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

Abstract

When faceting occurs, an initially planar surface converts to a ‘hill-and-valley’ structure, typically exposing low-Miller-index crystal faces of microscopic dimensions. The bcc 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 properties and reactivity of planar and faceted W(111) and W(211) covered by ultrathin films of metals (mainly Pd, Rh, Pt, Au) and non-metals (S, O). The measurements include ultrahigh-vacuum scanning tunneling microscopy, soft X-ray photoelectron spectroscopy (SXPS) using synchrotron radiation, Auger electron spectroscopy, low-energy electron diffraction, low-energy electron microscopy, and thermal desorption spectroscopy. The metal film growth and thermal stability have been investigated for coverages of 0–8 ML. The observed formation of three-sided pyramids with both {110} and {211} 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. 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 seen for Au films. These findings are discussed in terms of structural and electronic properties of bimetallic systems. The relevance to catalytic properties of a structure-sensitive reaction (acetylene cyclization over Pd–W) is also discussed.