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

Abstract

Active oxygen species formed on TiO2 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 TiO2 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 (O2-). On the contrary, under relatively weak UV light irradiation, 1 μW cm-2, only the long-lived species, O2-, was observed. The concentration of O2- on the TiO2 surface was saturated even under weak UV illumination, while the short-lived species was not. We suggest that O2- is deactivated mainly by an electron transfer to surface defects or vacancy sites on the TiO2 surface. This deactivation reaction proceeds almost independently of coexisting active species.