Discovery of Molecular Intermediates and Nonclassical Nanoparticle Formation Mechanisms by Liquid Phase Electron Microscopy and Reaction Throughput Analysis

Abstract

Formation kinetics of metal nanoparticles are generally described via mass transport and thermodynamics-based models, such as diffusion-limited growth and classical nucleation theory (CNT). However, metal monomers are commonly assumed as precursors, leaving the identity of molecular intermediates and their contribution to nanoparticle formation unclear. Herein, liquid phase transmission electron microscopy (LPTEM) and reaction kinetic modeling are utilized to establish the nucleation and growth mechanisms and discover molecular intermediates during silver nanoparticle formation. Quantitative LPTEM measurements show that their nucleation rate decreases while growth rate is nearly invariant with electron dose rate. Reaction kinetic simulations show that Ag₄ and Ag⁻ follow a statistically similar dose rate dependence as the experimentally determined growth rate. We show that experimental growth rates are consistent with diffusion-limited growth via the attachment of these species to nanoparticles. The dose rate dependence of nucleation rate is inconsistent with CNT. A reaction-limited nucleation mechanism is proposed and it is demonstrated that experimental nucleation kinetics are consistent with Ag₄²⁺ aggregation rates at millisecond time scales. Reaction throughput analysis of the kinetic simulations uncovered formation and decay pathways mediating intermediate concentrations. We demonstrate the power of quantitative LPTEM combined with kinetic modeling for establishing nanoparticle formation mechanisms and principal intermediates.