Hydrothermal synthesis of multiwall carbon nanotubes

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Masahiro Yoshimura Tokyo Institute of Technology, Materials and Structures Laboratory, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan (Received 10 February 2000; accepted 21 September 2000)

Multiwall open-end and closed carbon nanotubes with the wall thickness from several to more than 100 carbon layers were produced by a principally new method— hydrothermal synthesis—using polyethylene/water mixtures in the presence of nickel at 700–800 °C under 60–100 MPa pressure. An important feature of hydrothermal nanotubes is a small wall thickness, which is about 10% of the large inner diameter of 20–800 nm. Closed nanotubes were leak-tight by virtue of holding encapsulated water at high vacuum and can be used as test tubes for in situ experiments in transmission electron microscope (TEM). Raman microspectroscopy analysis of single nanotubes shows a well-ordered graphitic structure, in agreement with high-resolution TEM. The hydrothermal synthesis has the potential for producing multiwall nanotubes for a variety of applications. The fabrication of nanotubes under hydrothermal conditions may explain their presence in coals and carbonaceous rocks and suggests that they should be present in natural graphite deposits formed under hydrothermal conditions. Carbon nanotubes1 are among the most exciting new materials being investigated and synthesized because of their potential for use in new technologies and devices.2 In particular, multiwall nanotubes (MWNTs) are of interest because their size can be varied in a wide range, allowing for a variety of applications, including composites, hydrogen storage,3 nano-actuators,4 templates for nanorods/nanowires,5 the next generation of electronic nanodevices,6 and micro-electromechanical systems (MEMS).7 To realize the potential applications of nanotubes, their synthesis techniques must be improved to increase yields and decrease the fabrication cost, as well as to allow a better control over the nanotube diameter and wall thickness. Presently, the carbon arc and chemical vapor deposition (CVD) methods are the most widely used. However, high temperatures, electric fields, evaporation, and vacuum are not necessary conditions to prepare carbon nanotubes. Catalytic CVD synthesis can be conducted at 700 °C,8 and electrochemical synthesis of MWNTs was successfully accomplished at 600 °C in molten LiCl.9 Hydrothermal synthesis of materials has many advantages over other methods: it is environmentally benign, inexpensive, and allows for reduction of free energies for various equilibria. Supercritical water offers a different chemistry under pressure, sufficient density to dissolve a)

Address all correspondence to this author. e-mail: [email protected] Present Address: Drexel University, Department of Materials Engineering, Philadelphia, Pennsylvania 19104. J. Mater. Res., Vol. 15, No. 12, Dec 2000

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materials, a higher diffusivity than in liquid state, a low viscosity facilitating mass transport, and high compressibility allowing for easy