pubs.acs.org/Langmuir © 2010 American Chemical Society
Chemically Modified Superhydrophobic WOx Nanowire Arrays and UV Photopatterning Geunjae Kwak, Mikyung Lee, 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 January 4, 2010. Revised Manuscript Received March 17, 2010 A facile route is reported for the fabrication of superhydrophobic tungsten oxide (WOx) nanowire surfaces through the chemical adsorption of alkyltrichlorosilane with a static water contact angle (CA) of 163.5°. It is confirmed that CAs on the superhydrophobic surface decreased gradually under UV illumination because of the UV-assisted decomposition of alkyltrichlorosilane chemically adsorbed onto the surface. Superhydrophobic-superhydrophilic switching is also demonstrated by alternating self-assembled monolayer deposition and UV irradiation on the photopatterned nanowire surfaces. Furthermore, the superhydrophobic surface could be transformed selectively into a hydrophilic state by simply exposing the surface to UV through a shadow mask. These studies provide a relatively simple strategy for the design of superhydrophobic surfaces.
1. Introduction Surface-wetting behaviors are important characteristics of solid materials. Superhydrophobic surfaces on which water either does not adhere or only weakly adheres, with water CAs >150°, are generally obtained by a combination of high surface roughness and a water-repellant chemical coating. In particular, nanostructures such as ZnO,1-3 TiO2,4-6 etched silicon,7-9 and carbon nanotubes10,11 have been mainly used to generate superhydrophobic surfaces because of inherent surface roughness and their wide applications. A reversible wettability transformation between superhydrophobicity and superhydrophilicity has been obtained by applying external stimuli, including light irradiation,2-5,12 thermal treatment,13 and electric fields.14,15 Such stimulation of the surface can change the chemical states of surface atoms and induce reactions between surface atoms and molecules adsorbed on the surface. The alternating chemical state on the surface causes surface wettability to switch reversibly. WOx nanowire arrays have many applications in electrochromic devices, field-emission devices, gas sensors, and *Corresponding author. E-mail:
[email protected]. (1) Lee, J.; Kim, H.; Park, C.; Park, G.; Kwak, H.; Koo, S.; Sung, M. J. Phys. Chem. B 2003, 107, 8997. (2) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2004, 126, 62. (3) Kwak, G.; Seol, M.; Tak, Y.; Yong, K. J. Phys. Chem. C 2009, 11, 12085. (4) Sun, W.; Zhou, S.; Chen, P.; Peng, L. Chem. Commun. 2008, 5, 603. (5) Feng, X.; Zhai, J.; Jiang, L. Angew. Chem., Int. Ed. 2005, 44, 5115. (6) Wang, L.; Zhang, X.; Fu, Y.; Li, B.; Liu, Y. Langmuir 2009, 25, 13619. (7) Qi, D.; Lu, N.; Xu, H.; Yang, B.; Huang, C.; Xu, M.; Gao, L.; Wang, Z.; Chi, L. Langmuir 2009, 25, 7769. (8) Kim, T.; Tahk, D.; Lee, H. Langmuir 2009, 25, 6576. (9) Deng, T.; Varanasi, K. K.; Hsu, M.; Bhate, N.; Keimel, C.; Stein, J.; Blohm, M. Appl. Phys. Lett. 2009, 94, 133109. (10) Sethi, S.; Dhinojwala, A. Langmuir 2009, 25, 4311. (11) Pastin, S.; Okawa, D.; Kessler, B.; Rolandi, M.; Llorente, M.; Zettl, A.; Frechet, J. J. Am. Chem. Soc. 2008, 130, 4238. (12) Ichimura, K.; Oh, S.; Nakagawa, M. Science 2000, 288, 1624. (13) Li, C.; Guo, R.; Jiang, X.; Hu, S.; Hu, S.; Li, L.; Cao, X.; Yang, H.; Song, Y.; Ma, Y.; Jiang, L. Adv. Mater. 2009, 21, 4254. (14) Tian, D.; Chen, Q.; Nie, F.; Xu, J.; Song, Y.; Jiang, L. Adv. Mater. 2009, 21, 3744. (15) Lahann, J.; Mitragotri, S.; Tran, T.; Kaido, H.; Sundaram, J.; Choi, I.; Hoffer, S.; Somorjai, G.; Langer, R. Science 2003, 299, 371. (16) Pokhrel, S.; Simion, C. E.; Teodorescu, V. S.; Barsan, N.; Weimar, U. Adv. Funct. Mater. 2009, 19, 1767.
9964 DOI: 10.1021/la100022b
photocatalysts.16-20 The addition of wettability control to WOx nanowires promises to widen their applicability. However, the fabrication of superhydrophobic WOx surfaces is currently limited to film structures.21 Here, we report a simple method of fabricating WOx nanowire arrays that can be switched easily between superhydrophobic and superhydrophilic states. This method exploits the chemisorption of octadecyltrichlorosilane (OTS) to produce a self-assembled monolayer (SAM) on which the transition from superhydrophobicity to superhydrophilicity can be obtained by alternation between UV irradiation and SAM deposition. We also demonstrate a method of converting selected areas of a patterned area from superhydrophobic to superhydrophilic by employing selective UV illumination.
2. Experimental Section Fabrication of WOx Nanowire Arrays and Chemisorption of Alkyltrichlorosilanes on a Nanowire Substrate. Tungsten oxide nanowire arrays were grown on a tungsten substrate using a simple thermal evaporation method. W sheets (99.95% purity with a size of 10 10 1 mm3) were used as the substrates. WO3 powder with a purity of 99.99% was the source material for tungsten oxide nanowire growth and was placed at the edge of an alumina boat. The W substrate was kept top and center on the alumina boat. The source- and substrate-containing alumina boat was placed in the center of a furnace, and the temperature of the furnace was maintained at 1000 °C for 1 h. Although our synthesis method for WOx nanowires was not limited to the utilization of the W substrate, the substrates have been confined to thermally stable surfaces such as Mo and W because of the relatively high growth temperature. After growth, the substrate was rinsed with deionized water and dried in a stream of N2. Alkyltrichlorosilane was deposited onto the WOx nanowire surface by immersing the WOx samples in 3 mmol toluene solutions of hexyltrichlorosilane, (17) Abe, R.; Takami, H.; Murakami, N.; Ohtani, B. J. Am. Chem. Soc. 2008, 130, 7780. (18) Zhao, Z.; Miyauchi, M. Angew. Chem., Int. Ed. 2008, 47, 7051. (19) Baek, Y.; Song, Y.; Yong, K. Adv. Mater. 2006, 18, 3105. (20) Morales, W.; Cason, M.; Aina, O.; Tacconi, N. R.; Rajeshwar, K. J. Am. Chem. Soc. 2008, 130, 6318. (21) Wang, S.; Feng, X.; Yao, J.; Jiang, L. Angew. Chem., Int. Ed. 2006, 45, 1264.
Published on Web 04/06/2010
Langmuir 2010, 26(12), 9964–9967
Kwak et al.
Article
Figure 1. SEM images of WOx nanowire arrays grown on a tungsten substrate, (a) top and (b) tilt views. (c) TEM image of a WOx nanowire. The inset is a HRTEM image including a diffraction pattern of a WOx nanowire. (d) XRD pattern of the WOx (W18O49) nanowire arrays. dodecyltrichlorosilane, and octadecytrichlorosilane for 3 h at 4 °C. The samples were then washed in toluene to remove excess reactants and dried in a stream of N2.
UV-Stimulated Conversion of Wettability and Selective Wetting. For the UV-stimulated conversion of wettability, OTSmodified WOx samples were placed under a 300 W mercury lamp that generates both 185 and 254 nm wavelengths. The light intensity was maintained at 1 mW/cm2. The UV lamp directly illuminated the OTS-modified WOx samples at a working distance of 10 cm in air at room temperature. The relative humidity in all experiments was kept at 40%. OTS-treated WOx substrates were patterned by UV illumination through a polyester film mask. The photomask was placed on the modified WOx substrate. UV irradiation of the modified sample proceeded for 1 h under ambient conditions. Water droplets were dropped onto both the masked and exposed regions to confirm the selective wetting of the substrate. Characterization. The surface morphology, structure, and chemical states of the samples were examined by field-emission scanning electron microscopy (FESEM; JEOL, model JSM 330F), X-ray diffraction (XRD; Rigaku, model D-max1400), and X-ray photoelectron spectroscopy (XPS; nonmonochromatic Mg KR radiation, photon energy 1253.6 eV). The water CAs were measured with 5 μL of deionized water using a contact angle system (Kr€ uss, model DSA-10) under ambient conditions. The average value of five measurements at different positions on the sample was adopted as the apparent contact angle. The sliding water droplet was recorded with a high-speed camera (Fastcam, model Ultima 512) operating at 250 frames/s.
3. Results and Discussion WOx nanowire arrays were prepared using a simple catalystfree thermal evaporation method22 that produces arrays that have a high density of quasi-aligned nanowires (Figure 1a,b). The (22) Baek, Y.; Yong, K. J. Phys. Chem. C 2007, 111, 1213.
Langmuir 2010, 26(12), 9964–9967
nanowires had a typical length of ∼100 nm and diameters from 50 to 300 nm. We believe that our growth process is based on a typical vapor-solid (VS) mechanism. The length and diameter of the nanowires increased with increasing growth time and temperature. The TEM image of a single nanowire indicated that it had a smooth, rod-shaped structure (Figure 1c). A high-resolution TEM (HRTEM) lattice image of the nanowire and its corresponding selected-area electron diffraction (SAED) pattern are shown in the inset of Figure 1c. The electron diffraction pattern indicated that as-grown nanowires were single-crystalline in nature. The measured lattice spacing was 0.377 nm, which could be indexed as [010] of a monoclinic WOx (W18O49). These results are consistent with the X-ray diffraction (XRD) results (Figure 1d). OTS as a surface modifier was deposited onto the WOx nanowire surface using a simple dipping method (Figure 2a). The OTS molecules deposited at low temperature (4 °C) had a well-ordered monolayer on the surface.23 OTS modification has been applied as a standard procedure to produce superhydrophobic surfaces.24-26 The water CA of the as-grown WOx nanowire substrate was almost 0° (Figure 2a, left inset) because the low CA on the flat substrate (6.6°) favored water movement into the rough surface structures by a 3D capillary effect.27,28 The water droplet instantaneously soaked into the as-grown nanowire surface with a CA of
