TiO2 is by far the most widely studied transition oxide semiconductor for photochemical and photoelectrochemical applications. Thus it has become a sort of common model for research. It is a relatively abundant and stable material, with more a relatively good deal of resistance to photocorrosion than that of other commonly studied oxides. However, the larger band gap of conventional unmodified anatase TiO2 (3.2 eV) makes it less economical for practical visible light applications. On a longer run, research is required for visible light active semiconductor oxides that possess the same stability and versatility of TiO2.