Structural and tribological characterization of fluorinated graphene with various fluorine contents prepared by liquid-phase exfoliation

Abstract

The high strength and noble inertness of fluorinated graphene (FG) indicate very promising properties for its application in tribological applications to reduce friction and save energy, yet few works refer to its tribological performance, mostly due to the lack of an effective synthesis method and limited knowledge of FG. In this work, fluorinated graphene (FG) sheets with various fluorine contents are prepared from fluorinated graphite (FGi) by means of controllable chemical reaction with ethylenediamine (EDA) and liquid-phase exfoliation with N-methyl-2-pyrrolidone (NMP) in a one-pot synthesis. Transmission electron microscopy and atomic force microscopy analyses show that the obtained FG sheets possess large lateral size and ultrathin thickness (1.8–4.0 nm). Chemical characterizations indicate the C/F ratio can be readily tuned by adjusting the reaction temperature with EDA, which leads to defluorination and also substitution of a small amount of fluorine atoms by alkylidene amino groups. The tribological performance of FG samples as novel lubricant additives in base oil of polyalphaolefin-40 with different concentrations (0.1–0.4 mg mL⁻¹) is investigated. The tribological tests suggest that the addition of FG at optimum concentration can greatly improve the anti-wear property of the base oil and there exists a strong proportional relationship between anti-wear ability and fluorine content.

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1. Introduction

Fluorinated graphene (FG), as a new graphene derivative whose carbon skeleton is densely bound with fluorine atoms, has been a hotspot in theoretical and experimental studies since its first report in 2010.^1,2 Upon the covalent attachment of fluorine atoms, unlike its mother material graphene, FG with high fluorine content is a high quality insulator (room temperature resistance can reach 10¹² Ω for fully fluorinated graphene) with a wide bandgap varying with the level of fluorination; meanwhile, it has a low thermal conductivity leading to comparably good thermal stability and anti-oxidation ability.^1–10 Nevertheless, as the carbon skeleton remains intact, FG still inherits the mechanical properties of graphene. For example, fully fluorinated graphene exhibits a high Young's modulus (∼0.3 TPa) and is able to sustain strains of 15%.¹ Its novel structure and unusual optical, electrical and mechanical properties have expanded this exciting material into research areas such as photochemistry, electrochemistry, biology and electronic devices, exhibiting potential value in future applications.^8,11–17 Therefore, an effective synthesis method of FG is of great importance to fully display its performance and explore its other novel properties.

At present, the reported preparation strategies of FG can be divided into two types: (1) fluorination of graphene or graphene oxide (GO) utilizing fluorinating reagents such as F₂, XeF₂, HF, plasma (CF₄ and SF₆) or use of fluoropolymers.^1,14,18–22 FG sheets obtained by this type of strategy can reach different degree of fluorination, but the related procedures are complicated and high-cost, constraining large-scale production. (2) Exfoliation of fluorinated graphite (FGi) through mechanical cleavage or liquid-phase exfoliation.^13,23–28 Mechanical exfoliation can only prepare small amount of FG for fundamental study. The liquid-phase exfoliation method, as a typical top-down approach for preparing FG, has a potential for mass production of high-quality single- or few-layer FG sheets. During the process, chemical wet dispersion and ultrasonication cavitation can induce the isolation of dispersible FG sheets.²⁹ Consequently, this method is considered as a relatively nondestructive, simple and convenient route to prepare FG sheets.

On the other hand, before the discovery of FG, FGi has been used as solid lubricants.^30–34 Previous studies indicated that FGi has good friction-reducing effect due to its wide lamellar space and weak bonding energy of 9.36 kJ mol⁻¹ which results in lower shear strength than graphite.³⁵ As the building block of FGi, few-layer FG is proposed as a more efficient lubricant due to the fact that it combines the excellent properties such as nanoscaled size, strength, flexibility and laminar structure. In addition, it has been proved in current studies that the variation of fluorine coverage can significantly influence the optical, electronic, thermal and magnetic properties.^3,4,8,14 Selective tuning of fluorine coverage is expected to provide a feasible way to advance the understanding of properties and expand the application scope of this promising nanomaterial. Thus a method combining the above advantages and synchronously realizing the tunability of fluorine contents of FG is highly desirable to study the tribological properties of FG.

Considering all these factors in mind, we present a facile method to prepare few-layer FG samples with a stepwise decrease of fluorine coverage by using ethylenediamine (EDA) as reactive intercalation agent and then exfoliating in N-methyl-2-pyrrolidone (NMP) in a one-pot synthesis. The obtained FG sheets possess large lateral size and ultrathin thickness. Meanwhile, the prepared samples also present good dispersibility in oil-based fluids without further surface modification. Taking into account the properties of few-layer FG as mentioned earlier, we propose FG can be added into base lubricant oil to realize its potential as an additive for tribological applications. However, as far as we know, there are no related reports published to date. Considering the “like dissolves like” principle and the kinetic stability of FG in lubricant oil, polyalphaolefin-40 (PAO-40) has been chosen as the base oil without dispersant or surfactant to test the friction-reduction and anti-wear ability of FG.

2. Experimental details

2.1. Raw materials

FGi (F% = 51.29–51.47 atom%, Shanghai CarFluor Chemicals Co., Ltd.), NMP (Sinopharm Chemical Reagent Co., Ltd., ≥98%), EDA (Tianjin Rionlon Bohua Chemical Co., Ltd., 99%), and PAO-40 (Lanzhou Petrochemical Co., Ltd.) were used as received. Ultrapure water (>18 MΩ cm) was used for preparation and rinsing.

2.2. Preparation of FG with various fluorine contents

In a typical procedure, FGi (0.2 g) was added into 30 mL EDA and the mixture was transferred to a round-bottomed flask followed by heating at different temperature of oil-bath (the temperatures of mixture measured by the thermometer are 60, 90 and 120 °C) with constant stirring under a nitrogen atmosphere for 3 h. When the mixture was cooled down to room temperature, 100 mL NMP was introduced. Then, the obtained black dispersion was ultrasonicated at 180 W for 10 h. After standing for 5 h, the upper layer of liquid containing FG nanosheets was pipetted, followed by filtration and freeze drying. The FG consisting of few-layer sheets was collected and the corresponding samples were denoted as FG-60, FG-90 and FG-120.

2.3. Characterization techniques

The morphologies of FG samples were observed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai F30, acceleration voltage: 300 kV). The samples were obtained from the supernatant after the dried powder was re-dispersed in ethanol by ultrasonication and dropped onto holey carbon mesh grids (300 mesh). Atomic force microscopy (AFM, VEECO Nanoscope IIIa) was also employed to measure the thickness and microstructure. For imaging, the obtained supernatant was centrifuged at 3000 rpm for 5 min and then the solvent was replaced with ethanol. The new dispersion after ultrasonication was dropped onto a silicon wafer and dried overnight. Chemical structures and composition were analyzed by UV-Vis spectrophotometry (UV-2700, Shimadzu), powder X-ray diffraction (XRD, Rigaku D/Max-2400 X-ray diffractometer with Cu-Kα irradiation), Fourier-transformation infrared spectrometry (FT-IR, Bruker IFS 66 V/S, using KBr pellets) and X-ray photoelectron spectroscopy (XPS, PHI-5702, Physical Electronics, using monochromated Al-Kα irradiation). For these characterizations, the powder was directly used without further treatment.

2.4. Tribological characterization

Tribological tests in terms of friction coefficient and wear resistance for PAO-40 with additives were carried out on an oscillating reciprocating friction and wear tester (SRV-I, optimol, Germany). The machine was operated by ball-on-disk model, where an upper ball reciprocally slid against a stationary disk with an amplitude of 1 mm at a given oscillation frequency of 20 Hz. The ball (Φ 10 mm) and disk (Φ 24 × 8 mm) are made of GCr15 steel (AISI 52100) with Vickers hardness of 790–820. All tests were conducted at room temperature and the relative humidity was 40–45%. The morphology of the scar surface and wear volume of the disk was characterized by a MicroXAM 3D noncontact surface mapping microscope profiler.

3. Results and discussion

Fig. 1 illustrates the preparative processes of few-layer FG sheets. Due to the interlayer spacing in FGi and intramolecular N–H⋯F interaction between EDA molecules and fluorine atoms of FGi, EDA molecules can readily intercalate into the interlayers of FGi so as to weaken the Van der Waals interactions between the sheets. Subsequently, EDA molecules react with raw FGi at different temperature, which leads to reduction and chemical modification. The reaction mechanism may be related to the polarity of C–F bonds and nucleophilic substitution.^18,36,37 From a perspective of chemical kinetics, more activated molecules and effective collisions are present at elevated temperatures; therefore, the reaction process can be mild and controllable through the control of reaction temperature. After the intercalation of EDA, exfoliation occurs and the number of layers in the sheets is reduced. However, there are still many layers (>10 layers) stacked in each sheet. So NMP is further introduced as an exfoliation agent and it also can facilitate the dispersibility of FG due to the matching surface energy and strong dipole interaction with FG.^13,38 Through ultrasonication, few-layer FG sheets can be stably dispersed to form a dark colored suspension.

Fig. 1 Illustration for the preparation process of FG: (1) intercalation and reaction of EDA at 60, 90 and 120 °C. (2) Exfoliation by sonication at room temperature by introducing NMP.

The morphology and microstructure of the prepared few-layer FG sheets were initially identified by TEM and AFM observations. Fig. 2a and b show the two-dimensional transparent sheet-like structures with edges tending to be crumpled and folded. The selected area electron diffraction pattern (inset in Fig. 2a) shows a disordered hexagonal structure along with amorphous halos because of the presence of fluorine, which indicates that the FG sheets present polycrystalline structure and only partially inherit the parent structure of graphene. These results are corroborated by the HRTEM micrograph of the sheet edge (Fig. S1, ESI†). Moreover, the element analysis is also performed by the energy dispersive X-ray spectrum (inset in Fig. 2b), where C and F elements are detected. The fact that the sheet is a stack of few single layers was further confirmed by AFM. As shown in Fig. 2c–f, the obtained FG sheets are uniform and high quality with lateral dimension of 5 μm. The measured thickness was in the range of 1.8 to 4.0 nm. In view of the thickness of monolayer FG (0.783–0.87 nm) reported and interlayer spacing measured by XRD (Fig. S2, ESI†),² it can be estimated that the layer numbers are 2–5 layers. Overall, these results demonstrate that few-layer FG sheets have been effectively exfoliated and dispersed.

Fig. 2 TEM images (a and b) of few-layer FG-90 sheets; the insets in (a) and (b), respectively, show the selected area electron diffraction pattern and the energy dispersive X-ray spectrum. AFM images (c and d) and corresponding thickness analyses (e and f).

FT-IR spectra and XPS characterization were then performed to obtain the chemical structure and elemental composition information of FG-60, FG-90 and FG-120 samples. As shown in the FT-IR spectra (Fig. 3a), the strong absorption peaks at 1218 and 1350 cm⁻¹ are assigned to the stretching vibrations of covalent C–F bonds and C–F₂ bonds, respectively.^39,40 It is normally difficult to reduce CF₂ and CF₃ groups,^41,42 which may influence the fluorine contents of these samples. However, in this work, the intensity of the two peaks gradually decreases with the elevation of reaction temperature, and finally disappears for C–F₂ moieties. Meanwhile, a peak located at 1083 cm⁻¹ is ascribed to weakened C–F (C_sp²–F) bonds which are presumed to be softened by hyperconjugation of the fluorine orbitals with the conjugated π-scheme in a carbon sheet, which appears and becomes clear in FG-120.⁴³ There are two bands at 1645 and 3400 cm⁻¹, which can be respectively assigned to the C=C bonds and O–H/N–H stretching vibration (due to moisture and a small amount of attached secondary amine).¹⁸ The signal intensity of C=C bonds for all three samples is stronger than that of FGi (Fig. S3, ESI†). These all suggest the reduction of fluorine contents resulting from the loss of fluorine-containing functional groups and partial restoration of the polyaromatic structures, which is further supported by the following measurements of XPS spectra and UV-Vis spectra.

Fig. 3 (a) FT-IR spectra of FG obtained at different reaction temperatures. (b) XPS survey spectra of FG samples. High-resolution XPS spectra of F 1s (c) and C 1s of FG-60 (d), FG-90 (e) and FG-120 (f).

As shown in Fig. 3b, the detailed elemental composition information can be clearly acquired from the XPS survey spectra. With increasing reaction temperature, the intensity of the F 1s peak decreases, accompanied with an evident increase of the C 1s peak. The decrease of the fluorine contents suggests obvious defluorination during the reaction with EDA. In addition, a small amount of nitrogen element (<4 atom%) is detected in FG-90 and FG-120. Previous studies have demonstrated that amino groups possess a degree of chemical reactivity towards fluorinated single-walled carbon nanotubes, leading to partial substitution and reduction of fluorine.^36,44 However, in this work, defluorination is the dominant process and substitution is almost negligible. One possible explanation for the lower degree of functionalization is that the lower surface curvature of the basal plane of FGi reduces the reactivity.^39,45 As to the detailed process of defluorination, a possible mechanism is proposed in Scheme S1† and the related contents are depicted in ESI.† In the high-resolution F 1s spectra (Fig. 3c), the decrease of fluorine contents causes a shift of peak position from 688.8 to 688.3 eV.³⁹ Moreover, two peaks corresponding to two different types of carbon bonds can be seen in the high-resolution C 1s spectra (Fig. 3d–f). The peak at 284.4 eV is assigned to chemical bonds between carbon atoms while the peak at 289.9 eV with a lower intensity is assigned to the sp³ carbon atoms bonded to fluorine.^39,40 Assignment of the peaks and corresponding locations through deconvolution are summarized in Table 1. These analyses provide further support about the nature of the chemical bonds in different samples as also shown in the UV-Vis spectra (Fig. S4, ESI†). In the spectra, a characteristic broad band of the polyaromatic system at 275 nm gradually gains intensity while the band at 218 nm, characteristic of polyene structures due to C–F bonds, gradually weakens with increasing preparation temperature. Therefore, tuning of fluorine contents by adjusting the reaction temperature to obtain different C/F ratios is feasible and effective.

Table 1 Peak locations (eV) and assignments for FG-60, FG-90 and FG-120

Peak	(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	F atom (%)
Sample	C=C	C–C	C–CF	C–N	C–CF2	Csp^2–F	C–F	CF–CF2	C–F2	C–F3
FG-60	284.3	284.9	285.6	286.2	286.9	288.4	289.6	290.3	291.5	292.4	31.63
FG-90	284.4	285.0	285.8	286.2	286.9	288.5	289.5	290.3	291.5	292.4	17.67
FG-120	284.4	285.0	285.6	286.2	287.0	288.4	289.5	290.3	291.3	292.4	5.83

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Friction and wear are the two main causes for component failure and energy dissipation.⁴⁶ In order to improve the effective work of mechanical components with two surfaces in contact, it is still necessary and desirable to find better lubricants along with chemical additives. In this field, the dispersion stability is a primary issue when FG is used as lubricant additives. Fig. 4 shows the dispersion photos of the three samples (FG-60, FG-90 and FG-120) as additives in PAO-40 with various concentrations from 0.1 to 0.4 mg mL⁻¹, which show clear differences according to the color variation. It is noted that all the three samples can be stably dispersed in PAO-40 when the concentration is below 0.4 mg mL⁻¹ and there is no apparent precipitation observed at the bottom of the bottle even after standing for a long time. The nanoscaled size of FG sheets, the high viscosity of oil and similar polarity between them mainly contribute to this good stability. However, above 0.4 mg mL⁻¹, there is a degree of precipitation upon standing for a week. On the other hand, at the same concentration, samples with lower fluorine contents show a darker color, which also indicates the restoration of the polyaromatic structure of graphene. For comparison, without any treatment, micron size FGi was directly dispersed in the oil to observe its stability and subsequently test the tribological property (Fig. S5 and S6, ESI†).

Fig. 4 Dispersions of FG samples in PAO-40 with concentration ranging from 0.1 to 0.4 mg mL⁻¹. The top images show the dispersions after sonication and the bottom ones are the corresponding dispersions after standing for one week.

Fig. 5 shows the friction coefficient of pure PAO-40 under various applied loads as a function of time at a given frequency of 20 Hz. It can be seen that friction coefficient curve is stable under loads lower than 300 N; however, a drastic increase begins to appear at 702 and 344 s when the loads are raised up to 350 and 400 N and violent fluctuation subsequently appears so that the test has to be stopped. With the purpose of testing the friction-reduction and anti-wear ability of FG when it is added into PAO-40, the load of 350 N is chosen as the fit test load to explore the difference after the addition of FG.

Fig. 5 Evolution of friction coefficient with time at a given frequency of 20 Hz for PAO-40 at various applied loads.

The tribological properties of PAO-40 using the three FG samples as additives are shown in Fig. 6a–c. Apparently, under the same test conditions, the three samples have similar friction coefficient as that of pure PAO-40 and there is no remarkable reduction. Nevertheless, it is worth noting that the addition of FG makes the oil exhibit better friction stability and durability. For the oil with FG-60, the time of stable friction state can be prolonged from 816 s to the whole test period of 1800 s with an increase of concentration from 0.10 to 0.25 mg mL⁻¹. However, a further increase of concentration (higher than 0.25 mg mL⁻¹) leads to a decrease of the stable friction time. For the oils with FG-90 and FG-120, the friction coefficient curves present similar trends and the optimum concentrations are 0.30 and 0.35 mg mL⁻¹, respectively. These results suggest that the addition of FG can remarkably enhance the reliability of the base oil and the samples with higher fluorine contents exhibit better anti-wear effect. Fig. 6d summarizes these data and clearly shows the variation tendency of stable friction time at different concentrations for the three samples. In addition, the appearance of an optimum concentration is in agreement with previous reports about lubricant oil additives.^47–49

Fig. 6 Evolution of friction coefficient with time for PAO-40 containing FG-60 (a), FG-90 (b) and FG-120 (c) at various concentrations. (d) The time of stable friction state at various concentrations for the three FG samples.

To further estimate the anti-wear performance of the three samples, especially for FG-60 and FG-90 which exhibit the same stable friction time during the measurement period at respective optimum concentration, the wear surfaces were analyzed by a 3D profiler. Fig. 7a–c show the 3D morphology of the wear tracks on the disk at the optimum concentration of the three samples. It can be obviously seen that, even though at respective optimum concentration, the wear scar in the case of the base oil doped with 0.25 mg mL⁻¹ FG-60 is smoother than those in the cases of the base oil doped with 0.30 mg mL⁻¹ FG-90 and 0.35 mg mL⁻¹ FG-120. Especially for the oil doped with 0.35 mg mL⁻¹ FG-120, the wear scar consists of obvious furrows and grooves resulting from the unstable friction and vibration which begin to appear at about 1500 s (which can be seen in Fig. 5c). Likewise, as shown in Fig. 8, the wear volume and wear rate of the lower disk lubricated by the oil doped with FG-60 at optimum concentration are smallest, which further proves the fluorine content is a key factor influencing the anti-wear performance. On the other hand, the microtribological study on FG indicated that nanoscale friction exhibited 6-fold enhancement by fluorination of graphene,^50,51 yet our macrotribological investigation reveals that the addition of FG does not have much influence on reduction or enhancement of friction, but does have effect on the anti-wear performance. This divergence has long been observed in the micro/macro world due to the difference among testing conditions, experimental systems and tribological mechanism.^52,53

Fig. 7 Height profile images and the corresponding 3D optical microscopic images of wear tracks on the lower disk lubricated by PAO-40 doped with FG-60 (a), FG-90 (b) and FG-120 (c) at their respective optimum concentrations.

Fig. 8 Wear volume and wear rate for FG-60, FG-90 and FG-120 at their respective optimum concentration.

According to the performance of the base oil doped with FG, it is proposed that the lubrication action mechanism follows the mixed lubrication mechanism containing boundary lubrication and asperity contact.⁵⁴ Before the rupture of boundary oil film for pure base oil, the oil film plays a major role and keeps the stable friction state. Hence, there is no remarkable fluctuation for friction coefficient. On the other hand, there are still many micrometer/nanoscale ridges and valleys existing on the surface even though the ball and disk have been burnished, leading to the formation of asperity contact.⁵⁵ Then, the asperity contact becomes the dominant mechanism with increasing friction time and causes the drastic increase of friction coefficient. When the raw oil is doped with FG samples, the oil film still plays a major role during the lubrication corresponding to the unchanged friction coefficient. However, the difference is that the FG sheets can easily enter the contact area and form a protective layer, exhibiting excellent anti-wear performance. One possible mechanism is that the protective layer formed by effective physical adsorption can provide low resistance to shear owing to the nanoscaled size and extremely thin laminated structure, which can prevent the contact of friction surfaces and keep the friction process stable before the boundary film is recovered, and therefore prolong the lifetime of the stable state. Simultaneously, FG sheets can fill up the nanogaps and furrows of the rubbing surface and bear part of the load so that the wear losses decreased.^47,55 In addition, another possibility is that fluorine contents can influence the surface free energy and interactions between sheets. Nanoscale FG sheets with higher fluorine contents not only have wider lamellar space and weaker bonding energy within a sheet but also have less interaction and more relative slip between sheets under the shear stress.⁵⁶ Therefore, the difference of fluorine contents may lead to different optimum concentrations among the three samples. When the concentration exceeds a critical value, the excess additive may cause aggregation of FG sheets and metallic debris which induced the asperity contact, leading to even worse anti-wear performance.^54,57

4. Conclusions

In this work, an effective route is addressed to prepare high-quality few-layer FG sheets with various fluorine contents by reduction and liquid-phase exfoliation of FGi. The intercalation of EDA molecules facilitates the exfoliation and aids selective tuning of the fluorine coverage. According to the observation of morphology and microstructure, the obtained FG samples have nanoscale thickness, corresponding to the layer numbers of 2–5 layers. The characterization of chemical composition suggests the weakening of C–F bonds in the FG samples. The prepared FG samples show excellent dispersion stability in base oil. From tribological measurements, it is seen that the addition of FG at optimum concentration can prolong the friction time of the stable state and the effect can be enhanced with increase of fluorine content. This work suggests FG is an effective lubricant additive and holds great promise in lubrication engineering.

Acknowledgements

The authors thank the financial support from the National Natural Science Foundation of China (Grant no. 51375474), the “Project supported by the Science-Technology Foundation for Young Scientist of Gansu Province, China (Grant no. 145RJYA280)”, and the “Funds for Distinguished Young Scientists of Gansu Province”.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra10313k

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