Wavelength-Dependent Water Oxidation on Rutile TiO₂(110)

Understanding the microscopic mechanism of water photocatalysis on TiO₂ is of great value in energy chemistry and catalysis. To date, it is still unclear how water photocatalysis occurs after the initial light absorption. Here we report the investigation of the photoinduced water dissociation and desorption on a R-TiO₂(110) surface, at different wavelengths (from 250 to 330 nm), using temperature-programmed desorption and time-of-flight techniques. Primary photooxidation products, gas phase OH radicals and surface H atoms, were clearly observed at wavelengths of ≤290 nm. As the laser wavelength decreases from 290 to 250 nm, the relative yield of H₂O oxidation increases significantly. Likewise, photoinduced H₂O desorption was also observed in the range of 320-250 nm, and the relative yield of H₂O desorption also increases with a decrease in wavelength. The strong wavelength-dependent H₂O photooxidation and photodesorption suggest that the energy of charge carriers is important in these two processes. More importantly, the result raises doubt about the widely accepted photocatalysis model of TiO₂ in which the excess energy of charge carriers is useless for photocatalysis. In addition, the H₂O photooxidation is more likely initiated by nonthermalized holes and is accomplished on the ground state potential energy surface via a non-adiabatic decay process.