CVD synthesis and purification of multi-walled carbon nanotubes

Multi-wall carbon nanotubes (MWNTs) have been synthesized by catalytic chemical vapor deposition of acetylene over Fe loaded mesoporous silica. The raw product contains the aerogel support, catalyst particles, and a small amount of amorphous carbon (mostly as a coating on the catalyst nanoparticles) as impurities. Before these materials can be used in many applications, these impurities must be removed. A variety of purification processes, both chemical and physical, have been reported in the literature to remove metal catalyst, catalyst support and undesired carbonaceous phases from MWNTs. In this work we report on an effective purification method for the removal of the catalyst. The as-grown MWNTs are purified using a two-step purification procedure involving acid washing and oxidation in diluted air. Such oxidation steps burned the carbon coating on the catalyst particles, making them exposed to acid attack, while leaving MWNTs unaffected because of the higher stability of MWNTs against oxidation compared with amorphous carbon. MWNTs are characterized as-grown and purification by thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectrometry (EDS). It is observed that most of the mesoporous silica and metal particles have been removed from the as-grown MWNTs by these treatments. The carbon content of MWNTs material obtained after the purification is found to be more than 95 wt%. This procedure presents a nondestructive, commercially viable purification method for carbon nanotubes.