A Strategy for Preparing Modified Graphene Oxide with Good

May 2, 2017 - Herein, oleic diethanolamide borate (ODAB)-grafted graphene oxide nanosheets (GO) with good dispersibility in oil were successfully achi...
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A Strategy for Preparing Modified Graphene Oxide with Good Dispersibility and Transparency in Oil Zhi-Lin Cheng,* Wei Li, Pei-Rong Wu, and Zan Liu School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China ABSTRACT: The challenges of graphene as a high-performance additive applied in lubricant are its dispersibility and transparency. Herein, oleic diethanolamide borate (ODAB)-grafted graphene oxide nanosheets (GO) with good dispersibility in oil were successfully achieved. The structure analysis of the as-prepared ODAB-grafted GO (MGO) nanosheets verified that the molecular ODAB has bonded with the active groups of the GO nanosheets surface, as determined by characterizations. In addition, the tribological properties of the MGO nanosheets as lubricant additive applied in oil were evaluated by a friction-testing machine, and the morphologies of worn surfaces were observed by 3D laser scanning microscope. The results showed that the MGO nanosheets possessed excellent dispersibility and transparency in oil, while the friction coefficient and wear scar diameter (WSD) of the MGO-based oil with 0.02 wt % MGO content decreased to about 38.4% and 42.0% less than those of the base oil, respectively.

1. INTRODUCTION Over the recent decades, the increased public attention to and awareness of the protection of the environment have stimulated the development of lubricants.1 Finding lubricants with both high-performance and biodegradability has become the topic of interesting research. The key issue in formulating highperformance and biodegradable lubricants is the choice of suitable additives. Recently, many researchers have been focusing on the tribological properties of graphene oxide (GO), owing to its excellent chemical and physical properties.2−4 GO, as a ultrathin lamellar-structured material, easily shears between the contact surfaces, which promises its potential for becoming a good choice of lubricant additives.5 For example, Kinoshita et al.6 studied the tribological properties of GO monolayer nanosheets as an additive in water-based lubricants. The results showed a very low friction coefficient of approximate 0.05 and an inconspicuously worn surface after testing. Wu et al.7 developed the myristyltrimethylammonium bromide (TTAB)-modified GO hybrids and applied them as an additive in oil-in-water emulsions. The friction coefficient and the wear rate of the steel ball were decreased about 18% and 48% more than those of the emulsion without additive, respectively. However, solution of the problems with dispersibility and transparency was crucial for GO to be applied as a highperformance additive in lubricants. It was well-known that the GO nanosheets, as an inorganic material, are insoluble with oil. So, it is highly desirable to achieve a homogeneous solution of GO and oil.8 Fortunately, the abundant active oxygenous groups of the GO surface provide a platform for rich chemistry to occur both within the intersheet gallery and along the sheet edges, thus distinctly improving the dispersibility in oil. Therefore, the modification of the GO surface has become a strategy for solving the dispersibility issue. © XXXX American Chemical Society

Currently, the chemical modification of GO could not only improve the dispersion capability in oil but also might enhance the antiwear performance of oil. Lin et al.9 synthesized GO modified by stearic acid and oleic acid (MGP) by using chemical methods and added it into the 350 SN base oil as an additive. It was found that the wear resistance and load-carrying capacity of the oil containing only 0.075 wt % of the MGP were obviously improved. Choudhary et al.10 developed alkylated GO (GrO) by the coupling of alkylamine, and it exhibited improved dispersibility in hydrocarbon oil. The friction coefficient and wear scar diameter of the GrO-based oil reduced 26% and 9% more than those of the base oil, respectively. Mungse and Khatri.5 reported a chemical approach for selective inclusion of long alkyl chains on the edges and defects sites of GO nanosheets through an amide linkage. The resulting modified GO nanosheets could facilitate their stable dispersion in oil. Oleic diethanolamide borate (ODAB) was known to be an effective boundary lubrication additive. When incorporated into a lubricant, ODAB could exhibit an excellent antiwear and friction-reducing ability in the lubricants. More importantly, ODAB was also more readily biodegradable and possessed lower ecotoxicity than the other modifiers mentioned above.1 Therefore, ODAB was expected to be capable of improving the biodegradability of the lubricant. In addition, the presence of the long alkyl chain and active carboxyl group in molecular ODAB makes it an appropriate modifier for improving the dispersibility of GO in oil. So, a study based on the preparation Received: April 9, 2017 Revised: April 23, 2017 Accepted: April 27, 2017

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DOI: 10.1021/acs.iecr.7b01472 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of the fabrication of MGO.

Figure 2. FTIR spectra (a), XRD spectra (b), Raman spectra (c), and UV−vis spectra (d) of GO, ODAB, and MGO.

24 h, redundant thionyl chloride was separated by distillation. Then, 20 mL of ODAB and 2 mL of ethanol amine (ETA) were added, and the reaction temperature was elevated to 120 °C. After reacting for 4 h, the reaction mixture was washed with ethanol at least six times. Finally, the shiny black residue, namely, ODAB-modified GO nanosheets, was dried in an oven at 80 °C. The reaction mechanism is shown in Figure 1. 2.2. The Dispersibility of MGO in the Base Oil. MGO nanosheets were dispersed as additives into the 500 SN base oil by sonication for 10 min to obtain the transparent MGO-based oils with different MGO contents (0.01, 0.02, 0.03, and 0.04 wt %). After 8 days, the MGO-based oils remained transparent without any sedimentation on the bottom. 2.3. Characterizations and Tribological Testing. The tribological testing was performed in a MMW-1 four-ball machine (Jinan Chenda Ltd. Co.). The testing conditions were set at the rotating speed of 1200 rpm under a stable load (147 N) for the testing duration of 6 h and a constant load (200− 500 N) for 1 h. Every test was repeated three times. The wear scar diameter (WSD) was measured by optical microscope after testing. The obtained friction coefficient and wear scar diameter were the average of three measurements.

of the ODAB-modified GO hybrid and its dispersibility and tribological properties in oil is desirable. In this paper, a novel modifier based on ODAB was used to decorate the surface of GO nanosheets. The dispersibility and transparency of ODAB-modified GO nanosheets (MGO) in the base oil were studied. The structure of the MGO was determined by a series of characterizations. Finally, the tribological properties of MGO nanosheets as additives in the base oil were investigated. The worn surfaces were detected by a 3D laser scanning microscope and Raman spectrum. The mechanism of MGO in acting to reduce the friction coefficient and wear of the base oil was also proposed.

2. EXPERIMENTAL SECTION 2.1. Preparation of MGO. GO nanosheets with 1.42 nm thickness were prepared by an improved Hummers’ method.11,12 In order to introduce ODAB (Nantong Zhanyi Co. Ltd.), the carboxylic groups located on the edges of the GO molecules were activated by reacting with thionyl chloride.13 First, 1.0 g of GO was mixed with 15 mL of DMF and 60 mL of thionyl chloride under a nitrogen atmosphere. The reaction temperature was kept at 70 °C under continuous stirring. After B

DOI: 10.1021/acs.iecr.7b01472 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research The Fourier transform infrared (FTIR) spectrum was recorded on a Cary 610/670 micro infrared spectrometer in the range of 4000−750 cm−1 by a single reflection ATR attachment. The X-ray diffraction (XRD) data were recorded for a 2θ angle between 5° and 70° using a D8 ADVANCE XRD instrument. The Raman spectra were acquired on a Labram-1B (Dilor) confocal microscopy Raman spectrometer with 532 nm wavelength incident laser light. The UV−vis spectra were collected on a Cary 5000 spectrophotometer. The transmission electron microscope (TEM) images were recorded by a Tecnai 12 TEM. The SEM images were recorded by an S-4800 II field emission scanning electron microscope (Rili). The X-ray photoelectron spectroscopy (XPS) patterns were recorded by an ESCALAB 250Xi XPS. The wear scar micrographs were observed by an LSM 700 3D laser scanning microscope.

3. RESULTS AND DISCUSSION Figure 2 shows the FT-IR spectra, XRD spectra, Raman spectra, and UV−vis spectra of GO, ODAB, and MGO. In the FT-IR spectra, there appear some fresh peaks in the spectrum of the MGO in comparison to that of GO nanosheets. The bands at 2922 and 2855 cm−1 are attributed to the −CH3 and −CH2 stretching vibration, and the bands at 1404 and 880 cm−1 are assigned to the asymmetric and symmetric stretching of B(3)−O and B(4)−O bonds.14 All these fresh peaks originated from ODAB. Moreover, the intensity of the band at 1747 cm−1 (−COOH) is dramatically decreased, whereas the intensities of the bands at 1626 cm−1 (−CO−N−) and 880 cm−1 (B(4)−O) are increased. This should be ascribed to the condensation reaction between GO (−COOH) and ODAB (−BOH) by ETA. It proves that the surface of GO nanosheets was successfully decorated by ODAB molecules. As shown in Figure 2b, the XRD diffraction peak of GO at 2θ = 10.6° suggests the corresponding d-spacing of 0.83 nm.15 The XRD pattern of ODAB shows a broad peak at around 2θ = 19.2°, revealing that it is noncrystalline.16 As for the MGO, the pattern is similar to that of ODAB. The disappearance of the characteristic diffraction peak of GO nanosheets should be as a result of the basal plane of GO nanosheets coated by ODAB.17 The Raman spectrum of MGO reveals a G band at ∼1600 cm−1 deriving from the characteristic peak of the in-plane vibration of a layer of sp2 carbon atoms and a D band at around ∼1350 cm−1 coming from the defects or imperfections.18,19 More importantly, the intensity ratio (ID/IG) of the MGO is increased from 0.82 of GO to 1.10. This indicates that the average size of the sp2 carbon domains in MGO is decreased and the number is increased, suggesting the existence of a higher degree of defects.20 It could be due to the decoration of ODAB on the surface of GO. As shown in the UV−vis spectra (Figure 2d), GO nanosheets exhibit a strong absorption at 230 nm, which is attributed to the π−π* transitions in the polyaromatic (C−C bonds) system of GO layers, and a shoulder absorption peak at about 300 nm is assumed to be the n−π* transitions of C−O bonds.21 As for MGO, the peak of the C−C bond shifts to 210 nm and the peak of the C−O bond completely disappears. Such changes reveal the increasing of the π-conjugation network in the MGO.10 Figure 3 displays the TEM images of GO and the MGO. The self-made GO nanosheets consist of large leaflike nanosheets with a thinner thickness according to the lower contrast ratio. The morphology of the MGO is found to be more crumpled. It was clearly observed that a large number of black nanoparticles appear on the surface of GO nanosheets in the magnified

Figure 3. TEM images of GO (a) and MGO (b−d).

images. More importantly, after washing the MGO several times with ethanol, these nanoparticles still remained on the surface of GO. This confirms that the ODAB was grafted with the active groups of GO by bonding. Apparently, these ODAB nanoparticles are uniformly distributed on the GO surface, thus being favorable to prevent the secondary aggregation of GO nanosheets and improve the dispersibility in oil. Figure 4 shows

Figure 4. SEM images of GO (a) and MGO (b) and the elemental mapping images of MGO (c−f).

the SEM images of GO and MGO. The self-made GO nanosheets consist of the folding sheets with a large diameter. However, the morphology of MGO shows small pieces and the layered structure, which will play a significant role in lubrication (Figure 4b). As shown in Figure 4c−f, there is a uniform distribution of C, O, N, and B elements throughout the GO nanosheets. From this qualitative observation, the densely agglomerated GO nanosheets were significantly detracted, thus C

DOI: 10.1021/acs.iecr.7b01472 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. XPS survey spectrum (a) and C 1s XPS spectrum (b) of GO and XPS survey spectrum (c) and C 1s (d), N 1s (e), and B 1s (f) XPS spectra of MGO.

Figure 6. UV−vis spectra and dispersion photographs (inset) of MGO-based oil with different contents (a) and UV−vis spectra of 0.04 wt % MGObased oil as a function of time (b).

benefiting to the molecular insertion of ODAB into the basal plane of GO nanosheets. The XPS spectra of GO and MGO are shown in Figure 5. For the self-made GO nanosheets, only the presence of C and O is detected, whereas in the spectrum of MGO, the four signals of B, C, N and O appear. More importantly, the C/O

atomic ratio of GO (1.72) is much lower than that of MGO (3.09). This proves that the MGO nanosheets are deconvoluted into four peaks: C−C/CC (284.8 eV), C−O (286.8 eV), CO (287.9 eV), and O−CO (288.9 eV).22 In the XPS spectrum of MGO, the intensity of C−C/CC increases, whereas the intensity of C−O remarkably reduces compared to D

DOI: 10.1021/acs.iecr.7b01472 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 7. Average friction coefficient (a) and wear scar diameter (b) of MGO-based oil as a function of added content; (c) the friction coefficient curves of 500 SN base oil, ODAB-based oil (0.02 wt %), and MGO-based oil (0.02 wt %) depending on friction time; and (d) the PB value of MGObased oil (0.02 wt %) depending on MGO content.

coefficient and the WSD present a similar trend upon increasing the MGO content in the 500 SN base oil. They significantly decrease from 0.01 to 0.02 wt % and afterward increase from 0.02 to 0.04 wt %. The reason should be ascribed to the agglomeration of the excess MGO nanosheets at the higher content in oil, which hampers their uninterrupted supply to the contact interfaces.23 The lowest average friction coefficient and smallest WSD, 0.069 and 0.51 mm, respectively, are located at 0.02 wt % of MGO content. These values are about 38.4% and 42.0% lower than those of the 500 SN base oil (0.112, 0.88 mm), respectively. It is concluded that the MGO nanosheets, used as an additive, played a positive role in lowering the friction and wear of oil. Figure 7c shows the friction coefficient curves of the 500 SN base oil, the MGObased oil (0.02 wt %), and the ODAB-based oil (0.02 wt %) with repect to the friction time. The friction coefficients of the ODAB-based oil and MGO-based oil are obviously lower than that of the base oil. The average friction coefficients of the oils containing 0.02 wt % ODAB and MGO are up to 0.086 and 0.069, which decline by about 23.2% and 38.4% compared to those of the 500 SN base oil, respectively. Meanwhile, the result also indicates that the MGO nanosheets have a better antifriction and antiwear performance in oil. This is attributed to the unique layered structure of the MGO, which can provide easy shearing due to the weakly coordinated lamellas (van der Waals interaction) while they are under the tribo-stressed zone. Furthermore, the good dispersibility of MGO in the base oil ensures that enough of it is absorbed on the rubbing interface.5 Figure 7d shows the PB curve of the MGO-based oil depending on the MGO content in oil. Obviously, there is a remarkable increase in PB value upon increasing the MGO content in oil. The PB value of the MGO-based oil reaches its maximum at 0.02 wt % of MGO, which is about 46.2% higher than that of

that of GO, which is of a higher carbon content. The C 1s XPS spectra of GO and MGO could be due to the ODAB modification. In addition, two fresh peaks of N 1s and B 1s were successfully detected in the XPS survey spectra of MGO (Figure 5c). The N 1s XPS spectrum of MGO shows the C−N bond at 400.1 eV (Figure 5e). Moreover, the B 1s XPS spectrum of MGO shows two peaks of B(3)−O (191.7 eV) and B(4)−O (198.2 eV). This confirms the FT-IR results. The result of the detected C−N bond distinctly demonstrates that ODAB molecules are successfully grafted on the surface of GO by amide reaction. The UV−vis spectra and dispersion photographs (inset) of the MGO-based oils with the different MGO contents are shown in Figure 6a. The spectra of the MGO-based oil with different contents all show a strong absorption peak at 247 nm, which is attributed to the π−π* transitions of the graphitic C C bonds that originated from GO. With increasing the content of MGO in the base oil, the typical absorption peak of MGO becomes stronger, indicating that the dispersibility of the asprepared MGO still improves in oil even if the MGO is at the higher content. The photographs of the MGO-based oils with different contents can confirm that the as-prepared MGO nanosheets are of good oil solubility (Figure 6a, inset). To examine further the stability of the MGO in oil, the UV−vis absorbance of the MGO-based oil with 0.04 wt % MGO content over time is shown in Figure 6b. Within the standing time of 8 days, the typical absorption peak of the MGO in oil at 247 nm almost remains unchanged. These results can prove that the dispersibility and transparency of GO nanosheets are successfully achieved by the ODAB modification. Parts a and b of Figure 7 show the average friction coefficient and wear scar diameter (WSD) of the MGO-based oil as a function of added content at a load of 147 N. Both the friction E

DOI: 10.1021/acs.iecr.7b01472 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 8. (a) The oil temperature curves of the MGO-based oils with different MGO contents with respect to friction time and (b) the 500 SN base oil, MGO-based oil (0.02 wt %), and ODAB-based oil (0.02 wt %) as a function of friction time.

Figure 9. Average friction coefficient (a) and wear scar diameter (b) curves of the base oil, ODAB-based oil, and MGO-based oil versus the load.

Figure 10. 3D laser scanning micrographs and corresponding Raman spectra of worn surfaces tested in (a, d) 500 SN base oil, (b, e) ODAB-based oil (0.02 wt %), and (c, f) MGO-based oil (0.02 wt %).

the base oil. This indicates that the MGO nanosheets have a good load-carrying ability. Figure 8a shows the oil temperature curves of the MGObased oil with different contents. Obviously, when the content of MGO is 0.02 wt %, the oil temperature attains its minimum value. This is consistent with the results of the above friction coefficient study. As shown in Figure 8b, the oil temperature curves of the three oils all show a rapid rising prior to 7500 s, and afterward, the oil temperature curve of the MGO-based oil (0.02 wt %) keeps a good stability. The last oil temperature of the MGO-based oil (0.02 wt %) is about 9.2 and 11.8 °C lower

than those of the ODAB-based oil (0.02 wt %) and the 500 SN base oil. The cooling effect of the MGO nanosheets in oil is a valuable feature in practical application. As shown in Figure 9, the curves reflect the load-carrying capacity of the base oil, ODAB-based oil (0.02 wt %), and MGO-based oil (0.02 wt %) in the friction time of 1 h. With increasing the applied load, both the average friction coefficient and wear scar diameter of the MGO-based oil are increased. However, the average friction coefficient and wear scar diameter of the MGO-based oil rise more slowly in the range of the overall load compared to the base oil and ODAB-based F

DOI: 10.1021/acs.iecr.7b01472 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 11. Friction coefficient curves of ODAB and MGO added to (a) 150 SN base oil and (b)100 SN base oil as a function of time.

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ACKNOWLEDGMENTS This work was supported by the Talent Introduction Fund of Yangzhou University (2012), Key Research Project-Industry Foresight and General Key Technology of Yangzhou (YZ2015020),b Innovative Talent Program of Green Yang Golden Phoenix (yzlyjfjh2015CX073), Yangzhou Social Development Project (YZ2016072), Jiangsu Province Six Talent Peaks Project (2014-XCL-013), and Jiangsu Industrial−Academic−Research Prospective Joint Project (BY201606902). The authors also acknowledge the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. The data of this paper originated from the Test Center of Yangzhou University.

oil. At the higher applied load, the disparities of the average friction coefficient and wear scar diameter of the MGO-based oil are less than at the lower load. This suggests that the MGO nanosheets exhibit a better load-carrying capacity in oil. Figure 10 shows the 3D laser scanning micrographs of the worn surfaces after testing in the 500 SN base oil, ODAB-based oil (0.02 wt %), and MGO-based oil (0.02 wt %) at a load of 147 N. It can be clearly found that after a 6 h long time testing, the worn surface of the steel ball in testing the 500 SN base oil is the largest. Compared to other two oils, the worn surface of the MGO-based oil is much smoother. The Raman spectra of the worn surfaces indicate that the two characteristic peaks of GO at 1340 cm−1 (D band) and 1600 cm−1 (G band) are detected on the worn surface. This demonstrates that the MGO nanosheets in the base oil entered the contact interfaces with the aid of the base oil during sliding and absorbed in the worn surface, thus preventing damage due to the excellent wear resistant property of GO.17 To validate the results of the above testing, the friction performance of the MGO in base oils with low viscosity also was investigated at a load of 147 N, as shown in Figure 11. Obviously, the MGO nanosheets in the other two different base oils exhibit the same excellent friction-reducing ability. Furthermore, compared to the 150 SN and 100 SN base oils, the MGO in the 500 SN shows the lowest friction coefficient. It is ascribed to the high viscosity of the base oil being very helpful to improve the dispersibility and inhibit the agglomeration of MGO nanosheets in the base oil.12

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4. CONCLUSIONS The GO nanosheets have been successfully modified by a coupling reaction based on GO, ODAB, and ETA. The asprepared ODAB-grated GO (MGO) nanosheets with good dispersibility could create a transparent MGO-based oil. They exhibited a lower friction coefficient, higher PB value, and smaller wear scar diameter than those of the base oil. These results demonstrated that the MGO nanosheets as a lubricating additive possess perfect oil solubility, good antiwear ability, and excellent load-carrying capacity.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhi-Lin Cheng: 0000-0003-3164-5883 Notes

The authors declare no competing financial interest. G

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DOI: 10.1021/acs.iecr.7b01472 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX