Letter pubs.acs.org/NanoLett
Wettability of Graphene Rishi Raj,† Shalabh C. Maroo,‡ and Evelyn N. Wang*,† †
Device Research Laboratory, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ‡ Department of Mechanical and Aerospace Engineering, Syracuse University, Syracuse, New York 13244, United States S Supporting Information *
ABSTRACT: Graphene, an atomically thin two-dimensional material, has received significant attention due to its extraordinary electronic, optical, and mechanical properties. Studies focused on understanding the wettability of graphene for thermo-fluidic and surface-coating applications, however, have been sparse. Meanwhile, wettability results reported in literature via static contact angle measurement experiments have been contradictory and highlight the lack of clear understanding of the underlying physics that dictates wetting behavior. In this work, dynamic contact angle measurements and detailed graphene surface characterizations were performed to demonstrate that the defects present in CVD grown and transferred graphene coatings result in unusually high contact angle hysteresis (16−37°) on these otherwise smooth surfaces. Hence, understanding the effect of the underlying substrate based on static contact angle measurements as reported in literature is insufficient. The advancing contact angle measurements on mono-, bi-, and trilayer graphene sheets on copper, thermally grown silica (SiO2), and glass substrates were observed to be independent of the number of layers of graphene and in good agreement with corresponding molecular dynamics simulations and theoretical calculations. Irrespective of the number of graphene layers, the advancing contact angle values were also in good agreement with the advancing contact angle on highly ordered pyrolytic graphite (HOPG), reaffirming the negligible effect of the underlying substrate. These results suggest that the advancing contact angle is a true representation of a graphene-coated surface while the receding contact angle is significantly influenced by intrinsic defects introduced during the growth and transfer processes. These observations, where the underlying substrates do not affect the wettability of graphene coatings, is shown to be due to the large interlayer spacing resulting from the loose interlamellar coupling between the graphene sheet and the underlying substrate. The fundamental insights on graphene−water interactions reported in this study is an important step towards developing graphene-assisted surface coatings for heat transfer and microfluidics devices. KEYWORDS: Graphene, copper, silica, contact angle, wetting, molecular dynamics simulations, continuum model
G
raphene, the two-dimensional (2D) unit of threedimensional (3D) bulk material graphite, has received significant attention due to its extraordinary properties.1,2 It has captured the imagination of engineers for a variety of electronics, optical, sensing, microfluidics, manufacturing, and heat transfer applications. Moreover, graphene has also provided the scientific community an opportunity to experimentally investigate the unique characteristics of 2D materials. One of the most intriguing questions which otherwise is mostly irrelevant in the case of 3D materials is how the background substrate affects the properties of an atomically thin graphene coating. As an example, graphene coatings have been shown to shield the background substrate against chemical reactions with the ambient.3 Conversely, a recent study demonstrated that the substrate on which graphene rests strongly influences the chemical reactions occurring on top of the graphene surface.4 Another study showed that the first carbon layer on top of a silicon carbide surface acts as a buffer layer allowing the graphene layer to electronically behave like an isolated graphene sheet.5 © 2013 American Chemical Society
The fascinating 2D nature raises many interesting and pertinent questions which need to be addressed before graphene can be realized in practical applications. Here, we investigated the effect of the background substrate on the wettability of graphene, which has significant implications for graphene based surface-coatings for optical, microfluidics, manufacturing, and heat transfer applications. Graphite/ graphene−water interactions have been discussed in numerous papers in literature;6 few of the recent works have been directed toward answering these questions for graphene, however, the results are contradictory, and the effect of the background substrate remains unclear.6a−e,m While some studies report a negligible effect of the background surface on graphene−water interactions,6a−d Rafiee et al.6e proposed wetting transparency of graphene on silicon, gold, and copper. Recently, Shih et al.6m Received: December 17, 2012 Revised: February 1, 2013 Published: March 4, 2013 1509
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Figure 1. (a) Room temperature Raman spectra of HOPG and monolayer, bilayer, and trilayer graphene films on the SiO2 substrate. The evolution of contact angle, water droplet base radius, and height on (b) a monolayer graphene coating on a thermally grown SiO2 surface and (c) bare SiO2 surface. The inset in b shows the side-view image of the droplet with a 93° contact angle and a volume of ∼34 nL at 14 s. The dotted red line in the inset of b defines the graphene surface to avoid confusion with the reflection. Experimental results of (d) advancing contact angle, (e) receding contact angle, and (f) contact angle hysteresis for bare substrates and mono-, bi-, and tri-layered graphene on them. The corresponding values for HOPG are represented by black straight lines without symbols. The experimental uncertainty in the advancing and receding contact angles are ±2° and ±3°, respectively.
outlined the complexities associated with the wettability of graphene and proposed that the hypothesis of wetting transparency breaks down at extreme cases of underlying substrate wettabilities. These studies suggest that the role of the underlying substrate is not well-understood in literature. In this work, we investigated graphene−water interactions for substrates where van der Waals and electrostatic forces dominate (electrostatic forces are due to the presence of partial charges resulting from the covalently bonded atoms). Dynamic contact angle measurements are reported with graphene coatings on substrates with widely varying intrinsic wettability (∼0° contact angle on superhydrophilic copper to ∼40° contact angle on SiO2). The anomalies due to imperfections (edges, cracks, and grain boundaries) in as-grown and transferred graphene sheets on contact line behavior of microscopic droplets are explained by in situ imaging and surface characterization techniques. The results show that contact angle hysteresis on these graphene coated substrates is an important aspect that needs to be captured to clarify the discrepancies in literature. Furthermore, we determined that the advancing contact angle of water on monolayer graphene approaches the advancing contact angle on highly ordered pyrolytic graphite (HOPG), demonstrating a negligible effect of the underlying substrate wettability even for an atomically thin monolayer graphene coating. We used molecular dynamics simulations along with detailed theoretical calculations to confirm that monolayer graphene shields the majority of the interactions between water and the background substrates investigated in this study. Finally, the importance of contact angle hysteresis measurements and the understanding gained from the detailed theoretical analysis and molecular
dynamics simulations are discussed in the context of the results in literature to clarify the underlying physics. Experiments and Characterization. Monolayer graphene films synthesized by chemical vapor deposition (CVD) on copper foils7 were procured (Graphene Supermarket, NY). To understand the effect of the background substrate and the number of graphene layers on wettability, the CVD grown graphene was transferred8 to glass and ∼300 nm thick thermally grown oxide (SiO2) on silicon (Si), to obtain mono-, bi-, and tri-layered graphene sheets on these substrates (see section S1 of the Supporting Information for details). The Raman spectra of the transferred graphene sheets on the silica (SiO2) substrate, and HOPG were acquired using a confocal Raman microscope (CRM-200, WITec) and are shown in Figure 1a. Note that the curves have been arbitrarily shifted along the y-axis. The blue shift in the 2D peak and the decrease in the ratio of the 2D to G peak intensity as the number of graphene layers is increased are consistent with literature.9 Small or absent D peaks confirm the presence of high quality graphene films after the transfer process. The small D peaks in the bilayer and trilayer samples are likely created during the transfer of successive layers. Surface roughness measured via atomic force microscopy was found to be negligible so as to cause any significant hysteresis. The corresponding roughness ratio rmax (actual area by projected area, see section S1 of the Supporting Information for details) of monolayer graphene on the SiO2 substrate, which is a critical parameter that dictates the contact angle on rough surfaces,10 was only 1.004. Contact angle measurements on monolayer graphene on the SiO2 and on the bare SiO2 substrate are shown in Figure 1b and c, respectively. The contact angle quickly approached a 1510
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Figure 2. (a−d) Time-lapse images of water condensation and evaporation on a monolayer graphene on thermally grown SiO2 via environmental scanning electron microscope (ESEM). (a) Advancing contact angle mode during condensation stage. The droplets “A”, “B”, “C”, and “D” are examples of isolated droplets, while droplets “E” and “F” are coalesced droplets formed after the merging of multiple isolated droplets at the earlier stages of condensation. (b) A micrograph at the end of the condensation process which corresponds to the start of contact line pinning stage. (c) Image of the droplets in the contact line pinning stage during evaporation. While the contact angle of the droplet “B” in (c) is lower when compared to b, the base area is roughly constant signifying contact line pinning between these two frames. (d) A micrograph of the receding contact angle mode during the evaporation stage. As evident from the dotted circle, the contact area of the droplet “B” shrinks compared to the images in b and c. (e−f) Time-lapse optical microscope images of evaporating water droplet on a monolayer graphene transferred on the thermally grown SiO2 substrate. The droplet at the start of contact line pinning stage is shown in (e). Distortions of the contact line are pointed out by the arrows in (f). The cleanliness of the surface and the purity of the water used are confirmed by the spotless surface in (g) after complete droplet evaporation. (h) A 5× magnification scanning electron microscope image of a typical monolayer graphene on SiO2 surface. The corresponding image of the bare SiO2 surface is shown in the inset. (i−l) Raman spectroscopy of a 30 × 30 μm2 area of the monolayer graphene on SiO2 surface. Intensity maps of (i) 2D band, (j) G band, and (k) D band. (l) Intensity ratio of the 2D and G band. The low D peaks and high 2D to G peak ratio over the majority of area indicates high quality graphene. The solid arrows show sample locations where the graphene sheet is potentially absent resulting in negligible 2D and G intensity peaks in i and j and high D peak in the surrounding area in k. The dashed arrows indicate sample locations with multiple layers of graphene (I2D/IG ≤ 1).
However, the copper foil underneath the CVD graphene surface is pristine and unoxidized (and remains so even after exposure to ambient3b,c) due to the high temperature and H2 environment during the synthesis.7 Such copper surfaces free of contaminants and native oxide have been reported to be superhydrophilic with apparent contact angle of 0°.12 Accordingly, a contact angle value of 0° was used as the baseline for the bare copper surface in Figure 1d,e. Despite significant differences between the wettability of the bare substrates (θA,Cu = 0° (completely spreading), θA,SiO2/glass = 41°), the advancing contact angles (Figure 1d) approached the value on the bulk HOPG with the addition of just one layer of graphene on these substrates. The receding contact angle, however, was observed to be strongly affected by the number of layers of graphene sheets and the background substrate (Figure 1e). The receding contact angle value consistently increased with the number of graphene layers on glass and SiO2, while it was roughly constant but comparatively higher for graphene on copper. This observation, while surprising because bare copper is more wetting than SiO2 and glass, is due to the imperfections in graphene during the transfer processes as explained later. Nonetheless, the values of the advancing and receding contact
constant value during the liquid addition phase (constant advancing angle mode). Figure 1b shows that the advancing contact angle on the monolayer graphene surface was as high as 93 ± 2°. Once liquid addition was stopped (t = 26 s), the contact angle decreased due to evaporation while the contact line remained pinned (t = 26−78 s) as confirmed by the negligible decrease in the droplet base radius. The receding contact angle on monolayer graphene surface (t > 78 s) was 60 ± 3°, indicating significant contact angle hysteresis (Δθ > 30°). Similarly, the advancing and the receding contact angles on the bare SiO2 substrate were 41 ± 1° and 10 ± 2°, respectively. The evaporating droplets in both cases adopted a mixed mode toward the end, where both the contact angle and the base radius decreased simultaneously.11 However, this part of the droplet dynamics is not critical to the current work where we are interested in advancing and receding contact angles only. The dynamic contact angle measurements for all nine graphene samples (mono-, bi-, and tri-layered graphene on copper, silica, and glass), three bare substrates, and HOPG are summarized in Figure 1d−f. It should be noted that, due to the presence of the native oxide layer and contaminants upon exposure to ambient, we observed finite contact angles on the bare copper surface (θA,Cu = 86 ± 2°, θR,Cu = 72 ± 3°). 1511
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decreases significantly, such that the liquid is not in contact with the underlying substrate. Consequently, there is a significant reduction in contact angle hysteresis for bi- and tri-layered samples on SiO2 and glass as shown in Figure 1f. The hysteresis decreased by 11−12° when two additional graphene layers were added on SiO2 and glass substrates, which is in good agreement with the contribution of hysteresis calculated previously using the contact line fraction. The remaining value of contact angle hysteresis for trilayer graphene is similar to that on HOPG and is attributed to the presence of different groups (e.g., carboxyl and carbonyl turn these defect sites hydrophilic due to the presence of carbon−oxygen bonds14) attached to the edges and grain boundaries. The receding contact angle and the contact angle hysteresis values for graphene on copper, however, were roughly constant without any apparent effect of the number of graphene layers (mono-, bi-, and trilayer along with HOPG). Such behavior is expected since the defects that lead to contact line pinning and increased hysteresis are created only during the graphene transfer process and hence should not be present in the CVD grown graphene on copper. Our results indicate that the advancing contact angle on a graphene surface is a more appropriate representation of the actual surface material11b,15 and should be used for surface energy calculations. A constant contact angle value of ∼93° independent of the number of graphene layers and the type of background substrate was observed. These findings demonstrate the negligible effect of substrate (where van der Waals and electrostatic forces, due to the presence of partial charges resulting from the covalently bonded atoms, dominate) on the wettability of graphene coatings. Theory and Simulation. Molecular dynamics (MD) simulations were performed to provide molecular insight on the variation of water−substrate interactions due to the addition of intermediate graphene layers and the effect on the contact angle on substrates where dispersion forces (Cu) and electrostatic forces (SiO2−H) are dominant. The contact angle of a water droplet on copper and silicon dioxidehydroxylated (SiO2−H) substrates with a varying number of graphene layers on the surface was calculated. In particular, three cases per substrate were studied: (a) bare surface, i.e., no graphene layer, (b) surface with one graphene layer, and (c) surface with two graphene layers. The Cu-water interaction is due to van der Waals (short-range) forces only, whereas the interaction of water with the SiO2−H surface involves both electrostatic (long-range) and van der Waals forces (Si, O, and H interactions with water molecules). In addition, the contact angle on a graphite surface (six graphene layers) was also simulated. See section S1 of the Supporting Information section for the details of molecular dynamics simulation and the associated parameters. Figure 3a shows the simulation of a water droplet on the graphite (HOPG) surface. The initial simulation setup was a spherical water droplet of 6 nm diameter (2526 molecules, inset image at t = 0 ps) placed above the surface, and the simulation was performed for 1000 ps with a time step of 0.002 ps. The water molecules were coupled to a Berendsen thermostat16 to maintain the water temperature at 25 °C. The contact angle was determined (see section S1 of the Supporting Information for details) based on interface markers and a circular fit. The system equilibration was attained in the final 100−200 ps and was concluded from the minimal fluctuations in the contact angle as shown in Figure 3a. The effect of line tension on the
angles with three layers of graphene on any of these substrates were similar and close to that on a HOPG surface. To eliminate the effect of contamination on our findings, an experiment with cleaned monolayer graphene on the SiO2 substrate was performed in the enclosed chamber of an environmental scanning electron microscope (ESEM) as shown in Figure 2a−d. Charging from the electron beam was not a concern in these experiments due to the high electrical conductivity of the graphene layers. Dropwise condensation, which generally occurs on relatively less wetting substrates, was observed when water was condensed (0 to 1 min) by increasing the vapor pressure in the ESEM chamber. The droplets “A”, “B”, “C”, and “D” are examples of isolated droplets, while droplets “E” and “F” are coalesced droplets formed after merging of multiple isolated droplets at the earlier stages of condensation. Based on the surface inclination of 55°, the contact angle of the isolated droplets during the growth stage was within the range of 85−92°, which is in close agreement with advancing contact angle results reported in Figure 1d. These results suggest that the wetting behavior is comparable to bulk graphite during the advancing phase of the water on graphene. Once allowed to evaporate (1 to 3 min) by lowering the vapor pressure in the chamber, the droplets pinned (from Figure 2b and c) before receding (Figure 2d). Significant contact line pinning time and the associated decrease in contact angle supports the significant contact angle hysteresis on graphene coated surfaces. To investigate the mechanism associated with the large contact angle hysteresis, movement of the contact line during the pinning and receding phases was visualized in Figure 2e and f. Contact line distortions where the liquid front was pinned on defects is shown in Figure 2f. The clean surface without any noticeable residue after complete evaporation as in Figure 2g suggests that impurities were not a factor in the observed hysteresis. Moreover, no apparent heterogeneity or defects were observed even in the scanning electron micrographs (SEM) in Figure 2h. To investigate the nature of the defects that resulted in pinning, two-dimensional Raman mapping9 of a representative 30 × 30 μm2 area on the monolayer graphene on SiO2 surface with a spatial resolution of 300 nm was performed (Figure 2i−l). Interestingly, two locations (solid arrow) with negligible 2D and G peak and surrounded by high D peaks indicating the absence of graphene layer were identified.9 We attribute these small defects were generated during the graphene transfer process. Based on the maximum spatial resolution of 300 nm, the conservative estimate of the area fraction of these holes ranged between 0.01 and 0.04. Such defects (holes) where the water droplet can interact directly with the supporting substrate without any intermediate graphene layer introduce heterogeneity in the surface that result in contact line distortion as observed in Figure 2f. If these holes are assumed to be present in the form of a square array of circular defects, the contact angle hysteresis prediction based on Raj et al.,11b who demonstrated that the contact angle hysteresis is governed by the contact line fraction of the distorted contact line (f max) and could be significantly larger than the values predicted by the classical Cassie−Baxter equation,13 would vary between 7 and 14°. Considering that the square array of circular defects is an idealization and real defects are more tortuous, an even larger contact angle hysteresis can be expected for the same area fraction. With the increase in the number of graphene layers, the probability of defects in the total stack of graphene layers 1512
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Figure 3. (a) MD simulation of a water drop on a graphite (HOPG) surface simulated using six graphene layers. Variation of the drop contact angle with time >900 ps confirms equilibrium has been reached. The inset shows the initial setup (0 ps) of the computational domain is a 6 nm diameter water drop placed above the surface, followed by the determination of the contact angle of the equilibrated drop by identifying the interface using markers and curve fitting them. MD contact angle results of an equilibrated water drop on (b) Cu and (c) SiO2−H substrates coated with/without multiple graphene layers; the HOPG result is included for comparison. (Color scheme: C, cyan; Owater, red; Hwater, green; Cu, orange; Si, yellow. Nomenclature: Cu, copper; SiO2−H, hydroxylated silicon dioxide.)
matches well with the experimental results. Unlike copper, the graphene grain boundaries did not affect the wettability of the already oxidized background substrate, silica. As a result, there was an excellent agreement between experimental (Figure 1d) and simulated values (Figure 3c). Since the contact angle with a monolayer graphene was very similar to that on HOPG, no effect of additional graphene layers was observed either in experiments or in simulations. We also examined the validity of the continuum approximation for the few layered graphene systems discussed in this study. As per the continuum approximation, the discrete atomic configuration of a system is uniformly redistributed such that the bulk parameters can be easily calculated by integration schemes. A detailed theoretical derivation of the continuum and the penultimate interaction potential model was performed, and the contact angle prediction was compared with the molecular dynamics simulation results (see sections S2 and S3 of the Supporting Information, respectively). A penultimate interaction potential model implies a case where the graphene layers are not redistributed as a continuum along the direction normal to the 2D graphene plane and a direct summation of the interaction with individual graphene layers is instead performed. For each case, we obtained the short-range repulsion along with the attractive term from the fundamental LJ potential and identified the minima of the potential well to estimate the solid−liquid equilibrium distances, both for bare substrates and substrates coated with graphene. The calculated interfacial energy (Figure S3a) was then incorporated into the Young−Dupre equation20 to indirectly calculate the contact angle (see eq S14 of the Supporting Information). Comparison between the results for the bare copper substrate, HOPG, and mono-/bi-layered graphene on copper obtained through molecular dynamics simulations, the continuum model (2−8 potential, eqs S7 and S10), and the direct summation of the penultimate potential model (3−9 potential, eqs S8 and S11) demonstrated a close dependence of the contact angle on the solid−liquid equilibrium distance (Figure S3a and b). Due to the large interlayer spacing between the graphene layers as well as between a graphene layer and the substrate, the redistribution of the discrete graphene layers as a continuum via integration is inaccurate and under-predicts the solid−liquid equilibrium distances. As a result, the contact angle was overestimated in comparison to the MD values. Mean-
computed contact angle values was implicitly taken into account by adopting the water−carbon LJ potential parameters suggested by Werder et al.6h where the parameters were adjusted for the presence of line tension. Molecular simulation images of the equilibrated water drop on Cu surface with a varying number of graphene layers are shown in Figure 3b. As mentioned previously, a bare Cu substrate without any contamination and native oxide layer is known to be highly wetting.12 This result was also confirmed by our MD simulations where no apparent contact angle was observed due to complete spreading in the form of a thin film. It should be noted here that the LJ parameters for copper in our study were obtained from literature.17 However, the addition of a single layer of graphene resulted in a significant increase in the contact angle (∼70°). A second layer of graphene further increased the contact angle to 83.4°. The variation of the contact angle values in Figure 3b shows that additional graphene layers increase the contact angle value toward that on a graphite surface. The slight differences between the experimental advancing contact angle and the MD simulation results for one and two layers of graphene sheets can be attributed to the imperfections in practical graphene layers. Even though a perfect sheet of graphene has been demonstrated to be an excellent passivation layer,3 partial oxidation of the substrate underneath the grain boundaries of a CVD grown polycrystalline graphene sheet is known to occur3b,c under ambient conditions. As a result, in comparison to the simulated case of single crystal graphene on a perfect oxide free copper substrate (Figure 3b), the intrinsic wettability of practical CVD grown graphene on copper samples in experiments (Figure 1d) is lower due to the local regions of low surface energy copper oxide underneath the graphene grain boundaries. Figure 3c shows the molecular images of a water droplet on the SiO2−H surface. Thermally grown silicon dioxide surfaces are known to contain hydroxyl groups18 at the surface and hence have been emulated in our simulations. The partial charge values in the SiO2−H substrate were validated by computing the contact angle of the water droplet on the bare surface; the obtained value of 36.7° is in good agreement with experiments that we performed and in literature.19 On this surface, the addition of the first layer of graphene increased the contact angle similar to that on a graphite surface, which 1513
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would resemble very closely to that of graphite, even in the range of range of 30−90° for the bare substrate wettability. In summary, we analyzed substrates where van der Waals and electrostatic forces dominate to show that, irrespective of the intrinsic wettability, the large interlayer spacing resulting from the loose interlamellar coupling between the graphene sheet and the substrate minimizes the effect of the underlying substrate on the wettability of graphene coatings. The experimental results for monolayer graphene on oxygen-plasma cleaned silica surface in Shih et al.,6m however, demonstrate the potential of unobtrusive graphene coatings for surface wettability. Physical understanding of wettability on such plasma cleaned surfaces with free charges remains absent in literature and requires additional theoretical and modeling efforts to elucidate the underlying physics. Conclusions. We demonstrated that copper, glass, and silica substrates coated with an atomically thin 2D material graphene matches the wettability of 3D bulk graphite. The advancing contact angle was found to be independent of the number of layers of graphene sheets and was in good agreement with our molecular dynamics simulation and theoretical calculations. The receding contact angle, however, was governed by the defects in as-grown and transferred graphene sheets, leading to significant contact angle hysteresis. As a result, static contact angle measurements for wettability and surface energy characterization on such surfaces cannot solely be used. Continuum models for interfacial and adsorption energy calculations were shown to be inaccurate due to the underestimation of the solid−liquid equilibrium distances in graphene-coated substrates. The fundamental understanding of graphene−water interactions elucidated in this study is an important step towards developing graphene assisted surfacecoatings for heat transfer and microfluidics devices.
while, the equilibrium distance and contact angle values predicted using the direct summation over the layers via the penultimate 3−9 potential model showed excellent agreement with the MD results (see Figure S3a and b in the Supporting Information). Furthermore, the inaccuracies associated with the continuum models for graphene were demonstrated through the calculation of quantities other than the contact angle (see Figure S4 for water adsorption energy and graphene cleavage and exfoliation energy calculations). Our results which indicate that the underlying substrate has a negligible effect on the wettability of graphene-coated copper, silica, and glass substrates is now explained in the context of similar results in literature. For copper where van der Waals forces dominate, the experimental contact angle values for graphene coated copper in our work agrees with those reported in Rafiee et al.6e However, the contact angle used for bare copper substrates differs in the models, which lead to discrepancies in the interpretation of the substrate effect. While we demonstrated through the comparison of our simulation results (using fundamental LJ parameters from literature17) and previously reported controlled experiments12 that pristine copper underneath the CVD grown graphene is superhydrophilic with apparent contact angles of 0°, Rafiee et al.6e used the large experimental contact angle values (84−85°) on copper with native oxide layer and contaminates in ambient as the bare substrate wettability in their model. Accordingly, if the fundamental LJ parameters17 resulting in pristine copper substrate wettability12 was used, the effect of the underlying substrate on the wettability even for a monolayer graphene coating on copper would be negligible. In literature for monolayer graphene on glass and SiO2 (where both van der Waals and electrostatic forces due to partial charges dominate), static contact angle measurements were reported as between 48 and 54°6e and ∼64 and 85°,6m respectively. Owing to the defects generated during the graphene transfer process, unusually high contact angle hysteresis exceeding 30° with monolayer graphene on these substrates is expected, as we demonstrated in our work. Accordingly, by using the suggested11b,15 advancing contact angle measurements, we show a negligible effect of the background glass/SiO2 even with a monolayer graphene on these substrates. Finally, Shih et al.6m elucidated the effect of underlying substrate (van der Waals only) on the wettability of graphene by artificially tuning the simulation parameters such that the baseline contact angle values vary from 0° to 180° and demonstrated that wetting transparency6e breaks down, except when the bare substrate contact angle is in the range of 30−90°. Since only the attractive portion of the van der Waals interaction potential was modeled, the solid−liquid equilibrium distance was required to be a constant despite the variation in intrinsic substrate wettability. While such an approach allows a means to obtain a continuous variation in bare substrate wettability which otherwise is difficult in experiments or modeling, the solid−liquid equilibrium distance is a critical parameter governing wettability and decreases with increasing substrate wettability. For the substrates with intrinsic wettability in the range of 0−90°, the values of solid−liquid equilibrium distances calculated by including the short-range repulsion term in the interaction potential are lower than the used value of 5 Å6m (see Figure S3b in the Supporting Information). By including these considerations in the study of Shih et al.,6m the contact angle values on monolayer graphene coated substrates
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ASSOCIATED CONTENT
S Supporting Information *
Sample preparation and characterization techniques, experimental methodology, details of molecular dynamics simulations, derivation of theoretical model, and its comparison with MD results are discussed in this document. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Address: 77 Massachusetts Ave., 3-461B, Cambridge, MA 02139. E-mail:
[email protected]. Phone: (617)324-3311. Fax: (617)258-9346. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to acknowledge the support from the National Science Foundation through the Major Research Instrumentation Grant for Rapid Response Research (MRIRAPID). R.R. acknowledges fellowship support from Battelle’s National Security Global Business with Dr. Robert Carnes as the Director of Internal Research and Development Programs. The authors would to thank Nenad Miljkovic for help during condensation experiments. Part of this work was performed at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation under 1514
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(8) Li, X.; Zhu, Y.; Cai, W.; Borysiak, M.; Han, B.; Chen, D.; Piner, R. D.; Colombo, L.; Ruoff, R. S. Transfer of large-area graphene films for high-performance transparent conductive electrodes. Nano Lett. 2009, 9 (12), 4359−4363. (9) Yu, Q.; Jauregui, L. A.; Wu, W.; Colby, R.; Tian, J.; Su, Z.; Cao, H.; Liu, Z.; Pandey, D.; Wei, D. Control and characterization of individual grains and grain boundaries in graphene grown by chemical vapour deposition. Nat. Mater. 2011, 10 (6), 443−449. (10) Wenzel, R. N. Resistance of solid surfaces to wetting by water. Ind. Eng. Chem. 1936, 28 (8), 988−994. (11) (a) Kim, J. H.; Ahn, S. I.; Zin, W. C. Evaporation of water droplets on polymer surfaces. Langmuir 2007, 23 (11), 6163−6169. (b) Raj, R.; Enright, R.; Zhu, Y.; Adera, S.; Wang, E. N. Unified Model for Contact Angle Hysteresis on Heterogeneous and Superhydrophobic Surfaces. Langmuir 2012, 28 (45), 15777−15788. (12) Schrader, M. E. Ultrahigh vacuum techniques in the measurement of contact angles. III. Water on copper and silver. J. Phys. Chem. 1974, 78 (1), 87−89. (13) Cassie, A. B. D.; Baxter, S. Wettability of porous surfaces. Trans. Faraday Soc. 1944, 40, 546−551. (14) Rafiee, J.; Rafiee, M. A.; Yu, Z. Z.; Koratkar, N. Superhydrophobic to superhydrophilic wetting control in graphene films. Adv. Mater. 2010, 22 (19), 2151−2154. (15) Li, D.; Neumann, A. Surface heterogeneity and contact angle hysteresis. Colloid Polym. Sci. 1992, 270 (5), 498−504. (16) Berendsen, H. J. C.; Postma, J. P. M.; van Gunsteren, W. F.; DiNola, A.; Haak, J. R. Molecular dynamics with coupling to an external bath. J. Chem. Phys. 1984, 81 (8), 3684−3690. (17) (a) Zhu, S.-B. Interactions of water, ions, and atoms with metal surfaces. Surf. Sci. 1995, 329 (3), 276−284. (b) Zhu, S.-B.; Philpott, M. R. Interaction of water with metal surfaces. J. Chem. Phys. 1994, 100 (9), 6961−6968. (18) Martinez, N. Wettability of Silicon, Silicon Dioxide, and Organosilicate Glass; The University of North Texas: Denton, TX, 2009. (19) Osborne, K. L., III. Temperature-Dependence of the Contact Angle of Water on Graphite, Silicon, and Gold; Worcester Polytechnic Institute: Worcester, MA, 2009. (20) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed., with Applications to Colloidal and Biological Systems (Colloid Science); Academic Press: New York, 1992.
NSF award number ECS-0335765. CNS is a part of Harvard University. The authors would also like to thank Sean C. O’Hern and Prof. Rohit Karnik, Department of Mechanical Engineering, Massachusetts Institute of Technology and Emil Song and Prof. K. L. Wang, Department of Electrical Engineering, University of California, Los Angeles for their advice regarding improvements in graphene transfer process.
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DOI: 10.1021/nl304647t Nano Lett. 2013, 13, 1509−1515
