Determination of the surface chemistry of ozone-treated carbon fibers by highly consistent evaluation of X-ray photoelectron spectra

Abstract

Room-temperature ozone treatment is a promising technique for functionalization of carbon materials: it is fast, well controllable, and leads to oxygen uptakes of up to 15 at.% on carbon fiber surfaces. We investigated the surface chemistry of untreated and room-temperature ozone-treated polyacrylonitrile-based carbon fibers using X-ray photoelectron spectroscopy (XPS). By identifying two different nitrogen species (graphitic and pyridine-like) we developed a new constraint for fitting the C 1s spectra. This approach ensured high consistency between measured elemental concentrations and elemental concentrations calculated from fitted functional group peaks. It also revealed a different relative ordering of functional group concentrations than previously reported for ozone-treated carbons. The results of this new fitting procedure were cross-checked with chemical derivatization XPS measurements, resulting in a good correlation for hydroxyl and carbonyl groups. Examination of single fiber cross-sections using energy-dispersive X-ray spectroscopy in a transmission electron microscope showed that even after intense treatments oxygen can mainly be found in the topmost 40 nm–50 nm, stressing the importance of surface sensitive techniques such as XPS. Furthermore, XPS results suggested only a minor surface degradation caused by the ozone treatment. This was corroborated by atomic force microscopy results.