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Impact of N2 admixture on the synthesis of graphitic carbon nanoparticles using atmospheric-pressure microwave plasma

Atmospheric-pressure microwave plasma was utilized for catalyst-free gas-phase synthesis of graphitic carbon nanoparticles by adding CH4 to a mixture of N2 and He. The impact of N2 on the formation of the graphitic carbon nanoparticles was analyzed by increasing the N2 flow rate from 0 to 20 slpm and decreasing the He flow rate from 40 to 20 slpm simultaneously. The addition of N2 altered the morphology of the carbon nanoparticles to obtain flatter and larger shapes. Furthermore, carbon nanoparticles synthesized with N2 possessed more graphitic structures with higher C/O ratios and larger crystallite sizes than those synthesized without N2. The analysis of gas compositions at thermodynamic equilibrium states demonstrated that CN and HCN were the dominant precursor species when N2 was added. The presence of N atoms or N-containing species seem to have a synergistic effect combined with H atoms on the formation of the graphitic carbon nanoparticles.