photocatalysts

Photocatalysts are materials that accelerate chemical reactions upon light irradiation without being consumed in the process. Most studied photocatalysts are semiconductors that absorb photons with energy at least equal to their bandgap, generating electrons in the conduction band and holes in the valence band. These charge carriers migrate to the surface where they participate in redox reactions: holes can oxidize water or organic substrates to produce hydroxyl radicals, while electrons can reduce oxygen or other species. If both charge carriers are efficiently separated and retained at the surface, the catalyst can mineralize pollutants to inorganic end products such as CO2 and water. Recombination and surface recombination reduce efficiency, and strategies to suppress it are central to catalyst design.

Hundreds of materials have been explored as photocatalysts, with titanium dioxide (TiO2) being the most studied due to its stability, abundance, and non-toxicity. A limitation is its wide bandgap, which restricts activation to ultraviolet light. To harvest visible light, researchers employ doping with nonmetal or metal species, coupling with narrower-bandgap semiconductors (e.g., g-C3N4, WO3, ZnO) to form heterojunctions, or creating Z-scheme systems. Surface modifications, increased surface area, controlled crystallinity, and the use of co-catalysts such as platinum or nickel can improve charge separation and drive specific reactions. Carbon-based materials and graphene can act as conductive supports that facilitate electron transport.

Applications of photocatalysis include environmental remediation, such as degradation of dyes, pesticides, and pharmaceuticals in water, as well as self-cleaning surfaces. In energy-related fields, photocatalysts aim to drive water splitting for hydrogen production or the reduction of carbon dioxide to value-added fuels. Despite progress, challenges include relatively low quantum efficiencies under solar irradiation, limited stability under reactive conditions, and economic constraints for scale-up. Ongoing research focuses on developing durable, active, and selective photocatalysts capable of efficient operation under visible light.