New method of carbon onion growth by radio-frequency plasma-enhanced chemical vapor deposition

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Abstract

Large quantities of carbon onions with high purity were synthesized by radio-frequency plasma-enhanced chemical vapor deposition. The produced carbon onions are solid, clean and can be separated easily from the catalytic particles. The formation of onions is based on the formation of many cages in successive stages from the core to the surface. Around the edge of a carbon onion, discontinuous curved lines are shown in the high-resolution TEM image, reflecting the wavy behavior of carbon onions.

Introduction

After the discovery of carbon onions [1], which are made of concentric graphite spherical shells, many efforts have been made to create these onions and understand the formation mechanism of such materials. Carbon onions are expected to have good prospects for some aspects of composite materials, wear-resistant materials and magnetic storage media. These carbon onions are produced by DC arc-discharge [1]. Other ways to form carbon onions have been reported, such as high-energy electron irradiation of carbonaceous materials [2], or thermal treatment of fullerene black [3]. Onions can also be produced by shock-wave treatment of carbon soots [4], and plasma torch [5]. In these materials, carbon onions are by-products and are hardly separated from the product, which brings about difficulty in investigation and application for carbon onions. Therefore, searching for a new synthesis process, which could overcome the disadvantage of these techniques, is crucially important to the future application of the carbon onions. Here we report a new method of formation of carbon onions in macroscopic quantities by radio-frequency plasma CVD.

Section snippets

Experimental

The catalysts were prepared by impregnating silica gel with a solution of 0.02 M Co(NO3)2. The solution was stirred for 24 h and then dried at 150°C for 12 h, followed by a calcining at 500°C in flowing N2 for 4 h. Finally, the resulting samples were reduced at 700°C for 5 h in H2 gas environment to obtain the catalysts.

A ceramic boat containing 25 mg of catalyst was placed in the reaction chamber of the radio-frequency plasma CVD. A mixture of CH4 and H2 (H2:CH4=20:1) was introduced into the

Results and discussion

Fig. 1 is a typical TEM image of the high-percentage carbon spheres on the catalysts. The product contains no carbon tubes, only carbon spheres, which are solid, smooth and clean. The carbon spheres have an average diameter ranging from a few to several tens of nanometers, roughly the same as the diameter of multiwall tubes. Fig. 2 gives an HRTEM image of spherical particles, where the concentric graphitic layers are resolved clearly, related to the so-called carbon onion. From Fig. 2 we can

Conclusion

Carbon onions with closed-shell structure may exhibit unique physical and mechanical properties. We have established a processing route to synthesize carbon onions in macroscopic quantities by using radio-frequency plasma-enhanced CVD. The product is pure and not encapsulated on the surfaces of the catalysts. The significance of the present results is that the method of radio-frequency plasma-enhanced CVD has overcome the shortcomings of other techniques. The method described here might be used

Acknowledgements

The authors acknowledge financial support from the Natural Science Foundation of China (Grant No. 59972031).

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