  The effect of Fe and Ni catalysts on the synthesis of carbon nanotubes (CNTs)
using atmospheric pressure chemical vapor deposition (APCVD) was investigated.
Field emission scanning electron microscopy (FESEM) analysis suggests that the
samples grow through a tip growth mechanism. High-resolution transmission
electron microscopy (HRTEM) measurements show multiwalled carbon nanotubes
(MWCNTs) with bamboo structure for Ni catalyst while iron filled straight tubes
were obtained with the Fe catalyst. The X-ray diffraction (XRD) pattern
indicates that nanotubes are graphitic in nature and there is no trace of
carbide phases in both the cases. Low frequency Raman analysis of the
bamboo-like and filled CNTs confirms the presence of radial breathing modes
(RBM). The degree of graphitization of CNTs synthesized from Fe catalyst is
higher than that from Ni catalyst as demonstrated by the high frequency Raman
analysis. Simple models for the growth of bamboo-like and tubular catalyst
filled nanotubes are proposed.
