Nano-titanium dioxide (TiO2) as a good photocatalyst, its surface would produce highly active hole/electron couples in the light.O2 and H2O can be converted to free radical at these active sites.  These free radicals are very active. Its can degrade the organic matter in the air and a variety of bacteria to unharmful matters. The photocatalyst itself is like a natural material, non-toxic harmless, and it is not involved in the reaction but to provide a location and condition. So it is almost permanent, it is considered to be the best materials for air pollution control.
Another property of this material is the self-cleaning function. It’s also can be used as a self cleaning coating to protect the surface of buildings etc.