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21 March 1997

Chemical Physics Letters 267 (1997) 276-280

CHEMICAL PHYSICS LETTERS

Carbon nanotubes by the metallocene route

Rahul Sen a,b, A. Govindaraj a,b, C.N.R. Rao a,b,*

a CSIR Centre of Excellence in Chemistry and Materials Research Centre, Indian Institute of Science, Bangalore 560012, India b Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India

Received 31 December 1996

Abstract

Pyrolysis of rnetallocenes such as ferrocene, cobaltocene and nickelocene, is shown to yield carbon nanotubes and metal-filled onion-like structures. Pyrolysis of benzene in the presence of a metallocene gives high yields of nanotubes, the wall thickness of the nanotubes depending on the metallocene content. Pyrolysis of benzene in the absence of any metal however gives monodispersed nanospheres of carbon rather than nanotubes.

1. Introduct ion

Carbon nanotubes were initially discovered in the cathodic deposit during the arc-evaporation of graphite rods in an inert atmosphere [ 1 ]. Multiwalled nanotubes with a central tubule of 1-3 nm diameter, covered all around by graphite sheaths, are generally produced by this technique. Single-walled nanotubes have been prepared by using a catalyst such as Ni and Co during the arc-evaporation [2]. Carbon nan- otubes have also been prepared by decomposition of C2H 2 under inert conditions around 700°C over Fe/graphite [3], Co/graphi te [4] and Fe/s i l ica [5]. The presence of transition metal particles is found to be essential for the formation of nanotubes [6] and the diameter of the nanotube appear to be determined by the size of the metal particle, small nanoparticles producing single-walled nanotubes [7]. Various mod- els have been proposed for the role of transition metals in catalysing the growth of carbon nanofibres

* Corresponding author.

and nanotubes. According to one of the models, the hydrocarbon decomposes on the surface of the cata- lyst and the carbon so formed deposits itself as graphitic flakes on the rear face of the particles to yield carbon nanofibres in which the graphitic flakes are parallel to the face of the catalyst particle [8]. Dai et al. [7] propose a mechanism wherein carbon forms a hemispherical graphene cap ( y a r m u l k e ) on the catalyst particle and the nanotubes grow from such a yarmulke.

Since the metal particles play a crucial role in the formation of carbon nanotubes, we have examined the pyrolysis of ferrocene, cobaltocene and nicke- locene in a reductive atmosphere to explore whether metallocenes, containing both the transition metal and hydrocarbon fragments, would yield carbon nan- otubes. It was also of interest to investigate the pyrolysis of a hydrocarbon such as benzene in the presence of a small proportion of a metallocene to examine in what way the metal particles from the metallocene catalyse the formation of nanotubes. We have obtained interesting results pertaining to nan- otube synthesis by the metallocene route and report some of the highlights in this communication.

0009-2614/97/$17.00 Copyright © 1997 Published by Elsevier Science B.V. All rights reserved PII S0009-2614(97)00080-8

R. Sen et al. / Chemical Physics Letters 267 (1997) 276-280 277

2. Experimental

The pyrolysis of metallocenes and metallocene/ benzene mixtures was carried out in an atmosphere of Ar and n 2 at 900°C. Before carrying out these experiments, we have examined the pyrolysis of benzene in A r / H 2 atmosphere in the absence of any metal particles and also in the presence of Ni parti- cles. For this purpose, benzene was pyrolysed at ll40°C for lh in a stream of 75% Ar and 25% H 2 (in absence of any catalyst) at a gas flow rate of 50 sccm. The flow rate was monitored by UNIT mass flow controllers. Benzene vapour was also pyrolysed over Ni powder ( ~ 50 nm dia.), prepared by the polyol process [9], by passing benzene vapour mixed with a stream of 95% Ar and 5% H 2 over the metal powder at 900°C and a gas flow rate of 40 sccm for 2h .

Pyrolysis of the metallocenes (ferrocene, cobal- tocene and nickelocene) was carried out in a quartz tube located in a two-stage furnace system. In a typical experiment, a weighed quantity of a metal- locene was taken in a quartz boat, placed inside a furnace and a mixture of Ar and H 2 of the desired composition passed through the quartz tube. The furnace temperature was increased to 200°C and the metallocene vapour so generated was carried by the Ar -H 2 stream, into a quartz tube maintained at 900°C in a second furnace. The decomposition of the metallocene occurs in the second furnace. Pyrolysis of the metallocene-benzene mixtures was carried out by a similar procedure, by introducing benzene vapour alongwith the A r - H 2 mixture.

Carbon particles obtained from the pyrolysis ex- periments were observed with a LEICA $440i scan- ning electron microscope (SEM) and a JEOL 3010 transmission electron microscope (TEM).

3. Results and discussion

Fig. 1. (a) SEM image of monodispersed carbon nano-spheres obtained by the carbonization of benzene in an A r / H 2 mixture at

1140°C without any catalyst; (b) TEM image of the carbon spheres; (c) TEM image of carbon nanotubes obtained by the pyrolysis of benzene at 900°C in 5% H 2 and 95% Ar mixture over Ni powder (gas flow rate of 40 sccm).

Pyrolysis of benzene in a A r - H 2 atmosphere at 1140°C in the absence of a transition metal catalyst gives monodispersed nanospheres of carbon in the diameter range of 200-500 nm as revealed by the SEM and TEM images presented in Fig. l(a) and (b) respectively. Pyrolysis of benzene carried out in a Ar -H 2 atmosphere in the presence of Ni powder

( ~ 50 nm dia. particles) however gives carbon nan- otubes. In Fig. l(c), we show a TEM image of nanotubes obtained by the pyrolysis of benzene at 900°C in a stream of 5% H 2 and 95% Ar over Ni powder. These experiments clearly establish that the transition metal particles act as catalysts in the for- mation of nanotubes.

278 R. Sen et a l . / Chemical Physics Letters 267 (1997) 276-280

Instead of taking metal powders externally, to act as catalysts for the formation of nanotubes through the decomposition of benzene or other hydrocarbons, we have investigated the pyrolysis of the three metal- locenes containing both the transition metal and hy- drocarbon fragments. Interestingly, the pyrolysis of ferrocene in Ar or in Ar (75%)-H 2 (25%) mixture at 900°C yields nanotubes as can be seen from the TEM images in Fig. 2. The images in Fig. 2(a) and (b) are not different from those of nanotubes ob- tained by using transition metal catalysts. Pyrolysis of ferrocene at 900°C in an Ar-H 2 mixture (flow rate of 50 sccm), often gives rise to nanotubes completely filled by the iron metal as can be seen from the TEM image in Fig. 2(c). Pyrolysis of cobaltocene and nickelocene at 900°C in a stream of Ar (75%)-H 2 (25%) mixture (total flow rate of 50 sccm) similarly yield carbon nanotubes containing Co and Ni particles. The magnitude of incorporation of the metal depends on the flow rate of Ar-H 2 mixture and content of metallocene in the gas mix- ture. The fact that carbon nanotubes are produced by the pyrolysis of metallocenes, and not of benzene, underscores the essential role of the metal in the formation of the nanotubes. Furthermore, pyrolysis of metallocenes provides an alternative method of filling nanotubes with metals. In the literature, vari- ous solution methods for filling nanotubes with met- als have been reported [10,11]. However it would be difficult to obtain metal like iron and cobalt inside nanotubes by solution methods. In metallocene py- rolysis, we also obtain onion-like structures with the encapsulated metal particles as shown in Fig. 2(a). The high resolution image in Fig. 2(d) reveals the nature of metal encapsulated onion more clearly. The image clearly shows the graphitic fringes of the onion. Such encapsulated onions had earlier been obtained by arcing graphite alongwith the metals [12].

Having established that pyrolysis of metaliocenes yields carbon nanombes, we have investigated how the transition metals derived from the metailocenes can catalyse the formation of carbon nanotubes, by carrying out the pyrolysis of benzene in mixture with metallocene. We have indeed found that in the pres- ence of a small proportion of ferrocene, pyrolysis of benzene yields large quantities of carbon nanotubes. In Fig. 3(a) we show a typical SEM image of

Fig. 2. TEM images of carbon nanotubes obtained by the pyroly-

sis of ferrocene at 900°C in: (a) a 75% Ar /25% H 2 mixture at a flow rate of 25 sccm, (b) pure Ar at a flow rate of 25 sccm, and

(c) a 75% Ar /25% H 2 mixture at a flow rate of 50 sccm. (d) High resolution TEM image of an onion-like particle encapsulat- ing iron, obtained by the pyrolysis of ferrocene at 900°C in a 75% Ar/25% H 2 mixture at a flow rate of 50 sccm.

R. Sen et al. / Chemical Physics Letters 267 (1997) 276-280 279

at high flow rates of the gas mixture. The nanotubes obtained in these pyrolysis experiments are generally multiwalled as can be seen from the TEM image in Fig. 3(b). However, by increasing the flow rate and decreasing the ferrocene content, the wall thickness is considerably reduced, as can be seen from the TEM images in Figs. 3(c) and (d). By adjusting the relative concentrations of ferrocene and benzene in the vapour phase it should be possible to preferen- tially obtain single-walled nanotubes by this tech- nique. One way of doing this is to use dilute solu- tions of ferrocene in benzene as the starting material. The TEM images in Fig. 3(b)-(d) also demonstrate that the iron clusters generated from ferrocene act as nucleating centres for the formation and growth of the nanotubes. These conclusions are confirmed by the pyrolysis of benzene in presence of cobaltocene and nickelocene. In Fig. 4, we show the TEM im-

Fig. 3. (a) SEM image of carbon nanotubes obtained by the pyrolysis of a mixture of benzene and ferrocene at 900 ° in a

stream of 75% Ar /25% H 2 (total flow rate of 50 sccm). (b) TEM image of a typical nanotube obtained by this method. (c) and (d)

TEM images of nanotubes obtained by the pyrolysis of a mixture of benzene and ferrocene at 900°C in 85% Ar /15% H 2 mixture at

a gas flow rate of 1000 sccm.

nanotubes obtained by the pyrolysis of benzene in the presence of ferrocene at 900°C in 75% Ar-25% H 2. The yield of nanotubes is significantly enhanced

Fig. 4. TEM images of carbon nanotubes obtained by the pyroly-

sis of a mixture of benzene with (a) cobaltocene at 900°C in a stream of 75% Ar and 25% H 2 at a flow rate of 50 sccm and (b)

nickelocene at 900°C in a stream of 85% Ar and 15% H z at a flow rate of 1000 sccm.

280 R. Sen et a l . / Chemical Physics Letters 267 (1997) 276-280

ages of nanotubes obtained by the pyrolysis of ben- zene in the presence of cobaltocene and nickelocene.

The carbon nanotubes obtained in the present study have crystalline inner and outer layers and our various observations are in conformity with the yarmulke mechanism [7] rather than the one involv- ing bulk as well as surface diffusion of carbon in the metal particle [8,13]. It appears that bulk diffusion of carbon in the metal is insignificant in the process of formation of the nanotubes [14].

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