Crystallization and formation mechanisms of nanostructures

The synthesis of crystalline nanoparticles and nanostructures with well-defined sizes, morphologies and hierarchical structures is one of the grand challenges in nanoscience and nanotechnology today. While it is well known that particles with a huge range of morphologies and internal structures can be produced through careful control of reaction conditions and the use of additives such as polymers,¹ understanding of the mechanisms by which these structures develop remains poor. As such, although great progress has been made in the development of synthetic methods over the last few years, these routes remain largely empirical, such that production of target morphologies, size distributions and structures typically relies on time-consuming trial-and-error experiments. The situation becomes even more complex if the synthetic goal is hybrid nanoparticles of controlled size distribution and shape. The only solution to this problem is to apply experimental and theoretical methods to achieve a greater understanding of how nanoparticles nucleate, grow and assemble. Suitable techniques include time-resolved studies of nucleation and crystallization processes, ideally by in situ methods,^2–4 and state-of-the-art modeling methods. This will ultimately allow us to predict the outcome of nanoparticle growth and directed assembly – especially in the presence of additives – and thus further profit from it synthetically.

The last decade has seen a revolution in our understanding of both crystal nucleation and growth mechanisms, and it is now clear that in the formation of many crystals, both nucleation and growth occur via aggregation-based mechanisms. In recent studies, titration methods, ultracentrifugation⁵ and high resolution cryo transmission electron microscopy (TEM)⁶ have all been used to demonstrate the presence of stable clusters in solution prior to nucleation for many minerals including the biologically-important calcium carbonate and calcium phosphate. These so-called “pre-nucleation clusters” are not predicted according to classical nucleation theory, which only describes the existence of transient species prior to nucleation. Aggregation of these clusters, which is likely to be accompanied by structural and compositional changes, then gives rise to nucleation.⁶ In the case of calcium carbonate, the nucleated species is amorphous calcium carbonate (ACC) within which nucleation of a crystalline phase will ultimately take place. That the formation and the structure of pre-nucleation clusters can be affected by the presence of additives,⁷ such that crystal polymorphs may be selected at the earliest stages of crystallisation, suggests enormous potential for controlling crystallisation processes.

Post-nucleation, it is now also well-established that crystal growth often proceeds via the aggregation of precursor units, rather than by a classical ion-by-ion addition mechanism.^8–10 This can generate crystals with hierarchical structures and complex morphologies. Additives such as block-copolymers can facilitate this, but are not essential. Assembly can be non-oriented, leading to polycrystalline particles with no preferred orientation, or oriented, leading to “mesocrystals” where the crystallites can be arranged in crystalline register and the product particles behave as single crystals.⁸ A continuum of structures also exists between these two end-members.¹¹ Noting that mesocrystals are defined as “mesoscopically structured crystals”, meaning an assembly of nanocrystals with mutual order, such structures can also be generated on crystallisation of an assembly of amorphous nanoparticles, provided that a memory of the original amorphous particles is retained in the product crystal. Identification of a mesocrystal can be achieved according to the presence of a number of features including high porosity/large surface area, evidence of nano-scale units in the surface or internal structures as viewed from Scanning Electron or Transmission Electron Microscopy and demonstration of small building-blocks from line broadening in X-ray diffraction analysis. However, as post-assembly re-crystallisation of a mesocrystal can take place to give a true single crystal with large domain sizes, characterisation of the formation mechanism of particles which diffract as large-domain single crystals remains the only fail-safe method of determining whether they grow by an ion-by-ion or assembly mechanism.

It is also valuable to draw parallels between synthetic crystals, and their formation mechanisms, and crystals precipitated under biological control, namely biominerals such as bones, teeth and seashells. Biominerals are frequently characterized by remarkable morphologies and hierarchical structures,^12,13 suggesting that a strong synergy exists between these two research fields. It can therefore be expected that concepts operative in biomineralization can be applied to synthetic nanoparticle formation and vice versa. Indeed, there is currently intense interest in crystallisation from amorphous precursor phases, both from biological and synthetic perspectives. This was sparked by the observation that the spicules produced by sea urchin larvae form via amorphous CaCO₃ (ACC) which subsequently crystallises to give single crystals of calcite with identical tri-radiate morphologies to the initial amorphous phase.¹⁴ That Nature selects this growth mechanism rather than ion-by-ion growth suggests many potential benefits such as the absence of high ionic strengths, which can lead to high osmotic pressures, access to rapid mineralization rates and the ability to closely control the nucleation and growth processes.

The articles brought together in this special issue of Nanoscale describe recent developments in the understanding of solution-based crystal nucleation and growth mechanisms, and the application of this knowledge to the design and growth of novel nanomaterials. The issue is supported by a number of review articles which consider nucleation and growth mechanisms, and routes to controlling crystal morphologies. Vekilov considers the formation of crystals in solution via a 2-step nucleation mechanism analogous to that considered to take place during protein crystallisation, while Leite et al. consider aggregation-based mechanisms from the perspective of colloidal-chemistry. Addressing synthesis of nanoparticles, Kwon and Hyeon review the application of hot-injection and heating methods to the formation of uniform nanoparticles, while Wang and Yuan review the morphological control of metal nanoparticles in aqueous-based syntheses.

The focus on metal nanostructures continues in the full papers contributed by Xia, by Liz-Marzán et al., Polte et al., Guo and Searson, Kowalczyk et al. and Suber et al. where the themes vary from the fabrication of unusual morphologies, for example by assembling metal nanoparticles on the surfaces of liquid droplets, and stabilising these structures using dithiol crosslinks, as performed by Kowalczyk et al., to in situ monitoring and mechanisms of particle formation. Nanostructures of a range of other functional inorganics including PbS, PbSe, SnO and cobalt oxalate are considered in some of the other articles. While the articles by Bowen et al. and Shi et al. describe how reaction conditions and organic templates can be used to select the product particle morphologies, the potential for controlling particle properties via definition of their structures is illustrated by Sakaushi et al. who tune the optical properties of SnO by changing their morphologies from sheets to wires. Hall et al. prepare nanoparticles of the molecular magnet material, cobalt-Prussian Blue, using chitosan as a morphological structure-directing agent and show that they exhibit a sharp ferrimagnetic to paramagnetic transition at 16 K.

Characterisation of the mechanisms of particle aggregation is also essential for the development and ultimate control over this process. This is carried out using energy-dispersive XRD (EDXRD) techniques to determine the growth kinetics and mechanism of photocatalytically active hierarchical Bi₂WO₆ nanostructures in the article by Patzke et al. while Almeida et al. investigated the formation of α-Fe₂O₃ nanorods by analysing rapidly quenched samples using a combination of transmission electron microscopy, X-ray photoelectrin spectroscopy and FTIR spectroscopy. Moving towards the goal of programmed growth of pre-designed nanostructures, Qi et al. manufacture PbS/Au heterostructures where a single Au nanoparticle is deposited on a single horn of a PbS nanostar. Such particles are considered as future candidates for the assembly of anisotropic nanostructures.

The relationship between synthetic and bio-related systems is illustrated by articles focusing on the formation of calcium carbonate and calcium phosphate nanostructures. While calcium phosphate crystals with different morphologies and organisations are generated through the use of organic additives and structure-directing agents by Mann et al., Taubert et al., Zhai et al. and Ibsen and Birkedal, the formation and stability of amorphous calcium carbonate (ACC) is addressed by Sommerdijk et al. and Jiang et al. As an important potential precursor to crystalline structures, Jiang et al. report a method for producing large quantities of Mg-ACC, which can be stored for long periods when it is maintained in a dry state, while Sommerdijk et al. show that the stability of ACC particles in solution is related to their size, demonstrating rapid crystallization only when they achieve a critical size.

Finally, moving from inorganic to organic materials, two articles consider the formation of organic structures. Nam et al. describe the use of synchrotron grazing incidence angle X-ray diffraction (GIXD) to investigate the formation of the nanostructures of polythiophene/fullerene heterojunction films while layered, nanostructured inorganic/organic composites are generated by organization of an imidazolium salt in the interlayer space of α-cobalt hydroxide, a layered inorganic, through a two-step approach in the communication from Oaki et al.

In summary, the collection of articles presented in this themed issue of Nanoscale illustrate the rich, and creative range of synthetic approaches currently available to produce nanostructures of technologically- and biologically-important materials. Further, it is shown how application of characterisation techniques to probe intermediate structures and product materials can yield important information on their mechanisms of formation. Clearly, it is essential to continue to develop methods to enable in situ monitoring of nanoparticle formation under genuine experimental conditions. In combination with computer modeling, these studies will ultimately lead to a rigorous understanding of the principles underlying the formation of these nanostructures, and ultimately to the generation of general strategies for the rational synthesis of nanostructured materials with morphologies ranging from the simple to the complex, and structures based on the desired level of hierarchy.

Fiona C. Meldrum (Guest Editor), School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, UK LS2 9JT. E-mail: E-mail: F.Meldrum@leeds.ac.uk

Helmut Cölfen (Guest Editor), Department of Chemistry, Physical Chemistry, University of Konstanz, Universitätsstr. 10, Box 714D-78457, Konstanz, Germany. E-mail: E-mail: helmut.coelfen@uni-konstanz.de

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