Preparation of Titanium Oxide-Based Powders and Thin Films of High Photocatalytic Activities Using Solvothermal Methods

Abstract

Application of solvothermal methods to the synthesis of titanium-oxide based powders and thin films having high photocatalytic activities has been reviewed. Thermal treatment of titanium(IV) n-butoxide dissolved in alcohol under autogenous pressure (alcohothermal treatment) yielded nanocrystalline anatase-type titanium(IV) oxide (TiO2). Thermal treatment of oxobis(2,4-pentanedionato-O,O′)titanium in ethylene glycol in the presence of sodium acetate and a small amount of water yielded nanocrystalline brookite-type TiO2. Anatase TiO2 products were calcined at various temperatures and then used for photocatalytic mineralization of acetic acid in aqueous solutions under aerated conditions and dehydrogenation of 2-propanol under deaerated conditions. Almost all the anatase-type TiO2 samples showed the activities more than twice higher than those of representative active photocatalysts, Degussa P-25 and Ishihara ST01 in both reactions. A brookite TiO2 sample with improved crystallinity and sufficient surface area exhibited the hydrogen evolution rate almost equal to P-25. Solvothermal decomposition of titanium(IV) tert-butoxide in toluene in the presence of silica gel (SiO2) with continuous stirring yielded a TiO2–SiO2 composite. Solvothermally-synthesized TiO2–SiO2 composite exhibited higher photocatalytic performance in the oxidative removal of nitrogen oxides in air than that of the composite prepared by physical mixing or sol-gel method. Stable TiO2 sol was prepared from TiO2 powders synthesized by solvothermal method and transparent TiO2 thin films were successfully produced by dip-coating from the sol. These films exhibited much higher rate of malachite green decomposition compared with those prepared from a commercially available TiO2 sol.