In situ studies on controlling an atomically-accurate formation process of gold nanoclusters

Knowledge of the molecular formation mechanism of metal nanoclusters is essential for developing chemistry for accurate control over their synthesis. Herein, the top-down synthetic process of monodisperse Au-13 nanoclusters via HCl etching of polydisperse Au-n clusters ( 15 <= n <= 65) is traced by a combination of in situ X-ray/UV-vis absorption spectroscopy and time-dependent mass spectrometry. It is revealed experimentally that the HCl-induced synthesis of Au-13 is achieved by accurately controlling the etching process with two distinctive steps, in sharp contrast to the traditional thiol-etching mechanism through release of the Au(I) complex. The first step involves the direct fragmentation of the initial larger Aun clusters into metastable intermediate Au-8-Au-13 smaller clusters. This is a critical step, which allows for the secondary size-growth step of the intermediates toward the atomically monodisperse Au-13 clusters via incorporating the reactive Au(I)-Cl species in the solution. Such a secondary-growth pathway is further confirmed by the successful growth of Au-13 through reaction of isolated Au-11 clusters with AuClPPh3 in the HCl environment. This work addresses the importance of reaction intermediates in guiding the way towards controllable synthesis of metal nanoclusters.