TiO 2 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 TiO 2 photocatalysis. In this review, recent developments in the area of TiO 2 photocatalysis research, in terms of new materials from a structural design perspective, have been summarized. The dimensionality associated with the structure of a TiO 2 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 TiO 2 photocatalysis, and describe structurally controlled TiO 2 photocatalysts which can be classified into zero-, one-, two-, and three-dimensional structures. Furthermore, novel applications of TiO 2 surfaces for the fabrication of wettability patterns and for printing are discussed.

TiO2 photocatalysis: Design and applications

The detection methods and generation mechanisms of the intrinsic reactive oxygen species (ROS), i.e., superoxide anion radical (•O2–), hydrogen peroxide (H2O2), singlet oxygen (1O2), and hydroxyl radical (•OH) in photocatalysis, were surveyed comprehensively. Consequently, the major photocatalyst used in heterogeneous photocatalytic systems was found to be TiO2. However, besides TiO2 some representative photocatalysts were also involved in the discussion. Among the various issues we focused on the detection methods and generation reactions of ROS in the aqueous suspensions of photocatalysts. On the careful account of the experimental results presented so far, we proposed the following apprehension: adsorbed •OH could be regarded as trapped holes, which are involved in a rapid adsorption–desorption equilibrium at the TiO2–solution interface. Because the equilibrium shifts to the adsorption side, trapped holes must be actually the dominant oxidation species whereas •OH in solution would exert the reactivity...

Generation and Detection of Reactive Oxygen Species in Photocatalysis

Generations Yi Ma,† Xiuli Wang,† Yushuai Jia,† Xiaobo Chen,‡ Hongxian Han,*,† and Can Li*,† †State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian 116023, China ‡Department of Chemistry, College of Arts and Sciences, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, Missouri 64110, United States

Titanium dioxide-based nanomaterials for photocatalytic fuel generations.

A surface science perspective on TiO2 photocatalysis

This review focuses on the heterogeneous photocatalytic treatment of organic dyes in air and water. Representative studies spanning approximately three decades are included in this review. These studies have mostly used titanium dioxide (TiO 2 ) as the inorganic semiconductor photocatalyst of choice for decolorizing and decomposing the organic dye to mineralized products. Other semiconductors such as ZnO, CdS, WO 3 , and Fe 2 O 3 have also been used, albeit to a much smaller extent. The topics covered include historical aspects, dark adsorption of the dye on the semiconductor surface and its role in the subsequent photoreaction, semiconductor preparation details, photoreactor configurations, photooxidation kinetics/mechanisms and comparison with other Advanced Oxidation Processes (e.g., UV/H 2 O 2 , ozonation, UV/O 3 , Fenton and photo-Fenton reactions), visible light-induced dye decomposition by sensitization mechanism, reaction intermediates and toxicity issues, and real-world process scenarios.

Heterogeneous photocatalytic treatment of organic dyes in air and aqueous media

Effect of crystalline structure, anatase and rutile, on the production of OH and O2 − by TiO2 photocatalytic reaction was investigated. The OH radical free from the TiO2 surface was monitored by the fluorescence intensity of 2-hydroxyl terephthalic acid produced by the reaction with terephthalic acid. Superoxide radical was detected by the chemiluminescence probe method with luminol. Formation rate of OH with rutile photocatalysts was significantly lower than that with anatase photocatalysts. By the addition of H2O2, the formation rate of OH was significantly increased for rutile and for anatase mixed with rutile by 10–20%, while pure anatase showed an opposite tendency. We suggest that the adsorption structure of H2O2 on the rutile TiO2 surface is preferable to produce OH . In photocatalytic production of O2 −, rutile surpassed anatase in stabilizing the produced O2 −. On H2O2 addition, anatase surpassed rutile in the photocatalytic activity to produce O2 − from H2O2.

Photocatalytic reactivity for O2•- and OH• radical formation in anatase and rutile TiO2 suspension as the effect of H2O2 addition

Photoinduced reaction with TiO2 semiconductor photocatalysts was investigated by using nitroxide radicals as spin probes for ˙OH radicals. The effects of some additives such as I−, Cl−, ClO4−, methanol and 2-propanol on the photocatalytic decay of nitroxide radicals, 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidine-1-oxyl and 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl, were investigated. Among the additives, only iodide ions that can be oxidized by trapped holes prohibited significantly the decay of the nitroxide radicals, indicating that trapped holes oxidize nitroxide probe radicals. For several TiO2 photocatalysts, the apparent quantum efficiency of ˙OH radical formation was calculated and compared with those obtained by the other detection methods, such as DMPO spin trapping and terephthalic acid fluorescence methods. The experimental observations suggest that the photocatalytic oxidation should be caused more preferably by trapped holes or adsorbed ˙OH radicals, rather than the photoinduced valence band holes in semiconductor and the free ˙OH radicals in solution.

Kenta Yawata

Papers

Photocatalytic reactivity for O2•- and OH• radical formation in anatase and rutile TiO2 suspension as the effect of H2O2 addition

Photocatalytic ?OH radical formation in TiO2 aqueous suspension studied by several detection methods