HEOPTAHHHECKHE MATEPHAttbl, 2009, moM 45, № 11, c. 1320-1322
INFLUENCE OF NH3 ATMOSPHERE ON THE GROWTH AND STRUCTURE OF CARBON NANOTUBES BY THE ARC-DISCHARGE METHOD
© 2009 Y. Jiang* , H. Wang**, X. F. Shang**, Z. H. Li**, M. Wang**
*Shenyang university of technology, Shenyang, China **Zhejiang University, Hangzhou, China e-mail :jiangyuan1008@126.com Received 14.11.2008
Using the improved arc-discharge equipment, we selected graphite as electrode and prepared the multi-wall carbon nanotubes in an atmosphere of pure NH3 at pressure 0.02 MPa and discharge current of 70 A by the arc-discharge method. The influence of NH3 atmosphere on the growth and structure of carbon nanotubes was studied. By analyzing transmission electron microscope and scanning electron microscope and scanning electron microscope micrograph, we found that a large quantity of multi-wall carbon nanotubes can be grown on the top surface and in the inner of interlayer between central part and outer part in the cathode deposits. The results show that there is no evident significant difference in the shapes and the structures under nH3 atmosphere and other atmosphere such as He, H2 et al, only the existent positions. This study demonstrates that the arc-discharge method in NH3 atmosphere is one high efficiency method in the preparation of carbon nanotubes.
INTRODUCTION
Carbon nanotubes were first discovered in the cathode deposits of arc-discharge between two graphite rods by Iijima in 1991 [1]. The carbon nanotubes with helical periodic tubular structure are composed of rolled graphite. In general, they are formed by multilayer microtubule crystal whth both sides closed diameter ranging from 1nm to tens of nanometer, and the length approximately tens of micron [2-4]. Recently, along with the quick development of research on carbon nanotubes and nanomaterials, their broad applications have gradually emerged. First, there are enormous potentials for carbon nanotubes to be used as electron sources of filed emission, such as minitype electronic element [5, 6], minitype gear wheel [7], radar wave absorbing materials, etc. Secondly, some performances of carbon nanotubes, such as its intensity, electrical conductivity, photics and magnetic performances, can be improved using chemical methods, such as substitution, diazotization, oxygenation and deoxidization, to process on the surface or the inner of nanotubes. And it is expected to function as photoconducive materials, nonlinear optics material, new luminescent materials, soft ferromagnetic materials, the ideal molecule carriers and so on. How to effectively and largely prepare carbon nanotubes demanded by different research fields remains as a hot subject of research.
To prepare carbon nanotubes with different structures is the prerequisite condition for further studies of their properties and application. At present, common preparation methods include arc-discharge method, laser evaporation graphite method and chemical-vapor gas phase deposition method, in which the arc-discharge is a kind of main preparation one used to prepare typical one dimen-
sion material. Meanwhile, the arc-discharge method has been developed and used to prepare carbon nanotubes with special structures and performances. Through selecting a compound graphite rod containing the metal catalyst as the anode, we prepared single-walled carbon nanotubes (SWCNT) [8-11], double-walled carbon nanotubes (DWCNT) [12, 13] and multi-wall carbon nanotubes (MWCNT) [14] with the arc-discharge method in different atmosphere gases.
In this paper, we used the graphite as electrode and prepared carbon nanotubes with the arc-discharge method in the atmosphere of NH3 for the first time. By observing and analyzing transmission electron microscope (TEM) and scanning electron microscope (SEM) micrographs, MWCNT were observed in the cathode deposits.
EXPERIMENTS
We improved arc-discharge equipment with single-anode into the new multi-electrode arc-discharge equipment with doule-anode, which can be synchronously installed. The cathode is a fixed graphite hemisphere with a diameter of 50 mm. As for the anode, we utilized a high-purity graphite rod (with a diameter of 6 mm) which can move from the exterior into the interior of a preparation chamber. Thus above design can make two graphite rods discharge continuously, and consequently prepare much more carbon nanotubes once. Preparation process is described as follows:
First, after exhausting the preparation chamber by mechanical pump and Roots high vacuum pump, NH3 gas of certain pressure intensity was let in. Switching on the power and moving downwards the anode graphite rod, we adjusted the distance between the cathode and anode to be
INFLUENCE OF NH3 ATMOSPHERE ON THE GROWTH AND STRUCTURE OF CARBON
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1mm and as close to cathode as possible in order to induce the arc-discharge. The discharge current was kept 70-100 A until the graphite anode consumption were finished. After a sufficient water cooling, the prowdery sediments around the interior wall and the deposits adhered to cathode graphite rod was collected. Finally, we made specimen out of small quantity sediments and small quantity deposits in core area. Using transmission electron microscope (JEM-1200EX) and field emission scanning electron microscope (JSM-5510LV), we could observe, analyze and identify the prepared reaction products.
RESULTS AND DISCUSSION
The key point of arc-discharge is that graphite rod induces an electric arc in the atmosphere of NH3. The inducing arc distance NH3 atmosphere is less then 1mm, which is small comparing with that of 2-3 mm in other atmosphere [8, 9]. In the discharging process, black powdery things shoot at the container wall from discharge place. Synchronously we observe comparatively regular club-shaped deposits which diameter was little bigger than that of the anode. Figure 1 shows the optics photo of the cathode deposits that is prepared at discharge current 70 A and pressure 0.02 MPa in NH3 atmosphere. The section and front views of the cathode deposits are shown in Fig. 1a, 1b, respectively. The central area A and outer area C are silver gray with stiff texture. The interlayer area B between part A and C is black and comparatively soft.
We prepared specimen out of sediments matter on the container wall and deposits matter on the cathode in areas A, B, C, respectively. Through TEM observation, we discover that the powdery sediments adhered to the container wall didn't contain nanotubes, same as that by arc-discharge of other gases such as He, H2 et al. Therefore, it is unfavorable for carbon nanotubes to grow in the space of NH3 atmosphere. It is showen in Fig. 2. that the silver gray matter of A and C are amorphous carbon, while the inter-layer area B contains a great deal of carbon nanotubes. Under the condition of discharge current 70 A and NH3 atmosphere at pressure 0.02 MPa, it can be seen that there are a large quantity of multi-wall carbon nanotubes in the specimen and mostly exist as a single straight bundle which diameter about 10nm and length approximately 20-30 ^m. It can also be seen that there exist some hollow nanometer particles cling to exterior wall of carbon nanotubes, which are similar to the reaction products of carbon rod in arc-discharge of other gases (He, H2 et al). The differentia is that both the central area A and B of cathode deposits contain carbon nanotubes under arc-discharge of He, H2 et al atmosphere [3, 8]. However, only area B contains carbon nanotubes under NH3 atmosphere. From SEM image of the topmost cathode deposits, we can see that the top surface part A and part C are amorphous carbon. The SEM image of top surface part B under different amplifying ratios is shown in Fig. 3. It can be seen that there is a large quantity of fibroid carbon nanotubes produced.
Fig. 1. Optics photo of the cathode deposits in core and top position:
a - core part, b - top part.
The experimental results are analyzed as follows: With a stable arc-discharge after the electric arc occurs, the anode graphite rod vaporizes continuously due to high temperature, so the C atoms are ionized as the C ions. Subjected to electric field, these ions are attracted by the cathode and largely adhere to the cathode graphite to from carbon nanotubes. The electric arc temperature, as well as the density of C ions ionized by anode C atoms, with the arc-discharge of NH3 gases is lower compared with that of H2 gases. Because NH3 molecule is bigger than H2 molecule, the collision probability of NH3 molecule is also bigger than that of H2 molecule when moving to cathode. Consequently, the quantity of carbon nanotubes formed in the atmosphere of NH3 is less than that in the atmosphere of H2.
Fig. 2. TEM micrograph of the part B matter in the cathode deposits.
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CONCLUSION
Adopting the improved arc-discharge equipment, carbon nanotubes preparation is carried out in the atmosphere of NH3 at pressure 0.02 MPa and discharge current 70 A by the arc-discharge method. A large quantity of multiwall carbon nanotubes were grown on the top surface and in the inner of interlayer part between central part and outer part in the cathode deposits. From the TEM and SEM images, we observe that, for the generated multi-wall carbon nanotubes with arc-discharge method, there is no significant difference in the shapes and the structures under NH3 atmosphere and other atmosphere such as He, H2 et al, except positions. A great number of experiments prove that the arc-discharge method in the atmosphere of NH3 is one highly efficiency method in the preparation of carbon nanotubes.
The authors gratefully acknowledge the financial supports from the Zhejiang National Natural Science Foundation Under Grant No. and Y107394.
REFERENCES
1. Iijima S. Helical microtubules of graphitic carbon // Nature. 1991. V. 354. № 7. P. 56-58.
2. Ando Y. Carbon Nanotubes
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