Ordered ultra-thin cerium oxide overlayers on Pt(111) single crystal surfaces studied by LEED and XPS

The preparation of ordered ultra-thin cerium oxide overlayers on nonreconstructed Pt(111) single crystal surfaces is reported. Thin electron-beam evaporated CeO2 layers (d=21 Angstrom) decompose during heating in ultra-high vacuum (UHV) at T=1000 K and form domains of ordered PtCe2 and PtCe3 surface alloys. They exhibit a (2x2)+(2x2)R30 degrees pattern in low-energy electron diffraction (LEED). Oxidation under thermodynamically controlled conditions (p(O2)=6x10(-5) mbar, T=1000 K) leads to a CeO2 layer (d=6.3 Angstrom) which forms a (1.4x1.4) superstructure on Pt(111) 1x1. X-ray photoemission spectra (XPS) reveal a high concentration of Ce3+ ions. Deconvolution yields a deviation, x, of 0.21 from ideal stoichiometry. The deviation is lower (x=0.14) for an increased layer thickness (d=24 Angstrom) whilst the CeO2-x remains ordered with unchanged epitaxial relationships with respect to Pt(111). (C) 1998 Elsevier Science B.V.