Catalyst-free synthesis of multi-walled carbon nanotubes from carbon spheres and its implications for the formation mechanism

A route to producing multi-walled carbon nanotubes (MWCNTs) was reported, in which polyacrylonitrile microspheres (PANMSs) were assembled into one-dimensional strings of carbon spheres and transformed to carbon nanotubes at 1000 °C. It was found that the diameters of the MWCNTs are uniform and correlated with the size of the PANMSs used. Structures of intermediate products obtained by stopping the reaction have been examined by high-resolution transmission electron microscopy. The observations indicate that the formation of MWCNTs follows a different mechanism from the well-studied vapor–liquid–solid mode. It involves a direct self-assembly and solid-state structural transformation of PANMSs under the promotion of nitrogen atoms as shown by the X-ray photoelectron spectra of the resulting samples. On the basis of these observations, a sphere-string-tube mechanism was proposed for the MWCNT formation.