On the diffusion-controlled growth of multiwalled carbon nanotubes

Multiwalled carbon nanotubes were deposited by microwave chemical-vapor deposition on Fe, Co, and Ni catalyst layers. By adjusting the deposition temperature and time predominantly either a tubular structure or a bamboolike structure was obtained. The corresponding growth rates of these forms were determined. The activation energy of diffusion and the diffusion coefficient of carbon in the catalyst particles were evaluated to be as high as 0.3–0.6eV and (0.4–1)×10−10m2∕s for the tubular structure and 1.4–1.6eV and (0.6–1.8)×10−11m2∕s for the bamboolike structure. These data prove the occurrence of a liquid catalyst state during tubular growth and of a solid state in the case of bamboolike growth. It is likely that surface melting of the catalyst particles can also occur during the growth process. The tube growth is limited by time due to the metastable state of the catalyst particles.