High-yield preparation of ultrahigh-hydrophilicity MWCNTs by highly controllable oxidation with concentrated HNO3/HClO4 mixture

Abstract

Functionalization of carbon nanotubes (CNTs) by an oxidation method is the essential step for many applications of CNTs. Herein, concentrated HNO3/HClO4 mixture is used to oxidize multi-walled carbon nanotubes (MWCNTs). The results show that the MWCNTs functionalization level is readily adjusted by regulating both the HNO3/HClO4 molar ratio of 1:(0.5–2) and the reaction temperature. The results for the yield, chemical structure and morphology of the MWCNTs show that the oxidation takes place in two stages. In the initial stage, the intense introduction of oxygen-containing groups dominates the oxidation reactions thereby obtaining functionalized MWCNTs in an unexpectedly high yield of 110% and carbon conversion of >98%. The second stage is characteristic of an obvious decrease in the yield of products, the carbon conversion and apparent damage to the MWCNTs. The carboxyl, hydroxyl and ether-like linkages are the predominant functional groups introduced with a total content of 3–7 mmol/g. Moreover, the high-yield functionalized CNTs exhibit excellent aqueous hydrophilicity thereby achieving stable MWCNTs aqueous suspensions of >40 mg/mL. It is affirmed that, in the mixture, HNO3 and HClO4 play a role in oxidizing and increasing the oxidizing ability of the HNO3, respectively.