Design and Preparation of Ultra Highly Active Semiconductor Photocatalyst

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It has been clarified that the most decisive factor controlling the photocatalytic activity of semiconductor powders is surface area and crystallinity, on the basis of the investigation on the activity of commercial and laboratory-made titanium (IV) oxide powders. The larger the surface area becomes, the more the reaction substrate molecules are adsorbed on the surface, to result in the higher activity, while the higher the crystallinity of particles, the smaller the crystal defect inducing electron-hole recombination. The thus obtained hypothesis that semiconductor particles having large surface area and being high crystallinity should exhibit high photocatalytic activity, however, a little difficult to apply on the practical preparation of photocatalyst powders, because, in the conventional method of titanium (IV) oxide preparation, titanium hydroxide (or oxihydroxide) is prepared and then calcined to remove water. The higher-temperature calcination may induces crystallization but at the same time reduce the surface area, and vice versa. We have developed the novel method of titanium (IV) oxide preparation without the use of water ; (1) Hydrothermal Crystallization of Organic Media (HyCOM), (2) Thermal Decomposition (TD), and (3) Transfer Hydrolytic Crystallization in Alcohols (THyCA). The titanium (IV) oxide powders thus prepared had large surface area and high crystallinity of anatase crystallites. As expected from these physical properties, these powders showed the photocatalytic activity, under both ceroted and deceroted conditions, which is marked higher than that of highly active commercially available titanium (IV) oxide (Degussa P-25). Thus, we have confirmed the principle for the design of ultra-highly semiconductor photocatalyst and developed the novel synthetic methods.