Optimal levels of oxygen deficiency in the visible light photocatalyst TiO2−x and long-term stability of catalytic performance

Abstract The dependence of the visible light-responsive photocatalytic activity of oxygen deficient TiO 2 (TiO 2− x ) prepared by Ar/H 2 plasma surface treatment on the degree of oxygen deficiency ( x ) was assessed to determine the deficiency region associated with highest performance. The highest activity was obtained at x =0.06 (TiO 1.94 ). The maximum visible light activity for this material, estimated from the formaldehyde (HCHO) removal rate, was three times higher than that exhibited by nitrogen-doped TiO 2 (TiO 2− x N x ). The catalytic ability was found to decrease over the first week after fabrication of the material, after which it became stable, and the performance of TiO 2− x at this point was found to be nearly equal to that of TiO 2− x N x . The results of ab initio calculations of density of states for TiO 2− x suggest that new oxygen deficiency states emerge at almost the exact center between the valence and conduction bands when x >0.06, which increases the recombination rate between electrons and holes. Therefore the declining performance of TiO 2− x at larger x values is attributed to the emergence of new oxygen deficient states.