摘要
A superhydrophobic graphite surface has been fabricated through two facile physical steps, peeling and ultrasonicating. Peeling yields micron-scale roughening, and thus a highly hydrophobic surface is obtained. Further ultrasonicating results in a superhydrophobic surface with nanostructure embedded in microstructure. The nanostructure leads to networklike pores on the superhydrophobic film and convective Ostwald ripening is observed. Owing to their distinct resistance to liquid imbibition, contact angle hysteresis on hydrophobic and superhydrophobic surfaces is fundamentally different. Moreover, the adhesive force on a superhydrophobic surface grows with the contact time, and such aging effect is absent on hydrophobic graphite surface.
| 原文 | ???core.languages.en_GB??? |
|---|---|
| 文章編號 | 121601 |
| 期刊 | Applied Physics Letters |
| 卷 | 100 |
| 發行號 | 12 |
| DOIs | |
| 出版狀態 | 已出版 - 19 3月 2012 |
存取文件
引用此
- APA
- Author
- BIBTEX
- Harvard
- Standard
- RIS
- Vancouver
}
於: Applied Physics Letters, 卷 100, 編號 12, 121601, 19.03.2012.
研究成果: 雜誌貢獻 › 期刊論文 › 同行評審
TY - JOUR
T1 - Anomalous wetting on a superhydrophobic graphite surface
AU - Hong, Siang Jie
AU - Li, Yueh Feng
AU - Hsiao, Mu Jou
AU - Sheng, Yu Jane
AU - Tsao, Heng Kwong
N1 - Funding Information: Hong Siang-Jie 1 Li Yueh-Feng 1 Hsiao Mu-Jou 1 Sheng Yu-Jane 2 a) Tsao Heng-Kwong 1,3 b) 1 Department of Chemical and Materials Engineering, National Central University , Jhongli 320, Taiwan 2 Department of Chemical Engineering, National Taiwan University , Taipei 106, Taiwan 3 Department of Physics, National Central University , Jhongli 320, Taiwan a) Electronic mail: [email protected] . b) Electronic mail: [email protected] . 19 03 2012 100 12 121601 11 02 2012 08 03 2012 22 03 2012 2012-03-22T08:44:27 2012 American Institute of Physics 0003-6951/2012/100(12)/121601/4/ $30.00 A superhydrophobic graphite surface has been fabricated through two facile physical steps, peeling and ultrasonicating. Peeling yields micron-scale roughening, and thus a highly hydrophobic surface is obtained. Further ultrasonicating results in a superhydrophobic surface with nanostructure embedded in microstructure. The nanostructure leads to networklike pores on the superhydrophobic film and convective Ostwald ripening is observed. Owing to their distinct resistance to liquid imbibition, contact angle hysteresis on hydrophobic and superhydrophobic surfaces is fundamentally different. Moreover, the adhesive force on a superhydrophobic surface grows with the contact time, and such aging effect is absent on hydrophobic graphite surface. The graphite structure is composed of layers of hexagonally arranged carbon atoms. Due to the weak van der Waals type of bonds between the layers, the interplanar cleavage is facile, which gives rise to the excellent lubricative properties of graphite. The exfoliation of graphite can result in a single layer of graphite, so-called graphene. Graphite is an electrical conductor, a semimetal, and the electric conductivity is relatively high in crystallographic directions parallel to the hexagonal sheets. Because of high thermal conductivity and good chemical stability, graphite is commonly used as heating elements for electric furnaces, electrodes for arc welding, electrodes in batteries, and in air purification devices. 1–3 The wetting of solid surfaces by water droplets is ubiquitous in our daily lives as well as in industrial applications. Wettability in terms of the contact angle (CA) between solid-liquid-gas phases is one of the most important surface properties associated with materials. The CA of a flat solid ( θ ) is described by Young's equation, cos θ = ( γ sg - γ sl ) / γ lg , where γ sg , γ sl , and γ lg represent the interfacial tensions of solid-gas, solid-liquid, and liquid-gas, respectively. A surface is termed superhydrophobic when water CA exceeds 150°. 4 Recently, natural superhydrophobicity has attracted great interest owing to its importance in the inspired mimetic attempts, such as the self-cleaning property of Lotus or Allium leaves. 5 In general, the wetting behavior is governed by chemical composition and geometrical microstructures of the surface. The mechanisms responsible for the influence of surface roughness were explained by Wenzel 6 and later by Cassie and Baxter. 7 Wenzel assumed that the liquid filled up the grooves on the rough surface. Therefore, surface roughness can amplify hydrophobicity for θ > 90 ° or enhance hydrophilicity for θ < 90 ° . On the other hand, according to the Cassie and Baxter model, the superhydrophobic surface is considered as a surface consisting of two types of homogeneous patches that have different γ sl . Because the rough structure is mainly filled with air, the openings of the grooves correspond to nonwetting patches with θ = 180 ° . The wettability of water drops on graphite surfaces is somewhat controversial. The experimentally measured CAs are reported in the range of about 80° to 130°. 8,9 For example, the CA is ∼ 80 ° for highly ordered pyrolytic graphite, ∼ 92 ° for epitaxy graphene, and ∼130° for reduced graphene. On the other hand, the adsorption of water clusters on graphene by ab initio density functional theory indicates a graphene sheet is highly hydrophobic. Since the roughness effect can impart superhydrophobicity to the underlying substrates, the natural roughness of patterned surfaces modulated at submicron scale, such as carbon nanotube forests, 10 is believed to be capable of generating a superhydrophobic behavior. Using a plasma enhance chemical vapor deposition technique, vertically aligned and densely spaced carbon nanotubes are deposited on a SiO 2 surface with nickel catalyst islands. However, without further treatment, these surfaces are “hydrophilic” in the sense that a water drop is not stable and can completely impregnate the surface eventually. Further functionalization, such as conformal hydrophobic PTFE coating (−CF 2 -) or gold-thiol affinity (−CH 2 -), 10,11 is required to obtain superhydrophobicity. Recently, graphene-based coatings have been employed to control wetting properties of a variety of surfaces. The deposition of reduced graphene, which is ultrasonically dispersed in a suitable solvent after exfoliation from natural graphite flakes following oxidizing graphite into graphite oxide, onto the substrate can result in a superhydrophobic surface. 12 The adhesive nature of the graphene film is also reported. 9 However, many carbonyl, hydroxyl, or epoxy groups are unavoidably bonded on the surface of graphene prepared by chemical reduction processing, which results in adsorption of small molecules. In the present work, a superhydrophobic surface is fabricated on a graphite sheet through two facile physical steps, including micron-scale and nanoscale surface roughening. In addition, the anomalous wetting behaviors on the superhydrophobic graphite surface, such as Ostwald ripening, strong contact angle hysteresis (CAH), and aging, are investigated. The graphite sheets were purchased from NTC (IGS-743, 99.7%) and the CA is about 120° after a rinse in acetone to remove contamination. The shapes of sessile drops were recorded at an elapsed time of 10 s and analyzed at room temperature on a Krüss DSA10 CA measuring system. Reported CAs are the average of at least four measurements. As the sheet is ground by a 4000-grit sandpaper on a spinner, its root mean square roughness determined by atomic force microscopy reduces to about 125 nm, and the CA is lowered to 78.3°, as shown in Fig. 1(a) . Note that the surface has been rinsed by acetone after polishing and significant reflection is observed. The scanning electron microscope (SEM) image is illustrated in Fig. 1(b) , and this consequence reveals that the CA of a smooth graphite surface is close to 80°. Our first step to increase the CA by amplifying the roughness is to peel off the graphite substrate with an adhesive tape such as Scotch Magic TM . The adhesive tape must be adhered tightly to the graphite surface before peeling. As the outermost layers are removed, a fresh rough graphite surface is exposed, and the CA is increased to about 140°, as illustrated in Fig. 1(c) . The SEM image as shown in Fig. 1(d) reveals the existence of the micron-scale roughness. The adhesive surface of the transparent tape becomes completely black after peeling and the water CA on such a surface is also 140°. This result reveals that there is no polymeric residue on the freshly formed graphite surface. However, further treatment is required to elevate the CA. Ultrasonication (Ultrasonic LC 130/H, Elma, Germany) of the hydrophobic graphite substrate in acetone for about 15 min yields a superhydrophobic graphite with CA 160°, as demonstrated in Fig. 1(e) . Since its SEM image depicted in Fig. 1(f) is similar to that of a hydrophobic surface, the micron-scale roughness remains after sonication. The difference between Fig. 1(d) and 1(f) is insignificant. Nevertheless, the color of the solution turns slightly grayish during ultrasonication, indicating the appearance of graphite nanoparticles disintegrated from the substrate. Dynamic light scattering analysis (Zetasizer Nano ZS90) shows that the particle size is about 150 nm. Consequently, the superhydrophobicity of the graphite surface is induced by the formation of the nanoscale roughness in addition to micron-scale structure. The anomalous wetting behavior on graphite sheets with different roughness, including polished, hydrophobic, and superhydrophobic surfaces, can be first demonstrated by an air bubble adhered to the substrate immersed in water. Two types of bubbles are considered after immersion, residual bubbles and newly attached ones. During the sinking process, millimeter-sized bubbles are generally trapped on hydrophobic surfaces but detach easily on hydrophilic surfaces due to buoyancy. 13 As shown in Figs. 2(a), 2(c), and 2(e) , no bubble is seen on the polished surface, but flat bubbles are observed on hydrophobic and superhydrophobic surfaces. Such residual bubbles exhibit advancing (maximal) CA θ a (measured through the liquid), which are the same as those observed by the method of inflating a sessile drop. On the other hand, new bubbles released by a syringe can adhere to the polished hydrophilic surface but cannot be formed on hydrophobic surfaces, as illustrated in Figs. 2(b) and 2(d) . Since the newly formed bubbles display receding (minimal) CA θ r , 14 the difficulty of forming a new bubble implies that θ r on the hydrophobic surface is too small ( < 5 ° ) to resist buoyancy force. Nonetheless, a bubble of 2 μ l can attach to the superhydrophobic surface with the CA 74 . 2 ° , as illustrated in Fig. 2(f) . These results of θ r agree with the receding angle observed by the method of deflating a drop. Evidently, the graphite surface possesses strong CAH, Δ θ = θ a - θ r , even on polished surfaces. The presence of surface roughness further alters both θ a and θ r significantly. Although surface morphologies of hydrophobic and superhydrophobic graphite surfaces are similar, there exist nanostructural differences between them. Continuous injection of air or formation of tiny bubbles at a location away from the flat bubble adhered on the superhydrophobic graphite surface can result in the growth of the latter, as shown in Fig. 3(a) . This consequence is known as convective Ostwald ripening and indicates the presence of porous structure. 13 Because networklike pores in the superhydrophobic film remain nonwetted, they provide passage for gas flow between adhered bubbles. A large bubble grows spontaneously by absorbing smaller bubbles due to capillary pressure differences. For a newly attached bubble, the indirect deflation can also be seen in Fig. 3(b) by suction through a syringe at a position away from the bubble. During growth or shrinkage of the existing bubble, contact line pinning is shown and the CA varies between θ a and θ r . On the other hand, convective Ostwald ripening cannot be observed on hydrophobic graphite surfaces immersed in water, indicating the absence of the porous structure. Evidently, the formation of nanosized pores on superhydrophobic surfaces is achieved by the exfoliation of nanoparticles via ultrasonication in acetone. The extent of CAH implying adhesive performance can be vividly demonstrated by a drop clinging to the vertical wall. A clinging drop with the weight ρ gV is balanced by the pinning force due to wetting. The adhesive force is proportional to γ lg C ( cos θ b - cos θ f ) , where θ f and θ b represent front and back CAs, respectively. C is the circumference of the contact line and generally declines with increasing θ a . A maximum clinging drop before sliding exhibits θ b = θ r and θ f = θ a , and thus one has V max ∼ ( γ / ρ g ) C ( cos θ r - cos θ a ) . Evidently, the larger C Δ cos θ , the greater V max . As shown in Fig. 4 , the hydrophobic surface can sustain the largest clinging drop (∼61 μ l) while the superhydrophobic surface supports the smallest drop (∼16 μ l). Note that the advancing and receding angles observed in the clinging drop with the maximum volume agree with those determined by the methods of sessile drop and bubble. This consequence reveals that although surface roughening of the graphite sheet amplifies CAH Δ cos θ , the adhesive force is enhanced for hydrophobic surface but lessened for superhydrophobic surface because of C . The significant CAH on a smooth graphite surface ( Δ θ ≈ 60 ° ) may be realized by the formation of a dipole layer by water molecules adsorbed on graphite, which is probed by electrostatic force microscope. 15 That is, the intrinsic γ sl is substantially reduced after wetting. The stronger CA hystereses associated with hydrophobic ( Δ θ ≈ 140 ° ) and superhydrophobic surfaces ( Δ θ ≈ 85 ° ) are further resulted by surface roughness. The advancing angle is amplified by air pockets while the receding angle is altered by liquid impregnation. Since the intrinsic CA of a smooth graphite surface is less than 90 ° , indicating hydrophilic nature, the Wenzel model cannot be used to explain hydrophobic behavior of graphite. According to the Cassie-Baxter model, the area fractions of air pockets estimated from θ a are about 0.82 and 0.95 for hydrophobic and superhydrophobic graphite surfaces, respectively. However, the area fraction of grooves on a superhydrophobic surface is reduced to about 0.66, as estimated from its θ r . This consequence reveals that liquid impregnates some air pockets due to capillary condensation after the graphite surface has been wetted. While partial imbibition into small air grooves occurs at superhydrophobic surfaces, complete imbibition happens at hydrophobic surfaces. Therefore, θ r of hydrophobic surfaces approaches zero based on the Wenzel model. A drop can sit on a tilted plane because of CAH. The tilted drop can be achieved by inclination of a horizontal plane with a sitting drop or direct attachment of a drop onto the inclined plane. In general, both processes yield the same result because the characteristic time associated with droplet adhesion is small compared to the relaxation time of shape adjustment of a drop. As the superhydrophobic graphite substrate with a deposited drop of the volume 10 μ l is tilted, the drop remains attached even when the graphite substrate becomes vertical as shown in Fig. 4 . However, the drop fails to adhere directly to the tilted substrate with the sliding angle 30 ° or higher and rolls down. This consequence reveals that it takes quite a long time for a water drop to adhere to the superhydrophobic graphite surface. If the drop is kept in contact with the vertical graphite surface for 1 min and then released from the syringe, the drop is able to stay on the vertical wall. This result indicates the aging effect, which is absent for typical materials like silica glass, acrylic glass, copper, or Teflon. The aging effect on a superhydrophobic surface can be demonstrated through CAH. As shown in Fig. 5 , Δ θ grows with the contact time between droplet formation and deflation by a syringe. Note that the deflation is completed within a few seconds. While the advancing angle remains essentially the same, the receding angle decays with the contact time and reaches a steady value about 75 ° . This result indicates that it takes about 2 min for the drop to imbibe into air pockets on the superhydrophobic surface and for the adhesive force to reach its maximum. In conclusion, a superhydrophobic graphite surface has been fabricated through two facile physical steps. Peeling yields micron-scale roughening and further ultrasonication results in nanostructure embedded in microstructure. Anomalous wetting on such a surface is observed, including convective Ostwald ripening, strong CAH, and aging effect of adhesive forces. This research work is supported by National Science Council of Taiwan and Industrial Technology Research Institute of Taiwan. FIG. 1. Water contact angles and SEM images of polished graphite sheet (a), (b); hydrophobic graphite surface obtained by peeling (c), (d); and superhydrophobic graphite surface by further ultrasonication (e),(f). FIG. 2. Residual and newly attached air bubbles on polished graphite surface (a), (b); hydrophobic graphite surface obtained by peeling (c), (d); and superhydrophobic graphite surface by further ultrasonication (e), (f). FIG. 3. Convective Ostwald ripening on a superhydrophobic graphite surface. (a) Growth of an air bubble by absorbing tiny bubbles away from it. (b) Shrinkage of an air bubble by suction through a syringe away from it. Contact line pinning is clearly observed because of constant base diameter (BD). FIG. 4. The extent of contact angle hysteresis implying the adhesive performance is demonstrated by a drop clinging to the vertical wall, including (a) polished, (b) hydrophobic, and (c) superhydrophobic surfaces. The clinging drop with the maximum volume before sliding is given. FIG. 5. The variation of the advancing and receding contact angles with the contact time on a superhydrophobic graphite surface. The dynamic behavior of θ r reveals the aging effect of the adhesive force.
PY - 2012/3/19
Y1 - 2012/3/19
N2 - A superhydrophobic graphite surface has been fabricated through two facile physical steps, peeling and ultrasonicating. Peeling yields micron-scale roughening, and thus a highly hydrophobic surface is obtained. Further ultrasonicating results in a superhydrophobic surface with nanostructure embedded in microstructure. The nanostructure leads to networklike pores on the superhydrophobic film and convective Ostwald ripening is observed. Owing to their distinct resistance to liquid imbibition, contact angle hysteresis on hydrophobic and superhydrophobic surfaces is fundamentally different. Moreover, the adhesive force on a superhydrophobic surface grows with the contact time, and such aging effect is absent on hydrophobic graphite surface.
AB - A superhydrophobic graphite surface has been fabricated through two facile physical steps, peeling and ultrasonicating. Peeling yields micron-scale roughening, and thus a highly hydrophobic surface is obtained. Further ultrasonicating results in a superhydrophobic surface with nanostructure embedded in microstructure. The nanostructure leads to networklike pores on the superhydrophobic film and convective Ostwald ripening is observed. Owing to their distinct resistance to liquid imbibition, contact angle hysteresis on hydrophobic and superhydrophobic surfaces is fundamentally different. Moreover, the adhesive force on a superhydrophobic surface grows with the contact time, and such aging effect is absent on hydrophobic graphite surface.
UR - http://www.scopus.com/inward/record.url?scp=84859538717&partnerID=8YFLogxK
U2 - 10.1063/1.3697831
DO - 10.1063/1.3697831
M3 - 期刊論文
AN - SCOPUS:84859538717
SN - 0003-6951
VL - 100
JO - Applied Physics Letters
JF - Applied Physics Letters
IS - 12
M1 - 121601
ER -