In Situ XPS Studies of Perovskite Oxide Surfaces under Electrochemical Polarization

An in situ XPS study of oxidation-reduction processes on three perovskite oxide electrode surfaces was carried out by incorporating the materials in an electrochemical cell mounted on a heated sample stage in an ultrahigh vacuum (UHV) chamber. Electrodes made of powdered LaCr(1-x)Ni(x)O(3-delta) (x = 0.4, 1) showed changes in the XPS features of all elements upon redox cycling between formal Ni3+ and Ni2+ oxidation stoichiometries, indicating the delocalized nature of the electronic states involved and strong mixing of O 2p to Ni 3d levels to form band states. The surface also showed changes in adsorption capacity for CO2 upon reduction as a result of increased nucleophilicity of surface oxygen. Another perovskite oxide, La(0.5)Sr(0.5)CoO(3-delta), laser deposited as highly oriented thin films on (100) oriented yttria-stabilized zirconia (YSZ), also showed evidence of both local and nonlocal effects in the XPS features upon redox cycling. In contrast to LaCr(1-x)Ni(x)O(3-delta), redox cycling mainly affected the XPS features of cobalt with little effect on oxygen. This signifies reduced participation of O 2p states in the conduction band of this material. Small changes in surface cation stoichiometry in this film were observed and attributed to mobility of the A-site Sr dopant under polarization.