Comparative study of the growth of CNTs on stainless steel with and without the external catalyst

Multi-walled carbon nanotubes were synthesized on 316 stainless steel substrate by chemical vapor deposition through two different methods: 1-without use of any external catalyst and using ethylene as the carbon precursor and 2-using ferrocene as an external source of catalyst particles, dissolved in toluene, as the carbon precursor. Carbon nanotubes grown by the two methods were characterized by scanning and transmission electron microscopy and X-ray diffraction methods and were compared subsequently to determine certain characteristics of each method. Good coverage and homogeneity was observed in both cases. However, the carbon layer was thicker and denser in externally catalyzed samples. Two different mechanisms, namely, base and tip modes, were observed for the nanotubes growth, each with particular characteristics stemming from the synthesis methods. Surface nano-features and external catalyst behavior were found to have the dominant role in determining the morphology of carbon filaments in intrinsically and externally catalyzed samples, respectively.