Microstructural effects on electrical conductivity relaxation in nanoscale ceria thin films

Microstructure evolution and electrical conductivity relaxation kinetics in highly textured and nanocrystalline dense ceria thin films (∼65 nm) are reported in this paper. Highly textured films were grown on sapphire c -plane substrates by molecular beam synthesis (MBS) with orientation relationship (111) CeO₂ || (0001) Al₂O₃ and [1 1- 0] CeO ₂ || [12̄- 10] Al₂O₃. No significant structural changes were observed in highly textured films even after extensive annealing at high temperature. In contrast to MBS grown films, ceria films grown by electron beam evaporation at room temperature had polycrystalline structure with ∼10 nm grains, which grew to ∼30 nm upon annealing at 1173 K. Grain growth kinetics was self-limiting and the out-of-plane orientation was found to be substrate dependent. From conductivity relaxation measurements, oxygen exchange rate in highly textured thin films was found to be much slower than that in polycrystalline films. The response time for highly textured films to changes in P (O₂) from 1.07× 10-12 to 5.43× 10-10 Pa at 1148 K was 0.65 s, whereas that for polycrystalline films was 0.13 s under identical conditions. From temperature dependent experiments, activation energy for relaxation time was found to be similar, suggesting similar rate-limiting mechanisms in polycrystalline and highly textured films. The results highlight the importance of near-surface defects in controlling kinetics of oxygen incorporation into nanostructured oxides. In a broader context, the results maybe of relevance to designing catalytic surfaces in solid state ionic devices such as fuel cells.