Nanoscience volume 1 nanostructures through chemistry

In a classical category of crystalline materials, researcherscan classify the crystalline materials only into single crystals and polycrystals.Many researchers have observed ordered arra

Specialist Periodical Reports

Nanoscience

Volume 1: Nanostructures through Chemistry Edited by P O’Brien

Nanoscience Volume 1: Nanostructures through Chemistry

A Specialist Periodical Report

Victoria S Coker, University of Manchester, UK

Serena A Corr, University of Kent, UK

Mark Green, King’s College London, UK

Sarah Haigh, University of Manchester, UK

Hiroaki Imai, Keio University, Japan

Ian A Kinloch, University of Manchester, UK

Gerrit van der Laan, University of Manchester, UK and Diamond LightSource, UK

Jonathan R Lloyd, University of Manchester, UK

Mohammad Azad Malik, University of Manchester, UK

Ammu Mathew, Indian Institute of Technology Madras, India

Philip Moriarty, University of Nottingham, UK

Yuya Oaki, Keio University, Japan

Daniel Ortega, University College London, UK

Quentin A Pankhurst, University College London, UK and The RoyalInstitution of Great Britain

Arunkumar Panneerselvam, King’s College London, UK

Richard A D Pattrick, University of Manchester, UK

Carolyn I Pearce, Pacific and Northwest National Laboratory, USA

T Pradeep, Indian Institute of Technology Madras, India

Karthik Ramasamy, University of Alabama, USA

Neerish Revaprasadu, University of Zululand, South Africa

Anirban Som, Indian Institute of Technology Madras, India

N D Telling, Keele University, UK

Paulrajpillai Lourdu Xavier, Indian Institute of Technology Madras, IndiaRobert J Young, University of Manchester, UK

ISBN: 978-1-84973-435-6

DOI: 10.1039/9781849734844

ISSN: 2049-3541

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&The Royal Society of Chemistry 2013

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Thank you

DOI: 10.1039/9781849734844-FP005

Welcome to the first Edition of a new RSC SPR Nanoscience I would like tobegin by thanking all the authors for providing such interesting reading and

in time to meet our publication deadlines

This SPR will try each year to feature different and topical issues Itwould frankly be impossible to cover this enormous area each year withoutexcessive length or condensation of the content I hope some articles willappear on an annual basis where there is sufficient activity and interest Anew idea is to provide regional perspectives as in the chapter on India thisyear I am keen to commission an initial report on nanoscience in China aswell as other regional perspectives reflecting growth areas in contemporaryscience and engineering

I do hope that you enjoy the book and find it useful I am happy to receivesuggestions for contributions over the next few months

Paul O’BrienManchester

Nanoscience, 2013, 1, v–v | v

3 Recent development and application of mesocrystals 17

Mohammad Azad Malik, Neerish Revaprasadu and

Nanoscience, 2013, 1, vii–x | vii

Magnetic hyperthermia 60Daniel Ortega and Quentin A Pankhurst

3 Biocompatible magnetic colloids for hyperthermia 74

Recent developments in transmission electron microscopy and their

application for nanoparticle characterisation

89Sarah Haigh

1 Aberration corrected transmission electron microscopy 89

2 A potted history of advances in (ultra)high resolution SPM 117

3 Plucking, positioning, and perturbing atoms at silicon

5 ‘Dialling in’ dirac fermions and addressing atomic spins 137

Robert J Young and Ian A Kinloch

3 Case study of advances in characterisation: BaTiO3

nanoparticles

202

Arunkumar Panneerselvam and Mark Green

Phosphide and arsenide – containing quantum dots 235

Anirban Som, Ammu Mathew, Paulrajpillai Lourdu Xavier and

T Pradeep

4 Nano-bio interface, nanomedicine and nanotoxicity 266

Recent advances in mesocrystals and their related structures

Yuya Oaki and Hiroaki Imai*

DOI: 10.1039/9781849734844-00001

Noncalssical crystallization has attracted much interest in recent years In classicalmodels, crystalline materials were classified into single crystal and polycrystal Avariety of recent reports have showed mesocrystals as the intermediate statesbetween single crystal and polycrystal The present report focuses on mesocrystalsand their related architectures consisting of the unit crystals A variety ofmesocrystals and their related architectures were categorized by the ordered state

of the unit crystals These new superstructures have potentials for a variety ofapplications, 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 singlecrystal and polycrystal In nonclassical models, mesocrystals are defined asthe intermediate states between single crystal and polycrystal (Fig 1) Singlecrystal can be regarded as the regular continuous packing of unit cells Forexample, hexagonal prisms of quarts and cubes of table salt are typical singlecrystals The macroscopic faceted morphologies consist of a continuousarrangement of unit cells We cannot observe any intermediate ordered struc-tures between the macroscopic shape and the atomic arrangements (Fig 1a).The crystallographic direction is the same throughout the macroscopicshapes In contrast, polycrystals are a random aggregate of small single crystals.The crystallographic direction of each single crystal is not the same in theaggregate (Fig 1i) In a classical category of crystalline materials, researcherscan classify the crystalline materials only into single crystals and polycrystals.Many researchers have observed ordered arrangements of unit crystals thatare not simply assigned to a polycrystal.1–8The presence of a segmentalized unit

is not ascribed to a perfect single crystal The oriented architectures ofunit single crystals can be regarded as an intermediate structures betweensingle crystals and polycrystals (Fig 1c–e) Based on these facts, Co¨lfen andAntonietti proposed mesocrystal as a new category of crystalline materialsconsisting of oriented nanocrystals.1–3The colloidal crystallization of facetednanocrystals leads to the formation of mesocrystals The term of mesocrystalspread rapidly since the proposal of the concept A variety of review articlesrelated to mesocrystals have been published.4–8 In recent years, Zhou andO’Brien extended the concept of mesocrystals by addition of related structures.5,8Recent studies suggest nonclassical crystallization processes as well asthe structures and applications of mesocrystals The appearance of

Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan E-mail: oakiyuya@applc.keio.ac.jp, hiroaki@applc.keio.ac.jp

Nanoscience, 2013, 1, 1–28 | 1

prenucleation clusters is one of the most important findings in nonclassicalcrystallization behavior.9–13 In addition, the presence of precursor phasesand their roles for the subsequent crystallization have been studied in anattempt to understand nonclassical crystallization behavior.14,15

In the present article, we focus on the structures and applications ofmesocrystals In Section 1.2, the structure is reviewed using biominerals as atypical model of mesocrystal In Section 2, mesocrystals and their relatedstructures 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 urchinspine, and eggshell (Fig 2).16–20 In previous work, researchers tried todetermine whether or not the crystal structures of these biominerals aresingle crystal.21–30 Our group reported that carbonate-based biomineralspossess mesocrystal structures.16–19 At approximately the same time,Sethmann and coworkers reported on the presence of nanostructures inbiominerals.20We analyzed the nanoscopic structures of biominerals, such

as the nacreous layers, corals, echinoderms, foraminifers, and eggshells.These biominerals have unique macroscopic and micrometer-scalemorphologies (Fig 2a) Nanocrystals 20–100 nm in size are observed onmagnified scanning electron microscopy (SEM) and transmission electronmicroscopy (TEM) images regardless of the polymorphs, such as calcite andaragonite of calcium carbonate (CaCO3) (Fig 2b–e) The spotted electrondiffraction patterns are observed on these biominerals (Fig 3a,b) The peakbroadening originating from the miniaturization of the crystallites is notrecognized 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, areoriented in the same crystallographic directions Since the diffractionbehavior is the same as that of the single crystals, these biominerals wererecognized as single crystals in previous studies Based on electronmicroscopy and diffraction analyses, the biominerals form mesocrystal

Fig 1 Schematic illustrations of single crystal (a), polycrystal (i), and intermediate structures (b–h).

structures consisting of oriented nanocrystals with biological molecules The nanocrystals and the biological macromolecules can beregarded as the nanoscale bricks and mortar, respectively Since the nano-crystals are the building blocks for morphogenesis, living organisms canmake up a variety of macroscopic shapes with single crystalline orientationthrough biomineralization the first line in either of the columns and press therequired button.

macro-2 Mesocrystals and their related structures

Mesocrystals can be regarded as the intermediate state between singlecrystals and polycrystals In the present article, mesocrystal is defined as theoriented nanocrystals in the same crystallographic direction Recently, anumber of reports have shown a number of related structures to meso-crystals In this section, five types of mesocrystals and their related struc-tures are introduced The classification is based on the degree of theordering and the orientation of the unit crystals, even though the size andshape of the unit crystals are different

2.1 Oriented nanocrystals

As reported in detail in the reviews and in the literature, mesocrystal in thenarrow sense of the term is the assembly of oriented nanocrystals withorganic molecules (Fig 1d) A variety of oriented nanocrystals have been

Fig 2 Summarized SEM (a,b) and TEM (c–e) images of the biominerals investigated in this report (a) the macroscopic appearance (inset) and the SEM images of the characteristic morphologies (b) the magnified SEM images on the fractured surface, indicating the presence

of nanoscopic structures (c) the corresponding TEM images in the same scale as panel b (d) the TEM images of each nanocrystal exhibiting a specified facet (e) the high-resolution TEM images of the nanocrystals, showing that a nanocrystal is a single crystal Reprinted with permission from Wiley-VCH.17–19

reported in previous studies The formation of oriented nanocrystals ismediated 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.31–44Unit crystals with the adsorption of organic molecules are arranged in thesame crystallographic orientation Co¨lfen and co-workers reported onthe formation of the calcite CaCO3 mesocrystals in the presence ofpolystyrene sulfonate and its block polymers (Fig 4).31,32 The facetedrhombohedral shapes of calcite were changed to the morphologiesexposing the unusual crystal faces with an increase in the PSSconcentration Zhou and O’Brien reported the formation of the

NH4TiOF3mesocrystal in the presence of a surfactant (Fig 5).33,34Based

on a time-dependent observation, the particle-mediated crystallizationleads to the formation of mesocrystals Kato and co-workers reported on

Fig 3 TEM with SAED (a,b,d,e) and XRD (c) analyses of the nanostructures in biominerals (a,b) the TEM images of the assembly with the SAED spot pattern (insets) in the nacreous layer and eggshell, respectively (c) XRD profiles of the powdered samples to analyze the peak broadening (A: calcite single crystal (reference), B: a sea urchin (Heterocentrotus mammillatus), C: a sea urchin spine (Echinometra mathaei (Blainville)), D: the shell of a sea urchin (scientific name unknown)) The slight differences of the 2y values are ascribed to the doping of mag- nesium ions in biogenic calcite (d,e) TEM images of the assembled nanocrystals with a similar morphology to the each unit crystal and the schematic model of the crystallographic direction estimated from the dihedral angle (inset) Reprinted with permission from Wiley-VCH.17–19

Fig 4 SEM images of calcite mesocrystals synthesized in the presence of PSS (a–c) and poly(styrene-alt-maleic acid) (d,e) (a–c) morphological variations with an increase in the PSS concentration (d,e) trigonal calcite mesocrystals with triangular capped building blocks Reprinted with permission from Wiley-VCH 32

Fig 5 (a) Top and (b) cross-sectional SEM images of an NH 4 TiOF 3 mesocrystal particle (c) Low- and (f) high-magnification TEM images of an NH4TiOF3 mesocrystal, and (d) corresponding SAED pattern (e) Still images taken from the video, which show identical diffraction from different parts of an NH 4 TiOF 3 mesocrystal Reprinted with permission from the Royal Society of Chemistry.33

CaCO3thin films with a variety of morphologies Since the architecturesconsist of nanocrystals with the acidic macromolecules, a variety ofmorphologies with a specific crystallographic orientation can be formed(Fig 6).35–38 Yu and Co¨lfen reported the helical morphologies of BaCO3

through polymer-mediated crystallization (Fig 7).39 The oriented andspiral assembly of the unit crystals made up the helical shapes, whereas thetwisted morphologies were formed by the periodic changes of the growthdirection of each unit in our reports (see 2.4) It is noteworthy that the achiralnanocrystals form the chiral shapes through the formation of mesocrystals.Our group has reported on bridged nanocrystals (Fig 1c).16–19,45,46 Wefound that nanocrystals less than 100 nm in size were arranged with thesame 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 nanos-cale bridges.17 The spotted SAED pattern suggests that the resultantarchitectures had a single crystalline orientation (Fig 3) The oriented

Fig 6 SEM (a,b,e,f), optical microscopy (d), TEM (c) images of a variety of CaCO 3 -based thin-film composites (a–c) the thin film formed on the chitin matrix in the presence of calcification-associated peptide (CAP-1) extracted from the exoskeleton of a crayfish (d–f) rod- like mesocrystals formed on the oriented chitin matrices in the presence of PAA Reprinted with permission from Wiley-VCH 37,38

nanocrystals in biominerals can be interpreted as a bridged architecturewith the incorporation of biological macromolecules We also observed thatnanocrystals as the building blocks of the biomimetic materials are con-nected with each other (Fig 8c,d) Since the crystallographic orientationgradually vary with nonconformity or twin formation with the bridges, avariety of macroscopic morphologies can be generated from the nanocrys-tals, especially in terms of complex or curved shapes with a smooth surface.Since the nanocrystals are the building blocks, versatile macroscopicshapes can be formed with the assistance of organic molecules For example,the cone-shaped and hierarchical architectures of sulfates and chromates

Fig 7 SEM image (a) of the helical BaCO 3 crystals and its schematic illustration (b) Reprinted with permission from Nature Publishing Group 39

Fig 8 FETEM images of the oriented nanocrystals with the bridges (a,b) the nanoscale bridges observed on a sea urchin spine (a) and an eggshell (b), respectively (c,d) the connected nanocrystals of potassium sulfate (c) and potassium hydrogen phthalate (d), respectively Reprinted with permission from Wiley-VCH and the Chemical Society of Japan.17,45

were reported in the earlier works.40–43Our group has reported a variety ofhierarchically organized structures based on mesocrystals (Fig 9).43,44Theformation of mesocrystals from nanocrystals is ascribed to the models ofparticle-mediated assembly and bridged growth However, the formationmechanisms 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 widerange of scales Opal is a typical colloidal crystal with an orderedarrangement of silica particles.47 Photonic crystals have been developedfor the control of optical properties.48 A variety of supercrystals andsuperlattices consisting of nanoparticles are fabricated through self-assembly.49–64When the unit particles are an amorphous material and thecrystal lattices of each unit particle are not oriented, the colloidal assembly

is not regarded as a mesocrystal (Fig 1g) In contrast, colloidal crystals

Fig 9 Hierarchical architectures based on K 2 SO 4 (a–d) and CaCO 3 (e,f) mesocrystals formed

in the presence of PAA Reprinted with permission from Wiley-VCH and Nature Publishing Group.43,44

consisting of faceted nanocrystals have been reported (Fig 1e) Forexample, the ordered arrays of barium chromate, yttrium oxide, tungstenoxide, silver, and cadmium sulfide nanomaterials were mediated by organicmolecules (Fig 10).57–64 In nature, Tsukamoto and co-workers recentlyfound an ordered array of magnetite nanocrystals in a meteorite65(Fig 11).

Fig 10 TEM images of the oriented assembly of the nanomaterials (a) BaCrO 4 nanorods, (b) Y 2 O 3 nanorods,58(c) tungsten oxide nanorods,59(d) Ag polyhedrons,60(e) Ag cubes,61(f) CdS hexagonal prisms, 62 (g–i) CeO 2 63 Reprinted with permission from Nature publishing group, Royal Society of Chemistry, and the American Chemical Society.

Fig 11 SEM images of magnetite (Fe 3 O 4 ) colloidal crystals in the Tagish Lake meteorite The morphology is inset at the upper right in each image (a) Colloidal crystal with the bct structure composed of octahedral, crystalline nanoparticles of Fe 3 O 4 bounded by {111} faces (b) Colloidal crystal with the fcc structure The morphology of theconstituent particles is rhombic-dodecahedral, bounded only by {110} faces (c) Colloidal crystal with the fcc structure composed of particles bounded by {100}, {110}, and {311} faces Reprinted with permission from the American Chemical Society.65

The crystallographic direction of the unit particles is oriented in thesecolloidal crystals Therefore, these supercrystals are one of mesocrystalscomprised of the isolated nanoscale units These findings suggest that theself-assembled oriented architectures are easily formed by the facetedpolyhedral units with the surface modification by the organic molecules.The shapes of the unit crystals are involved in the geometrical packingstate.

2.3 Porous single crystal

Porous single crystal has a continuous single crystalline framework with aporous interior or occluded organic domains (Fig 1b) Meldrum andcoworkers have recently reported the calcite single crystal occluded with 13wt% of copolymer micelles ca 20 nm in size (Fig 12).66 The resultantsponge crystals showed the same mechanical strength as that of the biogeniccalcite Li and Estroff reported that single crystalline calcite was formedwith the occlusion of agarose gel (Fig 13).67–69 In addition, the networkstructures of the occluded organic molecules were visualized using anelectron tomography technique Qi and coworkers have shown the synth-eses of porous calcite single crystals using ordered arrangement of polymerlatex.70 These architectures are classified into not a perfect dense singlecrystal but a type of mesocrystals, namely porous single crystal It is inferredthat these single-crystalline structures are formed by the growth withexclusion of organic molecules

2.4 Periodic changes of the crystallographic directions in unit crystalsThe branched forms, dumbbell shapes, and curved and twisted morpholo-gies are observed in a variety of materials through self-organization.70

In these architectures, the unit crystals are arranged with the periodic

Fig 12 SEM images of calcite crystals precipitated in the presence of copolymer micelles (a,b) and their schematic representations (c,d) Reprinted with permission from Nature Publishing Group.69

changes of their crystallographic orientations (Fig 1f) The orderedarchitectures are neither a random assembly of the units nor singlecrystalline materials For example, Kniep and co-workers have reportedthat fluoroapatite with branched and dumbbell shapes is formed in gelatinmatrices (Fig 14).71,72 Since the growth of rod-shaped unit crystalproceeds with three-dimensional regular branching, the dumbbellmorphologies are obtained Yu and co-workers reported that dumbbellshaped barium carbonate crystals were obtained not in the gel matricesbut in the presence of polymers (Fig 15a,b).73They also showed that thebranched growth with the periodic changes of the crystallographicdirection led to the formation of the dumbbell shapes (Fig 15c–e) Whenthe unit crystals had the platy morphologies of calcium carbonate,

a similar growth behavior was observed in the polymer-mediatedcrystallization (Fig 16).74

Kato and co-workers have developed thin-film composites of CaCO3andorganic macromolecules.75–78 When CaCO3 crystals are grown on poly-(vinyl alcohol) matrices with the addition of poly(acrylic acid), relief struc-tures are obtained on the thin film They prepared calcite thin-film crystalswith the periodic changes of crystallographic orientations in the first step(Fig 17).77In the second step, the relief structures consisting of needlelike

Fig 13 SEM images of the calcite sponge crystals grown in an agarose gel (a–c) and in the presence of polymer microparticles (d,e) (a,b) the calcite crystal grown in an agarose gel after etching in water, (c) tomographic reconstructions of the agarose network inside of a section of a-sprepared calcite (d,e) the calcite crystals synthesized in the presence of polymer latex par- ticles with 380 nm in size after the dissolution of the polymer Reprinted with permission from the Royal Society of Chemistry, National Academy of Science (USA), and the American Chemical Society 67,69,70

Fig 15 Morphological evolution of the BaCO 3 crystals obtained in the presence of PMAA on a glass slip (a) the presence of quadrupolar structures as a defect event The insert shows a typical fragmented half of a dumbbell and a growing dumbbell (b) enlarged picture shows detailed structure of the dumbbells with a thin connecting bar (c–e) the schematic growth models Reprinted with permission from the American Chemical Society.73

PEG-b-Fig 14 SEM and TEM images of the fluorapatite–gelatin composites (a–d) SEM images illustrating subsequent states of the morphogeneses for fan-like (left frames) and fractal (right frames) growth mechanisms (e) SEM images of the half of a dumbbell aggregate viewed along the central seed axis Inset: Central seed exhibiting tendencies of splitting at both ends (small dumbbell) (f) TEM images of a fluorapatite–gelatine nanocomposite individual showing first states of branching in the fan-like growth series Reprinted with permission from Wiley- VCH 71,72

Fig 16 SEM image of the convex–concave calcite (a) and its proposed formation mechanisms (b) The primary blocks assemble to give flat, pseudo-symmetric mesocrystal structures When a certain size is exceeded, not only primary platelets, but also amorphous intermediates (spheres) are attracted By recrystallization of those species, bent crystalline structures without transla- tional order can develop Reprinted with permission from Wiley-VCH 74

Fig 17 SEM images (a–d) and their schematic representation (e,f) of the relief structures consisting of the CaCO 3 crystals grown on the PVA matrices in the presence of PAA after incubation for 8 h s the first step (a,b) and (c,d) 16 h (a,b) the first step providing the thin- film composites with the flat surface, (c,d) the second step leading to the self-organization of the relief structures Reprinted with permission from the American Chemical Society.77

crystals spontaneously formed on the thin-film crystals obtained in the firststep Since the c-axis directions as the growth direction of the needle-shapedunits periodically change, unique relief architectures are formed throughself-organization (Fig 17e,f).

Our group has prepared a variety of helical morphologies of unit crystalswith the twisted growth in a specific crystallographic direction (Fig 18).79–86The twisted morphologies of K2Cr2O7, H3BO3, K2SO4, CuSO4 5H2O, andaspartic acid are formed in gel matrices Since the unit crystals are notoriented in the same crystallographic directions, these architectures with theperiodic changes of the crystallographic direction can be defined as a relatedstructure of mesocrystals

In general, the morphologies of crystals change with an increase in thedriving force for crystallization (Fig 19).46,80 A faceted single crystal is

Fig 18 SEM images of the twisted morphologies consisting of the unit crystals (a) K 2 Cr 2 O 7 , (b) H 3 BO 3 , (c) K 2 SO 4 , (d) aspartic acid, (e) CuSO 4  5H 2 O, (f) the schematic models of the twisted assembly consisting of the unit crystals The crystallographic orientations are peri- odically changed with the growth in the axis Reprinted with permission from the American Chemical Society and Wiley-VCH.81,84,85

grown from a nucleus in a solution system at a low degree of saturation A randomly branching morphology is observed on crystalsgrown at a high degree of the supersaturation The morphology with therandom branches is a polycrystalline aggregate of the units A regularlybranching dendrite is observed through periodic crystal growth under theintermediate condition These morphological variations can be demon-strated on the crystal growth in gel matrices In general, the morphologicalvariation is observed on the crystal growth with an increase in the drivingforce for crystallization because the growing surface becomes unstable in adiffusion field In gel matrices, the decrease of the diffusion rate inducesthe diffusion-controlled condition for crystal growth Therefore, the dif-fusion-controlled condition is achieved by the increase in the gel density It

super-is inferred that the regular branching can lead to the formation ofdumbbell shapes and helical morphologies under diffusion-controlledconditions

2.5 Homogeneous but disordered assembly of spherical nanoparticles

In general, spherical particles easily form inhomogeneous and disorderedaggregates, namely polycrystals (Fig 1i) In contrast, the surface mod-ification of the nanoparticles leads to the formation of homogeneous andordered assemblies,87,88 such as supercrystals and superlattices, throughinhibition of aggregation (Fig 1e,g; also see 2.4) They are categorizedinto mesocrystals or related structures Herein, the homogeneous but

Fig 19 Morphological variation of crystals from polyhedral to dendritic shapes Schematic illustrations of the morphologies formed with changes of driving force (upper part), the rate- determining step and crystal structure related to the morphogenesis of these forms (middle part), and an experimental demonstration for the morphological evolution of Ba(NO 3 ) 2 crystals grown in gel matrices with the changes of gel density (lower part) Reproduced with permission from American Chemical Society and Chemical Society of Japan 46

disordered assembly can be defined as an intermediate state of theseassembled structures (Fig 1h) The assembly states are distinguished fromthe inhomogeneous and disordered aggregates because the secondaryparticles are not formed through the aggregation The primary nano-particles directly form the macroscopic object Since the secondary aggre-gates scattering visible light are not generated in the corresponding lengthscale, transparent macroscopic materials can be formed through thehomogeneous and disordered assembly of nanomaterials.

The homogeneous and disordered assembly can be observed on thenanoparticles (Fig 6) Ozin and co-workers reported that nanocrystalplasma polymerization leads to the formation of free-standing films (Fig.20a,b).89 The plasma treatment of the nanocrystal assembly leads to theremoval of organic ligands The formation of the surface amorphous phasecontributes to the attachment of each nanocrystal The resultant materialsinclude the homogeneous and disordered assembly of nanocrystals through-out the film Stucky and Ostomel reported on transparent free-standing filmconsisting of anatase nanocrystals with mesopores (Fig 20c,d).90 Ourgroup has reported the homogeneous and disordered assembly of nano-crystals consisting of titanium and tin oxides.87,88Nanocrystals 2–3 nm insize make up macroscopic bulk objects 1–5 mm in size The cracks and grain

Fig 20 Macroscopic and TEM images of the homogeneous disordered assembly consisting

of PbS (a,b), anatase TiO 2 (c,d) and SnO 2 nanocrystals Reprinted with permission from American Chemical Society, Royal Society of Chemistry, and Wiley-VCH.89,90

boundaries originating from the formation of secondary particles were notobserved in micrometer and submicrometer scales The TEM image with theSAED pattern indicates that the nanocrystals are closely packed without theformation of ordered structures (Fig 20e,f) The formation of a surfacehydrated layer on the nanocrystals facilitates the homogenous and dis-ordered assembly without the formation of inhomogeneous and disorderedaggregates.

Based on these reports, the homogeneous and stable dispersion ofnanomaterials in the liquid phase is a key to obtain the homogeneous anddisordered assembly of nanoparticles If aggregation proceeds in the dis-persion liquid, formation of the inhomogeneous and disordered assembly isinduced after the evaporation of the liquid phase The solvation of thenanomaterials in the dispersion liquid can inhibit the aggregation.88 Thehomogeneous dispersion states are condensed after the evaporation ofthe liquid phase These homogenous and disordered assemblies are notmesocrystal because all nanocrystals are not oriented in the same crystal-lographic direction However, the assembled state is distinguished from theinhomogeneous and disordered aggregates The assembly state can beregarded as the related structure of mesocrystals

3 Recent development and application of mesocrystals

Mesocrystals have potential for a variety of applications based on theirstructural features Since the unit crystals as the building blocks are oriented

in the same crystallographic direction, mesocrystals promise propertiessimilar to those of single crystals The interspace between each unit crystalserves as nanoscopic space for the introduction and reaction of guestmolecules Mesocrystals possess a high specific surface area originatingfrom the nanocrystals as the building blocks The mesocrystal typicallyexposes specific crystal faces on the surface of each nanocrystal unit.Although the random aggregates of nanoparticles show a high specificsurface area, the exposed crystal faces are not generally controlled in theaggregates Based on these structural features, the application of meso-crystals has attracted much interest in recent years In this section, we focus

on some recent reports of mesocrystal applications

3.1 Electrochemical properties

Mesocrystal structures can be applied to the active materials of ion batteries The crystallinity, specific surface area, and exposed crystalface have potential for the improvement of their performance The nano-sized building blocks contribute to shorten the diffusion distance of lithiumions The conductivity of ions and electrons can be improved by the singlecrystalline structure The high surface area and porous interior are bene-ficial for the reversible stability at a high charge-discharge rate

lithium-Niederberger and co-workers prepared lithium iron phosphate (LiFePO4)and lithium manganese phosphate (LiMnPO4) mesocrystals through anonaqueous route with the assistance of a microwave (Fig 21).91 Theresultant mesocrystals show the enhanced charge-discharge cycling perfor-mance (Fig 21e,f) Qi and co-workers reported the mesocrystals of

nanoporous anatase TiO2and their electrochemical properties as an anodematerial.92The anatase mesocrystals showed the charge-discharge reversiblestability at high charge and discharge rates (Fig 22) Our group preparedSnO nanoscale meshed morphologies with mesocrystal structures through

an aqueous solution route.93 The resultant SnO mesocrystals acted as theanode material of a lithium-ion battery The charge-discharge reversiblestability was better than that of the commercial powders and the flat plates(Fig 23) During the charge and discharge processes, the volume of theSnO crystals changed through the insertion and extraction of lithium ions.While the bulk SnO crystals collapsed during the charge and dischargereactions, the meshed mesocrystals maintained the morphologies after thecycling

properties Anatase mesocrystals ca 100 nm in size consisted oforiented nanocrystals 2–4 nm in size Methyl orange can be decomposedwith the resultant mesocrystals faster than commercial TiO2, namely P25(Fig 24).94Our group has shown the mesocrystalline nanosheets of rutileTiO2through an aqueous solution process (Fig 25).95 Rutile nanosheets

10 nm in thickness and 200–600 nm in width consisted of orientednanocrystals 2–3 nm in size The unit rutile nanocrystals were surrounded

by the combination of (101) and (110) faces or the (111) and (110) faces

In contrast to the case of a single crystalline nanosheet, the specificcrystal faces, such as the oxidation-preferred (111) or (101) planeand the reduction-preferred (110) plane are exposed on the surface

of the nanosheets Therefore, the oxidation reaction preferentiallyproceeded on the photocatalytic decomposition of methylene blue, anorganic dye

3.3 Host for guest molecules

As shown in Fig 1, mesocrystal structures have the nanoscopic spacebetween each nanocrystal We have proposed that organic molecules can beintroduced in the nanoscopic space of mesocrystal structures of bio-minerals and biomimetic materials.45,96,97When the organic dye moleculeswere introduced in the nanospace, the photoluminescence was observedwith excitation by UV light.44,96Photochemical reactions, such as dimer-ization and isomerization, were achieved in the nanospace.45Recently, wehave shown the hierarchical replication of biominerals with organic poly-mers from nanoscopic to macroscopic scales (Fig 26).97 After the

Fig 22 TEM images (a,b) and their electrochemical properties (c,d) of anatase TiO 2 crystals The inset of the panel (a) shows the SAED pattern (c) Rate capability of nanoporous mesocrystals and nanocrystals of anatase TiO 2 from C/20 to 2 C (1 C=170 mA/g) for five cycles (c) Cycling performance of nanoporous anatase TiO 2 mesocrystals with a current rate of

meso-1 C Reprinted with permission from the American Chemical Society.92

introduction and polymerization of pyrrole monomer, the composite ofpolypyrrole (PPy) and biomineral was formed The dissolution of bio-minerals led to the formation of the PPy hierarchical architectures Ingeneral, it is not easy to control the hierarchical morphologies of polymer

Fig 23 SnO mesocrystal and its electrochemical properties (a,b) SEM and TEM images, prespectively (c) the cycle performance from second cycle to the 20th cycle of SnO meshed mesocrystals and comparative SnO crystals at 0.1 A/g (d) Schematic illustrations of Liþinsertion/extraction of meshed plate, flat plate and commercial SnO Reprinted with permission from Elsevier 93

materials This approach has potential for the morphological control ofpolymer materials.

Gilbert and coworkers recently reported on the low surface area ofbiominerals consisting of nanocrystals.98 Based on the measurements,the surface area of sea urchin spine is comparable to that of space-filling macroscopic geologic calcite crystals The mesocrystal structures

of biominerals are different from those of synthetic materials They gested that the low specific surface area is ascribed to the presence of ACC

sug-in the nanospace In fact, organic molecules can be sug-introduced sug-in thenanospace from the solution or the liquid state However, the gas for themeasurement of the surface area is not introduced in the nanospace.The structures of the nanospace in mesocrystals remain unresolvedproblems

3.4 Repairing of mesocrystals

In nature, when biominerals, such as seashells, sea urchin spines, and teeth,are partially broken, living organisms repair the damaged parts This factindicates that living organisms can repair the mesocrystal structuresthrough controlled crystal growth Enamel is a mesocrystal consisting ofhydroxyapatite (HAp) nanorods oriented in the c axis direction Tang andco-workers studied the regeneration of the oriented HAp nanorods on theetched part of enamel When the etched enamel was immersed in simulatedbody fluid (SBF) without any organic molecules, flake-like particles weredeposited on the etched part In contrast, the oriented HAp nanorods weregrown through the deposition of HAp nanoparticles on the etchedpart when the specimen was immersed in the SBF with glutamic acid(Fig 27).99

We adopted the prismatic layer of a bivalve shell as a model for in vitrorepairing (Fig 28).100 The prismatic layer is comprised of calcite prismselongated in the c axis and an interprismatic organic framework In theoriginal prismatic layer, calcite polygonal columns 10–50 mm in diameter

Fig 24 Rod-like anatase mesocrystals and their photocatalytic activity (a–c) TEM images with the SAED pattern of the anatase TiO 2 rod-like mesocrystals, (d) photocatalytic activities

of the mesocrystal compared with the commercial standard TiO 2 (P25) Reprinted with mission from the Royal Society of Chemistry 94

per-and 150–250 mm in length were arranged per-and separated by interprismaticorganic walls (Fig 29) After etching of the calcite prism, the partial dis-solution proceeded in the area around 10–50 mm from the surface In vitro

Fig 25 Rutile mesocrystal nanosheets and their selective photocatalytic activities (a) SEM image, (b) TEM image with the SAED pattern, (c) a macroscopic view of nanosheet, (d) a single crystalline nanosheet surrounded by the (110) and (001) faces without the interior structures (e) the mesocrystal interior consisting of the bridged unit crystals exhibiting the (101) and (110) faces (f) the mesocrystal interior consisting of the bridged unit crystals exhibiting the (111) and (110) faces The pink, yellow, orange, and blue colors correspond to the (101), (111), (001) and (110) faces of rutile crystals, respectively The schematic models indicate that the different interior structures provide the different surfaces in the same nanosheet morphologies (g) the amount of the decreased MB (gray bars, –DMB) and the produced thionine (black bars, DThi) calculated from the changes of the peak area (h) the selectivity of the oxidation reaction in the photocatalytic decomposition of MB The selectivity can be estimated from the ratio of the decreased MB and the produced thionine (Thi), namely DThi/DMB Reprinted with permission from the Royal Society of Chemistry.95

repairing of the prismatic layer was achieved in an aqueous solution taining CaCl2and PAA (Fig 29) The calcite prisms were regenerated in thepartially dissolved parts In contrast, single crystals of calcite with arhombohedral habit were deposited from the precursor solution withoutaddition of PAA.

con-The partial regeneration of biominerals can be interpreted as the orientedgrowth of nanocrystals with the association of organic molecules Theepitaxial growth of mesocrystals leads to the in vitro repair of biominerals.Soluble organic molecules, such as glutamic acid and PAA, contribute tothe formation of mesocrystals in the repaired part When insoluble organic

Fig 26 SEM (a,b,d,e) and TEM (c,f) images of the original sea urchin spine (a–c) and its hierarchical replication to PPy (d–f) Reprinted with permission from the American Chemical Society.97

Fig 27 Cross-section SEM image (a) and the schematic models (b) of the enamel repair by using nanoparticles and glutamic acid Reprinted with permission from Wiley-VCH.99

macromolecules as the framework for the confined space remain during theregrowth process, the macroscopic morphologies are also reproducedthrough the growth of mesocrystals.

4 Conclusions and outlook

Mesocrystal can be defined as a new family of crystalline materials betweensingle crystal and polycrystal Single crystal consists of the regular con-tinuous packing of the unit cells The crystallographic direction is the samethroughout the macroscopic shapes In contrast, polycrystals are a randomaggregate of small single crystals The crystallographic direction of eachsingle crystal is not the same in the aggregates Recently, a variety ofintermediated architectures between single crystals and polycrystals havebeen reported In the present article, we have reviewed mesocrystals andtheir related architectures categorized by the degree of the ordering and theorientation of the unit crystals As in fundamental studies, recent reportshave suggested the potential applications of mesocrystals as, for example,electrodes, catalysts, and host materials On the other hand, many aspects

of mesocrystals remain unresolved, such as their formation mechanismsand structure-property relationships Understanding of mesocrystals andexploration of their functions are continuing challenges for materialsscientists

Fig 28 Schematic illustrations for the in vitro repairing of the prismatic layer: a) original prismatic layer consisting of interprismatic organic walls and calcite prisms, b) the partially- etched prismatic layer with the interprismatic organic walls, c) the regeneration of the etched part consisting of the oriented nanocrystals with an acidic organic polymer Reprinted with permission from Wiley-VCH.100

This work was partially supported by Grant-in-Aid for Scientific Research(No 22107010) on the Innovative Areas: ‘‘Fusion Materials’’ (Area no.2206) from the Ministry of Education, Culture, Sports, Science and Tech-nology (MEXT) and by Grant-in-Aid for Scientific Research for YoungScientist (A, No 22685022) (YO) from Japan Society of the Promotion ofScience

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Fig 29 SEM images of the original prismatic layer (a,b), the partially etched one (c,d), and the regenerated one (e,f) The left (a,c,e) and right panels (b,d,f) correspond to the top views and cross-sectional images, respectively The parts O, E, and R with the arrows indicate the original, the etched, and the repaired parts, respectively Reprinted with permission from Wiley-VCH 100

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Nanomaterials for solar energy

Mohammad Azad Malik,*aNeerish Revaprasaduband

Karthik Ramasamyc

DOI: 10.1039/9781849734844-00029

Colloidal synthesis of metal chalcogenides has been developed to be used asnanocrystal inks to produce high efficiency solar cells with the lower fabricationcosts Recent research on these materials has focused on the use of abundant and lowtoxicity elements such copper, iron, tin, lead and sulphur Several methods have beendeveloped for the synthesis of these materials and considerable progress has beenmade in controlling the size, shape and surface properties of the nanocrystals Thischapter will provide the most recent developments for the synthesis and use ofcolloidal nanocrystal inks for solar energy

1 Introduction

The projected world demand for energy in 2020 is 612 quadrillion Btu(B649  1018 J or 33 GW-yrs) The additional problem of carbon emissionmeans that it has become necessary to explore every technology that mayassist to achieve the production of energy in a sustainable way The sun isthe most abundant source of energy for the inhabitants of earth According

to one estimate, solar energy striking earth in one hour is more than totalenergy consumed on the planet in a year There are two routes for conversion

of sunlight into useful form of energy: the solar thermal approach wherebysolar energy is converted to heat and solar photovoltaic approach wheresemiconductors are used to convert solar radiations directly into electricity.The photovoltaic solar cell has been identified as one of the most promisingconversion devices for solar energy because it is clean and scalable However,solar cells still provide less than 0.1% of the world electricity as they are costlywhen compared to electricity generated through fossil combustion Photo-voltaics may, therefore, potentially ensure the transition towards a sustain-able energy supply system for the 21st century It is promising as it canprovide environmentally benign energy with no emissions and has thepotential to enhance energy security because of its global availability It issustainable and would promote economic and social welfare

Among the compound semiconductor materials, metal chalcogenidesemiconductor nanocrystals have been extensively studied and widely usedfor linear and nonlinear optical devices and photovoltaic solar cells The use

of these materials as nanocrystals for large-scale fabrication of films withapplications in solar energy conversion and other optoelectronic applications

is an emerging and important area in materials science Compared to the

a School of Chemistry, The University of Manchester Oxford Road, Manchester,

M13 9PL(UK) E-mail: azad.malik@manchester.ac.uk

b Department of Chemistry, University of Zululand, Private Bag X 1001, Kwa-Dlangezwa, 3886 South Africa

c Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, Alabama, 35487, USA

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