H2S-O2 혼합기체를 이용한 탄소나노튜브의 고수율 정제

정탁,1 한윤봉2*

1. LG Innotek, 2. 전북대 화학공학부

High Yield Purification Method of Carbon Nanotubes using H2S-O2 Mixture

Tak Jeong1 and Yoon-Bong Hahn2*

1. LG Innotek, 2. School of Chemical Engineering and Technology, Chonbuk National University, Chonju 561-756

INTRODUCTION

Carbon nanotubes were first synthesized in an arc discharge in a presence of a transition metal catalyst1,2 and have been produced by pulsed laser vaporazation3, arc discharge4, and chemical vapor deposition5. They present a wide variety of fascinating electronic and mechanical properties. The proposed future applications of this unique material include a good conductor6, quantum wires7, nano-devices7, field effect transistors8, nanoprobes, energy storage or conversion, and many others9.

The carbon nanotubes produced by all above methods generally contain large amount of impurities such as graphite, amorphous carbon and catalytic metal particles, resulting in a serious impediment for their detailed characterization. Although multi-walled carbon nanotubes can be purified by a variety of gas-phase oxidations,13-16 the use of a chemical purification process has not been demonstrated for single wall carbon nanotubes (SWNT). Attempts to use gas phase procedures for SWNT resulted in tube destruction. So efforts have been directed at modifying other techniques originally developed for multishell nanotubes.17,18

In this work, we present a simple and efficient method for purification of SWNT using both gas and liquid phase purification method. In order to eliminate catalytic metal particles, 3 M HCl solution was used. Thermal oxidation using hydrogen

* Author for correspondence. Email:ybhahn@moak.chonbuk.ac.kr.

sulfide and oxygen was followed to remove amorphous carbon and carbon nanoparticles. After purification, the purified SWNT were examined by SEM (scanning electron microscopy) equipped with an EDX (energy dispersive X-ray analysis system), and by Raman spectroscopy.

EXPERIMENTAL

The preparation of the SWNT used for this report was produced by the arc-discharge method. A graphite rod containing catalyst paticles (Ni:Co:Fe/1:1:1) powder was vaporized under a Helium atmosphere at 100 Torr. To remove metal residue particles, the raw sample (100 mg) of SWNT was suspended in 3 M HCl solution at room temperature and refluxed for 48 hrs. Then, the supernatant liquid was decanted off and the black sediment was resuspended in deionized water. This procedure was performed more than 3 times and the sample was dried at 150 ℃ for 24 hrs. In order to remove amorphous carbon and nanopaticles, 10 mg of SWNTs were then oxidized at 500 ℃ for 1 hr using hydrogen sulfide and oxygen. The gas ratios used for purification were 10 ml H2S/min and 20 ml O2/min which was the optimum ratio obtained in this work. The oxidized samples were suspended in 3 M HCl solution for 2 hrs to remove any metal particles that were still present after the oxidation process. After this, the samples were finally dried at 150 ℃ for 24 hrs and weighed for yield data.

RESULTS AND DISCUSSION

Typical micrographs of SEM taken at the raw SWNT sample are shown in Fig. 1 (a). As is well known, the coexistence of SWNTs, a large amount of amorphous carbon and metal particles embedded into the amorphous carbon are observed. The amount of SWNTs was estimated at less than 40 %. After acid treatment and thermal oxidation using H2S and O2, very high density of SWNTs were observed at random orientation with no carbon nanoparticles and amorphous carbon but with a few of metal particles, as shown in Fig. 1 (b). We believe that some of oxygen react with amorphous carbon, then result in formation of carbon dioxide or carbon mono-oxide and the lest of oxygen reacts with hydrogen sulfide and amorphous carbon, resulting in formation of carbon oxysulfide (COS) and water vapor. As a result, it appeared that hydrogen sulfide plays a role of a gas controling the rate of oxidation of oxygen as well as enhancing the removal of carbon particles.

Fig. 2 shows Raman spectroscopy of each step of the purification process. The strong peaks observed at 1590 cm-1 are related to the Raman-allowed phonon mode E2g which involves out-of-phase intra-layer displacement in graphite, while the peaks at around 1290 cm-1 are attributed to amorphous carbon.20 After acid treatment for 48 hr, the peak observed at 1590 cm-1 is somewhat reduced as shown in Fig. 2 (b). This might be explained by the fact that since carbon nanotubes stick to the catalyst surface very strongly, refluxing nanotubes in the presence of an oxidizing acid for a long time leads to breaking of nanotubes21 (probably at those places where the structure of their wall was not perfectly graphitized). However, after thermal oxidation for 1 hr using H2S and O2 mixture, the peak intensity at 1590 cm-1 is substantially increased, while the broad peak obseved at 1290 cm-1 is reduced (Fig. 2 (c)). The increase in the peak intensity at 1590 cm-1 and the relative decrease in the peak intensity at 1290 cm-1 mean that the thermal oxidation is quite effective and there is high dispersion of SWNTs which are highly graphitized. To ensure removing any metal particles remained from SWNT, we subjected them to further acid treatment for 2 hrs after thermal oxidation (Fig. 2 (d)). However, this did not yield a better Raman spectroscopy compared to Fig. 2 (c).

CONCLUSIONS

We developed a purification process of carbon nanotubes combining both acid treatment  and gas phase oxidation method which yield very high purity (>95 %). Thermal oxidation method using H2S and O2 was applied to preferentially remove amorphous carbon and nanoparticles. After the acid treatment and thermal oxidation at 500℃ for 1 h, high density single-walled nanotubes were observed with no carbon nanoparticles and amorphous carbon.

REFERENCES

1. S. Iijima, T. Ichihashi, Nature(London) 363, 603 (1993).

2. D. S. Bethune et al., Nature (London) 363, 605 (1993).

3. A. Thess et al., Science, 273, 483 (1996).

4. S. Iijima, Nature, 354, 56 (1991).

Fig. 1. SEM micrographs of (a) raw and (b) purified SWNTs.

(a)

(b)

Fig. 2. Raman spectra of SWNT of each purification step: (a)     raw sample, (b) after acid treatment for 48 hr, (c) after thermal   oxidation, and (d) after acid treatment for 2 hr.