Visualizing Ligand-Mediated Bimetallic Nanocrystal Formation Pathways with In Situ Liquid Phase Transmission Electron Microscopy Synthesis

Abstract

Colloidal synthesis of alloyed multimetallic nanocrystals with precise composition control remains a challenge and a critical missing link in theory-driven rational design of functional nanomaterials. Liquid phase transmission electron microscopy (LP-TEM) enables directly visualizing nanocrystal formation mechanisms that can inform discovery of design rules for colloidal multimetallic nanocrystal synthesis, but it remains unclear whether the salient chemistry of the flask synthesis is preserved in the extreme electron beam radiation environment during LPTEM. Here we demonstrate controlled in situ LP-TEM synthesis of alloyed AuCu nanoparticles while maintaining the molecular structure of electron beam sensitive metal thiolate precursor complexes. Ex situ flask synthesis experiments showed that nearly equimolar AuCu alloys formed from heteronuclear metal thiolate complexes, while gold-rich alloys formed in their absence. Systematic dose rate-controlled in situ LP-TEM synthesis experiments established a range of electron beam synthesis conditions that formed alloyed AuCu nanoparticles with similar alloy composition, random alloy structure, and particle size distribution shape as those from ex situ flask synthesis, indicating metal thiolate complexes were preserved under these conditions. Reaction kinetic simulations of radical-ligand reactions revealed that polymer capping ligands acted as effective hydroxyl radical scavengers during LP-TEM synthesis and prevented metal thiolate oxidation at low dose rates. In situ synthesis experiments and ex situ atomic scale imaging revealed that a key role of metal thiolate complexes was to prevent copper atom oxidation and facilitate formation of prenucleation cluster intermediates. This work demonstrates that complex ion precursor chemistry can be maintained during LP-TEM imaging, enabling probing nanocrystal formation mechanisms with LP-TEM under reaction conditions representative of ex situ flask synthesis.

Understanding Symmetry Breaking at the Single-Particle Level via the Growth of Tetrahedron-Shaped Nanocrystals from Higher-Symmetry Precursors

“…33,38,39 Interestingly, the growth pathway observed after ∼45 min of fluid flow, representing intermediate concentrations of ligand and Au precursor, showed the development of a tetrahedral nanoparticle product (T d ) from the original RD (O h ) seed (Figure 1b and Movie S2), suggesting a complex interplay between thermodynamic and kinetic factors being responsible for the symmetry reduction growth mechanism. Although the precise radiolytic chemistry inside a liquid cell experiment is difficult to quantify, 25,30,40 the particle growth reactions described above were imaged using identical electron dose rates, indicating that the observed differences are better attributed to energetic arguments rather than effects of the electron beam.…”

Revealing Reactions between the Electron Beam and Nanoparticle Capping Ligands with Correlative Fluorescence and Liquid-Phase Electron Microscopy

“…Several prior works by our group and others have demonstrated irreversible electron beam-induced aggregation of polymer ligand-capped nanoparticles during LP-TEM. 4,53,60 It is plausible that electron beam-induced intermolecular crosslinking and chain scission of polymer ligands will contribute to nanoparticle aggregation by reducing nanoparticle colloidal stability or covalently linking nanoparticles together. An important implication of this process is that irreversible electron beam-induced aggregation of nanoparticles is a kinetically controlled process driven by radical reactions, making it distinct from self-assembly, which is a reversible process driven by interparticle interactions.…”

“…68 A recent study by our group showed that PEG−SH ligands act as radical scavengers for hydroxyl radicals by hydrogen abstraction from the thiol groups and PEG chain. 53 In aqueous solutions of PEG and polyacrylamide, an uncharged polymer, intermolecular and intramolecular crosslinking prevails over degradation in deoxygenated solutions while main chain scission dominates with oxygen present. 36,69,70 Therefore, the molecular structure of the ligands is important in determining the successive radiation-…”

Visualizing Formation of High Entropy Alloy Nanoparticles by Aggregation of Amorphous Metal Cluster Intermediates

“…While prior work has suggested nucleation and growth as the mechanism for nanoparticle growth, there is a growing body of literature suggesting alternative mechanisms. Some researchers have suggested metal nanoparticle formation is simply controlled by precursor reduction reaction kinetics, 31 while others have suggested non-classical crystallization mechanisms, 32 including aggregative growth, [33][34][35][36][37][38] liquidliquid phase separation, 39 and prenucleation clusters, [40][41][42][43] underlie the formation of metal nanoparticles during solution phase synthesis.…”