Since Iijima and Ichihashi 1 and Bethune et al. 2 reported the discovery of single-walled carbon nanotubes (SWNTs), there has been much fundamental and applied research on their potential applications. Several methods have been established to synthesize SWNTs, such as arc discharge 3, laser ablation 4, catalytic decomposition of carbon-containing molecules such as hydrocarbon 5 and CO 6,7, etc. Multi-walled carbon nanotubes (MWNTs) and SWNTs can now be synthesized by these methods in commercial quantities, but it is still a challenge to synthesize carbon nanotubes with a certain number of walls. Double-walled carbon nanotubes (DWNTs) have been synthesized by electron irradiation 8 or heat-treatment from the 'peapod' nanotubes 9, hydrogen arc discharge method 10, and recently by CVD 11 and the floating catalysis method 12-14. These studies indicate that it is possible to control the number of walls in the synthesis process. In the present work, we designed a new process in which tetraethoxysilane (TEOS) and ferrocene were introduced as precursors and decomposed to form in situ SiO_2 and Fe composite particles that act as a catalyst for the formation of DWNTs.
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