Formation and Behavior of Singlet Molecular Oxygen in TiO 2 Photocatalysis Studied by Detection of Near-Infrared Phosphorescence

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