In situ FTIR studies of primary intermediates of photocatalytic reactions on nanocrystalline TiO2 films in contact with aqueous solutions

Multiple internal reflection infrared spectroscopy was applied to in situ investigations of surface intermediates of photocatalytic reactions on nanocrystalline TiO2 films in contact with aqueous solutions. UV irradiation in the presence of dissolved O-2 Caused the appearance of new bands peaked at 943, 838, and 1250-1120 cm(-1) together with intensity changes in other bands. Investigations of influences of the solution pH, the presence or absence of hole and electron scavengers, and isotopic H2O --> D2O exchange on the spectral changes have revealed that the primary step of photocatalytic 02 reduction is the formation of the surface peroxo species, Ti(02), giving the 943 cm(-1) band, probably with the surface superoxo species, TiOO-, as a precursor, in neutral and acidic solutions. The surface peroxo species is then transformed to the surface hydroperoxo, TiOOH, giving the 838 and 1250-1120 cm(-1) bands, by protonation in the dark. This is, to our knowledge, the first direct in situ spectroscopic detection of primary intermediates for the photocatalytic O-2 reduction in aqueous solutions. On the basis of the assignment, a possible reaction scheme for various processes of the photocatalytic O-2 reduction is proposed, which is in harmony with other spectral changes induced by the UV irradiation.