CVD synthesis of carbon nanotubes

Abstract

Reports of the synthesis and identification of carbon nanotubes, both multi-walled [1] and single-walled [2, 3], in arc-discharge soot products have excited great interest in the field of study of this newly discovered allotropic nanostructure of carbon. Extraordinary properties, such as electronic [4–6] and mechanical [7, 8] properties, have been demonstrated both theoretically and experimentally. Although it has been the primary method of producing carbon nanotubes, the arc-discharge technique suffers from a few drawbacks that might prevent it from becoming the best candidate for precisely controlled syntheses of carbon nanotubes of high quality. It is therefore important to search for new processing techniques, especially in the low temperature regime, which can produce high-quality carbon nanotubes in large quantities, while allowing control of the fabrication process with ease and precision. With these considerations in mind, a low-temperature chemical vapour deposition (CVD) apparatus has been set up in an effort to seek new processing routes for large-scale production of carbon nanotubes. Based on the early success of growing carbon fibers [9] using the pyrolytic decomposition of hydrocarbon gases, such as acetylene (C2H2), benzene (C6H6), carbon monoxide (CO), methane (CH4) etc., the CVD technique has become one of the preferred methods for fabricating carbon nanotubes in much lower temperature regimes than is possible with the arc-discharge technique. CVD methods utilize the pyrolytic decomposition of hydrocarbon gases at elevated temperatures in the range 600–1200 8C. Several researchers have reported successful syntheses of carbon nanotubes using CVD techniques recently. For example, Endo et al. [10] reported the observation of nanotubes in the pyrolytic product of benzene (C6H6) decomposition at about 1100 8C, Jose-Yacaman et al. [11] and Ivanov et al. [12] observed nanotubes and related nanostructures in the catalytic decomposition product of acetylene (C2H2) in the temperature range 1050– 1350 8C, and Jaeger and Behrsing [13] found similar structures in a mixture of natural gas, methane and benzene decomposition products. In this letter we report the synthesis of carbon nanotubes at about 750 8C using a CVD technique. The reactant gases employed were high-purity methane (CH4) and hydrogen (H2) gases, and ferrocene (C10H10Fe) was used as catalyst. The reaction was carried out in an electrical tube furnance. A quartz tube, used as the substrate for CVD, was placed inside the ceramic furnace tube of inner diameter about 3 cm and length about 80 cm. The temperature of the furnace was controlled by a Eurotherm unit. The furnace was first pumped to vacuum in order to remove the air inside the chamber, and then flushed with argon gas. When the temperature at the centre of the furnace tube reached about 900 8C, the argon gas was pumped out of the chamber before methane and hydrogen gases were allowed to flow into the chamber. The flow rates of methane and hydrogen gases were kept at about 150 ml miny1 and about 350 ml miny1, respectively, while the total pressure in the reaction chamber was kept at approximately 79.99 kPa. The furnace was heated to 1150 8C in about 15 min and was kept at this temperature for about 5 min to allow the following reaction to occur with the addition of ferrocene vapours: