Hydrothermal N-doped TiO2: Explaining photocatalytic properties by electronic and magnetic identification of N active sites

Abstract

N-doped TiO2 nanocrystals with high photoactivity in the visible range, were successfully synthesized by hydrothermal method, followed by thermal annealing at different temperatures (350–600 8C), in order to allow differential nitrogen diffusion into the TiO2 lattice. Optical and magnetic properties, studied by diffuse reflectance spectroscopy, electron paramagnetic resonance and X-ray photoelectron spectroscopy analysis, revealed that TiO2 was effectively doped. The thermal treatment induces insertion of nitrogen into TiO2 lattice in the form of nitride anion NÀ, detected as N by EPR, whose ionic character varies with the temperature of annealing. The amount of N increases till 450 8C, then it decreases. Similar trend was observed for the photomineralization of phenol under visible light irradiation (l > 385 nm): the photoactivity of N-doped samples becomes maximum for N–TiO2 annealed at 450 8C. The overall results suggest that the efficacy of the catalyst depends on the ability of NÀ centers to trap photogenerated holes. This effect lowers the rate of electron–hole recombination and allows the N (NÀ + h+) center acts as strong oxidizing agent.