ABSTRACT
Heterogeneous photocatalysis is the process of photochemical reactions on the surface of solid photocatalysts under the irradiation of light, thus light and a photocatalyst are the prerequisites to promote the photocatalytic redox reactions. Only when photons with sufficiently high energy are absorbed, semiconductor photocatalysts can further facilitate the oxidation and reduction processes [1,2]. At present, most photocatalysts are reported as n-type semiconductor oxides, which actually depend on their own optical characteristics. Some popular photocatalysts have been extensively studied, such as TiO2, ZnO, CdS, WO3, SnO2, Fe2O3, In2O3, and so on. TiO2 is the most popular photocatalyst for industrial applications because of its nontoxicity, natural abundance, high stability, and easy availability in various reaction systems, including liquid-solid, gas-solid, and liquid-gas-solid. Although different photocatalysts are matched with different catalytic activities, the photocatalytic principles of the compound semiconductors are similarly consistent, especially for two popular semiconductor oxides, ZnO and TiO2, which are both n-type oxide semiconductors with roughly the same band gap. This section will mainly discuss the basic principles of direct photocatalysis [3,4].
