Tuning Photoelectrochemical Performances of Ag−TiO₂ Nanocomposites via Reduction/Oxidation of Ag

Abstract

The effects of chemical states of Ag on the photoelectrochemical (PEC) properties of Ag−TiO 2 composites were investigated with Ag(0)−TiO 2 and Ag(I)−TiO 2 prepared by photoreduction-thermal treatment (PRT) method. The comparison of photoaction spectra of Ag(0)−TiO 2 and Ag(I)−TiO 2 showed that only the Ag(0) containing samples had notable photocurrent under visible light (in the range of 400−800 nm), which was attributed to the highly dispersed Ag(0), according to the DRS, XRD and XPS measurements. During the photocurrent spectra measurements of Ag(0)−TiO 2, it was demonstrated that Ag(0) was photoexcited because of plasma resonance in the visible light region, and charge separation was accomplished by the transport of photoexcited electrons from Ag(0) to the TiO 2 conduction band with the simultaneous formation of Ag(I), which could be partially reduced to the initial active Ag(0) state under the following UV light irradiation. Actually, it was the interconversion of Ag(0) and Ag(I) during the alternating irradiation that avoided the rapid decay of photocurrent and ensured a durable and stable visible light-induced photocurrent. In the case of visible light degradation of methyl blue (MB), however, Ag(0)−TiO 2 showed poorer photocatalytic activity than Ag(I)-containing ones. It was proposed that photoexcited Ag(I) rather than Ag(0) acted as active sites that were responsible for the enhanced photocatalytic abilities, whereas Ag(0) might contribute to the stability of the photocatalysts. Hence, the Ag−TiO 2 nanocomposites can exhibit different photoelectrochemical performances under visible light with the different chemical states of Ag. This work could have significance not only in the mechanism study but also in the attempts to improve the visible light-induced photoactivities of Ag−TiO 2, by tuning the chemical states of Ag species, in potential photoelectrochemical applications.