Solid state 13C and 19F NMR characterization of fluorinated charcoal

The preparation of CFx by elemental fluorination of charcoal was studied using solid state C-13 and F-19 NMR spectroscopy. F-19-C-13 CP/MAS NMR experiments determine the extent of fluorination vs reaction temperature. Three types of carbon species observed over the temperature range -80 to 350 degrees C were assigned to graphitic carbon (C), CF, and CF2 on the basis of chemical shift. These assignments were confirmed by measurement of cross polarization and dipolar dephasing time constants, T-CF and T-DD, respectively. The fluorinated carbons fully cross-polarize in tenths of milliseconds, while polarization transfer among graphitic carbon is slower and is explained by a two-component model. One component, with T-CF less than 1 ms, is assigned to sp(2) carbons adjacent to fluorinated carbons, viz., interfacial graphitic carbon. The other component, with T-CF on the order of milliseconds, is assigned to more remote carbon species, viz., bulk graphitic carbon. The concentrations of CF and CF2 found in the F-19-C-13 CP/MAS NMR experiments are confirmed by direct measurement of the F-19 NMR spectrum. NMR results are presented along with gravimetric and XPS results to provide new insight into fluorocharcoal structure. An average platelet size of 2-4 nm for the fully fluorinated charcoal is derived from these measurements and is proposed as representative of the graphitic carbon platelet size of the charcoal. The fluorination of charcoal is initiated by addition of fluorine to the surface of platelets. At the lowest fluorination temperature, -80 degrees C, a substance of formula CF0.16 is produced which is diamagnetic, as is CF1.1-1.2, carbon monofluoride, the white end-product from complete fluorination (350 degrees C). The low free electron density in these materials stands in stark contrast to that of the charcoal and CFx samples prepared at intermediate temperatures.