Complex structure of carbon nanotubes and their ...lc/nanotube-structure.pdf · carbon nanotubes...

JOURNAL OF APPLIED PHYSICS VOLUME 93, NUMBER 12 15 JUNE 2003

Complex structure of carbon nanotubes and their implicationsfor formation mechanism

Dan ZhouDepartment of Mechanical, Materials, and Aerospace Engineering, Advanced Materials Processingand Analysis Center, University of Central Florida, Orlando, Florida 32816

Lee Chowa)

Department of Physics, University of Central Florida, Orlando, Florida 32816

~Received 16 December 2002; accepted 19 March 2003!

Complex structures of carbon nanotubes formed from a conventional carbon arc-discharge methodhave been revealed by high-resolution transmission electron microscopy~HRTEM!. The branchingand the droplet growth phenomena observed by HRTEM indicate that the formation and growth ofcarbon nanotubes from an arc-discharge method cannot be simply processed from one end toanother. Based on the observation of the growth phenomena and the amorphous carbon residueinside the hollow core of carbon nanotubes, a two-step growth model of carbon nanotubes has beenproposed. ©2003 American Institute of Physics.@DOI: 10.1063/1.1573733#

eiorti-orirceal

n.cahep

nocotroo-ergsoanicohe

banlor

thva

ed.of

allelr-byof

cur-th

ofntobeuldssit is

om-our

e,ar-asednathehatem-ed.

on-cur--de,

ylin-th.eterwashe

I. INTRODUCTION

The discovery and synthesis1,2 of carbon nanotubes havstimulated extensive studies for their potential applicatbecause of the unique mechanical and electronic propeof this class of materials.3–7 In order to realize their promising technological utilization, nanotubes with a desired mphology and microstructure must be prepared. This requa control of the growth phenomena based on advanknowledge of the nucleation and growth mechanisms of cbon nanotubes. This advanced knowledge is also criticaestablishing the requirements for large-scale productio8,9

Therefore, a detailed study of the growth phenomena ofbon nanotubes will lead us to a better understanding of tformation and growth during a carbon arc-discharge evaration.

The formation and growth mechanism of carbon natubes prepared by an arc discharge has been the subjesome controversy, particularly with regard to their micrstructures. Based on high-resolution transmission elecmicroscopy ~HRTEM! observations of open-ended nantubes, Iijimaet al.10 proposed an open-ended growth modin which carbon atoms and small clusters from the dischaplasma add on to the reactive dangling bonds at the edgethe open-ended nanotubes. This open-ended growth msuggests that during the growth process, the ends of ntubes should be kept from closing by certain forces whare still unknown, and the walls of nanotubes are then ccentrically thickened layer by layer from the inside to toutside along the same tubular axis. In contrast, Saitoet al.11

proposed that during the discharge deposition, small carparticles were first formed on the surface of the cathode,these particles were elongated by an electrostatic force athe electric field just above the surface of the cathode to fothe carbon tips. The tips collect carbon atoms fromplasma, and continually grow forward as long as carbon

a!Electronic mail: [email protected]

9970021-8979/2003/93(12)/9972/5/$20.00

Downloaded 03 Jun 2003 to 132.170.140.216. Redistribution subject to A

nes

-esd

r-to

r-ir

o-

-t of-n

leof

delo-

hn-

ond

ngme-

por and ions are supplied and the electric field is sustainMeanwhile, the graphitization initiates from the bottomthe tips toward their end faces. Based on Saitoet al.’s11

model, nanotubes in the cathodic deposit should be parand lined up along the direction of the electric field. Unfotunately, a rich variety of growth phenomena revealedHRTEM characterizations indicate that the growth processnanotubes from an arc discharge is complicated, and therent growth models cannot explain those growbehaviors.12,13

A better understanding of the formation and growthcarbon nanoclusters is imperative, if tailoring nanotubes ia desired structure for their advanced application is toachieved. The basic elements of the growth dynamics shobe known in order to systematically optimize the proceparameters and control the nanotubes growth. Therefore,necessary to systematically investigate the growth phenena and microstructure of nanotubes, and to expandknowledge of their formation and growth. In this articlsome of the microstructures and growth morphologies of cbon nanotubes from arc discharges have been reported bon an HRTEM analysis. According to the growth phenomeobserved in this study, which show the sequential steps offormation of carbon nanotubes, a two-step growth model tassumes graphitization of the final structures from an assbly of amorphous or disordered initial structures is propos

II. EXPERIMENTAL PROCEDURES

The samples used in this study were produced by a cventional arc-discharge method, which was operated at arent density of 220 A/cm2 drawn by 27 V between the electrodes under a helium atmosphere of 500 Torr. The anobeing consumed during the discharge process, was a cdrical graphite rod 0.64 cm in diameter and 30 cm in lengThe cathode consisted of a graphite rod 0.95 cm in diamand 3 cm in length. The gap between the two electrodesmaintained at approximately 1 mm during the operation. T

2 © 2003 American Institute of Physics

IP license or copyright, see http://ojps.aip.org/japo/japcr.jsp

cersthahdly

-fes

boo

lebeintri

at

er-m-pli-Fig.m-

e inallsbond in

i-enther

rmo-

themthethe

uc-anyth

d bysets.dge

llsan-

ple

f thhft

o-the

9973J. Appl. Phys., Vol. 93, No. 12, 15 June 2003 D. Zhou and L. Chow

carbonaceous deposit on the surface of the cathode waslected as a slag, and samples for a HRTEM analysis wprepared by grinding the initial slag into powder and dispeing the powder in ethanol or acetone. After sonicatingpowder for approximately 5 min, the black suspension wdropped on a holey carbon thin film on a copper grid. Tsamples were then examined under a Hitachi H-8100 anJEOL 2000 FX HRTEMS operated at 200 keV, respective

III. RESULTS

The carbonaceous materials~slag! deposited on the cathode of an arc discharge are composed of a mixture of difent shapes of carbon nanotubes and nanoparticles besidegraphitic plates and amorphous carbon. Normally, carnanotubes consist of hollow carbon hexagonal networks ccentrically arranged around one another, and terminatedend caps in which the surface meet under specific angFigure 1~a! shows a HRTEM image of the carbon nanotucontaining multiple inner chambers, indicating that the cyldrical graphitic walls from different chambers are concencally lined up along different axes~labeled by arrows!. Theexistence of these chambers suggests that their nucle

FIG. 1. ~a! An HRTEM image shows that the nanotube contains multiinner chambers~labeled by arrows! lined up by different axes.~b! A com-plex internal structure of the nanotube showing that the morphologies oinner chambers are very flexible and depend on the confined space in wthey are formed.~c! An HRTEM image indicates that the exterior walls othe nanocluster grow along different directions. The inset images showedge dislocations in the graphitic walls.


ol-re-esea

.

r-thenn-bys.

--

ion

and formation processes are likely to occur from their outmost surface, which control the morphologies of the chabers, toward the inner in the confined space. A more comcated internal structure of a carbon nanotube, shown in1~b!, demonstrates that the morphologies of the inner chabers are very flexible and depend on the confined spacwhich they are formed. This suggests that the graphitic ware developed simultaneously through the entire carnanotube rather than initiated at one end and proceedeone direction. The arrows in Fig. 1~b! show the flexible cur-vatures of the inner graphitic walls, and the label ‘‘e’’ indcate that the confined space results in linking two differgraphitic walls together to avoid the dangling bonds ratthan closing the wall by a cap. Moreover, Fig. 1~c! showsthat the exterior walls of carbon nanocluster can also foalong different directions, with the morphology of the nancluster totally different from that of its innermost walls~la-beled ‘‘i’’ !. Again, this growth phenomenon suggests thatformation of the carbon nanocluster is likely to proceed frothe surface toward the center, and that the morphology ofcluster is determined by the outer surface rather than byinnermost layers of the cluster. Notice that the layer strtures of the carbon nanocluster are not perfect, and mdifferent types of defects can be found during the growprocess of the nanoclusters. Figure 1~c! also shows that thecarbon nanocluster has two-dimensional defects indicatearrows and their enlargement images are indicated as inThese defects are similar to the projected image of an edislocation.

Figure 2~a! shows that a smaller nanotube~labeled ‘‘B’’ !is grown out of the initial tubular base~labeled ‘‘A’’ !, whichindicates that the growth and formation of the tubular waare not symmetrical along one tube axis. Furthermore,

eich

he

FIG. 2. ~a! An HRTEM image shows that two nanotubes~A and B! areconstructed together.~b! An HRTEM image demonstrates that three nantubes~A, B, and C! are joined under certain angles. The label i indicatesmorphology of the innermost walls.


t

hc

lo

itce

rothfr

HR

o

ale

tic

3tinghettoibiti

thll

are

itic

hean

ra-,andcar-no-ess

rbonarecar-

in

side

in

that

raldon

ly-tic

no

pp

ar-

o-he

9974 J. Appl. Phys., Vol. 93, No. 12, 15 June 2003 D. Zhou and L. Chow

other HRTEM image, shown in Fig. 2~b!, demonstrates thathree carbon nanotubes~labeled ‘‘A’’, ‘‘B’’, and ‘‘C’’ ! can bejoined together to form a three-dimensional structure witsaddle surface, which has been predicted by theoreticalculations before.14–16 Notice that Fig. 2~b! clearly showsagain that the morphology of the nanocluster does not folthat of the innermost walls~labeled ‘‘i’’ ! of the cluster. Fur-thermore, instead of the continual bending of the graphlayers, two sets of graphitic walls meet together under atain angle@indicated by an arrow in Fig. 2~b!#. These growthphenomena indicate that an individual nanotube can gsimultaneously along different directions or axes, andgrowth process of carbon nanotubes appears to proceedthe outer surface of nanoclusters.

In contrast to connecting the nanotubes together, anTEM image, shown in Fig. 3~a!, illustrates a more compli-cated nanocluster structure, in which the nanocluster is cstructed by both tubular and polyhedral structures withdroplet morphology. This droplet morphology indicates ththe formation and growth of nanotubes and nanoparticfrom a carbon arc discharge occur simultaneously. Nothat the arrows shown in Fig. 3~a! indicate the joint angle ofthe nanotube and nanoparticle components. Figure~b!shows an HRTEM image of carbon nanotube, demonstrathat the outer diameter of the nanotube is suddenly chanby stopping the further growth of several outer walls. Tcontrast of the lattice image~indicated by ‘‘T’’! suggests thathe walls can be closed by linking two graphitic layersgether through continual bending, in which one wall contrutes a negative curvature and the other wall has a poscurvature. Notice that the growth of the outermost walls~la-beled by ‘‘o’’! of the nanotube passes smoothly throughshoulder, but the inner walls are stopped by joining two wa

FIG. 3. ~a! An HRTEM image shows that the carbon nanotubes and naparticle are connected with a droplet morphology.~b! An HRTEM image ofcarbon nanotube showing that the growth of the outermost walls~labeled‘‘o’’ ! of the tube passes through the shoulder, but the inner walls are stothere by joining two walls together.


aal-

w

er-

weom

-

n-atse

ged

--ve

es

together, which also suggests that the outermost wallsformed first.

Furthermore, the elongated carbon nanoparticles~labeledby ‘‘P’’ ! can be constructed by continuously bent graphlayers~indicated by arrows!, shown in Fig. 4~a!, illustratingthat the graphitic walls should be initially developed on tsurface of a carbon nanocluster rather than grown fromembryo. On the contrary, Fig. 4~b! shows an HRTEM imageof an undeveloped graphitic nanocluster~or an assembly!which may be quenched as it is. In this nanocluster, the gphitic walls ~indicated by arrows! are not well developedand the cluster consists mostly of disordered graphiteamorphous carbon. The observation of such a disorderedbon assembly supports the speculation that graphitic naclusters are developed through the graphitization procfrom the amorphous carbon assemblies produced by cadischarge plasma. The morphologies of the nanotubestherefore predetermined by the shapes of the amorphousbon assemblies.

More evidence to support this speculation is providedFig. 5. An HRTEM image of a nanotube@see Fig. 5~a!# dem-onstrates that amorphous carbon residue is left either inthe hollow core~indicated by arrows! or on the outer surface~labeled ‘‘S’’! of the nanotube. Further details are shownFig. 5~b!. The contrast of Fig. 5~b! indicates that the hollowcore of the tube is occupied by amorphous carbon, andthe discontinued inner walls~shown by arrows! have a fairlylower degree of graphitic order than that in the peripheregion. Notice that the label ‘‘d’’ clearly shows a projecteimage of edge dislocations in the layer structure of carbnanotubes.

Figure 6~a! shows that a cylindrical amorphous assembis just partially graphitized. According to this HRTEM image, in Fig. 6~a!, the assembly has well-developed graphi

-

ed

FIG. 4. ~a! An HRTEM image showing that the elongated carbon nanopticles ~labeled ‘‘P’’! are aligned by continuous bent graphitic layers~labeledby arrows!. ~b! An HRTEM image showing an undeveloped graphitic nancluster~or an assembly! carrying amorphous carbon. The arrows indicate tundeveloped graphitic layers in the assembly.


ar

laaf

t

he

tizc

nd

ra-as-as-

omearge

ob-ma-ul-

besbonrbon

rc-rentylin-cederuslingerthenperthe

en-e.ein-nceandptytheThes ofsize

itiesnet-

hetes,

theallelhe

es-nse-on-on-eshichbe-ricalpro-argebon

the

bofs aub

hera

9975J. Appl. Phys., Vol. 93, No. 12, 15 June 2003 D. Zhou and L. Chow

walls ~labeled ‘‘T’’ ! at the outer surface, but amorphous cbon ~labeled by ‘‘A’’ ! and graphitic layers~labeled by ‘‘G’’!are encaged inside the nanotube. Note that the graphiticers inside the tube have not been aligned along the tubeyet, and the graphitization process has been interruptedsome reasons. Figure 6~b! shows an HRTEM image of a joinof tow nanotube components~labeled by ‘‘T’’!. The image inFig. 6~b! demonstrates that following the morphology of tjoint, several graphitic layers~labeled by ‘‘G’’! have beendeveloped on the outermost surface, and then the graphition process has been stopped since there is amorphousbon ~labeled by ‘‘A’’ ! residing in between the nanotubes a

FIG. 5. ~a! An HRTEM image of a quenched nanotube showing thatdisordered carbon residue is left either inside the hollow core~indicated byarrows! or on the outer surface~labeled by ‘‘S’’!. ~b! An HRTEM imageindicates that the hollow core of the tube is occupied by disordered carand that the inner walls~shown by arrows! have a fairly lower degree ographitic order than that in the peripheral region. The label ‘‘d’’ illustratepair of edge dislocations on one side of the graphitic walls of the nanot

FIG. 6. HRTEM images shows~a! a cylindrical amorphous assembly, whicis just partially graphitized and~b! a joint of two nanotube components. Thouter graphitic walls are labeled by ‘‘T’’, and amorphous carbon and gphitic layers inside the tube are indicated by ‘‘A’’ and ‘‘G’’.


-

y-xisor

a-ar-

graphitic layers. These results simply indicate that the gphitic tubes were initiated from the amorphous carbonsemblies. The graphitization of these amorphous carbonsemblies toward the center has been interrupted for sreasons, such as quenching or fluctuation of the dischplasma.

IV. DISCUSSION

The rich variety of carbon nanotube structures asserved here by the HRTEM analysis suggests that the fortion of carbon nanotubes is a complex process. The difficties of interpreting the inner and outer structure of nanotucan be reduced by assuming that the formation of carnanotubes proceeds in two steps. First, amorphous caassemblies are formed on the surface of the cathode~high-temperature region in excess of 3500 °C) during the adischarge evaporation. These assemblies may have diffeshapes, such as plate, sphere, polyhedron, droplet, and cder, with random orientation, which depend on their surfaenergy and the local discharge conditions. Note that uncertain arc-discharge conditions, the cylindrical amorphoassemblies may be preferred. Second, during the cooprocess, the graphitization consisting of joining small layplane segments together to form larger plane areas androtating and translating these near-perfect layers into prostacking, occurs simultaneously on the assemblies fromsurface toward the interior region. The growth of the conctric graphite walls with cooling first occurs near the surfacThis is then followed by further alignment and joining of thinternal crystallites to the outer shell. Sometime the misjoing of the graphitic walls causes the edge dislocations. Sithe density of amorphous carbon assemblies is variedsmaller than that of graphite, in most of the cases, the emcores are left inside the graphitic nanoclusters whengraphitization starts on the surface of the assemblies.outer diameter of the tubes is determined by the diameterthe assemblies, and the inner diameter of the tubes or theof the hollow cores depends on the difference of the densbetween the amorphous carbon assemblies and graphiteworks. Notice that the growth tends to be confined within tboundaries of the primary carbon nanoclusters aggregawith most layer planes aligning parallel to the surface ofclusters. Those layer planes, which cannot be aligned parto the surface of the clusters, will cap the walls to form tinner chambers.

It is important to note that since the formation of thgraphitic walls may result from the graphitization of the asemblies, the introduction and distribution of the pentagoand heptagons in the hexagonal network will certainly dpend on the morphologies of the assemblies, which are ctrolled by their surface energy and the local discharge cditions. According to the growth model of carbon nanotubproposed here, it is not necessary to speculate a force wkeeps the ends of nanotubes open for forward growthcause carbon nanotubes are developed from the cylindamorphous carbon assemblies through the graphitizationcess. Furthermore, the fluctuation of the carbon arc dischmay be avoided if one can produce the needlelike car

n,

e.

-


eon

li

thrizllsea

lethrcefsreteso

aeim

omott

ud

ds-ro-lesd-the

n-

y,

ce

.

m.

9976 J. Appl. Phys., Vol. 93, No. 12, 15 June 2003 D. Zhou and L. Chow

assemblies in the nanometer scale at a high temperaturalternate methods, such as vapor condensation of carb17

plasma decomposition of hydrocarbon,18 and flames ofacetylene and benzene~or ethylene! premixed with oxygen,19

and then carbon nanotubes can be formed through a cooprocess.

V. CONCLUSION

Complex structures of carbon nanotubes produced byconventional arc-discharge method have been characteby HRTEM. In these structures, the growth of graphitic waof carbon nanotubes are not always concentrically alignThe morphologies of the inner chambers of nanotubesvery flexible and dependent upon the confined spacewhich they are formed. The branching and the dropgrowth phenomena have also been observed, indicatingthe formation and growth of carbon nanotubes from an adischarge method cannot be simply processed from oneto another. Furthermore, amorphous carbon has beenquently observed in the hollow cores of the nanotubes. Baon these growth phenomena and the amorphous carbondue inside the hollow core of some nanotubes, a two-sgrowth mechanism of carbon nanotubes has been propoFirst, the amorphous carbon assemblies with any kindmorphology, such as plate, sphere, polyhedron, droplet,cylinder with random orientation, which depends on thsurface energy and the local discharge conditions, are foron the cathode surface~high-temperature region!. Second,during the cooling process, the graphitization occurs frthe surface toward the interior of the assemblies. The mphologies of carbon nanoclusters appear to be solely demined by the shapes of the amorphous assemblies andthe needlelike assemblies, which result in the tubular strtures after the graphitization, are preferred under somecharge conditions.


by,

ng

eed

d.reintat-

ndre-edsi-p

ed.f

ndred

r-er-hatc-is-

ACKNOWLEDGMENTS

One of the authors~D. Z.! acknowledges S. Seraphin anB. Seraphin of the University of Arizona for valuable discusions and support, and J. C. Withers of Materials Electchemical Corp. Tucson, Arizona, for providing the sampfor this study. He also acknowledges support from the Avanced Materials Processing and Analysis Center andMaterial Characterization Facility at the University of Cetral Florida.

1S. Iijima, Nature~London! 354, 56 ~1991!.2T. W. Ebbesen and P. M. Ajayan, Nature~London! 358, 220 ~1992!.3M. S. Dresselhaus, Nature~London! 358, 195 ~1992!.4R. E. Smalley, Mater. Sci. Eng., B19, 1 ~1993!.5H. Dai, J. H. Hafner, A. G. Rinzler, D. T. Colbert, and R. E. SmalleNature~London! 384, 147 ~1996!.

6P. G. Collins, A. Zettl, H. Bando, A. Thess, and R. E. Smalley, Scien~Washington, DC, U.S.! 278, 100 ~1997!.

7M. M. Treacy, T. W. Ebbesen, and J. M. Gibson, Nature~London! 381,678 ~1996!.

8V. P. Dravid, X. Lin, Y. Wang, X. K. Wang, A. Yee, J. B. Ketterson, and RP. H. Chang, Science~Washington, DC, U.S.! 259, 1601~1993!.

9S. Seraphin, D. Zhou, and J. Jiao, Carbon31, 1212~1993!.10S. Iijima, P. M. Ajayan, and T. Ichihashi, Phys. Rev. Lett.69, 3100~1992!.11Y. Saito, T. Yoshikawa, M. Inagaki, M. Tomita, and T. Hayashi, Che

Phys. Lett.204, 277 ~1993!.12S. Seraphin, D. Zhou, and J. Jiao, Acta Microscopia3, 45 ~1994!.13D. Zhou and S. Seraphin, Chem. Phys. Lett.238, 286 ~1995!.14A. L. Mackay and H. Terrones, Nature~London! 352, 762 ~1991!.15T. Lenosky, X. Gonez, M. Teter, and V. Elser, Nature~London! 355, 333

~1992!.16L. A. Chernozatonskii, Phys. Lett. A172, 173 ~1992!.17M. Ge and K. Sattler, Science~Washington, DC, U.S.! 260, 515 ~1993!.18N. Hatta and K. Murata, Chem. Phys. Lett.217, 398 ~1994!.19J. B. Howard, K. das Chowdhury, and J. B. van de Sande, Nature~Lon-

don! 370, 603 ~1994!.


Complex structure of carbon nanotubes and their ...lc/nanotube-structure.pdf · carbon nanotubes...

Documents

Transcript of Complex structure of carbon nanotubes and their ...lc/nanotube-structure.pdf · carbon nanotubes...