Time-Dependent Behavior of Active Oxygen Species Formed on Photoirradiated TiO₂ Films in Air

Active oxygen species formed on TiO 2 film photocatalysts in air were examined by use of a chemiluminescence method. Under relatively strong UV light irradiation, 15 mW cm -2, two types of active oxygen species formed on the TiO 2 surface were observed after the interception of the excitation light, i.e., one with a shorter lifetime, ca. 3 s, and another with a longer lifetime, ca. 50 s, which may correspond to superoxide (O 2 - ). On the contrary, under relatively weak UV light irradiation, 1 μW cm -2, only the long-lived species, O 2 -, was observed. The concentration of O 2 - on the TiO 2 surface was saturated even under weak UV illumination, while the short-lived species was not. We suggest that O 2 - is deactivated mainly by an electron transfer to surface defects or vacancy sites on the TiO 2 surface. This deactivation reaction proceeds almost independently of coexisting active species.