifting the light absorption toward the visible range and reducing the e2/h1 recombination. For the preparation of multicomponent systems, solutions containing the different precursors are mixed together, and coprecipitation generally takes place by a pH adjustment. The different solubility of the components and the kinetic of precipitation influence the homogeneity of the coprecipitate. A heterogeneous precipitate is obtained when the precipitation rate is slow and the solution is poorly stirred; on the contrary, a more homogeneous product is obtained under a good mixing. Supported materials can be obtained by coprecipitanting the support simultaneously with the active metals or by adding the support separately. Both soluble precursors of the support or preformed solid can be introduced during the coprecipitation process. Some parameters that influence the properties of the materials prepared by precititation/coprecipitation are reported in Fig. 4.4. By using TiCl4 as the precursor, it was possible to tailor the content of the three TiO2 polymorphic phases (anatase, rutile, and brookite) by varying the initial TiCl4 amount, the HCl concentration, and, consequently, the solution pH and the [Cl]/[Ti] ratio [18,37
39]. When the initial precursor concentration was 0.15 M, high acidity (high [Cl]/[Ti] ratio) and TiCl4 concentration favored the formation of the rutile phase; as the pH increased, the percentage of the anatase phase raised, while an appropriate [Cl]/[Ti] ratio (17 # [Cl]/[Ti] # 35) was necessary to obtain also brookite (Table 4.1) [18]. By starting from a lower TiCl4 concentration (0.06 M), a mixture of brookite and rutile was obtained also for high [Cl]/[Ti] ratios. Bimetallic Au
Ru supported on TiO2 P25 samples were prepared by a deposition
precipitation method in the presence of urea and coprecipitating or sequentially depositing the metals, that is, Au and then Ru or vice versa [40]. The sample obtained

94

Titanium Dioxide (TiO2) and Its Applications

pH Composion

Solvent

Properes of the precipitate

Mixing rate

Concentraon

Temperature

Addives Addion order

Figure 4.4 Parameters that influence the precipitate properties.

Table 4.1 Crystal phase composition of TiO2 powders prepared by precipitation starting from TiCl4 by varying the precursor and HCl amounts. [Ti] mol/dm3

[Cl]/[Ti]

Anatase (%)

Brookite (%)

Rutile (%)

0.15 0.15 0.15 0.06

18 26 38 38

3.5

65.8 50.3

30.7 49.7 100 60.5

39.5

by depositing Ru after Au was the most active for catalytic CO oxidation. In this case, small bimetallic particles were formed with a Janus-type structure, that is, particles consisting of one side of Ru and the other Au, both interacting with TiO2. TiO2 in the form of nanotubes was particularly suitable not only as photocatalyst but also in Gr¨atzel-type solar cells [41], as sensor [42] and for biomedical applications [43]. Anatase TiO2 consisting of both nanotubes and nanorods has been prepared by precipitation from TiCl4 solutions by using an anodic alumina membrane containing cylindrical pores as template [44]. In particular, the TiO2 nanostructures were formed by two distinct parts: encompassing fragile tubules and compact rods inside the tubes (Fig. 4.5). The outer tubes were smooth, while the inner rods compact. Ag-doped TiO2 prepared by the coprecipitation method showed antibacterial properties toward the inactivation of Escherichia coli under visible-light irradiation [45]. Zr-doped TiO2 samples with different properties were prepared by homogenous coprecipitation by varying the synthesis parameters [46]. In particular, all the samples consisted of anatase except those with the highest Zr amount (10 mol.% Zr) that are totally amorphous, and the crystallinity decreased by increasing the Zr loading. Moreover, samples with a Zr content lower than 5.1 mol.% were spherical,

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Figure 4.5 SEM images of TiO2 nanostructures obtained by using an alumina membrane as template. SEM, Scanning electron microscopy. Source: Reprinted from I.-S. Park, S.-R. Jang, J.S. Hong, R. Vittal, K.-J. Kim, Preparation of composite anatase TiO2 nanostructure by precipitation from hydrolyzed TiCl4 solution using anodic alumina membrane, Chem. Mater. 15 (2003) 4633
4636 with permission, ©2003 American Chemical Society Publishing.

while clusters of irregular morphology were obtained at higher Zr contents. The Zr inclusion increased the specific surface area: samples prepared in the presence of Zr content lower than 5.1 mol.% Zr were mainly microporous, while those prepared with higher Zr contents were prevalently mesoporous. SiO2-supported TiO2 materials showed good catalytic and photocatalytic activity due to their high surface area, large pore volume, increased acidity, and surface hydroxyl group’s amount. The precipitation method was particularly suitable for the preparation of these samples as it allowed a good dispersion of TiO2 on SiO2 [47]. Moreover, by varying the SiO2/TiO2 ratio and the calcination temperature, it was possible to tailor the microstructure and the surface properties optimizing the photocatalytic activity. Ren et al. [48] prepared TiO2/SiO2 samples by a precipitation method starting from TiOSO4 and SiO2 and found that the photocatalysts with a TiO2/SiO2 ratio of 5% and calcined at 650 C were the most active in benzene conversion under UV irradiation. Nanostructured TiO2
ZnO composites synthesized via coprecipitation method starting from zinc acetate and titanium tetrachloride with different Ti/Zn ratios were active under UV- and visible-light irradiations for removing arsenic from industrial wastewater [49]. Quan et al. [50] compared the features of La
TiO2 photocatalysts prepared either by coprecipitation or with a sol
gel process starting from La(NO3)3  6H2O and TiCl4 in the presence of NH3 as the precipitating agent. The photoactivity was evaluated by using rhodamine B as the probe substrate. The samples prepared by the two methods exhibited similar particle size distribution but differences in surface area, morphology, pore size distribution, and photocatalytic activity. The catalysts prepared by sol
gel were mesoporous, while the pores of powders prepared by coprecipitation consisted both of mesopores and macropores. The photocatalytic activity of the samples obtained by coprecipitation was higher than that of those synthesized by the

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sol
gel method. This finding was explained by taking into account the structure of the anatase that appeared to be more regular and to the presence of surface Ti31 species that probably were more appropriate to induce the photoactivity.

4.2.1.3 Hydrothermal and solvothermal syntheses The hydrothermal and solvothermal methods consist in the preparation of photocatalysts at high temperature (higher than the solvent boiling point) in a closed system starting from aqueous/nonaqueous solutions [5,51
53]. The synthesis is indicated as a hydrothermal process if water is the solvent, while the term solvothermal is used if nonaqueous solvents are used. Both methods are considered very promising approaches for the preparation of nanomaterials [54], and steel pressure vessels (autoclaves) with or without Teflon coating which are equipped with a hermetic seal are used. The pressure inside the autoclave depends on temperature (that can be varied from 100 C to 800 C), volume of the solution, and presence of additives. The high-temperature and -pressure conditions enhance the solubility and reactivity of metal salts and complexes that are insoluble at ordinary temperature (,100 C) and pressure (,1 atm). For safety reasons the pressures should always be estimated beforehand [54]. The methods described earlier present many positive aspects such as the use of simple setups and operating conditions, the possibility of obtaining complex materials in a single stage, working at temperatures lower than those necessary for traditional solid-state reactions, and forming generally fairly crystalline samples without the need for subsequent postannealing treatment. For temperature and pressure values higher than the critical ones, some properties of the solvent, such as the dielectric constant, change dramatically. The dielectric constant of water decreases with increasing temperature and decreasing pressure (,10 under supercritical conditions). This results in a reduction of the solubility of the species present, which leads to the supersaturation of the solution, and nucleation and growth of the crystals is favored [55]. Moreover, not only ultrafine particles can be obtained by carrying out the hydrothermal synthesis in supercritical water, but also the possibility to vary the pressure and/or the temperature allows to optimize the morphology of the particles [56]. In the solvothermal process the working conditions can be milder than those reached in the hydrothermal one, due to the use of organic solvents, boiling point of which is lower than that of water. Moreover, precursors that are sensitive to water can be used [57,58]. The most common used organic solvents that sometimes act also as reagents are methanol, toluene, 1,4-butanediol, and amines. The crystal nucleation and growth of nanomaterials during both hydrothermal and solvothermal processes is influenced by the precursors type, the presence of additives, the reaction time, and the volume of the solution with respect to the volume of the autoclave [59
61]. The synthesis of nanostructured bare, doped, coupled TiO2 is one of the more explored solvothermal/hydrothermal reactions. Samples with different properties as

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crystallinity, morphology, crystalline phase, crystal facet, and particle size have been extensively prepared [62
67]. TiO2 nanotubes, nanorods, and nanowires used also in solar cells have been obtained by the hydrothermal method [68
72]. Cheng et al. prepared pure rutile and anatase nanoparticles from aqueous TiCl4 solutions by the hydrothermal process [73]. They observed that the basic conditions favored anatase while the acidic ones rutile; at higher temperatures, highly dispersed particles were formed, and the presence of some mineralizers such as SnCl4 and NaCl gave rise to a smaller average grain size, while that of NH4Cl to agglomeration. Successively, it was discovered that the type of TiO2 polymorph depended mainly on the concentration of TiCl4 while particle size on the reaction time [74]. In particular, the formation of rutile was favored when [TiCl4] 5 0.5 mol/dm3, while at lower concentrations a mixture of rutile and anatase with the percentage of rutile dependent on the reaction time was found. Hayashi et al. [75] synthesized hydrothermally TiO2 powders under different subcritical and supercritical water conditions starting from titanium(IV) tetraisopropoxide as the precursor. The photoactivity of the samples, compared for H2 production from aqueous methanol solution, was much higher than that of some commercial TiO2 photocatalysts. Powders obtained under supercritical water conditions were the most active due to the higher crystallinity and the smaller content of surface hydroxyls. Anatase TiO2 nanorods were synthesized by solvothermal method by using Ti (IV) isopropoxide as the precursor and benzyl alcohol as the solvent. Different amounts of acetic acid were added to control the morphology because it attaches certain TiO2 crystal facets favoring an anisotropic growth of particles. By varying the acetic acid/Ti(IV) isopropoxide molar ratio, anatase nanorods with different dimensions were obtained, in particular, samples about 13
17 nm long and 5 nm in diameter were prepared [71]. The performance of dye-sensitized solar cells containing these particles was higher than that of identical cells where the photoanode was TiO2 P25.

4.2.1.4 Sonochemical method In the sonochemical method, reactions are induced by the application of potent ultrasound radiations (20 kHz
10 MHz). The chemical effects of ultrasounds do not derive from the direct interaction with molecules but from acoustic cavitation, consisting of the formation and growth in the liquid of bubbles that reach an unstable size (some tens of mm) and collapse implosively (Fig. 4.6) [76,77]. The cavitational collapse releases the accumulated stored energy within a very short time generating intense local heating and pressure increase. The temperature can reach B5000K, while the local pressure can go up to 1000 atm [78]. These reaction conditions cannot be reached by other processes. Secondary effects can derive from the presence of chemically active species such as organic radicals that could form inside the bubbles and successively diffuse and react in the solution. Highly reactive H  and  OH radicals that are responsible for redox reaction can be produced

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Figure 4.6 Schematic representation of the process of acoustic cavitation. Source: Reprinted from H. Xu, B.W. Zeiger, K.S. Suslick, Sonochemical synthesis of nanomaterials, Chem. Soc. Rev. 42 (2013) 2555
2567 with permission, ©2013 Royal Society of Chemistry Publishing.

by ultrasonic irradiation of H2O [76]. For instance, the reduction of noble metal salts by H  radicals allows us to obtain very small metal particles without the need of a chemical reducing agent [77]. The advantages of the method consist in the mild synthesis conditions, short times, and low energy consumption. The sonochemical method has been extensively used for synthesizing materials with improved or particular properties [77], as a wide range of nanostructured metal oxides [76,79], and among them, a great attention was paid to TiO2 [80
86]. By varying the time of the treatment, the temperature, the irradiation frequency, and the ultrasonic intensity, it is possible to tune many features of the material. Zhu et al. [82] prepared TiO2 nanotubes and whiskers by ultrasonic irradiation starting from TiO2 raw powder in NaOH aqueous solution. Treatment at high sonication power (560 W) led to TiO2 whiskers, while lower powers (280 W) favored the formation of nanotubes. Guo et al. [83] synthesized anatase TiO2 particles by hydrolysis of titanium tetraisopropoxide in water/ethanol solution under high-intensity ultrasonic irradiation (20 KHz, 100 W/cm2) for 3 h. The crystallinity and the particle size of the product resulted dependent on the reaction temperature (Fig. 4.7). An almost amorphous structure was obtained at 40 C, anatase peaks started to appear at 70 C and became more intense at 90 C. After thermal treatment at 450 C for 2 h, the peaks appeared narrower and sharper. Anatase and rutile TiO2 nanoparticles and their mixtures were synthesized starting from different precursors by ultrasound irradiation, by changing the reaction temperature [84]. The particle sizes of the obtained materials are nanometric (,9 nm) and depended on the temperature. Yu et al. [80] prepared TiO2 nanoparticles consisting of anatase and brookite by hydrolysis of titanium tetraisopropoxide in pure water or EtOH
H2O solution. The samples showed a higher photoactivity than Degussa P25 for the oxidation of acetone. Finally, a low-frequency ultrasonication treatment of aqueous suspensions of TiO2 Degussa P25 induced the bandgap narrowing of the photocatalyst due to the formation of oxygen vacancies [87,88].

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Figure 4.7 Powder XRD patterns of TiO2 prepared at (a) 40 C, (b) 70 C, (c) 90 C, and (d) sample “c” calcined at 450 C for 2 h. XRD, X-ray diffraction. Source: Reprinted from W. Guo, Z. Lin, X. Wang, G. Song, Sonochemical synthesis of nanocrystalline TiO2 by hydrolysis of titanium alkoxides, Microelectron. Eng. 66 (2003) 95
101 with permission, ©2003 Elsevier Publishing.

4.2.1.5 Microwave irradiation Microwave-assisted method consists in the treatment of a dielectric material using microwaves as the source of energy (frequencies of microwave heating range between 300 MHz and 300 GHz) [89,90]. The energy from microwaves is converted into heat energy by interaction of the electromagnetic field with molecular dipoles of the materials. As microwaves can penetrate the material and supply energy, heat develops inside the material with consequent volumetric heating [91]. The rise in temperature is related to the amount of absorbed energy, and the capability of a material to transform the electromagnetic energy into thermal energy depends on its dielectric constant. In addition to volumetric heating, microwave heating presents other advantages such as (1) selective heating of the material, (2) rapid and noncontact heating, (3) quick start and stop, and (4) mild reaction conditions. The main reported disadvantages are related to the nonuniform microwave fields generated in most microwave ovens, which can involve the formation of superheating spots.

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The use of microwaves for the preparation of oxides has received considerable attention in the last years as an alternative method to conventional heating [92
97], and it has been largely applied to the synthesis of TiO2 with different structures [94
99]. The microwave method can be used in combination with others such as the sol
gel, hydrothermal, and solvothermic processes to improve both the surface and structural properties of some samples [94,95,100,101]. Corradi et al. [100] compared the conventional and microwave hydrothermal methods for the treatment of a 0.5 M TiOCl2 solution. A colloidal TiO2 nanoparticles suspension was prepared within 5 min to 1 h by using the microwave radiation, while 1
32 h were necessary with conventional synthesis methods. In both cases the samples contained rutile as the main crystallographic phase, but the samples prepared by the microwave hydrothermal process showed a higher crystallinity. Nanocrystalline pure anatase TiO2 samples, instead, were obtained by 3 h of microwave hydrothermal treatment of TiOCl2 aqueous solution in the presence of urea (molar ratio TiOCl2: urea 5 5) [95]. Hart et al. [96] prepared TiO2 thin films to be used in solar cell by both a microwave and conventional heat treatments of nanocrystalline anatase TiO2 obtained by a sol
gel method. The film was sintered to enhance the crystallization and the interparticle’s connection. Crystallization in a shorter time and a lower temperature were achieved with microwave irradiation, compared to conventional oven treatments. TiO2 nanotubes were obtained by Wu et al. [98] by microwave irradiation of both commercial and homemade nanopowders of TiO2 anatase, rutile, or mixed phase in NaOH aqueous solution. The influence of some experimental conditions as TiO2 type, NaOH concentration, reaction time was investigated. The obtained nanotubes had a hollow, multiwall, and open-ended central structure with diameters of 8
12 nm and lengths up to 200
1000 nm. The method resulted effective in preparing tubular materials to be used in photocatalytic, gas-sensing, optic, and biomedical materials.

4.2.1.6 Spray pyrolysis Spray pyrolysis is a synthesis method used mainly for preparing materials in the form of nanostructures, films, ceramic materials [102
104]. The process involves the spraying or injection by a nebulizer of the solution containing the precursor onto a hot surface inside a furnace. The high temperature allows the droplets precursor decomposition, the solute precipitation, and the drying to form the final desired material. An experimental setup is reported in Fig. 4.8. The samples features (particle size, shape) can be tuned by controlling some parameters such as spray energy (gas flow pressure, gas inlet to nebulizer), duration of spray, droplet size of the precursors, distance between the spray gun and the substrate, furnace temperature, and solvent type. Typical precursors are metal salts (chlorides, nitrates, sulfates, acetates), while water or alcohols are used as solvents. The method is suitable also for one-step preparation of loaded/doped and composite materials as several metal compounds can be contemporary added in the solution containing the precursor. The reagents are selected in such a way that products other than those desired are volatile at the deposition temperature.

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Figure 4.8 Spray pyrolysis process setup. Source: Reprinted from M. Okuya, K. Shiozaki, N. Horikawa, T. Kosugi, G.R. Asoka Kumara, J. Madara´sz, S. Kaneko, G. Pokol, Porous TiO2 thin films prepared by spray pyrolysis deposition (SPD) technique and their application to UV sensors, Solid State Ionics 172 (2004) 527
531 with permission, ©2004 Elsevier Publishing.

F-doped TiO2 powders were obtained by spray pyrolysis from an aqueous solution of H2TiF6 [105,106]. By varying the temperature of the process, it was possible to change the fluorine amount. The samples containing fluorine were more effective than bare TiO2 and commercial TiO2 P25 for the gas-phase photodegradation of acetaldehyde under both ultraviolet and visible light. TiO2 films employed in different applications have been obtained by spray pyrolysis by using different precursors and working conditions [107
110]. Porous anatase TiO2 films to be used in UV sensors were obtained by Okuya et al. [111] in air by using titanium(IV) oxyacetylacetonate as the precursor and in ethanol, 2-propanol, and 2-butanol as the solvents. Films with different surface morphology and grain size were obtained with the various solvents. The films prepared from ethanol solution were formed by several wedges, a smooth surface with large platelet particles was obtained in the presence of 2-butanol, while a mixture of wedges and platelets was formed with 2-propanol. Jiang et al. [110] investigated the influence of the solvent in the performance of TiO2 films for solidstate dye-sensitized solar cells, and they obtained analogous results. Smooth films were obtained in the presence of 2-propanol at 260 C, while a much rougher surface with the presence of pinholes was formed with ethanol. Moreover, the films prepared with 2-propanol revealed higher power conversion efficiencies.

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4.2.1.7 Impregnation Impregnation is a method for the preparation of loaded/doped or composite materials in the form of powders or films. The support can therefore be either a preformed solid or a surface of an appropriate material. The method requires that the active material is first deposited onto the support and then stabilized on it by solvent removal and calcination [1,112,113]. A certain volume of the solution containing the suitable precursor is contacted with the solid support. Depending on the solution volume, the process is referred to as (1) incipient wetness or dry impregnation and (2) wet impregnation. When the volume of the impregnating solution is equal or smaller than that of the pores of the support, the technique is denoted as incipient wetness. The precursor solution is sprayed on the support and is attracted to the pores by capillary action. When the interaction between precursor and support is weak, the method allows us to load the support with high precursor amounts and the maximum quantity of loading depends on the solubility of the precursor in the impregnating solution. It is possible to obtain a high quantity of surface species on a support by carrying out consecutive impregnation steps. If the interactions are not sufficiently strong, the drying step causes a redistribution of the impregnated species, and the coverage of the support is not homogeneous. There is talk of wet impregnation when the solvent is in excess with respect to the volume of the pores of the support. The solute arrives on the support only by diffusion, and the impregnation time is significantly longer than that needed for the dry impregnation. The system is stirred until the equilibrium is reached, then the excess of the solvent is removed by drying. The loading amount is related to the concentration of the solution and to the volume of the pores of the support. Sometimes a not uniform distribution of the species into the support is obtained [114,115]. TiO2
CeO2 nanocomposites were prepared by depositing different amounts of CeO2 on porous TiO2 by wet impregnation in the presence of cerium nitrate solutions [116]. The photodegradation of rhodamine B was carried out under visiblelight irradiation. The activity of the composite samples was higher than that of the bare TiO2. Cu
TiO2 photocatalysts were obtained by an improved wet impregnation method by using TiO2 Degussa P25 as the support and Cu(NO3)2 as the copper precursor [117]. TiO2 was impregnated with an aqueous Cu(NO3)2 solution, and the slurry was sonicated, stirred at 95 C, and dried before calcination that gave rise to CuO/TiO2. Moreover, a portion of the obtained powder was reduced in an H2/He environment at 300 C for 3 h to obtain Cu0/TiO2 or Cu2O/TiO2 samples. The obtained photocatalysts were used for the reduction of CO2 in a KHCO3 solution. TiO2 immobilized by an impregnation method on powdered activated carbon showed to be very active for the degradation of phenol under solar-light irradiation [118]. P
N junction was realized by depositing NiO onto TiO2 nanobelts by wet impregnation with a Ni(NO3)2 solution, and following calcination at 600 C [119]. The composite samples showed a higher photocatalytic degradation rate of methyl orange than the bare NiO nanoparticles and TiO2 nanobelts under both ultraviolet- and

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Figure 4.9 Schematic representation of NiO/TiO2 P
N junction. Source: Reprinted from J. Lin, J. Shen, R. Wang, J. Cui, W. Zhou, P. Hu, D. Liu, H. Liu, J. Wang, R. I. Boughton, Y. Yue, Nano-p
n junctions on surface-coarsened TiO2 nanobelts with enhanced photocatalytic activity, J. Mater. Chem. 21 (2011) 5106
5113 with permission, ©2011 Royal Society of Chemistry Publishing.

visible-light irradiations. The higher activity was explained by considering the increase of the charge separation rate due to the presence of the junction (Fig. 4.9). The impregnation route is a very effective method for the preparation of TiO2based heterostructures to be used as solar-cell electrodes, as photocatalysts for wastewater remediation, photocatalytic fuel production, sensors, and other semiconductor applications.

4.2.1.8 Deposition
precipitation In the deposition
precipitation method, the formation of a slightly soluble and catalytically active compound occurs on the surface of a suspended support by means of a chemical reaction, starting from a precursor present in the liquid phase [120]. By slowly adding the precipitant agent to the solution under vigorous mixing, the nucleation occurs exclusively on the support surface instead of in the solution. The nature of the support strongly affects the final dispersion of the precipitate onto the support. The method is suitable for the preparation of powdered TiO2-based photocatalysts and films. Ag/TiO2 photocatalysts were prepared by a deposition precipitation method by suspending TiO2 particles in an aqueous Ag(NH3)21 solution under stirring [121]. Hydrogen peroxide was added to reduce Ag1 to Ag. The samples containing about 2.0 wt.% of Ag were very effective in photocatalytic degradation of methyl orange under UV irradiation. Deposition
precipitation method is very effective for the preparation of catalysts with high dispersion of gold. Oros-Ruiz et al. [122] prepared Au/TiO2 powders containing different Au amounts by deposition
precipitation with urea by using TiO2 Degussa P25 as substrate and HAuCl4  3H2O as gold precursor. A solution containing different HAuCl4  3H2O amounts and urea was prepared, and then the TiO2 precursor was added. The obtained suspension was kept under stirring at 80 C

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Figure 4.10 Hydrogen production from water/methanol mixture in the presence of Au/TiO2 and bare TiO2 samples. Source: Reprinted from S. Oros-Ruiz, R. Zanella, R. Lo´pez, A. Herna´ndez-Gordillo, R. Go´mez, Photocatalytic hydrogen production by water/methanol decomposition using Au/ TiO2 prepared by deposition
precipitation with urea, J. Hazard. Mater. 263 (2013) 2
10 with permission, ©2013 Elsevier Publishing.

for 16 h, and then the support was recovered, washed with distilled water, and dried for 2 h at 80 C. Metallic gold was obtained for treatment in the presence of hydrogen at different temperatures. The photoactivity of the samples was evaluated for hydrogen production from water/methanol solutions under UV-light irradiation. The optimal temperature to produce active gold nanoparticles was 300 C, and the best Au amount was 0.5 wt.% (Fig. 4.10).

4.2.2 Preparation methods of TiO2 film In the previous sections the most common methods for the preparation of TiO2based photocatalysts both in the form of powders and films have been briefly described. In the following ones, some methods used only for preparing films will be presented. Photocatalytic films allow to overcome some drawbacks related to the use of powdered catalysts for which the separation from the reaction environment, the control of porosity, the coupling of materials with different functionalities are more difficult. TiO2-based films can be effective for the realization of self-cleaning and antifogging surfaces; in this case the wettability and the transparence are important properties, in addition to the photocatalytic ones. Furthermore, it is worth noting that films can also be used in electrocatalytic processes, and in that case, it is important to evaluate both their electrochemical and catalytic efficiency.

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4.2.2.1 Dip-coating The dip-coating technique is a wet deposition method that involves the deposition of a liquid precursor onto a substrate followed by a thermal treatment to obtain the desired material [123]. During that process the substrate is first immersed in the solution containing the precursor and then withdrawn at a constant speed [124]. The film is deposited thanks to the gravitational draining and the evaporation of the solvent. Successively, the coating is subjected to a heat treatment that allows the crystallization of the deposited material. The film thickness depends on the viscosity of the solution and the withdrawal rate. The advantage of this process is the simplicity of the equipment and the low cost, and the drawbacks, which may restrict its application, are the occurrence of the deposition at both sides of the substrate and the difficult to coat large surfaces. In addition, the bottom parts are thicker than up parts because of the gravitation. Pure anatase, brookite, or rutile TiO2 films were obtained by dip-coating by using TiCl4 as the precursor [125]. The dip-coating withdrawing rate was 96 mm/ min, and the thicknesses of the films were 40, 240, and 500 nm for rutile, brookite, and anatase, respectively. It is worth noting that, although the experimental conditions or the films preparation were very similar, the thickness of the three films resulted different as the intrinsic physical and morphological properties of the three TiO2 polymorphs were different. The samples were active for the degradation of 2propanol in a gas
solid system, and the brookite films exhibited a higher photoactivity than the rutile and anatase ones. TiO2-based films are widely used for photovoltaic applications [26]. SiO2/TiO2 films with different thickness prepared by dip-coating on silicon wafers were used as antireflection coatings for photovoltaic applications [126]. Tetraethoxysilane was used as SiO2 precursor and titanium isopropoxide as TiO2 precursor. By varying the number of dipping, it was possible to change the thickness and the refractive index of the films. The presence of an SiO2 sublayer allowed to decrease the refractive index. Nanocrystalline multilayered TiO2 films deposited by dip-coating onto ITO were used as under layer for developing the photoanode of dye-sensitized solar cells (DSSC). The photoconversion efficiency of the assembled solar cell was c. 7% [127].

4.2.2.2 Spin
coating Spin-coating is a process used to apply uniform thin films on flat supports by using a centrifugal force [128
130]. This technique is employed to realize self-cleaning hydrophobic or hydrophilic surfaces [131], for the preparation of antireflection coatings for solar cells [132] and by the microelectronics industry since it allows the deposit of smooth films [133]. In a typical procedure an excess amount of a solution containing the desired precursor is placed at the center of a circular surface, which is then rotated at high

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Figure 4.11 Steps of the dip-coating process.

speed in order to spread the fluid by centrifugal force. Uniform films of 1
10 μm in thickness are obtained and the thickness depends on spinning velocity and time, surface tension, and viscosity of the solution. Typical spin speeds range from 500 to 10,000 rpm. Generally, increasing the spinning speed decreases the thickness of the layer. Spin-coating results in a relatively planar surface. The process is carried out in four steps (Fig. 4.11): deposition, spin up, spin off, and evaporation. First, the precursor solution is deposited onto the support and then spin up and spin off ensue in sequence, while the evaporation phase occurs throughout the process. The solvent is removed partially during the spinning process due to evaporation and successively by heat treatment. The advantages are the shortness of the process, the small amounts of reagents, the homogeneity of the coating, and the possibility to prepare nanocomposite materials by successive spin depositions. Moreover, by adding metallic precursor to the solution, doped samples can be obtained. The spin-coating is preferred to dipcoating when it is necessary to deposit the coating only on one face of the support. Nanosized anatase TiO2 film with micro- and nanometer-scale hierarchical surface structure were obtained by spin-coating on the ITO glass in the presence of sodium dodecylbenzenesulfonate (DBS) with the rotation speed of 1000 rpm for 15 s and 3000 rpm for 30 s, followed by drying at 80 C and thermal treatment at 400 C for 2 h [134]. Thanks to the high roughness resulting from the hierarchical structure of the surface, the films exhibited superhydrophilic properties also without irradiation. The particular features of the surface are strictly linked to the DBS addition and encourage the practical application of TiO2 films. Cui et al. [135] obtained TiO2 thin films both by spin-coating and spray pyrolysis. Titanium tetraisopropoxide was used as Ti precursor, ethanol and water as solvents, and HNO3 as acidifying agent. The solution was spin-coated onto Pyrex glasses at 2050 rpm for 3 min. The resulting coatings were dried at room temperature and then calcined at 500 C. The films obtained by spin-coated showed a photocatalytic activity comparable to that of films prepared by spray pyrolysis. TiO2 and W/TiO2 thin films obtained starting from titanium(IV) ethoxide and tungsten(V) ethoxide by means of spin-coating onto alumina substrates used as sensors showed high ethanol-sensing performances [136]. TiO2 sol was obtained by a modified sol
gel method, and tungsten(V) ethoxide was used as a dopant in two different concentrations that led a nominal W/Ti atomic ratio of 5/33 and 10/33, respectively.

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4.2.2.3 Chemical vapor deposition Chemical vapor deposition (CVD) is a preparation method in which films are deposited onto a support from the vapor phase by the decomposition of reagents on the support surface [137
139]. The deposition of the film is controlled by a chemical reaction occurring on or near a heated support surface. The properties of coating can be tuned by varying the support temperature and type and the composition of the reaction mixture. The process takes place in a reaction chamber, inside which the reactant gases are introduced to decompose and react with the support to form the film. A typical CVD process consists of the following steps: (1) a predefined mix of reactant gases and diluent inert gases is introduced at a specified flow rate into the reaction chamber, (2) the gas species move to the support, (3) the reactants get adsorbed on the surface of the support, (4) the reactants undergo chemical reactions with the support to form the film, and (5) the gaseous by-products of the reactions are desorbed and evacuated from the reaction chamber. A schematic setup is reported in Fig. 4.12. The used precursors must be stable at room temperature and sufficiently volatile; moreover, they must react without the formation of side products. When metalloorganic compounds are used as precursors, the method is named metal organic CVD (MOCVD). The energy required to drive the reactions can be supplied simply by heating the substrate or by irradiating with UV light or a laser. If UV light is used, this is called photo-CVD. Laser CVD, instead, indicates the use of a coherent, monochromatic high-energy beam of photons to locally heat the substrate and to drive the deposition reaction. CVD is applied in the semiconductor industry to produce thin coating onto various supports that can be used as conductors, dielectrics, passivation layers, oxidation barriers, corrosion-resistant coatings, heat-resistant coatings, and epitaxial layers for microelectronics [138,139]. MOCVD process has been used for the preparation of TiO2 anatase supported on porous alumina [140]. Titanium tetraisopropoxide was

Figure 4.12 Schematic setup of a CVD system: (1) carrier gas, (2) precursor, (3) vertical furnaces, (4) quartz reactors, (5) catalyst support, and (6) cold trap. CVD, Chemical vapor deposition. Source: Reprinted from M. Bellardita, A. Di Paola, S. Yurdakal, L. Palmisano, Preparation of catalysts and photocatalysts used for similar processes, in: G. Marcı`, L. Palmisano (Eds.), Heterogeneous Photocatalysis: Relationships With Heterogeneous Catalysis and Perspectives, 2019, pp.25
56 with permission, ©2019 Elsevier Publishing.

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deposited inside the alumina pores under vacuum and then decomposed to give titanium dioxide. The photocatalytic activity of alumina-supported titanium dioxide film was similar to that of the commercial Degussa P25 TiO2 powder. Anatase TiO2 was deposited onto three different supports, activated carbon (AC), γ-alumina (Al2O3), and silica gel (SiO2), by CVD [141]. The properties of TiO2 depended on the support and the synthesis conditions, that is, evaporation temperature of precursor, flow rate of carrier gas, deposition temperature in the reactor. Among the three types of supports, SiO2 with higher surface hydroxyl groups and macropore surface area was the best one, and TiO2/SiO2 exhibited the highest photocatalytic activity for aqueous phenol degradation. Anatase TiO2 thin films deposited on silicon by a low-pressure MOCVD have been used as a new capacitor dielectric material in integrated circuits [142]. The dielectric constant of the obtained films ranged from 7 to 28 depending on the film thickness. Furthermore, annealing improved the quality of TiO2 with respect to leakage current density, dielectric constant (ε 5 70), and interface trap density. TiO2 was deposited by photo-CVD onto indium-tin-oxide coated polyethylene terephthalate film to prepare electrodes for dye-sensitized solar cells [143]. UVassisted CVD treatment drastically enhanced dye-sensitized photocurrent and improved photovoltage up to 750 mV. A film electrode bearing Ru complex yielded a solar energy conversion efficiency of 3.8%.

4.2.2.4 Physical vapor deposition Physical vapor deposition (PVD) represents a physical method for the deposition of thin layers of catalyst onto a substrate by high-temperature vaporization of a liquid or solid material that could contain also precursors of metal(s) [144]. Different types of substrates can be used, including electrically conductive materials. Nanocomposites coating can be obtained by PVD because individual elements can be evaporated and then codeposited onto the desired substrate. The procedure involves the (1) sputtering/evaporation of different components to produce a vapor phase by means of a high energy source as, for instance, an electron beam; (2) transport of the coating vapor to the support; (3) reaction between atoms of the metals and a suitable gas (such as oxygen) during the transport stage; (4) deposition of the coating at the support surface; (5) consolidation of the nanocomposite by heating under an inert atmosphere. The process is carried out in a vacuum chamber (1026 Torr) equipped with a cathodic arc source. Different techniques fall into the PVD methods, the most common are sputtering and evaporation, the less used are arc vapor deposition and pulsed laser deposition. The differences between these methods rely on the methodology of the evaporation of the metallic components and the plasma conditions during the deposition. The transition of a metallic component from solid to the vapor phase can occur by heating an evaporation source (as in cathodic arc) or by sputtering a target (as in magnetron sputtering). The arc deposition method uses higher energy input. Hydrophilic TiO2 films were obtained by electron-beam evaporation method using rutile TiO2 as the source material at different O2 pressures [145]. The TiO2 structure (anatase or rutile) and the UV-induced hydrophilicity of the films were

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related to the O2 pressure. The best hydrophilicity was observed on the wellcrystallized anatase TiO2 films obtained with 5 3 1025 Torr O2 pressure. The films deposited in the absence of O2 are not well crystallized and less hydrophilic. TiO2 films were deposited onto glass slides supports by DC magnetron sputtering by changing the sputtering current values and the deposition times [146]. The polycrystalline anatase films were obtained at a low sputtering current value, and they did not depend on the deposition time. The size of the crystallites increased by increasing the sputtering current and the deposition time, while only the first parameter influenced the crystallinity of TiO2. Thin films consisting of nanostructured TiO2 photocatalysts with a mixed-phase composition were obtained by reactive DC magnetron sputter deposition method [147]. By tuning some parameters as oxygen partial pressure, support bias, target power, deposition incidence angle, and postannealing treatment, it was possible to control the formation of phase and interface. Transparent TiO2 layers consisting of vertically aligned nanocolumns were deposited by PVD and used as electrodes for dye-sensitized solar cells [148]. The influence of some parameters as deposition angle was investigated. By using an electrode thickness equal to 500 nm, the solar cell efficiency varied from 0.6% and 1.04% at deposition angles ranging between 60 and 85 degrees. A power conversion efficiency of 2.78% was obtained when the electrode thickness was 3 μm.

4.3

Characterization techniques of TiO2

4.3.1 X-ray diffraction XRD is one of the most fundamental and important techniques for structural characterization of crystalline materials, including the TiO2-based ones [149,150]. Crystallinity percentage, crystal phase ratio, lattice parameters, facet parameters and their distribution, and average primary particle size of the TiO2-based materials can be determined by using this technique [66,151]. In the case of coupled-TiO2 or loaded/doped-TiO2 samples, also the presence of modifying species can be revealed, if the latter are present in a quite high amount (generally higher than 5% w/w) [116]. Samples can be analyzed as powders or films. X-rays could be generated when matter is irradiated by an electron beam with high energy [149]. Electrons are produced by a heated filament, and then they are accelerated under vacuum in the presence of a high electric field (20
60 kV) toward a metal target that acts as anode. The corresponding electric current is 5
100 mA. X-rays are highly penetrating because their energy is greater than 5 keV. von Laue suggested that wavelength (1
100 angstroms) of the X-rays is comparable to that of distances between the units that constitute the crystals cell, and in 1914 he was awarded the Nobel Prize in Physics for discovering XRD from the crystals. After pioneering work of von Laue, the technique was developed by Bragg and Bragg who were also awarded the 1915 Nobel Prize in Physics for their work in crystallography. The Bragg law links the observed scattering with reflections of equidistant planes within the crystal [152,153].

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Figure 4.13 The scattered X-ray beam from the atoms on two layers.

Some part of the incoming beam causes each scatterer to reemit as a spherical wave (Fig. 4.13). The scatterers are arranged symmetrically in crystalline solids, due to the repeating arrangement of atoms, with a distance of d, and these waves can be in sync only in directions where their path-length difference (2dsinθ) equals an integer multiple of the wavelength λ. In that case a part of the beam is deflected by an angle (2θ), producing a reflection spot in the diffraction pattern. Bragg’s law is reported in the following equation: 2dsinθ 5 nλ

(4.1)

where d is the distance between two diffraction planes, θ is the incident angle, n is an integer, and λ is the wavelength of the used X-ray beam. These specific directions appear as spots in the diffraction pattern. By using XRD, the average primary particle size (crystallite size) of TiO2-based materials in nanoscale can be determined by using the Scherrer equation. The particles are made up of many crystallites as agglomerates, and, consequently, XRD cannot provide any information on the size of the particles that could be determined in detail by means of electron microscopy techniques, namely, Scanning Electron Microscopy, Transmission Electron Microscopy, and Atomic Force Microscopy. The Scherrer equation can be written as the following equation: τ5

Kλ βcosθ

(4.2)

where τ is the average primary particle size (crystallite size) of sample, K is a shape factor (dimensionless and its value is 0.9 for spherical crystallites but varies with the actual shape of the crystallite), λ is the wavelength of q theffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi used X-ray ffi (i.e., 0.154 nm), θ is the Bragg angle, and β is the line broadening β 5

β 2exp 2 β 2s , where β exp and β s

are the measured full width at half maximum (FWHM) of the main XRD peaks of the sample and of a standard, respectively, to take account of instrumental broadening. To calculate the crystallinity, it is necessary to prepare a mixture of TiO2 and the standard

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with a mass ratio of 50%. The standard consists of a highly crystalline sample, the main peak of which must be close to the main peak of the sample whose crystallite size has to be calculated. The TiO2 crystallinity as mass percentages can be evaluated by following the procedure reported by Jensen et al. [154]. XRD diffractograms of the mixture of TiO2 and 100% crystalline CaF2 (50% by mass) used as an internal standard are recorded, and the areas of the 100% peaks of anatase (1 0 1), rutile (1 1 0 ), and CaF2 (2 2 0) are determined. Finally, the degree of crystallinity of phases and amorphous phases present in TiO2 is estimated by a comparison of the ratio between the peak areas of two TiO2 and CaF2 phases (1.25 for anatase and 0.90 for rutile). Therefore for rutile and anatase, Eqs. (4.3) and (4.4), respectively, can be used: Crystallinity of TiO2 rutile 5

1 rutile area ð1 1 0Þ 3 3 100% 0:90 CaF2 area ð2 2 0Þ

Crystallinity of TiO2 anatase 5

1 anatase area ð1 0 1Þ 3 3 100% 1:25 CaF2 area ð2 2 0Þ

(4.3)

(4.4)

Wang et al. [155] investigated another method to calculate the crystallinity of anatase TiO2, and the area of the (1 0 1) peak of anatase was compared with that of the (1 1 1) peak of CaF2 rather than with that of the (2 2 0) peak (Eq. 4.5): Crystallinity of TiO2 anatase 5

1 anatase area ð1 0 1Þ 3 3 100% 1:31 CaF2 area ð1 1 1Þ

(4.5)

Bellardita et al. [156] proposed different equations to evaluate the crystallinity degree of anatase and rutile TiO2 catalysts by XRD analysis from the ratio among the FWHM value of the main diffraction peaks of anatase or rutile and that of the (1 1 1) peak of CaF2 (Eqs. 4.6 and 4.7). In addition, the absolute crystallinity of brookite was also calculated from the ratio between the FWHM of the (1 2 1) XRD peaks of the brookite TiO2 and that of the (1 1 1) peak of CaF2 [157] (Eq. 4.8). Fig. 4.14 shows XRD patterns of 100% crystalline brookite, 100% crystalline CaF2, and homemade brookite prepared at 100 C. The homemade brookite showed a crystallity degree of 26.6%, and it increased after calcination treatments at different temperatures. The peaks used to determine the crystallinity are indicated [157]: Crystallinity of TiO2 rutile 5

1 FWHM CaF2 ð1 1 1Þ 3 3 100% 0:98 FWHM rutile ð1 1 0Þ

(4.6)

Crystallinity of TiO2 anatase 5

1 FWHM CaF2 ð1 1 1Þ 3 3 100% 1:15 FWHM anatase ð1 0 1Þ

(4.7)

Crystallinity of TiO2 brookite 5

1 FWHM CaF2 ð1 1 1Þ 3 3 100% 1:04 FWHM brookite ð1 2 1Þ

(4.8)

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Figure 4.14 XRD patterns of the mixtures constituted by 100% crystalline CaF2 and 100% crystalline brookite (A) or homemade brookite (B). XRD, X-ray diffraction. Source: Reprinted from M. Bellardita, A. Di Paola, B. Megna, L. Palmisano, Absolute crystallinity and photocatalytic activity of brookite TiO2 samples, Appl. Catal. B, 201 (2017) 150
158 with permission, ©2017 Elsevier Publishing.

The crystallinity degree influenced the photocatalytic activity for the 4nitrophenol degradation and the selective oxidation of 4-methoxybenzyl alcohol to p-anisaldehyde. In particular, the most crystalline samples were the most efficient for the oxidation of 4-nitrophenol or 4-methoxybenzyl alcohol, while the highest selectivity toward the synthesis of p-anisaldehyde was obtained in the presence of the least crystalline powders. Yurdakal and coworkers [158] prepared nanotube structured TiO2 on Ti plate, used as photoanode, by an anodic oxidation method in ethylene glycol. The samples were also characterized by the XRD technique, and the results are reported in Figs. 4.15 and 4.16 along with that of Ti plate for comparison. The anodes are named TiO2NT-x
y in which TiO2NT indicates the nanotube structured TiO2, x the anodic oxidation time used for the nanotube production, and y the calcination temperature of the anodes. Uncalcined TiO2NT sample did not present any XRD patterns since TiO2 was amorphous (its XRD patterns are not shown). Fig. 4.15 shows XRD patterns of anodes prepared at different calcination temperatures (400 C
600 C). The XRD peaks at 2θ 5 25.58, 38.08, 48.08, and 54.58 degrees can be attributed to anatase phase; those at 2θ 5 27.5, 36.5, 41.0, 54.1, and 56.5 degrees to rutile phase [159]; and those at 2θ 5 34.95, 38.25, 40.05, and 52.90 degrees to metal Ti [160]. The results of XRD analyses show that Ti/TiO2NT-30 min-400 and Ti/ TiO2NT-30 min-500 anodes contain only anatase and mainly anatase with traces of rutile, respectively, while Ti/TiO2NT-30 min-600 contains significant amounts of both anatase and rutile phases. By increasing the temperature of thermal treatment, XRD patterns of rutile, the most thermodynamically stable TiO2 phase, also increases [161].

Synthesis and characterization of titanium dioxide and titanium dioxide
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Ti

Ti Ti R

R

113

Ti

R

Ti

R

Ti/TiO2NT-30 min-600

Ti/TiO2NT-30 min-500

A A

A

A

Ti/TiO2NT-30 min-400 Ti Ti Ti Ti layer

Ti

Ti

20 25 30 35 40 45 50 55 60 65 70 75 80 2Ɵ (degrees)

Figure 4.15 XRD patterns of the Ti/TiO2-NT-30 min-Y anodes. A, Anatase; R, rutile; Ti, titanium; XRD, X-ray diffraction. ¨ zcan, V. Loddo, L. Source: Reprinted from S. Yurdakal, S, C¸etinkaya, M.B. Sarlak, ¸ L. O Palmisano, Photoelectrocatalytic oxidation of 3-pyridinemethanol to 3-pyridinemethanal and vitamin B3 by TiO2 nanotubes, Catal. Sci. Technol. 10 (2020) 124
137 with permission, ©2020 Royal Society of Chemistry Publishing.

Fig. 4.16 shows the XRD patterns of nanotube structured TiO2 photoanodes prepared at different anodic oxidation times (10 min to 6 h) but calcined at the same temperature (500 C). The main TiO2 phase on the anode surface is anatase along with traces amount of rutile. It is worth noting that the intensity of peaks attributable to anatase increased with the anodic oxidation time, and it could be related to the length of nanotubes (460
4500 nm) on the Ti plate surface. The above statement is valid only if the nanotube morphology is very similar and the thermal treatment temperature is the same. The loaded noble metals can be identified by XRD analysis if their quantity is not too low. Fu and collaborators [162] prepared TiO2 films loaded with noble metal (Pt, Au, or Pd) by the electrostatic self-assembly method, and their XRD

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A

Ti A

Ti

Ti A

Ti Ti

A

Ti/TiO2NT-6 h-500

Ti/TiO2NT-3 h-500

Ti/TiO2NT-2 h-500

Ti/TiO2NT-1 h-500

Ti/TiO2NT-30 min-500

Ti Ti/TiO2NT-10 min-500 Ti Ti

Ti Ti

Ti layer

20 25 30 35 40 45 50 55 60 65 70 75 80 2Ɵ (degrees)

Figure 4.16 XRD patterns of the Ti/TiO2NT-X-500 anodes. A, Anatase; R, rutile; Ti, titanium; XRD, X-ray diffraction. ¨ zcan, V. Loddo, L. Source: Reprinted from S. Yurdakal, S, C¸etinkaya, M.B. Sarlak, ¸ L. O Palmisano, Photoelectrocatalytic oxidation of 3-pyridinemethanol to 3-pyridinemethanal and vitamin B3 by TiO2 nanotubes, Catal. Sci. Technol. 10 (2020) 124
137 with permission, ©2020 Royal Society of Chemistry Publishing.

models are shown in Fig. 4.17. The TiO2 film deposited on a Ti wire is present as a mixture of anatase and rutile phases. The results obtained show that the Pt and Au nanoparticles are still in metallic form, while the Pd nanoparticles have been transformed into PdO. The diffraction peaks of the loaded species are shown in Fig. 4.17, and the corresponding 2θ values listed in Table 4.2.

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Figure 4.17 XRD patterns of pristine TiO2 film (A), Pt
TiO2 (B), Au
TiO2 (C), and PdO
TiO2 (D). A, Anatase; R, rutile; Pd, PdO; XRD, X-ray diffraction. Source: Reprinted from P. Fu, P. Zhang, J. Li, Simultaneous elimination of formaldehyde and ozone by product using noble metal modified TiO2 films in the gaseous VUV photocatalysis, Int. J. Photoenergy, 2012 (2012), Article ID 174862 with permission, ©2012 Hindawi Publishing. Table 4.2 The diffraction peak values and JCPDS numbers of PdO, metallic Pt, and Au used for TiO2 loading [162]. Name

Diffraction peak values at 2θ

JCPDS number

Pt Au

39.9 (1 1 1), 46.4 (2 0 0), 67.5 (2 2 0), 81.3 degrees (3 1 1) 38.3 (1 1 1), 44.4 (2 0 0), 64.7 (2 2 0), 77.5 (3 1 1), 81.9 degrees (2 2 2) 33.9 (1 0 1), 41.9 (1 1 0), 60.3 (1 0 3), 60.9 (2 0 0), 71.5 degrees (2 0 2)

4-0802 65-8601

PdO

41-1107

4.3.2 Scanning electron microscopy SEM is one of the most used techniques to obtain images of materials by scanning the surface with fast and focused electron beams [163]. The latter interact with the atoms of the materials allowing to determine the morphological and compositional properties of TiO2-based materials with high resolution (even higher than 1 nm)

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[164]. Detailed topographic and chemical composition analyses can be performed to study the influence of preparation methods on some important properties of the various TiO2 phases or composite materials in which TiO2 is present, which are used in photocatalytic, photoelectrocatalytic, photovoltaic, and sensor applications. SEM technique delivers three-dimensional imagines, while the TEM only two [163]. In addition, SEM analysis time is much shorter than that of TEM, but TEM has higher magnification power and resolution that are main demands for determining the crystallinity of the particles, the lattice structure, and the possible presence of defects that are essential, for instance, in photocatalytic processes. Therefore generally both methods are useful to get detailed information on the same material. Before performing the SEM analyses, nonconductive samples must be covered with a very thin gold layer with the sputtering deposition method. The microscope consists of an electron column, a scanning system, some detectors, a display, a vacuum system, and electronic controls (Fig. 4.18). An electron beam is obtained by an electron gun, where a tungsten wire is the source that is placed at the top of the column and the path of which is controlled by some electromagnetic lenses. The beam size and resolution are determined by the condenser, and the objective lens shift at a working distance the smallest spot size produced by the beam impinging the surface of the sample. When the electron beams interact with the sample, a series of electromagnetic radiations of electrons or photons occurs (see Fig. 4.19). In order to transform an image, both secondary electrons (SEs) and backscattered electrons (BSEs) accumulate by means of suited detectors and the amplified output signal appears in a monitor [165]. SEs are emitted from the atoms of the sample with phenomena that follow the inelastic scattering. The energy of these electrons is very low (about 3
5 eV), and, consequently, only those formed within a few nanometers of the sample surface are expelled out of the sample and can give a detailed high-resolution morphological information.

Figure 4.18 Scheme of an SEM instrument. SEM, Scanning electron microscopy.

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Figure 4.19 The radiations produced by the interaction of electron beam with the sample.

Conversely, BSEs that are scattered elastically backward give rise to single or multiple dispersion events and emerge from the sample surface with an energy greater than approximately 50 eV. As their energy is higher than that of the SEs, the information provided by their interaction with the matter derives from a deeper region of the sample and depends on its composition. Therefore a BSE image of heavy elements, which backscatter more efficiently, appears brighter than that of light elements. Another very important application of SEM is the elemental analysis of samples by detection of characteristic X-ray, which is produced during the electron
material interaction by energy-dispersive X-ray spectroscopy (EDS) or wavelength-dispersive X-ray spectroscopy (WDS). When an inner shell electron is released by an orbital of the sample, the vacancy created is occupied by an outer shell electron, and an X-ray with an energy equal to the gap between the two different shells of the excited atom of the sample is produced. Together with the characteristic X-rays, which produce well-defined lines in the EDS spectrum, continuous X-rays are obtained because the electrons of the primary beam decelerate due to the presence of the electromagnetic field of the atomic nuclei. The distribution of this loss of energy in the EDS spectrum is continuous and does not depend on the atomic number of the elements present in the sample. The lighter elements (i.e., H and He) of the material cannot be detected by EDS, while the other heavier elements can be determined both qualitatively and quantitatively. By decreasing the atomic mass of the element, the error percentage increases (from c. 0.1% to 5%) [166]. Other disadvantages of the EDS technique are the low peak-to-background ratio and the energy resolution. These limitations, because of which the identification and quantification of the trace elements is not easy, can be reduced by coupling the WDS technique with EDS. In fact, EDS allows us to examine the sample from a qualitative point of view, while WDS to define the details and to quantitatively determine the trace

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Titanium Dioxide (TiO2) and Its Applications

elements thanks to its greater sensitivity and resolution. In the following, only some examples reported in the literature of how the SEM technique can help in the study of TiO2-based materials will be reported. ¨ zcan and coworkers [167] prepared nanotube structured TiO2 on Ti layer as photoO anode by an anodic oxidation method carried out in ethylene glycol solution for different times. The anodes were crystallized by calcination at 500 C and their morphologic properties at top, bottom, and cross-sectional views were investigated by SEM observations (Figs. 4.20
4.22). The wall thickness, the internal diameter, and the length of nanotubes could be easily determined by SEM images. For instance, the values of Ti/ TiO2-4 h anode (4 h indicates the anodic oxidation time) resulted to be 35 nm, 47 nm, and 9.8 μm, respectively. Even if the layers are thick and large (5 cm 3 8 cm), the images of Figs. 4.20A, 4.21A, and 4.22 were taken directly and without covering them with gold. Fig. 4.22 shows SEM images of Pt-loaded nanotube structured layer by electrodeposition method. In this figure, cubic structure of metallic Pt nanoparticles and their sizes (c. 150 nm) along with their distribution can be also determined. In order to determine bottom and cross-sectional views of nanotubes (Figs. 4.20B and C and 4.21B), a part of nanotubes were scraped and covered with a thin layer of gold. Sun et al. [168] reported the preparation of three-dimensional dendritic TiO2 nanostructures by a hydrothermal method. Various ratios of titanium isopropoxide (TTIP),

Figure 4.20 SEM images of Ti/TiO2-4 h anode: (A) top view (200,000 3 ), (B) bottom view (100,000 3 ), and (C) cross-sectional view (20,000 3 ). SEM, Scanning electron microscopy. ¨ zcan, T. Mutlu, S. Yurdakal, Photoelectrocatalytic degradation Source: Reprinted from L. O of paraquat by Pt loaded TiO2 nanotubes on Ti anodes, Materials 11 (2018) 1715 with permission, ©2018 MDPI Publishing.

Figure 4.21 SEM images of Ti/TiO2-6 h anode: (A) top view (500,000 3 ) and (B) bottom view (120,000 3 ). SEM, Scanning electron microscopy. ¨ zcan, T. Mutlu, S. Yurdakal, Photoelectrocatalytic degradation Source: Reprinted from L. O of paraquat by Pt loaded TiO2 nanotubes on Ti anodes, Materials 11 (2018) 1715 with permission, ©2018 MDPI Publishing.

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hydrochloride acid (HCl), cetyltrimethylammonium bromide (CTAB), ethylene glycol (EG), urea, and water were used. In this way, TiO2 microspheres with high surface area containing nanorods (Fig. 4.23A
C), nanoribbons (Fig. 4.23D
F), and nanowires (Fig. 4.23G
I) building units were obtained, and their morphological properties were investigated by SEM observations. In particular, the images allowed to determine the

Figure 4.22 SEM images of Ti/TiO2-3 h-Pt anode at magnification: 200,000 3 (A) and 500,000 3 (B). SEM, Scanning electron microscopy. ¨ zcan, T. Mutlu, S. Yurdakal, Photoelectrocatalytic degradation Source: Reprinted from L. O of paraquat by Pt loaded TiO2 nanotubes on Ti anodes, Materials 11 (2018) 1715 with permission, ©2018 MDPI Publishing.

Figure 4.23 SEM images of 3D dendritic TiO2 microspheres with nanorods (A
C), nanoribbons (D
F), and nanowires (G
I). SEM, Scanning electron microscopy. Source: Reprinted from Z. Sun, J. H. Kim, Y. Zhao, F. Bijarbooneh, V. Malgras, Y. Lee, Y.-M. Kang, S. X. Dou, Rational design of 3D dendritic TiO2 nanostructures with favorable architectures, J. Am. Chem. Soc. 133 (2011) 19314
19317 with permission, ©2011 American Chemical Society Publishing.

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diameter of dendritic microspheres together with the width of their constituent nanostructures, and these values, for instance, for the sample shown in Fig. 4.23D are 3.8 and 10
20 nm, respectively. Pham et al. [169] prepared thin films by covering quartz substrates with a sol of Cu-doped TiO2 mixed with reduced graphene oxide (RGO) and used them for photocatalytic abatement of methylene blue under UV irradiation. Fig. 4.24 shows

Figure 4.24 EDS elemental mapping of 7.5-CTG-5 (7.5 wt.% of Cu and 5 wt.% of RGO) photocatalyst. Source: Reprinted from T.-T. Pham, C. Nguyen-Huy, H.-J. Lee, T.-D. Nguyen-Phan, T. H. Son, C.-K. Kim, E. W. Shin, Cu-doped TiO2/reduced graphene oxide thin-film photocatalysts: effect of Cu content upon methylene blue removal in water, Ceram. Int. 41 (2015) 11184
11193 with permission, ©2015 Elsevier Publishing.

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EDS mapping of 7.5-CTG-5 photocatalyst for C, O, Ti, and Cu atoms. The Cu and RGO content determined by the EDS spectrum reported in the sample code was 7.5% and 5% by mass, respectively, while those determined by the loss of mass into air from TGA were 7.20% and 6.37%, respectively.

4.3.3 Transmission electron microscopy TEM is another largely used and important electron microscopy technique that allows us to obtain high-resolution images of TiO2-based materials [170]. It was first investigated in the 1930s by Max Knoll and Ernst Ruska [171]. Ruska received the Nobel Prize in Physics in 1986 for his work in electronic optics and for the design of the first electron microscope. The basic principle is that an electron beam is transmitted through the material that is suspended on a grid. The image is enlarged and focused on a device such as a fluorescent or photographic screen or a sensor. TEM allows us to obtain a much higher resolution than optical microscopy since the wavelength of an electron is much less than that of light (UV or vis) [172]. In particular, the crystalline phase, the crystallinity, and the reticular structure of TiO2-based materials together with the possible presence of defects in them can be analyzed in detail in two dimensions using a TEM device. TEM instrument consists of a few main components: electronic gun, a series of electromagnetic lenses, electrostatic plates, imaging device, and vacuum system [172]. The electronic gun is the source of electrons and usually consists of a tungsten wire. The emission of electrons is obtained by applying a high voltage to the filament that produces a small current. Electromagnetic lenses and electrostatic plates allow to guide the beam. Imaging devices are used to obtain an image from the electrons that come out of the system. TEM analyses are performed at very low-pressure values (below 1024 Pa) by means of an efficient vacuum system. In this way, not only contaminants such as water adsorbed on the sample are eliminated, but also a high difference in the electrical potential between the cathode and the ground is achieved without producing an arc, and the frequency of impact of electrons with gas atoms is reduced [173]. Different applications of TEM by considering some selected studies are presented next. Liu and coworkers [174] prepared rutile, anatase, and brookite TiO2 nanoparticles in different morphologies by a hydrothermal method and by using TiB2 as precursor and analyzed their home prepared samples by means of SEM and TEM. Fig. 4.25 shows SEM, TEM, and high-resolution TEM (HR-TEM) images of rutile nanocones, anatase bipyramids, and brookite nanoparticles. SEM images (Fig. 4.25A) show the dandelion-like morphology of rutile TiO2 with a structure consisting of uniform nanocones, the diameters of which at the root and at the tip are c. 300 nm and only a few nm, respectively. The shape of the well-faceted crystals of anatase TiO2, instead, is a fairly truncated bipyramid (Fig. 4.25B). The surfaces (0 0 1) and (1 0 1) of the crystals can be picked out by combining SEM images and crystallographic symmetries of the anatase [175,176]. The average primary agglomerate size of brookite TiO2 is c. 30 nm (Fig. 4.25C). TEM images and selected area electron diffraction [insets in parts (D) and (F)] confirm the single-crystal characteristics of anatase, rutile, and

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Figure 4.25 SEM (A
C), TEM (D
F), and high-resolution TEM (G
I) images of the rutile (nanocones), anatase (bipyramids), and brookite (nanoparticles), respectively. The insets in parts (D) and (F) are the SAED patterns of the nanocones and the bipyramids. SAED, Selected area electron diffraction; SEM, scanning electron microscopy; TEM, transmission electron microscopy. Source: Reprinted from G. Liu, H.G. Yang, C. Sun, L. Cheng, L. Wang, G.Q. Lu, H.-M. Cheng, Titania polymorphs derived from crystalline titanium diboride, CrystEngComm 11 (2009) 2677
2682 with permission, ©2009 Royal Society of Chemistry Publishing.

brookite titania polymorphs. HR-TEM images identified the rutile, anatase, and brookite crystal phases by their characteristic lattice spaces of atomic planes that ˚ for rutile (1 1 0) [177], anatase have been reported to be 3.28, 2.41, and 3.56 A (0 0 4) [178], and brookite (0 1 2), respectively.

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Li and coworkers [179] prepared TiO2 nanorods by hydrothermal method and ZrO2-modified nanorod structured TiO2 nanocomposites (ZrO2/TiO2) by deposition
precipitation method. The TEM analyses of some of their samples are shown in Fig. 4.26. The lengths and diameters of nanorods are up to c. 1 μm and in the range 20
200 nm, respectively. The diameter of ZrO2 crystals in the 20 wt. %-ZrO2/TiO2 sample (20 wt.% nominal ZrO2-loaded one) is c. 10
20 nm, and it can be noticed that they are dispersed uniformly on the TiO2 nanorods (see Fig. 4.26C). EDS analysis confirms that Ti and Zr elements are in the composite (Fig. 4.26D). HR-TEM analysis indicates the presence of a nanorod surface with the lattice spacing of 0.35 nm that can be attributed to the (1 0 1) plane of the anatase phase (Fig. 4.26E). Moreover, the formation of a ZrO2 phase of about 20 nm in diameter and the lattice spacing of 0.30 nm on the nanoparticle is in accord with the value of the (1 0 1) plane reported in literature (Fig. 4.26F) [180]. Wu and coworkers [181] investigated Ga-doped and Pt-loaded TiO2
SiO2 composite (Pt/HGTS) in polystyrene, and the preparation of the material is illustrated in Fig. 4.27 together with the presentation of some TEM images. Fig. 4.27B shows a typical inverse opal structure because photonic polystyrene (PS) opal was used as the starting material in the catalyst preparation. The average diameter of the hexagonal macropores is c. 260 nm. The size of the loaded Pt nanoparticles on the surface of the framework is c. 4.4 nm, and the particles appear dispersed homogeneously (Fig. 4.27C). The HR-TEM image (inset, Fig. 4.27C) of the sample shows the lattice fringes of 0.27 and 0.35 nm due to the (1 1 1) planes of metallic platinum and the (1 0 1) planes of anatase phase of TiO2 [182]. The uniform distribution of Pt particles was identified by high-angle annular dark-field scanning TEM (HAADF-STEM) (Fig. 4.27D,E) and elemental mapping of the sample (Fig. 4.27F). The other elements (Ti, O, Si, and Ga) are also homogeneously distributed on the framework according to the mapping analysis (Fig. 4.27F). TEM images of pristine and Au-loaded TiO2 nanosheets with exposed (0 0 1) facets are shown in Fig. 4.28 [183]. It is very clear that the nanoparticles are in a nanosheet structure (Fig. 4.28A). The mean size and the thickness of the nanosheets are c. 40 and 5.4 nm, respectively. The parallel lattice spaces of the horizontal and vertical angle of well-crystallized TiO2 are 0.351 nm [corresponding to the (1 0 1) plane, Fig. 4.28B], and 0.235 nm [corresponding to the (0 0 1) plane, Fig. 4.28C], respectively. As shown in Fig. 4.28D, the Au nanoparticles with a diameter of c. 5 nm are present on the edge of the TiO2 nanosheets with highly exposed (0 0 1) facets. The same results were obtained also from the STEM images (Fig. 4.28E). From the observation of Fig. 4.28F, it can be seen that the TiO2 (0 0 1) and Au (1 1 1) planes are shared. The element mapping (Fig. 4.28G) and energy-dispersive spectroscopy (EDS) analysis (Fig. 4.28H) indicated the presence of Ti, O, and Au. This finding proves unequivocally that Au nanocrystals have been successfully grown on the edge of the TiO2 NSs. TEM has often been used to highlight the presence of heterojunctions in TiO2 composite materials. Chen et al. [184], for instance, prepared by a hydrothermal

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Figure 4.26 TEM images of TiO2 nanorods (A), 10 wt.%-ZrO2/TiO2 (B), and 20 wt.%-ZrO2/ TiO2 (C). EDS spectrum of TiO2/ZrO2 nanocomposite (D). HR-TEM images of TiO2 (E) and (F) ZrO2. HR-TEM, High-resolution TEM; TEM, transmission electron microscopy. Source: Reprinted from Z. Li, R. Wnetrzak, W. Kwapinski, J.J. Leahy, Synthesis and characterization of sulfated TiO2 nanorods and ZrO2/TiO2 nanocomposites for the esterification of biobased organic acid, ACS Appl. Mater. Interfaces, 4 (2012) 4499
4505 with permission, ©2012 American Chemical Society Publishing.

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Figure 4.27 (A) The illustration of the preparation process of Pt/HGTS. (B and C) TEM images of Pt/HGTS. (D and E) HAADF-STEM images of Pt/HGTS. (F) HAADF-STEM image of Pt/HGTS and the corresponding elemental maps for Pt, Ga, Ti, Si, and O. The theoretical Pt loading is 1 wt.%, and the inset of part (C) is the corresponding HR-TEM image of Pt/HGTS. HAADF-STEM, High-angle annular dark-field scanning transmission electron microscopy; TEM, Transmission electron microscopy. Source: Reprinted from S. Wu, X. Tan, J. Lei, H. Chen, L. Wang, J. Zhang, Ga-doped and Pt-loaded porous TiO2
SiO2 for photocatalytic nonoxidative coupling of methane, J. Am. Chem. Soc. 141 (2019) 6592
6600 with permission, ©2019 American Chemical Society Publishing.

method a hybrid catalyst in which heterojunctions between TiO2 nanosheets and NiO nanorods could be observed. The preparation process is summarized in Fig. 4.29A. The side length and the thickness of TiO2 nanosheets exhibit a rectangular outline and are c. 40 and 6 nm, respectively (Fig. 4.29B). The TEM image of TiO2/Ni(OH)2 is shown in Fig. 4.29C, and its diameter and length are 10 and 50
100 nm, respectively. Roots can be seen on both the upper and lower sides (0 0 1) of the TiO2 sheets. Numerous Ni(OH)2 nanorods are visible on each TiO2 nanosheet, and they were transformed into NiO nanorods by calcination to form 2D/1D TiO2 nanosheet/NiO nanorod heterojunction hybrids (Fig. 4.29D). The diameter and the length of the NiO nanorods are 5 and 20
40 nm, respectively. As shown from the HRTEM image in Fig. 4.29E, the lattice fringes about 0.35 and 0.21 nm are typical of the planes of anatase TiO2 (1 0 1) and NiO (2 0 0), respectively [185], suggesting the formation of well-defined TiO2/NiO

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Titanium Dioxide (TiO2) and Its Applications

Figure 4.28 (A) TEM images of TiO2. HRTEM images of TiO2 from the horizontal (B) and the vertical (C) angle of view. (D) TEM, (E) STEM, and (F) HRTEM images of Au
TiO2. (G) Element mapping of Ti, O, and Au of Au
TiO2. (H) EDS analysis of Au
TiO2. TEM, transmission electron microscopy. Source: Reprinted from Y. Cao, T. Wu, W. Dai, H. Dong, X. Zhang, TiO2 nanosheets with the Au nanocrystal-decorated edge for mitochondria-targeting enhanced sonodynamic therapy, Chem. Mater. 31 (2019) 9105
9114 with permission, ©2019 American Chemical Society Publishing.

heterojunction hybrids. The HAADF-STEM image in Fig. 4.29F and the elemental mappings of Ti (Fig. 4.29G), O (Fig. 4.29H), Ni (Fig. 4.29I), and their overlayered elemental mapping (Fig. 4.29J) also prove the formation of TiO2/NiO heterojunction hybrids.

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Figure 4.29 (A) The schematic diagram of the synthesis processes of TiO2/NiO heterojunction hybrids. TEM images of (B) TiO2 nanosheets, (C) TiO2@Ni(OH)2 hybrids, and (D) TiO2/NiO hybrids after 300 C calcination. (E) HRTEM; (F) HAADF-STEM; (G -I) the elemental mappings of Ti, O, and Ni; and (J) overlayered element mapping of Ti and Ni from TiO2/NiO hybrids. HAADF-STEM, High-angle annular dark-field scanning transmission electron microscopy; TEM, transmission electron microscopy. Source: Reprinted from J. Chen, M. Wang, J. Han, R. Guo, TiO2 nanosheet/NiO nanorod hierarchical nanostructures: p
n heterojunctions towards efficient photocatalysis, J. Colloid Interf. Sci. 562 (2020) 313
321 with permission, ©2020 Elsevier Publishing.

4.3.4 Brunauer
Emmett
Teller-specific surface area determination TiO2 is the most used and investigated photocatalysts for its high photocatalytic activity, low cost, and photocorrosion resistance [151,186,187]. In addition, TiO2 is nontoxic and chemically inert. The activity of TiO2 (and TiO2-based materials) depends on several factors, for instance, its bandgap energy, crystallinity, surface hydroxylation, and density of photoactive surface sites. The surface hydroxyl groups can be generally considered the photoactive sites that are responsible for the occurrence of the reactions, and they can be activated by photons, the energy of which is equal or higher than the bandgap of the photocatalyst [161,186]. Nevertheless, it should be noticed that only the surface hydroxyls that are transformed into radicals under irradiation are responsible for the photoactivity [188]. The other ones can only play a role indirectly because their presence influences the surface hydrophilicity and, consequently, the photoadsorption of the reacting species. It is, however, useful to

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determine an important parameter, that is, the specific surface area of the used photocatalysts although the photoactivity cannot be straightforwardly related only to the surface area values. Just as a simple example, the photoactivity of poorly crystallized TiO2 powders with high specific surface area may not be significant. Despite what said earlier about the meaning of the specific surface area values, their knowledge is essential in photocatalysis as in catalysis because they provide a useful knowledge of the solid surface, but it is necessary to analyze them together with other types of characterization. To determine the specific surface area of TiO2 powders and other materials the Brunauer
Emmett
Teller (BET) method is very often used [189]. This method is based on the physical adsorption of gas molecules on the solid surface and allows also to know the particle size and the pore size/volume distribution (textural properties of TiO2). The value of specific surface area per unit mass could be used only as a basis for determining a specific photocatalyst activity like the initial reaction rate (2r0), used for kinetic modeling in heterogeneous systems [190,191]:  ð 2 r0 Þ 5

   1 dn V dC 2 5 2 S dt S dt

(4.9)

where n, C, t, V, and S are moles of substrate, its concentration, irradiation time, volume of suspension (for solid
liquid systems), and the exposed surface area of catalyst (BET SSA 3 weight of catalyst), respectively. Another possibility consists in determining the reaction rate per gram of the used photocatalyst. Nevertheless, the weakness of both methods lays in the difficulty to know not only the real number of photoactive sites but also the amount of surface of catalyst actually irradiated.

4.3.4.1 Adsorption
desorption phenomena A fractional coverage of the surface, θ, ranging from 0 to 1, can be defined as in Eq. (4.10), and it is generally expressed in terms of volume of adsorbate (Eq. 4.11) [163,192]: θ5

ðoccupied number of adsorption sitesÞ ðavailable number of adsorption sitesÞ

(4.10)

θ5

V Vmon

(4.11)

where Vmon indicates the volume of adsorbate necessary to complete the coverage of the catalyst surface to obtain a monolayer. Two types of adsorption can occur on the catalysts surface, that is, physical (physisorption) and chemical (chemisorption) adsorptions [163,192,193]. BET surface area determination is based on physical adsorption due to week van der Waals forces or dipole
dipole interactions. The released energy during adsorption is low and close to the condensation enthalpy (from 220 to 240 kJ/mol). Physical adsorption is nonselective, reversible and the

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time of adsorption
desorption equilibrium is very short. Single- or multimolecular layers on the surface could form. Moreover, it generally occurs at low temperature, and the adsorption amount decreases by increasing the temperature.

4.3.4.2 Brunauer
Emmett
Teller isotherm Adsorption isotherms consist of the variation of fractional coverage (θ) with pressure [192,194,195]. Many factors as pressure, temperature, and properties of adsorbent and adsorbate influence the adsorbate amount and, generally speaking, the adsorption
desorption equilibrium. The monolayer volume (Vmon) and c (the model’s parameter) can be determined by the linearized BET equation (Eq. 4.12): p 1 c21 p 5 1 Vðp0 2 pÞ Vmon c Vmon c p0

(4.12)

where p0 is the vapor pressure on more than one layer of molecules of adsorbate that resembles a pure bulk liquid; c is a constant and it is large when the desorption enthalpy ðΔHdes  Þ of the monolayer is high with respect to the vaporization enthalpy (ΔHvap  Þ of the liquid adsorbate (Eq. 4.13): c 5 eðΔHdes



2ΔHvap  Þ=RT

(4.13)

Interaction between vapor molecules and surface is higher than the intermolecular interaction for high c values, and for this reason a Langmuir type of adsorption can be obtained under this condition at low-pressure values, while at high pressure a multilayer adsorption starts. On the contrary, the adsorbate molecules prefer to bind to each other for low c values, therefore the monolayer could form just at higher pressures. The BET specific surface area could be determined by using the following equation:  Surface area 5

 Vmon Na σ 22; 414

(4.14)

where Na is the Avogadro number and σ is the occupied area of an adsorbate molecule. For instance, σ value of an N2 molecule, which is the most used gas for BET measurements, is equal to 0.162 nm2 [163,195
197]. The obtained BET surface area value should be divided by the used amount of TiO2 to obtain this value in m2 per unit mass (g) of nanoparticles. The BET isotherms obtained theoretically for different c values are reported in Fig. 4.30. The coverage values increase continuously by increasing partial pressure since there is no limit to the adsorbate amount that could condense when multilayer coverage occurs.

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Titanium Dioxide (TiO2) and Its Applications

5 4.5 4 3.5

V/Vmon

3 2.5 2 1.5 1000 50

1

10

5

0.5

2

1

0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

p/p0 Figure 4.30 The obtained BET isotherm curves for different c values (from 1 to 1000). BET, Brunauer
Emmett
Teller.

When c is much higher than 1, the BET isotherm could be given as following equation: V 1 5 Vmon 1 2 ðp=p0 Þ

(4.15)

This simpler form is suitable for unreactive gases on polar surfaces in which c  102 because ΔH des is significantly greater than ΔH vap.

4.3.4.3 Brunauer
Emmett
Teller surface area determination BET-specific surface area of TiO2-based materials could be determined by following the adsorptive pressure using the vacuum-volumetric method in BET analysis [193,198,199]. In this method, pressure transducers display pressure variations with high accuracy during the adsorption
desorption process, and p/p0 values are determined under partial vacuum conditions. Usually, a mixture of N2 and He gases is used in the flow apparatus of a BET instrument as the adsorptive and nonadsorptive gases, respectively. The catalyst is cooled with liquid nitrogen during the analysis. A thermal conductivity detector monitors the extent of adsorption and desorption. A BET instrument should be calibrated before starting the analysis by means of a known volume of pure nitrogen in the absence of sample.

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4.3.4.4 The preparation of TiO2 samples The TiO2 surface and its pores are covered and blocked by impurities deriving from its precursor, solvent, and/or other species used for the preparation of the sample as water, HCl, NH3, ethanol. Therefore they should be removed to perform correctly the surface area determination. During this cleaning process, degassing proceeds by flowing an inert gas flow at moderate
high temperature (i.e., 250 C) or under vacuum [200]. Initially, the TiO2 catalyst is placed inside a glass cell that is wrapped in a heating mantle and connected to the outgas port of the machine during the degassing process. The most important point when doing the measurement is that the degassing temperature should be high enough to effectively remove the contaminant species from the surface of the powder without changing its morphology. BET-specific surface areas of TiO2 samples prepared at low or moderate temperature could result to be lower than their actual values when a too high degassing temperature is used. In particular, badly crystallized or mainly amorphous TiO2 samples could transform into well crystalline materials with higher particle size (due to aggregation) and lower surface areas [201,202]. Comparing XRD diffractograms and photocatalytic activity results before and after surface area determination of a sample is an excellent method to understand if the degassing treatment has induced changes in the structural properties and, consequently, in the surface area value.

4.3.4.5 Used gases for Brunauer
Emmett
Teller analysis As the adsorption phenomena between adsorbate and TiO2 surface increase by decreasing the temperature, BET analysis must be performed at low temperature. N2 adsorption at 77K (its boiling temperature) is the most widely used technique to determine the BET isotherm [192,200,203]. Nitrogen is cheap, abundant and it can be easily obtained from air. Other gases, such as argon, krypton, and hydrogen, could be also used for specific applications. Argon is an inert and nonpolar noble gas, and consequently, it is not influenced by surface charges, the adsorption
desorption equilibrium time is very short at 87K, and its atomic nature avoids the existence of possible orientation problems when adsorbs. Nevertheless, a full isotherm cannot be obtained with liquid nitrogen during the cooling process since boiling point of N2 is below the argon’s triple point. Another noble gas, krypton, can be used at 77K, instead, for measuring low surface areas and diameters of small pores of thin films.

4.3.4.6 Brunauer
Emmett
Teller instrument and its working principle A typical BET diagram used for surface area determination of materials by the volumetric method is shown in Fig. 4.31 [163]. After the degassing process a cylindrical Pyrex vessel containing the sample is placed in the analysis port, where liquid nitrogen in a dewar is used to cool the sample and maintain it at 77K. Nitrogen and

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Titanium Dioxide (TiO2) and Its Applications

Instrument manifold Data collector Nitrogen

P

P

Helium Sample cell Injection port Liquid nitrogen dewar

Figure 4.31 Schematic diagram of volumetric method apparatus. ¨ zcan, M. Bellardita, G. Palmisano, Source: Reprinted from S. Yurdakal, C. Garlisi, L. O (Photo)catalyst characterization techniques: adsorption isotherms and BET, SEM, FTIR, UV
vis, photoluminescence, and electrochemical characterizations, in: G. Marcı`, L. Palmisano (Eds.), Heterogeneous Photocatalysis: Relationships With Heterogeneous Catalysis and Perspectives, 2019, pp. 87
152 with permission, ©2019 Elsevier Publishing.

helium gases together are injected into the sample cell with a calibrated piston. During the BET analysis the adsorbed vapor amount on the catalyst can be measured by changing the pressure after the achievement of equilibrium. Therefore by using these values for different pressures, an adsorption isotherm is obtained, which allows the determination of surface area, pore size/volume, and their distribution. The instrument should be checked periodically with a reference material with known surface area and adsorption isotherm [199]. Pore size and its distribution in a material can be determined by the adsorption
desorption isotherm, and the most common used method is the numerical integration BJH (Barrett, Joyner, Halenda) [203]. The pore diameter distribution curve can be found by using the Kelvin equation (Eq. 4.16): ln

p 2γVm 52 p0 rk RT

(4.16)

where γ and Vm represent the surface tension and the molar volume of adsorbate, respectively. The relative pressure (p/p0) at which the capillary condensation of nitrogen inside the pores of a determined size occurs is related to the Kelvin radius (rk). This model hypothesizes that all pores on the surface are open and in cylindrical shape, and no intercommunication among them exists. The use of the Kelvin equation and the mathematical method BJH allows to build the integral pore volume curve f(d), where d 5 2rk. In such a way the distribution curve of the pores can be obtained.

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133

The followings discussed two examples of papers where nitrogen adsorption
desorption isotherms with analysis of BET-specific surface area and pore size/volume distribution for TiO2-based samples are reported. Li et al. [204] prepared mesoporous anatase TiO2 nanospheres with high surface area by a hydrothermal method (160 C for 16 h) by using titanium(IV) isopropoxide as the precursor. Then, the samples were subjected to calcination at 500 C for 2 h. The catalysts showed much higher photocatalytic activity than Degussa P25 for methylene blue degradation under UV irradiation. The adsorption
desorption isotherm and pore volume
pore diameter graphic of Degussa P25 and mesoporous TiO2 beads, determined by multipoint N2 adsorption technique, are shown in Fig. 4.32. The TiO2 beads show type IV isotherms with a sharp capillary condensation step at high relative pressures (p/p0  0.7
0.9) and H1 type hysteresis loops. This result indicates that the mesoporous TiO2 bead catalyst shows uniform pore size distribution and the sample has relatively large pore sizes. The specific surface areas of mesoporous TiO2 beads and Degussa P25 are c. 107 and 52 m2/g, respectively. The mesoporous TiO2 beads have a narrow pore size distribution from c. 0
10 nm centered at c. 5 nm, while Degussa P25 did not show porous character. Fang et al. [205] reported the preparation of TiO2 nanotubes pillared graphenebased macrostructures (TPGBM) for bisphenol A removal by both adsorption and photocatalysis under simulated solar source. TPGBM showed high bisphenol A adsorption capacity because its specific surface area, the accessibility to π
π adsorption sites, and the number of hydrogen bonds were higher with respect to the

Figure 4.32 Nitrogen adsorption
desorption isotherms of the mesoporous spherical TiO2 and Degussa P25 TiO2. Source: Reprinted from X. Li, M. Zou and Y. Wang, Soft-template synthesis of mesoporous anatase TiO2 nanospheres and its enhanced photoactivity, Molecules 22 (2017) 1943 with permission, ©2017 MDPI Publishing.

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Titanium Dioxide (TiO2) and Its Applications

Figure 4.33 Nitrogen adsorption
desorption isotherm of TPGBM photocatalyst. TPGBM, TiO2 nanotubes pillared graphene-based macrostructures. Source: Reprinted from Z. Fang, Y. Hu, J. Cheng, Y. Chen, Continuous removal of trace bisphenol A from water by high efficacy TiO2 nanotube pillared graphene-based macrostructures in a photocatalytically fluidized bed, Chem. Eng. J. 372 (2019) 581
589 with permission, ©2019 Elsevier Publishing.

graphene-based macrostructures. Fig. 4.33 shows N2 adsorption
desorption isotherm of TPGBM used for the determination of its specific surface area and pore size distribution. The BET-specific surface area of graphene-based macrostructures increased significantly in the presence of TiO2 nanotubes because they acted as pillars on graphene sheets by preventing agglomeration phenomena. BET-specific surface area of TPGBM, graphene-based macrostructures, and TiO2 nanotubes are 302.4, 204.6, and 125.8 m2/g, respectively. The pore size of the TPGBM is approximately distributed around 12 nm, which could be ascribed to the TiO2 nanotubes diameter in the TPGBM. The pore size range of 50
100 nm, instead, may correspond to the macroporous network of GBM. Generally, higher surface area of TiO2 nanoparticles implies higher activity, although how it has been highlighted at the beginning of this section, this parameter cannot be straighforwardly related to the photoactivity without considering many others. For instance, a calcination treatment induces a decrease of the surface area by sintering effect on the nanoparticles, but it increases the TiO2 crystallinity that is also important for the activity [151,187]. The photoactivity both per gram of photocatalyst and per square meter could increase in that case. Nevertheless, a very high calcination temperature (i.e., 800 C) could cause a decrease of the photoactivity due to elimination of surface hydroxyl groups, to a too drastic lowering of the surface area, and to the occurrence of a phase transition reaction.

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4.3.5 Diffuse reflectance spectroscopy The optical properties of TiO2-based materials can be studied by UV
vis spectroscopy [163,206]. For instance, the bandgap energy value of TiO2-based materials is one of the most important intrinsic electronic parameters of a semiconductor, and it is generally determined by means of diffuse reflectance measurements. Absorption, transmission, and reflection phenomena can take place after light irradiation, and they depend on the surface physicochemical and structural properties of the material and on the used wavelength [207]. The transmittance (T) is defined as the ratio between the light measured after interaction with the sample (I) and the incident intensity (I0) (I/I0), while the absorbance (A) as the natural logarithm of 1/T [208] (Eq. 4.17): A 5 kλ l 5 2 ln

I 1 5 ln 5 2 lnT I0 T

(4.17)

where kλ is the attenuation coefficient, a typical constant of the medium crossed by light, dependent on the wavelength λ, and l is the length of the path of light through the sample. Bandgap energy of TiO2-based materials can be determined by DRS technique. The optical band gap (Eg) is the minimum energy needed to promote an electron by irradiation from the top of the valence band (VB) level to the bottom of the conduction band (CB) edge [209]. This value is very important for TiO2-based materials used in many different areas such as photocatalysis, photoelectrocatalysis, and solar energy conversion [161]. The interpretation of UV
vis diffuse reflectance spectra is often difficult because the bands can be due to the d
d transitions, phonon absorption and emission, and excitation to or from color centers and/or defects and to exciton binding energies. The electronic transitions in TiO2-based materials after UV
vis-light irradiation can occur directly or indirectly [163,210]. A direct and an indirect transition can be described, respectively, as (1) the interaction between an electron and a photon when only photons excite the electrons and (2) as a transition due to the interaction of photon, electron, and phonon which require, at the same time, vibrations and energy transition from the crystal lattice (phonons). A phonon is defined as a unit of vibrational energy that rises from the oscillation of atoms within a crystalline lattice. The lattice vibration, due to the atoms thermal energy, generates mechanical waves. A packet of these waves can move inside the crystal with a fixed energy and momentum: they can be considered as particles and named phonons. Just as a photon is a quantum of electromagnetic energy or light, a phonon is a quantum of vibrational mechanical energy. From the shape of the diffuse reflectance spectrum, it is possible to distinguish the different transition types by mathematical elaborations based on the Tauc equation [211]:  n αhν 5 A hν2Eg

(4.18)

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Titanium Dioxide (TiO2) and Its Applications

where α, h, A, and v are the absorption coefficient, the Planck constant, the absorption constant, and the light frequency, respectively. The exponent n is related to the type of electronic transitions. In particular, n 5 1/2 or 3/2 for a direct allowed and forbidden transition, respectively, while n 5 2 or 3 for an indirect allowed and forbidden transition, respectively [212]. The intensity of the DRS can be expressed by the following Kubelka
Munk equation, considering an infinitely thick sample [213,214]: F ðRN Þ 5

ð12RN Þ2 α 5 S 2RN

(4.19)

The reflectance of an “infinitely” thick sample (RN) represents the ratio of the intensity of light reflected from a sample to the intensity of reflection from a standard sample, and S is the scattering coefficient. BaSO4 is the most used nonabsorbing blank sample. By introducing the coefficients α and S, both the absorption and scattering phenomena are considered in Eq. (4.19). If the absorption coefficient α is not dependent on the wavelength, F(RN) is proportional to it, and α can be substituted in Eq. (4.18) [215] giving rise to the following equation:   ðαhν Þ1=n 5 A hν 2 Eg 5 ½F ðRN Þhν 1=n

(4.20)

By plotting [F(RN) hν]1/n versus hν, it is possible to calculate the bandgap energy of a material by drawing a tangent line to the point of inflection of the curve: the hν value at the point of intersection of the tangent line with the horizontal axis is the Eg value [216] (see Fig. 4.34). The unit for hν is eV, and its relationship to the wavelength λ (nm) is hν 5 1239.7/λ. A thickness of TiO2-based samples in the range 1
3 mm is enough to perform a DRS analysis.

3

[F(R'∞ )hν]½

TiO2 Aeroxide P25 2

1

0 2.8

2.9

3

3.1

3.2

3.3

3.4

3.5

3.6

3.7

hv (eV) Figure 4.34 Bandgap energy determination from the Tauc plot obtained by DRS analysis for Aeroxide P25 TiO2. DRS, Diffuse reflectance spectroscopy.

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Figure 4.35 DRS analysis of pure TiO2 and 3, 10, and 15 wt.% GO/TiO2 composites calcined at 400 C. DRS, Diffuse reflectance spectroscopy. Source: Reprinted from M.S. Adly, S.M. El-Dafrawy, S.A. El-Hakam, Application of nanostructured graphene oxide/titanium dioxide composites for photocatalytic degradation of rhodamine b and acid green 25 dyes, J. Mater. Res. Technol. 8 (2019) 5610
5622 with permission, ©2019 Elsevier Publishing.

The determination of bandgap value of Aeroxide P25 TiO2 (c. 80% anatase, 20% rutile) by DRS analysis is shown in Fig. 4.34 [163]. Since TiO2 is an indirect semiconductor, n 5 2. Eg value of this sample was found to be 3.18 eV. In the absorption spectrum the bandgap corresponds to the point at which absorption begins to increase from the baseline, since this defines the minimum amount of energy that must be supplied to a photon to excite an electron across the bandgap. Adly and coworkers [217] prepared graphene oxide (GO)/TiO2 composites (3
15 wt.% with respect to TiO2) by hydrothermal method followed by a calcination treatment. DR spectra are shown in Fig. 4.35, and Eg values were calculated from Kubelka
Munk transformation. The bandgap of pure TiO2 is 3.16 eV, and the Eg values of the composites (calcined at 400 C) decrease by increasing the GO content; for 3, 10, and 15 wt.% GO/TiO2, the values are 3.06, 2.98, and 2.3 eV, respectively, due to the incorporation of graphene oxide into titania lattice. An interesting application of DRS was reported by Tobaldi and coworkers [218]. They prepared copper-loaded TiO2 samples (1
10 wt.% with respect to TiO2), showing tunable photochromic behavior under UVA or visible irradiation, by sol
gel method followed by calcination at 450 C (1
10 wt.% Cu
Ti450). Especially under UVA irradiation, Cu21 nanoparticles were reduced to Cu1, and then to metallic copper (Cu0). The induced photochromic behavior can be tuned by varying the irradiation time of the UVA or vis irradiation. This effect was investigated by DRS analysis

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Titanium Dioxide (TiO2) and Its Applications

Figure 4.36 DRS spectra of prepared samples, without exposed to irradiation. DRS, Diffuse reflectance spectroscopy. ˇ Source: Reprinted from D.M. Tobaldi, N. Rozman, M. Leoni, M.P. Seabra, A.S. Skapin, R.C. Pullar, J.A. Labrincha, Cu
TiO2 hybrid nanoparticles exhibiting tunable photochromic behavior, J. Phys. Chem. C 119 (2015) 23658
23668 with permission, ©2015 American Chemical Society Publishing.

immediately after irradiation of the photocatalysts for a determined time. Fig. 4.36 shows DRS spectra of pristine TiO2 (Ti450) and Cu-loaded TiO2 samples without UVA interaction. All of the samples showed the absorption band below 400 nm due to the Ti41
O22 metal
ligand charge transfer [219]. Moreover, an absorption was observed at c. 450 nm for the Cu
TiO2 samples. It was attributed to the electron transfer from the VB of TiO2 to that of CuxO clusters (intervalence charge transfer) [220] that exist around TiO2. The absorption tail extended by increasing Cu content, and this finding can be explained by considering the occurrence of interband absorptions in Cu2O [221]. The samples showed, in addition, another absorption band centered at c. 800 nm that was attributed to a d
d electronic transition due to the presence of Cu21 species [219,222]. The band intensity increased by increasing the Cu21 percentage. Fig. 4.37 reports the photochromism behavior of 1 wt.%Cu
Ti450 sample irradiated with UVA up to 1 min [218]. Under UVA irradiation, VB electrons of titania move to CB, and then they can migrate to the VB of the CuxO clusters, thus enhancing the absorption tail centered at B450 nm. They are also responsible for the reduction of Cu21 ions, and the appearance or enhancement (when already present) of the absorption tail in the 500
600 nm range (due to Cu2O). As a consequence of the Cu21 reduction, the absorption band due to Cu21 d
d electronic transition gradually decreases (Fig. 4.37, and its inset).

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139

Figure 4.37 DRS spectra of sample 1 wt.%Cu
Ti450 irradiated with UVA light up to 1 min. The inset shows the decrease of the absorption band due to Cu21 d
d transition at different times in the range 375
850 nm. DRS, Diffuse reflectance spectroscopy. ˇ Source: Reprinted from D.M. Tobaldi, N. Rozman, M. Leoni, M.P. Seabra, A.S. Skapin, R.C. Pullar, J.A. Labrincha, Cu
TiO2 hybrid nanoparticles exhibiting tunable photochromic behavior, J. Phys. Chem. C 119 (2015) 23658
23668 with permission, ©2015 American Chemical Society Publishing.

The absorption band related to Cu21 d
d electronic transition was quantitatively modeled by a Gaussian function, and the FWHM, the peak area, and its centroid (energy) was calculated [218]. The absorption bands at 500
600 nm (due to the reduced Cu2O) and at c. 450 nm (due to Cu21/TiO2 intervalence charge transfer) were also monitored. A spectrum was recorded after a certain irradiation time that depended on the sample under investigation. It was chosen as the baseline and subtracted from all the others (Fig. 4.38). Cu21 d
d transition in 1 wt.%Cu
Ti450 sample decreased c. 50% and 95% after 12 s and 10 min UVA irradiation, respectively. The results indicated that this effect can give precise information on the influence of exposure times on irradiation, and that even short exposure times can have a significant effect on tuning these values. Duarte and coworkers [223] prepared N-doped TiO2 films starting from metallic Ti layers using the reactive magnetron sputtering technique at different O2 and N2 flow rates. Fig. 4.39 shows the bandgap values of some films prepared at different oxygen flow rates, that is, 0.2, 0.6, and 3.5 sccm (standard cubic centimeters per minute). At the former one the film showed a double electronic transition related to the excitation states of N2p (2.2 eV) and Ti31 (0.6 eV), respectively. At the medium oxygen flow

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Titanium Dioxide (TiO2) and Its Applications

Figure 4.38 Bands due to the reduced Cu2O, and to Cu21/TiO2 intervalence charge transfer under UVA irradiation for the 1 wt.%Cu
Ti450 sample. The spectrum recorded after 60 min was used as the baseline. ˇ Source: Reprinted from D.M. Tobaldi, N. Rozman, M. Leoni, M.P. Seabra, A.S. Skapin, R.C. Pullar, J.A. Labrincha, Cu
TiO2 hybrid nanoparticles exhibiting tunable photochromic behavior, J. Phys. Chem. C 119 (2015) 23658
23668 with permission, ©2015 American Chemical Society Publishing.

Figure 4.39 Bandgap values of the prepared films deposited with 10 sccm of nitrogen flow rate along with different oxygen flow rates (0.2, 0.6, and 3.5 sccm). Source: Reprinted from D.A. Duarte, M. Massi, Incorporation of N in the TiO2 lattice versus oxidation of TiN: influence of the deposition method on the energy gap of N-doped TiO2 deposited by reactive magnetron sputtering, Mater. Res. 20 (2017) 549
554 with permission, ©2017 SciELO Publishing.

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rate (0.6 sccm), the electronic structure showed a single transition at 2.5 eV that is related to N2p states. At high oxygen flow rate (3.5 sccm), the energy gap increased to the value of the undoped anatase TiO2 (3.2 eV). The Fermi level of a powdered semiconductor could be determined by the Roy method [224]. Fermi level is very close to CB edge (below 0.1 V), and it is considered equal to the CB edge. By using the Eg and the CB edge values, also the VB edge can be easily determined. A detailed description of the Roy method is out of the aim of this section because it is an electrochemical characterization method. However, a mention on the subject is useful because it has been often used in the literature for determining parameters measured also by DRS [16,225,226].

4.3.6 Photoluminescence spectroscopy Luminescence is a phenomenon that implicates absorption and subsequent emission of light [163], and it includes fluorescence, phosphorescence, and photoluminescence (PL). A light emission at different energy (or wavelength) can occur if a suitable radiation excites a material. During this process a decay time depends on the characteristics of the sample. The emitted light is dispersed by a spectrograph and information on electronic properties, and the structure of the material can be obtained from the resulting spectrum [227]. Fluorescence phenomenon that is illustrated in Fig. 4.40 occurs when a chemical species absorbs a photon and is excited to a singlet electronic state, relaxes via mechanisms that are nonradiative, emits a lower energy photon, and then comes back to the ground

Figure 4.40 Scheme of electronic transitions in a chemical species after photons absorption. ¨ zcan, M. Bellardita, G. Palmisano, Source: Reprinted from S. Yurdakal, C. Garlisi, L. O (Photo)catalyst characterization techniques: adsorption isotherms and BET, SEM, FTIR, UV
vis, photoluminescence, and electrochemical characterizations, in: G. Marcı`, L. Palmisano (Eds.), Heterogeneous Photocatalysis: Relationships With Heterogeneous Catalysis and Perspectives, 2019, pp. 87
152 with permission, ©2019 Elsevier Publishing.

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Titanium Dioxide (TiO2) and Its Applications

electronic state. The emitted spectrum can be used to determine the chemical structures of molecules or species under investigation. The excited species (i.e., atoms or ions) could first undergo a nonradiative or optical transition to some intermediate level(s), before emitting fluorescence light, and a cascade of emission processes could also take place. These generally consist of a cascade of transitions to lower lying energy levels in which more than one fluorescence photon is emitted per each absorbed photon. Phosphorescence is similar to fluorescence, although the time for “absorption and emission” is much shorter in fluorescence (in nanosecond time scale) because just in phosphorescence emissions a change in the spin state of the electrons occurs. PL term is used to describe the “absorption emission” of light between different electronic energy levels in the material. Excitation by photon causes the semiconductor to turn into a higher electronic state, with subsequent decay of energy, relaxing to a lower energy level because a part of the excitation energy is converted into heat in the medium. The PL is due to the emission of light during the process. By the PL spectroscopy, it is possible to determine the energy of the bandgap and the composition of multiple layers, ascertain the presence of impurity levels, and establish recombination mechanisms [228
230]. PL consists of radiation emitted by solids (amorphous or crystalline) with enough energy, equal or higher to the bandgap energy, and, in particular, it is due to recombination processes of photoexcited electron/hole (e2/h1) pairs. It is worth noting that the lower the signal intensity, the lower the recombination rate of the photoproduced electron
hole pairs, and one could expect higher photoactivity of the material under examination [230
232]. However, the stronger the signal, the greater the number of defects and/or oxygen vacancies in the catalyst, and also in this case the higher the photocatalytic activity could be [233,234]. Consequently, it is not always possible to relate the photocatalytic activity directly to the PL spectra. Chen and coworkers [235] prepared TiO2 loaded with noble metals (Au, Pt, Pd, or Ag) by photoreduction method. In order to study the recombination behavior of the samples, PL spectra of the pristine and noble metal loaded TiO2 photocatalysts were recorded (Fig. 4.41). The intensity of them is different, although they appear similarly shaped. The emission peaks at 3.10 eV (c. 400 nm) were attributed to direct transitions (X1b!X1a: 3.45 eV and X1b!X2b: 3.59 eV) and phonon-assisted indirect transitions (X1b!Γ3, 3.19 eV; Γ1b!X2b, 3.05 eV; and Γ1b!X1a, 2.91 eV) of anatase TiO2 [236,237]. Moreover, oxygen vacancies and defects were invoked to explain the weaker signals at 2.80, 2.70, 2.56, and 2.34 eV [238]. The noble metals caused a decrease of the signal with respect to the bare TiO2. Their presence reduced the recombination of the electron
hole, increasing the number of photoproduced pairs on the TiO2 surface, where the photoreaction occurred [239]. Jia et al. [240] prepared carbon-modified and nitrogen-doped TiO2 (N-TiO2/C) catalysts by sol
gel method by using TiCl4 precursor, aqueous ammonia, and urea. The PL spectra of the pristine, N-doped, and/or C-modified TiO2 catalysts are reported in Fig. 4.42. The peak intensity of N-TiO2 is lower than that of pristine TiO2, and that of N-TiO2/C is much lower than both of them. These results suggest that the recombination of the photogenerated e2
h1 pairs was significantly inhibited in the N-TiO2/C sample, and this contributed to increase the photocatalytic activity.

Synthesis and characterization of titanium dioxide and titanium dioxide
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Figure 4.41 Photoluminescence spectra of TiO2 and Au-, Pt-, Pd-, Ag-loaded TiO2 (2 wt.%). Wavelengths range: 270
800 nm. Excitation wavelength of light: 250 nm. Source: Reprinted from Y. Chen, Y. Wang, W. Li, Q. Yang, Q. Hou, L. Wei, L. Liu, F. Huang, M. Ju, Enhancement of photocatalytic performance with the use of noble-metaldecorated TiO2 nanocrystals as highly active catalysts for aerobic oxidation under visiblelight irradiation, Appl. Catal. B, 210 (2017) 352
367 with permission, ©2017 Elsevier Publishing.

Figure 4.42 Photoluminescence spectra of TiO2, N-TiO2, and N-TiO2/C catalysts. Excited wavelength of light: 300 nm. Source: Reprinted from T. Jia, F. Fua, D. Yu, J. Cao, G. Sun, Facile synthesis and characterization of N-doped TiO2/C nanocomposites with enhanced visible-light photocatalytic performance, Appl. Surf. Sci. 430 (2018) 438
447 with permission, ©2018 Elsevier Publishing.

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Titanium Dioxide (TiO2) and Its Applications

Figure 4.43 Photoluminescence spectra of pristine TiO2 nanotubes and CdS nanoparticles for different CdS deposition times (5, 10, and 15 min) on TiO2 nanotubes. Excitation wavelength: 250 nm. Source: Reprinted from T. Hoseinzadeh, S. Solaymani, S. Kulesza, A. Achour, Z. Ghorannevis, S. ¸ T˘ ¸ alu, M. Bramowicz, M. Ghoranneviss, S. Rezaee, A. Boochani, N. Mozaffari, Microstructure, fractal geometry and dye-sensitized solar cells performance of CdS/TiO2 nanostructures, J. Electroanal. Chem. 830
831 (2018) 80
87 with permission, ©2018 Elsevier Publishing.

Hoseinzadeh and coworkers [241] prepared CdS decorated by radio-frequency magnetron sputtering on the top of open TiO2 nanotubes arrays. Fig. 4.43 shows PL spectra of the prepared catalysts. All samples show the emission peaks at c. 450 and 550 nm. The intensities of the PL peak decreased significantly by decorating TiO2 with CdS, and this reduction was related to the CdS thickness that in turn depended on the sputtering time. In particular, the results indicated that decorating the TiO2 nanotube structured with CdS nanoparticles with the frequency magnetron sputtering method for 15 min effectively reduced the recombination of the e2/h1 pairs, and, consequently, their lifetime increased significantly.

4.3.7 X-ray photoelectron spectroscopy XPS is a surface technique used to qualitatively and quantitatively analyze the composition and the electronic state of the elements making up various types of materials [149,242,243]. Films and powders of TiO2-based materials are studied, and almost all the elements present can be determined with the exception of hydrogen and helium. A pioneering contribution to the development of the XPS was made in the 1960s by Kai Siegbahn, who in 1981 received the Nobel Prize for Physics. XPS spectra are obtained by irradiating a sample with X-ray beams while simultaneously

Synthesis and characterization of titanium dioxide and titanium dioxide
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145

measuring the kinetic energy and number of emitted electrons from the top up to c. 10 nm of the sample under high vacuum. The energy of the X-ray beam used (i.e., Ex-ray 5 1486.7 eV for Al Kα) is known and the kinetic energies (Ekinetic) of the emitted electrons are measured by a detector. Consequently, the binding energy (Ebinding) of the emitted electrons is specific for each element and its orbital can be determined, according to an equation by E. Rutherford: Ebinding 5 Ex-ray 2 ðEkinetic 1 ΦÞ

(4.21)

where Φ is the work function of the spectrometer (a few eV) that is a constant depending on the used instrument. Only a few examples are reported next to illustrate how the technique can be used. Yurdakal et al. [244] prepared a poorly crystalline TiO2 rutile (HPRT) by a sol
gel method, and a fraction of it was loaded with metallic Pt (Pt-HPRT). HPrutile was also calcined at 400 C for 3 h to obtain rutile in a more crystalline form (HPRT-400). Commercial TiO2 (Degussa P25) was also loaded with Pt (Pt-P25) for the sake of comparison. Fig. 4.44 shows the XPS results of the main photoelectronic peaks of Pt-4f5/2 and Pt-4f7/2 as binding energies of the TiO2 catalysts loaded

P25

75.4 72.2 Pt-P25

70.5

73.7

Pt-HPRT-400 Pt-HPRT

66 67 68 69 70 71 72 73 74 75 76 77 78 79 80

Binding energy (eV) Figure 4.44 XPS spectra (Pt-4f) of bare and Pt-loaded TiO2 catalysts. XPS, X-ray photoelectron spectroscopy. Source: Reprinted from S. Yurdakal, S.O ¸ ¨ . Yanar, S. C ¸ etinkaya, O. Alago¨z, P. Yalc¸ın, L. ¨ zcan, Green photocatalytic synthesis of vitamin B3 by Pt loaded TiO2 photocatalysts, Appl. O Catal. B 202 (2017) 500
508 with permission, ©2017 Elsevier Publishing.

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with Pt. The results obtained show that Pt in the Pt-HPRT-400 sample exists mainly in metallic form (Pt0), while in Pt-P25 in cationic form (Pt21). In fact, the peaks of Pt-4f7/2 and Pt-4f5/2 at c. 70.5 and 73.7 eV, respectively, have been attributed to Pt0 and those at c. 72.2 and 75.4 eV to Pt21 [245]. The different result is due to the order in which these samples were calcined. The HPRT-400 sample was platinized after calcination at 400 C, while the Degussa P25 before calcination. Therefore, during the calcination of Pt-P25, the metallic platinum nanoparticles were oxidized to PtO. The peak values of the Pt-HPRT sample are not easily assignable, probably because the platinum is scattered on the high surface of the mainly amorphous support without forming aggregates of sufficient size to be analyzed by XPS. Kumar et al. [246] prepared Ag2O/rutile TiO2/polypyrrole composite, and the full scan XPS analysis along with that of rutile TiO2, used as comparison, is showed in Fig. 4.45A. Ti and O were detected for rutile TiO2, while Ag, N, and C in addition to Ti and O for the Ag2O/rutile TiO2/polypyrrole composite. The highresolution XPS spectra of the elements of the composite (Ti 2p, Ag 3d, N 1s, O 1s,

Figure 4.45 Full scan XPS analysis of TiO2 and Ag2O/rutile TiO2/polypyrrole composite (A) and high-resolution XPS spectra of Ti 2p, Ag 3d, N 1s, O 1s, and C1 in Ag2O/TiO2/ polypyrrole composite (B
F). XPS, X-ray photoelectron spectroscopy. Source: Reprinted from R. Kumar, Mixed phase lamellar titania-titanate anchored with Ag2O and polypyrrole for enhanced adsorption and photocatalytic activity, J. Colloid Interface Sci. 477 (2016) 83
93 with permission, ©2016 Elsevier Publishing.

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147

and C1s) are reported in Fig. 4.45B
F. The peak values of the Ti 2p3/2 and Ti 2p1/2 states at 458.8 and 464.5 eV, respectively, indicate the presence of Ti(IV) ions in the composite (Fig. 4.45B). The peak values of Ag 3d5/2 and Ag 3d3/2 states at 367.7 and 373.7 eV, respectively, indicate the presence of Ag(I) (as Ag2O) (Fig. 4.45C) [247]. In addition, O 1s peaks located at 530.3, 531.9, and 533.4 eV can be attributed to the Ti-O and adsorbed water (Fig. 4.45E) [248]. A strong peak at 399.6 eV corresponds to neutral nitrogen (
NH-group) in the polypyrrole (N 1s), while two low-intensity peaks at 400.6 and 397.9 eV are attributed to C
N 1 (positively charged nitrogen) and C 5 N 2 (deprotonated nitrogen) of polypyrrole (Fig. 4.45D) [249]. The C 1s signals at 284.2, 285.6, 286.2, 287.4, and 288.9 eV indicate the presence of C
C, C
H, C
N 1 , and C 5 N (Fig. 4.45F), confirming that the synthesis of the composite has been carried out successfully [249,250].

4.3.8 Thermal gravimetric analysis TGA is a method in which the mass loss of a material is followed by temperature changes (generally up to c. 1000 C) as a function of time [251,252]. By TGA analysis the amount of adsorbed water or other contaminants coming from the preparation of TiO2 can be determined, together with the quantities of surface hydroxyl groups that are responsible for the adsorption of oxygen during photocatalysis and, in general, for the hydrophilic behavior of TiO2. The differential thermal analysis (DTA) allows also to observe the phase transitions (i.e., anatase to rutile or amorphous to brookite phases). However, temperature-controlled XRD is a much more precise technique for the latter case. In the presence of TiO2 modified samples, other information can be obtained (for instance, the type of products derived from thermal decomposition) by TGA equipped with mass spectrometer or FT-IR as detectors. A TGA instrument contains an extremely precise balance with a sample holder positioned inside an oven that allows temperature control. The temperature can be raised to a constant speed, or different speed values can be chosen for fixed temperature ranges. During the analysis that can be performed under inert atmosphere (N2 or He), air, vacuum, or oxidizing/reducing gases, a constant temperature can be also applied for a certain time. TGA data are reported as a percentage of initial mass with respect to temperature or time. The derivative of the TGA curve (DTG) can be plotted to determine the inflection points useful for in-depth interpretations and DTA, and in this case the temperature difference between a sample and a reference is calculated as a function of temperature. DTA can also be used to study the processes in which heat is absorbed or released in such a way as to determine the temperatures of TiO2 transitions, as reported earlier. Yurdakal et al. [244] prepared a poorly crystalline TiO2 rutile (HPRT) by a sol
gel method, a fraction of which was loaded with metallic Pt (Pt-HPRT). HPRT was also calcined at 400 C for 3 h to obtain rutile in a more crystalline form (HPRT-400). Commercial TiO2 (Degussa P25), was also loaded with Pt (Pt-P25) for comparison aim, and TGA curves of various samples are reported in Fig. 4.46. The heating rate per minute for TGA analysis in an inert atmosphere (N2) was

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Titanium Dioxide (TiO2) and Its Applications

Mass loss (%)

100.0 98.0

(a)

96.0

(b) (c) (d)

94.0 92.0 90.0

(e) 88.0 86.0

(f) –0

50

100

150

200

250

300

350

400

450

500

550

600

650

700

t (°C) Figure 4.46 TGA analysis results of TiO2 samples. (a) Pt-HPRT-400, (b) Pt-P25, (c) Degussa P25, (d) HPRT-400, (e) Pt-HPRT, and (f) HPRT. TGA, thermal gravimetric analysis. Source: Reprinted from S. Yurdakal, S.O ¸ ¨ . Yanar, S. C ¸ etinkaya, O. Alago¨z, P. Yalc¸ın, L. ¨ Ozcan, Green photocatalytic synthesis of vitamin B3 by Pt loaded TiO2 photocatalysts, Appl. Catal. B 202 (2017) 500
508 with permission, ©2017 Elsevier Publishing.

10 C. The obtained curves could be examined in three regions [253
255]. The region between 30 C and 120 C belongs to physically adsorbed water (H2Ophys), while the regions between 120 C and 300 C and between 300 C and 600 C could be related to weakly (OHweak) and strongly (OHstrong) bonded hydroxyl groups, respectively, (OHtotal 5 OHweak 1 OHstrong). The results evaluated from TGA curves are given in Table 4.3. Both the poorly crystalline samples (HPRT and Pt-HPRT) prepared at low temperature have H2Ophys, OHweak, and OHstrong values very higher than those of the corresponding samples prepared at higher temperature and the commercial TiO2 catalysts. The home prepared TiO2 catalysts loaded with Pt have lower H2Ophys, OHweak, and OHstrong values than those of the naked ones, since the presence of Pt reduces the amount of hydroxyls on the catalyst surface. An opposite trend can be observed for Degussa P25. The above results have been explained considering the different forms in which platinum exists in the two types of samples, according to the XPS analyses (Fig. 4.44), that is Pt(0) and (Pt21), for TiO2-based HP samples and Degussa P25, respectively. Carata˜o and coworkers [256] prepared TiO2-filled polyvinylpyrrolidone (PVP) composite nanofibers by an electrospinning technique. DTA curve of heat flow of the sample is reported in Fig. 4.47. According to DTA analysis, the exothermic

Table 4.3 The mass loss (as percentage, w/w) values of the TiO2 samples determined from thermal gravimetric analysis curves. Catalyst

HPRT Pt-HPRT HPRT-400 Pt-HPRT-400 P25 Pt-P25

H2OPhys

OHweak

OHstrong

OHtotal

[30 C
120 C], (%)

[120 C
300 C], (%)

[300 C
600 C], (%)

[120 C
600 C], (%)

4.7 4.4 0.66 0.31 0.97 0.87

5.8 4.0 1.4 0.54 1.0 0.99

2.6 2.4 1.9 0.71 1.2 1.6

8.4 6.4 3.3 1.3 2.2 2.6

¨ zcan, Green photocatalytic synthesis of vitamin B3 by Pt loaded TiO2 photocatalysts, Appl. Source: Reprinted from S. Yurdakal, S.O ¸ ¨ . Yanar, S. C¸etinkaya, O. Alago¨z, P. Yalc¸ın, L. O Catal. B 202 (2017) 500
508 with permission, ©2017 Elsevier Publishing.

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Titanium Dioxide (TiO2) and Its Applications

Figure 4.47 DTA (left) and TGA (right) results of TiO2-filled PVP composite nanofiber. DTA, Differential thermal analysis; TGA, thermal gravimetric analysis. Source: Reprinted from B. Carata˜o, E. Carneiro, P. Sa´, B. Almeida, S. Carvalho, Properties of electrospun TiO2 nanofibers, J. Nanotechnol. 2014 (2014) Article ID 472132 with permission, ©2014 Hindawi Publishing.

peak at c. 427 C corresponds to the transformation of amorphous TiO2 into anatase phase, while the second peak c. at 788 C corresponds to the transformation of anatase into rutile phase. By considering the TGA curves (Fig. 4.47), the first mass loss (9.1%) occurred from room temperature to c. 72 C, and it was mainly due to humidity and to residual amounts of the solvents (acetic acid and ethanol) used in the preparation. The second mass loss (7.6%), from 72 C to 225 C, can be attributed to decomposition of Ti(IV) isopropoxide (TiO2 precursor) and degradation of PVP dehydration through a polymer side chain, while the last mass loss (36%) from 225 C to 424 C to crystallization of the TiO2 anatase phase and degradation of the polymer. Anaya-Esparza and coworkers [257] prepared TiO2
ZnO
MgO mixed oxide by a sol
gel method using magnesium, zinc nitrate di-tert-butoxide, and titanium(IV) butoxide as the precursors. The results of DTA and TGA/DTGA of the pristine TiO2 and TiO2
ZnO
MgO composites are reported in Fig. 4.48. The TiO2

ZnO (x %)
MgO (x%) samples are called T
Zx
Mx where x is the mass percentage of ZnO and MgO with respect to the whole sample. TGA has three main areas for mass loss; the first (about 15%) in the range 30 C
240 C corresponds to the desorption of the physically (30 C
80 C) and chemically (80 C
240 C) adsorbed water, the second between 240 C and 500 C (4%
8% for all materials) is mainly due to the decomposition of the hydroxides used as precursors [258], and the last in the range 500 C
700 C (mass loss is less than 1% for all samples) is attributed to the transformation of the sample into anatase and rutile phases. No more mass loss was observed above 700 C [259]. The endothermic effect in the derivative of thermogravimetric

Synthesis and characterization of titanium dioxide and titanium dioxide
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151

Figure 4.48 TGA/DTGA and DTA results of (A) pristine TiO2, (B) TiO2
ZnO(1%)
MgO (1%), (C) TiO2
ZnO(3%)
MgO(3%), and (D) TiO2
ZnO(5%)
MgO(5%) composites. DTA, Differential thermal analysis; TGA, thermal gravimetric analysis. Source: Reprinted from L.M. Anaya-Esparza, N. Gonza´lez-Silva, E.M. Yahia, O.A. Gonza´lez-Vargas, E. Montalvo-Gonza´lez, A. Pe´rez-Larios, Effect of TiO2-ZnO-MgO mixed oxide on microbial growth and toxicity against artemia salina, Nanomaterials 9 (2019) 992 with permission, ©2019 MDPI Publishing.

analysis/DTGA and DTA curve at 140 C is associated with the desorption of adsorbed water from the sample surface. The peak centered at 250 C and 350 C should be attributed to the transformation of the poorly crystalline phases and to the loss of structural water occluded in the interplanar regions and interstices of the crystal lattice [258]. The exothermic shoulder at c. 400 C in the DTA curve corresponds to the transformation (beginning of crystallization) of the amorphous TiO2
ZnO
MgO sample in the anatase phase, while the exothermic peak at 430 C
460 C could be attributed to the dehydroxylation/dehydration of the precursors [259]. The exothermic peak at c. 480 C
500 C in the DTA curve suggests that the transformation in the anatase phase, which is highlighted by XRD, is completed. At c. 700 C in the DTGA and DTA curves of the bare TiO2 sample, the rutile phase begins to appear (Fig. 4.48A). Subsequently, no significant thermal effects are detected even at 1000 C. In conclusion, the few examples described are useful only to give the nonexpert reader a general idea of the information that the TGA can provide. It is essential that the technique is associated with others in order to have the greatest possible correlations between the properties and behavior of TiO2 in its many fields of application.

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Titanium Dioxide (TiO2) and Its Applications

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Synthetic, natural and bioinspired dyes as TiO2 sensitizers in sustainable solar cells

5

Nunzio Genitori and Gaetano Di Marco Institute for Chemical and Physical Processes (IPCF), National Research Council, Messina, Italy

5.1

Introduction

The especially growing energy demand from modern society has increased the interest to find alternative sources to fossil fuels. Many states in the world, for both economic and environmental reasons, are investing in renewable and sustainable energy supplies. The International Energy Agency (IEA) [1], which coordinates the energy policies of member countries, has estimated that world consumption will double until 2050 from the current 435,000 petajoule (PJ) to over 807,000 PJ and world electricity production will increase from the current 16 thousand terawatt-hour (TWh) to about 46,000 TWh. The radiative energy coming from the sun is free and is very much taken into account by the scientific community also because it can guarantee a constant supply for about another 5 billion years (more than the Earth will continue to be inhabited). The average solar power supplied to the Earth every day is approximately 129,000 TW and the emitted electromagnetic radiation is in the ultraviolet to infrared range (200
3000 nm); for example, covering just 0.16% of the Earth’s surface, with solar energy conversion systems having a yield of 10%, 20 TW of power is obtained, over the global energy demand. In addition to the well-tested photoconversion systems such as silicon technology, thin films, other very interesting and still current alternatives are presented by the dye-sensitized solar cells (DSSCs). These are photoelectrochemical devices that capture light radiation by means of a photoanode, consisting of a thin layer of nanocrystalline semiconductor material, typically titanium oxide in the allotropic form of anatase, on which a dye is chemi-adsorbed to capture light. These cells have lower efficiencies (10%) than silicon cells but have significantly lower production costs and significantly sustainable essential materials.

5.1.1 Photovoltaic technology Radiative energy from the sun is related to nuclear fusion processes responsible for converting hydrogen into helium contained within it. Every second these reactions Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00005-5 © 2021 Elsevier Inc. All rights reserved.

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cause a loss of about 4000 kg of the solar mass which is currently about 2 3 1030 kg. The sun will continue to exist even after consuming the hydrogen until it will contract without emitting radiation to become a white dwarf. The amount of solar energy that reaches the Earth, however, depends on certain factors such as latitude, declination, weather conditions [2]. It is essential to understand the characteristics of solar radiation in order to correctly choose the components of a photovoltaic system. The density of solar radiation or irradiance is measured in kW/m2 while radiation measures its energy in kWh/m2. The solar irradiance is influenced by the amount of terrestrial atmosphere that the rays must pass before reaching the terrestrial surface. This parameter is referred to as air mass (AM) followed by a number [3]. The solar spectrum in the vacuum is indicated AM0, denotes the absence of atmosphere and is the spectrum that is conventionally measured at the distance of 1UA (astronomical unit, average Sun
Earth distance corresponding to 1.5 3 106 km), when the sun is at the Zenith the light passes through the least amount of Earth’s atmosphere and AM 5 1 namely, AM1 (see Fig. 5.1).

Zenith

AM 0

AM 1.0

Atm

osp

her

e

AM 1.5 48.2°

Figure 5.1 The air mass, visual representation. AM0: solar radiation in vacuum, before passing through the Earth’s atmosphere (1.353 kW/m2). AM1: light travels the least amount of atmosphere. AM1.5: global spectrum considered for standard photovoltaic tests (1 kW/m2).

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If now a ray that we indicate is collected at a different latitude, it will pass through an amount of air equal to (Eq. 5.1): AM 5

1 ; cosβ

(5.1)

where β is the angle between the incident rays and the Zenith. The formula for calculating the AM is valid in good approximation for β between 0 and 70 degrees. For higher angles, the curvature of the Earth must also be considered. The evaluation of the AM index has a precise function from a practical point of view. By crossing the atmosphere, in fact, the solar spectrum changes and also decreases the power that we can hope to receive. The irradiation outside the atmosphere at a distance of 1UA is 1367 W/m2. This quantity is called the solar constant. On Earth, the average radiation is about 1 kW/m2. The presence of the atmosphere therefore causes an attenuation of the useful energy. AM 1.5 means an angle, that is, 48.2 degrees displaced with respect to Zenith. AM1.5 is useful for representing the overall annual average taking into account that the Earth
Sun distance varies with the seasons. The specific value of 1.5 was selected in the 1970s for standardization purposes, based on an analysis of solar irradiation data in the United States. Since then, the solar industry has been using AM1.5 for all standardized tests or evaluation of terrestrial solar cells or modules, including those used in concentration systems.

5.1.2 Dye-sensitized solar cells “Third-generation” photovoltaic cells [4] are solar cells potentially capable of overcoming the limits of traditional cells: both “first generation” ones (based on crystalline silicon and used in common monocrystalline and polycrystalline panels) and “second generation” (based on the reduction of the costs of the first-generation cells through the use of “thin-film technologies,” used in current amorphous silicon panels or, more generally, in thin-film photovoltaic modules). Unlike previous generations, however, the third generation includes a variety of different approaches which theoretically have the potential to achieve the desired goal, and which include, among others, polymer cells and organic photovoltaics, hybrid cells, multijunction cells (used in concentration photovoltaics) and tandem, nanocrystalline cells, cells based on quantum dots. Among these, DSSCs have met with considerable success because they can use ecosustainable materials, contain production costs but also present varied colors, transparency, interesting for decorative, architectural applications, etc. A peculiar feature of these devices is the best efficiency in diffused light, therefore usable as indoor devices and used in sensors, home automation, devices to implement the Internet of Things. The DSSCs are sandwich-based consisting of a photoanode and a counter electrode (CE), which are generally used as a support glass made

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Titanium Dioxide (TiO2) and Its Applications

conductive by a transparent conductive oxide (TCO) film of tin oxide doped with fluorine (FTO), or tin oxide doped with indium (ITO), and an electrolyte placed between them to ensure contact. Titania, in anatase form, is the semiconductor that constitutes the anode of the device [5,6], which, subsequently sensitized with a synthetic (metalorganic, organic) or natural dye, will form the photoanode (chemical bond covalent between the orbitals π and the 3d orbitals, respectively, of the dye and Ti), while the CE usually based on Pt0 has the function of catalyzing the electrolyte reduction processes by the electrons coming from the previous electrode. However, the use of carbon materials in this catalytic activity seems to prove to be a valid alternative to the previous ones because they are cheaper, ecosustainable, more robust from a chemical and physical point of view. When the photoanode (TiO2/DHOMO) is exposed to light radiation, the dye (D) is photoexcited (TiO2/D LUMO) and an electron is injected into the conduction band (e2 CB) of the mesoporous nanocrystalline TiO2 film (TiO2/CB), the charges are separated at the interface between the dye and the semiconductor, consequently the oxidation of the dye molecules occurs (TiO2/D1) during the interorbital transition from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) Eqs. (5.2) and (5.3). The electron thus reaches the conductive oxide layer present on the glass support (ITO/FTO) through the semiconductor. Through an electrical connection, and by interposing an external load, an electron flow directed toward the CE is obtained. To allow the flow to be continuous and that the reaction does not stop, the electron is released to the electrolyte, which contains a redox pair. A reduction reaction then takes place through which the previously oxidized dye is reduced [7]. From an energy point of view, the LUMO level must be higher than that of the CB [2 0.5 V vs normal hydrogen electrode (NHE)], with an appropriate driving force that influences the charge intensity and therefore the current produced (Jsc). At the same time, HOMO must be low in such a way that the redox potential of the D/D1 pair is more positive than the redox potential of the electrolyte (generally I32/I2 0.4 V vs NHE) and easily reduce D1. In general, the Voc or open-circuit voltage of the cell is provided by the difference between the redox potential of the electrolyte and CB (see Fig. 5.2): TiO2 =DHOMO 1 hυ ! TiO2 =DLUMO

(5.2)

TiO2 =DLUMO 1 TiO2 =CB ! TiO2 =D1 1 e2 CB

(5.3)

Pt 1 ½I3 2 ! 3I 2

(5.4)

TiO2 =D1 1 3I 2 ! ½I3 2 1 TiO2 =D

(5.5)

e2 CB 1 TiO2 =D1 HOMO ! TiO2 =D

(5.6)

e2 CB 1 ½I3 2 ! 3I 2 1 TIO2 =CB:

(5.7)

Synthetic, natural and bioinspired dyes as TiO2 sensitizers in sustainable solar cells

173

è è

TiO2 è è

è

è

è

D+ / D*

TiO2+ Dye

ECB ,TiO2

EF ,TiO2 Vmax è

I3E(I3-/I-) è

I-

D0 / D+

Figure 5.2 Energy and electron transfer scheme in a DSSC: the recombination processes are indicated with violet arrows, the electronic transfer process are indicated with red arrows, the double blue arrow represents the maximum voltage (Voc) due to the difference between the quasi-Fermi level of the electrons in the conduction band of TiO2 and the redox energy of electrolytic mediator. DSSC, Dye-sensitized solar cell.

The electron, coming from the photoanode, does an electrical work with external load, and then returns to the cell through the CE. The electrolyte transports the positive charges toward the CE, and the redox couple is reduced on its surface [Eq. (5.4)] and, at the same time, I2 regenerates the oxidized dye making it ready to receive other photons and place other electrons in the photoelectrochemical circuit of the cell. Furthermore, Eqs. (5.6) and (5.7) show electronic recombination reactions known as back electron transfer and dark current processes that reduce cell performance, respectively, in fact the electrons involved do not work outside the DSSC. The thermodynamic and kinetic processes that determine the proper functioning of these devices must be in harmony with each other, for example, the dye should not exhibit radiative decay of the excited state and in any case, the speed of injection of the electrons toward TiO2/CB must be greater than the first, as well as Eq. (5.5) compared to Eq. (5.6) and consequently, Eq. (5.4) must also ensure the rapid regeneration of the redox pair to reduce the oxidized dye and compete with Eq. (5.6). In energy terms, the electronic excitation in the dye (by absorbing light) promotes the system in an excited LUMO state, simultaneously creating an electron deficient in conditions of low energy consumption (HOMO level). The electrons in these two states are separated by an enthalpy difference (ΔH): ΔH 5 ΔE 5 ELUMO
EHOMO ;

(5.8)

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Titanium Dioxide (TiO2) and Its Applications

while in the semiconductor the energy difference is between the conduction (ECB) and valence (EVB) bands ΔE 5 ECB 2 EVB :

(5.9)

The removal of the populations of states from their thermal equilibrium values implies a difference in their chemical potential (μ): Δμ 5 μLUMO
μHOMO

(5.10)

Δμ 5 μCB
μVB ðin the semiconductorÞ:

(5.11)

The contact between FTO, TiO2, and electrolyte in DSSCs plays a crucial role in the transfer of photoelectrons and in the recombination dynamics. The transport of the photoelectron extracted in the TiO2 conduction band (CB) must be effectively transferred to the TCO substrates in order to improve the efficiency of the cell. Therefore methods to suppress electronic recombination during the transfer process should be considered. There are mainly two recombination mechanisms: 1. The electrons recombine with the gaps. 2. Electrons reduce triiodide to iodide ion.

In reality these two chemical reactions occur simultaneously as competing reactions. Since the rate of reduction of dye molecules is two orders of magnitude faster than recombination with photoelectrons, the contribution of the previous mechanism can usually be overlooked. The dominant recombination reaction is represented as follows: ½I3 2 1 2e2 ! 3I 2

(5.12)

Since the electrolyte can penetrate through the mesoporous TiO2 layer up to the FTO substrates, originating dark current phenomena, one approach is to use an under layer placed at the FTO/photoanode interface. The transfer of electrons from a dye to the CB of a semiconductor generates an anode current given by the following equation: ð icb 5 nm F ket ðEÞNCB Dred ðEÞdE

(5.13)

where nm is the number of molecules present on the electrode surface, F is the Faraday constant, ket(E) is the electron transfer constant, NCB is the number of empty states in the CB and finally, Dred(E) is the Gaussian dye distribution of energy levels due to fluctuations in the reorganization energy of the external sphere. A factor of particular relevance in determining the magnitude of the anode current is the electron transfer constant ket. In the case of sensitizers based on coordination compounds, the absorption of a photon usually populates a singlet

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175

state—metal to ligand charge transfer (MLCT)1—from which charge injection into the CB or relaxation can occur in the triplet state (MLCT)3. This last transition is also able to inject the charge to the semiconductor; however, this process is in kinetic competition with other photophysical deactivation processes, that is, radiative and nonradiative deactivation in the ground state [8]. If the curves of the reagents and products cross at a point where ΔG0 5 2λ the transfer of electrons will be determined only by the electronic coupling. In many cases, the transfer of electrons is not active because in the CB there is a continuum of electronic states accessible to the injected electron, which, combined with the different vibrational levels of the other reaction product, that is, the oxidized dye, produces a multitude of possible reaction paths, so at least one channel is like being without an activation barrier and will be the preferred one for the reaction [9]. Some speed constants for the transfer of electrons from TiO2 excited dyes have been determined by time-resolved laser photolysis experiments. These speeds vary on eight orders of magnitude depending on the type of dye used. The fastest injection times are observed for dyes with a suitable anchoring group, such as carboxylate or phosphonate, through which the dye is firmly grafted onto the titania surface. The role of these groups is to provide a good overlap between the excited states of the sensitizer and the levels of the acceptor, or the 3d band of the semiconductor. The carboxylic groups are used to attach the Ru complex to the oxide surface and to establish a good electronic coupling between the ligand orbital π and the TiO2 orbital 3d. Since the electronic transition has an MLCT character, the optical excitation transfers the electron to a site from which electron injection to the semiconductor can take place promptly. Charge injections in the femtosecond time domain have been reported with molecules like these. On the other hand, the recombination between the injected electron and the oxidized dye is slower than at least six orders of magnitude. The main reason for this behavior derives from the fact that this process is generally characterized by a large driving force, often around 1.5 eV, and by a small reorganization energy, of the order of a few electronvolts. This limits recombination in the inverted region of Marcus, reducing its frequency by several orders of magnitude. Furthermore, electronic recapture involves an orbital mainly centered on Ru, whose electronic overlap with the TiO2 CB is small. The spatial contraction of the oxidation wave function of Ru (II) to Ru(III) further weakens the electronic coupling. The presence of an electric field at the coloring/semiconductor interface is also relevant in determining the charge separation underlying the photovoltaic effect. Although no band deflection should occur, due to the small size of the particles, a surface field is spontaneously created by the transfer of protons from the acid functions that anchor the dye, producing a layer of surface dipole. If the semiconductor is brought into contact with a protic solvent, the latter can also act as a proton donor. In aprotic media, such as those commonly used in DSSC, Li1 and Mg21 [10] can be used to positively charge the TiO2 surface. The local potential gradient from the dye to the positively charged semiconductor guides the injected electron in the desired direction. The same field inhibits electrons to some extent from the solid’s exit after injection.

176

5.2

Titanium Dioxide (TiO2) and Its Applications

Semiconductors

5.2.1 Bands formation The electrical properties of solids are influenced by the distribution of their electrons. A useful model for such a description takes into account “the approximation to strong bonds.” If we take into account a one-dimensional solid, consisting of an infinite alignment of atoms, each equipped with an orbital s available to form molecular orbitals, adding N atoms one after the other, an atom makes an orbital available at a certain energy, approaching a second atom, there is overlap, with the formation of binding and antibinding orbitals. The third atom overlaps the adjacent one and from these three atomic orbitals three molecular orbitals originate: one totally binding, one totally antibinding, and one intermediate, nonbinding, between contiguous particles. The fourth atom allows you to make the fourth molecular orbital. In this way, the field of energy covered by molecular orbitals is enlarged and more and more orbitals are introduced. Adding N atoms to the alignment, there will be N molecular orbitals to cover a finite width energy band; from this we can say that when N is infinitely large, the difference between the adjacent energy levels is infinitely small. We can imagine that such a band is made up of N distinct molecular orbitals, with that of minimum energy K 5 1 fully binding and that of maximum energy K 5 N fully antibinding between contiguous atoms. Four-dimensional solids take shape in three-dimensional solids. The band that originates from s-orbitals is called the s-band, if the atoms have p orbitals, a similar procedure leads to the p band. Where p orbitals enjoy higher energies than s, the p band will connect higher than the s, and there may be a band interval, an energy field to which no orbital corresponds.

5.2.2 The occupation of the orbitals Let us now consider the electronic structure of a solid formed by atoms each capable of contributing with an electron. There are N atomic orbitals and therefore N molecular orbitals packed in one apparently continuous band, and on the same merit, there are N electrons to be arranged. The band made up of the binding orbitals is also called the valence band, while the band made up of antibinding orbitals is called the CB. At temperature T 5 0 K there are N/2 molecular orbitals of lower energy and HOMO is called Fermi level. Unlike what happens for molecules, near the Fermi level there are vacant orbitals so that the excitation of the most energetic electrons requires very little energy. This means that a part of the electrons is in fact very mobile and gives rise to electrical conductivity. At temperatures above absolute zero the electrons can be excited by the simple thermal agitation of the atoms. The P population of the orbitals is given by the Fermi
Dirac distribution, a version of the Boltzmann distribution that takes into account the effect of the Pauli exclusion principle [Eq. (5.14)]: P5

1 : ðE 2 μÞkT 1 1

(5.14)

Synthetic, natural and bioinspired dyes as TiO2 sensitizers in sustainable solar cells

177

For higher energies the numerator is negligible and the previous Eq. (5.15) becomes: P 5 e2ð kT Þ : E2μ

(5.15)

This equation declines with the increase in energy. The electrical conductivity of a metallic solid decreases with increasing temperature while exciting a greater electrons number to vacant orbitals. This paradox is clarified where it is considered that the increase in temperature intensifies the thermal agitation of the atoms, making the shocks between the moving electrons and the atoms more likely. In other words, the electrons are diffused, deflected from their path inside the solid and carry the charge with less efficiency. When each of the atoms supplies two electrons, the 2N electrons complete the N orbitals of the S-band, the Fermi level is placed on top of the band (at T 5 0) and the next band is separated by an interval (Eg). As the temperature rises, the tail of the Fermi
Dirac distribution extends through the interval and the electrons populate the orbitals of the upper band. Now they are mobile and the solid is an electrical conductor. In reality, it is a semiconductor because the electrical conductivity depends on the number of electrons promoted beyond the range (number that increases with increasing temperature). If the range is wide, however, at ordinary temperature only a few electrons will be promoted, and the conductivity will remain close to zero, we thus define an insulator. Ultimately, the conventional distribution between insulators and semiconductors relies on the size of the band interval and does not constitute an absolute distinction like that between metals (incomplete bands at T 5 0) and semiconductors (complete bands at T 5 0). Another way to increase the number of charge carriers is to improve the semiconductivity of solids that consists in implanting foreign atoms in the otherwise pure material. If these droplet agents are capable of capturing electrons, we will find them from the full band, leaving gaps that will allow nearby electrons to move. This gives rise to p-type semiconduction, with p indicating that the gaps are positive with respect to the electrons present in the band. Alternatively, the dopant could cause excess electrons and these extra electrons would occupy alternately empty bands, providing n-type semiconductivity, where n denotes the negative charge of the carriers.

5.2.3 Titanium dioxide Semiconductors such as TiO2, ZnO, SnO2, and chalcogenides have been the subject of extensive investigations due to their wide application in energy storage and environmental remediation. They act as sensitizers to facilitate light reduction oxide processes due to their conductive electronic structure. In quantum physics theory, a photon with energy (hυ) that exceeds or corresponds to the prohibited band of a semiconductor can excite an electron to the CB leaving a gap with a positive charge to the valence band. These charges can be transferred to the external circuit to supply electric current or be used to catalyze a certain chemical reaction [11]. Titania

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Titanium Dioxide (TiO2) and Its Applications

is found in three natural allotropic forms: rutile, anatase, and brookite. Among these, the most stable form is rutile, which is in the equilibrium phase for any temperature. Although the shape of the rutile is more stable, anatase is perceived as more active when used in DSSCs, also due to its lower absorption in the UV region of the electromagnetic spectrum (bandgap energy is 3.00 eV for rutile, 3.13 eV for brookite, and 3.21 eV for anatase, respectively). The anatase is metastable and tends to convert to rutile on heating (Fig. 5.3). Hence, the phase constituents are strongly influenced by the synthesis processing method. DSSCs manufactured with rutile and anatase film of the same thickness were subjected to simulated solar lighting AM1.5. The results essentially show the same value as the open-circuit voltage (Voc), while the short-circuit photocurrent (Isc) of the anatase-based cell is 30% higher than that of the rutile-based cell. The difference in the short-circuit current is attributed to the lower amount of absorption of the dye by the rutile film, due to a relatively smaller specific surface area. The electron transfer rate of the rutile is generally slow in nature due to the low number of coordination associated with the packing density of the particles, which is identified by intensity modulation photocurrent spectroscopy. Various semiconductor oxides have an energy band structure similar to that of TiO2. Zinc oxide (ZnO) is a promising alternative to TiO2 because of the similar band structure and relatively high electron mobility (see Fig. 5.4). Tin dioxide (SnO2) is another interesting option that has two main advantages compared to TiO2: high mobility and wide bandwidth range. At 300K ambient temperature, the electron mobility of SnO2 is three orders higher than that of TiO2. The greater bandgap of SnO2 (3.8 eV), compared to TiO2 (3.2 eV), would create fewer gaps in the valence band under ultraviolet illumination, thus reducing the rate of degradation of the dye and improving stability at long time of DSSCs. A more positive band edge position facilitates the injection of electrons from photoexcited dye molecules. However, surprisingly, the performance of solar cells sensitized with

Ti O

(A) Figure 5.3 Two crystalline structures of TiO2: (A) rutile, (B) anatase.

(B)

Synthetic, natural and bioinspired dyes as TiO2 sensitizers in sustainable solar cells

179

E/V NHE -1 -0,5

TiO2

ZnO Zn2SO4

0 0,5

SnO2

1

ΔE= 3.2 eV 1,5

ΔE= 3.2 eV ΔE= 3.7 eV

2

ΔE= 3.8 eV 2,5 3 3,5 4

Figure 5.4 Energy gap of several semiconductors usable in the photoelectrochemical field.

SnO2-based dyes is much lower than that based on TiO2. The lower photovoltaic properties of SnO2 are ascribed to the faster electronic recombination dynamics resulting from a diffusion constant of electrons 100 times higher [12]. In addition, the lower isoelectric point of SnO2 (pH 4
5), compared with TiO2 (pH 6
7), inhibits the adsorption of the dye molecules with the acid carboxylic groups. To overcome the aforementioned problems, the SnO2 anode was coated with TiO2, ZnO, MgO, and Al2O3 (core
shell) significantly improving the efficiency of the photoconversion. The highest efficiencies of DSSCs were obtained by using Ru(II) complexes as the organometallic dye adsorbed on nanocrystalline TiO2, providing a conversion in terms of solar-electric efficiency of 10%
11%. The nanocrystalline morphology of the semiconductor metal oxide is not the only requirement that a photoanode must satisfy to guarantee a good collection of electrons; a key aspect of optimizing electron collection is the relationship between injection rates and recombination electron transfer reactions [13]. In order to obtain a better clarification on the mechanism of transfer of electrons from the photoexcited dye to the CB of the semiconductor, numerous studies have been conducted on the spatial distribution of the electronic density of the polypyridyl compounds of Ru(II). The high-energy conversion efficiency of the widely used N719 sensitizer and other Ru(II) polypyridyl complexes derives from the spatial separation of the donor LUMO orbital, which is closed on the TiO2 surface, and from the HOMO acceptor, resulting in an

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Titanium Dioxide (TiO2) and Its Applications

injection that it is much faster than recombination [14]. In natural dyes, however, this separation of the charge density is not present and the electron photoinjection recombination reaction with the oxidized dye becomes a significant contribution to the loss of efficiency of these devices. In these cases, the use of a compact layer between conductive glass and TiO2 or between TiO2 and the sensitizer is very useful to minimize the effect of the retro-recombination reaction on the overall efficiency of the photoconversion. Blocking layers of TiO2 subnanometer particles were used as a sublayer to block the recombination reaction with the back contact. Its function had previously been studied by Cameron who verified that the introduction of such particles on DSSCs has marginal effects on conversion efficiency. Other evidence of the effect of this compact layer on the recombination reaction has been observed using natural dyes as sensitizers. Gr¨atzel [15] has verified that the introduction of a compact layer induces an increase in overall efficiency from 1% to 1.6% in conditions of simulated solar lighting and more than doubled at lower intensities. Furthermore, the introduction of metal oxides, such as shellAl2O3, between the dye and the TiO2 film in order to block recombination has been studied. Al2O3 is a semiconductor oxide with a bandgap of 9 eV and a CB level of 24.45 V versus satured calomed elecrode. Its function is to induce a delay in the interfacial recombination dynamics, which leads to an improvement in device performance. Numerous studies have also reported that the treatment of nanocrystalline TiO2 with TiCl4 solutions (precursors of under and block layers) leads to a significant improvement in device performance. This treatment typically results in a significant increase in Jsc. The improvement in the performance of the device was attributed to the increase in the thickness of the film, the increase in dye absorption, and the activation of the postsintering FTO. It seems possible that higher Jsc values are due to the use of this as the interfacial recombination processes due to the formation by this treatment of a blocking layer are halved. All these treatments on the photoanode improve the efficiency of the devices, with an effect on the transport of electrons through the TiO2 film, thus preventing significant recombination losses in short-circuit conditions.

5.3

Dyes

In DSSCs the dye is responsible for its optical properties by absorbing a large portion of the light, it must be strongly anchored to the semiconductor, provide an effective injection of electrons in the CB of the semiconductor and have a high stability both from a chemical and physical point of view.

5.3.1 Synthetic dyes A class of compounds widely studied and used as a photoabsorbent element in DSSC cells is represented by the polypyridine complexes of Ru(II). Polypyridine complexes of metal ions d6, such as Ru(II), Os(II), and Re(I) show MLCT bands

Synthetic, natural and bioinspired dyes as TiO2 sensitizers in sustainable solar cells

Metal orbitals

181

Ligand orbitals

Molecular orbitals

p

σM*

s

σM* π*

πL*

d

πM MC

MLCT π

πL σL

σ LMCT

LC

Figure 5.5 Schematic orbital transitions in organometallic complexes.

[16] in the visible region; these transitions relate to the passage of an electron from the ground state πM to an excited state π L, thus originating charge transfer from the metal to the binders (Fig. 5.5). This phenomenon is particularly accentuated in the case of Ru(II), where there are lower energy MLCT bands and separated by ligand-centered or metal-centered charge transfers. The electrons pass from the state π L to the TiO2 CB, since the dye is anchored to the semiconductor by means of the presence of COOH groups [17]. It is therefore possible to change the energy of the MLCT transitions by varying the substituents present on the binders: the use of more electron-acceptor substituents lowers the energy of the state π L, while the opposite effect is obtained with electron-donor groups. The variation of the HOMO
LUMO gap is studied by means of cyclic voltammetry measurements. The electron transfer efficiency between the dye and the semiconductor can therefore be influenced both by energy factors (HOMO
LUMO gap) and by geometric factors determined, for example, by the different spatial arrangement of the
COOH groups responsible for anchoring the photo-absorber to TiO2. Some dyes can be connected directly to the surface, but it is more convenient to let smaller functional groups be responsible for the chemical bond with the surface. Dyes and adsorbates in general can bind to a surface through one or more of these groups called anchoring groups. At the interface, there is a well-defined adsorption geometry that ensures strong adsorption. The anchor assembly in a DSSC system must be stable with strong adhesion to the surface. Furthermore, the anchor should provide strong electronic coupling between a discrete electron donor orbital of the excited dye and the continuous electron-accepting semiconductor levels [18]. The ruthenium polypyridine complexes are based almost exclusively on the anchoring of the carboxyl groups on TiO2, providing a good electronic coupling to the dye
semiconductor interface. This anchor group undergoes desorption in the

182

Titanium Dioxide (TiO2) and Its Applications

presence of water, which is a huge disadvantage for DSSCs performances [19]. However, phosphonic acid (HPO3H2) is another anchoring group that can be used to bind a wide range of molecules to TiO2 surfaces. In fact, it has been shown that it binds more effectively than the former. The different types of adsorption are divided into molecular or dissociative adsorption. There are several bonding possibilities for an adsorbate that binds to a metal oxide surface through bonds between the adsorbed oxygen atoms and the surface metal atoms. The coordination can be mono-, bi-, or tri-dentate, depending on how many oxygen atoms the molecule uses to coordinate the surface metal atom. When there are many metal-oxygen bonds, the adsorption modalities can also be distinguished by the number of metal atoms involved (1M, 2M, etc.) [20]. In the adsorption process the adsorbate can induce surface relaxations due to a complete or partially restored coordination of the surface atoms involved in the adsorption. The type of bond with a metal surface can vary due to structural differences in the surface. Experimentally, the bond may also depend on environmental factors such as solvent or pH. It can also be expected that several binding modalities will be present simultaneously. Large adsorbates, like dye molecules, can bind through different anchoring groups. The binding of these separate anchor groups can limit the number of possible adsorption geometries [21]. Knowledge of bonding mode, geometry, and strength is fundamental for the development of efficient DSSCs devices. The interaction between the dye, the anchoring group, and the surface is of fundamental importance, since it determines the geometric and electronic coupling.

5.3.2 Natural dyes Natural dyes and their organic derivatives are the ideal candidates for DSSCs, aimed at reducing costs and respecting the environment as nontoxic, low-cost, renewable, and abundant. The possibility of obtaining the conversion of solar energy using natural dyes has been widely studied, suggesting a simple and economic approach based on the chemical and physical treatment of these dyes, avoiding any dangerous by-product. Particular attention has been paid to vegetable dyes such as chlorophylls, anthocyanins, betalaines, which can be easily extracted from fruit, leaves, flowers, and algae by means of aqueous solutions. Chlorophylls, anthocyanins, and betalaines are vegetable dyes extracted from flowers and fruit and easily used as dyes for DSSCs. The research activity is mainly aimed at selecting the right plant source to isolate the sensitizer and find the best operating conditions (concentration, pH and dye extraction techniques, size of TiO2 nanoparticles (NPs), thickness of the photoanode film, maceration time, electrolyte composition, etc.). Vegetable dyes are available in large quantities, easily and safely extractable but with a molecular structure already designed by nature; artificial ones, on the other hand, can be modulated in energy and absorption, intervening on the ligands and/or the central metal atom. A new class of bioinspired chromophores, on the other hand, tries to reconcile the natural characteristic with the artificial one, drawing inspiration from computational chemistry.

Synthetic, natural and bioinspired dyes as TiO2 sensitizers in sustainable solar cells

183

5.3.2.1 Anthocyanins Anthocyanins belong to the group of phytochemical flavonoids are responsible for the different colors of plants, including roots, stems, leaves, flowers, and fruits. Cranberry, black and red raspberry, black currant, cherry, Sicilian blood orange, aubergine, and red cabbage are just a few examples of the variety of vegetables that are rich in anthocyanins. Furthermore, in recent years an increasing attention has been paid to the exploitation of anthocyanins as multifunctional systems for the application in optoelectronics, for example, solar cells. Anthocyanins are derived from anthocyanidins in which there is at least a portion of pendant sugar (glyosidic form). The core of the anthocyanidins and anthocyanins is a 2-phenyl-benzopyryl chromophore (e.g., flavylium cation), which consists of an aromatic ring (A) linked to a heterocyclic ring (C) containing an oxygen atom which, in turn, is bound by a C-C bond to a third aromatic ring (B), therefore it can also be described as a C6C3-C6 skeleton [22] (see Fig. 5.6). The extended conjugation p, as well as the presence of positive charge and free -OH groups on the flavylium cation, allows the anthocyanins to absorb light in the visible region leading to a great variety of dyes. The difference between anthocyanins of different types is mainly due to: 1. the number of hydroxyl and/or methoxy groups in the molecule; 2. the nature and number of sugar units associated with the phenolic units; 3. the nature and number of aromatic or aliphatic acids attached to the sugar fraction.

The most common sugar is glucose, but galactose, arabinose, rhamnose, xylose, fructose, and others can also be present in the structure of anthocyanins. Furthermore, the sugar fraction can be acylated by aromatic acids, generally hydrocinnamic acids (coumarinic, caffeic, gallic, synapic, and ferulic acids) and sometimes by aliphatic acids (acetic, oxalic, malonic, malic, and succinic acids), aromatic and/or aliphatic acylation can take place in the same molecule, forming a polyacylated structure [23]. Generally, the acyl parts are bound to the C6 carbon atom in the sugar molecule. The naturally occurring anthocyanins typically have hydroxyl substrates in position 3 (always glycosylated, i.e., a hydroxyl group or other functional group is attached to a carbohydrate of another molecule) and in

Figure 5.6 (A) Chemical structures of 2-phenyl-1-benzopyrilium and anthocyanidins; (B) chemical reactions in Flavylium network.

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position 5 (occasionally glycosylated) of the flavylium cation, while the ring B (or 2-phenyl) has one or more hydroxyl or methoxy substituents. Situations may occur where in natural anthocyanins the 7-hydroxyl group is glycosylated or replaced by a methoxylic group, but they are quite rare. Mono- and diglycosylated anthocyanins in positions 3, 5, and even 4 are also rare compared to anthocyanins having typically hydroxyl substituents in positions 3 and 5. Due to this chemical complexity, there is a great variety of anthocyanins in nature and more of 20 anthocyanidins or aglycones currently known. However, only six anthocyanidins, that is, cyanidin (Cy), delphinidine (Dp), malvidine (Mv), pelargonidine (Pg), peonidine (Pn), and petunidine (Ptn) are common in higher plants. The number of hydroxyl and methoxy groups determines the intensity, the type (i.e., the absorption wavelength), and the color stability of the anthocyanins. Generally, the predominance of hydroxyl groups on the aromatic anthocyanin skeleton gives rise to an intense blue color while, when methoxy groups prevail, a red color is observed. Glycosyl substituents also influence color stability, probably due to a loss of coplanarity of the B ring with respect to the rest of the molecule, which causes a decrease in conjugation and stability. The chromatic stability of monoglycosides and anthocyanidin diglycosides remains higher than that of the corresponding aglycones. Therefore both hydroxylation and glycosylation influence the color properties of the dyes. The chromatic stability of cyanidin (i.e., cyanine without sugar fraction) is higher than that of malvidin but lower than that of malvidin 3glucoside. Furthermore, the color stability of mono- and di-glycosyl-anthocyanidins depends on the nature of the subsystem glycosyl and its anchoring on the anthocyanin skeleton. Studies on anthocyanins containing polysaccharides studied by Broennum-Hansen and Flink [24] reported that anthocyanins containing sambubiose disaccharide (a component of some glycosides) are more stable than monosaccharide anthocyanins containing glucose. A possible explanation could be attributed to the ability of the sugar portions to avoid degradation toward unstable intermediates such as phenolic acid and/or aldehyde compounds. For the use of anthocyanins as dyes in DSSCs, the structural variations mentioned above are fundamental. The choice is not a simple task, because the anthocyanin extracts are strongly influenced by the pH, the processing and storage temperature, concentration, dye aggregates, solvents, intensity and frequency of light, the presence of oxygen, metal ions, and other substances. All the flavylium compounds regardless of whether they are of natural or synthetic origin follow the same network of chemical reactions. This fortunate situation allows the use of the chemical knowledge obtained from the study of synthetic flavyl compounds to facilitate the understanding of natural ones, in particular of anthocyanins. On the other hand, natural compounds have been a source of inspiration for the design of new synthetic derivatives. The flavylium cation (AH1) is stable only at a very acidic pH because the two reactions in which it is involved, deprotonation to give the quinoidal base (A) and the hydration that leads to the hemiketal (B), depend on the pH. These two parallel reactions are in competition and occur on different time scales: the transfer of protons in microseconds and hydration from minutes to subseconds depend on the pH. When the pH changes from very acid solutions (where AH1 is the dominant species) to higher pH values,

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“A” appears as a kinetic product that usually attenuates with time, because it is not the most stable species at equilibrium. The hemiketal is involved in a tautomeric process that leads to the opening of the ring with the formation of a cis-chalcone shape. From the set of Fig. 5.6B it is easy to obtain the pH-dependent molar fraction of the species in the network [25]. The inspection of the absorption spectra is useful for identifying the shape and position of the bands of the flavylium cation at variable pH values. The quinoidal base absorbs around 490 nm while the transchalcone around 360 nm. Another characteristic aspect is that of assessing absorbance as a function of time, representing the trace corresponding to an acidity jump from pH 1.0 to 5.5 at 490 nm. The initial absorbance is due to the conversion of all flavyl cations to a quinoidal base and the equilibrium is equal to the remaining quinoidal base. An important step forward for understanding the chemistry of flavylium was the discovery made by Brouillard and Dubois in which only the hemiketal species was formed from an acid to a neutral environment through the hydration of the flavylium cation and not from the quinoidal base [26]. A strong research effort was made to highlight the effect of pH values with glucidic or acyl substituents and the solvent effect of anthocyanins on the spectral and chromatic characteristics of the dyes. An example of a study was carried out on the chromatic variation of cyanidin-3-glucoside (Cy-3-glc), in a wide pH range and for a long period of time in which it is seen that a spectral band moves along lengths longer wavelengths (bathochromic effect or shift to red) with an increase in pH (i.e., from 1.0 to 8.1) and a decreasing color intensity in the pH 1.0
5.0 range with a maximum intensity at pH 5 1.0. Furthermore, at pH values from 1.0 to 3.1, in which the anthocyanin is mainly found in the cationic form of the flavylium, the dye is stable for about 60 days. Above pH 3.1, color stability decreases with degradation to pH 6.0
7.0, due to the oxidation of Ct. By comparing the optical properties of the acylated and nonacylated glycosylated Pg derivatives, a variation of the molar extinction coefficient Ɛ also occurs, furthermore, depending on the structure of the acylation agent, bathochromic changes were also observed. The structure of sugar substitution is responsible for hypochromic shifts (blueshift). The nature of the solvent is also responsible for the bathochromism or hypochromic displacement in the absorption bands of the anthocyanins and, more interestingly, their ability to disperse the dyes depends on the chemical structure of the dye itself. Studies carried out on the influence of aromatic acyl structures on the color of anthocyanins and stability at pH values between 1.1 and 10.5 for a period of 98 days show that aromatic acyl groups influence color and improve stability anthocyanins in the explored pH range (1.1
10.5). The appearance of an intense blue/red coloring in anthocyanins has been attributed to a complexing reaction of the metal between the anthocyanins themselves and the small metal ions with the formation of very stable compounds. This fact is crucial for the complexation of the titanium ions located on the TiO2 surface. In this context, it has been shown that the chelation of the Ti(IV) ion by the catechol involves the formation of a coordinated ligand η2 (hapticity describes how a group of contiguous atoms of a ligand coordinate and a central atom) to the 3d metal. In this configuration, an improvement in the interface

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electron transfer rates between the catechol and titanium (IV) ions was observed. The first attempts, based on the complexing of the cyanidins with TiO2, were made by extracting the dye from the anthurium flower and in this case, a sudden decay of the photocurrent was found due to its degradation [27]. Cyanidin has been shown to adsorb strongly on TiO2 forming a highly stable complex, carrying charge transfer interaction between the catechol portion and the TiO2 surface. Despite the cyanine injects electrons to TiO2 very efficiently the values of quantum efficiency external (EQE) and photoconversion from DSSCs were quite low (19% and 0.56%, respectively) [28]. The diameter of TiO2 NPs significantly affects the photocurrent, which decreases with the increase in the size of the NPs (from 5 to 18 nm) [29]. A more systematic study was conducted to investigate the photosensitizing properties of over 20 different natural dyes (such as vegetables, bacteria, and insects) in DSSC [30]. Studies on the structural properties of different families of extracts of natural dyes (anthocyanins, chlorophylls, and carotenoids), have shown that anthocyanins, thanks to their hydroxyl and/or carboxyl groups, anchor strongly with TiO2 NPs, thus facilitating the transfer of electrons from the excited state of the dye to the CB of the semiconductor, compared to chlorophylls and carotenoids.

5.3.2.2 Betalains Betalains represent another class of water-soluble vegetable dyes of great interest for applications in DSSCs technology. In general, the coloring of these sensitizers varies between orange and red. For example, the prickly pear contains betanine and indicaxanthin and is orange, while the bonel variety of red beet contains betanin and vulgaxanthin, which show an intense red color. For a long time, betalains were incorrectly classified as flavocyanins (betaxanthine) and nitrogen anthocyanins (beta-cyanines). It was only in 1968 that Mabry and Dreiding attributed the name “betalaina” to both dyes, that is, the beet from which betayls were first extracted. Betalaine is mainly contained in red beets and betanin, which is a 5-O-b-glucoside betanidine containing a phenolic group and a cyclic amino group. Structurally, betalamic are derivatives of the main betalamic chromophore acid (I). In fact, all betaline molecules have a fraction of betalamic acid (I) with substituent R groups extending from the nitrogen atom (II). The nature of these substituent R groups determines whether or not the molecule belongs to the bethacyanins (i.e., betanidine and betanin) or the betaxanthin family. The glycoxidation of one of the substituent patterns in the C1 or C2 position in the benzene ring differentiates the individual beta-cyanine (III). Betaxanthins comprise various amino acid or amino conjugate groups of betalamic acid. A typical example is indicaxanthin, a betaxanthin containing proline (IV), one of the first betaletes identified chemically in the beet root. Thanks to the strong absorption in the visible part of the electromagnetic spectrum and the ability to act as an electron donor, beta positives have been considered good candidates in the “electron transfer of photons” processes. Betalaine have pHdependent redox properties. In addition, they show a dyeing strength (or color strength, i.e., the ability of a dye to keep its color mixed with another dye) up to three times higher than anthocyanins. The betalaine, in general, shows the pH

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stability in the range 3
7 which makes them particularly suitable for use in low acidity environment and in neutral pH conditions. In addition, they are less sensitive to hydrolytic cleavage when compared with anthocyanins. In this context, betalaine can be an alternative to less water-soluble anthocyanins, which under the same pH conditions lose their photochemical properties due to color changes. Despite their pH stability, at least in the pH range 3
7, betalaine is sensitive to temperature changes. In fact, in the combination of deglycosylation at high temperature (50 C
75 C) in acid conditions (pH , 6), as it is necessary to do for the hydrolytic cleavage of the aldmide bond and the decarboxylation of betalaine. A strategy to protect the aldmine bond resulting from the cleavage consists in the acylation of betaline with aromatic or aliphatic acid. Another critical parameter of betalaine, when used in DSSCs, is the redox potential whose values are 1.0 and 21.3 V (compared to NHE) for the ground and excited state, respectively. The latter shows a suitable energy level to favor a good electronic transfer to TiO2/CB. Furthermore, computational studies concerning the time-dependent density fluctuation (DFT and TDFT) have shown that during the HOMO
LUMO transition, the benzene ring loses its electron density while the dihydropyridine (DHP) ring gains the charge, indicating a transfer of electrons π-π . Since the DHP unit is presumed to be attached to the TiO2 surface via carboxyl groups, the excited state must be electronically coupled with the acceptor states of TiO2, which result in an effective charge injection. The strong bond of betalaine-TiO2 results in a faster electronic injection in TiO2/CB than in ZnO/CB due to the different symmetry. In fact, the TiO2 CB mainly includes unfilled d-orbitals, while ZnO has predominantly s-orbitals; therefore the overlap of the orbital π extended to betalaine is more efficient with the 3d orbitals of titanium than with the orbital s of the ZnO.

5.3.2.3 Chlorophylls Chlorophyll is a green pigment responsible for the green color of plants, it has the purpose of absorbing sunlight and triggering photosynthesis, the process by which oxygen is produced. Chlorophyll is indispensable for life to such an extent that plants that do not produce chlorophyll, such as mushrooms, survive only by implanting themselves on the roots of plants with green leaves or by using the humus produced by their decomposition. In chlorophyll the central atom is magnesium. Structurally, chlorophyll refers to a class of cyclic tetrapyrrole, similar to porphyrin and phthalocyanines. Unlike porphyrins, chlorophylls have a reduced pyrrolic ring and a phytol group (20 carbon atoms diterpene acetic alcohol, highly hydrophobic esterified to an acid side chain). Chl-d, and Chl-f, Chl-a (C55H72O5N4Mg, cyan), Chl-b (C55H70O6N4Mg, chartreuse), and Chl-c (C35H28O5N4Mg, blue-green), are actually recognized as the most common chlorophylls. In particular, Chl-a and Chl-b are the two most common chlorophylls in plants. Their concentration depends on the total amount of solar energy received per unit area (MJ/m2) of the plants. The main difference in the structure of Chl-a and Chl-b is based on the composition of a single side chain of tetrapyrrole which is a -CH3 in Chl-a (blue-green) and a -CHO in Chl-b (yellow-green). Chl-a and Chl-b have a highly stable polycyclic network with an

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alternating single and double bond structure (polyenes) which allow the delocalization of the orbitals, thus making the chlorophilic dyes ideal. Chemically, chlorophylls are unstable in both acidic and basic environments and have a strong tendency for aggregation and/or interaction with their molecular environment. Unlike the other derivatives of tetrapyrrole such as porphyrins and phthalocyanines, the production of which requires complicated and expensive ways of synthesis and purification, chlorophylls and their derivatives are economic and environmentally friendly compounds. In addition to natural abundance and low toxicity, and in addition to the ability to absorb solar energy in different regions of the electromagnetic spectrum, dyes of the tetrapirrolo-cyclic type (Chl) have some fundamental advantages compared to various inorganic and organic sensitizers. In fact, the excited states of the life of chlorophylls can be regulated by changing the central metals (Zn, Mg, Ni, Cu, Pd, Fe, etc.) and consequently, the LUMO orbital of the dyes can also be adjusted to adapt to the energy level of the CB of the mesoporous films. The absorption spectra of chlorophylls consist of two main bands, the Soret band located in the blue region, resulting from the electronic transition with the strength of the strongest oscillator (2.88 eV) and the Q band located in the red region, consisting of the Qx and Qy transitions at 1.87 and 2.14 eV, respectively. The positions and intensities of the Q-bands can be adjusted appropriately with the chemical modification of the macrocycle, moreover, the chlorophylls show values of ε up to 105/M/cm, among the highest observed for organic compounds. The first investigations on the photosensitization of TiO2 using chlorophyll derivatives were carried out by Kamat and collaborators [31]. In 1993 Kay and Gr¨aetzel published a comprehensive study on photosensitized colloidal TiO2 electrodes with chlorophylls and related derivatives, reaching EQE up to 30%. These low EQE values can be attributed to the structure of chlorophylls where the presence of alkyl groups instead of carboxylic or hydroxyl groups renders the chlorophylls unable to bind efficiently with the nanoporous TiO2 surface. Chl-a has a worse adsorption and a worse sensitization on TiO2 with respect to the chlorophyll derivatives in which the substituent is the carboxylic group, furthermore, long chains of alkenes give rise to strong steric hindrances which prevent an orderly arrangement of the chlorophyll molecules on the surface of the TiO2 molecule. Because of these effects, only chlorophylls with carboxyl groups could show effective electron injection into the mesoporous layer of TiO2. The position of the carboxylic group on the macrocyclic structure, with respect to the direction of the x- or y-axis, also influences the absorption characteristics and the molecular orbitals of the chlorophylls. Based on these considerations, Chl-c (c1 and c2) with functional groups -COOH appears to be the most favorable dye for DSSC applications.

5.3.2.4 Other vegetable dyes In addition to anthocyanins, betalaine and chlorophylls, other vegetable dyes (such as carotenoids, cochineal, quercetin, etc.) have been explored as sensitizers in DSSC. Also, in this case the presence of acids facilitates the sensitizing activity on the TiO2 surface, while the alkaline compounds favor its desorption with the consequent collapse of the cell performances. The molecular structure of the carotenoids

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determines their coloring, which goes from light yellow to red and can be modified in different ways, mainly through the cyclization of the terminal groups or through the introduction of oxygenated functional groups. The most surprising properties of carotenoids, such as chemical reactivity and light absorption properties, are due to the central part of the molecule, consisting of the alternation of single and double bonds. These bonds form a conjugated system in which the electrons p are delocalized along the entire length of the chain. The light collection capacity of the carotenoids in photosynthesis of higher plants to mediate the transfer of electrons, and their characteristic absorption of light in the range 380
520 nm, makes them potential sensitizers in photovoltaic applications and other artificial photochemical devices. As sensitizers in DSSCs technology, carotenoids seem capable of transferring electrons from excited states from the dye to the CB of the semiconductor. Carotenoids, like dyes in the DSSC, seem to be able to transfer electrons from excited beings from the dye to the CB of the semiconductor. Carotenoids, like dyes in the DSSC, seem to be able to transfer electrons from excited beings from the dye to the CB of the semiconductor. Carotenoids, like dyes in the DSSC, seem to be able to transfer electrons from excited beings from the dye to the CB of the semiconductor. Carotenoids, like dyes in the DSSC, seem to be able to transfer electrons from excited beings from the dye to the CB of the semiconductor. In addition, by studying the efficiency of injecting electrons from the carotenoid dye to the thyolic surface, the influence of carotenoid chain length, as well as the effects of the addition of surfactants (e.g. deoxycolic acid, a salt surfactant) were found to be the best carotenoid acid conversion efficiencies, with a number of double bonds married (n) ranging from 5 to 13 (in fact with n-7 cruades) appear to be the optimal length for DSSC applications. On the basis of these studies, a dependence was also assumed between the values of η and the singlet
triplet annihilation reactions of the dye molecules [32]. The lower photovoltaic performances of the crocin have been attributed both to the absence of carboxyl groups and to the presence of sugar in the portion of the molecular skeleton. On the one hand, the absence of carboxylic groups determines a low charge injection effect, on the other hand, the presence of glucosidic fractions hinders the absorption of the dye, due to the steric hindrance, on the surface of the TiO2 NPs. The dyes based on quercetin, alizarin, and luteolin, when assembled in DSSC, showed EQE values, in the wavelength range 580
600 nm, of 69%, 63%, and 59%, respectively. The best performances with quercetin are associated with a greater collection of light together with an important absorption intensity. In addition, the hydroxyl group on the C3 atom of the C ring of the quercetin molecule affects the distribution of the electron density and the molecular dipole moment improving its performance. The ability of the donor of the hydroxyl group increases the dipole moment of the dye and also increases in energy both HOMO and LUMO, favoring the separation of the charge.

5.3.3 Computational details Computer simulations are a useful means to deepen the mechanisms inherent in the dye
semiconductor interface. They help to make research more ecosustainable,

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indeed they tend to limit expensive experiments and environmental hazards. At a computational level, dyes can be studied starting from a quantum approach using density functional theory (DFT), time-dependent density functional theory methodologies; for dye
semiconductor interactions, on the other hand, there are different approaches to modeling and typical simulation parameters vary much more widely in surface and molecular absorption studies [33]. Due to this variability and limited experimental information on the interface structure, computational results tend to be less unanimous and their accuracy is more difficult to evaluate. The choice of the “exchange correlation” function has a strong effect on the calculation of the electronic structure. The use of generalized gradient approximations (PBE and PW91), as DFT applications, underestimates some structural information related to possible fault states present in TiO2. To overcome this criticality, hybrid functions such as B3LYP and PBE correct these trends and improve the above evaluations. Another approach, to improve the description of the electronic structure of metal oxides, is the DFT 1 U method that corrects the self-interaction error; in fact, conventional DFT cannot accurately describe the exchange energy terms and the Coulombian interactions in strongly localized electrons, such as d and f electrons in metals. A correct prediction of the dye absorption geometry on the semiconductor surface is an essential prerequisite for the theoretical description of the electronic interface structure and charge transfer characteristics. Most of the theoretical studies on dye adsorption have considered adsorption on the surfaces of anatase (101), a metastable form of TiO2. The LUMO positions of the dye and the CB of anatase are of considerable relevance for the physics of DSSC devices, because the energies of these levels influence the electronic injection efficiencies and thus the overall cell efficiency. It has been shown that the position of the LUMO orbitals of the adsorbed dyes in relation to the CB of TiO2 varies depending on the adsorption mode. The first step in the study of dye
surface interactions is to consider the adsorption of dyes through the anchor groups (mainly carboxylic groups, phosphinic and hydroxylic groups). Organic dyes often contain only one anchor group, however, bidentate linkages onto TiO2 substrate seem to be more effective than that of monodentate, for organic dyes with COOH anchor groups. This may be due to higher acidity for the dye molecules, for example, the optimized geometry of the flavylium dye, calculated at DFT level, shows a planarity consistent with the extended π conjugation involving dihydroxybenzene and the substituted benzopyrilium ring. In general, the presence of electron donor groups reduces the energy gap of HOMO
LUMO, which is consistent with the redshift experimentally observed in the absorption spectrum. At the same time (HOMO-1) and (HOMO) become closer in energy [34]. Theoretical studies, although they provide general guidelines, have confirmed a correlation between direction, dipole moment of the dye, and Voc value in the DSSC; it has been shown that if protons are transferred from the dye to the TiO2 surface, the dye causes a downshift of its CB. The computational analysis allowed to optimize the structure of natural dyes, allowing a selective and specific development of bioinspired synthetic dyes.

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5.3.4 Bioinspired The use of computational chemistry is a powerful means capable of optimizing the structures of dyes by functionalizing them in an appropriate way to make them more efficient in the collection of light, insertion of binders capable of enhancing the transitions, and the electronic transfer to the semiconductor (push
pull). DSSCs can profit from the concept of bioinspiration. While the anthocyanins available are more or less reduced to six more common structures, there are many possibilities to synthesize new flavyl derivatives designed to obtain better performance in photoelectrochemical devices. The most efficient synthetic compounds of flavylium are 40 -hydroxyflavyl and 7-diethylamino-30 , 40 -dihydroxyflavyl. In both cases, there are donors in positions 7 and 40 . In the case of 7-diethylamino-30 , 40 -dihydroxyflavyl, the reported efficiency was 2.1%, but a value of 3.5% has recently been obtained [35]. It has been shown that there are great advantages in observing natural flavyl and synthetic compounds as components of the same reality, due to the existence of a common network of chemical reactions. Knowledge of the chemical
physical behavior of flavylium derivatives is indispensable in the study of the properties of these compounds. For example, the flavylium cation is stable only in very acidic soils, which is not the most common situation in biological systems except in the stomach during part of digestion. On the other hand, natural flavyl compounds are an inspiration for the synthesis of new (synthetic) flavyl compounds. As for the latter’s applications, it has not been shown that they can replace natural ones, for example, as food additives or antioxidants, but this is a possible field for future research. What seems to be indisputable is the application of bioinspired flavylium compounds such as interesting photochromic systems or light absorbers in solar cells sensitized with dyes.

5.4

Other functional materials

5.4.1 Characteristics and performance of CEs An optimal CE [36] should possess the following qualities: high catalytic activity, high conductivity, high reflectivity, low cost, high surface, porous nature, optimal thickness, chemical, electrochemical and mechanical stability, good adhesion with TCO. The reference parameters for an ideal CE include 80% optical transparency, series resistance (RS) ,20 Ω sp21, and charge transfer resistance (RCT) about 2
3 Ω cm2. The overvoltage should be as low as possible with photocurrent density up to 20 mA/cm2. However, it is not easy for a material to achieve all these parameters simultaneously in most cases [37]. For example, RCT and RS with these characteristics can be achieved in carbon-based CE, but optical transparency is not ideal. The CE has important effects on the photovoltaic parameters of DSSCs devices. The theoretical maximum phototension of these devices is determined by the difference in energy between the redox potential of the mediator and the Fermi level of the metal oxide semiconductor on the photoanode and can only be obtained at zero current. Under load, however, the output voltage is usually lower than the open-circuit voltage. An

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effective CE should have good conductivity and have a high electrocatalytic activity for the reduction of the redox couple. The reduction reaction [Eq. (5.12)] on the FTO it is extremely slow. In DSSC devices, the transport of charge carriers from the photoanode to the CE must consider various resistances, including: the RS which includes the TCO glass resistance; the TCO/TiO2 contact resistance; the electron transport resistance in the TiO2 film; the charge transfer resistance; the charge recombination between the electrons in the TiO2 and I32 film in the electrolyte (RCT); 6. the charge transfer resistance at the CE/electrolyte interface and the charge transfer resistance at the interface TCO/electrolyte exposed.

1. 2. 3. 4. 5.

In DSSCs, the charge transfer resistance at the CE/electrolyte interface is often dominant between the different charge transfer resistors, and therefore the RCT often refers to this interface. Among these charge transfer resistors, the RS and the charge transfer resistance at the CE/electrolyte interface (RCT) are enslaved to the CE. The RS of a solar cell dominates the fill factor (FF) losses, especially in large commercial solar cells. A smaller RS will give a higher FF, which translates into high conversion efficiency. The catalytic activity of the CE can be explained in terms of current density (J), which is calculated from the charge transfer resistance (RCT) [Eq. (5.16)]: RCT 5

RT nFJ

(5.16)

where R, T, n, and F are the gas constant, the temperature, the number of electrons transferred in the reaction of the elementary electrode (n 5 2), and the Faraday’s constant, respectively. According to the composition of the material, CEs can consist of platinum, other metallic materials, carbon materials, transition metal compounds, conductive polymers, and composite materials. According to the bibliographic research based on ISI Web of Science and WIOP, carbon materials are the most commonly used CE materials in DSSCs, and the percentage of articles published on the total articles on CE is 23% and patents represent 47% of the total.

5.4.2 Characteristics and performance of electrolytes Traditional electrolytes consist of a redox pair and some additives dissolved in an organic solvent. As previously mentioned, the standard redox mediator is the I2/I32 pair, but other valid alternatives have been proposed. In particular, among these the cobalt complexes have allowed us to overcome the efficiencies of the reference redox pair, reaching efficiencies of up to 14.3%. Other redox mediators have also been proposed, such as (SCN)2/(SCN)32, (SeCN)2/(SeCN)32, Br2/Br32, sulfur-based systems, copper complexes, ferrocene derivatives, and stable nitroxide radicals. Furthermore, also the counterions of the redox mediators must be properly selected, since it has been shown that the photogenerated current decreases and the voltage increases with the increase of the radius

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of the cation, following the variation of the floatband (Ec) of the titania and the influence on the efficiency of electron injection that it exercises. Among the components that make up the electrolytic solution, various types of additives are also introduced, some of these have the effect of increasing the photovoltaic parameters. Heterocyclic compounds containing nitrogen and guanidinium thiocyanate are among the most frequently used. While the first increases the voltage of the cell by moving the titania’s Ec toward negative potentials and increasing the lifetime of the electrons, the second increases the photocurrent by moving the potential band toward positive potentials, thus increasing the efficiency of electron injection. Another category of substances added to electrolytic solutions is that of surfactants. As for organic solvents, the most widely used are acetonitrile, valeronitrile, 3-methoxypropionitrile, methoxyacetonitrile, ethylene carbonate, propylene carbonate, γ-butyrolactone, and N-methylpyrrolidone given their high dielectric constant and low viscosity. The worst disadvantage of organic solvents, as mentioned, is their high volatility, therefore to guarantee chemical and thermal stability, ionic liquids at room temperature (RTIL) as additives or solvents have been taken into consideration. However, RTILs often show high viscosity and therefore interventions are needed to improve their rheological characteristics to ensure effective ion transport [38].

5.5

Assembly and characterizations for DSSCs

5.5.1 Development of photoanodes and cathodes A first general cell assembly operation consists of drilling holes on the CE side for the subsequent introduction of the electrolyte during the sealing of the devices. The deposition of the TiO2 and catalyst pastes on glass supports is carried out by means of the screen printer procedure. This technique is capable of depositing films [39] in a more uniform way compared to other manual techniques such as the Doctor Blade. Post deposition of the electrodes, the plate is subjected to heat treatment for sintering [40] of the NPs. Sensitization is carried out by immersion of the mesoporous film in a solution whose theoretical concentration is calculated through the roughness factor or through the desorbing process of the dye starting from a known concentration. After development of CE, photoanodes and the characterizations relating to the individual components, the devices are assembled considering the average thickness of the components and spacing them by a sealant material. The photoanode and the CE are interfaced on a preheated press or through T-Cam. A liquid electrolyte is introduced into the device through a hole previously drilled.

5.5.2 Spectroscopic techniques 5.5.2.1 Raman spectroscopy: TiO2 From Raman spectrophotometry, the peaks in Fig. 5.7. are assigned as Eg, Eg, B1g, A1g 1 B1g, and Eg anatase phase mode at 142, 195, 395, 517, and 639 cm21, respectively [41]. The appearance of an intense peak at 142 cm21 means that there is a

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Titanium Dioxide (TiO2) and Its Applications

Intensity

Eg

Eg

B1g

A 1g+B1g

Eg

Raman shift (cm-1)

Figure 5.7 Active Raman mode in anatase nanoparticles.

certain degree of long-range order possessed by TiO2 nanocrystal. The most intense peak was attributed to the external vibration of the anatase structure. This peak has been attributed to the phononic confinement effect or to the surface pressure that is present in materials having nanoscale dimensions.

5.5.2.2 UV-vis and TiO2 emission spectroscopy In order for a dye to be candidates for DSSC applications, its light absorption should fall at wavelengths different from that of TiO2. In Fig. 5.8, as an example, the variation of the excitation wavelength from 560 to 600 nm of a dye is reported. As the spectra show, by exciting at 560 nm an emission peak is obtained at 700 nm with a shoulder at 745 nm. Exciting at 600 nm the blueshift of the peak is observed at 680 nm while the signal identified as shoulder remains fixed at 745 nm, in reality, the portion of the spectrum interesting for a dye to be candidate for use in DSSC is precisely the peak that does not change its position as the excitement changes. This peak corresponds to the excitation of the dye in the HOMO
LUMO transition in which a good part of the excited electrons are used for the injection of charge on the CB of the mesoporous layer of TiO2. The visible band is assigned to a charge transfer transition t2g-π which causes a shift in the electronic charge density from the thiocyanate groups to the titanium t2g orbitals. Therefore the LUMO has a higher charge density near the anatase binding site, allowing a good electronic coupling for the charge injection. The geometry of the complexation, mononuclear bidentate, has been deduced through cell studies known in the literature [42], in which a weak electric field is present at the semiconductor
electrolyte interface due to the absorbed dye molecules.

5.5.3 Cyclic voltammetry Voltammetric techniques are based on the measurement of current as a function of the potential applied during electrochemical processes. In classical voltammetry,

--- Excitation 560 nm TiO -dye --- Excitation 600 nm TiO -dye --- Absorbance TiO -dye

0.6

(A)

(B)

0.5

Absorbance

0.4 0.3 0.2 0.1 0.0 500

600

700

195

40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 800

Intensity (A.U.)

Synthetic, natural and bioinspired dyes as TiO2 sensitizers in sustainable solar cells

Wavelength (nm) Figure 5.8 MLCT characteristic absorption band for N719 dye in TiO2 matrix (A) and its emission spectra (B). MLCT, Metal to ligand charge transfer.

the potential supplied to the working electrode is varied linearly over time, while in the cyclic defined one, the applied voltage has a triangular waveform. In practice, a linear voltage ramp is applied to the working electrode for a time between t0 and t1, this is then inverted so as to bring the potential back to its initial value at time t2; the cycle can be repeated several times. As a result, a voltammogram is obtained that relates the trend of the current as a function of the potential to the working electrode for the oxidation/reduction of the chemical species analyzed. In particular, the faradic current is measured, that is, the one produced by the oxidation
reduction reactions of the redox species on the electrode surface; this is affected by the interference of the capacitive or charging current, consisting of the flow of charged molecules not linked to the redox reaction but to phenomena such as mass transport, migration, and convection. These phenomena are reduced by using a support electrolyte that migrates in place of the redox species and avoiding the agitation of the solution during the analysis. This technique is widely used because of the simplicity with which it is possible to obtain different information on the species analyzed and the reactions involved. Indicating with PA and PC, respectively, anodic peak (oxidation) and cathodic (reduction), the parameters of greatest interest are IPa/IPc, that is, the ratio between the peak currents, and EPa/EPc, that is, the separation between the respective potentials. If the reaction is reversible anode and cathode current have the same intensity and the IPa/IPc ratio is equal to one, while the difference between the potentials is equal to 57/mV at 25 C, with n corresponding to the number of electrons involved in

196

Titanium Dioxide (TiO2) and Its Applications

the reaction. On the contrary, for an irreversible reaction, the cathode and anodic peaks widen and separate more, up to the borderline case in which there is no return peak after oxidation.

5.5.4 Roughness and desorption factor The concentration of dye bound to TiO2 can be calculated through two ways: G

G

Through the theoretical calculation referred to the dye molecules that bind to TiO2 NPs; Through a practical method that involves desorbing the dye from the mesoporous layer.

The roughness factor is defined as the ratio between the total surface development and the external area of a layer of material, it takes into account the thickness of the latter and allows to determinate the absorbed dye concentration. Initially, the dimensional aspect of TiO2 NPs and the respective dye must be kept in mind. As an example, it is assumed that TiO2 NPs, 20 nm sized, are deposited on a surface of 1 cm2, that the profilometric analysis identified a thickness of 10 μm and the dye molecule has an area of 1 nm2. A theoretical example is that of not considering the NPs subjected to a sintering process, and therefore the particular interactions, but considering the TiO2
dye interactions on the whole surface of the mesoporous material. In 1 cm2 are present B2.5 3 1011 TiO2 NPs which multiplied by the surface of a single particle (B1.2 3 10211 cm2) given rise a value of B3 cm2, that is three times more than the apparent one (1 cm2). Considering that the thickness of 10 μm consists of 500 layers of NPs, the theoretical surface development corresponds to a roughness factor of B1500 cm2. The total surface area of a single TiO2 NPS is B1.2 3 103 nm2, while in 1 cm3 there are B1.2 3 1017, to which corresponds a surface development of B1.5 3 1020 nm2; if one molecule of dye is supposed to cover 1 nm2 of area, it can be assumed that its concentration in TiO2 layer is close to B2 3 1024 moles/cm3. Analyzing the following equation: α 5 θ 3 C;

(5.17)

where α is the reciprocal absorption length, θ is the optical absorption cross-section of the dye, and C is its concentration in the semiconductor film (Molarity). From the Lambert
Beer law: A 5 ε 3 l 3 C; ε5

(5.18)

A 5 cm2 =mol: lC

We rewrite the value of θ as:   θ 5 ε 3 1000 since the concentration is mol=L 5 cm2 =mol:

(5.19)

Synthetic, natural and bioinspired dyes as TiO2 sensitizers in sustainable solar cells

197

If a molecule has a ε of 104, θ 5 104 3 1000 5 107 cm2/mol, it is possible to calculate the length factor α considering the concentration C 5 2 3 1024 mol/cm3 in the following way: α 5 107 cm2 mol21 3 2 3 1024 mol cm23 5 2 3 103 cm21 : The absorption length of sensitizer (1/α) is:   1=α 5 1= 2 3 103 cm21 5 5 3 1024 cm 5 5 μm ðideal thicknessÞ: An alternative method to calculate light-harvesting efficiency (LHE) through absorbance measurements is: LHE 5 1 2 102αd

(5.20)

T 5 102A

(5.21)

If d 5 6 μm or 6 3 1024 cm, LHE corresponds to 0.93% or 93%. For example, a photoanode, made using materials with the above-mentioned characteristics and with a unitary absorbance value, has a quantity of dye equal to about 6 3 1016 molecules/cm2; if it is assumed that a dye molecule has a surface of about 1 nm2, it is concluded that 600 cm2 is the area covered by the dye, that is, 40% of the area at its disposal (1500 cm2). Another method is based on the desorption of the dye. This is obtained by immersing the photoanode into potassium hydroxide (KOH), tetramethylammonium hydroxide (TMAOH), or other solutions for about 2 h and then rinsed several times with an appropriate solvent. The amount of desorbate is analyzed by UV-visible absorption spectra and the results also provide useful information regarding dye concentration, desorption rate, and reliable reference to study the stability of DSSCs. To determine the ideal concentration in DSSC devices must be used a slow desorption using a diluted base. However, the diluted base treatment causes a blueshift in the absorption spectrum due to the deprotonation of the dyes [43].

5.5.5 Characteristic I-V curves The I-V curve for DSSC changes dramatically as the rate of change of the gradually polarizing power supply changes, while no significant changes are observed in that of the silicon cells (Fig. 5.9) [44]. The current expression with capacity performance is derived as: I 5 Iph 2 I0 ðexpð

q ðVðtÞ 1 IRs Þ dðIÞ dVðtÞ 1 ðCs 1 Csh ÞRs ðVðtÞ 1 1Rs ÞÞ 2 1Þ 2 1 Csh KTn Rsh dðtÞ dðtÞ

(5.22)

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Titanium Dioxide (TiO2) and Its Applications

Maximum power point

Current (mA)

Isc Imax

Power density

0

Voltage (V)

Vmax Voc

Figure 5.9 Characteristic I-V curves measured in photovoltaic devices.

where Rs is the resistance series, Rsh is the parallel resistance (shunt), T is the temperature, n is the diode factor, q is the elementary electric charge, K is the Boltzmann constant, Cs is the capacity series, Csh is the parallel capacity (shunt), and Iph is the photocurrent. The derivative of the voltage over time, dV(t)/d(t), is multiplied by the capacity Csh, as an algebraic term in Eq. (5.22). Therefore the scanning speed dV(t)/d(t) is one of the most important factors in determining the current output value I of the DSSC. What can be concluded directly from the equation is that the output value I would be distorted when the scanning speed changes. The greater the characteristic capacity in DSSCs, the greater the influence of the scanning speed on the measured I-V curve. If the cell capacity is zero, there will be no value containing d(t) in the equation and the scan speed will no longer affect the cell’s I-V curve. In the BandGap Fermi Level theory, the difference between the almost Fermi level of the mesoporous layer and the redox potential of the electrolyte determines the maximum voltage generated under lighting. It can be demonstrated with the following equation that: Voc 5

  KT ηφ0 ln q η3Ket½I3 2 

(5.23)

the open-circuit voltage varies with the concentration of iodide since the recombination reaction occurs between the electrons on the CB of TiO2 and I32. η represents the quantum yield of a photogenerated electron for the given photo incident flux (φ0), η0 the electron density on the TiO2 CB in the dark, while Ket reflects the speed of the recombination processes for the given triiodide concentration. Furthermore, the potential difference reaches its maximum Voc value, when the cell has the terminals isolated and

Synthetic, natural and bioinspired dyes as TiO2 sensitizers in sustainable solar cells

199

Rs

Rsh Isc

Jdark

+o V -o

Figure 5.10 Model of equivalent circuit in a solar cell.

ideally in these conditions the current density is zero. By setting J(V) 5 0 it is possible to obtain the open-circuit voltage which will have a logarithmic type trend, the open-circuit voltage increases logarithmically with the increase of the photocurrent and the intensity of the incident light. In reverse polarization the cell behaves like a photodetector; electrically it is equivalent to a current generator parallel to a diode. If subjected to radiation, it produces photocurrent proportional to the light intensity. In general, the equivalent circuit of a solar cell can be modeled starting from the photodetector model where the dissipation of power through the contact resistances and through the leakage currents around the device must be included (Fig. 5.10). If the device is lighted up and short-circuited, it gets the maximum deliverable current Isc. Another fundamental parameter is obtained in open-circuit conditions, when no current can flow in the cell and the open-circuit voltage Voc. Starting from these values it is possible to identify the maximum power peak (Pmax), that is, the point of the I-V characteristic that maximizes the product of the current for the voltage. For any intermediate load resistance value RL the circuit generates a potential ranging from 0 to Voc, which will correspond to a current I such that: V 5 RL 3 I:

(5.24)

The current is proportional to the illuminated area A, so we often refer to the current density: Jsc 5 Isc =A:

(5.25)

By comparing the solar cell with a battery, it is seen that for the latter the power that is supplied to the load is constant, while the solar cell provides a constant power for each level of illumination, while the potential difference depends on the load RL. The performance of a solar cell is also characterized by the FF defined as the ratio between the power density of Max and the product between the current short-circuit density for the open-circuit voltage: FF 5

Vmax Jmax : Voc Jsc

(5.26)

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Titanium Dioxide (TiO2) and Its Applications

This parameter indicates the ratio between the maximum power actually extractable and the maximum power ideally obtainable, that is, it gives us information on how much the real characteristic differs from the ideal one at its operating point. The maximum power density is given by the Vmax 3 Jmax, which graphically corresponds to the internal rectangle represented in Fig. 5.11. This parameter allows you to rewrite the maximum power output from the cell as: Pmax 5 Voc 3 Jsc 3 FF:

(5.27)

The photoconversion efficiency of solar cells is given by the following expression: η5

Pmax Jsc Voc FF 5 : Pin Pin

(5.28)

The Isc, Voc, FF, and η values are the key performance parameters of the solar cell [41]. This parameter indicates the ratio between the maximum power actually extractable and the maximum power ideally obtainable, that is, it gives us information on how much the real characteristic differs from the ideal one at its operating point. The maximum power density is given by the Vmax 3 Jmax, which graphically corresponds to the internal rectangle represented in the figure. This parameter allows you to rewrite the maximum power output from the cell as:   Jdark 5 J0 eqV=kBT 2 1 ;

(5.29)

where J0 is a constant. Under such conditions, the cell essentially behaves like a diode. On the other hand, when the cell is illuminated but not polarized (V 5 0) the

Maximum power point

0

Increasing

Voltage (V) (A)

Current (mA)

Current (mA)

Maximum power point

0

Decreasing

Voltage (V) (B)

Figure 5.11 FF variation based on Rs variations (A) and Rsh variations (B). FF, Fill factor.

Synthetic, natural and bioinspired dyes as TiO2 sensitizers in sustainable solar cells

201

Jsc is measured in short-circuit conditions. For this reason, the net current density flowing in a circuit powered by a solar cell is [Eq. (5.37)]:   JðVÞ 5 Jsc 2 J0 eqV=kBT 2 1 :

(5.30)

These dissipation phenomena are included through two parasitic resistors, Rs in series and Rsh in parallel. The first of these resistances is linked to the passage of current on the surface of the contacts. The second resistance is due to current losses through the cell and is a major problem in devices with limited rectification. High values of Rs and Rsh lead to a reduction of the FF as in Fig. 5.11.

5.5.6 Quantum efficiency: IPCE, APCE, and LHE Two important parameters for assessing the performance and quality of a photovoltaic cell are the external quantum efficiency (EQE) and the internal quantum efficiency (IQE). These two quantities are also commonly referred to as incident-photon-tocurrent efficiency (IPCE) and absorbed photon-to-current efficiency (APCE) acronyms, respectively. The EQE measures the ratio between collected electrons and incident photons, similarly the IQE is the ratio between collected electrons and absorbed photons. EQE and IQE are linked together with the LHE by the equation: IPCE 5 LHE 3 APCE:

(5.31)

While it is possible to obtain IQE values equal to 1 (i.e., all the absorbed photons are converted into electrons at a given wavelength), generally EQE reaches a maximum value close to 0.90. The integral, on all wavelengths, of EQE for the reference spectrum AM 1.5 is equivalent to the global current density Jsc. This value can be compared with the current density obtained under the solar simulator and their correspondence allows validating the measurements made. Experimentally EQE is obtained by measuring the density of short-circuit current Jsc (λ) generated with a monochromatic radiation of wavelength λ and irradiance φ; if Jsc is expressed in μA/cm2, λ in nm, and φ in W/m2: EQE 5 n ðelectrons generatedÞ=N  ðincident photonÞ n 5

Isc 3 t : q

(5.32) (5.33)

When Isc is short-circuit current, t is the time, q is the charge of electrons. Isc 3 t represents the total charge obtained from a solar cells (qtot) Therefore n 5 qqtot N 5

P 3 t ; Ev

(5.34)

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Titanium Dioxide (TiO2) and Its Applications

where P is the incident light power, t is the time, Ev is the photon energy, P 3 t represents the total incident light energy or Etot, therefore N 5

Etot : Ev

Substituting Eqs. (5.33) and (5.34) in Eq. (5.32)  EQE 5

t q P 3 t Ev

I 3

 5

I 3 Ev I h 3 c : 5 3 p q 3 λ P 3 q

(5.35)

When h is Plank constant, c is the speed of light, λ is the wavelength and q is the charge of an electron. The EQE or IPCE is: IPCE% 5

6624 3 10234 ðJ sÞ 3 29; 978 3 108 ðm s21 Þ IðA cm22 Þ 3 PðW cm22 Þ 1:6 3 10219 ðCoulombÞ 3 1029 m Isc ðAÞ 3 1241 3 100 (5.36) 3 100 5 PðWÞ

From IPCF measurements it is possible to obtain the theoretical value of shortcircuit current density (Isc). It is necessary to convert the solar irradiance value (W/m2) in A/cm2 (at AM1.5). The photon flux ф (s21 cm22) is extracted from the relationship: ф5

W m22 Ev

(5.37)

or W 3 λ ðmÞ : m2 3 hðj 3 sÞ 3 C ðm s21 Þ

(5.38)

For example, the solar irradiance at 400 nm is 1.114 W m22 and inserting these data in Eq. (5.38): ф5

1:114ðW m22 Þ 3 400 3 1029 ðmÞ 5 2:24 3 1018 = s=m2 6624 3 10234 ðJ sÞ 3 2:998 3 108 ðm=sÞ

5 2:24 3 1014 =s=cm2 Furthermore, ф 3 q provides the charge density in A/cm2, that is, 2.24 3 1014 (s cm22) 3 1.6 3 10219 (Coulomb) 5 3.58 3 1025 A/cm2 or 3.58 3 22 10 mA/cm2. 21

Synthetic, natural and bioinspired dyes as TiO2 sensitizers in sustainable solar cells

203

Therefore the product ф 3 q 3 EQE represents the theoretical value of Isc or ð EQE Isc 5

ф 3 q 3 EQE 3 dλ:

(5.39)

wavelength

5.5.7 Electrochemical impedance spectroscopy It is a useful method for studying the electrochemical properties of materials and their interfaces. Specifically, if carried out on DSSC, they provide information on the various important phenomena of transport, diffusion, and accumulation of charges that occur during cell operation: 1. The charge transport due to the electronic diffusion through TiO2 layer and to the diffusion of the ionic species in the electrolytic solution; 2. The charge transfer due to back electron transfer phenomena at the FTO/electrolyte interface, recombination at the TiO2/electrolyte interface and regeneration of redox species at the CE/electrolyte interface; 3. The charge accumulation in the capacitive elements in the cell, including the interfaces, the CB, and the surface states of the TiO2 mesoporous network.

EIS measurements are conducted by applying an alternating voltage with sinusoidal pattern to the system and by analyzing the response, which consists of an alternating current signal. The analysis is generally carried out through the use of a potentiostat and a frequency response analyzer. The applied alternating regime potential must have a very small amplitude, in order to consider the pseudolinear system under study. The frequency, however, can vary over a wide range of values, usually between MHz and a few mHz [45]. The main advantage of this technique consists in being able to obtain spectra in which distinct characteristic signals are highlighted, distributed over the frequency domain, associated with specific phenomena that occur simultaneously in the cell and which, therefore could not be studied individually. To observe the phenomena that occur in the cell, the measurements are carried out in the dark by imposing a continuous voltage equal to the Voc generated by the cell itself in standard lighting conditions (100 mW/cm22). An alternating potential, of 10 mV amplitude, variable in a frequency range between 100 kHz and 0.1 Hz is superimposed on this voltage. The impedance spectra of a DSSC are generally represented with two known representations. The Nyquist plot depicts the imaginary part of the impedance 2 Zv as a function of the real part of the impedance Z0 in complex space. The other type of representation is the Bode plot, in which the impedance module is plotted jZ j or the phase angle θ, as a function of the logarithm of the frequency. The latter graph, albeit of less immediate readings, can be useful because it explicitly shows the dependence on frequency. The obtained impedance spectra permit to observe different phenomena occurring simultaneously inside the cell through distinct signals observable in specific frequency ranges. In particular, we can distinguish arcs in the Nyquist diagram and peaks in the Bode diagram, which are located in three particular frequency ranges.

204

G

G

G

Titanium Dioxide (TiO2) and Its Applications

At frequency above 1 kHz, an arc (Z1) in the Nyquist plot and a peak in the Bode plot attributed to the charge transport to the CE are visible; at medium frequency (between 1 Hz and 1 kHz) a wider arc (Z2) in the Nyquist plot and another peak in the Bode plot attributed to electronic transport in the TiO2 layer and electron
hole recombinations are visible; at low frequency (below 1 Hz) a lateral arc (Z3) is visible in the Nyquist plot and a third Bode plot peak attributed to the diffusion of ionic species in the electrolyte.

A resistance is attributable to each arch, the value of which can be estimated as the diameter of the arch. Another resistance (R0) can be identified as the distance between the origin of the axes and the beginning of the high-frequency arc. However, the measurement can be carried out at any voltage value and the diagrams obtained for different applied potential values show different characteristic curves, which reflect the variation of the mechanisms that occur inside the cell.

5.5.8 Tafel electroanalysis In order to describe the Tafel polarization curve, it is necessary to first introduce the concept of exchange current. A representation of the current
potential relationship to a working electrode can be obtained by using the so-called exchange current j0. From the kinetic theory of the heterogeneous charge transfer process, it appears that, even in equilibrium conditions, two equal currents flow in the opposite direction. By definition, the cathode (positive) current jla, or the anodic current changed by sign jlc, which flows to an electrode in conditions of equilibrium, is called exchange current and indicated with j0. Strictly, it makes sense to speak of exchange current only when both the forms Ox and Red are present at a finite concentration, respectively, C0Ox and C0Red, in the electrolyzed solution, and therefore there exists a value of the potential, E 5 Eeq (COx 5 CRed), to which the current is zero. It is important to consider that the exchange current depends on the area of the electrode, the comparison between electrodes of different areas is significant only if the exchange current density (j0/cm2) is compared. The Tafel polarization curve is a powerful tool for studying the electrocatalytic activity of the CE, relating the intensity of electric current circulating in an electrochemical cell to the overvoltage (difference between the current electrode conditions according to the conditions of balance). It is possible to trace the Tafel polarization curve in DSSC by describing the curve to assuming to freeze the system at a given moment. If we want to represent on the cell\graph in Fig. 5.12 the initial condition in which there is only C0ox (green dot), any section and any point of the cell is always the same. The system is absolutely dynamic, seamless, we dissect the cell as if they were theoretical plates of a chromatographic column. By applying a potential E 5 Eq 1 V1 (positive potential), what happens in this first layer (what is in direct contact with the electrode) is that the current is so small that the concentration remains C0ox. Applying a potential of E 5 Eq instantly the Cox concentration is equal to that of Cred 5 1/2 C0ox in the first layer, the one in contact with the electrode. If you keep this potential activated (E 5 Eq), a part of the oxidized form from the second layer will begin to diffuse in the first because there is a concentration

Synthetic, natural and bioinspired dyes as TiO2 sensitizers in sustainable solar cells

(A)

e-

205

(B) jdl

V C0Ox

I3-

Platinum

Platinum

COx =CRed

3I-

COx =CRed

jdl/2

C 0Red E= Eq+V+

E= Eq

E= Eq-V-

Figure 5.12 (A) A cell showing two interfacing counter electrodes (CEs) for Tafel analysis. (B) The overall graph showing the two green curves together.

gradient. But what reaches the first layer and tends to increase C0ox cannot because there is always the potential applied, therefore everything that arrives is reduced instantly, because the concentration of the first layer can never be higher than 1/2 C0ox (that corresponding to the applied potential). If from the second layer begins to spread some oxidized species that tries to compensate for this gradient, the third layer tries to supply the second and the fourth supplies the third, and so on creating a diffusion front. Over time, increasingly distant layers from the electrode will be involved in the diffusion process. The goal is to have the maximum signal (in this case the maximum current value). The current is generated by the amount of electrolyte that reaches the electrode and is reduced. The condition in which is obtained the maximum measured current is when is applied a potential of E 5 Eq
V (negative potential), because in the first layer there will be practically a zero concentration of C0ox and therefore there will be the maximum gradient that will be possible to obtain, that is, between 0 and C0ox. In an analogous but inverse way in the figure, the same happens for the lower part of the graph. In DSSC, in the Tafel area, a greater slope offers a larger j0, reflecting a greater catalytic activity toward the reduction of the electrolytic mediator (triiodides). It is possible to calculate the exchange current through the following equation: j0 5

RT nFRct

(5.40)

where n is the number of electrons involved, j0 is the exchange current, R is the gas constant, T is the temperature, Rct is the charge transfer coefficient or charge transfer resistance, and F is the Faraday constant. Basically, only when the electronic transfer step is slow compared to the mass transfer, it will be possible to determine the exchange current and the electronic transfer coefficient Rct from a Tafel graph, of which it will generally be possible to obtain experimentally or only the cathode branch or only the anode branch.

206

Titanium Dioxide (TiO2) and Its Applications

Figure 5.13 (A) I-V curve of two CE sandwiches in the presence of electrolyte at 1 V, 21 V potential range; (B) Tafel plot. CE, Counter electrode.

The graphic extrapolation of a Tafel plot for a DSSC device will now be illustrated. In DSSC, the Tafel plot is a powerful means of qualitatively understanding the interactions at the electrolyte
CE interface, since, through the implementation of other techniques such as microscopic, optical transmittance, and thermogravimetric, it is possible to identify problems related to the formation of bundless, Brownian motions or problems related to the nature of the surfactant. Two CEs are interfaced, seal and liquid electrolyte is injected. Measurements with potential range 21, 11 with collection of 50 points. Light output 0 mW/cm2 for CE. After taking the measurement, a curve will be obtained as in Fig. 5.13A. Subsequently, the currents are reported in absolute value and the logarithm is made in base 10 of the currents in absolute value and is related to the area, for example, having an area of 0.196 cm2: (log(j0))/0.196 obtained the graph in Fig. 5.13B. By tracing a tangent to the inflection point of the curve and making it intersect with the straight line perpendicular to the abscissa plane at point 0, we obtain J0 that corresponds to the exchange current and is inversely proportional to the resistance to charge transfer [46].

5.6

Conclusions

This chapter illustrates the possible interactions of TiO2 with synthetic, natural, and bioinspired dyes by analyzing the current state of research in DSSC. The interaction between computational investigations and experiential assessment of charge transfer dynamics is the key to further developments in environmentally friendly DSSCs. The characteristics of the DSSC device are detailed, the determination of the typical parameters with a particular focus on some passages difficult to find in the literature

Synthetic, natural and bioinspired dyes as TiO2 sensitizers in sustainable solar cells

207

such as the roughness factor or the demonstration how the surface increase of the particles improves the interactions with the dye, the calculation of the IPCE of the LHE and APCE analytically. The UV-vis and excitation characteristics of a dye are also illustrated and how to qualitatively check the presence of the signal of interest. In detail, the characteristics of the other DSSC components such as electrolyte and CE, and the characterizations related to it such as the Tafel Plot, are explained and evaluated. The DSSCs have been appearing for about 30 years but despite this time they are still trying to maintain a strategic role in the field of sustainable development and energy. These devices are multicomponent systems in fact are made using different materials such as glass, plastics, conductive films, electrolytes in solution but also in solid-state, dyes of synthetic or natural origin, semiconductors, sealants, conductive pastes, metals, nanotubes, quantum dots, NPs, etc. Surely the less elements will be involved in the realization of devices, the simpler and more reliable will be the final product. It goes without saying that an important competence and multidisciplinary behavior are required in the management of the various sectors involved to finalize both the scientific and applicative aspects. Despite the complexity of implementation, the DSSCs are still very much studied and continue to be proposed and improved. They are more efficient especially when used in indoor environments, because they perform their function better in diffused light conditions and at the same time, they are able to last longer, just because they are less stressed by the action of external environmental agents. They still continue to be current and maintain their appeal in terms of architecture and positive environmental impact.

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467. [3] C.W. Allen, reprinted with corrections Astrophysical Quantities, 3rd ed., Athlone, London, 1976p. 125. [4] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Muller, P. Liska, et al., Conversion of light to electricity by cis-X2bis(2,20 -bipyridyl-4,40 -dicarboxylate)ruthenium(II) charge-transfer sensitizers (X 5 Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline titanium dioxide electrodes, J. Am. Chem. Soc. 115 (14) (1993) 6382
6390. [5] M.K. Nazeeruddin, S.M. Zakeeruddin, R. Humphry-Baker, M. Jirousek, P. Liska, N. Vlachoupoulos, et al., Acid 2 base equilibria of (2,20 -Bipyridyl-4,40 -dicarboxylic acid) ruthenium(II) complexes and the effect of protonation on charge-transfer sensitization of nanocrystalline titania, Inorg. Chem. 38 (26) (1999) 6298. [6] M. Gr¨atzel, S. Yanagida, in: K. Kalyanasundaram (Ed.), Dye sensitized solar cells, EPFL Press, 2010, pp. 45
76 (Chapter 2). [7] K. Kalyanasundaram, M. Gr¨atzel, Applications of functionalized transition metalcomplexes in photonic and optoelectronic devices, Coord. Chem. Rev. 177 (36) (1998) 347. [8] C.A. Bignozzi, S. Schoonover, F. Scandola, Molecular Level Artificial Photosynthetic Materials, 44, John Wiley & Sons, New York, 1997, pp. 42
48.

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296.

TiO2-based materials for photocatalytic hydrogen production

6

Maria Vittoria Dozzi and Elena Selli Department of Chemistry, University of Milan, Milan, Italy

6.1

Introduction

With the rapid depletion of fossil fuels and the increased concern about the global warming consequent to the accumulation of greenhouse gases and other air and water pollutants, extremely urgent is the development of technologies affording renewable energy, based on long-term sustainable and abundant energy sources. Conversion of solar energy into chemical energy in the form of so-called solar fuels, such as hydrogen in primis, certainly represents one of the most perspective strategies to solve present energy and environmental urgent problems. Hydrogen can be envisaged as an excellent energy carrier for the development of a low-carbon emission economy in the future. In fact, the chemical energy stored in the H
H bond can be released when it reacts with oxygen, yielding only innocuous water as a product, in a highly exothermic reaction. The calorific value of hydrogen per unit mass is much larger than that of gasoline and of natural gas, and its exploitation does not imply the emission of any greenhouse or pollutant gas. Although the technologies of energy production from hydrogen, such as fuel cells and internal hydrogen combustion engines, are already mature, the major problem that still needs to be solved for a large exploitation of hydrogen as an energy carrier remains its production. This is still largely based on fossil raw materials, the most important industrial process consisting in the energy-demanding high-pressure and high-temperature catalytic steam reforming of hydrocarbons. Water electrolysis would represent a sustainable alternative way to produce hydrogen from an abundant and low-cost starting material, provided that electricity obtained from renewable sources is employed. Alternatively, hydrogen can be produced from photocatalytic water splitting or reforming of organics, through the direct exploitation of the energy provided by solar radiation. For a full development of hydrogen economy, extensive research has been carried out in the last decades on the photocatalytic and photoelectrocatalytic splitting of water into H2 and O2, since Fujishima and Honda’s pioneering report of the photoelectrocatalytic water splitting on a TiO2 electrode [1]. As one of the earliest employed and most investigated n-type semiconductor photocatalysts, TiO2 has Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00022-5 © 2021 Elsevier Inc. All rights reserved.

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been widely used for this and other photo(electro)catalytic applications, essentially because of its outstanding stability also under irradiation. This chapter focuses on the research progresses in hydrogen production on TiO2-based photocatalysts, especially for hydrogen production from water photosplitting and photoreforming of organics. Emerging strategies to improve the activity in TiO2-based photocatalysts will be outlined, by addressing the key factors determining the efficiency of photocatalytic processes for hydrogen generation, including light absorption, charge separation, and surface reduction and oxidation reactions.

6.2

Photocatalytic water splitting with TiO2

Several excellent reviews have been published on the photocatalytic properties of TiO2 and its use in the photocatalytic splitting of water, which provide an exhaustive description of the fundamentals of this process and of its application in hydrogen production [2
10]. The photocatalytic splitting of water, as most reactions employing semiconductor photocatalysts, is usually initiated by the absorption of photons with energy equal to, or greater than, the semiconductor bandgap. This promotes an electron from the valence band (VB) of the semiconductor to its conduction band (CB), with the con1 sequent formation of an electron (e2 CB )
hole (hVB ) pair, as shown in Fig. 6.1 for the specific case of photocatalytic water splitting on TiO2. The so-produced e2 CB and h1 couple of charge carriers can either recombine, with the consequent loss of VB energy as heat or with emission of photons, or separate and initiate electron transfer reactions at the semiconductor surface. CB electrons, e2 CB , can transfer to acceptor species, having a reduction potential lower in energy than the CB minimum, while oxidation of electron donor species, with a potential higher in energy than the semiconductor VB, occurs through electron transfer to fill the h1 VB holes photogenerated in this latter. Both acceptor and donor species are adsorbed on the semiconductor surface. In the case of water

Figure 6.1 Schematic representation of the mechanism of water photocleavage over TiO2 semiconductor particles.

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213

cleavage, the electron acceptor species are protons (H1 ions from water), whereas water, or hydroxyl anions yielding hydroxyl radicals, are the electron donor species, according to the following reactions: 1 TiO2 1 hν ! e2 CB 1 hVB

(6.1)

2H1 1 2e2 CB ! H2

(6.2)

1 1 H2 O 1 2h1 VB ! O2 1 2H 2

(6.3)

Formally, the overall photocatalytic water splitting reaction is, therefore: H2 O 1 2hν ! H2 1

1 O2 2

ΔGr 5 237 kJ=mol

(6.4)

Because reaction (6.4) is accompanied by a positive Gibbs free energy change (i.e., it is a thermodynamically uphill reaction), the photocatalytic splitting of water may be regarded as a sort of artificial photosynthesis process, in which the energy of photons is converted and stored in the form of chemical energy within the H2 molecule. The redox potentials of reactions (6.2) and (6.3) are 20.41 and 10.81 V, respectively, at pH 7, 25 C, and 1 atm (see Fig. 6.2). Overall water splitting thus requires energy equal to 1.22 V, at least. On the basis of Planck’s law, this means that all photons with associated wavelength λ , 1100 nm, that is, the whole visible light spectral region, could in principle promote the water cleavage reaction with photocatalytic materials able to absorb light in this region and provide efficient separation of the photoproduced charges. TiO2 anatase –1

Potenal (V)

–0.5

- ECB

H2O

@ pH 7

4 H++4 e– → 2 H2

e-

0 0.5

2 H2O → O2+4 H++4 e–

hν

1 1.5

e–

2 2.5

+

EVB

Figure 6.2 Potential energy diagram of H2 production from photocatalytic water splitting over TiO2.

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Titanium Dioxide (TiO2) and Its Applications

Practically, the optimal semiconductor acting as photocatalyst in water cleavage should have a bandgap close to 2 eV in order to account, in addition to the thermodynamic water splitting potential, also for overpotential and ohmic drop losses [11]. Furthermore, to work as an effective photocatalyst in water splitting, a semiconductor should not only have a suitable bandgap but also CB and VB energy levels matching the potentials of reactions (6.2) and (6.3), that is, the CB edge energy should be more negative than the potential of the H1/H2 couple, and the VB edge energy should be more positive than the O2 evolution potential, on the electrochemical scale. TiO2 fulfills these requirements but has a relatively wide bandgap and thus absorbs light only below 400 nm. This limits its photoactivity only to UV light irradiation, which makes it able to exploit only a small portion (c. 4%) of the solar spectrum and represents the major limitation to its application in solar light harvesting and conversion. Other semiconductor oxides, for instance ZnO, with a slightly narrower bandgap compared to TiO2, would be possible candidates as photocatalysts for hydrogen production from water, but they can undergo photocorrosion (Zn21 dissolution) under illumination in contact with water. Alternatively, more stable, lower bandgap semiconductors, such as WO3 and Fe2O3, have a CB potential ECB lower than that required to evolve H2, a problem that could be partially circumvented by applying an electrical bias in a two-electrode cell. Thus mesoporous nanocrystalline TiO2 is still regarded as the best choice for the construction of photoanodes for all kinds of photoelectrochemical cells [12].

6.3

Development of sensitive TiO2-based photocatalysts for H2 generation

As a wide bandgap semiconductor, the biggest drawback of TiO2 as a photocatalyst thus consists in the fact that it can only absorb light in the ultraviolet region. Indeed, the bandgap of the anatase polymorph is 3.2 eV, corresponding to an absorption onset at 384 nm, while that of rutile is 3.02 eV, extending the absorption ability of this polymorph up to 410 nm. Much effort has been devoted in the last two decades to extend the light absorption ability of TiO2 to the visible region, either (1) by bandgap engineering, in the attempt of reducing it so that TiO2 would absorb visible light, which can be achieved by introducing other elements into the TiO2 lattice or through tailored preparation methods; or (2) by surface sensitization, exploiting visible light active materials as light harvesters, able to transfer electrons to the CB of TiO2, thus sensitizing its action in H2 production.

6.3.1 Bandgap engineering In the band structure of TiO2, O 2p orbitals contribute to the filled VB, while Ti 3d orbitals mainly contribute to the lower part of the unoccupied CB [8,13]. Upon doping TiO2 by replacing Ti41 with other cations, an impurity level could be

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215

introduced within the bandgap, which can act as either an electron acceptor or an electron donor, thus allowing doped TiO2 to absorb visible light. The possibility of obtaining visible light-responsive TiO2 by cation doping has been investigated for more than three decades [14,15]. Already in 1982, the photocatalytic splitting of water was investigated in the visible region (400
550 nm) on Cr31-doped TiO2 nanoparticles (NPs) and the visible light absorption of this material attributed to the photoinduced transition of Cr31 3d electrons into the CB of TiO2 [16]. More recent reports of photocatalytic water splitting occurring under visible light irradiation include the use of hydrothermally prepared Fe31-doped titania [17], titania nanotubes ion exchanged to introduce Pt, Ir, or Co [18], and Cr-doped TiO2 multilayer thin films with optical absorption energy as low as 2.1 eV [19]. The higher activity obtained by codoping TiO2 with two cations with different charges, for instance Ni21 and Nb51, was attributed to charge balancing effects increasing the photocatalyst stability [20]. Doping TiO2 with anions has been largely preferred to doping with cations, which would lead to the instability of the material and to quite localized states deep in the TiO2 bandgap acting as recombination centers of charge carriers [8]. Doping with anions could hardly affect the CB of TiO2, but it usually affects the VB and introduces impurity states within the bandgap at different energy levels depending on the dopant [21]. The position and the localization degree of these impurity states in the TiO2 structure govern not only the absorption of visible light, but other crucial properties in the performance of the photocatalyst, such as the mobility of the charge carriers and their recombination rate. Nitrogen-doped TiO2 [22] has most intensively been studied in the last decades, after the work of Asahi et al. [23], which gave birth to a new generation of semiconductor materials, mainly TiO2 doped with nonmetal elements. According to the calculations reported in that paper [23], the effect of N-doping consists in a rigid shift of the top of the VB to higher energies, but more accurate calculations evidenced that N 2p impurity states are almost fully localized on N atoms and lie few tenths of an eV above the VB [24]. Other first-row p-block elements [25], introducing intra-bandgap energy levels, as shown in Fig. 6.3, as well as sulfur and phosphorus, have been largely employed to dope TiO2. Many examples exist in the literature of N-doped TiO2 prepared by different techniques, which shows H2 production under visible light irradiation [26], as well as of TiO2 codoped with two of these nonmetal elements [27]. Also N-codoping with metals has been largely explored. For example, TiO2 codoped with N and In [28] or Ce [29] was reported to be active in H2 production under visible light irradiation. However, although this type of doping unequivocally extended TiO2 absorption toward the visible region, its efficiency in photocatalytic applications has been largely debated, mainly because the oxidizing species generated at intra-band states upon visible light illumination were found to be thermodynamically and kinetically unable to oxidize weak donors [30,31], with a consequent reduction of the efficiency of the whole photocatalytic process. Furthermore, besides the extended light absorption, other effects induced by TiO2 doping can determine its photocatalytic activity. For example, the specific surface area may largely vary upon TiO2 doping, or catalytic active sites may be

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Titanium Dioxide (TiO2) and Its Applications

Figure 6.3 Schematic representation of one-electron states of the substitutionally doped anatase TiO2. Source: Reprinted from C. Di Valentin, G. Pacchioni, Trends in non-metal doping of anatase TiO2: B, C, N and F, Catal. Today 206 (2013) 12
18, ©2013, with permission from Elsevier.

introduced, which may promote either the reduction of protons or the oxidation of water and/or the sacrificial reagents, hence enhancing the overall photocatalytic activity. Moreover, the increase in photocatalytic hydrogen production rate obtained upon doping TiO2 with both nitrogen and fluorine [32] was demonstrated to result from the high crystallinity of the bulk material and the formation of oxygen vacancies and surface defects consequent to doping with fluorine, while the visible light absorption increase at wavelengths longer than the TiO2 bandgap absorption onset, due to nitrogen doping, was demonstrated to be photocatalytically inactive [33]. In general, doped TiO2 suffers from instability and the introduced impurities would also act as recombination centers leading to photocatalytic activity decrease [14]. Therefore dopant-free TiO2 with visible light response received increasing attention in recent years [34
36] and several methods were successfully developed to produce dopant-free TiO2-based materials with extended absorption in the visible region. For instance, a radio frequency magnetron sputtering deposition method was introduced to synthesize visible light-responsive TiO2 thin films [37] containing oxygen vacancies, able to produce H2 or O2 using sacrificial reagents under irradiation at wavelengths above 420 nm. Defect levels originated from oxygen vacancies induced an onset of absorption at c. 440 nm of self-(Ti31)-doped TiO2 [38] produced by different syntheses. TiO2 materials highly active in photocatalytic hydrogen production were obtained by flame spray pyrolysis [39,40]. A combustion method was developed to synthesize TiO22x, with high stability in air and water under irradiation.

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The presence of Ti31 in the bulk, evidenced by the electron paramagnetic resonance (EPR) spectroscopy, was proposed to be responsible for the visible light absorption of this material [41]. Air-stable Ti31 was introduced into TiO2 by a low-temperature vacuum-activated method [42] or by a solvothermal method employing NaBH4 as reductant [35]. Oxygen vacancies can be regarded as responsible for the Ti 3d defect state in the bandgap of TiO2. Based on high-resolution scanning tunneling microscopy and photoelectron spectroscopy measurements [35], the Ti interstitials in the near-surface region were proposed to generate defect states in the bandgap, responsible for visible light absorption. A disorder-engineered TiO2, black in color by hydrogenation, consisting of crystalline TiO2 quantum dots or a nanocrystal core with a highly disordered surface layer ensuring visible and infrared light absorption showed substantial activity and stability in photocatalytic H2 production under sunlight [43].

6.3.2 Surface TiO2 sensitization Dye sensitization is a largely exploited strategy to make TiO2 active under visible light irradiation. In this case, light is not absorbed by TiO2 but by dye molecules adsorbed on the semiconductor surface, which are promoted into their electronically excited states, from which electrons can be injected into the semiconductor CB. In these dye-sensitized systems, the reduction reaction takes place on the semiconductor surface, while the oxidized dye molecules are usually regenerated by reaction with sacrificial reagents. Milestone in this field is the 1991 paper of O’Regan and Gr¨atzel [44], which originated the development of dye-sensitized solar cells and dye-sensitized photocatalytic reactions. Ruthenium-based dyes have been widely employed as sensitizers, after Gr¨atzel and coworkers found that water could be decomposed under visible light irradiation on TiO2 when Ru(bpy)321 was used as sensitizer [45,46]. Charge recombination and stability of Ru-based dye molecules were recognized to be fatal problems for dye-sensitized photocatalytic systems in the vast literature that followed, and various strategies have been explored to overcome them [47,48]. Besides Ru complexes, zinc porphyrin complexes [49] and platinum(II) complexes [50] were employed as sensitizers in photocatalytic H2 production under visible light, as well as less costly and toxic organic dyes. For instance, TiO2 particles modified by surface complexation with 8-hydroxyquinoline [51] showed excellent photocatalytic H2 production activity with EDTA as sacrificial reagent under visible light irradiation. Abe and coworkers investigated the H2 production reaction employing eosin Y as TiO2 sensitizer in aqueous tri-ethanolamine solution [52], as well as merocyanine and coumarin dye-sensitized TiO2 using iodide as an electron donor [53,54]. However, the dye-sensitized TiO2 photocatalytic systems usually suffer from activity loss due to the instability of the dye molecules in the photocatalytic reaction environment and to the limiting electron injection from the dye molecules to TiO2. These problems can be partly solved by tailored designing the ligands of the dyes, by covalently anchoring the dye molecules on the TiO2 surface or by encapsulating them in a porous material.

218

6.4

Titanium Dioxide (TiO2) and Its Applications

Separation of photogenerated charges in TiO2-based photocatalysts for H2 generation

Absorption of photons by semiconductor photocatalysts induces the promotion of electrons from the VB to the CB, photogenerating holes in the VB. The separation of such photogenerated charge couples is a crucial factor that determines the overall efficiency of the photocatalytic process. Therefore much attention has been devoted to increase the separation of charges photoproduced in TiO2 and in other semiconductor photocatalysts. This may be mainly achieved through the fabrication of junction structures, in particular phase junctions, heterojunctions, and Schottky junctions. Recently, photoproduced charge separation was found to be favored also by the exposure of different TiO2 anatase facets.

6.4.1 Charge separation in TiO2 phase junctions Anatase and rutile are the most common crystal forms of TiO2, with anatase usually exhibiting higher activity than rutile [55]. However, the mixed-phase structure exhibits higher photocatalytic activity than either anatase or rutile alone [56,57], the highest photoactivity in H2 evolution being obtained with mixed-phase TiO2 [58
60]. The synergistic effect in the mixed-phase structure of TiO2 was suggested to be due to efficient charge separation across the phase junction [61]. The mechanism of enhanced activity for mixed-phase structure of TiO2 has been widely investigated and is still object of debate. Based on results obtained with different types of analysis, the first hypothesis was that photopromoted electrons are expected to transfer from anatase to rutile, because the CB position of anatase is higher than that of rutile and rutile acts as the reduction site of the junction [62,63]. On the other hand, time-resolved photoacoustic spectroscopy revealed electron trapping sites located on average c. 0.8 eV below the CB edge of anatase [64], which may facilitate electron transfer from the CB of rutile to these trapping sites of anatase. This opinion found support in EPR experiments [65
67] and in the fact that the electron affinity of anatase is higher than that of rutile, in line with a favored transfer of photopromoted electrons from rutile to anatase [68]. Although the direction of charge transfer is still controversial, the effect of more efficient charge separation in the anatase/rutile phase junction, as well as in the junction of anatase with other TiO2 polymorphs [69
71], is commonly agreed, and the phase junction approach is a recognized strategy to enhance charge separation in TiO2 and other polymorphic semiconductor-based photocatalysts.

6.4.2 Charge separation in shape-controlled anatase TiO2 Also the shape of anatase TiO2 NPs may affect the photocatalytic hydrogen production rate. In fact, the equilibrium crystal morphology of anatase TiO2 consists in a slightly truncated tetragonal bipyramid, where the most thermodynamically stable {1 0 1} facets predominate on the exposed surface (Fig. 6.4A), whereas {0 0 1}

TiO2-based materials for photocatalytic hydrogen production

219

Figure 6.4 (A) Equilibrium crystal shape and (B) platelet-like morphology of anatase crystal with the indication of the types of exposed facets.

facets, due to their relatively higher surface energy, are expected to be more active, though representing a smaller portion of the surface, mainly due to a high density and a very strained configuration of surface undercoordinated Ti atoms [72]. Shapecontrolled anatase TiO2 may be produced in the presence of capping agents, which preferentially stabilize {0 0 1} facets during the crystal growth [73], with the consequent formation of platelet-like nanocrystals (Fig. 6.4B). Experimental evidence was obtained [74] that the {0 0 1} and {1 0 1} facets are the preferential oxidation and reduction sites, respectively, and that the copresence of specific crystal facets can positively contribute to the separation of photogenerated electron and hole couples [75], as shown in Fig. 6.5. In the first studies on photocatalytic hydrogen production on Pt-modified TiO2 anatase, NPs mainly exposing {0 0 1} facets were found to be extremely active [76], whereas results obtained in the presence of different percent amounts of exposed {0 0 1} facets [77], also in the presence of Au NPs [78], or with anatase

Figure 6.5 Schematic images of spatial separation of redox sites on anatase TiO2 particles with specific exposed crystal faces: decahedral particle with (A) a larger surface area of oxidation sites and smaller surface area of reduction sites and (B) a smaller surface area of oxidation sites and larger surface area of reduction sites. Source: Adapted with permission from N. Murakami, Y. Kurihara, T. Tsubota, T. Ohno, Shape-controlled anatase titanium(IV) oxide particles prepared by hydrothermal treatment of peroxo titanic acid in the presence of polyvinyl alcohol, J. Phys. Chem. C 113 (2009) 3062
3069. ©2009 American Chemical Society.

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Figure 6.6 Surface heterojunction between {0 0 1} and {1 0 1} facets based on the relative location of their band edges. Source: Adapted with permission from J. Yu, J. Low, W. Xiao, P. Zhou, M. Jaroniec, Enhanced photocatalytic CO2-reduction activity of anatase TiO2 by coexposed {0 0 1} and {1 0 1} facets, J. Am. Chem. Soc. 136 (2014) 8839
8842. ©2014 American Chemical Society.

single crystals with a predominance of {1 0 1} or {0 1 0} facets [79,80] point to the conclusion that anatase crystals with both {0 0 1} and {1 0 1} facets are most photoactive in hydrogen evolution. This suggests a synergistic effect between {0 0 1} and {1 0 1} facets of anatase TiO2 nanocrystals, resulting in lower charge carrier recombination, in line with the most well-established model represented in Fig. 6.6 [81]. In particular, anatase crystals with different coexposed facets can be envisaged forming “surface heterojunctions” with the selective migration of photogenerated holes and electrons toward {0 0 1} and {1 0 1} facets, respectively, being driven by the minimization of their respective energies. Of course, the optimal percent amount of {0 0 1} facets in anatase materials depends on the relative rates of the two simultaneous (reduction and oxidation) half reactions involved in the overall photocatalytic process, the optimal amount of {0 0 1} facets usually falling in the 49%
60% range. A similar type of investigation was extended to anatase with different exposed facets [82].

6.4.3 Noble metal nanoparticles deposition and Schottky junction fabrication Very early, Honda et al. provided experimental evidence that platinum was the H2 production site in platinum-loaded Pt/TiO2 photocatalysts [83] and, since then, Pt/TiO2 photocatalysts have been largely investigated for this application [56,84
86]. This is because platinum, and in general noble metals (NMs), is able to efficiently trap photopromoted electrons. The electron trapping ability of an NM is mainly determined by its work function, which is usually larger than that of TiO2. Therefore in NM-loaded TiO2, the electrons photopromoted into the semiconductor CB can transfer to the metal at the semiconductor
metal interface, which leads to a Schottky barrier formation (see Fig. 6.7) and efficient charge separation. The presence

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221

Figure 6.7 Scheme of semiconductor
metal interface, leading to the Schottky barrier formation. Source: Reproduced from S.J.A. Moniz, S.A. Shevlin, D.J. Martin, Z.-X. Guo, J. Tang, Visiblelight driven heterojunction photocatalysts for water splitting—a critical review, Energy Environ. Sci. 8 (2015) 731
759, with permission from The Royal Society of Chemistry.

of NMs on the TiO2 surface greatly increases its efficiency in photocatalytic hydrogen production, the NM not only providing the reduction site, but also serving as electron sink, inhibiting the recombination of photoproduced charges. Metals are able to trap electrons, the larger is their work function. Among NMs, platinum has the largest work function (5.12
5.93 eV) [87], together with the lowest activation energy for proton reduction [88]. Therefore Pt is generally accepted as the best proton reduction site in the photocatalytic production of hydrogen. Of course, the way platinum is loaded on the TiO2 and the size of the Pt particles on the TiO2 largely affect the activity of Pt/TiO2 photocatalysts in hydrogen production. For instance, a cold plasma method was developed for the preparation of Pt/TiO2 photocatalysts with significantly higher photoactivity in hydrogen production compared to those prepared by the impregnation method [89], due to a better contact between Pt atoms and the Ti41 and O22 ions on the TiO2 surface. Important is also the amount of Pt loading, which could be minimized by employing mesoporous TiO2 [90]. Both platinum- and gold-modified TiO2 photocatalysts, highly active in photocatalytic hydrogen production, have been successfully prepared by flame spray pyrolysis in one step [39,40]. Also gold is an efficient cocatalyst in photocatalytic hydrogen production [91
93], though with lower activity than that of Pt/TiO2 under similar experimental conditions [94,95]. A systematic investigation on the effects of the Au loading amount and Au particle size on the Au/TiO2 activity in photocatalytic H2 production revealed that 3
30 nm-sized Au NPs were most effective, with an activity of Au/TiO2 increasing with decreasing Au particle size, the optimal size of Au NPs being less than 10 nm [96].

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Furthermore, Au/TiO2 photocatalysts also exhibit interesting plasmonic effects, in which gold has a role different from that of a simple cocatalyst [97
99]. Indeed, photocatalytic H2 production can be driven upon visible light illumination on Au/TiO2 by surface plasmon resonance (SPR). The visible light activity of Au/TiO2 photocatalysts can be explained either by resonance energy transfer, based on the idea that SPR induces a local electromagnetic field that facilitates the formation of electron
hole pairs near the semiconductor surface, or in terms of an SPR-excited electron transfer from Au NPs to the CB of TiO2. Recent evidences obtained from EPR spectroscopy are in favor of the second mechanism [100] and indicate that impurity energy levels within TiO2 also play a role in trapping excited electrons [101]. Besides Pt and Au, other metals (Pd, Ag, Rh, Ru, Ir, Ni, Co, etc.) have been successfully employed as cocatalysts of TiO2 in the photocatalytic production of hydrogen [102
105]. However, in consideration of the high cost and scarce availability of precious metals, very active research is ongoing for the development of non-NM cocatalysts. For example, earth-abundant elements, such as Ni and Co, can be used in cluster form as alternative to NM cocatalysts for photocatalytic H2 production, though with a lower activity as compared to Pt/TiO2 [106]. Very promising in photocatalytic hydrogen production are also Cu-based TiO2 cocatalysts [107,108], with Cu species being present on TiO2 in different valence states, interchanging in the reaction conditions and under irradiation [109]. However, as better detailed later in this chapter, the action of such non-NM cocatalysts is different from that of NM NPs on the TiO2 surface, which is mainly consequent to the Schottky barrier formation.

6.4.4 Fabrication of heterojunctions Besides the abovementioned photogenerated electron
hole separation induced by surface NM NPs able to capture CB electrons, several other strategies have been employed to minimize a crucial drawback of TiO2 use as photocatalytic material, that is, the fast and undesired recombination of photoproduced electron
hole pairs, which strongly limits the overall efficiency of all photocatalytic processes [110,111]. In particular, TiO2 can be coupled with another metal oxide with suitable bandgap and edge positions, to enhance photoproduced charge separation [112,113]. By this way a semiconductor-based photocatalysts heterojunction is formed, in which both semiconductors are excited by photons to promote electrons in their CB and generate holes in their VB, respectively, followed by charge transfer from one semiconductor to the other, the direction of which depends on the relative positions of the CB and VB edges of the two semiconductors. At the same time, the use of visible light-responsive semiconductors with suitable band edges potentials to be coupled with larger bandgap semiconductor materials, such as TiO2, in heterojunction systems is an alternative, attractive strategy to extend the sensitivity of wide bandgap photocatalytic materials, such as TiO2, and improve their functionality. In fact, in these composite systems a more effective electron
hole separation together with a potential visible light sensitization may be achieved, ensuring longer lifetime of the photogenerated charges and a

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generally enhanced photocatalytic performance with respect to the individual components [114,115]. The three main types of heterojunction architectures are shown in Fig. 6.8, where the two components (A and B) can be either n-type or p-type semiconductors. In particular, the type I heterojunction consists of two semiconductors whereby the CB of component B is higher in energy than that of A, while the VB of B is lower in energy than that of A. Therefore holes and electrons will transfer and accumulate on component A. Type II junction relies on the transfer of photoexcited electrons from B to A due to the more negative CB position of B. Holes can travel in the opposite direction from the more positive VB of A to B, leading to all-round efficient charge separation and enhanced photocatalytic activity. Type III heterojunction is identical to type II except for the much more pronounced difference in VB and CB positions, which gives a higher driving force for charge transfer [114]. For instance, coupling TiO2 with WO3 was demonstrated to limit the undesired recombination of photogenerated charge carriers, with the photoexcited electrons being efficiently transferred from the TiO2 to the WO3 CB (Fig. 6.9A). However, this also results in an undesired decrease of their reducing power [116,117]. Moreover, Fe2O3 [118] and chalcogenide semiconductors, such as CdSe [119], characterized by smaller bandgaps compared to TiO2, were employed as sensitizers of transparent oxides to extend their photoactivity in the visible region. However, similarly to WO3, Fe2O3 is not a good candidate to preserve the potential of TiO2 for overall water splitting due to its too low CB edge energy [120], while chalcogenide materials, though having an ideal CB position for H2 generation [121], are usually unsuitable because of their instability under water oxidation conditions. For instance, CdS, a visible light-responsive n-type semiconductor, has been mostly investigated when coupled with TiO2 in CdS/TiO2 composite structures consisting in either CdS NPs on the TiO2 surface or TiO2 NPs on the CdS surface. Bulk CdS decorated with nanosized TiO2 showed high rate of H2 production under visible light, though in sulfide-containing aqueous solution [122]. If cocatalyst Pt was deposited on TiO2, higher activity was attained than upon Pt deposition on

Figure 6.8 Band alignment in types I, II, and III heterojunctions. Source: Reproduced from S.J.A. Moniz, S.A. Shevlin, D.J. Martin, Z.-X. Guo, J. Tang, Visible-light driven heterojunction photocatalysts for water splitting—a critical review, Energy Environ. Sci. 8 (2015) 731
759, with permission from The Royal Society of Chemistry.

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(B)

Potenal (V) versus SHE

(A)

Figure 6.9 (A,B) Charge carrier separation by injection of one electron in the CB of one semiconductor into the lower lying CB of a second semiconductor. (B) Sensitization of a large bandgap semiconductor by a visible-light-absorbing small bandgap semiconductor. CB, Conduction band. Source: Reprinted from P. Lianos, Review of recent trends in photoelectrocatalytic conversion of solar energy to electricity and hydrogen, Appl. Catal. B: Environ. 210 (2017) 235
254, ©2017, with permission from Elsevier.

CdS, indicating that electron transfer occurred from excited CdS to TiO2 that promoted photoinduced H2 production [123,124], as shown in Fig. 6.9B. Largely investigated have been also composite catalysts, obtained by coupling TiO2 with carbon-based materials, especially with carbon nanotubes [125] and with graphene. A very high specific surface area was obtained in this case, with high mobility of charge carriers and good mechanical strength [126,127], which favor photocatalytic H2 evolution [128]. Graphene as cocatalyst on TiO2 could act as an electron acceptor due to the lower potential of graphene/graphene2 couple as compared to the CB of TiO2 [129]. In this frame, BiVO4, with its relatively narrow bandgap of c. 2.4 eV and good stability, emerged as a promising candidate to broaden the photoactivity of TiO2 possibly without significant loss of reducing ability [130], since the BiVO4 CB edge is located only c. 0.2 eV below that of TiO2 [131]. By coupling titanium dioxide with bismuth vanadate in the TiO2/BiVO4 heterojunction, visible light sensitized TiO2 can be obtained [132]. Recently, the TiO2/BiVO4 heterojunction system was found to allow TiO2 sensitization for solar energy storage applications and has the potential to carry out overall water splitting under visible light, through a counterintuitive electron transfer mechanism that significantly deviates from the prediction based on the CB edges of the two isolate semiconductors [133]. In fact, evidence was obtained that highly reducing electrons, photopromoted in the BiVO4 CB under visible light irradiation, may transfer to the TiO2 CB, as shown in Fig. 6.10. A type II, instead of the predicted type I, band alignment (Fig. 6.8), or a defect-mediated charge transport pathway, may also be invoked to account for TiO2 activation [134
136]. The TiO2/BiVO4 heterojunction might thus in principle be exploited in stand-alone composite photoanodes in cells for complete solar water splitting.

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Figure 6.10 Proposed electron injection mechanism for the TiO2/BiVO4 heterojunction under visible light irradiation. Source: Reprinted from A. Polo, I. Grigioni, M.V. Dozzi, E. Selli, Sensitizing effects of BiVO4 and visible light induced production of highly reductive electrons in the TiO2/BiVO4 heterojunction, Catal. Today 340 (2020) 19
25, ©2020, with permission from Elsevier.

6.4.5 Loading cocatalysts on TiO2 The detrimental effects of the fast recombination of photogenerated electron
hole couples in TiO2 or of the lack of appropriate reaction sites can be attenuated employing composite systems obtained by loading proper oxidation and/or reduction cocatalysts on the semiconductor oxide. In a composite photocatalyst, the cocatalysts can provide trapping sites for the photogenerated charges and promote charge separation, thus enhancing the quantum efficiency, and also improve the photostability of the photocatalyst [137]. For instance, metal oxides and metal sulfides can serve as cocatalysts for H2 photocatalytic evolution on TiO2. In particular, Ni oxides/hydroxides were found most effective, with activity depending on the loading method. For instance, an NiO/TiO2 photocatalyst prepared by a single step sol
gel procedure was found to perform better than those prepared by impregnation [138]. The higher photocatalytic activity of the NiO/TiO2 photocatalyst [139] was attributed to a p
n junction between NiO and TiO2, NiO being a p-type semiconductor and TiO2 an n-type semiconductor. This creates an electric field that promotes charge separation between TiO2 and NiO. Ni hydroxide and hydrate were identified as the active species in H2 photocatalytic production [140], which also provide active sites for proton reduction [141]. Other oxides, such as boron oxide [142] and RuO2 [143], were also reported to be effective cocatalysts for H2 evolution once they are loaded on the surface of TiO2. Such cocatalysts might work as oxidation sites, promoting photocatalytic H2 evolution indirectly, by efficiently taking out of photogenerated holes. Electrons photopromoted in the CB of TiO2 are able to reduce these metal oxide cocatalysts to metals under the photocatalytic reaction conditions, so that both metals and metal oxides may coexist on the TiO2 surface during the photocatalytic reaction.

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Titanium Dioxide (TiO2) and Its Applications

Also sulfides or oxysulfide can be efficient cocatalysts in photocatalytic hydrogen production [144,145], though usually suffering from stability problems in aqueous solution. For instance, synergistic effects were observed when MoS2 and graphene were employed as TiO2 cocatalysts [146]. The modification of TiO2 with copper also represents a strategy of great interest due to the remarkable H2 evolution rates attained under irradiation and for the low cost of copper with respect to other cocatalysts such as platinum and gold. Starting from the pioneering work of Irie et al. [147], grafting of Cu21 ions mainly on rutile TiO2 was successfully applied to get visible light activation of the semiconductor without introducing impurity levels into the bulk of TiO2. Beneficial effects were attained not only in the photodecomposition of air pollutants (e.g., acetaldehyde) or in antibacterial applications under aerobic conditions [148], but also in H2 evolution under anaerobic conditions [103]. In fact, since both the redox potential of the Cu21/Cu1 couple and the potential of the CB of crystalline CuO are less negative than the CB of TiO2, CuO domains on the TiO2 surface may capture photoexcited electrons with the consequent partial reduction of copper and beneficial charge carriers separation [149], as depicted in Fig. 6.11 [150]. The activity of Cu-containing TiO2-based photocatalysts was found to be affected by the oxidation state of copper, the dispersion and morphology of metal and/or metal oxide NPs on the TiO2 surface, including the crystallinity [107], and size of CuO clusters [151], as well as by the phase composition of TiO2 [152]. Such key parameters can be modulated during the photocatalyst preparation, and innovative synthetic routes were developed to this aim, including the use of water-in-oil microemulsions [153] or solvothermal microwave procedures [150]. At the same time, too high copper loading and calcination temperature, which are generally employed to get a solid CuO/TiO2 heterojunction, favor the formation of large CuOx deposits, with relatively low amount of metallic copper and detrimental effects on photoactivity [154]. Several studies have shown that the presence of cuprous oxide (Cu2O) on the surface of materials such as TiO2 drastically enhances the H2 evolution rate with respect to that attained with the bare material. Again, Cu2O is a semiconducting

Figure 6.11 Mechanism of charge transfer in the CuO/TiO2 heterojunction. Source: Reprinted from S.J.A. Moniz, J. Tang, Charge transfer and photocatalytic activity in CuO/TiO2 nanoparticle heterojunctions synthesized through a rapid, one-pot, microwave solvothermal route, ChemCatChem 7 (2015) 1659
1667, with permission of Wiley.

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oxide (bandgap 2.0
2.5 eV) but, differently from the case of cupric oxide, the high H2 evolution rate, in this case, is unambiguously ascribed to the features of the formed oxide
oxide heterojunction. In fact, in line with the scheme reported in Fig. 6.12, the bandgap alignment of the two oxides improves the charge carrier separation and the H2 evolution capability also under visible light irradiation [155
157]. Also in this case, a partial formation of metallic copper may occur. Moreover, a recent study on photocatalytic tests and in situ EPR spectroscopic analysis pointed out that starting Cu/oxide materials (or precatalysts) undergo dramatic modifications during irradiation under anaerobic conditions so that TiO2 modified by surface impregnation of Cu21 ions or Cu2O particles undergoes a chemical transformation with the formation of metallic copper, playing a key role in improving the overall photocatalytic H2 evolution efficiency [158]. Hybrid TiO2-based materials modified with both Cu and Pt were found to photogenerate H2 with higher efficiency with respect to the monometallic counterparts [159], generally with the aim of obtaining alloys and also exploiting their plasmonic activation under visible light irradiation and aerobic oxidation conditions [160]. Alternatively, TiO2 was modified by combining the pregrafting Cu(II) species with platinum NPs deposition under mild conditions to avoid metal alloying and/or doping within the TiO2 lattice [161]. In this system, CuO nanoclusters appear to be intimately coordinated with surface Ti atoms in a surface structure that partially stabilizes pregrafted copper in metallic form, possibly acting as an electron-transfer bridge at the interface between CuO nanoclusters and TiO2, facilitating the transfer of photoexcited electrons from TiO2 toward Pt NPs, where H2 evolution occurs (see Fig. 6.13). Synergistic effects on photoactivity in H2 production were thus induced by the copresence of Cu nanoclusters obtained through mild grafting and Pt NPs on the TiO2 surface. Similar beneficial effects were successfully obtained also by applying the same wetphase-based synthetic route to TiO2 samples prepared by flame spray pyrolysis [162].

Figure 6.12 Photocatalytic H2 production mechanism on the Cu/Cu2O/TiO2 interface under UV
visible light irradiation. OPs, Oxidation products. Source: Reproduced from I. Tamiolakis, I.T. Papadas, K.C. Spyridopoulos, G.S. Armatas, Mesoporous assembled structures of Cu2O and TiO2 nanoparticles for highly efficient photocatalytic hydrogen generation from water, RSC Adv. 6 (2016) 54848
54855, with permission from The Royal Society of Chemistry.

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Figure 6.13 Electron transfer paths, induced by UV or visible light irradiation, in the Cu(II) pregrafted Pt/TiO2 materials, based on the CB and VB edges of TiO2, in relation to the redox potential of the Cu21/Cu1 couple. CB, Conduction band; VB, valence band. Source: Reprinted from M.V. Dozzi, G.L. Chiarello, M. Pedroni, S. Livraghi, E. Giamello, E. Selli, High photocatalytic hydrogen production on Cu(II) pre-grafted Pt/TiO2, Appl. Catal. B: Environ. 209 (2017) 417
428, © 2017, with permission from Elsevier.

6.5

Sacrificial agents in photocatalytic hydrogen production: from overall water splitting to biomass reforming

Many papers in the literature report that successful hydrogen production has been achieved via “water splitting” in the presence of various kinds of sacrificial reagents, such as methanol, amines, and sulfide. However, in this case the H2 production reaction cannot be considered as a true photocatalytic “water splitting” reaction, which, by definition, implies that water is simultaneously split into H2 and O2 in a stoichiometric 2:1 ratio under irradiation in the presence of a photocatalyst. In true water splitting the electrons used for proton reduction to yield H2 must be originally provided by the water oxidation half reaction, which implies simultaneous O2 evolution. If no O2 is evolved, the reaction is not water splitting but should rather be called “H2 production” or “H2 evolution” [8]. Another key point that needs to be underlined is that true water splitting is a thermodynamically uphill reaction, while these photocatalytic H2 production reactions in the presence of sacrificial reagents may be thermodynamically favorable. In such reactions, H2 is produced with no net solar energy conversion into the form of chemical energy. However, studies on hydrogen photocatalytic production from aqueous solution in the presence of sacrificial reagents are useful to get insight into the mechanism of the photocatalytic reaction, and also for the development of efficient photocatalytic systems for overall water splitting. In fact, the simultaneous oxidation and reduction of water is an overall four-electron transfer reaction, which is usually rather inefficient when pure water is employed. Using sacrificial electron donor molecules able to efficiently fill the holes photogenerated in the VB

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remarkably increases the H2 production rate, by overcoming the kinetic limitations of the oxidation process and allowing the researchers to probe the reductive cycle selectively [163]. Furthermore, the back reaction between simultaneously evolved H2 and O2 to give water cannot occur, although reduction reactions of H2 with the products of sacrificial reagents oxidation cannot be ignored. Sacrificial reagents can be both inorganic and organic electron donors. For instance, among inorganic electron donors, sulfide, S 22, and sulfite, SO22 3 , are very efficient hole scavengers that enhance the photocatalytic H2 generation by inhibiting electron
hole recombination, which are largely used in the presence of cocatalyst CdS [164]. However, their oxidation products are strongly reductant species, which are able to reduce protons to hydrogen, thus contributing to increase the overall H2 yield [165]. Various organic compounds, such as alcohols, organic acids, and hydrocarbons, are widely employed as hole scavengers in photocatalytic hydrogen production. For instance, addition of methanol as an electron donor irreversibly reacting with VB holes results in a higher quantum efficiency in hydrogen production, but to a lower amount of energy that can be stored in reaction (6.5) as compared to true water splitting, reaction (6.4) [166]. CH3 OHðlÞ 1 H2 OðlÞ ! 3H2 ðgÞ 1 CO2 ðgÞ

ΔGr 5 16:1 kJ=mol

(6.5)

Also in this case, the first oxidation product of methanol, the CH2OH radical, as well as the first oxidation products of other alcohols (ethanol, 2-propanol, butanol, polyvinyl alcohol, etc.), are all strongly reductant species able to efficiently transfer electrons generating the so-called current doubling effect [167]. This means that at least half of the detected H2 generated when methanol is employed as sacrificial reagent is formed through the action of photogenerated holes, not that of photopromoted electrons in the semiconductor. Also formate, one of the oxidation intermediates of methanol, when used as sacrificial electron donor would lead to the formation of the very strong reductant carbon dioxide radical anion CO2 2 . Thus in the presence of organic hole scavengers, such as methanol, the reaction would better be called “methanol photocatalytic reforming”. Biomass can also be regarded as a kind of sacrificial reagent in the photocatalytic reaction of biomass reforming yielding hydrogen, in addition to functioning as the H carrier/supplier. With the ultimate goal to convert naturally abundant cellulose and lignin to H2, organic compounds, such as alcohols, aldehydes, and organic acids, can be the model of biomass compounds in laboratory scale exploration of photocatalytic biomass reforming to produce H2. The traditional catalytic technologies for H2 production from biomass, such as thermal pyrolysis, steam/oxygen gasification, and supercritical water gasification, usually proceed under harsh conditions [168]. Photocatalytic reforming of biomass and its derivatives can produce H2 under mild conditions. However, the energy stored in H2 through photocatalytic biomass reforming would make the so-produced hydrogen as a “partial solar fuel” [8], a large portion of the solar energy being already stored in the chemical bonds of biomass. G

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Titanium Dioxide (TiO2) and Its Applications

Conclusion and perspectives

Titanium dioxide
based materials to be employed as photocatalysts in hydrogen production received enormous attention in the last two decades, and big steps forward in its potential use for solar light exploitation have been done, although its main limitation related to its light absorption properties still persists. Meanwhile, other different, also composite, photoactive materials have been developed and extensively investigated, such as perovskites [169] and hybrid materials [170], which, though presenting their own drawbacks, presently appear much more promising for solar light conversion. TiO2 still remains an ideal model of semiconductor-based photocatalyst for investigating the courses and mechanisms of photocatalytic reactions, which might be of great help in the design and synthesis of more efficient photocatalysts and systems. Furthermore, mainly due to its outstanding photostability, TiO2 still remains a very attractive photoactive basis material to be employed in devices for the separate evolution of hydrogen and oxygen from water photosplitting. Indeed, after the pioneering H-type cell first proposed in Fujishima and Honda’s paper [1], great advances have been done in the development of titanium dioxide
based materials to be employed in such devices mainly by Anpo’s group [171], who optimized an RF magnetron sputtering-based deposition technique to produce TiO2based engineered thin films for solar light harvesting and conversion. Recent advances in the field consist in the use of TiO2 nanotubes [172] and of other anodization techniques to produce robust and efficient TiO2-based photoelectrodes to be employed in cells for separate photocatalytic hydrogen production [173].

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C.G. Jothi Prakash and R. Prasanth Madanjeet School of Green Energy Technology, Pondicherry Central University, Pondicherry, India

7.1

Introduction

In general, energy harvesting and storage are manifested through renewable and nonrenewable sources that put forth the study of emission of greenhouse gases. Due to global energy demand and the drawbacks of nonrenewable resources, there is a need of alternative sources for cleaner energy production which is renewable and also environment friendly. The alternative energy production with renewable sources such as biomass, geothermal, wind, solar, and biofuel provide a clean and sustainable environment [1 3]. However, the energy production with renewable sources is limited by cost and conversion efficiency. Nanotechnology opened up to be a possible solution toward solving the energy crisis through manipulation of matter in the order of 1029 m. The control over dimensional properties of matter possesses unique phenomena that enable wider range of applications. Thus nanomaterials with distinct size-dependent properties make a firm impact in energy harvesting, conversion, and storage [4 6]. Dedicated research efforts are being carried out on size and morphology-dependent properties of nanomaterials to enhance the efficiency of energy harvesting, conversion, and storage. Specifically, morphology of nanostructures such as nanotubes, nanorods, nanowires, and nanopillars is being investigated to improve the performance of energy harvesting and conversion devices [7 11]. Such nanostructures provide a better performance via enhanced surface area, reactivity, porosity, and other good mechanical properties. This chapter summarizes the recent, state-of-the-art research on TiO2-based nanomaterials for energy harvesting and storage. The chapter is organized as follows: Section 7.1.1 provides an introduction about TiO2 nanomaterials in energy harvesting applications followed by introduction to storage applications in Section 7.1.2. Next, a detailed summary of energy storage applications using TiO2based nanomaterials as supercapacitor is presented in Section 7.2.1 with primary focus on nanostructures of TiO2 and its polymorphs. Section 7.2.2 is focused on analysis and summary of polymorphs of TiO2 and its nanostructures in batteries. The hydrogen storage and other forms of energy storage using TiO2 nanomaterials are discussed in Sections 7.2.3 and 7.2.4. Section 7.3 concludes with an overall Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00016-X © 2021 Elsevier Inc. All rights reserved.

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summary of this chapter and the future perspectives of current research scenario in TiO2 nanomaterials.

7.1.1 Titanium dioxide for energy harvesting Titanium dioxide is a white oxide ceramic with different crystalline forms as brookite, anatase, and rutile having orthorhombic and tetragonal crystal structure [12 14]. Titanium dioxide nanoparticles are being used as additives in cosmetics in the form of sunscreen and skin-care products. Moreover, it possesses abundant advanced applications in solar cells, biosensing, drug delivery, etc. [15 18]. TiO2 exhibits high tensile strength and anticorrosion under extreme conditions of temperature and humidity. Due to such exciting properties, it is being utilized in automotive and other industrial applications. Out of the diverse domestic and industrial applications, TiO2 remains a superior candidate for energy harvesting and storage applications in the past decades [19 22]. Harvesting energy using titania is attractive and promising due to less environmental pollution and comparatively low cost. But the synthesis and processing conditions make it less efficient while meeting the devices. In general, TiO2 nanomaterials are used for energy harvesting through photovoltaic, photoelectrochemical cells, and hydrogen production [23 25]. Due to the technical advancements in nanoscience and technology, the device processing and conversion efficiency improvement are being facilitated with morphology-dependent investigations. Specifically, the advantage of using TiO2 nanomaterials in various device architectures reduces the amount of active materials thus lowers the processing cost without any compromise in the conversion efficiency [26]. Hence, the assembly of nanomaterials is inevitable to improve the efficiency by tuning the electronic and optical properties. The wide bandgap (B3.2 eV) of TiO2 absorbs the UV region of the solar spectrum. However, surface association with noble metals opens up applications of TiO2 in solar photovoltaics and photocatalytic hydrogen production [27,28]. However, TiO2 has certain limitations of light absorption in visible region due to its high energy. Strategies like doping nonmetal such as N, S, F, and B and coupled doping system open up pathways for absorption of light in visible region [29]. Moreover, noble metals (Pt, Pd, Ag, and Au) deposition over TiO2 induces strong absorption in visible region and improves the photogenerated charge separation owing to the surface plasmon resonance [30]. Morphology tuning of titania is being devoted for enhancing the performance of energy harvesting and storage [31]. Furthermore, the energy conversion efficiency of this material system is also dependent on the structural, optical, and electronic properties of the material, which can be tunable in many ways.

7.1.2 Titanium dioxide for energy storage Energy storage through batteries and supercapacitor is of utmost importance due to the technological development and globalization. The energy storage technologies

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with rechargeable batteries and supercapacitor play a vital role in the roving and stationary applications [27,28,32]. The energy storage in batteries is more crucial in recent years that primarily focuses on the cycle life, stability, and the toxicity of battery materials. The toxic free and high cycle life batteries are targeted to provide a sustainable environment in the transportation. Recent research interest falls under the major categories mentioned earlier. The commercialization of energy storage devices boosted up the globalization by technological advancements which can power up mobile phones, laptops, and hybrid electric vehicles. The utilization of batteries in vehicles can replace the oil-driven automotive thus improving the sustainable environment [33,34]. The unique and fascinating properties of nanomaterials that possess large surface area, active surface sites, permeability attracted have the research interest toward enhancing the energy density and power density for a better storage device. Such an improvement in the previous properties would provide wider environmental compatibility, consumer distribution, low production cost, and high safety in the associated applications [35]. The hydrogen production and storage in the form of gas, liquid, and solid could be used as a renewable fuel that is compatible to the environment and does not produce any harmful emission. But, toady 96% of hydrogen production is through fossil fuel processing and the remaining 4% is through renewable energy resources [36]. The research in hydrogen production is of recent interest to researchers through electrolysis. The field of hydrogen energy is being explored to reduce the cost of production and to enhance the efficiency of those systems for greener environment. The photocatalytic water splitting is one such technique to produce hydrogen that utilizes semiconducting materials that are photoactive. Hence, the homogeneous TiO2 and composites of TiO2 nanomaterials are under research to obtain a cleaner, low-cost, and environment-friendly production of hydrogen [37]. This chapter is intended to provide an overall introduction and a technical summary to energy storage with TiO2 nanomaterials. The associated properties of TiO2 nanomaterials in the device assembly, efficiency of energy storage, and the shortcomings are discussed in further sections. Specifically, the significance of nanostructures such as nanotubes, nanowires, and nanorods and the polymorphs of TiO2 in energy storage performance are reviewed and summarized. A flowchart on approaches of nanostructure fabrication is presented in Fig. 7.1.

7.2

Energy storage applications

7.2.1 Supercapacitors Supercapacitors are similar to a capacitor that has very high capacitance that bridges the gap between batteries and capacitors. In this process, supercapacitors are investigated as viable energy storage technology. Supercapacitor covers three general categories based on the charging mechanism; faradaic supercapacitors (pseudocapacitor), nonfaradaic EDLC (electric double-layer capacitor), and hybrid supercapacitor [38]. Irrespective of mechanism, research is targeted to enhance the

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Figure 7.1 Schematic of nanostructure fabrication techniques.

capacity of energy storage. The device construction utilizes electrodes (anode and cathode), electrolyte, and a separator that isolates the electrodes. Moreover, the choice of active materials (electrodes and electrolyte) and its physical and electronic properties is critical to achieve high capacitance. High-surface area, electronic conductivity, and strong interconnectivity of active materials influence the charge storage properties [38]. The relationship between supercapacitor performance and physicochemical properties of device is presented in Fig. 7.2. In view of electronic conductivity, graphene-based materials are being investigated as active materials that present capacitance of about 232 F/g that gives rise to higher stability of 96% [39]. However, the synthesis methods for graphene hinder the large-scale production through cost. Hence, alternative materials are being investigated in the form of nanostructured materials and its composites that address surface area, electronic conductivity, and improved interfacial properties. The research of supercapacitors is aimed at enhancing energy density without any compromise in power density [40,41]. Such energy storage devices serve as energy sources in hybrid vehicles, backup systems, and other electronic devices [34]. The impact of nanostructures and TiO2 polymorphs on supercapacitor performance is detailed in the further section. Furthermore, the physicochemical properties that hinder the storage capacity are discussed in detail. The schematic representation of a supercapacitor is depicted in Fig. 7.3.

7.2.1.1 Significance of TiO2 polymorph (brookite, anatase, rutile) The polymorph of TiO2 is being investigated due to the superior electrical conducting properties. Moreover, orthorhombic and tetragonal crystal systems in brookite,

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Figure 7.2 Physicochemical properties of supercapacitor device.

Figure 7.3 Schematic of a supercapacitor.

anatase, and rutile polymorph present a varied performance in energy storage devices. The polymorph of TiO2 is depicted in Fig. 7.4. Annealing TiO2 in atmosphere and reductive environments at high temperature between 200 C and 800 C leads to crystallization and transformation of its polymorphs. Depending upon the temperature of annealing, the relative percentage of

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Figure 7.4 Polymorphs of TiO2 (A) brookite, (B) anatase, and (C) rutile.

anatase to rutile can be deduced. Moreover, the performance of those materials in associated applications can be deduced. In the case of supercapacitor electrodes, nanotubes fabricated and annealed at different temperature exhibit a varied capacity in charge storage. In a study conducted by Salari et al. [38], change in electrical conductivity is attained through annealing TiO2 nanotubes in air and argon atmospheres. The nanotube annealed in reductive environment shows the origin of rutile polymorph of TiO2. Furthermore, annealing in reductive environment alters the state from Ti41 to Ti31. The capacitance of titania annealed in air and argon atmosphere reads 30 and 521 µF/cm2. In an extensive study, TiO2 nanotubes annealed at high temperatures ranging from 450 C to 650 C. The crystallization process resulted with mixed phase of anatase and rutile with relative increase in percentage of rutile at high temperatures. The storage capacity of anatase TiO2 reads 1.1 F/g and that of rutile reads 7.6 F/g. The reason behind this enhancement is found to be dependent on the ratio of Ti31/ Ti41. During the transformation of Ti41 to Ti31, the vibrations of rutile dominate the suppression of anatase phase. Therefore the rutile polymorph of TiO2 exhibits higher conductivity and lower charge-transfer limitation for a better charge storage capacity [42]. The areal capacity of doped and undoped anatase titania reads 20.08 and 0.97 mF/cm2 at a current density of 0.05 mA/cm2. Thus electrochemical doping paves way for an ultrahigh conductivity through introduction of interstitial hydrogen ions and oxygen vacancies thereby increasing the storage capacity [43]. In a similar study, electrochemical doping of TiO2 nanotube structures enhanced the areal capacity by the influence of decrease in charge-transport resistance [44]. The conversion of TiO2 polymorph from anatase to rutile is dependent on certain crystallization conditions such as atmosphere, time, and temperature. Annealing in reductive atmosphere for varied time periods resulted with conversion of TiO2 polymorph from anatase to rutile. The annealing treatment for 5 h in argon atmosphere

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leads to partial oxygen loss resulting with increase in lower valence Ti31 ions. The areal capacitance is read as 1620 µF/cm2 with capacitance retention of 97% for testing over 1000 cycles [45]. In a different approach, Pan et al. [46] fabricated that randomly oriented and c-axis-oriented nanotubes show good capacitive behavior with areal capacitance of 0.32 and 11.44 mF/cm2. The c-axis-oriented nanotubes presented an excellent capacitance retention of 86% for 5000 cycles. The obtained enhancement is found to be dependent on the carrier density that arises due to the high concentration of oxygen vacancies. Rutile polymorph exhibits a superior conductivity, and reduced charge-transfer limitations make it a better candidate for energy storage applications. Moreover, due to high chemical stability and excellent coulombic efficiency, rutile TiO2 holds a promising position in energy storage applications [47]. The performance of TiO2 nanomaterials with brookite polymorph is deficit in supercapacitor applications due to its limitations in capability of storage driven by the ionic conductivity and mass transport. Furthermore, the most studied forms of TiO2 remain with anatase and rutile, while the studies of brookite polymorph of TiO2 are still scarce.

7.2.1.2 Significance of nanostructures (nanotubes, nanorods, nanowires) Nanostructures of TiO2 in the form of tubes, rods, and wires possess a varied performance in energy storage devices. High specific capacitance of nanostructured electrodes can be attained by tuning the morphology to attain higher specific surface area, electron transport, and ionic conductivity. One-dimensional (1D) nanotube array of TiO2 offers a highly ordered arrangement of pores that facilitate efficient mass transport through the pores. Nanotube electrodes possess better morphological properties rather than nanoparticles and its other morphologies such as flowers. In a study carried out by Kim et al. [48], the dependence of nanostructure is primarily focused to enhance the specific capacitance. The nanotube-based electrodes exhibited 4 times higher charge density than the nanoparticle-based electrodes. The areal capacitance of nanotubebased electrodes is read as 200 F/g at a discharge rate of 0.5 mA/cm2. It is evident that the crystallinity and the length of nanotubes favored the storage capacity by improved charge-transfer characteristics. Furthermore, the studies carried out by Patil et al. [49] in nanotube-based electrode with a diameter of 80 nm and 18 µm in length reveal a maximum specific capacitance of 33 F/g. In a study carried out by Raj et al. [31], the geometry of nanotubes exhibited a varied performance in specific capacitance. Thus porous nanotube electrode with diameter of 110 nm and length of 4.95 µm exhibits an areal capacitance of 14 11.1 mF/cm2 with a current density value ranging from 0.1 to 1 mA/cm2. Similarly, Salari et al. [50] observed capacitance in the range of 535 911 and 33 181 µF/cm2 at scan rates of 100 1 mV/s for nanotube and nanoparticle structures, respectively. The enhancement in specific capacitance is due to the high surface area and interconnectivity of active materials. The highly ordered tube structures provided direct pathway to the electron transfer resulting in a high

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capacitance supercapacitor electrode. Thulasi et al. [51] studied the supercapacitor performance in pristine nanotubes and ceria-deposited nanotubes. Pristine nanotube electrodes with a specific capacitance of B6.82 mF/cm2 are enhanced to B55.28 mF/cm2 by depositing 0.075 M ceria. In addition to open pores, the interaction of liquid electrolyte takes additional attention to a barrier-less ionic transport between the electrode and electrolyte. The interaction of electrolyte and its intrusion in porous materials is found to be dependent on enhancing the performance of a supercapacitor. The electrolyte-wetted area due to liquid intrusion increases the active surface area of electrode/electrolyte interface. Therefore the percentage of wetted surface area strongly influences ionic conductivity and mass transport from the electrolyte to nanostructured electrodes [52,53]. Nanotube electrodes in a physiological fluid-based electrolyte system exhibit a specific capacitance of 1700 µF/cm2 at a current density of 1.5 µA/cm2 [54]. The specific capacitance of nanotube-based electrode in 1.2 M LiPF6 electrolyte shows a high value of 5.12 mF/cm2 at a current density of 100 µA/cm2 [55]. The approach of posttreatment of nanotube electrodes such as hydrogenation and doping substantially improves the charge storage capacity. The enhancement by adopting such strategies improves the ionic conductivity and mass transport of the electrode materials. Hydrogenated nanotube electrodes exhibited a specific capacitance of 3.24 mF/cm2 at a scan rate of 100 mV/s. The increased density of donors and the surface hydroxyls improved the storage capacity with a maximum reduction of 3.1% after 10,000 cycles [56]. Similarly, Wu et al. [43] observed an enhancement in specific capacitance of 5.41 mF/cm2 at 0.05 mA/cm2 for hydrogenated nanotube electrode Whereas, the electrochemically doped nanotube electrode shows an average specific capacitance of 20 mF/cm2 at the same charge density. Electrochemical doping introduces oxygen vacancies and thus increases the electronic conductivity for better energy storage. Nanowire array electrode was utilized to study the asymmetric supercapacitor performance. The electrode exhibited a higher areal capacitance of 121 mF/cm2. In addition, the supercapacitor device provides a cell voltage of 2.3 V that is sufficient to illuminate an LED for 10 min. The report provides an evidence of nanowire array electrodes in the real-time applicability [57]. Similarly, Tang et al. [58] fabricated asymmetric supercapacitor with hydrogenated TiO2 nanowire arrays. An areal capacity of B258.7 mF/cm2 is observed with hydrogenated nanowire electrode and is high comparing with pristine nanowire electrode that exhibits 28.3 mF/cm2. This device gave rise to 2.4 V as the cell voltage that illuminated LED for a period of 40 min. The obtained results are ascribed to optimized electronic conductivity and better ionic transport through nanowire array electrodes. Xu et al. [59] fabricated TiO2 nanowire array over titanium aluminum alloy as supercapacitor electrodes. The electrodes exhibited an excellent specific capacitance of 60 mF/cm2 with capacitance retention of 97% even after 3000 cycles.

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Nanomaterials of TiO2 in electrochemical supercapacitors exhibit certain areal and specific capacitance values. The enhancement in specific capacitance of nanostructured electrodes is due to ionic conductivity through pores, surface area, and its inherent conductivity. Recent research is focused to enhance the ionic and electrical conductivity through doping, nanocomposite and hydrogenation strategies. The specific-capacitance performance by adopting earlier strategies shows improvement and consequently, mass transport is dependent on the structure of nanomaterial.

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7.2.2 Batteries Due to the advantages of high energy density, reasonable low production cost, cycle life, and stability and the ease of flexible design make batteries a potential candidate in energy storage applications. Currently, research is aimed at developing anode materials for batteries with reduced weight, miniaturization and hence to improve the performance of those devices [60]. In a typical Li-ion battery, Li-ion insertion and extraction occur at electrodes during charging/discharging process. The diffusion length of Li-ion and faster ion transport contribute to rate capability of batteries. The schematic of battery (Li-ion) and its working are depicted in Fig. 7.5. However, the applicability of TiO2 in commercial Li-ion batteries is hindered by the poor electronic conductivity and sluggish ion diffusion. Efforts are being devoted to increase the electronic conductivity and hence to improve the charge-transfer kinetics via fabricating nanostructured TiO2 materials [61]. Fabrication of such structured materials decreases the diffusion length of ion and hence enhances the faster Li transport ensuring a good cyclic stability. Hence, exploitation of anode materials with excellent reversibility and favorable long-term cycling stability is inevitable. The implication of TiO2 polymorph on electronic conductivity and ionic transport properties for the betterment of storage capacity is detailed later.

7.2.2.1 Significance of TiO2 polymorph (brookite, anatase, rutile) The performance of TiO2 polymorphs as anode materials in energy storage devices is widely investigated due to certain advantages. The advantages are accounted with superior reproducibility in ion insertion/removal, better stability in capacity

Figure 7.5 Schematic of a Li-ion battery.

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retention, enhanced electron transfer kinetics, and negligible change in volume during cycling [62]. For the first time, Reddy et al. [63] investigated nanocrystalline brookite TiO2 in Li-ion batteries. The electrochemical study exhibits a storage capacity of about 170 mAh/g that is found to decrease gradually upon cycling. It is reported that the obtained capacity is higher comparing with the other polymorphs. The decrease in capacity with cycling is attributed to partial amorphization of TiO2 during Li insertion and/or poor electronic conductivity. In an extensive study, the authors investigated the dependence of crystallite size of brookite in the charge storage performance [64]. The storage capacity appears to decrease with increase in crystallite size from 10 to 36 nm. It proposed to extend the study to establish an exact mechanism behind the observed storage capacities. In a study conducted by Lee et al. [65], brookite- and rutile-type TiO2 has been investigated for charge storage. The capacity of brookite read 100 mAh/g and for rutile it read 49 mAh/g. The electrochemical performance could not be related to particular property as the surface area and crystallite size of those polymorph varies. Nanostructured TiO2 with metastable anatase and stable rutile phase is being investigated as electrodes in batteries. The brookite polymorph of TiO2 appears to be more challenging for preparation. Due to the difficulty in synthesis, only a few studies are devoted toward its applicability in anode materials of charge storage devices [66]. The charge capacities of nanotubes and nanorods with anatase polymorph were observed to be 256 and 183 mAh/g. The lesser capacity of nanorods is attributed to the decrease in surface area [67]. Furthermore, it is concluded that at relatively low electrode densities, no fade in capacities is observed. Nanorods of anatase polymorph exhibited high initial capacity of 266 mAh/g and that dropped to 212 mAh/g in the consecutive cycle. The capacity presents a favorable cycling ability while charging and discharging. The small interconnected nanoparticles formed to provide a network structure addressed high specific surface area as well as nanopore structure for efficient charge transfer [68]. In a different study, Han et al. [69] fabricated hybrid anatase/rutile nanorods with [0 1 0] exposed facet. The nanorods were about 300 nm in diameter and varied length between 2 and 5 µm. The study demonstrated 99% of its initial capacity with a consecutive cycling of 100 charge discharge cycles. The study is aimed at comparison of commercially available P25 nanoparticles that exhibited a lower capacity compared with the presented hybrid nanorods. In addition, the hybrid nanorods have surface area typically less than the P25 nanoparticles. The complex behavior obtained here is attributed to the [0 1 0] exposed facet that contributed to enhancement in electrochemical properties [69]. This study brings in an insight into the development of anode materials focused on crystallinity and exposed facet. Rutile TiO2 nanorods grown along [0 0 1] axis is shown to have a reversible capacity of 242 mAh/g with an excellent cycle life. The preferentially oriented nanostructures of rutile polymorph are reported with a 97% retention after 50 cycles at different cycle rates [70]. Recent study focused on synthesis and performance analysis of mesoporous rutile TiO2 for charge storage performance. The increase in processing temperature of mesoporous TiO2 resulted with decrease in surface area

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in the range of 115.29 1.64 m2/g. The rutile TiO2 processed at 300 C exhibits a higher capacity of 457 mAh/g and higher surface area of 115.29 m2/g. The mesoporous rutile titania exhibits higher reversible charge/discharge high cyclic stability [71]. Here, the superior performance in storage capacity is attributed to surface area and mesoporous nature owing to increased active sites. Above all, the research reports explained the importance of developing nanomaterials with increased electrode/electrolyte contact area and a shortened ion diffusion length. The investigations were reported with possible ways to fabricate nanomaterials of TiO2 with its different polymorphs that resulted with an increase in the abovementioned properties. In addition, fabrication of nanostructures in a preferential crystal orientation further enhanced the storage capacity. The rate of ion insertion/extraction in the nanochannels facilitates enhanced storage capacity. Theoretical simulations based on preferential orientation resulted with ion diffusion coefficient of c-axis and ab-plane as 1026 and 10215 cm2/s. Hence, recent research interest is focused on enhancing the electronic conductivity, ionic diffusion thus enhancing storage capacity [72,73].

7.2.2.2 Significance of nanostructures (nanotubes, nanorods, nanowires) The technology of developing nanostructured TiO2 is still in progress to enhance the physical, chemical, and electrochemical properties. The primary research target is devoted to enhance the efficiency of charge storage by tuning the physicochemical properties. In this persist, porous nanostructured electrode gets additional attention due to its superior conductivity that enhances the kinetics during lithiation reaction. Nanotube structures of TiO2 serve as a better candidate owing to its short ion-diffusion pathways and tolerance to structural changes [74,75]. The self-organized nanotubes with control over nanotube morphology could be fabricated with cost-effective anodization technique that can go to a thickness of B260 µm with maximum tube diameter of B200 nm. By controlling the morphology and aspect ratio of nanotubes, the performance of energy storage devices could be tuned accordingly. Furthermore, thermal annealing of amorphous TiO2 under reductive atmosphere gives rise to oxygen-deficient polymorph which is believed to enhance the charge-transfer properties. Furthermore, such polymorph possesses higher rate capability compared to oxygen-rich TiO2 polymorph [75]. It is seen that organized nanotubes can increase the capacity to six fold at a current density of 10 C comparing with randomly oriented nanotubes. This performance is ascribed to good electrical contact between titanium substrate and the nanotube layer. In addition, the high surface area enables good contact between electrode and electrolyte thereby reducing the charge-transfer resistance [62]. Similarly, free standing nanotube membranes are utilized as anode materials in microbatteries that exhibit 184 mAh/g with a maximum loss of 6% for over 500 cycles [76]. Technically feasible methods for the fabrication of morphology-controlled nanostructured materials are detailed in Fig. 7.6. Anodization is used to induce native oxides over the metallic working electrode. However, nanotubes and porous thin films can be fabricated

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Figure 7.6 Schematic of electrochemical methods for nanostructure fabrication.

by tuning the electrochemical conditions. Electrochemical conditions typically cover the electrolyte part, applied potential, and duration of anodization. Furthermore, the synthesized nanopores and nanotubes are used as template to fabricate nanowires, nanorods, and nanofibers using electrodeposition technique. The details of electrodes and nanomaterials concerned with synthesis techniques are presented in Fig. 7.6. The charge storage kinetics in preferentially and randomly oriented TiO2 nanotube surface is studied. This study demonstrated an enhanced charge-transfer kinetics in [0 0 1] preferential orientation than a randomly oriented nanotube array. So, the orientation in this direction facilitates ion insertion and extraction thereby enhancing the reversibility and rate capability of charge storage [74]. In a similar study, Wei et al. fabricated nanotubes and the performance of storage capacity is investigated. The study exhibited 170 mAh/g reversible capacity with an excellent capacity retention of 97% for about 350 cycles [77]. Nanotubes fabricated at different voltages have been tested for energy storage. It is observed that nanotubes fabricated at 40 V exhibited a higher storage capacity of 198 mAh/g with good cyclic stability beyond 150 cycles. Meanwhile in this study, the significance of compact layer at nanotube/Ti foil interface is more focused as it hinders the transportation of ions. The critical mass ratio of compact layer to nanotubes is determined to be 0.14 beyond which the storage capacity tends to diminish [78]. Hence, it is obvious that the storage capacity of nanotubes is purely dependent on morphology and aspect ratio. Furthermore, the compact barrier layer too provides an insight of attaining the critical thickness to achieve higher storage capacity. SEM images of the nanostructures is depicted in Fig. 7.7. Higher reversible storage capacity of 180 mAh/g is achieved with nanorod-based microspheres. The fabricated microspheres are comprised with nanorods of length 250 and 50 nm diameter. The capacity is attributed to electron transport due to hierarchical conductive pathway through the rods’ structure [81]. TiO2 nanorods with

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Titanium Dioxide (TiO2) and Its Applications

Figure 7.7 SEM images of (A) nanotubes, (B) nanorods, and (C) nanowires [59,79,80].

diameter in the range of 40 50 nm and length of few micrometers drop its specific capacity from 290 to 94 mAh/g within 20 cycles and remain constant throughout the 100 cycles. The performance of charge storage is attributed to surface area, shortened ion diffusion path, and enhanced electrical conductivity [82]. Lee et al. [83] investigated the charge storage performance with nanorod and nanowire structures. It is observed that nanorods in the length of B2 4 µm exhibited a specific capacity of 146 mAh/g, whereas nanowires with typical length of .10 µm exhibited a specific capacity of 102 mAh/g. The performance in storage capacities is due to the diffusion rate of ions for rod and wire structures. The typical diffusion rate is read as 29.52 3 10215 and 8.61 3 10215 cm2/s for rod and wire morphology. Moreover, higher diffusion rate in rod structure is dependent on the length of the channel that facilitates reduced diffusion path and surface area that provides efficient electrode/electrolyte interface. In a study conducted by Shim et al. [84], hydrogenated titanate nanowire structures exhibit an excellent storage capacity of about 145 mAh/g even after 500 cycles. As discussed earlier, the observed results are attributed to the electrode/electrolyte active surface area and the diffusion path of ions. The reported capacity is closer to the capacity reported by Lee et al. [83] for nanorods with higher diffusion rate. In this approach a comparison of TiO2 polymorphs (TiO2-B and anatase) in the form of nanowire arrays has been tested for lithium ion storage capacity. The specific capacities of TiO2-B and anatase nanowire nanomaterials are read as 223 and 183 mAh/g, respectively. It is reported that annealing treatment at 700 C tends to loss its nanowire morphology [85]. These studies provide an insight of structure and morphology dependence in the performance of charge storage devices. Moreover, the capacity of charge storage could further be enhanced by adopting certain approaches such as doping and altering the polymorph of TiO2. Furthermore, fabrication of porous composite anode materials can enhance the electronic conductivity and hence the ionic transport. Ragone plot for electrochemical energy storage devices is presented in Fig. 7.8.

7.2.3 Hydrogen production and storage The increase in energy demand and global warming leads to challenges of nonreliance on fossil fuels for energy generation. Such a challenge can address issues

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Figure 7.8 Ragone plot of electrochemical energy storage devices [86].

related with the generation of energy and its transportation. In this hunt, it is proposed that hydrogen energy could be a possible replacement of fossil fuels. The common characteristics of hydrogen such as production from water, its storage in the form of liquid, gas, or solid, and its transportation over long distances make it more efficient than any other fuel. In addition, the compatibility with environment since its production to utilization does not produce any harmful gases and any other pollutants [87]. But, the production of hydrogen was dependent on fossil fuels that produce carbon monoxide and other harmful pollutants such as NOx/SOx. Hence, hydrogen production and storage from a clean source remain as primary target to address energy demand and environmental sustainability. Production of hydrogen from water through photocatalysis is being studied for more than two decades. Because of the availability, low cost, and chemical stability, TiO2 remains as a better candidate to produce hydrogen in a cleaner way. However, its bandgap (B3.2 eV) remains a major drawback as it is active in the UV region of solar spectrum that accounts to only 4%, while 50% of spectrum is in the visible region [87]. This reason initiated the incorporation of chemical and surface modifications to tune the bandgap of TiO2 to be active in the visible spectrum. The schematic of photocatalytic activity of water is depicted in Fig. 7.9. After a pioneered work from Fujishima and Honda, the area of photoelectrochemical water splitting for hydrogen production is largely investigated [88]. The research attempts to increase the region of absorption and suppress rapid recombination of electron hole pairs. In this approach the polymorph and nanostructures of TiO2 are being investigated for better charge separation and reduced recombination rate. For an effective photocatalysis the carrier diffusion length/ recombination lifetime should be much longer. Therefore the photo-driven carrier

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Titanium Dioxide (TiO2) and Its Applications

Figure 7.9 Schematic of water-based photocatalytic hydrogen production.

would have faster migration rate and lesser recombination rate. This brings in the investigation of hydrogen production on the basis of nanostructured materials that pave the way for faster migration of photogenerated carriers. In this context the assembly of building units in the form of nanostructures allows electrolyte diffusion, electron transfer pathway to provide functionalized chemical, physical, and biological properties [89 92]. In general, nanostructures are defined by the dimensions as 0D, 1D, 2D, and 3D [93]. Further classification of structures is detailed in Fig. 7.10. The morphological and dimensional properties of nanomaterials attracted enormous interest due to its superior performance in charge carrier transport and an unexceptional surface activity. Hydrogen production with TiO2 nanobelts is read as 2.41 m22/h, which is higher than commercially available P25 nanoparticle that is read as 1.91 m22/h [94,95]. In a comparison with 1D nanostructures, nanorods and nanowires produce a higher H2 due to close interparticle interaction with a controllable route for charge transfer. Such controlled morphologies facilitate highly conductive route, outstanding light-arrest, and a larger contact area of electrolyte for charge carrier collection [94,95]. Furthermore, doping of nonmetals brings in a strategy to lower the photothreshold of nanomaterials [96,97]. Among the nonmetals, N-doping is studied extensively to tune the absorption toward visible region [98]. Yang’s group fabricated mesoporous nanofiber morphology to study the significance of nonmetal doping effect on photocatalytic activity. The study demonstrated nearly 40 times enhancement in photocatalytic activity in comparison with untreated TiO2 [99,100]. Similarly, other nonmetals such as C, S, and Se could also be introduced in the lattice to narrow the bandgap and enhance the visible light absorption [101 104]. The

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Figure 7.10 Classification of nanostructures.

studies prove to be more focused to enhance photocatalytic hydrogen production activity by adopting the aforementioned surface and material engineering aspects. Such an engineering results with enlarging the accessible active surface area, bandgap reduction, and crystallinity optimization. A study presented by Dozzi et al. [29] investigated the effect of doping p-block elements on the photocatalytic activity of TiO2. A detailed investigation on the effect of photocatalytic activity of doping TiO2 with C, N, F, and B revealed the physicochemical properties involved in enhancing the efficiency of photocatalyst. The limitations of doping TiO2 with single nonmetal and codoping over photocatalytic activity are outlined. It is concluded that the surface effect of fluorination plays an important role to enhance the photo-induced oxidation reaction rate. Hence, N/F codoping with TiO2 modifies the surface properties with increased defect sites that is beneficial for adsorption of molecules. Moreover, it favors surface trapping of photogenerated electrons that results in enhanced carrier separation that results with increased photocatalytic activity. A similar study reported by Asahi et al. [105] investigated the significance of nonmetal doping in photocatalytic activity with density functional theory calculations. The comparison of density functional theory with experimental results is shown to have good correlation with doping of N, S, F, and B. The study is shown to report an enhanced visible light photocatalytic activity with N-doping. Recently, Xia et al. [106] presented an enhanced visible light photocatalytic activity with N-doped TiO2 nanotubes. Eventually, Kong et al. [107] demonstrated 17.7 times enhanced photocatalytic activity with N-doped TiO2, while few other researchers report conflicting results with non-metal doping that lead to debate on the contributory factors. However, doping TiO2 with fluorine induces surface traps that originate longlasting PL signal. Annealing at varied temperatures (500 C 700 C) suppresses the

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Titanium Dioxide (TiO2) and Its Applications

phase transformation from anatase to rutile in F-TiO2. Hence, it is concluded that the enhanced photoactivity arises due to the increase in surface traps. The timeresolved photoluminescence spectra are served as an evidence of how photocatalytic activity is related to the amount of surface trap sites [108]. From the previous discussion, it is evidenced that doping of F nonmetal and codoping enhance the photocatalytic activity through such an induced surface defect site. Furthermore, engineering the crystal facet will have significant impact on the carrier kinetics. The interfacial energy of TiO2 facets [0 0 1], [1 0 0], [1 0 1] are 0.90, 0.53, and 0.44 J/m2. Owing to the higher energy, [0 0 1] facet of anatase demonstrates a remarkable photocatalytic activity [109 113]. The effect of anatase nanocrystals with [0 0 1], [1 0 1], [0 1 0] exposed facets toward boosting photoefficiency is investigated by D’Arienzo et al. [114]. The study presented maximum H2 production for nanocrystal that displays the highest area of [0 0 1], the lowest area of [1 0 1], and significant area of [0 1 0] facets. From the observation, they suggested that the contiguous facets of [0 0 1], [1 0 1], and [0 1 0] form an effective heterojunction. This heterojunction drives the photogenerated electrons toward [1 0 1] and also to [0 1 0], while holes are driven toward [0 0 1] facet. Such an exposed facet arrangement is suggested for efficient charge separation thus attaining higher photoefficiency [114]. A similar enhanced photocatalytic activity with coexposed [0 0 1] and [1 0 1] facet has been reported by Jiaguo et al. [109]. However, introduction of excess electrons by oxygen vacancies and hydrogenation can increase the region of absorption from UV to visible [115]. The synergistic effect of crystal facet and oxygen vacancy engineering resulted with a higher H2 production than an untreated TiO2 [116]. In addition, noble metal nanoparticle deposition over TiO2 plays a vital role in photocatalytic hydrogen production. In this context, Chiarello et al. [30] investigated Pt- and Au-deposited TiO2 for hydrogen production. The observations present enough charge separation for the electron transfer to take place thereby enhancing the efficiency of hydrogen production. Similarly, recent studies with noble metal (Pt, Pd, Ag, and Au) deposition enhanced photocatalytic activity and hydrogen production [117,118]. The aforementioned obstacles of inferior absorption in visible region and recombination of electron hole pairs still impede the further development of TiO2 in hydrogen production. Numerous strategies have been adopted to modify the surface and nanomaterial properties resulting with a mere enhancement in photocatalytic activity. Hence, thorough insights over the photocatalytic mechanism and chargetransfer kinetics are indeed essential to overcome the efficiency limiting factors.

7.2.4 Others The energy harvesting, conversion, and storage are the critical aspects of current research [119]. Dye Sensitized Solar Cells (DSSCs) and perovskite solar cells (PSCs) have attracted considerable research interest because of its applicability toward sustainable environment. The electron transfer through TiO2 film and dye loading are more important in DSSCs. The electron transfer passes through two

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interfaces: (1) dye/TiO2 and (2) TiO2/fluorine doped tin oxide. Due to such electron transfer process, the interfacial engineering is concentrated to reduce the loss of photocurrent. Similar to energy storage devices of TiO2, the morphology and crystallinity of nanomaterials contribute to enhancement in photo-conversion efficiency (PCE). Nanostructures such as tubes, rods, wires, and fibers are widely investigated and reported so far [120 124]. Such 1D nanostructures facilitate direct pathways for the electron transfer and contribute to enhancement in PCE. A comparison study of morphology-dependent DSSC performance has been investigated by Desai et al. [119]. By comparing with nanorods, 3D nanoflowers exhibit higher surface area for dye loading and light absorption. The photo-electrochemical cell of nanorods is 5.82%, while nanoflowers exhibit 14.21%. Irrespective of nanostructure, the dye monolayer can insulate the surface of TiO2 from approaching of the electrolyte to prevent from the electron recombination. Hence, in-depth approach of dye-TiO2 interactions is utmost important to exploit and optimize the performance of DSSC. The crystallinity of TiO2 is also crucial for effective and efficient DSSC operation [125]. In DSSCs, anatase is mostly preferred because of its higher bandgap and band edge energy comparing with rutile. In a comparative study of anatase- and rutile-based TiO2, the open circuit voltage is the same at around 730 mV but the short-circuit current density of anatase is 30% lower than rutile polymorph. This resulted with energy conversion efficiencies as 5.6% and 7.1%, respectively. The obtained efficiency is attributed to total void volume that is large in rutile. This gives rise to lesser surface area per unit volume of 25% compared with anatase TiO2. Moreover, the lower photocurrent observed for rutile is due to the lesser amount of adsorbed dye. The slower electron transport in rutile layer arises owing to the interparticle connectivity which is confirmed with intensity-modulated photocurrent spectroscopy [126]. An extended investigation over band edge position of anatase and rutile polymorph in such cases is inevitable. In recent years, PSCs have gained a reasonable interest due to certain optoelectronic properties such as high absorption coefficient, large carrier diffusion length, high carrier lifetime, and tunable bandgap in the visible range [127 130]. Reducing the interface defects and tuning the composition of perovskites resulted with an improvement in efficiency from 3.8% to 22.7%. PSCs exhibit an impressive conversion efficiency; but long-term stability and production cost are found to be the key issues that remain as primary objectives for recent research. Furthermore, the decay in stability of PSCs in ambient atmosphere remains a hindrance in commercialization [131,132]. In PSC devices, mesoporous and planar structures are investigated. The presence of large voids in the scaffold layer paves the way for a better penetration of perovskite precursor solution. The highest conversion efficiency and repeatability so far reported are based on the mesoporous TiO2 [133]. In this regard, Ye et al. [134] observed enhancement in PCE by utilizing mesoporous TiO2 as the electron-transport layer (ETL). The significance of mesoporous TiO2 is compared with a compact TiO2 as ETL. The composite of compact and mesoporous TiO2 exhibits 15.12% conversion efficiency. The device retains over 89% of the initial PCE over 1000 h of storage in dry air demonstrating an excellent stability.

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Figure 7.11 Flowchart of thrust areas in nanomaterial synthesis and processing approaches.

Optimization of structural, mechanical, and optoelectronic properties of ETL paves the way for alteration in electron-transfer kinetics. In this regard, high surface area mesoporous TiO2 is being investigated to reduce the interfacial resistance between ETL and the perovskite. Such approach results with an enhancement in conversion efficiency as well as device stability in the open atmosphere.

7.3

Conclusion and outlook

Based on the research reports toward energy conversion and storage, certain properties of materials, surface, and the interface are given importance. Moreover, those properties significantly modify the transport behavior in the energy conversion and storage devices. Modifying the material properties by adopting the aforementioned fabrication and processing techniques resulted with enhancement in efficiency of energy conversion and storage. Regardless of the mechanism, it is clear that the interfacial properties (active surface area, wettability of electrolyte) of electrode and electrolyte contribute more toward the efficiency enhancement. A flowchart explaining the focus areas of TiO2 nanomaterials and its associated properties is depicted in Fig. 7.11. However, there are still challenges existing with production and stability of devices. Each research report experiences certain limitations and drawbacks that get additional attention. Hence, investigation of the transport properties with respect to material modifications needs to be improved. Although there are some devices that are commercially available, there is a demand for new devices that are highly efficient and stable in harsh conditions.

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Heat transfer by using TiO2 nanofluids

8

Vittorio Loddo1 and Giovanni Camera Roda2 1 Department of Engineering, University of Palermo, Palermo, Italy, 2 Department of Civil, Chemical, Environmental, and Materials Engineering, University of Bologna, Bologna, Italy

List of abbreviations BHT CHF CTAB CVD EG IEP Nu PG Pr PVD Ra Re SDBS SDS W

8.1

boiling heat transfer critical heat flux hexadecyltrimethylammonium bromide chemical vapor deposition ethylene glycol isoelectric point Nusselt number propylene glycol Prandtl number physical vapor deposition Rayleigh number Reynolds number sodium dodecyl benzene sulfonate sodium dodecyl sulfate water

Introduction

Heat transfer unit operations are essential in industrial processes. Very often they utilize liquids as heat transfer media, because of the relatively high convective heat transfer coefficient that can be obtained with liquids when operating at advantageous hydrodynamic conditions. However, some heat transfer properties of liquids, such as thermal conductivity and thermal diffusivity, are less favorable than those of solid metals or metal compounds. Therefore it is expected that the suspension of solid particles in a liquid may enhance heat transfer. However, some problems are observed in suspensions with milliparticles or microparticles, such as abrasion, clogging, and fouling, which are also a consequence of the poor dispersivity, and of the tendency to aggregation and sedimentation. Ultimately, these phenomena lead to a reduction of heat transfer performance and to an increase of the pressure drops. Nonetheless, recently the increased availability of a new class of materials, Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00015-8 © 2021 Elsevier Inc. All rights reserved.

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Titanium Dioxide (TiO2) and Its Applications

nanomaterials, made possible to direct the interest toward “nanofluids” that is a new type of suspensions with solids in the form of nanoparticles. Actually, they offer the opportunity of overcoming most of the drawbacks [1] encountered with larger particles. On the other hand, it is well known that the properties of a slurry largely depend on the size of the suspended particles. Moreover, some physical material properties of the same particles may change when they are nanosized. Indeed, solids in the micrometer scale generally have the same physical properties of their bulk form, whereas the same material, when used as nanopowders, exhibits different (often superior) thermal, optical, mechanical, electrical, and magnetic properties. The exceptionally high surface to volume ratio obtainable with nanoparticles is the main reason of these special properties. In practice, the design, fabrication, and applications of nanostructures and nanomaterials are the core of nanotechnologies, which, thanks to the accessibility to these new properties, are reforming the way materials can be utilized in many fields. Conductive heat transfer of a substance depends on the molecular agitation with the temperature which affects the energy and consequently the speed of molecular motion. With a nanofluid, heat can be transferred faster and more efficiently mainly thanks to the modification of the heat conductivity with respect to the fluid without the nanoparticles (base fluid). Table 8.1 shows the conductivities of some different materials [2]. In the last decades, there has been intensive research on the behavior of substances that contain extremely small particles. Nanotechnology is the science and engineering of working at the nanoscale, where the individual particles are 1
100 nm in size [2
4]. Generally, the procedure of preparation of nanofluids consists in suspending nanoparticles (average sizes below 100 nm) in conventional heat transfer liquids such as water, ethylene glycol, and oil. Although water is widely used as a thermo-vector liquid, under some points of view it is not a perfect base fluid. Fig. 8.1 shows the growing trend in the number of publications containing the terms “nanofluids or nanofluid” in the title [5]. This figure clearly illustrates that the research of nanofluids is growing really fast. Nanofluids may have important advantages over conventional heat transfer fluids. Indeed, very small amounts of nanoparticles, dispersed uniformly and stably suspended, can strongly enhance the thermal properties of fluids. Thermal properties generally increase with decreasing the particle size of the solid, allowing the use of nanofluids as thermo-vector fluids due to their enhanced thermal conduction and stability, which can be exploited in thermal convection and boiling heat transfer (BHT) applications. In the last years, different studies on thermal physical properties have been published [6
9], reporting also some potential benefits and applications of nanofluids. Among the different nanofluids that have been considered for heat transfer processes in thermal systems, TiO2 nanofluid presents some advantages. In fact, even if TiO2 does not have a high thermal conductivity compared to other materials (e.g., graphene), it is cheap and chemically stable and is currently considered safe for human being and animals. These features make TiO2 suitable for many large-scale applications, such as

Heat transfer by using TiO2 nanofluids

269

Table 8.1 Thermal conductivities of various materials and nanofluids. Material type

Material

Thermal conductivity [W/(m K)]

Thermal diffusivity (m2/s) 3 105

Solid metal

Silver Copper Aluminum Steel Carbon (nanotubes) Carbon (diamond) Carbon (graphite) Carbon (fullerenes) Silicon Alumina Titanium dioxide Sodium

429 401 237 45 2000

16.56 11.8 9.79 1.17

Solid nonmetal

Metallic liquid Base fluid

Nanofluid

Water Ethylene glycol (EG) Engine oil Water/TiO2 (0.75 v/v) Water/Al2O3 (1.5 v/v) EG/Al2O3 (3.0 v/ v) EG/water/Al2O3 (3.0 v/v) Water/CuO (1.0 v/v)

2300 110
190 0.4 148 40 4.8
11.8 72.3 0.613 0.253

1.2 0.287

0.0143

0.145 0.682 0.629 0.278 0.382 0.619

paint, food coloring, cosmetics and printing, and photocatalytic purification and synthesis of chemicals. Moreover, titanium dioxide nanoparticles show suitable temperature resistance and good dispersivity both in polar and nonpolar liquids, in particular, in the presence of specific dispersants [10
12]. Finally, TiO2 nanoparticles are easily produced in large industrial scale, which makes them convenient and appropriate for the utilization in thermo-vector fluids [13]. This chapter deals with heat transfer characteristics of TiO2 nanofluid. Also preparation, characterization, and influence of different parameters on the thermal conductivity of TiO2 nanofluids will be discussed. The aim is to give a critical survey of the various studies on this topic. Actually, there are several reviews that deal on the preparation, the properties, the possible applications, and the heat transfer

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Titanium Dioxide (TiO2) and Its Applications

Number of publications in web of science

1600 Number of publications 1400 1200 1000 800 600 400 200 0 1995

2000

2005

2010

2015

2020

Year Figure 8.1 Number of publications containing “nanofluids or nanofluid” in title found in “web of science” [5].

performance of the numerous nanofluids, which have considered so far. Usually, these presentations are quite dispersive because of the high amount of related publications. On the other hand, a selective and systematic view that focuses on the characteristic properties of a given nanofluid (specifically, TiO2 nanofluid) can be useful. So, this chapter aims to comprehensively illustrate the research progresses in the heat transfer applications of TiO2 nanofluids, and to discuss the open challenges and the possibilities of future developments.

8.2

Preparation and characterization of TiO2 nanofluids

Nanostructured materials can be prepared by different methods, thus giving the opportunity of obtaining nanofluids with different properties. Some common methods of preparation of nanoparticles and nanofluids are briefly presented. The description of the techniques that are used to characterize the stability of the nanofluids completes this section.

8.2.1 Nanoparticles preparation The order of magnitude of a nanometer corresponds approximately to the length of 10 hydrogen or 5 silicon atoms arranged in a line. Nanotechnology is not just a further miniaturization from micrometric to nanometric scale. In general, nanostructures and nanomaterials must be designed and manufactured to allow for a

Heat transfer by using TiO2 nanofluids

271

substantial improvement of the performance of systems adopting this new technology. The main methods used to prepare nanostructured solids are as follows: (1) high-energy ball milling; (2) physical vapor deposition (PVD); (3) chemical vapor deposition (CVD); (4) sol
gel methods; and (5) electrochemical deposition. The high-energy ball milling is a physical method of mayor industrial importance. It is also known as mechanical attrition or mechanical alloying and consists in the mechanical crushing of coarse-grained materials, carried out in rotating drums, by using hard steel or tungsten carbide balls of different size. This technology is employed even on a large scale in various industrial fields. Vapor phase deposition is appropriate for preparing thin films, multilayers, nanotubes, nanofilaments, or nanometer-sized particles. In PVD the solid phase is transformed into gaseous phase by means of physical transformation methods, then the material is deposited on a support thanks to cooling. The solid
gas transformation is accomplished by thermal evaporation (flame synthesis, resistive heating, and electron beam heating) or by spark erosion and sputtering (bombardment by means of atoms or ions to remove the target material) or by laser ablation or pulsed laser deposition (focusing a nanosecond pulse from a laser to the surface of a bulk target). The first step of a CVD consists in a thermal decomposition of gaseous species at high temperatures (500 C
1000 C) or in a chemical reaction between gaseous compounds. Then the gaseous products deposit onto a substrate. Often some suitable catalysts are used to enhance the rate of the chemical reactions. The use of ionized gases (i.e., plasmas) during PVD and CVD processes may improve the purity of the prepared solids and/or the chemical and physical properties with respect to the conventional methods. Nanocrystalline powder, wire, or rod feedstocks are generally prepared by mechanical milling or precipitation methods. Alternatively, also PVD processes can be adapted by the utilization of thermal spraying techniques, in which a spray of molten or semimolten material produced by plasma spraying (electrical thermal source) or by chemical combustion (flame spraying or high-velocity oxygen fuel spraying) is deposited onto a suitable support. Chemical synthesis may be carried out in solid, liquid, or gaseous state. In solidstate synthesis the precursors are mixed, ground, and then heated at temperatures sufficiently high to promote diffusion of the atoms and the formation of a product. Synthesis in liquid- or in gas-state can be carried out at much lower temperatures, thus resulting in grain grow inhibition and the easy obtainment of nanoscale systems. Sol
gel methods consist in a set of irreversibly chemical reactions occurring in a homogeneous solution of molecular reactant precursors (a sol) that produce a molecular three-dimensional polymer (a gel) and the subsequent formation of an elastic solid. Generally, a hydrolysis reaction followed by condensation polymerization is involved. Electrochemical deposition is also known as electrodeposition. It consists of a special electrolysis that allows the deposition of solid material onto an electrode. It is widely used for the production of metallic coatings (electroplating). The

272

Titanium Dioxide (TiO2) and Its Applications

production of nanocomposites occurs when the deposition is confined inside the pores of template membranes. The thermal degradation of the template generates nanorods or nanowires.

8.2.2 Preparation of nanofluids The techniques for the preparation of nanofluids are single-step and two-step methods. With the single-step method, synthesis and dispersion of the nanoparticles in the base fluid occur simultaneously. Different approaches are possible for this purpose, as (1) direct condensation into fluids of nanoscale powders from metallic vapor [14]; (2) physical process based on wet grinding with bead mills [15]; (3) chemical reduction to produce metallic nanoparticles in fluids [16]; and (4) optical laser ablation in liquid [17]. Generally, the base fluids for heat transfer applications, such as water, oil, or refrigerant, are not the liquid solutions used in the one-step method. Therefore the single-step method is suitable to produce nanoparticles but it is not the usual route for preparing directly heat-transfer nanofluids. In the two-step method the preparation of nanoparticles and their dispersion are separate processes. This method has been extensively used for the production at an industrial scale of TiO2 nanoparticles that are utilized also in the preparation of TiO2 nanofluids. TiO2 nanoparticles strongly interact with each other and have the tendency of aggregating. In order to improve the stability of the dispersion, the addition of dispersants with steric or ionic effects and the optimization of some operating conditions, such as pH and Zeta potential, could be necessary [18]. For the preparation of the dispersion, three physical means are proposed in the pertinent literature: (1) mechanical agitation, (2) ultrasonic waves, and (3) stirring bead milling. However, Hwang et al. [19] observed that the effectiveness of these methods is limited. They suggested a posttreatment based on a high-pressure homogenization, which allowed a substantial reduction of the particle size by at least one order of magnitude. Fig. 8.2 shows a scheme of the system used. Yang et al. [20] proposed an improved method to prepare nanofluids. They observed that, operating with a dispersion at high concentration, it is possible to separate the well-dispersed part from the bulk which is present after sedimentation and to dilute it with the base fluid till the desired concentration. The procedure is schematized in Fig. 8.3. Together with physical methods, the addition of surfactants is widely used to modify the properties of nanofluids [21]. Indeed, the organic groups of surfactants adsorbed onto the surface can prevent the aggregation of nanoparticles by forming steric hindrance or electrostatic repulsion. Fig. 8.4 shows a scheme of the typical sequence of operations adopted in a twostep method. The most used chemical dispersants for TiO2 nanofluid preparation are hexadecyltrimethylammonium bromide (CTAB), polyvinylpyrrolidone (PVP), sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), and acetic acid.

Heat transfer by using TiO2 nanofluids

273

Outlet reservoir

Inlet reservoir

Fixed geometry

Constant pressure

Pressure gauge

Figure 8.2 Scheme of the high-pressure homogenizer.

Sedimentation

Dilution to the desired concentration

Removal of well-dispersed nanofluid

Figure 8.3 Scheme of the optimizing dispersion method for nanofluid preparation.

Base fluid

Dispersant Mixing

pH-adjust

Nanostructured solid Ultrasonic dispersion

Nanofluid

Figure 8.4 Operations in a two-step method for nanofluid preparation.

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Titanium Dioxide (TiO2) and Its Applications

Generally, the base fluids are water (W), ethylene glycol (EG), propylene glycol (PG), ammonia
water mixture, refrigerants, mineral oils, etc. [22]. pH is of major importance to obtain a satisfactory dispersion of nanoparticle, so this parameter is adjusted taking into account also the measured value of the Zeta potential of the nanoparticles. In this way the aggregation is reduced thanks to the establishment of electrostatic repulsion. In general, the coupling of physical and chemical methods ensures enhanced stability of nanofluids. The two-step technique is particularly appropriate for oxide nanoparticles, while metallic nanoparticles are susceptible to oxidation in water and show a higher tendency to form agglomerates.

8.2.3 Parameters influencing the aggregation and stability of TiO2 nanofluids In nanoparticles a high percentage of atoms are on the surface (it can be assumed that about 50% of the atoms are on the surface in a particle of c. 4 nm). Moreover, nanoparticles may have such a high surface energy that the surface properties influence their behavior [23]. In aqueous medium, nanoparticles have the tendency to aggregate creating clusters and reducing the particle surface energy. This feature depends on different parameters, including surface functionalization, pH, and ionic strength. The particles may interact with each other by means of forces of different nature causing the aggregation and possibly the settling of the aggregates. These phenomena lead to a reduction of the stability of the nanofluids and, consequently, to fluid properties that can be indeterminate and variable. The stability can be monitored by controlling the size and shape of nanoparticles and the pH [24]. It can be enhanced by the addition of selected dispersants and surfactants [25]. Even the physical dispersion method of nanoparticles into the base fluid (ball milling, ultrasonication, and homogenization) may affect the stability [26,27].

8.2.4 Nanoparticle size measurements The size of nanoparticles is one of the most important factors that affect the possibility of aggregation and other important properties of a nanofluid. Therefore different techniques for the measurement of the size distribution of the nanoparticles have been developed which can be used on powders alone or in suspension in a liquid. One of the techniques for powders alone is the electron scanning microscopy that allows resolutions of few nanometers, whereas the use of transmission electron microscopy enables to extend the measurements till 1 nm or even a little bit beyond. For the determination of the size distribution in a liquid suspension, the most used technique is the dynamic light scattering one [28]. The particles in suspension are affected by Brownian motion, and the particle size is related to their relative velocity through the Stokes
Einstein equation.

Heat transfer by using TiO2 nanofluids

275

When a laser beam at known frequency impinges on the particles, scattering takes place and the fluctuation of the radiation depends on the particle’s velocity (smaller particles giving rise to more important fluctuation). Due to the Brownian motion, for the same temperature and viscosity, small particles move quickly, creating rapid changes in the intensity of scattering. The Brownian motion of the particles in the sample is then related to the particle size by means of well-known laws [29,30]. The measured value is reported as the equivalent diameter of a sphere that diffuses with the same velocity of the sample particle. The influence of particle size on the stability of TiO2 nanofluids is described in several publications. It depends both on the base fluid and on the dispersant used. The physical dispersion method too may influence the stability. For example, Fedele et al. [31] studied TiO2
water nanofluids and showed that the particle load (1%
35%) does not affect the measured mean particle size (72
76 nm), and after 1 h of sonication the mean particle size remained stable for c. 35 days (using acetic acid as dispersant, pH 5 1.86
3.07) thus indicating that no settling occurred. However, in the case of static solutions the mean particle size decreased slowly to 51 nm after the same time, due to a partial precipitation. Also Palabiyik et al. [32] found that the nanoparticle size (21 nm) was independent of the TiO2 load (0.25
2.4 vol.%) with propylene glycol as the base fluid. Different results were found by changing both the base fluid and the dispersant. He et al. [33] obtained nanofluids (mean size 20 nm) that remained stable for months with water as the base fluid, without any dispersant, with a TiO2 load of 0.24
1.18 vol.% and ultrasonication. The use of dispersants can increase the time of stability of the nanofluid. Indeed, Ghadimi et al. [12] showed that the stability of TiO2
water nanofluids can be increased up to 1 year by using SDS as dispersant.

8.2.5 Z-potential measurements Another parameter, which influences the stability of a nanofluid, is the Z-potential. The particles suspended in water may be electrically charged due to ionization phenomena or to the presence of charged species adsorbed onto their surface. Electrically charged particles are surrounded by ionic layers. The particles move together with the ionic double layer. The Z-potential is the potential at the double layer called also sliding plane. At higher potentials the electrostatic repulsion increases thus preventing the aggregation of the particles avoiding their sedimentation, and increasing the stability of the suspension. Values higher than the empirical limit of 30 mV are those over which a colloidal solution should be stable.

8.2.6 pH measurements The pH is another important factor for the stability of a suspension. When positive and negative charges on a surface are equivalent, the system is at its isoelectric point, IEP. The pH that identifies this condition is called zero point charge. The surface of the particles in a suspension at a pH more basic than the IEP will be negatively charged, whereas when the suspension is more acidic than the IEP the

276

Titanium Dioxide (TiO2) and Its Applications

surface will be charged positively. Therefore the stability of a nanofluid increases at pH far from the IEP, because in these conditions the surfaces of the particles are charged and tend to repel each other.

8.3

Heat conduction in TiO2 nanofluids

Heat conduction is one of the fundamental mechanisms of heat transfer. Numerous papers in the pertinent literature are on the thermal conductivity of nanofluids and on the influence of some parameters as temperature, nanoparticles’ size and shape, type, and concentration of surfactants. However, the mechanisms of heat transfer in nanofluids are not yet completely understood and some contradictory results have been obtained by various researchers. Indeed, the conductivity of a nanofluid is usually higher than the one of the base fluid [34
39] but the results do not always agree with a unique behavior [40,41]. Models have been developed to describe and then predict the effects that various parameters have on nanofluid thermal conductivity. Actually, specific models for heat transfer in TiO2 nanofluids are not common, but many models based on conventional approaches have been used, possibly by taking into account the peculiarities of the system. One of the first theoretical models was developed by Maxwell [42] that deduced the equation of the electrical conductivity for a twocomponent mixture. The Maxwell expression for the effective thermal conductivity of composite materials (keff) is shown in the following equation: keff


 kp 1 2kf 1 2φ kp 1 kf
 5 kf kp 1 2kf 2 2φ kp 1 kf

(8.1)

where kf is the conductivity of the base fluid, kp that of the particle, and φ is the volume fraction of nanoparticles. The Maxwell model presents some limitations. One of them is that it does not take into account the interactions between the particles that in general cannot be considered isolated. Therefore the application of the Maxwell equation is reliable only if the volume fraction of the suspended nanoparticles is sufficiently small to make negligible the interactions between them. Bruggeman [43] proposed a different relationship to evaluate the effective conductivity of composites at relatively high particle concentrations, which is based on the differential effective medium theory: keff with

pffiffiffiffiffi
 ð3φ 2 1Þ kp =kf 1 ½3ð1 2 φÞ 2 1 1 Δ 5 kf 4

 2 kp kp Δ 5 3ð12φÞ 1 ½3ð12φÞ21 1 8 kf kf

(8.2)

Heat transfer by using TiO2 nanofluids

277

However, thermal conductivity of nanofluids depends also on size and shape of particles. The shape is taken into account in Eq. (3.3), which resorts to the Hamilton and Crosser model [44]: keff 5 kf


 kp 1 ðn 2 1Þkf 1 ðn 2 1Þφ kf 2 kp
 kp 1 ðn 2 1Þkf 1 φ kf 2 kp

(8.3)

where n is the empirical factor defined as n 5 3/ψ, with ψ the sphericity (i.e., the ratio of the surface area of the hypothetical sphere, which has the same volume of the given particle, to the surface area of the particle). A model that has been used both for spherical and nonspherical particles is the one by Lu and Lin [45] who, by considering the pair interaction between aligned spheroids of a composite materials, obtained the following equation:
 keff 5 kf 1 1 aφ 1 bφ2

(8.4)

where a and b are coefficients that depend on the particle shape. The values of a and b for spherical particles are 2.25 and 2.27, respectively. The model by Xuan et al. [46], based on the Maxwell one, considers the effects of random motion, particle size, concentration, and temperature: keff

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
 ρp φC P;p kp 1 2kf 1 2φ kf 2 kp kB T
 1 5 kf 3πμr C 2kf kp 1 2kf 1 φ kf 2 kp

(8.5)

where ρp is the density of particles, CP,p is the particle-specific heat capacity, kB is the Boltzmann constant, T is the temperature, μ is the viscosity, and rC is the radius of the clusters. Liquid molecules close to a solid surface are known to form layered structures [47,48] that can affect the thermal properties of solid/liquid suspensions. So, Yu et al. [49] suggested a structural model of nanofluids that consists of solid nanoparticles, a bulk liquid, and solid-like nanolayers. Indeed, they assumed the liquid nanolayer around each particle and the particles form an equivalent particle. Based on effective medium theory, the equivalent thermal conductivity kpe of the equivalent particles can be expressed as follows [49]: kpe 5 kp

  2ð1 2 γ Þ 1 ð11β Þ3 ð1 1 2γ Þ 2 ð1 2 γ Þ 1 ð11β Þ3 ð1 1 2γ Þ

γ

(8.6)

where γ 5 klayer =kp is the ratio of nanolayer conductivity to particle conductivity and β 5 h=r is the ratio of the nanolayer thickness to the particle radius. So, the Maxwell equation can be modified as the following equation: keff


 kpe 1 2kf 1 2φ kpe 2 kf ð11β Þ3
 5 kf kpe 1 2kf 2 φ kpe 2 kf ð11β Þ3

(8.7)

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Titanium Dioxide (TiO2) and Its Applications

Table 8.2 Experimental data of thermal conductivity of TiO2 nanofluids: influence of the load [22]. Nanofluids

T ( C)

Loading (vol.%)

Particle size (nm)

Dispersant

keff/kf

TiO2
W [47] TiO2
W (pH 5 3) [48] TiO2
W [50] TiO2
W [51] A
TiO2
W [52] TiO2
EG [53] TiO2
PG [30] TiO2
EG [54]

RT RT

0.1
1 0.29
0.68

25 34





1.1
1.145 1.018
1.063

32 RT RT

1
4.3 1.2
2.6 0.99
4.35

27 40 27






1.02
1.108 1.036
1.054 1.034
1.108

RT RT 20

1
5 025
2.4 0.2
1

40 21 40






1.04
1.175 1.03
1.092 1.045
1.095

A, Anatase.

8.3.1 Influence of particle load Particle load is the most studied parameter affecting thermal conductivity. All the researchers agree that the conductivity increases with the particle load, but there is no consensus about how much it increases (see Table 8.2). The large differences are probably due to the different conditions in the various studies, such as size, shape, and surface area of the particles, and other factors, including the type of base fluid, the time of ultrasonication, the pH, the type of surfactant, and the temperature. Microcosmic factors too can be important, such as the state of charge of the surface, clustering, and dispersion of the nanoparticles.

8.3.2 Influence of temperature Of course, also the temperature may affect thermal conductivity. Several publications show an increase of the effective thermal conductivity ratio (keff/kf) of TiO2 nanofluids with the temperature [31,56
62]. This behavior could be explained by taking into account the Brownian motion and the micro-convection. However, there is not a general agreement. In fact, some authors [32,52,53,63
65] found that the thermal conductivity ratio is almost temperature independent, probably because the nanofluid conductivity increases with the same rate of the base fluid conductivity. An opposite behavior was reported by Duangthongsuk and Wongwises [66] who observed an adverse effect of the temperature increase on the effective thermal conductivity ratio (keff/kf) of TiO2 nanofluids. The different responses of thermal conductivities of TiO2 nanofluids to a temperature variation can be attributed not only to the different influence of temperature on the various base fluids (generally the thermal conductivity of liquid decreases with the temperature, with some important exceptions such as water and glycerol), but also to the effect of temperature on the micro-motion of nanoparticles

Heat transfer by using TiO2 nanofluids

279

(depending on size, surface charge, and allotropic form of TiO2). Table 8.3 shows the values of the ratio keff/kf reported in literature.

8.3.3 Influence of thermal conductivity of the base fluid In heat transfer the advantages of using a TiO2 nanofluid with respect to the base fluid depend also on the type of nanofluid. Indeed, it is likely that if the thermal conductivity of the base fluid is much lower than the one of the nanoparticles, the presence of nanoparticles would be more beneficial and the nanofluid would exhibit a higher effective thermal conductivity ratio. This expectation is confirmed by the experimental observations of several authors [63,67
69]. In fact, the increase of the thermal conductivity of water-based nanofluids (base fluid conductivity 5 0.608 W/ m/K at 298K) is lower than the one of the EG-based nanofluids (base fluid conductivity 5 0.2511 W/m/K at 298K), which, in turn, is lower than the one of PGbased nanofluids (base fluid conductivity 5 0.2010 W/m/K at 298K). Also, the results by He et al. [68] on binary EG/water base fluids are in agreement with this trend. Indeed, they showed that the enhancement of the effective thermal conductivity ratio (keff/kf) in TiO2 nanofluids is more important when the loading of the poor conducting species (EG) in the base fluid is higher.

8.3.4 Influence of particle cluster size and shape on thermal conductivity Particle size affects many different phenomena, such as motion of particles and the interactions due to van der Waals forces. In turns, these phenomena have an important role on thermal conductivity. In practice, the smaller are the particles the stronger becomes the energy transmission thus improving thermal conduction. These insights are generally confirmed in the pertinent literature where it is reported an increase of conductivity by decreasing of particle size. An exception to this behavior is reported by Warrier et al. [70], who found that the increase of the conductivity with respect to the base fluid is more important with larger nanoparticles. The shape too is a factor that may influence the thermal conductivity of TiO2 nanofluids, and for this reason it has been considered in some studies. Murshed et al. [37] compared TiO2 water-based nanofluids containing rod-like nanoparticles (10 3 40 nm) with those containing spherical nanoparticles (15 nm). Their results showed that with the same particle volume fraction, the enhancement of the thermal conductivity for rod-like particles was greater than the one obtained with spherical particles. Conversely, with EG- and water-based nanofluids Chen et al. [69] found no substantial difference in thermal conductivity enhancement for spherical (25 nm) and rod-like (10 3 100 nm) nanoparticles. After all, most of the results reported in the literature demonstrate that the shape of particles does not affect very much the effective thermal conductivity of TiO2 nanofluids. Anyhow, additional research would be advisable to clarify these conflicting results. Table 8.4 resumes some experimental results that are related to this topic.

Table 8.3 Experimental data of thermal conductivity of TiO2 nanofluids: influence of the temperature [22]: greenish data, positive effect; yellowish data, negligible; and pink data, negative effect. Nanofluids

T ( C)

Loading (vol.%)

Particle size (nm)

Dispersant

keff/kf

A
TiO2
W [54]

15
50 15
50 15
50 15
50 15
50 15
50 20.5
79 20.5
79 21.5
79 21.5
79 0
50 0
50 0
50 18
65 18
65 18
65 20
40 20
40 20
40 40
70 40
70 40
70 40
70 40
70

0.05 0.1 0.5 1 3 5 0.24 2.54 5.54 11.22 1 2 3 1 2 4 0.917 1.764 2.55 0.2 0.4 0.6 0.8 1

33 33 33 33 33 33 76 76 76 76 15 15 15 26 26 26 10 10 10 21 21 21 21 21



























1.02
1.045 1.03
1.055 1.04
1.075 1.075
1.11 1.12
1.175 1.135
1.215 1.012
1.177 1.031
1.247 1.083
1.256 1.213
1.332 1.011
1.06 1.053
1.128 1.138
1.196 1.031
1.1 1.046
1.133 1.11
1.196 1.0185
1.023 1.0392
1.0508 1.0559
1.0805 1.0248
1.027 1.0309
1.0339 1.0351
1.037 1.0395
1.0423 1.0452
1.0502

TiO2
W [29]

TiO2
EG [56] TiO2
W [55] TiO2
W [57] TiO2
W [58]

TiO2
W/EG (2:8) [59]

11.1
58.6

TiO2
AW [60]

20
40 20
40 20
40 20
40 20
90 20
90 20
90 20
90 20
90 20
90 10
40 10
40 10
40 20
80 20
80 20
80 13
55 13
55 13
55 13
55 32
47 32
47 10
70 10
70 10
70 10
70 10
70

TiO2
EG [61]

TiO2
W [51] TiO2
PG [30] TiO2
W [63]

TiO2
W [50] A 1 R
TiO2
W [61]

2 4 1 2 3 4 0.5 1 2 3 4 5 0.6 1.2 2.6 1 6 9 0.2 1 2 3 3.2 4.3 5 10 15 20 25

21 21 15 15 15 15 40 40 40 40 40 40 40 40 40 21 21 21 21 21 21 21 27 27 21 21 21 21 21
































1.052
1.064 1.095
1.119 1.035
1.05 1.064
1.097 1.093
1.119 1.126
1.151 1.020
1.024 1.047
1.061 1.085
1.090 1.153
1.162 1.214
1.213 1.263
1.276 1.014
1.01 1.031
1.037 1.058
1.065 1.0132
1.0134 1.0827
1.0772 1.0729
1.0982 1.004
1.003 1.025
1.022 1.042
1.045 1.074
1.072 1.08
1.084 1.05
1.099 1.0235
1.0307 1.0537
1.0556 1.0854
1.0906 1.1214
1.1221 1.1420
1.1433 (Continued)

Table 8.3 (Continued) Nanofluids

T ( C)

Loading (vol.%)

Particle size (nm)

Dispersant

keff/kf

A
TiO2
PG

10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 15
35 15
35 15
35 15
35 15
35

5 10 15 20 25 5 10 15 20 25 5 10 15 20 25 0.2 0.5 1 1.5 2

21 21 21 21 21 21 21 21 21 21 21 21 21 21 21 21 21 21 21 21























1.0353
1.0365 1.0618
1.0607 1.0900
1.0910 1.1251
1.1287 1.1501
1.1491 1.0197
1.0239 1.0444
1.0427 1.0717
1.0717 1.099
1.097 1.1148
1.1116 1.0298
1.0285 1.0533
1.0518 1.0806
1.0797 1.1079
1.1053 1.1225
1.1195 1.0293
1.0149 1.041
1.0325 1.0473
1.0393 1.0612
1.0532 1.0792
1.0616

A 1 R
TiO2
EG

A
TiO2
EG

TiO2
W [64]

A, Anatase; AW, Ammonia-Water; R, rutile.

Table 8.4 Experimental data of thermal conductivity of TiO2 nanofluids: influence of the conductivity of base fluid, size, and shape of particle clusters [22]. Nanofluids

T ( C)

Loading (vol.%)

Particle size (nm)

Dispersant

keff/kf

A 1 R
TiO2
PG [61]

10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 10
70 23 23 40
70 40
70 40
70 40
70 40
70

5 10 15 20 25 5 10 15 20 25 5 10 15 20 25 5 10 15 20 25 0.186
0.19 0.6 0.2 0.4 0.6 0.8 1

21 21 21 21 21 21 21 21 21 21 21 21 21 21 21 21 21 21 21 21 95 95
210 21 21 21 21 21






























1.0235
1.0307 1.0537
1.0556 1.0854
1.0906 1.1214
1.1221 1.1420
1.1433 1.0353
1.0365 1.0618
1.0607 1.0900
1.0910 1.1251
1.1287 1.1501
1.1491 1.0197
1.0239 1.0444
1.0427 1.0717
1.0717 1.099
1.097 1.1148
1.1116 1.0298
1.0285 1.0533
1.0518 1.0806
1.0797 1.1079
1.1053 1.1225
1.1195 1.019
1.086 1.0337
1.0113 1.0113
1.0433 1.0233
1.0456 1.0361
1.0471 1.0450
1.0500 1.0531
1.0684

A
TiO2
PG

A 1 R
TiO2
EG

A
TiO2
EG

TiO2
W [66] TiO2
EG/W (5:5)

(Continued)

Table 8.4 (Continued) Nanofluids

T ( C)

Loading (vol.%)

Particle size (nm)

Dispersant

keff/kf

TiO2
EG/W (6:4)

40
70 40
70 40
70 40
70 40
70 RT

0.2 0.4 0.6 0.8 1 0.5
5

RT RT RT RT RT RT

0.1
1.8 0.1
1.8 0.12
0.6 0.22
1.118 0.5
5 0.5
5

21 21 21 21 21 15 10 3 40 10 3 100 25 10 3 100 25 15 10 3 40
















1.0252
1.0668 1.0348
1.0765 1.0465
1.0842 1.0552
1.0987 1.0648
1.1190 1.05
1.3 1.08
1.33 1.009
1.136 1.017
1.145 1.005
1.039 1.017
1.062 1.05
1.3 1.08
1.33

TiO2
W [35] TiO2
EG [67] TiO2
W

A, Anatase; R, rutile; RT, room temperature.

Heat transfer by using TiO2 nanofluids

285

8.3.5 Influence of surfactant Surfactants are used to increase the stability of nanofluids. The thermal conductivity of surfactants is generally lower than the one of the base fluids, but usually the needed amounts are so low that they do not alter significantly the thermal conductivity. Indeed, Utomo et al. [41] showed that only at high loading the presence of surfactants could decrease the effective thermal conductivity of water-based TiO2 nanofluids. On the other hand, Murshed et al. [37] found that concentrations of 0.01
0.02 vol.% of oleic acid or CTAB improve the stability of TiO2 nanofluids dispersion without reducing their heat transfer coefficients. An important issue is to choose a surfactant able to enhance the stability of the suspension without significantly increasing the viscosity. Ling et al. [71] observed that the addition of low amounts of SDBS or OP-10 in TiO2 nanofluids induces a slight drop in viscosity. Table 8.5 reports some experimental results. Saleh et al. [56] found that the three kinds of surfactants, SDS, CTAB, and Span-80, improved the dispersion stability and the thermal conduction of TiO2 nanofluids. Yang et al. [62] demonstrated that the stabilities of nanofluids, obtained by adding surfactants (PEG1000 or PAA), contribute to increase significantly the thermal conductivity ratio, even if surfactants at low concentration have a theoretical small influence on the thermal conductivity.

8.3.6 Influence of ultrasonic treatment Among the physical methods used for increasing the stability of nanofluids, some researchers studied the effects of ultrasounds. Anyhow, Ismay et al. [72] found that 2 h of sonication increased even the thermal conductivity of water-based TiO2 nanofluids. Sonawane et al. [73] studied the effect of the sonication time on the thermal conductivity of TiO2 nanofluids by using three different base fluids, including water, EG, and PO. They found an optimal sonication time for achieving the maximum thermal conductivity that first increased with sonication time and then decreased. The optimal time was 60 min for all the three kinds of TiO2 nanofluids. This behavior was attributed to the micro-interactions of nanoparticles as Brownian motion and the intensification of micro-convection. Sonication times longer than 60 min gave rise probably to nanoparticles clustering with the corresponding reduction of the heat transport performance. Table 8.6 reports some experimental results.

8.4

Heat convection in TiO2 nanofluids

One aspect that must be taken into account in heat transfer with nanofluids is the fluid dynamic of these systems, that is, the convective mode of heat transfer. Colloidal and biological suspensions generally show non-Newtonian behaviors, but Wang et al. [74], in their measurements of viscosity by using three different

286

Titanium Dioxide (TiO2) and Its Applications

Table 8.5 Experimental data of thermal conductivities of TiO2 nanofluids: influence of the surfactant [22]. Nanofluids

T ( C)

Loading (vol.%)

Particle size (nm)

Dispersant

knf/kf

TiO2
W [54]

RT RT RT RT 20

0.05
1 0.05
1 0.05
1 0.05
1 1
4

33 33 33 33 15


Span80 CTAB SDS


1.0269
1.0788 1.0381
1.1056 1.0456
1.113 1.0456
1.1113 1.035
1.127

20

1
4

15

PEG1000

1.028
1.096

TiO2
W (pH 5 3) [60]

RT, Room temperature; SDS, sodium dodecyl sulfate.

Table 8.6 Experimental data of thermal conductivities of TiO2 nanofluids: influence of ultrasonication [22]. Nanofluids

T ( C)

Loading (vol.%)

Particle size (nm)

Sonication time (min)

keff/kf

TiO2
W [71]

RT

3
6

5

20

1.0757
1.1924

RT RT RT RT RT RT RT RT RT RT RT RT RT RT RT RT RT

3
6 3
6 3
6 3
6 3
6 3
6 3
6 3
6 3
6 3
6 3
6 3
6 3
6 3
6 3
6 3
6 3
6

5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5

40 60 80 100 120 20 40 60 80 100 120 20 40 60 80 100 120

1.0968
1.2081 1.1065
1.2226 1.1065
1.2048 1.1016
1.2000 1.1000
1.1935 1.0757
1.1924 1.0839
1.1924 1.0987
1.1974 1.1020
1.1974 1.0954
1.2007 1.0872
1.1924 1.0658
1.1711 1.0757
1.2007 1.1053
1.2072 1.0954
1.2072 1.0905
1.1859 1.0757
1.1941

TiO2
EG

TiO2
PO

RT, Room temperature.

methods on nanofluids, did not observe this behavior. Anyhow, the addition of nanoparticles to a base fluid increases the viscosity. The possible improvement of the heat transfer coefficient, h [W/(m2 K)], with respect to the base fluid, obtained with the nanofluid flowing in the heat transfer apparatus, can be compared with the improvement expected on the basis of the sole

Heat transfer by using TiO2 nanofluids

287

increment of the thermal conductivity. In this way, it is possible to understand if other mechanisms in addition to the variation of the thermal conductivity affect the heat transfer performance. It is generally supposed that the observed benefits for the heat transfer processes are greater than those expected by taking into account only the effects of the variation of the thermal conductivity. But this appraisal is not direct and it is probable that it could be somehow unreliable. As a matter of fact, only relatively few studies on this issue have been reported in the literature and the results are often inhomogeneous, probably due both to different operating conditions and the analysis of the results. Results are often reported in terms of heat transfer coefficients (or of the Nusselt number).

8.4.1 Forced convection Investigations of heat transfer in forced convection of nanofluids lead to some theoretical correlative formulas. However, it is evident that these relationships cannot predict the heat transfer coefficient of all kinds of nanofluids, because they do not take into account some peculiar characteristics of the different nanofluids. Moreover, it is not easy to estimate or to appreciate independently some of the parameters that appear in these formulas, such as the thermal conductivity, the kinematic viscosity, the size, and the space charge distribution in TiO2 nanofluids. For a fluid in motion in a heat transfer apparatus, the constitutive equation for heat transfer can be expressed by the Newton law: Q 5 hAs ΔT 5 hAs ðT b 2 T w Þ

(8.8)

where h is the heat transfer coefficient, As is the area of the interchange surface, and Tb and Tw are the bulk and the wall temperatures, respectively. The energy balance for the fluid between the inlet and the outlet can be written as follows: Q 5 WC p ΔT 5 WC p ðT in 2 T out Þ

(8.9)

where W is the mass flow rate, Cp is the specific heat, and Tin and Tout are the inlet and outlet temperatures, respectively. It follows that the heat transfer coefficient is expressed as follows: h5

WC p ðT in 2 T out Þ As ðT b 2 T w Þ

(8.10)

The pertinent dimensionless parameter is the Nusselt number, defined as follows: Nu 5

hDH k

where DH represents the hydraulic diameter, and k the thermal conductivity.

(8.11)

288

Titanium Dioxide (TiO2) and Its Applications

Many correlations of Nu for different nanofluids have been developed in recent years for various geometries of flow channels and a wide range of operating conditions (Reynolds number, Re, Prandtl number, Pr, and flow speed, u). Some results are reported in the following presentation. Hejazian et al. [75] studied the effect of nanoparticle volume fraction on the thermal behavior of TiO2/water nanofluid flowing in a horizontal tube, by using the single phase and mixture model approaches. They calculated the Nusselt number for turbulent flow in the tube by the following relationship:
hnf 5

ρCp

 nf

 AuðT in 2 T out Þ

πDLðT b 2 T w Þ

(8.12)

with Nunf 5

hnf D knf

where hf, D, knf, Tb, Tw, u, Cp, and L are the average values of convective heat transfer coefficient, the tube diameter, the thermal conductivity of the nanofluid, the bulk temperature, the wall temperature, the velocity of fluid, and the specific heat capacity, respectively. The correlation they found for the estimation of both water and the nanofluid average Nusselt numbers (4800 , Re , 30,500, 5.5 , Pr , 5.59, 0 , φ , 0.0025) is: Nu 5 0:00218Re1:0037Pr0:5 ð11φÞ154:6471

(8.13)

with Re 5

ρnf uDH μnf ;

Pr 5

μnf C P;nf knf

From these relationships, it is apparent that h increases with the particle loading. Abdolbaq et al. [76] studied a steady-state system, with incompressible Newtonian fluid in turbulent flow with constant thermo-physical properties of nanofluid, without the effect of gravity, and heat conduction in the axial direction. The geometry of the channel was rectangular. They found that for 0 , Re , 106 and 0 , φ , 0.03:
20:012 Nunf 5 0:023Re0:8Pr0:4 11Prnf ð11φÞ0:23 f

(8.14)

where the subscript f refers to base fluid. Chen and Cheng [77] analyzed, by computational fluid dynamics, the forced convective heat transfer of TiO2/water nanofluid by assuming a three-dimensional turbulent and steady incompressible flow with a heat convective boundary condition. The correlation they found (56,800 , Re , 94,748) is as follows: Nunf 5 0:026Re0:806Pr0:401 ð11φÞ0:5

(8.15)

Heat transfer by using TiO2 nanofluids

289

Nu and friction factor increased with the particle loading by 2% and 21%, respectively, for all the investigated values of Re. Celata et al. [78] reported the results of convective heat transfer tests on waterbased TiO2 (9 wt.%) and SiC (3, 6, 9 wt.%) nanofluids. Experiments were carried out in a two-loop test rig. The heat transfer was evaluated in a circular tube heated with uniform heat flux with flow regimes from laminar to turbulent. The correlation for laminar regime (Re , 2500) is as follows: 0:152 μ 1=3Þ ð Nu 5 1:86x  μW

(8.16)

with x 5

1 x 5 Gz DRePr

where x is the dimensionless axial length (inverse of the Graetz number); x is the distance from the inlet; and D is the diameter. For the transition and turbulent regimen (2500 , Re , 7000; 0.03 wt , φ , 0.09 wt) the following relationship was developed: Nu 5

f1 PrðRe 2 1000Þ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 1 1 12:7
Pr2=3 2 1
f =2

(8.17)

where f 1 5 ð1:58lnRe23:28Þ22 is the Fanning friction factor f1 5 f/4 and f the Darcy friction factor. Eiamsa-Ard and Kiatkittipong [79] studied the enhancement of thermal performance characteristics in a heat exchanger tube by considering different arrangements of multiple twisted tapes and different concentrations of TiO2 nanoparticles. Fig. 8.5 shows the different arrangements of twisted tapes. They found that the tube with the multiple twisted tapes showed superior convective thermal coefficient when compared with plain tube or the tube with a single twisted tape, due to multiple swirling flow and the presence of multilongitudinal vortices along the test tube. They developed different correlations for the various configurations. In particular, for TiO2, water nanofluids in a tube fitted with one to four cotapes: Nu 5 0:103Re0:618Pr0:4 ð11N Þ0:768 ð11φÞ0:438

(8.18)

Valid for 5000 , Re , 15,000 and 0.0007 , φ , 0.0021, with N, number of tapes. For a tube fitted with single tape, dual or quadruple counter tapes in parallel counter-swirl flow, it is as follows: Nu 5 0:104Re0:618Pr0:4 ð11N Þ0:789 ð11φÞ0:438

(8.19)

290

Titanium Dioxide (TiO2) and Its Applications

Figure 8.5 Scheme of different arrangements of dual/triple/quadruple twisted tapes [79].

and for tube fitted with quadruple counter tapes in cross counter-swirl flow: Nu 5 0:096Re0:618Pr0:4 ð11N Þ0:834 ð11φÞ0:468

(8.20)

In general, h depends on different factors, including k, Q, L, Cp, particle size and shape, loading and surfactants, and is affected also by the size of particles and clusters. Of course, in accordance with dimensional analysis, Re and Pr appear in any correlation of Nu. Moreover, some correlations include the effect of particle load and also viscosity and particle diameter.

8.4.1.1 Influence of particle loading and Re The most studied parameters in the field of convective heat transfer of TiO2 nanofluids are the particle volume fraction and Re. Almost all publications show a positive effect of the particle load and Re both in laminar and in turbulent conditions [75]. Actually, the improvement of heat transfer is nearly negligible at very low values of particle volume fraction (,0.5%), but at

Heat transfer by using TiO2 nanofluids

291

particle loading of 1.5 vol.% some studies report that heat transfer coefficient almost doubles [80]. On the same line, He et al. [68] found an increase of heat transfer coefficient with the particle load, with more pronounced effect in turbulent flow than in laminar flow. However, there is not a general consensus on the unconditional positive effect of nanoparticle load. In fact, Sajadi and Kazemi [81] measured minor effects of particle load on convection heat transfer coefficient for TiO2 nanofluids flowing in a horizontal tube in the turbulent regime. Other researchers observed that heat transfer coefficient is maximum at an optimal nanoparticles load. For instance, Duangthongsuk and Wongwises [82], who studied the convective heat transfer in a horizontal double-pipe heat exchanger, found that, in turbulent flow, the increase of heat transfer coefficient was 26% at 1 vol.% particle load, but at 3 vol.% the effect was negative with a 12% decrease with respect to the base fluid. Similarly, at 3 vol.% particle load, Pak and Cho [54] found that, for the same flow rate, the decrease of the heat transfer coefficient was 12% with respect to water base fluid.

8.4.1.2 Influence of particle size The effect of particle size has been also the subject of some works. Generally, it is argued that smaller dimensions could enhance Brownian motion, thus giving rise to enhanced heat transfer effects as stated by Goutam and Paul [83]. Another interesting outcome is that the effect of particle size on convective heat transfer coefficient seems more pronounced than the effect on thermal conductivity. Other researchers found that convective heat transfer is promoted by the decrease of particle size and by the increase of particle load and Re [84,85]. However, He et al. [68] showed that both in laminar and turbulent flow, TiO2 particle size has a negligible effect on convective heat transfer.

8.4.1.3 Influence of temperature Several studies indicate that temperature strongly affects the convective heat transfer coefficient of TiO2 nanofluids. For instance, Zhang et al. [86] found that the increase of average temperature raised the heat transfer coefficient of TiO2 nanofluids in an internally ribbed tube. Megatif et al. [87] too in their investigation on a binary nanoparticle mixture of TiO2 and carbon nanotube concluded that the heat transfer coefficient varies significantly both with the particle loading (12% from 0.1 to 0.2 wt.%) and temperature. Moreover, they found an enhancement of the convection heat transfer coefficient of c. 38% at a load of 0.2 wt.% and 32 C with respect to 25 C. The positive effect of temperature is showed also in other studies. For example, Srinivas and Venu Vinod [88] found that the effectiveness of the heat exchanger was improved by increasing the stirrer speed and the shell-side fluid temperature. The positive effect of temperature is explained by the enhancement of chaotic motion and the favorable change of the physical properties of nanofluid (e.g., the decrease of viscosity).

292

Titanium Dioxide (TiO2) and Its Applications

8.4.1.4 Influence of geometry of flow channel The geometry of the flow channel determines the fluid dynamics and therefore it represents another important factor that affects convective heat transfers of both base fluids and TiO2 nanofluids. Azmi et al. [89] investigated experimentally a system with structural modifications of the flow caused by the presence of twisted tape inserts in the tube. Their results showed that the heat transfer coefficient first increases then decreases by increasing not only the particle loading but also the twist ratio of the tape. The optimal values were 1 vol.% and 5 for particle loading and twist ratio, respectively, with a maximum enhancement of heat transfer coefficient of 81.1%. Zhang et al. [86] developed an experimental apparatus with data acquisition to study the heat transfer of TiO2 water nanofluids in a ribbed copper tube. Their results showed that, under the same Re at constant temperature, with a smooth tube, the heat transfer efficiency increases with the nanoparticles load but not with an internally ribbed tube. Eiamsa-Ard and Kiatkittipong [79] found that the utilization of TiO2 nanofluids in a system with multiple twisted tape inserts (see Fig. 8.5) is able to increase the heat transfer coefficient by 59% at 0.21 vol.% particle loading, but the combination of TiO2 nanofluids with multiple twisted tape inserted tubes can further increase the heat transfer coefficient of smooth tube with water up to 3.52 times. In this way, it would be possible to substantially reduce the size of the heat exchanger. However, a main drawback is the concurrent important rise of the friction factor (up to 11.7 times higher). Salimpour et al. [90] investigated the heat transfer performance of TiO2 nanofluids flowing in helically horizontal corrugated tubes. Fig. 8.6 illustrates the scheme of a tube. They found that the Nusselt number increases with the augmentations of the corrugation depth, e, and width, p, (especially at high Reynolds number) and with the reduction of the corrugation pitch, W. The nanoparticle load generates an incremental effect on heat transfer in corrugated tubes with a higher corrugation depth and width, while corrugation pitch has an adverse effect on heat transfer. Oon et al. [91] studied the heat transfer coefficient of TiO2 nanofluids in a horizontal tube where the cross section changes abruptly. The results indicated that in turbulent flow, the heat transfer coefficient increases significantly not only with the Re but also with the step height ratio of the tube section. They attributed this behavior to the promotion of turbulence in the nanofluid caused by the abrupt variation of the section. p

e

W

d1

Figure 8.6 Sketch of a typical helically corrugated tube [90].

Heat transfer by using TiO2 nanofluids

293

8.4.2 Natural convection Natural convection often plays an important role in heat transfer and it is the only convection mode that takes place in pump-free systems, such as those that can be encountered in solar energy storage, nuclear cooling system, and some heat pipes. Nanofluids also in these cases may enhance the efficiency of the heat transfer process again resorting to their higher thermal conductivity. In the last years, many articles on the application of nanofluids in natural convection systems have been published.

8.4.2.1 Factors influencing natural convection heat transfer of TiO2 nanofluids The extent of natural convection depends on several factors as the position and the inclination of the heater or cooler, the boundary conditions, the temperature difference that drives heat transfer, the geometry of the apparatus where heat transfer occurs, the roughness of the surfaces, etc. Most of these factors are grouped into the Rayleigh number (Ra), which is the relevant dimensionless parameter for natural convection. It is usually defined as follows: Ra 5

ρβΔTl3 g μα

(8.21)

in which α is the thermal diffusivity, β is the thermal expansion coefficient, l is the characteristic spatial dimension of the system in vertical direction, and μ is the dynamic viscosity. Basak and Chamkha [92] investigated the natural convection of nanofluids in the case of two kinds of boundary conditions: (1) hot and cold vertical walls in the presence of adiabatic horizontal walls and (2) hot bottom with cold side walls in the presence of an adiabatic top wall. They evaluated the local values of Nu, which, for nanofluids, resulted to be higher than those of the base fluid (water) for any value of Ra. Also the overall heat transfer rates were improved with nanofluids. In both the two different boundary conditions, the enhancement of heat transfer rates obtained with nanofluids was of the same order of magnitude when Ra # 104. This outcome was attributed to the dominance of viscous force. On the contrary, the heat transport rate due to nanofluids dramatically enhanced at Ra 5 105 due to the dominance of buoyancy force. Al-Hafidh and Zubaidy [93] studied the natural convection heat transfer of TiO2
water nanofluid in an annular geometry consisting of a partially heated enclosure between two horizontal coaxial cylinders filled with a porous media (silica sand). The problem was solved numerically and the solutions showed that for the cold cylinder at Ra 5 800 and a load of TiO2 nanoparticles of 0.5 vol.%, the average Nu increased by 450%. For hot cylinder Nu increased by 489.5%, under the same operating conditions. Other results demonstrated that the enhancement of heat transfer was even larger at higher Ra.

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Titanium Dioxide (TiO2) and Its Applications

¨ g˘ u¨t [94], who analyzed the Another study based on a mathematical model is by O effect of Ra, particle volumetric fraction, and length of the heater on the heat transfer of water-based Ag, Cu, CuO, Al2O3, and TiO2 nanofluids in an inclined square enclosure with a constant heat flux boundary condition. Heat transfer rate increases both with Ra and the particle loading, while it decreases by increasing the length of the heater. As far as the inclination of the heater is concerned, he found a gradual decrease of the heat transfer rate as the heater length increased for smaller inclination angles. The maximum heat transfer takes place at 30 C, and the minimum heat transfer takes place at 90 C.

8.4.2.2 Influence of nanoparticle type and load It has been seen that the type and load of nanoparticles determine the physical properties of nanofluids, such as the overall thermal conductivity and viscosity. On the other hand, these properties are fundamental for the onset and the importance of natural convection, which is therefore influenced both by the type and load of nanoparticles. Indeed, the high thermal conductivity of nanofluids promotes natural convection motions and this mechanism becomes more important for nanoparticles with higher thermal conductivity such as Ag and Cu with respect to Al2O3, TiO2. The mathematical model developed by Cho et al. [95] concerned the entropy generation and natural convection heat transfer efficiency for Cu
water, Al 2O 3
water, and TiO 2
water nanofluids, in a system consisting of an enclosure with wavy vertical side surfaces and flat end surfaces. They observed an increase of Nu and a reduction of the total entropy generation by increasing the nanoparticle loading. Cu
water nanofluid was the best performing. Furthermore, they demonstrated that the natural convection heat transfer performance can be maximized and the total entropy generation minimized by using appropriate nanofluids and geometry for the wavy surface. The results on the same kind of nanofluids carried out by Aminossadati and Ghasemi [96] agree with this observation. As far as the effect of nanoparticle load is concerned, the published results indicate a main outcome: the existence in natural convection of an optimal volumetric fraction which maximizes the heat transfer coefficient. Below this optimal value the heat transfer increases with the nanoparticle load, while beyond this value it decreases. Khadangi Mahrood et al. [97] studied the natural convection heat transfer of non-Newtonian nanofluids in a vertical cylinder. The results indicate that nanofluid at lower particle loading can strongly increase the natural heat transfer coefficient. On the contrary, the increase of the particle loading at values higher than 1 and 0.5 vol.% (for Al2O3 and TiO2, respectively) has adverse effects on heat transfer. Indeed, the heat transfer enhancement passes through a maximum that have an optimum nanoparticle load. The optimum load is 0.2 and 0.1 vol.%, for Al2O3 and TiO2, respectively.

Heat transfer by using TiO2 nanofluids

8.5

295

Boiling heat transfer of TiO2 nanofluids

In recent years, there has been a growing interest on heat transfer in fluids at boiling conditions and, more generally, in two-phase flow. The interest is motivated by their important applications in nuclear reactors and rockets and by the very high transfer rate which can be obtained in these “boiling” systems. BHT is a process where a phase change from liquid to vapor takes place. Two basic types of boiling heat systems exist: pool boiling and flow boiling. In the first systems the formation of the vapor bubbles occurs on a heating surface submerged in a pool, where at the beginning the liquid is quiescent, then it moves mainly thanks to the motions induced by the buoyant bubbles. The second is the boiling in a flowing stream of fluid, in which the heating surface generally is the wall of the channel confining the flow. In this last case the bubbles are well mixed throughout the liquid and there is the coexistence of both the phases in a two-phase flow [98]. There are several boiling regimes both in pool boiling and in flow boiling. Anyhow, in flow boiling the forced convection in the channel has a main role, whereas in pool boiling the buoyancy of the vapor bubbles is the principal mechanism for the motion of the fluid. The applications of BHT in modern industry are so important that many studies were deserved to clarify and identify its behavior and the underlying mechanisms. Boiling phenomena have not yet been fully understood due to the complexity and the relatively unsatisfactory reproducibility of the experiments. This is a consequence of the uncertainty and the variance of the surface conditions (i.e., surface roughness, absorption of gas, and the presence of foreign materials on the surface). The factors influencing BHT are really numerous. Many experimental and theoretical studies on these factors both for pool and flow BHT, including heater parameter (material, shape, size, and surface roughness) and fluid parameters, have been carried out [98
101]. In BHT too the performance can be improved by the addition of nanoparticles to the base fluid. One of the reasons of the improvement is that the presence of nanoparticles increases the surface available for the formation of the vapor bubbles. As a matter of fact, the existence of reentrant cavities promotes nucleation by trapping the vapor, thus enhancing both evaporation and heat transfer [102]. The BHT coefficient and the critical heat flux (CHF) quantify the performance of a BHT processes. The CHF represents the heat flux beyond which some adverse phenomena (for instance the formation of a vapor blanket onto the surface of the heat exchanger) appear that limit the same heat flux. Most of the experiments involving BHT with TiO2 nanofluids are carried out in pool-type systems. Nucleate pool boiling is a boiling type that occurs at surface temperature of c. 5 C higher than the saturation temperature of the liquid. This is an optimal condition as it allows high heat transfer rates with a small increase of temperature. Generally, two methods are adopted to increase the heat transfer rate in this case. The first one is to increase the nucleation sites by the treatment of a surface, the

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Titanium Dioxide (TiO2) and Its Applications

other consists in the utilization of nanofluids. The Rohsenow correlation [103] estimates the heat transfer coefficient for nucleate pool boiling: 0

1 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi C p ðT s 2 T sat Þ σ B q  C 5 Csf @ A hPrm μh g ρ 2 ρ g

(8.22)

where q is the heat flux, Csf is a coefficient (subscript sf stands for “surface fluid”), and m is a constant. Pioro [104] developed a different correlation: 0 hl B 5 0:075Csf @ k

10:66 q C h  i0:25 A r fg ρ0:5 g σg ρ2ρg

Prm

(8.23)

h   i0:5 where l is the pool boiling characteristic dimension: l 5 σ= g ρ2ρg , g is the gravitational acceleration, σ is the surface tension, and rfg is the latent heat of vaporization (subscript g refers to saturated vapor). Csf and m depend on the fluidheating surface combination. According to the pertinent literature [105
110], there are five main factors that influence the BHT performance of TiO2 nanofluids: nanoparticle type, nanoparticle loading, surface roughness, heater material, and ionic additive.

8.5.1 Influence of nanoparticle type The results of the published studies establish that the enhancement of BHT and CHF obtained with nanofluids varies with the type of nanoparticle. Ding et al. [111] studied the performance of TiO2 and Al2O3 nanofluids with water as base fluid. BHT increases with both kinds of nanofluids and raises with nanoparticle concentration. The most efficient nanofluid was that with TiO2 and this result was justified by the differences between nanoparticle properties. Conversely, the paper by Singh et al. [112] reports for the same particle load similar cooling rates for the different kinds of tested nanofluids (with TiO2, Al2O3, and SiO2 nanoparticles). According to the authors, this behavior is a consequence of the similar size of nanoparticles. Moreover, they found that with the nanofluids the shift from film boiling to transition boiling on the jet cooled surface occurred earlier than with pure water.

8.5.2 Influence of particle loading Celata et al. [78] found that the roughness of the heater surface increases by increasing the amount of TiO2 particles because of the deposition of nanoparticles

Heat transfer by using TiO2 nanofluids

q

Vapor bubble

297

Liquid microlayer

Heated surface

Figure 8.7 Schematic of the mechanism of micro layer deposition [11].

onto the surface. On the other hand, Mitra et al. [113] in their study of BHT in laminar regimen of TiO2
water nanofluid for a jet cooling system observed that the use of nanofluids could increase the cooling rate, but the increase of the particle concentration does not improve BHT. Moreover, they found that the use of nanofluid could induce a faster shift from film boiling to transition boiling. Other studies show an enhancement of BHT for small nanoparticle loadings which then diminishes at higher amount [114]. It could be argued that high nanoparticle concentrations could cause higher viscosities and smaller bubble size, thus inducing a reduction of the BHT rate, as suggested by Qi et al. [115]. In general, overheating could favor the BHT for different amounts of nanofluids. However, some papers report a decrease of BHT coefficient with the particle loading. Indeed, as observed by Kwark et al. [116], even if the nanoparticle deposition can increase nucleation, a decrease of the BHT coefficient is measured when the nanoparticle loading increases. The authors hypothesize a mechanism (Fig. 8.7) which assumes that, even if the deposition of nanoparticles increases the vaporization nucleus, the thermal resistance increases due to the thicker coating. Fig. 8.7 shows the mechanism of deposition.

8.5.3 Influence of surface roughness The surface roughness is considered the main factor for BHT and CHF enhancements. In almost all publication on the topic, it has been stated that nanoparticles settle on the heater surface. This deposition is responsible for the increase of the surface area and roughness as well as wettability, thus promoting nucleation by vapor trapping and evaporation. Kim et al. [105] measured the roughness of an NiCr wire after pool boiling for different contents of TiO2 nanoparticles in the nanofluid. They observed a significant increase of roughness after the pool boiling process due to particle deposition. The peak and valley structure formed by the deposition affected the vapor bubble growth. Salari et al. [117] too explained the obtained results on the basis of surface roughness. Their results showed an increase in BHT by increasing the nanoparticle

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Titanium Dioxide (TiO2) and Its Applications

amount and the heat flux. The deposition of nanoparticles intensified the bubble transport due to the increase of nucleation sites on the boiling surface.

8.5.4 Influence of the heater material Some researches considered also the effect of heater material on BHT. For example, Suriyawong and Wongwises [114] studied the nucleate pool BHT of water-based TiO2 nanofluids at a particle loading range of 0.000005
0.01 vol.%. They found that for copper surfaces, with roughness of 0.2 and 4 μm, the BHT coefficients increased 15% and 4%, respectively, at a load of 0.0001 vol.%. Higher loading had a detrimental effect on BHT coefficients. Aluminum surface, with the same roughness of 0.2 and 4 μm, demonstrated to give BHT coefficients higher by 30% and 27%, respectively. The BHT coefficient for a roughness of 4 μm was higher than for a roughness of 0.2 μm.

8.5.5 Influence of ionic additive Ionic species are used to improve the stability of the dispersion. During a pool BHT, they could create a more uniform and smooth structure on the heater surfaces, thus leading to a decrease in CHF enhancement. These insights were confirmed by Jung et al. in their studies of TiO2 water nanofluids in pool boiling at CHF conditions, by using nitric acid as the ionic additive. Some researches focused on the influence of ionic additive on the CHF of TiO2 nanofluids. Jung et al. [118] carried out an experiment with TiO2/water nanofluids to study the relationship between CHF onset in pool boiling and ionic additive (i.e., nitric acid). They found that ionic additive improved the dispersion stability but reduced the CHF increase obtained with nanofluids. They stated that the ionic additive created a more uniform and smoother structure on the heater surfaces, leading to a lower CHF improvement in TiO2 nanofluids.

8.6

Applications of TiO2 nanofluids

TiO2 nanofluids can be used in all devices that employ a liquid media for heat transfer [119]. Then the possible reasonable application is in heat exchangers. Heat transfer in heat exchanger takes place between two fluids, one at high temperature and the other at low, but without a direct contact between them because a solid material separates them. In theory, higher heat transfer rates in these devices can be simply obtained by using nanofluids, mainly because their heat transport properties are superior. The present chapter reports the correlations found by different researchers to evaluate heat transfer coefficients of systems with nanofluids. They can be used, for instance, for the design of the heat exchanger.

Heat transfer by using TiO2 nanofluids

299

A heat sink is a form of heat exchanger, which is adopted to absorb effectively the heat in excess of a system in order to maintain the optimal value of temperature or to avoid hazardous conditions. Heat sinks are generally made of conductive metals with air or liquids which cool the same metal. The higher the surface area in contact with air or cooling liquid, the more effective is the heat sink. But it is not possible to increase the surface beyond given limits, then nanofluids with higher thermal conductivity offer the possibility to increase the efficiency of the heat transfer apparatus. Several researchers studied these systems by using different nanofluids and in different conditions [120
123]. Another important application of TiO2 nanofluids is in the field of road haulage. Indeed, radiator (assimilable to a cross flow heat exchanger) is an important component of automobile. If there is the need to remove high amounts of heat, the size of radiator should be increased with an undesirable increase of both the volume and weight. In order to enhance the compactness and effectiveness of radiators, the use of coolants with the addition of nanoparticles on the base fluids might help. An automotive cooling system was studied by Hussein et al. [124] which analyzed the effects of volumetric flow rate, inlet temperature, and volumetric fraction on Nusselt number. They found that Nu increases as the volumetric flow rate, inlet temperature, and volume concentration are increased. For this application, Wadd et al. [125] investigated the performances of metal (Cu
water) and nonmetal (TiO2
water) nanofluids. The stability of TiO2
water nanofluid was higher than Cu
water nanofluid but the latter had a better thermal conductivity. TiO2 nanofluids have been proposed also for the cooling of electronic devices, which represents a challenging task due to compactness and high heat dissipation required by these systems. Different approaches can be used to enhance the thermal efficiency of electronic systems. One of them is to increase the thermal performance of the coolant with nanofluids that offer better thermal performance than base fluid. The selection of a nanofluid for computer cooling must be made considering the special needs of these systems in terms of thermal performance, economical aspects, and chemical and corrosion compatibility. In this field the literature presents comparisons between different nanofluids taking into account the obtainable thermal conductivity enhancement and heat flux as a function of the particle load [126,127].

8.7

Future investigations

During the drafting of the chapter, the huge amount of publications in the literature was evident in the form of articles and reviews on the preparation, the characterization, the properties, the performance, and the possible heat transfer applications of different types of TiO2 nanofluids. For this reason, it was not easy to present an allinclusive overview of the relevant information on their different aspects. Anyway, even if a lot of material has been published on the subject, in parallel the awareness of unsolved problems emerged. This means there is large room for

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Titanium Dioxide (TiO2) and Its Applications

further investigations on the subject of TiO2 nanofluids. The factual convenience of the utilization of TiO2 nanofluids has not yet been ultimately demonstrated. For instance, even it is well established that nanofluids show higher thermal conductivity than the base fluids, also their viscosity is higher. So, the search for nanofluids with improved heat transfer properties and limited increase of the viscosity must continue. As far as the theoretical and experimental approach to nanofluids, some aspects still need attention. Indeed, various authors adopted different evaluation approaches to analyze the results obtained in their experiments. The use of a unified standard procedure, with adequate evaluation methods of the properties of nanofluids, should overcome the inconvenience and be a guide for the optimization and preparation of nanofluids. Another important problem is related to long-term and high-temperature performance of the stability of the dispersion and the heat transfer properties. The data in literature seem not present a definite solution to the problem. Many authors showed that sedimentation and the adhesion of nanoparticles to the heat transfer surfaces are quite unavoidable in long-term runs. Therefore both time dependence of heat transfer parameters and their relationship to the sedimentation behaviors need to be deeply investigated. The stability of the nanoparticle dispersion can be ensured by the utilization of a dispersant, but even this solution is not without drawbacks, which need to be solved. Throughout the sections of this chapter, the advantages and the consequences of the use of surfactants have been considered with contrasting results. Although it has been established that their use is positive in both dispersion stability and nanofluid thermal conductivity, it seems that their influence on convective heat transfer (also in long-term performance) needs to be further analyzed. Another challenge for the engineering use of nanofluids is their high viscosity that rises the required pumping power. Then, in determining the heat transfer coefficients of nanofluid, also the extra consumption of energy should be considered. This aspect needs a global evaluation on particle amount, dispersion method, and surfactant types in order to obtain the optimum heat transfer performance with the minimum increase of pumping power. Finally, due to the complexity of crystal forms and composition of TiO2, different samples could exhibit enormous variations that make very difficult the design of the system. Moreover, TiO2 is easy colored and the thermal fluid could become opaque making difficult its observation and regulation. These drawbacks represent an opportunity to focus the research for future exploration.

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82.

TiO2 as white pigment and valorization of the waste coming from its production

9

Manuel Jesu´s Ga´zquez1, Silvia Marı´a Pe´rez Moreno2,3 and Juan Pedro Bolı´var2,3 1 Department of Applied Physics, Marine Research Institute (INMAR), University of Cadiz, Ca´diz, Spain, 2 Department of Integrated Sciences, Faculty of Experimental Sciences, University of Huelva, Huelva, Spain, 3 Research Centre of Natural Resources, Health and the Environment (RENSMA), University of Huelva, Huelva, Spain

9.1

Introduction

In 1791 William Gregor (1761
1817), a British clergyman and mineralogist, discovered titanium while studying the black magnetic sands from Menachan in Cornwall (England). Shortly after, in 1795, German chemist Martin Heinrich Klaproth (1743
1814) managed to isolate TiO2 from the mineral rutile, which he called tita (“Titan” in Greek). This name was explained by Klaproth as folnium, after Τιταν lows: “Whenever no mane can be found for a new fossil which indicates its peculiar and characteristic properties (in which situation I find myself at present), I think it best to choose such a denomination as means nothing of itself, and thus can give no rise to any erroneous ideas. In consequence of this, as I did in the case of Uranium, I shall borrow the name for this metallic substance from mythology, and in particular from the Titans, the first sons of the earth. I therefore call this new metallic genus TITANIUM; of which this titanium, is, indeed the first, but perhaps not the only species as is made probable by the following essay” (from Analytical Essays Towards Promoting the Chemical Knowledge of Mineral Substances). Thirty years later, in 1825, Jo¨ns Jacob Berzelius (1779
1848), considered one of the founders of modern chemistry, was the first person to isolate titanium. In addition, the first pigments (in the anatase form) were produced by mixing ilmenite (FeTiO3) with sulfuric acid, carrying out a hydrolysis process by adding calcium or barium sulfate. Thus, in 1916, the Titanium Pigment Corporation of Niagara Falls, New York, and the Titan Co. AS, Norway, simultaneously began the commercial production of titanium dioxide pigments. Later, in the 1940s, titanium pigments (in the rutile form) were obtained by using sulfuric acid. Then, in the 1950s, following the chloride route (developed by DuPont), this pigment was manufactured also in the rutile form, beginning the widespread use of the pigment. Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00011-0 © 2021 Elsevier Inc. All rights reserved.

312

Titanium Dioxide (TiO2) and Its Applications

Finally, the manufacture of Ti metal also started in the 1950 decade (following Kroll’s process), mainly due to the considerable advance of the aircraft industry.

9.1.1 Titanium minerals Titanium, symbol Ti and atomic number 22, is one of the transition metals in Group IVB of the periodic table, with relative atomic mass 47.867. Its main valence state is 41 , although can be 31 and 21 , which are less stable. Titanium is the ninth most abundant element in the Earth’s crust (with 0.64 w/w%) and the fourth most widely used metal after aluminum, iron, and magnesium, with an abundance of around 5 times smaller than iron and 100 times greater than copper [1]. Titanium is not free in nature and must be extracted by several processes. The titanium ores that are mined are ilmenite [mixed oxide of titanium and iron (FeTiO3 or FeO  TiO2)], rutile, anatase, and brookite, which have the same formula (titanium dioxide TiO2) and different crystalline structures. Other less common titanium oxide-bearing minerals are pseudobrookite (Fe2TiO5), perovskite (CaTiO3), geikielite [(Mg, Fe)TiO3], pyrophanite (MnTiO3), sphene or titanite (CaTiSiO5), and leucoxene (Fe2O3  TiO2) [2]. The properties of these titanium minerals are listed in Table 9.1. Leucoxene is not a true mineral, but a finely crystalline aggregate of rutile, anatase, or brookite with a percentage of TiO2 above 70%, which is the result of the weathering of ilmenite [3]. Table 9.1 Some properties of titanium minerals [2,4,5]. Name (formula)

% TiO2

Color

Crystal form

Hardness

Density (g/cm3)

Ilmenite (FeTiO3 or FeO  TiO2) Rutile (TiO2)

35
70

Black

Hexagonal

5.0
6.0

4.5
5.0

93
96.5

Tetragonal

6.0
7.0

4.2
4.3

Anatase (TiO2)

93
96.5

Tetragonal

5.0
6.0

3.8
3.9

Brookite (TiO2)

93
96.5

Orthorhombic

5.5
6.0

4.1
4.2

Perovskite (CaTiO3)

50
60

Monoclinic (pseudocubic)

5.5

4.2
4.5

Sphene or titanite (CaTiSiO5)

35
40

Dark-red, metallic gray, brownishred, yellowish Dark blue, black, reddish-brown, brown, honey yellow Brown, yellow-brown, yellow-orange, greenish-gray, black Black, brown, reddish-brown, yellow Brown, green, gray, yellow, black

Monoclinic

5.0
6.0

3.4
3.6

TiO2 as white pigment and valorization of the waste coming from its production

313

The most economically important titanium oxide is ilmenite, usually associated with magnetite, and is found in rocks and certain beach sands. In some cases the quality of ilmenite can decrease due to the substitution of titanium with aluminum, calcium, chromium, copper, magnesium, manganese, silicon, vanadium, and zinc in the ilmenite crystal structure [6]. Rutile is usually associated with sedimentary, igneous, and metamorphic rocks. Other titanium minerals, such as anatase and brookite, are closely related to the deposit of rutile [2]. While it is true that all these minerals are too widely dispersed, the main commercially important deposits are low and limited to certain locations of Canada, China, the United States, South America, Norway, and South of Australia [7]. As we can see in Table 9.2, around 93.6% of the world’s titanium resources correspond to the mineral ilmenite; the remaining percentage is mainly from rutile and, to a lesser degree, leucoxene. In addition, it is important to note that the production capacity of titanium dioxide pigments corresponds to around 1.3 Mt in both the United States and Europe, 3.2 Mt in China, and 5.7 Mt in the rest of the world.

9.1.2 Titanium ore purification Ilmenite and rutile are the most important mineral sources of titanium dioxide pigments; however, it is important to note that these minerals contain other metals in addition to titanium. Depending on the deposits, titanium minerals can contain impurities that must be removed before carrying out the industrial process to obtain titanium dioxide pigment. Nowadays, TiO2 pigment producers are requested to use titanium feedstock ores with higher TiO2 levels, with the aim of minimizing waste disposal costs and maximizing the capacity of pigment-processing equipment. When titanium dioxide pigment is produced by the sulfate route, the use of ilmenite mineral as raw material is needed, since rutile cannot be dissolved using sulfuric acid, as is detailed in Section 9.2. Typically, this mineral contains 30%
70% of TiO2, with the rest being mainly iron, which must be removed during the manufacturing process in order to guarantee the quality of the final product. Thus large amounts of by-product iron salts are produced. Therefore the demand for feedstock with high concentration of titanium dioxide has been met by the production of titaniferous “slags.” These titaniferous slags are coproducts of smelting processes where iron is removed from the ilmenite ore [9,10]. Typically, the feedstock for slag manufacture is ilmenite with a concentration of titanium dioxide below 50%, finally containing about 85% TiO2; however, through high-pressure acid leaching, titanium slag can be enriched to about 95% TiO2 [11]. On the other hand, high-grade TiO2 feedstocks, that is, with high percentage of TiO2, are necessary in chloride process plants. Thus while ilmenite sand can be directly used in the sulfate or chloride routes for manufacturing titanium dioxide, rutile and leucoxene cannot be attacked with sulfuric acid and only can be used directly in the chloride process. Therefore upgraded feedstocks, such as synthetic rutile (obtained by two basic processes, i.e., the Becher process and the Benilite process), synthetic rutile enhancement process (SREP) (modification of the Becher

Table 9.2 Mining production, reserves, and pigment production in 1000 t [8]. Ilmenite Mine production

United States Australia Brazil Canada China Germany India Japan Kenya Madagascar Mozambique Norway Senegal Sierra Leone Mexico Russia Saudi Arabia South Africa Ukraine United Kingdom Vietnam Other countries World total

2017

2018b

100 730 50 880 840
300
280 110 600 220 300



550 230
200 150

100 700 50 850 850
300
280 100 600 200 250



500 230
200 150

NA: Not available. a Yearend operating capacity. b Estimated. c US rutile production and reserves data are included with ilmenite.

Pigment productiona

Rutile Reserves

2000 250,000 43,000 31,000 230,000
850,00
54,000 40,000 14,000 37,000 NA



63,000 5900
1600 26,000 880,000

Mine production 2017

2018b

c

c

290



10
87

250



10
90

9

8

10 160


95 95

13

8 170


100 100

10

Reserves 2018
29,000



7400
13,000
880
NA 490


8300 2500

400 62,000

1370 260
104 3250 472 108 314





300 55 210
120 315
784

TiO2 as white pigment and valorization of the waste coming from its production

315

process involving the addition of a flux at the thermal treatment stage), and upgraded slag, can only be used in the chloride process [12
14]. Synthetic rutile (classified in thermal reduction, selective chlorination, and selective leaching) is produced by removing iron from ilmenite, which increases the TiO2 content to nearly 95% from about 50% [15]. Synthetic rutile can be used as a substitute for natural rutile [3].

9.2

Routes for the manufacture of titanium dioxide pigments (Pigment White 6)

About 93% of the titanium extracted in the world is used in the manufacture of TiO2 for pigment production [16]. Currently, commercial titanium dioxide pigment can be produced by two different routes: the sulfate process (about 40% of total TiO2 production) and the chloride process (about 60%). The anatase and rutile forms can be obtained by the sulfate route, while the chloride route can only yield the rutile form [17]. The sulfate process employs simpler technology compared to the chloride route, and it can use lower grade, cheaper ores; however, due to the acid treatment involved, a sulfate plant is more expensive to build than a chloride plant. In addition, the general perception is that the sulfate route is less environment-friendly; however, in the last decades the recycling and valorization of the wastes produced can make it as clean as the chloride route, where the amounts of generated wastes are lower. Nevertheless, due to such belief, the chloride process has dominated the pigment industry for the last few years. The choice between the two processes is based on several factors, such as the availability of specific raw materials, waste disposal costs, and requirements of final titanium dioxide grade. For example, the chloride process is the favorite form for use in coating and plastics, the two most common end-use markets, while the pigment from the sulfate process is preferred to use in selected paper products, manmade fibers, food products, pharmaceuticals, and cosmetics. Another aspect to consider is that the sulfate process can produce a volume of waste in the range of 8
12 t per ton of pigment, whereas the chloride process generates waste products in the range of 2
5 t per ton of pigment.

9.2.1 The chloride process The development of the chloride process started in the 1940s. The first chloride plant, owned and run by DuPont, began to produce in the early 1950s, and there are currently 23 chloride process plants operating in North America, Europe, Asia, and Australia [18]. The chloride process route accounts for 55%
60% of the 5.5 million tons/per year of pigment production worldwide, involving the use of gaseous chlorine to produce TiCl4, which is then converted to titanium dioxide by oxidation [5].

316

Titanium Dioxide (TiO2) and Its Applications

The chloride process includes a wide range of titaniferous feedstocks containing high-grade titanium dioxide, such as ilmenite sand, rutile, synthetic rutile, SREP, and upgraded slag. The selection of feedstock depends on the presence and percentage of heavy metal impurities, as they can influence the quality of the final product (e.g., whiteness and brightness). The chloride process, schematized in Fig. 9.1, is carried out in two main steps: first, the conversion of ore to titanium(IV) chloride and, second, the oxidation of titanium(IV) chloride. The process begins with the conversion of feedstock to titanium tetrachloride (vapor form) by chlorination in a fluidized bed reactor at 900 C
1000 C, using petroleum coke as a reducing agent. The main reactions are the following. 2TiO2 ðsÞ 1 3CðsÞ 1 4Cl2 ðgÞ ! 2TiCl4 ðgÞ 1 2COðgÞ 1 CO2 ðgÞ

(9.1)

TiCl4 ðgÞ 1 O2 ðgÞ ! TiO2 ðsÞ 1 2Cl2 ðgÞ

(9.2)

In the ores, oxygen is reacted with carbon to form carbon monoxide and dioxide (see Eq. 9.1). The titanium chloride vapor obtained is cooled down and recollected in its liquid form, which is possible thanks to the fact that its boiling point is higher than that of other metal chlorides. Then, it is boiled again and distilled to produce a purer product. At this stage, the nonvolatile chlorides and the unreacted coke and feedstock solids are removed from the gas stream and from the bottom of the chlorinator (see Fig. 9.2). For example, high boiling point chlorides such as CaCl2 and MgCl2 tend to remain in the bed in their liquid form, while SiO2 and ZrO2 tend not to chlorinate and accumulate in the bed in their solid form.

Figure 9.1 Diagram of the chloride process.

TiO2 as white pigment and valorization of the waste coming from its production

317

Figure 9.2 Diagram of the sulfate process.

Next, the liquid titanium chloride is transferred to an oxidation reactor, where it is mixed with oxygen at around 1500 C (in a plasma arc furnace or toluene-fired furnace), forming titanium dioxide and chlorine gas, as expressed by Eq. (9.2). This chlorine is recycled again in the first steps of the industrial process. The residual chlorine attached to the solid TiO2 is removed by hydrolysis and, finally, the pure titanium dioxide is taken through a conditioning step, where it is subjected to chemical surface treatments, milling, and drying. Currently, about 1 t of chlorine is required to produce 5
6 t of titanium dioxide pigment (depending on the percentage of impurities in the feedstock used).

9.2.2 Sulfate process The sulfate process was the first commercialized technology to obtain titanium dioxide pigment in 1916. The sulfate route represents 40%
45% of the total TiO2 production, and it involves the use of concentrated sulfuric acid. The feedstocks of this route are mainly ilmenite and titanium slag [5,17]. The sulfate process can be divided into three main steps: feedstock digestion with ore dissolution, formation of hydrated titanium dioxide, and, finally, formation of anhydrous titanium dioxide. The sulfate process, schematized in Fig. 9.2, begins with the acid digestion step of ilmenite or titanium slag (or a carefully controlled blend) with concentrated sulfuric acid (around 95%), water (to activate the reaction), and recycled acid (around 65%).

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Titanium Dioxide (TiO2) and Its Applications

The result of this step is a liquid effluent containing titanyl sulfate (TiOSO4) and iron sulfate (FeSO4) (see Eq. 9.3). FeTiO3 ðsÞ 1 2H2 SO4 ðaqÞ ! TiOSO4 ðaqÞ 1 2H2 OðlÞ 1 FeSO4 ðaqÞ

(9.3)

To ensure that all Fe is in dissolution, the liquor is passed through scrap metal to convert Fe31 to Fe21, which is called the “reduction step.” At this point, the resulting liquor is sent to a clarification tank, where the undissolved solids (mud) are separated from the solution by flocculation (decantation) and filtration. The titanium liquor is concentrated and hydrolyzed with steam in order to produce the precipitation of hydrated titanium dioxide (Eq. 9.4). TiOSO4 ðaqÞ 1 ðn 1 1ÞH2 OðlÞ ! TiO2 nUH2 OðsÞ 1 H2 SO4 ðaqÞ

(9.4)

After boiling for several hours, the liquor is cooled down to around 60 C, in order to precipitate the hydrated titanium dioxide. This step is essential for the control of the final crystal size and form (anatase or rutile) of the titanium dioxide, which is achieved through the addition of titanium-containing seed nuclei. Then, the titanium dioxide hydrate is separated from the liquor (commonly referred to as “strong” acid, 20%
25% H2SO4) through filtration using vacuum filters (known as Moore filters). The quantity of strong acid generated is more important when the iron content of the ilmenite ore is high. After filtration, the filtered TiO2 cake is washed with water to remove the remaining impurities, obtaining a solid phase containing TiO2 and a liquid phase usually called “weak acid solution.” Then, the hydrated titanium dioxide is sent to a calciner, where the titanium dioxide crystals grow to their final crystalline size, and the residual water and H2SO4 are removed (see Eq. 9.5). TiO2 nUH2 OðsÞ ! TiO2 ðsÞ 1 nUH2 OðgÞ

(9.5)

The dried titanium dioxide is sent to pigment finishing, involving any required milling and or chemical treatment, such as surface coating with silica or alumina. The milling process is carried out in two different steps. In the first step the particle is reduced to 75
100 μm and, after the second step, the final size is 0.2
0.4 μm, which is the optimal size for use as pigment. In addition, the coating steps are very important, as they improve both pigment durability and dispersibility. To produce the anatase form of the titanium dioxide, a small portion of the clarified liquor is neutralized with alkali to produce anatase microcrystals. Then, these microcrystals are introduced into the mother liquor, which is then hydrolyzed under carefully controlled conditions to produce anatase crystals. To produce the rutile form of the titanium dioxide, the clarified liquor is hydrolyzed in the presence of a specially prepared rutile seeding agent obtained by neutralizing a small portion of the mother liquor in the presence of hydrochloric acid or some other monohydric acid.

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Finally, both crystals formed are filtered, washed, calcined, and micronized. About 1 ton of feedstock (ilmenite or ilmenite 1 slag) is required to produce 0.5 tons of titanium dioxide pigment.

9.3

Properties and applications of Pigment White 6

In the last two decades, there has been an increased research interest in the properties and uses of titanium dioxide. In this sense, it is important to note that the number of manuscripts published up to the year 2000 was around 2800, which considerably increased to around 12,000 papers up to 2010 and 20,000 papers up to 2019 (data obtained from www.sciencedirect.com web page). This is due to the interesting and special physicochemical properties of TiO2 and its wide use. Pigment is defined as a group of relatively neutral, colorless, insoluble, and solid particles. Usually, the role of pigments is to change the color of the incident light on a surface, although they can also have other purposes, such as protection from solar radiation. It is currently commercialized in two different forms, rutile and anatase. Rutile is the most stable abundant form of titanium dioxide, showing a more compact structure than anatase, which gives rise to important differences in properties between these two crystalline forms. Rutile (melting point: 1825 C) has a higher refractive index (close to that of diamond), higher specific gravity, and greater chemical stability than anatase. Anatase has no specific melting point, as it is irreversibly transformed into rutile before melting. Nowadays, titanium dioxide is considered the most important white pigment in the market, due to its high efficiency in the dispersion of the incident light and its high opaque capacity and shine supplier. Thus titanium dioxide pigment provides exceptional whiteness. The following section analyzes the most important properties and main uses of titanium dioxide.

9.3.1 Properties In this section, we will define the main characteristics and properties of TiO2 pigments in order to understand their potential applications. A summary of the main properties of Pigment White 6 (PW6) will be described, such as its density, granulometry, mineralogy, solubility, refractive index, opacity, and ultraviolet (UV) absorption. In general, titanium dioxide is a white, solid, and inorganic substance that is thermally stable, nonflammable, poorly soluble, and not classified as hazardous according to the United Nations’ Globally Harmonized System of Classification and Labeling of Chemicals. Color index (CI) is a reference database jointly managed by the Society of Dyers and Colorists and the American Association of Textile Chemists and Colorists, where all pigments are classified with a name and number. This system provides information about which pigments have been used in the manufacture of colors and chemical components, and about the properties of these.

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Titanium dioxide primarily delivers outstanding whiteness, brightness, and opacity by scattering light. Therefore the main purpose of a white pigment is to optimize the scattering of visible light (as opposed to absorption), which can be carried out in three ways, that is, refraction, reflection, and diffraction, increasing the white opacity. Titanium dioxide has the highest refractive index of all known white pigments (. 2.4, see Table 9.3), which means it has the greatest opacity. Reflection and refraction are maximized by increasing the difference between the refractive index of the pigment and that of the polymer matrix or other materials in which it is dispersed. In this sense, titanium dioxide in the rutile phase shows the highest refractive index (2.7) and thus the highest refractive scattering (see Table 9.3). Therefore relatively low levels of the pigment are required to achieve a white opaque coating. For example, other pigmented materials using substances such as zinc oxide, china clay, or blanc fixe would require much larger quantities of pigment; this can cause “crowding,” reducing the light-scattering properties and, consequently, the physical performance of the product. In this sense, TiO2 is unique in that it combines a high refractive index with a high reflectance in the visible region of the spectrum, for both rutile and anatase crystal forms. Therefore there is currently no alternative pigment available in the market in enough quantities and showing similar properties that can match the opacity, hiding power, cost efficiency, inertness, or weather ability of TiO2.

Table 9.3 Characteristics of some typical white pigments. Pigments

Refractive index

Specific gravity

CI

White lead ( )

1.94
2.09

6.70
6.86

Zinc white (Chinese white or Zinc oxide) Zinc sulfide

2.08

5.5
5.6

2.37

4.0

Anatase

2.53

3.70
3.85

Rutile

2.71

3.7
4.2

Lithopone (30% of zinc sulfide)

1.84

4.3

Antimony oxide

2.08
2.35

5.6
5.7

China clay (known as bentonite or kaolin clay) Blanc fixe (barium sulfate)

1.57

2.6

1.6

4.0
4.4

CI Pigment White 1 no. 77597 CI Pigment White 4 no. 77947 CI Pigment White 7 no. 77975 CI Pigment White 6 no. 77891 CI Pigment White 6 no. 77891 CI Pigment White 5 no. 77115 CI Pigment White 11 no. 77052 CI Pigment White 19 no. 77004 CI Pigment White 21 no. 77120

Font: [19,20] ( ) CI PW2 No 77633 Lead Sulfate White, CI PW3 No 77630 Basic Lead Sulfate White. CI, Color index.

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In addition, both refraction and diffraction parameters must be extremely carefully controlled, not only in the composition but also in the physical dimension. Thus it is important to note that the particle size must be carefully controlled, since the optical theory (Mie theory) states that the highest diffractive scattering occurs with particle sizes that are approximately half that of the incident radiation to be scattered. This requires a primary particle size of approximately half the wavelength of the light to be scattered, which is half of 400
700 nm for visible light. Since the human eye is most sensitive to yellow-green light (wavelength about 0.55 μm), the theoretical optimum particle size of TiO2 pigments for coatings is between 0.2 and 0.3 μm in diameter. Thus most commercial products that are used as pigments have modal primary particle sizes within this range. Pigment-grade TiO2 is manufactured to optimize the scattering of visible light and, consequently, white opacity. In addition, this combination is a very important aspect to take into account by coating manufacturers, as it allows achieving a route to highly opaque and bright whites or tints at minimum film thicknesses.

9.3.2 Applications Titanium dioxide is the most versatile white pigment (usually called PW6 or CI 77891), as it is used in a wide range of applications, especially in paints and coatings (57%
58%), plastics and rubber (24%
25%), paper (12%), inks (6%); the remaining 3% correspond to synthetic fibers and the food, pharmaceutical, and cosmetic industries [21,22]. Due to the relatively low price of the raw material and its processing, titanium dioxide has been steadily gaining importance in the manufacturing processes over recent decades. The following sections describe all of these uses.

9.3.2.1 Coatings, plastics, and paints This section describes how the use of TiO2 is essential for opaque paints, coatings, and plastics. The success of this pigment lies in its excellent light-scattering capability, making its opacity unbeatable. In addition, it is important to note that TiO2 is not only present in white paint, as it is also added to color shades. Due to its light-scattering properties, anatase (pigment with lower quality than rutile) is mainly used in the manufacture of paints and, more specifically, indoor paints, since quality is less important for this use [23]. On the other hand, rutile is preferred to paints and plastics, especially those exposed to outdoor conditions [21]. Rutile and anatase pigments can be made more resistant to photodegradation by coating the pigment particles with, for example, alumina, silica, zirconia, or a combination of these. Usually, rutile pigments contain 1%
15% coating and anatase pigments contain 1%
5%, with the higher levels of the coating being used for applications such as flat (low-gloss) paints [24]. According to the American Society for Testing and Materials (ASTM)-D476-84 standard [25], there are four different types of titanium dioxide pigment [4] for surface coating, which are as follows: G

Type I (containing at least 94% titanium dioxide) is a titanium dioxide
anatase pigment used in white interior and exterior house paints.

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G

G

G

Titanium Dioxide (TiO2) and Its Applications

Type II (containing at least 92% titanium dioxide) is a titanium dioxide
rutile pigment mainly used in all types of interior paints, enamels, and lacquers. Type III (containing at least 80% titanium dioxide) is also a titanium dioxide
rutile pigment used mostly in alkyd and emulsion flat-wall paints. Type IV (containing at least 80% titanium dioxide) is another titanium dioxide
rutile used in exterior paints due to its excellent durability and gloss retention.

In Europe the grading system is defined by the International Standard ISO 591-1 [26], which is different from the classification given by the ASTM standard. Depending on the type and percentage of titanium dioxide, titanium pigments are classified into the following grades: Type A (anatase grades: A1 and A2) and Type R (rutile grades: R1, R2, and R3). A1 (uncoated anatase pigment) and A2 (coated anatase pigment) contain 98% and 92% TiO2, respectively, and R1 (uncoated), R2 (coated), and R3 (coated) contain 97%, 90%, and 80% TiO2, respectively. In most countries of the world, either the ASTM or the ISO standard is used. A third system, that is, the Japanese grading system (JIS K5116-1973), specifies four grades of titanium dioxide
rutile, three of which contain at least 92% titanium dioxide and the fourth contains a minimum of 82% [4]. Many researchers have studied the use of titanium dioxide as coating in several applications, based on its photocatalytic properties, discovered by Fujishima in 1967 [27]. For example, Lo et al. [28] tested its application as anode material in secondary lithium-ion batteries, Zhong et al. [29] used coated TiO2 in air filters to retain volatile organic compounds, and Yemmireddy and Hung [30] used it as antimicrobial coating on stainless steel surfaces where food is processed. In addition, TiO2-based photocatalyst shows great potential in the disinfection/inactivation of several harmful pathogens (e.g., Escherichia coli) in aqueous media [31
34]. On the other hand, and taking into account its antibacterial properties, titanium dioxide is also employed as coating in the acrylic resin used as a denture base material [35] in order to minimize the adhesion of food [36], bacteria, and fungi [37]. Titanium dioxide pigment is used to increase the opacity of plastic materials. The plastic industry, which consumes around 25% of the global TiO2 production, is the second largest user after the coating industry, which represents over 65% of the pigment used in the plastic industry. Plastic applications are divided into two groups. In the first group of applications, titanium dioxide is used in a transparent or translucent condition, whereas in the second group the opacity is fully necessary. In addition, the use of TiO2 in plastics minimizes the fragility and surface cracking that may occur with prolonged exposure to light. It is the most important pigment used in the manufacture of outdoor polyvinyl chloride (PVC) plastic products, conferring UV protection to the material. In this case the best crystalline form is rutile, which shall be under a surface coating formed by zirconium, silica, or aluminum, in order to minimize the photocatalytic effect of TiO2 in PVC degradation [38].

9.3.2.2 Printing inks and paper Titanium dioxide pigment is used in a wide variety of products, such as paper (direct addition to whiten and opacify the paper stock, and paper coatings), and as

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the main component of inks (letterpress, gravure, and lithographic printing). Titanium dioxide is extremely lightfast and has a high refractive index and a very high light-scattering and hiding capacity. It also has the highest opacity of all white pigments and an excellent brightening capacity. In addition, titanium dioxide is thermally stable, noncombustible, nearly insoluble in water, and resistant to weather and UV light. Therefore in the applications related to the manufacture of printing inks and paper, it is very difficult to find an alternative product with similar properties. In the manufacture of paper and paperboard, the end properties and quality mainly depend upon the resistance of the fiber net and the extent to which it is intertwined. In addition, the spaces between the fibers of cellulose must be filled with nonfibrous materials (inorganic additives), usually called pigments. One of the most widely used inorganic additives is titanium dioxide, as wet end filler, which is commonly employed to increase the opacity and the brightness of the final product. Therefore titanium dioxide pigment is widely used in the production of several types of paper, including lamination paper (decorative use), filled paper, and coated paper, to provide whiteness, brightness, and opacity as main properties. Thus, for example, in lamination paper (used as a substitute for wood and tile in countertops, furniture, and wallboards, among others), TiO2 is used in the first layer, mixed with plastic resin, where the decorative patterns are printed. One of the main roles of pigments is to prevent the decoloration of the product after prolonged exposure to sunlight and other environmental agents. In this sense, in order to ensure that the titanium dioxide pigment sticks to the fibers, flocculating agents and retention aides are added. Moreover, up to 6 wt.% of titanium dioxide can be added without decreasing the strength of the paper [39]. The paper sector accounts for approximately 12% of TiO2 consumption, which is around 130 kt annually, with lamination paper being especially relevant, representing around 80% of the total titanium dioxide consumption of the sector [22]. In addition, TiO2 has been used in toners, inks, and backings for inkjet printing substrates, improving the opacity and hiding power of printing inks and allowing to achieve very high print quality, with low abrasion and high printing speed. Thus, for example, white inks for packaging can be used as surface print or last layer on flexible packaging in plastic or aluminum films, obtaining optimal opacity. On the other hand, the hiding power of titanium dioxide is also crucial for uses which require a perfect contrast, such as barcode scanning. Moreover, titanium dioxide is used in labels (self-adhesive labels, wrap-around labels, lidding, shrink sleeve, in-mold labeling, etc.), due to its high opacity, toners, where it confers free flow and charge control, writing materials (colored pencils, finger paints, school tempera paints, modeling clays, etc.), and inks for leather, where titanium dioxide pigments are used as opacifier to helping printed textiles stand out. Finally, the usual concentrations of TiO2 in inks and related products are, for example, 50%
60% in white printing inks, 5%
10% in shaded inks, 3%
35% in pencils and similar products (3%
35%, up to 50% in correction fluids, 1
5% in toners, and up to 100% in artistic and recreation colors) [22].

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Another field in which titanium dioxide is widely used is flexible packaging, since the consistency and performance of white ink are crucial to the quality of the printed image. White ink should give enough hiding power to allow high-quality color printing, thus it is used in surface printing, reverse printing, and lamination structures. Furthermore, TiO2 pigments are suitable for use in various solvent-, water-, and oil-based inks and also in UV curable inks.

9.3.2.3 Pharmaceutical and cosmetic industries In the pharmaceutical industry, TiO2 meets all the safety requirements of medicines, including those set by the European, Japanese, and US pharmacopoeias, and its additive number (E-171) is also found in the food industry, since pharmaceutical manufacturers adhere to the same food additive standards governed by the European Food Standards Agency. In this industry, titanium dioxide pigment can be used in different ways: as a basic pigment (to increase whiteness), as coating (protection for photosensitive ingredients and also ingredients that may be vulnerable to UV light degradation), and in packaging (routinely incorporated in the packaging of medicines to extend shelf life and to prevent any premature degradation from moisture, heat, or light). For this type of application, nanometric particle size (less than 100 nm) is required. In addition, in recent years, the use of titanium dioxide in medical applications has been studied, showing an important role in the improvement of health care, especially cancer treatment, due to its excellent photocatalytic activity [40
43]. In addition, it is well known that sunlight may decompose active substances and excipients in pharmaceuticals, generating numerous formulation problems for the pharmaceutical compounds. For that reason, titanium dioxide pigment is widely used in packaging to protect against light transmission. Furthermore, titanium dioxide has also been tested to evaluate the protection of photolabile substances in photodecomposition processes. Thus, for example, this property of titanium dioxide has been analyzed in a photolabile nonsteroidal antiinflammatory drug substance called ketoprofen, which is a drug for transdermal delivery used in clinical practice to relieve pain in acute and chronic conditions. This drug has been involved in adverse photosensitivity reactions due to its instability under sunlight. Therefore titanium dioxide has been tested on the photostability of ketoprofen, demonstrating that it can photostabilize this drug [44]. In the cosmetic sectors, TiO2 or CI 77891 (as it is known in this market) is currently listed in Annex IV of the Cosmetics Regulation EC 1223/2009 (list of colorants allowed in cosmetic products) and Annex VI (list of UV filters allowed). Currently, only two mineral UV filters are allowed in cosmetics, that is, TiO2 and ZnO. Therefore the addition of titanium dioxide provides makeup with a sun protection factor, although relatively low (around 15), which can confer the sun protection upon a “side effect.” Sunburn is caused by sun radiation, known as UV radiation. Although UV radiation does not exceed 5% of the total energy that comes from the sun, its impact on organic molecules is very important, as they cause both photoaging and sun damage.

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This radiation is produced in three different wavelengths, called UV-A, UV-B, and UV-C, which have different wavelength ranges. UV-C is in the range of 190
290 nm, which is only transmitted in the air. On the other hand, UV-A and -B can generate different types of damage. UV-A, with longer wavelengths (320
400 nm), can cause cell damage, since it penetrates the skin, producing premature aging, skin decoloration, and wrinkles, whereas the effects of UV-B (290
320 nm) are visible on the skin surface, causing freckles, moles, sunburn, and cancer. In the cosmetic industry, sunscreens are usually classified into categories: chemical and physical (or mineral). Chemical sunscreens are composed of different chemical compounds that are combined to block the UV-A and UV-B rays, transforming UV rays into heat, which is subsequently released by the skin. On the other hand, physical sunscreens are manufactured with natural compounds, which settle on the skin, absorbing and deflecting UV radiation from the sun. The most obvious example of this natural compound is titanium dioxide pigment [45]. For example, as a curiosity, due to the high reflection power of titanium dioxide pigment, it is recommended not to apply makeup during a photo session, since it can make it difficult to control the brightness generated by it. Thus considering the previously described properties of titanium dioxide, it is important to note that TiO2 is widely used as a protector that absorbs and scatters both UV-A and UV-B radiation to protect the skin. Therefore titanium dioxide nanoparticles (particle size: 1
100 nm) are currently approved to be used as a UV filter in sunscreens. Coating titanium dioxide with silicon dioxide and aluminum (3.5 wt.%) can increase the photostability of titanium dioxide, reducing its photocatalytic activity by 99% [46]. Due to the extensive use of nanoparticles and considering their small size, the safety of titanium dioxide nanoparticles has been reevaluated in some studies where their use in cosmetic preparations or sunscreens is considered negligible [47,48]. In addition, and in view of all the evidence, the Scientific Committee on Consumer Safety demonstrated that titanium dioxide nanoparticles, used at a concentration of up to 25% as a UV filter in sunscreens, can be considered nonharmful for humans after application on a healthy, intact, or sunburnt skin. While it is true that producers of cosmetic products may not show this concentration on the label of the product, this limit must comply with the European legislation (Annex III of the EU regulation on cosmetic products; Regulation EC 1223/2009).

9.3.2.4 Textiles For several years, great effort has been made to immobilize TiO2 nanoparticles onto textile materials with the aim of producing smart textiles with multifunctional properties, such as UV protection [42], self-cleaning [49], enhanced durability [50], and antibacterial activity [18]. The antibacterial activity of titanium dioxide nanoparticles in textiles is based on the degradation of organic materials by photocatalytic reaction. Thus the antibacterial power of titanium dioxide nanoparticles is higher than other antimicrobial agents, such as silver, which makes it safer for the human skin [51,52].

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Wool is the most important natural animal fiber used in the textile industry, mainly due to its excellent properties, which are basically determined by the configuration of the fabric [53]. However, wool shows low photostability, high decomposition derived from insect action, and photo-yellowing [54], among other drawbacks. Most of these problems can be solved by coating the fiber with TiO2 [55,56]. Cotton is a very comfortable fabric to wear, easy to care, absorbs water and “breathes.” Thus when the body perspires, cotton fibers absorb the moisture and release it to the surface of the fabric, where it evaporates. On the other hand, it also shows some negatives aspects, such as sensitivity to mildew, and prolonged exposure to sunlight can damage it, apart from the fact that it easily becomes wrinkly and spotty. Cotton fiber can be blended with synthetic fibers, with the most common being the cotton/polyester blend, which reduces the appearance of wrinkles. In addition, anti-UV finish is also demanded in the manufacture of cotton fabric. As was previously described, titanium dioxide can reflect, scatter, or absorb UV rays. Therefore TiO2 has been tested in several studies with the aim of evaluating the behavior of the anti-UV ray finish of cotton [57
59]. The results obtained in treated fabric surfaces in these studies showed an exceptional anti-UV performance. Regarding synthetic fibers, there is an important parameter that provides information about the reflected light called “shine.” This parameter is mainly controlled by the concentration of TiO2 used in their manufacture. Thus a synthetic fiber is considered to be bright when the concentration of titanium dioxide is around 0.06%, semiopaque with a concentration of 0.3%, and opaque when the concentration is around 2% [20]. Another fundamental aspect related to fiber products is the static electricity generated by friction, which can cause skin damage. This problem can be solved by adding titanium dioxide nanoparticles with a concentration of 0.1%
0.5% to fiber resin; the resistance value is around 108
109 Ω cm [52].

9.3.2.5 Food industry Although titanium is not an essential element for humans beings, titanium dioxide is approved by the US Food and Drug Administration and FAO/WHO Codex Alimentarius as colorant E-171 in the United States and Europe [60,61], with the condition that the additive should not exceed 1 wt.% of the food and without the need to include it on the ingredient label. In the United States, it is used in the manufacture of candies, cookies, sweets, gum, yogurt, cottage cheese, milk, ice cream, etc. For example, the concentrations of up to 1% have been reported in food supplements and hard and soft panned candies, 0.02%
2% in icings, chewing gums, starch-molded confectionery and baked goods, and 0.05%
0.4% in savory snack foods [62]. Titanium dioxide provides certain optical properties, such as brightness and whitening, improving food presentation, for example, skimmed milk [63] and codfish [64]. It is poorly soluble and not readily absorbed by the body [65], and to date, its use in the food industry has not been shown to have negative health effects [66].

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Despite the aforementioned, in May 2016, the French authorities proposed the classification of TiO2 “as a category 1B (Carc1B) carcinogen.” While it is true that the Risk Assessment Committee of the European Chemical Agency concluded that this classification has not been scientifically justified, in September 2017, it was asserted that TiO2 “meets the criteria to be classified as suspected of causing cancer, falling into carcinogen category 2,” although specifically linked to the inhalation route (respirable particles) [22]. EU legislation regulating the use of and exposure to carcinogens generally does not distinguish between routes of exposure. Therefore even though the French authorities specifically indicate that titanium dioxide can be considered carcinogenic by inhalation, it is important to note that the EU regulatory framework regulating the use of and exposure to carcinogens generally does not distinguish between routes of exposure. Thus, on April 17th 2019, the French Government announced, following the recommendation of the French Food Safety Agency and the French Agricultural Research Institute, the suspension of the commercialization of foods containing titanium dioxide pigment (E-171). This decision shall come into force between January 1, 2020 and December 2020. The French ban applies to neither nonfood products, such as medications, cosmetics, and toothpastes, nor food contact materials (National Law Review, 2019). It is important to note that, to date, the E-171 colorant is still accepted in the rest of the EU, since, in most cases, TiO2 is used by the end user within a matrix, typically as a pigment, from which exposure to TiO2 via inhalation is highly improbable. Titanium dioxide nano-size is not approved for the food industry; however, the pigments used in foods do not have any specification in terms of particle size. Some researchers have reported that around 15%
36% of titanium dioxide particles are in the nanoscale [65,67,68]. Furthermore, in 2015, researchers from the Food and Environment Research Agency in the United Kingdom, the Food Institute at the Tu¨bitak Marmara Research Center in Turkey, and the RIKILT Institute of Food Safety in the Netherlands carried out a study on the oral consumption of nano- and larger particles of TiO2, showing no evidence of important internal exposure of the consumer to nanoparticles [69]. In addition, titanium dioxide pigment is widely used to protect foods, drinks. and supplements from premature degradation caused by the effect of light, thus extending the shelf life of the product [70,71]. Moreover, TiO2 packaging reduces E. coli contamination on food surfaces [72]. The most important food contact coatings are the following: food packaging adhesives, paper/paperboard in contact with aqueous/fatty foods, and food contact textiles/fibers, which can contain titanium dioxide pigments. TiO2 provides numerous advantages in food contact materials, such as UV protection properties and antibacterial and antimicrobial activities [73]. All this, combined with its low cost and high stability, makes it a perfect additive for this application.

9.4

Valorization of coproducts and wastes generated

One of the strategic lines of the EU policies aims at ensuring a “circular economy” through the efficient use of raw material, residues, and the generation of clean

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energy. For that reason, policies on waste management must be targeted to respect the hierarchy established by the three R’s strategy: reduction, reuse, and recycle. An important pillar of circular economy is the valorization of the waste generated in any industrial process. The valorization of waste could generate coproducts with high potential value, especially when the coproduct obtained can successfully compete with the properties of traditional raw materials or in the development of new materials for specific applications. In addition, it is important to take into account the environmental benefits of providing a proper waste recovery, as it represents an excellent management alternative. The next section describes the potential usefulness of some wastes generated in the two industrial processes (sulfate and chloride) of titanium dioxide manufacture.

9.4.1 Main wastes generated in the sulfate process Typical wastes generated by the sulfate process include, first, undissolved solids (mud) from the digestion step, which contain all the minerals present in the feedstock that are not attacked by sulfuric acid. These wastes are especially relevant when the ilmenite is used in the industrial process. Second, after the clarification step, the strong acid effluent is obtained, which contains all the iron in dissolution. Then, this effluent is pumped into batch cooling crystallizers, where the bulk of the iron sulfate is removed as solid ferrous sulfate heptahydrate (FeSO4  7H2O), commonly known as copperas, see Fig. 9.2. Once the copperas is separated, the resulting liquor is then preconcentrated in different evaporation steps, generating a recycled weak acid (liquid phase), which is used in the digestion step, and a precipitate of ferrous sulfate monohydrate (FeSO4  H2O). This material is usually referred to as filter salt and may be used as a by-product or neutralized and disposed of as waste. This material can also be obtained by drying ferrous sulfate heptahydrate, depending on the future applications of these coproducts. Lastly, the weak acid generated in the last washing step of TiO2 pulp is neutralized with the addition of lime or limestone, generating red gypsum (CaSO4  2H2O), which is separated by filtration, while the resulting clean water is partially recycled along the process. Copperas and monohydrate can be considered as by-products, since they are used in several applications with commercial value. Thus, historically, copperas, also called ferrous sulfate, green copperas, green vitriol, iron vitriol, and melanterite, is commercially used in several sectors. Their classic applications include water treatment (flocculants), agriculture (prevention of iron chlorosis in plants grown on iron-deficient soils), the pigment industry (raw material for iron oxide pigment), animal feedstuffs (iron supplement to produce hemoglobin in animals), and the manufacture of sulfuric acid (source of sulfur) [5]. In addition, copperas has been tested as agents to fight plagues of snails, which, especially in the early stages of plant growth, can damage the crops [74].

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Monohydrate is also used in agriculture and horticulture for the conditioning of alkaline soils and as raw material for the production of fertilizer for irondeficient soils, in the cement industry for the reduction of Cr(VI) to Cr(III) to prevent chromium dermatitis, and as a source of sulfur in the manufacture of sulfuric acid [5]. In the last decade, several studies at the laboratory scale have been carried out to evaluate the undissolved solid or mud and red gypsum. For example, undissolved solid has been tested as an additive in the manufacture of commercial ceramics [75], clay bricks [76], sulfur polymer cement [77], and Portland cement [78]. The possibility of recovering the remaining titanium by different routes has also been studied [79,80]. Red gypsum is used as an agricultural soil conditioner and for landscaping [81], and it can be also suitable for use as a settling retardant in cement [82], as substitute for natural gypsum by adding it to the clinker at a concentration of 3%
5%, without a loss of technical properties. Moreover, a mixture of undissolved solid and red gypsum has been tested as a new fire insulating material [83].

9.4.2 Main wastes generated in the chloride process In general, the wastes generated from the chloride process are less than those obtained from the sulfate route. The first residual solids and acids from the chloride process are generated in the chlorination step as an acid/solid slurry. Small amounts of titaniferous feedstock and coke remain unreacted within the chlorinator. The combined residual acids and solids are treated by a solid/liquid separation process. In the resulting residual solids, we can find a portion of unreacted material, which becomes part of the solid waste that forms in the bed of the reaction vessel and may be separated and reused as raw material feed. For example, unreacted coke is sometimes recovered and stockpiled for reuse within the process or for sale as a by-product. Alternatively, the entire residue can be disposed of as solid waste [5]. On the other hand, the acid solution may be neutralized with lime, sodium hydroxide, or other alkaline solution to render the residual metals insoluble, leading them to precipitate and stabilize. The solid metal chloride residue has applications as a by-product in the recovery of titanium metal scrap [84]. Alternatively, provided that the unreacted coke has been removed earlier in the process, the metal chloride residue stream can be used in its unneutralized form as a flocculant in water treatment applications after further processing. The aqueous filtrate from the neutralization filter is discharged, with treatment if necessary, to an appropriate surface water body [5]. In addition, a waste acid solution, usually called ferric or iron chloride waste acid, is also generated in the purification step when the metal chloride solids are acidified using water or residual HCl from the reaction scrubber. Ferric chloride can be used as a by-product in wastewater treatment, as it causes the precipitation of phosphorus, thus allowing its removal from the system [85].

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Titanium dioxide based nanomaterials: application of their smart properties in biomedicine

10

Giada Graziana Genchi Smart Bio-Interfaces, Istituto Italiano di Tecnologia, Pontedera, Italy

10.1

Introduction

Topic of over 100,000 reports in the scientific literature of the last 20 years, titanium dioxide (TiO2) is among the most investigated materials in different research areas, ranging from wastewater remediation to microbial control, energy storage, food preservation, and even more applications. Documented on average exceeding 1300 reports/year in the 2010 20 time frame (source: Scopus), titanium dioxide based nanomaterials represent a large subset of titanium dioxide based devices that are finding increasing applications also in the biomedical research. The huge interest in titanium dioxide is indeed motivated by its peculiar catalytic behavior in aqueous environment and under irradiation with light of appropriate wavelength [1]. This interest has also been expanded thanks to the advent of a multitude of fabrication and modification techniques enabling the achievement of a large variety of titanium dioxide nanomaterials, different for surface and bulk properties, such as shape, size, roughness, and photocatalytic and semiconductive behavior (the latter being typically improved by the larger surface to volume ratio of the nanoscale) [2]. For the possibility to achieve an intimate interaction with biological systems and to modulate biological response on the same dimensional scale, nanomaterials fabricated with titanium dioxide have been proposed for several biomedicine purposes, such as biosensing, drug delivery, and tissue engineering [3 6], but most of the literature explored passive properties of these versatile devices rather than their features tunable through an external source of stimulation, also termed “smart” properties. Nonetheless, the number of reports exploiting the smart properties of these nanomaterials in several areas of biomedical research justifies further efforts to increasing their safety and expanding their applications by hybridization with other materials. This chapter attempts at providing a concise overview of the smart properties of titanium dioxide nanomaterials and their composites, as well as of their application to selected areas of biomedical research, namely drug delivery, tissue engineering, and bionics. Advantages and drawbacks of the presented technological approaches will be mentioned, as well as future perspectives for these versatile nanomaterials. Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00002-X © 2021 Elsevier Inc. All rights reserved.

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10.2

Titanium Dioxide (TiO2) and Its Applications

Smart properties of titanium dioxide based nanomaterials

Well known for their peculiar responsiveness to light that enables their exploitation in several catalysis applications (ranging from water splitting to organic pollutant degradation), titanium dioxide based nanomaterials are the focus of intense interest in biomedical research since mid-2000s. Due to their semiconductive nature, these materials indeed support generation of charge carriers (hole/electron pairs) upon irradiation with light of particular wavelength [ultraviolet (UV) and, for instance, near-infrared (NIR) upon suitable nanomaterial modification] corresponding to the bandgap. Typically, the photogenerated holes in the valence band migrate to the surface of TiO2 where they can react with adsorbed water molecules and form hydroxyl radicals. Photogenerated holes and hydroxyl radicals oxidize surrounding organic molecules, whereas electrons in the conduction band contribute to reduction processes by reaction with molecular oxygen to produce superoxide anions. Hydroxyl radicals and superoxide anions represent reactive oxygen species (ROS) with high relevance due to their abundance in biological environment, and to their detrimental effect on biomacromolecules, with unfavorable impact on cell viability when largely exceeding the natural ROS buffering capabilities of biological systems. Besides light irradiation, ROS generation from titanium dioxide nanomaterials has also been documented upon stimulation with ultrasound (US), for reasons however yet to be fully ascertained [7]. Light irradiation and ultrasonication have thus become the main sources of physical stimulation of titanium dioxide based nanomaterials for the elaboration of innovative curative procedures, respectively termed photodynamic and sonodynamic therapy. The literature concerning applications of titanium dioxide nanostructures to biomedicine based on their inherent smart property of photo-/US-induced ROS generation is rich in examples showing antibacterial [8 10], antitumor [11 18], and wound healing [10,11] effects. Therapeutic outcomes were achieved by the development of complex nanoplatforms, also combining photodynamic effects to photothermal treatments [9,11] or radiotherapy [14], mostly for synergic treatment of bacterial infections or against tumor cell proliferation. The first evidence in the literature exploited the responsiveness to light and US of plain titanium dioxide nanomaterials, whereas later studies aimed at merging them with both organic and inorganic counterparts for the obtainment of unprecedented properties and functionalities. Titanium dioxide based nanomaterials were tested with a variety of cellular models, including breast cancer cells (4T1 [14] and MCF-7 cell lines [19,20]), primary fibroblasts [7,21], glioma cells (U87-MG [18] and U251 cell lines [22]), leukemia cells (HL-60 cell line [17]), and melanoma cells (B16F10 [11] and C32 cell lines [23]). Studies performed by considering murine animal models also confirmed the therapeutic efficacy of the proposed nanoplatforms, for instance (1) in contrasting bacterial infections sustained by both Gram-positive and Gram-negative microorganisms [8 10]; (2) in reducing size of tumors [14], xenografts included [15]; and (3) in promoting recovery from skin wounds [10] also in case of chronic,

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tumor-induced skin lesion [11]. In the following, relevant examples of advanced titanium dioxide based nanomaterials exploiting the generation of ROS for photodynamic and sonodynamic therapy will sequentially be recalled with a special focus on the putative functioning mechanisms, inferred from the observed biological responses in models in vitro and in vivo.

10.2.1 Advanced photodynamic therapy approached based on hybrid titanium dioxide based nanomaterials Photodynamic therapy is a promising antitumor treatment based on nanostructures working as active tumor-ablative tools per se, or as carriers of a medicament with activity triggered/potentiated by light of suitable wavelength and intensity. In both cases, photosensitizers (PSs) participate in reactions involving the production of free radicals (mostly ROS), eventually resulting in cell death. For inorganic nanostructures to work as PSs, they must hold specific optical absorption properties, responsiveness to wavelengths penetrating biological tissues (with maximum depth achievable by NIR light at 700 1000 nm), and biocompatibility. TiO2 nanoparticles can both work as photodynamic therapy tools themselves and as carriers due to their high biocompatibility and fast responsiveness to light, even ameliorating biocompatibility and selectivity of action of PSs of proven efficacy like porphyrin derivatives [7]. In this chapter, particular attention will be dedicated to advanced application of titanium dioxide nanomaterials, modified in order to overcome limitations inherent to their large bandgap (3.0 3.2 eV, which makes TiO2 nanostructures mainly responsive to high-energy light such as UV) and to the fast recombination of photoexcited electron and holes. Titanium dioxide nanocomposites with noble metal nanoparticles were, for instance, proposed for the possibility to exploit localized surface plasmon resonance phenomena, enabling responsiveness to light of higher wavelength than UV. Hotelectron injection and plasmon-induced resonance energy transfer are indeed supposed to be underlying the plasmonic enhancement of photocatalysis. Concerning hot-electron injection, the promotion of charge separation and transfer in titanium dioxide nanomaterials for the enhancement of photochemical ROS generation was demonstrated by hybridization with piezoelectric materials and application of a piezoelectric potential. In a study from Yu and co-workers, a multilayered coaxial heterostructured nanorod array was indeed fabricated with TiO2/BaTiO3/Au and tested as a skin antibacterial coating for photodynamic bacterial killing and wound healing applications, both in vitro and in vivo [10]. This enabled broad light absorption, high efficiency of separation of electron/hole pairs, and ROS generation. More in detail, hydroxyl radical generation was over 2.8-fold higher from TiO2/BaTiO3/Au nanocomposites with positive poling in comparison to control nanostructures. Singlet oxygen generation from ternary nanocomposites with positive poling was instead fourfold higher than from plain titanium dioxide nanoparticles, twofold higher than from dual nanocomposites, and 1.5-fold higher than from ternary nanocomposites without poling. When exposed to light with wavelength in the

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325 700 nm range, the ternary nanocomposites with positive poling exhibited the highest incident photoelectron conversion efficiency, demonstrating that the light absorbance from Au nanoparticles and the piezopotential from the barium titanate nanolayer contributed to the increase of photoelectrochemical performance and ROS generation. In vitro, Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus bacteria were exposed to ternary nanocomposites with positive poling and simulated sunlight irradiation (100 mW/cm2 for 40 min); in both cases, disinfection rate was exceeding 99%, and was almost constant after six repetitions of bacterial administrations/irradiations, thus proving the array gained photoactivated selfcleaning properties. On the other hand, mouse NIH-3T3 fibroblasts cultured on the ternary nanocomposite arrays exhibited a viability exceeding 90%, independently on light exposure or not. In vivo, full-thickness dermal wounds on female Balb/c mice were exposed to S. aureus, and wound closure was monitored over a period of 9 days. When animals were exposed to ternary coatings and simulated sunlight for 30 min, wound closure was exceeding 80%, whereas it was lower than 70% in control animals (only exposed to light irradiation). Hematoxylin and eosin staining of the wound region denoted incomplete healing and significant inflammation only in control animals. The putative mechanism for photoelectrochemical performance and ROS generation involving rutile TiO2/tetragonal BaTiO3 heterojunction and BaTiO3/Au Schottky junction is described in the following: sunlight exposure promotes hot electrons generated from the localized surface plasmon resonance to energy levels higher than that of the Schottky barrier, and their injection in the barium titanate layer, thereby facilitating the separation of electron/hole pairs. Under irradiation, holes migrate to the Au surface. Electrons generated in the barium titanate layer migrate toward the conduction band of titanium dioxide nanorods, whereas holes in the titanium dioxide nanorods migrate toward the valence band of the barium titanate layer. Charge transfer is influenced by the piezopotential of barium titanate. Indeed, poling imposes a positive polarization on the surface of barium titanate, leaving a remnant polarization and a piezopotential with positive charges facing Au and negative charges facing titanium dioxide. The piezopotential induces an upward band bending and increases the depletion region width in titanium dioxide and barium titanate while decreasing the band level of barium titanate. Electrons in the titanium dioxide nanorods interact with O2 in solution, thereby generating ROS, whereas holes on the surface of the barium titanate layer and of Au nanoparticles react with water to produce hydroxyl radicals. Free radicals interact with bacteria and induce their death. The mechanism is schematically reported in Fig. 10.1A. Another approach aiming at improving the efficacy of photodynamic therapies mediated by TiO2 nanoparticles consists in the implementation of upconversion processes, enabling responsiveness to NIR light, and therefore penetration of biological tissues at higher depth than in the case of UV light. These processes convert low-energy light (such as NIR) into higher energy light (such as UV and visible light). However, ROS generation upon exposure of TiO2 nanoparticles to NIR light requires high energy transfer efficiency, and this can occur when the

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Figure 10.1 (A) Putative piezophototronic and plasmonic mechanism at the base of the enhanced photodynamic antimicrobial effect of the ternary TiO2/BaTiO3/Au nanocomposite array developed for skin wound healing applications. (B) Putative mechanism at the base of the photodynamic induction of cancer cell apoptosis mediated by (NaYF4:Yb31, Tm31@NaGdF4:Yb31) at anatase TiO2 core/shell nanocomposites upon near-infrared light irradiation. BTO, Barium titanate; CB, conduction band; EF, Fermi level; VB, valence band. Source: (A) Adapted with permission from X. Yu, S. Wang, X. Zhang, A. Qi, X. Qiao, Z. Liu, et al., Nano Energy 46 (2018) 29 38. (B) Reproduced with permission from Z. Hou, Y. Zhang, K. Deng, Y. Chen, X. Li, X. Deng, et al., ACS Nano 9 (2015) 2584 2599. ©2015 American Chemical Society.

emission spectrum of upconversion nanoparticles hybridized to titanium dioxide nanoparticles overlaps the absorption spectrum of titanium dioxide nanoparticles themselves. An example of this approach is represented by a study from Hou and co-workers [24], proposing (NaYF4:Yb31, Tm31@NaGdF4:Yb31)@anatase TiO2 core/shell nanocomposites (denoted as UCNPs@TiO2 NCs) as a theranostic platform for tumor imaging and treatment. From an experimental viewpoint (NaYF4: Yb31, Tm31@NaGdF4:Yb31)@anatase, TiO2 core/shell nanocomposite phototoxicity in the dark was first demonstrated to be negligible on HeLa cell cultures. In vitro investigations on the nanocomposite uptake denoted activation of endocytic routes. Cell death through apoptosis occurred in a dose- and time-dependent manner upon light irradiation of nanocomposite-treated cultures. Investigations on protein expression of apoptosis markers indeed denoted (1) mitochondrial potential membrane disruption and organelle damage, (2) upregulation of proapoptotic protein Bak and downregulation of antiapoptotic proteins Bcl-xL and Bd-2, and (3) activation of caspases, mostly caspase 3. The activity of caspase 3 was also enhanced upon NIR-light irradiation in a mouse model bearing tumor tissue. Most importantly, tumors had a lower size when treated with nanocomposites after NIR-light irradiation than after UV-light irradiation, thus demonstrating the higher effectiveness of indirect UV stimulation of nanocomposites in tumor inhibition. The

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presence of Gb and Yb ions promises the use of nanocomposites as contrast agents for tumor treatment monitoring in magnetic resonance imaging procedures. In this composite system, both core and shell are designed to work as PSs, with the core acting as donor and the shell as acceptor. The putative mechanism of function of this system is schematically depicted in Fig. 10.1B and described in the following: upon irradiation at 980 nm the nanocomposite cores upconvert NIR light into UV light and transfer it to the titanium dioxide shell. This excites an electron from the valence band to the conduction band and promotes the generation of a hole on the valence band. The formation of the electron/hole pair triggers redox reactions resulting in ROS production. The photoinduced oxidative stress mediated by titanium dioxide then determines mitochondrial inner membrane permeabilization and depolarization, followed by upregulation of proapoptotic proteins (Bak and Bax) and downregulation of antiapoptotic proteins (Bcl-2, Bcl-xL, and Mcl-1). The increase of the ratio of proapoptotic to antiapoptotic proteins promotes the release of cytochrome c from the mitochondria to the cytoplasm, as well as the activation of the caspase cascade involving downregulation of procaspase 3 and upregulation of caspase 3. All of these events indeed result in apoptosis morphological hallmarks such as membrane blebbing and nucleus fragmentation. Altogether, the recalled examples well document the improvement of the photoresponsiveness properties of titanium dioxide based nanostructures by interface with other inorganic materials at the nanoscale. The punctual demonstration of the effects on different biological targets of light-activated hybrid nanostructures supports further investigations on these multifunctional nanoplatforms, and the elaboration of theranostic protocols for different pathological conditions.

10.2.2 Advanced sonodynamic therapy approached based on hybrid titanium dioxide based nanomaterials Alternative technique mainly proposed for tumor treatment, sonodynamic therapy involves the application of US to tissues treated with sonosensitizing agents for the generation of free radicals necessary for tumor cell killing. A first generation of these agents consisted in organic macromolecules such as phthalocyanines and porphyrin derivatives, showing high efficacy but also high systemic toxicity. For these reasons, a second generation of sonosensitizers has been proposed based on nanomaterials, among which TiO2 nanoparticles and their composites [19,20,22,23]. Among the first studies aiming at exploiting titanium dioxide nanoparticles as sonosensitizers for antitumor therapy, there is that one performed by Yamaguchi and co-workers. In this study, TiO2 nanoparticles were coated with polyethyleneglycol to exhibit higher colloidal stability in water solution and were administered to U251 glioblastoma cells. Cultures were then either treated with UV light (0.005 W/cm2, peak at 360 nm) or with US (1 W/cm2). The surviving cell fraction decreased by 90% after 60 min of UV irradiation and 50 s of US application. Upon administration of a ROS scavenger (glutathione), cultures were rescued from UV damage, whereas they exhibited very poor viability in the case of US exposure,

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thus suggesting that oxidative stress may be not the only exclusive mechanism at the base of the sonodynamic therapy (for reasons yet to be ascertained). Based on the results achieved by fluorescently staining apoptotic, necrotic, and viable cells, apoptotic cell death promoted by titanium dioxide nanoparticle-induced ROS may prevail in photodynamic treatment in comparison to sonodynamic treatment. The higher cytotoxic effects of ultrasonication in comparison to UV irradiation in cultures treated with nanoparticles may also be due to the significantly higher disruption of cell membrane integrity, as verified by fluorescent staining. Another study from Harada and co-workers proposed titanium dioxide nanoparticles as sonosensitizers for the treatment of melanoma both in vitro and in vivo [23]. Synergistic exposure of C32 melanoma cells to nanoparticles and US (up to 1 W/cm2 for 10 s, with 1 MHz frequency; 5 Hz burst rate; 50% duty cycle) was shown to determine a decrement of B40% 50% in viability and an increment of B10% in apoptotic cells, as compared to control cultures (both treated and untreated with nanoparticles) soon after sonication. Nanoparticle administration and sonication were also demonstrated to decrease melanoma cell xenograft size more than 50% compared to control conditions in nude mice after 3 weeks of observation, with no histological alterations in a number of organs. The reported examples of photodynamic and sonodynamic therapies mediated by titanium dioxide based nanoparticles well illustrate the leading role of these materials in triggering biological responses at a cell level, and in paving the way to in-depth validation studies on intact organisms, which are still largely missing.

10.3

Tissue engineering

Several attempts at applying titanium dioxide based nanomaterials to tissue engineering purposes have been conducted since the advent of these nanomaterials in biomedical research, proving successful mainly when used as rather passive devices. The scientific literature indeed is rich in examples of interaction of these materials with many cellular and tissue targets [5,6,25 27], but only a limited number of reports document on the exploitation of the smart properties of titanium dioxide nanomaterials for tissue engineering. Titanium dioxide nanostructures and their composites have indeed found their main applications in engineering of hard tissues such as bones and teeth, based on their mechanical properties and their intrinsic capability to generate ROS with highly desirable antibacterial effects. When titanium dioxide based nanomaterials were applied to other tissues, several adverse effects were instead documented, sometimes in contrast to what was found upon usage of these nanomaterials as passive devices. These effects were, for instance, found concerning adipose [28], cardiac [29], endothelial [30], hepatic [31], nervous [32], and pulmonary tissue [33], and circumscribed research efforts mostly to engineering of hard tissues. Among the very few examples of successful exploitation of the smart properties of titanium dioxide nanostructures to soft tissue engineering, there is an early study

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from Smith and co-workers [34], reporting on fabrication of arrays of titanium dioxide nanotubes, on their characterization in terms of nitric oxide scavenging property, and on their implantation into rat abdominal wall for evaluation of fibrotic tissue formation. Compared to control surfaces represented by pieces of grit-blasted titanium foil, arrays decreased nitric oxide in solution by B50% and diminished nitric oxide activity (as assessed by immunostaining for nitrotyrosine) in soft tissue in close proximity to the implant after 1 week from surgery. A significantly thinner fibrotic capsule was found around explanted arrays after 1 week and 6 weeks from surgery. Among the first studies to show application of the smart properties of titanium dioxide nanostructure in bone tissue engineering, there is that one performed by Liu and co-workers, exploiting TiO2 nanostructures as antibacterial agents and as scaffolds for osteogenic differentiation of mesenchymal stem cells [35]. In this study, arrays of TiO2 nanotubes were fabricated, decorated with TiO2 nanoparticles for increment of the surface area (compared to plain arrays), thermally treated for obtainment of a mixed crystalline composition with higher photoresponsiveness, and finally UV-irradiated for increment of the surface hydrophilicity and of the surface energy (compared to both plain Ti and nanotube arrays). The higher surface hydrophilicity and surface energy of these nanostructures decreased adhesion of Streptococcus mutans and Porphyromonas gingivalis in comparison to control substrates over a period of 8 days. The nanostructures decreased viability of the adherent strain to a larger extent than in the case of the planktonic strain. Transcriptional analysis was in agreement with the viability data: downregulation of genes encoding for bacterial adhesive proteins, glycosyltransferases B and C, was indeed found on nanoparticle-decorated arrays. The proliferation of mesenchymal stem cells diminished on arrays over 7 days of culture. Upregulation of marker genes of osteogenic differentiation (alkaline phosphatase, collagen 1, and osteocalcin) was also found within 14 days of culture on nanoparticle-decorated arrays exposed to UV light, suggesting suitability of these devices to dental implantation. In a study from Florez and co-workers, titanium dioxide nanoparticles were synthesized, doped with nitrogen in order to achieve responsiveness to visible light, and dispersed in a dental adhesive resin. Nanoparticles were tested for their effects against cariogenic biofilms of S. mutans with the goal of preventing caries recurrence in the field of dental restoration [36]. Nitrogen was chosen as doping agent as it introduces a midgap state at B2.47 eV above the valence band of TiO2 at 3.2 eV (for anatase), resulting in a narrowing of the bandgap and enabling electron photoexcitation with visible light (400 700 nm). The nanocomposite materials significantly decreased bacterial viability; however, their toxicity in the dark was comparable to that one obtained after both short- (3 h) and long-term (24 h) light irradiation. This may be ascribed to the very high percentage of dispersed nanoparticles (up to 80% v/v of resin), which denotes the need of further refinement in the nanocomposite fabrication process. Responsiveness of titanium dioxide nanoparticles to UV light was also exploited to improve hydrophilicity and osseo-integration of orthopedic devices in vivo. In a study from Hayashi and co-workers, titanium disks were coated with both anatase

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and rutile nanoparticles, thermally treated for nanomaterial immobilization and UV-irradiated before implantation in a rodent model [37]. The effects of UV-irradiated disk implantation on bone tissue were evaluated after 12 weeks at a histological level, denoting deposition and mineralization of trabecular bone similar to that one achieved in proximity of control disks (not irradiated with UV light), and no signs of inflammation. At a gene level, UV-irradiated disks determined an almost twofold upregulation of the expression of osteoblast differentiation markers, alkaline phosphatase, and runt-related transcription factor 2, thus suggesting that active bone remodeling process was ongoing even after 12 weeks from animal surgery. An antiinflammatory cytokine, interleukin 10, also underwent upregulation in bone tissue close to UV-irradiated implants compared to that one close to nonirradiated implants, thus suggesting higher mitigation of acute inflammation response and possible inhibition of osteoclast activity. As a final example of titanium dioxide based nanoplatform for dental applications, it is worth mentioning a recent investigation by Zhang and co-workers that demonstrated how nanomaterial modification with polydopamine enabled responsiveness to blue light and tooth whitening with full enamel preservation [38]. In this study, coating with increasing quantities of polydopamine promoted a shift of the absorption spectra to visible light wavelengths. Short-term blue-light irradiation (30 min) determined a two-level tooth whitening, whereas a long-term irradiation (4 h) promoted a 10-level whitening without any significant enamel demineralization. Oral administration of polydopamine-coated nanoparticles to mice did not produce any behavioral or histological alterations (in the heart, kidneys, liver, lungs, and spleen). Polydopamine-coated nanoparticles irradiated with blue light had stronger antibacterial effects against Gram-positive S. aureus compared to commercial antibacterial toothpaste. Lower antibacterial effects were instead found against Gram-negative Pseudomonas aeruginosa and E. coli. Altogether, these results support usage of titanium dioxide based tooth treatment for whitening and plaque control purposes.

10.4

Drug delivery

Titanium dioxide based nanostructures with morphology rich in porosities—such as nanotubes and nanotube arrays—have extensively been studied for the possibility of being loaded with high amount of drugs for spatiotemporally controlled delivery of their pharmaceutical payload to diseased target sites. The interest on these nanomaterials mainly arises from the ease of modification of their different geometrical properties (during both synthesis and postsynthetic processing), but also from the possibility to exert therapeutic effects by loading them with drugs and then triggering intrinsic, biologically relevant nanomaterial response or even drug release by application of suitable external stimuli. Following this novel approach, nanostructures are no longer passive containers/vehicles for medication: they actively deliver their pharmaceutical cargo and/or potentiate its effect in situ, thanks to their

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physicochemical properties enabling responsiveness to different sources of physical stimulation. If primary strategies devised for modulating drug loading and release consisted in dimension and roughness/porosity control, coating with films or filling with nanocarriers for delayed drug release (comprehensively reviewed in Ref. [39]), advanced strategies exploit responsiveness of plain and composite titanium dioxide nanostructures to light irradiation, US, magnetic fields, and pH. These strategies will be shortly presented in the following with the focus on drug loading, targeting, and subsequent release, taking advantage of the intrinsic or acquired physical properties of titanium dioxide based nanomaterials. Examples of synergic activity of drug payload and nanomaterials themselves will also be discussed. As described in the previous sections, the physical properties of titanium dioxide nanostructures enable responsivity to light of suitable wavelength (UV, or also NIR upon nanomaterials modification) resulting in ROS production applicable for microbial and tumor cell proliferation contrast. These properties have originally been investigated with plain nanoparticles, whereas later studies exploiting titanium dioxide based nanomaterials as drug delivery vectors demonstrated how these properties could be used for controlled release of the pharmaceutical cargo and for boosting of the therapeutic effects of the drug payload. In a study from Matijevi´c and co-workers [13], TiO2 nanoparticles were conjugated with Ru(II)(dcbpy)2Cl2 complexes and encapsulated into phospholipid vesicles. Conjugation aimed at obtaining higher photosensitivity than that of nanoparticles alone. Encapsulation aimed at higher colloidal stability in serumcontaining medium, as well as higher selectivity of the intended therapeutic effect (by drug-leakage prevention and enhancement of nanoparticle uptake by tumor cells) than that one of the nonencapsulated nanoparticles. Light with a wavelength of 632 nm and with intensity of 80 µW (He Ne laser as source) was chosen to investigate the rate of complex release from TiO2 NPs, along with light with a wavelength of 254 nm and with intensity of 120 µW (Hg lamp as source). Complex release was tested by dilution into physiological solution and by measuring absorbance at 310 nm. Release profiles at neutral pH demonstrated biphasic kinetics, with an initial burst followed by a lag phase and then complex degradation onset, and overall higher responsiveness to red light than to UV light at time points exceeding 24 h. In a study from Zhang and co-workers, TiO2 nanoparticles were, for instance, used for both drug delivery and photodynamic therapy purposes by covalent bonding to a chemotherapeutic drug (chlorambucil, via phenylboronic acid ester), and coating with a PS (zinc phthalocyanine) in order to obtain mTiO2-BCBL@ZnPc nanoparticles responsive also to NIR light [15]. Upon irradiation, the nanosystem generated ROS and hydrogen peroxide, which determined the phenylboronic ester linker cleavage. Drug release was negligible in the presence of concentrations of hydrogen peroxide in simulated physiological (20 nM) and microtumor environment (50 and 100 µM). NIR-stimulated drug release ranged from 30% to 80% in the 5 to 20 min time frame; it exceeded 87% and reached 100% when hydrogen peroxide was also administered at microtumor environment concentrations. This opened to the possibility of a spatiotemporally controlled release of the chemotherapeutic

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cargo in a cell model for breast cancer (MCF-7 adenocarcinoma cells), demonstrating dose- and irradiation time-dependent cytotoxic effects. In vivo, mTiO2-BCBL@ZnPc nanoparticles administered to rodents (rats and mice) both in single and in multiple doses demonstrated to be compatible with blood, liver, and kidney, thereby suitable to minimizing the typical hepatotoxic and nephrotoxic effects of the pharmaceutical cargo. This study well demonstrated advanced application of titanium dioxide nanoparticles with dual function as drug and photodynamic therapy vectors with targeted activity over time and space, highly desirable in the context of cancer treatment for minimization of the side effects on healthy tissues. Besides light, titanium dioxide nanostructures are well known for their responsiveness to US, proposed for several sonodynamic therapy approaches [40,41]. Careful investigations on sonication parameters for controlled drug release were performed, for instance, concerning power, duration, amplitude, and distance between US source and target. For instance, the Losic group documented US-induced release of indomethacin from polymeric micelle-loaded titanium dioxide nanotube arrays [40]. This study from Aw and co-workers demonstrated that full drug release could be achieved in a timeframe ranging from 5 min to 2 h upon application of US in a pulsatile manner. A more recent study from Melnitzer and Sosnik showed that hybrid TiO2/poloxamine nanoparticles could be loaded with antiparasitic nitazoxanide as a model lipophilic drug [41]. TiO2/poly(ethylene oxide)-b-poly(propylene oxide) nanoparticle loading with nitazoxanide reached B13% (w/w), and drug release in saline solution demonstrated a bimodal profile with 40% of drug detectable in solution after 1 h. US application to these hybrid nanoparticles enabled a significantly higher (. 6 3 ) ROS generation compared to plain nanoparticles, with potential application to biological systems for therapeutic purposes. Loading of nanomaterials with a pharmacological cargo useful for dual treatment of pathological conditions through targeted delivery and generation of ROS has been object of more recent studies. Among the first investigations reported in the literature involving titanium dioxide nanostructures and US for sonodynamic therapy, there is that one by Ninomiya and co-workers that modified nanoparticles with polyacrylic acid and with hepatitis B pre-S1/S2 protein for targeting of liver cells [42]. In this study, human hepatocellular carcinoma HepG2 cultures were administered nanocomposite particles (with B120 nm size) and treated with a low-power and short-duration sonication protocol (US with 1 MHz frequency, 50% duty cycle, 0.1 W/cm2 power for 30 s). Synergic exposure of cultures to both US and nanoparticles demonstrated doubling of the fluorescence associated with hydroxyl radical production in comparison to plain sonication. Proliferation of cell cultures after treatment with nanoparticles and exposure to US was clearly perturbed denoting decreased cell number, which was ascribable to apoptosis induction. A rodent model (Balb/c mouse) bearing subcutaneous xenografts was also exposed to sonication upon water immersion (by applying 1 W/cm2 power for 60 s, five times/day on days 0, 3, 6, 10, and 13 over 28-day observation). In agreement with cell culture results, relative tumor value remained constant upon synergic treatment of the animal model with both US and nanoparticles, whereas it significantly increased in

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control experiment classes. Full tumor regression could not be achieved likely due to the poor accumulation of nanoparticles in diseased liver cells and to the poor efficiency of the low-intensity US stimulation protocol. Analogous to the abovementioned research on drug release triggered by US from titanium dioxide nanocomposite devices, a study from the Losic group proposed titanium dioxide nanotube arrays as a nanoplatform to be modified by decoration with iron oxide nanoparticles in order to gain responsiveness to magnetic fields. In this context, titanium dioxide nanomaterials mainly worked as reservoirs for drug loading, and magnetoresponsiveness was acquired for temporally controlled release of the pharmaceutical cargo loaded within nanotube pores. In this study from Aw and co-workers, titanium dioxide nanotube arrays were modified with magnetite nanoparticles (dopamine-coated for higher stability in aqueous environment) and then loaded with three different types of micelles encapsulating indomethacin, as a water-insoluble model drug [43]. Burst release of the drug carriers occurred in all cases: 100% of the carriers were indeed released within 90 min from application of a static magnetic field, with modest differences ascribable to the different size of the micelles. Instead, release of the drug carriers governed only by diffusion did not exceed 20%. This study clearly demonstrates the emergence of unprecedented properties in titanium dioxide based nanomaterials by hybridization with other nanomaterials during the preparation of nanocomposites. Advances in the US-based approach are represented by the elaboration of multifunctional nanoplatforms accomplishing different duties such as drug delivery, imaging, and cancer cell killing. In this context an interesting nanoplatform for magnetically driven drug delivery, imaging, and sonodynamic therapy was developed by Shen and co-workers, who synthesized Fe3O4 NaYF4@TiO2 core shell nanocomposites, loaded them with doxorubicin as a chemotherapeutic drug, and coated them with hyaluronic acid to improve dispersion in water solution [19]. The nanocomposites demonstrated a pH-dependent drug release, and higher cytotoxic effects on MCF-7 breast cancer cells upon ultrasonication. Moreover, they underwent nuclear localization with promising cumulative therapeutic effects to doxorubicin. The sonication of cultures treated with empty vectors demonstrated that ROS were generated with nuclear localization. Luminescence imaging (with laser source at 980 nm) revealed higher emissions from subcutaneous S180 sarcoma-bearing mice exposed to magnetic fields than from those that were not exposed, thus demonstrating suitability of the developed nanoplatform to tumor diagnosis and targeted drug delivery. Ex vivo biodistribution imaging demonstrated early hepatic accumulation of the drug-loaded nanocomposites (within 12 h), and later accumulation/ retention on tumor site (within 24 48 h), whereas free doxorubicin underwent systemic accumulation and fast urinary excretion (within 12 h). The excised tumors from mice exposed to nanocomposites and US had a significantly lower size than those from control animals, thus demonstrating the higher effectiveness of synergistic nanoparticles/US delivery in tumor treatment. To conclude this section, drug delivery applications of titanium dioxide nanomaterials exploiting their responsivity to different environmental pH are also presented, in particular by mention to those studies aimed at the delivery of the

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chemotherapeutic drugs daunorubicin and cis-platinum [44,45]. In a first study from Zhang and co-workers, titanium dioxide nanoparticles were loaded with daunorubicin and tested in terms of pH-dependent drug release for the treatment of leukemia cells [44]. Drug encapsulation efficiency was shown to exceed 60%, and drug loading efficiency was slightly higher than 20%. Drug release was promoted by acidic conditions: B20% of the drug was released in neutral pH saline solution, whereas threefold and fourfold higher percentages of daunorubicin were, respectively, released at pH 5 6 and 5 in the same time frame, thus suggesting enhanced drug release in biological environments such as lysosomes and tumors (not shown). Administration to K562 cell cultures denoted higher expression of proapoptotic protein caspase 3 in comparison to control cultures (untreated, treated with free drug or with plain nanoparticles), in agreement with qualitative evidences on higher nuclear condensation and fragmentation in cultures treated with the nanocomposites. In a similar study from Liu and co-workers, titanium dioxide nanoparticles demonstrated tunable drug release capability under different pH values for ovarian cancer treatment purposes [45]. In this work, nanoparticles with B25 30 nm size were loaded with cis-platinum as a chemotherapeutic payload and coated with hyaluronic acid as a bioactive molecule for preferential delivery to cancer cells. Nanocomposite internalization was preliminarily demonstrated with A2780 human ovarian carcinoma cell cultures. The higher accumulation of nanoparticles in xenografts, as well as in liver tissue, was also shown at 6 h from intravenous administration to female Balb/c mice in comparison to other tissues. This study provided evidence of the suitability of titanium dioxide nanoparticles to pH-controlled drug release, but also of the need of developing more selective targeting strategies for spatial control of function of hybrid nanodevices based on titanium dioxide.

10.5

Other applications

Titanium dioxide based nanomaterials were also proposed as transducers for retinal cell stimulation or replacement of damaged retinal cells [46,47]. Based on the well-known photostimulated separation of charge carriers on titanium dioxide surfaces (leading to the formation of dipole moments and therefore to electric fields around the nanomaterials), UV light irradiated nanoparticles were indeed used to affect the opening of voltagegated ion channels in retinal horizontal cells. In particular, nanoparticle-treated retinal cultures showed significantly larger variations in the values of photoinduced current in the region of highest conductance for Na1 and K1 channels (230, 140 mV) than untreated cultures, with promising general application to fast I V dynamics monitoring in diseases related to voltage-gated ion channels [46]. Another study aimed at light response recovery in degenerated retina through photovoltaic neuronal stimulation mediated by titanium dioxide nanostructures. In a work from Tang and co-workers, arrays of densely packed, unidirectional rutile TiO2 nanowires (100 nm diameter and 2 µm length) were fabricated and densely decorated with Au nanoparticles (10 nm size) for implantation in a mouse model [47]. These arrays mimicked morphology, anisotropy, and function of rod and cone photoreceptors and had biologically relevant roughness and potential for color

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Figure 10.2 Subretinal implant of Au nanoparticle TiO2 nanowire arrays in mice for vision restoration. (A) array position on explanted eye bulb; (B) eye bulk appearance after 7 weeks from implantation; (C) eye bulk after 5 months from implantation; (D) immunostaining of Brn3a and ChAT markers of retinal ganglion cells on arrays after 5 months from implantation; (E) depiction of in vivo electrophysiology recording, with subretinal array implantation, and insertion of a multielectrode array into the primary visual cortex for light response recordings; (F) raster PSTHs of spikes from V1 neurons in blind mice (left), in blind mice at increasing times from array implantation (middle), and in wild-type mice (right). The purple shade area corresponds to light irradiation. PSTHs, Plots and poststimulus time histograms. Source: Reproduced with permission from J. Tang, N. Qin, Y. Chong, Y. Diao, Yiliguma, Z. Wang, et al., Nat. Commun. 9 (2018). Creative Commons license CC BY 4.0.

vision by tuning of spectral response. Characterization of the Au-decorated nanowire arrays demonstrated higher photoabsorption in the visible range compared to plain nanowire arrays, as well as higher absorption coefficient (0.5 µm21) for blue/green regimes than natural photoreceptors (0.02 0.0621 for green regime). The photogenerated current was .1000 pA in the case of UV light, and B100 pA in the case of blue and green light. The developed arrays supported retinal stimulation with high spatial resolution (50 µm). Size of the receptive field was similar to wild-type animals, and visual information could be processed by retinas interfaced with arrays (as demonstrated with light response inhibition by glutamatergic antagonists). Most importantly, functional and behavioral recovery of sensitivity to light was demonstrated in blind mice chronically implanted with Au TiO2 nanowire arrays (Fig. 10.2).

10.6

Conclusion and perspectives

This chapter has concisely presented the most relevant examples of applications of titanium dioxide based nanomaterials to biomedical research, where their smart properties proved particularly promising as concerns development of multifunctional

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platforms for cancer cell killing, microbial infection contrast, and drug delivery purposes. These examples offer invaluable insight into the mechanisms of action of titanium dioxide based nanomaterials in different biological systems, including intact organisms, and are far from exhausting the huge potentialities of these nanomaterials leveraging on their many physicochemical properties. Nanomaterial morphology, roughness, photocatalytic behavior, and even more properties can still be refined in order to match to the physicochemical properties and therapeutical needs of target tissues. In particular, soft tissue engineering can still be explored by further hybridization of titanium dioxide based nanomaterials with both organic and inorganic compounds, aiming at improving responsiveness to external stimuli while strictly maintaining biological safety. More studies on long-term interaction with these nanomaterials are still needed for a realistic application in biomedical context, as well as the elaboration of nanomaterial fabrication and modification strategies enabling the exertion of stringent spatiotemporally controlled theranostic activities.

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[19] S. Shen, X. Guo, L. Wu, M. Wang, X. Wang, F. Kong, et al., J. Mater. Chem. B Mater. Biol. Med. 2 (2014) 5775 5784. [20] S. Shen, L. Wu, J. Liu, M. Xie, H. Shen, X. Qi, et al., Int. J. Pharm. 486 (2015) 380 388. [21] C. Zhao, F.U. Rehman, H. Jiang, M. Selke, X. Wang, C.Y. Liu, Sci. China Chem. 59 (2016) 637 642. [22] S. Yamaguchi, H. Kobayashi, T. Narita, K. Kanehira, S. Sonezaki, Ultrason. Sonochem. 18 (2011) 1197 1204. [23] Y. Harada, K. Ogawa, Y. Irie, H. Endo, L.B. Feril, T. Uemura, et al., J. Control. Release 149 (2011) 190 195. [24] Z. Hou, Y. Zhang, K. Deng, Y. Chen, X. Li, X. Deng, et al., ACS Nano 9 (2015) 2584 2599. [25] G.G. Genchi, H. Nuhn, I. Liakos, A. Marino, S. Marras, A. Athanassiou, et al., RSC Adv. 6 (2016). [26] H. Gao, B. Li, L. Zhao, Y. Jin, Int. J. Nanomed. 10 (2015) 4009 4027. [27] J. Park, A. Mazare, H. Schneider, K. Von Der Mark, M.J.M. Fischer, P. Schmuki, Tissue Eng. Part C Methods, 22, 2016, pp. 809 821. [28] A.H. Silva, C. Locatelli, U.P.R. Filho, B.F. Gomes, R.M. De Carvalho Ju´nior, J.S. De Gois, et al., Toxicol. Ind. Health 33 (2017) 147 158. [29] X. Yu, F. Hong, Y.Q. Zhang, J. Hazard. Mater. 313 (2016) 68 77. [30] A.J. Leblanc, A.M. Moseley, B.T. Chen, D. Frazer, V. Castranova, T.R. Nurkiewicz, Cardiovasc. Toxicol. 10 (2010) 27 36. [31] L.M. Fadda, H. Hagar, A.M. Mohamed, H.M. Ali, Dose-Response 16 (2018) 1 9. [32] T. Lu, Y. Zhang, Y. Kidane, A. Feiveson, L. Stodieck, F. Karouia, et al., PLoS One 12 (2017) 1 19. [33] Y. Zhou, J. Ji, L. Ji, L. Wang, F. Hong, J. Biomed. Mater. Res. A 107 (2019) 2567 2575. [34] G.C. Smith, L. Chamberlain, L. Faxius, G.W. Johnston, S. Jin, L.M. Bjursten, Acta Biomater. 7 (2011) 3209 3215. [35] W. Liu, P. Su, S. Chen, N. Wang, J. Wang, Y. Liu, et al., Nanomedicine 10 (2015) 713 723. [36] F.L.E. Florez, R.D. Hiers, P. Larson, M. Johnson, E. O’Rear, A.J. Rondinone, et al., Mater. Sci. Eng. C 93 (2018) 931 943. [37] M. Hayashi, R. Jimbo, Y. Xue, K. Mustafa, M. Andersson, A. Wennerberg, Clin. Oral Implant. Res. 25 (2014) 749 754. [38] F. Zhang, C. Wu, Z. Zhou, J. Wang, W. Bao, L. Dong, et al., ACS Biomater. Sci. Eng. 4 (2018) 3072 3077. [39] M.S. Aw, M. Kurian, D. Losic, Biomaterials 2 (2014) 10 34. [40] M.S. Aw, D. Losic, Int. J. Pharm. 443 (2013) 154 162. [41] V.K. Melnitzer, A. Sosnik, Adv. Funct. Mater. 30 (2020) 1806146. [42] K. Ninomiya, C. Ogino, S. Oshima, S. Sonoke, S. Kuroda, Ultrason. Sonochem. 19 (2012) 607 614. [43] M.S. Aw, J. Addai-Mensah, D. Losic, J. Mater. Chem. 22 (2012) 6561 6563. [44] H. Zhang, C. Wang, B. Chen, X. Wang, Int. J. Mol. Sci. (2012) 235 242. [45] E. Liu, Y. Zhou, Z. Liu, J. Li, D. Zhang, J. Chen, et al., J. Nanomater. 2015 (2015). [46] X. Meshik, M. Choi, A. Baker, R.P. Malchow, L. Covnot, S. Doan, et al., Nanomed. Nanotechnol. Biol. Med. 13 (2017) 1031 1040. [47] J. Tang, N. Qin, Y. Chong, Y. Diao, Yiliguma, Z. Wang, et al., Nat. Commun. 9 (2018).

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Annachiara Berardinelli1,2 and Filippo Parisi3 1 Department of Industrial Engineering, University of Trento, Trento, Italy, 2 Center Agriculture Food Environment, University of Trento, Trento, Italy, 3 Department of Physics and Chemistry, University of Palermo, Palermo, Italy

11.1

Introduction

Titanium dioxide powders have extensively used in the food and personal care industry as additives [1]. Out of many white pigments, TiO2 is characterized by the highest refractive index and consequently is used in many applications as a white colorant, brightener for colors, and opacifying pigment. In its microcrystalline form, TiO2 can be found in different cosmetics and personal care products from toothpastes to hair color products, and it is ruled by the Cosmetics Regulation EC 1223/2009 Annex IV (list of colorants allowed in cosmetic products, reference number: 143) [2] and approved by the US Food and Drug Administration (FDA, 21 CFR 73.2575) [3]. In its nanoform, TiO2 is present in sunscreen products and day creams, lip balms, and foundations for its ability to protect against both UVB and UVA, visible light, and infrared radiation [1]. As physical filter, TiO2 can be found in the list of UV filters allowed in cosmetic products [2,4] and approved by the US FDA [5] with a maximum concentration in ready for use preparation of 25%. Titanium dioxide nanoparticles are characterized by unique technological properties as photoprotective capability, smooth application, and clear appearance [6]. However, due to their large surface area and to the potential occurrence of photocatalytic events, with the consequent production of reactive oxidizing species, basic investigation on the relationship between physicochemical properties, that is, particle size distribution, crystal structure, agglomeration, and aggregation, and its use in various formulations is still necessary. It is known that these properties are correlated to possible biological and environmental implications [7]. Safety concerns regarding their utilization in consumer products have been recently presented [8]. However, according to the report of Dre`no et al. [9], there is no evidence of carcinogenicity, mutagenicity, or reproductive toxicity after the dermal exposure to nano-TiO2. As a food additive, titanium dioxide is used essentially in the form of pure anatase and/or rutile, eventually coated with small amounts of alumina and/or silica with the aim of improving the technological properties of the product. It can be included in numerous industrial products such as confectionary, dairy products, Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00008-0 © 2021 Elsevier Inc. All rights reserved.

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cheese and fishery products, sauces, soups, and broths [10]. Chewing gum, sweets, and candies are the food products containing the highest level of TiO2 pigments with a mean particle size of 110 nm and a large distribution in terms of size [1]. Concerns about the use of nanoparticles in food products have recently increased as testified by the increments of the researches focused on the quantification of TiO2 in foods [11,12]. Research works concerning the risk assessment of titanium dioxide as food additives were also critically reviewed [13,14], and the reevaluation of the safety of TiO2 when used as food additive by European Food Safety Authority (EFSA) in 2016 [15] evidenced that the adsorption of both micro- and nanosized forms in the gastrointestinal tract is negligible and the additive does not raise a genotoxic concern. Because of its potentiality in replacing traditional food decontamination methods, mainly based on thermal technologies or the use of chemical biocides, titanium dioxide based photocatalysts are attractive for food industry. The TiO2 oxidative efficacy was tested toward different kinds of microorganisms such as Gram-negative and Grampositive bacteria, fungi, algae, viruses, and toxins, also in combination with other promising nonthermal technologies [16]. Reactive oxygen species (ROS) produced during the photocatalytic mechanism appears to be able to attack lipids, polysaccharides, proteins, and nucleic acids, leading to cell death. In particular, inactivation of microorganisms in drinking water [17], in drinks and fresh juices [18], and on fruits and vegetables surface [19] has been reported. More recently, TiO2 nanoparticles were immobilized onto different food packaging materials with the aim of assuring a better stability during storage [20] and as a tool for the degradation of ethylene, a naturally plant hormone responsible for the alteration of the senescence during postharvest processes of fresh fruits and vegetables [21].

11.2

Titanium dioxide as food additive

11.2.1 Titanium dioxide in food Titanium dioxide based pigments are widely used as food additives in food industry, in order to improve the product whitening and brightening properties. According to the Regulation (EC) No. 1333/2008 [22], Article 3, on the use of food additives, “food additive shall mean any substance not normally consumed as a food in itself, and not normally used as a characteristic ingredient of food, whether or not it has nutritive value, the intentional addition of which to food for a technological purpose in the manufacture, processing, preparation, treatment, packaging, transport or storage of such food results, or may be reasonably expected to result, in it or its by-products becoming directly or indirectly a component of such foods.” TiO2 can be found in several industrial products: confectionary (chewing gum, candies, chocolates, decorations, cereals, bakeries, and snacks), cheese and cheese products (unripened cheese, edible cheese rind, whey cheese), flavored fermented milk products, dehydrated milk and creams, ice cream, fruit and edible ices, noodles, seasonings and condiments, sauces, mustard, soups and broths, processed fish and

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fishery products, including molluscs and crustaceans, dietary food for special medical purposes and weight control. In the European Union, this white powder should be labeled as E171 and is authorized at quantum satis [22] while in the United States is labeled as INS17 and does not exceed 1 wt.% of the food [23]. TiO2 technological characteristics such as the insolubility in water, in organic solvents, and almost in aqueous alkaline media together with its thermal and chemical stability make the pigment particular suitable for food processing [24]. The quantification of TiO2 in commercial food products indicates that chewing gum, sweets, and candies are the products containing the highest level of TiO2 [1,11]. By analyzing the summary of the reported levels provided by industry and published by EFSA in the report “Re-evaluation of titanium dioxide (E 171) as food additive” (Appendix A, EFSA Journal, 2016) [15], the highest maximum levels of 16, 20, and 12 g/kg are respectively measured in chewing gum, decorations, and food supplements (capsules and tablets). According to the Commission Regulation (EU) No. 231/2012 [25] laying down specifications for food additives, titanium dioxide “consists essentially of pure anatase and/or rutile titanium dioxide which may be coated with small amounts of alumina and/or silica to improve the technological properties of the product.” The anatase crystalline structure was the only authorized form until 2006 for food applications. In film coating for food and supplement tablets and foodstuffs, rutile has been authorized to replace anatase in food products. Concerning the synthesis, the titanium rutile phase is typically obtained by the chloride process while the anatase grades can only be made by the sulfate process. About purity specifications the Commission Regulation rules that the loss on drying must be not more than 0.5% (105 C, 3 h) and the loss on ignition must be not more than 1.0% on a volatile matter free basis (800 C). Matter soluble in 0.5 N HCl must represent not more than 0.5% on an alumina and silica-free basis (not more than 1.5% for products containing alumina and/or silica on the basis of the product as sold), and water-soluble matter must be not more than 0.5%. Limits of 1 mg/kg are fixed for arsenic, cadmium, and mercury, and of 2 and 10 mg/kg, respectively, for antimony and lead. According to experimental data coming from different commercial food-grade TiO2 samples, particle sizes ranged between 106 6 38 and 132 6 56 nm, and based on the size distribution, nanosized particles were present in the percentage from 17% to 35% (Fig. 11.1). These food-grade TiO2 samples showed also some photocatalytic activity [12]. The state of the art of the analytical methods used for food formulations is not able to detect size in the range of 20 50 nm [11]. Low specific surface area of about 10 m2/g was also reported [26]. Recently, the EFSA reported that food additive E171 consists of microsized TiO2 particles, with the presence of nanosized TiO2 particle less than 3.2% by mass or 10% 39% by number size distribution and, consequently, E171 cannot be considered as a nanomaterial [15].

11.2.2 Influence of titanium dioxide on human health Current regulations about food additives do not include characterization of TiO2 particle size distribution. However, the use of TiO2 nanoparticles as food additive recently

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Figure 11.1 TEM images of food products containing TiO2 and size distribution. TEM, Transmission electron microscope. Source: Reproduced from R.J.B. Peters, G. van Bemmel, Z. Herrera-Rivera, H.P.F.G. Helsper, H.J.P. Marvin, S. Weigel, et al., J. Agric. Food Chem. 62 (2014) 6285 6293 with permission by ACS Publications.

produced concerns about the possible risks related to their oral exposure and an increasing interest in the quantification of the TiO2 presence in foods was testified by several research works [11,12]. Based on the Commission Recommendation, 2011/696/EU [27] on the definition of nanomaterial, a nanomaterial is a natural, incidental, or manufactured material containing particles, in an unbound state, as an aggregate, or as an agglomerate, and it is characterized for 50% (or more) of the particles in the size range 1 100 nm. As previously described, the recent reevaluation of titanium dioxide (E171) conducted by EFSA in 2016 [15] considers that this food additive mainly consists of microsized TiO2, and the presence of the nano-fraction is less than 3.2% by mass; in addition, the adsorption of both micro- and nanosized forms in the gastrointestinal tract is negligible, and the additive does not raise a genotoxic concern. However, the TiO2 food additive appeared to alter the release of gut bacterial metabolites of mice [28] and, based on the literature reviewed by Jovanovi´c [29], TiO2 could be adsorbed by the mammalian gastrointestinal tract after oral ingestion and bioaccumulate in the tissues.

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Scientists agree about the necessity of finding a reliable method for assessing and quantify the presence of TiO2 nanoparticles in food products. Main considerations and challenges regard the fact that foods are complex and variable matrices and sometimes incompatible with analytical procedures. There is not a single analytical method suitable for the identification, quantification, and the characterization of all food products [30,31]. Research works concerning the risk assessment of titanium dioxide as food additives were also critically reviewed. Based on these reviews and on the data gap and uncertainties characterizing the literature, it is not possible to declare that foodgrade TiO2 products are safe, and human risks due to long-term TiO2 particles ingestion should be deeply investigated. The estimated higher consumption of confectionary products by children (2 6 years old) than by adults appears a critical factor to be considered [13]. On the other hand, the cytotoxicity associated with these products seems not related to particle type and size distribution [14]. In April 2019, in keeping with the precautionary principle, the French Government signed a decree about a sale suspension of food products containing the additive E171 (titanium dioxide) at least for the period from January 1, 2020 to December 31, 2020 [32]. This decision was based on the opinion of the French Agency for Food, Environmental and Occupational Health and Safety (ANSES) that there was still not enough data available to carry out a proper assessment of the risks associated with the use of E171. The French ban does not apply to products such as medications, cosmetics, and toothpastes.

11.3

Titanium dioxide for food preservation

11.3.1 Antibacterial effects Titanium dioxide (TiO2) is attractive for food industry especially for its potentiality in replacing traditional decontamination methods mainly based on thermal technologies destroying microorganisms in a short time with a consequent loss in food organoleptic properties. Furthermore, for fresh fruits and vegetables, traditional methods consist of the use of chemical biocides and washing procedures. Washing with chlorine has been widely applied to reduce the number of food microorganisms and to prevent cross contaminations between clean and contaminated products [33]. However, the treatment efficacy was always questioned in terms of decontamination effectiveness and for the presence of toxic residues in fresh-cut products [34]. Chlorine washing procedures are also prohibited in some European countries [35]. Recently, different nonthermal alternative approaches have been proposed for food decontamination [36,37] and those based on oxidation processes were deeply investigated [38]. Among these new technologies, photocatalysis is considered a green promising alternative to traditional decontamination methods. Photocatalysis belongs to the so-called advanced oxidation processes along with cold atmospheric gas plasma techniques, ozonation, hydrogen peroxide, (photo)Fenton processes. All of these techniques are based on the oxidizing activity of ROS [39]. The well-known photocatalytic mechanism (Fig. 11.2) is characterized by the activation of TiO2 semiconductor particles by UVA light followed by the

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Figure 11.2 Photoinduced TiO2 mechanisms. Table 11.1 Effect of the microorganism characteristics on the treatment efficacy (semiconductor powder TiO2 tinder metal-halide lamp irradiation). Microorganism

Time (min)

Surviving ratio (%)

Saccharomyces cerevisiae

60 120 60 120 60 60 120

54 0 20 0 0 85 55

Escherichia coli Lactobacillus acidophilus Chlorella vulgaris

Source: Data from T. Matsunaga, R. Tomoda, T. Nakajima, H. Wake, FEMS Microbiol. Lett. 29 (1985) 211 214.

generation of holes (h1) and electrons (e2). These photogenerated charges react with water and dioxygen molecules to form mainly superoxide radicals (UO2 2 ), hydroxyl radicals (
OH), and hydrogen peroxide (H2O2) [40]. These active species trigger in turn the oxidation of organic compounds that constitute cellular constituents of microorganisms, such as lipids, polysaccharides, proteins, and nucleic acids [41]. The TiO2 photocatalytic decontamination efficacy is due to several variables mainly related to the microorganism characteristics, concentration, chemical nature of suspension medium, pH, temperature, and the ability of generating ROS [42]. Starting from pioneering research works conducted by Matsunaga et al. (Table 11.1) [43], TiO2-induced oxidation has been successfully performed on Gram-negative (especially on Escherichia coli) and Gram-positive bacteria, fungi, algae, viruses, and microbial toxins [16]. Differences in microorganisms’ cell wall complexity resulted in a more resistant response to photocatalytic treatments [44]. In fact, due to differences in the cell wall structure, Gram-positive bacteria (thick and complex peptidoglycan sheath outside of the inner membrane) appear in general more resistant to the photocatalytic action with respect to Gram-negative bacteria (triple-layer cell wall with an outer membrane, a thin single peptidoglycan middle layer, and a cytoplasmic inner membrane) [45]. Even if a higher number of studies were dedicated to the mechanisms of photocatalytic antimicrobial activity, a clear mechanism of action was not evidenced and

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Figure 11.3 Schematic diagram of ROS in photocatalytic killing of bacteria. ROS, Reactive oxygen species. Source: Adapted from H.M. Yadav, J.S. Kim, S.H. Pawar, Korean J. Chem. Eng. 33 (2016) 1989 1998, [46].

Figure 11.4 Scanning electron micrographs of pristine (A) and TiO2 photocatalytically treated (B) Escherichia coli. Source: Reproduced from Y. Gao, W. Zhang, P. Liu, Appl. Sci. 8 (2018) 945.

different theories have been proposed. However, as shown in Fig. 11.3, some common indications related to a first photocatalytic extracellular site oxidative damage (cell membrane and wall lipid peroxidation) can be extrapolated from the analysis of the literature. After this initial degradation, producing an increment in the cell permeability, the oxidative species can direct their action to the cytoplasmic structure and to intracellular components (enzymes, coenzymes, and nucleic acids) leading to cell death [41]. A great part of the researches conducted on food decontamination is focused on drinking water and the inactivation of E. coli (see Fig. 11.4), one of the measured indicators of fecal contamination in the analyses of water and wastewater samples [47 49]. Combinations with other nonthermal processes were extensively studied. Efficiency of TiO2 was tested for the inactivation of E. coli in natural fresh water combined with ozone (O3) and hydrogen peroxide (H2O2) [15] with H2O2 [17] and with pasteurization [50]. Due to the increased contact between the photocatalyst and the microbial cell, significant improvements on the photocatalytic action can be observed when suspended TiO2 is used, instead of immobilized on surfaces [45].

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The oxidative TiO2 photocatalytic power was also tested for the disinfection of wash waters from fresh vegetables industry, in order to propose an alternative to the use of chlorine-based chemicals. By means of a closed recirculation photocatalytic system, wash waters from different fresh-cut vegetables processes were successfully treated for the control of natural microflora and potentially pathogenic microflora (E. coli). Main results evidenced a significant influence on the disinfection efficacy of the wash water physicochemical characteristics (depending on the product), especially the turbidity and the presence of organic matter [51]. The bactericidal activity against E. coli, Salmonella typhimurium, and Bacillus cereus was assessed on water and on carrots [52], and the determination of the photocatalytic efficacy was evidenced during iceberg lettuce processes for E. coli, Listeria monocytogenes, Staphylococcus aureus, S. typhimurium, and natural microflora [53]. Literature is scarce in terms of possible changes in the quality of food products. Some studies showed increments in the total phenolic and total anthocyanin content in blueberries fruits [54] and an increased antioxidant activity in saffron stigmas (Crocus sativus L.) [55]. The antimicrobial activity of TiO2 photocatalysis was investigated also for microbial inactivation in drinks. Examples of recent works refer to total aerobic bacteria and coliforms in Korean rice-and-malt drink sikhye [18] and to E. coli ATCC 25922 in white grape juice [56]. The efficacy of the synergistic inactivation effect of TiO2 photocatalysis and high hydrostatic pressure technique was tested toward yeasts and molds, coliform bacteria, Pseudomonas, and B. cereus in freshly squeezed Angelica keiskei juice [57]; E. coli O157:H7 in orange juice [58]; and L. monocytogenes, S. aureus, E. coli O157:H7, S. typhimurium, and S. cerevisiae in apple juice [59]. Main results indicated a more pronounced cell membrane permeabilization and deformation following the combination of these two nonthermal methods. However, the application of the TiO2 photocatalysis technique could involve important negative oxidative reactions and possible undesired effects on the drink quality such as sensorial and nutritional properties that need to be measured in various treatment conditions.

11.3.2 Ethylene degradation The photocatalytic activity of TiO2 nanocomposites was also tested as a tool for the degradation of ethylene (C2H4) during postharvest handling of fresh fruits and vegetables, in order to prolong shelf life and reduce food losses (Fig. 11.5). Ethylene is a naturally occurring hormone that affects, by altering or accelerating, physiological mechanisms in plants such as ripening and senescence. Increase in softening, senescence, chlorophyll destruction, and in discoloration and browning of the vegetal tissues are some of the product detrimental effects following the exposure to ethylene during storage [60,61]. Several studies have described the efficacy of the TiO2 photocatalytic reaction in terms of ethylene oxidation into carbon dioxide and water under different atmosphere, humidity and temperature conditions [21,62,63]. The photocatalytic reaction of TiO2 to decompose ethylene was evidenced under different atmospheric storage conditions without excessively increasing the air temperature

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Figure 11.5 Ethylene scavenging process of titanium dioxide. Source: Reproduced from H. Guo, P. Warnicke, M. Griffa, U. Mu¨ller, Z. Chen, R. Schaeublin, et al., ACS Nano 13(12) (2019) 14337 14347 with permission by ACS Publications.

and producing disorder in tomato fruits [64]. The reduction of ethylene gas in the storage room by TiO2 photocatalytic oxidation was able to maintain the quality of mature green tomato fruits [65]. The treatment with mesoporous TiO2/SiO2 nanocomposites was efficient in terms of ethylene elimination and a slower epicarp color evolution with respect to control green tomato samples could be observed [66]. Potential effects of this technology were also demonstrated for the reduction of the ripening effect of kiwi [67] and papaya fruits [68] samples, in terms of preservation of their textural properties and improvement of the postharvest quality. In the food active packaging sector, different studies based on different composite films have been dedicated also to TiO2-based ethylene scavenger systems able to eliminate ethylene concentration in the package and extend the shelf life of fresh fruits and vegetables [69]. The photodecomposition of oriented polypropylene (OPP)-based film coated with 10% of TiO2 nanoparticles was tested in the packaging headspace of tomato fruits [70]. A TiO2 polyvinylpyrrolidone (PVP) film afforded similar results in a nanocompostite comprising also adsorbents such as zeolite and silica powders [71]. The photocatalytic control of the ethylene was also testified by research works conducted with polyethylene films with nano-Ag, kaolin, and TiO2 for the preservation of the physicochemical and physiological qualities of fresh strawberry [72], and with nano-Ag and TiO2 for the control of green mold decay and quality of Chinese bayberries [73]. A reduction of ethylene in Nam Dok Mai mango fruit cartons with TiO2 as an adsorbent was also shown; a significant extension of the shelf life at specific storage conditions was measured in terms of respiration rate, color changes, firmness, and chemical properties [74]. More recently, chitosan and TiO2 nanocomposite films were successfully tested with the aim of delaying the ripening process and changes in the quality of tomatoes [75].

11.3.3 Active packaging Given the well-explored antimicrobial and UV-blocking properties, metal oxide nanoparticles actually play a big role in the food and beverage active packaging sector. According to EC Regulations No. 1935/2004 [76] and No. 450/2009 [77] on

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active packaging for food and beverage applications, “active materials and articles” refer to new types of materials and articles able to actively maintain or improve the qualitative characteristics of the food in the pack. There is a growing interest in the immobilization of TiO2 nanoparticles onto food packaging materials, and different multifunctional materials were tested toward different types of microorganisms [20]. The term multifunctional refers to the fact that improvements in terms of packaging physico-mechanical properties or enhancements in the food organoleptic properties can be also observed, for example; these attributes are considered anyway fundamental for the product and environment preservation [78]. TiO2-coated OPP films appeared effective in the reduction of the E. coli growth on cut lettuce [79] and polyethylene (PE)-based film evidenced antibacterial activities toward both E. coli and S. aureus [80]. TiO2 incorporated into a low-density polyethylene polymeric matrix was tested toward Pseudomonas spp. and Rhodotorula mucilaginosa [81]. The inhibition of lactic acid bacteria and coliforms with a consequent better stability of short-ripened cheese was observed by using a high-density polyethylene/calcium carbonate (CaCO3) film containing TiO2 [82]. The nanosize form of titanium dioxide was also incorporated into biodegradable polymers and natural materials. The multifunctional properties of composites comprised TiO2 embedded in polylactic acid (PLA) [83,84] have been extensively reviewed, but some researches have been also published on other biopolymers such as cellulose acetate (CA) and polycaprolactone (PCL) [85]. One of the widely researched natural materials is chitosan, a linear cationic polysaccharide promising for food applications, because of its biodegradability characteristics, nontoxicity, and antimicrobial properties. The addition of TiO2 appears to be a strategy for enhancing mechanical, antimicrobial [86], and multifunctional properties [87] of the film composite. Concerning multifunctional properties, protein isolate/cellulose nanofibers with TiO2 and rosemary essential oil were also found to be active in the inhibition of spoilage and pathogenic bacteria in meat and in the improvement of its organoleptic properties [88]. For food-contact applications, safety concerns should be taken into consideration. In particular, the nanoparticles migration phenomenon into food from the surface of the packaging material should be deeply investigated and understood in order to cover the gap in literature and to assess toxicological effects [89].

11.4

Titanium dioxide in cosmetics and personal care products

Titanium dioxide has been used as a common additive in many cosmetics and personal care products. Its high refractive index among all of the white pigments makes it suitable for many applications such as a white colorant, brightener for colors, and opacifying pigment. TiO2 can be found in its microcrystalline form (range size of 150 300 nm) in the formulations of toothpastes (as a white pigment and as an abrasive); soaps, shampoos, shower gels, and depilatory products (as a pearlescent colorant and as an opacifier); cosmetics and skin care products (as a colorant), hair color (as an opacifier); and nail polish (as a colorant and as an opacifier)

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products. It is general opinion that the exceptional technical functions displayed by TiO2 are difficult to find in other pigments [1]. Because of its ability to reflect and absorb UV photons, TiO2 is also widely used as a physical filter in sunscreen products and day creams, lip balms, and foundations. It protects against both UVB and UVA, visible light, and infrared radiations. As a physical filter, TiO2 nanoparticles are present since the 1980s in cosmetic formulations because in this form the mineral particles are less visible after application (nano-TiO2, size lower than 100 nm) [1,8]. Rutile or a mixture of rutile and anatase phases can be found in commercial sunscreen products [90]. It was demonstrated that the TiO2 photoreactivity is a function of its crystal phase and, according to the results obtained by Turci et al. [91], the rutile form appeared to produce safer TiO2based sunscreen products. TiO2 in sunscreens is generally present as primary particles (5 20 nm), aggregates (30 150 nm), and, more rarely, agglomerates (1 100 µm). In fact, the formation of agglomerates in undesired, even if they can be formed directly on the skin surface during the product application. However, disaggregation can occur upon exposition to both natural and artificial light depending on photon intensity [92]. In terms of morphology and size distribution, an example of sunscreens characterization is shown in [93] in terms of TiO2 and ZnO size distribution (Fig. 11.6). Particle size is an important parameter affecting the UV protection capability of a sunscreen product. Decreasing TiO2 particle size widens its bandgap (quantum size effect), by shifting absorption spectrum toward the UVB range. TiO2 is often used together with ZnO in order to assure a broadband UV protection [94,95]. The physicochemical properties (crystal structure, shape, particle size, surface area, and surface modification) of both pigments influence the efficacy and the safety of the sunscreens. Titanium dioxide nanoforms are considered photoreactive and an

Figure 11.6 Characterization of TiO2 and ZnO particles extracted from sunscreens. (A) TEM images of ZnO and TiO2 particles; (B) Histogram showing the distribution of particle diameters. Source: Reproduced from M.J. Osmond-McLeod, Y. Oytam, A. Rowe, F. Sobhanmanesh, G. Greenoak, J. Kirby, et al., Part. Fibre Toxicol. 13 (2016) 44.

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increase in the production of ROS was observed [96]. Nanoparticles are in general characterized by differences in terms of physicochemical properties with respect to bigger particles. These differences, mainly related to surface properties and optoelectronic structure, affect the reactivity and, consequently, the bioactivity [97]. In general, and in order to increase the product stability, TiO2 nanoparticles are subjected to a surface coating procedure with silica, alumina, and/or other polymers in order to reduce the generation of ROS. Coating parameters such as thickness and chemical purity of the inert layers influence the photocatalytic mechanism and silica appeared one of the most effective materials to this purpose [98].

11.4.1 Regulations As a colorant in cosmetic products, titanium dioxide is actually listed in the Cosmetics Regulation EC 1223/2009 Annex IV (list of colorants allowed in cosmetic products, reference number: 143) [2] and purity criteria are described in Commission Directive 95/45/EC (E171) and its amendments [99]. The presence of this white pigment must be declared under the INCI name CI 77891. In the United States the pigment form is approved by the US FDA [3] and must be declared under the name titanium dioxide. As UV filter allowed in cosmetic products, titanium dioxide is listed in the Cosmetics Regulation EC 1223/2009 Annex VI (list of UV filters allowed in cosmetic products, reference number: 27 and 27a for its nanoform) [2] with a maximum concentration in ready for use preparation of 25%. Nanoparticles must respect additional specific conditions for applications and physicochemical characteristics listed in the Annex VI. Recently, based on the published opinion of March 7, 2017 of the EU’s Scientific Committee on Consumer Safety (SCCS/1580/16) [100], entry 27a of Annex VI to Regulation EC 1223/ 2009 [2] was amended by the Commission Regulation (EU) 2019/1857 of November 6, 2019 [4]. The following specific conditions are contemplated: rutile form, or rutile with up to 5% anatase, with crystalline structure and physical appearance as clusters of spherical, needle, or lanceolate shapes; median particle size based on number size distribution $ 30 nm and a volume-specific surface area # 460 m2/cm3. Allowing coatings are silica, hydrated silica, alumina, aluminum hydroxide, aluminum stearate, stearic acid, trimethoxycaprylylsilane, glycerin, dimethicone, hydrogen dimethicone, and simethicone. Conditions indicate that nano-TiO2 cosmetic products must not be used in applications that may lead to exposure of the end user’s lungs by inhalation. Face products containing nano-TiO2 coated with a combination of alumina and manganese dioxide should display a warning against the use on the lips. This is based on the potential harm that manganese dioxide can produce upon oral ingestion. The presence of TiO2 in sunscreens (up to a maximum concentration of 25%) is also ruled by the US FDA [5].

11.4.2 Safety of sunscreens Nanomaterial-based consumer products recently received big attention from the scientific community about the potential toxicity related to their application over the surface of the body. In fact, it is known that ROS can be generated when TiO2 or

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ZnO is exposed to ultraviolet radiation [101]. In order to quench the production of ROS, coating by alumina or silica and/or doping with manganese or vanadium are used and, according to EC 1223/2009 [2], the photocatalytic activity should be less than 10% than the corresponding noncoated or nondoped reference. As observed in food-grade products, titanium dioxide photochemical behavior and potential toxicological properties are highly influenced by its physicochemical characteristics [7]. Based on researches exploring the effect of sunscreens ZnO and TiO2 nanoparticles on in vitro 3D human skin models (KeraSkin) supported by in vivo results, metal oxides and their mixtures can be considered safe in terms of skin irritation (reversible skin damage) and skin corrosion (irreversible necrotic damage extending into the dermis) [102,103]. Concerning skin penetration, based on a scientific review published by the Australian Government, TiO2 nanoparticles are not able to penetrate the stratum corneum and the skin underlying layers (from in vitro and in vivo studies) [104]. Mainly due to this low probability of adsorption, potential risk for humans is considered extremely low [6]. In addition, evidence on acute and subchronic inhalation toxicity cannot support the safety of TiO2 nanomaterials in spray products [101]. The EU’s SCCS [105] considered that, on the basis of available information, the use of TiO2 nanoparticles in spray products cannot be considered safe when used as UV filter in sunscreens and personal care spray products at a concentration up to 5.5%.

11.5

Conclusion

As white pigment, titanium dioxide is widely used in foods, cosmetics, and personal care products. It is characterized by unique technological properties and is mainly present in its microcrystalline form even if a certain percentage of nanoforms particles are also present. Since 1980s, TiO2 nanoparticles are present as a physical filter in the formulations of sunscreen products because, in this form, they do not affect esthetically the product appearance. Ingestion of TiO2 nanoparticles could produce a potential risk for living beings. Several works review the safety assessment of titanium dioxide and agree on the fact that literature is characterized by a data gap concerning the presence of nanoparticles and their physicochemical characteristics. These properties are considered to play a big role for the human safety assessment and reliable detection; therefore quantification and characterization are fundamental aspects. Based on an opinion released by the French Food Safety Agency (ANSES) and starting from January 2020, France announced a ban of food products containing titanium dioxide in order to reduce the TiO2 exposure to workers, consumers, and the environment. On the other hand, the titanium dioxide photocatalytic mechanism was deeply studied as an alternative technique for food decontamination, and it is now considered a promising alternative to traditional decontamination methods. ROS generated during the mechanism were proven to be involved in the oxidation of many microorganism cellular constituents starting from the external wall to the cytoplasmic structure and the intracellular components. The decontamination efficacy was tested toward

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different microorganisms contaminating drinking water, wash waters from fresh vegetables industry, fruits and vegetables, and drinks, also in combination with ozone, hydrogen peroxide, and high hydrostatic pressure technique. Many variables affecting the process were explored and discussed such as the role of the reactive species, the characteristics of the microorganisms, and the liquid physicochemical characteristics. These studies were fundamental for the setting up of active packaging for food and beverage sector aiming at the reduction of microorganisms growing and for the ethylene control during postharvest handling of fresh fruits and vegetables with the aim of delaying the ripening process and changes in the product quality. However, possible negative effects on the food organoleptic quality are not extensively researched and should deeply investigated before a possible application of the technique together with the assessment of the phenomenon of nanoparticles migration into food from the surface of the packaging material.

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[71] K. Tanaka, J. Fukuyoshi, H. Segawa, K. Yoshida, Improved photocatalytic activity of zeolite- and silica-incorporated TiO2 film, J. Hazard. Mater. 137 (2006) 947 951. [72] F.M. Yang, H.M. Li, F. Li, Z.H. Xin, L.Y. Zhao, Y.H. Zheng, et al., Effect of nanopacking on preservation quality of fresh strawberry (Fragaria ananassa Duch. cv Fengxiang) during storage at 4 C, J. Food Sci. 75 (2010) C236 C240. [73] K. Wang, P. Jin, H. Shang, H. Li, F. Xu, Q. Hu, et al., A combination of hot air treatment and nano-packing reduces fruit decay and maintains quality in postharvest Chinese bayberries, J. Sci. Food Agric. 90 (2010) 2427 2432. [74] J. Chaishome, Y. Tunsathitsak, P. Boonyaritthongchai, S. Supapvanich, Proceedings of the International MultiConference of Engineers and Computer Scientists 2019 IMECS 2019, March 13 15, 2019, Hong Kong. [75] P. Kaewklin, U. Siripatrawan, A. Suwanagul, Y.S. Lee, Active packaging from chitosan-titanium dioxide nanocomposite film for prolonging storage life of tomato fruit, Int. J. Biol. Macromol. 112 (2018) 523 529. [76] EC, Regulation (EC) No 1935/2004 of the European Parliament and of the Council of 27 October 2004 on materials and articles intended to come into contact with food and repealing Directives 80/590/EEC and 89/109/EEC, Off. J. Eur. Union. (2004), L338/4. [77] EC, Commission Regulation (EC) No 450/2009 of 29 May 2009 on active and intelligent materials and articles intended to come into contact with food (text with EEA relevance), Off. J. Eur. Union. (2009), L135/3. [78] P. Xin, Z.C. Yun, J.Z. Zheng, Study on the Antibacterial Paper Coated by ZnO/MFC for Food Packaging, Appl. Mech. Mater. 731 (2015) 457 461. [79] C. Chawengkijwanich, Y. Hayata, Development of TiO2 powder-coated food packaging film and its ability to inactivate Escherichia coli in vitro and in actual tests, J. Food Microbiol. 123 (2008) 288 292. [80] Y. Xing, X. Li, L. Zhang, Q. Xu, Z. Che, W. Li, et al., Effect of TiO2 nanoparticles on the antibacterial and physical properties of polyethylene-based film, Prog. Org. Coat. 73 (2012) 219 224. [81] H. Bodaghi, Y. Mostofi, A. Oromiehie, Z. Zamani, B. Ghanbarzadeh, C. Costa, et al., Evaluation of the photocatalytic antimicrobial effects of a TiO2 nano- composite food packaging film by in vitro and in vivo tests, LWT—Food Sci. Technol. 50 (2013) 702 706. [82] M. Gumiero, D. Peressini, A. Pizzariello, A. Sensidoni, L. Iacumin, G. Comi, et al., Effect of TiO2 photocatalytic activity in a HDPE-based food packaging on the structural and microbiological stability of a short-ripened cheese, Food Chem. 138 (2013) 1633 1640. [83] S. Feng, F. Zhang, S. Ahmed, Y. Liu, Physico-mechanical and antibacterial properties of PLA/TiO2 composite materials synthesized via electrospinning and solution casting processes, Coatings 9 (2019) 525. [84] M. Kaseem, K. Hamad, Z.U. Rehman, Review of recent advances in polylactic acid/ TiO2 composites Materials 12 (2019) 3659. [85] J. Xie, Y.C. Hung, UV-A activated TiO2 embedded biodegradable polymer film for antimicrobial food packaging application, LWT—Food Sci. Technol. 96 (2018) 307 314. [86] X. Zhang, G. Xiao, Y. Wang, Y. Zhao, H. Su, T. Tan, Preparation of chitosan-TiO2 composite film with efficient antimicrobial activities under visible light for food packaging applications, Carbohydr. Polym. 169 (2017) 101 107. [87] X. Zhang, Y. Liu, H. Yong, Y. Qin, J. Liu, J. Liu, Development of multifunctional food packaging films based on chitosan, TiO2 nanoparticles and anthocyanin-rich black plum peel extract, Food Hydrocoll. 94 (2019) 80 92. [88] M. Alizadeh Sani, A. Ehsani, M. Hashemi, Whey protein isolate/cellulose nanofibre/ TiO2 nanoparticle/rosemary essential oil nanocomposite film: Its effect on microbial

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and sensory quality of lamb meat and growth of common foodborne pathogenic bacteria during refrigeration, Int. J. Food Microbiol. 251 (2017) 8 14. [89] D. Enescu, M.A. Cerqueira, P. Fucinos, L.M. Pastrana, Recent advances and challenges on applications of nanotechnology in food packaging. A literature review Food, Chem. Toxicol. 134 (2019) 110814. [90] A.S. Dussert, E. Gooris, J. Hemmerle, Characterization of the mineral content of a physical sunscreen emulsion and its distribution onto human stratum corneum, Int. J. Cosmet. Sci. 19 (1997) 119 129. [91] F. Turci, E. Peira, I. Corazzari, I. Fenoglio, M. Trotta, B. Fubini, Crystalline phase modulates the potency of nanometric TiO2 to adhere to and perturb the stratum corneum of porcine skin under indoor light, Chem. Res. Toxicol. 26 (2013) 1579 1590. [92] S.W. Bennett, D. Zhou, R. Mielke, A.A. Keller, Photoinduced disaggregation of TiO₂ nanoparticles enables transdermal penetration, PLoS One 7 (2012) e48719. [93] M.J. Osmond-McLeod, Y. Oytam, A. Rowe, F. Sobhanmanesh, G. Greenoak, J. Kirby, et al., Long-term exposure to commercially available sunscreens containing nanoparticles of TiO2 and ZnO revealed no biological impact in a hairless mouse model, Part. Fibre Toxicol. 13 (2016) 44. [94] D.G. Beasley, T.A. Meyer, Characterization of the UVA protection provided by avobenzone, zinc oxide, and titanium dioxide in broad-spectrum sunscreen products, Am. J. Clin. Dermatol. 11 (2010) 413 421. [95] S.L. Schneider, H.W. Lim, A review of inorganic UV filters zinc oxide and titanium dioxide, Photodermatol. Photoimmunol. Photomed. 35 (2018) 442 446. [96] L. Rowenczyk, C. Duclairoir-Poc, M. Barreau, C. Picard, N. Hucher, N. Orange, et al., Impact of coated TiO2-nanoparticles used in sunscreens on two representative strains of the human microbiota: Effect of the particle surface nature and aging, Colloids Surf., B: Biointerfaces 158 (2017) 339 348. [97] H. Shi, R. Magaye, V. Castranova, J. Zhao, Titanium dioxide nanoparticles: a review of current toxicological data, Part. Fibre Toxicol. 10 (2013) 15. [98] T.G. Smijs, S. Pavel, Titanium dioxide and zinc oxide nanoparticles in sunscreens: focus on their safety and effectiveness, Nanotechnol. Sci. Appl. 4 (2011) 95 112. [99] EC, Commission Directive 95/45/EC of 26 July 1995 laying down specific purity criteria concerning colours for use in foodstuffs. Off. J. L 226. [100] SCCS/1580/16. Scientific Committee on Consumer Safety 2916. SCCS Opinion on Additional Coatings for Titanium Dioxide (Nano Form) as UV-Filter in Dermally Applied Cosmetic Products. [101] F. Grande, P. Tucci, Titanium Dioxide Nanoparticles: a Risk for Human Health?, Mini-Rev. Med. Chem. 16 (2016) 762 769. [102] V.A. Miyani, M.F. Hughes, Assessment of the in vitro dermal irritation potential of cerium, silver, and titanium nanoparticles in a human skin equivalent model, Cutan. Ocul. Toxicol. 36 (2017) 145 151. [103] J. Choi, H. Kim, J. Choi, S.M. Oh, J. Park, K. Park, Skin corrosion and irritation test of sunscreen nanoparticles using reconstructed 3D human skin model, Environ. Health Toxicol. 29 (2014) e2014004. [104] Therapeutic Goods Administration, Literature review on the safety of titanium dioxide and zinc oxide nanoparticles in sunscreens, Sci. Rev. Rep. (2016). V1.1 August. [105] SCCS/1583/17. Scientific Committee on Consumer Safety SCCS Opinion on Titanium Dioxide (Nano Form) as UV-Filter in Sprays.

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Valeria De Matteis, Mariafrancesca Cascione and Rosaria Rinaldi Department of Mathematics and Physics “Ennio De Giorgi”, University of Salento, Lecce, Italy

12.1

Introduction

Since 1972, titanium dioxide nanoparticles (TiO2 NPs) have attracted increasing interest in the scientific community worldwide. In the last years, thanks to the exceptional physicochemical properties, these nanomaterials are being employed in a wide range of applications [1]. The high stability, anticorrosive, and photocatalytic properties have encouraged their use in personal care and cosmetic products [2], in topical sunscreen [3], water treatment [4], and as antibacterial agents [5]. In addition, the brightness has motivated their application as white pigments used for paints [6] and printing inks [7,8], medicines and additive [9]. Among all the properties showed by TiO2 NPs, the strong catalysis activity is by far the most interesting, and, for this reason a large use of TiO2 NPs in several industrial fields has occurred [10]. TiO2 is a semiconductor (SC) material. The electronic energy levels in an SC create energy bands, mainly indicated as valence band (VB) and conduction band (CB), separated by a bandgap. When an electron in VB adsorbs energy greater (or equal) than the bandgap, it can be protonated to CB, leaving a positive hole in CB: an electron hole (e h1) pair is generated and they may migrate toward the SC surface. In this path, some e h1 pairs could be captured by the defect sites or recombine releasing energy in the form of heat or photon. Only e h1 pairs that reach the SC surface, without being involved in these processes, could be involved in redox reactions with chemical species adsorbed on the SC surface, contributing to the photocatalytic reaction [11,12]. In the case of TiO2, its peculiar photocatalytic property is due to thermodynamic properties of its band structure. TiO2 is present in nature in four different crystal structures: tetragonal anatase, tetragonal rutile, orthorhombic brookite, and monoclinic TiO2(B); these forms show distinct photocatalytic activity, due to a different mass density and electronic band structures [13]. The most investigated TiO2 phase forms for photocatalytic applications are anatase and rutile; they present a bandgap equal to 3.20 and 3.02 eV, respectively. Consequently, if TiO2 is exposed to UV radiation (having λ 5 384 and 410 nm), e h1 carriers are generated and redox reactions can occur with chemical elements that are absorbed on the TiO2 surface such as water, nitric oxide (NO), sulfur oxide (SO), ammonia (NH3), hydrogen sulfide (H2S), hydroxide (OH2) ions, carbon monoxide (CO), and carbon dioxide (CO2) [14]. Although the e h1 pairs mobility is higher in rutile than anatase, the latter shows higher photocatalytic activity than rutile, due to a greater efficiency of the charge Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00010-9 © 2021 Elsevier Inc. All rights reserved.

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separation processes. In fact, anatase exhibits a better adsorption radiation capacity and a greater capability in the generation of active sites. Nevertheless, other parameters can be an important role in photocatalytic performance, such as active surface area, type of adsorption and substrate concentration, dimensions of particles, pH, and temperature [15,16]. The photocatalytic behavior of TiO2 was known since the early 19th century; however, only 50 years later, Fujishima and Honda proposed TiO2 powder for photoelectrolysis of water [17]. After UV exposure the created holes (h1) in VB oxidize H2O or OH2 ions to the hydroxyl radical (OH ); at the same time, the electrons promoted in CB reduce the adsorbed O2 species to superoxide (O2 ), triggering a series of chemical reactions that lead to the production of OH radicals. All these radicals strongly react, in non-selective manner, with organic substance, environmental pollutants, or harmful microorganisms, degrading them into H2O and CO2 [14]. This work suggested the possibility of exploiting the TiO2 catalytic capability to catalyze the oxidation of pollutants [18]. In several studies the TiO2 efficiency in water decontamination was analyzed [19], some of them highlighted the antibacterial and antiviral capabilities [20,21]. Besides air and water purifications, decontamination, and antibacterial exploitations, several fields of pure or doped TiO2 applications have been explored by researchers, ranging from “energy” [22], such as photovoltaics and photoelectrochromics [23], to environmental fields such as phocatalysis and sensor developments [24]. The possibility to reduce materials until the nanoscale, allows to modify their physico-chemical properties. In the case of TiO2 NPs, the high surface/volume ratio increases the active area, enhancing its photocatalytic efficiency [25]. In this optic, it is possible to control the movement of the h e pairs by means of quantum confinement, optimizing the transport capacity. In the last years, TiO2 NPs find application in numerous fields (Fig. 12.1), thanks to the ease production, their versatility, physicochemical properties, and the low cost. As early as 2011, Shukla and colleagues reported G

G

Figure 12.1 Applications of TiO2 NPs and the perspective in the near future. DSSC, Dye-sensitized solar cell; PACT, antimicrobial photodynamic therapy; PDT, photodynamic therapy. Source: From D. Ziental, B. Czarczynska-Goslinska, D.T. Mlynarczyk, A. GlowackaSobotta, B. Stanisz, T. Goslinski, et al., Nanomaterials 10 (2020) 387 [31].

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that TiO2 NPs were in the top five types of NPs used in commercial products [26]. Today, the annual production of TiO2 NPs amounts to 10,000 MT, and then they are the most fabricated nanomaterials [27]. The large use of TiO2 NPs in a wide range of applications has motivated the scientific community to evaluated the impact of this material on human health and environment [28 30]. In this chapter, we reported in detail some mechanisms of antimicrobial effects induced by TiO2 NPs; furthermore, the in vitro/in vivo toxicity following the NPs inhalation, ingestion, and absorption is described. Finally, their application in different surfaces and coatings is analyzed.

12.2

Antibacterial and antimicotic properties

12.2.1 Adverse effect of TiO2 on bacteria Many dangerous bacteria such as Escherichia coli, Clostridium difficile, Neisseria gonorrhoeae, Pseudomonas aeruginosa, Mycobacterium tuberculosis, Klebsiella pneumoniae, and Staphylococcus aureus induced several diseases due to infection pathways induction [32]. The common drugs against these microorganisms are antifungal and antibiotic agents; however, the increase of multiple drug-resistant strains induced scientists to look for some alternatives, such as metal and metal oxide NPs [33]. Compared with metal Ag-based systems, TiO2 exhibited considerable advantages against Gram-negative, Gram-positive bacteria and fungi [34]. Indeed, in the case of TiO2, there no need for substances release to active the biocidal action as instead is required for AgNPs, which are toxic due to silver ions (Ag1) release [35]. The antibacterial activity of TiO2 depends on their physicochemical properties such as shape, size, and crystalline structures. However, the generation of reactive oxygen species (ROS) following light irradiation is the mechanism responsible for bacteria death. Compared to bulk TiO2, the reduction of the size at nanoscale increases the surface area/volume ratio that enhances the interactions with the surrounding environment and, at the same time, the easy penetration of bacteria through physiological barriers such as cell wall and cell membrane is occurred. In general, bacteria reply to unknown agents by the use of enzymes production such as catalase and superoxide dismutase (SOD) that are typical biomolecules produced when antioxidant defense begin [36]. In addition, other molecules such as carotene and ascorbic acid help bacteria to inhibit lipid peroxidation, but when these pathways are overcome by ROS radicals, cell wall and cell membrane undergo fatal changes [37]. Cell wall has different composition depending on the types of microorganisms; Gram-positive bacteria cell wall is formed by peptidoglycan and teichoic acid that are organized in a layer structure [38], whereas Gramnegative wall is constituted by lipoproteins and lipopolysaccharides membrane that include peptidoglycan cover [39]. Chitin and polysaccharides are the common constituents of yeast and fungi’s wall [40]. Thus the effect of TiO2 NPs will be slightly different, depending on the type of microorganism.

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The Gram-negative E. coli is particularly susceptible to the TiO2 NPs but the data available in literature are in contrast; with this assumption a grown reduction of c. 70% was observed by Adams et al. [41] after exposure of 66 nm of TiO2 NPs at concentration of 5 g/L: the crystal structure was unknown. Another work reported 30% of E. coli growth decrease using the commercial P25 TiO2 NPs formed by mixed anatase and rutile phase (ratio 80:20) [42]. Lin et al. [43] reported a systematic study using five types of TiO2 NPs in a concentration range of 10 500 mg/L: anatase TiO2 10 nm (TiO2-NP 10A), 25 nm (TiO2-NP 25A), and 50 nm (TiO2-NP 50A); rutile TiO2 of 50 nm (TiO2-NP 50R); and mixed anatase and rutile 25 nm TiO2 (TiO2-NP25 AR). This work showed the strong link between toxicity and size/crystal phase of TiO2 NPs: the small anatase NPs were more toxic than grater ones inducing oxidative damage. In addition, the alkalinization of culture medium from 5 to 10 pH value and ionic strength (50 200 mg/L NaCl) reduced adverse effects on E. coli. The peroxidation triggered by TiO2 NPS was analyzed by Carre` and coworkers [44], which used 0.1 and 0.4 g/L of commercial TiO2 AEROXIDE P25 after UVA irradiation (315 400 nm) in order to analyze the E. coli degradation through lipids and proteins injury. The damage was observed by the evaluation of malondialdehyde levels that occurred during the photocatalytic activation. In addition, the scavenger activity demonstrated after SOD addition underlined the role of O22 radicals in the photocatalytic antimicrobial effect. An alteration of proteins and porins involved in stress response, bacterial metabolism, and transport was recorded after proteomic analysis. Albar et al. [45] demonstrated the alteration of gene and proteins expression involved in signaling, growth functions, ions equilibrium, and cell wall structure of P. aeruginosa PAO1 cells after the photocatalytic action triggered by TiO2-based nanocomposites. In addition, a strong decrease of the coenzyme-independent respiratory chains was observed suggesting the biocidal effect of TiO2. The effects of different kinds of Gram-negative (Salmonella enterica var. enteridis and E. coli) and Gram-positive bacteria (S. aureus, Bacillus cereus, Lactobacillus casei, Lactobacillus bulgaricus, Lactobacillus acidophilus, and Lactobacillus lactis) after exposure to two types of TiO2 NPs were analyzed by Ripolles-Avila et al. [46]. TiO2 was mentioned as NM101 (anatase form and with a size of 7 nm) and NM105 that were anatase rutile phases mix (80:20 wt/wt) with a size of 21 nm. The highest adverse effects of TiO2 NPs were against E. coli, reaching 2 3 log CFU/mL reduction. Many studies were focused on the effects of TiO2 on the second component of bacteria, beyond the cell wall, cell membrane, a semipermeable layer constituted by phospholipids [47]. The exposure to TiO2 provoked a loss of membrane integrity caused by ROS formation following the photoactivation processes with cell lysis as final step [48]. Different cell membranes characterized Gram-positive and -negative bacteria; one membrane covered by several layers of peptidoglycans is distinctive of Gram-positive, whereas an architecture formed by inner and outer membrane with a shell of peptidoglycan between them is constitutive of Gram negative. The outer membrane is exposed, thus, more liable to mechanical breakage due to the lack of

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peptidoglycan protective cover, such as in Gram-positive bacteria [39]. The genes involved in the expression of lipids constituting cell membrane are overexpressed following the interaction with TiO2 NPs; contrary, the genes codifying proteins involved in cell wall structure are down expressed. In general, the cell wall damage was balanced by the consolidation of cell membrane; the latter represents another barrier to contrast the oxidation processes induced by TiO2 [49]. Nevertheless, when the antioxidant elements induction can no longer act due to an imbalance respect to the peroxidation of membrane lipids, cell membrane collapses [50]. This process is also induced by mitochondria respiratory chain damage. Physiologically, mitochondria produce ROS in the electron transfer respiratory chain process, and they convert them in H2O2 and water by SOD and glutathione/catalase, respectively. When bacteria are exposed to several doses of TiO2, the high level of oxidative stress induces the mitochondrial respiratory chain disruption in the double membrane [51]. In particular, the action of TiO2 is connected by coenzyme A dimerization depletion [52]. The DNA injury induced the block of replication, transcription, and cell division caused by oxidative stress that triggered the Fenton reaction [53]. In addition, the presence of TiO2 NPs interferes with the transcriptional factors as well as the signaling molecules [54] (Fig. 12.2).

Figure 12.2 Toxicity effect of TiO2 NPs on microorganisms. Source: Reprinted from J. Hou, L. Wang, C. Wang, S. Zhang, J. Environ. Sci. (2018) [55] with permission from Elsevier, ©2019.

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12.2.2 Adverse effects of TiO2 on fungi Few studies were conducted on the antifungal effects of NPs. The effects of TiO2 against fungi are similar to those observed on bacteria. The photocatalytic action induced by TiO2 NPs upon irradiation with UV light led to produce free radicals, responsible of membrane oxidation and cell permeability [56]. The OH_ trapped hydrogen atoms from polysaccharides, constituting the cell wall with a consequent cleavage and cell death induction [57]. De Filpo et al. [58] used TiO2 NPs to treat Hypocrea lixii and Mucor circinelloides on eight types of wood that were often employed in cultural heritage. The two fungi were involved in rapid degradation of wood, and the application of TiO2 (80% anatase TiO2 and 20% rutile TiO2 with a surface area size of 50 and 15 m2/g) prevented the colonization of them due to their photocatalytic action. Ahmad et al. [59] explored the antifungal activity of the two TiO2 NPs crystalline forms (rutile and anatase) on Candida albicans, an opportunistic pathogenic yeast constituting human gut flora. The authors used three concentrations (50, 100, and 150 μg/mL) to incubate growing yeast cultures and obtained an inhibition effect that was stronger using anatase form (65% of growth inhibition vs 33% after rutile interaction).

12.3

Toxicity assessment on TiO2 NPs

12.3.1 Regulations Currently, the dynamics related to the toxicity of the nanomaterials, such as NPs, is studied due to controversial results founding in the literature. Furthermore, there is still no regulation for the use of NPs caused by this uncertain. In the United States the Food and Drug Administration together with the Environmental Protection Agency has regulated the use of NPs in the United States, whereas in Europe the “Registration, Evaluation, Authorization and Restrict on of Chemicals” (REACH) directs the chemical management. Nevertheless, in these documents, there is no clear differentiation between chemicals and nanomaterials [60,61], while the chemicals agency (ECHA) and European member states compile documents with the goal of registration of NPs in REACH. The National Institute for Occupational Safety and Health, in order to assure safe and healthful working conditions, has conducted a comparative systematic study about toxic effects induced by TiO2 exposure. The analysis performed taking into consideration epidemiological, in vivo and in vitro studies highlight the TiO2 exposure limits: 2.4 and 0.3 mg/m3 for fine and ultrafine TiO2, respectively, as time-weighted average concentrations for up to 10 h/day during a 40 h work week [62]. The widespread use of TiO2-based materials in several industrial applications has focused attention on its toxicity on living organisms. In particular, the International Agency for Research on Cancer classified the TiO2 NPs as potential carcinogenic factor from group 2B, which means it is probable toxic element for humans after several studies conducted in vitro and in vivo [63]. TiO2

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NPs are used in several different application fields, including food and beverage. In the United States the concentration of TiO2 must not exceed 1% in food products [64], while in Europe the at quantum satis principle is adopted [65,66].

12.3.2 Exposure route and biodistrubution The adverse effects of TiO2 NPs on living organisms were largely explored, due to the numerous applications in different fields. Therefore the different toxicity at cellular levels depends on physicochemical properties of NPs, such as charge, aggregation, size, sedimentation, and shape [67,68]. The principal routes of exposure are represented by inhalation, ingestion, and skin penetration.

12.3.2.1 Inhalation Following inhalation, TiO2 NPs are principally distributed in heart, spleen, lungs, and liver [69 71] perturbing the lipid and glucose metabolic pathways in mammalians [72]; the effects become more severe when the animal age increased; Horva´th et al. [73] studied a combined in vitro/in vivo approach: A549 cell lines (lung alveolar adenocarcinoma) were treated with TiO2 nanorods (15 3 65 nm). The same materials were used to expose rats by intratracheal instillation for 28 days in 5 and 18 mg/kg body-weight (bw) doses. A549 has undergone several oxidative stress and lysosomes injury; in animal models, high concentration of Ti was recorded in lungs and blood. In addition, many cytokines were secreted suggesting acute inflammation. Wang et al. [74] conducted a systematic study on toxicity induced by rutile TiO2 NPs (80 nm) and anatase (155 nm) administrated by intranasal instillation using a microsyringe every day (about 500 μg per mice) to study the accumulation in the brain of female mice by inductively coupled plasma mass spectrometry. The authors analyzed the oxidative stress, histopathology, and inflammation response after 2, 10, 20, and 30 days after instillation. The lipid peroxidation activation was particularly evident in mice exposed to anatase NPs as well as the increase of tumor necrosis factor alpha (TNF-α) and interleukin (IL-1-β) levels after 30 days: these data suggested how the crystalline structure induced different toxicity. Okada et al. [75] studied the inhalation of TiO2 NPs (P25) aerosols for 4 weeks at concentration of 4.1 mg/m3 to male Wistar rats. After 3, 30, and 73 days, the bronchoalveolar lavage fluid and serum were collected in order to perform cells and neutrophils count and surfactant protein (SP-D) analysis. SP-D is secreted into the alveoli by alveolar epithelial cells and Clara cells. The results shown an increase in neutrophil count at 3 and 30 days as well as an increase of SP-D levels. Bermudez et al. [76] evaluated the pulmonary toxicity induced by inhalation of TiO2 NPs (P25) on hamsters rats and mice up to 13 weeks. They recorded an increased number of macrophages and neutrophils and, as a consequence, a strong pulmonary inflammation. An interesting work [77] regarded the assessment of inhaled TiO2 NPs (188 6 0.36 nm) on the placental hemodynamic of pregnant Sprague-Dawley rats exposed for 6 days up to 525 6 16 μg of NPs concentration. NPs increased the placental vascular resistance and umbilical vascular

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responsiveness injury; after exposure to nano-TiO2, umbilical artery dilation was decreased in treated animals (30% 6 9%) compared to controls (58% 6 6%). A functional in vitro and in vivo study was performed by Rossi et al. [78] in order to evaluate acute effects induced on myocardial tissue of adult rats after exposure to TiO2 NPs (,50 nm). In vitro, the isolated cardiomyocytes exposed to TiO2 NPs at concentrations of 50, 25, and 5 μg/mL for different times (1, 2, and 5 h) exhibited a meaningful decrease of the action potential duration, sarcomere contraction reduction, and lower stability of membrane potential. In vivo, an increased cardiac conduction velocity and tissue excitability were recorded in healthy rats, after a single intratracheal administration of TiO2 NPs (2 mg/kg). Finally, based on the analysis by the use of computational modeling of the ventricular action potential, the authors assumed that TiO2 NPs adverse effects observed in vivo on rat cardiac tissue, may be related to a membrane leakage.

12.3.2.2 Ingestion NPs can be used to deliver oral drugs or in most of cases are present in beverage and food; [79] For this reason, it is necessary to quantify the toxic effects induced by NPs on the tissues of the gastrointestinal tract [80]. TiO2 NPs are commonly used as white food additive (E171) in several products such as sweets, candies, chewing gums, and puddings [81]. TiO2 are able to cross the intestinal epithelial barrier and to flow into the bloodstream; consequently, the long-term exposure elicits the TiO2 NPs accumulation in principal organs [82]. Several studies were conducted in vitro and in vivo in order to establish the acute and chronic effects of exposure to NPs. In a recent work, Guo et al. [83] investigated the TiO2 NPs in vitro toxicity in terms of ROS production, proinflammatory response (IL-8 and TNF-α cytokines production), nutrient absorption (iron, zinc, fatty acids), and brush border membrane enzyme function (intestinal alkaline phosphatase) and activity. The study was performed on Caco-2 and HT29-MTX cell lines, suitable as models of absorptive and goblet intestinal epithelial cells, respectively. As a result of acute or chronic TiO2 NPs exposure, both cell lines exhibited an increase of ROS and inflammatory cytokines levels; in addition, also intestinal alkaline phosphatase activity was increased. Furthermore, a reduction of nutrient absorption rate was reported, as a consequence of microvilli number decrease. In the light of these evidences, the authors have concluded that the chronic TiO2 NPs exposure, at concentrations normally used in food products, could compromise the intestinal barrier functions. Another in vitro study reported the microvilli damage after TiO2 NPs (,100 nm) on CaCo-2 cell model; the treatment with these NPs at concentration of 350 ng/mL for 24 h resulted in a 42% loss of intestinal villi [84]. Several studies to test TiO2 NPs cytotoxic effect were conducted in vivo on mice. Chen at el. [85] showed strong symptoms of toxicity (loss of appetite, passive behavior, trembling, and lethargy) in mice, in which TiO2 NPs (0, 324, 648, 972, 1296, 1944, 2592 mg/kg) were directly injected in mouse mouth at different time intervals (24, 48 h, 7, and 14 days). In addition, the authors recorded a significant

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accumulation of TiO2 principally in spleen, and in other major organs, such as liver, kidneys, and lungs. A work published by Wang et al. [86] pointed out to evaluate the spleen damage consequent to TiO2 NPs exposure. These NPs were administrated in the stomach at different concentrations (5, 50, and 150 mg/kg bw) for 30 days. The spleen extracted from mice exhibited congestion and proliferation of lymph nodule, probably due to a rise of ROS levels, a stronger lipid peroxidation, and an augmented expression of heme oxygenase-1 via the p38-Nrf-2 signaling pathway, recorded at cellular level. In the study performed by Mohamed [87], TiO2 NPs cytotoxic effects were evaluated on gastric cells isolated from mice, exposed to 5, 50, and 500 mg TiO2 NPs/kg bw by oral route for a period of 1, 7, and 14 days. The genotoxic effects and potential chronic gastritis, due to inflammation, apoptosis, and higher ROS level, were evident in treated cells compared with the control mice, in a dose dependent and time-dependent manner. In addition, histological observations revealed necrosis and inflammation. The authors concluded that these effects could be explained by TiO2 NPs accumulation, even at low doses of exposure (Fig. 12.3). Nogueira et al. [88] assessed the inflammatory response in small intestine of mice, following exposure to TiO2 (100 mg/kg bw), having micro- (MPTiO2, 260 nm) and nanosize (NPTiO2, 66 nm), by oral administration. After 10 days, duodenum, jejunum, and ileum were extracted from mice and analyzed in order to quantify cytokines levels, number of inflammatory cells, and amount to accumulated titanium TiO2. The experimental evidences showed that MPTiO2 and NPTiO2 trigger a Th1-mediated inflammatory response in mice small intestine, particularly in the ileum. Brun et al. [89] conducted in vivo and ex vivo experiments using agglomerates of 95% anatase TiO2 NPs nanopowder having a grain size of c. 132 6 3 nm and positive charge at concentration of 50 μg/mL TiO2 NPs for 48 h. The in vitro experiments were carried out on three gut epithelium models: a monoculture of Caco-2 cells (regular epithelium), a coculture of Caco-2 and HT29-MTX cells (mucus-secreting regular epithelium), and a coculture of Caco-2 and RajiB cells [follicle-associated epithelium (FAE)]. The authors reported the upregulation of TJP1 (ZO-1, a component of tight junctions) in Caco-2 cells after 6 and 48 h of incubation. Indeed, the upregulation of CTNNB1 gene, encoding β-catenin, was observed. The ex vivo studies highlighted the uptake through the FAE and regular ileum epithelium changing the paracellular ileum and colon epithelial permeability. After in vitro and ex vivo experiments the authors administrated 375 μg TiO2 NPs in each mice; this concentration corresponded to the daily intake in children. NPs were found agglomerated in the gut environment and, at the same time, boosted paracellular permeability alteration.

12.3.2.3 Skin penetration The TiO2 NPs are used in several products in cosmetic and personal care industries; it is the most commonly used material in dermal consumer products [90]. A great number of evidences suggest that the capability of skin penetration is very low for

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Figure 12.3 Histopathological analysis of TiO2 NPs treated group versus negative control. Li, Focal area of leukocyte infiltrations; M, mononuclear cells infiltrations; mn: mucosal; Mg, massively degenerated glands, massive mucosal and submucosal necrosis; Ne, necrosis of some glands; Se, submucosal edema; sn, Cb, submucosal congested blood vessel; sv, submucosal vasculitis. Source: Adapted from H. Ramadan, H. Mohamed, Food Chem. Toxicol. 83 (2015) 76 83.

inorganic micro-NPs; In particular, several researches have shown that these NPs are biologically inert for mammalians, seeing as they do not penetrate the skin [91,92]. The crossing rate results greater in the case of NPs: the anatase TiO2 NPs resulted more reactive [93,94] and able to cross the skin, especially in the presence of irritations or lesions [95,96]. For these reasons, although cosmetic and dermal products should be applied on healthy skin, it is sufficient the presence of microlesions on the epidermis permits particles to cross it [97]. Numerous in vitro and in vivo studies were conducted to quantify the penetration rate of TiO2 NPs both on undamaged and injured skins.

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According to the works made by Newman and colleagues [98], a negligible amount of TiO2 NPs penetrates healthy skin; this conclusion is corroborated by Sadrieh et al. [99]. Wu et al. [95] examined the penetration capability and potential toxicity of TIO2 NPs. The authors reported different results in the case of in vivo experiments respect to in vitro ones. In detail, when porcine skin was exposed to TiO2 NPs for 24 h, NPs did not cross the stratum corneum. At the same time, in in vivo experiment the pig ear was topically exposed to TiO2 NPs. After 30 days of treatment the NPs penetrated the stratum corneum, reaching the deeper epidermal layer; prolonging the exposure time to 60 days, the TiO2 NPs accumulated in the principal organs, inducing pathological changes. The undamaged and needle-abraded human skins were exposed to TiO2 NPs, suspended in synthetic sweat solution (1.0 g/L). In both cases, after 24 h, Crosera and coworker [100] did not find a significant amount of NPs crossedthrough the skin. Furthermore, in this work the cytotoxic potential of TiO2 NPs incubated with HaCaT keratinocytes after 24, 48 h, and 7 days, highlighted that these NPs weakly interacted with cells and their impacts become significant only after long-term exposure. In numerous in vitro studies the toxic potential of TiO2 NPs was studied on different skin cell models [101,102]. Wright and colleagues [103] performed a comparative study among different sized (F-TiO2, UF-TiO2, and H2TiO7) TiO2 particles, in order to evaluate the dependence of TiO2 cytotoxic effect on particles dimension. The experiments conducted in this work led to conclude that all tested TiO2 particles caused a rise in SOD production, in caspase 8, 9, and 11 levels. These effects were demonstrated to be dose dependent. In general, the increased ROS levels appeared the main mechanism behind toxic outcomes, until cell death by DNA damage in human epidermal and human dermal fibroblasts cells [104,105]. On the other hand, Zhao et al. [106] observed that using lower exposure dose, TiO2 NPs can triggered autophagy in primary human keratinocytes, protecting them against injury and cellular death. Yanagisawa and coworkers [107] demonstrated that the exposure to TiO2 NPs changed dermal barrier physiology, starting or promoting skin injury, such as atopic dermatitis, through increased T helper-2 immune responses. Albeit several studies were carried out to understand the toxic impact of TiO2 NPs, it is still unclear how these NPs may interact with other ingredients present in cosmetic commercial products and for personal care: this is the new frontier in the toxicology studies.

12.4

Antimicrobial surfaces

TiO2 was used to develop antimicrobial surfaces for both medical purpose and other industrial applications. In this field, biomaterials’ surfaces are known as materials subjected to bacterial colonization, through the first biofilm development that induces infections or material damage. To overcome this process, some

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antibacterial elements such as silver were used as well as the formation of superhydrophobic surface or fabrication of structured arrays. In a recent study [108], soft lithography and dip-pen nanolithography techniques were used with TiO2 addition (5% or 10% TiO2) in order to develop a Streptococcus mutans inhibitor surface on surgical-grade stainless-steel plates after UV irradiation. The micropatterned device exhibited a double effect on bacteria inhibition that was both chemical (TiO2) physical (micropatterned surface). In this way the reduction of S. mutans colonization was about 60%, proportionally increasing with TiO2 concentration (Fig. 12.4). The photocatalytic self-cleaning activity, due to the removal of microorganisms, is a technology widespread in the last years. The photocatalytic process directs the hydrophobic or hydrophilic properties; in this way, it is possible to couple both photocatalysis and photoinduced wettability. The production of ROS following the photocatalysis on the surface allows to use TiO2 to implement surfaces in the field that require disinfection or cleanness. Then, the TiO2-based surfaces are particularly suitable in medical buildings to eliminate fungi, bacteria,

Figure 12.4 SEM acquisitions regarding the bacteria colonization on different TiO2-treated surfaces: SS-TiO2 coated (A), SS-TiO2 micropatterned (B), SS polished (C), and unexposed SS polished (D). Scale bare is 5 μm whereas scale bar in the inserts is 10 μm. SEM, Scanning electron microscopy. Source: Adapted from S. Arango-Santander, A. Pelaez-Vargas, S.C. Freitas, C. Garcı´a, et al., Sci. Rep. 8 (2018) with permission of Springer Nature, ©2018.

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and organic compounds (VOCs). In general, the effectiveness of TiO2 surfaces can increase with composite layer formation by the use of Ag or carbon nanotubes (CNT); the latter exhibit high conductivity and electron storage capacity [109,110]. The surfaces constituted by Ag can be absorbed in the visible region of wavelength producing ROS; the electrons produce energy transfer to TiO2 generating electron and hole pair in order to have an additional surface reaction [111]. In addition, the contribution of the two elements (Ag TiO2) strongly enhanced the antimicrobial effect [112]. Won et al. [113] obtained a TiO2-based coating on a borosilicate transparent glass for application on touchscreens studying their antimicrobial effect both under UV irradiation in dark conditions. The principal goal of the work was to develop self-cleaning medical device to prevent the transmission of disease. The new surface was analyzed by atomic force microscopy to measure the hydrophilicity and roughness. The antimicrobial properties of this new surface were compared with a commercial counterpart, Corning Gorilla Glass (silver based) in the same conditions against the strain of E. coli ATCC 25922. Results showed that the Ag-doped TiO2 had better toxicological effect (80%) on bacteria than the commercial glass. The same principle was used to produce photocatalytic and superhydrophilic self-cleaning windows that were able to eliminate water and pollution on their surface [114]. Some of these experimental products became commercial such as San Gobain Bioclean, Pilkington Active, and Sunclean. Another application of TiO2 is the adding TiO2 powders in cementitious materials without damaging the performances or other treatments. The photocatalytic properties were important to eliminate the pollution (fungi, bacteria, and gas molecules) migration on concrete pavement and external building surfaces due to their flat nature that permit to obtain the maximum photocatalytic effect under solar irradiation [115]. Once the degradation process induced by TiO2 was finalized, the rain can eventually clean the surfaces; for this reason the application in urban context is very interesting. Chen and Poon [116] analyzed the removal of nitrogen oxides species (NOx) in order to assess if the internal properties of concrete, in particular the microstructure, influenced the organic removal process. The author concluded that the Ordinary Portland cement pastes exhibited lower photocatalytic activity compared to white cement pastes. In addition, the curing age was a critical factor in the light-induced phenomenon because a longer curing provoked a reduction of NOx ablation. This event can be justified by the cement pore occlusion that obstructed the propagations of photons. Janus et al. [117] investigated the antibacterial effects (against E. coli K12) of concrete plates with the presence of 10% of AEROXIDE TiO2 P25, nitrogen (N) modified or nitrogen and carbon (C,N) comodified titania photocatalysts carried out at 100 C, 300 C, and 600 C. Under irradiation a complete bacterial depletion was observed using TiO2/N, CMeOH-300, TiO2/N, CEtOH-100, TiO2/N, CisoPrOH-100, and TiO2/N-300 concluding how the functionalization with carbon or nitrogen together with the porosity and crystallinity influenced the effectiveness of antibacterial activity.

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Conclusion

Over the past decades, many studies have been focused on the broad applications of TiO2 NPs, ranging from medicine to cosmetics, as well as pharmaceutical, constructions, and chemical industry. TiO2 is an optimal material in the development of antimicrobial surfaces due to its ability to kill several kinds of Gram-positive and Gram-negative bacteria. Despite their exceptional properties, in particular their photocatalytic activity, there are still “dark sides” concern their toxic behavior in living organisms. In particular, the large application in consumer products exposes humans to adsorb TiO2 NPs by a different route of entry. Future challenges will involve the creation of reliable databases in which TiO2 NPs can be classified according to results deriving from toxicological experiments. The different types of NPs should be listed indicating the concentrations, the size, and the shape together with the animal models used. In this optic the scientific results could provide a strong help in order to develop certain legislation on the use of nanomaterials.

Conflicts of interest The authors declare no conflict of interest.

Acknowledgments V.D.M. kindly acknowledges Programma Operativo Nazionale (PON) Ricerca e Innovazione 2014 2020 Asse I “Capitale Umano”, Azione I.2, Avviso “A.I.M: Attraction and International Mobility” CUP F88D18000070001.

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[81] V.D. Matteis, M. Cascione, C.C. Toma, P. Pellegrino, L. Rizzello, R. Rinaldi, Cellular and molecular toxicology of nanoparticles, Advances in Experimental Medicine and Biology, Springer, Cham, 2018, pp. 1 19. [82] M.A. Vetten, N. Tlotleng, D.T. Rascher, A. Skepu, F.K. Keter, K. Boodhia, et al., Titanium dioxide nanoparticles: a review of current toxicological data. Part. Fibre Toxicol. 10 (2013). [83] Z. Guo, N.J. Martucci, F. Moreno-Olivas, E. Tako, G.J. Mahler, Titanium Dioxide Nanoparticle Ingestion Alters Nutrient Absorption in an In Vitro Model of the Small Intestine. NanoImpact 5 (2017) 70 82. [84] J.J. Faust, K. Doudrick, Y. Yang, P. Westerhoff, D.G. Capco, Food grade titanium dioxide disrupts intestinal brush border microvilli in vitro independent of sedimentation. Cell Biol. Toxicol. 30 (2014) 169 188. [85] J. Chen, X. Dong, J. Zhao, G. Tang, In vivo acute toxicity of titanium dioxide nanoparticles to mice after intraperitioneal injection. J. Appl. Toxicol. (2009). [86] J. Wang, N. Li, L. Zheng, S. Wang, Y. Wang, X. Zhao, et al., P38-Nrf-2 signaling pathway of oxidative stress in mice caused by nanoparticulate TiO2.Biol. Trace Elem. Res. 140 (2011) 186 197. [87] H. Ramadan, H. Mohamed, Estimation of TiO2 nanoparticle-induced genotoxicity persistence and possible chronic gastritis-induction in mice. Food Chem. Toxicol. 83 (2015) 76 83. [88] C.M. Nogueira, W.M. De Azevedo, M. Lucia, Z. Dagli, S.H. Toma, Z.D.A. Leite, et al., Titanium dioxide induced inflammation in the small intestine. World J. Gastroenterol. 18 (2012) 4729 4735. [89] E. Brun, F. Barreau, G. Veronesi, B. Fayard, S. Sorieul, C. Chane´ac, et al., Titanium dioxide nanoparticle impact and translocation through ex vivo, in vivo and in vitro gut epithelia. Part. Fibre Toxicol. 11 (2014). [90] T.A. Robertson, W.Y. Sanchez, M.S. Roberts, Are commercially available nanoparticles safe when applied to the skin? J. Biomed. Nanotechnol. 6 (2010) 452 468. [91] J. Lademann, H.-J. Weigmann, C. Rickmeyer, H. Barthelmes, H. Schaefer, G. Mueller, et al., Penetration of Titanium Dioxide Microparticles in a Sunscreen Formulation into the Horny Layer and the Follicular Orifice. Skin. Pharmacol. Appl. Skin Physiol. 12 (1999) 247 256. [92] J. Schulz, H. Hohenberg, F. Pflu¨cker, E. G¨artner, T. Will, S. Pfeiffer, et al., Distribution of sunscreens on skin. Adv. Drug Deliv. Rev. 1 (2002). [93] H. Shi, R. Magaye, V. Castranova, J. Zhao, Titanium dioxide nanoparticles: a review of current toxicological data. Fibre Toxicol. 10 (2013). [94] C. Jin, Y. Tang, F. Guang Yang, X. Lin Li, S. Xu, X. Yan Fan, et al., Cellular Toxicity of TiO2 Nanoparticles in Anatase and Rutile Crystal Phase. Biol. Trace Elem. Res. 141 (2011) 3 15. [95] J. Wu, W. Liu, C. Xue, S. Zhou, F. Lan, L. Bi, et al., Toxicity and penetration of TiO2 nanoparticles in hairless mice and porcine skin after subchronic dermal exposure. Toxicol. Lett. 191 (2009) 1 8. [96] C. Bennat, C.C. Mu¨ller-Goymann, Skin penetration and stabilization of formulations containing microfine titanium dioxide as physical UV filter. Int. J. Cosmet. Sci. 22 (2001) 271 283. [97] G. Xie, W. Lu, D. Lu, Penetration of titanium dioxide nanoparticles through slightly damaged skin in vitro and in vivo. J. Appl. Biomater. Funct. Mater. 13 (2015) e356 e361.

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Functionalization of glass by TiO2based self-cleaning coatings

13

Corrado Garlisi1,2, Gabriele Scandura1,2, Ahmed Yusuf1,2 and Samar Al Jitan1,2 1 Department of Chemical Engineering, Khalifa University, Abu Dhabi, United Arab Emirates, 2 Research and Innovation Center on CO2 and H2 (RICH Center), Khalifa University, Abu Dhabi, United Arab Emirates

13.1

Introduction

Self-cleaning coatings are increasingly becoming an integral part of our daily life. In particular the use of such coatings in building windows is advisable to improve the air quality in the surroundings of the buildings and reduce the costs related to cleaning and maintenance. Self-cleaning glass relies on the unique features of TiO 2 arising from the synergetic effect of photocatalytic activity and superhydrophilicity under UV irradiation. TiO2 is applied in the form of a thin film on the glass substrate, allowing for the degradation of the organic pollutants due to light-induced redox mechanism and the removal of the loosened contaminants from the surface by water sheets generated by the hydrophilic effect [1,2]. Currently, there are already numerous examples of commercial self-cleaning glasses. PPG Glass and Pilkington Glass are two of the most renowned manufacturer of self-cleaning glasses, which can also ensure solar control ability in some of their variants (i.e., Pilkington Activ Bronze and Blue), improving the energy efficiency of the building where these are installed [3,4]. In parallel to an exponential market growth of these products over the last years, an intense research activity has been devoted to enhancing the self-cleaning ability of the current coatings and adding further functionalities to the glass. In particular, a great effort has been made to extend visible-light harvesting and impart hydrophilic properties in the dark without resorting to UV illumination [5,6]. This chapter focuses on the basic principles behind the self-cleaning properties of TiO2, covering main applications of TiO2-coated glass and doping strategies for improving its self-cleaning ability. New tendencies are also discussed in the context of a growing interest toward TiO2-based multifunctional coatings combing selfcleaning, antireflection, and energy efficiency properties.

Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00009-2 © 2021 Elsevier Inc. All rights reserved.

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Titanium Dioxide (TiO2) and Its Applications

13.2

Main principle behind self-cleaning behavior

The self-cleaning effect of TiO2 thin films is due to two different mechanisms taking place contemporaneously on the surface under UV irradiation: superhydrophilicity and photocatalytic activity. This paragraph will focus on such unique properties of TiO2. Water droplets can form a thin aqueous film by spreading out on a superhydrophilic surface. This special condition of wettability depends on surface and interfacial energies. Surface wettability can be easily described by the contact angle (θ) measured between the liquid
gas and the solid
liquid interfaces when a liquid droplet adheres to a solid surface (Fig. 13.1). Table 13.1 shows a classification of materials based on θ. Contact angle is affected by several factors: surface energy, functional groups present on the surface, impurities, roughness, and porosity. The first theory on this topic was formulated in 1805 by Young [8]. He correlated the contact angle (θY) with the solid
gas (γ SG), solid
liquid (γ SL), and liquid
gas (γ LG) interfacial energies: cos θY 5

ðγ SG 2 γ SL Þ γ LG

(13.1)

Young’s equation is valid for a rigid, perfectly smooth, chemically homogenous solid surface (i.e., ideal surface) in thermodynamic equilibrium [9].

Figure 13.1 Wetting models of a droplet on solid substrate: (A) Young, (B) Wenzel, and (C) Cassie
Baxter. Source: Adapted from X. Feng, L. Jiang, Design and creation of superwetting/antiwetting surfaces, Adv. Mater. 18 (2006) 3063
3078 [7]. Reproduced with permission from Wiley.

Table 13.1 Different wettability regimes based on the water contact angle. Contact angle (degrees)

Wettability of the solid surface

θ,5 5 , θ , 90 90 , θ , 120 120 , θ , 150 θ . 150

Superhydrophilic Hydrophilic Hydrophobic Highly hydrophobic Superhydrophobic

Functionalization of glass by TiO2-based self-cleaning coatings

397

In 1936 Wenzel introduced a surface roughness factor (r) in Young’s equation. The factor r is the ratio between the real surface area and the apparent surface area. Wenzel’s equation can be written as follows: cos θW 5 r

  ðγ SG 2 γ SL Þ γ LG

(13.2)

Wenzel’s theory states that the roughness (r . 1) raises the surface wettability: θW , θY (higher hydrophilicity) when θY , 90 degrees, θW . θY (higher hydrophobicity) when θY . 90 degrees [10]. The model proposed by Cassie and Baxter in 1944 covers the effect of chemical heterogeneities on the equilibrium contact angle [11]. They assumed that air bubbles may be trapped below the liquid when θ . 90 degrees owning to a specific surface roughness. The apparent contact angle (θCB) will be given by the contributions of two phases: liquid
solid interface and liquid
vapor interface (Eq. 13.3) cos θCB 5 f1 cos θ1 1 f2 cos θ2

(13.3)

where f1, f2 and θ1, θ2 are the surface area fractions and the contact angles of phases 1 and 2, respectively (f1 1 f2 5 1). Notwithstanding Wenzel and Cassie
Baxter models are a good description of the contact angle on rough surfaces from a qualitative point of view, they do not rationalize quantitatively the wettability phenomenon. Indeed, the contact angle is not static but goes through hysteresis cycles between the advancing contact angle (θadv, maximum observed angle) and the receding contact angle (θrec, minimum observed angle). When inflated, θ increases until the droplet reaches a critical volume after which the drop starts receding (Fig. 13.2). Several experimental and theoretical works corroborated that both the contact angle and the contact angle hysteresis (Δθ 5 θadv
θrec) depend on the surface roughness [12
16]. Moreover, Δθ is also a function of the adhesion hysteresis [17
19] and liquid
solid molecular interactions [20]. Titanium oxide, the most applied photocatalyst due to its photostability, costeffectiveness, and nontoxicity [21,22], is a semiconductor material with a bandgap

Figure 13.2 (A) Schematic illustration of contact angle hysteresis. (B) Advancing and receding contact angles via increasing and decreasing the volume of droplet, respectively [2]. Source: Reproduced with permission from Elsevier.

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Titanium Dioxide (TiO2) and Its Applications

around 3.0
3.2 eV (depending on the polymorphic form). When TiO2 is irradiated by a wavelength shorter than 400 nm (i.e., UV light), electron-hole pairs are generated which, in turn, give rise to a series of highly reactive species such as superoxide anion, hydrogen peroxide, hydroxyl, and hydroperoxyl radicals [22]. The following reactions can take place in the presence of oxygen and water: TiO2 1 hν ! e2 1 h1

(13.4)

O2;ads 1 e2 ! O2 2

(13.5)

H2 O ! OH2 1 H1

(13.6)

H2 O 1 h1 ! OH 1 H1

(13.7)

1  O2 2 1 H ! HO2

(13.8)

HO2 1 e2 ! HO2 2

(13.9)

1 HO2 2 1 H ! H2 O2

(13.10)

These photogenerated species have a strong oxidation power toward several organic contaminants, including alkanes, ketones, alcohols, and carboxylic acids, which will be decomposed to CO2, H2O, or other basic products [23
26]. Titania photocatalysts can also degrade long-chain organic compounds and polymers [27]. This photocatalytic property results inherently in a self-cleaning effect since soiled compounds can be degraded over the surface exposed to the environment. The superhydrophilic character of TiO2 rises when it is deposited as thin film on a substrate (e.g., glass), or it is included in the blend for making concrete and paving blocks. A TiO2 film is usually hydrophilic with a water contact angle around 30
70 degrees. However, upon UV irradiation, the contact angle begins increasing, and, ultimately, the TiO2 surface becomes superhydrophilic showing a contact angle very close to 0 degrees. This phenomenon, which was observed adventitiously for the first time in 1995 at the Toto Inc. laboratories, is reversible [28]. In fact the material will return hydrophilic when stored in the dark, after maintaining a low contact angle for a certain time. Several techniques can be used to investigate the “switchable” wettability of TiO2 films. Contact angle measurements are useful from a macroscopic point of view, as described earlier. Wang et al. studied rutile TiO2 (1 1 0) single crystal by means of friction force microscopy [29]. The surface was homogeneously hydrophobic at the microscopic level before UV irradiation, and it exhibited both hydrophilic rectangular domains aligned along the [0 0 1] direction of the (1 1 0) crystal plane and hydrophobic areas due to UV light exposure. X-ray photoelectron spectroscopy (XPS) showed that UV irradiation creates surface oxygen vacancies at bridging sites, and, consequently, titanium oxidation state changes from 14 to 13 [30]. Arguably, these new defects (Ti31 sites) foster a dissociative water chemisorption

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making the domains more hydrophilic [31]. Water spreads out on the film surface because the microscopic hydrophilic domains are smaller than a liquid droplet. Thus, under UV illumination, the surface displays a macroscopic superhydrophilicity even if there are still hydrophilic/hydrophobic areas. Oxygen from the air will replace the chemisorbed hydroxyl groups while keeping the TiO2 film in the dark, and eventually the surface will revert back to its native hydrophobic (or hydrophilic) homogeneity. Fourier-transform infrared (IR) spectroscopy can distinguish between adsorbed molecular water and adsorbed dissociated H2O. As a result of UV exposure, the 1623 cm21 band (assigned to adsorbed molecular water) decays, whereas the 3695 cm21 band (chemisorbed H2O) grows [32]. Before irradiation, hydroxyl groups are bound to oxygen vacancies. During UV irradiation the chemical bond Ti
O
Ti is weaker because the photogenerated holes on the surface are trapped at lattice oxygen atoms. Then, adsorbed water molecules dissociate by breaking the Ti
O
Ti bond and forming new OH groups, which will be desorbed (at a certain rate) during dark conditions (Fig. 13.3). The crystal structure of TiO2 plays a significant role in the wettability phenomenon. For instance, in the case of rutile, the (0 0 1) plane takes longer to achieve the superhydrophilicity under UV light than plane (1 1 0) and (1 0 0) owning to the absence of reactive bridging site oxygen [34]. In order to quantitatively assess the photoinduced hydrophilic kinetics, a hydrophilic conversion rate definition is demanded. Using the slope of the contact angle versus irradiation time curve is inadvisable because this depends on the initial θ, which changes with surface energy [8], roughness [10], and dark storage time after reaching superhydrophilic under UV light (td) [33]. Plotting 1/θ versus irradiation time resulted in a straight line from the initial θ to the minimum value of θ (usually referred as the critical contact angle). Sakai et al. defined this slope as the hydrophilic conversion rate [kf (degree min)21], which depends on td but not on the initial θ [33]. The correlation between 1/θ and the logarithm of the dark storage time was

Figure 13.3 Reversible hydrophilic changes on a TiO2 film: (A) before UV irradiation, (B) during UV irradiation at the transition state, (C) after UV irradiation [33]. Source: Reproduced with permission from American Chemical Society.

400

Titanium Dioxide (TiO2) and Its Applications

also linear, indicating that the reverse process in the dark increases as the conversion from superhydrophilicity to hydrophilicity state proceeds. Moreover, a shoulder peak in O 1s XPS spectra rose with UV irradiation owing to dissociative water adsorption. The integrated intensity ratio of such peak to the O 1s main band was linear with the reciprocal of the contact angle (under illumination). The latter, thus, can also be used to estimate the amount of photoinduced surface OH groups. Hydroxyl groups can be classified according to their thermal desorption peak and stretching frequency observed by the temperature-programmed desorption [31] and the IR absorption spectroscopy [35], respectively. There are three types of hydroxyl groups: (1) molecularly adsorbed H2O, (2) dissociated adsorbed H2O, (3) OH groups bound to oxygen vacancies. However, XPS analysis cannot elucidate which kind of OH group is related to the photoinduced superhydrophilicity phenomenon. The reciprocal of the contact angle was found to be linear with the thermal desorption spectroscopy water peak integral (m/z 5 18) in the temperature region 200 C
270 C. This peak corresponds to the hydroxyl groups bound to oxygen vacancies [33]. Based on these experimental results, the TiO2 surface hydrophilic conversion rate is correlated with the rate for the reconstruction of surface hydroxyl groups at the oxygen defect sites (or oxygen vacancies) in response to UV light. This rate is a function of both intensity and wavelength of the incident light [33]. When plotting kf against incident UV intensity (I, expressed in mW/cm2) in a log
log graph, two different straight lines are obtained, having slopes equal to 1.03 (for I , 0.4 mW/cm2) and 0.46 (for I . 0.4 mW/cm2), respectively (Fig. 13.4A). This behavior is typical of the photocatalytic processes. The photoinduced hydrophilic

Figure 13.4 (A) Log
log plot of the hydrophilic conversion rate (kf) versus the incident UV intensity (I). Each line is the regression straight line for the UV intensity region lower and higher than 0.4 mW/cm2, respectively. (B) Action spectrum of the hydrophilic conversion rate (kf). The number of the incident photons was constant for all wavelengths. The inset shows the UV
Vis absorption fraction of TiO2 thin film, which was calculated by the substitution of transmittance and reflectance from 100% [33]. Source: Reproduced with permission from American Chemical Society.

Functionalization of glass by TiO2-based self-cleaning coatings

401

conversion, likewise the heterogeneous photocatalysis, takes place in the light-limited condition below the lower light intensity, and it is limited by the recombination of the photogenerated e2/h1 pairs at the higher light intensity. The dependence on the wavelength is very strong and matches with the TiO2 absorption spectrum (Fig. 13.4B). The last result confirmed that the TiO2 bandgap excitation is essential for the photoinduced hydrophilic conversion. Remarkably, the conversion rate toward superhydrophilicity increases with repeated UV illumination, and such effect is more evident on (0 0 1) face than (1 1 0) face, in the case of a rutile single crystal. This result was ascribed to a diverse formation of oxygen vacancies and to the resulting structural distortion by the chemisorbed H2O [36]. Both photocatalysis and photoinduced superhydrophilicity can happen at the same time on the same surface; both phenomena involve charge carriers and hydroxyl groups. Nevertheless, the two mechanisms are different. The electronic structure and photocatalytic oxidation activity of SrTiO3 are similar to TiO2, yet SrTiO3 films do not show any superhydrophilicity character under UV irradiation [37]. This fundamental result corroborates that the photocatalytic degradation of organic compounds does not induce the superhydrophilicity of TiO2. Conversely, few materials (e.g., WO3 and V2O5) exhibite the photoinduced hydrophilicity but no photocatalytic degradation of methylene blue adsorbed on surfaces under UV illumination [38]. As mentioned earlier, the photogenerated e2/h1 pairs (Eq. 13.4) can reduce/oxidize the TiO2 surface by following reactions (13.11) and (13.12) rather than (13.5)
(13.10). OðOÞ22 1 2h1 ! VðOÞ 1 0:5O2

(13.11)

Ti41 1 e2 ! Ti31

(13.12)

The creation of oxygen vacancies [V(O)] under UV irradiation enhances the H2O chemisorption on TiO2 surface yielding to the superhydrophilicity. In the case of SrTiO3, UV photons are not able to trigger the formation of V(O). As shown in Eq. (13.11), the photogenerated holes play a primary role in creating the oxygen vacancies and hence the photoinduced superhydrophilicity. In fact, kf decreases linearly when increasing the concentration of Na2SO3 (a typical hole scavenger) in the system [33]. To conclude this section a TiO2-based self-cleaning material is able to decompose organic contaminants, which will readily be washed out due to the photoinduced superhydrophilicity (Fig. 13.5). If the surface was only photocatalytically active, the overall pollutants degradation rate would be significantly lower. Indeed, a superhydrophilic surface hampers organic oily liquids adsorption. The superhydrophilic character also contributes to removing dust under water flow and avoids the water condensation on TiO2-coated window (antifogging effect) [39
41]. Thus the outstanding self-cleaning properties of a TiO2-coated glass are due to the synergy between two different mechanisms: photocatalysis and photoinduced superhydrophilicity (both activated by photons with a wavelength shorter than 400 nm).

402

Titanium Dioxide (TiO2) and Its Applications

Figure 13.5 Schematic diagram of the self-cleaning process on the TiO2 surface with (A) photocatalytic organic decomposition properties and (B) photoinduced superhydrophilic properties.

13.3

Applications of self-cleaning glass and main commercial products

Self-cleaning coatings leverage hydrophilicity and photocatalytic properties of TiO2 when exposed to UV light sources. They find application in the functionalization of a wide range of substrates including glass, plastics, ceramics, and concrete [42]. For instance, photocatalytic cement-based self-cleaning coatings have been demonstrated and tested [43], including the addition of nano-TiO2 to cement-based structures to improve their resistance to water permeability [44,45]. Hydrotec is a popular product in Japan by TOTO limited, which uses TiO2 thin films to produce self-cleaning tiles and paint. Their representative products are the white ceramic tiles for exterior walls and home surroundings [46]. Glazing is arguably the largest commercialization of self-cleaning coatings; hence, this paragraph will discuss commercialized hydrophilic self-cleaning coatings applicable to glaze products. This section aims to update and enlighten concerned and interested stakeholders on the state-of-the-art applications of self-cleaning coatings in the glass industry, by critically examining some selected available commercial products. Hydrophilic self-cleaning coatings can be used to modify the surface of ordinary glass to produce antifogging, antibiofouling, and antimicrobial surfaces [42]. Commercial glass products made of self-cleaning coatings can be categorized as either factory-finished or consumer-finished products. What differentiates these two types of products is where (factory vs home) and how (professional coating vs facile wiping) the self-cleaning coating is applied [3]. Table 13.2 reports the most popular self-cleaning glass products including manufacturer name, country, method of coating, product life span, and types of chemical that can damage the self-cleaning coatings (i.e., vulnerability). As can be seen from the table, most of the commercial products make use of TiO2, some others are polymer-based coatings which usually impart hydrophobic properties to the glass. However, the latter coatings go behind the scope of this chapter, so the reader is referred to references for further study [47,48].

Table 13.2 Some selected commercial glass products with self-cleaning coating [3]. Manufacturer

Country

Product

Photocatalyst/properties

Coating technique

Durability of coating

Vulnerability

Activ CLEAR/ BLUE/ NEUTRAL Neat glass (1v triple pane LoE-180) Self-cleaning glass

TiO2 (photocatalytic/ hydrophilic)

NA

TiO2 plus thin layer of SiO2 (photocatalytic/hydrophilic)

NA

External (facades, roof, and conservatories) Same as lifetime of the glass

Silicone and silicone products None

NA (antimold, antifungi, photocatalytic/hydrophilic)

Hightemperature pyrolytic spraying Permanent bonding to the glass surfaces NA

In excess of 15 years

None

10 years

None

NA

NA

Silicone and silicone products Abrasive cleaning solution

Factory-finished products Pilkington Group Limited Cardinal Glass Industries

United Kingdom

Fuyao Glass Industry Group Co. Limited Reflex Glass

China

United Kingdom

RE-FLEX/REFLEX1 Clear

Polymeric hydrophobic coating (antimold, antialgae growth, photocatalytic)

Press Glass, SA

Poland

TiO2 (photocatalytic/ hydrophilic)

Saint-Gobain Glass UK Limited PPG residential glass

United Kingdom

Active with TopGlass EcoPlus SGG BIOCLEAN/ TOTAL1 SunClean

United States

United States

Mineral material (photocatalytic/hydrophilic)

Fusion

Same as lifetime of the glass

TiO2 (photocatalytic/ hydrophilic)

Hightemperature formation

Same as lifetime of the glass

(Continued)

Table 13.2 (Continued) Manufacturer

Country

Product

Photocatalyst/properties

Coating technique

Durability of coating

Vulnerability

Viridian

Australia

TiO2 (photocatalytic/ hydrophilic)

Fusion

Same as lifetime of the glass

None

TOTO Ltd.

Japan

Renew with Viridian ThermoTech Clear Hydrotect (outdoor tiles and paint)

TiO2 (photocatalytic/ hydrophilic)

Spraying followed by thermal treatment

NA

NA

Consumer-finished products Balcony Systems Solutions Rain Racer Developments Ritec International

United Kingdom

BalcoNano

Hydrophobic

Spraying or wiping

3
10 years

Heavy cleaners

United Kingdom

Rain Racer

Polymer (hydrophobic)

3
4 years

None

United Kingdom

ClearShield

Polymeric resin (hydrophobic)

Spraying or wiping Spraying or wiping

Not specified

Abrasive cleaners or harsh chemicals

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13.3.1 Commercial self-cleaning glasses 13.3.1.1 Pilkington Activ Clear/Blue/Neutral by Pilkington Group Limited Pilkington Group is a renowned company in the self-cleaning glazing product industry. They produce varieties of glaze product ranging from Pilkington Activ Clear, Blue, and Neutral. Pilkington Activ is a hydrophilic self-cleaning thin film coating made of TiO2 which photocatalytically breakdowns organic contaminants in contact with its surface in the presence of UV radiation. The surface is smooth enough for the formation of a large sheet of rainwater which can easily wash away any loosely degraded organic and inorganic dust on the glass surface. This self-cleaning coating can also be applied to other Pilkington glass products in insulation glass units to provide more benefits which include solar control, noise reduction, fire protection, and thermal insulation. Pilkington Active Clear was the original self-cleaning glass product without any added features or benefits but with just a neutral tint. The application of self-cleaning features and solar control on glass led to other glaze products, namely, the Pilkington Activ Blue or Activ Neutral. The major difference between Blue and Neutral is just the degree of their tint which is blue and neutral color, respectively. The strength of glasses is not affected by carrying the Pilkington Activ coating, but it reduces the light and energy transmitted via the glass by 5%. For some viewing angles, greater reflectance can be observed than normal glass. The Pilkington Activ products are developed to last as long as the glass itself. The coating strongly adheres to glass without peeling, or breaking, or discoloring over time. Using silicone or silicone-based products to clean the coating can cause damage, and they may compromise its effectiveness for self-cleaning. To trigger its self-cleaning action, Pilkington Activ needs an activation time of about 5
7 days of solar irradiation. Pilkington Activ is mostly suitable for external use, too much dirt on the surface can reduce its effectiveness. Therefore there will be need for occasional washing to remove dust and reactivate its surface before it becomes fully functional again within few days [3,4].

13.3.1.2 Neat Glass produced by Cardinal Glass Industries The Neat Glass is a self-cleaning glass pane produced by Cardinal Glass Industries. This commercial glass relies on a self-cleaning coating made of TiO2 modified with SiO2. This TiO2 layer on the glass can absorb and react with UV rays and, in turn, degrade organic deposit on the glass surface. The SiO2 improves the hydrophilicity and smoothness, thereby making the surface easy to clean and dry. Neat Glass is produced through the sputtering process, which is one of the reasons behind its high visible transmittance and reduced reflectance. One common Neat Glass product is the 1v triple pane (see Fig. 13.6) low-emissivity (low-e) glass which has 70% visible transmittance and 20% reflectance. Neat Glass is majorly suitable for exterior use which includes facades, conservatories, and rooftops. They require occasional cleaning but are really easy to clean due to their good smooth and hydrophilic features [49,50].

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Figure 13.6 Schematic showing various layers and compartments in the Neat Glass triple pane low-e glass. (A) Primary seal (polyisobutylene—to prevent moisture and argon permeation), (B) secondary seal (silicone—for long-term adhesion), (C) spacer (stainless steel—to promote resistance to moisture and reduce stress on glass), and (D) desiccant (molecular sieve—for optimum moisture adsorption. Low-e, Low emissivity.

13.3.1.3 Self-cleaning glass by Fuyao Glass Industry Group Co. Ltd. UV Fuyao self-cleaning glasses are made from clear float glass coated with thin film which has antimold and antifungi capabilities in addition to the self-cleaning properties. The thin film is produced by high-temperature pyrolytic spraying process and possesses photocatalytic properties. When exposed to UV irradiation, it can degrade any organic contaminant in contact with its surface. The thin film hydrophilic properties enhance its cleaning ability by forming a large sheet of water which helps to wash away and degrade organic/loose inorganic pollutants. The selfcleaning coating by Fuyao can be combined in conjunction with other glaze qualities including noise and high-performance insulation and fire protection. Fuyao Glass Industries promised consumer a permanent photocatalytic performance even during the time with low irradiation such as nighttime and cloudy days. Their coating has no effect on the light transmittance of the float glass. Their self-cleaning coating requires rare cleaning for removal of excess degraded contaminant and improvement of photocatalytic activity [3,51].

13.3.1.4 SunClean by PPG residential Glass SunClean self-cleaning coating is a transparent thin film of TiO2, which is usually applied to glass at a very high-temperature yielding a strong and durable bond between the coating and glass. SunClean coating possesses both hydrophilic and photocatalytic surfaces, which makes it easy to clean and dry without leaving spots or streak. It can be combined with solar control glass to yield high energy
efficient glaze product in addition to its self-cleaning property. SunClean coating requires

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UV irradiation to be functional; thus it can only be used externally. The activated SunClean surface is able to degrade organic contaminants, but the coating does not easily let loose of inorganic deposits such as sand, paint, or cement runoff. Since it is for external use only, occasional cleaning, especially during the time of low precipitation, is required. The surface can form a large sheet of water for easy washing away of dirt and dust. SunClean coatings are designed to be durable as the glass, but the use of abrasive cleaning solutions can cause damages. An undamaged SunClean coating can block UV irradiation by 40% and improve the solar heat gain coefficient by 5% when compared with normal glass [3,52].

13.3.1.5 BIOCLEAN by Saint-Gobain Glass UK Ltd Bioclean is a self-cleaning coating sold with other Saint-Gobain glass products. When incorporated into most of Saint-Gobain glass products, it may yield other beneficial features such as solar control and fire protection, in addition to selfcleaning property. Bioclean is a transparent thin film of mineral material that possesses both photocatalytic and hydrophilic properties. Bioclean is usually fused with plain glass products in order to make the coating as durable as the glass itself. It requires UV radiation for activation, and it usually takes about 1
4 weeks for effective activation in order to maintain its self-cleaning activity. Bioclean is only suitable for external use. When it is fully radiated, the coating can degrade organic dirt and dust and prevent adherence of inorganic dirt on its surface. When it rains, the surface forms large sheet of water that can effectively wash away all degraded dirt. Like other self-cleaning products, Bioclean requires occasional cleaning during the dry weather. Bioclean glaze product must be inclined at least 10 degrees to horizontal for easy runoff of water. This coating is also susceptible to silicone products such as sprays, sealants, abrasives, or water-repellant cleaners. These substances have the tendency to form layer of film on it and eventually cause severe scratching, thereby reducing the hydrophilic feature of Bioclean. It is worthy to note that Bioclean integration into glass does not affect the mechanical, acoustic, and thermal integrity of the resulting low-maintenance glass products [3,53,54].

13.3.1.6 Renew by Viridian Glass Renew is a self-cleaning coating made up of thin, transparent, and pyrolytic TiO2 film which is usually fused to glass surfaces. Renew coating is very durable because of the fusion process employed to integrate it on glass surfaces. Renew can be incorporated in any of the Viridian products such as Viridian solar glass or thermal control glass (e.g., ThermoTech). Renew also utilizes UV radiation to degrade organic dirt and dust, and its hydrophilic properties ensure formation of large water sheet for effective washing of degraded dirt and loosely trapped inorganic dirt. Manual cleaning is required when there is no rain in order to keep the coating at optimal performance. Renew is required to be installed at about 10 degrees to the horizontal for better water runoff. Due to the need for UV radiation for activation, it is only suitable for external use such as facades, roofs, and conservatories [3,55].

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It is worthy to note that Renew has recently been withdrawn from the product range of Viridian Glass. The withdrawal was due to the company repositioning itself as a glass-processing business. Viridian hinted that the changes will allow them to be able to adapt easily to rapidly changing market [56].

13.4

Doped TiO2
based coatings for improved selfcleaning ability

13.4.1 Mechanism of doped-TiO2 coatings for glass To achieve a superhydrophilic state (i.e., θ , 5 degrees), exposure of TiO2 to visiblelight irradiation ( .400 nm) is necessary [57]. The kinetics of the decrease in water contact angle induced under visible-light exposure would be much faster than those induced under UV [58]. Therefore photocatalysts with strong activity under visiblelight irradiation must be developed for enhanced performance of self-cleaning coatings and for a better utilization of the solar spectrum (UV only covers 4%
6%). The main approach used to narrow the bandgap of a photocatalyst and consequently extend its light absorption ability into the visible range is metal [59,60] and/or nonmetal [61,62] doping. Other methods have also been utilized, and they include noble metal deposition [63,64], narrowing the optical bandgap by bandgap engineering [65], and modifying the photocatalyst surface by creating heterojunctions (sensitization with organic dyes) [65
67]. The enhanced activity of doped TiO2 photocatalysts has been attributed to different mechanisms including band-gap narrowing, defect formation (e.g., oxygen vacancies), and localized energy level formation [68]. In metal doping the metal cations usually lead to the reduction of Ti41 to Ti31, subsequently creating oxygen vacancies or defect states within the semiconductor matrix [69]. As a result, an electron-occupied level is formed below the conduction band, as shown in Fig. 13.7, and electrons are then excited from these newly

Figure 13.7 Mechanism of TiO2 photocatalysis. hv1: undoped; hv2: metal-doped; hv3: nonmetal-doped [70]. Source: Reproduced with permission from Elsevier.

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generated levels to the conduction band of TiO2. The energy state of the new level depends on the atomic number (Z) of the dopant used. As Z increases, the level shifts to a lower energy state [65]. Metal doping lowers the recombination of photoinduced charges by trapping electrons in these energy levels [71]. This is only true however if the dopant metals are used in small amounts [72]. Otherwise, these metals are extremely prone to function as charge recombination centers thereby reducing the efficiency of the metal-doped photocatalyst. Other drawbacks of metal doping include poor thermal stability and the possibility of photo-corrosion [73]. Among the various transition and noble metals, vanadium-doped TiO2 photocatalysts have been proven to be strongly active under visible-light irradiation (396
450 nm), whereby excited vanadium centers donate electrons to the TiO2 conduction band allowing for the photooxidation of surface-adsorbed species [74]. Silver-doped TiO2 was also prepared and tested as a self-cleaning coating [75]. Results show enhanced performance rates under both UV and solar illumination after the integration of silver nanoparticles into the TiO2 bulk. More specifically, the silver-doped TiO2 coating, deposited on a silicon substrate, exhibited a 94% dye degradation rate under UV and 91% under solar irradiation. Doping reduced the bandgap energy and consequently enhanced the photocatalytic activity and the absorption of visible light. In a study by Wang et al. [76] an iron-doped TiO2 coating was deposited on a polysulfone membrane and tested for the degradation of bisphenol A (BPA) under visible-light irradiation. The coated membrane exhibited enhanced self-cleaning ability with a BPA removal rate of 90.78% within 180 min of visible-light exposure. Improved wettability of iron-doped TiO2 was studied by Yu et al. [77], where results revealed the higher UV-induced hydrophilicity in 5 mol.% iron-doped TiO2 film (complete wetting after 4 h) in comparison to undoped TiO2 (complete wetting after 5 h). A study by Ratova et al. [78] reported that molybdenum (Mo) doping reduced the bandgap of TiO2 by 0.20 eV. Similarly, Mo was investigated as a potential dopant for lowering the bandgap energy of TiO2 and enhancing its activity under visible-light irradiation by Navabpour et al. [79]. Results showed an increase in photocatalytic activity under visible light when molybdenum (7 at.%) was introduced into TiO2 lattice. In the same study, results also showed an increase in wettability upon UV irradiation of the Mo-doped TiO2 coating. On the other hand, nonmetals, such as boron, carbon, nitrogen, or sulfur, reduce the bandgap by substituting oxygen in the TiO2 lattice, therefore creating new energy levels in the bandgap [69,80]. The position of these new energy levels is highly dependent on the atomic number of the nonmetal dopant. As such, boron, with its lower atomic number, forms levels that are high above the valence band (Fig. 13.8). Similarly, nitrogen, with its higher atomic number, forms levels that are just above the valence band. The most effective dopant used in lowering the bandgap of anatase-phase TiO2 is substitutional nitrogen [82]. Nitrogen-doped TiO2 is one of the most promising and widely-investigated visible-light (400
550 nm) active photocatalysts [57,62]. Nitrogen may be easily incorporated into the matrix of TiO2 due to its comparable atomic size with oxygen, high stability, and low ionization energy [83]. Moreover, studies have shown that doping with nitrogen prevents the thermal transformation of anatase TiO2 to the less active TiO2 phases, namely, rutile and brookite [84]. The photoinduced superhydrophilicity of the

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Figure 13.8 Mechanism of nonmetal doping of anatase TiO2. Source: Adapted form C. Di Valentin, G. Pacchioni, Trends in non-metal doping of anatase TiO2: B, C, N and F, Catal. Today, 206 (2013) 12
18 [81]. ©2011 Elsevier.

visible-light active N-doped TiO2 film was investigated by Premkumar et al. [57]. Exposing the partially hydrophobic N-doped TiO2 film to visible-light irradiation resulted in superhydrophilicity within 30 min. Under dark conditions, no change in hydrophilic conversion was observed. In a study by Irie et al. [85] the visible light
induced hydrophilicity of N-doped TiO2 films was reported to increase with increasing dopant loading levels. This correlation between enhanced wettability and increasing dopant concentration has been attributed to the increase in absorbed photons. On the other hand, the UV-induced hydrophilicity of N-doped TiO2 decreases with increasing dopant loading. As the concentration of dopant increases, the number of oxygen vacancies also increases. Both the dopant itself and the oxygen vacancies, at certain concentrations, act as charge recombination centers, consequently decreasing the hydrophilicity of the N-doped film [80,86]. The photocatalytic activity of N-doped TiO2 under visible-light was first reported in 1986 [87]. As illustrated in Fig. 13.9, the single-atom nitrogen impurities lead to the formation of localized centers within the bandgap of TiO2 a few tenths of an eV above the valence band. These centers result in a shift in the light absorption ability of TiO2 into the visible spectrum [88]. Electrons are then excited from the localized centers in the oxide bandgap to the conduction band to be scavenged by surface-adsorbed species. On the other hand, holes formed during charge separation remain at the localized centers and do not participate in the

Figure 13.9 Proposed mechanism for the processes induced by visible-light irradiation of nitrogen-doped titanium dioxide [88]. Source: Reproduced with permission from American Chemical Society.

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reductive/oxidative process. This poses a possible limitation on the photocatalytic activity of N-doped TiO2, which is primarily based on the simultaneous reduction and oxidation from electrons and holes, respectively. The mechanism proposed here holds true for N-doped TiO2 prepared via chemical methods. If a completely different technique is used for the synthesis process, different types of centers will most probably form and other mechanistic approaches must be followed [88]. It has also been reported that, in addition to the localized centers formed above the TiO2 valance band, N-doping also leads to the introduction of new centers c.1.3 eV below the conduction band [89]. However, this may be attributed to the presence of interstitial N or the poor crystallinity of the prepared N-doped TiO2 material. Different states of nitrogen, either in the TiO2 bulk or on the TiO2 surface, may be attained depending on the doping approach. During the preparation of N-doped TiO2, several nitrogen-containing products are formed, including substitutional N, NOx, and NHx. However, only substitutional N is responsible for the visible-light absorption and the molecular photoactivation of the TiO2 catalyst. All other chemical species are merely inert by-products of the synthesis technique [88]. Besides nitrogen, carbon has also been investigated as a promising nonmetal dopant for TiO2. Carbon poses some favorable photocatalytic features including strong electrical conductivity, large electron-storage capacity, ability to accept photogenerated electrons, absorption of visible-light radiation, and high adsorption of organic contaminants [90]. Similar to N-doping, carbon is doped into the TiO2 matrix by substituting oxygen atoms. This produces a new energy band just above the TiO2 valence band. Hence, the bandgap is lowered, and the absorption of light is shifted into the visible range [91]. In addition to increased visible-light absorption, C-doped TiO2 has been shown to exhibit conversion to a more hydrophilic state under both UV and visible-light illumination [92]. More specifically, the water contact angle of the 1.1 mol.% C-doped TiO2 thin film decreased from a value of 20 degrees before visible-light irradiation to a value of 13 degrees after irradiation. On the other hand, the water contact angle of the thin film with undoped TiO2 did not change. The hydrophilicity of the thin film was also investigated under UV light irradiation. The water contact angle of the 1.1 mol.% C-doped TiO2 thin film decreased from a value of 20 degrees before UV irradiation to less than 5 degrees after irradiation. The water contact angle of the thin film with undoped TiO2 also decreased to less than 5 degrees but with a higher hydrophilic conversion rate. Metal/nonmetal codoping (Fig. 13.10) is one strategy that might be utilized to enhance the self-cleaning ability of TiO2-based coatings. Herein, the synergetic effect of metal and nonmetal dopants significantly narrows the bandgap of TiO2 in addition to considerably lowering the recombination of photogenerated charges. The metal and nonmetal codopants form additional energy levels within the semiconductor bandgap. Under visible-light irradiation the photogenerated electrons will flow from the valence band of TiO2 to the metal-centered energy level as well as from the nonmetal energy level to the metal energy level and the conduction band of TiO2. As a result, the overall absorption of visible light increases [68]. Codoped V
N
TiO2 has been shown to exhibit a narrower bandgap of 2.30 eV when compared to undoped (3.13 eV) or singly doped TiO2 (V: 2.42 eV;

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Titanium Dioxide (TiO2) and Its Applications

Figure 13.10 Mechanism of metal/nonmetal doping of TiO2 for the visible-light photodegradation of pollutants.

N: 2.95 eV) [93]. As a result, the photocatalytic activity of the codoped TiO2 under visible-light illumination was reported to be considerably better. The integration of V and N into the TiO2 matrix leads to the generation of new energy levels near the conduction and valence bands, respectively. Therefore the combined effect of the metal and nonmetal doping significantly reduces the bandgap of TiO2. If the dopants are added at low concentrations, the newly formed energy levels may function as electron or hole traps improving the separation of photogenerated charges. The photocatalytic activity of V
N
TiO2 is thus enhanced due to both the narrower bandgap and the reduced charge recombination. In a study performed by Yan et al. [94], TiO2 was codoped with Ta and N. As a result, the TiO2 bandgap was reduced to 2.71 eV shifting the absorption of light into the visible range with an absorption maximum at c.460 nm. Codoping with Fe and N was also explored, and the resulting TiO2 photocatalyst revealed enhanced photocatalytic activity and higher hydrophilicity (complete wetting after 1 h) under visible light compared to single-doped and undoped TiO2 [58]. More importantly, the contact angle of the Fe
N
TiO2 coating only slightly increased after 1 h rest in the dark. A novel superhydrophilic coating of codoped Cu
F
TiO2 was immobilized on a glass substrate by Leyland et al. [95]. Fluorine was chosen as the main dopant of interest due to its ability to impart visible-light activity, thermal stability, and strong charge separation [96]. The addition of copper as a codopant resulted in a higher level of self-cleaning activity under visible light, which is attributed to the generation of new energy levels within the TiO2 bandgap. Surface wetting and contact angle measurements were also performed on the codoped Cu
F
TiO2 coating. Results show a reduction in contact angle and a strongly hydrophilic surface.

13.4.2 Synthesis strategies Various techniques have been used for the preparation of doped TiO2-based coatings. In general, these techniques may be categorized into either wet (brushing, dip coating, spray coating, spin coating, etc.) or dry (ion implantation, reactive

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magnetron sputtering, evaporation, etc.) deposition methods. The optical characteristics, the photoelectrochemical properties, and the surface wettability of the final catalyst are all highly dependent on the choice of preparation process and parameters. To enhance the wettability of doped-TiO2 coatings, the substrate, on which the coating is deposited, must be uniformly clean and contaminant-free.

13.4.2.1 Wet-deposition methods In wet-deposition methods a doped-TiO2 solution must first be prepared through chemical techniques, such as sol
gel or hydrothermal, before the deposition process. The sol
gel method is one of the simplest and most versatile modification strategies used for the incorporation of dopants into TiO2. Typically, a metal/ nonmetal precursor dissolved in alcohol is mixed with a TiO2 precursor dissolved in acid. After allowing for condensation and hydrolysis at room or elevated temperatures, the solution is dried (80 C
110 C), grounded, and calcined (200 C
600 C) to obtain the final catalyst. Depending on the choice of hydrolysis rate, solution pH, and solvent system, the doping level and the particle size may be controlled. Appropriate calcination conditions must be followed in order to remove surface organic residues and still maintain high dopant levels [65]. Titanium isopropoxide, tetrabutyl orthotitanate, and titanium tetrachloride are examples of TiO2 precursors commonly used during sol
gel synthesis [71]. The choice of metal/nonmetal precursor must be carefully considered as it can affect the photocatalytic activity and hydrophilicity of doped-TiO2. For instance, Bergamonti et al. [66] used the sol
gel method to prepare two N-doped TiO 2 coatings with different nitrogen sources. Results revealed higher activity and increased visible-light absorption when an acidic N-source was used as the dopant compared to when urea was used. In addition to that the coating doped with the acidic N-source exhibited a decrease in contact angle during light irradiation, while the one doped with the urea precursor showed no change in wettability. The enhanced performance of N-doped TiO2 prepared with an acidic N-source may be attributed to the formation of substitutional nitrogen in contrast to the interstitial nitrogen formed when urea was used as the nitrogen precursor. The hydrothermal method may also be used for the doping of metals/nonmetals. With this procedure a solution of TiO2 precursor and metal/nonmetal precursor dissolved in alcohol is heated at high temperatures (200 C
300 C) and in the presence of water [97]. After a certain period of time the powder is rinsed, dried, and calcined. Posttreatment calcination can greatly increase the photocatalytic activity by reducing the number of surface defects acting as centers for the recombination of electrons and holes [98]. After the preparation of the doped-TiO2 solution, the coating may be immobilized on a substrate via wet-deposition techniques, which include brushing, dip coating, spray coating, and spin coating. The most commonly used method among all the aforementioned techniques is spin coating. This is mainly attributed to the attractive features of spin coating which include its simple and low-cost implementation, its compatibility with other synthesis process, and its fabrication of a film

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Titanium Dioxide (TiO2) and Its Applications

with excellent thickness uniformity [99]. Although the other techniques are also considered simple and cheap, they are less commonly utilized due to their detrimental limitations. For instance, the brushing method is not yet feasible for large-scale application [66]; spray coating is typically only favored in applications that do not require extreme mechanical wear and abrasion [79]; and dip coating generally results in inhomogeneous and unreproducible coatings with clogged pores [100].

13.4.2.2 Dry-deposition methods In ion implantation, both metals and nonmetals are integrated into the bulk by bombarding TiO2 with high energy (150
200 keV) ions [101]. Although considered a highly reliable and a greatly effective doping technique, this approach is limited by the implantation fluence (1017
1018 ions/cm2), the implantation depth (a few µm), and the inhomogeneity of the dopant distribution [65,102]. Furthermore, ion bombardment leads to the production of defects in the TiO2 bulk and consequently lowers the photoconversion efficiency. However, this may be resolved by annealing out the defects through heat treatment [65]. According to Ghicov et al. [102], the photocurrent in the visible and UV range was enhanced by a factor of 8 after thermal annealing. Vanadium, chromium, nickel, manganese, and iron are all examples of transition metals that have been successfully doped into TiO2 via the ion implantation method [71]. Ion implantation was also used to dope TiO2 with a nonmetal (nitrogen) [102]. The resulting material exhibited an improved photocatalytic activity under both UV and visible irradiation. Reactive magnetron sputtering is an industrial process that may be utilized for large-area deposition. Moreover, high-quality doped-TiO2 films may be prepared even at low substrate temperatures [103]. Asahi et al. [82] obtained highly visiblelight active N-doped TiO2 by sputtering in an ambient atmosphere of nitrogen (40%) and argon. It was reported that doping with nitrogen reduced the bandgap of TiO2. Conversely, Chen et al. [104] also used reactive magnetron sputtering to prepare N-doped TiO2 films immobilized on aluminum sheets. However, for their reactive gas mixture, nitrogen and oxygen were used. The crystallinity, morphological properties, and photoactivity of the synthesized films were greatly affected by the nitrogen/oxygen ratio. More importantly, these films were photocatalytically active only under UV light.

13.5

Future tendencies: multilayer coatings for multifunctional glass

13.5.1 Multilayer structures for improved self-cleaning and antireflective ability The combination of TiO2 with other materials to form a layered structure is a promising strategy toward the improvement of the performance of self-cleaning coatings. In this area, stratified configurations combining TiO2 and SiO2 have received great

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attention for three main reasons: (1) improvement of the mechanical and thermal stability of the coating [105]; (2) formation of an efficient heterojunction at the interface between the two oxides, improving the charge separation and lightharvesting [106]; (3) the difference in refractive index (n) between TiO2 and SiO2 (n of TiO2B2.6 and n of SiO2B1.46 at 588 nm) boosts the antireflective properties of the coating, resulting in interference-type layered assemblies showing near-zero reflectance in a wide range of visible wavelengths [106,107]. One of the most significant studies in this area reports the deposition of sputtered five-layer TiO2/SiO2 coatings. Fig. 13.11A shows the photocatalytic performance of this multilayer, degrading almost 30% of phenol after 12 h of UV
vis. The decomposition of phenol was also confirmed by the decrease in the absorbance as shown in the inset of the same figure. Moreover, the functionalized glass showed hydrophobic properties (WCAB90 degrees) imparted by the amorphous TiO2 top layer [108]. The presence of mixed SiO2
TiO2 in the layers can also enhance the photoactivity of these systems as reported by Kim et al. [106]. Electrons generated in TiO2 can be easily trapped by mid-gap states in the SiO2
TiO2 layer, preventing e2/h1 recombination (Fig. 13.11B). Moreover, the deposition parameters can be tuned to impart a different porosity and, thus, different refractive index to the individual layers. In this area, Tao et al. [107] fabricated a self-cleaning and antireflective double-layer TiO2
SiO2 coating by surface sol
gel process. The inclusion of TiO2 into the SiO2 nanopores of the top-layer resulted in a decrease in the refractive index from 1.45 to 1.19, boosting the broadband antireflective properties of the coating (transmittance of c.97.7% in the wavelength range 400
1200 nm).

Figure 13.11 (A) Degradation of phenol by TiO2
SiO2 coating (named ARC, i.e., antireflective, in the figure) under UV
Vis irradiation. Absorbance spectra of the target pollutant are showed in the inset [108]. (B) The energy-level diagram of TiO2/SiO2 multilayer showing the Fermi level of TiO2 and trap states in the bandgap [106]. Source: (A) Reproduced with permission from Elsevier. (B) Reproduced with permission from Royal Society of Chemistry.

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Titanium Dioxide (TiO2) and Its Applications

The functionalized glass was quite hydrophilic (WCAB14 degrees in the dark) and degraded 85.1% of the target pollutant, that is, methylene blue, after 30 h of UV irradiation. Visible-light active semiconductors such as WO3, ZnIn2S4, Bi2MoO6, Bi2WO6 can be layered with TiO2 to compensate for the large bandgap of the latter [6,109,110]. In this field, Tian et al. [6] prepared double-layer Bi2MoO6/TiO2 coating able to degrade 85% of the target compound, that is, alizarin red, under visible irradiation. The favorable band alignment between the two semiconductors, which allowed the electron injection from the conduction band of Bi2MoO6 to that of TiO2, was the main reason behind the improved charge separation. The coating was also superhydrophilic in the dark state due to hierarchical flake-like morphology, characterized by a large porosity and roughness. The doping approach and multilayer strategy can be combined to further boost the visible-light activity, as demonstrated by Garlisi et al. [111] who prepared double-layer thin films consisting of N-doped and Cu-doped TiO2. This study aimed to optimize the layer arrangement in the coating in order to maximize the photocatalytic activity toward the degradation of a model volatile organic compound, that is, 2-propanol. It was found that the electron-mediated pathway provided a more decisive contribution to the degradation of 2-propanol compared to the hole-mediated pathway and that the bilayer with Cu-doping in the top layer (N
Cu
TiO2) exhibited a much higher reactivity compared to the inverted configuration (Cu
N
TiO2, Cu-doping in the bottom layer). The charge separation mechanism for the two bilayers is showed in Fig. 13.12. In the N
Cu
TiO2 sample the photogenerated electrons are trapped by the Cu1 dopants in the top layer and easily conveyed to the top surface of the stack. On the other hand, the electronmediated pathway is suppressed in Cu
N
TiO2 because electrons are conveyed through the Cu
TiO2 layer to the bottom surface of the stack in contact with the

Figure 13.12 Charge generation and separation mechanism for the bilayers N
Cu
TiO2 and Cu
N
TiO2 [111]. Source: Reproduced with permission from Elsevier.

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glass. The favorable stacking order of the doped layers in N
Cu
TiO2 thereby allowed more electrons to be scavenged by oxygen at the surface, boosting the formation of radical species, which was inhibited in the inverse configuration (i.e., Cu
N
TiO2).

13.5.2 Self-cleaning and energy-saving multilayer structures The development of energy-efficient glass for building applications has been a hot topic of research over the last years [112,113]. Metals such as Cu, Au, and Ag are widely used as energy-efficient materials for this purpose because of their ability to act as IR reflectors, shielding off the near-IR (NIR) portion of the solar radiation responsible for heating the building’s interior [114
116]. On the other hand, thermochromic materials such as vanadium oxide (VO2) are elective materials for next-generation smart windows. At a temperature of c.68 C (phase transition temperature, Tph), VO2 is able to change from monoclinic to tetragonal phase, switching from a transparent to a metallic state, in which is able to block the NIR radiation [117
119]. Some of the main challenges faced with the development of increasingly efficient smart windows are related to the poor transparency in the visible region of the traditional energy-efficient layers including metals and oxides mentioned earlier [120,121]. The visible-light transparency is usually improved by sandwiching the energy-efficient material between suitable dielectric layers. TiO2 is an elective dielectric material due to its high visible-light transmittance, high refractive index, mechanical stability [115,122]. Moreover, TiO2 is able to impart self-cleaning to the functionalized glass, thus making the glass multifunctional (i.e., energy efficient, self-cleaning, and antireflective at the same time). In a recent work, Dalapati et al. [115] prepared TiO2/Cu/TiO2 multilayer, where the typical brownish color of Cu was attenuated by TiO2 layers, significantly improving the aesthetic properties of the functionalized glass. The luminous transmittance of the coating was adjusted by varying the thickness of the TiO2 layers and the annealing temperature. In particular, the transmittance reached a maximum of 85% after thermal annealing at 500 C (Fig. 13.13A). A further increase in the annealing temperature was detrimental to the optical properties due to the diffusion of oxygen into the Cu layer, leading to the partial oxidation of Cu. As shown in Fig. 13.13B, the highest transparency of the coated glass annealed at 500 C was obtained by introducing a 20-nm Cu layer between two 50 nm TiO2 layers, making such a multilayer configuration an excellent candidate for application in smart glass. A novel TiO2/Si/Ag(Cr)/TiNx-layered structure has been deposited based on optical design and calculations [123]. TiN and Si layers served as protective layers to prevent the oxidation of the metallic core [i.e., Ag(Cr)] during the thermal treatment at 673K. The annealed glass reached a maximum of 85% at 550 nm due to the antireflective action of the TiO2 layer. The coated glass was mildly hydrophilic in the dark states but became superhydrophilic (θB5 degrees) after UV irradiation. As shown in the schematic in Fig. 13.13C, the multifunctional coatings allowed to transmit most of the visible irradiation, while reflecting most of the IR radiation.

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Titanium Dioxide (TiO2) and Its Applications

Figure 13.13 (A) Transmittance spectra of glass coated by TiO2/Cu/TiO2 treated at various temperatures. The inset displays the transmittance in the range 400
100 nm. (B) Photographs of the samples treated at various temperatures for different thicknesses: 20/20/20 nm and 50/20/50 nm) [115]. (C) Schematic of the working principle of the TiO2/Si/ Ag(Cr)/TiNx multilayer combining hydrophilic and energy efficiency properties [123]. Source: Reproduced with permission from (B) Nature Publishing Group and (C) Elsevier.

Moreover, it exhibited a self-cleaning ability under UV irradiation, supported by the efficient charge separation at the interface TiO2/Si. Various studies report a marked improvement of the VO2 transmittance when is introduced in a layered structure with TiO2. It has also been reported that use TiO2 helps improve the solar modulation ability of VO2, which is the difference in solar transmittance between the transparent state and metallic state [124]. Both doublelayer and triple-layer configurations lead to a significant improvement of the optical properties as demonstrated by Jin et al. [125], who obtained an increase in the luminous transmittance from 30.9% in single VO2 to 49.1% in two-layer ViO2/TiO2 and 57.6% in three-layer TiO2/VO2/TiO2. The same multilayer configurations have

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been also investigated by Zheng et al. [126], who did not obtain a significant improvement in the visible transmittance when switching from two- to three-layer configuration as shown in Fig. 13.14A. However, the TiO2/VO2/TiO2 exhibited an increase in the solar modulation ability (B2%) compared to VO2/TiO2. The three-layer system also exhibited a significant reduction in the Tph, which was 61.5 C. Self-cleaning properties of this multilayer structure were also assessed. Under UV irradiation the coated glass decomposed stearic acid and rhodamine B (RhB). The three-layer sample showed the highest photocatalytic activity toward both target molecules. Stearic acid was almost fully decomposed after 180 min of UV irradiation, while RhB was degraded by B60% after 120 min of illumination (Fig. 13.14B).

Figure 13.14 (A) Transmittance spectra of VO2/TiO2-based multilayers at 25 C and 90 C. (B) Photocatalytic activity toward RhB for the same samples [126]. (C) Photocatalytic decomposition of stearic acid on VO2/SiO2/TiO2 coatings, Pilkington Activ, and plain glass. (D) Quantum efficiency calculated for the same samples [23]. RhB, Rhodamine B. Source: (B) Reproduced with permission from American Chemical Society.

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Titanium Dioxide (TiO2) and Its Applications

The incorporation of SiO2 in VO2/TiO2 coating has been also studied with the aim of further improving the modulation of the incident radiation and visible-light transmittance of VO2-coated glass [127]. In this regard, Powell et al. prepared VO2/SiO2/TiO2 by chemical vapor deposition (CVD), studying the effect of different deposition times for SiO2 and VO2 layers on the energy-saving and selfcleaning performance. The deposition of the SiO2/TiO2 overlayer allowed to double the solar modulation ability and increase the visible transmittance by B30% compared to single-layer VO2 configuration. Regarding the self-cleaning ability, the VST-3 sample (higher deposition time, i.e., 3 min, for the VO2 and SiO2 layers), was more reactive than VST-1 sample (lower deposition time, i.e., 1 min, for the VO2 and SiO2 layers) and Pilkington Activ. The degradation of the stearic acid, which followed a zero-order kinetics, was studied by IR spectroscopy. Fig. 13.14C shows the reaction trend obtained by monitoring the area of the IR-active bands corresponding to C
H stretching modes. The low reactivity of VST-1 was attributed to the presence of a pure rutile phase. On the other hand, longer deposition times induced the formation of both rutile and anatase in VST-3. The quantum efficiency (ξ) was also computed for the different samples, and it was found that VST3 and the commercial glass showed the highest ξ, as displayed by the histogram in Fig. 13.14D.

13.6

Conclusion

In this chapter, fundamental principles of self-cleaning behavior along with the commercial applications of self-cleaning glass and strategies to improve their current performance have been presented. The combination of TiO2 with other materials to form multilayer structures has also been discussed as viable route to develop increasing efficient smart windows. The effectiveness of the latter approach mainly lies in the possibility of adding further properties to the selfcleaning products (i.e., energy-efficiency and antireflective ability) through a stratified structure that offers obvious advantages over the conventional singlelayer configuration. Doping of TiO2 is by far the most effective approach to extend the absorption of the self-cleaning coatings in the visible region and improve their hydrophilic properties in the dark. To achieve this the dopant amount to be introduced in the semiconductor should be carefully assessed: too low concentrations may not change the final performance compared to the undoped semiconductor, while too high concentrations may result in recombination centers for the photogenerated holes and electrons, negatively impacting on the kinetics of the degradation process and leading to an adverse photocatalytic performance. It should be also stressed that doping may not lead to any significant improvement in the photocatalytic performance despite the bandgap reduction and higher visible-light absorption. A severe alteration of the band structure of TiO2 may in fact result in a marked shift of the band edges with respect to the potential of both oxidation and

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reduction, which may bring about thermodynamic limitations. Another crucial point concerns the mechanism the improvement of the hydrophilic properties. The introduction of impurities usually alters the physical
chemical properties at surface level through the formation of defects, such as oxygen vacancies, which may create favorable conditions for water adsorption and spreading wetting. However, the same defects may also act as recombination centers for holes and electrons to the detriment of the photocatalytic performance. In this regard, it is evident that the two characteristic processes of TiO2 activated by the light, namely, the photocatalytic effect and the photoinduced wettability, occur both on the TiO2 surface under irradiation, and, depending on the surface properties, one can take place preferentially at the expense of the other. All these issues should be addressed in more detail in future studies in order to improve the design strategies of self-cleaning glasses.

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1376.

TiO2 as a source of titanium

14

Xingli Zou1, Zhongya Pang1, Li Ji2 and Xionggang Lu1 1 State Key Laboratory of Advanced Special Steel, School of Materials Science and Engineering, Shanghai University, Shanghai, P.R. China, 2 State Key Laboratory of ASIC and System, School of Microelectronics, Fudan University, Shanghai, P.R. China

14.1

TiO2 production from titanium minerals

Titanium metal is commonly produced from titanium minerals through TiO2 and TiCl4. Therefore titanium minerals will be introduced briefly, and then the process from titanium minerals to TiO2 will be described in detail. The reason why it is hard to convert TiO2 to Ti will be also discussed.

14.1.1 Production of titanium-rich slag from titanium minerals Ti is the fourth richer metallic element in the Earth’s crust, with the content of 0.6%, which is just next to Fe, Al, and Mg [1]. Ti exists widely in rocks, sand, seawater, plants, and many animals. Ti is not a rare metal in terms of reserves, but an abundant metallic element in the world. However, almost no pure metal Ti appears in nature due to its high chemical activity and easy combination with oxygen [2]. The predominant forms of Ti are mainly TiO2 and titanates, which are often associated with iron to form different minerals. At present, more than 140 titanium minerals with TiO2 content greater than 1% have been detected; however, only a few minerals can be directly utilized to extract Ti through the current technology [3]. Several typical titanium minerals are listed in Table 14.1. Ilmenite is the main source of titanium products, which accounts for B70% of the total global titanium minerals. The actual mass fraction of TiO2 in ilmenite varies from 42% to 64%. The composition of ilmenite changes greatly around the world due to the different metallogenic causes, metallogenic times, and weathering conditions [1,4]. Titanium concentrates are commonly obtained after the preliminary mineral processing, which can be directly used to produce TiO2 (pigment) through sulfuric acid process. However, this TiO2 production process has large acid consumption and commonly needs long time; therefore it was gradually replaced by chlorination process [1]. From the industrial point of view, regardless of the method used to prepare TiO2 or extract Ti, it is necessary first to achieve high-grade titanium-rich materials (hightitanium slag) after mining, that is, to do the enrichment of TiO2. Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00014-6 © 2021 Elsevier Inc. All rights reserved.

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Titanium Dioxide (TiO2) and Its Applications

Table 14.1 Properties of typical titanium minerals [3]. Mineral

Chemical formula

Theoretical content of TiO2 (%)

Density (g/cm)

Mohs hardness

Color

Rutile Anatase Brookite Arizonite Perovskite Ilmenite Leucoxene

TiO2 TiO2 TiO2 Fe2O3∙3TiO2 CaTiO3 FeTiO3 TiO2∙nH2O

100 100 100 60.01 58.75 52.64 /

4.2
4.3 3.9 4.1 4.25 4.1 4.5
5 3.5
4.5

6
6.5 5.5
6 5.5
6 / 5.5 5
6 4
5.5

Reddish/brown Brown Yellow/brown Russet Puce Black Greyish/brown

Many titanium-rich processes have been proposed and explored, which can be divided into two kinds of strategies: pyrometallurgical treatment and hydrometallurgical methods. Pyrometallurgical treatment method mainly includes electric furnace smelting process, plasma smelting process, selective chlorination, and other thermal reduction processes. Concerning the second method, acid leaching process, reduction rust (Becher method) process, ferric chloride leaching, and other chemical separation processes are collectively called hydrometallurgical method. Currently, the processes widely used in the industry mainly refer to electric furnace smelting process, reduction rust, and acid leaching processes. The titanium-rich products obtained by using the electric furnace smelting process are called titanium slag, while those obtained by other processes are called synthetic rutile. Electric furnace melting process is a very well developed and efficient manufacturing method, the flowchart of this process is shown in Fig. 14.1. The essence of this method is that ilmenite concentrate is mixed with the solid reducing agent of bitumen or petroleum coke in the electric furnace for reduction smelting. The iron oxide in ilmenite would be selectively reduced to metal iron, while TiO2 would be enriched in the slag. Titanium slag and by-product metal iron would be obtained through slag
iron separation. Electric furnace smelting process without the solid and liquid waste is relatively simple; the by-product metal iron can be directly used. Electric furnace gas can be recycled, increasing the process efficiency. The smelting process is mainly used to separate and remove iron, but it does not work well for noniron impurities. Titanium slag with TiO2 content above 90% is generally called titanium-rich slag or high-titanium slag, which is usually used as feedstock to produce titanium sponge and pure TiO2. By controlling the reduction degree of smelting process, acid dissoluble titanium slag can be obtained, which is usually used as raw material for preparing TiO2 pigment by sulfuric acid method. For reducing the rust process, coal is used as reducing agent and fuel to reduce the iron oxide at high temperature to form metal iron from ilmenite. Subsequently, the metal iron can be “rusted” into hydrated iron oxide (red mud) through a simple immersion process in an aqueous solution with a small amount of hydrochloric acid or ammonium chloride. The separation of TiO2 can be achieved by a cyclone separator. A pickling process can be performed for further purification of the enriched

TiO2 as a source of titanium

431

Petroleum coke

Bitumen

Ilmenite concentrate

Pulverizing

Pulverizing Dosing

Smelting

Furnace slag

Cast iron

Exhaust gas

Pulverizing

Magnetic separation

Iron slag

Pulverizing

Titanium slag

Figure 14.1 Production process from ilmenite to titanium slag.

TiO2. Acid leaching process has various treating parameters, but the basic principle is the same. Impurities such as iron, calcium, and magnesium in the ilmenite can be selectively leached out in dilute hydrochloric acid. Thus TiO2 can be enriched smoothly. Of course, many other processes have also been used to achieve TiO2 enrichment from ilmenite. In general, whether it is titanium slag or synthetic rutile, the purpose of these processes is to realize the enrichment of TiO2 for the subsequent efficient utilization.

14.1.2 Production of TiO2 from titanium-rich slag The TiO2-rich products (high-titanium slag) are commonly used to produce TiCl4, which can be further utilized to produce pure TiO2 (or TiO2 pigment) and metallic Ti. Currently, magnesium thermal reduction process (the Kroll process) is the main method for producing metallic Ti, which will be described in detail in the next section. Sulfuric acid method, as a traditional method for preparing TiO2, has a complex production route and high energy consumption. Therefore more attention has been paid to the chlorination process, and its detailed formation process is shown in Fig. 14.2. The TiO2-rich products are first chlorinated by chlorine gas to form crude TiCl4, which will be further purified to refined TiCl4. Subsequently, TiCl4 can be oxidized by oxygen gas to form TiO2 and Cl2 at high temperature, and Cl2 can be reinjected back into the chlorination process recycling it. The TiO2 pigment can

432

Titanium Dioxide (TiO2) and Its Applications

Titanium-rich products Cl2

Chlorination

Crude TiCl4

Wastes

Distillation TiCl4 Oxidation

O2

Cl2/TiO2 Gas-solid Separation TiO2 Modification TiO2 pigment

Figure 14.2 Titanium dioxide pigment formation process by chlorination method.

be produced after subsequent modification treatment. Summarizing, the production of TiO2 by chlorination process demonstrates the merit of less waste, but it requires high-grade and rich TiO2 stuffs as raw materials [4,5]. As discussed earlier, a complex multistep process is commonly needed to achieve the enrichment and purification of TiO2. In fact, titanium has a strong affinity for oxygen, it is difficult to directly convert TiO2 into metallic Ti. The thermochemical stability of oxides and chlorides can be presented by Ellingham diagrams, as shown in Fig. 14.3 [6]. In principle, TiO2 can be reduced by Mg, Ca, Al, Y, etc., with Mg and Ca as the most representative candidates, due to their strong reductive ability [7]. However, the removal of oxygen from TiO2 (or Ti suboxides) is still extremely challenging. Fig. 14.3C shows that there is a limit to reduce the oxygen content in Ti
O solutions. More specifically, it is difficult for Mg to reduce oxygen from TiO2 and Ti
O solutions to less than 0.5 wt.%, and Ca demonstrates a limit of 0.02 wt.% at 700 C [8,9]. Besides, the possible formation of undesirable alloys during the metallothermic reduction process is another factor that should be considered [6]. On the contrary, concerning chlorides, TiCl4 can be reduced easily by Na, Mg, Ca, K, Li, Al, etc., as demonstrated in Fig. 14.3B. The oxygen, including other impurities, can be preliminarily removed through the purification process of TiCl4, thereby achieving the effective reduction of TiCl4. Therefore metallic Ti is usually produced through an indirect process, that is, reduction of TiCl4.

TiO2 as a source of titanium

433

Figure 14.3 Ellingham diagrams for the formation of different oxides (A) and chlorides (B) [7]. (C) Ellingham diagrams for the comparison of oxygen potential in MgO, CaO, TiO2, TiO, and Ti
O solid solutions [6,8,9]. Source: Reproduced with permission from Y. Zhang, Z.Z. Fang, P. Sun, S. Zheng, Y. Xia, M. Free, JOM 69 (2017) 1861
1868, Springer Nature.

14.2

The Kroll process from TiO2 to Ti

The Kroll process from titanium minerals rutile (TiO2) and ilmenite to titanium metal will be introduced in detail in this section. Generally, titanium minerals or TiO2 is first carbon-chlorinated to form TiCl4, which would be then reduced to titanium metal by magnesium. The by-product MgCl2 would be electrolyzed to form Mg and Cl2 again. This multistep process is the main industrial process used to produce titanium from titanium dioxide and minerals. In 1940, titanium metal was successfully prepared via magnesium thermal reduction of TiCl4 presented by a Luxembourg scientist Kroll [10]. This method has become an industrial method for the production of titanium sponge up to now. The flowchart of this process is shown in Fig. 14.4. During the chlorination process the targeted valuable components in feedstock are titanium oxides, and their direct chlorination reactions are shown in reactions (14.1)
(14.8) [3]. Obviously, direct chlorination of TiO2 cannot occur spontaneously within the temperature range of 800 C
1300 C, as shown in reaction (14.1), where ΔGΘT value is positive. In contrast, low-valent titanium oxides (Ti3O5, Ti2O3, TiO) can react with chlorine gas directly (reactions 14.2
14.7). However, TiCl4 and TiO2 are more likely to be obtained through reactions (14.5)
(14.7) without catalyst due to their more negative ΔGΘT, and the resulted intermediate product TiO2 cannot continue to react with Cl2. Therefore the chlorination efficiency of low-valent titanium oxides is relatively low, and the direct chlorination rates of TiO, Ti2O3, and Ti3O5 are 50%, 25%, and 16.7%, respectively [3]. The reason is the high oxygen partial pressure, which usually causes the difficulty of direct chlorination of TiO2. TiO2 1 2Cl2 5 TiCl4 1 O2 ΔGΘ T 5 183; 800
59:4T 0 1 0 1 1 @ ATi3 O5 12Cl2 5TiCl4 1 @5AO2 3 6 ΔGΘ T

5 58; 615
28:85T

(14.1)

(14.2)

434

Titanium Dioxide (TiO2) and Its Applications

TiO2-rich products Supplementary Cl2 Chlorination

Petroleum coke

Cl2

Crude TiCl4 Distillation TiCl4 Supplementary Mg

Magnesiothermic reduction

MgCl2 Mg

Mg/MgCl2

Electrolysis Refining

Vacuum distillation

Titanium sponge

Figure 14.4 Preparation of titanium sponge through the Kroll process.

0 1 0 1 1 @ ATi2 O3 12Cl2 5TiCl4 1 @3AO2 2 4

(14.3)

ΔGΘ T 5
628
17:63T 0 1 1 TiO12Cl2 5TiCl4 1 @ AO2 2

(14.4)

ΔGΘ T 5
241; 578
26:25T 2Ti3 O5 12Cl2 5TiCl4 15TiO2 ΔGΘ T 5
566; 474 1 127:5T

(14.5)

2Ti2 O3 12Cl2 5TiCl4 13TiO2 ΔGΘ T 5
553; 914 1 109:8T

(14.6)

2TiO12Cl2 5TiCl4 1TiO2 ΔGΘ T 5
625; 090 1 112T

(14.7)

For this reason, carbon (commonly petroleum coke), as a reducing agent, was introduced to the chlorination process to promote the chlorination continuation (reactions 14.8
14.10) [3]. However, at the chlorinated temperature (,1100 C), carbon can hardly be used as a reducing agent directly, but its oxide CO (Boudouard

TiO2 as a source of titanium

435

reaction: C 1 CO2 5 2CO) works. The resulting chlorinated product is crude TiCl4. Due to the difference of the raw TiO2-rich materials (titanium slag, rutile, etc.), the impurities in crude TiCl4 are also different. These impurities mainly include SiCl4, FeCl3, VOCl3, AlCl3, TiOCl2, and some organic impurities. The performance of the TiCl4 and subsequent products may be harmed by these impurities. Therefore the purification of crude TiCl4 is necessary, which often involves complex distillation, vanadium removal, and other additional steps. Subsequently, the refined TiCl4 for magnesium thermal reduction process can be obtained. TiO2 1 2Cl2 1 C 5 TiCl4 1 CO2 ΔGΘ T 5
209; 970
62:1T

(14.8)

TiO2 12Cl2 12C5TiCl4 1 2CO ΔGΘ T 5
37; 422
239T

(14.9)

TiO2 12Cl2 12CO5TiCl4 12CO2 ΔGΘ T 5
382; 531
114:8T

(14.10)

A schematic illustration of the reaction vessel of the reduction process is shown in Fig. 14.5 [11]. The essence of magnesium reduction of TiCl4 is that the metal Mg reacts with TiCl4 to form MgCl2 and sponge-like solid Ti (Ti sponge). Subsequently, MgCl2, excess Mg, and Ti sponge can be separated by vacuum distillation. The recycling of Mg and Cl2 can also be achieved through the electrolysis of MgCl2. Currently, the processes for magnesium reduction and distillation are commonly combined to improve efficiency and reduce costs.

Figure 14.5 Scheme of the reaction vessel of titanium reduction process.

436

Titanium Dioxide (TiO2) and Its Applications

For the magnesium reduction process, the involved reactions are quite complex. Elemental titanium is a typical transition metal with many valences. Therefore stable intermediates and transition compounds of TiCl3 and TiCl2 are generated. The whole reduction process is related to the multiphases Mg
Ti
MgCl2
TiCl4
TiCl3
TiCl2 system, and the reactions would change with the variation of reaction conditions. In the case of excess TiCl4, the reactant is conducive to the formation of low-valent titanium chloride, as shown in reactions (14.12)
(14.14) [3], and the final product would be inevitably mixed with undesirable low-valent titanium chlorides. In the case of enough Mg, low-valent titanium chloride can be further reduced to metallic Ti, as shown in reactions (14.15) and (14.16) [3]. Therefore excess Mg should be guaranteed to achieve the complete reduction and further reduce the low-valent titanium chloride in the product. 0 1 0 1 @1ATiCl4 1 Mg 5 @1ATi1MgCl2 2 2

(14.11)

ΔGΘ T 5
231; 100 1 68T 2TiCl4 1Mg52TiCl3 1MgCl2

(14.12)

TiCl4 1Mg5TiCl2 1MgCl2 ΔGΘ T 5
364; 000 1 148T

(14.13)

2TiCl3 1Mg52TiCl2 1MgCl2

(14.14)

    2 2 TiCl3 1 Mg 5 Ti1MgCl2 3 3

(14.15)

TiCl2 1Mg5Ti1MgCl2 ΔGΘ T 5
98; 200
11T

(14.16)

In actual practice, magnesium thermal reduction of TiCl4 can be basically divided into three stages, that is, the initial stage, the titanium sponge formation stage, and the later stage. In the initial stage, the new supplementary TiCl4 vapor reacts with exposed liquid Mg violently, accompanied by the occurrence of gasphase reaction. However, the interaction region between Mg and TiCl4 is only limited to the surface of Mg, and the reaction demonstrates higher apparent activation energy and lower reaction speed at the lower temperature on the surface of Mg. Some impurities in Mg would be adsorbed by the rapidly generated Ti particles. Besides, a small amount of Ti particles may adhere to the reactor wall, while most of them would sink to the bottom together with MgCl2. Thus the primary titanium with high content of impurity can function as a purifying agent for magnesium. The metal Mg would be covered by the generated MgCl2 due to the surface tension, which would relax the surface tension of Mg and improve the infiltration condition

TiO2 as a source of titanium

437

of Mg. In the initial stage, the reduction rate is relatively slow and Mg consumption is about 5% [3]. In the reduction process, the liquid Mg surface is still covered by the generated MgCl2, which has been destroyed constantly due to the robust chemical reactions. The result is the reexposure of fresh Mg. The reactor pressure gradually increases as the increase of the MgCl2 content. Of course, the reaction speed can be effectively maintained through the regularly release of MgCl2. On the other hand, TiCl4 has been adsorbed on the surface of preliminary generated Ti particles with high chemical activity. Thus the activated TiCl4 may react with the reducing agent Mg, and the resulted Ti particles would deposit on these active spots. Commonly, the tip and corner of the crystal are the most active, and Ti would grow and develop along with these corners. Thus Ti product with spongy structure can be obtained due to the sintering and recrystallization processes. Relying on the automatic catalysis of titanium sponge and the decrease of the apparent activation energy, the newly generated Ti can be deposited on the preliminary Ti matrix, which would be gradually formed along the longitudinal and/or horizontal direction. During the formation process of sponge titanium, the reduction rate is relatively high, and Mg consumption is about 50%. At the later stage, the reaction speed gradually decreased and the titanium sponge was basically formed. With the decrease of “free Mg” (Mg commonly entering the pores of sponge titanium), the reaction speed mainly depends on the low diffusion speed of Mg in the pores. As producing Ti sponge, TiCl2 dissolved in MgCl2 would be reduced by the presented Mg in the titanium pores, leading to the gradual densification of Ti sponge in the middle of the reactor. The Mg consumption reaches B65%
70% in this stage, and the reduction operation should be finished timely. Besides Ti sponge, some residual Mg, unfinished chloride, and low-valent titanium chloride may exist in the reduced product. The composition is about 60% Ti sponge, 30% Mg, and 10% MgCl2. In addition, a small amount of TiCl2 and TiCl3 may also exist in the obtained product. These residues must be removed to obtain pure Ti sponge. In fact, the reduction products can be regarded as Mg
MgCl2
Ti ternary system. The different boiling points are the key factors for the separation of Mg and MgCl2 from Ti sponge. Table 14.2 shows the relative volatility (α) of Mg and MgCl2 to Ti. Obviously, the relative volatility of Mg/Ti and MgCl2/Ti is very large, which means high separation coefficient. Besides, the relative volatility of Mg/Ti is much higher than that of MgCl2/Ti, implying that the separation of Mg is more Table 14.2 Properties of Mg and MgCl2 in Mg
MgCl2
Ti system. Temp ( C)

Mgsaturated vapor pressure (Pa)

MgCl2saturated vapor pressure (Pa)

Ti-saturated vapor pressure (Pa)

Mg/Ti relative volatility (α)

MgCl2/Ti relative volatility (α)

900 1000

13,119 38,628

937 3310

3 3 1029 1.5 3 1028

4.37 3 1012 2.58 3 1011

3.12 3 101 2.21 3 1011

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Titanium Dioxide (TiO2) and Its Applications

easily than MgCl2. Therefore MgCl2 is the key for the distillation and separation processes. Although the separation coefficient of MgCl2/Ti is large enough, it is difficult to separate MgCl2 from the reduced products completely. This is mainly due to the presence of residual MgCl2 in the pores of sponge titanium. Under the condition of atmospheric distillation, MgCl2 is hard to be completely separated due to its high boiling point (1418 C), which is much higher than the high-temperature limit of reactor (1000 C). Therefore vacuum distillation separation is usually utilized to achieve the separation and improve the separation speed. After the series of abovementioned reactions, the resulting Ti sponge has been sintered into a whole stacking titanium sponge through the long-term high-temperature process. After crushing, screening, magnetic separation, manual selection, and mixing, it can be sold as commercial titanium sponge for subsequent utilization and processing, that is, Ti ingot, commercial Ti powder, and Ti sputtering targets for semiconductor products and liquid crystal devices, etc.

14.3

Electrolytic production of Ti from TiO2 in high-temperature molten salts

As introduced earlier, it is obvious that Ti production through the conventional Kroll process is an energy-intensive process, which leads to a high price of Ti. Accordingly, researchers are trying to find an alternative low-cost process to produce Ti. Many interesting works involving electrolytic production of titanium/(titanium alloys) from TiO2/(TiO2 and other metal oxides) in high-temperature molten salts have been reported in recent years, and these works will be described in detail in this section. Generally, solid TiO2 has been directly and electrochemically reduced to Ti in molten calcium chloride
based molten salts, which seems really promising for low-cost production of titanium. Electrolytic reduction of alumina into aluminum is the best known example of electrolysis industry. Alumina has been added into molten cryolite, and electrolysis can be carried out between a graphite-based anode and liquid aluminum cathode at high temperature (950 C
970 C). The aluminum can be obtained successfully through the reaction: 2Al2O3 1 3C 5 4Al 1 3CO2. Motivated by this green and high-efficiency process, the production of titanium metal/alloys via molten salt electrolysis (MSE) process attracted more attentions. Electrolysis of TiCl4 was once considered as the most promising method for extracting titanium [12]. However, the effective extraction of Ti by electrolytic reduction of TiCl4 is difficult to achieve due to (1) the existence of multivalent titanium ions [Ti(II), Ti(III), and Ti (IV)], (2) low solubility of TiCl4 in molten salts, (3) no suitable construction materials for the electrochemical cell, etc. [13]. In the year of 2000, Chen et al. [14] proposed the FFC (Fray
Farthing
Chen)Cambridge process for the extraction of Ti from TiO2 in molten CaCl2, which has become a research hot spot immediately. The FFC-Cambridge process is a versatile technology for the direct conversion of solid metal oxides into their corresponding

TiO2 as a source of titanium

439

Figure 14.6 Schematic illustration of the FFC-Cambridge process for the electrochemical reduction of solid metal oxide to solid metal in molten salt [15]. FFC, Fray
Farthing
Chen. Source: Reproduced with permission from G.Z. Chen, E. Gordo, D.J. Fray, Metall. Mater. Trans. B 35 (2004) 223
233, Springer Nature.

metals through electrochemical reduction process in molten salts, and a typical illustration of this process is shown in Fig. 14.6. Differently to the traditional Kroll process, the FFC-Cambridge process innovatively utilizes solid TiO2 as the feedstock for the direct preparation of Ti. In this process, porous TiO2 pellet served as a cathode, which can be electrolyzed against a graphite anode under a specific cell voltage. Commonly, the cell voltage is high enough to ionize the oxygen contained in the TiO2 without decomposing the electrolyte (i.e., CaCl2). As a result, the ionized oxygen ions (O22) would dissolve into molten salt and discharge at the anode, while Ti powder can be finally obtained at the cathode. This process can be represented by the following reactions. Overall reactions: nTiO2 1 2C 5 nTi 1 2COn

ðn 5 1 or 2Þ

(14.17)

Cathode reaction: TiO2 1 4e2 5Ti12O22

(14.18)

Anode reaction: nO22 1 C 5 COn 1 2ne2

ðn 5 1 or 2Þ

(14.19)

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Titanium Dioxide (TiO2) and Its Applications

The FFC-Cambridge process is based on the thermodynamic result that the decomposition voltage of solid TiO2 at high temperature is lower than that of molten CaCl2. Yet the achievement of the reaction will not come easy, in which two criteria need to be fulfilled. One is that the removal of the dissolved oxygen is possible at electrode potentials that are less than the potential for calcium deposition from the molten salt. Another criterion is that the oxide ions can dissolve into molten salt. In fact, the kinetic pathway of the cathode reaction is more complex than the above simple overall reaction (14.17). The fast discharged O22 and nearby Ca21 tend to combine with the TiO2, either chemically or electrochemically, to form perovskites (CaTiO3 and CaTi2O4) [16]. An in-depth investigation about the TiO2 reduction process was performed by Schwandt et al. [17]. Some calciumcontaining compounds such as CaTiO3 and CaTi2O4 can be observed in the reduction partway, and a reaction path based on their experimental results was proposed, that is, TiO2!Ti3O5/CaTiO3!Ti2O3/CaTiO3!TiO/CaTiO3!TiO/CaTiO3/ CaTi2O4!Ti. The existence of perovskites can reduce the porosity and slow O22 transport from the TiO2 cathode. The low O22 concentration may result in the oxidation of Cl2 (CaCl2) to Cl2 during the initial electrolysis stage [18]. Therefore appropriate CaO concentration can be added in the initial TiO2 reduction process. However, excess CaO may lead to the cathodic CaO saturation, which would slow or even stall the electrolysis [18]. In addition, it should be pointed that the solid CaTiO3 as feedstock can also be electro-reduced to Ti, which demonstrates faster electrolysis rate than TiO2 [19]. Even so, the mechanism of the cathodic electroreduction of TiO2 is complex [20,21]. During the reduction process, there are arguments for calcium deposition being an inevitable step, as shown in reaction (14.20) [22]. Suzuki et al. thus designed an electro-metallothermic reduction method based on the FFC-Cambridge process for the extraction of Ti, that is, Ono
Suzuki (OS) process. As shown in Fig. 14.7, the graphite and titanium mesh are used as anode and cathode, respectively. Different from the FFC-Cambridge process, CaCl2 with a small amount of Ca is utilized as electrolyte in this process. The electrolysis potential between the two electrodes is 3.0 V, which is higher than the decomposition voltage of CaO (1.66 V) and lower than the decomposition voltage of CaCl2 (3.2 V). As a result, TiO2 can be reduced to metallic Ti by Ca near the cathode and then deposited to the bottom of the cell. The reactions are demonstrated as reactions (14.20)
(14.22). The CaO generated from the parasitic reaction (14.21) can be cycled electrolyzed to form metallic Ca. Besides the OS process, some extraction processes, such as electronically mediated reaction (EMR) process [23], preform reduction process (PRP) [24], and Broken Hill Proprietary (BHP) billiton process [25] were also proposed. Even so, the FFC-Cambridge process is still universal in its applicability, and a great deal of research works has been persistently performed to produce metals and alloys through the MSE process. Cathode reaction: Ca21 1 2e2 5 Ca

(14.20)

TiO2 1 2Ca 5 Ti 1 2CaO

(14.21)

TiO2 as a source of titanium

441

Figure 14.7 Schematic illustration of the OS process [22]. OS, Ono
Suzuki. Source: Reproduced with permission from K. Ono, R.O. Suzuki, JOM 54 (2002) 59
61, Springer Nature.

Anode reaction: nO22 1 C 5 COn 1 2ne2

ðn 5 1 or 2Þ

(14.22)

Compared to the traditional Kroll process, the FFC-Cambridge process usually uses cheaper and safer TiO2 as feedstock to replace TiCl4. However, TiO2 is produced by the sulfuric acid process or chlorination process, which also causes environmental concerns. Thus some attempts trying to find a more sustainable and low-cost resource for the FFC-Cambridge process have been made, for example, titanium-rich slag was directly used as feedstock to synthesize ferrotitanium alloys [26]. Obviously, this process demonstrates great potential in the field of production of metals and alloys, such as Si nanoparticles [27], Ti5Si3 [28], and Ti3AlC2 [29], through a simple mixed metal oxide at a predefined ratio. Moreover, the FFC-Cambridge process is an alternative, simple, and more material efficient way to either regenerate spent titanium components without affecting their dimensions or recycle titanium scraps [17]. Another feature of the FFC-Cambridge process is that it is a direct solid-to-solid deoxidation process, and the electrolytic products retain the shape closely to the original shape of the oxide precursors. Fig. 14.8 shows the Ti
6Al
4V alloy with different shapes: hollow sphere, cup, and golf club head that are produced in molten CaCl2 by electroreducing their corresponding metal oxide precursor prepared using a one-step solid slip casting process. Oxygen contents in the Ti
6Al
4V components were typically below 2000 ppm. The maximum compressive stress and modulus of electrolytic Ti
6Al
4V alloy can reach 243 MPa and 14 GPa, respectively, matching with the

442

Titanium Dioxide (TiO2) and Its Applications

Figure 14.8 Electrolytically produced Ti
6Al
4V hollow sphere before (A) and after (B) polishing; (C) cross section of hollow sphere; cup-shaped Ti
6Al
4V before (D) and after (E) minor polishing, and (F) SEM image of the interior structure of the electrolytically produced Ti
6Al
4V alloy; (G) golf club head
shaped metal oxide precursor, (H) electrolytically produced miniature of Ti
6Al
4V hollow golf club head, the inserted photo is the commercial titanium golf club driver head; and (I) cross section of produced miniature golf club head [30]. Source: Reproduced with permission from D. Hu, W. Xiao, G.Z. Chen, Metall. Mater. Trans. B 44 (2013) 272
282, Springer Nature.

requirement for medical implants [30]. The combination of FFC-Cambridge process and advanced manufacturing concepts is more interesting. The products produced through the FFC-Cambridge process commonly possess porous powdered structure, which may also have potential to be used as the feedstock for additive manufacturing technology [e.g., three-dimensional (3D) printing] [31]. Extensive studies are conducted to achieve the industrial application of FFCCambridge process. Shortly after the proposal of this process, British Titanium PLC, founded as a Cambridge University spinout company, attempted to scale up the production of Ti, promoting grain yield from a few grams to around 1 kg of titanium dioxide per batch. After several designs and modifications of the electrolytic installations, the production quantities were further improved to 2.5
3.5 kg TiO2 per batch with a current efficiency of B28%. In addition, the cathodic reactor and anode can be replaced

TiO2 as a source of titanium

443

without cooling the cell to realize the continuous production. If multiple cathode
anode arrangements were taken, many tens or hundreds of kilograms of material per unit can be achieved [17]. However, the research is in abeyance due to some reasons. For the anode reaction of the FFC-Cambridge process, O22 would discharge at the graphite anode to release CO/CO2. The discharge of O22 on the graphite is thermodynamically more favorable than that of Cl2 [32]. However, the formation of Cl2 is possible at potentials below the thermodynamic decomposition potential of CaCl2 (3.2 V) at 900 C, which is the result of the reactive nature of the cathode [17]. As the generation of CO/CO2, significant polarization losses of graphite anode would release carbon particles, especially in the long-time electrolysis case, leading to the possible short circuits of the electrolytic cell. Moreover, CO2 is soluble as the form of CO22 in CaCl2, which may discharge at the cathode to produce carbon 3 deposition (reactions 14.23 and 14.24). Similarly, the Ca dissolved in CaCl2 would lead to some electronic conduction to the molten electrolyte. All these factors can lower the electrolytic current efficiency, commonly around B20%, which is a serious obstacle to the industrialization development of the FFC-Cambridge process. Anode side reaction: CO2 1 O22 5 CO22 3

(14.23)

2 22 CO22 3 1 4e 5 C 1 3O

(14.24)

With the aim to solve the anode problem, the solid oxide oxygen ion
conducting membrane (SOM) process was proposed by Prof. Pal to extract magnesium metal from magnesium oxide [33,34]. The schematic illustration of the SOM electrolytic cell is shown in Fig. 14.9. In this process the selectivity of solid oxide membrane has been used to control the flow of O22. The directional migration of

Figure 14.9 Schematic illustration of the SOM electrolytic cell [35]. SOM, Solid oxide oxygen ion
conducting membrane. Source: Reproduced with permission from X. Zou, X. Lu, Z. Zhou, W. Xiao, Q. Zhong, C. Li, et al., J. Mater. Chem. A 2 (2014) 7421
7430, Royal Society of Chemistry.

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Titanium Dioxide (TiO2) and Its Applications

Figure 14.10 (A) Solid oxide oxygen ion
conducting YSZ membranes with different specifications and (B) SOM-assisted molten salt electrolytic cell after electrolysis. SOM, Solid oxide oxygen ion
conducting membrane; YSZ, yttrium-stabilized zirconi.

O22 and the electrolytic reduction of metal oxides can be achieved under the external electric field. Nonoxygen ions hardly enter the anode reaction zone due to the existence of solid oxide membrane. Therefore an electrolytic potential higher than the molten salt electrolyte decomposition voltage can be applied naturally. The SOM-assisted electrolysis process is an environment-friendly method for the direct production of titanium metal, as well as titanium alloys [36]. In fact, the reaction mechanism of cathodic TiO2 is similar to the FFC-Cambridge process. When the direct current potential applied between the anode and the cathode exceeds the dissociation potential of TiO2, TiO2 can be reduced to Ti at the cathode. Moreover, with the introduction of predesigned multicomponent metal oxides as cathodic precursor, Ti alloys (such as Ti5Si3) can be in situ generated in high temperature [37,38]. The key feature of the SOM process is the use of a solid oxide oxygen ion
conducting yttrium-stabilized zirconia (YSZ) membrane, as shown in Fig. 14.10A, which separates the anode from the molten salt. A liquid medium such as tin or copper can be employed to transport the O22 through the YSZ membrane. The liquid metal served as anode is connected to the power through a conductor (such as Mo wire). Only O22 can be electro-migrated across the membrane and oxidized at the separated anode owe to the oxygen ions selectivity of the SOM. Therefore carbon deposition occurred in the FFC-Cambridge process can be avoided effectively. Fig. 14.10B is the photograph of a typical electrolysis cell for SOM-assisted process obtained after electrolysis. Obviously, the SOM-assisted electrolysis cell gives a clean molten salt without the carbon deposition. As a result, the current efficiency of the SOM process has also been improved. However, the durability and stability of the SOM tube need further investigations. Overall, the MSE method represented by the FFC process is promising for the extraction of Ti from TiO2. Compared to the Kroll process, MSE method has many advantages: short technologic process, easy operation, low manufacturing cost, absence of chlorination, etc. However, more investigations are still needed for its industrial applications.

TiO2 as a source of titanium

14.4

445

Electrodeposition of Ti in low-temperature liquid salts

In addition to the high-temperature MSE process, low-temperature electrolytic production of titanium from TiO2 and TiCl4 has also been investigated in recent years, which will be briefly introduced in this section. Commonly, the low-temperature electrolytic production of titanium mainly involves the electrodeposition of titanium in ionic liquids. Ionic liquid, as a new kind of green liquid salt, has attracted wide attention of researchers because of its excellent properties, such as good conductivity, stability, low energy consumption, and no pollution. Ionic liquids combine the advantages of aqueous solution and high-temperature molten salt. In addition, for ionic liquid, the corrosion of molten salt at high temperature and the damage of equipment can be avoided. Ionic liquid has a wide electrochemical window and avoids hydrogen evolution reaction, which gives huge advantage in electrodeposition of various metals and alloys at room temperature. Therefore it is really expected that Ti metal can be obtained through the ionic liquid electrodeposition. Many studies for electrodeposition of Ti in low-temperature ionic liquids have been made. As early as 1990, Carlin et al. [39] attempted to electrodeposit Ti in AlCl3-1-ethyl-3-methylimidazolium chloride (AlCl3-ImCl) ionic liquid with TiCl4 as the titanium source. The results show that Ti(IV) was reduced to Ti(III) and Ti(II) through two-step single-electron transfer, both of which show slow electron transfer kinetics. TiCl3 film was formed on the electrode finally, and no metal Ti could be observed. The electrochemical behavior of Ti in ionic liquid of AlCl3-1-ethyl-3-methylimidazolium chloride (AlCl3-EtMeImCl) was investigated by Tsuda et al. [40], and they found that TiCl3 insoluble film can be directly formed from Ti(0) under low current density, while Ti(0) was oxidized to Ti(IV) under high current density to form TiCl4 gas. In fact, the solubility of low-valent titanium ions in most ionic liquids is too low to achieve continuous Ti deposition. Besides, the insoluble TiCl3 with poor conductivity can be easily generated in the reduction process, which will hinder the electron transfer. l-Methyl-3-butyl-imidazolium bis(trifluoromethyl sulfone)imide [(BMIm)BTA] with highly dissolved TiCl4 was employed by Mukhopadhyay et al. [41], and ultrathin Ti coating with scale of 1
2 nm was successfully deposited on oriented pyrolytic graphite and Au electrodes. Besides, the transition from twodimensional clusters to 3D clusters of nano-Ti was first observed. Different titanium halides (TiCl4, TiF4, and TiI4) and ionic liquid electrolytes {1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide [(EMIm)Tf2N], 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide [(BMP)Tf2N], and trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide [(P14,6,6,6)Tf2N]} have been systematically investigated by Endres et al. [42], only some nonstoichiometric halides (TiCl0.2, TiCl0.5, and TiCl1.1) are formed, instead of metal Ti. According to these previous works, the electrodeposition of Ti from ionic liquids with various Ti precursors is extremely difficult. Besides electrolytic systems, the interaction of various ionic liquids with the substrate may influence the deposition process during the adsorption and electrolysis processes.

446

Titanium Dioxide (TiO2) and Its Applications

Figure 14.11 TiO2 pellet before (A) and after (B and C) electrolysis in AlCl3-BMIC [45]. Source: Reproduced with permission from X. Zhang, Y. Hua, C. Xu, Q. Zhang, X. Cong, N. Xu, Electrochim. Acta 56 (2011) 8530
8533, Elsevier.

Wu et al. [43] investigated the electrode/electrolyte interface (EEI) of TiCl4 in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide [Py1,4]TFSA, with a focus on the changes in the nanostructure at varying concentrations of the Ti precursor using atomic force microscopy, and the results reveal that the concentration of TiCl4 can influence strongly the interfacial EEI nanostructure. Obviously, the selection of suitable ionic liquid system is very important for the electrodeposition of titanium [44]. There have been already plenty of attempts to deposit Ti from its halides in various ionic liquids, but so far with only minor success. To circumvent the limitation of titanium halides, Ti deposition from TiO2 precursor in AlCl3-1-butyl-3-methylimidizolium (AlCl3-BMIC) was proposed by Zhang et al. [45], although voltammetry studies suggested that Ti(IV) can be reduced to metallic Ti, the reaction rate was quite slow, and only 12 wt.% of TiO2 was consumed after 48 h. Fig. 14.11 shows the photographs of TiO2 pellet before and after electrolysis. Berger et al. [46] utilized titanium isopropoxide as titanium source to electrodeposit Ti on Au(111) model electrodes in a custom-made guanidinium-based ionic liquid (N11N11NpipGuaTFSI); however, it is still challenging to effectively and continuously achieve the electrodeposition of Ti. In summary, although Ti electrodeposition in ionic liquids is feasible in some case, the continuous, large-scale, and controllable synthesis of Ti coating in ionic liquids is still hard to be achieved. The development of tailored titanium precursors and the selection of suitable ionic liquids systems are very important for the Ti electrodeposition, which requires further investigations.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (no. 51974181), the Shanghai Rising-Star Program (19QA1403600), and the Iron and Steel Joint Research Found of National Natural Science Foundation and China Baowu Steel Group Corporation Limited (U1860203); the authors also thank the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (TP2019041) and the CAS Interdisciplinary Innovation Team for financial support.

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188. [3] G. Deng, In Titanium Metallurgy (in Chinese), Metallurgical Industry Press, Beijing, 2010. [4] F. Froes, Titanium: Physical Metallurgy, Processing, and Applications, ASM International, Materials Park, OH, 2015. [5] M.J. Ga´zquez, J.P. Bolı´var, R. Garcia-Tenorio, F. Vaca, Mater. Sci. Appl. 5 (2014) 441. [6] Y. Zhang, Z.Z. Fang, P. Sun, S. Zheng, Y. Xia, M. Free, JOM 69 (2017) 1861
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28. [9] A.D. Mah, K.K. Kelley, N.L. Gellert, E.G. King, C.J. O’Brien, Thermodynamic Properties of Titanium-Oxygen Solutions and Compounds, US Department of the Interior, Bureau of Mines, Washington, DC, 1957. [10] W. Kroll, Trans. Electrochem. Soc. 78 (1940) 35. [11] O. Takeda, T. Uda, T.H. Okabe, Treatise on Process Metallurgy, vol. 3, Elsevier, London, 2013, pp. 995
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4965.

TiO2 in the building sector

15

Elisa Franzoni1, Maria Chiara Bignozzi1 and Elisa Rambaldi2 1 Department of Civil, Chemical, Environmental and Materials Engineering, Alma Mater Studiorum
University of Bologna, Bologna, Italy, 2 Centro Ceramico, Bologna, Italy

15.1

Introduction

According to the document “Science for Environment Policy” by the European Commission DG Environment News Alert Service (March 2011), constructions account for 24% of global raw materials removed from the earth. On the one hand, this gives an idea of the great environmental impact associated with the extraction, processing, transport, and installation of materials. Over last years a big effort was devoted to promote the development and use of more sustainable materials and manufacturing technologies. On the other, this suggests how impressive the benefit can be from innovating the performance of building materials, for our everyday life. In particular, multifunctional materials allow one to achieve higher functionality and a broader spectrum of desired properties, if compared to traditional materials. The great potential of these materials can be fruitfully exploited in the construction sector, owing to the large amounts of materials involved. In this context, the use of photoactive TiO2 is particularly appealing for the manufacturing of antipollution, self-cleaning, and antimicrobial surfaces in buildings, structures, and roads. Cement-based materials and ceramic tiles are obviously the most investigated substrates for the application of photocatalytic TiO2, due to their strategic role in buildings and constructions. However, other possible applications are getting more and more attention from the scientific community, namely, in geopolymers and in cultural heritage conservation.

15.2

TiO2 in cement-based materials

15.2.1 General goals of the use of TiO2 in cement-based materials Cement-based materials, namely, concrete and mortars, are omnipresent composite materials throughout the world. They are obtained by mixing a cement binder (ordinary Portland cement or blended cements), aggregates of different size (coarse aggregate, sand, and sometimes filler), water, and minor amounts of chemical admixtures. While concrete is used mostly for structural applications (buildings, Titanium Dioxide (TiO2) and its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00017-1 © 2021 Elsevier Inc. All rights reserved.

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Titanium Dioxide (TiO2) and its Applications

bridges, tunnels, infrastructures, etc.), cementitious mortars are generally used for rendering of vertical surfaces and for masonry joints. Paving blocks and elements constitute a further application of cement-based composites, being constituted by concrete and/or mortar, depending on the type of block/element. The wide diffusion of concrete in our everyday lives makes any improvement of this material strategic, especially considering that it is estimated that about a ton of cement is being produced for every human being per year and that 7% of the global carbon dioxide (CO2) emissions are due to cement manufacturing [1], as the production of 1 t cement involves the emission of average 0.7 t of CO2 into the atmosphere [2]. Over last decades an impressive research effort was devoted to improving this material from both the point of view of performance and environmental impact. A general improvement of the environmental impact of concrete is obtained by replacing its ingredients with more environment-friendly materials, such as industrial by-products, recycled materials, and waste streams. Supplementary cementing materials, such as fly ash, slag, and silica fume, as by-products deriving from other industrial sectors, may effectively replace part of Portland cement, reducing the overall impact of concrete [2]. On the other hand, the improvement of the performance of concrete is beneficial for both its applications and its environmental impact. In particular, an increase in mechanical strength of concrete involves smaller cross sections in structures and a general saving of material. Moreover, manufacturing a strong and durable concrete is of paramount importance to ensure a longer service life of the concrete structures [3], thus reducing the environmental and economic costs related to their maintenance and repair. Over the past decade, research has progressively focused on incorporating many different nanomaterials in the cement matrix, in order to develop cementitious materials with improved or even unique properties. In fact, a wide range of properties, including physical
mechanical, durability, workability, and rheological properties [4], can be improved through the inclusion of nano-scaled compounds, such as pozzolanic nanosilica, metallic oxides (nano-Fe2O3, nano-titania, nanoalumina, nano-MgO), and the salt nano-CaCO3 [5], into the cement matrix. The incorporation of photocatalytic TiO2 nanoparticles into concrete was initially proposed toward the end of the 1980s [6] and since then many literature papers investigated the performances that can be obtained, which are also summarized in some review papers [1,2,4,5,7
12]. The possible applications of photocatalytic cement-based materials as reported in the literature are summarized in Table 15.1. Considering the different applications of TiO2 that were proposed in cementitious materials and the various properties that were investigated, two main goals can be envisaged in the use of nano-TiO2 in cement-based composites: G

G

the exploitation of the photocatalytic behavior of nano-TiO2, for the creation of self-cleaning and antimicrobial surfaces in buildings, and for the creation of air-purifying and pollutionreducing structures and surfaces; the exploitation of TiO2 nanoparticles for the formation of a better packed microstructure of the cement matrix, aiming at an improvement of the compressive strength and durability of concrete [5].

TiO2 in the building sector

451

Table 15.1 Possible applications of photocatalytic cement-based materials as reported in the literature, according to [8]. Horizontal applications

Roofing tiles and panels Paving tiles and blocks Coating for pavement and roads Concrete pavement

Vertical applications

Masonry blocks Traffic divider elements Street furniture Retaining fair-faced elements Sound absorbent Finishing, coating, and plasters Indoor and outdoor paints Covering precast panels

Tunnels

Paints and renderings Concrete panels Concrete pavement Ultrathin white topping

The first approach addresses concrete as a functional material, while the second one addresses it as a structural material. TiO2 is usually incorporated in the concrete bulk, but it can also be introduced in the surface layer only, or even be applied over the surface as a coating. When incorporated in the concrete bulk, the photocatalytic behavior of the nano-TiO2 that is not reached by light is obviously not exploited. The uses of TiO2 in both functional and structural cementitious materials are described in the following sections.

15.2.2 Use of TiO2 for functional cement-based materials The applications of heterogeneous photocatalytic processes based on the use of semiconductor particles in the mixture of cement-based materials began toward the end of the 1980s [6] and received great attention in recent years, aiming at the enhancement of the esthetic durability of buildings and the reduction of environmental pollution. Among all, titanium dioxide is the most widely used semiconductor in cement-based materials, thanks to its relatively low cost, chemically stable nature, and compatibility with the cement matrix [8]. The introduction of TiO2 nanoparticles into cement-based materials is targeted at the exploitation of three main functionalities: G

G

G

air purifying (and water purifying) self-cleaning antimicrobial

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Titanium Dioxide (TiO2) and its Applications

15.2.2.1 Air-purifying cement-based materials A first and prominent functionality offered by cement-based materials incorporating TiO2 is their air-purifying ability, exploited in the so-called antismog or depolluting concrete, renders, and pavements. These materials are used in outdoor applications such as building fac¸ades, structures, roads, and tunnels, as well as for the removal of indoor pollution, possibly also associated with UV lamps in those applications lacking sunlight. Many laboratory studies demonstrated the effectiveness of photocatalytic TiO2 in the removal of VOCs (volatile organic compounds, such as aromatics, aldehydes, and toluene), nitrogen oxides (NOx), CO, SOx, and other gaseous pollutants. Among these pollutants, nitrogen oxides (NOx) recently raised a major concern [8]. In fact, while the concentration of sulfur oxides (SOx) in urban and industrial atmosphere drastically decreased in recent years thanks to the improvement in the quality of fuels [13], NOx concentration is still very high, especially in street canyons, owing to combustion engine exhausts [6]. Nitrogen oxides (NOx), a term mainly indicating the sum of nitric oxide (NO) and nitrogen dioxide (NO2), are among the main responsible for photochemical smog (a mix of hazardous chemicals formed in the atmosphere due to the interaction of sunlight with already present pollutants) and they can also generate acid rains [6]. The detrimental effect of NOx on the quality of life is well known, as these oxides may lead to bad effects on the human respiratory and immune systems [14] (e.g., emphysemas and bronchitis), and to alterations in plant regular metabolism [6]. Both EPA (Environmental Protection Agency) in the United States and EEA (European Environment Agency) in Europe established concentration limits for NOx in air. However, despite the maximum limit of 40 µg/m3 for NO2 annual mean concentration in air (200 µg/m3 limit for hourly mean) fixed by the European Directives [15], the actual concentration of NOx in almost all main European cities far exceeds this value, and, for example, an hourly average of about 300 µg/m3 was reported in London in July 2017 [16]. The situation in China is even more extreme, with NOx concentrations in large cities in the North China Plain exceeding 450 ppb in July 2013 [16]. For this reason, while technologies are under development for the reduction of these pollutants in the exhaust gas emissions, the exploitation of TiO2 for NOx abatement in air is presently considered as a promising route to reduce these pollutants [14]. Many laboratory studies provided experimental evidences about the depolluting effect of photocatalytic cement-based materials (among the others [7]), mostly focusing on the effectiveness in the abatement of NOx and VOCs, which are considered the most harmful gaseous compounds in polluted air. While VOCs are transformed mainly into H2O and CO2, the NOx abatement finally produces NO32 ions and then, by combining with calcium ions present in concrete, nitrites, and nitrates, harmless salts that can be easily washed away by rain [6,16]. Besides laboratory studies the photocatalytic behavior of concrete pavements and external building surfaces incorporating TiO2 was shown also in some onsite applications [7]. The application of a 10,000 m2 photocatalytic cementitious blocks pavement in a parking lane in Antwerp (Belgium) produced a significant decrease

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in the NOx concentration [14]. After washing and testing these same blocks 2 years after their application, an unaltered photocatalytic activity was found [17]. Three artificial street canyons were built in Guerville (France) and the walls were coated with photocatalytic cement-based mortar, showing that the NOx concentrations in the canyon were 36.7%
82.0% lower than the ones observed in the reference canyons without TiO2 [18]. A street in Bergamo (Italy) was partially paved with stones coated with a cement-based photocatalytic material, and the air surrounding the pavement was monitored for 2 weeks, highlighting an NOx concentration about 30%
40% smaller than the one in the portion of the street without photocatalytic pavement [14]. A tunnel in Rome (Italy) was coated with a photocatalytic cementbased paint and outfitted with an artificial UV lighting system, showing a NOx reduction .20% [14]. A field-test study was carried out in Copenhagen (Denmark), comparing photocatalytic concrete paving elements for solar light
driven air purification with ordinary concrete paving elements [19]. Over a 1-year period the daily average NO concentration measured by air-monitoring stations was maintained very low thanks to the presence of the photocatalyst, while the concentration of NO2 remained basically unaltered notwithstanding the photocatalyst presence. This seems to be due to the fact that photocatalytic oxidation of NO to nitrate proceeds in steps, involving the production of nitrous acid and NO2 as intermediates, so probably NO2 was contemporarily both eliminated and produced during the photocatalytic activity of the pavement [19]. In the selected location (Copenhagen), the best performance in terms of NOx reduction was achieved at the summer solstice, the monthly abatement being 22%, and the noon abatement being .45% [19]. Notably, some authors suggested that the limited reduction of NO2 (or even its increase [20]) must be taken very seriously, as the toxicity of NO2 is considerably greater (by conservatively a factor of 3) than that of NO and it can also act as a precursor for even more toxic atmospheric constituents (e.g., ozone and peroxyacyl nitrates) [16]. As shown earlier, many laboratory and onsite studies suggested a very positive contribution of TiO2-incorporating cementitious materials on the reduction of air pollutants. However, they also highlighted many open questions and critical aspects that need further investigation and that will be probably the future challenges to be tackled by researchers for a deep understanding of the depolluting behavior of this class of materials. For example, a study showed that different photocatalytic cementitious products provide a very different NOx degradation efficiency, with some products achieving 40% degradation, whereas others showing almost no effect [21]. This suggests that different factors play a role in the actual depolluting ability of cement-based materials incorporating TiO2 and these factors must be carefully considered in the perspective of applications in the construction field. Some of the main open questions are briefly summarized as follows.

Role of the climatic conditions The rate of NO conversion was found to vary considerably in relation to the air temperature and relative humidity [20], in particular, decreasing with increasing relative humidity (due to a competition between water and NO for catalytic sites at relative humidity higher than 10%) and increasing with increasing temperature (due

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Titanium Dioxide (TiO2) and its Applications

to higher diffusivity of the gaseous pollutants toward the photocatalytic surface) [19], although the size of TiO2 particles was highlighted as possibly playing a role in this aspect [20]. Moreover, the effectiveness of concrete with TiO2 depends on UV radiation with wavelengths in the range of 380
390 nm for activation, which represents only a small fraction of the available solar energy. This shortcoming boosted the development of a new generation of visible-light-responsive systems, but in the meantime TiO2 remains the dominant photocatalytic material used in practical applications, hence attention should be paid to the availability of suitable solar UV irradiation intensities in the application of these materials at various locations around the globe [22]. In fact, the often-quoted 3%
5% UV contribution is only achieved throughout the whole year at latitudes below 35 degrees, while at high latitudes the UV exposures are usually strongly reduced, although this is also somehow compensated by virtue of increased daylight hours in the long summer days [19].

Interactions with the cement matrix Several aspects must be considered when TiO2 is intermixed with cement-matrix composite materials rather than used as a powder or a slurry. The chemical nature of the matrix, the rate or dispersion or agglomeration of TiO2 nanoparticles, and the possible reduction of photoactivity due to the incorporation of TiO2 in the matrix are the most prominent ones. For its exploitation as a photocatalyst, TiO2 is required to have a particle size in the range of 10
20 nm, in order to provide surface sites for adsorption and to reduce recombination processes [23], hence controlling the agglomeration of the nanoparticles, and promoting their uniform dispersion in the material is of paramount importance for an efficient photoactivity. However, the chemistry of the cement environment can influence the dispersion behavior and adsorption properties of TiO2 in concrete, while its extremely high pH (12
13) may influence band edge positions in the semiconductor photocatalyst and redox processes taking place at the photocatalyst surface, thus influencing also the mechanisms of NOx photocatalytic degradation [8,23]. In fact, the highly alkaline cement environment has been pointed out to exert some inhibiting action toward photocatalytic degradation mechanisms compared to the ones occurring using TiO2 powder alone [6]. The instability of TiO2 particles in cement and their tendency to agglomerate were investigated in a study where both micron-size and nano-size particles were dispersed in Ca(OH)2 solutions at pH 5 12.5, mimicking the pore solution present in the cement matrix, where calcium hydroxide is released from the hydration reaction of the calcium
silicate phases of the cement (alite, C3S, and belite, C2S). The formation of clusters occurred, which is thought to be caused by the attractive electrostatic interactions promoted by ion
ion correlations, previously observed also for CsSsH (calcium silicate hydrate) gel [24]. Unexpectedly, micro-TiO2 generated smaller and better dispersed aggregates than nano-TiO2, the latter giving bigger clusters, more difficult to spread, so while nano-titania is generally supposed to perform better than micro-titania due to its much higher specific surface area, this is not necessarily the case in solid cement-based composites. This suggests that

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gaseous NOx, having small dimensions, can easily penetrate both in micro- and nano-TiO2 clusters, benefiting from the high specific surface area of the latter, while organic stains and contaminants, having larger size, will be likely more effectively decomposed by micro-TiO2, more easily accessible and also better dispersed in the matrix. In fact, the degradation of rhodamine B by the abovementioned cement-based specimens under UV light (a common test for the evaluation of the self-cleaning effect of photocatalytic concretes) was more efficient in the case of samples prepared with micro-TiO2 rather than with nano-TiO2, while the opposite happened for the degradation of NOx, which was higher for nano-TiO2 [24]. Due to the tendency toward agglomeration of TiO2 in cement paste, it is convenient to apply techniques promoting a uniform dispersion of these small clusters, the most common being ultrasonication and the use of superplasticizers. Superplasticizers are usually preferred, as they do not require specific equipment [5]. However, it was also suggested that organic admixtures must be selected carefully to not interfere with the photocatalytic activity of the products [8]. The immobilization of TiO2 by construction materials is another factor that can significantly reduce its photocatalytic activity, and in fact TiO2-cement mixtures were found to provide a lower effectiveness than TiO2 slurries, probably due to the reduction of active surface and the presence of ionic species, which contributed to the charge recombination [7]. Similarly, it was shown that TiO2-intermixed concrete provided a lower degree of NOx degradation with respect to TiO2 spraycoated concrete [4].

Durability of the photocatalytic activity Although the depollution effect of photocatalytic construction materials was widely demonstrated in laboratory studies and in some field-testing campaigns, the duration of the photoactivity in real-life applications is still debated and a standard testing procedure is under preparation by CEN (European Committee for Standardization) [17]. Exposed concrete is naturally affected by carbonation, a reaction occurring between calcium hydroxide present in the hardened cement matrix and gaseous CO2 provided by air. This reaction obviously starts at the concrete surface, where also the photocatalytic activity of TiO2 nanoparticles takes place, and it produces the precipitation of calcium carbonate and the subsequent densification of concrete microstructure, possibly occluding the nano-TiO2 in the surface region and causing a reduction of its photocatalytic efficiency [23]. Moreover, the accumulation of contaminants at the concrete surface may decrease its photocatalytic activity. This is particularly significant for concrete pavements and roads, where the horizontal flat configuration is ideal for the exposure of the photocatalyst to sunlight [4], but it may also cause an accumulation of dust and oil on the surface [7]. Dust accumulation and oil impregnation were reported to cause a partial loss of efficiency in removing NOx for TiO2-coated concrete, and a severe or even complete loss of performance for concrete containing TiO2 [8]. A report published by the Hong Kong Environmental Protection Department claimed that the photocatalytic activity of TiO2-coated paving blocks decreased significantly after 4-month exposure in a downtown area, due to the accumulation of contaminants on the blocks surface,

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Titanium Dioxide (TiO2) and its Applications

suggesting the need of periodic washing to maintain the depolluting effect [7], as highlighted also elsewhere [25].

Improvement strategies, by TiO2 doping and modifications To improve the efficiency of TiO2 in locations where the available UV intensity is low, a strong research has been devoted to visible light activation of TiO2. Nitrogendoped TiO2 was first reported in the Eighties, while increased levels of active N uptake by synergistic doping with transition metal ions has been recently observed [23]. Particularly encouraging results were obtained by (W,N)-codoped TiO2 [26]. Organomodified commercial nano-TiO2 particles were also embedded in the cement matrix, providing a more efficient NOx removal than those with nonmodified nanoTiO2 [4]. In other studies, a better performance of a hybrid nano-TiO2sSiO2 coating compared to nano-TiO2 coating was observed [5]. Photocatalysts supported on surface exposed quartz aggregates were also tested as an alternative to TiO2 dispersion in the cement matrix, with promising results and an enhancement in nitrate selectivity [16].

15.2.2.2 Water-purifying cement-based materials Although most of the studies were addressed to investigate the effectiveness of TiO2-incorporating cementitious materials in the reduction of air pollution in urban and industrial areas, their application for water purification was also proposed [27]. A large amount of storm water is delivered from roads and car parks, where it gets rich of vehicle pollutants, such as polycyclic aromatic hydrocarbons, mineral oils, and heavy metals. This polluted water may affect the top soil, ground water, and surface water, with detrimental environmental effects, and paving materials can offer good potentialities to mitigate this problem. Permeable concrete is a porous, pervious, and no-fines concrete that is produced by mixing cement, coarse aggregate (normally single sized), and water, with little or no sand. Its interconnected void network may allow up to 10 mm/s of water to flow through, which can then become ground water or be harnessed and reused. The limited compressive strength of permeable concrete makes its suitable mostly for paving footpaths and car parks, where the keeping of a high soil permeability is requested. Due to its high surface area, permeable concrete with incorporated TiO2 represents a good paving material to filter and purify the water by reducing pollutants such as nitrogen and phosphorus [27], provided that the release of TiO2 is prevented by an effective adhesion to the cementitious substrate.

15.2.2.3 Self-cleaning cement-based materials One of the most interesting functionalities offered by TiO2 and exploited in cement-based materials is its self-cleaning ability. Self-cleaning surfaces are able to reduce the costs associated with maintaining the clean appearance [8], the benefit being maximum when TiO2 is associated with the employment of white cement. Although one limitation in the addition of nano-TiO2 into concrete is the high cost of nanoparticles in comparison with the other ingredients of concrete [28], it must

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be considered that the costs of cleaning may be very high. For example, the cost to clean graffiti in Los Angeles city could amount to 100 million euro/year [10]. The so-called soiling, that is, the deposit affecting buildings surface, is generally composite in nature, being originated from the atmospheric aerosol pollutants and being constituted by small particles/dust and greasy deposits, which adhere to the surface by organic binders such as hydrocarbons and fatty acids [7]. In particular, the carboxylic groups of fatty acid molecules make them stick quite well to calcium ions present in concrete [7]. The self-cleaning effect provided by the presence of TiO2 nanoparticles over the concrete and mortar surface is due to both the redox reactions of adsorbed substances promoted by UV light and the photoinduced hydrophilicity [7,23], which act simultaneously. While the redox reaction allows the decomposition of organic matter, superhydrophilicity plays an additional and key role in the achievement of self-cleaning functions, as rainwater droplets spread over the surface under light exposure, generating a thin film of water in the space between the substrate and the dust, which helps the removal of dirt and deposits [6,8]. Superhydrophilicity not only has a direct self-cleaning action onto inorganic powder contaminants but also plays a role in keeping the TiO2 surface clean and accessible to light, hence maintaining the efficiency of photocatalytic decomposition of organic contaminants [8]. To describe the contribution of superhydrophilicity to the removal of dirt, some authors proposed the term “easy-cleaning” rather than the common “self-cleaning” behavior [23]. Concerning the self-cleaning performance of TiO2-incorporating concrete, the dispersion of the nanoparticles and their availability on the surface are basic factors in the success of the photocatalytic action, hence most of the remarks made for depollution are valid also in this context. The self-cleaning behavior of cement composites embedding nano-TiO2 was confirmed in several papers and under the framework of the European Projects PICADA [29] and LIGHT2CAT [30]. In laboratory testing the self-cleaning performance was mostly evaluated by measuring the rate of photocatalytic decomposition of an organic dye, usually rhodamine B [7] (which is also adopted in the Italian standard UNI 11259 [6]), but also Malachite green, methylene blue, or others [4]. The results obtained in laboratory studies evidenced a strong photocatalytic activity of cement-based materials, obtained by incorporating small amounts of nano-TiO2 (usually ranging from 0.5% to 10%) in Portland or white cement [4,7]. Several buildings were designed and built since 2000 employing concrete with photoactive white cement, the first one being the church Dives in Misericordia in Rome, which became one of the main symbols of self-cleaning concrete [12]. Afterward, other popular buildings employed photocatalytic concrete, including the music and arts city hall in Chambe´ry (France), the police central station in Bordeaux (France), the Air France building at the Roissy-Charles de Gaulle airport in Paris [8]. The self-cleaning performance was checked on-site, in some of these buildings. Six years after its construction, the church Dives in Misericordia in Rome exhibited an almost negligible darkening, which was assessed by comparing the internal and external surfaces, while the art city hall in Chambery showed no significant color change after 5 years [7,12].

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Although some results obtained by onsite monitoring confirm the effectiveness of photocatalytic concrete in terms of self-cleaning behavior, it is noteworthy that a gap still exists between laboratory studies and applications in the field. In fact, the self-cleaning ability in laboratory is assessed mainly by the discoloration test toward an organic dye under UV lighting, but this approach is quite far from the assessment of the actual self-cleaning action, which is a complex mechanism involving both photo-oxidation of organic matter and hydrophilic removal of dust and powders. More research seems necessary to quantify the self-cleaning effect, both in relation to the characteristics of materials and the local climatic parameters, such as rainfall frequency, relative humidity, and UV intensity.

15.2.2.4 Antimicrobial cement-based materials TiO2 exhibits photoinduced antimicrobial activity that can be exploited not only to fabricate self-disinfecting surfaces for buildings requiring a high level of hygiene (hospitals, schools, etc.) but also to limit biological growth on concrete surface, mainly owing to algae [7,8]. In fact, the growth of biofilm in building surfaces subjected to frequent wetting may cause a loss of esthetic value of concrete and an exacerbation of chemical deterioration mechanisms [7]. Moreover, biofilm contributes to keep the concrete surface region wet, boosting the physical
mechanical deterioration processes related to moisture, such as freeze
thaw cycles. Compared to other applications, less research work has been conducted in the field of antimicrobial concrete, so there is presently a lack of standard protocols to evaluate the light-induced antibacterial activity [7] and it is difficult to compare the performance of different product formulations. However, it was shown that cementitious substrates coated with a 10 wt.% dispersion of TiO2 powder experienced a decrease in the algae growth by 66% with respect to unprotected surface [8]. The doping of TiO2 with noble metals (i.e., Ag, Ni, Pt, Au, Cu, Rh, and Pd), oxides (i.e., ZnO, WO3, SiO2, and CrO3), or nonmetals (i.e., C, N, S, and P) was found to further increase the antimicrobial effectiveness [8].

15.2.2.5 Final remarks Commercial TiO2 products for photocatalytic concrete applications are already well established and available in the market, but the collaborations between industry and academia clearly suggest the need to enhance performance through fundamental understanding of photocatalysis in the cement environment [23].

15.2.3 Use of TiO2 for structural cement-based materials As a composite material, concrete is constituted by a dispersed phase (aggregate particles of different size) bound by a cement paste matrix, also including pores variable in size. The cement paste matrix has a multiphase and nanostructured nature, consisting of amorphous, crystalline, and semicrystalline phases in the micro- to nano-meter scale, and bound water. Calcium silicate hydrate (CsSsH),

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which is the main hydration product of Portland cement and is in the form of an amorphous
semicrystalline gel, and portlandite [Ca(OH)2], which is produced during hydration reactions, are themselves nano-sized materials [4]. Indeed, the specific surface area of CsSsH ranges between 7 3 107 and 7 3 108 m2/g of dry paste [2], while portlandite crystals have a thickness of less than 100 nm [4]. The performance of cement-based materials depends intensively on these nano-scale solids, such as CsSsH, and on the porosity, the latter including nano-sized gel and capillary pores (capillary pores are due to the excess mixing water used to provide concrete with a suitable flowability). Particularly important is the porosity at the interface-transition-zone (ITZ), that is, at the zone between cement paste and aggregate particles. The ITZ is the weakest part in the matrix, thus resulting a critical factor in determining the concrete performance [4]. The incorporation of nanomaterials in cement-based composites may have a remarkable influence on the modification of nano-sized CsSsH, improving the physical
mechanical, durability, workability, and rheological properties [9]; hence, the introduction of several nanoparticles into concrete, including pozzolanic nanomaterials (silica, etc.) and inert nanomaterials (Al2O3, MgO, CuO, ZnO, CaCO3, Fe2O3, nanoclays, nanofibers, graphene oxide, carbon nanotubes, and TiO2), was extensively investigated in recent years [4]. In this context the focus is not on the photocatalytic behavior of TiO2 (which is not exploited in the bulk of concrete, given the absence of light), but on its nanoparticle nature and its interactions with the cement matrix. In structural concrete, as in functional one, well-dispersed nanoparticles in the cement matrix are fundamental to achieve the targeted performances, although some agglomeration to the micro-scale in cement matrix seems unavoidable [9]. Ultrasonication and chemical dispersants have been successfully applied to reduce the size of these agglomerates [9,28]. Superplasticizers have been widely used as dispersing agents for nanomaterials in cement-based materials, by previously investigating the zeta potential of nanomaterial dispersions, and, for example, naphthalene-based superplasticizers were found to exhibit a good capability to disperse nano-TiO2 in water through mechanically stirring for 20 min [9]. It is believed that the effects of adding nano-TiO2 to concrete can be ascribed to two main causes: TiO2 acts as a superb filler and it modifies the cement hydration process [9]. The main effects of TiO2 addition are discussed in the following.

(a) Nano-TiO2 modifies the properties of concrete at the fresh state (rheology and hydration speed) Nano-titania, and nanoparticles in general, provides a seeding surface for the deposition of hydrates (nucleation) and therefore facilitates the hydration of ordinary Portland cement and mineral admixtures, that is, secondary cementitious materials [28,31], with significant implications for the hydration kinetics as well as the microstructure of the hardened cement paste [2]. In fact, when nano-TiO2 is added to the mix, the microstructure of the solid gel changes compared to pure CsSsH, with

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the appearance of small rods that are believed to be the CsSsH growing with the nucleus of TiO2 nanoparticles [2]. A first effect is that cement hydration process is accelerated [2,31,32], as a consequence of the early hydration of CsSsH. Shortening the time to initial set is usually undesirable, as it may interfere with transportation from the batch plant to the working site and with the inspection procedures, but a shorter setting time may be beneficial for rapid construction, for specific applications (e.g., pavements and shotcrete), or at low temperatures (where accelerators are usually required) [28,32]. A second effect is that nano-TiO2 can modify the rheological properties of cementitious materials [2], the fresh concrete mix generally becoming less workable and more cohesive [28]. In self-compacting concrete different percentages (ranging from 1% to 5%) of nano-TiO2 (average size 20 6 5 nm) made workability decrease (slump values, L-box ratios, and V-funnel flow time were reduced by 1.2%
7.5%, 1.3%
10.1%, and 4%
24%, respectively, for the addition of 1% and 5% nanoTiO2 in the mixes), but less bleeding and segregation were also noticed due to the addition of nano-TiO2 [2].

(b) Nano-TiO2 modifies the properties of hardened concrete (mechanical strength and durability) Nanoparticles, by influencing the cement hydration process, can enable the formation of a denser microstructure, thereby improving the mechanical properties and durability of cementitious materials [32]. Despite the extensive research, the exact mechanism governing strength development remains unclear [4]. In general, by expanding the particle size distribution through the addition of nanoparticles, the amount and size of the interstitial pores and voids decrease, probably because nanoparticles fill gaps between larger particles (e.g., cement and mineral admixtures), and therefore densify the packing of the material on the nano- to micro-scale [28] also reducing the ITZ porosity [4]. As a result, the strength increases and the permeability decreases [4,28]. The addition of nano-TiO2 with average particle size of 20 6 5 nm to a concrete mix with water to binder ratio of 0.38 (1%
5% cement replacement by nano-TiO2) made compressive, flexural, and splitting strengths increase with the amount of nano-TiO2, but only up to a substitution equals to 4% [2]. The 28-day compressive strength of cement mortars was found to increase with the percentage of nano-TiO2 (average size 25 nm), up to a 122% and 133% in specimens made with water to cement ratio of 0.4 and 0.6, respectively [2]. The addition of TiO2 was also found to improve the abrasion resistance of concrete [4], which is particularly important for pavements and roads. A reduction in the permeability of concrete was also observed when nano-titania is incorporated, with a correspondingly improved durability. In fact, the permeability reduction causes a lower absorption of aggressive agents, such as water (which is responsible for freeze
thaw cycles and also accelerates carbonation, leading to steel reinforcements corrosion), CO2, chlorides (responsible of the corrosion of steel by pitting), sulfates, and other deteriorating compounds [3]. Some authors showed

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that both total water absorption and capillary water absorption rates of concrete at 90 days curing can be reduced up to 25% when 5% nano-TiO2 is added to the mix [2]. A lower chloride penetration was also found in the same concrete, with expected benefits in terms of corrosion resistance [2]. Nanomaterials can be employed in cement-based materials also to enhance the resistance to crack initiation and propagation due to total shrinkage, which is the sum of drying, plastic, and autogenous shrinkage. This is very important, as the cracks are detrimental for both concrete strength and durability in aggressive environments. The total shrinkage of concrete can be reduced by the addition of nano-TiO2, which is thought to contribute to the reduction of the amount of mesopores (size 1.25
25 nm), playing a significant role in the total shrinkage of the paste [9]. Nano-TiO2 was shown to decrease the autogenous shrinkage (occurring as a result of self-desiccation during the hydration process), while its effect on the drying shrinkage (occurring as concrete dries) is quite contradictory [9].

15.2.4 Patents on cement-based materials with TiO2 The implementation of photocatalytic materials in combination with cement-based composites started in the early 1990s, and the first photocatalytic paving blocks and cement-based materials based on hydraulic binders with TiO2 were patented by Mitsubishi Materials Corporation (Japan) and Italcementi SpA (Italy), basically for self-cleaning and depolluting applications [6,8,24]. A comprehensive overview of the existing patents in the different application areas is provided in Ref. [33].

15.3

TiO2 in geopolymers

Geopolymers (and, more in general, alkali-activated materials, AAMs) are widely considered a third-generation cement, following lime used since ancient times, and ordinary Portland cement used since 19th century. In geopolymerization technology, many different kinds of alumino-silicate materials, such as kaolinite, feldspar, and many industrial wastes, are used as base materials and are activated by alkaline activators, such as sodium hydroxide, potassium hydroxide, sodium silicate, and potassium silicate. The chemical composition, mineralogical structure, fineness morphology, and glassy phase content of the base materials affect their reactivity and hence are key parameters in the development of stable and satisfactory geopolymers [34]. Due to the low temperature process and the use of industrial wastes in the formulation [35,36], the synthesis of geopolymers has significant environmental advantages, such as an 80% or greater reduction in CO2 emission compared to ordinary Portland cement [37]. The good resistance to high temperature and to aggressive environments is further advantage of geopolymers.

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An intensive research work has been carried out to enhance the strength and durability of geopolymers through the addition of various nanomaterials [1,38]. The incorporation of nano-TiO2 was found to accelerate the hydration process, resulting in more hydration products and a denser microstructure, as evidenced by the increase in the concentration of crystal phases at the hardened state [1]. The consequences are an increase in compressive strength and a general improvement of the microstructure, which, in turn, enhances the carbonation resistance of geopolymerbased materials [37,39]. A limited number of studies also investigated the photocatalytic behavior of TiO2-containing AAMs, highlighting good self-cleaning and antimicrobial potentialities, although several aspects still need deeper elucidation. A set of papers investigated the direct introduction of TiO2 nanoparticles in the geopolymer mix, considering formulations based on both metakaolin [40] and different waste materials [38,40
42] (mostly fly ash), with positive results in terms of photoactivity. However, some authors observed that the photocatalytic activity decreased with the number of experimental runs and with ageing of the photocatalytic material [40]. Other authors found that the photocatalytic activity strongly depends on the type of binder and the curing temperature, with a systematic decrease of photocatalytic activity and the occurrence of nonnegligible transport phenomena (and titania segregation) for curing at 60 C [43]. The incorporation of titanium dioxide in a metakaolin-based geopolymer matrix by ion exchange with a titania precursor was also proposed, as an alternative pathway for the preparation of photocatalytic geopolymers [44]. Some recent studies focused on the incorporation of TiO2 nanoparticles to prepare geopolymers with inhibition capacity against the growth of bacteria and algae, with good results [41,45]. In particular, fly ash geopolymer pastes containing 5 wt.% nano-TiO2 allowed one to reduce the algae and fungi formations to 54% and 24%, respectively, with respect to the control geopolymer [41]. Other interesting applications of TiO2 in geopolymers include the deposition of TiO2 coatings over geopolymer substrates [46] and the possibility of using porous geopolymers as supports for photocatalysts [42].

15.4

TiO2 in ceramic tiles

15.4.1 Ceramic tiles production Ceramic tiles largely evolved in the last 10 years, as their production technology faced several changes in order to introduce in the market products esthetically more attractive and more performing from the chemical, physical, and mechanical point of view. In order to understand the challenges that the use of TiO2 in ceramic tiles has to meet, and the importance of the results reached so far, it is thus very important to focus on tile production evolution. The main technological changes have been addressed to obtain tiles with negligible open porosity, in large format (up to 1 3 3 m2) and decorated with digital printing. The

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latter innovation really changed the glazing steps in tiles production, allowing to enormously enlarge the possibility of decorations on tile surface with esthetic effects comparable to those of natural materials (e.g., marbles, stones, and wood). According to ISO 13006 “Ceramic tiles—Definitions, classification, characteristics and marking” [47], tiles classification is based on manufacturing method (extruded tiles and dry-pressed tiles) and water absorption value (named Ev determined according to ISO 10545-3 [48]), thus leading to the groups summarized in Table 15.2. Currently, about 90% of tiles products belong to dry-pressed groups. Italy is the sixth producer in the world with a production of 416 millions of square meters of tiles in 2018 [50] and the second in Europe after Spain; however, the Italian leadership for tiles’ quality, technology, and esthetic innovation is well recognized worldwide. Therefore the Italian trend in the production of the different tile groups, reported in Fig. 15.1, is particularly significant to understand which products are presently most appealing and mostly requested by the global market. It can be observed that porcelain tiles (classified as BIa) are currently the leading products, whereas tiles belonging to other groups (BIb, BII, BIII) have experienced a decrease in their production, today representing only about 15% of the Italian production. Production cycles for the abovementioned groups are carried out differently according to the various raw materials selected for the tile design. Generally speaking, porcelain (BIa) and BIb tiles are single-fired products, which are fired at about 1250 C and 1150 C, respectively, whereas the other groups (BII and BIII) can be produced by single-firing or double-firing processes depending on the manufacturing plant. Single-firing tiles belonging to BII and BIII groups are usually fired in the temperature range of 1050 C
1150 C. In the case of double firing, two clearly

Table 15.2 Classification of ceramic tiles with respect to water absorption and shaping (based on Ref. [49]). Shaping A Extruded

B Dry pressed

Group I Ev # 3%

Group IIa 3% , Ev # 6%

Group IIb 6%Ev # 10%

Group III Ev . 10%

Group AIa Ev # 0.5%

Group AIIa-1a

Group AIIb-1a

Group AIII

Group AIb 0.5% , Ev # 3%

Group AIIa-2a

Group AIIb-2a

Group BIa Ev # 0.5%

Group BIIa

Group BIIb

Group BIIIb

Group BIb 0.5% , Ev # 3% a

Groups AIIa and AIIb are divided into two subgroups (Parts 1 and 2) with different product specifications. Group BIII covers glazed tiles only. There is a low quantity of dry-pressed unglazed tiles produced with water absorption greater than 10% mass fraction, which is not covered by this product group. b

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Titanium Dioxide (TiO2) and its Applications 400

Milion of square meter

350 300 250

Single firing

200

Double firing Porcelain les

150 100 50 0 2009

2010

2011

2012

2013

2014

2015

2016

2017

2018

Figure 15.1 Italian tiles production in the period 2009
18. Source: Data from Confindustria Ceramica (Italian Ceramic Manufacturers’ Association), Indagini statistiche sull’industria italiana, Artestampa Fioranese, Fiorano Modenese, MO, Italy, Anno 2019 [51].

distinguished firing processes are identified: the first firing (at 1050 C
1150 C) where the sintering of the ceramic body occurs, and the second firing (at 500 C
800 C) where glazing/decoration processes are carried out. Such temperature ranges are a function of select body raw materials and glaze compositions.

15.4.2 Exploitation of TiO2 in ceramic tiles The interest of the ceramic tile sector for the use of TiO2 has been mainly addressed to the exploitation of its photocatalytic behavior for the creation of self-cleaning, highly hydrophilic, and antimicrobial surfaces, even if it is well assessed that ceramic surfaces, thanks to their inorganic nature and vitrified surface, are not themselves susceptible to the growth of micro-organisms such as fungi, mold, or bacteria in normal conditions of use (i.e., with regular cleaning operations with detergents). Unlikely to what happens in building materials such as cement and paints where TiO2 is usually not involved in any thermal treatment but it is simply included in the product formulation, the introduction of TiO2 in ceramics shall take into account the firing process, which is fundamental for tile processing. Indeed, there are four polymorphs of TiO2, rutile, anatase, brookite, and TiO2 (B), the most thermodynamically stable of which is rutile. Both brookite and anatase (A) transform into rutile (R) in the temperature ranges of 500 C
600 C and 600 C
1100 C, respectively [52,53]. It is recognized that anatase exhibits the best photocatalytic properties, even if rutile is sometimes considered too [53]. The anatase to rutile transformation is based on a nucleation-growth mechanism: [54] during the thermal treatment, anatase crystallites

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can reach a critical nuclei size after which the rutile nucleation begins [55,56]. The anatase transformation into rutile depends on several factors, such as grain size of starting particles [57], crystallite dimension [58], atmosphere [59], and the presence of other atoms acting as dopants [60]. Anatase polymorphic transformation to rutile is thus a critical step when the ceramic process is considered, as the temperature range in which such transformation occurs is always included in the thermal history of ceramic body sintering process and can also be involved in thermal treatments for glazing and decoration. Thus the research for TiO2 exploitation in ceramic tiles has been mainly addressed to designing TiO2-based coatings/surface treatments (as in the examples of Figs. 15.2 and 15.3), although some research has also been carried out, including TiO2 in the ceramic body [61]. Figs. 15.2 and 15.3 represent a cluster of TiO2-rich spherical particles resulting after a sol
gel method application and agglomerated TiO2 nano-particles adhering to the ceramic surface by means of a low melting enamel, respectively. The functionalization of ceramic tile surfaces with TiO2 surface treatments has to tackle the following challenges: 1. 2. 3. 4.

the formulation of the treatment and relevant application method on ceramic tile surface; the firing temperature in order to bond the surface treatment to the ceramic body; the photocatalytic activity of the final product under UV and visible light; and the durability of the photocatalytic activity of the final product.

Pure TiO2 powders, with different content of anatase (85%
100%) and anatase crystallite size in the range of 23
140 nm, and a titania nanosuspension (100% anatase

Figure 15.2 Left: SEM micrograph of ceramic tile surface with a TiO2-based coating obtained by sol
gel method. Right: EDS microanalysis of the surface on the left.

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Titanium Dioxide (TiO2) and its Applications

Figure 15.3 Left: SEM micrograph of ceramic surface with titania powder coating adhering to the tile by means of a low melting enamel. Right: EDS microanalysis of the surface on the left.

with crystallite of 20 nm) were tested in Ref. [61]. In order to promote the stability of anatase phase after firing at high temperature (900 C, 1000 C, and 1200 C), TiO2
SiO2 mixes with the following stoichiometry Ti12xSixO2 with x 5 0, 0.1, 0.3, and 0.5 apfu were tested in Ref. [62]. It was discovered that no solid solution between crystalline TiO2 and SiO2 occurs, anyway silica addition shifts the polymorphic transformation A!R to higher temperatures, thus allowing anatase retention. In the investigated systems, the amorphous phase surrounding anatase/rutile particles decreased the photocatalytic performances and the best behavior was observed for firing at 900 C and 1000 C. An amount of 5 mol.% Nb2O5-doped-TiO2 nanoparticles were sprayed onto ceramic tiles creating, after firing at 600 C, 800 C, and 900 C, thin films with a thickness of about 250 nm and with self-cleaning properties [63]. According to literature data, the spray technique provides surface coatings having thickness from 0.1 to 3 µm [64
66]. A thin layer is advantageous, as it involves the use of less nanoparticles and, more important, it allows the formation of a transparent film with photocatalytic properties maintaining the esthetic characteristics of the ceramic tile. The introduction of Nb2O5 in the TiO2 lattice can stabilize the nanoparticles increasing the A!R transformation temperature of about 200 C. Another example of photocatalytic coating prepared by sol
gel method and applied on glazed ceramic tiles is based on TiO2/ZnTiO3 [65]. In this case, the subsequent thermal treatment at 800 C promotes a coating having thickness of 443 nm, tightly integrated with the glaze layer and with a higher photocatalytic activity than that of pure anatase TiO2.

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Finally, Ni-doped TiO2 applied by spray coating method and thermally treated at 550 C and 650 C has shown photocatalytic effect under visible light [64]. The UVsvis absorbance spectra show that the Ni-doped TiO2 thin film has a wavelength of maximum absorbance higher and a bandgap lower than the undoped TiO2. Although the state of the art of TiO2-based coatings for ceramic tiles is extensively developed, the durability issues are rarely taken into account, thus evidencing an important lack of knowledge. Moreover, in the research papers focused on TiO2-based coatings applied on ceramic tiles [61,63,65,66], the antibacterial activity is not usually ascertained, thus limiting their technological transfer. Nevertheless, it is well known that TiO2 nanoparticles are particularly attractive as antibacterial agents, even if their mechanism of attaching and destroying the bacterial cells is a topic still under discussion. Very recently, also the antiviral characteristics of colloidal TiO2 nanoparticles synthetized by sonochemical method [67] have been tested against Newcastle Disease Virus (NDV) that severely affects poultry. The results are promising, as antiviral activity was shown at a minimum dose of 6.25 µg/mL.

15.4.3 International patents on photocatalytic ceramic tiles Since 1997, when TOTO Ltd., Japan, discovered in its laboratories the photocatalytic phenomenon of photoinduced hydrophilic effect, many patents have been deposited by TOTO itself to cover a broad range of areas from the basic invention to the applied use of photocatalyst technology. Concerning photocatalytic building materials and in particular ceramic tiles, TOTO developed Hydrotect coating. Patents regarding Hydrotect technology that are owned by TOTO Ltd. (Japan), such as WO2001068786A1 (Hydrophilic member and method for manufacture thereof) [69], are granted after a license agreement to several ceramic tile companies. The TOTO Ltd. alone was found as applicant in more than 70 approved European and international patents on photocatalytic tiles and ceramic materials [70]. After these patents, other specific ones have been registered on photocatalytic coatings for ceramic tiles, such as WO2004094341A1 (A procedure for the realization of ceramic manufactures, in particular, porcelain stoneware tiles and trim pieces, with antipollution and antibacterial properties and products thereby obtained) [71] and WO20101464108 (Photocatalytic ceramic article and method for its production) [72]. The main differences among these patents essentially concern the application technology. Hydrotect coatings are produced by a sol
gel method and subsequent calcination at relatively low temperature (around 500 C). Other coatings are obtained by sol
gel process followed by calcination at relatively high temperature (around 1100 C) [71]. Other commercial photocatalytic tile surfaces are produced by using micrometric titania powder that adheres to the tile by means of a low melting enamel (around 700 C) [72].

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Titanium Dioxide (TiO2) and its Applications

15.4.4 Standards Six different ISO standards are presently available for fine ceramics (advanced ceramics, advanced technical ceramics) in order to evaluate (1) antibacterial activity of semiconducting photocatalytic materials (ISO 27447); [73] (2) photocatalytic activity of surfaces in an aqueous medium by degradation of methylene blue (ISO 10678); [74] (3) air-purification performance of semiconducting photocatalytic materials by the determination of removal of nitric oxide (ISO 22197-1) [75], acetaldehyde (ISO 22197-2) [76], and toluene (ISO 22197-3) [77]; and (4) self-cleaning performance of semiconducting photocatalytic materials by means of measurement of water contact angle (ISO 27448) [78]. The abovementioned standards are usually adopted for testing ceramic tiles too, however, as a standardized method to evaluate antibacterial activity for photocatalytic ceramic tile surfaces is needed, a new draft, ISO/DIS 17721-2 [79], specifically addressed to ceramic tile, is currently under development in the ISO Technical Committee 189 “Ceramic tiles” (ISO Working group 8: Antimicrobial properties of ceramic tile surfaces). The main differences between ISO 27447 and ISO/DIS 17721-2, under development, are as follows: G

G

G

Only the film adhesion method is reported in ISO/DIS 17721-2, thus removing glass cover method present in ISO 27447. Only 351BLB (black light blue) lamp is considered in ISO/DIS 17721-2 as UV light source, whereas in ISO 27447 light source is black light (BL) blue (BLB) or BL lamp with a wavelength of 351 nm. Antibacterial activity is expressed differently. Even if the antibacterial activity in both the standards is calculated on logarithm values of number viable bacteria in nontreated and treated specimens, under UV irradiation and in dark conditions, in ISO 17721-2 antibacterial activity can also be expressed in terms of percent reduction of recovered bacteria from photocatalytic treated test specimens after UV irradiation compared to that of recovered bacteria from photocatalytic treated test specimens in dark or nontreated test specimens in dark.

15.5

TiO2 in cultural heritage conservation

The use of photocatalytic TiO2 nanoparticles has been recently proposed for surface treatment of materials in cultural heritage, to fight the deterioration processes affecting stones, mortars, and bricks in polluted environments [80] (Fig. 15.4). The self-cleaning ability of titania is expected to actively reduce the local concentration of aggressive pollutants (mainly NOx and SOx, the latter causing the so-called black crusts) near the surface, to hinder biodeterioration, and to promote the removal of dust and dark deposits by rain. The application of TiO2 in the cultural heritage field is quite complex, because it is necessary to ensure that the conservation materials exhibit a good (esthetic, physicmechanical, chemical, etc.) compatibility and a satisfactory durability, avoiding any kind of defects. The esthetic constrain is particularly severe, as the white color of titania might induce unacceptable color changes; hence, the amount of nanoparticles deposited on the surface must be kept necessarily very limited.

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469

Figure 15.4 Dark layer of soiling and black crusts, over a limestone bas-relief exposed to polluted urban environment.

In the application of surface coatings to natural and artificial stones, two main approaches are followed, the first one involving the dispersion of titania nanoparticles in water repellents [81] and the second one involving a direct application of the nanoparticles to the surface [82], possibly with the aid of inorganic coupling phases. In the first approach the surface is expected to become superhydrophobic by exploiting the surface roughness provided by the nanoparticles, but many authors highlighted that titania catalyzes the degradation of the organic polymer matrix [83]. According to the second approach, colloidal aqueous suspensions of TiO2 nanoparticles (usually with the maximum concentration equals to 1 wt.%) are applied by spraying onto the surface of materials. The results obtained in terms of photocatalytic activity, usually evaluated through the discoloration of an organic dye and/or NO concentration reduction, are very encouraging, but three critical aspects must be considered. The first one is that the photocatalytic effectiveness depends on the porosity and roughness of the substrate [82], the second is that the hydrophilicity induced in the original material could increase the capillary absorption of water and hence trigger other decay processes, and the third one is the possible leaching of TiO2 nanoparticles, if not effectively attached to the surface, by rain [84]. Laboratory studies showed that stones treated with nano-TiO2 aqueous suspensions alone suffered a severe particle removal by simulated rain [84,85] and some studies performed on site confirmed that nano-TiO2 is removed by rain after shorttime exposure in the field [85]. Several routes were recently proposed to improve the adhesion of TiO2 nanoparticles to the original substrate materials, such as the combination of TiO2 with hydroxyapatite [86], the dispersion of titania inside a silica matrix [87,88], or the use of a silicatic coupling agent [85] (Fig. 15.5). Further studies were addressed to modify the structure and the composition of TiO2 in order to improve its

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Titanium Dioxide (TiO2) and its Applications

photocatalytic efficiency under solar light, for instance by increasing the oxide surface area, by harvesting the solar light through doping with transition metal ions (Cr31, Fe31) and nonmetal atoms (N, S, and C), by surface functionalization with suitable organic sensitizers, and by developing highly efficient nano-TiO2-based dispersions by nonaqueous synthesis in benzyl alcohol [88]. Trial testing about the effectiveness and durability of TiO2-based treatments in heritage materials are presently running in some monumental buildings [89], although the results are still quite limited. Beside the introduction of TiO2 nanoparticles in coatings, their incorporation into lime mortars and plasters to be used in the conservation of historical buildings and artifacts was proposed [90]. In particular, iron-doped TiO2 nanoparticles, ranging in Fe concentrations from 0.05 up to 1.00 wt.%, were synthesized by sol
gel and introduced in calcium hydroxide pastes, showing good photocatalytic performance and an enhancement of carbonation at both early and later stages [91]. This latter aspect is extremely important, as the slow hardening process of lime-based products is a drawback in their exploitation in the restoration and conservation fields.

15.6

Environmental and health concerns in the use of TiO2 in building materials

The benefits deriving from the incorporation of photocatalytic TiO2 into building materials are several and were briefly summarized earlier. The depolluting effect provided by titania brings obvious health and environmental benefits, and the reduction in the proliferation of fungi and bacteria, especially in indoor spaces, involves a further health benefit, due to the reduction of the associated mycotoxins [10]. The self-cleaning behavior of surfaces and the increased durability of cement-based materials turn out into a longer service life of buildings and structures, allowing to reduce the economic and environmental costs associated with maintenance and repair. In the case of heritage buildings, the use of self-cleaning coatings may help in preserving cultural heritage against deterioration and loss, thus contributing to preserve our national identities and economic prosperity, given the fact that tourism is a significant wealth source in many countries. Beyond the current excitement about the possibilities offered by photocatalytic building materials, there are reasonable concerns about unintended consequences [11]. The small size of nano-TiO2 and nanoparticles in general involves an environmental threat as they can be easily dispersed into the air and inhaled by living organisms [3]. Due to their larger surface area per mass unit compared to bigger particles, nanoparticles can harm human body through inhalation and absorption; the effect of nanoparticles could also affect ecological systems more easily and be more biologically active [1]. The risks may occur due to the exposure of workers, but also due to possible accidental release of the nanoparticles from building materials into air and water [1]. Leaching of nanoparticles that are not well incorporated

Figure 15.5 Istrian stone sculpture by Alfonso Leoni in the Department of Mathematics in Bologna (20th Century). The building is located in a zone with heavy traffic. (A) The sculpture immediately before the conservation intervention carried out in 2009 (darkening and deterioration are visible); (B) a detail of the sculpture immediately after the conservation intervention, which consisted in cleaning, consolidation, and spray application of a nanoTiO2 dispersion over a silicate coupling agent; (C) the same detail in 2012, when the stone appears unaltered, without deposits or darkening. Source: Details of the study are in E. Franzoni, R. Gabrielli, E. Sassoni, A. Fregni, G. Graziani, N. Roveri, et al., Performance and permanence of TiO2-based surface treatments for architectural heritage: some experimental findings from on-site and laboratory testing, in: Science and Art: A Future for Stone, Proceedings of the 13th Int. Congress on the Deterioration and Conservation of Stone, Paisley, UK, 6
10 September 2016, Volume II, pp. 761
768. Courtesy of Leonardo s.r.l., Bologna, Italy.

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Titanium Dioxide (TiO2) and its Applications

or adherent to the material surface can occur due to rain and wind [11]. The release of nanopowders may be due also to abrasive actions, such as in the case of roads, to construction and maintenance works and to demolition activities. During the treatment of demolition waste, crushing, landfarming, landfilling, and incineration could be the prevalent routes for the environmental release of nanoparticles [11]. Nano-TiO2 particles gave rise to some health problems during packing and production and it was believed that ongoing exposure to TiO2 causes cancer and inflammatory effects [4]. A risk assessment worksheet on nano-TiO2 released by DuPont showed that occupational exposure exceeded the acceptable limit in the packaging process [11]. Nanoparticles released from construction materials could pose a toxicological risk to microorganisms (which provide valuable ecosystem services, including primary productivity, nutrient cycling, and waste degradation), as well as to higher organisms via multiple mechanisms. TiO2 irradiated with UV light or sunlight produces ROS (reactive oxygen species), which causes inflammation, cytotoxicity, and DNA in mammalian cells, although the uptake by cell membranes may significantly differ depending on the morphology of TiO2 [11]. Symptoms from the exposure to nanomaterials such as nano-titania have been reported by several authors [2,3], and it was shown that the nano-sized particles of TiO2 have higher toxicity than the micro-sized ones. However, the toxicity of nanomaterials depends on a number of factors such as particle size, surface area, crystallinity, surface chemistry, and particle agglomeration tendency [1]. Different authors studied the effects related to the inhalation of TiO2 particles with a primary particle size ranging from 2 to 5 nm, reporting lung inflammation for a concentration of 8.8 mg/m3, so they recommend that the use of nanoparticles should be made with the same care already used for materials of unknown toxicity [10]. On the other hand, studies carried out in the field of coatings for surface protection of cultural heritage pointed out that TiO2 nanoparticles do not exhibit cytotoxic effects at the concentration used in that field but also highlighted the need of further studies [89,92]. The International Agency on Research on Cancer classified titanium dioxide in the Group “2B—possibly carcinogenic to humans,” as there is “inadequate evidence in humans” and “sufficient evidence in experimental animals,” pointing out the urgent need for research in this field. As far as indoor application is concerned, whether it is safe to apply photocatalytic nanomaterials is still an open question, in the light of the possible risks related to nanoparticles inhalation and health effects of by-products formed in incomplete photo-oxidation [7]. For the previous reasons, researchers, manufacturers, consumers, and government agencies have raised concerns regarding the health effects of engineered nanoparticles [1], and many authors suggested that construction materials should be designed to be “safe,” for example, by using appropriate durable coatings during manufacturing, improving matrix stability to minimize nanoparticles leaching, and adopting controlled construction and careful disposal practices and guidelines [11].

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TiO2 nanoparticles in self-cleaning fac¸ades and pavements should be strongly bound to the matrix and prevented from dispersing in the air or entering stormwater streams [11]. The environmental impact of titanium dioxide manufacturing is another aspect to consider. The high embodied energy and emissions connected with TiO2 manufacturing processes were highlighted by some authors, although they also pointed out that photocatalytic reactivity of TiO2 could offset initial higher environmental impacts in a life cycle perspective [93]. During the production of the white pigment TiO2 (rutile), SOx, sulfate waste and chloride waste are generated, and in fact the European Directive for the Ecolabel labeling of indoor paints and varnishes fixed limits for these kinds of waste during the production of TiO2-based pigments. However, titania can also allow the production of concrete with higher performance, thus allowing a considerable saving of material during construction; hence, a multicriteria analysis would be necessary to evaluate both the positive and negative environmental effects deriving from the use of photocatalytic TiO2. Notably, alternative production processes may be used for manufacturing TiO2 nanoparticles with lower embodied energy with respect to the current ones. Some authors also proposed the incorporation of waste latex paints (TiO2 content 2.5%
15%) into cement pastes [4], while others suggested the recovery of TiO2 from bauxite ore residues [94], improving the environmental sustainability of TiO2 employment. The current studies suggest that there is a strong need of research to support safe design, production, use, and disposal practices and associated recycling, reuse, and remanufacturing, in order to enhance the sustainability of nanotechnology in construction applications [11].

15.7

Conclusion and perspectives

Commercial TiO2 products for photocatalytic concrete applications and antibacterial ceramic tiles are already well established and available in the market. TiO2-based surface treatments for heritage material protection are commercially available too, although the data about their performance in the field are still limited. However, the publications discussed in this chapter and the collaborations between industry and academia clearly suggest the need to enhance the performance of TiO2 through fundamental understanding of photocatalysis in the environment of each specific materials. The interactions of TiO2 nanoparticles with the cement matrix and with the ceramic body or enamel have been investigated in many literature papers, but a deeper understanding seems necessary to develop fully efficient functional surfaces, able to provide buildings and structures with durable self-cleaning, depolluting, and antibacterial performances. The interactions of titania-embedding materials with the surrounding environment have been investigated as well, through on-site testing and monitoring campaigns, but also in this case the collection of more field data seems necessary, in order to be able to disclose and exploit the potential of these materials in the different climatic and environmental contexts.

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Titanium Dioxide (TiO2) and its Applications

The use of TiO2 has been also proposed as a component in the mix of cementitious and geopolymeric materials, thus exploiting the nanoparticle nature of this material rather than its photoactivity. It was shown that an increase in mechanical strength and durability can be achieved, but the results obtained are variable and several alternative nanoparticles have been proposed for the same purpose, thus the advantages and drawbacks of such addition should be further investigated, also in a life cycle analysis perspective. Finally, the possible health and environmental effects deriving from the processing, application, and disposal of building materials incorporating TiO2 nanoparticles are still under investigation. So far, almost all the literature studies highlighted the importance of a strong adhesion between titania nanoparticles and substrates, preventing nanoparticles from being released due to rain, wind, and maintenance operations.

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122. [41] S. Tuntachon, K. Kamwilaisak, T. Somdee, W. Mongkoltanaruk, V. Sata, K. Boonserm, et al., Resistance to algae and fungi formation of high calcium fly ash geopolymer paste containing TiO2, J. Build. Eng. 25 (2019) 100817. [42] X. Yang, Y. Liu, C. Yan, R. Peng, H. Wang, Geopolymer-TiO2 nanocomposites for photocatalysis: synthesis by one-step adding treatment versus two-step acidification calcination, Minerals 9 (11) (2019) 658. [43] A. Strini, G. Roviello, L. Ricciotti, C. Ferone, F. Messina, L. Schiavi, et al., TiO2-based photocatalytic geopolymers for nitric oxide degradation, Materials 9 (7) (2016) 513. [44] J.R. Gasca-Tirado, A. Manzano-Ramı´rez, P.A. Vazquez-Landaverde, E.I. Herrera-Dı´az, ´ valos, et al., Ion-exchanged geopolymer for photocaM.E. Rodrı´guez-Ugarte, J.C. Rubio-A talytic degradation of a volatile organic compound, Mater. Lett. 134 (2014) 222
224. [45] R.M. Gutie´rrez, M. Villaquira´n-Caicedo, S. Ramı´rez-Benavides, M. Astudillo, D. Mejı´a, Evaluation of the antibacterial activity of a geopolymer mortar based on metakaolin supplemented with TiO2 and CuO particles using glass waste as fine aggregate, Coatings 10 (2) (2020) 157. [46] L. Chen, K. Zheng, Y. Liu, Geopolymer-supported photocatalytic TiO2 film: preparation and characterization, Constr. Build. Mater. 151 (2017) 63
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[50] World Production and Consumption of Ceramic Tiles, seventh ed., ACIMAC Association of Italian Manufacturers of Machinery and Equipment for Ceramics (Ed.), 2019. [51] Confindustria Ceramica (Italian Ceramic Manufacturers’ Association), Indagini statistiche sull’industria italiana, Artestampa Fioranese, Fiorano Modenese, MO, Italy, Anno 2019. [52] J.-G. Li, T. Ishigaki, Brookite!rutile phase transformation of TiO2 studied with monodispersed particles, Acta Mater. 52 (2004) 5143
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1012.

TiO2 oxides for chromogenic devices and dielectric mirrors

16

Alessandro Cannavale1 and Giovanni Lerario2 1 Department of Sciences in Civil Engineering and Architecture, Polytechnic University of Bari, Bari, Italy, 2 CNR Nanotec, Institute of Nanotechnology, Lecce, Italy

16.1

TiO2 in electrochromic devices

The wide use of TiO2 ranges from pigments [1
3] and sunscreens [4,5] to paints [6,7], cosmetics [8,9], toothpaste [10,11], water splitting [8,12], photocatalysis [13
15], photovoltaics [16,17], and electrochromic (ECs) [18]. A strong impetus on research dealing with TiO2 has been given by the growing sector of nanoscience, reporting new, emerging properties of materials in the nanoscale due to shape and size, which strongly influence phonon and photon transport and surface-to-volume ratio. Large surface area dramatically affects the interfacial behavior and influences all the reactions in which this material is involved. TiO2 can show different structural arrangements. Anatase [19], brookite [20,21], and rutile [22,23] have been widely investigated for their performances. In each of these configurations, the Ti41 ions are surrounded by six O22 atoms, resulting in a TiO6 octahedron. Rutile shows slight orthorhombic distortion, whereas anatase shows a significant distortion and, consequently, a different symmetry [24]. As reported by Chen and Mao [17], rutile is the stable phase at higher temperatures; anatase and brookite are common in nanoscale samples, with a crystal structure that strongly depends on the preparation method. They also reported that anatase is a stable phase for nanoparticles smaller than 50 nm and can be transformed into brookite at temperatures higher than 973K. Larger bandgaps were reported for amorphous TiO2, compared to anatase, whereas they were narrower for rutile. It was observed that films deposited in oxidizing conditions are generally nonabsorbing [18]. Electrochromism is the ability of a material to undergo color modulation upon reversible redox reactions, due to the application of an external voltage. Spectral modulation of EC materials allows a smart tuning of the energy throughput in glazing, especially with reference to the visible and infrared wavelength ranges. In this way, a dynamic modulation of contrast may effectively reduce cooling loads in summer and enhance indoor visual comfort as well [25
28]. The EC properties of transition metal oxide have been thoroughly investigated by Granqvist in Ref. [18]. These materials show persistent and relevant modulation of transmittance, even at small voltages. Even if tungsten oxide is still the most widely investigated EC Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00003-1 © 2021 Elsevier Inc. All rights reserved.

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oxide, TiO2 is also a well-known cathodic coloring material. In fact, TiO2 shows higher coloration times and lower coloration efficiency compared to WO3 [29]. TiO2 films can be exploited in EC devices as cathodic materials, as a suitable intercalation host for small cations, such as H1 or Li1 [18]. The TiO2 film appears transparent in its as-prepared form and absorbing in the hydroxylated (or lithiated) form, as reported in the following, reversible redox reactions: TiO2 1 xH1 1 xe2 2TiO22x ðOHÞx TiO2 1 xLi1 1 xe2 2Lix TiO2 Such a modulation generally takes place in terms of absorbance modulation, rather than reflectance [30]. Several works have reported effective and fast modulation of absorbance (up to 90%) throughout the visible and infrared regions, due to lithium intercalation in the anatase lattice [17].

16.1.1 Deposition techniques An alternative kind of electrochromism was observed in nanocrystalline TiO2 electrodes modified with viologens and anthraquinones. Sung et al. reported the performance of viologen-modified periodic mesoporous nanocrystalline anatase [31]. Vlachopoulos et al. [32] studied viologen-modified TiO2 films with thickness ranging from 2 to 10 μm and high surface area (80
100 m2/g and porosity higher than 50%). Those films showed transmittance higher than 90% and were compatible with chromogenic uses. In this case a reflectance contrast ratio of 5 was observed, around 600 nm. Giannuzzi et al. [33] observed that among TiO2 polymorphs, monoclinic-phase TiO2(B) can be a promising candidate for EC use due to its fast lithium loading/extraction properties. Its crystal lattice shows a more open structure than anatase and rutile, with available channels for lithium intercalation/deintercalation. A coloration time of about 5 s was observed in EC devices embodying films of nanostructured TiO2(B), showing appreciable modulation of optical density starting from 0.5 V, with complete coloration at 2.0 V (Fig. 16.1). EC devices based on one-dimensional (1D) brookite nanoneedles (650 nm long and 30 nm wide) were reported by Patil et al. Such TiO2 material was grown on Indium Tin Oxide (ITO) substrates by means of hot-filament metal-oxide vapor deposition. Optical contrast of 67% and a coloration efficiency of 226 cm2/C were observed, confirming their suitability for the fabrication of TiO2-based smart windows (Fig. 16.2). TiO2 can be fabricated adopting different techniques: physical vapor deposition, chemical vapor deposition [34
36], spray pyrolysis [37
39], wet chemical techniques [40,41], doctor blading [42], and anodization [43]. A typical process is represented by reactive electron-beam evaporation or evaporation with concomitant exposure to an oxygen ion beam. Evaporated TiO2 films show an amorphous structure with grain size of about 1 nm in XRD analysis, if the substrate temperature was lower than 300 C. Anatase films were reported, on the other hand, for deposition (or annealing) at substrate temperature higher than 400 C.

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Figure 16.1 Transmittance spectra of mesoporous of anatase nanorods-based electrodes (above, left) and TiO2(B) nanorods-based electrodes (above, right) at different bias voltages. (below) TEM images of anatase nanorods (a) and of TiO2(B) nanorods (b) and scanning electron microscopy images of the same materials, respectively (c) and (d), in the same order. TEM, Transmission electron microscopy. Source: Adapted with permission from R. Giannuzzi et al., ACS Appl. Mater. Interfaces 6 (3) (2014) 1933
1943. ©2014, American Chemical Society.

Sorar et al. [44] reported the properties of 200-nm-thick TiO2 films, obtained by reactive dc magnetron sputtering, with a precise study dealing with the effect of deposition parameters on the EC properties of films. They reported that the Argon sputter gas pressure and the oxygen-to-argon ratio affected the coloration efficiency (Fig. 16.3). In detail, the most interesting results were obtained for pressures higher than 15 mTorr, showing coloration efficiency of about 24 cm2/C. On the other hand, the highest performance was obtained for films deposited with low substrate temperatures (25 C).

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Titanium Dioxide (TiO2) and Its Applications

Figure 16.2 Cyclo voltammetric graph for the TiO2 nanoneedles/ITO thin film upon coloration and bleaching, with a scan rate of 20 mV/s (A). Optical transmittance spectra showing colored/bleached states, in the range of 250
800 nm (B). Source: Reprinted from R. A. Patil et al., Efficient EC smart windows of one-dimensional pure brookite TiO2 nanoneedles, Solar Energy Mater. Solar Cells 147 (2016) 240
245. ©2016, with permission from Elsevier.

Figure 16.3 Spectral data reporting transmittance and reflectance of sputtered TiO2 films adopting different values of gas pressure (A) and temperature (B). Source: Reprinted from I. Sorar et al., Electrochromism of DC magnetron sputtered TiO2 thin films: role of deposition parameters, Solar Energy Mater. Solar Cells 115 (2013) 172
180. ©2013, with permission from Elsevier.

In a further work, the same group investigated the effect of thickness on the EC properties of films obtained by dc magnetron sputtering. They found out that 400nm-thick films reported the best coloration efficiency of 26.3 cm2/C. In this case, the substrate temperature was kept at 25 C and the chamber pressure was 25 mTorr, with an oxygen/argon ratio of 0.04. TiO2 can also be deposited by chemical solution deposition, starting from a solution of titanium diisopropoxide bis(acetylacetonate) and 2-methoxyethanol, with a volume ratio 1:3 [39]. The solution was deposited by spin-coating in two steps at

TiO2 oxides for chromogenic devices and dielectric mirrors

487

different rotation speed and time. The fabrication of thin films was then completed by final heating at 120 C for 10 min. Those films showed amorphous or anatase structure, according to the heating temperature. It was observed that the contrast at 550 nm increases with thickness: 28% for 0.1-μm-thick films and 35% for 0.3-μm-thick films. The latter, on the other hand, were fairly less transparent, in the bleached state, due to their thickness. The sol
gel method is a versatile alternative to obtain ceramic materials and oxide coatings [45,46]. A sol is typically formed after hydrolysis of specific precursors, such as metal akoxides M(OR)4, as follows: Hydrolysis: MðORÞn 1 nH2 O ! MðOHÞn 1 nROH Condensation: MðOHÞn ! MOn=2 1


n 2

H2 O

The coloration depends on the selected precursor: gray-colored states are obtained using titanium tetra-n-butoxide Ti(OBun)4, whereas blue-colored states can be obtained by adding acetic acid to the alkoxide precursor before the hydrolysis takes place [47]. After complete evaporation of solvents and polymerization, a gel is obtained. The obtained xerogels show amorphous structure by X-ray diffraction analysis but crystallize into TiO2 anatase around 673K [47]. TiO2 materials obtained by means of sol
gel technique can be deposited by spin-coating or dipcoating as well, even at large area. Drying and heat treatments eventually lead to the production of a dense ceramic material. Mesoporous and nanoporous TiO2 materials have been obtained, for example, using a hydrothermal method using tetraisopropoxide as a precursor [17]. Dinh et al. [29] reported the properties of TiO2 films obtained by a sol
gel dipping method. Coloration of these TiO2 films, deposited on a transparent conductive oxide, such as indium tin oxide, was obtained using an electrolyte solution of propylene carbonate, containing a LiClO4 salt. A mesoporous composite material, including TiO2 and reduced graphene oxide, was reported in 2018 by Zhi et al. [48]. They adopted an electrophoresis approach to obtain the novel film, showing better intercalation reversibility of lithium ions, larger specific surface area, and higher diffusion ion rate than pure TiO2. A solid-state TiO2-based EC device with high coloration efficiency in the infrared region (79.4 cm2/C at 1000 nm, with a bias of 1.6 V) and transmittance modulation of 27.3% (at the same wavelength) was recently reported by Zhang et al. [49]. They fabricated a solid-state device embodying TiO2 and a solid polyelectrolyte; both were obtained by the sol
gel technique. The polyelectrolyte, after the injection between the two substrates, was then cured for 12 h at 80 C, in a furnace. The transmittance modulation in the visible range was lower: about 16% at 560 nm. This point highlights the importance of the EC properties of TiO2, especially in the near-infrared range.

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Titanium Dioxide (TiO2) and Its Applications

Figure 16.4 Different electrochemical states of the device in (A) as-prepared, (B) colored (partially reduced), and (C) highly reduced conditions. Source: Reprinted from H. N. M. Sarangika et al., Low cost quasi solid state electrochromic devices based on F-doped tin oxide and TiO2, Solar Energy Mater. Solar Cells 23 (2020). ,https://doi.org/10.1016/j.matpr.2019.07.585.. ©2020, with permission from Elsevier.

Sarangika et al. [50] have recently reported an EC device embodying TiO2 on the work electrode of a quasi-solid-state device. The film was made by doctor blade technique, using a paste that was subsequently sintered at 450 C. They obtained films that were transparent, as prepared (Fig. 16.4), undergoing coloration as the consequence of simultaneous insertion of electrons and lithium ions, effecting a reduction of Ti41 to Ti31 and resulting in film coloration. Full coloration was obtained at 3.7 V in a device with the following architecture: Fluorine doped Tin Oxide (FTO) /8.1-μm-thick TiO2/polyethylene electrolyte/FTO. A contrast as high as 61.82 and a coloration time of 61.8 s were reported. Wu et al. [51] prepared a TiO2 nanoparticle dispersion and deposited it on an FTO conductive substrate by dip-coating at different lifting rates (1000, 2000, and 3000 μm/s). The as-obtained films were then sintered at 500 C for half an hour. Different sizes of nanoparticles were used in this study. Higher contrast was obtained with smaller nanoparticles, with red-shifts of reflectance peaks, depending on TiO2 nanoparticle size. A similar trend was observed for measurements of coloration efficiency, resulting 27.0, 20.7, and 16.9 cm2/C, for films obtained using nanoparticles with sizes of 5, 40, and 100 nm, respectively. They also reported the dependence of EC properties on the thickness and roughness of deposited films, strongly influenced by the dip-coating procedure.

16.2

TiO2 in photo-electrochromic devices

A photo-electrochromic (PEC) [52] device changes color upon light absorption, whereas ECs need an external bias to affect the same transmittance modulation. The first scientists who suitably coupled a TiO2-based ruthenium polypiridine-sensitized

TiO2 oxides for chromogenic devices and dielectric mirrors

489

nanocrystalline electrode to an evaporated WO3 EC film, on a counter electrode, were Bechinger et al. [53]. Unlike photochromic [54
56] films, generally used in lenses for sunglasses, in which light detection and coloration process take place within the same material, in PEC devices light absorption and coloration are two distinct processes. The dye-sensitized TiO2 semiconductor electrode plays the role of a photoanode, as it does in Gratzel cells [16,57,58]: it produces the photovoltage required to color the EC cathode. In short-circuit conditions and exposed to sunlight, electron move toward the counter electrode, via an external circuitry. The latter electrode, negatively charged, activates the insertion of lithium cations from the electrolyte. This simultaneous phenomenon activates reversible coloration of WO3. In the first PEC device the two electrodes were bonded using a Surlyn sealant and the gap was filled using a liquid electrolyte, containing iodine ions in propylene carbonate. Such ions were aimed at re-reducing the oxidized dye, activating an electron transfer from I2 in the solution. As explained by Gregg [59], in these first “separated architectures” for PEC devices, the I2 required in the electrolyte for high-power dye-sensitized cells was eliminated, not only to maximize photovoltage but also to increase the transparency of the device. Several materials and architectures have been reported in literature, for PEC cells. Li et al. [60] used polyaniline as EC material, with an opposite behavior compared to WO3, undergoing fair bleaching under illumination in short-circuit conditions. Afterward, Hsu et al. adopted an organic sensitizer for TiO2 and PProDot-Et2, as EC material. Researchers from the Fraunhofer Institute, in Germany, changed the device architecture, depositing both the TiO2 and the EC layer, on the same conductive substrate [61
63]. Only a thin layer of platinum was then deposited on the counter electrode, to catalyze the bleaching process in short-circuit conditions. Under 1-sun illumination, a contrast of 41% was measured, showing a complete kinetics in a couple of minutes (Fig. 16.5).

Figure 16.5 Coloration in open circuit (upper part) and bleaching in short circuit (lower part) of the new PEC device (left). Transmittance spectra of the PEC device in the colored and in the bleached states (right). PEC, Photo-electrochromic. Source: Reprinted from A. Hauch et al., New photoelectrochromic device A, Electrochim. Acta 46 (2001) 2131
2136. ©2001, with permission from Elsevier.

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Titanium Dioxide (TiO2) and Its Applications

Figure 16.6 SEM image of the layer configuration in PEC device. The upper part of the figure shows coloring processes, whereas bleaching is achieved under short-circuit conditions and is presented in the lower part of the figure. PEC, Photo-electrochromic; SEM, scanning electron microscopy. Source: Reprinted from U. O. Krasovec et al., Performance of a solid-state photoelectrochromic device, Solar Energy Mater. Solar Cells 84 (2004) 369
380. ©2004, with permission from Elsevier.

In 2005, Liao et al. reported a device with the “separated architecture,” initially adopted by Bechinger, using poly(3,4-ethylenedioxythiophene) as a cathodic EC material, with a coloration efficiency of 280 cm2/C and maximum visual contrast of 22% at 630 nm. Kraˇsovec et al. [64] reported devices with a size of 10 3 10 cm2 embodying a solid lithium ion conductor, based on ormolyte and a redox couple. A modulation of photopic transmittance of 63% was measured. The cross section of the device (Fig. 16.6) shows an architecture comparable to those that appeared in the works by Hauch et al. The ormosilane was adopted to obtain a solid-state electrolyte, combining the electrolytic properties of organic polymers to the chemical strength of its inorganic structure. A nanostructured frame-type TiO2 photoanode was also used by Wu et al. to activate the first photo-voltachromic device, disclosed in 2009, that is the first device capable of providing electricity generation by photovoltaic conversion as well as smart transmittance modulation, like a PEC cell. The very fast coloration kinetics (about 4 s) allowed to rely on the cell for energy harvesting for the rest of its operation time. It was the first of a series of publications dealing with novel architectures for multifunctional smart windows. In 2011 another group [65] reported a photo-voltachromic that reached 6.55% conversion efficiency by separating the areas for coloration and electricity generation on the counter electrode (Fig. 16.7). Other architectures appeared, afterward, aiming at higher efficiency and user controllability of photo-voltachromic devices [66,67].

TiO2 oxides for chromogenic devices and dielectric mirrors

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Figure 16.7 Geometry of the bifunctional counter electrode and electron micrograph images showing the spacing between ITO/WO3 and Pt patterns. Source: (A1) Reprinted with permission from A. Cannavale et al., ACS Appl. Mater. Interfaces 6 (4) (2014) 2415
2422. ©2014, American Chemical Society.

Figure 16.8 Layout and operation of a partly covered PEC device. PEC, Photoelectrochromic. Source: Reprinted from G. Leftheriotis et al., “Partly covered” photoelectrochromic devices with enhanced coloration speed and efficiency, Solar Energy Mater. Solar Cells 96 (2012) 86
92. ©2012, with permission from Elsevier.

Complete studies of the main figures of merit of PEC devices containing TiO2-nanostructured photoanodes were reported by Leftheriotis et al. [68,69]. They also reported devices embodying gel electrolytes [70], based on poly(methyl methacrylate), and the assessment of long-term performance of partly covered devices (Fig. 16.8), more recently [71]. Already in 2012, M. Green described the rapid diffusion of a new class of solar cells based on mixed organic
inorganic halide perovskites [72]. Some years before, Miyasaka

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Titanium Dioxide (TiO2) and Its Applications

et al. reported photovoltaic results for perovskites, dating back to 2007; such work described self-organization of CH3NH3PbBr3 perovskite in the nanoporous TiO2 of dyesensitized cells and reported an efficiency of 2.19% [73]. The architecture of the first perovskite solar cells (PSCs) was developed from dye-sensitized solar cells and employed a mesoporous TiO2 scaffold. Perovskite, in these cells, replaced the dye, in the role of light harvester. Moreover, organic
inorganic halide perovskite can conduct electrons and holes, and this fact led to further evolution of the device architecture toward more simple, planar PSCs [74,75]. As reported by Park et al., mesoporous TiO2 assists the charge carrier collection; in fact, the certified conversion efficiency for these cells was 22% in 2016, whereas it was 15.6% in planar cells [76]. Researchers are moving fast toward commercialization of these highly efficient cells. This step will require the development of high-efficiency and stable large-area solar modules, without relevant forms of hysteresis. As reported by Eperon et al. [77], PSCs can be fabricated at room temperature in a planar architecture, with a significant reduction of fabrication steps. In their work a customized dewetting of CH3NH3PbI32xClx absorber was developed to obtain small perovskite micro-islands to increase the device transparency. Islands were deposited between an n-type, compact TiO2 layer, and a p-type spiro-OMeTAD hole transporter [77,78]. Starting from these device evolutions, a new perovskite photo-voltachromic device was presented in 2015, including a semitransparent, planar perovskite
based solar cell and a solid-state EC film, in such a way that the photovoltaic component supplied the power to drive transmittance modulation [79]. Two external circuits connect the photoanode to the EC electrode and the gold cathode to the secondary electrode of the EC device.

16.3

TiO2 optical properties

The optical properties of TiO2 are not only at the basis of their use in chromogenic and photovoltaic devices but are also of fundamental importance for the realization of photonic devices. In fact, in this section, we will mainly focus on the modeling and applications of compact TiO2 layers. At first, we will focus on the optical properties of TiO2 compact layers in the visible range; then, we will introduce basic concepts of optics, useful for the understanding of the properties and modeling of a 1D photonic crystal, that is, distributed Bragg reflector (DBR). Finally, we will overview the applications of these photonic structures for lasing, sensing, nonlinear optics, and polariton systems. The TiO2 optical bandgap is about 3.4 eV. This property translates in almost no absorption of the TiO2 for wavelength above 400 nm; therefore it is transparent in the whole visible and near IR range. The optical response of a medium is modeled by its refractive index value n. The refractive index is, in general, a complex number, which also takes into account the dissipations (e.g., absorption) of the medium. Since we are considering a transparent compact layer, the refractive index of the TiO2 in the visible range is a real number. A plane wave propagating within a medium experiences its refractive index according to the equation E 5 E0 eikz with

TiO2 oxides for chromogenic devices and dielectric mirrors

493

Figure 16.9 TiO2 refractive index value as a function of the wavelength evaluated from ellipsometry measurements on an electron beam
evaporated TiO2 layer; k is the imaginary part and n is the real part of the refractive index. Source: This figure was produced by the authors, according to results reported in G. Lerario, D. Ballarini, A. Fieramosca, A. Cannavale, A. Genco, F. Mangione, et al., Light: Sci. Appl. 6 (2017) [80].

  k 5 2π=λ n, where E0 is the field amplitude, z is the direction of propagation, and λ is the wavelength. k is the wavevector and it also defines the wave phase velocity in the medium (vp 5 ω=k, where ω is the angular frequency). The TiO2 has a high refractive index value, compared to most of other transparent oxides. Depending on the deposition technique, the TiO2 refractive index can span from 2 to 2.5, in the whole visible range (see Fig. 16.9), and this property makes the TiO2 an extremely interesting material for applications in optics (e.g., light harvesting, antireflection coating). Hereafter, we will discuss in detail the modeling and applications of 1D photonic crystals incorporating the TiO2.

16.4

Modeling distributed Bragg reflectors

To give a clear and easy description of the DBRs, first we have to introduce a few basic concepts of optics. Let us consider a plane electromagnetic (EM) wave, hitting an interface between two nonmagnetic lossless dielectric media: Ei 5 E0 iki r , where ki is the incident wavevector. We can describe the reflected and transmitted signals using the EM field complex amplitudes coefficients r and t, which are complex-valued ratios of the incoming EM field: Er 5 rE0 ikr r and Et 5 tE0 ikt r , where kr and kt are the reflected and transmitted wavevectors, respectively. We are interested in the definition of the reflected and transmitted components of the EM wave and they depend on the EM field polarization. The simplest decomposition of the EM polarization at an interface is in the s- and p-polarized components of the field. In the case of s-polarized light, the electric field oscillates perpendicular to the incidence plane, while the p-polarized light has the electric field oscillating parallel to the incidence plane. Therefore we can describe two behaviors, according to the light polarization.

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Titanium Dioxide (TiO2) and Its Applications

From the momentum and energy conservation laws, one can derive the EM field complex amplitudes coefficients: rs 5

n1 cosϑi 2 n2 cosϑt n1 cosϑi 1 n2 cosϑt

rp 5

n2 cosϑi 2 n1 cosϑt n2 cosϑi 1 n1 cosϑt

ts 5

2n1 cosϑi n1 cosϑi 1 n2 cosϑt

tp 5

2n1 cosϑi n2 cosϑi 1 n1 cosϑt

where n1 is the refractive index of the “entrance layer” and n2 is the refractive index of the “exit layer,” and ϑi , ϑr , and ϑt are the angles of incidence, reflection, and transmission, respectively. The reflectance and transmittance values, that is, the fraction of the intensity reflected and transmitted through the interface, are: R 5 jr j2 T5

n2 cosϑt 2 jtj n1 cosϑi

A 1D stack of many pairs of dissipation-less layers with different refractive indexes, when opportunely designed, works as an efficient and energy tunable reflector. This kind of reflector is named DBRs [81]. Hereafter, we introduce the basic theoretical method used for modeling the optical response of these structures, that is, the transfer matrix method. We can define two directions of propagation along this words stream, the forward direction (from left to right) and the backward direction (from right to left). A field propagating along the forward direction will be marked with subscript f (Ef ), in the backward direction with subscript b ðEb Þ. Considering an EM field made by a forward and a backward plane component propagating through a medium, the total EM field is: EðzÞ 5 Ef eikz 1 Eb e2ikz In the transfer matrix formalism: 

0

Ef ð z Þ 0 Eb ðzÞ



 5P

Ef Eb



 5

eikz 0

0

e2ikz



Ef Eb



TiO2 oxides for chromogenic devices and dielectric mirrors

495

Defining δn 5 Ln kn the phase accumulated by the EM field in the nth layer with Ln thickness, the EM field reads: 

Ef ;n Eb;n



 5P

Ef Eb



 5

eiδn 0

0



e2iδn

Ef Eb



Let us now consider the interface between two semiinfinite layers with different refractive indexes and two incoming waves, one propagating in the forward direction and the other propagating in the backward direction as in Fig. 16.10. The equations describing the field components are: Eb1 5 r12 Ef 1 1 t21 Eb2 Ef 2 5 r21 Eb2 1 t12 Ef 1 Considering, as follows, from Fresnel equations, that rn21;n 5 2rn;n21 and tn21;n tn;n21 5 1 1 rn21;n rn;n21 , for both s- and p-polarized fields, we can rewrite the above equations as follows: 

Ef 1 Eb1

 5

1 t12



r12 1

1 r12



Ef 2 Eb2



and for generic nth and nth 2 1 layers: 

Ef ;n21 Eb;n21

 5

1 tn21;n



1 rn21;n

rn21;n 1



Ef ;n Eb;n



Figure 16.10 Schematic representation of a two-component (backward and forward propagating) field distribution at an interface between two different media. Source: This figure was produced by the authors, according to the modeling reported in S.J. Byrnes, (2016) 1
20 [82].

496

Titanium Dioxide (TiO2) and Its Applications

Therefore, putting together the propagation term and the interface term, the transfer matrix for a passage into a single layer and an interface reads: 

Ef ;n21 Eb;n21



 5 Mn

Ef ;n Eb;n

 5

1 tn21;n



1 rn21;n

rn21;n 1



e2iδn 0

Let us now consider a stack of N layers (where layer 0 and The transmitted amplitude is the EM field propagating out of forward direction (Ef ;N 5 t), and there is no backward wave direction (Eb;N 5 0); the reflected amplitude satisfies the Therefore we can define a transformation matrix:

0 eiδn



Ef ;n Eb;n



N are semiinfinite). the structure in the propagating in this condition Eb;0 5 r.

      t t 1 5 Mtot 5 M1 M2 . . . MN21 MN 0 0 r The reflectance and transmitted intensities are: R5

IR 5 jr j2 I

T5

IT nt cosϑt 2 jt j 5 I nr cosϑr

To obtain an efficient DBR, the key conditions are: G

G

a stack of two alternating media with high refractive index contrast; given the wavelength that has to be reflected, the optical paths (Ln nn ) in each layer constituting the DBR have to be a quarter of this wavelength.

Consequently, the layers’ thicknesses are sized considering the center wavelength and the refractive indexes of the two media constituting the structure. When these conditions are satisfied, the resulting reflectivity has a central flat maximum, the so-called optical stopband, which drops off in an oscillating fashion on either side of the band itself (Fig. 16.11B). The spectral bandwidth shape of the mirror is set by the refractive index difference between the materials and the number of deposited pairs. In this context the TiO2 is an excellent material for the realization of DBRs working in the visible range since it has almost no absorption (lossless) and high refractive index, and it can be used together with easy fabrication and cheap materials as the SiO2 (nSiO2 B1.49 in the visible range). Fig. 16.11 shows a draw of DBR stack (A) and TMM simulated reflectivity (B) considering five pairs of TiO2/SiO2. The reflectance condition of the DBR varies according to the angle of incidence, defining the reflectivity dispersion (for transverse EM modes) shown in Fig. 16.12. The bright central area defines the   evolution of the stopband in energy at different in-plane wavevectors kO 5 2π=λ n sinϑi , where ϑi is the angle of incidence of the

TiO2 oxides for chromogenic devices and dielectric mirrors

497

Figure 16.11 (A) Schematic draw of a DBR stack made by five pairs of layers with different refractive index deposited on a substrate. (B) Transfer matrix method (TMM) simulated reflectivity obtained by a DBR made with five pairs of TiO2/SiO2 deposited on a glass substrate. DBR, Distributed Bragg reflector.

Figure 16.12 Typical DBR reflectivity dispersion map. The optical stopband is defined by the bright central area. The green dashed line depicts the lightline; for higher in-plane wavevectors the system is in the total internal reflection regime. DBR, Distributed Bragg reflector.

incoming light; at increasing wavevector (i.e., angle of incidence) the reflection condition shifts the stopband toward higher energies due to the fact that the incoming light experiences a longer optical length within the layers constituting the DBR. The lightline—the dispersion of the light in vacuum—delimitates the cone accessible to propagating EM waves. Beyond the lightline, the system is in the total internal reflection regime: all the light incoming from the substrate with higher wavevector, for any given energy, is reflected back by the structure rear edge; this

498

Titanium Dioxide (TiO2) and Its Applications

condition is a consequence of the Snell’s law and depends on the index mismatch between the initial (substrate) and final media. As an example, if the substrate is made by glass and  the “final” semiinfinite layer is air, the critical angle is ϑc 5 sin21 nair =nglass .

16.5

Bloch surface waves and microcavity modes

DBRs are used for several applications, spanning from laser physics to sensing, and they are typically used for the possibility to sustain high-quality optical modes. The quality of an optical mode is as high as its dissipation is low. A common parameter to evaluate the quality of an optical mode is the quality factor (Q-factor), defined as the angular frequency times the energy in the  system  divided by the derivative of the energy dissipated in time (Q 5 ω U= dU ). dt Considering a Lorentian oscillator—which is the typical spectral linewidth of the modes we are going to investigate below, the energy in the system is U 5 U0 e2t=τ where U0 is the initial energy in the system and τ is the mode lifetime, from which we obtain Q 5 ωτ and, from the uncertainty principle, Q 5 ω=Δω where Δω is the mode spectral linewidth. Therefore the sharper is the mode linewidth, the lower is the energy dissipation and the higher is the quality factor. In this paragraph, we will discuss the properties of two particular kinds of optical modes that have weak energy dissipation and strong local enhancement of the EM field: the Bloch surface waves (BSWs) and the microcavity modes (MCMs). At first, we will focus on the detailed analysis of the peculiarities of the BSW modes. The BSW is an evanescent wave, lying in the total internal reflection regime within the stopband of a single DBR. Differently from other surface waves as surface plasmons, the BSW does not need for metals to be sustained, it is indeed generated by only using optical transparent materials and, therefore, it has very weak dissipation: the only dissipations, at least in principle, are due to its extraction from the system. The Qfactor of the BSW depends on the refractive mismatch between the dielectric media and, mainly, on the number of the dielectric pairs constituting the DBR: using lossless media, the quality factor increases with layer pair addition. In the reflectivity dispersion maps reported in Fig. 16.12 the BSW is the mode emerging from the critical angle and sustained within the stopband in the total internal reflection regime. The EM amplitude distribution along the device is depicted in Fig. 16.13. The energy dispersion of mode defines the group velocity of the    wave (vg 5 1=h¯ @E=@k ); by opportunely engineering the DBR layers thickness, the group velocity can reach values that are also higher than 2/3 the light speed in vacuum. The BSW modes propagate in the plane of the structure (the momentum vector is oriented along the orthogonal direction with respect to the DBR stack) and can have transverse electric (TE) or transverse magnetic (TM) polarization. Typically, TM modes have lower quality factor compared to the TE ones as a direct consequence of Fresnel laws. The electric field of the TE modes oscillates along the structure plane.

TiO2 oxides for chromogenic devices and dielectric mirrors

499

Figure 16.13 Field distribution along the DBR for a wavevector above the critical angle. The BSW is the bright mode at the center of the map and its maximum value is at the top of the structure, exponentially decreasing in air. The stopband is defined by the dark central area, where the field drops drastically in the first layers due to the high reflectivity. At the stopband edges, there are the leaky modes that penetrate deeper into the device. BSW, Bloch surface wave; DBR, distributed Bragg reflector.

Being evanescent modes, the collection of a BSW signal is made upon the use of near-field techniques, that is, scanning near-field optical microscopy or Otto prism coupling. Nevertheless, since these modes are also accessible to waves propagating into the substrate, an alternative collection system consists of refractive index matching between the substrate and a “collection material”. A common configuration is the Raether
Kretschmann prism coupling configuration, in which the substrate is kept in intimate contact with a glass prism upon an intermediate optical oil with refractive index matching the glass one. However, the latter technique does not allow for the observation of the real-space distribution of the EM field. To reconstruct this information the use of an immersion objective—matching the refractive index of the substrate—allows for the visualization of the whole space map field distribution [83]. Despite the research regarding BSWs can be still considered as “niche” research, many recent publications have demonstrated the great potential of this system. Thanks to the strong localization at the EM field at the DBR/air interface, the BSW energy dispersion is extremely sensitive to any refractive index perturbation at the DBR surface and, therefore, many studies focus on sensing application [84
88]. More recently, the BSWs have been applied for on-chip photonics. Inspired by the first experiment showing the guiding properties of the EM field through a BSW by patterning the DBR surface [89], many publications have demonstrated the possibility of engineering the EM field distribution in the device plane toward on-chip photonics. In this context, focusing and subwavelength focusing [90,91] and static manipulation of optical signals [92
94] have been demonstrated.

500

Titanium Dioxide (TiO2) and Its Applications

Recently, the strong exciton
photon coupling between a BSW and different excitonic materials—spanning from organic molecules to transition metal dichalcogenide monolayers—has been also demonstrated [95
97]. The resulting hybrid quasiparticles are named exciton
polaritons and their generation represents an important progress toward the dynamic manipulation of the optical signal through the optical nonlinear properties of the excitonic material, paving the way for onchip logic devices based on BSWs. Let us now compare the BSW peculiarities with the ones of a planar MCM. The MCMs are the optical modes of a Fabry
Perot resonator. Two DBRs facing each other, at a distance on the scale of the dimension of the stopband wavelength, generate an optical resonance with typical energy dispersion as in Fig. 16.14. The characteristic energy dispersion of a Fabry
Perot mode derives from the condition of constructive interference of the resonant waves after one round-trip; at nonorthogonal incidence of the external field the total wavevector is the vector sum of the orthogonal and longitudinal components, and the energy dispersion relation read: ¯hc Eph 5 ¯hωc 5 n

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 5 ¯hc kO2 1 k\ n

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2 πN kO2 1 L

where N is an integer, ¯h is the Planck constant, c is the light speed, kO is the inplane wavevector, L is the distance between the two mirrors, and n is the refractive index of the medium embedded into the structure. At the center of the stopband the MCM appears as a dip in the reflectivity spectrum and the EM field is confined in between the two DBRs.

Figure 16.14 Typical microcavity reflectivity dispersion map. Within the optical stopband (bright central area), the MCM appears as a dip in the reflection map. MCM, Microcavity mode.

TiO2 oxides for chromogenic devices and dielectric mirrors

501

It is important to notice that the separation of modes in a cavity (the free spectral range) is inversely proportional to the distance between the DBRs. Hence, compared to macroscopic cavities, microcavities sustain optical modes far apart in energy. Moreover, the MSC modes have a wider angular acceptance with respect to macroscopic cavity mode; the field is confined for a wide range of incidence angles and, therefore, the MCMs span over a wide range of in-plane group velocities. For an empty cavity the quality factor of an MCM is sensitive to both the DBRs. Similarly to the case of the BSWs, if the DBRs are opportunely tuned and made with lossless media, the MCM quality factor increases with layer pair addition. Microcavities have been used for a plethora of applications. The localization of the EM field in between the mirror can be used for emission enhancement and lasing [98
100]. One more application is the realization of vertical cavity surface emitting lasers [101]. MCMs have also been strongly coupled to excitonic media embedded in between the two DBRs [102
104]. A new intriguing frontier is the engineering of the system spin Hamiltonian through the use of materials with “exotic” optical properties (e.g., birefringence and optical chirality) [105
107]. For the sake of completeness, we mention that a wide variety of different photonic structures have been made by using TiO2, from micro-disk for whispering gallery modes [108], to 2D, 3D, and inverse opal photonic crystals [109
111].

16.6

Conclusion

This chapter contains an accurate—though nonexhaustive—discussion showing the versatility of the optical properties of TiO2. The characteristics of the material, in fact, allow it to be used as a cathodic EC material, capable of modulating its chromatic features, in a smart fashion. Furthermore, TiO2 is used not only as a photoelectrode in Gr¨atzel photovoltaic cells and in PEC devices but also as an n-type material in heterojunction solar cells and in the latest fast-growing perovskite-based photovoltaic technology. Moreover, the optical properties of TiO2 have also been studied in depth, with useful references to theory, to illustrate the relevance of this material in the design of DBRs and the realization of optically confined modes, as MCMs and BSWs.

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757.

TiO2 in memristors and resistive random access memory devices

17

Andrea Zaffora, Francesco Di Franco, Roberto Macaluso and Monica Santamaria Department of Engineering, University of Palermo, Palermo, Italy

17.1

Introduction

Titanium dioxide (TiO2) is currently used in a wide range of applications, from photocatalysis [1 3] to solar energy conversion [4 6] and micro- and nanoelectronics [7 10]. One of the most recent applications of TiO2 thin films is as an oxide layer in memristor and in redox-based resistive random access memory (ReRAM) devices. The former is a circuit element (memory resistor) postulated by Chua in 1971, which relates the charge, q, to the magnetic flux, ϕ [11], while the latter is a particular class of nonvolatile memories in which redox reactions at materials interfaces and nano-ionic transport processes play a key role [12]. These types of devices are worldwide considered as the next-generation nonvolatile memories and are also ideally suitable for beyond von Neumann computing, analog circuits, and artificial neuromorphic operations. In 2008 Williams’ group at HP published the paper that represented a breakthrough in memristors and ReRAM fields, as they stated, “The missing memristor was found” [13]. In fact, they related memristor characteristics to a ReRAM device based on 5 nm titanium dioxide thin film containing one layer of insulating TiO2 and one layer of substoichiometric TiO22x, sandwiched between two Pt electrodes. Since then different systems can be considered as memristors or memristive systems and ReRAMs belong to this class. How does a ReRAM work? A ReRAM is generally built by a capacitor-like metal/insulator/metal (MIM) structure, where an insulating or high resistive material (e.g., oxide or chalcogenide) is sandwiched between two electronic conductors (e.g., metals, transparent conducting oxides, and TiN). These MIM cells can be electrically switched between at least two different resistance states after an initial electroFORMING cycle that is usually required to activate the switching property. By applying appropriate programming or write voltage pulses, a cell that is initially in its high-resistance (OFF or HRS) state can be SET to a low-resistance (ON or LRS) state and, then, RESET back into the OFF state. ON and OFF states represent the Boolean “1” and “0,” respectively, and are used for digital information storage and operations [12,14]. SET and RESET processes can be triggered applying different voltage polarities (bipolar switching), or also with the same polarity but different voltage amplitudes (unipolar switching). Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00020-1 © 2021 Elsevier Inc. All rights reserved.

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The characteristics of ReRAM devices (switching type, magnitude of SET voltage, VSET and RESET voltage, VRESET, etc.) strictly depend on the materials used in the cells, that is, electrodes and insulating layer. In particular, the characteristics of the insulating layer (which usually is an oxide) have a direct influence on device properties and performances. Thus controlling the switching layer properties through the fabrication process is crucial to tailor the device on a specific application. Since TiO2 was historically correlated to the memristor devices and there is a wide literature on the electronic and the crystallographic structure of TiO2 thin films, as well as on fabrication methods, it can be considered as a prototypical material to provide deeper insight into the mechanisms behind memristive switching [15]. In this chapter, we want, first, to give some fundamentals on resistive switching and several types of ReRAMs [electrochemical metallization memories (ECMs), valence change memories (VCMs), and thermochemical memories (TCMs)]. Then, several fabrication methods to reliably produce TiO2 thin films will be described and the correlation between fabrication and oxide characteristics will be highlighted. For every fabrication method, a focus on TiO2-based ReRAM performances will be done.

17.2

Fundamentals on resistive switching

Working principle of memristors and ReRAMs is based on resistive switching phenomenon, that is, a change in the resistance of the insulating layer, also called solid electrolyte. Two types of resistive switching can be usually distinguished, that is, filamentary switching and area-dependent switching. In the former case the switching in the resistance of the solid electrolyte is due to the formation/disruption of conducting filament/filaments that can be metallic or formed by reduced oxide, depending on the material used in the device. In the case of area-dependent switching, resistance change is supposed to take place uniformly at the interface between the solid electrolyte and a “reactive” metal electrode. Uniform resistance switching can be recognized by area-dependent LRS resistance and programming current, in contrast to area-independent switching in filamentary-type devices [16]. Depending on the working principle of the ReRAM cells, three different types of devices can be recognized: ECMs, VCMs, and TCMs. The latter use symmetric electrodes and are characterized by the formation and dissolution of filaments due to Joule heating generated by the high current flowing into the device. Among the ReRAMs, TCMs have the lowest prospects for industrial realization. This is due both to their large power consumption and to their intrinsic unipolar switching, which makes their practical application complicated. For the sake of brevity, we report here the switching mechanism related to ECM and VCM cells, but interested readers will find further details on this complex topic in specialized literature [14,17 19].

17.2.1 Electrochemical metallization memories ECMs are devices composed by a simple metal/solid electrolyte/metal junction with some peculiarities: they consist of an active electrode (AE) (typically Ag or Cu, but

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also other electrodes can be used), an electrochemically inert counter electrode (CE) (typically Pt, Ir, or W but also some electronically conductive materials such as ITO or TiN can be used since it is supposed they are unable to exchange mobile ions with the electrolyte), and a thin solid electrolyte able to conduct ions, which is sandwiched between the electrodes [20,21]. A schematic representation of an ECM cell, in its initial state (HRS), is shown in Fig. 17.1A. Let consider now an ECM cell formed, for instance, by a Cu/oxide/Pt structure. If a sufficient voltage is applied to the Cu AE (orange electrode in Fig. 17.1), it is possible to electrochemically oxidize it generating Cu1/21 ions that solubilize into the solid electrolyte. After the migration of the Cu ions into the electrolyte sustained by the presence of high electric field strengths across the thin film, Cu1/21 ions reach the Pt inert electrode (gray electrode in Fig. 17.1) and start to be reduced at that electrode. This leads to the electro-crystallization of Cu at the Pt surface. Because of the high current densities (BMA cm22) and high electric field (B108 V/m21), the deposit forms as a conducting Cu filament with a diameter typically varying between 5 and 10 nm. After the metal filament has grown sufficiently far to make a (electronic) contact to the opposite Cu electrode, the cell switches to the ON state (Fig. 17.1B), which is characterized by a sudden current increase in the I V characteristic. The cell retains the ON state unless a sufficient voltage of opposite polarity is applied, and the electrochemical dissolution of the metal filament leads to the RESET process, that is, the switching from the LRS to the (initial) HRS with the consequent decreasing of the current. This switching mechanism is widely accepted since the observation of the formation of metallic filament in different ECM cells by in situ Transmission Electron Microscopy observation [22 25]. Many parameters can influence the cell performances, such as the nature of active and inert electrodes [26,27], moisture (that can affect the migration of metal ions across the electrolyte [28,29] or can contribute in originating an electromotive force inside the cell [30]), and temperature and composition of the oxide used as solid electrolyte [31,32]. Advantages of ECM cells are low SET voltages (0.2 1 V), low SET/RESET currents (down to pA range) with low power consumption and large (typically 106)

Figure 17.1 Schematic representation of the two resistance states in ECM cells: (A) pristine state and HRS, (B) ON state. Orange electrode: active electrode, gray electrode: inert electrode. ECM, Electrochemical metallization memory; HRS, high-resistance state.

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OFF-to-ON resistance ratio [32,33], which are excellent parameters in the applications of memory devices. Unfortunately, these systems suffer from instability of the metallic filament that leads to insufficient retention (i.e., stability of the LRS and HRS over time after the resistive switching transitions) of the LRS, especially for resistances above B15 kΩ [33].

17.2.2 Valence change memories VCMs are two-electrode devices as ECM cells, but typically the metal electrodes are chosen to have two Schottky barriers at the electrode/electrolyte interfaces with different heights. Low work function metals (and also easily oxidable), such as W, Ti, Ta, Hf, are usually used for the Schottky contact that, during working operation, becomes ohmic [34]. For this junction, typically the same metals are used as the metal component of the oxide. The other junction usually employs Pt as electrode, or other high work function metals (e.g., Ir and Au) can be selected for the higher Schottky contact [35]. In some cases, symmetric cells with Schottky electrodes are used, for example, Pt/Oxide/Pt, and the FORMING process (reduction) is believed to ensure an ohmic contact at one of the interfaces [36]. The resistive switching mechanism relies on the motion of mobile ionic defects under the applied electric field across the oxide. The most widely accepted model is based on (double) positively charged oxygen vacancy defects, VO , but the motion of metal cations has been evidenced and their role cannot be excluded [37]. In this case, metal cations can be either interstitial ions or move on cation vacancy sites. Conducting filament in VCM can be of metallic nature, but usually, it is composed of strongly reduced metal oxide with almost metallic conductivity. A typical VCM cell is shown in Fig. 17.2A in its pristine state. The oxygen vacancies are introduced in the switching layer during the initial FORMING step, which results in the formation of a filamentary region with a high concentration of oxygen vacancies between the two electrodes (see Fig. 17.2B). GG

Figure 17.2 Schematic representation of the resistance states in VCM cells: (A) pristine state, (B) ON state, and (C) OFF state. Blue electrode: low work function metal (ohmic contact), gray electrode: high work function metal (Schottky contact). Light blue spheres: oxygen vacancies, violet spheres: reduced metal ions. VCM, Valence change memory.

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However, during the RESET process, the conducting filament is only partially dissolved (in contrast with the complete dissolution of the metallic filament in ECM cells) (see Fig. 17.2C). Between the tip of the filament and the Schottky electrode (gray electrode in Fig. 17.2C) a thin “disk” is formed, less conductive than the filament (plug), but a bit more conductive with respect to the surrounding oxide matrix. Thus the Schottky barrier at that interface with, for instance, Pt electrode determines the overall resistance. During SET process the disk is reduced and becomes as conductive as the other part of the filament (ON state) while, during RESET process, it is oxidized and/or may also increase in thickness, defining the high resistive (OFF) state [21]. A typical I V sweep of a VCM cell is reported in Fig. 17.3. VCM cells exhibited much better retention than ECM cells. They are also able to provide intermediate resistance states (but not so finely distinctive as in ECM cells) and show low switching times (i.e., fast switching memory). Disadvantages are the higher switching voltages and currents (i.e., higher power consumption) and the lower OFF/ON ratio that varies typically between 10 and 1000 [21]. As a final comparison, the performance parameters of the different types of ReRAM cells are reported in Table 17.1, although it is not straightforward

Figure 17.3 Typical I V sweeps of a VCM cell. The SET process has a positive voltage polarity, while the RESET process exhibits a negative voltage polarity. VCM, Valence change memory. Table 17.1 Performance parameters of electrochemical metallization memory (ECM) and valence change memory (VCM) cells. Parameter

ECM

VCM

Max switching speed (ps) Min switching energy (fJ) Endurance (no. of cycles) Data retention (years at 85 C)

7500 [38] 18 [38] 1010 [41] .10 [41]

85 [39] 115 [40] 1012 [41] .10 [41]

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Titanium Dioxide (TiO2) and Its Applications

comparing the performances of different types of ReRAMs since they strongly depend on employed electrodes and oxides, electrodes area, oxide thickness, operating temperature, operating voltages, and, even, on the particular measurement setup.

17.3

TiO2 in memristors and resistive random access memories: fabrication methods and performances

Insulating layer (called also active layer) of a ReRAM device is generally a metal oxide, which is sandwiched between two metallic electrodes in a classical MIM structure. Among the metal oxides the most used are Ta2O5 [36,42,43], HfO2 [44 46], SiO2 [47], ZnO [48 50], SrTiO3 [51 53], and TiO2. The latter is of particular interest in the ReRAM world, being TiO2-based system one of the first semiconductor devices to be identified as memristor [13]. Furthermore, TiO2 has several properties, such as a wide bandgap (between 3.05 and 3.2 eV [54,55]), high thermal stability, and high dielectric constant, that make it prone to be used in electronic devices. Moreover, TiO2 is easy to prepare and its stoichiometry can be precisely tailored, offering a variety of possible chemical phases: anatase, rutile, and brookite. TiO2 is usually a nonstoichiometric oxide (TiO22x), crucial characteristic for the use as active layer in resistive switching-based devices. This characteristic is due to point defects that are typically classified into two types: (1) titanium interstitials, Tii or Tii (depending on the charge of point defect), and (2) oxygen vacancies, VO [56]. Due to these peculiar characteristics, TiO2 was widely studied from the researchers of the field and it is, nowadays, one of the most used materials in resistive switching memories. Table 17.2 summarizes some of the performance parameters of TiO2-based cells with a comparison with other metal oxides that have shown the best switching properties (e.g., Ta2O5, HfO2, and SiO2). Several fabrication methods have been used to reliably produce TiO2 thin films with precise material properties for resistive switching-based devices. In the following, we will explore some of the most used physical and chemical methods and one recent and unconventional electrochemical method to deposit/grow Ti oxide thin films highlighting the relationship between material properties, controlled by fabrication route, and final device performances. GGG

GGGG

GG

17.3.1 Anodizing Electrochemical oxidation, namely, anodizing, is a wet electrochemical process, carried out at low temperature, which allows to grow an oxide of tailored composition and properties on the surface of valve metals (e.g., Ti, Al, Ta, Nb, Hf, and W) and valve metal alloys. The metal substrate is oxidized by applying an anodic polarization generating metal cations that react with oxygen anions coming from the electrolyte. Both types of ions migrate in a solid matrix; this migration is triggered and sustained by the high electric field (order of 106 107 V/cm) present across the oxide layer. A sketch of the anodizing process is shown in Fig. 17.4.

Table 17.2 Performance comparison of different resistive random access memory (ReRAM) devices. ReRAM cell

SET voltage (V)

RESET voltage (V)

HRS/LRS ratio

Endurance (no. of cycles)

Retention (s)

TiN/TiO2/Pt [57] Ti/TiO2/(Pt, Cu) [58,59] Pt/Ta2O52x/TaO22x/Pt [36] Ta/Ta2O5/Pt [43] Ti/HfOx/TiN [60] Hf/HfO2/TiN [61,62] Mo/SiOx/Au [63] TiN/SiO2/p-Si [64]

0.6 0.8 0.4 1.2 1.1 0.45 0.65 0.8 1.2 0.1 1.2 21.2 1.2

21 to 21.1 20.5 to 21.6 1.9 20.55 to 20.65 20.8 to 21.5 20.2 to 20.8 1.2 21.5

~10 80 100 ~10 10 20 .100 .100 .100 .100

104 4.5 3 103 1012 106 108 1010 107 105

104 n.a. 105 104 n.a. 105 104 104

HRS, High-resistance state; LRS, low-resistance state; n.a., not available.

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Titanium Dioxide (TiO2) and Its Applications

Figure 17.4 Sketch of anodizing technique in a three-electrode electrochemical cell with WE, RE, and CE. Inset: ions migration during the anodic oxide growth. CE, Counter electrode; RE, reference electrode; WE, working electrode.

Through a careful choice of the anodizing conditions, it is possible to finely tailor anodic oxide nature (crystalline or amorphous) and its thickness, while the anodizing bath and base alloy compositions have a direct influence on the composition of the grown oxides. Concerning the electrochemical bath, it is important to consider (1) the possible incorporation of electrolyte species into the oxide during the growth and (2) the role of electrolyte pH and aggressive ions in determining the morphology of the growing layer. In fact, different morphologies can be obtained through the anodizing process, from barrier films (i.e., compact films with uniform thickness) to porous films and nanostructured oxides by correctly selecting the process parameters. The growth of anodic TiO2 layers was largely studied in the last decades, due to its peculiar characteristics. In fact, if the formation voltage exceeds B10 V, anodic titania shows an amorphous crystalline transition [65]. Crystalline regions are characterized by a higher electronic conductivity with respect to amorphous counterpart, thus enabling water oxidation with oxygen gas evolution inside the oxide layer. The consequent increased pressure can lead to the formation of flaws inside the oxide and, even, to the film breakdown. Therefore it is important to avoid or, at least, delay the crystallization of TiO2 for its possible use in electronic devices. Several strategies have been proposed in the literature to hinder or delay the onset of crystallization during anodizing of Ti as, for instance, the incorporation of

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foreign species into the oxide that helps in keeping amorphous the outer part of the grown layer [65,66]. Another strategy consists in the incorporation of foreign atoms from the metal substrate through the anodizing of Ti-based metal alloys. In fact, it has been demonstrated that alloying Ti with Si or Al leads to a delay in the crystallization phenomenon with the formation of amorphous oxide films up to quite high formation voltages. In the case of Ti Si alloys, the resulting oxide is layered due to the presence of Si only in the inner part of the oxide since Si metallic cations have a lower transport number with respect to Ti41 ion [65,66]. By changing anodizing process parameters, it is also possible to fabricate organized nanostructures, such as TiO2 nanotubes (NTs). In order to form NTs oxide layers, electrochemical oxidation is usually carried out by applying a constant potential value, between 1 and 30 V, in aqueous electrolyte or between 5 and 150 V if nonaqueous electrolytes are used, containing a few percentages (0.1 1 wt.%) of fluoride ions [67]. Nonaqueous electrolytes are used because, in this way, smooth tubes without any inhomogeneity (ripples) can be obtained, with a much higher aspect ratio. On the other hand, the presence of fluorides in the electrolyte is necessary leading to the formation of water-soluble TiF622 species that help in the development of NT array. Among the other technological applications, anodic oxides have been used, in the last two decades, in resistive switching-based memories as solid electrolytes [43,46,68 71]. Best device performances were reached with anodic Ta oxide, showing superior endurance and data retention properties, as well as a multilevel programming capability [43]. This demonstrated how the anodizing process is an effective way for the fabrication of tailored oxides, designed to work as active layers in ReRAM devices, due to the formation of very compact, high density, flaw-free oxides, with a very sharp metal/oxide interface and perfect adhesion. Furthermore, anodic oxide growth is the fast process (with respect to other deposition techniques) and can be performed with low power consumption. More important, in the case of ReRAM devices, is that the metal substrate can be directly used as bottom electrode of the cell, although other strategies could be adopted to have a different bottom electrode. Despite anodizing is an effective and a valuable route to fabricate compact and reliable TiO2 thin films, just a few works have been devoted to studying the memristive behavior of barrier-type anodic titania [58,59,72 78] and in nanostructured form [79 81]. The first demonstration of memristive behavior was reported by Miller et al, studying TiO2 thin film grown by anodizing a Ti foil with potentiostatic method at 30 V in water/glycerol solution 0.27 M in NH4F [75]. Cell was completed with Ag paste which is, doubtless, not suitable for real devices but can anyway work as top electrode. In that case, resistive switching was detected for nonannealed samples with a quite good cyclability and VSET and VRESET lower than 1 V. In contrast, annealed samples did not show good resistive switching properties probably due to the crystallization of the samples and to extra oxygen vacancies created at the top of the TiO2 layer, inducing the formation of ohmic junctions at both electrodes. Anodizing Ti foils approach was followed also by other research groups. Yin et al. reported resistive switching behavior of anodic (annealed) TiO2

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grown to 20 V in acidic electrolyte, using Pt as top electrode, showing good data retention (up to 103 s) and HRS/LRS ratio as high as one order of magnitude [78]. In that case a VCM-like switching mechanism with the formation and disruption of conducting filaments formed by oxygen vacancies was proposed. Diamanti et al. studied barrier TiO2 layers grown in different acid baths coupled with Cu top electrode, showing that best performances could be reached with films grown up to 25 V, with a ROFF/RON ratio of B30 [77]. Yu et al. prepared Ag/TiO2/Ti devices with anodic oxides grown in electrolyte containing 0.25 wt.% NH4F and ethylene glycol and formation voltage of 60 V, studying also the effect of the anodizing time [76]. ECM-like switching mechanism was proposed by the authors with the formation of Ag metallic filaments, triggered by the oxidation of Ag top electrode. Despite resistive switching was shown, not so good cyclability was reached with the resistance ratio that decreased from 27 to 1.5 after the first switching cycle. Diamanti et al. also performed an interesting characterization with conductive atomic force microscope which showed that oxide properties are not homogeneous at the surface with resistive switching spots inserted in a low conducting matrix and located mostly at grain boundaries [73,82]. A step further toward real devices was performed by Aglieri et al., which studied the memristive behavior of anodic TiO2 grown on e-gun deposited Ti on glass substrates [58,72]. MIM structure for ReRAM was completed with Cu top electrode patterned by optical lithography, with pad dimensions between 1 and 10 3 10 μm2. Authors demonstrated the capability of anodic titania for resistive switching-based memories showing switchable devices without any forming process, a resistance OFF/ON ratio up to 80 and the possibility of multilevel switching. Interestingly, 29 nm anodic titanium oxide had higher HRS than 8 nm anodic oxide due to (1) a higher oxide thickness (tailored by a higher formation voltage) and (2) to a reduction in the concentration of oxygen vacancies due to a longer anodizing time. Recently, an extensive characterization regarding the use of anodic TiO2 thin films in resistive switching-based devices was carried out by Chen et al. exploring the possibility to use electrochemical oxidation for preparing oxides that emulate biological synaptic functions in order to build neural networks for artificial intelligence systems [59]. As major results, authors demonstrated resistive switching properties of Pt/anodic TiO22x/Ti structures with low SET and RESET voltages and the possibility to have different (multilevel) resistance states, by adjusting the current compliance or the VRESET, with low switching times as down as 90 ns. The possibility to finely tune the conductance state by applying sequences of pulsed voltage stresses demonstrated also the successful fabrication of a synapsis using anodic oxide. The main challenge related to anodizing as fabrication technique in ReRAM fields is the integration of this wet process in the classical CMOS fabrication chain.

17.3.2 Atomic layer deposition Atomic layer deposition (ALD) is one of the most used techniques for depositing thin films for a wide range of applications, especially in semiconductor industry where the continuous miniaturization of the devices led to the need of a control at

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atomic level of thin-film deposition. ALD bases on a sequence of self-limiting surface reactions, repeated N times [83]. In a typical deposition carried out by ALD, two or more coreactants are sequentially pulsed into the reactor, followed by a reactor purge step. This reaction scheme can be repeated many times with different processing times. This sequence is called deposition cycle (or growth per cycle), and the film thickness directly depends on how many deposition cycles are carried out. It is noteworthy to mention, regarding the growth by ALD, that the reacting species are allowed to react only with the accessible surface active sites, which are consumed as the reaction proceeds. This makes the ALD process “self-limiting” because the reaction stops when the substrate is depleted of active (accessible) sites, regardless of the presence of excess species in the reactor. A scheme of an ALD deposition is shown in Fig. 17.5. The greatest advantage of ALD with respect to other vacuum deposition techniques is its self-limiting nature. In this way, monolayers can be deposited with high reliability, allowing for a nanometer control of the thin-film thickness. Moreover, if the substrate is completely homogeneous (i.e., uniformity of accessible reaction sites), ALD allows a uniform deposition on the substrate. More detailed review papers can be found in literature [83 86]. Regarding the fabrication of TiO2 thin films by ALD, several routes have been reported during the last decades involving many types of titanium precursors, for example, halides (TiCl4, TiI4, TiF4), alkoxides, alkylamides, or also heteroleptic precursors [86]. As oxygen source, water is surely the most used but also ozone, H2O2, or plasmas have been studied and successfully applied. All the different crystalline forms of TiO2 can be obtained by ALD with proper deposition conditions, although often two or more crystal structures coexist and form a mixed-phase material. The deposition parameters that mostly influence the crystal structure of titania thin films are deposition temperature, the choice of precursors and oxygen sources, the substrate material, and the film thickness. Usually, low deposition temperatures lead to amorphous films, while anatase and rutile structures are frequently obtained with intermediate and high (above 300 C) temperature, respectively [86]. The growth of the rutile phase can be of interest, for instance, for charge storage devices because of the highest dielectric constant than the other two thermodynamically

Figure 17.5 Sketch of ALD process using self-limiting surface chemistry. ALD, Atomic layer deposition.

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Titanium Dioxide (TiO2) and Its Applications

stable phases [87]. Depending on the surface of the substrate, TiO2 thin film can start growing as amorphous or crystalline, even at temperatures where usually crystalline films form. For instance, the presence of a native oxide on the surface of typical Si substrate can lead to the formation of amorphous TiO2 layer with the beginning of crystallization only when the film thickness exceeds a critical value [88]. In contrast, the removal of the native oxide from Si can promote the formation of a crystalline film with a “island growth.” Regarding application in resistive switching-based devices, the control of the microstructure and phase of titania thin films is essential since these characteristics determine the ionic and electronic conductivity of the whole ReRAM cell. Owing to its excellent controllability and reproducibility down to atomic level, ALD is an effective way to produce thin films for ReRAM devices [89 95]. For these reasons, ALD was used by Kwon et al, in 2010, for their seminal paper on atomic structure of conducting filaments in TiO2 memories [89]. Through careful inspection with high-resolution transmission electron microscope, it was shown that conducting filaments in Pt/TiO2/Pt systems are composed by so-called Magne´li phases, represented by the formula TinO2n21 (with n 5 4 or 5). TiO2 samples that were not undergone to electroforming step were composed by a metastable brookite structure, rather than anatase or rutile crystallographic structures. This was a direct consequence of deposition conditions, since pristine samples were far from the thermodynamic stability condition, despite the deposition was carried out in a plasma atmosphere at temperature of 250 C (plasma-enhanced ALD, PEALD). In contrast, conducting filaments were formed by Magne´li phases, defective structures derived from rutile phase having a metallic conductivity near room temperature [96]. Therefore the effect of deposition temperature of TiO2 thin films and variation of deposition cycles of PEALD led to an improvement of the memory performances. Jeong et al. reported how it is possible to change the I V characteristics of Al/TiO2/Al cells just by changing the film deposition temperature of PEALD [97]. The same authors also demonstrated that it is important the substrate of the PEALD process. In fact, the performances of a flexible 8 3 8 crossbar memory array of Al/TiO2/Al/TiO2/Al device with polyethersulfone as substrate were better than those of structures fabricated on conventional Si/SiO2 substrate, when TiO2 was deposited at low temperature (80 C) [98]. Furthermore, Park et al. reported how ALD is an effective technique in reliably controlling the stoichiometry of titania thin films, thus tailoring resistive switching characteristics through layer oxygen vacancies [91]. Systematic control of the stoichiometry of TiOx films could be achieved by adjusting the reactions between the precursor [i.e., titanium(IV) tetraisopropoxide] and the plasma-activated reactant gas molecules (i.e., O2 1 N2 mixture). Not only the concentration of oxygen vacancies could be controlled but also the crystallinity of the resulting titania layers resulting in nanocrystalline anatase TiOx with low content of oxygen vacancies or in amorphous TiOx with more oxygen vacancies [91]. Regarding the switching mechanism, the authors found that VSET is dependent on the concentration of oxygen vacancies, decreasing with the reduction of VO2U, whereas the VRESET was found to be not dependent on the concentration of oxygen vacancies. ALD also demonstrated its versatile nature when was used for the

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homogeneous fabrication of multilayers of several binary oxides. For instance, Giovinazzo et al. studied the memristive behavior of Pt/TiOx:AlOy/Pt symmetric devices, fabricated by ALD at low temperature (100 C) where Al2O3 single layers were periodically inserted into TiO2 films [90]. Al2O3 acted as dopant, leading to the reduction of the oxygen ion mobility with a quadratic dependence of the VSET and VRESET on dopant concentration. The variation of doping concentration in the TiO2 film obtained by ALD enabled also to tune the resistance values and, thus, the HRS/LRS ratio. These studies demonstrate how ALD and PEALD are among the most reliable and promising deposition techniques for the fabrication of future ReRAM devices. However, a more detailed understanding of the growth of thin films is needed.

17.3.3 Sputtering Sputtering is, so far, the most used deposition technique both in industry and academic research. This physical method can be employed to deposit metals or oxides onto any substrate. The sputtering process is schematically reported in Fig. 17.6. Noble gas ions, typically Ar, are accelerated in the deposition room, with a voltage comprised between 50 and 1000 V, and hit the surface atoms of the target of the material we want to deposit on the substrate. Therefore surface atoms of the target are scattered backward due to collisions in a phenomenon called “back-sputtering” or simply “sputtering.” There are several types of sputtering systems: (1) DC, (2) RF (radio frequency), (3) magnetron diode, and (4) ion beam sputtering [99]. In DC sputtering the target material is at the cathode (negative potential) and the substrate is at the opposite side at the anode. The applied voltage between the plates leads to the ignition of a plasma discharge at pressure between 1021 and 1023 mbar, and thus the positively charged ions are push toward the target. The accelerated particles sputter off the material atoms, which deposit onto the substrate. The discharge is maintained as the electrons continuously ionize new ions by

Figure 17.6 Sketch of sputtering technique.

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Titanium Dioxide (TiO2) and Its Applications

collisions with sputter gas. When the conductivity of the target material is low (e.g., in the case of a dielectric material such as SiO2 or AlOx), a high-frequency plasma discharge must be applied, thus avoiding the buildup of a surface charge of positive ions on the front side of the insulating material [99]. RF-sputtering systems are then used for the deposition of insulator films directly from dielectric targets. The deposition rates are, however, lower than DC sputtering. A step further was done with magnetron sputtering. The magnets are organized as one pole is positioned at the central axis of the target and the second pole is formed by a ring of magnets around the outer edge of the target. Therefore the electrons (emitted from the target surface as the consequence of the ion bombardment) are trapped, increasing the probability of atoms ionization. The increased ionization efficiency leads to the formation of a dense plasma in the target region [100]. This gives higher sputtering rates and, therefore, higher deposition rates at the substrate. The disadvantage of this method is an inhomogeneous target erosion. Whenever a reactant gas is added to Ar, for example, O2, the technique is called reactive sputtering and is typically used for the deposition of metal oxides through a careful choice of the Ar/O2 pressure that directly influence the composition of the oxide layers. Physical and chemical properties of TiO2 films can be tailored changing several parameters during the sputtering process, such as applied voltage, reactive gases pressure, deposition time and temperature, and target substrate distance. Wang et al. reported that power and bias voltage are the parameters that mainly affect the structure and properties of TiO2 films deposited on 5083 aluminum alloy substrate [101]. Anatase phase resulted to be the main phase of titania layer and the anatase content increased by increasing the sputtering power and the applied voltage. Moreover, the increase of these two parameters led also to bigger particles but to more rough thin layers. As stated, it can be possible to adjust the stoichiometry of TiO2 films by changing, for instance, the O2 pressure during the deposition. This suits perfectly ReRAM active layer fabrication because the performances of ReRAMs are directly related to the composition of the oxide layer. Reddy et al. studied how the oxygen partial pressure in reactive sputtering can affect the electrical properties of 100 nm TiO2 thin films [102]. By changing the pO2 between 5.6% and 6.4%, there was a steep reduction in conductivity of titania, from 50 to 0.01 S/cm. In fact, at low pO2, concentration of oxygen vacancies in TiO2 increases leading to an enhanced conductivity. Sputtering technique is, therefore, a versatile method to reliably deposit TiO2 thin films, and it is widely reported how is effective for preparing titania layers for reproducible and endurable ReRAM devices [37,57,103 117]. Bousoulas et al. reported the correlation between resistive switching properties of titanium oxide and fabrication process studying the influence of oxygen flux during thin film sputtering at room temperature on cell performances [112]. In particular, for Au/Ti/ 45 nm TiOx/Au ReRAM cells, an increase in the oxygen flux during sputtering procedure led to an increase in the oxygen content in the titania layer. This meant a reduction in the concentration of oxygen vacancies with a consequent worsening in cell performances that happened also in the presence of a too high concentration of

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oxygen vacancies. Moreover, the concentration and distribution of oxygen vacancies had a direct impact on the conducting filament diameter, in terms of sensitivity of the conducting paths (high OFF/ON ratio). Regoutz et al. studied the behavior of 25 nm sputtered TiO2-based cells, highlighting the effect of thermal posttreatment on the characteristics of titania layer and, consequently, on the performances of the memory [113]. In that case, TiO2 was deposited by reactive magnetron sputtering at room temperature resulting in an amorphous layer that, after the annealing process at 600 C in an N2/O2 mixture, became crystalline, in anatase phase. The FORMING voltage resulted to be not dependent on thermal posttreatment, while the switching behavior, both in SET and RESET processes, was completely different due to the different structures and, mostly, on different oxygen contents (i.e., film stoichiometry), leading to a higher resistance ratio and lower power consumption. The switching behavior was ascribed to the formation/rupture conducting filaments formed by oxygen vacancies (VCM-like). On this matter, it is necessary to cite the paper wrote by Wedig et al. (Prof. Waser’s research group) where the motion of nanoscale ions in memristive systems (formed by Pt/metal oxide deposited by reactive sputtering/metal cells) was studied [37]. The authors stated that the resistive switching in TaOx, HfOx, and TiOx can be triggered not only by the motion of oxygen vacancies (as commonly thought) but also by the migration of metal cations, leading to the formation of a highly reduced conducting filament. A transition from VCM switching to ECM-like switching could be even reached by introducing an intermediate layer of amorphous carbon in the cells. This transition in switching mode was then described by Ge and Chaker in their study on Ti/TiO2/ Nb-SrTiO3 ReRAMs with TiO2 deposited by RF magnetron sputtering [111]. In that case the transition was due to the low concentration of oxygen vacancies, consequence of an annealing process in oxygen atmosphere, which did not lead to any resistive switching. The switching was instead reached by exploiting the oxidation of Ti electrode leading to an ECM-like switching mode, enabling an improvement of the performances of ReRAM cell, such as a higher ON/OFF ratio, smaller electronic leakage current in the HRS, and lower SET and RESET current. The main challenge related to sputtering as fabrication technique in ReRAM fields regards the relatively small deposition rates and a too low economic process efficiency.

17.4

Conclusions and perspectives

Memristors and ReRAMs are considered among the most promising devices for the new generation of nonvolatile memories and neuromorphic computing. In this frame, TiO2 was, and is, widely studied as possible oxide layer/solid electrolyte in the MIM structures that compose resistive switching-based electronic devices. In this work, we provided a brief overview of the use of TiO2 in memristors and ReRAM devices, highlighting the relationship between fabrication method and device performances. For the scope, we presented some of the fabrication methods of titania layer for ReRAMs, for example, anodizing, ALD, and sputtering. All

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these techniques demonstrated their potentialities as reliable ways to fabricate TiO2 thin films, with the possibility of a fine-tuning of morphological and electronic properties of the oxide. We also provided the main challenges to be faced in order to further improve the reliability of these techniques.

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Applications of TiO2 in sensor devices Giuseppe Mele1, Roberta Del Sole1 and Xiangfei Lu¨2 1 Department of Engineering for Innovation, University of Salento, Lecce, Italy, 2 School of Water and Environment, Chang’An University, Xi’an, P. R. China

List of abbreviations (AChE) (a.c.) (d.c.) (EIS) (ENMs) (FETs) (GC) (GOx) (LOD) (LSPR) (MAPbI3) (MIT) (MIPs) (MWCNTs) (NMs) (NPs) (NRs) (NSs) (NSA) (NWs) (NTs) (OFET) (OP) (PEC) (BAMPPV) (PANI) (PAHs) (POMs) (PPy) (PPyA) (PVP) (PPS) (QDs) (RH) (Ra)

Acetylcholinesterase Alternating current Direct current Electrochemical impedance spectroscopy Engineered nanomaterials Field-effect transistors Glassy carbon Glucose oxidase Limit of detection Localized surface plasmon resonance Methylammonium lead halide perovskite Molecular imprinting technology Molecularly imprinted polymers Multiwalled carbon nanotubes Nanomaterials Nanoparticles Nanorods Nanosheets Nanosheets array Nanowires Nanotubes Organic field-effect transistor Organophosphorus pesticides Photoelectrochemical Poly(2,5-bis(N-methyl-N-hexylamino)phenylene vinylene) Polyaniline Polycyclic aromatic hydrocarbons Polyoxometalates Polypyrrole Polypyrrolepropylic acid Poly(vinyl pyrrolidone) Porous polycrystalline silicon Quantum dots Relative humidity Resistance of gas sensors in the reference gas (usually air)

Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00004-3 © 2021 Elsevier Inc. All rights reserved.

18

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(Rg) (RTILs) (SPE) (SMO) (4S) (SERS) (SPR) (TNPs) (UV) (VCCs) (VOCs)

18.1

Titanium Dioxide (TiO2) and Its Applications

Resistance of gas sensors in the reference gas (target gases) Room-temperature ionic liquids Screen-printed electrode Semiconductor metal oxide Speed, sensitivity, selectivity, and stability Surface-enhanced Raman scattering Surface plasmon resonance TiO2 nanoparticles Ultraviolet Volatile chlorinated hydrocarbons Volatile organic compounds

Introduction

In the last decades a growing demand for the identification, quantification, and monitoring of chemical and biological compounds in different fields and with various purposes has been seen. The control of an increasing number of chemical and biological substances with application in environmental protection, food safety, clinical diagnosis, biomedical monitoring, and industrial control is becoming essential. Their quantification at always lower levels of detection as well with faster response times are some of the significant issues that need to be achieved. In this context the development of technologically advanced sensor devices that enable one to satisfy all the previous requests is a crucial issue. Nowadays, tremendous progress in this research field has been made, and numerous kinds of sensors are known that led to different sensors classification. Thus various names are common to define some of them, such as chemical sensors and biosensors; electrochemical, photoelectrochemical (PEC), and optical sensors; sensor array or multielement sensor system; and lab-on-a-chip sensor. Nevertheless, in general, a sensor is an electronic device in which the main components are the sensing material or receptor, the transducer, and the detector. The sensing material is devoted to interact with the analyte giving a signal variation such as temperature, mass, conductivity, work function, optical characteristic, reaction energy, which is received by the transducer that converts one of the variations mentioned earlier into an electric signal (such as capacitance, inductance, and resistance), and finally the detector amplifies the signal, which is usually proportional to the analyte concentration, and the circuit to which the sensor is connected gives rise to the sensing signal utilizing an appropriate data-processing system. It is worth reminding that electric signals are either current or voltage and for each we can measure the magnitude, frequency, and phase [1]. A simplified scheme of a semiconductor sensor is reported in Fig. 18.1. The basic parameters that need to be considered in order to achieve useful sensors are [2] high sensitivity, good selectivity, fast response time, fast recovery time, long-term stability, easy production, and easy miniaturization. In some papers the definition of “4S” characteristics, namely speed, sensitivity, selectivity, and stability, is used as important parameters to evaluate the performance of a sensor [3].

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Figure 18.1 Simplified scheme of a semiconductor sensor.

Sensitivity is generally defined as the quantitative change in the sensor’s response signal for a measurable unit change in the quantity of the analyte. Sometimes it is arbitrarily used sensitivity or limit of detection (LOD) where detection limit is the lowest concentration of the analyte that can be detected by the sensor under given conditions, particularly at a given temperature. In fact, often high sensitivity usually implies a rather low LOD. For a chemical sensor the selectivity is generally rather low. However, the introduction of sensor arrays allows overcoming the poor selectivity with the use of model-based and model-free multicomponent analyses. On the other hand, in biosensors, selectivity is achieved by the choice of the recognition element based on the biomolecular interaction process or alternatively by using molecularly imprinted polymers (MIPs)-based sensors where selectivity can be satisfied by synthetic polymers that mimic the biosensor behavior. Sensitivity can also be defined as Ra/Rg for reducing gases or Rg/Ra for oxidizing gases, where Ra stands for the resistance of gas sensors in the reference gas (usually the air), and Rg stands for the resistance in the reference gas containing target gases. Both Ra and Rg have a significant relationship with the surface reaction(s) taking place [4]. Response time is the time for a sensor to reach 90% of the total signal starting from the contact with the analyte, while the recovery time is defined as the time employed from the removal of the analyte to return to 90% of the original baseline signal [5]. In order to improve the previous requirements, different strategies have been taken into consideration over time. Primarily, the receptor process, as well as the transducer process, can be enhanced. Secondarily, the shape of the device, data processing, or other elements can be modified. The working process of the sensing material could be quite different for sensors of different types and purposes. Sensors can be made of different materials such as metal oxides, polymers, macromolecules, biomolecules, alloys, hybrid materials, ceramic materials, multiphase materials, and so on. In this context the significant advances in nanotechnology are giving an important aid in the progress of the sensor field. The inherent properties of nanomaterials (NMs), for example, super small size and large surface
volume ratio, can considerably improve the sensitivity of the sensor. It is well known that

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microstructure, as well as the morphology of the receptor (including its size, dimensionality, pore structure and volume, specific surface area, exposed surface facets, and crystalline phase), can significantly influence the sensor performance. Thus the structure can be hugely varied, ranging from zero-dimensional nanocrystals, one-dimensional (1D) nanowires (NWs) or nanofibers, two-dimensional (2D) nanosheets or nanoplates till three-dimensional (3D) hierarchical microstructures. On the other side, many transducer processes can be used in a sensor device to achieve a specific aim. Mainly electrochemical or optical transducers are used. Electrochemical techniques measure the variation of electrical current, potential, conductance, or impedance [electrochemical impedance spectroscopy (EIS)] at the interface of the analyte and the electrode, while optical sensors such as luminescent/fluorescent, colorimetric, surface plasmon resonance (SPR), electrochemiluminescence, surface-enhanced Raman scattering (SERS) cause variation of reflected or emitted light. An important class of sensing materials is the semiconductor metal oxides (SMOs). The SMOs are generally classified taking into account their energy difference and electronic configuration in transition metal oxides (Cr2O3, NiO, Fe2O3, etc.) and nontransition metal oxides (pre-nontransition metal oxides Al2O3, etc., and post-nontransition metal oxides ZnO, SnO2, TiO2, etc.) [3,6]. Among them the progress made in the sensor field for titanium dioxide used as sensing material will be thoroughly explored in this chapter. TiO2-based sensors have been widely applied in fields such as industrial manufacturing, aerospace, ocean exploring, environmental protection, resource investigation, medical diagnosis, and bioengineering and many efforts are focused on the ways that allow to tailor and organize TiO2 crystallites at the nanometric scale with the scope to improve their sensing performances [7]. This chapter mainly outlines state-of-the-art modification strategies in optimizing the performance of TiO2 in view of its technological application for the realization of sensor devices, including the introduction of intrinsic defects and foreign species into the TiO2 lattice, morphology and crystal facet control, and the development of unique mesocrystal structures. The band structures, electronic properties, and chemical features of the modified TiO2 nanomaterials are clarified along with details regarding their photocatalytic performance and various target analytes, based on different target analytes, such as gas substances, biomolecules, and environmentally relevant analytes; a detailed overview of latest innovative TiO2-based sensors is described.

18.2

Titanium dioxide in sensor field: principles and mechanisms of action

As a representative metal-oxide semiconductor, TiO2 has been extensively studied and considered as one of the most promising materials for gas detection due to its high chemical and mechanical stabilities, harsh environment tolerance, environment-friendly characters, and catalytic properties.

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Titanium dioxide
based materials had garnered extensive scientific interest since 1972 when Fujishima and Honda discovered photocatalytic splitting of water on a TiO2 electrode under ultraviolet (UV) light [8]. Semiconductor properties of TiO2 have been widely used 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, photocatalysis, to sensors [9
12]. Moreover, TiO2 presents high stability and environmental resistance as well as environment-friendly properties, making it one of the most attractive sensing material for sensors [13,14]. Among the three primary polymorph forms of anatase, brookite, and rutile of TiO2 with bandgaps of 3.2, 3.02, and 2.96 eV, respectively, in sensing field, rutile TiO2 and anatase TiO2 are the most studied polymorphs due to their higher stability than brookite. TiO2 crystal is electron rich and belongs to the n-type semiconductor. When gas absorbs onto the TiO2 surface, it could release electrons into TiO2, leading to the increase or decrease of resistance of TiO2 materials, the typical sensing mechanism of TiO2-based gas sensor. Furthermore, the conductivity property can be modified by doping other elements (especially metal elements) into TiO2 materials. By controlling the doping pattern, such as doping dosage and heating temperature, the n-type TiO2 materials can be transformed to p-type. Different from n-type, the resistance of p-type TiO2 will increase when contacting gases. Moreover, biocompatibility and environment-friendly of TiO2 make it a right candidate for the immobilization of biomolecules. Besides, TiO2 forms coordination bonds with the amine and carboxyl groups of enzymes and maintains the enzyme’s biocatalytic activity. Finally, due to the electron-accepting character of TiO2 the electrons produced by the reaction between biomolecules and analyte can be harvested by TiO2. The injected electrons can be transferred to the outer circuit, which can be used to detect the reaction. With all the aforementioned merits, TiO2 is one of the most competitive candidates for biosensors. Although TiO2 possesses the desired performance in utilizing UV light, its overall solar activity is still minimal because of a wide bandgap (3.0
3.2 eV) that cannot make the use of visible light or light of longer wavelength. This phenomenon is a deficiency for TiO2 with respect to its potential application in visible light photocatalysis and PEC devices, as well as photovoltaics and sensors. 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 electrical properties of TiO2, resulting in improved photocatalytic activity [15].

18.2.1 Mechanism of sensing The main components of the sensors are elements that respond to changes in physical or chemical properties, which are converted to electrical signals by the transducers (Fig. 18.2).

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Figure 18.2 Types of TiO2-based sensors or biosensors.

The main mechanisms of sensing related with devices based on TiO2 are summarized as follows.

18.2.1.1 Resistive-type gas sensors (chemiresistors) Specific categories of conducting material produce a change in their electrical resistance in response to an interaction with gases and vapors. TiO2 is one of the most typical semiconductor materials that can be engineered for sensing purpose involving resistance measurements related with simple electronic apparatus. This prompted the development of simple and inexpensive gas sensors based on the measurement of the device resistance. The name chemiresistor is ascribed to this kind of sensor. A recent trend in this field is based on the application of various nanomaterials in chemiresistor manufacturing. Polycrystalline SMOs interact with oxygen to form active oxygen species that alter the electrical charge at the grain surface. By reaction with combustible gases, active oxygen species are depleted, which results in an alteration of the resistance of the device [16]. As reported by Miller and coworkers, this kind of mechanism involves p
n and n
n potential barrier manipulation, n
p
n response type inversions, spill-over effects, synergistic catalytic behavior, and microstructure enhancement [17]. Different typologies of gas sensors are used for two-point direct current (d.c.) and alternating current (a.c.) conductivities measurement parallel to the surface as a function of the temperature and the partial pressures of oxidizing and reducing gases [18]. Mixed conductivity sensors consisting undoped TiO2 thin layers and two- and four-point arrangements of Pt contacts to determine d.c. and a.c. conductivities (e.g., as a function of O2 partial pressure at high temperatures) and Schottky-barrier sensors, drawn in Fig. 18.3, consisting front contacts (metal electrode/TiO2) formed

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Figure 18.3 A conventional resistive sensor based on Schottky barrier effect [20].

Figure 18.4 A typical SPR configuration and setup for biological and chemical sensing [22]. SPR, Surface plasmon resonance.

by low-temperature evaporation of Pt or Pd onto single-crystalline TiO2 (rutile phase) and SnO2 ohmic Zr back contacts can be used [19].

18.2.1.2 Optical sensing Nanomaterials based on localized SPR (LSPR) phenomena are revealing to be a key solution for several applications, namely, those of optical biosensing (Fig. 18.4). This effect is generally studied, examining the photocatalytic activity of Au nanoparticles (NPs) on TiO2 films. The resonant behavior is due to the lightinduced collective displacement of conduction electrons with respect to the positive ionic background in the nanoparticle, which results in a restoring force due to surface polarization [21]. The main reasons are mostly related to their high sensitivity,

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Titanium Dioxide (TiO2) and Its Applications

with label-free detection, and to the simplified optical systems that can be implemented.

18.2.1.3 Photoconductive devices TiO2 nanoparticles are commonly used as precursors to make functional nanocomposites in photoelectric devices. For that is concerning this kind of device, surface defects at grain boundaries of TiO2 nanoparticles may behave as deep electron traps that cause additional carrier recombination, resulting in a severely negative effect on the photoelectric performance of TiO2 nanomaterials. The photoconductivity of semiconductor materials mainly depends on the free carrier concentrations, which depend on the competition between the photogenerated excess carriers and charge recombination, while photoconductive materials can increase the electrical conductivity by the absorption of photons. Thus the study of the photoconductivity of new materials has drawn considerable attention considering their potential applications in various areas such as in gas sensors [23] as well as for selective detection of pollutants in aqueous solutions [24], and the great efforts focused on modifying the surface of TiO2 nanoparticles are also reasonable.

18.2.1.4 Photoelectrochemical sensing As a newly emerged and promising analytic method for the detection of biological substances, PEC biosensor has the features of simple instrument, low cost, and easy miniaturization, which is well suited for rapid and high-throughput bioanalysis. Moreover, it owns potentially higher selectivity because of the separated and different energy forms of the excitation source and detection signal resulting in reduced background signals. Hence, PEC biosensor has attracted considerable research interests. Obviously, photoactive materials act as a vital factor in the analytical performances of PEC biosensors. TiO2 nanomaterials are the most promising and effective materials in PEC biosensing, attributing to their excellent biocompatibility, chemical, and mechanical stability, strong UV
vis absorption, and low cost [25]. Schematic representation of a brand new cost-effective photo electrochemical sensing activity of self-organized TiO2 nanotubes via electrochemical anodization process vis. sodium-hydroxide soaking treatment to form sodium-modified TiO2 has been reported in Fig. 18.5 [26].

18.3

Gas sensors

A gas sensor is a kind of device that can transfer the component or concentration information of gas into electric signals or some other signals that can be easily detected. TiO2, as well as ZnO, SnO2, α-Fe2O3, NiO, and Cu2O, possess extensive applications as gas sensors due to their high sensitivity, low cost, and simplicity. To boost their excellent sensing performance and meet the growing demand for

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Figure 18.5 Schematic representation of a brand new cost-effective photo electrochemical sensing activity of self-organized TiO2 nanotubes via electrochemical anodization process vis. sodium-hydroxide soaking treatment to form sodium-modified TiO2 [26].

applications, a series of strategies have been developed, such as the surface morphology engineering and function manipulation. Recently, the controlled morphology with exposed high-energy facets and the facet-dependent sensing properties have attracted much attention. Because of its abundant unsaturated active sites, the crystal planes with high surface energy usually serve as a promising platform for gas sensing. Recent advances have involved the design of engineered crystal structures with exposed high-energy facets for producing materials having enhanced sensing performances. The relationship between dangling bonds density and gassensing properties can be considered as one significant factor in evaluating the superior sensing surface of these SMOs. Hierarchical and hollow oxide nanostructures are very promising gas sensor materials due to their high surface area and well-aligned nanoporous structures with less agglomerated configurations. The literature data clearly show that hierarchical and hollow nanostructures increase both the gas response and response speed simultaneously and substantially. This can be explained by the rapid and effective gas diffusion toward the entire sensing surfaces via the porous structures [27]. SMOs used as gas-sensing materials, apart from large surface-to-volume ratios, well-defined and uniform pore structures are particularly desired for improved sensing performance. The role of some key structural aspects in porous gas sensors,

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Titanium Dioxide (TiO2) and Its Applications

such as grain size and agglomeration, pore size, or crack-free film morphology, is very important for the sensing mechanism. New synthesis concepts, for example, the utilization of rigid matrices for structure replication, allow one to control these parameters independently, providing the opportunity to create self-diagnostic sensors with enhanced sensitivity and reproducible selectivity [28]. The use of nanostructures for gas sensing is attributed to the high specific area, change of resistance on exposure to gases, and high photoconduction abilities, while the photon-carrier collection abilities and antireflectance qualities are vital for solar photovoltaic cells. Nanostructures have also been used as coating pigments to prevent corrosion of facilities, reduce urban heat islands and energy consumptions, due to the nearinfrared reflective characteristics [29]. This paragraph is focused on explaining how different parameters can affect the 4S performances of TiO2 gas sensors, also giving an insight into the dopant or impurity induced variations in the SMO materials used for gas sensing. In fact, dopants enhance the properties of SMOs for gas sensing applications by changing their microstructure and morphology, activation energy, electronic structure, or bandgap of the metal oxides. In some cases, dopants create defects in SMOs by generating oxygen vacancy or by forming solid solutions. These defects enhance the gas sensing properties. Different nanostructures (nanowires, nanotubes, and heterojunctions) other than nanopowders have also been used to this scope [30]. Gas sensors can be opportunely engineered with the aim to detect the presence of various gases, including combustible, flammable, and toxic gases harmful to organic life. TiO2 gas sensor can detect different gases, including oxidative gas such as O2, NO2, and reductive gas, such, for example, H2, CO, NH3, H2S, including volatile chlorinated hydrocarbons (VCCs) generating change in the resistance. Usually, the microscopic reactions between these gases and TiO2 surface are much different, the sensing mechanism is more complicated, and the sensor performance could be affected by many factors. However, the sensing mechanism can be recognized by the following two processes: receptor process and transducer process. TiO2 thin films for gas sensors can be prepared by various methods such as magnetron sputtering technique, electric field aerosol-assisted chemical vapor deposition, and ultrasonic spray pyrolysis [31]. Solid-state gas sensors based on nanocrystalline metal-oxide thin films can provide a faster response with real-time analysis capability, higher spatial resolution, simplified operation, and lower running costs compared with conventional methods such as chemiluminescence and infrared spectrometry. Their sensitivity and selectivity are dependent on operating temperature, film thickness, porosity, and grain size and can be increased by doping with metals.

18.3.1 H2O (humidity) A humidity sensor can convert the ambient moisture variation into an electrical signal (conductance or capacitance) variation. These kinds of sensors, opportunely engineered as part of an electronic device, are capable of detecting or controlling

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the ambient humidity. The ceramic humidity sensor can be classified into conductance type and capacitance type depending on the electrical signal variation. The conductance type can be subclassified into ionic type and electronic type depending on the conduction carrier. The mechanism of sensing involved in the ionic conduction type humidity sensor implicates the following steps: G

G

G

At low relative humidity (RH) a few water vapor molecules are chemically adsorbed on the grain surface to form hydroxyl groups. The hydroxyl groups dissociate, providing mobile protons (H1). When more water is physically adsorbed on the hydroxide surfaces, it can form an icelike cluster of hydrogen-bonded water molecules. In this stage the dominant conduction carrier is H3O1. At higher RH the H3O1 ion is hydrated into H1 and H2O in the presence of enough adsorbed water, and each water molecule is singly bonded with one hydroxyl group, so the dominant conduction carrier H1 can be easily moved, increasing the conductivity of the sensor.

For an excellent humidity-sensitive characteristic, the conductance of the sensor must be small in low-moisture atmospheres. The ceramic humidity sensor must be sintered to produce a porous structure to allow water vapor to pass easily through the pores. The adsorbed water vapor condenses within the capillaries between the grain surfaces. A quantity of defect lattice sites and “nonlattice” oxygen atoms at oxide surfaces can also increase the water molecule adsorption sites: thereby increasing the humidity-sensitive characteristics of the sensor. The electrical properties of sensors in a moist atmosphere can be analyzed by both d. c. and a.c. techniques. The d.c. versus time curves measured on charging and discharging processes can be used to identify whether the conduction carrier is an ion, electron, or both. It can also be used to determine which is the dominant conduction carrier. Porous ceramic based on pure TiO2 doped with a few molar percent K2CO3 was used as humidity sensor. The conductance versus RH sensitivity of the sensor sintered at 1473K was as high as four orders of magnitude in the range of 15%
95% RH at 200 Hz and 298K. The sensors were reversible without repeated high-temperature thermal desorption processes, and the electrical properties were determined by both d.c. and a.c. analysis techniques under different RH conditions. The sensors could be polarized similar to electrolytes in a charging process as a result of the electrode and water molecular polarization effects, and the effects were enhanced with increasing RH. The conduction carriers of the sensors in a moist atmosphere were ions and electrons, and the dominant conduction carriers were ions. By complex impedance plots with a “non-Debye” capacitor concept, an equivalent circuit model was established, which can simulate well all the electrical properties of the sensors in the range of 65%
95% RH for all measured frequencies (5 Hz to 13 MHz) [32]. TiO2 synthesized via a solvothermal route in the form of single-crystalline TiO2 nanowires was found to display excellent humidity sensing abilities as functional materials in the humidity sensor application. With RH increased from 5% to 95%, about one and a half orders of magnitude change in resistance was observed in the TiO2 nanowire-based surface-type humidity sensors. The photogenerated charge

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transfer and PEC properties of the TiO2 nanowires, based on the experimental data from the electric field
effected photocurrent action spectrum and Mott
Schottky measurements [33], exhibited unique electronic properties, for example, favorable charge-transfer ability, negative-shifted appearing flat-band potential, existence of abundant surface states of oxygen vacancies, and high-level dopant density in comparison with commercial TiO2 P25 nanoparticles [34]. Resistive-type humidity sensors were produced by photopolymerization of polypyrrole (PPy) and TiO2 nanoparticles (TNPs) in the form of composite thin films (TNPs/PPy) supported onto an alumina substrate. The electronic properties related to the humidity sensing mechanism of TNPs/PPy composite thin films investigated studying activation energy and impedance spectroscopy, showed the highest sensitivity, smaller hysteresis, and best linearity [35]. Another humidity sensor that exhibited excellent sensing characteristics, such as an ultrafast response and recovery times, good reproducibility, linearity, and environmental stability, was based on LiCl-doped TiO2 nanofibers with poly(vinyl pyrrolidone) (PVP) nanofibers as a sacrificial template has been fabricated through electrospinning and calcination [36].

18.3.2 Dihydrogen (H2) Nowadays, the production of hydrogen safety sensors working under technically specified conditions of temperature, pressure, RH, power consumption, and lifetime is of paramount importance. Also, in this type of sensors, the morphology, compositional modification of TiO2 as well as the detection parameters are essential to design functional gas sensors. Primary categories include catalytic sensors, electrochemical sensors, resistive palladium, palladium alloy sensors, field-effect transistors (FETs), and semiconductor metal-oxide sensors. Many efforts are oriented to reduce the gaps in the performance of hydrogen sensing technologies that are thus identified and areas recommended for future developments [37,38]. Highly ordered, vertically oriented TiO2 nanotubes were prepared and successfully used as hydrogen sensors at low temperatures [39]. These self-organized nanotube arrays were grown by anodic oxidation of a titanium foil in an aqueous solution that contains 1 wt.% HF at 293K. A potential ramp at a rate of 100 mV/s was applied, increasing from the initial open-circuit potential to 20 V, and maintaining constant the final potential of 20 V during the anodization process. The fabricated TiO2 nanotubes were annealed in a dry air atmosphere at 773K for 3 h to obtain high sensitivity to hydrogen. The TiO2 nanotubes, grown with this morphology, were approximately 1 μm in length and 90 nm in diameter, and two platinum pads were used as electrodes on the TiO2 nanotube arrays for the sensor construction. The hydrogen sensing characteristics of the sensor were analyzed by measuring the sensor responses [(I 2 I0)/I0] in the temperature interval of 293K
423K. The sensitivity of the sensor was approximately 20 for 1000 ppm H2 exposure at room temperature and increased with increasing temperature. The sensing mechanism of the TiO2 nanotube sensor could be explained with the chemisorption of H2 on the highly active nanotube surface. The hydrogen

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sensitivity of the synthesized nanotubes increases nearly 10-fold, from 10 to 18, with increasing temperature (298K
423K) for 100 ppm H2. Nanocrystalline powders of TiO2 and TiO2:Cr (0.1
10 at.% Cr) obtained by flame spray synthesis have been used as starting materials for the preparation of gas sensors. SEM images demonstrate the agglomeration of crystallites into spherical grains. Gas sensing characteristics of TiO2:Cr nanosensors upon interaction with H2 are recorded in a self-assembled experimental system. Detection of hydrogen is carried out over the concentration range of 50
3000 ppm at the temperatures extending from 473K to 673K. It is demonstrated that nanomaterials based on TiO2:Cr are attractive for ultimate sensor applications due to a substantial decrease in the operating temperature down to 483K
523K. At a certain level of doping (about 5 at.%), a reversal of the sensor response from that of n-type to that of p-type semiconductor is seen [40]. TiO2 nanotubes, prepared by anodization, are highly sensitive to hydrogen; for example, cycling between nitrogen atmosphere and 1000 ppm hydrogen a variation in measured resistance of 103 is seen for 46 nm diameter nanotubes at 563K. The hydrogen sensors are reversible and have response times of approximately 150 s. Field emission scanning electron microscopy and glancing angle X-ray diffraction were used to study the surface morphology and crystal structure of the nanotubes. Titania nanotubes prepared using anodization and annealed in an oxygen atmosphere at a temperature of 773K were found highly sensitive to hydrogen. The nanotube sensors contain both anatase and rutile phases of titania and showed appreciable sensitivity toward hydrogen at temperatures as low as 453K. The sensitivity increased drastically with temperature showing a variation of three orders in magnitude of resistance to 1000 ppm of hydrogen at 673K. The response time decreased with increasing temperature at 563K, full switching of the sensor took approximately 3 min. Results were highly reproducible with no indication of hysteresis. The results showed that these sensors are capable of monitoring hydrogen levels from 100 ppm to 4%. At 563K, nanotubes with smaller pore diameters (46 nm) showed higher sensitivity to hydrogen compared to larger pore diameter samples (76 nm). The sensors showed high selectivity to hydrogen compared to carbon monoxide, ammonia, and carbon dioxide. Although the sensor was sensitive to high concentrations of oxygen, the response time was high, and the sensor did not wholly regain the original condition. The hydrogen sensitivity of the nanotubes is due to hydrogen chemisorption onto the titania surface, where they act as electron donors [41]. A flexible gas sensor of hydrogen based on a TiO2 thin film on polyimide foil capable of room-temperature operation with a response of 104 for 1% H2, bending 1000 times over diameter of 10 mm without reducing its performance, has been recently prepared [42].

18.3.3 Dioxygen (O2) The interaction of O2 with TiO2 surfaces plays a key role in many technologically important processes such as catalytic oxidation reactions, chemical sensing, and photocatalysis. While O2 interacts weakly with fully oxidized TiO2, excess electrons are often present in TiO2 samples. These excess electrons can be originated

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Titanium Dioxide (TiO2) and Its Applications

from intrinsic reducing defects (oxygen vacancies and titanium interstitials), doping, or photoexcitation and form polaronic Ti31 states in the bandgap near the bottom of the conduction band. Oxygen adsorption involves the transfer of one or more of these excess electrons to
an O2 molecule
at the TiO2 surface. This results in an adsorbed superoxo OU2 or peroxo O22 species or molecular dissociation and 2 2 formation of two oxygen adsorbed atoms (2 3 O22). Oxygen adsorption is also the first step toward oxygen incorporation, a fundamental reaction that strongly affects the chemical properties and charge-carrier densities; for instance, it can transform the material from an n-type semiconductor to a poor electronic conductor [43]. Different (two) types of TiO2-based oxygen sensors operating at different temperatures with different detection principles have been tested. At high temperatures, TiO2 devices can be used as thermodynamically controlled bulk defect sensors to determine oxygen over an extensive range of partial pressures. Their intrinsic behavior can be controlled by carefully directed doping with tri- or pentavalent cations. At low temperatures, it has been found that Pt/TiO2 Schottky diodes make extremely sensitive oxygen detection possible. The latter show reversible shifts of current-voltage curves, which are determined by interface states formed by chemisorbed oxygen [19]. Electronic conductivity, mixed conductivity, and Schottky-barrier sensors based upon the oxides SnO2 and TiO2 which are modified or contacted by the metals Pd and Pt can be used to detect gases such as CO, CH4, H2, and O2. The response signals of these different types of sensors result from changes in surface and bulk conductivities or in Schottky-barrier heights, which are measured using different geometric arrangements of metal (Pd or Pt) contacts. The atomistic understanding of sensing mechanisms and corresponding sensor structures is deduced from comparative spectroscopic and electrical measurements. Studies showed as reliable sensor properties can only be achieved either by forming stable metal/oxide interfaces or by adjusting a stable dopant distribution. When O2 adsorption works at low temperatures, the oxygen can be chemisorbed as O22 or O2 (the latter may be formed by dissociative adsorption at platinum and a subsequent spill-over effect to TiO2). Thin-film structures with ohmic Pt back contacts (prepared at high temperatures) and with microstructured Pt Schottky contacts with large three-phase boundaries subsequently prepared at low temperatures permitted the preparation of Schottky-barrier sensors with stable dopant distributions in the subsurface regions [44]. Titanium dioxide thin films prepared by using the sol
gel method and subsequently doped with Nb2O5 were used in oxygen sensing applications. In this case the chemical composition of the resultant film sensor surface was essentially stoichiometric with carbon as the dominant impurity at the surface. Measurements of electrical resistance permitted the detection of oxygen at concentrations of 1 ppm to 1%. Doping resulted in a 40% increase in the oxygen gas sensitivity of the thin films at operating temperature, as low as 463K. TiO2 films deposited by the sol
gel method were found to be stoichiometric [45]. TiO2 thin-film prepared by a sol
gel process deposited on an alumina rod by dipcoating of a titania solution was used as resistive oxygen sensors. In particular, linear relations between the dependence of the logarithm of the resistance lg(R) and the logarithm of the oxygen partial pressure of O2 lg(pO2) with temperature-dependent

Applications of TiO2 in sensor devices

541

slopes and response time decrease with increasing temperature [46]. The response time of this kind of sensor was shorter for an atmosphere change from oxygen rich to lean than that from oxygen lean to rich. Furthermore, the response time was reduced by the presence of a Pt coating on the TiO2 surface. Sensor response at moderate temperatures of TiO2:Nb and ZnO thin-film has been reported. These materials showed a rapid and reliable response of photoconductivity to oxygen pressure changes in the region of 1023
1 atm. The response of steady-state photoconductivity to changes in oxygen partial pressure (1 atm) has been quantitatively studied in thin-film polycrystalline at 353K
393K. The magnitude of photoconductivity varied as a square root of illumination intensity regardless of oxygen pressure. Both materials showed a fast response to oxygen, although in different pressure ranges. ZnO was more sensitive to lower oxygen pressures, while titanium dioxide worked better at pressures close to 1 atm [47]. Looking for practical applications, some work has been focused on the evaluation of microfabricated Pt-doped titanium oxide thin film sensors’ capability to discriminate different combustion conditions of a real gasoline engine. Sensor devices such as zirconia-based lambda probes used in the combustion control for fuel injection engines suffer high production costs. A cheaper mass fabrication method consists thin-film platinum-doped TiO2 sensors, performance of which was validated under real working conditions. A lambda probe and the Pt-doped TiO2 thin film sensor fabricated onto ceramic alumina substrates opportunely arranged in a dedicated experimental bench enabled simultaneous data acquisition investigated under different operative temperatures. A simple threshold-based ON/OFF data analysis allows satisfactory responses of Pt-doped TiO2 sensor to discriminate the amount of O2 under rich and lean combustion conditions in relatively short response times [48]. Binary nanosized V2O5
TiO2 thin-film sensors made by using the sol
gel technique, after annealing at 773K, gave a monoclinic polycrystalline structure of 0.8-mm thickness consisted of average grain size in the range of 3
5 nm. The sensor properties of these materials, investigated in the temperature range of 473K
623K permitted the detection of oxygen from 10 ppm up to 20.9% with a response time of a few minutes. Interestingly, these sensors were fond relatively insensitive to ozone [49]. An oxygen gas microsensor based on nanostructured sol
gel TiO2 thin films with a buried Pd layer was developed on a silicon substrate. A sandwich TiO2 square board with an area of 350 μm 3 350 μm was defined by both wet etching and dry etching processes. A pair of 150 nm Pt micro interdigitated electrodes with a 50 nm Ti buffer layer was fabricated on the board by a lift-off process. The sensor chip was tested in a furnace with changing the O2 concentration from 1.0% to 20% by monitoring its electrical resistance where stability and sensitivity (0.054 with deviation 2.65 3 1024 and hysteresis is 8.5%) were demonstrated after several testing cycles [50].

18.3.4 CO2 Monitoring the level of CO2 in closed spaces is required in technological applications or human activities. Commonly high sensitivity detection materials for CO2 work at a temperature over 573K. CO2 gas sensors based on Cr-doped thin films

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Titanium Dioxide (TiO2) and Its Applications

working at operating temperatures close to the room temperature and at atmospheric pressure were obtained by radio frequency magnetron TiO2 reactive sputtering. The undoped films contain a mixture of anatase and rutile phases. With the increase of Cr content, the crystallite size decreases, and the films become pure rutile for a 4 at.% Cr concentration. These TiO2-based sensors are more sensitive to CO2 for higher Cr concentration, the optimum operating temperature approaching to the room temperature, determining, in fact, low energy consumption. These films were able to detect CO2 at temperatures nearer to room temperature under atmospheric conditions. The doped film TiO2:Cr 3/3 produced an increase in the sensor response of about nine times compared with the undoped film in an atmosphere of 10% CO2 at 328K. The improvement of the sensing properties was ascribed to the presence of the rutile phase, which has a higher dielectric constant than anatase; the increase of the oxygen vacancies number; the increase of the active surface area and the decrease of the crystallite size determining better accessibility of the adsorbed O2 and/or CO2; and the presence of Cr31 which increases the power of interaction of the LUMO states of TiO2 matrix with the adsorbed concentrated on the film surface. Furthermore, the surface-active area in front of CO2 increases, as the films become rougher for higher Cr contents. The increase of Cr31 percentage enhances the power of interaction with the adsorbed O2 and/ or CO2. The sample TiO2:Cr 1/3, despite its lower sensitivity to CO2 than the TiO2:Cr 3/3-doped film, produced the faster response (81 s) and recovery(63 s) times [51]. TNPs in pure anatase phase were synthesized by a sol
gel method with a particle size distribution within the range 7.5
10.5 nm. TNPs and Al electrodes deposited on porous polycrystalline silicon (PPS) substrate by electron beam evaporation at 298K gave polycrystalline silicon wafers with CO2 sensing properties. Electrical measurements demonstrated the semiconducting behavior of thin film-devices in which conductivity was increased on exposure to CO2 gas. The gas sensitivity, studied on exposure to 10% CO2 gas, was explained on the basis of change in the surface conductivity of thin-film devices and deduced within the frame work of the band theory. The adsorbed gases produced appropriate donor or acceptor levels within the bandgap of material at the film surface consisting different grains with different orientations and nonuniform pore size distribution in porous layer with an average diameter of 5 μm. The device with TNPs thin film (Al/Si/PPS/TNPs/Al) was more sensitive and had better response and reversibility in comparison with the device without TNPs thin film (Al/Si/PPS/Al). The sensitivity factor of Al/Si/PPS/TNPs/Al was obtained in the range of 2
2.32 and response time 95 s, recovery time 110 s, and percentage of reversibility 86 toward 10% CO2 [52].

18.3.5 NH3 An ultrasensitive nanostructured sensor capable of detecting 50 ppt of NH3 gas in the air has been prepared enchasing nanograins of a p-type conductive polymer, polyaniline (PANI) on an electrospun n-type semiconductive TiO2 fiber surface. The resistance of the p
n heterojunctions combining with the bulk resistance of PANI nanograins can function as electric current switches when NH3 gas is absorbed by PANI nanoparticles. This kind of sensor was 1000 times more sensitive than the best PANI sensor reported in the literature [53].

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Harb et al. fabricated an organic FET (OFET) based on the bottom contact of a PANI or PANI/TiO2 nanocomposite as an active layer and SiO2 as an insulating layer to be used for ammonia gas sensing applications. The OFET sensors exhibited a change in the drain current when exposed to NH3. TiO2 nanoparticles with different weight percentages (0
50 wt.%) were added to dope PANI and enhance charge carrier transport, although the response of both the PANI OFET sensor and PANI/TiO2 OFET sensor has reached saturation value at almost the same period. The response of the PANI/TiO2 transistor is (2.5), which is much higher than that of PANI (0.17). The response of the sensor device fabricated with PANI/TiO2 was 15 times greater than that with a device fabricated using pristine PANI [54].

18.3.6 CO The detection and measurement of the concentration of gases such as carbon monoxide (CO) and hydrogen in the ambient are important from the viewpoint of industrial and automobile exhaust pollution control. On the other hand, CO is considered one of the main air pollutants that is harmful to public health and environment by the US Environmental Protection Agency [55]. Gas sensors designed for this scope have been realized by using TiO2-based semiconducting oxide material as a reliable and rugged CO and H2 working in the temperature range of 773K
1073K. Significant change in the sensing characteristic of the anatase modification of TiO2 was observed when admixed with an insulating second phase, such as Al2O3 or Y2O3. In the case of TiO2
10 wt.% Al2O3, the sensor response was found to be exclusively dependent on the hydrogen concentration alone; the presence of CO or CO2 did not affect the sensitivity. On the other hand, the sensor based on TiO2
10 wt.% Y2O3 showed increased sensitivity to CO and decreased sensitivity to H2, compared to that of the undoped TiO2. The addition of elemental iron, in small concentration, to the two-phase mixture of titania and yttria seemed to further improve the sensitivity and selectivity of the latter to CO [56]. More details about the mechanism of sensing based on CO adsorption and CO/O2/H2O coadsorption were reported by Hsu et al. [57]. Nanostructured-mixed Ti/Fe oxide thin films were prepared by using radio frequency magnetron sputtering
assisted annealing. The composite film was dense and compacted with 20
30 nm TiO2 grains. The sensor performance was affected by the number of Fe insets, that is, the larger the abundance of Fe, the higher the response. The response was: 310% for the film with eight insets of Fe and 60% for the film with only two insets to 15 ppm of CO at a working temperature of 573K. The higher performance is ascribed to the nanosized structure of the film [58]. TiO2 nanomaterials are assembled into a sensing electrode and fixed in a sealed chamber where also used as a resistive sensor for CO detection. For a specific review, see Ref. [59]. TiO2 nanotubes grown using an electrochemical method and La0.8Sr0.2 Co0.5Ni0.5O3 (LSCNO) perovskite film combined to form a p
n type heterojunction structure for use in a carbon monoxide CO sensor at 473K for a 400 ppm concentration were made (see Fig. 18.6) [57]. An array of TiO2-based sensors opportunely engineered has been used to measure the concentrations of a gaseous mixture of CO and O2 at high temperatures.

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Titanium Dioxide (TiO2) and Its Applications

Figure 18.6 (A) Schematic for electrochemical growth apparatus. (B) Schematic diagram for the current voltage and the ethanol response measurements of the TiO2 nanotube samples. (C) Schematic diagram for the current voltage and the CO response measurements of the TiO2/LSCNO heterojunction film samples [57].

TiO2 was doped with La2O3 and different levels of CuO providing orthogonal information over specific gas concentration ranges. Responses of the sensor arrays were analyzed using kernel ridge regression modeling to determine the concentrations of CO and O2 in gas mixtures at 873K. Two variations of a two-sensor combination related respectively with an La2O3-doped TiO2 and with a second sensor in the array CuO
La2O3-doped TiO2 sensor, doped with different levels of copper. Different arrays of sensors (identified as I and II) were used to this scope. The sensor array I, with 2 wt.% CuO, was made by mechanical mixing of commercial metal-oxide powders, while the sensor array II, with 8% of CuO deposited onto the 10% La2O3
TiO2, was fabricated using the chemisorption hydrolysis method. The sensor array I with 2% CuO provided a high degree of orthogonality for O2 and CO at very low levels of both CO and O2. Sensor array II with 8% CuO has high orthogonality at low levels of CO for a wide range of O2 concentrations. Sensor array I was used to extract the concentrations of CO and O2 in a gas mixture over ranges of 2%
10% O2 and 250
1000 ppm CO, while the sensor array II with 8% CuO sensors to extract concentration over the range of 0
1000 ppm CO in the presence of 2%, 5%, and 10% O2 [60].

18.3.7 NO2 As one of the greenhouse pollutants, NO2 exhibits toxicity and may cause acid rain and photochemical smog. Gas NO2 sensors are extensively used, in order to monitor the concentration of NO2, which is necessary for controlling NO2 emission in the atmosphere. Among all sensing materials, TiO2 has aroused considerable interest due to its extraordinary chemical stability, resistance to harsh atmospheric conditions, easy oxygen adsorption on its surface, simplicity to prepare, and low production cost. Based on the n-type semiconductor, the sensing mechanism is the changes in concentration of oxygen adsorbed on the surface of TiO2 before and after exposure to NO2 gas. During this process, NO2 acts as a scavenger for photogenerated electrons resulting in decreasing the photocurrent. At present, there are following sensors based on TiO2 for detecting NO2 gas: (1) pure TiO2, including TiO2 thin film [61], TiO2 nano-tubular [62], and TiO2 nanowires

Applications of TiO2 in sensor devices

545

[63]; (2) Al-doped TiO2 [64,65]; (3) Au-doped TiO2 [66,67]; (4) Cr-doped TiO2 [68]; (5) TiO2/ZnO nanowire [69]; and (6) Sol
gel graphene/TiO2 nanoparticles [70]. Among the sensor materials above, all the sensing mechanisms mainly depend on the conductivity of TiO2 under UV-irradiation. The changes in conductivity are caused by the increasing or decreasing amount of the charge carriers. Thus the challenges in the area of NO2 gas sensors are shifted onto the fabrication of sensing electrodes as smallscaled and highly crystallized materials. For the pure TiO2 the thickness of the film is often required to be below 10 nm. Furthermore, only the process of producing electronic devices satisfied the condition, such as the atomic layer deposition method, radiofrequency sputtering, reactive magnetron sputtering, and screen printing technology. For the Al-doped TiO2, Al doping retards the anatase to rutile transformation temperature. It reduces grain growth, which leads to the increase of the electronic and ionic conductivity of TiO2, allowing using these sensors at 873K and 1073K. For Au-doped TiO2 the addition of Au nanoparticles decreases the response and recovery times of the sensors: it allows for reaching response times of 0.5 min at 513K, especially at lower temperatures (,523K). For Cr-doping of TiO2, Cr results in improvement of the high-temperature NO2-sensing capability in the temperature range of 573K
773K and concentrations between 25 and 100 ppm. Moreover, the cross-sensitivity to CO has been reduced by doping, as demonstrated during the mixed gas detection of NO2. For the TiO2/ZnO NW heterostructure film, it exhibited a response of approximately seven toward 5 ppm of NO2 at an operating temperature of 473K with faster response kinetics. For sol
gel graphene/TiO2 nanoparticles, its calibration curve has a good linear relationship (R2 5 0.984) according to the changes in resistance when exposed to NO2 in the range of 70
1750 ppb. Table 18.1 summarized some of the advanced NO2 sensors based on TiO2 and their corresponding operating conditions.

18.3.8 Volatile organic compounds Since the sensing capability of semiconducting metal oxides was demonstrated in the 1960s, solid-state gas sensors based on these materials have attracted considerable attention from both scientific and practical points of view. Because of the promising Table 18.1 Operation conditions of TiO2-based NO2 sensors. Sensor materials

Operation temperature (K)

Detection limits (ppm)

TiO2 thin film [61] TiO2 nano-tubular [62] TiO2 nanowires [63] Al-doped TiO2 [64] Au-doped TiO2 [66] Cr-doped TiO2 [68] TiO2/ZnO nanowire [69] Sol
gel graphene/TiO2 nanoparticles [70]

Room temperature, UV 573
773 Room temperature, UV 673
1073 453
673 523 523 Room temperature, UV

100
500 10
100 100 50
200 1 2
50 2
20 0.07
1.750

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Titanium Dioxide (TiO2) and Its Applications

characteristics for detecting toxic gases and volatile organic compounds (VOCs) compared to conventional techniques, these devices are expected to play a key role in environmental monitoring, chemical process control, personal safety, and so on soon. Therefore in recent years, intensive studies have been conducted to improve their sensing performances, particularly to increase the sensitivity and detection limit of such devices. This can be accomplished by using metal-oxide nanostructures with various shapes such as nanoparticles, nanowires, nanorods, and nanotubes having sizes in the nanometer range. Owing to the high surface-to-volume ratios and consequently, the large number of surface sites exposed to target gas, nanostructured metal oxides enable a gas-sensing layer interaction and hence a higher sensitivity in comparison with conventional materials. Recent developments related to the sensing properties of conductometric solid-state sensors based on nanostructured semiconducting metal oxides for the detection of some common VOCs include acetone, acetylene, benzene, cyclohexene, ethanol, formaldehyde, n-butanol, methanol, toluene, and 2-propanol [71]. Mesoporous indium-doped TiO2/WO3 nanohybrid with high surface area and ordered pore structures is prepared by using the hard template of Santa Barbara Amorphous-15 via the nanocasting process. Gas sensors based on mesoporous nanohybrid material offer advantages in designing highly sensitive, reliable, and reproducible sensors with short response/recovery times. The TiO2/WO3 nanocomposite showed a high response of 48 toward detecting n-butanol (50 ppm) gas at 483K. The response increases by 2.65-folds with the 1 wt.% In doping in TiO2/WO3 related to the advantages of mesoporous nanohybrid materials in improving the absorption efficiency of an n-butanol gas molecule sensor surface which facilitates in easier adsorption, rapid transmission, and quick dissociation across the sensor surface. Interestingly, the In
TiO2/WO3 sensor showed a maximum response of 127.2 at 473K along with a response of 2.2 s, 3 s of recovery, and a detection limit of 1 ppm of n-butanol [72].

18.3.8.1 Ethanol TiO2 nanobelts prepared by a hydrothermal process and manipulated by an acid-corrosion procedure with the formation of Ag
TiO2 heterostructures on TiO2 nanobelts surface by photoreduction were used in the detection of ethanol vapor. The electrical conductivity measured varying temperatures of sensors based on the four nanobelt samples (TiO2 nanobelts, Ag
TiO2 nanobelts, surface-coarsened TiO2 nanobelts, and surface-coarsened Ag
TiO2 nanobelts), they all displayed improved sensitivity, selectivity, and short response times for ethanol vapor detection, in comparison with sensors based on other oxide nanostructures. Importantly, the formation of Ag
TiO2 heterostructures on TiO2 nanobelts surface and surface coarsening of TiO2 nanobelts were found to lead to apparent further enhancement of the sensors sensitivity, as well as a decrease of the optimal working temperature. That is, within the present experimental context, the vapor sensor based on surface-coarsened Ag
TiO2 composite nanobelts exhibited the best performance. The sensing mechanism was interpreted on the basis of the surface depletion model, and the improvement by oxide surface engineering was accounted for the chemical sensitization mechanism. This work provided a practical approach to the enhancement of gas sensing performance by 1D oxide nanomaterials. TiO2 nanobelt surface might be

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coarsened by an acid-corrosion procedure, and Ag-TiO2 composite nanoscale heterostructures were produced by depositing silver nanoparticles onto the TiO nanobelt surfaces by photoreduction. The sensing activity of these four nanobelt materials in the detection of ethanol vapor was then examined and compared at controlled temperatures. It was found that surface coarsening and formation of Ag-TiO2 nanoscale heterostructures (which results from the deposition of Ag nanoparticles onto the TiO2 nanobelts surface) led to the apparent improvement in the vapor sensing sensitivity as well as a diminishment of the optimal working temperature. The sensing mechanism based on surface-coarsened TiO2 nanobelts to ethanol vapor by surface depletion model has been elucidated [73]. A titanium precursor, H-exchanged titanate nanobelts, was used to prepare nanosized anatase TiO2 with various morphologies by hydrothermal method. Cetyltrimethylammonium bromide and ethylenediaminetetraacetic acid tetrasodium salt (EDTA-4Na1) played critical roles in synthesizing the nanorods and nanopolyhedrons. In particular, nanorods, nanobelts, and nano-polyhedrons exhibit rapid response and recovery time to ethanol [74]. Hierarchical nanostructures with much increased surface-to-volume ratio based on TiO2/Ag0.35V2O5 produced by relatively simple processes shown in Fig. 18.7 have been used as a resistive gas sensor for ethanol detection.

Figure 18.7 Fabrication of TiO2/Ag0.35V2O5-branched nanoheterostructures and characters of the heterostructures. (b
e) SEM images at three different magnifications of TiO2 nanofibers (b,c) and TiO2/Ag0.35V2O5 branched nanoheterostructures (d,e), where a great many small branches extend out of the fiber backbones [75].

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Titanium Dioxide (TiO2) and Its Applications

The ethanol sensing performance of this device shows a lower operating temperature, faster response/recovery behavior, better selectivity, and about 9 times higher sensitivity compared with pure TiO2 nanofibers. The enhanced sensitivity of the TiO2/Ag0.35V2O5-branched nanoheterostructures was attributed to the extraordinary branched hierarchical structures and TiO2/Ag0.35V2O5 heterojunctions, which can result in a change of resistance upon ethanol exposure [75]. In Fig. 18.8, the proposed sensing mechanism diagram of TiO2/Ag0.35V2O5 nanoheterostructures has been reported. The fabrication of engineered multifunctional devices printed on flexible substrates, with multisensing capability, is important in view of practical fields of applications, such as wearable electronics, soft robotics, interactive interfaces, and electronic skin design, revealing the vital importance of precise control of the fundamental properties of metal-oxide nanomaterials. A new strategy for producing low-cost, scalable, flexible, and multifunctional devices for UV detection and ethanol sensing, which is based on printed TiO2 nanoparticles with laser-tunable properties, has been proposed by Dubourg and Radovi´c [76]. One of these techniques consists to realize a TiO2 active layer deposited on the top of silver interdigitated electrodes. The screen printing is used for the patterning of silver interdigitated

Figure 18.8 (A) Schematic band structure of TiO2/Ag0.35V2O5 heterojunction exposed in air and ethanol gases (qΦ: energy barrier); (B) Sensing model of the TiO2/Ag0.35V2O5 nanoheterostructured sensor in air (Steps 1
3) and in ethanol (Steps 4
5). Modified from the Source: ©2016 Nature Y. Wang, L. Liu, C. Meng, Y. Zhou, Z. Gao, X. Li, et al., A novel ethanol gas sensor based on TiO2/Ag0.35V2O5 branched nanoheterostructures, Sci. Rep. 6 (2016) 33092. doi:https://doi.org/10.1038/srep33092.

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549

electrodes and the active layer based on anatase TiO2 nanoparticles, whereas the laser processing is utilized to fine-tune the UV and ethanol-sensing properties of the active layer. Another one was adapted for the posttreatment of the active layers on a flexible substrate permitting the simultaneous and selective sintering of TiO2 patterned films in a single step. A precisely aligned TiO2 nanowire-based ethanol sensor has been fabricated using e-beam lithography and designed as shown in Fig. 18.9. The optimized operating temperature of the TiO2 nanowire sensor was 573K with the rise and recovery times reduced to 3.2 and 17.5 s, respectively, and the corresponding sensing response (ΔR/R0) was approximately 21.7% for the lowest ethanol injection mass of 0.2 μg [77].

Figure 18.9 Device fabrication process: (A) thermal oxidation of bulk Si wafer; (B) deposition of Cr/Au for contact electrodes; (C) deposition of Cr/Au on the backside of the Si subtract for the microheater; (D) e-beam lithography and TiO2 deposition followed by the lift-off process; (E) illustration of electrode and sensing film design on Si chips; and (F) photo images of the TiO2 nanowire array bridging at the integrated electrodes [77].

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Titanium Dioxide (TiO2) and Its Applications

18.3.8.2 Acetone New photoconductive nanocomposites based on TiO2/polyoxometalates (POMs) have demonstrated improved photoconductivity and gas sensing properties for acetone gas. The surface defects at the grain boundaries of TiO2 nanoparticles significantly affect the photoelectric properties of TiO2 nanomaterials. The interface modification on grain boundaries of commercial TiO2 P25 by using K5[PW11TiO40] or K7[PW10Ti2O40] onto titanium-substituted POMs produced a class of TiO2/POM nanocomposites exhibiting superior photoelectric properties compared with pure TiO2 nanoparticles. In particular, TiO2/PW11Ti exhibited a high gas sensing performance for acetone [23].

18.3.8.3 Formaldehyde A Cd-doped TiO2
SnO2 composite has been used as a sensor for formaldehyde gas detection. The maximum sensitivity was found to be 32 under a formaldehyde gas concentration of 200 ppm and the optimum operating temperature to be 593K. The response and recovery times were estimated to be 25 and 17 s, respectively [78].

18.3.8.4 Trimethylamine A flexible gas sensor operating at low temperature has been fabricated using TiO2 membrane nanotubes prepared by anodization method. This was used to detect trimethylamine within a concentration range of 40
400 ppm. The TiO2 membrane nanotubes have an average diameter of 100 nm with a length of 12 μm. TiO2 membrane nanotubes were supported over a flexible substrate, onto which array interdigitated Au electrodes (see Fig. 18.10). The sensing characteristics were investigated by measuring the electrical resistance of the sensor in a test chamber as a function of gas concentration [79].

Figure 18.10 A trimethylamine flexible gas sensor based on TiO2 membrane nanotubes [79].

Applications of TiO2 in sensor devices

551

18.3.8.5 Toluene Fig. 18.11 shows toluene gas sensor based on TiO2 nanoparticle
decorated 3D graphene-carbon nanotube nanostructures working at room temperature [80]. This provides an effective example representing how SMO gas-sensitive materials based on TiO2 can be arranged in order to design smart sensor devices having practical applications.

Conclusion In this paragraph the basic sensing mechanism and transduction principle of different gases, various fabrication methods used in the preparation of sensors are discussed, and the advantages inherent to nanosized TiO2-based materials have been summarized. It is expected that sensor arrays based on nanosized TiO2-based materials would make promising candidates in sensing applications [59]. At present, the important application fields are industrial and automotive sector, the food industries (gas sensors are used here for control of fermentation processes), domestic sector (for the detection of humidity, CO2, and combustible gases), the medical sector (inpatient monitoring and diagnostics), and security fields (to detect the traces of explosives) [81]. The understanding of the facet-dependent properties of SMO will assist in and guide the fabrication of more excellent gas sensors in the future [82].

Figure 18.11 Smart sensor for toluene detection [80].

552

18.4

Titanium Dioxide (TiO2) and Its Applications

Biosensors

The semiconductor property of TiO2 allows one to selectively detect, in its proximity, the presence of redox molecules, which becomes highly conductive upon reduction to suboxides, titanium carbide (TiC), or titanium. Starting from this specific feature of TiO2, researchers explored the utility of various TiO2 material mainly nanostructure one, as sensors for highly sensitive and quantitative biorecognition of many redox biomolecules ranging from small molecules, such as glucose, cholesterol, uric acid, nucleotides or biomarkers to proteins, bacteria, or cancer cells [83,84]. Quantitative or semiquantitative detection of biological or biochemical processes is of extreme importance for medical, biological, and biotechnological applications. However, converting the biological information to a quickly processed electronic signal is challenging due to the complexity of connecting an electronic device directly to a biological environment. The measurement principle of a biosensor depends on its sensing mechanisms. Typical biosensor models are electrochemical biosensors based on amperometric or potentiometric as sensing signals and optical biosensors based on SPR and fluorescence as sensing signals. Elements and selected components of a typical biosensor have been shown in Fig. 18.12. Electrochemical biosensors provide an attractive means to analyze the content of a biological sample due to the direct conversion of a biological event to an electronic signal. To reach this aim, redox mediators such as protein enzyme or inorganic catalytic materials are used, giving enzyme or nonenzyme electrochemical sensors, respectively. Over the past decades, several sensing concepts and related devices have been developed ranging from the most common traditional techniques, such as cyclic voltammetry (CV), chronoamperometry, chronopotentiometry, EIS, and various FET to promising novel approaches, such as NWs, NT, nanosheets

Figure 18.12 Elements and selected components of a typical biosensor [85].

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array (NSA), or magnetic nanoparticle-based biosensing. Additional measurement techniques have been shown useful in combination with electrochemical detection such as the electrochemical versions of SPR, optical waveguide light mode spectroscopy, ellipsometry, quartz crystal microbalance, scanning probe microscopy, and photocurrent of photoactive materials. The signal transduction and the general performance of electrochemical sensors are often determined by the surface architectures that connect the sensing element to the biological sample at the nanometer scale. The most common surface modification techniques, the various electrochemical transduction mechanisms, and the choice of the recognition receptor molecules all influence the ultimate sensitivity of the sensor. New nanotechnology-based approaches, such as the use of engineered ion-channels in lipid bilayers, the encapsulation of enzymes into vesicles, polymersomes, or polyelectrolyte capsules, provide additional possibilities for signal amplification. Many aspects of a biosensor must be taken into consideration such as the importance of the precise control over the delicate interplay between surface nano-architectures, surface functionalization, and the chosen sensor transducer principle, as well as the usefulness of complementary characterization tools to interpret and optimize the sensor response [85]. Biosensor fields are showing particular interest in NWs due to rapid response, small size, and high sensitivity and portability. Moreover, newer NWs with particularly powerful, robust, and economically feasible platforms may provide high current amplification and sustain an enhanced signal-to-noise ratio among all the detection methodologies owing to their excellent sensitivity, label-free, real-time response for bio- and chemical molecule detection. Due to their one dimensionalities, the electrical conductivity through NWs is greatly affected by the biological/ chemical species adsorbed on their surface. Hence, NWs have been used for the integration/immobilization of biosensing devices for clinical, environmental, and industrial applications. NWs-based FETs have attracted considerable interest in developing innovative biosensors using NWs of different materials (i.e., semiconductors and polymers). NWs-based FETs provide significant advantages over the other bulk or non-NWs nanomaterial-based FETs. As the building blocks for FET-based biosensors, 1D NWs offer excellent surface-to-volume ratio and are more suitable and sensitive for sensing applications. During the past decade, FET-based biosensors are smartly designed and used due to their excellent specificity, sensitivity, and high selectivity. In addition, they have the advantage of low weight, low cost of mass production, small size, and compatible with commercial planar processes for large-scale circuitry [86]. Micro- and nanoelectromechanical systems, including cantilevers and other small-scale structures, have been studied for sensor applications. Accurate sensing of gaseous or aqueous environments, chemical vapors, and biomolecules have been demonstrated using a variety of these devices that undergo static deflections or shifts in resonant frequency upon analyte binding. In particular, the biological detection of viruses, antigens, DNA, and other proteins is of great interest. While the majority of currently used detection schemes are reliant on biomarkers, such as fluorescent labels, time, effort, and chemical activity could be saved by developing an ultrasensitive method of label-free mass detection. Micro- and nanoscale sensors

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have been effectively applied as label-free detectors with recent efforts toward integrating these sensors in microfluidic systems as well.

18.4.1 Glucose Glucose is the source of energy for the living cells and metabolic intermedia; it is critically associated with the public health and the breakdown of glucose homeostasis in the human body correlated with diabetes. Thus the development of analytical methods for monitoring glucose levels has become one of the significant challenges in food security and clinical diagnosis [87]. Traditionally, enzyme glucose sensors are based on glucose oxidase (GOx) enzyme since the 1950s. GOx enzyme catalyzes the glucose oxidation that can be transformed into gluconic acid and H2O2 by the assistant of oxygen. The decrease of oxygen value can be measured. Enzyme immobilization determines the sensing performance of glucose sensors and depends on the morphology of nanomaterials. A new approach was used to directly grow uniform and highly ordered TiO2 NSA on a low-cost, flexible carbon cloth substrate simultaneously fulfilling precise TiO2 nanostructure tailoring and crystal phase control. The unique vertically erected TiO2 NSA/carbon cloth with hierarchical structures was directly explored as electrode for enzyme immobilization and biosensing applications without suffering any influences of insulating binders usually used to fix nanomaterials on conductive substrates during sensor fabrications. Efficient direct electron transfer was successfully achieved for GOx immobilized on the TiO2 NSA/carbon cloth, which produces a stable, mediator-free glucose sensor with good selectivity, high-sensitivity (52 μA/mM/cm2), low response time (,5 s), and low detection limit (23.4 μM, S/N 5 3) [88]. Recently, Guo et al. fabricated plasmon-enhanced biosensors to detect glucose, and lactose using a cost-effective approach has the potential to enhance the sensing performance of NW-based FET biosensors without the addition of enzyme. Periodically patterned Au nanorods (NRs) in TiO2 nanocavities (Au NRs@TiO2) were fabricated via magnetron sputtering followed by a thermal dewetting process. This innovative Au NRs@TiO2 heterostructure was used as a plasmonic sensing platform for PEC detection of glucose and lactose. Higher sensing properties to other Au nanoparticle-based sensors were obtained because (1) LSPR generated at Au/TiO2 interfaces enhanced sensitivity of glucose (lactose) amperometric detection; (2) periodic Au nanocrystals in TiO2 nanocavities accelerated charge separation and transfer rate, especially under monochromatic blue light irradiation; and (3) discrete planar architectures comprising Au NRs immobilized on TiO2 substrates significantly improved stability and reusability of the sensors [89]. Many efforts have been addressed in developing small and reliable sensors that would allow continuous glucose monitoring in diabetic patients. To this aim the continuous glucose monitoring in the tear fluid has been considered. Yao et al. realized a contact lens with an integrated amperometric glucose sensor, in which the glucose sensor was constructed by creating microstructures on a polymer substrate, which was subsequently shaped into a contact lens. Titania sol
gel film was

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applied to immobilize GOx, and Nafion was used to decrease several potential interferences. The titania sol
gel film was very efficient for retaining the GOx activity and preventing the enzyme detaching from the film [90]. A facile method has been developed to synthesize the rhizobia like self-support NiNPs/TiO2NW as nonenzymatic composite electrode for glucose detection via hydrothermal treatment of Ti foil in an NaOH solution, Ni ion exchange, and subsequent annealing under H2/Ar. The NiTiO3 has been transferred into metallic Ni species and TiO2 by a phase separation process in reductive atmosphere. This sensor shows high activity and perfect chemical and structural stability [91]. An ultrasensing PEC glucose biosensor has been constructed from the bioderived nitrogen-doped carbon sheets
wrapped titanium dioxide nanoparticles synthesized by the sol
gel method, followed by the covalent immobilization of GOx on them. The biosensor exhibited a good charge separation, highly enhanced and stable photocurrent responses with switching PEC behavior under the light (λ . 400 nm) and a good photocurrent response to the detection of glucose concentrations (0.05
10 μM) [92]. A nonenzymatic PEC biosensor based on ternary nanocomposites of Au/CuS/TiO2 (Au/CuS/TiO2) has been fabricated with a highly ordered TiO2 nanotube arrays (TiO2 NTs) prepared by anodization of Ti foils, and CuS NPs and Au NPs were deposited on TiO2 NTs by the successive ionic layer adsorption and reaction method. The sensor exhibited excellent PEC behavior as a glucose sensor, in human serum samples, under white light illumination. The fabricated Au/CuS/TiO2 nonenzymatic PEC sensor showed brilliant catalytic activity, favorable selectivity, good reproducibility, and long-term stability for glucose detection under optimized conditions. The linear range was 0.1
3 μM (R 5 0.9942) with a detection limit of 0.03 μM (S/N 5 3) [93].

18.4.2 DNA and biomarkers DNA sequences detection is of enormous research interest due to its broad applications in molecular diagnostics, genetics therapy, environmental monitoring, forensic analysis, early screening of cancers, etc. Generally, a standard analytical method has some limitations, such as reduced sensitivity, time-consuming, expensive, and complicated instruments. Since DNA concentration is often at a very low level in biological samples, it is necessary to explore ultrasensitive, selective, simple, and inexpensive method for detecting specific DNA sequences. The schematic of the elements and the single components typically arranged to design a DNA biosensor have been summarized in Fig. 18.13. Fan et al. presented a PEC platform for ultrasensitive detection of DNA based on a cosensitization strategy. TiO2/CdS:Mn hybrid structure was prepared by successive adsorption and reaction of Cd21/Mn21 and S22 ions on the surface of TiO2 film and then was employed as matrix for immobilization of hairpin DNA probe, whereas large-sized CdTe
COOH quantum dots (QDs) and small-sized CdTe
NH2 QDs as photocurrent signal amplification elements were successively labeled on the terminal of a hairpin DNA probe to achieve a sensitization effect.

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Figure 18.13 Elements and the single components comprised in a typical DNA-based biosensor [94].

The DNA sensor has an ultralow detection limit of 27 aM and a wide linear range from 50 aM to 50 pM. In this case the synergy effect of TiO2-NTs/CdS:Mn/CdTe cosensitized structure and signal amplification of SiO2@Ab2 conjugates were used. As the matrix for the sensing electrode, TiO2
NTs/CdS:Mn/CdTe cosensitized structure could significantly enhance the photocurrent intensity due to its excellent properties of adequate absorption of light energy, ultrafast electron transfer, and effective inhibition of charge recombination [25]. Problems of fouling and fragility of complex engineered electrochemical devices for the detection of biomolecules were solved through the design of sandwichstructured electrodes endowed with a photoactive top layer. The fine-tuning of the system, composed of a highly ordered distribution of silver nanoparticles between bottom silica and a top titania layer, confers multifunctional properties to the device for a complex biomedical challenge: dopamine detection [95]. Wu et al. designed a new electrochemical biosensor based on catalyzed hairpin assembly target recycling and cascade electrocatalysis [cytochrome c (Cyt c) and alcohol oxidase (AOx)] for signal amplification for highly sensitive detection of microRNA (miRNA). Porous TiO2 nanosphere was synthesized, which could offer more surface area for Pt NPs enwrapping and enhance the amount of immobilized DNA strand 1 (S1) and Cyt c accordingly. This biosensor provided a sensitive detection of miRNA-155 from 0.8 fM to 1 nM with a relatively low detection limit of 0.35 fM [96].

18.4.3 Pesticides Over the last decades the widespread use of persistent pesticides, often toxic for humans and the environment, for crop protection is well known. Thus the development of reliable, sensitive, low cost, and rapid detection systems is of great interest. Different biosensors with TiO2 have been recently designed for pesticide detection.

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Organophosphorus pesticides (OP) are toxic for humans, and most animals were acting through the inhibition of the enzymes in neurosynapsesacetylcholinesterase (AChE), inducing an accumulation of the enzyme acetylcholine in the human body with eventual damages in the nervous system. Cui et al. developed a highly stable electrochemical acetylcholinesterase (AChE) biosensor for the detection of OP through the adsorption of AChE on chitosan, TiO2 sol
gel, and reduced graphene oxide (RGO)
based multilayered immobilization matrix with a mesoporous nanostructure. Accurate detection of dichlorvos, used as OP model, in cabbage juice samples was proved. Incorporation of chitosan and electrodeposition of a chitosan layer into/on the TiO2 sol
gel makes the gel mechanically stable providing an efficient platform for AChE immobilization. The detection linear range measured for dichlorvos was from 0.036 μM (7.9 ppb) to 22.6 μM, with a LOD of 29 nM (6.4 ppb) and a total detection time of about 25 min [97]. A biosensor for monitoring of p-nitrophenyl substituted OP in the aqueous system that was developed using a functional nanocomposite, which consists of elastin-like-polypeptide-organophosphate hydrolase, bovine serum albumin, TiO2 nanofibers, and carboxylic acid
functionalized multiwalled carbon nanotubes (MWCNTs). TiO2 NFs were employed to enrich organophosphates in the nanocomposite due to its strong affinity with phosphoric group in OP, while carbon nanotube was used to enhance the electron transfer in the amperometric detection. This sensor works within a wide linear range, giving a fast response (less than 5 s) and LODs (S/N 5 3) as low as 12 and 10 nM for methyl parathion and parathion, respectively [98]. A visible-light-activated PEC biosensor based on acetochlor’s ability to inhibit glucose oxidase (GOx) activity was fabricated for the detection of the pesticide acetochlor in vegetables and fruit. NH2-MIL-125(Ti)/TiO2 nanocomposite (MIL stands for Materials from Institute Lavoisier), as a new functional material, was used for the immobilization of GOx by using chitosan as the dispersion matrix. The GOx/CS/NH2-MIL-125(Ti)/TiO2/glassy carbon electrode immersed in phosphate buffers (0.1 M, pH 7.0) containing different concentrations of acetochlor at room temperature for 10 min, and the electrode was transferred to a glass cell containing 10.0 mL (0.1 M, pH 7.0) phosphate buffer containing 0.6 mM glucose when acetochlor was added to a phosphate buffer solution containing glucose, the activity of GOx was inhibited thus causing a photocurrent drop that was inversely proportional to the acetochlor concentration. A corresponding analytical method was developed. The inhibition of the photocurrent was proportional to the concentration of acetochlor in the range from 0.02 to 1.0 nM and in the range from 10 to 200 nM with a detection limit of 0.003 nM (S/N 5 3) [99]. Inspired by the PEC properties of TiO2 nanotube arrays and their application as a super vessel for immobilizing biomolecules, an inhibition-effect PEC biosensor for the determination of asulam based on the in situ generation of CdS QDs on TiO2 NTs using an enzymatic reaction was constructed. Horseradish peroxidase (HRP) enzyme was covalently assembled on the inner wall of TiO2 TNs, which exhibited excellent electrochemical and catalytic properties [100].

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Titanium Dioxide (TiO2) and Its Applications

18.4.4 Cholesterol derivatives Cholesterol is an essential component of mammalian cell membranes, a steroid hormone, cell signaling, and vitamin D. The ratio of esterified cholesterol and free cholesterol form is 70:30 present in a human blood sample. High cholesterol accumulation or low level of cholesterol concentration in blood is correlated to a different number of diseases. An amperometric nonenzymatic cholesterol biosensor based on Cu2O
TiO2 hybrid nanostructure was developed starting from TiO2 NTs obtained via anodization of titanium foils and successively decorated with Cu2O NPs by chemical bath deposition. CV and the amperometric response of the fabricated electrode exhibited high catalytic activity toward the cholesterol oxidation and fivefold increase in the sensitivity compared to pristine [101]. The exploitation of the beneficial characteristics of graphene and TiO2 NWs to achieve good selectivity and high sensitivity for cholesterol detection was made by Komathi et al. with the fabrication of an electrochemical PEC dual-mode cholesterol biosensor based on graphene sheets, interconnected-graphene-embedded titanium nanowires 3D nano stocks. First the electrode was fabricated by the reaction between functionalized graphene and grapheme-embedded titanium nanowires. Subsequently, cholesterol oxidase was immobilized using chitosan as the binder [102]. Electrospinning appears to be the ultimate technique to generate biocompatible and biodegradable polymer/metal-oxide nanofibers such as TiO2 for highly sensitive biosensing applications. This technique permitted the deposition of a 3D porous nanofibrous mat network on the surface of a sensor transducer, which provides a large global pore volume, predictable pore size distribution, and tunable interconnected porosity. A surface-modified and aligned mesoporous anatase titania nanofiber mat opportunely engineered for esterified cholesterol detection was fabricated by this technique. The electrospinning and porosity of mesoporous TiO2 nanofibers were controlled by the use of polyvinylpyrrolidone as a sacrificial carrier polymer in the titanium isopropoxide precursor. The functional groups such as
COOH and
CHO were introduced on TiO2 nanofibers surface via oxygen plasma treatment making the surface hydrophilic. Cholesterol esterase and cholesterol oxidase were covalently immobilized on the plasma-treated surface of nanofibers. The high mesoporosity (B61%) of the fibrous film allowed enhanced loading of the enzyme molecules in the TiO2 nanofibers mat [103].

18.4.5 H2O2 H2O2 is extensively used in various industrial processes such as paper bleaching, disinfection and sterilization, food processing, and pharmaceutical applications. It is one of the significant members of reactive oxygen species (ROS). It is a marker for oxidative stress, which has been implicated in various neurodegenerative disorders such as Alzheimer and Parkinson’s diseases. Furthermore, it is a by-product of a large number of oxidases such as glucose oxidase, lactate oxidase, galactose

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oxidase, and xanthine oxidase [104]. Thus along with the development of biosensors enable to detect H2O2 produced upon a redox process and used to measure indirectly other analytes, the direct detection of H2O2 is an important issue that will be considered in this paragraph. A novel PEC biosensing platform coupling with enzyme-induced biocatalytic precipitation was developed by Li et al. with TiO2 TNs arrays coated with polydopamine and HRP. The polydopamine was incorporated into the sensor system not only as a sensitizing agent to alter the optical properties of TiO2 NTs arrays and enhance the visible light absorption, but also as a bonding resin. This biosensor exhibited an excellent sensitivity as well as selectivity for H2O2 in a detection range from 1 nM to 50 M, with a detection limit of 0.7 nM [105]. An impedimetric-based biosensor was successfully realized with gold nanoparticles entrapped within TiO2 particles. The surface of TiO2 was previously modified with an amine terminal group to create sites for hemoglobin immobilization on its surface enable to quantify H2O2 by using an alternative impedance method in which the biosensor exhibited a wide linear range response between 1 3 1024 and 1.5 3 1022 M and a LOD of 1 3 1025 M without a redox mediator [106]. Hollow TiO2-modified RGO microspheres were synthesized and then be used to immobilize hemoglobin. The hollow TiO2-based microspheres enhance the immobilization efficiency of proteins and facilitate the direct electron transfer, which results in better H2O2 detection performance, such as a wide linear range and the extremely low detection limit. Finally, the hollow microspheres ensure long-term biosensor stability [107].

18.4.6 Urea Urea is a small vital biomolecule that exists in nature. It is the chief end product of protein metabolism and it can act as an important indicator of liver and kidney function. Sensitive biosensors for urea were developed, such as a 3D hierarchical nano-ZnO/TiO2 on the conductive fluorinated-tin oxide layer. The biosensor was fabricated by reactive d.c. magnetron sputtering of ZnO on a precovered TiO2 surface with polyvinyl alcohol as an omissible polymer in a pattern of parallel strips following by PVA omission via annealing process, which resulted in an efficient porous media for urease enzyme immobilization. TiO2 was selected as the substrate for (1) its ability to promote electron transfer, (2) affording an electrostatic repulsion layer on the biosensor surface for the anionic interference at the biological media, and (3) enhancement of urea biosensing. In this way an impedimetric biosensor showing high sensitivity for urea detection within 5
205 mg/dL and LOD as 2 mg/dL for real-time analysis was fabricated [108]. Combining the surface modification and molecular imprinting technology (MIT), a novel PEC sensing platform with excellent photochemical catalysis and molecular recognition capabilities was established for the detection of uric acid based on the magnetic immobilization of Fe3O4@C nanoparticles onto magnetic glassy carbon electrode and modification of molecularly imprinted TiO2 film on Fe3O4@C [109].

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Titanium Dioxide (TiO2) and Its Applications

Recently, a magnetic beads urease/graphene oxide/titanium dioxide
based biosensor was integrated with a wireless measurement system and microfluidic measurement system. The fluidic microchannel system showed the measurement reliability while the wireless measurement system work exhibited the feasibility for the remote detection of urea, but it requires refinement and modification to improve stability and precision [110].

18.4.7 Glutamate The development of biosensors for the determination of glutamate has been of great research interest for the past 25 years due to its importance in biomedical (for instance, as energy and nitrogen source, excitatory neurotransmitters) and food (flavor enhancer) fields. Glutamate oxidase and glutamate dehydrogenase were the most common enzymes used in the preparation of glutamate biosensors. An optical biosensor based on the immobilization of glutamate dehydrogenase in titanium dioxide sol
gel matrix has been developed. The transparent titania matrix, prepared by vapor deposition method, was also suitable to encapsulate carboxy seminaphthorhodamine-1-dextran as the fluorescent dye for optical detection of glutamate in water and biological samples [111]. The same technique was adopted to prepare an array-based optical biosensor for the simultaneous determination multianalyte, including glucose, urea, and glutamate, in serum samples through the coimmobilization of the three different enzymes urease, glucose dehydrogenase, and glutamate dehydrogenase in the titania sol
gel matrix [112]. An amperometric microelectrode for measuring L-glutamic acid, based on the oxygen storage and release capacity of cerium oxides, was realized starting from a nanocomposite of oxygen-rich ceria and titania nanoparticles dispersed within a semipermeable chitosan membrane that was coimmobilized with the enzyme glutamate oxidase on the surface of a Pt microelectrode. The oxygen delivery capacity of the ceria nanoparticles embedded in a biocompatible chitosan matrix
facilitated enzyme stabilization and operation in oxygen-free conditions. The biosensor was tested in cerebrospinal fluid [113].

18.4.8 Bacteria (Escherichia coli, etc.) A piezoelectric quartz crystal electrode obtained by photo-deposition of nano-Ag at TiO2 coated was developed for detecting Escherichia coli in environmental water with the enhancement of 3.3 times for binding of complementary DNA thanks to the nano-Ag coating [114]. TiO2 NWs, due to their excellent chemical and photochemical stabilities and negligible effect on protein denaturation, are useful sensing material for FET-based immunosensor. A TiO2 NW bundle microelectrode-based FET immune-sensor was also effective for the detection of Listeria monocytogenes. TiO2 nanowire bundle was prepared through a hydrothermal reaction of alkali with TiO2 powder and connected to gold microelectrodes with mask welding. Monoclonal antibodies were immobilized on the surface of a TiO2 nanowire bundle to capture L. monocytogenes

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precisely. Impedance change caused by the nanowire
antibody
bacteria complex was measured and correlated to bacterial numbers. The detection limit was 102 cfu/mL of L. monocytogenes in 1 h without significant interference from other foodborne pathogens such as E. coli, Salmonella typhimurium, and Staphylococcus aureus [115].

18.4.9 Other analytes Finally, to conclude our TiO2-based biosensor survey, some interesting biosensors for target analytes (lactate, antirabbit immunoglobulin G, thrombin, and ErbB2 antigen) which cannot be included in the previous paragraphs will be herein described. Gold nanoparticles
deposited polyaniline
TiO2 nanotube for SPR was an effective engineered system capable of enhancing the PEC biosensing for lactate detection [116]. Because of the SPR-enhanced effect of Au NPs, the electrochromic performance of PANI, and excellent conductivity and biocompatibility of the composite, this method showed a dynamic range of 0.5
210 μM, sensitivity of 0.0401 μA/μM, and a detection limit of 0.15 μM. The encapsulation of biomolecules (antibody) via the electropolymerization of pyrrole propylic acid, a self-made low-conductivity polymer, on TiO2-nanowire (NW)-based FETs is presented. Pyrrole propylic acid monomer was electropolymerized onto a patterned TiO2 NW surface for an abutment reaction. The initial step was agglutination of TiO2 NW by using the hydrothermal method onto the exposed gated micro-region. After electrochemical polymerization of pyrrole propylic acid with the antibody, the antirabbit immunoglobulin G was detected in the concentration range of 119 pg/mL to 5.95 ng/mL, the LODs were 23.96 A/(ng/mL) at the applied voltage of 5 V [117]. A conductance-based immunosensor was constructed based on an antibody/ conducting polymer/TiO2 nanowire film. TiO2 NWs were made by the hydrothermal synthesis method and directly spin-coated on a gold microelectrode
patterned surface. Conducting polymer polypyrrolepropylic acid (PPyA) and antibody composite films were electrochemically polymerized on patterned NWs using pyrrole propylic acid and antirabbit IgG (1 Ab) mixture solutions. The surface properties in the designed immuno-sensor during the preparation processes of NWs, PPyA(PPa/TiO2 NWs), antirabbit IgG (PPyA-1 Ab/TiO2 NWs immunosensor), and the measurement of rabbit IgG (2 Ab/PPyA-1 Ab/TiO2 NWs immunosensor) were recorded and expressed by the changes in current
voltage (I
V). The devices designed in this study can detect 2 Ab within a linear range of 11.2
112 μg/mL, and the detection sensitivity and R-square are 2 0.64 A/(g/mL) and 0.896, respectively [118]. An aptasensor for thrombin with high visible-light activity was facilely fabricated based on graphitic carbon nitride/TiO2 (g-C3N4/TiO2) PEC composite. Crystallization of TiO2 nanoparticles (NPs) and their strong interaction with g-C3N4 contributed to the high photocurrent intensity under visible-light irradiation. Carboxylfunctionalized thrombin aptamers were first successfully bound to the g-C3N4/TiO2modified electrode, as proven by PEC test and EIS analysis. Ascorbic acid was utilized as the electron donor for scavenging photo-generated holes and inhibiting

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Titanium Dioxide (TiO2) and Its Applications

Figure 18.14 Schematic illustration of C3N4/TiO2 preparation (A) and the PEC aptasensor fabrication process (B) [119].

light-driven electron
hole pair recombination. The specific recognition between thrombin aptamer and thrombin led to the linear decrease of photocurrent with the increase of logarithm of thrombin concentration in the range of 5.0 3 10213 to 5.0 3 1029 mol/L with a detection limit of 1.2 3 10213 mol/L [119]. In Fig. 18.14 the C3N4/TiO2 preparation and the PEC aptasensor fabrication process have been shown. A selective, reproducible, and stable microfluidic immunosensor for the detection of breast cancer molecules via antigen
antibody interactions was fabricated. The sensor utilized a new immunoelectron made of hierarchical graphene foam modified with electrospun carbon-doped nanofibers. TiO2 allowed the sensitive detection of the ErbB2 antigen, at the minute (1.0 fM) to higher (0.1 μM) concentrations, overexpressed in breast cancer cells. The sensor interfaced with two electrochemical measurement methods, impedance and pulse voltammetry, was used to detect the ErbB2 antigen [120].

18.5

Sensors for environmental applications

Chemical pollution, due to high toxicity and detrimental effects on human health and the environment, has induced in recent years enormous social concerns over chemical contaminants in the environment and foods. Contaminant detection via portable sensing devices, which encompass the demands of low cost and the potential for online environmental monitoring and food safety applications, is of great

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interest. Besides TiO2-based sensors for gas detection, or biosensors for pesticide detection, previously described in this chapter, various detection systems for chemical contaminants have been developed, and they will be encompassed in the following paragraphs emphasizing organic pollutants (phenolics and pesticides), dyes, and heavy metals. In addition, the advantages of using MIT in TiO2-based sensors for selective chemical pollutant detection will also be discussed. A significant challenge for water/wastewater treatment is water-quality monitoring due to the extremely low concentration of specific contaminants, the lack of fast pathogen detection, as well as the high complexity of the water/wastewater matrices. Innovative sensors with high sensitivity and selectivity and fast response are in great need. Recent advances in nanotechnology offer leapfrogging opportunities to develop next-generation water supply systems [121]. The TiO2 sensors can be used for lab-based analyses, on-line and on-site determination of organic pollutants in wastewater. Various applications of TiO2 nanomaterials in photocatalytic and photoelectrocatalytic monitoring of aggregative organic parameters such as total organic carbon and chemical oxygen demand, as well as individual organic compounds in aqueous solution have been developed over the years [122,123].

18.5.1 Detection of organic pollutants Important classes of persistent organic pollutants such as polycyclic aromatic hydrocarbons (PAHs), phenols, pesticides have been successfully detected by using innovative TiO2 sensors. The electrochemical behavior of TiO2 nanostructures is contingent on their crystalline structure, surface properties, and textural properties, which include specific surface area, pore-volume, pore dimensions, and distribution. For instance, TiO2 NTs have garnered significant research interest due to their ease of preparation, high orientation, extensive surface area, high uniformity, and excellent stability. TiO2 NTs decorated with metal particles and enzymes have been employed for the development of electrochemical sensors and biosensors for the detection of a variety of species [124]. QD-modified TiO2 NTs lowered the detection limits of PAHs to the level of picomole per liter based on fluorescence resonance energy transfer. CdTe QDs were prepared on TiO2 NTs with pulse electrodeposition, and a singledrop optical sensor was prepared with the CdTe QD
modified TiO2 NTs [125]. TiO2 sensors opportunely engineered for the detection of phenolic pollutants have been developed [126
128]. An electrochemical sensor, based on CV measurements, for the sensitive and convenient determination of 4-chlorophenol, was developed based on TiO2-modified graphene nanoparticle casting onto screen-printed carbon electrodes. A facile hydrothermal method was performed to prepare the novel TiO2
graphene oxide nanoparticles. TiO2-modified graphene film exhibited a distinctly higher activity for bimodal-response detection for 4-chlorophenol than the pristine graphene oxide film [126].

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A highly sensitive and selective PEC aptasensor for determination of 3,30 ,4,40 tetrachlorobiphenyl (PCB77) by immobilizing aptamer on N-doped TiO2 nanotubes was developed by Fan et al. To improve the analytical performance of the PEC sensor, the complementary DNA-functionalized CdS QDs were introduced onto N-doped TiO2 NTs by hybridization. In addition to PCB77, owing to high affinity of the aptamer to PCB77, PCB77-aptamer complexes were formed by being bound of PCB77, while DNA-CdS QDs were released from the sensing surface. The complexes lead to the photocurrent decrease and enhance the photocurrent decrease, playing the role of signal amplification. The photocurrent change was utilized to detect PCB77 quantitatively [127]. Nanomaterials composed of TiO2 nanoparticles and nitrogen-doped graphene, prepared by a one-pot thermal treatment method, displayed enhanced PEC performances compared with pure TiO2. The enhancement was ascribed to the introduction of nitrogen-doped graphene, which effectively restrains the recombination of photoinduced electron
hole pairs, improves charge transfer, and extends photoresponse to visible light. The PEC aptasensor was established for sensitive and selective detection of bisphenol A with the assistance of bisphenol A aptamer. In particular, the femtomolar sensitivity of such a bisphenol A PEC aptasensor can be achieved owing to the introduction of nitrogen-doped graphene, with a wide linear range from 1 fM to 10 nM and low LOD of 0.3 fM [128]. Recently various nonenzyme TiO2-based sensors have been successfully developed for pesticide contaminants, which are considered the most dangerous of environmental contaminants, for instance, for parathion or methyl parathion [129
133], chlorpyrifos [134,135], diazinon [136], fipronil [137], and atrazine [138] pesticides. Di-branched triphenylamine dye
sensitized TiO2 nanocomposites with good photostability for sensitive PEC detection of parathion, as a model pesticide, was prepared. The dye with triphenylamine as electron donor, thiophene as electron transfer π-bridge, and acrylic acid as both acceptor and anchoring groups was synthesized and coupled with TiO2 nanoparticles for the highly sensitive PEC assay and exhibited good anchoring stability in neutral buffer solutions. The sensor showed a wide linear range from 2 3 10 to 1224 3 1026 g/mL and an extremely low LOD of 5.6 3 10213 g/mL [129]. A visible light
sensitized PEC sensing platform based on the perylene-3,4,9, 10-tetracarboxylic acid coated on the nano-TiO2, forming a heterojunction as the photoactive element, has been realized. The fabricated derivative PEC sensor showed good performances for methyl parathion detection with a rapid response, instrument portable and straightforward, low LOD (0.08 nmol/L), and good selectivity against other pesticides and possible interferences. It was successfully applied in green vegetables [130]. A PEC sensor for the determination of methyl parathion was developed based on a nanocomposite photoactive species prepared by solvothermal method and then electrochemical deposition technique to form RGO/TiO2/CdS nanocomposite onto a modified ITO electrode [131]. CuO
TiO2 hybrid nanocomposites were decorated on the glass carbon electrode and used for the the detection of methyl parathion that occurred in a wide dynamic

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detection range from 0 to 2000 ppb with a lower LOD of 1.21 ppb by differential pulse voltammetry (DPV) measurements [132]. 2D TiO2 nanosheets (NSs) modified with Au NPs, and mono-6-thio-β-cyclodextrin monolayers were arranged to realize an electrochemical sensing interface on a glassy carbon electrode surface with a relatively large active surface area. This configuration permitted an excellent electron-transfer speed, and electrocatalytic activity of Au NPs with TiO2 NSs and the selective recognition properties of the cyclodextrin derivative, as the prepared sensor could remarkably improve the electrochemical response signals [square-wave voltammetry (SWV)] of trace methyl parathion pesticide. In particular, the sensor based on AuNP/TiO2NS hybrids and SH-b-CD was capable of detecting methyl parathion achieved in a relatively wide range of concentrations from 1.5 to 60.0 nM with a low detection limit of 0.05 nM (0.012 ppb) [133]. A PEC sensor based on the synergistic contributions of an ultrathin BiOCl nanosheet and anchored TiO2 nanoparticles is used to detect chlorpyrifos. The heterostructure showed a distinct improvement in photocurrent intensity compared with pristine BiOCl and TiO2 attributable to the separation and transfer of photogenerated charge carriers, which benefit from intimate interfacial interactions between the ultrathin BiOCl nanosheets and TiO2 nanoparticles. Based on the robust photocurrent signal, this sensor was established for the sensitive and selective detection of chlorpyrifos demonstrating many advantages such as a wide linear range of 1
12 mM, a low detection limit of 0.11 mM, S/N 5 3, and remarkable convenience for its detection in the green vegetable analysis [135]. A voltammetric sensor for diazinon pesticide based on electrode modified with TiO2 nanoparticles
covered MWCNT nanocomposite was successfully used on real samples, including city piped water and agricultural well water [136]. A novel immobilization-free PEC aptasensor to atrazine has been proposed and compared to traditional immobilization-based PEC aptasensors. The immobilization-free synergistic signal
amplified method is based on aptamer
graphene complex and deoxyribonuclease I. The limit of the detection for atrazine by the present immobilization-free PEC aptasensor is estimated to be as low as 12.0 fM, and the linear range is from 50.0 fM to 0.3 nM. At the same time, this immobilization-free PEC aptasensor exhibits excellent stability and selectivity and has been successfully applied to the analysis of real samples (see Fig. 18.15) [138].

18.5.2 Detection of dyes Colorants, which are in high demand in manufacturing industries such as foods, pharmaceuticals, cosmetics, and textiles, often cause environmental pollution. The photocatalytic degradation of organic dyes using semiconductor materials has resulted in being an efficient method for the degradation of pollutants. In this paragraph, TiO2-based composites used for the development of electrodes employed in the simultaneous detection and degradation of organic dyes are described.

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Titanium Dioxide (TiO2) and Its Applications

Figure 18.15 Illustration for the construction of the immobilization-free PEC aptasensor [138].

Wu et al. demonstrated that TiO2
Au Janus micromotors could obtain energy from the photocatalytic degradation of dyes in aqueous solutions without the requirement for any additional reagents. These micromotors exhibited excellent reusability in the degradation and detection of methyl blue, cresol red, and methyl orange [139]. Ahirwar et al. fabricated hierarchically mesoporous NiO/TiO2/dextran catalyst nanostructure, synthesized with green, acid-free, reliable controlled over shape procedure. The mesoporous NiO/TiO2/dextran exhibited significant photochemical degradation and sensing capability for malachite green oxalate dye [140]. While graphene/TiO2 Ag-based composites were synthesized through a facile chemical
thermal route and used as sensitive electrode materials for amaranth electrochemical detection and degradation, the electrode with the best performance in terms of sensitivity was the Au/GTA-10 electrode. It had considerably larger sensitivity (24 mA/M), in comparison with that of Au/GTA-5 and Au/GTA-15 electrode (8 and 12 mA/M, respectively). The LOD was the same for all modified electrodes (LOD 5 1 3 1027 M; S/N 5 3). The Au/GTA-10 electrode was also tested at the electrochemical degradation of amaranth, from aqueous solutions. The degradation reaction followed first-order kinetics with a half-time of degradation of 203 min [141]. A SERS sensor was developed for repeatable detection of Rhodamine 6G as a probe of organic molecules. Vertically oriented TiO2 NT arrays were grown by ultrafast anodic oxidation of flexible titanium foils and then decorated with Ag NPs. The

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substrates were employed in an SERS optofluidic device, consisting a polydimethylsiloxane cover irreversibly sealed to the silver-coated TiO2 NTs. The substrates were employed in a SERS optofluidic device, consisting a polydimethylsiloxane cover irreversibly sealed to the silver-coated TiO2 NTs, able to detect Rhodamine molecules in ethanol over a wide range of concentrations down to 10214 M, taking advantage of both electromagnetic and chemical enhancements [142].

18.5.3 TiO2 in molecular imprinting technology Over the past 40 years, TiO2 has focused continuously growing worldwide concern due to its novel characteristics such as large specific surface area, powerful oxidation strength, and chemical stability. Nevertheless, its poor selectivity restricted its promising application in specific detection and selective removal of target pollutants in mixing solutions, especially the low concentration of highly toxic pollutants. Importantly, concerning the formation of selective sites with the memory of a template, MIT thus can specifically recognize and selectively remove the template and their structural analogs. An SPR TiO2-based sensor combined with a 75 6 5-nm-thick MIP nanofilm as a recognition element has been used for the selective detection of Sudan dyes [143]. With the appealing characteristics of the combination of TiO2 and MIT, the TiO2-based MIPs can selectively detect and photodegrade targets and are widely utilized in chemical sensors, solid-phase extraction, and artificial antibodies [144]. A highly selective and sensitive PEC sensor was fabricated for fast and convenient detection of PCB 101 in environmental water samples with a low detection limit of 1.0 3 10214 mol/L based on single-crystalline TiO2 nanorods. The MIT combined with PEC sensor configuration permitted the preparation of a highly selective and picomolar-level PEC sensor for PCB 101 detection in environmental water samples based on single-crystalline TiO2 nanorods. The high selectivity could be attributed to the high-quality expression of the MI sites on the rigid and smooth surface of single-crystalline TiO2 nanorods [145]. A PEC sensor based on hierarchically branched TiO2 nanorods modified with MIP was constructed for sensitive and efficient detection of chlorpyrifos TiO2 that was grown directly on fluorine-doped tin oxide substrate by the hydrothermal method and employed as a matrix for immobilization of MIP. p-Aminothiophenol and chlorpyrifos were assembled on the surface of hierarchically branched TiO2 nanorods by the formation of hydrogen-bonding interactions through electropolymerization in the MIP preparation process. The proposed sensor offered a promising platform for application in detecting chlorpyrifos with excellent sensitivity and selectivity, low interference, and high stability in the linear range from 0.01 to 100 ng/mL with a low detection limit of 7.4 pg/mL [146]. Recently, electrochemical sensing of bisphenol A on facet-tailored TiO2 single crystals engineered by inorganic-framework MI sites was prepared. The highenergy {0 0 1}-exposed TiO2 single crystals have specific inorganic-framework molecular recognition ability versus bisphenol A with linearly correlated to the

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Titanium Dioxide (TiO2) and Its Applications

BPA concentration from 10.0 nM to 20.0 μM (R2 5 0.9987), with a low detection limit of 3.0 nM (S/N 5 3) [147]. Poly(levodopa), as a biocompatible polymer, was used in developing a molecularly imprinted sensor, was electropolymerized on electrodeposited TiO2 NPs
modified glassy carbon electrode surface, and was applied for selective and sensitive determination of imidacloprid pesticide. The sensor response to imidacloprid was investigated by using SWV, CV, and DPV techniques. The sensor showed a wide linear range of 2
400 μM, a completely low detection limit of 0.3 μM, and a limit of quantitation of 1 μM by SWV measurements that are very acceptable in comparison to other reported imidacloprid sensors [148].

18.5.4 Metal ions detection Selective detection of metal ions, especially heavy metals such as mercury or lead, is critical for environmental monitoring, as these are highly toxic and common pollutants. Metha et al. described a novel approach based on the methodology to “kill waste by waste.” First the toxic metals such as Pb ions were detected using fluorescence carbon QDs. After impregnation of TiO2 into the same Pb-carbon QDs composite, it was further used for photodegradation of harmful industrial dyes. The carbon QDs have been designed as a nanosensor for Pb ion detection, and its fluorescence is effectively quenched with good sensitivity (0.070 μM) and selectivity. This Pb-carbon QDs solution was further immersed in TiO2 by wet impregnation method to fabricate Pb-carbon QD-TiO2 nanocomposite with a change in the energy gap (3.2
2.8 eV) making the composite active in visible light irradiation. The degradation efficiency achieved approximately 100% mark for red X-3BS dye, and 1.8 μmols of CO2 evolution was observed in 60 min [149]. CuO
TiO2 NSs were prepared by a wet-chemical process using reducing agents in alkaline medium, and selective Fe(III) uptake on CuO
TiO2 nanostructure was proven [150]. Wang et al. prepared a dual-emitting fluorescence probe for rapid and ultrasensitive detection of Fe(III) by coating CdSe semiconductor QDs onto the surface of carbon nanodot
doped TiO2 microspheres. The as-prepared nanoprobe exhibits the corresponding dual emissions at 436 and 596 nm for carbon nanodots and CdSe, respectively, under a single excitation wavelength. The blue fluorescence of the carbon nanodots is insensitive to Fe(III), whereas the orange emission of the CdSe semiconductor QDs is functionalized to be selectively quenched by Fe(III). The intensity ratio of I436/I596 shows an excellent linear relationship with the concentration of Fe(III) in the range of 1029 to 1025 M [151]. Functionalized monolayers on mesoporous silica and titania nanoparticles for colorimetric mercuric sensing was prepared through immobilization of an azobenzene-coupled receptor onto titania nanoparticles via sol
gel or hydrolysis reactions [152]. Another colorimetric chemosensor based on TiO2/poly(acrylamideco-methylene bisacrylamide) nanocomposite was prepared for visual detection of trace levels of Hg and Pb ions. The surface modification of synthesized TiO2 nanoparticles was made by using methacryloxypropyltrimethoxysilan (MAPTMS),

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which provided a reactive C
C bond that polymerized the acrylamide and methylene bisacrylamide [153]. Kang et al. prepared a highly selective and sensitive reversible electrochemical sensor for Cu(II) ions detection based on hollow TiO2 spheres modified by fluorescein hydrazine-3,6-diabetic acid [154]. A nanocomposite of hollow TiO2 microspheres and plasma polyacrylic acid modified with rhodamine was used as an electrochemical sensor for the ultrasensitive and selective detection of Cu(II) in water. A gold electrode was first modified with hollow TiO2 nanospheres [155].

18.6

Fabrication of nanoscale sensors and future prospects

Nanosized metal-oxide semiconductors have their versatile applications in different areas such as catalysts, sensors, photoelectronic devices, and highly functional and useful devices. Mechanism of toxicity of metal-oxide nanoparticles can occur by different methods such as oxidative stress, coordination effects, nonhomeostasis effects, and genotoxicity. Factors that affect the metal-oxide nanoparticles were size, dissolution, and exposure routes [156]. There is growing evidence showing that if engineered nanomaterials (ENMs) are released into the environment, there is a possibility that they could cause harm to aquatic microorganisms. Among the diverse effects triggering their toxicity, the ability of ENMs to generate ROS capable of oxidizing biomolecules is currently considered a central mechanism of toxicity. Photoactive ENMs, including fullerenes and semiconducting metal oxides, such as TiO2, CuO, CeO2, ZnO, and Al2O3, can generate ROS when illuminated. It has been demonstrated that these ENMs, the most prominent being TiO2, can activate molecular oxygen radicals, 1 O2 and OU2 2 , which belong, together with OHU , to the biologically most potent ROS [157]. Soft nanoimprinted titanium dioxide substrates decorated with methylammonium lead halide perovskite (MAPbI3) crystals that were fabricated by controlling the perovskite precursor concentration and volume during spin coat processing combined with the use of hydrophobic TiO2 templates. The patterned growth was demonstrated with different perovskite crystallization methods. The controlled assembly of two MAPbI3 nanomaterials has been demonstrated, first a nanocomposite formed between the perovskite and a hole conducting polymer poly2,5-bis(N-methyl-N-hexylamino)phenylene vinylene, and a second formed from perovskite crystals using standard solution
based MAPbI3 growth methods. Both types of MAPbI3 crystals were fabricated on hydrophobic TiO2 nanotemplates composed of nanowells or grating patterns. Patterned areas as large as 100 μm 3 100 μm were achieved. This represents an attractive route to developing nanopatterned and small-area perovskite substrates for applications in photovoltaics, X-ray sensing/detection, image sensor arrays, and others [158]. The fabrication of optically active inorganic nanomaterials with chiral superstructures attracts attention because of their potential applications in chemical

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Titanium Dioxide (TiO2) and Its Applications

sensing and nonlinear optics. Wang et al. presented a facile way to prepare TiO2 nanofibers, in which the nanocrystals are helically arranged into a chiral superstructure. Notably, the chiral superstructure shows strong optical activity due to the difference of absorbing left- and right-handed circularly polarized light. This particular optical activity resulted from electron transition from the valence band to the conduction band of TiO2 through a vicinal effect of helically arranged TiO2 nanocrystals [159]. Peng et al. reported a simple, environment-friendly synthetic route for mesoporous anatase TiO2 nanospindles and successfully applied this method to obtain in situ grown TiO2 nanospindles/graphene oxide composite. After a thermal reduction at 673K, holes are created in the RGO sheets through a photocatalytic oxidation mechanism. The resulting TiO2/holey RGO composites may overcome the original impermeability of graphene sheets and find applications in catalysis, energy conversion/storage devices, and sensors [160]. A novel methodology for a one-step synthesis and deposition of TiO2/Fe3O4/Ag nanocomposites on the polyester surface using the sonochemical technique was developed. Physicochemical effects of ultrasound irradiation arise from extreme conditions induced during acoustic cavitation allow the in situ formation of superparamagnetic, bio, and photoactive nanomaterials with a size of about 40 nm on the fabric surface. The synergy between iron oxide and silver nanoparticles resulted in enhanced photocatalytic activity of TiO2 by the separation of electron
hole pairs [161]. A novel electrode based on glassy carbon (GC) has been prepared and characterized electrochemically for application in electroanalytical chemistry. In particular, a GC screen-printed electrode (SPE) has been modified with nanostructures, namely, MWCNTs and TiO2 nanoparticles, and combined with a new generation of ecofriendly room-temperature ionic liquids (RTILs). The green RTILs here used are suitable for the immobilization of enzymes on the electrode surface and, additionally, facilitate the kinetics of electron transfer due to their intrinsic electrical conductivity. Upon the evaluation of these newly modified electrodes, an improvement in terms of electrochemically active area concerning the electrodes was reported previously. The modified SPEs were then used as substrates for the construction of two enzymatic biosensors for analytical applications: the first is an enzymatic biosensor based on alcohol dehydrogenase for the analysis of ethyl alcohol; the second biosensor is based on lipase enzyme and has been tested for the analysis and the classification of extra-virgin olive oil. The performances of the here-projected sensors appear comparable with biosensors having similar finalities. It is here envisaged that such a kind of electrodes could represent the starting tool for the construction and the definition of new portable devices for screening and field analyses [162]. With three models in commercial use, artificial retinas are the most concrete hope to restore sight to blind patients, notably those affected with retinitis pigmentosa. However, existing architectures are costly to produce, while the restored visual acuity remains below the legal threshold for blindness. Furthermore, the complexity of current systems with tethered application-specific integrated circuits requires complex surgeries, with the risks of complications and

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failures. In the search for new nanomaterials, it is demonstrated that, when placed in contact with photoreceptors (control mouse retinas) or directly with bipolar cells (rhodopsinP23H mouse retinas, a model of retinitis pigmentosa), films of vertically aligned anatase TiO2 nanotubes can drive the activity of the retinal network for stimulation frequencies up to the video rate (25 Hz), in response to short (5
20 ms), small (50
100 μm) light spots. Acting as endless arrays of electrodes, these films should allow fine-tuning of prosthetic stimulations, through modulation of the spot size, duration, and precise localization over the implant surface [163]. There has been an emerging interest to rationally design such membranes for a broad spectrum of appealing applications ranging from lightweight, foldable electronic devices to highly efficient separation and sensing systems. Freestanding ultrathin nano-membranes can be fabricated via an assortment of bottom-up selfassembly strategies [164]. Nanomaterials based on LSPR phenomena are revealing to be an excellent solution for several applications, namely, those of optical biosensing. The main reasons are mostly related to their high sensitivity, with label-free detection, and to the simplified optical systems that can be implemented. In particular, the optical sensing capabilities were tailored by optimizing LSPR absorption bands of nanocomposite Au/TiO2 thin films. These were grown by reactive d.c. magnetron sputtering. The main deposition parameters changed were the number of Au pellets placed in the Ti target, the deposition time, and d.c. applied to the Ti
Au target. Furthermore, the Au NPs clustering, a key feature to have biosensing responses, was induced by several postdeposition in-air annealing treatments at different temperatures and investigated via SEM analysis. Results showed that the Au/TiO2 thin films with a relatively low thickness (B100 nm), revealing concentrations of Au close to 13 at.%, and annealed at temperatures above 873K, had the most welldefined LSPR absorption band and thus, the most promising characteristics to be explored as optical sensors. The NPs formation studies revealed an incomplete aggregation at 573K and 773K and well-defined spheroidal NPs for higher temperatures. Plasma treatment with Ar led to a gradual blue shift of the LSPR absorption band, which demonstrates the sensitivity of the films to changes in the dielectric environment surrounding the NPs (essential for optical sensing applications) and the exposure of the Au nanoparticles (crucial for higher sensitivity) [165].

18.7

Conclusion

An attempt has been made in this chapter to review the current status and future prospects of selective sensing using TiO2-based materials. Although many metal oxide
based sensors are already available, yet detecting of various analytes in low concentrations remains a challenge. Various morphologies of nanostructured materials, due to their unique properties, offer a possibility that selective sensors could be fabricated and many progresses, in the near future, will concern the development of biosensors and sensors for environmental applications.

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83.

TiO2 photocatalysis for environmental purposes

19

Olga Sacco1, Vincenzo Vaiano2 and Diana Sannino2 1 Department of Chemistry and Biology “A. Zambelli”, University of Salerno, Fisciano, Italy, 2 Department of Industrial Engineering, University of Salerno, Fisciano, Italy

19.1

General overview on air and water pollution

Air and water pollution is a serious problem related to the growth of earth population and the industrial development conditions. Indeed the most problematic sources of pollution are of anthropic origin with respect to those natural ones. After the recognition as common goods of the humanity of air and water, the different regulations present in most of the countries’ legislations have evolved, individuating the definitions and the methods to monitor and control the pollution. In general, air pollution occurs when there is an introduction in earth’s atmosphere of foreign elements or substances that have a harmful character or are present in a concentration level that could degrade the environment or its pleasant character, but more accurate individuation is necessary to protect the environment by rules [1]. A definition that could be applied for law requirements is “any modification of the normal composition or physical state of atmospheric air, due to the presence in the same of one or more substances in quantity or with characteristics such as: to alter the normal environmental and health conditions of the air, to constitute a danger or direct or indirect prejudice for human health; to compromise the recreational activities and the other legitimate uses of the environment; to alter the biological resources and the ecosystems and the public and private material goods.” This definition comprises the influences of the quality and the quantity of pollutants on the natural environment and their direct effects on the air characteristics. General classes of atmospheric pollutants can be distinguished as functions of the properties of the substances, and they are gaseous compounds, such as carbon oxide, sulfur dioxide, nitrogen oxide, methane, ozone, volatile organic hydrocarbons, and chlorofluorocarbons, or the atmospheric pollutants can be present as a particulate matter of very fine size, both organic and inorganic in origin, comprising carbon, complex organic chemicals, sulfates, nitrates, mineral dust, and water. Moreover, biological molecules and particulates can be emitted into the atmosphere. The noxious effects of atmospheric pollutants range from irritations, diseases, allergies, and even death to human beings; meanwhile they can be harmful to other living and nonliving organisms damaging also the natural or man-made environment. It is worthwhile to mention the combustion of fossil fuels as a source of air pollution. The application of combustion to get heat and electrical and mechanical Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00007-9 © 2021 Elsevier Inc. All rights reserved.

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Titanium Dioxide (TiO2) and Its Applications

energy is the cause of introduction of 85% of airborne respirable particulate pollution and almost all sulfur dioxide and nitrogen oxide emissions to the atmosphere [2]. However, there are other sources (of anthropic origin or natural) of pollution for volatile organic compounds (VOCs), defined as the class of organic substances that have a high vapor pressure at ordinary room temperature due to a low boiling point. VOCs are of several kinds and are substances of anthropic origin or natural. Mankind sources are regulated to avoid their long-term health effects. An example of pollutants emitted by combustion processes and industrial use of solvents (road transport, nonindustrial combustion, industrial combustion plants, other means of transports, use of solvent, and other products for a large city) is reported in [3]. It is possible to note that road transport strongly affects the emission of pollutants; meanwhile, they are more controlled in industrial combustion plants. A high contribution to VOCs emission is given by the use of solvents and other chemical products. In the urbanized areas the highest source of pollution is related to transportation, but volatile compounds are also produced by the use of solvents and other products. Most of the methods applied to control gaseous pollutant emissions are based on techniques for the recovery or destruction of polluting substances, treating the polluted streams at the pollution sources, and exploit four basic processes: absorption, adsorption, condensation, and combustion. The choice of technology adopted depends on the kind of pollutant to remove, depollution efficiency, the chemical
physical moieties of the polluted gas flow, and specific characteristics of the point of emission and release into the atmosphere. These techniques can include the following: G

G

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G

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G

G

G

G

G

G

G

G

G

condensation cryocondensation regenerative adsorption on active carbon, zeolite, polymer dry and semidry lime absorption gas scrubbing with acid, alkali, wet lime, and water Biofilter, biotrickling filter (degradation by biological organisms as techniques) membrane processes thermal combustion catalytic combustion flaring Photooxidation ionization selective noncatalytic reduction (SNCR) selective catalytic reduction (SCR)

These processes can be used also in combination to achieve the permitted emissions limits of several pollutants in the environments. As an example, to limit the emissions into the atmosphere from vehicles, catalytic oxidation SNCR, SCR, and filtration units could be placed on board; meanwhile, condensation and biofiltration are more suitable for the treatment of emission from fixed plants [4]. However, after the emissions of pollutants in the atmosphere, only selected processes, such as the photooxidation, can be cheaply

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applied for the purification to improve the quality of urban atmosphere that is strongly affected by the pollution, especially in the hours of the day. The quality of urban air infers strongly the purity of indoor air, but also other sources of contamination have to be taken into consideration in the closed environments. It must be taken into account that most of the people spend most of their time in closed environments, and indoor air quality (IAQ) refers to the air quality within and around buildings and structures, especially as it relates to the health and comfort of building residents [5]. According to the Environmental Protection Agency, the air indoors where we spend as much as 90% of our time can be more polluted than city smog. Moreover, highly airtight buildings, to improve the comfort and warming of the environments, could lead to an increase of pollutants causing illness for occupants [6]. The most relevant pollutants in closed environments are asbestos, biological pollutants, carbon monoxide (CO), formaldehyde/pressed wood products, lead (Pb), nitrogen dioxide (NO2), pesticides, radon (Rn), indoor particulate matter, VOCs [5]. In particular, biological contamination, in terms of bacteria and viruses, arises from the poor ventilation of the indoor environment. A general good housekeeping, and maintenance of heating and air-conditioning equipment, is very important to limit the contamination of indoor air. Apart from particulate pollutants, that require filtration, gaseous indoor pollutants can be eliminated by oxidation processes. From this consideration arises the interest with regard to the application of techniques that involve the oxidation of organic and inorganic compounds at low temperature, without the supply of external energy to promote thermal processes. With regard to the water pollution the accumulation of organic compounds in natural waters is mainly caused by the development and extension of chemical processes developed for organic synthesis and processing [7]. However, particularly, in the places in which the natural resources are limited, the progressive growth of industrial areas increased the adverse impacts on water resources. For this reason, water use and reuse have become major concerns. So, it is important to develop valid and affordable technologies for water and wastewater treatment. The principal reasons of water pollution are industrial discharges (even in low quantities), excess use of pesticides, agrochemicals (fertilizers), and landfilling domestic wastes. Generally, classes of compounds of interest include solvents, volatile organics, chlorinated volatile organics, dioxins, dibenzofurans, pesticides, PCBs, chlorophenols, asbestos, heavy metals, and arsenic compounds. In particular, the contaminants of major concern are 4-chlorophenol, pentachlorophenol, trichloroethylene (TCE), perchloroethylene (PCE), ethylene dibromide, vinyl chloride, ethylene dichloride, methyl chloroform, p-chlorobenzene, and hexachlorocyclopentadiene, carbamazepine, flumequine, ibuprofen, and sulfamethoxazole [8]. Different routes are studied in order to remove the hazardous chemical compounds from the environment. Specifically, conventional wastewater treatment is based on various mechanical, biological, physical, and chemical processes. In fact, this is a combination of many operations like filtration, flocculation, biological treatment, chemical sterilization, and the elimination of particles in suspension. The biological treatment is the ideal process (natural decontamination). The physical
chemical processes like coagulation and flocculation require the use chemical reagents (aluminum

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Titanium Dioxide (TiO2) and Its Applications

chloride or ferric chloride, polyelectrolytes, etc.), and, as consequences, the decontamination process generates large amounts of sludge, transferring the contaminants problems from the liquid to the solid phase. Recently, drastic change of regulations on wastewater disposal and quality has required the development of more effective processes; in this contest, different solutions have been explored: ion exchange, ultrafiltration, reverse osmosis, chemical precipitation, and electrochemical technologies. Each of these treatment methods has advantages and disadvantages. These technologies can be applied successfully to remove pollutants that are partially removed by conventional methods, for example, biodegradable organic compounds, suspended solids, colloidal substances, phosphorus and nitrogen compounds, heavy metals, dissolved compounds, microorganisms, thus enabling recycling of residual water [9]. Special attention was paid to electrochemical technologies, because they have advantages such as versatility, safety, selectivity, possibility of automation, environmentally friendly, and require low investment costs [10]. More in detail, concerning the treatment of wastewater containing organic pollutants, different procedures have been developed: G

G

G

Nondestructive: Based on physical processes of adsorption, removal, stripping, etc. Biological destructive: Based on biological processes using active mud. Oxidative destructive: Based on oxidative chemical processes (WO—“wet oxidation”, WAO—“wet air oxidation”; CWAO—“catalytic wet air oxidation”; SWA—“supercritical water oxidation.”)

As an alternative to these methods, advanced oxidation processes (AOPs) are very interesting, since they are based on the generation of oxidizing hydroxyls radicals in mild conditions, such as ambient temperature and pressure.

19.2

General remarks on advanced oxidation processes

As pointed earlier, the traditional methods used to remove VOC gaseous emissions are mainly based on thermal oxidation, catalytic oxidation, or internal combustion engine [11]. These technologies are able to transform VOCs into less harmful compounds, such as carbon dioxide, water, and hydrochloric gas. The thermal oxidation units generally operate at very high temperature (from 760 C up to 870 C) and at high gas residence time (about 1 s) [11]. The internal combustion engine operates in the same way as thermal oxidation units but, generally, the gaseous stream contains high VOCs concentrations in a manner to use the organic compounds as fuel [11]. Catalytic oxidation units are instead based on the use of a catalyst to accelerate the oxidation reaction rate, with the adsorption of the reactants on the catalyst surface followed by the reaction between oxygen and adsorbed VOCs [11]. As it is well known, the use of a catalyst allowed to realize the process at temperatures lower than those required in a conventional thermal oxidation process. However, it is worthwhile to note that both thermal and catalytic oxidation units present some disadvantages. For example, in the case of halogenated VOCs, the exhaust stream must be further treated by

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scrubber systems to mitigate the release of acid vapors in the atmosphere [12]. It is very important to consider that the oxidation processes are inefficient for low VOCs concentrations (in the range between 0.1 and 10 mg/m3) [13]. The removal of water pollutants is currently achieved by means of separation processes (such as sedimentation, flocculation, filtration, electrocoagulation, and adsorption on active carbon [14]) or biological oxidation processes [15]. However, it must be considered that separation processes imply the increase of the concentration of pollutants in sludges or solids that must be disposed of elsewhere [16], whereas biological oxidation
based technologies are not able to remove many biorecalcitrant pollutants [17]. On the other hand, chemical oxidation processes, such as chlorination and ozonation, could be utilized for water and wastewater treatment, but they do not reduce the total organic carbon (TOC) content in an effective way, and, additionally, they can generate hazardous by-products (such as dichloromethane) [18]. In this perspective the application of AOPs may be an effective alternative to the traditional methods for removing pollutants both in water and in gaseous phase since AOPs can transform a wide variety of contaminants in harmless compounds, avoiding the need of further separation and posttreatment processes [19,20]. Actually, AOPs can be considered “environmental-friendly” technologies for the removal of different pollutants because they do not transfer pollutants from one phase to the other (as in chemical precipitation and adsorption), and they do not produce hazardous sludge [21
23]. The efficiency of AOPs is mainly due to the generation of OH radicals at very mild conditions. OH radicals are unselective and powerful oxidizing species (E0 5 2.80 V), and therefore they are able to degrade indiscriminately the target pollutants [19,24]. However, more in general, the degradation efficiency of AOPs is due to the generation of reactive oxygen species (ROS) [25]. ROS are characterized by a very high reactivity, promoting the degradation of a wide variety of pollutant categories [25]. The most common AOPs for environmental purposes include G

G

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H2O2 1 UV (direct photolysis) [26] H2O2 1 Fe21/31 (homogeneous Fenton) [27] H2O2 1 Fe/support (heterogeneous Fenton) [28] H2O2 1 Fe21/31 1 UV (photo-Fenton) [29] O3 (direct ozone feeding) [30] O3 1 UV (photo-ozone feeding) [31] O3 1 catalysts (catalytic ozone feeding) [32] H2O2 1 O3 [33] TiO2 1 UV (heterogeneous photocatalysis) [34].

Among the different AOPs, heterogeneous photocatalysis is considered an interesting option in the field of water and wastewater treatment, decontamination of gaseous streams, as well as synthesis of chemicals [35
40]. In fact it can be considered a “green” technology [40] since this process operates at low temperature and pressure, with consequent low cost and significantly low energy consumption [34]. Heterogeneous photocatalysis is based on the simultaneous action of the light source and a catalyst that must have semiconducting properties [35].

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Titanium Dioxide (TiO2) and Its Applications

The photoexcitation of semiconductor particles by means of light with energy (hv) greater than the semiconductor bandgap (Eg) induces the promotion of an electron (e2) from the valence band to the conduction band, leaving a hole (h1) in the valence band (Fig. 19.1) [41]. The photogenerated h1 and e2 diffuse to the semiconductor surface and react with the molecules of pollutant adsorbed on the photocatalyst surface leading to their degradation by means of oxidative and/or reductive pathways [42]. Starting from the photocatalytic water splitting reaction studied by Fujishima and Honda in 1972 [43], heterogeneous photocatalysis has gained considerable attention in the recent years in different research areas, such as environmental and energy fields [44,45]. In this perspective the photocatalytic properties of several semiconducting materials have been extensively studied [46]. Among the different proposed semiconductors, TiO2 is the most widely used in a lot of photocatalytic applications because of its chemical stability, long durability, nontoxicity as well as low cost [42]. From a crystallographic point of view, TiO2 has three stable phases with little differences in bandgap: anatase (Eg 5 3.2 eV), rutile (Eg 5 3.02 eV), and brookite (Eg 5 3.14 eV) [47,48]. In the case of photocatalytic oxidation reactions, it was extensively reported that TiO2 in anatase phase evidences photocatalytic efficiency higher than the rutile phase [49]. On the other hand, the combination between rutile and anatase phases is able to increase the TiO2 photocatalytic activity [50,51]. For these reasons the most commonly used commercial photocatalyst is TiO2 P25, which is composed of 75% anatase and 25% rutile [50]. Considering its Eg value, TiO2 can be excited only by ultraviolet (UV) light. Under irradiation the TiO2 surface generates e2
h1 pair (Eq. 19.1) [42], and the electron is excited from the VB to the CB, with the simultaneous generation of h1 in the VB [52]. The e2 in the conduction band can react with the molecular oxygen dissolved inside water (in the case of water pollutants degradation) or with oxygen

Figure 19.1 Schematic illustration of the mechanism related to the excitation of a semiconductor under light irradiation [42].

TiO2 photocatalysis for environmental purposes

UV light

589

O2 O2•–

e–

Electron

+ Organic compound CO2 + H2O

3.2eV

h+

Hole TiO2

OH• H2O

Figure 19.2 Schematic mechanism of TiO2 photocatalysis [22].

in air (in the case of removal of VOCs) generating superoxide radicals (O2 2) (Eq. 19.2). Simultaneously, the h1 reacts with adsorbed OH2 forming OH (Eq. 19.3). G

G

TiO2 1 hv ! e2 1 h1

(19.1)

e2 1 O2 ! O2 


(19.2)

h1 1 OH2 ! OH

(19.3)

A schematic picture of the photoactivation of TiO2 is reported in Fig. 19.2. The photogenerated h1 and OH participate in the oxidation mechanism of organic molecules adsorbed on the TiO2 surface; meanwhile, e2 in the conduction band typically participates in reduction processes (such as the generation of superoxide radicals) [42]. The photocatalytic routes may result in the total mineralization of organic compounds into CO2 and H2O [47,53]. The following sections of the chapter are focused on different applications of TiO2 photocatalysis for environmental purposes. G

19.3

TiO2 photocatalysis for the removal of volatile organic compounds from gaseous stream

VOCs are widely used in (and produced by) both industrial and domestic activities [54]. This extensive use results in their occurrence in aquatic, soil, and atmosphere environments [55]. Many VOCs are toxic, and some are considered to be carcinogenic, mutagenic, or teratogenic [56]. However, the most significant problem related to the emission of VOCs is centered on the possible production of photochemical oxidants, for example, ozone and peroxyacetyl nitrate [57]. Tropospheric

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Titanium Dioxide (TiO2) and Its Applications

ozone, formed in the presence of sunlight from NOx and VOC emissions, is toxic to humans, damaging to crops, and is implicated in the formation of acid rain [58,59]. Emissions of VOCs also contribute to localized pollution problems of toxicity and odors. Many VOCs are implicated in the depletion of the stratospheric ozone layer and may contribute to global warming. As a result of all these problems, VOCs have drawn considerable attention in the last decade. Approximately 50% of the US Environmental Protection agency (EPA’s) list of priority pollutants is composed of VOCs. The Clean Air Act of 1990 called for a 90% reduction in the emission of 189 toxic chemicals in 8 years, 70% of these being VOCs [60]. Therefore there is currently a great deal of interest in developing processes which can destroy these compounds, and since a large number of the VOCs are oxidizable, chemical oxidation process can be looked upon as a viable method. As pointed in the previous section of the chapter, the application of heterogeneous catalytic oxidation technology to air pollution control is well established. Examples are automotive exhaust treatments [60] and catalytic incineration [61]. In general, these catalysts operate at elevated temperatures. Therefore photocatalytic oxidation of organic compounds in the gas phase appeared to be a promising process for remediation of air polluted by VOCs since a photocatalyst can operate at room temperature and ambient pressure. Along with the formation of e2
h1 pairs under UV irradiation with the consequent formation of ROS, the photocatalytic process for VOCs degradation includes several phenomena, such as the adsorption of VOCs on the TiO2 surface, the photocatalytic degradation of the target pollutant, and the desorption of products or intermediates [62]. It must be considered that the incomplete mineralization of VOCs generates some intermediates that can occupy the catalyst active site, worsening the photocatalytic activity [63]. In addition, the photocatalytic performances of TiO2 are influenced by several parameters: relative humidity, air flow rate, UV light intensity, and the initial concentration of VOCs to be removed [64]. Specifically, a key factor is the presence of water molecules in the gaseous stream since they act as resources for OH , which in turn can oxidize the pollutants. Therefore the mineralization of some VOCs into CO2 cannot be achieved without the presence of water vapor [47,65]. However, it was found that, at high water concentration, the water molecules adsorb on the photoactive sites of the TiO2, decreasing the photocatalytic reaction rate [66,67]. Also the air flow rate strongly affects the photocatalytic performances [36,53,68]. In detail, at low air flow rate, the VOCs removal was enhanced by increasing the flow rate, indicating that the mass transfer phenomena from the gaseous phase to the catalyst surface worsen the VOCs degradation. At intermediate air flow rates the change in flow rate does not induce considerable effect on VOCs removal rate, meaning that kinetics of the photocatalytic surface reaction is the controlling step of the process. On the other hand, at high air flow rates, the increase of flow rate decreases the contact time between the gas phase and the catalyst, thereby decreasing the VOCs removal rate [20]. With regard to the UV light intensity at the external surface of the photoreactors, it is well known that the photocatalytic oxidation performances are enhanced with the increase of light intensity [47,69] since the generation of ROS is enhanced [70]. However, it must be G

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taken into account that the recombination of charge carriers as well as the low interfacial charge transfer rates reflect into low quantum yields [71]. The concentration of VOCs to be removed by means of TiO2 photocatalysis is also an important aspect that it is necessary to consider. For all the treated VOCs the photocatalytic efficiency is enhanced by decreasing the initial concentration of contaminants, mainly due to the limited adsorption capacity of photoactive sites [20]. On the other hand, when the initial contaminant concentration of VOCs is high, the by-product can be strongly adsorbed on the TiO2 surface, inducing the deactivation of the photocatalyst [72]. Therefore TiO2 photocatalysis may be suitable only for the treatment of gaseous streams at low VOCs concentration [20]. Several VOCs, such as formaldehyde, trichloroethene, toluene, benzene, and methyl ethyl ketone were tested in a heterogeneous photocatalytic process using TiO2-based catalysts [73
75]. However, it must be considered that commercial TiO2 samples can be activated only by devices emitting UV light. In order to overcome this main drawback, several research studies focused on the modification TiO2 properties in order to enhance the light absorption and, consequently, the photocatalytic activity under visible-light irradiation [76]. In particular, the introduction of foreign atoms into the crystal lattice of TiO2 can modify the electronic and the optical properties of the semiconductor [77]. This type of modification is generally called “doping,” and a suitable dopant element for TiO2 should be able to increase the VB position or to introduce mid-gap states, without modifying the CB one in a manner to reduce the Ebg value. Moreover, the chosen dopant should minimize the electron
hole recombination phenomena, enhancing the photocatalytic activity of the semiconductor [77,78]. In this context, N-doped TiO2 sample was the most investigated visible-light active photocatalyst, and several review papers deal with the chemical
physical properties of N-doped TiO2, explaining the possible mechanisms linked to the improved photocatalytic activity under visible and solar light [79,80]. Consequently, research studies evaluated the feasibility of applying N-doped TiO2 in the depuration of air polluted by VOCs at a low ppb concentration [76]. For instance, this type of photocatalyst showed remarkable photocatalytic activity in the removal a wide range of VOCs (benzene, toluene, ethyl benzene, and o,m,p-xylenes) using a lamp emitting in the visible range (400
720 nm) with a removal efficiency between 23% and 96% [76]. In addition, benzene exhibited the lowest degradation efficiency, whereas the degradation efficiency of toluene was nearly three times higher than that one of benzene [76]. These results underline that the photocatalytic activity is also depending on the chemical structure and properties of the VOC that it is requested to remove from gaseous streams.

19.4

TiO2 photocatalysis for indoor air purification

The indoor air is basically any volume of air in a confined environment at least partially separated from outside air, in a manner that the physical
chemical parameters, such as gas composition, pressure, temperature, are different from those

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Titanium Dioxide (TiO2) and Its Applications

prevailing at the external environment. The levels of pollutants in the confined places are usually higher than the ambient concentrations outside, and this can not only be found mainly in the buildings but also in underground garages, vehicles, aircraft, storehouses, etc. A large variety of pollutants can be found in the indoor air, as indicated in Paragraph 1. In the top 10 list of air indoor pollutants [81] appears the radon, for which ventilation is required to decrease the level of concentration or walls work is required to block the emission at the source (the buildings’ walls made by tuff materials), then there are cigarettes smoke and biological emissions from pets, humans, mold at the walls, that in part could be diminished with cleaning of the walls. At the fourth position, CO is found, emitted by the smokes of home fires devoted to cooking and heating, and following there are NO2, which comes from many of the same sources as carbon monoxide, and VOCs that are emitted from many different sources such as paint, cleaning substances, pesticides, glue, printers and photocopiers, permanent markers, and certain building materials. Finally, we found respirable particles, such as formaldehyde, mainly formed by emissions from sources such as the aging of pressed wood containing ureaformaldehyde resins in building materials and furniture, pesticides, and asbestos. Emission sources control, ventilation, and air cleaning are the three important strategies to improve IAQ [82]. The aftertreatment of polluted gaseous streams is often not possible in the indoor air, since the emission points can be diffused in the overall internal environment. The polluted air can be treated in two ways: creating a flux of air to be purified by means of a pump or a fan or exploiting the surface of the walls and furniture to have a “photocatalytic effect” trough the functionalization with titania. As a consequence, some of these pollutants contributions can be reduced, profiting of TiO2 photocatalysis. As pointed earlier, TiO2 photocatalysis is well stated in the research field starting from the pioneer work of Fujishima and Honda [43] on water splitting up to the thousands of manuscripts and patents involving the applications for water splitting, water treatment, air purification, and self-cleaning of surfaces [83]. The number of scientific publications dedicated to photocatalytic air treatment however is lower with respect to devoted to the water treatment, but there are more patents inherent to air treatment. This points out to a higher interest in the application of photocatalysis for air treatment purposes.

19.4.1 TiO2 photocatalysis with forced air Patents on gas-phase photocatalytic conversion to CO2 of organic compounds started from the mid-1980s, demonstrating the ability to treat vaporized organic compounds [84] and to deodorize rooms [85
87]. Photocatalysis appears well suited for the purification of indoor air because the low concentrations of pollutants measured in indoor air enables continuous operation without reaching the saturation of the surface of the applied photocatalysts.

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In addition, there is the possibility of use of photocatalysis for both VOCs mineralization and bacterial disinfection [88]. Indoor air treatment foresees the use of apparatuses in which air is circulated. Such systems are mainly composed of a blower or an air pump, a particulate filtering unit, a source of UV light, and a photocatalyst fixed on a substrate. So, heterogeneous photocatalysis is applied in the air-conditioning systems employing several types of photoreactors usually at fixed bed of photocatalyst. In general the main parameters that control the indoor air polluted treatment are mass transfer, presence of particulates, adsorption of contaminants, contact time, degradation of specific contaminants or by-products, humidity effects, deactivation of the photocatalyst, the utilization of photons, and the adhesion of photocatalyst to substrates. Since the low concentration is to be treated, mass transfer of pollutants toward the photocatalyst surface should be assured. This is realized in active air photopurifiers generating a forced air circulation around the photocatalyst, usually placed in form of fixed bed; even if in some cases, fluidized bed reactor can also be used [89]. There are numerous and several placing of the flow around the catalyst, and this depends mainly from the kind of fixed bed used. The photocatalysts are often formed on a solid substrate; the most used are ceramic porous honeycombs but also corrugated surfaces or fibrous materials. In the case of ceramic honeycomb the flow is parallel to the regular channels of the macrostructure, and the main limiting phenomenon is the illumination of the surfaces. For the latter cases the air flow could be parallel to the surfaces or perpendicular. In some cases the photocatalysts can be placed also on the blades of fan, resolving well the problem of mass transfer, but the high shear rate can wear away the film of photocatalysts, decreasing the photoactivity. The presence of particulate that could deposit on the photocatalyst has to be avoided. This is usually obtained by inserting filtering elements before the air comes into contact with the photocatalytic surfaces. The moieties of adsorption of contaminants, which is the first step in the photocatalytic process, are very relevant to the efficiency obtained in the removal of contaminants. To promote adsorption the use of composite structures comprising an inert, adsorptive domain, coexisting with photocatalytic phases is found. The composite is so composed by an adsorbent phase or phases, which are not able to give the photocatalytic reaction, but through the so-called Adsorb and Shuttle effect [90], the pollutants firstly adsorbed on the inert sites and then diffused to the photocatalytic fraction of the composite material to be converted in harmless compounds. Activated carbons, carbon nanotubes, zeolites, and also fibrous materials have been used as adsorbent materials in the composite structures. When the concentration of contaminants temporarily surpasses the maximal concentration that can be handled by a specific photocatalytic device, the coupling of the photocatalytic part to an adsorption module could be useful to have a controlled released of adsorbed pollutants as source for the photocatalyst. This assures also that the concentration of contaminants in the gas cannot go below a predesigned threshold.

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Titanium Dioxide (TiO2) and Its Applications

The contact time is very relevant, but the operation at an optimal contact time, considered also with respect to the irradiation of the adsorbed substances, is challenging due to the fact that concentration of contaminants varies from place to place and as a function of time. Degradation of pollutants is a feature of any photocatalytic device, in particular organic substances, NO, CO, or by-products of gas-phase oxidation could be converted essentially by changing the composition of photocatalysts or by coupling other devices, for instance, thermal devices. In particular, to promote CO conversion, gold/titania photocatalyst can be used, or to obtain the conversion of ozone, manganese oxide can be coupled to the titania [91]. The changing in the relative humidity may have a relevant influence on the photocatalytic efficiency, since the water molecules are involved in the photocatalytic mechanism as competitive adsorbents, OH sources, and reaction products. The control of relative humidity is difficult, and this restricts the predictability of the performances of photocatalytic air purifying devices. To limit the sensitivity of photocatalytic devices to the variations in humidity, the coating with MoOx of the titania surface was proposed [92]. Deactivation of the photocatalyst is a relevant problem in practical applications, and it is intensely discussed in the patents rather than in scientific papers. This difference is related to the fact that the real conditions of photocatalytic operations are much more “complicated” with respect to the “clean” model systems traditionally studied. Real operations foresee the facing with complex mixtures of pollutants and the presence of inactivating compound for the photocatalyst. Deactivation studies require long time tests that are often accelerated in the literature. Volatile siliconcontaining compounds (VSCCs) are present in indoor environments, due to their presence in the formulations of silicone sealants, and in personal care products, in low concentrations (typically 0.01 ppm, much lower than the concentration of VOC of 1 ppm). Some studies have evidenced that with metal doping that repels or attracts siloxanes only onto the doped areas is possible to obtain the photocatalyst free to release hydroxyl radicals even in the presence of VSCCs [93]. Another way to fight the deactivation by siloxanes is to realize a labyrinthous layer of silicon particles on the surface of the photocatalyst to limit the diffusion of the higher size VSCCs with respect to VOCs. The addition of noble metals to convert sulfur compounds that act as poison for the photocatalysts (such as SOx and H2S to less noxious sulfates) or to convert VSCCs is reported in literature [94]. Photon transfer to the photocatalyst has a major influence on photocatalytic activity, and several studies are devoted to this theme. There are two general approaches to optimize the irradiation by photons: one regards the increase of the efficiency to convert absorbed photons into charge carriers inhibiting the electron
hole recombination and the other to decrease the number of photons that are not useful because they are not absorbed or do not reach the photocatalyst surface. The main prevention to the recombination of charge carrier is the interaction of photogenerated electrons with adsorbed oxygen, already present in anyhow air stream to be treated, to give superoxide radical anion. However, charge separation is enhanced if electron sinks are located on the surface of the photocatalyst, such as domains of noble metals [95] or even carbon nanotubes [96]. It must be considered

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that, kinetically, recombination is a second-order process with respect to charge carrier’s concentration, since both charge carriers are involved; meanwhile the formation of hydroxyls radicals is a first-order process, due to the interaction with the photogenerated holes with hydroxyl or water. This gives rise to a trend of photocatalytic activity with the photon flux, in which a linear increment is found, but above a certain flux, the quantum efficiency deviates from the linear trend. Optimization of the photon flux on the exposed area of the photocatalyst is so a strategy to improve the photocatalyst activity. On the other hand, photons are not to be dispersed but conveyed on the photocatalyst surface, and many dispositions can be found to resolve this issue [97]. In general it is necessary to realize short distance between the lamps and the photocatalyst. This depends on the geometry of the lamps that in most of the case can surround or be in the center of the photocatalyst bed [98]. However, another possibility is to cover the exterior part of UV-fluorescent lamps with a layer of photocatalyst or to encompass the lamp with a flexible glass fiber mesh with the photocatalyst [99]. The UV light can be given also by optical fibers [100] that could be coated with the photocatalyst or not. It must be considered that the air purifier apparatus have to be covered to avoid the leak of UV radiation to the extern, since the radiation is damaging to the health. The use of reflector protects from the radiation leaks but also reflects the UV radiation again toward the photocatalyst, increasing the photon flux. Last but not least is the problem of the photocatalyst adhesion to the supports, an issue that deserves importance. The support is usually made of UV-transparent or nonporous or fibrous substrates that could be realized of oxides, polymer metals, paper tissues, and many patents use simple impregnation [101]; so the adherence of the photocatalyst worsens with time, unless prevention is undertaken. The use of organic binders to improve the adhesion is not resolving since they could be consumed by the photocatalytic action, and the system loose the “glue” for the photocatalyst. It has been reported that [102] the photocatalyst better adheres to optical fibers if a sol
gel process of deposition is adopted. In general, sol
gel obtaining
deposition process is prone to give a good adhering of the photocatalyst to the substrate. Organotitanate precursors form very robust nanocrystalline films when a following by thermal treatment is applied [103], even if the specific surface area is generally low. The other way to get adhesion of the photocatalyst to the surface is to use spray pyrolysis, chemical vapor deposition, or sputtering [104]. Organosilane polymers, forming inorganic bonds, have been suggested as TiO2 binders [105]. Up to now the photocatalyst composition has not been discussed but is the great importance to the efficiency of an air purifier. There are many formulations that start from nanoparticulate titanium dioxide up to doped titania with N, P, or S to improve the capacity of natural light harvesting. The objective to increase the photoefficiency could be therefore to shift the photocatalyst absorption toward longer wavelengths, thus changing the kind of photons that are able to give the photooxidation, such as the latter case in which the presence of heteroatoms in the titania lattice decreases the bandgap of the titania, by far the most used photocatalyst in real applications. For instance titanium oxynitrides (TiOn2xNx) [106] performed

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Titanium Dioxide (TiO2) and Its Applications

better in the presence of visible light. For this reason an economic advantage is present if visible-light sources can be used with respect to UV sources; however, the overall photoactivity could be worst. The dopant ions were claimed in some patents to be located at oxygen sites, interstitial sites, or at the grain boundaries of the titanium dioxide particles. A patent on an air purifying device operating by both visible light and UV light was reported, based on these photocatalysts [107]. Another way to obtain relevant photoactivity with visible light is the use of sensitizers, such as example ruthenium pyridine complexes, analogous to that used by Gratzel in dye-sensitized solar cells (DSSCs) or artificial and natural cyanines (as those found in blackberry extract). The stability of these photocatalysts is by far known under visible but even UV light, since the majority of patents use commercially available UV-A or UV-Clamps. Considering the latter application of artificial sources of UV radiation, it must be pointed out that the photoactivity of doped titania under visible light is inferior to that of undoped titania under UV irradiation, and so the doped titania is more suitable for air purification in the environment, reported in the following.

19.4.2 TiO2 photocatalysis in indoor environments Indeed the other approach for indoor air treatment applies photocatalytic surfaces such as photocatalytic painted walls, photocatalytic concretes or transparently coated windows, profiting of natural air convection, or on the circulation of air promoted by air-conditioning systems. Since a large variety of VOCs are converted into CO2 and water but this process requires large surface areas, chemical photooxidation processes are practiced profiting of the walling and of surfaces of the buildings or infrastructures. The mechanism of photocatalytic oxidation of VOCs involves three steps: transfer of contaminants from the air bulk to the photocatalytic surface, adsorption on the photocatalyst and generation of ROS by light absorption, and finally, the degradation of VOCs by the reactive species formed on the surface, desorption of the products, and their diffusion to bulk. For the application of photocatalysis in closed environments, a TiO2 photocatalyst can be therefore incorporated into walling materials, decreasing the concentration of indoor air pollutants by exploiting the UV part of the solar light, which usually contains about the 5% of UVA and UVB radiations. The oxygen needed for the photoreaction is never limiting to that, since in the air it is largely disposable. As mentioned before, a photocatalyst can be incorporated into a concrete, and it is particularly used in the case of white cement, added to maintain the aesthetic characteristics of concrete structures, preventing discoloring and fouling by VOCs or organic compounds. This effect belongs to the class of self-cleaning materials, in which the photocatalytic method does the best. The spontaneous removal of organic dirty from the surfaces can be obtained, and simply spraying water on the surface of the objects will induce dust and dirt falling. TiO2
concrete composites can assure that their aesthetic characteristics will be unchanged over time, and moreover, they will act to reduce NOx and SOx pollution [108
111].

TiO2 photocatalysis for environmental purposes

597

Another method of realization of a photocatalytic surface is to obtain a coating with a photocatalytic thin film on a wall, or onto glass, or on the furniture surface. As a consequence, TiO2 can be combined into coatings and paints, plastics and fabrics, pavement blocks, wood-based panels, gypsum boards, ceiling tiles, ceramic tiles, and wallpapers in a home or any other building [112]. Some efforts have been devoted to determine how much are active in the selfcleaning surfaces of the photocatalytic composites. Reliable and suitable quantitative methods for the determination of the self-cleaning activity have been established. To date, two standard methods are widely recognized. The first is based on photo bleaching of a dye aqueous solution, in particular methylene blue (MB), when its solution is placed into contact with the photocatalyst layer, and the second on photodegradation of a solid fatty deposit, usually stearic acid, placed directly on the photocatalytic surface. Further methods have been reported in the literature [113], but these previously mentioned are comprised in official methods reported by the various national and international organizations for the standardization of tests. The presence of TiO2 lead also to other effects, such as the purification of the air: not only harmful organic substances such as formaldehyde and benzene can be abated, but also ammonia, nitrogen oxides, sulfur dioxide, carbon monoxide are transformed, respectively, to gaseous nitrogen, nitrates (entrapped and washed out by water), to sulfates (also them washable by water), to CO2 [114]. The abatement is effected also for allergens. Moreover, a deodorizing function is explicated, removing cigarette smell, odor of toilet, smell of garbage, body odor of the animal, and the smell of mud. Furthermore, TiO2 photocatalysis induces a sterilization function: it has a bactericidal effect on several pathogens, such as Escherichia coli and Staphylococcus aureus, capturing and killing the floating agents in the air. This function consists not only into the oxidation of the walls of bacteria and cysts, and of virus but also into the breaking down of the harmful compounds released by the dead bodies of bacteria. Moreover, the functionalization with TiO2 of the surfaces can induce an antimildew performance: the coating or the presence of TiO2 avoids the formation of mold and algae and prevents the adhesion of the incrustations. For practical applications the photocatalyst should be exposed on a flat surface receiving sunlight or an appropriate artificial light, containing UV radiations, but several recent patents have described the ways to shift the photocatalytic activity toward longer wavelengths, with the aim to increase the percentage of solar photons that may be utilized by the photocatalyst. This is typically realized by incorporating nitrogen [115,116], carbon [117], or sulfur [118] into TiO2 lattice, so it is affirming the use of photocatalysts based on doped titania. The N-dopant ions should be located either at oxygen sites, interstitial sites, or at the grain boundaries of the titanium dioxide particles. The photoactivity under visible is relevant compared to that of titanium dioxide under the same conditions, which is almost nil. For example, carbon as a dopant on the surface of the photocatalyst particles [117] is reported or the possibility of application of titanium oxynitrides (TiOn2nNn) that performed better in the presence of visible light [106]. The synthesis of this photocatalyst occurs by a liquid-phase reaction between an organotitanate compound dissolved in polar solvent and ammonia [119].

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Titanium Dioxide (TiO2) and Its Applications

Another possible Vis-light-driven photocatalyst has been patented and is composed by photosensitizers belonging to the class of those used by Gr¨atzel et al. in their DSSCs [120], ruthenium pyridine complexes, and TiO2. These photosensitizers could be both synthetized and natural in origin, such as cyanines found in blackberry extract. However, it must be pointed out that the stability of the photosensitizer under solar irradiation is limited, because they are prone to photodegradation with the UV components. An issue related to photocatalytic application is the adhesion of the TiO2 photocatalyst to the surface, that is resolved or by incorporating the TiO2 into the mass of substrate, typically 5
10 wt.%, or generating stabile layer through the use of binding agents. The requirements for a good binding are (1) the ability of support the catalyst particles firmly, but permitting the TiO2 particles to work by leaving exposed a high part of their surface area; (2) the chemical and physical stability against the strong photocatalytic activity of TiO2; (3) the final structure should have a wide specific surface area necessary for the adsorption and reaction of pollutants; (4) to be harmless to the environment. As example of self-cleaning properties conferred to ceramic tiles and to take advantage of the solar radiation, an N-doped TiO2 thin film has been realized on ceramic tiles [121]. N-doped TiO2 films have been obtained by sol
gel deposition using titanium tetraisopropoxide, urea as doping source, and polyethylene glycol as binder and acetylacetone as a chelating agent, to obtain better dispersion and adhesion of the final TiO2 particles onto the ceramic substrates after calcination [121]. The photocatalytic properties induced by N-doped TiO2 film have been tested in the discoloration of aqueous solution of MB under the light of blue LEDs as visible-light source (Fig. 19.3). Raw tile and tile functionalized with undoped TiO2 (tile K) evidenced similar activity due to photolysis phenomena, under irradiation with visible light. The presence of N-doped TiO2 (tile U) on the tile surface led to a relevant higher discoloration activity accomplished by high TOC removal (up to 87%). Therefore N-doped TiO2 film on the ceramic tiles gives a better exploiting of the visible light and as a consequence of the solar spectrum. The TiO2 functionalized tiles were able also to give conversion of nitrogen oxides to harmless compounds. Tiles functionalized with titanium dioxide (TiO2) for the elimination of air pollutants, prepared by the dip-coating process with titanium alkoxide solution followed by calcination, removed efficiently NOx at very low concentration (0.40 ppm NO 1 0.15 ppm NO2) in air in continuous flow reactors according to the standard method [122], as shown in the Fig. 19.4. The photocatalytic oxidation of the various pollutants based on TiO2 coatings or composites is affected by several parameters such as temperature, moisture and oxygen, light wavelength/intensity, contaminants concentration and kind, surface velocity, residence time, and catalyst loading. In particular, temperature influences both the kinetic reaction of oxidation and the adsorption of airborne VOCs on the photocatalyst and the substrate support [112]. The kinetics is governed by the Arrhenius temperature dependence of the kinetic constant k: k 5 f ðexp 2 EA =RTÞ

(19.i)

TiO2 photocatalysis for environmental purposes

599

Figure 19.3 MB discoloration as a function of visible-light irradiation time [121]. MB, Methylene blue.

Figure 19.4 Influence of total flow rate (Q) on NOx photocatalytic conversion with a plate reactor under UV irradiation [122] (AIDIC/CET is kind acknowledged for the reproduction of the figure).

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Titanium Dioxide (TiO2) and Its Applications

where EA is an apparent activation energy, T is the TiO2 temperature, and R the gas constant. Hence, the temperature increase enhances the photooxidation inducing an increase of the kinetic constant. By contrary the adsorption of an air pollutant on the photocatalyst surfaces would progressively drop with an increase of temperature [123]. The adsorption equilibrium constant K has a dependence from the reaction temperature as follows: K 5 f ðexp 2 DH=RTÞ=T 1=2

(19.ii)

where ΔH is the adsorption enthalpy change due to the adsorption of pollutant. The adsorption of a species is an exothermic reaction, so the value of K will be higher at lower temperatures. The rate of a photocatalytic reaction is of the Langmuir
Hinshelwood kind [74], r 5 k KP=ð1 1 KPÞ

(19.iii)

where P is the partial pressure of the adsorbing species. As a result of both trends, the rate of TiO2 photooxidation before increases with the temperature then declines, since the desorption becomes more favored. In general the application of photocatalytic composites on the surfaces is poorly affected by the variation of temperature, since the process occurs within the variations of the ambient temperature. The effect of moisture is also important and depends to a large degree on the kind of photocatalyts and coupled material. A high humidity will mean that there will be a large adsorption of water molecules, which could prevent the adsorption of pollutants on the TiO2. If the relative humidity is too low, there will be a lack of hydration of the photocatalyst and a decreased photoefficiency due to the low generation of the OH radicals, which in turn oxidize the pollutants [124]. So a sufficient relative humidity level, that is, the existence of an optimum concentration of water is found for several VOCs. In the case of indoor photocatalytic coatings, predicting the photoactivity is almost difficult, since the variability of the operative conditions in the closed environments. G

19.5

TiO2 photocatalysis for the removal of organic pollutants from water and wastewater

In the same way describe before for the degradation of pollutants in gaseous phase, the main goal of TiO2-based photocatalysis is to oxidize organic pollutants in water via hydroxyl radicals (OH ), thereby converting the constituents of the organic pollutants into relatively harmless organic or inorganic molecules. Even if there are G

TiO2 photocatalysis for environmental purposes

601

alternative and innovative technologies to photocatalysis such as wet oxidation or nanofiltration [125], TiO2-based photocatalysis could compete because it minimizes energy consumption and optimizes the operating conditions effective in the mineralization of the pollutants and the reduction of the TOC dissolved in water. Otherwise, the aim of TiO2-based photocatalysis could be to reduce the toxicity of the effluent before the conventional biological wastewater treatments, making the process more effective [125]. As in the case for gaseous phase, the application of TiO2 photocatalysts for water treatment requires that the process should be performed in mild conditions (room temperature and pressure), and it should be able to achieve the complete mineralization without secondary pollution [126]. In addition, the photocatalyst must have a good stability after several repetitive cycles with low costs for operations [126]. With regard to the mechanism, when the surface of the TiO2 particles (suspended in the water to be treated) is excited by UV light, the e2 in the CB participates in the reduction processes thanks to the reaction with dissolved oxygen in water to produce O2 2. Simultaneously, the h1 in the VB can diffuse to the TiO2 surface and react with adsorbed water molecules, forming OH [126]. In the case of water and wastewater treatment, it is recognized that the OH is the most important active species in the photocatalytic oxidation reactions of water pollutants [127]. In addition, the influence of physical parameters governing the kinetics of TiO2 photocatalysis for the degradation of water pollutants is similar to what was found for the VOCs removal. In particular the photocatalytic performances are strongly influenced by excitation wavelength, radiant flux, and initial pollutant concentration [128]. Additional considerations should be made about the influence of the amount of photocatalyst particles used in the process since, in the case of water purification, the most used photoreactor configuration is in slurry mode, which implies the dispersion of the semiconductor in the liquid medium [129,130]. With regard to this last aspect, it is extensively reported that the initial photocatalytic reaction rates are directly proportional to the mass (m) of the photocatalyst, indicating a full catalytic regime. On the other hand, above a certain value of m, the rate becomes constant and independent of m. In other words, this limit means that the TiO2 particles are totally exposed to the UV light. Therefore, generally speaking, the optimal mass of catalyst has to be chosen in order to avoid an excess of catalyst and to ensure a total absorption of photons suitable for the chosen photoreaction [128]. Several water pollutants (such as haloalkanes, aliphatic alcohols, carboxylic acids, alkenes, aromatics, haloaromatics, herbicides, pesticides, pharmaceuticals, and dyes) are degraded by photocatalysis [131,132]. In recent years, much attention has been devoted to regulate the use of pesticides and pharmaceuticals. However, these types of pollutants are continuously detected in water bodies and negatively affect the municipal water treatment plants [133], indicating a need for alternative water treatment methods able to degrade pesticides and pharmaceuticals in order to preserve drinking water [132]. Photocatalysis has emerged as a potential alternative to traditional water treatment methods such as UV light irradiation. G

G

G

602

Titanium Dioxide (TiO2) and Its Applications

Figure 19.5 Photocatalytic activity under visible light using N-doped TiO2 [115].

Several papers reported studies regarding the modification of TiO2-based photocatalysts to increase the performances [134]. The photocatalytic activity of TiO2 is typically studied under UV irradiation, but, given the Ebg value of TiO2, it can only use only a small fraction (about 5%) of the solar energy that actually reaches the Earth surface [135,136]. Therefore the increase of the photoactivity of TiO2 under the visible light is still a major research focus [126]. Different strategies have been proposed in order to extend the visible-light response of TiO2 [137]. It is reported that metal-doped TiO2 was effective in the removal of different pesticides from water bodies [132]. Moreover, N-doped TiO2 is the most commonly studied visible-light active photocatalyst [115,116,138
140]. This photocatalyst was found active in the removal of several organic pollutants (including organic dyes, pharmaceuticals, and pesticides) and in the E. coli inactivation under visible or solar light irradiation [115,141]. For example, Fig. 19.5 reports the comparison between the results about the degradation of MB and methyl orange (MO) dyes in the presence of white and blue LEDs, as light sources of a slurry photoreactor. For both the tested dyes a significant decrease in relative concentration was achieved. Simultaneously to the decolorization process, a significant TOC removal (up to 97%) was obtained, evidencing that the photocatalyst is also able to mineralize the target pollutant.

19.6

Conclusion and future perspectives

Gaseous and water pollutants removal by means of heterogeneous photocatalysis appears as promising solution with a great potential application since many toxic pollutants are

TiO2 photocatalysis for environmental purposes

603

totally mineralized or oxidized into harmless final chemical compounds. Moreover, with respect to the traditional methods, heterogeneous photocatalysis has the advantage of not generating sludge to be disposed. Among the different semiconductors, TiO2 is the most studied photocatalyst. TiO2 photocatalysis offers interesting advantages since the process can be carried out in mild conditions (room temperature and ambient pressure). Moreover, no additional chemical reactants are required (only oxygen from the air or dissolved in water), and the system is applicable also when the pollutants are present at very low concentrations. However, their slow transfer to the TiO2 functionalized surfaces yields the rate of mineralization very low both in air and in water. Adsorptionphotocatalyst hybrids are promising bifunctional materials, based on an adsorbent and a photocatalyst able to solve this problem, combining the singular properties of the two compounds. As a result, a synergetic effect arises leading to an increase of the photocatalytic reaction rate due to the accumulation and transfer of pollutants from the adsorbent sites to the semiconductor through the interface present between the two phases. Despite the great efforts of the past decades, desirable photocatalytic efficiency is yet to be achieved to a level suitable for large-scale applications. The utilization of solar energy is currently limited because of the light absorption properties the TiO2 catalyst (only 5% of the solar spectrum can be used since TiO2 is activated only by UV light). Therefore the formulation of innovative TiO2-based photocatalysts in order to maximize the photocatalytic activity under visible light is strongly necessary in order in order to make photocatalysis a commercially technology to be used in practical applications. A successful approach is the combination of photocatalysis with other depollution technologies. In particular, traditional wastewater treatment methods should be successfully implemented by coupling photocatalysis with them in the removal of recalcitrant and noxious compounds such as pesticides and pharmaceuticals. Further explorations about the possibility of combined use of TiO2-based technologies with other technologies (such as biological treatments for water pollutants removal and adsorbing materials in the case of gaseous depollution) will be of future breakthrough for the application of photocatalysis. Furthermore, photocatalysis-based water disinfection will play an important role because of the strong biocidal properties of TiO2-based materials. Possible new frontiers are the design and the engineering of a multifunctional photocatalyst able to associate the characteristics of excellent visible-light photocatalytic activity, high adsorption capacity, high stability, and separability in the case of water treatment. In the literature about wastewater depollution a limitation remains because the most of papers are focused on batch-scale photocatalytic reactor, and there is the need of the development of photocatalytic reactors operating in continuous mode or the design of prototypes effective in both photocatalytic degradation and particles recovery after treatment.

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8.

Fine chemistry by TiO2 heterogeneous photocatalysis

20

Giuseppe Marcı`1, Elisa I. Garcı´a-Lo´pez2 and Leonardo Palmisano1 1 “Schiavello-Grillone” Photocatalysis Group, Department of Engineering (DI), University of Palermo, Palermo, Italy, 2 Department of Biological, Chemical and Pharmaceutical Sciences and Technologies (STEBICEF), University of Palermo, Palermo, Italy

20.1

Introduction

Heterogeneous photocatalysis is a technology that has been widely applied to degrade organic and inorganic pollutants in both gas and liquid phases [1]. The growing interest in this technology is due to the possibility of exploiting sunlight as a renewable energy source and inexpensive, harmless, and recyclable photocatalysts such as TiO2 [2]. This chapter will report only heterogeneous processes, where the solid photocatalyst by absorbing light promotes redox processes that involve organic species adsorbed on its surface. It is important to underline that the reactions considered occur at ambient temperature and pressure. The application of photocatalysis for synthetic chemistry and fuel production [3] is less common than for the degradative processes that generally occur with a low selectivity due to the presence of strongly oxidizing radicals formed under irradiation conditions, especially in the presence of oxygen. However, photocatalytic studies aimed to obtain selectively a high value product or for synthetic scopes have recently grown because of the sustainability of the photocatalytic technology in comparison to other chemical approaches [4]. Papers on selective oxidation [5] and reduction [6,7], coupling [8], functionalization [9], and C C and C N bonds formation [10] have appeared in literature. They demonstrate that it is possible to obtain appreciable selectivity in photooxidation and photoreduction processes compared to the conventional ones, which are often carried out in more drastic experimental conditions. Furthermore, the use of low-cost UV LED radiation sources instead of mediumpressure UV Hg lamps, the possibility to use sunlight, and the coupling of photocatalysis with membrane technology that allows the separation of the catalyst and/or the product(s) from the reacting mixture can improve the feasibility of the photocatalytic process under examination. Another advantage of the photocatalytic process over traditional ones derives from the possible replacement of toxic organic solvents with harmless solvents, possibly with water. However, the application of photocatalysis in industrial synthetic chemistry is still a challenge more than a reality and much effort is needed to reach significant outcomes. Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00012-2 © 2021 Elsevier Inc. All rights reserved.

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20.2

Titanium Dioxide (TiO2) and Its Applications

Reactions of partial oxidation

It is well known that TiO2 heterogeneous photocatalytic oxidations are generally unselective, and consequently studies and applications concern usually pollutants abatement for outdoor and indoor environment purifications. The most frequently used photocatalyst is polycrystalline TiO2 in the anatase phase both as a powder and as a film, but some works have also been carried out with the rutile and brookite phases [11,12]. In order to increase the photocatalytic activity, TiO2 was loaded or doped with various transition metals [13] or rare earths [14], and nonmetallic species such as N [15], C [16], S [17], and F [18]. In the following section, attention will be focused on photocatalytic partial oxidation in the presence of bare TiO2. However, it must be considered that it is not possible, especially in aqueous systems, to avoid the occurrence of the reaction of complete degradation that leads to complete oxidation of the substrates. The latter process can be minimized and often the starting reagent is not completely transformed because the reaction is stopped to avoid overoxidation of the partial oxidation products. In some cases, it is useful to separate the product from the photocatalyst by means of a membrane before its subsequent oxidation.

20.2.1 Oxidation of alcohols to aldehydes Partial oxidation of alcohols to the corresponding aldehyde is of great industrial importance. Generally, these reactions are carried out in harmful organic solvents in the presence of chemical oxidants at high temperatures and pressures of oxygen, and large quantities of hazardous waste are produced [19,20]. Heterogeneous photocatalysis has been successfully used for partial oxidation of alcohols in both gaseous [21,22] and liquid systems using organic solvents [23,24], and in some cases high yields have been obtained. The use of water, the green solvent par excellence is rare. Only a few examples on the use of heterogeneous photocatalysis for the selective oxidation of alcohols to aldehydes in aqueous suspension of TiO2 are briefly described later. For instance, Palmisano et al. compared the performances of several homeprepared (HP) and commercial TiO2 samples for the selective oxidation of aromatic alcohols to the corresponding aldehydes in aqueous medium [5]. In particular, they studied the partial oxidation to the corresponding aldehyde of benzyl alcohol (BA) and 4-substituted benzyl alcohols, that is, 4-methoxybenzyl alcohol (4-MBA), 4-methylbenzyl alcohol (4-MeBA), and 4-nitrobenzyl alcohol (4-NBA). It turned out that the crystallinity of the photocatalyst was probably the most important feature that addressed the photoactivity toward selective oxidation. Indeed, the highest conversion rates of alcohol were found for commercial catalysts which were more crystalline, but the selectivity was quite low (c. 8% in the case of TiO2 P25 and c. 16% in the case of TiO2 Merck) for the partial oxidation of 4-MBA toward the corresponding aldehyde. On the contrary, the least oxidizing

Fine chemistry by TiO2 heterogeneous photocatalysis

611

catalysts were the least crystalline among the tested catalysts and they were the best samples for selective oxidations. For instance, the highest selectivity toward the 4-methoxybenzaldehyde formation was c. 42% in the presence of an HP TiO2 anatase sample and c. 60% 72% when two different HP TiO2 rutile samples were used as photocatalysts. Interestingly, all these last samples showed a very low crystallinity. An increase in crystallinity caused a significant decrease in selectivity. Notably, the intermediates detected, the selectivity values toward the aldehyde, and the reaction rate also depended on the type of substituent group. The highest selectivity toward the aldehyde formation was observed for 4-MBA oxidation. Unfortunately, once formed, aldehyde can undergo further oxidative attacks on the aromatic ring, giving rise to a decrease in selectivity. Saccharides deriving from lignocellulose hydrolysis are one of the most abundant biomass-derived platform molecules. The oxidation of sugars with six carbon atoms gives 5-hydroxymethyl-2-furfural (HMF), which can be further oxidized to 2,5-furandicarboxaldehyde (FDC) and 2,5-furandicarboxylic acid (FDCA) that are precursors for the fabrication of biopolymers. Yurdakal et al. [25] studied the photocatalytic selective partial oxidation of HMF to FDC in water solution in the presence of some TiO2 HP samples. For comparison purposes a commercial TiO2 P25 sample was also tested. A scheme of this reaction is reported in Fig. 20.1. Table 20.1 reports the selectivity showed by the various samples toward the formation of FDC. Commercial TiO2 P25 sample showed a good activity for the photooxidative conversion of HMF, but it was much less selective toward the FDC formation (12%) with respect to the HP ones. Indeed, these last HP catalysts resulted poorly crystallized, and they gave rise to a selectivity toward FDC more than twice H O

O C

H

C

H OH

O

TiO2 Light

5-Hydroxymethyl-2-furfural (HMF)

C H

O

O C H

2,5-Furandicarboxaldehyde (FDC)

Figure 20.1 Photocatalytic partial oxidation of HMF to FDC.

Table 20.1 Selectivity toward FDC at 20% of HMF photocatalytic conversion in aqueous suspension of various TiO2 samples. Photocatalyst

Selectivity (%)

P25 HP-brookite HP-rutile HP-anatase

12 21 25 23

HP, Home prepared.

612

Titanium Dioxide (TiO2) and Its Applications

(21% 25%) than that of commercial and well-crystallized catalyst. However, fully amorphous specimens have proven inactive. This study indicates also that the used TiO2 crystalline allotropic phase (brookite, B, rutile, R, and anatase, A) did not seem to have a strong influence on the global selectivity of the process. Another example of alcohol partial oxidation is the photocatalytic oxidation of glycerol that was performed in aqueous suspensions of some HP and commercial TiO2 photocatalysts [26]. The most important glycerol oxidation products were reported to be glyceraldehyde (GAD), 1,3-dihydroxyacetone (DHA), formic acid (FA), and carbon dioxide. Contrary to what obtained for the partial oxidation of aromatic and furanic alcohols, results of glycerol oxidation indicated that commercial samples are more performing for both reaction rate and selectivity. This finding confirms, as suggested by the authors, that the crystallinity of the samples is not the only parameter correlated with their selectivity. In particular, the interaction of the glycerol structure with the catalyst surface and consequently the adsorption and desorption of reagents and products occur to a different extent with respect to the aromatic alcohols. Probably the fact that HP sample surfaces are generally much more hydroxylated than commercial ones results in a stronger interaction with the glycerol OH groups, thus favoring the mineralization of adsorbed intermediates.

20.2.2 Hydroxylation of aromatic compounds The hydroxylation of aromatics is of great interest for the chemical industry but the typical processes used for their production generate massive quantity of wastes. The conversion of benzene to phenol is probably the most interesting reaction among the photocatalytic hydroxylations: phenol is extensively employed, for instance, as a disinfectant and to synthesize resins. Molinari et al. [27] proposed an interesting example of phenol synthesis from benzene. They used a photocatalytic membrane reactor, where the simultaneous reaction and product separation occurred thanks to a hydrophobic membrane. Three different commercial TiO2 samples were used as photocatalysts. In this system, benzene acted both as reagent and as extracting organic solvent. Fig. 20.2 reports a scheme of the photocatalytic setup used. The influence of the pH of aqueous TiO2 suspensions, initial amount of benzene, and catalyst loading were the parameters investigated in the batch tests. The production and separation of phenol indicated that the better pH value resulted to be 3.1. Indeed, a better control of formation and extraction of oxidation intermediates at that pH value was obtained, although the flux of phenol in the benzene phase changed only very slightly (1.16 6 0.11 mmol/h/m2). Palmisano et al. [28] studied the hydroxylation of different benzene derivatives in order to understand how the substituent group affects the selectivity to hydroxylated compounds. It was observed that TiO2 photocatalyzed hydroxylation of aromatics was regioselective in the presence of an electron donor group (EDG) in the aromatic ring, as reported in Fig. 20.3. In such cases, OH radicals attack the aromatic ring obtaining only the ortho- and para-isomers. This would suggest that the photocatalytic hydroxylation could take place upon a weak photoadsorption mode

Fine chemistry by TiO2 heterogeneous photocatalysis

613

Figure 20.2 Photocatalytic membrane reactor used for benzene to phenol oxidation (1: batch reactor, 2: UV lamp, 3: magnetic stirrer, and 4: peristaltic pump). Source: Reported with permission from R. Molinari, A. Caruso, T. Poerio, Direct benzene conversion to phenol in a hybrid photocatalytic membrane reactor, Catal. Today 144 (2009) 81 86. ©2009 Elsevier.

Figure 20.3 Hydroxylation of aromatic compounds containing an electron donor as the NH2 group (EDG) or an electron withdrawing (EWG) group as the NO2. Source: Reported with permission from G. Palmisano, M. Addamo, V. Augugliaro, T. Caronna, A. Di Paola, E.I. Garcı´a-Lo´pez, et al., Selectivity of hydroxyl radical in the partial oxidation of aromatic compounds in heterogeneous photocatalysis, Catal. Today 122 (2007) 118 127. ©2007 Elsevier.

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Titanium Dioxide (TiO2) and Its Applications

on TiO2 surface. On the contrary, in the absence of an electron donor group or in the presence of an electron-withdrawing group (EWG), the attack was completely unselective. Notably, the reported behavior did not depend on the irradiation source or the type of TiO2 used. Total oxidation of the substrate has always been observed together with hydroxylation of the aromatic ring, most significantly for aromatic compounds that did not contain groups of electron donors. Moreover, when an electron donor group was present, rather high selectivity values into the hydroxylated species (up to 75% with a 70% conversion) were reached. The selectivity in the hydroxylation of aromatic compounds containing an EDG can be explained by examining the possible formulas of the radical intermediates.

20.2.3 Epoxidation of alkenes In literature, only very few papers report the photocatalytic epoxidation of alkenes in the presence of TiO2-based photocatalyst. The use of bare TiO2 is not reported for this reaction. Consequently, we will discuss in this section only the partial oxidation of propene to obtain propene oxide (PO) in the presence of mixed catalyst containing TiO2. This reaction is of paramount importance in chemical industry where the classical methods to produce PO are based on the chlorohydrin and on the hydroperoxidation processes [29]. Nevertheless, they suffer from several disadvantages; indeed, the first one cannot be considered as a green process due to the utilization of chlorine and the production of hazardous by-products, while the economy of the second one depends on the valorization of the by-products. The epoxidation process can be performed also by using hydrogen peroxide [30] or oxygen [31] as the oxidant species. In both last cases, titanium-silicalite-1 (TS-1)-based material is used as the catalyst. Compared with the classical processes, these two last methods are advantageous from the environmental point of view; however, they are often carried out at high temperature and high pressure. Therefore the opportunity to develop a more environmentfriendly process, such as the photocatalytic one, is highly desirable. Unfortunately, very few papers report the photocatalytic propene epoxidation with molecular oxygen under mild conditions. It has been reported that highly dispersed TiO2 on silica, alumina, or zeolite exhibits a good activity for the partial oxidation of propene [32]. TiO2 SiO2 samples containing low amount of TiO2 were active for propene partial oxidation in the presence of oxygen and at room temperature (c. 50 C) in a batch photocatalytic reactor. Low pressures of O2 (30 Torr) and propene (15 Torr) and an Xe lamp as the irradiation source were used. The maximum selectivity toward PO formation in the presence of TiO2 SiO2 obtained by Murata et al. [33] was c. 60% for a propene conversion of 4.4%. Interestingly, the propene conversion increased by increasing the content of TiO2 in the mixed catalyst reaching a maximum value of c. 32% for a titanium loading of 8.3%. Regrettably, Murata et al. observed also that at any increase of propene conversion corresponded a decrease of the selectivity toward the propene oxide formation. This fact indicates that the overoxidation of the species produced must be carefully considered in all oxidative processes and its extent will be more significant at higher conversions. As reported by Murata et al. [34], the oxygen-derived species responsible for photocatalytic epoxidation in the presence of

Fine chemistry by TiO2 heterogeneous photocatalysis

615

TiO2/SiO2 appears to be O2 3 , which can be generated by the reaction between O2 and photo-formed O2 of the tetrahedral and highly dispersed coordinated titanium oxide on the surface (Ti-O4). Subsequently, the photo-produced ozonide radical attacks the double bond C 5 C of the olefin, providing the corresponding epoxy. Unfortunately, dispersed photoactivated Ti-O4 species can also produce O2 and olefin radicals, promoting undesirable side reactions with alcohol and ketone formation. Shiraishi et al. [35] carried out the photocatalytic epoxidation of several cyclic and linear alkenes by using Ti-containing silica with a hexagonal MCM-41 structure as the photocatalyst. The photoreaction was carried out in a tubular reactor by irradiating the system with light at λ . 280 nm and in the presence of an atmosphere of O2 saturated with acetonitrile (MeCN), or in the absence of the latter. Notably, the authors found that a simple addition of acetonitrile to the photocatalytic system significantly improved the selectivity toward the formation of epoxide and hypothesized that MeCN acted in the so-called shield effect by suppressing the side reactions. In particular, after 12 h of irradiation, the selectivity of some linear alkenes toward the corresponding epoxides reached 99% but, unfortunately, this process showed a low olefin conversion (about 12% as the maximum) and required light UV at approximately 300 nm for activating the catalyst.

20.3

Reactions of partial reduction

The reduction reactions in this section are those in which the double and triple carbon carbon bonds have been reduced to the corresponding single or double bond, respectively. The photocatalytic reduction of the C 5 O double bond of a carbonyl group will also be analyzed. Furthermore, the most widely studied reactions among the photocatalytic reductions are those involving nitro groups, in which a nitrous group is formed by adding two electrons and two protons, and eliminating a water molecule. The further reduction of nitrous species causes the formation of hydroxylamines or amines. Photocatalytic reductions play a significant role and often follow a different reaction path compared to direct reduction, with the advantage of good selectivity.

20.3.1 Hydrogenation of double and triple carbon carbon bonds Hydrogenation of alkenes and alkynes is very important reaction in chemical industry. Manley et al. reported that the hydrogenation of C 5 C double bond in maleimides took place in the presence of TiO2 Degussa P25 [36]. They found that the reaction gave poor performance in acetonitrile (MeCN), probably due to the absence of an efficient hole scavenger. On the contrary, when the reaction was carried out in methanol (MeOH), the performance versus the hydrogenation reaction increased, but it was observed the opening of the ring with the formation of the corresponding amide. Consequently, a good compromise was reached by using as

616

Titanium Dioxide (TiO2) and Its Applications

solvent an MeCN/MeOH 9:1 mixture. Indeed, under those optimized conditions, several commercially maleimides were selectively reduced to the corresponding succinimides in very good yields (see Fig. 20.4). Kinoshita et al. [37] investigated UV-photoinduced phenol ring hydrogenation over metal-loaded TiO2 photocatalyst without the use of gaseous hydrogen (H2). In particular, they studied the influence on the ring hydrogenation of various parameters such as the type and the amount of metal cocatalyst loading TiO2 and the kinds of solvents and hole scavengers used. The highest yield to cyclohexanol was found when Rh-loaded TiO2, water, and oxalic acid were contemporaneously used as photocatalyst, solvent, and hole scavenger, respectively. This study seems to indicate that phenol was first hydrogenated to cyclohexanone via keto enol tautomerism of cyclohexenol and then hydrogenation of cyclohexanone to cyclohexanol occurred. Phenol adsorption onto the photocatalyst was a key factor for the ring hydrogenation (Fig. 20.5). Another very interesting reaction is the semihydrogenation of alkynes to cisalkenes. Anyway, the possibility to perform this reaction under mild condition in the absence of free hydrogen (H2) and poisons (lead and quinolone) is still a challenge. Fukui et al. [38] prepared a photocatalyst based on TiO2 with two functions (visible light responsiveness and semihydrogenation activity) obtained by using 2,3-dihydroxynaphthalene (DHN) and a copper (Cu) cocatalyst, respectively. The resulting photocatalyst (DHN/TiO2-Cu) showed high performance for diastereoselective semihydrogenation of alkynes to cis-alkenes in water acetonitrile solution under visible light irradiation without the use of free H2 and poisons. Notably, alkynes having reducible functional groups were converted to the corresponding alkenes with the preservation of the functional groups. The presence of water in the solvent was of fundamental importance. Indeed, the addition of water to acetonitrile changed the amount of alkynes adsorbed on the photocatalyst and it was a key factor determining the rate of hydrogenation. The rate-determining step of this reaction seems to be a thermal catalytic semihydrogenation process over the

Figure 20.4 Photocatalytic reduction of the C 5 C bond in (substituted) maleimides. Source: Reported with permission from D.W. Manley, L. Buzzetti, A. MacKessack-Leitch, J.C. Walton, Hydrogenations without hydrogen: titania photocatalyzed reductions of maleimides and aldehydes, Molecules 19 (2014) 15324 15338. ©2017 MPDI.

Fine chemistry by TiO2 heterogeneous photocatalysis

617

Figure 20.5 Hypothesized pathways of photocatalytic hydrogenation of phenol to cyclohexanol under UV light irradiation in water suspension of Rh TiO2 and in the presence of oxalic acid as hole scavenger. Source: Reported with permission from A. Kinoshita, K. Nakanishi, R. Yagi, A. Tanaka, K. Hashimoto, H. Kominami, Hydrogen-free ring hydrogenation of phenol to cyclohexanol over a rhodium-loaded titanium(IV) oxide photocatalyst, Appl. Catal. A 578 (2019) 83 88. ©2019 Elsevier.

Cu cocatalyst; indeed, the semihydrogenation and hydrogen evolution occurred competitively on Cu metals, and the former became predominant at slightly elevated (328K) temperatures (see Fig. 20.6). In this work, authors used the DHN/TiO2 Cu photocatalyst for the semihydrogenation under visible light irradiation of various alkynes and the results are shown in Table 20.2. Three internal alkynes were converted into the corresponding cis-alkenes in high yields (entries 1 3). Interestingly, in these reactions only cis-alkenes were produced. Terminal alkynes were also hydrogenated to the corresponding terminal alkenes in high yields (entries 4 7).

20.3.2 Reduction of carbonyls Benzhydrol (hydroxydiphenyl methane) and its derivatives are important intermediates in the pharmaceutical industry to prepare antihistamines, antihypertensive, and antiallergenic agents. The conventional method to obtain benzhydrol is the catalytic reduction of benzophenone under high pressure of pure hydrogen in the presence of solvent and very expensive palladium-based catalyst. The possibility to carry out this reduction by photocatalysis is very attractive. Albiter Escobar et al. reported a very promising study where they tested the photocatalytic reduction of benzophenone to benzhydrol by using, as photocatalysts, some HP titanium dioxide samples, obtained by a hydrothermal method, and commercial TiO2 P25 [39]. For comparing purpose the photochemical reaction in the absence of catalyst was also tested.

618

Titanium Dioxide (TiO2) and Its Applications

Interestingly, the photochemical conversion of benzophenone was higher if compared with that obtained under heterogeneous photocatalytic conditions, but the yield toward benzhydrol drastically decreased due to the formation of benzopinacol

Figure 20.6 Hypothesized photocatalytic semihydrogenation pathways of alkynes over DHN/TiO2-Cu under visible light irradiation. Source: Reported with permission from M. Fukui, Y. Omori, S. Kitagawa, A. Tanaka, K. Hashimoto, H. Kominami, Visible light-induced diastereoselective semihydrogenation of alkynes to cis-alkenes over an organically modified titanium(IV) oxide photocatalyst having a metal co-catalyst, J. Catal. 374 (2019) 36 42. Copyright 2019 Elsevier. Table 20.2 Photocatalytic results obtained from the hydrogenation of various alkynes in water acetonitrile suspensions of DHN/TiO2 Cu under visible-light irradiation. Entry

Substrate

Product

Time (h)

Conv. (%)

Yield (%)

1

4

.99

.99

2

24

.99

.99

3

20

.99

93

4

4

.99

86

5

18

.99

70

6

7

.99

85

7

5

.99

90

Source: Reported with permission from M. Fukui, Y. Omori, S. Kitagawa, A. Tanaka, K. Hashimoto, H. Kominami, Visible light-induced diastereoselective semihydrogenation of alkynes to cis-alkenes over an organically modified titanium(IV) oxide photocatalyst having a metal co-catalyst, J. Catal. 374 (2019) 36 42. ©2019 Elsevier.

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619

Figure 20.7 Photocatalytic and photochemical reduction of benzophenone.

as the main product, probably obtained via coupling of radical species formed under UV irradiation. On the contrary, in the presence of TiO2, isopropanol (as electron donor), acetonitrile as solvent, and UV light, the photocatalytic reduction to benzhydrol proceeded. This last insight indicates that both conversion and selectivity depend on the initial concentration of benzophenone and amount of catalysts. In particular, the optimal conditions to obtain high selectivity and yield toward benzhydrol (yield 70%) were reached when the concentration of benzophenone was c. 0.5 mM and the amount of catalyst 2 g/L. The main products obtained from the benzophenone reduction via photocatalytic or photochemical route are illustrated in Fig. 20.7. Manley et al. [36] studied the photocatalytic reduction of aromatic and heteroaromatic aldehydes to the corresponding primary alcohols in the presence of TiO2 P25 suspended in acetonitrile. Interestingly, also in these cases pinacols were formed as by-products. Authors concluded that the product distribution is affected both by the amount of the photocatalyst and of methanol that was used as cosolvent. The best results in terms of alcohol yield were reached when a dispersion of 1 g/L of TiO2 P25 in an MeCN/MeOH 9/1 mixture was irradiated for 20 h with UV-A light. Under these conditions, several aldehydes were reduced to the corresponding alcohols (see Table 20.3).

20.3.3 Reduction of N-containing functional groups Functionalized amines are very important industrial chemicals that are used for the production of pharmaceuticals, dyes, agro compounds, and resins. Unfortunately, anilines are generally obtained via catalytic hydrogenation of nitro-aromatics under high pressure of H2 and high temperature which induce also the reduction of other

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Titanium Dioxide (TiO2) and Its Applications

Table 20.3 Primary alcohol yields obtained from the photocatalytic reductions of some aldehydes after 20 h of irradiation with UV-A light in the presence of TiO2 P25 (1 g/L) and MeCN/MeOH 9/1 mixture. Aldehyde

Yield (%) to the corresponding alcohol

Benzaldehyde 2-Naphthaldehyde 4-MeO-benzaldehyde 4-Cl-benzaldehyde

63 95 56 78

Figure 20.8 Photocatalytic reduction of various nitrobenzenes.

functional groups present in the molecules. Photocatalytic reduction of nitroaromatics in the presence of TiO2 has been applied to obtain the corresponding anilines [40 45] (Fig. 20.8). The published results conclude that the conversion of the nitro-aromatics and the selectivity toward the corresponding anilines depend on different parameters such as the wavelength of irradiation chosen and the presence of additives. TiO2 is largely adopted for this purpose, either the commercial Degussa P25 or various homemade samples, usually prepared via hydrolysis of a titanium tetra-alkoxy derivative followed by thermal decomposition and calcination. Chen et al. studied the photocatalytic reduction of nitrobenzene in the presence of homemade titanium dioxide. The reactivity was carried out in the presence of methanol or ethanol as sacrificial electron donor obtaining the complete conversion of the substrate and c. 90% of yield to aniline (aniline was the main product formed) after 6 h of irradiation [40]. The optimal experimental conditions were reached with a TiO2 loading in the dispersion of 4.0 g/L, initial pH of reaction solution equal to 4.0, and methanol as solvent/sacrificial agent. Imamura et al. studied the photocatalytic reduction of nitrobenzene and nitrobenzenes differently substituted (containing for instance a vinyl group, or a chlorine or bromine atom) in TiO2 suspensions of 2-propanol and under various experimental conditions [41]. In these reactions, nitrobenzenes were almost completely reduced to the corresponding anilines and contemporaneously a stoichiometric amount of 2-propanol was oxidized to acetone. Interestingly, a highly chemoselective nitro group reduction was observed.

Fine chemistry by TiO2 heterogeneous photocatalysis

621

Fig. 20.9 resumes the most important results obtained in terms of nitrobenzenes conversion and selectivity to anilines. The high efficiency of this reduction process depends on the efficiency of an irreversible hole scavenger obtained through the presence of a sacrificial agent. As reported in the abovementioned work, an alcoholic solvent can help, but it can be considered also the formation of the corresponding oxidized (mainly carbonyls) derivatives as by-products [7]. A solution to this drawback would be the substitution of the alcoholic solvent with water in which a traceless hole scavenger is added. In this contest, Imamura et al. [7] proposed the use of formic and oxalic acids, which give only gaseous carbon dioxide as the final by-product. Authors performed the photocatalytic reduction of nitrobenzenes to corresponding aminobenzenes in aqueous suspensions of TiO2-containing hole scavengers under various conditions. In particular, photocatalytic reduction of m-nitrobenzenesulfonic acid in the presence of formic acid as the hole scavenger under deaerated conditions gave m-aminobenzenesulfonic acid (m-ABS) with high yield ( .99%). As an important result, it can be considered that no reoxidation of m-aminobenzenesulfonic acid occurred in acidic conditions. The high yield toward m-ABS was explained by considering that, apart from its role as hole scavenger, formic acid was also claimed to increase the selectivity, avoiding undesired by-processes (e.g., reoxidation). Indeed, HCOOH had the ability to protonate the reduced amino group to NH31, limiting the probability for the product to interact with the surface of

TiO2

Conversion 99%

hν

Selecvity 98%

TiO2

Conversion 92%

hν

Selecvity 98%

TiO2 hν

TiO2 hν

Conversion 99% Selecvity 82%

Conversion 86% Selecvity 99%

Figure 20.9 Photocatalytic reduction of nitrobenzenes to the corresponding anilines in 2-propanol suspensions of TiO2 under deaerated conditions [40].

622

Titanium Dioxide (TiO2) and Its Applications

TiO2 positively charged in acidic suspensions. It is also useful to underline that the authors observed a linear correlation between the amount of adsorbed m-nitrobenzenesulfonic acid and the yield of m-aminobenzenesulfonic acid. This finding suggested that the ability of TiO2 to adsorb m-nitrobenzenesulfonic is one of the key factors for the efficient photocatalytic reduction of m-nitrobenzenesulfonic acid to the m-aminobenzenesulfonic acid. In the same work, authors report the aminonitrobenzenes reduction to the corresponding diaminobenzenes in the presence of oxalic acid as hole scavenger. Interestingly, high yields of m-aminobenzenesulfonic acid and diaminobenzenes were obtained even in the presence of oxygen. The previous work indicates that the surface properties of the photocatalyst are a determining parameter for photocatalytic reductions. Indeed, the charge, if any, of titanium dioxide heavily affects the adsorption of the considered substrates, thereby influencing the associated photocatalytic activity. Another example is the photocatalytic reduction of 4-nitrophenol that was performed by Ahn et al. using arginine-modified TiO2 (Arg-TiO2) nanoparticles as photocatalyst [42]. The catalysts were prepared simply by adding an arginine solution to an aqueous TiO2 suspension. The only identifiable reduction product of 4-nitrophenol was 4-aminophenol. It was hypothesized that the terminal amino groups of the arginine monolayer created a positive TiO2 surface charge over a wide range of pH values. Consequently, the rate of 4-nitrophenol reduction proceeded very fast at pH 5 9, due to an improved adsorption of the Ar OH hydroxyl group which is deprotonated at this pH. Moreover, the increased selectivity of the process was attributed to the presence of the arginine monolayer. Wang et al. synthesized 2-alkylbenzimidazoles by TiO2 photocatalysis. In this case the photocatalytic reduction of ortho-dinitrobenzenes in the presence of TiO2 and in ethanol led to 2-methylbenzimidazole with a high yield (96%) [43]. According to the authors (see Fig. 20.10), this heterocyclic product was formed by the reduction of one of the two nitro groups present in ortho-dinitrobenzenes to give ortho-nitroaniline, with the concomitant oxidation of ethanol (the solvent) to acetaldehyde. Condensation of the amino group and the carbonyl moiety followed, and then further reduction and dehydration allowed the formation of the final product, probably with the involvement of a hydroxylamine intermediate. A systematic investigation on this synthetic method was performed by introducing different substituents in the aromatic ring (e.g., methyl, ethoxy, and ethoxycarbonyl), and by varying the alcoholic solvent used. Notably, the expected benzimidazole has always been obtained with very good yields ( .70%). The reduction of nitro groups was also carried out using modified titania photocatalysts, as for instance N-doped TiO2 [44,45]. Aromatic nitro compounds were chemoselectively reduced to the corresponding amines by using N-doped TiO2 and potassium iodide as photocatalysts in the presence of methanol [44]. The N-doped TiO2 photocatalyst used in this work was prepared by the sol gel technique in the presence of urea as the nitrogen source. Interestingly, N-doped TiO2 showed higher photocatalytic activity if compared with that of pure TiO2 and it was also active under UV-filtered solar light irradiation. The effect of the additives in the reduction of o-nitrophenol as a model reaction was checked out by comparing the yields with

Fine chemistry by TiO2 heterogeneous photocatalysis

623

Figure 20.10 Photocatalytic formation of benzimidazoles on TiO2 particles starting from ortho-dinitrobenzenes and ethanol. Source: Reported with permission from H. Wang, R.E. Partch, Y. Li, Synthesis of 2alkylbenzimidazoles via TiO2-mediated photocatalysis, J. Org. Chem. 62 (1997) 5222 5225. ©1997 ACS.

and without additives. Potassium iodide and sodium iodide were found to be the most effective additives. Under optimized conditions a reduction rapidly occurred (about 20 min) with yields .90% even in the case of nitro compounds substituted with different functional groups such as amino, hydroxy, carbonyl, and carboxylic groups, as well as halogen atoms, which were selectively reduced. A different reaction course was observed by Wang et al. [45] in the selective reduction of aromatic nitro compounds to the corresponding symmetrically substituted azo derivatives (see Fig. 20.11). Also N-doped TiO2 nanoparticles were used as the photocatalyst, and urea was the N-source during the sol gel preparation of the material [45]. The presence of an acid was mandatory for the reaction. In particular, the best

624

Titanium Dioxide (TiO2) and Its Applications

Figure 20.11 Photocatalytic reduction of nitroarenes to azo compounds with the yields reported in Ref. [45].

performance was obtained by using formic acid, but different acids, including nitric, hydrochloric, and acetic acids, gave also good results. Various azo compounds containing additional reducible substituents (e.g., halogen atoms, carboxylic, and phenol functions) were synthesized in a single step in the presence of N-doped TiO2 catalyst. The conversion occurred in few minutes and in a clean way and afforded the desired products with high yield at room temperature. Notably, the photocatalyst could be reused for at least four times with only a negligible loss of performance.

Fine chemistry by TiO2 heterogeneous photocatalysis

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The heterogeneous photocatalytic nitroaromatic compounds reduction in the presence of an organic additive was performed using various semiconductors, including TiO2 [46]. Irradiation in the presence of cyclohexene gave rise to a complex mixture of reaction products, as azoxybenzene, azobenzene, aniline, and a coupling product, namely N-(2-cyclohexenyl)aniline. The contribution of the heterogeneous semiconductor played an important role in the relative product distribution, which was observed to depend on both competitive adsorption desorption equilibria of the involved reagents and intermediates on the solid surface and on the different reducing power of the photoexcited solid [46]. Several papers in the literature deal with the mechanistic details of the reductive process. Among the others, it is important to mention the photoreduction of 4-nitroaniline (4-NA) to p-phenylenediamine (PPD) over TiO2 in the presence of methanol [47]. The main process of these 4-NA to PPD reduction can be described as reported in Fig. 20.12A and the mechanism proposed in Fig. 20.12B.

Figure 20.12 Photocatalytic photoreduction of 4-NA to PPD (A) and mechanism by using a solid photocatalyst (B). Source: Reported with permission from W. Wu, L. Wen, L. Shen, R. Liang, R. Yuan, L. Wu, A new insight into the photocatalytic reduction of 4-nitroaniline to p-phenylenediamine in the presence of alcohols, Appl. Catal. B: Environ. 130 131 (2013) 163 167. ©1997 Elsevier.

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Titanium Dioxide (TiO2) and Its Applications

20.4

Reactions of alkylation

20.4.1 Reactions of addition Cermenati et al. demonstrated the benzylation of benzyl silanes, phenylacetic acids, and alkyl aromatics. The reaction occurred by irradiation of a TiO2 suspension in CH3CN and under inert atmosphere, in the presence of benzyltrimethylsilane, its 4-methoxy derivative, or 4-methoxyphenylacetic acid as the donors and maleic acid, maleic anhydride, or related nitriles as the acceptors. The corresponding benzylsuccinic acid derivatives were obtained via electron/hole transfer and fragmentation of the benzyl radical cation (such a cleavage is too slow with 4-methoxytoluene) [48]. The same reaction was also carried out under solar light irradiation exploiting a TiO2 suspension of maleic anhydride and (4-methoxybenzyl)trimethylsilane in CH3CN [49]. Under these conditions the expected benzylated succinic acid derivative was obtained in a higher scale. The reaction scheme for the processes in both conditions is reported in Fig. 20.13. The formation of C C bonds by using carboxylic acids as the substrates in the presence of TiO2 has been reported under dry, anaerobic conditions with high yields of dimers from coupling of the radicals arising via decarboxylation of the reagent. Upon addition of an electron-deficient alkene to the reaction mixture, efficient alkylations were achieved. In particular, the reaction of phenoxyacetic acid with maleic anhydride or maleimides afforded chromene derivatives (through a cascade process) in addition to the expected addition products (Fig. 20.13B). The proposed mechanism, involving oxidation of the starting acid and decarboxylation, was supported by EPR spectroscopic evidence, as well as by deuterium labeling experiments [50]. Radical addition of tertiary amines to α,β-unsaturated lactones can be performed with high yields (up to 90%) under heterogeneous conditions, in the presence of TiO2, as reported in Fig. 20.14. The yield can be suitably tuned by modifying the (A)

O TMS +

O

(B)

O

O

COOH

tBu

MeO

+

O

O TiO 2 (5 g/L) hν 42 h

TiO 2 (1.4 g/L) Solar irradiation 10 h O O MeO O 65% selectivity

N

O H tBu

O H

N O 57% selectivity

Figure 20.13 Photocatalytic conjugate addition reactions: reduction of nitroarenes to azo compounds with the respective yields, obtained in Ref. [48] for part (A) and Ref. [49] for part (B).

Fine chemistry by TiO2 heterogeneous photocatalysis

627

Figure 20.14 Reaction of (5R)-menthyloxy-2[5H]furanone with N-methylpyrrolidine, and the maximum selectivity to the products, observed in Ref. [51].

Figure 20.15 Selective carbamoylation of heteroaromatic bases, as reported in Ref. [53].

experimental parameters. According to the proposed mechanism, the initiation and termination steps took place at the surface of the semiconductor via interfacial electron transfer. The amines were used both as reactant and solvent, the excess was recycled by distillation and the inexpensive TiO2 easily removed by filtration [51].

20.4.2 Substitution reactions in aromatic compounds The functionalization of various heterocyclic bases induced by artificial irradiation or natural sunlight occurred in a heterogeneous liquid solid system in the presence of TiO2 with higher yield compared to a homogeneous regime under the same experimental conditions [52 56]. This approach resulted in a smooth and environmentfriendly method for the photochemical functionalization of these heterocyclic bases under mild conditions. As an example, the reaction of a benzodiazine, as the quinoxaline (1,4-diazanaphthalene) in the presence of formamide, is reported in Fig. 20.15. The heterocyclic bases included amides [52,53], a variety of oxygenated derivatives as ethers and acetals [54] and aldehydes [55]. An oxidant, either hydrogen peroxide or oxygen, and sulfuric acid were required. Alternatively, the reaction of quinaldine (2-methylquinoline) among other heterocyclic bases, also in the presence of formamide, reported in Fig. 20.16A resulted in a clean and high yielding carbamoylation process, where the selective activation of the formyl C H group was achieved [52,53]. A series of ethers could also be used for the generation of α-oxy radicals, including both cyclic ethers such as tetrahydrofuran and tetrahydropyran, and acyclic ones as diethyl ether derivatives [54 56]. Of particular interest is the reaction with trioxane (1,3,5-trioxane), because the products could be subjected to hydrolysis giving rise to the corresponding heterocyclic aldehydes, resulting in an indirect formylation reaction, as reported in Fig. 20.16B [54]. In the same manner, acyl radicals were obtained from aliphatic aldehydes by irradiating these substances in the presence of TiO2 [55]. The initially formed

628

Titanium Dioxide (TiO2) and Its Applications

(A)

O TiO2

O H

NH2

NH2

+

N

hν H2O2 , H2SO4

Yield 100 %

N

(B) TiO2

O O

O

+

hν

N

H2O2 , CF3COOH

N

O O

O

Yield 40 %

Figure 20.16 Photocatalytic selective carbamoylation of heteroaromatic bases in air by an amide (A) or an ether (B), as reported in Refs. [53,54], respectively.

O

N

(70%)

N TiO2

+

O

hν , H2O2 , CF3COOH N N

N N

(30%)

Total yield 72% Figure 20.17 Example of a functionalization reaction of a heteroaromatic base (quinoxaline) with an aldehyde (ethylbutyraldehyde) in the presence of TiO2, as reported in Ref. [55].

radicals could undergo decarbonylation to yield the corresponding alkyl radicals. No decarbonylation occurred when starting from acetaldehyde, but with primary aldehydes significant amounts of alkylated derivatives were obtained, and this finding indicated that acyl radicals decarbonylated before trapping. Acylated products accounted only for a small fraction of the products, as reported in Fig. 20.17, upon shifting to secondary aldehydes and were detected only in traces in the case of tertiary aldehydes [55]. The cyanomethylation of benzene has been also performed in the presence of Pd/TiO2 under visible light irradiation, where the formation of phenylacetonitrile derivatives was obtained. According to the authors, Pd was not only involved in the C C bond forming step but also acted as electron scavenger enabling, moreover, the H2 evolution during the reaction [57]. Perfluoroalkylated products were obtained from perfluoroalkyl iodides and an aromatic derivative (benzene, naphthalene, and benzofuran). Perfluoroalkyl radicals were obtained by a photocatalytic reaction in the presence of TiO2 [58]. The

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629

Figure 20.18 TiO2-photocatalytic aromatic perfluoroalkylation mechanism proposed by Iizuka et al. Source: Reported with permission from M. Iizuka, M. Yoshida, Redox system for perfluoroalkylation of arenes and α-methylstyrene derivatives using titanium oxide as photocatalyst, J. Fluor. Chem. 130 (2009) 926 932. ©2009 Elsevier.

mechanism proposed by the authors is reported in Fig. 20.18. The radicals, formed by a mono-electronic reduction of the iodides with loss of I2, reacted with the aromatic ring giving rise to an adduct that was oxidized by TiO2 activated by light, deprotonated, and eventually transformed into the perfluoroalkylated product. Authors claim that iodide was not reduced by electrons because the electron/hole recombination was faster. Methanol, which traps the holes (Eq. 3) was essential to react with the iodide. Then, the perfluoroalkyl radical can react with p-xylene to form an arenium radical, which was successively oxidized to an arenium cation by the holes (Eq. 4).

20.4.3 Reactions of carbonyl alkylation Heterogeneous photocatalysis also offers easy access to α-carbonyl radicals, a class of electron-poor intermediates generated from the corresponding halides (often bromides) via mono-electronic reduction and subsequent loss of the halide anion. The formed radicals were trapped by electron-rich olefins [59,60]. An example of these types of reactions is the combination of a carbonyl (generally an aldehyde) with a secondary amine. In this reaction, after the addition step, an α-amino radical adduct

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Titanium Dioxide (TiO2) and Its Applications

O

O

O

H5C2O

OC2H5

N H

Br

+

·

HCl

O

O

(20 mol.%)

TiO2, hν visible

H O

N

H5C2O H

OC2H5 5

O

5

Figure 20.19 Enantioselective alkylations using MacMillan’s chiral secondary amine and TiO2 as photocatalyst, as reported by Cherevatskaya et al. [59]. O N

+

C=N

Ts

+

H2O

TiO2 hν, dioxane

N

N

Ts

H Yield 82 %

Figure 20.20 Visible light mediated heterogeneous C H functionalization of tertiary amines giving rise to α-amino amides by the three-component Ugi-type reaction of N,N-dimethylaniline with p-toluenesulfonylmethyl isocyanide and H2O in the presence of TiO2 [62].

was formed, which was oxidized to the corresponding iminium cation. The final product resulted an α-alkylated carbonyl compound. The reaction scheme is reported in Fig. 20.19. Apart from radicals, heterogeneous photocatalysis can also be exploited to have access to other kinds of intermediates, including cations. Recently, the activation of tertiary amines for the formation of iminium cations, which have then been trapped in nucleophilic addition reactions, has been exploited also in the TiO2 photocatalytic alkylation process [59 62]. Several semiconductors, including metal oxides such as TiO2, have been used to perform amine α-alkylation reactions. This technology allowed an easy separation of the solid photocatalyst that can be also recycled and reused. The process was based on the oxidation of the substrate to the corresponding radical-cation. The electron generated on the conduction band reduces the O2 forming the superoxide anion O2 2 , a highly reactive species able to abstract a hydrogen atom from the amine radical cation giving rise to an iminium ion. The latter species is trapped by a nucleophile resulting in a dehydrogenative coupling reaction. Several C C and C P bonds were successfully formed, including nitro-amines, aminocarbonyls (carboxyls), amino-nitriles, and amino-phosphonates in high chemical yield. These compounds are formed by reaction with nitronate anions, enamines (possibly organocatalytically generated from a carbonyl and a secondary amine) or enols, cyanide, and phosphite derivatives, respectively. The same approach can be applied to obtain a variety of α-amino amides. Indeed, the Ugi-type, three-component reaction, involving the amine, an isonitrile and water, reported in Fig. 20.20, is conceived by following the same mechanism [62].

Fine chemistry by TiO2 heterogeneous photocatalysis

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631

Conclusion

Heterogeneous photocatalysis in the presence of TiO2 can be used for reactions of organic synthesis. The nonexhaustive examples presented in this chapter suggest that a proper attention to the experimental parameters could allow to exploit the advantages of this technology. The use of TiO2, a safe, recyclable, and stable material working under mild temperature and pressure conditions, often using green solvents, allows one to consider seriously heterogeneous photocatalysis for synthetic purposes as an alternative to less sustainable industrial processes. The use of TiO2 allows an easy separation of the organic product(s) from the reaction mixture, simplifying the experimental procedures.

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[29] S.B. Shin, D. Chadwick, Kinetics of heterogeneous catalytic epoxidation of propene with hydrogen peroxide over titanium silicalite (TS-1), Ind. Eng. Chem. Res. 49 (2010) 8125 8134. [30] G. Wu, Y. Wang, L. Wang, W. Feng, H. Shi, Y. Lin, et al., Epoxidation of propylene with H2O2 catalyzed by supported TS-1 catalyst in a fixed-bed reactor: experiments and kinetics, Chem. Eng. J. 215 216 (2013) 306 314. [31] J. Huang, T. Takei, H. Ohashi, M. Haruta, Propene epoxidation with oxygen over gold clusters: role of basic salts and hydroxides of alkalis, Appl. Catal. A: Gen. 435 436 (2012) 115 122. [32] A. Maldotti, A. Molinari, R. Amadelli, Photocatalysis with organized systems for the oxofunctionalization of hydrocarbons by O2, Chem. Rev. 102 (2002) 3811 3836. [33] C. Murata, H. Yoshida, J. Kumagai, T. Hattori, Active sites and active oxygen species for photocatalytic epoxidation of propene by molecular oxygen over TiO2 SiO2 binary oxides, J. Phys. Chem. B 107 (2003) 4364 4373. [34] C. Murata, T. Hattori, H. Yoshida, Electrophilic property of O32 photoformed on isolated Ti species in silica promoting alkene epoxidation, J. Catal. 231 (2005) 292 299. [35] Y. Shiraishi, M. Morishita, T. Hirai, Acetonitrile-assisted highly selective photocatalytic epoxidation of olefins on Ti-containing silica with molecular oxygen, Chem. Commun. (2005) 5977 5979. [36] D.W. Manley, L. Buzzetti, A. MacKessack-Leitch, J.C. Walton, Hydrogenations without hydrogen: titania photocatalyzed reductions of maleimides and aldehydes, Molecules 19 (2014) 15324 15338. [37] A. Kinoshita, K. Nakanishi, R. Yagi, A. Tanaka, K. Hashimoto, H. Kominami, Hydrogen-free ring hydrogenation of phenol to cyclohexanol over a rhodium-loaded titanium(IV) oxide photocatalyst, Appl. Catal. A 578 (2019) 83 88. [38] M. Fukui, Y. Omori, S. Kitagawa, A. Tanaka, K. Hashimoto, H. Kominami, Visible light-induced diastereoselective semihydrogenation of alkynes to cis-alkenes over an organically modified titanium(IV) oxide photocatalyst having a metal co-catalyst, J. Catal. 374 (2019) 36 42. [39] E. Albiter Escobar, M.A. Valenzuela Zapata, S. Alfaro Herna´ndez, S.O. Flores Valle, ´ ngeles, et al., Photocatalytic reduction of benzophenone O. Rı´os Berny, V.J. Gonza´lez A on TiO2: effect of preparation method and reaction conditions, J. Mex. Chem. Soc. 54 (3) (2010) 133 138. [40] S. Chen, H. Zhang, X. Yu, W. Liu, Photocatalytic reduction of nitrobenzene by titanium dioxide powder, Chin. J. Chem. 28 (2010) 21 26. [41] K. Imamura, S. Iwasaki, T. Maeda, K. Hashimoto, B. Ohtani, H. Kominami, Photocatalytic reduction of nitrobenzenes to aminobenzenes in aqueous suspensions of titanium(IV) oxide in the presence of hole scavengers under deaerated and aerated conditions, Phys. Chem. Chem. Phys. 13 (2011) 5114 5119. [42] W.Y. Ahn, S.A. Sheeley, T. Rajh, D.M. Cropek, Photocatalytic reduction of 4-nitrophenol with arginine-modified titanium dioxide nanoparticles, Appl. Catal. B: Environ. 74 (2007) 103 110. [43] H. Wang, R.E. Partch, Y. Li, Synthesis of 2-alkylbenzimidazoles via TiO2-mediated photocatalysis, J. Org. Chem. 62 (1997) 5222 5225. [44] H. Wang, J. Yan, W. Chang, Z. Zhang, Practical synthesis of aromatic amines by photocatalytic reduction of aromatic nitro compounds on nanoparticles N-doped TiO2, Catal. Commun. 10 (2009) 989 994.

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[45] H. Wang, X. Yang, W. Xiong, Z. Zhang, Photocatalytic reduction of nitroarenes to azo compounds over N-doped TiO2: relationship between catalysts and chemical reactivity, Res. Chem. Intermed. 41 (2015) 3981 3997. [46] A. Maldotti, L. Andreotti, A. Molinari, S. Tollari, A. Penoni, S. Cenini, Photochemical and photocatalytic reduction of nitrobenzene in the presence of cyclohexene, J. Photochem. Photobiol., A 133 (2000) 129 133. [47] W. Wu, L. Wen, L. Shen, R. Liang, R. Yuan, L. Wu, A new insight into the photocatalytic reduction of 4-nitroaniline to p-phenylenediamine in the presence of alcohols, Appl. Catal. B: Environ. 130-131 (2013) 163 167. [48] L. Cermenati, M. Mella, A. Albini, Titanium dioxide photocatalysed alkylation of maleic acid derivatives, Tetrahedron 54 (1998) 2575 2582. [49] L. Cermenati, C. Richter, A. Albini, Solar light induced carbon carbon bond formation via TiO2 photocatalysis, Chem. Commun. (1998) 805 806. [50] D.W. Manley, R.T. McBurney, P. Miller, R.F. Howe, S. Rhydderch, J.C. Walton, Unconventional titania photocatalysis: direct deployment of carboxylic acids in alkylations and annulations, J. Am. Chem. Soc. 134 (2012) 13580 13583. [51] S. Marinkovic, N. Hoffmann, Semiconductors as sensitisers for the radical addition of tertiary amines to electron deficient alkenes, Int. J. Photoenergy 5 (2003) 175 182. [52] T. Caronna, C. Gambarotti, L. Palmisano, C. Punta, F. Recupero, Sunlight induced functionalisation of some heterocyclic bases in the presence of polycrystalline TiO2, Chem. Commun. (2003) 2350 2351. [53] T. Caronna, C. Gambarotti, A. Mele, M. Pierini, C. Punta, F. Recupero, A green approach to the amidation of heterocyclic bases: the use of sunlight and air, Res. Chem. Intermed. 33 (2007) 311 317. [54] T. Caronna, C. Gambarotti, L. Palmisano, C. Punta, F. Recupero, Sunlight-induced reactions of some heterocyclic bases with ethers in the presence of TiO2: a green route for the synthesis of heterocyclic aldehydes, J. Photochem. Photobiol. A 171 (2005) 237 242. [55] T. Caronna, C. Gambarotti, L. Palmisano, C. Punta, M. Pierini, F. Recupero, Sunlightinduced functionalisation reactions of heteroaromatic bases with aldehydes in the presence of TiO2: a hypothesis on the mechanism, J. Photochem. Photobiol. A 189 (2007) 322 328. [56] C. Gambarotti, C. Punta, F. Recupero, T. Caronna, L. Palmisano, TiO2 in organic photosynthesis: sunlight induced functionalization of heterocyclic bases, Curr. Org. Chem. 14 (2010) 1153 1169. 1153. [57] H. Yoshida, Y. Fujimura, H. Yuzawa, J. Kumagai, T. Yoshida, A heterogeneous palladium catalyst hybridised with a titanium dioxide photocatalyst for direct C C bond formation between an aromatic ring and acetonitrile, Chem. Commun. 49 (2013) 3793 3795. [58] M. Iizuka, M. Yoshida, Redox system for perfluoroalkylation of arenes and α-methylstyrene derivatives using titanium oxide as photocatalyst, J. Fluor. Chem. 130 (2009) 926 932. [59] M. Cherevatskaya, M. Neumann, S. Fueldner, C. Harlander, S. Kuemmel, S. Dankesreiter, et al., Visible-light-promoted stereoselective alkylation by combining heterogeneous photocatalysis with organocatalysis, Angew. Chem. Int. Ed. 51 (2012) 4062 4066. [60] D. Ma, S. Zhai, Y. Wang, A. Liu, C. Chen, Synthetic approaches for C-N bonds by TiO2 photocatalysis, Front. Chem. 7 (2019) 635.

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Catalytic applications of TiO2

21

Salvatore Scire`1, Roberto Fiorenza1, Marianna Bellardita2 and Leonardo Palmisano2 1 Department of Chemical Sciences, University of Catania, Catania, Italy, 2 Department of Engineering, University of Palermo, Palermo, Italy

21.1

Introduction

Titanium was discovered independently by the British chemist William Gregor in 1791, from whom the name gregorite, and by the German chemist M.H. Klaproth in 1793, who named it Titanium after the Titans of Greek mythology, “the incarnation of natural strength.” However, the element was not successfully isolated until 1910. Titanium dioxide (TiO2) was later discovered in 1821, but it was massproduced only since 1916 to be used as a white pigment. Titanium has an atomic weight of 47.87 amu, and it is the ninth most abundant element on earth (24% of the crust). Titanium occurs mainly in the minerals ilmenite, leucoxene, and rutile. Titanium has excellent engineering properties, having low density (60% lower than steel) but high mechanical resistance (as steels and twice that aluminum), exhibiting low corrosion in most mineral acids and chlorides. It is nontoxic and a suitable material for medical implants (see Chapter 10: Titanium Dioxide-based Nanomaterials: Application of Their Smart Properties in Biomedicine). Titanium dioxide, also known as titania, is the naturally occurring oxide of titanium; and anatase, rutile, and brookite are its three main crystalline forms [1
3]. It is the most used white pigment in the world in paints, enamels, textiles, fibers and plastics, sunscreens, and food. The superiority of TiO2 as a white pigment is due to its high refractive index and light-scattering ability. These properties give rise to a very good brightness in addition to covering power. It has been estimated that TiO2 is used in two-thirds of all pigments (see Chapter 11: TiO2 in Food Industry and Cosmetics). Another important use of titania is as catalyst. Catalysis has a key role in most of industrial chemical processes, providing enhancement in terms of activity, selectivity to desired products, and energy savings. Thermocatalysis and photocatalysis are different catalytic approaches that can be applied to both energy and environmental fields. Sometimes over TiO2-based systems, a synergetic effect of the two processes, in the so-called photo-thermocatalysis, has been observed, with highly positive advantages. Titanium Dioxide (TiO2) and Its Applications. DOI: https://doi.org/10.1016/B978-0-12-819960-2.00006-7 © 2021 Elsevier Inc. All rights reserved.

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Titanium Dioxide (TiO2) and Its Applications

TiO2 is a semiconductor of n-type, and it is well known for its high (photo) chemical stability and high ultraviolet absorption. The latter properties allow its use in various catalytic and photocatalytic applications. It is undoubtedly the most famous and used photocatalyst due to several advantages, namely availability, low price, nontoxicity, and resistance in alkaline and acid media. Its application as a classical thermocatalyst has been more limited, in so as TiO2 has generally a low surface area with reduced ability to adsorb reagents. Moreover, it is often hard to separate from reaction media. Notwithstanding these drawbacks, its active role as support for other oxides or metals in several catalytic reactions has been widely reported in the literature since decades. Several reactions of industrial relevance involve, in fact, titania-based catalysts, namely metal/TiO2 or metal oxide/ TiO2 composites, for hydrogenations (HYD), hydrodesulfurizations (HDS), selective oxidations or reductions, and Fischer
Tropsch processes. As an example, V2O5/TiO2 are among the best catalysts for the selective catalytic reduction (SCR) of NOx with ammonia, which is a key technology for NOx abatement in power plant exhausted gases. Being more acidic than alumina, TiO2 is more resistant to SOx and is not easily sulfated. This is also quite important in the Claus process, for H2S to S partial oxidation, and in the HDS reactions. Finally, sulfate-modified titania is considered as one of the few superacids known for heterogeneous Friedel
Crafts acylations.

21.2

Titania as catalytic support: role of the strong metal
support interaction

Metal oxides are used as support for catalysts or as catalysts themselves. The use of them as support improves the dispersion and the stabilization of the active species [4,5]. Moreover, some oxides (TiO2, V2O5, Nb2O5, Ta2O5) are able to undergo a strong interaction with metal nanoparticles (NPs), the so-called strong metal
support interaction (SMSI), resulting in special properties and modified catalytic activity of the resulting system [6]. Indeed, the control of the interaction degree between the support and the active metal requires pre- and posttreatments to achieve the desired loading, size, electronic and bulk structure of the metal. The choice of the treatment conditions is crucial to attain optimal catalytic performance. Since first reports of Tauster et al. on the SMSI effect [7,8], there is a debate on which is the predominant cause of this phenomenon, namely electronic or geometric effects [4,6,9,10]. SMSI has been often invoked to explain and control the structural properties and, hence, the performance of metal/titania catalysts for reactions as ester hydrogenolysis, carbon monoxide oxidation, and transesterification processes. Generally, the incapsulation model has been proposed to explain the behavior, with electronic effects providing an important contribution. It is well documented that the decoration in metal
titania systems is due to partial reduction of the titania support, followed by surface diffusion of TiOx, species onto the metal crystallite surfaces [11].

Catalytic applications of TiO2

639

For metal-supported catalysts it is really a challenge to discriminate between effects of metal cluster size and cluster
support interactions on physicochemical properties and catalytic activity [12]. In this context, Au/TiO2 has been considered a model catalyst and often investigated by several characterization techniques and in situ activity measurements to provide insights into the effects of the metal and the support on adsorption, morphological, electronic, and catalytic properties [13]. In order to have SMSI, redox treatments at high temperatures by molecular oxygen or hydrogen are required. For instance, Tang et al. [14] invoked the classical SMSIs between gold NPs and titanium dioxide in Au/TiO2. This hypothesis was supported by the suppression of CO adsorption, the occurrence of electron transfer from TiO2 to Au, and the gold encapsulation by a TiOx overlayer after high-temperature reduction. The behavior appeared similar to that observed for platinum group metals over TiO2. In the SMSI state, Au/TiO2 exhibited improved stability toward CO oxidation. SMSI can be claimed also for Au supported over other reducible oxides (CeO2 and Fe3O4) and other group IB metals (Cu and Ag) over titania. One of the most important examples of application of the SMSI effect is the reaction of HYD with H2 of organic compounds over metals/titania systems [1]. In this case, in fact, the hydrogen molecule, activated on the noble metal, can spillover the titania support, which acts as a reservoir and can provide H1 to the organic reagent, thus enhancing the HYD activity of the system (Fig. 21.1). Other examples of the SMSI effect are discussed in the fourth paragraph, where the main reactions involving TiO2-based catalysts are examined. Interestingly, Zhang et al. [15] recently found that it is possible to achieve SMSI also without high-temperature treatments using a wet-chemistry methodology (wcSMSI). In this case, SMSI can be obtained by employing a redox interaction between the metal and the titanium precursors in aqueous solution. The occurrence of the SMSI was evidenced by electronic interaction between Au and TiO2 with a TiOx layer covering Au NPs, and resulted in lower CO adsorption on Au and

Figure 21.1 SMSI effect during reduction treatment by H2. The gray circles represent metallic nanoparticles, whereas the blue parts represent the TiO2 support [1]. SMSI, Strong metal
support interaction. Source: Reprinted from L.E. Oi, M.-Y. Choo, H.V. Lee, H.C. Ong, S.B.A. Hamida, J.C. Juan, Recent advances of titanium dioxide (TiO2) for green organic synthesis, RSC Adv. 6 (2016) 108741
108754 with permission, ©2016 Royal Society of Chemistry Publishing.

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Titanium Dioxide (TiO2) and Its Applications

improved O2 activation, which accelerated the oxidation of CO. Moreover, the oxide overlayer stabilized the Au NPs, avoiding sintering.

21.3

The role of defects on catalytic performances

The study of the impact of defects on the structural, optical, and electronic properties is an important task in so as defects are essential factors for defining the catalytic activity of an oxide material [16]. Therefore many efforts have been made to engineer defective structures and to understand the influence of defects on the catalytic performance of TiO2-based systems [17
23]. Oxygen vacancies can be formed by reduction processes. The surface and bulk reduction also achieves the formation of color centers, with a variety of TiO2 colors from white to yellow, gray, blue, deep purple, and black [24,25]. The as-obtained defective TiO2 NPs showed common features. The morphology is characterized by a crystallized lattice core surrounded by an amorphous shell [24], which allows the formation of the Ti
H and O
H bonds and contributes to the midgap states and the color change, commonly associated with the introduction of Ti31 ions. Anyway, depending on the synthesis of defective TiO2 nanomaterials, Ti31 ions are not easily observed experimentally [24]. The same consideration also occurs for the OH and Ti
H species. These intrinsic defects result in a n-type semiconductor with high conductivity, whereas when the proportion of O atom in TiO2 rises, the titanium dioxide may be identified as a defective p-type semiconductor taking holes as the main charge carriers. Although the oxygen vacancies occur naturally in materials, higher density of them can be generated by treatment under vacuum [26] or under reducing conditions [27], by using hydrogen plasmas [28], combustion [29], or solvothermal methods [30]. High temperatures (400 C
700 C), systems under vacuum, multistep operations, and long times are involved in these methods. Recently, to increase the number of defects of TiO2 and other materials, the “in liquid” laser irradiation technique has been proposed as an easy method [31
34]. Particularly for photocatalytic applications, the presence of defects has a positive effect under solar or visible light irradiation. Reduced TiO2 (TiO22x) containing Ti31 species or oxygen vacancy has been demonstrated, in fact, to exhibit visible light absorption [35]. Furthermore, theoretical calculations show that a high concentration of vacancies could induce a vacancy band of electronic states just below the conduction band [36]. Due to the fundamental importance of the TiO2 defects engineering, some studies focused on the reactions on the surface with the lowest thermodynamic energy. Rutile TiO2(110), in particular, is a good sample to investigate the mechanisms of adsorption, diffusion, dissociation, and product formation (Fig. 21.2) [37
39]. Under actual conditions, TiO2 rutile (110) usually contains oxygen vacancies [40], and molecular and dissociative adsorption can deeply modify the electronic properties of its surface. O2 and H2O are among the most important species to induce these effects [41,42], and consequently they play an important role in the catalytic and

Catalytic applications of TiO2

641

Figure 21.2 Photoinduced dissociation of O2 on rutile TiO2(110) [37]. Source: Reprinted from N.G. Petrik, G.A. Kimmel, Photoinduced Dissociation of O2 on Rutile TiO2(110), J. Phys. Chem. Lett. 1 (2010) 1758
1762 with permission, ©2011 American Chemical Society Publishing.

photocatalytic reactions. Furthermore, O2 and H2O can reduce the number of oxygen vacancy defects of TiO2 by dissociation, restoring the surface stoichiometry [43
45]. In the literature, there are different results concerning the temperature at which O2 dissociates. Some authors [46,47] used temperature-programmed desorption and electron loss spectroscopy characterization techniques. The results indicated that O2 did not dissociate on the reduced TiO2 surface at temperatures lower than 150K. Onda et al. [48] suggested that oxygen vacancies which were formed after electron irradiation can be removed by O2 adsorption at 100K. Wendt et al. [44] agreed also that O2 can dissociate at low temperatures (about 120K). Upon O2 dissociation at an oxygen vacancy, one O atom fills the vacancy, whereas the other one diffuses away. From the earlier considerations it is clear that some physical
chemical properties of TiO2, such as light absorption, surface adsorption, or charge carrier separation, can be tuned by the disordered arrangement of intrinsic atoms. Therefore oxygen vacancies and defect states play a key role in directing catalytic and photocatalytic properties of TiO2, then providing an innovative and attractive methodology to obtain titania-based materials able to be used for a variety of applications.

21.4

Main reactions involving titania-based catalyst

In the different fields of heterogeneous catalysis, TiO2-based catalysts have been used for many applications as supports or active catalysts. Hereinafter, the most important catalytic reactions involving TiO2-based catalysts are presented and discussed. Special attention is also devoted to the recently investigated combination of thermo- and photocatalysis, whose application is strongly expanding in the last few years.

21.4.1 NOx removal 21.4.1.1 Selective catalytic reduction of NOx V2O5
TiO2 composites were undoubtedly the most investigated catalysts in the SCR with ammonia of nitrogen oxides (N2O, NO, N2O3, NO2, N2O4, and N2O5,

642

Titanium Dioxide (TiO2) and Its Applications

usually grouped as NOx) [49]. The removal of NOx, formed during the combustion processes, has a great environmental importance due to their crucial role in the formation of smog and acid rains, ozone depletion, and evolution of greenhouse gases. The SCR occurs according to Eqs. (21.1) and (21.2): 4NO 1 4NH3 1 O2 ! 4N2 1 6H2 O

(21.1)

2NO2 1 4NH3 1 O2 ! 3N2 1 6H2 O

(21.2)

For these reactions, the bare TiO2 has shown only negligible catalytic activity even at relevant reaction conditions. Over V2O5
TiO2 systems, instead, it was verified the occurrence of an activation of the sites on the vanadia surface (support effect), which strongly enhanced the catalytic performance [50,51]. It must be pointed out that vanadia is active both in the reduction of NOx and in the unwanted oxidation of SO2 to SO3, which can lead to catalyst deactivation by the formation of ammonium sulfate deposits. Therefore the vanadia content of industrial SCR V2O5
TiO2 catalysts must be kept as low as possible. V2O5
WO3
TiO2 or V2O5
MoO3
TiO2 catalysts were then introduced. In these catalysts, the presence of WO3 or MoO3 suppresses the loss of activity reducing the vanadia content [52
54]. With the incorporation of WOx, some positive effects as increased thermal stability, lower activity for sulfur oxidation, and improved SCR activity were also attained [50,55]. These enhancements were related to a more significant (1) ammonia adsorption capacity, (2) number of strong Brønsted and Lewis acid sites, and (3) reducibility of vanadia sites [56,57]. Temperature Programmed Reduction (TPR), Raman, and activity measurements indicated that the modification of vanadia morphology with larger surface vanadia domains on TiO2 supports in the presence of WOx sites accounted for the improved performance [58,59]. However, Raman spectroscopy analysis was not sufficient to obtain thorough structural information because the blueshift of the V 5 O vibration was quite modest, indicating that surface vanadia sites could be oligomerized in the presence of WO3. The strong Raman band due to surface tungsten oxide sites, in fact, masked the weak band of the surface vanadia sites, and to draw a definitive conclusion was hard. A direct observation of the molecular structures of the surface vanadia sites on TiO2 in the presence of tungsten oxide sites by means of solid-state 51V NMR spectroscopy demonstrated the oligomerization of surface vanadia [60,61]. Kobayashi et al. [62] focused on alternative TiO2
SiO2 and V2O5/TiO2
SiO2 systems. The mixed oxide TiO2
SiO2, compared to bare TiO2, displayed a significantly stronger acidity, higher Brunauer
Emmett
Teller (BET) surface area, and lower crystallinity of anatase. These characteristics give rise to a good thermal stability and a higher vanadia dispersion on the TiO2
SiO2 support up to high loadings (15 wt.%) of V2O5. The SCR activity and N2 selectivity were found to be superior over vanadia loaded on TiO2
SiO2 with 10
20 mol% of SiO2 compared to the pure TiO2, and this was explained by considering the presence of highly dispersed vanadia on the supports and big amounts of adsorbed NH3. With increasing

Catalytic applications of TiO2

643

silica content, it was verified a decrease of activity in the oxidation of SO2 to SO3, and this is a positive feature for industrial SCR catalysts. These effects were strongly dependent on the existence of vanadium species with oxidation state close to V41 on TiO2
SiO2, whereas X-ray photoelectron spectroscopy (XPS) investigation indicated that V51 predominated on TiO2. In conclusion, vanadia on Ti-rich TiO2
SiO2 is a suitable SCR catalyst for sulfur-containing exhaust gases when the amount of SiO2 is low, and this catalyst not only has an excellent de-NOx activity but also has a low performance for SO2 oxidation. In recent years, the focus on the SCR reaction was addressed to the decrease in the reaction temperatures. The 100 C
200 C range has been mainly investigated because it reflected a high stability of catalysts associated with an easier control of NOx [63]. Under these conditions, TiO2-supported manganese oxides exhibited superior catalytic performance, including high NOx conversion and resistance to SO2 poisoning [64,65]. Attention has been paid to look into the role of MnOx loading and dispersion on the properties of MnOx/TiO2 catalysts. Qi and Yang [66] studied the manganese oxide supported on TiO2 (Degussa P25) and found that NO conversion increased until 10 at.% of Mn loading, whereas after this level, a detrimental effect was observed. Ettireddy et al. [51] investigated the structure and the catalytic properties of various loadings of manganese on diverse phases of TiO2 with different surface areas, that is, Hombikat anatase (239 m2/g), Kemira rutile (54 m2/g), and Degussa P25 (52 m2/g, 80% anatase, 20% rutile). Their data showed that the NO conversion and N2 selectivity increased up to approximately the monolayer (16.7 at.%) of manganese on the Hombikat anatase, and then decreased. The same Hombikat anatase catalyst with supported MnOx was found very active and selective for SCR reaction, due to the high specific surface area and the strong interaction between MnOx and the support. It was established that the high dispersion of isolated or polymeric manganese oxide species plays a significant role. Zhuang et al. [63] compared the structure and the catalytic properties of anatase and rutile supported MnOx catalysts synthetized by impregnation. For the TiO2 rutile supported MnOx catalysts, the major species of manganese oxide was found to be Mn41, whereas when anatase was used as support, in addition to Mn41 also Mn31 was found as a minor species. Small amounts of undecomposed Mn-nitrate were present in both types of catalysts. Under the SCR reaction conditions, Mn31 species on anatase were oxidized to Mn41 and induced the anatase
rutile phase transformation. Being these Mn41 cations dispersed on the support which has a rutile shell
anatase core structure and a concentration close to that of the MnOx/TiO2 rutile catalyst, the relation between rate constant and MnOx loading was similar for anatase and rutile supports.

21.4.1.2 Catalytic oxidation of NOx The NOx can be also removed by catalytic oxidation [67]. In recent years, the catalytic ozonation technology, involving OH radicals, has been applied to lowtemperature denitrification. TiO2 has been employed as a catalyst for the catalytic ozonation (O3/NO molar ratio . 1.5) [68
70], because its surface is rich of surface hydroxyls, although their density strongly depends on the preparation methods of G

644

Titanium Dioxide (TiO2) and Its Applications

the oxide. The catalytic oxidation of NO on TiO2 was also investigated by Jogi et al. [71], who report as the presence of TiO2 strongly enhanced the efficiency of oxidation of NO to NO2 and N2O5 at 100 C in the presence of O3. The role of the TiO2 catalyst was related with the catalytic destruction of O3 producing surfacebound oxygen atoms that are accountable for the improved oxidation of NO2. As discussed earlier, reduced TiO2 attracted the attention of researchers in different research fields because of the presence of surface defects [20,31]. Han et al. [72] found a higher NO conversion at 60 C over black reduced TiO2 with respect to the unreduced one. Over black TiO2 the deep oxidation of NO to HNO3 was favored, together with a higher sulfur resistance. Wei et al. [73] and Wang et al. [74] reported that surface hydroxyl groups increased by reduction of TiO2. Moreover, hydroxyl groups can be formed by the interaction of H2O with the TiO2 anatase (101) surface and subsurface oxygen vacancies [75]. Therefore reduced TiO2 can be considered a promising system to promote the catalytic ozonation and a valuable alternative to the classical catalysts used for the NO oxidation.

21.4.2 Deacon process Chlorine is a key chemical element in the obtainment of numerous products for daily living, either directly as component or, more widely, as intermediates. The Cl2 production was often used as an indicator of the development status of the chemical industry of a country [76]. Most of the annual chlorine production worldwide (B50 Mton, more than 95%) is attained by electrolysis in mercury, diaphragm, or membrane cells (chlor-alkali process). After using, 50% of the Cl2 can be transformed into HCl or chloride salts [77], but HCl is a noxious compound and has a small market. There has been, consequently, a great interest in efficient and green methods for reconverting HCl present in waste streams into Cl2. A reversible and exothermic useful reaction for recycling chlorine is the gas-phase catalytic oxidation of HCl with air or oxygen:
4HCl 1 O2 #2Cl2 1 2H2 O ΔH0 5 2 28:4 kJ mol21 HCl : Henry Deacon industrialized this process, which is one of the first known catalytic processes, more than 140 years ago by using copper chloride as catalyst [78,79]. Compared to electrolytic processes, the previous reaction requires a lower electrical and thermal power. More recently, the Sumitomo Chemical developed an effective reaction route by using RuO2 supported on rutile TiO2 as the catalyst [80,81]. The application of Sumitomo process can be considered as a real breakthrough to recover Cl2 from HCl and an important stage toward sustainable and green chemistry. After that, a growing interest to find new heterogeneous catalysts for HCl oxidation was stimulated. For instance, Bayer industrialized for this process Ru-based catalysts on SnO2 cassiterite [82], which presents a common structure with rutile TiO2. This type of structure allowed the epitaxial growth of RuO2 on the support and appeared to favor high Cl2 yields [83].

Catalytic applications of TiO2

645

The corrosion resistance of the catalyst is one of the most important properties in the Deacon process. The very high and variable price of Ru imposes to search active catalysts with a low Ru content, but stable under the drastic conditions of the reaction [84]. In order to get more insight on the Sumitomo/Deacon process on TiO2-supported RuO2 catalysts, Seitsonen and Over [85] carried out density functional theory (DFT) calculations. The recombination of two adjacent chlorine atoms on the catalyst surface is the rate-determining step of the reaction that proceeded via a onedimensional Langmuir
Hinshelwood mechanism. Oxygen activation is the true rate-determining step in the HCl oxidation on TiO2(110), because the dissociative adsorption of O2 is highly endothermic (.200 kJ/mol). Furthermore, the calculations indicated that the presence of small quantities of Ru in the outermost surface induces the activation of oxygen on TiO2, supporting the suitability of TiO2 as an active support for the preparation of efficient Deacon catalysts. Lopez et al. [86] also used RuO2 supported on rutile TiO2. The characterization of the catalyst by X-ray diffraction (XRD) and XPS showed that after the reaction the bulk structure was not significantly modified and only a limited chlorination occurred. Ab initio thermodynamics calculations suggested that the initial state of the RuO2(110) surface was partially overoxidized, and both oxygen and chlorine were present on the catalyst surface after the reaction. The Deacon reaction was described by DFT with a five steps Mars
van Krevelen-type mechanism consisting of (1) hydrogen abstraction from HCl, (2) recombination of atomic chlorine, (3) hydroxyl recombination, (4) water desorption, and (5) dissociative oxygen adsorption. Cl2 formation increased by rising the O2 to HCl ratio of the feed. The authors ascribed these findings to the lower recombination energy of chlorine atoms into Cl2 at high coatings and to the faster surface reoxidation due to higher partial pressures of O2. Interestingly, Hevia et al. [87] investigated the Deacon process comparing RuO2 supported on different phases of TiO2 (anatase, rutile and P25 which consists of both phases, that is, about 80% anatase and 20% rutile). As expected, RuO2/TiO2rutile showed the highest activity at ambient pressure in a continuous flow fixedbed reactor. The authors focused on the reaction mechanism by carrying out pump
probe experiments between O2 and HCl (or in the reverse order), and they concluded that the reaction took place mainly in the presence of adsorbed species. Cl2 production, in fact, was highly dependent on both the oxygen and chlorine coverage. One of the advantages of the use of TiO2 as support for the Deacon process is the possibility to exploit also its photoactivity. Rath et al. [81] investigated the Deacon process through a photo-thermocatalytic route and determined the steadystate Cl2 production rates by using a fixed-bed gas-phase reactor equipped with UV light-emitting diodes. They found stable Cl2 production rates for commercial anatase TiO2 (Hombikat UV100). Notably, in the photothermal test the reaction rate increased linearly with temperature from 21 C to 140 C, due to the enhanced desorption rate of water that was produced contemporary to Cl2 from oxygen reduction (Fig. 21.3). A comparison of different TiO2 crystalline structures showed that the photoactivity of anatase was higher than that of rutile. Finally, IR measurements

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Titanium Dioxide (TiO2) and Its Applications

Figure 21.3 Photocatalytic Cl2 production over TiO2.

indicated that the chemisorbed HCl chlorinates the surface of TiO2, and this was suggested as the first step of the photo-assisted reaction mechanism. The photocatalytic process allows to operate at lower temperature than that reached in the catalytic Deacon process.

21.4.3 Reactions with sulfur-rich compounds TiO2-based catalysts were also widely used for industrial processes that involve sulfur-containing compounds, as in HDS and Claus processes, exploiting the high resistance of titania under sulfur-rich atmospheres [88
90].

21.4.3.1 Hydrodesulfurization processes Due to more stringent regulations, the reduction in the maximum allowable sulfur content in fuels is a mandatory purpose for oil industries. HDS is the main route to achieve this requirement. Nowadays, the most used catalysts in HDS consist of supported MoS2 on alumina promoted by cobalt (or nickel) [91,92]. Better systems using different supports are required for improving the HDS performance [93]. In addition to alumina, ZrO2 [94], SiO2 [95], and TiO2 [96] are other supports for HDS catalysts. Titania supported systems, in particular, exhibited high activities in HDS but also some disadvantages, chiefly the low surface area that hindered their industrial application. Palcheva et al. [97] overcome this drawback by synthetizing high surface area TiO2 nanotubes through an alkali hydrothermal method. Nanotube TiO2-supported catalysts were employed for NiW (Ni3/2PW12O40) HDS catalysts, pointing to a more significant thiophene conversion (about doubled). This HDS activity was also related to the higher amount of W oxysulfide entities interacting with Ni sulfide particles, due to some electronic properties of the TiO2 nanotubes, as confirmed by XPS analysis. The use of supported precious metals is another strategy for the deep HDS. In fact, starting with a feed containing low sulfur amounts (200 ppm or less) supported precious metals exhibited higher HYD activities than metal sulfides [98]. Nevertheless, noble metals in the presence of sulfur-containing molecules could

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transform into metal sulfides, which are less active. Among the noble metals, platinum supported on acidic materials was found to be the most resistant one to sulfur poisoning [99], because the acid support generates electron-deficient metallic particles that are resistant to sulfur. However, acidic supports favor the formation of cracking products and low cetane numbers for diesel. In this contest, the combination of titania with alumina in mixed oxides has been investigated as support for Ptor Pd-based catalysts [100,101]. A Pd
Pt catalyst supported on alumina
titania with an equimolar composition was the most active for HDS. Moreover, Nun˜ez et al. [102] evaluated the HYD of biphenyl in the presence of sulfur by using Al2O3
TiO2-supported Pd
Pt catalysts, changing the Ti content in the carrier. They assumed that the presence of titania increased the HYD activity as SMSI effects change the electronic properties of the active phase. Moreover, the presence of titania improved the sulfur resistance of the metallic particles throughout the HYD process. Aguirre-Gutie´rrez et al. [100] showed that the addition of Pd and Ni to the molybdenum/alumina
titania (NiMo/AT) support produced highly active sulfided catalysts for HDS of the refractory 4,6-dimethyldibenzothiophene (4,6DMDBT) by enhancing the HYD and HDS routes. It was observed a synergistic effect of Pd and Ni on the Mo/AT catalyst, higher when both metals were incorporated into Mo/AT catalyst. The presence of Pd favored the HDS activity for the NiMo system, whereas the alumina
titania support played an important role for incorporating Pd into the active phases. Following this perspective, Saih and Segawa [103] focused on the ultradeep HDS of dibenzothiophene derivatives over Mo/TiO2 and Mo/TiO2
Al2O3 catalysts. They stated as the Mo species are uniformly spread out on the surface of titania up to 6.6 wt.% MoO3 loading. Above this value, some aggregation of Mo occurs, leading to the formation of bulk MoO3. Below 6.6 wt.% MoO3 loading, Raman spectroscopy data showed that the supported Mo species possess a highly distorted octahedral MoO6 structure. In the sulfidation conditions used (673K, 5% H2S/95% H2), Mo species supported on TiO2 are better sulfided than on alumina, as demonstrated using XPS. This finding can be explained by considering the lower interaction between titania and Mo species. However, at relatively higher TiO2 loadings (11 wt.%), Mo/TiO2
Al2O3 exhibited sulfidability similar to that of Mo/TiO2. The authors performed HDS tests, confirming as the sulfide catalysts supported on TiO2
Al2O3 (11 wt.% TiO2) were more active than those supported on the single oxides.

21.4.3.2 Claus process The Claus process is one of the most known industrial inorganic processes, aiming to the decrease of the emission of SOx and to the transformation of H2S into sulfur (21.3). During the process, CS2 and COS are formed if hydrocarbon impurities enter the system. Therefore catalysts active also for CS2 and COS hydrolysis are necessary. 2H2 S 1 SO2 ! 3=8S8 1 2H2 O

(21.3)

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Titanium Dioxide (TiO2) and Its Applications

Both alumina and titania catalysts were employed for the previous reactions. However, titania appeared more effective than alumina [104,105]. Therefore TiO2based composites were the most investigated materials for the Claus process reactions. Chun et al. [106] investigated the selective catalytic oxidation (SCO) of H2S to elemental sulfur in the presence of TiO2/SiO2 catalysts. Results pointed to a good resistance of TiO2 to sulfation and sulfidation, which are known to be primarily responsible for the catalysts deactivation in this type of processes. Interestingly, at low reaction temperatures the catalyst deactivation, due to the sulfur deposit on the surface, caused a drop in H2S conversion, without affecting the selectivity to sulfur. The decrease of the TiO2/SiO2 acidity by K2O doping leads to a strong reduction of the H2S conversion, whereas the addition of B2O3 was uninfluential. The authors concluded that H2S selectively oxidized on acidic sites, whereas the reverse Claus reaction occurred on basic sites. An innovative approach was reported by Clark et al. [107]. They demonstrated that Ti31 cations on TiO2 catalysts substantially improved the CS2 conversion activity (21.4), and this finding was explained by considering a dual-center adsorption model at adjacent Ti31
Ti41O groups for CS2 over titania. CS2 1 2H2 O ! 2H2 S 1 CO2

(21.4)

The Ti31 sites were formed in the catalyst, in situ during the Claus reactions, being the H2S a reducing agent for Ti41. A macroscopic evidence of the presence of reduced sites was the color change of TiO2 as shown in Fig. 21.4. Surface Ti31

Figure 21.4 Color changes of TiO2 under the Claus reaction conditions [107]. Source: P.D. Clark, N.I. Dowling, M. Huang, Role of Ti31 in CS2 conversion over TiO2 Claus catalyst, Appl. Catal. A 489 (2015) 111
116 with permission, ©2015 Elsevier Publishing.

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and oxygen bound to Ti41 constituted the Lewis acid
base centers, which promoted the adsorption
activation process of CS2. Titania is more effective than alumina for the conversion of both CS2 and COS under Claus process conditions at 320 C [104]. Very recently, Kazemi et al. [108] tested the Au NPs supported on Ln-modified TiO2 on Claus catalytic combustion process of the tail gas, with the aim to carry out the oxidation of CO, H2, and H2S at lower temperature. The addition of lanthanum increased the thermal stability of the catalyst. Furthermore, they found that the addition of lanthanides with multiple oxidation states (e.g., Eu21 and Eu31) improved the activity of Au/TiO2, whereas lanthanides with only one oxidation state (e.g., La31) had the reverse effect. Additionally, when sulfur species were present in the Claus tail gas, the poison of the catalysts occurred at low temperatures, whereas a higher reaction temperature reduced the sulfate poisoning. Ultimately, the increase of the Au amount reduced the amount of sulfate, because the electron-rich Au donated electrons to TiO2, thus favoring the decomposition of sulfate into SO2. The use of lanthanides as doping agents of TiO2 was also studied by Sui et al. [109] who used nanofibers of La(III)-doped TiO2 as a novel Claus catalyst. A nanofibrous xerogel was prepared with a one-pot sol 2 gel process, by incorporating La (III) into titanium-oxo-acetate complexes, followed by calcination. The nanofibers formed a connected network as they joined together, so the xerogel showed a great macroporosity. These La(III) doped TiO2 samples exhibited a higher activity than La(III)-TiO2 xerogels prepared by impregnation, pointing out that the catalytic activity is positively influenced by the dispersion of the lanthanum species on TiO2.

21.4.4 Direct synthesis of hydrogen peroxide Hydrogen peroxide (H2O2) is a compound with a high oxidation potential and shows a low selectivity attacking the most varied types of species. Consequently, it can be successfully used for the treatment of wastewater polluted with organic compounds [110,111]. H2O2 is an environmentally friendly chemical as it does not decompose into noxious species, but only into oxygen and water [112]. Today, H2O2 is produced by HYD and subsequent oxidation of an alkyl anthraquinone [113]. Nevertheless, this route has relevant matters associated, as the cost of the solvent and the necessity to replace anthraquinone due to HYD. Furthermore, the process is economically feasible only on a large scale, whereas the in situ production of H2O2 for direct use in the industries could reduce the shipping costs. The reaction between H2 and O2 was firstly reported in 1914 on a Pd catalyst [114]. Afterward, some additional studies have been pioneered in industrial laboratories. Initially, H2
O2 mixtures in the explosive region were investigated, but recent studies have been addressed on the use of dilute H2
O2 mixtures, far from the explosive regime [115]. It was described that the addition of acid and bromide was beneficial for the H2O2 yields, and solutions of more than 35 wt.% H2O2 were prepared by reacting H2
O2 over Pd catalysts at high pressures [116]. The enhancement of the catalyst

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Titanium Dioxide (TiO2) and Its Applications

efficiency was achieved by tuning the Pd ensembles changing the properties of the support or by forming Pd-based alloys. In this contest, titanium dioxide was used as valuable support. Edwards et al. [117] investigated the synthesis of H2O2 from H2 and O2 in the presence of TiO2 supported Au
Pd systems. The bimetallic Au
Pd catalysts gave significantly higher activity compared to the monometallic Pd/TiO2 and Au/TiO2 ones. Au
Pd particles were characterized by a core
shell structure, with Pd mainly on the surface. The highest yields of H2O2 were obtained with uncalcined catalysts, but these were not very stable and lost both metals during their use. On the contrary, samples which were calcined at 400 C showed no loss of activity after being used multiple times. These catalysts displayed a low activity for CO oxidation at 25 C, whereas those effective for low-temperature CO oxidation were virtually inactive for the obtainment of H2O2 from H2 oxidation. The authors concluded that a synergistic effect occurred when Pd was added to Au for the TiO2-supported catalysts. Notably, the sites possessing high activity and high selectivity for H2 oxidation were different from those in the Au/TiO2 catalysts. The same research group [118] claimed the role of supports and promoters on bimetallic Au
Pd catalysts. In particular, they established that the reactivity for the support materials investigated was in the following order: carbon . silica . TiO2 . Al2O3. Transmission electron microscopy (TEM) studies revealed that impurities of carbon are present in the silica and the Au
Pd alloys preferably interact with them, and therefore the catalytic performance resulted nearly equal to that of the carbon-supported Au
Pd catalysts. The increased activity for the silica and carbon supported catalysts gave rise to a higher H2 selectivity toward H2O2 formation, and it was explained by considering the surface composition and the size distribution of the NPs. The influence of some promoters on the rate of H2O2 synthesis was also studied, and it was reported that the addition of Br2 and PO432 ions was detrimental, whereas an acidic CO2 aqueous solution was beneficial. Finally, the H2O2 direct synthesis from H2 and O2 over Au
Pd/TiO2 catalysts was described in a further work [119]. The authors focused their study both on the variation of the process efficiency and HYDdecomposition rates when the reaction temperature approached the room temperature and water was chosen as the solvent. These mild and environmentally friendly experimental conditions allow us to save energy for heating or cooling and avoid further extraction/purification of the formed H2O2. Unfortunately, the change of the experimental conditions for the synthesis of H2O2 employing room temperature and an aqueous system instead of methanol 2 water at 2 C caused a marked decrease in H2O2 yield. The authors correlated this behavior to the increased H2O2 degradation and to the decreased solubility of H2. The reduced H2 solubility, in fact, strongly hinted the HYD of O2. Ouyang et al. [120] studied the direct synthesis of H2O2 using Pd/TiO2 to understand the origin of the active sites. They demonstrated that some of surface Pd atoms are oxidized by the adsorbed oxygen with the help of TiO2 already at 283K, forming Pd
PdO ensembles. The electronic structure of the surface Pd atoms changed dynamically under different pressures. The authors assumed that H2O2 synthesis occurred at the interfaces of the Pd and PdO domains (Fig. 21.5).

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Figure 21.5 The reaction mechanism of H2O2 synthesis on Pd/TiO2 catalyst [120]. The symbol O indicates the favoured reactions, the symbol x the inhibited one. Source: L. Ouyang, P. Tian, G. Da, X.C. Xu, C. Ao, T. Chen, R. Si, J. Xu, Y.-F. Han, The origin of active sites for direct synthesis of H2O2 on Pd/TiO2 catalysts: Interfaces of Pd and PdO domains, J. Catal. 321 (2015) 70
80 with permission, ©2015 Elsevier Publishing.

The electrochemical synthesis can be another possibility for the in situ production of H2O2 via the oxygen reduction reaction [121
123]. Dos Reis et al. [123] focused in particular on carbon-supported TiO2
Au hybrid materials. Interestingly, they found that the activity significantly depended on the shape of TiO2 particles and TiO2
Au loading. The use of TiO2 colloidal spheres led to higher activities than TiO2 wires. The optimal results were obtained with TiO2
Au(3 wt.%), which exhibited better performances than commercial Pt/C catalyst.

21.4.5 Fischer
Tropsch synthesis The Fischer
Tropsch synthesis (FTS) allows to turn natural gas, coal, and biomass into chemicals and fuels via syngas, with the formation of olefins and paraffins of different sizes. Cobalt-based catalysts were largely employed in FTS due to their high activity and selectivity toward the formation of long-chain paraffins and only a slight activity for water
gas shift (WGS) [124]. Different supports were investigated for Co catalysts, namely SiO2, Al2O3, and TiO2. It was reported that different forms of silica, TiO2, and Al2O3 have a strong interaction with the cobalt precursor leading to a significant dispersion of Co and limited reducibility [125]. Hinchiranan et al. [126] studied the influence of the addition of small amounts of TiO2 to silicasupported cobalt catalysts. They found that the presence of titania improved the catalytic activity of Co/SiO2 by favoring the interaction between silica and cobalt, and by increasing the tendency of the cobalt supported on SiO2 to be dispersed and reduced. The high activity of titania-based catalysts was explained by considering the presence of a higher number of bridged-type adsorbed CO species, which were

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Titanium Dioxide (TiO2) and Its Applications

easily dissociated into oxygen and carbon. Interestingly, the impregnation method used for obtaining the Co/TiO2 catalysts did not significantly influence properties and performance. In the same contest, Feyzi et al. [127] prepared by sol
gel a series of Co 2 Fe catalysts supported on TiO2
SiO2. They investigated the effect on the catalytic performance of different factors as (Co 2 Fe) wt.%, pH, Co/Fe molar ratios, calcination conditions, and the presence of promoters. The results showed that 35 wt.% (Co
Fe)/TiO2
SiO2 promoted with 1.5 wt.% of Cu was the optimal catalyst. Vannice and Garten [128] found that the Ni/TiO2 system is a valuable candidate for the FTS reaction, providing higher molecular weight paraffin hydrocarbons compared to other Ni/metal oxides catalysts (Ni/Al2O3, Ni/C, Ni/ SiO2). Moreover, Ni/TiO2 was also sensibly more stable than the corresponding Ni/ Al2O3 system. Storsæter et al. [129] focused on Co and Co
Re supported on γ-Al2O3, α-Al2O3, SiO2, and TiO2 systems, prepared by incipient wetness impregnation. They found that the size and shape of the cobalt particles strongly depended on the pore size of the support. In particular, Co3O4 was present as clusters of small particles on the small pore of γ-Al2O3 and SiO2, and as single particles on the bigger pore of TiO2 and α-Al2O3. Thus authors hypothesized that the size of Co3O4 agglomerates increases only when the pore size increases up to a certain dimension. Rhenium was supported in addition to cobalt over the substrate surface, but at high concentrations it occupied the cobalt-containing positions. Furthermore, a relationship was found between the C51 selectivity and the amount of external water for the three supports, the selectivity increasing on raising conversion and amount of water. The catalysts that were most influenced by the presence of water were the TiO2-supported ones, whereas those that were the least influenced were the γ-Al2O3-supported ones. By comparing the C51 selectivities for the unpromoted catalysts at dry conditions, a slight difference among the different supports was discovered. Nevertheless, the silica- and titania-supported catalysts showed significantly higher C51 selectivity than the γ-alumina ones when Rhenium was used as a promoter. This finding was ascribed to a better contact between Re and Co on the silica and titania systems than on γ-alumina. Morales et al. [130] studied by diffuse reflectance infrared spectroscopy (DRIFTS) the adsorption of CO and H2 chosen as the probe molecules on Mnpromoted Co/TiO2 FTS catalysts. Manganese turned out to be closely related to the FTS Co active surface sites. CO bound linearly to the surface metal sites by increasing the MnOx loading. Manganese also reduced the extent of Co
TiO2 interactions, improving the Co dispersion and the H2 chemisorption. Moreover, FTS tests (1 bar and 220 C) at high MnOx loadings showed an increase in C51 and olefins selectivities. It was concluded that MnOx species induced both electronic and structural effects in the catalysts, which can account for a higher metal dispersion and lower HYD activity, and definitely for a higher FTS performance. On the other side, Abrokwah et al. [131] investigated the role of the titania support on FTS using cobalt, iron, and ruthenium (12 wt.%) catalysts in a siliconmicrochannel microreactor operating at atmospheric pressure and various temperatures. (Fig. 21.6). The sol
gel catalyst layer was evenly coated in the

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Figure 21.6 Schematic representation of the Fischer
Tropsch synthesis performed in silicon-microchannel microreactor [131]. Source: R.Y. Abrokwah, M.M. Rahman, V.G. Deshmane, D. Kuila, Effect of titania support on Fischer-Tropsch synthesis using cobalt, iron, and ruthenium catalysts in silicon-microchannel microreactor, Mol. Catal. 478 (2019) 110566 with permission, ©2019 Elsevier Publishing.

microchannels of Si-microreactors by means of a modified closed channel infiltration method. H2-TPR profiles pointed to different SMSI of each metal with TiO2, stronger with Fe
TiO2 and Co
TiO2 than with Ru
TiO2. The XRD studies suggested that Ru
TiO2 consisted of mixed anatase and rutile phases. The absence of rutile phase in Co
TiO2 and Fe
TiO2 significantly influenced not only the FTS activity in the 150 C
300 C range for CO conversion and hydrocarbon selectivity but also the stability of the catalysts which decreased. The overall reactivity and stability were in the order: Ru
TiO2 . . Fe
TiO2 . Co
TiO2.

21.4.6 Water
gas shift reaction The WGS reaction, CO 1 H2 O#H2 1 CO2 , is a fundamental reaction for providing hydrogen [132]. The most used industrial catalysts for the WGS (mixtures of Fe
Cr or Zn
Al
Cu oxides) are pyrophoric and long and tricky activation steps are necessary. In the last years, many works have focused on Au NPs supported on oxides such as CeO2 and TiO2, which resulted efficient catalysts [133,134]. It has been reported that noble metals supported on TiO2 are promising low-temperature WGS catalysts. They, in fact, present a high redox capability and induce the formation of defects (oxygen vacancies) through reduction of titanium cations with CO [1,135]. The oxygen vacancies were able to dissociate H2O producing H2, to regenerate surface oxygen sites, and create hydroxyl groups [136]. The surface properties of TiO2 can be tuneable by coupling with other transition metals or rare earth oxides. Panagiotopoulou and Kondarides [137] found that CO conversion at temperature ,300 C increased significantly when Pt was dispersed on TiO2. The turnover frequency (TOF) of CO and the apparent activation energy exponentially grew and considerably dropped, respectively, when the primary crystallite size of TiO2 decreased. In the same line, other noble metals as rhodium, ruthenium, or palladium supported on TiO2 were highly performing for the WGS reaction [135,136]. Among noble metals supported on TiO2, the CO conversion and

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Titanium Dioxide (TiO2) and Its Applications

TOF for the WGS reaction have been found to follow the order Pt . Rh . Ru . Pd [1]. The catalytic activity depends on the different crystalline forms of TiO2 also for the WGS reaction. Lida and Igarashi [138] observed that the Pt/TiO2-rutile catalyst has a relatively high activity at low temperature. The superior catalytic activity was attributed to the SMSI between Pt and TiO2. Moreover, the dispersion of Pt is larger on Pt/rutile than on Pt/anatase samples. The authors explained the better dispersion with a high degree of lattice matching, thanks to which a better Pt
TiO2 interaction occurred, and the agglomeration of Pt particles during the calcination avoided. Park et al. [139] studied the catalytic performance of gold, copper, and platinum NPs dispersed on CeOx/TiO2(110) surfaces for the WGS reaction. Small coatings of ceria on TiO2(110) led to unusual coordination mode, which favors the formation of Ce31 species. At high ceria coverage on TiO2(110), the surface exhibited two types of morphology. The first one was similar to that observed at low ceria coverage with diagonal arrays of ceria dimers. The second type consisted of a compact array of ceria particles with structures different from those of CeO2(111) or CeO2(110). Gold, copper, and platinum exhibited a different dispersion on the CeOx/TiO2(110) surfaces after annealing. The average height and diameter of the metal particles were found to increase in the following order: Pt , Cu , Au. Regarding the catalytic activity, the M/CeOx/TiO2(110) surfaces present a very high WGS activity, with the order Au/CeOx/TiO2(110) , Cu/CeOx/TiO2(110) , Pt/ CeOx/TiO2(110). In the M/CeOx/TiO2(110) systems, there was a strong coupling of the chemical properties of the added metal and the mixed-metal oxide. The authors concluded that the final reaction steps occurred at the oxide
noble metal interface after the adsorption and dissociation of water on the oxide and CO on the metal NPs. Gold, ceria, and titania were also investigated by Rodriguez et al. [140], who found that pure Au(111) was not catalytically active for the WGS, whereas gold surfaces covered by 20%
30% of ceria or titania NPs had comparable activity to Cu(111) or Cu(100). In TiO22x/Au(111) and CeO22x/Au(111) water dissociated on the O vacancies of the oxide, CO adsorbed on near Au sites, and the further reaction steps occurred at the metal
oxide interface. Finally, in a recent work a photothermal route, under the combined use of light and heat, was investigated by Caudillo-Flores et al. [141], employing ruthenium
anatase solids with noble metal loadings from 1 to 10 wt.%, and testing the H2 production by photoreforming of methanol driven by WGS reaction. They characterized the samples that showed a core
shell structure having a hexagonal closepacked metallic Ru core and a RuO2-type shell structure. The best thermo-photo performance was obtained with a 5 wt.% Ru/TiO2 catalyst. AA pointed out the fundamental role of the ruthenium
anatase interface in the activation of carboncontaining species that were subsequently transformed by means of a WGS reaction. Activity was directly related to the promotion of this step by light and heat with a mechanism that provides for methanol oxidation on titania sites and the subsequent evolution of the formed species at the interface to generate H2 and CO molecules.

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21.4.7 CO2 methanation One of the most severe issues of our days is the rising level of CO2 in the atmosphere. The transformation of CO2 into chemicals is considered as a sustainable method to mitigate this problem. This route usually involves high temperatures to speed the CO2 transformation to intermediates, and then requires extra energy input [142,143]. Consequently, the use of solar energy represents an intriguing approach. Several papers report the reduction of CO2 to chemicals by water using solar energy. CO2 intermediate transformations involving the participation of protons have been described [144,145]. To increase the photocatalytic yields, the photothermal approach was recently applied to this reaction. In particular, Wang et al. [146] examined a photothermal-coupled device designed to obtain a high-achievement CO2 methanation by H2O over carbon dots (CDs) drafted Cu/TiO2(Cu/TiO2-C). The catalysts showed a very high promotion of CH4 production under UV irradiation at temperature . 150 C, but a poor photo-promotion at room temperature. In situ DRIFTS and isotopic-label temperature-programmed surface reaction suggested that the CO2 methanation process over Cu/TiO2-C consisted of two main steps: CO2 was firstly reduced by Cu(I) (Cu2O) to CO (CO2 1 Cu2O ! CO 1 2CuO), and then CO was reduced by H2O by the WGS process (Fig. 21.7). The role of CDs is to act as electron storage to maintain Cu(I) (from Cu(II) to Cu(I)) avoiding the formation of Cu(0) at high temperature and to trap the holes that are photo-produced by the UV irradiated TiO2. The synergic

Figure 21.7 The possible mechanism for CO2 reduction by H2O over Cu/TiO2-C under irradiation. There are two main processes: one is the CO2 reduction by Cu2O above 150 C (lower part), and the other one is the in situ regeneration of Cu2O from CuO under UV irradiation (upper part) [146]. Source: K. Wang, R. Jiang, T. Peng, X. Chen, W. Dai, X. Fu, Modeling the effect of Cu doped TiO2 with carbon dots on CO2 methanation by H2O in a photo-thermal system, Appl. Catal. B 256 (2019) 117780 with permission, ©2019 Elsevier Publishing.

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effect of temperature and UV light did not occur on the Cu/TiO2 sample in the absence of CDs. In the latter case, Cu(II) was mostly reduced to Cu(0) by H2 pretreatment while a scarce cycle Cu(I)/Cu(II) was displayed under UV irradiation. In the same contest, Li et al. [147] prepared Rh/TiO2 catalyst for the photothermal reduction of carbon dioxide. In particular, they claimed the synergic effect between photo- and thermo-mechanisms in plasmon-driven catalysts. They showed as a new indirect illumination technique, jointly with the thermal profiles of the catalyst, can explain the role of nonthermal effects in plasmon-enhanced methanation of CO2 on Rh/TiO2. They found that the nonthermal methane production rate presented a linear dependence on the top surface temperature, differently from the exponential one found in thermal catalysis. Furthermore, the apparent quantum efficiency from the nonthermal contribution was not correlated with the light intensity, but it showed a linear dependence on the top surface 200 C
350 C temperature range. The enlightenment of nonthermal effects in the Rh/TiO2 plasmonic photocatalyst indicated that this methodology may be applied for the quantitative estimation of thermal and nonthermal light effects in plasmon-enhanced catalytic reactions. The synergic effect between light and heat accelerated the CH4 formation both at low and fair temperatures. Heat influenced negatively the light-driven reaction above B350 C because the reverse reaction of CH4 reforming became competitive in the presence of light. Kho et al. [148] tested Ni/ceria
titania catalysts for the CO2 photothermal reduction. The catalysts showed a higher absorption in the Vis
NIR region than those in the absence of nickel. In particular, a greater absorption in the Vis and NIR region was found for ceria-rich samples compared with the titania-rich ones. This finding was explained by taking into account the higher extent of Ni dispersion that induced a huge heating of the catalyst bed and in turn promoted CO2 methanation activity. Moreover, ceria made CO2 adsorption easier and, consequently, the formation of surface defects during the Ni prereduction stage. Titania, on the contrary, was detrimental for the occurrence of Ni dispersion and CO2 adsorption, but its presence stabilized the oxygen vacancy-type defects within the support, thus better keeping the CO2 adsorption capacity during the photothermal methanation. Finally, in a more conventional method, Inoue et al. [149] studied the thermal CO2 methanation on co-sputtered Ru
TiO2 and Ru
ZrO2. The results showed that the particle sizes of deposited Ru decreased by the co-sputtering technique. The CO2 methanation tests indicated that for Ru
TiO2/TiO2 and Ru
ZrO2/TiO2 methane yields in the 160 C
240 C range were higher than those for Ru/TiO2, leading to lower reaction temperatures. Improved activities of these co-sputtered catalysts were induced by smaller Ru NPs. The authors emphasized that the deposition of smaller Ru particles was maintained for the Ru
TiO2/TiO2 and Ru
ZrO2/TiO2 catalysts, even though the reaction occurred at 360 C.

21.4.8 Biofuels production Environmental issues related to the consumption of fossil fuels are directing research toward renewable and environmentally friendly sources [150]. Among them, biodiesel, also known by the acronym FAME (fatty acid methyl ester), or

Catalytic applications of TiO2

657

bio-oils produced by pyrolysis of biomasses, is considered sustainable, economical, and technically feasible [151].

21.4.8.1 Transesterification of triglycerides Transesterification of triglycerides in vegetable oils is mostly used to obtain biodiesel. In this contest, the heterogeneous catalysts are preferred to the homogeneous ones, due to the intrinsic disadvantages of the latter as the difficulty to separate the catalyst from the reaction mixture and to purify the raw materials. For these reasons titanium dioxide as support found a wide
applicability [152]. Gardy et al. worked with sulfated loaded TiO2 SO22 =TiO 2 as a solid superacidic catalyst which had 4 very good performances compared to other sulfated metal oxides [153]. Wen et al. [154] synthetized MgO
TiO2 (MT) mixed oxides by sol
gel to convert waste cooking oil into biodiesel. The best performing catalyst was the sample 1:1 (molar ratio) Mg/Ti. The best yield of FAME was 92.3% obtained with a 50:1 methanol:oil molar ratio, catalyst amount of 10 wt.%, reaction time 6 h, and temperature 160 C. Moreover, it was observed that the catalytic activity of MT decreased slowly in the recycling process. The yield of FAME increased a little up to 93.8% with respect to the value of 92.8% obtained with the fresh catalyst due to the higher specific surface area and average pore diameter. The presence of TiO2 induced the formation of defects in the magnesia lattice because Ti ions replaced the Mg ones, improving the catalyst stability. Salinas et al. investigated the transesterification of canola oil by using a K2O/ TiO2 catalyst for production of biodiesel [155]. They report that the most active catalysts were those for which the adsorption and desorption of CO2 were the highest. A complete conversion to methyl esters was observed with a catalyst 20% K-loaded under atmospheric pressure and without any pretreatment. In another work, they examined the same reaction with a hydro-treated TiO2 supported potassium catalyst [156]. The calcination at different temperatures transformed the catalyst into a titanate form of the oxide and its activity increased.

21.4.8.2 Upgrading of pyrolysis oils Bio-oils are also interesting alternatives to fossil fuels for sustainable biofuels production. In this contest, microalgae are promising feedstock for biofuels as well as for chemicals, food, cosmetics, and healthcare [157,158]. Compared to consolidate routes such as the transformation of algal fatty acids to biodiesel, the pyrolysis of algae can advantageously convert the algal biomass [159] by depolymerizing organics at 400 C
600 C in the absence of oxygen. However bio-oils have high oxygen contents, and if used as bio-fuels, can cause some engine compatibility problems, instability, and worse cold flow properties [160]. In addition, the high O2 content in bio-oils blends could increase the NOx emissions. Then upgrading these bio-oils is required. Catalytic fast pyrolysis involves the conversion of primary pyrolysis vapors to less oxygenated liquid fuels by removing oxygen as CO, CO2, and H2O. The

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Titanium Dioxide (TiO2) and Its Applications

catalysts can be mixed with biomass in the pyrolysis reactor or in a downstream reactor [161]. Acidic metal oxides (as alumina), sulfated metal oxides (as SO22 4 =TiO2 ), and transition metal oxides (as cerium oxide and titania) have been investigated for this purpose [162
164]. TiO2 and CeO2 catalysts increased the amount of water, gas, and solid products, where carboxylic acids as acetic acid were transformed into gasoline-range organics by elimination of O2 as water and CO2, while they reduced the yields of liquid and organic products [165]. Aysu et al. [166] studied the effects of titania-based catalysts on in situ pyrolysis of Pavlova microalgae. The bio-oil yield was 20% and was enhanced with the utilization of Ni/TiO2 (22.55 wt.%) at 500 C. The highest values of the produced biooils were found by supplying energy in the 35
37 MJ/kg range. H-NMR and GC
MS analyses indicated that the titania catalysts extended the type and amount of aliphatic and aromatic compounds and decreased the presence of oxygenates in the bio-oils. The TiO2 catalyst supported with Ni, instead, showed the greatest activity towards the formation of aliphatics (60%) and reduced the coke formation, due to the higher Ni disposability on the surface, in comparison to Co and Ce, and to a significant Ni
TiO2 interaction. Another approach to reduce the oxygen content in bio-oils involves the use of specific and selective catalysts for deoxygenation [167]. The properties of the resulting bio-fuels possess similarity to petroleum fuel [168]. The deoxygenation process over TiO2 supported catalysts required high H2 pressures under which the unsaturated products decreased and the stability of the catalysts improved [169]. Kubiˇcka et al. [170] used rapeseed oil as feedstock and studied the influence of nickel and molybdenum interactions with TiO2 on the size distribution of the pores and the clusters of the active phase as well as the deoxygenation performance. They measured that the conversion was 100% with high selectivity toward hydrodeoxygenated products. Recently, He et al. [171] focused on the hydrodeoxygenation reaction of guaiacol on Pt
Mo/TiO2 and Pt
Mg/TiO2 catalysts. The Pt
Mo/TiO2 resulted in about 97% conversion and the most important hydrodeoxygenation product was cyclohexane (73.4% yield). Nevertheless, the dehydration and hydrogenolysis route were suppressed by the presence of Mg.

21.4.9 Dehydrogenations, selective oxidations, and hydrogenations Over the years, TiO2 was efficiently used as support for different redox reactions, namely (oxy)dehydrogenations, selective oxidations, and HYDs. Furthermore, in the last years due to the increasing importance of TiO2 as photocatalyst, these reactions were also investigated by combined photo-thermo-catalytic approaches [172].

21.4.9.1 (Oxy)dehydrogenations The composites of VOx with TiO2 were among the most employed systems in oxydehydrogenation reactions [173
177]. In particular, the V2O5/TiO2 was one of the best catalysts for the oxidative dehydrogenation (ODH) of C2
C4 alkanes to the

Catalytic applications of TiO2

659

corresponding olefins. Corma et al. [177] found that the most selective catalysts in propane ODH were VOx supported on basic metal oxides. On the contrary, Arena et al. [174] and Martin Aranda et al. [175] reported that the reactivity of V2O5 on various supports was much higher in the presence of amphoteric oxides, and the highest reactivity was observed on TiO2-supported samples. Moreover, the acid
base properties of the support addressed the dispersion and the reducibility of the active phase. Christodoulakis et al. [178] analyzed the influence of the support (ZrO2 or TiO2) and vanadia loadings (1.2
10 wt.%) on the structure and the catalytic performance for propane ODH. Both in situ Raman spectroscopy and catalytic activity experiments were carried out. The main differences of the structure of VOx entities in the two supports originated from changes in the vanadia dispersion that appeared to be higher for V2O5/TiO2 compared to V2O5/ZrO2 at the same VOx density. The reactivity studies discovered that under the same experimental conditions, ODH rates expressed per V atom, were influenced by the type of support as they depended on the VOx density. This is an elegant example of how in catalysis the support can also have an equally important influence of the supported species on the rate of the reaction under examination. K2O
TiO2 and K2O
TiO2
ZrO2 were used for dehydrogenating ethylbenzene into styrene. In the presence of a mixture of K2O and CO2, the reactivity and the selectivity of the investigated reaction over TiO2
ZrO2 improved [179], whereas the addition of only K2O to the TiO2
ZrO2 was detrimental for the activity, due to the effect of K2O to neutralize the acidic sites of the TiO2
ZrO2 system [180]. The presence of CO2 was beneficial as it acts both as oxidant and diluent. A fascinating photothermal approach was applied in the dehydrogenation of 2-propanol over TiO2 by Brinkley and Engel [181]. The difference between (110) and (100) planes of TiO2 was systematically investigated (Fig. 21.8). It was found

Figure 21.8 Schematic representation of the (100) surface side and face view and (110) surface side and face view. The solid spheres are Ti atoms, whereas the hollow spheres are lattice oxygen atoms. The letter “b” denotes bridging oxygen atoms [181]. Source: D. Brinkley, T. Engel, Evidence for Structure Sensitivity in the Thermally Activated and Photocatalytic Dehydrogenation of 2-Propanol on TiO2, J. Phys. Chem. B 104 (2000) 9836
9841 with permission, ©2020 American Chemical Society Publishing.

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Titanium Dioxide (TiO2) and Its Applications

that the reaction proceeded through thermally and photocatalytic pathways and acetone and propene were produced. The total reaction probability for an incident 2propanol molecule Preaction 5 Pacetone 1 Ppropene on the (110) surface was 0.03 in the absence of light, whereas that of incident acetone was less than 0.01. In the presence of light with hν . 3.2 eV, Ppropene and Pacetone were B 0 and 0.15, respectively. Notably, the reaction proceeded quickly for T , 180K, indicating that it was not thermally activated. Both thermally activated and photocatalytic pathways, instead, were observed on the (100) surface. However, the branching ratio was different than that on the (110) surface. In fact, the (110) surface showed a high selectivity for the photocatalytic pathway, whereas the (100) one showed a high selectivity for the thermal reaction. This finding can be explained by considering the different adsorption site geometry on these surfaces. For the (100) sites, bridging oxygens were closer to Ti41 binding sites than on the (110) plane, enabling an easier proton transfer, which is essential for the thermal reaction pathway. The photocatalytic pathway prevails at the (110) surface because hydrogen abstraction proceeds faster from the cation deriving from hole trapping than through proton transfer.

21.4.9.2 Selective oxidations of alkanes, alcohols, and aromatics The V2O5/TiO2 system was also found to be a good catalyst for the selective oxidations of alkanes, alcohols, and aromatics [182]. The surface structure of TiO2 permits the development of the Lewis acidity. Moreover, its redox properties modified by V2O5 lead to electronic interaction with V2O5 species [183]. The vanadium oxide loading plays an essential role for the oxidation activity, and it has been hypothesized that a single redox surface site is involved in the kinetically significant steps. The formation of crystalline V2O5 appeared to be detrimental for the activity [182,184]. In this contest, it has been reported that acetic acid can be obtained by butane and butene oxidation over supported vanadia catalysts [185,186]. n-Butenes were oxidized to acetic acid and/or acetaldehyde at low temperatures (200 C
300 C) and in the presence of water vapor [187]. AA describes the oxyhydrative scission mechanism for this reaction in which ketones are formed by oxyhydration of olefins and successively the C
C bond scission occurred. When the V2O5
TiO2 catalyst was employed, the activity was influenced and the reducibility or lattice oxygen mobility gave rise to a more facile oxidation/oxydehydrogenation of the adsorbed species [188]. V2O5/TiO2 exhibited also a high conversion and selectivity to dimethylmethane in the oxidation of methanol under mild experimental conditions [189]. A good performance for the oxidation of 1,2-dichlorobenzene has been also reported [190]. Bulushev et al. [191] focused on the partial oxidation of toluene to benzaldehyde and benzoic acid over vanadia/titania catalysts. In particular, the authors prepared bare and K-doped vanadia/titania. The ratio of different vanadia species was tuned by adding the catalyst to diluted HNO3, which removed only the bulk vanadia and polymeric vanadia species, maintaining the monomeric ones. AA supposed that the catalytic activity and selectivity to benzaldehyde and benzoic acid was due to both

Catalytic applications of TiO2

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monomeric and polymeric monolayer species, whereas bulk V2O5 and TiO2 were significantly less active. Hence, the specific rate in toluene oxidation decreased when the vanadium concentration in the samples increased beyond that corresponding to the monolayer, because of the partial blockage of active species by bulk crystalline V2O5. Addition of potassium diminished the acidity resulting in a decrease of the rate of toluene oxidation, probably because the amount of active monolayer vanadia decreased also, leading to the formation of nonactive K-doped monomeric vanadia species and KVO3. Keranen et al. [192] prepared V2O5/TiO2/SiO2 catalysts by means of atomic layer deposition (ALD), testing them for the selective oxidation of o-xylene. The ALD produced well-dispersed vanadia species, allowing also to attach them not only on the titania surface species but also on the uncovered silica support. The vanadia species deposited by impregnation were more tightly supported on titania, but also V2O5 crystallites that decreased the surface area were present. Catalysts prepared by ALD with submonolayer titania coverages showed a high number of strong Lewis acid sites. When high titania contents were present, instead, the acidity was hindered better in the catalysts prepared by ALD than in those prepared by impregnation. The authors highlighted as o-xylene conversion was more significant on the ALD catalysts where a high dispersion occurred than on the impregnated analogs. In the same contest, Maurya et al. [193] investigated the oxidation of 4fluorotoluene on vanadia
titania in a gas
solid system. The authors highlighted the role of the preparation method and established that vanadia
titania samples prepared by the sol
gel technique were more active than the corresponding impregnated samples for the oxidation of 4-fuorotoluene to 4-fuorobenzaldyde. For the mixed oxides with low vanadia loadings prepared by the sol
gel technique, titania was mainly in the rutile phase, whereas when the vanadia loadings were higher, the anatase phase was detected. In the latter case, vanadium was present not only in substitutional positions but also as dispersed species on the surface, due to the high specific surface area of the titania support. Moreover, the catalysts with lower vanadia loadings showed only Lewis acidity with high acid strength, whereas both Lewis and Bro¨nsted acidity were disclosed at higher loadings. The more significant activity of sol
gel vanadia
titania was due to the higher dispersion of vanadia on the support surface and the higher Lewis acidity. The increase of gas hourly space velocity led to higher selectivity toward benzaldehyde as the further oxidation to fluorobenzoic acid virtually did not occur. Gasior et al. [194] studied the o-xylene and the isopropanol oxidation on V2O5
TiO2 oxides. The catalyst was prepared by decomposing vanadia on anatase and rutile TiO2. Higher activity and selectivity toward o-xylene oxidation, lower reduction degree of the catalyst in the stationary state of catalytic reaction, and lower dehydration
dehydrogenation ratio were showed by the anatase-containing catalysts with respect to the corresponding rutile-containing samples. The properties of the rutile-containing catalysts were similar to those of pure vanadia, whereas a monolayer structure of vanadia formed on the anatase surface that presented centers for isopropanol dehydrogenation and low acidity.

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Titanium Dioxide (TiO2) and Its Applications

Chang et al. described the partial oxidation of methanol to H2 (CH3OH 1 1/2O2 ! 2H2 1 CO2) over Au/TiO2 [195]. TEM characterization showed as Au/TiO2 samples prepared by deposition
precipitation exhibited hemispherical gold particles, firmly adherent to the TiO2 support. The catalytic activity strongly depended on the gold particle size that was affected by the pH and the calcination temperature. XPS demonstrated that gold was present in three states in uncalcined catalysts, that is, metallic (Au0), cationic (Auδ1), and Au2O3, whereas when the catalysts were calcined at 573K only metallic (Au0) was detected. The catalyst prepared at pH 5 8 and the uncalcined catalysts exhibited the highest activity for H2 generation. The authors observed that the products distribution was strictly related to the partial pressure of O2. No CO was detected when the O2/CH3OH molar ratio in the feed was 0.3. Both hydrogen selectivity and methanol conversion increased with the reaction temperature.

21.4.9.3 Selective oxidations of heteroaromatic compounds Kong et al. [196] focused on the selective oxidation of thiophene by H2O2 at 333K using titanium silicalite. Interestingly, they found that frameworks of titanium species were the active sites for thiophene oxidation, whereas the thiophene stability was due to the conjugated π-electrons of its ring. The role played by framework titanium was thought to be the interruption of the conjugation during the reaction, leading to the breakage of the aromaticity. The latter process has a key role for the oxidation of thiophene under mild experimental conditions. The selective aerobic oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-furandicarboxylic acid (FDCA) by using Pt supported on TiO2 and ZrO2 catalysts was studied by the group of Ait Rass et al. [197]. HMF, produced by fructose or glucose dehydration, is an important product for the synthesis of many derivatives with several applications, as pharmaceuticals, antifungal compounds, and polymer precursors. HMF is, in fact, the starting material for FDCA, which has been recently proposed as a greener monomer for the production of an alternative polymer to poly-ethyleneterephtalate [198]. The preparation of furanic-modified amine-based curatives for polyureas, hybrid epoxy- and urea
urethanes, and polyester polyols for the manufacture of coatings providing corrosion and flame resistance can be cited among the other applications of FDCA [199]. For these reasons, the SCO of HMF to FDCA is an intriguing reaction of industrial interest. The work by Ait Rass et al. pointed that with the Pt/TiO2 catalyst, the reaction stopped without the addition of a homogeneous base because the pH value became too acidic and carboxylic products were deposited on the catalyst. In moderately alkaline aqueous solutions, instead, the Pt-based catalyst were efficient. 2,5-Diformylfuran (DFF) and 5hydroxymethyl-2-furancarboxylic acid (HMFCA) were formed in a first step. Subsequently, they turned into 5-formylfuran carboxylic acid (FFCA) and at the end into FDCA with very good yields (Fig. 21.9). The oxidation of the aldehyde moiety in FFCA was the limiting step. Furthermore, the authors showed that the replacement of 4 equivalents of the weak NaHCO3 base by 2 equivalents of the stronger Na2CO3 base produced a more

Catalytic applications of TiO2

663 O O OH

HO

O O OH 5-Hydroxymethylfurfural (HMF)

O

O

5-Hydroxymethyl-2-furancarboxylic acid (HMFCA) O

O O

HO 5-Formil-2-furancarboxylic acid (FFCA)

O

O O

O HO

OH 2-5-Furandicarboxylic acid (FDCA)

2-5-Diformylfuran (DFF, FDC)

Figure 21.9 Reaction pathway for aqueous HMF oxidation to FDCA over Pt/TiO2 catalyst. HMF, 5-Hydroxymethylfurfural; FDCA, 2,5-furandicarboxylic acid.

active system for the transformation of FFCA into the fully oxidized FDCA. However, the final yield of FDCA was lowered due to the formation of some byproducts. Notably, the activity and the reuse of Pt/TiO2 was increased by the addition of Bi. Pt/ZrO2 catalyst was slightly less active than Pt/TiO2. The same reaction was examined by Casanova et al. [200] comparing Au/TiO2 and Au/CeO2 catalysts. They found that Au/CeO2 was more active and selective than Au/TiO2. They reported a reaction mechanism for FDCA formation where the rate-limiting step of the reaction was the oxidation of the alcohol functionality of HMFCA. When the reaction was carried out in a two temperature steps procedure, the substrate degradation was reduced, and the lifetime of the catalyst increased. Lolli et al. [201] also investigated the HMF to FDCA reaction through both catalytic and photocatalytic route employing a Pt/TiO2
SiO2 composite. The catalyst was found to be very active in the liquid-phase oxidation of HMF at neutral pH, and the most important products were DFF, FFCA, and FDCA. The introduction of gold further enhanced the photo- and thermo-activity.

21.4.9.4 Olefin epoxidation TiO2 on SiO2 was the first utilized heterogeneous system, patented by Shell in 1971 [202], for liquid-phase olefin epoxidation reactions. Since then, numerous publications can be found, pointing to the high epoxidation activity of Ti(IV) species on the silica surface. For example, Muller et al. [203] synthetized titania
silica mesoporous oxides with various amounts of methyl groups that were covalently bound. They started from methyl-trimethoxysilane and tetramethoxysilane by applying a sol
gel method and a subsequent supercritical extraction with CO2 at low temperature. The as-synthetized aerogels were highly stable as the Si
C bonds broke with a significant rate only at t . 400 C. These materials were used as catalysts for the epoxidation of olefins and allylic alcohols. The epoxidation activity of the olefins was lowered by increasing the methyl content, whereas that of the allylic alcohols increased. Cyclohexenol epoxidation proceeded with 98% selectivity at 90% conversion after 10 min. The intrinsic basicity of inorganic salts and bases used as additives strongly influenced the catalytic behavior of the modified aerogels. According to the authors, different Ti complexes formed upon interaction with the reactant and the basic

664

Titanium Dioxide (TiO2) and Its Applications

additive, and their presence changed the stability of the catalyst and reactivity. In the same line, Sanz et al. [204] developed an original method for the synthesis of TS-1 zeolite with hierarchical porosity. They crystallized SiO2
TiO2 xerogels that were formerly imprinted with silanized protozeolitic units (Fig. 21.10). The organic functionalization inhibited the growth and the aggregation of the zeolitic crystals during the hydrothermal crystallization, and a hierarchical porosity was obtained. TS-1 materials presented a significant porosity within the micro/mesoporous range, which allowed a better diffusion of the reagent species with respect to the TS-1 ones prepared by conventional methods. Nevertheless, the modified TS-1 zeolite showed to have a high hydrophilic character, which was a drawback for its potential application in oxidation reactions with H2O2. In fact, water molecules, which could be adsorbed on the zeolite surface, prevented the access of the substrates to the titanium active sites. On the contrary, the catalytic activity of the hierarchical TS-1 zeolites in olefin epoxidation in the presence of alkyl hydroperoxides as the oxidant species was remarkable. The authors explained these results with a greater availability of Ti active sites, thanks to the presence of a secondary porosity. Recently, Smeets et al. [205] synthetized titanosilicates through the nonhydrolytic sol
gel method. The materials had large surface areas and volumes of mesopores, making them very useful for the conversion of olefins, such as cyclohexene. The catalysts did not show an important epoxidation activity in the presence of high quantities of water, whereas they were highly active in organic solvent. In fact, water induced only the unwanted radical allylic oxidation route. To restore the

Figure 21.10 Preparation of hierarchical TS-1 zeolites from silanized protozeolitic units imprinted in SiO2
TiO2 xerogels [204]. Source: R. Sanz, D.P. Serrano, P. Pizarro, I. Moreno, Hierarchical TS-1 zeolite synthesized from SiO2TiO2 xerogels imprinted with silanized protozeolitic units, Chem. Eng. J. 171 (2011) 1428
1438 with permission, ©2011 Elsevier Publishing.

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665

epoxidation efficiency of such amorphous catalyst, the authors increased the surface hydrophobicity of catalysts by the addition of methyl groups. However, the asmodified catalysts were not homogenous, the concentration of surface-active sites was lower, and the epoxide yield did not improve in the presence of water. Methylation by postgrafting, instead, allowed to restore the direct mechanism of epoxidation also in the presence of water because the surface hydrophobicity prevented the inactivation of the active sites. Qi et al. [206] synthetized manganese oxides supported on γ-Al2O3, amorphous SiO2, MCM-41, and TiO2 by means of an impregnation method and investigated the epoxidation of alkenes with 30% H2O2 in NaHCO3 aqueous solution. The MnO/γ-Al2O3 showed superior epoxidazing activity of styrene with respect to other supported MnOx. Jafarpour et al. [207] in an original way prepared TiO2 NPs and a cobalt Schiff base complex (Fig. 21.11), which exhibited a synergistic effect on the visible-light photocatalytic aerobic oxidation of many olefins without the presence of a reducing agent. Furthermore, the activity did not change, and the complex did not decompose after the catalyst was reused up to five times. Finally, Yang et al. [208] studied the epoxidation of olefins with H2O2 and TiO2 with a combined photo-thermo-catalytic approach. They discovered that anatase TiO2 is a good catalyst for the epoxidation of cyclooctene with H2O2 at room temperature. However, the catalyst deactivated when an excess of H2O2 was used, due to the formation of inactive side-on Ti-η2-peroxide species on the surface of TiO2. Isotope labeled resonance UV Raman spectroscopy and kinetics studies confirmed this explanation. Notably, the epoxidation reaction dramatically accelerated under UV light irradiation because the inactive peroxide species were removed and the TiO2 surface sites that were active for the cyclooctene epoxidation were regenerated. This multicatalytic approach is an intriguing route for efficient epoxidation reactions on TiO2 under mild experimental conditions.

Figure 21.11 Aerobic olefin oxygenation catalyzed by CoL2@TiO2 nanohybrid [207]. Source: M. Jafarpour, H. Kargar, A. Rezaeifard, A synergistic effect of a cobalt Schiff base complex and TiO2 nanoparticles on aerobic olefin epoxidation, RSC Adv. 6 (2016) 79085 with permission, ©2016 Royal Society of Chemistry Publishing.

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Titanium Dioxide (TiO2) and Its Applications

21.4.9.5 Selective oxidation of ammonia to nitrogen Another interesting redox reaction was the SCO of ammonia to nitrogen investigated by Long and Yang [209] through Fe2O3
Al2O3, Fe2O3
TiO2, Fe2O3
ZrO2, and Fe2O3
SiO2 catalysts prepared by a sol
gel method. They found that the Fe2O3
TiO2 catalysts, synthetized starting from iron sulphate (with a significant amounts of sulfate species adsorbed on the catalyst surface), gave a higher selectivity for N2 than those deriving from nitrate.
More than 92% of N2 was obtained
by 22 and 20 wt.% Fe at using the 10 wt.% Fe2 O32TiO2 SO22 O 2TiO SO 2 3 2 4 4 400 C
450 C. The authors proposed a reaction mechanism in which NH3 was first oxidized to NO, and then NO was reduced to N2 by unreacted NH3 adsorbed spe
cies through SCR. Sulfate species on Fe2 O32TiO2 SO22 induced an increase of 4 the surface acidity and thus the SCO performance.

21.4.9.6 Hydrogenations TiO2-supported metal catalysts (Pd, Pt, Re, Ni) were also applied for HYD reactions, taking advantages of both acidity and SMSI of titania in providing high selectivity to hydrogenated products and low coke formation. Indeed, anatase, rutile, and mixed phases, combined with different metals, have been reported to exhibit good performances with different feedstocks [1]. In fact, the occurrence of a spillover effect of hydrogen, activated on the metal sites and able to migrate over the TiO2 surface, can strongly enhance the HYD capability of these noble metal/TiO2 systems. Activity and selectivity were also found to depend on the type of metal supported on TiO2. As examples, HYD of alkadienes over Pd/TiO2 gave the related alkene with high selectivity [210], whereas Pt/TiO2 was effective for the HYD of stearic acid to the corresponding stearyl alcohol [211]. Moreover, Ni/TiO2 was used for HYD of benzaldehyde and acetophenone to toluene/benzene and phenylethanol, respectively [212,213]. Also, in this case the high HYD activity of Ni was ascribed to the SMSI interaction with TiO2, which boosted Ni dispersion and activated hydrogen by spillover. Finally, several bimetallic/TiO2 systems were investigated, such as Pt
Re and Pt
Ir, which showed a further improvement of the HYD performance [1,211].

21.5

Conclusion and outlooks

Together with its well-recognized photocatalytic activity, TiO2 demonstrated to be a versatile metal oxide suitable for a series of important industrial catalytic reactions. In fact, despite some intrinsic drawbacks as the low surface area, the properties of titania can undergo appropriate modifications by means of different approaches. This allows to obtain and finely modulate specific textural and surface properties, thus matching the desired performance of the catalytic systems in term of activity, selectivity, and stability.

Catalytic applications of TiO2

667

In particular, it was displayed that titania can be efficaciously modified with metals or metal oxides in order to improve the performance. The SMSI occurring with noble metals, as Pt, Pd, Ru, Au, provides higher dispersion and stability of the metallic particle over titania. The presence of another oxide often results in a change of the acid/base properties which are fundamental for some reactions. Very interesting appears also the possibility to prepare titania structures with a specific porosity, higher surface area and well-organized morphologies. Moreover, the different polymorphs of TiO2 provided different physicochemical properties that influence in a different way the previously reported interactions (metal
support and chemical reagents/products
support), thus strongly affecting the final catalytic performance. Finally, it must be stressed that the use of titania also allows to exploit its photoactivity and this appears important in designing photothermal processes, which represent an important advancement for various industrial applications requiring greener approaches. The possible synergy between thermo- and photocatalysis appears, in fact, important to improve the overall performance of the TiO2-based systems. These studies entailing additional efforts, will be a hot field in the heterogeneous catalysis of 21st century.

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172.

Index

Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively. A A549 cell lines, 379 AAMs. See Alkali-activated materials (AAMs) Abrasion, 267 268 Absorbed photon-to-current efficiency (APCE), 201 Absorption edge of rutile, 36 37 Absorption emission of light, 142 AC. See Activated carbon (AC) Acetic acid, 272 274 Acetone, 550 Acetonitrile (MeCN), 615 616 Acetylcholinesterase (AChE), 557 Acid leaching process, 430 431 Acidic N-source, 413 Acoustic cavitation, 97 98, 98f Activated carbon (AC), 108 Active electrode (AE), 508 509 Active layer. See Insulating layer Active packaging, 361 362 Addition reactions, 626 627, 626f Adsorption on active carbon, 587 Adsorption desorption phenomena, 128 129 Advanced oxidation processes (AOPs), 357, 586 589 Adverse effect of TiO2 on bacteria, 375 377 of TiO2 on fungi, 378 AE. See Active electrode (AE) Aerogels, 88 89 Ag-doped TiO2, 94 95 Ag/TiO2/Ti devices, 515 516 Air mass (AM), 170, 170f Air pollution, 583 586 Air-purifying cement-based materials, 452 456

durability of photocatalytic activity, 455 456 improvement strategies, by TiO2 doping and modifications, 456 interactions with cement matrix, 454 455 role of climatic conditions, 453 454 AlCl3 1-butyl-3-methylimidizolium (AlCl3BMIC), 445 446 AlCl3 1-ethyl-3-methylimidazolium chloride (AlCl3-EtMeImCl), 445 446 Alcohol oxidase (AOx), 556 Alcohols, 229 oxidation to aldehydes, 610 612 selective oxidations of, 660 662 ALD. See Atomic layer deposition (ALD) Aldehydes, oxidation of alcohols to, 610 612 Algae, 354, 358 Alkali-activated materials (AAMs), 461 462 Alkanes epoxidation, 614 615 selective oxidations of, 660 662 Alkylation reactions addition reactions, 626 627, 626f carbonyl alkylation reactions, 629 630 substitution reactions in aromatic compounds, 627 629 Alzheimer disease, 558 559 AM. See Air mass (AM) American Society for Testing and Materials (ASTM), 321 322 Ammonia (NH3), 542 543, 637 638 Amperometric microelectrode, 560 Analytes, 561 562 Anatase, 16, 18, 30, 177 180 films, 484 polymorphic transformation to rutile, 465

682

Anatase (Continued) structural, optical, and electrical properties, 34t TiO2, 94 nanoparticles, 98 thin films, 108 Anchoring groups, 180 182 Angelica keiskei juice, 360 Anode reaction, 439 441, 443 Anodic oxides, 515 Anodization, 252 253 Anodizing, 512 516, 514f ANSES. See French Agency for Food, Environmental and Occupational Health and Safety (ANSES) Anthocyanins, 182 186 Antibacterial and antimicotic properties adverse effect of TiO2 on bacteria, 375 377 of TiO2 on fungi, 378 Antibacterial effects, 357 360 Antibody composite films, 561 Antimicrobial cement-based materials, 458 surfaces, 383 385 bacteria colonization on TiO2-treated surfaces, 384f Antismog concrete, 452 AOPs. See Advanced oxidation processes (AOPs) AOx. See Alcohol oxidase (AOx) APCE. See Absorbed photon-to-current efficiency (APCE) Aqueous environments, 553 554 Argon, 131 sputter gas pressure, 485 Aromatic compounds hydroxylation of, 612 614, 613f substitution reactions in, 627 629 Aromatics, selective oxidations of, 660 662 Asbestos, 585 ASTM. See American Society for Testing and Materials (ASTM) at quantum satis principle, 378 379 Atomic layer deposition (ALD), 516 519, 517f, 661 Au NRs. See Gold nanorods (Au NRs) Au/TiO2, 639 Azo compounds, 624

Index

B B3LYP functional, 68 69, 74 BA. See Benzyl alcohol (BA) Bacillus cereus, 360 Back electron transfer, 173 Back-sputtering. See Sputtering Backscattered electrons (BSEs), 116 117 Bacteria, 560 561 adverse effect of TiO2 on, 375 377 Band bending, 41 Band flattening mechanism of TiO2, 41f Bandgap of brookite, 36 37 energy of TiO2-based materials, 88 issue, 68 70 Bands formation, 176 Batteries, 250 254 significance of nanostructures, 252 254 significance of TiO2 polymorph, 250 252 BBO. See Bridge bonded oxygen (BBO) BBOVs. See Bridge bonded oxygen vacancies (BBOVs) Becher process, 313 315, 430 Benilite process, 313 315 Benzene, 628 629 Benzofuran, 628 629 Benzyl alcohol (BA), 610 Betalains, 182, 186 187 Bethe Salpeter equation, 70 BHLYP hybrid functional, 78 BHT. See Boiling heat transfer (BHT) Bi2MoO6, 416 Bi2WO6, 416 Binary nanosized V2O5 TiO2 thin-film sensors, 541 Bio-oils, 657 Bioclean by Saint-Gobain Glass UK Ltd, 407 Biodistribution, 379 383 Biofuels production, 656 658 transesterification of triglycerides, 657 upgrading of pyrolysis oils, 657 658 Bioinspiration, 191 Biological destructive procedure, for wastewater treatment, 586 Biological pollutants, 585 Biomarkers, 555 556 Biomass, 229 Biomolecules, 553 554

Index

Biosensors, 528, 532f, 552 562. See also Gas sensors; Sensors for environmental applications analytes, 561 562 bacteria, 560 561 cholesterol derivatives, 558 DNA and biomarkers, 555 556 elements and single components, 556f fields, 553 glucose, 554 555 glutamate, 560 H2O2, 558 559 pesticides, 556 557 urea, 559 560 Bisphenol A (BPA), 409 BiVO4, 224 Black crusts, 469f Blended cements, 449 450 Bloch surface waves (MSWs), 498 501 Bode plot, 203 204 Boiling heat transfer (BHT), 268 of TiO2 nanofluids, 295 298 heater material, 298 ionic additive, 298 nanoparticle type, 296 particle loading, 296 297 surface roughness, 297 298 Boltzmann distribution, 176 177 Boron, 409 410 Boron oxide, 225 BPA. See Bisphenol A (BPA) Bragg’s law, 110 Bridge bonded oxygen (BBO), 23 Bridge bonded oxygen vacancies (BBOVs), 23 Broken Hill Proprietary billiton process, 440 Brookite, 16, 18, 177 180 Brunauer Emmett Teller (BET) analysis, used gases for, 131 instrument and working principle, 131 134 isotherm, 129 130 surface area, 642 643 BET-specific surface area determination, 127 134 determination, 130 BSEs. See Backscattered electrons (BSEs) Building materials, concerns in the use of TiO2 in, 470 473

683

Bulk defects, 22 Bulk doping, 41 42 C C-doped TiO2, 411 CA. See Cellulose acetate (CA) CaCl2, 438 439 Caco-2 cells, 381 Cadmium sulfide (CdS), 223 224 CAGR. See Compound annual growth rate (CAGR) Calcium carbonate (CaCO3), 362 Calcium silicate hydrate gel (C S H gel), 454 455, 458 459 CAM-B3LYP hybrid functional, 78 Cancer therapy, 346 347 Candida albicans, 378 Candies, 353 354 Capacitance, 528 Carbamazepine, 585 586 Carbon, 409 411, 434 435 Carbon dioxide (CO2), 541 542, 612 emissions, 450 methanation, 655 656, 655f Carbon dots (CDs), 655 656 Carbon monoxide (CO), 543 544, 585, 592 Carbon nanotubes (CNT), 383 384 Carbonyl alkylation reactions, 629 630 Carbonyl reduction, 617 619 Cardinal Glass Industries, 405, 406f Carotenoids, 188 189 Cassie Baxter model, 397 Catalase, 375 Catalytic applications of TiO2 defects role on catalytic performances, 640 641 reactions involving titania-based catalyst, 641 666 aerobic olefin oxygenation, 665f biofuels production, 656 658 CO2 methanation, 655 656, 655f deacon process, 644 646 direct synthesis of hydrogen peroxide, 649 651 FTS, 651 653, 653f hierarchical TS-1 zeolites, 664f hydrogenations, 666 NOx removal, 641 644 olefin epoxidation, 663 665

684

Catalytic applications of TiO2 (Continued) (oxy)dehydrogenations, 658 660 reactions with sulfur-rich compounds, 646 649 selective oxidations of alkanes, alcohols, and aromatics, 660 662 selective oxidations of ammonia to nitrogen, 666 selective oxidations of heteroaromatic compounds, 662 663 water gas shift reaction, 653 654 titania as catalytic support, 638 640 Catalytic fast pyrolysis, 657 Catalytic oxidation of NOx, 643 644 Catalytic wet air oxidation (CWAO), 586 Cathodes, 193 reaction, 439 441 CaTi2O4, 440 CaTiO3, 440 CB. See Conduction band (CB) CDs. See Carbon dots (CDs) CE. See Counter electrode (CE) Cell voltage, 438 439 Cellulose acetate (CA), 362 Cement hydration process, 460 Cement-based materials applications of photocatalytic, 451t goals of use, 449 451 patents on cement-based materials with TiO2, 461 of TiO2 use for functional, 451 458 of TiO2 use for structural, 458 461 Cement-based materials, 449 Ceramic tiles, TiO2 in ceramic tiles production, 462 464 classification, 463t exploitation of TiO2 in ceramic, 464 467 International patents on photocatalytic ceramic tiles, 467 Italian tiles production, 464f standards, 468 Ceria, 654 Cetyltrimethylammonium bromide (CTAB), 118 120, 272 274, 547 CH3NH3PbBr3 perovskite, 491 492 Characteristic I-V curves, 197 201 Characterization techniques of TiO2, 109 151

Index

BET-specific surface area determination, 127 134 diffuse reflectance spectroscopy, 135 141 photoluminescence spectroscopy, 141 144 SEM, 115 121 TEM, 121 126 TGA, 147 151 XPS, 144 147 XRD, 109 114 Charge separation in shape-controlled anatase TiO2, 218 220 in TiO2 phase junctions, 218 Chemical sensors, 528 Chemical sunscreens, 325 Chemical vapor deposition (CVD), 107 108, 270 271 Chemical vapors, 553 554 Chemiluminescence, 536 Chemiresistors, 532 533 Chewing gum, 353 354 CHF. See Critical heat flux (CHF) Chitin, 375 Chlorella vulgaris, 358t Chloride process, 315 317 wastes generated in, 329 Chlorine, 644 washing procedures, 357 4-Chlorophenol, 563, 585 586 Chlorophylls, 182, 187 188 Chl-a, 187 188 Chl-b, 187 188 Cholesterol derivatives, 558 Chromium (Cr), 414 Chronoamperometry, 552 553 Chronopotentiometry, 552 553 CI. See Color index (CI) CI 77891, 364 Circular economy, 327 328 Claus process, 647 649 “Clean” model systems, 594 Clogging, 267 268 Clostridium difficile, 375 CNT. See Carbon nanotubes (CNT) Coal, 430 431 Coatings, 321 322 Codoped V N TiO2, 411 412 Collection material, 499

Index

Color index (CI), 319 Coloration, 487, 489f Commercial glass products, 402 with self-cleaning coating, 403t Commercial self-cleaning glasses Bioclean, 407 Neat Glass produced, 405, 406f Pilkington Activ Clear/Blue/Neutral, 405 Renew, 407 408 Self-cleaning glass, 406 SunClean, 406 407 Commercial TiO2 samples, 610 Commission Directive 95/45/EC (E171), 364 Commission Regulation (EC) No. 1333/ 2008, 354 355 Commission Regulation (EU) No. 231/2012, 355 Compound annual growth rate (CAGR), 4 5 Compression/bending tests, 45 Computational chemistry, 191 Computational modeling of cutting-edge materials, 76 80 of titania nanoparticles, 78 80 Computer simulations, 189 190 Concrete, nano-TiO2 modifying the properties of, 459 460 Condensation, 487 Conduction band (CB), 135, 174, 212, 373 Conductive heat transfer, 268 Contact angle, 396 hysteresis, 397f Conventional resistive sensor based on Schottky barrier effect, 533f Copper (Cu), 417 cocatalyst, 616 Copperas, 328 Coprecipitation, 90 96 Core shell structure, 26 Corning Gorilla Glass, 383 384 Cosmetics, TiO2 in, 362 365 regulations, 364 safety of sunscreens, 364 365 Cosmetics Regulation EC 1223/2009 Annex IV, 353, 364 Cotton, 326 Counter electrode (CE), 171 172, 508 509 characteristics and performance of, 191 192

685

Coupled TiO2, 87 88 Critical heat flux (CHF), 295 Crocus sativus L., 360 Crystal, TiO2, 531 Crystallinity, 111 112 C S H gel. See Calcium silicate hydrate gel (C S H gel) CTAB. See Cetyltrimethylammonium bromide (CTAB) Cu-doped TiO2, 411, 416 417 Cultural heritage conservation, TiO2 in, 468 470 CuO TiO2 hybrid nanocomposites, 564 565 Cuprous oxide (Cu2O), 226 227 Current doubling effect, 229 Cu TiO2 photocatalysts, 102 Cutting-edge materials, computational modeling of, 76 80 CV. See Cyclic voltammetry (CV) CVD. See Chemical vapor deposition (CVD) CWAO. See Catalytic wet air oxidation (CWAO) Cyanidin (Cy), 184 185 Cyclic voltammetry (CV), 194 196, 552 553 Cytochrome c (Cyt c), 556 Cytotoxicity, 357, 472 D Dark current processes, 173 DBR. See Distributed Bragg reflector (DBR) Deacon process, 644 646 Defectivity of TiO2, 19 22 bulk defects, 22 interfacial defects, 21 22 line defects, 21 point defects, 19 20 Defects role on catalytic performances, 640 641 Delphinidine (Dp), 184 185 Density functional theory (DFT), 68, 189 190 Density of states (DOS), 74 Deodorizing function, 597 Depolluting concrete. See Antismog concrete Depollution effect, 455 Deposition techniques, 484 488 Deposition precipitation method, 103 104

686

Derivative of TGA curve (DTG), 147 Desorption factor, 196 197 Device fabrication process, 549f DFF. See 2,5-Diformylfuran (DFF) DFT. See Density functional theory (DFT) DFTB method. See Tight-binding density functional theory method (DFTB method) DHA. See 1,3-Dihydroxyacetone (DHA) DHN. See 2,3-Dihydroxynaphthalene (DHN) DHN/TiO2 Cu photocatalyst, 617 DHP. See Dihydropyridine (DHP) Diamond anvil cells techniques, 42 1,4-Diazanaphthalene, 627 Dielectric mirrors, 483 506 Differential thermal analysis (DTA), 147 Diffuse reflectance infrared spectroscopy (DRIFTS), 652 Diffuse reflectance spectroscopy (DRS), 88, 135 141 2,5-Diformylfuran (DFF), 662 Digital print” of semiconductor, 37 38 Dihydrogen (H2), 538 539 Dihydropyridine (DHP), 186 187 1,3-Dihydroxyacetone (DHA), 612 2,3-Dihydroxynaphthalene (DHN), 616 Dioxygen (O2), 539 541 Dip-coating, 105 Direct solid-to-solid deoxidation process, 441 442 Direct synthesis of hydrogen peroxide, 649 651 Dislocations. See Line defects Distributed Bragg reflector (DBR), 492, 497f field distribution along, 499f modeling, 493 498 reflectivity dispersion map, 497f DNA, 555 556 Doctor Blade, 193 Doped TiO2, 87 88, 215 216 Doped TiO2 based coatings for improved self-cleaning ability mechanism of doped-TiO2 coatings for glass, 408 412 synthesis strategies, 412 414 Doped-TiO2 coating mechanism for glass, 408 412 metal/nonmetal doping mechanism, 412f nonmetal doping mechanism of anatase TiO2, 410f

Index

TiO2 photocatalysis mechanism, 408f DOS. See Density of states (DOS) Double carbon carbon bonds hydrogenation, 615 617, 616f DRIFTS. See Diffuse reflectance infrared spectroscopy (DRIFTS) DRS. See Diffuse reflectance spectroscopy (DRS) Drug delivery, 345 349 Dry impregnation, 102 Dry-deposition methods, 414. See also Wetdeposition methods Drying, 487 DSSCs. See Dye-sensitized solar cells (DSSCs) DTA. See Differential thermal analysis (DTA) DTG. See Derivative of TGA curve (DTG) Durability of hardened concrete, 460 461 of photocatalytic activity, 455 456 Dye-sensitized solar cells (DSSCs), 105, 169, 171 175, 596 assembly and characterizations, 193 206 development of photoanodes and cathodes, 193 spectroscopic techniques, 193 194 Dye(s), 180 191 bioinspiration, 191 computational details, 189 190 detection, 565 567 natural dyes, 182 189 sensitization, 217 sensitizers, 80 synthetic dyes, 180 182 E E171, 354 356 “Easy-cleaning” behavior, 457 EC devices. See Electrochromic devices (EC devices) ECHA, 378 379 ECMs. See Electrochemical metallization memories (ECMs) Economic aspects of TiO2, 3 6 EDG. See Electron donor group (EDG) Edge-type dislocations, 21, 21f EDLC. See Electric double-layer capacitor (EDLC)

Index

EDS. See Energy-dispersive X-ray spectroscopy (EDS) EDTA-4Na1. See Ethylenediaminetetraacetic acid tetrasodium salt (EDTA-4Na1) EEA. See European Environment Agency (EEA) EEI. See Electrode/electrolyte interface (EEI) EFSA. See European Food Safety Authority (EFSA) EG. See Ethylene glycol (EG) e h1 pairs mobility, 373 375 Eigenvalue gap, 69 70 Einstein model, 46 EIS. See Electrochemical impedance spectroscopy (EIS) Electric double-layer capacitor (EDLC), 243 244 Electric furnace gas, 430 smelting process, 430 Electric signal, 528 Electrical properties of TiO2, 33 35 Electrochemical biosensors, 552 553 Electrochemical deposition, 270 272 of Ti in low-temperature liquid salts, 445 446 Electrochemical impedance spectroscopy (EIS), 203 204, 530, 552 553 Electrochemical metallization memories (ECMs), 508 510, 509f, 511t Electrochemical method, 543 Electrochemical oxidation. See Anodizing Electrochemical techniques, 530 Electrochromic devices (EC devices), 483 488 deposition techniques, 484 488 Electrochromism, 483 484 Electrocoagulation, 587 Electrode/electrolyte interface (EEI), 445 446 Electrodeposition. See Electrochemical deposition Electrolysis of TiCl4, 438 water, 211 Electrolytes, characteristics and performance of, 192 193

687

Electrolytic production of Ti from TiO2, 438 444 Electrolytic reduction of alumina into aluminum, 438 Electromagnetic lenses, 121 Electromagnetic wave (EM wave), 493 Electron donor group (EDG), 612 614, 613f Electron paramagnetic resonance spectroscopy (EPR spectroscopy), 217 Electron-hole pairs, 397 398 recombination, 229 Electron-transport layer (ETL), 259 260 Electron-withdrawing group (EWG), 612 614, 613f Electronic conductivity, 540 excitation in dye, 173 174 gun, 121 properties of TiO2, 32 33 Electronic structure calculations on TiO2, 68 76 bandgap issue, 68 70 excess electrons in TiO2, 70 71 interstitial Ti species, 74 75 oxygen vacancies, 72 74 photoexcited carriers, 75 76 Electrons, 410 411 Electrospinning, 558 Electrostatic plates, 121 Element mapping, 123 Elemental titanium, 436 Elongated nanoparticles, 27 EM wave. See Electromagnetic wave (EM wave) Enantioselective alkylations, 630f Energy harvesting and storage, 241 Energy storage applications, 243 260 batteries, 250 254 supercapacitors, 243 249 Energy-dispersive X-ray spectroscopy (EDS), 117 118, 123 Energy-saving multilayer structures, 417 420 Engineered nanomaterials (ENMs), 569 ENMs. See Engineered nanomaterials (ENMs) Environment-friendly materials, 450

688

Environmental and health concerns in use of TiO2, 470 473 Environmental Protection Agency (EPA), 452 “Environmental-friendly” technologies, 587 EPA. See Environmental Protection Agency (EPA); US Environmental Protection agency (EPA) Epoxidation of alkenes, 614 615 EPR spectroscopy. See Electron paramagnetic resonance spectroscopy (EPR spectroscopy) EQE. See External quantum efficiency (EQE) Escherichia coli, 94 95, 358, 358t, 360, 375, 560 561, 597 inactivation, 602 Ethanol, 546 549 Ethylene (C2H4), 360 degradation, 360 361 dibromide, 585 586 dichloride, 585 586 scavenging process of TiO2, 361f Ethylene glycol (EG), 118 120, 272 274 Ethylenediaminetetraacetic acid tetrasodium salt (EDTA-4Na1), 547 ETL. See Electron-transport layer (ETL) European Environment Agency (EEA), 452 European Food Safety Authority (EFSA), 354 355 EWG. See Electron-withdrawing group (EWG) Excess electrons (and holes) in TiO2, 70 71 “Exchange correlation” function, 189 190 Exchange current, 204 Excitonic effects, 70 Exploitation of TiO2 in ceramic, 464 467 ceramic surface with titania powder coating, 466f ceramic tile surface with TiO2-based coating, 465f Exposed concrete, 455 Exposure route, 379 383 External quantum efficiency (EQE), 185 186, 201 F F-doped TiO2 powders, 101 FA. See Formic acid (FA)

Index

Fabrication of heterojunctions, 222 224 methods, 512 521 of nanoscale sensors, 569 571 Fabry Perot mode derives, 500 FAE. See Follicle-associated epithelium (FAE) Fast switching memory, 511 FDA. See US Food and Drug Administration (FDA) FDC. See 2,5-Furandicarboxaldehyde (FDC) FDCA. See 2,5-Furandicarboxylic acid (FDCA) Feldspar, 461 Fermi level, 37, 141, 176 Fermi Dirac distribution, 176 177 FF. See Fill factor (FF) FFC. See Fray Farthing Chen (FFC) FFC-Cambridge process, 438 444, 439f FFCA. See 5-Formylfuran carboxylic acid (FFCA) Fill factor (FF), 192 Filter salt, 328 Filtration, 587 Fischer Tropsch synthesis (FTS), 637 638, 651 653, 653f Flavylium cation, 184 185 Flocculation, 587 Flow channel, geometry of, 292 Flumequine, 585 586 Fluorescence phenomenon, 141 142 Fluorine, 412 413 Fock exchange energy, 68 Follicle-associated epithelium (FAE), 381 Food additive, TiO2 as, 353 354 in food, 354 355, 356f TiO2 influence on human health, 355 357 Food industry, 326 327 titanium dioxide based photocatalysts for, 354 titanium dioxide based pigments for, 354 355 Food preservation, TiO2 for active packaging, 361 362 antibacterial effects, 357 360 ethylene degradation, 360 361 photoinduced TiO2 mechanisms, 358f

Index

Forced air, TiO2 photocatalysis with, 591 600 Forced convection, 287 292 geometry of flow channel, 292 particle loading and Re, 290 291 particle size, 291 temperature, 291 Formaldehyde, 550, 585 Formic acid (FA), 612 FORMING process, 510 511 5-Formylfuran carboxylic acid (FFCA), 662 Fouling, 267 268 Fourier-transform IR spectroscopy, 399 Fray Farthing Chen (FFC), 438 439 “Free Mg”, 437 French Agency for Food, Environmental and Occupational Health and Safety (ANSES), 357 Frenkel defects. See Interstitial defects Fresnel laws, 498 FTS. See Fischer Tropsch synthesis (FTS) Full width at half maximum (FWHM), 110 111 Functional cement-based materials, TiO2 use for, 451 458 air-purifying cement-based materials, 452 456 antimicrobial cement-based materials, 458 self-cleaning cement-based materials, 456 458 water-purifying cement-based materials, 456 Functional materials characteristics and performance of CEs, 191 192 characteristics and performance of electrolytes, 192 193 Functionalized amines, 619 620 Functionalized monolayers, 568 569 Fungi, 354, 358 adverse effect of TiO2 on, 378 2,5-Furandicarboxaldehyde (FDC), 611 2,5-Furandicarboxylic acid (FDCA), 611, 662, 663f Fuyao Glass Industry Group Co. Ltd. UV, 406 FWHM. See Full width at half maximum (FWHM)

689

G GAD. See Glyceraldehyde (GAD) γ-alumina (Al2O3), 108 Gas sensors, 534 551. See also Biosensors; Sensors for environmental applications CO, 543 544 CO2, 541 542 dihydrogen (H2), 538 539 dioxygen (O2), 539 541 H2O (humidity), 536 538 NH3, 542 543 NO2, 544 545 operation conditions of TiO2-based NO2 sensors, 545t VOCs, 545 551 Gaseous environments, 553 554 GC. See Glassy carbon (GC) Geikielite ((Mg,Fe)TiO3), 312 Generalized gradient approximation (GGA), 68 Geopolymers, TiO2 in, 461 462 GGA. See Generalized gradient approximation (GGA) Glass functionalization by TiO2-based selfcleaning coatings doped TiO2-based coatings for improved self-cleaning ability, 408 414 multilayer coatings for multifunctional glass, 414 420 principle behind self-cleaning behavior, 395 401 self-cleaning glass applications and commercial products, 402 408 self-cleaning process on TiO2 surface, 402f Glassy carbon (GC), 567 Glucose, 554 555 Glucose oxidase (GOx), 554 Glutamate, 560 Glutamate dehydrogenase, 560 Glutamate oxidase, 560 Glyceraldehyde (GAD), 612 GO. See Graphene oxide (GO) Gold (Au), 221, 417, 654 Gold nanoparticles deposited polyaniline TiO2 nanotube, 561 Gold nanorods (Au NRs), 554 GOx. See Glucose oxidase (GOx) Grading system, 322

690

Gram-negative bacteria, 354, 358, 376 Gram-negative E. coli, 376 Gram-positive bacteria, 354, 358, 375 376 Graphene oxide (GO), 137 Graphene-based materials, 244 Gratzel cells, 488 489 Gr¨atzel-type solar cells, 94 Gut epithelium models, 381 H H-exchanged titanate nanobelts, 547 HAADF-STEM. See High-angle annular dark-field scanning TEM (HAADFSTEM) Hamilton and Crosser model, 277 Hamiltonian, 501 Hardened concrete, modification of properties of, 460 461 HCOOH, 621 622 HDS. See Hydrodesulfurizations (HDS) Heat conduction in TiO2 nanofluids, 276 285 particle cluster size and shape on thermal conductivity, 279 284 particle load, 278 surfactants, 285 temperature, 278 279 thermal conductivity of base fluid, 279 ultrasonic treatment, 285 Heat convection in TiO2 nanofluids, 285 294 forced convection, 287 292 natural convection, 293 294 Heat sink, 299 Heat transfer applications of TiO2 nanofluids, 298 299 characteristics, 269 270 coefficient, 287 future investigations, 299 300 preparation and characterization of TiO2 nanofluids, 270 276 thermal conductivities of materials and nanofluids, 269t unit operations, 267 268 Heat treatments, 487 Heater material, 298 Heavy metals, 456 Heritage materials, 470

Index

Heteroaromatic compounds, selective oxidations of, 662 663 Heterogeneous photocatalysis, 587 588, 593, 609, 629 630 alkylation reactions, 626 630 partial oxidation reactions, 610 615 partial reduction reactions, 615 625 Heterojunctions, 67, 76 77 fabrication of, 222 224 Hexachlorocyclopentadiene, 585 586 Heyd Scuseria Ernzerhof functional (HSE functional), 69 High-angle annular dark-field scanning TEM (HAADF-STEM), 123 High-energy ball milling, 271 High-quality doped-TiO2 films, 414 High-resolution TEM images (HR-TEM images), 121 122 High-temperature molten salts, 438 444 High-titanium slag, 430 432 HMF. See 5-Hydroxymethyl-2-furfural (HMF) HMFCA. See 5-Hydroxymethyl-2furancarboxylic acid (HMFCA) Hollow TiO2-modified RGO microspheres, 559 Home-prepared TiO2 samples (HP TiO2 samples), 610 HP TiO2 samples. See Home-prepared TiO2 samples (HP TiO2 samples) HR-TEM images. See High-resolution TEM images (HR-TEM images) HRS/LRS ratio, 515 516 HSE functional. See Heyd Scuseria Ernzerhof functional (HSE functional) HT29-MTX cells, 381 Human health, TiO2 influence on, 355 357 Humidity, 536 538 Hybrid functionals, 68 69 Hydration speed, 459 460 Hydrocarbons, 229 Hydrochloric acid (HCl), 118 120, 644 Hydrodesulfurizations (HDS), 637 638, 646 647 Hydrogen, 131, 211 production and storage, 254 258 sensitive TiO2-based photocatalysts for H2 generation, 214 217

Index

bandgap engineering, 214 217 surface TiO2 sensitization, 217 separation of photogenerated charges in TiO2-based photocatalysts, 218 227 Hydrogen defects (H defects), 24 25 Hydrogen peroxide (H2O2), 357 359, 558 559, 649 650 Hydrogenations (HYD), 637 639, 649, 666 of double and triple carbon carbon bonds, 615 617, 616f Hydrolysis, 88 89, 487 Hydrometallurgical method, 430 Hydrophilic self-cleaning coatings, 402 Hydrophilic TiO2 films, 108 109 Hydrothermal syntheses, 96 97 Hydroxyl groups, 399 Hydroxyl radicals (
OH), 40, 338, 357 358, 600 601 Hydroxylation of aromatic compounds, 612 614, 613f 5-Hydroxymethyl-2-furancarboxylic acid (HMFCA), 662 5-Hydroxymethyl-2-furfural (HMF), 611, 662 Hypocrea lixii, 378 I IAQ. See Indoor air quality (IAQ) Ibuprofen, 585 586 IEA. See International Energy Agency (IEA) IL-1-β. See Interleukin (IL-1-β) Ilmenite (FeTiO3), 311 313 concentrate, 430 Imaging device, 121 Impedimetric-based biosensor, 559 Impregnation, 102 103 Impurities, 430 431 In vitro 3D human skin models, 365 Incapsulation model, 638 Incident-photon-to-current efficiency (IPCE), 201 Incipient wetness, 102 Indoor air purification, TiO2 photocatalysis for, 591 600 Indoor air quality (IAQ), 585 Indoor particulate matter, 585 Inductance, 528 Infrared spectrometry, 536 Ingestion, 380 381

691

Inhalation, 379 380 INS17, 354 355 Insulating layer, 512 Intensity of DRS, 136 Interleukin (IL-1-β), 379 380 Internal quantum efficiency (IQE), 201 International Agency for Research on Cancer, 378 379, 472 International Energy Agency (IEA), 169 International patents on photocatalytic ceramic tiles, 467 Interstitial defects, 19, 21 22 Interstitial Ti species, 74 75 Intrinsic defects, 71 Ionic additive, 298 Ionic liquid, 445 446 electrolytes, 445 446 Ionic liquids at room temperature. See Room-temperature ionic liquids (RTILs) Ionized gases, 271 IPCE. See Incident-photon-to-current efficiency (IPCE) IQE. See Internal quantum efficiency (IQE) Iron, 414 Iron oxide, 430 Iron sulfate (FeSO4), 318 Island growth, 517 518 ISO 22197 1, 468 ISO 22197 2, 468 ISO 22197 3, 468 ISO 27447, 468 ISO 27448, 468 ISO-10678, 468 ISO/DIS 17721 2, 468 Istrian stone sculpture, 471f Italian tiles production, 464f K K2O TiO2, 659 K2O TiO2 ZrO2, 659 Kaolinite, 461 Kelvin equation, 132 KeraSkin, 365 Ketoprofen, 324 Klebsiella pneumoniae, 375 Kroll process, 431 432, 438 from TiO2 to Ti, 433 438

692

Kroll process (Continued) reaction vessel of titanium reduction process, 435f titanium sponge preparation through, 434f Krypton, 131 Kubelka Munk function, 37, 136 L La0.8Sr0.2Co0.5Ni0.5O3 perovskite (LSCNO perovskite), 543 Lactobacillus acidophilus, 358t Lambert Beer law, 196 197 Langmuir Hinshelwood kind, 600 Large-sized CdTe COOH quantum dots, 555 556 La TiO2 photocatalysts, 95 96 Lattice distortion, 26 LDA. See Local density approximation (LDA) Leaching of nanoparticles, 470 472 Lead (Pb), 585 Leucoxene (Fe2O3
TiO2), 312 Lewis acidity, 661 Li-ion insertion and extraction, 250 Light-harvesting efficiency (LHE), 197 LIGHT2CAT, 457 Limit of detection (LOD), 529 Line defects, 21 Lipid peroxidation activation, 379 380 Liquid salts, 445 446 Listeria monocytogenes, 360, 560 561 Loading cocatalysts on TiO2, 225 227 Local density approximation (LDA), 68 Localization, 75 Localized surface plasmon resonance (LSPR), 533 534, 554 LOD. See Limit of detection (LOD) LoE glass. See Low-emission glass (LoE glass) Low-emission glass (LoE glass), 405 Low-temperature electrolytic production, 445 LSCNO perovskite. See La0.8Sr0.2Co0.5Ni0.5O3 perovskite (LSCNO perovskite) LSPR. See Localized surface plasmon resonance (LSPR) Luminescence, 141 Luminous transmittance, 417

Index

M m-aminobenzenesulfonic acid (m-ABS), 621 622 MacMillan’s chiral secondary amine, 630f Magne´li phases, 518 519 Magnetic beads urease/graphene oxide/ titanium dioxide based biosensor, 560 Magnetic nanoparticle-based biosensing, 552 553 Malvidine (Mv), 184 185 Manganese (Mn), 414 MAPbI3. See Methylammonium lead halide perovskite (MAPbI3) MAPTMS. See Methacryloxypropyltrimethoxysilan (MAPTMS) Maxwell expression for effective thermal conductivity of composite materials, 276 MB dyes. See Methyl blue dyes (MB dyes) 4-MBA. See 4-Methoxybenzyl alcohol (4MBA) MCMs. See Microcavity modes (MCMs) 4-MeBA. See 4-Methylbenzyl alcohol (4MeBA) Mechanical agitation, 272 Mechanical alloying. See High-energy ball milling Mechanical attrition. See High-energy ball milling Mechanical properties of TiO2, 42 45 Mechanical strength of concrete, 460 461 MeCN. See Acetonitrile (MeCN) Memory resistor (Memristor), 507 fabrication methods and performances, 512 521 ALD, 516 519, 517f anodizing, 512 516, 514f sputtering, 519 521, 519f working principle, 508 Memristor. See Memory resistor (Memristor) MeOH. See Methanol (MeOH) Mesoporous anatase TiO2 nanospheres, 133 Mesoporous indium-doped TiO2/WO3 nanohybrid, 546 Metal akoxides (M(OR)4), 487 Metal ions detection, 568 569 Metal nanoparticles, 638 Metal organic CVD (MOCVD), 107 Metal oxide/TiO2 composite, 637 638

Index

Metal oxides, 638 Metal to ligand charge transfer (MLCT), 174 175 Metal/insulator/metal structure (MIM structure), 507 Metal/nonmetal codoping, 411, 412f Metal/TiO2 composite, 637 638 Metallothermic reduction process, 431 432 Methacryloxypropyltrimethoxysilan (MAPTMS), 568 569 Methanol (MeOH), 615 616, 628 629 photocatalytic reforming, 229 4-Methoxybenzyl alcohol (4-MBA), 610 Methyl blue dyes (MB dyes), 602 Methyl chloroform, 585 586 Methyl orange dyes (MO dyes), 602 Methylammonium lead halide perovskite (MAPbI3), 569 4-Methylbenzyl alcohol (4-MeBA), 610 MgCl2, 436 437 Mg MgCl2 Ti system, 437t MI polymers (MIPs), 567 Micro-electromechanical systems, 553 554 Microalgae, 657 Microcavity modes (MCMs), 498 501 Microcavity reflectivity dispersion map, 500f Microorganisms, 354 characteristics effect, 358t toxicity effect of TiO2 on, 377f MicroRNA (miRNA), 556 Microwave irradiation, 99 100 Mie theory, 321 MIM structure. See Metal/insulator/metal structure (MIM structure) Mineral oils, 456 MIPs. See MI polymers (MIPs) miRNA. See MicroRNA (miRNA) MIT. See Molecular imprinting technique (MIT) Mixed conductivity, 540 sensors, 532 533 MLCT. See Metal to ligand charge transfer (MLCT) MO dyes. See Methyl orange dyes (MO dyes) Mo wire, 444 MOCVD. See Metal organic CVD (MOCVD)

693

Molecular imprinting technique (MIT), 559, 567 TiO2 in, 567 568 Molybdenum/alumina titania (NiMo/AT), 646 647 Monohydrate, 329 Monolayer volume, 129 Monomer addition from solution, 93 Moore filters, 318 Morphologies of TiO2, 26 28 MPTiO2, 381 MSE process, 440 MSWs. See Bloch surface waves (MSWs) Mucor circinelloides, 378 Mucus-secreting regular epithelium, 381 Multifunctional materials, 449 cement-based materials, TiO2 in, 449 451 ceramic tiles, TiO2 in, 462 468 cultural heritage conservation, TiO2 in, 468 470 environmental and health concerns in use of TiO2, 470 473 geopolymers, TiO2 in, 461 462 Multilayer coatings for multifunctional glass multilayer structures for improved self-cleaning and antireflective ability, 414 417 self-cleaning and energy-saving multilayer structures, 417 420 Multiwalled carbon nanotubes (MWCNTs), 557 Mycobacterium tuberculosis, 375 N N-(2-cyclohexenyl)aniline, 625 N-doped TiO2, 410 411, 416 417, 590 591, 598, 622 624 films, 139 141 N-doping, 256 257, 411 4-NA. See 4-Nitroaniline (4-NA) Nano-indentation, 45 Nano-TiO2, 460, 472 cosmetic products, 364 modifying the properties of concrete, 459 460 modifying the properties of hardened concrete, 460 461 Nano-titania, 459 460, 472

694

Nanocrystalline powders, 539 Nanoelectromechanical systems, 553 554 Nanofluids, 46, 268 270 Nanomaterials (NMs), 461, 529 530, 533 534, 564 based on LSPR phenomena, 571 of TiO2, 249 Nanoparticles (NPs), 78, 182, 215, 363 364, 378 380, 459 460, 472, 533 534, 545 546 in food products, 354 preparation, 270 272 size measurements, 274 275 Nanorods, 545 546 Nanoscale sensor fabrication, 569 571 Nanosheets array (NSA), 552 554 Nanosized metal-oxide semiconductors, 569 Nanostructured frame-type TiO2 photoanode, 490 Nanostructured-mixed Ti/Fe oxide thin films, 543 Nanostructures, 339 Nanotechnology, 241, 268 Nanotubes (NTs), 515, 545 546, 552 553 electrodes, 247 248 TiO2, 539 Nanowires (NWs), 545 546, 552 553 array electrode, 248 249 NWs-based FETs, 553 Naphthalene, 628 629 National Institute for Occupational Safety, 378 379 Natural convection, 293 294 factors influencing natural convection heat transfer of TiO2 nanofluids, 293 294 nanoparticle type and load, 294 Natural dyes, 182 189 anthocyanins, 183 186 betalains, 186 187 chlorophylls, 187 188 Natural rutile crystals, 13 4-NBA. See 4-Nitrobenzyl alcohol (4-NBA) Near-infrared (NIR), 338, 417 Neat Glass produced by Cardinal Glass Industries, 405, 406f Neisseria gonorrhoeae, 375 Newton law, 287 Ni-doped TiO2, 467 “Niche” research, 499

Index

Nickel (Ni), 414 NiMo/AT. See Molybdenum/ alumina titania (NiMo/AT) NIR. See Near-infrared (NIR) Nitric oxide (NO), 452 4-Nitroaniline (4-NA), 625 4-Nitrobenzyl alcohol (4-NBA), 610 Nitrogen, 131, 409 410 Nitrogen dioxide (NO2), 452, 544 545, 585, 592 operation conditions of TiO2-based NO2 sensors, 545t Nitrogen oxides (NOx), 452 removal catalytic oxidation, 643 644 selective catalytic reduction, 641 643 Nitrogen-doped TiO2, 215 NMs. See Nanomaterials (NMs); Noble metals (NMs) Noble metals (NMs), 220 nanoparticles deposition, 220 222 “Non-Debye” capacitor concept, 537 Nonaqueous electrolytes, 515 Nondestructive procedure, for wastewater treatment, 586 Nonenzymatic PEC biosensor, 555 NPs. See Nanoparticles (NPs) NPTiO2, 381 NSA. See Nanosheets array (NSA) NTs. See Nanotubes (NTs) Nucleate pool boiling, 295 296 Nucleation, 92 93 Nusselt number, 288 NWs. See Nanowires (NWs) Nyquist plot, 203 204 O ODH. See Oxidative dehydrogenation (ODH) OFET. See Organic FET (OFET) Olefin epoxidation, 663 665 One-dimensional (1D) brookite nanoneedles, 484 Ono Suzuki process (OS process), 440, 441f OP. See Organophosphorus pesticides (OP) OPP. See Oriented polypropylene (OPP) Optical band gap, 135 TiO2 of, 492 493

Index

Optical biosensor, 560 Optical electron transfer, 42 Optical properties of TiO2, 35 39, 492 493 Optical sensing, 533 534 Orbitals, occupation of, 176 177 Ordinary Portland cement, 449 450 Organic acids, 229 Organic dyes, 190 Organic FET (OFET), 543 Organic pollutants detection, 563 565 TiO2 photocatalysis for organic pollutants removal, 600 602 Organomodified commercial nano-TiO2 particles, 456 Organophosphorus pesticides (OP), 557 Organotitanate precursors, 595 Oriented polypropylene (OPP), 361 OS process. See Ono Suzuki process (OS process) Ostwald ripening, 93 Oxidation of alcohols to aldehydes, 610 612 Oxidative dehydrogenation (ODH), 658 659 Oxidative destructive procedure, for wastewater treatment, 586 Oxidative TiO2 photocatalytic power, 360 Oxide layer in memristor, 507 (Oxy)dehydrogenations, 658 660 Oxygen (O2), 398 399, 614 615, 640 641 oxygen-to-argon ratio, 485 photoinduced dissociation, 641f vacancies, 23 24, 72 74 Oxysulfide, 226 Ozone (O3), 359 P p-chlorobenzene, 585 586 p-phenylenediamine (PPD), 625 PAHs. See Polycyclic aromatic hydrocarbons (PAHs) Paints, 321 322 PANI. See Polyaniline (PANI) Paper, 322 324 Parkinson’s disease, 558 559 Partial oxidation reactions, 610 615 epoxidation of alkenes, 614 615 of HMF to FDC, 611f

695

hydroxylation of aromatic compounds, 612 614, 613f oxidation of alcohols to aldehydes, 610 612 selectivity toward FDC, 611t Partial reduction reactions, 615 625 hydrogenation of double and triple carbon carbon bonds, 615 617, 616f hypothesized pathways of photocatalytic hydrogenation, 617f hypothesized photocatalytic semihydrogenation pathways, 618f N-containing functional groups reduction, 619 625 photocatalytic formation of benzimidazoles, 623f photocatalytic reduction of benzophenone, 619f of nitroarenes, 624f of nitrobenzenes, 620f, 621f photocatalytic results, 618t photochemical reduction of benzophenone, 619f primary alcohol yields, 620t reduction of carbonyls, 617 619 Particle cluster size and shape on thermal conductivity, 279 284 Particle load(ing), 278, 290 291, 296 297 Particle size, 291, 363 364 Patents on cement-based materials with TiO2, 461 Pauli exclusion principle, 176 177 Pavlova microalgae, 658 Pb-carbon QD-TiO2 nanocomposite, 568 PBE functional. See Perdew Burke Ernzerhof functional (PBE functional) PBE0 functional, 68 69 PCE. See Perchloroethylene (PCE); Photoconversion efficiency (PCE) PCL. See Polycaprolactone (PCL) PE. See Polyethylene (PE) PEALD. See Plasma-enhanced ALD (PEALD) PEC devices. See Photo-electrochromic devices (PEC devices) Pelargonidine (Pg), 184 185 Pentachlorophenol, 585 586 Peonidine (Pn), 184 185

696

Perchloroethylene (PCE), 585 586 Perdew Burke Ernzerhof functional (PBE functional), 68 Perfluoroalkylated products, 628 629 Permeable concrete, 456 Perovskite (CaTiO3), 312, 491 492 photo-voltachromic device, 492 Perovskite solar cells (PSCs), 258 259, 491 492 Personal care products regulations, 364 safety of sunscreens, 364 365 TiO2 in, 362 365 Pesticides, 556 557, 563, 585 Petajoule (PJ), 169 Petroleum coke, 434 435 Petunidine (Ptn), 184 185 PG. See Propylene glycol (PG) pH measurements, 275 276 of system, 90 Pharmaceutical and cosmetic industries, 324 325 Phenols, 563, 612 Phosphonic acid (HPO3H2), 180 182 Phosphorescence, 142 Photo-conversion efficiency (PCE), 258 259 of solar cells, 200 Photo-electrochromic devices (PEC devices), 373 375, 488 492, 491f, 528, 565, 567 Photo-induced superhydrophilicity, 401 Photo-thermocatalysis, 637 Photoanodes, 172, 193 Photocatalysis, 357, 401 with forced air, 592 596 for indoor air purification, 591 600 in indoor environments, 596 600 for organic pollutants removal from water and wastewater, 600 602 TiO2, 589f for VOC removal from gaseous stream, 589 591 Photocatalyst deactivation, 594 Photocatalytic Cl2 production, 646f Photocatalytic effect, 592 Photocatalytic films, 104 Photocatalytic formation of benzimidazoles, 623f

Index

Photocatalytic hydrogen production, 212 sacrificial agents in photocatalytic hydrogen production, 228 229 sensitive TiO2-based photocatalysts for H2 generation, 214 217 separation of photogenerated charges in TiO2-based photocatalysts, 218 227 Photocatalytic membrane reactor, 613f Photocatalytic reductions, 615 of benzophenone, 619f of nitroarenes, 624f of nitrobenzenes, 620f, 621f Photocatalytic reforming of organics, 229 Photocatalytic routes, 589 Photocatalytic TiO2 nanoparticles, 450, 468 Photocatalytic water splitting with TiO2, 212 214 Photochemical reduction of benzophenone, 619f Photoconductive devices, 534 Photoconversion systems, 169 Photodynamic therapy, 339 Photoelectrochemical devices, 169 Photoelectrochemical sensing, 534 Photoexcited carriers, 75 76 Photoexcited charge carriers separation in titania nanocomposites, 76 78 Photoexcited electron hole couple, 76 Photoinduced electron transfer, 41 42 Photoluminescence spectroscopy (PL spectroscopy), 88, 141 144 Photon(s), 38 39 photon-induced behavior, 39 42 transfer, 594 595 Photosensitization of TiO2, 41 42 Photosensitizers (PSs), 339 Photovoltaics, 169 171, 373 375 Phthalocyanines, 187 188 Physical vapor deposition (PVD), 108 109, 270 271 PICADA, 457 Pickling process, 430 431 Pigment White 6 (PW6), 319 applications, 321 327 characteristics, 320t properties, 319 321 Pilkington Activ Bronze and Blue, 395 Pilkington Activ Clear/Blue/Neutral by Pilkington Group Limited, 405

Index

Pilkington Glass, 395 Pilkington Group Limited, 405 PJ. See Petajoule (PJ) PL spectroscopy. See Photoluminescence spectroscopy (PL spectroscopy) PLA. See Polylactic acid (PLA) Planar nanoparticles, 27 Planck’s law, 213 Plasma-enhanced ALD (PEALD), 518 519 Plastic(s), 321 322 industry, 322 PO. See Propene oxide (PO) Point defects in TiO2, 19 20 Polarons, 71 Pollutants degradation, 594 Pollution, 451 452 Poly(levodopa), 568 Polyaniline (PANI), 542 Polycaprolactone (PCL), 362 Polycondensation reactions, 88 89 Polycrystalline SMOs, 532 Polycyclic aromatic hydrocarbons (PAHs), 456, 563 Polyethylene (PE), 362 Polylactic acid (PLA), 362 Polyoxometalates (POMs), 550 Polypyridine complexes of Ru(II), 180 182 Polypyrrole (PPy), 538 Polypyrrolepropylic acid (PPyA), 561 Polysaccharides, 375 Polystyrene (PS), 123 Polyvinyl chloride (PVC), 322 Polyvinylpyrrolidone (PVP), 148 150, 272 274, 361 POMs. See Polyoxometalates (POMs) Porcelain tiles (BIa), 463 Pore diameter distribution curve, 132 Porous ceramic, 537 Porous polycrystalline silicon (PPS), 542 Porous TiO2 nanosphere, 556 Porphyrins, 187 188 Porphyromonas gingivalis, 344 Potassium hydroxide (KOH), 197 Potassium iodide, 622 624 Powdered TiO2-based materials deposition precipitation method, 103 104 hydrothermal and solvothermal syntheses, 96 97

697

impregnation, 102 103 microwave irradiation, 99 100 precipitation and coprecipitation, 90 96 preparation methods of, 88 104 sol gel process, 88 90 sonochemical method, 97 98 spray pyrolysis, 100 101 PPD. See p-phenylenediamine (PPD) PPG Glass, 395 PPG residential Glass, 406 407 PPS. See Porous polycrystalline silicon (PPS) PPy. See Polypyrrole (PPy) PPyA. See Polypyrrolepropylic acid (PPyA) Precipitation, 90 96 Precursor materials, 93 Preparation methods, 88 109 of powdered TiO2-based materials, 88 104 of TiO2 film, 104 109 Pressed wood products, 585 Printing inks, 322 324 Propene (15 Torr), 614 615 Propene oxide (PO), 614 615 Propylene glycol (PG), 272 274 PS. See Polystyrene (PS) PSCs. See Perovskite solar cells (PSCs) Pseudobrookite (Fe2TiO5), 312 Pseudomonas, 360, 362 P. aeruginosa, 375 PAO1 cells, 376 PSs. See Photosensitizers (PSs) Pure TiO2 powders, 465 466 PVC. See Polyvinyl chloride (PVC) PVD. See Physical vapor deposition (PVD) PVP. See Polyvinylpyrrolidone (PVP) PW6. See Pigment White 6 (PW6) Pyrolysis oils, upgrading of, 657 658 Pyrometallurgical treatment method, 430 Pyrophanite (MnTiO3), 312 Q Quantum efficiency, 201 203 Quantum satis, 354 355 Quantum size effect, 33 R (5R)-menthyloxy-2[5H]furan reaction, 627f Radiative energy, 169 170

698

Radiator, 299 Radical addition of tertiary amines, 626 627 Radon (Rn), 585 Raether Kretschmann prism coupling configuration, 499 RajiB cells, 381 Raman spectroscopy, 17, 193 194 REACH. See Registration, Evaluation, Authorization and Restrict on of Chemicals (REACH) Reaction energy, 528 involving titania-based catalyst, 641 666 aerobic olefin oxygenation, 665f biofuels production, 656 658 CO2 methanation, 655 656, 655f Deacon process, 644 646 direct synthesis of hydrogen peroxide, 649 651 FTS, 651 653, 653f hierarchical TS-1 zeolites, 664f hydrogenations, 666 NOx removal, 641 644 olefin epoxidation, 663 665 (oxy)dehydrogenations, 658 660 reactions with sulfur-rich compounds, 646 649 selective oxidations of alkanes, alcohols, and aromatics, 660 662 selective oxidations of ammonia to nitrogen, 666 selective oxidations of heteroaromatic compounds, 662 663 water gas shift reaction, 653 654 with sulfur-rich compounds, 646 649 Claus process, 647 649 hydrodesulfurization processes, 646 647 photocatalytic Cl2 production, 646f Reactive magnetron sputtering, 414 Reactive oxygen species (ROS), 338, 354, 359f, 364 365, 472, 558 559, 587 Recombination, 594 595 Red gypsum (CaSO4
2H2O), 328 Redox mediators, 192 193 Reduced graphene oxide (RGO), 120 121, 557 Reduced titania, 70

Index

Reduction step, 318 Reflectance of “infinitely” thick sample, 136 value, 494 Reflection, 320 Refraction, 320 Refractive index (RI), 3 value of TiO2, 493f Registration, Evaluation, Authorization and Restrict on of Chemicals (REACH), 378 379 Regular epithelium, 381 Relative humidity (RH), 537 Renders, 452 Renew by Viridian Glass, 407 408 ReRAM devices. See Resistive random access memory devices (ReRAM devices) Resistance, 204, 528 Resistive random access memory devices (ReRAM devices), 507 508 fabrication methods and performances, 512 521 ALD, 516 519, 517f anodizing, 512 516, 514f sputtering, 519 521, 519f performance comparison, 513t working principle, 508 Resistive switching, 508 512 ECMs, 508 510, 509f, 511t VCMs, 510 512, 510f, 511t Resistive-type gas sensors, 532 533 humidity sensors, 538 Response time, 529 Reversible hydrophilic changes of TiO2 films, 399f RGO. See Reduced graphene oxide (RGO) RH. See Relative humidity (RH) RhB. See Rhodamine B (RhB) Rheological properties of TiO2, 46 Rheology, 459 460 Rhodamine B (RhB), 418 419, 457 RhodopsinP23H mouse retinas, 570 571 Rhodotorula mucilaginosa, 362 RI. See Refractive index (RI) Rohsenow correlation, 295 296 Room-temperature ionic liquids (RTILs), 192 193, 567 ROS. See Reactive oxygen species (ROS)

Index

Roughness factor, 196 197 RTILs. See Room-temperature ionic liquids (RTILs) Ruthenium pyridine complexes, 598 Ruthenium(IV) oxide (RuO2), 225 Rutile, 16, 18, 29, 177 180, 311, 313, 319, 483 anatase polymorphic transformation to, 465 structural, optical, and electrical properties, 34t TiO2, 640 641, 641f nanoparticles, 98 S S-band, 176 Saccharides, 611 Saccharomyces cerevisiae, 358t Sacrificial agents in photocatalytic hydrogen production, 228 229 Saint-Gobain Glass UK Ltd, 407 Salmonella typhimurium, 360, 560 561 Scanning electron microscopy (SEM), 88, 115 121 Scherrer equation, 16 17, 110 111 Schottky electrode, 510 511 Schottky junction fabrication, 220 222 Schottky-barrier sensors, 540 Science for Environment Policy, 449 Scientific Committee on Consumer Safety (SCCS/1580/16), 364 SCR. See Selective catalytic reduction (SCR) Screen-printed electrode (SPE), 567 Screw-type dislocations, 21, 21f SDBS. See Sodium dodecyl benzene sulfonate (SDBS) SDS. See Sodium dodecyl sulfate (SDS) Secondary electrons (SEs), 116 117 Sedimentation, 587 Selective catalytic reduction (SCR), 584 585, 637 638 of NOx, 641 643 Selective noncatalytic reduction (SNCR), 584 585 Selective oxidations of alkanes, alcohols, and aromatics, 660 662 of ammonia to nitrogen, 666

699

of heteroaromatic compounds, 662 663 Self-cleaning behavior, 457 cement-based materials, 456 458 coatings, 395, 402 commercial glass products with, 403t by Fuyao Glass Industry Group Co. Ltd. UV, 406 glass, 395 applications and commercial products, 402 408 commercial self-cleaning glasses, 405 process on TiO2 surface, 402f structures, 417 420 Self-trapping energy, 71, 76 SEM. See Scanning electron microscopy (SEM) Semiconductor metal oxides (SMOs), 530, 534 535 polycrystalline, 532 Semiconductors, 176 180 bands formation, 176 occupation of orbitals, 176 177 sensor, 529f titanium dioxide, 177 180 Sensing mechanism, 531 534 optical sensing, 533 534 photoconductive devices, 534 photoelectrochemical sensing, 534 resistive-type gas sensors, 532 533 Sensitive biosensors, 559 Sensitivity, 529 Sensor(s), 528 530 biosensors, 552 562 for environmental applications, 562 569 construction of immobilization-free PEC aptasensor, 566f detection of dyes, 565 567 detection of organic pollutants, 563 565 metal ions detection, 568 569 TiO2 in molecular imprinting technology, 567 568 gas sensors, 534 551 sensor field, TiO2 in, 530 534 mechanism of sensing, 531 534 Separated architectures, 488 490 SERS. See Surface-enhanced Raman scattering (SERS)

700

SEs. See Secondary electrons (SEs) Silica gel (SiO2), 108 SiO2-supported TiO2 materials, 95 Silver (Ag), 417 silver-doped TiO2, 409 Singlet oxygen, 40 Skin penetration, 381 383 Small-sized CdTe NH2 QDs, 555 556 Smart materials, 337 Smart properties of titanium dioxide based nanomaterials, 338 343 Smart sensor for toluene detection, 551f SMOs. See Semiconductor metal oxides (SMOs) SMSI. See Strong metal support interaction (SMSI) SNCR. See Selective noncatalytic reduction (SNCR) Snell’s law, 496 498 SOD. See Superoxide dismutase (SOD) Sodium dodecyl benzene sulfonate (SDBS), 106, 272 274 Sodium dodecyl sulfate (SDS), 272 274 Sodium iodide, 622 624 Soft nanoimprinted titanium dioxide, 569 Soiling, 457, 469f Solar constant, 171 Solar fuels, 211 Solar spectrum, 170 Sol gel method, 88 90, 270 271, 487, 555 Solid oxide oxygen ion conducting membrane process (SOM process), 443 444 electrolytic cell, 443f SOLID oxide oxygen ion conducting YSZ membranes, 444f SOM-assisted electrolysis process, 444 SOM-assisted molten salt electrolytic cell, 444f Solid-state gas sensors, 536 TiO2-based EC device, 487 51 V NMR spectroscopy, 642 Solvothermal syntheses, 96 97 SOM process. See Solid oxide oxygen ion conducting membrane process (SOM process) Sonochemical method, 97 98 SPE. See Screen-printed electrode (SPE)

Index

Spectroscopic techniques, 193 194 characteristic I-V curves, 197 201 cyclic voltammetry, 194 196 electrochemical impedance spectroscopy, 203 204 quantum efficiency, 201 203 Raman spectroscopy, 193 194 roughness and desorption factor, 196 197 Tafel electroanalysis, 204 206 UV-vis and TiO2 emission spectroscopy, 194 Speed, sensitivity, selectivity, and stability characteristics (“4S” characteristics), 528 Sphene (CaTiSiO5), 312 Spherical nanoparticles, 27 Spin coating, 105 106 SPR. See Surface plasmon resonance (SPR) Spray pyrolysis, 100 101 Sputtering, 519 521, 519f Square-wave voltammetry (SWV), 565 SREP. See Synthetic rutile enhancement process (SREP) Standards for ceramic tiles, 468 Staphylococcus aureus, 360, 375, 560 561, 597 Stearic acid, 418 419 Stirring bead milling, 272 Streptococcus mutans, 344, 383 384 Strong acid, 318 Strong metal support interaction (SMSI), 638 640, 639f, 652 653 Structural cement-based materials, use of TiO2 for, 458 461 Structural properties of TiO2, 14 18 Substitution defects, 20 reactions in aromatic compounds, 627 629 functionalization reaction of heteroaromatic base, 628f photocatalytic selective carbamoylation of heteroaromatic bases, 628f Sugar, 183 184 Sulfamethoxazole, 585 586 Sulfate process, 315, 317 319, 317f wastes generated in, 328 329 Sulfides, 226 Sulfur, 409 410 Sulfur oxides (SOx), 452

Index

Sulfuric acid method, 431 432 Sumitomo Chemical, 644 Sumitomo/Deacon process, 645 Sunburn, 324 325 SunClean by PPG residential Glass, 406 407 Sunscreens, 325 safety of, 364 365 Supercapacitors, 243 249 significance of nanostructures, 247 249 significance of TiO2 polymorph, 244 247 Supercritical water oxidation (SWA), 586 Superhydrophilicity, 457 of TiO2 surfaces, 40 41 Superoxide anions, 338 Superoxide dismutase (SOD), 375 Superoxide radicals (
O22), 357 358 Superplasticizers, 455 Supported materials, 93 Supported TiO2, 87 Surface decoration, 80 Surface defectivity, 22 25 H defects, 24 25 O vacancies, 23 24 Ti defects, 24 Surface distortion, 26 Surface plasmon resonance (SPR), 222, 530 configuration, 533f Surface roughness, 297 298 factor, 397 Surface stresses, 26 Surface-enhanced Raman scattering (SERS), 530, 566 567 Surfactant protein (SP-D), 379 380 Surfactants, 285 SWA. See Supercritical water oxidation (SWA) Sweets, 353 354 “Switchable” wettability of TiO2 films, 398 399 SWV. See Square-wave voltammetry (SWV) Synchrotron high-pressure XRD, 42 Synthesis strategies, 412 414 dry-deposition methods, 414 wet-deposition methods, 413 414 Synthetic dyes, 180 182 Synthetic rutile enhancement process (SREP), 313 315

701

T Tafel electroanalysis, 204 206 Tafel plot, 206 Tafel polarization curve, 204 Tauc equation, 135 136 TCE. See Trichloroethylene (TCE) TCMs. See Thermochemical memories (TCMs) TCO. See Transparent conductive oxide (TCO) TEM. See Transmission electron microscopy (TEM) Temperature, 278 279, 291 Terawatt-hour (TWh), 169 Tetraisopropoxide (TTIP), 90 Tetramethylammonium hydroxide (TMAOH), 197 Tetrapyrrole, 187 188 Textiles, 325 326 TGA. See Thermal gravimetric analysis (TGA) Thermal conductivity, 267 268 of base fluid, 279 Thermal convection, 268 Thermal diffusivity, 267 268 Thermal gravimetric analysis (TGA), 88, 147 151 Thermal properties of fluids, 268 Thermo-vector fluids, 268 Thermochemical memories (TCMs), 508 Thermodynamic properties of TiO2, 29 32 Thin films, 507, 512, 515 518 technologies, 171 TiO2, 540 “Third-generation” photovoltaic cells, 171 Tight-binding density functional theory method (DFTB method), 78 79 Time-dependent hybrid DFT approaches (TD hybrid DFT approaches), 78 Time-resolved absorption spectroscopy, 39 Tin dioxide (SnO2), 177 180 Tin oxide doped with fluorine (FTO), 171 172 Tin oxide doped with indium (ITO), 171 172 Tissue engineering, 343 345 Titania, 67, 75, 171 172, 177 180 heterojunctions and nanoparticles, 76 80, 654

702

Titania (Continued) computational modeling of titania nanoparticles, 78 80 separation of photoexcited charge carriers in titania nanocomposites, 76 78 Titanite, 312 Titanium (Ti), 311, 429, 637 alloys, 438 defects, 24 halides, 445 446 metal, 429 minerals, 312 313, 312t ore, 312 ore purification, 313 315 slag, 430 production process from ilmenite to, 431f sponge preparation through Kroll process, 434f Ti 6Al 4V alloy, 441 442, 442f TiO2 production from titanium minerals, 429 432 electrodeposition of Ti in lowtemperature liquid salts, 445 446 electrolytic production of Ti from TiO2, 438 444 Ellingham diagrams, 433f Kroll process from TiO2 to Ti, 433 438 properties of typical titanium minerals, 430t TiO2 production from titanium-rich slag, 431 432 titanium-rich slag production from titanium minerals, 429 431 titanium-rich slag, 430 Titanium carbide (TiC), 552 Titanium dioxide (TiO2), 4, 13, 67, 87, 177 180, 337, 353, 507, 637 638 applications, 3 4 bulk modulus of nanocrystalline and bulk TiO2 phases, 43t cocatalysts, 222 in cosmetics and personal care products, 362 365 dependence of TiO2 bulk modulus on particle size, 45f electrical properties, 33 35

Index

electronic properties, 32 33 emission spectroscopy, 194 for energy harvesting, 242 for energy storage, 242 243 engineering, 214 215 excitation mechanisms, 35f as food additive, 353 357 for food preservation, 357 362 global TiO2 capacity, 4f market, 4 6 mechanical properties, 42 45 morphologies, 26 28 nanoforms, 363 364 nanoparticles, 353 nanowires, 560 561 optical properties, 35 39 oxidative efficacy, 354 pellet, 446f photocatalytic behavior, 373 375 photon-induced behavior, 39 42 pigments, 311 312 powders, 353 preparation and characterization of TiO2 nanofluids, 270 276 applications of TiO2 nanofluids, 298 299 nanoparticle size measurements, 274 275 nanoparticles preparation, 270 272 parameters influencing aggregation and stability of, 274 pH measurements, 275 276 preparation of nanofluids, 272 274 Z-potential measurements, 275 preparation methods, 88 109 preparation of TiO2 samples, 131 producers, 4 Raman vibrations of rutile, anatase, and brookite TiO2, 18f rheological properties, 46 routes for manufacture of titanium dioxide pigments, 315 319 chloride process, 315 317 sulfate process, 317 319 structural properties, 14 18 structures, 14f and defects, 18 26 structural data of crystalline TiO2 phases, 15t

Index

of TiO2 phases, 14 16 techniques used for TiO2 structural analysis, 16 18 thermodynamic properties, 29 32 TiO2-based ethylene scavenger systems, 361 TiO2-based materials, 87 TiO2-based photocatalysts, 212 TiO2-based sensors, 532f TiO2-coated OPP films, 362 TiO2-containing coatings, 3 4 TiO2-nanowire-based FETs, 561 TiO2-photocatalytic aromatic perfluoroalkylation mechanism, 629f TiO2/Ag0.35V2O5 nanoheterostructures, 547f, 548 TiO2/Cu/TiO2 multilayer, 417, 418f TiO2/Si/Ag(Cr)/TiNx-layered structure, 417 418 TiO2/VO2/TiO2, 418 419 TiO2(B), 16 TiO2 CeO2 nanocomposites, 102 titanium dioxide based materials, 531 titanium dioxide based nanomaterials, 337 advanced photodynamic therapy approach, 339 342 advanced sonodynamic therapy approach, 342 343 applications, 349 350 drug delivery, 345 349 smart properties of, 338 343 tissue engineering, 343 345 titanium dioxide based pigments, 354 355 world consumption, 5f X-ray patterns of TiO2 crystalline phases, 17f Titanium dioxide nanoparticles (TiO2 NPs), 373 375, 538, 542 applications, 374f toxicity assessment of, 378 383 toxicity effect on microorganisms, 377f Titanium dioxide nanotubes pillared graphene-based macrostructures (TPGBM), 133 134 Titanium isopropoxide (TTIP), 118 120 Titanium oxide, 397 398, 433 434 superhydrophilic character, 398

703

Titanium oxynitrides (TiOn nNn), 597 Titanium tetrachloride (TiCl4), 93, 431 435 magnesium thermal reduction, 436 437 Titanium-silicalite-1 (TS-1), 614 615 Titanium(II) chloride (TiCl2), 436 Titanium(III) chloride (TiCl3), 436 Titanium(IV) chloride, 316 Titanyl sulfate (TiOSO4), 318 TMAOH. See Tetramethylammonium hydroxide (TMAOH) TNF-α. See Tumor necrosis factor alpha (TNF-α) TOC. See Total organic carbon (TOC) TOF. See Turnover frequency (TOF) Toluene, 551 Total organic carbon (TOC), 587 TOTO Ltd., 467 Toxicity assessment of TiO2 NPs exposure route and biodistribution, 379 383 ingestion, 380 381 inhalation, 379 380 skin penetration, 381 383 regulations, 378 379 Toxins, 354, 358 TPGBM. See Titanium dioxide nanotubes pillared graphene-based macrostructures (TPGBM) Transducer processes, 530 Transesterification of triglycerides, 657 Transmission electron microscopy (TEM), 88, 121 126 Transmittance, 135 value, 494 Transparent conductive oxide (TCO), 171 172 Transparent oxides, 493 Transparent TiO2 layers, 109 Transverse electric polarization (TE polarization), 498 Transverse magnetic polarization (TM polarization), 498 Treatment efficacy, 357 microorganism characteristics effect, 358t Trichloroethylene (TCE), 585 586 Triglycerides, transesterification of, 657 Trimethylamine, 550 flexible gas sensor, 550f

704

Triple carbon carbon bonds hydrogenation, 615 617, 616f TTIP. See Tetraisopropoxide (TTIP); Titanium isopropoxide (TTIP) Tumor necrosis factor alpha (TNF-α), 379 380 Tungsten oxide (WO3), 416 Turnover frequency (TOF), 653 654 2D TiO2 nanosheets, 565 U U parameter, 68 Ultrasensing PEC glucose biosensor, 555 Ultrasonic treatment, 285 waves, 272 Ultrasound (US), 338 Ultraviolet (UV), 338 absorption, 319 light, 531, 595 radiation, 324 325 UV-A wavelength, 324 325 UV-B wavelength, 324 325 UV-C wavelength, 324 325 UV-vis spectroscopy, 194 Uniform resistance switching, 508 Urea, 559 560 US. See Ultrasound (US) US Environmental Protection agency (EPA), 452, 590 US Food and Drug Administration (FDA), 353 UV. See Ultraviolet (UV) V Vacancy defects, 19 20 Vacuum system, 121 Valence band (VB), 32 33, 135, 176, 212, 373 Valence change memories (VCMs), 508, 510 512, 510f, 511t Valorization of coproducts and wastes generation, 327 329 Vanadium, 414 Vanadium oxide (VO2), 417 419 Vanadium pentoxide (V2O5) V2O5/TiO2 system, 660 V2O5/TiO2/SiO2 catalysts, 661 Vapor phase deposition, 271

Index

VB. See Valence band (VB) VCCs. See Volatile chlorinated hydrocarbons (VCCs) VCMs. See Valence change memories (VCMs) Vegetable dyes, 182, 188 189 Vinyl chloride, 585 586 Viridian Glass, 407 408 Viruses, 354, 358 Viscosity, 46 Visible-light active semiconductors, 416 Volatile chlorinated hydrocarbons (VCCs), 536 Volatile organic compounds (VOCs), 452, 545 551, 583 584, 589 590 acetone, 550 ethanol, 546 549 formaldehyde, 550 TiO2 photocatalysis for VOC removal from gaseous stream, 589 591 toluene, 551 trimethylamine, 550 Volatile silicon-containing compounds (VSCCs), 594 Voltammetric techniques, 194 196 Volumetric method, 131 132, 132f VSCCs. See Volatile silicon-containing compounds (VSCCs) VST-1, 420 VST-3, 420 W WAO. See Wet air oxidation (WAO) Wastewater, organic pollutants removal from, 600 602 Water (H2O), 272 274, 460 461, 536 538, 640 641 droplets, 396 electrolysis, 211 organic pollutants removal from, 600 602 pollutants, 601 pollution, 583 586 purification, 601 splitting, 213 water-purifying cement-based materials, 456 Water gas shift reaction (WGS reaction), 651 654

Index

Wavelength-dispersive X-ray spectroscopy (WDS), 117 118 WAXS. See Wide-angle X-ray scattering (WAXS) Weak acid solution, 318 Wenzel model, 397 Wenzel’s equation, 397 Wenzel’s theory, 397 Wet air oxidation (WAO), 586 Wet impregnation, 102 Wet oxidation (WO), 586 Wet-deposition methods, 413 414. See also Dry-deposition methods Wettability regimes based on water contact angle, 396t Wetting models, 396f WGS reaction. See Water gas shift reaction (WGS reaction) Wide-angle X-ray scattering (WAXS), 17 WO. See Wet oxidation (WO) WO2001068786A1, 467

705

WO2004094341A1, 467 WO20101464108, 467 Wool, 326 X X-ray diffraction (XRD), 16, 88, 109 114 X-ray photoelectron spectroscopy (XPS), 88, 144 147, 398 399, 642 643 Xerogels, 88 89 Y Yttrium-stabilized zirconia membrane (YSZ membrane), 444 Z Z-potential measurements, 275 Zinc oxide (ZnO), 177 180 Zirconium-doped TiO2 samples (Zr-doped TiO2 samples), 94 95 ZnIn2S4, 416