Molecular Origin of Superlubricity between Graphene and a Highly

Subscriber access provided by Bibliothèque de l'Université Paris-Sud

Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Molecular Origin of Superlubricity between Graphene and Highly Hydrophobic Surface in Water Jinjin Li, Wei Cao, Jianfeng Li, Ming Ma, and Jianbin Luo J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00952 • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 18, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Letters

Molecular Origin of Superlubricity between Graphene and Highly Hydrophobic Surface in Water

Jinjin Li#*, Wei Cao#, Jianfeng Li, Ming Ma*, and Jianbin Luo State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, China

Corresponding authors： *To whom all correspondence should be addressed. Jinjin Li E-mail: [email protected] Ming Ma E-mail: [email protected] #J.J.

L. and W. C. contributed equally.

The authors declare no competing financial interests.

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Graphene is efficient to provide ultralow friction after the formation of incommensurate interface, but is limited to dry contact conditions and specific lattice structures. In this letter, a new strategy was proposed to achieve the superlubricity of graphene through the creation of a sliding interface between graphene and highly hydrophobic surface of a self-assembled fluoroalkyl monolayers (SAFMs) in water. The superlow friction coefficient of µ = 0.0003 was obtained, demonstrating the extremely low shear stress between graphene and hydrophobic SAFMs in water. Molecular dynamics (MD) simulation shows that a nanometer-thick water layer is intercalated between graphene and hydrophobic SAFMs, and the weak interactions between water molecules and graphene provide a small energy barrier for water molecules sliding on graphene, which contributes to superlubricity. This finding reveals how to form a superlubricity interface by water intercalation, which has implications for minimizing the friction of layered materials and hydrophobic surfaces in water.

Table of Contents Graphic

2


Page 2 of 26

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Main text Friction and wear are ubiquitous in mechanical sliding systems, where the two compressed surfaces slide against each other at both the nano- and macroscales.1-2 In the past decade, many attempts have been made to reduce friction and wear for saving energy, including the formation of solid coatings or liquid films on the friction surfaces. The diamond-like carbon (DLC) coatings and layered materials, such as molybdenum disulfide (MoS2), graphene, and graphite,3-8 were widely studied recently for solid lubrication owing to their weak interlayer interactions. The friction would disappear or reduce to near-zero under some specific conditions, which is recognized as solid superlubricity.9-10 For instance, the superlubricity has been

observed

at

homogeneous

interfaces,3

MoS2

graphite/graphene

interfaces,11

graphite/boron nitride heterojunctions,12 and gold particle/graphite interfaces.13 Besides the solid superlubricity, the treated friction surfaces with aqueous solution can also minimize friction either by the formation of boundary molecular layers or fluid lubricating films. The hydrated salt ions, charged polymer, and biological macromolecules, have all been reported to achieve superlow friction coefficients of less than 0.001 when they are bound to mica and slide in water.14-17 Moreover, the surfactant micelles and polymer brushes bound to silica (SiO2) can also cause a significant reduction in friction when the sliding occurs in aqueous environments.18-22

No matter which method is used, the key factor for the efficient friction reduction is the formation of a sliding interface with an extremely low shear stress in the contact zone. For 3



example, the incommensurate lattice contact formed between the 2D layered materials,23-24 coulomb repulsion formed at the contact between DLC coatings,25 hydration layer formed between the mica surfaces,26 and self-assembled monolayer formed between the SiO2 surfaces,27 can all make the shear stress of the original sliding interfaces reduce 1 – 4 orders of magnitude, and thereafter lead to the achievement of superlubricity. Thus, how to build a sliding interface with the extremely low shear stress is critical for the design of the superlow friction systems. Although the layered materials are very efficient to provide the extremely low friction by the formation of the incommensurate contact, the friction would be greatly increased when the lattice contact is in the commensurate state or sliding against common non-layered materials,11, 23, 28-29 which limits its application in lubrication. In this letter, we created a sliding interface between graphene and hydrophobic surface in water to overcome these limitations, which magically exhibited the extremely low shear stress according to the observed superlow friction coefficient (µ = 0.0003). The friction behavior and mechanism dominating the extremely low shear stress were then studied by both the atomic force microscopy (AFM) and MD simulation.

The SiO2 probe was first fabricated by gluing a SiO2 microsphere (R = 11.5 µm) to the end of a rectangular tipless AFM cantilever (Figure 1a), and then the highly hydrophobic surface of the probe was formed by immersing it in the mixture of 1H,1H,2H,2Hperfluorodecyltriethoxysilane (FDTS) and absolute ethanol for self-assembly (Figure 1b and c). After the adsorption for 2 hours, the probe was rinsed by ethanol, and then X-ray photoelectron spectroscopy (XPS) was used to detect the chemical component on the probe 4


Page 4 of 26

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


surface. A clear peak of 688.8 eV was observed in Figure 1d, consistent with the binding energy of F 1s in the C–F bond. Moreover, the comparison of the peak areas of F 1s and Si 2p (Figure S1) gave the F/Si ratio of 17.5, which is in a good agreement with the molecular stoichiometric ratio of FDTS. It confirms that the FDTS molecules are successfully self-assembled on the probe (forming a monolayer), yielding a SAFMs probe. The topography and contact angle of the self-assemble molecular layer was measured on a flat SiO2 substrate, as shown in Figure 1f. A uniform monolayer was adsorbed on the SiO2 with the surface roughness of 0.3 nm and water contact angle of 109°, which indicates that the original SiO2 surface becomes highly hydrophobic after the self-assemble. The multiple graphene layers were mechanically exfoliated from the highly ordered pyrolytic graphite (HOPG) to provide an atomically smooth, graphene surface before the measurements. AFM was performed in the contact model to measure the normal and frictional forces between graphene surface and SAFMs probe when they were immersed in water (Figure 1e).

5



Figure 1 Preparation and characterization of the SAFMs probe. (a) SEM image of the fabricated SiO2 probe. (b) Molecular structure of FDTS. (c) Diagram of the self-assembled FDTS monolayer formed on the SiO2 probe, yielding a SAFMs probe. (d) XPS spectrum of F 1s on the surface of SAFMs probe (white circle in (a)). (e) Diagram of the SAFMs probe sliding against the graphene surface in water for AFM measurement. (f) AFM topography of the SAFMs on the SiO2 surface in tapping model, giving the roughness of the monolayer of 0.3 nm. Inset shows the water contact angle of the SAFMs.

When the SAFMs probe was approaching the graphene surface, a long-range repulsive force, which exhibited the exponentially increase with the separation reduction, was observed when the separation was less than 90 nm. This observed repulsion agreed well with the feature of the typical double electrical layer force, and was reproducible in all contact points. Moreover, the 6


Page 6 of 26

Page 7 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lack of a jump into contact suggests that the van der Waals (vdW) force was screened, and therefore, the exponentially decaying repulsive force was analyzed solely according to the double electrical layer force. Because the graphene surface is uncharged and the probe surface is hydrophobic after the formation of SAFMs, it is different from the two similarly charged surfaces producing the double electrical layer force.30 The only explanation for the repulsion is the presence of charge on the SAFMs probe during compression, which was in line with the zeta potential measurements on the hydrophobic solids (negatively charged).31-32 Although the origin of the charge was not clear, but could be attributed to the adsorption of hydrated ions on the SAFMs to attain a zeta potential. Here, we tried to fit the normal force profile with the Poisson−Boltzmann (PB) equation (under boundary conditions of constant charge),30, 33 and the parameters gave the surface charge density of σ = -e/(15.8 nm2) on the SAFMs and Debye screening length of κ-1 = 42 nm, corresponding to a 1:1 electrolyte concentration of 5×10-5 M (the presence of ions with the extremely low concentration in water may arise from the dissolved ambient CO2).

Figure 2 Normal force (Fn)/R measured in water as a function of separation when the SAFMs 7



probe approached the graphene surface and then retraced. The approach and retrace speed of the probe was 200 nm/s. Dotted line is the fitting line of the normal force curve with PB equation, giving the surface charge density of σ = -e/(15.8 nm2) on the SAFMs and Debye screening length of κ-1 = 42 nm. Inset shows the enlarged force profile within the separation range of 0 – 15 nm. The SAFMs probe was observed to push away from the graphene surface when Fn/R reduced to 0.47 mN/m.

When the SAFMs probe was detached from the graphene surface, there was not any adhesive force observed between graphene and SAFMs. Instead, the probe was pushed away from the graphene surface when the normal force reduced to 5.4 nN (Fn/R = 0.47 mN/m), with the separation suddenly jumping to 8.4 nm. After that, the normal force profile was almost the same as that when the probe was approaching the graphene surface, which is indicative of the same surface charge density and Debye length as approaching. This result implies that the SAFMs probe and graphene surface was able to separate under a very low pressure in the presence of the double electrical repulsive force surrounding the contact zone, which gives an inference that there should be water molecules intercalated between graphene and SAFMs probe to eliminate the adhesion between them. Even if the SAFMs probe approached and retraced for many times, the force profiles were always unchanged (the same as in Figure 2), confirming that the SAFMs attached onto the probe was very stable during the multiple measurements.

8


Page 8 of 26

Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The frictional force between graphene and SAFMs probe was measured as the probe slid against the graphene surface in water under a constant normal load. Figure 3a shows the variation of frictional forces as functions of normal loads measured in the loading process. The frictional forces increased slightly with increasing loads and presented approximately a linear relationship within the load range of 0 – 250 nN. The offset frictional force at zero load was close to zero, which indicates that the adhesion between graphene and SAFMs probe was very small (consistent with the force profile in Figure 2). Through the linear fitting to these frictional data, the friction coefficient, equal to the slope of fitting line, was got as μ = 0.0003 ± 0.0001, which enters the superlubricity regime.10 The maximal contact pressure for this superlubricity was approximately 14.5 MPa when the applied load reached 250 nN, estimated by P = 4Fn/πD2, where D is the diameter of contact zone (D ≈ 147 nm according to the Hertz contact mechanics). The dependence of friction properties on the sliding speed was analyzed through the comparison of frictional behaviors under different sliding velocities, as shown in Figure 3b. The frictional force had a slight increase with the load increasing under all velocities, and maintained extremely low values within the whole load range of 0 – 250 nN. The friction coefficients of them fluctuated in a small range of 0.0003 – 0.0004, which indicates that the sliding velocity has little influence on the friction coefficient. However, there was a clear increase of frictional force with the increase of velocity when the normal loads were fixed to constant. Figure 3b inset shows the detailed variation of frictional force as the sliding velocity varied over a wide range of 500 nm/s – 50 µm/s when the normal load was set to 99 nN. The

9



frictional force exhibited an evident logarithmic correlation with the sliding velocity over the whole region, which confirms that the friction can be influenced by the sliding velocity.

Figure 3 Friction properties between graphene and SAFMs probe in water. (a) Frictional force measured when the SAFMs probe slid against the graphene surface in water as a function of load under a constant velocity of 4 µm/s. The slope of the linear fitting line to the data gives the friction coefficient of µ = 0.0003 ± 0.0001. Inset shows the diagram of SAFMs probe sliding against the graphene surface in water during the friction measurement. (b) Frictional force measured when the SAFMs probe slid against the graphene surface in water as a function of load under three sliding velocities (4.0, 7.8, and 15.6 µm/s). The friction coefficients of them fluctuated in a small range of 0.0003 - 0.0004. Inset shows relationship between the sliding velocity and frictional force under the constant applied load of Fn = 99 nN. All the error bars are standard deviations of frictional forces extracted from 6 independent friction loops at the 10


Page 10 of 26

Page 11 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


same conditions.

Usually, the superlubricity of graphite or graphene is easily achieved as they sliding against layered materials or specific metal crystals by the formation of the incommensurate lattice contact.4, 11, 13, 23 However, the extremely low friction when the graphene (or graphite) slides against the common materials, such as polymer, plastic, and silicon, was difficult to be achieved because there are not uniform lattices on their surfaces to form the incommensurate contact. In the present work, the extremely low friction (μ = 0.0003) was achieved as the hydrophobic SAFMs slide against the graphene surface in water. It indicates that there produced a sliding interface with extremely low shear stress between graphene and SAFMs probe in the presence of water. To reveal the origin of the extremely low shear stress as the SAFMs probe slides against the graphene in water, MD simulations were performed, as depicted in Figure 4a. SAFMs covered on the SiO2 probe were built by attaching FDTS molecules to the SiO2 surface randomly with the chains perpendicular to the surface. The graphene surface was composed by four graphene sheets, where the lowest sheet was rigid. The system was then packed with water molecules between graphene and SAFMs. Thereafter, the structure and force field of SAFMs were verified by analyzing the surface roughness and water contact angle. Based on the simulation of a water droplet on the SAFMs, a contact angle of ~106° was obtained, which is comparable to that measured in experiments (Figure 1f inset). After relaxing the surface in vacuum, the root mean square (RMS) roughness of the SAFMs adsorbed on SiO2 was calculated to be 0.24 nm, which is also in agreement with the value measured by AFM. 11



The load capacity of water between graphene and SAFMs was explored at various separations (Figure 4b). The initial structure contains the SAFMs and graphene surface in a large water reservoir under the atmospheric pressure of 1 atm. After equilibrium at room temperature, the oscillation of the repulsive force was observed as a function of separation after the separation was decreased to d < 2 nm. Because of the hydrophobic properties of SAFMs, the repulsive force mainly comes from the packing effect of water molecules on the atomically smooth graphene surface,34 ensuring the water molecules being still confined between graphene and SAMFs under a normal compression. The maximal load capacity of water is found to be 28 MPa at the separation of 1.1 nm, for a double-layered water structure trapped in the contact zone (Figure S2). It is slightly higher than the pressure (~5 MPa) reported for water between the hydrophilic mica and hydrophobic fluoropolymer film with a contact angle of ~122°.35 The load capacity decreased sharply as the probe further approached the surface and finally the water molecules were squeezed out. The attractive force appeared when there were no layered water structures confined in the space (the separation was less than 0.8 nm). This simulation indicates that there exists a trapped water layer between graphene and SAFMs under a pressure of less than 28 MPa. Thus, when the SAFMs probe slides against graphene surface in water under our experimental pressures (< 14.5 MPa), there exists a nanometer-thick water layer intercalated between graphene and SAFMs (cannot be squeezed out), which may play the dominate role on the achievement of extremely low friction.

12


Page 12 of 26

Page 13 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4

MD simulation of the SAFMs probe sliding against graphene surface. (a) Atomistic

model in the friction simulations. From top to down are amorphous SiO2, FDTS molecules, water molecules, and four graphene layers. (b) Normal load and pressure (dividing the normal force by contact area) between graphene and SAFMs as a function of separation in water. There was an attractive force between graphene and SAFMs at the separation of less than 0.8 nm (red arrow). (c) Relationship between the shear stress under the normal loads in (b) and separation, showing the independence of shear stress on separation and load. The sliding velocity was set to 10 m/s. (d) Structure factor of the confined water molecules close to graphene surface for different number of water layers. (e) Relationship between shear stress and sliding velocity, including the experimental (1 – 50 µm/s) and simulation (104 – 106 µm/s) velocities. The experimental data were fitted by the logarithmic equation of   a  bln(vs ) , where a =3.1 kPa , and b = 7.9 kPa·s/m.

The detail of the sliding process was revealed by pulling the SAFMs probe along the 13



graphene surface under constant normal pressures and corresponding water contents confined between graphene and SAFMs. It was observed that the water molecules intercalated between graphene and SAFMs were dragged by the SAFMs probe (Movie S1), which indicates that the shear plane appeared in the water/graphene interface. Thus, it can be inferred that the shear stress of the water/graphene interface is lower than that of SAFMs/water interface and water layer under this simulation condition. This result was verified by comparing the shear stress of the three shear planes, i.e., τ = 0.2 MPa for water/graphene interface, τ = 8.1 MPa for SAFMs/water interface, and τ = 4.9 MPa for water layer (τ = ηv/D, where D is the separation when the double-layered water structure are confined, η is the water viscosity at 300 K). The extremely low shear stress of the water/graphene interface mainly arises from the very weak vdW interaction between graphene and water molecules, leading to the water molecules sliding on the graphene surface easily with very small energy barrier. While the much larger shear stress between water and SAFMs mainly comes from the roughness of SAFMs surface (not atomically smooth), which would produce walls to trap water molecules in the sliding direction.

The shear stress was found to hardly depend on the separation and normal pressure when the water was highly confined, i.e. D < 2 nm (Figure 4c). The average friction stress calculated from simulations was 0.21 ± 0.05 MPa for different separations as the moving velocity of SAFMs probe was set to 10 m/s. This value is comparable to those reported for water intercalated between two graphene nanosheets (~ 0.1 MPa when the shear velocity is 10 m/s).36 Figure 4d gives the structure factor of water, showing that the hexagonal close-packed water structures near the graphene surface had no correlation with the number of layers, and could 14


Page 14 of 26

Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sustain until the layered wierater structures disappeared in the contact zone, which may be the reason why the shear stress is independent on the separation and pressure. Moreover, under the normal pressures studied in the simulations, the close-packed water molecules, also called ice,37 were not observed due to the small confined pressures. Thus, the friction stress would not change much with the variation of normal loads, which is in accordance with the experimental results (Figure S3).

The shear velocity dependence on friction was performed for a range of 0.01~1 m/s (Figure 4e), which were several orders larger than those in AFM measurements, because the frictional force under small velocity is difficult to obtain due to the small signal-to-noise ratio.38 According to the relationship between the frictional force and velocity in Figure 3b inset, the shear stress in the range of 1 – 50 µm/s can be obtained as a function of velocity (Figure S4), which shows a logarithmic correlation with the sliding velocity. The shear stress in large velocity (0.01~1 m/s) can be extrapolated by this logarithmic relationship, which is just in the range of the simulated shear stresses (Figure 4e). Thus, it is evident that the experimental results are in good agreement with the simulations, confirming that the shear plane appears in the water/graphene interface, which results in an extremely low frictional energy dissipation. In this case, the frictional process involves the thermal actuation process as the water molecules sliding on the graphene,38-39 which is the reason why the shear stress depends logarithmically on the sliding velocity.

From the above analyses, the key factor for the achievement of extremely low friction is the 15



existence of a nanometer-thick water layer intercalated between graphene and SAFMs, which can provide the extremely low shear stress as the water molecules slide on the graphene surface with very small energy barrier. Therefore, the preconditions to achieve the extremely low friction is that the water molecules cannot be squeezed out under the applied pressures. From Figure 4b, the maximal bearing pressure to trap water molecule in the contact zone between graphene and SAFMs is approximately 28 MPa, which is higher than the experimental values (< 14.5 MPa), and thus ensure the robustness of extremely low friction state during sliding. The mechanism for the water molecules being intercalated between graphene and SAFMs mainly originates from the vdW forces which involve the water-water and water-graphene interactions.40 Although there is a nanometer-thick water layer intercalated between graphene and SAFMs, the MD simulations show that the shear plane appears in the water/graphene interface rather than among water molecules, which is different from the typical hydration lubrication where the shear plane is the hydration layer.26 It denotes that the shear stress between graphene and water molecules may be lower than that among water molecules. This finding demonstrates that the water molecules can be successfully intercalated between graphene and hydrophobic surface, and meanwhile are very efficient to lubricate the graphene and hydrophobic surface, which provide a new approach to create the superlubricity interface of them in water.

In summary, the present study reported that the extremely low friction coefficient of µ = 0.0003 could be achieved as the hydrophobic SAFMs probe slides against the graphene surface in water at the contact pressures of less than 14.5 MPa. There is a nanometer-thick water layer 16


Page 16 of 26

Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


intercalated between graphene and SAFMs, which is in the liquid phase and robust enough to ensure the water molecules not being squeezed out under the applied pressures. The weak interaction between the water molecules and graphene surface dominates the appearance of extremely low friction, because it provides a small energy barrier for water molecules sliding on the graphene surface. Our finding reveals that the superlubricity can be achieved between graphene and hydrophobic surface by water intercalation, which has important implications for the design of efficient water lubrication with layered materials and hydrophobic surface.

EXPERIMENTAL METHODS Materials. Monodispersed SiO2 microspheres (R = 11.5 µm) were provided by Nano-Micro, Co., Ltd. HOPG substrate with a ZYA quality (mosaic spread = 0.4°) was supplied by Shanghai NTI, Co., Ltd. Water was taken from the nano water purification system with the resistivity of 18.2

MΩ·cm-1

for

the

experiments.

Absolute

ethanol

and

1H,1H,2H,2H-

perfluorodecyltriethoxysilane were purchased from Aladdin , Co., Ltd. The tipless cantilever (TL-CONT) with the rectangular shape and spring constant of 0.02 – 0.77 N/m was used, and one SiO2 microsphere was glued to the end of the cantilever by the epoxy glue (forming a SiO2 probe). The multiple graphene layers (> 5 layers) were mechanically exfoliated from the HOPG substrate, and then attached on a SiO2 substrate to provide an atomically smooth, graphene surface.

SAFMs probe preparation and characterization. 1H,1H,2H,2H-perfluorodecyltriethoxysilane was first mixed with absolute ethanol with a 17



volume ratio of 5:1. Thereafter, the SiO2 probe was immersed in the mixture for 2 hours, and then it was picked out and rinsed by ethanol. The chemical composition of the self-assemble molecular layer on the probe was detected using XPS (PHI Quantera II). The topography of SAFMs on the SiO2 surface was captured in water on the AFM (Icon) in peak force mode by using a sharp nitride lever tip. The contact angle of the SAFMs was measured using the video based optical angle measuring system OCA25 after injecting a water droplet (1 µL) on the SAFMs.

AFM force measurement. Before the force measurement, the spring constant of the SAFMs probe was calibrated by the frequency method, and the lateral detector sensitivity was calibrated by the improved wedge calibration method.41-42 The forces, including normal and frictional forces, were measured on AFM (MFP-3D) in the contact mode. Water (about 200 µL) was dripped on the freshly cleaved graphene surface, and then the probe stated to approach the graphene until the probe was fully immersed. The normal forces between graphene and probe were measured in water when the probe approached graphene surface with a moving speed of 200 nm/s and then retraced with the same speed. The frictional forces were measured when the probe slid against the graphene surface in water at the constant applied load. The relationship between frictional force and normal load was obtained by increasing the normal load from 0 to 250 nN with an interval of 10 nN (loading process). The scanning area was set in the range of 0.6 × 0.6 μm2 – 5 × 5 μm2 (it was divided into 25 parts equally when the frictional forces were measured in the loading process), and the scanning speed was varied between 0.5 and 50 μm/s. According to the lateral voltage difference in the friction loops, the frictional 18


Page 18 of 26

Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


force could be obtained by multiplying the lateral detector sensitivity (6 friction loops in the same load were averaged). All the force measurements were performed at the room temperature of 26 ℃.

MD simulation. We first built a model containing a self-assembled FDTS monolayers covered on SiO2 probe.43 An amorphous SiO2 slab was created from annealing,44 with the face size of A = 4×4 nm2. FDTS molecules were inserted onto the SiO2 surface randomly with the chain perpendicular to the surface. The silanol groups on FDTS were attached to the Si atoms on the SiO2 surface, similar to those in experiments (Figure 1c). The interactions between these Si and O atoms on FDTS were strong enough to adsorb the FDTS molecules onto the surface. The coverage was calculated to be 3.8 molecules/nm2, which is consistent to a defect-free, closely packed SAFMs. The graphene model was consist of four graphene sheets with the side length of 4 nm, where the lowest sheet was rigid. The system was then packed with water molecules in the middle of the graphene and SAFMs. The number of water molecules was determined by the load capacity calculation. The force field for FDTS was AMBER force field,45 and the force fields for water, graphene, and SiO2, and the interactions between them, including vdW and Columbic interactions were the same as that shown in Ref. 46. Periodic boundary conditions in all directions were implemented. In the simulations, the upper part of the SiO2 was set as a rigid group. The normal and lateral forces were added to the upper rigid part (~1 nm) of SiO2. The temperature was stabilized at the room temperature by thermostating the lower part of SiO2 and the middle graphene sheet. In the load capacity calculations, we immersed the system in a water reservoir, and then set 19



the pressure to 1 atm by two rigid pistons. The SAFM was then pressed by a spring, leaving the water being squeezed out. After that, the structure was extracted at various distances to calculate the normal force on SAFM discretely. The water content was recorded for the later frictional simulations at different separations. In the friction simulations, the rigid SiO2 probe was pulled along the graphene surface under constant normal pressures by the spring (spring constant = 10 N/m) within the velocity range of 0.01 - 10 m/s. Given that the velocity was several orders larger than that in experiments, the stick-slip dynamics in MD simulations with a high speed could approach the fast equilibration of the system. The frictional forces were then calculated by averaging the lateral forces with Fs  kP(vt  x) , where v is the sliding velocity,

kP is the lateral spring constant, and x is the center-of-mass displacement of the

SiO2 probe. At least two steady state stick-slip periods were simulated for the small velocity studied in this work. The standard error was analyzed by the running average of frictional force. To ensure the consistency between MD simulation and experiment, the normal pressure and shear stress (τ = Fs/A) were mainly calculated in the MD simulations.

ACKNOWLEDGEMENT The work is financially supported by NSFC of China (51775295 and 51527901), and Foundation from State Key Laboratory of Tribology (SKLT2019C01).

20


Page 20 of 26

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ASSOCIATED CONTENT Supporting information. XPS result of the silica probe after the self-assemble of FDTS molecules, density and structure factor of the double-layered water structure confined between SAFMs and graphene, shear stress as a function of normal load, and shear stress as a function of sliding velocity. Movie of the SAFMs probe sliding along the graphene surface under constant normal pressures and corresponding water contents confined between graphene and SAFMs.

REFERENCE (1) Holmberg, K.; Erdemir, A. Influence of Tribology on Global Energy Consumption, Costs and Emissions. Friction 2017, 5, 263-284. (2) Williams, J. A. Friction and Wear of Rotating Pivots in Mems and Other Small Scale Devices. Wear 2001, 251, 965-972. 21



(3) Li, H.; Wang, J.; Gao, S.; Chen, Q.; Peng, L.; Liu, K.; Wei, X. Superlubricity between MoS2 Monolayers. Adv. Mater. 2017, 29, 1701474. (4) Kawai, S.; Benassi, A.; Gnecco, E.; Soede, H.; Pawlak, R.; Feng, X.; Muellen, K.; Passerone, D.; Pignedoli, C. A.; Ruffieux, P.; Fasel, R.; Meyer, E. Superlubricity of Graphene Nanoribbons on Gold Surfaces. Science 2016, 351, 957-961. (5) Berman, D.; Deshmukh, S. A.; Sankaranarayanan, S. K. R. S.; Erdemir, A.; Sumant, A. V. Macroscale Superlubricity Enabled by Graphene Nanoscroll Formation. Science 2015, 348, 1118-1122. (6) Feng, X.; Kwon, S.; Park, J. Y.; Salmeron, M. Superlubric Sliding of Graphene Nanoflakes on Graphene. ACS Nano 2013, 7, 1718-1724. (7) Erdemir, A.; Eryilmaz, O. L.; Fenske, G. Synthesis of Diamondlike Carbon Films with Superlow Friction and Wear Properties. J. Vac. Sci. Technol. A 2000, 18, 1987-1992. (8) Martin, J. M.; Donnet, C.; Lemogne, T.; Epicier, T. Superlubricity of MolybdenumDisulfide. Phys. Rev. B 1993, 48, 10583-10586. (9) Li, J.; Luo, J. Advancements in Superlubricity. Sci. China-Technol. Sci. 2013, 56, 28772887. (10)Erdemir, A.; Martin, J.-M., Superlubricity. Elsevier Academic Press, New York, 2007. (11)Liu, Z.; Yang, J.; Grey, F.; Liu, J. Z.; Liu, Y.; Wang, Y.; Yang, Y.; Cheng, Y.; Zheng, Q. Observation of Microscale Superlubricity in Graphite. Phys. Rev. Lett. 2012, 108, 205503. (12)Song, Y.; Mandelli, D.; Hod, O.; Urbakh, M.; Ma, M.; Zheng, Q. Microscale Superlubricity in Graphite/Hexagonal Boron Nitride Layered Heterojunctions. Nat. Mater. 2018, 17, 894-899. 22


Page 22 of 26

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(13)Cihan, E.; Ipek, S.; Durgun, E.; Baykara, M. Z. Structural Lubricity under Ambient Conditions. Nat. Commun. 2016, 7, 12055. (14)Raviv, U.; Klein, J. Fluidity of Bound Hydration Layers. Science 2002, 297, 1540-1543. (15)Raviv, U.; Giasson, S.; Kampf, N.; Gohy, J. F.; Jerome, R.; Klein, J. Lubrication by Charged Polymers. Nature 2003, 425, 163-165. (16)Chen, M.; Briscoe, W. H.; Armes, S. P.; Klein, J. Lubrication at Physiological Pressures by Polyzwitterionic Brushes. Science 2009, 323, 1698-1701. (17)Goldberg, R.; Schroeder, A.; Silbert, G.; Turjeman, K.; Barenholz, Y.; Klein, J. Boundary Lubricants with Exceptionally Low Friction Coefficients Based on 2D Close-Packed Phosphatidylcholine Liposomes. Adv. Mater. 2011, 23, 3517-3521. (18)Drobek, T.; Spencer, N. D. Nanotribology of Surface-Grafted PEG Layers in an Aqueous Environment. Langmuir 2008, 24, 1484-1488. (19)Kampf, N.; Wu, C.; Wang, Y.; Klein, J. A Trimeric Surfactant: Surface Micelles, Hydration-Lubrication, and Formation of a Stable, Charged Hydrophobic Monolayer. Langmuir 2016, 32, 11754-11762. (20)Li, J.; Zhang, C.; Cheng, P.; Chen, X.; Wang, W.; Luo, J. AFM Studies on Liquid Superlubricity between Silica Surfaces Achieved with Surfactant Micelles. Langmuir 2016, 32, 5593-5599. (21)Li, J.; Luo, J. Nonlinear Frictional Energy Dissipation between Silica-Adsorbed Surfactant Micelles. J. Phys. Chem. Lett. 2017, 8, 2258-2262. (22)Li, J.; Luo, J. Normal and Frictional Force Hysteresis between Self‐Assembled Fluorosurfactant Micelle Arrays at the Nanoscale. Adv. Mater. Interf. 2018, 5, 1700802. 23



(23)Dienwiebel, M.; Verhoeven, G. S.; Pradeep, N.; Frenken, J. W. M.; Heimberg, J. A.; Zandbergen, H. W. Superlubricity of Graphite. Phys. Rev. Lett. 2004, 92, 126101. (24)Dienwiebel, M.; Pradeep, N.; Verhoeven, G. S.; Zandbergen, H. W.; Frenken, J. W. M. Model Experiments of Superlubricity of Graphite. Surf. Sci. 2005, 576, 197-211. (25)Erdemir, A. The Role of Hydrogen in Tribological Properties of Diamond-Like Carbon Films. Surf. Coat. Technol. 2001, 146, 292-297. (26)Klein, J. Hydration Lubrication. Friction 2013, 1, 1-23. (27)Ren, S. L.; Yang, S. R.; Zhao, Y. P. Micro- and Macro-Tribological Study on a SelfAssembled Dual-Layer Film. Langmuir 2003, 19, 2763-2767. (28)Li, J.; Ge, X.; Luo, J. Random Occurrence of Macroscale Superlubricity of Graphite Enabled by Tribo-Transfer of Multilayer Graphene Nanoflakes. Carbon 2018, 138, 154-160. (29)Li, J.; Gao, T.; Luo, J. Superlubricity of Graphite Induced by Multiple Transferred Graphene Nanoflakes. Adv. Sci. 2018, 5, 1700616. (30)Israelachvili, J. N., Intermolecular and Surface Forces. Academic Press, New York, 1991. (31)Donaldson, S. H., Jr.; Das, S.; Gebbie, M. A.; Rapp, M.; Jones, L. C.; Roiter, Y.; Koenig, P. H.; Gizaw, Y.; Israelachvili, J. N. Asymmetric Electrostatic and Hydrophobic-Hydrophilic Interaction Forces between Mica Surfaces and Silicone Polymer Thin Films. ACS Nano 2013, 7, 10094-10104. (32)Drechsler, A.; Petong, N.; Zhang, J. F.; Kwok, D. Y.; Grundke, K. Force Measurements between Teflon Af and Colloidal Silica Particles in Electrolyte Solutions. Colloids Surf., A 2004, 250, 357-366. (33)Ben-Yaakov, D.; Andelman, D. Revisiting the Poisson-Boltzmann Theory: Charge 24


Page 24 of 26

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Surfaces, Multivalent Ions and Inter-Plate Forces. Phys. A (Amsterdam, Neth.) 2010, 389, 2956-2961. (34)Giovambattista, N.; Rossky, P. J.; Debenedetti, P. G., Computational Studies of Pressure, Temperature, and Surface Effects on the Structure and Thermodynamics of Confined Water. Annu. Rev. Phys. Chem. 2012, 63, 179-200. (35)Rosenhek-Goldian, I.; Kampf, N.; Klein, J. Trapped Aqueous Films Lubricate Highly Hydrophobic Surfaces. ACS Nano 2018, 12, 10075-10083. (36)Falk, K.; Sedlmeier, F.; Joly, L.; Netz, R. R.; Bocquet, L. Molecular Origin of Fast Water Transport in Carbon Nanotube Membranes: Superlubricity Versus Curvature Dependent Friction. Nano Lett. 2010, 10, 4067-4073. (37)Zhu, Y.; Wang, F.; Bai, J.; Zeng, X. C.; Wu, H. Compression Limit of Two-Dimensional Water Constrained in Graphene Nanocapillaries. ACS Nano 2015, 9, 12197-12204. (38)Ma, M. D.; Shen, L.; Sheridan, J.; Liu, J. Z.; Chen, C.; Zheng, Q. Friction of Water Slipping in Carbon Nanotubes. Phys. Rev. E 2011, 83, 036316. (39)Mueser, M. H. Velocity Dependence of Kinetic Friction in the Prandtl-Tomlinson Model. Phys. Rev. B 2011, 84, 125419. (40)Pertsin, A.; Grunze, M. Water-Graphite Interaction and Behavior of Water near the Graphite Surface. J. Chem. Phys. B 2004, 108, 1357-1364. (41)Varenberg, M.; Etsion, I.; Halperin, G. An Improved Wedge Calibration Method for Lateral Force in Atomic Force Microscopy. Rev. Sci. Instrum. 2003, 74, 3362-3367. (42)Green, C. P.; Lioe, H.; Cleveland, J. P.; Proksch, R.; Mulvaney, P.; Sader, J. E. Normal and Torsional Spring Constants of Atomic Force Microscope Cantilevers. Rev. Sci. Instrum. 25



2004, 75, 1988-1996. (43)Roscioni, O. M.; Muccioli, L.; Mityashin, A.; Cornil, J.; Zannoni, C. Structural Characterization of Alkylsilane and Fluoroalkylsilane Self-Assembled Monolayers on SiO2 by Molecular Dynamics Simulations. J. Phys. Chem. C 2016, 120, 14652-14662. (44)Valle, R. G. D.; Andersen, H. C. Molecular Dynamics Simulation of Silica Liquid and Glass. J. Chem. Phys. 1992, 97, 2682-2689. (45)Cornell, W. D.; Cieplak, P.; Bayly, C. I.; Gould, I. R.; Merz, K. M.; Ferguson, D. M.; Spellmeyer, D. C.; Fox, T.; Caldwell, J. W.; Kollman, P. A. A Second Generation Force Field for the Simulation of Proteins, Nucleic Acids, and Organic Molecules. J. Am. Chem. Soc. 1995, 117, 5179-5197. (46)Li, J.; Cao, W.; Wang, Z.; Ma, M.; Luo, J. Origin of Hydration Lubrication of Zwitterions on Graphene. Nanoscale 2018, 10, 16887-16894.

26


Page 26 of 26

Molecular Origin of Superlubricity between Graphene and a Highly

Recommend Documents