In Situ IR Spectroscopy Unveils Defects-Induced Changes in the Concentration of Surface-Available Photoholes in TiO2 Nanoparticles

Unraveling the effect of defects on the concentration of surface-available photoexcited charges is of great importance for semiconductors in photocatalysis. Herein, by using in situ diffuse-reflectance infrared (IR) spectroscopy, the concentration of surface-available photoholes on TiO2 nanoparticles with tunable bulk/surface defects was quantified by an adsorbate (methanol) surface elementary reaction kinetic analysis method. Reaction kinetics showed that the oxidation of the adsorbed methanol species under light irradiation was a first-order reaction. Therefore, the concentration of surface-available photoholes can be calculated from the rate constants of adsorbed methanol on the TiO2 surface. By carefully comparing the specific reaction rate constants on the TiO2 surface with various bulk/surface defect ratios, we demonstrated for the first time that increasing the bulk defect concentration relative to the surface defects decreases the concentration of surface-available photoholes. This trend implicates different effects of bulk and surface defects on band bending and subsequent interfacial hole transfer processes in TiO2 nanoparticles. These insights not only reveal the mechanistic role of surface and bulk defects in photocatalysis but also highlight that in situ IR spectroscopy combined with reaction kinetic analysis has enormous potential in elucidating photocatalytic mechanisms.