Langmuir 1985,l)741-747
741
Surface Chemical Properties of Ag/Pt( 111): Comparisons between Electrochemistry and Surface Science M. T. Paffett,* C. T. Campbell, and T. N. Taylor Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Received April 15, 1985 The growth modes and interaction of vapor-deposited Ag on a clean Pt(ll1)surface have been monitored by Auger electron spectroscopy (AES),low-energy electron diffraction (LEED), work function measurements, thermal desorption mass spectroscopy (TDMS), and chemisorption of H2and CO. The AES data indicate that at 1260 K Ag grows in a uniform monolayer up to about one monolayer and that at larger coverages three-dimensional island growth occurs (i.e., Stranski-Krastanov mechanism). In the submonolayer region above -260 K, LEED data indicate that Ag grows in p(lX1) islands one atom thick. In the second layer Ag growth assumes a Ag(ll1) lattice spacing, rotationally commensurate with the underlying Pt(ll1) substrate. The change in work function (A4) indicates that the annealed Ag/Pt(lll) surfaces at BAg 2 2.0 achieve a work function value more negative than smooth Ag(lll), consistent with the microscopic surface roughness for three-dimensional island growth. An irreversible increase in the work function at -225 K indicates that Ag adatoms became mobile and form larger islands at this temperature and that vapor deposition at temperatures below this results in much rougher surfaces, concomitant with lower work functions. Analysis of the TDMS data for Ag/Pt(lll) indicates that for BAg < 1.0, Ag desorbs in a single state with an activation energy (Ed) of 70.6 kcal mol-'. For B k > 1.0, a second multilayer desorption peak appears with Ed equal to the sublimation energy of Ag (66.8 kcal mol-'). Due to the island growth mechanism, Ag causes simple one-to-one site blocking of the Pt(ll1) surface for H2and CO chemisorption. Comparisons with the underpotential decomposition of Ag onto Pt electrodes indicate that the underpotential contains roughly equal contributions from differences in metal-metal bond stability and metal-solution bonding (free energy of immersion). The blocking of hydrogen chemisorption by adsorbed Ag observed here has a strong electrochemical analogue.
I. Introduction The modification of metal surfaces by deliberate addition of another metal component is an area of active research with many parallels in the fields of heterogeneous catalysis and electro~hemistry.'-~ Controlled submonolayer deposits of a variety of metals can be electrochemically deposited onto certain metal substrates by underpotential metal deposition (UPD).' At the gas-solid interface, modification of the metal substrate is usually studied by vapor deposition of the second metal. In both cases the metal adatom imparts a change in chemical reactivity to the substrate surface. Ultrahigh vacuum (UHV) surface techniques give a detailed atomic-level picture of the chemistry occurring a t gas-solid interfaces, which is not easily obtained in the analogous electrochemical case. From the present study of the influence that Ag adatoms have on the chemical reactivity of the P t ( l l 1 ) surface, specific comparisons will be made with the underpotential deposition of Ag on Pt electrodes.
11. Experimental Section The experimental arrangement has been previously des~ribed.49~ The AES data were taken with the retarding grid analyzer and electron beam associated with the LEED optics. The sample normal was oriented at an angle yi = 45' with respect to the beam. The average acceptance angle for detection of secondary electrons was estimated at 30' from the sample normal for this configuration. AES spectra of the Ag/Pt(lll) interface were usually recorded at -450-475 K to minimize CO adsorption and subsequent C buildup. The change in work function was measured by using the onset energy of secondary electron emission from the sample (biased-10 V with respect to ground) using the LEED
electron beam at 200 eV and
