Nitric acid oxidation of vapor grown carbon nanofibers Original Research Article

Vapor grown carbon nanofibers (Pyrograf III™) with 100–300 nm diameters and ∼10–100 μm lengths were oxidized in 69–71 wt.% nitric acid (115 °C) for various times (10 min to 24 h). These fibers were remarkably oxidation-resistant. XPS (O1s) showed that the surface atomic oxygen percent increased from 6.3 to 18.3–22.5% for 10–90 min oxidations followed by a drop to 14–15% after 10–24 h oxidations. No damage was observed by TEM. Little change in surface area was observed by N2 BET but CO2-DR measurements exhibited an increase from 20–25 m2/g to 41–73 m2/g after 10–90 min of oxidation followed by a decrease to 35–22 m2/g after 10 h, consistent with the XPS findings. Shallow ultramicropore formation could account for the surface area increase. NaOH titrations showed ∼3-fold increase in surface acidic functions (∼27 to 76 μmol/g) occurring after 10 min of oxidation. Then this level remained constant through 24 h of oxidation. XPS (C1s), O1s) confirmed that carboxyl groups were removed and ester, anhydride, quinoid and phenolic hydroxyls appeared upon HNO3 oxidation. Oxidized fibers dispersed when shaken in water, demonstrating wettability had increased. A model for this oxidation behavior is proposed.