Fluorographane (C1HxF1-x-δ)n: Synthesis and Properties

Fluorographane (C1HxF1-x-δ)n was obtained from graphene by hydrogenation via the Birch reaction with consequent fluorination of the resulting graphane. Fluorographane exhibits fast heterogeneous electron transfer rates and hydrophobicity, which increase with increasing fluorination.

Hydrogenated graphene is a material with many interesting properties. 1 Hydrogenation of graphene introduces a band gap, which can be tuned from 0 to 3.7 eV for graphene and fully hydrogenated graphene (graphane), respectively. 2 Hydrogenated graphene exhibits fluorescence and paramagnetism, 3 properties that are not seen in graphene.1 In addition, hydrogenated graphenes exhibit fast heterogeneous electron transfer rates. 4 The properties of hydrogenated graphene can be tuned by the level of hydrogenation. [5][6][7][8][9] In similar manner fluorographene (C 1 F 1 ) n shows large band-gap which is tunable based on level of fluorination. 10 Fluorographene 11 shows fluorescence and enhanced electrochemical properties [12][13][14] and its 3D analogue, fluorographite found way to electrochemical application decades ago. 15 In order to add additional vectors to tune the properties of hydrogenated graphene, one can consider covalently bonding a simple electronegative element to the graphane backbone. Since most of the properties of hydrogen are similar to those of halogens, recently performed theoretical studies on fluorinated graphane showed that the incorporation of fluorine to graphane would lead to additional opening of the band gap 16 and that such material exhibits a large piezoelectric effect. 17 Fluorination of materials in general is an excellent way to tune their catalytic properties. 18,19 To best of our knowledge, no experimental report on the synthesis and properties of fluorographane has been published. Here, we report the synthesis of fluorographane via a two-step method, first involving the creation of C-H bonds in a graphene framework with consequent fluorination of the resulting hydrogenated graphene to create fluorographane. We report detailed characterization of fluorographane via combustible elemental analysis, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy/Xray energy dispersive spectroscopy, and infrared spectroscopy along with the heterogeneous electron transfer rates at various (C 1 H x F 1-x-δ ) n by cyclic voltammetry. We will show that while it is challenging to fluorinate graphite and graphene, which requires the use of high temperatures (~200-400 o C), the hydrogenated graphene (graphane) is highly reactive and significant fluorination of graphane proceeds even at atmospheric pressure fluorination with F 2 /N 2 mixture for 1 h. Very high content of fluorine in fluorographane can be obtained at longer fluorination times and higher pressures.
Synthesis of hydrogenated graphene was performed via Birch reduction process. Graphite was oxidized to graphite oxide (GPO) using the permanganate route (Hummers) 20 ; GPO was reduced using hydrazine and hydrogenated via Birch method.3 The resulting hydrogenated graphene (graphane) of composition 45.54 % at. of C, 49.81 % at. of H, 4.40 % at. of O and 0.26% at. of N of summary formula C 1 H 1.09 was then exposed to various fluorination conditions: hydrogenated graphene was exposed to a fluorine/nitrogen gas mixture (20 vol.% F 2 ) at a pressure of 1 bar for 1 h; 5 bar for 24 h, and 5 bar for 24 h with consequent fluorination at 12 bar for another 24 h. The resulting materials, labeled accordingly as CHF [1h:1bar], CHF [24h:5bar], and CHF [24+24h:5+12bar] were characterized in detail.
We found that significant fluorination occurs at partial pressures of F 2 as low as 0.2 bar for 1 h and that F 2 1 bar for 24 h saturates graphane so that a consequent increase of pressure and time does not lead to a significant increase in the fluorine content in graphane, reaching a F/C ratio of 0.75. We performed a detailed characterization of the fluorine content as well as of the morphology of the resulting fluorographanes.
According to a combustible elemental analysis, the fluorographane materials prepared by this method contained for CHF H 0.13 F 0.77 , and C 1 H 0.14 F 0.73 , respectively. Delta (δ) in (C 1 H x F 1-x-δ ) n stands for the remaining elements, mostly O and traces of N, which were introduced during the synthesis. It is obvious from the results that with the increased time and pressure of fluorination, there is a significant increase in the amount of fluorine at the expense of the amount of hydrogen; one can expect substitution reaction occurring during the fluorination of graphane. It can be also seen that fluorination proceeds at very mild conditions (0.2 bar F 2 , 1 h, room temperature) where the F/C ratio is 0.17 and dramatically increases to the saturation point at higher pressures of 1 bar (24 h) to an F/C ratio of ~0.75. Further increase of pressure and time did not lead to higher content of F in graphane. Schematic of the proposed structure of fluorographane is shown in Scheme S-1 (ESI).   Table S-1). This can be explained by partial etching of carbon atoms and formation of perfluorinated terminal carbon atoms. Note that XPS cannot provide direct evidence of a C-H bond. We also performed high resolution XPS spectra measurement on F 1s peak (see Figure S-3). The results confirmed the presence of C-F band formation. Slight difference between F/C ratios as determined by various methods originates from different sensitivities of these methods. Combustible analysis takes in account whole sample composition, while XPS is surface sensitive, taking in account only few atomic layers whilst SEM/EDX provides analysis of small portion of the sample.
We performed FTIR measurements of the fluorographanes, which indicate the presence of both C-H and C-F bonds ( Figure S   We have studied the electrochemical behavior of fluorographanes. For any electrochemical application, it is important to determine heterogeneous electron transfer of the material. We used ferro/ferricyanide as an electrochemical probe (Fig. 3 increases. This trend is similar to the trend observed for fluorinated graphites and graphenes, where the HET rates increased with an increased amount of fluorine in the structure. 22,23 In addition, we wish to demonstrate that with increased fluorination of fluorographane, the hydrophobivity of the material increases, as shown in In conclusions, we have for the first time successfully prepared fluorographanes with varied ratios of H and F. We used Birch method for preparation of hydrogenated graphene (graphane) followed with fluorination of the graphane. This new member of graphene family shows fast heterogeneous electron transfer properties. We expect that fluorographane will find variety applications. Changes in the fluorine concentration can be used for development of surface coating with tailored hydrophobic properties.

Acknowledgements:
The project was supported by Czech Science Foundation (Project