The W(111) surface is morphologically unstable when covered by monolayer films of certain metals (including Pt, Pd, Rh, Au), and develops nanoscale 3-sided pyramidal facets with mainly [112] faces upon annealing. In the present work, we focus on the structure, electronic properties and reactivity of planar and faceted W(111) and W(211) covered by ultrathin films of metals (0 - 8 ML, mainly Pt, Pd, Rh) and non-metals (S, O). The measurements include UHV-STM, soft XPS (SXPS) using synchrotron radiation, Auger spectroscopy, LEED, and thermal desorption spectroscopy. The observed formation of 3-sided pyramids with both [110] and [112] facets, as induced by 1 ML of overlayer metal, is predicted also by recent first-principles calculations of surface energetics. The faceting is caused by an increased anisotropy in surface free energy that occurs for the film-covered surfaces. The adsorption of S induces a different reconstruction with nanoscale texturing of the surface. At coverages above 1 ML, SXPS data indicate that interfacial alloys are formed upon annealing films of Pt and Pd, but not Au. These findings are discussed in terms of structural and electronic properties of bimetallic systems. The relevance to the surface chemistry of a structure-sensitive reaction (acetylene cyclization to benzene over Pd/W) is also discussed.
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@footnote 1@Supported in part by US DOE, Office of Basic Energy Sciences @footnote 2@Collaborators include C.-H. Nien, J.J. Kolodziej, K. Pelhos, I. Abdelrehim, H.-S. Tao, R. Barnes (Rutgers); J. Keister, J.E. Rowe (NC State); J. Eng, J.G. Chen (EXXON); C-T. Chan (HKUST)