High-yield synthesis of carbon nano-onions in counterflow diffusion flames

High-yield synthesis of carbon nanotubes (CNTs) and nano-onions (CNOs) on a catalytic nickel substrate using counterflow diffusion flames was investigated. With ethylene fixed at 5%, methane varied from 15% to 45% in the upper flow, and air supplied in the lower flow, only a moderate CH₄ concentration (25%) could yield high density CNTs. However, when oxygen was increased to 50% in the lower flow, only CNOs were synthesized. An increase in methane concentration from 15% to 45% led to a higher yield and a greater diameter (ranging from 5 to 60 nm) of CNOs. To examine the role of mixed fuel, it was observed that as ethylene was removed and only 45% methane and nitrogen were supplied in the upper flow, no CNOs could be generated. While, as methane was increased to 50% or 55%, high-yield CNOs were synthesized and the yield increased with methane concentration. Note that the key parameters influencing the formation and yield of CNOs are both the oxygen and fuel concentrations. There was a critical threshold value of oxygen concentration, 30%, for onset of CNOs synthesis. Also, the critical threshold value of methane concentration for onset of CNOs formation decreased with increasing oxygen concentration or ethylene concentration.