A green and economical vapor-assisted ozone treatment process for surface functionalization of carbon nanotubes

Abstract

We report herein a green and economical strategy for oxidative modification of carbon nanotubes (CNT) by a facile ozone treatment in the presence of solvent vapor (H₂O, 30% H₂O₂ solution and ethanol). This procedure is able to introduce a considerable level of oxygen groups with a dominant population of carboxyl groups at near ambient temperature. The optimization study of the process provides the reaction temperature and time, as well as the source and partial pressure of the vapor, key parameters determining the oxidation efficiency. The process efficiency and sustainability are assessed to compare with conventional oxidative treatment (e.g., nitric acid oxidation). Ozone treatment assisted by water or H₂O₂/H₂O vapor can introduce abundant oxygenated groups on the sidewalls of CNT, and the process with H₂O₂/H₂O vapor can deliver a much higher selectivity towards carboxyl species on the sidewalls of CNT. With the appropriate functionalization temperature (40 °C) and incremental partial pressure of vapor, the surface defect and oxygen content on the CNT could be dramatically increased. The ozone treated CNT can be used for diverse applications such as metal-free catalyst and support; the modified CNT display higher activity for the ring-opening reaction of styrene oxide than HNO₃ treated CNT and develop strong interaction for anchoring iron oxide nanoparticles even up to 350 °C. This ozone treatment in the presence of vapor could also be accomplished by using compressed air as the carrier gas, which can be considered an efficient and eco-friendly functionalization process.