Sensitivity of Structural and Electronic Properties of Gold–Thiolate

Nov 1, 2012 - With the recent successful synthesis of an increasing number of atomically precise thiolate–gold nanoclusters (NCs), there are growing...
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Sensitivity of Structural and Electronic Properties of Gold−Thiolate Nanoclusters to the Atomic Composition: A Comparative X‑ray Study of Au19(SR)13 and Au25(SR)18 Daniel M. Chevrier,† Mark A. MacDonald,† Amares Chatt, and Peng Zhang* Department of Chemistry, Dalhousie University, Halifax, NS, Canada, B3H 4R2

Zhikun Wu and Rongchao Jin Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States 15213 ABSTRACT: With the recent successful synthesis of an increasing number of atomically precise thiolate−gold nanoclusters (NCs), there are growing demands for a better understanding of their atomic structures and structure−property relationships in response to composition change. Herein, we present X-ray experimental results on structural and electronic properties of a recently synthesized gold−thiolate nanocluster, Au19(SR)13, in comparison with the structurally well-known Au25(SR)18 reference. The atomic structure of Au19(SR)13 was first probed with extended X-ray absorption fine structure (EXAFS). A multishell EXAFS fitting method (consisting of one Au−S and three Au−Au shells) allowed for a detailed structural comparison of the two nanoclusters, which revealed a sizable increase of the bond distances between the surface−surface and surface−staple Au atoms upon the decrease of Au atoms in the nanocluster. X-ray photoemission experiments and ab initio calculations further demonstrated the sensitive change of electronic properties of the nanoclusters when the cluster composition was varied, that is, the Au19(SR)13 nanocluster has a differently shaped and positively shifted Au 5d band relative to Au25(SR)18. These experimental findings highlight the sensitivity of the local structure and electronic properties of thiolate-gold nanoclusters to even a small change of the metal core from Au13 to Au11.

1. INTRODUCTION Understanding of thiolate-stabilized gold nanocluster (Au NC) structure has significantly improved over the past few years, owing to the growing contribution in this field from many different directions of research.1 Improving and fine-tuning experimental conditions for synthesizing Au NCs has allowed researchers to closely control the formation of many atomically precise clusters.2−8 The isolation and purification of a series of precise Au NCs has enabled fundamental studies that probe the unique electronic, optical, and physical properties.9−12 Uncovering the total structures of [Au 1 0 2 (SR) 4 4 ], 1 3 [Au38(SR)24],14 and [Au25(SR)18]11,15,16 was a significant turning point when it comes to understanding the core− surface interface for Au NCs. The presence of surface staple or semiring units was an important insight into the distinctive surface environment of thiol-capped Au NCs.17,18 Being able to correlate the local structure of Au NCs with chemical and physical properties has opened up new opportunities in theoretical calculations as well as optical and catalysis applications.19−21 On the topic of uncovering the structure of newly isolated clusters, new approaches in computational chemistry have been able to aid the understanding of structural evolution and determination of many Au NCs.22−24 More recently, Jiang was able to predict the staple motif arrangement on the surface of © 2012 American Chemical Society

Au25(SR)18 and Au38(SR)24 with a selection process termed “staple fitness”.25 The only stipulation is one must be able to determine favorable core structures before you begin the “staple fitness” procedure. Jiang was able to use this method to predict two structural scenarios for the recently isolated Au19(SC2H4Ph)13 NC by Wu et al.8 Experimental determination of the local structure of Au NCs without available crystal structures can be a difficult and exhaustive process if the right characterization techniques are not available to the researcher. Alternatively, the X-ray spectroscopy approach, in association with ab initio calculations when needed, has been found useful in uncovering the unique structural environment of many Au NCs.26−31 This work aims to use X-ray photoemission spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) analyses to experimentally probe the structure and properties of the Au19(SC2H4Ph)13 NC in order to test the recently predicted structural model from Jiang,25 as well as unveil the structureelectronic property relationships from a small change in NC composition. Received: September 20, 2012 Revised: October 29, 2012 Published: November 1, 2012 25137

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Figure 1. (a) Experimental FT-EXAFS of Au19(SR)13 (black line) with simulated two-shell fit (red dotted line). Structural compositions for scenario 1 (b) and scenario 2 (c).

2. EXPERIMENTAL: MATERIALS AND METHODS The synthesis and isolation of anionic [Au 25 (SR) 18 ] − (Au25(SR)18 is used throughout this work for simplicity) and neutral Au19(SR)13 NCs have been reported earlier in the literature.7,8,32 See references for detailed synthetic procedures. Au L3-edge XAFS measurements of Au19(SR)13 and Au25(SR)18 NCs were collected in transmission mode at the Hard X-ray MicroAnalysis (HXMA) beamline of the Canadian Light Source (Saskatoon, SK, Canada) and the PNC/XSD beamline of the Advanced Photon Source (Argonne National Laboratory, Argonne, IL). Powdered samples were sealed in a plastic sample vial (Au19(SR)13) or packed in a kapton pouch (Au25(SR)18) and measured using a Si(111) monochromator and a Rhodium mirror. Au foil was measured simultaneously with each scan (transmission mode) for the edge energy calibration. XPS measurements of the valence band were conducted at the Spherical Grating Monochromator (SGM) beamline of the Canadian Light Source (CLS). All samples were prepared by depositing a thin layer of NCs onto a Si(111) wafer. Samples were then loaded into the SGM vacuum chamber, operated using a 700 eV excitation energy, a base pressure of 7.5 × 10−10 Torr, and a temperature of 85 K. Binding energy calibration was conducted following the standard procedure.28 XAFS data analysis was performed with the WinXAS 3.1 software package using a similar data reduction procedure reported in earlier work.30,31 Theoretical phase and scattering amplitudes used for fitting experimental data were obtained using the FEFF8.2 computational package. The model used to obtain scattering paths was the crystal structure of [Au 25 (SR) 18 ] −. 15 FT-EXAFS fitting of Au 19 (SR) 13 was performed following two separate procedures: two-shell fitting (1 Au−S and 1 Au−Au) and multishell fitting (1 Au−S and 3 Au−Au). For two-shell fitting, a short k-range (2.5−10 Å−1) was found useful to minimize the contribution from long distance Au−Au bonds (e.g., aurophilic bonding), thus providing more reliable information on the fitting results of the first-shell Au−Au bonding. Multishell fitting required a longer k-range of 2.5−11.5 Å−1 to incorporate the three Au−Au bonding environments. Theoretical coordination numbers were fixed at calculated values from model clusters (see section 3.1). The amplitude reduction factor (S02) was determined by fitting Au foil with a fixed CN of 12. A value of 0.9 was obtained and used in all FT-EXAFS fitting procedures. Error calculations were obtained using the double χ2 method.33 Projected angular momentum density of states (l-DOS) simulations of the Au19(SR)13 and Au25(SR)18 NCs were performed with the FEFF8.2 computational program. The

coordinates of Au 19 (SR) 13 are borrowed from recent literature,25 whereas coordinates for Au25(SR)18 are from its crystal structure.11 For our simulation purposes, the capping thiolate molecule is simplified with a methyl group instead of a phenylethyl group.

3. RESULTS AND DISCUSSION 3.1. Au Core Composition of Au19(SR)13. From recent theoretical predictions of the new Au19(SR)13 NC by Jiang,25 the only two possible Au core compositions are Au11 or Au12 with a combination of single and double staple motifs on the surface (shown in Figure 1b and c). Scenario 1 consists of two single staples and three double staples on the Au11 core. For the 13 sulfur atoms in the staple motif, only 10 (4 from the 2 single staples and 6 from the 3 double staples) of them directly bind to the Au core surface, resulting in a Au11 core model with 1 Au atom in the center and 10 Au on the surface. Scenario two consists of five single staples and one double staple on the Au12 core. Using the same method as the Au11 calculation, there should be 12 sulfur atoms directly binding to the Au12 surface, giving a Au12 core model with all the 12 Au atoms on the surface and no center Au inside. From these structural predictions, theoretical coordination numbers for Au19(SR)13 can be compared to our EXAFS fitting results. The theoretical calculation for scenario 1 is as follows: the Au−S CN is [(10 surface Au atoms × 1 bond to S) + (8 staple Au atoms × 2 bonds to S)]/19 total Au atoms = 1.37. For the first-shell Au−Au core CN, [(10 surface Au atoms × 1 bond to center Au) + (1 center Au atom × 10 bonds to surface Au)]/19 total Au atoms = 1.05. Now we turn to scenario 2: Au−S CN is [(12 surface Au atoms × 1 bond to S) + (7 staple Au atoms × 2 bonds to S)]/19 total Au atoms = 1.37, which is the same for scenario 1. For the Au−Au interaction in Au12, without a center Au the first Au−Au shell should come from the bonding between the nearest surface Au atoms. From theoretical results, the number of nearest neighboring atoms for stable Au12 structures should be in the range of 4−5 (i.e., Ih, Oh, D5h symmetry),29,34 yielding a theoretical first-shell Au−Au CN = (12 surface Au atoms × 4−5 bonds to surface Au)/19 = 2.5− 3.2. From these calculations, it is clear that the calculated firstshell Au−Au CN is determined by the Au core composition and the number of surface Au atoms in the core. Even when the Au core composition varies by one atom, there should be significant change to the first-shell Au−Au CN. With the two proposed Au core compositions for Au19(SR)13, we employed a standard two-shell fitting procedure (Au−thiolate and Au−Au core bonding) to determine which structural scenario was most prevalent in FT-EXAFS fitting 25138

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structure (XANES) is shown in Figure 2a with all spectra normalized and overlapped for close comparison. There is a small difference in white-line intensity between NC samples. Au19(SR)13 and Au25(SR)18 both have higher white-line intensities than the bulk, indicating more d-band vacancies because of small core sizes and molecular-like interactions (Au(I)−thiolate) on the surface. Near-edge features after the absorption edge do not change significantly for both measured NCs, partly from a similar coordination environment on the NC surface. k-Space oscillations of the measured samples are presented in Figure 2b with a k3-weighting for each spectrum. Oscillations from Au25(SR)18 and Au19(SR)13 NCs are similar in frequency and in shift. Au foil k-space illustrates fcc oscillation patterns which are absent in Au NC samples due to their small core sizes. A k-range of 2.5−11.5 Å−1 was used for the FT-EXAFS of each sample as seen in Figure 2c. From a qualitative look at the FT-EXAFS spectra, Au19(SR)13 has scattering patterns similar to Au25(SR)18 with a large contribution from Au−S bonding and a multishell Au−Au environment, as seen in our previous work from the EXAFS analysis of Au25(SR)18.31 To further probe the structure of Au19(SR)13, we continued with a Au−Au multishell EXAFS fitting procedure as seen in Figure 3c. A detailed procedure for fitting the multishell bonding environment of Au25(SR)18 has been reported recently by our group.31 The fitting results from this previous work for Au25(SR)18 are shown in Table 2 for a close comparison of Au− Au bonding with Au19(SR)13. We have also considered a comparison with another structurally well-known reference, Au38(SR)24. However, the fused icosahedral core structure of Au38(SR)24 makes its multishell EXAFS analysis very different from that of Au19(SR)13 and Au25(SR)18, in that Au38(SR)24 has three special Au atoms located at the interface of the two icosahedral cores.14 Such interfacial Au atoms are different from the regular center and surface Au atoms and are absent in Au19(SR)13 and Au25(SR)18. Therefore, a close comparison with all three Au NCs using a similar multishell EXAFS analysis procedure would be impractical. The k-range was extended for multishell FT-EXAFS fitting to increase the number of total independent parameters to account for a good fit of all four shells. Fitting results are presented in Table 2, where Au− thiolate (Au−S), Au−Au center−surface bonding (Au−Au1), Au−Au surface−surface bonding (Au−Au2), and long-range aurophilic bonding (Au−Au3) make up the multishell fitting region. Closely comparing the bond lengths for each system in Figure 3d, there is little change in Au−S and center−surface Au−Au bonding. However, we observe that for the other two Au−Au shells, Au19(SR)13 has increasingly longer bond lengths

results (shown in Figure 1). Since the calculated Au−S CN is identical for both scenarios, the resulting CN value for Au−Au core bonding will be the deciding factor from the standard twoshell fitting results. Figure 1b and c illustrates the two structural scenarios for Au19(SR)13. The experimental FT-EXAFS of Au19(SR)13 with simulated fit is shown in Figure 1a with results shown in Table 1. From standard two-shell fitting, we obtain a Table 1. Preliminary Fitting Results from Two-Shell Fitting of Au19(SR)13a Au19(SR)13 shells

CN

scn 1

scn 2

R/(Å)

σ2/(Å2)

E0/(eV)

Au−S Au−Au

1.3(1) 1.0(5)

1.37 1.05

1.37 2.5−3.2

2.32(1) 2.84(6)

0.0004(2) 0.012(9)

−1(1) −1(1)

a

Theoretical coordination numbers are shown in bold for each scenario.

Au−S CN of 1.3(1) corresponding with both scenarios. The Au−Au CN, however, is reported to be 1.0(5) with a bond length of 2.84(6) Å. This low CN value for Au−Au indicates the presence of center−surface bonding and well supports the Au11 core model predicted in scenario 1 in Figure 1b. In contrast, the theoretical CN value (2.5−3.2) of first-shell Au− Au for scenario 2 (i.e., Au12) is significantly different from the experimental EXAFS results. Scenario 1 appears to be the best candidate for Au19(SR)13. As can be seen from this standard two-shell fit, the difference in core composition for Au19(SR)13 can be determined from our FT-EXAFS results, therefore, demonstrating the usefulness of EXAFS CN analysis of the Au core structure for thiolateprotected NCs. It should be noted that in Jiang’s theoretical work, a total of 11 isomers were predicted for the Au11 scenario.25 However, most of these isomers have the same number of surface (10) and center (1) Au atoms, thus they are not distinguishable from regular EXAFS analysis, which is a limitation of EXAFS-based structural analysis on these NCs. To understand more on the Au−Au bonding structure of Au19(SR)13, a multishell fitting procedure is employed in the next section with in-depth structural comparisons with the wellknown and previously studied Au25(SR)18 NC.31 The best candidate structure for Au19(SR)13, as determined by Jiang (Figure 3a), is a defective icosahedral core (Au11) with three double and two single staples on the surface. This model will be further explored in our multishell FT-EXAFS fitting and discussion of electronic properties. 3.2. Site-Specific Structural Analysis of Au19(SR)13. Au L3-edge XAFS was collected for Au foil, Au25(SR)18, and Au19(SR)13 (R = −C2H4Ph). X-ray absorption of near-edge

Figure 2. (a) XANES comparison of measured Au NCs and Au foil reference at the Au L3-edge. (b) k-Space oscillations of measured Au NCs and Au foil for reference. Spectra are vertically adjusted for comparison. (c) FT-EXAFS of Au NCs and Au foil using a k-range of 2.5−11.5 Å−1. 25139

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Figure 3. Comparison of Au19(SR)13 (a) and Au25(SR)18 (b) total structure and core structure. Multishell EXAFS fitting of Au19(SR)13 (c). Bond distances are plotted in d for close comparison between NCs, Au25(SR)18 (red) and Au19(SR)13 (black striped) .

Table 2. EXAFS Fitting Results from the Multishell Fitting Procedure of Au19(SR)13 and Au25(SR)18ba Au19(SR)13

Au25(SR)18

shells

CN

R/(Å)

σ2/(Å2)

E0/(eV)

CN

R/(Å)

Au−S Au−Au1 Au−Au2 Au−Au3

1.37 1.05 1.89 1.89

2.33(1) 2.83(1) 3.04(2) 3.37(3)

0.0006(1) 0.004(1) 0.01(2) 0.03(1)

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

1.44b 1.44b 1.92b 2.88b

2.32b 2.82b 2.97b 3.14b

a Coordination numbers are fixed using the theoretical values calculated from cluster models (shown in bold). E0 shift for Au−Au shells are correlated. bEXAFS parameters for Au25(SR)18 were borrowed from the literature.30

from surface−surface to aurophilic interactions. Significantly longer Au−Au surface−surface and aurophilic bonding could be attributed to the defective icosahedral core (see Figure 1b) where the structure is more relaxed and disordered than the highly symmetric icosahedral core in Au25(SR)18 NCs. From our multishell fitting, we can further exclude scenario 2 as a possible candidate since our fitting shows distinct center− surface and surface−surface bonding. The mixed variation of single and double staples could also promote longer aurophilic bond lengths due to vacancies on the cluster surface affecting the capping interactions from staple units. Indeed, the determined Au19(SR)13 NC structure (scenario 1, Figure 3a) is not as symmetric as its Au25(SR)18 relative, considering the defective icosahedral core and variety of surface staple units. This lack of structural symmetry could lead to variable sitespecific electronic properties in Au19(SR)13. Therefore, we turned to study the electronic properties in the following section. 3.3. Sensitivity of the Electronic Properties. Traditionally, XPS/UPS valence band data have been useful to understand the size-dependent evolution of electronic properties of transition metal nanocrystals when investigating their structure−property relationship. Interestingly, the two nanocluster systems studied here, Au19(SR)13 and Au25(SR)18, have nearly the same size (∼1 nm), but different composition, thus providing opportunities to evaluate the sensitivity of the composition effect on the electronic properties of thiolate−gold nanoclusters. In Figure 4, the valence band data are presented for Au19(SR)13 and Au25(SR)18. Elevated intensity at ∼10 eV is due to background in both samples. A ∼0.2 eV shift in higher energy is visible for d-band centroid of Au19(SR)13. This shift in higher energy is also consistent with our XANES results, where Au19(SR)13 NCs have slightly higher white-line intensity (see Figure 1). Negishi et al.35 reported earlier that an “initial-state effect” is the main factor determining the relative band energy

Figure 4. Experimental XPS valence band of Au25(SR)18 and Au19(SR)13 with lines of best fit.

shift for different sizes of Au-thiolate nanoclusters. A smaller Au core will shift the emission band to higher energy. In our case, the smaller Au11 core could cause this slight energy shift through this “initial-state effect”. Another possible explanation is the anionic [Au25(SR)18]− could be shifted to lower energy due to the negative charge resting on the cluster. Interestingly, the valence band profile of Au19(SR)13 appears to be clearly different from that of Au25(SR)18. At a first glance, a sizable d-band narrowing effect is apparent for Au19(SR)13 NCs. The smaller core of Au19(SR)13 (Au11 vs Au13) could explain the narrowing effect as it has been seen in previous gas phase studies of metal clusters.36 The longer Au−Au bonding in Au19(SR)13 NCs could also contribute to this band narrowing effect. The same narrowing was also observed from d-DOS simulations of the Au38SR24 NC.30 A closer inspection of the X-ray data further reveals the difference of the spectral shape, that is, the Au25(SR)18 NCs show an extra feature in the region of 2−3 eV, which is nearly absent in the spectrum of Au19(SR)13. Smalley et al.36 has also observed a similar feature (2−3 eV range) for gaseous phase clusters where the intensity of this feature is sensitive to the composition of the clusters such as Au6−, Au8−, and Au20−. A small composition change for these gas phase clusters can significantly change the valence band structure. Our findings point out that this feature is also sensitive to the compositional change of gold−thiolate nanoclusters, similar to these previous observations.36,37 25140

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two examined NCs to the atomic composition revealed by XPS and ab initio calculations. These findings highlight the sensitivity of the local structure and electronic behavior of thiolate−gold nanoclusters to even a small change of the metal core from Au13 to Au11 and point out the significance of the “composition effect” in the regime of small thiolate−gold nanoclusters instead of the “nanosize effect” widely used for regular nanoscale materials.

To further elucidate the electronic properties of Au19(SR)13 NC from a site-specific perspective, ab initio calculations were performed to examine the Au 5d projected density of states (dDOS) of the nanocluster from each representative atomic site. Figure 5a displays the calculated d-DOS from the breakdown of

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax and phone: (902) 494-3323. Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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Figure 5. (a) FEFF simulated d-DOS of the Au19(SR)13 NC from a site-specific perspective. d-DOS simulations comparing identical sites on both Au19(SR)13 (yellow) and Au25(SR)18 (red) NCs for (b) double staple Au, (c) surface Au bonded to double staple, and (d) center Au. All spectra are calibrated to their own calculated Fermi energy.

ACKNOWLEDGMENTS 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. Ning Chen from the CLS HXMA beamline and Dr. Tom Regier from the SGM beamline is also acknowledged. PNC/XSD facilities at the Advanced Photon Source and research at these facilities are supported by the U.S. Department of Energy (DOE)−Basic Energy Sciences, a Major Resources Support grant from NSERC, the University of Washington, Simon Fraser University, and the Advanced Photon Source. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. DOE Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02- 06CH11357. Technical assistance from Dr. Robert Gordon at the PNC/XSD facilities is also acknowledged.

the Au19(SR)13 cluster into five unique Au sites: Au in double staple, Au in single staple, surface Au bonded to double staple, surface Au bonded to single staple, and the center Au. A dramatic change in d-DOS is seen in our calculations when comparing staple sites to other sites. The narrow d-DOS for structure for surface staple sites reflects the expected molecularlike behavior of the staple Au in the NCs. Interestingly, the surface site that bonds to double staples shows a narrower band than surface sites bonded to single staples. This observation could be reasoned by considering the arrangement of surface Au atoms on the cluster. From the predicted model, single staples are connected to surface sites adjacent to vacancies on the defective icosahedral core. From this d-DOS calculations, the Au19(SR)13 NC appears to have electronic properties that significantly change from site to site in the cluster, which could offer potential opportunities for catalytic applications. Comparative results of the d-DOS shape of the double-staple Au, surface Au (binding to the double-staple), and the center Au site from Au19(SR)13 and Au25(SR)18 are provided in Figure 5b, c, and d, respectively. Simulated d-DOS of Au25(SR)18 is reproduced from previous work.18 It is clearly seen that the dDOS of the surface and center Au sites from Au25(SR)18 are broadened compared to the corresponding Au sites from Au19(SR)13. This observation can be interpreted by the defective Au11 core structure in Au19(SR)13, which has less Au−Au bonding interactions than the Au13 core in Au25(SR)18. Therefore, the d-DOS calculations consistently support the experimental findings on the XPS valence bands (Figure 4) from a theoretical perspective.

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REFERENCES

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CONCLUSION In conclusion, we have presented a series of findings on the unique structural and electronic properties of the thiolate−gold nanocluster, Au19(SR)13, based on X-ray spectroscopy experiments and ab initio calculations. Important experimental results include (i) supporting evidence of the structural model of Au19(SR)13 predicted by the “staple fitness” method, (ii) increased bond length for surface−surface and aurophilic Au− Au bonds, and (iii) sensitivity of Au 5d band structure of the 25141

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp309283y | J. Phys. Chem. C 2012, 116, 25137−25142