In Situ Studies of the Temperature-Dependent Surface Structure and Chemistry of Single-Crystalline (001)-Oriented La<sub>0.8</sub>Sr<sub>0.2</sub>CoO<sub>3−δ</sub> Perovskite Thin Films

Perovskites are used to promote the kinetics of oxygen electrocatalysis in solid oxide fuel cells and oxygen permeation membranes. Little is known about the surface structure and chemistry of perovskites at high temperatures and partial oxygen pressures. Combining in situ X-ray reflectivity (XRR) and in situ ambient pressure X-ray photoelectron spectroscopy (APXPS), we report, for the first time, the evolution of the surface structure and chemistry of (001)-oriented perovskite La<sub>0.8</sub>Sr<sub>0.2</sub>CoO<sub>3−δ</sub> (LSC<sub>113</sub>) and (La<sub>0.5</sub>Sr<sub>0.5</sub>)<sub>2</sub>CoO<sub>4+δ</sub> (LSC<sub>214</sub>)-decorated LSC<sub>113</sub> (LSC<sub>113/214</sub>) thin films as a function of temperature. Heating the (001)-oriented LSC<sub>113</sub> surface leads to the formation of surface LSC<sub>214</sub>-like particles, which is further confirmed by ex situ Auger electron spectroscopy (AES). In contrast, the LSC<sub>113/214</sub> surface, with activities much higher than that of LSC<sub>113</sub>, is stable upon heating. Combined in situ XRR and APXPS measurements support that Sr enrichment may occur at the LSC<sub>113</sub> and LSC<sub>214</sub> interface, which can be responsible for its markedly enhanced activities.