Role of Ag Loading on the Concentration of Surface-Reaching Photoexcited Holes in TiO2 Nanoparticles

The effect of noble metal cocatalyst loading on the concentration of surface-reaching photoexcited charges on semiconductors is an interesting and important issue in photocatalysis. Herein, through in situ diffuse-reflectance infrared spectroscopy (DRIFTS), an adsorbate (methanol) surface elementary reaction kinetic analysis method has been exploited to track the concentration of surface-reaching photoholes in a series of Ag/TiO2 nanoparticles with varied Ag loading. The photocatalytic oxidation of adsorbed methanol species on the Ag/TiO2 surface proceeds via first-order reaction kinetics. Thus, the measured reaction rate constants directly reflect the concentration of surface-reaching photoholes under light irradiation. By determining the specific reaction rate constants for methanol oxidation on the Ag/TiO2 surface, we discover that the concentration of surface-reaching photoholes under light irradiation exhibits a volcano-shaped dependence on the Ag loading. Under the optimized Ag loading, it could be found that the concentration of surface-reaching photoholes on Ag/TiO2 surface is nearly 1.5 times greater than that of the pristine TiO2 surface. These results provide, for the first time, unambiguous experimental evidence for the significance of modification with suitable noble metal cocatalysts on regulation of surface-reaching photoexcited charges in semiconductor-based photocatalysis.