关键词: 纳米团簇, 合金, 晶体学, 表面化学, 结构

Abstract:

Atomically precise pieces of metallic matter with nanometer dimensions, which are called nanoclusters, have attracted special research interest as a frontier in nanoscience research. These nanoclusters exhibit unique properties that make them suitable for widespread applications in fields like medical treatments and catalysis. Studies in nanoclusters have been greatly benefited from the use of advanced instrumentation, especially adaptation of mass spectrometry (e.g., matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) and electrospray ionization mass spectrometry (ESI MS)). However, mass spectrometry could not elucidate the bonding between metals and ligands; therefore, single-crystal X-ray diffraction (SC-XRD) analysis has been used. SC-XRD is significant for the development of the nanocluster range in terms of revealing the precise structure of nanoclusters and fully understanding the structure-property relationship. Furthermore, understanding the nature of nanocluster surface has provided possibility to embellish nanocluster surface and to improve their performance. Nowadays, alloy nanoclusters play an important role in catalysis, biology, and materials science. Researchers have synthesized and predicted the alloy structure composed of silver and nickel in ultra-small size (Ag₄Ni₂(DMSA)_4, (DMSA = meso-2, 3-dimercaptosuccinic acid)). However, no precise crystal structure has been reported. Herein, we report the crystal structure of the Ag-Ni alloy nanocluster Ag₄Ni₂(SPhMe₂)₈. The structure was further confirmed by SC-XRD, X-ray photoelectron spectroscopy (XPS), MALDI-TOF MS, ESI MS and thermo gravimetric analysis (TGA) measurements. The stability experiment suggested that the Ag₄Ni₂ nanocluster could be stable in ultra-small sizes. This research on Ag-Ni alloy nanoclusters will contribute to the understanding of the alloy in ultra-small sizes. Specifically, based on the structure determination by SC-XRD, the structure of Ag₄Ni₂(SPhMe₂)₈ could be divided into three layers: upper and lower layers with Ni(SPhMe₂)₄ complexes constituting a parallelogram, and the middle layer with four silver atoms constituting a parallelogram like a sandwich. The Ag―Ni, Ag―S and Ni―S bond distances were 0.31–0.32, 0.23–0.24, and 0.22–0.23 nm, respectively. XPS analyses revealed that the Ag/Ni/S atomic ratio was 5.19/2.55/10.28, consistent with the corresponding expected ratio of 4/2/8 in Ag₄Ni₂(SPhMe₂)₈. In addition, the Ag 3d_3/2 and Ag 3d_5/2 binding energy peaks were located at 375.0 and 369.0 eV, respectively, and the Ni 2p_1/2 and Ni 2p_3/2 are located at 871.50 and 853.90 eV, respectively. Moreover, combined with ESI, the Ag 3d_3/2 and Ag 3d_5/2 binding energies of Ag₄Ni₂(SPhMe₂)₈ were close to the +1 valences, according to previous reports. Meanwhile, the spectra of Ag₄Ni₂(SPhMe₂)₈ illustrated that the valence of nickel was +2. Additionally, the MALDI-TOF mass spectrum was in good agreement with the ESI results. Weight loss upon heating was used to confirm the percentage of organic material in nanoclusters (66.31% weight loss was observed in TGA, consistent with the 66.67% loss calculated according to the formula). In the liquid state, the UV-Vis spectra showed no change after exposure to oxygen for a few weeks. Meanwhile, we used UV-Vis spectroscopy at temperatures under 80 ℃ to test the stability of the Ag₄Ni₂(SPhMe₂)₈. The absorption peaks were almost identical with each other, suggesting high stability of the Ag₄Ni₂(SPhMe₂)₈. Our study proves that small-sized alloy also has the possibility of diversification, which will play an important role in the synthesis of alloy nanoclusters. Moreover, this research on Ag-Ni alloy nanoclusters will contribute to the understanding of alloys in ultra-small sizes.

Key words: Nanocluster, Alloy, Crystallography, Surface chemistry, Structure