Mesocrystals

Mesocrystals are a very interesting form of colloidal crystal, as they extend the so far known colloidal crystals with spherical building units to those with non-spherical building units. This offers new possibilities of superstructme formation due to the anisotropic particle shape of the nanoparticle building units [113]. Thus, mesocrystals are colloidal crystals but with extended possibihties for their self-assembled superstructure, offering new handles for crystal morphology control. 

Although the above system has been very weU investigated, the formation process of the mesocrystal is still not fully revealed. This is essentially true for most reported mesocrystals. Although precursor nanoparticles have already been foimd experimentally [122] and dipole fields are suggested to be responsible for their almost perfect alignment [117,123], the full growth mechaiusm has so far only been reported for two examples. 

Mesocrystals can even be formed without additive, as foimd for the related cobalt oxalate dihydrate [126]. Copper oxalate crystals were composed of nanometer building units and strings of nanodomains oriented along the principal axis of the particle were detected, revealing that the lateral and basal faces of the precipitate are composed of stacked nanoparticle layers with a thickness of 5-7 nm. 

A time-resolved HRSEM study revealed the formation mechanism of the mesocrystal [126]. Poorly crystalline primary particles (10 nm) first aggregate to form secondary particles (23 nm). The latter subsequently aggregate to form polydisperse elongated particles. These elongated particles also ag- 

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H. Colfen, M. Antonietti, Mesocrystals inorganic superstructures by highly parallel crystallization and controlled alignment. Angew. Chem. Int. Ed 44 (2005) 5576. 

Liu and Zeng [88] showed that [001]-oriented petal-like Ti02 mesocrystals could be grown onto MWCNTs in aqueous solution (Fig. 16.6(c)). These examples show how in the presence of CNTs unusual morphologies for Ti02 nanocrystals could be prepared, with clear relevance for their photocatalytic and/or photoanode activity due to both different crystallographic faces exposed and different type of interface with nanocarbons on which they are supported. 

Liu, B. Zeng, H. C., Carbon Nanotubes Supported Mesoporous Mesocrystals of Anatase Ti02. Chem. Mater. 2008,20 2711-2718. 

Wang, T.X., Colfen, H., and Antonietti, M. "Nonclassical crystallization Mesocrystals and morphology change of CaC03 crystals in the presence of a polyelectrolyte additive".. Am. Chem. Soc. 127(10), 3246-3247 (2005). 

In contrast to mesoscale assemblies, mesocrystals are considered intermediates on the formation pathway to single crystals (Colfen and Yu, 2005). The full growth mechanism has been reported for very few mesocrystals, two of... 

Barium sulfate (BaS04) has an industrial relevance due to its whiteness, inertness, high specific gravity, and optical properties, such as opacity to UV rays and X-rays [164-167]. It is mainly used as a radio-contrast agent, filler in plastics, extender in paints, coatings and additive in pharmaceutical products, and in printing ink. Nowadays, interest on this material has been renewed with the development of methods to produce nanosized particles, supraparticles of a well-controlled number of nanoparticles and mesocrystals that mimic biomineralization processes. 

Figure 16. Schematic demonstration of the formation of single crystals, mesocrystals, and polyorystalline aggregates in the presence of DHBCs. Reproduced from [96] by permission of the American Chemical Society.

Fig. 3 A one-step annealing process allows for the formation of metal oxide mesocrys-tals containing (a) a single-metal oxide, (b) nanocomposite mesocrystals or (c) solid solution mesocrystals. Reproduced with permission from ref. 17. Copyright 2014 Nature Publishing Group. ...

Colfen, H., Antonietti, M. (2008). Mesocrystals andNonclassical Crystallization. John Wiley Sons, Chichester, U.K. 

Figure 7 Schematic representation of classical and nonclassical crystallization. Pathway (a) the classical crystallization pathway where nucleation clusters grow to a primary nanoparticle, which is amplified to a single crystal. Pathway (b) the primary nanoparticles form an iso-oriented crystal, which can fuse to form a single crystal. Pathway (c) the primary nanoparticles can be stabilized by an additive and can form a mesocrystal. Pathway (d) amorphous particles or liquid droplets are formed, which can transform to complicated morphologies. (Reproduced from Ref. 7. John Wiley Sons, Ltd, 2008.)...

Stabilization of mesocrystals inhibiting fusion to a single crystal. 

Zhao Y, Lu Y, Lin LN, et al. Synthesis of superparamagnetic CaC03 mesocrystals for multistage dehvery in cancer therapy. Smah November 2010 6 2436 2. 

Keywords Bio-inspired mineralization - Double hydrophilic block copolymers Mesocrystals Non-classical crystallization Oriented attachment Self-assembly... 

Other mesocrystal systems were also recently reported. CaCOa precipitation in the presence of a PSS additive yielded mesocrystals with a morphology that could be systematically changed by variation of the PSS/CaCOs ratio [122]. At higher CaCOs concentrations, even the symmetry of the morphology was broken and convex-concave shaped mesocrystals with a central hole were obtained (Fig. 17). 

Fig. 18 Concept of DHBCs, their interaction with mineral surfaces and possibilities for particle stabilization or self-assembly, a Nanoparticle stabilization, b unordered particle aggregation, c oriented attachment, d ordered aggregation/mesocrystal formation, e arrangement of nanoparticles around a template. Reproduced from [151] with permission of the Materials Research Society...

The most interesting observation was made when an amino acid was crystallized with one of the above-mentioned chiral DHBCs. The formation of facetted superstructures composed of highly ahgned nanoparticle building units could be observed for OL-alanine, which was the first description of the mesocrystal concept [123]. 

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