J. Cent. South Univ. (2016) 23: 2800-2808

DOI: 10.1007/s11771-016-3343-2

Scalable preparation of functionalized graphite nanoplatelets via magnetic grinding as lubricity-enhanced additive

JI Hai-bin(季海滨), HE Zhi-wei(何志伟), SONG Sha-sha(宋沙沙), ZHAO Zeng-dian(赵增典)

School of Chemical Engineering, Shandong University of Technology, Zibo 255000, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract: Graphite nanoplatelets were prepared by a novel magnetic-grinding method using self-made equipments. Under a variant magnetic field, magnetic needles collided at a high rotating speed and exfoliated pristine graphite into graphite nanoplatelets with high efficiency. The obtained graphite nanoplatelets are highly crystalline, and the thickness is less than 10 nm. Moreover, the surface area could reached 738.1 m²/g with a grinding time of 4 h. Silanized graphite nanoplatelets can disperse well in SG 15W-40 engine oil and serve as lubricant additive. Tribological results indicate that the friction coefficient and wear-scar of the friction pairs are lower than 76% and 41%, respectively, by adding 1.5‰ (mass fraction) of silanized graphite nanoplatelets. Notably, the functionalized graphite nanoplatelets can realize large-scale production and commercial application.

Key words: magnetic grinding; graphite nanoplatelets; silanization; lubricant additives

1 Introduction

Since the first isolation of a single graphene sheet in 2004 [1], graphene nanoplatelets have attracted enormous attention due to their exceptional properties e.g., high surface area, high elastic modulus, excellent electrical and thermal conductivity, and good chemical stability [2-4]. In the recent years, many applications of graphene nanoplatelets such as supercapacitors, nanoelectronic devices, and strain sensors have been reported [5-6]. Moreover, graphite nanoplatelets(GNS) also exhibit excellent performance when being used as additive in various materials [7-9]. Friction reduction and anti-wear properties of lubricants can also be enhanced by the addition of GNs [10-13].

However, the complex preparation process of GNs prohibited its commercial application [14]. Among the reported synthetic methods, chemical vapor deposition (CVD) is one of the important methods [15]. According to recent investigation, large-area graphene films could be grown on different substrates via deposition of different hydrocarbon sources [16]. CVD growth is a well-established method to synthesize high-quality graphene; but, this method is time-consuming and can hardly meet the needs of large-scale industrial production. The chemical reduction of graphite oxide (GO) is another widely reported method. Using strong oxidant, e.g., HNO₃ or KMnO₄, graphite can be oxidized to GO and the subsequent reduction of GO will be carried out with the help of reducing agent like hydrazine hydrate or NaBH₄ after exfoliation in solution [17-19]. While the use of strong oxidant leads to severe damage in the carbon basal plane, which cannot be reversed by reduction [20]. Recently, ESWARAIAH et al [21] prepared highly conducting GNs by exfoliation of GO under solar radiation, which is a simple method of GO reduction, providing much promise for replacing tedious production steps and dangerous reagents. Moreover, micro-mechanical cleavage is also a very promising method for preparing GNs. JEON et al [22] prepared edge-carboxylated graphite (ECG) by ball-milling in a stainless-steel capsule using dry ice and pristine graphite as raw materials and the prepared edge-carboxylate graphene showed good dispersion performance in different solvents.

In the present work, we reported a novel and facile magnetic grinding method to prepare functionalized graphite nanoplatelets (FGNPs). The chemical and structural properties of the FGNPs were measured by SEM, TEM, SAED, XRD, TG, FTIR, BET, etc. The prepared FGNPs were further modified chemically by N-dodecyltriethoxysilane (DTES). The modified FGNPs (MFGNPs) show strong synergistic effects when it is used as lubricant additive, which was confirmed by standard four-ball friction tests, and absorbance test demontrasted that the suspension property of modified FGNPs is better than FGNPs obviously.

2 Experimental sections

2.1 Chemicals and materials

Pristine graphite (180 μm with purity of 99.9%) was purchased from Qingdao Ruisheng Graphite Co., Ltd.. Grinding media were SUS304 magnetic steel needles of dimensions d0.5 mm×5 mm and magnetic flux density of 8×10^-4 T. SG 15W-40 engine oil was supplied by SINOPEC Group. All other chemicals (AR) were purchased from Shanghai Chemical Reagents Company and used directly without further purification.

2.2 Materials preparation

2.2.1 Preparation of FGNPs

FGNPs were prepared by magnetic grinding using self-made equipments. As shown in Fig. 1(a), stainless steel needles and pristine graphite were mixed in a milling chamber. Below the milling chamber, four NdFeB permanent magnets were installed on an electrically driven rotating disc to produce a rotating magnetic field inside of the milling chamber, which causes the grinding media to flow. Friction, collision, and shear forces created by the needles rotating at high speed exfoliate the pristine graphite with extremely high efficiency. Typically, graphite and the stainless steel needles (mass ratio of 1:20) were filled into the grinding chamber. The rotational frequency was fixed at 1800 rad/min for a grinding time interval of 30 min. The rotational direction was reversed after every grinding interval to achieve a uniform and even shape of the samples. After grinding, the mixture was filtered by a sieve (180 mesh) to separate the FGNPs from the stainless steel needles. 1 mol/L HCl was used to remove the metallic impurities. The final FGNPs were obtained after filtrating and drying at 80 °C.

Fig. 1 Schematic diagram of magnetic grinding method (a) and preparation of FGNPs and silanization of FGNPs (b)

2.2.2 Modification of FGNPs

DTES was used to modify the as-prepared FGNPs in order to improve their dispersion in base oil (SG 15W-40 engine oil). Figure 1(b) gives a schematic for the modification process of FGNPs using DTES. FGNPs (1 g) produced by grinding for 4 h and DTES (0.33 g) were added into toluene (100 mL). After refluxing at 100 °C for 15 h, the mixtures were filtered and the solid residues were washed with acetone to remove residual DTES, resulting in the modified FGNPs (MFGNPs).

2.3 Material characterizations

2.3.1 Chemical and structural characterization of FGNPs

The morphology of the samples was observed by scanning electron microscopy (SEM, Sirion 200, FEI, the Netherlands). Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were obtained by using the instrument type JEM-2010 (JEOL, Japan) operated at an accelerating voltage of 200 kV. The crystal structure of the powder samples was characterized by X-ray diffraction (XRD, D8 Advance, Bruker AXS, Germany) using CuK_α radiation (λ=0.15418 nm) at a scan rate of 2°/min and step-size of 0.1°. Raman spectra were obtained using a He-Ne laser (325 nm) as excitation source on a conventional Raman spectrometer (Alpha 300S, HORIBA Jobin Yvon, France). Nitrogen-sorption measurements were carried out on an ASAP2020M system (Micromeritics, USA). Thermal stabilities of the pristine graphite and the FGNPs were studied by thermogravimetry (TG, DSC Q100, USA). Oxygen- containing functional groups on the sample surface were tested by Fourier-transform infrared spectroscopy (FT-IR, Nicolet 5700, Thermo Nicolet, USA). The suspension stability of the prepared samples is reflected in their absorbance recorded by an ultraviolet-visible spectrophotometer (Shimadzu UV-2450, Shimadzu Corporation, Japan) at 600 nm.

2.3.2 Tribological tests

The standard four-ball friction test (MRS-10A, Jinan Yihua Tribological Testing Technology Co. Ltd., China) was used to determine the friction-reduction and anti-wear properties of the MFGNPs used as lubricant additive. Before the tests, the prepared MFGNPs were added to SG 15W-40 engine oil and the mixture was stirred at 200 r/min at 80 °C for 1 h to form a homogeneous system. The test balls were made of GCr15A-bearing steel (AISI 52100) with diameter of 12.7 mm and hardness of HRC64. The steel balls and other test components of the machine were washed with petrol before each test. The tests were carried out according to the ASTM D4172-82 standard. The rotational speed was set at 1200 r/min under a load of 147 N, and the temperature was maintained at 75 °C.

3 Results and discussion

3.1 Preparation of FGNPs

As shown in Fig. 1(a), stainless steel needles rotate at an extremely high speed under the rotating magnetic field. Powerful shearing forces are produced by collisions of the grinding media with the rigid walls. Besides, the interactions among the stainless steel needles can also create strong forces. Thus, friction, collision, and shear forces created by the rotating needles exfoliate the pristine graphite with extremely high efficiency. These forces, especially the shearing force, are strong enough to break the C—C bonds and to destroy the network of graphite, which in turn, creates unstable edges with large surface energy [23]. After opening the lid of the grinding chamber, the sample is suddenly exposed to air. CO₂ and O₂ are adsorbed easily by the extremely active surfaces of carbon, turning into carboxylates (—COO^-). Under the effect of H₂O (moisture) and O₂ contained in air, further parts of the unstable carbon edges and the carboxylate groups turn into carboxylic acids (—COOH), hydroxylic (—OH), and hydroperoxylic (—OOH) functional groups to reach an equilibrium state [22]. It is evident that the smaller the size of the FGNPs, the larger the number of oxygen-containing functional groups at their edges.

3.2 Physical and chemical properties of FGNPs

SEM images of pristine graphite and of FGNPs are shown in Fig. 2. Large flakes with thickness of about 45 μm can be found in Fig. 2(a). After grinding for just 2 h, all large flakes turned into many thinner platelets with irregular edges (Fig. 2(b)). Due to the non-covalent interactions, these thin platelets are stacked each other to form larger aggregates to decrease their surface energy. When FGNPs were ground for 4 h and 6 h (Figs. 2(c) and (d)), the size and thickness were further reduced and the assembly of the nanoplatelets became more serious. Figure 2(e) clearly shows the presence of nanoplatelets. The corresponding SAED pattern (Fig. 2(e) inset) exhibits a number of well-defined sharp peaks with 6-fold symmetry, suggesting that the as-prepared FGNPs are extremely thin under preservation of the crystal structure [24]. The high-resolution TEM image (Fig. 2(f)) shows that the thickness of the FGNPs is less than 5.5 nm.

Fig. 2 SEM images of pristine graphite (a) and ground for 2 h (b), 4 h (c), 6 h (d) and TEM image (e) and (inset) SAED pattern of FGNPs ground for 4 h (a), and high-resolution TEM (HR-TEM) image (f)

The specific surface areas of FGNPs obtained from grinding for different times are presented in Fig. 3. After grinding for only 2 h, the Brunauer-Emmett-Teller (BET) surface area increased sharply from 1.7 to 702.2 m²/g, i.e. an increase of more than four hundred times compared to pristine graphite, indicating that the magnetic grinding method is highly efficient in exfoliating large flakes of graphite. Notably, the BET surface area decreases gradually when the grinding time exceeds 4 h. The longer the grinding time, the smaller the prepared FGNPs, and the more serious their agglomeration, resulting in the decrease of the specific surface area at longer grinding time. After grinding for 6 h, the BET surface area remains at about 600 m²/g, which means that exfoliation and aggregation have reached an equilibrium state.

Fig. 3 BET surface data of graphite ground for different hours

FT-IR spectra were used to determine the existence of oxygen-containing functional groups in FGNPs. The aromatic C=C stretching peak at 1578 cm^-1 can be found in pristine graphite (Fig. 4(a)), and new peaks appear in the FGNPs, indicating the GNs have been successfully modified. The characteristic absorption peaks of oxygen-contained groups appear, such as the peak of —OH groups at 3400 cm^-1. The strong absorption peak at 1734 cm^-1 is attributed to the —C=O stretching vibration in carboxylic acid. The peaks at 1047 and 1465 cm^-1 are designated to C—O and C—OH stretching vibration, respectively [25-26]. As shown in Fig. 4(e), the survey spectrum of FGNPs ground for 4 h reveals the presence of C, O, and the O content is found to be 7%-8% (mole fraction) from the spectra collected at different locations. The C 1s spectra (Fig. 4(f)) show the presence of C—C/C=C at 284.6 eV, C—OH at 286.5 eV, C=O at 287.9 eV, and C(O)OH at 288.9 eV, and the peak at 285.6 eV attributes to defect caused in the process of grinding. The predominant peak in the C 1s spectrum of FGNPs is hydroxyl bonding (C—OH), followed by carbonyl bonding (C=O) and carboxyl bonding (C(O)OH). To further illustrate the presence of oxygen-containing functional groups in FGNPs, thermo- gravimetric analysis (TGA) was used to detect the thermal stability of pristine graphite and FGNPs. From Fig. 4(b), one can see that pristine graphite present outstanding thermal stability in the entire investigated temperature range of 30-700 °C. In contrast, FGNPs start to lose mass above 100 °C, owing to the loss of oxygen-containing functional groups and release of CO₂ and H₂O [22, 27-28], leading to a mass loss of ~ 5%. FGNPs loss more mass with the increase of grind time because oxygen-containing functional groups was decomposited. Both FT-IR and TGA analyses demonstrate that the graphene nanoplatelets can adsorb different gases (CO₂, O₂, and moisture contained in air) to form oxygen-containing functional groups.

Figure 4(c) show the XRD patterns of pristine graphite and FGNPs for different grinding time (2, 4, 6, 8 h). The strong (0, 0, 2) reflection at 26.02° of pristine graphite indicates that the d-spacing of adjacent carbon layers is approximately 0.34 nm, corresponding to the typical layered structure of graphite. Meanwhile, the (0, 0, 2) peak around 26° becomes wider and weaker when the grinding time increases from 2 h to 6 h. The probable reason may be that the size decrease of pristine graphite and the stacking of ultra-fine sheets of FGNPs result in the broadening of the peak [29]. No obvious peak was observed when the magnetic grinding process was performed for 8 h, indicating that the hexagonal crystal structures are seriously damaged. With the increasing in the grinding time, the interlayer distance (d_2h=3.36 , d_4h=3.37 , d_6h=3.42 , d_8h=3.49 ) increases from 3.36 to 3.49 . This subtle difference can be interpreted as edge-expansion of graphite layers induced by the increasing number of oxygen-containing functional groups at the edges of FGNPs.

The powder samples were further characterized by Raman spectroscopic measurements (Fig. 4(d)). The pristine graphite showed the D and G bands at 1321 cm^-1 and 1576 cm^-1 [30]. In contrast, the broadening and increasing in strength of the G band in the case that the material was ground for 2 h and 4 h may be ascribed to the effective reduction in size of the in-plane sp² domains. Furthermore, the positions of the two bands exactly match with those of graphite. The intensity of the D band of the materials ground for 2 h and 4 h gained a massive increase possibly attributed to the generation of a large number of edges and defects by the grinding process. The intensity ratios of the D to the G band (I_D/I_G) of pristine graphite and graphite ground for 2 h and 4 h are 0.612, 1.2167, and 1.632, respectively. The increase of I_D/I_G indicates a larger number of defects from oxygen- containing groups in the ground graphite material, which is consistent well with the XRD results.

Fig. 4 FT-IR spectra of pristine graphite, FGNPs ground for 4 h and modified FGNPs (a), TGA of pristine graphite and FGNPs ground for 2, 4, 6 h (b), powder XRD patterns of all investigated types of FGNPs and pristine graphite (c), Raman spectra of pristine graphite and FGNPs ground for 2 h and 4 h (d), XPS survey spectrum (e) and high resolution C 1s spectra (f) of FGNPs ground for 4 h

3.3 Modification and anti-wear application of FGNPs

The micron-sized graphite flakes could be effectively exfoliated into ultra-thin nanoplatelets having oxygen-containing functional groups (—COOH, —OH). In order to improve the suspension stability in lubricants, long-chain hydrocarbons could be grafted onto the FGNPs through a silanization reaction (Fig. 1(b)). The FGNPs prepared by grinding of 4 h were selected for their high BET surface area (738.1 m²/g, see Fig. 3). In the FT-IR spectra of MFGNPs (Fig. 4(a)), the presence of new peaks at 2956, 2921 and 2852 cm^-1 are ascribed to asymmetric methyl (—CH₃), asymmetric and symmetric stretching vibrations of methylene (—CH₂—), respectively [31-32], confirming the silanization of FGNPs; otherwise, there still have peaks of residual oxygen-containing functional groups. The suspension stability of the prepared samples was evaluated by their absorbance relative to base oil at 600 nm. After 30 min of centrifugation at 4000 min^-1, the samples with lower suspension stability partly deposit onto the bottom and therefore exhibit lower light absorbance. After centrifugation, the color of base oil with pristine graphite was nearly the same as that of pure base oil, meaning that nearly all the pristine graphite deposited to the bottom of the centrifuge tube due to the big particle size of pristine graphite. The color of base oil with FGNPs became lighter, which demonstrated that the FGNPs were heavily agglomerated in base oil. In contrast, no clear change could be observed by naked eye in base oil with MFGNPs after centrifugation. Obviously, the improvement could be attributed to the grafting of hydrophobic DTES.

Since the ultra-thin morphology and high strength, the MFGNPs can efficiently improve the tribological properties of lubricant oil. Lubricants prevent direct contact of friction pairs by forming a separating film [33]. However, this protective film can be destroyed once the working pressure exceeds the load-carrying capacity. After the addition of MFGNPs, the protective film is strengthened since the more powerful carbon component serves as spacer between the metallic friction pairs with higher mechanical robustness. As shown in Fig. 5, the average friction coefficient (AFC) and wear scar diameter of the steel ball tested in base oil without MFGNPs are 0.1170 and 482 μm, respectively. When added 1‰ of MFGNPs, the AFC of the base oil can reduce to 0.0642, corresponding to a decrease of 45.1%. This significant reduction in the friction coefficient of base oil could be ascribed to the unique layered structure and self-lubrication of MFGNPs. When the concentration increases to 1.5‰, a larger number of layered MFGNPs deposit on the rubbing surfaces and the AFC drops to 0.0282, which is significantly lower (decrease by 75.9%) than that of base oil (0.1170), and no obvious change forms when MFGNPs concentration increases to 2‰. The presence of the MFGNPs in base oil formed a tribological thin film that can hardly be destroyed under the same load conditions, thus the rubbing surfaces are well protected. As a result, the wear-scar diameter of the steel balls gets improvement to 353 and 284 μm for the two samples with the addition amounts of 1% and 1.5‰ and corresponding to a decrease of about 26.8 % and 41.1%. Meanwhile, the AFC of the base oil shows less variation with time after the addition of MFGNPs (Fig. 5(a)), illustrating the MFGNPs have excellent anti-wear properties and can easily form a better continuous protective film due to their small size and extremely thin layered shape. We can conclude that after adding MFGNPs, a better working environment could be created for friction pairs.

Fig. 5 (a) Friction coefficient and wear scar of base oil containing MFGNPs at different concentrations (a1-0‰; a2-1‰; a3-1.5‰; a4-2‰); (b) absorbance data of four samples before and after centrifugation at 4000 r/min for 30 min (b1- Pure base oil and base oil containing; b2-Pristine graphite; b3- FGNPs; b4-MFGNPs and corresponding photographs of these samples providing their optical appearance, namely color and brightness)

Fig. 6 Bench test result of base oil and oil with MFGNPs:

The influence of MFGNPs lubricant additive on the oil consumption and vehicle exhaust was tested by the car bench experiment. During the car operation, the engine burns too much oil and creates much exhaust. Besides, the reciprocating movement renders the piston seal much relax. MFGNPs lubricant additive can not only make the piston and gear run more smoothly, but also repair the scars on the surface of piston, resulting in the engine piston more compact. After the addition of MFGNPs lubricant additive, the fuel burns much more sufficiently in the combustion chamber, and produce less carbon monoxide/oxynitride (Fig. 6). As shown in Fig. 7, fuel consumption increase with the increase of output power, but after the addition of MFGNPs lubricant additive the fuel consumption is lower than just added basic lubricant under the same output power. The results demonstrate that the MFGNPs lubricant additive effectively improves the engine piston seal, causing that the engine could provide more power.

Fig. 7 Fuel consumption data of base oil and oil with MFGNPs in bench test

4 Conclusions

In summary, FGNPs are easily prepared by magnetic grinding. The method is simple to operate, high in performance, suitable for commercial production, and cost-effective. Friction, collision, and especially shear- type forces formed by rotating stainless steel needles efficiently smash and exfoliate pristine graphite. The silanized FGNPs (MFGNPs) could disperse well in base oil and effectively reduce the friction coefficient and wear-scar of friction pairs when used as a lubricant. Without tedious steps or complex and dangerous chemical reagents, the self-made device can provide 50 g (or more) of FGNPs every 4 h. Therefore, this eco- friendly method provides a very promising approach to realize large-scale production of FGNPs.

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(Edited by YANG Hua)

Foundation item: Project(ZR2011BL005) supported by the Natural Science Foundation of Shandong Province, China

Received date: 2015-10-28; Accepted date: 2016-02-01

Corresponding author: ZHAO Zeng-dian, Professor; Tel/Fax: +86-533-2781987; E-mail: zhaozengdian@sdut.edu.cn