Grain growth, microstructure and surface modification of textured CeO 2 thin films on Ni substrate

Abstract

CeO 2 films were prepared from solutions of different concentrations (0.05–1.0 M) on textured Ni substrates. Homogeneous nucleation and growth of CeO 2 nanocrystals <8 nm occurs upon calcination at 350–500 °C. At the heating and sintering stage, the homogeneous growth is inhibited in favor of the development of grains with (0 0 l) texture. The grain growth normal to the film surface is well described by a stretched exponential function with a relaxation time of up to 60 min and with Kohlrausch exponent values of less than unity. The increase in grain size is accompanied by the relaxation of the microstrain. During the relaxation time, the grain coarsening is controlled by surface diffusion characterized by an activation energy as low as 0.6 eV. At the relaxation time, the surface morphology is strongly concentration dependent. The surface morphology changes from separated (agglomerated) grains to a continuous grain configuration as the concentration increases from 0.05 M to 0.8 M. After the relaxation time, both the grain size normal to the film surface and the lateral grain size continue to grow, and the grain configuration continues to change. These processes are concentration dependent. In the films with a nominal thickness >170 nm (>0.8 M), the transition to classical curvature-driven grain-growth kinetics is evident (at 1000 °C, below the literature value of 1100 °C). The decrease in the Kohlrausch exponent for these thick films suggests that the grain coarsening through grain boundary migration is responsible for the stretched regime of grain-size relaxation.