Article pubs.acs.org/JPCC
Insights into the PhCC/Au Interface Qing Tang and De-en Jiang* Department of Chemistry, University of California, Riverside, California 92521, United States ABSTRACT: The phenylethynyl group (PhCC) has attracted attention recently as a new way to passivate gold surfaces, but little is known of the interfacial structure and bonding. Here we employ density functional theory to investigate the organogold interfaces between PhCC and the flat Au(111) surface as well as between PhCC and a model gold nanocluster (Au20). Although isolated PhCC prefers the three-coordinate hollow site on the perfect Au(111) surface, formation of the PhCCAuadatomCCPh motif becomes energetically preferred when the Au adatom is present, resembling the RSAuSR motif on Au(111). However, distinct from the thiolate/Au interface, the PhCCAuadatom−CCPh staple motif features π-bonding of the CC bond to the substrate Au atom in addition to the σ-bonding of the terminal carbon to the gold adatom (or the staple gold atom). Geometry optimization and simulated annealing further show that this new type of staple motif is also the preferred bonding mode of the PhCC groups on the Au20 cluster. The novel PhCCAuCCPh motif with π-bonding to the Au substrate is expected to be a key feature of the PhCC/Au interface for gold clusters, nanoparticles, and surfaces. This insight will help structural elucidation and chemical understanding of both self-assembled monolayers of PhCC groups on gold surfaces and PhCC protected gold nanosystems.
1. INTRODUCTION Gold nanoclusters, nanoparticles, and surfaces functionalized by organic ligands, particularly thiolate (RS-), have sparked huge research interest in recent years.1−5 The presence of these ligands not only stabilizes the Au nanostructures but also generates interesting interfacial properties, leading to electronic,6,7 magnetic,8 optical,9,10 catalytic,11 biological,12 and sensing13 applications. The structures, stabilities, dynamics, and bonding characteristics of the Au−thiolate interface have been widely studied through both experiments14−19 and theoretical calculations 20−24 over the past several years. The SR[AuSR]x oligomers or the staple motifs have been a key feature of the gold−thiolate interface and an important hypothesis for structure prediction for thiolated gold nanoclusters.25,26 Besides the broadly used thiolate ligands, the modification of gold surface by a new class of organic protecting ligands such as aryl radicals27−29 and terminal alkynes30 (RCC, where R is usually an aromatic group) by virtue of the formation of strong Au−C anchor bonds has been gaining attention. These ligands act as viable alternatives to thiolates for forming protective layers on gold, leading to the so-called “organogold” structures due to the bonding of C atoms directly with Au. By using terminal alkynes as the stabilizers [e.g., phenylacetylene (PA-H) or 9-ethynyl-phenanthrene (EPT-H)], Tsukuda and co-workers have synthesized a series of organogold clusters with precisely defined chemical compositions, such as Au 34 (PA)16 , Au 43(PA)22, Au54(PA) 26, Au 30 (EPT) 13 , and Au35(EPT)18. These organogold clusters were formed by direct ligation of the alkyne ligands to the preformed polyvinylpyrrolidone (PVP)-stabilized Au clusters.31−33 Spectroscopy characterizations revealed that the terminal H of alkynes is deprotonated and the resulting alkynyl carbon (RCC) © XXXX American Chemical Society
is bound to the Au cluster surfaces, evidenced by the weakening of the CC stretching band as well as the acidification of the aqueous phase after ligand exchange. Among the prepared organogold clusters, Au54(PA)26 was found to be magic, having higher stability than the other compositions.32 Despite the above experimental advances, little is known of how these terminal alkynyls are bonded to gold in the organogold clusters. Understanding the Au−alkynyl interfacial bonding would be of great fundamental importance to elucidate the formation process of the organogold clusters as well as the accompanying changes in electronic and optical properties of the Au clusters upon surface passivation. Moreover, these alkynyl ligands could form self-assembled monolayers on gold surfaces just as thiolates do.34,35 In fact, a very recent experimental report by Zaba and co-workers showed that the aliphatic alkynes [HCC(CH2)nCH3, n = 5, 7, 9, 11] can indeed form highly ordered self-assembly monolayers on the Au(111) substrate.36 These experimental progresses inspired us to examine the interfacial bonding of alkynyl groups on gold surfaces and clusters. In this work, we employ first-principles density functional theory (DFT) to explore the interfacial structures between the phenylethynyl group (PhCC), a simple and representative aromatic alkyne ligand, and gold. We consider both a flat surface, Au(111), and a cluster, Au20. We seek to understand how the phenylethynyl groups are chemically bonded to these Special Issue: Current Trends in Clusters and Nanoparticles Conference Received: September 2, 2014 Revised: October 1, 2014
A
dx.doi.org/10.1021/jp508883v | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
Figure 1. (a) Four possible surface sites for PhCC adsorption. Here only the top two layers of Au(111) surface are shown. (b−e) correspond to the optimized geometry structures (side view) for surface-bound PhCC/Au(111) at the top, bridge, hcp hollow, and fcc hollow sites, respectively; only the topmost surface layer is shown. Au, green; C, gray; and H, white. Same color scheme is used in figures below.
two systems and whether some interfacial motifs can emerge at the organogold interface.
Table 1 shows the binding energies corresponding to the four adsorption sites, which are defined as the energy of the
2. COMPUTATIONAL METHOD The DFT computations were performed by using the Vienna ab initio simulation package (VASP).37 The ion−electron interaction is described with the projector augmented wave (PAW) method.38 Electron exchange-correlation is represented by the functional of Perdew, Burke, and Ernzerhof (PBE) of generalized gradient approximation (GGA).39 A cutoff energy of 450 eV was used for the plane-wave basis set. The Au(111) surface slab was modeled in a rectangular 3 × 2√3 unit mesh with four-layer thickness. The bottom two layers were kept fixed at the optimized bulk positions during all the computations, and the thickness of the vacuum layer was set to be 17 Å. For adsorption of PhCC ligands (often shortened as PhCC in this paper for brevity), a 3 × 2√3 superstructure of Au(111) slab (with lateral dimensions of a = 10.2 Å and b = 11.8 Å) was used when adsorbing one PhCC group, while a larger superstructure (with lateral dimensions of a = 15.3 Å and b = 17.7 Å) was used when adsorbing two PhCC groups. The Brillouin zone was sampled by a Monkhorst−Pack k-point mesh of 4 × 4 × 1 grid. To model the cluster surface, the Au20 cluster was placed in a 25 × 25 × 25 Å3 cubic box, and only the Γ-point was used for k-point sampling. The convergence threshold for structural optimization was set to be 0.02 eV/Å in force. Simulated annealing at 350 K was performed by first-principles (that is, DFT-based) molecular dynamics via the Nóse thermostat with a time step of 1 fs, and the total simulation time was 10 ps.
Table 1. Binding energy (B.E.), Au−C Bond Length (rAu−C), CC Bond Length (rCC), and the Height of the Terminal C Atom above the Surface Au Layer (H) of PhCC on Au(111) at Different Adsorption Sites site
B.E. (eV)
rAu−C (Å)
rCC (Å)
H (Å)
top bridge hcp hollow fcc hollow
2.70 2.99 3.02 3.11
1.979 2.154 2.251 2.242
1.229 1.253 1.257 1.261
1.979 1.525 1.406 1.329
sum of the isolated two components [PhCC and Au(111)] relative to the adsorbed PhCC:Au(111) system (hence, a positive number means a favorable binding). Clearly, the terminal phenylethynyl carbon prefers to occupy the fcc hollow site forming 3-fold coordination (3.11 eV), whereas the top site is energetically the least favorable (2.70 eV). The strongest binding at the fcc site also corresponds to the shortest adsorption height of the terminal C of PhCC above the plane of the uppermost surface Au (1.329 Å). Such a large binding energy around 3 eV indicates that the PhCC group is strongly bonded to the Au(111) surface. For comparison, we also performed the same level computations on the adsorption of phenylmethanethiol group (PhCH2S) on Au(111), wherein the binding energy is about 1.78 eV. The significantly larger binding energy for PhCC than for thiolate can be attributed to the shorter and stronger Au−C bond than the Au−S bond. Therefore, PhCC might provide stronger protection of the Au surface than thiolate. The formation of the Au−C bond is accompanied by slight elongation of the C C bond (the CC bond length in the phenylacetylene molecule is 1.214 Å), indicating the weakening of the CC bond upon chemisorption onto the Au(111) surface. 3.2. Interaction between PhCC and Au(111) with a Au Adatom. We now turn to the question of the interfacial structures between PhCC and Au(111) with one Au adatom [Au−Au(111)]. This is motivated by the fact that the creation of Au adatoms on Au(111) is facile experimentally and that the alkanethiolate species would form the staple motifs (RSAu SR) on Au(111) in the presence of Au adatoms.14 Can the PhCC ligands form similar PhCCAuCCPh motifs on
3. RESULTS AND DISCUSSION 3.1. Interaction between the PhCCGroup and the Au(111) Surface. We first consider the adsorption process of one PhCC ligand on the Au(111) surface. Four types of high-symmetry sites for binding PhCC were considered (Figure 1a): the top site, the bridge site, the hcp hollow site, and the fcc hollow site. For all these sites, the PhCC ligand is bonded to the Au(111) surface via σ bonding (Figure 1, panels b−d), and the terminal C forms chemical bonds with one, two, and three Au atoms when adsorbed onto the top, bridge, and hollow sites, respectively. B
dx.doi.org/10.1021/jp508883v | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
and 2.39/2.71 Å (AusurfaceC), respectively (Figure 3b). We found that the π staple motif (Figure 3) is energetically more stable than the σ staple motif (Figure 2) by about 0.84 eV, indicating the much greater stability of the π staple motif due both to the CC/Au π interaction and the Ph/surface vdW interaction. Figure 4 shows a simple energetic plot for the formation of the PhCCAuadatomCCPh π motif on Au(111), compared
[Au−Au(111)]? On the basis of the bonding characteristics of linear RSAuSR motifs (paring of two RS− groups via the Au adatom), we constructed the PhCCAuCCPh motif from two PhCC groups and one Au adatom on Au(111). In the initial structure, the terminal alkynyl C atom forms one bond with the Au adatom and another with the underlying surface Au atom. Figure 2 presents the optimized structure of
Figure 2. Optimized structure of the PhCCAuadatomCCPh motif on Au(111). Au adatom is highlighted in blue.
Figure 4. Schematic showing the energetic stability of formation of the PhCCAuadatomCCPh π motif relative to the isolated Au adatom and PhCC groups on Au(111). (The energy is compared at the DFT-PBE-D3 level).
the PhCCAuadatomCCPh complex on Au(111). We found that the formation of PhCCAuadatom−CCPh staple motif on Au(111) signficantly increases the binding energy of PhC C to 3.48 eV per molecule, as compared to the binding energy of an isolated PhCC on Au(111) (3.17 eV) or on AuAu(111) (3.28 eV, PhCC group is located atop the Au adatom). To test if the staple-like PhCCAuCCPh motif in Figure 2 is dynamically stable, we further applied first-principles molecular dynamics (MD) based on DFT to perform simulated annealing of the PhCCAuCCPh complex on Au(111) at 350 K. During the MD simulation, we found a staple motif with a different mode of bonding on Au(111). In this structure (Figure 3), the two PhCC molecules are parallel to the Au surface in a nearly flat-lying geometry, and the CC bond of each PhCC has noticeable π-type bonding with the underlying surface Au in addition to the σ bond of the terminal C to the Au adatom. We call this novel motif the “π” staple motif, to differentiate it from the staple motif in Figure 2, which we call the σ staple motif since it involves only the bonding of the terminal C atom to the two Au atoms. In the π motif, the close spatial distance between the phenyl ring and the Au surface implies that the role of the van der Waals (vdW) forces should not be ignored. Hence, we used the recently developed DFT-D3 method40 to include dispersion correction to examine the structure and energetic properties of this adsorption model. In the optimized structure, the Au−C bond lengths associated with the σ and π bonding patterns are 2.00 Å (AuadatomC)
with one Au adatom and two PhCC groups (the two PhCC ligands are adsorbed at the fcc hollow sites). Obviously, forming the π staple motif has a significantly large energy gain of 1.58 eV, which provides the thermodynamic force to drive the formation of π staple motifs at the interface. Therefore, different from the upright bonding of isolated PhCC on a perfect Au(111) surface, with the involvement of Au adatoms, the flat-lying PhCCAuadatomCCPh motif should be the dominating interfacial structure on Au(111). Overall, the above findings show that the adatom-mediated interfacial mode of PhCC groups on Au(111) resembles that observed on the thiolate-passivated Au surface. However, different from the RSAuSR staple motifs formed via the Au−S bonds, the PhCCAuadatomCCPh staple motif consists of both the σ bonding of the terminal C to the staple Au atom and the π bonding of the unsaturated CC bond to a surface Au atom. 3.3. PhCC on the Au20 Cluster. The above understanding of the interfacial structures of PhCC on Au(111) surface prompted us to further investigate the surfacechemical bonding of PhCC ligands to Au nanoclusters. We chose the Au20 cluster to model the adsorption of the PhCC group, because Au20 with the Td symmetry and high stability is structurally analogous to a small fragment of bulk face-centered cubic gold, and each of the four faces resembles the (111) surface of bulk gold.41 We tested about 10
Figure 3. (a) Top and (b) side views of the PhCCAuadatomCCPh π staple motif on Au(111). C
dx.doi.org/10.1021/jp508883v | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
staple motif evolved out during the MD simulation (Figure 6) and in fact is energetically more stable than the PhCCAu CCPh σ staple motif (Figure 5d) by 0.20 eV. In the π staple motif, the σ bond between the terminal C and the staple Au is 1.99 Å in length, while the Au−C bonds between Au and CC are 2.27 and 2.32 Å in length. Different from on Au(111) where the formation of the π staple motif has been largely driven by the vdW force between the phenyl ring and the gold surface, here one sees that on small gold clusters such as Au20, the CC/Au π interaction drives the formation of the π staple motif. 3.4. Bonding in the π Staple Motif. We have now found that the π motif is the preferred interfacial bonding mode for PhCC on Au(111) and the Au20 cluster. To gain more insight into the electronic interaction in the π staple motif, we show in Figure 7 the bonding character of PhCC on
different sites on vertices, edges, and faces of Au20 and found that the PhCC ligand prefers to be covalently bonded to the vertex of the Au20 pyramid than the other sites. This behavior is very different from that on Au(111) where PhC C prefers the high-coordinate fcc hollow site. At the favorable vertex site, the optimized Au−C bond length is 1.957 Å, and the corresponding binding energy is 2.85 eV (Figure 5a). We note that this vertex mode of bonding has been recently found in a Au8 cluster protected by both 1,3bis(diphenylphosphino)propane (dppp) and pyridylethynyl ligands.42
Figure 5. (a) Optimized structure of PhCC covalently bonded to pyramidal Au20 cluster. (b−d) correspond to the optimized structures of two PhCC groups adsorbed on Au20 with the vertex−vertex, bridge−vertex, and bridge−bridge (or σ staple) binding configurations, respectively.
When two PhCC ligands are adsorbed on Au20, the favorable binding configuration corresponds to the two PhC C groups located on the bridge site near the vertex of Au20 pyramid (Figure 5d). This adsorption configuration is similar to the PhCCAuadatomCCPh σ staple motif on Au(111) (Figure 2), and the vertex Au atom resembles a Au adatom on Au(111). After forming the PhCCAu−CCPh motif, the Au20 pyramid becomes slightly distorted, and the Au−Au bond right below the Au vertex is broken and elongated to 3.97 Å (dashed line in Figure 5d). The binding energy here is at 3.64 eV per PhCC group. In comparison, the adsorption configuration with the two PhCC groups located at the vertex site of Au20 (Figure 5b) or the asymmetric bridge-vertex site (one PhCC group is located above the bridge site near the vertex, while the other PhCC group is atop the vertex, Figure 5c) has much weaker binding (2.90 and 3.12 eV per PhCC, respectively). To further test the stability of the PhCCAuCCPh motif on Au20, we performed DFT-based simulated annealing at 350 K for 10 ps. As found on Au(111), the PhCCAuCCPh π
Figure 7. Charge density difference for (a) PhCC/Au(111) and (b) PhCC/Au20 interfaces at contour levels of 0.003 e/Å3. The blue and red isosurfaces correspond to the regions of electron accumulation and depletion, respectively.
Au(111) and Au20 by calculating the charge density difference upon π staple formation. It is clear that the bonding at the PhCC/Au(111) and PhCC/Au20 interfaces bear much resemblance: in both cases, electron is depleted from the dz2like orbitals of the substrate Au atoms and accumulated at the π-bonding regions (between the CC bond and the substrate Au atom); depleted from the staple Au atom and accumulated
Figure 6. Simulated annealing of the PhCCAu20 complex with an initial bridge-vertex configuration at 350 K. For the final structure, both the front and the side views are shown on the right panel. D
dx.doi.org/10.1021/jp508883v | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
Figure 8. Likely structures of three PhCC groups on Au20: (a) dimeric π motif and (b) monomeric π motif plus a vertex position.
at the σ-bonding regions (between the staple Au atom and the terminal C atom). 3.5. Implication of the PhCCAuCCPh Staple Motifs on Self-Assembled Monolayers of PhCC on Gold. The insights from above suggest that the PhCCAu CCPh π staple motifs should serve as the prevalent protecting units at the organogold interface with the PhCC ligands. This conclusion has immediate implications on the selfassembly process of PhCC ligands to form surface protective layers. For example, on Au(111), the formation of self-assembly monolayer (SAM) depends on the balance between the PhCCAu substrate interaction and the vdW interactions between the tail phenyl rings. In a low coverage regime, the PhCC molecules should be anchored to Au surface mainly via the π staple motif. At high coverage, however, the van der Waals interaction between adsorbates may win over the CC/ Au π interaction. In this case, the σ staple motif may become energetically more favorable. This picture can also apply to the generic case of SAMs of alkylethynyl (RCC) groups on gold,36 but more detailed study on the SAM formation would require extensive exploration of the superstructures of σ and π RCC−Au−CCR motifs on Au(111) as a function of coverage. Moreover, it is well-known that mobility of adsorbed ligands is important in SAM formation. The small energetic differences between hollow and bridge sites (Table 1) suggest a high mobility of the PhCC ligands on Au(111) surface, which is conductive to SAM formation. 3.6. Implication of the PhCCAuCCPh Staple Motifs on the Structures of the PhCC-Protected Organogold Clusters. More excitingly, the idea of the π staple motifs at the organogold interface now allows one to predict the structures for atomically precise organogold clusters such as Au34(PhCC)16 and Au54(PhCC)26. The staple hypothesis has been very successfully applied in predicting structure for thiolated gold nanoclusters.26,43 For the thiolated gold nanoclusters, longer staple motifs are common, such as the dimeric motif, RSAuSRAuSR. This has inspired us to further investigate the feasibility of forming dimeric π staple motifs at the organogold interfaces. On the basis of the monomeric π staple motif in Figure 6, we constructed a dimeric π motif on the Au20 cluster by placing another PhCC− group to one of the bridge Au atoms (Figure 8a). In this structure, the joined bridge Au (or the second staple Au) forms π bonding with the middle PhCC ligand and σ bonding with the third PhCC ligand. For comparison, we also considered the
configuration in which the added PhCC is bonded to the Au vertex (Figure 8b). Total energy computations showed that the dimeric π motif is more stable than the π motif-vertex configuration by about 0.16 eV (at the DFT-PBE-D3 level). Thus, in addition to the monomeric staple motif, the dimeric π motif could also exist as an interfacial motif. One can then partition an organogold cluster such as Au54(PhCC)26 into a gold core protected by staple motifs: such as Au41 protected by 13 PhCCAuCCPh motifs or Au40 protected by 10 PhCCAuCCPh and 2 PhCCAuCCPhAuCCPh motifs. We are currently working on predicting structures for these organogold clusters based on the staple motifs at the interface. 3.7. Implication for Other Ligands. Our discussion in this work focuses on the PhCC ligands where the CC/Au π interaction dominates the PhCC−Au interfaces. Besides PhCC, it is also of interest to consider possible π interaction for other ligands such as PhCCS or PhCH2S. At low ligand coverage, it might be possible to form an interfacial structure on the Au(111) surface or gold nanoclusters where the CC/Au π interaction or phenyl/Au π interaction appears in addition to the S/Au bonding of the S AuS staple motif. However, at high coverage, the ligand− ligand van der Waals interaction may overwhelm and prevent such interfacial π interaction.
4. SUMMARY AND CONCLUSIONS We have used DFT-based geometry optimization, molecular dynamics, and simulated annealing to examine the organogold interfacial structures of PhCC ligands on the Au(111) surface as well as on the Au20 cluster. The PhCC group with a terminal sp-hybridized carbon forms strong covalent bond with the underlying Au substrate. On the perfect Au(111) surface, the PhCC prefers to absorb onto the fcc hollow site through the terminal alkynyl carbon, forming an upright absorption configuration. With the involvement of a Au adatom, the PhCCAuadatomCCPh staple motif forms in which two terminal C atoms are joined by one Au adatom, while the CC bond interacts with a substrate Au atom via π interaction. This novel π-bonding motif, strengthened by the van der Waals (vdW) interaction between the phenyl ring and the Au(111) surface, is energetically more favorable than isolated PhCC groups adsorbed on Au(111) or the σ staple motif. The π-bonding PhCCAuCCPh motif is also the preferred interfacial structure on the Au20 cluster, even without E
dx.doi.org/10.1021/jp508883v | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
(13) Kumar, S. S.; Kwak, K.; Lee, D. Amperometric Sensing Based on Glutathione Protected Au25 Nanoparticles and Their pH Dependent Electrocatalytic Activity. Electroanalysis 2011, 23, 2116−2124. (14) Maksymovych, P.; Sorescu, D. C.; Yates, J. T., Jr. Gold-AdatomMediated Bonding in Self-Assembled Short-Chain Alkanethiolate Species on the Au(111) Surface. Phys. Rev. Lett. 2006, 97, 146103. (15) Kautz, N. A.; Kandel, S. A. Alkanethiol/Au(111) Self-Assembled Monolayers Contain Gold Adatoms: Scanning Tunneling Microscopy before and after Reaction with Atomic Hydrogen. J. Am. Chem. Soc. 2008, 130, 6908−6909. (16) Zeng, C.; Liu, C.; Chen, Y.; Rosi, N. L.; Jin, R. Gold−Thiolate Ring as a Protecting Motif in the Au20(SR)16 Nanocluster and Implications. J. Am. Chem. Soc. 2014, 136, 11922−11925. (17) Azubel, M.; Koivisto, J.; Malola, S.; Bushnell, D.; Hura, G. L.; Koh, A. L.; Tsunoyama, H.; Tsukuda, T.; Pettersson, M.; Häkkinen, H.; Kornberg, R. D. Electron Microscopy of Gold Nanoparticles at Atomic Resolution. Science 2014, 345, 909−912. (18) Yang, H.; Wang, Y.; Huang, H.; Gell, L.; Lehtovaara, L.; Malola, S.; Häkkinen, H.; Zheng, N. All-Thiol-Stabilized Ag44 and Au12Ag32 Nanoparticles with Single-Crystal Structures. Nat. Commun. 2013, 4, 2422. (19) Antonello, S.; Perera, N. V.; Ruzzi, M.; Gascon, J. A.; Maran, F. Interplay of Charge State, Lability, and Magnetism in the Moleculelike Au25(SR)18 Cluster. J. Am. Chem. Soc. 2013, 135, 15585−15594. (20) Hakkinen, H. The Gold-Sulfur Interface at the Nanoscale. Nat. Chem. 2012, 4, 443−455. (21) Grönbeck, H.; Häkkinen, H. Polymerization at the Alkylthiolate−Au(111) Interface. J. Phys. Chem. B 2007, 111, 3325−3327. (22) Pei, Y.; Gao, Y.; Zeng, X. C. Structural Prediction of ThiolateProtected Au38: A Face-Fused Bi-icosahedral Au Core. J. Am. Chem. Soc. 2008, 130, 7830−7832. (23) Jiang, D. E.; Overbury, S. H.; Dai, S. Structure of Au15(SR)13 and Its Implication for the Origin of the Nucleus in Thiolated Gold Nanoclusters. J. Am. Chem. Soc. 2013, 135, 8786−8789. (24) Pei, Y.; Lin, S.; Su, J.; Liu, C. Structure Prediction of Au44(SR)28: A Chiral Superatom Cluster. J. Am. Chem. Soc. 2013, 135, 19060− 19063. (25) Jiang, D. E.; Tiago, M. L.; Luo, W.; Dai, S. The “Staple” Motif: A Key to Stability of Thiolate-Protected Gold Nanoclusters. J. Am. Chem. Soc. 2008, 130, 2777−2779. (26) Jiang, D. E. The Expanding Universe of Thiolated Gold Nanoclusters and Beyond. Nanoscale 2013, 5, 7149−7160. (27) Laurentius, L.; Stoyanov, S. R.; Gusarov, S.; Kovalenko, A.; Du, R.; Lopinski, G. P.; McDermott, M. T. Diazonium-Derived Aryl Films on Gold Nanoparticles: Evidence for a Carbon−Gold Covalent Bond. ACS Nano 2011, 5, 4219−4227. (28) Jiang, D. E.; Sumpter, B. G.; Dai, S. Structure and Bonding between an Aryl Group and Metal Surfaces. J. Am. Chem. Soc. 2006, 128, 6030−6031. (29) Mirkhalaf, F.; Paprotny, J.; Schiffrin, D. J. Synthesis of Metal Nanoparticles Stabilized by Metal-Carbon Bonds. J. Am. Chem. Soc. 2006, 128, 7400−7401. (30) Zhang, S.; Chandra, K. L.; Gorman, C. B. Self-Assembled Monolayers of Terminal Alkynes on Gold. J. Am. Chem. Soc. 2007, 129, 4876−4877. (31) Maity, P.; Tsunoyama, H.; Yamauchi, M.; Xie, S. H.; Tsukuda, T. Organogold Clusters Protected by Phenylacetylene. J. Am. Chem. Soc. 2011, 133, 20123−20125. (32) Maity, P.; Wakabayashi, T.; Ichikuni, N.; Tsunoyama, H.; Xie, S. H.; Yamauchi, M.; Tsukuda, T. Selective Synthesis of Organogold Magic Clusters Au54(CCPh)26. Chem. Commun. 2012, 48, 6085−6087. (33) Maity, P.; Takano, S.; Yamazoe, S.; Wakabayashi, T.; Tsukuda, T. Binding Motif of Terminal Alkynes on Gold Clusters. J. Am. Chem. Soc. 2013, 135, 9450−9457. (34) McDonagh, A. M.; Zareie, H. M.; Ford, M. J.; Barton, C. S.; Ginic-Markovic, M.; Matisons, J. G. Ethynylbenzene Monolayers on Gold: A Metal-Molecule Binding Motif Derived from a Hydrocarbon. J. Am. Chem. Soc. 2007, 129, 3533−3538.
the help of the vdW interaction. Plot of the electron density difference clearly shows the π-bonding between the CC triple bond and the surface Au atom. Hence our work indicates that the PhCCAuCCPh π-bonding motif should be the main interfacial feature of the PhCC ligands on gold. This insight will greatly facilitate structural understanding of PhCCprotected Au nanoclusters as well as stimulate future experimental study of self-assembled monolayer of PhCC ligands on gold surfaces.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +1-951-827-4430. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the University of California, Riverside startup fund. We thank Prof. Ludwig Bartels for helpful discussion. 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 DE-AC0205CH11231.
■
REFERENCES
(1) Negishi, Y.; Nobusada, K.; Tsukuda, T. Glutathione-Protected Gold Clusters Revisited: Bridging the Gap between Gold(I)-Thiolate Complexes and Thiolate-Protected Gold Nanocrystals. J. Am. Chem. Soc. 2005, 127, 5261−5270. (2) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Structure of a Thiol Monolayer-Protected Gold Nanoparticle at 1.1 Angstrom Resolution. Science 2007, 318, 430−433. (3) Heaven, M. W.; Dass, A.; White, P. S.; Holt, K. M.; Murray, R. W. Crystal Structure of the Gold Nanoparticle [N(C 8 H 17 ) 4 ][Au25(SCH2CH2Ph)18]. J. Am. Chem. Soc. 2008, 130, 3754−3755. (4) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103−1169. (5) Maity, P.; Xie, S.; Yamauchi, M.; Tsukuda, T. Stabilized Gold Clusters: From Isolation toward Controlled Synthesis. Nanoscale 2012, 4, 4027−4037. (6) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Groenbeck, H.; Hakkinen, H. A Unified View of Ligand-Protected Gold Clusters as Superatom Complexes. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 9157−9162. (7) Yi, C.; Tofanelli, M. A.; Ackerson, C. J.; Knappenberger, K. L. Optical Properties and Electronic Energy Relaxation of Metallic Au144(SR)60 Nanoclusters. J. Am. Chem. Soc. 2013, 135, 18222−18228. (8) Zhu, M.; Aikens, C. M.; Hendrich, M. P.; Gupta, R.; Qian, H.; Schatz, G. C.; Jin, R. Reversible Switching of Magnetism in ThiolateProtected Au25 Superatoms. J. Am. Chem. Soc. 2009, 131, 2490−2492. (9) Yau, S. H.; Varnavski, O.; Goodson, T. An Ultrafast Look at Au Nanoclusters. Acc. Chem. Res. 2013, 46, 1506−1516. (10) Yu, Y.; Luo, Z.; Chevrier, D. M.; Leong, D. T.; Zhang, P.; Jiang, D. E.; Xie, J. Identification of a Highly Luminescent Au22(SG)18 Nanocluster. J. Am. Chem. Soc. 2014, 136, 1246−1249. (11) Zhu, Y.; Qian, H.; Zhu, M.; Jin, R. Thiolate-Protected Aun Nanoclusters as Catalysts for Selective Oxidation and Hydrogenation Processes. Adv. Mater. 2010, 22, 1915−1920. (12) Liu, J.; Yu, M.; Ning, X.; Zhou, C.; Yang, S.; Zheng, J. PEGylation and Zwitterionization: Pros and Cons in the Renal Clearance and Tumor Targeting of Near-IR-Emitting Gold Nanoparticles. Angew. Chem., Int. Ed. 2013, 52, 12572−12576. F
dx.doi.org/10.1021/jp508883v | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
(35) Ford, M. J.; Hoft, R. C.; McDonagh, A. Theoretical Study of Ethynylbenzene Adsorption on Au(111) and Implications for a New Class of Self-Assembled Monolayer. J. Phys. Chem. B 2005, 109, 20387−20392. (36) Zaba, T.; Noworolska, A.; Bowers, C. M.; Breiten, B.; Whitesides, G. M.; Cyganik, P. Formation of Highly Ordered SelfAssembled Monolayers of Alkynes on Au(111) Substrate. J. Am. Chem. Soc. 2014, 136, 11918−11921. (37) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (38) Blochl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979. (39) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (40) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104. (41) Li, J.; Li, X.; Zhai, H.-J.; Wang, L.-S. Au20: A Tetrahedral Cluster. Science 2003, 299, 864−867. (42) Kobayashi, N.; Kamei, Y.; Shichibu, Y.; Konishi, K. ProtonationInduced Chromism of Pyridylethynyl-Appended [core+exo]-Type Au8 Clusters. Resonance-Coupled Electronic Perturbation through πConjugated Group. J. Am. Chem. Soc. 2013, 135, 16078−16081. (43) Pei, Y.; Zeng, X. C. Investigating the Structural Evolution of Thiolate Protected Gold Clusters from First-Principles. Nanoscale 2012, 4, 4054−4072.
G
dx.doi.org/10.1021/jp508883v | J. Phys. Chem. C XXXX, XXX, XXX−XXX
