“Wash and go”: sodium chloride as an easily removable catalyst support for the synthesis of carbon nanotubes

Abstract

A sodium chloride supported cobalt catalyst was found to be active in the synthesis of carbon nanotubes by a CCVD method.

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Introduction

Carbon nanotubes (CNTs) are widely considered as promising materials both in nanochemistry and nanoelectronics.^1–3 New generations of composite materials, semiconductor circuits or TV screens based upon CNT technology are under construction and very near to being realized.⁴ However, in order to appear on the market and later in the households, these products as well as their components must be cheap enough.

In contrast to the laser ablation⁵ and arc discharge⁶ techniques catalytic chemical vapour deposition (CCVD) is able to produce nanotubes in industrial quantities. In the laboratory scale C₂H₂ or C₂H₄ as carbon sources, cobalt, iron or nickel as catalyst particles, and high surface area SiO₂, Al₂O₃ or zeolite supports are commonly used for the synthesis of multiwalled nanotubes (MWNTs).^7–11 After the synthesis process nanotubes must be removed from the catalyst. This means strong acidic (HF in the case of zeolites) or basic (concentrated NaOH in the case of Al₂O₃ and SiO₂) treatment in most of the cases. Applying a water soluble catalyst support could make the purification step easier controllable, more economical, and environmentally friendly.

Experimental

Sodium chloride (2 g, ACROS) was ground into a fine powder in a mortar, and impregnated with a methanolic solution of 0.1198 g of Co(NO₃)₂·6H₂O (Vel). The materials were sonicated in an ultrasound bath for 10 min. The solvent was evaporated under vacuum at 80 °C by a rotary evaporator. The sample was dried at 120 °C for 5 h and ground again into a fine powder.

1 g of the catalyst prepared this way was placed in a quartz boat and slipped into a horizontal quartz tube reactor with a diameter of about 5 cm. After purging the system by N₂ at room temperature the reaction was carried out at 700 °C for 30 min in a 10 ∶ 100 C₂H₂–N₂ mixture with a total flow rate of 330 ml min⁻¹. The original light violet colour of the catalyst changed to dark grey during the reaction, caused by the appearance of carbon species on the surface. The amount of carbon deposite was determined by thermal analysis on a Netzsch STA 409PC Luxx thermogravimeter. The resulting carbon forms were characterised with a Philips XL20 scanning electron microscope before, and with a Philips Tecnai transmission electron microscope after removing the sodium chloride support by washing with distilled water.

Results and discussion

Results of the TG analysis showed a weight loss of 2.6% in the 300–700 °C temperature range, which is attributed to the burning of different carbon species. This means about 200% carbon deposit relative to the weight of the cobalt particles.

SEM investigations verifyed the presence of nanotubes in the sample. Carbon nanotubes winding from the surface of catalyst particles can be seen in Fig. 1.

Fig. 1 SEM picture of the carbon nanotubes formed on a NaCl supported Co catalyst, before purification.

Fig. 2a and 2b show the TEM pictures of the purified sample. Next to the encapsulated cobalt particles well graphitized MWNTs with rather large diameter (up to 30 nm) can be observed. Coiled carbon nanotubes were also formed.

Fig. 2 TEM images of CNTs grown on the surface of a NaCl supported Co catalyst, after dissolving the support material.

In spite of its low specific surface area, melting point (about 810 °C), and unfavourable thermal conductivity¹² sodium chloride can support dispersed cobalt particles suitable for building of CNTs. In order to avoid the formation of small metal islands stemming from the aggregation of metal particles,¹² one has to use low cobalt concentrations and to avoid the preliminary reduction step.

Employing the very reactive C₂H₂ as the carbon source makes possible the use of a synthesis temperature of 100 °C below the melting point of the support.

Conclusion

In contrast with the results of other authors¹² we have proved the possibility of CNT synthesis by CCVD method on cobalt particles supported on sodium chloride. The support material can be quickly and easily dissolved and completely removed by distilled water. Optimisation of catalyst preparation and reaction conditions will be directed towards the development of a high performance and highly recyclable CCVD catalyst system producing carbon nanotubes with low defect walls and a more uniform size distribution. Reduction should be sought in the total number of graphite coated cobalt particles and additional carbon forms produced during the reaction, because current post-synthetic purifying treatments are mostly incapable of removing them.

Acknowledgements

This work was supported by the European RTN NANOCOMP (n° HPRN-CT-2000-00037).

References

1. Ph. Avouris, Acc. Chem. Res., 2002, 35, 1026.
2. P. J. F. Harris, Carbon Nanotubes and Related Structures - New Materials for the Twenty-First Century, Cambridge University Press, Cambridge, 1999.
3. Carbon Nanotubes: Synthesis, Structure, Properties and Applications, ed. M. S. Dresselhaus, G. Dresselhaus and P. Avouris, Springer-Verlag, Berlin, 2001.
4. J. L. Kwo, Meiso Yokoyama, W. C. Wang, F. Y. Chuang and I. N. Lin, Diamond Relat. Mater., 2000, 9, 1270.
5. C. Journet, W. K. Maser, P. Bernier, A. Loiseau, M. Lamy de la Chapelle, S. Lefrant, P. Deniard, R. Lee and E. Fischer, Nature, 1997, 388, 756.
6. A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y. H. Lee, S. G. Rinzler, D. T. Colbert, C. Xu, D. Tomanek, J. Fischer and R. E. Smalley, Science, 1996, 273, 483.
7. A. Weidenkaff, S. G. Ebbinghaus, Ph. Mauron, A. Reller, Y. Zhang and A. Züttel, Mater. Sci. Eng.: C, 2002, 19, 119.
8. K. Hernadi, A. Fonseca, J. B. Nagy, A. Siska and I. Kiricsi, Appl. Catal. A: General, 2000, 2, 245.
9. N. Nagaraju, A. Fonseca, Z. Kónya and J. B. Nagy, J. Mol. Catal., 2002, 181, 57.
10. K. Hernadi, A. Fonseca, J. B. Nagy, D. Bernaerts, A. Fudala and A. A. Lucas, Zeolites, 1996, 17, 416.
11. D. Gournis, M. A. Karakassides, T. Bakas, N. Boukos and D. Petridis, Carbon, 2002, 14, 2641.
12. B. H. Liu, J. Ding, Z. Y. Zhong, Z. L. Dong, T. White and J. Y. Linm, Chem. Phys. Lett., 2002, 358, 96.

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