Effect of growth conditions on B-doped carbon nanotubes

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Odile Stéphan Laboratoire de Physique des Solides, Universite Paris-Sud, 91405 Orsay, France

David L. Carroll Department of Physics, University of Wake Forest, Winston Salem, North Carolina 27109-7507 (Received 4 April 2006; accepted 7 September 2006)

The modified arc-discharge technique was used for the growth of boron-doped multiwalled carbon nanotubes. A variety of weight percentages of boron and sulfur were mixed (0.5–15 wt%) with graphite powder and packed in the consumable anode. Transmission electron microscopy, Raman spectroscopy, thermogravimetric analysis (TGA), and electron energy loss spectroscopy (EELS) were used to characterize the samples. EELS indicated a small percentage of boron present (15 wt% boron was used in the precursor mixture, the irregularities obtained were substantial. Other elements were considered to avoid the formation of these irregularities. Namely, the addition of sulfur to the boron/graphite precursor powder mixture was primarily to act as a doping promoter since it is known to enhance the growth of CNTs without being incorporated into the lattice.14 Sulfur is also known to enhance the graphitization of carbon below 2000 °C.16 The growth of CNTs when sulfur is introduced in the growth conditions can alter the diameters of the tubes obtained and therefore result in better control of the NT production.17 Moreover, when sulfur is used during the arc-discharge method, it results in an increased yield of the CNTs.18 Here, by adding sulfur to the precursor mixture (when >15 wt% boron), the irregularities observed inside the dark gray shell collected from the cathode after the arc-discharge were significantly reduced. Also, an increase in the quantity of black material collected from the dark grey shell was observed, possibly promoting a “smother” arc-discharge during growth with a better yield. A. TEM analysis

The TEM images of the nanotubes obtained from this study showed that they, in general, have hollow cores and diameters from 10 up to 30 nm. The morphology of samples was rather similar to that of undoped MWCNTs.1 Figure 1 shows TEM images from the NTs obtained after using 15 wt% boron in the sacrificial anode. Figure 1(a) shows a low-magnification TEM image with a large number of small nanoparticles in the form of polyhedral carbon clusters aggregated among the NTs, which is common when this growth technique is used. Tubes with different diameters and hollow cores are present, which is similarly observed for undoped MWCNTs. Figure 1(b) shows a high-resolution TEM image of a NT tip obtained from the experiment using 15 wt% boron in the anode. Again the morphology of the NT tip is similar to that of undoped MWCNTs. Figure 1(c) shows a TEM image of long NTs with diameters ∼5 nm whereas the typical length of boron free MWCNTs is up to ∼1 ␮m.19 Thus, the only observed difference from undoped arcgrown MWCNTs was the slight increase in lengths of several NTs of up to ∼5 ␮m. The increase in length when boron is present has been previously observed and explained by first-principles sta