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.