Unique Bonding Properties of the Au36(SR)24 Nanocluster with FCC

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Unique Bonding Properties of the Au36(SR)24 Nanocluster with FCCLike Core Daniel M. Chevrier,† Amares Chatt,† Peng Zhang,*,† Chenjie Zeng,‡ and Rongchao Jin‡ †

Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States

‡

S Supporting Information *

ABSTRACT: The recent discovery on the total structure of Au36(SR)24, which was converted from biicosahedral Au38(SR)24, represents a surprising finding of a face-centered cubic (FCC)-like core structure in small gold−thiolate nanoclusters. Prior to this finding, the FCC feature was only expected for larger (nano)crystalline gold. Herein, we report results on the unique bonding properties of Au36(SR)24 that are associated with its FCC-like core structure. Temperature-dependent X-ray absorption spectroscopy (XAS) measurements at the Au L3-edge, in association with ab initio calculations, show that the local structure and electronic behavior of Au36(SR)24 are of more molecule-like nature, whereas its icosahedral counterparts such as Au38(SR)24 and Au25(SR)18 are more metal-like. Moreover, site-specific S K-edge XAS studies indicate that the bridging motif for Au36(SR)24 has different bonding behavior from the staple motif from Au38(SR)24. Our findings highlight the important role of “pseudo”Au4 units within the FCC-like Au28 core in interpreting the bonding properties of Au36(SR)24 and suggest that FCC-like structure in gold thiolate nanoclusters should be treated differently from its bulk counterpart. SECTION: Physical Processes in Nanomaterials and Nanostructures

T

Nevertheless, structural features like an FCC-ordered Au core or Au-SR bridging motifs are not likely to be selected for predicting Aun(SR)m NC structure. It was only recently shown that an FCC-like core and bridging thiolate structures could exist for Aun(SR)m NCs when Zeng et al. transformed Au38(SCH2CH2Ph)24 NCs (Au38(SR)24) into Au36(SPh-t-Bu)24 NCs (Au36(SR)24) through a ligand-exchange process with resultant successful single crystal growth of Au 36 (SPh-t-Bu) 24 NCs. 25,26 It should be noted that Au36(SPh)23 NCs were previously identified by Dass et al.; however, no crystal structure was reported.27 The structure of Au36(SPh-t-Bu)24 consists of an FCC-like Au28 core with 12 bridging thiolates (12 × −S(R)−) and 4 dimeric staple motifs (4(−S(R)−Au−S(R)−Au−S(R)−). The structural transformation dramatically changes the core:staple Au ratio, going from 23:15 for Au38(SR)24 to 28:8 for Au36(SR)24, via a disproportionation mechanism.26 It was also demonstrated that Au25(SR)18 NCs could undergo a similar ligand-exchange induced transformation to form Au28(SR)20 NCs with an FCC-like core and a mixture of bridging and staple motifs.28 A remarkable finding for Au36(SR)24 was the increase in the optical gap from 0.9 eV (Au38(SR)24) to 1.7 eV (Au36(SR)24). This difference in optical gap could suggest that Au36(SR)24 is more molecule-like than Au38(SR)24, despite having an FCC-like core. Therefore, these

hiolate-stabilized gold nanoclusters (Aun(SR)m NCs) have been widely studied in recent years, with remarkable progress made on the development of synthetic protocols,1−4 understanding electronic/structural properties,5−9 and application-based technologies.10−12 The total structure determination of Au102(SR)44,13 Au38(SR)24,14 and Au25(SR)18 NCs15,16 accelerated our understanding of these materials through unveiling unique non-face-centered cubic (FCC) core structures and Au−S surface “staple” motifs, which helped account for the molecule-like optical properties (i.e., discrete electron energy levels and photoluminescence) and their extraordinary stability.17−20 The unique Au−S staple motif consists of short Au+−SR oligomers (monomeric or dimeric) that are anchored to the surface by thiolate molecules at the end of the short oligomer. Au atoms in the staple are, therefore, displaced from the surface of the NC. Based on these new experimental findings of the dominant staple motif, other models for describing the nature of thiolate−Au bonding in Aun(SR)m NCs were put aside, such as head-on (thiolate bonded to 1 surface Au) or bridging (thiolate bonded to 2 surface Au) motifs.21 Besides the aforementioned Aun(SR)m NCs, many other Aun(SR)m NCs have been isolated but are without an obtainable crystal structure. For unknown structures, “rules” have been developed to predict the core and staple composition (e.g., the # of monomeric and dimeric staples) based on electron shell closings,22 divide-and-protect,23 and staple-fitness methodologies.24 The combination of these rules have been proven useful for prediction and rationalization of Aun(SR)m NCs. © 2013 American Chemical Society

Received: August 25, 2013 Accepted: September 10, 2013 Published: September 10, 2013 3186

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Figure 1. (a) Depiction of surface and internal structure for Au36S24 (Au atoms − gold; S atoms − red and magenta). (b) Offset FT-EXAFS of Au36 and Au foil (k-range: 3−16 Å−1), and (c) back-transformed FT-EXAFS (R-filter: 2.4−2.9 Å) of Au36 Au−Au core bonding with comparison to Au28 FCC-like core model (y-scale for EXAFS of the theoretical FCC model is rescaled to account for the thermal vibration effect). Best fits for Au36 (d) roomtemperature and (e) low-temperature FT-EXAFS spectra (k-range: 3−13.7 Å−1).

EXAFS. The structure of Au36 is depicted in Figure 1a with isolated layers to illustrate the FCC-ordered Au28 core (staple Au sites are removed) and its inner tetrahedral component. Two different Au−S bonding modes are also shown and are known as the staple and bridging motifs, where Au atoms are held in the core for bridging motifs. Figure 1b displays the Fouriertransformed EXAFS spectrum (FT-EXAFS) of Au36 (k-range of 3.0−16.0 Å−1 used for transformation) at low temperature (90 K), with Au foil shown underneath to serve as bulk FCC Au. On an important side note, a deeper understanding of Au−Au bonding environment can be achieved with EXAFS when the sample is of high purity and is measured at low temperatures to obtain high quality oscillations up to the high k region (i.e., k = 16.0 Å−1). This is evident in FT-EXAFS spectra when scattering shells are well resolved and the high-R shell intensities are enhanced. Before quantitative EXAFS fitting results are shown, we present a qualitative approach to quickly identify FCC Au−Au bonding structure in Aun(SR)m NCs. To do this, the backtransformed FT-EXAFS signal from the Au−Au bonding environment of the Au core in Au36 is isolated and compared with simulated structural model of an FCC Au28 core shown in Figure 1c. Overlapping back FT-EXAFS oscillations produces an almost perfect match of phase and frequency between Au36 and an FCC Au28 core model. Similar scattering peaks are also

latest discoveries of an FCC-like core pose an interesting question about the effect of the core on the bonding properties and electronic structure of Aun(SR)m NCs. In particular, how does the FCC-like core behave in relation to icosahedral-like core structures found for other Aun(SR)m NCs such as Au25(SR)18 or Au38(SR)24? Herein, we investigate the remarkable bonding and electronic properties of Au36(SR)24 using X-ray absorption spectroscopy and ab initio calculations. Comparisons are drawn to Au38(SR)24 NCs as they have almost identical elemental composition to Au36(SR)24 but contrasting structural properties such as icosahedral-like versus FCC-like cores and staple versus bridging motifs. The Au28 FCC-like core is highlighted in this work as the structural and electronic properties are studied from a sitespecific perspective. Our results reveal smaller pseudo-Au4 units within the core largely contribute to the molecule-like electronic properties of Au36(SR)24, which is consistent with the experimental findings from Au L3-edge X-ray absorption nearedge structure (XANES) and temperature-dependent extended X-ray absorption fine structure (EXAFS). The role of the Au−S bonding motif on the electronic properties is also examined from the sulfur perspective using S K-edge XANES and ab initio calculations. Our analysis on Au36(SR)24 (Au36 for short) begins with an investigation of the Au−Au bonding properties by Au L3-edge 3187

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observed in the FT-EXAFS for Au36 and bulk FCC Au (indicated with asterisks). These initial observations indicate the Au−Au bonding environment of the Au36(SR)24 core is indeed highly FCC-like. For a deeper understanding of the FCC-like core local structure, we move to EXAFS fitting results. A three-shell EXAFS (Au−S, Au−Au1, and Au−Au2) fit was performed for the Au36 spectra at both room temperature (RT) and low temperature (LT) to obtain quantitative structural parameters for each shell’s environment (k-space spectra are shown in Figure S1). FT-EXAFS spectra and their respective best fits are shown in Figure 1d,e with fitting results summarized in Table 1. Upon inspection of the Au36(SR)24 crystal structure,

Scheme 1. Representation of the (a) FCC-like core (in Au36) and (c) Biicosahedral Core (in Au38) with the Shorter Au−Au Bonding Frameworks Shown in Panels b and d, Respectivelya

Table 1. Au L3-Edge EXAFS Fitting Results for Au36 Measured at Room Temperature and Low Temperature temperature (K)

bond type

coordination number

bond length (Å)

σ2 × 10−3 (Å2)

E0 shift (eV)

295 (room)

Au−S Au−Au1 Au−Au2 Au−S Au−Au1 Au−Au2

1.33 2.06 2.56 1.33 2.06 2.56

2.322(2) 2.732(4) 2.89(1) 2.329(2) 2.746(3) 2.951(5)

3.9(1) 8.0(3) 18(2) 2.6(1) 3.7(9) 10.3(6)

0.0(5) 0.0(5) 0.0(5) 2.0(5) 2.0(5) 2.0(5)

90 (low)

a

Central Au sites not bonded to thiolate ligands are shown as larger spheres in panels b and d.

temperature, with the Au−Au2 shell having a more pronounced expansion of 2.1%. The expansion of metal−metal bonding with decreasing temperature is an uncommon property for bulk FCCordered metals, although it has been observed in a few studies with larger Au nanoparticles.33,34 Our previous temperaturedependent EXAFS study on Au25 showed noticeable contraction of the first-shell Au−Au bond at low temperature, illustrating the metallic behavior of the core for Au25(SR)18.30 Returning to the EXAFS results on Au36, structural parameters from the Au−Au1 shell mainly correspond to the short Au−Au bonding found within and nearby the pseudo-Au4 clusters mentioned above. It is interesting to note that there are 4 Au atoms located in the center of the core also forming a tetrahedral shape (Figure 1a, right), which are the only Au atoms not bonded to any thiolate ligand. Nonetheless, these 4 central Au atoms are in fact not tightly bonded to each other being, on average, 2.95 Å apart and are therefore represented by the longer Au−Au2 shell. Furthermore, slight Au−Au expansion is seen within the small Au clusters at low temperature, 2.732(4) to 2.746(3) Å, while significantly more expansion is experienced between the clusters and on the surface of the core going from 2.89(1) to 2.951(5) Å. This physicochemical observation is an intriguing property for Au36, exemplifying the more molecule-like behavior of its FCClike core. This property could also be considered useful for novel applications involving negative thermal expansion materials. After identifying the unique pseudo-Au4 environment in the Au36 core framework, the electronic structure was then investigated with XANES and projected angular moment-density of states (l-DOS) from both Au and S perspectives to probe the influence of such Au clusters on the molecule-like behavior of Au36. Au L3-edge XANES for Au38, Au36, and Au foil are presented in Figure 2a. Absorption edge positions (E0) for Au36 and Au38 are both at higher energy than Au foil. This is generally caused by S− Au+−S interactions that occur on the surface of Aun(SR)m NCs. Interestingly, a noticeable decrease in the white-line intensity (the first resonance feature following the edge jump, indicated by arrow) is seen for Au36 compared to Au38; albeit, the white-line

calculation of the ideal Au−S coordination number (CN) gave a value of 1.33 and was therefore fixed for the three-shell fitting process. Due to the complex variety in Au−Au bonding environments, a multishell EXAFS fit accounting for each of these scattering paths was not obtainable. However, preliminary fitting of the Au−Au environment consistently lead to two distinct Au−Au scattering paths which could provide an excellent fit from 1.5 to 3.2 Å, encompassing the strongest single scattering paths (including Au−S). Organizing all Au−Au bond length data into a histogram confirmed these two distributions of Au−Au bonding where ideal CN values could be calculated and used in the three-shell EXAFS fitting method (Figure S2). As a result, Au−Au1 and Au−Au2 shells account for Au−Au bonding between ca. 2.72−2.86 Å and ca. 2.86−3.1 Å, respectively. Previously reported EXAFS fitting of Au38(SR)24 and Au25(SR)24 NCs (Au38 and Au25 for short) at room temperature showed short Au−Au bonding in the core at 2.789(7)29 and 2.80(1)30 Å, respectively, whereas the first Au−Au shell bond length for Au36 (Au−Au1) is much shorter at 2.732(4) Å. The shorter Au−Au bonding framework representing the Au−Au1 shell for Au36 is illustrated in Scheme 1, with comparison to the biicosahedral Au38 core. Interestingly, short Au−Au bonding (RAu−Au < 2.86 Å) is more localized for Au36 than for the Au38 with smaller Au−Au bonding occurring within and nearby pseudo-Au4 tetrahedral units. These pseudo-Au4 units are highlighted with red bonds in Scheme 1. Complete evolution of the pseudo-Au4 unit with Au−Au bond length is further shown in Figure S3. The fact that shorter metallic bonding occurs as separate small clusters in the FCC-like core can help account for the much shorter bond lengths found from EXAFS results at room temperature. Recently, the important role of Au4 units in Aun(SR)m NCs has been brought to attention by Jiang,31 proposing that these tetrahedral units could be the smallest Au core structures for Aun(SR)m NCs. Cheng et al.32 also used Au4 core units as the foundation for superatom-networks to explain the stability of certain Aun(SR)m NCs. Another important finding from EXAFS analysis is the expansion of bond lengths for the two Au−Au shells at low 3188

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intensity of Au36 is almost as low as Au foil. The Au L3-edge white-line originates from electronic transition from occupied 2p to unoccupied 5d state, where a less intense white-line corresponds to higher 5d electronic density. The significant difference between Au36 and Au38 white-line intensities indicates valence levels are more highly occupied in Au36 than Au38. This is somewhat surprising since it has been known that Au will lose delectron to S due to the metal−ligand charge transfer, and the S/ Au ratio is higher in Au36(24/36) than in Au38(24/38). The unexpected difference in electronic structure between the measured Au36 and Au38 could be due to the fact that small pseudo-Au4 units that comprise the Au36 core are approaching the limit of observable nanosize effects. As was reported in the literature, the net nanosize effect of Au (without contribution from ligand−metal charge transfer) results in an increase of Au delectron density due to the lesser extent of s-p-d rehybridization (i.e., less intra-atomic electron flow from d to s/p state).35 Therefore, this decrease in core size (e.g., pseudo-Au4 units vs Au23 biicosahedral core) should cause the valence d-orbitals to become more highly occupied. To verify this nanosize effect, we simulate the Au L3-edge XANES of the central Au site of Au36 and Au38 (i.e., site not bonding to surface thiolate in order to exclude the metal−ligand charge transfer effect). Simulated Au L3-edge XANES (Figure 2b) shows the same trend of white-line intensity as that in Figure 2a, which further confirmed our hypothesis. This supports the idea that smaller Au clusters are governing the electronic properties to be more molecule-like for Au36, more so than the S ligand. To further probe the influence of the Au28 FCC core on the electronic properties from a site-specific perspective, the l-DOS calculations were performed on all unique Au sites in Au36. Since we are interested in the effect of the Au28 FCC-like core on the bonding and electronic properties of Au36, l-DOS calculations are shown for the 5d level and are henceforth referred to as d-DOS. Figure 2c presents the site-specific d-DOS analysis of the Au36 structure. A gradual narrowing of the 5d level is apparent when Au atoms locations are closer to the surface. This indicates the electronic structure of each Au site transitions from metallic-like to atomic-like going from the central sites (band (i)) to staple sites (band (vi)). Besides this metallicmolecular band structure trend, also observed in studies for other Aun(SR)m systems,36,37 the d-DOS illustrates the subtle differences between surface sites and bridging sites (bands (ii) to (v)) where the Au−S CN is 1 and 2, respectively.

Figure 2. (a) Au L3-edge XANES of Au36, Au38 and Au foil. (b) Au L3edge near-edge simulation of central Au site in Au36 and Au38. (c) Sitespecific interpretation of d-DOS band structure for Au36 including (i) central, (ii) bridging surface, (iii) dimeric staple surface, (iv and v) bridging, and (vi) dimeric staple Au sites. (d) d-DOS comparison of an ideal Au28 FCC-ordered cluster from bulk Au (Au−Au distance adjusted to match that of Au36) with actual Au28 FCC-like core from Au36 with S atoms removed.

Figure 3. (a) S K-edge XANES of Au36 and Au38, (b) simulated S K-edge XANES for apical bridging and staple S, and (c) simulated S K-edge XANES of apical bridging site with corresponding calculated DOS band structure. 3189

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Bands (ii) and (iii) represent the two unique surface sites with slightly different environments, where site (ii) is bonded to a bridging motif and site (iii) is bonded to a dimeric staple. Other Au sites are found on the surface of the Au28 core but are involved in bridging thiolate bonding where the Au−S CN = 2. These Au sites are represented by bands (iv) and (v). For bridging sites, Au atoms are displaced from the core more than corner sites in band (iii) due to stronger Au−S bonding resulting in further narrowing of their 5d bands compared to other surface sites. The different distance of bridging Au to the surface causes the appearance of a shoulder in band (iv), since these particular Au sites are held more closely to the surface. The nature of these bridging Au sites in conjunction with the smaller pseudo-Au4 units in the core (which do not share similar Au sites) would contribute to the molecule-like electronic structure of Au36. Additional d-DOS simulations compare the bare Au28 core (i.e., without thiolate-gold bonding effect) from Au36 with an ideal Au28 core from bulk gold, shown in Figure 2d. Simulated d-DOS bands are overlapped for similar sites on both models. Evidently, the bridging thiolates have a dramatic effect on the corner Au sites as the band is significantly narrower (i.e., more moleculelike) than the ideal structure (Figure 2d, top). In addition, the central Au site in the nanocluster shows a narrower d-DOS than that in the ideal structure; again, influencing the more moleculelike electronic properties for Au36. In the above discussions, the electronic structure of Au36 has been probed from the Au perspective. We now turn to the S Kedge XANES spectra of Au36 and Au38 to examine the effect of the thiolate-bonding motif on the electronic structure. Experimental XANES spectra in Figure 3a show the overlap of Au36 and Au38. The most important finding from this comparison is the distinct broadening of the first near-edge feature for Au36. In fact, previous S K-edge XANES studies on Au25, Au38 and Au144 NCs showed no broadening of this feature, only an increase in the preedge feature at 2.471 keV.38 To understand the origin of this broadening, XANES simulations (Figure 3b) were performed for each S site on Au36 and Au38. The structure of staple and bridging motifs both have two different S sites (−Se(R)−Au−Sa(R)−Au− Se(R)−), which will be referred to as edge S (Se) and apical S (Sa). Simulated S XANES for thiolate ligands on the edge of each motif had almost identical spectra for either bridging or staple motif on either NC (Figure S3). The apical S atom in the dimeric staple motif for both Au36 and Au38 are also similar in structure although the spectrum for Au36 is slightly broader, as shown in Figure 3b. More significantly, the apical S atom in the bridging motif has a near-edge spectrum much broader than any other S site due to an additional near-edge feature higher in energy from the absorption edge (shown with dotted line), which could contribute to the broader near-edge feature seen from the experimental XANES. To understand the origin of the S K-edge XANES linebroadening of Au36, the l-DOS components that influence the postedge features in XANES spectra are presented for the apical bridging S in Figure 3c. It is observed that S d-DOS resonance for the apical bridging S is positioned in the same region where the additional near-edge feature appears for the apical bridging S, suggesting the line-broadening of S K-edge XANES (Figure 3a) should be related to the Au−S bonding associated with the unoccupied S d-state. This observation implies that the Au−S bonding related to the S d-state is sensitive to the local structural difference between the apical S in the bridging and the staple motif. Moreover, this finding of near-edge broadening for Au36

could be a method to detect the presence of bridging motifs or perhaps the FCC-like core structure for Aun(SR)m NCs without known structures. A more conclusive mechanism waits for further experimental and theoretical investigations. In summary, XAS measurements in conjunction with ab initio calculations demonstrate the unique bonding properties for Au36(SR)24 with FCC-like core from both the gold core and gold−thiolate surface motif perspectives, when comparing with icosahedral-like nanoclusters with similar composition. These findings indicate that the molecule-like gold−gold bonding behavior of Au36(SR)24 is largely determined by tightly bonded pseudo-Au4 units within the FCC-like Au28 core, and that the bonding of its gold−thiolate bridging motif differs from that of the staple-like motif. This work suggests that the FCC-like gold− gold packing structure in small gold−thiolate nanoclusters should be treated differently from its bulk counterpart, albeit they share the FCC structural feature.

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ASSOCIATED CONTENT

S Supporting Information *

Materials and methods, experimental k-space spectra for Au36, Au36 Au−Au bonding data, schematic evolution of pseudo-Au4 units, and additional S K-edge XANES simulations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: 1-(902)-494-3323. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

P.Z. would like to acknowledge funding support from Dalhousie University and NSERC in the form of discovery grants. R.J. acknowledges research support from the Air Force Office of Scientific Research under AFOSR Award No. FA9550-11-1-9999 (FA9550-11-1-0147) and the Camille Dreyfus Teacher-Scholar Awards Program. The Canadian Light Source (CLS) is financially supported by NSERC Canada, CIHR, NRC, and the University of Saskatchewan. The synchrotron technical support by Dr. Yongfeng Hu and Aimee MacLennan from the CLS SXRMB is acknowledged. PNC/XSD facilities at the Advanced Photon Source and research at these facilities are supported by the U.S. Department of Energy − Basic Energy Sciences, a Major Resources Support grant from NSERC, the University of Washington, the Canadian Light Source, and the Advanced Photon Source. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. The authors are also thankful for the technical assistance from Dr. Robert Gordon at the PNC/XSD facilities.

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