Growth Mechanism, Microstructure, and Surface Modification of Nanostructured CeO2 Films by Chemical Solution Deposition

The evolution from a partially oriented granular microstructure to a dense epitaxial one in CeO2 films deposited from chemical solutions on Y2O3-stabilized cubic ZrO2 (YSZ) has been investigated using cross-section transmission electron microscopy (XTEM), electron energy-loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), X-ray diffraction (XRD), and X-ray reflectometry (XRR). In crystallization from chemical solutions, undercoolings are typically high enough to promote homogeneous and heterogeneous nucleation with equal probability. The desired texture is then transmitted from heterogeneously nucleated epitaxial grains throughout the volume of the film upon sintering. Crystallization of CeO2 under the reducing atmospheric conditions of Ar/5 % H2 results in a nanometric granular microstructure with a high concentration of C impurities decorating grain boundaries and interstitial cavities, which unexpectedly prevails after high-temperature annealing. Post-processing in oxidizing conditions removes C impurities and promotes grain growth resulting in a fully epitaxial film, as well as stabilizing the otherwise energetically prohibitive polar (001) planes. Misfit stresses in the post-processed epitaxial films are completely relieved by interfacial dislocations with b = (a/2) < 110 > and b = (a/2) < 011 > Burgers vectors. A mechanism that considers impurity-induced grain-boundary blocking and the stabilization of (001) planes via surface oxidation is proposed. Processing conditions to obtain high-quality CeO2 buffer layers can be adapted to YBa2Cu3O7-coated conductors.