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.