Can OH Radicals Diffuse from the UV-Irradiated Photocatalytic TiO2 Surfaces? Laser-Induced-Fluorescence Study

Diffusion of OH radicals from UV-irradiated TiO2 surface to the gas phase was successfully detected using a laser-induced-fluorescence technique for various types of TiO2 powders. The diffusion time of OH radicals was found to vary with the types of TiO2 powders and to be affected by the heat treatments of these powders, depending on the treatment temperatures. The diffusion mechanism was discussed based on the characteristic OH-LIF intensities for individual TiO2 powders and the observations of OD-LIF after the exposure of D2O vapors over the TiO2 powders. The quantum yield of OH radicals diffused from the TiO2 surface was estimated to be about 5 × 10-5 by comparing the OH-LIF intensities produced by the 266-nm photolysis of HNO3.