Theoretical Verification of Photoelectrochemical Water Oxidation Using Nanocrystalline TiO2 Electrodes

摘要

Mesoscopic anatase nanocrystalline TiO2 (nc-TiO2) electrodes play effective and efficient catalytic roles in photoelectrochemical (PEC) H2O oxidation under short circuit energy gap excitation conditions. Interfacial molecular orbital structures of (H2O)3 &OH(TiO2)9H as a stationary model under neutral conditions and the radical-cation model of [(H2O)3&OH(TiO2)9H]⁺ as a working nc-TiO2 model are simulated employing a cluster model OH(TiO2)9H (Yamashita/Jono’s model) and a H2O cluster model of (H2O)₃ to examine excellent H₂O oxidation on nc-TiO₂ electrodes in PEC cells. The stationary model, (H₂O)₃&OH(TiO₂)₉H reveals that the model surface provides catalytic H₂O binding sites through hydrogen bonding, van der Waals and Coulombic interactions. The working model, [(H₂O)₃&OH(TiO₂)₉H]⁺ discloses to have a very narrow energy gap (0.3 eV) between HOMO and LUMO potentials, proving that PEC nc-TiO₂ electrodes become conductive at photo-irradiated working conditions. DFT-simulation of stepwise oxidation of a hydroxide ion cluster model of OH⁻(H₂O)₃, proves that successive two-electron oxidation leads to hydroxyl radical clusters, which should give hydrogen peroxide as a precursor of oxygen molecules. Under working bias conditions of PEC cells, nc-TiO₂ electrodes are now verified to become conductive by energy gap photo-excitation and the electrode surface provides powerful oxidizing sites for successive H₂O oxidation to oxygen via hydrogen peroxide.