Unveiling origins of defect peaks in carbon materials by analyzing oxygen and non-hexagonal rings in isotropic pitch-based carbon fiber using Raman, infrared, X-ray photoelectron spectroscopy, and density functional theory calculations

Computational spectroscopic analyses of defects in carbon materials have been widely conducted, but a detailed understanding of structures containing oxygen, non-hexagonal rings, and sp3C remains limited due to an insufficient number of calculated models. In this work, isotropic pitch-based carbon fiber was analyzed by experimental and simulated Raman, infrared, and X-ray photoelectron spectroscopy (XPS) using density functional theory calculations as an example to analyze general carbon materials prepared mainly at high temperatures (1473 K or higher). One of the origins of generally reported peaks for sp3C in carbon materials at ca. 285 eV of C1s XPS spectra was unveiled as carbon atoms surrounded by three rings including at least one heptagon, one octagon, or an even larger vacancy defect, if the origin of the peak at ca. 285 eV is not charged-up sp3C, C–N, or adventitious carbon. The peaks at ca. 1500–1550 cm−1 in Raman spectra were unveiled to originate from C=C in hexagonal rings influenced by non-hexagonal rings and oxygen-containing functional groups such as cyclic ether. It was revealed that the main defects in isotropic pitch-based carbon fiber heated at 1873 K or higher were edges (one and two adjacent sp2C–H on one aromatic ring), cyclic ethers, and non-hexagonal rings (pentagon, heptagon, and/or octagon). Graphical abstract