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. File list (2) download file view on ChemRxiv AuCumanuscript_ChemRxiv.pdf (1.69 MiB) download file view on ChemRxiv Supplementary Materials_chemRxiv.pdf (1.08 MiB)

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

“… ,, Since all particles imaged after this point showed dendritic growth behavior, it is reasonable to assume that the mixing and exchange of flowed HAuCl 4 solution had completed by ∼60 min, consistent with previous reports. ,, 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 b 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, ,, 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.…”

Visualizing Electron Beam-Capping Ligand Reactions for Controlled Nanoparticle Imaging with Liquid Phase Transmission Electron Microscopy

“…Prior works have established that alcohols, such as isopropanol and tert-butanol, and other organic molecules can act as hydroxyl radical scavengers to mitigate radiation damage to organic molecules during LP-TEM imaging. 50,[56][57][58][59] We added 1 M tert-butanol in DI water as a radical scavenger to test its impact on electron beam damage of BPEI in the presence of silver nanoparticles (Figure 6). Similar to the experiment without any nanoparticles, there was no visible change in fluorescence intensity in the image regions exposed for 30 seconds and 1 minute at a dose rate of 0.274 MGy/s.…”

“…Several prior works by our group and others have demonstrated irreversible electron beam induced aggregation of polymer ligand capped nanoparticles during LP-TEM. 4,57,64 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 selfassembly, which is a reversible process driven by interparticle interactions.…”

“…72 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. 57 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. 40,73,74 Therefore, the molecular structure of the ligands are important in determining the successive radiation driven reactions of polymeric capping ligands during LP-TEM experiments.…”

Dipeptide Nanostructure Assembly and Dynamics via in Situ Liquid-Phase Electron Microscopy

“…Indeed, success in LCTEM experiments heavily depends on the flux (e – Å –2 s –1 ) and fluence (e – Å –2 ) used to observe a given nanoscale process. − Solvated soft materials typically experience radiation damage by undergoing secondary reactions with radicals produced from solvent radiolysis, causing a perturbation in the chemistry and dynamics of the nanomaterials under investigation. − The analysis of the liquid-cell chips after imaging ( i.e. , post-mortem analysis) can be performed to verify morphology and the extent of the electron beam effects in the observed dynamic process. ,, Specifically for polymers and peptides, we have reported the use of matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS) to characterize the extent of electron beam damage at the chemical level. , Here, we observe FF assemblies in real-time via LCTEM and examine the effect of the electron beam through post-mortem surface characterization via time-of-flight secondary ion mass spectroscopy (ToF-SIMS).…”