Effect of polymer and solvent on purification and cutting of single-wall carbon nanotubes

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Abstract

Following our recent report that it is possible to purify and cut SWNTs by using a saw-edged homogenizer in a monochlorobenzene (MCB) solution of polymethylmethacrylate (PMMA) [Appl. Phys. A 71 (2000) 451], we report here our investigation of the effect of PMMA and MCB on the purification and cutting of SWNTs by thermogravimetric analyses. Our results show that MCB helps the a-C aggregation states to homogenize during the ultrasonic treatment. The PMMA helps to make bundles of SWNTs thinner and accelerates both the cutting and damaging of them.

Introduction

Single-wall carbon nanotubes (SWNTs) [1] have attracted a great deal of attention because of their unique properties and potential applications. However, as-grown SWNTs obtained by laser ablation or arc-discharge are impure and tangle together to form mats. This means that pure, short, and individual SWNTs have to be prepared before they can be applied in fields such as molecular-scale electrical devices and their intrinsic properties can be studied. Recently, Shelimov et al. [2] proposed a method of purifying SWNTs by ultrasonically assisted filtration. Their results showed that the purified nanotubes are shorter than pristine samples, but nanotube bundles become thicker and still tangle together. Liu et al. [3] purified SWNTs by using HNO3 and then cut them by sonication in a 3:1 mixture of concentrated H2SO4/HNO3 solution for 24 h. However, even in aqueous suspensions stabilized with the help of surfactant, the cut tubes were in the form of bundles, containing up to a few dozen individual nanotubes [4]. In addition, we think this method is not good from the viewpoints of safety and the environment. Further research on obtaining short and thin bundles of SWNTs or individual nanotubes is required.

Recently, we reported that we could purify and cut SWNTs by using a saw-edged homogenizer in a monochlorobenzene (MCB) solution of polymethylmethacrylate (PMMA) followed by filtration [5]. We showed that the number of short and thin bundles of SWNTs was increased by using this method. In this Letter, we confirm the effect of an MCB solution of PMMA on purification and cutting SWNTs by using ultrasonic processing and further investigate the functions of PMMA and MCB in this process by thermogravimetric analyses (TGA).

Section snippets

Experimental

The SWNTs used in this study were synthesized by Nd:YAG laser ablation. The target was prepared by pressing mixtures of powder graphite, Ni, and Co. The concentrations of Ni and Co were 0.3 at.%. The laser ablation conditions were the same as those previously reported [6], [7]. About 1 mg of as-grown SWNTs was mixed with about 4 ml of MCB solution containing 2% PMMA (hereafter called PMMA–MCB solution). Then the mixtures were stirred with a magnetic stirrer at 1300 rpm for 1 h followed by

Results

The scanning electron microscopy (SEM) image of Filtrate (MCB) (Fig. 1a) showed there were many impurities coexisting with SWNT-bundles; the SWNTs were tangled together. We could not find the tips of the SWNT-bundles easily. Fig. 1b shows the SEM images of Filtrate (PMMA–MCB). It is clear that impurities of a-C and graphite particles were mainly removed and SWNT-bundles were shortened and distributed separately. The ends of shortened SWNT-bundles can be observed clearly. This means that the

Discussion

All these results indicate that a-C mainly burned between 400 and 530 °C and that the SWNTs mainly burned above 530 °C when as-grown SWNTs were treated with MCB or PMMA–MCB solution. The burning of carbonaceous material in SWNTs has been changed mainly due to MCB. One reason is that the aggregation states of a-C can be homogenized during ultrasonic treatment. This leads to a-C burning in a narrow temperature range and a-C and SWNTs can burn in two distinct steps. Another reason is that the MCB

Conclusion

Our results show that MCB changes the burning of carbonaceous in SWNTs and makes a-C and SWNTs burn in two distinct steps, since MCB helps to homogenize the a-C aggregation states during the ultrasonic treatment. We suspect that a chemical reaction between MCB and a-C and/or SWNTs is another reason. PMMA mainly helps to make bundles of SWNTs thinner and accelerates the cutting of SWNTs.

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