Effective utilization of spray pyrolyzed CeO 2 as optically passive counter electrode for enhancing optical modulation of WO 3

Abstract

Nanocrystalline cerium oxide (CeO 2) thin films were deposited onto the fluorine doped tin oxide coated glass substrates using methanolic solution of cerium nitrate hexahydrate precursor by a simple spray pyrolysis technique. Thermal analysis of the precursor salt showed the onset of crystallization of CeO 2 at 300 °C. Therefore, cerium dioxide thin films were prepared at different deposition temperatures from 300 to 450 °C. Films were transparent ( T ~ 80%), polycrystalline with cubic fluorite crystal structure and having band gap energy (Eg) in the range of 3.04–3.6 eV. The different morphological features of the film obtained at various deposition temperatures had pronounced effect on the ion storage capacity (ISC) and electrochemical stability. The larger film thickness coupled with adequate degree of porosity of CeO 2 films prepared at 400 °C showed higher ion storage capacity of 20.6 mC cm − 2 in 0.5 M LiClO 4 + PC electrolyte. Such films were also electrochemically more stable than the other studied samples. The Ce 4+/Ce 3+ intervalancy charge transfer mechanism during the bleaching–lithiation of CeO 2 film was directly evidenced from X-ray photoelectron spectroscopy. The optically passive behavior of the CeO 2 film (prepared at 400 °C) is affirmed by its negligible transmission modulation upon Li + ion insertion/extraction, irrespective of the extent of Li + ion intercalation. The coloration efficiency of spray deposited tungsten oxide (WO 3) thin film is found to enhance from 47 to 53 cm 2 C − 1 when CeO 2 is coupled with WO 3 as a counter electrode in electrochromic device. Hence, CeO 2 can be a good candidate for optically passive counter electrode as an ion storage layer.