Aqueous sols of TiO2 (anatase, particle radius 25 Å) were excited with (347.1 nm)-laser light and the reaction of valence-band holes with halide ions (X = I−, Br−, Cl−) was investigated. Hole transfer takes place within the duration of the (10 ns)-laser pulse and results in the formation of anion radicals according to the sequence:
The quantum yield of X increases in the order Cl < Br < I, attaining 0.8 for I at pH 1. It is affected by pH, halide concentration and the presence of a protective agent for the sol.
RuO2 deposited onto TiO2 enhances markedly Cl and Br -formation, but has no effect on the yield of I.
Laser-photolysis investigation of halide oxidation were also carried out with colloidal Fe2O3 (particle radius 600 Å). For I2−formation, the quantum yield exceeds 0.9 indicating almost quantitative hole scavenging by iodide.