Particle size dependence and model for iron-catalyzed growth of carbon nanotubes by thermal chemical vapor deposition

The catalytic particle size dependence of chemical vapor deposition growth of multiwall carbon nanotubes was systematically investigated using two different molecules, ${\mathrm{C}}_2{\mathrm{H}}_2$ and ${\mathrm{C}}_{60},$ as carbon feedstock gases. In the particle size range between 25 and 500 nm, the use of ${\mathrm{C}}_2{\mathrm{H}}_2$ leads exclusively to growth of carbon nanotubes. The nanotube diameters increase with increasing catalytic particle sizes but do not scale 1:1. In contrast, nanotube formation from ${\mathrm{C}}_{60}$ is observed only if the particle sizes are sufficiently small with an optimum between 20 and 30 nm. For catalyst samples with considerably larger diameters, ${\mathrm{C}}_{60}$ is transformed into a nontubular deposit. A growth model is given that explains the different behavior.