Differences in the catalyst removal from single- and double-walled carbon nanotubes

Single-walled carbon nanotubes (SWCNTs) and double-walled carbon nanotubes (DWCNTs) synthesized by a catalytic chemical vapor deposition method showed partially incorporated metal catalysts inside the graphene walls. In order to remove the metal catalysts, acid and thermal treatments were successively carried out. The methods for effective catalyst removal in SWCNTs and DWCNTs were examined by means of thermogravimetric analysis, electron microscopy, and electron paramagnetic resonance. The DWCNTs showed distinctly different metal catalyst removal behavior from that of SWCNTs due to the double-wall structure. The acid treatment is less efficient for catalyst removal from DWCNTs, while catalysts in SWCNTs are effectively removed by acid treatment. Additional thermal treatment is quite effective to remove metal catalysts from DWCNTs.

► Distinguished metal catalyst removal behavior of DWCNTs comparing with SWCNTs.
► Catalysts in DWCNTs are effectively removed by sequential acid and thermal treatments.
► EPR is used for macroscopic examination on metal catalyst existence.
► Results imply that double-wall structure may disturb acid penetration into CNT walls.