Progressive and invasive functionalization of carbon nanotube sidewalls by diluted nitric acid under supercritical conditions

Distinctive direct sidewall functionalization of multi-wall carbon nanotubes (MWCNTs) has been carried out using dilute nitric acid (HNO3) under supercritical water (SCW) conditions. The functionalization proceeded invasively from the outer to the inner graphitic layers of the MWCNTs as the reaction progressed. The resulting nanotube-derived material was comprised of a functionalized amorphous carbon sheath and the remaining inner nanotube covered with the sheath. The functional groups induced in the sheath region included alcoholic hydroxyl groups, which endowed the nanotube-derived material with hygroscopicity. The reaction pathways for the functional group formation involved hydration and dehydration processes as well as HNO3 oxidation in the SCW medium. The results of this work have demonstrated the effectiveness of the SCW regime on the direct chemical modification of CNT sidewalls.