Loading effects of silver oxides upon generation of reactive oxygen species in semiconductor photocatalysis

Superoxide anion radical (O-2(-center dot)) and OH radical generations in suspensions of Ag metal-, Ag2O-, or AgO-loaded TiO2 and BiVO4 photocatalysts in alkaline conditions ( pH 12.0) were examined by means of a luminol chemiluminescence ( CL) technique and a spin-trapping fluorescence one in which terephthalic acid reacts with an OH radical to afford the highly fluorescent 2-hydroxyterephthalic acid (TAOH), respectively. The observed luminol CL intensity was remarkably enhanced by the AgO loading on TiO2 as well as BiVO4. This can be explained by enhancement of O-2(-center dot) production on the AgO-loaded photocatalysts caused by the synergetic effects on the thermocatalytic activity upon the AgO surface and the efficient electron-hole separation at the photocatalyst/AgO interface. On the other hand, loading effects of AgO on the TAOH formation were not so significant compared to those on the CL observation, though the TAOH formation rates for the TiO2 samples were much larger than those for the BiVO4 ones by about three orders of magnitude. The properties of O-2(-center dot) and OH radical generations on these photocatalysts are discussed on the basis of the luminol CL kinetics and approximate band edge positions of TiO2, BiVO4, and silver oxides.