REPRODUCIBILITY OF THE COLORATION PROCESSES IN TIO2 FILMS

Transparent titanium oxide coatings produced by the sol-gel dipping technique exhibited pronounced electrochromic properties. The reproducibility of coloration cycles in TiO2 films was investigated as a function of time. A saturation-like behaviour of colour centres produced by charge injection in the form of Li+ ions was found. X-ray analysis revealed that TiO2 films changed to LixTiO2 during the electrical coloration process. 1 M LiClO4-propylene carbonate was used as the Li+-supplying electrolyte. The crystal structure of coloured films changed back to the initial state when the films were bleached, even though a large quantity of lithium atoms still remained inside the bleached films. We propose that a fraction of the lithium cations will bind to optically inactive sites, possibly the grain boundaries of the TiO2 film, in the bleaching process, and will not contribute to the coloration. Scanning electron microscopy and X-ray diffractometry, ellipsometry, UV-visible-near-NIR spectroscopy and X-ray photoelectron spectroscopy were used for characterization. The stability of coloured TiO2 films under zero applied field was good for as-deposited films, in contrast, crystalline films were easily bleached under the same conditions. Sol-gel as-deposited TiO2 films exhibit electrochromism and indicate that TiO2 layers could soon be very promising for electrochromic applications.