Enhancement of microplasma-based water-solubilization of single-walled carbon nanotubes using gas bubbling in water

The authors have previously proposed a novel technique for the preparation of water-soluble carbon nanotubes (CNTs) using microplasma generated by a pulsed streamer discharge in water. This paper describes an improvement in the method of the microplasma-based CNT solubilization process by the use of gas bubbling in water. Oxygen, argon and nitrogen were used as bubbling gas in order to clarify the effects of the gas species on the single-walled CNT (SWCNT) solubilization efficiency. Ultraviolet-visible absorption spectra of the SWCNT suspensions revealed that the SWCNT solubility was increased by more than two times by using gas bubbling together with microplasma treatment. No significant difference was observed among the three gas species tested. Fourier transform infrared (FTIR) spectroscopy and x-ray photoelectron spectroscopy (XPS) analysis showed that the number of -OH groups, introduced on the SWCNT surface by the microplasma treatment, was increased by gas bubbling. Optical emission measurements also showed that the number of highly oxidative oxygen and hydrogen radicals, which were generated by the microplasma, was also increased by gas bubbling. These results indicate that gas bubbling has positive effects on microplasma-based SWCNT solubilization as a result of enhanced radical formation and functionalization of the SWCNT surface.