Tracking the Concentration of Surface-Reaching Photoexcited Holes in TiO2 Nanoparticles: Anatase versus Rutile

Tracking the concentrations of surface-reaching photoexcited charges in photocatalysis is of great importance to fundamental science. Herein, through combining in situ and time-resolved diffuse-reflectance infrared spectroscopy (DRIFTS), we report an adsorbed methanol species surface elementary reaction kinetic analysis method. This strategy allows us to track the concentration of surface-reaching photoholes in well-defined anatase TiO2 nanoparticles, and compare it with that of rutile TiO2 nanoparticles. The photocatalytic oxidation of adsorbed methanol species on different TiO2 crystal phase surface follows first-order reaction kinetics. Thus, the corresponding rate constants of adsorbed methanol species have been acquired, which could directly mirror the concentration of surface-reaching photoholes under light irradiation. Based on the specific value of the reaction rate constant of adsorbed methanol species on different TiO2 crystal phase surface, for the first time, it can be concluded that concentration of surface-reaching photoholes in anatase is nearly 3.3 times greater than that of rutile under light irradiation. These results newly illustrate why anatase usually exhibits higher photocatalytic activity than rutile. Meanwhile, this work highlights that probing molecules in a surface elementary reaction kinetic analysis based on in situ and time-resolved DRIFTS could be utilized as an effective method to track the concentration of surface-reaching photoexcited charges in photocatalysis as well as other similar kinds of catalytic processes.