Proton conductivity of columnar ceria thin-films grown by chemical vapor deposition

Columnar thin films of undoped ceria were grown by metal-organic chemical vapor deposition. The films, deposited on Pt-coated MgO(100) substrates, display a columnar microstructure with nanometer scale grain size and similar to 30% overall porosity. Through-plane (thickness mode) electrical conductivity was measured by AC impedance spectroscopy. Proton conduction is observed below 350-400 degrees C, with a magnitude that depends on gas-phase water vapor pressure. The overall behavior suggests proton transport that occurs along exposed grain surfaces and parallel grain boundaries. No impedance due to grain boundaries normal to the direction of transport is observed. The proton conductivity in the temperature range of 200-400 degrees C is approximately four times greater than that of nanograined bulk ceria, consistent with enhanced transport along aligned grain surfaces in the CVD films.