Transformation of multi-walled carbon nanotubes to amorphous nano-spheres, micron-size rods and flakes by oxidative sulfation reaction

This study unveils a chemical functionalization provoked phase transformation in the naphthalene modified multi-walled carbon nanotubes (MWCNTs) and the associated formation of amorphous carbonaceous particles of various size and morphology. Functionalized nanotubes were prepared by a sulfonation-oxidization and a highly corrosive oxidative sulfation reaction then characterized by means of thermogravimetry/mass spectrometry, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy and Raman spectroscopy. In spite of the small change in the composition of sulfonation and sulfation reaction mixtures, conventional sulfonation with oleum resulted in lower sulfonation and higher oxidative conversion of MWCNTs accompanied with a mediated exohedral amorphization of the nanotubes, induced by the yield of sp3 hybridization defect introduction. On the other hand, the newly applied oxidative sulfation reaction provided severe oxidative etching, cutting and shortening of MWCNTs with a subsequent high conversion of sulfate derivatisation of oxidized functionalities, which induced complete collapse and phase transformation of the nanotubes. Amorphization and phase transformation of oxidative sulfated MWCNTs have led to the formation of highly dispersed nano- and micron-size amorphous carbon spheres, rods and flake-like structures. Water based colloidal systems of excellent stability were prepared from the oxidative sulfated carbonaceous material. Consequently, the employed highly corrosive sulfation reaction is capable to provide highly functionalized carbonaceous nano and micro-size particles from MWCNTs.

Research highlights
► Agitation in oleum gives low sulfonation and high oxidation yield of modified MWCNTs.
► Oxidation related hybridization defects provoke exohedral amorphization of MWCNTs.
► Mixed acid based oxidative sulfation provides superior efficacy of MWCNT sulfation.
► Oxidative sulfation leads to amorphization and complete phase transformation of CNTs.
► Sulfate functionalized carbonaceous particles exhibit slow sedimentation in water sol.