Role of Au4 Units on the Electronic and Bonding

Article pubs.acs.org/JPCC

Role of Au4 Units on the Electronic and Bonding Properties of Au28(SR)20 Nanoclusters from X‑ray Spectroscopy Daniel M. Chevrier,† Chenjie Zeng,‡ Rongchao Jin,‡ Amares Chatt,† and Peng Zhang*,† †

Department of Chemistry, Dalhousie University, Halifax, Nova ScotiaB3H 4R2, Canada Department of Chemistry, Carnegie Melon University, Pittsburgh, Pennsylvania United States

‡

ABSTRACT: The emergence of FCC-ordered core structures in thiolate-protected gold nanoclusters has motivated researchers in this area to reconsider the possibility of nonicosahedral core frameworks. With the recent elucidation of a four-membered FCC-ordered core series (Au20(SR)16, Au28(SR)20, Au36(SR)24, and Au44(SR)28), it is now of significant interest to understand how their electronic and bonding properties are influenced by the cluster composition and how they differ from, or may be linked to, their icosahedral counterparts. Of recent, attention has been focused on the presence of small Au4 units, which comprise the majority of the known FCC-ordered thiolate-protected gold nanocluster core structures. Herein, the Au28(SR)20 nanocluster with a 20 Au atom FCC-ordered core is investigated with temperature-dependent X-ray absorption spectroscopy experiments and ab initio simulations to elucidate the bonding and electronic properties of the second member of the FCC-ordered core series. By comparing the results of Au28(SR)20 with its larger FCC-ordered relative, Au36(SR)24, and its icosahedral counterpart, Au25(SR)18, the significant role of Au4 core units on the bonding properties of FCC-ordered gold nanoclusters is highlighted in this work. “magic series” of four Aun(SR)m NCs differentiating by Au8(SR)4 units.11,12 These newest members have core structures predicted to be similar (Au44(SR)28) or resembling (Au20(SR)16) the Au core framework of Au36(SR)24 and Au28(SR)20. It is interesting to note that the total structure of Au30S(SR)18 NCs has revealed a core structure almost identical to Au28(SR)20 NCs synthesized with a tert-butyl thiolate ligand instead of tert-butyl benzene thiolate.13 Aside from the unexpected FCC arrangement of Au atoms in the core, attention has been shifted to smaller tetrahedral Au4 or vertexsharing bitetrahedral Au7 clusters within the core in order to identify the implication these fundamental core units have on the nucleation/formation and composition-dependent bonding/electronic properties of Aun(SR)m NCs.14−16 Furthermore, Au42+ and Au73+ clusters within Au28(SR)20 NCs have been described and interpreted with the divide-and-protect model and the noble gas superatom theory as basic 2e− and 4e− superatoms, respectively.17 In continuation from our previous study on Au36 NCs,18 Au28(SR)20 NCs were examined with temperature-dependent X-ray absorption fine structure (XAFS) spectroscopy experiments and ab initio simulations of valence electronic structure. Our findings demonstrate the valence electronic structure is

1. INTRODUCTION Thiolate-protected gold nanoclusters (Au NCs) synthesized with atomic precision (giving rise to the chemical formula Aun(SR)m to describe their composition) have emerged as promising nanomaterials for a new class of catalysts and sensing/imaging agents.1−5 Before these materials reach a time of practical application, an understanding of Au n (SR)m composition-property relationships will indefinitely benefit the researcher’s ability to predict and tailor the structure in order to achieve the desired properties.6 This is a difficult task since the optoelectronic and physicochemical properties of Aun(SR)m NCs have been shown to be largely dependent on the composition with many unique Au sites (core, surface, bridging and staple) affecting such properties. In addition to composition, the combination of various structural components such as different core sizes or surface Au-SR protecting units could potentially influence Aun(SR)m properties. The synthesis and total structure elucidation of Au36(SR)24 and Au28(SR)20 NCs (where R = Ph-tert-Bu) introduced the possibility of Aun(SR)m NCs accommodating core structures with face centered cubic (FCC)-ordered geometry similar to larger Au nanoparticles (diameter > ∼2 nm) and the bulk.7,8 The appearance of bridging thiolate motifs on the FCC-ordered core was also a first for Aun(SR)m NCs, which were originally predicted as the thiolate bonding motif for larger Au nanoparticles and Au(111) surfaces. 9,10 Shortly after, Au44(SR)28 and Au20(SR)16 NCs were isolated forming a © 2014 American Chemical Society

Received: September 14, 2014 Revised: November 21, 2014 Published: December 17, 2014 1217

DOI: 10.1021/jp509296w J. Phys. Chem. C 2015, 119, 1217−1223

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The Journal of Physical Chemistry C

Figure 1. (a) Representation of the Au28(SR)20 NC with core structures, (b) simulated Au L3-edge EXAFS spectra of each unique Au site, and (c) comparison of the average simulated and experimental (measured at 90K) Au L3-edge EXAFS.

coordination number of 12. For multishell EXAFS fitting, coordination numbers (CN) were fixed according to the Au25(SR)18 and Au28(SR)20 crystal structures.8,19,20 A k-range of 3−11.25 Å−1 was used for the Fourier transformation of all kspace data to R-space. A fitting window of 1.5−2.9 and 1.5−3.2 Å was used for 2-shell and 3-shell fits, respectively. E0 shift values were correlated and theoretical CN values were fixed for EXAFS fitting to reduce the number of free running parameters, justifying the incorporation of up to three scattering shells. Theoretical phase and scattering amplitudes for Au−S and Au−Au scattering paths were calculated using the FEFF8.2 computational package21 and each Au NC model. Reported uncertainties for EXAFS fitting results were computed from off-diagonal elements of the correlation matrix, which were weighted by the square root of the reduced chisquared value obtained for each simulated fit.22 The amount of experimental noise was also taken into consideration for each Fourier transformed R-space spectrum from 8 to 25 Å. Projected angular momentum density of states (l-DOS) spectra were simulated for each Au site in Au25(SR)18 and Au28(SR)20 NCs (the R group was simplified to one C atom for calculation purposes) using the FEFF8.2 computation package21 with only the d-DOS results presented herein. FEFF8.2 code is an ab initio calculation method that uses self-consistent real-space multiple scattering calculations including important factors such as core-hole effects, full multiple scattering and real-space relativistic Green’s function.

fundamentally different from its similar sized, icosahedral counterpart Au25(SR)18 because of the unique local structure of smaller Au4 units in the core of Au28(SR)20. A multishell extended X-ray absorption fine structure (EXAFS) fitting analysis was then conducted on Au28(SR)20 to examine the local structure of short Au−Au within Au4 units and longer Au−Au bonding in the core. By measuring the Au28(SR)20 NC at both low (90 K) and room temperature (300 K), thermal contraction of longer Au−Au bonding was detected, consistent with our previous experiments on Au36(SR)24 NCs. This result suggests Au4 units in the Au core play an important role in understanding the electronic properties and the bonding properties of FCC-ordered Au NCs.

2. MATERIALS AND METHODS Au28(SPh-tert-Bu)20 NCs were synthesized via ligand-exchange induced transformation using Au 25 (SC 2 H 4 Ph) 18 − NCs (counterion: +N(C8H17)4) as the precursor material. Briefly, Au25(SC2H4Ph)18− was reacted with excess amounts of tertbutyl benzene thiol in toluene at 80 °C. After ∼2 h of mixing, a high yield of transformed Au28(SPh-tert-Bu)20 NCs were collected. The synthetic details and complete characterization of the reaction product have been published elsewhere.8 Au L3-edge XAFS measurements were collected in transmission mode at the PNC/XSD beamline of the Advanced Photon Source (Argonne National Laboratory, Argonne, IL) using a Si(111) monochromator and a Rhodium mirror. Powdered samples of Au25(SR)18− and Au28(SR)20 NCs were prepared for measurement by drop-casting the sample from a toluene solution onto kapton film, dried and then sealed. Enough Au NC material was transferred onto the kapton film until an ample absorption edge jump (Δμ0 > ∼0.5) was obtained. Multiple measurements were collected for each XAS experiment to ensure reproducibility of fine structure oscillations. Measurements were conducted at 300 K under ambient conditions and at 90 K using a helium-cooled cryostat chamber. Background subtraction, energy calibration, X-ray absorption near-edge structure (XANES) normalization and EXAFS fitting were all performed using the WinXAS 3.1 software package. The amplitude reduction factor (S02) was fixed at 0.90 for Au L3-edge EXAFS fitting, which was determined by fitting the Au−Au scattering of a Au foil reference with a fixed Au−Au

3. RESULTS AND DISCUSSION 3.1. Bonding/Electronic Properties of Au28(SR)20 and Comparison of Aun(SR)m NC Core Structures. As mentioned in the previous section, Au28(SR)20 NCs (Au28) were synthesized from Au25(SR)18 NCs (Au25) using a ligandexchange protocol that dramatically reconstructed the icosahedral Au13 core and surface structure of Au25. The resulting structure of Au28 as determined by X-ray crystallography consists of an FCC-like core composed of 20 Au atoms and 8 Au atoms (smaller atoms) located in four double staple motifs (−S(R)−Au−S(R)−Au−S(R)−) (Figure 1a, top).8 The remainder of the thiolate ligands bond with three Au atoms on each end of the 20 Au atom core (Figure 1a, center). As a result, these 6 Au atoms are pulled away from the two central Au sites (shown as purple), leaving a 14 Au atom alternative 1218


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bridge, bridge-surface, center−center, surface-center and staplesurface. The k-space EXAFS spectra of Au28 and Au25 NC samples measured at 300 K are shown in Figure 3a. The spectra are comparable in oscillation pattern and intensity but with noticeable changes in oscillation frequency. Identical k-ranges of 3.0−11.25 Å−1 were used to acquire the Fourier-transformed (FT) R-space for each Au NC sample shown in Figure 3b. Au− S scattering peaks are similar in position and intensity for both Au NCs, which is consistent with the average coordination number (CN) for Au−S bonding being 1.44 for Au25 and 1.43 (((14 Au each bonded to 2 S) + (12 Au each bonded to 1 S))/ 28 Au) for Au28 (Figure 2). Although Au28 contains Au-SR double staple motifs (planar semirings) and Au-SR bridging motifs (which can also be thought of as bent semirings), fitting results from a two-shell refinement (shown in Figure 3c and Table 1) reveal the Au−S bonding distance and structural disorder (Debye−Waller factor, σ2) are close in value to Au25, which only contains double staple units on its surface. The scattering feature that precedes the dominant Au−S peak for Au28 (around 1.2 Å) is likely due to the difference in Au−S scattering environments since this feature is also present in the simulated EXAFS of Au28 (Figure 1c). Structural deviations of the Au28 NC from Au25 are more noticeable with the Au−Au scattering shell from the two-shell fit in Figure 3c. Using a similar Au−Au scattering path for the EXAFS fit of each Au core,24 the shortest Au−Au bonding for Au28 is on average 2.73(2) Å in length compared to 2.80(1) Å for the icosahedral core of Au25. The determined Au−Au bond length is very close to what Zeng et al. had reported as the shortest Au−Au bond type (2.74(3) Å).8 This short Au−Au bond length is a common signature of small FCC-ordered Au cores or smaller Au4 tetrahedral units.7,12,25,26 The structural disorder from the Debye−Waller factor for the shortest Au−Au scattering path is also less for Au28 than Au25, suggesting lower structural disorder of Au−Au bonding within the FCC-like core of Au28. This is another indication that the shortest Au−Au scattering shell (Au−Au1) originates from the two staggered bitetrahedrons (Au7 core units), which contain tightly bonded Au4 tetrahedral units. The relative S/Au ratio is similar for each Au NC: 0.71 for Au28 and 0.72 Au25. Thus, only considering the electron withdrawal from the thiolate ligands in Au−S bonding from πelectron backdonation (Au 5d to S 3p), it is expected that the Au 5d electron occupancy would be similar for each Au NC. Moreover, the ratio of theoretical Au+/Au0 (using the general rule that Au atoms bonding to two thiolate molecules are expected to have a Au+ oxidation state)27,28 is similar for each Au NC system (0.50 for Au28 and 0.48 for Au25). In this regard, Au25 and Au28 NCs should have a similar valence electronic structure. Presented in Figure 4 are Au L3-edge XANES spectra of Au28, Au25, and Au foil, for energy calibration purposes and additional comparison. It is immediately obvious that the whiteline feature (promotion of Au 2p electrons to vacant Au 5d valence levels) for Au28 is much lower than Au25 despite the almost identical S/Au and Au+/Au0 criteria mentioned above. Instead, the white-line intensity is more similar to Au foil with a shift in the absorption edge (E0) position to lower energy (inset). Both results indicate Au28 has a substantially higher 5d electron density than Au25. On the nanoparticle or nanocluster size regime, however, higher occupation of Au 5d valence orbitals can originate from under-coordinated Au sites or smaller clusters of Au, decreasing the amount of s-p-d

core structure (Figure 1a, bottom). Additionally, considering Au−Au bond lengths shorter than 2.88 Å, four Au4 or two Au7 units are revealed as the core constituents (connected with red bonds). A preliminary investigation of the local structure is conducted to confirm consistency with the total structure of Au28(SR)20 NCs and to compare with the Au25 precursor. First, Au L3-edge EXAFS simulations were conducted on the Au28 NC by simulating the scattering environment of each Au site in order to create an average EXAFS signal. Site-specific scattering environments and the average simulated R-space EXAFS spectrum are depicted in Figure 1, parts b and c, respectively. From these spectra, the origin of the most dominant EXAFS scattering peaks (∼1.5−3.5 Å) can be identified. The first feature for surface, bridge and staple sites is from Au−S scattering (Figure 1b). The second scattering feature is from the shortest Au−Au bonding that occurs within the Au20 core. This scattering feature is seen for center and surface Au sites. The last scattering feature that will be examined herein is a longer Au−Au bonding type that can be found for center, surface, bridge and some staple sites. Between the average simulation and the experimental EXAFS (90 K), there is good agreement with the position of the Au−S (1.9 Å not phase-corrected) and Au−Au (2.45 and 2.75 Å not phase-corrected) scattering peaks. The relative intensities of Au−S and Au−Au peaks are also comparable. These similarities in the EXAFS reassure the measured Au28 NC contains Au local structural features expected from the crystal structure. Longer range Au−Au scattering features are more intense for the simulated EXAFS since the simulation is effectively conducted at 0 K without thermal dampening of the EXAFS signal or a limited mean free path for the scattered photoelectron.23 A k1weighting is used for the simulated and experimental EXAFS for this reason. The theoretical coordination (CN) values for each of the scattering paths identified above can then be calculated from the bond distance distribution, shown in Figure 2. From the two distinct Au−Au bonding distributions (2.7−

Figure 2. Bond length distribution for Au28(SR)20 and representative EXAFS scattering shells.

2.8 and 2.9−3.15 Å), two Au−Au scattering shells can confidently be assigned in the EXAFS fit. Depictions of Au28 in Figure 2 indicate the first Au−Au scattering shell (Au−Au1) will account for Au−Au bonding in Au4 or Au7 core structures exclusively. The second Au−Au scattering shell (Au−Au2) encompasses a variety of Au−Au bonding environments throughout the Au NC including surface−surface, bridge− 1219


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Figure 3. Au L3-edge (a) k-space, (b) FT EXAFS, and (c) fitted EXAFS spectra of Au25 and Au28.

Table 1. Two-Shell EXAFS Fitting Results for Au28(SR)20 and Au25(SR)18 NCs at Room Temperature (300 K) Au25(SR)18

a

a

scattering shell

CN

Au−S Au−Au1

1.44 1.44

Au28(SR)20

R (Å)

σ (Å )

ΔE0 (eV)

CN

2.321(5) 2.80(1)

0.0033(2) 0.009(1)

−1(1) −1(1)

1.43 1.71

2

2

a

R (Å)

σ2 (Å2)

ΔE0 (eV)

2.314(9) 2.73(2)

0.0035(4) 0.007(1)

−1(2) −1(2)

Coordination numbers were fixed according to the expected value determined from the total structure of Au25(SR)18 and Au28(SR)20 NCs.8,19,20

Figure 4. Au L3-edge XANES of Au28, Au25, and Au foil (inset, whiteline region).

hybridization and approaching a more atomic-like Au electronic structure (5d∼10) instead of bulk Au (5d∼9.6).29 From our initial inspection of the Au28 local structure from EXAFS fitting, the Au−S bonding on the surface is similar on average for both Au NCs while each Au core has contrasting bonding properties likely related to the different core structures (Au13 icosahedral core versus FCC-ordered core containing smaller Au4 core structures). To examine the influence of the core structures on the valence electronic properties, namely the bound electronic states in d-DOS (density of states), ab initio simulations were conducted from a site-specific perspective to elucidate differences in valence electronic structure for each Au NC system. The simulated d-DOS spectrum for the center Au (center of Au7 core unit (purple atoms), Figure 1a), surface Au (surface of Au7 core unit), bridge Au and staple Au are displayed in Figure 5. Simulations were also performed for Au25, which has three unique Au sites instead of four for Au28. Stacked plots of the simulated d-DOS are presented for each specific site in Figure 5 with all spectra corrected to their respective calculated Fermi energy (dotted line). Starting from the outermost staple Au sites for both systems, the 5d bands of bound electronic states are comparable in overall width and fine structure features. This narrow and sharp 5d band is understandably the localization of 5d electrons from molecular −S(R)−Au+−S(R)− staple-like motifs on the surface of each Au NC.24,30 The Au28 NC

Figure 5. Site-specific d-DOS simulations of Au25 and Au28 NCs. Each spectrum is corrected to their own calculated Fermi energy (dotted line).

contains a special bridging Au site that has, so far, only been identified in Au NCs with FCC-like cores. In our previous study on the Au36(SR)24 NC (Au36), which contains FCCordered kernel of 28 Au atoms, we found the bridging Au site to resemble the molecule-like electronic character of staple Au sites, despite having short Au−Au bonding with the rest of the FCC-ordered core.18 Once again, the d-DOS simulation of the bridging site for Au28 (compared with both staple and surface sites in Figure 5) has a bandwidth and structure comparable to staple Au sites. A Bader-charge analysis by Knoppe et al. also classified these bridging Au sites as more similar to the electronic structure of staple Au sites.17 Now examining the surface of each Au core structure (icosahedron surface vs Au7 core unit surface), the surface Au 1220


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Figure 6. (a) Au L3-edge FT R-space spectra of Au28 collected at 90 and 300 K. Simulated EXAFS three-shell fit of Au28 at (b) 90 K and (c) 300 K with (d) comparison of temperature-dependent EXAFS fitting results between Au28 and Au36.18

previous work on Au36 in order to examine the effect of composition and size on Au NCs with FCC-like cores. Our initial EXAFS study on Au28 presented in section 3.1 employed two-shells to specifically fit comparable scattering environments with Au25 (Au-SR and short Au−Au core bonding). The local structure of Au28 can be further analyzed by including a second Au−Au scattering shell (Au−Au2) that accounts for longer Au−Au bonding between Au4 units and on the surface of the Au core (see Figure 2). Longer aurophilic bonding between staple Au and surface Au was not detected due to the low average CN (∼0.5). Correlating the total structure and the Au−Au bond distance distributions, our EXAFS analysis can now examine the bonding properties of Au28 with temperature and can be correlated with our previous EXAFS fitting results on Au36 NCs. We first compare the three-shell EXAFS fitting results of Au28 at 90 K (low temperature, LT) (Figure 6a) to check for consistency with the average bond distances expected for each scattering environment. Figure 6b presents the EXAFS spectrum of Au28 at 90 K with simulated three-shell fit and fitting results in Table 2. Consistent with the structure determined by X-ray crystallography, bond distances determined from EXAFS fitting (2.328, 2.73, and 2.99 Å) are within experimental or statistical error from the total structure. After confirming the validity of the three-shell EXAFS fitting procedure, the same fit was conducted for the EXAFS spectrum of Au28 collected at 300 K (room temperature) (Figure 6c) to examine the effect of temperature on the bonding properties of the FCC-like core. Comparing the fitting results of Au28 at 90 and 300 K, there is a striking consistency with our previous study on the larger FCC-ordered core system Au36. A direct comparison of Au28 and Au36 bond distances from EXAFS

sites for Au28 are slightly narrower in width with some change in the fine structure of the 5d band. The greatest difference in the d-DOS is evident for the central Au sites of Au28 where the valence electron density is more localized into a narrower 5d band. A narrower 5d band for a larger Aun(SR)m and Au core is unexpected suggesting the local environment of the tightly bonded Au4 units can modify the electronic properties considerably. From both the unbound (XANES) and bound (d-DOS) electronic perspectives, the unique core structure of Au28 has a prominent influence on the electronic properties; namely, XANES results indicate higher 5d electron occupancy and d-DOS simulations show more localized 5d energy levels for Au28 compared to Au25. This is consistent with its FCCordered core relative, Au36, where the combination of bridging Au sites (which are similar in valence electronic structure to staple Au sites) and Au4 core units lead to more localized 5d electron density when compared to a Au NC of similar size and icosahedral core. 3.2. Effect of Composition on the Bonding Properties of Aun(SR)m NCs with FCC-Ordered Core Structures. In our previous work on the first discovered Au NC with FCC-like core, Au36, XAFS experiments and simulations were used to investigate how the FCC-ordered core, in contrast with an icosahedral core of similar size (Au38(SR)24), influenced the bonding and electronic properties of Au NCs.18 The presence of Au4 units within the FCC-like core offered an explanation for the molecule-like temperature-dependent bonding behavior, which exhibited thermal contraction of the longer Au−Au metallic bonding in the FCC-like core. This section continues our investigation on the role of Au4 units in directing the bonding properties for Au28 with comparisons drawn to our 1221


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The Journal of Physical Chemistry C Table 2. EXAFS fitting results of Au28(SR)20 at various temperatures using three scattering shells temperature (K)

scattering shell

CNa

R (Å)

σ2 (Å2)

ΔE0 (eV)

90 (LT)

Au−S Au−Au1 Au−Au2 Au−S Au−Au1 Au−Au2

1.43 1.71 2.09 1.43 1.71 2.09

2.328(6) 2.73(1) 2.99(3) 2.322(9) 2.73(2) 2.93(3)

0.0028(3) 0.006(1) 0.015(4) 0.0033(4) 0.005(1) 0.011(4)

2(1) 2(1) 2(1) 1(2) 1(2) 1(2)

300 (RT)

properties are not sensitive to cluster size/composition change, differing from the size-dependent behavior commonly observed for nanoscale materials.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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Coordination numbers were fixed according to the expected value determined from the total structure of Au28(SR)20 NCs. a

ACKNOWLEDGMENTS P.Z. acknowledges 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. PNC/SXD facilities at the Advanced Photon Source (APS) (Argonne National Laboratory) 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 CLS, and the APS. Use of the APS and Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC0206CH11357. The authors are thankful for the technical assistance provided by Dr. Robert Gordon at PNC/XSD facilities.

results is shown Figure 6d. The shorter Au−Au bonding of Au4 units again appears to be impervious to the temperature change, remaining around 2.73(2) Å in average distance for Au28. The longer Au−Au bonding, however, is sensitive to a change in temperature displaying thermal contraction from 2.99(3) to 2.93(3) Å. The shift in Au−Au scattering with temperature is also evident when directly comparing the Rspace spectra in Figure 6a. The magnitude of the thermal contraction is slightly less than that found for the Au36 core (1.7% versus 2.1%) but still significant in comparison to the thermal expansion of larger Au nanoparticles and bulk Au.31 We propose the thermal contraction of Au28 occurs through a similar mechanism to Au36 where Au−Au bonds within Au4 units are more rigid than Au−Au bonds on the surface on the NC core and between Au4 units. As a result, these short Au−Au bonds are resistant to temperature-induced structural changes and longer Au−Au bonds contract with temperature, possibly to conserve stability of the Au NC by counteracting the high surface energy (i.e., large curvature and many unique Au sites) and/or thermal vibrations of the thiolate ligands. Specifically, there are three potential locations through which Au−Au contraction could occur: (i) between Au7 (or Au4) core units, (ii) bridging Au atoms, and/or (iii) staple Au atoms. Further experiments and theoretical studies should be pursued for FCC-ordered Au NCs in order to determine the structural changes that occur from a site-specific perspective with response to varied temperature conditions. Finally, temperature-dependent EXAFS results support the important role of Au4 in directing not only the electronic properties, but also the bonding properties of Aun(SR)m NCs with a FCC-ordered core.

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REFERENCES

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CONCLUSION In summary, X-ray spectroscopy experiments and ab initio calculations have been utilized to examine the bonding and electronic properties of Au28(SR)20 NCs, with FCC-ordered core structure, in comparison with its icosahedral counterpart Au25(SR)18 (similar composition) and its FCC-ordered core relative Au36(SR)24 (considerably different composition). It was found that the Au4 unit within Au28(SR)20 controls its electronic and bonding properties in these studies, which consistently accounts for its differing electronic behavior from Au25(SR)18 and identical temperature-dependent bonding properties with Au36(SR)24. These findings are useful toward better understanding the structure−property relations of gold− thiolate NCs with regards to the cluster composition and core geometry perspectives. In particular, our results imply that Aun(SR)m NCs with FCC-ordered core structures may represent a special category of nanomaterials whose bonding 1222


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Role of Au4 Units on the Electronic and Bonding

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