Article pubs.acs.org/Langmuir
Wetting of Graphene Oxide: A Molecular Dynamics Study Ning Wei, Cunjing Lv, and Zhiping Xu* Applied Mechanics Laboratory, Department of Engineering Mechanics and Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China ABSTRACT: We characterize the wetting properties of graphene oxide by performing classical molecular dynamics simulations. With oxygen-containing functional groups on the basal plane, graphene becomes hydrophilic and the water contact angle decreases with their concentration, c. The concentration dependence displays a transition at c ≈ 11% as defined by the interacting range of hydrogen bonds with oxidized groups and water. Patterns of the oxidized region and the morphological corrugation of the sheet strongly influence the spreading of water droplets with their lateral spans defined by corresponding geometrical parameters and thus can be used to control their behavior on the surface. These results are discussed with respect to relevant applications in graphene oxide-derived functional materials and offer a fundamental understanding of their wetting and flow phenomena.
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INTRODUCTION Graphene has attracted much interest recently not only because of its outstanding intrinsic properties but also because of the broad spectrum of materials derived from it through chemical or physical functionalization.1,2 Graphene oxide (GO), a native product from the exfoliation and restacking treatment (e.g., the Hummer’s method) to graphite,3 is one of the most representative examples. GO has been successfully synthesized and fabricated into macroscopic materials in the forms of fibers, papers, and films and has found a vast number of applications in functional materials,4,5 coatings,6 and separation membranes.7−10 Possible controls in the chemical functionalization and microstructures of these materials enable an unexpected potential for rational material design and optimization. For example, with the reduction through chemical, thermal, or electrochemical methods, the concentration of oxygen-containing groups in GO can be tuned,2 and the microstructures of GO-based bulk materials can be optimized through techniques such as wet spinning.11 One of the key properties of GO-based materials in the aforementioned applications is their wetting behavior. It was reported that graphene is a neutral material with a water contact angle (WCA) θc,G measured within the range of 87− 127°.12−14 However, GO from typical treatment displays hydrophilic properties with θc,GO ≈ 30−60°,8,15 which thus can be successfully dispersed in solution. The hydrophilic nature plays an important role in modulating the structure and properties of GO and, as a result, its applications. For example, the interlayer distance of GO increases when immersed in water.16,17 A capillary forms between oxidized regions in GO and was proposed to act as a spacer to tune the porous interlayer structures.7 Cooperative hydrogen bonds (H bonds) between GO sheets regulate the mechanical properties of GO paper and its nanocomposites.18,19 Moreover, a recent study showed that physicochemical environments (pH, salinity, or temperature, etc.) can further modify the conformation of GO © 2014 American Chemical Society
sheets through their influence on the hydrophilic surface functional groups.20 In spite of this experimental evidence that highlights its critical importance, the correlation between the wettability of GO and its structural characteristics (e.g., chemistry, microstructures) has not yet been identified, which prohibits deeper insights into both understanding the material properties and developing potential applications. The present work focuses on the wetting properties of GO by considering the effects of both its chemical functionalization (e.g., concentration of oxygen-containing groups,2,21 spatial distribution of oxidized regions22−24) and structural characteristics (morphological corrugation of the GO sheet25). We find that the hydrophilicity of GO increases with c. The WCA decreases from 98° for graphene to 26.8° for GO with c = 20%, a typical value identified in recent experiments.21 Textured patterns of oxidized regions are explored, and the results show that functional groups adjacent to the contact line control the wetting behavior. Moreover, the out-of-plane wrinkling of the GO sheet can impede the lateral spreading of a water droplet on it by pinning the contact line at morphological peaks, leading to quantized wetting states.
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MODELS AND METHODS In this study, we perform molecular dynamics (MD) simulations to explore the wetting properties of GO with various concentrations of oxygen-containing groups, texture patterns, and morphological corrugation. In our model of GO, both hydroxyl and epoxy groups are considered in this work, following the Lerf−Klinowski model that is consistent with recent experimental evidence.26 The long-lived quasi-equilibrium state was reported to be rich in hydroxyl groups,21 which is the focus here. Epoxy groups are also investigated and show Received: February 6, 2014 Revised: March 6, 2014 Published: March 10, 2014 3572
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Figure 1. (a) Water contact angles (WCAs, θc,GO) of monolayers and bilayer GO with hydroxyl (−OH) and epoxy groups measured as a function of their concentration c. In addition to randomly distributed functional groups, a regular pattern is also explored for hydroxyl groups. The prediction of θc from the Cassie−Baxter mixing model (eq 1) is also plotted for comparison, which features a transition at c ≈ 11% (details in the text). The inset shows two typical time evolution profiles of measured WCA in equilibrium. (b) Distribution of water density in the droplet plotted in units of its bulk value, ρ0. The liquid−vapor interface is defined as the contour line at ρ0/2 (the red line).
which extends with a length of 40 nm (the y direction) and a width of 2.1 nm (the x direction). Periodic boundary conditions are applied in these two directions. Our analysis of the water structure intercalated between GO sheets show that it approaches the bulk phase after the interlayer distance exceeds 1.4 nm,34 and thus a width of 2.1 nm could exclude the size effect in that dimension. Accordingly, a cylindrical droplet with a diameter of 5.27 nm (1588 water molecules) is placed above the GO surface at the beginning of simulations before the droplet spreads into a stabilized state. The size effect is assured to be avoided by comparing WCA values calculated from models with 1588, 2255, and 3493 water molecules. WCAs are measured by fitting density profiles in the yz plane. From our MD simulation results, a water isochore is calculated by the time-averaged water number density through a dense spatial mesh with a grid spacing of 0.05 nm. The liquid−vapor interface is defined as the contour line with a density level at half of the bulk value, as labeled in Figure 1b. The WCA is then calculated following the Werder method.35 The cross-section profile of the liquid−vapor interface is fitted into an arc, where θc,GO is calculated as the contingence angle at the basal plane (i.e., z = 0). It is worth noting that the distributed functional groups and slow diffusion of the droplet on GO introduce technical difficulties in the convergence of WCA measurements. To resolve this issue, we introduce a criterion by tracking the time evolution of θc,GO (Figure 1a, inset). In MD simulations, the WCA is measured in a series of time periods, each elapsing for 0.25 ns. The reported values of WCAs in this work are averaged values after the convergence is reached. It should be noted that the time to convergence τc increases with c. For instance, τc is 0.5 ns for pristine graphene and 3.5 ns for GO with c = 25%.
similar wetting effects. The concentration of oxidized groups c is defined as nO/nC, where nO and nC are the number densities of oxygen-containing groups and carbon atoms, respectively. A typical value of c is ∼20% for GO.21 Further reduction could yield a lower concentration (13.9−15.9%) in the reduced graphene oxide (RGO).2 The distribution of oxidized groups is set to be either random or regular in our models, as will be specified in the following discussions. The all-atom optimized potentials for liquid simulations (OPLS-AA) is used for GO, which is able to capture essential many-body terms in interatomic interactions, including bond stretching, bond angle bending, and van der Waals and electrostatic interactions.27,28 Following previous studies on similar systems, the rigid SPC/E model is used for water molecules.29,30 The SHAKE algorithm is applied for the stretching terms between oxygen and hydrogen atoms to reduce high-frequency vibrations that require a shorter time step for numerical integration. The interaction between water and GO includes both van der Waals and electrostatic terms. The former is described by the 12−6 Lennard-Jones potential between oxygen and carbon atoms with parameters ε = 4.063 meV and σ = 0.319 nm.30 The van der Waals forces are truncated at 1.0 nm, and the long-range Coulomb interactions are computed by using the particle−particle particle-mesh (PPPM) algorithm with an accuracy of 10−4.31 Our MD simulations are carried out by using the large-scale atomic/molecular massively parallel simulator (LAMMPS).32 All equilibrium simulations are performed at a constant temperature of 298 K by using the Nosé−Hoover thermostat. The time step for integrating equations of motion is set to be 0.5 fs. To explore the effect of morphological corrugation, we study separately flat and wrinkled structures. In the former case, the basal plane of the GO sheet is constrained and the atomic structures of functional groups are relaxed. A comparative model consisting of unconstrained GO supported by an adhesive substrate is also investigated, which suggests that the constraint has a negligible effect because the overall planar configuration of GO is maintained, which is the situation where GO is deposited on a substrate or stacked multilayers. To overcome the size limitation for WCA evaluation, we adopted a quasi-2D setup. This approach has several advantages, including the elimination of line tension due to curvature in the Young’s equation compared to that of dome-shape droplets and thus a low requirement for the size of the system to be modeled.33 A slab configuration is constructed for the water−GO system,
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RESULTS Effects of Chemical Functionalization. After the contact between GO and water is established, water molecules interact with pristine graphene region through van der Waals forces, whereas electrostatic interactions play an important role in the oxidized region, forming an interfacial H-bond network. It is interesting that although oxygen-containing groups are intercalated between water and the carbon basal plane in GO, the shorter interacting range of H bonds compared to those of van der Waals interactions leads to a similar distance from the basal plane to the first water layer (WL1), whether or not the graphene sheet is functionalized. Moreover, in WL1 with an 3573
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Figure 2. (a) Apparent WCAs calculated as a function of the distance d between two oxidized regions on GO. R0 is the cross-sectional radius of the initial cylindrical droplet isolated from the surface. The simulation results (dots) agree well with predictions from eq 2 by assuming a constant volume of the droplet (the solid line) for d/R0 < 6, beyond which the droplet detaches from oxidized groups. WCA θc,GO = 33° for GO with uniformly distributed hydroxyl groups is plotted for reference (the dashed line). (b, c) Simulation snapshots of a droplet on GO patterned with separated oxidized regions (b) and a single oxidized region covered by the droplet (c).
γLV) needs to be used. Here, γSV and γSL are the interfacial tension values at the solid−vapor and solid−liquid phases, respectively.36 In the spirit of the Cassie−Baxter mixing model,37 the relation between the WCA and the concentration of oxygen-containing groups is
average distance of ∼0.35 nm from the basal plane as the interphase, water molecules prefer to lie in parallel to the basal plane in the pristine graphene region. For water molecules residing further from the GO sheet, a more random distribution is identified for their orientations. As c increases, more H bonds could form between water molecules in WL1 and GO, and thus their orientations become more ordered. According to the Young−Dupré equation, Wa = γLV(1 + cos θc,GO), WCA decreases while Wa increases.36 Here Wa is the work of adhesion at the liquid−solid interface, and γLV is the interfacial tension between the liquid and vapor phases. The results for WCA measured as a function of c are summarized in Figure 1. Randomly distributed hydroxyl and epoxy groups show similar results, although the WCA for GO with epoxy groups is slightly lower as a result of its lower charge (qO = −0.39e on the oxygen atom) compared to that of the hydroxyl group (qO = −0.53e).27 This gentle dependence between WCA and qO can be understood from the Young−Dupré equation that shows that although Wa increases when the electrostatic interaction is enhanced, the Wa−θc,GO dependence is weakened at low values of θc,GO. The wettability of GO increases with c, and the WCA decreases from θc,G = 98.4° for pristine graphene to θc,GO = 26.8° for GO with 20% hydroxyl groups. The WCA is even lower for multilayer GO as a result of the enhanced droplet−GO interaction (results for bilayer GO in Figure 1a). The experimentally measured values for the WCA of GO membranes (c ≈ 10−20%) agree well with our simulation results here.8,15 However, it should be noted that the WCA measured here for the nanoscale water droplet may be sensitive to the spatial distribution of hydroxyl groups, as evidenced by our simulation results for different samples (error bars in Figure 1a). To clarify this point further and assess the reliability of the predicted θc,GO−c relationship, we perform additional MD simulations with regular patterns of hydroxyl groups, with an ordered “superlattice” pattern of hydroxyl groups on the underlying hexagonal graphene lattice. Here, four samples of GO with c = 3.125, 6.25, 12.50, and 25.0% are investigated, and a trend similar to that of the random models is observed. On the basis of this evidence, we confirm that the results shown in Figure 1 are quantitatively reliable in discussing the wetting properties of GO. To characterize the wetting properties of materials, a spreading parameter S is often used.36 According to the Young−Dupré equation, we can thus define S = γLV (cos θc − 1) for S < 0. For S > 0, the water droplet spreads completely on the solid surface and an alternative definition S = γSV − (γSL +
⎞ ⎛ S cos θc,GO = (1 − f )cos θc,G + f ⎜⎜ + 1⎟⎟ ⎠ ⎝ γLV
(1)
where θc,G = 98.4° is the intrinsic WCA of the pristine graphene region in GO and f is the areal fraction of the oxidized region. With the definition of c, we simply assume f to be proportional to c (i.e., f = k(c/2), where k is a constant). The factor of 2 in the denominator arises by considering the GO sample to be oxidized on both sides. It should be noted that this assumption of f for GO with atomic-scale roughness is tricky because it depends on the range of interaction between the water molecules and the surface (and functional groups if present), which varies further when the concentration changes, especially when c is high. By fitting our MD simulation results using eq 1, we find that k is very close to 1.0 for c < 11%, which suggests that the areal fraction of the oxidized region equals the concentration on each side. Least-square fitting of the data obtained from MD simulations yields S/γLV = 6.62 (Figure 1a), and the prediction from eq 1 can well capture the dependence of WCA on c in this low-concentration region (c < 11%) but deviates significantly from our MD results at higher c. This deviation could be explained by the failure of definition f = c/2. By considering the H-bond distance lHB ≈ 0.3−0.35 nm, the interacting ranges of neighboring oxidized groups will overlap at a critical concentration of ccr = 8−11.2%, which agrees well with our MD simulation results. As a result, the fitting should be updated beyond ccr that gives k = 0.24 (Figure 1a). From the discussion above, we conclude that the wetting properties of GO are enhanced as c keeps increasing, and the WCA reaches 7.9° at c = 25% (a linear extrapolation gives zero WCA at c = 30%) as a result of the hydrophilic property of hydroxyl groups. In experiments, the highest c reported is about 41%, and the lowest measurable WCA for GO is thus limited instead of approaching zero.21 Patterns of Oxygen-Containing Groups. Recent experiments and atomistic simulations showed phase separation between the pristine and oxidized regions in GO, with a typical size of 1 to 2 nm for each of them.22−24,38 Accordingly, in addition to the local concentration of functional groups, the wetting properties of GO may also depend on the distribution 3574
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Figure 3. Schematic plot of a droplet on a wrinkled surface. For a droplet with the same volume, there is an extra liquid−vapor interface (among A2, A3, and air; the dashed line) if the contact line resides in the valleys instead of the peaks. For wrinkled GO sheets, the WCA is measured with respect to the plane consisting of peaks (i.e., for A0).
Figure 4. Simulation snapshots of a water droplet on wrinkled graphene (a, b) and GO (c−h) for a = 0.5 nm, λ = 7.93 (a, c), a = 0.5 and 3.0 nm (b, d), and a = 0.1 nm, λ = 3.9, 2.8, 2.1, 1.0 nm (e−h).
patches of oxidized regions in GO could effectively control the spreading behavior of the water droplet on it. Morphological Corrugation. The morphology of a GO sheet is nonplanar because functionalization breaks its sp2 intrasheet C−C bonding nature. Ripples or wrinkles are widely identified in experimental characterization.3,40 The effect of morphological corrugation on the wetting behavior of GO is studied here by both theoretical models and MD simulations. Here, the concentration c of hydroxyl groups is set to be 10%, which is required to stabilize the system with a corrugation wavelength of only a few nanometers. We model the wrinkled profiles of graphene or GO as z = a sin by in the yz plane, where a is the corrugation amplitude, b is the wavenumber 2π/λ, and λ is the wavelength. In this study, we tune a and b (or λ) with respect to the following discussions. We now consider a water droplet with lateral span l covering multiple wrinkle wavelengths (i.e., l = nλ where n is an integer) and explore its spreading behavior. A schematic plot of the model is shown in Figure 3 for a water droplet with size comparable to the wrinkles. In this situation, the droplet prefers to reside in the valley to reduce the liquid−vapor interfacial energy because there is an extra interface (dashed lines in Figure 3) when the contact line of a droplet is located at the peaks. The results in Figure 4a−d show that for both graphene and GO with a = 0.5 nm the droplet is constrained by the corrugation and the relation l = nλ still holds, whereas for a = 0.1 nm (Figure 4e−h) the constraint is weakened and the droplet spreads with different values of l that depend on λ. The morphological corrugation thus must be included in the prediction of WCAs, which is absent in eq 2. For wrinkled GO sheets, the WCA is measured with respect to the plane consisting of peaks in wrinkled GOs. Equation 2 can be modified by including the water content in the valleys (A1 in Figure 3) as
of these regions. We explore this effect by arranging discrete hydroxyl functionalized regions in a quasi-2D model (Figure 2). Within the oxidized region, we keep the spatial distribution of hydroxyl groups as uniformly random with c = 20%. The droplet initially covers both the two oxidized regions and the pristine channel between them, and the distance between adjacent oxidized regions d or the width of the pristine channel is tuned subsequently in our MD simulation step by step (Figure 2b). The results summarized in Figure 2 show that by increasing d, the contact line moves with the oxidized patterns. The apparent WCA θc,a decreases for d < 14.64 nm, whereas for d > 14.64 nm the droplet detaches from hydroxyl groups and the WCA converges to a constant value of ∼33°. This observation is due to the H bonds formed between hydroxyl groups and water that pins the contact line.39 We also study the effect of hydroxyl groups under the cover of a droplet by constructing an oxidized region that is narrower than the lateral span of a droplet (Figure 2c). By tuning the oxidized pattern, we find that the droplet profile remains unchanged. This pinning effect plays an important role in defining the wetting behavior of GO and well explains our simulation results that only the chemical functionalization close to the contact line of the droplet contributes to the wetting. By assuming the water volume to remain a constant (πR02 per unit width) and the shape of droplet on surface to be circular (i.e., the volume is d2(θc,a − sin θc,a cos θc,a)/4 sin2 θc,a, we can derive an equation relating the WCA to d 4π sin 2 θc,a d2 = (θc,a − sin θc,a cos θc,a) R 02
(2)
where R0 is the cross-sectional radius of the initial cylindrical droplet that is isolated from the surface. The prediction by eq 2 agrees well with our simulation results in Figure 2. With this minimal quasi-2D model, one could expect that the discrete 3575
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containing groups that impede contact line movement (Figure 5c).39 The discrete nature of allowable wetting states of water on wrinkled GO suggests an abrupt transition of the lateral span of a droplet while its size is changed. When water is added to the droplet, the WCA will thus increase first and then decreases after the contact line crosses an energy barrier and reaches another peak. To verify this argument, we perform additional simulations with the water content in the droplet gently increased, with λ = 1.03 nm and a = 0.3 nm. The simulation results in Figure 6 shows that as the number of water molecules in the droplet increases from 1753 to 1819, the lateral span of the water droplet jumps from 9λ to 10λ.
(3)
Following this equation, we calculate WCAs for a water droplet spanning a length of nλ and plot them in Figure 5a,b as open
Figure 6. Transition of lateral span (open circles) of a water droplet on corrugated GO by adding water content. The parameters for the MD simulation results are λ = 1.03 nm and a = 0.3 nm.
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DISCUSSION Wetting properties characterize the interfacial interaction between liquid and solid, which help us to understand capillary and flow phenomena on the microscopic scale.7,41 In a recent study of flow between pristine graphene regions in GO, a capillary pressure of ∼500 bar was predicted at an interlayer distance of 0.6 nm.7 The WCA measured in their work for graphene is 107°, indicating a much weaker interfacial adhesion than that for the water−GO interface. Thus, on the basis of our MD simulation results here, an enhanced capillary force could be expected for water constrained between GO sheets. The pinning force induced by the oxidized groups could impede water flow in the channel, reducing the interfacial boundary slip and flow enhancement as a result of the electrostatic interaction and H bond formed with water molecules. Moreover, the pattered nature of oxidized groups on GO allows the formation of discrete capillaries that could act as spacers to tune the interlayer distance between neighboring GO sheets.7,22−24 A designed pattern of oxidized regions on graphene could be used to control its apparent WCA. The pinning force induced by the functional groups controls the contact line of the droplet and thus the wetted area. This mechanism could be combined with the coffee-ring effect to deposit particles on desired patterns during liquid evaporation. By introducing a gradient in the spatial distribution of oxidized groups on GO, one could drive a droplet in the direction where c increases. These concepts could be further combined with the morphological effect to find interesting applications in nanoscale material assembly and functional devices.42−45 Although the continuum wetting model can well explain our simulation results on the nanoscale, our MD simulations are limited by the length scale it
Figure 5. (a) Apparent WCAs of a droplet containing 1588 water molecules on corrugated GO with a = 0.1 nm, c = 10%, and λ ranging from 1.03 to 3.93 nm. Both allowable values predicted by eq 3 and those measured in MD simulations are plotted as open (thin and thick) and filled squares, respectively. The symbols for simulation results and theoretical predictions for the occupied states (thick open squares) are linked by lines. The dashed line shows the apparent WCA of flat GO. (b) The same as for plot a but with 3493 water molecules for a larger amplitude a = 0.3 nm and concentrations of c = 5% (blue) and 10% (red). (c) Snapshots of a water droplet on GO with λ = 1.79 nm. The contact line resides at peaks on the wrinkle with deviations caused by the local pinning effect, as can be seen from the positions of the contact line (p) and its adjacent peaks (I−III).
symbols in comparison to our MD simulation results (filled symbols). The WCA values are higher than those for flat GOs (dashed lines) and oscillate with λ (Figure 5a). This observation is confirmed by another set of simulations with a = 0.3 nm and c = 5 and 10% (Figure 5b) and suggests that at a fixed wavelength there exists a set of quantized wetting states defined by l = nλ for the droplet to spread on a wrinkled GO sheet, which is constrained by a local energy barrier defined by the water content and strength of the pinning effect. The MD results in general agree with theoretical predictions from eq 3 with a slight deviation caused by the pinning effect by oxygen3576
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(12) Taherian, F.; Marcon, V.; van der Vegt, N. F. A.; Leroy, F. What is the contact angle of water on graphene? Langmuir 2013, 29, 1457− 1465. (13) Shih, C.-J.; Wang, Q. H.; Lin, S.; Park, K.-C.; Jin, Z.; Strano, M. S.; Blankschtein, D. Breakdown in the wetting transparency of graphene. Phys. Rev. Lett. 2012, 109, 176101. (14) Li, Z.; Wang, Y.; Kozbial, A.; Shenoy, G.; Zhou, F.; McGinley, R.; Ireland, P.; Morganstein, B.; Kunkel, A.; Surwade, S. P.; Li, L.; Liu, H. Effect of airborne contaminants on the wettability of supported graphene and graphite. Nat. Mater. 2013, 12, 925−931. (15) Chen, Y.; Guo, F.; Jachak, A.; Kim, S.-P.; Datta, D.; Liu, J.; Kulaots, I.; Vaslet, C.; Jang, H. D.; Huang, J.; Kane, A.; Shenoy, V. B.; Hurt, R. H. Aerosol synthesis of cargo-filled graphene nanosacks. Nano Lett. 2012, 12, 1996−2002. (16) Talyzin, A. V.; Hausmaninger, T.; You, S.; Szabo, T. The structure of graphene oxide membranes in liquid water, ethanol and water-ethanol mixtures. Nanoscale 2014, 6, 272−281. (17) Talyzin, A. V.; Luzan, S. M.; Szabó, T.; Chernyshev, D.; Dmitriev, V. Temperature dependent structural breathing of hydrated graphite oxide in H2O. Carbon 2011, 49, 1894−1899. (18) Compton, O. C.; Cranford, S. W.; Putz, K. W.; An, Z.; Brinson, L. C.; Buehler, M. J.; Nguyen, S. T. Tuning the mechanical properties of graphene oxide paper and its associated polymer nanocomposites by controlling cooperative intersheet hydrogen bonding. ACS Nano 2011, 6, 2008−2019. (19) Gao, Y.; Liu, L.-Q.; Zu, S.-Z.; Peng, K.; Zhou, D.; Han, B.-H.; Zhang, Z. The effect of interlayer adhesion on the mechanical behaviors of macroscopic graphene oxide papers. ACS Nano 2011, 5, 2134−2141. (20) Whitby, R. L. D.; Gun’ko, V. M.; Korobeinyk, A.; Busquets, R.; Cundy, A. B.; László, K.; Skubiszewska-Zięba, J.; Leboda, R.; Tombácz, E.; Toth, I. Y.; Kovacs, K.; Mikhalovsky, S. V. Driving forces of conformational changes in single-layer graphene oxide. ACS Nano 2012, 6, 3967−3973. (21) Kim, S.; Zhou, S.; Hu, Y.; Acik, M.; Chabal, Y. J.; Berger, C.; de Heer, W.; Bongiorno, A.; Riedo, E. Room-temperature metastability of multilayer graphene oxide films. Nat. Mater. 2012, 11, 544−549. (22) Wilson, N. R.; Pandey, P. A.; Beanland, R.; Young, R. J.; Kinloch, I. A.; Gong, L.; Liu, Z.; Suenaga, K.; Rourke, J. P.; York, S. J.; Sloan, J. Graphene oxide: Structural analysis and application as a highly transparent support for electron microscopy. ACS Nano 2009, 3, 2547−2556. (23) Erickson, K.; Erni, R.; Lee, Z.; Alem, N.; Gannett, W.; Zettl, A. Determination of the local chemical structure of graphene oxide and reduced graphene oxide. Adv. Mater. 2010, 22, 4467−4472. (24) Pacilé, D.; Meyer, J. C.; Fraile Rodríguez, A.; Papagno, M.; Gómez-Navarro, C.; Sundaram, R. S.; Burghard, M.; Kern, K.; Carbone, C.; Kaiser, U. Electronic properties and atomic structure of graphene oxide membranes. Carbon 2011, 49, 966−972. (25) Zang, J.; Ryu, S.; Pugno, N.; Wang, Q.; Tu, Q.; Buehler, M. J.; Zhao, X. Multifunctionality and control of the crumpling and unfolding of large-area graphene. Nat. Mater. 2013, 12, 321−325. (26) Lerf, A.; He, H.; Forster, M.; Klinowski, J. Structure of graphite oxide revisited. J. Phys. Chem. B 1998, 102, 4477−4482. (27) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. Development and testing of the opls all-atom force field on conformational energetics and properties of organic liquids. J. Am. Chem. Soc. 1996, 118, 11225−11236. (28) Shih, C.-J.; Lin, S.; Sharma, R.; Strano, M. S.; Blankschtein, D. Understanding the pH-dependent behavior of graphene oxide aqueous solutions: A comparative experimental and molecular dynamics simulation study. Langmuir 2011, 28, 235−241. (29) Chen, C.; Ma, M.; Jin, K.; Liu, J. Z.; Shen, L.; Zheng, Q.; Xu, Z. Nanoscale fluid-structure interaction: flow resistance and energy transfer between water and carbon nanotubes. Phys. Rev. E 2011, 84, 046314−7. (30) Xiong, W.; Liu, J. Z.; Ma, M.; Xu, Z.; Sheridan, J.; Zheng, Q. Strain engineering water transport in graphene nanochannels. Phys. Rev. E 2011, 84, 056329−7.
captures. The pinning effect could be weakened for larger droplets, and more complicated microstructures such as crumples25 may further modulate the wetting and flow phenomena at interfaces with GO.
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CONCLUSIONS We have explored the wetting properties of graphene oxide by combining molecular dynamics simulations and theoretical analysis. The oxygen-containing groups on graphene lead to hydrophilism, and the water contact angle decreases with its concentration c, featuring a transition at c ≈ 11%. The oxidized patterns and wrinkles formed naturally in graphene oxide modify the wetting and spreading behaviors remarkably through the pinning effect and quantized wetting states as revealed here. These findings characterize the wetting properties of graphene oxide and identify opportunities in engineering capillary, flow, and wetting phenomena on the surfaces and interfaces of graphene oxide-derived materials.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China through grants 11222217 and 11002079 and Tsinghua University Initiative Scientific Research Program 2011Z02174. The simulations were performed on the Explorer 100 cluster system at Tsinghua National Laboratory for Information Science and Technology.
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REFERENCES
(1) Geim, A. K.; Novoselov, K. S. The rise of graphene. Nat. Mater. 2007, 6, 183−191. (2) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The chemistry of graphene oxide. Chem. Soc. Rev. 2010, 39, 228−240. (3) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Preparation and characterization of graphene oxide paper. Nature 2007, 448, 457−460. (4) Xu, Z.; Sun, H.; Zhao, X.; Gao, C. Ultrastrong fibers assembled from giant graphene oxide sheets. Adv. Mater. 2013, 25, 188−193. (5) Hu, X.; Xu, Z.; Gao, C. Multifunctional, supramolecular, continuous artificial nacre fibres. Sci. Rep. 2012, 2, 767. (6) Moon, I. K.; Kim, J. I.; Lee, H.; Hur, K.; Kim, W. C.; Lee, H. 2D graphene oxide nanosheets as an adhesive over-coating layer for flexible transparent conductive electrodes. Sci. Rep. 2013, 3, 1112. (7) Nair, R. R.; Wu, H. A.; Jayaram, P. N.; Grigorieva, I. V.; Geim, A. K. Unimpeded permeation of water through helium-leak-tight graphene-based membranes. Science 2012, 335, 442−444. (8) Sun, P.; Zhu, M.; Wang, K.; Zhong, M.; Wei, J.; Wu, D.; Xu, Z.; Zhu, H. Selective ion penetration of graphene oxide membranes. ACS Nano 2013, 7, 428−437. (9) Huang, H.; Song, Z.; Wei, N.; Shi, L.; Mao, Y.; Ying, Y.; Sun, L.; Xu, Z.; Peng, X. Ultrafast viscous water flow through nanostrandchannelled graphene oxide membranes. Nat. Commun. 2013, 4, 3979. (10) Sun, P.; Zheng, F.; Zhu, M.; Song, Z.; Wang, K.; Zhong, M.; Wu, D.; Little, R. B.; Xu, Z.; Zhu, H. Selective trans-membrane transport of alkali and alkaline earth cations through graphene oxide membranes based on cation-π interactions. ACS Nano 2014, 8, 850− 859. (11) Xu, Z.; Gao, C. Graphene chiral liquid crystals and macroscopic assembled fibres. Nat. Commun. 2011, 2, 571. 3577
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(31) Hockney, R. W.; Eastwood, J. W. Computer Simulation Using Particles; Taylor & Francis: New York, 1988. (32) Plimpton, S. Fast parallel algorithms for short-range molecular dynamics. J. Comput. Phys. 1995, 117, 1−19. (33) Rafiee, J.; Mi, X.; Gullapalli, H.; Thomas, A. V.; Yavari, F.; Shi, Y.; Ajayan, P. M.; Koratkar, N. A. Wetting transparency of graphene. Nat. Mater. 2012, 11, 217−222. (34) Wei, N.; Peng, X.; Xu, Z. Breakdown of fast water transport in graphene oxides. Phys. Rev. E 2014, 89, 012113−012118. (35) Werder, T.; Walther, J. H.; Jaffe, R. L.; Halicioglu, T.; Koumoutsakos, P. On the water-carbon interaction for use in molecular dynamics simulations of graphite and carbon nanotubes. J. Phys. Chem. B 2003, 107, 1345−1352. (36) de Gennes, P.-G.; Brochard-Wyart, F.; Quere, D. Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves; Springer: New York, 2004. (37) Cassie, A. B. D.; Baxter, S. Wettability of porous surfaces. Trans. Faraday Soc. 1944, 40, 546−551. (38) Zhou, S.; Bongiorno, A. Origin of the chemical and kinetic stability of graphene oxide. Sci. Rep. 2013, 3, 2484. (39) Wang, F.-C.; Wu, H.-A. Pinning and depinning mechanism of the contact line during evaporation of nano-droplets sessile on textured surfaces. Soft Matter 2013, 9, 5703−5709. (40) Qiu, L.; Zhang, X.; Yang, W.; Wang, Y.; Simon, G. P.; Li, D. Controllable corrugation of chemically converted graphene sheets in water and potential application for nanofiltration. Chem. Commun. 2011, 47, 5810−5812. (41) Stroberg, W.; Keten, S.; Liu, W. K. Hydrodynamics of capillary imbibition under nanoconfinement. Langmuir 2012, 28, 14488− 14495. (42) Layani, M.; Gruchko, M.; Milo, O.; Balberg, I.; Azulay, D.; Magdassi, S. Transparent conductive coatings by printing coffee ring arrays obtained at room temperature. ACS Nano 2009, 3, 3537−3542. (43) Jin, Z.; Sun, W.; Ke, Y.; Shih, C.-J.; Paulus, G. L. C.; Hua Wang, Q.; Mu, B.; Yin, P.; Strano, M. S. Metallized DNA nanolithography for encoding and transferring spatial information for graphene patterning. Nat. Commun. 2013, 4, 1663. (44) Patra, N.; Wang, B.; Král, P. Nanodroplet activated and guided folding of graphene nanostructures. Nano Lett. 2009, 9, 3766−3771. (45) Hernández, S. C.; Bennett, C. J. C.; Junkermeier, C. E.; Tsoi, S. D.; Bezares, F. J.; Stine, R.; Robinson, J. T.; Lock, E. H.; Boris, D. R.; Pate, B. D.; Caldwell, J. D.; Reinecke, T. L.; Sheehan, P. E.; Walton, S. G. Chemical gradients on graphene to drive droplet motion. ACS Nano 2013, 7, 4746−4755.
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Wetting of Graphene Oxide: A Molecular Dynamics Study Ning Wei, Cunjing Lv, and Zhiping Xu* Applied Mechanics Laboratory, Department of Engineering Mechanics and Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China ABSTRACT: We characterize the wetting properties of graphene oxide by performing classical molecular dynamics simulations. With oxygen-containing functional groups on the basal plane, graphene becomes hydrophilic and the water contact angle decreases with their concentration, c. The concentration dependence displays a transition at c ≈ 11% as defined by the interacting range of hydrogen bonds with oxidized groups and water. Patterns of the oxidized region and the morphological corrugation of the sheet strongly influence the spreading of water droplets with their lateral spans defined by corresponding geometrical parameters and thus can be used to control their behavior on the surface. These results are discussed with respect to relevant applications in graphene oxide-derived functional materials and offer a fundamental understanding of their wetting and flow phenomena.
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INTRODUCTION Graphene has attracted much interest recently not only because of its outstanding intrinsic properties but also because of the broad spectrum of materials derived from it through chemical or physical functionalization.1,2 Graphene oxide (GO), a native product from the exfoliation and restacking treatment (e.g., the Hummer’s method) to graphite,3 is one of the most representative examples. GO has been successfully synthesized and fabricated into macroscopic materials in the forms of fibers, papers, and films and has found a vast number of applications in functional materials,4,5 coatings,6 and separation membranes.7−10 Possible controls in the chemical functionalization and microstructures of these materials enable an unexpected potential for rational material design and optimization. For example, with the reduction through chemical, thermal, or electrochemical methods, the concentration of oxygen-containing groups in GO can be tuned,2 and the microstructures of GO-based bulk materials can be optimized through techniques such as wet spinning.11 One of the key properties of GO-based materials in the aforementioned applications is their wetting behavior. It was reported that graphene is a neutral material with a water contact angle (WCA) θc,G measured within the range of 87− 127°.12−14 However, GO from typical treatment displays hydrophilic properties with θc,GO ≈ 30−60°,8,15 which thus can be successfully dispersed in solution. The hydrophilic nature plays an important role in modulating the structure and properties of GO and, as a result, its applications. For example, the interlayer distance of GO increases when immersed in water.16,17 A capillary forms between oxidized regions in GO and was proposed to act as a spacer to tune the porous interlayer structures.7 Cooperative hydrogen bonds (H bonds) between GO sheets regulate the mechanical properties of GO paper and its nanocomposites.18,19 Moreover, a recent study showed that physicochemical environments (pH, salinity, or temperature, etc.) can further modify the conformation of GO © 2014 American Chemical Society
sheets through their influence on the hydrophilic surface functional groups.20 In spite of this experimental evidence that highlights its critical importance, the correlation between the wettability of GO and its structural characteristics (e.g., chemistry, microstructures) has not yet been identified, which prohibits deeper insights into both understanding the material properties and developing potential applications. The present work focuses on the wetting properties of GO by considering the effects of both its chemical functionalization (e.g., concentration of oxygen-containing groups,2,21 spatial distribution of oxidized regions22−24) and structural characteristics (morphological corrugation of the GO sheet25). We find that the hydrophilicity of GO increases with c. The WCA decreases from 98° for graphene to 26.8° for GO with c = 20%, a typical value identified in recent experiments.21 Textured patterns of oxidized regions are explored, and the results show that functional groups adjacent to the contact line control the wetting behavior. Moreover, the out-of-plane wrinkling of the GO sheet can impede the lateral spreading of a water droplet on it by pinning the contact line at morphological peaks, leading to quantized wetting states.
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MODELS AND METHODS In this study, we perform molecular dynamics (MD) simulations to explore the wetting properties of GO with various concentrations of oxygen-containing groups, texture patterns, and morphological corrugation. In our model of GO, both hydroxyl and epoxy groups are considered in this work, following the Lerf−Klinowski model that is consistent with recent experimental evidence.26 The long-lived quasi-equilibrium state was reported to be rich in hydroxyl groups,21 which is the focus here. Epoxy groups are also investigated and show Received: February 6, 2014 Revised: March 6, 2014 Published: March 10, 2014 3572
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Figure 1. (a) Water contact angles (WCAs, θc,GO) of monolayers and bilayer GO with hydroxyl (−OH) and epoxy groups measured as a function of their concentration c. In addition to randomly distributed functional groups, a regular pattern is also explored for hydroxyl groups. The prediction of θc from the Cassie−Baxter mixing model (eq 1) is also plotted for comparison, which features a transition at c ≈ 11% (details in the text). The inset shows two typical time evolution profiles of measured WCA in equilibrium. (b) Distribution of water density in the droplet plotted in units of its bulk value, ρ0. The liquid−vapor interface is defined as the contour line at ρ0/2 (the red line).
which extends with a length of 40 nm (the y direction) and a width of 2.1 nm (the x direction). Periodic boundary conditions are applied in these two directions. Our analysis of the water structure intercalated between GO sheets show that it approaches the bulk phase after the interlayer distance exceeds 1.4 nm,34 and thus a width of 2.1 nm could exclude the size effect in that dimension. Accordingly, a cylindrical droplet with a diameter of 5.27 nm (1588 water molecules) is placed above the GO surface at the beginning of simulations before the droplet spreads into a stabilized state. The size effect is assured to be avoided by comparing WCA values calculated from models with 1588, 2255, and 3493 water molecules. WCAs are measured by fitting density profiles in the yz plane. From our MD simulation results, a water isochore is calculated by the time-averaged water number density through a dense spatial mesh with a grid spacing of 0.05 nm. The liquid−vapor interface is defined as the contour line with a density level at half of the bulk value, as labeled in Figure 1b. The WCA is then calculated following the Werder method.35 The cross-section profile of the liquid−vapor interface is fitted into an arc, where θc,GO is calculated as the contingence angle at the basal plane (i.e., z = 0). It is worth noting that the distributed functional groups and slow diffusion of the droplet on GO introduce technical difficulties in the convergence of WCA measurements. To resolve this issue, we introduce a criterion by tracking the time evolution of θc,GO (Figure 1a, inset). In MD simulations, the WCA is measured in a series of time periods, each elapsing for 0.25 ns. The reported values of WCAs in this work are averaged values after the convergence is reached. It should be noted that the time to convergence τc increases with c. For instance, τc is 0.5 ns for pristine graphene and 3.5 ns for GO with c = 25%.
similar wetting effects. The concentration of oxidized groups c is defined as nO/nC, where nO and nC are the number densities of oxygen-containing groups and carbon atoms, respectively. A typical value of c is ∼20% for GO.21 Further reduction could yield a lower concentration (13.9−15.9%) in the reduced graphene oxide (RGO).2 The distribution of oxidized groups is set to be either random or regular in our models, as will be specified in the following discussions. The all-atom optimized potentials for liquid simulations (OPLS-AA) is used for GO, which is able to capture essential many-body terms in interatomic interactions, including bond stretching, bond angle bending, and van der Waals and electrostatic interactions.27,28 Following previous studies on similar systems, the rigid SPC/E model is used for water molecules.29,30 The SHAKE algorithm is applied for the stretching terms between oxygen and hydrogen atoms to reduce high-frequency vibrations that require a shorter time step for numerical integration. The interaction between water and GO includes both van der Waals and electrostatic terms. The former is described by the 12−6 Lennard-Jones potential between oxygen and carbon atoms with parameters ε = 4.063 meV and σ = 0.319 nm.30 The van der Waals forces are truncated at 1.0 nm, and the long-range Coulomb interactions are computed by using the particle−particle particle-mesh (PPPM) algorithm with an accuracy of 10−4.31 Our MD simulations are carried out by using the large-scale atomic/molecular massively parallel simulator (LAMMPS).32 All equilibrium simulations are performed at a constant temperature of 298 K by using the Nosé−Hoover thermostat. The time step for integrating equations of motion is set to be 0.5 fs. To explore the effect of morphological corrugation, we study separately flat and wrinkled structures. In the former case, the basal plane of the GO sheet is constrained and the atomic structures of functional groups are relaxed. A comparative model consisting of unconstrained GO supported by an adhesive substrate is also investigated, which suggests that the constraint has a negligible effect because the overall planar configuration of GO is maintained, which is the situation where GO is deposited on a substrate or stacked multilayers. To overcome the size limitation for WCA evaluation, we adopted a quasi-2D setup. This approach has several advantages, including the elimination of line tension due to curvature in the Young’s equation compared to that of dome-shape droplets and thus a low requirement for the size of the system to be modeled.33 A slab configuration is constructed for the water−GO system,
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RESULTS Effects of Chemical Functionalization. After the contact between GO and water is established, water molecules interact with pristine graphene region through van der Waals forces, whereas electrostatic interactions play an important role in the oxidized region, forming an interfacial H-bond network. It is interesting that although oxygen-containing groups are intercalated between water and the carbon basal plane in GO, the shorter interacting range of H bonds compared to those of van der Waals interactions leads to a similar distance from the basal plane to the first water layer (WL1), whether or not the graphene sheet is functionalized. Moreover, in WL1 with an 3573
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Figure 2. (a) Apparent WCAs calculated as a function of the distance d between two oxidized regions on GO. R0 is the cross-sectional radius of the initial cylindrical droplet isolated from the surface. The simulation results (dots) agree well with predictions from eq 2 by assuming a constant volume of the droplet (the solid line) for d/R0 < 6, beyond which the droplet detaches from oxidized groups. WCA θc,GO = 33° for GO with uniformly distributed hydroxyl groups is plotted for reference (the dashed line). (b, c) Simulation snapshots of a droplet on GO patterned with separated oxidized regions (b) and a single oxidized region covered by the droplet (c).
γLV) needs to be used. Here, γSV and γSL are the interfacial tension values at the solid−vapor and solid−liquid phases, respectively.36 In the spirit of the Cassie−Baxter mixing model,37 the relation between the WCA and the concentration of oxygen-containing groups is
average distance of ∼0.35 nm from the basal plane as the interphase, water molecules prefer to lie in parallel to the basal plane in the pristine graphene region. For water molecules residing further from the GO sheet, a more random distribution is identified for their orientations. As c increases, more H bonds could form between water molecules in WL1 and GO, and thus their orientations become more ordered. According to the Young−Dupré equation, Wa = γLV(1 + cos θc,GO), WCA decreases while Wa increases.36 Here Wa is the work of adhesion at the liquid−solid interface, and γLV is the interfacial tension between the liquid and vapor phases. The results for WCA measured as a function of c are summarized in Figure 1. Randomly distributed hydroxyl and epoxy groups show similar results, although the WCA for GO with epoxy groups is slightly lower as a result of its lower charge (qO = −0.39e on the oxygen atom) compared to that of the hydroxyl group (qO = −0.53e).27 This gentle dependence between WCA and qO can be understood from the Young−Dupré equation that shows that although Wa increases when the electrostatic interaction is enhanced, the Wa−θc,GO dependence is weakened at low values of θc,GO. The wettability of GO increases with c, and the WCA decreases from θc,G = 98.4° for pristine graphene to θc,GO = 26.8° for GO with 20% hydroxyl groups. The WCA is even lower for multilayer GO as a result of the enhanced droplet−GO interaction (results for bilayer GO in Figure 1a). The experimentally measured values for the WCA of GO membranes (c ≈ 10−20%) agree well with our simulation results here.8,15 However, it should be noted that the WCA measured here for the nanoscale water droplet may be sensitive to the spatial distribution of hydroxyl groups, as evidenced by our simulation results for different samples (error bars in Figure 1a). To clarify this point further and assess the reliability of the predicted θc,GO−c relationship, we perform additional MD simulations with regular patterns of hydroxyl groups, with an ordered “superlattice” pattern of hydroxyl groups on the underlying hexagonal graphene lattice. Here, four samples of GO with c = 3.125, 6.25, 12.50, and 25.0% are investigated, and a trend similar to that of the random models is observed. On the basis of this evidence, we confirm that the results shown in Figure 1 are quantitatively reliable in discussing the wetting properties of GO. To characterize the wetting properties of materials, a spreading parameter S is often used.36 According to the Young−Dupré equation, we can thus define S = γLV (cos θc − 1) for S < 0. For S > 0, the water droplet spreads completely on the solid surface and an alternative definition S = γSV − (γSL +
⎞ ⎛ S cos θc,GO = (1 − f )cos θc,G + f ⎜⎜ + 1⎟⎟ ⎠ ⎝ γLV
(1)
where θc,G = 98.4° is the intrinsic WCA of the pristine graphene region in GO and f is the areal fraction of the oxidized region. With the definition of c, we simply assume f to be proportional to c (i.e., f = k(c/2), where k is a constant). The factor of 2 in the denominator arises by considering the GO sample to be oxidized on both sides. It should be noted that this assumption of f for GO with atomic-scale roughness is tricky because it depends on the range of interaction between the water molecules and the surface (and functional groups if present), which varies further when the concentration changes, especially when c is high. By fitting our MD simulation results using eq 1, we find that k is very close to 1.0 for c < 11%, which suggests that the areal fraction of the oxidized region equals the concentration on each side. Least-square fitting of the data obtained from MD simulations yields S/γLV = 6.62 (Figure 1a), and the prediction from eq 1 can well capture the dependence of WCA on c in this low-concentration region (c < 11%) but deviates significantly from our MD results at higher c. This deviation could be explained by the failure of definition f = c/2. By considering the H-bond distance lHB ≈ 0.3−0.35 nm, the interacting ranges of neighboring oxidized groups will overlap at a critical concentration of ccr = 8−11.2%, which agrees well with our MD simulation results. As a result, the fitting should be updated beyond ccr that gives k = 0.24 (Figure 1a). From the discussion above, we conclude that the wetting properties of GO are enhanced as c keeps increasing, and the WCA reaches 7.9° at c = 25% (a linear extrapolation gives zero WCA at c = 30%) as a result of the hydrophilic property of hydroxyl groups. In experiments, the highest c reported is about 41%, and the lowest measurable WCA for GO is thus limited instead of approaching zero.21 Patterns of Oxygen-Containing Groups. Recent experiments and atomistic simulations showed phase separation between the pristine and oxidized regions in GO, with a typical size of 1 to 2 nm for each of them.22−24,38 Accordingly, in addition to the local concentration of functional groups, the wetting properties of GO may also depend on the distribution 3574
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Figure 3. Schematic plot of a droplet on a wrinkled surface. For a droplet with the same volume, there is an extra liquid−vapor interface (among A2, A3, and air; the dashed line) if the contact line resides in the valleys instead of the peaks. For wrinkled GO sheets, the WCA is measured with respect to the plane consisting of peaks (i.e., for A0).
Figure 4. Simulation snapshots of a water droplet on wrinkled graphene (a, b) and GO (c−h) for a = 0.5 nm, λ = 7.93 (a, c), a = 0.5 and 3.0 nm (b, d), and a = 0.1 nm, λ = 3.9, 2.8, 2.1, 1.0 nm (e−h).
patches of oxidized regions in GO could effectively control the spreading behavior of the water droplet on it. Morphological Corrugation. The morphology of a GO sheet is nonplanar because functionalization breaks its sp2 intrasheet C−C bonding nature. Ripples or wrinkles are widely identified in experimental characterization.3,40 The effect of morphological corrugation on the wetting behavior of GO is studied here by both theoretical models and MD simulations. Here, the concentration c of hydroxyl groups is set to be 10%, which is required to stabilize the system with a corrugation wavelength of only a few nanometers. We model the wrinkled profiles of graphene or GO as z = a sin by in the yz plane, where a is the corrugation amplitude, b is the wavenumber 2π/λ, and λ is the wavelength. In this study, we tune a and b (or λ) with respect to the following discussions. We now consider a water droplet with lateral span l covering multiple wrinkle wavelengths (i.e., l = nλ where n is an integer) and explore its spreading behavior. A schematic plot of the model is shown in Figure 3 for a water droplet with size comparable to the wrinkles. In this situation, the droplet prefers to reside in the valley to reduce the liquid−vapor interfacial energy because there is an extra interface (dashed lines in Figure 3) when the contact line of a droplet is located at the peaks. The results in Figure 4a−d show that for both graphene and GO with a = 0.5 nm the droplet is constrained by the corrugation and the relation l = nλ still holds, whereas for a = 0.1 nm (Figure 4e−h) the constraint is weakened and the droplet spreads with different values of l that depend on λ. The morphological corrugation thus must be included in the prediction of WCAs, which is absent in eq 2. For wrinkled GO sheets, the WCA is measured with respect to the plane consisting of peaks in wrinkled GOs. Equation 2 can be modified by including the water content in the valleys (A1 in Figure 3) as
of these regions. We explore this effect by arranging discrete hydroxyl functionalized regions in a quasi-2D model (Figure 2). Within the oxidized region, we keep the spatial distribution of hydroxyl groups as uniformly random with c = 20%. The droplet initially covers both the two oxidized regions and the pristine channel between them, and the distance between adjacent oxidized regions d or the width of the pristine channel is tuned subsequently in our MD simulation step by step (Figure 2b). The results summarized in Figure 2 show that by increasing d, the contact line moves with the oxidized patterns. The apparent WCA θc,a decreases for d < 14.64 nm, whereas for d > 14.64 nm the droplet detaches from hydroxyl groups and the WCA converges to a constant value of ∼33°. This observation is due to the H bonds formed between hydroxyl groups and water that pins the contact line.39 We also study the effect of hydroxyl groups under the cover of a droplet by constructing an oxidized region that is narrower than the lateral span of a droplet (Figure 2c). By tuning the oxidized pattern, we find that the droplet profile remains unchanged. This pinning effect plays an important role in defining the wetting behavior of GO and well explains our simulation results that only the chemical functionalization close to the contact line of the droplet contributes to the wetting. By assuming the water volume to remain a constant (πR02 per unit width) and the shape of droplet on surface to be circular (i.e., the volume is d2(θc,a − sin θc,a cos θc,a)/4 sin2 θc,a, we can derive an equation relating the WCA to d 4π sin 2 θc,a d2 = (θc,a − sin θc,a cos θc,a) R 02
(2)
where R0 is the cross-sectional radius of the initial cylindrical droplet that is isolated from the surface. The prediction by eq 2 agrees well with our simulation results in Figure 2. With this minimal quasi-2D model, one could expect that the discrete 3575
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containing groups that impede contact line movement (Figure 5c).39 The discrete nature of allowable wetting states of water on wrinkled GO suggests an abrupt transition of the lateral span of a droplet while its size is changed. When water is added to the droplet, the WCA will thus increase first and then decreases after the contact line crosses an energy barrier and reaches another peak. To verify this argument, we perform additional simulations with the water content in the droplet gently increased, with λ = 1.03 nm and a = 0.3 nm. The simulation results in Figure 6 shows that as the number of water molecules in the droplet increases from 1753 to 1819, the lateral span of the water droplet jumps from 9λ to 10λ.
(3)
Following this equation, we calculate WCAs for a water droplet spanning a length of nλ and plot them in Figure 5a,b as open
Figure 6. Transition of lateral span (open circles) of a water droplet on corrugated GO by adding water content. The parameters for the MD simulation results are λ = 1.03 nm and a = 0.3 nm.
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DISCUSSION Wetting properties characterize the interfacial interaction between liquid and solid, which help us to understand capillary and flow phenomena on the microscopic scale.7,41 In a recent study of flow between pristine graphene regions in GO, a capillary pressure of ∼500 bar was predicted at an interlayer distance of 0.6 nm.7 The WCA measured in their work for graphene is 107°, indicating a much weaker interfacial adhesion than that for the water−GO interface. Thus, on the basis of our MD simulation results here, an enhanced capillary force could be expected for water constrained between GO sheets. The pinning force induced by the oxidized groups could impede water flow in the channel, reducing the interfacial boundary slip and flow enhancement as a result of the electrostatic interaction and H bond formed with water molecules. Moreover, the pattered nature of oxidized groups on GO allows the formation of discrete capillaries that could act as spacers to tune the interlayer distance between neighboring GO sheets.7,22−24 A designed pattern of oxidized regions on graphene could be used to control its apparent WCA. The pinning force induced by the functional groups controls the contact line of the droplet and thus the wetted area. This mechanism could be combined with the coffee-ring effect to deposit particles on desired patterns during liquid evaporation. By introducing a gradient in the spatial distribution of oxidized groups on GO, one could drive a droplet in the direction where c increases. These concepts could be further combined with the morphological effect to find interesting applications in nanoscale material assembly and functional devices.42−45 Although the continuum wetting model can well explain our simulation results on the nanoscale, our MD simulations are limited by the length scale it
Figure 5. (a) Apparent WCAs of a droplet containing 1588 water molecules on corrugated GO with a = 0.1 nm, c = 10%, and λ ranging from 1.03 to 3.93 nm. Both allowable values predicted by eq 3 and those measured in MD simulations are plotted as open (thin and thick) and filled squares, respectively. The symbols for simulation results and theoretical predictions for the occupied states (thick open squares) are linked by lines. The dashed line shows the apparent WCA of flat GO. (b) The same as for plot a but with 3493 water molecules for a larger amplitude a = 0.3 nm and concentrations of c = 5% (blue) and 10% (red). (c) Snapshots of a water droplet on GO with λ = 1.79 nm. The contact line resides at peaks on the wrinkle with deviations caused by the local pinning effect, as can be seen from the positions of the contact line (p) and its adjacent peaks (I−III).
symbols in comparison to our MD simulation results (filled symbols). The WCA values are higher than those for flat GOs (dashed lines) and oscillate with λ (Figure 5a). This observation is confirmed by another set of simulations with a = 0.3 nm and c = 5 and 10% (Figure 5b) and suggests that at a fixed wavelength there exists a set of quantized wetting states defined by l = nλ for the droplet to spread on a wrinkled GO sheet, which is constrained by a local energy barrier defined by the water content and strength of the pinning effect. The MD results in general agree with theoretical predictions from eq 3 with a slight deviation caused by the pinning effect by oxygen3576
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captures. The pinning effect could be weakened for larger droplets, and more complicated microstructures such as crumples25 may further modulate the wetting and flow phenomena at interfaces with GO.
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CONCLUSIONS We have explored the wetting properties of graphene oxide by combining molecular dynamics simulations and theoretical analysis. The oxygen-containing groups on graphene lead to hydrophilism, and the water contact angle decreases with its concentration c, featuring a transition at c ≈ 11%. The oxidized patterns and wrinkles formed naturally in graphene oxide modify the wetting and spreading behaviors remarkably through the pinning effect and quantized wetting states as revealed here. These findings characterize the wetting properties of graphene oxide and identify opportunities in engineering capillary, flow, and wetting phenomena on the surfaces and interfaces of graphene oxide-derived materials.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China through grants 11222217 and 11002079 and Tsinghua University Initiative Scientific Research Program 2011Z02174. The simulations were performed on the Explorer 100 cluster system at Tsinghua National Laboratory for Information Science and Technology.
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