The role of pre-nucleation clusters in the crystallization of gold nanoparticles

The syntheses of metal nanoparticles by reduction in apolar solvents in the presence of long chain surfactants have proven to be extremely effective in the control of the particle size and shape. Nevertheless, the elucidation of the nucleation/growth mechanism is not straightforward because of the multiple roles played by surfactants. The nucleation stage, in particular, is very difficult to describe precisely and requires in situ and time-resolved techniques. Here, relying on in situ small angle X-ray scattering (SAXS), X-ray absorption spectroscopy (XAS) and high-energy X-ray diffraction (HE-XRD), we propose that ultra-small gold particles prepared by reduction of gold chloride in a solution of oleylamine (OY) in hexane with triisopropylsilane do not follow a classical nucleation process but result from pre-nucleation clusters (PNCs). These PNCs contain Au(iii) and Au(i) precursors; they are almost stable in size during the induction stage, as shown by SAXS, prior to undergoing a very fast shrinkage during the nucleation stage. The gold speciation as a function of time deduced from the XAS spectra has been analyzed through multi-step reaction pathways comprising both highly reactive species, involved in the nucleation and growth stages, and poorly reactive species acting as a reservoir for the reactive species. The duration of the induction period is related to the reactivity of the gold precursors, which is tuned by the coordination of OY to the gold complexes, while the nucleation stage was found to depend on the size and reactivity of the PNCs. The role of the PNCs in determining the final particle size and structure is also discussed in relation to previous studies. The multiple roles of OY, as the solubilizing agent of the gold salt, the ligand of the gold complexes determining both the size of the PNCs and the reactivity of the gold precursors, and finally the capping agent of the final gold particles as oleylammonium chloride, have been clearly established. This work opens new perspectives to synthesize metal NPs via metal-organic PNCs and to define new synthesis routes for nanoparticles that may present structure and morphologies different from those obtained by the classical nucleation routes.

The polyol process: a unique method for easy access to metal nanoparticles with tailored sizes, shapes and compositions

After about three decades of development, the polyol process is now widely recognized and practised as a unique soft chemical method for the preparation of a large variety of nanoparticles which can be used in important technological fields. It offers many advantages: low cost, ease of use and, very importantly, already proven scalability for industrial applications. Among the different classes of inorganic nanoparticles which can be prepared in liquid polyols, metals were the first reported. This review aims to give a comprehensive account of the strategies used to prepare monometallic nanoparticles and multimetallic materials with tailored size and shape. As regards monometallic materials, while the preparation of noble as well as ferromagnetic metals is now clearly established, the scope of the polyol process has been extended to the preparation of more electropositive metals, such as post-transition metals and semi-metals. The potential of this method is also clearly displayed for the preparation of alloys, intermetallics and core-shell nanostructures with a very large diversity of compositions and architectures.