pubs.acs.org/Langmuir © 2009 American Chemical Society
Improved Hydrophobicity of Carbon Nanotube Arrays with Micropatterning Si Hong Lu,†,§ Ma Han Ni Tun,†,§ Zhiwei Joshua Mei,†,§ Guo Hao Chia,†,‡ Xiaodai Lim,‡ and Chorng-Haur Sow*,‡ †
Dunman High School, 10 Tanjong Rhu Road, Singapore 436895, and ‡Department of Physics, Blk S12, Faculty of Science, National University of Singapore, 2 Science Drive 3, Singapore 117542. §These authors contributed equally to this work Received May 20, 2009. Revised Manuscript Received July 11, 2009 The hydrophobicity of vertically aligned multiwalled carbon nanotubes (MWCNTs) was improved through the creation of a parallel array of microwalls via a laser pruning technique. Changes to the hydrophobic nature of the patterned MWCNTs due to artificially induced roughness through variations in both the widths of the walls and the distance between adjacent walls, channel width, were investigated. The sample became more hydrophobic whenever water droplets landed on one microwall 7 or 13 μm in width. However, when a droplet bridged two microwalls, the surface became less hydrophobic. The optimal superhydrophobic MWCNT surface corresponded to a parallel array of microwalls with a width of 13 μm and a channel width of ∼50 μm. Such findings could possibly serve as value-add for further developments in the creation of water-repelling CNT surfaces via micropatterning.
1. Introduction Superhydrophobic surfaces inspired by plants such as the lotus leaf1 and insects such as the water strider2 have captured the attention of researchers for their potential applications in fields ranging from antisticking of snow on various surfaces3 to antibiofouling paints for boats.4 With the use of the contact angle (CA, R), the angle between the surface and the water meniscus near the line of contact (measured through a droplet), as an indicator of the wettability of the surface, a surface was deemed to be hydrophobic if 90° e R < 150°5 or superhydrophobic if R > 150°.6 While the basic idea of superhydrophobicity had been developed by Young, Wenzel, Cassie, and Baxter decades ago, it was the publication by Onda et al.7 presenting both theoretical and experimental results on the wettability of fractal surfaces that marked the start of an explosion in the number of articles published about this topic.8 In recent years, different techniques for creating hydrophobic surfaces have been developed. The range of techniques includes mimicking and using biological structures as templates,9 chemical functionalizations of the surface,3 and physical modifications.10 In the area of physical modification, both top-down and/or bottom-up approaches have been utilized for the fabrication of roughness on either intrinsic *To whom correspondence should be addressed. Phone: (þ65) 65162957. Fax: (þ65) 67776126. E-mail:
[email protected]. (1) 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. (2) Gao, X.; Jiang, L. Nature 2004, 432, 36. (3) Kako, T.; Nakajima, A.; Irie, H.; Kato, Z.; Uematsu, K.; Watanabe, T.; Hashimoto, K. J. Mater. Sci. 2004, 39, 547. (4) Scardino, A.; De Nys, R.; Ison, O.; O’Connor, W.; Steinberg, P. 11th International Congress on Marine Corrosion and Fouling, San Diego, CA, 2002, 221. (5) Burton, Z.; Bhushan, B. Nano Lett. 2005, 5, 1607. (6) Sun, T. L.; Wang, G. J.; Feng, L.; Liu, B. Q.; Ma, Y. M.; Jiang, L.; Zhu, D. B. Angew. Chem., Int. Ed. 2004, 43, 357. (7) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. (8) Roach, P.; Shirtcliffe, N. J.; Newton, M. I. Soft Matter 2008, 4, 224. (9) Sun, M. H.; Luo, C. X.; Xu, L. P.; Ji, H.; Qi, O. Y.; Yu, D. P.; Chen, Y. Langmuir 2005, 21, 8978. (10) Zheng, Q. S.; Yu, Y.; Zhao, Z. H. Langmuir 2005, 21, 12207.
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hydrophobic materials or coating the rough surfaces with a hydrophobic layer.8 Recently, Bhushan et al.11 conducted an analysis of how hierarchical roughness was used to optimize biomimetic superhydrophobic surfaces. From their analysis, they proposed that one of the important factors for the distribution of optimized roughness on a water repellent surface was that the asperities at each scale level should have small widths and large distances between them, i.e., a small value for the spacing factor, Sf, where the value of Sf was defined as the ratio of the width of the asperities to the distance between them. However, this requirement was limited by some critical value of Sf, providing the structures were able to support the weight of the water droplet. Using samples comprised of asperities and valleys made of singlecrystal silicon (Si), coated with a hydrophobic 1,1,2,2-tetrahydroperfluorodecyltrichlorosilane (PF3) self-assembled monolayer, the critical value of Sf was found to be between 0.083 and 0.111.12 While many materials such as PDMS13 and silicon14,15 have been widely used in the study of roughness on the hydrophobic effect, one other interesting material for studying such a phenomenon is carbon nanotubes (CNTs). With intrinsic hydrophobic properties and nanoscale surface roughness, CNTs show great potential in applications such as self-cleaning surfaces. By a bottom-up approach, Lin et al.1 reported the improved intrinsic hydrophobic nature of aligned CNTs by growing them into similar structures mimicking those found on lotus leaves. Another piece of work via the bottom-up approach was presented by Li et al.,16 in which improvement of the hydrophobicity of aligned CNTs was achieved through the growth of honeycomb-patterned aligned CNTs by pyrolysis of iron phthalocyanine. Such surfaces were found to exhibit a CA of (11) Nosonovsky, M.; Bhushan, B. Ultramicroscopy 2006, 107, 969. (12) Bhushan, B.; Nosonovsky, M.; Jung, Y. C. J. R. Soc. Interface 2007, 4, 643. (13) Chen, Y.; He, B.; Lee, J. H.; Patankar, N. A. J. Colloid Interface Sci. 2005, 281, 458. (14) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (15) Zhu, L.; Feng, Y. Y.; Ye, X. Y.; Zhou, Z. Y. Sens. Actuators, A 2005, 130131, 595. (16) Li, S. H.; Li, H. J.; Wang, X. B.; Song, Y. L.; Liu, Y. Q.; Jiang, L.; Zhu, D. B. J. Phys. Chem. B 2002, 106, 9274.
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Figure 1. (a) Schematic of the laser pruning system and (b-e) FESEM images of (b) unpatterned MWCNTs and parallel arrays of (c) 7, (d) 13, and (e) 21 μm MWCNT microwalls created using the laser pruning technique.
163.4 ( 1.4°. Applying a similar approach, Zhang et al.17 recently showcased their creations of superhydrophobic surfaces made up of single-walled carbon nanotube (SWCNT) pillars. With previous studies on superhydrophobic systems highlighting the importance of both micro- and nanoscale roughness in the formation of such surfaces, double structural roughness of the surface was formed. The nanoscale roughness was induced by the intertwined SWCNTs at the top of the pillars, while SWCNT pillars grown on a nanosphere templated substrate constituted roughness at the microscale level. In the case of top-down approaches, Huang et al.18 and Cho et 19 al. modified surfaces of CNTs to improve the intrinsic hydrophobicity of the material. By coating a layer of a zinc oxide (ZnO) thin film on CNT arrays, Huang et al. created stable superhydrophobic surfaces. With alternation of ultraviolet (UV) radiation and dark storage, a reversible change in the wettability of the hybrid system from superhydrophobic to hydrophilic was achieved. Replacing the coating material, Cho et al. coated methane (CF4) onto CNT surfaces via glow discharge plasma at low pressures. They observed that the treated CNTs remained floating on water for several months because of a drastic reduction in the surface free energy of the plasma-treated CNTs from 27.04 to 1.3210-7 mN m-1. Using a combination of both topdown and bottom-up approaches, Sun et al.20 demonstrated the preparation of a superhydrophobic anisotropic aligned CNT film by chemical vapor deposition (CVD) on silicon substrates with quadrate micropillar arrays prepared by photolithography. By varying the spacing between the micropillars with a height of 10 μm, they were able to observe both hydrophobic and hydrophilic surfaces. However, with the introduction of fluorinated SAM of [(2-perfluorooctyl)ethyl]trimethoxysilane, such spacedependent wetting properties were no longer observed. Through their work on simulation, Varnik et al.21 reported their findings on scaling effects in microscale fluid flows at rough solid surfaces. It was determined from their work that flow instability was not controlled by the Reynolds number alone. Rather, surface roughness/channel geometry provided a new route for tuning qualitative features of the flow. By varying distances between the obstacles, they found fluid flows switched between stable and (17) Zhang, L.; Resasco, D. E. Langmuir 2009, 25, 4792. (18) Huang, L.; Lau, S. P.; Yang, H. Y.; Leong, E. S. P.; Yu, S. F.; Prawer, S. J. Phys. Chem. B 2005, 109, 7746. (19) Cho, S. C.; Hong, Y. C.; Uhm, H. S. J. Mater. Chem. 2007, 17, 232. (20) Sun, T.; Wang, G. J.; Liu, H.; Feng, L.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc. 2003, 125, 14996. (21) Varnik, F.; Raabe, D. Model. Simul. Mater. Sci. Eng. 2006, 14, 857.
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unstable flows. Although their work was based on the study of fluid flow, the idea of the possible existence of an optimal choice of distance between the obstacles motivated us to conduct a combination of top-down and bottom-up approaches in the investigation of the wettability of aligned multiwalled carbon nanotubes (MWCNTs) with micropatterns. In particular, we studied the hydrophobic nature of a patterned MWCNT array comprised of parallel arrays of microwalls. To do so, both the channel width (A) between the microwalls and the structure width (B) of the microwalls were varied and the respective contact angles were measured. Parallel arrays of microwalls made out of an aligned array of MWCNTs were created via a laser pruning technique.22 The laser pruning technique involved the use of an optical lens to focus a laser beam onto a spot with a diameter of ∼5 μm. By fixing the position of the laser spot while moving the MWCNT sample, we pruned or burned out parallel arrays of microwalls of the MWCNT arrays due to the intense heat produced by the focused laser beam. The height of these microwalls was 10-30 μm, while the width of the microwalls was 7, 13, or 21 μm. The channel width, wall-to-wall distance, between the microwalls was varied at an A/B width ratio of 2-6. The contact angle was measured after patterning. The average size of the water droplets used was 100 ( 30 μm. From the results obtained, the hydrophobic nature of the sample was much improved whenever water droplets landed on one microwall 7 μm in width. However, when a droplet bridged two microwalls, the surface became less hydrophobic. Thus, to ensure that the water droplets mostly landed on one microwall, an optimal superhydrophobic MWCNT surface corresponded to a parallel array of microwalls with a width of 13 μm and a channel width of ∼50 μm. Such findings could possibly serve as value-add for further developments in the creation of water-repelling patterned MWCNT surfaces.
2. Experimental Section 2.1. Carbon Nanotube Growth. The aligned MWCNTs were synthesized using a plasma-enhanced chemical vapor deposition (PECVD) chamber, and the growth details were reported previously.23-25 Precleaned N-typed silicon [5 mm5 mm, (100) Si] (22) Lim, K. Y.; Sow, C. H.; Lin, J. Y.; Cheong, F. C.; Shen, Z. X.; Thong, J. T. L.; Chin, K. C.; Wee, A. T. S. Adv. Mater. 2003, 15, 300. (23) Zhu, Y. W.; Cheong, F. C.; Yu, T.; Xu, X. J.; Lim, C. T.; Thong, J. T. L.; Shen, Z. X.; Ong, C. K.; Liu, Y. J.; Wee, A. T. S.; Sow, C. H. Carbon 2005, 43, 395. (24) Wang, Y. H.; Lin, J.; Huan, C. H. A.; Chen, G. S. Appl. Phys. Lett. 2001, 79, 680. (25) Bell, M. S.; Teo, K. B. K.; Lacerda, R. G.; Milne, W. I.; Hash, D. B.; Meyyappan, M. Pure Appl. Chem. 2006, 78, 1117.
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Figure 2. Cross-sectional view showing points of contact between a water droplet and (a) an unpatterned MWCNT surface, (b) one microwall, and (c) two microwalls. Insets are higher-magnification images of the point of contact between the water droplet and the microwall(s).
Figure 3. Comparison of mean contact angles with the water droplets that landed on different numbers of microwalls on different patterned MWCNTs arrays. The widths of the microwalls are (a) 7, (b) 13, and (c) 21 μm. The dotted lines represent upper and lower limits of the error in the contact angle of the unpatterned MWCNT array. substrates containing a native oxide layer were coated with a thin layer of iron (Fe) film. The Fe film coating catalyzed the growth of MWCNTs during the synthesis process. The film was coated using a magnetron sputtering system (RF Magnetron Denton Discovery 18) with a sputtering rate of 4 nm/min for 6.00 min. 2.2. Contact Angle Measurement. Contact angle (CA, R) measurement of the water droplets was conducted using a custombuilt contact angle measurement system. After the sample had been mounted onto the stage (Figure S1 of the Supporting Information), distilled water droplets with a typical diameter of 100 ( 30 μm were sprinkled onto the sample surface, using a commercially available plant sprinkler. Images were captured using a sideways microscope and lens (Opten International) together with video capturing software (WinDVD). For the sake of consistency, all images were captured with the magnification maintained at 8. Using the snapshot function in WinDVD, images of water droplets were obtained from the videos and the respective CAs measured. 12808 DOI: 10.1021/la9018025
2.3. Further Characterization. Further studies of the samples were conducted using a field emission scanning electron microscope (FESEM, JEOL 6700F).
3. Results and Discussion 3.1. Creation of MWCNT Microwalls. A laser pruning system as shown in Figure 1a was used to create parallel arrays of microwalls from vertically aligned MWCNTs (Figure 1b).22 A MWCNT sample was placed in a microscope focused laser beam system. Localized cutting of the MWCNTs was achieved via a focused incident laser beam (wavelength of 660 nm and initial beam size of 3 mm) on the sample. By moving the sample via a computer-controlled stage with respect to the laser spot (spot size of ∼5 μm), we created a parallel array of microwalls of various widths. Channels were cut using a laser power of 50 mW at a velocity of 150 μm s-1. Panels c-e of Figure 1 show examples of parallel arrays of microwalls with widths of 7, 13, and 21 μm, Langmuir 2009, 25(21), 12806–12811
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Figure 4. Schematic showing CAs of a water droplet landing on (a) unpatterned MWCNTs, (b) one microwall, and (c) two microwalls. Panel c shows possible distortions in the shape of a water droplet when it lands across two microwalls as the droplet seeks to maintain a spherical shape over both microwalls.
respectively, created using the laser pruning system as seen under the FESEM. In the course of the investigation, systematic studies on the effect of microwalls on the hydrophobicity of the MWCNTs were conducted by varying the ratio of the channel width (A) to the width of the microwalls (B), as well as the width of the microwalls, B, as indicated in Figure 1c. 3.2. Effect of Micropatterning on the Hydrophobicity of MWCNTs. Wetting of a solid surface usually involved three different interfacial boundary surfaces, namely, solid-air (free solid), solid-liquid, and liquid-air (free liquid) interfaces. With each interface having its own specific energy content, wetting with its accompanying change in the extent of each interface will result in variations to the net specific energy content of the system. As a water droplet spreads, both the wetted area under the droplet and the free liquid surface over it thereby increase. The former released energy, while the latter consumed energy. The difference in these energies would be the one that determines the wetting characteristics of the solid.26 When a water droplet lands on the MWCNT surface, because the MWCNT surface has a higher surface energy, the droplet tends to maintain a nearly spherical shape on the MWCNTs. Water droplets landing on unpatterned MWCNTs (Figure 2a) and one microwall (Figure 2b) have only one point of contact, whereas water droplets landing on two microwalls (Figure 2c) have two points of contact. The contact angle of water droplets landing on one microwall can be observed to be greater than that of unpatterned MWCNTs. Panels a-c of Figure 3 show the measured mean CAs of water droplets that landed on one microwall or across two microwalls, on different patterned MWCNTs. The horizontal axis in these graphs corresponds to the A/B ratios for these samples. The widths of the microwalls were 7, 13, and 21 μm. For the sake of comparison, the measured upper and lower limits for the CA of water droplets that landed on unpatterned MWCNTs are shown in Figure 3. The graphs show that for the same structure width, the largest CA was obtained when the droplet landed on one microwall. The CA decreased when the droplet landed across two microwalls. In Figure 3a, the CA for a water droplet landing on one microwall was at least 10% greater than that for two microwalls. Furthermore, with the exception of microwalls with a width of 21 μm, the CA of water droplets landing on one microwall, with widths of 7 and 13 μm, was greater than the CA of water droplets landing on unpatterned MWCNTs. As such, with reference to Figure 4, when a water droplet lands on two microwalls, the water droplet attempts to maintain a spherical shape about both points. However, the attempt of the droplet bridging both structures causes the water droplet to deviate from a spherical shape, resulting in a reduced contact angle (Figure 3) as compared to that of unpatterned MWCNTs or (26) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988.
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droplets landing on one microwall. To determine the value of the CA for water droplets landing on an unpatterned surface and one microwall, a tangential line from the contact point between the droplet and the surface of the MWCNTs was drawn as shown in panels a and b of Figure 4. For the case of a water droplet landing on two microwalls, a horizontal line connecting the two microwalls on which the droplet landed was drawn in addition to the tangential line as shown in Figure 4c. The CA was determined to be the internal angle from the tangent to the MWCNT surface/ horizontal line between the two microwalls. As the water droplet that was supported by two microwalls undergoes evaporation, distortion of the shape of the droplet is observed (Figure 5). Such a phenomenon was similar to that observed for water droplets evaporating from surfaces of lotus leaves as reported by Cheng et al.,27 where the adhesive nature of the water droplet caused it to maintain the same base area while reduction in droplet volume occurred in the form of a decrease in the contact angle. Such an observation was consistent with that shown in Figure 5a,b where a reduction in the measured CA, from 108.8° to 87.4°, was observed as the water droplet evaporated. Furthermore, the sudden decrease in the CA was consistent with the ellipsoidal cap model for evaporation that was proposed for water evaporation on a polymethylmethacrylate polymer by Erbil and Meric.28 In addition to what was previously observed for the evaporation of a water droplet from superhydrophobic surfaces of lotus leaves, water droplets were found to possess the ability to “jump” from two microwalls onto one microwall as shown in panels b and c of Figure 5. The jumping of the water droplet results in a spike in contact angle from 87.4° to 124.8°. In other words, such a phenomenon of water droplet jumping from two microwalls to one microwall would cause a percentage increase of 42.9% in the measured CA. This was attributed to the overpowering of the surface tension effect over the adhesive forces between the CNTs and the water droplet. 3.3. Optimal Microwall Width Size When the Water Droplet Lands on One Microwall. From the results presented in Figure 3, a higher CA was obtained when the water droplet landed on one microwall. Hence, systematic studies of the CA of the droplets landing on one microwall of different widths were conducted. Figure 6 shows the effect of variations in the width of the microwall on the CA of water droplets of similar sizes landing on one microwall. When the width of the microwall (Figure 6b,c) was smaller than the contact length of unpatterned MWCNT surface (Figure 6a), the contact spatial extent would be smaller. A higher contact angle was achieved, as there was less interaction between the surface and water molecules. If the width of the (27) Cheng, Y. T.; Rodak, D. E. Appl. Phys. Lett. 2005, 86, 3. (28) Erbil, H. Y.; Meric, R. A. J. Phys. Chem. B 1997, 101, 6867.
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Figure 5. Cross-sectional view showing variations in CA as a water droplet stretching across two microwalls undergoes evaporation. (a) Water droplet just landing on two microwalls. (b) Distortion of the shape of a water droplet and hence CA as the droplet evaporates. (c) The same water droplet “jumping” over to one microwall.
Figure 6. Water droplets of similar volume landing on (a) an unpatterned sample or a microwall with a width of (b) 7, (c) 13, or (d) 21 μm. The double-headed arrows in each panel indicate the contact length for the unpatterned surface. The measured CAs are shown in top right corner of each image.
Figure 7. Comparison of contact angles when a water droplet lands on one microwall for different dimensions. The horizontal line indicates the CA of unpatterned MWCNTs.
microwall (Figure 6d) was similar to or greater than the typical contact spatial extent for unpatterned MWCNTs, the CA would be similar to or smaller than that of a droplet landing on an unpatterned MWCNT surface considering the standard deviation. To verify the proposed explanation for the results shown in Figure 6, CAs of water droplets of similar volume with different areas of contact with the MWCNTs were compared. To change the area of contact, microwalls of different widths (7, 13, and 21 μm) were pruned. As shown in Figure 7, the CA was largest when the water droplet landed on one microwall with a width of 7 μm. When a water droplet of a particular size landed on the surface of unpatterned MWCNTs, it occupied a point of contact with diameter x. Given a water droplet of similar volume, reduction in the area on which it could land would cause the water droplet to behave like a perfect drop with a CA value of 180°, whereas an increase in the area of contact would stretch the droplet and lead to a reduction in the measured CA, as shown for the CA of a water droplet measured on 13 and 21 μm microwalls (Figure 7). 12810 DOI: 10.1021/la9018025
Figure 8. Percentage of water droplets landing on one microwall with respect to the width of the microwall, B.
3.4. Probability of a Water Droplet Landing on One Microwall. The effectiveness of the patterned MWCNTs as a hydrophobic surface depended on both the CA of the water droplet and the probability of the water droplet landing on one microwall, where the CA was greatest. Figure 8 showed that the percentage of the water droplet landing on one microwall varies with the size of the channel width of the MWCNT sample for water droplets with an average diameter of 100 ( 30 μm used in the experiments. This percentage was obtained via calculation of the number of water droplets that landed on one microwall as a percentage of the total number of water droplets landing on one microwall, across two microwalls, or between microwalls. Even though the CA for MWCNT samples with microwalls with a width of 7 μm was higher than that of microwalls with a wdith of 13 μm, the probability of a water droplet landing on one microwall was much lower. Therefore, we found that the most effective MWCNT sample had a channel width of ∼50 μm and a 13 μm microwall with an ∼70% probability of a water droplet landing Langmuir 2009, 25(21), 12806–12811
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on one microwall. Microwalls with a width of 21 μm were not included in this study since the measured CA for this sample was lower than that of unpatterned MWCNTs. Using the values obtained, the optimal Sf value, the ratio of the width of microwalls to the distance between them, was found to be ∼0.26. This Sf value was greater than those proposed by Bhushan et al.12 The difference could be attributed to several things. First, MWCNTs possessing intrinsic hydrophobic properties were used in our experiments as compared to PF3-coated single-crystal Si. Second, the microwalls created on the MWCNTs were in the form of microridges instead of the cylindrical pillars created by Bhushan et al. Third, the strands of interlacing MWNTs protruding out of both microwalls and the channels created in our experiments provided additional roughness at the nanoscale as compared to flat surfaces on both asperities and valleys studied by Bhushan. Such a combined effect of nano- and microscale roughness on the hydrophobicity of surfaces was also reported by Zhang et al.17 Because of the difference in the nature of materials used, the critical value proposed on the basis of our study using MWCNTs could possibly serve as a value-add for further developments in
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the creation of MWCNT-based water-repelling surfaces and aquatic devices.
4. Conclusion In conclusion, through micropatterning of MWCNTs, the hydrophobicity of the MWCNTs could be increased. The MWCNTs were found to become more hydrophobic when a water droplet landed on one microwall with a width of 7 μm. However, when a droplet bridged two microwalls, the surface became less hydrophobic. Thus, to ensure that the water droplets mostly landed on one microwall, the most optimal superhydrophobic MWCNT surface corresponded to a parallel array of microwalls with a width of 13 μm and a channel width of ∼50 μm. Acknowledgment. We acknowledge the funding support of MOE-ARF Grant R-144-000-211-112. Supporting Information Available: Schematic of the CA measurement system. This material is available free of charge via the Internet at http://pubs.acs.org.
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