Facet Recognition and Molecular Ordering of Ionic Liquids on Metal Surfaces

Ionic liquids are widely used as solvents and reaction media due to low volatility, stability up to high temperature, and large dipole moment. Emergent applications also aim at the anisotropic growth of metal nanostructures in ionic liquids through facet-selective interactions although the governing mechanisms remain poorly understood. We employed a combination of quantum mechanical and classical simulations to analyze the structure and energetics of the self-assembly of ionic liquids on metal surfaces from single ion pairs to multilayers, using the example of 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIM]­[ES]) on the crystallographic {111}, {100}, and {110} facets of gold. Adsorption is controlled by the interplay of soft epitaxy, ionic interactions, induced charges, and steric effects related to the geometry of the cation and anion. These factors lead to characteristic molecular patterns on individual surfaces. Binding energies are similar irrespective of surface coverage and only slightly increase from {111} to {100} and {110} surfaces due to stronger surface corrugation and higher induced charge. The results explain specific experimental observations and aid in understanding particle growth in ionic liquid media. A mechanistic hypothesis for the formation of anisotropic gold nanorods in the presence of silver ions is made, in which silver retards the growth along {100} and {110} facets through underpotential deposition.