Purification and functionalisation of multi-walled carbon nanotubes

•Strong oxidation caused MWCNTs damaging, i.e. tube breaking and shortening.•MWCNTs functionalisation results in highCOOH group concentration.•Catalysts impurities concentration decreased by 90 wt% after treatment. Purifying and functionalising multi-walled carbon nanotubes (MWCNTs) are important processes for providing property modifications, improving handling and enhancing reactivity. The removal of impurities allows avoiding synthesis process interferences and the formation of by-products. During the process it is possible to change the MWCNT properties, e.g. providing enhanced hydrophilicity and better chemical reactivity. In this work, a two-step procedure of MWCNT functionalisation with non-oxidative and oxidative agents is presented. Firstly, hydrochloric acid was used to remove catalyst impurities, which remain after the synthesis process. Secondly, nitric acid was employed to oxidise the MWCNT surfaces and incorporate carboxylic groups onto the surface.