Combined catalyst system for preferential growth of few-walled carbon nanotubes

The effect of a combined catalyst system on the synthesis of carbon nanotubes (CNTs) from methane (CH4) was investigated using molybdenum trioxide (MoO3) as a conditioning catalyst and molybdenum-doped iron supported on magnesia as the main catalysts. Without the conditioning catalyst, only single-walled CNTs with diameters smaller than 2 nm were formed on the main catalyst. With the conditioning catalyst, double-walled CNTs and few-walled CNTs with larger hollow diameters than 2 nm were produced with much higher yields. The combination of the two kinds of Mo-containing catalyst would more effectively transform CH4 into reactive species related to the early stage of nanotube formation on the main catalyst, resulting in the increase of the yield, diameter and layer number of the CNTs.