Site Sensitivity of Interfacial Charge Transfer and Photocatalytic Efficiency in Photocatalysis

Understanding structural effects and reaction mechanisms of photocatalysts is of great interest and importance. Herein we report a comprehensive study of photocatalytic oxidation of methanol on various anatase TiO2 nanocrystals by means of combined in situ and time-resolved characterizations and density functional theory calculations. Surface site and resulting surface adsorbates were observed to strongly affect the surface band bending/bulk-to-surface charge migration processes and the interfacial electronic structure/interfacial charge transfer processes. TiO2 nanocrystals predominantly enclosed by the {001} facets expose a high density of reactive fourfold-coordinated Ti sites (Ti4c) at which CH3OH molecules dissociate to form the CH3O adsorbate (CH3O(a)Ti4c). CH3O(a)Ti4c exhibits the localized density of states almost at the valence band maximum of TiO2 surface, which facilitates the interfacial hole transfer process; meanwhile, CH3O(a)Ti4c with a high coverage greatly promotes the upward surface band bending, which facilitates the bulk-to-surface hole migration process. Consequently, among all the observed surface adsorbates, CH3O(a)Ti4c exhibits the highest photocatalytic oxidation rate constant. Consequently, TiO2 nanocrystals predominantly enclosed by the {001} facets are most active in photocatalytic methanol oxidation reaction. These results unambiguously exemplify the surface structure effect in photocatalysis and elucidate the underlying mechanism.