Mesocrystals are 3D ordered nanoparticle superstructures, often with internal porosity, which receive much recent research interest. While more and more mesocrystal systems are found in biomineralization or synthesized, their potential as material still needs to be explored. It needs to be revealed, which new chemical and physical properties arise from the mesocrystal structure, or how they change by the ordered aggregation of nanoparticles to fully exploit the promising potential of mesocrystals. Also, the mechanisms for mesocrystal synthesis need to be explored to adapt it to a wide class of materials. The last three years have seen remarkable progress, which is summarized here. Also potential future directions of this reaserch field are discussed. This shows the importance of mesocrystals not only for the field of materials research and allows the appliction of mesocrystals in advanced materials synthesis or property improvement of existing materials. It also outlines attractive research directions in this field.

A simple hydrothermal method has been developed for the systematic synthesis of lanthanide orthophosphate crystals with different crystalline phases and morphologies. It has been shown that pure LnPO(4) compounds change structure with decreasing Ln ionic radius: i.e., the orthophosphates from Ho to Lu as well as Y exist only in the tetragonal zircon (xenotime) structure, while the orthophosphates from La to Dy exist in the hexagonal structure under hydrothermal treatment. The obtained hexagonal structured lanthanide orthophosphate LnPO(4) (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) products have a wirelike morphology. In contrast, tetragonal LnPO(4) (Ln = Ho, Er, Tm, Yb, Lu, Y) samples prepared under the same experimental conditions consist of nanoparticles. The obtained hexagonal LnPO(4) (Ln = La --> Tb) can convert to the monoclinic monazite structured products, and their morphologies remained the same after calcination at 900 degrees C in air (Hexagonal DyPO(4) is an exceptional case, it transformed to tetragonal DyPO(4) by calcination), while the tetragonal structure for (Ho--> Lu, Y)PO(4) remains unchanged by calcination. The resulting LnPO(4) (Ln = La --> Dy) products consist almost entirely of nanowires/nanorods with diameters of 5-120 nm and lengths ranging from several hundreds of nanometers to several micrometers. Europium doped LaPO(4) nanowires were also prepared, and their photoluminescent properties were reported. The optical absorption spectrum of CePO(4) nanowires was measured and showed some differences from that of bulk CePO(4) materials. The possible growth mechanism of lanthanide phosphate nanowires was explored in detail. X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, electron diffraction, infrared absorption spectra, X-ray photoelectron spectroscopy, optical absorption spectra, and photoluminescence spectra have been employed to characterize these materials.

Porous magnesium hydroxide nanoplates were prepared directly from commercial bulk magnesium oxide crystals by a simple hydrothermal treatment. These thin plates would aggregate into large spherical particles. The platelike morphology was retained after calcination, and porous magnesium oxide nanoplates were obtained. These plates have a wormhole-like porous structure with high surface area. The obtained materials exhibit bimodal pore size distributions in the mesoporous domain. The aggregation of the nanoplates gives rise to large mesopores with a size of about 36 nm. In addition, each plate has small wormhole mesopores with a size of about 3.7 nm. The growth of magnesium hydroxide nanoplates occurred through a dissolution−recrystallization process. X-ray diffraction and electron diffraction, transmission electron microscopy, scanning electron microscopy, and nitrogen sorption have been employed to characterize these nanoplates. Such porous nanoplates with high surface area and high crystallinity have many promising applications. Moreover, bismuth oxide nanoplates were also produced following a similar method. The formation mechanism of such mesostructures without the use of a template is also discussed.

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Novel hierarchical ZnO nanostructures, porous ZnO nanobelts, and nanoparticle chains are prepared from a precursor of synthetic bilayered basic zinc acetate (BLBZA) nanobelts. BLBZA nanobelts are obtained by a simple synthetic route under mild conditions. X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, infrared spectroscopy, and thermal analysis are used to characterize the BLBZA nanobelts and ZnO nanostructures. The obtained BLBZA precursor consists of a lamellar structure with two interlayer distances of 1.33 and 2.03 nm, exhibits a beltlike morphology, and has widths of 200 to 600 nm, thicknesses of 10 to 50 nm, and lengths of up to 50 μm. Refluxing an aqueous dispersion of BLBZA nanobelts at 120 °C for 12 h leads to the formation of well‐defined hierarchical ZnO nanostructures. The time‐dependent shape‐evolution process suggests that spindlelike ZnO particles form first, and then the ringlike nanosheets grow heterogeneously on the backbone of these spindles. In addition, calcination in air can remove ligand molecules and intercalated water molecules from BLBZA nanobelts, resulting in the formation of porous ZnO nanobelts and nanoparticle chains. The BLBZA nanobelts serve as templates during the transformation to form ZnO beltlike nanoparticle chains without morphological deformation. Photoluminescence results show that both the as‐synthesized hierarchical ZnO nanostructures and porous ZnO nanobelts show a narrow and sharp UV emission at 390 nm and a broad blue–green emission at above 466 nm when excited by UV light.

In this paper, Tb(OH)3 and Y(OH)3 single‐crystalline nanotubes with outer diameters of 30–260 nm, inner diameters of 15–120 nm, and lengths of up to several micrometers were synthesized on a large scale by hydrothermal treatment of the corresponding oxides in the presence of alkali. In addition, Tb4O7 and Y2O3 nanotubes can be obtained by calcination of Tb(OH)3 and Y(OH)3 nanotubes at 450 °C. X‐ray diffraction (XRD), field‐emission scanning electron microscopy, transmission electron microscopy (TEM), electron diffraction (ED), energy‐dispersive X‐ray spectroscopy (EDS), thermogravimetry, and differential scanning calorimetry (DSC) have been employed to characterize these nanotube materials. The growth mechanism of rare earth hydroxide nanotubes can be explained well by the highly anisotropic crystal structure of rare earth hydroxides. These new types of rare earth compound nanotubes with open ends have uses in a variety of promising applications such as luminescent devices, magnets, catalysts, and other functional materials. Advantages of this method for easily realizing large‐scale production include that it is a simple and unique one‐pot synthetic process without the need for a catalysts or template, is low cost, has high yield, and the raw materials are readily available. The present study has enlarged the family of nanotubes available, and offers a possible new, general route to one‐dimensional single‐crystalline nanotubes of other materials.

Besides the classical atom/ion/molecule based mechanism, nonclassical crystallization provides a nanoparticle-based crystallization pathway toward single crystals. However, there is a lack of experimentally established strategies for engineering a range of crystalline microstructures from common nanoparticles by nonclassical crystallization. We demonstrate that a commercial random copolymer polyelectrolyte poly(4-styrene sulfonate)-co-(maleic acid) (PSS-co-MA) considerably guides crystallization of calcium carbonate (CC) with a high versatility. The bioinspired nonclassical crystallization protocol yielded a series of calcite microstructures. Calcite single crystals obtained at low supersaturation show a pseudo-dodecahedral shape with curved faces, whereas increasing supersaturation generated calcite mesocrystals with pseudo-octahedral shapes and scalloped surfaces. Further increase of supersaturation induced the formation of polycrystalline multilayered and hollow spheres. In the initial growth stage of all these microstructures, amorphous CC nanoparticles formed as the early product. Remarkably, microparticles with minimal primitive (P)-surface were captured as the prominent intermediate indicative of liquidlike behavior. Moreover, nanogranular structures exist broadly in the as-synthesized crystals. These results demonstrate that the polyelectrolyte can effectively stabilize the amorphous CC nanoparticle precursors, impart control over the evolution from amorphous precursors via a liquid aggregate through P-surface intermediates to the final crystals, and thus allow the morphogenesis. Simple variation of calcium and polyeletrolyte concentrations enables a systematic control over the size and morphology of particles among pseudo-dodecahedra, pseudo-octahedra, multilayered spheres, and hollow spheres, which are expressed in a morphology diagram. A unifying nanoparticle aggregation formation mechanism was suggested to explain the morphogenesis by the combination of nonclassical crystallization and surface area minimization principles.

Platonic solids [1] are the five polyhedrons with equivalent facets composed of congruent convex regular polygons, in which the same number of facets meet at every vertex. The octahedron, with eight facets, is a platonic shape composed of triangles, and the dodecahedron, with 12 facets, is the only one with pentagonal facets. In nature, pyrite frequently occurs as octahedral or dodecahedral crystals. Certain viruses and radiolaria also routinely take the form of these regular polyhedral shapes.[2] A number of synthetic inorganic crystals with platonic shapes have been reported recently, such as gold nanocrystals, [3a] mesoporous silicate dodecahedra and octahedra, [3b,c] [4k] and from sea urchins. [4l,m] The mechanisms of crystal growth of these calcite pseudo-dodecahdra are additive adsorption on the {011} faces, [4l, n] the inhibiting effect of additives on step-growth, [4c] or the combination of the two functions.[4] Calcite mesocrystals with pseudo-octahedral morphology [5a,b] and otoconia-like morphology [5c] were also described.However, a unified route towards more than one platonic calcite shape based on one same mechanistic approach has not yet been revealed. Herein, we present a nonclassical crystallization approach towards calcite with a platonic shape based on nanoparticle aggregation. We show that a commercial co-polyelectrolyte can be used to manoeuvre the crystallization of CaCO 3 into meso-crystalline calcite structures with curved surfaces and platonic shapes. Significantly, the intermediates are consistent with rhombohedral primitive (P)-surface morphology (see Supporting Information Figure S1)-one of the mathematical minimal surfaces, which were found in nature and in synthetic bicontinuous mesophases. [6a,b, 7] The most striking example is the skeleton of Cidaris rugosa, in which self-assembled 3D networks have a continuous curvature. The importance of the P-surface structure was also recognized in other sea-urchin skeletal plates and has been connected with their functions in nutrient permeation, stress distribution, and their unique optical properties. [6,8] The platonic calcite crystals were generated by a simple gas-diffusion method [9] in the presence of poly(4-styrenesulfonate-co-maleic acid) (PSS-co-MA). This polymer was chosen as it combines the maleic acid, which stabilizes {011} faces of calcite, [4e] with the mesocrystal-inducing properties of PSS [10] in one molecule. Pseudo-dodecahedral crystals were obtained for a polymer concentration of 0.1 g L À1 and [CaCl 2 ] = 1.25 mm after 2 weeks crystallization.