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Hydrated alkali atoms on copper(111): A density functional theory study

Abstract:

We present a systematic computational study of submonolayer coverage of alkali atoms (Na, K, Cs) on Cu(111) surface hydrated from 1 to 6 water molecules. Our calculations show that water molecules preferentially bind to the adsorbed alkali ion and that a gradual detachment of the alkali from the Cu(111) surface is found as the hydration increases. This decoupling of the alkali from the Cu(111) surface results in a linear decrease of the charge transfer to the substrate. The orientation of the water dipoles pointing toward the surface leads to a gradual increase of the work function of the substrate as the number of coordinated water molecules increases from 1 to 4. Beyond 5 coordinated water molecules, the alkali adatom becomes saturated, and water adsorption sets in on the Cu(111) surface with the expected decrease in the work function of the system, as measured in two-photon photoemission spectroscopy (2PPE) experiments. From the detailed analysis of the orientation of the water electric dipoles, we were able to understand the experimentally observed initial increase of work function upon hydration and its subsequent decrease after saturation of alkali sites with water molecules. From the calculated energetics, we gauge the relative strengths of the alkali-Cu(111), alkali-water, and water-Cu(111) interactions as we move across the alkaline group. We found an excellent linear correlation between experimental water desorption temperatures and our computed water-alkali binding energies on Cu(111).