Surface and electrosurface characterization of surface-oxidized multi-walled N-doped carbon nanotubes

In our experiments, N-doped multi-wall carbon nanotubes (CNTs) were produced by chemical vapor deposition (CVD method). The bamboo-shaped carbon nanotubes were treated with various oxidizing agents, namely H2O2, HNO3, HClO4 and a binary mixture of H2SO4/HNO3 acid. The purity of the CNTs was tested with thermo-gravimetric methods.TEM and FTIR microscopy was used to investigate the relative content of oxygen-containing functional groups formed on the CNT surfaces. For the characterization of the electric double layer (EDL) of the surface-functionalized carbon nanotubes, the zeta-potential of the CNTs samples was measured as a function of pH and electrolyte concentration (for 1–1, 21–12 and 31–13 electrolytes). It has been shown that the electrokinetic behavior of CNTs essentially resembles the behavior of lyophobic colloids: (a) the ζ (CKCl) curves reveal a maximum or plateau region; (b) the addition of CaCl2 leads to a sharp decrease in the ζ-potential; (c) the increase of trivalent counterions leads to/causes a charge reversal of the surface. A correlation between the sedimentation stability of CNTs suspensions and the zeta-potential of nanotubes has been found.