Elsevier

Chemical Physics Letters

Volume 282, Issues 5–6, 23 January 1998, Pages 429-434
Chemical Physics Letters

Purification of single-wall carbon nanotubes by ultrasonically assisted filtration

https://doi.org/10.1016/S0009-2614(97)01265-7Get rights and content

Abstract

An efficient method for purification of single-wall carbon nanotubes (SWNT) synthesized by the laser-vaporization process has been developed. Amorphous and crystalline carbon impurities and metal particles are removed from SWNT samples by ultrasonically-assisted microfiltration. Sample sonication during the filtration prevents filter contamination and provides for a fine nanotube–nanoparticle suspension throughout the purification process. The process generates SWNT material with purity of more than 90% and yields of 30–70%, depending on the quality of the starting material. Nanotubes in purified samples are shorter than in pristine samples due to some sonication-induced nanotube cutting. Nanotube bundles in purified samples are also substantially thicker due to spontaneous nanotube alignment.

Introduction

Single-wall carbon nanotubes (SWNT) were first synthesized in an arc discharge in a presence of a transition metal catalyst 1, 2. Since then significant efforts have been directed at optimizing conditions for the arc production of SWNT 3, 4, 5, 6, 7. For example, a modification of the arc discharge process allowing production of high-quality nanotube material within a restricted region of the apparatus has recently been reported [8]. However, the overall nanotube yield for this process remains relatively low. Other methods for SWNT synthesis have also been developed. To date the most efficient SWNT production method is the laser vaporization of graphite/transition metal catalyst targets in a heated oven [9], which can produce nanotube yields of >70% [10]. SWNT syntheses by metal-catalyzed disproportionation of carbon monoxide [11]and catalytic decomposition of acetylene [12]have also been reported. The main impurities produced together with carbon nanotubes in all the above processes are multishell carbon nanocapsules (`buckyonions'), both empty and filled with the transition metal, amorphous carbon nanoparticles and fullerenes. These impurities have to be removed in order for one to be able to study intrinsic properties of carbon nanotubes.

Methods developed for the purification of multi-wall carbon nanotubes, such as gas-phase oxidation [13]or liquid-phase oxidation with strong oxidants [14], proved to be inapplicable to SWNT. Larger tube curvatures and correspondingly higher chemical reactivity compared to multi-wall tubes result in SWNT being destroyed prior to nanocapsules. It has been reported that arc-generated SWNT can be purified by oxidation in air if the material is refluxed in water prior to the oxidation [15]. However, we were unable to reproduce this result. A method for SWNT purification by liquid-phase oxidation with mild oxidants has recently been developed [16]. The oxidation is highly efficient in removing amorphous carbon from the sample, but does not remove multishell nanocapsules. Removal of the latter requires an additional filtration step. Also, a thorough multistep cleaning is required to rid purified samples of all the oxidation products because of their high affinity for nanotubes.

Bandow et al. have reported a procedure for one-step SWNT purification by microfiltration in an aqueous solution in the presence of a cationic surfactant [17]. Using this procedure they have purified a sample containing ∼76% SWNT to >90% SWNT. The advantage of this method is that nanocapsules and amorphous carbon are removed simultaneously and nanotubes are not chemically modified. However, the procedure reported by Bandow et al. (multiple filtrations with sample resuspension after each filtration) can be readily implemented only for dilute and relatively pure samples and is prohibitively slow and inefficient if large quantities of low-purity material are to be purified. Large-scale purification requires a continuous filtration process where the material is efficiently suspended throughout the filtration and the filter surface does not get contaminated by deposited material. In the purification procedure described below we have achieved these conditions by combining filtration with ultrasonication. A similar approach to purification of multi-wall carbon nanotubes has recently been reported [18]. The ultrasonically assisted filtration technique allowed us to purify up to 150 mg of SWNT soot in 3–6 h with the purity of the resulting material of >90%.

Section snippets

Experimental

SWNT-containing soot was synthesized by a two-laser vaporization of graphite targets containing 1 at.% Ni and 1 at.% Co in a 1200°C oven [10]. The soot was suspended in toluene and the suspension was filtered to extract soluble fullerenes. The toluene-insoluble fraction was then resuspended in methanol. A typical concentration of the raw material in the suspension was 1 g/l. The suspension was then transferred into a 47 mm filtration funnel. A 25.4 mm ultrasonic horn was inserted into the

Results

Fig. 1 presents electron microscope images of a raw material (a, b) and the same material after purification (c, d). The raw soot contains bundles of aligned nanotubes (`nanoropes'), but also a significant amount of amorphous carbon particles and metal particles embedded into the amorphous carbon. The amount of SWNT can be estimated at ∼50%. An average particle size is 100–200 nm, while most nanotubes are several microns to tens of microns long. This provides for a very efficient separation of

Discussion

A variety of purification methods, both chemical and physical, has been reported for multi-wall carbon nanotubes. The chemical methods rely on a relatively small difference in reactivities between multi-wall tubes and multi-shell capsules to eliminate the latter, consuming most of the former (up to 99%) at the same time. SWNT appear to be more chemically reactive than both multi-wall tubes and multi-shell capsules. Therefore oxidative treatments are successful only in removing amorphous carbon

Conclusions

We have developed an ultrasonically-assisted filtration method that allowed us to purify 100 mg quantities of raw SWNT material and achieve nanotube purity of >90% with a very little loss of nanotubes. Ultrasonication applied to samples during the filtration maintains the material in suspension and prevents cake formation on the surface of the filter. Ultrasonication also generates some nanotube ends and cut nanoropes and facilitates nanorope assembly into `superropes'.

Acknowledgements

This work was supported by the National Science Foundation (grants DMR-95-22251 and DMR-97-11785), the Advanced Technology Program of the State of Texas (grant 003604-047) and the Robert A. Welch Foundation (grant C-0689).

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