Two Growth Mechanisms of Thiol-Capped Gold Nanoparticles Controlled by Ligand Chemistry

Thiols play an important role in the synthesis of well-defined nanoparticles (NPs) with tailored properties, but their effects on the formation kinetics of NPs are still under investigation. Here, we used in situ small-angle X-ray scattering (SAXS)/UV-vis spectroscopy and time-dependent transmission electron microscopy (TEM) to elucidate the role of thiols in the formation process of gold NPs (AuNPs) by changing the adding sequence between thiol ligand and reducing agent. Through quantitative analysis of in situ SAXS/UV-vis and TEM, detailed information on size, size distribution, the number of particles, optical properties, and the size evolution was obtained. Two different growth mechanisms of monodisperse AuNPs controlled by thiol ligand are exhibited: (i) thiol plays a dual role as a digestive ripening etchant and as a stabilizing ligand in the presence of a weak phosphine ligand. The digestive ripening mechanism involving the dissolution of bigger particles and subsequent deposition of monomers onto existing small NPs is responsible for producing narrowly dispersed NPs. (ii) Thiol acts as a strong stabilizing agent; in this case, the formation rate constant is quite slow, thus limiting the growth rate of NPs. Therefore, diffusion-limited growth mechanism is proposed for obtaining narrowly dispersed NPs with a diameter of 5.6 nm (12%). Our findings demonstrate that the formation of nearly monodisperse AuNPs with controllable size distribution could be realized by different growth mechanisms in the presence of thiol ligand.