Large-Scale Synthesis of Boron Nitride Nanotubes with Iron-Supported Catalysts Ching-Yuan Su, † Wen-Yi Chu, † Zhen-Yu Juang, † Ko-Feng Chen, † Bing-Ming Cheng, ‡ Fu-Rong Chen, † Keh-Chyang Leou, †,§ and Chuen-Horng Tsai* ,†,§ Department of Engineering and System Science, National Tsing Hua UniVersity, Hsinchu, Taiwan, National Synchrotron Radiation Research Center, Taiwan, and The Center for Nano Science and Technology in the UniVersity System of Taiwan, Hsinchu, Taiwan ReceiVed: May 16, 2009; ReVised Manuscript ReceiVed: June 14, 2009 Boron nitride nanotubes (BNNTs) were synthesized in a large scale with iron-supported catalysts (Fe/SiO 2 -Al 2 O 3 ) at low temperatures (900 °C) in a plasma-assisted chemical vapor deposition system. The structural morphology, chemical composition, and optical and photoluminescence properties of BNNTs were characterized. The obtained BNNTs are crystalline and with tubular structures, and the preferential zigzag arrangement of BNNTs was discovered for the first time for BNNT grown at low temperature (<1000 °C). The O 2 additives were found to affect the growth yield significantly and it was attributed to the enhanced dehydrogenation from the catalyst surface and improved balancing of excess H radicals during our synthetic process. In addition, results from elemental-mapping revealed that the boron dissolved in iron tends to precipitate and react with nitrogen to form a BN sheet on the catalyst surface. Introduction Boron nitride nanotubes (BNNTs) were theoretically predicted to be a stable nanostructure in 1994 1 and were successfully synthesized in 1995. 2 BNNT is a structural analogue of well- known carbon nanotubes (CNT) created by substituting the C atoms in a curved graphitic sheet with B and N atoms. However, BNNTs exhibit remarkable chemical and physical properties very different from CNTs; for instance, according to electronic property calculations, BNNTs have a wide band gap of about 5 eV that is independent of its diameter and chirality. 3 Moreover, the oxidation resistance of BNNTs is much higher than that of CNTs (up to 800 °C), 4,5 and other advantageous properties such as chemical stability, 6 high thermal conductivity (∼ 350 W mK -1 ), 7 and mechanical strength (∼ 1.1-1.3 Pa) 8,9 have also been proposed. Thus, BNNTs have great potential for applica- tions in electronics, 10,11 optoelectronics, 12-14 energy storage, 15,16 and nanoelectromechanical systems. 17,18 Several synthesis techniques have been developed for BNNT growth, including arc discharge, 19 laser ablation, 20 ball-milling, 21 plasma-jet, 22 substitution reactions with CNTs as templates, 23,24 and chemical vapor deposition (CVD). 25 However, most of these synthesis methods were carried out above 1000 °C (from 1100 to 2700 °C) and contained undesirable impurities (e.g., amor- phous B particles and BN bulky flakes) in the as-grown products. To address this and related issues, several approaches have been proposed recently to synthesize high-quality BNNTs at tem- peratures lower than 1000 °C by catalytic CVD assisted with plasmas. In 2005, Wang et al. demonstrated the growth of multiwalled BNNTs at 600 °C with iron (Fe) catalyst by plasma- enhanced pulsed-laser deposition (PE-PLD). 26 In 2008, Guo et al. developed a process to synthesize BNNTs on nickel (Ni) coated oxide (SiO 2 ) substrates at a temperature of 800 °C in a microwave plasma-enhanced CVD (MPCVD) system. 27 These two works provide feasible routes for synthesizing crystalline (hexagonal) BNNTs at temperatures lower than 1000 °C. With a plasma-assisted method, BN species (B x H y , NH z , BNH, etc.) were formed, obtained sufficient energy, and were synergistically combined to generate reactive conditions for growing BNNTs at lower temperatures. Plasma assistance seems to be a useful and perhaps necessary factor for growing BNNTs at low temperatures. Nevertheless, to create high-quality and high-yield BNNTs, all synthesis parameters (e.g., processing atmosphere, temperature, plasma power, etc.) must be optimized for this approach (i.e., catalytic CVD with plasma assistance). In 2007, Wang et al. proposed a different growth method: 28 BNNTs were synthesized on a catalyst-free anodic aluminum oxide (AAO) template by MPCVD below 520 °C. However, the as-grown BNNTs had an amorphous structure, and the unique properties based on crystalline BNNTs could be lost. Therefore, it is worthwhile to investigate the growth mechanism of BNNTs. Although it is especially important to understand the catalytic mechanism of lower temperature metal-catalyst synthesis with plasma assistance, few studies have addressed this area. In addition, it is important to address the large-scale produc- tion of high quality of BNNTs, which is necessary for practical applications. In 2005, a pioneering group lead by Bando developed a synthetic route based on a thermal-CVD process (MgO and B powder were mixed and heated to approximately 1300 °C). The as-produced B 2 O 2 vapor was reacted with NH 3 to form BNNTs in the cold zone of the furnace. 29 So far, this method is the most successful route for yielding high-quality, high-purity, and large-quantity multiwalled BNNTs. Recently, Oku’s group synthesized large amounts of BNNTs by annealing Fe 4 N/B powders at 1000 °C in a nitrogen atmosphere. 30 In this work, a simple purification process was also conducted to obtain high purity BNNTs. These works imply that the catalytic CVD method may lead to the large-scale production, low-temperature growth, and high-purity synthesis of BNNTs. Catalytic CVD growth with transition metals on a supported powder, such as alumina (Al 2 O 3 ), silica (SiO 2 ), magnesia (MgO), and titania * To whom correspondence should be addressed: E-mail: d947108@ oz.nthu.edu.tw (C.-Y.S.); chtsai@ess.nthu.edu.tw (C.-H.T.). † National Tsing Hua University. ‡ National Synchrotron Radiation Research Center. § The Center for Nano Science and Technology in the University System of Taiwan. J. Phys. Chem. C 2009, 113, 14732–14738 14732 10.1021/jp904583p CCC: $40.75  2009 American Chemical Society Published on Web 07/06/2009 Downloaded by CY Su on August 13, 2009 Published on July 6, 2009 on http://pubs.acs.org | doi: 10.1021/jp904583p