Detection of the intermediates of colloidal TiO2-catalysed photoreactions

Electrons and positive holes have been produced in colloidal TiO2 particles upon illumination with near-u.v. light. In the absence of adsorbed reactants, the charge carriers recombine. In the presence of an adsorbed reactant for holes [such as SCN– or poly(vinyl alcohol)], an excess of long-lived electrons remains on the particles and finally reacts with a reactant in the bulk solution. In the presence of an adsorbed reactant for electrons (such as a platinum deposit or methyl viologen), an excess of long-lived holes remains and finally reacts with an oxidizable compound. The processes observed include the reduction of water to form H2, the reduction of tetranitromethane and halothane and the oxidation of Br– and various organic compounds. Methyl viologen, MV2+, was found to be reduced to the radical MV˙+ and also to be oxidized, to a lesser extent, to form a fluorescing compound. The absorption spectra of the long-lived excess of electrons and positive holes trapped at the interface were measured using flash photolysis.