Structural characterization of exfoliated graphite nanofibers

Structural characterization of exfoliated graphite nanofibers (EGNFs) with transmission electron microscopy (TEM) and high-angle annular dark-field–scanning TEM (HAADF–STEM) indicates exfoliation has led to structural expansion along the fiber axis, with discrete domains of graphitic nanocones separated by gaps ranging from 50 to 500Å. Image contrast in HAADF–STEM demonstrates that structural expansion dominates over chemical etching. Raman spectroscopy indicates the EGNF is more graphitic than the precursor, and the disappearance of the characteristic defect (D) peak with multi-wavelength excitation is inconsistent with the presence of amorphous carbon. The highly expanded EGNF structure oxidizes at two distinct rates at 750°C in CO2, leading to a highly-disordered graphitic fiber, with apparent collapse of the expanded structure as no gaps or discrete graphitic domains are observed after oxidation. Variation in the heat input per intercalant mass during thermal shock leads to changes in fiber morphology, including the extent of fiber expansion, the number of defects and pores observable within the fiber via TEM, and the surface area measured by nitrogen adsorption.