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

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 ((1)O2) 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 (1)O2 as well as experiments using externally generated (•)OH and (1)O2 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.