Clarification of Mechanisms for Surface Photoreactions and Improvement of Photocatalytic Activity for Visible-light Responsive Metal Oxides

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With an aim to improve the activity of TiO_2 and related metal-oxide photocatalysts for solar water splitting and photodecomposition of harmful organic compounds, we have studied molecular mechanisms of surface photoreactions on TiuO_2 (rutile) and explored new active visible-light responsive metal-oxide photocatalysts. At first, we have succeeded in preparing atomically flat and stable n-TiO_2 (rutile) (110) and (100) surfaces by a novel method of HF-etching and annealing at 550℃. This success has for the first time enabled us to reveal interesting crystal-face dependences of important properties of TiO_2 (rutile), such as the flat-band potential (U_<fb>), the hole reactivity, and the photoluminescense (PL) spectrum which arises from a precursor (called surface-trapped hole, s.t.h.) of photooxidation reaction of water on TiO_2. In addition, we have clarified that the occurrence of the photooxidation reaction of water on the atomically flat TiO_2 (110) and (100) surfaces has led to their roughening on an atomic scale together with weakening of the PL intensity. These results have been explained well by our recently proposed mechanism that the photooxidation reaction of water on TiO_2 is initiated by a nucleophilic attack of water (Lewis base) to the surface-trapped hole (s.t.h.) (Lewis acid), giving strong support to the mechanism. In order to investigate further the validity of the mechanism, we have also studied the mechanism for the water photooxidation reaction on nitrogen-containing metal oxides, such as N-doped TiO_2 and TaON, and found that the reaction proceeds by the same mechanism as on TiO_2 as well. Furthermore, we have explored new visible-light responsive metal-oxide photocatalysts and found that mixed metal oxides such as BiCu_2VO_6 and BiZn_2VO_6 show higher activity that BiVO_4 that is known as one of the most active visible-light driven photocatalysts.