Evolution from superatomic Au₂₄Ag₂₀ monomers into molecular-like Au₄₃Ag₃₈ dimeric nanoclusters

Abstract

Hierarchical assembly of nanoparticles has been attracting wide interest, as advanced functionalities can be achieved. However, the ability to manipulate structural evolution of artificial nanoparticles into assemblies with atomic precision has been largely unsuccessful. Here we report the evolution from monomeric Au₂₄Au₂₀ into dimeric Au₄₃Ag₃₈ nanoclusters: Au₄₃Ag₃₈ inherits the kernel frameworks from parent Au₂₄Ag₂₀ but exhibits distinct surface motifs; Au₂₄Ag₂₀ is racemic, while Au₄₃Ag₃₈ is mesomeric. Importantly, the evolution from monomers to dimers opens up exciting opportunities exploring currently unknown properties of monomeric and dimeric alloy nanoclusters. The Au₂₄Ag₂₀ clusters show superatomic electronic configurations, while Au₄₃Ag₃₈ clusters have molecular-like characteristics. Furthermore, monomeric Au₂₄Ag₂₀ catalysts readily outperform dimeric Au₄₃Ag₃₈ catalysts in the catalytic reduction of CO₂.

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Introduction

Nanoparticle assembly in nanoscience and nanotechnology is important because of the exceptional properties produced that can serve for fundamental investigations and new applications.^1–9 Despite the impressive advances in the assembly of artificial nanoparticles, the precise assembly of nanoparticles with atomic-scale manipulation remains a great challenge. This hinders researchers from creating and optimizing the functionality of nanomaterials. The key requirement for achieving such assemblies is the formation of truly atomically defined particles. Metal nanoclusters with absolutely precise formulae and atomic structures have provided access to currently challenging issues regarding conventional nanoparticles.

In a series of seminal studies, gold or silver nanoclusters with icosahedral structures have been demonstrated to be basic building blocks forming the hierarchical assembly of nanoclusters with elegant structures. For example, Au₃₈(SR)₂₄ with a double-icosahedral Au₂₃ kernel is assembled from two icosahedral Au₁₃ units in a coplanar manner.^10,11 [Au₂₅(PPh₃)₁₀(SR)₅Cl₂]²⁺ is evolved from two Au₁₃ icosahedra sharing one vertex and [Au₃₇(PPh₃)₁₀(SR)₁₀X₂]⁺ is formed by three Au₁₃ icosahedra sharing vertices in a linear form.^12,13 Recently Ag₆₁(dpa)₂₇(SbF₆)₄ was found to contain four linear vertex-sharing Ag₁₃ icosahedra.¹⁴ Similarly, the cores of both Au₂Ag₄₂(SAdm)₂₇(BPh₄) and [Au₂Ag₄₈(S-tBu)₂₀(Dppm)₆Br₁₁]Br(BPh₄)₂ are composed of two icosahedral Ag₁₃ units.¹⁵ Besides icosahedral M₂Au₃₆(PET)₂₄ was reported from the structural fusion of [HMAu₈(PPh₃)₈]⁺ and [MAu₂₄(PET)₁₈]⁻.¹⁶ Ag₂Au₅₀(SR)₃₆ was synthesized from two Au₂₅(SR)₁₈ units assembled with two Ag atoms in a hand-in-hand mode.¹⁷ These studies are exciting in the rational design and fabrication of tailored structures and reveal that the perfect nanocluster assembly deserves more efforts to tailor the functionality on an atom-by-atom basis.^18–21

In this work, we successfully synthesized monomeric Au₂₄Ag₂₀(C₁₂H₁₃)₂₄Cl₂ and Au₂₄Ag₂₀(C₉H₇)₂₄Cl₂ nanoclusters with similar kernels comprised of a hollow Au₁₂ icosahedron surrounded by fullerene-like Ag₂₀. The two Au₂₄Ag₂₀ monomers are further fused along different pathways into the Au₁₂@Ag₁₉-Au-Au₁₂@Ag₁₉ kernels and finally form Au₄₃Ag₃₈(C₁₂H₁₃)₃₆Cl₁₂ and Au₄₃Ag₃₈(C₉H₇)₃₆Cl₉ dimeric nanoclusters, respectively. The Au₂₄Ag₂₀ monomers and Au₄₃Ag₃₈ dimers offer a novel platform for atomic manufacturing on alloy nanoclusters to construct harmonious structures and unveil currently elusive properties such as electronic structures and catalytic properties.

Results and discussion

As shown in Fig. 1, based on a “cluster to cluster” strategy, four nanoclusters can be synthesized. AuSMe₂Cl and CH₃COOAg precursors were reduced by C₄H₁₄BN in the presence of 4-tert-butylphenylacetylene and sodium methoxide. Au₂₄Ag₂₀(C₁₂H₁₃)₂₄Cl₂ (abbreviated as Au₂₄Ag₂₀-1) was formed in a 12 hour reaction and then transformed into Au₄₃Ag₃₈(C₁₂H₁₃)₃₆Cl₁₂ (abbreviated as Au₄₃Ag₃₈-1) in another 12 hour reaction. With 2-methylphenylacetylene as the ligand and diphenylsilane as the reducing agent, Au₂₄Ag₂₀(C₉H₇)₂₄Cl₂ (denoted as Au₂₄Ag₂₀-2) was first obtained and further converted to Au₄₃Ag₃₈(C₉H₇)₃₆Cl₉ (denoted as Au₄₃Ag₃₈-2).

Fig. 1 Synthetic scheme of monomeric Au₂₄Ag₂₀(C₁₂H₁₃)₂₄Cl₂ and Au₂₄Ag₂₀(C₉H₇)₂₄Cl₂ and dimeric Au₄₃Ag₃₈(C₁₂H₁₃)₃₆Cl₁₂ and Au₄₃Ag₃₈(C₉H₇)₃₆Cl₉. C₁₂H₁₃ = deprotonated 4-tert-butylphenylacetylene, C₉H₇ = deprotonated 2-methylphenylacetylene. H atoms are omitted for clarity.

Electrospray ionization mass (ESI-MS) and matrix-assisted laser desorption/ionization time of flight mass (MALDI-TOF-MS) spectrometry were carried out to confirm the cluster formula. The major peaks of monomers at m/z = 10 995 and 9719 were assigned to [Au₂₄Ag₂₀(C₁₂H₁₃)₂₄Cl₂ + 2Cs⁺]²⁺ and [Au₂₄Ag₂₀(C₉H₇)₂₄Cl₂ − 2e]²⁺ (Fig. S1†), respectively. The mass peaks of dimers at m/z = 18 500 and 16 916 were related to Au₄₃Ag₃₈(C₁₂H₁₃)₃₅Cl₁₂ and Au₄₃Ag₃₈(C₉H₇)₃₅Cl₉ (Fig. S2;† note that one alkyne ligand for Au₄₃Ag₃₈ was removed during the measurements), respectively. The total structures of the four clusters were determined by X-ray crystallography (Tables S1–S4†). Among them, the two monomers have identical kernels of Au₁₂@Ag₂₀, which can be viewed as a hollow Au₁₂ icosahedral structure surrounded by a fullerene-like Ag₂₀ shell (Fig. 2A). The kernels of dimers are generated by the fusion of two Au₁₂@Ag₂₀ units of monomers in a mode of Au₁₂@Ag₁₉–Au–Au₁₂@Ag₁₉ (Fig. 2). This is completely different from previous cases, in which the building blocks of nanoclusters are mainly based on icosahedral units.

Fig. 2 (A) The kernel anatomy of Au₂₄Ag₂₀ containing a hollow Au₁₂ icosahedron surrounded by fullerene-like Ag₂₀ and the kernel anatomy of Au₄₃Ag₃₈ fused from two Au₁₂@Ag₂₀ units sharing a Au atom that replaces a Ag atom. The assembly pathways of (B) Au₄₃Ag₃₈-1 and (C) Au₄₃Ag₃₈-2 dimers evolved from the Au₂₄Ag₂₀ monomers.

In terms of the monomers, the average Au–Au bond lengths of the Au₁₂ icosahedral cores are very close (2.786 Å for Au₂₄Ag₂₀-1 and 2.785 Å for Au₂₄Ag₂₀-2). As shown in Fig. S3,† the average Au–Ag bond lengths between the Au₁₂ core and Ag₂₀ shell are 2.878 Å in Au₂₄Ag₂₀-1 and 2.868 Å in Au₂₄Ag₂₀-2, respectively, and both are shorter than those of bulk Au or bulk Ag, implying the strong metal bonding between the Au₁₂ core and Ag₂₀ shell. Of note, the Au–Au distances between the Au₁₂ core and outer Au₁₂ shell and between the Ag₂₀ shell and outer Au₁₂ shell are longer than those of bulk Au and Ag, indicating the weak interaction between the outermost Au atoms and Ag₂₀ shell. Obviously, the surface arrangements of the two Au₂₄Ag₂₀ nanoclusters are different. Au₂₄Ag₂₀-1 has four binding types on the Ag₅ faces (Fig. S4A–D†), while Au₂₄Ag₂₀-2 has three binding structures on the Ag₅ faces (Fig. S4E–G†), although both Au₂₄Ag₂₀ nanoclusters exhibit such coordination modes on the staples: 12 as η₃-μ₁(Au), μ₂(Ag), and μ₂(Ag) and 12 as η₂-μ₁(Au) and μ₂(Ag). The different binding motifs also lead to distinguishable steric arrangements of two Au and two Cl atoms at the top of the Au₂₄Ag₂₀ nanoclusters: symmetrically upward to form two parallel staples for Au₂₄Ag₂₀-1 (upper panel of Fig. 3); distinctively twisted to form two crossed staples for Au₂₄Ag₂₀-2 (lower panel of Fig. 3). The specific staples of the two nanoclusters might be acting as identification cards for identifying changes that have taken place in subsequent polymerizations.

Fig. 3 (A) Two enantiomers of Au₂₄Ag₂₀-1 (upper panel); two enantiomers of Au₂₄Ag₂₀-2 (lower panel). (B) The specific staples and corresponding steric configurations of Au₂₄Ag₂₀: two parallel staples and two Cl arrangements for Au₂₄Ag₂₀-1 (upper panel); two crossed staples and two Cl arrangements for Au₂₄Ag₂₀-2 (lower panel).

With respect to the dimers, their kernel structures are alike. The average Au–Au bond lengths of the Au₁₂–Au–Au₁₂ core are 2.790 Å in Au₄₃Ag₃₈-1 and 2.789 Å in Au₄₃Ag₃₈-2. The lengths of the Au–Au bonds directly connected to the central gold are the shortest in the whole core (Fig. S5†). The average Au–Ag lengths between the Au₁₂–Au–Au₁₂ core and Ag₃₈ shell are 2.891 Å in Au₄₃Ag₃₈-1 and 2.911 Å in Au₄₃Ag₃₈-2, suggesting the weak interaction between the Au core and Ag shell, which differs from the monomeric cases. Additionally, the average Au–Au distances between the Au₁₂–Au–Au₁₂ core and outer Au₁₈ shell are 2.856 Å in Au₄₃Ag₃₈-1 and 2.883 Å in Au₄₃Ag₃₈-2, suggesting strong a Au–Au force on the outer shell. The Au–Ag distances between the Ag₃₈ shell and outer Au₁₈ shell are longer than 3 Å for both Au₄₃Ag₃₈ nanoclusters and are consistent with the monomers. Moreover, clearly discernible surface motifs are presented in the Au₄₃Ag₃₈ clusters: Au₄₃Ag₃₈-1 has four binding types on the Ag₅ faces with the coordination modes (Fig. S6†): 22 as η₃-μ₁(Au), μ₂(Ag), and μ₂(Ag) and 14 as η₂-μ₁(Au) and μ₂(Ag); Au₄₃Ag₃₈-2 has five binding types on the Ag₅ faces with the coordination modes (Fig. S7†): 24 as η₃-μ₁(Au), μ₂(Ag), and μ₂(Ag) and 12 as η₂-μ₁(Au), μ₂(Ag). Both dimers contain 36 alkyne ligands, but Au₄₃Ag₃₈-1 has 12 Cl and Au₄₃Ag₃₈-2 has only 9 Cl. In addition, both dimers have six four-coordinated Cl atoms, but Au₄₃Ag₃₈-1 has another six one-coordinated Cl atoms and Au₄₃Ag₃₈-2 has another three two-coordinated Cl atoms. More interestingly, the specific staples as identification cards can also be found in the two dimeric nanoclusters (Fig. 4): the parallel staples on Au₄₃Ag₃₈-1 and the crossed staples on Au₄₃Ag₃₈-2.

Fig. 4 (A) Fusion of isomeric Au₂₄Ag₂₀ into Au₄₃Ag₃₈ containing chiral units. (B) The distinct staples and corresponding steric configurations of the Au₄₃Ag₃₈ nanoclusters: two parallel staples and two Cl position for Au₄₃Ag₃₈-1 (upper panel); two crossed staples and two Cl position for Au₄₃Ag₃₈-2 (lower panel).

It is worth pointing out that the unit cell of each Au₂₄Ag₂₀ monomer comprises two enantiomers, and hence the monomers are racemic (Fig. 3A). One Au₂₄Ag₂₀ isomer rotates 180° and then fuses with the other isomer by four or six Cl linkages to form Au₄₃Ag₃₈, in which two fusion units are chiral and the total Au₄₃Ag₃₈ clusters are thus mesomeric (Fig. 4A). Remarkably, Au₄₃Ag₃₈-1 is assembled from two chiral units along the diagonal direction, while Au₄₃Ag₃₈-2 is evolved from the vertical fusion of two chiral units. During the fusion process, each fusion unit of Au₄₃Ag₃₈-1 shows 5 alkyne rearranged, 6 alkyne detached and 4 Cl added (upper panel of Fig. 4A). However, when the fusion process happened to Au₄₃Ag₃₈-2, one fusion unit has 5 alkyne relocated, 6 alkyne left and 3 Cl fixed, whereas the other fusion unit shows 6 alkyne relocated, 6 alkyne and one Cl fell off, and another 3 Cl appended (lower panel of Fig. 4A).

We next compared the electronic properties of monomeric and dimeric nanoclusters. The UV-vis absorption spectra of Au₂₄Ag₂₀ show four apparent peaks, while those of Au₄₃Ag₃₈ show five absorption peaks (Fig. S8†). It implies different electronic structures between monomers and dimers. We further performed density functional theory (DFT) calculations on the electronic configuration of these nanoclusters. From Fig. 5A, it is obvious that the Kohn–Sham (KS) orbital diagram of the Au₂₄Ag₂₀ monomer is similar to that of the orbitals of atoms, with the characteristics of s, p, d, and f atomic orbitals. Considering that Au₂₄Ag₂₀ has 18 valence electrons, it just forms an electron shell of 1s²|1p⁶|1d¹⁰, which is identical to the KS electron density diagram of typical superatomic Au₂₅(SR)₁₈⁻ (Fig. S9A and B†).²² Therefore, Au₂₄Ag₂₀ can be classified into the category of superatomic clusters. Surprisingly, Au₄₃Ag₃₈ and Au₃₈(SR)₂₄ exhibit similar molecular orbital types (Fig. 5B, S9C and D†): bonding orbitals σ and π; anti-bonding orbitals σ* and π*.²³ Obviously, these molecular orbitals can be viewed as linear combinations of different atomic orbitals of Au₂₄Ag₂₀ in different ways (head-to-head combination to obtain σ orbitals and side-to-side combination to form π orbitals). It is noted that due to the influence of the outer layer ligand, the molecular orbital diagram of the overall structure of Au₄₃Ag₃₈ and the orbital diagram of the inner core structure are different, e.g., the molecular orbital of the inner core has a δ orbital (Fig. S9D†). Nevertheless, this does not affect our qualitative judgement on the properties of the molecule. Overall, the configurations that can form superatoms are not only icosahedral but also diversified, and even alloy clusters also have superatomic or molecular-like characteristics.

Fig. 5 The Kohn–Sham orbital diagrams of (A) Au₂₄Ag₂₀ and (B) Au₄₃Ag₃₈. Simplified calculations with methyl as a substituent. The subscript in the molecular orbital symbol indicates the type of atomic orbital that is linearly combined into molecular orbitals. The head-to-head overlap of p atomic orbitals leads to bonded (σ_p–p) and anti-bonded molecular orbitals, where the electron density is centered along the internuclear axis, making them σ orbitals. The π_d–d and orbitals are a linear combination of two d atomic orbitals side-to-side. HOMO: highest occupied molecular orbital. LUMO: lowest unoccupied molecular orbital.

More importantly, the inherent ability of electron transport in monomeric and dimeric nanoclusters is readily differentiated in their electrochemical impedance spectra (Fig. S10†). The semicircular diameters of the Au₂₄Ag₂₀ clusters are smaller than those of the Au₄₃Ag₃₈ clusters, meaning that the electron transport in the monomers might be faster than that in the dimers. Given the inherent advantages of employing the cluster catalysts in electronic properties, we envisioned the CO₂ reduction reaction (CO₂RR) involving the electron transfer process to explore the catalytic properties of the monomers and dimers. We were pleased to observe different reactivities of the four nanocluster catalysts for the CO₂RR and the high performances were achieved in the monomers. From the linear scanning voltammetry (LSV) curves of Au₂₄Ag₂₀ and Au₄₃Ag₃₈ in CO₂-saturated solutions of 0.5 M KHCO₃, the monomers showed higher current density than the dimers (Fig. 6A). Meanwhile, the CO partial current density was arranged in descending order as Au₂₄Ag₂₀-1 > Au₂₄Ag₂₀-2 > Au₄₃Ag₃₈-1 > Au₄₃Ag₃₈-2 (Fig. 6B). Notably, the monomers showed much higher faradaic efficiency (FE) toward CO than the dimers in the voltage range from −0.4 to −0.8 V (Fig. 6C and D). 90% CO FE was obtained at a voltage of −0.5 V over the Au₂₄Ag₂₀ catalysts. The results were partially related to their atomic-packing structures (individual-core vs. dual-core) and surface motif arrangements (parallel vs. crossed).^24,25 Further studies are ongoing to elucidate the roles of superatomic and molecular-like electronic properties on cluster catalysis, which so far remain elusive.

Fig. 6 (A) LSV curves of the Au₂₄Ag₂₀ and Au₄₃Ag₃₈ cluster catalysts in a CO₂-saturated 0.5 M KHCO₃ solution. (B) The corresponding CO partial current density. (C) Faradaic efficiency for CO₂RR products obtained on Au₂₄Ag₂₀. (D) Faradaic efficiency for CO₂RR products obtained on Au₄₃Ag₃₈.

Conclusions

In summary, we have successfully implemented the evolution of racemic Au₂₄Ag₂₀ monomers into mesomeric Au₄₃Ag₃₈ dimers and mapped out significant differences in steric configurations and electronic structures between the monomeric and dimeric nanoclusters. Our studies show that the Au₂₄Ag₂₀ monomers exhibit more efficient reactivity than the Au₄₃Ag₃₈ dimers for CO₂ reduction processes. This work not only provides a strategy for hierarchical assembly of metal nanoclusters to tune their structure and functionality, but also provides a paradigm of the monomeric and dimeric alloy nanoclusters to find applications in challenging chemical reactions.

Data availability

The X-ray crystallographic structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre (CCDC) with deposition numbers (CCDC: 2129383 for Au₄₃Ag₃₈(C₁₂H₁₃)₃₆Cl₁₂; CCDC: 2129384 for Au₂₄Ag₂₀(C₉H₇)₂₄Cl₂; CCDC: 2129385 for Au₄₃Ag₃₈(C₉H₇)₃₆Cl₉; CCDC: 2129386 for Au₂₄Ag₂₀(C₁₂H₁₃)₂₄Cl₂).

Author contributions

Y. Z. conceived the project. J. X. synthesized the nanoclusters and grew the crystal. L. X. and Y. P. conducted the calculations. X. C. and S.T. did the catalytic tests. A. T. and X. L. analyzed the crystal data. All authors wrote the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge financial support from National Natural Science Foundation of China (22125202; 91961121) and Programs for High-Level Entrepreneurial and Innovative Talents Introduction of Jiangsu Province.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2129383–2129386. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1sc07178e

‡ These authors contributed equally to this work.

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