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

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 °C), 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 N−, detected as N by EPR, whose ionic character varies with the temperature of annealing. The amount of N increases till 450 °C, then it decreases. Similar trend was observed for the photomineralization of phenol under visible light irradiation (λ > 385 nm): the photoactivity of N-doped samples becomes maximum for N–TiO2 annealed at 450 °C. The overall results suggest that the efficacy of the catalyst depends on the ability of N− centers to trap photogenerated holes. This effect lowers the rate of electron–hole recombination and allows the N (N− + h+) center acts as strong oxidizing agent.