Ionic adsorbates on metal surfaces



We present a detailed study of the ionicity of Cs and I adsorbates on a Cu(111) surface and of the consequences of this ionicity for changes in the work function, Δϕ. In particular, a novel aspect of the present work is that we model coverage changes and we are able to determine how coverage increases affect the ionicity and the Δϕ. We use wavefunctions for finite clusters to model the adsorption, where normally only isolated adsorbates are studied. Techniques to simulate coverage were developed. We find and explain unexpected features for the adsorption of I, an excellent electron donor. For I adsorbed at a three-fold site, we confirm earlier results, for I at an on-top site, that the adsorbed Iodine is dominantly anionic with a minor covalent donation from I to the surface. For the initial increases of coverage, we find that Δϕ has a stronger than linear dependence on the number of adsorbates. We are able to identify the main origin of this super-linear behavior of Δϕ with coverage as changes in the way that the substrate charge polarizes in response to the presence of a higher coverage of adsorbates than to an isolated adsorbate. In particular, the polarization of surface charge at the periphery of the I adsorbate, a kind of pillow effect, is suppressed by the higher density of adsorbates. We present results for different charges of a cluster model for I/Cu that may be relevant for the interpretation of electrochemical measurements of charge flow induced by adsorption of ionic species. Our work presents information relevant for the adsorption of electron donors and acceptors on metal surfaces in general. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 000:000–000, 2010