Selective carboxylation versus layer-by-layer unsheathing of multi-walled carbon nanotubes: new insights from the reaction with boiling nitrating mixture

Abstract

We have studied the oxidation of multi-wall carbon nanotubes (MWCNTs) by boiling them in a nitrating mixture composed of conc. HNO₃/H₂SO₄ (v/v = 1/3). By analysis of the morphology and surface physicochemistry of the oxidation products as a function of MWCNT treatment time, we have revealed two interrelated phenomena. Firstly, the most outer walls were becoming more functionalized with carboxylic groups to the point of quasi-saturation where, secondly, oxidized MWCNTs could be desheathed uncovering the yet non-functionalized wall. These phenomena were manifested by the periodic-like nature of functionalization and de-functionalization. In the products of MWCNT oxidation – the number of graphitized MWCNT walls was determined by HR-TEM while quantification of oxygen functionalities was performed via Boehm titration. The above techniques coupled with the analysis of zeta potential and Raman spectroscopy allowed us to propose a pseudo-1^st order kinetic model for MWCNT oxidation translatable to other sp²-C allotropes. The findings mean that prolonged oxidation does not necessarily yield nanotubes of higher levels of functionalization. The final outcome is of great relevance in all fields of MWCNT applications from medicine to sensors to nanomaterials engineering.