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

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 EDTA4–, 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.