Photocatalysis is a light driven process in which a semiconductor catalyst absorbs photons to trigger chemical reactions, most often oxidation and reduction at its surface. When illuminated with light of sufficient energy, the photocatalyst generates electron hole pairs. These charges migrate to the surface and participate in redox reactions with adsorbed molecules such as water, oxygen or pollutants.

Titanium dioxide is the most widely studied photocatalyst because it is chemically stable, non toxic and relatively inexpensive. However, it primarily absorbs ultraviolet light, which is a small fraction of solar radiation. A major research focus is shifting its activity into the visible range. Strategies include doping with metal or nonmetal ions, coupling with narrow band gap semiconductors, depositing noble metals that trap electrons, and sensitizing with dyes or quantum dots.

Photocatalysis is explored for environmental remediation, including degradation of organic pollutants in water and air, self cleaning surfaces, and antimicrobial coatings. It is also central to solar fuel production, where water splitting yields hydrogen and oxygen or where carbon dioxide is reduced to energy rich products such as carbon monoxide, methane or methanol. Achieving high efficiency requires optimizing light absorption, charge separation, surface reaction kinetics and reactor engineering.

Researchers investigate nanostructured materials to increase surface area and control charge transport. Advanced characterization techniques track charge carrier dynamics and identify active sites. Computational studies guide the design of new photocatalysts with tailored band structures. Remaining challenges include low quantum efficiencies under sunlight, catalyst deactivation and scalability, but progress continues toward practical solar driven chemical technologies.