Insights into the sequential oxidation mechanisms on the surface of carbon nanotubes by photoinduced force microscopy

The oxidation process of carbon nanotubes (CNTs) is a critical step in their industrial applications. Understanding the surface oxidation mechanism of CNTs enables precise control of the surface properties and fine-tuning of CNT device performance. In this study, photo-induced force microscopy (PiFM), which integrates infrared spectroscopy and atomic force microscopy (AFM), was used for the first time to visualize the oxidative functional groups on single-walled carbon nanotube (SWCNT) bundles treated with a KMnO₄/H₂SO₄ solution. SWCNTs grown by enhanced direct injection pyrolytic synthesis (eDIPS) were the primary focus in this study. The PiFM imaging revealed the presence of oxidative functional groups including epoxides, alcohols, ketones and carboxylic acids on the surface of SWCNT bundles. The distribution of epoxides and other functional groups showed distinct differences, suggesting that a continuous chemical reaction involving epoxide cleavage and alcohol oxidation takes place during the oxidative treatment. The heterogeneity observed in each functional group also indicates that the surface functionalization and debundling of CNTs occurred simultaneously during the reaction. Furthermore, the functional group distribution of another type of as-grown SWCNT synthesized by the water assisted super-growth method (SG-CNTs) was investigated. PiFM mapping results revealed that epoxides and alcohols were uniformly distributed on the as-grown SG-CNTs. This study elucidates the surface oxidation mechanism on CNTs and the differences in oxidation susceptibility depending on the CNT growth methods.