Identification of surface composition and chemical states in composites comprised of phases with fluorite and perovskite structures by X-ray photoelectron spectroscopy

摘要

X-ray photoelectron spectroscopy (XPS) has been used to explore the cation rearrangement between the surface and bulk of grains and the surface chemical states of Ce, Mn, Co, Sr, and O ions in the single phase CeO_2-based solid solutions and in the two phase (100-x)La_(0.6)Sr_(0.4)CoO_3-xCeO_2 (LSCCx) and (100-x)La_(0.7)Sr_(0.2)MnO_3-xCeO_2 (LSMCx) composites. The well-resolved Ce 3d_(5/2) and Ce 3d_(3/2) spin-orbit components were determined to study the Ce~(4+) to Ce~(3+) electronic transition at the surface. The surface of the LSCCx and LSMCx (x = 8-40 mol%) composites is depleted in cerium. Both Ce~(4+) and Ce~(3+) cations were revealed. The surface fraction of Ce~(4+) cations increases with the rise in CeO_2 content in both systems, but it is higher by about a factor of two in the LSCC composites. A strong enrichment of the surface by Co cations was observed for the composites LSCCx (x = 8-40 mol%), implying that the presence of cerium could facilitate the Co surface segregation. A slight Sr surface enrichment compared to the nominal stoichiometry was found for La_(0.6)Sr_(0.4)CoO_3, CeO_2-based solid solutions, LSCC57, and LSMCx (x = 10-57) composites. In contrast, the surface of the LSCCx (x = 10-37) composites is slightly depleted in strontium cations. The O Is spectra of the LSCC and LSMC composites contain several contributions associated with the lattice oxygen related to the phases with the perovskite and fluorite structures as well as with different surface states. Their contributions vary with CeO_2 content and a chemical origin of the transition metal cations on the B-site in the perovskite structure. The Ce_2(CO_3)_3-based phase seems to exist at the surface of grains and crystallites in the LSCC57 and LSCM composites.