Interaction of Water Molecule with Au(111) and Au(110) Surfaces under the Influence of an External Electric Field

Abstract

The interaction of water monomers with a gold surface is investigated using density functional theory (DFT) to develop a better understanding of the response of a water molecule to an imposed electric field at the surface. Two gold surface orientations, Au(111) and Au(110), are studied. Multiple unique stable adsorption positions of water molecules are identified for each surface orientation, and the results are validated against existing theoretical and experimental data. The values of the adsorption energies do not vary by more than 0.06 eV, which suggests that the potential energy surface of the water molecule interacting with the gold electrode is relatively smooth. The projected density of states and the difference charge density analyses reveal that the interaction mechanism between the water molecule and the gold electrode is a partial exchange of charge rather than a chemical bonding. A normal electric field of magnitude between ±5.0 × 109 V/m is applied and its effect on the geometry and orientation of the water molecule is analyzed. The change in the geometry of the water molecule in response to the applied electric field shows a strong nonlinearity and asymmetry with respect to the magnitude and direction of the applied field. The interaction between the water monomer and Au electrode with/without the electric field is explained in terms of the interplay of Au–O, Au–OH, and electrostatic interactions. There is a significant difference between the dielectric response of the water molecule on the Au(111) and Au(110) surface that is related to the strength of the adsorption energy of the water monomer to both surfaces.