Multi-and Single-Walled Boron Nitride Nanotubes Produced from Carbon Nanotubes by a Substitution Reaction
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ABSTRACT Boron nitride (BN) multi-walled (MWNT) or single-walled (SWNT) nanotubes were synthesized from carbon MWNT and SWNT templates, respectively, under heating of C nanotubes together with boron trioxide in a flowing nitrogen atmosphere. B and N atom substitution for C atoms in the nanotubular shells during C oxidation by the B2 0 3 vapor in the N 2 flow is thought to underlie the formation mechanism. Structural and chemical BN MWNT/SWNT analyses were performed by means of high-resolution electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS), respectively. In particular, BN MWNT shell structure and defects, and electron irradiation stability were studied. Finally, the prospects of using the substitution technique for the synthesis of other advanced nanostructures made of BN (nanorods, nanoplates and nanocones) are discussed. INTRODUCTION BN nanotubes (NT), first predicted to exist by theoreticians [1, 2], were discovered in 1995 [3]. The BN NT was calculated to have a wide band gap of -5.5 eV and was considered to have stable insulating properties independent of diameter and chirality in contrast to conventional nanotubes made of carbon [1]. The observed Young's modulus of a BN NT, 1.22±-0.24 TPa, exceeds those of all other known insulating fibers [4]. Thus, a BN NT may serve as a perfect nanoinsulating tubular shield for any conducting material encapsulated within. Different experimental techniques have been found applicable for the BN NT synthesis [5-9]. However, until recently there have been significant BN NT yield limitations. Very recently, the present authors have discovered a simple, low cost and large-scale method for synthesis of BN and B-C-N MWNT and SWNT: a substitution chemical reaction during which B and N atoms substituted for C atoms under C NT heating with boron trioxide in a flowing nitrogen atmosphere [10-121. The present paper reports on new findings with respect to the substitution reaction technique and HRTEM analyses of the resultant BN nanostructures. EXPERIMENT CVD-grown C MWNT and bundles of C SWNT (Carbolex, USA) were heated with B201 in a flowing N, atmosphere as described in our recent papers [10-12]. The optimal temperaturetime parameters were adjusted based on our previous results [10-12]. Synthesis of BN and B-CN MWNT was carried out at 1773 K over 30 min, whereas synthesis of BN and B-C-N SWNT was performed at 1623 K over 30 min. The extracted product was mixed with CCI4 and a few drops of the resultant mixture were dripped onto a 03mm carbon-coated-copper grid for HRTEM and EELS analyses. HRTEM was carried out by means of a field emission highresolution transmission electron microscope JEM-3000F (JEOL). EELS was performed using a parallel detector spectrometer Gatan-666 with a nanobeam focused down to 1.6 nm in diameter. 27 Mat. Res. Soc. Symp. Proc. Vol. 593 ©2000 Materials Research Society
RESULTS BN MWNT shell arrangement Figures l a-c show the HRTEM images of a starting C MWNT and those of BN MWNT synthesized through a substitution reaction. Diffraction
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