pubs.acs.org/Langmuir © 2009 American Chemical Society
Bioinspired Holographically Featured Superhydrophobic and Supersticky Nanostructured Materials Sung-Gyu Park,† Jun Hynk Moon,‡ Seung-Kon Lee,† Jaewon Shim,† and Seung-Man Yang*,† †
National Creative Research Initiative Center for Integrated Optofluidic Systems, Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 335 Gwahangno, Yuseong-gu, Daejeon, 305-701 Korea and ‡Department of Chemical and Biomolecular Engineering, Sogang University, 1 Shinsu-dong, Mapo-gu, Seoul, 121-742 Korea Received September 22, 2009. Revised Manuscript Received November 11, 2009 In this Letter, we present an intriguing method for fabricating polymeric superhydrophobic surfaces by reactive-ion etching of holographically featured three-dimensional structures. Using the proposed strategy, we generated both lotus and gecko surfaces by simply controlling the incident angle of the laser beam during holographic lithography. The adhesion force of the gecko-state superhydrophobic surfaces was the highest yet reported for an artificial superhydrophobic surface. The well-controlled patterns enable an in-depth understanding of superhydrophobic and superadhesive surfaces. In particular, the present observations provide direct evidence of a high adhesive force resulting from surface-localized wetting, which is quite different from previously suggested mechanisms.
Introduction Some natural surfaces, such as superhydrophobic lotus leaves and supersticky gecko foot pads, exhibit amazing surface properties.1,2 The wettability and adhesion of liquids on solid surfaces are governed mainly by two parameters: chemical composition and surface morphology. Over the years, considerable effort has been devoted to fabricating bioinspired superhydrophobic and supersticky surfaces by various methods including self-assembly,3-5 electrochemical deposition,6 electrospinning,7,8 photolithography,9-12 and templating methods.13,14 However, there is still the lack of full understanding how the surface structures endow either superhydrophobicity, supersticky property, or sometimes both at the same time. Superhydrophobic surfaces can be classified into “lotus” (or Cassie) and “gecko” states.15 The lotus state3-12,16-20 is *To whom correspondence should be addressed. Telephone: þ82-42-3503962. Fax: þ82-42-350-5962. E-mail:
[email protected].
(1) Sun, T. L.; Feng, L.; Gao, X. F.; Jiang, L. Acc. Chem. Res. 2005, 38, 644. (2) Li, X. M.; Reinhoudt, D.; Crego-Calama, M. Chem. Soc. Rev. 2007, 36, 1350. (3) Srinivasan, S.; Praveen, V. K.; Philip, R.; Ajayaghosh, A. Angew. Chem., Int. Ed. 2008, 47, 5750. (4) Yuan, J. K.; Liu, X. G.; Akbulut, O.; Hu, J. Q.; Suib, S. L.; Kong, J.; Stellacci, F. Nat. Nanotechnol. 2008, 3, 332. (5) Genzer, J.; Efimenko, K. Science 2000, 290, 2130. (6) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z. Q.; Jiang, L.; Li, X. Y. J. Am. Chem. Soc. 2004, 126, 3064. (7) Lim, J.-M.; Yi, G.-R.; Moon, J. H.; Heo, C.-J.; Yang, S.-M. Langmuir 2007, 23, 7981. (8) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338. (9) Wagterveld, R. M.; Berendsen, C. W. J.; Bouaidat, S.; Jonsmann, J. Langmuir 2006, 22, 10904. (10) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (11) He, B.; Patankar, N. A.; Lee, J. Langmuir 2003, 19, 4999. (12) Gao, L. C.; McCarthy, T. J. Langmuir 2006, 22, 2966. (13) Cho, W. K.; Choi, I. S. Adv. Funct. Mater. 2008, 18, 1089. (14) Jin, M. H.; Feng, X. J.; Feng, L.; Sun, T. L.; Zhai, J.; Li, T. J.; Jiang, L. Adv. Mater. 2005, 17, 1977. (15) Wang, S.; Jiang, L. Adv. Mater. 2007, 19, 3423. (16) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (17) Blossey, R. Nat. Mater. 2003, 2, 301. (18) Li, H. J.; Wang, X. B.; Song, Y. L.; Liu, Y. Q.; Li, Q. S.; Jiang, L.; Zhu, D. B. Angew. Chem., Int. Ed. 2001, 40, 1743. (19) Chiou, N. R.; Lui, C. M.; Guan, J. J.; Lee, L. J.; Epstein, A. J. Nat. Nanotechnol. 2007, 2, 354. (20) Gu, Z. Z.; Uetsuka, H.; Takahashi, K.; Nakajima, R.; Onishi, H.; Fujishima, A.; Sato, O. Angew. Chem., Int. Ed. 2003, 42, 894.
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characterized by a low sliding angle (SA), whereas the gecko state shows high adhesive properties. Previous studies indicate that creation of a sealed air pocket is one of the main factors in generating the high adhesive force of gecko states.14,21 However, exploiting superhydrophobic and superadhesive properties remains a challenging issue in terms of physical and chemical interactions as well as structural diversity. Here, we present an intriguing method for fabricating polymeric superhydrophobic surfaces by reactive-ion etching (RIE) of holographically featured 3D structures. Using the proposed strategy, we generated both lotus and gecko surfaces by simply controlling the incident angle of the laser beam during holographic lithography (HL).22-27 These well-controlled patterns enable an in-depth understanding of superhydrophobic surfaces and can be used in various applications including handling of infinitesimal quantities of liquids,13,14,28 drag reduction in microfluidics,29,30 and localized reactions.31,32
Experimental Section Prism Holographic Lithography. We prepared the SU-8 photoresist (PR) by mixing SU-8 resin (150 wt % to solvent) and a cationic photoinitiator (triarylsulfonium hexafluorophosphate, 1.3 wt % to resin) in solvent (γ-butyrolactone (GBL)).24 A 12 μm thick PR film was obtained by spin-casting the solution onto a (21) Guo, Z. G.; Liu, W. M. Appl. Phys. Lett. 2007, 90, 223111. (22) Campbell, M.; Sharp, D. N.; Harrison, M. T.; Denning, R. G.; Turberfield, A. J. Nature 2000, 404, 53. (23) Maldovan, M.; Thomas, E. L. Nat. Mater. 2004, 3, 593–600. (24) Lee, S.-K.; Park, S.-G.; Moon, J. H.; Yang, S.-M. Lab Chip 2008, 8, 388. (25) Kang, J.-H.; Moon, J. H.; Lee, S.-K.; Park, S.-G.; Jang, S. G.; Yang, S.; Yang, S.-M. Adv. Mater. 2008, 20, 3061. (26) Park, S.-G.; Lee, S.-K.; Moon, J. H.; Yang, S.-M. Lab Chip 2009, 9, 3144. (27) Jang, J. H.; Ullal, C. K.; Maldovan, M.; Gorishnyy, T.; Kooi, S.; Koh, C. Y.; Thomas, E. L. Adv. Funct. Mater. 2007, 17, 3027. (28) Aussillous, P.; Quere, D. Nature 2001, 411, 924. (29) Cottin-Bizonne, C.; Barrat, J. L.; Bocquet, L.; Charlaix, E. Nat. Mater. 2003, 2, 237. (30) Steinberger, A.; Cottin-Bizonne, C.; Kleimann, P.; Charlaix, E. Nat. Mater. 2007, 6, 665. (31) Sun, T. L.; Tan, H.; Han, D.; Fu, Q.; Jiang, L. Small 2005, 1, 959. (32) Shiu, J. Y.; Chen, P. Adv. Funct. Mater. 2007, 17, 2680.
Published on Web 11/24/2009
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Figure 1. Controlling 3D nanostructures by tilting the truncated triangular pyramidal prism with respect to the perpendicularly incident laser beam. (a) Geometrical features of the prism and the wave vectors when the incident laser beam impinges perpendicularly to the top surface of the prism. (b) SEM image of the resulting 3D SU-8 interference pattern. Inset shows the iso-intensity surface of the 3D interference pattern. (c) Cross-sectional view of the SEM image. (d) Changes in the wave vectors when the prism is tilted at an angle of θ with respect to the perpendicularly incident laser beam. k0 changes to k00 due to this tilting. k0 and k00 are in the same plane (red shaded plane). (e) SEM image of the 3D SU-8 interference pattern when the prism is tilted by an angle of 0.8° with respect to the perpendicularly incident laser beam. Inset shows the iso-intensity surface of the 3D interference pattern. The transient layer was designated as the region between line elements a and b in the (111) plane. (f) Cross-sectional view of the SEM image in (e). glass substrate and subsequent softbaking at 95 °C for 20 min to evaporate the solvent. A 325 nm He-Cd laser beam (50 mW, Kimmon) with a diameter of 1 cm after passing through a beam expander was incident onto a top-cut fused silica prism with a refractive index of 1.48 at 325 nm. After laser exposure followed by the postexposure bake (PEB), a 3D nanostructure was obtained after developing with propylene glycol methyl ether acetate (PGMEA) and rinsing with 2-propanol. CF4 Reactive-Ion Etching. 3D nanostructures were etched and chemically modified using 13.56 MHz RF reactive-ion etching equipment (RIE, Vacuum Science). CF4 gas was introduced into the chamber at a flow rate of 200 sccm (standard cubic centimeters per minute) and the base pressure was kept at 0.95 Torr while the RF power was maintained at 100 W in each experiment. In the CF4 plasma etching, physical and chemical etching processes occur simultaneously. Therefore, the energetic inert ions are used to sputter off the 3D nanostructured surfaces. In addition, the reactive species such as fluorine radicals and other excited neutrals etched the surface by chemical reaction.33 Characterization. The morphologies of the 3D nanostructures fabricated by HL were investigated by field emission scanning electron microscopy (FE-SEM; Hitachi S-4800) and tapping mode atomic force microscopy (AFM, Veeco, Dimension 3100). Water contact angles (CAs) were measured using contact angle goniometry (KRUSS, DSA 10-Mk2). The CA values were measured at four points on each sample using 5 μL water droplets; the mean values are reported in this paper. Close inspection of the SEM images indicated that, in the tilted surface subjected to RIE for 18 and 30 min, about 18 and 23 air cavities were formed per 10 10 μm2 area, respectively. The densities of air cavities were thus approximately 1.8 105 and 2.3 105 mm-2, respectively. An X-ray photoelectron spectrometer (XPS, VG Microtech, ESCA (33) Vora, K. D.; Holland, A. S.; Ghantasala, M. K.; Mitchell, A. Proc. SPIE 2004, 5276, 162.
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2000) equipped with a Mg KR photon source was used to analyze the chemical composition at the surface of the cross-linked SU-8 thin films before and after RIE treatment.
Results and Discussion Figure 1a illustrates the geometrical features of the truncated triangular pyramidal prism used in the HL, and the interfering beams generated when the incident laser beam impinges at an angle perpendicular to the top surface of the prism. Three beams, designated by wave vectors k1, k2, and k3, are generated by refraction from the three side surfaces of the prism, and a central beam of wave vector k0 travels through the top truncated surface of the prism. These four beams recombine at the bottom of the prism, which sits on top of a SU-8 PR substrate. Figure 1b shows the theoretical iso-intensity surface and an experimental SEM image of the 3D interference pattern generated by the four interfering beams. The resulting 3D pattern has a face-centered cubic (FCC) lattice structure, in which the motifs at lattice points (i.e., periodically repeated units) are elongated in the [111] direction with a length of about 750 nm and connected by bridges to their nearest neighbors as shown in Figure 1b and c. If the prism is tilted by an angle of θ away from the perpendicular incidence of the laser beam as depicted in Figure 1d, the central beam of wave vector k0 is tilted to k00 and the three beams designated by wave vectors k1, k2, and k3 are changed to k10 , k20 , and k30 , respectively. Specifically, when the tilting angle is 0.8°, the wave vectors of the four beams without and with the tilting are {k0, k1, k2, k3} = 2πn/λ {[0.000 0.000 -1.000], [-0.315 -0.182 -0.932], [0.315 -0.182 -0.932], [0.000 0.363 -0.932]} and {k00 , k10 , k20 , k30 } = 2πn/λ {[-0.009 0.000 -0.999], [-0.309 -0.179 -0.934], [0.309 -0.179 -0.934], [0.000 0.357 -0.934]}, respectively, where n is the refractive index of the fused silica prism DOI: 10.1021/la9035826
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Figure 2. RIE effect on the untilted surface morphology of 3D structures generated by HL. (a) Theoretical 3D interference pattern when the incident laser beam impinges perpendicularly to the top surface of the prism. SEM images of 3D nanosturctures after (b) 6 min, (c) 9 min, and (d) 21 min of RIE. (e) 20° tilted view of the SEM image of 3D nanostructures after 21 min of RIE. (f) AFM image of (d).
(n = 1.48) and λ is the wavelength of the laser (λ = 325 nm). It can be noted in Figure 1e that transient layers appear periodically on the FCC lattices, as designated by the region between the line elements a and b in the (111) plane. The SEM image in Figure 1f shows that the resulting FCC lattice is tilted with respect to the substrate surface and the transient layer corresponds to the region where the top (111) plane partially overlaps with the underneath (111) plane. Such transient layers were generated solely due to the tilting of the incident beam and appeared repeatedly. In the case of θ = 0.8°, the width of the transient layer was about 3 μm with a 10 μm period as noted from Figure 1e and f. In particular, we can control the period of the transient layer and the inclination of the 3D lattice along the [111] direction by changing the tilting angle (see Figure S1 in the Supporting Information). To form a superhydrophobic surface, we applied RIE directly on the holographically featured 3D pattern. CF4 RIE changes the 3D surface morphology and transforms the SU-8 surface into substances with low surface free energy, such as chemically inert fluorinated groups. The presence of fluorine atoms at the surface was confirmed by X-ray photoelectron spectroscopy (see Figure S2 and Table S1 in the Supporting Information). The modified SU-8 surface after 9 min CF4 RIE has the composition of 51.7% C, 16.1% O, and 32.2% F, which is considerably different from 81.7% C and 18.3% O of the as-prepared SU-8 surface. Further 9 min CF4 RIE does not change appreciably the chemical composition but changes the surface morphology. Further analysis of the functional groups on the surface was performed using a highresolution C 1s spectrum (see Figure S3 in the Supporting Information). The C 1s spectrum for the as-prepared SU-8 consists of two components. One at 284.5 eV indicates the presence of hydrocarbons (C-C and C-H), and the other at 1470 DOI: 10.1021/la9035826
286.2 eV arises from C-O. In the spectrum of the modified SU-8 surface, a new peak appears at 289.1 eV, which is assigned to C-F.34 Figure 2 shows the surface morphology of untilted FCC lattices subjected to various etching times. In the as-prepared surface, the motifs (the apex of the triangle) are connected by bridges. As the etching time is increased, the volume fraction of this structure decreases, with reductions in both the bridge width and the radius of the motifs (Figure 2b). After 9 min of RIE, about 10-20 nm scale bumps appear on both the 200 nm motifs and the top bridges (Figure 2c). The SEM images of the structure after 21 min of RIE (Figure 2d and e) disclose that the tips of motifs in the surface layer are extremely sharp. Close inspection of the surface reveals an average tip diameter of about 40 nm (Figure 2e). This is clear in the AFM image of the surface morphology (Figure 2f), which shows that RIE results in conical-shaped motifs. The formation of such sharp tips is a unique feature of applying a combination of HL and RIE. This kind of shaping can be attributed to the fact that, due to the sinusoidal intensity profile of the laser interference patterns, the central core of each motif is more highly cross-linked than the outer region, leading to faster etching in the outer region.35 The structures with dual scale roughness and needlelike structures like those produced using our method can drastically increase superhydrophobicity by minimizing the contact area of a water drop on the surface.36 In contrast to RIE of the untilted FCC lattice, RIE of the tilted FCC lattice generates a distinct morphology (see Figure S4 in the (34) Ge, J.; Kivilahti, J. K. J. Appl. Phys. 2002, 92, 3007. (35) Moon, J. H.; Ford, J.; Yang, S. Polym. Adv. Technol. 2006, 17, 83. (36) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Zhai, J.; Song, Y. L.; Liu, B. Q.; Jiang, L.; Zhu, D. B. Adv. Mater. 2002, 14, 1857.
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Supporting Information). The different etching behavior of the tilted FCC lattice is due to the presence of the transient regions. Since the new (111) layer starts growing in the transient region, the bridges connecting the motifs are narrow and the motifs are short. Examination of a cross-sectional image of the unetched tilted structure discloses that the motifs of the first layer will block the direct etching of the motifs in the third layer as well as the bridges connecting motifs in the second and third layers. Therefore, regarding the second layer motifs in the transient region, there is anisotropic etching of the bridges between the second and third layers: one or two of each set of three bridges connecting the motifs between the second and third layers is delayed by the selective shadow effect of the first layer. However, this type of anisotropic etching effect does not occur in the untilted surface. After one or two bridges among the three connecting the motifs between the second and third layers are disconnected, the motifs collapse as displayed in Figure S4 in the Supporting Information. Collapsed motifs in the transient region can be found after 18 min of RIE, whereas the motifs in the outer region remain stable. The density of the collapsed pillars is estimated about 1.8 105 mm-2 after 18 min of RIE and it increases to 2.3 105 mm-2 after 30 min of RIE. Next, we measured the CA and SA of the untilted and tilted samples as a function of the etching time, and the CA results are plotted in Figure S5 in the Supporting Information. The area fraction of air in the surface was calculated by using CassieBaxter’s law.37 cos θr ¼ f ð1 þ cos θs Þ -1
ð1Þ
where f is the area fraction of the solid/water interface and θr and θs denote the CAs of a water droplet on rough and smooth surfaces, respectively. This equation indicates that a high area fraction of air contributes greatly to the enhancement of hydrophobicity. In the case of the untilted surfaces, the CA reaches 160° after 9 min of RIE due to dual scale roughness and then remains over 160° with further etching. A CA of 160° corresponds to an increase of about 60° over the CA of the cross-linked SU-8 smooth surfaces subjected to the same RIE conditions (see Figure S6 in the Supporting Information). These high CAs are attributed to the dual scale roughness and the array of sharp tips with short period (650 nm).9,36 We calculated the air fractions based on Figures S5 and S6 in the Supporting Information. For example, the untilted and smooth surfaces showed a CA of 160° and 99° after 9 min of RIE, respectively. The air fraction was calculated to be 0.92 based on the Cassie-Baxter equation. The calculated air fractions remained higher than 0.92 after 9 min of RIE. In the SA measurements, the SA of a 5 μL water droplet on the untilted surface subjected to 9 min or more of RIE was less than 3° (see Videos S1 and S2 in the Supporting Information), indicating that the surface was highly slippery. Therefore, the etched untilted surface resembles the lotus state. Compared to the untilted sample, the CA at a given etching time was lower for the tilted surface and as etching progressed the CA slowly saturated to a maximum value. Specifically, the available contact surface area of the transient region is higher than that of the outer surface as directly noted from Figure 1b and e, and this difference of surface morphology remains unchanged during RIE. Therefore, the high contact area of the transient region is responsible for the lower contact angle of the tilted surface. Contrary to the behavior of the untilted sample, in SA measurements of the tilted samples, the droplet did not roll off or (37) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546.
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slide even when the sample was upside down. We found that the tilted surface after 30 min of RIE could hold a water droplet weighing up to 13 mg with a contact diameter of 1.2 mm when the surface was upside down and that the water droplet did not fall off during vibration of the sample (see Video S3 in the Supporting Information). Therefore, the etched surface of the tilted sample showed gecko-state superhydrophobicity, characterized by a high CA and high adhesion force. The maximum adhesion force was normalized to be 0.01 N/cm2. Although this value is only 0.1% of that of a gecko foot pad (10 N/cm2),38,39 it represents the highest adhesion force for water yet reported for an artificial superhydrophobic surface.13,14,21 We also investigated wetting characteristics depending on the period of the transient region (see Figures S7 and S8 in the Supporting Information). The surface morphologies when the prism tilting angle was higher than 1.5° were different from those with the prism tilting angle of 0.8°. All 3D nanostructured surfaces with the prism tilting angle higher than 1.5° showed a hydrophobic wetted state with low CAs (
