Ozone-Mediated Functionalization of Multi-Walled Carbon Nanotubes and Their Activities for Oxygen Reduction Reaction

The functionalization of multi-walled carbon nanotubes (MWCNTs) by ozone treatment has been systematically investigated by using Raman spectroscopy, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), organic elemental analysis (OEA) and Boehm titration. The results showed that the functionalization process occurred at defective sites (opened mouths, tube caps, debris, etc.) before opening caps and truncating walls, and finally the graphitic structure was deteriorated. The surface oxygen content first increased with the treatment time but kept at around 8.0 wt% after 5 h. The analysis of the distribution of oxygen-containing groups revealed that phenolic hydroxyl was gradually converted to carboxyl and lactone. The carboxyl was found to play a pivotal role to reduce the over-potentials when we used the functionalized MWCNTs as the catalyst for oxygen reduction reaction (ORR).