**7. Conclusions and future prospect**

The surface of TiO2 can be modified by oxocomplexes of the first transition metals (MOCs/ TiO2) with the loading amount precisely controlled by using the CCC technique. Among the MOCs/TiO2, Fe2O3-, Co2O3- and NiO-surface-modified TiO2 possess unique physicochemical properties such as strong visible-light absorption and the excellent reduction ability of O2. Spectroscopic experiments and first-principles DFT simulation have revealed that the surface modification with the MOCs raises the VB maximum of TiO2 due to the formation of plural metal–O–Ti interfacial bonds. Surface-to-bulk and/or bulk-to-surface interfacial electron transfer induced by visible-light absorption enhances charge separation. This novel coupling system consisting of MOCs and TiO2 would be promising as the "solar environmental catalyst."

The standard potentials of multiple-electron ORRs (*E*<sup>0</sup> (O2/H2O2) = +0.695 V and *E*<sup>0</sup> (O2/H2O) = +1.229 V versus SHE) are much more positive than that of one-electron ORR. Therefore, the hybridization of appropriate electrocatalysts for the multiple-electron ORR can impart visiblelight activity to many metal oxide semiconductors with *E*g < 3 eV. The effectiveness of this approach has recently been verified in the Pt NP-WO3 (Eg = 2.7 eV) [43] and Cu(acac)2-BiVO4 (Eg = 2.4 eV) hybrid systems [44], where Pt NP and O2-bridged Cu complex work as excellent electrocatalysts for multiple ORRs, respectively.

As a future subject, we further suggest the importance of the effective use of the infrared ray occupying 52% of the solar energy for the catalytic reactions (Figure 1). For example, Co2O3/ TiO2 exhibits high levels of photocatalytic and thermocatalytic activities [29], whereas Mn2O3/ TiO2 exhibits a high thermocatalytic activity for the oxidation of organic compounds [45]. Further, MOCs/TiO2 with the VB maximum level (or the oxidizing ability of the VB holes) finetuned by the loading amount may open up the application of MOCs/TiO2 to "green" and selective chemical transformations [46–48].
