Photochemical reactions of amorphous and crystalline titanium dioxide powder suspensions in water

Abstract

Photochemical and photophysical properties of crystalline and amorphous TiO₂ colloids in water are compared. Crystallinic TiO₂ colloids show a broad emission with a maximum at 375 nm, which corresponds to the band gap of the semiconductor, and the material absorbs anionic, cationic and neutral molecules at pH 5.6. Instantaneous formation of primary electron-transfer products is observed on laser excitation; these subsequently decay away in short time periods (µs). Doping with small amounts of metal-oxide impurities increases the steady-state photolysis yields of products. Aqueous solutions of amorphous TiO₂ colloids do not luminesce and do not adsorb cations and neutral molecules. However, anions such as EDTA^4–, I^– or SCN^– are adsorbed at the surface of these particles. Photoexcitation of methyl viologen, MV²⁺, with amorphous TiO₂ samples leads to a growth of MV⁺ formation which lasts many microseconds. This behaviour is contrary to that observed with crystalline TiO₂.