Role of pre-nucleation clusters on the crystallization of gold nanoparticles

The syntheses of metal nanoparticles by reduction in apolar solvent in 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 the surfactant. 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 results of 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 undergo a very fast shrinkage during the nucleation stage. The gold speciation as a function of time deduced from the XAS spectra have been analyzed through multi-steps reaction pathways comprising both highly reactive species, involved in the nucleation and growth stages, and poorly reactive species acting as 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 with previous studies. The multiple roles of OY, solubilizing agent of the gold salt, ligand of the gold complexes determining both the size of the PNCs and the reactivity of the gold precursors, and finally 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.