16 April 1999

Ž .Chemical Physics Letters 303 1999 467–474

Continuous production of aligned carbon nanotubes: a step closerto commercial realization

R. Andrews a, D. Jacques a, A.M. Rao a,b,), F. Derbyshire a,c, D. Qian c, X. Fan d,E.C. Dickey c, J. Chen e

a Center for Applied Energy Research, UniÕersity of Kentucky, Lexington, KY 40506, USAb Department of Physics and Astronomy, UniÕersity of Kentucky, Lexington, KY 40506, USA

c Department of Chemical and Materials Engineering, UniÕersity of Kentucky, Lexington, KY 40506, USAd Solid State DiÕision, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

e Department of Chemistry, UniÕersity of Kentucky, Lexington, KY 40506, USA

Received 28 January 1999; in final form 23 February 1999

Abstract

Ž .High-purity aligned multi-walled carbon nanotubes MWNTs were synthesized through the catalytic decomposition of aferrocene–xylene mixture at ;6758C in a quartz tube reactor and over quartz substrates, with a conversion of ;25% of thetotal hydrocarbon feedstock. Under the experimental conditions used, scanning electron microscope images reveal that theMWNT array grows perpendicular to the quartz substrates at an average growth rate of ;25 mmrh. A process of thisnature which does not require preformed substrates, and which operates at atmospheric pressure and moderate temperatures,could be scaled up for continuous or semi-continuous production of MWNTs. q 1999 Elsevier Science B.V. All rightsreserved.

1. Introduction

The synthesis of aligned multi-walled carbon nan-Ž .otubes MWNTs via the catalytic pyrolysis of hy-

drocarbons has been realized by several researchw xgroups around the world 1–12 . Most of these re-

searchers report the use of preformed substrates. Forexample, high-purity aligned MWNTs were pro-duced by the catalytic decomposition of 2-amino-

) Corresponding author. Fax: q1 606 323 2846; e-mail:rao@pop.uky.edu

4,6-dicholoro-s-triazine at 10008C between tracksgenerated by laser etching cobalt films supported on

w xsilica 2 . Large arrays of aligned MWNTs weregrown normal to nickel-coated glass substrates be-low 6668C by the plasma-enhanced hot-filamentchemical-vapor deposition of a mixture of acetylene

w xand ammonia 5 . In this Letter, we describe a methodfor producing bulk quantities of high-purity alignedMWNTs through the catalytic decomposition of aferrocene–xylene mixture at temperatures as low as;6508C. Our synthesis method does not requirepreformed substrates. In principle, it is simple andinexpensive. It can be operated at atmospheric pres-sure and moderate temperatures and gives very high

0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2614 99 00282-1

( )R. Andrews et al.rChemical Physics Letters 303 1999 467–474468

selectivity to multi-walled nanotubes. A process ofthis nature could be easily scaled up and used forcontinuous or semi-continuous production.

A desirable goal is to be able to produce single-Ž .wall nanotubes SWNTs by a similar continuous

w xprocess. Cheng et al. 8 have demonstrated thatthiophene serves as a growth promoter for producinglong SWNT bundles from the ferrocene-catalyzeddecomposition of benzene at 10008C in a stream ofhydrogen. Recently, the Rice group mentioned thatthey had not been able to reproduce the experimentalfindings reported by Cheng et al. suggesting that thegrowth parameters for long SWNT bundles are still

w xnot well defined 13 . Nevertheless, it seems mostprobable that it is only a matter of time beforeSWNT bundles will also be produced by a continu-ous economical process, similar to the one reportedin this Letter.

2. Experimental

Approximately 6.5 mol% of ferrocene was dis-Žsolved in xylene to obtain a feed solution with

.;0.75 at % FerC ratio and was fed continuouslyŽinto a two-stage tubular quartz reactor diameter,

. Ž .;34 mm using a syringe pump Fig. 1 . FerroceneŽ .sublimation temperature, ;1408C has been shownto be a good precursor for producing Fe catalystparticles which can seed nanotube growth, and xy-

lene was selected as the hydrocarbon source since itŽ .boils boiling point, ;1408C below the decomposi-

Ž . w xtion temperature of ferrocene ;1908C 14 . Theliquid feed is passed through a capillary tube andpreheated to ;1758C prior to its entry into thefurnace. At this temperature, the liquid exiting thecapillary, is immediately volatilized and swept intothe reaction zone of the furnace by a flow of argonwith 10% hydrogen. Various parameters, such as the

Ž .furnace temperature 650–10508C , ferrocenerxylene ratio and feed rate, total reaction time, andsweep gas flow rate were adjusted to determine thegrowth conditions for high purity aligned MWNTsŽ .Table 1 . After the reaction, the preheater and thefurnace were allowed to cool to room temperature inflowing argon. Carbon deposits were formed on thewalls of the quartz furnace tube and on plain quartz

Ž .substrates microscope slides that were placed withinthe furnace to act as additional sites for nanotubegrowth. The substrates were weighed before andafter each run to determine the quantities of nan-otubes produced at different sites within the reactor.

Ž .Scanning electron microscope SEM and transmis-Ž .sion electron microscope TEM images of the nan-

otubes deposited on the substrates allowed us tomonitor the quality of MWNTs grown under differ-ent operating conditions. The walls of the reactortube were also scraped clean of nanotube materialand weighed. At all times, the reactor was operated

Fig. 1. Schematic of the reactor used for the nanotube synthesis. The ferrocene–xylene liquid feed is injected into the preheater stage using asyringe pump. Using a mass flow controller, ArrH gas is introduced at the entrance of the preheater to sweep the reactant vapors into the2

hot zone of the furnace.

( )R. Andrews et al.rChemical Physics Letters 303 1999 467–474 469

Table 1Operating conditions for three reactor runs with the furnace temperature held at ;6758C, 0.75 at % FerC ratio and run duration ;2 h

Run ArrH Ferrocene–xylene Ferrocene–xylene Nanotube2

flow rate feed rate partial pressure production rate2Ž . Ž . Ž . Ž .sccm mlrh mbar mgrcm rh

A 675r75 5 20 0.3B 675r75 1 4 0.08C 1688r188 2.5 4 0.15

at 1Y of H O pressure above atmospheric pressure to2

prevent any influx of oxygen.

3. Results and discussion

At ;10508C, a small amount of MWNT materialwas formed at the entrance of the furnace and mostof the ferrocene–xylene feed was converted to amor-phous carbon which deposited on the available sur-faces: tube walls, substrates, and the water-cooled

Ž .condenser see Fig. 1 . However, when the reactortemperature was lowered to ; 6758C, copiousamounts of aligned MWNTs were produced withhigh selectivity on all surfaces inside the quartz tube:it was estimated that the conversion of the carbon inthe feed to MWNTs was ;25%. Under these condi-tions no carbon was deposited on the condenser andonly unconverted ferrocene and xylene were col-lected.

Typical SEM images obtained from MWNTs de-posited on the quartz substrates and on the reactorwalls are shown in Fig. 2. Fig. 2a shows a SEMimage of aligned MWNTs growing normal to thesubstrate surface. Elemental analyses of the alignedMWNT array near the tip and root regions indicatethat Fe catalyst particles are present at either ends ofthe MWNTs. If this is the case, it is possible thatnanotubes can grow upwards from particles thatremain on the substrate and also behind particles thatmove away from the substrate at the growing tip ofthe nanotubes. In the present study, we find thatnanotube growth begins only after Fe has been de-posited on the substrate. The role of the ferrocene isto provide a source of Fe in the vapor phase whichthen deposits in the form of fine particles or clusterson the various surfaces that are present in the fur-nace. Nanotubes can then nucleate and grow fromthe metal sites, this mechanism does not preclude thepossibility of nucleation and growth on catalyst sites

in the vapor phase, and indeed it seems possible thatsuch a mechanism may be operative in the laser andelectric arc systems. However, in this research the

Ž .presence of the substrate or walls of the quartz tubeappears to be essential to nanotube formation, andthe yield in any given experiment is determined bythe surface area that is available in the hot zone ofthe reactor. The growth of the nanotubes is probablyinitiated normal to the substrate surface due to thedirection of the carbon concentration gradient, whencarbon will be first deposited on the exposed uppersurface of the metal particle and diffuse through andover the metal to precipitate at the opposite face inthe form of nanotubes. Once initial nanotube growthis established, growth will tend to continue in thissame direction and may well be reinforced by thepresence of surrounding nanotubes, where ‘crowd-ing’ will limit the possibilities for nanotube propaga-tion in other directions.

Surprisingly, the MWNT arrays can be easilypeeled off the quartz substrates while retaining their

Ž .parallel orientation Fig. 2b . To the naked eye, thetip and root surfaces look black and shiny grey incolor, respectively. Expanded views of the tips andthe roots of peeled MWNTs are shown in Fig. 2c and2e, respectively. From Fig. 2d, it is clear that there isa high degree of alignment between adjacent

Ž .MWNTs. Under higher magnification Fig. 2f , theroots of the peeled MWNTs exhibit open ends sug-gesting that in the predominant growth mode, the Fenanoparticles detach from the substrate and travel atthe head of the growing nanotube. Examination byhigh resolution TEM confirms that Fe nanoparticlesare present at the tips of the MWNTs.

It should be noted that under all operating condi-tions listed in Table 1, with a xylene partial pressureof ;4 mbar, the quartz substrates were coated onboth sides with only MWNTs. However, when a

Ž .higher partial pressure 20 mbar of xylene was used

( )R. Andrews et al.rChemical Physics Letters 303 1999 467–474470

amorphous carbon deposits were found in the mate-rial that was scraped off the reactor walls in the hotzone. At low partial pressure, the nanotube produc-tion rates as determined from the substrate deposits

only are listed in Table 1. They represent the lowerboundaries for the rates of nanotube production.

Fig. 3 depicts a typical high resolution TEMimage of a MWNT obtained using the synthesis

Ž . Ž . Ž .Fig. 2. SEM images Hitachi 3200N, 5 kV of as-grown MWNTs on a quartz substrate a and a peeled film of as-grown MWNTs b .Ž . Ž . Ž . Ž .Expanded views of the tip c , the middle section d , and the root e of the peeled MWNTs film. Higher magnification Hitachi 5900 of

Ž .the root region reveals the open ends of the MWNTs f .

( )R. Andrews et al.rChemical Physics Letters 303 1999 467–474 471

Ž .Fig. 2 continued .

conditions listed in Table 1 for runs B and C. Theseimages confirm the presence of MWNTs in thesample with the dominant tube diameter in the range20–25 nm. It is interesting to note that the MWNTsproduced with a FerC ratio of 0.75 at % showed thepresence of the Fe catalyst inside the core of theMWNTs. Lowering the FerC ratio by a factor of 10produced longer and thinner nanotubes and the Fecatalyst was absent inside the core of the nanotubes.Microdiffraction patterns obtained on individualMWNTs show the presence of 002 reflections con-

Žfirming a high degree of structural order inset in.Fig. 3 .

The results of this and other investigations havehelped to elucidate the growth mechanism of carbonnanotubes. It seems inescapable that nanotube forma-tion is a specific case or extension of what is alreadyknown, and that nanotubes are routinely formed in

Ž .many systems even under non-optimal conditionsand have simply been overlooked. A recent exampleis the occurrence of nanotubes at the core of vapor

w xgrown carbon fibers 15,16 .

( )R. Andrews et al.rChemical Physics Letters 303 1999 467–474472

Ž .Fig. 2 continued .

It is well known that different carbon structurescan be formed by the reaction of gaseous carbon-containing precursors over metal surfaces, particu-larly Fe, Co, and Ni. In these reactions, whichgenerally are operated at temperatures up to about11008C, a distinction should be made between thecarbon that is formed by interaction with the metaland that which is co-produced. For high selectivity,

it is important to confine the reaction to the immedi-ate vicinity of the metal surface. Carbon atoms orfragments produced by the catalytic decompositionof the precursor can be extremely mobile on metalsurfaces, and vice versa. Hence, carbon atoms candiffuse rapidly over and through the metal, driven bya temperature or concentration gradient, to a locationwhere they assemble into an ordered structure. To

( )R. Andrews et al.rChemical Physics Letters 303 1999 467–474 473

Ž .Fig. 3. HRTEM image Phillips CM 200, 200 kV showing theŽ .multilayered structure of a single MWNT. Inset: Typical 002

electron diffraction spots observed in a microdiffraction pattern.

minimize the incidence of reactions away from themetal, the rate of carbon production must be limited.This can be achieved through an appropriate selec-

Žtion of the precursor compound relating to its ther-.mal stability and partial pressure , the catalyst metal,

and the reaction temperature. As demonstrated in ourexperiments, under a given set of conditions, a re-duction in xylene partial pressure has this very ef-fect, reducing the production of ‘amorphous’ carbonand increasing the selectivity to nanotubes.

Ž .Over metal foils millimeter dimensions , highlycrystalline graphite films, oriented parallel to thesurface, can be formed by solid solution and subse-

w xquent precipitation 17,18 . With supported metalŽ .particles tens of nanometers in diameter , carbon

filaments can be formed with diameters of the orderw xof tenths of a micrometer 19 . In the proposed

mechanism, carbon is deposited on the particle sur-face in contact with the gas phase and diffusesthrough or over the metal to precipitate at the oppo-site surface. The metal particle is carried at the headof the growing filament. In going from a foil to asmall particle the essential change in the carbonstructure is that the precipitated graphitic or

graphite-like basal planes grow parallel to the metalsurface in the former case, and tangentially to it inthe latter case, forming the axis of the fiber. Aroundthe fiber circumference, it appears that the graphiticbasal planes approximate to the curved surface as aseries of small planar crystallites. It is suggested thatthe change in form between foil and particle is dueentirely to the dimensions and shape of the metal,where the small size and curvature of the metalparticles necessitate growth away from the surface.The question then remains as to why the structurechanges from a series of planar crystallites to closedtubes as the metal particle size decreases. An article

w xby Endo et al. 16 may provide an important clue.They show that the interlayer spacing increases asthe ‘tube’ diameter is reduced, passing down throughthe series of carbon filaments to nanofibers to nan-otubes. The increase in the interlayer spacing innanotubes with smaller diameter is indicative ofstrain imposed by the increasing curvature. Lookingat the result another way, it could be that as themetal particle size is progressively reduced belowthat employed to form fibers or filaments, a point isreached where the radius is so small that the imposedstrain will necessitate a transition from approximat-ing to a curved surface to adopting a tubular form:that is, as multi-walled or, ultimately, single-wallednanotubes.

4. Conclusions

In conclusion, it should be noted that a process ofthis nature could be easily scaled up and used for thecontinuous or semi-continuous production of multi-walled nanotubes. Based upon our results, it seemsmost probable that it is only a matter of time beforenanotubes are produced cheaply and in quantity foraround the same cost as carbon fibers, and probablyless. Only a few months ago, Prof. Rick Smalleypredicted that in the next 10 years, an economicalprocess to produce single-wall nanotubes in ton

w xquantities from the gas phase will be discovered 20 .While we have not yet produced single-wall nan-otubes by our method, the potential certainly exists,and the availability of multi-walled nanotubes maybe more than adequate for many practical applica-

( )R. Andrews et al.rChemical Physics Letters 303 1999 467–474474

tions. There is every reason to believe that we havetruly stepped on the road to commercialization.

Acknowledgements

The authors wish to acknowledge financial sup-port from the NSF MRSEC Grant DMR-9809686;and valuable discussions with Prof. R.C. HaddonŽ .Department of Chemistry, University of Kentucky ,

ŽProf. S. Sinnott Department of Chemical and Mate-.rials Engineering, University of Kentucky and Dr.

Ž .S. Bandow Meijo University, Japan .

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