Wettability Control and Water Droplet Dynamics on SiC−SiO2 Core

pubs.acs.org/Langmuir © 2010 American Chemical Society

Wettability Control and Water Droplet Dynamics on SiC-SiO2 Core-Shell Nanowires Geunjae Kwak, Mikyung Lee, Karuppanan Senthil, and Kijung Yong* Surface Chemistry Laboratory of Electronic Materials, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea Received March 29, 2010. Revised Manuscript Received May 20, 2010 We present a simple method for fabricating superhydrophobic SiC-SiO2 core-shell nanowire surfaces via the facile dip-coating of alkyltrichlorosilanes. Water droplets displayed a variety of shapes with varying surface energies on the nanowire surfaces, which could be modified through chemisorption of alkyltrichlorosilanes with variable carbon chain length. The effects of UV irradiation on the superhydrophobic nanowire arrays were also investigated. UV light efficiently decomposed the chemisorbed molecules, and the superhydrophobic surface gradually converted into a hydrophilic surface with increasing UV exposure. The water droplet impact behavior on the modified surfaces was studied to test the stability of the superhydrophobicity under dynamic conditions.

1. Introduction Lotus leaves exhibit extreme water-repellent and self-cleaning properties.1 Nature achieves such fascinating superhydrophobic properties through a combination of reduced surface energy via chemical coating and enhanced surface roughness. Considerable effort has been applied toward the fabrication of biomimetic superhydrophobic surfaces. In particular, one-dimensional nanomaterials, such as ZnO,2-7 TiO2,8-10 WOx nanowires,11,12 and carbon nanotubes,13-17 have been widely tested for fabricating superhydrophobic surfaces due to their inherent surface roughness and unique properties. Nanostructures with low surface energies reduce water droplet contact area and prevent penetration of water into the interstices between the nanostructures, leading to a large contact angle (CA) and a small CA hysteresis (CAH). The interplay between controllable parameters, such as nanostructure spacing, diameter, height, and surface energy, controls the wettability transition criteria. Reduced surface energy and reduced water droplet contact area on a surface indicates enhanced *Corresponding author: e-mail [email protected].

(1) Koch, K.; Bhushan, B.; Barthlott, W. Soft Matter 2008, 4, 1943. (2) Wang, L.; Zhang, X.; Fu, Y.; Li, B.; Liu, Y. Langmuir 2009, 25, 13619. (3) Li, G.; Chen, T.; Yan, B.; Ma, Y.; Zhang, Z.; Yu, T.; Shen, Z.; Chen, H.; Wu, T. Appl. Phys. Lett. 2008, 92, 173104. (4) Saleema, N.; Farzaneh, M. Appl. Surf. Sci. 2008, 254, 2690. (5) Badre, C.; Pauporte, T.; Turmine, M.; Lincot, D. Nanotechnology 2007, 18, 365705. (6) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2004, 126, 62. (7) Kwak, G.; Seol, M.; Tak, Y.; Yong, K. J. Phys. Chem. C 2009, 113, 12085. (8) Feng, X. J.; Zhai, J.; Jiang, L. Angew. Chem., Int. Ed. 2005, 44, 5115. (9) Jhang, X.; Jin, M.; Liu, Z.; Tryk, D. A.; Nishimoto, S.; Murakami, T.; Fujishima, A. J. Phys. Chem. C 2007, 111, 14521. (10) Sun, W.; Zhou, S.; Chen, P.; Peng, L. Chem. Commun. 2008, 5, 603. (11) Wang, S.; Feng, X.; Yao, J.; Jiang, L. Angew. Chem., Int. Ed. 2006, 45, 1264. (12) Kwak, G.; Lee, M.; Senthil, K.; Yong, K. Appl. Phys. Lett. 2009, 95, 153101. (13) Sethi, S.; Dhinojwala, A. Langmuir 2009, 25, 4311. (14) Pastin, S.; Okawa, D.; Kessler, B.; Rolandi, M.; Llorente, M.; Zettl, A.; Fr
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hydrophobicity. Previous studies18-21 have demonstrated changes in the superhydrophobic behavior that result from variations in geometric parameters (nanopost spacing, diameter, and height). However, experimental control over geometric parameters generally requires strict conditions and complex processing techniques. Hydrophobic chemical coatings have been tested as an approach to the fabrication of artificial superhydrophobic surfaces. Lowering the surface energy through chemical modification can enhance the hydrophobicity of a surface with fixed geometric parameters. Simple chemical coating methods have been used to fabricate superhydrophobic surfaces composed of nanomaterials. However, such studies22 have not extensively investigated the tunable parameters that may alter surface energy and wettability. Systematic changes in the surface energy of nanostructures may be an efficient means for control over surface wettability. Also, in practical application, although superhydrophobicity should be stable under dynamic rather than stationary droplet conditions, the stability of artificial superhydrophobic surfaces with different surface energies have not been studied under dynamic conditions. SiC nanowires are wide-band-gap semiconductors and have shown excellent stability under harsh conditions due to their high mechanical strength, high chemical stability, and low induced activity.23-26 The incorporation of water-repellent properties into SiC surfaces is expected to expand the scope of their applications. To date, few studies have described the fabrication of superhydrophobic SiC nanostructures with CAs as large as 160°.27,28 In this paper, we report a novel and simple method for fabricating superhydrophobic core-shell SiC-SiO2 nanowire (20) Kwon, Y.; Patankar, N.; Choi, J.; Lee, J. Langmuir 2009, 25, 6129. (21) Wang, Z.; Lopez, C.; Hirsa, A.; Koratkara, N. Appl. Phys. Lett. 2007, 91, 023105. (22) Albert, J. N.; Baney, M. J.; Stafford, C. M.; Kelly, J. Y.; Epps, T. H. ACS Nano 2009, 12, 3977. (23) Zhou, W. M.; Wu, Y. J.; Kong, E. S. W.; Zhu, F.; Hou, Z. Y.; Zhang, Y. F. Appl. Surf. Sci. 2006, 253, 2056. (24) Pan, Z.; Lai, H. L.; Au, F. C. K.; Duan, Z.; Zhou, W.; Shi, W.; Wang, N.; Lee, C. S.; Wong, N. B.; Lee, S. T.; Xie, S. Adv. Mater. 2000, 12, 1186. (25) Shen, G.; Bando, Y.; Ye, C.; Liu, B.; Golberg, D. Nanotechnology 2006, 17, 3468. (26) Wu, Z. S.; Deng, S. Z.; Xu, N. S.; Chen, J.; Zhou, J.; Chen, J. Appl. Phys. Lett. 2002, 80, 3829. (27) Niu, J. J.; Wang, J. N.; Xu, Q. F. Langmuir 2008, 24, 6918. (28) Niu, J. J.; Wang, J. N. J. Phys. Chem. B 2009, 113, 2909.

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Figure 1. (a) SEM image of core-shell SiC-SiO2 nanowires. (b) TEM image of a core-shell SiC-SiO2 nanowire. The crystalline SiC core appears darker than the amorphous SiO2 sheath in this imaging mode. The selected area electron diffraction (SAED) pattern (SiC core) corresponds to a cubic SiC crystal structure. No diffraction pattern was observed from the amorphous SiO2 shell layer.

arrays through the chemisorption of an octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM). A wettability control study was conducted on SiC nanostructures with different surface energies through the adsorption of SAMs with various alkyl chain lengths. The effects of UV irradiation on the superhydrophobic SiC-SiO2 nanowire arrays were investigated because the photodecomposition of OTS molecules adsorbed onto the surface was found to locally alter the surface energy. Moreover, the dynamics of water droplet impact on superhydrophobic surfaces were investigated to characterize the influence of surface energy on the drop impact transition. Practical applications require that superhydrophobicity be maintained under dynamic conditions.

2. Experimental Section A. Fabrication of Core-Shell SiC-SiO2 Nanowire Arrays and Chemisorption of Alkyltrichlorosilanes on Nanowire Substrates. Core-shell SiC-SiO2 nanowires were grown on a silicon substrate by carbothermal reduction of tungsten oxide (WO3) with graphite using a NiO catalyst.29 The silicon substrates were immersed in the Ni(NO3)2/ethanol solution (0.01 M). After drying in air, growth was carried out at 1100 °C for 2 h under a constant Ar flow of 500 sccm. After cooling to room temperature, the silicon substrate was observed to be covered with a white deposit. After growth, the substrate was rinsed with deionized water then dried under N2 flow. Alkyltrichlorosilane was deposited onto the nanostructured surfaces by immersing the substrates in 3 mmol of toluene solutions of hexyltrichlorosilane (HTS, C = 6), dodecyltrichlorosilane (DTS, C = 12), or octadecyltrichlorosilane (C = 18) for 3 h at 4 °C. At low temperatures (4 °C), the alkyltrichlorosilane molecules display well-ordered monolayers on the surfaces.30 The substrates were subsequently washed with ethanol to remove excess reactants then dried under N2 flow for further analysis. The water contact angle and intensities of X-ray photoelectron spectroscopy (XPS, nonmonochromatic Mg KR radiation, photon energy 1253.6 eV) peaks were measured as a function of the SAM carbon chain length.

B. UV-Stimulated Conversion of Wettability and Selective Wetting. The wettability of the prepared surfaces was tuned by exposure to UV light: OTS-modified SiC-SiO2 nanowire samples were placed under a 300 W mercury lamp, which yielded 185 and 254 nm illumination. The light intensity was maintained at 1 W/cm2. The OTS-coated SiC-SiO2 nanowire samples were (29) Senthil, K.; Yong, K. Mater. Chem. Phys. 2008, 112, 88. (30) Lee, H.; Kim, D.; Cho, J.; Hwang, M.; Jang, Y.; Cho, K. J. Am. Chem. Soc. 2008, 130, 10556.

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directly illuminated at a working distance of 10 cm. The UVstimulated conversion of wettability was investigated by measuring the water contact angle and XPS peaks as a function of UV irradiation time, from 0 to 60 min. C. Characterization. The surface morphology, structure, and chemical states of the nanowire samples were examined by field-emission scanning electron microscopy (FESEM, JEOL, JSM 330F) and XPS. The water CAs were measured using 5 μL droplets of deionized water using a contact angle measurement system (Kr€ uss, DSA-10) under ambient atmospheric conditions. Advancing and receding contact angles were measured with the dispensing needle embedded in the sessile drop. The dynamic impact behavior of droplets and the sliding motion of the water droplets were recorded using a high-speed camera (Fastcam, Ultima 512) operated at 2000 and 250 frames/s, respectively.

3. Results and Discussion A large quantity of nanowire products was obtained by the simple heating method. The as-grown sample was composed of uniform nanowires 20-50 nm in diameter and several tens of micrometers in length (Figure 1a). The surface showed an open mesh of nanowires lying parallel on a substrate with a high roughness, as shown in Figure 1. According the previous study,31 this open mesh structure showed the most superhydrophobic behavior in various surface structures, including smooth surface, assembly of spherical nanoparticles, vertically aligned nanowire array, and mesh structure of nanofibers. The internal structure of the nanowires was investigated by transmission electron microscopy (TEM). The TEM image shown in Figure 1b showed that the nanowires were composed of core SiC and shell SiO2. Typically, the thickness of the shell layer was 20 nm, and the diameter of the core fell in the range 10-30 nm. The selected area electron diffraction (SAED) spectrum from the core SiC was indexed as a cubic β-SiC structure, whereas no diffraction pattern was observed from the amorphous shell SiO2 layers (Figure 1b, insets). The surface morphology and density of SiC-SiO2 nanowires depended on the growth parameters, such as NiO catalyst concentration, growth temperature, and time.29 The outer nanowire SiO2 shell layer acted as a protective layer for the core SiC nanowires and assisted the chemical binding of SAM molecules to the surface by condensation reactions. Alkyltrichlorosilane surface modifiers were deposited on the as-prepared (31) Shang, H. M.; Wang, Y.; Takahashi, K.; Cao, G. Z. J. Mater. Sci. 2005, 40, 3587.

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Figure 2. Water contact angles (CAs) on chemically modified nanowire surfaces as a function of the alkyl chain length of SAM molecules: (0) static contact angle, (b) advancing contact angle, and (O) receding contact angle. Inset: contact angle hysteresis (CAH) as a function of the alkyl chain length.

SiC-SiO2 nanowire surfaces. Dynamic water CA measurements were conducted on the modified SiC-SiO2 nanowire surfaces as a function of SAM carbon chain length. As shown in Figure 2, the water CAs (static CA, advancing and receding CAs) of SAM-treated nanowire surfaces increased monotonically with increasing SAM alkyl chain length. The HTS treated surface showed 89.5° and 54° for advancing and receding CAs, respectively. The contact angle hysteresis (CAH), which is the difference between the advancing and receding contact angles, was 35.5°. Increasing the alkyl chain length caused an increase in both the advancing and receding CA of water. Also, decreases in CAH were observed with increased alkyl chain length (the inset of Figure 2). The DTS-modified surface showed advancing/ receding CAs of 145°/123° with a CAH of 22°, whereas for OTSmodified surface both advancing (163.5°) and receding (162°) water CAs were almost the same with a CAH of 1.3°. The exceedingly low CAH and large receding CA caused the nonadhesion behavior between solid/liquid interfaces. These results indicate that longer alkyl chain lengths decrease the surface energy of the modified nanowire surfaces. The OH group that was inherently present on the shell SiO2 layer of the nanowires imparted a superhydrophilic state with a very low CA on the asgrown nanowire surfaces (C = 0). The OTS-modified surface (C = 18) supported a water droplet that was nearly spherical, with a static CA of 164°. This superhydrophobicity could be explained by the Cassie model. The contact area between the water droplet and the sample surface consisted of two parts: the OTS-coated SiC-SiO2 nanowire surface and air pockets. Impact dynamic studies supported this model, which will be discussed later in the article. According to the Cassie model,32 water CAs are directly influenced by the surface fraction of solid ( f1) versus air pockets ( f2), where the sum of these two parameters is 1:

The decreased surface energy produced by the OTS coating indicated a low solid surface fraction ( f1) and a high air surface fraction ( f2), yielding a high water CA. In this Cassie state, capillary effects and the air pockets supported the droplet and prevent the droplet from penetrating the nanostructure.33,34 The stability of the superhydrophobic state was also investigated. The OTS-modified SiC-SiO2 nanowires showed durable superhydrophobicity, in which no apparent changes in CA were observed after 1 month under ambient atmospheric conditions with exposure to sunlight (data not shown). The chemical binding states of the modified nanowire surfaces were examined by XPS C 1s, O 1s, and Si 2p core level spectra (see Supporting Information, Figure S1). Figure S1a shows the C 1s XPS spectra for the various alkyltrichlorosilane molecules tested. The C 1s peaks at 284.8 eV were ascribed to the carbon atoms in the aliphatic chain (C-C), consistent with the data presented in a previous study of OTS.35 As expected, the C 1s XPS peak intensity increased as the alkyltrichlorosilane carbon chain length increased. In contrast, the XPS intensities of both the O 1s (532.6 eV) and Si 2p (102.8 eV) peaks assigned to the outer SiO2 shell of the nanowire decreased gradually with the increased chain length (Figures S1b and S1c). These results showed that alkyltrichlorosilane molecules with longer carbon chains shielded the nanostructures from exposure to the exterior. Interestingly, the coating molecules could be effectively removed by irradiation with intense UV light (300 W). We measured the changes in CA and XPS spectra to confirm the UVenhanced wettability transformation of OTS-coated SiC-SiO2 nanowires as a function of UV irradiation time under ambient conditions. Figure 3 shows that the CAs decreased monotonically from 164° to 0° with increasing UV illumination time. After UV irradiation in excess of 45 min, the CA approached 0°, similar to

cos θ
¼ f1 cos θ - f2

(33) Tuteja, A.; Choi, W.; McKinley, G. H.; Cohen, R. E.; Rubner, M. F. MRS Bull. 2008, 33, 752. (34) Gao, L.; Fadeev, A. Y.; McCarthy, T. J. MRS Bull. 2008, 33, 747. (35) Lin, M. H.; Chen, C. F.; Shiu, H. W.; Chen, C. H.; Gwo, S. J. Am. Chem. Soc. 2009, 131, 10984.

(32) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546.

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ð1Þ

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consideration for practical applications. In real applications, superhydrophobicity must persist under dynamic, not only stationary, droplet conditions. When droplets impinge on a textured surface, the transition criteria of wetting states depend on the balance of wetting pressure (Pw) and antiwetting pressure (Pa).12,18,19,38,39 When Pw was larger than Pa, the droplet struck the surface in a wetting state. The Pw is given by Pw ¼

Figure 3. Water CAs of the OTS-treated nanowire surfaces vs. exposure time to 185 and 254 nm UV light at 1 mW/cm2.

the CA of the as-grown SiC-SiO2 nanowires. The gradual conversion into a hydrophilic surface indicated that intensive UV light effectively decomposed the OTS molecules adsorbed onto the surface. UV radiation combined with ambient oxygen to produce UV-assisted decomposition of the OTS alkyl chains.36,37 The OTS alkyl chains decomposed through attack from the OH radicals and atomic oxygen produced by the UV-catalyzed dissociation of ozone, which was photogenerated in air. The water CA, both advancing and receding CAs, decreased monotonically with increasing UV exposure time (data not shown). The magnitude of CA hysteresis increased with increasing UV irradiation time. This increase in hysteresis may result from an increase in interaction of the water droplet with the surface due to the photodecomposition of the adsorbed OTS molecules. The UV-enhanced decomposition of the alkyl chains was characterized by XPS analysis. Figure S2a (see Supporting Information) shows XPS spectra of C 1s as a function of UV irradiation time. The peak intensity decreased with increasing UV irradiation time and saturated after exposures in excess of 45 min. This result suggests that UV irradiation gradually reduced the alkyl chain length of the OTS molecules chemisorbed on the nanowire surfaces. After termination of the photochemical reaction, the alkyl chains were found to be completely decomposed, and only residual carbon remained on the surfaces. Figures S2b and S2c show respectively the Si 2p and O 1s XPS spectra as a function of UV irradiation exposure time. The intensities of the Si 2p and O 1s peaks increased upon UV irradiation. These changes indicate that the nanowire surface covered with the SAM became exposed due to decomposition of the OTS molecules. The carboncontaining silicate material will possibly be formed on the surface during photodecomposition of OTS molecules. However, in the current study, there are no peaks observed for C 1s and Si 2p corresponding to Si-C bonds (283.5 and 100.3 eV, respectively), and C 1s and Si 2p peaks are nearly symmetric centered at 284.6 and 102.8 eV corresponding to C-C bonds (aliphatic chain) and Si-O bonds (outer shell of the nanowire), respectively (Figure S2 in Supporting Information). We believe that this is because the SiO2 shell layer is too thick to detect the core SiC in XPS. Also, these results indicate that the C signal originating from carbon-silicate material is too weak to detect, if any. Although most previous studies investigated hydrophobicity in stationery states, the stability of the superhydrophobicity under dynamic conditions (e.g., impinging water droplets) is a key (36) Ye, T.; Wynn, D.; Dudek, R.; Borguet, E. Langmuir 2001, 17, 4497. (37) Cox, R.; Patrick, K.; Chant, S. Environ. Sci. Technol. 1981, 15, 587.

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1 2 FV 2

ð2Þ

where F and V are the density and the velocity of the impinging liquid, respectively. For SiC-SiO2 nanostructured surface with a maximum spacing between nanowires (D, ∼300 nm), the maximum value of Pa is calculated as the Laplace pressure of the maximum deformation of the water-air interface between nanowires Pa ¼ - 2γLV cos θA =D

ð3Þ

where γLV is the surface energy of the water at the liquid-vapor interface (0.073 N/m) and θA is the advancing CA of the water droplet on the flat surface. For a flat OTS-modified and UVirradiated silica surface, the measured θA were ∼120° and 12°, respectively. We studied the impact dynamics of droplets impinging on the OTS-modified and UV-irradiated nanowire surfaces to understand how the superhydrophobic coating influenced the dynamic shape of the water droplets. Figure 4a shows sequential snapshots of water droplets with diameters of 1 mm and velocities of 0.52 m/s impinging on the OTS-modified surfaces, recorded using a high-speed camera operated at 2000 frames/s. The calculated Pa value of 243.13 kPa from eq 3 was significantly greater than the calculated Pw (135.2 Pa) from eq 2. The droplet clearly bounced off the OTS-modified substrate without penetrating the nanostructure. The air pockets and capillary forces supported the droplet throughout the impact event. Finally, the droplet rested on the surface and maintained a high contact angle without undergoing a transition to a wetted state, which suggested the formation of a solid-air-liquid interface. However, all superhydrophobic surfaces do not completely perform the same behavior of bouncing off surface. When the surface does not have sufficiently low surface energy or procure enough geometrical spacing between the nanoscale structures, the wetting state of a droplet can be partially pinned at the contact area.12,19 In contrast, on UV-irradiated nanowire surfaces, the calculated Pa of UV irradiated one has a negative value and acts as a wetting pressure. In this case, two wetting pressures (Pw, Pa) caused a droplet to wet the surface. Thus, the water droplet appeared to soak into the texture without bouncing or vibrating and subsequently spread out within a few milliseconds on the surface (Figure 4b). These results indicate that restoration of the superhydrophilic surface favored penetration of the water droplet into the surface structure by 3D capillary effects.40,41 Sliding of a water droplet on the chemically modified/UVtreated surfaces was directly observed by dropping the water droplet onto slightly tilted substrates placed in a row (tilt angle of 5°). Photographs of the sliding droplets were recorded using a (38) Bartolo, D.; Bouamrirene, F.; Verneuil, E.; Buguin, A.; Silberzan, P.; Moulinet, S. Europhys. Lett. 2006, 74, 299. (39) Reyssat, M.; P
epin, A.; Marty, F.; Chen, Y.; Qu
er
e, D. Europhys. Lett. 2006, 74, 306. (40) Bico, J.; Tordeux, C.; Qu
er
e, D. Europhys. Lett. 2001, 55, 214. (41) Bico, J.; Thiele, U.; Qu
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e, D. Colloids Surf., A 2002, 206, 41.

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Figure 4. Photographs of water droplets impinging on modified and irradiated to 185 and 254 nm UV light at 1 mW/cm2 nanowire arrays.

suggests many potential applications in biological, chemical, and electronic devices.

4. Conclusions In summary, we have presented a facile method for fabricating superhydrophobic SiC-SiO2 nanowire surfaces with a static water CA of 164°. The high roughness and low surface energy provided by the particular geometry of the nanostructure and alkyltrichlorosilane coating, respectively, contributed to the superhydrophobicity. The surface energy of the nanowire substrates, which determined the wetting state of the water droplet, could be systematically modified through chemisorption of alkyltrichlorosilanes with varying carbon chain lengths and by UV-assisted photodecomposition of the SAM molecules on the nanowire surfaces. Also, we studied the stability of the fabricated superhydrophobic surface under dynamic conditions for practical applications and characterized the influence of surface energy on the wetting transition through measurements of water droplet impact dynamics. A droplet on the OTS-modified surface bounced cleanly on the surface, whereas the droplet spread out on the UV-exposed or as-grown nanostructure surfaces. This study presents guidelines for the design of stable nanostructured surfaces that are antiwetting with respect to water and are self-cleaning. Figure 5. Sequential images of a water droplet sliding on the patterned tilted substrates arranged in a row. The two right-most samples were chemically modified superhydrophobic surfaces, and the left surface was a UV-exposed surface with both 185 and 254 nm wavelengths and 1 mW/cm2.

high-speed camera at 250 frames/s. All three samples were OTStreated. The left-most sample was extensively UV-irradiated to convert the surface into a superhydrophilic state. The water droplet very easily slid off the two right-hand OTS-modified surfaces, whereas the droplet immediately spread and appeared to soak into the left-most surface as soon as it reached the UV exposed area (Figure 5) (see Supporting Information video for details). This extreme wettability conversion of the nanostructured surface

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Acknowledgment. This work was supported by Grant RTI0401-04 from the Regional Technology Innovation Program of the Ministry of Commerce, Industry, and Energy (MOCIE), and Korean Research Foundation Grants funded by the Korean Government (MOEHRD) (KRF-2008-005-J00501). Supporting Information Available: XPS spectra from the OTS-modified nanowires as a function of the SAM alkyl chain length and XPS spectra for OTS-modified nanowires after UV irradiation during various times; video of sliding water droplet on the patterned SiC-SiO2 nanowire surface. This material is available free of charge via the Internet at http://pubs.acs.org.

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