**2. Design for solar environmental catalysts**

The requirements for the highly active TiO2-based "solar environmental catalyst" are described below. As shown in Figure 1, the excitation from the VB electrons to the CB needs UV-light irradiation, occupying only 3% of the incident sunlight. From a viewpoint of the effective use of the sunlight, the response to the visible-light occupying 45% of the solar energy (Figure 1) should be particularly imparted to anatase TiO2 and anatase–rutile-mixed TiO2 (Requirement 1). However, even if TiO2 can be endowed with the visible-light activity, it is usually much smaller than the UV-light activity. Therefore, the inherent excellent UV-light activity of TiO2 should be compatible with the visible-light activity (Requirement 2). For high levels of visible and UV-light activities to be achieved, the enhancement of the ORR is crucial because it is usually the rate-determining step in the TiO2-photocatalyzed reactions [21] (Requirement 3).
