NMR and EPR studies of room temperature highly fluorinated graphite heat-treated under fluorine atmosphere

摘要

Semi-ionic carbon fluoride obtained by reaction of graphite with a gaseous mixture of IF_5,HF and F_2 was modified by a post-fluorination at various temperatures in the range 100-680 deg C.The resulting materials were studied by~I9F-NMR and EPR both at room temperature and at different experimental temperatures from 200 to 360 K and from 100 to 473 K for NMR and EPR experiments,respectively.~19F-NMR gives information about the residual intercalated iodine fluoride species(IF_5,IF_6~-and IF_7)such as the temperature of their removal from the host fluorocarbon matrix and their mobility into the interlayer space.The nature of the C-F bonding was investigated by~19F-NMR,MAS/~13C-NMR,FT-IR spectroscopy and X-ray diffraction.The C-F bonding changes from semi-ionic to a mainly covalent character at a fluorination post-treatment temperature close to 450 °C.This transformation coincides with the removal of the iodine fluoride species from the host structure.The densities of the paramagnetic dangling bonds were studied by EPR;this technique allows also to investigate the local environment of the dangling bonds in the fully fluorinated samples(obtained with fluorination post-treatment temperatures higher than 450 °C).As a matter of fact,an hyperfine structure was observed for these samples;it results from interaction of the dangling bond electron with six neighboring fluorine nuclei.The role of the intercalated species(iodine fluoride compounds and HF)on the mechanisms of the extra-fluorination is also discussed.