Photochemical Transformation of Carboxylated Multiwalled Carbon Nanotubes: Role of Reactive Oxygen Species

Abstract

The study investigated the photochemical transformation of carboxylated multiwalled carbon nanotubes (COOH-MWCNTs), an important environmental process affecting their physicochemical characteristics and hence fate and transport. UVA irradiation removed carboxyl groups from COOH-MWCNT surface while creating other oxygen-containing functional groups with an overall decrease in total surface oxygen content. This was attributed to reactions with photogenerated reactive oxygen species (ROS). COOH-MWCNTs generated singlet oxygen (¹O₂) and hydroxyl radical (^•OH) under UVA light, which exhibited different reactivity toward the COOH-MWCNT surface. Inhibition experiments that isolate the effects of ^•OH and ¹O₂ as well as experiments using externally generated ^•OH and ¹O₂ separately revealed that ^•OH played an important role in the photochemical transformation of COOH-MWCNTs under UVA irradiation. The Raman spectroscopy and surface functional group analysis results suggested that ^•OH initially reacted with the surface carboxylated carbonaceous fragments, resulting in their degradation or exfoliation. Further reaction between ^•OH and the graphitic sidewall led to formation of defects including functional groups and vacancies. These reactions reduced the surface potential and colloidal stability of COOH-MWCNTs, and are expected to reduce their mobility in aquatic systems