In the classical picture of crystalization, nucleation and growth occur via monomer-by-monomer addition, first to sub-critical clusters that form via thermal fluctuations and then to kink sites on the surfaces of super-critical particles. In contrast, recent observations of crystallizing systems have revealed a rich set of “hierarchical” pathways involving higher-order species. To understand the thermodynamic and kinetic sources of such pathways, we are using in situ TEM and AFM to observe crystallization of calcium carbonate, iron oxide and aluminum oxide. Here we present results that highlight the importance of interfaces in pathway selection. We observed sub-critical clusters and their transition to stable nuclei using high-speed, atomically-resolved AFM to image formation of aluminum hydroxide films at the mica-water interface. We find hydrolyzed Al-precursor species are concentrated by the negative mica surface and that fluctuating aluminum hydroxide clusters form within this precursor-rich region. The cluster populations drop off exponentially with size and subcritical clusters show dynamic fluctuations, consistent with classical predictions, but nucleation of stable islands is not a rare event as expected in the classical picture. In situ TEM studies show all four common phases of calcium carbonate can form directly, but crystalline phases can also form indirectly from amorphous precurors, which provide an interface for nucleation of the crystals that then grow by consuming the precursor. These results are consistent with the classical picture in which the high supersaturation needed to overcome the free energy barrier to calcite formation opens the path to all phases, while surfaces provide a low energy barrier. Cryo-, ex situ, and high temperature in situ (S)TEM on the iron oxide system shows that heating of ferrihydrite (Fh) precursors in pure solutions produces well faceted hematite (Hm), but addition of oxalate ligands leads to a hierarchical pathway. First, Hm nanoparticles nucleate sporadically on Fh, but the adsorption of oxalate inhibits their growth and causes subsequent particles to form within about 1 nm of existing Hm surfaces. These then assemble to form spindle-shaped aggregates of coaligned particles with a constant, reproducible shape. These findings reveal the extent to which crystallization in these systems can be understood from the standpoint of classical descriptions and delineate aspects that require alternative descriptions.

<i>In situ</i> look at interfacial controls over nucleation and growth of nanostructured materials

2019 Spring National Meeting

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In situ look at interfacial controls over nucleation and growth of nanostructured materials