Article pubs.acs.org/JPCC
Unraveling the Atomic Structures of the Au68(SR)34 Nanoparticles Wen Wu Xu and Yi Gao* Division of Interfacial Water and Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China S Supporting Information *
ABSTRACT: The atomic structure prediction of thiolate-protected gold nanoparticle (RS-AuNP) Au68(SH)34 is performed based on the “divide and protect” concept and experimental studies on 14 kDa RS-AuNPs. Four low-lying energy isomers, Iso1− Iso4, were identified by the density-functional theory. Our results indicate the most stable structure Iso2 adopts the C2v Au50 core with Marks-decahedral (m-Dh) Au18 inner core. The calculated HOMO−LUMO gap is 0.74 eV, which is very close to that of Au67(SR)352−. Further analysis suggests the 14 kDa RS-AuNPs might be not only the turn point between the fused core structure and the compact core structure but also the turn point between the one-shell inner core structure and the multishell inner core structure. The threshold number of Au atoms in bulk-like RS-AuNPs is evaluated as ∼263 based on the linear fitting of the HOMO−LUMO gaps of various RS-AuNPs including Au68(SR)34. The research on the medium-sized Au68(SR)34 establishes a bridge between smaller and larger RS-AuNPs, which is beneficial for us to better understand the structures of the RS-AuNPs.
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INTRODUCTION The thiolate-protected gold nanoparticles (RS-AuNPs)1−9 have attracted significant research interest in the past decade due to the breakthroughs in the determination of the atomic structure of Au102(p-MBA)44 (p-MBA = p-mercaptobenzoic acid, SC7O2H5)10 and Au25(SCH2CH2Ph)18−11−13 from X-ray crystallography. Subsequently, the structures of several RSAuNPs such as Au133(SPh-t(p)Bu)52,14,15 Au38(SCH2CH2Ph)24,16 Au36(SPh-tBu)24,17 Au30S(S-tBu)18,18 Au28(SPh-tBu)20,19 Au24±1(SAdm)16,20 Au20(SPh-tBu)16,21 and Au18(SC6H11)14,22 etc. have been successfully characterized experimentally. Besides the experimental crystallization, the structures of RS-AuNPs such as Au20(SR)16,23 Au24(SR)20,24 Au38(SR)24,25 Au44(SR)28,26 Au68(SR)32,27Au144(SR)60,28 and etc. have been predicted theoretically. Although much progress has been achieved in this area, there are few investigations on the larger-sized RS-AuNPs (ie. Au130(SR)50,29 Au137(SR)56,30 etc.) and medium-sized ones with core mass (Au and S atoms) of 14 kDa due to the difficulty of purification and crystallization. As medium-sized nanoclusters, the knowledge of 14 kDa species is beneficial to understand the size evolution of RSAuNPs31 and further to establish a bridge between smaller RSAuNPs such as Au25(SR)18−11−13 and Au38(SR)2416,25 and larger ones such as Au102(SR)44,10 Au133(SR)52,14,15 and Au144(SR)6028 (theoretical structure). Earlier laser desorption ionization (LDI) and X-ray scattering studies by Whetten and co-workers suggested that the 14 kDa RS-AuNPs with a Marksdecahedral (m-Dh) motif have ∼70 core atoms (Au).32−34 Recently, Dass considered that the broad LDI peaks with a fwhm (full width at half-maximum) of 0.3 kDa resulted in much difficulty to identify the number of core atoms precisely.35 © XXXX American Chemical Society
Consequently, via a powerful matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) technique, Dass successfully assigned the 14 kDa species with the molecular formula Au68(SR)34.35 Besides of the intense peak of Au68(SR)34 in MS, some other lower peaks of Au68(SR)33, Au68(SR)32, Au68(SR)30, and Au64(SR)30 have also been observed. Further investigation by Dass and coworkers shows that the MS peaks of larger nanoclusters such as Au68 and Au102 decrease in intensity after 6 h.30 These phenomena demonstrate that it is formidable to purify the Au68(SR)34 nanocluster, which has greatly hindered an in-depth study and understanding of the characterization of the Au68(SR)34 nanocluster. Fortunately, with the development of the experimental technique, recently significant progress has been made in Au67(SR)352−,36 Au64(SR)32,37 and Au68(SR)32,38 three kinds of 14 kDa RS-AuNPs. The high-yield synthesis and isolation of the pure Au67(SR)352− nanocluster, as well as the theoretical structure prediction based on first-principles, have been presented by Nimmala and co-workers.36 Zeng et al. have obtained a medium-sized gold nanocluster of atomic precision formulated as Au64(S-c-C6H11)32 through direct synthesis.37 Azubel et al. have presented the structure determination of Au68(3-MBA)32 at atomic resolution by a powerful aberrationcorrected transmission electron microscopy (AC-TEM) technique,38 which no longer requires a single crystal of RSAuNPs. In this paper, we will focus on the structure determination of Au68(SR)34. Since having the same shell-closing electron count Received: April 1, 2015 Revised: May 26, 2015
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DOI: 10.1021/acs.jpcc.5b03128 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C 3439 and similar molecular weight between Au67(SR)352− and Au68(SR)34, the joined experimental−theoretical investigations on the Au67(SR)352− nanocluster35 provide us a slice of clues to investigate the Au68(SR)34 nanocluster on the theoretical side. We construct a series of structures of the Au 68 (SR) 34 nanocluster following the “divide and protect” (D&P) concept.40 The basic idea is to divide a RS-AuNP into groups such as [Au]a+a′[Au(SR)2]b and [Au2(SR)3]c, where a, a′, b, and c are integers. [Au]a represents the inner Au core, and the [Au]a+a′ core satisfies the condition that the number of core “surface” Au atoms (a′) equals the sum of end points of the exterior motifs (2b + 2c); that is, each core surface Au atom is protected by one end point of the staple motif. The predicted formulations of Au 6 8 (SR) 3 4 are (i) [Au] 1 7 + 3 4 [Au(SR)2]17[Au2(SR)3]0, (ii) [Au]18+32[Au(SR)2]14[Au2(SR)3]2, (iii) [Au]19+30[Au(SR)2]11[Au2(SR)3]4, (iv) [Au]20+28[Au(SR)2]8[Au2(SR)3]6, (v) [Au]21+26[Au(SR)2]5[Au2(SR)3]8, and (vi) [Au]22+24[Au(SR)2]2[Au2(SR)3]10, respectively. Since having higher relative energy and lower symmetry, the structures of Au68(SR)34 with formulations (v) and (vi), which have been presented in Figures S1 and S2 (Supporting Information) (Iso5 and Iso6), are not discussed in detail here. Following the predicted formulations (i)−(iv), the four lowlying energy isomers Iso1−Iso4 with high-symmetry [Au]a inner cores and [Au]a+a′ cores were identified and optimized using the DFT code as implemented in the Dmol3 (version 7.0) software package.41,42 The generalized gradient approximation with the Perdew−Burke−Ernzerhof (PBE)43 functional and the double numeric polarized (DNP) basis set were employed. The theoretical powder X-ray diffraction (XRD) curve is calculated using the Debye formula I (s ) =
∑∑ i
j≠i
⎛ Bs 2 ⎞ sin(2πdij) cos θ exp⎜ − ⎟f f (1 + α cos(2θ )) ⎝ 2 ⎠ i j 2πdij
where s is the diffraction vector length and θ is the scattering angle, satisfying s = 2 sin θ/λ. The λ and α are determined by the experimental setup and are set to be 0.1051967 nm and 1.01, respectively. B is the damping factor, which reflects thermal vibrations, and is set to be 0.03 nm 2 . The corresponding atomic numbers are used for the scattering factors f i. dij is the distance between atoms i and j. The atomic distance dij used in the calculation is taken from the optimized structure of clusters.
Figure 1. Two orthogonal views of the DFT-optimized atomic structure of the neutral Au68(SR)34 nanocluster. The [Au]a (a = 17− 20) inner core (column (i)), [Au]a+a′ (a + a′ = 51−48) core (column (ii)), and the complete structures (column (iii)) of neutral Iso1−Iso4 are presented. The Au atoms in the inner core, shell, and staples (−RS−Au−RS− or −RS−Au−RS−Au−RS−) are in red, olive, and wine, respectively, and S atoms are in yellow. The H atoms (R) are not shown.
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core with m-Dh symmetry. The energies of Iso1 with a hollow m-Dh Au17 inner core and Iso4 with a m-Dh Au48 core are 1.16 and 2.74 eV higher than that of Iso2, respectively, as shown in Table 1, indicating that the structures of Iso1 and Iso4 are not energetically favorable. Neutral Iso2 with the m-Dh Au18 inner core and C2v Au50 core is the most stable among these four isomers. Furthermore, we examine the stability of different charge states of Iso1−Iso6 for the Au68(SR)34 nanocluster, which is shown in Figure S2 (Supporting Information). We find that Iso2 with different charge states is the most stable. Through the comparison of the HOMO−LUMO (H−L) gaps of Iso1−Iso6 with different charge states, as shown in Table S1 (Supporting Information), the neutral Iso2 has the largest gap (0.74 eV), which is consistent with that of Au67(SR)352−. The above results manifest that neutral Iso2 is the optimal structure for Au68(SR)34. The optical absorption spectra of four neutral isomers, Iso1− Iso4, were computed by the time-dependent DFT (TDDFT) method, as shown in Figure 2. We find that the shapes of the
RESULTS AND DISCUSSION Two orthogonal views of the [Au]a (a = 17−20) inner core, [Au]a+a′ (a + a′ = 51−48) core, and the complete structures of neutral Iso1−Iso4 for the Au68(SR)34 nanocluster are presented in Figure 1. The symmetry of the [Au]a inner core and [Au]a+a′ core, the relative energies, and H−L gap of Iso1− Iso4 are listed in Table 1. The characterization of the theoretical structure of Au67(SR)352− is also included for comparison.30 From Figure 1 and Table 1, we find that the Iso1 has the same hollow m-Dh Au17 inner core as Au67(SR)352−, and Iso2 has a compact m-Dh Au18 inner core, which has one more central atom than the hollow Au17 inner core. With the increase of the number of atoms of the inner core, the symmetry becomes lower, such as Au19 of Iso3 with C2v and Au20 of Iso4 with C2. With the addition of a [Au]a′ atom shell on the [Au]a inner core, it forms a [Au]a+a′ core. It can be seen from Table 1 that the Au51−49 cores of Iso1−Iso3 are C2v structure, and both Iso4 and Au67(SR)352− have the same Au48 B
DOI: 10.1021/acs.jpcc.5b03128 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Table 1. Symmetry of the [Au]a (a = 17−20) Inner Core and [Au]a+a′ (a + a′ = 51−48) Core, the Relative Energies, and HOMO−LUMO (H−L) Gaps of Neutral Iso1−Iso4 for the Au68(SR)34 Nanoclustera isomers Au67(SR)35 Iso1 Iso2 Iso3 Iso4 a
2−
symmetry of [Au]a
symmetry of [Au]a+a′
relative energies/eV
H−L gap/eV
Au17/m-Dh Au17/m-Dh Au18/m-Dh Au19/C2v Au20/C2
Au48/m-Dh Au51/C2v Au50/C2v Au49/C2v Au48/m-Dh
1.16 0 0.82 2.74
0.75 0.64 0.74 0.70 0.63
The characterization of the theoretical structure of Au67(SR)352− is also included for comparison.36
Figure 2. Theoretical optical absorption spectra of neutral Iso1−Iso4 of Au68(SH)34. The black curve denotes TDDFT-computed spectra obtained from the individual optical transitions (red vertical lines).
simulated absorption curves of Iso1−Iso4 are different, suggesting that the optical absorption spectrum of Au68(SR)34 is very sensitive to the core structure [Au]a+a′ (a + a′ = 48−51), and can be used to characterize the structures of different isomers. Additionally, the simulated XRD curves of neutral Iso1−Iso4 are shown in Figure 3. Overall, the main peak locations of Iso1−Iso4 are consistent with each other, although a slight shift can be observed.
Figure 4. Presentation of A = (n − m)/(m + n) with the increase of the size of RS-AuNPs (red solid circle), where m and n denote the number of short [−RS−Au−RS−] staple motifs and long staple motifs except [−RS−Au−RS−], respectively. The horizontal axis denotes the normalized molecular weight of RS-AuNPs, where R = SCH2CH2Ph. (a) The structures with fused and compact [Au]a+a′ cores are in blue and wine, respectively. (b) The structures with one-shell and multishell [Au]a inner cores are in blue and wine, respectively. The 14 kDa RS-AuNPs (Au67(SR)352− and Au68(SR)34) are emphasized in an olive square frame.
+ n) with the increase of the size of RS-AuNPs, where m and n denote the number of short [−RS−Au−RS−] staple motifs and long staple motifs except [−RS−Au−RS−], respectively. We can clearly see from Figure 4 that the value of A changes from positive to negative with the increase of the size of RS-AuNPs except Au18(SR)14, which includes one Au4(SR)5 tetramer, one Au2(SR)3 dimer, and three Au(SR)2 monomers. There is a turn point at the 14 kDa RS-AuNPs (Au67(SR)352− and Au68(SR)34). The long staple motifs dominate when the size of RS-AuNPs is smaller than 14 kDa ones; on the contrary, the short [−RS− Au−RS−] staple motifs become more prevalent. In the meanwhile, the structures of [Au]a+a′ cores give us more information on the structural evolution of RS-AuNPs. The Au cores of Au 67 (SR) 35 2− , Au 68 (SR) 34 , Au 102 (SR) 44 , and Au 14 4 (SR) 60 are compact structures, while, except Au25(SR)18−, the others with a smaller size than 14 kDa RSAuNPs are face- or edge-fused structures, as shown in Figure 3(a). In addition, the change from one-shell to multishell Au inner cores in RS-AuNPs is also given in Figure 3(b), where the Au68(SR)34, Au102(SR)44, and Au144(SR)68 have multishell [Au]a inner cores and the other smaller RS-AuNPs have oneshell [Au]a inner cores. The above analyses on the structures of
Figure 3. Simulated X-ray diffraction (XRD) curves of neutral Iso1− Iso4 of the Au68(SH)32 cluster. The calculated XRD curves of Iso1− Iso4 are displayed in red, blue, wine, and olive, respectively.
Having the optimal structure of neutral Iso2 among these four isomers, we pour our attention into the various RS-AuNPs that have been resolved by experiments or predicted by theory, including Au10(SR)10,44 Au18(SR)14, Au20(SR)16, Au24(SR)20, Au25(SR)18−, Au28(SR)20, Au36(SR)24, Au38(SR)24, Au44(SR)28, Au67(SR)352−, Au68(SR)34 (neutral Iso2), Au102(SR)44, and Au144(SR)60, and so on.10−27 Figure 4 presents A = (n − m)/(m C
DOI: 10.1021/acs.jpcc.5b03128 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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threshold number of Au atoms in bulk-like RS-AuNPs is also evaluated as about 263, which is close to the results of Jin and Negishi et al. and consistent with the SPR experiments.
various RS-AuNPs demonstrate that the investigation on medium-sized 14 kDa RS-AuNPs (Au 67 (SR) 35 2− and Au68(SR)34) is significant to understand the change of the Au (inner) core and the staple motifs with the increase of size. The threshold number of free electrons,45 which support the surface plasmon resonance (SPR) of RS-AuNPs, is a criterion for distinguishing between the molecular-like and bulk-like structures of RS-AuNPs. The SPR peaks at about 500 nm have been observed for Au 32 9 (SR) 8 4 , 4 6 , 4 7 Au 3 33 (SR) 79 , 4 8 Au∼500(SR)∼120,49 and Au∼940±20(SR)∼160±450 nanoparticles. Due to a lack of knowledge of the structures of RS-AuNPs with the number of Au atoms larger than 144, it is difficult to calculate the threshold number of free electrons precisely. Here we provide the value of HOMO−LUMO gap Eg versus the number of Au atoms (N−1/3) in RS-AuNPs to evaluate the threshold number of Au atoms in bulk-like RS-AuNPs, as shown in Figure 5. By the linear fitting of the H−L gaps of
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ASSOCIATED CONTENT
* Supporting Information S
The structures of Iso5 and Iso6, the energies and HOMO− LUMO gaps of Iso1−Iso6 with different charge states, and the Cartesian coordinates of Iso1−Iso4. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03128.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS W.W.X. is supported by China postdoctoral science foundation (Y419022011). Y.G. is supported by the startup funding from Shanghai Institute of Applied Physics, Chinese Academy of Sciences (Y290011011), National Natural Science Foundation of China (21273268), “Hundred People Project” from Chinese Academy of Sciences, and “Pu-jiang Rencai Project” from Science and Technology Commission of Shanghai Municipality (13PJ1410400). The computational resources utilized in this research were provided by Shanghai Supercomputer Center, National Supercomputing Center in Tianjin and Supercomputing Center of Chinese Academy of Sciences in Beijing.
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Figure 5. Calculated HOMO−LUMO gaps (Eg) of various RS-AuNPs (red solid circle) versus N−1/3 (N denotes the number of Au atoms of RS-AuNPs). The blue straight line denotes the linear fitting of the H− L gaps. The 14 kDa RS-AuNPs (Au67(SR)352− and Au68(SR)34) are emphasized in an olive square frame.
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various RS-AuNPs including Au68(SR)34, we obtain that the threshold number of Au atoms in bulk RS-AuNP is about 263. This value is slightly larger than the newly reported results of Negishi and co-workers,51 in which Au cores of Au187(SR)6852 and larger clusters have bulk-like electronic structures and fcc geometric structures. Most recently, Jin presents the optical energy gaps of the reported RS-AuNPs as a function of gold core size N. Their results demonstrate that the clusters can be categorized into two sets with reasonable linearity when Eg is plotted against N−1/3.53 According to the empirical relation obtained by Jin, the range of threshold number of Au is from 157 to 332. The threshold number 263 obtained by us locates in this range.
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CONCLUSIONS In conclusion, we have presented a set of isomer structures of Au68(SR)34 based on the D&P approach and density-functional theory calculations. We find that neutral Iso2 with a m-Dh Au18 inner core and C2v Au50 core is the most stable among these four isomers. The HOMO−LUMO gap of neutral Iso2 (0.74 eV) is very close to Au67(SR)352− (0.75 eV). When taking into account the various RS-AuNPs besides Au68(SR)34, the research on the medium-sized Au68(SR)34 establishes a bridge between smaller and larger RS-AuNPs, which is beneficial for us to better understand the structures of the RS-AuNPs. The D
DOI: 10.1021/acs.jpcc.5b03128 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpcc.5b03128 J. Phys. Chem. C XXXX, XXX, XXX−XXX
