Oriented Aggregation: Formation and Transformation of Mesocrystal Intermediates Revealed

Abstract

Oriented aggregation is a special case of aggregation in which nanocrystals self-assemble and form new secondary single crystals. This process has been suggested to proceed via an intermediate state known as the mesocrystal, in which the nanocrystals have parallel crystallographic alignment but are spatially separated. We present the first direct observations of mesocrystals with size and shape similar to product oriented aggregates by employing cryo-TEM to directly image the particles in aqueous suspension. The cryo-TEM images reveal that mesocrystals not only form but also transform to the final single crystal product while in the dispersed state. Further, high-resolution cryo-TEM images demonstrate that the mesocrystals are composed of spatially separated and crystallographically aligned nanocrystals.

Formation and Restacking of Disordered Smectite Osmotic Hydrates

“…Because transformation of osmotic to crystalline hydrates involves assembly of individual smectite 2:1 layers in solution, it can be classified as an example of oriented attachment-based crystal growth (OA) (Penn and Banfield, 1998). Such OA has been reported as a crystal-growth mechanism for many poorly soluble nanoparticulate minerals, including oxides and sulfides (Penn et al, 2001;Huang et al, 2003), and has been observed in suspensions of nanoparticles with a range of morphologies (Yuwono et al, 2010;Frandsen et al, 2014). The case of assembly of 2:1 smectite layers is distinct from other growth processes described as OA because the attachment process is readily reversible due to the unusually low strength of layerÀlayer interactions (because attachment does not involve the formation of covalent or ionic bonds).…”

“…The flash freezing approach has been proven to preserve the structure of hydrated biological macromolecules such as ribosomes for which independent atomic-resolution structures exist (see Cheng, 2015, and references therein). Cryo-TEM has previously been used to determine the structure of microbial mineral precipitates (Comolli et al, 2011) and the structure of fragile, extended aggregates of Fe oxyhydroxide nanoparticles (Yuwono et al, 2010;Frandsen et al, 2014;Legg et al, 2014). Cryo-TEM was used by Segad et al (2012) to compare the aggregate structures of Na-and Ca-equilibrated montmorillonite suspensions, but without investigating the osmotic hydrate regime.…”

Trends in mica–mica adhesion reflect the influence of molecular details on long-range dispersion forces underlying aggregation and coalignment

“…OA has been inferred for metals (10), semiconductors (11), and insulating oxides (6); it has been directly observed via liquid phase transmission electron microscopy (LP-TEM) (5), and the evolution of particle distributions during OA has been captured with cryogenic TEM (12). OA is highly dependent on solution conditions, including pH, ionic strength, and temperature, and is marked by two important stages.…”

“…In the first stage, particles approach one another but do not make contact; rather they remain separated by an intervening solvent layer of O(1) nm in thickness (5). This solvent-separated state can occur on such an extensive scale that a kinetically stabilized particle array-sometimes referred to as a mesocrystal-is formed in which particles are crystallographically coaligned, but not in contact (12). In the second stage, the attachment barrier associated with the layer is overcome and particles fuse via a sudden "jump-to-contact" (5).…”

Nanoparticle-mediated nonclassical crystal growth of sodium fluorosilicate nanowires and nanoplates

“…Although the aggregation-based crystallization has been regarded as the actual crystallization mechanism for many inorganic crystals, however, few direct evidences exist that support the existence of such an intermediate stage. 19 Herein we report the observation of the primary nanoparticles in Na 2 SiF 6 , their self-assembly into loose aggregates, and subsequent crystallographic fusion and crystallization to generate final crystalline nanoplate or nanowire/nanorod. The complex Na 2 SiF 6 superstructures were also observed to form through similar processes.…”