XPS and Sem Characterization of Pan-Based Carbon Fibers Treated in Oxygen and Nitric Oxide Plasmas

Abstract

ABSTRACTUnsized, type II carbon fibers were treated for 2 to 60 minutes in either an oxygen or nitric oxide gas plasma and were investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). XPS revealed plasma treatments increased both oxygen and nitrogen functionality on the fiber surface. O2 plasma treatments showed a progressive increase of fiber surface oxidation from alcohol to carboxyl species, increasing the total surface oxygen from 3 atomic % to 46% after 60 minutes. N(1s) spectra for these fibers showed a broad peak centered at ca. 400.4 eV with a shoulder at ca. 398.2 eV, which most likely is attributed to nitrogen remaining from the polyacrylonitrile (PAN) precursor material. NO treatments only increased the total surface oxygen to 33%, but introduced additional nitrogen species onto the fiber surface compared to O2 plasmas. N(ls) spectra for the NO plasma treated fibers showed, in addition to the two nitrogen species mentioned above, a higher binding energy peak at ca. 405.5 eV, which is assigned to -NO2 species. This peak increased to a maximum intensity after 5 minutes of treatment and then decreased upon further treatment, even though the total nitrogen intensity remained constant at approximately 6%. Subsequent increases in the two lower binding energy species at 400.4 eV and 398.2 eV suggest -NO2 surface species were reduced to species such as amides, nitriles, and amines. Reducing the NO plasma power levels from 25 Watts to 5 Watts resulted in the same three peaks in the N(ls) spectra but required 30 minutes for the -NO2 species to reach a maximum intensity. Scanning electron micrographs (x 10,000 magnification) of fibers treated up to 5 minutes revealed that plasma treatments did not significantly change surface morphology. Small bumps on the surface appeared after longer treatments as the plasma began etching the fiber surface, but even 60 minutes of treatment did not significantly alter the overall fiber diameters.