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
/C
₂
H
₂
/CO
₂
medium (CNTs
_N2/C2H2/CO2
) are open-ended or porous ones and the impurity amount adhered on CNTs
_N2/C2H2/CO2
or CNTs
_He/C2H2/CO2
is smaller than that adhered on CNTs
_N2/C2H2
or CNTs
_He
. TEM characterization results show that the wall of CNTs
_N2/C2H2/CO2
is partly damaged because of being oxidized, which led to its diameter is smaller than that of CNTs
_N2/C2H2
. OES diagnosis results show that active radicals including C
₂
and CH are the precursors of CNTs formation while CO
₂
acts as the etching species to eliminate amorphous carbon in the presence of CO
₂
. 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.