High specific surface area carbon nanotubes from catalytic chemical vapor deposition process

doi:10.1016/S0009-2614(00)00558-3

Chemical Physics Letters

Volume 323, Issues 5–6, 23 June 2000, Pages 566-571

https://doi.org/10.1016/S0009-2614(00)00558-3 Get rights and content

Abstract

A carbon nanotube specimen with a carbon content of 83 wt.% (95 vol.%) and a specific surface area equal to 790 m²/g (corresponding to 948 m²/g of carbon) is prepared by a catalytic chemical vapor deposition method. The nanotubes, 90% of which are single- and double-walled, are individual rather than in bundles. High-resolution electron microscopy shows a diameter distribution in the range 0.8–5 nm and Raman spectroscopy shows a high proportion of tubular carbon. Both techniques reveal a maximum in the inner wall diameter distribution close to 1.2 nm.

Introduction

Non-bundled carbon nanotubes (CNT) with a high specific surface area may be interesting for such applications as electromechanical actuators [1] and as media for H₂ storage [2]. Ye et al. [2] have indeed reported that the H₂ adsorption capacity of cut single-walled carbon nanotubes (SWNT) exceeded 8 wt.% at 40 bar at 80 K. The increase in adsorption at high pressures was attributed to the separation of the individual SWNTs in the bundle, thus increasing the specific surface area.

In this Letter, we report the synthesis of high specific surface area CNTs by a catalytic chemical vapour deposition (CCVD) method which could be efficient for the low-cost large-scale production of CNTs. In previous works, the present laboratory has reported the synthesis of composite powders containing well-dispersed CNTs by the selective reduction in H₂–CH₄ of solid solutions between one or more transition metal oxides and a non-reducible oxide such as Al₂O₃[3], MgAl₂O₄[4] or MgO [5]. The reduction produces very small metal (Fe, Co, Ni and their alloys) nanoparticles at temperatures higher than 800°C. The decomposition of CH₄ over the freshly formed nanoparticles prevents their further growth resulting in the very strong proportion of SWNTs and double-walled carbon nanotubes (DWNT) compared to other forms of carbon. Other researchers have also reported the formation of SWNTs on metal nanoparticles supported on graphite flakes or oxide substrates based on Al₂O₃ and SiO₂ by the catalytic decomposition of carbon monoxide [6] or hydrocarbons 7, 8, 9, 10.

MgO presents the advantage over other substrates that it can be easily dissolved by a mild acid treatment. Recently, we have reported [5] the synthesis of SWNTs and DWNTs with diameters in the range 0.5–5 nm by the reduction of Mg_0.9Co_0.1O. After the reduction, MgO and part of the Co catalyst were dissolved in 37% HCl at room temperature which allowed the CNTs to be separated without damage. The carbon content was 64.5 wt.% (87.5% vol.) and the remaining Co particles were either at the tube tips or encapsulated by a few graphene layers which protected it from dissolution [5]. In order to increase the yield and the purity, we have increased the specific surface area of the precursor oxide and varied its cobalt content. Specimens containing 83 wt.% carbon (i.e. 95 vol.%) and with a specific surface area of 790 m²/g (corresponding to 948 m²/g of carbon) are obtained in the present work.

Section snippets

Experimental

Mg_1−xCo_xO (x=0.025, 0.05, 0.10, 0.15 and 0.20) solid solutions were prepared by combustion synthesis [11]. The experimental conditions were adjusted to prepare monophased products with a high specific surface area. Nitrates of magnesium and cobalt in the appropriate ratios were mixed with urea and the mixture was introduced into a furnace maintained at 550°C, producing the desired powders in less than 5 min. The formation of a single phase was verified by X-ray diffraction and the specific

Results and discussion

The carbon content in the composite powder (C_n) increases with cobalt content. A plateau is observed between R5 and R10 (Fig. 1). ΔS follows a similar evolution, the plateau being less marked (Fig. 1). Its origin remains unclear. SEM observations (images not shown) confirm that the quantity of CNTs increases with cobalt content, but it was noted that individual CNTs rather than bundles are obtained for lower cobalt contents. However, this is difficult to quantify. The carbon content in the

Conclusions

A CNTs specimen with a carbon content of 83 wt.% (95 vol.%) and a specific surface area equal to 790 m²/g (corresponding to 948 m²/g of carbon) has been synthesized. The specific surface area is the highest reported value to the best of our knowledge. The selective reduction in H₂–CH₄ of Mg_1−xCo_xO produces CNTs–Co–MgO powders. MgO and a fraction of the Co particles are dissolved by a mild acid treatment that does not damage the CNTs. More than 90% of the CNTs are SWNTs and DWNTS with diameters

Acknowledgements

The authors thank Dr. E. Flahaut for discussions and Mr. L. Datas for his assistance in the HREM observations, which have been performed at the Service Commun de Microscopie Electronique à Transmission - Université Paul-Sabatier.

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High specific surface area carbon nanotubes from catalytic chemical vapor deposition process

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Chem. Phys. Lett.

Chem. Phys. Lett.

Chem. Phys. Lett.

Zeolites

Science

Appl. Phys. Lett.

J. Mater. Res.

J. Mater. Chem.