Unraveling Molecular-Scale Nanoparticle Formation Mechanisms by Liquid Phase Transmission Electron Microscopy

Abstract

Understanding nanoparticle nucleation and growth is essential for controlling size and shape and composition. This thesis explores the synthesis and growth mechanism of metallic nanoparticles using liquid phase transmission electron microscopy (LPTEM). Systematic wet synthesis and LPTEM were performed to elucidate the formation mechanism of high-entropy alloy (HEA) nanoparticles. Direct real-time observations with LPTEM imaging showed that the HEA nanoparticles exhibited cluster-cluster aggregation during synthesis, which suggested metal cluster intermediates as an important synthetic handle for multi-metallic nanoparticle synthesis. The role of molecular intermediates during nanoparticle synthesis is not widely studied and their contribution to nanoparticle formation is unclear. LPTEM and reaction kinetic modeling were combined to establish the nucleation and growth mechanisms of silver nanoparticles. Quantitative LPTEM measurements of nanoparticle growth rate and nucleation rate were performed as a function of the electron dose rate. Reaction kinetic simulations established the concentration of 11 silver intermediate species as a function of the electron dose rate. Experimental data fitting enabled uncovering the critical intermediate species during nucleation and growth. Experimental growth rates aligned with a diffusion-limited growth mechanism, where the predominant species contributing to nanoparticle growth were Ag- and Ag4. The nucleation kinetics were consistent with aggregation of Ag42+ clusters at millisecond time scales. This study highlights the power of combining experimental LPTEM study and kinetic modeling in unraveling molecular scale nanoparticle formation mechanisms. Another important factor to consider in nanoparticle formation is temperature. Systematic synthesis experiments showed that gold nanoparticle size increased as a function of temperature. Temperature dependent LPTEM study on the gold nanoparticle growth was conducted over a range of electron dose rates. We observed that the effect of temperature on gold nanoparticle growth rate depended on the electron dose rate. At relatively high dose rate the nanoparticle growth rate was not significantly impacted by temperature, while at lower dose rate increasing temperature increased nanoparticle growth rate. We consider several factors controlling nanoparticle growth kinetics as a function of temperature and dose rate, including gold monomer concentration, monomer diffusion coefficient, and gold solubility. This work establishes foundational understanding for utilizing LPTEM to investigate temperature dependent nanoparticle formation mechanisms.