Growth mechanism and structural control of catalytically grown carbon nanotubes using nano-sized metal particles

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In this project, we successfully produced high purity carbon nanotubes in a catalytic chemical vapor deposition method through an exact understanding of the critical function of nano-sized metal particles during nanotube growth, and have carried out their structural characterizations.The summarized research results are as follows :(1) We have developed the method of synthesizing high purity double walled carbon nanotubes (DWNTs) through an understanding of long-discussed growth mechanism of carbon nanotubes in a catalytic chemical vapor deposition (CVD) method. Specifically, we have clarified the importance of supporting materials because their size and compositions directly contributed to the formation of nano-sized metal particles for growing carbon nanotubes.(2) We found that the reaction temperature of 850℃ was the optimum condition for preferentially growing thin DWNTs (ca. 0.8 nm) over single walled carbon nanotubes (SWNTs).(3) When DWNTs were thermally treated at 2000℃ in argon, … More the adjacent outer shells were structurally merged to form a coalesced nanostructure.(4) DWNT-derived buckypaper exhibited higher resistance to oxidation, and furthermore, had large fraction of nano-sized pores below 1 nm.(5) We demonstrate that the incorporation of boron (B) atoms between double walled carbon nanotubes (DWNTs) during thermal annealing (1400 -1600℃) results in covalent nanotube "Y" junctions, DWNT coalescence and the formation of flattened multi-walled carbon nanotubes (MWNTs). These processes occur via the merging of adjacent tubes which is triggered by B interstitial atoms.(6) By using the mixed gases (methane + sulfur) instead of methane, we found that the obtained nanotubes exhibited the increased shell numbers, possibly due to the increased size of metal particles on the outer surface of supporting materials.(7) We successfully attached fluorine atoms to the sidewall of outer shell in DWNTs through direct reaction with fluorine gas at 200℃ for 5 hrs. The stoichiometry of the fluorinated DWNTs is CF_<0.30>, based on XPS study, while double-layered structure is sustained after fluorination according to HR-TEM observation. By tuning electronic property of only outer shell and controlling dispensability of DWNTs without disrupting double-layered concentric shell morphology through fluorination, we envisaged these chemically modified DWNTs to be useful in the fabrication of novel sensors, nanocomposites and electronic devices. Less