Carbon Nanotubes Grown by RF Heating and their Morphological and Structural Properties

Multiwall and singlewall carbon nanotubes were synthesized on Fe-Co/CaCO₃ and a Fe-Co/MgO catalyst systems, respectively, by using two different catalytic chemical vapor deposition methods, external furnace (EF) heating and radio frequency (RF) excitation. The carbon nanotubes synthesized with radio frequency excitation have a smaller outer diameter, fewer layers (smaller outer/inner diameter ratio), and better crystalline properties, compared to the nanotubes grown with external furnace heating. The radio frequency process was found to be responsible for a faster growth rate of the carbon nanotubes over longer periods of time due to a higher localized heating. These findings can be explained by the skin currents induced in the metallic catalytic clusters, which keep the catalysts active for longer periods of time and diminishes the amount of non-crystalline carbon formed in the synthesis process. A direct correlation between the frequency of the electromagnetic field and the morphology of the nanotubes was also found.