Spray deposited CeO2-TiO2 counter electrode for electrochromic devices

Optically passive thin films of CeO2-TiO2 mixed oxides with molar ratio of Ce/Ti of 0.05 were deposited by the spray pyrolysis technique (SPT) on a glass and fluorine-doped tin oxide (FTO)-coated glass substrates. Precursor solution containing cerium nitrate hexahydrate (Ce(NO3)(2)a <...6H(2)O) and titanium tetraiso-propoxide (Ti((OPr)-Pr-i)(4)) having different volumetric proportions (0-5 vol% of Ti) in methanol were used. These films were characterized for structural, morphological, molecular, optical, electrochromic and colourimetric analysis. CeO2-TiO2 films deposited at 400 (a) C were found to be polycrystalline with cubic fluorite crystal structure. Transformation from polycrystalline to amorphous phase was observed with increasing TiO2 content. The band centred at 539 cm(-1) is assigned to Ce-O stretching vibration and the two medium intensity bands assigned to (Ti-O) and (Ti-O-Ti) stretching modes at 798 and 451 cm(-1), which confirms the mixed CeO2 and TiO2 phases. The band gap energy decreases (E (g)) from 3.45 eV for pristine CeO2 to 2.98-3.09 eV for CeO2-TiO2 films. The ion storage capacity (ISC) of CeO2-TiO2 thin film with 3 vol% Ti (Ce-Ti-3 sample) was found to be 26 mC cm(-2) and electrochemical stability up to 30,000 cycles in 0.5 M LiClO4-PC electrolyte. The optically passive behaviour of CeO2-TiO2 thin film is confirmed by its negligible transmission modulation (Delta T 2.5%) upon Li+ ion insertion/extraction, irrespective of the extent of Li+ ion intercalation. The optical modulation of sputter deposited electrochromic WO (3) thin film was found to be enhanced from 56 to 61% with rapid increase in colouration efficiency (CE) from 42 to 231 cm(2) C-1 when CeO2-TiO2 is coupled as a counter electrode with WO3 in an electrochromic device (ECD). On reduction of WO3 thin film with CeO2-TiO2 as counter electrode, the CIELAB 1931 2 (a) colour space coordinates show the transition from colourless to the deep blue state (L (au) = 88.07, a (au) = -2.37, b (au) = 24.59 and L (au) = 40.32, a (au) = -1.16, b (au) = -5.65) with steady decrease in relative lightness. Yxy and L (au) a (au) b (au) coordinates signify CeO2-TiO2 films and it also exhibits the application as counter electrode in electrochromic smart windows in which they are able to retain their transparency under charge insertion/extraction.