Nanochemistry: Discovery of molecular intermediates and non-classical mechanisms in nanoparticle formation

October 7, 2024 - Classical nucleation theories only represent the complex reality of nanochemistry to a limited extent, as researchers from HI ERN and the University of Maryland (USA) demonstrate in their current work. Instead, they propose a reaction-limited nucleation mechanism. The article was recently published in the journal "Small Structures".

Metallic nanoparticles are often used as heterogeneous catalysts, electrocatalysts, and optical materials. Their functional properties result from their surface and atomic structure, composition, and shape. While the colloidal synthesis method is relatively simple, the underlying nanochemistry still puzzles and fascinates researchers.

The formation of metal nanoparticles is generally described by mass transport and thermodynamic models such as diffusion-limited growth and classical nucleation theory. However, metal monomers are usually assumed to be precursors, so the identity of the molecular intermediates and their contribution to nanoparticle formation remain unclear.

Researchers at HI ERN, Friedrich-Alexander-Universität Erlangen-Nürnberg, the University of Maryland (USA), and the University of Washington (USA) are using liquid phase transmission electron microscopy (LPTEM) and reaction kinetic modeling to determine the nucleation and growth mechanisms and identify molecular intermediates during the formation of silver nanoparticles. LPTEM allows special reaction control because the electron beam can directly influence the reaction conditions. Quantitative LPTEM measurements show that the nucleation rate decreases while the growth rate is almost independent of the electron dose rate. Reaction kinetic simulations show that the concentrations of Ag₄ clusters and Ag^- ions correlate with experimentally determined growth rates.

Nanochemie: Entdeckung molekularer Zwischenprodukte und nichtklassischer Mechanismen in der Nanopartikelbildung — LPTEM and correlative simulations can be used to determine non-classical reaction mechanisms.

The team led by Andreas Hutzler (HI ERN), in collaboration with Prof. Taylor Woehl’s research group (University of Maryland, USA) demonstrated that nucleation and growth cannot be described using classical nucleation theory. Instead, the scientists propose a reaction-limited nucleation mechanism that describes particle formation by aggregating Ag₄²⁺ clusters. A reaction throughput analysis of the complex reaction network of 200 chemical reactions involved has revealed relevant formation and decay pathways that mediate intermediate concentrations and thus contribute significantly to understanding the reaction conditions.

Quantitative LPTEM, in combination with kinetic modeling, is powerful for identifying nanoparticle formation mechanisms and the most important intermediates. The scientists demonstrated this with their joint work. The results also make it clear that classical nucleation theories do not easily reflect the complex reality of nanochemistry—a finding that could significantly impact the understanding of nanocatalysts and their synthesis approaches.

The results were recently published in the renowned journal "Small Structures".

Original publication

Jiayue Sun, Birk Fritsch (shared), Andreas Körner, Mehran Taherkhani, Chiwoo Park, Mei Wang, Andreas Hutzler, Taylor J. Woehl
Discovery of Molecular Intermediates and Nonclassical Nanoparticle Formation Mechanisms by Liquid Phase Electron Microscopy and Reaction Throughput Analysis
Small Structures, 2400146, https://doi.org/10.1002/sstr.202400146