This study describes synthesis and detailed characterization of 2D and 3D mesocrystalline films produced by self‐assembly of iron oxide (magnetite) truncated nanocubes. The orientational relations between nanocrystals within the superlattice are examined and atomistic models are introduced. In the 2D case, two distinct superstructures (i.e., translational order) of magnetite nanocubes can be observed with p4mm and c2mm layer symmetries while maintaining the same orientational order (with [100]magnetite perpendicular to the substrate). The 3D structure can be approximated by a slightly distorted face‐centered cubic (fcc) superlattice. The most efficient space filling within the 3D superstructure is achieved by changing the orientational order of the nanoparticles and following the “bump‐to‐hollow” packing principle. Namely orientational order is determined by the shape of the nanoparticles with the following orientational relations: [001]SL||[310]magnetite, [001]SL||[301]magnetite, [001]SL||[100]magnetite. Overall the presented data provide a fundamental understanding of a mesocrystal formation mechanism and their structural evolution. Structure, composition, and magnetic properties of the synthesised nanoparticles are also characterized.

Applications in the fields of materialss ciencea nd nanotechnology increasingly demand monodisperse nanoparticles in size and shape. Up to now,n og eneral purification procedure exists to thoroughly narrow the size and shape distributions of nanoparticles.H ere, we show by analytical ultracentrifugation (AUC)a sa na bsolute and quantitative high-resolutionm ethod that multiple recrystallizations of nanocrystals to mesocrystalsi savery efficient tool to generate nanocrystals with an excellent and so-far unsurpassed size-distribution(PDI c = 1.0001)a nd shape. Similart o the crystallization of molecular building blocks, nonclassical recrystallization removes "colloidal" impurities(i.e.,n anoparticles,w hich are different in shape and size from the majority) by assembling them into am esocrystal. In the case of nanocrystals, this assembly can be size-and shape-selective, since mesocrystals show both long-range packing ordering and preferable crystallographic orientation of nanocrystals. Besides the generation of highly monodisperse nanoparticles, these findings provide highly relevant insights into the crystallization of mesocrystals.

Silica biomorphs are extraordinary inorganic superstructures formed via autocatalytic co‐precipitation and bottom‐up self‐assembly of alkaline‐earth carbonates and silica. However, they show no inherent functionality except for their striking textural motifs and curved morphologies. This work presents strategies to magnetize silica biomorphs, thus creating thermally stable ceramic microswimmers with unique elaborate shapes. This is achieved by growing super paramagnetic magnetite mesocrystals on and around the complex curved surfaces of biomorphs, while keeping their morphology and maintaining mesocrystal integrity. Selective mesocrystal formation on certain parts of the substrates is induced by chemical modification of the biomorph surface, increasing the loading of magnetite on the silica–carbonate structures and, in suitable cases, rendering them able to respond to external magnetic fields and move as microswimmer entities. In this way, the complex ultrastructure of silica biomorphs is successfully used as a template for functional ceramics. Furthermore, selective dissolution of the carbonate core from the biomorphs leads to hollow magnetic structures that could be filled with actives, thus serving as microcarriers with considerable loading capacity.

This letter describes the formation and detailed characterization of iron oxide mesocrystals produced by the directed assembly of superparamagnetic iron oxide-truncated nanocubes using the slow evaporation of the solvent within an externally applied homogeneous magnetic field. Anisotropic mesocrystals with an elongation along the direction of the magnetic field can be produced. The structure of the directed mesocrystals is compared to self-assembled mesocrystalline films, which are formed without the influence of a magnetic field. The remarkable structural difference of mesocrystals produced within the external magnetic field from those self-assembled without field indicates that the specific nanoparticle ordering within the superstructure is driven by competing of two types of anisotropic interactions caused by particle shape (i.e., faceting) and orientation of the magnetic moment (i.e., easy axes: <111>magnetite). Hence, these findings provide a fundamental understanding of formation mechanisms and structuring of mesocrystals built up from superparamagnetic nanoparticles and how a magnetic field can be used to design anisotropic mesocrystals with different structures.

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.