Atomic mechanisms of gold nanoparticle growth in ionic liquids studied by in situ scanning transmission electron microscopy.

Elementary atomic mechanisms underlying nanoparticle growth in liquids are largely unexplored and mostly a subject of conjectures based on theory and indirect experimental insights. Direct, experimental observation of such processes at an atomic level requires imaging with single-atom sensitivity and control over kinetics. Although conventional liquid-cell (scanning) transmission electron microscopy ((S)TEM) enables nanoscale studies of dynamic processes, the visualization of atomic processes in the liquid phase is inhibited owing to the liquid film thickness and its encapsulation, both limiting the achievable spatial resolution. In contrast, by using thin, free-standing ionic liquid nanoreactors, this work shows that the mechanisms controlling and triggering particle growth can be uncovered at an atom-by-atom level. Our observations of growing particle ensembles reveal that diverse growth pathways proceed simultaneously. We record Ostwald ripening and oriented particle coalescence tracked at the atomic scale, which confirm the mechanisms suggested by theory. However, we also identify unexpected growth phenomena and more intricate coalescence events which show competing mechanisms. The diversity of the observed growth processes thus illustrates that growth reactions in liquids, on the atomic scale, are much more complex than predicted by theory. Furthermore, this work demonstrates that free-standing ionic liquids enable (sub-)Ångström resolution imaging of dynamic processes in liquids with single-atom sensitivity, thus providing a powerful alternative approach to conventional liquid-cell (S)TEM.