Fast ion transport in nanoscaled thin film cerium oxide

Dense CeO₂ layers of 350 nm thickness, with columnar grains of size 25 nm, were RF-sputtered on silicon. ¹⁸O isotope exchange in the temperature range from 200 to 575 °C showed surface exchange at CeO₂ as the rate-determining step, while diffusion through the polycrystalline thin layer was fast. The surface exchange coefficients controlled by surface or grain boundary processes were found to be k_s = 2.7 × 10^{- 8} exp (frac(- 0.3 eV, k T)) cm s^{- 1} or k_gb = 1 × 10^{- 9} exp (frac(- 0.3 eV, k T)) cm s^{- 1}. Comparison with literature data on doped ceria revealed that surface exchange could be dominated by grain boundary processes. A lower limit for the diffusion coefficient was determined as 10^{- 15} cm² s^{- 1} at 575 °C.