Insights into Growth Kinetics of Colloidal Gold Nanoparticles: In Situ SAXS and UV–Vis Evaluation

Abstract

Understanding the formation process and kinetics is of vital importance for preparing monodisperse colloidal nanocrystals with controllable size and morphology in a predictive way. However, in-depth understanding of nucleation and growth mechanisms is impeded due to the lack of reliable and complete in situ experimental data from molecular precursors to colloidal nanocrystals. Herein we used in situ UV–vis and synchrotron-based time-resolved in situ small-angle X-ray scattering to monitor the reduction kinetics of gold salt and the fast nucleation and growth kinetics of gold nanoparticles synthesized in stopped flow microfluidics through the reduction of a gold precursor by a morpholine–borane complex in the presence of oleylamine or the combination of oleylamine and oleic acid ligands. Our method enables obtaining detailed information on the evolution of size, size distribution, the number of particles, and monomer concentration by probing the original reaction solution over time. Through quantitative analysis of in situ SAXS and UV–vis data, complex growth trajectories involving the coalescence of small particles, intraparticle growth within coalesced nanoparticles, and surface-reaction limited focusing of size-distribution events could be identified in the synthesis of monodisperse Au NPs for both cases. When an oleic acid ligand is present in the system, it can cause the dissolution of larger particles, followed by the growth of small ones at the expense of larger ones at the initial growth stage before coalescence event. The polydispersity of 20% is found to be a quantitative indicator for the transition of the intraparticle growth event to the focusing of a size distribution mechanism for both cases. In the later focusing process, the polydispersity can be significantly narrowed from 15% to 9%, while the particle size keeps almost constant. By quantitative analysis of the growth process, we can conclude that nucleation and the growth process are dependent on ligand–NP binding affinity. We believe that our findings can further advance the understanding of the growth kinetics and mechanisms of oleylamine-capped metal nanocrystals.