A theoretical investigation on photocatalytic oxidation on the TiO2 surface

The TiO2 photocatalytic oxidation mechanism was theoretically investigated by using long-range corrected time-dependent density functional theory (LC-TDDFT) with a cluster model of the anatase TiO2(001) surface. We found that LC-TDDFT with the cluster model quantitatively reproduces the photoexcitations of the TiO2 surface by calculating the electronic spectra of a clean TiO2 surface and one with oxygen defects. We calculated the electronic spectra of a molecularly adsorbed TiO2 surface for the adsorptions of phenol, methanol, and methane molecules as typical organic molecules. We obtained the surprising result that the main peak of the phenol-adsorbed TiO2 surface, which overlaps with the main peak of the clean TiO2 surface, corresponds to charge transfers from the phenol molecule to the TiO2 surface. This indicates that the TiO2 photocatalytic oxidation proceeds through direct charge transfer excitation from the substrate molecules to the TiO2 surface. In contrast, we found slight and no charge transfer for methanol and methane adsorption, respectively, in agreement with the experimental findings for their reactivities. In light of these results, we propose a new mechanism for heterogeneous TiO2 photocatalytic oxidations.