Environmental photo(electro)catalysis: Fundamental principles and applied catalysts

Starting with a detailed introduction of the fundamental concepts that are most relevant to photocatalytic and photoelectrocatalytic systems and processes, this chapter reviews the recent research and development of semiconductor-based photocatalyst materials that are applicable to environmental remediation purposes. A wide variety of TiO2particles and/or films has been studied during the past 30 years as the most stable and powerful photocatalysts toward the degradation of various organic pollutants. The photocatalytic performance of other semiconductor materials such as ZnO, SnO2, WO3, Fe2O3, and CdS has also been intensively investigated. A general limitation concerning the efficiency for any photocatalytic process is the recombination of the photogenerated charge carriers, i.e., of electrons and holes, following bandgap illumination. Considerable efforts have been made to suppress the recombination hence enhancing the charge-carrier separation and the overall efficiency by means of, e.g., coupling of different semiconductors with desirable matching of their electronic band structures, or incorporation of noble metal nanoclusters onto the surface of semiconductor photocatalyst particles. Modification of the physicochemical properties, e.g., particle size, surface area, porosity, and/or crystallinity of the semiconductor materials and optimization of the experimental conditions such as pH, illumination conditions, and/or catalyst loading, during photocatalytic reactions have also been elaboratively addressed to achieve high reaction rates or yields. In order to utilize the solar energy more efficiently, i.e., to extend the optical absorption by photocatalysts into the visible light range, numerous research groups have contributed to developing novel visible-light-active photocatalysts. With the application of semiconductors with narrower bandgap such as CdS, Fe2O3, and WO3being a straightforward choice, doping of wide bandgap semiconductors as exemplified by TiO2has been the most popular technique to enhance their optical absorption abilities. Both theoretical and experimental evidence have been accumulated to support that such-developed semiconductor materials can serve as highly efficient photocatalysts.