Article pubs.acs.org/Langmuir
Microwetting of Supported Graphene on Hydrophobic Surfaces Revealed by Polymerized Interfacial Femtodroplets Shuhua Peng,†,‡ Detlef Lohse,§ and Xuehua Zhang*,†,‡,∥ †
Department of Chemical and Biomolecular Engineering and ‡School of Chemistry, University of Melbourne, Parkville, VIC 3010, Australia § Physics of Fluids Group, Department of Science & Technology, Mesa+ Institute, and J. M. Burgers Centre for Fluid Dynamics, University of Twente, 7500 AE Enschede, The Netherlands ∥ School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne, VIC 3001, Australia ABSTRACT: Understanding the wettability of graphene is the crucial step toward the design and control of graphene-based surface in contact with liquids. In this work, the static microwettability of a supported single layer graphene (SLG) immersed in water or alcoholic aqueous solutions is revealed by the morphological characterization of the polymerized interfacial femtoliter droplets. As expected, the contact angle of the femtoliter droplets on the SLG in water is in between that on the underlying silanized silicon and that on graphite (HOPG). However, the wettability of femtoliter droplets on the SLG demonstrates a unique dependence on the compositions of the surrounding liquid medium: Their contact angle on SLG becomes much larger than that on both graphite and on silanized silicon, once short-chain alcohol molecules are present in the surrounding medium. To account for this finding, we hypothesize two scenarios to rationalize the effect of alcohol on the microwettability on SLG. The understanding elucidated in this study may allow for improved control of the interaction between graphene and the surrounding liquid environment and facilitate applications in which graphene is in contact with liquids, such as in microfluidics and in lab-on-chip systems.
■
INTRODUCTION
So far the wetting properties of a micron or submicron scale on a graphene-covered substrate remain unclear, although understanding of the microwetting behavior of droplets is crucial to control interfacial events. Static microwettability is frequently revealed by the morphology of microscopic droplets. Among several techniques for the morphological characterization of droplets, an approach developed in our recent work is to produce droplets with a volume of less than 10−13 L, i.e., 100 fL (femtodroplets in brief), by a solvent exchange technique and then convert the femtodroplets at the solid−liquid interface to polymeric lenses by in situ polymerization. 10 The morphological features of the polymeric lenses are closely related to the wetting properties of the femtodroplets of the precursor monomer at the interface.10,11 It has been shown that the contact angle of a microscopic droplet is often different from their macroscopic counterparts.12 The contact angle of a macroscopic drop at thermodynamic equilibrium is defined by Young’s equation. As the drop size reduces to the microscopic scale, Seemann et al. revealed the diverse wetting morphologies on microstructured surfaces from polymer liquid morphology that were “frozen” by lowering the
A single layer of graphene supported by a solid substrate represents a novel material that possesses fascinating properties due to the top surface of atomically thin graphene and the presence of the underlying substrate. The intense research interest in graphene-covered surfaces arises due to their extraordinary electrical, thermal, and mechanical properties with the wide range of potential applications of graphene-based materials in energy conversion, environmental process, and biomedicine.1−3 Wettability is a phenomenon that dictates various fundamental and applied processes, such as the adsorption of biomolecules or cells, condensation and growth of liquid droplets, heterogeneous catalysis, and corrosion. The wettability of a supported single layer graphene (SLG) has been explored on a macroscopic scale.4−8 Recently it was argued that a monolayer of graphene was sufficient to shield off the majority of the interactions between water and the underlying substrates and hence to dictate the wettability of the interface.9 On the contrary, Rafiee et al. reported “wetting transparency”,7 meaning that a monolayer of graphene does not alter the wetting behavior of the underlying substrate, whereas Shih et al. demonstrated that graphene is not entirely “wetting transparent”.4,6 © XXXX American Chemical Society
Received: June 11, 2014 Revised: July 29, 2014
A
dx.doi.org/10.1021/la5022774 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
temperature below the glass transition temperature.13 Checco et al. demonstrated that the contact angle of submicron droplets exhibits nonlinear dependence on their droplet size, which cannot be solely attributed to the effect of line tension.14,15 The microdroplet morphology showed extreme sensitivity to the heterogeneities of the substrate on molecular scale, which was described by the weak heterogeneity theory.14−16 The effect of line tension is only pronounced for nanoscopic droplets16,17 and is too weak to account for the contact angle of our microscopic droplets. In this study, we explore the microwettability of femtodroplets on SLG supported by a hydrophobic surface and multilayer graphene immersed in a liquid phase. The results reveal that the microwetting on SLG is different from both the underlying hydrophobic substrate and graphite. The SLG is not completely “wetting transparent” at the microscopic scale. Furthermore, the microwettability on SLG shows unique dependence on the compositions in the surrounding liquid phase. This intriguing microwetting property may be potentially utilized to control the interfacial behavior of supported single layer of graphene.
■
Figure 1. Schematic diagram showing the procedure of the formation and polymerization of femtodroplets on OTS-Si and SLG simultaneously in water: (a) 50% ethanol aqueous solution saturated with the precursor monomer (solution 1) was injected into the fluid cell containing OTS-Si substrate which was partially coated with SLG. (b) Femtodroplets of the precursors were produced at the interface during the solvent exchange. (c) The morphological features of the femtodroplets were fixed by in situ photopolymerization. (d) Polymerized femtodroplets were ready for characterization at ambient conditions.
EXPERIMENTAL SECTION
Chemicals and Materials. SLG coated with about a 500 nm PMMA layer was purchased from ACS Material LLC (Trivial Transfer Graphene). One side polished silicon wafers were from Mitsubishi Silicon. Highly oriented pyrolytic graphite (HOPG) (ZYB, SPI) was freshly cleaved immediately before use. Octadecyltrimethylchlorosilane (OTS, >90%), monomer (1,6-hexanediol diacrylate), and photoinitiator (2-hydroxy-2-methylpropiophenone) were from Sigma. Organic solvents like chloroform (AnalaR), toluene (AnalaR), and ethanol (100%) were from Merck Pty Ltd. All chemicals were used without further purification unless otherwise specified. Preparation of Hydrophobic OTS-Silicon (OTS-Si) Substrates. The hydrophobic surface is silicon wafers with an adsorbed layer of OTS.12,18 Polished silicon wafers were cleaned in piranha solution [H2SO4 (70%):H2O2 (30%)] at 75 °C for 20 min. It is important to minimize exposure of OTS to water as this causes polymerization and consequently a heterogeneous interface. All glassware used for the solution was dried for 2 h under 120 °C in oven. The silicon was dried at 120 °C for 1.5 h and soaked in 0.5 vol % OTS in toluene for about 2 h in a sealed dry container at room temperature. After 12 h, the OTS-Si substrates were rinsed with chloroform, sonicated in toluene and ethanol, dried with nitrogen, and then stored in a clean container. Before use, the OTS-Si was cleaned in ultrasound in ethanol and water. Transfer of SLG onto OTS-Si Substrate. SLG was transferred onto OTS-Si surface by following the procedures reported in the literature.19 SLG coated with PMMA layer was directly immersed into Milli-Q water to obtain transparent floating SLG/PMMA stack on the surface of water. A small amount of ethanol (about 1 mL) was added into water (20 mL) to improve the wetting of the liquid phase and help the spreading of SLG on OTS-Si. The SLG/PMMA film was placed onto OTS substrate while positioning the film with a needle. After drying it under vacuum for several hours, the SLG/PMMA film was heated at 180 °C in air for over 30 min to enable the flattening of SLG on the OTS-Si surface. The PMMA was finally removed with an acetone bath overnight. Raman spectroscopy (Horiba Xplora system, Ar+ laser, λ = 532 nm, ×50 magnification objective lens) with an Olympus BX41 microscope was employed for the characterization of the transferred SLG. Characterization of Polymerized Femtodroplets. The procedure of the formation and polymerization of femtodroplets on OTS-Si and SLG simultaneously by solvent exchange method was illustrated in Figure 1. The solvent exchange was carried out inside a homemade fluid cell with a glass window and two outlets for the solvent exchange.
The substrate was immobilized on the bottom of the cell, facing up close to the window. A 50% ethanol/water solution was used as the first solution (solution 1) that was saturated with the precursors required for photopolymerization including monomer and photoinitiator. The femtodroplets of the precursors were produced at the interface of the solid and aqueous solution after the first solution was exchanged by pure water or low concentration ethanol aqueous solutions (5% and 10%, respectively). The cell was then placed under UV light. After about 10 min of polymerization, the surface was then taken out for the morphological characterization. The polymerized femtodroplets with photopolymerized microlenses prepared in water or the water−alcohol mixtures were imaged by using a MFP-3D atomic force microscope (Asylum Research, Santa Barbara, CA). The cross-sectional profiles of the microlenses were extracted from the AFM topography images.
■
RESULTS Characterizations of Surfaces and Femtodroplets. The characterization by Raman spectrum was conducted to ensure the single layer and the high quality of the transferred graphene. Figure 2a shows a representative Raman spectrum with the typical peaks of SLG supported by OTS-Si. The spectrum includes a 2D-band located at ∼2677 cm−1, a G-band at ∼1584 cm−1, and a D-band at ∼1340 cm−1. The 2D peak can be well fitted with a single Lorentzian with a full width at halfmaximum (fwhm) of 37.8 cm−1, which was consistent with spectroscopic features of single-layer graphene previously reported.8 Disorder and defects in SLG are quantitatively analyzed by the intensity ratio between the disorder-induced D-band and the Raman-allowed G-band, i.e., ID/IG. A higher ID/IG value indicates more defects in SLG.20,21 It is generally accepted that a ratio of 0.3 and lower represents a high quality of the single B
dx.doi.org/10.1021/la5022774 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Figure 2. Raman spectra of single layer graphene (SLG) on the hydrophobic OTS-Si surface. (a) Representative Raman spectrum on SLG. (b) Raman intensity ratio ID/IG of three SLG samples; ID and IG are the intensities of peaks D and G, respectively. Five different locations were measured on three samples labeled as square, circle, and triangle.
layer of graphene.22 Raman spectra were measured on several random spots in the clean SLG sample, and a range of ID/IG values are plotted in Figure 2b. All the obtained ID/IG values were smaller than 0.3 for the three different samples, demonstrating the homogeneity and continuity of the whole transferred SLG film. The characterization by AFM shows that the root-meansquare (RMS) roughness of 5 × 5 μm2 areas of the bare OTS-Si substrate was 0.4 nm and of SLG was 2.6 nm. Such roughness of transferred graphene is comparable to the value reported in the literature,9 confirming the high quality of our transferred SLG. The polymerized femtodroplets on OTS-Si and SLG surfaces were produced by solvent exchange and subsequent in situ photopolymerization as described above. More details can be found in our previous work.10,11 The whole process is performed with a substrate partly consisting of OTS-Si and partly of SLG on OTS-Si. Femtodroplets of the monomer precursors on OTS-Si and SLG were simultaneously produced by one single solvent exchange process, which eliminates any effects from variations in the formation process on the droplet morphology. The obtained polymerized microlenses were first examined by optical microscope in a reflection mode and then imaged by AFM. The difference in the morphology of femtodroplets on SLG and on underlying OTS-Si can be unambiguously attributed to the intrinsic difference in the microwettability of these two surfaces, as the droplets were produced and fixed in the same process and thus under exactly identical conditions. In Pure Water. Figure 3 shows the optical images of the polymerized femtodroplets on OTS-Si (a, b) and on SLG (a, c) obtained in pure water. The boundary dividing OTS-Si (left) and SLG on OTS-Si (right) is clearly visible in Figure 3a. The circular, symmetric interference patterns from the polymerized femtodroplets on OTS-Si are Newton rings due to the interference between the reflected light from the surface of the nontransparent supporting substrate and the curved top surface of polymerized droplets. The spacing and color of these rings are determined by the morphology, radius of curvature of the droplet for given materials of droplets, and the substrate.23 The symmetric rings demonstrate that the shape of the polymerized femtodroplet is spherical cap, i.e., like a microscopic lens.
Figure 3. (a) Optical image of microlenses on OTS-Si (left half) and on SLG (right half). The red arrows point to two lenses with the same lateral diameter. Higher magnification images of microlenses on OTSSi surface (b), SLG surface (c), and HOPG (d). Scale bar: 50 μm.
The optical images of the same resolution in Figure 3a can resolve the Newton rings on OTS-Si, but not on SLG. The more densely spaced rings on SLG are only detected with a higher resolution image in Figure 3c, reflecting that femtodroplets on SLG are more curved than that on OTS-Si. In addition, some of Newton rings are distorted on SLG, indicating presence of heterogeneities on the surface. As a comparison, femtodroplets were also produced and cured on HOPG (a model for bulk graphene) in Figure 3d. AFM images of the polymerized femtodroplets on the three substrates OTS-Si, SLG, and HOPG (Figure 4a−c) show that their lateral diameters range from 5 to 20 μm and their heights from 0.3 to 1 μm. Their equivalent volume is from 1 to 100 fL. The representative cross-sectional profiles of a 10 μm wide femtodroplet show that the height of the droplet on OTS-Si is 540 nm, slightly shorter than typically 590 nm on SLG or 560 nm on HOPG. The plot of the height H versus the lateral diameter L in Figure 4d shows that the height of femtodroplets is increasing with increasing their lateral diameter on all substrates. For the full range of the sizes, the femtodroplets on SLG are noticeably higher than those on OTS-Si, but slightly lower than on HOPG. The data can be fitted with a linear relation H = cL with slope c = 0.0368 on OTS-Si, 0.0513 on SLG, and 0.0567 on HOPG. Such linear behavior corresponds to a constant contact angle of the respective droplets with assumed spherical cap shape, irrespective of their size. C
dx.doi.org/10.1021/la5022774 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Figure 4. Morphology of femtodroplets that were produced and polymerized at the solid−water interfaces: (a)−(c) show AFM height images and sectional analysis of the representative polymerized femtodroplets on different substrates (a, OTS-Si; b, SLG; c, HOPG); (d) and (e) show plots of height and the microscopic contact angle versus the lateral diameter of the droplets on three substrates, and the dotted lines in (e) indicate the contact angles calculated from the slope of linear fitting in (d); (f) shows a sketch of the geometry of the femtodroplet, giving the used notation: L is the lateral diameter of the femtodroplet, H is height, R is radius of curvature, and θ is the contact angle on the droplet side.
respectively, to alter the wettability of the droplet on the surfaces by changing the interfacial tension at the droplet surface and that between the bulk liquid phase and solid surface. The microwettability in the mixtures of water and alcohol is provided in Figure 5, showing the AFM images and the representative cross-sectional profiles of the femtodroplets that are polymerized in aqueous ethanol solutions. The representative cross-sectional profiles show that a 9 μm wide droplet in 5% ethanol aqueous solution is typically 330 nm high on OTSSi and 390 nm on HOPG, which are about half of the droplet on SLG (580 nm). This significant difference on SLG was also observed in 10% ethanol solution shown in Figure 5d−f: a 10 μm wide droplet is typically 280 nm high on OTS-Si and 330 nm on HOPG, only less than half of a droplet on SLG (670 nm). This clearly shows that the femtodroplets wet SLG the least once ethanol is present in the bulk liquid phase. The plots in Figure 6 show the full range of height and microscopic contact angle as a function of lateral diameter. The contact angles of femtodroplets on OTS-Si and on HOPG are consistently smaller than those on SLG in both 5% and 10% ethanol solutions. The averaged microscopic contact angle is 5.5° on OTS-Si and 5° on HOPG, as shown in Figure 7, which is only about one-third of 15° on SLG.
This microscopic contact angle is used to quantify the morphology of femtodroplets, i.e., the static microwettability, on three substrates. Given a spherical-cap shape, the contact angle θ of the droplets directly follows from the lateral diameter L and the droplet height H (both taken from AFM images): L /2 tan θ = = R−h
H L 1 4
−
H 2 L
( )
=
c 1 4
− c2
(1)
The above three slopes H/L = c = 0.0368, 0.0513, and 0.0567 thus imply contact angles of 8.4°, 11.7°, and 12.9°, respectively. However, when looking in more detail, we find a possible slight dependence of θ on L in particular for HOPG, which is shown in Figure 4e. In any case, the contact angle for SLG lies in between that for OTS-Si (8°−9°) and that for HOPG (10°− 13°), reflecting that the microwettability of droplets on SLG lies in between HOPG and OTS-Si with the microscopic contact angle on SLG lower than that on OTS-Si. In Water−Alcohol Mixtures. This situation however dramatically changes when we use mixtures as the surrounding liquid phase. Indeed, the very same approach applied in this work to characterize the static microwettability allows us to demonstrate the significant effect of the chemical composition of the liquid phase. A simple organic solvent, ethanol, was added into the water at volume ratios of 5% and 10%, D
dx.doi.org/10.1021/la5022774 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Figure 5. AFM height images and representative cross-sectional profiles of polymerized droplets that were produced and polymerized in 5% (a−c) or 10% (d−f) ethanol aqueous solutions, respectively. The substrates were OTS-Si (a, d), SLG (b, e), and HOPG (c, f).
We finally compared the contact angle of femtodroplets in tert-butanol aqueous solution. As shown in Figure 8 for the femtodroplets in 5% tert-butanol, the same effects were observed: the contact angle of femtodroplets was much higher on SLG than on both OTS-Si and HOPG, demonstrating the general trend that the femtodroplets wet the least on SLG in the presence of short chain alcohol in the liquid phase.
wetting properties of the supported graphene. As comparison, the physical roughness on HOPG and on SLG is very different, whereas the microscopic contact angles on them in water were just slightly different. This reflects that the carbon nature of the top surface plays a dominant role in the microwettability. The result that microscopic contact angles on SLG are different from those on OTS-Si in water clearly shows that the femtodroplets can “see” the materials under graphene, as expected. Such effect from the underlying materials on wetting is consistent with the report that the interfacial adhesion is affected by the subsurface properties of layered solids25 and with the theoretical analysis that graphene can partly screen the molecular interactions from the underlying substrate.6 Why does the microscopic contact angle on SLG become so large in the presence of short-chain alcohol in surrounding liquid phase? We speculate two scenarios to account for the microwettability on SLG. The first may be due to some creeping of the small alcohol molecules between graphene and the supporting substrate. Indeed, several papers have revealed
■
DISCUSSION The above results clearly show that single layer graphene influences the microwettability of the surface in contact with water. Such influence may originate from two following aspects. On the one hand, graphene has rendered the chemical composition of hydrocarbon chain of OTS molecules to carbon. This may smoothen out any possible physical and chemical heterogeneities of the self-assembled monolayer of silane, as observed on rough surfaces.24 On the other hand, transferred SLG itself may introduce chemical and physical heterogeneities of various sources, which could complicate the E
dx.doi.org/10.1021/la5022774 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Figure 6. Plot of height and microscopic contact angle versus the lateral diameter of polymerized femtodroplets on different surfaces. The droplets were produced and polymerized in 5% (a, b) and 10% (c, d) ethanol aqueous solution. The dotted lines in (b) and (d) indicate the contact angles calculated from the linear fitting in (a) and (c), respectively.
only occurs on SLG, but not on HOPG. An alternative scenario would be the preferential absorption of alcohol on SLG, but not on HOPG. Such preferential adsorption of ethanol on graphene (with the C2H5− group toward the graphene and the −OH group toward the liquid) has been indicated theoretically,29 as well as the application of graphene as adsorbents to separate alcohols from water.30−32 Either scenario would cause less affinity of oil droplets to the surface, so that the femtodroplets are more strongly curved on SLG than on OTS-Si or HOPG, also explaining the increase of the microscopic contact angle of the oil droplet on SLG when changing water to aqueous ethanol solutions.
■
Figure 7. Microscopic contact angle on three surfaces as a function of ethanol concentration in the liquid phase.
CONCLUSIONS In summary, the static microwettability of a single layer of supported graphene is revealed by the morphological characteristics of the polymerized interfacial femtodroplets. The microwettability on SLG demonstrates a unique dependence on the chemical composition of the surrounding liquid phase. In water, the contact angle of femtodroplets on a SLG lies in between that on the supporting silanized silicon and that on graphite. The femtodroplets “see” both graphene and the underlying substrate. In contrast, the contact angle of monomer femtodroplets on SLG in aqueous ethanol solution is much larger than that for both on the silanized Si and the HOPG case, suggesting that the ethanol may creep under the SLG or that there is a preferential adsorption of the alcohol on graphene, but not on HOPG. The findings in this study will improve the fundamental understanding of intriguing interfacial properties of graphene supported by a substrate as well as device design in which graphene is in contact with a liquid environment. A final note is that this work also demonstrates the flexibility of our protocol for the characterization of wettability on microscopic scale. The surrounding liquid media can be conveniently switched from one to the other. The complications from the microdroplet formation dynamics are overcome by production and fixation of femtodroplets simultaneously
Figure 8. Height versus lateral diameter of polymerized femtodroplets produced in 5% aqueous tert-butanol solutions on OTS-Si and SLG. The results of polymerization in 5% aqueous ethanol solution are also plotted as comparison.
the entrainment of molecules between a solid and a single layer of graphene.26−28 In our three substrates, compared to the van der Waals interactions between graphene layers on HOPG, the interactions between OTS layer on silicon and graphene were weaker, which might imply that the invasion of small molecules F
dx.doi.org/10.1021/la5022774 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
(17) Berg, J. K.; Weber, C. M.; Riegler, H. Impact of negative line tension on the shape of nanometer-size sessile droplets. Phys. Rev. Lett. 2010, 105. (18) Zhang, X. H.; Ducker, W. Interfacial oil droplets. Langmuir 2008, 24, 110−115. (19) Suk, J. W.; Kitt, A.; Magnuson, C. W.; Hao, Y. F.; Ahmed, S.; An, J. H.; Swan, A. K.; Goldberg, B. B.; Ruoff, R. S. Transfer of CVDgrown monolayer graphene onto arbitrary substrates. ACS Nano 2011, 5, 6916−6924. (20) Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R. Perspectives on carbon nanotubes and graphene Raman spectroscopy. Nano Lett. 2010, 10, 751−758. (21) Dong, J.; Yao, Z.; Yang, T.; Jiang, L.; Shen, C. Control of superhydrophilic and superhydrophobic graphene interface. Sci. Rep. 2013, 3. (22) Liang, X. L.; Sperling, B. A.; Calizo, I.; Cheng, G. J.; Hacker, C. A.; Zhang, Q.; Obeng, Y.; Yan, K.; Peng, H. L.; Li, Q. L.; Zhu, X. X.; Yuan, H.; Walker, A. R. H.; Liu, Z. F.; Peng, L. M.; Richter, C. A. Toward clean and crackless transfer of graphene. ACS Nano 2011, 5, 9144−9153. (23) Illueca, C.; Vazquez, C.; Hernandez, C.; Viqueira, V. The use of Newton’s rings for characterising ophthalmic lenses. Ophthalmic Physiol. Opt. 1998, 18, 360−371. (24) Singh, E.; Thomas, A. V.; Mukherjee, R.; Mi, X.; Houshmand, F.; Peles, Y.; Shi, Y. F.; Koratkar, N. Graphene drape minimizes the pinning and hysteresis of water drops on nanotextured rough surfaces. ACS Nano 2013, 7, 3512−3521. (25) Loskill, P.; Hahl, H.; Faidt, T.; Grandthyll, S.; Muller, F.; Jacobs, K. Is adhesion superficial? Silicon wafers as a model system to study van der Waals interactions. Adv. Colloid Interface Sci. 2012, 179, 107− 113. (26) Xu, K.; Cao, P.; Heath, J. R. Graphene visualizes the first water adlayers on mica at ambient conditions. Science 2010, 329, 1188− 1191. (27) Mu, R.; Fu, Q.; Jin, L.; Yu, L.; Fang, G.; Tan, D.; Bao, X. Visualizing chemical reactions confined under graphene. Angew. Chem., Int. Ed. 2012, 51, 4856−4859. (28) Cheng, M.; Wang, D.; Sun, Z.; Zhao, J.; Yang, R.; Wang, G.; Yang, W.; Xie, G.; Zhang, J.; Chen, P.; He, C.; Liu, D.; Xu, L.; Shi, D.; Wang, E.; Zhang, G. A route toward digital manipulation of water nanodroplets on surfaces. ACS Nano 2014, 8, 3955−3960. (29) Lazar, P.; Karlicky, F.; Jurecka, P.; Kocman, M.; Otyepkova, E.; Safarova, K.; Otyepka, M. Adsorption of small organic molecules on graphene. J. Am. Chem. Soc. 2013, 135, 6372−6377. (30) Bi, H.; Xie, X.; Yin, K.; Zhou, Y.; Wan, S.; He, L.; Xu, F.; Banhart, F.; Sun, L.; Ruoff, R. S. Spongy graphene as a highly efficient and recyclable sorbent for oils and organic solvents. Adv. Funct. Mater. 2012, 22, 4421−4425. (31) Jang, A. R.; Jeon, E. K.; Kang, D.; Kim, G.; Kim, B. S.; Kang, D. J.; Shin, H. S. Reversibly light-modulated dirac point of graphene functionalized with spiropyran. ACS Nano 2012, 6, 9207−9213. (32) Joo, P.; Kim, B. J.; Jeon, E. K.; Cho, J. H.; Kim, B. S. Optical switching of the Dirac point in graphene multilayer field-effect transistors functionalized with spiropyran. Chem. Commun. 2012, 48, 10978−10980. (33) Ma, A.; Xu, J.; Zhang, X.; Zhang, B.; Wang, D.; Xu, H. Interfacial nanodroplets guided construction of hierarchical Au, Au-Pt, and Au-Pd particles as excellent catalysts. Sci. Rep. 2014, 4.
under identical experimental conditions. This approach has the potential to be applied for the characterization of the crystallographic facets of particles,33 microfibers, or the local wettability of micropatterns on a surface.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (X.H.Z.). Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
REFERENCES
X.H.Z. and S.H.P. gratefully acknowledge the support from the ARC Future Fellowship Scheme (FFT120100473) and a Postdoctoral Fellowship from Australian Renewable Energy Agency (ARENA). D.L. acknowledges support from an ERC Advanced Grant and from FOM.
(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric field effect in atomically thin carbon films. Science 2004, 306, 666−669. (2) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008, 321, 385−388. (3) Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. The electronic properties of graphene. Rev. Mod. Phys. 2009, 81, 109−162. (4) Shih, C. J.; Wang, Q. H.; Lin, S. C.; Park, K. C.; Jin, Z.; Strano, M. S.; Blankschtein, D. Breakdown in the wetting transparency of graphene. Phys. Rev. Lett. 2012, 109. (5) Xu, K.; Heath, J. R. Wetting contact with what? Nat. Mater. 2013, 12, 872−873. (6) Shih, C. J.; Strano, M. S.; Blankschtein, D. Wetting translucency of graphene. Nat. Mater. 2013, 12, 866−869. (7) Rafiee, J.; Mi, X.; Gullapalli, H.; Thomas, A. V.; Yavari, F.; Shi, Y. F.; Ajayan, P. M.; Koratkar, N. A. Wetting transparency of graphene. Nat. Mater. 2012, 11, 217−222. (8) Li, Z. T.; Wang, Y. J.; Kozbial, A.; Shenoy, G.; Zhou, F.; McGinley, R.; Ireland, P.; Morganstein, B.; Kunkel, A.; Surwade, S. P.; Li, L.; Liu, H. T. Effect of airborne contaminants on the wettability of supported graphene and graphite. Nat. Mater. 2013, 12, 925−931. (9) Raj, R.; Maroo, S. C.; Wang, E. N. Wettability of graphene. Nano Lett. 2013, 13, 1509−1515. (10) Zhang, X. H.; Ren, J. M.; Yang, H. J.; He, Y. H.; Tan, J. F.; Qiao, G. G. From transient nanodroplets to permanent nanolenses. Soft Matter 2012, 8, 4314−4317. (11) Yang, H.; Peng, S.; Hao, X.; Smith, T. A.; Qiao, G. G.; Zhang, X. Surfactant-mediated formation of polymeric microlenses from interfacial microdroplets. Soft Matter 2014, 10, 957−964. (12) Zhang, X. H.; Quinn, A.; Ducker, W. A. Nanobubbles at the interface between water and a hydrophobic solid. Langmuir 2008, 24, 4756−4764. (13) Seemann, R.; Brinkmann, M.; Kramer, E. J.; Lange, F. F.; Lipowsky, R. Wetting morphologies at microstructured surfaces. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 1848−1852. (14) Checco, A.; Schollmeyer, H.; Daillant, J.; Guenoun, P.; Boukherroub, R. Nanoscale wettability of self-assembled monolayers investigated by noncontact atomic force microscopy. Langmuir 2006, 22, 116−126. (15) Checco, A.; Guenoun, P.; Daillant, J. Nonlinear dependence of the contact angle of nanodroplets on contact line curvature. Phys. Rev. Lett. 2003, 91. (16) Quere, D. Surface wetting - Model droplets. Nat. Mater. 2004, 3, 79−80. G
dx.doi.org/10.1021/la5022774 | Langmuir XXXX, XXX, XXX−XXX
