Thermal stability of oxygen functionalization in v-CNTs by low kinetic energy ion irradiation

Vertically aligned multiwalled carbon nanotubes synthetized by catalytic chemical vapor deposition are irradiated with low kinetic energy oxygen ions to graft oxygen functional groups at their surface. Subsequently, the thermal stability of these functional groups is investigated by heating the sample progressively to 200, 300, 400 and 500 °C. X-Ray photoelectron spectroscopy (XPS) is used to analyze the atomic concentration and chemical configuration at the sample surface after each stage. Scanning and transmission electron microscopies are used to observe the morphology of the nanotubes after the thermal treatment. The solvent-free functionalization is carried out in 10 min reaching an oxygen atomic concentration of 17.2 at.%, with no impurities introduced. At the end of the thermal treatment, the relative oxygen content drops to 5.6 at.%. XPS analysis indicates that four oxygen groups produced by the ion irradiation: epoxide, hydroxyl, carbonyl, and carboxyl groups, with the epoxides being the least stable and the carbonyls the most stable. Combining low kinetic energy ion-irradiation with a post-thermal anneal is shown to be an effective route to simultaneously control both sample oxygen content and the ratio of different oxygenated functional groups.