Role of plasma treatments on the functionalization and sorption ability of carbon nanotubes

Carbon nanotubes were oxygen-functionalized using maleic anhydride under different cold plasma types, including radio frequency or bipolar pulsed discharges, with either capacitively coupled or dielectric barrier configurations, and with or without sample holder vibration. The samples were characterized using Fourier-transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, N 2 adsorption test, and zeta potential measurements. The results show that using bipolar pulsed plasma in a capacitive coupled reactor without vibration are the parameters that promote the highest oxygen attachment to the CNT surface, which is 2.3 times the oxygen atomic content of untreated CNT. Other combinations also provided functionalization to varying degrees, along with samples’ drying and preservation of the nanoparticles’ structure. Batch sorption experiments evaluated the use of functionalized nanotubes in methylene blue removal from water. For the sample treated with the best set of plasma parameters, after 1 h the sorption capacity was about 33% higher compared to untreated nanotubes, and after 3 h, the removal efficiency reached 99%. Sorption isotherms also indicate that treated samples presented adsorption mechanisms more complex than untreated CNT, including multilayer and cooperative interactions.