Formation and behavior of singlet molecular oxygen in TiO2 photocatalysis studied by detection of near-infrared phosphorescence

Singlet molecular oxygen (O-1(2)), which was produced with a significant yield, was detected from a powder-type TiO2 photocatalyst irradiated with a 355-nm laser pulse by monitoring its near-infrared phosphorescence at 1270 nm. Lifetime measurements for the O-1(2) produced at TiO2 (Degussa P25) in various environments, such as in air and in suspensions of H2O, D2O, and ethanol, indicated that quenching takes place mainly at the TiO2 surface in the absence of reactants. Quantum yields for O-1(2) generation were measured for ten commercial TiO2 photocatalysts in air and ranged from 0.12 to 0.38, while the lifetimes ranged from 2.0 to 2.5 mu s. Since the quenching by the TiO2 surface is quite fast, the formation and decay processes of O-1(2) have not been distinguished from the recombination of the photogenerated electron-hole pairs. The values of quantum yield suggest that O-1(2) may contribute to the oxidation of some organic molecules at the irradiated TiO2 surface.