Carbon nanotubes contain residual metal catalyst nanoparticles even after washing with nitric acid at elevated temperature because these metal nanoparticles are sheathed by several graphene sheets.

It is demonstrated that multiwalled (MWCNT) and single-walled (SWCNT) carbon nanotube materials contain residual metal impurities (Fe, Ni, Co, Mo) even after prolonged periods of "washing" with concentrated nitric acid at temperature of 80 degrees C. Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) reveals that this is because such metal impurities are intercalated in the nanotube channel (in the case of MWCNT) or in the "bamboo" segment of the nanotube (in the case of "bamboo"-like MWCNT), or they create graphene sheet protected metal core/shell nanoparticles (in the case of SWCNT). TEM/energy-dispersive X-ray spectroscopy (TEM/EDS) elucidate that residual metal impurities presented in "washed" carbon nanotube materials are in some cases in the form of metal alloys or that there can be several different pure metal nanoparticles presented in one CNT material. It is shown by thermogravimetric analysis that "washing" with concentrated nitric acid removes up to 88% (w/w) of metal catalyst nanoparticles from as-received carbon nanotubes and that such removal has in some cases a significant effect on the electrochemical reduction of hydrogen peroxide.