Sol-gel processed cerium oxide and mixed cerium-titanium oxide films as passive counter electrodes for transmissive electrochromic devices

Abstract

Thin films of CeO₂ and mixed oxide CeO₂-TiO₂ with a Ce/Ti mole ratio of one have been obtained by an alcohol based sol-gel process. The precursor materials for the latter comprises a Ce(III) salt, CeCl₃.7H₂O and titanium propoxide in ethanol. The mixed oxide film is characterized by the prevalence of amorphicity, though morphological studies have

indicated the presence of CeO₂ nanograins. Pure cerium oxide film exhibits crystalline nature with the average grain size in the nanorange. In agreement with the higher levels of Li⁺ ion intercalation in an amorphous film in comparison to crystalline film, a superior inserted/ extracted charge in the mixed oxide film is revealed from the electrochemical investigations. The films prepared from an aged sol are observed to exhibit a much higher ion insertion capacity and transparency as compared to the films deposited using as-prepared sol. The ion insertion capacity obtained for an 800 nm thick film of mixed oxide is of the order of 18.36 mC cm^-2 at a frequency of 0.0011 Hz. The presence of well-defined grains of about 89 nm dimension in the CeO₂-TiO₂ film has been determined by the AFM studies. The measurements on the films for electrochromic applications such as "smart windows" reveal fast coloration-bleaching kinetics for WO₃ in conjunction with the mixed CeO₂-TiO₂ film and affirm its suitability as passive counter electrode in electrochromic devices.