Carboxylate groups have recently been explored as a new type of ligand to protect superatomic copper and silver nanoclusters, but little is known of the interfacial structure and bonding. Here, we employ density functional theory to investigate the interfaces of a model carboxylate group, CH3COO, on the coinage metal surfaces and clusters. We found that μ2-CH3COO is the most preferred binding mode on the three M(111) surfaces (M = Cu, Ag, and Au), while μ3-CH3COO is also stable on Cu(111) and Ag(111). The saturation coverage was found to be about seven CH3COO groups per nm2 for all surfaces. CH3COO has the strongest binding on Cu and weakest on Au. Moving from the flat surfaces to the icosahedral M13 clusters, we found that the eight-electron superatomic [M13(CH3COO)6] nanoclusters also prefer the μ2-CH3COO mode on the surface. The icosahedral kernel in [Cu13(CH3COO)6] and [Ag13(CH3COO)6] was well maintained after geometry optimization, but a larger deformation was found in [Au13(CH3COO)6]. Given the broad availability and variety of carboxylic acids including amino acids, our work suggests that carboxylate groups could be the next-generation ligands to further expand the universe of atomically precise metal clusters, especially for Cu and Ag.

Ligand-protected coinage metal nanoclusters find applications in microelectronics,1 electrochemistry,2 energy conversion,3 pharmaceutical chemistry,4 sensing,5,6 and catalysis.7,8 Great efforts toward synthesis and characterization have been devoted to establishing their structure–property relationships. Generations of ligands from phosphines9 and thiolates10–13 to alkynyls14 and carbenes15,16 have been used in protecting coinage metal nanoclusters. Meanwhile, first-principles density functional theory (DFT) has provided insights into many fundamental questions about the structure, interfaces, formation, and reactions.17–19 

The confluence of efforts in the past decade has led to the new field of atomically precise nanochemistry, epitomized by ligand-protected coinage metal clusters. One important and common theme in this new field is the interfacial structure and bonding of ligands on the cluster surface and the other is the impact of the curvature and cluster size on the interfacial motifs.20 A case in point is the famous staple motif of RS–Au–SR on the gold clusters10,21 and surfaces.22 The analogous RCC–Au–CCR motif was also later found,23,24 opening up a new avenue of research that leads to many new structural, optical, and catalytic insights.25,26 Hence, exploring new ligands is a long-lasting thrust in the field of atomically precise nanochemistry.

Although carboxylates as a hard base were less explored for protecting soft acid ions of coinage metals, self-assembly monolayers (SAMs) of n-alkanoic carboxylates on copper and silver surfaces were investigated in the 1990s.27–29 Recently, researchers were also interested in carboxylate chemistry on gold nanoparticles30 and gold surfaces.31 Lately, a new all-carboxylate-protected superatomic silver nanocluster, [Ag8(pfga)6]6−, was discovered,32 which has generated great interest in using carboxylic acids, including amino acids, for synthesizing coinage metal nanoclusters.33 The pfga (perfluoroglutarate) ligand is a special chelate ligand with fluorination in the middle and two carboxylate groups at the ends. To fully take advantage of the great variety and availability of carboxylate ligands for protecting coinage metals, one wonders what the expected binding mode should be for a typical or a more general carboxylate group at the interface with a coinage metal cluster or surface.

The recent discovery of carboxylate-protected Ag nanoclusters and the previous work of carboxylates on coinage metal nanoparticles and surfaces prompted us to examine in detail the structure and energetics of the carboxylate–metal interface for both a flat surface and a nanocluster for a general carboxylate group. To this end, herein we employ first-principles density functional theory (DFT) to first explore the interfacial structures of a model carboxylate group (namely, the acetate or CH3COO group) on the (111) surfaces of Cu, Ag, and Au. Then, we will probe the acetate group on the icosahedral M13 clusters of Cu, Ag, and Au from the superatomic perspective.34 Below, we first introduce the computational method.

The structure and energetics of the acetate group on the (111) surfaces of Cu, Ag, and Au were studied by using the Vienna ab initio simulation package (VASP).35 The ion–electron interaction is described with the projector augmented wave (PAW) method.36 Electron exchange and correlation are represented by the functional of Perdew–Burke–Ernzerhof (PBE) of the generalized gradient approximation (GGA).37 A cutoff energy of 450 eV was used for the plane-wave basis set. The convergence criteria were 10−4 eV in energy and 0.02 eV/Å in force for all optimizations. As shown in Table I, the optimized bulk lattice parameters of fcc Cu, Ag, and Au show good agreement with the experiment. Then, the M (111) (M = Au, Ag, and Cu) surface slabs were modeled in a rectangular 3 × 2√3 supercell with four atomic layers (16 atoms per layer), and their parameters are summarized in Table I. The thickness of the vacuum layer between slab images was set to be 17 Å. The bottom two layers of the slab were fixed at the optimized bulk positions during structural relaxation. The Brillouin zone of the surface supercell was sampled by a Monkhorst–Pack k-point mesh of a 4 × 4 × 1 grid. Bader charge analysis was done with the implementation by Henkelman et al.38 

TABLE I.

Bulk lattice parameters of the fcc metals and the surface lattice parameters of the rectangular 3 × 2√3 supercell of their (111) surfaces.

Lattice parameters (Å)CuAgAu
Expt. bulk 3.62 4.16 4.17 
DFT bulk 3.57 4.08 4.12 
DFT surface 10.27 × 8.89 11.81 × 10.23 11.79 × 10.27 
Lattice parameters (Å)CuAgAu
Expt. bulk 3.62 4.16 4.17 
DFT bulk 3.57 4.08 4.12 
DFT surface 10.27 × 8.89 11.81 × 10.23 11.79 × 10.27 

Structural optimizations of the acetate-protected M13 nanoclusters were carried out via the quantum chemistry program Turbomole V6.5.39 The TPSS (Tao–Perdew–Staroverov–Scuseria) functional40 was used for electron exchange and correlation, with the def2-TZVP orbital and auxiliary basis sets. Effective core potentials that have 19 valence electrons were used for Ag and Au.41 The convergence criteria were 3 × 10−5 eV for energy and 5 × 10−3 eV/Å for force.

The van der Waals interaction was included via the DFT-D3 approach with zero damping:42 for the VASP, the coefficients used for the PBE functional are s6 = 1.0, sr,6 = 1.217, and s8 = 0.722, and for Turbomole, the coefficients used for the TPSS functional are s6 = 1.0, sr,6 = 1.166, and s8 = 1.105.

The goal of the present work is to computationally assess the preferred binding mode and its associated binding energy of the acetate group on the coinage-metal surfaces and clusters. We started with a low-coverage scenario where there is only one CH3COO group on the surface supercell. We first computed the binding energy, defined as the energy needed to dissociate the CH3COO group as a radical into the gas phase away from the surface; we used this energy reference to avoid the complication if one were to study the desorption of a charged group from a surface with periodic boundary conditions. As shown in Table II and Fig. 1, the μ2 mode of the carboxylate O atoms anchoring on two neighboring metal sites is the preferred mode on all three surfaces. The μ3 motif on a surface M3 triangle (where one O atom of the COO moiety is at the bridge site of two neighboring metal atoms) is slightly higher in energy than μ2 and also a local minimum on Cu(111) and Ag(111). The μ3 motif is not stable on Au(111) and relaxes to the μ2 mode. Comparing just the common μ2 mode on the three surfaces, one can see that the CH3COO group binds to Cu(111) strongest, followed by Ag(111) and then Au(111). The stronger CH3COO–Cu(111) interaction is also reflected in the shorter Cu–O distance [2.00 Å; Fig. 2(a)]. Interestingly, the Ag–O and Au–O distances [Figs. 2(c) and 2(e)] are very close, but the CH3COO–Ag(111) interaction is 0.46 eV stronger than the CH3COO–Au(111) interaction.

TABLE II.

Binding energies (in eV) of a single CH3COO group, defined as the energy needed to dissociate the CH3COO group as a radical into the gas phase away from the surface, on the supercells of the (111) surfaces of Cu, Ag, and Au (see Table I for the supercell dimensions).

Binding modeCuAgAu
μ2 2.88 2.50 2.04 
μ3 2.76 2.44 ⋯ 
Binding modeCuAgAu
μ2 2.88 2.50 2.04 
μ3 2.76 2.44 ⋯ 
FIG. 1.

Bonding geometries of μ2-CH3COO (left) and μ3-CH3COO (right) on the (111) surfaces: (a) and (b) Cu(111); (c) and (d) Ag(111); and (e) Au(111). Color code: H, white; C, gray; O, red; Cu, green; Ag, blue; and Au, yellow.

FIG. 1.

Bonding geometries of μ2-CH3COO (left) and μ3-CH3COO (right) on the (111) surfaces: (a) and (b) Cu(111); (c) and (d) Ag(111); and (e) Au(111). Color code: H, white; C, gray; O, red; Cu, green; Ag, blue; and Au, yellow.

Close modal
FIG. 2.

Charge density difference for (a) μ2-CH3COO and (b) μ3-CH3COO on Ag(111) (Ag atoms in blue). Magenta (electron accumulation) and yellow (electron depletion) isosurfaces are at a contour level of 0.003 e/Å3.

FIG. 2.

Charge density difference for (a) μ2-CH3COO and (b) μ3-CH3COO on Ag(111) (Ag atoms in blue). Magenta (electron accumulation) and yellow (electron depletion) isosurfaces are at a contour level of 0.003 e/Å3.

Close modal

To further shed light on the interfacial bonding, we analyzed the charge transfer between CH3COO and the metal surfaces. First, we used Bader charge analysis and obtained the partial atomic charges on CH3COO. After summing up the atomic charges, we obtained the molecular charges of μ2-CH3COO and μ3-CH3COO on the three surfaces. As one can see from Table III, the molecular charge correlates with the binding energy: the stronger the bonding, the more negative the CH3COO group. Using CH3COO–Ag(111) as an example, the charge-density-difference plot (Fig. 2) clearly shows that electrons move from the metal surface and the C–O bonds (yellow) to the O–Ag bonds and O atoms (magenta). In the case of μ2-CH3COO, the C–C bond also accepts some transferred electrons [Fig. 2(a)].

TABLE III.

Molecular charge (in e) of the CH3COO group on the (111) surfaces of Cu, Ag, and Au for the two different binding modes.

Binding modeCuAgAu
μ2 −0.65 −0.65 −0.50 
μ3 −0.63 −0.62  
Binding modeCuAgAu
μ2 −0.65 −0.65 −0.50 
μ3 −0.63 −0.62  

The binding-energy trend can be further explained by the soft–hard acid–base theory from a molecular perspective and by the electronegativity from the ligand–surface interaction perspective. Carboxylate is a hard base, so it has the weakest interaction with the softest acid of the three, which is gold.43 On the surface, the carboxylate group will gain some electrons from the surface (Table III and Fig. 2); Au has the greatest electronegativity of the three and the least tendency to give away electrons to the carboxylate group and hence the weakest interaction. Unfortunately, we have not found any previous experimental work comparing all three coinage metals for the same carboxylate ligand(s) in terms of the binding strength. One difficulty we envision is the decomposition of the carboxylate group during the desorption process. In fact, a recent experimental study found that the carboxylate groups from adsorption of pyridine dicarboxylic acid on Cu(111) undergo decarboxylation during desorption with a measured activation energy of ∼1.9 eV.44 This is consistent with our finding of the binding energy of ∼2.8 eV of a carboxylate group on Cu(111); in other words, the decarboxylation reaction tends to happen already before the desorption of the complete ligand due to the lower activation energy of decarboxylation than the binding energy of the carboxylate ligand.

Both SAMs and ligand-protected metal nanoclusters usually have a complete layer of ligands covering the metal surfaces. Therefore, we next examine how the binding energies and interfacial motifs vary with the CH3COO coverage. We computed the differential binding energy as the energy needed to desorb just the newly adsorbed CH3COO group off the surface. As shown in Fig. 3, the binding energies of CH3COO groups in the coverage of 1–5 nm−2 are rather constant for each metal surface. The saturation coverage is similar for the three surfaces (∼6.6 nm−2). Beyond this coverage, the binding energies turn negative, meaning that desorption is spontaneous and downhill in energy.

FIG. 3.

Differential binding energy (DBE) vs the coverage of CH3COO on the (111) surfaces of Cu, Ag, and Au. DBE is defined as the energy needed to dissociate just one CH3COO group as a radical into the gas phase away from the surface at the specific coverage.

FIG. 3.

Differential binding energy (DBE) vs the coverage of CH3COO on the (111) surfaces of Cu, Ag, and Au. DBE is defined as the energy needed to dissociate just one CH3COO group as a radical into the gas phase away from the surface at the specific coverage.

Close modal

The adsorption configurations of CH3COO groups with increasing coverage on Cu(111) are shown in Fig. 4. One can see that the μ3 motif starts to appear when the coverage reaches about half of the saturation coverage [Fig. 4(c)]. At the saturation coverage, there is one μ3 and five μ2 CH3COO groups; due to the steric effect, the five μ2 CH3COO groups are slightly tilted [Fig. 4(e)].

FIG. 4.

Adsorption configurations of CH3COO with increasing coverage on Cu(111): (a) 2, (b) 3, (c) 4, (d) 5, and (e) 6 CH3COO groups per surface supercell.

FIG. 4.

Adsorption configurations of CH3COO with increasing coverage on Cu(111): (a) 2, (b) 3, (c) 4, (d) 5, and (e) 6 CH3COO groups per surface supercell.

Close modal
FIG. 5.

Adsorption configurations of CH3COO with increasing coverage on Ag(111): (a) 2, (b) 3, (c) 4, (d) 5, (e) 6, (f) 7, and (g) 8 CH3COO groups per surface supercell.

FIG. 5.

Adsorption configurations of CH3COO with increasing coverage on Ag(111): (a) 2, (b) 3, (c) 4, (d) 5, (e) 6, (f) 7, and (g) 8 CH3COO groups per surface supercell.

Close modal
FIG. 6.

Adsorption configuration of full coverage of CH3COO groups on Au(111). The dashed rectangle denotes the surface supercell.

FIG. 6.

Adsorption configuration of full coverage of CH3COO groups on Au(111). The dashed rectangle denotes the surface supercell.

Close modal
FIG. 7.

DFT-optimized structures of all-carboxylate-protected M13 nanoclusters: (a) [Cu13(CH3COO)6], (b) [Ag13(CH3COO)6], and (c) [Au13(CH3COO)6].

FIG. 7.

DFT-optimized structures of all-carboxylate-protected M13 nanoclusters: (a) [Cu13(CH3COO)6], (b) [Ag13(CH3COO)6], and (c) [Au13(CH3COO)6].

Close modal

The structures of CH3COO groups on Ag(111) are shown in Fig. 5. Interestingly, the μ3 motifs become more frequent especially in the intermediate coverages [Figs. 5(c) and 5(d)]. One reason for this more frequent appearance of the μ3 motifs is that the μ2 and μ3 motifs are much closer in energy on Ag(111) than on Cu(111) (Table II). At the saturation coverage, however, all the carboxylate groups are in the μ2 mode on Ag(111); this is because now all the surface Ag atoms can be utilized to anchor the carboxylate group groups [Fig. 5(g)]. In the case of Au(111), the picture is simpler because only the μ2 motif is stable on it, even at high coverages; in the full coverage (Fig. 6), all surface Au atoms are bonded to carboxylates.

The interfacial structure and energetics of CH3COO on M(111) surfaces (M = Cu, Ag, and Au) explored above now provide us a basis to further investigate the chemical bonding of CH3COO ligands on nanoclusters. The centered icosahedral M13 was chosen as the kernel because of its common appearance in ligand-protected metal clusters. Since there are 12 metal atoms at the surface of the icosahedral M13 core, six CH3COO groups are needed to fully protect the surface metal atoms in the μ2 mode. Then, according to the superatom model,34 an anionic cluster of the composition of [M13(CH3COO)6] would be an eight-electron superatom. To test this design idea, we explored a few different arrangements of the six ligands on each M13 core, and the most stable structures are shown in Fig. 7. We found that the μ2 mode is preferred on the icosahedral M13 kernel of Cu and Ag; we tested cases with mixed μ2 and μ3 CH3COO groups on Cu13 and Ag13 initially and found that the μ3 CH3COO group changed to μ2 after relaxation. The optimized [Cu13(CH3COO)6] and [Ag13(CH3COO)6] nanoclusters are similar in structure, and their icosahedral cores are well maintained after structural relaxation. Hence, both [Cu13(RCOO)6] and [Ag13(RCOO)6] could be viable targets for synthesis. In contrast, the icosahedral shell of [Au13(CH3COO)6] opened up after geometry optimization, indicating that such a geometry is unstable.

The computed HOMO–LUMO gap of 1.06 eV for [Cu13(CH3COO)6] and 1.32 eV for [Ag13(CH3COO)6] is similar to other eight-electron superatoms in the (1S)2(1P)6 configuration.45,46 Indeed, the double degenerate HOMOs show the P character (the three 1P orbitals split into a nondegenerate HOMO-1 and double-degenerate HOMO), while the triple degenerate LUMOs show clear D character (Fig. 8).

FIG. 8.

Orbital energy diagram and frontier orbitals of [Cu13(CH3COO)6].

FIG. 8.

Orbital energy diagram and frontier orbitals of [Cu13(CH3COO)6].

Close modal

In the structures of [Cu13(CH3COO)6] and [Ag13(CH3COO)6] in Fig. 7, each surface metal atom is coordinated by one O atom from a CH3COO group. In general, the coordination number of O atoms around a surface Ag or Cu atom depends on both the surface coverage of ligands and the electron count of the cluster (hence the oxidation state of the metal atoms). Higher coverages of ligands and higher oxidation states of surface metal atoms can lead to higher coordination numbers.47 While keeping the electron count constant at 8, we tested the coverage effect by examining 7, 8, and 9 CH3COO groups on Ag13. We found that the AgO2 coordination (two CH3COO groups coordinate one Ag atom) is indeed present at these high coverages (Fig. S1 in the supplementary material), but the AgO3 coordination either changed to AgO2 or was higher in energy. We expect that the AgO3 coordination mode may be stabilized if we lower the total electron count (in other words, making the surface Ag atoms more positively charged). The same can be expected for Cu as well, and a detailed further study is warranted.

Another point to note is that the focus of the present work is on the ligand–metal interface, so we have not explored the global minima of the [M13(CH3COO)6] clusters beyond the assumed icosahedral core. To assess if this assumption is sound, we have tested a couple of popular minima of bare M13 (Cs and C2 symmetry) in addition to the icosahedral structure.48 The comparison is shown in Fig. S2 in the supplementary material. We found that the [M13(CH3COO)6] cluster with the icosahedral core is much more stable than the other two cores for both Cu and Ag. Given our focus on the interface and the results in Fig. S2, we think that using the icosahedral M13 core is a good initial model to simulate carboxylate-protected Ag and Cu clusters.

We have investigated the interfaces between a model carboxylate group, CH3COO, and the coinage metal surfaces and clusters by first-principles density functional theory. The μ2-CH3COO mode was found to be the most stable configuration on all three M(111) surfaces (M = Cu, Ag, and Au), while the μ3-CH3COO mode is also stable on Cu(111) and Ag(111). The binding energy of CH3COO correlates with the amount of charge transfer from the metal surface to the adsorbate; CH3COO binds with Cu(111) the strongest with the shortest interfacial Cu–O bonds and the largest amount of negative charge (∼−0.65e). A distinct and similar saturation coverage of about 7 CH3COO groups per nm2 was found on all three surfaces. At the saturation coverage on the Ag(111) and Au(111) surfaces, all surface metal atoms are coordinated by μ2-CH3COO, while μ3-CH3COO modes appear in some intermediate coverages on Cu(111) and Ag(111). Geometry optimization of monolayer-protected icosahedral M13 clusters with six CH3COO ligands found that the eight-electron superatomic [Cu13(CH3COO)6] and [Ag13(CH3COO)6] nanoclusters are stable and maintain the icosahedral kernel, but a larger deformation was found in the case of Au. Our work provides insights into the interface between carboxylate groups and coinage metals that could help design and synthesize new atomically precise coinage-metal clusters.

See the supplementary material for (1) DFT-optimized structures of [Ag13(CH3COO)7]2−, [Ag13(CH3COO)8]3−, and [Ag13(CH3COO)9]4− and (2) comparison of the stability of [Cu13(CH3COO)6] and [Ag13(CH3COO)6] clusters with different core structures.

This work was sponsored by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Catalysis Science Program. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Supplementary Material