Faceting induced by ultrathin metal films: structure, electronic properties and reactivity

When faceting occurs, an initially planar surface converts to a 'hill-and-v alley' structure, typically exposing low-Miller-index crystal faces of micr oscopic dimensions. The bce W(111) and Mo(111) surfaces are morphologically unstable when covered by monolayer films of certain metals (including Rh, Pd, Ir, Pt, Au), and they become 'nanotextured' upon annealing at T > 700 K : the surface is covered by nanoscale three-sided pyramids with mainly {211 } facets. In the present work, we focus on the structure, electronic proper ties and reactivity of planar and faceted W(111) and W(211) covered by ultr athin films of metals (mainly Pd, Ph, Pt, Au) and non-metals (S, O). The me asurements include ultrahigh-vacuum scanning tunneling microscopy, soft X-r ay photoelectron spectroscopy (SXPS) using synchrotron radiation, Auger ele ctron spectroscopy, low-energy electron diffraction, low-energy electron mi croscopy, and thermal desorption spectroscopy. The metal film growth and th ermal stability have been investigated for coverages of 0-8 ML. The observe d formation of three-sided pyramids with both {110} and {211} facets, as in duced by 1 ML of overlayer metal, is predicted also by recent first-princip les calculations of surface energetics. The faceting is caused by an increa sed anisotropy in surface free energy that occurs for the him-covered surfa ces. At coverages above 1 ML, SXPS data indicate that thin film alloys are formed upon annealing films of Pt and Pd; surface alloy formation is not se en for Au films. These findings are discussed in terms of structural and el ectronic properties of bimetallic systems. The relevance to catalytic prope rties of a structure-sensitive reaction (acetylene cyclization over Pd-W) i s also discussed. (C) 1999 Elsevier Science B.V. All rights reserved.