Acta Phys. -Chim. Sin. ›› 2018, Vol. 34 ›› Issue (7): 762-769.doi: 10.3866/PKU.WHXB201801084

Special Issue: 原子水平上精确控制纳米簇和纳米粒子

• ARTICLE • Previous Articles     Next Articles

In situ Study of Formation Kinetics of Au Nanoclusters during HCl and Dodecanethiol Etching

Lina YANG1,2,Li HUANG1,Xueyang SONG1,Wenxue HE1,Yong JIANG1,Zhihu SUN1,*(),Shiqiang WEI1,*()   

  1. 1 National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, P. R. China
    2 School of Food Science and Engineering, Hefei University of Technology, Hefei 230009, P. R. China
  • Received:2017-12-06 Published:2018-03-26
  • Contact: Zhihu SUN,Shiqiang WEI E-mail:zhsun@ustc.edu.cn;sqwei@ustc.edu.cn
  • Supported by:
    The project was supported by the National Key Research and Development Program of China(2017YFA0402800);the National Natural Science Foundation of China(11475176);the National Natural Science Foundation of China(U1632263);the National Natural Science Foundation of China(21533007);the National Natural Science Foundation of China(11621063)

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 Aun (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 (RAu-Au(c-p) and RAu-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

MSC2000: 

  • O643