Matthew Hauwiller1 Layne Frechette1 Matthew R. Jones2 Justin Ondry1 Phillip Geissler1 A. Alivisatos1

1, University of California-Berkeley, Berkeley, California, United States
2, Rice University, Houston, Texas, United States

The emergent properties of colloidal nanomaterials are dependent on their shape and exposed facets, so mechanistic understanding of atomic formation and removal is critical. Non-equilibrium synthetic methods are powerful tools for making energetically unfavorable shapes and facets, but studying these processes is challenging. Liquid cell Transmission Electron Microscopy (TEM) enables single nanoparticle dynamics to be monitored in their native liquid environment with the necessary spatial and temporal resolution to observe shape and facet evolution. Studying non-equilibrium etching of gold nanocrystals has provided insight into fundamental formation mechanisms of nanocrystals with high-energy facets.
Graphene liquid cell TEM encapsulates small pockets of liquid between graphene sheets for imaging using a transmission electron microscope. Oxidative etching of gold nanocrystals in the graphene liquid cell was induced through a combination of pre-loaded iron chloride and oxidative species generated by electron beam induced radiolysis. Control of the chemistry in the graphene liquid cell pockets allowed the nanocrystal dynamics and mechanisms to be related back traditional synthetic techniques. The electron beam dose rate controlled the rate of atom removal, and the initial concentration of iron chloride established the potential of the oxidative etching.
Pre-synthesized gold nanocubes and nano-rhombic dodecahedra (RDD), with {100} and {110} surface facets respectively, were oxidatively etched while monitoring the effect of chemical potential on the facet trajectories. Both the cubes and RDD transformed to intermediate tetrahexahedra (THH) shapes with {hk0} surface facets. When etching the cubes in this non-equilibrium regime, lower initial concentrations of iron chloride led to intermediate {hk0} facets with lower h/k values. However, etching the RDD at differing initial iron chloride concentrations led to same intermediate THH with {hk0} facets of h/k = 2.5. Monte Carlo simulations corroborated the role of chemical potential in controlling the facets for the cubes but not the RDD. Zero temperature kinetic models show that removing a 6-coordinated edge atom on the nanocrystals reveals 7-coordinate inner atoms for cubes but 6-coordinate inner atoms for RDD. Therefore, chemical potential controls the facets for cubes by modulating the probability ratio of removing inner versus edge atoms. This fundamental understanding of kinetically-driven shape transformations will aid efforts to make nanocrystals with high-energy facets.
Through these in situ TEM studies, the formation of non-equilibrium nanocrystal structures were watched at the single particle level in solution, and the mechanisms of etching were elucidated. This mechanistic understanding of nanocrystal etching will hopefully inform future synthetic efforts to control facets and structures for energetically unfavorable shapes.