Nanostructured TiO2 semiconductor electrodes modified with surface attached viologens: applications for displays and smart windows

Viologen derivs. adsorbed on TiO2 surfaces in nanocryst.-nanoporous electrodes have been studied with respect to electrochromic phenomena. The transparent nature and the high internal surface area of these electrodes are very beneficial for this purpose. Two different viologens were studied giving rise to blue, green, and violet coloration. For one of the viologens the authors showed the possibility to vary the color for the same viologen/TiO2 electrode from transparent to green or violet depending on the applied potential. Depending on the applied potential different optical densities were obtained. Optical d. changes from 0.1 up to 1 and coloration efficiencies above 100 cm2 C-1 with a max. value of 440 cm2 C-1, were obtained. The switching times for these electrodes for both coloration and bleaching were faster than 1s for a transmittance change larger than 60%, using 0.5M LiClO4 in acetonitrile as electrolyte. Preliminary stability tests were performed. One electrode has been cycled 654 times without substantial decline in coloration efficiency.