Rational Design for Monodisperse Gallium Nanoparticles by In Situ Monitoring with Small-Angle X-ray Scattering.

Colloidal chemistry is a well-known synthetic platform for producing size-uniform nanoparticles. However, the optimization of each material system still relies on a tedious trial-and-error approach in a multiparametric space, commonly referred to as design-of-experiments. This process is particularly laborious for emerging material classes for which only a handful of syntheses have been reported. Alternative approaches for the rational design of colloidal nanoparticles involve studying the reaction with in situ methods, thereby revealing the true underlying rules for the synthesis of monodisperse nanoparticles. Here, we focus on highly promising but little-studied colloidal gallium nanoparticles, using synchrotron-based small-angle X-ray scattering as a highly suitable in situ monitoring technique. We investigate the intertwined effects of process temperature, concentration of reactants, and the sterics of surface ligands during the hot-injection synthesis of gallium colloids. For quantitative comparison, we provide a description of gallium synthesis through the timestamps of partially overlapping reaction, nucleation, and growth stages. Our results reveal the key role of surface ligands in balancing the kinetics of nucleation and growth, as well as in enabling colloidal stability during the synthesis. Furthermore, we demonstrate that the large overlap between the nucleation and growth stages does not preclude the formation of monodisperse gallium nanoparticles. Our in situ experiments suggest several possible strategies for achieving size-uniform colloidal nanoparticles, thus enabling a rational design for the peculiar system of liquid metal nanodroplets and offering insights that can be extended to other monodisperse colloids prepared via hot-injection synthesis.