Liquid crystal engineering of carbon nanofibers and nanotubes

Carbon 43 (2005) 2431–2440

www.elsevier.com/locate/carbon

Liquid crystal engineering of carbon nanofibers and nanotubes

Christopher Chan a, Gregory Crawford a, Yuming Gao a, Robert Hurt a,*,Kengqing Jian a, Hao Li a, Brian Sheldon a, Matthew Sousa a, Nancy Yang b

a Division of Engineering, Box D, Brown University, Providence, RI 02912, USAb Sandia National Laboratories, Livermore, CA, USA

Received 1 October 2004; accepted 26 April 2005

Available online 1 July 2005

Abstract

Four high-aspect-ratio carbon nanomaterials were fabricated by template-directed liquid crystal assembly and covalent capture.

By selecting from two different liquid crystal precursors (thermotropic AR mesophase, and lyotropic indanthrone disulfonate) and

two different nanochannel template wall materials (alumina and pyrolytic carbon) both the shape of the nanocarbon and the graph-

ene layer arrangement can be systematically engineered. The combination of AR mesophase and alumina channel walls gives plate-

let-symmetry nanofibers, whose basic crystal symmetry is maintained and perfected upon heat treatment at 2500 �C. In contrast, AR

infiltration into carbon-lined nanochannels produces unique C/C-composite nanofibers whose graphene planes lie parallel to the

fiber axis. The transverse section of these composite nanofibers shows a planar polar structure with line defects, whose existence

had been previously predicted from liquid crystal theory. Use of solvated AR fractions or indanthrone disulfonate produces plate-

let-symmetry tubes, which are either cellular or fully hollow depending on solution concentration. The use of barium salt solutions

to force precipitation of indanthrone disulfonate within the nanochannels yields continuous nanoribbons rather than tubes. Overall

the results demonstrate that liquid crystal synthesis routes provide molecular control over graphene layer alignment in nanocarbons

with a power and flexibility that rivals the much better known catalytic routes.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Carbon nanofibers; Carbon nanotubes; Mesophase pitch; Electron microscopy; Crystal structure

1. Introduction

Much of the excitement surrounding new carbon

nanomaterials can be traced to their directional proper-

ties, which arise through precise orientation of thegraphene layers [1,2] that are the anisotropic building

blocks of all sp2-hybridized carbon forms [3]. A long-

term goal in carbon synthesis is to develop techniques

for systematic control of graphene layer arrangement

0008-6223/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.carbon.2005.04.033

* Corresponding author. Tel.: +1 401 863 2685; fax: +1 401 863 9120.

E-mail address: [email protected] (R. Hurt).

in order to fabricate materials and nanomaterials with

crystal structures preprogrammed for specific applica-

tions [3].

In 1995, Rodriguez et al. published an article entitled:

‘‘Catalytic Engineering of Carbon Nanostructures’’ [4],describing the synthesis of three nanofiber types: ‘‘tubu-

lar’’ nanofibers (multi-wall nanotubes), ‘‘platelet’’

nanofibers, whose graphene layers lie perpendicular to

the fiber axis; and ‘‘herringbone’’ nanofibers with a tilted

layer arrangement [4]. More recent work has also dem-

onstrated the related cup-shaped nanofibers also with

titled layer arrangement [1,5]. These platelet, herring-

bone, and cup-shaped nanofibers are inferior to thetubular nanofibers in mechanical strength and conduc-

tivity, but contain exposed graphene edge sites that

mailto:[email protected]

Fig. 1. Four high-aspect-ratio carbon nanomaterials fabricated here

by liquid crystal assembly and covalent capture. The lines show the

local mean orientation of the graphene planes.

Fig. 2. Two liquid crystalline systems used in nanofiber and nanotube

synthesis. Panel A: AR mesophase, a thermotropic LC that shows

surface anchoring states that vary systematically with substrate [11];

Panel B: indanthrone disulfonate, a lyotropic LC whose rod-like

aggregates in aqueous solution anchor parallel to substrates (Panel C),

and align as concentration increases (Panel D). The parallel alignment

of the rods on substrates leads to edge-on alignment of the molecular

disks at the periphery of the carbon artifact.

2432 C. Chan et al. / Carbon 43 (2005) 2431–2440

make them attractive for complementary applications

such as catalysis [6,7], substrates for covalent functionali-

zation, and Li intercalation electrochemistry [8]. The

catalytic route derives its flexibility from the ability of

catalytic nanoparticles to decompose vapor-phase or-

ganic species and precipitate graphitic carbon from par-ticular crystal facets with defined orientations that

depend on catalyst formulation and reaction conditions.

In recent years, the catalytic approach has been exten-

sively applied for synthesis of high-aspect-ratio nanocar-

bons due to its flexibility in structure control, relatively

mild synthesis conditions, and good process scalability.

The goal of the present article is to demonstrate that

liquid crystal routes offer similar flexibility. By control-ling the interactions between discotic molecules and

template surfaces, liquid crystal assembly can be directed

to produce a variety of new high-aspect-ratio carbon

nanomaterials with varying graphene layer arrange-

ments. The first attempt to fabricate nanocarbons in this

way used AR mesophase, the discotic naphthalene poly-

mer, in combination with anodic alumina templates to

produce ‘‘orthogonal’’ carbon nanofibers with graphenelayers perpendicular to the fiber axis [9]. This structure

reflects the original (uncarbonized) discotic assembly

[9], which, of all possible structures, is the only one that

achieves the desired edge-on molecular anchoring on the

inner alumina surfaces while also avoiding elastic direc-

tor strain [9]. These nanofibers have the same basic crys-

tal symmetry as the catalytic platelet nanofibers, but the

graphene layers are smaller and ‘‘meander’’ around themean orientation in a manner that is typical of low-tem-

perature carbons derived from, or through, mesophases

[3]. Recently Konno et al. [10] reported platelet structure

nanofibers from PVC and PVA polymer precursors in

the presence of anodic alumina templates. They also re-

port that tunnel-etch aluminum (metal) substrates give

nanofibers with parallel orientation [10], demonstrating

the ability to set fiber structure through selection ofthe template wall material. This particular switch was

not predictable since the only report of anchoring state

on Al metal is edge-on (reported for an aluminum foil

[11]), which is inconsistent with the observed parallel

alignment. More work is needed to understand the inter-

actions of polyaromatic compounds with complex metal

surfaces, which may differ greatly in surface oxidation

state or presence of adsorbed species.The present paper describes the synthesis and struc-

ture of four high-aspect-ratio carbon nanomaterials fab-

ricated by liquid crystalline (LC) precursors (see Fig. 1).

It will be seen that the carbon crystal structure is a direct

consequence of the selection of LC precursor and the

channel wall composition, which together direct the ori-

entation of graphene layers at the channel wall. In the

case of nanoscale materials, the wall alignment can bethe deciding factor that establishes the bulk crystal

structure of the fibrous carbon form.

2. Experimental

2.1. Materials

This study uses both a thermotropic and a lyotropic

liquid crystal precursor for nanocarbon synthesis (see

Fig. 2). AR mesophase (HP grade, Mitsubishi Gas

C. Chan et al. / Carbon 43 (2005) 2431–2440 2433

Chemical) is a well-known carbon precursor made by

the polymerization of naphthalene. It has a distribution

of molecular weights spanning 400–2000 Daltons with a

mean of approximately 700 Daltons. It softens between

250 and 300 �C into a homogeneous discotic nematic

liquid crystal phase that has been described as primarilythermotropic, but with some lyotropic nature due to

the broad distribution of molecular weights [12]. In an

attempt to make hollow forms (tubes), experiments were

also carried out with soluble fractions of AR mesophase

in pyridine and quinoline solvents. The resulting solu-

tions are rich in polyaromatic material but no longer

exhibit liquid crystallinity both due to dilution and to

the lower mean molecular weight of the soluble fractioncompared to the parent material.

The second LC precursor is an aqueous solution of

the ammonium salt of indanthrone disulfonate. This is

a new carbon precursor used first only recently in our

laboratory to fabricate ordered all-edge surface carbon

thin films [13]. This LC precursor is synthesized by

introducing sulfonic acid groups on the periphery of

indanthrone (also ‘‘indanthrene’’), a polyaromatic dyeof planar discoid shape (see Fig. 2). In such amphiphilic

discotic molecules, the disk peripheries are hydrophilic,

but the polyaromatic faces remain hydrophobic, and

in aqueous solution the planar molecules stack face-

to-face to provide favorable local environments for both

faces and edge groups [14,15] (see Fig. 2). In indan-

throne disulfonate solutions, this face-to-face stacking

is extensive, leading to rod-like aggregates with aspectratio near 200 having approximate diameters of 1.5 nm

and mean lengths of 300 nm [15]. Proton dissociation

imparts negative charge to the aggregates, which aids

in their dispersion [15], and above 4–5% these solutions

form lyotropic liquid crystalline phases in which the rod-

like aggregates align by self-exclusion and electrostatic

repulsion [15]. Dried films of indanthrone disulfonate

have a density of 1.69 g/cm3 by picnometry [16] andXRD analysis reveals crystallinity with a d-spacing of

0.336 nm in the p-stacks and molecular order parame-

ters from 0.86 to 0.91 [16]. For the present work, indan-

throne disulfonate was acquired as an ammonium salt

solution from the firm Optiva (South San Francisco),

which uses the solutions for fabrication of thin film or-

ganic polarizers.

2.2. Procedures

The syntheses involved solution infiltration into com-

mercial nanochannel alumina (Whatman Anodisc 47),

which has been used extensively in the past as a template

for fabrication of nanofibers/tubes from polymers

[17,18], metals [17], organic molecules [19] and carbons

[20,21]. The goal here is to use nanochannels not onlyfor shape control, but also to direct the molecular struc-

ture of the material through polyaromatic/alumina sur-

face interactions. For some syntheses, a thin CVD

carbon coating was applied to the inner and outer alu-

mina surfaces using reactant mixtures of acetylene

(100 sccm) and ammonia NH3 (400 sccm) at 900 �C for

12 hours. Most nanocarbon samples were prepared by

spontaneous capillary infiltration at 300 �C followedby slow heating (4 �C/min) to 700 �C and a 10 min iso-

thermal hold time. Low temperatures were chosen for

the initial carbonization in order to gain insight into

the initial orientation of the graphene layers in the LC

state with only limited possibilities for high-temperature

rearrangement. Some nanocarbon samples were further

heat treated at 2500 �C in a custom rapid thermal

annealing device described elsewhere [22], whose peakheating approaches 1000 K/s. The alumina templates

were removed by 0.4 M NaOH etching as described pre-

viously [9] and the samples characterized for structure

by electron microscopy and X-ray diffraction.

3. Results and discussion

3.1. Platelet-symmetry nanofibers

Figs. 3 and 4 show new characterization results on the

platelet-symmetry LC-derived nanofibers first reported in

[9]. These nanofibers have already been shown by

HRTEM to have a platelet morphology (edge-on orienta-

tion with the graphene layers in most fibers lying approx-

imately perpendicular to the fiber long axis [9]) and arethus held together with axial Van der Waals forces. Here,

they show evidence of the expected low rigidity (Fig. 3A

and B) as well as transverse fissures after handling and

reheating (Fig. 3C). The group of nanofibers in Fig. 3

shows that most have fissures lying strictly perpendicular

to the fiber axis, but some fibers show tilted fissures

strongly suggesting tilted graphene layers (Fig. 3C). The

tilted fibers do not have herringbone structure, which re-quires a central kink, but rather a platelet structure in

which an asymmetric tilt relative to the fiber axis is super-

imposed on all the layers uniformly. The origin of the tilt

is unknown, but it is possible that the two closely related

anchoring states (strictly perpendicular vs. tilted) have

similar surface free energies, or liquid crystal ‘‘anchoring

energies’’, and thus small asymmetric stresses associated

with volatile release or carbonization shrinkage couldcause tilt in a minority of the fibers.

Arrays of these fibers show both top and bottom car-

bon films due to wetting of the alumina template face.

One film is typically of submicron thickness, while the

other film (often on the infiltration side) has a thickness

that depends upon the amount of excess AR in the ori-

ginal formulation. The thick film can be mostly removed

by scraping prior to carbonization, and then both thinfilms can be removed by controlled air oxidation at

520 �C for 10–40 min.

Fig. 3. Platelet-symmetry or ‘‘orthogonal’’ carbon nanofibers [9]

fabricated by AR mesophase infiltration into nanochannel alumina

followed by heating. A: free standing array, B: polished transverse

section of array, C: SEM of the 200 nm diameter fibers showing

numerous transverse fissures after handling and re-heating in 1% H2/

He in the presence of iron nitrate. Some of the nanofibers have tilted

fissures (‘‘T’’) suggesting tilted graphene layers, while most are strictly

perpendicular (‘‘P’’). Note that both perpendicular and tilted types are

seen by HRTEM.


Fig. 4 shows XRD and HRTEM analysis of the

platelet-symmetry nanofibers before and after 2500 �Cheat treatment. The initial fibers prepared at 700 �Cshow short meandering graphene layers typical of alow-temperature carbon from a liquid crystal precursor

[3,9]; a structure that reflects the molecular positions in

the liquid crystalline phase at the point of solidification

[9]. After 2500 �C heat treatment, the essential platelet

structure is preserved in the interior of the fiber and

the short, meandering fringes are greatly lengthened

and straightened. We observe both strictly perpendicu-

lar and tilted arrangements by HRTEM in both the

low-temperature and high-temperature samples. Heat

treatment is thus not the primary cause of the tilt. The

existing data set is not sufficient to state with statistical

significance whether heat treatment changes the relative

proportions of the two fiber types.

Application of the Warren equation [23] to theXRD 002 line broadening yields about 2 nm for the

700 �C sample and 43 nm for the 2500 �C sample. Near

the fiber edge, there is a clear evidence of surface

reconstruction similar to that seen by Lim et al. [24]

during annealing of catalytically produced platelet

nanofibers. The driving force for this surface recon-

struction is believed to be the minimization of surface

energy by elimination of free edge sites (danglingbonds). Without removal of the reconstructed zone

by etching or oxidation, the potentially large number

of active edge sites are likely to be unavailable for most

chemical reaction processes.

3.2. C/C-composite nanofibers

We hypothesized previously [9] that the platelet struc-ture is selected because it represents the minimum free

energy state for a discotic liquid crystal in a confined

nanocylinder, where the preferred anchoring state is

edge-on (planar). It should therefore be possible to fab-

ricate alternate nanofiber types by switching the anchor-

ing state to face-on (homeotropic), provided the new

anchoring state is strong enough to overcome the elastic

strain that is unavoidable in that configuration. Our ap-proach here is to coat the inner wall surface with a mate-

rial known to produce face-on (homeotropic) anchoring

on flat substrates. Only a few materials have been re-

ported to produce face-on anchoring of AR mesophase:

Pt, Ag, mica, polyimide, and carbon [11]. Here, we chose

to work with carbon, which can be deposited on the in-

ner alumina surfaces by well-known CVD methods [25].

Fig. 5 shows C/C composite nanofibers produced byinfiltrating AR-mesophase into nanochannel alumina

pre-coated with a thin (approx. 5 nm) CVD carbon

layer, then followed by a second carbonization at

700 �C. Fig. 5 clearly shows that CVD pre-coating does

indeed switch the graphene layer arrangement from per-

pendicular to parallel with respect to the fiber axis (see

panel C). Since the CVD layer is much thinner than

the channel radius (5 nm vs. 100 nm), the confinementgeometry is not significantly affected, and the switch

must reflect the new wall anchoring state. It is interest-

ing that the CVD and mesocarbon layers are not clearly

distinct in panel C. This is not entirely unexpected;

although their synthesis routes are different, both have

been subject to similar peak temperatures, which plays

an important role determining graphene layer diameter

and defect density. These CVD/mesocarbon compositenanofibers appear to be straighter and thus stiffer (Fig.

5A and B) than the platelet symmetry nanofibers

Fig. 4. Crystal structure characterization of as-prepared (700 �C) and annealed (2500 �C) platelet symmetry liquid-crystal-derived nanofibers. High

temperature treatment preserves and perfects the platelet symmetry (either strictly perpendicular or tilted) in the fiber interior, but also leads to

surface reconstruction to avoid free edges at the fiber periphery. This particular example image shows a tilted structure, but the high-temperature

annealed samples also contain strictly perpendicular fiber types, also of high crystallinity.

Fig. 5. Carbon/carbon composite nanofibers made by AR mesophase infiltration of CVD-carbon-lined alumina templates. A. FE-SEM image

showing fully dense nanofibers of uniform diameter, B. TEM image showing straight and uniform nanofibers, C. comparison of unfilled carbon

nanotubes (top) with the composite nanofibers (bottom). The dominant alignment of graphene layers in the mesocarbon interior is parallel to the

fiber axis following the parallel alignment in the outer CVD carbon ring.



(Fig. 3A and B), which is likely related to the dominant

parallel orientation of the graphene layers.

These CVD/mesocarbon composite nanofibers show

a fascinating transverse texture on fracture surfaces

(Fig. 6). Each of the fracture surfaces examined shows

concentric structure in an outer annular zone, givingway to parallel alignment in the center. Many of the fi-

bers have non-circular cross sections and in these cases

the molecular planes in the center region appear to lie

along the major dimension of the transverse section.

The non-circularity may be due to anisotropic carbon-

ization shrinkage. The apparent graphene layer struc-

ture is illustrated in Fig. 6B. It is important to note

that the region of concentric alignment includes not onlythe CVD layer, but also a significant portion of the inte-

rior mesophase-derived carbon. Considering the meso-

carbon alone, its texture is a known liquid crystal

confinement pattern referred to as ‘‘planar polar with

line defects’’ (PPLD), where the line defects are of

strength +1/2 (+p disclinations). The existence of PPLD

has been predicted [26,27] but not to our knowledge ob-

served experimentally prior to this study. Theoreticalpredictions of the PPLD structure include the Monte

Fig. 6. Transverse structure of the CVD/mesocarbon composite

nanofibers. A: Typical fracture surfaces, B: Sketch of the graphene

layer orientational pattern, which corresponds to a known liquid

crystal confinement pattern classified as Planar Polar with Line Defects

(PPLD). Note that the line defects are not at the CVD/mesocarbon

interface (approx. 5 nm from edge) but rather within the mesocarbon.

Carlo study by Chiccoli et al. [26], where PPLD is ob-

served in submicron cavities when homeotropic surface

anchoring is strong (as might be expected for AR on car-

bon [28]). The molecular dynamics study of Bradac et al.

[27] predicts a variety of transverse textures in cylindri-

cally confined LCs depending on anchoring strengthand elastic constants. The structures include planar po-

lar (PP), planar radial (PR), escape radial (ER) and

PPLD. The PPLD structure is predicted to be favored

for certain conditions when surface anchoring is strong.

It may be said that strong anchoring favors a uniform,

defect-free outer layer, and since this geometry (homeo-

tropic anchoring in a cylinder) requires defects due to

infinite curvature at the center, the defects are internal-ized as two +p disclinations.

Several related structures have been observed experi-

mentally. If the PPLD line defects are brought to the

periphery one obtains the simple planar polar structure,

which as been experimentally observed by Crawford

et al. [29] in <0.4 lm cavities filled with the rod-like

liquid crystal 5CB. Fathollahi and White [30] observed

the relaxation of the flow-induced mesophase micro-structures in a uniform set of 720 lm diameter (L/D >

25) capillaries subjected to identical flow conditions.

Upon annealing at 300 �C, the microstructure relaxes

to a pair of +p disclinations with a radial orientation

of the discotic planes near the outer tube surface. This

structure can be mapped onto that of Fig. 5 by a 90�rotation of all layers. This related structure [30] is not

expected here since it would require edge-on anchoringon nanochannel walls. It is a candidate structure for

the uncoated alumina templates, but is not observed.

Rather at the nanoscale, the edge-anchored discs prefer

instead to flip out of plane to produce the platelet struc-

ture (Fig. 3), which avoids the +p disclinations and

indeed all elastic strain. We offer the following explana-

tion for the Fathollahi and White texture. At larger

length scales in their supramicron cavities, rearrange-ment times are longer and the low curvature reduces

the driving force for the out-of-plane flip. As a result,

it does not occur over experimental time frames. Instead

the flow-induced alignment parallel to the cavity axis re-

mains metastable and annealing produces only rear-

rangements within the transverse plane—a limited

two-dimensional free energy minimization that leads to

the observed texture. Fig. 7 shows that 2500 �C annealingimproves crystallinity of the composite nanofibers, while

maintaining the preferred parallel orientation. Both car-

bon components show large increases in fringe length.

3.3. Platelet-symmetry nanotubes

In order to explore whether liquid crystalline routes

can be used to make hollow forms (tubes), precursorswith very high volatile yields or solvent fractions were

sought whose vaporization would produce hollow

Fig. 7. HRTEM fringe images of the C/C composite nanofibers after

rapid heating to 2500 �C. The parallel orientations are maintained in

both regions and the overall crystallinity greatly increased.


spaces within the nanochannels. Most attractive are

solution systems, where controlled solvent evaporation

early in the heating process may lead to thin adherent

organic films on the inner channel walls, which can be

converted by carbonization into unique carbon nano-

tube varieties. Our first attempt to make carbon tubesused soluble fractions of AR mesophase in pyridene

and quinoline solvents. The procedure was otherwise

the same as for the melt-processed platelet nanofibers.

Figs. 8 and 9 show that evaporation of the solvent pro-

duces open-ended tubes (Fig. 8C)—some fully hollow

and some with cellular structure. When high-concentra-

tion (7.5 wt.% in quinoline) solutions are employed, the

Fig. 8. Cellular carbon nanotubes (A,B,C) and fully hollow carbon nanotub

structure in (D) is the result of lower polyaromatic concentration (1 wt.%) i

cellular structure is especially pronounced (Fig. 8A).

The hollow regions clearly appear to be vesicles, some

of which show nearly regular spacing, and the overall

structure can be described as an one-dimensional solid

cellular foam. Increasing the drying time at a tempera-

ture at 80 �C (below the 113 �C boiling point of quino-line) had little effect on the foam structure (Fig. 8B).

Reducing the AR concentration to 1 wt.% greatly sup-

presses this foam structure and produces instead hollow

nanotubes (Fig. 8D). These tubes have thin wall, how-

ever, which show some inter-tube fusion after template

removal leading to some structures much larger than

the 200 nm channels. The 1D cellular foam thus appears

to arise at high concentration where saturated condi-tions readily occur during drying leading to precipita-

tion within and across the bulk channel rather than at

a later stage as a thin film drying on inner wall surfaces.

Fig. 9 shows perpendicular graphene layer alignment

in the thin wall sections of the cellular tubes. These are

platelet-symmetry tubes (structure C in Fig. 1) and are

hollow relatives of the platelet-symmetry nanofibers

(structure A in Fig. 1). The formation mechanisms arenot necessarily identical, however. The solvated pitch

fractions are isotropic both in solution and upon drying

and thus their initial assembly is not governed by liquid

crystal theory. We wish here only to propose two possi-

bilities: (1) the isotropic fractions transform into meso-

phase during carbonization and align edge-on by the

es (D) produced by solution processing of AR mesophase. The hollow

n quinoline solution compared to 7.5 wt.% for (A).

Fig. 9. Perpendicular graphene layer arrangement in the thin wall

sections of cellular carbon nanotubes produced from high-concentra-

tion solutions of AR mesophase in pyridine.


same anchoring mechanism as the original whole AR,

and (2) the alignment occurs by a non-LC mechanism

driven by maximization of p stacking. Even without

an LC phase, mobile polyaromatic compounds below

their decomposition temperatures will typically stack

to maximize p–p bonds, which are the strongest non-

covalent interactions in the system. Liquid crystalline

phases show long-range order, but at these nanometriclength scales, only short range ordering is necessary to

produce the molecular structure in Fig. 9, which can

then captured by carbonization. Indeed polyaromatic

p-stacking with edge-on orientation is a common assem-

bly pattern seen in organic thin films and in the ultrafine

channels of mesoporous silica [31]. It requires liquid

crystalline phases only when the alignment must propa-

gate over long length scales (�10 nm).A more well-defined synthesis route to platelet-sym-

metry tubes uses capillary infiltration of ammonium

indanthrone disulfonate aqueous solutions, which form

true liquid crystalline phases at concentrations above

4 wt.%. The starting solutions lose most of their mass

upon drying, leading to the formation of a thin film

(2–12 molecular layers) on the inner surfaces of the

100 nm radius cylindrical template channels.Fig. 10 shows that infiltration of indanthrone disulf-

onate solution followed by drying and 700 �C treatment

produces monodisperse carbon tubes of 100 nm radius

and 60 lm length, which form free standing ordered ar-

rays upon template etching (see Fig. 10A). Using

12 wt.% indanthrone disulfonate, the tubes show a cellu-

lar structure with hollow cavities separated by internal

membranes. The cellular structure can be suppressed al-most entirely by reducing solvent concentration from

12% to 2%. The lower concentration produces tubes

with thinner walls and almost no internal structure.

High resolution fringe images reveal the same platelet-

symmetry in these thin-walled carbon nanotubes (Fig.

10B). The tube wall structure consists of short (2–3

nm) graphene layers, similar to AR-derived carbons pre-

pared at the same temperature. This crystal structureimplies that the rods orient parallel to the channel axis

during drying. This is not unexpected, as the rod length

is comparable to the channel diameter, making parallel

orientation much more favorable, especially in the

curved thin liquid films that coat the inner nanochannel

walls during drying. The dried solid film of rod-like

supramolecules is then covalently captured by thermal

polymerization with accurate translation of the molecu-lar order into an arrangement of linked graphene layers.

Note that the success of this covalent capture scheme

could not be predicted a priori, as many thermal carboni-

zation processes destroy supramolecular order in the

organic precursor and/or alter the overall form of the

carbon body though re-softening and volatile product

release [3]. In separate thermogravimetric experiments,

we found that bulk samples of indanthrone disulfonatebegin to decompose at around 300 �C, at a point where

the material retains most of its optical anisotropy [13].

As heating continues, sharp features remain intact

through 700 �C indicating an all-solid-state carboniza-

tion path, and the final bulk carbons show multi-domain

anisotropy [13].

These platelet symmetry tubes are mechanically sta-

ble when made from 7% or 12% solution concentration.These solutions retain a reasonably low viscosity allow-

ing nanotube micropatterns to be written by pro-

grammed injection using capillary tube pens [32]. In

contrast, the fully hollow tubes made from 2% solution

are very fragile outside the template. More work is

needed on processing and stability of the thin-walled

hollow variety, but it is likely that their primary use will

be inside the alumina template rather than free standing.

3.4. Carbon nanoribbons

Finally, we report the ability to form long, continu-

ous carbon nanoribbons by a slight modification of

the synthesis procedure. Dipping the filled nanochannel

membrane in 10 wt.% BaCl2 solutions prior to drying

and carbonization leads not to tubes but to ribbons orstrips of 60 lm in length, 200 nm in width with rectangu-

lar cross section (Fig. 11). In separate experiments, we

Fig. 10. Large, thin-walled carbon nanotubes formed by capillary infiltration of indanthrone disulfonate solutions into nanochannel alumina

followed by thermal covalent capture at 700 �C and template removal. A: intact tube array, showing the cellular nature of the tubes made when

solution concentration is high (12 wt.%). B: high-resolution TEM image shown the perpendicular graphene layer orientation in the walls of 2 wt.%

tubes. C: sketch of the platelet-symmetry tubes with the molecular disk size exaggerated for visibility.


observed rapid formation of fibrous precipitates when

indanthrone disulfonate solutions were injected throughfine glass tubes into a BaCl2 solution bath. We therefore

believe that in the template synthesis, the divalent bar-

ium ion cross-links the negatively charged aggregates

and reduces their solubility, leading to early precipita-

tion within the nanochannels instead of deposition on

the inner wall surfaces during drying. HRTEM shows

these ribbons to have a more random crystal structure

suggesting that the rod-like molecular aggregates were

Fig. 11. Carbon nanoribbons or strips fabricated by barium chloride

precipitation of indanthrone disulfonate within the channels of anodic

alumina templates. A: precipitation chemistry and sketch, B/C:

example ribbons after carbonization and template removal.

not highly oriented in the solution at the point of precip-

itation prior to drying.In summary, we believe the tubular structure (Fig. 10)

first forms during drying in the form of tubular precur-

sor film that assembles on the inner channel walls as a

curved solvent meniscus recedes. Addition of barium

salts bypasses this mechanism by forcing the organic

precursor out of solution in the filled channel before

drying occurs. In this case, the precursor is not forced

to precipitate on the curved channel wall, but is free toadopt its characteristic preferred precipitate morphol-

ogy, which is not tubular but ribbon-like.

4. Conclusions

Liquid crystals provide a powerful and flexible route

to new high-aspect-ratio carbon nanomaterials. The pre-

cursor/template pair can be intelligently selected to

establish desired graphene layer arrangements at the

carbon/template interface, and at the nanoscale, this

alignment propagates inward a sufficient distance to dic-

tate the overall structure of the material. To date wehave demonstrated platelet-symmetry nanofibers, C/C-

composite nanofibers with graphene layers parallel to

the fiber axis, platelet-symmetry tubes both cellular

and open, and carbon nanoribbons with a lesser degree

of crystalline order. The platelet and composite nanofi-

bers are similar, but not identical to, the well-known

platelet and tubular nanofibers synthesized by catalytic

routes. The LC-derived version offer the followingadvantages: (1) their degree of crystallinity can be varied


from extremely low (including quenched or partially car-

bonized mesophase) to high by adjusting the tempera-

tures of formation and annealing, (2) they can be

easily grown in well-ordered arrays, (3) they are free of

metallic catalyst residues. The need for a sacrificial tem-

plate, however, is a significant disadvantage for bulksynthesis. For this reason, the LC-derived nanofibers

are most attractive in high-value, array-based devices.

More work is needed on the mechanical and thermal

stability of the platelet symmetry tubes both within the

template and free standing. More work is also needed

to identify surface treatment procedures for the edge-

on forms to remove reconstructed layers and access

the potentially abundant active sites.

Acknowledgements

This work was supported by the National Science

Foundation through a Nanoscale Interdisciplinary Re-

search Team (NIRT) Grant at Brown University,

CMS-0304246, and by the Electric Power Research

Institute, Dr. A. Mehta project manager. The authors

would like to thank Daniel Morris for the Fe-doped

fiber images, Michael Paukshto of Optiva Inc. for theindanthrone samples, and Essie Yamoah and Bevan

Weissman for technical contributions in the laboratory.

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Liquid crystal engineering of carbon nanofibers and nanotubes

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