oil repellency and work of adhesion of liquid droplets on graphene oxide and graphene surfaces

Surface & Coatings Technology 205 (2011) 4554–4561

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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t

Water/oil repellency and work of adhesion of liquid droplets on graphene oxide and graphene surfaces Chien-Te Hsieh ⁎, Wei-Yu Chen Department of Chemical Engineering and Materials Science, Yuan Ze Fuel Cell Center, Yuan Ze University, Taoyuan 320, Taiwan

a r t i c l e

i n f o

Article history: Received 25 October 2010 Accepted in revised form 28 March 2011 Available online 3 April 2011 Keywords: Graphene Graphene oxide Superhydrophobicity Work of adhesion Water/oil repellency

a b s t r a c t The study examines the water/ethylene glycol (EG) repellency of graphene and graphene oxide sheets prepared by the chemical exfoliation of natural graphite powders. The graphene nanosheets were produced by reducing graphene oxide with EG under microwave irradiation. The graphene sheets were assembled into a thin paper, and a facile fluorination was used to coat a thin fluorine layer over the graphene paper. The graphene oxide paper is generally hydrophilic, whereas without aid of any fluorination, the resulting graphene paper displays superhydrophobicity (contact angle: 150.1 ± 2.3°) and low fraction in contact with solid (12.2%). Such low solid fraction may be attributed to the air pocket trapped in (i) the interspaces between graphene powders and (ii) the flake-like voids between graphene sheets, referred to as the Cassie state. The EG repellency of graphene paper can be significantly improved by surface fluorination. Taking into account Young–Duprè's equation incorporated with the Cassie parameter, the Wad values of the graphene papers for water repellency were found to fall in the region of 9.62–12.5 mJ/m2. The low Wad value between the droplets and the graphene surface can be ascribed to the fact that porous graphene sheets offer an air cushion to repel the drop penetration, inducing the low work required for the movement of droplets on graphene paper. On the basis of the results, this study offers fundamentals on the water and EG repellency of graphene and graphene oxide surfaces. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Graphene-based material has attracted extensive attention from both experimental and theoretical scientific communities due to its extraordinary properties, such as high surface-area to volume ratio and electronic transport character [1–3]. Graphene is either a twodimensional (2-D) nanostructure of carbon arranged in a honeycomb network or an unrolled single-walled carbon nanotube [4]. The structure of graphene can be regarded as a counterpart of graphite with well-separated 2-D aromatic sheets composed of sp2-bonded carbon atoms. Generally, graphene sheets can be prepared by exfoliation of a bulk graphene crystal to a dispersion of individual atomic-layer graphene sheets, and the reassembling process results in layered nanosheet products [5]. It is recognized that the π-stacked graphene sheets can be exfoliated (i.e., exfoliation energy: 61 meV/C atom) by a manipulation of chemical functionalization [6–8]. One of the most popular methods is the Hummers method that consists of the oxidation of graphite powder in the presence of concentrated mineral acids and oxidizing agents, thus forming graphene oxide materials [9]. The resulting graphene oxide would provide a way to functionalize graphene sheets through adequate chemical reduction

⁎ Corresponding author. Tel.: + 886 3 4638800x2577; fax: + 886 3 4559373. E-mail address: [email protected] (C.-T. Hsieh). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.128

of exfoliation [10]. Accordingly, this development of fabrication procedure proves that the productivity of graphenes gradually approximates to a large-scale production. Recently, a number of research groups have devoted themselves to applying graphenes in a variety of technological applications including methanol oxidation [11], lithium-ion battery [2,5,12,13], solar cell [14], transparent conducting film [15], and so on. To inspect their applicability in technological fields, surface wettability of graphene sheets plays a vital role in determining the compatibility with the desired environment. For example, onedimensional carbon materials attached with noble nanoparticles (e.g., Pt or Pt-based alloys) have been considered as efficient electrochemical catalysts, showing high activity toward acid electrolyte [16,17]. Regarding the preparation of Pt-graphene hybrids for fuel cell application by chemical reduction route, the dispersion and bonding of nanocatalysts depend strongly on the hydrophilicity of graphenes, resulting from the ionic interaction between Pt ions and surface oxides (e.g., carboxylic groups) on graphene surface. Another example is that of the supercapacitor assembled with carbon-based electrodes, which also requires surface hydrophilicity toward hydration molecules, thus creating excess surface coverage available for the formation of a double layer [18]. Accordingly, knowing the water and oil repellencies of the graphene surface is the fundamental that provides useful information about its various potential applications. However, there are a few reports that focus on hydrophobic/hydrophilic

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behavior and on the analysis of adhesion work on graphene and graphene oxide surfaces. In this study, a facile approach was proposed to synthesize graphene papers using epoxy resin and nylon filter as binder and substrate respectively. Contact angles (CAs) of water and ethylene glycol (EG) droplets were used to examine their water and oil repellencies respectively. To thoroughly examine the surface repellency, the fluorinated graphene papers were compared according to the CAs and the work of adhesion (Wad) of water and oil droplets. The results shed some light on the water and oil repellency of the resultant graphene and graphene oxide papers, and how the fluorination of graphene affects hydrophobic behavior. 2. Experimental The procedure for preparing graphene oxide powders was based on a modified Hummer method. Initially, 5 g natural graphite (NG) and 2.5 g NaNO3 were poured into 115 mL concentrated H2SO4. The graphite slurry was placed in an ice bath for 2 h. The mean size of the graphite precursor ranged from 5 to 10 μm. Then, 15 g KMnO4 was slowly added into the graphite slurry, and the slurry was stirred by a magnetic bar for 2 h. The slurry was heated to 35 °C and kept at this temperature for 0.5 h. The graphite slurry was gradually diluted with distilled water (230 mL), and the slurry was then heated to 98 °C and kept at this temperature for 1 h. After that, 30 vol% H2O2 (15 mL) was poured into the slurry until the color of the mixture tended to become dark yellow. The as-prepared GO powders were re-dispersed in distilled water, and 1 vol% HCl solution was added into the aqueous mixture for replacing SO2− ions by Cl− ions. Finally, the as-prepared 4 graphite oxide powders were exfoliated to form graphene sheets in an ultrasonic bath for 0.5 h, thus giving a colloidal dispersion of graphene oxide sheets in water.

Fig. 2. FE-SEM images of the edges of (a) GN and (b) GN-F1 papers.

Fig. 1. FE-SEM images of GO sheets with (a) low and (b) high magnifications, and GN sheets with (a) low and (b) high magnifications.

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The graphene oxide papers were made by filtration of the resulting colloid through a nylon filter paper 47 mm in diameter and 0.45 μm in nominal pore size (Whatman), followed by air drying. To reinforce the adhesion, a spin coater was used to uniformly disperse dilute epoxy resin on the GO cake/filter paper composites. The epoxy resin was a liquid mixture that contained 5 wt.% epoxy resin and 95% toluene. The epoxy-containing solution was well coated over the graphene oxide cake, using a two-stage spin coating. The coating process consisted of (i) the first stage at 500 rpm for 5 s, and (ii) the second stage at 3000 rpm for 10 s. The graphene oxide paper was then peeled from the nylon filter after drying in air at 70 °C. The backside of graphene oxide paper was used for the CA measurement to avoid interference from the epoxy resin. The graphene oxide paper was designated to GO paper. The fabrication of graphene papers is very similar to that of the GO papers. The major difference was to prepare the graphene paper after chemically reducing the resulting graphene oxide powders. The asreceived graphene oxide powders (3 g) were then well mixed with the EG solution. The graphene oxide slurry, placed into a microwave oven, served as a reactor for the chemical reduction of graphene oxide under microwave irradiation. The microwave power and treated time were set at 720 W and 3 min, respectively. The microwave-assisted approach is capable of producing graphene sheets dispersion in EG solution. Similarly, the graphene papers were formed by the above cake-filter method, using epoxy resin and nylon filter as binder and substrate respectively. Here, the graphene sheet paper was nominated as GN paper. To lower their surface energies, a liquid mixture consisting of perfluoroalkyl methacrylic copolymer (Zonyl 8740, DuPont Co.) and distilled water (7/3 in v/v), was adopted to coat the GN papers using a

(002)

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NG (101)

(004)

(002)

(101)

GO

GN

10

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2θ (degree) Fig. 4. Typical XRD patterns of natural graphite, graphene oxide, and graphene samples. The peak intensity of NG powders has been divided by twenty.

spin coater. This recipe of the F-containing solution has been used elsewhere too [19,20]. The coating process involved the following steps: (i) the spinning speed of the first-stage coating process was set at 500 rpm for 5 s, and (ii) the second-stage speed was raised to 3000 rpm and maintained for 10 s. After that, the moisture was evaporated from the GN papers at 105 °C in an oven overnight, inducing a fluorocarbon coating on the surface of the GN papers. Two weight ratios of F-copolymer solution to the GN papers were set at 0.5:99.5 and 2:98, namely, GN-F1 and GN-F2 papers respectively. The morphology of graphene samples was characterized by using field-emission scanning electron spectroscope (FE-SEM, JEOL JSM5600) and high-resolution transmission electron microscope (HRTEM, JEOL, JEM-2100). The microstructures of graphene samples were characterized by Raman spectroscopy (Renishaw Micro-Raman spectrometer) and X-ray diffraction (XRD, Shimadzu Labx XRD-6000) spectroscopy, using Cu Kα radiation (λ = 0.15418 nm). The chemical

Intensity (a.u.)

G band

D band NG

GO GN 2000

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Raman shift Fig. 3. HR-TEM images of (a) graphere oxide and (b) graphene sheets.

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(cm-1)

Fig. 5. Raman spectra of natural graphite, graphene oxide, and graphene samples. The peak intensity of NG powders has been divided by twenty.

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composition and functional group distribution of the graphene were analyzed using X-ray photoelectron spectroscopy (XPS). The XP spectra were recorded with a Fison VG ESCA210 spectrometer and Mg Kα radiation. The deconvolution of the spectra was performed using a non-linear least squares fitting program with a symmetric Gaussian function. Two types of liquids, deionized water (surface tension: 72.3 mN/m) and EG (surface tension: 45.2 mN/m), were used to evaluate the water and oil repellency of the GN and GO papers. An optical CA meter was applied to measure the CAs of liquid droplets on the graphene papers. Each droplet was dropped onto the surface from a distance of 5 cm by vibrating the syringe. A 3 μL liquid drop was deposited on the GN surface using a micropipette. Five drops of water were placed at different locations on a horizontal carbon surface, and five readings were then taken. The derivation of the CAs measured in this study was within 2.4°. All CA measurements were performed at ambient temperature. 3. Results and discussion Fig. 1(a)–(d) shows the FE-SEM micrographs of the resulting graphene papers with low and high magnifications. The GN paper is composed of a number of loose graphene nanosheets generating fluffy agglomerates whereas the GO paper exhibits a dense structure consisting of curled graphene stacking sheets. The magnified views, as observed in Fig. 1(d), clearly show that the graphene nanosheets are of multilayer type with wrinkles and folds, forming a large number of nanovoids and nanocavities. This result demonstrates that the modified Hummer method is able to create two-dimensional nanosheets, originated from the exfoliation of NG precursor. The FE-SEM images for the edges of GN and GN-F1 papers are provided, as shown in Fig. 2(a) and (b), respectively. It has shown that both the graphene sheets adhere to filter paper, forming the composite papers. Basically, the sheets are well dispersed over the porous filter paper, thus creating 2-D network. We also observe that after the fluoropolymer coating, the F-coated graphene sheets still keep wrinkle morphology on the filter papers, generating roughened surfaces.

HR-TEM analysis was conducted to further characterize the microstructures, as shown in Fig. 3(a) and (b). The HR-TEM images clearly show the flake-like shape of GO and GN samples. The graphene nanosheets with an area of several square micrometers appear like transparent silk. This shows that both the graphene samples are fully exfoliated into nanosheets with microsized wrinkles by the modified Hummer method. Corrugation and scrolling are part of the intrinsic nature of graphene nanosheets, originated from the fact that thermodynamic stability of 2-D membrane structure is via bending [21,22]. Thus, this corrugation significantly induces the existence of nanovoids and nanocavities. Typical XRD patterns of pristine NG, GO and GN papers are shown in Fig. 4. The (002) peak of natural graphite takes place at 26.6°, indicating that the interlayer distance, d002, is approximately 0.334 nm, obtained by Bragg's equation. This d002 value for the graphite precursor gives an interlayer space close to highly oriented graphite carbon (0.335 nm) [23]. As for the GO paper, the (002) peak obviously shifts to 11.6°, showing that interlayer spacing increases to 0.762 nm. This increase in the interlayer distance is ascribed to oxidizationinduced expansion, thereby suggesting the presence of residual oxygencontaining functional groups or other structural defects [2]. After the microwave-assisted reduction, the (002) peak of the GN paper disappears, but the XRD pattern shows very weak and broad peak near 23.1°. This change in lattice structure may originate from the graphene oxide powders that were reduced and exfoliated into a monolayer or few-layer states [4]. Basically, this exfoliation degree of graphene oxide possibly depends on the different types of pristine graphite crystals. It is worth noting that the influence of epoxy additive on lattice analysis seems to be minor since no peaks or lumps were found about the epoxy resin. Raman spectroscopy is a powerful approach to characterize the graphite degree of carbon-based materials. Fig. 5 collects the Raman spectra of pristine NG, GO and GN papers, showing different crystalline structures. The Raman band observed at 1580 cm− 1 can be attributed to the stacking of the graphite hexagon network plane (G band), whereas the band at 1350 cm− 1 is assigned either to amorphous carbon or to deformation vibrations of a hexagonal ring (D band) [24–26]. The intensity ratio of the intensities of the D and G

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Binding Energy (eV) Fig. 6. Survey XP spectra of different graphene papers: (a) GO, (b) GN, (c) GN-F1, and (d) GN-F2.

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(a)

Intensity (a.u.)

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Binding energy (eV) Fig. 7. XPS C 1s peak of graphene papers and its functional group distributions: (a) GO, (b) GN, and (c) GN-F1.

280

Binding energy (eV)

Intensity (a.u.)

peaks (ID/IG) provides useful information about the graphite degree and lattice distortion of carbon-based materials [27,28]. The ID/IG ratio has the following order: GN (0.92) N GO (0.76) N NG (0.12). This significant increase in ID/IG ratio as compared to NG, reflects a decrease in size of in-plane sp2 domains and partially disordered crystal structure of graphene nanosheets [12]. XPS was used to analyze the composition of the oxygen functionalities on the resulting graphene papers, as shown in Fig. 6. The C 1s, O 1s, F 1s peaks of the scan spectra have binding energies of ca. 284.6, 533.5, and 629.0 eV, respectively [18]. Quantitative analysis was performed to evaluate the atomic ratios, and the results demonstrate: (i) the graphene oxide powder possesses very high oxidation level, (ii) the graphene can be chemically reduced by the microwave reduction in EG solution, and (iii) the fluorine content onto graphene surface can be improved by the fluorination treatment. The broad C 1s peak ranging from 280 to 292 eV in the XP spectra may contain peaks contributed by several carbon-based functional groups that have different binding energies. These binding energy peaks have been identified as C or C–H at 284.6 eV, C–O at 286.7 eV, C=O at 288.4 eV and O–C=O at 289.7 eV [18,20]. The C 1s peak of each carbon can be deconvoluted using a peak synthesis procedure in which Gaussian peak shape was assumed to fit each component with a fixed binding energy. Fig. 7(a) and (b) shows the C 1s peaks of GN and GO papers, respectively. It is of interest that the resulting graphene surface is obviously oxygenated by oxide functionalities, i.e., C–O, C=O, and O– C=O groups, since there is a weak peak for C=C or C–H groups in the C 1s spectrum of GO paper. This can be attributed to the fact that a large amount of surface oxide groups are chemically attached to the defect or edge sites of the basal planes. This result indicates that the oxygen content upon chemical oxidation is mainly contributed by the formation of C–O (35.2%) and C=O (45.1%) groups. Fig. 7(c) depicts the C 1s peak of GN-F1 paper, indicating that the functional group distribution consists of CF3 at 294.7 eV, CF2 at 292.1 eV, CF at 289.5 eV, C–CF at 288.2 eV, and C–C at 285.6 eV. The distribution on GN-F1 surface is collected as follows: CF3 (~ 2.0%), CF2 (~ 18.7%), CF (~12.9%), C–CF (~16.6%), and C–C (~49.8%). This result proves that the presence of fluorine copolymer, consisting of perfluoroalkyl groups such as difluoromethyl or trifluoromethyl groups, is capable of binding with the graphene surface. To inspect the wettability, each paper was adopted to examine the repellency by measuring its static CAs with water and EG droplets. Fig. 8(a)–(d) shows cross-view photographs of water droplets on GO, GN, GN-F1 and GN-F2 surfaces respectively. It is apparent that the GO paper displays a poor water repellency, i.e., CA of water: 52.3 ± 2.0°, whereas the other three papers shows a hydrophobicity, i.e., GN (150.1 ± 2.3°), GN-F1 (149.2 ± 2.4°) and GN-F2 (145.8 ± 2.3°). The affinity of the GO paper toward the water drop can be attributed to the existence of surface oxides, such as carboxylic and phenolic groups, leading to the surface hydrophilicity [1,3]. The hydrophilic surface on the GO paper tends to be wetted easily by the water droplet. This is because the GO paper possesses expanded layers, consisting of hydrophilic oxygenated graphene sheets, bearing oxygen functional groups on their basal planes and edges [29], as demonstrated by the XPS results (see Fig. 7). Without any surface fluorination or fluorine coating, the GN paper displays a better repellency against the water droplet (i.e., CA of water N 150°) than the other two F-coated GN papers. It is recognized that water repellency is an interfacial behavior that generally depends on the combination of surface chemistry and roughness on a multiple scale [20,30]. This ideal combination for superhydrophobicity has been observed in lotus leaf and artificial structures. An electron dispersive X-ray spectrometer (EDS) was also applied to analyze the

Intensity (a.u.)

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Fig. 8. Cross-sectional views of water droplets on (a) GO, (b) GN, (c) GN-F1, and (d) GN-F2 papers.

atomic composition of various carbon papers. As expected, the GN paper exhibits a low O/C ratio of ca. 5.8%, which is identical with the XPS analysis. The EDS result also demonstrates the presence of fluorine content in both F-coated GN papers, i.e., 0.8 and 1.7% for GN-F1 and GN-F2 papers respectively. Previous studies have pointed out that the wettability of F-coated flat surfaces is hydrophobic with a water CA of 100–120° [31], and the CA for a water droplet on flat graphite surface is about 84–86° [32]. In comparison, the GN paper shows an improved repellency toward the water droplet, i.e., superhydrophobicity (CA N 150°). This improvement can be attributed to the fact the GN paper basically displays a two-dual roughened surface that hierarchically combines the microand the nanoscaled structures of the surface. As observed in Fig. 1(d), the graphene powders offer a primary roughness whereas the flakelike voids between the nanosheets generate a secondary roughness. Thus, the GN paper, incorporating two-tier roughness and non-polar character (e.g., C=C or C–H), makes it difficult for the water drop to wet the few-layer graphene surface. This situation allows an air pocket into the roughened structure, referred to as the Cassie model [33]. The existing air film in the graphene surface is capable of providing a floating force to resist water penetration, thus inducing super water repellency. Additionally, both GN-F1 and GN-F2 papers display a slight decay of water CAs. This is presumably due to the fact that the fluorination layers may partially fill with or cover some nanovoids of the graphene surface. Cross-sectional views of EG droplets on different papers are shown in Fig. 9(a)–(d). The CAs of EG show are in the following order: GN-F2 (125.3 ± 2.3°) N GN-F1 (112.2 ± 2.2°) N GN (110.1 ± 2.2°) N GO (24.2 ± 2.0°). This result indicates that the GO paper is wettable to the polar solvent with low surface tension, whereas the GN paper displays a better repellency to the oil droplet. This result can

be attributed to the presence of hydrophobic C=C or C–H groups, attached to edges or defects of graphene sheets. The GN paper, consisting of a large number of hydrophobic sheets, offers a multiple hydrophobic architecture against the EG drop. As for both F-coated GN papers, the fluorine coating shows a positive effect on the enhancement of oil repellency. Generally, the CAs of EG on F-coated flat and graphite surfaces are 91.2° and 56.0° respectively. It is known that trifluoromethyl group (CF3)-terminated surfaces possess the lowest surface tension, approximately ~6 mN/m [34–36]. Recently, several groups have developed oleophobic surfaces with unique structures, such as overhang topography [37], re-entrant structure [38] and twotier sphere stacking layer [19]. By incorporating the hierarchical nanosheets with the fluoropolymer coating, the GN-F2 paper represents a promising combination of repellency to water and low surface tension liquids. On the basis of the results, the condition for the resulting GN papers shows incomplete wetting at solid–liquid interface for both water and EG drops, referred to as the Cassie state. Accordingly, the Cassie– Baxter model is adopted to describe both water and oil repellencies in case of GN papers. The relationship between the apparent CA (Θ*) observed on a rough surface and the CA (Θ) obtained on a flat surface is based on the same chemical composition [20,36]. The Cassie–Baxter equation can be formulated as cosΘ
= Φ1 cos Θ−Φ2

ð1Þ

where Φ1 and Φ2 (= 1 − Φ1) are the area fractions of a droplet in contact with solid surface and air respectively, on the GN paper. Accordingly, the Φ1 values of GN paper for both water and EG repellencies are measured as 12.2% and 42.1% respectively. The surface fractions (Φ1) are collected and listed in Table 1. The quantity

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Fig. 9. Cross-sectional views of EG droplets on (a) GO, (b) GN, (c) GN-F1, and (d) GN-F2 papers.

of the Φ2 value represents an area fraction for air pocket trapped in (i) the interspaces between graphene powders and (ii) the flake-like voids between graphene sheets, thus leading to the incomplete wetting behavior. The existing air cushion between droplets and graphene surface makes water droplets easily roll off a surface with extremely low adhesion. Due to its super water repellency, moisture cannot easily condense or form dew on the graphene surface, thus maintaining a dry surface. When the drop is slid on a tilted surface, the gravitational force on a tilted surface can be considered a critical force for driving the drop movement. Thus, the energy for the rolling drop traveling a certain distance depends significantly on the microscopic Wad. Based on the Young–Duprè equation, the Wad value on a flat surface can be written as [19,39] Wad = γL ð1 + cosΘÞ

ð2Þ

where γL is the surface tension of liquid drops. Since the droplet sits partially on the resulting GN papers, the Wad value is a function of wetted solid fraction, determined from the Cassie–Baxter model. Thus, Eq. (2) for describing the repellent behavior on the GN papers can be rewritten as [39] Wad = Φ1 γL ð1 + cosΘÞ

ð3Þ

Table 1 The Cassie parameters and the work of adhesion between the droplets and the resulting GN papers. Sample type

GN GN-F1 GN-F2

H2O repellency

EG repellency

Φ1 (%)

Wad (mJ/m2)

Φ1 (%)

Wad (mJ/m2)

12.2 12.9 15.9

9.62 10.2 12.5

42.1 39.9 27.1

29.7 28.1 19.1

comparison, the Wad values of the GN papers are approximately two times higher than those of F-coated carbon nanotube (CNT)-based paper and fabric [20], implying that a smaller work is required for the movement of water drop on the CNT-based surfaces. This is attributed to the fact that the CNT forest offers a number of “nanotips” to repel the liquid wetting, referred to as the Cassie state. In case of EG repellency, the high magnitude of Wad value reflects that the EG drops tend to stick to the GN paper because this paper easily suffers oil contamination. However, surface fluorination on the GN paper efficiently reduces the Wad value from 29.7 (GN) to 19.1 mJ/m2 (GN-F2). Accordingly, the fluorination provides a facile pathway to tune the repellency of graphene papers against liquid droplets with low surface tension. 4. Conclusions

The above equation reflects that a higher area fraction of a liquid drops in contact with a solid surface (or a lower wetted area fraction), resulting in a lower work of adhesion [20,39]. Table 1 collects the Wad for the drops sliding on various GN papers according to Eq. (3). For the water repellency, all GN papers display small Wad values, ranging from 9.62 to 12.5 mJ/m2. In

This paper investigated the water and EG repellencies on different graphene papers, produced through the chemical exfoliation of natural graphite powders. The graphene papers were composed of irregularly-ordered graphene nanosheets, analyzed by HR-TEM, FESEM, XRD, Raman spectroscopy and XPS. Generally, the GO paper

C.-T. Hsieh, W.-Y. Chen / Surface & Coatings Technology 205 (2011) 4554–4561

showed a hydrophilic behavior whereas the GN paper exhibited super repellency toward water droplet, i.e., maximal CA of water: 150.1 ± 2.3° and low Φ1 value: 12.2%. Such low Φ1 value originated from a higher fraction for air pocket trapped in (i) the interspaces between graphene powders and (ii) the flake-like voids between graphene sheets, referred to as the Cassie state. A modified Young–Duprè equation, incorporated with the Cassie parameter, was used to estimate the Wad between droplets and graphene surface. The Wad values of the GN papers for water repellency were found to fall in the region of 9.62–12.5 mJ/m2. The low adhesion between graphene and water drop can be attributed to the fact that the existing air layer provides a high fraction of vapor–solid interface, thus inducing the movement of the sliding drop. For EG repellency, the facile fluorination approach efficiently improved the repellency of the GN paper against EG contamination. On the basis of the above results, this study offers a fundamental analysis on the water and oil repellency of graphene and graphene oxide surfaces. However, future work is required to clarify (i) how the exfoliation degree and graphene reduction affects the water/oil repellency and (ii) how the implantation of oxides or amino groups influences the deposition of metallic nanocatalysts on graphene sheets for electrochemical devices. Acknowledgements The authors acknowledge the financial support from National Science Council of Republic of China under the contracts NSC 992120-M-155-001 and NSC 99-2221-E-155-078. References [1] D. Li, M.B. Müller, S. Gilje, R.B. Kaner, G.G. Wallace, Nat. Nanotechnol. 3 (2007) 101. [2] C. Wang, D. Li, C.O. Too, G.G. Wallace, Chem. Mater. 21 (2009) 2604. [3] J.I. Paredes, S. Villar-Rodi, A. Martínez-Alonso, J.M.D. Tascón, Langmuir 24 (2008) 10560. [4] S. Wang, Y. Zhang, N. Abidi, L. Cabrales, Langmuir 25 (2009) 11078. [5] E. Yoo, J. Kim, E. Hosono, H.S. Zhou, T. Kudo, I. Honma, Nano Lett. 8 (2008) 2277.

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