Photocatalysis uses light to accelerate chemical reactions at the surface of a semiconductor catalyst, typically without being consumed in the process. When the photocatalyst absorbs photons with energy equal to or greater than its band gap, electrons are excited from the valence band to the conduction band, leaving behind positively charged holes. These charge carriers migrate to the surface, where they drive redox reactions with adsorbed molecules.

A central research goal is improving how efficiently light energy is converted into useful chemical transformations. Titanium dioxide is a widely studied photocatalyst due to its stability, low cost and non toxicity, but it mainly absorbs ultraviolet light, which is a small fraction of sunlight. To extend activity into the visible range, researchers tailor the band structure by doping with metal or nonmetal elements, forming solid solutions, or creating heterojunctions between different semiconductors. Such strategies promote broader light absorption and more effective separation of electrons and holes, reducing their recombination.

Nanostructuring is another key theme. Designing photocatalysts as nanoparticles, nanorods, nanosheets or porous frameworks increases surface area and provides more active sites. Interfaces between different phases, such as anatase and rutile in titanium dioxide, can enhance charge transfer. Cocatalysts like noble metals or transition metal oxides are often deposited on the photocatalyst surface to facilitate specific reactions, for example hydrogen evolution from water.

Applications include solar driven water splitting, carbon dioxide reduction to fuels, air and water purification, and self cleaning surfaces. Current challenges focus on raising quantum efficiency under visible light, improving stability and scalability, and understanding interfacial reaction mechanisms at the atomic level to guide rational design of next generation photocatalytic materials.