Titanium dioxide is a photo catalyst, once it is illuminated by light with energy higher than its band gaps, the electrons in TiO2 will jump from the valence band to the conduction band and the electron (e-) and electric hole (h+) pairs will form on the surface of the photocatalyst. The negative electrons and oxygen will combine into O2 - the positive electric holes and water will generate hydroxyl radicals. Since both are unstable chemical substances, when the organic compound falls on the surface of the photocatalyst it will combine with O2 - and OH- respectively, and turn into carbon dioxide (CO2) and water (H2O).