Invited review
TiO2 photocatalysis: Design and applications

https://doi.org/10.1016/j.jphotochemrev.2012.06.001Get rights and content

Abstract

TiO2 photocatalysis is widely used in a variety of applications and products in the environmental and energy fields, including self-cleaning surfaces, air and water purification systems, sterilization, hydrogen evolution, and photoelectrochemical conversion. The development of new materials, however, is strongly required to provide enhanced performances with respect to the photocatalytic properties and to find new uses for TiO2 photocatalysis. In this review, recent developments in the area of TiO2 photocatalysis research, in terms of new materials from a structural design perspective, have been summarized. The dimensionality associated with the structure of a TiO2 material can affect its properties and functions, including its photocatalytic performance, and also more specifically its surface area, adsorption, reflectance, adhesion, and carrier transportation properties. We provide a brief introduction to the current situation in TiO2 photocatalysis, and describe structurally controlled TiO2 photocatalysts which can be classified into zero-, one-, two-, and three-dimensional structures. Furthermore, novel applications of TiO2 surfaces for the fabrication of wettability patterns and for printing are discussed.

Highlights

► TiO2 photocatalysts from the viewpoint of structural design and applications are summarized. ► We classified TiO2 photocatalysts into zero- to three-dimensional structures. ► The dimensionality of the structure of a TiO2 photocatalysts affects its properties and functions. ► New applications of TiO2 surfaces for wettability patterns and for printing is also described.

Introduction

The development of photocatalysis has been the focus of considerable attention in recent years with photocatalysis being used in a variety of products across a broad range of research areas, including especially environmental and energy-related fields (Fig. 1) [1], [2], [3], [4]. Following on from the water splitting breakthrough reported by Fujishima and Honda in 1972 [5], the photocatalytic properties of certain materials have been used to convert solar energy into chemical energy to oxidize or reduce materials to obtain useful materials including hydrogen [5], [6], [7], [8] and hydrocarbons [9], and to remove pollutants and bacteria [10], [11], [12], [13], [14], [15], [16], [17], [18] on wall surfaces and in air and water [1], [2], [3], [4], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]. Of the many different photocatalysts, TiO2 has been the most widely studied and used in many applications because of its strong oxidizing abilities [23], [35], [36], [37], [38], [39] for the decomposition of organic pollutants [25], [26], superhydrophilicity [40], chemical stability, long durability, nontoxicity, low cost, and transparency to visible light.

The photocatalytic properties of TiO2 are derived from the formation of photogenerated charge carriers (hole and electron) which occurs upon the absorption of ultraviolet (UV) light corresponding to the band gap (Fig. 2) [1], [3], [19], [41], [42]. The photogenerated holes in the valence band diffuse to the TiO2 surface and react with adsorbed water molecules, forming hydroxyl radicals (radical dotOH) (Fig. 3) [3]. The photogenerated holes and the hydroxyl radicals oxidize nearby organic molecules on the TiO2 surface. Meanwhile, electrons in the conduction band typically participate in reduction processes, which are typically react with molecular oxygen in the air to produce superoxide radical anions (O2radical dot).

In addition, TiO2 surfaces become superhydrophilic with a contact angle of less than 5° under UV-light irradiation (Fig. 4) [40]. The superhydrophilicity is originated from chemical conformation changes of a surface [42]. The majority of the holes are subsequently consumed by reacting directly with adsorbed organic species or adsorbed water, producing radical dotOH radicals as described above. However, a small proportion of the holes is trapped at lattice oxygen sites and may react with TiO2 itself, which weakens the bonds between the lattice titanium and oxygen ions. Water molecules can then interrupt these bonds, forming new hydroxyl groups (Fig. 5). The singly coordinated OH groups produced by UV-light irradiation are thermodynamically less stable and have high surface energy, which leads to the formation of a superhydrophilic surface.

The construction of TiO2 nano- or micro-structures with interesting morphologies and properties has recently attracted considerable attention [43] and many TiO2 nanostructural materials, such as spheres [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], nanorods [66], [67], [68], [69], [70], [71], [72], [73], [74], fibers [75], [76], [77], [78], [79], [80], [81], [82], [83], [84], [85], [86], [87], [88], [89], tubes [28], [90], [91], [92], [93], [94], [95], [96], [97], [98], [99], [100], [101], [102], [103], [104], [105], [106], [107], [108], sheets [109], [110], [111], [112], [113], [114], [115], [116], [117], [118], [119], [120], [121], [122], [123], [124], [125], [126], [127], [128], [129], [130], [131], [132], [133], [134], [135], [136], [137], [138], [139], [140], [141], [142], and interconnected architectures [143], [144], [145], [146], [147], [148], [149], [150], [151], [152], [153], [154], [155], [156], [157], [158], [159], [160], [161], [162], have been fabricated. Nanostructured TiO2 materials are widely used not only in photocatalysis, but also in dye-sensitized solar cells (DSSCs) [163], [164], [165], lithium-ion batteries [166], [167], and electrochromic displays [168].

It is well known that there are many factors which can exert significant influence on photocatalytic performance, including the size, specific surface area, pore volume, pore structure, crystalline phase, and the exposed surface facets. Thus, the development of performance improvements by adjusting these factors remains the focus of photocatalysis research. Structural dimensionality is also a factor which can affect the photocatalytic performance and also has a significant impact on the properties of TiO2 materials (Fig. 6). For example, a sphere with zero dimensionality has a high specific surface area, resulting in a higher rate of photocatalytic decomposition of organic pollutants [55]. One-dimensional fibers or tubes have advantages with regard to less recombination because of the short distance for charge carrier diffusion [28], light-scattering properties [169], and fabrication of self-standing nonwoven mats [170]. Zero- and one-dimensional structures have been well developed and will be discussed in greater detail in the following sections. Two-dimensional nanosheets have smooth surfaces and high adhesion [118], [131], whereas three-dimensional monoliths may have high carrier mobility as a result of their interconnecting structure and be used in environmental decontamination. Choosing TiO2 materials with the appropriate dimensionalities enables us to take full advantage of the unique properties offered by TiO2 materials.

In this review, recent research in the field of TiO2 photocatalysis has been reviewed from the perspective of both structural design and novel applications. We will initially introduce TiO2 photocatalysts possessing spheres as a zero-dimensional structure, fibers and tubes as one-dimensional structures, nanosheets as a two-dimensional structures, and interconnected architectures as three dimensional structures. We will then proceed to discuss the fabrication of wettability patterns and their application for the offset printing plate as a new application of TiO2 photocatalysis. Finally, a conclusion of the reviewed research will be provided together with a brief future perspective.

Section snippets

TiO2 spheres (zero-dimensional)

Nano- or micro-structured TiO2 spheres are the most widely studied and used in TiO2-related materials. Useful and interesting properties derived from their unique structures have been reported in a great many publications [51], [52], [56], [57], [59], [60], [63], [64], [65]. These TiO2 spheres usually possess a high specific surface area and a high pore volume and pore size, with these properties increasing the size of the accessible surface area and the rate of mass transfer for organic

Wettability patterning using TiO2 photocatalysts

Multiple opportunities still exist for the practical application of TiO2 photocatalysis to a variety of different fields, especially in the environmental and energy fields. TiO2 surfaces exhibit a strong oxidizing ability for the decomposition of organic molecules, and superhydrophilicity [1], [3], [19], [21], [184], [185], and these properties can be used for generating wettability patterns. Wettability patterns have been used in many fields such as offset printing [186], [187], [188] and in

Conclusions

An overview of recent significant publications in the field of TiO2 photocatalysis, especially from the perspective of the design and new applications of TiO2 materials, has been provided. The review initially highlights the structural design of TiO2 materials regarding their dimensional classification. In the zero-dimensional structure of spheres, TiO2 materials have a high specific surface area which and are typically produced according to the hydrothermal and/or electrospray methods to

Acknowledgments

This work was supported by a Grant-in-Aid for Scientific Research (B) and for Challenging Exploratory Research (No. 21654043) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, a Kurata Research Grant, and the Nippon Sheet Glass Foundation for Materials Science and Engineering.

Kazuya Nakata was born in 1977 in Sapporo, Japan. He received his BSc (2000) from Shizuoka University, and his MSc (2002) and PhD (2005) in science from Tokyo Metropolitan University under the supervision of Professor Masahiro Yamashita. In 2005, he joined Tohoku University as a research fellow of the Japan Society for the Promotion of Science (JSPS), and then joined the Massachusetts Institute of Technology in 2006 as a JSPS research fellow. He has been a full-time researcher in the

References (201)

  • A. Mills et al.

    An overview of semiconductor photocatalysis

    J. Photochem. Photobiol. A

    (1997)
  • D.A. Tryk et al.

    Recent topics in photoelectrochemistry: achievements and future prospects

    Electrochim. Acta

    (2000)
  • G.K. Mor et al.

    A review on highly ordered, vertically oriented TiO2 nanotube arrays: fabrication, material properties, and solar energy applications

    Sol. Energy Mater. Sol. Cells

    (2006)
  • K. Hashimoto et al.

    TiO2 photocatalysis: a historical overview and future prospects

    Jpn. J. Appl. Phys.

    (2005)
  • A. Fujishima et al.

    Electrochemical photolysis of water at a semiconductor electrode

    Nature

    (1972)
  • A. Kudo et al.

    Heterogeneous photocatalyst materials for water splitting

    Chem. Soc. Rev.

    (2009)
  • A. Ryu

    Recent progress on photocatalytic and photoelectrochemical water splitting under visible light irradiation

    J. Photochem. Photobiol. C

    (2010)
  • T. Inoue et al.

    Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders

    Nature

    (1979)
  • R. Cai et al.

    Photokilling of malignant cells with ultrafine TiO2 powder

    Bull. Chem. Soc. Japan

    (1991)
  • C. McCullagh et al.

    The application of TiO2 photocatalysis for disinfection of water contaminated with pathogenic micro-organisms: a review

    Res. Chem. Intermed.

    (2007)
  • K. Sunada et al.

    Bactericidal and detoxification effects of TiO2 thin film photocatalysts

    Environ. Sci. Technol.

    (1998)
  • K. Sunada et al.

    Bactericidal activity of copper-deposited TiO2 thin film under weak UV light illumination

    Environ. Sci. Technol.

    (2003)
  • E.J. Wolfrum et al.

    Photocatalytic oxidation of bacteria, bacterial and fungal spores, and model biofilm components to carbon dioxide on titanium dioxide-coated surfaces

    Environ. Sci. Technol.

    (2002)
  • A. Fujishima et al.

    TiO2 Photocatalysis: Fundamentals and Applications

    (1999)
  • M.R. Hoffmann et al.

    Environmental applications of semiconductor photocatalysis

    Chem. Rev.

    (1995)
  • A. Fujishima, D.A. Tryk, M.S. In: A.J. Bard, S. Licht (Eds.), Encyclopedia of Electrochemistry, Vol. 6: Semiconductor...
  • K. Nakata et al.

    Visible light responsive electrospun TiO2 fibers embedded with WO3 nanoparticles

    Chem. Lett.

    (2011)
  • K. Nakata et al.

    Fabrication and photocatalytic properties of TiO2 nanotube arrays modified with phosphate

    Chem. Lett.

    (2011)
  • K. Nakata et al.

    Electrospun fibers composed of Al2O3-TiO2 nanocrystals

    J. Ceram. Soc. Japan

    (2009)
  • P.V. Kamat

    Photochemistry on nonreactive and reactive (semiconductor) surfaces

    Chem. Rev.

    (1993)
  • A. Heller

    Chemistry and applications of photocatalytic oxidation of thin organic films

    Acc. Chem. Res.

    (1995)
  • J. Peral et al.

    Heterogeneous photocatalysis for purification, decontamination and deodorization of air

    J. Chem. Technol. Biotechnol.

    (1997)
  • Y. Nosaka et al.

    Singlet oxygen formation in photocatalytic TiO2 aqueous suspension

    PCCP

    (2004)
  • Y. Nosaka et al.

    Photocatalytic [radical dot]OH radical formation in TiO2 aqueous suspension studied by several detection methods

    PCCP

    (2003)
  • A. Jańczyk et al.

    Singlet oxygen photogeneration at surface modified titanium dioxide

    J. Am. Chem. Soc.

    (2006)
  • Y. Nosaka et al.

    Behavior of superoxide radicals formed on TiO2 powder photocatalysts studied by a chemiluminescent probe method

    PCCP

    (2002)
  • A.Y. Nosaka et al.

    Photoinduced changes of adsorbed water on a TiO2 photocatalytic film as studied by 1H NMR spectroscopy

    J. Phys. Chem. B

    (2003)
  • R. Wang et al.

    Light-induced amphiphilic surfaces

    Nature

    (1997)
  • H. Sakai et al.

    Local detection of photoelectrochemically produced H2O2 with a “Wired” horseradish peroxidase microsensor

    J. Phys. Chem.

    (1995)
  • K. Ikeda et al.

    Photocatalytic reactions involving radical chain reactions using microelectrodes

    J. Phys. Chem. B

    (1997)
  • X. Chen et al.

    Synthesis of titanium dioxide (TiO2) nanomaterials

    J. Nanosci. Nanotechnol.

    (2006)
  • H. Bai et al.

    Hierarchically multifunctional TiO2 nano-thorn membrane for water purification

    Chem. Commun.

    (2010)
  • J.S. Chen et al.

    Ellipsoidal hollow nanostructures assembled from anatase TiO2 nanosheets as a magnetically separable photocatalyst

    Chem. Commun.

    (2011)
  • J.S. Chen et al.

    TiO2 and SnO2@TiO2 hollow spheres assembled from anatase TiO2 nanosheets with enhanced lithium storage properties

    Chem. Commun.

    (2010)
  • J.S. Chen et al.

    Constructing hierarchical spheres from large ultrathin anatase TiO2 nanosheets with nearly 100% exposed (001) facets for fast reversible lithium storage

    J. Am. Chem. Soc.

    (2010)
  • Y. Dai et al.

    Synthesis of anatase TiO2 nanocrystals with exposed {001} facets

    Nano Lett.

    (2009)
  • W.Q. Fang et al.

    Hierarchical structures of single-crystalline anatase TiO2 nanosheets dominated by {001} facets

    Chem. Eur. J.

    (2011)
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    Kazuya Nakata was born in 1977 in Sapporo, Japan. He received his BSc (2000) from Shizuoka University, and his MSc (2002) and PhD (2005) in science from Tokyo Metropolitan University under the supervision of Professor Masahiro Yamashita. In 2005, he joined Tohoku University as a research fellow of the Japan Society for the Promotion of Science (JSPS), and then joined the Massachusetts Institute of Technology in 2006 as a JSPS research fellow. He has been a full-time researcher in the photocatalyst group at the Kanagawa Academy of Science and Technology (KAST) since December 2007. In September 2010, he also joined in the Organic Solar Cell Assessment Project at KAST. He has also been a visiting associate professor at the Tokyo University of Science since January 2011. In 2012, he received a Sano award for young scientists from the electrochemical society of Japan.

    Akira Fujishima was born in 1942 in Tokyo, Japan. He received his BSc (1966) from the Yokohama National University, and his MSc (1968) and PhD (1971) in engineering from The University of Tokyo. He became a lecturer at Kanagawa University in 1971 and then a lecturer at The University of Tokyo in 1975. After serving as an associate professor (1978) and professor (1986), he became a professor at The University of Tokyo Graduate School of Engineering in 1995. He was appointed as the Chairman of the Kanagawa Academy of Science and Technology (KAST) and Director of the Functional Materials Research Laboratory of the Central Japan Railway Company in 2003. He was appointed as professor emeritus of The University of Tokyo and later became a special university professor emeritus of The University of Tokyo in 2005. He served as the chairman for the Chemical Society of Japan from 2006 to 2007 and has been the Director of the China Research Center at the Japanese Science and Technology Agency (JST) since 2008. He has been the President of Tokyo University of Science since 2010. He received the Asahi Prize in 1983, the Chemical Society of Japan Award in 2000, the Purple Ribbon Medal (Shijuhosho) in 2003, and in 2004 he received the Japan Prize and the Japan Academy Prize and was named an Honorable Citizen of Kawasaki City. He also received the Imperial Invention Award and Kanagawa Culture Award in 2006. He received a Cultural Contributor in 2010.

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