Hydrated Alkali Atoms on Copper(111): A Density Functional Theory Study

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)