Mesocrystals : Ordered Nanoparticle Superstructures

Mesocrystals-Ordered Nanoparticle Superstructures By Rui-Qi Song and Helmut Colfen'k mesocrystal with perfect mutual order of its Mesoc:r)'stalsare 3Dord~rednan~pa~icl~s~perstrlJcture~, often with internal subumts becomes obvious. The scattering and light polarization porosity,whkh receive interest.• While more and more rnuc~r(!Centr~search properties of mesocrystals are the reason, mesQCrystalsystemsare folJnd I":biomine~alizatio~ or. ~nthesb:ed,their why mesocrystals were synthesized already ,potential as material ~till.ne,ecJsto, be eJ(plor,ed. ,It needs to,berevealed, \Vhkh many decades ago, but were not recognized new chemkal and physkal properties~~iseJromthE!mes9crystal structure, or as such and were usually interpreted as howt~eych~nge ,bYJ,l1e ~rderedaggregati(mof nanoparicles to fully exploit single crystals. This makes literature research for mesocrystals a difficult task. the ,t,he mec:,hanisms. formeso promi~i'ng, potEln~alormes()(;rystal~~Als(), Available early reports on mesocrystals were crystal~ynthesis l'leedt9 be eXplorfld ~adaptit to a \yide class of rna~erials. summarized in [11; However, in the last The ,last three years havElsee~ rel1'larkable progress,whkh is sUmmarized three years, a rapidly increasing number of here. Also potential future directi()ns.ofthis reaserch field are discussed~This mesocrystal systems have been reported shows th,e importanceof~esocrystals notQ":lyfor the field of materials due to increased analytical capabilities and . resear~hand ,allows the" ~ppli~ion ,of me so crystals" in advanced materials the introduction of the mesocrystal and mesoscale organization concepts.l231 These synthesis or propertyhnprovement of existing materials. It alsooutlinflS concepts highlight nonclassical crystalliza· attractive resear~h diregionsil1 this field. tion as a nanoparticle·based crystallization pathway toward single crystals besides the classical atomjionjmolecule-based mechanism. This includes the oriented aggregation mechanism 1. Introduction of nanoparticles toward single crystals pioneered by Penn and Banfield 10 years agof4-61or grain growth after grain rotation and Mesocrystals are experiencing rapidly increasing attention of coalescence in a polycrystalline materialFI Nonclassical crystal chemists and physicists in the last years. In particular for lization has been discussed and reviewed in several recent materials chemists, mesocrystals offer unique new opportunities overviewsP,2,8-141 and the basic characteristics are known. for materials design due to their mesoscopic structure. The However, while the mechanism of oriented attachment has notation "mesocrystal" is an abbreviation for a mesoscopically been revealed, mesocrystals can form by various mechanisms and structured crystal, which is an ordered superstructure of crystals consequently, relatively little is yet known about the processes with mesoscopic size (1-1000nm), Often, the crystalline subunits leading to mesocrystal formation, In addition, the number of are in perfect 3D mutual order resulting in the difficulty to known mesocrystal systems has much increased spanning from distinguish a mesocrystal from a single crystal as the mesocrystal biominerals to functional materials, The fact that mesocrystals shows an identical scattering pattern and behavior in polarized are found for biominerals, which are evolutionary highly light to a single crystal. 111erefore, it is difficult to defme the optimized materials, shows that obviously, nonclassical crystal border between a single and mesocrystalline substance. lization is an advantageous crystallization pathway, which is Practically this can be done by the coherence length in scattering worth to apply for the synthesis of advanced materials with because single crystals usually exhibit coherence lengths improved properties. Also, mesocrystals were shown to be > 100 nm while mesocrystals typically have smaller ones. Another intermediates between a classical single crystal and a polycrystal possibility to distinguish a mesocrystal from a single crystal is without mutual orientation of its subunits and an extended view microscope images, which show subunits with mesoscopic size on crystallization with a continuous structural transition between for a mesocrystal. The analytical challenge to correctly identify a single and polycrystal arisesYSI Making the building units of a 3D·aligned mesocrystal smaller via clusters to ions will be the transition of a mesocrystal to a single crystal. On the other hand, if the mutual order of the nanocrystals is continuously brought out [*] Dr. H. C6lfen, R.-Q. Song of crystallographic register until mutual order is lost, the Max-Planck·lnstitute of Colloids and Interfaces, Colloid Chemistry, mesocrystal will transform to an unoriented polycrystal (see also Research Campus Golm Fig. 5).f151 These are just two examples and we therefore try to Am Muhlenberg, D·14424 Potsdam (Germany) catch this exciting development and progress by reporting on E-mail: [email protected]:de mesocrystals in the literature between 2006 and 2008 to close the 12 001: 10.1002/adma.200901365 gap to the initial overviews,[2, 1 1301 2. Mesocrystal Formation Mechanisms Several different mechanisms were reported for the formation of mesocrystals in the recent years and it is likely, that further mechanisms will follow in the future. With the present knowledge base in the literature, we can think of four basic mechanisms to form a mesocrystal, which are displayed in Figure 1. Combinations of these mechanisms are also possible. 2.1. Nanoparticle Alignment by an Organic Matrix An obvious mechanism for mesocrystal formation would be to fill compartments with crystalline matter oriented by a structured organic matrix or align nanoparticles along an organic matrix (Fig. la). The nanoparticles could either be generated first and then attach to the organic matrix and get oriented or they could alternatively directly nucleate on the organic matrix, which could serve as a heterogenous nucleation site. Numerous examples of highly ordered macromolecular structures are especially known for biomacromolecules so that this mechanism can be considered to be especially relevant in biomineralization processes. Indeed, a matrix-mediated nanoparticle growth on a fibrous organic matrix was for instance suggested for the growth of corals.f1 6J A highly oriented matrix which can be used for the purpose ofnanoparticle alignment to a mesocrystal is chitin, which is for example found in the organic matrix of nacre. In an elegant work Kato and coworkers demonstrated that a peptide CAP-l from a crayfish with a chitin binding motif, a phosphoserin unit for promoting Dplni:lrtrn,i>nt of nstitute of CoU;ids the initial formation of amorphous CaC03 (ACC) and an acidic .• :.' binding domain for CaCOl led to a mesoClystal film after ~nd ·1·n~erfac~s.;Sh~:Jec~lvecr,~i nucleation of ACC, crystallization and nanoparticle alignment on her Ph, P ..in Physi!=alChernistry . a film composed of parallel a-chitin microfibrils. [17J Here, calcite ;'fr~rnthe.~hineseACadetr1y .~f; nanoparticles were aligned along the chitin fibrils. Alignment S<;ienc~s.jn~002:A~~rJiv~ ...... " ,', along an ordered chitin matrix can even work with less :yearsas ar7se~~ch fellowat j sophisticated molecules compared to CAP-l like simple poly Sun Yat:Senl.1niversity, ·sh~. \i (acrylic acid) (PAA).f1RJ Here, a modified chitin phenylcarbamate ", joitled Helmu~ F<;ilfe~'s.group was used to form a nematic liquid crystal. After conversion to a gel in 200.7.Herresearchirivolves film, stretching, cleavage of the carbamoyl groups and drying, a design and syntheslsof new 9rganic-lnorg~~ichybrid chitin film with the chains elongated parallel to the direction of materials, particul~rly using bl()ck copolym7rdriven elongation was formed. Rod-like calcite mesocrystals were crystallization through nanoparticle ~elf-assembly. formed in a CaC03 crystallization essay elongated with the c-axis parallel to the direction of film elongation. It is assumed that PAA molecules bind to the oriented chitin backbone and thus facilitate oriented nucleation and the growth of calcite. order-even without any additives (Fig. 1b). This mechanism for Related to nanoparticle alignment along fibers is the mesocrystal formation heavily relies on particle interaction forces mesocrystal formation in a gel matrix, which can also lead to as well as on external forces, which have recently been reviewed the orientation of the formed nanocrystallites (e.g., the for the formation of higher order nanoparticle structuresplJ fluorapatite mesocrystals in a gelatin gel matrix, see Section The ordering physical fields/forces like electricp2.23] mag 5.1.2). It should be noted that gels are also discussed in netic, [24J or dipole fields[23] as well as polarization forces have to be biomineralization processes such as for example the silk hydrogel anisotropic to allow for an ordering of the nanoparticle units. The in the formation of nacreY 9) Nacre aragonite tablets are known to nanoparticles in turn also have to be anisotropic with respect to be mesocrystals. [20) their interaction potentials so that an ordering can take place. Such anisotropy could for example be oppositely charged counter faces of a crystal, a magnetic or dipole moment along one 2.2. Ordering by Physical Fields nanooystal axis or differences in the polarizability along different crystallographic directions. The anisotropy could already be In the majority of known cases, mesocrystals form nanoparticles present in

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