Controlled self-organization of nanoparticles can lead to new materials. The colloidal crystallization of non-spherical nanocrystals is a reaction channel in many crystallization reactions. With additives, self-organization can be stopped at an intermediary step-a mesocrystal-in which the primary units can still be identified. Mesocrystals were observed for various systems as kinetically metastable species or as intermediates in a crystallization reaction leading to single crystals with typical defects and inclusions. The control forces and mechanism of mesocrystal formation are largely unknown, but several mesocrystal properties are known. Mesocrystals are exiting examples of nonclassical crystallization, which does not proceed through ion-by-ion attachment, but by a modular nanobuilding-block route. This path makes crystallization more independent of ion products or molecular solubility, it occurs without pH or osmotic pressure changes, and opens new strategies for crystal morphogenesis.

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Mesocrystals: Inorganic superstructures made by highly parallel crystallization and controlled alignment

CuO nanostructures: Synthesis, characterization, growth mechanisms, fundamental properties, and applications

In this review, we highlight particle based crystallization pathways leading to single crystals via mesoscopic transformation. In contrast to the classical mechanism of atom/molecule mediated growth of a single crystal, the particle mediated growth and assembly mechanisms are summarized as “non-classical crystallization”, including exiting processes like oriented attachment and mesocrystal formation. Detailed investigations of non-classical crystallization mechanisms are a recent development, but evidence for these pathways is rapidly increasing in the literature. A major driving force for these investigations originates from biomineralization, because it seems that these crystallization routes are frequently applied by natural organisms. We give a non-exhaustive literature survey on these two mechanisms with a focus on recent examples and studies, which are dedicated to a mechanistic understanding. Furthermore, conditions are introduced for which these non-classical crystallization mechanisms can be expected, as they are always an alternative reaction pathway to classical crystallization.

Oriented attachment and mesocrystals: Non-classical crystallization mechanisms based on nanoparticle assembly

The latest advances in oriented attachment controlled morphosynthesis and crystal growth of various technically important inorganic materials have been reviewed with the focus on how to generate inorganic micro-/nanostructured materials based on the so-called oriented attachment mechanism. The overview about the basic crystallization principles nowadays falls into two types, i.e., one is the classical crystal growth mode, which is via atom-by-atom additions to an existing nucleus or dissolution of unstable phases and reprecipitation of more stable phases, and the other occurs through particle based aggregation modes involving the process of mesoscopic transformation. The systematic analysis of the particle based aggregation mechanism of oriented attachment in controllable synthesis of functional inorganic materials will be described in particular. Several fashions of attachment are undertaken in the already explored reaction systems, with nanoparticles or nanoribbons as primary building units to form 1D, 2D or 3D structures, and heterostructures. The mechanism of oriented attachment could happen in systems with addition of organic additives or without, demonstrating that organic additives are not the essential factor for this kind of growth mode, which shed new light to intensive understanding of this particular phenomenon. With organic additives, i.e., reactions in organic solvents or in aqueous solution, oriented attachment events can occur too. Current developments in oriented attachment, including the basic principles and potentials with specific examples, indubitably reinforce the understanding of detailed interaction mechanisms between inorganic nanoparticles and their subsequent high order self-assembly mechanism, which are definitely promising for rationally designing various kinds of inorganic materials with ideal hierarchy, controllable length scale, and structures in solution-based systems.

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Recent advances in oriented attachment growth and synthesis of functional materials: concept, evidence, mechanism, and future

In this work, hierarchical WO3 hollow shells: dendrites, spheres and dumbbells have been successfully synthesized by simply calcining the acid-treated PbWO4 and SrWO4 as precursors at 500 °C for 2 h. These hollow structures were characterized using X-ray diffraction, scanning and transmission electron microscopies, Brunauer–Emmet–Teller surface area and diffuse reflectance spectroscopy, respectively. Possible formation mechanisms for hollow WO3 shells consisted of tiny nanoplatelets were investigated in detail. Comparing with commercial WO3 particles, all the obtained hollow shells with larger BET surface areas showed enhanced photocatalytic activities for the degradation of organic contaminants under visible light irradiation.

Hierarchical WO3 Hollow Shells: Dendrite, Sphere, Dumbbell, and Their Photocatalytic Properties

Three-dimensionally oriented aggregation of a few hundred nanoparticles into monocrystalline architectures

Complex 3D dendritic nanoarchitectures of copper hydroxide built up of ultrathin nanosheets have been synthesized progressively under nearly sustained supersaturation of impurity-free nutrients that are supplied statically from the natural oxidation of copper metal. Adjacent dendritic nanoarchitectures can further expand and eventually self-organize/merge themselves into impressive bicontinuous networks through the reaction-volume-controlled growth in formamide aqueous solution. When the simplest amino acid, glycine, was used as organic additive, high-order, shell-ornamented dendritic nanoarchitectures can be fabricated accordingly. A clear perspective is shown here that more complex nanostructured materials could be chemically synthesized under nearly sustained supersaturation and further decorated when biological molecules are used as templates.

Morphosynthesis and Ornamentation of 3D Dendritic Nanoarchitectures

A simple, low-temperature strategy has been developed for the low-cost and large-area fabrication of ZnO nanoarrays on a zinc substrate by the natural oxidation of zinc metal in formamide/water mixtures. The one-step, wet-chemical approach has exhibited well-controlled growth of highly oriented and densely packed ZnO nanoarrays with large-area homogeneity and consisting of nanorods or nanowires with predictable morphologies, such as tunable diameters and identical lengths. The chemical-liquid-deposition process, an analogue to the widely used chemical-vapor-deposition technique, has been used for the near-room-temperature production of ZnO nanoarrays through continuous supply, transport, and thermal decomposition of zinc complexes in a liquid phase.

Xiaoqiong Shao

Papers

Three-dimensionally oriented aggregation of a few hundred nanoparticles into monocrystalline architectures

Morphosynthesis and Ornamentation of 3D Dendritic Nanoarchitectures

Near‐Room‐Temperature Production of Diameter‐Tunable ZnO Nanorod Arrays through Natural Oxidation of Zinc Metal