Colloidal synthesis of metal alloy nanoparticles is challenging because metal precursors reduce at different rates, leading to phase separation even for miscible metals. Recent work has explored the use of bimetallic precursor-ligand complexes for promoting alloying; however, the underlying nanochemistry of their conversion to nanocrystals remains unclear. In this study, we explore the formation mechanism of alloyed AuCu nanocrystals using liquid phase transmission electron microscopy (LP-TEM) and solution chemistry. Electron beam radiolysis was used to create reducing radicals that converted aqueous solutions of heteronuclear AuCu metal thiolate precursors to alloyed nanocrystals and visualized their nucleation and growth in real time. Atomic resolution TEM and energy dispersive x-ray spectroscopy measurements of the LP-TEM synthesized nanocrystals demonstrated that nanocrystals synthesized under low beam current conditions had statistically indistinguishable composition (~70 atomic% Au) and alloy structure as those formed by flask synthesis. Atomic resolution TEM imaging identified sub-nanometer prenucleation clusters as intermediate species. Nanocrystals formed at relatively large electron beam currents were aggregated and nearly pure gold. Reaction kinetic simulations showed that hydroxyl radicals formed by electron beam radiolysis oxidized metal thiolate complexes, copper atoms, and capping ligands at this condition. Taken together, these results demonstrated that the metal thiolate species encouraged simultaneous reduction of gold and copper by localizing them within the same molecule and/or by modifying their reduction potentials. Due to localization of Au and Cu within metal thiolates, the metal atoms rapidly aggregated in nanocluster intermediates, preventing intermetal electron transfer reactions between copper atoms and gold ions.

Visualizing ligand mediated formation pathways of alloyed nanocrystal formation with liquid phase transmission electron microscopy

ACS Fall 2021

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