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Ammonia-Induced Size Convergence of Atomically Monodipserse Au Nanoclusters 6

Ting Huang, Li Huang, Wenxue He, Xueyang Song, Zhihu Sun, Yong Jiang, Guoqiang Pan, and Shiqiang Wei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12422 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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The Journal of Physical Chemistry

AmmoniaSize Convergence mmonia-Induced Monodipserse Au6 Nanoclusters anoclusters

of

Atomically

Ting Huang, Li Huang, Wenxue He, Xueyang Song, Zhihu Sun,* Yong Jiang,* Guoqiang Pan, and Shiqiang Wei National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, P. R. China Abstract: Developing effective synthetic protocols for atomically monodisperse Au nanoclusters is pivotal to their fundamental science and applications. Here, we present a novel synthetic protocol toward atomically monodisperse [Au6(PPh3)6]2+ nanoclusters (abbreviated as Au6), via ammonia-induced size convergence from polydisperse Aux (x=6−11) nanocluster mixture. The analogous ammonia-induced size conversion reactions starting from individually prepared Au7 and Au9 nanoclusters to Au6 were traced by time-dependent UV-vis absorption and electrospray ionization mass spectra. It is observed that in both cases the size conversion is achieved through gradual release of the ion-molecule complex [NH4AuPPh3Cl]+ from the larger Au nanoclusters until the formation of thermodynamically stable Au6 nanoclusters with the stability against the etching reaction. The role of ammonia ions in this size-convergence synthesis is to accelerate the depletion of [Au(PPh3)]+ fragments from the PPh3-protected Au nanoclusters, by the formation of the stable complex [NH4AuPPh3Cl]+.

* Corresponding authors. E-mail: [email protected]; [email protected]

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Introduction Metal nanoclusters consisting of a few to tens of metal atoms possess unique molecular-like structure and discrete electronic energy levels, rendering them diverse applications in catalysis, sensing, imaging, labelling, electrochemistry, etc.1-4 Among them, ultrasmall nanoclusters with metal atom nuclearity less than ten5-11 have higher surface-to-volume ratio than the larger ones and exhibit fantastic properties due to the extreme size sensitivity of the physiochemical in this size region. For both fundamental science and technical applications, it is desired to obtain atomically monodisperse nanoclusters with good control over the atom nuclearity and composition, which depends critically on the design and implementation of solution-based synthetic chemistry.12-14 Among a diversity of metal nanoclusters, gold nanoclusters have been a representative class and studied most extensively. A large number of Au nanoclusters (from Au6 to Au144) have been successfully synthesied.2-3 Nevertheless, synthetic obstacles still remain because most synthetic procedures suffer from the production of mixed clusters of different sizes. Synthetic protocols that lead to exclusive formation of one product are highly desired to avoid complicated post-synthetic purification steps. Tremendous efforts have been made to develop synthetic strategies for atomically monodisperse Au nanoclusters. Starting from the well-known Brust-Schiffrin method,15 the type of reducing agents, ligands, precursor/reductant and ligand/precursor molar ratios are factors frequently optimized to mediate the kinetic growth stage of nanoclusters.4, 16 For instance, McKenzie et al. selectively synthesized Au11(PPh3)7Cl3 and [Au11(PPh3)8Cl2]Cl (PPh3 = triphenylphosphine) nanoclusters by controlling the precursor/reductant ([AuPPh3Cl]/[NaBH4]) molar ratio at 5:1 and 1:4, respectively.17 Pettibone and Hudgens show that controlling the molar ratio of diphosphine ligand/precursor at specific values could yield monodisperse nanoclusters of distinct nuclearity in the range from Au8 to Au10.18 Another synthetic way is to add some specific agent (like NaOH) into the reaction system, so as to tune the reducing ability of the reductant and to guide the reaction routes.19 Precursor engineering has also been proposed in nanocluster synthesis,20 since a good precursor can simplify the ACS Paragon Plus Environment

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synthesis processes and can offer more controllability in targeting the desired nanocluster products. We found that atomically monodisperse [Au8(PPh3)7]2+ nanoclusters could be directly synthesized by using Au(PPh3)2Cl, instead of commonly used AuPPh3Cl, as the precursor.21 Recently, post-synthetic size-focusing been emerged as an efficient “top-down” methodology to converge preformed polydisperse nanoclusters into atomically monodisperse ones, by using a proper etching agent such as thiol (SR) and hydrochloric acid (HCl) depending on the specific reaction systems. Size convergence by thiol etching has been widely used for synthesis of a large number of thiolated-gold nanoclusters with gold atom nuclearity ≥ 25.4 Diphosphine-protected Au13 nanoclusters could be obtained through HCl etching of polydisperse cluster mixture.22-24 For producing smaller nanoclusters, phosphine could act as an etching agent, as exemplified by the conversion of [Au9(PPh3)8]3+ into [Au8(PPh3)8]2+ in the presence of free PPh3,25-26 whose role is to deplete [Au(PPh3)]+ fragments from the clusters. Besides, Au nanoclusters could convert from one structural type to another, by the addition reaction of mononuclear gold species to preformed cluster species.1,

3, 24-25

A good example is the size conversion of

[Au11(PMe2Ph)10]3+ into [Au13(PMe2Ph)10Cl2]3+ by the addition of Au(PMe2Ph)Cl.1 In spite of these progresses, synthesis of sub-nanometer monodisperse Au nanoclusters continues to be the topic of ongoing research. In this work, we report a novel ammonia-induced approach for the exclusive synthesis of atomically monodisperse [Au6(PPh3)6]2+ nanoclusters, avoiding the nontrivial multi-step conversions from [Au9(PPh3)8]3+ to [Au8(PPh3)8]2+ and then [Au8(PPh3)8]2+ to [Au6(PPh3)6]2+ nanoclusters.27-28 This synthesis is achieved via two distinct reaction stages: the first kinetic reduction stage and the second ammonia-induced size-converge stage. In the first stage, a narrowly size-distributed Aux (x=6, 7, 8, 9, and 11) nanoclusters mixture was obtained by controlling the low molar ratio (0.1 eq.) of reducing agent (NaBH4) to precursor (AuPPh3Cl). In the second stage, these preformed nanoclusters mixture was size-converged exclusively into atomically monodisperse Au6 nanoclusters in the aqueous ammonia circumstance. To unravel the role of aqueous ammonia in the size convergence step, ACS Paragon Plus Environment


independent experiments between aqueous ammonia and separately prepared Au7 and Au9 nanoclusters were carried out and monitored by time-dependent UV-vis absorption and mass spectrometry. It turns out that during the size convergence step, the role of ammonia is to accelerate the depletion of [Au(PPh3)]+ fragments from the clusters by the formation of the stable ion-molecule complex [NH4AuPPh3Cl]+, thus help converge the Aux mixture into atomically monodisperse Au6 nanoclusters.

Experimental Chemicals: Chloroauric acid (HAuCl4, Alfa Aesar), triphenylphosphine (PPh3, 99%, Alfa Aesar), sodium borohydride (NaBH4, 98%, Aldrich), aqueous ammonia (25 wt%, Aldrich), hydrochloric acid (HCl, 30 wt%, Aldrich). All solvents were obtained from Aldrich and employed as received without further purification. Synthesis of [Au6(PPh3)6]2+ nanoclusters: The precursor AuPPh3Cl was prepared according to the method reported in the literature.29 In 10 ml of ethanol, 0.2 mmol of AuPPh3Cl was dissolved, to which NaBH4 (0.02 mmol) was added and subjected to reaction for 3 h under rapid stirring at room temperature. Then to this solution, 10 ml of ammonia (25 wt%) aqueous solution was subsequently added and allowed to react for another 72 h at 318 K. After the solution was centrifuged at 15000 rpm for 5 min, the supernatant was evaporated to dryness. The obtained reddish-brown solid was re-dissolved in 2 ml ethanol and collected by centrifugation/evaporation again. This step was repeated three times. Finally, ~14 mg solid of Au nanoclusters was collected, giving a reaction yield of ~15% (Au atom basis). The obtained end product ([Au6(PPh3)6]2+) was redispersed in ethanol for further mass spectrometry and UV-vis absorption measurements. Evolutions of Au7 and Au9 nanoclusters in the aqueous ammonia circumstance: To synthesize the [Au7(PPh3)7]2+ nanoclusters, HCl (1500 µL, 30 wt%) aqueous solution was added to the precursor (Au(PPh3)2Cl, 2 mmol) solution in 100 ml ethanol, and NaBH4 (10.5 mmol) was subsequently added into the reaction solution, with the reaction vessel wrapped by tinfoil. Under vigorous stirring the reaction lasted for 48 h at 308 K. After removal of insoluble solid by centrifugation, ACS Paragon Plus Environment

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the obtained light yellow solution was evaporated to dryness and then washed in 10 ml of ethanol. This washing procedure was repeated three times until the final light yellow product ([Au7(PPh3)7]2+, ~319 mg) was collected. The [Au9(PPh3)8]2+ nanoclusters were prepared according to the methods reported in the literature,17, 28 through reducing the AuPPh3NO3 precursor. In order to provide the same AuPPh3Cl circumstance, a certain amount of AuPPh3Cl precursor was added to the following aqueous ammonia-induced reaction of Au9. All the previously obtained Au7 (0.1 mmol) and Au9 nanoclusters (0.1 mmol, 0.1 mmol AuPPh3Cl) were dissolved in ethanol (10 ml) and aqueous ammonia (10 ml, 25 wt%) solution for 72 h at 318 K. During the reaction process, the solution colours of Au7 (Au9) nanoclusters changed gradually from light yellow (dark red brown) to reddish brown. The rude products in these solutions were intermediately characterized by mass spectrometry and UV-vis absorption spectra. Characterizations: Electrospray ionization

mass spectrometry (ESI-MS)

measurements were carried out in the positive ionization mode using a Bruker MicroTOF mass-spectrometer. After dissolving the Au nanoclusters in a mixture of 0.1% formic acid and acetonitrile/H2O (volume ratio of 1:1), 10 µL of the solution was injected directly into the chamber at a rate of 100 µL/min. The ionization process was controlled at a temperature of 100 °C and nitrogen drying gas of 1 µL/min was used in the ESI source. A capillary voltage of 4 kV and a cone voltage of 80 V were used during the measurement. All spectra were obtained in the reflection mode of the time-of-flight (TOF) mass spectrometer equipped with multi-step detection for obtaining maximum sensitivity, with the isotopic resolution over the entire m/z range of 800 to 2000. UV-vis absorption spectra were collected in the transmission mode by using a Purkinje General TU-9001 spectrometer over the wavelength range of 250−800 nm. The background absorption was corrected by pure ethanol. X-ray photoelectron spectroscopy (XPS) measurements were carried out by using a VG Thermo ESCALAB 250 spectrometer operated at 120 W. The binding energies were referenced to carbon 1s at 284.4 eV. The Au L3-edge X-ray absorption near-edge



structure (XANES) spectra were obtained in transmission mode at the 1W1B beamline of Beijing Synchrotron Radiation Facility (BSRF), China. Results and Discussions Our synthetic protocol for the [Au6(Ph3)6]2+ nanoclusters involves two main stages as schematically shown in Scheme 1. In stage I, the ethanol solution of AuPPh3Cl precursor (0.2 mmol, 10 ml of ethanol), obtained according to the widely used method,29 was reduced by NaBH4 (0.02 mmol) for 3 h at room temperature. The originally transparent colourless solution gradually became dark reddish-brown over time during the reduction course. Then in stage II, to the obtained solution, 10 ml of aqueous ammonia (25 wt%) was added and the mixture was stirred at 318 K for 72 h. With prolonged reaction time, the solution colour gradually changed from dark reddish-brown to light reddish-brown. After centrifugation of the solution at 15000 rpm for 5 min to remove the insoluble solid, the supernatant was evaporated to dryness and then re-dissolved in 2 ml ethanol. This centrifugation/evaporation procedure was repeated three times, until the final product of a brown-red solid was obtained (~14 mg, 15% yield).

Scheme 1. Schematic diagram showing the ammonia-induced synthesis of monodisperse Au6 nanoclusters in two stages.

The products in the above two stages were identified by a combination of ESI-MS and UV-vis absorption spectra. The ESI-MS analysis for the product in stage I shows some distinct peaks located in the m/z region of 1000−2000 (Fig. 1(a), bottom). The multiple sets of signals located at m/z 1290, 1377, 1422, 1575, 1609, and 1837 can be well assigned to: [Au9(PPh3)8]3+ (m/z = 1290.08), [Au6(PPh3)6]2+ (m/z = 1378.10), [Au11(PPh3)8]3+ (m/z = 1422.66), [Au8(PPh3)6]2+ (m/z = 1575.75), [Au7(PPh3)7]2+ (m/z


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=1608.42), and [Au8(PPh3)8]2+ (m/z =1838.04), respectively. The [Au11(PPh3)8]3+ is a fragment of [Au11(PPh3)8Cl2]+ by dissociating two Cl− ion ligands during the ESI-MS measurement, possibly because the sampling cone voltage (80 V) was not optimized to suppress the in-source fragmentation. This dissociation does not affect other smaller cation clusters, since they do not have Cl− ion ligands. At m/z < 1000, there are two intense peaks at 721 and 953, corresponding to the unreacted precursors in the forms of [Au(PPh3)2]+ and [Au2(PPh3)2Cl]+. These identifications could also be supported convincingly by the excellent match between experimental data and the calculated isotope patterns for these species with the assigned formula, as exemplified in Fig. 1(b) for typical nanoclusters of [Au6(PPh3)6]2+, [Au7(PPh3)7]2+ and [Au9(PPh3)8]3+.

Figure 1. (a) ESI-MS for the NaBH4 reduction product of AuPPh3Cl, and the final product after reaction with ammonia for 72 h. (b) Comparison between the experimental data (black) of ESI-MS and the calculated isotope patterns (red) for [Au6(PPh3)6]2+, [Au7(PPh3)7]2+ and [Au9(PPh3)8]3+ nanoclusters. (c) UV-vis absorption spectra for the NaBH4 reduction product and the final product after reaction with ammonia.

The obtained Au nanoclusters were also identified by the UV-vis absorption spectra, which act as a simple fingerprint to monitor the size and size distribution of nanoclusters. Absorption peaks at around 308 and 412 nm (Fig. 1(c)) are visible, which could be assigned to mixed Aux nanoclusters,1, 3 in agreement with the ESI-MS ACS Paragon Plus Environment


analysis. Moreover, the absence of the surface plasmon resonance peak at 530 nm characteristic of Au nanocrystals (>2 nm) implies that no large nanoparticles were formed. Therefore, the ESI-MS and UV-vis spectra show the obtainment of relatively narrowly dispersed mixture of PPh3-protected Aux (x=6, 7, 8, 9, and 11) nanoclusters as the product of the AuPPh3Cl precursor reduced by NaBH4. Similarly, Pettibone and Hudgens reported the formation of PPh3-protected AuN nanoclusters with gold atom nuclearity 8≤ N ≤13 from the reduction of AuClPPh3 (0.02 mmol) by NaBH4 (0.093 mmol) for 5 days in both methanol and ethanol solvent systems.25 After treating the Aux mixed nanoclusters in ammonia aqueous solution for 72 h, the ESI-MS shows that in the m/z region of 1000−2000 only a dominant signal at m/z = 1377 corresponding to [Au6(PPh3)6]2+ remains (Fig. 1(a)), while the other peaks related to Au7, Au8, Au9, and Au11 are very weak. At the same time, a new peak at m/z 853 appears, which could be assigned to a new Au(I)-PPh3 complex of [Au3(PPh3)]+ released from the Aux nanoclusters. The dominant formation of [Au6(PPh3)6]2+ nanoclusters could be further confirmed by the UV-vis absorption spectrum (Fig. 1(c)). The optical absorption peaks characteristic of [Au6(PPh3)6]2+ nanoclusters are observed at 330, 452 and 476 nm, consistent with peak positions reported in the literature.1, 3 From the above results, it can be concluded that in our synthetic protocol of ammonia-induced production of [Au6(PPh3)6]2+ nanoclusters, stage I involves the formation of narrowly distributed Aux (x=6, 7, 8, 9, and 11) nanocluster mixtures, and stage II is the ammonia-induced size convergence of the Aux nanocluster mixtures into monodisperse [Au6(PPh3)6]2+ nanoclusters. With respect to stage I, the obtained narrowly distributed Aux nanocluster mixtures are intimately related to the low molar ratio of reducing agent to the precursor (0.1 equivalent of NaBH4 per mole of gold) and the relatively short reduction time (3 h). Increasing the molar ratio of NaBH4 to AuClPPh3 and the reduction reaction time would increase the gold atom nuclearity of the nanoclusters.25 As for the ammonia-induced stage II, this is the first report that ammonia could also aid in the size-convergence synthesis of gold nanoclusters. Obviously, the role of ammonia is different from that of NaOH in mediating the ACS Paragon Plus Environment

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growth of Au25(SR)18 nanoclusters,19 where NaOH, added during the reduction reaction, is used to decrease the reduction ability of NaBH4 and accelerate the etching ability of SR ligands by the deprotonation of thiol groups. It should be remarked that [Au6(PPh3)6]2+ could resist the etching of ammonia and it is more stable than other PPh3-protected Aux (x=7, 8, 9, and 11) in this environment; however, this does not mean that Au6 is always more stable than Au7−Au11. [Au6(PPh3)6]2+ could also convert into [Au8(PPh3)7]2+ in the presence of reactive Au(I)-PPh3 complexes as we have reported recently.21 This once again shows that Au nanoclusters could convert from one structural type to another under a proper chemical conditions,1,

3-4, 24-25

which provides a controllable way for selective synthesis of a specific nanocluster.

Figure 2. Time-dependent ESI-MS (a)/(c) and UV-vis spectra (b)/(d) in the evolution process from Au9/Au7 nanoclusters to Au6 nanoclusters upon reaction with ammonia. The inset in (a) shows the comparison between the experimental data and the calculated isotope pattern for [NH4AuPPh3Cl]+ complex.



In the ammonia-induced stage II, the Au7−Au11 nanocluster mixtures eventually disappear, while only the [Au6(PPh3)6]2+ nanoclusters and the Au(I)-PPh3 complexes (Au1, Au2, and Au3) are the finally remaining species. Since the ESI-MS and UV-vis spectra could not show how these Au7−Au11 nanoclusters are connected to the end product of Au6, there is an issue unclarified: Do they eventually convert to Au6 or dissolve into the solution as Au(I)-PPh3 complexes? To detect the fate of these intermediate nanoclusters, instead of the difficult task of isolating them from the ethanol solution, we independently prepared two typical Aux nanoclusters (Au7 and Au9) and then explored their evolutions in the same ammonia circumstance. The individual evolution pathway were then monitored by time-dependent ESI-MS and UV-vis spectra. The synthesis of individual [Au9(PPh3)8]3+ and [Au7(PPh3)7]2+ nanoclusters is described in detail in the experimental section. For the reaction between [Au9(PPh3)8]3+ and ammonia where a certain amount of AuPPh3Cl precursor was added to provide the same AuPPh3Cl circumstance, the solution colour changes gradually from dark green to reddish brown as the reaction proceeded. During the ESI-MS measurements, the Au(I) complex of [Au(PPh3)2]+ at m/z 721 appeares as a very strong peak due to its significant conductivity and masks the peaks of other species. To better detect the evolution of the Au(I) complexes and nanocluster products over the m/z range of 400−2000, the time-dependent ESI mass spectra were measured separately in two m/z ranges of 400−800 and 800−2000, as shown in Fig. 2(a). Seen from the mass spectra in the m/z range from 800 to 2000, the initially prepared [Au9(PPh3)8]3+ nanoclusters show clear signals at 1290. It is gradually weakened as the reaction prolongs, and at the same time the [Au6(PPh3)6]2+ peak at 1377 is gradually enhanced. Finally at 72 h, the [Au9(PPh3)8]3+ peak disappears completely and the [Au6(PPh3)6]2+ peak becomes very sharp, together with other signals related to the Au(I)-PPh3 complexes such as [Au2(PPh3)3]+. More insights into the size-conversion process could be given by the ESI-MS measurements in the mass ranges of m/z 400-800. It is observed that as the reaction ACS Paragon Plus Environment

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proceeds, a new peak at m/z 512 becomes gradually intensified. This peak could be assigned to a complex with the formula of [NH4AuPPh3Cl]+ (calculated m/z 511.75), whose calculated isotope pattern is in excellent agreement with the experimental data as shown in the inset of Fig. 2(a). [NH4AuPPh3Cl]+ could be regarded as cation-anion complex formed by NH4+ and AuPPh3Cl molecule. Moreover, this reaction process was simultaneously characterized by the UV-vis absorption spectra (Fig. 2(b)). The [Au9(PPh3)8]3+ nanoclusters exhibit characteristic peaks at 314, 375 and 443 nm, identical to those reported in the literature.1, 3 As the reaction going on, these optical features are gradually weakened and the characteristic peaks (330, 452 and 476 nm) of [Au6(PPh3)6]2+ are progressively intensified. Finally, the spectral feature resembles that of the pure [Au6(PPh3)6]2+ nanoclusters. These results indicate that ammonia can indeed aid in the size-conversion from [Au9(PPh3)8]3+ into [Au6(PPh3)6]2+ nanoclusters, through the release of Au(I) complexes like [NH4AuPPh3Cl]+ and [Au2(PPh3)3]+. Similar size-conversion occurs in the reaction between [Au7(PPh3)7]2+ and aqueous ammonia. This is evidenced by the direct observation of the gradually weakened ESI peak of [Au7(PPh3)7]2+ at m/z 1609 and the enhanced [Au6(PPh3)6]2+ signal at m/z 1377 with increasing reaction time (Fig. 2(c)). Like in the case of [Au9(PPh3)8]3+, the appearance and gradual enhancement of the [NH4AuPPh3Cl]+ peak during the whole reaction time could be observed. The evolution of time-dependent UV-vis absorption curves is in agreement with the ESI-MS observations: the initial peak of [Au7(PPh3)7]2+ at 345 nm gradually shifts towards the characteristic peak of [Au6(PPh3)6]2+ at 330 nm with the shoulder peak of [Au7(PPh3)7]2+ at 412 nm finally fading away; simultaneously, the characteristic peaks of [Au6(PPh3)6]2+ at 452 and 476 nm become progressively well-defined (Fig. 2(d)). These results indicate that Au7 could also be transformed to Au6 by aqueous ammonia by depleting the [NH4AuPPh3Cl]+ complex. We also conducted supplementary experiments on the reaction between Au7 and ethanol solution of ammonium acetate (CH3COONH4). The time-dependent UV-vis absorption spectra monitoring this reaction process (Fig. S1) show similar evolutions of the characteristic bands (330, ACS Paragon Plus Environment


452, and 476 nm) of Au6 clusters, as that for the reaction between Au7 and aqueous ammonia (Fig. 2(d)). This confirms the key role of NH4+ in this size-conversion synthesis. (a) 3

(b)

+

[Au(PPh3)2]

1.2

(c)

AuPPh3Cl AuPPh3Cl + ammonia

1.2

AuPPh3Cl +

1

[NH4AuPPh3Cl]

0.8

Au 4f7/2

0.4

0 400

Normalized µ(E)

85.6 85.4

2

Counts (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Au(PPh3)Cl Au(PPh3)Cl + ammonia

11925

0.8

0.4

AuPPh3Cl + ammonia

800

1200 1600 2000 m/z

0.0 92

90 88 86 84 Binding Energy (eV)

82

0.0 11910 11930 11950 11970

E (eV)

Figure 3. (a) ESI-MS, (b) XPS, and (c) XANES spectra for the precursor AuPPh3Cl before and after reaction with aqueous ammonia. We note that the [NH4AuPPh3Cl]+ complex could be observed during the reaction between the precursor AuPPh3Cl and aqueous ammonia under the same reaction conditions as for synthesizing Au6 nanoclusters. The reaction products were examined by ESI-MS and XPS. The ESI-MS in Fig. 3(a) shows the presence of the [NH4AuPPh3Cl]+ peak at m/z 512, besides the intense peak of [Au(PPh3)2]+ at m/z 721. In addition, the Au 4f7/2 XPS spectrum of the AuPPh3Cl precursor is peaked at the binding energy of 85.4 eV, lying between the binding energies of metallic Au (84.0 eV) and the Au(I) (86.0 eV). After reaction with aqueous ammonia, the XPS peak is shifted to a higher binding energy of 85.6 eV (Fig. 3(b)). We also measured the Au L3-edge XANES spectra of AuPPh3Cl before and after the addition of aqueous ammonia, which reflect the changes in the electronic structure of the Au atoms.30 As shown in Fig. 3(c), the XANES spectrum of the AuPPh3Cl precursor displays a wide dump at around 11925 eV in the white-line region which arise from the electron transitions from the Au 2p3/2 to the unoccupied 5d5/2, 3/2 states. Upon reaction with aqueous ammonia, the white-line peak is remarkably intensified. Therefore, both XPS and XANES measurements suggest the higher oxidation states of Au atoms in


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[NH4AuPPh3Cl]+ than in AuPPh3Cl, possibly caused by the charge transfer from gold to nitrogen atoms. Based on these results, we could draw a picture on the ammonia-induced size-focusing from mixed Aux to Au6 nanoclusters, as shown schematically in Fig. 4. The wet-chemistry synthesis of nanoclusters is dynamic in reaction solution, involving both the growth of nanoclusters and the digestion or etching of newly formed clusters.19, 31 In the synthesis of PPh3-protected nanoclusters, PPh3 plays a dual role as protecting surfactant and etching agent.25 The role of PPh3 as an etching agent is interpreted as depleting [Au(PPh3)]+ fragments from the clusters by the formation of the stable monomeric [Au(PPh3)2]+ species.25-26 At the same time, the labile Au(I)-PPh3 complexes could also aggregate on preformed Au nanoclusters, thus promoting the growth of the cluster nuclearity.1,

3, 24-25

The balance of these two

competitive processes often leads to a reaction equilibrium. In this work, the reaction equilibrium is shifted by addition of NH4+ into the solution. NH4+ reacts with the [Au(PPh3)]+ fragments to form the stable ion-molecule complex [NH4AuPPh3Cl]+ which is depleted from the clusters, resulting in the reduced nuclearity of the remnant nanoclusters. Due to the loss of Au(I)-PPh3 motif that is crucial in stabilizing the nanocluster skeleton, the remaining part of Aux nanocluster is unstable and tend to undergo slow structural reconstruction/reorganization process, like those in the transformation from larger particles into Au25 nanoclusters.4, 32-33 The role of aqueous ammonia in this size-convergence synthesis is to accelerate the depletion of [Au(PPh3)]+ fragments from the PPh3-protected Au nanoclusters. This is significantly different from the thiol-etching of phosphine-protected Au nanoclusters where the thiol-for-phosphine ligand exchange occurs and the final products are ligated by thiols.22, 34-36



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NH4+

NH3⋅H2O

Etching NH4

+

+

Aux

Au6

NH4+ Au

[NH4AuPPh3Cl]+ complex

P

Figure 4. Schematic illustration of the effect of ammonia on converting the Aux nanoclusters into Au6 nanoclusters, through depletion of [NH4AuPPh3Cl]+ ion-molecule complex. Finally we briefly discuss why [Au6(PPh3)6]2+ could survive the etching reaction. The basis of this size-focusing methodology is the stability property of different nanoclusters under a certain condition, which selectively crashes those less-stable nanoclusters but permits the robustest one to survive. The underlying principle of this size-focusing process is primarily related to the peculiar stability of certain sized nanoclusters, that is, “survival of the robustest”, reminiscent of the natural selection law “survival of the fittest”.37 What determines the nanocluster’s stability is a major issue not completely understood. We hypothesize that the stability of [Au6(PPh3)6]2+ against etching is related to its unique atomic structure, namely, an edge-sharing bi-tetrahedral (Au4) structure. The larger Aux nanoclusters prefer centered polyhedral geometries where the peripheral gold atoms are interacted with each other by Au(I)⋯Au(I) aurophilic interaction.1, 3 As first proposed by Mingos et al.1 and later proved by many nanoclusters,4, 28, 38 Au4 tetrahedron serves as a basic building block for construction of stable Au nanoclusters. In [Au6(PPh3)6]2+, the Au-Au distances are 2.65 Å in the shared edge, 2.67 Å in the terminal edges, and 2.76–2.84 Å in the other eight edges.1 These Au–Au distances are significantly shorter than those in the larger centered polyhedral Aux-PPh3 nanoclusters. For instance, in [Au8(PPh3)8]2+, the Au– Au distances involving the central gold atom are 2.64–2.72 Å, and the Au–Au distances on the periphery are 2.83–2.96 Å; in [Au11(PPh3)8Cl2]+, the Au–Au distances range from 2.67–3.09 Å.1 Due to the shorter Au–Au distances in


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[Au6(PPh3)6]2+, the direct Au–Au bonding is much stronger than the weak Au(I)⋯Au(I) aurophilic attraction whose strength is in the order of tens of kJ mol−1, comparable with hydrogen bonds. Thus, [Au6(PPh3)6]2+ could survive the etching reaction, different from the other clusters where the [Au(PPh3)]+ fragments are relatively easier to lose.

Conclusion In conclusion, we have developed a novel ammonia-induced protocol for synthesis of monodisperse [Au6(PPh3)6]2+ nanoclusters. The synthetic protocol includes two reaction stages: the kinetic reduction stage and the aqueous ammonia-induced size convergence stage. In stage I, the AuPPh3Cl precursor was reduced by NaBH4 of 0.1 equiv, yielding a mixture of Aux (x=6, 7, 8, 9, and 11) nanoclusters. In the presence of excess aqueous ammonia solution, the preformed nanocluster mixture is converged into monodisperse Au6 nanoclusters (stage II). Parallel experiments on the separately prepared Au7 and Au9 nanoclusters indicate that the size convergence induced by ammonia is achieved through depletion of ion-molecule complex [NH4AuPPh3Cl]+ from the PPh3-protected Au nanoclusters. This is the first report on the role of ammonia in synthesis of ultrasmall Au nanoclusters and presents a new synthetic approach in a controllable way.

Supporting Information Experimental details of the reaction process on the reaction between Au7 nanoclusters in the ethanol solution of ammonium acetate, and the time-dependent UV-vis spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments This work was financially supported by the National Key Research and Development Program of China (No. 2017YFA0402800), National Natural Science Foundation of China (Grant No. 11475176, U1632263, 21533007, 11435012, and U1732116), and ACS Paragon Plus Environment


the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (11621063). The authors are grateful to BSRF for the synchrotron radiation beamtime.

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