Effect of different gas medium on carbon nanotubes synthesis by arc discharge

Carbon nanotubes (CNTs) were synthesized in different gas medium by arc discharge. Optical emission spectroscopy (OES) was used in situ to investigate the formation mechanism of CNTs. Field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM) were used to characterize CNTs. FE-SEM observation reveals that a few CNTs synthesized in N-2/C2H2/CO2 medium (CNTsN2/C2H2/CO2) are open-ended or porous ones and the impurity amount adhered on CNTsN2/C2H2/CO2 or CNTsHe/C2H2/CO2 is smaller than that adhered on CNTsN2/C2H2 or CNTsHe. TEM characterization results show that the wall of CNTsN2/C2H2/CO2 is partly damaged because of being oxidized, which led to its diameter is smaller than that of CNTsN2/C2H2. OES diagnosis results show that active radicals including C-2 and CH are the precursors of CNTs formation while CO2 acts as the etching species to eliminate amorphous carbon in the presence of CO2. A possible formation process of CNTs is proposed on the basis of the results of OES diagnosis and FE-SEM as well as TEM observation.