Analysis on controlling factors for the synthesis of carbon nanotubes and nano-onions in counterflow diffusion flames.

It is important to identify the dominant factors for governing the growth of carbon nanotubes (CNTs) and nano-onions (CNOs). A diffusion flame of a gas mixture of methane-ethylene was used as the carbon and heat sources and Ni as the catalyst for the synthesis of CNTs and CNOs. The effects of CH4/C2H4 ratio in the fuel side and oxygen concentration in the oxidizer side for counterflow diffusion flames were investigated. It was found that oxygen concentration can greatly affect the morphologies of synthesized products with a threshold of 30% distinguishing the formation of CNO or CNT. CNOs were fabricated at higher oxygen concentrations (30%, 40%, 50%), and CNTs were synthesized only at lower oxygen concentrations (21%, 30%). The fuel composition has minor effects on the morphologies except for the threshold value of oxygen concentration (30%). More carbon sources are required for the synthesis of CNOs than for CNTs, but the temperature requirements are similar (1140-1160 K for CNTs, 1070-1160 K for CNOs). The nanostructures were synthesized as long as the fuel concentration is sufficiently high regardless of the oxygen concentration. Higher fabrication tendency was found for ethylene as fuel to form nanostructures than for methane.