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

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Abstract
The preparation of ordered ultra-thin cerium oxide overlayers on nonreconstructed Pt(111) single crystal surfaces is reported. Thin electron-beam evaporated CeO 2 layers ( d = 21 A ̊ ) decompose during heating in ultra-high vacuum (UHV) at T = 1000 K and form domains of ordered PtCe 2 and PtCe 3 surface alloys. They exhibit a (2 × 2) + (2 × 2)R30° pattern in low-energy electron diffraction (LEED). Oxidation under thermodynamically controlled conditions ( po 2 = 6 × 10 −5 mbar, T = 1000 K) leads to a CeO 2 layer ( d = 6.3 A ̊ ) which forms a (1.4 × 1.4) superstructure on Pt(111) 1 × 1. X-ray photoemission spectra (XPS) reveal a high concentration of Ce 3+ 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 A ̊ ) whilst the CeO 2− x remains ordered with unchanged epitaxial relationships with respect to Pt(111).