pubs.acs.org/Langmuir © 2009 American Chemical Society
Wettability-Controllable Super Water- and Moderately Oil-Repellent Surface Fabricated by Wet Chemical Etching Tae-il Kim, Dongha Tahk, and Hong H. Lee* School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742, Korea Received January 10, 2009. Revised Manuscript Received April 2, 2009 A facile fabrication method is presented for a super-repellent surface, in which a silicon wafer is etched with a wet chemical method and treated by a fluorinated self-assembled monolayer. This surface is composed of harshly rough nanostructures and highly dense nanoholes. The contact angle of both water and oil with the surface is larger than 150°. The self-cleaning capability of the surface allows for the removal of sticky powders with glycerin droplet. Any desired part(s) of the super-repellent surface can be turned superamphiphilic by simply exposing the desired part(s) to ultraviolet light.
Introduction Controlling the wetting behavior of a liquid on solid surfaces1-4 may find many applications in various fields, such as microfluidics,5 and in unconventional lithography, as in the patterning of carbon nanotube (CNT)6-8 and colloidal particles.9,10 Theoretical11-13 and experimental14-19 studies on controlling wettability have been presented for producing a superhydrophobic surface. Examples of fabricating a superhydrophobic surface include use of CNT,14 transfer printing,15 etching,16 imprinting,17 layer-by-layer deposition,18 and molding.19 In contrast, there have been only a handful of studies on a super-repellent surface, in which the surface is super-repellent with respect to both water and oil. Solvent-induced phase separation of a polymer blend,20 electrodeposition,21,22 spreading nanoparticles,23 plasma polymer *To whom correspondence should be addressed. E-mail: honghlee@ snu.ac.kr. (1) Xia, F.; Jiang, L. Adv. Mater. 2008, 20, 2842. (2) Gleiche, M.; Chi, L. F.; Fuchs, H. Nature 2000, 403, 173. (3) Courbin, L.; Denieul, E.; Dressaire, E.; Roper, M.; Ajdari, A.; Stone, H. A. Nat. Mater. 2007, 6, 661. (4) Kim, T.; Kwon, S.; Lee, J.; Lee, H. H. Appl. Phys. Lett. 2006, 89, 173115. (5) Kim, P.; Lee, S. E.; Jung, H. S.; Lee, H. Y.; Kawai, T.; Suh, K. Y. Lab Chip 2006, 6, 54. (6) Lee, M.; Im, J.; Lee, B. Y.; Myung, S.; Kang, J.; Huang, L.; Kwon, Y. -K.; Hong, S. Nat. Nanotechnol. 2006, 1, 66. (7) Xia, Y.; Yang, P.; Sum, Y.; Wu, Y.; Maywes, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353. (8) Tsukruk, V. V.; Ko, H.; Peleshanko, S. Phys. Rev. Lett. 2004, 92, 165502. (9) Zhang, X.; Sun, B.; Friend, R. H. Nano Lett. 2006, 6, 651. (10) Yang, S.-M.; Jang, S. K.; Choi, D.-G.; Kim, S.; Yu, H. K. Small 2006, 2, 458–475. (11) Denny, M. W. Science 2008, 320, 886. (12) Quere, D. Annu. Rev. Res. 2008, 38, 71. (13) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457. (14) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. Adv. Mater. 2002, 14, 1857. (15) Kim, T.; Beak, C.; Suh, K. Y.; Seo, S.; Lee, H. H. Small 2008, 4, 182. (16) Xia, Y.; Zhu, L.; Hess, D. W.; Wong, C. P. Nano Lett. 2007, 7, 3388. (17) Lee, W.; Jin, M. K.; Yoo, W. C.; Lee, J. K. Langmuir 2004, 20, 7665. (18) Zhai, L.; Cebci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (19) (a) Choi, S.-J.; Suh, K. Y.; Lee, H. H. J. Am. Chem. Soc. 2008, 13, 6312. (b) Jeong, H. E.; Lee, S. H.; Kim, J. K.; Suh, K. Y. Langmuir 2006, 22, 1640. (20) Zie, Q.; Zu, J.; Feng, L.; Jiang, L.; Tang, W.; Luo, X.; Han, C. C. Adv. Mater. 2004, 16, 302. (21) Feng, H.; Wang, S.; Zi, J.; Tang, Z.; Jiang, L. J. Phys. Chem. C 2008, 112, 11454. (22) Xi, J.; Feng, L.; Jiang, L. Appl. Phys. Lett. 2008, 92, 053102. (23) (a) Hikita, M.; Tanaka, K; Nakamura, T.; Kajiyama, T.; Takahara, A. Langmuir 2005, 21, 7299. (b) Steele, A.; Bayer, I.; Loth, E. Nano Lett. 2009, 9, 501.
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film24 and re-entrant surface25 were used to produce a superrepellent surface. However, electrodeposition methods are timeconsuming (∼3 days21 or repeated process22), and polymer structures by phase separation20 or plasma polymerization,24 nanocomposited particles,23 microhoodoo structures25 (silicon disk arrays), and electrospun fibers25 are susceptible to matting in some harsh environments. In this work, we present a method that we developed for fabricating a super water- and moderately oil-repellent robust surface structure on a large area (4 in. wafer) with a wet chemicaletching method that does not involve any sophisticated experimental setup. In general, fabrication of a hierarchical structure involves spreading nanoparticles23 or electrospun fibers25 on microstructures or two-step molding.19a In contrast, simple wet etching leads to a hierarchical structure consisting of nano- and microscale features. This surface is super-repellent to both oil and water, with the contact angle larger than 150°. Any part of the fabricated surface can be made amphiphilic simply by exposing the part to ultraviolet (UV) light. Self-cleaning of the surface can be realized with oil.
Experimental Section The procedure involved in the chemical wet etching is illustrated in Figure 1. The silicon wafer was first cleaned by nitrogen blowing, ultrasonification for 6 min each in trichloroethylene (TCE) and methanol, rinsing with deionized (DI) water, and blow drying by nitrogen. The cleaned wafer (Figure 1a) was baked on a hot plate at a temperature between 100 and 120 °C for 10 min to completely remove any residual water and solvent on the wafer. Gold was then deposited by thermal evaporation. Nanoclusters of gold formed when the deposition was carried out at 10-5 torr at a deposition rate of 0.5 nm/s (Figure 1b). The mean diameter of the clusters ranged from 10 to 20 nm, and the height was 3-4 nm (Figure 1e). A mixture solution of hydrofluoric acid (HF), hydrogen peroxide (H2O2), and DI water in the ratio of 1:5:10 by volume was used to etch the silicon surface. The parts covered by nanoclusters are etched selectively because of catalytic etching (24) Coulson, S. R.; Woodward, I. S.; Badyal, J. P. S.; Brewer, S. A.; Willis, C. Chem. Mater. 2000, 12, 2031. (25) (a) Tuteja, A.; Choi, W.; Ma, M.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Science 2007, 318, 1618. (b) Tuteja, A.; Choi, W.; Ma, M.; McKinley, G. H.; Cohen, R. E.; Rubner, M. F. MRS Bull. 2008, 33, 752. (c) Tuteja, A.; Choi, W.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 18200.
Published on Web 4/29/2009
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Figure 1. (a-d) Schematic representation of preparing the sample for wet chemical etching. (e) AFM image of Au nanoclusters on silicon wafer. The average height of the nanocluster is 4.5 nm, and the diameter is in the range of 20 nm. (f ) Cross-sectional SEM image of 40 min etched silicon wafer. Panel f is enlarged in panel g. promoted by metal-assisted etching26 (Figure 1c) at room temperature. This process is based on metal-seed-induced excessive local oxidation and dissolution of silicon substrate. The etched silicon surface was then treated with a self-assembled monolayer (SAM) material of (tridecafluoro-1,1,2,2-tetrahydrooctyl)-trichlorosilane [CF3-(CF2)5-CH2CH2SiCl3] (FOTCS) to lower the surface energy of the etched silicon (Figure 1d). Crosssectional scanning electron microscopy (SEM) images in panels f and g of Figure 1 show that the silicon surface treated with the etching solution for 40 min has nanoholes and is very rough.
Fabrication of a Super Water- and Moderately OilRepellent Surface. A wafer was cleaned by nitrogen blowing, ultrasonification for 6 min each in TCE and methanol, and rinsing deionized water and blow drying by nitrogen. The cleaned wafer was baked on a hot plate at a temperature between 100 and 120 °C for 10 min to fully remove any residual water and solvent on the wafer. Onto this wafer, a gold layer that has ∼3 nm thickness was thermally deposited by evaporation at a pressure of ∼10-5 torr. The deposition rate was kept at less than 5 A˚/s to produce Au nanoclusters. A wet chemical etching of silicon wafer was carried out at room temperature in a mixed solution of HF, H2O2, and DI water in the volume ratio of 1:5:10. The etched silicon surface was treated in an Ar chamber with a material of low surface energy, which, in this case, is a SAM material of FOTCS, to lower the surface energy. UV Illumination for the Amphiphilic Surface. Through a patterned SUS mask, a super water- and moderately oil-repellent (26) Peng, K.; Hu, J.; Wu, Y. Y.; Fang, H.; Xu, Y.; Lee, S.; Zhu, J. Adv. Funct. Mater. 2006, 16, 387.
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surface was selectively illuminated by UV (Vision Semichem Corp. AH-1700) for 10 min. Self-Cleaning Effect. On the flourinated SAM-treated substrate (etched for 40 min), a droplet (1 mL volume) of glycerin was dispensed and then tilted by 10°. Scanning Electron Microscopy (SEM). Images were taken using high-resolution SEM (Philips XL30FEG). Samples were coated with a 4 nm Pt layer prior to analysis to prevent charging. Contact Angle Measurements. A Rame-Hart goniometer (Mountain Lakes) equipped with a camera was used to measure the contact angles of drops of 3-500 μL in volume. The volume of water droplet can be measured precisely. However, the droplet size of the other liquids could only be estimated, because of high viscous properties.
Results and Discussion To examine the evolution of the nanostructure as the time of etching increases, cross-sectional SEM and AFM images were taken for those samples etched for various periods of time, and the results are shown in Figure 2. As the etching time increases from 10 min (Figure 2a) to 80 min (Figure 2f ), the depth of the nanoholes increases and the surface root-mean-square (rms) roughness also remarkably increases from 40.04 to 287.4 nm. For example, the nanohole depth increases from 1.5 to 10 μm as the etching time is increased from 10 to 60 min. The contact angles and contact angle hysteresis that various liquids make with the etched silicon surface are shown in Figure 3. These are DI water, glycerine (Samchun Chemical), DOI: 10.1021/la900106s
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Figure 2. Cross-sectional SEM and AFM images of etched silicon wafer with deposited Au nanoclusters: (a) 10 min, (b) 20 min, (c) 30 min, (d) 40 min, (e) 60 min, and (f ) 80 min of etching. The depth of etched nanoholes and roughness of surface increases with time. The rms roughness is 40.04, 97.23, 102.2, 166.4, 205.1, and 287.4 nm, respectively.
triethanolamine (TEA, Sigma-Aldrich), and lubricating oil (Mastumura Oil Corp. Neovac MR-200). For all of the liquids, the contact angle increases with an increasing etching time, reflecting the increase in both the surface roughness and hole depth. The liquids with relatively high surface tension, i.e., DI water (72.7 mN/m) and glycerine (63.4 mN/m), cannot penentrate readily into small diameter holes (∼200 nm), and the contact angle becomes larger than 150° even with relatively short etching time. As shown in Figure 3e, the contact angle increases linearly with the etching time in these cases. The liquids with relatively lower surface tension, such as TEA (46.0 mN/m)27 and lubrication oil (35 mN/m), however, tend to wet the etched surface unless the etching progresses further to produce deep holes and to make the surface rougher. Only after more than 80 min of etching would the surface become surface-repellent, with a contact angle larger than 150°. As shown in the SEM images in Figure 2, wet etching for 40 min leads to a random porous structure, while etching for 30 min leads to a vertically oriented porous structure. The reentrant texture25 created by the random porous structure with water, glycerin, and oil droplets causes the system to behave differently. Because of this re-entrant curvature, contact angles of liquid increased radically and the contact hysteresis decreased (27) Vazquez, G.; Alvarez, E.; Rendo, R.; Romero, E.; Navaza, J. J. Chem. Eng. Data 1996, 41, 806.
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significantly in 40 min of etching. However, other liquid droplets with surface tension lower than 35 mN/m, such as iso-octane (18.77 mN/m), isopropyl alcohol (IPA) (21.79 mN/m), and toluene (28.53 mN/m), readily wetted despite the 80 min etch surface. Although the structure represents a highly rough surface, low-surface-tension liquids do not sustain re-entrant curvature on the surface. Sliding contact angles with 80 min etched wafer are displayed in Table 1. This highly repellent substrate has quite a low sliding angle with water and glycrtine droplets. A modified Cassie-Boxter’s relationship28 can be used to explain the increase in the contact angle with an increasing etching time. The relationship is cos θ ¼ rφ φs cos θ þ φs -1
ð1Þ
where θ* is the apparent contact angle on the etched (patterned) surface, θ is the equilibrium contact angle on the flat surface, rφ is the roughness of the wetted area, and φs is the area fraction of the liquid-air interface occupied by the rough surface. The fractional area φs decreases with an increasing etching time, and therefore, the contact angle on the etched surface, θ*, can become larger than 90°, even if the intrinsic contact angle θ is smaller than 90°. (28) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546.
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Figure 5. In situ images for self-cleaning demonstration with a glycerin droplet (arrows). (a) Surface with adsorbed aluminum hydroxide particles. These particles remained on the surface even after nitrogen blowing. (b-d) Images of the path cleaned by a rolling glycerin droplet in 3, 5, and 7 s after the droplet was dispensed. The tilting angle is 10°.
Figure 3. (a) Contact angle images of DI water, glycerine, TEA, and oil. These liquids have different properties in terms of viscosity and surface tension. (b) Contact angle hystreresis with etching time. Table 1. Sliding Angles with Diverse Liquid Droplets on the 80 min Etched Wafer
sliding angle (deg)
DI water
glycerine
TEA
oil
∼0
6
58
15
functional group of the SAM is broken by the light, exposing surface oxygen ions. The dark patches in Figure 4 are the parts of the super-repellent silicon surface that were transformed to superamphiphilic surface by the DUV treatment. Oil, glycerine, and water are seen to wet the patches, but they form droplets on the nontreated super-repellent surface. One advantage that a superhydrophobic surface offers is that it allows for self-cleaning of the surface with water. A superrepellent surface should provide such an advantage with respect to both water and oil. A self-cleaning capability is demonstrated in Figure 5 for the super-repellent surface. The etched silicon surface (40 min of etching) was first contaminated with aluminum hydroxide particles, and then the surface was blown with compressed nitrogen to remove particles that are free to move. Onto this contaminated surface, a glycerine droplet was placed (Figure 5a). When the surface was tilted by 10°, the droplet descended on the surface, and the snapshots taken after 3, 5, and 7 s are shown in panels b-d of Figure 5. As shown by the images, the self-cleaning capability is clearly realized.
Summary
Figure 4. Image of super-repellent surface (4 in. wafer) etched for 80 min. The nine rectangular patches were exposed to DUV through a SUS mask. DI water, oil, and glycerin droplets have a high contact angle (>150°) on a super-repellent surface, but these liquids readily wet the exposed patches (dark brown patches).
In summary, we have presented a new facile method for a super water- and moderately oil-repellent surface fabricated with wet chemical etching and treated by fluorinated SAM. The suface consisting of nanoholes is super-repellent to both water and oil, with the contact angle being larger than 150°. The super-repellent surface can be transformed to an amphiphilic surface simply by exposing those desired parts of the surface to DUV. This controllable super-repellent surface should have potential applications in water harvest, anti-biofouling, and a highly lubricated surface with extremely low friction. The self-cleaning of the surface contaminated with sticky meterial can easily be realized with the super-repellent surface.
Any part of the amphiphobic surface realized by the etching can be made superamphiphilic by exposing the part to deep UV (DUV) (λ ∼ 254 nm). This change occurs because the end
Acknowledgment. The authors thank Dr. Jeonggon Son for AFM measurements. This work is supported by KRF-2008-314D00234.
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