Microstructure of nanocrystalline undoped and doped cerium oxide thin films and their electrical and optical properties

Cerium oxide (CeO₂) is an exceptional material for electrochemical applications whose electrical, optical and catalytic properties are strongly related to the lattice defects, which can be controlled by impurities, ambient atmosphere and microstructure. The goal of this research is to investigate the electrical and optical properties of CeO₂ thin films and correlate them with microstructure.; The nanocrystalline CeO₂ films have been obtained by a polymeric precursor spin coating technique. The grain size (d_g) depends upon the annealing temperature and is in the range of 6–300 nm when the temperature is changed from 400–1200°C. A variety of characterization techniques including X-ray diffraction, impedance spectroscopy, Raman scattering and optical transmission have been used for the evaluation of the properties. From the optical measurements the energy band gap, the refractive index and the extinction coefficient have been determined and correlated with microstructure. The electrical conductivity has been studied by impedance spectroscopy as a function of temperature and oxygen activity. For undoped CeO₂, the transition from extrinsic to intrinsic type of conductivity has been observed as d_g decreases which appears to be related to a decrease an enthalpy of oxygen vacancy formation resulting increasing electronic conductivity. For nanocrystalline Gd-doped CeO ₂ (d_g < 50 nm), the electrical conductivity has extrinsic character and the ionic conductivity increases as d_g decreases, which is related to reduction of the activation energy for the ion mobility. The results showed that defect equilibria for nanocrystalline CeO₂ are related to the microstructure and its electrical and optical properties can be effectively controlled by controlling its microstructure.