Production of CNts by the Catalysed Vapour Phase Pyrolysing process

Production of carbon nanotubes by the catalysed vapour phase pyrolysing process

Al. Darabont1, K. Kertész1, C. Neamţu2, Zs. Sárközi1, L. Tapasztó1,

L.P. Biró3, Z.E. Horváth3, A.A. Koós3, Z. Osváth3, Z. Vértesy3

1Babeş-Bolyai University, Faculty of Physics, str. Kogălniceanu nr.1, Cluj-Napoca, 3400, Romania

2National Institute for Research and Development of Isotopic and Molecular Technologies, 71-103 Donath St., P.O. Box 700, ClujNapoca, 3400, Romania

3Hungarian Academy of Sciences, Research Institute for Technical Physics and Materials Science, P.O. Box 49, H-1525, Budapest, Hungary

Abstract. This paper confirmes that single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), as well as bundles of well aligned MWCNT films can be obtained simultaneously by injecting a solution of ferrocene in benzene into a reaction furnace in Ar atmosphere. We have also justified that using a solution of ferrocene in thiophene, the reaction products contain Y-junction CNTs. There are data concerning the home made experimental set-up used. The reaction product was analysed on the basis of TEM, STM, FESEM and XRD studies. Besides the CNTs, the reaction product contains as byproducts Fe and Fe3C, which can be eliminated by the described purification process. Some CNTs are partially filled with Fe-catalyst.

Introduction

Since the discovery in 1991 [[1]] of the carbon nanotubes (CNTs), they have been promising candidates for various applications. Thus, they are effective emitters for field emission due to he large current density at low threshold voltages [[2]]. The small size and high toughness make them suitable for new advanced scanning probes [[3]]. As CNTs are generally metallic or semiconducting, depending on the helicity and diameter [[4]], they can be used to construct nanoelectronic devices. In particular the Y and T shaped CNT junctions are considered likely to be the basic building units for this purpose. The CNTs are up to 100 times stronger than steel, able to withstand repeated bending, buckling and twisting, which results in building lightweight metal-matrixes [[5], [6]] and polymer composites [[7]]. They have also potential use as molecular pressure sensors [[8]] and chemical sensors [[9]].

There are many methods of producing CNTs, as electric arc discharge [[10]], laser evaporation [[11]], chemical vapour deposition (CVD) [[12], [13]], plasma-enhanced CVD [[14]] etc. Amongst the CVD methods, pyrolysis of hydrocarbons in the presence of a metal catalyst constitutes a simple and efficient process [[15]]. Generally, two methods are used to introduce the carbon source material into the pyrolysis furnace: either as a vapour in a gas stream (Ar) [15] or by liquid injection using an atomizer (sprayer) [[16]]. We used the latter method to produce CNTs at laboratory level.

Experimental

The scheme of the home made experimental spray-pyrolysis set-up used for the synthesis of CNTs is represented in Fig. 1. This set-up uses the single step synthetic route, which involves the spray-pyrolysis of ferrocene-benzene or ferrocene-thiophene solutions in an Ar atmosphere. The essential parts are: a) an 1 meter long quartz tube (the reactor) with ~20 mm inner diameter, which supports well the 1000ºC temperature, and at the same time plays the role of support for the catalyst particles resulting from the active solution and for the final product; b) a sprayer (atomizer) of the active solution, which consists of a glass nozzle (capillary) with 0.65 mm inner diameter at the end, surrounded by another glass tube with 2 mm inner diameter also at the end. The outer glass tube of the nozzle directs the carrier gas (Ar) flow around the nozzle. We consider that the surface area between the inner- and outer tube has a decisive role in the spraying process of the solution. In our case this area was 3.14 mm2.

At the beginning of each experiment the quartz tube is flushed with Ar, to eliminate the oxygen from the reaction chamber. Then the tube is gradually preheated to temperatures between 750–975ºC and the Ar flow-rate is set at the desired value. The ferrocene-benzene or ferrocene-thiophene solution is introduced into the sprayer and pulverized into the reaction chamber by the Ar gas flowing around the nozzle. The flow-rate of the solutions is adjusted. The final product (the carbonaceous material deposited on the wall of the reactor chamber) is analysed by means of Transmission Electron Microscopy (TEM), Scanning Tunnelling Microscopy (STM), Field Emission Scanning Electron Microscopy (FESEM) and X Ray Diffraction (XRD).

We have performed growth experiments at different values of furnace temperature, solution concentration, and solution flow-rate. The values of the investigated parameters are resumed in Table 1. In addition, we investigated as carbon source material also thiophene (sample S14), because in the literature this material is known as promoting Y-junction growth of CNTs [[17]]. The parameters of this growth process were: 1 ml/min solution flow-rate, 3 g ferrocene/ 50 ml thiophene solution concentration, 875ºC.

Results and Discussion

Systematic ana-lysis of TEM images of the samples makes possible to determine the most suitable values of the investigated parameters. These are: 875–925°C temperature range, ~1 ml/min ferrocene-benzene solu-tion flow-rate, ~3 g ferrocene in 50 ml benzene catalyst concentration, while in all experiments a constant Ar flow-rate of 500l/h was maintained. TEM and FESEM images of the most characteristic samples are given in Fig. 1–3. The TEM images also indicate that the samples, beside the CNTs, contain impurities as: Fe, Fe3C, and a small amount of amorphous carbon (as byproducts). A considerable part of byproducts may be removed by heating the samples first in dilute nitric acid for several hours, than in distilled water, finally the samples are washed with distilled water. The purification process do not affect the catalyst particles encapsulated in the CNTs (compare Fig. 1 a) with b)). Fig. 2 (FESEM image of sample S9) indicates that the sample contains large areas of aligned CNT films. The diameters of MWCNTs decrease with increasing growth temperature. It is probable, that the SWCNTs visibles in STM appears as the result of an overgrow of fine particles and fibrous materials which appears on the top of the aligned nanotube films, and this overgrown material contains the SWCNTs [[18]]. Fig. 3 gives a characteristic TEM image of sample S14, obtained using thiophene as carbon source material and ferrocene as catalyst. Thus, we have obtained Y-type CNTs, which confirm that the presence of sulphur atoms promote the formation of ramified CNTs [17].

Conclusions

The used spray-pyrolysis method yields both MWCNTs and SWCNTs. The prepared samples contain also bundles of well aligned CNTs. Probable the SWCNTs are included in the overgrown material which appears on the top of the aligned CNT films. Pyrolysis of thiophene with ferrocene yields Y-junction CNTs.

The purification of the samples in hot, dilute nitric acid and in distilled water eliminates the byproducts as Fe, Fe3C deposited between the tubes, but the impurities encapsulated in the tubes are not affected. The spray-pyrolysis method is a promising technique for production of large quantities of CNTs.

REFERENCES

[1]. S. Iijima, Nature 354 (1991) 56.

[2]. Q. H. Wang, I. D. Corrigan, J. Y. Dai, R. P. H. Chang, A. R. Krauss, Appl. Phys. Lett. 70 (1997) 3308.

[3]. H. Y. Dai, J. H. Hafner, A. G. Rienzler, D. T. Colben, R. F. Smally, Nature 384 (1996) 147.

[4]. T. W. Ebbesen, H. J. Lezec, H. Hiuru, J. W. Bennett, H. F. Gaemi, T. Thio, Nature 382 (1996) 54.

[5]. S. Dong, X. Zhang, Trans. Nonferrous Metal Soc. China 9 (1999) 457.

[6]. T. Kuzumaki, K. Miyazawa, H. Ichinose, K. Ito, J. Mater. Res. 13 (1998) 2445.

[7]. P. Calvert, Nature 399 (1999) 210.

[8]. J. R. Wood, I. L. D. Wagner, Appl. Phys. Lett. 76 (2000) 2883.

[9]. J. Kong, N. R. Franklin, C. W. Zhon, M. G. Chapline, S. Peng, K. Cho, H. Y. Dai, Science 287 (2000) 622.

[10]. T. W. Ebbesen, P. M. Ajayan, Nature 358 (1992) 220.

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

[12]. M. Joseyacaman, M. Mikiyoshida, L. Rendon, J. G. Santiesteban, Appl. Phys. Lett. 62 (1993) 657.

[13]. M. Endo, K. Takeuchi, K. Kobori, K. Takahashi, H. W. Kroto, A. Sarkar, Carbon 33 (1995) 873.

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

[15]. C. Journet, T. Bernier, Appl. Phys. A 67 (1998) 1.

[16]. R. Sen, A. Govindaraj, C. N. R. Rao, Chem. Phys. Lett. 267 (1997) 276.

[17]. F. L. Deepak, A. Govindaraj, C. N. R. Rao, Chem. Phys. Lett. 345 (2001) 5.

[18]. Ch. Singh , M. S. P. Shaffer, A. H. Windle, Carbon 41 (2003) 359.