titania sol and gel were prepared through the hydrolysis of Ti (OC~4H~9)~4 in acidic water solution. The sol and gel are sensitive to UV ilght and turn into blue-purple on irradiation. electron spin resonance (ESR) measurement showed that the photochromism was due to the presence of titanium (Ⅲ) ions in the …Ti－O－Ti－O… 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 were attributed to the transition of 3d electron of a trivalent titanium in certain environment.

Key words: TITANIUM DIOXIDE, SOL-GEL PROCESS, PHOTOCHROMISM, ELECTRON SPIN RESONANCE