Photocatalysts are materials that induce photocatalytic reaction under photoirradiation. A general definition of photocatalysis, a conceptual name of photocatalytic reactions, is a chemical reaction induced by photoabsorption of a solid material, or “photocatalyst,” which remains unchanged during the reaction. Therefore, “photocatalyst” should act catalytically, i.e., without change, under light. Although molecules or metal complexes dissolved in solution or in the gas phase, not solid materials, can drive such photoinduced reactions without change during the reactions, they are called “photosensitizer” but not “photocatalyst.” photoinduced reactions are, in principle, initiated by photoabsorption, i.e., excitation of electrons in a material, followed by electron transfer from/to a reaction substrate. The photoexcited electrons can reduce a substrate adsorbed on the surface of a photocatalyst, and positive holes, electron deficiencies, can oxidize an adsorbed substrate, if...