Cerium oxides and cerium-platinum surface alloys on Pt(111) single-crystal surfaces studied by scanning tunneling microscopy

Three-dimensional clusters of metallic Ce are deposited on Pt(111) surfaces by thermal evaporation in ultrahigh vacuum. Different reactions occur upon heating of the sample in ultrahigh vacuum and during the exposure of oxygen that lead to distinct well-ordered surface phases. Their geometric structures are determined by scanning tunneling microscopy (STM). A two-dimensional ordered surface alloy of Pt5Ce is obtained by annealing at 1000 K. It forms two different incommensurate overlayers on Pt(111) with long-distance spacings in the moire patterns (13.7 and 14.8 Angstrom). Hexagonally shaped two-dimensional islands are formed upon heating the Pt(111) single crystal with a submonolayer coverage of Ce at 900 K. They are localized at step edges. STM reveals a local 2x2 structure with respect to Pt(111). It is assigned to a precursor state of alloy formation. Ordered CeO2 phases result from annealing the alloy at 1000 K in oxygen. The surface structure is consistent with oxygen-terminated fluorite-type CeO2(111). During decomposition of CeO2 at 900 K, an oxide structure is identified which is attributed to an ultrathin ordered phase of surface-Ce2O3.