Covalent Functionalization of Fluorinated Graphene and Subsequent

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Covalent Functionalization of Fluorinated Graphene and Subsequent Application as Water-based Lubricant Additive Xiangyuan Ye, Limin Ma, Zhigang Yang, Jinqing Wang, Honggang Wang, and Shengrong Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10579 • Publication Date (Web): 29 Feb 2016 Downloaded from http://pubs.acs.org on March 5, 2016

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ACS Applied Materials & Interfaces

Covalent Functionalization of Fluorinated Graphene and Subsequent Application as Water-based Lubricant Additive

Xiangyuan Yeab, Limin Maa, Zhigang Yanga, Jinqing Wanga*, Honggang Wanga, and Shengrong Yanga*

a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, P. R. China. b University of Chinese Academy of Sciences, Beijing, 100080, P. R. China.

* Corresponding author, [email protected] (J. Wang) or [email protected] Fax: +86 931 8277088 Tel.: +86 931 4968076 1

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Abstract: Although the fluorinated graphene (FG) possesses numerous excellent properties, it can not be really applied in aqueous environments due to its high hydrophobicity. Therefore, how to achieve hydrophilic FG is a challenge. Here, a method of solvent-free urea melt synthesis is developed to prepare the hydrophilic urea modified FG (UFG). Some characterizations of transmission electron microscopy (TEM), atomic force microscopy (AFM), Fourier transfer infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and thermo gravimetric analysis (TGA) demonstrate that the urea molecules can covalently functionalize the FG and the hydrophilic UFG can be prepared. According to the tribological tests run on an optimal-SRV-I reciprocation friction tester, it can be found that the anti-wear ability of water can be largely improved by adding the appropriate UFG. When the concentration of UFG aqueous dispersion is 1 mg/mL, the sample of UFG-1 has the best anti-wear ability with a 64.4% decrease of wear rate compared with that of the pure water (UFG-0), demonstrating the prepared UFG can be used as a novel and effective water-based lubricant additive. Keyword: fluorinated graphene (FG), functionalization, hydrophilicity, water-based lubricant additive, friction, wear

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1. Introduction As the youngest graphene derivative, fluorinated graphene (FG) has been attracted immense research interest due to its excellent performances1-14. The FG inherits excellent mechanical properties of the graphene even though the fluorination disrupts the Van der Waals forces between the FG sheets, therefore, the FG can be utilized to enhance the mechanical properties of polymers15. Based on the insulating property of FG6, 10, 14, the polyimide/FG hybrids have been prepared and displayed the low dielectric constant16. In addition, the low surface free energy of the C-F bond enables FG to tailor the wetting characteristics of a surface17. All the above-mentioned excellent properties of FG are closely related to the fluorine on the surface of FG nanosheets, however, it is also the fluorine leading to the predicament that FG can not be widely used in aqueous environments. Water is the most common and environmentally friendly solvent in research laboratory and industry. Many nanomaterials have been dispersed in water to enhance the properties of water as additives or to strengthen polymers as nanofillers which were prepared using water as the solvent. For examples, MoS218, fullerene19 and monolayer graphene20 have been successfully applied as effective water-based lubricant additives to improve the tribological properties of water. Carbon nanotubles21 and graphene nanosheets22-23 also can be utilized as nanofillers to enhance the properties of poly(viny alcohol) by using a simple water solution processing method. The crux that the above-mentioned nanomaterials can play important roles in enhancing various properties under aqueous environments is based 3



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on the prerequisite that the nanomaterials are hydrophilic and can be homogeneously dispersed in water. Although the FG possesses outstanding properties24, it can not be dispersed well in water due to the low surface free energy of the C-F bond, leading to the limited applications of the FG in aqueous environments. Therefore, modifying the hydrophobic FG to get the hydrophilic FG is very significant to broaden the application area of the FG. Before implementation of high-performance FG in aqueous environments, practical challenges must be addressed. Firstly, selecting the material that can react with the FG is important to replace the fluorine effectively. Additionally, in order to get the good dispersibility of FG in water, it’s also important to change the surface wettability of FG from hydrophobicity to hydrophility, since the homogeneous dispersion of nanomaterials is a guarantee to effectively reinforce the matrix materials. According to the reaction of fluoronanotube with urea25, it can be speculated that urea can partially replace the fluorine to covalently functionalize the FG by controlling the reaction conditions. At the same time, it is worth noting that the amino of the urea is a hydrophilic group, which means the urea modified FG (UFG) can be dispersed well in water. Therefore, in this work, the FG is firstly functionalized by urea to get the UFG, and then the UFG is dispersed in ultrapure water by ultrasonication. Based on this, as a typical application of the UFG in aqueous environments, the UFG is regarded as a novel lubricant additive to improve the tribological properties of the water. Experimental results indicate that covalent functionalization of the FG by urea is successful, and the prepared UFG can be 4


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dispersed uniformly in water and employed as a water-based lubricant additive. 2. Experimental section 2.1 Materials Fluorographite (FGi) was purchased from Shanghai CarFluor Ltd. and used as provided. Urea with 99% purity was purchased from Xilong Chemical Co., Ltd. N-methyl-2-pyrrolidone (NMP) was purchased from Sinopharm Chemical Reagent Co., Ltd. Ultrapure water (>18 MΩ·cm) was used in this work. 2.2 Preparation of FG According to our previous report15, the preparation of FG was described as follows: 100 mg FGi was dispersed in 20 ml NMP and the mixture was heated at 60 o

C for 2h. Then, the ultrasonication was carried out to further exfoliate FGi. The

resultant product was filtered and freeze-dried to get the FG. 2.3 Preparation of UFG As depicted in Figure 1, the UFG was prepared from the FG by using solvent-free urea melt synthesis. In a typical synthesis, the calculated amounts of the FG and urea (mass ratio of FG: urea=1: 500) were mixed and ground in a mortar. Then the mixture was placed into a three-neck flask and heated at 150 oC to melt the urea. After the complete melt, the mixture was further stirred at 150 oC for another 4 h under nitrogen gas flow. Then, the product was extensively washed with ultrapure water and ethanol, and collected by a Millipore Fluoropore PTFE filter membrane. Lastly, the resultant product was filtered and freeze-dried for 48 h to get the UFG.

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Figure 1. Reaction of the FG with urea.

2.4 Preparations of FG and UFG aqueous dispersions In order to investigate the dispersibility of the FG and the UFG in water, meanwhile, to test the tribological properties of the UFG as water-based lubricant additive, two kinds of aqueous dispersions of FG and UFG were respectively prepared. 10 mg of the FG was dispersed by ultrasonication (180 W) for 8 h in 5 mL ultrapure water to form FG aqueous dispersion with the concentration of 2 mg/mL (FG-2). The calculated amounts of the UFG (0 mg, 2.5 mg, 5.0 mg, 10 mg and 20 mg) were respectively dispersed by ultrasonication (180 W) for 8 h in 5 mL ultrapure water to form various UFG aqueous dispersions (0 mg/mL, UFG-0; 0.5 mg/mL, UFG-0.5; 1 mg/mL, UFG-1; 2 mg/mL, UFG-2; 4 mg/mL, UFG-4). 2.5 Characterizations Transmission electron microscopy (TEM, Tecnai G2 TF20, operated at 200 kV) and atomic force microscopy (AFM, Bruker, Multimode 8) were applied to observe the morphology and microstructure of the FG and the UFG. Herein, the obtained FG and UFG were firstly dispersed in ethanol via ultrasonication, and then the prepared dispersions were respectively dropped on Cu grids and silicon substrates to get the samples for TEM and AFM observations. Fourier transfer infrared spectroscopy 6


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(FTIR) was recorded between 400 and 4000 cm-1 on a Bruker IFS66v/S FTIR spectrometer as the samples were milled with KBr and pressed as tablets. X-ray photoelectron spectroscopy (XPS, PHI-5702, Physical Electronics) was performed by using monochromated Al-Ka irradiation. Thermo gravimetric analysis (TGA, STA 449C, Germany) was performed under nitrogen flow at a heating rate of 10 oC/min from 30 oC to 800 oC. Macrotribological tests were run on an optimal-SRV-I (Germany) reciprocation friction tester and at least three repeated tests were performed. The machine was operated by a ball-on-block configuration, where an upper ball reciprocally slides against a stationary disk with an amplitude of 1 mm at a normal load of 50 N and a given oscillation frequency of 15 Hz. The ball (φ = 10 mm) and disk were made of GCr15 steel (AISI 52100). The wear volume of the disk and the depth of the wear scar were characterized by a MicroXAM 3D noncontact surface mapping microscope profiler and each data point was carried out through five measurements. The scanning electron microscope (SEM) (JEOL-5600LV) was utilized to observe the morphology of the worn surface.

3. Results and discussion 3.1 Characterization of the FG and the UFG

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Figure 2. TEM images of the FG (a) and UFG (b), AFM images of the FG (c) and UFG (d).

The prepared FG and UFG are identified by TEM and AFM characterizations. As shown in TEM images of the FG (Figure 2a) and UFG (Figure 2b), the obtained FG and UFG nanosheets are apparently transparent with the size of several micrometers. Compared with flat surface of the FG, the UFG exhibits typically rough morphology which is attributed to the bonding of urea on the surface of FG nanosheet. The AFM images and height profiles of the FG and UFG are presented in Figure 2c and Figure 2d, and both of them have the high aspect ratio (the ratio of lateral size to thickness). Based on the fact that the thickness of the FG monolayer is 0.67-0.87 nm12,26, it is calculated that the thickness of 3.8 nm for FG nanosheet corresponds to the layer numbers of 3-5, demonstrating the prepared FG possesses multiple layered 8


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microstructure, which also can be confirmed by observation of the high resolution TEM images (Figure S1a). After modification, a slight increasement of 0.3 nm in thickness can be differentiated in contrast with FG and the UFG sheets also present multiple layered microstructures (Figure S1b). At the same time, in order to illustrate the samples are pure, the Energy-Dispersive Spectrums (EDS) of the FG and the UFG are given in Figure S2.

Figure 3. FTIR spectra of the FG, urea and UFG.

FTIR spectra of the FG, urea and UFG are shown in Figure 3. Apparently, the peak located at 1212 cm-1 can be ascribed to the stretching vibration of the C-F covalent bonds, which is a typical peak of the FG27. For the urea, the characteristic peak located at 780 cm-1 is normally assigned to the CO deformation mode coupled with the antisymmetric NH2 torsional mode. In the FTIR spectrum of the UFG, these two typical peaks at 1212 cm-1 and 780 cm-1 can be clearly found, meanwhile, the stretching mode band of the C-F bonds in the UFG spectrum is significantly weakened compared with that of the FG, which illustrates the fact that substantial 9



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amount of fluorine in FG has been removed by the reaction between FG and urea25.

Figure 4. The XPS survey spectra (a) and N1s fine spectra (b) of the FG and UFG samples. The inserted Table in Figure 4a summarizes the chemical compositions of the FG and UFG, while the inserted figure in Figure 4b gives the N1s spectra of the FG and UFG.

The XPS characterization is further applied to explore the chemical composition and the chemical environment information of elements in the samples of FG and UFG. As clearly shown in Figure 4a, the distinctions between the FG and the UFG are the chemical composition and the peak intensity of the elements. Compared with the FG, a new peak of the N1s emerges at the 400 eV on the XPS spectrum of the UFG; meanwhile, the peak intensity of the F1s at the 690 eV of the UFG is apparently weaker than that of the FG, the changes also can be reflected by data in the inserted Table of Figure 4a, which summarizes the chemical compositions of the FG and UFG. Meanwhile, the binding energy of N1s peak of the UFG is obviously higher than that of the urea, indicating different N bonding configurations existed in the UFG, as shown in Figure S3a. The peak of C–F (CF2) bound of the UFG loses intensity 10


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compared with that of the FG, which means the departure of fluorine, as shown in Figure S3b. The results suggest the urea molecules have partially replaced fluorine of the FG. The N1s fine XPS spectra of the UFG sample is measured and shown in Figure 4b. Clearly, there are two peaks observed at different binding energies, which indicates there are two chemical states of nitrogen existed. In detail, the peak located at 399.8 eV is attributed to the bond of O=C-N resulting from the urea. Another peak located at 400.7 eV is assigned to the bond of C-C-N coming from the reaction between urea and FG28. These results sufficiently demonstrate the urea molecules have covalently functionalized the FG to form the UFG.

3.2 Thermal properties of the UFG

Figure 5. TGA curves of the FG, the urea and the UFG samples.

The thermal properties for the samples of FG, urea and UFG are measured in a nitrogen flow at a heating rate of 10 oC/min from 30 oC to 800 oC and the results are exhibited in Figure 5, indicating that the thermal decomposition process of the UFG is 11



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tightly related to the FG and urea. One-step decomposition of the FG starts from 500 o

C due to removing the fluorine from the FG in the form of CF429. While the urea

presents multistep decomposition processes including the vaporization and various decomposition of the urea ranging from 140 oC to 420 oC30-31. As the product of the reaction between FG and urea, the UFG also presents multistep decomposition. 3.3 Tribological properties of the UFG as water-based lubricant additive It is well known that the FG can not be dispersed in water due to its high hydrophobic property, which largely restricts the application of the FG. Therefore, the modification of FG with some hydrophilic functional groups to improve its dispersibility in water is the key to break through this obstacle. As shown in Figure 6, the FG and UFG aqueous dispersions with the same concentration of 2 mg/mL are respectively prepared. Obviously, the FG can not disperse and remain on top of water even after 4 h of sonication, while the visible homogeneous and dark dispersion is formed for UFG, which means the modification of FG by urea has greatly changed its surface wettability in water from hydrophobility to hydrophilicity. This is a significant research progress of FG because the good dispersibility of the UFG in water endows it with promising applications in aqueous environments.

Figure 6. Dispersibility of the FG and UFG in water. 12


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Based on the good dispersibility of the UFG in water, the UFG can be employed as a new water-based lubricant additive. Through testing tribological properties of the UFG aqueous dispersions with different concentrations, it is found that the anti-wear ability of water after addition of UFG can be largely improved, demonstrating the UFG is an effective water-based lubricant additive. As shown in Figure 7(a1, b1, c1, d1, e1), the wear tracks are significantly changed by the addition of the UFG, namely, the size of the wear tracks displays a trend of first decrease and then increase with increasing the content of the UFG, and the wear track is smallest when the concentration of the UFG is 1 mg/mL (UFG-1). At the same time, the morphology of the worn surfaces is observed and provided in Figure 7(a2, b2, c2, d2, e2). Compared with that of the pure water, the addition of UFG in water can reduce the parallel plowing grooves in the sliding direction and the plowing groove on the worn surfaces is slightest for the sample of UFG-1. On the other hand, the height profile of the wear tracks also can be used to reflect the tribological properties of the UFG as water-based lubricant additive. As shown in Figure 7(a3, b3, c3, d3, e3), the height variation is similar to that of the wear track, and the sample of UFG-1 presents the minimum variation. Namely, the UFG can be applied as a water-based lubricant additive to reduce the wear rate, and the sample of UFG-1 displays the best anti-wear ability.

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Figure 7. SEM micrographs of the wear tracks (a1-f1) and the worn surfaces (a2-f2) for UFG-0, UFG-0.5, UFG-1, UFG-2, UFG-4 and FG-2; height profile measurements of the wear tracks for UFG-0 (a3), UFG-0.5 (b3), UFG-1 (c3), UFG-2 (d3), UFG-4 (e3) and FG-2 (f3).

Herein, in order to further demonstrate the fact that the dispersibility of a 14


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material in medium is the crucial factor to decide the tribological property of the additive, the tribological performances of the FG-2 are also investigated for comparison. As shown in Figure 7(f1, f2, f3), the wear track, worn surface and height profile for the FG-2 dispersion are almost indistinguishable from that of pure water (UFG-0). Therefore, the poor dispersibility of the FG in water approves the fact that the FG can not be used a water-based lubricant additive directly. After the addition of UFG nanosheets in water, obvious reduction of friction coefficients can be attained although they are still high compared with that of the pure water, as shown in Figure 8. Like hexagonal boron nitride (h-BN)32, the prepared UFG is a notified insoluble material in water and the friction coefficient can be slightly reduced by repeated exfoliation and deposit of the UFG nanosheets on the worn surface during sliding. For comparison, the change trends of friction coefficient for the steel ball-steel block contact in dry sliding condition, as well as the GO-1 and UFG-1 aqueous dispersions with the same concentration of 1mg/mL are also measured and provided in Figure S4, indicating that the friction coefficient of GO and UFG dispersions presents the similar change trend. On the other hand, the anti-wear property of water can be greatly improved upon the addition of UFG; specially, the sample of UFG-1 has the optimal anti-wear ability with the minimum wear rate of 11.7 µm3/N·m, which is a 64.4% decrease compared with the sample of UFG-0 (32.9 µm3/N·m). The decrease of the wear rate is attributed to the fact that the UFG can enter the water-boundary lubricated layer to reduce the direct contact zone between the specimen and the counter face. For comparison, the tribological behaviors of the 15



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pure urea solutions with two different concentrations are also tested, proving that the addition of urea can not change the tribological property of water, as shown in Figure S5.

Figure 8. The histogram of friction coefficient and wear rate of the FG and UFG aqueous dispersions.

In order to demonstrate that the UFG can form a tribofilm between the specimen and the counter face, the energy-dispersive spectrum analysis is carried out to render the atomic percentages on the worn surfaces of UFG-0 and UFG-1, and the results are summarized in Table 1. Apparently, the fluorine is found on the worn surface of the UFG-1, suggesting the formation of UFG tribofilm, which is responsible for the reduction of wear rate noticed during the tribological test. Correspondingly, the characteristic distribution of elements in the section of the worn surfaces for the samples of UFG-0 and UFG-1 further confirm the fact that the UFG tribofilm effectively enhances the tribological properties of the water, as shown in Figure S6 and Figure S7 in the Supporting Information. 16


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Table 1. Atomic percentage of elements detected on the worn surface.

4. Conclusions In this paper, the FG has been covalently functionalized by the urea via part replacement of fluorine, and such replacement can change the surface wettability of FG from hydrophobicity to hydrophility. The huge transformation endows the prepared UFG with the good dispersibility in water, which allows the UFG as an effective lubricant additive to enhance the tribological property of the water. Tribological tests demonstrate that the sample of UFG-1 has the best anti-wear ability with a 64.4% decrease of wear rate compared with that of the pure water (UFG-0). Collectively, we have successfully modified the hydrophobic FG by using cheap urea to achieve the hydrophilic UFG, which opens up a bright application prospect for the FG nanosheets in aqueous environments.

Notes The authors declare no competing financial interest.

Acknowledgment This work was supported by the National Natural Science Foundation of China 17



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(Grant Nos. 51375474 and 51205385), 863 Plan (Grant No. 2015AA034602), and the ‘‘Funds for Young Scientists of Gansu Province (145RJYA280)’’ scheme.

Supporting Information High resolution TEM image of the FG; the Energy-Dispersive Spectrums of the FG and the UFG, the XPS N1s spectra of the urea and the UFG; the friction coefficients of steel ball-steel block contacts in dry sliding, UFG-1 and GO-1 conditions; tribological properties of the urea solutions; element mapping in a section of the worn surface.

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Covalent Functionalization of Fluorinated Graphene and Subsequent

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