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Lian Ouyang, Zujin Shi, and Zhennan Gu College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China (Received 13 March 2003; accepted 16 July 2003)

The compound growth of single-walled carbon nanotube (SWCNT) and multiwalled carbon nanotube (MWCNT), which formed a nanotube cable, was achieved by the chemical vapor deposition of natural gas on an Fe catalyst supported on SiO2–Al2O3 hybrid materials at 950 °C. The microstructure of nanotubes was characterized by high-resolution transmission electron microscopy (HRTEM). The SWCNTs encapsulated inside MWCNTs can be two, three, or even more in quantity with a diameter range from 1.0 nm to 2.0 nm. The diameter of SWCNT is controlled by the size of the catalyst nanoparticles. Some bundles of SWCNT and double-walled nanotubes were also found. The possible mechanism of compound growth is briefly discussed.

I. INTRODUCTION

The discovery of carbon nanotube has stimulated extensive theoretical and experimental research.1 Most experimental research is focusing on synthesizing carbon nanotube that has high yield and is well aligned. So far, the main methods used to synthesize carbon nanotube can be classified into three classes: arc-discharge,2,3 laser ablation, 4 and catalytic decomposition of carboncontaining gas on the surface of certain transition metals chemical vapor deposition (CVD). In view of practical applications, which require yields and quality of nanotubes, catalytic decomposition seems to be a promising and potential method, although the first two methods can provide a high-quality nanotube. Significant progress has been made in controlling the growth of multiwalled carbon nanotubes (MWCNTs) using CVD methods. Long and well aligned MWCNTs on large-scale substrates have been synthesized.1,5–8 However, the growth of single-walled carbon nanotube (SWCNT) in a large-scale and ordered architecture remains a challenging task although SWCNTs exhibit unique mechanical, electronic, and magnetic properties. Some have reported the bulk growth of SWCNT since the first development of a CVD synthesis approach to high-quality SWCNTs using methane.9 Several hydrocarbon feedstocks were used to a)

Address all correspondence to this author. Present address: Max-Planck-Institut fu¨ r Metallforschung, Heisenbergstr. 3, D-70569 Stuttgart, Germany. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 10, Oct 2003

sythesize the carbon nanotube, such as CVD of benzene,10 ethylene,11 or methane/hydrogen mixture.12 Peigney et al.13 reported a mixture of single- and multiwalled nanotubes resulting from decomposition of CH4 at 1050 °C on an alumina-supported Fe catalyst. A great effort has also been made to produce bulk SWCNTs that indicate that the type of support materials for the catalyst play a key role in the type of SWCNT.14 More recently, direct synthesis of long SWCNT strands has been achieved by optimized catalytic CVD.15 In this paper, we used an Fe catalyst supported on SiO2–Al2O3 hybrid materials and natural gas