Single walled 0.4 nm carbon nanotube bundles prepared by pyrolysis of n -hexane catalyzed by ferrocene
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Single walled 0.4 nm carbon nanotube bundles prepared by pyrolysis of n-hexane catalyzed by ferrocene Qixiang Wang, Guoqing Ning, Fei Wei, and Guohua Luo Department of Chemical Engineering, Tsinghua University, Beijing 100084, China.
ABSTRACT Bundles of single walled 0.4 nm carbon nanotubes were prepared by chemical vapor deposition using n-hexane as carbon source catalyzed by ferrocene with additives of thiophene at 1373 K. 8 cm long rope of single walled carbon nanotubes with diameters of 0.8-1.18 nm was also synthesized. The products were characterized by scanning electron microscopy, high resolution transmission electron microscopy and Raman spectroscopy.
INTRODUCTION Single walled carbon nanotubes (SWNTs) with diameters about 0.4 nm, which are probably close to the theoretical limit of free standing SWNTs, have unusual properties such as superconductivity [1,2]. The smallest SWNTs have been associated with the smallest fullerenes, with a nanotube diameter of 4 Å corresponding to those of the C20 structure [3]. SWNTs with such small diameters have been successfully fabricated by pyrolyzing tripropylamine molecules inside the inert channels of zeolite AlPO4-5 (AFI) single crystals [4,5]. The characterization of these smallest SWNTs has presented some difficulties [6]. In the catalytic chemical vapor deposition (CCVD) of hydrocarbons, the presence of metal nanoparticles is essential for the formation of the nanotubes and also to control the diameter of the nanotubes to some extent [6,7]. Organometallic precursors such as metallocenes and iron pentacarbonyl, which can serve as a dual source of the both carbon and the metal nanoparticles, have been used to synthesis multiwalled carbon nanotubes (MWNTs) and SWNTs [8]. In this study, the pyrolyzing of n-hexane catalyzed by ferrocene (dicyclopentadienyl iron) with additives of thiophene was used, and we have observed the 0.4 nm SWNTs in the SWNT bundles produced. This is clearly shown in the high resolution transmission electron microscopy (HRTEM) images.
EXPERIMENTAL DETAILS The CCVD process schematic diagram is shown in figure 1. A horizontal quartz tube with a diameter of 25 mm (i.d.) serves as the high temperature CCVD reactor. A reactant mixture of n-hexane with ferrocene and thiophene additives is used as sources of the both carbon and metal nanoparticles. A known quantity (100 ml) of n-hexane with about 0.01 g/ml ferrocene and about 0.03 ml/ml thiophene is placed in a flask. The flow rate of the liquid reactants is controlled by a mass flow controller. The reactant is first heated to about 363 K at the first furnace. The reaction
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Figure 1. Pyrolysis apparatus for the synthesis of SWNTs by pyr
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