Photocatalytic characterization of oxygen vacancy TiO$_{2}$ prepared with Ar/H$_{2}$ plasma surface treatment

It has been observed that oxygen vacancy TiO2 (TiO$_{2-x}$) prepared with Ar/H2 plasma surface treatment have an optimal process condition for visible light photocatalytic activity. However it may depend on the plasma process system. Therefore, it is desirable to describe the optimal condition with a more general term, for example the x value of TiO$_{2-x}$. In addition the reason why the optimal condition exists is unclear. In this study, we carried out the measurement to find out the dependence of visible light photocatalytic activity on the x value and ab initio calculation of density of states (DOS) of TiO$_{2-x}$. The dependence of visible light photocatalytic activity of TiO$_{2-x}$ on the x value has been estimated experimentally. When the x value was 0.069, the visible light photocatalytic activity was optimal. The results of the calculation have suggested a new oxygen vacancy state would emerge at a nearly center between the valence band and the conduction band for larger x values. This result implies that the recombination rate between electrons and holes would increase, resulting in less photocatalytic activity for the larger x value.