Recent studies have suggested that the pathways of crystal nucleation can be more complex than predicted by classical theories, and may involve phenomena such as the aggregation of prenucleation clusters or the formation of liquid-like precursors. In order to better understand these pathways for crystal nucleation, we are seeking to directly image the precursors to crystal nucleation. These structures are generally too small or to scarce to be seen with most experimental methods, but recent advances with in situ atomic force microscopy (AFM) now allow surface adsorbates to be imaged under fluids with near atomic resolution. In this study we are utilizing in situ AFM to investigate the epitaxial nucleation and growth of aluminum hydroxide (gibbsite) films on the muscovite mica surface. Preliminary results indicate that molecular clusters of aluminum hydroxide rapidly adsorb to the mica surface in the presence of AlCl3 aqueous solutions. At low-temperature conditions, we are able to image single-molecule adsorbates. The adsorbate population varies with pH and Al3+ concentrations in a manner that is consistent with adsorption of Al(OH)2+ monomers. Upon heating to temperatures in excess of 60 °C, the adsorbates form a dense nanostructured film with liquid-like properties, and islands of gibbsite subsequently crystallize within this concentrated adsorbate layer. The liquid-like film can be preserved for many hours at lower temperatures, but undergoes rapid crystallization at higher temperatures. This suggests a two-step nucleation process, where the presence of an interface induces highly localized concentrations of reactants, but nucleation of the crystal within this concentrated layer is controlled by the kinetics of bond formation.

Molecular-scale imaging of gibbsite nucleation at the mica-water interface using in situ atomic force microscopy

253rd National Meeting (2017)

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