Comparative Study of Catalytic Activity of Iron and Cobalt for Growing Carbon Nanotubes on Alumina and Silicon Oxide

Alumina is a key material to activate catalyisis of carbon nanotube growth. Although alumina is known to prevent aggregation of metal nanoparticles and to promote vertically aligned carbon nanotube growth, details of the interaction of alumina with metal are still unclear. We investigated how alumina functions to activate Fe and Co catalysts comparing acetylene and ethanol as the carbon source from the viewpoint of nanoparticle formation. Catalyst metals diffuse into alumina and precipitate as nanoparticles. Thus, a critical alumina thickness of similar to 20 nm is necessary to retain the metal atoms deposited on the surface. However, in an ethanol ambience, highly dense nanotubes were not obtained with Fe on alumina due to Fe dissolving into the alumina. The length of the grown carbon nanotubes is determined by the stability of catalyst nanoparticles against ripening. Thus, the Fe catalyst forms short nanotubes on silicon oxide where the morphology of Fe nanoparticles changes rapidly and forms submillimeter-long nanotubes on alumina by which Fe nanoparticles are stabilized. Diffusion of metal atoms on the surface and in the host alumina determines the catalytic activity and lifetime of metal catalysts.