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Fullerenes, Nanotubes andCarbon NanostructuresPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/lfnn20

Synthesis, Properties andApplications of Helical CarbonNanotubesA. Szabó a , A. Fonseca a , J. B. Nagy a , A. Volodin b

, C. Van Haesendonck b , L. P. Biró c & J.‐F. Colomerc

a Facultés Universitaires Notre Dame de la Paix ,Namur, Belgiumb Laboratorium voor Vaste‐Stoffysica enMagnetisme , Katholieke Universiteit Leuven ,Leuven, Belgiumc Research Institute for Technical Physics andMaterials Science , Budapest, HungaryPublished online: 06 Feb 2007.

To cite this article: A. Szabó , A. Fonseca , J. B. Nagy , A. Volodin , C. VanHaesendonck , L. P. Biró & J.‐F. Colomer (2005) Synthesis, Properties and Applicationsof Helical Carbon Nanotubes, Fullerenes, Nanotubes and Carbon Nanostructures,13:S1, 139-146, DOI: 10.1081/FST-200039237

To link to this article: http://dx.doi.org/10.1081/FST-200039237

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Synthesis, Properties and Applications ofHelical Carbon Nanotubes

A. Szabo, A. Fonseca, and J. B. Nagy

Facultes Universitaires Notre Dame de la Paix, Namur, Belgium

A. Volodin and C. Van Haesendonck

Laboratorium voor Vaste-Stoffysica en Magnetisme, Katholieke

Universiteit Leuven, Leuven, Belgium

L. P. Biro and J.-F. Colomer

Research Institute for Technical Physics and Materials Science,

Budapest, Hungary

Abstract: Helical carbon nanotubes were produced on silica-supported Co catalysts by

chemical vapour decomposition of acetylene. Coiled nanotubes were examined. They

have been found to be of various shapes, diameter, and pitch. Some of them are

extremely long, up to 5mm, with regular helices. The average outer diameter of coils are

about 30–50 nm, the pitch is in the range 50–200 nm and the length about 1–1.4mm.

The helix-shaped windings of the helical carbon nanotubes reveal characteristic mechan-

ical resonances,which are determined by the elastic modulus,mass, shape, and dimensions.

Keywords: Carbon nanotubes, chemical vapor deposition, helices, supported catalysts

INTRODUCTION

Helical carbon nanotubes were predicted by Dunlap (1) and Ihara et al. (2) The

coiling of the proposed structures originated from the regular insertion of

pentagons and heptagons in the hexagonal honeycomb network.

Address correspondence to A. Szabo, Facultes Universitaires Notre Dame de la

Paix, 61 Rue de Bruxelles, Namur B-5000, Belgium. E-mail: antonia.fonseca@

fundp.ac.be

Fullerenes, Nanotubes, and Carbon Nanostructures, 13: 139–146, 2005

Copyright # Taylor & Francis, Inc.

ISSN 1536-383X print/1536-4046 online

DOI: 10.1081/FST-200039237

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Relatively large amounts of curved and coiled nanotubes were produced

by catalytic vapour decomposition of hydrocarbon. Their interesting, unusual,

and various shapes stimulated several studies on the modelling of the coiling

mechanism (3, 4). Recently Biro et al. (4) showed that specific wrappings

of the structures with nonhexagonal and hexagonal rings lead to a new

varieties of toroidal, coiled, screwlike, and pearl-necklace-like carbon nano-

architectures. The pentagons and heptagons are important parts of these struc-

tures. Their very specific arrangement is the basis for the formation of regular

helical coils.

The helical carbon nanotubes are described by the coil diameter and the

pitch. These nanotubes could be used in composites because they would be

better anchored in their embedding matrix than straight nanotubes. Their

shape would favour a better load transfer to the matrix than in the case of

straight tubes, and possibly easier infiltration. They could be attractive

objects for nano-electro-mechanical devices, because of their special electri-

cal and mechanical properties (5).

In present work we report the synthesis of coiled carbon nanotubes on

silica-supported cobalt catalysts produced by catalytic vapour decomposi-

tion of acetylene. We were interested in the influence of the catalyst

preparation methods and in the influence of metal loading on the quality of

carbon deposit and the formation of coiled nanotubes.

EXPERIMENTAL

Preparation Methods of the Catalysts

Ion-Adsorption Precipitation Method

The appropriate amount of Co(CH3COO)2. 4H2O (Aldrich) was dissolved in

distilled water; 3 g of silicagel (Merck part. diam. 15–40mm) as added to the

solution under stirring. The mixture was stirred about 20 min. The pH was

controlled and adjusted to pH 9 using ammonia solution (Vel). After 48 hr

the pH of the as-prepared mixture was controlled and set again to 9, just

before the filtration. Then the precipitate was washed with distilled water,

dried at 1208C overnight, and ground.

Sol–Gel Method

This technique is based on the hydrolysis of tetraethoxysilane (TEOS-Acros)

in an acidic or basic environment; 10 ml of ethanol (Fluka), 10 ml of TEOS

and 15 ml of 1.5 M solution of Co(CH3COO)2. 4H2O (Aldrich) were mixed.

The mixture was stirred for 20 min. A few drops of conc. HF (Vel) were

added to promote the reaction and the stirring was continued for another

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20 min. The as-prepared gel was dried at 608C overnight, then ground and

calcined for 1 hr at 4508C in air.

The carbon nonotube growth reactions were carried out in a fixed bed

flow reactor using acetylene as carbon source and N2 as inert gas; 1 g of

catalyst was introduced to the reactor at 7008C for 30 min. Gas flow rate

was 30 ml/min of C2H2 in 300 ml/min of N2. TEM (Tecnai, Philips), SEM

(XL Series, Philips), HRTEM were used to characterize the products. The

carbon deposit was calculated as follows:

Carbon deposit ð%Þ ¼ 100mtot � mcat

mcat

;

where mtot is the mass of total product and mcat is the mass of corrected

catalyst.

RESULTS AND DISCUSSION

The first part of our results is summarised in Table 1. Here we compare the

products obtained on the catalyst prepared by Method (1) with different Co

loadings (2.5–12.5 wt.%). The results are based on the TEM microscope

observations.

In the case of 2.5 wt.% metal content, the product was poor in carbon

nanotubes and coils. Mainly amorphous carbon and catalyst particles were

found in the samples. In the other cases, the catalysts had a good activity.

The surface of the catalyst was totally covered by CNTs. We observed

several morphologies of nanotubes such as tubes differently curved, helical

tubes, springs, and spirals. The coiled tubes were examined. Generally, the

pitch varied from 50–200 nm and the coil diameter was in the range of 50–

150 nm. We observed several coils with length up to 5mm. These long coils

were usually intercoiled with themselves, and in some cases they formed inter-

esting structures. The total carbon deposit was about 40–60% and the amount

of coiled nanotubes was approximately 5% (Fig. 1).

Most of the tubes (Fig. 1) do not contain metal particles and their tips are

open. In some cases we observed coils that, after some coiling, continue as

Table 1. Carbon deposit and quality, for catalysts prepared by Method (1)

Wt.% of Co 2.5 5 10 12.5

Coiled tubes þ þ þ þ þ þ þ þ þ þ

Other tubes þ þ þ þ þ þ þ þ þ þ þ þ þ þ

Carbon deposit (%) 40 50 60 60

% of coils 0.5 15 10 6

Helical Carbon Nanotubes 141

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straight tubes. Probably the activity of the catalyst changed during the reaction

and this is the reason why the form of the tube changed. The pitch and coil

diameter generally do not change noticeably.

In the samples produced on the catalysts prepared by Method 2 (sol–gel),

there are high amounts of CNTs, among which some are coiled nanotubes

(Fig. 2). Based on TEM pictures we made an approximate estimation of the

amount of coils in these samples. It was found to be about 15%. The carbon

deposit was about 40%. The diameter of the tubes was in the range of 15–

30 nm. The pitch values were found in the range of 30–130 nm and the coil

diameters were 35–100 nm. There were some exceptions, i.e., pitch up to

1,100 nm, coil diameter up to 300 nm. The coiled carbon nanotubes found,

in these samples were very regular (Fig. 3). There are more tubes like

“telephone wires” or springs in the product. Most of the helical tubes were

long coils.

Some of the coils were suspended in ethanol and transferred to silicon

plates for the measurements of their mechanical properties (6). After adsorp-

tion on an oxidized silicon substrate, the coiled CNTs form three-dimensional

structures with freely suspended windings (Fig. 4). The helix-shaped windings

Figure 1. Low magnification TEM image of coils produced on the catalyst prepared

by Method (1).

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reveal characteristic mechanical resonances, which are determined by the

elastic modulus, mass, shape, and dimensions. The suspended windings are

resonantly excited in situ at the fundamental frequency by an ultrasonic trans-

ducer connected to the substrate. When the tip of the atomic force microscope

(AFM) is positioned above the winding, the cantilever is unable to follow its

fast oscillations. Nevertheless, an oscillation-amplitude dependent signal is

generated due to the nonlinear force-to-distance dependence (7). The helix-

dependence shaped CNTs can be used as convenient mechanical resonant

sensors.

CONCLUSION

Helical carbon nanotubes were synthesised on silica-supported Co catalyst by

chemical vapour decomposition of acetylene. Catalysts were prepared by sol–

gel and ion-adsorption precipitation methods. In the latter case, the metal

loading varied from 2.5 to 12.5 wt.%. The carbon products grown on them

were examined by different microscopic techniques. The coiled nanotubes

produced possess various morphologies: slightly curved, spirals, springs, or

Figure 2. Low magnification TEM image of coils produced on the catalyst prepared

by Method (2).

Helical Carbon Nanotubes 143

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Figure 3. TEM images of different shapes of coiled carbon nanotubes found in the samples produced on the

catalyst prepared by Method (2).

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“telephone wire”-like tubes. Products grown on the first type of the catalysts

were rich on springs with regular helicity that had the pitch of about 30 nm and

coil diameter about 50 nm. The proportion of helical tubes was about 15%, in

the samples. In the second type, the amount of coils was about 5% and they

presented more a sinusoid-like shape, with large helicity, with the pitch

about 50 nm, coil diameter 50 nm. Generally, the tubes and the coils grown

on the sol–gel catalysts have been found thinner and more regular than in

the case of the ion-adsorption precipitation method.

ACKNOWLEDGMENTS

The authors acknowledge the E.C. (RTN contract NANOCOMP HPRN-CT-

2000-00037), the Belgian program PAI (P5/1), Region of Wallonia

(SYNATEC convention No. 0014526), and OTKA (Grant T043685) for

financial support.

REFERENCES

1. Dunlap, B.J. (1992) Connecting carbon tubules. Phys. Rev. B, 46: 1933.2. Ihara, S., Itoh, S., and Kitakami, J. (1993) Helically coiled cage forms of graphitic

carbon. Phys. Rev. B, 48: 5643.3. Fonseca, A., Hernadi, K., Nagy, J.B., Lambin, Ph., and Lucas, A.A. (1996) Growth

mechanism of coiled carbon nanotubes. Synth. Met., 77: 235.4. Biro, L.P., Mark, G.I., Koos, A.A., Nagy, J.B., and Lambin, Ph. (2002) Coiled

carbon nanotube structures with supraunitary nonhexagonal to hexagonal ringratio. Phys. Rev. B, 66: 165405.

Figure 4. AFM image of a suspended coiled nanotube bridging a step formed by the

edge of a gold pad on a silicon substrate (a). Vertical displacement of the AFM tip,

which is positioned above the coiled nanotube (see white arrow in (a)), as a function

of the excitation frequency (b). Model of a suspended helical carbon nanotube (c),

used for calculations (B � 0.5mm; 2r � 20 nm; 2R � 0.5mm; n � 4 is the number

of windings; E is the elastic modulus � 0.2 TPa; r is the density �1,300 kg/m3).

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5. Ahlskog, M., Seynaeve, E., Vullers, R.J.M., Van Haesendonck, C., Fonseca, A.,Hernadi, K., Volodin, J.A., and Nagy, J.B. (1999) Ring formations from catalyti-cally synthesized carbon nanotubes. Chem. Phys. Lett., 300: 202.

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7. Volodin, A., Van Haesendonck, C., Tarkiainen, R., Ahlskog, M., Fonseca, A., andNagy, J.B. (2001) AFM detection of the mechanical resonances of coiled carbonnanotubes. Appl. Phys. A., 72: S75.

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