The
gaseous S doping reaction taking place between SO2 and
CNTs is also found to be capable of purifying CNTs. As listed in Table
1, the residue content in SCNT-S (0.087 wt%) and SNCNTs (1.505 wt%) is
significantly less than CNTs (0.984 wt%) and NCNTs (2.08 wt%). When
CNTs are treated with SO2-containing gas at high
temperature, the carbon layers coated on metal catalyst particles would
be preferentially burned off by the oxidation catalyzed by metal
species,29 which is in favor of removing metal
particles and improving the product purity.