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

Abstract

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.

Figure 1

Synthesis of Ggallium nanoparticles, monitored by in situ small-angle X-ray scattering. (a) Synthesis scheme, illustrating the two reaction stages: transamination of the gallium precursor with a long-chain secondary alkylamine, followed by thermolysis and formation of gallium nanoparticles. (b) Schematic of the experimental setup. The in situ reactor is a tailor-made three-neck flask equipped with Kapton windows, magnetic stirrer, condenser, heating band, and automated syringe pump. During the entire synthesis, small-angle X-ray scattering can be recorded continuously.

Figure 2

Fitting the in situ small-angle X-ray scattering curves. (a) Full description of the synthesis of gallium nanoparticles as derived from in situ SAXS measurements. The process includes reaction, nucleation, and growth as three stages, which overlap in time. Gallium nanoparticles are considered as a primary structure and agglomerates of gallium nanoparticles as a secondary structure. (b) Evolution of SAXS patterns (circles) during the synthesis of gallium nanoparticles (synthetic conditions: 0.37 mmol of Ga precursor, [DOA]:[Ga] molar ratio of 30, and a growth temperature of 230 °C). Red lines indicate the fitted function using a spherical form factor and a hard sphere structure factor. Green lines indicate the fitted function using a power law. The data are cascaded for better visibility. (c–h) Extracted parameters from in situ SAXS data fits in (b): average diameter of Gallium nanoparticles (c), relative polydispersity using Schulz–Zimm distribution (d), normalized reaction yield (e), normalized number of gallium particles (f), the volume fraction of Ga within the agglomerates (g), and the fractal dimension of the agglomerate structures (h).

Figure 3

Tuning the growth temperature of gallium nanoparticle synthesis. (a) Duration of reaction, nucleation, and growth stages for the syntheses, carried out at 220, 230, and 240 °C (all other conditions are kept constant: 0.74 mmol of Ga precursor, [DOA]:[Ga] molar ratio of 30). The characteristic times of the three stages are extracted from SAXS fittings as follow: (b) onset and end of the nucleation stage as 0 and 90% of the normalized number of gallium particles, (c) onset of the growth stage and end of the reaction stage as 0% of the normalized reaction yield, and (d) end of the growth stage as 90% of the normalized total volume of gallium nanoparticles. (e) The duration of the three stages and (f) the size of gallium nanoparticles for the syntheses at different growth temperatures. Data are color coded for growth temperature of 240 °C (in orange), 230 °C (in green), and 240 °C (in blue).

Figure 4

Tuning the amount of secondary amine. (a) Timestamps of reaction, nucleation and growth stages for the syntheses, carried out with various amount of dioctylamine (all other conditions are kept constant: 0.37 mmol of Ga precursor, T_growth of 230 °C). (b–e) Effects of different dioctylamine-to-Ga-precursor molar ratios on (b) size, (c) size distribution, (d) nucleation rate, and (e) growth rate of gallium nanoparticles. (f) Schematic illustration of the role of secondary amine on relative kinetics of nucleation and growth stages of the synthesis.

Figure 5

Reaching monodisperse gallium nanoparticles. (a) Timestamps of reaction, nucleation and growth stages for the syntheses, carried out with different secondary amines and without an amine (all other conditions are kept constant: 0.37 mmol of Ga precursor, T_growth of 230 °C). (b,c) Size and size distribution of gallium nanoparticles, prepared with and without a secondary amines (DOA is dioctylamine and DDA is didodecylamine). (d–f) Triune strategies for monodisperse gallium nanoparticles: (d) Normalized reaction yield of gallium nanoparticles, showing improved growth kinetics in the presence of long-chain alkylamines; (e) volume fraction of the agglomerated particles as a function of the gallium diameter, showing improved colloidal stability for didodecylamine; and (f) correlation of polydispersity and the temporal overlap between the nucleation and growth stages, showing more effective separation of nucleation and growth for didodecylamine. Data are color-coded for didodecylamine (pearly purple) and for dioctylamine (in turquoise).