Invited Review
Modifications on reduced titanium dioxide photocatalysts: A review

https://doi.org/10.1016/j.jphotochemrev.2017.05.003Get rights and content

Highlights

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    A general and critical review on the further modifications on reduced TiO2(TiO2-x).

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    The synergistic effects of non-metal elements and Ti3+ are discussed.

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    TiO2-x compositing with metal-based materials has been summarized.

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    Graphene and other carbon materials are introduced to the further modification of TiO2-x.

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    TiO2-x materials with special facets exposure and ordered structure controlling are employed.

Abstract

A large variety of reduced titanium dioxide (TiO2-x) materials have been reported recently. Reduced TiO2, usually resulting from the removal of oxygen atoms or hydrogen incorporation, is proved to be efficient for achieving highly photocatalytic performance including photodegradation of organic compounds, hydrogen generation from water splitting, CO2 reduction for CH4 evolution, solar cells, etc. To further improve the properties and activities of TiO2-x, a combination of the Ti3+ self-doping and other traditional modifications like nonmetals doping has been proposed in the past decades. This paper provides a general and critical review on the further modifications on reduced TiO2 samples, including non-metal elements (N, B, S, F and I) doping, noble-metal (Au, Pt, Pd and Ag) and iron-group metal (Fe, Co and Ni) grafting, metal oxide compositing, carbon (nanotubes and graphene) and carbon-based-material compositing, special facets exposure (mainly dual {001}-{101} and {111}-{110} facets) of TiO2-x and ordered structure controlling of TiO2-x. These modifications enhance the physical and/or chemical properties of the reduced TiO2, or create new features for the modified TiO2-x samples, which finally leads to the enhancement of photocatalytic performance. Key examples such as N-doping, Au grafting and graphene-based compositing are discussed carefully, and the mechanisms for solar light enhancement, electron transfer and charge separation are also investigated. Finally, some challenging issues on TiO2-x catalysts are also proposed to encourage new approaches for preparation of TiO2-x catalysts with efficiently photocatalytic performance.

Introduction

Titanium dioxide (TiO2) is one of the most investigated semiconductors since the discovery of its water splitting property in 1970s [1]. Owning to the abundance, non-toxicity and chemical stability, TiO2 shows great applications in catalysis, cosmetic, paints, antibacterial agents, lithium ion batteries (LIBs), dye-sensitized solar cells (DSSCs), etc [2]. However, the white color and wide bandgap (3.2 eV for anatase phase) of pristine TiO2 deeply limit its applications to the UV part of solar light spectrum [3]. Only a small part of the solar energy (∼5%) can be well utilized by pristine TiO2 and most of the solar energy is wasted. Even the photons are absorbed by the TiO2 catalyst, the photogenerated electrons and holes will quench quickly because of the charge recombination. Therefore, it is necessary to extend the absorption edge of solar spectrum to the visible region and reduce the recombination of electron-hole pairs to enhance the photocatalytic activity of TiO2.

Recently, reduced TiO2 (TiO2-x) materials have received much attentions, owing to the high photocatalytic and photoelectrochemical (PEC) performance [4], [5], [6], [7], [8], [9], [10]. The material was reported to show unique properties such as extended absorption edge of solar light and colorful appearance (ranging from yellow [11], [12], [13], red [14], blue [15], [16], [17], grey [18], [19], [20], [21] to black [22], [23], [24], [25] color), along with structural changes on the surface and/or in the bulk: generation of Ti3+ species and oxygen vacancies, formation of disordered layers on the surface, improvement of Ti-H species or Ti-OH bonds, narrowed bandgap of the catalyst, modified electron density, etc.

Until now, plenty of efforts have been devoted to study the properties and applications of TiO2-x materials. Compared to the pristine TiO2 catalyst, TiO2-x nanomaterials have shown improved photocatalytic activities under solar light irradiation. However, further modification of TiO2-x photocatalyst and enhancement of its photocatalytic performance are required, to match the rising demand for energy and environmental protection. Therefore, new approaches are developed for the further enhancement of the photocatalytic performance of TiO2-x photocatalyst.

In this review, we try to discuss the further modifications on reduced TiO2 photocatalysts, and the mechanism of the obtained modified TiO2-x, including non-metal or metal doping, noble metal deposition, metal oxide composition, carbon-based composite, special facets exposure, morphology controlling, etc. These modifications improve the physical and/or chemical properties of TiO2-x, or create some new features for the photocatalysis. It is hoped that this review would be helpful for understanding the current development of reduced TiO2 photocatalysts, and also encourage further modifications to improve the photocatalytic activities.

Section snippets

Preparation of TiO2-x photocatalysts

In general, reduced TiO2 can be prepared either by the reduction of Ti(IV) precursors or the oxidation of low-oxidation-state titanium compounds. Until now, a variety of methods have been reported for the synthesis of reduced TiO2, including calcination under H2 atmosphere [4], [11], [26], vacuum activation [13], [27], [28], metal reduction [10], [20], [29], [30], electrochemical reduction [9], [31], [32], [33], UV irradiation [34], [35], [36], plasma treatment [25], [37], [38], [39], [40],

Doping with nitrogen

Doping with non-metal elements is a traditional modification way for TiO2 material to improve its photocatalytic performance [116]. Early in 2001, Asahi et al. demonstrated the band gap narrowing and enhanced reactivity under visible light for nitrogen-doped TiO2 catalyst [117]. Then Burda et al. increased the concentration of nitrogen doping in TiO2 to 8% via direct amination of TiO2 nanoparticles [118]. The introduction of N contributes to the narrowing of band gap of TiO2, which is caused by

Noble-metal-based TiO2-x materials

It is reported that low concentration of Ti3+ facilitates the formation of localized oxygen vacancies and inhibit electron mobility [45], [94]. These localized defect sites act as recombination centers for photogenerated electron-hole pairs and thus restrain the photocatalytic activities of TiO2 samples. Noble metal nanoparticle, such as Au, Pt and Pd are usually used as the co-catalyst over the TiO2 (M-TiO2), promoting the activities by surface plasmon resonance (SPR) and the separation of

Carbon-based TiO2-x materials

Carbon is one of the most fascinating elements in the periodic table. It is the base of lives all over the earth. The atoms of carbon can be bonded in different ways. The best-known forms of carbon are graphite, diamond and amorphous carbon. In addition, fullerene, carbon nanotube and graphene are newly found materials in recent decades. Based on the various properties of carbon-based materials, the combination of carbon and TiO2-x will contribute to new features and enhanced performance.

TiO2-x with special morphology

The morphology of TiO2-x samples is one of the key factors for determining the performance of the catalysts [186]. Until now, plenty of morphologies have been reported, such as TiO2-x nanotube arrays (NTAs), TiO2-x nanosheets with special facets exposure, mesoporous TiO2-x, core-shell or yolk-shell structured TiO2-x, TiO2-x inverse-opals, etc. Besides, special facets exposure of TiO2 make significant changes to the physical and chemical properties of TiO2 catalysts.

Summary and outlook

The modification methods of TiO2-x material have been discussed in this review, for the further improvement of photocatalytic performance of TiO2 photocatalysts. Nitrogen was proved to be an efficient non-metal element for the doping of TiO2 samples. The synergistic effect between N and Ti3+ species is investigated during the formation of themselves, as well as in the photocatalysis process. Except for nitrogen, doping with other non-metal elements were also discussed, such as boron, sulfur,

Notes

The authors declare no conflict of interests.

Acknowledgements

This work has been supported by National Nature Science Foundation of China (21577036, 21377038, 21237003 and 21677048), the National Basic Research Program of China (973 Program, 2013CB632403), State Key Research Development Program of China (2016YFA0204200), “Chenguang Program” supported by Shanghai Education Development Foundation and Shanghai Municipal Education Commission (14CG30), the Science and Technology Commission of Shanghai Municipality (16JC1401400) and the Fundamental Research

Wenzhang Fang is a PhD student in East China University of Science and Technology (ECUST, Shanghai, China). He received his bachelor degree in applied chemistry from ECUST and then joined Prof. Jinlong Zhang’s group in 2011, focusing on the research of nanomaterials. In 2014, he worked as joint-PhD student in Institut de recherches sur la catalyse et l'environnement de Lyon (IRCELYON, Lyon, France) for two years under the supervision of Prof. Stephane Daniele and Dr. Shashank Mishra, funded by

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    • Cited by (0)

    Wenzhang Fang is a PhD student in East China University of Science and Technology (ECUST, Shanghai, China). He received his bachelor degree in applied chemistry from ECUST and then joined Prof. Jinlong Zhang’s group in 2011, focusing on the research of nanomaterials. In 2014, he worked as joint-PhD student in Institut de recherches sur la catalyse et l'environnement de Lyon (IRCELYON, Lyon, France) for two years under the supervision of Prof. Stephane Daniele and Dr. Shashank Mishra, funded by China Scholarship Council (CSC). His current interests include the design, characterization and applications of nanomaterials such as TiO2 photocatalysts, graphene, up-conversion materials, etc.

    Dr. Mingyang Xing is the Associate Professor, supervisor of postgraduate in the School of Chemistry and Molecular Engineering, East China University of Science and Technology (ECUST). He obtained his Doctoral Degree in 2012 from ECUST, and then worked in University of California, Riverside as a visiting scholar for one year. His research focuses on the preparation of functional nanomaterials and applications to the environmental and energy filed. He has published more than 50 papers in SCI journals in these areas, which have been cited more than 2400 times.

    Prof. Dr Jinlong Zhang obtained his Bachelor Degree in 1985 and his Ph.D. in 1993 from East China University of Science and Technology, Shanghai, China. Professor Zhang joined the East China University of Science and Technology in 1993. He worked in Osaka Prefecture University as a Postdoctor (JSPS) collaborated with Prof. Anpo from 1996 to 2000. He became a full professor in 2000. He has published over 350 original papers, which were cited more than 12000 times (H-index: 56). He is currently on the Editorial Boards of “Applied Catalysis B: Environmental” and “Scientific Reports”. He is also the associate editor of “Res. Chem. Intermed”. His research interests include photocatalysis, mesoporous materials, and materials science. He has been selected as the “Most Cited Chinese Researchers” in China by “Elsevier” in 2014, 2015 and 2016.

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