Thermal stability of cobalt oxide thin films and its enhancement by cerium oxide

Thermal stability of Co3O4 and CeOx-Co3O4 thin films prepared by various "dry " physical techniques is inves-tigated. A model system is represented by well-ordered Co3O4(1 1 1)/Ir(1 0 0), more realistic and industrially relevant Co3O4 thin films were grown by magnetron sputtering and direct oxidation of cobalt. In all cases, combined systems with non-continuous cerium oxide overlayer were prepared and compared to pristine oxides. The thin films were subjected to thermal annealing under vacuum and monitored by X-ray photoelectron spectroscopy (XPS), microscopy (STM or SEM) was used to observe the surface morphology. Thermal reduction of Co3O4 spinel structures to CoO and, eventually, metallic Co depends substantially on layer morphology and type of substrate. Thermal stability of the well-ordered model system is significantly higher than for the high surface area oxides deposited by magnetron sputtering or grown by direct oxidation. In the reduction process, inevitable in many applications utilizing Co3O4, cerium adlayer can act as an efficient stabilizing component, protecting cobalt oxide against reduction and decomposition. There is a temperature dependent synergistic interplay between cerium and cobalt oxides via oxygen transfer involving Co2+/Co3+ and Ce4+/Ce3+ redox pairs. Thermal activation can lead to a formation of surface mixed spinel with higher thermal stability in reducing environments.