Mechanistic Study of Visible-Light-Induced Photodegradation of 4-Chlorophenol by TiO 2− x N x with Low Nitrogen Concentration

TiO2−x Nx powders with low N-doping concentrations ( 0 . 0 2 1 < 𝑥 < 0 . 0 4 9 ) were prepared by annealing commercial TiO2 (P-25) under an NH3 flow at 550°C. Regardless of UV or visible case, the photoactivities of the samples decreased as x increased, and TiO1.979N0.021 showed the highest activity for the 4-chlorophenol (4-CP) decomposition under the visible-light irradiation. The visible-light response for N-doped TiO2 could arise from an N-induced midgap level, formed above the valence band (O 2p). Electron spin resonance (ESR) measurements and the radical scavenger technologies gave the combined evidence that the active species ( • O H and O 2 • − ) are responsible for the photodecomposition of 4-CP over TiO2−xNx under the visible irradiation. A possible photocatalytic mechanism was discussed in detail.