Oxygen functionalization of MWCNTs in RF-dielectric barrier discharge Ar/O₂ plasma

The oxygenation of multi-wall carbon nanotubes (MWCNTs) was performed via a radio frequency dielectric barrier discharge (RF-DBD) in an Ar/${\text{H}}_2\text{O}$ plasma mixture. The relative intensity of the Ar/${\text{O}}_2$ plasma species was characterized by optical emission spectroscopy (OES). The effects of treatment time, RF power and oxygen gas percentage on the chemical composition and surface morphology of MWCNTs were investigated by means of x-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and field emission scanning electron microscopy (FE-SEM). The results of FTIR and XPS revealed the presence of oxygen-containing functional groups on the MWCNTs treated in an Ar/${\text{O}}_2$ plasma at an RF power of 50 W and pressure of 400 Pa. The amount of oxygen functional groups (C=O, C–O, and O–COO) also increased by increasing treatment time up to 6 min, but slightly decreased when treatment time was increased by 10 min. The increase of oxygen gas percentage in the plasma mixture does not affect the oxygen content in the treated MWCNTs. Meanwhile, MWCNTs treated at high power (80 W) showed a reduction in oxygen functional groups in comparison with low RF power conditions. The Raman analysis was consistent with the XPS and FTIR results. The integrity of the nanotube patterns also remained damaged as observed by FE-SEM images. The MWCNTs treated in RF-DBD using the Ar/${\text{O}}_2$ plasma mixture showed improved dispersibility in deionized water. A correlation between the OES data and the observed surface characterization for an improved understanding of the functionalization of MWCNTs in Ar/${\text{O}}_2$ plasma was presented. (paper)