Cation-Induced Dipole Moments on Single-Crystal Metal Surfaces

The properties of the electric double layer are largely determined by ions adsorbed on the electrode surface. To elucidate the role of cations in particular, we have studied hydrated Ca2+ and K+ ions adsorbed on Pt(111), Au(111), and Ag(111) using density functional theory. In all the investigated cases, the cations are not chemisorbed on the metal electrode. A hybrid solvent model, combining implicit solvent with explicit water, was employed to accurately capture solvation effects. In this model, explicit water molecules form stable solvation cages around the ions, stabilizing their charge. These charges are balanced by the countercharge induced on the metal. The resulting dipole moment produces a dipole potential, which is determined by the solvation shell, the chemical interaction with the metal, and the screening properties of the metal. Notably, Ag(111) shows distinctive behavior, as its large electronic spillover results in smaller dipole moments. The dipole potential is seldom measured but can be related to the familiar electrosorption valency. While K+ practically retains its charge upon adsorption, Ca2+ shows a true partial charge transfer, resulting in a fractional electrosorption valency.