НЕОРГАНИЧЕСКИЕ МАТЕРИАЛЫ, 2007, том 43, № 5, с. 546-550
УДК 546.26
The Influence of Different Atmosphere Gases on the Growth and Structure of Double-Walled Carbon Nanotubes
© 2007 r. Z. H. Li*, M. Wang**, B. Yang**, Y. B. Xu**
*Department of Mechanics Zhejiang University, Hangzhou, China **Department of Physics Zhejiang University, Hangzhou, China e-mail: miaowang@css.zju.edu.cn Received 31.03.2006
To clarify the impact of different atmosphere gases on the growth and structure of double-walled carbon nanotubes (DWCNT), we used a graphite rod containing some catalyst as our anode and prepared the DWCNT with the arc-discharge method in an atmosphere of pure Ar, pure H2, and the mixture (l : 1) of Ar and H2 at different pressures, respectively. The Fe family metal-sulfide composite (FeS, NiS, CoS) used as the catalyst was mixed with high-purity graphite powder in a certain proportion and was then enclosed into a drilled hole in the anode. By altering the discharge conditions and examining the product of the DWCNT using high-resolution electron microscopy, we found the optimum growth condition to be the atmosphere of mixed (1 : 1) Ar and H2, and the ratio (FeS : NiS : CoS : C = 1 : 1 : 1 : 15 wt. %) for the catalyst of the Fe family metal-sulfide composite.
INTRODUCTION
After being discovered by Iijima [1] in 1991, carbon nanotubes (CNT) became a popular topic and research subject among scientists worldwide. As a frontier material, CNT is often the topic of research and applications in nanostructure electronic devices, scanning tunneling microscopes (STM), energy-storage materials, and high-strength composites. In some fields, such as STMs and cathode materials of field emission devices (FED), research and applications have moved into the practical phase [2, 3].
Recently a new form of CNT, the double-walled carbon nanotubes (DWCNT) consisting of two concentric cylindrical graphene layers, has attracted increased attention [4-9]. This is due to the fact that DWCNTs are the ideal and simplest candidates for performing both theoretical and experimental investigations of the CNT's other than single-walled carbon nanotubes (SWCNT) [10]. Because DWCNT is also an advanced material, it's important to know the optimum conditions for its synthesis.
It has been reported that in the atmosphere of He, SWCNT can be prepared with the arc-discharge method, using CoS as catalyst. The experiment result shows that this process can only provide a minute quantity of DWCNT [11].
Hutchison et al. [4] first reported the preparation of DWCNTs as the dominant products of the arc-discharge technique with a catalyst mixture of Ni, Co, Fe, and S. Selective production of high-purity DWCNTs in scale up is not easy, even with the promising chemical-vapor deposition method [8, 9].
To clarify the impact of different atmosphere gases on the growth of DWCNT, we used graphite and Fe family metal-sulfide composites as our anode and grew
Fig. 1. General views of bundles of double-walled nanotubes (DWCNTs).
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Fig. 3. HRTEM image of one DWCNT.
Fig. 2. Magnified image of Fig. 1.
DWCNT with the arc-discharge method in the atmosphere of Ar, H2 and separately in the mixture of Ar and H2. Through altering the preparation process and through comparing the results of HRTEM observation and analysis, we have found the optimum growth condition of DWCNT.
EXPERIMENTS
In this work, we adopted the arc-discharge method [12] to obtain DWCNT in different atmosphere gases. The mixed Fe family metal-sulfide composite (FeS, NiS, CoS) is used as a three-element catalyst mixed with high-purity graphite powder in certain proportion (FeS : NiS : CoS : C = 1 : 1 : 1 : 15 wt. %). The anode was a graphite rod (6.0 mm in diameter and 50 mm in length) with a drilled channel (3.2 mm in diameter and 30 mm in length) filled with the catalyst powder. As for the cathode, we utilized a high-purity graphite rod (with a diameter of about 10 mm) which is movable from the exterior to the interior. After exhausting the chamber, the vacuum valve was turned off and the gas was let in up to certain pressure between 13.33-80 kPa. We switched on the power and then adjusted the distance between the cathode and anode when the arc-discharge was taking place in the low-pressure gas. The discharge current was kept between 50-70 A. The whole run was completed in a few minutes. After full water cooling, we collected the reaction product around the interior wall and the cathode and then changed the (Ar, H2, Ar /H2) gases, the pressure, and the discharge current to start a new run. Finally, we utilized HRTEM to observe and analyze every reaction product. Thus, we could find the optimum growth conditions.
RESULTS AND ANALYSIS
In the experiment, we selected different atmosphere gases. Ar, H2, and their mixture with other conditions were unchanged. The results showed that in the atmosphere of Ar, there is nearly no DWCNT, but there is SWCNT; in the atmosphere of H2 there is a small quan-
tity of DWCNT; while in the atmosphere of Ar and H2 mixture, we could get the highest DWCNT fraction in the product. A general view of the specimen containing DWCNT is shown in Fig. 1. This figure shows the HR-TEM image of the sample prepared in the atmosphere of Ar and H2 mixture at the partial pressures 13330/13330 Pa and discharge current 50 A. We can see from the general view that there are bundles of double-walled nanotubes (DWCNT). Their outer diameter
Fig. 4. Separate DWCNT with varying diameter.
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Fig. 5. The third layer of a CNT is visible.
is about 3-4 nm and the length of tubes in bundle is about 1 ^m. Figure 2 is a more-magnified image of Fig. 1. The HRTEM micrograph reveals that most of the tubules had a specific structure and consisted of two concentric cylindrical layers of graphene. The regions of distortion in the DWCNT's are marked by large arrows.
In Fig. 3, one DWCNT with outer tube diameter of 3.3 nm and inner tube diameter of 2.5 nm were observed with the interlayer distance on about 0.40 nm, which is 17% greater than the interlayer distance of multiwalled nanotubes (MWNTs) (0.34 nm) [10, 13]. Amorphous carbon materials are observed both inside and outside the tube. A single DWCNT with varying diameters is shown in Fig. 4 and a DWCNT with a third layer is visible in Fig. 5. Some forms of amorphous carbon material are observed both inside and outside the tube. At the arrow in Fig. 6 amorphous carbon materials are observed outside the tube. In Fig. 7, DWCNT has a rounded double-layer terminating cap at the end. In
Fig. 7, DWCNT has a rounded double-layer terminating cap at the end.
Figure 8 is the Raman spectrum of a DWCNT excited by a 488 nm laser. The peak G at 1580 cm-1 is caused by the vibration parallel to the graphite slice, i.e. the walls of the DWCNT corresponding to the E2g vibration mode of the graphene sheet. Peak D at 1350 cm-1 probably corresponds to the local vibrations around the disorders in DWCNTs or to the sp2 hybrid peak of amorphous carbon. In the low-frequency area of the Raman spectrum, we can not find the peak of the breathing mode for DWCNT [9, 14, 15]. This is because the diameter of DWCNT is 3-4 nm and the low-frequency characteristic peak, according to its diameter, is between 60-80 cm-1, while the wave number of the present Raman apparatus is beyond 100 cm-1.
In the experiment, we selected different atmosphere gases. Ar, H2, and their mixture with other conditions were unchanged. The results showed that in the atmosphere of Ar, there is nearly no DWCNT, but just SWCNT; and in the atmosphere of H2 there is a small quantity of DWCNT; while in the atmosphere of Ar and H2 mixture, we could get the highest DWCNT fraction in the product. From these experimental results, a growth mechanism of the DWCNT can be speculated as follows. Since hydrogen plays an important rate in the preparation of DWCNT, it seems that some hydrocarbons are formed at first from C atoms and H atoms in the arc, and them decomposed to produce DWCNTs. Moreover, because the temperature of the hydrogen arc and the density of C ions are high compared with the arc in other gases, high-grade carbon nanotubes can more easily be obtained on from the carbon monomer and dimmer, C and C2, which also exist in the electric arc.
Fig. 6. Amorphous carbon material is observed outside the
tube, by the arrow. Fig. 7. DWCNT with rounded end cap.
Ramam Shift, cm 1 Fig. 8. Raman spectrum of the DWCNTs product.
Why is Ar better than He in the growth of DWCNT? Due to its larger mass the cooling effect of Ar is weaker than He. We think that with Ar the hydrocarbons can stay longer at high temperature so that most of them will be decomposed.
CONCLUSION
When we use the Fe family metal-sulfide composite as a catalyst and carry out our preparation in the atmosphere of H2, the DWCNT content begins to rise. In the atmosphere of the Ar and H2 mixture, the DWCNT content in the product increases dramatically. From the HRTEM images, we can see that most CNTs have double walls, and those with three or more layers have been scarcely been observed. Moreover, when we use the Fe family element as a catalyst and discharge in H2, no DWCNT is discovered. Therefore, to prepare DWCNT, the Fe family metal-sulfide composite and the atmosphere of mixture gas Ar and H2 mixture gas at partial pressures 13330/13330 Pa are the optimum growth conditions.
The Project Supported by National Natural Science Foundation of China Under Grant Nos 60271009.
REFERENCES
1. Iijima S. Helical Microtubules of Graphitic Carbon // Nature. 1991. V. 354. № 7. P. 56-58.
2. Saito Y, Hata K, Takakura A. et al. Field Emission of Carbon Nanotubes and its Application as Electron Sources of Ultra-High Luminance Light-Source Devices // Physica B. 2002. V. 323. № 1-4. P. 30-37.
3. Saito Y, Mizushima R, Hata K. Field Ion Microscopy of Multiwa
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