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Carbon nanospheres andtubules obtained by thepyrolysis of hydrocarbonsA. Govindaraj , Rahul Sen , B. VenkataNagaraju & C.N.R. RaoPublished online: 14 Nov 2010.

To cite this article: A. Govindaraj , Rahul Sen , B. Venkata Nagaraju &C.N.R. Rao (1997) Carbon nanospheres and tubules obtained by the pyrolysisof hydrocarbons, Philosophical Magazine Letters, 76:5, 363-368, DOI:10.1080/095008397178977

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PHILOSOPHICAL MAGAZINE LETTERS, 1997, VOL. 76, NO. 5, 363± 367

Carbon nanospheres and tubules obtained by the pyrolysisof hydrocarbons

By A. Govindaraj ² , Rahul Sen² , B.Venkata Nagaraju ³and C. N. R. Rao ² ³ §

² CSIR Centre of Excellence in Chemistry and Materials Research Centre,Indian Institute of Science, Bangalore 560012, India

³ Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064,India

[Received 6 January 1997 and accepted 20 July 1997]

AbstractMonodispersed nanospheres of carbon are obtained by the pyrolysis of

methane or benzene at high temperatures in the presence of hydrogen withoutthe use of any catalyst. Carbonization of these hydrocarbons over transitionmetal powders (for example Ni) or supported metal catalysts, however, yieldscarbon nanotubes.

Kang and Wang (1996) have recently reported the formation of monodispersednanosized carbon spheres by the catalytic carbonization of methane at 1100 ë C overmixed-valent oxide catalysts. These authors (Wang and Kang 1996) have alsoreported the formation of spiral carbon tubes by the same reaction carried out ata slightly lower temperature of 950 ë C. The carbon tubes are highly twisted withmany nodes, the inner diameter being determined by the size of the original sphericalcore. These tubes seem to be di� erent from the carbon nanotubes belonging to thefullerene family (Iijima 1991, Rao et al. 1995). Wang and Kang (1996) point out thatthe key role of the oxide catalyst in the carbonization reaction is to release oxygenwhich then abstracts hydrogen atoms from methane giving rise to carbon. Carbonnano® bres with interesting structures have also been obtained by decomposition ofhydrocarbons on metal particles (Rodriguez et al. 1995).

We have been examining the pyrolysis of methane and benzene in a hydrogenatmosphere and have obtained monodispersed spherical carbon particles by thepyrolysis of methane and benzene in the absence of any catalyst. However, if thepyrolysis is carried out under reductive conditions over powders of the transitionmetals or supported transition-metal catalysts, we obtain carbon nanotubes as wellas onion-like structures with encapsulated metal particles.

Pyrolysis of CH4 (C6H6) was carried out in a quartz ¯ ow-tube reactor located ina horizontal tube furnace. The ¯ ow rate of the gases was monitored with UNIT mass¯ ow controllers. CH4 (C6H6) was mixed with hydrogen in a desired ratio and pyro-lysed at 1140 ë C for 1 h in the absence of any catalyst. Pyrolysis was also carried outover Ni powder or a Ni± AlPO4 catalyst at 700 ë C for 2 h in the case of methaneand at 900 ë C for 2 h in the case of benzene. Ni powder (about 50 nm in diameter)was prepared by the polyol process (Fievet et al. 1989), wherein 5 g of polyvinyl

0950± 0839/97 $12 ´ 00 Ñ 1997 Taylor & Francis Ltd.

§ Author for correspondence.

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364 A. Govindaraj et al.

Fig. 1

(a) SEM image of monodispersed carbon nanospheres obtained by the carbonization ofbenzene in an Ar± H2 mixture at 1140ë C without catalyst; (b) TEM image of carbonspheres obtained by pyrolysis of benzene in Ar± H2 mixture at 1140ë C without catalyst;(c) high-resolution TEM image showing the graphitic flakes: the graphitic fringes arenot continuous.

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pyrrolidone (PVP) and 2.5 g of nickel acetate were dissolved in 150 ml of ethyleneglycol and re¯ uxed in an argon atmosphere for 5 h. The resulting Ni powder washeated in air at 750 ë C to burn away the PVP and then treated with hydrogen at700 ë C. Ni supported on AlPO4 was prepared by the impregnation of AlPO4 with theappropriate amount of nickel acetylacetonate in methanol solution followed bycalcination at 400ë C for 16 h. The loading of Ni was 12%. The Ni± AlPO4 catalystwas reduced in hydrogen at 700 ë C. The carbon particles, obtained by pyrolysis, wereexamined by a Leica S440i scanning electron microscope and a JEOL 3010 transmis-sion electron microscope operating at 300 kV.

In ® g. 1(a) we show a typical scanning electron microscopy (SEM) image of themonodispersed carbon spheres with diameters in the range 200± 500 nm obtained by

C nanospheres and tubules from hydrocarbons 365

Fig. 2

(a) SEM image of monodispersed carbon nanospheres obtained by the carbonization of CH4and hydrogen at 1140ë C without catalyst; (b) TEM image showing the spherical shapeof the particles.

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366 A. Govindaraj et al.

Fig. 3

(a) TEM image of carbon nanotubes obtained by the pyrolysis of 90% CH4 ± 10% H2 mixtureover Ni powders at 700ë C. The inset shows a HREM image of one such tube. Thegraphitic planes are at an angle to the tube axis and seem to form a conical structure.(b) TEM image of a tubule obtained by pyrolysis of 95% CH4 ± 5% H2 mixture overNi± AlPO4 catalyst at 700ë C.

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the pyrolysis of benzene at 1140 ë C in a stream of hydrogen (25%) and argon (75%)(in the absence of any catalyst) at a gas ¯ ow rate of 50 sccm. The spheres are similarto those reported by Kang and Wang (1996). In ® g. 1(b) and (c) we show transmis-sion electron microscopy (TEM) images of these carbon spheres at low and highmagni® cations. The high-resolution image in ® g. 1(c) shows that the surface of thecarbon spheres has a wavy graphitic layered structure. These layers do not form aclosed shell and are composed of many ¯ akes. Pyrolysis of 75% CH4 and 25% H2

under similar conditions also yields monodispersed spherical carbon particles asshown in ® g. 2. The SEM and TEM images are comparable with those obtainedwith benzene (® g. 1). It is noteworthy that the carbon spheres shown in ® gs. 1 and 2where obtained without the use of any catalyst. Furthermore, the temperature atwhich the carbon spheres are obtained is not far from that employed by Kang andWang (1996) in the presence of oxide catalysts.

Pyrolysis of benzene or methane over Ni particles under reductive conditionsyields carbon nanotubes. It appears that metal particles are essential to preparenanotubes from hydrocarbons (Rodriguez et al. 1995). In ® g. 3(a) we show theTEM image of carbon nanotubes produced by the pyrolysis of 90% CH4 and10% H2 (total ¯ ow rate, 50 sccm) over Ni powder at 700 ë C for 2 h. In the inset ofthe ® gure, we show the high-resolution electron microscopy (HREM) image of ananotube. The image shows graphitic planes which are at an angle to the tube axisand form a conical structure. Pyrolysis of 95% CH4 ± 5% H2 under similar conditionson the Ni± AlPO4 catalyst (total ¯ ow rate, 100 sccm) also gave carbon nanotubes asshown in ® g. 3(b). These nanotubes are of the type described by Iijima (1991) andRao et al. (1995).

Pyrolysis of benzene or acetylene in a H2 ± Ar mixture over Ni or Co powder at900 ë C or 700 ë C for 2 h also gave carbon nanotubes (Ivanov et al. 1994, Hernadi et al.1996). By exposing a mixture of 10 wt%Ni and graphite to a hydrogen plasma at900 ë C for 5 h, we have obtained amorphous carbon ® bres, the process probablyinvolving hydrocarbon intermediates.

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Iijima , S. , 1991, Nature (L ond.), 354, 56.Ivanov, V., Nagy, J. B., Lambin, Ph., Lucas, A., Zhang , X. B., Zhang, X. F., Bernaerts,

D., Van Tendeloo, G., Amelinckx , S., and Van Landuyt, J., 1994, Chem. Phys.Lett., 223, 329.

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