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Surface Gradient Material: From Superhydrophobicity to Superhydrophilicity Xi Yu, Zhiqiang Wang, Yugui Jiang, and Xi Zhang* Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed NoVember 19, 2005. In Final Form: March 23, 2006 We have developed a simple method to fabricate a gradient from superhydrophobicity to superhydrophilicity. It is based on the controlled self-assembled monolayer consisting of a thiol molecule on a gold surface and the amplifying effect of the wetting property on a rough surface. Using a relatively dilute HS(CH2)11CH3 solution (0.05 mmol/L), we found that the density of molecules on the surface can be controlled by varying the immersion time. Slowly adding the dilute solution to the container holding the rough gold substrate will lead to a density gradient along the surface. After the complementary adsorption of HS(CH2)10CH2OH, the surface exhibits a gradient from superhydrophobicity to superhydrophilicity. The slope of the gradient can be conveniently tuned by varying the speed of addition. CassieBaxter and Wenzel equations are employed to explain this special property based on the rough structure and the molecular composition gradient that have been determined by XPS. This kind of material would provide a larger oriented driving force for many important biological and physical processes and might have potential applications in water droplet movement, oriented axonal specification of neurons, protein adhesion, and so on.
Surface gradient materials,1 of which the surface physical and chemical properties change continuously along the materials, have generated much interest for many practical applications in recent years, such as biomolecular interaction investigations, cell motility studies, microfluidics fabrication, and so on. All sorts of gradient, such as chemical composition,1a,b,2-7 morphology,8 density,9 and nanowire length,10 have been experimentally realized though different techniques, including diffusioncontrolled vapor deposition,1a cross diffusion,2 corona discharge,3 the use of microfluidic devices,6 scanning tunneling microscopy,5 gradual immersion method,7 spatial gradients of electrochemical potential4,10 and temperature,8 and so on. Among them, the wettability gradient is one of the most interesting. The wetting property gradient along the surface can allow water droplet movement along the gradient.1a,b However, the range of the wettability gradient is still limited to common hydrophobicity and hydrophilicity (i.e., the surface contact angles change from about 110 to 20°). Superhydrophobicity with contact angles larger than 150°11 and superhydrophilicity with contact angle smaller * Corresponding author. E-mail:
[email protected]. (1) (a) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 256, 1539. (b) Daniel, S.; Chaudhury, M. K.; Chen, J. C. Science 2001, 291, 633. (c) Ruardy, T. G.; Schakenraad, J. M.; van der Mei, H. C.; Busscher, H. J. Surf. Sci. Rep. 1997, 29, 1. (2) Liedberg, B.; Tengvall, P. Langmuir 1995, 11, 3821. (3) Jeong, B. J.; Lee, J. H.; Lee, H. B. J. Colloid Interface Sci. 1996, 178, 757. (4) Terrill, R. H.; Balss, K. M.; Zhang, Y.; Bohn, P. W. J. Am. Chem. Soc. 2000, 122, 988. (5) Fuierer, R. R.; Carroll, R. L.; Feldheim, D. L.; Gorman, C. B. AdV. Mater. 2002, 14, 154. (6) Dertinger, S. K. W.; Jiang, X.; Li, Z.; Murthy, V. N.; Whitesides, G. M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12542. (7) (a) Udea-Yukoshi, T.; Matsuda, T. Langmuir 1995, 11, 4135. (b) Morgenthaler, S.; Lee, S.; Zu¨rcher, S.; Spencer, N. D. Langmuir 2003, 19, 10459. (c) Tomlinson, M. R.; Genzer, J. Chem. Commun. 2003, 1350. (8) Zhang, J.; Xue, L.; Han, Y. Langmuir 2005, 21, 5. (9) Wu, T.; Efimenko, K.; Vlceˇk, P.; Sˇ ubr, V.; Genzer, J. Macromolecules 2003, 36, 2448. (10) Sehayek, T.; Vaskevich, A.; Rubinstein, I. J. Am. Chem. Soc. 2003, 125, 4718. (11) (a) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395. (b) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; S, Y.; Liu, B.; Jiang, L.; Zhu, D. AdV. Mater. 2002, 14, 1857. (c) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (d) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z.; Jiang, L.; Li, X. J. Am. Chem. Soc. 2004, 126, 3064. (e) Lafuma, A.; Que´re´, D. Nat. Mater. 2003, 2, 457.
than 10°12 are two extreme wetting property cases, and they have attracted extensive interest because of their intriguing promise as self-cleaning materials. Fabricating a gradient surface with this wetting property that can change from superhydrophobic to superhydrophilic remains a challenge. Herein we attempt to introduce a simple method to fabricate a gradient surface with a wetting property that can change from superhydrophobic to superhydrophilic continuously by utilizing the controlled adsorption of a thiol self-assembled monolayer on a rough gold surface. Usually, surface roughness can amplify the intrinsic contact angle. In other words, roughness will make a hydrophilic surface more hydrophilic and a hydrophobic surface more hydrophobic.11,13 Hence, to achieve a superhydrophobic-to-superhydrophilic gradient, a rough surface is a prerequisite. Herein, we obtained a rough gold surface by engaging static potential gold electrodeposition on a flat gold substrate at -200 mV (vs Ag/ AgCl) in a HAuCl4/ H2SO4 mixed electrolyte solution for 40 min. Scanning electron microscopy (SEM) was used to characterize the morphology of the deposited gold structure. As shown in Figure 1, many microscale gold clusters are distributed on the surface randomly. Meanwhile, nanoscale gold structures can be observed on and between the gold clusters. Actually, the structure of the deposited gold is fractal-like with a dimension of between two and three. The roughness factor R is defined as the ratio of the real surface area (S) to the projected surface area (S0) of the gold surface, and the real surface area can be measured from O-atom electrosorption voltammograms.14 Using this electrochemical method, we identified that the roughness factor of the as-prepared rough gold surface is around 7.2. This kind of rough gold substrate has been proven to be suitable for the realization of superhydrophobicity.15 (12) (a) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kijima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (b) Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 Photocatalyst, Fundamentals and Applications; BKC: Tokyo, 1999. (13) (a) Bico, J.; Tordeux, C.; Que´re´, D. Europhys. Lett. 2001, 55, 214. (b) McHale, G.; Shirtcliffe, N. J.; Aqil, S.; Perry, C. C.; Newton, M. I. Phys. ReV. Lett. 2004, 93, 036102. (14) (a) Go´mez, M. M.; Garcı´a, M. P.; Fabia´n, J. S.; Va´zquez, L.; Salvarezza, R. C.; Arvia, A. J. Langmuir 1996, 12, 818. (b) Finot, M. O.; Braybrook, G. D.; McDermott, M. T. J. Electroanal. Chem. 1999, 466, 234.
10.1021/la053133c CCC: $33.50 © 2006 American Chemical Society Published on Web 04/07/2006
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Figure 1. SEM images of deposited gold structures: (a) microscale gold structures distributed on the surface and (b) a magnified image of the gold clusters.
Then the as-prepared rough surface was further modified with thiol molecules by immersing the substrate in a 1 mmol/L HS(CH2)11CH3 ethanol solution. In this way, a self-assembled monolayer forms on the gold surface by chemisorption. We found that after being modified with HS(CH2)11CH3 during 30 s of immersion the surface can easily reach the standard of superhydrophobicity with a contact angle larger than 150° and a tilt angle smaller than 5°. The tilt angle reflects the difference between advancing and receding contact angles (contact angle hysteresis), and the low tilt angle indicates that water droplets roll off easily. The wetting property shows hardly any change even after immersing the substrate in a 1 mmol/L HS(CH2)10CH2OH solution for 10 min, which may suggest that in such a short time the HS(CH2)11CH3 molecule have formed a relatively complete monolayer and have left few sites for the adsorption of the complementary molecule, HS(CH2)10CH2OH. This is consistent with the fact that for a 1 mmol/L HS(CH2)11CH3 ethanol solution the adsorption can be completes in less than 1 min.15 It is known that when the concentration of the thiol solution decreases the rate of adsorption will decrease correspondingly. Spencer and co-workers have fabricated a hydrophobic-to-hydrophilic gradient on a flat surface utilizing this property.7b In our experiment, we first modified the rough gold surface with HS(CH2)11CH3 solution while varying the concentration of the modifying solution and immersion time. Then, the substrate was immersed in 1 mmol/L HS(CH2)10CH2OH for 10 min. At lower concentration and shorter immersion time, the HS(CH2)11CH3 molecules may not occupy all of the space on the gold surface, so the HS(CH2)10CH2OH molecules can adsorb on the unoccupied sites. An immersion time of 10 min in 1 mmol/L HS(CH2)10CH2OH solution was selected to make sure that the HS(CH2)10CH2OH molecule can complement the unoccupied sites completely. We use contact angle measurements to monitor this adsorption process. Figure 2 gives contact angles of the surface at different modifying solution concentrations and immersion times. We found that when the concentration of the HS(CH2)11CH3 solution decreased to 0.05 mmol/L the adsorption dynamics of the decane thiol molecules will decrease dramatically. It will take nearly 10 min for the rough gold surface to adsorb enough HS(CH2)11CH3 molecules for superhydrophobicity to occur, which is remarkably different from that of the 1 mmol/L solution. Therefore, a 0.05 mmol/L decanethiol solution is the proper concentration because it provides relatively slow adsorption dynamics and facilitates the realization of a gradient from superhydrophobic to superhydrophilic. (15) (a) Brady, R. M.; Ball, R. C. Nature 1984, 309, 225. (b) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. (c) Yu, X.; Wang, Z. Q.; Jiang, Y. G.; Shi, F.; Zhang, X. AdV. Mater. 2005, 17, 1289. (d) Jiang, Y. G.; Wang, Z. Q.; Yu, X.; Shi, F.; Xu, H. P.; Zhang, X.; Smet, M.; Dehaen, W. Langmuir 2005, 21, 1986.
Figure 2. (a) Contact angle of the rough surface after being modified with an HS(CH2)11CH3 solution at different concentrations and different immersion times. All measurements are conducted after the substrates are further modified with HS9(CH2)10CH2OH solution for 10 min. (b) Scheme of the preparation process of the gradient surface.
As shown in Figure 2b, we put the as-prepared rough surface in a tube with its back standing against the wall of the tube. Then a 0.05 mmol/L HS(CH2)11CH3 ethanol solution was added slowly to the tube, and the upper edge of the solution increased gradually as time elapsed. In this way, positions along the latitude of the substrate would correspond directly to a continuously changing immersion time (i.e., the immersion time decreased gradually from the lower part to the upper part along the substrate, which means that different position of the substrate will possess different wettabilities). At the same time, we tuned the addition speed to make sure that it would take 10 min for the surface of the solution to reach the upper edge of the substrate. Then the substrate was immersed in a 1 mmol/L HS(CH2)10CH2OH solution for 10 min to adsorb the complementary molecule. As predicted, the asprepared substrate did exhibit a gradient property. As shown in Figure 3, the contact angles change continuously along the surface from larger than 150° to smaller than 10° in about 3 cm, indicating that we have successfully realized a gradient from superhydrophobic to superhydrophilic. Furthermore, we can tune the slope of the gradient conveniently by varying the addition speed (Figure 4a) because different rising speeds of the thiol solution will make the same positions correspond to different immersion times and then different contact angles. Higher addition speed will lead to a smaller slope, and lower addition speed produce a larger one. To confirm if there indeed exists a monolayer composition gradient along the substrate, we made spatially resolved XPS measurements to provide a semiquantitative analysis. As shown in Figure 5, the ratio of the amount of carbon to oxygen decreases continuously and evenly along the altitude of the surface. For the superhydrophobic part, the ratio is much larger than that of
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Figure 3. Photograph of the water contact angles along the gradient surface. The photograph was combined with four continuous photographs along the substrate because the view angle of the contact angle measurement system is not wide enough. The volume of the water droplets was kept at ∼3 µL.
Figure 4. Contact angle as a function of position along the gradient surface. (a) The gradient surface can exhibit different slopes with varying adding speed. (b) Advancing and receding contact angles along the surface.
Figure 5. Ratio of the carbon and oxygen atomic concentrations as a function of the positions along the surface whose wetting properties change from superhydrophobic to superhydrophilic.
the superhydrophilic part, which means that superhydrophobic part contains many more HS(CH2)11CH3 than HS(CH2)10CH2OH molecules. However, the hydrophilic part must contain more HS(CH2)10CH2OH than HS(CH2)11CH3, leading to a lower ratio of carbon to oxygen as indicated by the XPS data. Although carbon is a poor choice of element to use for XPS, it can provide a relative indicator for reflecting the surface change in composition. To understand the mechanism behind the change in the wetting property, we described the wetting property on the rough surface using two models. The first one is the Wenzel equation: cos θr ) r cos θ, in which θ, r, and θr represent the intrinsic contact angle of the surface, the roughness, and the contact angle on the rough surface, respectively, which means that the roughness will make a hydrophilic surface more hydrophilic and a hydrophobic surface more hydrophobic.17 The second one is the CassieBaxter equation, cos θCB ) f1 cos θ - f2, in which θ is the contact (16) (a) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (b) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 6560. (17) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988.
angle on a flat surface and f1 and f2 are the fractions of the solid surface and the air in contact with the liquid, respectively (i.e., f1 + f2 ) 1).18 It indicates that air trapped beneath the water can dramatically intensify the hydrophobicity. The hysteresis between advancing and receding contact angles is used to distinguish these two states. If the contact angle hysteresis is large, then it is Wenzel state; if it is small, then it is the Cassie state.11 In our case, the contact angle hysteresis of the superhydrophobic section of the surface is as small as 3° (Figure 3b), indicating that in this section the superhydrophobicity is mainly due to the air trapped beneath the water droplet. This fractal-like structure is believed to benefit from the realization of trapped air and superhydrophobicity.15 As pointed out by Que´re´ et al., with the increase in hydrophilicity on a flat surface, there exists a transition from the Cassie-Baxter state to the Wenzel state on a rough surface. Indeed, we have observed such a transition state. As shown in Figure 4b, from about 5 mm of the substrate, the receding angle became very small, showing large hysteresis, and this is an indication of the Wenzel state.11,17 As indicated by the XPS data, the relative amount of HS(CH2)10CH2OH increases from the superhydrophobic to the superhydrophilic part, which means that the wetting property of the monolayer becomes more and more hydrophilic along the substrate; therefore, according to the Wenzel equation, the apparent contact angles will decrease correspondingly. For the section covered with many HS(CH2)10CH2OH molecules, roughness will enhance the hydrophilicity as described by the Wenzel equation. On the electrodeposited gold surface, as mentioned before, the fractal-like structure with a roughness factor of 7.2 has a large surface area and is rough enough that the absorption of water will occur on the rough surface as a result of the 3D capillary effect.13 The surface also exhibits superhydrophilicity. Water droplets cannot move on this gradient surface. Indeed, we found that if the water droplet is large enough, it can form an asymmetric shape at the moment it lies on the surface because of the gradient, but it cannot move. We believe that the large hysteresis of the contact angle is responsible for this point, and we found that this can be partially overcome by substrate vibration, (18) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546.
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which imposes a periodic force on the droplet, according to Chaudhury’s work.19 In summary, we have developed a simple method to fabricate a superhydrophobic-to-superhydrophilic gradient. It is based on the controlled self-assembled monolayer of thiol molecules on a gold surface and the amplifying effect of the wetting property on a rough surface. This method allows us to fabricate a largescale superhydrophobic-to-superhydrophilic gradient with various gradient slopes. It is greatly anticipated that this superhydrophobic-to-superhydrophilic gradient will provide a larger oriented driving force for many important biological and physical processes and will have potential applications in water droplet movement,1a,b (19) (a) Daniel, S.; Chaudhury, M. K. Langmuir 2002, 18, 3404. (b) Daniel, S.; Sircar, S.; Gliem, J.; Chaudhury, M. K. Langmuir 2004, 20, 4085.
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oriented axonal specification of neurons,6 protein adhesion, and so on. Acknowledgment. The research was supported by the National Natural Science Foundation of China (20334010, 20473045, 20574040, 50573042) and the National Basic Research Program of China (2005CBF24400). We thank Professor Zhishan Bo, Miss Yang Han, and Bo Zhu for their help with the SEM and XPS measurements. Supporting Information Available: Experimental details of gold substrate preparation, SEM image acquisition, X-ray photoelectron spectroscopy, and roughness factor calculation. This material is available free of charge via the Internet at http://pubs.acs.org. LA053133C
