Nucleation in Confinement: A Pathway to Size-Controlled Ultrasmall Icosahedral Silver Nanoparticles

The size and structural control of ultrasmall metal nanoparticles (NPs) remain a key area of research, as their unique nanoscale properties can significantly impact their utility and applications in industry. Among the different synthesis strategies, the formation of large clusters of metal precursors in solution, which favors a confined nucleation, has been less explored. Here, we report a simple synthesis protocol of ultrasmall silver NPs with an icosahedral structure, a mean diameter of 3 nm, and low polydispersity (down to 6%), through the reduction of AgNO3 by trialkylsilanes in a solution of oleylamine (OY) and hexane. Small- and wide-angle X-ray scattering (SAXS, WAXS) and pair distribution function (PDF) analysis reveal that the Ag NP size and structure remain constant regardless of Ag and reducing agent concentrations or temperatures up to 120 °C. The lack of correlation between the final NP size and the initial Ag concentration, temperature, and reaction rate, despite variations spanning 2 orders of magnitude, does not align with a classical nucleation/growth mechanism of isolated monomers in solution. SAXS analysis of the precursor solutions revealed the presence of {OY–Ag(I)–NO3}n clusters, which interact repulsively in solution. The final Ag particle size is closely tied to the size of the Ag(I) clusters. This suggests that size control is governed by a nucleation process involving Ag atoms within the clusters. The separation of nucleation and growth appears to be achievable through nucleation confined within the repulsively interacting {OY–Ag(I)–NO3}n clusters. Only two factors have been found to decrease the mean NP size: increasing the OY concentration or substituting AgNO3 with silver trifluoro-sulfonate or trifluoro-acetate, which reduces the mean size, in agreement with smaller initial Ag(I) clusters. The advantage of this synthesis route is the very high concentration of metals that can be achieved without degrading the morphological and structural quality of the final particles.