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

Abstract

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 Au₁₃ 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 Au₁₃ 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 Au_n clusters into metastable intermediate Au₈–Au₁₃ smaller clusters. This is a critical step, which allows for the secondary size-growth step of the intermediates toward the atomically monodisperse Au₁₃ 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 Au₁₃ through reaction of isolated Au₁₁ 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.