ESR study on the visible photocatalytic mechanism of nitrogen-doped novel TiO2: Synergistic effect of two kinds of oxygen vacancies

The visible photocatalytic mechanism of nitrogen-doped novel TiO2 was studied by means of electron spin resonance spectroscopy (ESR). It was found that, under visible light irradiation, the concentration of single-electron-trapped oxygen vacancy (SETOV, Vo•) of novel TiO2 remained unchanged, but that of nitrogen-doped novel TiO2 increased and returned to original state when the light was turned off. This implies that, aside from Vo• in bulk of nitrogen-doped novel TiO2, oxygen vacancy without trapped electron (Vo••) was formed on its surface. Vo•• as a surface electron trap captured photogenerated electron from the bulk to generate extra Vo•, carrying out photocatalytic reaction on the surface. At the same time, nitrogen doping product NO was chemically adsorbed on the vicinity of Vo•• and inhibited the attack of oxygen, allowing Vo•• to remain stable in air. The synergistic action of the two kinds of active structures, i.e., bulk Vo•–NO–Ti and surface Vo••–NO–Ti, accounted for the visible photocatalytic activity of N-doped novel TiO2.