Faceting induced by ultrathin metal films on W(111) and Mo(111): Structure, reactivity, and electronic properties

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(lll) surfaces are morphologically unstable when covered by films greater than or equal to 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 theta(c). Formation of pyramidal facets also occurs when W(111) or Mo(lll) 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(lll) 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 substrate: 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(lll) 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 synchrotran 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.