SOLID-STATE C-13 AND F-19 NMR CHARACTERIZATION OF FLUORINATED CHARCOAL

The preparation of CFx by elemental fluorination of charcoal was studi ed 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 tempe rature. Three types of carbon species observed over the temperature ra nge -80 to 350 degrees C were assigned to graphitic carbon (C), CF, an d CF2 on the basis of chemical shift. These assignments were confirmed by measurement of cross polarization and dipolar dephasing time const ants, 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 assign ed to more remote carbon species, viz., bulk graphitic carbon. The con centrations of CF and CF2 found in the F-19-C-13 CP/MAS NMR experiment s are confirmed by direct measurement of the F-19 NMR spectrum. NMR re sults 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 measu rements and is proposed as representative of the graphitic carbon plat elet size of the charcoal. The fluorination of charcoal is initiated b y addition of fluorine to the surface of platelets. At the lowest fluo rination temperature, -80 degrees C, a substance of formula CF0.16 is produced which is diamagnetic, as is CF1.1-1.2, carbon monofluoride, t he white end-product from complete fluorination (350 degrees C). The l ow free electron density in these materials stands in stark contrast t o that of the charcoal and CFx samples prepared at intermediate temper atures.