pubs.acs.org/Langmuir © 2010 American Chemical Society
Superhydrophobic Cylindrical Nanoshell Array Yong-Bum Park, Maesoon Im, Hwon Im, and Yang-Kyu Choi* Department of Electrical Engineering, KAIST, Daejeon 305-701, Republic of Korea Received March 4, 2010. Revised Manuscript Received April 23, 2010 A superhydrophobic property was demonstrated on a cylindrical poly crystalline silicon nanoshell array due to its geometrical properties, even without a hydrophobic coating. The proposed structure showed superior water-repellency compared to a conventional pillar structure with an identical structural dimension. This superhydrophobic property is attributed to an air pillar that exists in the nanoshell. Through the calculation of capillary pressure, the stability of the air pillar was confirmed. Furthermore, a droplet impinging test was conducted on the fabricated cylindrical nanoshell array to verify the robust Cassie state of the proposed structure under a dynamic condition.
Recently, superhydrophobic (SHP) surfaces have attracted an enormous amount of attention related to their practical applications. Areas include microfluidics in biotechnology, impermeable textiles, fog-resistance applications, and self-cleaning surfaces. Intensive studies have been made to realize a SHP surface through the formation of microstructures1-3 or nanostructures.4,5 The wetting behavior of a liquid droplet on these textured surfaces follows the Cassie and Baxter (henceforth the Cassie) or the Wenzel model.6 However, most of these structures require a coating with low surface energy chemical groups to realize a SHP surface.1-4 Moreover, for practical applications, such micro- or nanostructured surfaces must maintain their SHP property even under dynamic conditions (e.g., impinging5,7 or squeezing8 of the droplet on the surface), which is closely related to a robust Cassie (nonwetting) state.8,9 This study proposes a three-dimensional structure, as schematically shown in Figure 1a. This is termed cylindrical nanoshell array, which by its geometrical merit shows a water contact angle (CA) of 166° and contact angle hysteresis (CAH) of 5° without a chemical treatment or a hydrophobic coating. Figure 1b shows a photograph of a water droplet on the cylindrical nanoshell array. This remarkable SHP property originates from the physical property of the cylindrical polycrystalline silicon (poly-Si) nanoshell structure: a small liquid/solid interface thereby shows a high CA (according to the Cassie model). Air trapping sites inside the nanoshell contribute significantly to this remarkable SHP property by forming air pillars. The stability of this air pillar was confirmed by the calculation of the capillary pressure inside the nanoshell structure. Theoretical CA values for the Cassie and Wenzel states were extracted from the CA value predicted by the minimum Gibbs free energy. These CA values were then com*To whom correspondence should be addressed. E-mail:
[email protected]. ac.kr. Telephone: þ82-42-350-3477. Fax: þ82-42-350-8565.
(1) Nosonovsky, M.; Bhushan, B. Nano Lett. 2007, 7, 2633–2637. (2) Yeh, K.-Y.; Chen, L.-J.; Chang, J.-Y. Langmuir 2008, 24, 245–251. (3) Bhushan, B.; Jung, Y.-C.; Koch, K. Langmuir 2009, 25, 3240–3248. (4) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701–1705. (5) Kwak, G.-J.; Lee, M.-K.; Senthil, K.; Yong, K.-J. Appl. Phys. Lett. 2009, 95, 153101. (6) He, B.; Patankar, N. A.; Lee, J.-H. Langmuir 2003, 19, 4999–5003. (7) Bartolo, D.; Bouamrirene, F.; Verneuil, E.; Buguin, A.; Silberzan, P.; Moulinet, S. Europhys. Lett. 2006, 74, 299–305. (8) Kwon, Y.-J.; Patankar, N. A.; Choi, J.-K.; Lee, J.-H. Langmuir 2009, 25, 6129–6136. (9) Nosonovsky, M. Langmuir 2007, 23, 3157–3161.
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Figure 1. (a) Three-dimensional schematic image of the cylindrical nanoshell array. (b) Photograph of a water droplet on the SHP cylindrical poly-Si nanoshell array surface. (c) Process flow of the SHP cylindrical nanoshell array.
pared with the empirical values to verify that the droplet on the fabricated cylindrical nanoshell array is in the Cassie state. Moreover, this structured surface maintains its SHP property under dynamic conditions, according to a droplet impinging test. The fabrication process of the cylindrical poly-Si nanoshell array, which uses the concept of a novel sublithographic patterning known as “spacer lithography”,10 is schematically shown in Figure 1c. First, a sacrificial oxide was deposited by plasma-enhanced chemical vapor deposition (PECVD). Afterward, an array of PECVD oxide pillars with a height of 2 and 4 μm were delineated on the silicon wafer by conventional photolithography and reactive ion etching (RIE). These oxide pillars served as a sacrificial support for the cylindrical nanoshell to morph at the sidewall and, hence, determine the height of the cylindrical nanoshell (Hshell). Subsequently, 80 nm of poly-Si was deposited by low-pressure chemical vapor deposition (LPCVD) on the oxide pillar array. The poly-Si was then etched by RIE with an 80 nm target to reveal the top surface of the oxide pillars as shown in the second schematic in Figure 1c. In order to reduce the thickness of the deposited poly-Si layer at the sidewall of the oxide pillars, oxide pillars with a positive (10) Choi, Y.-K.; Lee, J.-S.; Zhu, J.; Lee, L. P.; Bokor, J. J. Vac. Sci. Technol. 2003, 21, 2951–2955.
Published on Web 05/04/2010
DOI: 10.1021/la100911s
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Figure 2. SEM images of a cylindrical nanoshell array. (a) Top view. (b) Tilted view to show the sub-30 nm thickness of the cylindrical shell. (c) Cross-sectional view. (d) Tilted view of the array. The inset photo shows a water droplet (∼10 μL) with a CA of 166° on a cylindrical shell array.
slope were intentionally fabricated in the first step, as shown in Figure 1c. Hence, the thickness of the deposited poly-Si layer at the sidewall of the oxide pillars was reduced from 80 nm to approximately 30 nm after the poly-Si RIE process. Next, the oxide pillars left inside the poly-Si nanoshell array were removed by buffered oxide etchant (BOE). Thereafter, they remained as a form of cylindrical nanoshell array with a sub-30 nm sidewall thickness. Figure 2 shows SEM images of a cylindrical nanoshell array fabricated with a height of 2 μm. The diameter (D) and thickness (T) of the cylindrical nanoshell were measured to be 1 μm and 30 nm, respectively. The pitch length between the cylinders was 3.5 μm. The inset of Figure 2d shows a water droplet (10 μL) on the cylindrical nanoshell array. Its CA is 166° without application of a chemical treatment on the surface. The CAH value, which is also a critical parameter in the characterization of a waterresistant surface, was found to be 5°, as shown in Supporting Information Figure S1c, satisfying the criteria for a SHP surface.11 Such a low CAH was obvious because the geometrical properties of the cylindrical nanoshell intrinsically reduce the solid/water interface. As a consequence, the decreased adhesion force of the surface causes the droplet to roll off, even at a slightly tilted angle. In order to observe the relationship between adhesion force and the nanoshell thickness further, the CAH value of a cylindrical nanoshell array with a much thicker sidewall poly-Si (thickness of 400 nm) was also measured. The SEM image of a fabricated 400 nm thick poly-Si sidewall is shown in Supporting Information Figure S1d. Although the static CA of this sample was 151°, its CAH was measured as 52°, as shown in Supporting Information Figure S1f, which is significantly greater than that of a nanoshell with a sub-30 nm thickness. The increased CAH is attributed to the increase in the adhesion force by the larger solid/ water interface. This indicates that the CAH value can be lowered by reducing the sidewall thickness of the cylindrical nanoshell. It is worthwhile to note that the poly-Si sidewall thickness can be controlled with a nanometer scale precisely by virtue of the aforementioned LPCVD process. Figure 3a shows the structural advantages of the cylindrical shell over conventional pillarlike structures at a height of 2 μm. (11) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644–652.
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The figure shows the CA change as ΔH=Hinit - Hoxide changes from 0 to 2 μm, which corresponds to the structural transformation of the pillar to a cylindrical shell. This was achieved by gradually hollowing out the sacrificial oxide pillar surrounded by the poly-Si shell. For a fair comparison of the surface material, a poly-Si pillar array was fabricated. The CA measured on this poly-Si pillar (without an oxide surface) corresponds to the CA at ΔH=0 μm in Figure 3a. ΔH=2 μm corresponds to the state at which the oxide pillar was fully etched, thereby leaving only the poly-Si nanoshell. Accordingly, the CA increased from 102° to 166° as the structure transformed from a poly-Si pillar array to a poly-Si nanoshell array, that is, ΔH increased from 0 to 2 μm. When ΔH = 2 μm, only the cylindrical poly-Si nanoshell remained; the structured surface entered the regime of superhydrophobicity, revealing the geometrical superiority of the cylindrical shell to a conventional pillar or post.1-3 Such a SHP property can be understood by considering the geometrical effect in the Cassie equation,12 cos θCapp ¼ rf f cos θpoly;flat - ð1 - f Þ
ð1Þ
where θC app is the apparent CA of the Cassie model, rf is the ratio of the actual area to the projected area in liquid-solid contact, f is the area fraction of solid-liquid contact, and θpoly,flat is the CA on a flat poly-Si surface. The CA is increased by minimizing the solid fraction, f, and maximizing the air fraction, (1 - f ), thereby increasing the air trapping site under the droplet.13 With a simple geometrical calculation, the area fraction of f per unit area of the cylindrical nanoshell (0.78%) was found to be an order of magnitude smaller than that of the pillar (6.2%) under the aforementioned dimension, proving its geometrical superiority. The gradual increase of CA by an increment of ΔH is attributed to the increase in rf f. A very important aspect of the nanoshell structure is that it can trap air inside the nanoshell. For convenience, this trapped air is termed an air pillar. Stability of the air pillar is critical for sustainable hydrophobicity. Here, the capillary pressure of water (12) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546–551. (13) Huang, X.-J.; Kim, D.-H.; Im, M.; Lee, J.-H.; Yoon, J.-B.; Choi, Y.-K. Small 2009, 5, 90–94.
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pressure in eq 2, showing a repelling force for water. Otherwise, when cos θpoly,flat is positive, the nanoshell experiences an absorbing force for water by the positive capillary pressure. However, trapped air inside the nanoshell may also prevent it from wetting inside the nanoshell.15,16 Figure 3b shows the negative capillary pressure for an air pillar with a radius ranging from 1 nm to 4 μm. Moreover, Pla decreases as the air pillar radius shrinks, indicating that water-repellency of the air pillar can be improved by decreasing the radius. The capillary pressure for the fabricated poly-Si nanoshell with an air pillar radius of approximately 500 nm was calculated to be -24.9 kPa, where its absolute value is a few tens of times larger than the critical hydraulic pressure for microscaled reliefs (Pc ≈ 300 Pa),17 indicating the stability of the air pillar. Here, only the stability of the air pillar is confirmed, whereas the stability of the Cassie state will be empirically verified later in this paper. The heights (Hshell) in Figure 1a of 2 and 4 μm were then fabricated to investigate the wetting behavior with respect to the height of the cylinder. The apparent CA measured for each height is plotted in Figure 3c, as marked by the blue squares in this figure. Theoretical CAs according to the Cassie (dashed line) and the Wenzel (solid line) model for the two structures are also included in the figure for comparison with the empirical data. The Cassie model followed eq 1, while the Wenzel model followed the equation18 cos θW app ¼ r cos θpoly;flat
Figure 3. (a) Water CAs of a cylindrical shell filled by an oxide pillar as a function of ΔH with a range of 0 μm e ΔH e 2 μm. The CA increased from 102° to 166° as the structural transformation moved from a poly-Si pillar to a poly-Si cylindrical shell. (b) Capillary pressure inside the cylindrical nanoshell with an air pillar radius ranging from 1 nm to 4 μm (1 nm e r e 4 μm). (c) Empirical CA measured at Hshell =2 and 4 μm. The theoretical CA values of the Cassie and Wenzel model in a range of 1 μm e Hshell e 4 μm are also included for comparison.
on the nanoshell was calculated in an effort to confirm the stability of the air pillar. The capillary pressure is noted as Pla in the inset of Figure 3b. It is governed by the following equation:14 Pla ¼
2γsl cos θpoly;flat r
ð2Þ
Here, γsl is the surface tension of water and r is the radius of the air pillar. For reference, the average CA on a flat poly-Si was measured as θpoly,flat = 98°. Depending on the sign of Pla, the wettability of the air pillar can be determined. When the value of cos θpoly,flat is negative, the air pillar will have a negative capillary (14) Yun, K.-S.; Yoon, E.-S. Biomed. Microdevices 2005, 7, 35–40.
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ð3Þ
where θW app is the apparent CA of the Wenzel model and r is the ratio of the actual area of liquid-solid contact to the projected area on the horizontal plane. Here, when f=1 in eq 1, rf=r and the Cassie equation becomes a Wenzel equation. In calculating the theoretical CA value, the minimum Gibbs free energy of the structured surface is considered to deduce the thermodynamically favored state and corresponding CA value. Hence, areal Gibbs free energy was computed by eq S(1) in the Supporting Information.19 Subsequently, θapp at which the minimum Gibbs free energy exists was extracted for the Wenzel (complete wetting) and the Cassie C (partial or nonweting) state as θW app and θapp, respectively. (See section S2 in Supporting Information for further details.) Theoretical CA values based on the Wenzel model with the minimum Gibbs free energy were 116.3° and 138.3° when Hshell was 2 and 4 μm, respectively. On the other hand, the theoretical CA value based on the Cassie model was 170.5° for both heights. The empirical CAs for Hshell of 2 and 4 μm were measured to be 166° and 167°, respectively. These values are in good agreement with the theoretical CA values based on the Cassie model, indicating that the droplet is in the Cassie state. The stability of the Cassie state under the dynamic wetting condition was investigated with the fabricated cylindrical nanoshell array. The stability of the Cassie state can be determined by observing the wetting transition. For instance, the surface has no robust Cassie state if a transition from the Cassie to the Wenzel state is induced by external force applied onto the droplet, such as releasing the drop from a certain height,7 pushing the droplet,20,21 or vibrating the droplet.22 Studies on the wetting transition from (15) Ishino, C.; Okumura, K. Eur. Phys. J. E 2008, 25, 415–424. (16) Bormashenko, E.; Bormashenko, Y.; Whyman, G.; Pogreb, R.; Stanevsky, O. J. Colloid Interface Sci. 2006, 302, 308–311. (17) Zheng, Q.-S.; Yu, Y.; Zhao, Z.-H. Langmuir 2005, 21, 12207–12212. (18) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994. (19) Marmur, A. Langmuir 2003, 19, 8343–8348. (20) Patankar, N. A. Langmuir 2004, 20, 7097–7102. (21) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457–460. (22) Bormashenko, E.; Pogreb, R.; Whyman, G.; Erlich, M. Langmuir 2007, 23, 6501–6503.
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Figure 4. Images of a free-falling water droplet rebounding from the cylindrical nanoshell array with Hshell of 2 μm. A droplet roughly 2 mm in diameter was released 1 cm above the surface.
the Cassie to Wenzel state (e.g., through calculation of critical hydraulic pressure17) are of great importance in elucidating the stability of SHP surface.23,24 Herein, droplet impinging test was performed as a means to empirically verify the stability of the Cassie state. A droplet free-fall test was conducted on the cylindrical nanoshell array when Hshell was 2 and 4 μm to confirm the stability of the Cassie state and to observe whether the kinetic energy of the freefalling droplet was sufficient to make the transition. Figure 4 shows the images of the droplet (released at a height of 1 cm) impinging on the structure with Hshell of 2 μm. The figure shows the free-falling droplet rebounding on the cylindrical nanoshell array, which demonstrates the nonwetting behavior of the surface even under a dynamic wetting condition. This rebounding phenomenon25 can be understood by the fact that the compressed air inside the nanoshell gained repelling pressure; that is, it acted as a spring to repel the liquid and prevent (23) Sbragaglia, M.; Peters, A. M.; Pirat, C.; Borkent, B. M.; Lammertink, R. G. H.; Wessling, M.; Lohse, D. Phys. Rev. Lett. 2007, 99, 156001. (24) Barbieri, L.; Wagner, E.; Hoffmann, P Langmuir 2007, 23, 1723–1734. (25) Li, X.-Y.; Ma, X.-H.; Lan, Z. Langmuir 2010, 26, 4831–4838.
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it from wetting the nanoshell. The same phenomenon was observed when Hshell was 4 μm. This experimental result verifies the stability of the cylindrical nanoshell array for maintaining the Cassie state even under dynamic conditions. In summary, the superior SHP properties of the cylindrical nanoshell array compared to that of the conventional pillarstructured array were demonstrated. Interestingly, the proposed structure showed excellent SHP properties, along with a robust Cassie state, without an additional coating with low surface energy materials. This remarkable SHP property originates from the air pillar inside the nanoshell. The negative capillary pressure in the nanoshell verified the stability and the water-repellent property of this air pillar. Additionally, the estimated CA values for the Cassie and Wenzel states were compared with the empirical values. Finally, a droplet impinging test was conducted to ensure the stability of the Cassie state under a dynamic wetting condition. Acknowledgment. This work was partially supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Ministry of Education, Science and Technology (MEST) (No. 2009-0083079). It was also partially supported by the National Research and Development Program (NRDP, 20090065615) for the development of biomedical function monitoring biosensors. This work was also sponsored by the Korean Ministry of Education, Science and Technology. M. Im is thankful for the financial support of Brain Korea 21 Project, the School of Information Technology, KAIST, in 2009. The authors would like to thank Ju-Hyun Kim, Chunghee Park, and Professor Hoon Huh for generous support in imaging with a high-speed camera. Supporting Information Available: Measured CA and CAH values of a cylindrical nanoshell array with sidewall thicknesses of 30 and 400 nm are provided to observe the relationship between CAH and the nanoshell thickness. A SEM image for cylindrical nanoshell array with each of the thickness is provided as well. Also, the detailed calculation of Gibbs free energy for the Cassie state and Wenzel state is provided to predict the CA value. This material is available free of charge via the Internet at http://pubs.acs.org.
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