Surface chemical properties of silver/platinum(111): comparisons between electrochemistry and surface science

Abstract

The growth modes and interaction of vapor-deposited Ag on a clean Pt(111) 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 H/sub 2/ and CO. The AES data indicate that at greater than or equal to 260 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 approx. 260 K, LEED data indicate that Ag grows in p(1 x 1) islands one atom thick. In the second layer Ag growth assumes a Ag(11) lattice spacing, rotationally commensurate with the underlying Pt(111) substrate. The change in work function (..delta..phi) indicates that the annealed Ag/Pt(111) surfaces of theta/sub Ag/ greater than or equal to 2.0 achieve a work function value more negative than smooth Ag(111), consistent with the microscopic surface roughness for three-dimensional island growth. An irreversible increase in the work function at approx. 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. Analysismore » of the TDMS data for Ag/Pt(111) indicates that for theta/sub Ag/ 1.0, a second multilayer desorption peak appears with E/sub d/ equal to the sublimation energy of Ag (66.8 kcal mol/sup -1/). Due to the island growth mechanism, Ag causes simple one-to-one site blocking of the Pt(111) surface for H/sub 2/ and 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.« less