PNNL

Abstract

Despite the major advancements in colloidal metal nanoparticles synthesis, a quantitative mechanistic treatment of the ligand’s role in controlling the rates of nucleation and growth still remains elusive. In this work, we conducted a mechanistic investigation and kinetic modeling of the role of trioctylphosphine (TOP) in controlling the size of Pd nanoparticles in different solvents using in-situ small angle x-ray scattering (SAXS). In both pyridine and toluene, slow nucleation which overlapped fast growth was observed under various synthetic conditions demonstrating the significant deviation of the particle formation pathway from the classical LaMer mechanism. We developed a novel kinetic model that, for the first time, accounts for both the nucleation and growth events through simultaneous fitting of number of nanoparticles (nucleation event) and their diameter (increase in number of atoms in nanoparticles from both nucleation and growth events) measured from in-situ SAXS. We show that the binding of TOP to both the Pd precursor and surface of Pd nanoparticles controls the nucleation and growth rates and is necessary to capture the evolution of diameter and number of particles during synthesis. The kinetic model was used to predict the synthetic conditions to control the Pd nanoparticle size from 1.1 to 4.6 nm, and the experimental results showed an excellent quantitative agreement. Additionally, our model was used to quantitatively explain the effect of ligand/metal ratio on the final size of Pd and Au nanoparticles reported in the literature. More importantly, we demonstrate that the final nanoparticle size can be determined using a single unique kinetic descriptor: Growth-to-Nucleation rate ratio to the power of 1/3, which is model independent, and remarkably applies to all the conditions studied in this work and several results from the literature despite the very different ligands, solvents and concentrations used. Our kinetic model, while simple, can provide a powerful tool for predictive synthesis of colloidal nanoparticles.