The differentiation of classical and non-classical crystallization concepts as well as the formation and definition of mesocrystals were described in detail by Helmut Cölfen and Markus Antonietti.61,62 Important aspects linked to this work are briefly summarized in Chapters 1.4.1 and 1.4.2.
1.4.1 Classical and non-classical crystallization
The classical crystallization pathway postulates an ion-by-ion or single-molecule attachment, finally forming a critical crystal nucleus. This process is highly influenced by the respective solubility products as well as diffusion limitations. The nuclei grow to primary nanoparticles, which grow further via ion-by-ion attachment and unit cell replication to form single crystals (see Figure 1.10, pathway (a)). In recent years, increasing evidence has been found for the occurrence of an alternative crystallization pathway, the so-called ‘non-classical’
crystallization. Two different scenarios are distinguished, both potentially initiated with amorphous or liquid precursors (see Figure 1.10).
Figure 1.10: Schematic illustration of classical and non-classical crystallization. (a) Classical crystallization pathway, (b) oriented attachment pathway leading to iso-oriented crystals, (c) mesocrystal formation via mesoscale assembly.63
12 The first pathway (pathway (b) in Figure 1.10) is a process occurring in absence of any additives. Primary nanoparticles can aggregate and mutually arrange, which is the so-called oriented attachment mechanism, leading to iso-oriented crystals. These crystals can subsequently merge into a common crystallographic register and form a single crystal. The driving force is, on the one hand, a gain in energy due to a decrease of surface energy, associated with reducing the amount of high-energy surfaces when particles fuse, and on the other hand, a win of entropy, caused by the release of small molecules like water that were adsorbed onto the surface. A competing process during oriented aggregation is Ostwald ripening, which refers to the growth of large particles at the expense of smaller particles. This process becomes even dominant in case of a high molecular solubility in the respective continuous phase and high interface energy of the crystal.61-63
The second non-classical crystallization pathway (see pathway (c) in Figure 1.10) involves adsorption of additives like surfactants or polymers on the primary nanoparticles, stabilizing the individual nanoparticles temporarily. Subsequent mesoscale assembly of the nanoparticle building units can be triggered by these additives via an additive-mediated oriented aggregation mechanism, leading to the formation of mesocrystals. The term ‘mesocrystal’, also known as mesoscopically structured crystal, is defined as a nanocrystal superstructure with 3-dimensional (3D) mutual order of the building units and with external crystal faces.
These crystals may subsequently fuse to form iso-oriented crystals and finally single-crystals.
The modular crystallization route can result in crystals with highly oriented nanoparticle building units, in one or two dimensions (Figure 1.11),64 and even three dimensions (Figure 1.12).65,66 Mesocrystals possess characteristic scattering and birefringent properties of single crystals, but consist of individual building units.61,62
Figure 1.11: TEM images of arrangements of prismatic BaCrO4 nanoparticles in chains (left) and a rectangular superlattice of BaCrO4 nanoparticles (right; 2-dimensional aggregation) prepared in a reverse microemulsion.
Right image: The arrow displays dislodged particles, revealing the prismatic morphology of individual particles.
The electron diffraction pattern in the inset shows the superimposition of reflections from zone axes approximately parallel to the (110) direction. The scale bar is in both images 50 nm.62,64
13
Figure 1.12: SEM images of progressive stages of the growth via self-assembly of fluorapatite aggregates in a gelatin gel (morphogenesis): from elongated hexagonal-prismatic seeds (a) through dumbbell shapes (b) to spherical shapes (c). The surface of the spheres shown in (c) is composed of needle-like subunits.62,65
Due to the difference of composition and porosity between mesocrystals and single crystals, mesocrystals can possess a variety of (potential) applications. Besides advantageous mechanical properties, which are exploited by Nature for biominerals, they may possess enhanced catalytic, optical or electronic properties, depending on the nanoparticles used.61,62
1.4.2 Mesocrystal formation
Different mechanisms for mesocrystal formation are known.61,62 The alignment of particles by means of an organic scaffold occurring via a matrix-mediated particle growth, as observed for example in bone, was mentioned in Chapter 1.2.1. Mesocrystals, however, are reported to form mainly via other, more dynamic mechanisms involving three different formation principles (some concepts are summarized in Figure 1.13), starting from nanoparticles in solution, which aggregate and mutually align in a crystallographic order. It is worth mentioning that it is very likely that more principles for mesocrystal formation exist.
Figure 1.13: Illustration of some principal concepts for three-dimensional alignments of nanoparticle building units to form a mesocrystal. (a) Alignment of nanoparticles by means of physical fields or mutual alignment of identical crystal faces; (b) epitaxial growth via mineral bridges; (c) nanoparticle alignment by spatial constraints.61
The first formation principle involves ordering of particles by directional external physical fields (Figure 1.13a), such as magnetic, electric, dipole and gravitational fields as well as polarization forces. Prerequisite for this kind of ordering is an anisotropic characteristic of the
14 nanoparticles with respect to their interaction potentials. Examples include the formation of BaTiO3 mesocrystals in an external electric field (Figure 1.14a)67 and oriented assemblies of maghemite nanocubes by means of an external magnetic field (Figure 1.14b and c).68
Figure 1.14: (a) SEM image of dumbbell-like aggregation of BaTiO3 nanoparticles in the presence of an external electric field.67 (b, c) Superlattices composed of maghemite nanocubes were found to form by slow drying of a concentrated toluene-based maghemite nanocube dispersion. (b) Low-magnification TEM image.
Scale bar is 1 µm. (c) Enlargement of a superlattice and selected area diffraction patterns on atomic scale (inset upper right) and on mesoscale (inset lower left), indicating a high degree of orientational order. Scale bar is 500 nm.68
Besides external physical fields, nanoparticles can also align through contact forces via oriented attachment. In close contact, van der Waals forces act as principal attractive forces.
Short-range repulsive forces can occur e.g. due to the presence of a polymer layer of the particles, governing the particles to support the mutual orientation of two identical (or complementary) crystal faces, so that these faces can finally fuse. A very controlled alignment of nanoparticles can be induced by face-selective polymers. This method can lead to a helical arrangement of BaCO3 crystals (Figure 1.15a).69
Figure 1.15: (a) SEM image of helical BaCO3 nanoparticle superstructure formed after two weeks in the presence of polyethyleneglycol-b-[(2-[4-dihydroxyphosphoryl]-2-oxabutyl)-acrylate ethyl ester]. (b) Scheme of the proposed mechanism for helix formation.69
The phosphonated racemic double hydrophilic block polymer selectively adsorbs on the (110) crystal faces of the anisotropic nanoparticles, leading to a restriction of the crystal growth in this direction as well as a selective sterical stabilization of this face. Consequently, particle