TiO2 nano-particles based photochromic composite films

Photochromic TiO2-based composite films, with a particle size of 30-40 nm, were prepared using a titanium tetra-isopropoxide and a transparent urethane resin. The transparent films turned blue upon UV irradiation. They were color-saturated in about 20 min, then bleached reaching the initial transparency after two hours in a dark room.