Nitrogen-substituted TiO₂: investigation on the photocatalytic activity in the visible light range

Nitrogen-doped titanium oxides are attractive materials for the degradation of organic pollutants in water due to their photocatalytic activity in the visible light range. The evolution of the photocatalytic properties was studied on a number of TiO_xN_y powder samples where x varied from 2 to 0 (TiO₂ to TiN) through increasing the nitrogen content (y = 0–1). X-ray diffraction and Raman spectroscopy showed that an anatase type TiO_xN_y was obtained at low nitrogen contents (<2 wt%). With increasing nitrogen content a structural transition from anatase to cubic TiN was observed. Electron spin resonance measurements of the TiO_xN_y samples confirmed the presence of unpaired electrons and defects for the TiO_xN_y materials with low nitrogen content (<2 wt%). The photo-induced activity of the TiO_xN_y materials was evaluated under VIS illumination of solutions containing methylene blue as an organic probe. The TiO_xN_y samples exhibited an improved photocatalytic activity under visible light illumination compared to TiO₂ at nitrogen levels lower than 2 wt%. A photocatalytic activity could not be detected at nitrogen levels higher than 10 wt% and after conversion of the TiO_xN_y into the cubic phase. Optimum photocatalytic activity in the visible range can be achieved at nitrogen levels lower than 2 wt% for TiO_xN_y materials.