Metal oxide-based photocatalysis : fundamentals and prospects for application 9780128116333, 0128116331, 9780128116340

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Metal Oxide-Based Photocatalysis

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Metal Oxides Series

Metal Oxide-Based Photocatalysis Fundamentals and Prospects for Application

Adriana Zaleska-Medynska Series Editor

Ghenadii Korotcenkov

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 © 2018 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-811634-0 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisition Editor: Kayla Dos Santos Editorial Project Manager: Andrae Akeh Production Project Manager: Swapna Srinivasan Cover Designer: Miles Hitchen Typeset by SPi Global, India

Contents

List of contributors

ix

1

Introduction Adriana Zaleska-Medynska 1.1 Introduction

1

Fundamentals of metal oxide-based photocatalysis Anna Goła˛biewska, Kobyla nski Marek P., Adriana Zaleska-Medynska 2.1 Principles of metal oxide-based photocatalysis 2.2 Requirements of metal oxides 2.3 Role of noble metals in photocatalysis References Further reading

3

2

3

4

Metal oxide photocatalysts Ewelina Grabowska, Martyna Marchelek, Marta Paszkiewicz-Gawron, Adriana Zaleska-Medynska 3.1 Synthesis of metal oxides used in photocatalysis 3.2 Unitary metal oxides (advantages, disadvantages, achievements) 3.3 Nanostructured MeIOx/MeIIOx heterojunction photocatalysts 3.4 Metal/MeOx photocatalysts 3.5 Composite-based photocatalysts 3.6 Outlook References Further reading Application of metal oxide-based photocatalysis Bajorowicz Beata, Kobyla nski Marek P., Malankowska Anna, Mazierski Paweł, Nadolna Joanna, Pieczy nska Aleksandra, Adriana Zaleska-Medynska 4.1 Water treatment 4.1.1 Types of contamination 4.1.2 Photocatalyst immobilization 4.1.3 Photocatalytic reactors 4.1.4 Effect of reaction parameters on the photocatalytic process 4.1.5 Scale-up process and prospects References

1

3 23 34 38 49 51

51 65 123 140 150 155 157 209 211

211 212 213 214 216 220 220

vi

Contents

Further reading 4.2 Hydrogen production 4.2.1 Basic principle of photocatalytic hydrogen generation by water splitting 4.2.2 Strategies for achieving photocatalytic water splitting 4.2.3 Brief overview of metal oxide semiconductors in photocatalytic water splitting 4.2.4 Chemical additives for H2 and O2 production enhancement 4.2.5 Metal oxide co-catalysts for photocatalytic water splitting 4.2.6 Evaluation of photocatalytic water splitting 4.2.7 Photoreactor design 4.2.8 Conclusions and prospects References Further reading 4.3 Air depollution and volatile organic compound (VOC) removal using different photocatalysts 4.3.1 The mechanism of pollutant photo-oxidation 4.3.2 Photodisinfection of pathogens 4.3.3 Factors that impact the photocatalytic oxidation process 4.3.4 Immobilization techniques 4.3.5 Photoreactor design for photocatalytic air purification 4.3.6 Conclusions and future prospects References 4.4 Hydrocarbon generation (CO2 reduction) 4.4.1 Fundamental aspects for photocatalytic reduction of CO2 4.4.2 Photocatalyst design for photocatalytic CO2 reduction 4.4.3 Important factors for photocatalytic reduction of CO2 4.4.4 Photoreactors for CO2 reduction 4.4.5 Conclusions and prospects References 4.5 Photochemical transformation of specific compounds 4.5.1 Types of photocatalysts 4.5.2 Role of external conditions 4.5.3 Synthesis of organic compounds 4.5.4 Reactors for phototransformation 4.5.5 Scaling-up process and prospects References 4.6 Medical applications: Application in photodynamic therapy 4.6.1 Phocatalysts 4.6.2 Light penetration 4.6.3 Conclusions and perspectives References 4.7 Applications in construction, building materials, and textiles 4.7.1 Introduction

222 223 223 224 225 226 227 228 229 232 233 237 237 239 243 244 245 246 248 248 252 252 256 258 259 261 262 265 267 270 272 274 275 276 279 280 283 284 285 287 287

Contents

4.7.2 4.7.3 4.7.4 4.7.5 4.7.6

4.8 4.8.1 4.8.2 4.8.3

4.9 4.9.1 4.9.2 4.9.3 4.9.4 4.9.5

5

vii

Superhydrophilicity mechanism Exterior construction materials Interior construction materials New applications Perspectives, problems, and limitations References Further reading Photoelectrocatalysis (PEC) for energy generation Photoelectrocatalysts for solar energy conversion PEC hydrogen generation Conclusions and prospects References Further reading Integration and coupling of heterogeneous photocatalysis with other advanced oxidation processes Photocatalytic ozonation Photoelectrocatalysis Heterogeneous photo-Fenton and photoelectro-Fenton methods Sonophotocatalysis Conclusions and prospects References Further reading

Outlook and prospects Adriana Zaleska-Medynska

Index

288 290 293 294 295 296 297 297 297 309 312 312 319 319 320 325 327 332 334 335 340 341 345

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List of contributors

Pieczy nska Aleksandra University of Gdansk, Gdansk, Poland Malankowska Anna University of Gdansk, Gdansk, Poland Bajorowicz Beata University of Gdansk, Gdansk, Poland Anna Goła˛biewska University of Gdansk, Gdansk, Poland Ewelina Grabowska University of Gdansk, Gdansk, Poland Nadolna Joanna University of Gdansk, Gdansk, Poland Martyna Marchelek University of Gdansk, Gdansk, Poland Kobyla nski Marek P. University of Gdansk, Gdansk, Poland Marta Paszkiewicz-Gawron University of Gdansk, Gdansk, Poland Mazierski Paweł University of Gdansk, Gdansk, Poland Adriana Zaleska-Medynska University of Gdansk, Gdansk, Poland

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Introduction Adriana Zaleska-Medynska University of Gdansk, Gdansk, Poland

1.1

1

Introduction

Nano- or microparticles of metal oxides irradiated by an appropriate light spectrum (in the ultraviolet and visible range) may serve as a source of active oxygen species as well as a source of photogenerated electrons and holes. These unique properties (so called photocatalytic properties) may be used in a wide range of environmental friendly processes, such as: (1) pollutant degradation or removal from the gas and aqueous phases (water, wastewater and air treatment technologies); (2) pollutant degradation at the surface of building materials (self-cleaning surfaces); (3) conversion of solar energy into energy of chemical bonds (via water photosplitting or CO2 conversion into useful hydrocarbons); (4) conversion of solar energy into electric energy (solar cells); (5) photochemical transformation of specific chemical compounds.

Some undoubted advantages of the photocatalytic process are the ability to carry it out in ambient conditions of temperature and pressure, the possibility of using solar light to drive chemical reactions, and the possibility to reuse photocatalyst particles. Hydroxyl radicals, as well as other active oxygen species produced at the surface of irradiated metal oxide particles, could be a powerful and green oxidant employed to degrade organic pollutants and to remove inorganic compounds and microorganisms present in water, wastewater, or air. On the other hand, the mechanism of the photocatalytic process is quite complicated, the efficiency of most of the processes mentioned above is not too high at present, and moreover, the photoactivity of metal oxide particles is affected by a wide spectrum of factors. Notwithstanding this, the immense value of heterogeneous photocatalysis lies in its great potential to use solar energy to conduct chemical reactions and to convert solar energy into energy of chemical bonds. Efficient utilization of solar light for water splitting to generate hydrogen (clean energy carrier) and for artificial photosynthesis (CO2 photoconversion) seems to be a Holy Grail for the chemical community. Metal oxide photocatalysts could also be employed to produce energy via photoelectrochemical processes. This book presents the principles and fundamentals of metal oxide-based heterogeneous photocatalysis and highlights the current developments and future potential of applications of unitary metal oxides, as well as their hybrids with noble metal nanoparticles and other semiconductors. The book consists of three main chapters, presenting the mechanisms of metal oxide-based photocatalysts excitation, the most important requirements for metal oxides used in photocatalysis, preparation methods Metal Oxide-Based Photocatalysis. https://doi.org/10.1016/B978-0-12-811634-0.00001-9 © 2018 Elsevier Inc. All rights reserved.

2

Metal Oxide-Based Photocatalysis

of metal oxides used in photocatalysis, advantages, disadvantages, and achievements of the most important metal oxides (TiO2, WO3, ZnO, Fe2O3, Ta2O5, CuO) and the most important applications (water treatment, hydrogen production, air treatment, chemical synthesis, medical applications and applications in construction). We hope that this book summarizes and organizes the knowledge in the field of metal-oxide based photocatalysis and that it can also be used in the classroom for undergraduate and graduate students focusing on heterogeneous photocatalysis, sustainable chemistry, energy conversion and storage, nanotechnology, chemical engineering, environmental protection, optoelectronics, sensors, and surface and interface science.

Fundamentals of metal oxide-based photocatalysis

2


ski Marek P., Adriana Zaleska-Medynska Anna Goła˛biewska, Kobylan University of Gdansk, Gdansk, Poland

2.1

Principles of metal oxide-based photocatalysis

The discovery of metal oxide-based photocatalysis was published in 1972 by Fujishima and Honda, who demonstrated that photoirradiation of a TiO2-covered electrode immersed in an aqueous electrolyte led to the production of O2 and H2 from TiO2 and Pl electrodes, respectively (so-called Honda-Fujishima effect) [1]. Notwithstanding this, some reports describing the ability of TiO2 particles to produce photocatalytic oxidation of organic compounds had been presented prior to this discovery. Thus, the ability of ZnO and TiO2 to generate photocatalytic degradation of organic compounds in the gas phase was announced in 1961 [2] and 1964 [3], respectively, and the photocatalytic oxidation of tetralin (1,2,3,4-tetrahydronaphthalene) in aqueous TiO2 suspension was reported in 1964 [4]. Since 1977, when Frank and Bard explored the possibility of employing TiO2 to decompose cyanide in aqueous suspension [5], there has been increasing interest in the environmental applications of such a process.

2.1.1 Mechanism of photocatalytic reactions In contrast to metals, which possess a continuum of electronic states, semiconductors have an empty region in which there are no accessible energy levels that promote the recombination of an electron and a hole produced by photoactivation of a solid [6]. This empty region, which extends from the top of the filled valence band to the bottom of the vacant conduction band, is called the bandgap. Thus, the electronic structure of semiconductors is characterized by a filled valence band (VB) and an empty conduction band (CB). When a semiconductor is irradiated by a photon of energy hv that equals or surpasses the bandgap energy (Eg), an excited electron (e
) moves from the VB to the CB, leaving a hole (h+) behind (Eq. 2.1) [7]. The excited-state CB electrons and the VB holes can (1) recombine and be deactivated, (2) be trapped in metastable surface states, or (3) react with surrounding species. In the absence of a suitable scavenger or a surface defect state, photogenerated charge carriers can recombine and be deactivated radiatively (release of light) or nonradiatively (release of heat) (Eq. 2.2). If a relevant scavenger or a surface defect state is available to trap the electron or hole, recombination is avoided and e
/h+ pairs can react with electron donors and electron acceptors adsorbed on the surface of the semiconductor or within the circumfluent electrical double layer of charged particles, Metal Oxide-Based Photocatalysis. https://doi.org/10.1016/B978-0-12-811634-0.00002-0 © 2018 Elsevier Inc. All rights reserved.

4

Metal Oxide-Based Photocatalysis

as shown in Fig. 2.1. The VB holes are powerful oxidizers (+1.0 to +3.5 V vs NHE [normal hydrogen electrode], depending on the semiconductor and pH) and the CB electrons are good reducers (+0.5 to
1.5 V vs NHE) [8]. For TiO2 (the most used metal oxide in heterogeneous photocatalysis), the redox potential for photogenerated holes is +2.53 V vs NHE (at pH 7), while the redox potential for CB electrons is
0.52 V [9]. In subsequent reactions, oxygen, as a primary electron acceptor, generates superoxide radicals (O2 
) (Eq. 2.3) that undergo further reactions to produce hydroxyl ( OH) radicals (Eqs. 2.5, 2.6), which are extremely powerful oxidants [10]. Meanwhile, positive holes oxidize OH
or H2O at the semiconductor surface to produce OH radicals (Eqs. 2.4, 2.8, reprinted from [11]). Afterwards, the OH radicals oxidize organic compounds (e.g., organic pollutants) resulting in their mineralization and the formation of CO2, H2O, and eventually simple inorganic compounds containing heteroatoms (Eq. 2.9, reprinted from [11]). l

l

l

+ Semiconductor + hv ! hVB + e
CB

(2.1)

+ + e
hVB CB ! energy

(2.2)


e
CB + O2,ad ! O2

(2.3)

Fig. 2.1 Formation of active species at the surface of a semiconductor (photocatalysts) after light absorption (hv  Eg): (i) initiation of an oxidative pathway of electron donors (D) by VB holes; (ii) initiation of a reductive pathway of electron acceptors (A) by CB electrons. Based on A.L. Linsebigler, G. Lu, J.T. Yates Jr, Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758; M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96; M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, P.S. Dunlop, J.W. Hamilton, J.A. Byrne, K. O’Shea, A review on the visible light active titanium dioxide photocatalysts for environmental applications, Appl. Catal. B Environ. 125 (2012) 331–349.

Fundamentals of metal oxide-based photocatalysis

5

O2 
+ H + ! HO2 

(2.4)

2HO2  ! H2 O2 + O2

(2.5)

H2 O2 + e
!  OH + OH


(2.6)

+ + H2 O !  OH + H + hVB

(2.7)

+ + OHads !  OH hVB

(2.8)



OH + pollutants !!! H2 O + CO2

(2.9)

If the oxidation potential of the VB holes is not sufficient to form OH radicals, O2 
is the dominant oxidant. This might be a major obstacle for the practical application of VB holes, because O2 
has a much lower redox potential than OH ½E0 ðO2 
, 2H + =H2 O2 Þ ¼ 0:91V; E0 (OH , H+/H2O) ¼ 2.31 V vs NHE) [12]. The ability of a semiconductor to undergo the transfer of photoinduced electrons to adsorbed species on its surface is a function of the positions of the CB and VB edges and the redox potential of the adsorbate. The potential level of the acceptor molecules or species thermodynamically has to be placed below (i.e., more positive than) the lower edge of the semiconductor CB. The potential level of the donor has to be located higher than (i.e., more negative than) the upper edge of the semiconductor VB to be able to provide an electron to the vacant hole. The correlation between the band-edge positions of the TiO2 photocatalyst and the relevant redox couples at pH 7 is summarized in Fig. 2.2, while the value of the bandgap and the band-edge positions for the most common metal oxides are presented in Section 2.2 (Table 2.4.). For hydrogen production (in a water-splitting process), the bottom level of the CB should be more negative than the reduction potential of H+/H2 (0 V vs NHE), while the top level of the VB should be more positive than the oxidation potential of O2/H2O (1.23 V) for efficient hydrogen and oxygen production from water by photocatalysis, as shown in Fig. 2.3. Even the band-edge positions of TiO2 are suitable for water photodecomposition, where only TiO2 becomes inactive because of the presence of a large overpotential for the production of H2 and O2 on the surface of TiO2 [6]. Linsebigler et al. indicated that maintained photodecomposition of water in the presence of TiO2 is possible under the following experimental conditions [6]: l

l

1. Use of a photoelectrochemical cell with a TiO2 anode and a metal cathode (e.g., Pt) and a small, externally applied electrical potential (>0.25 V) [6]. In this system, the photoexcitation of TiO2 pumps electrons from its VB into its CB. The electrons then move through the external circuit to the Pt cathode where H2 is produced, while O2 is formed on the TiO2. 2. Use of TiO2 powder modified by metal nanoparticles (e.g., Pt) for H2 generation and by metal oxide particles (e.g., RuO2) for O2 production. In this system, irradiation of TiO2 results in the injection of electrons into the Pt particles and holes into the RuO2 particles. The trapped charges reduce water to hydrogen at the Pt surface and the trapped holes oxidize water to oxygen at the RuO2 surface due to a significant reduction in the overpotential for H2 and O2 formation by the presence of Pt and RuO2, respectively.

6

Metal Oxide-Based Photocatalysis

Fig. 2.2 Potentials for various redox processes that occur on the surface of TiO2 at pH 7. Reproduced with permission from J. Kou, C. Lu, J. Wang, Y. Chen, Z. Xu, R.S. Varma, Selectivity enhancement in heterogeneous photocatalytic transformations, Chem. Rev. 117 (2017) 1445–1514.

Fig. 2.3 Mechanism of photocatalytic water splitting for hydrogen production over semiconductor particles. Based on M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production, Renew. Sust. Energy Rev. 11 (2007) 401–425.

Fundamentals of metal oxide-based photocatalysis

7

3. Introduction into the reaction environment of a sacrificial species, which removes one of the formed products and results in the shift of the reaction equilibrium toward further water photosplitting. Thus, it could be predicted that if the sacrificial agent is oxidized by the product from the reaction with holes (e.g., O2), maintained H2 production will be observed [6].

The efficiency of converting solar energy to hydrogen (i.e., photocatalytic watersplitting) on a single semiconductor is still limited owing to the following reasons [66]: 1. Quick back reaction between photogenerated e
/h+ pairs: CB electrons can react with VB holes followed by energy release (via radiative or nonradiative decay); 2. Backward reaction: splitting of water molecules into hydrogen and oxygen increases energy, so the backward reaction, i.e., the recombination of hydrogen and oxygen into water, proceeds easily; 3. Limited number of metal oxides excited by visible light: The bandgap of most stable semiconductors (e.g., TiO2) is greater than 3.2 eV, so only ultraviolet (UV) light can be utilized for hydrogen production. The solar spectrum reaching ground level contains only about 4% of UV light and about 50% of visible light. Thus, the inability to absorb light from the visible region limits the efficiency of solar-driven photocatalysis to produce hydrogen [66].

Titanium dioxide has high photostability, is chemically inert, and has relatively high oxidation efficiency under UV light (λ < 387 nm), the energy of which exceeds the TiO2 bandgap (from 3.0 to 3.2 eV for the rutile and anatase forms, respectively). This makes TiO2 a very popular and commonly used semiconductor in heterogeneous photocatalysis. However, the application of metal oxide-based photocatalysis on a large scale requires photocatalysts with high reactivity under visible light (λ > 400 nm) to be able to use renewable solar energy. The five main approaches to forcing photocatalytic activity from wide-bandgap semiconductors (e.g., TiO2) under visible light irradiation are (1) doping, (2) sensitization, (3) charge transfer complex formation, (4) coupling with a narrow-bandgap semiconductor, and (5) addition of noble metal nanoparticles. According to the Kisch classification, metal oxide-based photocatalysis has two reaction modes: direct and indirect [67]. In direct photocatalysis, light excitation (hv) results in the generation of an e
/h+ pair localized at the semiconductor surface, followed by the two interfacial electron transfer (IFET) reactions with donor and acceptor species (Figs. 2.1 and 2.3). In indirect photocatalysis, electrons or holes that appear at the surface of the semiconductor originate from an adsorbed molecule (e.g., semiconductor photosensitization).

2.1.1.1 Doped semiconductors The activity of a doped semiconductor under visible light is due to a new energy level produced in the bandgap of the semiconductor by the dispersion of dopant atoms (cations or anions) in the crystal structure of the semiconductor (Fig. 2.4). Transition metals, rare earth metals, noble metals, and poor metals can be used for cation doping. Doping with transition metals tunes the electronic structure and shifts the light absorbed from UV to visible light, thus improving the photocatalytic activity of wide-bandgap semiconductors. The observed redshift is caused by the charge transfer between the d electrons and the CB or the VB of the semiconductor. The transition metal creates a new electron state inside the electronic structure of the wide-bandgap

8

Metal Oxide-Based Photocatalysis

Fig. 2.4 Mechanism of visible light-induced excitation through the metal- or nonmetal-doped widebandgap semiconductor.

semiconductor by capturing the excited electrons from the VB of the semiconductor and preventing the recombination of charge carriers [63]. As metal ions are incorporated into the TiO2 lattice, impurity energy levels are formed in the bandgap of TiO2, as indicated by Eqs. 2.10, 2.11 [66]: Mn + + hv ! Mðn + 1Þ + + e
CB

(2.10)

+ Mn + + hv ! Mðn
1Þ + + hVB

(2.11)

where M and Mn+ represent metal and metal ion dopant, respectively. The energy level of Mn+/M(n
1)+ should be less negative than that of the CB edge of TiO2, while the energy level of Mn+/M(n+1)+ should be less positive than that of the VB edge of TiO2 [66]. Depending on the redox energy state of the doping transition metal, the substitution of metal ions into the TiO2 (or other metal oxide) introduces an intraband state close to the CB or VB edge, inducing visible light absorption at subbandgap energies [68]. The additional benefit of doping with transition metal is the enhanced trapping of electrons, which inhibits the recombination of electrons and holes during illumination [6]. However, the type of transition metal, its content, and the microstructural characteristics of the metal oxide are the main factors which affect the effectiveness of the photodegradation of miscellaneous compounds induced by visible light irradiation [63,69,70]. The next popular approach for changing the optical response of the semiconductor to visible light is to dope with anionic nonmetals, such as N, C, S, B, P, and I [71]. Compared to doping with cations, the role of nonmetallic anions as recombination centers might be minimized. For nonmetal-doped TiO2, the impurity states are near the VB edge but do not act as charge carriers. Both theoretical [72] and experimental [73] studies revealed that for N-doped TiO2, the N 2p localized states were just above the top of the O 2p VB.

Fundamentals of metal oxide-based photocatalysis

9

However, the oxidation power and mobility of photogenerated holes in the electronic state of isolated N are lower than those in the VB of TiO2. Thus, in the case of N-doped TiO2, superoxide radicals are responsible for organic compound decomposition rather than OH radicals [74]. Doping by S, C, and P also induces mid-bandgap levels leading to visible light absorption. Doping with F does not shift the TiO2 bandgap but it does enhance surface acidity and cause formation of reduced Ti3+ ions owing to the charge compensation between F
and Ti4+.

2.1.1.2 Semiconductor photosensitization In semiconductor photosensitization, photoexcited electrons or holes appear in the CB or the VB of the semiconductor, respectively, from the excited state of organic molecules adsorbed on the surface of the semiconductor. This type of photoinduced electron transfer is considered a weak-type interaction between a semiconductor band and the surface molecule and is known as the Sakata-Hiramoto-Hashimoto (SHH) mechanism [75] (Fig. 2.5). According to the SHH model, photosensitization results from photoinduced electron transfer and depends on the type of semiconductor [76]. In n-type semiconductors, electrons are injected into the CB, with possible contribution of the empty surface state (Fig. 2.5A). When the energy of the highest occupied molecular orbital (HOMO) of the photosensitizer is sufficiently low, hole injection into the VB of a p-type semiconductor is observed (Fig. 2.5B). The indirect photosensitization process, which involves excitation of the adsorbed dye followed by rapid electron injection from the excited state of the dye into the CB of the semiconductor, is privileged for covalently bound photosensitizer due to the fact

Fig. 2.5 Energy diagrams showing indirect photosensitization of (A) n- and (B) p-type semiconductors according to the SHH mechanism: (A) under photoexcitation of an adsorbed dye, electrons are transferred from the excited dye to the CB of the semiconductor; (B) under photoexcitation of the adsorbed dye, holes are injected from the excited dye to the VB of the semiconductor. Based on T. Sakata, K. Hashimoto, M. Hiramoto, New aspects of electron transfer on semiconductor surface: dye-sensitization system, J. Phys. Chem. 94 (1990) 3040–3045; W. Macyk, K. Szaciłowski, G. Stochel, M. Buchalska, J. Kuncewicz, P. Łabuz, Titanium(IV) complexes as direct TiO2 photosensitizers, Coord. Chem. Rev. 254 (2010) 2687–2701.

10

Metal Oxide-Based Photocatalysis

that it allows only for unsteady interaction between chromophore and bands or surface states of the semiconductor. Thus, the excited dye molecules transfer electrons into the CB of the semiconductor (e.g., TiO2), while the dye itself is converted into its cationic radical. The semiconductor acts only as a mediator for transferring electrons from the photosensitizer to the substrate on the semiconductor surface, which acts as an electron acceptor, and the VB of the semiconductor stays uninfluenced. In this action, the energy of the lowest unoccupied molecular orbital (LUMO) of the dye molecules is supposed to be more negative than that of the CB of the semiconductor. The injected electrons move quickly to the surface of the semiconductor where they are scavenged by O2 to form O2 
and the hydrogen peroxide radical OOH. These reactive species can also be disproportionate to yield the hydroxyl radical OH (Fig. 2.6A). In addition to the above-mentioned species, 1O2 can also be formed under some reaction conditions. Oxygen has two singlet excited states above the triplet ground states. This makes 3 O2 an efficient quencher for almost all other excited states. 1O2 is produced by quenching the excited state of the photosensitizer with 3O2, as summarized in Eqs. 2.12, 2.13, where S refers to the photosensitizer. l

l

S ∗ + 3 O2 ! 1 S + 1 O2

(2.12)

S∗ + 3 O2 ! S  + + O2 


(2.13)

3

The subsequent radical chain reaction leads to the degradation of the dye. The time for electron injection between the semiconductor and the adsorbate depends on the nature of the sensitizer, the semiconductor, and their interaction [11]. Some frequently used dyes and their absorption wavelength maxima are listed in Table 2.1.

Fig. 2.6 Indirect photosensitization: (A) formation of oxygen active species at the surface of the photosensitized semiconductor; (B) formation of hydrogen at the surface of the photosensitized semiconductor (dye-mediated water splitting). Based on S.G. Kumar, L.G. Devi, Review on modified TiO2 photocatalysis under UV/visible light: selected results and related mechanisms on interfacial charge carrier transfer dynamics, J. Phys. Chem. A 115 (2011) 13211–13241.

Table 2.1

Dyes used for photocatalysis Water splitting

Dye Thionine (TH+) Toluidine Blue (Tb +) Methylene Blue (MB) New methylene Blue Azure A Azure B Azure C

Phenosafranin (PSF) Safranin-O (Saf-O/SO) Safranin-T (Saf-T/ST) Neutral Red (NR) Fluorescein

Pollutant degradation

Solar cells

λmax (nm)

Class

Dye

λmax (nm)

Class

Dye

λmax (nm)

Class

596

Thiazines

Chrysoidine R

455

Azo

Cumarine 343

480

Cumarines

630

Thiazines

Acid Red 29

510

Hemicyanine, HC-1

535

Hemicyanine

665

Thiazines

Acid Red 14

515

Chromotrope 2R Azo

Thienylflourene

538

Fluorene

650

Thiazines

Reactive Red 22

538

Azo

4-N,NDimethylamine-benzaldehyde

406

N,NDiametylamines

635 647 620

Thiazines Thiazines Thiazines

Acid Red 27 Reactive Red 198 Acid Red 2

521 518 430

Azo Azo Azo

470 491 450

Thiazines Indolynes N,NDiametylamines

520

Phenazines

Acid Red 18

507

Azo

Phenothiazine, Z4 Indoline, Ind 1 N,NDimethylamine-cyanoacetic acid Porphyrin

400–650

Porphyrines

520

Phenazines

Acid Red 176

519

Zinc carboxyphtholocyanine

680

Zn-complex

520

Phenazines

Reactive Red 15

500

Chromotrope 2R Azo

N3

380

Ru-complex

534

Phenazines

Basic Blue 41

600

Azo

Black Dye

605

Ru-complex

490

Xanthenes

Acid Yellow 17

400

Azo

N719

540

Ru-complex Continued

Continued

Table 2.1

Water splitting

Pollutant degradation

Dye

λmax (nm)

Class

Dye

Erythrosin

530

Xanthenes

Erythrosin B

525

Xanthenes

Reactive Yellow 17 Acid Yellow 23

Rhodamin B (Rh.B) Rose Bengal

551

Xanthenes

550

Xanthenes

545

Pyronine Y (PY) Eosin Rhodamin 6G Acridine Orange (AO) Proflavin (PF)

Solar cells

λmax (nm)

Class

Dye

λmax (nm)

Class

426

Azo

C101

547

Ru-complex

455

Azo

Rhododendron

540

410

Azo

Violet

546

419

Azo

Tangerine peel

446

Xanthenes

Reactive Yellow 14 Reactive Yellow 145 Acid Orange 10

Natural (flowers) Natural (flowers) Natural (fruit)

480

Azo

Shiso

440

Natural (leaves)

514 524 492

Xanthenes Xanthenes Acridines

Acid Orange 7 Acid Orange 8 Acid Orange 12

485 490 488

Azo Azo Azo

Bixa arellana L.

474

Natural (seeds)

444

Acridines

Reactive Orange 16 Direct Yellow 12

496

Azo

395

Azo

Direct Blue 160

570

Azo

Direct Red 80

580

Azo

Acridine Yellow (AY) Fusion

442

Acridines

545

Crystal Violet

578

Triphenyl methane deriv. Triphenyl methane deriv.

Malachite Green

625

Methyl Violet

580

Triphenyl methane deriv. Triphenyl methane deriv.

Congo Red

524

Azo

Acid Brown 14

465

Azo

Reactive Black 5 Acid Black 1 Acid Blue 80 Acid Blue 25 Reactive Blue 4 Reactive Blue 19 Alizarin Red Basic Violet 3 Acid Blue 7 Gentian Violet Acid Blue 1 Acid Blue 9 Acid Yellow 73 Rhodamine-B Sulforhodamine B

597 618 626 600 596 592 342 550 625 536 630 625 490 550 565

Azo Azo Anthraquinone Anthraquinone Anthraquinone Anthraquinone Anthraquinone Triarylmethane Triarylmethane Triarylmethane Triarylmethane Triarylmethane Xanthenes Xanthenes Xanthenes

Modified from M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production, Renew. Sust. Energy Rev. 11 (2007) 401–425; A.R. Khataee, M.B. Kasiri, Photocatalytic degradation of organic dyes in the presence of nanostructured titanium dioxide: Influence of the chemical structure of dyes, J. Mol. Catal. A Chem. 328 (2010) 8–26; N.A. Ludin, A.M. Al-Alwani Mahmoud, A. Bakar Mohamad, A.A.H. Kadhum, K. Sopian, N.S. Abdul Karim, Review on the development of natural dye photosensitizer for dye-sensitized solar cells, Renew. Sust. Energy Rev. 31 (2014) 386–396.

14

Metal Oxide-Based Photocatalysis

2.1.1.3 Charge transfer interaction Another way to modify a wide-bandgap semiconductor (such as TiO2) for visible light absorption is the formation of a charge transfer (CT) complex between the semiconductor and the surface adsorbate, neither of which absorbs visible light [77–81]. The mechanism of CT complex-mediated visible light excitation is different from that of common dye photosensitization. In CT sensitization, the electron is photoexcited directly from the ground-state adsorbate to the CB of the semiconductor, without the involvement of the excited state of the adsorbate. The strong interaction between the semiconductor CB and a surface molecule can be represented by the Creutz-Brunschwig-Sutin model [82–84]. For strong electronic coupling, the interaction between a ligand and the surface of the semiconductor results in the formation of a surface coordination species (Fig. 2.7A and B). New energy levels are formed with the appropriate arrangement of molecular orbitals. For an n-type semiconductor (Fig. 2.7A), a bonding orbital is created via interaction of the HOMO level of the surface ligand with the empty surface state. Typically, the resulting surface state, with HOMO and LUMO characteristics, is situated at the surface ligand (electron donor) and constitutes a part of the CB. In this case, excitation of the surface complex leads to optical electron transfer from the surface molecule to the CB. The opposite occurs when the p-type semiconductor interacts with the electron acceptor (Fig. 2.7B). The simple surface complexes of TiO2 have three basic structures: (1) monodentate structure with the organic ligands engaging one coordination site of TiO2, (2)

Fig. 2.7 Energy diagrams showing direct photosensitization of (A) n- and (B) p-type semiconductors according to the Creutz-Brunschwig-Sutin model (LMCT: ligand-to-metal charge transfer; MLCT: metal-to-ligand charge transfer; Eg: bandgap energy; EF: Fermi level). Based on T. Sakata, K. Hashimoto, M. Hiramoto, New aspects of electron transfer on semiconductor surface: dye-sensitization system, J. Phys. Chem. 94 (1990) 3040–3045; W. Macyk, K. Szaciłowski, G. Stochel, M. Buchalska, J. Kuncewicz, P. Łabuz, Titanium(IV) complexes as direct TiO2 photosensitizers, Coord. Chem. Rev. 254 (2010) 2687–2701.

Fundamentals of metal oxide-based photocatalysis

15

bidentate chelating structure with the ligand occupying two coordination sites, and (3) bidentate bridging structure composed of a chelating ligand binding two neighboring Ti centers. The bidentate surface complexes form only if the ligand owns at least two donor groups or one group containing two donor atoms. The opportunity for bidentate complex formation does not preclude complexation via the monodentate mode [76]. The following substances are able to form a CT complex with the surface of TiO2: catechol [85], 8-hydroxyorthoquinoline [86], 1,1-binaphthalene-2,2-diol [87], chlorophenol [88], 2,4,5-trichlorophenol [89], and calixarene [90].

2.1.1.4 Composite semiconductors Visible light can be utilized by coupling a wide-bandgap semiconductor with a narrow-bandgap semiconductor. In this system, visible light photons are absorbed by the narrow-bandgap semiconductor (Eg < 3 eV) resulting in the formation of e
/h+ pairs. If the CB level of the narrow-bandgap semiconductor is more negative than that of the wide-bandgap semiconductor, electrons from the narrow-bandgap semiconductor are injected into the wide-bandgap semiconductor (Fig. 2.8).

Fig. 2.8 Mechanism of visible light-induced excitation through the heterojunction between wide- and narrow-bandgap semiconductors. Based on S.G. Kumar, L.G. Devi, Review on modified TiO2 photocatalysis under UV/visible light: selected results and related mechanisms on interfacial charge carrier transfer dynamics, J. Phys. Chem. A 115 (2011) 13211–13241; K. Rajeshwar, N.R. de Tacconi, C.R. Chenthamarakshan, Semiconductor-based composite materials: preparation, properties, and performance, Chem. Mater. 13 (2001) 2765–2782.

16

Metal Oxide-Based Photocatalysis

The energy gap between the corresponding band levels guarantees the transport of charge carriers from one particle to its neighbor and favors the separation between the e
/h+ pairs. The photogenerated electrons transferred to the CB of the wide-bandgap semiconductor are involved in the formation of O2 
and subsequently H2O2 and HO2  . If the oxidation potential of the photogenerated holes is greater than +2.27 V, then OH radicals are formed via HO
or H2O oxidation. However, in most cases, OH is not generated by visible light-responsive semiconductors because of an inadequate VB position. Visible light-responsive photocatalysts have been successfully obtained by combining TiO2 with Ag2O [13,91], Cu2O [32], Fe2O3 [92], ZnO with Fe2O3 [36], and Ag2O [93]. l

2.1.1.5 Semiconductor modified by noble metal nanoparticles In a hybrid nanostructure consisting of plasmonic metal nanoparticles and a widebandgap semiconductor, the incident visible light excites the localized surface plasmon resonance (LSPR) in the metal nanocrystals. The transfer of plasmonic energy from the plasmonic metal nanoparticles to the semiconductor results in the appearance of e
/h+ pairs in the semiconductor. The energy concentrated in the localized plasmonic oscillations is transferred to the semiconductor via three main mechanisms: (1) direct electron transfer (DET), (2) local electromagnetic field enhancement (LEMF) of the semiconductor charge separation process, and (3) resonant energy transfer (RET) [94]. These mechanisms, and the role of noble metal nanoparticles in heterogeneous photocatalysis, are discussed in Section 2.3. In the DET mechanism, electrons are transferred from the plasmonic metal to the CB of the semiconductor if the metal particle is connected with the semiconductor surface. The alignment of the band-edge positions of the semiconductor and the Fermi level of the plasmonic metal influences the DET mechanism. Electrons from the metal can be injected into the semiconductor at energies below Eg if the electronic energy levels match. DET occurs when the LSPR in the nanoparticles is excited by visible light, followed by decoherence of LSPR, leaving a number of hot electrons for injection into the semiconductor matrix (Fig. 2.9A). Hot electrons are electrons whose distribution can be fundamentally described by the Fermi function, but with an elevated effective temperature [95]. An efficient way to catch hot electrons is to form a Schottky barrier with an appropriate semiconductor. The efficiency of the transfer of hot electrons, with energy higher than the Schottky barrier energy, into the semiconductor depends on their energy (Fig. 2.9B). Generation of plasmonic hot electrons has been confirmed for TiO2 combined with Au, Ag, or Pt nanoparticles [96–98], ZnO with added Au nanocrystals, Au nanoparticles seated on CeO2, Au on WO3, and Al2O3 with added Au and Ag nanoparticles [99]. Similar to the other above-mentioned mechanisms, the injected electrons move quickly to the surface of the semiconductor, where they are trapped by O2 to form O2 
and OOH, and if they are disproportionate, OH is obtained (Fig. 2.9A). l

l

Fundamentals of metal oxide-based photocatalysis

17

Fig. 2.9 Schematic illustration of (A) mechanism of visible light-induced excitation of plasmonic nanoparticles at the surface of a wide-bandgap semiconductor and (B) energy band diagram of Schottky barrier formed at metal/n-type semiconductors interface. Based on S. Mukherjee, F. Libisch, N. Large, O. Neumann, L.V. Brown, J. Cheng, J.B. Lassiter, E.A. Carter, P. Nordlander, N.J. Halas, Hot electrons do the impossible: plasmoninduced dissociation of H2 on Au, Nano Let. 13 (2013) 240–247; A. Bumajdad, M. Madkour, Understanding the superior photocatalytic activity of noble metals modified titania under UV and visible light irradiation, Phys. Chem. Chem. Phys. 16 (2014) 7146–7158.

2.1.2 Kinetics of photocatalysis [100] The photodegradation rate of a chemical compound on the surface of a metal oxidebased semiconductor follows the Langmuir-Hinshelwood (L-H) mechanism. In accordance with the L-H model, given by Eq. (2.14), the rate of photocatalytic reaction (r) is proportional to the fraction of surface coverage by organic substrate (θx), where: kr is the reaction rate constant, C is the concentration of organic species and K is the Langmuir adsorption constant [101]: r¼


dC kr KC ¼ kr θ x ¼ dt 1 + KC

(2.14)

The appropriateness of Eq. (2.14) relies upon the following presumptions: (1) the reaction system is in dynamic equilibrium; (2) the reaction occurs on the surface of photocatalysts; and (3) there are no limits for the competition between byproducts and reactive oxygen species for the metal oxide active surface sites [101]. Recently, Mills et al. [102] reviewed ten proposed reaction mechanisms. The reaction processes that support the reaction mechanisms, described below, are given in Table 2.2. Table 2.3 presents some of the common assumptions and their associated rate equations.

18

Metal Oxide-Based Photocatalysis

Table 2.2 No.

Key reaction processes

Process

Core processes Generation G TiO2 + hν ! h+ + e
Recombination B1 h+ + e
! heat
 B2 OH =hs+ + e
! H2 O=OH
= > O2
s + heat Trapping
 T1 h + + H2 O=OH
= > O2
s ! OH =hs+ Interfacial electron transfer R1 h+ + P ! products R2 OH =hs+ + P ! products R3 e
+ O2 ! O2
Other processes T2 R4 R5 B3 T3 T4 T5

OH l ! h+ + (H2O/OH
/O2
)s h+ + P ! P+ h+ + P+ ! products e
+ P+ ! P h+ + T ! T+ e
+ T ! T
hs+ + T∗ ! T∗ +

Ratea

k0ρ kr[h +][e
]  kr1 OH =hs+ ½e
 kT1[h+] kP[h+][P]   kP OH =hs+ ½P kO2 ½e
½O2  k_T1[OH l] kP1[h+][P] kP2[h+][P+] kb[e
][P+] kT3[h+] kT4[e
]   kT5 hs+

Adsorption L1 L1 L2

Pbulk + S Ð P Pbulk + S ! P P! Pbulk + S

[P] ¼ [S]OKL[P]sol/(1 + KL[P]sol) kads[P]sol[S] Kdes[P]

a Unless stated otherwise, all concentrations are surface concentrations and are in units of mol m
2 and all rates are in units of mol m
2 s
1. Bulk solution concentrations are identified by the subscript “sol,” e.g., [P]sol, in the Langmuir equation. Thus, it is often assumed that KL[P]sol ≪ 1, so [P] ¼ [S]oKL[P]sol, i.e., [P] ∝ [P]sol. If P is nonspecifically adsorbed, then [P] ∝ [P]sol for all values of [P]sol. Reproduced with permission from A. Mills, C. O’Rourke, K. Moore, Powder semiconductor photocatalysis in aqueous solution: an overview of kinetics-based reaction mechanisms, J. Photochem. Photobiol. A Chem. 310 (2015) 66–105.

2.1.3 Generation and separation of charge carriers The recombination rate of photoexcited e
/h+ pairs straightforwardly affects the reaction rate of any photochemical process that appears on the surface of the metal oxide. Thus, the recombination of photoexcited e
/h+ pairs should be suppressed for the charge transfer process on the semiconductor surface to be efficient. Recombination can be suppressed by trapping the photogenerated electrons, the photogenerated holes, or both, thus extending the lifetime of the separated electrons and holes to more than a fraction of a nanosecond [110].

Table 2.3

Kinetics model and associated rate equations

Rate r¼

X1 X2 ρθ ½P 1 + X2 ½P

r¼


X1 X2 ½Psol
 1 + X2 ½Psol =ρ

X1 X2 ρ½Psol  r¼
1 + X2 ½Psol r ¼ αρθ ½P ¼


X1 ρθ ½Psol  1 + X2 ρθ + X3 ½Psol

!   X2 ρ 1=2 r ¼ ½PX1 1+
1 c ½P  !   X2 ρ 1=2 1+
1 c r ¼ ½PX1 ½P     X2 ½P 2 + ð4X1 ½PÞ= 1 + ð1 + 8X1 ½PÞ1=2  r¼ 2 1 + ð1 + 8X1 ½PÞ1=2 0 0 0 111=2 1  1=2 4 1 + ð 1 + 8X ½ P  Þ ρ X 1 3 B B B CC C @
1 + @1 + @    2 AA Ac 1=2 X2 ½P 2 + ð4X1 ½PÞ= 1 + ð1 + 8X1 ½PÞ

Processes

Type

Ref.

X2 ¼ (kp/k
T1) but at low ρ, ν ¼ 1 and X1 ¼ k0, and at high ρ, ν ¼ 0.5 and X1 ¼ kT1(k0/kr)1/2

G, B1, T1, R2, T2

[103]

X1 ¼ (kT1k0/kT3); X2 ¼ (kT4kp/kr1k0) [S]0KL X1 ¼ (k0[S]0kp/kT1) and X2 ¼ KL, and assuming kp[S]0/kT1 ≪ 1

G, B2, T1, R2, T3, T4, L1 G, T1, R1, L1

X1 ¼ αKL[S]0; θ ¼ constant, (where: 0.5  θ  1; X2 ¼ α/kdes; X3 ¼ KL0

(G, B1, T3, R1) + L1, L1’, L2’

X1 ¼ 2kpkO2[O2]/kr and X2 ¼ k0/X1

G, B1, R1, R3

Langmuir-Ollis Langmuir (case III; ER mechanism) Emeline (Langmuir mechanism) Salvador (Direct Transfer, DT mechanism) Ollis disrupted adsorption model (nonequilibrated adsorption) Quadratic Gerischer (case: α)

X1 ¼ kpkO2[O2]/2kr1 and X2 ¼ 2k0/X1

G, B2, T1, R2, R3

[105]

X1 ¼ δγ=βθ ¼ kP1 kb =ðkO2 ½O2 kP2 Þ X2 ¼ δβ=α ¼ ðkP1 kO2 ½O2 =kr1 ; X3 ¼ k0

G, B2, T1, R3, R4, R5, B3

Salvador (indirect transfer, IT mechanism) Marin (quadratic with back reaction)

[104]

[105]

[106]

[107]

[108]

Continued

Table 2.3

Continued

Rate 0

1

!1=2   X3 ½P + ðX1 X4 ρÞð1
αÞ @ 4αX4 X3 X1 ρ r¼ 1+
1A 2X4 ðX3 ½P + ðX1 X4 ρÞð1
αÞÞ2  c X1 X2 X5 ρ½Psol + 1 + X6 ½Psol + X2 X5 ½Psol  ½Psol X1
1 + ð1 + X2 ρÞ1=2  r¼ 1 + X3
1 + ð1 + X2 ρÞ1=2 + X4 ½Psol
!!1=2 1 c ½ P  1 + K ½ P  L sol sol A 1 + aρ + b½Psol @
1 1 +
2 1 + aρ + b½Psol 0 r¼
!!1=2 1


 c½Psol 1 + KL ½Psol @ A 2 1 + KL ½Psol + 1 + aρ + b½Psol
1 + 1 +
2 1 + aρ + b½Psol


X1’ ρ



0

Processes

Type

Ref.

0 α
¼ 1=ð1 + X2 ½P Þ
1 +X6 ½Psol = 1 + X6 ½Psol + X2 X5 ½Psol ; X1 ¼ k0 ,X2 ¼ kP0 =kT1 ,X3 ¼ kP kO2 ½O2 , X4 ¼ kr1 ,X5 ¼ ½S0 KL0 X6 ¼ KL

G, B2, T1, R1, R2, R3, L1

Combined Salvador (IT and DT combined)

[109]

X1 ¼ kP kT1 ðkO2 ½O2 =ð2kr kTS ÞÞKL ½S0 ; X2 ¼ 4kr k0 =ðkT1 kO2 ½O2 Þ; X3 ¼ kP kT1 ðkO2 ½O2 =ð2kr kT5 ÞÞ=kdes ; X4 ¼ KL
 a ¼ X1 X2 ;b ¼ KL0 1
X2 ½S0 ; c ¼ 4X2 KL ½S0 ,X10 ¼ k0 and X2 ¼ kP =kT1 ,X1 ¼ k0 =ðkdes Þ;KL0

G, B1, T1, R2, R3, T5, L1, L1’, L2’ (G, B1, T1, R1, R2, R3, T2) + L1, L1’, L2’

Revised Ollis disrupted adsorption (RODA) model






DT modified Ollis disrupted adsorption model

Reproduced with permission from A. Mills, C. O’Rourke, K. Moore, Powder semiconductor photocatalysis in aqueous solution: an overview of kinetics-based reaction mechanisms, J. Photochem. Photobiol. A Chem. 310 (2015) 66–105.

[109]

Fundamentals of metal oxide-based photocatalysis

21

UV-irradiation of the TiO2 particles results in the formation of e
/h+ pairs only in the outer surface of the semiconductor due to the low penetration depth (160 nm) of UV light [111]. The recombination of electrons and holes is impeded when the nearsurface electric field causes the photogenerated charge carriers to move in different directions. Constant illumination results in the eradication of this electric field, that is, band flattening. Band flattening occurs when (1) holes accumulate at the surface resulting in a band shift at the surface, or (2) electrons accumulate in the bulk region causing a band shift in the bulk (Fig. 2.10). Therefore, the Fermi level (EF), which is the average energy of the electrons in the system, turns more negative in the bulk. Commonly, the energy of the surface states is fixed, especially if the capacitance of the Helmholtz layer is much higher than that of the depletion layer [111]. The dynamic of reactions that appear on the surface of irradiated TiO2 is shown in Fig. 2.11. Some photoinduced processes that occur inside or at the surface of the TiO2 particle can last from femtoseconds to microseconds [111]. On the basis of timeresolved photoacoustic spectroscopy (TRPAS), Hupp et al. [112] assumed that about 60% of all trapped e
/h+ pairs in TiO2 recombine in about 25 ns, releasing 154 kJ mol
1 of energy as heat. The electron-trapping sites in TiO2 were estimated to be at energy levels about 0.2–0.9 eV below the CB edge [112–115]. Review reports [6,111] have shown that sample preparation, reaction temperature, charge trapping, interfacial charge transfer, and the intensity of the excitation light can affect e
/h+ recombination. Photogenerated charge carriers are efficiently trapped by bulk and surface trapping states as well as by degradable adsorbates or sacrificial reagents [6,111]. Surface or bulk irregularities that appear in the crystal lattice of the semiconductor during preparation can serve as surface or bulk electron-trapping sites, as shown in Fig. 2.12. These irregularities are related to surface electron states, the energy of which varies from that of the band in the bulk semiconductor. Therefore, the charge

Fig. 2.10 Schematic diagram of photoinduced band flattening of pristine TiO2: (A) photogeneration of e
/h+ pairs in the space charge layer; (B) band flattening caused by accumulation of holes at the surface resulting in a band shift at the surface; (C) band flattening caused by accumulation of electrons in the bulk region resulting in a band shift in the bulk. Reproduced with permission from J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo, D.W. Bahnemann, Understanding TiO2 photocatalysis: mechanisms and materials, Chem. Rev. 114 (2014) 9919–9986.

22

Metal Oxide-Based Photocatalysis

Fig. 2.11 Photoinduced reaction in TiO2 and the corresponding time scale. Reproduced with permission from J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo, D.W. Bahnemann, Understanding TiO2 photocatalysis: mechanisms and materials, Chem. Rev. 114 (2014) 9919–9986.

carriers captured in such like states are placed on an exact site on the surface or in the bulk. The assemblage of the bulk and surface traps relies upon the dissimilarity in energy between the trap and the CB bottom edge and the reduction in entropy as the electron is trapped [6]. In the hydroxylated n-type TiO2 the photoexcited electrons must move from the surface to the bulk followed by their delocalization over various Ti sites, due to the upward band twisting [111]. Holes can be trapped at a bridging O2
or can be transferred to surface-bound OH
anions, resulting in the formation O
or OH centers, respectively [111]. The electron scavenger used most often to prolong the lifetime of photogenerated holes is adsorbed O2, which quickly reacts with an electron to become a superoxide ion [116]. For photogenerated holes, the scavenger molecules most often used include methanol, ethanol, propanol, glycerol, and surface hydroxyl groups [116]. l

l

Fundamentals of metal oxide-based photocatalysis

23

Fig. 2.12 Surface and bulk electron trapping. Reproduced with permission from A.L. Linsebigler, G. Lu, J.T. Yates Jr, Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758.

2.2

Requirements of metal oxides

The capability of a semiconducting material to transfer photoinduced electrons to an adsorbate on its surface is a function of its band energy positions and the redox potentials of the adsorbed species. The appropriate potential level of the acceptor molecules or species thermodynamically must be more positive than (located below) the potential level of the CB of the semiconductor. The potential level of the donor must be more negative than (located above) the potential level of the VB of the semiconductor to donate an electron to a vacant hole [6]. The bandgap values and the band-edge positions of most common metal oxides used in heterogeneous photocatalysis are presented in Table 2.4.

2.2.1 Surface chemistry of metal oxides The nature and number of defects is often the key to the chemical reactivity of metal oxides. For photocatalytic reactions, the surface defect state traps the electron or hole, thus inhibiting the recombination process and favoring the subsequent redox reactions that occur at the surface of the metal oxide. According to the Terrace-Ledge-Kink model (TLK), there are five types of defects at the crystal surface (see Fig. 2.13):

▪ ▪ ▪ ▪ ▪

steps—one-dimensional defects in the form of steps; kink sites—point defects in steps; terraces—two-dimensional defects, such as stacking faults or twin planes, in the bulk solid; adatoms—the atom moved into any position; vacancy—the atom missing from the terrace.

24

Metal Oxide-Based Photocatalysis

Table 2.4 Bandgap values and positions of the VB and CB for selected metal oxides

Bandgap value, Eg (eV)

Upper edge position of valence band (V)

Lower edge position of conduction band

Ref.

Al2O3

1.3 1.2 7.0

– – +3.64

– – –

[13,14] [15–17] [18]

ɣ-Al2O3

7.6

+3.47

–

[18]

Bi2O3

2.8 2.6 2.04 2.48 3.2 2.79 3.24 2.81 2.61 3.4 3.0 2.1 2.0 2.17 1.83 2.2 1.94 2.1 3.1 2.2 0.14 4.6 4.8 4.56 4.7 4.5 4.9 4.8 4.67 3.6 2.65 2.8 2.8

+3.13 +2.76 +2.47 +2.78 – – +2.71 +2.775 +2.57 – – – – –
6.235 –
6.85 – – – – – – – – – – – – – +2.3 +2.17 +2.2

+0.33 +0.16 +0.43 +0.30 – –
0.55
0.035
0.04 – – – – –
4.405 –
4.91 – – – – – – – – – – – – –
0.35
0.63
0.6

[19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32–34] [35] [36,37] [35] [38] [37] [39,40] [29] [41] [42] [43] [43] [44] [45,46] [47] [43] [48] [49] [50] [51]

Metal oxide

Chemical formula

Silver (I) oxide Aluminum oxide Aluminum oxide Bismuth (III) oxide

Ag2O

Cerium (IV) oxide

α-Bi2O3 CeO2

Chromium (III) oxide Copper (IV) oxide

Cr2O3

Iron(III) oxide

α-Fe2O3

Cu2O

Fe2O3 Magnetite Gallium (III) oxide

Fe3O4 Ga2O3 α-Ga2O3 β-Ga2O3

Indium(III) oxide

ɣ-Ga2O3 In2O3

Fundamentals of metal oxide-based photocatalysis

Table 2.4

25

Continued Upper edge position of valence band (V)

Lower edge position of conduction band

Ref.

Metal oxide

Chemical formula

Bandgap value, Eg (eV)

Manganese (IV) oxide Niobium (V) oxide

MnO2

0.26–0.28

–

–

[52]

Nb2O5

3.2 3.1 3.1 2.95 3.5 3.4 3.55 2.6

+2.75 +2.20 +2.30 +2.43 – – – –

–0.45 –0.90 –0.8 –0.52 – – – –

[53] [54] [55] [56] [23,57,58] [59] [60] [61]

TiO2

3.5 3.8 3.2

+3.9 +4.1 +3.1

+0.4 +0.3
0.1

[62] [63] [63]

TiO2

3.0

+3.0

0

[63]

V2O5

2.3 2.7 2.7 3.2 3.2 3.37 5.0

– – – – +3.0 – +4.0

– – – –
0.2 –
1.0

[64] [37] [37] [6,40] [63] [36] [65]

Nickel(II) oxide

NiO

Ruthenium (IV) oxide Tin(IV) oxide Titanium (IV) dioxideanatase Titanium (IV) dioxiderutile Vanadium (V) oxide Tungsten (III) oxide Zinc(II) oxide Zirconium (IV) oxide

RuO2 SnO2

WO3 ZnO ZrO2

In addition to surface defects, bulk defects affect some metal oxide properties, such as conductivity. The most important types of bulk and surface defects on TiO2 are discussed below. The bulk structure of reduced TiO2
x crystals can contain various defects, such as doubly charged oxygen vacancies and Ti3+ and Ti4+ interstitial and planar defects, such as crystallographic shear planes [117]. The structure of these defects differs with oxygen deficiency and may be controlled by changing the temperature, gas pressure, and impurities during preparation. Ti interstitial defects are thought to be the dominant defect in the region from TiO1.9996 to TiO1.9999 (from 3.7 1018 to 1.3 1019 missing O atoms cm
3) [118].

26

Metal Oxide-Based Photocatalysis

Fig. 2.13 TLK model of defects at the surface of a single crystal. Reproduced with permission from G.E. Brown, V.E. Henrich, W.H. Casey, D.L. Clark, C. Eggleston, A. Felmy, D.W. Goodman, M. Gr€atzel, G. Maciel, M.I. McCarthy, K.H. Nealson, D.A. Sverjensky, M.F. Toney, J.M. Zachara, Metal oxide surfaces and their interactions with aqueous solutions and microbial organisms, Chem. Rev. 99 (1999) 77–174.

Surface defects, such as step edges, oxygen vacancies, line defects, impurities, and crystallographic shear planes, can be produced at the surface of TiO2 during preparation or posttreatment procedures (see Fig. 2.14). Some imperfections, e.g., vacancies, introduce changes into the electronic structure, e.g., a bandgap feature 0.8 eV below EF. Step edges at the surface of TiO2 (Fig. 2.14A) are created by sputtering and annealing in an ultrahigh vacuum. Fischer et al. [122] showed that the average increase in terrace area has a T1/4 dependence at temperatures above 800 K. Oxygen vacancies at the surface of TiO2 (Fig. 2.14B) may be formed by thermal annealing, sputtering, electron bombardment, UV irradiation, and also by some impurities, such as Ca and H (Fig. 2.14B). On the other hand, oxygen vacancies can be regenerated easily by exposing the reduced TiO2 sample to oxygen. In a photocatalytic reaction, the metal oxide particles on the surface must be taken into consideration in a solvent-containing system. Metal oxide particles suspended in H2O are amphoteric [7]. Hydroxyl groups on the surface of TiO2 undergo the following acid-base equilibrium reaction: s pKa1

> TiOH2 + ! > TiOH + H + s pKa2

> TiOH + ! > TiO
+ H +

(2.15) (2.16)

where >TiOH represents the “titanol” surface group, pK s a1 is the negative log of the microscopic acidity constant for the first acid dissociation (Eq. 2.15), and pK s a2 is that for the second acid dissociation (Eq. 2.16) [7]. The zero point of charge point of zero charge (pHzpc) is expressed as [7]
s  s pHzpc ¼ 1=2 pKa1 + pKa2

(2.17)

The surface acidity constants for Evonik P25 (formerly Degussa P25) are pKa1s¼ 4.5 and pKa2s¼ 9 for a pHzpc ¼ 6.25 [123]. This pHzpc value for TiO2 indicates that interactions with cationic electron donors and electron acceptors for heterogeneous

Fundamentals of metal oxide-based photocatalysis

27

Fig. 2.14 Images of TiO2 surface showing different types of defects: (A) step edges running parallel to [111] and [001] directions, with a kink site at the [111] step edge (marked ‘K’) [119]; (B) scanning tunneling microscope (STM) image of a TiO2 (110) surface showing point defects assigned to O vacancies [119]; (C) STM image of string growing out of the upper terrace (line defects) [120]; (D) STM image of a pair of crystallographic shear planes (CSPs) running in the [335] direction across reconstructed terraces [121]. Reproduced with permission from U. Diebold, The surface science of titanium dioxide, Surf. Sci. Rep. 48 (2003) 53–229; E. Yagi, R.R. Hasiguti, M. Aono, Electronic conduction above 4 K of slightly reduced oxygen-deficient rutile TiO2
x, Phys. Rev.B, 54 (1996) 7945–7956; S. Fischer, A.W. Munz, K.-D. Schierbaum, W. G€ opel, The geometric structure of intrinsic defects at TiO2(110) surfaces: an STM study, Surf. Sci. 337 (1995) 17–30.

photocatalytic activity will occur when pH > pHzpc, while interactions with anionic electron donors and acceptors will occur when pH < pHzpc [7].

2.2.2 Morphology of photocatalyst materials Critical review of the recent literature on the application of metal oxides in heterogeneous photocatalysis clearly shows that apart from their modification (discussed in Sections 2.1.1.1–2.1.1.5), designing metal oxides with exposed reactive facets and hierarchical morphologies is the main approach to enhancing their activity because

28

Metal Oxide-Based Photocatalysis

of the improved charge carrier separation kinetics and structural stability. The distinct active sites, active facets, and associated adsorption-desorption capacity with respect to reagents affect the photocatalytic performance of the metal oxide. Metal oxide nano- or microparticles can be synthesized as wires, rods, tubes, cubes, spheres, hollow spheres, plates, and flowers, as shown in Fig. 2.15. The size and shape of a particular metal oxide can be managed by varying the preparation method (e.g., sol-gel, solvothermal, microemulsion, and electrochemical techniques) as well as the preparation conditions (e.g., precursor type and concentration, pH, type of solvent, presence of structure-directing agents and mineral acids, temperature, electrolyte content, and applied voltage in electrochemical techniques) [124]. The morphology of a metal oxide is also characterized by porosity, including pore size distribution (microporous, 50 nm), pore shape (regular or irregular), nature of the pores, and pore volume [124]. The morphological features of hierarchical structures are achieved mainly by the presence of templates (soft or hard) or structure-directing agents (e.g., surfactants, alkylamines, Pluronic-type polymers, graft copolymers, biopolymers, crown ethers, and ionic liquids) [124,125]. Miscellaneous competitive processes, e.g., the generation of primary nuclei in larger concentrations, Ostwald ripening (including

Fig. 2.15 Illustrations of different metal oxide morphologies with representative electron micrographs. Based on M. Basu, A.K. Sinha, M. Pradhan, S. Sarkar, A. Pal, T. Pal, Monoclinic CuO nanoflowers on resin support: recyclable catalyst to obtain perylene compound, Chem. Commun. 46 (2010) 8785–8787; S.M. Mousavi, A.R. Mahjoub, R. Abazari, Green synthesis of ZnO hollow sphere nanostructures by a facile route at room temperature with efficient photocatalytic dye degradation properties, RSC Adv. 5 (2015) 107378–107388; C. Wang, J. Shi, X. Cui, J. Zhang, C. Zhang, L. Wang, B. Lv, The role of CO2 in dehydrogenation of ethylbenzene over pure α-Fe2O3 catalysts with different facets, J. Catal. 345 (2017) 104–112; Z. Gu, M.P. Paranthaman, J. Xu, Z.W. Pan, Aligned ZnO nanorod arrays grown directly on zinc foils and zinc spheres by a low-temperature oxidization method, ACS Nano 3 (2009) 273–278; G. Zhang, W. Guan, H. Shen, X. Zhang, W. Fan, C. Lu, H. Bai, L. Xiao, W. Gu, W. Shi, Organic additives-free hydrothermal synthesis and visible-light-driven photodegradation of tetracycline of WO3 nanosheets, Indust. Eng. Chem. Res. 53 (2014) 5443–5450; P. Mazierski, J. Nadolna, W. Lisowski, M.J. Winiarski, M. Nischk, T. Klimczuk, A. Zaleska-Medynska, Effect of irradiation intensity and initial pollutant concentration on gas phase photocatalytic activity of TiO2 nanotube arrays, Catal. Today 284 (2017) 19–26.

Fundamentals of metal oxide-based photocatalysis

29

dissolution and recrystallization), oriented attachment and growth, and assembling and interlacing of nanostructures as a result of dipole-dipole interactions, are performed in the assemblage of each nanostructure unit [126]. The effects of the method and conditions of preparation on the morphology and activity of selected metal oxides are discussed in Chapter 3. A hollow structure, such as a hollow sphere (TiO2, WO3, and ZnO), allows multiple reflections of incident photons within the interior cavity, promoting maximum photon absorption [127–129]. In a porous structure, macropores act as a light transfer path for the introduction of incident photon flux onto the inner surface of the structure, resulting in deeper penetration of the photocatalyst [130]. The crystal structure of metal oxides is defined by the atomic arrangement of the basic unit cell, different locations of band edges, various adsorption capacities for oxygenated species, diverse acid-base behavior that affects the carrier transfer pathways, and the redox potential of photogenerated e
/h+ pairs [131]. Moreover, each polycrystalline material is characterized by facets with different levels of interaction between the molecules or ions and the surface [132]. The common understanding of the atomic structure of a crystal surface is that the facets with a higher content of undercoordinated atoms are usually more reactive in heterogeneous reactions [133]. Crystal shapes and exposed facets for the most common metal oxides are given in Table 2.5. Of the three TiO2 polymorphs, anatase, brookite, and rutile, anatase is the most preferred for heterogeneous photocatalysis. The anatase polymorph is characterized by (001), (100), and (101) facets with average surface energies of 0.90, 0.53, and 0.44 J m
2, respectively. In addition to its high surface energy, anatase has a low atomic coordination (unsaturated) number of exposed atoms and the wide bond angle of TidOdTi promotes the superior activity of the (001) facet [132]. Moreover, the (101) and (001) facets on TiO2 anatase serve as efficient reducing and oxidizing sides, respectively [168]. The wurtzite phase of ZnO comprises polar facets from Zn-terminated (0001) and O-terminated (000-1) facets with positive and negative charge, respectively. The presence of an internal electric field between facets with opposite charges conducts the migration of electrons and holes toward the positive and negative polar planes, respectively. In theory, the coordinatively unsaturated environment of Zn ions on the (0001) plane attracts more hydroxyl anions and oxygen through chemisorption or physisorption, which promotes the generation of oxidative species, such as H2O2 or OH radicals. The Zn-(0001) facet has the highest likelihood for chemisorption and the highest surface energy, resulting in its higher reactivity than that of the O-(000-1) and nonpolar (10-10) surfaces [132]. Because data on the dependence of crystal facet activity in WO3 is limited, we can only assume that orthorhombic WO3 with (001) facets generates more OH and O2 
, resulting in faster degradation of pollutants [169]. l

2.2.3 Grain size effects Many authors have confirmed the significance of the size of semiconductor particles with respect to photocatalytic activity. In general, the photocatalyst grain should be small while the surface area should be large. For small-grain photocatalysts,

Table 2.5 Metal oxide Anatase TiO2

Different shapes of metal oxide crystals Other theoretically possible crystal shapes

Experimentally produced faces

Surface energy of facets

(101) [134–136] (001) [137–139] (010) [140] (110) [139] (103) [141] (105) [142] (106) [140,143] (107) [142] (201) [144,145] (401) [144] (301) [136]

(110): 1.09 J m
2 (001): 0.90 J m
2 (010): 0.53 J m
2 (101): 0.44 J m
2 (103): 0.93 and 0.83 J m
2 for faceted and smooth configurations, respectively (105): 0.84 J m
2

Rutile TiO2

(110) [146,147] (011) [148] (001) [149] (111) [150,151]

(110): 15.6 meV au
2 (100): 19.6 meV au
2 (011): 24.4 meV au
2 (001): 28.9 meV au
2

Brookite TiO2

(210) [152–154] (201) [154] (101) [154] (111) [153,155,156] (110) [155,156] (100) [156,157] (001) [152,153,155,156,158] (010) [156] (011) [156] (001) [160] (200) [160] (100) [160,161] (002) [160]

(100): 0.88 J m
2 (010): 0.77 J m
2 (001): 0.62 J m
2 (011): 0.74 J m
2 (101): 0.87 J m
2 (111): 0.72 J m
2 (210): 0.70 J m
2 [159]

WO3

(001) 1.74 J m
2 [162] (100) 1.69 J m
2 [162] (010) 1.69 J m
2 [162] Continued

Table 2.5 Metal oxide ZnO

Continued Other theoretically possible crystal shapes

Experimentally produced faces

Surface energy of facets

(0001) [163,164] (0110) [163,165] (0001) [163,165] (0111) [163] (1010) [164,166] (1120) [164,166] (2110) [165,166] (4223) [166]

(1010) [166] (1120) [166] (2110) [166] (4223)

0.941 J m
2 1.503 J m
2 1.508 J m
2 1.863 [166]

CuO

(001) [167] (010) [167] (100) [167] (011) [167] (101) [167] (110) [167]

(001) 4.23 J m
2 [167] (010) 1.04 J m
2 [167] (100) 1.73 J m
2 [167] (011) 1.22 J m
2 [167] (101) 2.51 J m
2 [167] (110) 2.69 J m
2 [167]

Reproduced with permission from G. Liu, H.G. Yang, J. Pan, Y.Q. Yang, G.Q. Lu, H.-M. Cheng, Titanium dioxide crystals with tailored facets, Chem. Rev. 114 (2014) 9559–9612; X.-Q. Gong, A. Selloni, First-principles study of the structures and energetics of stoichiometric brookite TiO2 surfaces, Phys. Rev. B 76 (2007) 235307; M. Ramamoorthy, D. Vanderbilt, R.D. King-Smith, First-principles calculations of the energetics of stoichiometric TiO2 surfaces, Phys. Rev. B 49 (1994) 16721; M. Lazzeri, A. Vittadini, A. Selloni, Structure and energetics of stoichiometric TiO2 anatase surfaces, Phys. Rev. B 63 (2001) 155409; D. Su, X. Xie, S. Dou, G. Wang, CuO single crystal with exposed {001} facets-A highly efficient material for gas sensing and Li-ion battery applications, Sci. Rep. 4 (2014) 5753.

34

Metal Oxide-Based Photocatalysis

Fig. 2.16 Quantum size effect on semiconductor bandgap for ZnO. Based on A.L. Linsebigler, G. Lu, J.T. Yates Jr, Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758; T.J. Jacobsson, T. Edvinsson, Absorption and fluorescence spectroscopy of growing ZnO quantum dots: size and band gap correlation and evidence of mobile trap states, Inorg. Chem. 50 (2011) 9578–9586.

photogenerated electrons and holes are easily transported from the bulk to the surface and the surface charge transfer rate is enhanced because of the higher quantity of available adsorbates at the surface of the photocatalyst. If the diameter of the semiconductor particle is 1010 K s
1), providing a unique environment for chemical reactions. These extraordinary conditions permit access to a range of chemical reaction space normally not available, thereby allowing the synthesis of a wide variety of unusual nanostructured materials [85,86]. There are many advantages of the sonochemical method, such as increased reaction speed and output, as well as more efficient energy usage, etc. Moreover, this technique allows the use of crude technical reagents and may cause the activation of metal and solids. During the synthesis of metal or metal oxides, the main advantage of sonochemical methods is that no chemical reducing agent is needed. Sonochemical irradiation of both water and organic solvents produces radicals which can act as reducing agents. However, while very small nanoparticles may be obtained with sonochemical synthesis, it tends to produce only spheres, which limits the use this method in applications that require shape tuning [87,88]. In addition, the sonochemical method is considered to be a green approach, because water is the major liquid component used in most sonochemical reactions. The free radicals H and OH are generated in the aqueous solution under ultrasonic irradiation [89]: l

l

H2 O + ultrasonication ! H +  OH

(3.1)

These free radicals are highly unstable, and they can either recombine into H2O or produce H2 and H2O2. They can also produce HO2  by combination with O2. These strong oxidants and reductants are utilized for various sonochemical reactions in aqueous solutions [85]: H + H ! H2 

OH +  OH ! H2 O2

(3.2) (3.3)

60

Metal Oxide-Based Photocatalysis

A possible formation mechanism for CuO/CuO2/Cu nanoparticles was presented as [90]: CuðCH3 COOHÞ2  H2 O + H2 O ! Cu2 + + CH3 COO


(3.4)



H ðO Þ + Cu2 + ! Cu +

(3.5)



H ðO Þ + Cu + ! Cu0

(3.6)

Cu2 + + OH
! CuðOHÞ2

(3.7)

Cu + + OH
! CuðOHÞ

(3.8)

CuðOHÞ2 + CuðOHÞ ! CuO=Cu2 O Δ 400° C under N2




(3.9)

Suslick and coworkers for the first time prepared nanostructured metals using the sonochemical method [91]. They studied the effects of different cavitation conditions on the synthesis of amorphous iron. Suslick’s group showed that, by changing the experimental parameters that control the conditions of bubble collapse, different chemical environments could be created. Despite the fact that the sonochemical method usually produces spherical particles, a few research groups successfully obtained other shapes, such as silver nanoplates and gold nanorings [92]. Sonochemical synthesis may be combined with other available synthetic methods, such as hydrothermal [93]. In the case of metal oxides used in photocatalysis, composites were very often achieved by this method. Examples include TiO2 [94], SnO2 [95], NiO [96,97], ZnO [98], TiO2/WO3 [99], Zn2SnO4-SnO2 [100], Cu2OCu [101], Cu2O-rGO [102], and ZnO-Ce [103].

3.1.7 Electrochemical method In a typical electrochemical synthesis, the reactant dissolved in the electrolyte is deposited as a solid thin film or coating of the product on the electrode. Further, a solid-liquid interface facilitates the growth of conformal coatings on substrates of arbitrary shape, especially if a suitably shaped counter electrode is employed to provide uniform polarization. Consequently, the activity of the reactant decreases as the reaction proceeds. The two important parameters that determine the course of the reaction are the deposition current and the cell potential, and either one can be controlled as a function of time during the reaction. When designing an electrochemical experiment, it is important to choose the electrode (inert or reactive), electrolyte (concentration and composition of the solution), cell (divided or undivided), temperature, and pH of the process. Electrosynthesis may be used to obtain products that are very difficult to achieve in conventional chemical synthesis. This is due to the very high potential gradient (105 V cm
1) around the electrode within the electric double layer, and the fact that electrochemical synthesis takes place close to this electrode. The composition of the film product can be controlled by varying the bath composition.

Metal oxide photocatalysts

61

Moreover, the electrochemical synthesis is a low-temperature process limited by the boiling point of the electrolyte, which, however, can be raised by using molten salts. During the experiments, kinetic control can be achieved by controlling the current passing through the cell. Also, by choosing the applied cell potential, thermodynamic control can be exercised. In general, the synthesis is simple to perform, and the equipment is cheap and easily available. The weaknesses of the method include poorly ordered products that make unequivocal structural characterization difficult. The electrochemical methods used for metal oxide preparation include: l

l

Anodic oxidation (Fig. 3.6A) of metal species in electrolytes, such as aqueous solution containing fluorides, organic-based electrolytes containing fluorides, or hot phosphate/glycerol mixtures. Anodization of some metals and alloys (such as Ti, Nb, Ta, Zr, Hf, W, and Al) leads to the formation of compact oxide layers, porous oxides in the form of nanopores, or nanotubes (ordered or disordered layers). In general, the morphology and structure of porous layers are strongly affected by the electrochemical conditions (particularly the anodization voltage) and the solution parameters (in particular the HF concentration, the pH, and the water content in the electrolyte). Similar to the case of TiO2, this method was also implemented for many other metals, such as Zr, Nb, W, Ta, and Hf. In the cases of Ti, Hf, and Zr, anodization leads to the formation of metal oxide nanotubes, while for W, Nb, and Ta it leads to the growth of porous oxide layers. Cathodic electrodeposition (Fig. 3.6B) is a two-stage process: (1) cathodic electrodeposition of a metal oxyhydroxide gel film from aqueous solution that contains a metal precursor and (2) subsequent heat-treatment of this gel film results in the formation of metal oxide film [104]. Reduction may take place at the working electrode without the possibility of

Fig. 3.6 Schematic diagram for the (A) anodic oxidation, (B) cathodic electrodeposition, and (C) anodic electrodeposition processes. Based on J.M. Macak, H. Tsuchiya, A. Ghicov, K. Yasuda, R. Hahn, S. Bauer, P. Schmuki, TiO2 nanotubes: self-organized electrochemical formation, properties and applications, Curr. Opin. Solid State Mater. Sci. 11 (2007) 3–18.

62

l

Metal Oxide-Based Photocatalysis

dissolution of the metallic substrate into the electrolyte, and the oxides can be obtained on a vast variety of metallic substrates (such as aluminum and its alloys, magnesium alloys, stainless steel, and galvanized and Zn-plated steels) by cathodic electrodeposition [105]. In this process, the rates of electrochemical reactions, and the nucleation and growth can be controlled by changing the current density, potential, bath temperature, and reaction medium. There are three possible cathodic reactions: reduction of water, dissolved oxygen, and nitrate ion. The first reaction is accompanied by hydrogen gas evolution, whereas the remaining two reactions produce soluble products. H2 evolution has a detrimental effect on the morphology of cathodically electrodeposited nanostructures. The bubbles formed in the process provide a dynamic template for the particles being deposited on the electrode surface [105,106]. Hence, nanoparticle formation is favorable at low hydrogen evolution rates, while nanorod or nanowire formation is favorable at high hydrogen formation rates. The electrochemical synthesis of TiO2 thin films via cathodic deposition leads to the formation of a TiO(OH)2xH2O gel film on cathodic substrates from aqueous peroxo-titanium solutions. An aqueous solution of TiOSO4 in the presence of NO3
ions is usually used as a substrate. When the substrate is cathodized at potentials below about
0.9 V, nitrate is reduced to generate OH
ions. The electrochemically generated base traps soluble Ti species to form a Ti hydroxide gel film on the electrode. The deposited film is then heated for crystallization of TiO2. Recently, cathodic electrodeposition of thin films of various metal oxides from aqueous solutions was successfully achieved, such as ZrO2 [107], Y2O3 [108], Nb2O5 [109], CeO2 [110], Mn2O3 [106], and ZnO [111]. Anodic electrodeposition (Fig. 3.6C) of TiO2 from TiCl3 solution or Ti(IV)-alkoxide stabilized solutions. Two types of anodic electrodeposition solutions have been reported: those that use and do not use ligand anions (oxalate, EDTA, etc.) as complexing species for metal ions. There are only a few published reports regarding the production of metal oxides by this method: CuO [112], CoO [113], and Ir(III) and Ir(IV) oxyhydroxides [114].

Often, the product comes with impurities that appear amorphous under X-ray. Additionally, electrodeposition can only be carried out on conducting substrates [115–117]. The electrochemical method has been applied to synthesize a variety of metal oxides: TiO2 [118,119], Au/TiO2/WO3 [120], ZnO [121], Fe2O3 [122], ZnO/ CuO [123], carbondots/TiO2, and Co-doped ZnO/rGO. Ordered nanotubes made of TiO2 mixed with a second metal oxide and perpendicularly aligned to the substrate surface may be prepared by one-step anodic oxidation of titanium alloys [119,124,125].

3.1.8 Dip-coating method Dip coating represents the oldest commercial coating process among the various wet chemical methods for thin film deposition. In general, the dip-coating process consists of the following steps [126]: l

l

l

Immersion: the substrate is immersed in the solution of the coating material at a constant speed (preferably jitter-free); Start-up: the substrate is kept in the solution for a time before the removal process begins. Deposition: A thin layer deposits itself on the substrate while the latter is pulled up. The withdrawal is carried out at a constant speed to avoid any jitters. The pulling speed determines the thickness of the coating, that is, a faster withdrawal gives a thicker coating;

Metal oxide photocatalysts l

l

63

Drainage: excess liquid is allowed to drain from the surface; Evaporation: the solvent evaporates from the liquid to leave a thin layer. For volatile solvents, such as alcohols, the evaporation starts already during the deposition and drainage steps.

To date, the dip-coating method has been combined with other approaches, such as ultra sonification [127]. Very often, a sol-gel precursor solution is used in the dipcoating process. The reason is that, by modifying the size and structure of the inorganic species in the sol together with the solvent(s), the sol-gel offers the most possibilities to influence the film properties [128]. The following metal oxides that could potentially be used in photocatalysis have been obtained by dip coating or sol-gelderived coatings: TiO2 [129], MxCoyOz with M ¼ Mn, Cu, Ni [130], ZnO [131,132], and multilayered graphene-metal oxides [133].

3.1.9 Liquid-phase deposition Liquid-phase deposition (LPD) is a unique soft solution process and it is performed by very simple procedures. This process allows thin films of metal oxide or hydroxide to be formed on the substrate through the ligand-exchange (hydrolysis) equilibrium reac
tion of metal-fluoro complex species MFx ðx
2nÞ
and the F
consumption reaction with boric acid (H3BO3, a F
scavenger). In the treatment solution, MFx ðx
2nÞ
is hydrolyzed following the ligand-exchange equilibrium reaction [134,135]: MFx ðx
2nÞ
+ nH2 O ¼ MOn + xF
+ 2nH + ðdeposition reactionÞ

(3.10)

The equilibrium reaction is shifted toward the right-hand side by the addition of boric acid or aluminum metal as F
ion scavenger, which readily reacts with F
and forms a stable complex as follows: H3 BO3 + 4H + + 4F
¼ HBF4 + 3H2 O ðF
consumption reactionÞ

(3.11)

The addition of the F
scavenger leads to the consumption of free F
ions, and the ligand-exchange reaction is accelerated. The main advantage of this synthetic method over gas-phase processes is the freedom from using special equipment, such as vacuum systems. Moreover, it can be easily applied to various substrates with a large surface area or complex morphology [135]. The LPD process is a simple way to produce various metal oxides: TiO2 [136,137], SnO2-TiO2 [138], ZnO [139], WO3/TiO2 [140], Fe2O3 [141], SrTiO3 [142], and h-MoO3nH2O [143].

3.1.10 Nanosphere lithography Nanosphere lithography (NSL) is a technique used for generating single layers of nanoscale features that are hexagonally close packed or in similar patterns. This method may produce regular and homogenous arrays of nanoparticles with different sizes and with precisely controlled spacings. Moreover, it is inexpensive and simple to

64

Metal Oxide-Based Photocatalysis

Fig. 3.7 Nanosphere lithography process. Based on P. Colson, C. Henrist, R. Cloots, Nanosphere litography: a powerful method for the controlled manufacturing of nanomaterials, J. Nanomater. 2013 (2013) 19 pages, Article ID 948510.

implement [144]. NSL consists of two main stages: mask preparation followed by nanostructure production (Fig. 3.7) [145]. At the beginning, the flat substrate undergoes a chemical treatment to enhance its hydrophilic character, and then it is coated with a suspension containing monodisperse spherical colloids (e.g., polystyrene). Next, upon drying, self-organization of a monolayer or bilayer of hexagonal-close-packed (HCP) features takes place. The thus-formed mask is then used to selectively pattern the substrate via deposition of the material of interest through the interstices of the ordered beads. The subsequent removal of the mask (lift-off ) by sonication in an appropriate solvent, or by stripping, leaves an array of ordered nanodots on the surface of the substrate. An annealing step is sometimes necessary to crystallize the sample and/or induce a crystallographic phase change [145].

Metal oxide photocatalysts

65

Nanosphere lithography has been used to prepare some metal oxides like TiO2 [146], NiO [147], and ZnO [148] (NSL and hydrothermal growth).

3.2

Unitary metal oxides (advantages, disadvantages, achievements)

Metal oxide-based heterogeneous photocatalysts could be applied to a wide range of applications in environmental technologies, which are mentioned in Section 2.1 and described in detail in Chapter 4. The current chapter is focused on selected groups of the most promising photocatalysts, such as titanium dioxide (TiO2), tungsten trioxide (WO3), zinc oxide (ZnO), iron (III) oxide (Fe2O3), tantalum pentoxide (Ta2O5), and copper oxide (CuO), with particular emphasis on the correlation among preparation methods, synthesis conditions, crystal structures, electronic properties or surface properties, and photocatalytic activity under UV or visible light irradiation in several typical reactions.

3.2.1 TiO2 (structure, surface properties, and photocatalytic activity of pristine and doped TiO2) 3.2.1.1 Structure and surface properties Titanium dioxide (TiO2) is a semiconductor which belongs to the family of transition metal oxides. It has attracted much interest, with a wide range of applications, such as photovoltaic cells, gas sensors, pigments, and photocatalysis [149]. Besides the four polymorphs of TiO2 found in nature (tetragonal anatase, orthorhombic brookite, tetragonal rutile, and monoclinic TiO2), two additional high-pressure forms have been synthesized starting from rutile: TiO2(II) which has the PbO2 structure, and TiO2 (H) with the hollandite structure [149]. The primary source and the most stable form of TiO2 is rutile. In contrast, anatase and brookite are metastable, transforming to rutile under calcination (typically 600–700°C) [150]. In all three forms, the titanium (Ti4+) atoms are coordinated to six oxygen (O2
) atoms, forming TiO6 octahedra (Fig. 3.8). The three crystal structures differ by the distortion of each octahedron and by the assembly patterns of the octahedral chains. The rutile unit cell contains two Ti atoms (at {0, 0, 0} and {½, ½, ½}) positions), and four oxygen atoms that form a distorted octahedron around Ti. The anatase unit cell contains four Ti atoms (at {0, 0, 0}, {½, ½, ½}, {0, ½, ¼}, and {
½, 0,
¼}), and eight oxygen atoms that form a distorted TiO6 octahedron around each Ti cation [151].

3.2.1.2 Photocatalytic activity Titanium dioxide is typically an n-type semiconductor due to oxygen deficiency, with the bandgaps of 3.2 eV for anatase, 3.0 eV for rutile, and  3.2 eV for brookite. The valence band (VB) of TiO2 is composed of the 2p orbitals of oxygen hybridized with

66

Metal Oxide-Based Photocatalysis

Fig. 3.8 Crystalline structures of titanium dioxide: (A) anatase, (B) rutile, and (C) brookite. Reproduced with permission from M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, P.S. Dunlop, J.W. Hamilton, J.A. Byrne, K. O’Shea, A review on the visible light active titanium dioxide photocatalysts for environmental applications, Appl. Catal. B Environ. 125 (2012) 331–349.

the 3d orbitals of titanium, while the conduction band (CB) is composed only of the 3d orbitals of titanium. The CB energy level (ECB ¼
0.51 V at pH 7) lies slightly above the reduction potential of oxygen (E0 ðO2 =O2 
Þ ¼
0:33V), which is a ubiquitous and predominant oxidant (or electron acceptor) in the environment. The photoinduced reduction of O2 produces superoxide radical anion (O2 
) and hydroperoxyl radical (E0 ðO2 =HO2  Þ ¼
0:45V at pH 7). The latter is further reduced to hydrogen peroxide (E0(HOO /H2O2 ¼ +1.007 V at pH 7). On the other hand, the TiO2 VB (EVB ¼ + 2.69 V at pH 7) is far lower (more positive) than the oxidation potentials of most organic and inorganic compounds (electron donors). The highly oxidative holes can be transferred to the surface-adsorbed water or hydroxide groups (i.e., titanol group) to form the surface-bound hydroxyl radical ( OHsurf) (or free hydroxyl radical OHfree after desorption from the surface) [152]. For many applications of TiO2, the particle size, crystal structure and phase, porosity, and surface area influence the photoactivity. The good properties of nano TiO2 are due to its low dimensionality and quantum size effect. TiO2 nanocrystals have several advantages over their bulk counterpart because of their high surface-to-volume ratio, increased number of delocalized carriers on the surface, improved charge transport and lifetime afforded by their dimensional anisotropy, and the efficient separation of photogenerated holes and electrons [153–155]. Because of these factors, a variety of methods have been used for TiO2 preparation, such as sol-gel, hydrothermal, solvothermal, anodic oxidation, microwave-assisted, hard template, reverse microemulsion, direct oxidation, nonhydrolytic sol-gel, sonochemical, chemical and physical vapor deposition, electrodeposition, and ionic liquid-assisted methods [151,156]. Physically modified TiO2, in the form of nanoparticles, nanotubes, nanorods, nanofibers, nanoflowers, and other morphologies have shown different degrees of l

l

l

Metal oxide photocatalysts

67

Fig. 3.9 Schematic illustration of the structural dimensionality of TiO2 materials with the expected properties. Based on K. Nakata, A. Fujishima, TiO2 photocatalysis: design and applications, J. Photochem. Photobiol. C: Photochem. Rev. 13 (2012) 169–189.

photoactivity improvement, because the structural dimensionality is one of the most important factors that affect the photocatalysis (see Fig. 3.9). For example, a sphere with zero dimensionality has a high specific surface area, resulting in a high rate of photocatalytic decomposition of organic pollutants [157]. One-dimensional fibers or tubes benefit from reduced charge recombination, because of the short distance for charge carrier diffusion, good light-scattering properties, and the capacity to fabricate self-standing nonwoven mats [158]. Two-dimensional nanosheets have smooth surfaces and high adhesion, whereas three-dimensional monoliths may have high carrier mobility as a result of their interconnecting structure and can be used in environmental decontamination [159]. TiO2 has been applied as a photocatalyst for the degradation of organic contaminants and xenobiotic compounds in both water and gas phases, as well as for the decolorization of dyes, inactivation of microorganisms, hydrogen production, and photoconversion of CO2. TiO2 spheres (zero-dimensional) are the most widely studied materials because they possess a high specific surface area, high pore volume, and large pore size, which increase the size of the accessible surface area and the rate of mass transfer for organic pollutant adsorption [160–162]. Furthermore, these structural features increase the light-harvesting capabilities of these materials, because they allow the maximum amount of light to access the interior. TiO2 spheres are generally prepared from a

68

Metal Oxide-Based Photocatalysis

titanium alkoxide, such as titanium tetraisopropoxide or titanium tetrabutoxide in the presence of a polymer, to provide a porous structure, and sometimes with or without the addition of an acid to accelerate the reaction [157]. The TiO2 spheres thus obtained may be further treated by hydrothermal methods to produce porous or hierarchical structures. Moreover, the synthesized spheres can have different structures that depend on the annealing temperature [162]. The TiO2 spheres formed at 400°C showed the highest photocatalytic activity for decomposition of organic molecules, because of their high specific surface area and highly crystalline form [162]. Also, the formation of hollow structures is a strategy to obtain highly photoactive materials, because a hollow structure not only implies a high specific surface area but also enables multiple diffractions and reflections of light [161,163,164]. The obtained sphere-in-sphere samples had high photocatalytic activity, likely because of the multiple scattering and reflections of light within the TiO2 spheres that extend the light path length [161]. It is known that anatase TiO2 crystals with exposed high-energy {001} facets have high photocatalytic activity. Therefore, TiO2 spheres with such facets exhibited enhanced photocatalytic activity, which can be attributed to their three-dimensional (3D) hierarchical structures, exposed {001} facets, and more active sites than either 1D or 0D architectures [165]. The high specific surface area, high pore volume, suitable pore size, spherical core-shell structure, and exposed high-energy facets of these materials make them attractive candidates for photocatalytic applications. A second group of TiO2 materials consists of one-dimensional structures, such as fibers and tubes. Based on the literature data, it can be concluded that in fibers and tubes, the higher surface-to-volume ratio enables a reduction in the hole-electron recombination rate and increases the interfacial charge carrier transfer rate. Furthermore, TiO2 nanotubes possess a number of other attractive properties, such as potentially enhanced electron percolation pathways and light conversion, as well as improved ion diffusion at the semiconductor-electrolyte interface [166]. The morphological structure of anodized TiO2 nanotubes, as well as their photocatalytic activity, can be modified by changing the preparation conditions, such as anodization time, applied voltage, temperature, Ti foil roughness, calcination parameters, and the electrolyte composition (including fluoride concentration, solvent, water content, pH, viscosity, conductivity, and organic additives). Increasing the length of the TiO2 nanotubes improves the photocatalytic performance because of the increased specific surface area. An exhaustive and consistent work on TiO2 nanotubes as photocatalysts is not available at present, due to the large number of influencing parameters: tube characteristics (crystallinity, length, diameter, and more detailed geometric and compositional effects), reactant concentration, hydrodynamic conditions, light intensity, wavelength distribution, etc. Based on the literature, it seems that different crystalline structures are induced in the TiO2 nanotubes (i.e., anatase or rutile) according to the heat treatment temperature and atmosphere, and the electrical properties change from semiconducting to semimetallic. There is a temperature limit (500–800°C depending on the preparation procedure) above which the sintering of particles and collapse of the nanotube structure occur, and any benefits associated with an ordered nanostructure are lost [167]. Moreover, electrochemically prepared TiO2 nanotubes are amorphous, and some reports indicate the presence of nanocrystallites in the tube wall,

Metal oxide photocatalysts

69

particularly if anodization is carried out at higher voltages [168,169]. In acidic electrolytes (e.g., H2SO4), depending on the anodization conditions, the formed oxide film may consist of anatase, a mixture of anatase and rutile, or rutile crystallites [170]. However, claims of the presence of crystallites in the as-formed oxide nanotubes should be examined very critically, as amorphous TiO2 seems to be particularly prone to electron beam-induced crystallization [171]. TiO2 nanosheets, with thickness of 1–10 nm and a lateral size from the submicrometer level to tens of micrometers, display excellent adhesion to substrates and can be used for photocatalytic decomposition of organic molecules. A pioneering work in the preparation of anatase TiO2 with exposed {001} facets was reported by Yang et al., who synthesized single crystallites with a high percentage of {001} facets (initially 47%) from the hydrothermal reaction of a mixture of TiF4 and HF [172]. With the help of 2-propanol, the percentage of {001} facets was improved to 64%, and the resulting anatase crystals were reported to generate more active hydroxyl radicals ( OH) upon UV irradiation compared to commercial Degussa P25. Zhang et al. obtained anatase TiO2 single-crystals and observed that the photocatalytic activity increased proportionally as the level of reactive {001} facets was continuously tuned from 27% to 50% [173]. The photocatalytic performance of TiO2 nanosheets was examined in the decomposition of methyl orange, 2-propanol and methylene blue [174,175]. Due to the high percentage of exposed {001} facets, the samples showed excellent photocatalytic efficiency. Moreover, it has been reported that TiO2 particles with specific exposed crystal faces, namely, decahedral anatase with {101} and {001} exposed crystal faces and dodecahedral rutile with {110} and {101} exposed crystal faces, also showed excellent photocatalytic activity despite the large particle size ( 1 μm) [176]. Murakami et al. indicated that the different energy levels of the conduction and valence bands, which are determined by the type and arrangement of the constituent atoms, drive the electrons and positively-charged holes to different exposed crystal faces, resulting in a decrease in the back-reaction rate by predominant progress of reduction and oxidation on each crystal face [177]. Consequently, during the last few years much effort has been directed toward developing modified titania to maximize the utilization of solar radiation. Among these attempts, doping has been the common approach for improving the photoresponse of TiO2 photocatalysts in both the UV and visible regions. These methods include metal and nonmetal doping, co-doping (with metal-metal, metalnonmetal, and nonmetal-nonmetal), and doping with various elements restricted to the tridoping system. When doping with metallic or nonmetallic elements, the doped ions are either incorporated into the bulk TiO2, or highly dispersed on the surface as clusters or mononuclear complexes [178]. l

Nitrogen-doped TiO2 Nitrogen can be easily introduced into the TiO2 structure, due to its comparable atomic size with oxygen, small ionization energy, and high stability. In N-doped TiO2, elemental nitrogen permeates to the lattice of TiO2 and substitutes for a lattice oxygen atom to form nitride (TidN) or oxynitride (OdTidN) arrangements. Such doping

70

Metal Oxide-Based Photocatalysis

can introduce energy states above the valence band of titania due to the mixing of the N 2p with O 2p states, or create a new mid-gap state by N incorporation [179,180]. It was confirmed that oxygen vacancies, stabilized by the presence of nitrogen as a result of charge compensation, might act as color centers and impart a visible-light response [181]. Indeed, the oxygen vacancies stabilized by nitrogen doping could induce visible-light absorption [182]. Various doping technologies have been reported, such as the nitrification of TiO2, the oxidation of TiN, ion implantation of TiO2, and the sol-gel method [183]. Asahi and co-workers explored for the first time the visible light activity of N-doped TiO2 produced by sputter deposition of TiO2 under an N2/Ar atmosphere, followed by annealing under N2 [184]. Since then, many reports have been devoted to investigating the structural, electronic, and optical properties of N-doped TiO2, understanding the underlying mechanisms, and improving the photocatalytic and self-cleaning efficiency under visible and solar light. There remain uncertainties over the mechanisms of N-doping in improving light absorption and photocatalytic efficiency of TiO2. Literature data show that: l

l

l

oxygen vacancies, stabilized by nitrogen as a result of charge compensation, might act as color centers, imparting a visible light response [185] nitrogen is bound to hydrogen as NHx species in interstitial sites [186] energy states in a nitrogen doping configuration were formed inside the titania bandgaps, accounting for the red shift of the energy gap and the induced visible-light photocatalytic activity [180]

However, none of the available studies demonstrated the exact chemical states of N species that are responsible for the acquired visible-light photocatalytic activity of TiO2. The visible-light photocatalytic activity of N-doped TiO2 samples has been tested in the degradation of pharmaceuticals and pollutants, such as 4-chlorophenol, formic acid, trichloroethylene, hydroquinone, acetaldehyde, stearic acid, benzene, and carbon monoxide [180,187–190]. N-doped titania exhibited superior visible-light activity compared to pure TiO2. It was concluded that N doping leads to the appearance of mid-bandgap states, as well as the generation of a manifold of surface states that are located close to the valence-band edge. The presence of these energy levels induces efficient photocatalytic activity. N-doped TiO2 nanoparticles were synthesized via an ultrasonic-assisted impregnation reaction method for photocatalytic oxidative desulfurization [191]. The N-doping reduced the crystallite size, increased the specific surface area of TiO2, and enhanced the visible light absorption.

Sulfur-doped TiO2 Doping sulfur into the TiO2 lattice is more difficult than for nitrogen due to its larger ionic radius. Insertion of cationic sulfur (S6+) into the lattice is chemically favorable over the anionic form (S2
). This behavior can be qualitatively rationalized by the low solubility of S2
ions in the titania lattice, due to the relatively larger ionic radius of S2
compared to that of O2
and the concomitant increase of the TidS bond formation energy. Sulfur was detected as hexavalent (S6+), tetravalent (S4+), or sulfide (S2
), depending on the synthetic method of S-doped TiO2 or sulfur precursor, leading

Metal oxide photocatalysts

71

to intra-gap impurity states between the VB and CB [192]. Experimental results revealed substitution of Ti4+ by S6+ to form TidOdS bonds for cationic S-doping, and reduced crystalline grain size due to the smaller ionic radius of S6+ compared to Ti4+. In anionic S-doping, the substitution of S2
by O2
to form OdTidS bonds could be achieved, and S2
substitution increased the crystalline grain size in anionic S-doped TiO2 samples due to a larger electronegativity difference. In cationic S-doped TiO2 photocatalysts, chemisorbed hydroxyls (OHads
) and photoinduced holes (h+) play a major role in photocatalysis, while electrons (e
) and h+ play nearly the same role for anionic S-doped TiO2 photocatalysts. Besides narrowing the energy bandgap of TiO2, the anatase-to-rutile ratio could also be modified by introducing S into TiO2 [193]. Moreover, it was confirmed that the S atoms are not only incorporated into the crystal lattice of TiO2 as S4+ to replace Ti4+, but also anchored on its surface in the form of SO42
[194]. The higher photocatalytic activity was mainly due to the synergistic effects of S4+ and SO42
. The S4+ substituted for Ti formed an impurity level below the CB of TiO2, extending the light response into the visible region. SO42
anchored on the surface was favorable for trapping photo-induced electrons. It therefore suppressed the recombination of e
/h+ and consequently promoted the formation of hydroxyl radicals. Many strategies have been adopted to synthesize S-doped TiO2 photocatalysts, from the oxidative annealing of TiS2, to catalyzed hydrolysis, hydrothermal and solvothermal synthesis, chemical modification of titanium tetraisopropoxide using thiourea, as well as the sol-gel and co-precipitation methods [192,194–197]. The resulting S-TiO2 photocatalysts revealed improved photocatalytic decomposition of 2-propanol, phenol, 2-chlorophenol, MB, and methyl orange under visible light irradiation [198]. The absorption of visible light by these samples was explained by the mixing of O 2p and S 3p states. Moreover, with sulfur doping, photocatalysts with small crystal size and a high content of anatase phase were obtained.

Carbon-doped TiO2 Carbon doping has several potential advantages: (1) as one of the many possible electronic materials, carbon presents metallic conductivity, (2) it has a large electronstorage capacity and can accept the photon-excited electrons to improve the separation of photogenerated carriers, (3) it also has a wide range of visible light absorption at the wavelength of 400–800 nm, thus facilitating charge transfer from the bulk TiO2 to the surface region, where the desired oxidation reaction takes place. During carbon doping, the elemental carbon is believed to permeate the lattice of TiO2, substituting a lattice O atom to form a OdTidC bond, produce a hybrid orbital just above the VB of TiO2, and enhance the visible light absorbance [199]. A number of effective strategies have been developed to engineer C/TiO2 nanoparticles including the solgel method with carbon precursors, direct burning of titanium metal in a natural gas flame, annealing TiO2 under CO gas flow at high temperature, and thermal oxidation of TiC [199]. Synthesized C-TiO2 hollow spheres exhibited narrow pore size distribution, controlled shell thickness, high crystalline anatase content, and superior visible-light photocatalytic activity for degrading Rhodamine B (RhB) under visible

72

Metal Oxide-Based Photocatalysis

light irradiation. C-TiO2 nanoparticles also demonstrated improved photocatalytic performance for the mineralization of gaseous toluene under visible light irradiation, in comparison to commercial P25 TiO2 [200]. The doped carbon can exist as graphitelike carbon, interstitial carbonate species, and substitutional carbon in the oxygen sites. The first two carbon states serve as photosensitizers. The substitutional carbon leads to the appearance of mid-bandgap states in TiO2 by introducing additional electronic states just above the VB, which are directly responsible for the red shift of the absorption edge in the UV-Vis absorption spectrum.

Fluorine-doped TiO2 F-doping is also useful for improving the visible-light photocatalytic activity of TiO2 and for stabilizing the most reactive (0 0 1) facets of anatase TiO2 [201,202]. Based on the literature, F-doping causes the reduction of Ti4+ to Ti3+, creating the mid-bandgap states [203]. Moreover, (1) fluorine doping can enhance the surface acidity, increasing the adsorptivity for reactant molecules on the catalyst surface; (2) the doped fluorine atoms promote the formation of oxygen vacancies like F and F+ centers, which are responsible for the appearance of visible-light photocatalytic activity and provide sites for the formation of active species (superoxide and hydroxyl radicals). Additionally, the doped fluorine atoms increase the photogenerated electron mobility from the inner region to the surface and initiate redox reactions [204]. Highly crystalline flower-like F-TiO2 nanostructure exhibited high photoelectrochemical activity for water splitting and 4-nitrophenol degradation under UV/visible light illumination compared to Degussa P25 [205]. F-doped TiO2 powders synthesized by spray pyrolysis demonstrated higher photocatalytic activity for the decomposition of gas phase acetaldehyde under UV/visible light illumination compared to Degussa P25 [204]. Porous F-doped TiO2 microspheres showed enhanced photoactivity for 4-chlorophenol degradation, due to the excitation of extrinsic absorption bands by oxygen vacancies rather than the excitation of intrinsic absorption band of bulk TiO2 [202].

Boron-doped TiO2 Taking a more theoretical approach, Xu et al. studied the band structure of nitrogen (N)-, carbon (C)-, and boron (B)-doped TiO2 by a first-principles quantum mechanical calculation performed using the local-density approximation (LDA) [206]. The resulting bandgaps were 2.33, 2.44, 2.85, and 2.47 eV for N-, C-, B-doped TiO2 and pure TiO2, respectively. Their study showed that three 2p bands of impurity atoms are located above the valence-band maximum and below the Ti 3d bands. Also, the fluctuations become more intense with a decreasing number of impurity atoms. Their results did not show any obvious bandgap narrowing. Therefore, the cause of visible light absorption might be the isolated impurity atom 2p states in the bandgap, rather than the bandgap narrowing. Geometric and electronic structures of B-doped TiO2 photocatalyst were estimated by calculation with a plane-wave-based pseudopotential method [207]. It was found that B-doped anatase is much more efficient and stable than pristine TiO2. Since then, boron has emerged as a nonmetallic dopant that enhances the visible light response. However, some controversial results have

Metal oxide photocatalysts

73

been reported as the preparation method strongly affects the properties of the materials (such as the optimum level of boron doping, the oxidation state, and the position of B in the anatase crystal) and hence their activity and the doping process itself. It was found that boron doping can improve the visible light absorption significantly, although that does not always translate to significant visible light photocatalytic activity. This may be caused by the promotion of structural defects with doping (e.g., formation of Ti3+) that lead to charge recombination [208,209]. It was confirmed that TiO2 modified with boron oxides is very effective for photocatalytic decomposition of water under UV light [210]. Moreover, the photocatalytic efficiency can be improved by doping TiO2 simultaneously with boron atoms and nickel oxide (Ni2O3), thus extending the absorption spectrum to the visible region [208]. The prediction made by Xu et al. was confirmed experimentally, showing that boron modification can result in absorption of visible light and that B-TiO2 can be active in the presence of visible light [211]. B-TiO2 photocatalysts were prepared by sol-gel or surface impregnation methods using H3BO3 and (C2H5O)3B as boron sources. It was found that only the samples prepared by surface impregnation (e.g., grinding of boric acid triethyl ester with pure TiO2 and subsequent calcination at 450°C) were active under visible light. In contrast, annealing at 700°C with large amounts of boron dopant (0.75, 1.0, and 3.0 g of boron powder) resulted in the crystallization of HBO3 and rutile titania, decreasing the photocatalytic activity. The surface impregnation method allowed the introduction of all modifying moieties onto the surface of TiO2 particles, and these moieties could play a significant role in the surface photocatalytic reaction. The effect of boron content, calcination temperature, and TiO2 source used during preparation on the photoactivity was also investigated [212]. B-doped TiO2 also has potential application for four recalcitrant pesticides (diuron, o-phenylphenol and terbuthylazine) under simulated solar irradiation. It has been used for the degradation of: methylene blue, metoprolol, endocrine disrupting compounds (2,4-dichlorophenol (DCP), bisphenol-A (BPA)), nonsteroidal antiinflammatory drugs (ibuprofen (IBU) and flurbiprofen (FLU)), and atrazine [213–217]. The incorporation of boron improved the photocatalytic activity of TiO2 mainly under visible light illumination, which is likely due to the decreased bandgap energy and more developed surface area. The formation of surface-phase junctions between anatase and rutile nanoparticles enables effective interparticle electron transfer and results in more efficient charge separation. Also, the doped boron provides charge traps, which can mediate oxidative electron transfer.

Iodine-doped TiO2 Iodine is another potential possibility to enhance the visible-light absorption by altering the electronic structure of TiO2 photocatalysts. Fu et al. reported the synthesis, characterization, and electronic structure of multivalent iodine (I7+/I
)-doped TiO2 [218]. It was suggested that the recombination of photogenerated electron-hole pairs is inhibited due to the electron trapping action of the I sites [219]. Experimental studies have determined the maximum absorption edge to be 550 nm for the lattice I-doped TiO2, whereas an extended absorption up to 800 nm was observed for the surface

74

Metal Oxide-Based Photocatalysis

I-doped TiO2 [220]. The ultra-long light response of I-doped anatase TiO2 was attributed to the much lower energy for the photon excitation pathways from the occupied states of IdOdTi structure just above the VB to the unoccupied states of IdOdI structure below the CB in the bandgap. The low levels of occupied states just above the VB, together with the favorable surface structure, were responsible for the photoactivity beyond 600 nm.

Transition metal-doped TiO2 Doping TiO2 with transition metals, such as Cr, Cu, Mn, W, V, Ni, and Fe, enhances the visible light absorption and reduces the recombination rate of photogenerated electrons and holes [221]. As a result of transition metal doping, impurity energy levels are created in the bandgap, which result in visible-light absorption. Photocatalytic activities are also improved by electron transfer between TiO2 and transition metal ions. Transition metals, such as Ni2+, Zn2+, Cr3+, and Fe3+, can be easily incorporated into the crystal lattice of TiO2 because of their similar ionic radii (0.72, 0.74, 0.76, and ˚ , respectively, while that of host Ti4+ is 0.75 A ˚ ) [222]. The photocatalytic activ0.69 A ities of these doped TiO2 samples were evaluated in the degradation of aniline blue (AB) under UV/solar light. Mn2+ (0.06 at.%)-TiO2 showed enhanced activity, which is attributed to the synergistic effect in the bicrystalline framework of anatase and rutile. In the case of Cr-doping, the bandgap energy is shifted by 2.00 eV [223]. It was also explained that by the excitation of an electron of Cr3+ into the CB of TiO2, Cr3+ can act as a photogenerated hole trapper since the energy level for Cr3+/Cr4+ is above the VB edge of TiO2. The trapped holes in Cr4+ can migrate to the surface-absorbed hydroxyl ions to produce hydroxyl radicals. RhB degradation was enhanced by 25% over 10% Cr3+-doped titania, as compared to bare TiO2 [223]. CrdTi mixed oxide was used to photodegrade Orange G dye (an azo dye), with pseudo first-order kinetics and a degradation efficiency of 70% in 300 min [221]. In another study, Cr-doped titania electrodes were used to degrade Acid Red G (another azo dye) [224]. The photocatalytic efficiency was remarkably enhanced by the incorporation of Cr3+ and this was attributed to the improved charge separation in these systems. The photocatalytic activity of nanostructured titania powder doped with 3 and 6 mol% of Cr3+ was improved for the degradation of crystal violet [225], owing to the formation of oxygen vacancies with Cr doping. It was shown that Cu2+ doping of TiO2 catalyst increased the photocatalytic degradation of two typical azo dyes used in the dyeing industry, namely Acid Orange 7 (AO7) and tartrazine (Tart) [226]. The behavior of Cu2+ was explained on the basis that Cu2+ may scavenge electrons to form Cu+, which in turn enhances the oxidation of the substrate. Photocatalytic oxidation of phenol was performed over Cu-doped TiO2 prepared by a sol-gel method in the presence of nitric or sulfuric acid [227]. It was explained that the oxygen-deficient surface could, in principle, stabilize Cu+ species against thermal oxidation to Cu2+ and probably their surface segregation. The photocatalytic activity clearly indicates that surface Cu+ may act as the photoactive species, leading to improved photoactivities compared to the undoped and Cu2+-doped samples.

Metal oxide photocatalysts

75

Among the aforementioned metals, Fe is deemed to be a worthy dopant for increas˚ ) close to ing the photocatalytic performance of TiO2. Fe3+ has anionic radius (0.64 A 4+ ˚ that of Ti (0.68 A) and can therefore be incorporated into the TiO2 lattice. It is reported that Fe dopant can facilitate the separation of photogenerated electrons and holes. Fe3+ also traps photogenerated holes to form Fe4+, which reacts with the surface-adsorbed hydroxyl ions to produce hydroxyl radicals and O2
in the surface lattice. Alternatively, Fe4+ can react with photogenerated electrons, thereby affecting the photocatalytic activity [221,228]: TiO2 + hν ! e
+ h +

(3.12)

Fe3 + + h + ! Fe4 +

(3.13)

Fe3 + + e
! Fe2 +

(3.14)

Fe2 + + O2 ðadsÞ ! Fe3 + + O2


(3.15)

Fe2 + + Ti4 + ! Fe3 + + Ti3 +

(3.16)

Ti3 + + O2 ðadsÞ ! Ti4 + + O2


(3.17)

Fe4 + + e
! Fe3 +

(3.18)

Fe4 + + OH
ðadsÞ ! Fe3 + +  OH ðadsÞ

(3.19)

However, when the concentration of Fe3+ ions is too high, they can act as recombination centers for the photogenerated electrons and holes, resulting in decreased photocatalytic activity: Fe4 + + e
! Fe3 +

(3.20)

Fe2 + + h + ! Fe3 +

(3.21)

Fe2 + +  OH ! Fe3 + + OH


(3.22)

Under visible light irradiation, the excitation of Fe-TiO2 arises from the electronic transition from the dopant energy level (Fe3+/Fe4+) to the CB of TiO2. Because the t2g level of the Fe3+ 3d orbital is above the VB of TiO2, Fe3+ can absorb a photon with a wavelength exceeding 400 nm to produce Fe4+ and an electron in the CB of TiO2. The CB electron further reacts with adsorbed O2 to form O2
, while Fe4+ reacts with a surface hydroxyl group to produce a hydroxyl radical. Fe3 + + hν ! Fe4 + + e


(3.23)

e
+ O2 ðadsÞ ! O2


(3.24)

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Metal Oxide-Based Photocatalysis

Fe3+ doping effectively improved the photocatalytic activity under both UV and visible light irradiation [228]. Under visible light irradiation, Fe3+ doping introduces a new energy level (Fe3+/Fe4+) above the valence band, resulting in enhanced visible light absorption and improved photocatalytic activity. The effects of oxygen vacancy and surface hydroxyl group density on the photocatalytic activity of Fe3+-doped TiO2 were also investigated [229]. The results indicated that the adsorbed hydroxyl group significantly influenced the photocatalytic activity. Moreover, a small amount of Fe3+ can act as photogenerated holes to trap photogenerated electrons, and inhibit the electron-hole recombination. The active species ( OH and  O2
) formed on the surface of TiO2 could also enhance the photocatalytic activity. TiO2 doped with 0.10% Fe3+ demonstrated the optimal photocatalytic degradation of MB under simulated sunlight. Photodegradation in the presence of an Mn2+ ion can be represented by the following mechanism [230]: l

+ Mn2 + + e
CB ! Mn

(3.25)

Mn + + O2 ðadsÞ ! Mn2 + + O2 


(3.26)

+ ! Mn3 + Mn2 + + hVB

(3.27)

Mn3 + + OH
! Mn2 + +  OH

(3.28)

Alternatively, Mn3+ can also trap CB electrons, or Mn+ can trap VB holes to retain the half-filled electronic structure of Mn2+: + ! Mn2 + Mn + + hVB

(3.29)

2+ Mn3 + + e
CB ! Mn

(3.30)

Mn2 + + Ti3 + ! Mn + + Ti4 + ðelectron trapÞ

(3.31)

Mn2 + + O
! Mn3 + + O2
ðhole trapÞ

(3.32)

Because Mn3+ and Mn+ are less stable than Mn2+, there is a tendency for the trapped charges to transfer from Mn3+ and Mn+ to the interface: Mn + + O2 ! Mn2 + + O2 
ðelectron releaseÞ

(3.33)

Mn3 + + OH
! Mn2 + +  OH ðhole releaseÞ

(3.34)

Mn2+-doped TiO2 was used for photocatalytic degradation of methyl orange under solar light [231]. Mn2+-TiO2 (0.06%) showed the highest photocatalytic activity, due to the synergetic effect observed in the mixed phase and its unique half-filled electronic configuration. TiO2 photocatalyst powders with various Mn doping were synthesized by the sol-gel method [232]. They exhibited red shift in the absorption edge

Metal oxide photocatalysts

77

with increasing Mn content, leading to the appearance of the mid-bandgap states. The enhanced optical absorbance caused strong photocatalytic activity under visible light illumination during the degradation of MB. Six transition metal ions, namely V, Mn, Fe, Cu, Ce, and W, were doped into TiO2. The effects of the doping on charge trapping, recombination, interfacial transfer, and photocatalytic activity were studied [233]. The photocatalytic activity was examined based on the degradation of bisphenol A in an aqueous solution under UV light. The doped Fe, Cu, and V ions improved the photocatalytic activity, whereas the Mn, Ce, and W ions had the opposite effect. The photocatalytic activity changed in the order of Fe/TiO2 > Cu/TiO2 > V/TiO2 > W/TiO2 > Ce/TiO2 > Mn/TiO2. The different influences of the metal ions are associated with their energy levels, coordination numbers, and electronegativity. Those authors explained that (1) Fe and Cu ions inhibit defectmediated annihilation, facilitating interfacial charge transfer, (2) Mn ions trap holes and electrons and therefore quickly consume charge carriers via intra-atomic relaxation, (3) Ce and W ions, which have high coordination numbers and electronegativity, strongly bond the O2
radicals and thus limit the charge utilization as well as the photocatalytic performance. V5+ is known to undergo the following reactions [234]: V5 + + e
! V4 +

(3.35)

V4 + + O2 ðadsÞ ! V5 + + O2 


(3.36)

V5 + + h + ! V6 +

(3.37)

V6 + + OH
! V5 + +  OH

(3.38)

V5 + + dyeðadsÞ ! V4 + + dye

(3.39)

V3 + + O2 ðadsÞ ! V4 + + O2 


(3.40)

while V4+ traps holes and electrons to form V5+ and V3+ respectively [235]. ˚ ) similar to that of host Ti4+ (0.75 A ˚ ), Since Zn2+ has an ionic radius (0.74 A 2+ 4+ Zn can easily substitute Ti ions in the TiO2 lattice without distorting the crystal structure. Zn2+ as a dopant can act as follows [222]: Zn2 + + e
! Zn +

(3.41)

Zn + + O2 ðadsÞ ! Zn2 + + O2 


(3.42)

Zn + + h + ! Zn2 +

(3.43)

Zn2 + + h + ! Zn3 +

(3.44)

Zn3 + + OH
! Zn2 + +  OH

(3.45)

Zn3 + + e
! Zn2 +

(3.46)

78

Metal Oxide-Based Photocatalysis

The inclusion of Zn2+ ions as a dopant in TiO2 can thus generate O2 
and OH radicals, which can take part in subsequent reactions to effectively accelerate the dye degradation. Zn2+-TiO2 with 0.06% Zn2+ showed 100% dye degradation of Congo red [234]. The higher activity was attributed to the generation of excess hydroxyl radicals due to the prolonged separation of charge carriers, smaller crystallite size, and larger surface area. l

Noble metal-doped TiO2 Noble metals including Au, Ag, Pt, Pd, and Rh have been reported as very efficient dopants for the visible-light activation of TiO2 photocatalysts, thus improving their performance [236,237]. The Fermi levels of these noble metals are lower than that of TiO2, which results in the effective transfer of the photogenerated electrons from the CB of TiO2 to the metal particles. This electron trapping process significantly reduces the electron-hole recombination rate, which results in stronger photocatalytic reactions. Silver-doped TiO2 has recently been developed for enhancing the photocatalytic efficiency. Researchers achieved highly efficient electron-hole separation by forming a Schottky barrier in the Ag-TiO2, thus enhancing its photocatalytic activity. Ag doping of titania introduced two additional electron-accepting species, namely Ag+ and metallic silver (Ag0). Moreover, the presence of Ag results in a larger specific surface area, and thus more reactive sites are available to take part in the photoreactions. In addition, doping TiO2 with Ag retards the rate of electron-hole pair recombination by enhancing the charge carrier separation. Another explanation for the role of metal silver on the catalyst surface is that the noble metal improves the quantum yields by accelerating the removal and the transfer of electrons from the catalyst to molecular oxygen. It was found that Ag could modify the TiO2 anatase surface, while TiO2 defects were produced when Ag was introduced into the lattice sites, such as oxygen vacancies [238]. Ag-doped TiO2 is also more efficient than undoped TiO2 for the photocatalytic degradation of acid red 88 (AR88) and MB under UV light irradiation [239,240]. Moreover, the calcination temperature exhibits a marked influence on the microstructures and photocatalytic activity of Ag-TiO2
x samples. With increasing calcination temperature, the visible light absorption capacity, pore volume, and BET specific surface areas of the Ag-TiO2
x samples decrease, and the crystallite size increases [241].

Lanthanide-doped TiO2 TiO2 photocatalysts activated by rare earth (RE) elements, which have shown tremendous potential as dopants, have not only red-shifted absorption but also improved photocatalytic activity and increased anatase-to-rutile transformation temperature. Additionally, materials (such as TiO2) modified by RE3+ ions usually possess luminescent properties. Thus, besides classical UV-excited emission, these materials can also display upconversion luminescence. Moreover, because the f-orbitals of the lanthanide ions can form complexes with various Lewis bases, the substrates are concentrated onto the TiO2 surface. Furthermore, modification with RE3+ ions prevents

Metal oxide photocatalysts

79

electron-hole recombination. Doping TiO2 with RE metals that have incompletely occupied 4f orbitals, such as neodymium (Nd), has been investigated to improve the photocatalytic activity [242,243]. It was inferred that the increase in photocatalytic activity was due to the transition of 4f electrons in RE ions, which enforced the optical absorption of the photocatalysts and supported the separation of photogenerated electron-hole pairs. It was also concluded that the presence of a 4f level in Nd ions helped to decrease the TiO2 energy bandgap by allowing charge transfer between the TiO2 VB/CB and the 4f level of Nd ion [243]. Among the TiO2 nanoparticles doped with lanthanide ions (La3+, Nd3+, Sm3+, 3+ Eu , Gd3+, and Yb3+), Gd3+/TiO2 has the lowest bandgap and particle size and also the highest surface area and pore volume, thus explaining its highest photocatalytic activity [244]. Additionally, holmium doping decreased the crystalline size and increased the specific surface area [245]. Photodegradation experiments with methyl orange showed that Ho doping improved the photocatalytic activity of TiO2. This may be attributed to the synergetic effects of the large surface area, small crystallite size, lattice distortion, and increased charge imbalance of holmium-doped TiO2. Series of Nd3+/Er3+, Nd3+/Eu3+, Eu3+/Ho3+-TiO2, Er3+-TiO2, Yb3+-TiO2 and Er3+/Yb3+-TiO2 photocatalysts and Y3+, Pr3+, Er3+ and Eu3+-modified TiO2 were prepared by the sol-gel (SG) and hydrothermal (HT) methods [246–248]. The introduced RE metals formed metal oxides (RE2O3) at the TiO2 surface. The results demonstrated that all RE-doped samples (such as Eu3+/Ho3+ co-doped titania) exhibited higher photocatalytic activity than P25 in phenol degradation under visible light, whereas Nd3+/ Eu3+ co-doped TiO2 showed one of the highest activities in the degradation of both phenol and acetic acid under UV and visible light among all of the RE-doped samples. Action spectra analysis of the selected samples clearly showed that RE-doped TiO2 could be excited under visible light in the range from 420 to 450 nm [246]. The primary mechanism for the visible light sensitization was probably the availability of highly adsorptive sites, increased BET surface area, decreased crystallite size, and inhibited electron-hole recombination [247]. It can be summarized that RE3+ doping during the sol-gel and hydrothermal syntheses of TiO2 resulted in the blue shift of the absorption edges of TiO2. This could be attributed to movement of the CB edge above the first excited state of RE3+. Single- and co-doped photocatalysts containing Fe3+, Cr3+, La3+, or Eu3+ were evaluated for the degradation of 4-nitrophenol under UV-visible light (330 nm < λ < 800 nm) [53].

Co-doped TiO2 Among all co-doped materials, N-F co-doping has attracted many researchers, due to the similar structural properties of the two dopants. In addition, the co-doped structure retains the advantages of high visible light response from N-doping and significant charge separation by the F-doping. Additionally, surface fluorination inhibits the anatase-to-rutile phase transformation and favors the incorporation of N ions into the TiO2 crystal lattice, creating oxygen vacancies and inducing the formation of Ti3+ ions [249]. The resulting nanoparticles exhibited high surface area, high porosity, mesoporosity, and a low degree of agglomeration. The co-dopants of N and F may

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Metal Oxide-Based Photocatalysis

hinder the recombination of photogenerated electron-hole pairs, making the material active in visible light and increasing the photo quantum efficiency. Besides studies of N- and S-doped TiO2, a recent focus is the N-S co-doped TiO2, which can further extend the absorption range into the visible light region and improve the photocatalytic efficiency, evidently from the synergistic effects of the multiple ions. Compared to single doping with N or S, the N-S co-doped titania with optimal N and S concentrations displayed excellent photocatalytic activity under visible light [250] as a result of the decreased electron-hole recombination process. Moreover, photocatalytic activity measurement showed that the N-S co-doped TiO2 nanowires with high quantum efficiency have the best photocatalytic performance for atrazine degradation under visible light irradiation, due to (1) the synergistic effects of N and S doping in narrowing the bandgap, separating electron-hole pairs, and increasing the photoinduced electrons; and (2) elevated anatase-to-rutile transformation temperature of above 600°C [251]. The preparation of mesoporous B-N co-doped TiO2 with significantly enhanced visible light absorption and photocatalytic activity was also reported [252,253]. These observations were attributed to the presence of a new OdTidBdN structure formed on the surface of the photocatalyst, which is highly active in collecting and separating the charge carriers. Due to the charge compensation effect of the nitrogen and boron anions, no new recombination centers of electrons and holes (such as Ti3+) were formed, which inhibits the decline of photocatalytic activity. N-I co-doping caused a change in the bandgap value varying from 3.11 to 2.34 eV [254]. Photocatalytic experiments for the simultaneous reduction of Cr(VI) ions and oxidation of benzoic acid showed a higher activity for all co-doped catalysts than the undoped TiO2. EPR studies showed that N doping creates two kinds of paramagnetic nitrogen species: (1) nonphotoactive NO centers, and (2) highly photoexcited Nb  species which are correlated with the photogenerated electrons through the path Nb
! Nb  + e
. The Cr(VI) reduction efficiency of N-I-TiO2 was correlated with the concentration of Nb  species and the Eg values. Doping with iodine ions promotes the formation of photoinduced surface Ti3+ ions, which react with the adsorbed Cr(VI) and/or O2 to form radicals, such as O2 
, HO2 
, and OH, that support the reduction and oxidation mechanisms. Additionally, EPR experiments showed the stabilization of oxygen trapped holes TiO4+-O

) created by the reaction of anatase holes with lattice oxygen O2
. The concentration of such stabilized trapped holes detected in the N-I catalysts was inversely correlated with the oxidation rates. The C-N co-doped TiO2 was synthesized by a one-pot hydrothermal method in the presence of glycine. The product was found to have improved photocatalytic activity, due to the large surface area, heterostructure formation, strong absorption in the visible range, and efficient separation of the charge carriers [255]. Simultaneous doping (co-doping) with anions and transition metal ions may modify the conductivity and optical properties of titania. Furthermore, such co-doping may introduce new electronic states that lie closely to the CB or VB of titania [256]. Cobalt and sulfur co-doped titania photocatalysts of various compositions were synthesized [257]. The cobalt concentration was found to have a significant effect on the structure, optical, and photocatalytic properties of the resulting titania nanoparticles. l

l

l

Metal oxide photocatalysts

81

A synergistic effect from the co-doping enhanced the photocatalytic efficiency in degrading crystal violet, malachite green, procion blue MXR, alizarin red, and phenol under both UV (with cutoff filter λ < 380 nm) and visible light irradiations (with cutoff filter λ > 420 nm). The beneficial effects of S and Fe(III) co-doping were also noted [258]. Fe-S codoped TiO2 exhibited much higher photocatalytic activity for phenol degradation under visible light irradiation. The Fe-S codoped TiO2 photocatalysts also showed small crystallite size, large specific surface areas, as well as more surface-adsorbed water and hydroxyl groups, leading to a narrow bandgap. Moreover, Fe3+ cations were doped into the TiO2 lattice, while SO42
anions were adsorbed on the surface of TiO2 and trapped the photogenerated electrons, thereby promoting the separation of photogenerated holes and electrons.

Tri-doped TiO2 Recently, tri-doped TiO2 has been found to further improve the visible light photocatalytic activity. However, there have been only a handful of such examples: l

l

l

l

l

Nb-Sb-C tri-doped TiO2: After doping, mid-bandgap states from C 2p appeared, and Sb 5s and Nb 4d states were introduced. In addition, rutile TiO2 co-doped with (Nb, Sb, C) and (2Nb, C) had much stronger visible light absorption than that with C mono-doping and (2Sb, C) co-doping [259]. Mo-Sb-S tri-doped TiO2: Its high photocatalytic activity was attributed to the synergetic effects of the large specific area, high crystallinity and crystalline size, porous structure, and intense absorption in the visible light region. Furthermore, the nonmetal S incorporated into the bulk phase of TiO2 leads to the appearance of the mid-bandgap states, while the metals of Mo and Sb effectively suppresses the recombination of electrons and holes [260]. Nb-N-S tri-doped TiO2: Red shift of the absorption edge and improved visible light absorption were observed due to the additional electronic states in the bandgap of TiO2 induced by the dopants. The N 2p and S 3p states formed new defect levels at the top of the VB and the Nb 4d state appeared at the bottom of the CB [261,262]. Fe-N-S-tri-doped TiO2: In this tri-doped TiO2 catalyst, Fe3+ was incorporated into the lattice of TiO2 through titanium atom substitution. N may exist in the forms of substitutional N (OdTidN) and interstitial N (TidOdN). S was also incorporated into the lattice of TiO2 through substitution of the titanium atom and exists in the forms of both S6+ and S4+. The enhanced visible light photocatalytic activity of FeNS-TiO2 was mainly attributed to the small crystallite size, high anatase crystallinity, intense light absorption in the visible region, and high separation efficiency of photoinduced charge carriers. After tri-doping with Fe, N, and S elements, electrons can be promoted from the VB to the mixed level, and from the localized N 2p level to the CB. Furthermore, the photoinduced electrons can also be promoted from the localized N 2p level to the mixed level. Finally, the photoinduced electrons in the CB can migrate to the mixed level and subsequently transfer to the surface of the nanomaterials, thereby enhancing the separation efficiency of photoinduced charge carriers. As a result, more photoinduced electrons and holes (e
/h+) can participate in the photodegradation process [263,264]. Fe-N-C-tri-doped TiO2: Its highest photocatalytic activity can be attributed to the synergetic effects of the higher BET surface area, faster separation of electron-hole pairs, more

82

l

l

l

l

Metal Oxide-Based Photocatalysis

numerous surface-absorbed hydroxyl groups (dOH), stronger absorption in the visible light region, and optimum Fe/Ti weight ratio [265]. N-F-Ta tri-doped TiO2: The strong absorption of visible light was related to the isolated levels consisting of N 2p, π*NdO formed above the VB, and Ti3+ and oxygen vacancy states formed below the CB. Moreover, at a low tri-doping level, the surface area was enlarged and the separation of electron-hole pairs was improved. In contrast, a higher tri-doping level introduces too many impurity atoms into the lattice of TiO2. This affects the crystal growth and introduces more defects, thus causing the recombination of photoexcited electron-hole pairs. Moreover, the co-existence of Ta and N narrowed the bandgap by forming fully occupied N 2p-Ta 5d hybridized states, which enhanced the visible light absorption and do not act as recombination centers [266]. N-B-F tri-doped TiO2: The high visible light photocatalytic activity mainly originated from the synergistic effects of the tri-doping and the large specific surface area. The N-B-codoping changed the bandgap of TiO2 from 3.2 to 2.78 eV, which facilitated the absorption of visible light. F-doping, on the other hand, improved the electron transfer in the process [267]. Sm-N-P tri-doped TiO2: The material’s photoefficiency was enhanced due to the synergetic effect of more efficient inhibition of the recombination of photogenerated electrons and holes, increased absorption of visible light, the presence of surface hydroxyl groups, larger specific surface area, as well as improved surface textural properties [268,269]. Pr-N-P-tri-doped TiO2: The P-doping played a dominant role in both inhibiting the crystal growth and improving the surface textural properties and amount of hydroxyl groups. Meanwhile, Pr played a key role in increasing the pore diameter and pore volume, as well as inhibiting the anatase-to-rutile phase transformation. As a result, the tri-doped TiO2 has enhanced photon absorption in the UV region, lowered indirect recombination of photogenerated carriers, and even larger pore volume [270].

3.2.2 WO3 (structure, surface properties and photocatalytic activity of pristine and doped WO3) 3.2.2.1 Structure and surface properties Tungsten oxide has become one of the most investigated functional metal oxides in many fields including photocatalysis, electrochemistry, and phototherapy, due to its strong solar spectrum absorption (12%, with Eg ¼ 2.5 – 2.8 eV) and better electron transport (ca. 12 cm2 V
1 s
1) compared with TiO2 (0.3 cm2 V
1 s
1). Moreover, many oxygen-deficient tungsten oxides (WOx dendrites > dumbbells. Zhang et al. demonstrated a facile synthesis of hierarchical WO3 coreshell microspheres, in which the core was composed of aggregated nanoparticles encapsulated by a hierarchical shell layer that was self-assembled from ultrafine nanoplates with a thickness of about 15 nm [302]. The microspheres thus obtained showed superior photocatalytic activity for the degradation of RhB under visible light irradiation. The photocatalytic activity enhancement was related to a higher degree of crystallization (reduced recombination of the photoexcited e
/h+) and increased surface area (i.e., more surface active sites). WO3 has been tested as a photocatalyst for the degradation of various dyes (RhB, MB, indigo carmine, Congo red, and methyl orange [43,278,279,281,282,292, 303–311]), phenols [312–314], and other organic compounds [315–321]. Based on the literature data, the degradation of organic dyes over WO3 can take place by two paths: (1) true photocatalysis where radiation on the photocatalyst promotes an electron from its VB to the CB to form an electron-hole pair and (2) the photosensitization process, in which the radiation excites an electron from the dye, and then this electron is injected to the CB of the semiconductor oxide, as represented in Fig. 3.11. Moreover, the formation of OH radicals in the WO3 suspension takes place through two paths. The first is reductive with the participation of electrons of the CB of WO3: 2WO3 + hν ! 2WO3 ðh + Þ + 2WO3 ðe
Þ

(3.47)

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Metal Oxide-Based Photocatalysis

Fig. 3.11 Mechanisms of dye degradation in the presence of WO3 photocatalysts: (A) photocatalysis process and (B) photosensitization process. Based on D.S. Martı´nez, A. Martı´nez-De La Cruz, E.L. Cu
ellar, Photocatalytic properties of WO3 nanoparticles obtained by precipitation in presence of urea as complexing agent, Appl. Catal. A Gen. 398 (2011) 179–186.

WO3 ðe
Þ + O2 ! WO3 + O2 


(3.48)

WO3 ðe
Þ + O2 
+ H + ! WO3 + HO2 

(3.49)

HO2  + H + ! H2 O2

(3.50)

H2 O2 + e
!  OH + OH


(3.51)

The other path is direct oxidation of hydroxide ions by the holes generated in the VB of WO3 during the charge separation (oxidative path): WO3 ðh + Þ + OH
+ H + ! WO3 +  OH + H +

(3.52)

Mesoporous WO3 with high surface area and porous structure was also used for the photocatalytic conversion of methane into methanol [318,319]. It was explained that OH radical groups on the photocatalyst surface were mainly responsible for the enhanced photocatalytic process in the selective oxidation of methane to methanol, while a larger number of free OH radicals favors the formation of ethane. Moreover, the enhanced photocatalytic activity was due to the low recombination rate of e
and h+ [315]. Pure WO3 nanomaterials are usually not efficient photocatalysts because their low charge mobility results in a high electron-hole recombination rate and thus

Metal oxide photocatalysts

87

significantly diminished efficiency of the photocatalytic reaction. In fact, this is one of the biggest obstacles in the application of WO3 as a practical photocatalyst. Several approaches have been developed to improve its solar energy conversion efficiency, such as substitutional doping of WO3 with anions or cations.

Nitrogen-, sulfur-, and carbon-doped WO3 Due to the relatively large bandgap (2.6–3.0 eV), WO3 mainly absorbs in the near ultraviolet and blue regions of the solar spectrum. It has been found that doping WO3 with elements like carbon, sulfur, and nitrogen enhance the photoresponse under visible irradiation [322,323]. By comparing the effect of these anion dopants through density of states (DOS) calculations, nitrogen has been identified as the most promising dopant because its p states mix with O 2p states, resulting in the appearance of mid-bandgap states in tungsten oxide. Based on the literature data, there are two ways to dope WO3 with nitrogen, namely sputtering or calcination in the presence of ammonia [324,325]. In those synthesis routes, N2 was used as the nitrogen source, and the N-doping reduced the energy bandgap efficiently. S2
doping may be difficult to carry out due to the large formation energy required for substitutionally forming WdS bonding instead of WdO bonding, because its ˚ ) is significantly larger than that of O2
(1.22 A ˚ ). Considering that ionic radius (1.7 A the ionic radius of W6+ is close to that of Ti4+, the substitution of W6+ by S6+ is more favorable than replacing O2
with S2
, and subsequently WdOdS bonds are formed. Moreover, S-doping, which converts W6+ to W5+ (according to XPS measurements), resulted in increased oxygen adsorption on the surface, so that the number of oxygen vacancies also increases. It is expected that effective modification of WO3 could be realized by S-doping. [326]. A theoretical calculation indicated that the incorporation of carbon dopant can improve the photogenerated carrier migration pathways and reduce the recombination rate [327]. The changes of band structure, distortion of WO6 octahedron, and lattice defects help to improve the photocatalytic performance. The new level increases the migration pathway of photogenerated carriers and reduces their recombination rate. As a result, C-doped WO3 showed better photocatalytic performance in degradation of RhB. Moreover, the experiments of radical quenchers confirmed that h+ has the main influence on RhB degradation.

Transition metal-doped WO3 Metal-doped semiconducting oxides are potential materials for photocatalysts and photoelectrochemical conversion applications because of their unique physicochemical properties. Due to the enhanced absorption of visible light and effective separation and transformation of the photo-excited electrons and holes, Fe-doping has gained more and more attention for narrowing the bandgap of WO3 [328,329]. Density functional theory (DFT) calculation revealed that the formation of an impurity band in the bandgap narrows the bandgap of Fe-doped WO3 nanostructures [328]. Moreover, an appropriate amount of Fe3+ doping could enhance the separation and transformation of the photo-excited electrons and holes. Fe-doped WO3 hollow

88

Metal Oxide-Based Photocatalysis

nanospheres with an optimal Fe content (5.25%) had outstanding activities compared to pure WO3. At a lower Fe concentration, Fe3+ ions are mainly doped in the surface of the samples, and they could trap and transfer electrons and holes to inhibit their recombination. Experiments with different scavengers showed that most of the photocatalytic degradation of RhB was caused by OH radicals that were mainly produced by Fe3+ and hole. Another study examined the effect of different transition metals (Fe, Co, Ni, Cu, and Zn) at different concentrations on the photocatalytic activity of WO3 for splitting of water into hydrogen and oxygen under UV laser irradiation [330]. Fe, Co, Ni, Cu, and Zn were present in the forms of Fe2O3, CoO, NiO, Cu2O, and ZnO, respectively. A significant change in the photocatalytic activity of WO3 toward hydrogen generation was observed when using transition metals as dopants. Under UV illumination, the electrons were excited from the VB of WO3 formed by the 2p orbitals of oxygen to the CB formed by the 4f orbitals of tungsten. These excited electrons were then transferred from the CB of the host (WO3) to the CB of the doped metal oxides formed by the “3d” orbitals of the transition metal dopants. Also, Zn, Cu, and Zn-Cu co-doped WO3 possessed high photocatalytic activity [331]. The Mo-doped WO3 nanowires exhibited high photocatalytic activity for the decomposition of MB in aqueous solution [332]. A small amount of Mo-doping (optimal atomic ratio of Mo to W: 0.07) enhanced the photocatalytic activity of WO3 nanowires, due to the fact that a small amount of molybdenum ions can act as photogenerated holes to trap the electrons and inhibit the hole-electron recombination. By doping Mo in WO3, the CB and VB positions were shifted negatively by 2.25 V, leading to a CB edge position that was negative enough for H+ ions to be reduced thermodynamically [333]. l

Lanthanide-doped WO3 Lanthanide-doped WO3 showed improved visible light response, with increased photocatalytic activities and photocurrents for water splitting and photocatalytic degradation processes. Yb incorporation reduced the charge transfer resistances and increased the donor densities in WO3 photocatalysts. The improvement in photocurrent density was attributed to additional conductive carrier paths and increased oxygen vacancies due to the substitution of W6+ by Yb3+ and electron enrichment by the 4f orbital configuration of Yb3+ [334]. Eu3+-WO3 nanoparticles were found to be composed of puncheon and catenary shapes after being pretreated with pH, pressure, and surfactant [335]. The photocatalytic activities of the nanoparticles were evaluated by decomposition of RhB, and Eu3+-WO3 nanoparticles were more efficient than WO3 and TiO2 under sunlight illumination. A series of Dy-doped WO3 nanopowders were also synthesized [336]. Their BET surface area changed insignificantly with the Dy-doping, while the absorption spectrum range and absorbance were improved. The experimental results indicated that Dy-doping not only improved the light absorption of WO3, but also enhanced its photoactivity and photostability. The doped Dy3+ can give an electron to O2 adsorbed on the surface of WO3 to form O2 
and Dy4+. Then Dy4+ tends to receive a photogenerated electron in the CB of WO3 to form Dy3+ and thereby inhibits it from recombination with photogenerated holes.

Metal oxide photocatalysts

89

In summary, it may be concluded that tungsten oxide semiconductors play a notable role in the field of photocatalysis. While a vast range of WO3 nanostructures have already been reported in the literature, it is still very important to continue the exploration of innovative synthesis routes of nanostructured WO3, as well as investigating the mechanism of growth control.

3.2.3 ZnO (structure, surface properties and photocatalytic activity of pristine and doped ZnO) 3.2.3.1 Structure and surface properties Zinc oxide (ZnO) is one of the most important II–VI semiconductors because of its interesting and unique characteristics, including a wide bandgap (3.37 eV), a large exciton binding energy (60 meV), physical and chemical stability, biocompatibility, nontoxicity, high photosensitivity, and piezoelectric and pyroelectric properties [337]. These unique properties have rendered ZnO indispensable in applications, such as solar cells, thin-film transistors, laser diodes, piezoelectric and optoelectronic applications in surface acoustic wave devices, transparent conductive contacts, and ultraviolet lasers [337]. In recent years, its photocatalytic properties were also extensively studied [338,339]. In pure ZnO, the presence of zinc interstitial, oxygen vacancy, and zinc on oxygen antisite makes it a natural n-type semiconductor. It appears as white hexagonal crystals or white powder known as zinc white [337]. ZnO has well-defined crystal structures, usually the cubic rocksalt structure, hexagonal wurtzite, or cubic zinc blende (Fig. 3.12). The ZnO wurtzite structure has the highest thermodynamic stability

Fig. 3.12 (A) Rock salt (cubic), (B) zinc blende (cubic), and (C) wurtzite (hexagonal) structure models of ZnO. Reproduced with permission from K.M. Lee, C.W. Lai, K.S. Ngai, J.C. Juan, Recent developments of zinc oxide based photocatalyst in water treatment technology: a review, Water Res. 88 (2016) 428–448.

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Metal Oxide-Based Photocatalysis

among the three structures and therefore is the most common one. ZnO can also form a hexagonal wurtzite crystal structure at ambient pressure and temperature, with the two lattice parameters of a ¼ 0.3296 nm and c ¼ 0.52065 nm [340]. This ZnO hexagonal wurtzite space grote structure belongs to the P63mc space group and exhibits a noncentrosymmetric structure, which causes ZnO to be piezoelectric and pyroelectric. Wurtzite ZnO consists of atoms forming hexagonal-close-pack sublattices, in which each Zn2+ sublattice contains four Zn2+ ions and is surrounded by four O2
ions and vice versa, coordinated at the edges of a tetrahedron [341]. The zinc blend ZnO can be stabilized only by growth on cubic structures. A rocksalt structure can also be yielded under high pressure, and thus it is quite rare [338]. ZnO nanomaterials with various morphologies, such as nanorods, nanowires, nanotubes, nanobelts, nanoribbons, tetrapods, towerlike, complicated hierarchical microstructures, and hollow structures/nanoparticles (including quantum dots), have been prepared by different methods: evaporative decomposition of solution [342], solid state reaction [343], wet chemical synthesis [344], aerosol [345], ultrasonic [346], micro-emulsion [347], sol-gel [348], hydrothermal [349], and conventional ceramic fabrication [350] methods. The preparation techniques and deposition parameters have been known to exert strong influence on the particle size, shape, and/or physical properties of ZnO. Nanostructured ZnO has a particle size in the nanoscale with higher surface area, and its high surface-to-volume ratio may offer better physicochemical properties. In view of this, the hydrothermal technique is a simple and cost-effective approach for the growth of ZnO structures at mild conditions [351–355]. Moreover, during the growth process, the size, shape, and physical properties of ZnO particles depend on the growth temperature and time, alkaline source, precursor concentration, mineralizers, inorganic electrolytes, templates, and the pH of the solution [356–359]. Zn(II) can exist as Zn2+, Zn(OH)2, ZnðOHÞ3
, and ZnðOHÞ4 2
in aqueous solution, with the concentration ratio depending on the pH of the reaction medium. In general, under alkaline conditions, a relatively large quantity of ZnðOHÞ4 2
will act as seeds for the nucleation of ZnO growth units. Additionally, various additives/capping agents, such as surfactants, polymers, and organic molecules, have been used to control the morphology of ZnO nanostructures, leading to the formation of different nanostructures [360–364]. Different surfactants, such as cetyltrimethylammonium bromide, tetraethylammonium bromide, tetraoctylammonium bromide, sodium dodecyl sulfate, and hexamethylenetetramine, were extensively used for the preparation of a wide variety of ZnO nanostructures [363]. Most of these surfactant additives provide specific shapes to the ZnO particles and control their defect structure. It has also been reported that modifying the ZnO nanoparticle surface with organic molecules improves the photocatalytic performance to a remarkable extent. ZnO nanostructures were also synthesized in the presence of cetylpyridinium chloride (CPC) as a shapedirecting agent to form single-phase hexagonal wurtzite ZnO nanostructures [365]. Moreover, the presence of CPC favors the formation of pyramidal-shaped nanoparticles, whereas roughly spherical nanoparticles were formed in the absence of CPC. The bandgap and defect emissions of ZnO nanostructures were strongly dependent on the concentration of CPC. Apart from the different synthetic methods, the morphologies and photocatalytic performances of ZnO nanostructures were also

Metal oxide photocatalysts

91

dramatically affected by the use of different zinc salts, such as zinc chloride, zinc acetate, zinc sulfate, and zinc nitrate [366–368]. To obtain ZnO nanomaterials with different morphologies, such as nanoflakes, spherical nanoparticles (SNPs), and nanorods, the concentration of the capping agent is a key factor [369]. The photocatalytic activity follows the order of spherical > rodlike > flake-like. The high activity of the spherical shape was due to the formation of a nonfaceted morphology with a high surface area, a small crystalline size distribution, and high concentrations of electron (oxygen vacancies) and hole traps (oxygen interstitials) [369]. During photocatalysis, the defects and oxygen vacancies become centers to trap the photogenerated electrons on the surface. Oxygen vacancy is the most important factor and the prevalent defect in most semiconductors including ZnO. It is well-known that the existence of defects in a semiconductor lead to a corresponding defect energy level in the bandgap. ZnO has rich defect chemistry, and many kinds of defects can exist in ZnO nanocrystals, such as zinc vacancy (VZn), oxygen vacancy (VO), zinc interstitial (Zni), oxygen interstitial (Oi), and antisite oxygen (OZn). The presence of different defects could either improve or reduce the photocatalytic activity, depending on the type and location of native defects [370]. Besides the VB and CB, some energy levels of the defects can also store the photogenerated electrons and holes. When photogenerated electrons and holes in these defect energy levels recombine, it is known that different kinds of light emission can be distinguished in the photoluminescence (PL) spectra. On the other hand, these electrons and holes can also be trapped by oxygen and surface hydroxyl species. As a consequence, active species are also formed in these defect energy levels, and therefore the photocatalytic performance of ZnO is affected by these defects to some extent. As discussed above, the defects are very important for both the PL and the photocatalysis of ZnO. Recently, there have been efforts to understand the correlation between the photocatalytic activities of ZnO and its defect-related surface characteristics. The intensity of the defect-induced emission band decreases as the shape of the ZnO particles change from spheres to rods, which in turn influences their photocatalytic activity [371]. Moreover, it was observed that a lower concentration of nonradiative defects resulted in a higher photocatalytic activity, and the defect concentration on the surface of ZnO nanostructures strongly influences the photocatalytic degradation of organic dyes [372].

3.2.3.2 Photocatalytic activity Although the wide bandgap of ZnO greatly limits light absorption in the ultraviolet (λ < 380 nm) and seriously limits the photocatalytic efficiencies, a few works have reported that its photocatalytic activity is better than Evonik P25 titania (P25) or synthesized TiO2 [373–375]. On the other hand, ZnO normally suffers from the intrinsic drawback of photocorrosion, which greatly restricts its use as an efficient photocatalyst in wastewater treatment. The photocorrosion occurs in four steps as follows [338]: +
O2
surface + hVB ! Osurface

(3.53)

92

Metal Oxide-Based Photocatalysis 2
+ O
+ 3hVB ! 2 O
O2
surface + 3O





+ ! O2 O
O2
+ 2hVB

(3.54) (3.55)

2Zn2 + ! 2Zn2 + ðaqÞ

(3.56)

+ The overall reaction of ZnO photocorrosion : ZnO + 2hVB 2+ ! Zn + ½O2

(3.57)

It was observed that ZnO-based photocatalysis suffers from the following drawbacks: (1) ZnO does not absorb the visible portion of the solar spectrum, while UV light is expensive for bandgap excitation; (2) rapid recombination of the charge carriers slows down the degradation reactions at the semiconductor-liquid interface; (3) after the reaction, it can be difficult to recover ZnO powder from the suspension by conventional filtration; (4) the tendency to aggregate during catalytic reactions and the susceptibility to corrosion under UV light [376]. Most photocatalytic studies of ZnO were conducted under UV irradiation [377–382], but studies under visible light conditions have also been investigated [368,383–385]. Other studies compared the photocatalytic activity of ZnO with other semiconductor photocatalysts (SnO2, ZnS, and CdS), proving that ZnO is generally a better photocatalyst under visible light than other semiconducting metal oxides [386]. Recently, photodegradation based on the photocatalytic reaction of ZnO has been proposed as an efficient method for the removal of pollutants including dyes (orange II, direct yellow, remazol black B, remazol brilliant blue R, RhB, MB, and malachite green) [351,355,365,380,387–399]. ZnO nanosheets with increased BET surface area and rich oxygen-vacancy defects were fabricated for degradation of Rhodamine B under visible light illumination [387]. The rich oxygen vacancies improved the visible light absorption of the ZnO nanosheets, leading to enhanced photocurrent and photocatalytic activities. The photocatalytic degradation of malachite green by ZnO nanomaterials (nanoparticles and flowers) was studied [388]. The effects of various parameters, such as initial dye concentration, catalyst loading, solution pH, and light source, on the degradation efficiency were also investigated. The experimental results showed that hydrothermally synthesized flower-shaped ZnO nanorod bundles exhibited excellent photocatalytic activity under solar light. Hexagonal wurtzite ZnO with spherical or flower-like structures was synthesized in the presence of different surfactants (CTAB, AOT, or PEG) by a co-precipitation method [390]. The photocatalytic efficiency of MB degradation was enhanced in the presence of ZnO nanopowders synthesized with these surfactants. ZnO nanostructures with different morphologies, including nanorods, nanospheres, and nanosheets, were prepared in another study [396], with the nanorods showing the highest photocatalytic activity under visible and UV-visible light irradiation, as well as excellent photostability and reusability. Additionally, mechanism studies by trapping different active species showed that hydroxyl radicals ( OH), photoinduced holes (h+), and superoxide anion radicals ( O2
) were involved in the photocatalytic process.  O2
played a major role under l

Metal oxide photocatalysts

93 l

visible light irradiation, whereas OH was the main active species under UV light irradiation. The photocatalytic elimination of phenolic compounds has been tested in the presence of ZnO photocatalysts [349,384,400–405]. Fabricated rod-like ZnO nanostructures showed good photocatalytic performance for phenol degradation, which could be ascribed to its special structural feature [349]. Highly photoactive ZnO samples for 2-chlorophenol (2-CP) degradation were prepared by using hyper zinc-accumulating plants [384]. The degradation efficiency of 2-CP reached 96.93% with the ZnO nanoparticles under simulated sunlight irradiation for 120 min. The authors explained that both  O2
and OH had certain effects on the photocatalytic degradation of 2-CP, but  O2
occupied the dominant position. The effects of various operating parameters, including solution pH, photocatalyst dosage, and initial phenol concentration on phenol degradation were investigated in the presence of ZnO nanosheets immobilized on montmorillonite [402]. Characterization results showed that the degradation efficiency of phenol under UV light was higher with the ZnO nanosheets/montmorillonite composites (88.5%) than with ZnO photocatalyst alone (56%). ZnO films fabricated by the low-temperature atomic layer deposition (LT-ALD) method, and ZnO nanowires grown on stainless steel mesh were used for phenol and 4-nitrophenol degradation [403,405]. In other studies, ZnO photocatalysis under UV light irradiation degraded persistent organic pollutants, such as metamitron [406], and the pharmaceutical compounds of amoxicillin, ampicillin, cloxacillin, and tetracycline [407,408]. l

Nitrogen-, sulfur-, and carbon-doped ZnO Cation doping of ZnO usually produces additional defects or impurities, which may act as recombination centers. In contrast, doping with nonmetals with small radii are more promising, because they easily substitute for lattice oxygen sites of ZnO or occupy interstitial sites. Nitrogen as a dopant has attracted the most attention, because of the similar ionic radii of nitrogen and oxygen, the similar N 2p and O 2p energy states, high solubility, and also low formation energy among nonmetal dopants. There are two main routes of nitrogen doping in ZnO: replacement of oxygen by nitrogen (substitutional sites) or occupation of interstitial sites [409]. Depending on the nitrogen source and preparation methods, different properties can be observed for N-doped ZnO. N-doped ZnO prepared from ammonium hydroxide possessed a higher specific surface area, which is attributed to the controlled nucleation and growth of crystallites in the presence of nitrogen [410]. At the same time, when using NH4NO3 as the nitrogen source, a smaller specific surface area was observed due to the collapse of small pores in the ZnO sample during calcination [411]. Nitrogen-doped ZnO materials were also obtained by the sol-gel method with various N/Zn ratios using urea as the nitrogen source [412]. Variable specific surface area, crystallite size, and Eg were obtained with different amounts of nitrogen doping. The photocatalytic activity of N-doped ZnO catalysts was evaluated for the degradation of mixed herbicides (2,4D and picloram) under visible radiation (λ  400 nm). The 30% N-doped ZnO material showed higher visible-light activity compared with pure ZnO. N-doped ZnO

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nanoparticles were also prepared using melamine as the nitrogen source, and found to exhibit improved photocatalytic performance for the degradation of methyl orange dye under simulated daylight in comparison with the undoped ZnO [413]. In S-doped ZnO, the number of oxygen vacancies also significantly affects the photoactivity. The electronic structure, optical properties, and photocatalytic activity of S-doped ZnO were investigated using DFT calculations [414]. It was reported that the physical properties of S-doped ZnO are influenced by the different substitutional sites of S in the ZnO lattice. For SO-doped ZnO (configurations with the substitution of O by one and two S atoms), the S 3p states are located above the VB and mixed with the O 2p states, leading to bandgap narrowing. The absorption coefficient of the introduced native defects (VO and VZn) shows different features in the low energy range. The existence of VO in SO-doped ZnO leads to a strong absorption in the UV range and VZn plays a critical role in visible-light absorption, which may improve the photocatalytic activity of ZnO in the UV-visible light range. Also, N-S-C tri-doped ZnO particles were synthesized [415]. Incorporating these dopants in appropriate quantities reduced the crystal size, increased the surface area, and improved the photoactivity by enhancing the light absorption and the electron/hole transport. Carbon is the second most popular nonmetal for doping ZnO. Enhancement of the photoefficiency in carbon-doped ZnO may be attributed to three causes [416]. First, the presence of carbon on the ZnO surface enhanced the pollutant adsorption on the surface. Second, C-ZnO showed stronger UV absorption in comparison to pure ZnO. Third, the surface oxygen vacancies in the ZnO nanostructure induced new energy levels below the CB of ZnO, which can scavenge the photo-excited electrons and avoid electron-hole recombination. XPS analysis demonstrated the formation of ZndC and ZndOdC bonds, as well as adsorbed carbonate or carbon dioxide in the ZnO lattice [417]. The band structure and DOS for pure and C-doped ZnO were calculated by the DFT method. The results demonstrated that the localized C 2p energy levels appeared near the Fermi level. Therefore, electrons in the VB of C-doped ZnO transferred to these new states and then to the CB of ZnO as a result of visible-light excitation. Additionally, it was observed that C4
with a larger ionic radius (260 pm) is substituted instead of the smaller O2
(140 pm) [418].

Transition metal-doped ZnO To hinder the undesired recombination of photogenerated holes and electrons, the doping of ZnO with foreign ions/impurities was used to change the coordination environment of host metal ions in the lattice and modify the electronic band structure via the introduction of localized electronic energy levels within the bandgap states (elevation of the VB maximum or lowering of the CB minimum). Metal doping counters the recombination problem by enhancing the charge separation between electrons and holes. The generation of hydroxyl radicals and active oxygen species will also be greatly increased due to the enhanced charge separation efficiency. Moreover, the structural, optical, chemical, electrical, and magnetic properties of ZnO can be tuned by the addition of selected cationic dopant. Cation-doped ZnO has a lower bandgap energy compared to undoped ZnO. When cationic dopants are introduced as

Metal oxide photocatalysts

95

impurities in the ZnO crystal lattice, extra energy levels are created. Several locations are possible for the cationic dopant: the ZnO surface, Zn sites within the lattice of ZnO crystal, and the interstitial sites of the ZnO in the crystal lattice [419]. Also, anionicdoped ZnO photocatalyst can exhibit enhanced photocatalytic degradation performance compared to pure ZnO. In recent years, doped ZnO has exhibited faster photodegradation of many organic pollutants. ZnO has been doped with elements, such as Al, Sb, Mn, Ni, Co, Ag, Fe, Li, and Zr, to enhance the photocatalytic efficiency. The incorporated elements are usually isomorphic to zinc ions, such as Cu2+, Ni2+, Co2+, and Mn2+. The radius of Mn2+ is nearly equal to that of Zn2+ with the same cationic charge, so it can easily replace Zn2+ in the ZnO lattice without any significant crystal changes. It is known from the literature that the addition of Mn precursors during the preparation of ZnO can affect the final product morphologies, form smaller particles and crystallites with higher surface area, and enhance the photocatalytic activity [420–423]. One important effect of Mn doping is related to the increased defect concentrations, especially of oxygen vacancies, which act as charge-carrier traps to separate photo-induced electron/hole pairs and increase their lifetime [424,425]. Alternatively, if the Mn doping concentration is higher than the optimum value, the average distance between trapped carriers is reduced and the Mn dopant sites can act as efficient recombination centers to decrease the photocatalytic activity [420]. It was also observed that, at high concentrations, Mn2+ ions prefer to react with lattice oxygen to form MnOx species instead of substitution to the Zn2+ sites [424]. Cu2+ can simply penetrate the substitutional sites of the ZnO lattice and modify the absorption and emission spectra into the visible light region. The effective Co2+ ionic radius is slightly smaller than Zn2+, therefore, Co2+ can be easily substituted to Zn2+ sites without any significant distortion in the lattice structure. Fe doping can strongly influence the nanostructural morphologies and reduce the crystallite and particle sizes to enlarge the surface area. Moreover, it induces defects, especially additional oxygen vacancies. The incorporation of rareearth elements in ZnO modifies the electronic structure and extends the absorption to visible light due to (1) the formation of localized impurity level band by RE metals, (2) potential fluctuation introduced by the ionized impurities, and (3) charge transfer between the ZnO VB or CB and the 4 f and 5 d states of RE metals [426]. The increase in optical bandgap energy by Mn2+ doping was attributed to the Moss-Burstein effect, which was caused by electrons generated from the oxygen vacancies [427]. The incorporation of Co in the ZnO lattice leads to a shift in the optical absorption edge toward the visible light region, as well as the bandgap energy narrowing due to the s-d and p-d exchange interactions between the localized d electrons of the Co2+ ions and ZnO band electrons [428–431]. Co doping, as in the case of Mn, hinders the growth of ZnO nanoparticles and produces smaller particles with higher effective surface area [430]. Also, the presence of Fe dopants has a strong influence on the nanostructural morphologies, and usually reduces the crystallite and particle sizes while increasing the surface area [432,433]. Fe-doped ZnO nanostructures can have ferromagnetic properties and be gathered by simple magnetic separation from aqueous solutions after the photodegradation process [434]. Fe dopants can exist in different oxidation states, namely Fe2+, Fe3+, and Fe4+ [434,435]. The redox potential of Fe3+/Fe2+ is 0.771 V,

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which is between the VB and CB of ZnO and acts as a charge-carrier trap. The photocatalytic degradation of waterborne pollutants over Fe-doped ZnO nanostructures can be described as follows [433]: ZnO + hv ! e
+ h + ðcharge pair generationÞ

(3.58)

2+ ðelectron trapÞ Fe3 + + e
CB ! Fe

(3.59)

Fe2 + + O2 ðadsÞ ! Fe3 + + O2 
ðelectron releaseÞ

(3.60)

+ ! Fe4 + ðhole trapÞ Fe3 + + hVB

(3.61)

Fe4 + + OH
! Fe3 + +  OH ðhole releaseÞ

(3.62)

Noble metal-doped ZnO Modification of ZnO by noble metals is another effective method to promote the photocatalytic performance because noble metals can modify the optical and electron properties of ZnO and thereby influence the photocatalytic activity. The results showed that even though doping Pd into ZnO decreases the BET surface area, it improves the activity for the degradation of methyl orange [436]. The Pd2+-doping introduced a new electronic energy level within the bandgap states to serve as a shallow trap for charge carriers. The charge carriers were more efficiently separated by increasing the Pd2+ content from 2% to 3%. However, the efficiency suddenly decreased at high concentrations. The introduction of Ag into ZnO nanostructures has received great attention as a way to produce photocatalysts active under visible light [437–441]. Ag doped in the ZnO crystal lattice has a few roles. Ag ions act as an electron acceptor in ZnO and can be used to generate p-type conductivity in pristine ZnO [442]. Ag creates acceptor levels inside the ZnO energy bandgap and decreases the Eg value. In addition, the Ag dopant prefers to substitute on the Zn site under both O-rich and Zn-rich environments and a new acceptor state is created above the VB for AgZn-doped ZnO. Therefore, electrons can be excited from the VB to this gap state, which leads to a visible-light absorption edge. Due to the larger radius of Ag+ than Zn2+, it may be difficult to substitute Zn ions by Ag ions. Therefore, the incorporation of Ag in ZnO beyond a critical value leads to the aggregation of Ag atoms at the grain boundaries and the formation of separated particles and clusters [443]. Nevertheless, Ag is one of the most promising dopants, as it alters the surface of fabricated ZnO by decreasing the total volume of the particle, which in turn decreases the recombination probability. The result is more available carriers for oxidation/reduction on the surface.

Lanthanide-doped ZnO Eu-doping of hierarchical ZnO micro/nanospheres increased the light absorption ability and caused a redshift in the absorption [444]. Under sunlight irradiation, Eu-ZnO exhibited higher photocatalytic activity for the degradation of phenol than pure ZnO

Metal oxide photocatalysts

97

and commercial TiO2. These authors also studied the photocatalytic performance in the presence of charge-carrier scavengers and identified OH radicals as the main active species responsible for phenol degradation. The formation of OH radicals and the degradation mechanism for Eu-doped ZnO were described in the following reactions [444]: l

l

+ ZnO + hv ! ZnO e
CB + hVB




(3.63)

2+ Eu3 + + e
CB ! Eu

(3.64)

Eu2 + + O2 ! Eu3 + + O2 


(3.65)

O2 
+ H + !  OOH

(3.66)



OOH + H + + e
CB ! H2 O2

(3.67)


H 2 O 2 + e
CB ! OH + OH

(3.68)

+ + H2 O !  OH + H + hVB

(3.69)

+ + OH
!  OH hVB

(3.70)

Lanthanide-doped ZnO nanoparticles (Ln ¼ La, Nd, and Sm) were studied, and the photocatalytic activity with different dopants and concentrations were compared with pure ZnO [445,446]. The degradation rate over Ln-doped ZnO increases with increasing La, Nd, and Sm concentrations up to an optimal concentration and then decreases. Nd-doped ZnO was found to be the most active photocatalyst. It was concluded that the type and amount of loading, small particle size, and separation of charge carriers (e
/h+) have the major effect on photocatalytic activity of Ln-doped ZnO. Also, cerium doping increases the photocatalytic activity of ZnO due to the co-existence of Ce3+ and Ce4+ [447–449]. Ce doping (with Ce4+ ions incorporated into the lattice position of Zn2+ ions) reduces the crystallite and particle sizes and increases the surface area. The doping also caused blueshift or redshift in the optical absorption edge of ZnO, which leads to increased photocatalytic efficiency [448]. Moreover, a photodegradation mechanism over Ce4+-doped ZnO photocatalysts was proposed. Under illumination, the Ce4+ ions absorb photoexcited electrons and change into Ce3+. Next, the Ce3+ ions react with O2 molecules to produce O2 
radicals and Ce4+ ions, while the photogenerated holes react with water to produce OH radicals. In summary, intense research efforts have led to significant progress in ZnO-based photocatalysis under UV/visible/solar irradiation. Nevertheless, for diverse photocatalytic applications, many fundamental disadvantages of ZnO need to be overcome, such as the fast recombination of photogenerated excitons, lack of resistance to photoinduced corrosion, dissolution at extreme pH values, aggregation of particles because of structural instability, and a large bandgap that remains inert in visible light. Moreover, most of the current literature deals with dye removal, while the

98

Metal Oxide-Based Photocatalysis

photocatalytic degradation of emerging contaminants, such as pharmaceutical compounds, is little investigated or reported. Additionally, most photocatalytic investigations are carried out using powdered ZnO suspensions in aqueous media, even though the separation of the catalyst particles from the solution after treatment can be a problem in practical applications. Preparing new photocatalysts by incorporation of other elements is also an important challenge in the development of ZnO photocatalyst. Nevertheless, this doping strategy appears to be a good solution to modify the physical (i.e., crystallite size and surface area), electronic (i.e., bandgap energy and defect concentration), and morphological features of ZnO for photocatalytic applications.

3.2.4 Fe2O3 (structure, surface properties and photocatalytic activity of pristine and doped Fe2O3) 3.2.4.1 Structure and surface properties Fe2O3 is a transition metal oxide which has different stoichiometric and crystalline structures, including w€ ustite (FeO), hematite (α-Fe2O3), maghemite (ν-Fe2O3), and magnetite (Fe3O4). α-Fe2O3 is the most thermodynamically stable crystal phase among iron oxides. Thanks to its suitable Eg value (2.0–2.2 eV) and high chemical stability, α-Fe2O3 has become an excellent photoelectrode material for electrochemical water splitting [450,451], an anode material in lithium ion batteries [452,453], and a photocatalyst for the degradation of organic pollutants [454–459]. Various researchers have utilized α-Fe2O3 nanostructures for water treatment [460], as chemical sensors [461], and for biomedical applications [462]. α-Fe2O3 has a hexagonal crystal system with each iron atom surrounded by six oxygen atoms (Fig. 3.13). In hematite, the iron and oxygen atoms are arranged in a corundum structure, which is a trigonal-hexagonal scalenohedral with space group R-3c, lattice parameters of a ¼ 5.0356 nm, c ¼ 13.7489 nm, and six formula units per unit cell [463]. In α-Fe2O3, the oxide ions (O2
) are arranged along the (0 0 1) plane of a hexagonal closed-packed lattice, whereas two-thirds of the octahedral interstices are occupied by the cations (Fe3+) in the (0 0 1) basal planes. The tetrahedral sites are unoccupied. It was found that the ferromagnetic low-temperature phase of γ-Fe2O3 can be easily transformed into the more stable antiferromagnetic α-Fe2O3 when annealed at around 500°C [464]. A literature study revealed that the performance of α-Fe2O3 nanomaterials is strongly dependent on the shape and size of the particles. The physicochemical properties of the product in turn depend strongly on the fabrication conditions, especially the material origin, concentration, solution pH, as well as the mode of thermal treatment used (annealing temperature, atmosphere, and rate of heating/cooling). Moreover, the surface area plays an important role in determining the photocatalytic activity, so researchers have attempted to reduce the size of photocatalyst particles and enhance the photocatalytic properties by producing hematite in a nanoscale powder form using different methods, such as sol-gel [465], hydrolysis [466], co-precipitation [467,468], hydrothermal methods [469–471], solvothermal methods [468,472], IL-assisted synthesis [473,474], thermal decomposition [475], combustion

Metal oxide photocatalysts

99

Fig. 3.13 Crystal structure of α-Fe2O3. From http://www.crystallography.net/cod/.

methods [476], and a combination of reflex condensation and hydrothermal methods [477]. Up to now, a variety of α-Fe2O3 structures have been fabricated, such as rhombohedra [478], nanoparticles [479], nanocubes [480], nanorings [481], nanowires [482], nanorods [483,484], nanotubes [485], nanofibers [465], nanoflakes [486], nanopeanuts [458], microcages [487], microflowers [488], cauliflower [489], hollow core/shell structures [454], and hierarchical nanostructures [490]. Among the various morphologies, two- and three-dimensional (2D and 3D) hierarchical architectures of Fe2O3 composed of nanoscale building blocks have aroused considerable interest owing to their novel properties. Thus, the effects of experimental conditions on the formation of nanostructures were investigated by many researchers, and possible formation mechanisms were proposed for particular structures [489,491–498]. Table 3.1 schematically illustrates the formation mechanisms of hollow spindles, flower-like structures, multishelled hollow spheres, cauliflower, dendric shapes, nanoplatelets, and nanosheets. l

Fe2O3 solid spindles and hollow spindles were synthesized by a template-free hydrothermal method [492]. In the first step of reaction, hematite nanocrystals nucleate in a solution due to the solvent-mediated hydrolysis of Fe3+. The nanocrystals so formed are unstable and tend spontaneously aggregate because of the high surface energy. During the aggregation process, adjacent nanocrystals are arranged in an orderly way by an oriented-attachment mechanism, so that the particles share a planar interface in a common crystallographic

100

Metal Oxide-Based Photocatalysis

Table 3.1

Formation mechanism of different α-Fe2O3 structures

Type of nanostructure

Mechanism formation

Ref. [493]

α-Fe2O3 hollow spindles

[495]

Flowerlike α-Fe2O3

[496]

Multishelled α-Fe2O3 hollow spheres

[490]

Cauliflower-like α-Fe2O3

Metal oxide photocatalysts

Table 3.1

101

Continued

Type of nanostructure

Mechanism formation

Ref. [497]

Dendritic α-Fe2O3

[498]

α-Fe2O3 nanoplatelets

[499]

α-Fe2O3 nanosheet

Based on J. Huang, M. Yang, C. Gu, M. Zhai, Y. Sun, J. Liu, Hematite solid and hollow spindles: selective synthesis and application in gas sensor and photocatalysis, Mater. Res. Bull. 46 (2011) 1211–1218; D. Zhu, J. Zhang, J. Song, H. Wang, Z. Yu, Y. Shen, A. Xie, Efficient one-pot synthesis of hierarchical flower-like α-Fe2O3 hollow spheres with excellent adsorption performance for water treatment, Appl. Surf. Sci. 284 (2013) 855–861; Y. Liu, C. Yu, W. Dai, X. Gao, H. Qian, Y. Hu, X. Hu, One-pot solvothermal synthesis of multi-shelled α-Fe2O3 hollow spheres with enhanced visible-light photocatalytic activity, J. Alloys Compd. 551 (2013) 440–443; X.-L. Cheng, J.-S. Jiang, C.-Y. Jin, C.-C. Lin, Y. Zeng, Q.-H. Zhang, Cauliflower-like α-Fe2O3 microstructures: toluene–water interface-assisted synthesis, characterization, and applications in wastewater treatment and visible-light photocatalysis, Chem. Eng. J. 236 (2014) 139–148; G. Bharath, N. Ponpandian, Hydroxyapatite nanoparticles on dendritic α-Fe2O3 hierarchical architectures for a heterogeneous photocatalyst and adsorption of Pb(ii) ions from industrial wastewater, RSC Adv. 5 (2015) 84685–84693; A.A. Ayachi, H. Mechakra, M.M. Silvan, S. Boudjaadar, S. Achour, Monodisperse α-Fe2O3 nanoplatelets: synthesis and characterization, Ceram. Int. 41 (2015) 2228–2233; T.-W. Sun, Y.-J. Zhu, C. Qi, G.-J. Ding, F. Chen, J. Wu, α-Fe2O3 nanosheet-assembled hierarchical hollow mesoporous microspheres: microwave-assisted solvothermal synthesis and application in photocatalysis, J. Colloid Interface Sci. 463 (2016) 107–117.

102

l

l

l

l

l

Metal Oxide-Based Photocatalysis

orientation. The CTAB ions coordinated preferentially with the surface of precursor Fe atoms and led to the formation of solid spindles. During the following hydrothermal reaction, the nanoparticles comprising the solid spindles decomposed and recrystallized. The dissolution primarily occurred in the spindles’ interior because of the high surface energy of the nanocrystals, while the exterior shells had relatively lower surface energy because of the stabilization of oxalate ions. Thus, hollow spindles were formed. Flower-like α-Fe2O3 hollow spheres were obtained by the one-pot solvothermal method in a glycerol-ethanol system [494]. It was found that the volume ratio of glycerol/ethanol and reaction time have a significant effect on the morphology of the products. The role of glycerol was to provide single-bond OH groups to coordinate with Fe3+ to form Fe(III)-glycerol complexes as nucleation centers. The α-Fe2O3 nanoparticles obtained by hydrolysis quickly aggregate to form microspheres because of the high surface energy and subsequently formed due to the oriented growth mechanism. With prolonged reaction time, a complete hierarchical structure assembled by nanosheets is produced by the growth of crystals. The hollow structures are formed via Ostwald ripening, and the inner part of the solid spheres is gradually dissolved and recrystallized on the outer shell to finally form hollow spheres. α-Fe2O3 hollow spheres of multiple porous shells were fabricated by a solvothermal method using carbon spheres as templates [495]. Surface charges on the rough, hydrophilic shell of the carbon microspheres facilitated the deposition of the Fe2O3 precursor. After a calcination process, hollow spherical frames of relatively uniform size were formed. Moreover, the individual cage-like spheres were porous and made up of nanosized particles. At the same time, ring-like structures made of a few nanoplates were observed inside the sphere, which could be explained by the size shrinkage and phase separation effects, leading to the multishelled structure in the final nanospheres. Cauliflower-like α-Fe2O3 microstructures were synthesized through a one-step toluenewater biphasic interfacial reaction route [489]. Firstly, the reaction of Fe(acac)3 (acac ¼ acetylacetonate) with ammonia produced crystalline nuclei, which quickly grew into small nanoparticles. However, the obtained nanoparticles were unstable at the toluene/water interface and they rapidly assembled into bigger aggregates to decrease their surface energy. Nanoparticles on or near the surface of the aggregates grew slowly, and as a consequence large crystals resided on the surface of the aggregates with smaller ones in the cores. Due to the high surface energy, the smaller crystallites in the cores dissolved and diffused outwards, while the large ones served as starting sites for the subsequent recrystallization process and continued crystal growth. Polyvinylpyrrolidone (PVP) was added to the reaction system as a crystal face inhibitor, as it might be selectively adsorbed onto specific planes to facilitate oriented nucleation and promote anisotropic growth along the (0 0 1) direction. Dendritic α-Fe2O3 was prepared by hydrothermal synthesis [496]. During the process, the FeðCNÞ6 3
ions as a precursor slowly form Fe3+. The Fe3+ is hydrolyzed to form FeOOH or Fe(OH)3, which is decomposed to form dendritic α-Fe2O3. The formation of the dendritic architecture starts from the six crystallographic directions of the seed. The central trunk was grown along the < 1010> direction and subbranches were created along the < 0110> and < 1010> directions to form the hierarchical and symmetric dendritic α-Fe2O3 structure. The formation mechanisms of α-Fe2O3 nanoplatelets during hydrothermal processing and thermal treatment have been explained [497]. Dissolved Fe3+ ions formed Fe(H2O)63+ complexes with water. After addition of NaOH, brown iron hydroxide (FeOOH) particles were formed by hydrolysis of the iron complexes. In the next step, α-FeOOH nuclei aggregated to generate the primary 1D goethite nanostructure, and finally α-FeOOH nanorods with a lower surface potential were formed under the direction of urea molecules. When the hydrothermal

Metal oxide photocatalysts

l

103

temperature was increased to 200°C, the urea partially decomposed and liberated the α-FeOOH nanorods, which attracted each other and agglomerated along their sides, leading to directed agglomeration to form nanoplatelet-like α-FeOOH. Afterwards, dehydration and crystallization were carried out by thermal treatment at 450°C. α-Fe2O3 nanosheet-assembled hierarchical hollow mesoporous microspheres were prepared by thermal transformation of similarly structured microspheres of a precursor [498]. First, Fe3+ ions were reduced by ethylene glycol to form Fe2+ ions under microwave-assisted solvothermal conditions, eventually forming the Fe(OH)2(HOCH2CH2OH) complex. Then, according to the Ostwald ripening process, smaller nanoparticles of the precursor continued to grow into bigger ones. As the reaction time passed, the nanoparticles started to form nanosheets, and the nanosheets self-assembled to form hierarchically nanostructured, solid microspheres. A void gradually formed in the core of the microsphere, as the nanosheets in the core dissolved and transferred to the outer shell and recrystallized there.

3.2.4.2 Photocatalytic activity Aspects of Fe2O3, such as the shape, size, and structures that affect its photocatalytic activity, were investigated by researchers. Nanostructured iron oxide with different morphologies and phase compositions (α-Fe2O3 and Fe3O4) were obtained by a solid-state reaction route [499]. Samples containing a mixture of α-Fe2O3 and Fe3O4 showed better photocatalytic activity than pure α-Fe2O3. The higher photoefficiency was attributed to the higher transfer rate of electrons and holes generated during the photoreaction of α-Fe2O3 to the VB of Fe3O4, which limits the recombination rates. α-Fe2O3 nanoparticles in a wide size range from about 170 nm to about 2 μm were synthesized by a hydrothermal process and their photocatalytic properties were studied [500]. The α-Fe2O3 powders with smaller crystallites showed higher efficiency in photocatalytic dye degradation compared with those with larger crystallites or commercially available Degussa P25. Zhou et al. synthesized α-Fe2O3 nanorods by thermal dehydration, and compared their photocatalytic activity with microrods [501]. The authors reported a higher degradation rate for nanosized α-Fe2O3 than for the corresponding micron-size rods. Higher FedO bond stretching frequencies were proposed as a key factor behind the enhanced photocatalytic activity. The effect of surface morphology of different α-Fe2O3 nanostructures (such as microflowers, nanospindles, nanoparticles, and nanorhombohedra) on the photocatalytic activity was reported [477,502,503]. The best photocatalytic activity was observed for the samples with the highest surface area and porosity. Similar surface area effects were also reported for flower-like α-Fe2O3 nanostructures synthesized by a biphasic interfacial reaction route [504]. The results showed that nanoflowers had better photocatalytic activity than commercial α-Fe2O3 powders due to the increased crystallinity and surface area. Also, α-Fe2O3 hollow microspheres prepared by solvothermal and hydrothermal methods showed higher photoefficiency than α-Fe2O3 nanoparticles [454,456,470]. α-Fe2O3 hollow spindles and spheres were prepared respectively by Li et al. and Xu et al., with enhanced photocatalytic degradation efficiency due to the larger specific surface area, which means more

104

Metal Oxide-Based Photocatalysis

unsaturated surface coordination sites being exposed to the solution [471,473]. The hollow microsphere facilitates the electron-hole transport and lowers the recombination rate. It also allows multiple reflections of visible light within the interior to improve the light utilization, leading to an increased quantity of OH radical for the photocatalytic reaction. As mentioned before, Fe2O3 is a promising candidate for photocatalytic applications due to its narrow bandgap of about 2.0–2.2 eV. Further, hematite absorbs light up to 600 nm, collects up to 40% of the solar spectrum energy, is stable in most aqueous solutions (pH > 3), and is one of the cheapest semiconductor materials available. Additionally, α-Fe2O3 powder can be easily separated from solution using a magnetic field. Therefore, iron oxide nanostructures with shape-dependent catalytic properties are becoming important materials for degrading various pollutants, such as dyes (RhB [477,505–517], MB [455,469,497,518–522], methyl orange [474,488,523–526], Congo red [459,489,494,527], indigo carmine [528], methyl violet [496], rose bengal [475,529]), phenols [454,491,530–532], and salicylic acid [456,457,498,533]. The enhanced photodegradation of dyes in the presence of α-Fe2O3 photocatalysts may be attributed to the photosensitization of the dye molecules followed by semiconductor-initiated photocatalysis. In the proposed mechanism, first the photosensitization of the dye takes place and then charge transfer occurs from the lowest unoccupied molecular orbital (LUMO) level of the dye to the CB of the photocatalyst [527]. Then, α-Fe2O3 generates electron-hole pairs upon photoirradiation. The electrons eject from the VB of α-Fe2O3 to the CB, leaving behind holes in the VB. Electrons from α-Fe2O3 generate intermediate superoxide radicals (O2 
) by reacting with chemisorbed oxygen on the catalyst surface and oxygen in the aqueous solution. O2 
combines with H+ from water dissociation to form hydroperoxyl radicals ( HO2), which may form H2O2. The hydroxyl radicals (OH
) are formed on the surface of the catalyst by the reaction between photogenerated holes (h+) and adsorbed water (H2O). These reactive radicals and intermediate species react with organic compounds, such as Congo red(CR) dye, and degrade them into nontoxic organic compounds [527]: l

l

CR + hν ! CR

(3.71)

CR + α
Fe2 O3 ! CR + + α
Fe2 O3 e
CB + α
Fe2 O3 + hν ! α
Fe2 O3 e
CB + hVB







(3.72) (3.73)

H2 O ! H + + OH


(3.74)


e
CB + O2 ! O2

(3.75)

+ ! HO2  O2 
+ Haq

(3.76)

HO2  + HO2  $ H2 O2 + O2

(3.77)

Metal oxide photocatalysts

105

+ 
hVB + H
aq ! OH

(3.78)

CR =CR + + ðO2 
, HO2  , H2 O2 , OH
Þ ! Degradation products

(3.79)

The photodegradation of 2,6-dimethylphenol (2,6-DMP) in the iron oxide system with oxalic acid under UV irradiation was investigated [530], including the effects of various parameters, such as the amount of photocatalyst, initial concentration of oxalic acid, and pH value. The results indicate that the photodegradation of 2,6-DMP depends strongly on the initial concentration of oxalate, the initial pH, and the presence of oxygen. This dependency was attributed to the formation of dissolved Fe-oxalate complex in the solution and the adsorbed Fe-oxalate on the surface of natural iron oxide. Fe2O3 nanoparticles with different hydroxyl contents were synthesized to enhance the photocatalytic activity for Cr(VI) reduction and 4-CP oxidation under visible light irradiation [532]. The authors suggested that the synergistic effects of Cr(VI) reduction and 4-CP oxidation over Fe2O3 photocatalysts can promote the separation of photogenerated charge and utilization of active oxidant components, such as OH, H2O2, O2 
, and h+, in the photocatalytic process. In an additional application, α-Fe2O3 also showed good performance in the photocatalytic conversion of aniline to azobenzene [534] and acrylamide polymerization [535]. The mechanism of acrylamide formation is a photopolymerization reaction, which starts with the formation of holes in the VB of α-Fe2O3, followed by the formation of acrylamide radicals, and the subsequent recombination of the growing macro-radicals. l

Nitrogen-, sulfur-, and carbon-doped Fe2O3 The poor conductivity and extremely short hole diffusion length ( 4 nm) hamper the charge transport in the semiconductor and increase the probability of charge recombination. Therefore, the photocatalytic activity of hematite frequently suffers from a high recombination rate of the photogenerated electron-hole pairs on the oxide surface, which significantly restricts its practical applications. Much effort has been focused on addressing this problem by doping with metallic and nonmetallic elements to promote the separation of photogenerated carriers and improve the performance of α-Fe2O3. It was claimed that sulfur (S2
) doping on α-Fe2O3 controls the electron-hole recombination by creating a trap state (separate band) between the VB and CB to delay the recombination process, so that the photogenerated charge carriers could be better utilized for the redox process [536]. S-N co-doped hematite nanostructures have been synthesized by a co-precipitation technique and evaluated for the degradation of RhB [537]. Superior photocatalytic activity was observed and explained by the fact that sulfur and nitrogen create the trap states to control the electron-hole recombination. Moreover, parameters, such as particle size, surface area, formation of OH radicals, and (110) plane, have important roles in determining the degradation efficiency over sulfur-doped materials. Many attempts have been made to dope α-Fe2O3 with fluorine and nitrogen to enhance the photocatalytic activity [538]. By analyzing the band structures of pure

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Metal Oxide-Based Photocatalysis

and doped hematite, it was shown that significant acceptor levels were induced by N dopants, whereas F dopants created shallow donor levels and Fermi energy entered the CBs. Moreover, these authors proposed that N, as a p-type dopant, helps to produce hydrogen in visible light and the n-type dopant of F inhibits the recombination of photoproduced electron-hole pairs.

Metal-doped Fe2O3

α-Fe2O3 can be converted easily to a p-type semiconductor by doping with the appropriate impurity ions, such as Mg2+, Cu2+, and Zn2+. Incorporation of Cu2+ ions in Fe3+ sites is expected to give rise to a charge imbalance in hematite, which can be compensated by one or more of the following mechanisms: (1) conversion of Fe3+ to the Fe2+ state, (2) filling of oxygen vacancies, and (3) creation of cation vacancies [539]. Co2+-doped Fe2O3 nanoparticles with enhanced photocatalytic activity have been reported [540]. The results show that the Co2+ ions were well doped within the lattices of Fe2O3 and that they suppress the formation of the less stable γ-Fe2O3 instead of the most stable α-Fe2O3 at 450°C. Gallium-doped α-Fe2O3 with different molar ratios of Ga/Fe were prepared by a parallel flow co-precipitation method [541]. Doping Ga3+ into α-Fe2O3 increases the specific surface area, the separation efficiency of photoinduced charges, and the formation rate of OH radicals during the photocatalytic reaction. Sn doping inhibits the particle growth of hematite and enhances the visible light harvesting and e
/h+ separation [542]. Compared to pure hematite, the doped photocatalysts showed better activity during degradation of MB under visible light irradiation. l

Noble metal-doped Fe2O3

Doping α-Fe2O3 with noble metals, such as platinum (Pt) and gold (Au), has been carried out [543,544]. The influence of Au doping on the photoelectrochemical, structural, optical, and morphological properties of Fe2O3 was studied [545]. The photocatalytic activity was evaluated by degradation of salicylic acid under sunlight irradiation. The Au-Fe2O3 samples exhibited about 45% more degradation of pollutants than pure Fe2O3. The effect of the Au particle size (2–20 nm) on the photocatalytic activity was also investigated [544]. The kinetic rate constant of RhB degradation increases for smaller Au particles. This may be attributed to the highly dispersed α-Fe2O3 nanocrystals, with well-distributed small Au nanoparticles, allowing effective interactions between the reactants and photocatalysts. Moreover, the metal nanostructures in metal-doped α-Fe2O3 act like optical antennas to concentrate the light energy near the surface. Therefore, α-Fe2O3 nanocrystals near the Au/αFe2O3 interface encounter a much more intense bandgap excitation by this surface plasmon-enhanced electric field, which can generate more photoexcited electrons for the photocatalytic reaction. Pt/α-Fe2O3 catalysts were prepared and tested for the degradation of MB. The Pt nanoparticles embedded in the α-Fe2O3 matrix were 3.9 nm in average size and located at the defective positions. The photocatalytic activity increases with increasing surface Pt atoms because of the higher charge separation. α-Fe2O3 particles of

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different shapes and Pt-doped α-Fe2O3 nanocrystals were synthesized, and demonstrated enhanced photocatalytic activity during the degradation of different dyes [491]. Among all the nanostructured semiconductors, Pt-doped hematite nanorods showed the highest photoefficiency toward the decolonization of MB and Congo red under sunlight illumination. Substantial progress has been made in the degradation of organic dyes and other organic pollutants in water by using α-Fe2O3 powders. But they frequently suffer from activity decline because of the electron-hole charge recombination on the oxide surface, which can occur within nanoseconds. Some other obstacles can also hinder the wide application of iron oxide nanoparticles for the photocatalysis of toxic compounds: (1) difficult separation of catalyst materials after the treatment process, and (2) low quantum-yield of the treatment process, which restricts the kinetics and efficiency. Considerable efforts have been made to enhance the photocatalytic activity, such as by decreasing the photocatalyst size to increase surface area.

3.2.5 Ta2O5 (structure, surface properties and photocatalytic activity of pristine and doped Ta2O5) 3.2.5.1 Structure and surface properties Tantalum oxide (Ta2O5) is one of the most important transition metal oxides because of its extraordinary physical and chemical properties, including high dielectric and refractive coefficients and excellent photoelectric performance. Moreover, Ta2O5 has certain advantages over other materials due to its low cost, nontoxicity, availability, relatively high efficiency, and structural stability. It is utilized in applications, such as photovoltaic devices, electronics, and antireflective layer material [546]. Ta2O5 is a semiconducting material that also has shown photocatalytic activities [547,548]. However, due to the wide bandgap (Eg ¼ 4.0 eV), Ta2O5 photocatalysts can only absorb UV light, which accounts for only 4% of the total sunlight, thereby greatly restricting their practical applications. Several polymorphs of Ta2O5 have been reported, and they can be divided into lowtemperature (L-Ta2O5) and high-temperature (H-Ta2O5) Ta2O5. The most common polymorphs in L-Ta2O5 are orthorhombic β-Ta2O5 and hexagonal δ-Ta2O5. Stephenson et al. reported an orthorhombic β-Ta2O5 with 22 Ta atoms and 55 O atoms [549]. Aleshina and Loginova later pointed out inconsistencies in the β-Ta2O5 data and proposed a new orthorhombic structure which consists of 4 Ta atoms and 10 O atoms [550]. To date, mesoporous Ta2O5 is often prepared by a ligand-assisted templating method involving the complex formation of organic metal precursor with surfactant molecules, followed by hydrolyzation-induced precipitation or gelation of the product [551,552]. Additional methods include: sol-gel [547], combined sol-gel process with surfactant-assisted template [553], and SiO2-reinforcement method [554]. Nanosized Ta2O5 powder can be prepared by the solvothermal reaction of tantalum pentabutoxide (TPB) or tantalum oxide gel [548,555]. However, these precursors are not easy to obtain and are also very expensive. Ta2O5 nanowires [556] and

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Metal Oxide-Based Photocatalysis

nanoflowers [557] were recently prepared hydrothermally using hydrofluoric acid, which, however is impractical for green syntheses and industrial applications. As an alternative to TiO2, Ta2O5 has been successfully applied in the photodegradation of dyes [21,558–560]. Ta2O5 films have been fabricated on a tantalum substrate via a hydrothermal method [21]. The results showed that the morphology of Ta2O5/Ta monolith, which was controlled by the crystallization temperature and duration, had great influence on the photocatalytic activity. Samples with small particles on the top surface, or with tip-shaped top surfaces, could effectively suppress the irradiative recombination and exhibited higher photocatalytic activity. It was explained that the separation of electron-hole pairs was enhanced by the hierarchical structure of Ta2O5 films on a Ta substrate (Fig. 3.14).

3.2.5.2 Photocatalytic activity 1D Ta2O5 nanorods were hydrothermally synthesized with the assistance of polyethylene glycol (PEG) and Sr(OH)2 [558]. The different Ta2O5 samples exhibited differing photocatalytic activities. The authors observed a direct correlation between the aspect ratio and photocatalytic efficiency. Ta2O5 nanorods with a length-diameter ratio of  18 were the most active photocatalyst. These authors also proposed a growth mechanism for the Ta2O5 nanorods. In the presence of Sr(OH)2, Ta2O5 dissolved and was hydroxylated to form Ta2O5xH2O or Ta(OH)5 complex. In the

Fig. 3.14 Scheme of the separation and transfer of electron-hole pairs over hierarchical Ta2O5 films on Ta substrate under irradiation. Based on J. Li, W. Dai, G. Wu, N. Guan, L. Li, Fabrication of Ta2O5 films on tantalum substrate for efficient photocatalysis, Catal. Commun. 65 (2015) 24–29.

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second step, PEG adsorbed to this complex via hydrogen bonding and van der Waals forces. The chains so formed controlled the nucleation and growth of the nanorods by favoring the formation of elongated nuclei. PEG continuously adsorbed on the nuclei surfaces, and its different affinities for different facets caused preferential growth in specific directions and the formation of 1D structure. In the presence of PEG, the single Ta2O5 crystal lattice formed preferentially landed on the {0 0 1} faces of Ta2O5, so nanorods gradually formed along this direction. Prolonged reaction resulted in longer nanorods. Tantalum oxide thin films prepared at room temperature on quartz and indium tin oxide (ITO) substrates by pulsed reactive direct current (DC) magnetron sputtering (the pulsing frequency varied between 5 and 100 kHz) were used for RhB degradation [561], and the sample prepared at 50 kHz showed the highest photocatalytic activity.

N-, S-, C-, and F-doped Ta2O5 Recently, nitrogen-doped Ta2O5 was reported as a visible light-sensitive photocatalyst, capable of decomposing organic compounds under UV and visible light irradiations. To date, N-doped Ta2O5 photocatalysts of different shapes (such as Ta3N5 nanoparticles, TaON ellipsoids, Ta3N5 nonwoven cloth, and mesoporous Ta3N5 microspheres) have been reported [562]. N-doped Ta2O5 could decompose gaseous 2-propanol (IPA) under visible and UV light. It showed high photocatalytic activity that depended on the nitrogen concentration and the wavelength of the irradiated light [563]. N-doped Ta2O5 obtained by ammonia gas treatment at 700°C was used for photocatalytic decomposition of gaseous toluene under artificial solar light and pure visible light [564], showing very stable performance and long lifetime after being reused several times. The higher photocatalytic performance was attributed mainly to the large number of defect states and oxygen vacancies. N-doped Ta2O5 also exhibited excellent photocatalytic activity for hydrogen evolution and CO2 reduction under visible light irradiation (λ  410 nm), due to the larger surface area and controlled morphology [565]. H2 evolution over N-Ta2O5 by water splitting in aqueous solution under visible light irradiation was reported [566]. The N doping into Ta2O5 induced a redshift in the optical absorption edge from 320 to 500 nm, resulting in the absorption of visible light. Moreover, the N doping caused a change in the material conduction from n-type to p-type, resulting in a cathodic photoresponse. Among nonmetal dopants, such as S, N, C, and F in semiconductors, sulfur can tune the electronic band structure and optical absorption properties of the material. Sulfur doping not only extends the absorption region for visible light, but also boosts the oxidation capability of the photoinduced holes even at a very low concentration. Mesoporous sulfur (S)-doped Ta2O5 nanocomposites were synthesized through the sol-gel reaction of tantalum chloride and thiourea in the presence of a copolymer as a structure-directing agent [567]. XPS analysis detected S6+ species, due to the substitution of Ta5+ by S6+. Despite of the currently available strategies to promote the photocatalytic activity over tantalates, the direct application of Ta2O5 as a photocatalyst with adequate activity requires more work. While a number of publications have discussed pure

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Metal Oxide-Based Photocatalysis

and doped Ta2O5, many aspects of their optical and microelectronic properties and their modifications after doping remain unclear or are given different interpretations. Moreover, practical applications of Ta2O5 photocatalyst are still limited, mainly because of the low photocatalytic activity caused by the low quantum efficiency and narrow light-response range. Additionally, most reported Ta2O5-based mesoporous materials are bulk and/or micron-sized. As a result, they showed poor dispersibility and sedimented in the reaction system, reducing the light harvesting and reactant accessibility. Therefore, well-dispersed, crystalline Ta2O5 mesoporous submicrospheres are expected to be a better photocatalyst. Nevertheless, the fabrication of well-dispersed, crystalline Ta2O5 is still a challenge because of the fast hydrolysis rates and sensitive nature of the tantalum precursors, as well as the high crystalline temperature of tantalum oxide.

3.2.6 CuO (structure, surface properties and photocatalytic activity) 3.2.6.1 Structure and surface properties As an important p-type semiconducting metal oxide with a narrow bandgap of 1.2–1.5 eV, CuO was extensively used in various applications, including energetic materials (EMs), bio-sensors, magnetic storage media, gas-sensors, electronics, solar cells, and so on, because of its outstanding photoconductive and photochemical properties [568]. Copper oxide has also been widely used as a heterogeneous catalyst in many important chemical processes, such as the degradation of nitrous oxide, selective catalytic reduction of nitric oxide with ammonia, and oxidation of carbon monoxide, hydrocarbon, and phenol in supercritical water [569]. The superhydrophobic properties of CuO nanostructures make these materials promising candidates in Lotus effect self-cleaning coatings (antibiofouling), surface protection, textiles, water movement, microfluidics, and oil-water separation [570]. Very recently, this oxide has been explored as a new class of materials for photocatalytic processes [571–579]. Contrary to the usual rock-salt structure of other 3d transition-metal monoxides, the CuO crystal structure is monoclinic with C2/c symmetry and has four formula units per unit cell [568]. The Cu2+ ion is at the center of inversion symmetry in a single fourfold site 4c (1/4, 1/4, 0) and the oxygen ions occupy site 4e (0, y, 1/4) with y ¼ 0.416(2) (Fig. 3.15). Bourne et al. proved that CuO has no phase transition at high pressures and temperatures, in contrast to other metal oxides, in which crystal phase transitions can occur during annealing and cooling [580]. The electronic bandgap of CuO corresponds to the difference between the energy levels of the top of the VB derived from the Cu 3d orbital and the bottom of the CB derived from the Cu 3d orbital [568]. Several authors have reported on the positions of the CB and VB edges of CuO [581,582]. Their results indicate that the VB and CB are located at
4.80 to
5.22 eV and
3.23 to
3.80 eV with respect to vacuum, respectively. It is also interesting that the morphology of CuO nanostructures affects their bandgap. Yang and He prepared nanostructured CuO with various shapes by controlling the reactants, reaction

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111

Fig. 3.15 Crystal structure of CuO. From http://www. crystallography.net/cod/.

temperature, and reaction duration [583]. The bandgaps of the flower-, boat-, plate-, and ellipsoid-like CuO products were estimated by the authors to be 1.425, 1.429, 1.447, and 1.371 eV, respectively. CuO nanoribbons or nanorods with different sizes have also been synthesized by controlling the molar ratio of NaOH to Cu(NO3)2, reaction temperature, and concentration of the starting NaOH solution [584]. The absorption edge of the prepared CuO was blue-shifted to shorter wavelength with reduced size and changed morphology, confirming that not only the morphology, but also the size of the CuO nanocrystals affect the bandgap. The unique physical and chemical properties of CuO strongly depend on its morphology, size, specific surface area, and especially the preparation method. Thus, numerous methods have been recently developed to synthesize CuO nanostructures with diverse morphologies, sizes, and dimensions: pulsed laser deposition [585], electrodeposition [586], successive ionic layer adsorption and reaction method (SILAR) [587], chemical vapor deposition [588], sol-gel method [589], thermal oxidation [590], and solvothermal method [591], etc. CuO nanostructures with large surface areas and potential size effects can possess superior physical and chemical properties that differ remarkably from those of their micro or bulk counterparts. Additionally, compared with other metal oxide nanostructures, such as TiO2, ZnO, SnO2, and WO3, CuO nanostructures have more interesting magnetic and superhydrophobic properties [592]. A series of CuO nanostructures have been synthesized, namely nanocubes, nanoneedles, nanorods, nanoflowers, nanosheets, nanotubes, nanowires, nanofiber balls, hollow microspheres, spindle and urchin-shaped nanoparticles, and core-shell structure [568,569,579]. In most cases, the hydrothermal synthesis starts with the formation of the intermediate compound Cu(OH)2. In the next step, synthesis control and manipulation of CuO nanostructures with various shapes is possible by choosing different solutions and adjusting the concentrations of precursors. To obtain different CuO nanostructures, several surfactants and structure-directing agents are normally added, in conjunction with reaction temperature adjustment, reaction time, and pH value to control the morphology, growth, size, and dimensions of CuO nanoparticles (Fig. 3.16).

112

Fig. 3.16 See the legend on opposite page.

Metal Oxide-Based Photocatalysis

Metal oxide photocatalysts

113

3.2.6.2 Photocatalytic activity Because of its relatively low-cost, CuO is a promising candidate for degrading organic pollutants. At the same time, it should be pointed out that H2O2 is often added to the reaction solution to enhance the photocatalytic efficiency of CuO. As a better electron acceptor than O2, H2O2 can be quickly reduced and converted to OH radicals after trapping the photogenerated electrons from the surface of photocatalysts. This not only offers more active radicals but decreases the electron-hole recombination rate and thereby increases the hole utilization during the photocatalytic process. It was proved that, in the absence of H2O2, CuO is not an effective photocatalyst for degrading organic pollutants because of its inability to produce a large amount of OH radicals [593]. This is due to the fact that the VBs of CuO are more negative than the redox potential necessary for the generation of OH. Thus, CuO cannot create hydroxyl radicals under illumination and has lower oxidative activities for the degradation of organic pollutants. It was found that CuO hollow spheres exhibited photocatalytic activity under visible light irradiation only when H2O2 was added as a source of OH [594]. Results obtained for CuO nanostructures (flower-, boat-, plate-, and ellipsoid-like) also showed that no degradation occurred after 15 h of light irradiation in the absence of H2O2 [583]. A number of studies have focused on the dependence of the photocatalytic activity of CuO on the presence of H2O2 [595–598]. The photocatalytic degradation of dyes by CuO in the presence of H2O2 under light irradiation was proposed to occur via an adsorption-oxidation-desorption mechanism [583]. First, dye and H2O2 molecules are adsorbed on the surface of the CuO nanostructure. CuO absorbs a photon and electrons (e
) in its VB can be excited to the CB, with simultaneous generation of the same number of holes (h+) in the VB. Second, the e
and h+ pairs can be captured by H2O2 molecules to form HO, HOO, or  O2
radicals, which are highly oxidative and cause the destructive oxidation of the organic dye. Third, small molecules produced by the dye degradation are desorbed from the CuO surface, and the catalyst is recovered. As mentioned before, various physical parameters, such as particle size, shape, composition, and structure, can affect the properties of CuO nanostructures. Thus, CuO l

l

l

l

l

Fig. 3.16 (A) Schematic representation of the synthesis of CuO using alcohols, EG, and nonionic polymeric surfactants, (B) schematic growth of CuO nanostructures with diverse morphologies, (C) schematic illustration of the growth of CuO nanostructures under different pH conditions. (A) Based on R. Srivastava, M.A. Prathap, R. Kore, Morphologically controlled synthesis of copper oxides and their catalytic applications in the synthesis of propargylamine and oxidative degradation of methylene blue, Colloids Surf. A Physicochem. Eng. Asp. 392 (2011) 271–282; (B) based on J. Liu, J. Jin, Z. Deng, S.-Z. Huang, Z.-Y. Hu, L. Wang, C. Wang, L.-H. Chen, Y. Li, G. Van Tendeloo, Tailoring CuO nanostructures for enhanced photocatalytic property, J. Colloid Interface Sci. 384 (2012) 1–9; (C) based on J. Xiang, J. Tu, L. Zhang, Y. Zhou, X. Wang, S. Shi, Self-assembled synthesis of hierarchical nanostructured CuO with various morphologies and their application as anodes for lithium ion batteries, J. Power Sources 195 (2010) 313–319.

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nanostructures with various morphologies have recently been used as photocatalysts to degrade organic pollutants. The photocatalytic degradation efficiency between commercial CuO and CuO nanowires were compared [599]. CuO nanowires showed the highest photoactivity due to the 1D nanostructure, with an average diameter of less than 15 nm. Photodegradation in the presence of CuO nanowires and nanoleaves indicated that the former exhibited the best photocatalytic activity, which is attributed to the highest exposure of {0 0 2} crystallographic surfaces, the polycrystalline structure, and the small particle size [600]. CuO nanostructures with various morphologies (1D nanoseeds and nanoribbons, 2D nanoleaves, and 3D shuttle- and shrimp-like structures and nanoflowers) were hydrothermally synthesized with PEG as structuredirecting reagent [569]. Photocatalytic results showed that the morphology of CuO samples affects their photoactivity, and the structures (nanoflowers, nanoleaves, and nanosheets) displayed higher photocatalytic activity than nanoseeds and shuttle- and shrimp-like structures with the same amount of exposed {1 0 0} facets because of their larger surface area. However, the 1D CuO nanoribbons with a relatively smaller surface area exhibited the best performance, because of the exposed {
1 2 1} crystal plane with high surface energy. The photocatalytic activity of flower-, boat-, plate-, and ellipsoidlike CuO nanostructures can be influenced by the time and temperature of hydrothermal treatment using PEG and NH3H2O [583]. The degradation rates are in the order of plate-like > flower-like  boat-like > ellipsoid-like > commercial CuO. Cupric oxide nanostructures, such as spherical, vesicular, nanosheets, and platelets, were also obtained by changing the reaction parameters, namely, the precipitating/reducing agent, complexing agent, and the reaction temperature [601]. When used for the decomposition of dyes under sunlight illumination, the photocatalytic degradation rates changed in the order of: spherical, vesicular, sheet-like, and platelet-like morphologies. In another study, the photocatalytic properties of CuO nanorods, nanoleaves, and nanosheets for the degradation of Congo red (an organic dye) were examined [602]. The CuO nanorods provided a high density of surface active sites for the reaction. CuO nanococoons with a unique morphology and high crystallinity were used effectively in degradation reactions because they might promote effective electron/hole separation and generate a large number of oxy radicals [603]. Copper oxide was used for the degradation of phenol, p-aminophenol, and 4-nitrophenol [571–573,575]. The degradation rate of p-aminophenol under solar irradiation was enhanced when CuO was loaded on natural clinoptilolite zeolite and in the presence of hydrogen peroxide or potassium bromate. Photocatalytic performance of the CuO hexapod nanostructures toward the degradation of phenol was examined under visible light [572]. Also, CuO core-shell nanospheres possess superior capacity for the photocatalytic decomposition of phenol [571] mainly because of the unique core-shell nanostructure, high specific surface area, and fine crystalline structure.

Doped CuO Doping the CuO nanostructures has been proven to be a promising strategy for enhancing the photocatalytic performance of CuO. Feather-like Cd-doped CuO nanostructures were synthesized and the Cd2+ was proven to enter the crystal lattice

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115

of CuO and substituted Cu2+ without destroying the crystal structure [604]. The optical bandgap of CuO slightly increased from 1.50 to 1.57 eV with increasing Cd doping levels from 0 to 8.5 wt.%, and Cd2+ doping led to improved photocatalytic activity. Cu2+ ions in the lattice structure can also be replaced by Zn2+ ions because of their ˚ , respectively) [605]. Doping Zn into CuO leads similar ionic radii (0.73 and 0.74 A to the formation of shallow levels inside the bandgap, causing a red shift in the absorption spectra compared to undoped CuO. The Zn-CuO nanoflowers exhibited a significant enhancement in photocatalytic activity compared to CuO nanorods. Ce-doped CuO nanoleaves were synthesized by a simple and low-cost chemical method [606]. The photodegradation efficiency was 5.2 times higher than that of bare CuO. Ce3+ incorporated in the CuO lattice was oxidized to Ce4+ by releasing electrons, which reacted with the adsorbed O2 to produce  O2
superoxide radicals. Then, as an electron scavenger or acceptor, Ce4+ is transformed back to Ce3+ by absorbing photoexcited electrons from the CB of CuO. Cerium-loaded CuO nanoparticles may also be obtained by a simple precipitation-thermal decomposition method [607]. The photocatalytic activities of pure, commercial, and Ce-doped CuO photocatalysts were studied for the degradation of azo dyes under UV light irradiation. The Ce-loaded CuO displayed superior photoactivity compared to bare CuO due to the enhanced electronhole separation. Several studies of Sn doping of CuO have been reported. Nevertheless, the photocatalytic activity was not examined. Sn-doped CuO has been synthesized by a hydrothermal method to achieve the incorporation of Sn ions into the crystal structure of CuO [608]. Sn-doped CuO nanotubes were obtained by the thermal oxidation of Cu/SnO2 core-shell nanowires in air [609]. A study of the photocatalytic activity of Sn-doped CuO nanoparticles found that the Sn doping affected the structural and optical properties of CuO, leading to a redshift of the bandgap (from 1.33 to 1.18 eV) [610]. The Sn doping led to the emergence of a new reactive surface species that was responsible for the higher degradation efficiency. In conclusion, with the development of nanotechnology, many CuO nanostructures with various morphologies and sizes have been fabricated. However, most of these reaction systems can only produce one or two morphologies. In particular, the CuO nanostructures obtained are mostly used to enhance existing applications. A systematic study on the one-pot synthesis of CuO nanostructures with various morphologies is urgently needed and CuO nanostructures for waste water treatment are still lacking. Moreover, the growth mechanisms of these various CuO nanostructures are still not fully understood.

3.2.7 Other metal oxides (NiO, Cr2O3, RuO2, perovskite oxides, etc.) NiO is a p-type semiconducting oxide with a wide bandgap (3.6–4.0 eV). It has aroused considerable interest due to its applications in various fields, such as gas sensors, electrochromic devices, catalysis, battery cathodes, magnetic materials, fuel cell electrodes, and also dye-sensitized solar cells. The CB and VB of NiO

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(
0.5 and +3.1 V, respectively) are highly suitable for water splitting and photocatalytic processes. In addition, NiO acts as a promoter for the generation of OH radicals. Over the past few years, various well-defined hierarchical NiO nanostructures, such as spheres assembled by nanopetals [611], nanosheets [612], nanowires, or nanorods [613], have been obtained successfully by sol-gel [614], precipitation [615], hydrothermal [616], or microwave-assisted hydrothermal methods [617]. Using thermal decomposition, sol-gel, hydrothermal, and emulsion nano-reactor (ENR) methods, NiO particles in the forms of flower, wood, plate, and spheres were synthesized [618]. The samples obtained by the ENR method were highly crystalline, spherical, and had excellent monodispersity and the best photocatalytic activity. NiO samples in rods, wafers, cotton-, and flower-like shapes were synthesized via the hydrothermal reaction of nickel nitrate with four different kinds of alkalis (namely sodium hydroxide, ammonia solution, urea, and triethanolamine) combined with a subsequent calcination process [616]. The characters of the NiO photocatalysts affect their activity in the degradation of MB. The sample with good crystallinity and small particle size (100 nm) possesses more shallowly trapped holes to react with chemisorbed oxhydryl OH
or H2O to generate OH radicals, exhibiting a high photocatalytic activity. Furthermore, in this UV/NiO suspension, the photocatalytic oxidation process of MB occurs via the attack by OH radicals. In comparison, the NiO sample with even higher crystallinity and bigger particle size (>200 nm) possesses more deeply trapped holes (hV+B) to react directly with physisorbed organisms, exhibiting a low photocatalytic activity. Therefore, in this system the MB is oxidized by direct reaction with hV+B. NiO thin films with different thicknesses were tested as photocatalysts for the degradation of methyl green dye from wastewater under UV light [619]. As the film becomes thicker from 30, 50, to 80 nm (with particle size of 8, 10, and 12 nm, respectively), the photoefficiency decreases. This can be explained on the basis of the agglomeration of the particles with longer deposition time, which decreases the number of active sites and the photocatalytic activity. The photocatalytic degradation of RhB was also investigated in the presence of NiO samples [620,621]. Oriented NiO thin films were prepared on LaAlO3 (100), (110), and (111) substrates by pulsed laser deposition [621]. The RhB degradation increases in the order of (100) < (110) < (111), which is consistent with the order of OH generation. Cerium oxide has been investigated for multiple applications, such as photocatalysis, electrolyte material for solid oxide fuel cells, material with high refractive index, ceramic materials, oxygen gas sensors, cosmetics, and as an insulating layer on silicon substrates [622]. Nanocrystalline CeO2 has been prepared by sol-gel process, sonochemical synthesis, gas condensation, solvothermal and hydrothermal synthesis, microwave, combustion synthesis, and template-assisted precipitation [623,624]. Unfortunately, these methods tend to be complex and require expensive raw materials so the cost of production is very high and industrial production is difficult to realize. CeO2 and titania share some features, such as a wide bandgap, nontoxicity, and high stability. Due to its unique 4f electron configuration, CeO2 has been frequently selected as a component to prepare complex oxides or as a dopant to improve titania-based catalysts’ performance. As for pure CeO2, it has been investigated under UV or Vis irradiation for degrading dyes and phenols [622,624–626]. A mesoporous l

l

l

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CeO2 photocatalyst showed a high surface area and a blue shift in its light absorption with respect to bulk CeO2 in the UV-Vis spectra [627]. Under visible light, both bulk and mesoporous CeO2 showed faster photocatalytic degradation of dye than TiO2 P25 from Degussa. More recently, CeO2 was prepared by the precipitation method [625]. Under visible light irradiation (λ > 420 nm), dye molecules were absorbed on its surface through the bidentate coordination of oxygen atoms from the sulfonate group. Dye self-sensitization was the main mechanism for the degradation process, in which excited dye molecules inject electrons into the Ce 4f orbitals and the injected electrons are scavenged by oxygen on the catalyst surface to produce O2 
active species. Therefore, CeO2 could serve as an excellent mediator to transfer electrons coming from the photo excited dye. Ceria-coated activated carbon photocatalysts were used for degrading 4-chlorophenol (4-CP) in aqueous solution under UV irradiation [626]. The synthesized samples exhibited higher photocatalytic activity compared with commercial titania P25, which can be ascribed to the enhanced adsorption activity. Ruthenium dioxide (RuO2), a transition-metal oxide with rutile-like structure, has been investigated for its unique properties, such as high chemical stability, electrical conductivity, and excellent diffusion barrier properties. The CB and VB positions of RuO2 are both lower than those of TiO2, so they are expected to have band overlap [628]. Moreover, RuO2 is a very powerful oxidation catalyst and well-known for its application in heterogeneous catalysis. Molecules, such as CO, O2, NH3, NO, methylene, and methane, adsorb well on the RuO2 (110) surface via the undercoordinated Ru atoms, which is a prerequisite for catalytic reactions. Additionally, the smaller RuO2 nanoparticles have higher activity due to the numerous active sites with increased specific surface area [629]. However, small RuO2 nanoparticles tend to become deactivated due to the occurrence of particle sintering during the catalytic process at elevated temperatures [630]. Chromium oxide (Cr2O3) is a p-type semiconductor with a wide bandgap (3 eV), high electrical conductivity, as well as high thermal and chemical stability. Therefore, it is widely used in various applications including photocatalysis [631]. The most common method for producing Cr2O3 involves deoxidizing an alkali dichromate with sulfur, carbon, or ammonium chloride; other methods include solution-combustion synthesis, laser-induced deposition, microwave, and hydrothermal reduction [631–633]. The spherical Cr2O3 particles synthesized by a microwave refluxing method were investigated as a photocatalyst to degrade methyl orange [631]. The Cr2O3 nanoparticles exhibited good photocatalytic properties, and particles heattreated at 800°C had better performance than the untreated sample and that treated at 400°C. Perovskite-type oxides, originated from CaTiO3, are a family of oxides having the general formula ABO3 (where A is a rare or alkaline earth metal and B is a first row transition metal), wherein cations with a large ionic radius coordinate to 12 oxygen atoms and occupy A-sites and cations with a smaller ionic radius are six-coordinate and occupy B-sites [634]. The perovskite crystal structure provides a good framework in which to tune the bandgap values to enable visible-light absorption and the band edge potentials to suit the needs of specific photocatalytic reactions. Furthermore, lattice distortion in perovskite compounds strongly influences the separation of

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photogenerated charge carriers. The perfect structure of the octahedral connection results in a cubic lattice. However, the real perovskite ABO3 exhibits lattice distortion to varying degrees (the octahedra are tilted around their centers), resulting in the transformation of crystal phases in the following sequences with increasing octahedral rotations: orthogonal, rhombohedral, tetragonal, monoclinic, and triclinic phases. Some groups of perovskite materials, such as titanate, tantalate, vanadium- and niobium-based perovskites and ferrite perovskites, have shown visible-light activity [635]. The structure, size, and potential applications of perovskite oxide materials are strongly influenced by the synthesis process and, therefore, a lot of research is aimed at both its processing and application. Most of the perovskite photocatalysts are still prepared by conventional solid-state reactions, in which the produced particles are typically on a micron scale, which limits their commercial applications due to lower light absorption in the visible region and short lifetimes of the excited states. Several alternative routes have been reported for the synthesis of perovskite oxides, such as combustion synthesis, sol-gel method, thermal decomposition of bimetallic compounds, sonochemical method, microemulsion method, polymerizable complex method, the polyvinyl alcohol (PVA) route, electrospinning method, co-precipitation method, and microwave-assisted method [635]. Among these methods, the facile hydrothermal method is a dominant tool for the synthesis of anisotropic nanoscale materials. ˚ ) with an indirect SrTiO3 is a well-known simple cubic perovskite (Pm3m, a ¼ 3.9 A bandgap of 3.1–3.7 eV, and it has been widely studied as an important n-type semiconductor because of its various outstanding physical properties (stability, wavelength response, and current-voltage response) [636]. Furthermore, it is a promising candidate for efficient photocatalysis. Based on the total DOS, the bands of SrTiO3 are classified into three parts: (1) the lower-energy side consisting of O 2s + Sr 5p + Ti 4p + Ti 4s hybrid orbitals, (2) the middle part of the bands corresponding to the VB consists of O 2p + Sr 5s + Ti 3d hybrid orbitals, and (3) the bottom of the CB formed by the Ti 3d orbital. Thus, the highest occupied molecular orbital (HOMO) and LUMO are composed of the hybrid of O 2p + Sr 5s + Ti 3d and the Ti 3d orbitals, respectively [637]. All these properties depend on its crystal structure and morphology. Therefore, there are many synthesis methods to obtain pure and doped SrTiO3, including the sol-gel method, hydrothermal synthesis, the polymeric precursor method, the solid-state reaction, and the micro-emulsion method [635]. Because the photoactivity of heterogeneous catalysts is influenced by their crystal phase, size, surface area, and crystallinity, controlling the size and shape of SrTiO3 structures is critically important for evaluating their shape-dependent photoreactivity and developing SrTiO3-based high-performance photocatalysts. Huang et al. reported the precise control of SrTiO3 nanoparticle morphology in basic aqueous solutions under different reaction conditions, including NaOH concentration, reaction time, and temperature [638]. It was reported that the formation of the cubic SrTiO3 depended on the NaOH concentration because agglomeration of strontium titanate nanoparticles at higher NaOH concentration promoted the growth of the cubic phase. The photoactivities of the samples were investigated by the degradation of crystal violet dye under UV light irradiation. The SrTiO3 produced using 3 M NaOH, a reaction time of 72 h, and a hydrothermal

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temperature of 130°C was found to have optimal photocatalytic activity. Dong et al. synthesized porous and solid SrTiO3 spheres using a free-template hydrothermal method with titanate nanotubes or nanowires and SrCl2 as precursors [639]. The photocatalytic activity of the porous SrTiO3 spheres was evaluated in the degradation of RhB under UV illumination (λ ¼ 254 nm). After 20 min of irradiation, 100% of the RhB was degraded. BaTiO3, a wide-gap metal oxide having perovskite structure, has attracted extensive attention due to its excellent photocatalytic performance in the splitting of water into H2 and O2 [640–642]. Meanwhile, CaTiO3 perovskite has been considered a promising material for photoelectrochemical water splitting because it has a similar band structure to TiO2, matches the energy levels of water splitting, and has no defects or recombination centers [643–645]. NiTiO3 has been widely investigated for its suitable bandgap (Eg ¼ 2.18 eV) and unique photoresponse in the visible light region for photocatalysis [646,647]. NiTiO3 has a structure in which both Ni and Ti prefer octahedral coordination with alternating cation layers occupied by Ni and Ti alone. CdTiO3 is regarded as an intelligent material with promising applications because of its excellent dielectric, ferroelectric, pyroelectric, piezoelectric, photorestrictive, magnetorestrictive, and electro-optical properties and it has also been used as a photocatalyst [648,649]. The high photocatalytic activity of tantalates (ATaO3, A ¼ Na, K, Ag, Li) is most often attributed to their suitable CB level, which consists of Ta5d, and the efficient carrier delocalization caused by the proper distortion of TaO6 connections [650,651]. Of all tantalate perovskites, NaTaO3 and KTaO3 are the most efficient photocatalysts. However, work on the development of UV-Vis- and visible-lightdriven KTaO3-based photocatalysts is uncommon. At room temperature, NaTaO3 is orthorhombic with space group Pcmn [651]. The Ta cation is coordinated with six oxygen anions to form a tilted octahedron, in which Na cations are located in the central position among these octahedra. The calculated lattice constants are ˚ , b ¼ 5.510 A ˚ , and c ¼ 7.750 A ˚ . NaTaO3 is not active under visible light a ¼ 5.428 A irradiation because its VB predominantly consists of O2p orbitals whose potential energy levels are located at a deep position of about 3 V versus the normal hydrogen electrode (NHE) [652]. Hu et al. investigated the influence of the synthesis method (sol-gel or solid-state) on the structure and photoactivity of NaTaO3 [651]. Using SEM analysis, they observed that samples synthesized by the sol-gel method (SG) had irregular surface morphology with a size of 30–50 nm, while samples obtained by solid-state method (SS) were cube-like with a size of 2–3 μm. Their photocatalytic activity was measured in the water-splitting reaction. The H2 evolution was much greater with SG (2050 μmol h
1 g
1) than with SS (13 μmol h
1 g
1). The high photoactivity of SG was explained by the fact that the sample had a larger surface area and therefore a more active surface than the SS samples. There are two reasons why monoclinic NaTaO3 is better for applications in photocatalytic reactions. First, because phonons are involved in the gap transition, the recombination rate for the photo-induced electron-hole pairs is smaller than that in orthorhombic NaTaO3. Second, the monoclinic phase has a larger number of effective states available for the photo-induced electrons and holes. The influence of NaOH concentration on the formation of

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NaTaO3 with cubic morphology during hydrothermal process (120°C, 12 h) was also reported [653]. The Eg of the obtained NaTaO3 was estimated to be about 3.96 eV. The photoactivity was measured during gaseous formaldehyde and RhB solution degradations under UV light irradiation. In both cases, NaTaO3 photocatalysts showed relatively high activity. Formaldehyde and RhB were completely decomposed over 30 min and 3.5 h of UV light irradiation, respectively. KTaO3 (space group Fd3m) has interesting dielectric, photoconductive, and optical properties, as well as nonlinear performance at low temperatures [652]. For KTaO3, the bands between
17.45 and
15.72 eV are mainly contributed by O 2s, Ta 6s, and Ta 5d, while the second region below the Fermi level, between
11.19 and
10.65 eV, is only contributed by K 3p. The VBs lying between
5.51 eV and the Fermi level are mainly due to O 2p states hybridized with Ta 5d, which means the existence of a covalent-type bond between the O and Ta. Octahedral nanocrystalline KTaO3 particles were obtained using the hydrothermal method [654]. The edge size of the nanoparticles was approximately 500 nm and the bandgap was 4.06 eV. It was suggested that the large bandgap may make it more suitable for photonic and optoelectronic applications and other functional devices. Similar to tantalum-based photocatalysts, niobium-based perovskites (ANbO3, A ¼ Na, K, Ag, Cu) show good photocatalytic activity. NaNbO3 and KNbO3 have bandgap energy values of about 3.08 and 3.14 eV, respectively, and hence they show excellent photocatalytic properties under UV irradiation [635]. Sodium niobate is a well-known perovskite oxide that possesses attractive physical properties, such as low density and high sound velocity, and it is useful for ferroelectric and piezoelectric applications [655]. Moreover, NaNbO3 is nontoxic and highly stable, and therefore it can be used in photocatalytic processes. The basic perovskite NaNbO3 has a cubic structure with the space group of Pm3
m. It is stable only at high temperature (>913 K), while at room temperature the common phase of NaNbO3 is an anti˚ , b ¼ 5.566 A ˚, ferroelectric orthorhombic phase (space group of Pbcm, a ¼ 5.506 A ˚ and c ¼ 15.52 A) [656]. Additionally, valence band tops of cubic and orthorhombic NaNbO3 are constructed from O 2p orbitals, and they are located at similar energy levels. At the same time, the bottoms of conduction bands are significantly different. The energy level of the conduction band bottom in cubic NaNbO3 is lower than that in orthorhombic NaNbO3, which is directly related to the fact that the variant octahedral ligand field of orthorhombic NaNbO3 changes NbdO bond lengths and OdNbdO bond angles of the basic cubic crystal structure [656]. Alkali niobates have been synthesized via the solid-state reaction of alkali metal carbonates and Nb2O5 [657]. Over a temperature range varying from room temperature to 1000 K, several phases of NaNbO3 can be formed, such as tetragonal and cubic structures, while highertemperature calcination of alkaline niobates can lead to the undesirable volatilization of alkaline species. Despite the fact that cubic, tetragonal, and orthorhombic NaNbO3 are all constructed from the basic perovskite unit, most of the reported photocatalytic investigations were carried out using the orthorhombic form. NaNbO3 with special morphologies, such as nanowires, nanorods, or plate-like structures, have been shown to exhibit higher photoactivity in comparison with samples prepared by the solid-state reaction method. Li et al. studied photocatalytic hydrogen evolution over NaNbO3

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samples prepared by three different methods: the solid-state reaction (SSR), hydrothermal (HT), and polymerized complex (PC) methods [658]. The bandgaps of the synthesized NaNbO3 samples were estimated from the optical absorption spectra to be about 3.3–3.4 eV. From the DOS measurement, Li et al. observed that the bottom of the CB is located at 0.3–0.4 eV, which is more negative than the H+/H2 potential of 0 eV. They simultaneously observed the potential of the VB top (+2.94 eV), which is more positive than the O2/H2O potential (+1.23 eV). Based on these calculations, they suggested that NaNbO3 can be successfully used in the water splitting process. Its photocatalytic activity in H2 evolution from both aqueous methanol solution and pure water under UV irradiation was investigated. The authors reported that the sample prepared by PC with the smallest particles exhibited the highest photocatalytic activity in both reactions. This size dependence may be explained by the fact that smaller particles have shorter diffusion distances for the photogenerated electrons to travel to reach the surface reaction sites, hence increasing the rate of electrons reaching these sites. Li et al. reported that, along with the crystal size, the crystal structure (particularly crystallographic symmetry) also affects the photocatalytic properties of sodium niobate [656]. They synthesized cubic NaNbO3 (c-NaNbO3) and orthorhombic NaNbO3 (o-NaNbO3) by furfural alcohol-derived polymerization-oxidation (FAPO) process and the polymerized complex (PC) method, respectively. The photocatalytic activity was measured in H2 evolution from aqueous methanol solution and CO2 photoreduction in the gas phase. In both cases, Li et al. observed that c-NaNbO3 showed nearly twice the photoactivity of o-NaNbO3. The difference in the photoactivity may be due to the different electronic structures of the two phases of NaNbO3, which is caused by the variant octahedral ligand field. Moreover, the high symmetry in c-NaNbO3 means that the photogenerated electrons have a lower effective mass and higher migration ability. Potassium niobate is a perovskite with low toxicity and high stability under light irradiation. It presents temperature-dependent crystalline phases, including orthorhombic, tetragonal, and cubic. The cubic phase of KNbO3 is crystallized at temperatures higher than 437°C, while the orthorhombic phase is formed at 200°C. KNbO3 has been mainly studied in photocatalytic water splitting and dye photodegradation [659–661]. KNbO3 in the orthorhombic phase has Cm2m space group with lattice constants of a ¼ 0.5695, b ¼ 0.5721, and c ¼ 0.3973 nm; the cubic phase KNbO3 has Pm3m space group with a lattice constant of a ¼ 0.4022 nm; while tetragonal KNbO3 has P4mm space group [662]. Yan et al. prepared KNbO3 with cubic (c-KNbO3) and orthorhombic (o-KNbO3) phases by hydrothermal process, and tested the obtained powders in photocatalytic water splitting [663]. Both c- and o-KNbO3 showed higher photoreactivity than commercial bulk KNbO3 powders. c-KNbO3 also showed a higher rate of H2 generation (1242 μmol h
1 g
1) than o-KNbO3 (677 μmol h
1 g
1). Yan et al. calculated the bandgaps of o-KNbO3 and c-KNbO3 to be 3.25 and 3.24 eV, respectively, suggesting that they have different electronic structures. c- and o-KNbO3 have the same VB level, but the bottom of the CB could be different. Differences in active sites between these two phases have an influence on the separation, mobility, and lifetime of the photogenerated charges, and thus also on the photocatalytic activities. The photoreactivity of cubic, orthorhombic, and tetragonal KNbO3 (t-KNbO3)

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microcubes was also investigated by Zhang et al. in hydrogen evolution from aqueous methanol under UV light [661]. The bandgap values of hydrothermally synthesized o- and t-KNbO3 microsized cubes were 3.15 and 3.08 eV, respectively. A comparison of all three KNbO3 phases shows that the photoactivity follows the order cubic > orthorhombic > tetragonal. These results are similar to data obtained by Yan et al. The higher symmetry in the crystal structure of cubic KNbO3 led to higher photocatalytic activity. Moreover, Zhang et al. reported that photogenerated electrons cannot be transferred isotropically in o-KNbO3 and t-KNbO3. Jiang et al. investigated the influence of KOH concentration, reaction temperature, and reaction time on the KNbO3 morphologies, including nanowires, nanotowers, nanocubes, and nanorods [664]. The photocatalytic activity of these structures was measured in the decomposition of RhB under UV-Vis irradiation. Jiang et al. reported that the photoactivity strongly depended on the KNbO3 microstructure in the order of nanocubes (89%) > nanowires (71%) > nanorods (54%) > nanotowers (41%). AVO3-type oxides have been reported for most divalent elements A. For alkaliearth elements (except Mg) their structures are three-dimensional frameworks consisting of corner-shared VO6 octahedra (regular or slightly distorted). Among them, extensive attention has been focused on silver vanadium oxide nanomaterials due to their potential applications in photocatalysis [665–667]. In vanadium oxides, the unique hybridized VBs (V 3d, O 2p, and Ag 4d orbitals) lead to a narrow bandgap and highly dispersed VBs. For example, both α-AgVO3 and β-AgVO3 possess an intense absorption band in the visible light region (bandgap 2.3–2.5 eV), making them potential visible-light-sensitive photocatalysts. β-AgVO3 nanowires longer than 300 μm and with Eg ¼ 2.25 eV have been synthesized by a hydrothermal method [667]. Under visible light irradiation, about 64% of RhB dye was effectively photodegraded in the presence of β-AgVO3 nanowires, which was mainly attributed to the efficient absorption of sunlight and reduced electron-hole recombination due to the high degree of crystallization. Additionally, iron-based semiconductors (AFeO3), such as BiFeO3, LaFeO3, and YFeO3, have also drawn increasing attention in recent years as narrow-bandgap, visible-light photocatalysts [668,669]. Among them, lanthanum orthoferrite (LaFeO3) nanostructures are one of the most common perovskite-type oxides and promising materials for efficient visible-light photocatalysis due to the narrow bandgap energy and unique optoelectronic properties [670,671]. LaFeO3 microspheres synthesized via citric acid-assisted hydrothermal method exhibit good photocatalytic activity for the degradation of Rhodamine B in aqueous solution under visible light illumination, with complete degradation observed within 3 h [670]. LaFeO3 materials with different morphologies, such as cubes, rods, and spheres, were synthesized by a hydrothermal process [671]. The visible-light photocatalytic activity of the prepared nanostructures was evaluated by decolorization of RhB in aqueous solution. The enhanced activity of LaFeO3 nanospheres compared to other nanostructures and commercial Degussa P25 TiO2 can be ascribed to the high specific surface area, unique pore size distribution, and smaller particle size. Perovskite alkaline earth stannate with the general formula ASnO3 (A ¼ Ca, Sr, and Ba) has very interesting properties. Among them, SrSnO3 has been applied as a

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photocatalyst, and its high photocatalytic activity can be attributed to the spatial structure of SrSnO3. In this structure, the three-dimensional network of corner-sharing SnO6 octahedra can help charge carriers to move more easily, and the octahedral tilting distortion also has a positive effect on local charge separation [672]. SrSnO3 nanoparticles with peanut-like morphologies were synthesized by a wet chemical reaction, and the photocatalytic activity for hydrogen evolution was investigated under UV light irradiation. The peanut-shaped nanoparticles exhibited a much higher photocatalytic activity compared to SrSnO3 powder synthesized by a solid-state reaction. This was attributed to the higher structural order of the nanoparticles caused by the formation of a carbonate-free pure phase, as well as their higher surface area resulting from the small particle size.

3.3

Nanostructured MeIOx/MeIIOx heterojunction photocatalysts

Strictly appropriate project composites comprising two or three different materials or phases can effectively facilitate charge separation and charge carrier transfer, thereby significantly improving the photocatalytic and photoelectrochemical efficiency [673]. We can identify two different heterojunction systems: (1) the p-n semiconductor system, and (2) non-p-n heterojunction system. Generally, when p- and n-type semiconductors are in contact with each other, a p-n junction is formed. At the interfaces in the space-charge region some diffusion of e
/h+ occurs. The resulting electrical potential permits the electrons and holes to transfer from one semiconductor to the other. When irradiated by photons with equivalent or higher energy to the bandgap position in the heterostructure, the built-in electric field leads to quick separation of the photogenerated charges. In the resultant electric field, the electrons may be transferred to the more negatively positioned CB of the n-type material, while the holes migrate to the VB of the p-type photocatalyst. The utilization of this kind of heterojunction results in: (1) quick and effective charge separation, (2) fast charge transfer, (3) a longer lifetime of photogenerated carriers, and (4) the separation of locally incompatible reduction and oxidation reactions [674]. A diagram of such a p-n type heterojunction is illustrated in Fig. 3.17. Furthermore, the non-p-n type heterojunction has been applied to enhance the photocatalytic properties of materials. This strategy involves a composite with staggered bandgaps. The heterojunction of two semiconductors with matched potentials of energy bands forms a tightly bonded structure. Due to this fact, an internal field is produced and the barrier between two materials may thus be reduced [674]. When two different semiconductors are in direct contact, the resultant heterojunction can be classified into three groups, depending on the energy bandgap position of each metal oxide (Fig. 3.18). In Type I composites, the VB of semiconductor A (VB (A), with a narrower bandgap) is higher than that of semiconductor B (with a wider bandgap), while the CB of A(CB(A)) is lower than the CB of photocatalyst B. During the photoexcitation process, the electrons can be transferred from CB(B) to CB(A). Moreover, sufficient contact will cause the holes to be transferred from VB(A) to

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Fig. 3.17 Schematic diagram of the separation of the photogenerated charge carriers in the p-n heterojunction. Reproduced with permission from H. Wang, L. Zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu, X. Wang, Semiconductor heterojunction photocatalysts: design, construction, and photocatalytic performances, Chem. Soc. Rev. 43 (2014) 5234–5244.

Fig. 3.18 Different types of photocatalytic heterojunctions in binary composites. Type I: the electrons are transferred from CB(B) to CB(A), at the same time the holes are transferred from VB(A) to VB(B), causing the accumulation of all charge carriers on the semiconductor A (high recombination efficiency). Type II: efficient charge carrier separation because of the optimum band positions, with the electrons and holes transferred suddenly in opposite directions. Type III: the broken-gap situation, in which the electrons transfer from CB(B) to CB(A) and the holes migrate from VB(A) to VB(B). Reproduced with permission from R. Marschall, Semiconductor composites: strategies for enhancing charge carrier separation to improve photocatalytic activity, Adv. Funct. Mater. 24 (2014) 2421–2440.

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VB(B), leading to the accumulation of all charge carriers on the semiconductor A. Therefore, this kind of heterojunction will not help with the charge carrier separation [675]. A Type III composite provides electron transfer from CB(B) to CB(A) and hole migration from VB(A) to VB(B). Such heterojunctions may occur because of favorable energy positions of the CBs, or due to the band bending in the heterostructure (and probably interface) to induce an inner electric field. This kind of relative position of the bandgaps is called the broken-gap situation. In Type II heterojunctions, the same processes of electron and hole transfer occur. Furthermore, the charge carrier separation is efficient because of the optimum band positions of each semiconductor, which can lead to enhanced photocatalytic activity [675]. Moreover, the structural type of the prepared composite plays a crucial role in the charge transfer. When the semiconductor with accumulated electrons is encapsulated by the second photocatalyst, the e
will be trapped and cannot react. This mechanism may occur for core-shell composites and layer-by-layer surfaces. The preparation of different types of metal oxide composites leads to a correlation between the surface acidity and observed higher photocatalytic reactivity. The mechanism is based on the acceptance of holes generated during irradiation by surface hydroxyl groups. Furthermore, the oxidation of adsorbed molecules may occur. It is clear that the heterojunction should be carefully designed to match the desired application. Metal oxide composites can be fabricated by various methods including: (calcination-induced) phase transformation [676–678], impregnation-deposition [679,680], dip coating [681,682], liquid-phase deposition [140,683,684], hightemperature solid-state reaction [685], hydrothermal or solvothermal methods [686–688], electro-deposition [689–691], and sol-gel methods [692–695]. The proposed methods were described in detail in Section 3.1 (synthesis of metal oxides used in photocatalysis). In recent years, the majority of the literature has been related to the modification of TiO2 with other semiconductors. The TiO2-based heterojunction photocatalysts can have improved photocatalytic properties, because they possess high dye adsorption capacity, extended light absorption range, enhanced charge separation, and promoted mass transfer.

3.3.1 Different crystal phases based heterojunction In order to achieve highly efficient charge carrier separation, the multiphase heterojunction of semiconductors has been proposed. The electronic structure of a semiconductor depends on its atomic arrangement and thereby the crystal structure. For the same photocatalyst in different crystal phases, various energy bandgaps and bandgap positions may be observed [696]. The heterojunctions may be created when two semiconductors, each having suitable VB and CB positions, form a composite. The most commonly used structure is based on different TiO2 phases (anatase, rutile, and brookite). The diverse phases have various photocatalytic properties, however the cause of this has not been clearly elucidated. The utilization of anatase, rutile, and brookite TiO2 depends on the type of photocatalytic reaction [697]. Previous investigations suggested the existence of catalytic hot spots at the anatase-rutile interface

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that lead to effective charge carrier separation and enhanced hydroxyl radical generation [698,699]. It is believed that the different phases with varied surface properties could be utilized to increase the photocatalytic yield. In the Introduction, we mentioned that the anatase structure has the widest application as a photocatalyst, due to its lower recombination rate and higher adsorption of organic substrates during photocatalysis. However, the anatase phase has a larger bandgap than rutile, which could reduce the light range that can be absorbed by the photocatalyst. On the other hand, it is known that the maximum of the VB level in the anatase phase could be higher in energy than the redox potentials of adsorbed molecules. However, it was found that the formation of superoxide species during a photocatalytic process was higher for rutile TiO2. As a consequence, the higher activity of anatase should not be attributed to the photoinduced reduction process. Moreover, one study explained the different formation mechanisms of hydroxyl radicals on anatase and rutile surfaces [700]. It was proved that the lower activity of the rutile structure comes from the lower ability of holes to effectively and irreversibly drive the oxidation of alcohol scavengers [701]. Zhang et al. investigated the formation process of OH radicals during photocatalytic degradation of coumarin-3-carboxylic acid. In anatase TiO2, OH was formed from the trapped holes, while in rutile TiO2 the Ti-peroxosites (TidOOdTi) arising from two trapped holes play the role of catalyst to generate OH radicals from water (see Fig. 3.19). These conclusions could significantly contribute to the optimization of the photocatalytic activity of TiO2 polymorphs for various applications [702]. l

l

l

Fig. 3.19 Possible mechanisms of lOH generation for (A) anatase and (B) rutile. TiO2 corresponds to H2O2 absorbed on the TiO2 surface. Reproduced with permission from J. Zhang, Y. Nosaka, Mechanism of the OH radical generation in photocatalysis with TiO2 of different crystalline types, J. Phys. Chem. C 118 (2014) 10824–10832.

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Based on the literature, it was supposed that by combination of the two phases, rapid electron transfer from rutile to anatase can occur at their interface [703–705]. Researchers suggested that forming a composite with rutile can extend the light absorption wavelength to the visible region. The composite also forms a built-in electric field at the interface, which facilitates the interfacial charge transfer [706]. In an anatase-rutile composite under ultraviolet light illumination, the electron was found to transfer from the CB of anatase to the rutile phase, which has a relatively more positive CB edge (see Fig. 3.19A). On the other hand, under visible light irradiation, the electron migrates from the rutile CB to anatase lattice trapping sites, as the absorption edge of rutile extends to 3.0 eV (see Fig. 3.19B). Fig. 3.20 presents the charge carrier mechanisms in the anatase-rutile mixed phase [698]. However, the interaction within the multiphase structure of TiO2 may be even more complicated, and the electronic arrangement in the composite interface is still under investigation. The Degussa (Evonik) P25 is one of the best-known commercially-available TiO2 powders. It consists of mixed anatase and rutile phases. Ohtani et al. isolated the anatase and rutile phases from P25, and concluded that a slight difference in bandgap energy could enable stronger photoabsorption by the rutile phase. Additionally, it was shown that the treatment of isolated phases leads to a slight change in the surface properties and in the photocatalytic properties. Different photocatalytic reactions (decomposition of acetic acid and acetaldehyde, dehydrogenation of methanol, and oxygen liberation) were carried out on the original P25 and reconstructed P25 with isolated anatase and rutile particles. Furthermore, the results cast doubt on the belief that the core-shell type or interconnected anatase-rutile composite structures exhibit high efficiency in photocatalytic degradation due to synergetic effect from the

Fig. 3.20 Charge carrier mechanisms operating in the anatase-rutile mixed phase: (A) Conventional carrier transfer from anatase to rutile under UV light, (B) Electron migration from rutile CB to anatase lattice trapping sites under visible light, as evidenced by EPR. Reproduced with permission from D.C. Hurum, A.G. Agrios, K.A. Gray, T. Rajh, M.C. Thurnauer, Explaining the enhanced photocatalytic activity of Degussa P25 mixed-phase TiO2 using EPR, J. Phys. Chem. B 107 (2003) 4545–4549.

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co-existence of anatase and rutile [707,708]. Nonetheless, improvements in the photocatalytic activity of TiO2 composites with different phases continues to be reported in some studies. The brookite phase is also used in multiphase composite materials. Binary composites like anatase-brookite and rutile-brookite structures are formed to obtain highly photoactive, stable, and recyclable materials [709–711]. Likewise, ternary phase composites, such as the anatase-rutile-brookite structure, were also developed. Kaplan et al. used a sol-gel method to synthesize a highly uniform ternary material consisting of 43% anatase, 24% rutile, and 33% brookite. These researchers confirmed that, while single-phase photocatalysts only display low to moderate rates of mineralization, the multiphase composite could completely convert bisphenol A to CO2 and H2O under UV light. The composite also showed the highest photocatalytic activity for MB degradation under ultraviolet irradiation. Contrary to the claim by Ohtani et al., the authors concluded that the enhanced photoactivity can be associated with the synergy among the three phases [709,712]. Su et al. also indicated that significant synergetic effect between rutile and anatase in porous TiO2 film enhanced the photocatalytic activity, in comparison to either pure or polymorph structure [713]. They claimed that this synergistic effect originates from the crystallite size and density of defects in each phase, together with their intimate contact, the band alignment, and also the interfacial structure when an optimized ratio of the phases was used in the composite structure [713].

3.3.2 Nano- and microstructured TiO2-WOx heterojunction photocatalysts One of the most promising methods to enhance the visible light absorption of TiO2 and minimize the electron-hole recombination process is to combine it with other semiconductors, such as WO3. The goal is to alter the overall band structures and induce visible light absorption by reducing the effective energy bandgap, as we mentioned earlier in this chapter (Fig. 3.16). The absorption edge of bare WO3 is very narrow and therefore it fails to utilize photons from the major portion of the solar spectrum. Composites based on the TiO2-WO3 system are the most common type I heterojunction used in photocatalysis. During ultraviolet irradiation, the photoexcited electrons and holes are transferred to the CB and VB of WO3, respectively. Consequently, there is no enhancement of photocatalytic activity. One of the problems in the utilization of bare metal oxides is their tendency to undergo photocorrosion. To solve this problem, significant efforts have been devoted to enhance the photocatalytic properties by manipulating the nanostructures or morphology of metal oxides. Besides coating with polymers and surface doping with other species, the electrospinning technique has also been used to prepare hierarchically porous TiO2-WO3 composite nanofibers with a bicontinuous interior and an outer shell. The semiconductors were coupled with self-assembled polystyrene-block-poly(ethylene oxide) in a one-step strategy. During synthesis, the block copolymer serves as a structure template to ensure selective distribution of TiO2 and WO3. The TiO2 semiconductor acts as a frame, while WO3 fills the gaps in the fiber skeleton. The prepared

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composite showed enhanced photocatalytic degradation of acetaldehyde under visible light irradiation. The improvement of photocatalytic properties is attributed to the extended absorption edge to the visible region and the inhibition of recombination of photogenerated carriers [714]. The sol-gel method can also be used to prepare the TiO2-WOx composite, although for this type of heterojunction no significant extension of light absorption range was noted. The presence of WOx improves the single-electron transfer chemistry. It was observed that the distinct wavelength depends on the anatase-rutile ratio, which could be changed during composite synthesis by the sol-gel method. The simultaneous preparation of WOx inhibits the conversion of TiO2 to rutile in the bulk. The highest photoactivity was attributed to the better absorption properties of the composite, or the longer lifetime of “active” holes as a result of the hydroxyl chemistry [715]. Highly dispersed amorphous WOx can form shallowly trapped sites, due to its similar band structure with TiO2. The photocatalytic activity of the WO3-TiO2 heterostructure under UV light irradiation has been shown to be mainly due to the enhanced charge separation efficiency, instead of the extension of light absorbance by highly dispersed amorphous WO3. The electron transport between TiO2 and WOx and the subsequently improved charge separation are attributed to the strongly electron-withdrawing property of tungsten oxide in a highlydispersed state. However, the higher photocatalytic activity under visible irradiation mainly arises from improved light harvest [716]. Further investigation of the TiO2WO3 coupled system demonstrated that the photocatalytic activity is only increased in the presence of electron acceptor species, such as dichromate anions whose redox potential is lower than the CB edge of tungsten oxide. Since the transferred electrons could not reduce adsorbed oxygen, they mainly recombined with the holes, which resulted in poor pollutant mineralization. However, the trapped electrons in the CB of tungsten oxide are able to reduce Cr(VI), causing bichromate reduction under aerobic conditions [717]. It was shown that 17-a-ethinylestradiol could be directly oxidized by the photogenerated holes at the TiO2-WO3 semiconductor surfaces, by comparing the HOMO level of the organic compound and the semiconductor VB edges [718]. When TiO2 was doped with 2.7 wt.% WO3, the photocatalytic Pb2+ conversion into metallic atoms in a synthetic aqueous solution revealed a reduction capacity of 74.7%. It was found that the reaction constant increased to reach a maximum value with decreasing kinetic parameters, while the mass transfer coefficient showed the opposite trend [719]. Frequently, the photocatalytic activity of these composites is investigated using various dyes as organic substrates for degradation. It was reported that the photocatalytic activity of TiO2-WO3 composite in the degradation of phenol and methyl orange under UV irradiation can be tuned by adjusting the structural and morphological properties of the electron-hole separator component (i.e., the tungsten oxide material). The study concluded that controlled shape manipulation plays a major role in enhancing the photocatalytic decomposition performance [720]. It is well known that interconnected nanostructures enhance the photocatalytic activity by improving the electron-hole transfer. On the other hand, the structural disorder in a nanopowder photocatalyst can also enhance the scattering of free electrons and reduce the charge mobility [721].

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To obtain photocatalysts with red-shifted absorption edge and improved charge separation/transfer, TiO2 nanotubes can be loaded with WO3 nanoparticles [722,723]. The degradation rate of methyl orange increased with the sample crystallinity, and became lower with the appearance of rutile TiO2 [722]. The same researchers modified anatase TiO2 nanotubes by WO3 nanoparticles to improve the methyl orange degradation reaction, and the highest degradation rate (about 95% within 2 h under UV light irradiation) was observed for the nanotubular composite with 10% W/Ti atomic ratio. The synthesized WO3 nanoparticles decorated both the inside and outside walls of highly porous TiO2 nanotubes. The resultant material displayed high photocatalytic efficiency in the RhB degradation reaction, and there was no obvious decrease in the removal rate of organic compound after ten cycles of reuse. The WO3@TiO2@WO3 heterostructures also exhibited higher photocatalytic activity because of the highly porous structure, large specific surface areas, and excellent properties at the interface between WO3 nanoparticles and TiO2 nanotubes [724]. Typically, the heterojunction not only facilitates effective separation of photogenerated carriers, but also enhances the surface OH groups and surface acidity, thus improving the overall photocatalytic process [725]. In the degradation of 4-chlorophenol, the new photocatalyst exhibited higher activity, mainly due to hindered charge carrier recombination and the formation of a more acidic surface [723]. Furthermore, a focus in studying the incorporation of TiO2 with metal oxides is improving the electronic or photoelectrochemical properties of TiO2. Composite materials with unique organized nanostructures and enhanced electrochromic activity have been developed [726,727]. The data indicate that the improved TiO2 electrodes may also be used in dye-sensitized solar cells. Nanostructured semiconductor electrodes can be used for photoelectrochemical water splitting to obtain clean energy from abundant resources. It has been reported that the WO3 (core)-TiO2 (shell) nanorods exhibit the light harvesting properties of the core material and the surface effects of the shell material. This quasi core-shell nanostructure could modify the surface properties of the nanorods, such as the flat band potential [728]. The effects of the amount of WO3 and the heat treatment temperature on the structure and photocatalytic properties were discussed, and the highest hydrogen production in the water splitting process was observed for the sample with 2% WO3. Three coating layers were applied to the WO3-TiO2 photoanode that contained anatase with high crystallinity [729]. The mechanism of charge transfer in WOx-TiO2 composite was explained [730]. The prepared nanostructured TiO2-WO3 bicomponent films were used in hydrogen generation and organic compound degradation processes. The photoelectrocatalysis using the photocatalytic material as the photoanode provided faster dye degradation, a greater degree of mineralization, and improved hydrogen generation, compared to pure oxide electrode [731]. The combination of TiO2 with tungsten oxide helps to extend the light absorption of TiO2-based photocatalyst toward the visible range, and eliminates the rapid recombination of excited electrons/holes during photoreaction. Vacuum activation and calcination processes have been used to treat the composites with different amounts of tungsten oxide. The calcined samples contained more tungstate in the hexavalent form, while the composites activated in vacuum contained more element in the

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reduced form. During the photocatalysis, W5+ species are formed at the heterojunction due to photogenerated electron transfer from TiO2 to W6+. Further, the electrons were transferred to the absorbed oxygen molecules on the photocatalyst surface, leading to the oxidation of W5+ to W6+. When WOx semiconductor is deposited on the TiO2 surface, the extension of light absorption into the visible region was noted, and the charge recombination was effectively inhibited. Superior activity for decomposing methyl orange and phenol was observed for the composite activated in vacuum with an optimal dosage of 4% WO3 [730]. Besides the liquid phase, the TiO2-WO3 photocatalysis system can also be applied to energy storage devices in the gaseous phase. TiO2 generates reductive energy under UV light, which can be stored in some other semiconductors in the composite structure. The ionic conductivity of the medium or the composite film surface was found to be important, because the protons generated at TiO2 from water oxidation are intercalated into WO3 [732]. Therefore, the TiO2-WO3 composite can be used as an energy-storage photocatalyst for applications ranging from corrosion protection to a possible bactericide and also photochromism. Another investigation considered the degradation of gas-phase diethyl sulfide under visible light. Diethylsulfide can adsorb and react preferentially on Ti4+ and further on TidOH surface sites. This process can lead to different degradation pathways, due to the confirmed presence of sulfur-containing and partially oxidized by-products. The deposition of surface sulfate species causes competition reactions that are responsible for the on-stream deactivation of the photocatalysts toward diethyl sulfide elimination. Modifying the TiO2 nanotubes with 4 wt.% of WO3 enhanced the removal efficiency of organic compounds, and improved the resistance against sulfate deactivation [733].

3.3.3 Nano- and microstructured TiO2-ZnO heterojunction photocatalysts Quite a few studies have focused on the design and synthesis of TiO2-ZnO nanocomposites, such as composite nanoparticles [734,735], nanofilms [736–738], and many other hybrid nanostructures [725,739,740]. The enhanced photocatalytic efficiency could depend on the synergic effect of coupling the two photocatalysts of nanocrystalline TiO2 and nanosized ZnO. The bandgap energies of the metal oxides are similar to each other, although the CB of ZnO is a bit more negative. The electron derived from ZnO could transfer to TiO2 semiconductor. Unfortunately, the VB edge positions of ZnO and TiO2 are often almost the same, and the hole transfer may not be significant. Nevertheless, it is common knowledge that the incorporation of TiO2 with ZnO can profoundly improve the charge isolation compared with pure single semiconductors. This composite could have wide applications, such as in water and air purification systems, chemosensors, UV-Vis blockers, self-cleaning surfaces, photonic crystals, organic synthesis, and solar cells. Considering the relative positions of the bandgap levels, a lower recombination rate and longer lifetime for e
/h+ can be achieved. The composite forms a type II heterojunction, which enriches the photoinduced electrons in ZnO and confines the holes in TiO2. It is supposed that because of the higher electron mobility in ZnO in

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comparison to TiO2 (about 115
155 vs 10
5 cm2 V
1 s
1, respectively), the ZnO particles or clusters may serve as good electron trappers [741]. A two-step facile hydrothermal method was applied to obtain 3D urchin-like TiO2 nanospheres with 1D ZnO nanospindles [742]. This ZnO-TiO2 structure showed enhanced photocatalytic activity in the decomposition reactions of methyl orange and nitrophenol. The more efficient separation of photogenerated charges is mainly attributed to the intimate bonding in the ZnO-TiO2 surface heterostructure [742]. In another study, ZnO-TiO2 mixed metal oxides constructed of hierarchically nanostructured hollow spheres showed high photocatalytic efficiency. The hybrid particles of ZnO-TiO2 can work more efficiently as a photocatalyst in comparison to pure semiconductor spheres. Further, the composition of elements as well as shell thickness of these hollow structures can be manipulated by tuning the concentration of metal oxide precursors [743]. Microreactors based on the ZnO-TiO2 heterojunction were also synthesized, in which the nanorod arrays showed rapid and highly efficient photocatalytic activities and high durability during continuous recycling [744]. It was shown that the addition of ZnO enhanced the activity significantly. Moreover, sulfating ZnO-TiO2 with sulfuric acid resulted in an enhanced degradation ratio compared with the original ZnO-TiO2. It was explained that the increased surface acidity causes favorable transfer of the photogenerated electrons in the CB to the surface, which improved the separation of photogenerated e
/h+ [745]. The TiO2-ZnO photocatalyst in a film state has several advantages: the absence of conglomeration, easy reclaim after reaction, and particularly high surface activity. The film structure may be prepared by several methods, such as thermal chemical vapor deposition [746], hydrolysis deposition [747], spray pyrolysis [748], radiofrequency magnetron sputtering [749], and sol-gel method [738,750]. The photocatalytic activity of the composite film for dye degradation under UV irradiation showed a linear relationship with the value of Ti/(Ti + Zn) [751]. To explore the ZnO-functionalized TiO2 nanotube arrays, the two-step anodization combined pyrolysis strategy was used [752]. The resultant ZnO nanocrystals, when distributed on the TiO2 nanotubes, served as favorable hole channels and receptors for the efficient separation of photoexcited charge carriers. Moreover, the ZnO nanorods grafted on vertically aligned TiO2 nanotubes were also formed by a seedinduced hydrothermal process. The flower-like ZnO clusters were directly grafted on the tops of TiO2 nanotubes, forming a large number of aggregated lead wires [753]. A slight change in the bandgap absorption edges was observed, because the ZnO nanorods work as favorable hole channels and receptors for the efficient separation of the e
and h+. The composite material with a graft amount of 0.65 mg cm
2 has a broader optical absorption range under the illumination of 365-nm UV light, and the photoelectric conversion efficiency was enhanced to 23.6%. The removal rate of bisphenol was almost 2.3 times faster than that on pure TiO2 nanotubes. Furthermore, the stability of ZnO nanorods was promoted after coupling with TiO2. Stable repeated cycles of bisphenol A removal were achieved, because the h+ on ZnO nanorods retarded the photocorrosion of ZnO [753]. Additionally, there are even composites based on two semiconductors with triple nanostructures that could improve charge isolation [754].

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3.3.4 Nano- and microstructured TiO2-SnO2 heterojunction photocatalysts The SnO2-TiO2 thin film heterojunction improved the hydrophilicity and photocatalytic activity of the heterostructure due to the slow TiO2 particle growth [755–757]. The TiO2 was doped with SnO2 thin films using a sol-gel method and deposited onto the glass surface. Besides the photocatalytic activity in the dye degradation reaction, the self-cleaning properties of the fabricated thin films were also evaluated. It was concluded that the TiO2 sample doped with 1 mol% SnO2 thin films had the highest photocatalytic and self-cleaning properties under fluorescent irradiation [757]. Another report also claimed that the doping of SnO2 into TiO2 can improve the hydrophilicity and photocatalytic activity [756]. For the sample with 10% SnO2, a thicker film helps to reduce the contact angle on the composite films. Moreover, the doping of TiO2 with SnO2 did not affect the visible light transmittance [756]. Because of the one-dimensional morphology, nanofibers show great advantages for solving the problem of charge separation. Therefore, hybrid nanofibers were prepared and characterized with SnO2-rich beads and pure TiO2 (composed of rutile and anatase structures). The existence of SnO2 promotes the formation of the rutile phase, and the synergistic effect between rutile and anatase phases strengthens the separation of the photogenerated carriers. After irradiation, the photogenerated electrons in the anatase phase are more likely to transfer to the rutile and SnO2 phases because of the position of the energy bands. Furthermore, the photogenerated h+ tends to gather in the anatase and rutile phases, because of their higher VB levels [758]. The stable and easily reusable TiO2-SiO2 material features high specific surface area, efficient charge recombination process, and high photocatalytic activity under daylight and UVA light sources [759]. A composite with high mixing quality and high concentration of heterojunction was produced by metal organic chemical vapor synthesis. The sample so obtained was transformed into an aqueous colloidal dispersion using formic acid to adjust the particles’ surface charge. The data showed that the produced nanoparticles have dehydroxylated surfaces. The large amount of TiO2-SnO2 heterojunction enhanced the cross section for interparticle charge separation. Moreover, substantially increased concentrations of electrons and hole centers were observed [760]. The results show the importance of the preparation method for obtaining suitable, highly active photocatalysts. In a binary TiO2-SnO2 nanofiber heterostructure network, the semiconductors are completely exposed to the surface, which results in full utilization of the photogenerated holes and electrons during the photocatalysis. The observed RhB degradation activity reached 100% within 45 min [761]. Using a different synthesis route, the plasma-enhanced chemical vapor deposition method was used to obtain bicomponent TiO2-SnO2 particulate film. Increased separation of photogenerated carriers was observed, which leads to the formation of reaction intermediates that further take part in the degradation pathway [762]. A promising approach to create TiO2-SnO2 photocatalyst is using ZnO nanowires as a sacrificial template. Further, the vertically aligned SnO2 nanotube arrays can be coated with a thin TiO2 layer. The electron recombination lifetime in the hybrid TiO2-SnO2 nanotubes is significantly longer than those in TiO2 nanotubes, ZnO nanowires, and films

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of sintered TiO2 nanoparticles. The recombination kinetics of photogenerated electrons in different photoanode materials lead to the conclusion that these hybrid anodes present the possibility of using redox mediators with faster kinetics in systems that otherwise would be hampered by fast recombination of electrons [763]. Moreover, solar cells using TiO2-coated multilayered SnO2 hollow microspheres as dyesensitizer exhibited an efficiency of about 5.65%, which is remarkably higher than that measured for TiO2-nano-SnO2 [764,765]. Recently, TiO2-SnO2 nanocomposites were prepared by a gaseous detonation method. It was found that the SnO2 concentration is negatively related to the grain size of the samples, while the rutile phase of TiO2 is positively correlated with the photocatalytic properties [766]. TiO2SnO2 double-shelled hollow spheres were successfully synthesized by a two-step liquid-phase deposition method using carbon spheres as templates. The composite structure showed higher photocatalytic activity in the decomposition of dye in comparison to bare TiO2 and SnO2. Moreover, no significant loss inactivity was observed in the recycling experiments [767]. The researchers investigated the effects of film thickness and the amount of SiO2 and SnO2 co-doping into TiO2 nanocomposite films on the existing phases, crystallite size, photocatalytic reaction, and hydrophilicity [755]. The crystallinity of anatase phases, the crystallite size, and photocatalytic reactions of the SnO2 and SiO2 co-doped TiO2 films decreased with increasing SiO2 concentration. The synthesis at less than 600°C indicates that a larger quantity of added SiO2 seems to inhibit the grain growth and formation of the anatase TiO2 phase. The photocatalytic efficiency decreased with higher SiO2 content, when the amount of SnO2 was fixed at 1–3 mol%. The photocatalysts exhibited superhydrophilicity (self-cleaning effect) under UV irradiation for 30 min [755].

3.3.5 Nano- and microstructured TiO2-CuO and TiO2-Cu2O heterojunction photocatalysts Copper oxide semiconductors are commonly used as a modifier of TiO2 in photocatalysis. CuO and Cu2O are narrow-bandgap semiconductors, with Eg 1.3 and 2.0 eV, respectively. Therefore, the coupling of copper oxides with TiO2 results in better charge separation and utilization of visible light during photocatalysis. A material with 3D mesoporous networks of Cu2O and TiO2 nanoparticles was reported as a highly active photocatalyst for hydrogen generation from water [768]. The observed high efficiency was due to the highly accessible porous surface in the assembled structures, which exposes a large fraction of anatase TiO2 and Cu2O nanoparticles to electrolytes. Moreover, the photocatalyst had a small grain size for the constituent nanocrystals, leading to excellent activity for H2 evolution and reduction of protons under UV-Vis light. The reported results showed excellent activity of the mesoporous Cu2O-TiO2 catalyst containing 1.5 wt.% of copper, as it reached an average hydrogen evolution rate of 542 μmol h
1 (and apparent quantum efficiency of 13.5% at λ ¼ 365 nm) under UV-visible light illumination (360  λ  780 nm) [768]. It was confirmed that the composite could store multiple electrons in the thin film, and could split water even after the light was turned off for several hours [769]. TiO2-Cu2O

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samples with different morphologies were prepared by various methods and examined for the photocatalytic degradation of organic dyes [770–772]. The photocatalytic activity in RhB decomposition was related to the impregnation time of the copper nitrate solution during synthesis. The activity of TiO2-CuxO synthesized with 2 h of impregnation displayed the best result (4.9 times as high as that of pure TiO2 nanotubes). The high decomposition efficiency was attributed to the synergistic effects of initiating visible light absorption and the matched energy band positions of Cu2O, CuO, and TiO2 [773]. In the case of CuO, it was found that the metal oxide content in the heterostructure could improve the hydrogen generation. Additionally, the effect of thermal treatment conditions, like the temperature and treatment time on the crystalline structure, surface area, quantum yield, and the bandgap positions were investigated. An ionic liquid-assisted hydrothermal process was able to improve the hydrogen evolution in the water/ethanol system to 8670 μmol/g under illumination from a xenon/mercury lamp. The results highlight the importance of the preparation procedure to design photocatalysts with the required properties. The enhancement may be attributed to the largest BET surface area, the highest quantum yield, and adequate surface morphology in the sample [774]. A high efficiency in the water splitting process was also observed for the P25 modified with CuO. The hydrogen evolution rate reached 10.2 mL min
1 (18,500 μmol h
1 g
1 catalyst) over the CuO-TiO2 with an optimal Cu content of 9.1 mol% from an aqueous solution containing 10 vol% methanol [775]. A study was published on how different Cu species (Cu(0), Cu(I), and Cu(II)) influence the composite structures and their roles in photocatalytic hydrogen generation. It was concluded that the positive charges in the Cu center affect the photo-induced charge transfer and catalyst activity significantly. What is more, the Cu(I) probably promotes photocurrent generation, while Cu(II) negatively influences this process. Meanwhile, the Cu(0) content is beneficial for transferring photoinduced electrons and enhancing the separation efficiency of the produced carriers [776].

3.3.6 Nano- and microstructured TiO2-α-Fe2O3 heterojunction photocatalysts The Fe2O3 hematite exhibits high resistance to corrosion, therefore, it has been extensively used in many fields, including as a photoanode for the photo-assisted electrolysis of water. The material is a narrow-bandgap semiconductor with an energy gap of about 2.2 eV. Moreover, it is well known that iron oxide shows excellent magnetic properties and can be conventionally used as a visible light-driven photocatalyst because of its stability. TiO2 can be coupled with the Fe2O3 semiconductor. Unfortunately, the recombination of photogenerated charge carriers is quite high, the lifetime is very short ( Pt-D-WO3 ≫ Pt-N-WO3. Experiments involving the addition of scavengers for OH radicals, holes, and excited electrons to the solution indicated that OH and h+ were the main oxidants in the degradation process, in which OH radical generation could be attributed to oxidation of H2O by the holes. l

3.4.4 Plasmonic photocatalysts based on Pd nanoparticles Palladium on a TiO2 surface creates a Schottky junction between the metal and the semiconductor, which acts as a sink for photogenerated electrons. Plasmonic metal@TiO2 core-shell structures with well-controlled size, shape, and shell thickness were prepared using a two-step surface modification process [890]. The superior activity was attributed to the synergistic effects between the large surface area with high crystallinity and well-defined morphology with controllable optical properties

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from the plasmonic effects. The Pd core serves as a sensitizer to absorb resonance photons, facilitating light harvest by enhancing light absorption. Moreover, it was explained that during this process, valence electrons of the metal undergo collective oscillation upon interaction with incident light, exciting the electrons from the ground state to the SPR state (which has an energy between 1.0 and 4.0 eV with respect to the Fermi level of the Pd metal) [891]. These photoexcited electrons are directly injected into the nearby TiO2 CB, due to the formation of a Schottky barrier at the metalsemiconductor interface. It was also suggested that the hot spots (where plasmon decay produces intensely hot electrons) appear on the sharp edges and corners of TiO2, where the electric field intensity of SPR is several times higher than that of the incident photon [892]. The effects of Pd nanoparticles with different shapes (spherical and cubic) on different commercial titania-based catalysts were investigated [893]. It was proven that nanocomposites based on Pd spheres were more efficient for all three photocatalytic experiments (hydrogen production, and phenol and oxalic acid degradation), whereas the Pd cube-based composites possessed lower efficiency. The differences in the photoactivity were attributed to the possible different electron affinities of the Pd nanoparticles, which are affected by the exposed facets of the nanocrystals. TiO2 nanocrystals with exposed (001) facets were decorated with Pd nanoparticles by a photoreduction deposition route [894], and the produced Pd-TiO2 samples exhibited greatly enhanced activity and stability in the photocatalytic selective alcohol oxidations under UV light irradiation in aqueous medium. The high photoactivity was attributed to the synergistic promoting effects of the following factors. First, the exposed (001) facets with higher surface energy facilitated the activation of reactant molecules and the photocatalytic oxidation. Second, the increased oxygen vacancies and enhanced O2 adsorption on (001) facets could trap photoelectrons and thus inhibit their recombination with holes. Third, the decoration with Pd nanoparticles effectively reduces the photocharge recombination rate by capturing photoelectrons. Moreover, the obtained results showed that the photogenerated holes, photoelectrons, and O2 
radicals were the main active species involved in the oxidation reactions. Deposition of Pd onto the surface of WO3 nanorods resulted in enhanced photodegradation under visible light [895]. It was observed that an increase in the weight percentage of Pd shifted the absorption edge of WO3 from 426 to 500 nm. Thus, the absorption edge of WO3 can be controlled by the Pd content. Moreover, the weight percentage of deposited Pd played an important role in controlling the bandgap and the PL peak intensity that determines the electron-hole recombination. Pd-ZnO photocatalysts were prepared by a solvothermal method. During the reaction, hexamethylenetetramine (HMT) can be converted into ammonia and formaldehyde simultaneously, which were used as the precipitant for the fabrication of ZnO and the reducing agent for the formation of metal Pd, respectively [896]. The prepared samples showed good photocatalytic performance and high stability, due to the effective separation of electron-hole pairs and the high specific surface of the samples. Meanwhile, Pd-ZnO showed increased UV emission in the PL spectra, which means that the optical bandgap of ZnO was widened by the loaded Pd particles due to the

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Burstein-Moss (BM) effect. Moreover, the decrease of visible emission may be caused by a reduction of the oxygen vacancies during the crystallization process of ZnO in the presence of PdCl2.

3.5

Composite-based photocatalysts

It was already mentioned in Section 3.3 that the coupling of two different materials may transfer e
from the excited semiconductor with a small bandgap into the other material, when the positions of the CB are properly aligned. Moreover, this favors the separation of photogenerated charge carriers and consequently improves the photocatalytic efficiency. The wider-bandgap semiconductors (e.g., TiO2, WO3, ZnO, and SnO2) could be used to obtain heterojunctions with visible photoresponse using a material with narrower bandgaps. What is more, these multisemiconductor devices can absorb a larger fraction of the solar spectrum, which helps with the excitation of the semiconductor and thus the photoinduced generation of electrons and holes. It is also known that the photocatalytic heterojunctions should be carefully designed with regards to the desired application [897]. The bismuth-based semiconductors, a large family that includes several phases in the BidTidO system (e.g., Bi2Ti4O11, Bi2Ti2O7, Bi4Ti3O12, Bi20TiO32, and Bi12TiO20), recently have drawn much interest because of their specific physical properties and various technological applications. Moreover, bismuth-based materials, such as Bi2O3, BiVO4, BiFeO3, BiOI, and BiOBr, show strong absorption in the visible region, and they have been applied in environmental fields as a visible light-active material. Bi4Ti3O12, Bi2WO6, BiVO4, Bi2MoO6, and other similar semiconductors usually have a layered Aurivillius structure, that is (Bi2O2)2+ layers intergrown with metal oxide layers along the c axis [898]. Generally, bismuth-based materials can be classified as binary oxides, sulfides, multicomponent oxides, and oxyhalides. Different types, structures, preparation methods, and various photocatalytic applications have been reported for Bi-based materials. It was found that composites with this kind of semiconductor exhibited remarkable photocatalytic reactivity for the degradation of pollutants in both the aqueous (dyes, aromatic pollutants, pharmaceutical compounds, etc.) and gas phases (volatile organic compounds and CO2) using various light sources with different wavelengths. What is more, Li et al. newly suggested that in the Bi-based semiconductor/TiO2 system, the degradation process of dyes under visible light irradiation is synergistic by combining the photosensitization and photocatalysis [899]. The band positions of bismuth materials (e.g., ECB ¼
0.32 eV and EVB ¼ 2.39 eV for Bi2MoO6) are in good alignment with those of TiO2 (ECB ¼
0.29 eV, EVB ¼ 2.91 eV) to form a heterojunction photocatalyst [900]. Furthermore, these authors confirmed that Bi3+-containing compounds have narrow bandgaps and exhibit high photocatalytic activity in visible light because of the hybridized O 2p and Bi 6s2 VBs [901–903]. Even the empty 6s orbital of Bi5+ results in higher efficiency for Bi5+-based materials in the photocatalytic process [904]. Bismuth titanates are semiconductor materials that include various phases with hybridized Bi2O3 and TiO2 units. Their crystal structures are constructed with connected

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BiOn and TiOn polyhedra with different n values. In this kind of photocatalyst, the VB consists of a 6s2 filled orbital and an O 2p orbital, and the CB consists of a Ti 3d empty orbital [898]. This electronic structure leads to optimal position of energy bands in the composites with regard to other wide-bandgap semiconductors, such as TiO2, to obtain visible-light photoactive materials. In Table 3.2, some types of heterojunctions

Table 3.2 Examples of composite photocatalysts and model for the degradation process Metal oxide TiO2

Model degradation compound Bi2O3

Bi2S3

Bi2WO6

Bi2MoO6 BiVO4 BiOI BiOBr BiOCl Bi2Ti2O7 Bi12TiO20 MoS2

NiS CuS CdS

PbS Ag2S

Photoelectrochemical measurements, dyes, aldehydes, alcohols, phenolic compounds, aromatic compounds, pharmaceutical compounds, Cr(VI) Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), photoconversion, dyes, phenolic compounds, aromatic compounds, Cr(VI), CO2 reduction Photoelectrochemical measurements, dyes, inorganic compounds (ammonia), aldehydes, organic acids, phenolic compounds, disinfection Photoelectrochemical measurements, dyes, phenolic compounds, aromatic compounds Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), dyes Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), dyes, phenolic compounds Dyes, aldehydes, phenolic compounds Phenolic compounds, photocatalytic water splitting (hydrogen production) Dyes Dyes, aromatic compounds Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), solar cells, dyes, phenolic compounds Dyes, oxidation-reduction of methanol (hydrogen production), photocatalytic water splitting (hydrogen production) Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), solar cells, dyes, alcohols Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), solar cells, dyes, alcohols, alkenes, phenolic compounds, CO2 conversion to methane Photoelectrochemical measurements, solar cells, dyes, phenolic compounds Photocatalytic water splitting (hydrogen production), dyes, phenolic compounds Continued

152

Table 3.2

Metal Oxide-Based Photocatalysis

Continued

Metal oxide

Model degradation compound In2S3

CdSe

PbSe CdTe

Ag2O

ZnFe2O4 SrTiO3 g-C3N4

GaN InN Ta3N5 WO3

Bi2O3 Bi2S3 Bi2WO6 Bi2MoO6 BiVO4 BiOI BiOBr BiOCl Bi2Ti2O7 Bi12TiO20 MoS2 NiS CuS CdS

PbS Ag2S In2S3 CdSe

Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), dyes, thioethers, herbicides, phenolic compounds, Cr(VI) Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), solar cells, dyes, phenolic compounds Photoelectrochemical measurements, phenolic compounds Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), solar cells, dyes, phenolic compounds, aromatic compounds Mercury elimination, photocatalytic water splitting (hydrogen production), dyes, phenolic compounds, pharmaceutical compounds Dyes, esters, phenolic compounds, CO2 reduction, Cr(VI), Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), solar cells, dyes, CO2 reduction Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), photoconversion, dyes, aldehydes, alkenes, phenolic compounds, pharmaceutical compounds, CO2 reduction, Cr(VI), disinfection Solar cells, na Photocatalytic water splitting (hydrogen production) Dyes, phenolic compounds, aromatic compounds Photoelectrochemical measurements, dyes Photoelectrochemical measurements, dyes na Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), dyes Dyes, pharmaceutical compounds na Dyes, alcohols, organic acids, pharmaceutical compounds na na na na na Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), dyes, pharmaceutical compounds, Cr(VI) na na na na

Metal oxide photocatalysts

Table 3.2

Continued

Metal oxide

Model degradation compound PbSe CdTe Ag2O ZnFe2O4 SrTiO3 g-C3N4 GaN InN Ta3N5

ZnO

153

Bi2O3 Bi2S3 Bi2WO6 Bi2MoO6 BiVO4 BiOI BiOBr BiOCl Bi2Ti2O7 Bi12TiO20 MoS2

NiS CuS CdS

PbS Ag2S In2S3 CdSe PbSe CdTe

na na Dyes na na Photocatalytic water splitting (hydrogen production), hydrogen production from amine, dyes, aldehydes, phenolic compounds na na na Dyes, pharmaceutical compounds Solar cells, dyes Dyes Dyes Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), dyes Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), dyes Photocatalytic water splitting (hydrogen production), dyes, phenolic compounds Dyes na Dyes Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), hydrogen production from H2S, dyes, phenolic compounds, aromatic compounds, disinfection na Photocatalytic water splitting (hydrogen production), dyes Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), solar cells, dyes, organic acids, aromatic compounds, disinfection Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), solar cells, dyes Photoelectrochemical measurements, solar cells, dyes Photoelectrochemical measurements, dyes Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), dyes Photoelectrochemical measurements Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), solar cells, dyes, phenolic compounds Continued

154

Table 3.2

Metal Oxide-Based Photocatalysis

Continued

Metal oxide

Model degradation compound Ag2O ZnFe2O4 SrTiO3 g-C3N4 GaN InN Ta3N5

Dyes, phenolic compounds, pharmaceutical compounds, Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), solar cells, dyes, disinfection Photoelectrochemical measurements, dyes Photoelectrochemical measurements, dyes, phenolic compounds, CO2 conversion, Cr(VI) Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production), dyes, alcohols Photoelectrochemical measurements, photocatalytic water splitting (hydrogen production) na

with Bi-based semiconductors are included. In most single Bi-based semiconductors, the bottom of the CB is positive (>0 eV), and therefore they cannot directly photocatalyze H2 generation or CO2 reduction. However, the production of Z-scheme junction using Bi-based semiconductors with a suitable (i.e., with sufficiently negative CBs) photocatalyst is a promising method for photocatalytic H2 evolution and CO2 reduction (see Table 3.1). In the coupled photocatalysts, the photogenerated e
in the Bi-containing compounds can migrate to the VB of the other semiconductor and scavenge the holes, thereby prolonging the lifetime of the photoinduced electrons [898]. We already mentioned that metal oxides are the most common materials explored for photocatalysis. However, most of them suffer from inefficient light absorption due to their large bandgaps (the O2p orbital is located at ca. 3.0 eV) and poor optoelectronic properties (short electron-hole lifetimes and low carrier mobility). Due to these facts, other band-engineering and nanostructuring strategies have been developed to overcome the limiting factors of metal oxides, such as by coupling with nitrides. The nitride-containing photocatalysts (i.e., C3N4, GaN, InN, and Ta3N5) possess high absorption coefficients and high charge carrier mobility, which result in excellent photon absorption and charge carrier extraction for efficient solar fuel conversion. The chemical bonds in III-nitrides are strongly ionic, therefore the surface states are located mostly near the band edges and prevent them from becoming on-radiative combination centers [905]. Possible heterojunctions using nitrides in photocatalytic degradation processes are presented in Table 3.2. One of the most commonly used photocatalysts in this group is polymeric graphitic carbon nitride, g-C3N4. Characterized by a delocalized conjugated structure, g-C3N4 may be coupled with a widebandgap semiconductor to improve the charge separation in photocatalysis. The connections between the two semiconductors serve as electron migration paths. As a consequence, it promotes the charge separation and induces a synergistic effect

Metal oxide photocatalysts

155

for improved photoactivity. Improved separation efficiency of the photogenerated charge carriers was observed for composites with wide-bandgap photocatalysts, for example, TiO2, ZnO, and SrTiO3 [906–910]. Nevertheless, not all materials can form a heterostructure with g-C3N4. The most important prerequisite to form a g-C3N4-based heterostructure with effective visible light excitation is that the other semiconductor should have an appropriate structure that helps to create a coupling hybridization. Moreover, as already mentioned, the composite should create a built-in electric field in the binary structure that improves the carrier transfer between the materials. The g-C3N4-based nanocomposites are commonly used in various fields, such as photodegradation of nitrogen oxides, organic contaminant photodegradation, photocatalytic hydrogen evolution, conversion of CO2 to methane fuel, and oxygen reduction reaction (see Table 3.2) [911]. When visible light is utilized during the photocatalytic degradation process, excited electrons in the HOMO of the carbon nitride would transport to the LUMO of C3N4 (π-π* transition). Chemical interaction in the binary system is observed when the LUMO potential of C3N4 (
1.1 eV) is lower than the CB edge of the other semiconductor in the composite [912]. Further, the excited-state electrons on carbon nitride can directly inject into the CB of the other component in the heterostructure. In addition, the composites could also perform as supercapacitors, and therefore many investigations are concerned with the photoelectrochemical measurements [911]. Nevertheless, the reason for the photocatalytic activity in C3N4-based composites is still controversial and not completely clear. Therefore, more studies are required to promote the general understanding of the enhancement mechanism. In summary, it is very important to synthesize various composites and search for their enhanced properties. Proposing a composite with high efficiency in photocatalytic reactions is a difficult task, due to the problems in predicting the carrier transfer between two semiconductors. The construction of proposed high-quality heterostructures without defect is also a core technology that needs to be realized in the future.

3.6

Outlook

TiO2, WO3, ZnO, Fe3O3, Ta2O5 and CuO are some of the best-known semiconductor photocatalysts used due to their bandgap and distinct electronic structure (unoccupied conduction band and occupied valence band). One of the main shortcomings of these photocatalysts is the recombination of photogenerated charge carriers, which leads to decreases of photoefficiency. In view of this, various methods, including metal/nonmetal doping and decoration of the semiconductor with plasmonic metallic nanoparticles, have been used to improve the light adsorption and charge transport and, as a result, to improve the photocatalytic properties. The advantages and disadvantages of these strategies are summarized in Table 3.3.

The advantages and disadvantages of different types of semiconductor modifications

Type of modification Nonmetal doping

Advantages l

Nonmetal doping can create a mid-gap state acting as an electron donor or acceptor in the bandgap of the semiconductor, which introduces lower bandgap and shifts the optical absorption of the semiconductor into the visiblelight region

Disadvantages l

l

l

Transition metal ion doping

l

l

Noble metal decorating

l

l

l

l

l

l

l

l

l

l

l

Doping of nonmetals into the semiconductor lattice usually results in the formation of oxygen vacancies in the bulk, which can act as recombination centers of photo-induced e
/h+ pairs, which limit the visible light photoefficiency, Low stability of nonmetal doped samples, doping process involves a long thermal treatment at high temperatures, which is unfavorable in energy, and which generates undesirable gaseous byproducts Expensive ion-implantation facilities requirement in their preparation Poor photocatalytic activity because of their thermal instability Quantum efficiencies under visible light are much lower than those under UV light Dopants produce impurity and/or vacancy levels in the bandgap, which serve as the recombination centers for the photogenerated charge carriers

High concentration decorating could lead to formation of metal clusters, blocking the surface of the semiconductor, and reducing light absorption and photocatalytic efficiency Negatively charged metal particles can act as recombination centers trapping holes Au nanoparticles have the tendency to aggregate over time Ag nanoparticles may undergo oxidation to Ag2O

Metal Oxide-Based Photocatalysis

l

Improvement of photocatalytic activity under visible irradiation due to enhancement of the adsorption region from UV to visible light due to charge-transfer transition between the d-electrons of the transition metals and the conduction band (CB) or valance band (VB) of semiconductor Metal-ion-doped semiconductor could improve the redox potential of the photogenerated radicals, Enhances quantum efficiency via inhibition of the recombination of photogenerated electrons and holes as the ions act as electron traps Electron-hole pair separation due to the Schottky barrier formed at the metal-semiconductor interface Shift of the absorption region from UV to visible-light due to the unique surface plasmon resonance Deposition of some nanoparticles (especially Pt and Pd) on the surface of the semiconductor increases its stability

156

Table 3.3

Metal oxide photocatalysts

157

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Further reading [1] J. Yuan, C. Chen, Y. Hao, X. Zhang, B. Zou, R. Agrawal, C. Wang, H. Yu, X. Zhu, Y. Yu, Z. Xiong, Y. Luo, H. Li, Y. Xie, SnO2/polypyrrole hollow spheres with improved cycle stability as lithium-ion battery anodes, J. Alloys Compd. 691 (2017) 34–39. [2] D. Zhang, Z. Cheng, J. Cheng, F. Shi, X. Yang, G. Zheng, M. Cao, Hydrothermal preparation and characterization of sheet-like (KxNa1
x)NbO3 perovskites, Ceram. Int. 42 (2016) 9073–9078. [3] J.M. Macak, H. Tsuchiya, A. Ghicov, K. Yasuda, R. Hahn, S. Bauer, P. Schmuki, TiO2 nanotubes: self-organized electrochemical formation, properties and applications, Curr. Opin. Solid State Mater. Sci. 11 (2007) 3–18. [4] M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, P.S. Dunlop, J. W. Hamilton, J.A. Byrne, K. O’Shea, A review on the visible light active titanium dioxide photocatalysts for environmental applications, Appl. Catal. B Environ. 125 (2012) 331–349. [5] K. Nakata, A. Fujishima, TiO2 photocatalysis: design and applications, J. Photochem. Photobiol. C: Photochem. Rev. 13 (2012) 169–189. [6] http://www.crystallography.net/cod/. [7] J. Xiang, J. Tu, L. Zhang, Y. Zhou, X. Wang, S. Shi, Self-assembled synthesis of hierarchical nanostructured CuO with various morphologies and their application as anodes for lithium ion batteries, J. Power Sources 195 (2010) 313–319.

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Application of metal oxide-based photocatalysis

4

Bajorowicz Beata, Kobylan´ski Marek P., Malankowska Anna, Mazierski Paweł,
ska Aleksandra, Adriana Zaleska-Medynska Nadolna Joanna, Pieczyn University of Gdansk, Gdansk, Poland

4.1 Water treatment In the presence of metal oxides, heterogeneous photocatalysis allows for the degradation or mineralization of pollutants in the aqueous phase via reaction with active oxygen species formed at the surface of the irradiated photocatalysts (Section 2.1.1). Organic and inorganic pollutants dissolved or suspended in the aqueous phase may also react directly with holes or electrons generated during semiconductor excitation. Photocatalytic reaction in the aqueous phase can lead to mild or total oxidation of organic compounds, dehydrogenation, hydrogen transfer, O2 18
O2 16 and deuterium-alkane isotopic exchange, metal deposition, and water detoxification [1]. The efficiency of pollutant degradation depends on the following factors: l

l

l

l

l

l

l

l

photocatalyst type and properties (Chapters 2 and 3) pollutant type and concentration photocatalyst loading pH temperature irradiation spectrum and intensity dissolved oxygen concentration presence of additional substances (suspended matter, scavengers, etc.)

Similar to classical heterogeneous catalysis, the overall photocatalytic process of pollutant degradation can be broken down into five main independent steps: (1) transfer of reactants from the aqueous phase to the surface of the photocatalyst; (2) adsorption of at least one of the reactants at the surface of the photocatalyst; (3) photocatalytic reaction in the adsorbed phase; (4) desorption of the product(s), and (5) removal of the products from the interface region [1]. Both modified and nonmodified TiO2, WO3, ZnO, Fe3O4, Cu2O, and Ta2O5 metal oxide photocatalysts have been employed as photocatalysts during the photocatalytic water treatment process, however, TiO2 is the most frequently used semiconductor in this process.

Metal Oxide-Based Photocatalysis. https://doi.org/10.1016/B978-0-12-811634-0.00004-4 © 2018 Elsevier Inc. All rights reserved.

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4.1.1

Types of contamination

Problematic substances in wastewater that can be photodegraded during the water treatment process include organic matter and/or different trace contaminants. Moreover, industrial wastewater may also contain heavy metals, organic and inorganic compounds, pesticides, and other deleterious contaminants.

4.1.1.1 Removal of trace metals Trace metals including lead, chromium, and mercury are considered to be highly hazardous to health and thus, the removal of these toxic metals is important for human health and water quality. Water treatment-based photocatalytic processes include the removal of heavy metals, such as lead, cadmium, mercury, chromium, arsenic, copper, and nickel. The photoreduction ability of photocatalysis has also been used to recover expensive metals, such as platinum, gold, and silver, from industrial effluent [2]. Heavy metals can be removed from wastewater as small crystallites deposited on the surface of the photocatalyst according to the redox process [1]: n Mn + + H2 O ! M0 + nH + + O2 4

(4.1)

Under identical conditions, the reactivity pattern was established as [1]: Ag > Pd > Au > Pt ≫ Rh ≫ Ir ≫ Cu ¼ Ni ¼ Fe ¼ 0

(4.2)

4.1.1.2 Removal of inorganic compounds Some inorganic compounds are sensitive to photochemical transformation on the photocatalytic surface. A wide range of inorganic species, such as nitric oxide, bromate or chlorate, halide ions, palladium, rhodium, and sulfur species, can be photodegraded in the presence of photocatalysts, while cyanide, ammonia, thiocyanate, nitrates, nitrites, and metal salts, such as HgCl, AgNO3, and organometallic compounds, can be removed from water by a photocatalytic process [2]. TiO2-sensitized photosystems for the removal of toxic inorganics include [3]: O2 + 2CN
! 2OCN


(4.3)

5O2 + 4H + + 4CN
! 2H2 O + 4CO2 + 2N2

(4.4)

5O2 + 6NH3 ! 2N2 + N2 O + 9H2 O

(4.5)

4.1.1.3 Destruction of organics Photocatalysis has been used for the transformation of organic compounds (carboxylic acids, alcohols, phenolic derivatives, and chlorinated aromatics) into harmless products including carbon dioxide, water, and simple mineral acids. Water contaminated

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213

by oil substances can be treated with high efficiency by photocatalytic reaction. Herbicides and pesticides that may contaminate water, for example 1,1,1-trichloro-2,2-di (4-chlorophenyl)ethane (DDT), have also been successfully removed [2]. The complete mineralization (photodegradation of organic compounds to CO2, H2O, and asso
ciated inorganic components, such as SO2
4 , HCl, HBr, and NO3 ) of a variety of aromatic and aliphatic chlorinated hydrocarbons has been reported [4]. Oxidation photoreactions in the presence of photocatalysts include [4]: CH3 COOH + 2O2 ! 2CO2 + 2H2 O

(4.6)

CCl4 + 2H2 O ! CO2 + 4H + + 4Cl


(4.7)

2CHCl3 + 2H2 O + O2 ! 2CO2 + 6H + + 6Cl


(4.8)

2HOC6 Cl5 + 7O2 ! 4HCO2 H + 8CO2 + 10HCl

(4.9)

4.1.2

Photocatalyst immobilization

The TiO2 photocatalyst is one of the most popular photocatalysts used in photocatalytic water treatment reactors. In the past, much research was carried out on a suspension of fine powdered TiO2 (slurry system). However, posttreatment removal of TiO2 can be a time-consuming and costly process [5] and, therefore, filtration and re-suspension of the photocatalyst powder should be avoided in any wastewater treatment process. The immobilization of TiO2 on a substrate is attractive due to its simple recovery. One method to produce a photocatalyst that is easy to separate after the photocatalytic process involves anchoring the photocatalyst particles onto readily removable supports. Photocatalysts have been coated onto a variety of surfaces, such as glass, ceramics, silica gel, polymers, metals, fibers, zeolites, activated carbon, cellulose, alumina clays, reactor walls, membranes [6], and magnetic particles [7]. When magnetic particles (magnetite, Fe3O4) are employed in the water treatment process, the photocatalyst can be captured downstream with a magnet once the process is complete. An additional SiO2 layer should be applied between the magnetite and photocatalyst surfaces because direct contact of the photocatalyst with the iron oxide core leads to lower photocatalytic activity [7]. However, for the same amount of photocatalyst, a suspension offers a higher active surface area than a thin film. To overcome this problem, nanostructured thin films with high porosity are desirable. The template-assisted method usually involves a pore-directing agent to control the porosity. However, removal of the template thin films often results in poor mechanical stability of the system. Moreover, template-assisted growth is time consuming and often involves several steps. Electrochemical methods have been recently reported to produce TiO2 nanotubes. The anodic oxidation of titanium foil in an adequate medium affords a porous TiO2 surface coating. This approach is fast, facile, and allows precise control of the porosity [8]. Fig. 4.1. presents the images of (A) uncoated silica particles and ones coated with titania by the sol-gel method

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Fig. 4.1 (A) Uncoated silica particles and silica particles coated with titania by the sol-gel method [10]; (B) TiO2 nanotube arrays on a Ti foil plate [8]. Reproduced with permission from M. Nischk, P. Mazierski, M. Gazda, A. Zaleska, Ordered TiO2 nanotubes : the effect of preparation parameters on the photocatalytic activity in air purification process, Appl. Catal. B Environ. 144 (2014) 674–685, https://doi.org/10.1016/j. apcatb.2013.07.041; M. Adams, N. Skillen, C. Mccullagh, P.K.J. Robertson, Development of a doped titania immobilised thin film multi tubular photoreactor, Appl. Catal. B Environ. 130–131 (2013) 99–105, https://doi.org/10.1016/j.apcatb.2012.10.008. Copyright © 2012, 2013 Elsevier B.V.

and (B) TiO2 nanotube arrays on a Ti foil plate. Detailed syntheses of thin films and oriented nanostructures were described in Section 3.1.7. From a practical point of view, the ideal support for photocatalysis must satisfy several criteria [9]: (1) (2) (3) (4)

strong adherence between the photocatalyst and support; high photocatalytic activity of the material after the attachment process; a high specific surface area; strong adsorption affinity toward the pollutants.

4.1.3

Photocatalytic reactors

Photocatalytic reactors for water purification processes can be classified into two main configurations, depending on the location of the photocatalysts: (1) slurry-type photocatalytic reactors with suspended photocatalyst particles, and (2) reactors with the photocatalyst immobilized onto an inert carrier [11]. The light source can be positioned inside or outside the reactor. Sunlight irradiation can be used as an outside

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Fig. 4.2 Schematic diagram of (A) a photocatalytic membrane reactor with an immersed light source and (B) a solar photocatalytic reactor system. Based on (A) W.-Y. Wang, A. Irawan, Y. Ku, Photocatalytic degradation of Acid Red 4 using a titanium dioxide membrane supported on a porous ceramic tube, Water Res. 42 (2008) 4725–4732, https://doi.org/10.1016/j.watres.2008.08.021, (B) J. Herrmann, Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants, Catal. Today 53 (1999) 115–129, https://doi.org/10.1016/S0920-5861(99)00107-8.

irradiation source. Fig. 4.2 illustrates photocatalytic reactors irradiated with an immersed light source and sunlight. The difference between the slurry-type photocatalytic reactors and photocatalyst-immobilized reactors is that the former requires a posttreatment separation process for the recovery of photocatalyst particles, while the latter allows continuous operation. The most important factors in configuring a photocatalytic reactor are the total irradiated surface area of the photocatalyst per unit volume and the light distribution within the reactor [12]. Slurry-type photocatalytic reactors usually exhibit a high total surface area of photocatalyst per unit volume, while reactors with photocatalysts immobilized onto inert carriers are often associated with mass transfer limitation [9]. Thus, until recently, the slurry photocatalytic reactor was still the preferred configuration. Photocatalyst particles can be separated by settling tanks or an external cross-flow filtration system. Photocatalytic reactors with membrane systems are a promising solution toward solving posttreatment separation of the photocatalyst particles [9]. As illustrated in Fig. 4.3, photocatalytic membrane reactors can be generalized as exhibiting (1) irradiation of the membrane module with an immobilized photocatalyst, and (2) irradiation of a feed tank containing a photocatalyst in suspension [13]. Different geometries of the reactor and

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Fig. 4.3 Configuration of photocatalytic membrane reactors: (A) slurry-type photocatalytic membrane reactors (photoreactors with a photocatalyst in a liquid suspension) and (B) immobilized photocatalytic membrane reactors (photoreactors with the photocatalyst immobilized in/on the membrane). Based on W. Zhang, L. Ding, J. Luo, M.Y. Jaffrin, B. Tang, Membrane fouling in photocatalytic membrane reactors (PMRs) for water and wastewater treatment: a critical review, Chem. Eng. J. 302 (2016) 446–458, https://doi.org/10.1016/j.cej.2016.05.071.

configurations of the radiation source are possible [14]. The photocatalytic membrane reactors allow continuous operation of the slurry-type reactor without any loss of photocatalyst particles. The membrane acts as a physical barrier against the photocatalyst particles and pollutants. The membrane module in the photocatalytic membrane reactors with immobilized photocatalysts functions as a support for the photocatalyst particles and a barrier against the different organic pollutants in the treated water. In these reactors, the photocatalytic reaction takes place on the surface of the membrane or within its pores. In photocatalytic membrane reactors, two of the main operational issues are the transmembrane pressure and membrane fouling, which determine both the filtration rate and operating costs [14]. The treatment costs increase for small particle and colloidal size of the photocatalysts [9].

4.1.4

Effect of reaction parameters on the photocatalytic process

The influence of the five physical parameters that affect photocatalytic activity, presented by the temporal reaction rate r, is displayed in Fig. 4.4.

4.1.4.1 Photocatalyst loading The concentration of the photocatalysts in the photocatalytic water treatment system affects the overall photocatalysis reaction rate. In heterogeneous photocatalytic reactions, the increase in photodegradation is proportional to the catalyst loading. The optimum photocatalyst concentration must therefore be determined to avoid the

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Fig. 4.4 Influence of the different physical parameters that affect the kinetics of photocatalysis, reaction rate r: (A) mass of photocatalyst m; (B) wavelength λ; (C) temperature T; (D) initial concentration c of reactant; (E) radiant flux Φ. Based on J.-M. Herrmann, Photocatalysis fundamentals revisited to avoid several misconceptions, Appl. Catal. B Environ. 99 (2010) 461–468, https://doi.org/10.1016/j.apcatb.2010.05.012.

use of excess photocatalyst and ensure total absorption of efficient photons [15]. When the amount of photocatalyst increases above the saturation level, the light photon absorption coefficient usually decreases radially. The excess photocatalyst particles create a light screening effect that reduces the surface area of the photocatalysts being irradiated, thereby decreasing photocatalytic efficiency [9]. Fig. 4.4A

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demonstrates that the reaction rate r is proportional to the mass m of the photocatalyst before reaching a plateau due to maximum photon absorption by the photocatalytic matrix [16].

4.1.4.2 Wavelength The variation of the reaction rate as a function of the wavelength follows the absorption spectrum of the photocatalyst and corresponds to the photocatalyst bandgap energy (Fig. 4.4B) [1].

4.1.4.3 pH The photocatalytic water purification process is highly dependent on the pH as it affects the charge on the photocatalyst particles, size of the aggregates, and position of the conductance and valence bands [9]. The photocatalyst surface can be protonated and deprotonated under acidic and alkaline conditions, respectively [17]: MOH + H + ! MOH2+

(4.10)

MOH + OH
! MO
+ H2 O

(4.11)

The operating pH affects the isoelectric point and the surface charge of the photocatalyst used. The point of zero charge is a condition in which the surface charge is zero or neutral and lies in a pH range of 4.5–7.0, depending on the photocatalyst used. At the point of zero charge there are no electrostatic forces at the surface of photocatalysts and thus, interactions between the photocatalyst particles and water contaminants are at a minimum. When the operating pH is lower than the point of zero charge, the surface charge for the photocatalyst becomes positively charged, thereby increasing electrostatic attraction toward the negatively charged compounds. Such polar attractions between the photocatalysts and charged anionic organic compounds can intensify adsorption onto the photocatalyst surface [9].

4.1.4.4 Temperature An increase in reaction temperature results in increased photocatalytic activity. However, at reaction temperatures >80°C, there is an increase in the rate of the recombination process of charge carriers and a decrease in adsorption of the organic compounds on the photocatalyst surface. Conversely, a reaction temperature 400 nm). Other parameters, including charge separation, mobility, the lifetime of photoinduced electron-hole pairs, and overpotentials, also affect the efficiency of hydrogen production from photocatalytic water splitting. Fig. 4.5 depicts the most important processes in the photocatalytic generation of oxygen and hydrogen, comprising: (1) absorption of photons, (2) generation of electron-hole pairs, (3) recombination of the excited charges, (4) separation of the excited charges, (5) migration of the charges, and (6) surface chemical reactions between these carriers with water and other molecules. Notably, water splitting into hydrogen and oxygen is an energy increasing process, thus, the backward reaction of hydrogen and oxygen to water can proceed easily. Numerous efforts have been made to promote efficiency and enhance the visible light response to make photocatalytic hydrogen production more feasible. Among them, the addition of electron donors and carbonate salts, noble metal loading, co-catalyst loading, metal ion doping, anion doping, dye sensitization, use of composite semiconductors, and metal ion-implantation have been thoroughly investigated and some were found to be useful in enhancing hydrogen production.

4.2.2

Strategies for achieving photocatalytic water splitting

There are two primary approaches to achieving photocatalytic water splitting (Fig. 4.6). The first method, whereby water is decomposed into H2 and O2 via a single semiconductor (Fig. 4.6A), requires that the photocatalyst should have a suitable thermodynamic potential for photocatalytic water splitting, a narrow bandgap to utilize

Fig. 4.5 Main processes in photocatalytic water splitting. Based on R. Abe, Recent progress on photocatalytic and photoelectrochemical water splitting under visible light irradiation, J. Photochem. Photobiol. C Photochem. Rev. 11 (2010) 179–209, https://doi.org/10.1016/j.jphotochemrev.2011.02.003.

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Fig. 4.6 Primary approaches to achieve overall photocatalytic water splitting. (A) One-step photoexcitation systems and (B) two-step photoexcitation systems. Based on K. Maeda, K. Domen, Photocatalytic water splitting: recent progress and future challenges, J. Phys. Chem. Lett. 1 (2010) 2655–2661, https://doi.org/10.1021/jz1007966.

visible photons, and photocorrosion stability. Thus, the number of suitable semiconductors for one-step photoexcitation systems is limited [4–6]. Another approach inspired by natural photosynthesis, namely a two-step photoexcitation mechanism using two different semiconductors (Fig. 4.6B), has therefore been developed [7]. In this system, photocatalytic water splitting is divided into half-reactions, one for H2 evolution and the other for O2 evolution, which occur in the presence of a shuttle redox couple (Red/Ox) in the solution. In the presence of the semiconductor responsible for H2 evolution, the photogenerated electrons reduce water to H2 and holes in the VB oxidize the reductant (Red) to an oxidant (Ox). The oxidant is reduced back to the reductant by electrons generated over a semiconductor responsible for O2 evolution, whereby the photogenerated holes oxidize water to O2. The two-step photoexcitation system holds several advantages. First, more lowbandgap photocatalysts with either a water reduction or oxidation potential can be used on one side of the system. Second, in this approach, visible light is utilized more efficiently than in the conventional one-step system. Two-step photoexcitation allows the separation of H2 and O2 production via various types of separators, such as membranes. Thus, this system allows inhibition of the backward reaction, whereas during one-step water splitting, it is impossible in principle to achieve separate H2 and O2 production. However, the two-step system requires twofold more photons than the one-step system to decompose water into H2 and O2.

4.2.3

Brief overview of metal oxide semiconductors in photocatalytic water splitting

A suitable metal oxide semiconductor for overall water splitting should have a bandgap of at least 1.23 eV and exhibit resistance to photocorrosion. In addition to TiO2, other metal oxide semiconductors, such as ZnO, Nb2O5, ZrO2, α-Fe2O3,

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WO3, CuO, Cu2O, Bi2O3, and their modifications or coupling with TiO2 (binary composites), have been used for photocatalytic H2 and O2 generation [8,9]. However, compounds such as WO3 (photoactive under visible light) can only be used as O2 evolution photocatalysts under visible light and no H2 production is detected due to a low conduction band level. On the other hand, CuO (photoactive under visible light) can only be employed as a sensitizer for composite semiconductors because it does not afford adequate photoactivity with the sole use of CuO [10–12]. Zhu and Zach [13] proposed other metal oxides comprising three or four elements, such as potassium hexaniobate (K4Nb6O17), perovskite (SrTiO3, ACa2Nb3O10, A ¼ Hr, K), K4Ce2M10O30 (M ¼ Ta, Nb), titanates and tantalates, which can be applied as good and stable semiconductors for photocatalytic water splitting. An extensive list of metal oxide-based semiconductors has been presented by Chen et al. [9].

4.2.4

Chemical additives for H2 and O2 production enhancement

One of the problems encountered in the use of metal oxide semiconductors in photocatalytic water splitting and other photocatalytic reactions is the recombination of photogenerated electron-hole pairs. To enhance photocatalytic electron-hole separation, a strategy based on the addition of electron donors (sacrificial reagents or hole scavengers) has been developed. Organic compounds are the most commonly employed electron donors for photocatalytic water splitting because they can be oxidized by photogenerated holes to improve the hydrogen generation rate. Among them, methanol, ethanol, EDTA, lactic acid, and formaldehyde have been widely investigated [14–19] and have demonstrated improved ability to generate hydrogen. It was revealed that both the type of electron donor and its concentration play an important role in the production of hydrogen from water [14]. The research group [14] studied the effect of four electron donors on the efficiency of hydrogen production in the presence of a TiO2 photocatalyst. The efficiency of hydrogen generation decreased in the order: EDTA < methanol < ethanol < lactic acid. Other studies [17] have demonstrated that oxalic acid, formic acid, and formaldehyde act as electron donors and that their decomposition was related to hydrogen generation.
3+/ To enhance H2 production, inorganic ions, such as IO
Fe2+, and S2
/ 3 /I , Fe 2
2
SO3 , were used as sacrificial reagents [20–23]. For example, S /SO2
3 can inhibit photocorrosion of CdS photocatalysts and other sulfides. An IO
3 /I
shuttle redox mediator (electron acceptor) is often used in the two-step photoexcitation system (Z-scheme) [21,22,24]. In this system, I
can scavenge holes, allowing the reduction of protons, via electrons, to hydrogen. Simultaneously, IO
3 reacts with the electrons to form I
and thus, holes can oxidize water to oxygen (Fig. 4.6B). Conversely, photo
catalytic hydrogen and oxygen production in the presence of IO
3 /I shuttle redox mediators occurs without consumption of a sacrificial reagent. The addition of carbonate salts significantly enhances photocatalytic splitting of water into H2 and O2. For example, the addition of Na2CO3 was proven to be beneficial for photocatalytic hydrogen and oxygen production over various

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Photocatalytic hydrogen and oxygen production over ZrO2 and in the presence of various additives Table 4.1

Production rate (μmol/h) Type of additive

H2

O2

None NaOH Na3PO4 Na2CO3 NaBO2 Na2HPO4 NaHCO3 Na2SO4 NaCl HCl H3PO4 H2SO4

72 242 228 378 164 129 607 112 91 46 65 85

36 129 113 190 84 65 319 56 48 19 33 39

Based on K. Sayama, H. Arakawa, Effect of carbonate addition on the photocatalytic decomposition of liquid water over a ZrO2 catalyst, J. Photochem. Photobiol. A Chem. 94 (1996) 67–76, https://doi.org/10.1016/1010-6030(95)04204-0.

semiconductors, such as TiO2, Ta2O5, and ZrO2 [25]. Na2CO3 increased the production of hydrogen and oxygen over Pt-modified TiO2 [26]. Table 4.1 lists hydrogen and oxygen production in the presence of ZrO2 photocatalysts mediated by the presence of various carbonate salts.

4.2.5

Metal oxide co-catalysts for photocatalytic water splitting

Photocatalytic hydrogen and oxygen production is also improved by the presence of H2 evolution and/or O2 evolution co-catalysts that separate photogenerated charge carriers, create active sites for H2 or O2 evolution, and enhance the stability of the metal oxide semiconductor. Recently, noble metals, such as Ru [27], Rh [23,28], Pd [29], Pt [30,31], Au [32], and Ag [33], have been widely investigated as efficient co-catalysts for photocatalytic hydrogen evolution. Several metal oxides, such as NiO [34,35], NiOx [36,37], CuO [38,39] Cu2O [40], RuO2 [41,42], IrO2 [43,44], CoOx [45], MnOx [46], FeOx [46], and Co3O4 [47], have also been applied as co-catalysts for this process. Notably, the latter four co-catalysts have been used for photocatalytic O2 production, while the others were used as H2 evolution co-catalysts. Because metal oxide semiconductor co-catalysts improve photocatalytic hydrogen and oxygen production, their roles should be highlighted: (1) these co-catalysts may decrease the activation energy or overpotential needed for H2 or O2 evolution reactions, (2) they are able to participate in electron-hole separation processes at the

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co-catalyst/semiconductor interface, and (3) they also inhibit photocorrosion and improve the stability of the semiconductors [48]. On the other hand, the amount of co-catalyst and its size and structure can affect the efficiency of hydrogen and oxygen evolution in photocatalytic water splitting. Thus, the optimal amount of co-catalyst gradually enhances the efficiency of photocatalytic water splitting by facilitating charge separation. However, an excess of the co-catalyst may: (1) cover the surface-active sites and consequently block their contact with sacrificial reagents or water, (2) inhibit light absorption by the semiconductor and the generation of photogenerated electrons and holes, and (3) act as charge recombination centers that cause a decrease in hydrogen and oxygen generation [48]. Furthermore, the size of the co-catalysts has significant impact on photocatalytic water splitting, so that co-catalysts of smaller size have a larger surface area and more active sites, resulting in improved hydrogen and oxygen production. A lower recombination of charge carriers is also observed in co-catalysts composed of smaller particles [6,48].

4.2.6

Evaluation of photocatalytic water splitting

Two main factors should be considered when evaluating the ability of metal oxide semiconductors to produce hydrogen and oxygen from photocatalytic water splitting: the rate of H2 and O2 production (photocatalytic activity) and the photocatalytic stability. Different photocatalytic set-ups, irradiation types, light sources (Xe and Hg lamps), and amounts of metal oxide semiconductors have been reported in different works. This makes direct comparison of the different reported results highly challenging. However, the most commonly used units for the rate of H2 and O2 evolution (μmol/h and μmol/h g) afford a measurable comparison between different semiconductors under similar experimental conditions [9]. The quantum yield and/or apparent quantum yield is an important property for the evaluation of the photocatalytic activity in the water splitting reaction. The overall quantum yield and apparent quantum yield (for H2 and O2 evolution) are defined by Eqs. (4.16), (4.17), respectively [9]: Overall quantum yield ðQY, %Þ ¼

Number of reacted electrons  100% Number of absorbed photons

(4.16)

Number of reacted electrons  100% number of absorbed photons 2  number of evolved H2 molecules ¼ Number of incident photons  100%

Apparent quantum yield ðAQY, %Þ ¼

¼

4  number of evolved O2 molecules Number of of incident photons  100%

(4.17)

Notably, the apparent quantum yield is smaller than the total quantum yield because the number of absorbed photons is usually smaller than that of incident light.

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4.2.7

229

Photoreactor design

The selection of a photoreactor usually depends on the experimental conditions and the type of metal oxide semiconductors (one- or two-step photoexcitation). Slurry and batch-type photoreactors are the most popular for hydrogen and oxygen production from water splitting. The simplest construction of a photoreactor for photocatalytic water splitting, namely the slurry-type photoreactor, is illustrated in Fig. 4.7. This type of photoreactor is used for powder photocatalysts and usually consists of a Pyrex chamber equipped with a stirrer and external light source. In some versions, the slurry-type photoreactor is connected to a gas chromatograph for in-situ measurements of the afforded products (H2 and O2) [49,50]. The batch-type reactor is another photoreactor used in photocatalytic water splitting over metal oxide semiconductors. In this reactor, which is most commonly used for hydrogen and oxygen production, the photocatalyst occurs in the form of a slurry. Fig. 4.8 displays a typical batch-type photoreactor based on [51]. This type of photoreactor can be made from stainless steel, Pyrex, quartz, and borosilicate glass. The stirrer inside the reactor prevents deposition and accumulation of the photocatalyst. Moreover, the photoreactor is equipped with a water jacket to maintain the reaction process at a set temperature. In this set-up, the irradiation source is located above the photoreactor and light passes through a window. Batch-type photoreactors can vary in geometry and dimensions, rector material, window material, cooling system, and light source. More examples of batch-type photoreactor set-ups are compiled in Table 4.2. An appropriate photoreactor set-up maximizes hydrogen and oxygen production via photocatalytic water splitting over metal oxide semiconductors. For example, light distribution determines the local photon absorption rate and thus, the hydrogen and oxygen production rate. On the other hand, the reactor and window materials determine the amount of light transmission and therefore affect the water splitting efficiency.

Fig. 4.7 Slurry-type photoreactor for photocatalytic water splitting. Based on Z. Xing, X. Zong, J. Pan, L. Wang, On the engineering part of solar hydrogen production from water splitting: photoreactor design, Chem. Eng. Sci. 104 (2013) 125–146, https://doi.org/10.1016/j. ces.2013.08.039.

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Fig. 4.8 Batch-type photoreactor for photocatalytic water splitting. Based on C. Huang, W. Yao, A. T-Raissi, N. Muradov, Development of efficient photoreactors for solar hydrogen production, Sol. Energy. 85 (2011) 19–27, https://doi.org/10.1016/j.solener. 2010.11.004. Table 4.2 Basic design elements of the batch-type photoreactors for photocatalytic water splitting Reactor materials

Window materials

Light source

Stainless steel [51] Pyrex [52] Quartz [54]

Quartz [51] Fused silica [51] Aclar, Kynar PVDF, PET, PVC [51]

Xenon lamp [49,51] Mercury lamp [53] Halogen lamp [55]

The main disadvantage of the these photoreactors is the backward reaction. Thus, to overcome this, a twin-type photoreactor was developed as illustrated in Fig. 4.9. This photoreactor [55], comprising two chambers separated by a proton exchange Nafion membrane (Fig. 4.9), has been widely used in two-step photoexcitation systems (Z-scheme photocatalytic water splitting). The Pyrex photoreactor has a cylindrical shape. In this system, H2 and O2 photocatalysts were placed in different chambers. Before the process, the membrane was cleaned by different acid and alkali solutions. The generated hydrogen and oxygen were collected separately via an online sampling loop by switching the valves alternatively [56]. During the process, the photocatalysts were stirred and irradiated with the same 500-W halogen lamp in front

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Fig. 4.9 Twin-type photoreactor for photocatalytic water splitting. Based on C.-C. Lo, C.-W. Huang, C.-H. Liao, J.C.S. Wu, Novel twin reactor for separate evolution of hydrogen and oxygen in photocatalytic water splitting, Int. J. Hydrogen Energy. 35 (2010) 1523–1529, https://doi.org/10.1016/ j.ijhydene.2009.12.032.

of the twin photoreactor. Conventional Z-scheme photocatalysts, Pt/SrTiO3:Rh and WO3, were used as H2 and O2 photocatalysts, respectively [55]. Fe2+ and Fe3+ were added to the reaction solution as electron-transfer mediators. The ratio of evolved H2 and O2 agreed with the stoichiometric ratio (2:1) of hydrogen and oxygen in water. Thus, in this photoreactor, the improved H2 yield was attributed to the prevention of the water splitting backward reaction. Various research groups [13,57,58] next proposed the use of photoreactors based on TiO2 thin films. Two types of such photoreactors are displayed in Figs. 4.10 and 4.11. A thin film photoreactor comprising Ti foil and a TiO2 film on one side, and Pt on the other side, was reported [58]. This was mounted on a H-type glass photoreactor that separated the two aqueous solutions. Light irradiation was carried out with a highpressure lamp through the quartz window of the reaction cell (Fig. 4.10). This photoreactor consisted of two water phases separated by a TiO2/Ti/Pt photocatalyst and a proton-exchange membrane. Chemical bias between the two compartments was created using a 1.0 M NaOH aqueous solution at the TiO2 side and a 0.5 M H2SO4 aqueous solution at the Pt side. Fig. 4.10 H-type photoreactor for H2 and O2 production from photocatalytic water splitting. Based on M. Kitano, K. Tsujimaru, M. Anpo, Decomposition of water in the separate evolution of hydrogen and oxygen using visible light-responsive TiO2 thin film photocatalysts: effect of the work function of the substrates on the yield of the reaction, Appl. Catal. A Gen. 314 (2006) 179–183, https:// doi.org/10.1016/j.apcata.2006.08.017.

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Fig. 4.11 Photoreactor for hydrogen and oxygen generation used by Selli et al. [57].

In the work reported by Selli et al. [57], photocatalytic hydrogen and oxygen production was studied in two chambers made of plexiglass that allowed the separate production of H2 and O2 at the two different sides of the photoactive TiO2/Ti material. One side of the cell was irradiated with a UV lamp through a window. The TiO2/Ti was mounted between the two chambers of the cell, with the TiO2 layer exposed to the irradiation source. A cation exchange membrane was placed over the Ti disc separating the two compartments of the system. Two aqueous solutions (NaOH and H2SO4) were used to generate a small electrochemical current to ensure the passage of electrons (Fig. 4.11). At irradiation wavelengths 1.0 N, while their production was negligible when the two compartments were filled with pure water. In summary, photoreactors equipped with membranes are effective in attaining sustainable hydrogen and oxygen production and preventing the backward reaction between H2 and O2.

4.2.8

Conclusions and prospects

Hydrogen generated from water splitting remains an innovative and potential route to produce green fuel using solar light. However, for this process to hold potential for application in future technology, several issues need to be resolved. One of the main problems is the low quantum efficiency of the reaction, which is not sufficient for practical application. Therefore, the development of photocatalysts with remarkable activity and visible-light-responsive stable materials should be continued. Despite the two primary approaches to achieve photocatalytic water splitting, new strategies for efficient separation of photogenerated electrons and holes in powder semiconductors are highly desirable. Another problem that should be solved is the scaling-up process

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and separation of powder photocatalysts after the process. Despite these shortcomings, photocatalytic water splitting is very promising and further research should be conducted.

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[44] R. Abe, K. Shinmei, K. Hara, B. Ohtani, Robust dye-sensitized overall water splitting system with two-step photoexcitation of coumarin dyes and metal oxide semiconductors, Chem. Commun. (2009) 3577–3579, https://doi.org/10.1039/B905935K. [45] R. Li, Z. Chen, W. Zhao, F. Zhang, K. Maeda, B. Huang, S. Shen, K. Domen, C. Li, Sulfurization-assisted cobalt deposition on Sm2Ti2S2O5 photocatalyst for water oxidation under visible light irradiation, J. Phys. Chem. C 117 (2013) 376–382, https://doi.org/ 10.1021/jp310138b. [46] L. Liu, Z. Ji, W. Zou, X. Gu, Y. Deng, F. Gao, C. Tang, L. Dong, In situ loading transition metal oxide clusters on TiO2 nanosheets As Co-catalysts for exceptional high photoactivity, ACS Catal. 3 (2013) 2052–2061, https://doi.org/10.1021/cs4002755. [47] J. Zhang, M. Grzelczak, Y. Hou, K. Maeda, K. Domen, X. Fu, M. Antonietti, X. Wang, Photocatalytic oxidation of water by polymeric carbon nitride nanohybrids made of sustainable elements, Chem. Sci. 3 (2012) 443–446, https://doi.org/10.1039/C1SC00644D. [48] J. Ran, J. Zhang, J. Yu, M. Jaroniec, S.Z. Qiao, Earth-abundant cocatalysts for semiconductor-based photocatalytic water splitting, Chem. Soc. Rev. 43 (2014) 7787–7812, https://doi.org/10.1039/C3CS60425J. [49] R.S. Khnayzer, L.B. Thompson, M. Zamkov, S. Ardo, G.J. Meyer, C.J. Murphy, F. N. Castellano, Photocatalytic hydrogen production at titania-supported Pt nanoclusters that are derived from surface-anchored molecular precursors, J. Phys. Chem. C 116 (2012) 1429–1438, https://doi.org/10.1021/jp206943s. [50] P.A. Mangrulkar, M.V. Joshi, S.P. Kamble, N.K. Labhsetwar, S.S. Rayalu, Hydrogen evolution by a low cost photocatalyst: Bauxite residue, Int. J. Hydrogen Energy 35 (2010) 10859–10866, https://doi.org/10.1016/j.ijhydene.2009.10.075. [51] C. Huang, W. Yao, A. T-Raissi, N. Muradov, Development of efficient photoreactors for solar hydrogen production, Sol. Energy. 85 (2011) 19–27, https://doi.org/10.1016/j. solener.2010.11.004. [52] J.-J. Chen, J.C.S. Wu, P.C. Wu, D.P. Tsai, Plasmonic photocatalyst for H2 evolution in photocatalytic water splitting, J. Phys. Chem. C 115 (2011) 210–216, https://doi.org/ 10.1021/jp1074048. [53] J.C. Escudero, R. Simarro, S. Cervera-March, J. Gim
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Further reading [1] K. Sayama, H. Arakawa, Effect of carbonate addition on the photocatalytic decomposition of liquid water over a ZrO2 catalyst, J. Photochem. Photobiol. A Chem. 94 (1996) 67–76, https://doi.org/10.1016/1010-6030(95)04204-0. [2] Z. Xing, X. Zong, J. Pan, L. Wang, On the engineering part of solar hydrogen production from water splitting: photoreactor design, Chem. Eng. Sci. 104 (2013) 125–146, https://doi. org/10.1016/j.ces.2013.08.039.

4.3 Air depollution and volatile organic compound (VOC) removal using different photocatalysts Air pollution presents a major worldwide public health challenge because of its prevalence and emission by many different sources [1]. Indoor and outdoor air pollution is caused by chemical contaminants including (1) inorganic and organic compounds (NOx, SOx, CO, and VOC), (2) pathogens (viruses, bacteria, and fungi), and (3) particulate matter (PM) [2]. VOCs mainly comprise alkenes, alkanes, aromatics, alcohols, esters, and carboxylic acids, with typical indoor levels being five to ten times higher than outdoor readings [3,4]. Indoor contaminants are produced from a variety of sources including combustion processes, emissions from adhesives and building materials, solvents, disinfectants, glues, deodorants, fuels, leather, smoking, chlorinated water, and combustion products [1,5,6]. Flooring can also emit VOCs, such as xylene, benzene, and toluene, while large amounts of formaldehyde are emitted from components of building interiors and furniture as well as from heat treatment and combustion. This latter compound is a dangerous VOC and a known carcinogen. Indoor pollution also includes biological contaminants, such as mold and pollen from cooling and heating systems, cooking, and materials used in buildings (glue, paint, and solvents) [5]. Finally, VOCs generated outdoors may also contaminate the indoor atmosphere via air exchange; the reverse may also occur. Outdoor VOCs originate from both anthropogenic and natural sources. Anthropogenic sources include industrial discharge, automobile exhaust fuel combustion products, petrochemical solvents, evaporated fuels, and biogenics, while natural sources include secondary metabolic reactions of vegetation [7]. Outdoors, formaldehyde is emitted from both natural (forest fires, solid wood, excretion of animals) and anthropogenic sources (motor vehicles, coal processing industries, chemical plants, and biomass combustion), as well as from oxidation of many VOCs [4,8]. Air pollutants released from fossil fuel combustion, such as coal, oil, and gas, include CO, NOx, SO2, VOCs, PM, and lead [9]. Inorganic oxides and VOCs may travel long distances to produce secondary pollutants, such as ozone and acid rain, and are also responsible for global warming [5]. Outdoor air contamination also includes gaseous ozone, a major component of photochemical smog. Benzene and toluene react with atmospheric ozone, NOx, and hydroxyl radicals to form photochemical smog, formaldehyde, and other reactive compounds [4]. Odor can be caused by the presence of S-containing compounds (H2S) and some bacteria [10]. Table 4.3 lists selected results of photocatalysts used for photodegradation of

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Examples of photocatalysts used for the photodegradation of chemical contaminants in air

Table 4.3

Pollutant

Photocatalyst

Efficiency

Light source

Ref.

Acetaldehyde

g-C3N4/WO3

UV at 1.0 mW/cm2

[11]

Acetylene

TiO2

CO

N-TiO2 Sn2+-TiO2 RuO2/TiO2/Pt

Cyclohexane

Pt/TiO2

Ethanol

P25-graphene

Formaldehyde

ZnO

25% conversion

H2S

MnOx-CeO2 WO3/TiO2

90% removal 70% conversion

NOx

TiO2

16% conversion

Fe/TiO2

38% conversion

Pt/TiO2

30% conversion

a-Fe2O3

20% removal

NO2

Mo/TiO2 NTs

77% removal

NO

83% conversion

NO and SO2

Au/CeO2TiO2 BiOI/Al2O3

Trichloroethylene

Fe2O3-TiO2

100% conversion for both NO and SO2 95% conversion

UV at 10 mW/cm2 Visible light Visible light UV at 1.0 mW/cm2 UV LEDs at 90 mW/cm2 UV at 25 mW/cm2 UV at 3.6 mW/cm2 Vis light at 35.5 mW/cm2 UV Xe lamp UV at 3.3 mW/cm2 UV 365 at 1.0 mW/cm2 Vis light at 35.8 mW/cm2 Vis light 420 nm at 0.7 mW/cm2 Vis light at 58 mW/cm2 UVA at 1.25 mW/cm2 UV or visible light Vis light at 6 mW/cm2

[12]

Benzene

Higher conversion than single component 85% mineralization 72% conversion 27% conversion 100% conversion 100% conversion 95% conversion

Cu-TiO2

no data

V-TiO2

80% removal

Toluene

UV and visible light UV at 1.0 mW/cm2 Visible light

[13] [14] [15] [16] [17] [18]

[19] [20] [21] [22] [23]

[24] [25] [26]

[27] [28] [29]

Application of metal oxide-based photocatalysis

239

chemical contaminants in air published over the past few years. Modified or doped TiO2 was the most commonly studied photocatalyst for photodegradation of chemical contaminants in air. A section in this chapter contains an overview of pollutant degradation mechanisms, pathogen photodisinfection, and the most important factors that affect the photocatalytic oxidation process. Finally, a reactor design for photocatalytic air purification as well as its prospects are proposed with the goal of overcoming challenges and stimulating further research in this promising field.

4.3.1

The mechanism of pollutant photo-oxidation

4.3.1.1 Photocatalytic removal of NOx and SOx The aim of NOx photo-oxidation is to transform NO into HNO3 via HNO2 and NO2 formation with the aid of photocatalysis [10]. NO is more likely to convert into NO2 than HNO3 because of the considerable amount of oxygen present in the atmosphere [30]. The mechanism can be described by the following equations [10,31]: NO + HO ! HNO2

(4.18)

HNO2 + HO ! NO2 + H2 O

(4.19)

NO2 + HO ! HNO3

(4.20)


NO + O
2 ! NO3

(4.21)

HNO2 ! H + + NO
2

(4.22)


2NO + O
2 + 3e ! 2NO2

(4.23)

HNO3 ! H + + NO3


(4.24)

3NO2 + 2OH
! 2NO3
+ NO + H2 O

(4.25)

In the first step, holes and electrons produce “active” hydroxyl and oxygen radicals that in turn participate in oxidation reactions (Eqs. 4.18–4.20, 4.21) for hydroxyl and oxygen radicals, respectively). The photocatalyst surface eventually becomes saturated with HNO3. Lasek et al. suggest that the NOx cycle is terminated by HNO3 removal from the photocatalytic surface [30]. The HNO3 quickly diffuses into photocatalyst films of different thicknesses where it accumulates. Due to a reduced increase in the surface density of HNO3, the apparent oxidation rate of NO2 is lower in thicker films, and thus, more time is required to deactivate thicker photocatalyst films [32,33]. Some photoselective catalytic reduction processes employ reductive reagents, such as CO, hydrocarbons, and NH3, to convert NO to N2. SOx is another air pollutant that can lead to acid rain, vegetation damage, building corrosion, as well as respiratory conditions, such as bronchoconstriction and asthma

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Metal Oxide-Based Photocatalysis

exacerbation [10]. The major reaction pathways responsible for surface sulfate formation can be described by the following equations: 2
SO2 +  O 2
2 ! SO4

(4.26)

SO2 +  OH
! HSO3


(4.27)

SO2 can be oxidized to sulfide and sulfate surface species, with the latter being more thermodynamically stable [10,34]. Photodecomposition of SOx can follow other routes [26]: SO2 + h + + 2H2 O ! H2 SO4 + 2H +

(4.28)

SO2 +  OH ! H2 SO3

(4.29)

2
SO2 +  O
2 ! SO4

(4.30)

4.3.1.2 Photodegradation of VOCs VOCs are considered as a group of major environmental air pollutants and include toluene, xylene, formaldehyde, trichloroethylene (TCE), ethylbenzene, acetone, 1-butanol, butylaldehyde, and 1,3-butadiene. Photo-oxidation of VOCs can generate many oxidation by-products/intermediates that exist in both the gas phase and on the photocatalyst surface. Some of these by-products can be even more toxic than their parent compounds [35,36] and thus, the generation of intermediates is one of the main concerns associated with the application of photocatalysis. Furthermore, the generation of by-products is one of the main causes of photocatalyst surface deactivation. Formaldehyde is one of the best-known VOCs, whose concentration is usually at ppbv to ppmv levels in polluted air. The products and intermediates from the photooxidation of formaldehyde can follow [10]: HCHO +  OH !  CHO +H2 O

(4.31)



CHO +  OH ! HCOOH

(4.32)



+
CHO +  O
2 ! HCO3 ð +H Þ ! HCOOOH ð +HCHOÞ ! HCOOH

(4.33)

HCOOOHð
H + Þ ! HCOO
ð +  OHÞ ! H2 O +  CO2


(4.34)

HCOO
ð +h + Þ !  CO2
+ H +

(4.35)



CO2
+½O +  OH + h


+

! CO2

(4.36)

Formic acid is the most common by-product of formaldehyde photo-oxidation in the gaseous phase. The yield of formic acid decreases with increasing humidity

Application of metal oxide-based photocatalysis

241

levels [37]. Chlorinated VOCs, such as TCE and tetrachloroethylene (PCE), have been widely used in industry as solvents for cleaning and degreasing [38]. The photooxidation by-products of chlorinated VOCs are more toxic and hazardous than those of other VOCs. The generated OH radicals can react with TCE to generate Cl radicals [39]: l

l

Cl2 C ¼ CHCl +  OH ! Cl2 C
CHClOH

(4.37)

Cl2 C
CHClOH ! Cl2 C ¼ CHOH + Cl

(4.38)

The Cl radicals are highly reactive species and can react with TCE [39]: Cl2 C ¼ CHCl + Cl ! Cl2 HC
CClCl

(4.39)

Cl2 HC
CClCl + O2 ! Cl2 HC
CCl2 OO

(4.40)

2Cl2 HC
CCl2 OO ! 2Cl2 HC
CCl2 O + O2

(4.41)

2Cl2 HC
CCl2 O ! Cl2 HC
COCl + Cl

(4.42)

Cl2 HC
COCl + O2 ! COCl2 + CO2 + HCl

(4.43)

Dichloroacetylchloride is produced as an intermediate and can form phosgene (COCl2), CO2, and HCl. Prolonged illumination leads to the decomposition of phosgene to carbon dioxide. Other PCE and TCE oxidation intermediates include dichloroacetyl chloride (DCAC), chloroform (CHCl3), trichloroacetyl chloride (TCAC), carbon monoxide, and carbon tetrachloride [10,38]. The optimization of O2 and humidity can reduce the amount of these toxic intermediates [40]. Aromatic compounds, such as benzene, toluene, and xylene, are highly significant, because they are highly toxic and carcinogenic [41]. Toluene weakly adsorbs onto the photocatalyst surface and can be oxidized to benzaldehyde. This adsorbs onto the photocatalysts surface more strongly, leading to the formation of ring-opening products, such as carboxylic acids and aldehydes. Toluene photo-oxidation intermediates, such as benzaldehyde, benzoic acid, and small concentrations of benzyl alcohol and phenol, have also been reported [10,42]. The main intermediate from the oxidation of benzene is phenol but the final products are CO2, CO, and H2O [43]. In summary, different reaction pathways have been proposed and various by-products have been identified in the degradation of VOCs.

4.3.1.3 Photocatalytic removal of CO Carbon monoxide (CO) is a toxic air pollutant. CO photo-oxidation to CO2 is often the last step of most VOC photocatalytic oxidation processes occurring on the photocatalyst surface [44]. Photocatalysts can oxidize CO under UV irradiation through the reactions [45]:

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(a) Oxidization of CO

CO + h + ! CO

(4.44)

(b) Reduction of O

O 2 + e
! O 2


(4.45)

+ O
2 + h ! 2O

(4.46)

O + e
! O


(4.47)

(c) Combined reaction

CO + O
! CO
2 !! CO2

(4.48)

The oxidation and reduction reactions (Eqs. 4.44, 4.47) can be combined to form Eq. 4.48 with CO2 as the final product.

4.3.1.4 Photocatalytic deodorization Odor can be caused by the presence of S-containing compounds (H2S) and some bacteria. Odor-containing sulfur, such as dimethyl sulfide (DMS), dimethyl disulfide (DMDS), and methyl mercaptan, can be emitted from sewage disposal plants and fermentation treatment of animal feces in the livestock industry. The decomposition of DMS and DMDS leads to the formation of SO2 and formic acid. However, the reported products extracted from the bead after the reaction was complete were formic and acetic acid [46]. Hydrogen sulfide is a toxic, corrosive, and malodorous compound with damaging effects even at low concentrations. The major reaction steps in H2S removal under irradiation are described in the following equations [10]: H2 S + h + ! H2 S + ! HS + H + 

OH + H2 S + ! HS + H2 O

(4.49) (4.50)

HS + O2 ! HSOO

(4.51)

HSOO + O2 ! SO2

(4.52)

This pathway leads to the formation of sulfates as the final reaction products that accumulate at the photocatalyst surface and consequently cause on-flow deactivation. Another mechanism that describes the hydrogen evolution processes of the photocatalytic decomposition of hydrogen sulfide is [47]: H2 S + OH
! HS
+ H2 O

(4.53)

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243

2HS
+ 2h + ! 2S + 2H +

(4.54)

2H + + 2e
! H2

(4.55)

4.3.2

Photodisinfection of pathogens

Photocatalysis has been shown to exhibit the capacity to disinfect a variety of pathogens including fungi, bacteria, and viruses [48,49]. Photodisinfection can result in one of two outcomes: pathogen inactivation (disablement) or lysis (death) via ROS. Photocatalyst films exhibit deodorizing, bactericidal, and self-cleaning functions under UV light irradiation [50]. The photocatalysis of pathogens potentially involves one or more of four features: (1) disinfection, (2) inhibition of proliferation, (3) degradation of the produced toxins, and/or (4) degradation of the by-products of these organic decompositions [2]. Gram-negative bacteria have a cell membrane comprising three layers (an outer membrane, a peptidoglycan, and a cytoplasmic membrane), while Gram-positive bacteria have only two layers (a peptidoglycan and a cytoplasmic membrane) [2]. Except for viruses, other pathogens have similar cell structures. Most research now indicates that the destruction of the cell membrane is an important process for inactivation [51]. However, this may also be followed by degradation of the internal cellular components in the form of the interior genome (DNA and RNA), which causes cell lysis. The photocatalytic mechanism for the disinfection of pathogens (with the exception of viruses) is as follows [50–52]: (1) The outer membrane (for Gram-negative bacteria) comprises ROS that establish permeability and expose the cytoplasmic membrane. (2) The ROS can pass through the peptidoglycan to reach the inner layer. (3) Subsequently, the ROS can reach and degrade the internal cellular components causing cell death (Fig. 4.12).

The inactivation of viruses by ROS involves photodegradation of the protein capsid, resulting in decreased infectability, followed by the destruction of the genome [53,54].

Fig. 4.12 Schematic diagram of the stages of the photocatalytic bactericidal mechanism leading to cell lysis: (A) the outer membrane is compromised by the ROS, (B) the ROS can pass through the peptidoglycan to reach the inner layer, (C) the ROS can reach and degrade the internal cellular components causing cell death. Reproduced with permission from H. Ren, P. Koshy, W.-F. Chen, S. Qi, C.C. Sorrell, Photocatalytic materials and technologies for air purification, J. Hazard. Mater. 325 (2017) 340–366.

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Photocatalysis has been shown to exhibit the capacity to disinfect a variety of pathogens, including bacteria, fungi, and viruses. The inactivation of Escherichia coli, Staphylococcus aureus, Staphylococcus epidermidis, Anabaena, Edwardsiella tarda, Klebsiella pneumoniae, Legionella pneumophila, Pseudomonas aeruginosa, Bacillus subtilis, Serratia marcescens, Bacillus cereus, Lactococcus lactis, Micrococcus luteus, Aspergillus niger, Candida vini, MS2 bacteriophage, Penicillium citrinum, Avian influenza, Virus A/H5N2, and MS2 bacteriophage in the gas phase [2,50,55,56].

4.3.3

Factors that impact the photocatalytic oxidation process

The photocatalytic oxidation process is strongly dependent on the operating and process conditions: (1) type and concentration of the pollutant; (2) air humidity; (3) light intensity and spectrum; (4) flow rate/residence time; (5) amount of photocatalyst per active area; (6) temperature; and (7) photocatalyst deactivation and reactivation [4,5,35,57]. The reaction rates of VOCs depend on the pollutant [5]. Moreover, the photodegradation rates of each pollutant vary due to different absorption rates and reaction schemes. VOCs, such as toluene, benzene, xylene, and ethylbenzene, display various photosteady states and reaction rates. Generally, the higher soluble compounds tend to possess higher reaction rates. Thus, conversion rates are of the order: alcohols and glycol ethers >aldehydes, ketones, and terpene hydrocarbons >aromatic and alkane hydrocarbons >halogenated aliphatic hydrocarbons [58]. The influence of the pollutant concentration plays an important role during the photo-oxidation process and different pollutant concentrations lead to different reaction rates. Generally, this influence is significantly higher at lower concentration ranges. For higher VOC concentrations, the number of VOC molecules that can be adsorbed on the photocatalyst surface, and subsequently oxidized, increases, thereby boosting the reaction kinetics. However, the reactive species + active sites/pollutant ratio decreases and consequently, more VOCs can leave the reactor without undergoing degradation. In addition, the high amount of intermediates produced during the photocatalytic process can reduce mineralization and/or occupy part of the active sites, impeding the oxidation progress [36]. The relationship between the pollutant concentration and reaction rate follows the Langmuir-Hinshelwood model, which includes the reaction occurring between both reactants at their adsorption equilibrium: R ¼ kθR θO2 ,ads

(4.56)

where, θR and θO2, ads are the fractions of R and oxygen adsorbed onto the photocatalysts surface, k is the reaction constant [35]. For monolayer adsorption, the fractional coverage of R is defined as: θR ¼ q=qs ¼ m=mmax ¼ K ½R=1 + K ½R

(4.57)

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245

where, K is the adsorption equilibrium coefficient, m is the amount adsorbed per unit weight or volume of photocatalyst, mmax is the maximum amount adsorbed, q and qs are the total numbers of adsorbed and adsorption sites per unit weight or volume of the semiconductor nanoparticles, respectively [35]. The details of the models used to describe the kinetics of these photocatalytic reactions are described in Section 2.1.2. The water adsorbed onto the photocatalyst surface can react with a hole to generate an OH radical, which in turn oxidizes the pollutants. Therefore, water vapor plays a significant role in these photodegradation reactions. In the absence of water, photodegradation of some chemical compounds (formaldehyde and toluene) can be significantly delayed and total mineralization to CO2 is inhibited. The undue water vapor on the photocatalyst can lead to a reduction in the reaction rate because water molecules can occupy the active sites of the reagents on the surface and the hydrophilic effect at the surface prevails over the oxidizing effect [59]. Increasing the light intensity, or using a light source with a shorter wavelength, enhances the reaction rate and photocatalytic efficiency [36]. Another parameter that influences the photocatalytic efficiency is the flow rate of contaminated air. Most works have reported flow rates between 1.0 and 5.0 L/min [57]. Generally, as the air flow rate increases, the time spent by pollutant molecules inside the reactor decreases, thereby leading to a reduction in pollution adsorption and lower degradation efficiency. Conversely, a higher air flow rate enhances the mass transfer coefficient between the air and photocatalyst surface, resulting in higher degradation efficiency [60]. During the adsorption process, pollutant coverage of the photocatalyst surface decreases progressively with increasing temperature [35]. The optimum temperature for the photooxidation process ranges between 20°C and 80°C. Another parameter that influences the photocatalytic efficiency is the amount of photocatalyst. Due to the increased weight, the photocatalyst displays more porosity and a higher surface area. These factors provide a larger specific surface area and cause higher adsorption, leading to higher degradation efficiency. However, this increase in efficiency reaches a maximum limit that depends on the mass transfer and light penetration limitations, after which, an increase in the mass of the available photocatalyst does not lead to an increase in photo-oxidation efficiency [35]. l

4.3.4

Immobilization techniques

For easy recycling, the photocatalyst powder should be immobilized onto a solid support, such as glass, fiber, or stainless steel. The photocatalyst surface density on the support is an important factor because it directly alters the available number of active sites as well as the surface area. Increasing the photocatalyst layer thickness results in lower adsorption competition between the reactants, a higher surface area, and a higher degree of pollutant removal and mineralization [36]. The ideal support should possess several properties including a porous structure, stability under irradiation, high surface area, and high adsorptive affinity toward the pollutant. The photocatalyst can be immobilized on a support by dip-coating, spraying, sol-gel, thermal, and chemical vapor deposition methods. The dip-coating method is a highly popular method for

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Metal Oxide-Based Photocatalysis

deposition of very thin films from a photocatalyst suspension, by controlling the dip-coating time. This method has been widely used due to its low cost and flexibility with a wide range of shapes. To prevent the agglomeration of nanoparticle photocatalysts as well as to increase the surface area, photocatalysts are first dispersed in a solvent (silane, ethanol, and methanol) and subsequently ultrasonicated. Spraying is another technique in which small droplets of the photocatalyst suspension are sprayed onto the surface of the support and the excess photocatalyst suspension is removed by air spraying after a set period of time [3]. In the thermal method, the photocatalyst is sprinkled onto the support and subjected to thermal treatment. Finally, in the chemical vapor deposition method, the support is exposed to a gaseous atmosphere of the photocatalyst precursor at high temperature, which decomposes at the support [10].

4.3.5

Photoreactor design for photocatalytic air purification

A photocatalytic reactor for air purification should comprise (1) a high specific surface area of photocatalyst, (2) a light source irradiating directly on the photocatalytic surface, (3) high mass transfer, (4) a low pressure drop, and (5) a long residential time. Generally, photocatalytic air purification is carried out in plate, annular, honeycomb monolith, and fluidized bed reactors (Fig. 4.13) [2,10,35,61]. The plate reactor (Fig. 4.13A) is the simplest type of photoreactor used for photooxidation of pollutants, in which the source of irradiation can be located inside or outside the reactor. For plate reactors with an inner source of irradiation, a lamp is placed at the upper part of the reactor [10]. In the second type, photoreactors are equipped with a quartz window that allows the light to pass from the lamp into the photocatalyst sample. These reactors are made of stainless steel, plexiglass, or a polycarbonate material resistant to UV light, and can be a square or rectangular box [62]. The annular reactors are composed of cylindrical tubes made of Pyrex, with the photocatalyst coated on the inner wall of the outer cylindrical tubes (Fig. 4.13B). The light source is located at the central part of the cylindrical tube. The airflow is provided along the axial direction through the annulus between the lamp and the tube. The annular and plate reactors are not designed for high air throughput and are therefore not suitable for commercial application. Honeycomb monolith reactors comprise a number of channels of circular or square cross section (Fig. 4.13C). The photocatalyst nanoparticles are coated onto the inner walls of the channels and the irradiation source is located in front of the channels. Fluidized bed reactors comprise a transparent container filled with a photocatalyst bed with the light source located outside the reactor. The treated airstreams pass through the container filled with the photocatalyst bed (Fig. 4.13D) and makes good contact with the photocatalyst [62]. This type of reactor allows high throughput and a low pressure drop, however, it is difficult to control and tends to suffer from photocatalyst losses in the entrained air. Photocatalytic reactions in the gas phase were mainly investigated in reactors with average volumes ranging from

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247

Fig. 4.13 The main types of gas-phase photocatalytic reactors: (A) plate reactor, (B) annular reactor, (C) honeycomb monolith reactor, and (D) fluidized bed reactor. Based on H. Ren, P. Koshy, W.-F. Chen, S. Qi, C.C. Sorrell, Photocatalytic materials and technologies for air purification, J. Hazard. Mater. 325 (2017) 340–366; Y. Boyjoo, H. Sun, J. Liu, V.K. Pareek, S. Wang, A review on photocatalysis for air treatment: from catalyst development to reactor design, Chem. Eng. J. 310 (Part 2) (2017) 537–559; J. Mo, Y. Zhang, Q. Xu, J.J. Lamson, R. Zhao, Photocatalytic purification of volatile organic compounds in indoor air: a literature review, Atmos. Environ. 43 (2009) 2229–2246; T. Ochiai, A. Fujishima, Photoelectrochemical properties of TiO2 photocatalyst and its applications for environmental purification, J. Photochem. Photobiol. C Photochem. Rev. 13 (2012) 247–262.

5 cm3 to 120 dm3. Other types of photocatalytic reactors used for photocatalytic degradation, such as foam packed-bed, multiannular, and multiplate reactors [10], have also been designed. Nowadays, indoor air purification products are commercially available and usually comprise a fan or air pump, a particulate filter or electrostatic precipitator, a light source, photocatalyst, and an optional activated carbon filter and ionizer generator [3,63] (Fig. 4.14). Commonly, the photocatalyst is fixed on a substrate, either in a honeycomb-type construction to reduce pressure drop, or in a three-dimensional porous structure [61,63]. All commercial air purification devices use UV light sources, particularly UVA.

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Metal Oxide-Based Photocatalysis

Fig. 4.14 Schematic diagram of a photocatalytic indoor air purification device: (A) fan, (B) particulate filter, (C) photocatalyst, (D) light source, (E) activated carbon filter, and (F) ionizer. Changes may vary from product to product. Based on T. Ochiai, A. Fujishima, Photoelectrochemical properties of TiO2 photocatalyst and its applications for environmental purification, J. Photochem. Photobiol. C Photochem. Rev. 13 (2012) 247–262; Y. Paz, Application of TiO2 photocatalysis for air treatment: patents’ overview, Appl. Catal. B Environ. 99 (2010) 448–460.

4.3.6

Conclusions and future prospects

Air purification is becoming increasingly important because dangerous levels of air pollution are becoming common in urban areas. The photocatalytic oxidation process is strongly dependent on the air flow rate/residence time, type and concentration of pollutant, humidity, and light source and intensity. A higher light intensity, shorter light source wavelength, lower pollution concentrations, and higher residence times favor the photocatalytic removal of pollutants. However, there is limited knowledge on the deactivation mechanisms under different operating conditions, regeneration methods, generation of intermediate products associated with health risks, and long-term performance of photocatalytic systems in real-life situations. Visible light absorbing materials are predicted to be the most important components in wide-scale technology. Additionally, solar-driven or low-powered UV lamp-irradiated (LEDs) photoreactors are crucial for broader-scale application of photocatalytic processes. The LEDs are low-power and low-cost irradiation sources that reduce power consumption and the cost of photocatalytic processes.

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[39] S. Rodrigues, K.T. Ranjit, S. Uma, I.N. Martyanov, K.J. Klabunde, Visible-light photooxidation of trichloroethylene by Cr–Al-MCM-41, J. Catal. 230 (2005) 158–165. [40] H.-H. Ou, S.-L. Lo, Photocatalysis of gaseous trichloroethylene (TCE) over TiO2: the effect of oxygen and relative humidity on the generation of dichloroacetyl chloride (DCAC) and phosgene, J. Hazard. Mater. 146 (2007) 302–308. [41] A.M. Ferrari-Lima, R.P. de Souza, S.S. Mendes, R.G. Marques, M.L. Gimenes, N.R. C. Fernandes-Machado, Photodegradation of benzene, toluene and xylenes under visible light applying N-doped mixed TiO2 and ZnO catalysts, Catal Today 241 (Part A) (2015) 40–46. [42] V. Augugliaro, S. Coluccia, V. Loddo, L. Marchese, G. Martra, L. Palmisano, M. Schiavello, Photocatalytic oxidation of gaseous toluene on anatase TiO2 catalyst: mechanistic aspects and FT-IR investigation, Appl. Catal. Environ. 20 (1999) 15–27. [43] W. Wang, Y. Ku, Photocatalytic degradation of gaseous benzene in air streams by using an optical fiber photoreactor, J. Photochem. Photobiol. A Chem. 159 (2003) 47–59. [44] A. Linsebigler, G. Lu, J.T. Yates, CO Photooxidation on TiO2(110), J. Phys. Chem. 100 (1996) 6631–6636. [45] A. Nishimura, T. Hisada, M. Hirota, M. Kubota, E. Hu, Using TiO2 photocatalyst with adsorbent to oxidize carbon monoxide in rich hydrogen, Catal Today 158 (2010) 296–304. [46] H. Nishikawa, Y. Takahara, Adsorption and photocatalytic decomposition of odor compounds containing sulfur using TiO2/SiO2 bead, J. Mol. Catal. A Chem. 172 (2001) 247–251. [47] A.P. Bhirud, S.D. Sathaye, R.P. Waichal, J.D. Ambekar, C.-J. Park, B.B. Kale, In-situ preparation of N-TiO2/graphene nanocomposite and its enhanced photocatalytic hydrogen production by H2S splitting under solar light, Nanoscale 7 (2015) 5023–5034. [48] C. Guillard, T.-H. Bui, C. Felix, V. Moules, B. Lina, P. Lejeune, Microbiological disinfection of water and air by photocatalysis, C. R. Chim. 11 (2008) 107–113. [49] A. Vohra, D.Y. Goswami, D.A. Deshpande, S.S. Block, Enhanced photocatalytic disinfection of indoor air, Appl. Catal. Environ. 64 (2006) 57–65. [50] K. Sunada, T. Watanabe, K. Hashimoto, Studies on photokilling of bacteria on TiO2 thin film, J. Photochem. Photobiol. A Chem. 156 (2003) 227–233. [51] O.K. Dalrymple, E. Stefanakos, M.A. Trotz, D.Y. Goswami, A review of the mechanisms and modeling of photocatalytic disinfection, Appl. Catal. Environ. 98 (2010) 27–38. [52] A.-G. Rinco´n, C. Pulgarin, Bactericidal action of illuminated TiO2 on pure Escherichia coli and natural bacterial consortia: post-irradiation events in the dark and assessment of the effective disinfection time, Appl. Catal. Environ. 49 (2004) 99–112. [53] L. Zan, W. Fa, T. Peng, Z.-K. Gong, Photocatalysis effect of nanometer TiO2 and TiO2coated ceramic plate on Hepatitis B virus, J. Photochem. Photobiol. B Biol. 86 (2007) 165–169. [54] R. Nakano, H. Ishiguro, Y. Yao, J. Kajioka, A. Fujishima, K. Sunada, M. Minoshima, K. Hashimoto, Y. Kubota, Photocatalytic inactivation of influenza virus by titanium dioxide thin film, Photochem. Photobiol. Sci. 11 (2012) 1293–1298. [55] H.A. Foster, I.B. Ditta, S. Varghese, A. Steele, Photocatalytic disinfection using titanium dioxide: spectrum and mechanism of antimicrobial activity, Appl. Microbiol. Biotechnol. 90 (2011) 1847–1868. [56] A. Kubacka, M.S. Diez, D. Rojo, R. Bargiela, S. Ciordia, I. Zapico, J.P. Albar, C. Barbas, V.A.P. Martins dos Santos, M. Ferna´ndez-Garcı´a, M. Ferrer, Understanding the antimicrobial mechanism of TiO2-based nanocomposite films in a pathogenic bacterium, Sci. Rep. 4 (2014) 4134. ˆ ngelo, L. Andrade, L.M. Madeira, A. Mendes, An overview of photocatalysis phenom[57] J. A ena applied to NOx abatement, J. Environ. Manage. 129 (2013) 522–539.

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4.4 Hydrocarbon generation (CO2 reduction) Anthropogenic CO2 emissions, mostly from combustion of fossil fuels, have been widely accepted as the main source of global warming. The reduction of CO2 has recently been regarded as an important research area in chemical technology, not only for solving problems resulting from environmental pollution but also for finding ways to maintain carbon resources that are being depleted by fossil fuel combustion [1]. Carbon dioxide is a very stable molecule and is the final combustion product of all carbonaceous fuels [2]. The high fuel consumption in modern society will lead to an energy crisis if suitable alternative energy sources cannot be found. One possible solution is artificial photosynthesis, whereby CO2 is recycled into useful chemicals and energetic organic fuels (methane and methanol) using water (hydrogen source) and solar energy (driving energy source) [3]. Thus, photocatalytic CO2 conversion offers a promising way for low cost, clean, and environmentally friendly production of fuels by solar energy under relatively mild conditions with lower energy input.

4.4.1

Fundamental aspects for photocatalytic reduction of CO2

CO2 photocatalytic conversion into a variety of useful hydrocarbons, including CO, CH3OH, HCHO, CH4, and HCOOH, can be realized in the liquid or gas phase [2]. The generated electrons and holes migrate to the surface of the semiconductor nanoparticles or a co-catalyst loaded on the surface of the semiconductor. A large fraction of electron-hole pairs recombine together with the resultant energy released as

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Fig. 4.15 Schematic illustration of the basic mechanism of photocatalytic reduction of CO2 with H2O on a semiconductor photocatalyst by suitable redox co-catalysts. Based on K. Li, X. An, K.H. Park, M. Khraisheh, J. Tang, A critical review of CO2 photoconversion: catalysts and reactors, Catal. Today 224 (2014) 3–12; M. Tahir, N.S. Amin, Advances in visible light responsive titanium oxide-based photocatalysts for CO2 conversion to hydrocarbon fuels, Energy Conv. Manag. 76 (2013) 194–214.

heat and photons [4]. In the last step, the photogenerated electrons reduce CO2 into hydrocarbons (fuel), while the holes oxidize H2O into O2 (Fig. 4.15). An optimal CO2 photoreduction system usually requires: (1) the redox potential of the photogenerated VB hole to be sufficiently positive for the hole to act as an electron acceptor; (2) the redox potential of the photogenerated CB electron to be more negative than that of the CO2/reduced product redox couple (Fig. 4.16); (3) reactants, such as CO2 and CO3 2
, that can be adsorbed onto the catalyst; and (4) a photocatalyst that is not prone to corrosion and does not afford any toxic byproducts [5]. The major disadvantage of this process is the low efficiency of the CO2 photocatalytic reduction process, ascribed mainly to the low visible light absorption of the semiconductors. Fig. 4.16 displays the band-edge positions as well as VB and CB potentials of the photocatalysts (oxides and other semiconductors) used in the photoconversion of CO2. Thermodynamically, the photocatalysts, which can catalyze the reduction of carbon dioxide with water, should possess a CB edge higher or more negative than the redox potential of the reaction. On the other hand, the VB edge should be lower or more positive than the redox potential for the oxidation of H2O to O2 (0.82 V) [4]. Notably, the photogenerated electrons in the CB edge of all the candidate semiconductors do not possess enough driving force to carry out the one-electron reduction of CO2 to CO2 
.

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Fig. 4.16 Schematic representation of valence and conduction band potentials as well as bandgap energies of selected semiconductor photocatalysts relative to the redox potentials at pH 7 of compounds involved in CO2 reduction. Based on S. Xie, Q. Zhang, G. Liu, Y. Wang, Photocatalytic and photoelectrocatalytic reduction of CO2 using heterogeneous catalysts with controlled nanostructures, Chem. Commun. 52 (2016) 35–59; X. Chang, T. Wang, J. Gong, CO2 photo-reduction: insights into CO2 activation and reaction on surfaces of photocatalysts, Energy Environ. Sci. 9 (2016) 2177–2196; O. Ola, M.M. Maroto-Valer, Review of material design and reactor engineering on TiO2 photocatalysis for CO2 reduction, J. Photochem. Photobiol. C Photochem. Rev. 24 (2015) 16–42.

The reduction of CO2 with H2O into hydrocarbon fuels, such as CH4 and CH3OH, is an uphill reaction with a higher positive change in Gibbs free energy. This makes CO2 reduction highly challenging [6]. For example: CO2 + 2H2 O ! CH3 OH + 3=2O2 △G0 ¼ + 702:2 kJ=mol CO2 + 2H2 O ! CH4 + 2O2 △G0 ¼ + 818:3 kJ=mol







(4.58) (4.59)

The conversion process to potential fuels is endothermic and therefore requires very high amounts of energy. The high input energy afforded by incident light is used to overcome these reaction barriers to break C]O bonds and form CdH bonds [7]. The free energy change for the conversion of one H2O molecule into H2 and ½O2 under standard conditions is 237.2 kJ/mol. Possible reactions and various products related to CO2 reduction and the corresponding standard redox potentials acquired from thermodynamic data are presented in Table 4.4. The final oxidation state of the carbon atom

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255

Redox potentials for CO2 reduction with reference to NHE at pH 7 [3,6,7]

Table 4.4

Product name of product O2 and H+ CO2 
HCO2H CO HCHO H2 CH3OH CH4

Oxygen and hydrogen ion Anion radical carbon dioxide Formic acid Carbon monoxide Formaldehyde Hydrogen Methanol Methane

Reactions

E0 (V)

H2O + 2h+ ! ½O2 + 2H+

+0.82

CO2 + e
! CO2 



1.9

CO2 + 2H+ + 2e
! HCO2H CO2 + 2H+ + 2e
! CO + H2O CO2 + 2H+ + 4e
! HCHO + H2O 2H+ + 2e
! H2 CO2 + 6H+ + 6e
! CH3OH + H2O CO2 + 8H+ + 8e
! CH4 + 2H2O


0.61
0.53
0.48
0.41
0.38
0.24

in the products is determined by the specific reaction pathway and the number and rates of electrons transferred between the generated carriers and the species in solution or gas phase in the reaction system [7]. The mechanism of carbon dioxide reduction must have multiple stages during which electrons and protons are transferred. The reduction potential is too high (
1.9 V) for single electron transfer to CO2 to produce CO2 
; this is the primary obstacle for CO2 photoreduction [3]. Several competitive CO2 reduction pathways exist, however, more importantly, water reduction takes place as a competitive reaction. Water is also oxidized as a redox couple [3]. The generation of methane and methanol (8e
and 6e
reduction, respectively) is thermodynamically more feasible because of their less negative redox potentials [4]. The H2 reduction potential level is more positive than the reactions forming HCOOH (2e
reaction), CO (2e
reaction), and HCHO (4e
reaction), but more negative than those forming CH3OH (6e
reaction) and CH4 (8e
reaction). Therefore, the photocatalyst, which aids carbon dioxide reduction, may also catalyze the reduction of H2O to H2. Water reduction consumes H+ and the rate of CO2 reduction is suppressed. Thus, the reduction of H2O to H2 is kinetically more favorable and competes strongly with CO2 reduction. In addition, if methane is the main target, the formation of CH4 needs 8e
, which would be more difficult than the formation of hydrogen from water, a 2e
reduction. In summary, the reduction of carbon dioxide shows great potential, not only for solving problems resulting from environmental pollution, but also for finding ways to maintain carbon resources. However, at present one of its greatest limitations is its low conversion efficiency due to (1) low solubility of CO2 in H2O, (2) competition with water reduction to hydrogen, (3) mismatching between the absorption ability of the photocatalysts and the solar spectrum, and (4) back reactions during CO2 reduction [8]. Therefore, attaining high selectivity and satisfactory production yields is challenging [3].

256

4.4.2

Metal Oxide-Based Photocatalysis

Photocatalyst design for photocatalytic CO2 reduction

Many photocatalyst structural parameters can affect the kinetics of processes including photoabsorption, electron-hole separation, and surface reaction [4]. It is very difficult to compare the photoactivity of different nanomaterials investigated by different research groups. However, it is generally agreed that several aspects can be studied to increase the photoconversion of CO2. In order to be efficient, the semiconductor nanoparticles must exhibit several characteristic features including high crystallinity, small particle size, and a suitable bandgap (Table 4.5). The optimal photocatalyst for photocatalytic CO2 reduction should absorb efficiently over the whole wavelength range of the solar spectrum. Thus, various surface modification techniques, including metal deposition, heterojunction construction, and doping with metals and nonmetals, were investigated. The classification of photocatalysts (oxides and other semiconductors) used for visible and UV light CO2 photoreduction are summarized in Fig. 4.17. The efficient photoconversion of carbon dioxide and water vapor into hydrocarbons under visible light was mostly achieved by (1) doping (N [10–14], B [15], I [16], Cr [17], and N-C [18]), (2) surface modification (Cu [14,19], Pt [10,20], Pd [20], Au [10,20–22], Ag [10,23,24], Rh [21], and NiO [25,26]), and (3) sensitization (CuO [27,28], Cu2O [27], Co3O4 [25], g-C3N4 [29], BiOI [30], CdS [31], Bi2S3 [32], CdSe [33], AgBr [34], ZnTe [35], and dye [36,37]). Doping delays fast charge recombination and introduces defect states (interband states and mid-gap levels). Narrowing of the bandgaps after doping the semiconductor with nonmetals is ascribed to the mixing of dopant p states with O 2p states to form a new VB [8]. On the other hand, photocatalyst surface modification can greatly enhance carbon dioxide adsorption and activation as well as lower the barriers for subsequent CO2 reduction processes [6]. In addition, the interface between the semiconductors and co-catalyst is also critical for the transportation and separation of charge carriers, Table 4.5

Desirable properties of semiconductor nanoparticles [9]

How to achieve the property Suitable bandgap High crystallinity Small nanoparticle size

Presence of co-catalysts

Property

Effect

Light absorption High mobility of charge carriers High surface area High surface to volume ratio Sites for CO2 adsorption Long lifetime of charge separation Higher mobility of charges

Enhance photoactivity More efficient charge separation Higher adsorption rate Higher light absorption Possibility of chemical reactions Higher photocatalytic activity Higher absorption

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257

Fig. 4.17 Summary of photocatalysts used in photocatalytic CO2 reduction under UV and visible light irradiation.

while the co-catalyst affects the selectivity of the reduction products [8]. Doping and/ or surface modification are effective in increasing the spectral response of the photocatalysts by narrowing the electronic properties and altering the optical responses [38]. Another effective method to enhance light absorption and charge separation is photosensitization. In this process, electrons produced in the CB of the sensitizer under visible light are transferred to the CB of a wide-band semiconductor enabling it to function as a catalyst under visible light [39]. Different types of sensitizers, such as quantum dots (QDs), coupling semiconductors, dyes, and novel sensitizers, were investigated. The visible light response of the semiconductors can be easily adjusted in the presence of QDs. Moreover, QDs can utilize hot electrons to generate multiple charge carriers when excited by a single high energetic photon, thereby leading to an increased amount of charge carriers [40]. To date, CdSe QDs/Pt/TiO2 [33], PbS QDs/ Cu/TiO2 [41], and carbon dots/Cu2O [42] systems have been employed in the photoconversion of CO2 into light hydrocarbons. Similarly, dyes can inject electrons into the CB of the semiconductor.

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Metal Oxide-Based Photocatalysis

Electron transfer efficiency between the dye sensitizer and the photocatalysts depends on many factors: (1) the dye visible light adsorption efficiency; (2) the LUMO level of the dye and the CB edge of the semiconductor; and (3) the population of low-lying ligand fields in the presence of adsorbents, such as water vapor and oxygen [43]. The use of dyes, such as [Ru(Bpy)3]2+ [tris(2,20 -bipyridyl) ruthenium(II) chloride)] [36], chrysoidine G [37], tolylene-2,4-diisocyanate [37], rhodamine B, and [Cu (bpy)2]+ [44], in CO2 photoreduction, has been investigated. Many types of semiconductor photocatalysts, such as AgBr-TiO2 [34], Cu2O/SiC [45], Bi2S3/ CdS [35], and Ag3PO4/g-C3N4 [46], have been designed and have exhibited enhanced carbon dioxide photoreduction compared to their single components [47]. Among the various studied sensitizers, enzymes [carbon monoxide dehydrogenase (CODH)] were reported as efficient sensitizers for photocatalysts [48,49]. These enzymes are homodimeric and comprise buried active sites in each [4Fe-4S] cluster. Thus, electrons can enter or leave the enzyme cluster freely [38]. In summary, numerous new techniques and novel sensitizers have been investigated, however, the yields afforded by currently available photocatalysts are still not sufficient for practical application. Nowadays, studies are focused on enhancing the properties of wide bandgaps in photocatalysts by utilizing visible solar irradiation, increasing e
/h+ pair transport, and reducing the recombination rate.

4.4.3

Important factors for photocatalytic reduction of CO2

It is very difficult to compare the efficiency of CO2 reduction for photocatalysts studied by different research groups. However, it is generally agreed that several reaction conditions can increase the photoconversion of CO2. The temperature, pressure, H2/CO2 ratio, pH, and sacrificial agent are all important factors. In an aqueous reaction system, a photocatalyst is dispersed in water under CO2 atmosphere. The gas phase reaction is more widely applied and the concentrations of gaseous CO2/H2O, as well as the structures of the semiconductors, significantly affect both activity and selectivity [43]. Generally, CO2 reactivity increases with an increase in H2O/CO2 ratio and an excessive amount of H2O suppresses the reaction [43,50,51]. The optimized molar H2O/CO2 ratio for the photoconversion of CO2 was established as 5.0 [52]. The solubility of carbon dioxide is relatively low in water (2 g/L), indicating that it is thermodynamically more favorable to reduce water than CO2, thus, an alkali (NaOH, Na2CO3, and NaHCO3) acts as a solute to enhance CO2 solubility [53,54]. A holetrapping sacrificial reagent can consume photogenerated holes and lengthen the lifetime of photogenerated electrons, thereby leading to an increase in reaction efficiency [55]. When water is used as reductant, the amount of hydrocarbon products is low. Triethylamine, isopropyl alcohol, triethanolamine, and dimethylformamide are used in the aqueous phase as sacrificial electron donors to improve the CO2 photoconversion efficiency [56]. On the other hand, methanol [57,58], methane [59], hydrogen sulfide [5,60], and hydrogen [61,62] have been extensively studied as potential sacrificial electron donors in the gas phase. The pH value also plays an important role

Application of metal oxide-based photocatalysis

259

in determining the preferred reaction pathway and afforded hydrocarbons as well as CO2 solubility in the aqueous system [6]. The photoreduction of CO2 is more favorable under basic conditions and, during its reduction, carbonates may also be produced. The lower pH and higher proton concentration are slower than the theoretical reduction potential of CO2 and enhance H2 production [63]. Generally, photoreduction of CO2 is tested at room temperature because solubility decreases with increasing temperature [53]. However, pressure and temperature have significant influence over gas phase reactions because an increase in temperature and pressure improves the concentration of the reactants as well as the collision frequency and diffusion rate, resulting in an increased reaction rate [5,51,53]. The optimum temperature was determined as 293–353 K [64]. Increasing the CO2 pressure also increases the CO2 concentration in aqueous solvent and improves product selectivity. In addition, in the gas phase, increasing the pressure of CO2 and H2O can also enhance the combination of the reactant with the active sites of the photocatalysts [5,43]. Ethylene and methane were not produced at ambient CO2 pressure but were detected under high CO2 pressures (2.5 MPa) [43].

4.4.4

Photoreactors for CO2 reduction

Generally, CO2 photoconversion is carried out using two systems: (1) two phases (gassolid or liquid-solid photocatalysts) and (2) three phases (gas-liquid-solid photocatalysts) [53]. The photoreactor must have uniform light distribution throughout the entire system to achieve highly efficient CO2 reduction. The configuration of the particles in a photoreactor is an important factor that can influence the overall photoreduction of CO2. Photocatalysts can be generally tested in a fluidized or fixed bed reactor (Table 4.6). In the fluidized bed reactor the photocatalyst particles are suspended in a fluid-like state to ensure highly dispersed particles and are agitated by magnetic stirring to prevent photocatalyst sedimentation [8]. In this type of reactor, the source of irradiation can be located inside or outside the photoreactor (Fig. 4.18A and B). The carbon dioxide is bubbled via the reactor to maintain good interaction in the reagent mixture. In addition, the photocatalyst, lamp, source efficiency, amount of water, shape and dimensions, as well as the design of the reactor, may differ in various systems [8,38]. Fluidized bed reactors, with high photocatalyst loading, simple construction, well-mixed photocatalyst suspensions, and violent solid motion, also provide high mass and heat transfer rates. The separation of photocatalyst particles from the reaction mixture is a major disadvantage and increases the operational cost. Furthermore, the penetration depth of the irradiation into the reaction medium can also be limited due to strong light absorption and irradiation scattering by the mixture of reactants, therefore, a large proportion of catalyst surface area might remain inactive [53,63]. Moreover, fluidized bed reactors are eroded by abrasion of the photocatalyst particles and attrition of the catalyst [5]. For fixed bed reactors the photocatalyst particles are immobilized onto supports [e.g., plate (Fig. 4.18C)], fibers (Fig. 4.18D), and monoliths (Fig. 4.18E)] and placed inside the photoreactor [53].

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Metal Oxide-Based Photocatalysis

Table 4.6 The advantages and disadvantages of fluidized and fixed bed reactors [53,63] Type of reactors Fluidized bed

Fixed bed

Advantages

Disadvantages

(1) Simple construction, (2) Well-mixed photocatalyst suspension, (3) High photocatalyst loading, (4) Efficient mass and heat transfer rates, (5) Feasibility for large capacity, (6) The temperature gradients inside the beds can be reduced through vigorous movements caused by the solid passing through the fluids (7) Potentially lower capital needed for a large-scale (1) Continuous operation,

(1) The separation of catalyst particles is difficult, (2) Additional cost during separation of fine catalyst particles, (3) Low light utilization efficiency due to absorption and scattering of the light by the reaction medium, (4) Erosion by abrasion of particles and attrition catalyst.

(2) Photocatalyst separation not required, (3) Configuration can be modified, (4) Photocatalyst in powder or pellets could be used, (5) Low pressure drop, (6) Suitable for reaction at normal temperature and pressure, (7) Gas-photocatalyst or liquidphotocatalyst phases.

(1) Low surface area to volume ratios, (2) Mass transfer limitation, (3) Photocatalyst fouling or catalyst washout, (4) In continuous flow reactors the yield rates of hydrocarbons can be difficult to measure, (5) Spatial distance between light and photocatalysts.

The advantage of fixed-bed systems is the low pressure drop, which allows them to operate at reduced operating costs [5]. The photoconversion efficiency in this system is limited by a lower contact area exposed to light and hence, lower conversion and yield rates. Moreover, in continuous flow reactors the yield rates of the afforded hydrocarbons can be difficult to measure. Among the available fixed-bed designs, optical fiber and monolith reactors are the most commonly used due to their larger volumes. The optical fiber reactor can deliver light efficiently and uniformly and exhibits higher conversion and yield rates. However, this system displays lower adhesion strength and a relatively low surface area and only 20%–30% of the reactor volume is available for photoreaction [63]. The monolith reactor exhibits high feed rates, high conversion and selectivity, as well as higher volume to surface area ratios. Moreover, the monolith can be irradiated from the front and back [63]. The drawback of these reactors is a low light efficiency due to the opacity of the monolith channels,

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261

Fig. 4.18 Main types of photocatalytic reactors for CO2 photoconversion: fluidized bed reactors with source of irradiation located (A) inside and (B) outside the reactor; fixed bed (C) plate, (D) optical-fiber, and (E) monolith photoreactors. Based on K. Li, X. An, K.H. Park, M. Khraisheh, J. Tang, A critical review of CO2 photoconversion: Catalysts and reactors, Catal. Today, 224 (2014) 3–12; M. Tahir, N.S. Amin, Advances in visible light responsive titanium oxide-based photocatalysts for CO2 conversion to hydrocarbon fuels, Energy Conv. Manag. 76 (2013) 194–214p; O. Ola, M.M. Maroto-Valer, Review of material design and reactor engineering on TiO2 photocatalysis for CO2 reduction, J. Photochem. Photobiol. C Photochem. Rev. 24 (2015) 16–42; G. Liu, N. Hoivik, K. Wang, H. Jakobsen, Engineering TiO2 nanomaterials for CO2 conversion/solar fuels, Solar Energy Mater Solar Cells 105 (2012) 53–68, M. Tahir, N.S. Amin, Recycling of carbon dioxide to renewable fuels by photocatalysis: prospects and challenges, Renew. Sustain. Energy Rev. 25 (2013) 560–579.

moreover, the photocatalysts need to be coated by the dip coating method and only reflector-type light sources can be employed [63]. We therefore conclude that gasphase systems offer more flexibility than fluidized bed reactors.

4.4.5

Conclusions and prospects

CO2 photoconversion reactions not only provide an alternative way to produce sustainable fuels but also convert waste CO2 into valuable chemicals, thereby mitigating environmental pollution due to the greenhouse effect. However, the efficiency of solar-to-chemical energy conversion is too low for commercial application. Moreover, the efficiency of currently available photocatalysts is not sufficient for practical use. The development of novel photocatalysts with high reaction selectivity, high

262

Metal Oxide-Based Photocatalysis

chemical and physical stability, significant activity, and wide spectral response under sunlight irradiation is crucial for broader-scale utilization. In addition, a fundamental understanding of the electron transfer dynamics, reaction pathways, and reaction intermediates is required. Currently, the photoconversion CO2 process has only been investigated at laboratory scale in small reactors with average volumes ranging from 7 to 1180 cm3. The efficiency of CO2 photoconversion can be increased by operating at optimum process conditions and designing photoreactors that can efficiently utilize direct solar irradiation. Further research concerning solar-to-chemical energy conversion should focus on the potential and economics of solar reactors and their design.

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4.5 Photochemical transformation of specific compounds Photocatalytic processes can also be employed for photochemical synthesis of specific compounds. The main difference between photodegradation and phototransformation is that the objective of the former process is compound mineralization, while that of the latter is to afford new molecules with high selectivity and yield via photoreduction and photo-oxidation. During the photodegradation process, the ROS display crucial roles in the mineralization of the organic compounds, usually in the presence of water. However, if the aim of the reaction is to synthesize new molecules, decomposition and random reactions between molecules must be avoided. Thus, choosing suitable solvents (toluene and its derivatives, acetonitrile, water, and ethanol), as well as a neutral atmosphere (argon, nitrogen) can be important to produce new molecules without by-products via photocatalytic synthesis.

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Moreover, this reaction can be carried out in both aqueous (oxidation of alcohols [1]) and gaseous (oxidation of ammonia [2]) phases. Photochemical transformation is a very interesting method of synthesizing new compounds and changing the structure of organic molecules. In this reaction the applied photocatalyst can be active under visible light and thus, sunlight energy can be utilized as the energy source during the reaction. Such transformations are possible for compounds characterized by their ability to absorb irradiation directly or by reaction with a reactive species formed after photoexcitation of the photocatalyst [3]. Metal oxides are one of the most commonly used semiconductor materials in this field. There are three main paths that can be used to afford the products via photochemical transformation with metal oxides as photocatalyst: (1) Direct reaction of the reactant with the photogenerated electrons (photoreduction). (2) Direct reaction of the reactant with the photogenerated holes (photo-oxidation). (3) Reaction of the reactants with intermediates created during photo-oxidation and photoreduction.

Fig. 4.19 demonstrates that the most important stage in all the processes is light absorption by the semiconductor and the subsequent generation of reactive electron-hole pairs. Thus, the reagent molecules can be directly transformed into new compounds with photogenerated electrons (photoreduction) or holes (photooxidation). However, during these reactions the production of other reactive species (radicals, carbocations, carbanions) is also possible. Subsequently, intermediates could react with each other or with substrate molecules to create new molecules. The key to the synthesis of a particular molecule is the appropriate choice of synthetic parameters, especially the type of photocatalyst, reaction conditions (temperature, pH, solvent, and atmosphere), and a modified substation [4,5].

Fig. 4.19 Schematic mechanism of photochemical transformation.

Application of metal oxide-based photocatalysis

267

The most important challenge in the photocatalytic transformation process is to achieve high selectivity, yield, and conversion. According to Yuan et al. [1], most of these parameters can be defined as: Conversion ð%Þ ¼ Yield ð%Þ ¼

Ci
Cf  100 Ci

Cp  100 Ci

Selectivity ð%Þ ¼ Cp ðCi
Cf Þ  100

(4.60)

(4.61) (4.62)

where Ci is the initial concentration, Cr is the concentration of the reactant after photoreaction, Cf is the concentration of the product after photoreaction. Table 4.7 presents examples of chemical reactions with different photocatalysts and includes selectivity and conversion values for each reaction. One of the aims of studying phototransformation reactions is to reduce the cost of production of new compounds and to create more environmentally friendly synthetic processes. Therefore, many studies focus on the creation of a good photocatalyst that works under visible light. This requires photocatalyst modification and optimization of the reaction conditions.

4.5.1

Types of photocatalysts

TiO2, ZnO, Al2O3, WO3, Fe3O4, and CeO2 are the most common metal oxides used as photocatalysts in photochemical transformation reactions [14], with TiO2 being the most popular. These solid materials can be used in various types of organic reactions: (1) (2) (3) (4) (5) (6) (7)

oxidation of aromatics—oxidation of benzene [15]. oxidation of alcohols–oxidation of pentanol, benzyl alcohol [1]. epoxidation of alkenes—epoxidaton of 1-decen [16]. addition reactions—synthesis of anilides from nitrobenzenes [17]. polymerization of methyl methacrylate [18]. cyclization of N-methylmaleimide [19]. isomerization of norbornadiene [20].

There are two main approaches to increase the selectivity of the phototransformation process: (1) modification of the photocatalyst and (2) modification of the external conditions of the phototransformation reaction. The main factors influencing the final results of the photochemical transformation are presented in Fig. 4.20. The photocatalyst can be present in different polymorphic forms and this is one of the most important factors that influences the results of the phototransformation reaction. Several reports indicate that the yield and selectivity of the reaction depend on the polymorphic form of TiO2 [4,14,21,22], namely anatase, rutile, or brookite (Section 3.2.1). The main difference between these structures is in their acidity.

268

Table 4.7

Examples of photocatalytic reactions employed for chemical synthesis Substrate

Product

Conversion (%)

Selectivity (%)

Ref.

TiO2 WO3/TiO2 Cu-Pd/TiO2 MoOx/TiO2 Pd/CeO2 Au/CeO2 Au/CeO2 Nb2O5 Cu/Nb2O5 Sn-Pd/Al2O3

4-Methoxybenzyl alcohol Felluric acid Nitrate Cyclohexane p-Fluoro benzyl alcohol Styrene oxide Azobenzene Benzyl alcohol Benzyl alcohol Nitrate

4-Metoxybenzaldeyhde Vanillin Ammonia Benzene p-Fluoro benzaldehyde Styrene Hydroazobenzen Benzaldehyde Benzaldehyde Nitrogen

50 4.3 76 15 13 20 40 19 24 69

58 24.7 78 65 65 88 78 93 98 90

[6] [7] [8] [9] [10] [11] [11] [12] [12] [13]

Data from J. Kou, C. Lu, J. Wang, Y. Chen, Z. Xu, R.S. Varma, Selectivity enhancement in heterogeneous photocatalytic transformations, Chem. Rev. 117 (2017) 1445–1514.

Metal Oxide-Based Photocatalysis

Photocatalyst

Application of metal oxide-based photocatalysis

269

Fig. 4.20 Most important factors that influence the selectivity of the photocatalytic process. Modified from J. Kou, C. Lu, J. Wang, Y. Chen, Z. Xu, R.S. Varma, Selectivity enhancement in heterogeneous photocatalytic transformations, Chem. Rev. 117 (2017) 1445–1514.

Hence, an anatase surface exhibits higher Lewis acidity than a rutile surface. Thus, the synthesis of an aromatic amino compound is more selective for rutile species [23], while the generation of O2 
species is higher for anatase [22]. Moreover, the selectivity of the phototransformation reaction strongly depends on the crystal facet of the photocatalyst. For TiO2 the {001} and {110} facets are the most selective because unselective OH radicals can be avoided [24]. The possibility of the adsorption of molecules at the surface of different types of facets is a key role in phototransformation [24]. Additionally, mixing different forms of a semiconductor could be useful for the phototransformation process. Mixed-valence vanadium oxide is a good example. This compound demonstrates that a structure comprising different types of vanadium oxide species (VO2, V2O5, and V6O13), in which the V4+ and V5+ states are present, is characterized by high selectivity toward the oxidation of 3-hexanol [25]. The particle size of the photocatalyst also affects the phototransformation efficiency. Generally, smaller-sized nanoparticles with the highest surface areas exhibit the highest photocatalytic activity. Thus, in the oxidation of benzyl alcohol and its derivatives, ZnO nanorods display higher selectivity and yields than commercial ZnO (selectivity >90% for ZnO nanorods and Au > Ni > Pd > Cu > Ag. However, where the selectivity of the reaction is the main factor used to compare the metals, they could be in the order: Pt > Ni > Au > Cu > Pd > Ag (Table 4.8). Photocatalyst modification with metals and metal oxides can be very useful because noble metal-modified photocatalysts can absorb irradiation from the visible region (Section 3.2.1.). Such modifications reduce the cost of large-scale production of the chemicals and could be a solution to achieve higher selectivity in phototransformation reactions [27]. Additionally, materials that are not active under UV and visible light after modification are able to react with organic compounds [5]. Moreover, photocatalyst modification is usually necessary to avoid mineralization of some molecules and to selectively favor oxidation to afford new compounds [30]. However, modification by noble metal nanoparticles could be insufficient and materials could still be inactive under visible light.

4.5.2

Role of external conditions

The external conditions of the phototransformation process are as important as the choice of photocatalyst. The most significant factors are the solvent, substances added to the solvent, and substances added to the solvent as co-catalyst. Other important external factors include pH, gas atmosphere, reaction time, amount of photocatalyst, and temperature [31]. A choice of good solvent for phototransformation could influence the final yield, selectivity, and structures of the afforded molecules. Table 4.9

Conditions, conversions, and selectivity of the most important photocatalytic transformations

Reaction

Photocatalyst

Atmosphere

Solvent

Conversion (%)

Selectivity (%)

Ref.

Oxidation of glycerol Oxidation of p-nitro benzyl alcohol Oxidation of benzyl alcohol Oxidation of p-chloro benzyl alcohol Oxidation of cyclohexane Hydrogenation of azobene Reduction of nitrate to nitrogen

TiO2 Pd/CeO2

air O2

Water Trifluorotoluene

20 13

96 65

[32] [10]

Pt-TiO2 CdS/TiO2

Ar O2

Acetonitrile Trifluorotoluene

99 30

99 70

[29] [33]

Ag/TiO2 Au-CeO2 Sn
Pd/ Al2O3 Pt/TiO2 TiO2

O2 Ar air

Water IPA Water

9 33 69

63 79 90

[34] [11] [13]

Ar N2

Ethanol Dodecane

100 90

80 96

[35] [17]

Ag/TiO2

N2

Isopropanol solution

99

97

[36]

Conversion of m-nitrotoluene Addition of 1octanethiol on 1-octene Reduction of nitrobenzene

Application of metal oxide-based photocatalysis

Table 4.9

Data from J. Kou, C. Lu, J. Wang, Y. Chen, Z. Xu, R.S. Varma, Selectivity enhancement in heterogeneous photocatalytic transformations, Chem. Rev. 117 (2017) 1445–1514.

271

272

Metal Oxide-Based Photocatalysis

lists examples of the most important reactions under different atmospheric and solvent conditions The solvent properties that affect these reactions include polarity, viscosity, and acidity. Thus, changing each of these properties could influence the yield and selectivity of the photoreaction. Contrary to photodegradation, the aim of photochemical transformation is to synthesize new molecules. The synthesis of organic compounds very often requires organic solvents because of their limited solubility in water. The presence of water in the reaction solution plays a crucial role in the photocatalytic process because, in an aqueous solution, the water is a source of hydroxyl radicals that are generated during the reaction of water with holes. Thus, because these reactive  OH radicals can favor the unselective reaction, avoiding their generation is often the key step toward achieving a good final product [31]. On the other hand, using organic solvents could dramatically change the reaction mechanism by eliminating OH radicals, and consequently increasing the selectivity of the photoreaction [37]. The absence of ROS, which are present in the water solution, allow the organic compounds to react directly with photoexcited holes and electrons. For example acetonitrile, commonly used as a solvent for organics syntheses, cannot react with photogenerated holes and consequently cannot be a reactant in the photochemical transformation reaction [38]. This example indicates that using organic solvents during photosynthesis could dramatically change the mechanism of the reaction [15]. On the other hand, organic solvent molecules could behave like scavengers and react with the photoexcited electrons and holes. The concentration of organic compounds in the solvent solution also influences the phototransformation reaction because organic reactions are very sensitive toward changes in the viscosity and acidity of the solution. Therefore, molar ratios of substances used as solvents are highly significant in this type of reaction, and manipulation of the solvent composition could be used to produce products with the highest yield and selectivity [39,40]. Moreover, organic solvents exhibit characteristic cut-off wavelengths (Table. 4.10) that could influence the whole phototransformation process.

4.5.3

Synthesis of organic compounds

The application of metal oxides in the photocatalytic process shows potential as a synthetic method for new organic molecules. Photocatalysts play a crucial role during phototransformation, and thus their presence is necessary at the beginning of the reaction. The key step in the general mechanism of synthesis via a photocatalytic route is the creation of radicals or other reactive species that can react with the photoinduced electrons or holes [23,41]. Moreover, the newly created organic radicals can in turn react with hydroxyl radicals to form other substances that are necessary to create new bonds [42]. Generally, in contrast to photodegradation, the synthesis of organic compounds from photochemical processes with applied metal oxides as photocatalysts is

Application of metal oxide-based photocatalysis

Table 4.10

273

Value of cut-off wavelength for different solvents

Solvent

Cut-off wavelength (λ/nm)

Water Acetonitrile n-Hexane Ethanol Methanol Cyclohexane Diethyl ether 1,4-Dioxane Methylene chloride Chloroform Tetrahydrofuran Acetic acid Ethyl acetate Carbon tetrachloride Dimethyl sulfoxide Benzene Toluene Pyridine Acetone

185 190 195 204 205 215 215 230 230 245 245 250 255 265 277 280 285 305 330

Data from Y. Su, N.J.W. Straathof, V. Hessel, T. Noe¨l, Photochemical transformations accelerated in continuous-flow reactors: basic concepts and applications, Chem. Eur. J. 20 (2014) 10562–10589.

carried out in microscale. The new organic substances can be synthesized by photocatalytic reduction and oxidation. Subsequently, the process proceeds via cyclization, addition, alkylation [43], or a combination of some of these processes [44,45]. The most popular photocatalyst applied to these synthetic reactions is TiO2, which can be used in the cyclization of N-methylmaleimide and N,N-dimethylaniline and in the synthesis of L-pipecolinic acid from L-lysine [46]. It is possible to obtain new structures in the intermediate and cyclization reactions [47–50], thereby creating new bonds in molecules, such as CdC [51], CdO [52], CdN [23], N]N [53], and SdO [17]. A possible mechanism for CdC coupling was presented for the photocatalytic synthesis of 2,3-butanediol from ethanol, as shown in Fig. 4.21A. The reaction was carried out in the presence of water with Pt-TiO2 as photocatalyst. A 300 W high pressure lamp was used as the light source. The CdN bond was created from the cyclization of halogen-substituted aniline under UV light in the presence of oxygen, with acetonitrile as solvent and TiO2 as photocatalyst, as shown in Fig. 4.21B. The mechanism for the creation of a NdN bond from the reduction of 2,2-dinitrobiphenyl in isopropanol solution, under UV light and Ar atmosphere, with TiO2 as photocatalyst is illustrated in Fig. 4.21C.

274

Metal Oxide-Based Photocatalysis

Fig. 4.21 Possible mechanism for the creation of a (A) CdC, (B) CdN, and (C) N]N bond during photochemical synthesis. Based on J. Kou, C. Lu, J. Wang, Y. Chen, Z. Xu, R.S. Varma, Selectivity enhancement in heterogeneous photocatalytic transformations, Chem. Rev. 117 (2017) 1445–1514.

4.5.4

Reactors for phototransformation

Continuous-flow and capillary reactors are the most commonly used reactors in photochemical transformation processes [46]. The appropriate design of a reactor could increase the yield of the reaction and decrease the time for photocatalytic transformation to be completed. The combination of a photocatalyst with a reactor with a significantly developed area results in a highly effective photocatalytic reaction. In contrast to photodecomposition, phototransformation is carried out in a microreactor. At laboratory scale, a batch reactor with a photocatalyst suspension is often used [54]. However this poses many problems, such as maintenance of temperature and mass transport, when the batch reactor dimension is increased [55]. These problems are not connected with the microreactors, which allow better penetration of light through the reactor and better control of the phototransformation process. A comparison of the results of the photoalkylation of benzylamine, carried out in both a batch reactor and a flow microreactor, is presented in Table 4.11. Application of flow reactors could reduce the time of the reaction from hours to minutes in contrast to processes in batch [55].

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Comparison of the reaction carried out in a batch reactor and microreactor

Table 4.11 Reactor type

Photocatalyst

Batch reactor Microreactor

Pt-TiO2 TiO2

Source of the light

Yield (%)

Time of reaction

400 W mercury lamp UV LEDs

84

4h

84

6–150 s

Based on Y. Su, N.J.W. Straathof, V. Hessel, T. Noe¨l, Photochemical transformations accelerated in continuous-flow reactors: basic concepts and applications, Chem. Eur. J. 20 (2014) 10562–10589.

Fig. 4.22 Photocatalytic microflow reactor for N-alkylation of amines. Based on D. Cambi
e, C. Bottecchia, N.J.W. Straathof, V. Hessel, T. Noe¨l, Applications of continuous-flow photochemistry in organic synthesis, material science, and water treatment, Chem. Rev. 116 (2016) 10276–10341.

This therefore determines the choice of the reactor and the reaction conditions. The highest reaction rate of photochemical transformation was observed for processes in which microflow reactors were used. The final products were afforded in 70%–80% yield after a short reaction time (1 min). A schematic illustration of a microreactor used for photochemical transformation is presented in Fig. 4.22. The photocatalyst layer is between 300 and 1000 μm thick and 500 μm in height [46,55]. In this type of reactor, the photocatalyst is immobilized on the microreactor features (membranes or channel walls). Finally, the light source also plays a very important role in phototransformation. An emitted light should be characterized by a high photon flux, however, to avoid degradation of products the intensity of the irradiation must not be too high. According to literature, LEDs and mercury lamps could be used as the light source for photocatalytic processes carried out in a microreactor [55]. For systems that employ bare and modified metal oxides, scarce literature on the application of sun energy in phototransformation is available to date.

4.5.5

Scaling-up process and prospects

The photochemical transformation based on a metal oxide photocatalyst plays a crucial role in the synthesis of organic compounds. Manipulations of external conditions as well as photocatalyst structures should afford high selectivity, conversions, and

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yields. Additionally, visible light can be used in the photoreaction to create new molecules, thereby significantly reducing the cost of synthesis of important compounds. The majority of studies have focused on pristine and modified TiO2 application and phototransformation and this semiconductor was characterized with high selectivity and substrate conversion. The most promising material for both photo-oxidation and photoreduction was established as noble metal-modified TiO2. The presence of other metals at the photocatalyst surface or in its structure highly increased the selectivity of the photoreaction, thus, this type of material shows great potential for application in phototransformation reactions. Photochemical synthesis of organic compounds with application of metal oxides as photocatalyst could be very promising to afford new molecules by creation of new bonds, such as CdC, CdN, N]N, and CdO bonds. However, few examples of metal oxide photocatalyst applications in such photoreactions have been reported to date. Generally, applications are limited to simple reactions where the products and substrates have similar formulae. Additionally, phototransformation based on metal oxide photocatalysts are carried out at laboratory scale and information on the development of large-scale phototransformation reactions based on metal oxide photocatalysts is scarce. Therefore, the development of this field of photocatalysis could be rather challenging. Future research should focus on the creation of new, cheap materials that can be active under visible light, achieve high selectivity and yields, and increase the field of metal oxide photocatalyst applications.

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4.6 Medical applications: Application in photodynamic therapy Photodynamic therapy (PDT) is attracting much attention as an alternative treatment for various types of cancer. Compared to conventional cancer therapy, such as chemotherapy, surgery, and radiotherapy, PDT has the benefit of being a minimally invasive localized therapy that allows precise control over dosage and time of treatment [1]. It also promotes fast healing of healthy tissue to reduce long-term morbidity and improve the

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quality of life of the patient [2]. Furthermore, PDT is also widely applied to noncancerous treatments, including viral, bacterial, parasitic, and fungal infections [1]. Photodynamic therapy usually comprises three key components: (1) a photocatalyst, responsible for selectivity of the diseased tissue, (2) a light source to activate the photocatalyst, and (3) tissue oxygen [3]. In the biomedical literature, photocatalysts used in photodynamic therapy are generally termed as photosensitizers [1,4,5]. The combination of these three elements leads to the formation of ROS, subsequently resulting in diseased cell destruction. Metal oxide nanomaterials have the ability to generate ROS in the presence of light irradiation, and therefore they can overcome most of the limitations of classic organic photocatalysts. Importantly, metal oxide nanoparticles are relatively stable with changes in pH and temperature. Moreover, nonbiodegradable metal oxide particle photocatalysts can be used repeatedly. Finally, the surface of metal oxide nanoparticles can be modified with appropriate functional groups to alter its physicochemical properties, thereby improving its biodistribution [6]. However, metal oxide nanoparticles also pose potential risks in biomedical applications. Thus, nanosafety must be addressed and resolved in order to develop nontoxic, biocompatible, and cost-effective photocatalysts based on metal oxide nanoparticles [1].

4.6.1

Phocatalysts

Photocatalysts play an important role in the development and application of modern PDT treatment. An ideal photocatalyst should exhibit a high singlet-to-triplet intersystem crossing efficiency, negligible dark-toxicity, minimized phototoxic damage to normal tissue, minimal self-aggregation tendency, and maximum absorption at long wavelengths [5–7]. Currently, various photocatalysts are being explored for PDT application. These can be classified into two main groups: organic and inorganic photocatalysts including metal oxide nanoparticles [8]. Traditional organic photocatalysts are mainly based on porphyrin, chlorin, and their derivates. However, these exhibit two major limitations, namely, poor solubility in aqueous solution and a small absorption cross-section comparable to that of small-molecule dyes [9]. An alternative approach is the use of inorganic metal oxides that, in a photodynamic process, can be divided into active and passive nanoparticles [1]. Thus, active metal oxide nanoparticles act as direct photocatalytic agents, whereas passive metal oxide nanoparticles are used as carriers for molecular photocatalysts. The photocatalytic mechanism in the presence of metal oxide nanoparticles is reviewed in Chapter 2. The design of both active and passive semiconductor metal oxide nanoparticles for PDT must consider various aspects, from the structure and size of the nanoparticles to postcoating of the nanoparticles to extend their lifetime in vivo.

4.6.1.1 Active metal oxide nanoparticles The most common metal oxide nanoparticles used as photocatalysts in PDT are TiO2, ZnO, and their composites. Table 4.12 displays various metal oxide particles used as photocatalysts in photodynamic therapy [10–16]. Among these, TiO2 is the most

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281

Overview of metal oxide nanoparticles used as direct photocatalyst in PDT

Table 4.12

Metal oxide photocatalyst

Modifier of metal oxide

Excitation wavelength (nm)

TiO2 TiO2 Black TiO2 ZnO TiO2 ZnO

– N – Er/Yb/Gd Pt –

340–440 >400 808 980 365 320–400

TiO2

CdS

390–425

Application

Ref.

Skin cancer Leukemia Bladder cancer Liver cancer Cervical cancer Liver adenocarcinoma Leukemia

[10] [11] [12] [13] [14] [15] [16]

widely studied due to its excellent biocompatibility and unique photocatalytic activity. TiO2 nanoparticles can be photoexcited under UV light irradiation to generate charge carriers, which subsequently react with the surrounding molecules to produce powerful oxidative radicals that induce cancer cell apoptosis or necrosis. The successful use of TiO2 nanoparticles in PDT has been reported for various types of cancer, such as leukemia, hepatocarcinoma, human cervical adenocarcinoma, and nonsmall cell lung cancer. The use of TiO2 in PDT affords several advantages: (1) the photoactivity of TiO2 is related to its surface area and thus, the efficiency of the process is in the nanoregime, (2) the production cost of TiO2 is very low and production is highly reproducible as compared to traditional photocatalysts, (3) the surface of the TiO2 photocatalyst can be easily functionalized allowing the attachment of tumor-specific antigens. However, the main drawback in the clinical application of TiO2 nanoparticles for PDT is their high bandgap energy level that requires activation with harmful UV light irradiation. One way of solving this problem involves doping TiO2 with metals or nonmetals to shift the absorption onset of TiO2 to longer, nontoxic wavelengths [11]. For example, nitrogen-doped titanium dioxide nanoparticles produced more ROS and exhibited a higher killing effect on human cervical carcinoma (HeLa) cells by visible light photodynamic therapy as compared to pristine TiO2 nanoparticles [17]. Coupling TiO2 with other semiconductor nanoparticles is another promising approach. Efficient Fe3O4-TiO2 composites for magnetic resonance imaging and potential photodynamic therapy were successfully prepared. It was suggested that TiO2-based photocatalysts were more stable than traditional organic photocatalysts due to their small size and antiphotodegradable properties [18]. Zinc oxide semiconductors are another important type of metal oxide that can be used as photocatalytic agents in PDT because of their low toxicity, unique electronic and optical properties, and simplicity of fabrication [19]. The photodynamic effect of 20-, 60-, and 100-nm ZnO nanoparticles on SMMC-7721 hepatocellular carcinoma cells has been studied [20]. The highest cancer cell cytotoxicity was observed for

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the 20-nm nanoparticles at the lowest concentration (2.5 μg/mL) under UV irradiation. On the other hand, at the highest concentration (10 μg/mL), all three sizes of nanoparticles exhibited 100% reduction in cell activity but different degrees of dark toxicity [20]. In another study, the photocatalytic effect of rod-like zinc oxide nanoparticles in human head and neck squamous cell carcinoma cell lines in vitro was investigated. After 15 min of UVA-1 irradiation the vitality of the cancer cell lines was reduced to 65% and 35 % in the presence of ZnO nanoparticles at concentrations of 0.2 and 2 μg/mL, respectively [21]. These observations clearly indicate the size and dose dependence of ZnO nanoparticles on their photodynamic activity.

4.6.1.2 Passive metal oxide nanoparticles Passive metal oxides have the potential to enhance stability, bioavailability, and aqueous solubility of the photocatalyst and to deliver photocatalysts to the target sites [22]. Table 4.13 displays various metal oxide particles used as carriers for organic photocatalysts in photodynamic therapy [23–29]. Metal oxide application improves the photocatalyst accuracy and efficacy when treating tumor sites. Importantly, several studies have indicated that the conjugation of traditional photosensitizers with metal oxide nanoparticles significantly increases photocatalyst selectivity toward tumor sites over healthy tissue [1]. Nitrogen-doped TiO2 nanoparticles (N-TiO2) conjugated with aluminum phthalocyanine chloride tetrasulfonate (Pc) were reported as excellent photocatalysts in PDT. The photokilling efficiency of N-TiO2-Pc was about tenfold higher than that of Pc. Furthermore, cellular uptake was greatly improved by its metal

Overview of metal oxide nanoparticles used as carriers for traditional organic photocatalysts in PDT

Table 4.13 Metal oxide carrier

Organic photocatalyst

Excitation wavelength (nm)

Fe3O4 ZnO

Porphyrin Photofrin

465 625

ZnO

meso-tetra (o-amino phenyl) porphyrin Tetra sulphonatophenyl porphyrin protoporphyrin IX meso-tetra(4sulfonatophenyl) porphyrin dihydrochloride zinc phthalocyanine

450

TiO2 ZnO Fe3O4

MnO2

410 635 400

660

Application

Ref.

Skin cancer Larynx cancer Breast tumor

[23] [24]

Rheumatoid arthritis Skin cancer Nonsmall cell lung cancer

[26]

Cervical cancer

[29]

[25]

[27] [28]

Application of metal oxide-based photocatalysis

283

oxide carrier [30]. Chlorin e6 (Ce6)-conjugated Fe3O4 nanoparticles with a 20-nm diameter were successfully designed and developed for gastric cancer photodynamic therapy. The prepared Ce6-Fe3O4 exhibited good biocompatibility, high water solubility, and remarkable photodynamic efficacy upon irradiation [31].

4.6.2

Light penetration

The choice of light source depends mainly on the depth of penetration that it should reach. Moreover, the chosen wavelength must be adjusted according to the absorption spectrum of the metal oxide photocatalyst. The appropriate wavelength should give a maximal ROS yield at maximal depth [32]. The penetration depth of light depends on the type of tissue because both the transport scattering and optical absorption coefficients of the excitation light vary from tissue to tissue. It was found that the penetration depth of long-wavelength near-infrared (NIR) light is more than twofold greater than that of short-wavelength ultraviolet-visible light [5]. Generally, metal oxide photocatalysts are excited by short-wavelength UV-Vis light, for which light penetration into the skin is only a few millimeters deep [33]. There are three major drawbacks in traditional PDT that employs UV-Vis light irradiation to excite metal oxide photocatalysts: (1) ineffective treatment of deep-seated cancer cells due to limited tissue penetration of the UV-visible light, (2) nonspecific tissue damage by the influence of natural light on the photocatalyst, (3) photodamage of healthy tissue by harmful UV light irradiation [34–36]. Maximum light penetration in tissue can be achieved in the NIR range (700–1000 nm) [37]. In this regard, upconversion nanoparticles (UCN) have great potential to overcome the limitations of light penetration in traditional PDT and act as light transducers to emit light of shorter wavelength for metal oxide photocatalyst activation under NIR light. Furthermore, the light emitted by UCN is nonblinking and nonbleaching, less scattered, and capable of deep tissue penetration because the excitation in the NIR region is within the optical “transparency” window of biological tissue [38]. NIR-activated photocatalysts based on TiO2 shells modified with NaYF4:Yb3+ and Tm3+@NaGdF4:Yb3+ UCN cores, which convert NIR light into UV irradiation to excite the TiO2 shell, have been designed. It was observed that TiO2-coated UCN can efficiently generate ROS under 980-nm NIR laser excitation to induce cancer cell apoptosis in vitro (Fig. 1 of [35]). Another study developed a nanostructure comprising NaYF4:Yb,Tm UCN as core and a ZnO shell for therapeutic application of deepseated tumors. The unique core-shell combination of UCN and ZnO can utilize UCN emission for the sensitization of ROS generation with the assistance of a ZnO semiconductor as photocatalyst. This obviates complicated methods for loading the photocatalyst [36]. Furthermore, well-defined photocatalysts in which the UCN core was coated with a thin layer of TiO2 shell of uniform size and shape, was designed (Fig. 4.23). Under NIR excitation, TiO2 photoinduced by upconverted UV light resulted in the formation of three types of ROS: superoxide anions, hydrogen peroxide, and hydroxyl radicals for effective cell killing. Notably, compared to previous

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Metal Oxide-Based Photocatalysis

Fig. 4.23 Scheme of deep penetrating near-infrared (NIR) light excitable upconversion nanoparticles (UCN) modified TiO2 shell for treatment of solid tumors. Reproduced with permission from S.S. Lucky, N. Muhammad Idris, Z. Li, K. Huang, K.C. Soo, Y. Zhang, Titania coated upconversion nanoparticles for near-infrared light triggered photodynamic therapy, ACS Nano 9 (2015) 191–205.

nanoconstructions, the afforded TiO2-UCN photocatalyst allows controllable loading of a sufficient amount of TiO2, preventing any photocatalyst leakage [39], and therefore ensuring repeatable PDT results both in vitro and in vivo [40].

4.6.3

Conclusions and perspectives

The application of metal oxides in the field of PDT is a very promising avenue for future medical breakthroughs. Metal oxide-based PDT can be used to treat various diseases including liver, lung, cervical, bladder, larynx, breast, and skin cancer, as well as leukemia. Most traditional organic photocatalysts exhibit disadvantages, such as low tumor selectivity, poor solubility in aqueous solution, and undesirable pharmacokinetics. However, application of appropriate metal oxides to carry the photocatalysts is expected to resolve these problems. Moreover, metal oxide nanoparticles can also directly generate ROS upon UV irradiation and act as photocatalysts, inducing an exceptional photokilling effect against cancer cells; these effects were studied both in vitro and in vivo. On the other hand, TiO2 and ZnO nanoparticles can only be activated by UV light irradiation. Thus, they have a limited penetration depth and are harmful to healthy cells and tissue. However, these limitations have been overcome by recent progress in the combination of metal oxides with upconverted particles, improving the penetration depth of NIR light to tissues without causing photodamage. Finally, the use of nanoparticle-based photocatalysts is in its infancy and there is still much to be explored. Notably, metal oxide nanoparticles tend to accumulate in tissue, and thus a detailed mechanism for exclusion should be

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determined. Optimization of the photocatalyst dose and a better understanding of nanoparticle behavior in vivo has raised hope for the successful development of metal oxide nanoparticles for application in photodynamic therapy.

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4.7 Applications in construction, building materials, and textiles 4.7.1

Introduction

Titanium dioxide is one of the most popular engineering and industrial materials in everyday life. For the past 60 years, rutile-type titanium dioxide has been used as a pigment for paintings, cosmetics, and other products. However, chalking has been observed in titanium dioxide-containing white paint after long-term outdoor exposure. Nowadays, to avoid the degradation of organic binders, TiO2 pigments are often coated with different oxides (zirconia, silica, and alumina) that reduce the generation of radicals and oxides [1,2]. Since the early 1990s, photocatalysis has been applied to building materials (ceramic tiles, blocks, glass, and paint) with self-cleaning properties, air and water purification, and antibacterial functions. Under UV light, these compounds exhibit two photochemical phenomena on their surface that make them suitable for application in construction materials: (1) redox reaction of the adsorbed substances and (2) high water-wettability [3]. The mechanism of the photocatalytic redox reactions was described in Chapter 2. The mechanism of photocatalytic

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List of possible applications of photocatalytic construction and textile materials [1]

Table 4.14

Horizontal applications l

Concrete pavements

l

Paving blocks and paving plates Coating systems for pavements and roads (white toppings, selflevelling mortars) Roofing tiles Roofing panels Cement-based tiles

l

l

l

l

Vertical applications l

l

l

l

l

l

l

l

Tunnels

Indoor and outdoor paints

l

Finishing coatings, plasters and other final rendering cement-based materials Permanent formworks Masonry blocks Sound-absorbing elements for buildings and roof applications Traffic divider elements Street furniture Retaining fairfaced elements

l

l

l

Textiles

Paints and renderings

l

Curtains

l

Concrete panels Concrete pavements Ultrathin white toppings

l

Tents T-shirts Upholstery

l

degradation in the presence of a photocatalyst immobilized on self-cleaning surfaces and textiles was examined on a variety of substances ranging from coffee, wine, juice, pigments, and make-up stains to bacteria, algae, viruses, and fungi. A broad list of possible applications of photocatalytic construction and textile materials is summarized in Table 4.14 [1].

4.7.2

Superhydrophilicity mechanism

The water contact angle can be used to measure the degree of water repellence on the surface of a specific material. This angle is lower for a hydrophilic surface than for one that is hydrophobic. For glass and other inorganic materials, the water contact angle ranges between 20 and 30 degrees [1]. TiO2 surfaces under UV light irradiation become superhydrophilic at contact angles