Paul O’Brien is at present the Chair of Inorganic Materials Chemistry at the University of Manchester and acting as Head of the School of Materilas. He completed his PhD in Inorganic Chemistry at University College Cardiff in 1978 and has worked in the University of London at Chelsea, Queen Mary and Imperial Colleges, and as a visiting Professor at Georgia Tech (1995-99). The recipient of numerous awards, he received the first Peter Day Award of the Royal Society of Chemistry in 2009.

Nanoscience Volume 1: Nanostructures through Chemistry

A Review of Recent Literature

By P. O’Brien

The Royal Society of Chemistry

Copyright © 2013 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84973-435-6

Contents

Preface Paul O’Brien, v,
Recent advances in mesocrystals and their related structures Yuya Oaki and Hiroaki Imai, 1,
Nanomaterials for solar energy Mohammad Azad Malik, Neerish Revaprasadu and Karthik Ramasamy, 29,
Magnetic hyperthermia Daniel Ortega and Quentin A. Pankhurst, 60,
Recent developments in transmission electron microscopy and their application for nanoparticle characterisation Sarah Haigh, 89,
Extracellular bacterial production of doped magnetite nanoparticles Richard A D Pattrick, Victoria S Coker, Carolyn I Pearce, Neil D Telling, Gerrit van der Laan and Jonathan R Lloyd, 102,
Atom-technology and beyond: manipulating matter using scanning probes Philip Moriarty, 116,
Graphene and graphene-based nanocomposites Robert J Young and Ian A Kinloch, 145,
Metal oxide nanoparticles Serena A Corr, 180,
Recent advances in quantum dot synthesis Arunkumar Panneerselvam and Mark Green, 208,
Nanoscience in India: a perspective Anirban Som, Ammu Mathew, Paulrajpillai Lourdu Xavier and T. Pradeep, 244,

CHAPTER 1

Recent advances in mesocrystals and their related structures

Noncalssical crystallization has attracted much interest in recent years. In classical models, crystalline materials were classified into single crystal and polycrystal. A variety of recent reports have showed mesocrystals as the intermediate states between single crystal and polycrystal. The present report focuses on mesocrystals and their related architectures consisting of the unit crystals. A variety of mesocrystals and their related architectures were categorized by the ordered state of the unit crystals. These new superstructures have potentials for a variety of applications, such as electrode and catalyst materials.

1 Introduction to mesocrystals and nonclassical crystallization

1.1 Crystalline materials – Two categories: classical and nonclassical

In classical models, crystalline materials have been classified into single crystal and polycrystal. In nonclassical models, mesocrystals are defined as the intermediate states between single crystal and polycrystal (Fig. 1). Single crystal can be regarded as the regular continuous packing of unit cells. For example, hexagonal prisms of quarts and cubes of table salt are typical single crystals. The macroscopic faceted morphologies consist of a continuous arrangement of unit cells. We cannot observe any intermediate ordered structures between the macroscopic shape and the atomic arrangements (Fig. 1a). The crystallographic direction is the same throughout the macroscopic shapes. In contrast, polycrystals are a random aggregate of small single crystals. The crystallographic direction of each single crystal is not the same in the aggregate (Fig. 1i). In a classical category of crystalline materials, researchers can classify the crystalline materials only into single crystals and polycrystals.

Many researchers have observed ordered arrangements of unit crystals that are not simply assigned to a polycrystal. The presence of a segmentalized unit is not ascribed to a perfect single crystal. The oriented architectures of unit single crystals can be regarded as an intermediate structures between single crystals and polycrystals (Fig. 1c–e). Based on these facts, Cölfen and Antonietti proposed mesocrystal as a new category of crystalline materials consisting of oriented nanocrystals. The colloidal crystallization of faceted nanocrystals leads to the formation of mesocrystals. The term of mesocrystal spread rapidly since the proposal of the concept. A variety of review articles related to mesocrystals have been published. In recent years, Zhou and O’Brien extended the concept of mesocrystals by addition of related structures.

Recent studies suggest nonclassical crystallization processes as well as the structures and applications of mesocrystals. The appearance of prenucleation clusters is one of the most important findings in nonclassical crystallization behavior. In addition, the presence of precursor phases and their roles for the subsequent crystallization have been studied in an attempt to understand nonclassical crystallization behavior.

In the present article, we focus on the structures and applications of mesocrystals. In Section 1.2, the structure is reviewed using biominerals as a typical model of mesocrystal. In Section 2, mesocrystals and their related structures are summarized with recent papers. In Section 3, the applications of mesocrystals are introduced on the basis of recent reports.

1.2 Biominerals – A model of mesocrystals

Mesocrystal is found in biominerals, such as the nacreous layer, sea urchin spine, and eggshell (Fig. 2). In previous work, researchers tried to determine whether or not the crystal structures of these biominerals are single crystal. Our group reported that carbonate-based biominerals possess mesocrystal structures. At approximately the same time, Sethmann and coworkers reported on the presence of nanostructures in biominerals. We analyzed the nanoscopic structures of biominerals, such as the nacreous layers, corals, echinoderms, foraminifers, and eggshells. These biominerals have unique macroscopic and micrometer-scale morphologies (Fig. 2a). Nanocrystals 20–100 nm in size are observed on magnified scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images regardless of the polymorphs, such as calcite and aragonite of calcium carbonate (CaCO3) (Fig. 2b–e). The spotted electron diffraction patterns are observed on these biominerals (Fig. 3a,b). The peak broadening originating from the miniaturization of the crystallites is not recognized on the XRD pattern (Fig. 3c). In addition, each unit crystal is found to be arranged in the same direction in TEM images (Fig. 3d,e). These facts indicate that the nanocrystals, as the building blocks, are oriented in the same crystallographic directions. Since the diffraction behavior is the same as that of the single crystals, these biominerals were recognized as single crystals in previous studies. Based on electron microscopy and diffraction analyses, the biominerals form mesocrystal structures consisting of oriented nanocrystals with biological macro-molecules. The nanocrystals and the biological macromolecules can be regarded as the nanoscale bricks and mortar, respectively. Since the nanocrystals are the building blocks for morphogenesis, living organisms can make up a variety of macroscopic shapes with single crystalline orientation through biomineralization. The first line in either of the columns and press the required button.

2 Mesocrystals and their related structures

Mesocrystals can be regarded as the intermediate state between single crystals and polycrystals. In the present article, mesocrystal is defined as the oriented nanocrystals in the same crystallographic direction. Recently, a number of reports have shown a number of related structures to mesocrystals. In this section, five types of mesocrystals and their related structures are introduced. The classification is based on the degree of the ordering and the orientation of the unit crystals, even though the size and shape of the unit crystals are different.

2.1 Oriented nanocrystals

As reported in detail in the reviews and in the literature, mesocrystal in the narrow sense of the term is the assembly of oriented nanocrystals with organic molecules (Fig. 1d). A variety of oriented nanocrystals have been reported in previous studies. The formation of oriented nanocrystals is mediated by the assembly of the particles.

For example, a variety of mesocrystals, such as CaCO3, BaSO4, Fe2O3, and TiO2, were synthesized in the presence of organic molecules. Unit crystals with the adsorption of organic molecules are arranged in the same crystallographic orientation. Cölfen and co-workers reported on the formation of the calcite CaCO3 mesocrystals in the presence of polystyrene sulfonate and its block polymers (Fig. 4). The faceted rhombohedral shapes of calcite were changed to the morphologies exposing the unusual crystal faces with an increase in the PSS concentration. Zhou and O’Brien reported the formation of the NH4TiOF3 mesocrystal in the presence of a surfactant (Fig. 5). Based on a time-dependent observation, the particle-mediated crystallization leads to the formation of mesocrystals. Kato and coworkers reported on CaCO3 thin films with a variety of morphologies. Since the architectures consist of nanocrystals with the acidic macromolecules, a variety of morphologies with a specific crystallographic orientation can be formed (Fig. 6). Yu and Cölfen reported the helical morphologies of BaCO3 through polymer-mediated crystallization (Fig. 7).39 The oriented and spiral assembly of the unit crystals made up the helical shapes, whereas the twisted morphologies were formed by the periodic changes of the growth direction of each unit in our reports (see 2.4). It is noteworthy that the achiral nanocrystals form the chiral shapes through the formation of mesocrystals.

Our group has reported on bridged nanocrystals (Fig. 1c). We found that nanocrystals less than 100 nm in size were arranged with the same crystallographic orientation in a number of CaCO3-based bio-minerals, such as nacreous layers, coral, sea urchin spines, and eggshells (Fig. 2). As shown in Fig. 8, these nanocrystals were connected via nanoscale bridges. The spotted SAED pattern suggests that the resultant architectures had a single crystalline orientation (Fig. 3). The oriented nanocrystals in biominerals can be interpreted as a bridged architecture with the incorporation of biological macromolecules. We also observed that nanocrystals as the building blocks of the biomimetic materials are connected with each other (Fig. 8c,d). Since the crystallographic orientation gradually vary with nonconformity or twin formation with the bridges, a variety of macroscopic morphologies can be generated from the nanocrystals, especially in terms of complex or curved shapes with a smooth surface.

Since the nanocrystals are the building blocks, versatile macroscopic shapes can be formed with the assistance of organic molecules. For example, the cone-shaped and hierarchical architectures of sulfates and chromates were reported in the earlier works. Our group has reported a variety of hierarchically organized structures based on mesocrystals (Fig. 9). The formation of mesocrystals from nanocrystals is ascribed to the models of particle-mediated assembly and bridged growth. However, the formation mechanisms of the complex macroscopic shapes remain unclear issues.

2.2 Supercrystals and superlattices – Ordered assembly of nanocrystals

An ordered arrangement of particles, colloidal crystals, is found in a wide range of scales. Opal is a typical colloidal crystal with an ordered arrangement of silica particles. Photonic crystals have been developed for the control of optical properties. A variety of supercrystals and superlattices consisting of nanoparticles are fabricated through self-assembly. When the unit particles are an amorphous material and the crystal lattices of each unit particle are not oriented, the colloidal assembly is not regarded as a mesocrystal (Fig. 1g). In contrast, colloidal crystals consisting of faceted nanocrystals have been reported (Fig. 1e). For example, the ordered arrays of barium chromate, yttrium oxide, tungsten oxide, silver, and cadmium sulfide nanomaterials were mediated by organic molecules (Fig. 10). In nature, Tsukamoto and co-workers recently found an ordered array of magnetite nanocrystals in a meteorite (Fig. 11). The crystallographic direction of the unit particles is oriented in these colloidal crystals. Therefore, these supercrystals are one of mesocrystals comprised of the isolated nanoscale units. These findings suggest that the self-assembled oriented architectures are easily formed by the faceted polyhedral units with the surface modification by the organic molecules. The shapes of the unit crystals are involved in the geometrical packing state.

2.3 Porous single crystal

Porous single crystal has a continuous single crystalline framework with a porous interior or occluded organic domains (Fig. 1b). Meldrum and coworkers have recently reported the calcite single crystal occluded with 13 wt% of copolymer micelles ca. 20 nm in size (Fig. 12). The resultant sponge crystals showed the same mechanical strength as that of the biogenic calcite. Li and Estroff reported that single crystalline calcite was formed with the occlusion of agarose gel (Fig. 13). In addition, the network structures of the occluded organic molecules were visualized using an electron tomography technique. Qi and coworkers have shown the syntheses of porous calcite single crystals using ordered arrangement of polymer latex. These architectures are classified into not a perfect dense single crystal but a type of mesocrystals, namely porous single crystal. It is inferred that these single-crystalline structures are formed by the growth with exclusion of organic molecules.

2.4 Periodic changes of the crystallographic directions in unit crystals

The branched forms, dumbbell shapes, and curved and twisted morphologies are observed in a variety of materials through self-organization. In these architectures, the unit crystals are arranged with the periodic changes of their crystallographic orientations (Fig. 1f). The ordered architectures are neither a random assembly of the units nor single crystalline materials. For example, Kniep and co-workers have reported that fluoroapatite with branched and dumbbell shapes is formed in gelatin matrices (Fig. 14). Since the growth of rod-shaped unit crystal proceeds with three-dimensional regular branching, the dumbbell morphologies are obtained. Yu and co-workers reported that dumbbell shaped barium carbonate crystals were obtained not in the gel matrices but in the presence of polymers (Fig. 15a,b). They also showed that the branched growth with the periodic changes of the crystallographic direction led to the formation of the dumbbell shapes (Fig. 15c–e). When the unit crystals had the platy morphologies of calcium carbonate, a similar growth behavior was observed in the polymer-mediated crystallization (Fig. 16).

Kato and co-workers have developed thin-film composites of CaCO3 and organic macromolecules. When CaCO3 crystals are grown on poly-(vinyl alcohol) matrices with the addition of poly(acrylic acid), relief structures are obtained on the thin film. They prepared calcite thin-film crystals with the periodic changes of crystallographic orientations in the first step (Fig. 17). In the second step, the relief structures consisting of needlelike crystals spontaneously formed on the thin-film crystals obtained in the first step. Since the c-axis directions as the growth direction of the needle-shaped units periodically change, unique relief architectures are formed through self-organization (Fig. 17e,f).

Our group has prepared a variety of helical morphologies of unit crystals with the twisted growth in a specific crystallographic direction (Fig. 18). The twisted morphologies of K2Cr2O7, H3BO3, K2SO4, CuSO4 · 5H2O, and aspartic acid are formed in gel matrices. Since the unit crystals are not oriented in the same crystallographic directions, these architectures with the periodic changes of the crystallographic direction can be defined as a related structure of mesocrystals.

In general, the morphologies of crystals change with an increase in the driving force for crystallization (Fig. 19). A faceted single crystal is grown from a nucleus in a solution system at a low degree of super-saturation. A randomly branching morphology is observed on crystals grown at a high degree of the supersaturation. The morphology with the random branches is a polycrystalline aggregate of the units. A regularly branching dendrite is observed through periodic crystal growth under the intermediate condition. These morphological variations can be demonstrated on the crystal growth in gel matrices. In general, the morphological variation is observed on the crystal growth with an increase in the driving force for crystallization because the growing surface becomes unstable in a diffusion field. In gel matrices, the decrease of the diffusion rate induces the diffusion-controlled condition for crystal growth. Therefore, the dif- fusion-controlled condition is achieved by the increase in the gel density. It is inferred that the regular branching can lead to the formation of dumbbell shapes and helical morphologies under diffusion-controlled conditions.

2.5 Homogeneous but disordered assembly of spherical nanoparticles

In general, spherical particles easily form inhomogeneous and disordered aggregates, namely polycrystals (Fig. 1i). In contrast, the surface modification of the nanoparticles leads to the formation of homogeneous and ordered assemblies, such as supercrystals and superlattices, through inhibition of aggregation (Fig. 1e,g; also see 2.4). They are categorized into mesocrystals or related structures. Herein, the homogeneous but disordered assembly can be defined as an intermediate state of these assembled structures (Fig. 1h). The assembly states are distinguished from the inhomogeneous and disordered aggregates because the secondary particles are not formed through the aggregation. The primary nanoparticles directly form the macroscopic object. Since the secondary aggregates scattering visible light are not generated in the corresponding length scale, transparent macroscopic materials can be formed through the homogeneous and disordered assembly of nanomaterials.

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