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The restructuring of S/W(111) surfaces has been studied using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). Dosing W(111) with a saturation coverage of H 2 S followed by annealing to >800 K causes the substrate to reconstruct, forming a structure with (4× 4) periodicity. In addition, the terrace-step configurations restructure and form triangular domains with a preferential orientation. These domains coalesce and enlarge, and also form multiple steps when the surface is heated to T>1000 K. The low reactivity of sulfided W(111) to high exposures of oxygen demonstrates that the surface is passivated by sulfur. Adsorption of S onto a faceted Pd/W(111) surface causes the facets to disappear, restoring the surface's planar form upon annealing. The resulting features are dominated by a (2× 2) structure. A size-mismatch mechanism, based on charge transfer between S/W and coadsorbed Pd/S on W, has been proposed to explain the formation of (4× 4) and (2× 2) structures, as well as the transition between these structures.

We have studied ultrathin films of transition and noble metals on Mo(111) and W(111) using Auger spectroscopy, LEED, thermal desorption spectroscopy (TDS) and scanning tunneling microscopy (STM). The atomically rough, open bcc(111) surfaces are morphologically unstable when covered by films ≥ 1 monolayer thick of certain metals, i.e. they form faceted structures. For example, using a UHV STM to study Pd/W(111) , we find that the Pd-covered W(111) surface becomes completely faceted to three-sided {211} pyramids upon annealing, for Pd coverages greater than a critical coverage θc. Formation of pyramidal facets also occurs when W(111) or Mo(111) surfaces are dosed with Pt, Au, Ir, Rh, oxygen or sulfur. In contrast, monolayer films of Ti, Co, Ni, Cu, Ag and Gd do not induce massive reconstruction or faceting on W(111) and Mo(111) surfaces. The faceting appears to be thermodynamically driven but kinetically limited: faceting is caused by an increased anisotropy in surface free energy that occurs for the film-covered surfaces. An interesting correlation has been observed for both substrates: faceting occurs for overlayer elements having Pauling electronegativities greater than 2.0, suggesting that surface electronic effects are controlling the structural instability of both Mo(111) and W(111) . Structure sensitivity in a model catalytic reaction, n-butane hydrogenolysis, is observed over planar and faceted Pt/W(111) . We have also used soft x-ray photoemission spectrosocopy (SXPS) based on synchrotron radiation methods to characterize the bimetallic interface; for Pt, Pd and Au on W(111) , we find that substrate core level shift effects associated with interface formation are substantial, while those associated with faceting are rather subtle.