Stabilization of polar solid oxide surfaces: competition between adsorption and reconstruction

Multi-cationic spinel compounds are solids that exhibit polar faces. X-ray Photoelectron spectroscopy revealed the main phenomena allowing the stabilization of these faces, carried out as a function of the material treatment, particularly the cooling rate after thermal treatment at a high temperature (1200degreesC). This study showed that, whatever the cooling rate, each sample is subject to a significant hydroxylation that reduces the polarity. Nevertheless, it appears that the hydroxyl group content at the surface is a strong function of the cooling rate. Indeed, whereas quenched materials are subject to high levels of hydroxylation, slowly cooled samples are sparingly hydroxylated. This phenomenon corresponds to the fact that, during a slow cooling down, a change of the cationic composition in the topmost layers of the solids is able to occur, inducing a decrease of the polarity. Moreover, this segregation comes with a change of the grain surface morphology, leading to less polar faces. Copyright (C) 2002 John Wiley Sons, Ltd.