Discovery of molecular intermediates and non-classical nanoparticle formation mechanisms by liquid phase electron microscopy and reaction throughput analysis

The 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. Here we utilize liquid phase transmission electron microscopy (LPTEM) and reaction kinetic modeling to establish the nucleation and growth mechanisms and discover molecular intermediates during silver nanoparticle formation. Quantitative LPTEM measurements showed that their nucleation rate decreased while growth rate was nearly invariant with electron dose rate. Reaction kinetic simulations showed that Ag4 and Ag- followed a statistically similar dose rate dependence as the experimentally determined growth rate. We demonstrate that experimental growth rates are consistent with diffusion limited growth via attachment of these species to nanoparticles. Dose rate dependence of nucleation rate was inconsistent with CNT. We propose a reaction limited nucleation mechanism and demonstrate 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. The work demonstrates the power of quantitative LPTEM combined with kinetic modeling for establishing nanoparticle formation mechanisms and the principal intermediates.