Minghua Li, Thomas P. Beebe, and Chin-Pao Huang. University of Delaware, Newark, DE
Functionalization of carbon nanotubes (CNTs) is critical for enhancing its application, processability and performance in various fields. Changes of surface characteristics can also affect the fate, transport and effects of these particulates in the environment. A clean and simple wet chemical oxidation method involving only ozone and ultrasound (US) processing in water was used to functionalize single-walled CNTs (SCNTs). SCNTs were disintegrated, oxidized and even shortened by O3 or O3/US treatment for 24 hr. After treatments, the solubility and aqueous phase stability of SCNTs were improved. Oxygen functional groups were formed on the SCNTs surface. Surface oxygen to carbon ratio increased by more than 600% after 72 hr of O3 treatment based on the X-ray photoelectron spectroscopy (XPS) results. XPS showed evidence for surface C-OH, C=O and COOH groups as oxidation products. The effective particle length was reduced from initial 5000 to 300 and to 150 nm, respectively after 24 hr of O3 and O3/US treatments, indicating SCNTs shortening during the oxidation process. The surface potential of SCNTs decreased from 5 to -29 mV (at pH 2) after 2 hr O3 US treatment. Results of sedimentation and turbidity measurements were consistent with that of XPS analysis. An oxidation pathway was proposed and results were fitted with various kinetic models to identify the pertinent oxidation kinetic parameters. The oxidation state of the SCNTs can be controlled by the O3/US exposure time without any additional treatments; this aqueous phase oxidation offers the potential for use as a low-cost CNTs surface process.