Atomistic Simulations of the Surface Coverage of Large Gold

Article pubs.acs.org/JPCC

Atomistic Simulations of the Surface Coverage of Large Gold Nanocrystals Takieddine Djebaili,† Johannes Richardi,*,† Stéphane Abel,‡ and Massimo Marchi‡ †

Laboratoire des Matériaux Mésoscopiques et Nanométriques (LM2N), UMR CNRS 7070, Université Pierre et Marie Curie, bât F, BP 52, 4 Place Jussieu, 75252 Paris Cedex 05, France ‡ Commissariat à l’Energie Atomique et aux Energies Alternatives, DSV/iBiTEC-S/SB2SM/LBMS & CNRS UMR 8221, Saclay, France S Supporting Information *

ABSTRACT: Here, the adsorption of alkanethiols (from ethane to dodecanethiol) on icosahedral gold nanocrystals with diameters up to 10 nm is studied by molecular dynamics simulations in a vacuum. The surface coverage of the nanocrystals obtained in the simulations is in good agreement with experimental data. We show that the average surface per adsorbed thiol does not markedly depend on the nanocrystal size and ligand and is only about 10% lower than the value observed on a flat Au(111) surface. We observe two different molecular organizations of the thiolates on the edges and in the centers of the nanocrystal facets. The incompatibility between both organizations explains the fact that the formation of self-assembled monolayers usually observed on flat Au(111) surfaces is hindered for nanocrystals smaller than 6 nm. We also show that the organization of thiolates on the edges is at the origin of the lower average surface per adsorbed thiol found for the nanocrystal.

1. INTRODUCTION Because of their unique size-tunable properties, gold nanocrystals (AuNC) are currently studied for various applications including catalysis, electronic and photonic devices, and biomedical sensors.1−4 Thus, their good biocompatibility lets them also be good candidates for therapeutic drug delivery in cancer diagnostics and therapy.3 To prevent the aggregation of the AuNC, organic ligands are used to stabilize them.1 Among these ligands, we can cite alkanethiols, amines, and phosphines. These ligands are able to form compact monolayers at the nanoparticle surface due to the high affinity between the NC gold atoms and the ligand head groups. The formation of these ligand monolayers on solid gold surfaces has been widely studied in the literature.5−10 It was found that these monolayers are usually highly ordered molecular films called self-assembled monolayers (SAM) (see Figure 1). Figure 1A shows that the thiol head groups form a hexagonal overlayer structure denoted by (√3 × √3)R30°. The alkane chains are tilted by an angle of about 30° with respect to the surface normal.5 DFT calculations and STM experiments11,12 have shown that the adsorbed thiolates are between the three gold atoms, but the geometry of adsorption is better described by a shifted bridge position. Classical simulations often yield a hollow position as preferred positions for the thiol atoms (see Figure 1), which is a drawback of the interaction model we use. Several theoretical and experimental studies7,13−20 have shown the importance of surface reconstruction for the © 2013 American Chemical Society

Figure 1. Self-assembled monolayer on a flat Au(111) surface (A) and on the edge of NC facets (B) as obtained by classical simulations: Positions of the SH groups of the alkanethiols are shown by yellow spots.

formation of SAMs with the presence of adatoms and vacancies. A very recent simulation study allowing surface reconstruction9 indicates that the vacancies and adatoms may form islands and we might obtain a slightly perturbed (√3 × √3)R30° lattice. However, one has to be very careful with these first results, which have to be confirmed in the future. Experiments have shown significant differences between the monolayers of alkanethiols formed on gold nanocrystals (NCs) and flat surfaces.21−27 Elemental analysis using transmission electron microscopy for alkanethiol-derivated gold clusters with diameters of 2 nm gives a lower average surface per adsorbed Received: April 8, 2013 Revised: July 12, 2013 Published: July 24, 2013 17791

dx.doi.org/10.1021/jp403442s | J. Phys. Chem. C 2013, 117, 17791−17800

The Journal of Physical Chemistry C

Article

thiol on nanoparticles (15.2−17.2 Å2/thiol)22 as compared to the Au(111) surface (21.5 Å2/thiol).5 However, the formation of ordered molecular assemblies is still observed for NCs, but the transition to a liquid-like state is found at significantly lower temperature than for SAMs on Au(111) surfaces. To understand these experimental results, several simulation studies have been carried out.28−30 In simulations of Au NCs coated with alkanethiols in vacuum,28,29 it was shown that the adsorption site occupancies on the facets of the NCs are markedly different from those observed on a flat gold surface. It was also found that the ordered molecular monolayer on NCs transit to a liquid-like state at lower temperatures than for the flat Au(111) surface, in good agreement with the experiment data.22,23 In other simulations,30 the influence of the solvent hexane on the adsorption of alkanethiols on gold NCs has also been studied. The dynamics and structure of monolayers of alkanethiol on gold NCs have been studied in more detail in several simulation papers.31−33 It was shown that the highly asymmetric arrangement of ligand bundles around large NCs could be attributed to the curvature of the particles.33 In several simulation and theoretical studies,28−30,34−36 the number of thiol molecules attached to NCs in excess of ligand molecules was determined. They have shown that a significantly higher surface coverage is observed for nanoparticles in comparison to a flat Au(111) surface, which is in good agreement with experiment.22,25 In particular, very recent experimental studies using various experimental techniques confirm this result for NC sizes larger than 10 nm.37−42 However, the origin of the higher surface coverage on nanoparticles is still discussed.34−36 Two explanations have been proposed in the literature to explain the high coverage of the alkanethiol on gold NCs: it was postulated that adsorption sites at edges and corners on the NC surface may allow a higher density of alkanethiols with respect to those of the terraces.24 A second explanation takes into account the curvature of the NC surface, which gives more space to explore for flexible molecules such as alkanethiols.36 Therefore, it has been proposed that the increase in the number of adsorbed thiol molecules is due to the gain in entropy induced by more conformational freedom. The knowledge of the actual origin of the higher surface coverage for NCs is important. Thus, if the first effect is predominating, we may expect the surface coverage to depend only slightly on the alkane chain length of the ligands, which is not expected in the second case. The aim of this paper is to explain and quantify the differences between the monolayers of alkanethiols observed on gold NCs and on flat surfaces. To achieve this goal, we carried out molecular dynamics (MD) simulations of the adsorption of alkanethiols with different chain lengths on AuNCs with sizes from 1 to 10 nm. These simulations are a step onward since previous simulation studies were carried out with NCs with diameters smaller than 5 nm. The interest of the investigation of large NCs is that we can expect the terraces to be sufficiently extended so that we should observe formation of SAMs such as on a flat gold surface. Following the evolution of the ligand film with decreasing NC size we can investigate the transition from this SAM to the monolayer usually observed on small NCs. The paper is organized as follows. In sections 2 and 3, we present our model and the simulation method used in this study. In section 4, we will first examine the time evolution of the adsorption of thiols on AuNCs. Then, the average surface per thiol obtained by simulations is compared to experimental and theoretical data of the literature. To better understand the

higher surface coverage, we will investigate the preferred adsorption sites and their occupancies on NCs. A distinction of the adsorption site occupancies on the edges and in the centers of the NC facets will be made which reveals the presence of two different molecular organizations of the thiols. Besides the usual SAM in the center of the facets (Figure 1A), a second molecular zigzag organization of the thiol head groups is observed on the edges (Figure 1B). These observations will be finally combined to a simple geometrical model to explain the simulation results and to extrapolate the surface coverage for NC with unlimited NC size. It has long time been discussed whether the molecules in SAMs finally adsorbed are intact thiols (physicosorption) or thiolate groups (chemisorption). While this question is still open for nanocrystals, recent experimental and theoretical studies on gold surfaces indicate the presence of thiolate.11,43 Here, we use a unified model and the hydrogenes are not explicitly treated. Therefore, the only distinction between the thiols and the thiolates is the strength of interaction between the head groups and the gold atoms. For the sake of convenience, we will speak in the following of thiols, which does not exclude the presence of thiolates on the nanocrystals.

2. MODEL To construct our model of AuNCs, we used the interaction model proposed in ref 30 where the SH, CH2, and CH3 groups of the alkanethiol molecules are represented by a single interaction site (united atom model). The sites interact with each other by a Lennard-Jones (LJ) pair potential. LJ interactions within an alkane chain are only calculated, when the interaction sites are separated by more than three bonds. The LJ parameters εij and σij are also taken from ref 30 by applying the Lorentz−Berthelot mixing rules (εij = √εiiεjj and σij = 0.5(σii + σjj)). The intramolecular interaction for the alkane chain were modeled with a simple Ryckaert and Belleman energy function:44 U bond =

∑ bonds

U bend =

∑ bends

1 k1(r − r0)2 2

1 k 2(cos θ − cos θ0)2 2 5

U torsion =

∑ ∑ ηn cosn ϕ torsions n = 0

(1)

where r, θ, and ϕ are the site−site distance, the bend angle, and the dihedral angle. The interaction parameters r0, k1, θ0, k2, and ηn are taken from ref 44 and are provided in Table S1 of the Supporting Information, which gives all the interaction and geometric parameters used in our study. The AuNC is described as an icosahedral atomic cluster and to reduce the computational time the NC is considered as rigid fixing the relative positions of the gold atoms. The icosahedral shape was chosen, since this form is widely observed for gold NCs.45−47 The icosahedra also exposes only (111) facets which allows a good comparison with the flat Au(111) surface. We simulated an idealized nanocrystal without any defects, adatoms, and vacancies. It would be important to study the role of surface restructuration in the future. Since we are interested in the adsorption of thiol molecules on the gold NC, the Au−SH interaction must be well described.48 We use the LJ potential proposed by Pool et al.30 It is based on the interaction potential 17792



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Figure 2. (A) Snapshots of the initial and final configuration (after 90 ns) of a 6.8 nm NC in an excess of butanethiol. (B) Time evolution of the number of adsorbed thiol molecules for different alkane chain lengths (NC diameter: 6.8 nm).

with a number of thiol molecules significantly larger than the one obtained from average surface per thiol usually found for NCs (16−18 Å2). After each simulation it was checked that an excess of nonadsorbed thiol molecules was in the box. The system was allowed to evolve at 300 K. To avoid interactions between periodic images of the NCs, a sufficiently large box has been chosen. A time step of 1.0 fs is applied, and the temperature is kept constant using a Nose−Hoover thermostat51 with the time constant of 0.4 ps. The equilibration takes much time, and a sufficiently long simulation run of 30 to 180 ns is necessary to obtain a stable number of thiols adsorbed on the gold NC, in particular for long alkane chains and large NCs. We have checked that the number of adsorbed thiols and all derived properties are well stabilized. In particular, transitions of head groups from one adsorption site to another are very rare events, but we have controlled that they occur several times during our simulations. To check the dependence of the results with the excess of thiol molecules, simulations were carried out with different thiol concentrations. We did not observe a significant variation of the simulation results as a function of the thiol number. In Table S2 of the Supporting Information, the parameters of simulated systems discussed in the following such as the number of gold atoms and thiols and the box size are given. For this work, we examine the adsorption of ethane, butane, octane, and dodecanethiol. For ethane and butanethiols all available NC sizes for closed icosahedra up to 10 nm were studied, whereas for the two longer alkanethiols we restricted the simulations to AuNCs with diameters of 1.7, 2.6, 3.6, 5.0, 6.8, and 10.1 nm. A summary of all the simulations carried out in this study is given in Table S2. The structural results were obtained and averaged

between a Au(111) surface and the SH and CHx groups of alkanethiols derived by Hautman and Klein49 from quantum chemical calculations. Within the Hautman−Klein model, the Au(111) plane is handled as a flat surface. To carry out atomistic simulations, Pool et al.30 described the Au(111) slab as an assembly of gold atoms and fitted the parameters of a position averaged LJ interaction to the Hautman−Klein potential. Because of the Boltzmann factor applied during the averaging, the energies and, thus, the LJ parameters depend on the temperature. So we used the values of εij and σij obtained at 300 K. Please note that the Morse potential for the Au−SH interaction used in former simulations28,29,50 yields an attraction almost twice as strong compared to the Hautman− Klein potential.30 All LJ potentials are truncated at a cutoff distance of 12.0 Å.

3. SIMULATION METHOD To set up the simulation systems and do the analysis of the trajectories, we used the simulation code NATOMOS developed in our laboratory. This program was initially written to carry out simple molecular dynamics and Monte Carlo simulations of small molecular system (including metallic NCs, flexible ligands, solvents). Since in this work, we simulate large AuNCs (up to 60 000 atoms), we decided to use the highly optimized GROMACS 4.5.5 MD package51 to carry out the simulations with a large number of computer cores. Within the statistical accuracy of the methods NATOMOS and GROMACS yield the same energetical and structural results as shown in Figure S1 of the Supporting Information. To study the adsorption of alkanethiols, a bare gold NC was put in the center of the simulation box. Then, the box was filled 17793



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chain length may be explained by more favorable van der Waals interaction or an entropy increase induced by more conformational freedom. We cannot exclude that the smaller number of dodecanethiol adsorbed is related to a very slow saturation of the monolayer for this thiol as observed in Figure 2B. Please note that in all the figures hereafter results for identical alkanethiols are always labeled by the same symbols. Our results for small NC ( 7 nm. Figure 7 shows that re = 3.2 is a good estimate from NC diameters larger than 2 nm. However, rc markedly increases for NC diameters smaller than 5 nm. Nevertheless, eq 3 may give good estimates for the number of adsorbed thiols in this case, since for small NCs the number of adsorption site in the center is much less than the ones on the edges. We have compared the average surface per adsorbed thiol obtained from the simulation results and the values

Figure 8. (A) Frequencies of SH groups belonging to a SAM usually found on Au(111) surfaces (with more than 3 members). (B) Frequencies of SH groups belonging to a zigzag SAM on the edges (with more than three members). Results for the total icosahedron surface and for the edges and centers of its facets.

compared to 4.7 nm observed in the facets center (see Figure S4). Our model gives an energy of adsorption for a propanethiol molecules in a SAM on Au(111) around −0.86 eV. This is actually between the case of the adsorption of a propanethiol

Figure 9. Average surface per adsorbed thiol: prediction of theoretical model and simulation results for nanocrystal diameters between 0 and 10 nm (A) and 0 and 200 nm (B). 17797



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agreement with experimental data from elemental analysis using TEM22 and thermogravimetry.25 Our results are usually in good agreement with other simulation studies,22,30,35 except for two theoretical studies34,36 which use specific approximations such as the description of the nanoparticles as spherical objects. We have examined the adsorption of these alkanethiol on the different AuNC regions. The arrangement of the thiols exhibits two characteristic organizations at the edges and at the center of the NC facets. In particular, we find a higher occupation of the sites at the edges, which explains the reduced surface per thiol observed in NC with respect to that of a flat gold surface. Since both types of molecular organizations are incompatible, the formation of SAMs as those on a flat Au(111) surface are hindered especially for small NCs. In spite of increasing the conformational entropy of flexible molecules, the curvature of the NC seems to play here a minor role. Thus, we find that the surface coverage does depend only slightly on the alkane chain length. For AuNCs of sizes much larger than 10 nm, on the basis of our results we have also proposed a theoretical model to estimate the surface per adsorbed thiol. This property is difficult to address directly by atomistic simulation alone. The surface coverage predicted by the theory for NC sizes between 10 and 20 nm are in agreement with very recent mass spectrometry data.37 The zigzag organization of thiols on the edges of large Au nanocrystal as revealed by our simulations needs to be probed by new experiments. Finally, other properties such as the tilt angles and the dependence of the simulation results on the interaction models will be thoroughly investigated in our future works. Our study is a first step to better understand surface coverage of large metallic nanocrystals. It gives the results obtained for a classical interaction model. In the future, we plan to refine our model by introducing the ligand polarization, formation of Au−S bonds (chemisorption), and the possibility of a restructuration of the surface.

obtained with eqs 2 and 3 (Figures 9A and 9B show the same curves for two different ranges of diameters). As shown in Figure 9A, eq 3 correctly predicts the smaller surface per thiol observed for NCs compared to Au(111) surfaces. To better understand this results, the average surface per thiol on the edges is calculated from the number of thiol on the edges (ns,e/re) and the NC edge surface Se and is shown in Figure 9A,B. Se is obtained from the difference of the total surface of a facet (see eq 2) and the surface marked by the red line in Figure 5A which separates the adsorption sites on the edges and the centers. For large NC sizes the average surface per thiol is 16 Å2, which is about 3/4 of the value of 21.5 Å2 found in the facet centers and on Au(111) surfaces. The factor 3/4 is explained by the fact that the ratios re and rc predict a 2 times higher occupation of the edges, while the ratio between the surface and the number of adsorption sites increases only by about a factor 1.5 on the edges with respect to the center. Figure 9A also shows that up to 10 nm the small variation of the average surface per thiol with NC size is due to a decrease in the surface per thiol on the edges. In Figure 9B, eq 3 is used to predict the evolution of the surface per thiol for NC diameters much larger than those accessible in atomistic simulations. Figure 9B shows that even at NC sizes of 50 nm the average surface per thiol is still 5% lower than the value for the flat Au(111) surface. Little and contradictory information exists for the surface coverage of NCs larger than 10 nm. It is difficult to determine precisely by experiments, since the amount of ligand is small with respect to the quantity of NC material. Using inductively coupled plasma−mass spectroscopy, Hinterwirth et al.37 have determined an average surface of 15.9 ± 1.7, 17.5 ± 0.5, and 18.9 ± 0.7 Å2/thiol on large gold NPs (13.2−26.2 nm) for 3-mercaptopropanoic, 11-mercaptoundecanoic, and 16-mercaptohexadecanoic acid, respectively. Different surfaces of 12.8,38 21.8,39 and 20.139 Å2/thiol for 3mercaptopropionic, 6-mercaptohexanoic acid, and 11-mercaptoundecanoic acid, respectively, were measured using other experimental techniques such as inductively coupled plasma optical emission spectrometry (ICP-OES, NC size: 10−100 nm)38 and X-ray photoelectron spectroscopy (XPS, NC size: 13 nm).39 It will be important in the future to understand the origin of these differences. Since the acid group is at the end of the alkane chain and the surface coverage seems to be mainly determined by the interaction between the headgroup and the gold surface, we may compare the experimental data to our results. For this comparison the surface of the NC must be calculated from that of a sphere with the corresponding diameter (see discussion of Figure 4A). Our theory then predicts an average surface varying from 18 to 19 Å2 per thiol, when the NC size increases from 10 to 20 nm. This is in good agreement with the results by Hinterwirth et al.37 However, we do not observe a decrease in the surface coverage with the alkane chain length as in the experiments which may be explained by the formation of bundles for long alkyl thiols (see above).

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ASSOCIATED CONTENT

S Supporting Information *

Tables S1−S7 and Figures S1−S4. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.R.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed using HPC resources from GENCI[CCRT/CINES/IDRIS] (Grant 2012-[x2012086946]) and the CCRE (Université Pierre et Marie Curie). Marie-Paule Pileni and Nicolas Goubet from LM2N and Frederik Tielens from LCMCP are kindly acknowledged for fruitful discussions.

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5. CONCLUSIONS In this report, we have carried out atomistic simulations of the icosahedral AuNC with diameters up to 10 nm with different alkanethiols (ethanethiol to dodecanethiol). We find that when the icosahedral surface is used to estimate the NC surface accessible for adsorption, the average surface per thiol only slightly varies on the NC size up to 10 nm. The surface coverages here obtained for NC size up to 5 nm are in

REFERENCES

(1) Daniel, M.-C.; Astruc, D. Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum-Size-Related Properties, and Applications Toward Biology, Catalysis, and Nanotechnology. Chem. Rev. 2004, 104, 293−346. (2) Sardar, R.; Funston, A. M.; Mulvaney, P.; Murray, R. W. Gold Nanoparticles: Past, Present, and Future. Langmuir 2009, 25, 13840− 13851.

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Electron Hopping Studies of Alkanethiol Stabilized Gold Clusters. J. Am. Chem. Soc. 1995, 117, 12537−12548. (22) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Self-Assembled Monolayers on Gold Nanoparticles. Chem.Eur. J. 1996, 2, 359−363. (23) Badia, A.; Cuccia, L.; Demers, L.; Morin, F.; Lennox, R. B. Structure and Dynamics in Alkanethiolate Monolayers Self-Assembled on Gold Nanoparticles: A DSC, FT-IR, and Deuterium NMR Study. J. Am. Chem. Soc. 1997, 119, 2682−2692. (24) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Infrared Spectroscopy of Three-Dimensional Self-Assembled Monolayers: NAlkanethiolate Monolayers on Gold Cluster Compounds. Langmuir 1996, 12, 3604−3612. (25) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; et al. Alkanethiolate Gold Cluster Molecules with Core Diameters from 1.5 to 5.2 Nm: Core and Monolayer Properties as a Function of Core Size. Langmuir 1998, 14, 17−30. (26) Pradeep, T.; Sandhyarani, N. Structure and Dynamics of Monolayers on Planar and Cluster Surfaces. Pure Appl. Chem. 2002, 74, 1593−1607. (27) 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 Å Resolution. Science 2007, 318, 430−433. (28) Luedtke, W. D.; Landman, U. Structure, Dynamics, and Thermodynamics of Passivated Gold Nanocrystallites and Their Assemblies. J. Phys. Chem. 1996, 100, 13323−13329. (29) Luedtke, W. D.; Landman, U. Structure and Thermodynamics of Self-assembled Monolayers on Gold Nanocrystallites. J. Phys. Chem. B 1998, 102, 6566−6572. (30) Pool, R.; Schapotschnikow, P.; Vlugt, T. J. H. Solvent Effects in the Adsorption of Alkyl Thiols on Gold Structures: A Molecular Simulation Study. J. Phys. Chem. C 2007, 111, 10201−10212. (31) Rapino, S.; Zerbetto, F. Dynamics of Thiolate Chains on a Gold Nanoparticle. Small 2007, 3, 386−388. (32) Ghorai, P. K.; Glotzer, S. C. Molecular Dynamics Simulation Study of Self-Assembled Monolayers of Alkanethiol Surfactants on Spherical Gold Nanoparticles. J. Phys. Chem. C 2007, 111, 15857− 15862. (33) Lane, J. M. D.; Grest, G. S. Spontaneous Asymmetry of Coated Spherical Nanoparticles in Solution and at Liquid-Vapor Interfaces. Phys. Rev. Lett. 2010, 104, 235501. (34) Jimenez, A.; Sarsa, A.; Blazquez, M.; Pineda, T. A Molecular Dynamics Study of the Surfactant Surface Density of Alkanethiol SelfAssembled Monolayers on Gold Nanoparticles as a Function of the Radius. J. Phys. Chem. C 2010, 114, 21309−21314. (35) Olmos-Asar, J. A.; Rapallo, A.; Mariscal, M. M. Development of a Semiempirical Potential for Simulations of Thiol-gold Interfaces. Application to Thiol-protected Gold Nanoparticles. Phys. Chem. Chem. Phys. 2011, 13, 6500−6506. (36) Tambasco, M.; Kumar, S. K.; Szleifer, I. Quantitatively Modeling the Equilibrium Properties of Thiol-Decorated Gold Nanoparticles. Langmuir 2008, 24, 8448−8451. (37) Hinterwirth, H.; Kappel, S.; Waitz, T.; Prohaska, T.; Lindner, W.; Lämmerhofer, M. Quantifying Thiol Ligand Density of SelfAssembled Monolayers on Gold Nanoparticles by Inductively Coupled Plasma−Mass Spectrometry. ACS Nano 2013, 7, 1129−1136. (38) Elzey, S.; Tsai, D.-H.; Rabb, S. A.; Yu, L. L.; Winchester, M. R.; Hackley, V. A. Quantification of Ligand Packing Density on Gold Nanoparticles Using ICP-OES. Anal. Bioanal. Chem. 2012, 403, 145− 149. (39) Ivanov, M. R.; Haes, A. J. Anionic Functionalized Gold Nanoparticle Continuous Full Filling Separations: Importance of Sample Concentration. Anal. Chem. 2012, 84, 1320−1326. (40) Techane, S. D.; Gamble, L. J.; Castner, D. G. Multitechnique Characterization of Self-Assembled Carboxylic Acid-Terminated Alkanethiol Monolayers on Nanoparticle and Flat Gold Surfaces. J. Phys. Chem. C 2011, 115, 9432−9441.

(3) Lim, Z.-Z. J.; Li, J.-E. J.; Ng, C.-T.; Yung, L.-Y. L.; Bay, B.-H. Gold Nanoparticles in Cancer Therapy. Acta Pharmacol. Sin. 2011, 32, 983− 990. (4) Maeda, K.; Okabayashi, N.; Kano, S.; Takeshita, S.; Tanaka, D.; Sakamoto, M.; Teranishi, T.; Majima, Y. Logic Operations of Chemically Assembled Single-Electron Transistor. ACS Nano 2012, 6, 2798−2803. (5) Ulman, A. Formation and Structure of Self-Assembled Monolayers. Chem. Rev. 1996, 96, 1533−1554. (6) Schreiber, F. Structure and Growth of Self-assembling Monolayers. Prog. Surf. Sci. 2000, 65, 151−257. (7) Luque, N. B.; Santos, E.; Andres, J.; Tielens, F. Effect of Coverage and Defects on the Adsorption of Propanethiol on Au(111) Surface: A Theoretical Study. Langmuir 2011, 27, 14514−14521. (8) Tielens, F.; Costa, D.; Humblot, V.; Pradier, C.-M. Characterization of ω-Functionalized Undecanethiol Mixed Self-Assembled Monolayers on Au(111): A Combined Polarization Modulation Infrared Reflection−Absorption Spectroscopy/X-ray Photoelectron Spectroscopy/Periodic Density Functional Theory Study. J. Phys. Chem. C 2008, 112, 182−190. (9) Longo, G. S.; Bhattacharya, S. K.; Scandolo, S. A Molecular Dynamics Study of the Role of Adatoms in SAMs of Methylthiolate on Au(111): A New Force Field Parameterized from Ab Initio Calculations. J. Phys. Chem. C 2012, 116, 14883−14891. (10) 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, 1103−1170. (11) Tielens, F.; Santos, E. AuS and SH Bond Formation/Breaking during the Formation of Alkanethiol SAMs on Au(111): A Theoretical Study. J. Phys. Chem. C 2010, 114, 9444−9452. (12) Maksymovych, P.; Sorescu, D. C.; Yates, J. T. Methanethiolate Adsorption Site on Au(111): A Combined STM/DFT Study at the Single-Molecule Level. J. Phys. Chem. B 2006, 110, 21161−21167. (13) Woodruff, D. P. The Interface Structure of N-alkylthiolate Selfassembled Monolayers on Coinage Metal Surfaces. Phys. Chem. Chem. Phys. 2008, 10, 7211−7221. (14) Roper, M. G.; Skegg, M. P.; Fisher, C. J.; Lee, J. J.; Dhanak, V. R.; Woodruff, D. P.; Jones, R. G. Atop Adsorption Site of Sulphur Head Groups in Gold-thiolate Self-assembled Monolayers. Chem. Phys. Lett. 2004, 389, 87−91. (15) Cossaro, A.; Mazzarello, R.; Rousseau, R.; Casalis, L.; Verdini, A.; Kohlmeyer, A.; Floreano, L.; Scandolo, S.; Morgante, A.; Klein, M. L.; et al. X-ray Diffraction and Computation Yield the Structure of Alkanethiols on Gold(111). Science 2008, 321, 943−946. (16) Mazzarello, R.; Cossaro, A.; Verdini, A.; Rousseau, R.; Casalis, L.; Danisman, M. F.; Floreano, L.; Scandolo, S.; Morgante, A.; Scoles, G. Structure of a CH3-S Monolayer on Au(111) Solved by the Interplay Between Molecular Dynamics Calculations and Diffraction Measurements. Phys. Rev. Lett. 2007, 98, 016102. (17) Maksymovych, P.; Sorescu, D. C.; Yates, J. T. Gold-AdatomMediated Bonding in Self-Assembled Short-Chain Alkanethiolate Species on the Au(111) Surface. Phys. Rev. Lett. 2006, 97, 146103. (18) Kondoh, H.; Iwasaki, M.; Shimada, T.; Amemiya, K.; Yokoyama, T.; Ohta, T.; Shimomura, M.; Kono, S. Adsorption of Thiolates to Singly Coordinated Sites on Au(111) Evidenced by Photoelectron Diffraction. Phys. Rev. Lett. 2003, 90, 066102. (19) Torres, E.; Blumenau, A. T.; Biedermann, P. U. Mechanism for Phase Transitions and Vacancy Island Formation in alkylthiol/ Au(111) Self-assembled Monolayers Based on Adatom and Vacancyinduced Reconstructions. Phys. Rev. B 2009, 79, 075440. (20) Wano, H.; Uosaki, K. In Situ Dynamic Monitoring of Electrochemical Oxidative Adsorption and Reductive Desorption Processes of a Self-Assembled Monolayer of Hexanethiol on a Au(111) Surface in KOH Ethanol Solution by Scanning Tunneling Microscopy. Langmuir 2005, 21, 4024−4033. (21) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G. Monolayers in Three Dimensions: NMR, SAXS, Thermal, and 17799



Article

(41) Lanterna, A. E.; Coronado, E. A.; Granados, A. M. When Nanoparticle Size and Molecular Geometry Matter: Analyzing the Degree of Surface Functionalization of Gold Nanoparticles with Sulfur Heterocyclic Compounds. J. Phys. Chem. C 2012, 116, 6520−6529. (42) Xia, X.; Yang, M.; Wang, Y.; Zheng, Y.; Li, Q.; Chen, J.; Xia, Y. Quantifying the Coverage Density of Poly(ethylene Glycol) Chains on the Surface of Gold Nanostructures. ACS Nano 2012, 6, 512−522. (43) Lavrich, D. J.; Wetterer, S. M.; Bernasek, S. L.; Scoles, G. Physisorption and Chemisorption of Alkanethiols and Alkyl Sulfides on Au(111). J. Phys. Chem. B 1998, 102, 3456−3465. (44) Dubbeldam, D.; Calero, S.; Vlugt, T. J. H.; Krishna, R.; Maesen, T. L. M.; Smit, B. United Atom Force Field for Alkanes in Nanoporous Materials. J. Phys. Chem. B 2004, 108, 12301−12313. (45) Marks, L. D. Experimental Studies of Small Particle Structures. Rep. Prog. Phys. 1994, 57, 603. (46) Ascencio, J. A.; Gutiérrez-Wing, C.; Espinosa, M. E.; Marín, M.; Tehuacanero, S.; Zorrilla, C.; José-Yacamán, M. Structure Determination of Small Particles by HREM Imaging: Theory and Experiment. Surf. Sci. 1998, 396, 349−368. (47) Wang, Y.; Teitel, S.; Dellago, C. Melting and Equilibrium Shape of Icosahedral Gold Nanoparticles. Chem. Phys. Lett. 2004, 394, 257− 261. (48) Häkkinen, H. The Gold-sulfur Interface at the Nanoscale. Nat. Chem. 2012, 4, 443−455. (49) Hautman, J.; Klein, M. Simulation of a Monolayer of Alkyl Thiol Chains. J. Chem. Phys. 1989, 91, 4994−5001. (50) Lal, M.; Plummer, M.; Richmond, N. J.; Smith, W. Solvation of Metal Nanoparticles in a Subcritical−Supercritical Fluid: A Computer Simulation Study. J. Phys. Chem. B 2004, 108, 6052−6061. (51) Hess, B.; Kutzner, C.; Spoel, D.; van der Lindahl, E. GROMACS 4: Algorithms for Highly Efficient, Load-Balanced, and Scalable Molecular Simulation. J. Chem. Theory Comput. 2008, 4, 435−447.

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Atomistic Simulations of the Surface Coverage of Large Gold

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