Photochromism of titania sol and gel

Titania sol and gel were prepared through the hydrolysis of Ti (OC4H9)(4) i n acidic water solution. The sol and gel are sensitive to UV light and turn into blue - purple on irradiation. Electron spin resonance (ESR) measureme nt showed that the photochromism was due to the presence of titanium (III) ions in the . . . Ti-O-Ti . . . networks. The titanium (III) ions could be oxidized by the oxygen in the atmosphere, and cause the slow color fading. The absorption peaks in the optical absorption spectra of the titania gel w ere attributed to the transition of 3d electron of a trivalent titanium in certain environment.