Orientation and morphological evolution of catalyst nanoparticles during carbon nanotube growth

We examined the structure, morphology, and orientation of catalyst nanoparticles used for seeding and growing multiwall carbon nanotubes (MWCNTs) by plasma enhanced chemical vapor deposition in CH₄/H₂ gas mixtures. Iron catalyst nanocrystals are converted to Fe₃C in CH₄/H₂ plasmas and the MWCNTs grow from Fe₃C nanocrystals. Initially faceted and equiaxed catalyst nanocrystals are distorted and elongated significantly once a tubular CNT structure is formed around the catalyst particles. Eventually, catalysts deform into elongated tear-drop shapes. Once this morphology forms, CNT structures produced are straight and have uniform diameters. Surprisingly, the Fe₃C nanocrystals located inside the base of well-graphitized nanotubes do not exhibit a preferred orientation relative to the nanotube axis. Catalyst nanocrystals in a variety of orientations relative to the nanotube axis still produce well-graphitized nanotubes with similar diameters and structures.