Influence of aging and composition of the precursor sol on the properties of CeO2–TiO2 thin films for electrochromic applications

Sol–gel derived spin coated CeO2–TiO2 thin films with two different compositions have been prepared using precursor sols based on ceric ammonium nitrate and titanium isopropoxide in ethanol. The aging effect of the sol on different properties of the films has been investigated by subjecting the films to optical, electrochemical and structural analysis by employing characterization techniques such as UV–visible spectrophotometry, cyclic voltammetry, multiple step chronoamperometry, X-ray diffraction, FT-IR, electron microscopy, differential thermal analysis and energy dispersive X-ray analysis. The films are nanocrystalline as shown by the TEM study. The optically passive nature of the films shows their potential as passive counter electrodes. Good electrochemical reversibility under lithium ion insertion, studied for a large number of cycles confirms that the films are promising candidates as ion storage layer in electrochromic devices. The performance characteristics of the electrochromic devices have improved with the aging of the precursor sol employed for the deposition of the CeO2–TiO2 films. High diffusion coefficient and thereby faster kinetics of the insertion/extraction reactions in Ce/Ti (1:1) films shows the suitability of this composition in systems wherein dynamic response is important. However, higher ion storage capacity of Ce/Ti (1:2) films has led to their usefulness in applications wherein the transmission modulation of the overall device is the most significant. In the present study, the device comprising the CeO2–TiO2 (1:2) film has exhibited the highest transmission modulation, thereby this composition of the ion storage layer is considered to be the most promising from the viewpoint of its application in transmissive electrochromic devices.