Photo-induced charging effect and electron transfer to the redox species on nitrogen-doped TiO2 under visible light irradiation

Energy levels for sub-band structures of the nitrogen-doped TiO2 (N-TiO2) and photo-excitation mechanism for the visible light response were investigated by photo-electrochemical and spectroscopic measurements. It was demonstrated that the photo-excitation from the N-doping level of N-TiO2 causes the accumulation of electrons into the sub-band level at the potential energy of ca. +0.35 V vs. NHE (pH 2.5) under visible light irradiation. Subsequently, electron transfer does not take place from the photo-charged N-TiO2 under visible light irradiation into such redox species as methyl viologen (MV2+), H+, Cu2+ ions, but into O2 molecules, Pt4+, Ag+ and Au3+ ions by way of the sub-band level. These findings shed light on a mechanism for the photocatalytic reactions on the N-TiO2 under visible light irradiation.