Solution-Phase Structure and Bonding of Au38(SR)24 Nanoclusters

Dec 9, 2010 - Understanding the atomic structure and structure−property relationship of Au−thiolate clusters is essential for advancing their fund...
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J. Phys. Chem. C 2011, 115, 65–69

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Solution-Phase Structure and Bonding of Au38(SR)24 Nanoclusters from X-ray Absorption Spectroscopy Mark A. MacDonald and Peng Zhang* Department of Chemistry and Institute for Research in Materials, Dalhousie UniVersity, Halifax, NoVa Scotia, B3H 4J3, Canada

Ning Chen Canadian Light Source, Saskatoon, Saskatchewan, S7N 0X4, Canada

Huifeng Qian and Rongchao Jin Department of Chemistry, Carnegie Mellon UniVersity, Pittsburgh, PennsylVania 15213, United States ReceiVed: July 30, 2010; ReVised Manuscript ReceiVed: NoVember 18, 2010

Understanding the atomic structure and structure-property relationship of Au-thiolate clusters is essential for advancing their fundamental studies and applications. However, a precise determination of the solutionphase structure and bonding of these clusters, albeit of its paramount importance in synthesis, processing, and applications, is still lacking. We report here solution-phase X-ray absorption spectroscopy (XAS) studies of thiolate-protected Au38 nanoclusters. A significant bond expansion associated with an increase of Au d-electron density of states (DOS) has been observed upon solvation of Au38 in toluene. These findings are accounted for by projected DOS calculations, which further illustrate the unique electronic behavior of Au38 from a site-specific perspective. The impact of these findings on tuning the properties of Au-thiolate clusters is also discussed. 1. Introduction

2. Experimental Methods

Recently, intense research has been directed at the study of quantum-sized gold-thiolate nanoclusters.1-5 Understanding the atomic structure and structure-property correlation of these clusters is critical for both fundamental studies6-10 and technological applications.11-13 Since the chemical synthesis and a variety of potential applications of these clusters are conducted in solution, studies on the evolution of their atomic structure and properties upon solvation are of significant importance and interest. Recent studies by Toikkanen et al. have shown that solvation effect(s) can substantially influence the surface reactivity of Au38 clusters;14 however, precise determination of the solution-phase structure and variation of bonding of these clusters upon solvation is still lacking. Herein, we present a Au L3-edge X-ray absorption spectroscopy (XAS) study, including extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), and ab initio calculations of the electronic properties of the Au-thiolate cluster Au38 in solution and the solid phase. We observed a significant expansion of the Au-S and Au-Au bonds associated with an increase of Au d-electron density of states (DOS) for Au38 upon solvation. These findings are accounted for by ab initio calculations of projected DOS, which further illustrate the unique electronic behavior of Au38 from a site-specific perspective. Finally, we discuss the impact of these findings on tuning the properties of Au-thiolate clusters. * To whom correspondence should be addressed. E-mail: peng.zhang@ dal.ca.

2.1. Au38 Synthesis. Details of the synthesis of Au38(SCH2CH2Ph)24 clusters have been reported elsewhere.15 Briefly, 0.5 mmol of HAuCl4 · 3H2O and 2.0 mmol of glutathione (GSH) were mixed in 20 mL of acetone at room temperature under vigorous stirring. After ∼20 min, the mixture (a yellowish cloudy suspension) was cooled to ∼0 °C in an ice bath (∼20 min); then a fresh solution of NaBH4 (5 mmol, dissolved in 6 mL of cold Nanopure water) was rapidly added to the suspension under vigorous magnetic stirring. After ∼20 min, black Aun(SG)m nanoclusters were spontaneously precipitated out of solution and stuck to the inner wall of the flask. The clear acetone solution was decanted, and 6 mL of water was added to dissolve the Aun(SG)m nanoclusters. The aqueous solution of Aun(SG)m was then mixed with 0.3 mL of ethanol, 2 mL of toluene, and 2 mL of PhCH2CH2SH. The diphase solution was heated to 80 °C and maintained at this temperature (in air atmosphere). The Aun(SG)m nanoclusters were found to transfer from the water phase to the organic phase in less than 10 min. The thermal process was allowed to continue for ∼40 h at 80 °C. Over this prolonged etching process, the initial polydisperse Aun nanoclusters were size focused to monodisperse Au38(SCH2CH2Ph)24. The organic phase was thoroughly washed with ethanol (or methanol) to remove excess thiol. The Au38(SCH2CH2Ph)24 nanoclusters were separated from Au(I)SR side product by extraction with dichloromethane or toluene (note, Au(I)-SR is poorly soluble in almost all solvents). The molecular purity of as-prepared Au38(SCH2CH2Ph)24 nanoclusters was confirmed by ESI-MS (Figure S4, Supporting Information) and UV-vis spectroscopy.

10.1021/jp1102884  2011 American Chemical Society Published on Web 12/09/2010

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2.2. XAS Measurement, Fitting, Simulation, and l-DOS Calculation. The Au L3-edge EXAFS measurements were conducted in transmission mode at the HXMA beamline of the Canadian Light Source (CLS) operated at 2.9 GeV. A Si (111) monochromator crystal was used in conjunction with rhodium mirrors. For solid-state measurements a toluene solution of Au38 was drop cast onto Kapton tape and folded until ample signal was observed. Solution-phase measurements were recorded by dispersing Au38 in toluene in a Teflon sample cell. The stability of samples in toluene was examined by UV-vis, which showed that the clusters remain unchanged before and after EXAFS measurements. Data processing was performed using the WinXAS program16 and following standard procedures.17,18 Simulation of EXAFS phase and amplitude and calculation of angular momentum density of states (l-DOS) were performed using the FEFF819 program code and the crystalline structure of Au38 previously reported.20 Only one carbon atom per sulfur was included in the model to save computing time. Detailed l-DOS data are presented in Table S1, Supporting Information. The fitting procedure was performed on the k3-weighted FTEXAFS from 3 to 12.3 Å-1. Fitting was performed using a nonlinear least-squares fit in R space. A FT-EXAFS R window of 1.5-3.2 Å was used for the fitting. Theoretical values of Au-S and Au-Au coordination number were determined from the total structure of Au38. One group of Au-S bonds was determined to have a total coordination of 1.26, while two groups of Au-Au bonds can be identified in the Au38 cluster. The first group is the shorter Au-Au bonds with a total CN of 3.3, whereas the second group is longer ones with a total CN of 3.5. The S02 was fixed at 0.9 (deduced from the fitting of Au foil), and all E0 values were correlated. All other parameters (R and σ2) were allowed to run free. According to standard EXAFS analysis practices, the maximum number of independent variables (N) in EXAFS refinement is

N)

2∆k∆R π

where N represents the maximum number of independent variables and ∆k and ∆R represent the k and R ranges, respectively. This produces an Nmax ) 10, well above the Nindependent ) 7 used in our EXAFS fitting. 3. Results 3.1. EXAFS Analysis of the Au38 Structure. Figure 1 shows the experimental k space and Fourier-transformed (FT) EXAFS data along with expansions highlighting the difference of Au-S and Au-Au bonding between the solid and the solution-phase Au38 clusters. The oscillation frequency of EXAFS has been known to be sensitive to the bond lengths surrounding the absorbing atom(s) in a material.21 Qualitative comparison of the EXAFS (Figure 1a) of the solution-phase (in toluene) and solid samples clearly reveals an increase in overall oscillation frequency and decrease in intensity upon cluster solvation, corresponding to a general increase in bond length(s). This effect can also be observed in the FT-EXAFS showing a shift of the first-shell Au-S and Au-Au features to a higher value of R (Figure 1b). In order to obtain quantitative structural information, refinement of the FT-EXAFS (Figure 2, Table 1) was performed based on the recently determined total structure of the Au38 cluster20 using one shell of Au-S and two shells of Au-Au (Figure 2). In comparison to the solid sample we find a general bond

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Figure 1. Solid state (s) and solvated (---) (a) X(k)k3 and (b) FT[X(k)k3] and zoom ins of the Au-S (c) and Au-Au (d) regions of the FT[X(k)k3] spectra for Au38. The solvation-induced expansion of Au-S and Au-Au bonds can be qualitatively observed by the shift of the FT-EXAFS features to higher values in R space. The more significant shift in the high-k region (a) implies a larger solvationinduced shift for Au-Au bonding (d) than Au-S (c).

Figure 2. Absolute (0) and imaginary (O) components of the experimental FT[X(k)k3] for (a) solution-phase and (b) solid-state Au38. Refinement of the absolute (s) and imaginary (---) spectra reveals a solvation-induced bond expansion of approximately 1% when the clusters are dispersed in toluene.

TABLE 1: EXAFS Fitting Data of Solution-Phase and Solid-State Au38a bond Au38 liquid Au-S Au-Au Au-Au Au38 solid Au-S Au-Au Au-Au

CN

R (Å)

∆R (Å)

1.26 3.3 3.5 1.26 3.3 3.5

2.346(3) 2.820(7) 3.032(10) 2.325(4) 2.789(7) 2.965(12)

0.021 0.031 0.067

σ2 (Å2) × 10-3

E0 (eV)

2.3(1) 9.7(6) 12.7(1) 1.5(2) 7.4(8) 10.8(14)

3.7(6) 3.7(6) 3.7(6) 1.0(5) 1.0(5) 1.0(5)

a The CNs obtained from the total structural data were fixed, and Eo was correlated to be equal. All other variables were extracted from the refinement. It is noted that the uncertainties were determined using the residual of the fit, mainly reflecting the quality of the fitting. Therefore, the R uncertainties (0.003-0.012 Å) are generally smaller than the normal uncertainty of 0.01-0.02 Å for EXAFS bond distance analysis.

lengthening of 0.02-0.07 Å (Table 1) upon cluster solvation. The shorter Au-Au bonds (∼2.8 Å) show lengthening to a much lesser extent than the longer bonds. Results on the total structure of Au38 indicate that the shorter Au-Au bonds are mainly associated with the interaction between the central Au atoms and the icosahedral surface Au atoms, whereas the longer bonds come from the surface-surface and surface-staple Au interactions.20 Therefore, it is understandable that the outer-shell

Structure and Bonding of Au38(SR)24 Nanoclusters

J. Phys. Chem. C, Vol. 115, No. 1, 2011 67 SCHEME 1: Illustration of the Representative Atomic Sites of Au38 and Au25 Used for l-DOS Calculationsa

Figure 3. Simulated Au L3-edge FT-EXAFS of each atomic site in Au38. All sites are labeled as per Scheme 1. All spectra were generated using then FEFF 8 program code and the recently reported total structure of Au38. The feature at approximately 2 Å corresponds to Au-S bonds, while all features at higher values of R correspond to Au-Au bonding.

Au-Au bonds experience a more pronounced structural change caused by solvation than the center-surface Au bonds. The simulated EXAFS spectra in Figure 3 illustrate the sitespecific coordination environments of each atomic site within Au38. The feature at 1.8 Å is attributed to Au-S bonding and thus is only observable for surface and staple environments (Figure 3). All other features from 1.8 to 3.8 Å are a result of Au-Au single-scattering paths. It can be seen that for sites with relatively high Au-Au coordination numbers (i.e., sites 1 and 2) the EXAFS spectra resemble that of bulk gold, while surface sites have a mixture of short and long Au-Au interactions. Aurophilic interactions between staple sites and surface sites normally occur around 3.5 Å and are quite significant in this particular nanocluster. In general, Au-Au coordination numbers are quite low for surface sites versus core sites; thus, aurophilic interactions could play a role in the observed solvation-induced structural changes. A bond expansion of 0.02 Å upon solvation has been previously found in solution-phase EXAFS studies of a mercury(II) chloride compound, which was likely a result of the strong solvation of the Hg(II) cation by the solvent (dimethyl sulfoixde).22 In our case, it is unlikely that the solvent (toluene) directly interacts with gold atoms in Au38 to cause bond expansion; however, it is plausible that through the aromatic interaction between the -CH2CH2Ph moiety in the thiolate ligand and the solvent molecule (CH3Ph) the Au-S and Au-Au bond distance is expanded in order to achieve a more favorable ligand-solvent interaction to stabilize the system. Nevertheless, a conclusive mechanism has to await further studies. 3.2. Au L3-Edge XANES Results and l-DOS Calculations. The near-edge region of an XAS experiment can be very useful in probing the electronic structure of gold nanomaterials.10,18,21,23 In an L3-edge experiment the XANES region is used to provide information regarding Au d-hole counts (dhole + delectron ) 10).21 Electronic transition dipole selection rules allow L3-edge XANES to selectively probe transitions between occupied 2p and unoccupied 5d states near the Fermi level. As a result, higher d-hole counts (more unoccupied d-DOS near the Fermi level) correspond to more intense X-ray absorption near the absorption threshold E0. Upon comparison of the solvated and solid-state Au38 XANES (Figure 4), we observe a noticeable increase of Au 5d-electron density (i.e., decrease of d-hole counts or

a The red spheres represent sulfur ligands, while yellow represents gold. All other atoms (carbon, hydrogen) have been omitted for clarity.

Figure 4. (a) XANES spectra and (b) white line of solid state (s) and solvated (---) Au38. The decrease in white line intensity upon cluster solvation in toluene corresponds in an increase of d-band occupation.

decrease of X-ray absorption intensity near E0) for the solutionphase Au38. By conducting site-specific calculations of the Au d-DOS19 we find the same trend in d-electron counts for every atomic site across the cluster (Scheme 1) upon 1% bond expansion (see Figure 5). The observed increase of d-electron

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Figure 5. Calculated 5d and 6s electron count distribution across each atomic site of Au38 (9), lattice-expanded Au38 (b), and Au25 (∆). A 1% lattice expansion was used to simulate the effect of solvation upon the structure of Au38, while Au25 shows the effect of cluster size upon the electronic properties of similar atomic sites. The experimental d-electron counts were determined using normal procedures (ref 18).

density upon solvation of Au38 can be accounted for by considering the d-charge transfer associated with Au-S bonding. For the solid sample, a shorter Au-S bond corresponds to more efficient d-charge transfer from Au to S than in longer bonds. As a result, the solvated sample containing longer Au-S bonds exhibits higher d-electron density (lower d-hole counts). To gain a deeper understanding of the electronic properties of Au38 ab initio l-DOS calculations were performed using the FEFF 8 code. The l-DOS calculations employ a self-consisted real Green’s function that has been found to be in good agreement with modern calculations of band structure, even when relativistic effects are considerable.19 Figure 5 shows the calculated d- and s-electron counts of the original and expanded Au38 model clusters. As a reference, the results for Au25 are also presented. A survey of the d- and s-electron counts indicates that staple Au atoms in both Au38 and Au25 clusters (Scheme 1, sites 6-8) show lower d- and higher s-electron counts than nonstaple Au (Scheme 1, sites 1-5). It has been suggested that the Au-S interaction involves electron donation from the S lone-pair electrons to Au 6s states and a back-donation from Au 5d to S 3d states.24 The calculation results in Figure 5a and 5b clearly demonstrate the significance of this donation/backdonation bonding mechanism in the staple motif by showing the Au 5d count decrease (donation) and 6s count increase (backdonation). In comparison, such a bonding feature is not evident from the d- and s-electron counts of nonstaple Au sites, most likely due to perturbation of metal-metal bonding among these Au atoms. Close inspection of the d-DOS of Au38 and Au25 indicates that the former shows slightly higher average d-electron counts than the latter (Figure 5a). Notably, the Au site (site 2) in the fusion plane of the biicosahedral Au23 core shows substantially higher d-electron density than all other sites of the two clusters, which can be understood by the lower Au-Au coordination numbers and absence of direct Au-S interactions, signifying the unique d-electron behavior of Au38 associated with this site (absent in Au25). Moreover, Au38 possesses higher s-electron counts at each atomic site than Au25 and shows a general trend of increased s-electron counts upon bond expansion. The only exception is the monomeric staple Au site which shows a small decrease of

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Figure 6. Calculations of average (a) and site-specific (b) d-DOS for representative atomic sites of Au38 with (---) and without (s) 1% lattice expansion. The 1% lattice expansion was used to simulate the effect of solvation upon cluster structure. The observed change in d-DOS can be clearly seen for core, bridging, surface, and staple environments. The DOS for each atomic site in the cluster may be found in the Supporting Information (Figure S5).

s-electron counts upon lattice expansion, implying the sensitivity of the staple Au 6s states to the local environment (Figure S1, Supporting Information). Upon 1% bond expansion of Au38, the low-energy side of the d band shifts toward the Fermi edge while the high-energy side remains essentially unchanged (Figure 6). This causes a narrowing of the overall d-DOS and a small shift of the center of the d-DOS toward the Fermi level (Figure 6a). Such a narrowing and shifting effect is also observed at each atomic site shown in Figure 6b, suggesting the potential tunability of the d-electronic behavior of Au at each site via solvation effects. Inspecting the shape of the d-DOS at each representative site (Figure 6b) indicates that the core Au atom is of metallic nature (widest d band) and the staple Au is of molecular nature (very sharp d band). All other Au sites are of metallic nature but showing significantly narrowed d bands versus that of the core Au. Moreover, a comparison of the d-DOS shape of Au38 and Au25 shows a narrowing for Au25 to a similar level as in solvated Au38 (Figure S2, Supporting Information), implying that solvation effects can modify the d-band shape as efficiently as size effects. The solvation-induced bond expansion and electronic property change presented here is unique for Au-thiolate clusters, presumably due to the extremely high surface-to-volume ratio of Au38 (87% Au on surface) and the existence of staple bonding. The solvation-caused change of structure and electronic behavior of Au38 reported in this work, albeit of its relatively small extent, has been consistently observed by both experimental and theoretical approaches and should be correlated to the critical dependence of the stability and reactivity of Au38 on dispersing solvents.14 It is envisioned that by controlling the solvation process of the clusters via selection of the protecting ligand (-SR), solvent, and additive solute, more pronounced structural and property changes could possibly be achieved. Remarkably, the change of structure and bonding could potentially influence a variety of other properties such as chemical stability,25 ferromagnetism,26 and homogeneous catalysis,11 which are normally sensitive to the variation of electronic behavior of the clusters. Particularly, the predicted d-band shift and narrowing may directly impact the catalytic behavior of transition-metal clusters, which was found to be very sensitive to Au-Au bond distance and the d-band character.27 It is also worth noting that the solution-phase XAS technique offers a valuable opportunity to follow these structural and property changes in an in situ manner and can be extended to S K-edge measurement (pre-

Structure and Bonding of Au38(SR)24 Nanoclusters liminary S K-edge EXAFS results shown in Figure S3, Supporting Information, and NEXAFS reported in ref 10). 4. Conclusion We presented X-ray absorption spectroscopy studies of the local structure and electronic behavior of solution-phase Au-thiolate cluster Au38 and ab initio calculations of their electronic properties from a site-specific l-DOS perspective. Our work highlights the importance of solvation effects on the structure and bonding of Au-thiolate clusters and implies the tunability of their properties by controlling solvation process; it also systematically illustrates the site-specific l-DOS properties of Au38 and demonstrates the donation/back-donation Au-S bonding mechanism in the cluster. Furthermore, this work demonstrates the unique potential of XAS in precisely detecting the solution-phase structure and bonding of Au-thiolate clusters, which can be potentially extended to studies of various dynamic processes in solution. Acknowledgment. P.Z. is thankful for funding from NSERC Canada and Dalhousie University. R.J. acknowledges financial support from Carnegie Melon University and the Air Force Office of Scientific Research (AFOSR). The Canadian Light Source (CLS) is financially supported by NSERC Canada, CIHR, NRC, and the University of Saskatchewan. The synchrotron technical support by Dr. Y. Hu from the CLS SXRMB beamline is also acknowledged. Supporting Information Available: Experimental and data processing details as well as supporting figures and tables. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Nanocrystal gold molecules. AdV. Mater. 1996, 8 (5), 428– 433. (2) Templeton, A. C.; Wuelfing, M. P.; Murray, R. W. Monolayer protected cluster molecules. Acc. Chem. Res. 2000, 33 (1), 27–36. (3) Jin, R. C. Quantum sized, thiolate-protected gold nanoclusters. Nanoscale 2010, 2 (3), 343–362. (4) 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 (14), 5261–5270. (5) 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 (5849), 430–433. (6) Pei, Y.; Gao, Y.; Zeng, X. C. Structural prediction of thiolateprotected Au-38: A face-fused bi-icosahedral Au core. J. Am. Chem. Soc. 2008, 130 (25), 7830–7832. (7) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Gronbeck, H.; Hakkinen, H. A unified view of ligand-protected gold clusters as superatom complexes. Proc. Natl. Acad. Sci. U.S.A. 2008, 105 (27), 9157–9162.

J. Phys. Chem. C, Vol. 115, No. 1, 2011 69 (8) Nardelli, A.; Fronzoni, G.; Stener, M. Theoretical Study on the X-ray Absorption at the Sulfur K-Edge in Gold Nanoparticles Protected by Thiolates. J. Phys. Chem. C 2009, 113 (33), 14844–14851. (9) Lopez-Acevedo, O.; Tsunoyama, H.; Tsukuda, T.; Hakkinen, H.; Aikens, C. M. Chirality and Electronic Structure of the Thiolate-Protected Au-38 Nanocluster. J. Am. Chem. Soc. 2010, 132 (23), 8210–8218. (10) MacDonald, M. A.; Zhang, P.; Qian, H. F.; Jin, R. C. Site-Specific and Size-Dependent Bonding of Compositionally Precise Gold-Thiolate Nanoparticles from X-ray Spectroscopy. J. Phys. Chem. Lett. 2010, 1 (12), 1821–1825. (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 (17), 1915–1920. (12) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Gold Nanoelectrodes of Varied Size: Transition to Molecule-Like Charging. Science 1998, 280 (5372), 2098–2101. (13) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Selective Colorimetric Detection of Polynucleotides Based on the Distance-Dependent Optical Properties of Gold Nanoparticles. Science 1997, 277 (5329), 1078–1081. (14) Toikkanen, O.; Carlsson, S.; Dass, A.; Ronnholm, G.; Kalkkinen, N.; Quinn, B. M. Solvent-Dependent Stability of Monolayer-Protected Au38 Clusters. J. Phys. Chem. Lett. 2010, 1 (1), 32–37. (15) Qian, H. F.; Zhu, Y.; Jin, R. C. Size-Focusing Synthesis, Optical and Electrochemical Properties of Monodisperse Au-38(SC2H4Ph)(24) Nanoclusters. ACS Nano 2009, 3 (11), 3795–3803. (16) Ressler, T. WinXAS: a program for X-ray absorption spectroscopy data analysis under MS-Windows. J. Synchrotron Radiat. 1998, 5, 118– 122. (17) Zanchet, D.; Tolentino, H.; Alves, M. C. M.; Alves, O. L.; Ugarte, D. Inter-atomic distance contraction in thiol-passivated gold nanoparticles. Chem. Phys. Lett. 2000, 323 (1-2), 167–172. (18) Simms, G. A.; Padmos, J. D.; Zhang, P. Structural and electronic properties of protein/thiolate-protected gold nanocluster with “staple” motif: A XAS, L-DOS, and XPS study. J. Chem. Phys. 2009, 131 (21), 214703. (19) Ankudinov, A. L.; Ravel, B.; Rehr, J. J.; Conradson, S. D. Realspace multiple-scattering calculation and interpretation of x-ray-absorption near-edge structure. Phys. ReV. B 1998, 58 (12), 7565–7576. (20) Qian, H.; Eckenhoff, W. T.; Zhu, Y.; Pintauer, T.; Jin, R. Total Structure Determination of Thiolate-Protected Au38 Nanoparticles. J. Am. Chem. Soc. 2010, 132 (24), 8280–8281. (21) Zhang, P.; Sham, T. K. X-ray studies of the structure and electronic behavior of alkanethiolate-capped gold nanoparticles: The interplay of size and surface effects. Phys. ReV. Lett. 2003, 90 (24), 245502. (22) Akesson, R.; Persson, I.; Sandstrom, M.; Wahlgren, U. Structure And Bonding Of Solvated Mercury(Ii) And Thallium(Iii) Dihalide And Dicyanide Complexes By Xafs Spectroscopic Measurements And Theoretical Calculations. Inorg. Chem. 1994, 33 (17), 3715–3723. (23) van Bokhoven, J. A.; Miller, J. T. d electron density and reactivity of the d band as a function of particle size in supported gold catalysts. J. Phys. Chem. C 2007, 111 (26), 9245–9249. (24) Park, Y. S.; Whalley, A. C.; Kamenetska, M.; Steigerwald, M. L.; Hybertsen, M. S.; Nuckolls, C.; Venkataraman, L. Contact chemistry and single-molecule conductance: A comparison of phosphines, methyl sulfides, and amines. J. Am. Chem. Soc. 2007, 129 (51), 15768–15769. (25) Reimers, J. R.; Wang, Y.; Cankurtaran, B. O.; Ford, M. J. Chemical Analysis of the Superatom Model for Sulfur-Stabilized Gold Nanoparticles. J. Am. Chem. Soc. 2010, 132 (24), 8378–8384. (26) Negishi, Y.; Tsunoyama, H.; Suzuki, M.; Kawamura, N.; Matsushita, M. M.; Maruyama, K.; Sugawara, T.; Yokoyama, T.; Tsukuda, T. X-ray magnetic circular dichroism of size-selected, thiolated gold clusters. J. Am. Chem. Soc. 2006, 128 (37), 12034–12035. (27) Miller, J. T.; Kropf, A. J.; Zha, Y.; Regalbuto, J. R.; Delannoy, L.; Louis, C.; Bus, E.; van Bokhoven, J. A. The effect of gold particle size on Au-Au bond length and reactivity toward oxygen in supported catalysts. J. Catal. 2006, 240 (2), 222–234.

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