YANG Lina , HUANG Li , SONG Xueyang , HE Wenxue , JIANG Yong , SUN Zhihu ,＊,WEI Shiqiang ,＊

1 National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, P. R. China.

2 School of Food Science and Engineering, Hefei University of Technology, Hefei 230009, P. R. China.

Abstract: Gold nanoclusters are promising materials for a variety of applications because of their unique “superatom” structure, extraordinary stability, and discrete electronic energy levels. Controlled synthesis of well-defined Au nanoclusters strongly depends on rational design and implementation of their synthetic chemistry. Among the numerous approaches for the synthesis of monodisperse Au nanoclusters, etching of pre-formed polydisperse clusters has been widely employed as a top-down method.Understanding the formation mechanism of metal nanoclusters during the etching process is important. Herein, we synthesized monodisperse Au13(L3)2(SR)4Cl4 nanoclusters via an etching reaction between polydisperse 1,3-bis(diphenylphosphino)propane (L3)-protected polydisperse Aun (15 ≤ n ≤ 60) clusters and a mixed solution of HCl/dodecanethiol (SR). The Au13 product, with a mean size of (1.1 ± 0.2)nm, shows pronounced step-like multiband absorption peaks centered at 327, 410, 433, and 700 nm. The synthetic protocol has a suitable reaction rate that allowss for real-time spectroscopic studies. We used a combination of in situ X-ray absorption fine structure (XAFS) spectroscopy, UV-Vis absorption spectroscopy, and matrix-assisted laser desorption ionization mass-spectrometry (MALDI-MS) to study the kinetic formation process of monodisperse Au13 nanoclusters.Emphasis was given to the detection of reaction intermediates. The study revealed that the size-conversion of the Au13 nanoclusters can be divided into three stages. In the first stage, the polydisperse Au15–Au60 clusters, covering a wide m/z range of 3000-13000, are prominently decomposed into smaller Au8-Au11 (within a m/z range of 3000–4000) species owing to the etching effect of HCl. They are immediately stabilized by the absorbed SR, L3, and Cl- ligands to form metastable intermediates, as indicated by the high intensity of the Au-ligand coordination peak at 0.190 nm as well as the low intensity of the Au–Au peaks (0.236 and 0.288 nm) in the Fourier-transform (FT) EXAFS spectra. In the second stage, these Au8–Au11 intermediates are grown into Au13 cores. The experimental X-ray absorption near-edge spectra,totally different from that of Au(I)-SR polymer, could be well reproduced by the calculated spectrum of the Au13P8Cl4 cluster.The Au-ligand coordination number (1.0) obtained from the EXAFS fitting is much closer to the nominal values in Au13(L3)2(SR)4Cl4 (0.92) than to that in Au(I)-SR polymers (2.0), suggesting that majority of the Au atoms are in the form of Au13 clusters. The driving force for this growth process is primarily the geometric factor to form a complete icosahedral Au13 skeleton through the incorporation of Au(I) ions or Au(I)-Cl oligomers pre-existing in the solution. In the third stage,the composition of the clusters is nearly unchanged as indicated by the MALDI-MS and the UV-vis spectra; however, their atomic structure undergoes rearrangement to the energetically stable structure of Au13(L3)2(SR)4Cl4. During this structural rearrangement, the central-peripheral and peripheral-peripheral Au–Au bond lengths (RAu-Au(c-p) and RAu-Au(p-p)) decrease from 0.272 to 0.267 nm and 0.295 to 0.289 nm, respectively, resulting in considerable structural distortion of the original icosahedral Au13 skeleton. This distortion is also reflected by the slightly increased disorder degree of the Au-Au bonds from 0.00015 to 0.00017 nm2. This work expands our understanding of the kinetic growth process of metal nanoclusters and promotes design and synthesis of metal nanomaterials in a controllable manner.

Key Words: Au nanoclusters; Size-convergence; Kinetic process; In situ spectroscopy; Etching; HCl;Dodecanethiol

1 Introduction

Gold nanoclusters consisting of a few to tens of core Au atoms have size-dependent molecular-like structure and discrete electronic energy levels, which render them diverse potential applications in a rich variety of areas such as catalysis, optical devices and imaging1–9. For both basic and application researches, the availability of well-defined nanoclusters is crucial, which depends critically on the design and implementation of solution-based synthetic chemistry. Gold thiolate (SR) nanoclusters6,7are most widely studied due to their extraordinary stability and unique structures, and their formation is significantly influenced by the kinetics of the reaction process,as exemplified by the synthesis of monodisperse Au25clusters via deliberate control over the reaction conditions10. The critical importance of kinetic control of the reaction process is to obtain a suitable size distribution of the intermediate complexes or sizemixed clusters that allow for spontaneous size convergence into a monodisperse product11–13. Understanding how the final nanoclusters product evolves from the reaction solution is critical in developing more cluster synthetic strategies, and deserves further researches.

Among a number of synthetic approaches for monodisperse Au nanoclusters, etching pre-formed polydisperse clusters has been widely employed as a top-down method, such as Au25nanorods etched from triphenylphosphine (PPh3)-protected polydisperse Au nanoparticles (1–3.5 nm)14,15, and Au38etched from Glutathionate (SG)-protected Aun (38 ＜ n ＜ 102)13or Aun(10 ＜ n ＜ 39)16mixed clusters. Hydrochloric acid (HCl) has also been evidenced to possess the etching ability to converse polydisperse diphosphine-stablized clusters into monodisperse Au13 clusters17,18. Currently, the prevailing mechanism describing SR-etching of nanoclusters is the release of Au(I)complex to the solution until the remaining cores reach a stable size13,19,20. A good example of this phenomenon is the etching reaction of Aun(SG)mclusters with free GSH under aerobic conditions, where Aun(SG)m (n ≥ 25) clusters were etched into Au25(SG)18 clusters, while Aun(SG)m (n ＜ 25) clusters were completely oxidized to Au(I)-SG complexes20. Besides, direct liberation of the same-sized particle as the target cluster in a single step was also hypothesized21. In the case of HCl-etching,it involves the fragmentation of the initial larger clusters into smaller intermediate clusters22. The driving force of the size convergence is phenomenologically interpreted as the “survival of the most robust”2,23, but what happens in the convergence process and how this process is related to the structure and properties of the end product is scarcely known. To fill these knowledge gaps, real-time studies on the nanoclusters conversion processes at the molecular level is of great interest24.The main obstacle lies in the existence of various intermediates with complex atomic/electronic structures that could not be isolated from the reaction solution. In order to overcome these barriers, in situ methods by a combination of various probing techniques with sensibility to transient intermediate species in solution are specially required22,25–30.

Herein, we report an investigation on the kinetic formation process of an unprecedented monodisperse Au13(L3)2(SR)4Cl4 nanocluster by a combination of in situ X-ray absorption fine structure (XAFS), UV-Vis absorption and mass spectra. The Au13nanocluster is synthesized through an etching reaction with a mixture solution of HCl and dodecanethiol, starting from polydisperse 1,3-bis(diphenylphosphino)propane (L3)-protected polydisperse Aun clusters. The synthetic protocol has a suitable reaction rate allowing for convenient real-time spectroscopic studies, and particular emphasis will be given to the detection of reaction intermediates by a combination of in situ XAFS and UV-Vis absorption spectra. It is found that the formation of the monodisperse Au13(L3)2(SR)4Cl4nanoclusters is realized in an etching/growth/rearrangement manner including three reaction stages, namely, HCl + HSR induced etching of the initial Aun(15 ≤n ≤ 60) precursor to Au8-Au11intermediates, size-growth of these intermediate clusters to Au13cores, and final structural rearrangement into the stable configuration of the Au13end product.

2 Experimental

2.1 Sample preparation

The monodisperse Au nanoclusters were synthesized by reaction of HCl and HSR mixture with preformed L3-capped Aun clusters, as reported in our previous work22. L3-capped monodisperse Aunclusters were synthesized by NaBH4(15 mg,0.04 mmol) reduction of Au2(L3)Cl2 (70 mg, 0.08 mmol) in dichloromethane solvent at room temperature for 3 h. After removal of the solvent, the residue was filtered for several times to remove insoluble materials. The filtrate was evaporated to dryness and then redissolved in 12 mL ethanol, to which a mixture of dodecanethiol (50 μL, 0.2 mmol) and HCl (200 μL,2.4 mmol) was added to etch the clusters.

2.2 In situ XAFS and UV-Vis absorption measurement

The Au L3-edge XAFS spectra were measured at the BL14W1 beamline of the Shanghai Synchrotron Radiation Facility(SSRF). The storage ring of SSRF was operated at 3.5 GeV with a maximum current of 240 mA in the top-up mode. A Si(111)monochromator crystal was used in conjunction with a harmonic rejection mirror. XAFS measurements were conducted in a quick-scan transmission mode (1 min per scan). The reactive solution was continuously circulated along a microtube by peristaltic pump and flowed into a deliberately designed in situ cell for in situ XAFS measurement28. The UV-Vis absorption spectra were recorded in transmission mode with a TU-9001 spectrometer in the wavelength range of 300-800 nm, and the background absorption was corrected by pure ethanol.

2.3 Transmission electron microscopy (TEM) and matrix-assisted laser desorption ionization mass-spectrometry (MALDI-MS) measurement

TEM images were obtained with a JEM-2100F system, for which the samples were prepared by dropping the reaction solution onto copper grids directly and drying in air. Mass spectra were measured with a Bruker Autoflex III mass spectrometer in positive linear mode. The Au cluster samples were mixed with matrix trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenyldidene] malononitrile (DCTB) in the molar ratio of 1 : 1000. Then one or two microliters of the mixture was applied to the sample plate and air-dried. In our measurements,the range of m/z was tuned from 2000 to 20000.

3 Results and discussion

Fig. 1 (a) Synthetic scheme, (b) TEM images, (c) UV-vis absorption spectrum and (d) MALDI mass spectrometry of the as-obtained Au clusters.The inset of (c) shows the digital photo of the solution of the Au clusters.

The synthesis of our monodisperse nanoclusters involves two primary stage as schematically shown in Fig. 1a. First,polydisperse L3-protected Aunclusters were synthesized as precursors by reduction of Au2(L3)Cl2 with NaBH4. Second, a mixture of HSR and HCl solution was added into the L3-protected Aunclusters and drove the size-convergence process,during which the solution color gradually changed from dark brown to light green. The reaction was completed over a sufficiently long period (8 h). Fig. 1b shows the TEM image of the end product, indicating the monodisperse clusters with mean size of (1.1 ± 0.2) nm. The UV-Vis absorption spectrum of the obtained nanoclusters as displayed in Fig. 1c shows pronounced step-like multiband absorption peaked at 327, 410, 433, and 700 nm. The distinct optical absorption and the absence of the 530 nm peak imply high purity and high yield of the as-synthesized nanoclusters, in good agreement with the TEM measurement.

To further characterize the size and identify the component of the Au clusters, MALDI mass spectrometry was employed by using the DCTB matrix for minimization of fragmentation during lase desorption. Seen from the MALDI mass spectrometry in Fig. 1d, a peak at m/z = 4335 is observed and is assigned to the intact Au13(L3)2(SR)4Cl4. On the lower mass side of the intact cluster ion, a very strong peak appears at m/z = 4101.This peak has a spacing of 233 relative to the intact cluster,corresponding to the loss of a AuCl unit from Au13(L3)2(SR)4Cl4due to laser-induced fragmentation. Although great care was taken to minimize fragmentation during the MALDI-MS measurement, it was still inevitable to generate fragments due to laser ionization, as frequently observed in literature12,31–33.Taking into account the unique UV-Vis absorption spectral shape for this nanocluster that is different from those for any known clusters, we believe that a new type of Au nanoclusters is obtained. Other than the Au13P10X2, Au13P9X3 and Au13P8X4 (X= halogen)17,18,25,34clusters that have been discovered before,the new cluster has an unprecedented component of Au13P6X6(X = S, Cl). A point worthy of note is that, compared with the solely HCl-etched final product of Au13(L3)4Cl417,Au13(L3)2(SR)4Cl4obtained in this work could be considered as a ligand-exchange product of replacing two L3 ligands in Au13(L3)4Cl4by four SR ligands. Regardless of the same core size of these two Au13clusters, their color and UV-Vis absorption spectra are totally different. The color of the Au13(L3)2(SR)4Cl4in ethanol solution is greenish (Fig. 1c inset), in contrast to the brownish orange of Au13(L3)4Cl4.

In order to study the formation mechanism of Au13(L3)2(SR)4Cl4nanoclusters, time-dependent UV-Vis absorption,MALDI-MS and XAFS spectra were combined to monitor the reaction course. Shown in Fig. 2a are the time-dependent UVVis absorption spectra and the color of the solution at typical reaction times. The optical absorption spectrum for the starting material demonstrates a wide hump at around 500 nm. At 10 min,a noticeable decrease of the hump is observed; and at 30 min the spectral profile shows a featureless decay. As the reaction goes on to 1 h, there starts to appear a subtle absorption peak at about 400 nm, which becomes progressively distinct with increasing reaction time. From 2 h on, step-like multiband absorption bands peaked at 327, 410, 433, and 700 nm become gradually pronounced and reach saturation at 8 h or longer.

The MALDI mass spectra at different reaction times are displayed in Fig. 2b. The starting material covers a wide size range of m/z 3000–13000 (the inset of Fig. 2b), corresponding to a mixture of Au15–Au6022. Strikingly, within the initial 10 min,the wide size distribution is narrowed and divided into two regions covering roughly m/z 6000–8000 and 3000–4000. The m/z 6000–8000 size range stands for a mixture of relatively larger clusters (Au31–Au39), while the peaks of m/z 3000–4000 could be assigned to Au8–Au11 clusters. Considering that HCl could solely induce such a similar fragmentation process22, this step is mainly attributed to the etching of HCl. These Au31–Au39clusters persist until the reaction time reaches 1 h; and at this time, the peak at m/z = 4101 becomes the strongest. As the reaction proceeds to 2 h, the Au8–Au11peaks vanish, and only the peaks at m/z 4101 and 4335 are present, indicating complete size-convergence into Au13clusters at this time.

Fig. 2 In situ UV-Vis absorption spectra (a) and MALDI mass spectra (b) at different reaction times.The inset of (a) shows the solution color at typical reaction times, and the inset in (b) shows the MALDI-MS of the starting material at a wide mass range.

Next, in situ XAFS measurements at Au L3-edge were performed to monitor the evolutions of clusters during the sizeconversion process. The time-dependent X-ray absorption nearedge structure (XANES) spectra in Fig. 3a display that the spectrum of the starting material exhibits a peak at 11936 eV(labeled by an arrow), a characteristic of face-center-cubic (fcc)structured Au35,36. After addition of HCl + HSR, this peak is remarkably decreased in intensity at the reaction time of 10 min,almost smeared out at 30 min, and disappears completely at 1 h.The white-line peak at around 11926 eV in the XANES spectrum, corresponding to the 2p3/2→5d5/2,3/2electronic transition of Au atoms, is significantly intensified immediately after the addition of HCl + HSR. The Fourier transformed (FT)EXAFS k2χ(k) curves in Fig. 3b also show that immediately after the start of reaction (within 10 min), the Au-ligand (Au-S/P/Cl)peak at 0.190 nm is intensified rapidly, while the Au-Au peaks(0.236 and 0.288 nm) are significantly damped. With continued reaction, the Au-Au peaks are gradually weakened. Quantitative structural parameters around Au atoms during the size conversion reaction have been obtained through a least-squares curve-fitting of the EXAFS data using the ARTEMIS module37of IFEFFIT package38. The fitting included an Au-ligand, and two Au-Au pairs standing for the central-peripheral and peripheral-peripheral Au―Au bonds in an complete or incomplete icosahedron (labelled by Au-Au(c-p) and Au-Au(pp), respectively). The obtained coordination number (CN), bond distance (R) and Debye-Waller factor (σ2) are shown in Fig. 3c–e as a function of time.

Based on the above observations, the size conversion pathway of Au13clusters could be roughly divided into three stages as schematically presented in Fig. 4. At the first stage within the first 30 min, the polydisperse Au15–Au60clusters with low ligand coverage, as suggested by the small Au-ligand CN of 0.44, are prominently etched into smaller Au8–Au11 metastable intermediates. The Au8–Au11intermediates smaller than the target Au13clusters attained are mainly the fragments of larger Aun(15 ≤ n ≤ 60) clusters in the HCl environment (step 1 in Fig.4)22, in sharp contrast to SR-etching cases where the product size is reached via gradual liberation of Au(I) complex13,19,20.This can be inferred by the decreased peripheral-peripheral Au-Au CN, along with the increased central-peripheral Au-Au CN in the first 1 h, since breaking of larger cluster would yield more surface Au atoms to increase the peripheral-peripheral Au-Au CN (Fig. 3c). Subsequently, these active small Au8–Au11clusters are immediately stabilized by the absorbed SR, L3and Clligands to form metastable intermediates as evidenced by the quickly increased Au-ligand CN (from 0.44 at 0 min to 0.72 at 30 min). These etching-produced smaller intermediate clusters are quite essential, since they provide possibility for the following size convergence and structural rearrangement to achieve monodisperse and stable Au13nanoclusters.

Fig. 3 (a) Time-dependent XANES spectra, and (b) EXAFS spectra. (c) Coordination number CN, (d) bond distance R, and(e) Debye-Waller factor σ2 against reaction time extracted from EXAFS curve-fitting.Au-Au(c-p) and Au-Au(p-p) represent the central-peripheral and peripheral-peripheral Au―Au bonds in an complete or incomplete icosahedron, respectively.

Fig. 4 Schematic illustration of the polydisperse-to-monodisperse conversion process of Aun nanoclusters.

The second stage, covering the reaction time from 30 min to 2 h, is mainly dominated by the size focusing of these metastable Au8-Au11intermediates into Au13cores (protected by SR, L3and Cl-ligands) in a secondary growth manner. The driving force of this growth process might be the geometric factor to form complete icosahedral Au13skeleton, through incorporating the Au(I) ions or Au(I)-Cl oligomers pre-existing in the solution(step 2 in Fig. 4). Indeed, this growth manner has been used in the growth of Au13(PMe2Ph)10Cl2clusters from an ethanol solution of Au11(PMe2Ph)1039, and also from Au11(L3)4Cl2to Au13(L3)4Cl4in our previous work22. It should be noted that this second-growth step leads to a high-yield of the cluster product,significantly different from the low-yield of the purely SR-etching cases13,19,20in which a large portion of Au atoms are dissolved to form Au(I) species in solution. Evidence to this point could be found from in-depth analysis of the XAFS spectra. We compare in the inset of Fig. 3a the XANES spectrum at the reaction time of 2 h with the spectrum of Au(I)-SR polymer40and the calculated spectrum of a Au13P8X4 clusters34using the ab initio multiple-scattering code FEFF8.2041. Due to the similarity between the component of the Au13(L3)2(SR)4Cl4cluster with that of Au13P8Cl4, it could be roughly assumed that it also adopts the icosahedral configuration as in Au13P8Cl418,34.Evidently, the experimental spectrum at 2 h, totally different from that of Au(I)-SR polymer, could be well reproduced by the calculated spectrum for the Au13P8Cl4cluster. On the EXAFS side, the Au-ligand CN (1.0) obtained from EAXFS fitting at 2 h is much closer to the nominal values in Au13(L3)2(SR)4Cl4(0.92) than to that in Au(I)-SR polymers (2.0)40, suggesting the majority of the Au atoms in the form of Au13cluster, in agreement with the XANES analysis.

After the completion of size-convergence into Au13cores at 2 h, the reaction enters the third stage. In this stage, the composition of the clusters keeps nearly unchanged, but their atomic structure undergoes a rearrangement process toward the energetically stable structure. The previously absorbed SR, L3and Cl-ligands unnecessarily occupy the optimal surface positions. In order to minimize the energy to reach a thermodynamically stable configuration, a rearrangement of the Au13 skeleton and the ligand shell occurs (step 3 in Fig. 4).Concurrently, the electronic states of these intermediates are greatly modified as indicated by the optical absorption spectra shown in Fig. 2a. The electronic structure modification arising from atomic arrangement was also observed in the structure transformation of Au38(PET)24to Au36(TBBT)24induced by ligand-exchange of phenylethanethiol (PET) using a bulky tertbutylbenzenethoil (TBBT) ligand33. The EXAFS curve-fitting show that during the structure rearrangement process, the Au―Au bond displays a trend of contraction. At the beginning of forming Au13skeletons, both the RAu-Au(c-p)and RAu-Au(p-p)(0.272 and 0.295 nm) of the Au13icosahedron are within the typical range (0.271–0.279 nm and 0.285–0.295 nm) of Au13 clusters as previously reported.17,34,39After the structure rearrangement, the RAu-Au(c-p)and RAu-Au(p-p)decrease to 0.267 and 0.289 nm, respectively, suggesting a considerable structural distortion away from the original icosahedral Au13skeleton. The gradually enlarged distortion is also reflected by the slightly increased disorder degree σ2of the Au―Au bonds, from 0.00015 to 0.00017 nm2. These disorder degrees of Au―Au bonds are most possibly attributed to the sterically bulky nature of the L3ligands that causes a distortion of the Au13 skeleton, in analogy to the extra structural disorder triggered by the bulky TBBT ligands in the TBBT-for-PET ligand-exchange of Au38(PET)2433.

4 Conclusions

In summary, kinetics study have been conducted by a combination of in situ UV-Vis absorption, XAFS and mass spectra to probe the formation process of monodisperse Au13(L3)2(SR)4Cl4 nanocluster converted from polydisperse Aun clusters. It is found that the cluster formation is achieved in an etching/growth/rearrangement manner including three distinct reaction steps. (1) The initial polydisperse Aun(15 ≤ n ≤ 60)clusters are etched by HCl to form much smaller Au8–Au11intermediate clusters protected by SR, L3, and Cl-ligands. (2)These intermediate species undergo a size growth to Au13 cores,by incorporating the pre-existing Au(I) ions or Au(I)-Cl oligomers in the solution. (3) Finally, a slow structure rearrangement of the Au13skeleton and the covering ligand shells occurs toward formation of the stable final structure of Au13(L3)2(SR)4Cl4clusters. These findings enrich our understanding on the etching mechanism during nanoclusters formation and can guide our way towards the design and synthesis of nanomaterials in a controllable manner.

Acknowledgment: The authors are greatly thankful to the BL14W1 beamline at SSRF for the synchrotron radiation beamtime.