Effect of Humidity and Water Intercalation on the Tribological Behavior

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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Effect of Humidity and Water Intercalation on the Tribological Behavior of Graphene and Graphene Oxide Taib Arif,† Guillaume Colas,† and Tobin Filleter*,† †

Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario M5S 3G8, Canada

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S Supporting Information *

ABSTRACT: In this work, the effect of humidity and water intercalation on the friction and wear behavior of few-layers of graphene and graphene oxide (GO) was studied using friction force microscopy. Thickness measurements demonstrated significant water intercalation within GO affecting its surface topography (roughness and protrusions), whereas negligible water intercalation of graphene was observed. It was found that water intercalation in GO contributed to wearing of layers at a relative humidity as low as ∼30%. The influence of surface wettability and water adsorption was also studied by comparing the sliding behavior of SiO2/GO, SiO2/Graphene, and SiO2/SiO2 interfaces. Friction for the SiO2/GO interface increased with relative humidity due to water intercalation and condensation of water. In contrast, it was observed that adsorption of water molecules lubricated the SiO2/SiO2 interface due to easy shearing of water on the hydrophobic surface, particularly once the adsorbed water layers had transitioned from “ice-like water” to “liquid-like water” structures. Lastly, an opposite friction trend was observed for the graphene/SiO2 interface with water molecules failing to lubricate the interface as compared to the dry graphene/SiO2 contact. KEYWORDS: nanotribology, graphene, graphene oxide (GO), hydration, water intercalation, friction, wettability, adhesion

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INTRODUCTION Microscale molybdenum disulfide (MoS2) and carbon-based materials (graphite, diamond-like carbon (DLC)) are well established lubricating materials with contrary frictional behavior/dependence on the presence of water molecules.1 The lubricating property of DLC and graphite are highly dependent on water molecules.2−4 Water molecules adsorb on the outer surface5 as well as intercalate within graphite, weakening the surface basal plane interaction and facilitating easier shearing of the basal planes.2 MoS2 (thick) is highly lubricious under vacuum which makes it favorable for space applications. However, MoS2 loses its lubricious properties with the introduction of water due in part to oxidation.1,6 Both graphite and MoS2 consist of layered 2D structures which upon discovery have attracted wide range of possible applications in composites,7 electronics,8 additives in liquid lubricants9 and as solid coatings to minimize friction.10−12 Graphene11,13 and 2D-MoS210,14 are generally recognized as highly effective solid lubricants and exhibit ultralow coefficients of friction.15 The interaction of graphene and 2D-MoS2 with water molecules is different than that of bulk materials (e.g., graphite) and hence can change their frictional behavior/ dependence on water.19 Tribological studies on graphene, in particular, have enhanced the understanding of some of the unique mechanisms governing friction at the atomic scale. These mechanisms include thickness effects,14 electron− phonon coupling,13 local commensurability,16 and puckering effects.17,18 To exploit these atomic-scale mechanisms further, chemically altered 2D structures have been synthesized. © XXXX American Chemical Society

Modified graphene such as graphene oxide (GO) have additional functional groups (-hydroxyl and -epoxide) on the carbon basal plane changing the interlayer interaction from Van der Waal’s to hydrogen bonds.20 These interlayer hydrogen bond networks dictate GO’s mechanical properties and can be sensitive to the environmental conditions.15,20 Water molecules from the surrounding environment can intercalate within the GO layers and have been shown to change the existing hydrogen bond network. The water molecules bond with the functional groups which ultimately alters both the interaction between GO sheets and the GO’s structure (topography).20−25 Water molecules can diffuse in and out between GO layers,22 however complete removal of water molecules from within GO is difficult even under dry conditions.26,27 Studies have shown the explicit dependence of various mechanical properties (out of plane stiffness, Young’s modulus, and storage modulus) of GO on intercalated water.20,21,23 Interestingly, mechanical properties of GO generally increase initially with increased water until an optimum water content is reached, followed by reduction of mechanical properties at higher humidity. Atomistic simulations have demonstrated that the optimal water content can vary for GO by the type and density of the functional groups.20 In the case of graphene, recent atomistic simulations have highlighted how the reactive edges of the basal plane can resist Received: March 6, 2018 Accepted: June 12, 2018 Published: June 12, 2018 A

DOI: 10.1021/acsami.8b03776 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. Normalized thickness of multilayer graphene and graphene oxide (GO) flakes as a function of humidity. The inset schematics illustrate the water intercalation mechanism within GO due to the presence of functional groups at low and high RH. Upper right inset: GO thickness measurements relative to the silicon wafer by Gaussian fitting at 80% humidity.

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water from intercalating within atomic scale graphene.19 However, there has been no experimental work reported studying water intercalation within few layers of graphene to the best of our knowledge. In addition to the water intercalation mechanism, adsorption of water molecules on the surface can also influence the tribological properties.5 Tocci et al.28 have shown, using atomistic simulations, that adsorption of water can increase the COF for graphene. Excessive adsorption, or condensation, of water molecules from the surrounding environment is also known to form a water meniscus around the asperities within mechanical contacts. Therefore, it is essential to highlight the effect of water meniscus on AFM tribology experiments which emulate single asperity sliding contacts. Water exhibits very different rheological properties within sub-nanoconfined gaps as compared to bulk.29 Experiments have shown that an increase in the confinement of water can result in a drastic increase in viscosity as compared to bulk.29−33 The slipping behavior of liquid molecules can also be affected by the surface chemistry and morphology.34,35 The presence of oxygen functional groups on graphene’s basal plane has been reported to change its wettability from hydrophobic to hydrophilic.36,37 Moreover, water molecules have also been shown to slip and move even on motionless solid surfaces.38−40 Recent studies have highlighted the attractive tribological properties of carbon-based 2D materials and some of the unique mechanisms which dictate and affect their frictional behavior. One unique mechanism associated with the carbonbased 2D materials is the intercalation of water molecules within the layers, affecting the material’s structure and properties. However, there remains a gap in understanding the influence of water intercalation mechanism on the tribological behavior. Herein, we first look into the extent of water molecule intercalation within graphene and GO by controlling the environmental humidity and measuring the thickness at steady state. Next, the correlation of the water intercalation mechanism to the tribological behavior for both the materials was investigated. Friction force microscopy (FFM) was performed to study the frictional and wearing behavior on graphene and GO while varying humidity. In addition, the study also highlights the role of surface wettability and water adsorption on friction by comparing SiO2/SiO2, GO/SiO2, and graphene/SiO2 interfaces.

RESULTS AND DISCUSSION

Water Intercalation in Graphene and GO. Figure 1 compares the change in thickness of multilayer GO and multilayer graphene as a function of relative humidity (RH). The experiment was initiated from 10%RH and increased in increments of ∼10−20% up to 80%RH. The thickness was normalized (Figure 1−yaxis) by dividing the current thickness of films at the given humidity (Hi) with the thickness at 10% humidity (H10%RH). It was observed that a change in RH from 10−80% was found to result in minimal thickness change for multilayer graphene (less than 15%). This is consistent with previous studies by Levita et al.19 in which atomistic simulation highlighted that the very reactive edges of graphene plane (particularly the zigzag edge) can adsorb fragments of water molecules (−H and −OH) preventing water from intercalating. However, for GO, it was observed that an increase in relative humidity leads to a monotonic increase in the thickness of GO to approximately double that of the original thickness. At RH10%, the GO (∼2 layers) initially had a thickness of 1.3 nm which doubled to 2.6 nm at RH80% due to the intercalation of water molecules between GO layers. This increasing trend is in general agreement with a previously reported thickness increase of ∼30%.22 The larger change in thickness reported herein is evidence of greater water intercalation partially due to the longer total hydration time of ∼14 h as compared to previous reports using shorter durations (∼1 h).22 In addition, the type and coverage of the functional groups can also affect the water intercalation in GO. Within the GO interlayers, there are regions along the basal plane which are highly oxidized or weakly/nonoxidized.22 The highly oxidized areas are hydrophilic and therefore desirable sites for water to form hydrogen bonds with GO. The GO studied herein was prepared using the Hummers method which has been shown to form a complex oxidized pattern on GO and offers easier hydration as compared to the GO prepared by the Brodies method used for the previous hydration studies.22 Once the water molecules intercalate within the GO, they contribute to hydrogen bonding which dominates the interaction between the GO layers. Initial water molecules bond to the functional groups forming interlayer and intralayer bonds as illustrated in the Figure 1 schematic. Once all the B

DOI: 10.1021/acsami.8b03776 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

point after which an increase in humidity (above RH30%) results in a change of the adsorbed structure. Further adsorption of water molecules evolves into “water-like” liquid on top of the existing “ice-like” layers of water and continues to grow with humidity.43 The adsorption and evolution of water structure on the SiO2 surface alters the sliding SiO2/SiO2 interface by changing the contact from SiO2/SiO2 (low humidity) to SiO2/“ice-like” water (20−30RH%) followed by SiO2/liquid water (high humidity). This change in the sliding interfaces reflects on the friction force with the drastic drop observed after 20−30%RH. It is also observed that adsorbed water is more lubricious as a “water-like” liquid structure than as an “ice-like” solid structure. Depending on the mating materials at the interfaces, evidence of “ice-like” water structure has been shown to lubricate sliding contacts.44 It is also important to highlight the shearing behavior of water against an AFM tip due to the change in the nature of the contact at high humidity. Young et al.29 showed that the increase in a sample’s surface hydrophobicity lowers the interfacial viscous force between the tip and water (sandwiching water between the sample and tip surface). The reason for the reduction in viscous interfacial force is due to a corresponding increase in slip length, hence allowing water slippage at the surface.29 Water contact angle (WCA) measurements confirm SiO2 to have a borderline hydrophilic surface, which can facilitate easier shearing of water against the tip. Therefore, the lubricating behavior observed for the SiO2/SiO2 interface with an increase of humidity is due to (i) adsorption of water molecules, which changes the SiO2/SiO2 interface to a relatively lubricious SiO2/“liquid-like” water interface and (ii) that the SiO2/“liquid-like” water interface is facilitated with easy shearing behavior of water which is dependent on SiO2’s surface wettability. The overall friction behavior of graphene as a function of humidity shown in Figure 2 displays a combination of two distinctive trends which can be separated into two regimes, (i) The low/intermediary humidity regime (5−50%RH) with very minimal increase in friction force from ∼5.9 to 8.2 nN, and (ii) The high humidity regime (50−70%RH), where friction force increases from ∼8.2 to 16.5 nN. It should be noted that only a small dependence on normal load was observed due to the limited range of normal loads studied herein. In the case of graphene, this contrary trend of an increase in friction was observed as compared to SiO2, despite the fact that graphene exhibits a similar WCA to that of SiO2. In general, the friction at the SiO2/graphene interface is much lower as compared to SiO2/SiO2. Similar to SiO2, for graphene, it has been reported that adsorption of water forms an organized monolayer (icelike) followed by bulk-like water (liquid) evolving from the second layer of adsorbed water. The thickness of “ice-like” water adsorbed on graphene (monolayer) is thinner than that on SiO2 (∼3 layers).43 From Figure 2 it is apparent that in the case of graphene the introduction of water molecules (as either a solid or liquid layer) is not as lubricious as dry graphene itself and hence friction increases at high humidity as the water layer forms. In fact, it is notable that at high humidity the friction for both graphene and SiO2 approaches a similar level (∼25 nN) albeit from different directions. The increase of friction due to water adsorption of graphene is supported by recent atomistic simulations of friction of 2D materials in the presence of water. Tocci et al. highlighted the importance of free energy of the water contact layer using MD simulations.28 Their simulations showed an increase in friction coefficient for graphene as a

functional groups are occupied, the water molecules within the layers start to bond with other water molecules (water−water hydrogen bonds) causing them to cluster.20 This clustering behavior between the GO’s layers causes swelling, structural disturbances (protrusion/roughness), and a reduction in the interaction between the GO’s basal planes.20,23 Structural topographic disturbances in the form of protrusions were observed on GO’s surface which confirms that a water intercalation mechanism is taking place within the GO subsurface (Supporting Information, SI, Figure S1). The protrusions were first observed at 30%RH and continued to form as humidity increased (Figure S1). The likelihood of water intercalating under the bottom GO layer (between GO and SiO2) is significantly lower than water intercalation between GO sheets. For water to penetrate between the SiO2 wafer and the layered material, an extremely high humidity (RH > 95%) regime must be maintained for much longer duration (days).41,42 No such protrusions were observed forming on graphene’s surface while graphene roughness only increased from 122 pm (10%RH) to 144 pm (80%RH) (Figure S2). This suggests that the small increase in the thickness of graphene is primarily due to adsorption of water molecules on the top graphene surface. Effect of Water Adsorption and Surface Wettability on Friction. In this section, the effect of only water adsorption by increasing relative humidity on friction for different surface wettability is investigated. It focuses on two surfaces, SiO2, and graphene, for which water only adsorbs and does not intercalate into the material as demonstrated in the previous section. Figure 2 illustrates the dependence of surface

Figure 2. Average friction force of a beaded SiO2 tip in contact with SiO2 and graphene surfaces. The left inset shows the measured water contact angle measurements. The right insets are schematic drawings of a sliding tip at different humidity conditions to highlight the change of contact from solid/solid to solid/water.

wettability on friction under different relative humidity (FFM experiments were repeated twice for all data sets for repeatability). It is observed that there is a general decreasing trend in friction for the SiO2/SiO2 interface with an increase of humidity. The 20%RH mark acts as a threshold after which an increase of humidity results in a significant decrease of friction. Literature has reported the effect of humidity on the adsorption of water on SiO2 surface highlighting the significance of the 20−30%RH mark. At low humidity, water molecules begin to bond onto the SiO2 surface forming an “ice-like” solid structure.43 By 20−30%RH, 2−3 layers of water molecules can settle on to the SiO2 surface maintaining the “ice-like” structure. The 20−30%RH acts as a transitioning C

DOI: 10.1021/acsami.8b03776 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. A) Comparison of the average friction force between beaded and sharp SiO2 tip against GO sample and beaded SiO2 tip against graphene for different normal loads as a function of relative humidity. Inset: SEM images of beaded and sharp tips. B) and C) Tapping mode AFM images of GO flakes before and after wear (scale bar: 1 μm). D)−F) Schematics of the different stages in GO due to water intercalation and humidity leading to wear. The schematics highlight the increase of interlayer distance (red arrows), water clustering effect causing protrusions in GO’s topography (green arrow), and delamination of GO (blue arrows).

significant difference observed for both the friction and wear behavior of GO and graphene finds its origin from multiple factors as discussed in the following paragraphs. The observed higher friction of GO as compared to graphene is attributed to several factors including (i) the tip/ surface interaction potential, (ii) the puckering effect, (iii) surface roughness, and (iv) the water meniscus effect. First, GO has a higher surface interaction potential corrugation as compared to graphene. This can result in higher energy dissipation during sliding for GO as compared to graphene and is reflected in the higher friction force.46 Second, in addition to the surface interaction potential, it was earlier established in Figure 1 that GO exhibited a significant increase in thickness due to water intercalation whereas graphene had very minimal change. Excessive water intercalation within GO has previously been reported to reduce mechanical properties,20,23 particularly the out-of-plane stiffness.21 This reduction in the out-of-plane stiffness with higher humidity would result in an increased puckering effect, which can increase the overall contact area, resulting in increased friction.47 Third, it was observed that GO had higher surface roughness as compared to graphene (Figure S2). GO roughness was observed to be more dependent on humidity, increasing from 141 pm (10%RH) to 187 pm (80%RH). The higher increase in surface roughness for GO (33%) as compared to graphene (18%) was likely due to the combined influence of heterogeneous adsorption of water molecules on the top surfaces and water intercalation within GO layers disrupting its structure. Lastly, the change of humidity can also cause the formation of a water meniscus at the tip/sample contact where the distances are less than twice the Kelvin radius.48 The size/volume of the meniscus is dependent on the contact radius. For a large tip radius, the

result of water adsorption. The study reported that the potential energy landscape corrugation of the adsorbed water on graphene was dependent on the free energy rather than the water/solid interaction strength alone.28 Even though both materials had similar water contact angles and similar adsorbed water structure, they exhibited different frictional behavior due to the difference of the corrugated energy landscape. While the current experimental setup does not permit directly imaging the “ice-like” structure, or directly measuring the adsorbed water layer, indirect evidence such as the increase in the thickness of graphene due to adsorption of water (Figure 1) and the increase in surface roughness of both graphene and GO with increasing humidity (Figure S2) provides evidence to suggest the adsorption of a solid “ice-like” structure. Effect of water intercalation on the friction and wear of graphene and GO. The effect of the water intercalation on friction and wear of graphene and GO were studied using FFM at controlled levels of RH. Figure 3 illustrates the dependence of friction on relative humidity for SiO2 beaded tips sliding in contact with multilayer graphene and GO surfaces (FFM experiment were performed twice for repeatability). An increase of humidity resulted in an increase of friction for GO from ∼20 nN in dry conditions (5%RH) to ∼40 nN in humid conditions (30%RH). In the case of the SiO2/graphene interface, the friction was found to be much lower as compared to SiO2/GO.45 Comparing the wear behavior, it was also observed that GO repeatedly wore off from the surface when the humidity was higher than 30%RH. The onset of wear at 30%RH for GO was also confirmed using a sharp SiO2 tip (sharp Si tip with native SiO2 oxide layer). Contrary to GO, Graphene did not wear at high humidity and remained in a wear-free regime for all experiments. The D

DOI: 10.1021/acsami.8b03776 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Effect of humidity on pull off force for A) GO/SIO2 and SIO2/SIO2 (on GO sample), B) Graphene/SIO2 and SIO2/SIO2 (on graphene sample) interfaces using the beaded SiO2 tip.

allow water molecules to easily bond onto the GO outer surface and increase capillary forces.52 The increase in adhesion observed at the SiO2/GO (tip and GO top layer) interface coupled with reduction in interaction at GO’s subsurface between GO’s basal-plane/basal-plane could consequently contribute to delamination (Figure 3F−blue arrows). Lastly, clustering of intercalated water molecules within GO has been reported to form localized protrusions (Figure 3E−green arrows) in the GO’s surface.22 These protrusions can change the contact of a sliding tip along the GO plane and possibly act as stress concentration points to initiate wear.

volume of water meniscus forming around the contact area is larger.49 Once the meniscus is formed, it can be dragged along with the tip which can contribute to friction.50 Binggeli et al.51 reported that the strength of the capillary meniscus is dependent on the hydrophobicity of the surface, exhibiting stronger bonding for hydrophilic surfaces as compared to hydrophobic surfaces.51 Water contact angle (WCA) measurements on GO and graphene confirmed that GO is hydrophilic (θc,GO ≈ 45°) while graphene is hydrophobic (θc,G ≈ 91°) (cf. Table on Figure 3A), which is in agreement with previous studies.36,37 Therefore, since GO forms a stronger bonding meniscus as compared to graphene, higher energy is required to slide the bead. To illustrate the significance of the meniscus on the friction of GO, FFM was also performed using a SiO2 sharp tip (∼60 nm tip radius) sliding on GO (Figure 3A). The average friction force measured was relatively constant (∼10 nN) and did not appear to significantly depend on the humidity as opposed to that observed with the beaded tip. Consequently, the beaded tip would exhibit the formation of a much larger water meniscus which would in turn induce a higher friction. The increased propensity for wear of GO as compared to graphene can also be explained by several factors including (i) the reduction of basal plane interaction between the GO sheets due to water intercalation, (ii) an increase in adhesion at the contact due to meniscus formation, and (iii) localized protrusions from the GO surface due to excessive water intercalation (Figure S1). Depending on the coverage of functional groups on the GO’s basal plane, there are limited locations for water molecules to bond. Once all the locations are occupied, intercalation of additional water molecules initiates clustering within the gallery space.20 The clustering affects the interlayer distance20 and the GO’s structure/ roughness.22 The increase of interlayer distance and the change of hydrogen bonds from inter/intralayer bonds to water−water bonds will collectively reduce the GO’s basal-plane/basal-plane interaction. Consequently, delamination will likely initiate locally where water clustering pockets are formed, i.e., where the basal-plane/basal-plane interaction is the weakest (Figure 3F−blue arrows). Simultaneously, an increase of humidity will contribute to meniscus formation which affects the adhesion between the sample and tip. Measurements of the effect of humidity on adhesion of SiO2/GO and SiO2/graphene interfaces (Figure 4) revealed that SiO2/GO interface experienced a greater change of adhesion (more than 20%) as compared to SiO2/graphene (∼9%) over the same humidity range (5−40%RH). The higher increase in adhesion seen for SiO2/GO is likely due to the presence of oxygen functional groups on the GO surface. Those functional groups would

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CONCLUSIONS In this work, the effect of humidity and water intercalation on the tribological behavior of graphene and graphene oxide (GO) was investigated using friction force microscopy (FFM). In addition, the influence of surface wettability and water adsorption on the sliding behavior of the tip and water molecules was investigated by comparing FFM measurements of SiO2/SiO2, SiO2/Graphene, and SiO2/GO interfaces. Thickness measurements illustrated that water molecules easily intercalate between GO layers increasing the thickness by up to 100% as well as affecting its topography (roughness and protrusions). Considerably less change in thickness was observed for graphene due to some water adsorption, while no significant water intercalation was observed. This highlights the influence that functional groups have on the occurrence of water intercalation. FFM measurements revealed that an increase in humidity will reduce friction between the SiO2/SiO2. The reduction in friction is attributed to the change in contact with the sliding interface in 3 stages, from (i) solid/solid (SiO2/SiO2) to (ii) solid/ice to (iii) solid/water (SiO2/water). This change highlights the importance of (i) water adsorption and (ii) shearing behavior of water on the SiO2 surface. It was also confirmed that water shears easily on the more hydrophobic SiO2 surface as compared to the hydrophilic GO surface. Graphene however exhibited a contrary friction behavior to that of SiO2 with two distinctive trends that can be separated into two regimes. It was found that in the low/intermediary humidity regime (5−50%RH) the behavior is dominated by water adsorption and in the high humidity regime (50−70% RH) by meniscus/capillary condensation for graphene. This highlights that the surface wettability alone is not enough to lubricate a contact using water, but rather the lubrication in the presence of water is dependent on the mating materials at the interface and their initial sliding regime. E

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determined as half the difference between the forward and backward lateral force. To convert the lateral voltage signal to lateral force, the trace and retrace voltage signals were multiplied by lateral sensitivity, torsional stiffness, and 1/h2, where h is the height of the AFM tip. Adhesion experiments were performed with a maximum normal force of 1000 nN. The dwell time was set at 1 s for each adhesion measurement, and the results were averaged over 30 measurements. Humidity Control and WCA Measurements. A humidity control box was used to control the environment around the contact. The temperature for all the experiments was constant at 23 °C ± 1 °C. The humidity was controlled by mixing dry and wet nitrogen gas (nitrogen was of 99.9% purity). Wet nitrogen was obtained by passing the gas through water. The desired humidity was achieved by controlling the ratio of the gases. The humidity control box was equipped with a humidity sensor (Honeywell HIH 4000) which was aligned with the height of the surface sample to monitor the environment constantly. For each humidity condition, the environment was allowed to stabilize thoroughly to ensure no drift. To track the water intercalation, the thickness of the GO films was measured using tapping mode AFM imaging as the difference in height between the flat GO film and the silicon wafer. A sharp tip silicon cantilever with the resonance frequency of 320 kHz and the spring constant of 42 N/m was used for imaging. The humidity was allowed to stabilize before measuring the thickness of the flake. The images were processed using the built-in Asylum software functions to correct any influence of vertical drift and were plane-fit with a first-order polynomial to correct any sample tilt. WCA measurements were performed using a custom designed experimental setup at room condition (22 °C and approximately 35%RH). The measurements were performed by drop casting 6 μL of DI water onto the sample surface and repeated at least 5 times.

FFM measurements also revealed a more significant dependence of friction and wear on humidity for GO as compared to graphene. The overall difference of frictional trends was attributed to several factors, of which, the presence of functional groups is the primary dictating factor. This highlights the importance and influence of functional groups on nanoscale friction. Comparing the wearing behavior of both the materials, GO was found to wear repeatedly at humidity higher than 30%, irrespective of the tip radius. The repeated wearing of GO for both the sharp and beaded tip is attributed to the water intercalation within GO as the primary wear mechanism with a secondary contribution from the meniscus formation.

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MATERIALS AND METHODS

Sample Preparation and Material Characterization. The three types of samples prepared for this study were SiO2, graphene and GO. For SiO2, an N-doped Si wafer was cleaned in an ultrasonic bath in ethanol and methanol for 10 min. A GO solution was synthesized by stirring 1 mg of GO powder (Cheaptube, Inc.) in 20 mL of DI water at 300 rpm for 3 weeks. The solution was centrifuged at 3000 and 5000 rpm for 5 min to separate the thicker unexfoliated GO from exfoliated GO. 6 μL of the GO solution was then drop cast on the Si wafer. Graphene samples were prepared by mechanically exfoliated using HOPG graphite (SPI Supplies) onto the Si wafer. Raman and XPS characterizations were performed on bulk GO flake and HOPG (SI: Section S2). The C/O ratio for GO used in this study was measured via XPS to be ∼2.4 with a high level of epoxide and hydroxyl surface functionalization. AFM Measurements. AFM imaging, FFM, and adhesion experiments were performed using an Asylum Research MFP 3D AFM. Beaded tips for FFM and adhesion experiments were custom built. The silica spherical beads (Polyscience Inc.) used were first cleaned in an ultrasonic bath using ethanol and methanol for 10 min. The beads were attached to tipless silicon cantilevers (APPNano) with PC-super epoxy using a custom built micromanipulator stage under an optical microscope. SEM (Hitachi SU3500) was used to measure the bead diameter. The beaded tips had a diameter of ∼14 μm. Thickness measurements were performed by imaging in tapping mode AFM using stiff (k ≈ 42 N/m) and sharp tip silicon cantilevers (Nanoworld). The thicknesses of GO and Graphene in Figure 1 were measured relative to their respective SiO2 substrate surfaces. From the AFM topography (height) images, the thickness of the GO or graphene material was measured as the difference in the mean of a Gaussian fit to the GO or graphene surface height measurements as compared to that of the SiO2 surface height measurements. For each experiment, a new sample was prepared and placed in a customized humidity control box, and the box was filled with dry nitrogen to achieve low humidity. Experiments were then initiated from the low humidity regime (10%RH) and increased in increments of ∼10−20% up to 80%RH. Prior to imaging at any given environmental condition, the humidity was constantly monitored and allowed to stabilize (less than 0.1% change in humidity) which took approximately 20−25 min. For FFM and adhesion experiments the normal deflection sensitivity calibration was performed by indenting the tip on a clean silicon wafer to acquire the normal cantilever displacement and voltage signals. Lateral deflection sensitivity calibration was performed using the test probe method.53 Cantilever normal and torsional stiffness was calibrated using Saders method.54,55 For the beaded tip the normal stiffness was measured to be k ≈ 4 N/m and the torsional spring constant to be ∼1.89 × 10−8 N.m. FFM was performed with a scan rate of 4 μm/s. Commercial silicon sharp tips with low stiffness (k ≈ 2.8 N/m) were used from Nanoworld for the sharp tip FFM measurements. FFM was performed using line-by-line scanning of the AFM probe laterally (90° scan angle) over the sample surface and measuring the lateral force acting on the cantilever in both the forward and backward scanning directions. The friction force was

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b03776.

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Complementary experimental studies; AFM studies show the effect of water intercalation and humidity on the topography of graphene and GO surfaces (Section S1); XPS and RAMAN spectra characterizing the materials (Section S2); and additional references (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: fi[email protected] (T.F.). ORCID

Tobin Filleter: 0000-0003-2609-4773 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the Ontario Ministry of Research and Innovation Early Researcher Award; the Erwin Edward Hart Endowed Professorship; the Natural Sciences and Engineering Research Council of Canada (NSERC); and the Canada Foundation for Innovation (CFI). SEM analysis was carried out at Ontario Center for the Characterization of Advanced Materials (OCCAM). The authors would also like to thank Dr. B. Hatton for assistance with WCA measurements, Dr. R. Sodhi for assistance with XPS measurements, and Dr. P. M. Sudeep for assistance with Raman measurements. F

DOI: 10.1021/acsami.8b03776 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.8b03776 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX