Self-Assembled Graphene Film as Low Friction Solid Lubricant in

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Self-Assembled Graphene Film as Low Friction Solid Lubricant in Macroscale Contact Pu Wu,†,‡ Xinming Li,△,‡ Chenhui Zhang,*,† Xinchun Chen,† Shuyuan Lin,§,∥ Hongyan Sun,⊥ Cheng-Te Lin,⊥ Hongwei Zhu,*,§,∥ and Jianbin Luo*,† †

State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, P. R. China Department of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong SAR, China § State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P. R. China ∥ Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, P. R. China ⊥ Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, P. R. China △

S Supporting Information *

ABSTRACT: Promoted by the demand for solid lubricants, graphene has been proved to be a promising material for potential applications in reducing friction and wear. Here, a novel lubricating system where graphene sliding against graphene is developed to achieve low friction in macroscale contact. And the large area graphene film used here were prepared by a unique self-assembly technique based on Marangoni effect. Low friction coefficient of about 0.05 is obtained, and it is demonstrated that the film thickness, applied normal load and annealing process all have important influences on the tribological properties of graphene. The expedient fabrication procedure of large-area graphene film with excellent transferability and high-performance frictionreducing behaviors of the developed lubricating system both have a promising perspective in engineering applications. KEYWORDS: self-assembled graphene film, Marangoni effect, friction, wear, solid lubricant

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INTRODUCTION

literatures have been dedicated to the tribological behavior of graphene in both atomic-scale and macroscale. Studies on nanotribology have already proved graphene’s capability in reducing friction and wear at the atomic-scale and the so-called “superlubricity” is achieved as a result of incommensurate contact.14−21 On the other hand, studies at the macroscale have more practical importance because they provide meaningful solutions to industries where friction and wear give rise to large amounts of energy dissipation and material losses.22 Berman et al. have obtained superlubricity in macroscale when graphene prepared by chemical exfoliation of highly oriented pyrolytic graphite is used in combination with nanodiamond particles

Graphene, a two-dimensional material discovered in 2004, has been extensively investigated in the recent decade and demonstrates great potential for a wide range of applications in optoelectronic devices,1−3 sensors,4 energy devices,5,6 and lubricants.7 Especially, graphene is qualified to be an effective solid lubricant owing to its exceptional properties, including notable mechanical strength, ultrathin film thickness, atomically smooth surface, and high chemical stability.8−12 Easy shear capability owing to its atomically smooth surface and lamellar structure could effectively reduce friction,7 and the ultrathin feature makes graphene a potential lubricating candidate for microcosmic application, such as microelectromechanical systems.13 Meanwhile, graphene is chemically inert and impervious to water and gases, thus being able to protect sliding surfaces from corrosion and oxidation.12 So far, plenty of © 2017 American Chemical Society

Received: March 31, 2017 Accepted: May 29, 2017 Published: May 29, 2017 21554

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effect. The tribological properties of the graphene film transferred to tribopair in macroscale contact are investigated. Besides, the influences of the film thickness, applied normal load and annealing process on the tribological properties of graphene film are also studied. In a word, the expedient fabrication procedure of large-area graphene film with excellent transferability and the high-performance friction-reducing behaviors of the developed lubricating system both have a promising perspective in engineering applications.

and diamondlike carbon.23 To meet the practical demand, it is extremely necessary to develop unique preparation methods for large-area, high-quality, easily transferred and low-cost graphene, as well as to build new lubricating systems with low friction and wear. In general, different methods are commonly used to prepare large-area graphene film for tribological studies in macroscale contact, including chemical vapor deposition (CVD) and solution-based process. As for the as-grown graphene film on Ni foil prepared by CVD, it could achieve low friction coefficient of about 0.03, however the friction coefficient substantially increased to 0.12 while the graphene film was transferred from Ni foil to SiO2 substrate.24 Recently, it was found that the friction coefficient of as-grown graphene film on Ni foil decreased from 0.59 to 0.11 with the test environment changed from dry nitrogen to 45% relative humidity air.25 Besides, the CVD grown graphene film transferred to stainless steel substrates exhibited great wear resistance in a hydrogen atmosphere with a friction coefficient of about 0.22.26 Even though the CVD grown graphene film showed impressively low friction coefficient and antiwear property, the complicated process when transferring it from the substrate to other material surfaces, the high cost, and the intricate preparation process pose an obstacle to the practical applications. For solution-processed graphene prepared by chemical exfoliation, it could also serve as an effective lubricant by dropping graphene-containing ethanol solution on the sliding surfaces, and the lowest friction coefficient was about 0.15 in both air and dry nitrogen atmospheres.27,28 Although the solutionprocessed graphene has low cost and simple preparation process, the graphene in the ethanol solution could not achieve complete coverage of the sliding surfaces. For the graphene film prepared by a self-assembly approach via covalent interactions using 3-aminopropyltriethoxysilane (APTES) as a chemical linker on silicon substrates, the obtained friction coefficient ranged from 0.13 to 0.52.29−32 Despite the tightly bonding between the graphene film and the APTES molecular layer, the complex process and relative high friction retard its applications as being a solid lubricant at the macroscale. In addition, electrodynamic spraying process (ESP) and electrophoretic deposition (EPD) are also used for preparing large-area graphene film. The ESP processed based graphene film showed a friction coefficient of about 0.11,33 while the EPD graphene film exhibited low friction coefficient of about 0.05 owing to its low surface roughness (0.59 nm).34 Even though the graphene film prepared by EPD showed excellent friction reducing the ability, the devices needed for the preparation process were sophisticated and the substrate must be electro-conductive, which may place restrictions on its practical use. The challenge for these large-area graphene films is to balance the tribological performance and the complexity in the preparation process. Besides, the majority of current research focused on the lubricating system where bare ball sliding against graphene film in macroscale contact, mainly owing to the difficulty in preparation of large-area graphene film on the ball surface. At present, it needs to devote more efforts to develop effective processing techniques for engineering large-area graphene films in low-friction lubricating systems. Here, a novel lubricating system where graphene sliding against graphene is developed to achieve low friction, which is first studied in macroscale contact so far. And large-area graphene film with superb transferability was prepared by a unique and simple self-assembly technique based on Marangoni

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EXPERIMENTAL PROCEDURE

Synthesis of Electrochemical Exfoliated Graphene Flakes and Self-Assembled Graphene Film (SGF). Graphene sheets used in this study were prepared by an efficient electrochemical exfoliation method and the raw material was natural flake graphite. Before the electrochemical reaction, the holder filled with flake graphite was placed in an aqueous electrolyte containing H2SO4 and KOH (pH 12). Then, 1 A current was applied to the system to start the fast intercalation/exfoliation reaction. After several hours, the color of the electrolyte would turn into black because of the production of fewlayer graphene. The graphene samples were collected by filtration and washed with deionized water. The graphene sheets were redispersed in ethanol for film self-assembly. The graphene ethanol solution was injected on the surface of the deionized water, and the graphene flakes would bind with each other because of the Marangoni effect and π−π interaction, thus forming large-area SGF on the surface of the deionized water with high structural uniformity and superb transferability. The details of the preparation process of SFG could be found in our previous work.35,36 Tribological Tests Procedure. The tribopair used in the experiments were 4 mm-diameter silicon nitride ball (Beijing Sinoma Synthetic Crystals Co. Ltd.) with the mean roughness Rq = 20 nm and a silicon wafer (Kaihua Sinovel Silicon Material Co. Ltd.) with the mean roughness Rq = 2 nm measured by the 3D profilometer. After self-assembly processes, the large-area SGF formed on the deionized water surface was transferred to the surface of the Si3N4 ball and Si wafer. The details of the transfer process of SFG could be found in our previous work.35 The morphology and structural properties of SGF were studied by means of an optical microscope, atomic force microscope (AFM), scanning electron microscopy (SEM) and transmission electron microscope (TEM). Meanwhile, the chemical composition analysis of SGF was evaluated by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Tribological tests were performed on a reciprocating type of homemade tribometer under the ball-on-plate contact configuration. The details of this tribometer could be found in our previous work.37 Figure S1 shows the schematic diagram of this tribometer. The normal load was applied by a precision displacement stage (PI, M-414.3VG) and measured by a precision force sensor. The friction force sensor used in this tribometer was based on the double leaf cantilever design. For all the measurements in this work, a cantilever with a lateral spring constant of k = 1000 N/m was used. And the deformation of this cantilever was measured by a dual-frequency laser interferometer system (Agilent, 5517C, 10705A) with the nonlinearity error less than 4.2 nm. Besides, a rectangular bender piezo actuator (PI, PL140) with high precision was used to provide reciprocating movement. During the friction tests, the applied normal load was set to be 5 to 300 mN, resulting in the Hertzian maximum contact pressure of 0.14 to 0.56 GPa. The sliding length was 2 mm and the sliding speed was 0.4 mm/s. All measurements were carried out under ambient conditions. After friction tests, the analysis of wear scars and tracks was conducted by SEM, 3D profilometer and Raman spectroscopy.

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RESULTS AND DISCUSSION Characterization of SGF. In this work, the features of graphene film should be characteristic of large-area, simple preparation process and superb transferability. Here, the

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ACS Applied Materials & Interfaces graphene film which is self-assembled at the liquid/air interface based on Marangoni effect is applied to tribological study (Figure 1a). Particularly, SGF can be easily transferred to Si3N4

conducted under the same experimental conditions, and the plotted friction coefficient values are the average of all the results obtained for each case. As can be seen in Figure 2a, interestingly, the friction coefficient was effectively reduced in the case where SGF slid against SGF. After a quite short running-in period, the friction coefficient decreased to about 0.09, which was 6 times lower than that of bare tribocounterfaces and 2.5 times lower than that of bare Si3N4 ball sliding against Si wafer with transferred SGF. While there was no SGF on the tribopair, the friction coefficient reached a value of about 0.56 for the rest of the test after the first running-in period, mainly because of the poor lubricating properties of Si3N4 and Si, as well as the harsh lubricating conditions in solid dry contact. When SGF was only transferred to the Si wafer, as can be seen from the friction coefficient curve, a minor runningin behavior was also observed in the beginning of the friction tests and the sliding system exhibited improved tribological properties with a reduced friction coefficient of about 0.24, which was one time lower than that of the bare tribocounterfaces. The friction force decreased markedly while SGF was transferred to the Si wafer, it is speculated that the friction reduction with one-side SGF covering originated from the low resistance to shear between the lamellae of SGF and the subsequent transfer of exfoliated lamellae of SGF from Si surface to the counterpart surface of the Si3N4 ball. These results proved that the lubricating system where SGF sliding against SGF had excellent capability of reducing friction. Besides friction reduction, the SGF also provide excellent wear-resistance for the sliding surfaces. Figure 2c−2k presents the SEM images, 3D surface morphologies, and cross-sectional profiles of the wear tracks produced on the Si wafers for the three lubricating systems. It could be seen from Figure 2f and 2i that the wear track on the bare Si wafer was up to 55 nm in depth. In comparison, it was found that the wear tracks of the Si substrates were much shallower in the cases where SGF was transferred to only the Si wafer (Figure 2g) or to the both sides of the tribopairs (Figure 2h). Comparing the depth of wear tracks (Figures 2j and 2k) with the thickness of SGF, that is 3− 5 nm measured by an ellipsometer, it could be reasonably concluded that the SGF still remained intact there and was not worn out, indicating the remarkable antiwear properties of SGF. Additionally, the widths of the wear tracks were about 20.3 and 21.7 μm (Figures 2d and e), respectively, when SGF were transferred to the Si wafer or to the both sides of the tribopairs, whereas the width of the wear track was 25.4 μm (Figure 2c) in the case where no SGF existed on the tribopairs. Film Thickness Dependence of Friction Coefficient. To further determine the influence of film thickness on friction coefficient, tribological measurements in macroscale contact were conducted by the same reciprocating-type tribometer under 5 mN load for 1200s. To obtain SGF with different thickness on the surfaces of tribopair, the times of SGF transfer to tribopair were changed from 1 to 6. For instance, when the transfer times was 2, it meant that the substrate was dried off by an infrared lamp after the first time of transfer, and then another transfer process was performed on the same specimen. The corresponding thickness of SGF with transfer times from 1 to 6 was presented in Figure S3, which was measured by an ellipsometer. Figure 3a shows the thickness dependence of friction coefficient. In the lubricating system where SGF sliding against SGF, it was observed that the friction coefficient gradually decreased with increasing transfer times and was roughly equal to each other, while the transfer times exceed 4.

Figure 1. (a) Self-assembly process of graphene film at the liquid/air interface. (b) Optical image of the Si3N4 ball and Si substrate with transferred SGF. (c) SEM image and (d) AFM image of SGF on Si substrate. (e) TEM image and (f) AFM image of graphene flakes.

balls and Si wafers with good uniformity and fairly good adhesion (Figure 1b). After transferred to Si wafer and Si3N4 ball, several measurements were conducted to analyze the structure of SGF. It could be concluded that the SGF had a large dimension with graphene flakes well-distributed on the substrate (Figures 1c and S2a). Atomic force microscopy (AFM) topography image (Figures 1d and S2b) indicated that graphene flakes were linked and stacked with each other during the self-assembled process on the surface of the deionized water on account of π−π interactions. It was also found that there are some small areas of the Si substrate not covered with SGF after the transfer process (Figure 1d). Figure 1e shows the TEM image of graphene flakes, which clearly demonstrated the layered structure. The thickness of graphene flakes was ∼2 nm, representing 2−3 layers of graphene (Figure 1f). Moreover, the film thickness of SGF was about 3−5 nm, which was confirmed by an ellipsometer (Sentech Instruments GmbH, SE850DUV). Tribological Properties of SGF. To study the tribological properties of the self-assembled graphene film in macroscale contact, friction tests including three tribopairs were conducted at 5 mN normal load in ambient environment by a homemade reciprocating-type tribometer (Figure S1): (1) bare Si3N4 ball against bare Si wafer, (2) bare Si3N4 ball against SGF transferred to Si wafer, and (3) SGF transferred to Si3N4 ball against SGF transferred to Si wafer. Figures 2a and 2b show the variations of friction coefficients for all the three cases. At least five measurements were 21556

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Figure 2. (a) Friction coefficient curves, (b) average friction coefficients, and (c−e) SEM images, (f−h) 3D surface morphologies, and (i−k) crosssectional profiles of wear tracks on Si substrates for Si3N4 against Si, Si3N4 against graphene−Si and graphene−Si3N4 against graphene−Si, respectively.

Figure 3. (a) Friction coefficient as a function of graphene film thickness for Si3N4 against graphene−Si and graphene−Si3N4 against graphene−Si. Schematics of (b) surface coverage rate and (c) “puckering effect” for different film thickness.

The saturation value of the friction coefficient was about 0.07, which was 22% lower than that with one-time transferred layer. As for the lubricating system, where bare Si3N4 sliding against Si wafer with transferred SGF, it shows the same trend with the case where SGF was transferred to the both sides of the tribopair. The lowest friction coefficient was about 0.14 when the transfer time exceeded four, whereas the highest friction

coefficient was about 0.24 when there was only one layer of transferred SGF on the Si wafer. To understand the mechanism of this phenomenon, we investigated the surface coverage rate of SGF on Si wafer by means of contact angle meter. It was found that the contact angle of water and SGF increased from 69.2° to 80.0° with the transfer times from 1 to 4 and kept stable at about 80.4° and 80.3° when the transfer times were 5 and 6, respectively, which 21557

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Figure 4. (a) HRTEM image of the annealed SGF on Si wafer (transfer times = 4). (b) Friction coefficient as a function of load for Si3N4 against graphene−Si (unannealed), graphene−Si3N4 against graphene−Si (unannealed), and graphene−Si3N4 against graphene−Si (annealed). Schematic of friction process under different loads for (c) graphene−Si3N4 against graphene−Si and (d) Si3N4 against graphene−Si.

amorphous areas existed within the SGF. In addition, the dspacing values of the graphene layers varied between 0.36 and 0.38 nm. Figure 4b shows the values of friction coefficient changed with a variable normal load in two different lubricating systems. In the case of the unannealed SGF tribopair, it could be observed that the friction coefficient was the lowest (∼0.07) under lower applied normal load (5 and 10 mN). With normal load increasing from 10 to 15 mN, the friction coefficient increased from ∼0.07 to ∼0.09. Further increase of load to 20 and 25 mN resulted in the friction coefficient significantly increasing to about 0.13. It was presumed that the out-plane deformation of SGF and the real contact area between the shearing surfaces both increased with the increase of normal load, resulting in the increase of friction coefficient. On the other hand, at the higher normal load, the SGF transferred to tribopair might have been worn out partly and lost its capacity of fully covering the sliding surfaces, thus leading to a relatively high friction coefficient (Figure 4c). While the SGF was annealed, the trend of friction coefficient with respect to the normal load was the same as unannealed SGF. However, the lowest friction coefficient decreased from ∼0.07 to ∼0.05. In the meantime, as for the lubricating system where bare Si3N4 ball sliding against unannealed SGF transferred to Si wafer, interestingly, the friction coefficient was about 0.14−0.15 when the normal load was in the range of 5−300 mN. It was believed that graphene could transferf from plate to the ball under both low and high normal load, thus resulting in the stable friction coefficient (Figure 4d). To better understand the origin of these differences, the wear tracks were further analyzed by SEM, 3D surface profilometer and Raman spectroscopy. Figures 5a and 5b present the SEM image and corresponding surface profile of the wear track produced on Si wafers in the lubricating system, where bare Si3N4 ball sliding against SGF transferred to Si wafer under 300 mN load. It can be clearly seen that most areas of the wear track

could demonstrate that the surface coverage rate of SGF increased with the increase of transfer times and reached a stable state when the transfer times exceeded 4 (Figure S4). We believe that the increased surface coverage rate of SGF was an important reason for the decrease of friction coefficient with the increase of film thickness (Figure 3b). Besides, the same trend found in graphene lubricants also has been reported in several previous studies in the nanoscale,38−40 which attributed this phenomenon to the “puckering effect”. It was believed that the puckered region would form at the front edge of ball-film contact area and additional work was needed to move the puckered SGF region forward, thus increasing the friction coefficient (Figure 3c). Whereas, the impact of “puckering effect” might be diminished owing to the large contact area and imperfect morphological and crystal structure of SGF in the macroscale. Load Dependence of Friction Coefficient and Wear. In addition to the thickness dependence of friction coefficient, the influence of variation of the applied normal load on the tribological property was evaluated by the same tribometer. To improve the tribological performance of the lubricating system, where SGF sliding against SGF in macroscale contact, the SGFs on Si3N4 ball and Si wafer were annealed to exhaust the oxygen inherent in SGF transferred to the tribopair in consideration of enhancing the interaction between the SGF and the substrate and reducing the surface roughness. The SGFs on Si3N4 ball and Si wafer were annealed for 3 h at 500 °C in a tube furnace under the mixed gas of H2 (400 sccm) and Ar (800 sccm). The cross-sectional TEM-specimen of the annealed SGF on Si was prepared by the FIB in situ lift-out technique. Metallic layers of Cr and Pt were sequentially deposited on the SGF surface before the FIB process to protect the graphene film. Figure 4a shows the high-resolution TEM (HRTEM) image of the SGF on Si wafer, which revealed that the film thickness was ∼18 nm (transfer times = 4). It could be seen that graphene layers stacked with each other in the same direction and some 21558

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Figure 5. (a, e, i) SEM images and (b, f, j) cross-sectional profiles of wear tracks on Si substrates, (d, h, l) SEM images of contact areas on Si3N4 ball for Si3N4 against graphene−Si, graphene−Si3N4 against graphene−Si (unannealed), and graphene−Si3N4 against graphene−Si (annealed). (c, g, k) Raman spectra of corresponding areas on Si substrate and Si3N4 ball.

on the Si wafer were badly worn out with wear depth up to 80− 185 nm (Figure 5b). And the SGF also had been transferred to the surface of the Si3N4 ball (Figure 5d). Furthermore, in order to investigate the sliding-induced disordering and bonding state of C-network, Raman spectroscopy was employed to examine the SGF after the friction tests.41,42 Figure 5c shows the Raman spectra of the wear track on Si wafer and transferred film on the Si3N4 ball. It can be observed there exist a D peak at 1350 cm−1, a G peak at 1580 cm−1, a weak 2D peak at 2700 cm−1, and a D + D′ peak at 3080 cm−1 in the spectrum, which was similar to that observed in the initial SGF (line 1). The sliding process resulted in an increase of D peak intensity (line 2, ID/IG = 1.41 compared to ID/IG = 1.12 of the initial SGF), suggesting that the SGF became defected and disordered under high normal load. And in the area where the substrate was badly worn out, the Raman spectra signal was quite weak and the D peak intensity also increased (line 3), indicating there was still some SFG left in the wear track. Despite the high load of about 300

mN, the friction coefficient was about 0.15, only about onefourth that of bare Si3N4 ball sliding against bare Si wafer, demonstrating that the SGF could still play an important role in improving the lubricating performance under high normal load. And the Raman spectra of SGF (line 4) which was transferred to the surface of the Si3N4 ball during the friction test was similar to that of SGF in the wear track on Si wafer. Figures 5e and 5f show the SEM image and the corresponding surface profile of the wear track on Si wafer under 20 mN load in the lubricating system where unannealed SGF sliding against unannealed SGF in macroscale contact. It could be seen that the maximum depth of wear track was up to 25 nm, thus it was reasonable to conclude that the SGF on the Si wafer were worn out partly when compared with the original film thickness of SGF (∼16 nm measured by ellipsometer). And the friction coefficient increased to a higher value of ∼0.13 compared with that of ∼0.07 (5 mN normal load). Besides, it could be seen that the SGF on the Si3N4 ball was also partly 21559

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Figure 6. (a) Schematic of contact areas on the Si3N4 ball and Si wafer, (b) Cross-sectional TEM image of the tribo-film on the Si3N4 ball, HRTEM images of areas (c) near Si3N4, (d) in the middle of the tribo-film, and (e) near Cr. (f) HRTEM image of the tribo-film on Si wafer for graphene− Si3N4 against graphene−Si (annealed).

Si3N4 ball during the sliding process. HRTEM images (Figures 6c−e) present the typical structures of the areas near the Si3N4 ball/graphene film interface, in the middle of the graphene film, and near the top surface of the graphene film. It could be seen that these areas consist of both layers of graphene stacked with each other and amorphous matrix of carbon. Raman spectroscopy was used to examine the disorder of C-network by considering the appearances of defect-activated D and split-G peaks. The occurrence of a D peak of the initial SGF and an increase of the D peak intensity after the sliding process (Figure 5k) suggested that the electrochemical exfoliated process would lead to growth defects of graphene flakes, and the slidinginduced defects were also introduced during the friction tests. Besides, Figure 6e shows that there are also graphene stacks which had lost the parallel structure when compared to the original annealed SGF (Figure 4a), and some graphene films had been highly curved while still maintaining a layer-like structure. Similar phenomenons have been observed during the friction process when graphene acted as solid lubricant.23,43 In addition, the d-spacing values of the graphene layers was 0.38 nm. Figure 6f presents the HRTEM image of the tribo-film in the contact area on Si wafer. It could be found that the film thickness was about 8.5 nm, and there also exist a composite structure with a layered structure mixed with amorphous carbon. The d-spacing value of the graphene-like layers was once again about 0.38 nm. It is important to note here that the d-spacing values of the graphene nanolayers detected in the transfer layers on both Si3N4 ball and Si wafer surfaces were larger than that of pristine graphite with d(002) = 0.33 nm. The increase in the interlayer spacing may be attributed to the introduction of oxygen-containing functional groups during the electrochemical exfoliated process, and it may also be inferred that the adsorption of H and OH in the graphene layers during the friction test would impose repulsive interactions, which also resulted in the increase in the lattice spacing.25 The graphene film on both Si3N4 ball and Si wafer was believed to play a decisive role in reducing friction and wear in the lubricating system of graphene sliding against graphene. In the previously reported studies, Kim et al. studied the tribological properties of graphene oxide nanosheet (GONS)

worn out due to the high normal load (Figure 5h). The Raman spectra of the wear areas on Si wafer and Si3N4 ball are presented in Figure 5g. Line 2 shows the Raman spectrum of the shallower position in the wear track. The D peak intensity increased obviously (ID/IG = 1.37 compared to ID/IG = 1.09 of initial SGF) during the sliding tests, suggesting that the SGF became defective and disordered after a mechanical rubbing action. Line 4 represents the Raman spectrum of SGF in the contact area on the Si3N4 ball, which shows the same trend like line 2. Moreover, it could be found weak Raman spectra signal (lines 3 and 5) in the positions where Si wafer and Si3N4 ball were badly worn out, indicating there was still some SFG left in the wear track. Figures 5i and 5j show the SEM image and the corresponding surface profile of the wear track on Si wafer under 20 mN load in the lubricating system where self-mated annealed SGF sliding in macroscale contact. The SGF on the Si wafer was not worn out when comparing the maximum depth of the wear track (∼13 nm) with the original film thickness (∼18 nm measured by ellipsometer). And it can be seen from Figure 5l that the SGF on the Si3N4 surface was also not worn out. Owing to the full coverage by SGF on the tribopair, the friction coefficient was as low as ∼0.09, which was 31% lower than that of unannealed SGF under the same normal load. The Raman spectra of the wear track on the Si wafer and Si3N4 ball are presented in Figure 5k. It could be found that the SGF in the contact area also became defective and disordered during the sliding process (lines 2 and 3). The improved tribological behavior of the annealed SGF resulted from the lower content of defects and oxygen functionalities (Figure S5a and S5b), the smoother surface (Figure S6), and the stronger interaction between the SGF and the substrate. The SGF in the contact area was investigated by TEM after the friction tests. The cross-sectional TEM samples were prepared by the FIB in situ lift-out technique from the worn surfaces of Si wafer (Figure 5i) and Si3N4 ball (Figure 5l). Figure 6b shows the low magnification cross-sectional TEM image of the tribo-film on the Si3N4 ball (Figure 5l), which revealed that the film thickness was 400 ± 40 nm. It could be concluded that SGF has been transferred from the plate to the 21560

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coating fabricated by ESP method, and it is believed that the low-friction behavior of the GONS coatings was attributed to the GONS transferred to the counter surface which remained stable throughout the sliding process.33 Bhowmick et al. found that graphene transfer layers formed on the Ti−6Al−4 V counterface during the sliding process in humid air led to lower friction coefficient than that in N2.24 In our study, it is found that both the contact areas on the Si3N4 ball and Si wafer were fully covered by the tribo-film after friction test in the lubricating system of self-mated annealed graphene, and thus shear occurred between the tribo-film throughout the sliding process, resulting in low friction coefficient and antiwear properties. The lowest friction coefficient obtained in this work was ∼0.05 in the lubricating system of self-mated annealed SGF when the transferred time was 4 and the applied normal load was below 15 mN. Figure S7 and Table S1 present the friction coefficients reported in the literatures so far when graphene films except for as-grown graphene served as a solid lubricant without using any other lubricants in macroscale contact. It could be seen that 0.05 was one of the lowest value. Besides, the wear of the substrate was nearly negligible in this lubricating system when SGF was transferred for only 1 time under 5 mN load. The remarkable friction-reducing and antiwear performance could be ascribed to the contribution of low shear resistance between the neighboring lamellae of SGF, and importantly, the full coverage of tribopair by SGF. As shown in the Figure 4c, both the tribopair surfaces were fully covered by SGF during the sliding process under low normal load, thus leading to low friction coefficient. And the SGF would be worn out while normal load increased, thus resulting in higher friction coefficient. As for the lubricating system where bare Si3N4 ball sliding against SGF, the friction coefficient was higher than that of SGF sliding against SGF under low normal load, mainly because the tribopair surfaces could not be fully covered by SGF. However, it is found that the SGF could play an efficient role as long as it existed in the contact area, therefore the friction coefficient could remain stable even under high normal load because of the formation of transferred graphene film on Si3N4 surfaces (Figure 4d). Besides, the experiments conducted in this work are all under ambient and here it is necessary to note that the environment conditions such as atmosphere species,25,26 vacuum,44 and temperature45 would have a significant effect on the tribological properties of graphene.

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04599. Schematics of tribometer and sliding system, SEM image and AFM image of SGF, Raman spectra and XPS spectra characterization, 3D surface morphologies characterization, and comparison of the friction coefficient of graphene film as solid lubricants in macroscale contact (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xinming Li: 0000-0002-7844-8417 Chenhui Zhang: 0000-0002-9514-0027 Cheng-Te Lin: 0000-0002-7090-9610 Author Contributions ‡

P.W. and X.L. contributed equally to this work. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is financially supported by National Natural Science Foundation of China (51335005, 51527901, 51321092, 51402060, 51672150) and National Key Basic Research Program of China (2013CB934200).

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REFERENCES

(1) Li, X.; Tao, L.; Chen, Z.; Fang, H.; Li, X.; Wang, X.; Xu, J.; Zhu, H. Graphene and Related Two-Dimensional Materials: StructureProperty Relationships for Electronics and Optoelectronics. Appl. Phys. Rev. 2017, 4, 021306. (2) Li, X.; Zhu, M.; Du, M.; Lv, Z.; Zhang, L.; Li, Y.; Yang, Y.; Yang, T.; Li, X.; Wang, K.; et al. High Detectivity Graphene-Silicon Heterojunction Photodetector. Small 2016, 12, 595−601. (3) Chen, Z.; Li, X.; Wang, J.; Tao, L.; Long, M.; Liang, S.; ANG, L. K.; Shu, C. C. T.; Tsang, H. K.; Xu, J. Synergistic Effects of Plasmonics and Electrons Trapping in Graphene Short-Wave Infrared Photodetectors with Ultrahigh Responsivity. ACS Nano 2017, 11, 430−437. (4) Shi, J.; Li, X.; Cheng, H.; Liu, Z.; Zhao, L.; Yang, T.; Dai, Z.; Cheng, Z.; Shi, E.; Yang, L.; et al. Graphene Reinforced Carbon Nanotube Networks for Wearable Strain Sensors. Adv. Funct. Mater. 2016, 26, 2078−2084. (5) Li, X.; Lv, Z.; Zhu, H. Carbon/silicon Heterojunction Solar Cells: State of the Art and Prospects. Adv. Mater. 2015, 27, 6549−6574. (6) Li, X.; Zhu, H.; Wang, K.; Cao, A.; Wei, J.; Li, C.; Jia, Y.; Li, Z.; Li, X.; Wu, D. Graphene-on-Silicon Schottky Junction Solar Cells. Adv. Mater. 2010, 22, 2743−2748. (7) Berman, D.; Erdemir, A.; Sumant, A. V. Graphene: A New Emerging Lubricant. Mater. Today 2014, 17, 31−42. (8) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene. Science 2008, 321, 385−388. (9) Stankovich, S.; Dikin, D. A.; Dommett, G. H.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-Based Composite Materials. Nature 2006, 442, 282−286. (10) Li, X.; Xie, D.; Park, H.; Zeng, T. H.; Wang, K.; Wei, J.; Zhong, M.; Wu, D.; Kong, J.; Zhu, H. Anomalous Behaviors of Graphene

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CONCLUSIONS In summary, we have developed a novel lubricating system where SGF sliding against SGF in macroscale contact to achieve low friction. In particular, the lubricating system exhibited remarkably friction-reducing and antiwear properties with the friction coefficient lowered to about 0.05 because of the low resistance to shear between the neighboring lamellae of SGF and the full coverage of tribopair by SGF. Besides, in this lubricating system, the friction coefficient decreased with the increase of film thickness because of the weakening of “puckering effect”. It is found that the friction coefficient would increase under high normal load due to the breakage of SGF, and the annealing process would greatly enhance the tribological performance of SGF. In a word, the large-area selfassembled graphene film based on Marangoni effect sliding in macroscale contact shows dramatically improved lubricating properties and has a promising potential for real application. 21561

DOI: 10.1021/acsami.7b04599 ACS Appl. Mater. Interfaces 2017, 9, 21554−21562

Research Article

ACS Applied Materials & Interfaces Transparent Conductors in Graphene-Silicon Heterojunction Solar Cells. Adv. Energy Mater. 2013, 3, 1029−1034. (11) Gu, W.; Zhang, W.; Li, X.; Zhu, H.; Wei, J.; Li, Z.; Shu, Q.; Wang, C.; Wang, K.; Shen, W.; et al. Graphene Sheets From WormLike Exfoliated Graphite. J. Mater. Chem. 2009, 19, 3367−3369. (12) Bunch, J. S.; Verbridge, S. S.; Alden, J. S.; Van Der Zande, A. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Impermeable Atomic Membranes From Graphene Sheets. Nano Lett. 2008, 8, 2458−2462. (13) Pu, J.; Mo, Y.; Wan, S.; Wang, L. Fabrication of Novel Graphene-Fullerene Hybrid Lubricating Films Based on Self-assembly for MEMS Applications. Chem. Commun. 2014, 50, 469−471. (14) Zeng, X.; Peng, Y.; Lang, H.; Liu, L. Controllable Nanotribological Properties of Graphene Nanosheets. Sci. Rep. 2017, 7, 41891. (15) Li, S.; Li, Q.; Carpick, R. W.; Gumbsch, P.; Liu, X. Z.; Ding, X.; Sun, J.; Li, J. The Evolving Quality of Frictional Contact with Graphene. Nature 2016, 539, 541−545. (16) Filleter, T.; McChesney, J. L.; Bostwick, A.; Rotenberg, E.; Emtsev, K. V.; Seyller, T.; Horn, K.; Bennewitz, R. Friction and Dissipation in Epitaxial Graphene Films. Phys. Rev. Lett. 2009, 102, 86102. (17) Kawai, S.; Benassi, A.; Gnecco, E.; Söde, H.; Pawlak, R.; Feng, X.; Müllen, K.; Passerone, D.; Pignedoli, C. A.; Ruffieux, P.; et al. Superlubricity of Graphene Nanoribbons on Gold Surfaces. Science 2016, 351, 957−961. (18) Kim, D.; Park, S.; Hong, S. W.; Jeong, M. Y.; Kim, K. H. The Periodicity in Interfacial Friction of Graphene. Carbon 2015, 85, 328− 334. (19) Li, P. F.; Zhou, H.; Cheng, X. Investigation of a Hydrothermal Reduced Graphene Oxide Nano Coating on Ti Substrate and its Nano-Tribological Behavior. Surf. Coat. Technol. 2014, 254, 298−304. (20) Berman, D.; Erdemir, A.; Zinovev, A. V.; Sumant, A. V. Nanoscale Friction Properties of Graphene and Graphene Oxide. Diamond Relat. Mater. 2015, 54, 91−96. (21) Deng, Z.; Klimov, N. N.; Solares, S. D.; Li, T.; Xu, H.; Cannara, R. J. Nanoscale Interfacial Friction and Adhesion on Supported Versus Suspended Monolayer and Multilayer Graphene. Langmuir 2013, 29, 235−243. (22) Perry, S. S.; Tysoe, W. T. Frontiers of Fundamental Tribological Research. Tribol. Lett. 2005, 19, 151−161. (23) Berman, D.; Deshmukh, S. A.; Sankaranarayanan, S. K.; Erdemir, A.; Sumant, A. V. Macroscale Superlubricity Enabled By Graphene Nanoscroll Formation. Science 2015, 348, 1118−1122. (24) Kim, K.; Lee, H.; Lee, C.; Lee, S.; Jang, H.; Ahn, J.; Kim, J.; Lee, H. Chemical Vapor Deposition-Grown Graphene: the Thinnest Solid Lubricant. ACS Nano 2011, 5, 5107−5114. (25) Bhowmick, S.; Banerji, A.; Alpas, A. T. Role of Humidity in Reducing Sliding Friction of Multilayered Graphene. Carbon 2015, 87, 374−384. (26) Berman, D.; Deshmukh, S. A.; Sankaranarayanan, S. K.; Erdemir, A.; Sumant, A. V. Extraordinary Macroscale Wear Resistance of One Atom Thick Graphene Layer. Adv. Funct. Mater. 2014, 24, 6640−6646. (27) Berman, D.; Erdemir, A.; Sumant, A. V. Few Layer Graphene to Reduce Wear and Friction on Sliding Steel Surfaces. Carbon 2013, 54, 454−459. (28) Berman, D.; Erdemir, A.; Sumant, A. V. Reduced Wear and Friction Enabled by Graphene Layers on Sliding Steel Surfaces in Dry Nitrogen. Carbon 2013, 59, 167−175. (29) Li, P. F.; Zhou, H.; Cheng, X. Nano/Micro Tribological Behaviors of a Self-Assembled Graphene Oxide Nanolayer on Ti/ Titanium Alloy Substrates. Appl. Surf. Sci. 2013, 285, 937−944. (30) Ou, J.; Wang, J.; Liu, S.; Mu, B.; Ren, J.; Wang, H.; Yang, S. Tribology Study of Reduced Graphene Oxide Sheets on Silicon Substrate Synthesized via Covalent Assembly. Langmuir 2010, 26, 15830−15836. (31) Mungse, H. P.; Tu, Y.; Ichii, T.; Utsunomiya, T.; Sugimura, H.; Khatri, O. P. Self-Assembly of Graphene Oxide on Silicon Substrate via

Covalent Interaction: Low Friction and Remarkable Wear-Resistivity. Adv. Mater. Interfaces 2016, 3, 1500410. (32) Ou, J.; Wang, Y.; Wang, J.; Liu, S.; Li, Z.; Yang, S. Self-Assembly of Octadecyltrichlorosilane on Graphene Oxide and the Tribological Performances of the Resultant Film. J. Phys. Chem. C 2011, 115, 10080−10086. (33) Kim, H.; Penkov, O. V.; Kim, D. Tribological Properties of Graphene Oxide Nanosheet Coating Fabricated by Using Electrodynamic Spraying Process. Tribol. Lett. 2015, 57, 27. (34) Liang, H.; Bu, Y.; Zhang, J.; Cao, Z.; Liang, A. Graphene Oxide Film as Solid Lubricant. ACS Appl. Mater. Interfaces 2013, 5, 6369− 6375. (35) Li, X.; Yang, T.; Yang, Y.; Zhu, J.; Li, L.; Alam, F. E.; Li, X.; Wang, K.; Cheng, H.; Lin, C. T.; et al. Large-Area Ultrathin Graphene Films by Single-Step Marangoni Self-Assembly for Highly Sensitive Strain Sensing Application. Adv. Funct. Mater. 2016, 26, 1322−1329. (36) Li, L.; Li, X.; Du, M.; Guo, Y.; Li, Y.; Li, H.; Yang, Y.; Alam, F. E.; Lin, C.; Fang, Y. Solid-Phase Coalescence of Electrochemically Exfoliated Graphene Flakes into a Continuous Film on Copper. Chem. Mater. 2016, 28, 3360−3366. (37) Xu, H.; Zhang, C.; Wu, P. Design and Testing of MicroTribometer with Macro Applied Load. Sci. China: Technol. Sci. 2016, 59, 1666−1672. (38) Lee, C.; Li, Q.; Kalb, W.; Liu, X.; Berger, H.; Carpick, R. W.; Hone, J. Frictional Characteristics of Atomically Thin Sheets. Science 2010, 328, 76−80. (39) Smolyanitsky, A.; Killgore, J. P.; Tewary, V. K. Effect of Elastic Deformation on Frictional Properties of Few-Layer Graphene. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 35412. (40) Zeng, X.; Peng, Y.; Lang, H. A Novel Approach to Decrease Friction of Graphene. Carbon 2017, 118, 233−240. (41) Cançado, L. G.; Jorio, A.; Martins Ferreira, E. H.; Stavale, F.; Achete, C. A.; Capaz, R. B.; Moutinho, M.; Lombardo, A.; Kulmala, T. S.; Ferrari, A. C. Quantifying Defects in Graphene via Raman Spectroscopy at Different Excitation Energies. Nano Lett. 2011, 11, 3190−3196. (42) Luo, Z.; Yu, T.; Ni, Z.; Lim, S.; Hu, H.; Shang, J.; Liu, L.; Shen, Z.; Lin, J. Electronic Structures and Structural Evolution of Hydrogenated Graphene Probed by Raman spectroscopy. J. Phys. Chem. C 2011, 115, 1422−1427. (43) Bhowmick, S.; Banerji, A.; Alpas, A. T. Friction Reduction Mechanisms in Multilayer Graphene Sliding Against Hydrogenated Diamond-Like Carbon. Carbon 2016, 109, 795−804. (44) Marchetto, D.; Feser, T.; Dienwiebel, M. Microscale Study of Frictional Properties of Graphene in Ultra High Vacuum. Friction 2015, 3, 161−169. (45) Zhang, Y.; Dong, M.; Gueye, B.; Ni, Z.; Wang, Y.; Chen, Y. Temperature Effects on the Friction Characteristics of Graphene. Appl. Phys. Lett. 2015, 107, 011601.

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DOI: 10.1021/acsami.7b04599 ACS Appl. Mater. Interfaces 2017, 9, 21554−21562