Constructing perfect cubic Ag–Cu alloyed nanoclusters through selective elimination of phosphine ligands

Abstract

The aspiration of chemists has always been to design and achieve control over nanoparticle morphology at the atomic level. Here, we report a synthesis strategy and crystal structure of a perfect cubic Ag–Cu alloyed nanocluster, [Ag₅₅Cu₈I₁₂(S-C₆H₃^2,4(CH₃)₂)₂₄][(PPh₄)] (Ag₅₅Cu₈I₁₂ for short). The structure of this cluster was determined by single-crystal X-ray diffraction (SCXRD) and further validated by X-ray photoelectron spectroscopy (XPS), inductively coupled plasma (ICP), Energy-dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), and ¹H and ³¹P nuclear magnetic resonance (NMR). The surface deviation of the cube was measured to be 0.291 Å, making it the flattest known cube to date.

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Regulating nanoparticle morphology has always been a prominent topic in current research, as the physical and chemical properties of nanoparticles are closely tied to their shape and structure.^1–5 For instance, spherical nanoparticles and rod-shaped nanoparticles exhibit distinct behaviours in terms of optical properties, electron transport, and catalytic reactions.^6–8 With advancements in synthesis technology, researchers can now tune nanoparticle morphology using various approaches.^9–13 For instance, Mirkin's group reported the photoinduced transformation of Ag nanospheres into triangular nanoprisms.¹² Yang's group achieved cubic Ag nanoparticles through the use of mercaptan as a directing agent.¹³ However, studying ordinary nanoparticles at the atomic level directly presents challenges due to their larger size and complex structure. Fortunately, metal nanoclusters have emerged as a swiftly advancing category of nanomaterials characterised by their atomically precise structures and diverse applications.^14–29 Furthermore, the morphology of metal nanoclusters, such as metal–metal bonds and metal–ligand interactions, can be precisely controlled through ligand exchange,^30–35 metal doping,^36–40 and other techniques.^41,42 Most importantly, single-crystal X-ray diffraction enables us to obtain atomic-level interface information, including the arrangement of doped metals within nanoclusters and the bonding modes between peripheral ligands and metals.

The synthesis of metal nanoparticles with polyhedral shapes, such as nanocubes, has always posed challenges compared to thermodynamically stable nanospheres. We have observed that Zheng's group recently synthesized an almost perfect silver nanocube.⁴³ The main reason for the deviation from a perfect cube is the displacement of metal atoms at the eight vertices from their ideal positions within the cube. Structural analysis suggests that this deviation is most likely caused by the coordination of triphenylphosphine ligands with the vertex Ag atoms. If the dissociation of the phosphine ligands can be achieved, it is theoretically possible to bring the displaced Ag atoms back to the vicinity of their ideal positions, thus realizing the construction of a perfect cube. Notably, introducing copper atoms into Ag₂₉(SSR)₁₂(PPh₃)₄ nanoclusters not only enhances their stability but also modifies their luminescent properties and catalytic activities.^44,45 By fine-tuning the doping methods of copper atoms, precise control over the size and composition of copper-based nanoclusters can be achieved.^46–49 In essence, copper atom doping imparts novel properties and functionalities to metal nanoclusters, thereby broadening their application scope and contributing to the advancement of nanotechnology.

In this study, we introduced Cu atoms during the synthesis of (R′ = ⁿBu or Ph, Ag₆₃ for short) nanoclusters, where the Cu atoms occupied the vertices of the cube due to the vertex effect.⁵⁰ By utilizing the difference in bonding energies between Cu–P and Ag–P bonds,⁵¹ we achieved the dissociation of the ligands at the vertex positions, leading to the synthesis of the [Ag₅₅Cu₈I₁₂(S-C₆H₃^2,4(CH₃)₂)₂₄][(PPh₄)] nanocluster (abbreviated as Ag₅₅Cu₈I₁₂). The surface deviation of this cluster has been measured to be 0.291 Å, making it the metal nanocube with the highest level of smoothness achieved to date.

The Ag₅₅Cu₈I₁₂ nanocluster was prepared via the “One-pot method”. Details of synthesis are provided in the ESI.† Briefly, the Ag₅₅Cu₈I₁₂ nanocluster was synthesized by the reduction of a mixture of AgNO₃, PPh₃, PPh₄Br, HS-C₆H₃^2,4(CH₃)₂, and CuI in the mixed solvent of CH₃OH and CH₂Cl₂ at room temperature. NaBH₄ was used as the primary reducing agent, while triethylamine was added to reduce the reactivity of NaBH₄. The reaction was conducted at room temperature and lasted for 12 hours. After completion, the crude product was washed with an excess of methanol and subsequently subjected to crystallization in a mixture of dichloromethane and n-hexane. Ag₅₅Cu₈I₁₂ crystallizes in a monoclinic system with C12/m1 group. SCXRD analysis of Ag₅₅Cu₈I₁₂ showed one PPh₄⁺ counterion in the system, indicating that the NC bears a −1 charge. The details of crystal data and refinement are provided in Table S1 (ESI†). It should be noted that due to its high symmetry, the complete PPh₄⁺ molecule was not resolved. Further confirmation of PPh₄⁺ was attained through the utilization of nuclear magnetic resonance (NMR) analyses, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and energy-dispersive X-ray spectrum (EDX).

The optical properties of Ag₅₅Cu₈I₁₂ and Ag₆₃ were compared. As shown in Fig. 1a and b, Ag₅₅Cu₈I₁₂ showed three main absorption peaks located at 353, 413, 503, and 827 nm, which are distinct from those of Ag₆₃ (located at 325, 415, 470, 580 and 840 nm). X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectrum (EDX) further verify the existence of Ag, Cu, S, P, C and I atoms (Fig. 1d and Fig. S2 and S3, ESI†). The Cu 2p XPS spectrum shows that the binding energies of Cu 2p_3/2 and Cu 2p_1/2 are at 932.8 and 952.6 eV (Fig. S2b, ESI†), respectively. The Auger Cu LMM spectrum was further utilized to verify that the copper atoms in Ag₅₅Cu₈I₁₂ exhibit a Cu(I) oxidation state (Fig. S2c, ESI†).⁵² Furthermore, the Ag 3d_5/2 binding energy can be deconvoluted into two distinct peaks, as depicted in Fig. S2d and e (ESI), suggesting the simultaneous presence of Ag(i) and Ag(0) within the Ag₅₅Cu₈I₁₂ nanocluster. These peaks, at approximately 368.89 eV and 367.95 eV, correspond to Ag(I) and Ag(0),⁵³ respectively, and are represented by a blue curve and a turquoise curve. The ratio of these peaks is found to be 2.6 : 1. Furthermore, XPS, EDX and inductively coupled plasma (ICP) measurements were performed to validate the ratio of Ag/Cu in the bi-metallic Ag₅₅Cu₈I₁₂ nanoclusters (Table S2, ESI†), and the results perfectly matched the theoretical value (55/8 of Ag/Cu), which is close to the ratio of 55/8 obtained from the crystal analysis results. TGA result of the product shows a weight loss of about 33.83%. The theoretical weight loss for the cluster is 30.36% (excluding solvent), with the additional loss attributed to the solvent. The results from SQUEEZE indicate the removal of approximately 388 electrons. Considering the varying electron counts at different positions and the solvent used in crystallization, we deduced the presence of about 2.4 dichloromethane molecules and 1.88 n-hexane molecules. The theoretical weight loss calculated from the SQUEEZE results is approximately 33.42%, which is in good agreement with the TGA findings (Fig. S4, ESI†). In order to further confirm the formula and valence state of Ag₅₅Cu₈I₁₂, we performed nuclear magnetic resonance (NMR) analysis (Fig. 1c and Fig. S5–S7, ESI†). ³¹P NMR spectra of Ag₅₅Cu₈I₁₂ were also recorded, further verifying the existence of a phosphine ligand in the structure (Fig. 1c and Fig. S5, ESI†). In the ¹H NMR spectrum of Ag₅₅Cu₈I₁₂ (Fig. S6 and S7, ESI†), the peaks at 6.70–7.90 ppm (92 H) are assigned to –C₆H₅ in Ph₄P⁺ and –C₆H₃ in –S-C₆H₃^2,4(CH₃)₂ the peaks at 1.65–1.80 ppm (144 H) are assigned to –CH₃ in the –S-C₆H₃^2,4(CH₃)₂ ligands. According to the proton ratio (C₆H₅ + C₆H₃:CH₃ = 92 : 144), we determined that the ratio of [Ag₅₅Cu₈I₁₂(SC₈H₉)₂₄]⁻ to PPh₄⁺ is 1 : 1. All the aforementioned results indicate that the precise formula of obtained nanocluster is [Ag₅₅Cu₈I₁₂(S-C₆H₃^2,4(CH₃)₂)₂₄][(PPh₄)]. In contrast to Ag₆₃, the distinguishing factor between the two lies in their oxidation states and the consequent variation in the number of free electrons. Ag₆₃ exhibits 26 free electrons, whereas Ag₅₅Cu₈I₁₂ possesses 28 free electrons. As demonstrated in Fig. S8 (ESI†), the energy scale spectra of Ag₅₅Cu₈I₁₂ and Ag₆₃ have been analyzed, revealing that the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in Ag₅₅Cu₈I₁₂ (E_g = 1.18 eV) is smaller compared to that in Ag₆₃ (E_g = 1.24 eV). This reduction in the energy gap may be attributed to the presence of two additional electrons in Ag₅₅Cu₈I₁₂.

Fig. 1 The UV-vis spectra of Ag₅₅Cu₈I₁₂ (a) and Ag₆₃ (b); (c) ³¹P-NMR spectrum of Ag₅₅Cu₈I₁₂ and (d) XPS result of Ag₅₅Cu₈I₁₂.

The crystal structure is composed of an Ag₅₅Cu₈ metal kernel, 24 μ₃-S ligands, and 12 μ₄-I ligands, with one PPh₄⁺ group as counterions (Fig. 2a and Fig. S9, ESI†). Specifically, for Ag₅₅Cu₈I₁₂, eight Cu atoms were substituted for the vertex site of the eight Ag atoms coordinated to the phosphine ligand in the mentioned Ag₆₃ nanocluster (Fig. 2a and b, blue dotted line). Additionally, in comparison to the Ag₆₃ nanocluster, the two sulfur ligands located at the center of the (100) facet of the nanocube were replaced by two iodine atoms, while maintaining a similar coordination mode (Fig. 2a and b, indicated by black dotted lines). The remaining 24 thiolate ligands are present on the edges of the cube, as depicted by the red dotted lines in Fig. 2a and b.

Fig. 2 (a)–(h) Comparison of Ag₅₅Cu₈I₁₂ and Ag₆₃ nanoclusters: (a) and (b) Framework structures of Ag₅₅Cu₈I₁₂ and Ag₆₃, respectively; (c) and (d) Cubic metal Frames of Ag₅₅Cu₈I₁₂ and Ag₆₃, respectively; (e) and (f) comparing the deviations in the edges of the cubic structures between Ag₅₅Cu₈I₁₂ and Ag₆₃; (g) and (h) comparing the angular deviations on the edges of the cubic structures between Ag₅₅Cu₈I₁₂ and Ag₆₃. Ag = dark green, Cu = orange, S = yellow, I = brown, P = magenta. For clarity, carbon and hydrogen atoms are omitted.

The results of the single-crystal analysis clearly demonstrate that the Ag₅₅Cu₈I₁₂ cluster exhibits a closer approximation to a perfect cube compared to the Ag₆₃ nanocluster (Fig. 2a–h). All the data regarding bond lengths in the clusters can be found in Table S3 (ESI†). In the main text, we only discuss the key data that determine whether the cluster exhibits a perfect cubic structure. In the Ag₅₅Cu₈I₁₂, the bond lengths between the vertex Cu atom and the three adjacent Ag atoms are 2.851 Å, 2.921 Å, and 2.915 Å, respectively (Fig. 2c). In Ag₆₃, the bond lengths between the vertex silver atom and the neighboring silver atoms are 3.938 Å, 3.953 Å, and 3.892 Å, respectively (Fig. 2d). These values are significantly larger than the Ag–Ag (2.880 Å) van der Waals radius. This indicates closer proximity and stronger bonding compared to Ag₆₃, where the vertex silver atom protrudes outward due to phosphine ligand coordination. The analysis of the deviation of two clustered cube structures compared to the standard cube is depicted in Fig. 2e and f. In the case of Ag₅₅Cu₈I₁₂, the distance between Cu1–Ag1–Cu2, representing one edge of the cubic-like structure, is 0.276 Å from the corresponding edge of the standard cube, while for Ag₆₃, it is 0.757 Å. Similarly, the other edges were also evaluated (Fig. S10, ESI†). The results indicate that the Ag₅₅Cu₈I₁₂ cube exhibits a surface deviation closer to that of the standard cube compared to Ag₆₃ (0.291 Å vs. 0.723 Å). As shown in Fig. 2g, for Ag₅₅Cu₈I₁₂, the cubic framework is constructed by 12 Ag and 8 Cu. Every edge of the cube is formed by connecting two Cu atoms and one Ag atom. Taking the Cu1–Ag1–Cu2 unit as an example, the bond lengths of Cu1–Ag1 and Ag1–Cu2 are 4.122 Å and 3.998 Å, respectively.

Notably, the sum of the Cu1–Ag1 and Ag1–Cu2 bond lengths (8.120 Å) in the case of Ag₅₅Cu₈I₁₂ is almost equivalent to the Cu1–Cu2 distance (8.101 Å) with a difference of 0.019 Å (Fig. 2g and Table S4, ESI†). The bond angle of Cu1–Ag1–Cu2 is 172.17°, indicating that Cu1, Ag1, and Cu2 are almost in a straight line, deviating by only 4.35% from 180° (Fig. 2g and Table S5, ESI†). In contrast, for Ag₆₃, the Ag1′–Ag3′ bond length is 9.514 Å, which is significantly shorter than the sum of the bond lengths Ag1′-Ag2′ (4.838 Å) and Ag2′–Ag3′ (4.796 Å), with a difference of 0.12 Å (Fig. 2h and Table S4, ESI†). Furthermore, the Ag1′–Ag2′–Ag3′ bond angle is 161.89°, deviating by 10.06% from 180° (Fig. 2h and Table S5, ESI†). This difference indicates that Cu1, Ag1, and Cu2 in Ag₅₅Cu₈I₁₂ are more nearly in a straight line compared to Ag1′-Ag2′-Ag3′. The comparison was extended to the remaining edges of the cubic metal kernel, leading to the conclusion that Ag₅₅Cu₈I₁₂ is closer to the standard cubic structure (Tables S4 and S5, ESI†). This exciting result demonstrates the possibility of customizing nanocluster morphology through metal doping. Furthermore, we assessed the stability of the two nanoclusters and found that the shape-modified Ag₅₅Cu₈I₁₂ exhibited higher stability (Fig. S11, ESI†).⁴³ Meanwhile, the packing modes of these two nanoclusters exhibit distinct differences in their crystal lattices, as shown in Fig. S12 (ESI†).

In summary, our work focuses on the synthesis and precise structural characterization of the flattest nanocube achieved to date. The formula of this nanocluster, [Ag₅₅Cu₈I₁₂(S-C₆H₃^2,4(CH₃)₂)₂₄][(PPh₄)], was determined by single-crystal X-ray diffraction (SC-XRD) and further validated through UV-Vis, XPS, ICP, EDX, ³¹P NMR, and ¹H NMR analyses. By combining the vertex effect of metal doping and the weak Cu-P interaction, we successfully achieved directional modulation of the Ag₆₃ nanocluster morphology. Notably, the morphologically modified clusters exhibited significant differences in terms of UV-vis absorption and stability compared to their pre-modulation counterparts. The directed control of metal nanoparticle cluster morphology holds great potential in enhancing the performance and expanding the applications of nanoclusters, while deepening our understanding of the structure-performance relationship in nanomaterials.

S.W. designed the project, analyzed the data, and wrote the manuscript. L. T. and Q. H. carried out experiments, analyzed the data and revised the manuscript. B. W., Z. Y., C. S. and G. F. assisted in the synthesis of samples.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

S. W. acknowledges the financial support provided by the National Natural Science Foundation of China (22171156 and 21803001), the Taishan Scholar Foundation of Shandong Province, and Startup Foundation of Qingdao University of Science and Technology.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Fig. S1–S9 and Tables S1–S5 for the crystal structure, XPS, EDX, NMR and UV-vis results of nanoclusters. CCDC 2283567. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3cp04224c

‡ These authors contributed equally to this work.

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