Photocatalysis is using light as a catalyst to increase the rate of a photoreaction. Most known photoreactions begin when light creates a free radical in the reaction system. Light can have a lot of energy and is able to break chemical bonds. For example some things fade or lose their color when left in sunlight. When light breaks the chemical bonds, free radicals are produced. A free radical is a molecule, atom or an ion that needs an electron. The free radical will take electrons from other molecules or atoms and this starts chemical reactions.