Photocatalysis is a process that uses light energy to speed up a chemical reaction. It causes a change in a reaction pathway, helps lower the amount of energy needed for a reaction to start, and therefore increases the reaction rate. Photocatalysis allows reactions that would not happen under regular conditions to take place. This process is useful for solving environmental problems, like removing pollutants from air or water. It's also being studied to create renewable energy.

For photocatalysis to happen, you need a special material called a photocatalyst, which can absorb light. When the material absorbs light, its electrons get energized and move from one energy level to another. If the material is too good at conducting electricity (like a metal), or if it doesn’t let electrons move at all (like an insulator), photocatalysis won’t work. Photocatalysts are semiconductor materials, in which the difference between energy levels is appropriate for the electrons to be promoted from one orbital to another thanks to light energy. Another thing that must happen is adsorption of a reactant on the surface of the photocatalyst. Photoexcited electrons create charge imbalance. Due to this charge difference, molecules are attracted to photocatalyst surface and adsorb to it. In effect bonds in the reactant molecules are more easily destroyed. Products of photocatalysis can further decompose or react with other molecules.