Direct observation of one-electron oxidation processes of aromatic sulfides in TiO2 colloidal solution by laser flash photolysis

Abstract

A laser flash photolysis technique has been employed to clarify the oxidation reaction processes of 4-methylthiophenylmethanol (MTPM) and 2-phenylthioethanol (PTE) in a colloidal TiO₂ aqueous solution. The transient absorption spectra attributed to the radical cations of the substrates were observed during the 355 nm laser irradiation of the colloidal TiO₂ aqueous solution in the presence of the substrates. The oxidation process on the TiO₂ surface and the free OH radical mediated oxidation process were found to depend on the substrate concentration. The oxidation reaction rate constants (k_ox) determined at low substrate concentrations in the colloidal TiO₂ aqueous solution were quite consistent with those obtained in the bulk solution. The significant variations in the spectral shape and formation rates of the radical cation clearly suggest two oxidation processes with the valence band and/or surface-bound OH radicals and the free OH radicals.