Design and Preparation of Ultra Highly Active Semiconductor Photocatalyst

通过对工业生产和实验室自制的氧化钛粉末的光催化活性的研究，阐明了控制半导体粉末光催化活性的最决定性因素是比表面积和结晶度。表面积越大，反应底物分子在表面吸附越多，活性越高，而颗粒的结晶度越高，引起电子-空穴复合的晶体缺陷越小。由此得出的假设是，具有大表面积和高结晶度的半导体颗粒应该具有高的光催化活性，然而，在实际光催化剂粉末的制备中应用起来有点困难，因为在传统的氧化钛制备方法中，先制备氢氧化钛（或氧化氢氧化钛），然后煅烧除去水。高温煅烧可以诱导结晶，但同时减少了表面积，反之亦然。我们开发了不用水制备氧化钛的新方法；(1)有机介质的水热结晶（HyCOM），(2)热分解（TD），(3)醇中的转移水解结晶（THyCA）。所制得的氧化钛粉体具有表面积大、锐钛矿结晶度高的特点。正如这些物理性质所预期的那样，这些粉末在碳化和脱脂条件下都显示出光催化活性，这明显高于高活性的市售氧化钛（IV） （Degussa P-25）。从而确定了超高半导体光催化剂的设计原理，并开发了新的合成方法。

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.