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. 2023 Feb 8;6:24. doi: 10.1038/s42004-023-00817-5

Atomically precise copper dopants in metal clusters boost up stability, fluorescence, and photocatalytic activity

Yifei Zhang 1,2,#, Jingjing Zhang 2,3,#, Zhiwen Li 2, Zhaoxian Qin 2,3,✉, Sachil Sharma 2,4, Gao Li 2,3,✉
PMCID: PMC9908894  PMID: 36755056

Abstract

The structurally precise alloy nanoclusters have been emerged as a burgeoning nanomaterial for their unique physical/chemical features. We here report a rod-like nanocluster [Au12Cu13(PPh3)10I7](SbF6)2 (Au12Cu13), which was generated through a transformation of a [Au9(PPh3)8]3+ intermediate in the presence of CuI, unveiled by time-dependent UV-vis spectroscopy, electrospray ionization mass spectrometry as well as single crystal X-ray diffraction. Au12Cu13 is comprised of two pentagonal bipyramids Au6Cu units and a pentagonal prism Cu11 unit, where the copper and gold species are presented in +1 and 0 chemical states. The Cu-dopants significantly improved the stability and fluorescence (quantum yield: ~34%, 34-folds of homo-Au25(PPh3)10Br7). The high stability of Au12Cu13 is attributed to the high binding energy of iodine ligands, Au-Cu synergistic effects and its 16-electon system as an 8-electron superatom dimer. Finally, the robust Au12Cu13 exhibited high catalytic activity (~92% conversion and ~84% methyl formate-selectivity) and good durability in methanol photo-oxidation.

Subject terms: Nanoparticles, Nanoparticle synthesis, Optical materials, Photocatalysis

Copper doping of atomically precise gold nanoclusters is a useful strategy to tune their chemical and physical properties, but Au–Cu nanocluster alloys tend to exhibit poor stability. Here, a [Au12Cu13(Ph3P)10I7](SbF6)2 cluster is prepared and shown to display enhanced stability and fluorescence in comparison to homonuclear cluster [Au25(PPh3)10Br7](SbF6)2, in addition to promising photocatalytic activity for methanol oxidation.

Introduction

In the famous lecture in 1959 with the title “There is plenty of room at the bottom”, R. Feynman predicted the beginning of the research at the atomic level1. His prediction has been come true in a real sense with the emergence of research in the area of magic nanoclusters, where we can make nanoscale things at the atomic level in a highly controlled fashion2. Such functionalized nanoscale materials at the atomic level actually gained significant importance in technology progression due to their tunable physical and chemical properties3,4. Serving as an important part of such nanomaterials, the metal nanoclusters “MnLm”, where n and m represent the number of metal atoms and surficial protecting ligands (L), respectively, have been highly promising in both fundamental research and practical applications, such as sensing5, photo-luminescence6, catalysis4,7,8 and bio-imaging9,10, owing to their unique and modulating physicochemical properties.

Metal atom doping is an important strategy to tune the structure and properties of metal nanoclusters11–15. Recently, the copper species doped into gold nanoclusters to generate Au-Cu alloy composites can well tailor their electronic structures and in turn to boosting the intrinsically physical and chemical properties, especially in luminescence and catalysis13,16–21. For example, Jin et al. have prepared an Au@Cu alloy nanocluster with a 71.4% quantity yield (QY) of fluorescence18. We found that the copper atoms doped in M25(SC2H4Ph)18 clusters, primarily enhanced the benzaldehyde-selectivity in the oxidation of styrene20. Unfortunately, Au-Cu alloy nanoclusters tend to show less stable under external environments21–23 (e.g., light irradiation, oxidizer, and thermal conditions) due to the nature of CuI and Cu0 species, corroborated by the basis of Jellium Model12. Therefore, copper clusters and Au-Cu alloy clusters were often acquired under extreme conditions and stored in cool and dark place filled with inert atmosphere. The poor stability of Cu-based clusters becomes a burning issue and is still a big challenge for their applications.

Focused on the tunable properties and the improvement of stability of Cu doped gold clusters, we, herein, designed a strategy to prepare the rod-like [Au12Cu13(Ph3P)10I7](SbF6)2 (abbreviated as Au12Cu13, hereafter) alloy cluster based on the self-assemble behavior of [Au9(PPh3)8](SbF6)3 (Au9 in short) clusters in the presence of CuI, which was further monitored by a serious of time-dependent measures including ultraviolet-visible (UV-vis) spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and single crystal X-ray diffraction (SCXRD) to conclude the assembling process/mechanism of Au12Cu13 nanocluster. To our surprised, although Au12Cu13 cluster shares similar structure with the rod-like [Au13Ag12(PPh3)10Cl8](SbF6)14 and [Au25(PPh3)10Br7](SbF6)2 (noted as Au25 hereafter) clusters obtained in similar way, the dopant of Cu atoms in this alloy cluster has boosted its stability and photoluminescence significantly. The synergistic effects of Au and Cu atoms and strong Cu-I bonds may be the reason for the strong and high fluorescence of Au12Cu13 clusters. Finally, the Au12Cu13 cluster-based photo-catalyst was prepared based on the high stability and unique photo-properties, which shown excellent catalytic activity in the selective photo-oxidation of methanol.

Results

Synthesis and structure determination

The alloy nanoclusters with composition of [Au12Cu13(Ph3P)10I7](SbF6)2 were prepared in a step-by-step synthetic strategy. Briefly, a clear solution obtained upon mixing Ph3PAuCl and AgSbF6 was reduced by NaBH4 giving a dark-brown mixture, which was further reacted with CuI to produce the target clusters. To figure out the progress of alloy cluster formation, we firstly monitored the reaction solution by UV-vis spectroscopy and ESI-MS before the CuI addition. The obtained red-brown mixture gave five obvious peaks at 315, 350, 378, 443, and 511 nm in UV-vis spectrum (Fig. 1a), similar to that of the well-known [Au9(PPh3)8]3+ clusters24. And four main mass peaks at 1290.45 Da, 1705.90 Da, 1822.5 Da, and 2054.49 Da in the scale of m/z 200–10,000 Da, assigned to the compositions of [Au9(PPh3)8]3+ (m/z (z = 3) 1290.33 Da calcd.), [Au8(PPh3)7]2+ (m/z (z = 2) 1705.85 Da calcd.), [Au9(PPh3)7Cl]2+ (m/z (z = 2) 1822.3 Da calcd.), and [Au9(PPh3)8SbF6]2+ (m/z (z = 2) 2054.37 Da calcd.), respectively, were detected in positive ion mode of ESI-MS (Fig. 1b and Supplementary Figure 1), which confirmed the existence of [Au9(PPh3)8]3+ clusters in the obtained mixture. Note that such fragmentations are usual owing to the dissociation during ESI ionization of these phosphine protected metal nanoclusters14. All these results suggested that Au9 cluster is dominant and served as starting material/precursor for the further construction of Au12Cu13 cluster.

Fig. 1. The conversion of Au9 to Au12Cu13 clusters.

Fig. 1

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a UV-vis spectrum (in CH2Cl2) and b positive ion mode ESI-MS of Au9 cluster. Conversion of Au9 to Au12Cu13 clusters monitored by c time-dependent UV-vis spectra and d time-dependent ESI-MS.

Next time-dependent UV-vis spectroscopy (TD-UV-vis) and ESI-MS were applied to monitor the construction process of Au12Cu13 cluster originated from the reaction of Au9 cluster and CuI in solution. As presented in Fig. 1c, the intensity of characteristic absorption peaks of Au9 cluster at 315 and 443 nm decreased within initial 10 min, accompanied with the color deepening after CuI addition. Further, the peaks at 352, 434, 508, and 655 nm increased gradually in TD-UV-vis, indicating that the Au9 cluster was transforming into other clusters. Figure 1d showed that the mass signals corresponding to Au9 cluster disappeared quickly, and a series of new peaks (Supplementary Table 1) were shown up, demonstrating that Au9 cluster reacted with CuI rapidly generating some metastable clusters, such as [AuCu(Ph3P)2I]+, [Au3(Ph3P)3CuI]+, [Au6(Ph3P)6]+, [Au8Cu(Ph3P)8I]2+ and [Au9Cu(PPh3)8I2]2+. These metastable clusters were further aggregated with each other to furnish Au25-xCux clusters during the time-consuming thermodynamic process. Similar to the silver-doped AgxAu25−x nanoclusters15, the number of Cu dopants in M25 cluster ranges from 1 to 9, which is lower than that of the final Cu atoms (13) in the tested crystal sample. It may indicate that the Cu-doping process keeps going on during the crystallization process and the Au12Cu13 should be the most robust one in the final products14.

The composition and purity of the obtained Au12Cu13 crystals were manifested by ESI-MS and X-ray photoelectron spectroscopy (XPS). As shown in Fig. 2a, the ESI-MS spectrum exhibited an intense peak at m/z ~3350.45 Da (z = 2, calcd. m/z ~3350.44 Da for Au12Cu13P10C180H150I7, deviation: 0.01 Da) in the scale from m/z ~1400 to 7000, corresponding to [Au12Cu13(PPh3)10I7]2+ cluster with high molecular purity. The isotopic pattern was found to be in exact agreement with simulated one and the peaks separation of m/z ~ 0.5 Da, confirming the +2 charged state of Au12Cu13 nanocluster. These results demonstrated that the cluster specie in crystals is pure [Au12Cu13(PPh3)10I7]2+ instead of Au/Cu alternation and the Au12Cu13 cluster exhibit a 16-electron system (i.e. 25 (metal atoms) – 7 (I atoms) – 2 (charge) = 16).

Fig. 2. Physical property of the Au12Cu13 nanocluster.

Fig. 2

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a ESI-MS with isotopic pattern (insert). b Wide scan XPS. c Au 4 f and d Cu 2p XPS spectra of Au12Cu13 nanocluster.

The bimetal nature of Au12Cu13 cluster was also characterized by the wide scan XPS, Fig. 2b. The binding energy (BE) of Au 4f7/2 in Au12Cu13 can be deconvoluted to Au0 (highlighted in orange) and AuI species25 (Fig. 2c). Taking note of previous investigations, the final-state hole-shielding effect arising from extra atomic relaxation acts in opposite to electron donating effect of phosphine ligands, resulting in the increase of binding energy26–28. An unsymmetrical BE peak of Cu 2p3/2 near 933.1 eV was presented in Cu 2p XPS (Fig. 2d), excluding the existence of CuII species (934.2 eV for Cu 2p in CuII)29. And the Cu 2p3/2 peak also can be deconvoluted into two peaks, corresponding to Cu0 species at 932.5 eV and CuI at 933.1 eV30. For validation, Auger electron spectrum showed a broad peak at 571.0 eV with a 1.0 eV positive shift with respect to the value of CuI (570.0 eV) and a weak peak to Cu0 at 568.0 eV (Supplementary Figure 2), indicating the presence of both Cu+ and Cu0 species in Au12Cu13. And the Cu0 species, generally, should be attributed to the shared vertex Cu atom, while the CuI one should belong to the peripheral and end vertex Cu atoms bonded directly with I atoms according to SCXRD analysis (vide infra). To the best of our knowledge, the Au12Cu13 cluster in present work is the first case of AuCu alloy cluster, where the Cu species exists both in +1 and 0 of chemical states, without any partial occupancy.

Based on the results of ESI-MS and UV-vis, we put forward explanation that the Au12Cu13 cluster should be prepared via the transformation of Au9 clusters upon a reaction with CuI. Thus, we conceived a sequence of experiments for the transformation of Au9 clusters into another M25 clusters, including rod-like Au25-yAgy(PPh3)10Cl7 (Au25-yAgy) and homo Au25(PPh3)10Br7 (Au25) in the presence of AgCl and KBr, respectively. Further, with respect to the self-assembly of these clusters, their structural analyses are of the essence. Therefore, the crystals of Au12Cu13 and Au25 were grown by slow vapor diffusion of diethyl ether into a CH2Cl2 solution of clusters at ~10 °C over two weeks and characterized by SCXRD. Of note, the Au25-yAgy nanoclusters have been extensively studied14,31.

SCXRD analysis revealed that Au12Cu13 and Au25 clusters crystallize in P21/n and P21/m space groups, respectively. They share similar rod-like framework, as illustrated in Supplementary Figure 3. The metal core of Au12Cu13 can be divided into one Cu11 and two Au6Cu1 units (Fig. 3a). Two Au6Cu1 units bind with the Cu11 unit through Au-Cu metal bonds, generating an Au12Cu13 framework (Fig. 3b). The obtained Au12Cu13 metal kernel/core is directly ligated by ten PPh3 ligands through Au-P bonds and seven I- ions via Cu-I bonds furnishing the full structure of Au12Cu13 cluster. In detail, five I anions are coordinated to ten Cu atoms in Cu11 unit taking a µ2-bridging coordination mode (i.e., one I anion per two Cu atoms of Cu11 unit with an average Cu-I bond length of ~2.586(2) Å), and the remaining two I anions are bonded to the vertex Cu atoms, Fig. 3c. In another way, the rod-like Au12Cu13 cluster could be deemed as the fusion of two icosahedron Au6Cu7 units by sharing a Cu vertex like the well-known rod-like M25 (M = Au, Ag, Cu) clusters14,15,32. Of note, Au12Cu13 cluster is different from CuxAu25–x(PPh3)10(PhC2H4S)5Cl2 with an eclipsed arrangement in structure, where the copper atoms partially occupy the top and M11 sites and bonded with chlorine or thiolate ligands33.

Fig. 3. Configuration of Au12Cu13 cluster.

Fig. 3

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a, b Anatomy of Au12Cu13 cluster. c Side view and d top view. Color code: Au, yellow; Cu, blue and cyan; I, red; P, pink. Note that the other atoms are omitted for clarity.

In a typical way, the core structure of Au12Cu13 and Au25 can be regarded as a four-layer cylinder32, Supplementary Figure 4. It is worth noting that the two neighboring pentagons of layer II and layer III in Au12Cu13 showed a staggered arrangement and adopt a torsion angle of 12.1o (Fig. 3d). In comparison, no twisting is observed in Au25 cluster (Supplementary Figure 4b), indicating that the Cu atoms cause the torsional stress in Au12Cu13 cluster, which is plausibly attributed to the different atomic radii of Au (~144 pm) and Cu (~128 pm). Both Au12Cu13 and Au25 kernels are protected by triphenylphosphine and halide (I/Br) ligands in similar coordination patterns. The iodide anions and phosphine ligands coordinate selectively to Cu and Au atoms in Au12Cu13 cluster, respectively, due to the different electro negativities of Cu and Au.

Furthermore, the various M-M, M-P and M-X (M = Au/Cu, and X = I-/Br-) bond distances are summarized in Supplementary Table 2. The Auc-Aup and Aup-P bond lengths (c and p denote central and peripheral) of Au12Cu13 and Au25 are found comparable. And the average Cuc-Cup (2.717(2) Å) and Cup-Cup (2.761(2) Å) bond distances in Cu11 unit of Au12Cu13 were found to be substantially smaller than the corresponding Auc-Aup (2.878(2) Å) and Aup-Aup (2.940(3) Å) in Au25 cluster. Interestingly, compared with these structures of Au12Cu13, Au25 and Au13Ag12 clusters, they all can deemed as two vertex-sharing icosahedrons with a metal center, which further promote us to associate them with the uncomplete-icosahedral Au9 cluster with an Au center, the starting materials as well. In short, the Cu atoms in system not only lead the reconstruction and assemble of Au9 clusters into Au12Cu13 clusters, but also caused the structural distortion in the Au12Cu13 clusters.

Stability of Au12Cu13 clusters

The optimum stability of metal nanocluster is essential for their use in various applications, and it is previously reported that the Cu dopants in gold nanoclusters often caused the instability to the alloy clusters22,34. Therefore, we examined the stability of Au12Cu13 vis-á-vis that of Au25 clusters in solution. As shown in Fig. 4a, Au12Cu13 in a CH2Cl2 solution give three obvious absorption features at 448, 508 and 655 nm in the range of 400 to 800 nm, which is notably much distinct from those of Au25 (419, 470, 526 and ~660 nm), indicating reasonable perturbation of electronic structure upon Cu doping.

Fig. 4. Stability of Au12Cu13 clusters.

Fig. 4

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a UV-vis spectra of Au12Cu13 and Au25 nanoclusters. Stability tests of Au12Cu13 and Au25 clusters: b, c under sunlight and d in air.

The stability of Au12Cu13 and Au25 clusters in CH2Cl2 was evaluated under irradiation of sunlight via time-dependent UV-vis spectroscopy using crystal samples. As depicted in Fig. 4c, UV-vis profile of homo-Au25 decreased obviously in 30 min and almost disappeared after 60 min indicating the poor stability of homo-Au25 cluster in CH2Cl2. Whereas the profile Fig. 4b assigned to Au12Cu13 clusters kept intact illustrating Au12Cu13 cluster is higher stability than corresponding homo-Au25 clusters owing to the dopant of Cu atoms. Furthermore, the stability of Cu species in Au12Cu13 cluster have been improved significantly in solution exposed to air since these UV-Vis profiles in Fig. 4d remained overlapping over 7 hours and no precipitation formed, which is unusual even for CuI complexes. Hence the Au12Cu13 with high stability in solution under light or in air is achieved, which is not observed in other copper doped M25 nanoclusters22. Although the rod-shaped clusters of Au12Cu13 and Au25 share the similar configuration and a 16-electron system (i.e. the dimer of 8-electron superatom)35, the composition in core differs suggesting the synergistic effects between Au and Cu may be an important reason for the high stability of Au12Cu13 cluster. Secondly, the ligated I- ions with large radius (γ: ~ 2.2 Å) at waist of Au12Cu13 cluster exhibit higher coordinate capability than Br- (γ: ~1.96 Å) in Au25 cluster.

Fluorescence property

We next investigated the fluorescence (FL) property of Au12Cu13 nanocluster. The excitation curve from 400 nm to 700 nm in Fig. 5 shares the similar profile with UV-vis spectrum of Au12Cu13 monitored by 774 nm, suggesting the fluorescence of Au12Cu13 generated from the metal core cluster and different from small metal-organic complexes with similar ligands, whose fluorescence need to be pumped by UV lower than 400 nm. Interestingly, the intensity of fluorescence has been enhanced significantly once the Cu atoms were doped. An obvious broad fluorescence emission centered at 774 nm was noticed in the wavelength range from 660 to 850 nm, excited at λex ~ 470 nm as depicted in Fig. 5, which is broader than that of Au25 clusters (700–850 nm, Supplementary Figure 5). Further, the maximum emission wavelength of Au12Cu13 at λem ~ 774 nm was found red shifted with respect to Au25 clusters (λem ~ 750 nm, λex ~ 470 nm) and Au12Ag13 (λem ~ 739 nm, λex ~ 430 nm) clusters14. Besides, the excitation spectrum monitored at 774 nm emission (Fig. 5a, blue dash line) was found almost identical to the absorption spectrum of Au12Cu13 cluster (Fig. 5a, red line), and Stokes shift of Au12Cu13 cluster was determined as 304 nm close to that of Ag atoms doped Au12Ag13 cluster (309 nm)14 and smaller than that of Au25 cluster (330 nm). The quantum yield (QY) of Au12Cu13 was determined to be ~34% by an absolute method, which is significantly higher than Au12Ag13 (QY: ~26%)14 and Au25 clusters (~1%, by absolute method) and close to the highly fluorescent [Au12Ag13(PPh3)10(SR)5Cl2]2+ nanocluster (~40%)15, where the 13 Ag was doped in the center position similar to our Cu-centered Au12Cu13 cluster.

Fig. 5. Fluorescence property of Au12Cu13 clusters.

Fig. 5

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a Excitation and emission spectra of Au12Cu13 cluster in solution. b Temperature-dependent fluorescence of Au12Cu13 clusters in solution.

Furthermore, the FL lifetime of Au12Cu13 is up to 900 ns deduced from Supplementary Figure 6, and an almost linear fading tendency was detected indicating the nature of monoexponential FL decay dynamics of Au12Cu13 cluster in solution. These results illustrated the synergistic effect of Cu and Au atoms in Au12Cu13 clusters plays a key role in the drastic enhancement of fluorescence. We further investigated the temperature-dependent fluoresce (TDFL) of Au12Cu13 clusters at 277 and 77 K, Fig. 5b. An obvious enhancement of about 10-fold was achieved when the cluster solution was cooled to 77 K from 277 K, and FL emission kept unchanged, suggesting no-radiative transmittance Au12Cu13 at low temperature decreased obviously. TDFL analysis proves that FL of Au12Cu13 may be originated from the same excited state and the electron structure of Au12Cu13 is not temperature dependent18.

Catalytic test in photo-oxidation of methanol

The structurally well-identified and highly stable Au12Cu13 clusters with characteristic absorption features and excellent fluorescence can serve as a promising photocatalyst towards oxidation of methanol to methyl formate as both gold and copper species are considered as excellent candidates for this transformation14,29,36,37. Thus, we tested Au12Cu13 clusters as catalyst for photo-oxidation of methanol. The catalytic experiment details were given in Experimental section. In brief, ~0.5 wt% of Au12Cu13 nanocluster was firstly loaded on surface of TiO2, followed by treatment of Au12Cu13/TiO2 samples with amorphous Al2O3 using atomic layer deposition (ALD) technique to grow the shell/cage of Al2O3 (100 ALD cycles) around the Au12Cu13 cluster at 150 °C38, where Au12Cu13 clusters keep inert. The photocatalytic results are depicted in Fig. 6 and Supplementary Figure 7. It is worthy to note that no methanol conversion was detected when the light and catalysts were absent, demonstrating that the photo-oxidation occurred over the Au12Cu13/TiO2 catalysts. And it is found that methanol conversion was gradually improved, and methyl formate-selectivity was gradually decreased with the increasing reaction temperatures (from 25 to 45 °C), as shown in Fig. 6a. The highest methyl formate formation rate over Au12Cu13/TiO2 is evaluated to be ~10.6 mmol g−1 h−1 at 40 °C, which is 4-folds of that over TiO2 (Fig. 6b) and substantially higher than previously reported photocatalysts, Supplementary Table 339.

Fig. 6. Catalytic performance of Au12Cu13/TiO2 in the photo-oxidation of methanol.

Fig. 6

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a Catalytic performance as a function of temperature on TiO2 (P25) and Au12Cu13/TiO2. b Formation rate of methyl formate (MF) over TiO2 and Au12Cu13/TiO2 catalysts. c Durability test of the Au12Cu13/TiO2 catalysts at 25 °C. Reaction conditions: ~ 20 mg catalysts, λ = 365 nm, methanol (1.0 v %) and O2 (0.5 v %) balanced with N2 at the flow rate of 20 mL min−1.

Further, the durability of Au12Cu13/TiO2 catalyst for the photooxidation of methanol was tested. Significantly, during the whole process (~40 h), the cluster catalyst gave rise to a constant ~92% methanol conversion and a ~84% methyl formate-selectivity with no appreciable loss of activity, as shown in Fig. 6c. These results strongly indicate the robust nature of the Au12Cu13 composite, holding promise of outstanding catalytic activity for the prolonged period of time.

We have developed a strategy to acquire the rod-like [Au12Cu13(PPh3)10I7](SbF6)2 nanocluster, which can also be extended to the preparation of [Au25(PPh3)10Br7](SbF6)2 and [Au25-yAgy(PPh3)10CI8](SbF6) nanoclusters. A plausible mechanism for the Au12Cu13 formation that Au12Cu13 clusters were constructed by the dimerization of metastable intermediates like [Au8Cu(Ph3P)8]2+ and [Au9Cu(Ph3P)8I]2+generated from the reaction of Au9 clusters and CuI was presented and demonstrated by UV-vis spectroscopy, ESI-MS and single crystal X-ray diffraction technologies. The obtained Au12Cu13 cluster exhibits high stability in solution and outstanding photo-luminescence character with QY ≈ 34%, which laid the foundation for the applications in photoluminescence and photo-catalysis. Furthermore, the Au12Cu13 clusters showed good catalytic performance in the photo-oxidation of methanol toward methyl formate and good durability. The present work deepens the understanding of the assembling mechanism of clusters and provide the future guidelines for the highly controllable synthesis of functionalized alloy nanoclusters.

Methods

Synthesis of M25 clusters

Typically, Ph3PAuCl (25 mg) reacted with AgSbF6 (17.2 mg) in 4 mL of CH2CL2 and methanol (v/v = 1), and the obtained clear solution was reduced by NaBH4 solution (2 mg dissolved in ice-cold methanol) at 0 oC, giving a dark brown solution. After ~24 h of stirring, CuI (9.5 mg) was added into the mixture and kept stirring at 0 °C. The solution was filtrated when the characteristic peaks at 352, 434, 508 and 655 nm appeared. The filtrate was further evaporated to dryness under vacuum, which was further washed twice with a mixture of hexane and dichloromethane (v/v = 5:1). The pure clusters were finally extracted with a mixed solution of CH2Cl2 and CH3OH (v/v = 1). A black block crystal of Au12Cu13 was obtained via a slow diffusion of diethyl ether into the dichloromethane solution of clusters over two weeks. Yield: 15.25 mg, 50.5% (based on Au). The Au25 (yield: ~58%) and Au25-xAgx (yield: ~49%) clusters were obtained through a similar method by using KBr (11 mg, 0.05 mmol) and AgCl (7 mg, 0.05 mmol) to replace CuI, respectively. The detailed characterizations of the clusters are given in in the SI.

Catalytic performance evaluation

The Au12Cu13/TiO2 catalyst was prepared by the method of atomic layer deposition; see details in the SI. The oxidations of methanol to methyl formate were carried out in a home-made continuous-flow aluminum alloy reactor with a rectangle quartz window on the top. A 500 W high-pressure mercury lamp (CEl-LAM 500) with a wavelength of 365 nm was employed as the light source, installed above the quartz window of the reactor with the light intensity of 18.6 mW cm−2. The reaction temperatures were controlled at 25–40 °C using a cooling water circulation. 20 mg of catalysts were uniformly coated on the glass’s surface, which is fully exposed to light irradiation. A gas mixture containing 1.0 v% methanol, 0.5 v% O2 balanced with N2 at the flow rate of 20 mL·min−1 was introduced into reactor that is bubbled through a liquid methanol in a flask. The effluent was examined by an on-line Agilent 7820 with a thermal conductivity detector (TCD) and a flame ionization detector (FID).

Further details

See Supplementary Methods for details on X-ray crystallographic structural determinations, additional nanocluster characterizations, and preparation of the Au12Cu13/TiO2 catalyst.

Supplementary information

Peer Review File (3.2MB, pdf)
Supplementary Information (526KB, pdf)
42004_2023_817_MOESM3_ESM.pdf (130.8KB, pdf)

Description of Additional Supplementary Files

Supplementary Data 1 (2.4MB, cif)
Supplementary Data 2 (1.5MB, cif)

Acknowledgements

We are grateful for the provision of beam time at the BL17B beamline of the National Facility for Protein Science (NFPS), Shanghai Synchrotron Radiation Facility (SSRF) Shanghai, China.

Author contributions

Z.Q. and G.L. conceived and designed the project. J.Z., Y.Z., Z.Q., and Z.L. carried out the experiments. Z.Q., S.S., and G.L. wrote the manuscript, and all authors analyzed the data and discussed the results.

Peer review

Peer review information

Communications Chemistry thanks the anonymous reviewers for their contribution to the peer review of this work. Peer reviewer reports are available.

Data availability

Crystallographic data for [Au12Cu13(Ph3P)10I7](SbF6)2 and [Au25(Ph3P)10Br7](SbF6)2 is deposited in The Cambridge Crystallographic Data Centre (CCDC) as CCDC-1965910 and CCDC-1966985 (Supplementary Data 1 and 2). The data supporting the findings of this study are available within this article and its Supplementary Information. Extra data are available from the corresponding author upon reasonable request.

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

These authors contributed equally: Yifei Zhang, Jingjing Zhang.

Contributor Information

Zhaoxian Qin, Email: qinzhaoxian@yeah.net.

Gao Li, Email: gaoli@dicp.ac.cn.

Supplementary information

The online version contains supplementary material available at 10.1038/s42004-023-00817-5.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Peer Review File (3.2MB, pdf)
Supplementary Information (526KB, pdf)
42004_2023_817_MOESM3_ESM.pdf (130.8KB, pdf)

Description of Additional Supplementary Files

Supplementary Data 1 (2.4MB, cif)
Supplementary Data 2 (1.5MB, cif)

Data Availability Statement

Crystallographic data for [Au12Cu13(Ph3P)10I7](SbF6)2 and [Au25(Ph3P)10Br7](SbF6)2 is deposited in The Cambridge Crystallographic Data Centre (CCDC) as CCDC-1965910 and CCDC-1966985 (Supplementary Data 1 and 2). The data supporting the findings of this study are available within this article and its Supplementary Information. Extra data are available from the corresponding author upon reasonable request.

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