Solid State ¹³C and ¹⁹F NMR Characterization of Fluorinated Charcoal

The preparation of CF x by elemental fluorination of charcoal was studied using solid state 13 C and 19 F NMR spectroscopy. 19 F− 13 C 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 °C were assigned to graphitic carbon (C), CF, and CF 2 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 CF 2 found in the 19 F− 13 C CP/MAS NMR experiments are confirmed by direct measurement of the 19 F 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 °C, a substance of formula CF 0.16 is produced which is diamagnetic, as is CF 1.1 - 1.2, carbon monofluoride, the white end-product from complete fluorination (350 °C). The low free electron density in these materials stands in stark contrast to that of the charcoal and CF x samples prepared at intermediate temperatures.