The growth of crystalline nanoparticles (NPs) generally involves three processes: nucleation, growth, and shape evolution. Among them, the shape evolution is less understood, despite the importance of morphology for NP properties. Here, we propose a symmetrybased kinematic theory (SBKT) based on classical growth theories to illustrate the process. Based on the crystal lattice, nucleus (or seed) symmetry, and the preferential growth directions under the experimental conditions, the SBKT can illustrate the growth trajectories. The theory accommodates the conventional criteria of the major existing theories for crystal growth and provides tools to better understand the symmetry-breaking process during the growth of anisotropic structures. Furthermore, complex dendritic growth is theoretically and experimentally demonstrated. Thus, it provides a framework to explain the shape evolution, and extends the morphogenesis prediction to cases, which cannot be treated by other theories.
One aspect of the
research on mesocrystals nowadays focuses on
applications, whereby such applications demand mesocrystals with a
tunable size. To achieve this task, more effort needs to be undertaken
to understand how mesocrystals form, which parameters influence mesocrystal
formation, and which kind of structure results from the nanoparticle
assembly. Within this communication, we demonstrate for faceted mesocrystals
assembled from iron oxide nanocubes stabilized by oleic acid that
the proper choice of crystallization conditions in the gas phase diffusion
setup is essential to achieve this task. The appropriate choice of
substrate, dispersion and destabilizing agents, additive, nanocrystal
concentration, crystallization kinetics, and duration allows growing
faceted iron oxide mesocrystals with sizes ranging from a few micrometers
up to almost a millimeter. By these findings supported by light and
scanning electron microscopy, we show that in this system, heterogeneous
nucleation is the predominant mechanism for mesocrystal formation
on a solid substrate. Additionally, other surfactants than oleic acid
can also act as molecular additives to support mesocrystal growth.
These findings should be transferable to tune the size and quality
of other self-assembled mesocrystals.
Nanoparticle assemblies
with long-range packing order and preferred
crystallographic orientation of building blocks, i.e., mesocrystals, are of high interest not only because of their unique physical
properties but also due to their complex structure and morphogenesis.
In this study, faceted mesocrystals have been assembled from the dispersion
of truncated cubic-shaped iron oxide nanoparticles stabilized by oleic
acid (OA) molecules using the nonsolvent “gas phase diffusion
technique” into an organic solvent. The effects of synthesis
conditions as well as of the nanoparticle size and shape on the structure
and morphogenesis of mesocrystals were examined. The interactions
of OA-capped iron oxide nanoparticles with solvent molecules were
probed by analytical ultracentrifugation and double difference pair
distribution function analysis. It was shown that the structure of
the organic shell significantly depends on the nature and polarity
of solvent molecules. For the nonpolar solvents, the interaction of
the aliphatic chains of OA molecules with the solvent molecules is
favorable and the chains extend into the solvent. The solvation shell
around the nanoparticles is more extended in nonpolar and more compact
in polar solvents. There is a clear trend for more spherical particles
to be assembled into the fcc superlattice, whereas
less truncated cubes form rhombohedral and tetragonal structures.
The observed changes in packing symmetry are reminiscent of structural
polymorphism known for “classical” (atomic and molecular)
crystals.
Mesocrystals are nanostructured materials consisting of individual nanocrystals having a preferred crystallographic orientation. On mesoscopic length scales, the properties of mesocrystals are strongly affected by structural heterogeneity. Here, we report...
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