关键词: Au纳米团簇, 尺寸转换, 动力学过程, 原位谱学技术, 刻蚀, 盐酸, 十二硫醇

Abstract:

Gold nanoclusters are promising materials for a variety of applications because of their unique "superatom" structure, extraordinary stability, and discrete electronic energy levels. Controlled synthesis of well-defined Au nanoclusters strongly depends on rational design and implementation of their synthetic chemistry. Among the numerous approaches for the synthesis of monodisperse Au nanoclusters, etching of pre-formed polydisperse clusters has been widely employed as a top-down method. Understanding the formation mechanism of metal nanoclusters during the etching process is important. Herein, we synthesized monodisperse Au₁₃(L₃)₂(SR)₄Cl₄ nanoclusters via an etching reaction between polydisperse 1, 3-bis(diphenyl-phosphino)propane (L₃)-protected polydisperse Au_n (15 ≤ n ≤ 60) clusters and a mixed solution of HCl/dodecanethiol (SR). The Au₁₃ product, with a mean size of (1.1 ± 0.2) nm, shows pronounced step-like multiband absorption peaks centered at 327, 410, 433, and 700 nm. The synthetic protocol has a suitable reaction rate that allowss for real-time spectroscopic studies. We used a combination of in situ X-ray absorption fine structure (XAFS) spectroscopy, UV-Vis absorption spectroscopy, and matrix-assisted laser desorption ionization mass-spectrometry (MALDI-MS) to study the kinetic formation process of monodisperse Au₁₃ nanoclusters. Emphasis was given to the detection of reaction intermediates. The study revealed that the size-conversion of the Au₁₃ nanoclusters can be divided into three stages. In the first stage, the polydisperse Au₁₅–Au₆₀ clusters, covering a wide m/z range of 3000-13000, are prominently decomposed into smaller Au₈-Au₁₁ (within a m/z range of 3000–4000) species owing to the etching effect of HCl. They are immediately stabilized by the absorbed SR, L₃, and Cl^- ligands to form metastable intermediates, as indicated by the high intensity of the Au-ligand coordination peak at 0.190 nm as well as the low intensity of the Au–Au peaks (0.236 and 0.288 nm) in the Fourier-transform (FT) EXAFS spectra. In the second stage, these Au₈–Au₁₁ intermediates are grown into Au₁₃ cores. The experimental X-ray absorption near-edge spectra, totally different from that of Au(Ⅰ)-SR polymer, could be well reproduced by the calculated spectrum of the Au₁₃P₈Cl₄ cluster. The Au-ligand coordination number (1.0) obtained from the EXAFS fitting is much closer to the nominal values in Au₁₃(L₃)₂(SR)₄Cl₄ (0.92) than to that in Au(Ⅰ)-SR polymers (2.0), suggesting that majority of the Au atoms are in the form of Au₁₃ clusters. The driving force for this growth process is primarily the geometric factor to form a complete icosahedral Au₁₃ skeleton through the incorporation of Au(Ⅰ) ions or Au(Ⅰ)-Cl oligomers pre-existing in the solution. In the third stage, the composition of the clusters is nearly unchanged as indicated by the MALDI-MS and the UV-vis spectra; however, their atomic structure undergoes rearrangement to the energetically stable structure of Au₁₃(L₃)₂(SR)₄Cl₄. During this structural rearrangement, the central-peripheral and peripheral-peripheral Au–Au bond lengths (R_Au-Au(c-p) and R_Au-Au(p-p)) decrease from 0.272 to 0.267 nm and 0.295 to 0.289 nm, respectively, resulting in considerable structural distortion of the original icosahedral Au₁₃ skeleton. This distortion is also reflected by the slightly increased disorder degree of the Au-Au bonds from 0.00015 to 0.00017 nm². This work expands our understanding of the kinetic growth process of metal nanoclusters and promotes design and synthesis of metal nanomaterials in a controllable manner.

Key words: Au nanoclusters, Size-convergence, Kinetic process, In situ spectroscopy, Etching, HCl, Dodecanethiol