Bulk Synthesis of Helical Coiled Carbon Nanostructures
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Bulk Synthesis of Helical Coiled Carbon Nanostructures Wei Wang1 and A. M. Rao2 1 School of Materials Science and Engineering, Clemson University, Clemson, SC 29634, U.S.A. 2 Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, U.S.A. ABSTRACT Three dimensional helical coiled carbon nanostructures including coiled nanowires and nanotubes were synthesized at ambient pressure using a thermal chemical vaporization deposition (CVD) process in which xylene and acetylene were used as the primary carbon source. A bi-metal catalyst formed from a mixture of ferrocene and indium isopropoxide served as the seed to initiate the growth of these helical coiled nanostructures. The as-synthesized coiled nanowires and nanotubes are pure without the presence of amorphous carbon nanoparticles. By precisely controlling the atomic ratio of In / (Fe+In), coiled nanowires or nanotubes can be synthesized exclusively. The diameters of the as-grown coiled nanowires vary from several tens to several hundreds nanometers, whereas the diameters of the coiled nanotubes are around 20 nm. The structure of coiled nanowires and nanotubes were determined using scanning electron microscopy and transmission electron microscopy. The key novel aspects of this research are: (i) to synthesize helical coiled nanotubes or nanowires exclusively by controlling the bi-catalyst concentration of Fe and In, and (ii) no preformed substrates are required which implies that our process is amenable for scaled-up synthesis of these nanostructures. INTRODUCTION Three dimensional carbon nanostructures such as helical coiled carbon nanowires (HCNWs) and carbon nanotubes (HCNTs) have attracted much attention recently. Due to their fascinating morphologies, they are expected to exhibit unique structural, mechanical and electrical properties which may enable realization of potential devices such as the electromagnetic wave absorbers, nanosprings for microelectromechanical systems (MEMS) and nanoelectromecahnical systems (NEMS). An energetically stable coiled nanotube structure was predicted by Itoh et al. [2] soon after the pioneering discovery of the multi-walled carbon nanotubes (MWNTs) by Ijima in 1991 [1]. One year later, the HCNTs were experimentally observed by the catalytic decomposition of acetylene over cobalt [3]. Since then, several groups have reported the synthesis of HCNTs using chemical vapor deposition (CVD) on preformed substrate [4-7] which severely limits the amount of material that can be produced. Furthermore, these HCNTs co-exist with regular MWNTs which making it difficult to access the HCNTs from the soot. To the best of our knowledge, synthesis of soot which predominantly contains HCNTs is still a challenge task. HCNW (also called carbon nanocoils) is another form of helical coiled carbon nanostructure which can also be prepared in a CVD process [8-11]. Pan et al. [8] prepared HCNWs from a thermal decomposition of acetylene over Fe catalyst coated indium-tin-
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