Fluorine-Rich, Hydrophobic Graphite Fluoride with Improved Charge Transport/Storage Properties Produced by Gamma Irradiation

Abstract

It is known that fluorine-containing carbon materials are highly insulating and exhibit low dielectric permittivity due to the presence of covalent C–F bonds and F atoms with small polarizability. These electrical properties can be improved by defluorination (F loss) and by partial restoration of the aromatic character of the carbon networks. Contrary to this knowledge, we show herein that γ-irradiation of graphite fluoride (CF)n improves its conductivity and charge transport/storage properties while preserving the F content. It is found that the crystallinity and specific surface area decrease by γ-irradiation, but the platy morphology, composition, surface functional, thermal stability, and optical band gap are maintained. Comparing to the nonirradiated one, the sample irradiated at 400 kGy shows increased conductivity (10–9 vs 10–10 S·cm–1) and shorter relaxation time (0.3 vs 0.9 ms), consistent with the decreased apparent activation energy (76.3 vs 82.4 kJ·mol–1). Meanwhile at 200 kGy, the dielectric permittivity increases to ∼6 (from 4.2) with the loss tangent close to the nonirradiated sample. These findings are attributed to the variation of effective dimension of charge carriers which is optimized (depending on the properties considered) at 200/400 kGy and at 50 °C but not at elevated temperatures. The chemical-free, ambient-temperature tuning of electrical properties by γ-irradiation is also demonstrated by the calculated refractive index (up to 2.4, temperature-independent from RT to 200 °C) and the dielectric heating coefficient, which varies by a factor of 2 at the same temperature range.