Controlled surface functionalization of carbon nanotubes by nitric acid vapors generated from sub-azeotropic solution

Abstract

Controlled surface modification of nanocarbons is crucial for their use in applications. The paper deals with the functionalization of carbon nanotubes (CNTs) with HNO3 vapors. Sub-azeotropic HNO3 + H2O + Mg(NO3)2 solution is used for the generation of nitric acid vapors. Because this approach allows tuning the HNO3 concentration in the vapor phase, the effect of its variation on the surface chemistry and structural properties of the CNTs is investigated. A combination of analytical techniques is applied to evaluate oxidation extent of the CNT surface, selectivity towards the formation of carboxyl groups compared with other oxygenated functionalities, and CNT integrity. The comparison with liquid-phase functionalization in H2SO4 + HNO3 mixture (1 : 3–3 : 1 v/v), conventionally utilized for oxidizing CNTs, shows that vapor-phase functionalization affords greater surface oxygen uptakes and higher selectivity towards the formation of carboxyl groups, with smaller tube damage; more importantly, it evidences that, regardless of the method and conditions chosen, the selectivity towards carboxyl groups increases linearly with the surface oxygen concentration. The presented results prove that the product of HNO3 concentration in the vapor-phase (25–93 wt%) and vapor-phase functionalization duration (0.5–5 h) controls the surface oxygen concentration. A simple rate model is proposed to account for its increase. Copyright © 2015 John Wiley & Sons, Ltd.