Controlled growth of epitaxial CeO2 thin films with self-organized nanostructure by chemical solution method

Abstract

Chemical solution deposition is a versatile technique to grow oxide thin films with self-organized nanostructures. Morphology and crystallographic orientation control of CeO₂ thin films grown on technical NiW substrates by a chemical solution deposition method are achieved in this work. Based on an enhanced understanding of the effect of oxygen partial pressure during film crystallization, a strong texture can be obtained on the surface of the CeO₂ films annealed at temperatures as low as 900 °C followed by a two-step annealing procedure. Crystallization at high temperature (e.g., 1100 °C) in a reducing atmosphere leads to the formation of an oxygen deficient CeO_2−x phase coexisting with a small amount of a polycrystalline CeO_1.67 phase. Further surface phase and texture analysis by an electron backscattering diffraction technique reveals that the off-stoichiometric CeO_2−x phase retains a fluorite structure but exhibits an alternative in-plane texture with eight fold symmetry on the surface. According to phase and texture stability studies, these off-stoichiometric phases gradually transform back to fully oxidized CeO₂ with a 45° rotated cube texture during storage in ambient air. Moreover, the morphology of the CeO₂ thin films is controlled by precisely regulating the film thickness and crystallization temperature. A temperature-induced transition from the commonly observed granular grain to an atomically flat surface is found in the CeO₂–NiW constitution. Cross-sectional transmission electron microscope observation also reveals that this phenomenon is mainly attributable to the surface re-organization, which is strongly associated with the critical film thickness, crystallization temperature, reducing ability of the crystallization atmosphere as well as the interface properties.