Flame synthesis of carbon nanostructures using mixed fuel in oxygen-enriched environment

The effects of key parameters, namely oxygen concentration, mixed fuel, and sampling positions, on the formation of carbon nano-onions (CNOs) and carbon nanotubes (CNTs) were investigated in oxy-fuel inverse diffusion flames. Particular focus was put on the intermediate species in connection with the synthesis of CNOs and CNTs. Three patterns of carbon nanostructures were observed: CNTs only, CNOs only, and CNTs/CNOs cogeneration. An appropriate temperature range in the synthesis of CNTs was identified to lie between 400 and 1,000 C, whereas the temperature range for the synthesis of CNOs was higher, within 800-1,250 C. A threshold value of oxygen concentration, 30 %, existed for onset of CNO synthesis. Gas composition analysis indicated that no carbon nanomaterial was formed at low CO and C₂H₂ concentration as well as low substrate temperature (lower than 400 C). Compared with the synthesis condition of CNTs only, the C₂H₂ concentration was higher for the onset of CNTs/ CNOs cogeneration, whereas the CO concentration was maintained at the same level. Additionally, the critical C₂H₂ concentration for the onset of CNOs only was found to be 0.4 %. A large quantity of CNOs was observed for C₂H₂ concentration greater than 0.4 % and CO concentration greater than 4 %.