Aligned carbon nanotube growth under oxidative ambient
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Bingqing Wei Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180 (Received 21 June 2001; accepted 9 August 2001)
Carbon nanotubes (CNTs) are always produced under a reductive ambient with hydrogen present using the chemical vapor deposition method. Oxidative media, such as carbon dioxide and oxygen, could damage the tubular structures by opening the nanotube ends or etching the tube walls. Here we report the synthesis of aligned defective, but clean, CNTs in the presence of water vapor. The tube walls were found broken as well as the tube ends. CNTs with a large amount of exposed broken sites on their tube walls have potential applications in many areas such as energy storage.
I. INTRODUCTION
Carbon nanotubes (CNTs) have shown unique and attractive mechanical,1 electrical,2 and thermal3 properties since their discovery in 1991.4 Researchers throughout the world have recently become interested in exploring possible applications of CNTs in many fields. Depending on their diameter and helicity, CNTs could be semiconductive or metallic and behave as one-dimensional quantum wires.5 The hollow cavity inside the tube is ideal to store hydrogen or other molecular due to capillary suction.6 Field emitters using CNTs as the cathode show stable and large current under lower voltages (about 3 V/m).7,8 Other aspects of CNTs, such as their ability to act as chemical9 and molecular sensors,10 have also been reported. Recently, large-area, self-oriented carbon nanotube arrays have been successfully produced,11 which enable in-depth study on the anisotropic behavior of organized nanostructures. Patterned aligned nanotube arrays are potential candidates of the cold cathode for flat panel displays (FPD) due to their excellent field-emission properties.12 CNTs are generally synthesized by the conventional chemical vapor deposition (CVD) method, in which carbon atoms (clusters) decomposed from a carbon source form tube structures at the surfaces of catalyst particles. However, there is always a large amount of impurities, such as amorphous carbon, in as-grown CNT samples. They exist either among the entangled CNTs or adhere to their tube walls and tips and cannot be easily removed.
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 11, Nov 2001
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Impurities have a negative effect on further utilization of CNTs. Post-treatments such as boiling with acid would remove amorphous carbon and catalyst particles, but the original alignment of CNTs is inevitably damaged. CNTs are synthesized in flowing H2; that is, in a reductive ambient. Oxidative media, such as oxygen and carbon dioxide, would etch and strip off the graphitic layers of nanotubes.13,14 In this paper, we report the synthesis of aligned CNTs in an oxidative ambient by introducing water vapor into the reaction furnace. We found that amorphous carbon has been removed effectively. High-resolution transmission e
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