photocatalyst

A photocatalyst is a substance that uses light to accelerate a chemical reaction without undergoing a permanent chemical change itself. In photocatalysis, photons excite a semiconductor material, creating charge carriers that drive redox reactions on the material’s surface.

When a photocatalyst absorbs light with energy at least equal to its band gap, electrons are promoted from the valence band to the conduction band, leaving behind holes. The excited electrons and holes migrate to the surface, where they can participate in reduction and oxidation reactions. Electrons can reduce oxygen or other species, while holes can oxidize water, hydroxide, or organic contaminants. Reactive oxygen species formed during these processes can oxidize pollutants, convert pollutants to less harmful products, or drive other useful transformations such as water splitting.

Common photocatalysts are semiconductors. Titanium dioxide (TiO2) is the most widely used due to stability, abundance, and relative safety, though its wide band gap limits visible-light activation. Other materials include zinc oxide (ZnO), cadmium sulfide (CdS), tungsten trioxide (WO3), bismuth vanadate (BiVO4), graphitic carbon nitride (g-C3N4), and various perovskites. Researchers employ strategies such as doping, coupling (heterojunctions), sensitization, and surface modifications to extend light absorption into the visible range and improve charge separation.

Applications span environmental and energy fields. Photocatalysts are used for degrading organic pollutants in water and air, sterilizing surfaces, self-cleaning coatings, and solar fuel production such as hydrogen generation from water and CO2 reduction. Challenges include rapid recombination of charge carriers, limited visible-light activity for many materials, stability under operating conditions, and scalable, cost-effective implementation.