Solid State Communications 150 (2010) 311–315

Contents lists available at ScienceDirect

Solid State Communications

journal homepage: www.elsevier.com/locate/ssc

Synthesis of multiwall carbon nanotubes by chemical vapor deposition offerrocene alone

Ravi Bhatia ∗, V. PrasadDepartment of Physics, Indian Institute of Science, Bangalore-12, India

a r t i c l e i n f o

Article history:Received 15 August 2009Received in revised form30 October 2009Accepted 12 November 2009 by A. PinczukAvailable online 25 November 2009

Keywords:A. Carbon nanotubeB. Chemical vapor depositionC. Electron microscopyD. Magnetic property

a b s t r a c t

Multiwall carbon nanotubes (MWNTs) filled with Fe nanoparticles (NPs) have been synthesized bythermal chemical vapor deposition of ferrocene alone as the precursor. The MWNTs were grown atdifferent temperatures: 980 and 800 ◦C. Characterization of as-prepared MWNTs was done by scanningand transmission electronmicroscopy, and X-ray diffraction. The transmission electronmicroscopy studyrevealed that Fe NPs encapsulated in MWNTs grown at 980 and 800 ◦C are spherical and rod shaped,respectively. Room-temperature vibrating sample magnetometer studies were done on the two samplesup to a field of 1 T. Themagnetization versusmagnetic field loop reveals that the saturationmagnetizationfor the two samples varies considerably, almost by a factor of 4.6. This indicates that Fe is present indifferent amounts in the MWNTs grown at the two different temperatures.

© 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon nanotubes (CNTs) are sp2 hybridized molecular nanos-tructures, which are thought to be derived from fullerenes [1,2]. Itis the unique and exceptional combination ofmechanical and elec-trical properties, combined with their high length-to-diameter ra-tio (greater than 106), that make them an important material forfuture technology [3]. Attempts have been made to use CNTs inthe fabrication of devices like field emission displays, field emit-ters, flow meters, batteries and carbon nanotube field effect tran-sistors [4–8]. These molecular nanostructures are proposed to bean efficient hydrogen storage material [9]. Carbon nanotube cylin-drical membranes have been reported to be used as filters forthe elimination of multiple components of heavy hydrocarbonsfrom petroleum and for the filtration of bacterial contaminants ofsize less than 25 nm from water [10]. Recently, CNT bundles havebeen proposed to be a good material for low-temperature sens-ing [11]. Arc discharge, laser ablation, chemical vapor deposition(CVD), etc., have been employed to produce CNTs [1,12,13]. Differ-ent precursors have been used to obtain different morphologies ofthe CNTs. Carbon sources in liquid phase like tetrahydrofuran, hex-ane, xylene, benzene, pyridine, etc., with ferrocene (FeCp2) as a cat-alyst have been used for fabricating different architectures of CNTs

∗ Corresponding author. Tel.: +91 80 2293 2313; fax: +91 80 2360 2602.E-mail address: bhatia.phy@gmail.com (R. Bhatia).

0038-1098/$ – see front matter© 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2009.11.023

[14]. Some hydrocarbons in gaseous phase like methane, ethylene,acetylene, etc., have also been used to produce CNTs [15–17]. CVDis a popular method for the synthesis of CNTs due to its large scala-bility, low cost and lower processing temperature requirements. Inearlier studies, the production of Fe nanoparticles (NPs) enclosedin carbon shells, carbon-encapsulated Fe3C NPs, metallic Fe andFe203 NPs had been reported by decomposition of FeCp2 using dif-ferent methods such as the laser assisted CVD method, photolyticdissociation method, plasma production method, etc. [18–21]. Inaddition, an expensive and quite complicated approach, involvingmany steps, for the growth of CNTs by thermal decomposition ofFeCp2 has been reported [22].In this paper we report on a simple one-step method of synthe-

sizing MWNTs of fairly good quality and yield using FeCp2 aloneas the starting material. The synthesis technique adopted by us issimple and quite inexpensive ($ 200–300) in comparison to otherreported methods. The advantage of using FeCp2 alone is that theprecursor acts as both the catalyst and carbon source material.Growth temperature is found to be one of the most important pa-rameters that greatly affects the magnetic properties of Fe-filledMWNTs.

2. Experimental details

The schematic of the set-up employed for the synthesis ofMWNTs is shown in Fig. 1. The whole apparatus consisted ofa single-zone furnace attached with a temperature controller. A

312 R. Bhatia, V. Prasad / Solid State Communications 150 (2010) 311–315

Fig. 1. Simple set-up used to synthesize MWNTs.

quartz tube (diameter ∼10 mm and length ∼500 mm) closed atone end was placed inside the single-zone furnace with 250 mg ofFeCp2 [(C5H5)2Fe; mp: 174–176 ◦C, bp: 249 ◦C] (supplied by SigmaAldrich). After evacuating the tube, the furnace was switchedon and the temperature was allowed to rise. Fig. 2 shows thetemperature profile of the furnace. The temperature of the furnaceincreased linearly with time and attained the desired temperature(980 or 800 ◦C) in 35 min. Fluctuations in temperature at thecentral zone (10 cm away from both the ends) are very small∼5 ◦C(maximum) and there is a considerable temperature gradient fromthe central part to either ends of the furnace. The open end ofquartz tubewas connected to an empty bladder to collect the gasesevolved from the reaction tube. When the temperature reachedthe desired temperature the bladder was removed and anotherbladder filled with Ar gas was connected. It should be noted thata continuous flow of Ar was not supplied in the reaction tube. Thefurnace was then kept at 980 ◦C for another 1 h. Afterwards thefurnace was allowed to cool to room temperature. MWNTs werefound to be deposited on a selected portion on the inner wall ofthe tube in the form of a black material and were scrapped off asproduct. Scanning electron microscopy (SEM) using a Sirion high-resolution scanning electronmicroscope equippedwith a Schottkyfield emitter was used to study the overall morphology of thesamples. Transmission electron microscopy (TEM) was used in anFEI Technai F30 electron microscope for the detailed structuralstudy of MWNTs filled with Fe NPs. The crystalline structure ofMWNTswas studied by X-ray diffraction (XRD) using a BRUKER D8Advance x-ray diffractometer. Magnetization studies were carriedout on Lakeshore Model 7400 vibrating sample magnetometer(VSM).

3. Results and discussions

The formation of MWNTs in the present work depends onthe thermal fragmentation of FeCp2 molecules in the reactor. TheFeCp2molecule is reported to be stable at a temperature lower than497 ◦C [23]. The growth mechanism of MWNTs can be understoodusing the time versus temperature plot of the furnace, which isshown in Fig. 2. As the temperature rises above 249 ◦C (at pointB), FeCp2 vapors are produced. The fragmentation of FeCp2 startswhen the temperature in the reactor crosses 497 ◦C (at point C).The different bonds in FeCp2molecules break in a sequence accord-ing to their increasing binding energies. The binding energy of C–Hbond is the lowest, so the hydrogen atom first leaves themolecularstructure of FeCp2. Then the C–C bonds and Fe–C bonds are broken

Fig. 2. Time versus temperature graph of the furnace employed.

Fig. 3. X-ray diffraction pattern of MWNT sample CNT980.

subsequently in a very small fraction of time [24]. Thus the cat-alytic Fe particle is available for the growth ofMWNTs at a temper-ature around 500 ◦C. At this moment carbon clusters (dimers andtrimers) start covering the Fe particles in tubular form and thus theformation of carbon nanotubes begins, as illustrated in theoreticalstudies reported by Endo [25]. MWNTs are found to be depositedat selected portions away from the central part on the inner wallsof quartz tube, which is due to the temperature difference fromthe center to the ends of furnace. MWNTs have been produced attemperatures above 800 ◦C in one direct step. The MWNT samplesprepared at 980 and 800 ◦C are referred to as CNT980 and CNT800for simplicity.The XRD pattern of CNT980 is shown in Fig. 3. It shows a typi-

cal peak for carbon nanotubes at 25.89◦ which corresponds to the(002) plane of graphite. Another peak is observed at 44.54◦ thatcorresponds to Fe (110). Two more peaks have been noticed at43.59◦ and 42.65◦, which correspond to Fe3C (102) and C (100),respectively.Free-standing samples that were simply scrapped from the in-

ner wall of tube were examined under SEM. Fig. 4 shows the SEMimages ofMWNTs grown at the different growth temperatures.Weobserved a significant effect of presence of Ar gas in the synthesis ofMWNTs. An inert gas environment gives MWNTs of nice morphol-ogy. SEM images of MWNTs prepared at 980 ◦C in the presence ofAr gas are shown in Fig. 4(a) and (b). From the images it is observed

R. Bhatia, V. Prasad / Solid State Communications 150 (2010) 311–315 313

Fig. 4. SEM micrographs of (a) and (b) MWNTs grown at 980 ◦C in the presence of an inert environment, (c) and (d) MWNTs grown at 980 ◦C in the absence of an inertenvironment, (e) and (f) MWNTs grown at 800 ◦C in the absence of an inert environment.

that the MWNT sample grown in an inert atmosphere has a lesseramount of amorphous carbon (a-C). MWNTs samples grown in theabsence of Ar contain a-C in large amounts and lack any kind ofalignment as well. Thus the presence of Ar gas also supports theformation of a good-quality MWNT sample in our system. Also, at-tempts were made to synthesize MWNT samples at temperaturesless than800 ◦C; thesewere examinedunder SEM. The electronmi-crographs indicated the presence of a few carbon nanotubes alongwith lumps of a-C. Thus the synthesis of MWNTs at a process tem-perature below 800 ◦C is not favored to get high yield and betterMWNTs samples. SEM images of CNT980 and CNT800 are shownin Fig. 4(c) and (d). The comparison between the SEM images ofMWNTs grown at 980 and 800 ◦C suggests that the MWNT sampleprepared at 980 ◦C is better than that prepared at 800 ◦C.The TEM images ofMWNT samples are shown in Fig. 5. The TEM

images show that Fe particles are encapsulatedwithin theMWNTs,which is in support of the XRD data. The interlayer spacing of theconcentric walls of the MWNTs is found to be 0.341 nm, which isalmost the same as the spacing between the two graphene layers ingraphite. A careful observation of the TEM images indicates that theMWNTs in CNT980 contain spherical Fe NPs whereas the MWNTsin CNT800 contain rod-like Fe NPs, as shown in Fig. 5(a) and (b),

respectively. Fig. 5(c) and (d) show HRTEM images of MWNTs en-capsulated with spherical and rod-shaped particles, respectively.The average diameter of the spherical Fe particles is 12 nm and thedimensions of the rod-like Fe particles are ∼25 nm diameter and∼150 nm length. Thus an effect of temperature on the size of cat-alytic particles is observed, which in turn affects the diameter ofthe MWNTs. Therefore, the diameter of the MWNTs prepared atdifferent temperatures varies significantly. Fig. 5(e) and (f) showHRTEM images of MWNTs from CNT980 and CNT800. The aver-age outer diameter of the MWNTs is∼35 nm in CNT980 while forCNT800 it is ∼70 nm. The variation of diameter of MWNTs syn-thesized at different reaction temperatures can be understood asfollows. After the thermal fragmentation of FeCp2 molecules intoFe and the elemental form of carbon, the Fe particle clusters retainbigger sizes at lower temperatures. As the temperature is increasedfrom800 to 980 ◦C, the Fe particle size reduces andhence the diam-eter of MWNTs produced at higher temperatures is less than thatat lower temperatures.Fig. 6 shows themagnetization versusmagnetic field for the two

samples at room temperature up to a magnetic field of 1 T. BothCNT980 and CNT800 attain saturation magnetization (Ms) at 1 T.The source of magnetization in both the samples is, obviously, thepresence of catalytic Fe particles, as previous studies reveal that

314 R. Bhatia, V. Prasad / Solid State Communications 150 (2010) 311–315

Fig. 5. TEMmicrographs of (a) MWNTs grown at 980 ◦C (CNT980), (b) MWNTs grown at 800 ◦C (CNT800), (c) an MWNT encapsulated with spherical Fe NPs, (d) an MWNTwith rod-like Fe NPs, (e) an MWNT of diameter 35 nm, (f) an MWNT of diameter 70 nm.

CNTs are diamagnetic in nature [26,27]. The value ofMs for CNT980is 1.91 emu/g, whereas for CNT800 it is 8.87 emu/g, and the aver-age coercivity (Hc) for the two samples is almost ∼640 Oe, whichis more than the reported values in Fe-filledMWNTs [28,29]. How-ever, the values of Ms and Hc for bulk Fe are 222 emu/g and 1 Oeat room temperature, respectively. The difference in the value ofMs for the two samples is probably due to the fact that CNT980contains MWNTs with smaller Fe particles whereas CNT800 con-tains MWNTs with larger Fe particles. In previous studies [30], it isalso reported that the value of Ms decreases with decreasing par-ticle size, which is in agreement with our results. The magnetiza-tion study suggests that the Fe content is more in CNT800 than inCNT980. This result is in accordance with our TEM study. The ratioof residual magnetizations of CNT800 to CNT980 is∼4.5.

4. Conclusions

In summary, MWNTs were synthesized by CVD using FeCp2 asthe carbon source and as well as the catalyst. The TEM study in-dicates that the shape and size of Fe NPs embedded inside theMWNTs changes with the process temperature, because of whichtwo important effects are observed: (1) the diameter of theMWNTsvaries with the process temperature, which is due to the change

Fig. 6. Hysteresis loops of samples (a) CNT980 and (b) CNT800.

in size of catalytic Fe NPs, and (2) the Fe content is present indifferent amounts in MWNT samples grown at different temper-atures, which in turn alters the magnetic properties. This makes usconclude that the process temperature is one of the most impor-tant parameterswhich can greatly influence both the diameter andmagnetic properties of Fe-filled MWNTs.

R. Bhatia, V. Prasad / Solid State Communications 150 (2010) 311–315 315

Acknowledgements

The authors are grateful to the Institute Nanoscience Initiative,Indian Institute of Science, Bangalore, for the TEM and SEM char-acterization. Special thanks are due to Prof. Reghu Menon, for hiskind support of this work.

References

[1] S. Iijima, Nature 354 (1991) 56.[2] H.W. Kroto, J.R. Sheath, S.C. O’Brien, R.F. Curl O’Brien, R.E. Smalley, Nature 318(1985) 162.

[3] M.M.J. Treacy, T.W. Ebbesen, J.M. Gibson, Nature 381 (1996) 678.[4] A.A. Talin, K.A. Dean, J.E. Jaskie, Diam. Relat. Mater. 10 (2001) 265.[5] W.A. de Heer, J.M. Bonard, K. Fawth, A. Chatelain, L. Forro, D. Ugarte, Adv.Mater. 9 (1997) 87.

[6] S. Ghosh, A.K. Sood, N. Kumar, Science 299 (2003) 1042.[7] G. Che, B.B. Lakshami, E.R. Fisher, C.R. Martin, Nature 93 (1998) 346.[8] S.J. Wind, J. Appenzeller, R. Martel, V. Decycke, Ph. Avouris, Appl. Phys. Lett.80 (2002) 3817.

[9] S.M. Lee, K.S. Park, Y.C. Choi, Y.S. Park, J.M. Bok, D.J. Bae, et al., Synth. Met. 113(2000) 209.

[10] A. Srivastva, O.N. Srivastva, S. Talapatra, R. Vajtai, P.M. Ajayen, Nat. Mat. 3(2004) 610.

[11] P. Mahanandia, L.T. Singh, K.K. Nanda, Rev. Sci. Inst. 19 (2008) 053909.

[12] A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, et al., Science 273 (1996)483.

[13] Z.F. Ren, Z.P. Huang, J.W. Xu, J.H. Wang, P. Bush, M.P. Siegel, et al., Science 282(1998) 1105.

[14] P. Mahanandia, K.K. Nanda, Nanotechnology 19 (2008) 155602.[15] J. Kong, A.M. Cassell, H. Dai, Chem. Phys. Lett. 292 (1998) 567.[16] L. Yuan, K. Saito, W. Hu, Z. Chen, Chem. Phys. Lett. 346 (2001) 23.[17] Y.H. Mu, A.K.M.F. Kibria, K.S. Nahm, Synth. Metals 122 (2001) 443.[18] K. Elihn, F. Otten, M.B. Oman, P. Heszler, F.E. Kruis, H. Fissan, et al., Appl. Phys.

A 72 (2001) 29.[19] K. Elihn, L. Landstrom, O. Alm, M. Boman, P. Heszler, J. Appl. Phys. 101 (2007)

034311.[20] H.H. Song, X.H. Chen, Chem. Phys. Lett. 374 (2003) 400.[21] C.H. Chou, J. Phillips, J. Mater. Res. 7 (1992) 2107.[22] A. Barreiro, S. Hample, M.H. Rummeli, C. Kramberger, A. Gruneis, K. Bider-

mann, et al., J. Phys. Chem. B 110 (2006) 20973.[23] E.O. Fischer, H. Grubert, Chem. Ber. 92 (1959) 2302.[24] K. Elihn, K. Larsson, Thin Solid Films 458 (2004) 325.[25] M. Endo, K. Takeuchi, K. Kobori, K. Takahashi, H.W. Kroto, A. Sarkar, Carbon

13 (1995) 873.[26] A.S. Kotosonov, S.V. Kuvshov, Phys. Lett. A 229 (1997) 377.[27] A.A. Ovchinnikov, Phys. Lett. A 195 (1994) 95.[28] B.C. Satishkumar, A. Govindaraj, P.V. Vanitha, A.K. Raychaudhuri, C.N.R. Rao,

Chem. Phys. Lett. 362 (2002) 301.[29] J. Cheng, X.P. Zou, G. Zhu, M.F. Wang, Y. Su, G.Q. Yang, et al. Solid State

Commun. (in press).[30] S. Ganagopadhyay, G.C. Hadjipanayis, B. Dale, C.M. Corensen, K.J. Klabunde, V.

Papaefthymiou, Phys. Rev. B 45 (1992) 9778.