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Abstract: The morphology, structure, and porosity of graphite nanofibers (GNFs) can be significantly modified through simple exposure of the GNFs to acids, followed by a high temperature treatment. This treatment leads to chemical etching, fiber rupture and/or partial exfoliation. The degree of these somewhat competing processes is dependent upon acid exposure conditions and thermal treatment temperature. As the temperature of thermal treatment is increased, both the surface area and pore volume of the modified GNFs are significantly increased. Low-temperature treatments (600–800°C) tend to favor the formation of micropores, whereas higher temperatures (>800°C) lead to the formation of mesopores. An extended heat treatment at 1000°C after acid exposure produces a drastic increase of mesopores and a pronounced change in fiber morphology, evidenced by X-ray diffraction and electron microscopy. This ability to select the operative pore size of graphitic nanofibers may lead to applications of these materials for energy storage, size-selective catalysis, as well as separation techniques where a finely controlled graphitic pore structure is desired. [Copyright &y& Elsevier]

Abstract: Nanocrystalline diamond (NCD) was observed after reactive ball milling of anthracite coal with cyclohexene, a high-temperature (1400°C) thermal anneal, and a 4M HCl treatment followed by a 10M NaOH treatment. A crystalline carbon region was also observed when the thermal anneal was omitted. This crystalline region is highly unstable and converts to NCD and carbon onions via electron irradiation in the TEM. X-ray diffraction, Raman, ash content, and temperature-programmed oxidation (TPO) data suggest that tetrahedral amorphous carbon is formed during milling, iron carbides are formed during the thermal anneal step, and both the HCl and NaOH purification steps lead to changes in carbon structure. NaOH oxidizes metal carbides and this process may contribute to NCD formation. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]