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International audience; Fluorine–graphite intercalation compounds were prepared from highly oriented pyrolytic graphite (HOPG) and natural graphite under high pressure of fluorine (16 MPa) at room temperature and were analyzed by means of X-ray diffraction. The rate of intercalation under high pressure of fluorine was higher than that under ordinary pressure. The compositions of the compounds obtained from HOPG and natural graphite were estimated by the weight change before and after fluorination to be C3.41F and C5.62F, respectively. They were blue–black compounds composed of two different phases. One phase is a conventional stage II CxF with the repeat distance of ca. 0.93–0.96 nm which was the same compound as that obtained under ordinary pressure of fluorine, and the other was also a stage II with a different repeat distance of ca. 1.03–1.18 nm. The fluorine species intercalated between the graphene layers can easily move between both phases and the phase transition occurs periodically. The decomposition rate of fluorine-intercalated HOPG was extremely low and it shows a long-term stability in air for 26 years.

Thin films of boron-substituted carbons have been synthesized by chemical vapor deposition. X-ray diffraction measurements showed that the d(002) value depends both on B at.% in the deposits and on the reaction temperature. Addition of boron increases the crystallinity up to a certain boron concentration, but beyond this critical value, on the contrary, the crystallinity deteriorates as the boron content increases. 11B-NMR indicated that isolated boron atoms are dominant at low boron concentration in the deposits, and that the number of boron-to-boron connected atoms increases rapidly when the boron content is higher.

The effect of CF 4 -plasma enhanced fluorination on the surface modification of carbon blacks has been examined using XPS. Three different types of carbon blacks have been studied: a thermal black, a furnace black and a high electrical conducting black. The analysis of the XPS spectra of fluorinated carbon black samples indicates that all fluorine atoms, fixed at the surface and in a subsuperficial zone of the particles, are covalently linked to carbon atoms. The influence of the physicochemical properties and morphology of these three types of carbon blacks on the fluorination reaction has also been investigated. The proportion of different types of fluorinated carbon atoms, i.e. on one hand CF x surface and border groups of graphitic bulk domains for which the planar configuration of the graphene layers is preserved together with the sp 2 character of C, i.e. structures of type I, on the other hand polyalicyclic perfluorinated structures in which sp 3 C form puckered layers similar to those of covalent fluorographites, i.e. structures of type II, and also the F/C ratio of the fluorinated groups are related to the surface morphology and depend on the microstructural organization of particles. When the microstructure ordering and graphitic character of the carbon increase, the size of the ordered graphitic domain also increases. At the same time the density, the size of defects and proportion of protonated sp 3 C entities bridging the graphene layers decrease. As a consequence, the proportion of carbon atoms, potentially able to form perfluorinated CF 2 and CF 3 groups, decreases. The relative contribution of those groups is appreciably higher in fluorinated compounds which are derived from carbon blacks with a lower structural order.

The deintercalation process of fluorine species from fluorine-graphite intercalation compounds (F-GICs) has been investigated by evolved gas analysis. The controlled transformation rate thermal analysis method was used. In this way, the rate of production of the gaseous phase is permanently controlled at low pressure, so that both temperature and pressure gradients can be lowered at will, avoiding the overlap of the successive steps of the reaction. These experiments have been carried out on stage-1 F-GICs with C F ratios ranging from 3 to 4, which were obtained from high-temperature treated carbon fibres (ex-pitch) fluorinated at room temperature with P F 2 bar in the presence of gaseous HF. The thermal analysis exhibits three parts: the first two parts correspond to the deintercalation of fluorine and HF; and the third part (> 460 °C) to the degradation of the carbonaceous skeleton. The activation energies associated with the different parts have been determined by the rate-jump method. A kinetic study of the deintercalation/decomposition processes has been proposed. The first (deintercalation) part is apparently accompanied by a post-deintercalation reaction of evolving fluorine with carbon atoms of defects leading to the formation of covalent CF bonds. The deintercalation is achieved during the second part which can be described by a two-dimensional interfacial advancement process. The apparent activation energy of this deintercalation part has been evaluated to be 144 kJ mol−1. The degradation part, with 240 kJ mol−1 activation energy, can be ascribed to a two-dimensional nucleation and growth of nuclei mechanism.

The fluorination of a furnace carbon black (N115 from Degussa) with gaseous F2, diluted in N2, at room temperature was studied by XPS. The influence of the reaction parameters (% of F2 in the gaseous mixture, duration of contact) and activation (O2 at 450 °C) of the carbon black on the amounts of fluorine fixed, in the bulk and at the surface and subsurface zone of the carbon particles has been investigated. By analyzing the C1s and F1s peaks, different forms of C and F were evidenced and their relative contributions in fluorinated black determined. Information concerning the nature of the CF bond formed and the effect of the fluorination on the structure of the pristine carbon black is given. It is shown that the fluorinated product obtained is of polyphasic nature. Fluorinated islands of limited thickness alternate with non-fluorinated zones that have kept their original structure. F is mainly covalently bonded to reactive C atoms of the surface and subsurface shell of the particles.

A critical review of the literature relative to the chemical characterisation of fluorinated carbon compounds by X-ray photoelectron spectroscopy is proposed. Guidelines for identification and assignation of the different components which are individualized by fitting of the C 1s and F 1s peaks are given. It is suggested that in fluorocarbons, the chemical shifts of the C and F core levels depend not only on the nature of the CF bond (ionic, semi-ionic or covalent), the number of F atoms linked to the C atom and the chemical environment of the CF group (β effect), but also on the structure and conformation of the C atom skeleton in the fluorinated material. The types of fluorinated structural unit that are formed vary as a function of the conditions of fluorination (temperature, eventual presence of traces of HF or fluoride acting as catalysts) and of the structure and graphitic character on the starting material. Since the experimental values of the binding energy of the different types of CF bonds proposed by the authors differ to a slight extent, some fluorinated polymers and fluorographites of known composition have been analysed by XPS. The binding energies of the components of the C 1s and F 1s peaks, the energy separation ΔCiCr of the former relative to the component (Cr) corresponding to hydrocarbon contamination and the energy separation ΔCiFi between the corresponding components of the C 1s and F 1s peak have been determined and correlated to the nature of the chemical and structural environment of the involved atom.

A high resolution transmission electron microscopy study of fluorine-intercalated carbon fibers has been carried out on two types of host materials: high-temperature treated pitch-based and PAN-based fibers. TEM images and corresponding Fourier transforms have shown that only stage-1 compounds and graphitic-type domains are detected in the experiment conditions. In particular, no interlayer spacing corresponding to higher staged materials have been noticed. The observed results can be correlated to the ratio of semi-ionic to covalent FC bonding previously determined by XPS. The disorder mostly depends on the nature of the host fiber and on the amount of mobile species that may react with the graphene layers during the deintercalation process, which is favored by the high-vacuum conditions.

The intercalation of fluorine in various types of carbon fibers (PAN-based or pitch-based, asreceived or high-temperature treated) has been investigated at room temperature in the presence of gaseous HF. Stage-1 compounds with C2.5F to C4F compositions are obtained for 10 bar F2 pressures, whereas lower pressures (1 bar F2) lead to stage-2 compounds. Although in higher stages (≥2) the electrical conductivity is generally larger than in the pristine fiber, in stage-1 compounds a drastic increase of resistivity is observed, ρ being more than one order of magnitude larger than that of the starting material. Finally, fluorine-intercalated GICs have been found appropriate to investigate the effects of disorder and reduced dimensionality.