supramolecular synthesis of graphenic mesogenic materials

MacromolecularChemistry and PhysicsReview

Supramolecular Synthesis of Graphenic Mesogenic Materials

Fei Guo, Robert Hurt*

Supramolecular chemistry offers new routes for creation of carbon and graphene-based mate-rials with precise control of structure. This approach stacks, aligns, polymerizes, or otherwise directs the assembly of precursor molecules into non-covalent superstructures or macro-molecular intermediates that are then chemically converted to carbon. A subset of this fi eld uses liquid crystal phases, or mesophases, which exhibit ori-entational molecular order somewhere along the chemical trajectory to carbon. Covalent capture of that orientational order followed by heteroatom removal leads to “mesogenic graphenic materials”. This review article addresses these car-bons and focuses on new methods and materials that use well-defi ned molecular precursors. It discusses polyaromatic compounds as starting materials, assembly modes in the melt and solvent phases, and potential applications.

1. Introduction

Carbon atoms can be assembled into an amazing variety of one-, two-, and three-dimensional structures, and this fl exibility has given the sixth element a special place in the nanotechnology revolution of the last 25 years. Carbon materials — those solids consisting primarily of the ele-ment carbon — include graphite, diamond, graphene, carbon nanotubes, fullerenes, carbon black, pyrolytic carbon, glassy carbon, carbon fi bers, cokes and chars from pyrolyzed hydrocarbon fuels or biomass, and a wide variety of related materials. With the exception of diamond these materials are all based on planar sheets of sp 2 -hybridized carbon and form the large family of “graphenic carbons”. There are numerous ways to synthesize graphenic carbons including thermal carbonization of solid or liquid precur-sors, [ 1 , 2 ] chemical vapor deposition, [ 3–6 ] arc synthesis using carbon or graphite electrodes, [ 7 , 8 ] catalytic vapor–liquid–solid synthesis, [ 9 , 10 ] fl ame synthesis, [ 11 , 12 ] and graphite

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F. Guo , Prof. R. Hurt School of Engineering and Institute for Molecular and Nanoscale Innovation (IMNI), Brown University, Providence, Rhode Island 02912, USA E-mail: [email protected]

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exfoliation. [ 13–15 ] These methods provide a powerful tool kit to create all-carbon structures, but most do not give the precise control over fi nal structure that is the trademark of organic synthesis, or they yield well-defi ned carbons but at low yield with much ill-defi ned material byproduct (e.g., arc synthesis of nanotubes or fullerenes).

An emerging approach for creating defi ned carbon structures is supramolecular synthesis . Beginning with defi ned molecular precursors, this approach stacks, aligns, polymerizes, or otherwise directs the assembly of the molecules into non-covalent superstructures or macromolecular intermediates that are subsequently converted to carbon. The chemical conversion to carbon might involve thermal or surface-assisted polymeriza-tion; [ 2 , 16 , 17 ] controlled removal of heteroatoms, or thermal or oxidative cyclodehydrogenation. [ 16 , 18 ] The results are all-carbon materials whose macroscopic form, carbon–carbon bonding, and/or graphenic layer arrangement and orientation have been controlled or directed in “bottom-up” fashion by supramolecular chemistry. Supramolecular assembly can occur on surfaces that template a 2D poly-merization [ 16 ] or carried out in the melt phase, [ 17 ] or solu-tion. [ 2 , 18 ] For our purposes, we regard a given procedure as an example of supramolecular synthesis, rather than a simple condensed-phase carbonization, if the chemical

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MacromolecularChemistry and Physics

Fei Guo is currently a Ph.D. candidate in Chemi-cal, Biochemical, and Environmental Engineer-ing (CBE Program) at Brown University in Providence, Rhode Island, USA. He received his B.S. degree from Dalian University of Technol-ogy in 2006 and M.S. degree from Zhejiang University, China, in 2008. He is lead author on two recent publications in Advanced Materials and ACS Nano on graphene-based materials from liquid crystal precursors.

Robert Hurt is a Professor of Engineering at Brown University in Providence, Rhode Island, and Director of Brown’s Institute for Molecular and Nanoscale Innovation (IMNI). He received a Ph.D. from the Massachusetts Institute of Tech-nology in 1987, and before coming to Brown in 1994, held posts at Bayer AG in Leverkusen, Germany and Sandia National Laboratories in Livermore, California. He has served as an Edi-tor for the scientifi c journal Carbon , as Techni-cal Program Chair for the International Carbon Conference in 2004, and as Graffi n Lecturer of the American Carbon Society.

pathway passes through some defi ned supramolecular state, either a 1D π stack, [ 2 ] a phase with orientational molecular order, [ 2 , 17 ] or a covalently constructed oligomer of defi ned chemical structure. [ 16 ] Within this fi eld are a variety of routes with differing degrees of control of chemical structure along the polymerization/carboniza-tion pathway, as will be discussed.

A subset of the supramolecular approach to carbon or graphene involves liquid crystalline phases as intermedi-ates (Figure 1 ). Liquid crystal phases are ordered liquids that are intermediate between crystalline solids and conventional, isotropic liquids. Liquid crystals are thus referred to as “mesophases” (intermediate phases), and solid materials that originate from such phases are “mes-ogenic materials”. Mesogenic materials typically retain some aspect of the mesophasic order originally estab-lished in the liquid phase. The two main advantages of passing through a liquid crystalline phase on the route to graphenic carbon are as follows:

(1) The ability to generate long-range orientational order. In the liquid crystal phase, precursor molecules have high molecular mobility and can propagate infor-mation on molecular orientation from one part of the material to another over long length scales. This property can be exploited to produce highly ordered materials if a method can be identifi ed to capture and stabilize the orientational order before or during the chemical conversion to carbon.

(2) The ability to achieve complex but well-defi ned supramolecular confi gurations that represent equi-librium states in the precursor or intermediate. Again, high molecular mobility allows the precur-sors to choose minimum free energy confi gurations, and these can be directed by selection of boundary conditions (at interfaces or in microconfi nement) or electromagnetic fi elds. These equilibrium superstruc-

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tures can be elegant liquid crystal textures, which can be quantita-tively described and in some cases predicted a priori. At the least, they are reproducible and do not depend on complex and diffi cult-to-control dynamic processes involving chem-ical kinetics or catalytic surfaces.

Figure 1 . Optical textures in liquid crystal phases used as precursors to carbon mate-rials: (a) free surface (gas–liquid interface) of naphthalene homopolymer melt phase [ 24 ] under the refl ectance mode with crossed-polars and half-wave retarder plate (b) calam-itic (rod-like) nematic phase seen in chromonic liquid crystals used to synthesize verti-cally aligned graphene layer arrays. [ 2 ] Transmission mode, crossed polars. Adapted with permission from refs. [ 2 , 24 ] Copyright 2000, American Institute of Physics publications; Copyright 2011, John Wiley & Sons, Inc.

This paper reviews the literature on the use of liquid crystal phases in the supramolecular assembly of defi ned carbon materials and graphene super-structures. The paper covers precursors, assembly modes in the melt and solvent phases, and potential applications, and in closing attempts to give outlook for this exciting fi eld.

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1.1. Liquid Crystal Precursors and Phases

The fi rst requirement for liquid crystal supramolecular synthesis is the formation of an orientationally ordered fl uid phase. It is advantageous if the precursor is an extended, 2D, highly conjugated molecule that already possess the structural kernel of a graphene layer, which is the desired end product. Some linear liquid crystal pre-cursors without this graphenic kernel can be converted to carbon, but the major chemical transformation required along with low yield and high gas volume make it diffi cult

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∗ Note, however, that the thermotropic transition from the nematic (Figure 2 j) to the isotropic (Figure 2 k) phase upon heating is not always observable in practice, especially for carbonaceous mesophase (Figure 2 c), as thermal decomposition often occurs at lower temperatures than the nematic–isotropic phase transition. [ 24 ] Decomposition, therefore, occurs before or contemporaneously with the phase transition unless the precursor has unusually high chemical stability. [ 25 ]

to capture and retain supramolecular order. There are two basic approaches for fl uid phase alignment of these graph-enic kernels; (i) the melt-phase processing of thermotropic discotic liquid crystals, and (ii) solution phase processing lyotropic chromonic liquid crystals.

1.2. Thermotropic Discotic Liquid Crystals

Approximately 10% of organic compounds for liquid crystal phases, but the vast majority are rod like (calamitic), rather than disk like or plank like. Of the many planar aromatic molecules, only a small minority form liquid crystal phases, and the total number of known discotic liquid crystal (LC) systems is limited (Figure 2 ). Large regular molecules like hexabenzocoronene (Figure 2 b) would seem ideally suited as graphenic precursors, but they pack effi ciently into solid crystals whose high-melting point prevents observation of lower-temperature liquid crystal phases. [ 19–21 ] Introduc-tion of aliphatic side chains (Figure 2 a) reduces melting points and aids in LC formation, but aliphatic material has a low carbon yield, and generates large volumes of gaseous pyrolysis byproducts that make it unattractive as a carbon precursor. Compounds with aromatic cores and aliphatic peripheries (Figure 2 a) have been successfully employed as discotic liquid crystals, when used in the molecular form, [ 22 ] but have not, to our knowledge, been transformed into carbon materials.

A method to circumvent the problem of high-melting solid crystals is the use of multicomponent mixtures (Figure 2 c). Blending reduces melting points by disrupting packing in the solid phase, and reveals underlying liquid crystal phases. [ 20 ] Most liquid-crystal-derived carbon materials fabricated to date have used mixture precur-sors. It would be historically inaccurate to suggest that blending of pure polyaromatic compounds was discov-ered as a method to make these discotic liquid crystals. In fact, the fi rst discotics were observed in organic phases from natural sources, namely, coal or petroleum tar and pitch, where are intrinsically mixtures of polyaromatic compounds of different size (molecular weight) and shape. [ 21 , 23 ] The multicomponent nature of these nat-ural materials is necessary for liquid crystal formation in this class of compounds (those consisting primarily of unsubstituted polyaromatic cores), but the role of the mixtures in suppressing high-temperature crystal phases was pointed out only much later. [ 20 ] The discotic liquid crystal mixture phase is conventional referred to as “car-bonaceous mesophase”, “mesophase-pitch” or sometimes simply “mesophase” in the carbon materials literature. Its history has been reviewed previously, [ 24 ] and it is the basis for important technological materials that include mesophase-pitch-derived carbon fi bers, mesocarbon microbeads (MCMBs), needle cokes, and some forms of graphite.



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These precursors and phases (Figure 2 a, c, i–k) are all thermotropic liquid crystals—they are processed in the melt phase and undergo order–disorder transitions upon heating. ∗ The mixtures used to suppress solid crystalline phases also give rise to composition dependence [ 21 ] and also suppress formation of columnar phases (Figure 2 i), and the order in carbonaceous mesophase is nematic— a statistically fl uctuating preference in molecular orienta-tion along a single vector, the director , but with no posi-tional order in the molecular centers of mass (Figure 2 j).

Recent developments have used simpler, better-defi ned phases made from polymerizing single-component mono-mers, such as naphthalene, [ 25 , 26 ] or methyl-naphthalene, [ 27 ] though of course these mixtures continue to have broad molecular weight distributions and the oligomers have a distribution of molecular structures. Much of the modern research in this area has focused on well-defi ned single-component phases, and the creation of ordered structures through well-defi ned directed assembly approaches. It is this latter work that will be the main focus of the present paper.

1.3. Lyotropic Chromonic Liquid Crystals

A more recent development are carbons fabricated from chromonic liquid crystals, in which rigid disk-like or plank-like molecular precursors stack noncovalently into supramolecular rods that align in solution. Chromonic liquid crystals, or “chromonics” have been known since 1915 [ 28 ] and in the last two decades their properties and applications have received increasing attention. [ 29–34 ] Chromonic phases are exhibited by a number commer-cial water-soluble dyes and drugs, and the last decade has seen the molecular design of new chromonics (see collec-tion of molecular structures in the work of Tam-Chang and Huang [ 29 ] ).

A chromonic nematic phase, which again is a phase with orientational but no positional order, is shown in Figure 2 l. Chromonics can form higher-order phases, that include hexagonal, chiral, and lamallar phases [ 29 , 30 ] with the principal phases being nematic and, at higher con-centration, the hexagonal M phase. In the M phase, the rods or columns still exist and are aligned, but also fall into a hexagonal spatial array. [ 30 ] Examples of commer-cially available molecules that exhibit chromonic phases

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Figure 2 . Disk-like or plate-like precursor molecules and their liquid crystal phases. (a) hexa-alkoxy or hexa-alkyl benzoates of triphenylene engineered to form thermotropic liquid crystal phases. [ 22 ] (b) Hexabenzocorenene, a large, planar polyaromatic compound that does not form liquid crystal phases but rather high-melting solid crystalline phases. In pure form, such compounds either melt directly into isotropic liquid phases or decompose. (c) Carbonaceous mesophase, the polyaromatic mixture that exhibits thermotropic liquid crystal phases due to melting point depression associated with its many components. [ 20 ] Carbonaceous mesophase derived from thermal treatment of lower-molecular weight hydrocarbon feedstocks is a practical precursor for a range of commercially important carbon materials made through liquid crystal synthesis. (d,e) Planar core-periphery amphiphiles that form chromonic liquid crystals phases (l). (f–h) Additional molecular cores that form chromic liquid crystals if sulfonated to impart water solubility. (i–k) Thermotropic discotic liquid crystal phases that include the columnar (i), nematic ( j), and isotropic (k) phases. (l) The chromonic lyotropic nematic liquid crystal phase formed by self-repulsion of rod-like supramolecular stacks in aqueous solution. The (l) image adapted from molecular simulations reported in the work of Chami and Wilson [ 37 ] (d–h). Note that the π -stacked columns in chromonics are not always a single molecule in width, but can be aggregates.

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∗ The homeotropic anchoring state has also been achieved for chromonics, but only on modifi ed substrates, such as glass treated with silane coatings containing long alkyl chains. [ 45 ]

are shown in Figure 2 d, e. These rigid, planar conjugated molecules with charged hydrophilic peripheral groups are water soluble [ 2 , 17 , 29 , 35 , 36 ] but have a hydrophobic core or face. The combination of a hydrophobic face and hydrophilic periphery make these compounds complex amphiphiles that stack face-to-face in aqueous solution. The aggregate structures and driving forces are not com-pletely understood, [ 29 ] but in addition to hydrophobic forces, the stacking is likely favored by π – π interactions comprising dispersion forces and also electrostatic attrac-tion associated with the electronegative heteroatoms that are common in these molecules and induce in-plane polarization. The molecular stacks can grow to hundreds of nanometers in length and give rise to rod-like liquid crystals at concentrations where viscosity remains low enough for easy processing (Figure 2 l). The stacking process is thought to be “isodesmic”, meaning that the process of adding or removing a molecule from a rod/stack has a free energy (estimated at 5–10 kT) that is inde-pendent of the rod/stack length. [ 30 , 37 ] By the isodesmic model, rods exist at all concentrations, even below the nematic/isotropic (N/I) transition point, but their length distributions shift to higher values as concentrations increase. Interestingly, DNA and RNA can be thought of as “side-chain chromonic stacks” [ 30 ] in which the H-bonded base pairs form a column that winds helically as plate-like rungs of a spiral ladder.

Some of the chromonic-forming structures have been observed to convert to carbon upon heating with reten-tion of the basic supramolecular structure. These mol-ecules include the polysulfonated forms of the organic dye core structures in Figure 2 f–h. [ 2 ] This ability to retain order is key for synthesizing mesogenic carbons, but the precursor structures or properties that give rise to this ability have not been systematically studied. Logically, there must be a competition between thermal decompo-sition and cross-linking reactions on the one hand, which serve to covalently capture the supramolecular order, and melting, vaporization, or plastic deformation asso-ciated with gas evolution on the other hand that intro-duce disorder. From our experience, the precursors with larger fractions of aromatic carbon, increased degrees of ring condensation, and structures with a single cen-tral extended, space-fi lling conjugated core (rather than aryl–aryl-linked single rings) are more likely to preserve supramolecular order on carbonization.

These LC phases, although made of molecular disks, are fundamentally different from discotic thermotropics. The discs exist as supramolecular rods, and are highly charged colloidal particles that are stabilized and by elec-trostatic repulsion. Their self-avoidance leads to excluded volume entropic effects, which at high-aspect ratio give rise to orientational order as described by the statistical theory of Onsager. [ 38 , 39 ] These liquid crystal phases under



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order-disorder transitions upon concentration/dilution, and are thus classifi ed as lyotropic liquid crystals. While the phase transitions of Onsager hard-rod systems are independent of temperature, [ 38 , 39 ] chromonic phase behavior show some temperature dependence, which may refl ect the disk-rod stacking equilibrium, which is believed to have an enthalpic component and be tempera-ture dependent. [ 29 , 31 ] The behavior of these systems is also known to depend on the native counterions or other salts present in the aqueous system. [ 29 , 31 ]

2. Methods for Directing Assembly

Liquid crystal ordering occurs spontaneously, but without the imposition of external control, thermal fl uctuations or uncontrolled fl ow processes cause the director (the vector defi ning the mean molecular orientation) to meander ran-domly, which limits the length scale of the unidirectionally ordered zones (Figure 1 a, b). For many applications in mes-ogenic materials, we are interested in uniform, long-range crystal order and in pre-specifi ed directions, so methods are needed to direct the supramolecular assembly. The primary tools for directing assembly are surfaces, confi ne-ment, fl ows, and electric or magnetic fi elds.

Surfaces or phase boundaries set the local director ori-entation in liquid crystal phases –a phenomenon referred to as “surface anchoring”. [ 40–42 ] Thermotropic discotic liquid crystals show a variety of anchoring states: “home-otropic” or face-on; “planar” or edge-on (Figure 3 a), and tilted. These anchoring states vary with substrate in a systematic way, and this gives the designer of mesogenic materials a tool for directing the assembly, at least in the near-surface region. [ 43 ] Chromonic liquid crystals are lyo-tropic systems, and being solvated, their primary interac-tion with surfaces is the entropic effect associated with interfacial excluded volume (Figure 3 c). The effect is geo-metric only, and gives rise to an anchoring state where the long axis is parallel to the substrate plane. ∗ This is the planar or “tangential” or “side-on” state for rods (Figure 3 c and Figure 4 ), and the homeotropic or face-on state for disks. This state is the same for all substrates unfortu-nately, which removes one tool from the kit of the mes-ogenic materials designer — the ability to direct assembly by substrate selection. One can still control the rod orien-tation within the substrate plane, either by surface tex-turing with aligned channels [ 44 ] or directional rubbing [ 30 ] or by fl ow (vida infra).

Confi nement is a powerful tool for creating complex structures in mesogenic materials. Here surface anchoring

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Figure 3 . Liquid crystal surface anchoring and confi nement. (a) Sketch of various dis-cotic surface anchoring states: “planar” or edge-on, “homeotropic” or face-on. (b) Various order modes for discotic liquid crystals confi ned in cylindrical geometry. [ 47 ] (c) Anchoring states including the tilted state. Also shown is the interfacial excluded volume associ-ated with the edge-on or planar anchoring state.

typically sets the director in the near-surface region as above, but the confi ning geometry gives rise to a variety of curved director fi elds and defects or singularities as the surface order attempts to propagate inward (Figure 3 b). These 2D or 3D director fi elds are ultimattely the result

Figure 4 . Assembly mechanism for mesogenic graphene structures formed from chro-monic liquid crystals. Top sketch shows water-soluble disks with hydrophobic cores and hydrophilic peripheries undergoing π -stacking in aqueous phase to form supramo-lecular rods. Above a threshold concentration (N/I transition) the rods align into a nematic liquid crystal phase (optical image, crossed polars). Mayer bar coating of the rods followed by drying produces a uniaxially ordered organic fi lm that can be directly converted to graphenic carbon by thermal treatment. The results are vertically aligned graphene layer arrays whose layer orientations can be observed by high-resolution TEM following substrate thinning. Adapted with permission from ref. [ 2 ] Copyright 2011, John Wiley & Sons, Inc.

of a free energy minimization [ 46 , 47 ] including terms for (i) departure from the preferred anchoring state, if any, (ii) curvature in the director fi eld com-prising bend, splay, twist, and higher order terms, and (iii) defect generation where phases may become isotropic to avoid infi nite director curvature.

Flows and fi elds are widely used to align liquid crystal phases. Because dis-cotic mesophases have high viscosities, the use of fi elds is typically impractical and fl ow is often the main determinant of orientation. Flow-induced forces are large and the timescales for thermal relaxation to erase the fl ow-induced order are long. Uniaxial extensional fl ow (elongational fl ow) aligns rods uniaxi-ally, as is common in fi ber spinning. The same fl ow aligns disks with their planes parallel to the fl ow, but with a variety of director orientations perpendicular to the fl ow, giving rise to a variety of transverse structures in mesophase-based carbon fi bers. [ 48 ] Biaxial exten-sional fl ow or “squeezing fl ow” orders disks perpendicular to the squeezing

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direction and can be used to homogene-ously align discotic LCs. [ 2 ] Shear fl ows induce more complex behavior that include tumbling, but under come con-ditions can be used to uniaxially order the supramolecular rods of chromonic liquid crystals to form ordered graph-enic structures. [ 2 ]

Figure 4 shows an example of fl ow ordering to achieve uniaxial orientation. A chromonic liquid crystal in aqueous solution is coated onto a substrate using the Meyer bar technique, and then con-verted to “vertically aligned graphene layer arrays” (VAGLAs) by direct thermal carbonization. The Meyer bar is a cylin-drical metal rod helically wound with a fi ne metal wire, and when placed on a fl at surface creates a series of fi ne fl uid channels bounded by the substrate below and adjacent wires on the sides. Pulling the Meyer bar across the sub-

strate sends a defi ned volumetric fl ow of liquid through the channels to produce a uniform thin liquid fi lms on the downstream side. The fl ow is a combination of uniaxial extension and shear, and aligns the chromonic rods, or other rod-like molecules such as collagen, parallel


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Figure 5 . A selection of graphenic mesogenic materials. (a) Mesocarbon microbeads (MCMB). (b, c) A variety of nanofi ber types that have been demonstrated by confi ning mesophase (naphthalene homopolymer) in aluminum oxide nanochannels. (d) Rod-like liquid crystal phases formed carbon nanotubes form rod-like liquid crystal phases, either in liquid crystal host solvents or alone. [ 52 ] (e) The sketch of the VAGLA structure made by chromonic liquid crystals assembly on substrates.

to the pulling direction, as shown in Figure 4 . For this technique to work, the deposited liquid must dry to an ordered organic fi lm before thermal fl uctuations cause a relaxation in the fl ow-induced order. [ 2 ]

3. Graphenic Mesogenic Materials and Their Applications

The formation of carbon materials from liquid crystal precursors or intermedi-ates has a long history, and includes such important industrial materials as mesophase-pitch-based carbon fi bers, needle coke, and graphite anodes used in electric arc steelmaking. The primary function of the liquid crystalline phase is to establish long-range orientational crystal order in the carbon body with important implications for directional strength, stiffness and conductivity. Mesocarbon microbeads (Figure 5 a), of interest in battery electrode applica-tions, [ 49 ] are a good example of a com-mercial material made by liquid crystal confi nement. Carbonaceous mesophase often fi rst appears as a discontinuous droplet phase in an isotropic continuous phase, and here the liquid–liquid phase boundary serves as a spherical con-fi nement chamber that sets the beau-tiful symmetric director fi eld seen in Figure 5 a. The polyaromatic molecules anchor edge-on at this interface, and
the 3D spherical geometry typically leads to the bipolar or “Brooks–Taylor” confi guration shown, though other pat-terns have been observed. [ 49 , 50 ]
Figure 5 b,c show a variety of nanofi ber types that have been demonstrated by confi ning mesophase naphthalene homopolymer, [ 17 ] hexa(4-dodecylphenyl) benzene, and hexa(4-dodecylphenyl)-peri-hexabenzocoronene [ 1 ] in alu-minum oxide nanochannels followed by carbonization and template removal by etching with NaOH. [ 1 , 47 ] Discotic polyaromatics in the melt phase anchor edge-on on the inner alumina surfaces leading to the platelet-symmetry fi bers shown on the far left of Figure 5 c. Changing the wall chemistry to carbon fl ips the anchoring state to face-on yielding the structure to the immediate right, which resembles a multiwall carbon nanotube but does not pos-sess a core and instead shows a continuous liquid crystal director pattern in its cross-section.



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Capillary infi ltration of chromonic precursors into alu-mina nanochannels followed by drying and carbonization gives rise to carbon nanotubes with the unique crystal symmetry [ 17 ] shown in Figure 3 c (the hollow structure). These imbedded carbon nanotubes have inner surfaces that consist entirely of graphene edge sites and are essen-tially a 3D, high-surface-area version of the VAGLA fi lms, of interest as catalyst supports or chemisorption-based capture or detection. Under some conditions, carbon nano tubes form rod-like liquid crystal phases, either in liquid crystal host solvents or alone [ 51 , 52 ] and can produce dry deposits in which liquid crystal textures are easily dis-cernable (Figure 5 d). Finally, Figure 5 e shows the VAGLA structure made by chromonic liquid crystals assembly on substrates, discussed in Section 3.

Beyond simple fi lms and fi bers, lyotropic liquid crystals with aromatic cores have been used to assemble more

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complex structures, such as multilayer fi lms, [ 53 , 54 ] aligned microribbons, [ 44 ] chimney structures, [ 55–57 ] helically wrapped tubes, [ 58 ] and spherical vesicles, [ 59 ] although these structures have not yet been chemically converted to graphenic carbon.

Potential applications of supramolecular carbons and graphene structures include catalysis, [ 60 , 61 ] electrodes, [ 62 ] Li-ion batteries. [ 49 , 63 ] Most carbon fi bers are currently fabricated from polyacrylonitrile (PAN) or rayon, though mesophase-pitch-based fi bers are preferred in some appli-cations where various strength and fl exibility characteris-tics are desired.

The mesogenic nanocarbons , or graphene superstruc-tures, are a relatively new class of carbon materials. The potentially attractive features of these materials include high-active site density associated with exposed graphene edge sites, easy access to interlayer spaces for intercalating agents, and the ability to create optical, electronic, and chemical properties that are directional and amenable to micropatterning. Most of the applica-tion potential for these materials relates to one or more of these material features. The nanofi ber forms occupy a similar application space with carbon nanotubes and may fi nd use in reinforcing agents or in composites, or as conductive additives to electrode structures or polymers requiring EMI shielding. The platelet-symmetry fi bers are of interest as catalyst supports or battery electrodes due to high-active site density or easy access to interlayer spaces. The nanocarbons made from thermotropic discotic precursors in microconfi nement have the disadvantage in large-scale application that they require a high-value sac-rifi cial template.

In contrast, the chromonic-derived materials do not require templates or stabilization, and can be processed easily and cheaply if the molecular precursors become widely available. For this reason there may be more interest in their further development toward products. The anisotropic organic fi lms that are intermediates in VAGLA synthesis have potential applications as linear polarizers, optical compensators, retarders, alignment layers and color fi lters. [ 29 ] The carbonized VAGLAs retain some of these properties and are particularly stable to temperature and fl uid environment. They also become electrically conductive [ 35 ] and conductivity increases with increasing annealing temperature to approximately 2000 S cm − 1 . [ 35 ]

Figure 6 shows a number of potential applications for VAGLA fi lms. They are of interest as thin conducting and/or polarizing fi lms (Figure 6 a), and thickness can be set at values down to 10 nm by adjusting concentration in solu-tion (Figure 6 b). At 10 nm the constituent layers become narrow graphenic ribbons, and the lateral (here vertical) confi nement may alter their electronic properties as is seen in monolayer graphene. [ 16 ] The fi lms can also be

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locally sheared to establish patterns visible only in polar-ized light (Figure 6 c, d). The rafts of graphene edge sites at the fi lm surface impart a high chemical activity, which can be exploited to render the surfaces superhydrophilic (Figure 6 e) under conditions where conventional carbon fi lms are less hydrophilic (Figure 6 f, g). The same chemical reactivity could be used for high-density functionalization with other chemical moities for applications as biological substrates for implants or cell growth guidance. Another set of potential applications makes use of the easy access to interlayer spaces between the vertical graphene rib-bons (Figure 6 i). The intercalation of lithium ions into these spaces can occur directly at the top surface, which conventional carbons allow access only at defect sites or grain boundaries (Figure 6 h). Lithiation of VAGLA fi lms produces expansion stresses in plane (Figure 6 i) unlike conventional fi lms where the expansion is out of plane, and this provides a method for measuring those stresses through membrane defl ection (Figure 6 i). Because the dif-fusion length scales are short and the entry to the inter-layer spaces are numerous, VAGLA fi lms would be promise as high-discharge-rate electrodes. Other applications men-tioned are transparent conductive fi lms for solar cells, [ 35 ] macroelectronics and microelectronics, [ 36 ] Z -directional nanopores. [ 2 ]

4. Comparison of Thermotropic and Lyotropic Assembly Routes

The thermotropic and lyotropic routes to mesogenic carbon materials are profoundly different and have dis-tinct advantages and disadvantages. The advantages of the thermotropic route are as follows:

(i) The ability to set the surface anchoring states by selection of substrates or confi ning surfaces. Ther-motropic LC phases show substrate-dependent sur-face anchoring states, where lyotropic phases show substrate-independent anchoring determined by the effects of geometry on interfacial excluded volume and interfacial entropy.

(ii) The availability of low-cost precursors in the form of mesophase pitch from thermal treatment of heavy hydrocarbons or naphthalene polymerization.

(iii) Low heteroatom content in most precursors.

The major disadvantage of the thermotropic route is the need for stabilization to achieve covalent capture of the discotic order established in the precursor phase. Simple heating of thermotropic discotic phases typically leads to gas evolution and disruption of both the phys-ical form of the supramolecular order as the gas escapes from the highly viscous melt phase. Typical results are swelling, foam formation, and loss of fi brous geometry


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Figure 6 . Potential applications for vertically aligned graphene layer arrays (VAGLAs) fabricated from chromonic liquid crystals. (a, b) Trans-parent conducting polarizing thin fi lms, whose thickness can be tuned by adjusting the concentration of precursor in solution. (c, d) Hidden polarization-active micropatterns produced by local shear fl ow using an artist's brush. (e–g) Superhydrophilic surfaces formed by func-tionalization of the active upper surface. Panels f and g show contact angles relative to conventional carbon fi lms subjected to the same treatment. [ 74 ] (h, i) High-rate alkali-ion battery electrodes that exploit the open interlayer spaces and short diffusion paths offered by the VAGLA structure. ( j) catalyst supports, in which active nanoparticles are tethered or bound to graphene edge sites.

and orientational molecular order. This can be counter-acted by nanoconfi nement, [ 47 ] but this requires sacrifi cial templates, or by low-temperature oxidative stabilization. Divalent oxygen atoms cross-link the polyaromatic mol-ecules in the glassy solid phase and captures the desired molecular order in a form that is stable upon subsequent heating. Oxidative stabilization is a proven technology carried out in carbon fi ber manufacture, but is slow and adds considerably to the cost of the materials production.

The lyotropic (chromonic) route has a different set of advantages:

(i) Simple “green” processing at room temperature using aqueous phases.

(ii) Elimination of the need for oxidative stabilization or confi ning sacrifi cial templates in many systems. In many cases, the Van der Waals forces between mol-ecules in the organic solid phases are suffi cient to



Macromol. Chem. Phys. 2012,© 2012 WILEY-VCH Verlag G

prevent rearrangement during heating. In these cases, it is the solvent (water) evaporation that captures the molecular order in the LC state.

(iii) The ability to set orientational order or order pat-terns with simple shear fl ows established by brushes, blades, or Meyer bars.

The signifi cant processing advantages (i) and (ii) sug-gest that the lyotropic route will be attractive for large-scale roll-to-roll manufacturing if the precursor costs are acceptable.

A limitation of the lyotropic route is the irregular and heteroatom-rich precursors that have been demonstrated to date. The poor space-fi lling properties and nitrogen and oxygen content of the aromatic cores (Figure 2 ) as well as the sulfur associated with sulfonation are not desirable in many applications seeking high conductivity or high strength in graphenic carbon materials. This disadvantage

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DOI: 10.1002/macp.201100600mbH & Co. KGaA, Weinheim

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may be removed by future work on other molecular cores specifi cally chosen as precursors to engineered graphenic carbons.

5. Outlook

The supramolecular synthesis of carbons and graphene structures is an exciting young fi eld with many opportu-nities. The use of liquid crystalline materials as precursors or intermediates adds a new dimension to this fi eld–the ability to systematically set graphene layer orientation patterns. Much of the excitement in the supramolecular synthesis of carbon and graphene lies in the ability to use well-defi ned molecular precursors that can be polymerized to produce graphene ribbons or other graphenic carbon structures with precise control of atomic positions (see the recent work of Müllen and co-workers [ 1 , 16 ] ). The liquid crystalline route has not yet achieved that degree of con-trol, but the development of new precursors that are both liquid crystalline and designed for effi cient, space-fi lling polymerization (“mesogen monomers”) could allow the attractive features of the two approaches to be combined. An obstacle to this desirable goal is the inherent diffi -culty in forming thermotropic discotic liquid crystal from large unsubstituted polyaromatic compounds, whose regular packing leads to high-melting solid crystals rather than ordered fl uids. [ 19 ] The chromonic route shows more promise in this regard, because the amphiphilic π -stacking is compatible in principle with a wide variety of different polyaromatic cores structures. Can we design molecules that will form chromonic liquid crystal phases and also undergo orderly lateral polymerization to fi ll space and produce high-quality graphenic carbons or arrayed nano-ribbons? This is a challenge for the future. We may also be able to improve our ability to fabricate extended graphenic structures using new chromonic phases based on larger aromatic cores than those synthesized to date, as there does not appear to be a fundamental limitation on core size in the chromonic assembly process. [ 30 ]

Finally, the fi eld of mesogenic carbon materials now has a new precursor at its disposal, graphene oxide. This single-atom thick sheet of oxidized graphene can be proc-essed in the aqueous colloidal phase [ 37 ] and through chem-ical or thermal reduction can be converted into a variety of graphene-based materials. [ 64 ] There is an emerging fi eld that seeks to create new 3D carbon architectures through the assembly of sheet-like building blocks. [ 65–70 ] Very recently, three independent laboratories have been reported that concentrated graphene oxide suspensions form discotic, lyotropic liquid crystal phases. [ 71–73 ] At the time of writing there is reason to anticipate that mono-layer graphene oxide will become a new, giant molecular

0

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liquid crystal precursor for new graphene superstructures and engineered carbon materials.

Acknowledgements : The authors would like to thank Professor Brian Sheldon and Dr. Amartya Mukhopadhyay of Brown University for technical contributions on VAGLA battery applications, Dr. Pavel Lazarev and Gregory King of Carben Semicon (South San Francisco) for materials and for technical discussions, and Prof. Klaus Müllen for the invitation to contribute to this special issue.

Received: October 28, 2011 ; Revised: January 25, 2012; Published online: ; DOI: 10.1002/macp.201100600

Keywords: assembly; graphenic; liquid crystal; mesogenic; supramolecular


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