A brief review for fluorinated carbon: synthesis, properties and applications

2.1 Structure of fluorinated carbon

Typically, there are three types of fluorinated carbon as shown in Figure 1. The structure characteristics of fluorinated carbon mainly include C-F bonds and F/C ratios, which can determine the physical/chemical features of materials. Therefore, it is significantly important to deeply understand the structure for tuning the properties of fluorinated-carbon.

Figure 1

The schematic diagrams of some typical fluorinated carbons with an F/C ratio of 1:1. (a) F-C₆₀ view onto; (b) F-CNT; (c) F-graphene top view; (d) F-C₆₀ view across; (e) F-CNT; (f) F-graphene side view

For C-F bonds of fluorinated carbon, the extremely high electronegativity of fluorine endows the C-F bonds varying from ionic bonds, through semi-ionic bonds, to covalent bonds, which results in more electrostatic character in the C-F bonds [26]. It was found that the ionic C-F

bonds are typically only formed when the percentage of F in fluorinated carbon is extremely small (e.g. x<0.05 in CF_x for graphite fluoride), which makes no sense for its applications, especially for energy storage. The semi-ionic C-F bonds are of great significance to the properties of fluorinated carbon. Sato et al. demonstrated the existence of semi-ionic C-F bonds in first stage fluorine-graphite intercalation [27]. Moreover, Zhu et al. confirmed that the formation of semi-ionic C-F not only enhanced the discharge capacity and plateau but also provided good conductivity for significant power density [28]. Covalent C-F bonds own the characteristics of sp₃ C, where the angle of F-C-C is greater than 90^∘ and the length of the neighboring C-C bond is about ∼ 0.153 nm. So, a high covalent C-F ratio might destroy the conductive network of conjugate double bonds [29]. Meanwhile, the covalent C-F includes C-F, CF₂, and C-F₃, the ratio of those bonds also have an effect on the properties of fluorinated carbon. Thus, reasonable tuning of the type of C-F bonds is one of the key factors in the regulation of fluorinated carbon properties.

The semi-ionic C-F bonds are essentially covalent, which differs from bonding formation and bond length [29]. In terms of theoretical calculation, Zhou et al. calculated the shifts in F1s energy level for various fluorine modes on graphene utilizing the core-excited pseudopotential technique [15]. Figure 2 exhibits the relationship between the level shift calculated for an isolated F and the C-F bond length. The shorter the bond, the higher the shift. And the authors concluded that the ionictiy of the F-C bond depends strongly on the local chemical environment. The calculation shows that the core binding energy chemisorption of fluorine on graphene is about 3.5 eV and an intermediate core layer binding energy between the F anion in graphite and the covalent F-C bond at the edge of graphene is yielded by the F in the isolated F-C species. When the bonds loss some ionicity, they are closed to a highly fluorinated region of graphene. In the case of F-C bonds forming orderly, the semi-ionic and semi-covalent F-C bonds packed domains of fluorinated graphene. Meanwhile, the C-F bonds with the length larger than 1.5 Å can be also attributed to semi-ionic bonds with diverse interaction strength.

Figure 2

F-C bond length (ordinates) and F1s core-level energy shift (abscissa) of fluorine species chemisorbed on graphene in various bonding environments. From left to right, F bonded to edge C atoms of an armchair-type graphene edge, energetically most stable F-C tetramers, trimers, and dimers, and an individual fluorine species chemisorbed on a pristine region of graphene (bottom) and nearby a tightly packed agglomerate of F-C bonds on graphene (top). The color (dash) and the number of the vertical segments and open circles are used to establish the correspondence between core-level energy shift and the fluorine species drawn in the ball-and-stick illustration. The height of the vertical segments is used to show the length of the F-C bonds in the various bonding environments. The F1s core-level binding energies calculated from DFT are referred to that as F in single F-C species (8) chemisorbed on a pristine region of graphene. The core-level energy shift of an F anion in bilayer graphene is also reported on the right (vertical dashed segment). Reprinted with permission from [15]

Additionally, precise control of the F/C ratio of fluorinated carbon is also significant for tuning the bandgap and electrical conductivity as well as understanding the structural transformation such as type of bonds. In order to design the F/C ratios, many factors should be considered including fluorination time and temperature, fluorination agents (e.g. F₂, CF₄, XeF₂ and etc.), and carbon source (e.g. CNT, graphene graphite and etc.). Yu et al. reported that the F/C ratio of fluorinated graphene was controlled by the treatment time of CF₄-plasma [30]. Besides, Ahma et al. presented that higher fluorination temperature could result in a higher F/C ratio under a certain range [31]. Meanwhile, different F/C ratios could be controlled by direct fluorination with F₂ and indirect fluorination with TbF₄ [31].

2.2 Preparation methods of fluorinated carbon

Carbon is a precursor to synthesize fluorinated carbon materials. The main preparation methods including direct gaseous fluorination, wet chemical, and plasma fluorination were developed to synthesize fluorination carbon.

2.3 Properties of fluorinated carbon

Fluorinated carbon, owing to the properties of chemically stable, non-toxic, non-flammable, non-corrosive, excellent solvent-resistant, weather-resistant, as well as superhydrophobicity, is a promising insulating, waterproof and lubricating materials [33, 34, 35, 36]. The low surface energy of fluorinated carbon results in a good lubricity. Moreover, it is difficult to break C-F bonds at high temperature, high pressure and different gaseous due to the large bonding energy, which endows and excellent lubricating properties under high temperature, high speed and high load conditions [34]. In addition, fluorinated carbon exhibits the highest theoretical specific capacity (865 mA h g⁻¹) for primary lithium battery, tunable bandgap, and good thermal stability and conductivity [37, 38, 39]. Fluorinated carbon with such remarkable properties has been widely researched and applied in the field of self-cleaning, lubricants, super-hydrophobic coating and energy storage, et al. Here, we give a simple comparison of the performances for fluorinated carbon and carbon materials applied in different fields in the Table 2. Compare to the properties of carbon and fluorinated carbon, CF_x materials possess a lower friction coefficient than carbon,which demonstrates that the CF_x materials have longer wear lives, lower friction coefficients and superior load-carrying capacities due

to the weakened interlamellar van der Waals forces. The surface tension and energy are calculated by the contact angle. The higher contact angle signifies that the materials possess a lower surface energy. Hence, the large contact angle of CF_x materials provide better lubricity than carbon materials.

3.1 The reasonable design of carbon sources

In terms of carbon sources, a series of carbonaceous nanomaterials were investigated for fluorination such as zero-dimensional (0D) fluorinated fullerene C₆₀ [51, 52], one-dimensional (1D) carbon nanotubes and carbon nanofibers [11, 53, 54], as well as two-dimensional (2D) graphene [19, 55].

Fullerene C₆₀ is particularly fascinating because of its highly conjugated molecular structure, high electron-accepting and transporting properties. Grafting of strongly electronegative fluorine to C₆₀ has a significant effect on its electronic properties, and thus it is of great significance for the research of C₆₀ fluorination. Due to the abundance of active sites of C₆₀, the reaction with fluorine results in a composition range from C₆₀F₂ to C₆₀F₆₀ [56]. Attributing to the steric hindrance of the molecular surface during the fluorination process, it is considered that the compound composed of C₆₀F₄₈ with the highest fluorine content synthesized by the direct fluorination of F₂ [57]. Further fluorination inclined to cause internal fracture at the carbon skeleton level. Matsuo et al. reported that the synthesis of fluorinated fullerenes with compositions between C₆₀F₁₅ and C₆₀F₄₇ using F₂ [51]. Furthermore, different fluorinating agents also successfully employed to synthesize highly fluorinated C₆₀, such as XeF₂ (C₆₀F₆₀) [58].

Carbon nanotubes (CNTs) possess excellent mechanical, optical, and electrochemical properties. Meanwhile, there are many deficiencies on the surface of them [59, 60, 61, 62, 63]. Fluorination is one of the efficient approaches to modifying CNTs [64]. Mickleson et al. investigated the fluorination of single-walled carbon nanotubes (SWCNTs) at different temperatures using F₂ [65]. It demonstrated that a temperature above 150^∘C is essential for the formation of C-F bonds. Meanwhile, the obtained fluorinated SWCNTs transferred to an insulated state under temperatures above 250^∘C, while the structure collapsed when the temperature exceeds 500^∘C. Furthermore, An et al. calculated that the F/C ratio of fluorinated SWCNTs could reach up to 1 from 0.25 with increasing temperature [66]. Recently, Li et al. presented two series of F-CNTs are prepared from ordinary CNTs and graphitized CNTs [53]. It is confirmed that the graphitization treatment of CNTs could increase the structural stability under high-temperature fluorination. The resulted FGCNTs possessed aweaker C-F bonding leading to higher performance.

Graphene exhibits tremendous potential in advanced electronic devices due to its unique electronic properties [67, 68, 69, 70, 71, 72, 73]. On the other hand, the zero band gap of graphene restricts its application [74, 75]. It is believed that the fluorination of graphene-based materials is an efficient technique to open the bandgap by transforming the structure of the chemical bonds from sp² to sp³ configuration. Theoretically, the bandgap of fluorinated graphene could be tuned from 0 to 3.8 eV by controlling the fluorination degree with different C-F bonding characters [38]. The high bandgap of ∼ 3.8 eV also reported to a fluorinated graphene practically [76]. Liu et al. investigated the different F/C ratios of fluorinated graphene concluding that the bandgap increasing with F/C ratios [77].

3.2 The control of fluorination conditions

The fluorination conditions, such as fluorination temperature, fluorination time, gas flow rate and fluorinating agent, also determine the properties of fluorinated carbon [78]. Ahmad et al. investigated the effect of different fluorination temperatures and fluorinating agents (F₂ and TbF₄) on fluorinated carbon preparation [31]. After the temperature of fluorination increased, higher homogeneity of the materials were obtained from the indirect fluorination of TbF₄ in comparison with the direct fluorination using F₂. This can be interpreted by the slow kinetics of TbF₄ decomposition, which allows for the continuous addition of fluorine to the carbon matrix and makes the fluorination process more gradual. Struzzi et al. used a plasma-assisted method to fluorinate vertically oriented multi-walled carbon nanotubes (VCNTs) in a magnetron sputtering chamber with mixed gas (Ar/F₂=95:5) atmosphere [79]. The role of high dilution fluorine in the precursor gas mixture was investigated by studying the nanotube structure change and the electrical properties during plasma processing. Then, the authors used CF₄ plasma to fluorinate VCNTs and studied the influence of different plasma parameters (power, time and distance) on the fluorination reaction [80]. Similarly, Nebogatikova et al. used the HF aqueous solution to fluorinate graphene suspension to investigate the influence of suspension composition, fluorination time, temperature and thermal stress on the crushing process of graphene [81]. Bulusheva et al. produced fluorinated double-walled carbon nanotubes (DWCNTs) by using fluorine F₂ (200^∘C), room temperature gaseous BrF₃ and CF₄ radio frequency plasma, respectively [82]. The fluorinated DWCNTs produced by F₂ and BrF₃ have smaller per-fluorinated regions and shorter fluorinated chains. CF₄ plasma induced different adsorbing groups, including single or paired fluorine atoms and -CF₃ group. These results imply that fluorinating agents could partly decide the structure of fluorinated carbon.

Overall, different reaction conditions are applied in different applications, correspondingly. For example, in the case of the lithium-ion primary battery, if the higher energy density requires then the higher fluorination degree should be chosen. But for the higher power density, a F/C ratio of 0.7-0.9 requires. When the fluorinated carbon materials are applied in the field of superhydrophobic surface(insulation property), the higher F/C ratio requires and the corresponding fluorination conditions including reaction temperature, time, fluorine concentration should choose the large values.

Figure 3

(a) Schematic of FCNT primary lithium/sodium battery, galvanostatic discharge curves of FCNT and FGCNT at different current density of 10, 100, 500, 1000, 2000 mA g⁻¹ for primary lithium battery. Reprinted with permission from [53]. (b) Schematic illustration of the preparation of FG nanosheets by the intercalation and exfoliation, the galvanostatic discharge curves at different discharge rates and the Ragone plot of FG. Reprinted with permission from [91]. (c) The galvanostatic discharge curves, Ragone plot and gravimetric energy densities at different discharge rates. Reprinted with permission from [28]

4.1 Primary lithium-ion battery

Fluorinated carbon (CF_x) firstly used as cathode materials for primary lithium batteries in 1972 [83], and Matsushita Electric Co. in Japan firstly commercialized Li/CF_x batteries in 1975 [84]. Fluorinated carbon used as cathode materials for primary lithium batteries have attracted extensive attention because of its various unique properties, including high energy density (2180 Wh kg⁻¹ for x=1), average operating voltage, long shelf life, operation stability and wide range of operating temperature [10, 53, 85, 86]. More important, fluorinated carbon owns the highest theoretical specific capacity (865 mA h g⁻¹ for x=1) among primary lithium battery systems [87, 88]. Note that the type of C-F bond and F/C ratio are closely related to the electrochemical properties of fluorinated carbon. Thus, recently, tremendous efforts have been made to achieve the theoretical capacity of fluorinated carbon in the respect of tuning the type of C-F bond and F/C ratio by designing the raw materials and fluorination conditions [89, 90].

Feng et al. investigated deeply fluorinated multi-wall carbon nanotubes (F-MWCNTs), where the F/C ratio is controlled to be approximate to 1 [47]. The as-obtained F-MWCNTs was able to support the high discharge rate up to 5C delivering a maximum power density of 7114.1 W kg⁻¹ and a high energy density of 1923 W h kg⁻¹, owing its conductive networks and the intrinsic fast rate capability of one-dimensional nanostructures. Later, they reported a fluorographene, solvothermal exfoliated of fluorinated graphite by acetonitrile and chloroform [91]. As a cathode of primary lithium battery, it exhibited a remarkable discharge rate performance because of good Li⁺ diffusion and charge mobility through nanosheets. The fluorographene with semi-ironic C-F bonds delivered a high specific capacity of 520 mA h g⁻¹ at 1^∘C, and a maximum power density of 4038 W kg⁻¹ at 3^∘C. Recently, Feng et al. developed fluorinated calcinated macadamia nutshell (F-cMNS) delivering a specific capacity exceeding 900 mA h g⁻¹, associated with a discharge potentials exceeding 3.0V(vs. Li/Li⁺) and an unprecedented energy density of 2585 W h kg⁻¹ [92]. Such unique electrochemical performance endowed by the vigorous destruction of the periodic carbonaceous lattice of the basal plane, which substantially alters the electron distribution within the C-F bonds. Thus, the structural features of fluorinated carbon have a significant influence on electrochemical performance. In addition, although the theoretical capacity of CF_0.95 sample was calculated as 847 mA h g⁻¹, Ahmad et al. synthesized the fluorinated carbon nanodiscs (CND) to push the theoretical limit capacity to a high one of 1180 mA h g⁻¹ [31]. The reinforcement effect of the central discs and the presence of a shell of LiF allowed the diffusion of the lithium ions through the sheet edges or the surface cracks, leading to the extra-capacity. The authors explained that the formation of Li₂F⁺ species play a vital role in the achievement of the extra-capacity.

Surface modification of the fluorinated carbon is an effective pathway to improve the performance of Li/CF_x batteries. Carbon coating CFx(x=1) materials delivered a higher energy and power density [93]. When the fluorinated carbon materials are treated by a ball milling with urea, the maximum power density reaches 10309 W kg⁻¹ at 5000 mA g⁻¹ [94]. The ball milling effect enlarged the interlayer distance, decreased particle size and increased surface area of CF_x materials, which results in improving the electrochemical reaction activity, decreasing the reaction resistance and promoting the lithium ions diffusion. Furthermore, the composition of fluorinated carbon and oxides like CF_x-mSiO₂ also delivered a significantly higher power density of 9689 W kg⁻¹ [28].

4.2 Other batteries

Except for primary lithium batteries, fluorinated carbon is also widely investigated in the field of solar cells, lithium/sodium-ion batteries, and lithium-sulfur batteries [95, 96, 97, 98, 99]. Jeon et al. reported an edge-selectively fluorinated graphene nanoplatelets (FGnPs) with a maximized charge polarization and an enhanced chemical stability, which demonstrates superb stability/cycle life for dye-sensitized solar cells (FF: 71.5%; Jsc: 14.44 mA cm⁻²; Voc: 970mV; PCE: 10.01%) and lithium-ion batteries (650.3 mA h g⁻¹ at 0.5^∘C, charge retention of 76.6% after 500 cycles) [95]. Liu et al. employed the home-made fluorinated mesocarbon microbeads (F-MCMB) for sodium-ion batteries [99]. Binder-free of F-MCMB film electrode delivered an initial discharge capacity of 647.8 mA h g⁻¹ and obtained a good lifespan of over 65 cycles at 0.05^∘C. The results demonstrated that micro-structure design could improve the electrochemical performance of Na/CF_x cells. In terms of lithium-sulfur batteries, Vizintin et al. synthesized fluorinated reduced graphene oxide (F-rGO) used as an interlayer additive supported by a glass fiber separator in lithium-sulfur batteries, which can block the diffusion/migration of polysulfides from the porous positive electrode to the metallic lithium electrode and to prevent the redox shuttle effect [98]. The results illustrated that the additive of F-rGO resulted in better reversibility of the sulfur electrochemical reaction and capacity retention upon cycling.

Although fluorinate carbon materials are applied and developed widely in the field of primary lithium ion batteriy, the unique materials also are utilized in other fields like biomedicines, gas sensors, electronics devices and microwave absorption devices [43, 100, 101, 102, 103, 104, 105].

4.3 Biomedicine

In recent years, due to the largest electronegativity of the fluorine atom, the strong polarity of the C–F bond is expected to induce biological responses, especially, 50% of drugs contain fluorine element [101]. Compared to other carbon materials without fluorine, FC_x materials have higher paramagnetism and can be used as a contrast agent of magnetic resonance imaging (MRI) in biological fields [102]. Since the distribution of fluorine in the human body is very small, the contrast medium is very ideal, so the observed signal is stable and shows an excellent degree of specificity. Then, the conjugated structure makes the fluorinated carbon ready for the loading of the aromatic drugs [103]. All these properties make FC_x a potentially valuable candidate for imaging, drug delivery, and photothermal responses. Sun et al. developed fluorinated carbon fiber (FC) based nano-carrier with good biocompatibility, high drug-loading capacity and enhanced photo-thermal performance [104]. As a novel nano-carrier,

Figure 4

(a) Schematic of EFGnPs-F for highly stable inverted perovskite solar cells. Reprinted with permission from [96]. (b) Schematic of FFGF for lithium/potassium storage and the corresponding rate performance. Reprinted with permission from [97]. (c) Schematic of F-rGO as an interlayer in Li-S batteries. Reprinted with permission from [98]. (d) The first discharge/charge curves of Na/F-MCMB cell at a rate of 0.05C. Reprinted with permission from [99]

it shows low toxicity and exhibits excellent cancer therapy effects deriving from a good combination of chemotherapy and photothermal therapy (Figure 5a).

Figure 5

(a) Preparation and application of fluorinated carbon fiber oxide (FCO) as a novel nanocarrier. Illustration of synthesis of FCO by activation and oxidation and loading of DOX and intracellular release combined with photothermal therapy of cancer cells. Reprinted with permission from [104]; schematic illustration of (b) the FG-based gas sensor. Reprinted with permission from [107]; (c) the preparation of the NFC/FCNT composite films. Reprinted with permission from [43]; (d) the preparation of F-h-MWCNTs through direct heating fluorination. Reprinted with permission from [110]

4.4 Gas sensor

Gas monitoring technology has recently become a critical issue since they are some of the most common air pollutants, such as toxic gas handling plants and laboratories, urban areas filled with vehicles and plants that generate energy by combustion of fossil fuels [105]. Thus, the development of a gas sensor is an important goal that will reduce damage to human life. The development of a gas sensing material is the most important step in the development of such a gas sensor. In the past few years, nanoscale materials such as carbon materials, have been used as gas sensing materials, due to high specific surface area for enhanced gas adsorption, excellent electrical properties for high sensitivity and chemical stability for resistance to acidic and high-temperature conditions [106]. Zhang et al. showed monolayer fluorinated graphene (FG) synthesized by a controllable plasma treatment that could be applied as a gas-sensing material [107]. The FG-based gas sensor exhibited better performance for ammonia detection compared to pristine graphene, in which the fluorine species and defects in FG play key roles in the function of the gas sensor. According to the density functional theory simulation results, the superior sensing performance of FG was attributed to enhanced physical absorption due to the C-F covalent bonds on the surface of FG (Figure 5b).

4.5 Electronics device

Carbon has excellent electronic, optical and mechanical properties, which be used in new-generation devices. For example, the high mobility of charge carriers has aroused great interest in the field of high-speed electronics. In addition, due to the unique combination of high conductivity and optical transparency, graphene is a promising photoelectric application material, such as display, photovoltaic cells, and light-emitting diodes [108]. Since fluoridation is one of the most effective chemical methods to modify and control physicochemical properties in a wide range, fluorination of new forms of carbon materials is of great interest. The tunable electronic structure in fluorinated carbon is sufficiently utilized in electronic devices.Wang et al. firstly used fluorinated carbon nanotube (FCNT) as a thermally conductive filler to prepare composite film with nano fibrillated celluloses (NFCs) [43]. The presence of FCNT induces the excellent thermal conductivity, favorable electrical insulation property, enhanced toughness, reliable mechanical strength, and flexibility. These outstanding comprehensive properties guaranteed that the prepared composite film containing FCNT has promising applications in the heat dissipation of electronic devices (Figure 5c).

4.6 Microwave absorption

The demand for broadband microwave absorbing materials is increasing to prevent electromagnetic radiation from most modern electronic devices, such as computers and mobile phones. Due to their unique tubular nanostructure with extremely large aspect ratio, CNTs possess larger excluded volume, a smaller percolation threshold, and a lower additive amount, which endows CNTs with great potential as lightweight microwave absorbers [109].

In order to reduce electromagnetic interference (EMI), Liu et al. synthesized a novel fluorinated multi-walled carbon nanotubes (F-MWCNTs) with skin-core heterostructure to possess a minimal reflection loss up to −69 dB at a thickness of 1.5 mm [110]. During the fluorination process, the preformed CF₂ bonds prevent the F₂ gas permeating into the inner tubes, which results in an intact structure of the inner tubes. The intact inner tubes provide a good attenuation of electromagnetic waves. However, the outer skin is the fluorinated layer with good impedance matching, which helps the transmission of electromagnetic waves (Figure 5d).