One of our projects deals with ultrathin films of metals on metals; these are model systems for bimetallic catalysts, and we are searching for relations between the surface structure and the surface chemical reactivity. We find that the atomically rough, open bcc(111) surfaces are morphologically unstable when covered by films one monolayer (ML) thick of certain metals, i.e., they form faceted structures. To characterize these phenomena, we have studied ultrathin films of transition and noble metals on Mo(111) and W(111) using Auger Spectroscopy, LEED, thermal desorption spectroscopy (TDS), soft x-ray photoemission spectroscopy (SXPS), field emission microscopy (FEM), and scanning tunneling microscopy (STM). For example, using a UHV STM to study bimetallic 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 ~ 1 ML. Formation of pyramidal facets also occurs when W(111) or Mo(111) surfaces are dosed with Pt, Au, Ir, Rh, or oxygen. 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.

As part of this program to study structure-reactivity relationships in the morphologically-unstable W(111) system, we are examining known structure sensitive reactions, acetylene tricyclization to benzene and n-butane hydrogenolysis. Work in progress focuses on the reactivity of planar and faceted Pt/W(111) and Pd/W(111). Finally, high resolution SXPS using synchrotron radiation provides insights into electronic factors that
influence faceting at the bimetallic interface. Recent theoretical descriptions of morphological instability provide new insights into these processes.

References:

1.	T.E.Madey, C.-H.Nien, K.Pelhos, J.J.Kolodziej, I.M.Abdelrehim, H.-S.Tao	Faceting induced by ultrathin metal films: structure, electronic properties and reactivity	Surf. Sci. 1999, V.438, P.191-206
2.	J.J.Kolodziej, T.E.Madey, J.W.Keister, and J.E.Rowe	Photoelectron spectroscopy studies of growth, alloying, and segregation for transition-metal films on tungsten (211)	Phys.Rev. B, 2000, V.62, No.8, P.5150-5162