Optimal levels of oxygen deficiency in the visible light photocatalyst TiO sub(2-x) and long-term stability of catalytic performance

The dependence of the visible light-responsive photocatalytic activity of oxygen deficient TiO sub(2) (TiO sub(2-x)) prepared by Ar/H sub(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 sub(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 sub(2) (TiO sub(2-x)N sub(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 sub(2-x) at this point was found to be nearly equal to that of TiO sub(2-x)N sub(x). The results of ab initio calculations of density of states for TiO sub(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 sub(2-x) at larger x values is attributed to the emergence of new oxygen deficient states.