Elsevier

Chemical Physics Letters

Volume 296, Issues 1–2, 30 October 1998, Pages 195-202
Chemical Physics Letters

Catalytic growth of single-wall carbon nanotubes from metal particles

https://doi.org/10.1016/S0009-2614(98)01024-0Get rights and content

Abstract

Single-walled carbon nanotubes (SWNTs) have been synthesized at milligram per hour rates by the catalytic decomposition of both carbon monoxide and ethylene over a supported metal catalyst known to produce larger multiwalled nanotubes. Under certain conditions, there is no termination of nanotube growth, and production appears to be limited only by the diffusion of reactant gas through the product nanotube mat that covers the catalyst. Further development of the catalyst geometry to overcome the diffusion limitation may allow bulk catalytic production of SWNTs by supported metal catalysts.

Introduction

Single-wall carbon nanotubes (SWNTs) have been previously produced in high yield by laser vaporization of a graphite rod doped with Co and Ni [1]. These high-quality samples have for the first time enabled experimental confirmation of the structurally dependent properties predicted for carbon nanotubes 2, 3. High-quality SWNTs have also been generated by arc evaporation of a graphite rod doped with Y and Ni [4]. These techniques allow production of only gram quantities of SWNTs. Another way to synthesize nanotubes is by catalytic decomposition of a carbon-containing gas by nanometer-scale metal particles supported on a substrate. The carbon feedstock molecules decompose on the particle surface, and the resulting carbon atoms then diffuse through the particle and precipitate as a part of nanotube from one side of the particle. This procedure typically produces imperfect multiwalled nanotubes in high yield [5]. Its advantage is that it is relatively simple and can be scaled to produce nanotubes by the kilogram [6]. Clearly, a method of making high-quality SWNTs by catalytic decomposition could lead to economic production of SWNTs in bulk.

There are two reports of SWNTs produced by catalytic decomposition of stable gas-phase carbon-containing molecules on pre-formed catalytic particles. Peigney et al. reported a mixture of single- and multiwalled nanotubes resulting from decomposition of CH4 at 1050°C on an alumina-supported Fe catalyst [7]. They do not report the relative amounts of single- and multiwalled nanotubes. Dai et al. reported the growth of SWNT and a small amount of double-wall nanotubes (DWNTs) by disproportionation of CO on alumina-supported Mo particles at 1200°C [8]. The nanotubes had diameters ranging from 1 to 5 nm and were 100 nm to microns in length, but grew in low yield and appeared by transmission electron microscopy (TEM) observation to be more defective than SWNT formed by the laser vaporization or arc techniques.

We report here the production of high-quality SWNTs, in some cases including DWNT, in yields much larger than previously reported by catalytic decomposition of carbon-containing precursor gases. Under certain conditions, it appears that SWNTs can grow continuously to arbitrary length. Our results demonstrate a means for nucleating and growing nanotubes only from the smallest of the supported catalyst particles, which produce SWNTs, while deactivating the larger particles so that no multiwalled nanotubes are produced. This allows the growth exclusively of SWNTs from catalyst systems previously thought to produce only larger-diameter multiwalled nanotubes.

Section snippets

Experimental

SWNTs are grown by passing carbon-containing gases (CO or C2H4) at elevated temperatures over nanometer-size metal particles supported on larger (10–20 nm) alumina particles. Two different metal catalysts have been used, one containing pure Mo, the other containing Fe and Mo in a ratio of 9:1. Both catalysts are made using a method described previously [5]. Briefly, fumed alumina (Degussa) is stirred with methanol, and to the resulting slurry is added a methanol solution of metal salts (ferric

Results

The production of SWNTs by the disproportionation of CO over alumina-supported Mo particles has been greatly improved since an initial report by this laboratory [8]. The catalyst is 34:1 alumina:Mo by mass. The reaction is carried out at 850°C under a flow of 1200 sccm of CO at 900 Torr. The resulting material, which consists of SWNT very monodisperse in diameter (0.8–0.9 nm), is shown in Fig. 1. Particles of the fumed alumina support, 10–20 nm in size, are also visible in this and subsequent

Discussion

The most remarkable aspect of this series of experiments is that, in every case, only SWNTs and sometimes DWNTs are produced with diameters in the range 0.5–3 nm. We see no 5–20 nm diameter multiwalled nanotubes that are typically produced by supported catalyst particles. We believe that the key difference responsible for these effects is that we have arranged for the growth reaction rate to be limited by the supply of carbon to the catalyst particles, whereas the multiwalled nanotube growth is

Conclusions

We have demonstrated the ability to grow nanotubes by catalytic decomposition of C2H4 and CO only from the small particles in a supported catalyst system, leading to the growth of SWNTs and deactivation of multiwalled nanotube growth by encapsulation of larger particles. For certain conditions, nanotubes can apparently be grown to arbitrary length, but become limited by the diffusion of reactants to the catalyst particles. This problem has been solved for the production of multiwalled nanotubes

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