Photoelectrochemistry with Colloidal Semiconductors; Laser Studies of Halide Oxidation in Colloidal Dispersions of TiO₂ and α‐Fe₂O₃

Aqueous sols of TiO 2 (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: magnified image 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. RuO 2 deposited onto TiO 2 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 Fe 2 O 3 (particle radius 600 Å). For I 2− formation, the quantum yield exceeds 0.9 indicating almost quantitative hole scavenging by iodide.