Transparent two-phase composite oxide thin films with high conductivity

Abstract

New types of transparent thin films based on CeO2–Al2O3 and SiO2 and ion-doped CeO2 composite oxides have been developed by sol-gel process. The films have two phases: a host phase; CeO2 or ion-doped CeO2; and a guest phase Al2O3 or SiO2, so called binary composites. This is the unique character to distinguish from other transparent films, e.g. CeO2–SnO2, TiO2 and ZrO2 single phase (solid solution), for electrochromic applications. Film conductivity was determined by impedance analysis and electrochemical property was investigated by cyclic voltammetry. The conductivity of the composite films is in the range of 10−2–0.2 S/cm in the temperature range of 500–700°C. Binary films containing 30% alumina and 50% silica with thickness 35 and 30 nm have the highest amount of charge exchange for 1.0 Li/Ce and 0.7 Li/Ce, respectively. After 380 cycles the charge capacity of the CeO2Al2O3 reached about the same value as started in the beginning. The films show a high transmittance for visible light. Unusual properties for these composite thin films have been implemented due to the special thin film nano-structure and two-phase regions and interfaces. The ionic transport and insertion/extraction mechanisms in these new composite film materials are discussed based on a proposed physical model. These composite films have a broad application in various electrochemical devices, such as electrochromic and fuel cell devices.