Photoelectrochemical behavior of nanostructured TiO2 thin-film electrodes in contact with aqueous electrolytes containing dissolved pollutants:: A model for distinguishing between direct and indirect interfacial hole transfer from photocurrent measurements

A model for describing the photoelectrochemical behavior of nanostructured TiO2 thin-film electrodes in contact with aqueous electrolytes containing dissolved pollutant species is presented. The model correlates the steady-state photocurrent, the illumination intensity, and the specific photooxidation mechanism for dissolved pollutant species in competition with water molecules. For physical events, the model considers the existence of hole trapping at surface states and the specific hole-transfer mechanisms taking place at the semiconductor-electrolyte interface (direct, via photogenerated valence-band free holes or indirect, via photogenerated surface-bound OH radicals or both). The model has been applied to the study of the photooxidation of aqueous solutions of formic acid and methanol. In agreement with previous results concerning massive polycrystalline electrodes, methanol is found to be photooxidized via indirect hole transfer, whereas formic acid is photooxidized mainly with the participation of valence-band free holes.