Coadsorption of water and chlorine on Ag(110): Evidence for adsorbate-induced hydrophilicity

Abstract

Coadsorption of H2O and Cl was studied on a Ag(110) surface under conditions of ultrahigh vacuum with thermal desorption spectroscopy, low energy electron diffraction, and electron stimulated desorption ion angular distribution. The experiments were conducted over the temperature range of 100–650 K for water coverages ranging from zero to several multilayers and chlorine coverages θCl of 0–0.75 monolayers (ML). Water adsorption is stabilized by chlorine; the thermal desorption peak for water interacting with chlorine, called the α2 state, shifts to higher temperature by 25–40 K from the α1 state for desorption from the clean surface. A c(2×2) bilayer for H2O forms for coadsorption with less than 0.25 ML of Cl. The surface solvation number (SSN), defined as the number of stabilized water molecules per chlorine atom, varies from 13 to about 4 as θCl increases from 0 to 0.25. The unusually large SSN and the c(2×2) structure is evidence that Cl(a) promotes water adsorption to the metal surface itself in an effect called adsorbate-induced hydrophilicity. Coadsorption with higher chlorine coverages produces a p(4×3) structure for 0.25<θCl<0.4 and a c(4×4) structure for 0.4<θCl<0.5. Chlorine interacts directly with water in these structures in the form of surface solvation seen in previous studies of water coadsorption. The p(4×3) and c(4×4) patterns are evidence that coadsorbed water alters the distribution of chlorine on the surface. These results are interpreted in terms of the balance of forces among the two adsorbed species and the surface.