Semiconductor photocatalysts have recently attracted much interest because of their possible applicability to detoxification of environmental pollutants [1] and solar-energy conversion. [2] Among the photocatalysts, TiO 2 is believed to be the most promising presently known material because of its superior photoreactivity, nontoxicity, long-term stability, and low price. The photocatalytic activity of TiO 2 depends on various parameters, including crystallinity, impurities, surface area, and density of surface hydroxy groups; however, the most significant factor is its crystal form. [3] TiO 2 is usually used as a photocatalyst in two crystal structures: rutile and anatase. Anatase generally has much higher activity than rutile. [4] More interesting is the fact that the activity of P-25 (Degussa), which consists of anatase and rutile (4/1 w/w), exceeds that of pure anatase in several reaction systems. [3b, 5] Indeed, P-25 has frequently been used as a benchmark for photocatalysts.However, the origin of the high photocatalytic activity of P-25 remains unclear. Here we report on the fundamental mechanism and present a highly active photocatalyst film designed on the basis thereof. Thin films of photocatalysts not only serve as models of particulate systems but also aid in the development of their applications. [1b] Addition of 1-phenyl-1,3-butanedione (BzCH 2 Ac) to a solution of Ti(OC 4 H 9 ) 4 in methanol led to a red shift of the absorption peak for the p ± p* transition of BzCH 2 Ac from 310 to 360 nm, owing to chelation of Ti 4 . After hydrolysis, the resultant sol was used to form a gel film in which the chelate bonds were kept intact. Significant lower solubility of the gel film in alcohol was induced by photoexcitation of the p ± p* absorption band. [6] Patterned (pat-) TiO 2 films were prepared by utilizing this phenomenon. The dimensions of the patterning are expressed by the width w of the stripes of the TiO 2 film and their spacing s. Figure 1 shows a 3D surface-structure photograph of a sample formed on quartz by using a photomask with slits of 0.2 mm in width. Regularly spaced, Figure 1. Three-dimensional surface-structure photograph of pat-TiO 2 (A)/ quartz (w s 0.2 mm). 0.2 mm wide stripes of TiO 2 film with a thickness of about 65 nm are present on the substrate with a spacing of 0.2 mm (TiO 2 /quartz; w s 0.2 mm).X-ray diffraction (XRD) patterns are shown in Figure 2 A for a sputter-deposited TiO 2 (sp-TiO 2 ) film (a) and a sol ± gel TiO 2 (sg-TiO 2 ) film overlaid on the sp-TiO 2 film (b). In pattern (a), the diffraction peaks from the (110) and (211) Figure 2. A) X-ray diffraction patterns of the sp-TiO 2 film (a) and the sgTiO 2 /sp-TiO 2 film (b). B) Plots of (ahn) 1/2 vs photon energy for the sg-TiO 2 (a) and sp-TiO 2 (b) films. planes of rutile are observed at 2 q 27.4 and 54.38, respectively. The peak intensity ratio I(211)/I(110) is much smaller than that of randomly oriented rutile powder (ca. 0.6) [7] and suggests that the sp-TiO 2 film has a preferred orientation towards the [001] ...

A major challenge in heterogeneous photocatalysis is the need to increase charge separation efficiency of a photocatalyst, under illumination, without any applied electrical potential. Regularly patterned TiO 2 films were formed on a SnO 2 -film coated soda lime glass substrate by using an organically modified sol-gel method. This bilayer-type photocatalyst exhibited a very high photocatalytic activity for gas-phase reactions. Efficient interfacial electron transfer from the TiO 2 overlayer to the SnO 2 underlayer was demonstrated by labeling and visualizing the reduction sites with Ag particles.Much attention has recently been focused on the development of thin films of semiconductor photocatalysts represented by TiO 2 . They are important because of their possible applicability to detoxification of environmental pollutants. 1 Among the photocatalysts, TiO 2 is believed to be the most promising presently known material, due to its great capacity for oxidation, nontoxicity, and long-term stability. On the other hand, window glass (soda lime (SL) glass), an indispensable constituent of buildings and automobiles, is quite attractive as a substrate for the photocatalytic films. The resultant window glass enables air purification in the living space of human beings by utilizing both solar and illuminating light as the excitation energy source. 2 The essential issue involved in photocatalysis is to increase its quantum efficiency by suppressing the recombination of photogenerated charge carriers. 3 First, the crystallinity of TiO 2 must be improved. Sol-gel TiO 2 films have been formed on SLglass with SiO 2 film in order to inhibit Na + ion diffusion from SL-glass into TiO 2 . It has thus been shown that the sol-gel TiO 2 films have good anatase crystallinity, which provides a high photocatalytic activity. 4 In typical TiO 2 photocatalytic reactions, reduction and oxidation concurrently take place at adjacent reaction sites, which in turn gives rise to unique products. 5 However, this process also has an inherent drawback, i.e., it causes the recombination of the charge carriers. In addition, the excited electrons must be separated spatially from holes, because the characteristic time required for reduction (∼µs) is generally much greater than that for oxidation (∼100 ns). 6 Although efficient charge separation can be achieved within the space-charge layer generated in the anodically biased TiO 2 electrode, 7 this approach is inaccessible to gas-phase reactions.

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Ag clusters (mean diameter = 1.5 nm, standard deviation = 0.37 nm) were photodeposited on TiO(2) particles in a highly dispersed state. The loading of a small amount of the Ag clusters (0.24 wt %) dramatically enhanced both the activity for the TiO(2) photocatalytic reduction of nitrobenzene and the product selectivity of aniline. The essential action mechanism of the Ag clusters is discussed.