Clean double-walled carbon nanotubes synthesized by CVD

doi:10.1016/S0009-2614(02)01862-6

Chemical Physics Letters

Volume 368, Issues 3–4, 17 January 2003, Pages 299-306

https://doi.org/10.1016/S0009-2614(02)01862-6 Get rights and content

Abstract

High quality double-walled carbon nanotubes (DWNTs) were synthesized by decomposition of methane over cobalt (Co) nanoparticles supported on porous MgO nanoparticles. The growth was significantly influenced by catalyst concentration and MgO type. Catalysts with 2.5–5 wt% Co loaded MgO (pore size: ∼4 nm) efficiently grow DWNTs with diameters of 2–4 nm, high graphitization, clean surfaces, and clean inside channels. The preliminary separation of DWNTs from MgO, Co and other carbonaceous nanoparticles has been carried out. Energy dispersive X-ray spectroscopy and X-ray diffraction were employed to analyze the chemical composition of the materials before and after purification.

Introduction

Carbon nanotubes [1] have attracted a great deal of interest because of their promising properties and potential applications. Single-walled tubes (SWNTs) can be either metallic or semiconducting or semimetallic, depending on their microstructures [2], [3], [4], [5]. However, the physical and chemical properties of multi-walled carbon nanotubes (MWNTs) are more complicated due to the interaction or coupling between the constituent layers [6], [7]. For investigating the interaction or coupling behavior between different layers, DWNTs are the ideal and simplest candidates for performing both theoretical and experimental investigation. Saito et al. [8] have shown theoretically that the inner and outer layers in a DWNT can be metal–metal, metal–semiconducting, or semiconducting–semiconducting pairs, and that the formation of the DWNTs are determined by the spacing between the two layers rather than by the chiralites. The directions in which easy motion can occur between the inner and outer layers in DWNTs are found to depend on the chiralities of the two layers [6], [7], [8], which will affect the mechanical and transport properties of the DWNTs. Experimentally, Flahaut et al. [9] and Bacsa et al. [10] have prepared mixtures of DWNTs and SWNTs by reducing Mg_1−xCo_xO solid solutions in a H₂–CH₄ atmosphere, the Mg_1−xCo_xO was synthesized by combustion of metal nitrates and urea. The fraction of DWNTs is about 50% in the mixed product, and the diameter of the DWNTs is about 0.5–5 nm [9]. Hutchison et al. [11] have synthesized DWNTs by arc discharge between two graphite electrodes in an atmosphere of Ar and H₂, and the graphite anode was drilled a channel and filled with catalyst of Ni, Co, Fe, and S powder. The outer diameter of the DWNTs is in the range of 1.9–5 nm. Bandow et al. [12] have prepared DWNTs by heating peapod (C₆₀ capsulated inside SWNTs) [13], [14] at 1200 °C, the C₆₀ chains inside the SWNTs coalesce to form the second (inner) layer resulting in the formation of DWNTs. In this case, the outer diameter of the DWNTs are controlled by the diameter of the original SWNTs. Ci et al. [15] have produced DWNTs by pyrolizing C₂H₂ on floating iron catalyst at 900–1100 °C, and the growth of the DWNTs is strongly dependent on the sulfur content, which can promote the formation of the DWNTs. Ren et al. [16] have synthesized DWNTs by using CH₄ as carbon source, H₂ as carrier gas, ferrocene ((C₅H₅)₂Fe) as catalyst precursor and thiophene (C₄H₄S) as sulfur source at 1100 °C.

Although DWNTs have been synthesized by a variety of methods, synthesis of clean and highly crystalline DWNTs is still a great challenge. Most of the DWNTs reported previously are either coated on the outside surface or filled inside the inner channel with amorphous carbon, which is a significant drawback for measuring the intrinsic properties of DWNTs. In this Letter, we present a method for synthesizing DWNTs with a clean outer surface and inner channel, and the tubes have a narrow distribution of small diameters. The separation of DWNTs from the catalyst and catalyst support is also described. The structure of the double-walled tubes is characterized by electron microscopy. The extremely clean surface and inside channel together with the high degree of graphitization make our DWNTs ideal both for investigating the interlayer interactions and for exploring the possible applications in of nanotubes in nanoelectronics.

Section snippets

Experimental

The experiments were carried out in a conventional horizontal tube furnace with a quartz tube (of 46 mm inner diameter) as the reaction chamber. The catalyst used for growing the DWNTs is prepared by impregnating MgO powder with cobalt nitrate ethyl alcohol solution. MgO powders (Nantek) used as catalyst support have a specific surface area (BET) of ∼ $400 m^{2} / g$ and a crystallite size of 3–4 nm and an average pore diameter of 3 nm. For loading the Co catalyst onto the MgO powder, a certain amount

Results and discussion

Growth of nanotubes has been observed with all the different Co concentrations, however, the yield and the uniformity of the nanotubes are significantly affected by the Co concentration, as shown in Fig. 1. At Co concentrations ⩽1.5 and ⩾7 wt%, a few of very fine carbon nanotubes are formed and the density is extremely low, as shown in Figs. 1a and d. But the reasons for low nanotube yields at low and high Co concentrations are different. It is found that, after growth, low Co concentration

Conclusions

In summary, we have successfully produced very uniform and clean DWNTs from Co particles dispersed on porous MgO nanoparticles. Both the surface area and pore size of the MgO nanopartilces affect the growth of DWNTs. Very uniform DWNTs have been prepared by using catalyst of Co 2.5–5 wt% loaded on MgO nanoparticles with pore size of 3 nm. The DWNTs have a very narrow diameter distribution of 2–4 nm. These high-quality DWNTs will facilitate the fundamental research on their properties and

Acknowledgments

This work is partly supported by The US Army Natick Soldier Systems Center under grants DAAD16-00-C-9227 and DAAD16-02-C-0037, partly by DoE under a grant DE-FG02-00ER45805, and partly by NSF under a grant ECS-0103012.

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Clean double-walled carbon nanotubes synthesized by CVD

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgments

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