Lifetime and diffusion coefficient of active oxygen species generated in TiO2 sol solutions

Active oxygen species generated by photoexcitation of TiO2 sol solutions were directly observed by the single-shot near-field heterodyne transient grating method. Transient responses were compared in the presence and absence of various kinds of scavengers such as sodium azide, dimethyl sulfoxide, and superoxide dismutase, and three components of the transient responses were assigned to hydroxyl radicals, hydrogen peroxide, and superoxide radicals. The diffusion coefficients of each active oxygen species were obtained by the analysis of the transient responses, and it was revealed that they were 1-3 orders smaller than those for molecules with a similar size. It was proposed that this is because the active oxygen species are under equilibrium between free radicals and adsorbed species on a TiO2 surface. Furthermore, it was suggested that the adsorption equilibrium for each species was varied depending on the pH of the solutions.