Mixed Ionic–Electronic Conduction in NiFe2O4–Ce0.8Gd0.2O2−Δ Nanocomposite Thin Films for Oxygen Separation

Abstract

NiFe2O4–Ce0.8Gd0.2O2−δ (NFO/CGO) nanocomposite thin films were prepared by simultaneously radio-frequency (RF) magnetron sputtering of both NFO and CGO targets. The aim is the growth of a CO2-stable composite layer that combines the electronic and ionic conduction of the separate NFO and the CGO phases for oxygen separation. The effect of the deposition temperature on the microstructure of the film was studied to obtain high-quality composite thin films. The ratio of both phases was changed by applying different power to each ceramic target. The amount of each deposited phase as well as the different oxidation states of the nanocomposite constituents were analyzed by means of X-ray photoelectron spectroscopy (XPS). The transport properties were studied by conductivity measurements as a function of temperature and pO2. These analyses enabled (1) selection of the best deposition temperature (400 °C), (2) correlation of the p-type electronic behavior of the NFO phase with the hole hopping between Ni3+–Ni2+, and (3) following the conductivity behavior of the grown composite layer (prevailing ionic or electronic character) attained by varying the amount of each phase. The sputtered layer exhibited high ambipolar conduction and surfaceexchange activity. A 150 nm-thick nanograined thin film was deposited on a 20 μm-thick Ba0.5Sr0.5Co0.8Fe0.2O3−δ asymmetric membrane, resulting in up to 3.8 mL min−1 cm−2 O2 permeation at 1000 °C under CO2 atmosphere.