Effect of Carrier Gas on the Growth Rate, Growth Density, and Structure of Carbon Nanotubes
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M11.31.1
Effect of Carrier Gas on the Growth Rate, Growth Density, and Structure of Carbon Nanotubes Yoke Khin Yap 1*, Vijaya Kayastha 1, Steve Hackney 2, Svetlana Dimovski 3 and Yury Gogotsi 3 1
Department of Physics, Michigan Technological University, Houghton, MI 49931, USA. 2 Department of Materials Science and Engineering, Michigan Technological University, Houghton, MI 49931-1295, USA. 3 Department of Materials Science and Engineering and A. J. Drexel Nanotechnology Institute, Drexel University, Philadelphia, PA 19104, USA. * Email:[email protected] ABSTRACT We attempt to understand the fundamental factors that determine the growth rate of carbon nanotubes. In a series of experiments on growing multiwall carbon nanotubes (MWNTs) by thermal chemical vapor deposition, we found that the addition of carrier gas and the type of carrier gas can change the growth rate, growth density, and structures of MWNTs. We explain these results based on the dissociative adsorption of C2H2 on Fe nanoparticles and the vapor-liquid-solid (VLS) growth model. Finally, high-density, vertically aligned MWNTs were grown when decomposition and segregation rates of carbon were balanced.
INTRODUCTION Carbon nanotubes (CNTs) [1, 2] are among the promising materials for applications in future nanotechnology. Chemical vapor deposition (CVD) is a convenient technique for growing both singlewall, and multiwall CNTs. However, it is still fundamentally unclear on the factors that will determine the growth rate and structure of carbon nanotubes. Sustaining the growth of carbon nanotubes to unlimited lengths is still technologically impossible. Why does the growth of carbon nanotubes stop or saturate under certain growth conditions? What are the factors that cause the formation of bamboo-like multiwall carbon nanotubes (MWNTs) that are often produced by CVD techniques? We will attempt to answer some of these questions in this paper. The well-accepted growth mechanism of CNTs by thermal CVD involves the decomposition of hydrocarbon gas on the surface of the catalyst, the diffusion of carbon into the catalyst until saturation, and subsequent segregation of carbon from the catalyst as a tubular structure. The catalytic decomposition of hydrocarbon gases is known as dissociative adsorption [3-5]. The subsequent steps can be described by using the vaporliquid-solid (VLS) model [6, 7]. Ideally, CNTs will continue to grow if every carbon atom that adsorbed on the catalyst’s surface can be added to CNTs. This is valid if the catalyst remains active. Such an ideal condition has not been achieved due to the lack of control over the decomposition, diffusion, and segregation processes.
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Here we report our original finding on controlling the decomposition of C2H2 on the Fe catalyst during the growth of MWNTs. We found that the use of carrier gases can either suppress or enhance the decomposition rate of C2H2 on Fe nanoparticles. Furthermore, the growth density and the structure of these MWNTs vary when different carrier gases are used. These results
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