Journal of Photochemistry and Photobiology C: Photochemistry Reviews
Invited reviewTiO2 photocatalysis: Design and applications
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 (OH) (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 (O2−).
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 OH 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
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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.