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(1 11) and W(111) using Auger spectroscopy, LEED, thermal desorption spec troscopy (TDS) and scanning tunneling microscopy (STM). The atomically rough, open bcc(lll) surfaces are morphologically unstable when cover ed by films greater than or equal to 1 monolayer thick of certain meta ls, 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 c ompletely faceted to three-sided {211} pyramids upon annealing, for Pd coverages greater than a critical coverage theta(c). Formation of pyr amidal facets also occurs when W(111) or Mo(lll) surfaces are dosed wi th Pt, Au, Ir, Rh, oxygen or sulfur. In contrast, monolayer films of T i, Co, Ni, Cu, Ag and Gd do not induce massive reconstruction or facet ing on W(111) and Mo(lll) surfaces. The faceting appears to be thermod ynamically driven but kinetically limited: faceting is caused by an in creased anisotropy in surface free energy that occurs for the film-cov ered surfaces. An interesting correlation has been observed for both s ubstrate: faceting occurs for overlayer elements having Pauling electr onegativities greater than 2.0, suggesting that surface electronic eff ects are controlling the structural instability of both Mo(lll) and W( 111). Structure sensitivity in a model catalytic reaction, n-butane hy drogenolysis, is observed over planar and faceted Pt/W(111). We have a lso used soft x-ray photoemission spectrosocopy (SXPS) based on synchr otran radiation methods to characterize the bimetallic interface; for Pt, Pd and Au on W(111), we find that substrate core level shift effec ts associated with interface formation are substantial, while those as sociated with faceting are rather subtle.