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Fabrication of Superhydrophobic Surfaces of n-Hexatriacontane H. Tavana,† A. Amirfazli,*,‡ and A. W. Neumann*,† Department of Mechanical and Industrial Engineering, UniVersity of Toronto, 5 King’s, College Road, Toronto, Ontario, Canada M5S 3G8, Department of Mechanical Engineering, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G8 ReceiVed March 22, 2006. In Final Form: May 6, 2006 Superhydrophobic surfaces of n-hexatriacontane were fabricated in a single-step process. The low surface energy of n-hexatriacontane together with the randomly distributed micro- and nanoscale roughness features guarantees very large contact angles and a small roll-off angle for water drops. The advantage of n-hexatriacontane superhydrophobic surfaces is their stability in the sense that they are impervious to chemical reactions and retain their wetting characteristics over a long period of time, as confirmed by XPS analysis and contact angle measurements.
Many examples of superhydrophobicity can be found in nature such as in the leaves of Lotus (nelumbo nucifera) and Lady’s mantle (Alchemilla mollis).1,2 On such surfaces, a water drop beads off completely and removes dirt and debris as it rolls off the surface. This self-cleaning property is known as the “Lotus effect”, which is imparted by a combination of roughness on the micrometer and/or nanometer scale and generally an intrinsically hydrophobic material. Superhydrophobic surfaces have recently attracted a great deal of interest because of their potential applications in microfluidic devices as well as in developing new waterproof coatings, stain-resistant finishes, and self-cleaning materials. Techniques such as plasma etching,3,4 electrodeposition,5 laser treatment,6 sol-gel processing,7 anionic oxidation,8 and chemical etching9,10 have been used to fabricate superhydrophobic surfaces by tailoring the surface topography. Most of these techniques are costly, time-consuming, and might involve complicated multistage processes and application of several treatments to attain desirable roughness and hydrophobicity. Furthermore, some superhydrophobic surfaces are made of polymer materials that might be vulnerable to oxidation, heat, and different solvents. In this study, a straightforward method is used to prepare superhydrophobic surfaces in a single-step process. Silicon wafers 〈100〉 (Silicon Sense, Nashua, NH) were selected as the substrate. The procedure to clean the substrates was described previously.11 The coating material is n-hexatriacontane (C36H74) that was obtained from Sigma-Aldrich Co. (Oakville, Ontario, Canada) at a purity of g99.5%. It is a crystalline n-alkane * Corresponding authors.
[email protected]. † University of Toronto. ‡ University of Alberta.
E-mail:
[email protected];
(1) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1-8. (2) Otten, A.; Herminghaus, S. Langmuir 2004, 20, 2405-2408. (3) Olde Riekerink, M. B.; Terlingen, J. G. A.; Engbers, G. H. M.; Feijen, J. Langmuir 1999, 15, 4847-4856. (4) Minko, S.; Mu¨ller, M.; Motornov, M.; Nitschke, M.; Grundke, K.; Stamm, M. J. Am. Chem. Soc. 2003, 125, 3896-3900. (5) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2005, 21, 937-943. (6) Khorasani, M. T.; Mirzadeh, H.; Kermani, Z. Appl. Surf. Sci. 2005, 242, 339-345. (7) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. AdV. Mater. 1999, 11, 1365-1368. (8) Tsujii, K.; Yamamoto, T.; Onda, T.; Shibuichi, S. Angew. Chem., Int. Ed. Engl. 1997, 36, 1011-1012. (9) Thieme, M.; Frenzel, R.; Schmidt, S.; Simon, F.; Hennig, A.; Worch, H.; Lunkwitz, K.; Scharnweber, D. AdV. Eng. Mater. 2001, 3, 691-695. (10) Qian, B.; Shen, Z. Langmuir 2005, 21, 9007-9009. (11) Tavana, H.; Lam, C. N. C.; Friedel, P.; Grundke, K.; Kwok, D. Y.; Hair, M. L.; Neumann, A. W. J. Colloid Interface Sci. 2004, 279, 493-502.
Figure 1. SEM images of an n-hexatriacontane surface at magnifications of (a) 1000× and (b) 5000×.
with a density of ∼0.959 g/cm3 and a melting point of ∼76°C.12 n-Hexatriacontane was selected for two key reasons: (i) It is a hydrophobic material that can form films with a low surface energy of ∼20 mJ/m2.13 (ii) It consists of saturated chains of molecules, making it less susceptible to chemical reactions such as oxidation. The silicon substrate lends itself to compatibility with MEMS devices. Physical vapor deposition (PVD) was used as the film preparation method.14 The substrates were mounted inside a vacuum chamber. The n-hexatriacontane crystals were deposited in a heater-equipped cell at the bottom of the chamber. The pressure of the chamber was set to 1.3 ×10-5 Torr to reduce unwanted gas-phase reactions and improve film uniformity across the wafer. Gradually, the temperature of the cell was raised until the crystals were completely liquefied and then evaporated. The deposition rate on the substrates was ∼5 nm/min, which is equivalent to ∼1.15 µg/min for a coated area of 240 mm2. Figure 1 shows the scanning electron microscope (SEM) images of the same area of a coated surface at two different magnifications. It is seen that the surface texture comprises randomly distributed clusters of n-hexatriacontane, with lateral sizes from hundreds of nanometers to several micrometers. It has been suggested that a random distribution of rough structures might yield superior hydrophobicity compared to that of orderly textured features.15 (12) Vand, V. Acta Crystallogr. 1953, 6, 797-798. (13) Neumann, A. W. AdV. Colloid Interface Sci. 1974, 4, 105-191. (14) Briand, D.; Mondin, G.; Jenny, S.; van der Wal, P. D.; Jeanneret, S.; de Rooij, N. F.; Banakh, O.; Keppner, H. Thin Solid Films 2005, 493, 6-12. (15) O ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777-7782.
10.1021/la0607757 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/25/2006
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Langmuir, Vol. 22, No. 13, 2006 5557
Figure 2. AFM image of the superhydrophobic surface of n-hexatriacontane over a 20 µm × 20 µm area and a typical section analysis corresponding to the image. The images were acquired as 512 × 512 pixel images at a scan rate of 1 to 2 Hz. Imaging was accomplished using diving board TESP tips with integral square-pyramidal tips. All data were plane fit and flattened prior to analysis using the Nanoscope software.
Figure 3. Wide-scan spectrum of the n-hexatriacontane surface shows only one peak corresponding to C 1s, and no traces of silicon or oxygen are observed.
To obtain quantitative information about the roughness features, the n-hexatriacontane surfaces were analyzed by atomic force microscopy (AFM) (Veeco Instruments, New York) in tapping mode. Figure 2 shows an AFM image for an area of 20 µm × 20 µm and the corresponding section analysis. From three independent analyses of different areas, the root-mean-square (rms) roughness on the surface and the average maximum peakto-valley distance were obtained as ∼0.62 µm and ∼0.9 µm, respectively. X-ray photoelectron spectroscopy (XPS) (SPECS Scientific Instruments, Florida) analysis was performed to establish the elemental composition of the coated surface. The spectrometer was equipped with a monochromatic Al KR (hν ) 1486.6 eV) X-ray source of 250 W at 12.5 kV. The kinetic energy of the photoelectrons was determined with a hemispheric analyzer set to a pass energy of 192 eV for wide-scan spectra and 48 eV for high-resolution C 1s spectra. The angle-resolved XPS spectrum was recorded for a takeoff angle of 90° with respect to the horizontal, yielding information for a depth of ∼7-10 nm from the film. Figure 3 shows the wide-scan spectrum of the n-hexatriacontane surface. Only one peak is observed that corresponds to C 1s. The high-resolution scanning also showed a similar peak. The elemental composition ratio corresponding to the wide-scan spectrum is [C]:[O]:[Si] ) 99.9:0.1:0.0. This
Figure 4. Output of ADSA-P from a sessile drop experiment with water on a superhydrophobic surface of n-hexatriacontane. The advancing and receding angles, contact radius, and volume of the drop are shown as a function of time.
means that n-hexatriacontane molecules cover the entire surface, leaving no traces of the bare silicon substrate. The small amount of oxygen might be due to the presence of impurities in the coating material. Overall, it is concluded that the surface is covered only by saturated hydrocarbons. As such, surfaces should be chemically stable. This point will be further examined below. The wetting properties of the n-hexatriacontane-coated surfaces were studied by dynamic contact angle measurements with distilled water. The contact angles were determined by sessile drop experiments11 using axisymmetric drop shape analysisprofile (ADSA-P) methodology.16 Figure 4 shows an output of ADSA-P from these experiments. The advancing and receding angles, contact radius, and volume of the drop are shown as a function of time. The contact angle results from three experiments are summarized in Table 1. It is seen that the average advancing and receding contact angles are very large (i.e., 170.9 and 165.0°, respectively). Hence the average contact angle hysteresis is only (16) Lahooti, S.; Del Rio, O. I.; Cheng, P.; Neumann, A. W. Applied Surface Thermodynamics; Marcel Dekker: New York, 1992; Chapter 10, pp 441-507.
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Table 1. Reproducibility of Advancing and Receding Contact Angles of Water on the Superhydrophobic Surfaces of n-Hexatriacontanea
a
run
θa (deg)
θr (deg)
1 2 3 mean
170.4 ( 1.0 171.8 ( 1.1 170.6 ( 1.3 170.9 ( 0.9
164.2 ( 1.5 166.0 ( 1.2 164.7 ( 1.4 165.0 ( 0.9
The errors are the 95% confidence limits.
Figure 5. Advancing sessile drop of water on a superhydrophobic surface of n-hexatriacontane. The contact diameter of the drop is 0.33 cm.
∼6°. An advancing sessile drop of water on the n-hexatriacontane surface is shown in Figure 5. These results confirm the superhydrophobicity of the n-hexatriacontane surfaces. The small contact angle hysteresis suggests that water does not penetrate significantly into the spaces among the protrusions on the surface. This point is further examined below. Depending on the morphology and chemistry of a surface, two distinct types of wetting behavior might be observed: (i) Wenzel model or (ii) Cassie model.17,18 In the Wenzel regime, the liquid penetrates into the troughs of the surface texture and generally yields a large contact angle hysteresis. However, in the Cassie regime the liquid drop sits on a composite surface that comprises solid and air pockets. Normally a small hysteresis is observed in this regime.19 The Wenzel model is described by the following relation
cos θw ) r cos θY
(1)
where θw is the apparent contact angle on the rough surface (Wenzel angle), θY represents the intrinsic contact angle of the same material, and r is the roughness ratio (i.e., ratio of actual to apparent surface area). The Cassie model is described by the following equation
cos θC ) -1 + Φs(1 + cos θY)
(2)
where θC is the apparent contact angle on the rough surface (Cassie angle) and Φs is the fraction of the solid surface in contact with the liquid. The switch from the Wenzel regime to the Cassie regime requires the contact angle to be larger than a certain value (i.e., the threshold contact angle (θ*)). It is obtained by equating the above equations:20 (17) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988-994. (18) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546-551. (19) Mohammadi, R.; Wassink, J.; Amirfazli, A. Langmuir 2004, 20, 96579662. (20) Lafuma, A.; Que´re´, D. Nat. Mater. 2003, 2, 457-460.
θ* ) cos-1
[
]
Φs - 1 r - Φs
(3)
To examine which regime the contact angles in Table 1 belong to, the geometrical parameters r and Φs were calculated. To calculate the roughness ratio (r), the length and the width of the clusters were simply approximated from a high-magnification SEM image (15000×) knowing the length scale of the image. In addition, the height of the clusters was approximated as the rms mean roughness from AFM measurements. Knowing the length, width, and height of the clusters, we calculated the actual area of the solid surface. Dividing this value by the apparent surface area yielded r. This procedure was repeated for three different images, and an average value of 1.3 was obtained for the roughness parameter. To determine Φs, the length and the width of each n-hexatriacontane cluster were determined as above. The apparent area of the clusters in each image was then obtained by adding up the areas of the individual clusters. This procedure was repeated for three different images and yielded an average value of ∼0.12 for Φs. Using the values for r and Φs, the threshold contact angle θ* is ∼138.2°. It is evident that the measured advancing and receding angles are much larger than the threshold contact angle, confirming that they belong to the Cassie regime. An estimate of the contact angle of water on the superhydrophobic surfaces of n-hexatriacontane can be made from the Cassie equation using the value of Φs ) 0.12 and the water contact angle on the smooth surfaces of this material (i.e., θY ) 104.6° 13). This gives a value of ∼156° for θc. This value is about 15° lower than the experimental contact angle, meaning that the Cassie equation underestimates the contact angles. Similar observations are also reported in the literature.5,21 The exact reason is not known. However, a close look at the highmagnification SEM image of n-hexatriacontane (Figure 1) shows that the clusters have different heights. It might be that the water drop sits only on the taller clusters and does not make contact with the shorter ones. If this is the case, then Φs would be smaller than 0.12, and the contact angle predicted by the Cassie equation would be closer to the experimental value. The measurement of the “roll-off” angle (i.e., the angle of an inclined surface at which a drop of water rolls off the surface) confirmed the above conclusion. Water drops with a volume of 19.8 µL and a contact radius of 0.89 mm easily roll off of the superhydrophobic n-hexatriacontane surfaces at an angle of ∼3° (Supporting Information). It is noted that the roll-off angle and the contact angle hysteresis show a difference of 3°. It has been suggested that the roll-off angle does not necessarily equal the contact angle hysteresis because the rolloff angle also depends on the volume and contact radius of the drop.22,23 To examine the chemical stability of produced surfaces, contact angles of water were measured on n-hexatriacontane surfaces that had been left in a Petri dish in the laboratory environment for about one year. The contact angles were reproduced with an accuracy of (2°, indicating the longevity of the surfaces. The reproducibility of the XPS analysis (lack of O peak) of these surfaces also confirmed this point. In summary, stable superhydrophobic surfaces of n-hexatriacontane were developed by a simple process. The randomly distributed roughness features that vary in size from hundreds (21) Neelesh, B. H.; Patankar, A.; Lee, J. Langmuir 2003, 19, 4999-5003. (22) Furmidge, C. G. L. J. Colloid Sci. 1962, 17, 309. (23) Extrand, C. W.; Kumaga, Y. J. Colloid Interface Sci. 1995, 170, 515521.
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of nanometers to several micrometers combined with the hydrophobic nature of n-hexatriacontane yields superhydrophobic behavior. The surfaces have a stable chemistry as well as longlasting wetting characteristics. Acknowledgment. This research was supported by the Natural Science and Engineering Research Council of Canada (NSERC), a University of Toronto Fellowship (H.T.), and an Ontario
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Graduate Scholarship (H.T.). We thank Dr. C. M. Yip for the AFM measurements. Supporting Information Available: A video clip showing a sessile drop of water rolling off of a superhydrophobic surface of n-hexatriacontane tilted at ∼3°. This material is available free of charge via the Internet at http://pubs.acs.org. LA0607757
