Article pubs.acs.org/JPCC
Facet Recognition and Molecular Ordering of Ionic Liquids on Metal Surfaces Kshitij C. Jha,† Hua Liu,† Michael R. Bockstaller,‡ and Hendrik Heinz*,† †
Department of Polymer Engineering, University of Akron, Akron, Ohio 44325, United States Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
‡
S Supporting Information *
ABSTRACT: 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-3methylimidazolium 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.
1. INTRODUCTION Ionic liquids are widely used as solvents due to the low volatility, stability up to high temperatures, and high dipole moment.1,2 Ionic liquids also present a green alternative to cytotoxic surfactants such as cetyl trimethyl ammonium bromide (CTAB).3−8 The ionic character, polarity, and ability of ionic liquids to form networks can also be exploited in the synthesis of inorganic nanostructures.4,5,9−12 Anisotropic gold nanostructures, for example, have applications in sensing, imaging, and optoelectronic devices and are especially attractive for medical imaging due to low cytotoxicity and tunable NIR absorbance.13−21 The control of shape and size of gold nanostructures is thus an interesting material science problem, and considerable progress has been made toward the reductive synthesis in aqueous surfactant solutions13,22−25 and in waterfree ionic liquids.9−12 The growth mechanism of anisotropic metal nanostructures in surfactant solutions likely involves capping of crystal facets with surfactants.23−29 However, the role of additional stabilizers has not been fully explored, and a universal theory of anisotropic growth has yet to be developed.24,30,31 Experimental studies of the formation of metal nanostructures in ionic liquids suggest that oligo-molecular networks of ionic liquids may provide directionality to the self-assembling metal but also do not explain the growth mechanism.4,5,9−12 Adsorption of various peptides and ionic surfactants on surfaces of gold in colloidal solutions was shown to involve soft epitaxial interactions28,32−34 and contributions by induced charges.34 However, specific information on ionic liquids is not available, © 2013 American Chemical Society
and challenges to understand growth and control shape persist because the direct observation of reaction and stabilization processes at the metal interfaces exceed the capabilities of current laboratory instrumentation such as TEM, NMR, FTIRATR, XPS, SPR, AFM, and UV−vis. Insight into the basic interactions contributing to the adsorption and ordering of ionic liquids on metal surfaces can be obtained using quantum mechanical and all-atom molecular simulation.27−29,32−51 In this contribution we employed density functional theory (DFT) and molecular dynamics (MD) calculations with the Interface Force Field48 to evaluate the assembly of 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIM][ES]) on gold {111}, {100}, and {110} surfaces for coverage from single ion pairs to multilayers, including adsorption geometry, charge density, and adsorption energies. Contributions to the attraction to the metal surface, changes in the arrangement of ionic liquid molecules as a function of surface coverage, and the formation of networks on the nanometer scale are explained in comparison to experimental observations.9−12 The results aid in understanding metal nanostructure growth in ionic liquids and support a hypothesis on the role of silver ions as a shape-directing agent in the formation of gold nanorods. The cations and anions of [EMIM][ES] will also be called “molecular ions” besides the chemical nomenclature to reflect Received: April 1, 2013 Revised: November 14, 2013 Published: November 25, 2013 25969
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Figure 1. Optimized geometry and electron density contour across three planes for an ion pair of [EMIM][ES] adsorbed on a Au {111} surface at ∼50% surface coverage in vacuum according to DFT. (A) Plane 1 is located near the plane of the imidazolium ring, Plane 2 midway between the ring plane and the top layer of surface atoms (d/2), and Plane 3 one-third of the distance between the ring plane and the top layer of surface atoms above the metal surface (d/3). (B) The ion pair aligns parallel to the surface and optimizes long-range Coulomb interactions with periodic neighbor molecules. Thereby, polarizable atoms (S, O, N, C) retain a tendency to coordinate epitaxial sites as well as surface atoms. (C) Covalent bonding in the ring plane. (D) Minor covalent interactions between the ion pair and the metal (note different scale for the electron density in panels c, d, e). (E) A fingerprint of induced charges near the metal surface. Results are shown using the revPBE and PBE density functionals (see Figure S1 in the Supporting Information for additional results with the older LDA functional).
the similarity to surfactants and organic ions. The terms “ion pair” and “molecule” will be used equivalently for an electroneutral unit, consistent with conventions for ions and charged molecules (e.g., proteins, DNA).
periodic simulation box (Figure 1 and Figure S1, Supporting Information). Three different density functionals were employed to characterize bounds of reliability, including revPBE, PBE, and LDA. Geometry optimization of the [EMIM][ES] molecule indicates a parallel orientation of the imidazolium ring to the {111} surface using all functionals (Figure 1A,B). The presence of localized covalent bonds in the ring plane leads to high contour charge densities >2 e/Å3 (Figure 1C).52−56 The electron density is particularly higher at the nitrogen atoms due to higher electronegativity and lower at
2. RESULTS AND DISCUSSION 2.1. Surface Interaction with a Single Ion Pair. Density functional theory allows the examination of the adsorption geometry and electronic structure of single ion pairs. We focus on the gold {111} surface at 50% effective surface coverage in a 25970
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Figure 2. Structural arrangement of [EMIM][ES] on {111}, {100}, and {110} surfaces of gold as a function of surface coverage from 50% to 200% in side and top view according to molecular dynamics simulation. (A) On the {111} surface, the ion pairs adsorb flat-on and tend to avoid top-layer surface atoms. Yet Coulomb forces between cations and anions significantly weaken the coordination of epitaxial sites. At high surface coverage, an approximately alternating order of cations and anions can be observed. (B) On the {100} surface, adsorption proceeds similar to that on {111} surfaces although a notable fraction of imidazolium ions is tilted with respect to the surface. At 100% surface coverage, the average coordination number of imidazolium and sulfate ions is about four, whereby one neighbor is of the same kind. (C) On the {110} surface, the linear arrangement of epitaxial sites leads to characteristic upright orientation of the imidazolium rings at 100% surface coverage. At higher coverage, molecular cations and anions in the first layer continue to align with major and minor grooves of this surface. Rectangular dotted lines indicate the periodic repeat unit in the simulation. Molecular layers on the top are shown in a thicker line width than layers below, and metal atoms in different layers are shown with increasing radius toward the surface.
the connected carbon atoms due to lower electronegativity. These charge balances are reflected in the force field for
[EMIM][ES] (Figure S2 and Section S3, Supporting Information). The majority of polarizable atoms (S, O, N, C) 25971
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2.2. Surface Interaction with Multiple Ion Pairs. Several classical force fields for metal−organic interfaces represent noncovalent (soft epitaxial) and electrostatic interactions well and can be applied to larger systems.29,32−34,39,42,48,49,51,64 In particular, Lennard-Jones parameters for fcc metals42 alone and embedded in various classical force fields (CHARMM, CVFF, PCFF) have shown a wide range of experimentally meaningful results, including facet preferences of peptides and surfactants, adsorption mechanisms, as well as guidance in the rational design of nanocrystals and of organically protected metal nanoparticle catalysts.28,29,32−34,48−50 We have employed these models in the form of the INTERFACE-PCFF force field48 to analyze adsorption of ionic liquids as a function of surface coverage on {111}, {100}, and {110} surfaces of gold using molecular dynamics simulation (Figure 2). The main contributions to adsorption of [EMIM][ES] include soft epitaxy with electrostatically controlled ordering and packing imperfections of the molecular ions, as well as induced charges, i.e., the same contributions as in DFT calculations on smaller systems. The analysis of equilibrium conformations and packing on the surface indicates a preference for flat-on conformations of the imidazolium rings on {111} and {100} surfaces (Figure 2A,B), the possibility of upright orientations on {100} surfaces, and a substantial amount of upright orientations on {110} surfaces (Figure 2C). “Flat-on” conformations of imidazolium rings on {111} surfaces have also been observed by SERS analysis of methylimidazole on a copper substrate,65 DFT calculations (Section 2.1), and other previous studies.27,32,33,50 Successive deposition of [EMIM][ES] from single ion pairs up to multilayers proceeds similarly on the different facets, as can be seen from representative snapshots and the progression of the adsorption energy (Figure 3). The adsorption energy was calculated relative to bulk [EMIM][ES] liquid and a clean gold surface as a reference state. At a surface coverage of {111}.26,73 We tested this trend further by computation of the stabilization of individual Ag atoms (not ions) when deposited on different facets of gold (Figure 5).49 The results indicate a better geometric fit and higher adsorption
Possible sources of uncertainty include the computation of the image potential a posteriori, the reliability of force fields, covalent bonding, and conformation sampling. However, we expect near-quantitative accuracy in structures and adsorption energies, and the results are consistent with various related experimental and computational findings. 2.3. Relation to Nanostructure Growth and Shape. Understanding the structure and energetics of ionic liquids on gold surfaces also provides insight into the origin of the formation of equilibrium (sphere-like) and nonequilibrium (rod-like) gold nanocrystals in reductive synthesis using ionic liquid solvents.9−12 In particular, the simulation results help evaluate previous hypotheses that relate anisotropic growth to facet-selective adsorption of [EMIM][ES]. Figure 4 compares the typical surface facet characteristics of spherical and rod-like particles that have been obtained in ionic liquids.9−12 One important question in the context of the formation of nonequilibrium-shaped particles, and one that can be uniquely addressed by computer simulations, is whether or not the shape control could be related to the thermodynamic stabilization of low density facets (such as {110} and {100}) due to selective adsorption of ionic liquids species. The nearly equal interaction energies of [EMIM][ES] with the three major facets according to the simulation indicate about equal thermodynamic stability of these facets when immersed in ionic liquids (Figure 3). This observation is supported by experimental results that have shown that near-spherical particles of low aspect ratio, and including a mixture of {111}, {100}, and {110} facets, are formed from tetrachloroaurate upon reduction with ascorbic acid in ionic liquids (Figure 4A).11 In the absence of shape regulation by other means, the nanoparticles were shown to subsequently grow to larger diameters with a majority share of {111} facets, in agreement with the low surface area-to-volume ratio and predominance of {111} crystallographic directions on nearspherical particles.49 Similar results are also seen on other metals.4,5
Figure 5. Computed binding energies of single silver atoms (blue) on {110}, {100}, and {111} surfaces of gold with the INTERFACE-PCFF force field. The trend in decreasing attraction {110} > {100} > {111} is the same as the decrease in underpotential of silver(I) ions, in support of preferred silver deposition onto {110} and {100} surfaces. 25974
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freezing) was particularly notable for the first molecular layer and layering effects extended up to the third molecular layer as seen by periodic changes in adsorption energy as a function of surface coverage. The preferred conformation of imidazolium ions is flat-on on {111} surfaces and partially upright on {100} surfaces, and characteristic upright orientations were observed on {110} surfaces at monolayer coverage. Induced charges are a major contribution to adsorption that account for about 25% of the total adsorption energy in the first molecular layer and for more than 50% of the total adsorption energy in the second and subsequent molecular layers. Similar thermodynamic preferences of adsorption of [EMIM][ES] toward different facets agree with the formation of near-spherical particles in experiment and may not be strong enough to explain the formation of anisotropic gold nanostructures. Therefore, we rather suggest that silver underpotential deposition on {110} and {100} facets upon addition of Ag(I) is a likely driver for observed nanorod growth in {111} directions. We propose a tentative growth mechanism that involves slowing deposition of further gold on the silvercontaining facets which are similarly stabilized by the ionic liquid and preferred deposition of further gold toward terminal {111} facets. The role of ionic liquids appears to lie mostly in the stabilization of existing facets, and future studies are required to answer more specific questions of nanostructure growth.
strength of single silver atoms in the same order {110} > {100} > {111} as the stabilization of Ag(I) ions by UDP. Stronger adsorption thus enables the transfer of more electron density from silver to gold. Ultimately, these observations suggest a hypothetical mechanism for nanorod growth. (1) First, silver preferentially deposits on {110} and {100} facets and likely remains in the metal nanostructure formed. The presence of silver subsequently slows down the deposition of further gold atoms because the surface energy of silver is 15% lower compared to gold.42,74 (2) The ionic liquid likely stabilizes silver-containing facets and silver-free facets similarly well. This happens because the contributions to adsorption are strong and not significantly metal specific (>25 kcal/mol ion pair). Soft epitaxial interactions, no matter how imperfect, remain strong because the lattice spacing of Ag and Au is nearly identical (0.2% difference)33 and the lower surface energy of Ag weakens adsorption only slightly.28,32,42 Interionic interactions and induced charges are similar on Ag and Au, whereby induced charges could be even stronger on silver according to the position of the image plane.34 (3) Finally, the reduction of further Au(III) precursor and deposition of gold atoms would be favored on facets of higher Au purity. It is thus preferred on twinned {111} facets at the end of nanorods in agreement with rod growth observed in TEM images.9 This growth hypothesis is certainly only a first attempt to achieve consistency among observations, physical chemical principles, and simulation data. More specific evidence in future studies will be needed for further validation. We also note that the above discussion is specific to nanorod formation in ionic liquids, and a strong influence of solvents on particle formation can be expected (see Section S2, Supporting Information, for details). In summary, the simulations reveal that the role of short chain ionic liquids such [EMIM][ES] as reaction media for the controlled growth of shape-anisotropic gold nanoparticles lies predominantly in electro-epitaxial stabilization. No significant bias toward anisotropic growth can be attributed, for example, to “facet-selective” binding of ionic liquid species, which has been proposed in some experimental studies. The simulations rather point to the role of “shape-regulating coreagents” such as Ag(I) salts in altering the shape evolution of nanoparticles by retarding the growth of {100} and {110} facets. In this context, one interesting area for future research to pursue is the control over underpotential deposition of Ag(I) ions under different conditions that appear to play an important role during the shape evolution of gold nanostructures in ionic liquid and other solvent media. In situ monitoring of the deposition process and 3D resolved elemental analyses of the rods formed could also provide further valuable insight.
4. COMPUTATIONAL METHODS 4.1. DFT Calculations. We employed a small Au unit cell containing 54 atoms of dimension 0.8652 × 1.4985 × 0.7064 nm3 to analyze adsorption of single [EMIM][ES] ion pairs on the Au {111} surface using DFT. The cell was periodic in the x and y directions and extended in the z direction to a total height of 2 nm to create a metal slab of a thickness of three atomic layers (0.7064 nm3) terminated by a {111} surface. A single ion pair of [EMIM][ES] was placed onto the Au {111} surface in the position of minimum energy identified by classical MD simulation with the force field (Section 4.2).75 The surface coverage was approximately 50% due to periodic boundary conditions. Similarly sized unit cells were employed to test adsorption on Au {100} and {110} surfaces. Subsequently, quantum mechanical energy minimization was carried out, resulting in atomic displacements up to 0.8 Å. To test convergence, three additional start configurations were prepared using different orientation and displacements of cation and anion of ∼1 Å across the xy plane. The minimum energy decreased by about 10%, and convergent end conformations were obtained, showing that different start conformations proved necessary to identify a global energy minimum under the constraints of the finite size surface. The structure of lowest energy was employed to analyze the adsorption geometry of the [EMIM][ES] ion pair and the contour charge density across three different planes parallel to the surface (Figure 1 and Figure S1, Supporting Information). One plane was positioned through the molecular plane at distance d from the top atomic layer of the metal, another plane equidistant between the top atomic layer of the metal and the ion pair at d/2, and a third plane at one-third of the distance d from the top atomic layer of the metal. We employed three different density functionals for comparison, including the revised PBE, PBE, and the LDA approximation with PAW pseudopotentials, and carried out at least 100 steps of energy minimization in each case.76,77 Simulations involved plane
3. CONCLUSION The interaction of ethylmethylimidazolium ethyl sulfate with various gold surfaces was investigated for surface coverage ranging from single ion pairs to multilayers using DFT and classical molecular dynamics methods. Three primary energetic contributions are shown to determine the adsorption energy, i.e., imperfect soft epitaxy in combination with Coulomb interactions between neighbor ions as well as induced charges, which may be summarized as electro-epitaxial stabilization. Covalent bonding was found to be small. The binding strength is in the range of −30 to −40 kcal/mol per ion pair and attraction slightly stronger on {110} and {100} surfaces compared to {111} surfaces. Very low mobility (surface 25975
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Figure 6. Calculation of the adsorption energy per [EMIM][ES] ion pair (“molecule”) on gold surfaces relative to the liquid state. (A−C) Simulations using three boxes were employed, including the gold−ionic liquid interface for a given surface coverage (A), the bulk ionic liquid (B), and the same gold surface in contact with vacuum (C). (D) The adsorption energies were calculated from the average energies of the individual simulations for all molecules and on a per molecule basis. The example illustrates the calculation of adsorption energies for monolayer coverage of the {110} surface with x = 6 and N = 30.
waves with 4 × 2 × 1 k points and an energy cutoff at 400 eV using the program VASP.78,79 The LEV00 and TETR tools were used for visualization.80 The results show that the form of the exchange-correlation function somewhat affects the density of gold and the equilibrium distance d of [EMIM][ES] (lowest d for LDA). The electronic structure at the interface is similar, and a shorter distance d expectedly leads to slightly increased electron densities at the planes at d/2 and d/3 (Figure 1 and Figure S1, Supporting Information). Uncertainties of the reported distances and electron densities for revised PBE are smaller than the differences caused by use of another density functional. The choice of three atomic layers had no effect on our analysis; however, dynamic simulations of gold surfaces on significant time scales would require at least six atomic layers of Au to justify the existence of a “surface” rather than atomically thin layers with residual reactivity. 4.2. Classical Molecular Dynamics Calculations. 4.2.1. Models. Models of gold {111}, {100}, and {110} surfaces with a cross-sectional area of ∼2 × 2 nm2 and a vertical thickness of the metal slab of ∼2 nm were built using the metal unit cell from X-ray data42,71 and the graphical user interface of Materials Studio.75 Models of the {100} surface (equal to the {001} surface in the fcc lattice) were prepared from a 3D periodic 5 × 5 × 5 superstructure of the unit cell with the dimensions 2.039 × 2.039 × 2.039 nm3. The super cell was extended in the z dimension to 50 nm to create a metal slab of
2 nm thickness terminated by a {001} surface in contact with a vacuum slab of 48 nm thickness. Models of {111} and {110} surfaces were constructed similarly using customized 3D periodic rectangular cells that yield the desired cleavage plane in z direction. The smallest repeat units of these customized rectangular cells were obtained from the original super cell by appropriate rotation of the Cartesian coordinate system and analysis of the atomic coordinates in the new coordinate system without changes in relative atomic positions and 3D periodicity.42,48 The super cells for {111} and {110} surfaces correspond to 7 × 4 × 3 and 5 × 7 × 7 superstructures with the dimensions 2.019 × 1.998 × 2.119 nm3 and 2.039 × 2.019 × 2.019 nm3 before extension to 50 nm in the z direction. In this first study of gold−ionic liquid interfaces, we assume nonreconstructed models of all surfaces. Specifically,√3 × 22 surface reconstruction on {111} surfaces and 20 × 5 reconstruction on {100} surfaces are disregarded, as they require a length >8 nm that is several times larger than the simulation cell.81 We also do not consider 1 × 2 surface reconstruction of {110} surfaces, which complicates the computation of the image potential.81,82 Specifically reconstructed surfaces could be investigated in a separate study. Molecular models of the ion pairs of [EMIM][ES] and of the gold−ionic liquid interfaces were built using the graphical interface of Materials Studio.75 We prepared models with increasing surface coverage from single ion pairs of [EMIM][ES] to multilayers on each of the {111}, {110}, and {100} 25976
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average energies. The three trajectories of lowest average energy were chosen to report representative structures (Figure 2), average energies, and standard deviations (Figure 3). The energies after annealing were lower than before annealing and convergent among multiple trajectories within 34%). The requirement of annealing to reach convergent equilibrium configurations indicates considerable energy barriers for diffusion and relaxation of surface-adsorbed ionic liquids. The same protocol was applied to bulk [EMIM][ES] liquid and the neat metal surface to obtain reference values for the computation of adsorption energies (Figure 6). For the calculation of the adsorption energy of a single silver atom on the gold {110}, {100}, and {111} surfaces, we carried out molecular dynamics simulations at 298 K in the adsorbed state on the surface and in the desorbed state (>2 nm away from the surface in vacuum) for 1 ns. The difference in average energies corresponds to the adsorption energy reported in Figure 5. The numerical uncertainty is
