Photokatalysis

Photocatalysis is a process by which a catalyst absorbs light and accelerates a chemical reaction without undergoing permanent chemical change itself. In heterogeneous photocatalysis, a semiconductor catalyst absorbs photons with energy at least equal to its band gap, generating electron–hole pairs. The charge carriers migrate to the surface, where electrons participate in reduction reactions and holes drive oxidation reactions. Recombination of electrons and holes can limit efficiency, so many strategies focus on improving charge separation and extending light absorption.

Common photocatalysts include titanium dioxide (TiO2), zinc oxide (ZnO), cadmium sulfide (CdS), and graphitic carbon nitride (g-C3N4). TiO2 is widely studied due to chemical stability and safety, but its large band gap confines activity mainly to ultraviolet light. Approaches to improve performance include doping with nonmetal elements, forming solid solutions or heterojunctions with other semiconductors, and loading cocatalysts to promote surface redox reactions. These modifications aim to harvest visible light and enhance charge separation.

Applications span environmental and energy domains. Photocatalysis is used for degradation of organic pollutants in water and air, disinfection, and sterilization. It also enables solar-driven fuel production, such as splitting water to generate hydrogen and reducing carbon dioxide to value-added fuels. In organic synthesis, photocatalysis can drive selective transformations under mild conditions.

History and development began with the discovery of photoelectrochemical water splitting on TiO2 by Fujishima and Honda in 1972, which spurred extensive research into semiconductor photocatalysts, reactor designs, and material engineering. Challenges include limited quantum efficiency, reliance on UV light for many catalysts, catalyst stability, and scale-up considerations. Ongoing research seeks new materials, novel architectures, and sustainable implementations.