Behavior of superoxide radicals formed on TiO2 powder photocatalysts studied by a chemiluminescent probe method

A luminol chemiluminescence (CL) probe method was successfully applied to the investigation of the superoxide radical (O(2)(-.)) formed on photoirradiated TiO(2) powders. For several kinds of commercially available TiO(2) photocatalysts and their calcined samples, the amount of O(2)(-.) produced at the steady state was measured and found to increase with the secondary particle size or the degree of aggregation. The decay of O(2)(-.) for the non-calcined samples was as long as several hundred seconds and obeyed second order kinetics, indicating that disproportionation is the main deactivation pathway. For some TiO(2) photocatalysts, oxidative species such as OH(.) radicals are suggested to exist for 1 s after irradiation. On the other hand, reductive photoinduced electrons may remain for several seconds after irradiation since the decay of O(2)(-.) starts after a delay of more than 1 s. On calcination at temperatures up to 1173 K, O(2)(-.) decays by a mechanism other than disproportionation, suggesting the prolonged lifetime of an oxidative species that can react with O(2)(-.).