Letter Cite This: J. Phys. Chem. Lett. 2017, 8, 5338-5343
pubs.acs.org/JPCL
Two-Way Transformation between fcc- and Nonfcc-Structured Gold Nanoclusters Hongwei Dong,§,†,‡ Lingwen Liao,§,† and Zhikun Wu*,† †
Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, China ‡ University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *
ABSTRACT: Precisely tuning the structure of nanomaterials, especially in a two-way style, is challenging but of great importance for regulating properties and for practical applications. The structural transformation from nonfcc to fcc (face center cubic) in gold nanoclusters has been recently reported; however, the reverse process, that is, the structural transformation from fcc to nonfcc, not to mention the two-way structural transformation between fcc and nonfcc, remains unknown. We developed a novel synthesis method, successfully fulfilled the two-way structure transformation, and studied the stability of gold nanoclusters with different structures. Additionally, a novel gold nanocluster was synthesized and structurally resolved by single-crystal X-ray crystallography. This work has important implications for structure and property tuning of gold nanoclusters and might open up some new potential applications for gold nanoclusters.
U
formation, and compared the stability of the involving gold nanoclusters, which will be detailed below. The recently reported Au44(2,4-DMBT)26 (where 2,4DMBTH = 2,4-dimethylbenzenethiol) nanoclusters64 was chosen as the starting material for the two-way structure transformation due to the medium size of the gold nanocluster and its lower stability compared with some other size-closed gold nanoclusters (e.g., Au44(TBBT)28,41 where TBBTH = 4tert-butylbenzenethiol). It is known that the surface ligand influences the structure of gold nanoclusters9,10 and that most cyclohexanethiolated gold nanoclusters bear fcc structures.34,38,42 Thus cyclohexanethiol (CHTH = cyclohexanethiol) was employed to trigger the transformation. (For experimental details, see the Supporting Information.) The UV−vis/NIR spectrum of the as-obtained gold nanoclusters exhibits five blunt but distinct peaks at 365, 460, 550, 690, and 780 nm, as shown in Figure 1A, which has not been previously reported and indicates that the as-prepared gold nanoclusters are novel. Electrospray ionization mass spectrometry (ESI−MS), a powerful and well-recognized technique,9,10,23,39,40,43,65 was employed to determine the exact molecular formula of the as-prepared gold nanoclusters. Without the addition of cesium acetate (CsOAC), no rational signals in both positive and negative ionization modes were observed in the mass spectra, implying that the novel nanoclusters might be charge neutral. After the addition of CsOAC, a dominant peak at 5807.92 Da was detected in the
ltrasmall thiolated gold nanoparticles (so-called gold nanoclusters)1−12 have received extensive interest in recent years due to their well-defined compositions and structures9,10 and potential applications.7,13−28 In current nanocluster research, it is of major importance to synthesize novel gold nanoclusters and solve their structures by singlecrystal X-ray crystallography.9,10 However, the controlled synthesis and structure unraveling of gold nanoclusters are still challenging, which handicap the subsequent studies and practical applications. It is known that gold nanocrystals generally adopt fcc structures; however, the first several resolved structures of gold nanoclusters, such as Au102(pMBA)44 (p-MBAH = p-mercaptobenzoic acid, SC7O2H5),29 Au25(SCH2CH2Ph)18,30,31 and Au38(SCH2CH2Ph),32 are icosahedron-based (nonfcc) structures. Only recently were fccstructured27,33−45 and even hexagonal close-packed (hcp)structured46−48 gold nanoclusters reported. It is known that nonfcc-structured gold nanoclusters can be transformed into fcc-structured ones;33,49,50 however, the reverse process, that is, an fcc to nonfcc structure transformation (or the two-way structure transformation between fcc and nonfcc structures), has not been reported so far to the best of our knowledge. This kind of structure transformation, which is somewhat like a solid-phase transformation of a bulk metal,51−56 is not only interesting but also important to understand the structures and tune the properties of gold nanoclusters. This two-way structure transformation even might have some potential applications in shape memory,57−59 chemical sensing,60−62 data storage,63 and so on. Here we developed a novel synthesis method, successfully fulfilled the two-way structure trans© XXXX American Chemical Society
Received: September 15, 2017 Accepted: October 17, 2017 Published: October 17, 2017 5338
DOI: 10.1021/acs.jpclett.7b02459 J. Phys. Chem. Lett. 2017, 8, 5338−5343
Letter
The Journal of Physical Chemistry Letters
Figure 2. Pair of enantiomers in the unit cell of the Au43(CHT)25 crystal. Color labels: yellow = S; gray = C; white = H; others = Au.
Figure 1. UV−vis/NIR absorption (A) and ESI−MS (B) spectra of Au43(CHT)25.
mass spectrum acquired in positive ionization mode, which is assigned to the [Au43(CHT)25Cs2]2+ species (calcd: 5807.89 Da; deviation: 0.03 Da). The assignment was confirmed by the good match between the experimental and calculated isotopic patterns (see Figure 1B). Thus it is concluded that the asprepared gold nanocluster has a formula of Au43(CHT)25 after deducting the two Cs+ ion adducts. Single-crystal X-ray crystallography (SCXC) confirmed this formula and revealed its structure. Interestingly, the nominal shell closing electron count (n*) of the Au43(CHT)25 nanocluster is also 18 (n* = NvA − M − z = 43 × 1 − 25 − 0 = 18),64,66−68 the same as that of the starting material Au44(2,4-DMBT)26.64 In addition, to the best of our knowledge, Au43(CHT)25 represents the second reported nanocluster with a nominal 18-electron shell closure. The Au43 nanocluster crystallized in a triclinic P-1 space group, and the unit cell comprises a pair of enantiomers (Figure 2 and Figure S1), similar to the case of Au44(2,4-DMBT)26. Below, the left-handed enantiomer was chosen for the structure analysis. In general, the Aux(SR)y nanocluster is composed of kernel and staples, and the kernel is made of pure gold atoms with the shortest Au−Au bond length. In this view, the kernel of Au43(CHT)25 consists of 29 Au atoms packed in an fcc fashion (see Figure 3 and Figure S2). The staples of Au43(CHT)25 consist of four Au2(SR)3 dimers, six Au(SR)2 monomers, and one bridging SR thiolate (see Figure 3). The Au−Au bond length in the Au29 kernel of Au43(CHT)25 averages to 2.862 Å, which is comparable to the Au−Au bond length (2.878 Å) in bulk gold and indicates that the Au29 kernel
Figure 3. Anatomy of the Au43(CHT)25 structure: (A) Au29 kernel, (B) six Au(SR)2 monomers, (C) the arrangement of six Au(SR)2 on the kernel, (D) four Au2(SR)3 dimers, (E) the arrangement of four Au2(SR)3 on the kernel, (F) the bridging SR thiolate, and (G) Au43(SR)25. For clarity, the C and H atoms in panels A−E and G are omitted. Color labels: yellow and orange = S; gray = C; olive = Au.
adopts similar structure as bulk gold. However, the lengths of the Au−Au bonds connecting the kernel and the staples are somewhat stretched (averaged to 3.079 Å). The Au−S bonds give an overall average of 2.324 Å (standard deviation: 0.042 Å), showing no considerable difference between the average Au−S bond length in the Au2(SR)3 dimer and that in the Au(SR)2 monomer (2.291 vs 2.295 Å, respectively). However, the Au−S bonds connecting the staples and the kernel are stretched with an average of 2.363 Å, which is consistent with the length (2.368 Å, average) of the Au−Sbr bonds (Sbr means the sulfur in only the bridging thiolate). The average Au−S−Au bond angle is 92.552° (standard deviation: 8.110°). The Au− S−Au bond angles in the Au2(SR)3 dimers range from 91.200 to 106.500°, whereas the Au−S−Au bond angles related to the Au(SR)2 monomer vary from 76.800 to 103.700°. Notably, the only Au−S−Au bond angle associated with the bridging thiolate is 89.900°. For the Au44(2,4-DMBT)26 precursor, in brief, it is composed of a Au29 kernel capped by six Au2(SR)3 dimers, three Au(SR)2 monomers, and two bridging SR thiolates.64 In particular, the kernel consists of one face-fused 5339
DOI: 10.1021/acs.jpclett.7b02459 J. Phys. Chem. Lett. 2017, 8, 5338−5343
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The Journal of Physical Chemistry Letters
not successful under the investigated conditions (see Figure S5), indicating that Au44(TBBT)28 is far more stable than Au43(CHT)25. It is known that Au44(TBBT)28 is more thermostable than Au44(2,4-DMBT)26 on the basis of our previous work.64 Herein, we further demonstrated that Au44(2,4-DMBT)26 is more thermostable than Au43(CHT)25 (see Figure 5). Thus the stability sequence of the three size-
bi-icosahedral Au23 and one special Au6 bottom cap, and the detailed structure analysis can be found in our previous work.64 On the basis of the above analysis, the essential structural difference between Au43(CHT)25 and Au44(2,4-DMBT)26 lies in Au43(CHT)25 possessing a fragmented fcc structure whereas Au44(2,4-DMBT)26 does not. Thus the structure transformation from Au44(2,4-DMBT)26 to Au43(CHT)25 is one from nonfcc to fcc. Next, we investigated the reverse transformation. Au43(CHT)25 was etched by excess 2,4dimethylbenzenethiol at 80 °C (for experimental details, see the Supporting Information) and monitored by UV−vis/NIR spectroscopy, which reveals that the characteristic peaks of Au43(CHT)25 gradually vanish and that peaks resembling those of Au44(2,4-DMBT)26 slowly manifest (see Figure 4).
Figure 5. Thermostability of Au43(CHT)25 (A) and Au44(2,4DMBT)26 (B) monitored by UV−vis/NIR spectroscopy.
closed nanoclusters is Au44(TBBT)28 > Au44(2,4-DMBT)26 > Au43(CHT)25, and, indeed, Au44(TBBT)28 is far more stable than Au43(CHT)25, which can possibly explain why the transformation between Au43(CHT)25 and Au44(TBBT)28 is irreversible. Note that the three gold nanoclusters in solution are all stable at room temperature for at least 3 days. The twoway structure transformation between Au44(2,4-DMBT)26 and Au43(CHT)25 (Au44(TBBT)28) indicates that the thermostability of Au 4 4 (2,4-DMBT) 2 6 an d Au 4 3 (CHT) 2 5 (Au44(TBBT)28) is comparable, and the structure transformation process can be regarded as a balance. With the excess of the ligand, the balance will move to the according ligand protected nanocluster (e.g., when CHTH is excessive, Au43(CHT)25 will be predominantly produced). It is known that the fcc structure is a robust one for gold nanoclusters, and thus the higher stability of the nonfcc-structured Au44(2,4DMBT)26 compared with the fcc-structured Au43(CHT)25 indicates that the ligand plays a more important role than the structure in stabilizing the nanoclusters in this study and that 2,4-DMBT is a better protecting ligand than CHT for gold nanoclusters. The electronic and steric factors should be considered for the ligand effects, and based on the current results and some previous reports of the transformation from nonfcc to fcc structure,33,49,50 it can be concluded that aromatic ligands generally provide better protection than nonaromatic ligands. It is worth noting that the ligand contributes to the form of staples, which also influences the stability of metal
Figure 4. Time-dependent UV−vis/NIR absorption spectra of Au44(2,4-DMBT)26 after etching by cyclohexanethiol (A). Timedependent UV/vis/NIR absorption spectra of Au43(CHT)25 after etching by 2,4-dimethylbenzenethiol (B).
Indeed, the isolated product exhibits a superimposable UV− vis/NIR absorption spectrum with that of Au44(2,4-DMBT)26, and the ESI analysis confirmed that the transformed product is Au44(2,4-DMBT)26 (see Figure S3), which indicates that Au43(CHT)25 can also be transformed back to Au44(2,4DMBT)26. Thus, for the first time, a two-way structure transformation between nonfcc and fcc in gold nanoclusters was observed. Note that the yield of Au43(CHT)25 in the transformation from Au44(2,4-DMBT)26 to Au43(CHT)25 is ∼60%, and the yield of Au44(2,4-DMBT)26 in the reverse transformation is near 68%, indicating that the transformation is rather complete and neat. Interestingly, it is revealed that the transformation between Au44(2,4-DMBT)26 and Au44(TBBT)28 is also reversible (see Figure S4), further verifying the feasibility of the two-way structure transformation between nonfcc and fcc in gold nanoclusters. The mass spectrometry indicates that the ligand was completely replaced because the M/Z values are well consistent with the expected ones; see Figure 1 and Figure S3. Additionally, it is found that Au43(CHT)25 can be transformed to Au44(TBBT)28, whereas the reverse process is 5340
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nanoclusters, which might endow them with some new potential applications.
nanoclusters. However, because of the limited comparable cases, it is difficult to make a conclusion for the influence of staple type in this work. Such a thorough investigation should be conducted in the future by both experimental and theoritical researchers. Except for the differences in stability and absorption, the three structures also show remarkable differences in emission, as shown in Figure 6. The fluorescence
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b02459. Synthesis details, crystallization, characterization, Figures S1−S5, and Table S1. (PDF) X-ray crystallographic data of Au43(CHT)25. (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zhikun Wu: 0000-0002-2711-3860 Author Contributions §
H.D. and L.L. contributed equally to this work
Notes
The authors declare no competing financial interest.
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Figure 6. Fluorescence spectra of the Au44(TBBT)28, Au44(2,4DMBT)26, and Au43(CHT)25 nanoclusters dissolved in dichloromethane.
ACKNOWLEDGMENTS We thank Natural Science Foundation of China (nos. 21222301, 21528303, 21603234, 21771186, 21171170), National Basic Research Program of China (grant no. 2013CB934302), the Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (2014FXCX002), Hefei Science Center, CAS (user of potential: 2015HSC-UP003), the CAS/SAFEA International Partnership Program for Creative Research Teams, and the Hundred Talents Program of the Chinese Academy of Sciences for financial support.
mechanism is rather complex, and there are several recent studies20,69−71 on uncovering the luminescence fundamentals of gold nanoclusters, such as the pathways of aggregationinduced emission (AIE),20,70 charge transfer,71 and direct electron donation.71 Lee et al. revealed that the rigidity of gold shell was responsible for the luminescence enhancement;72 we also found that ligand,71,73 structure,27 and doping74,75 are some factors that influence the fluorescence properties of metal nanoclusters. For some summaries, see Zheng’s76 and Aikens’69 works. It is worth noting that in 2016 Jin’s group reported a two-way transformation between two fcc-structured Au28 nanoclusters;38 however, the two-way transformation between a nonfcc and fcc structure (even a one-way transformation from an fcc structure to a nonfcc structure) has not been previously reported to the best of our knowledge. Such transformations are not only interesting but also might provide strategies for tuning the properties of gold nanocluster in a reverse way and designing devices in shape memory,57−59 chemical sensing,60−62 data storage,63 and so on. In summary, a novel gold nanocluster was successfully obtained by a ligand-inducing method and structurally resolved by single-crystal X-ray crystallography, and the transformations and the comparison of some properties among Au44(2,4DMBT)26, Au43(CHT)25, and Au44(TBBT)28 were investigated. The significance and novelty of this work lie in: (1) the twoway structure transformation in gold nanoclusters and the oneway transformation from an fcc structure to a nonfcc structure were demonstrated for the first time; (2) a novel gold nanocluster Au43(CHT)25, which represents the second gold nanocluster with a nominal 18-electron structure, was synthesized and structurally resolved; (3) the stability comparison among Au44(2,4-DMBT)26, Au43(CHT)25, and Au 44 (TBBT)28 (Au 44 (TBBT) 28 > Au 44 (2,4-DMBT)26 > Au43(CHT)25) indicated that the ligand had a more important influence than the structure on the stability of gold nanoclusters in this study; and (4) it was shown that the two-way structure transformation can reversibly tune the properties of the gold
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DOI: 10.1021/acs.jpclett.7b02459 J. Phys. Chem. Lett. 2017, 8, 5338−5343
Letter
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DOI: 10.1021/acs.jpclett.7b02459 J. Phys. Chem. Lett. 2017, 8, 5338−5343
