Fabrication of Super-Oil-Repellent Dual Pillar Surfaces with Optimized

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LETTER pubs.acs.org/Langmuir

Fabrication of Super-Oil-Repellent Dual Pillar Surfaces with Optimized Pillar Intervals Takashi Fujii, Yoshitaka Aoki, and Hiroki Habazaki* Division of Materials Chemistry, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan

bS Supporting Information ABSTRACT: Hierarchical dual pillar surfaces with optimized pillar intervals are fabricated by a novel combined process of the oblique angle magnetron sputtering deposition of Al Nb alloys and their anodizing. The pillar intervals are controlled by the deposition angle and cell size of a scalloped substrate for oblique angle deposition. Anodizing of the deposited pillar surfaces develops a nanopillar oxide layer, producing the hierarchical dual pillar surfaces. After being coated with a fluoroalkyl phosphate layer to reduce the surface free energy, hierarchical surfaces with submicrometer pillar intervals greater than 400 nm show super liquid repellency even for hexadecane with a low surface tension of 27.5 mN m 1, although the submicrometer pillar surfaces with smaller submicrometer pillar intervals and without nanopillars were not super-oil-repellent. In contrast, the dual pillar surfaces show superhydrophobicity regardless of the submicrometer pillar intervals. Thus, the present study demonstrates the importance of the pillar intervals (gap size between pillars) to realize the superoleophobicity.

’ INTRODUCTION Controlling the wettability of a solid surface is of crucial importance in daily life and industrial applications. In particular, superhydrophobic surfaces, which have water contact angles of greater than 150°, have attracted considerable attention because of their many potential applications in self-cleaning,1 antifreezing,2 antisticking of snow,3 anticorrosion,4 and antibiofouling.5 Superhydrophobic surfaces are often observed in nature from plant leaves6 to water strider legs,7 which inspired the fabrication of artificial superhydrophobic surfaces. Surfaces that repel water are not usually oil-repellent. Thus, superhydrophobic surfaces can be contaminated with oil, losing their superior properties. The wettability of a solid surface is controlled by two factors: its chemical composition and its topographical structure. The chemical composition determines the contact angle of a liquid on a flat surface, and the surface topography (roughness) enhances the wetting. For super liquid repellency, roughness must be introduced onto the liquid-repellent flat surface (θflat > 90°) because the maximum contact angle on a flat surface is 120°.8 When the contact angle of a liquid on a flat surface is less than 90°, surface roughening reduces the contact angle.9 The difficulty of producing super-oilrepellent surfaces arises from the low surface tension of oils (γl e 35 mN m 1) as compared with that of water (γl = 72.8 mN m 1).10 To obtain a value of θflat greater than 90°, the surface tension (γs) of a solid surface of less than ∼20 mN m 1 is needed for water, and for an organic liquid with γl = 20 30 mN m 1, γs is reduced to less than ∼6 mN m 1. Only a CF3 group with a theoretical surface tension of 6 mN m 1 satisfies the requirement for super liquid repellency in low-surface-tension organic liquids. However, as a matter of fact, the surfaces with a CF3-terminated layer were still oleophilic for some organic liquids with γl < 30 mN m 1.11 r 2011 American Chemical Society

Recently, the importance of a third factor (i.e. a re-entrant12 or an overhanging13 structure) has been shown in the fabrication of superoleophobic surfaces. In these cases, superoleophobicity has been realized even if the surface is intrinsically oleophilic (θflat < 90°). The re-entrant and overhanging structures are able to prevent water from penetrating the cavities of rough surfaces. As a consequence, a composite interface with air pockets in the cavities, referred to as the Cassie Baxter state, was maintained and high contact angles of greater than 150° were reported for some organic liquids. However, superoleophobicity has been achieved mainly for liquids with γl values greater than 35 mN m 1, such as rapeseed oil. Although even surfaces with switchable superoleophobicity14 and self-healing superoleophobicity15 have been reported, examples of superoleophobicity for liquids with γl values of less than 30 mN m 1 are still very limited.13 17 Here, we report the synthesis of superoleophobic hierarchical surfaces with a contact angle of greater than 150°, even for hexadecane (γl = 27.5 mN m 1). The hierarchical surfaces that we have produced by a combined process of oblique angle magnetron sputtering deposition (OAD) and anodizing have dual-scale pillar morphology. The produced oxide surfaces have been coated with a monolayer of fluoroalkyl phosphate to reduce the surface tension. It is now well accepted that hierarchical, multiscale rough surfaces, often found in plant leaves and insect wings and legs, are of crucial importance for superhydrophobicity.18 Nosonovsky has explained the role of multiscale roughness in terms of the pinning of water droplets, which tend to penetrate the cavities Received: June 30, 2011 Revised: August 22, 2011 Published: August 24, 2011 11752

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Figure 1. Schematic illustration showing the fabrication of the superoil-repellent dual scale pillar surfaces by a combined process of OAD, anodizing, and FAP coating.

between the pillars, by the introduction of nanoconvex surfaces on micrometer-sized pillars.19 This work shows that optimized dual pillar morphology can also be useful in achieving superoleophobicity even when the contact angle on a flat surface is less than 90°. The fabrication process of dual pillar surfaces is illustrated in Figure 1. Submicrometer pillar Al Nb alloy films were deposited on a scalloped aluminum substrate (Figure S1 in Supporting Information) by OAD. Owing to an enhanced self-shadowing effect on the scalloped substrate, isolated pillar films with a gap between pillars were developed. The pillar morphology was controlled by the cell size of the scalloped cells and the deposition angle with respect to the substrate normal. Then, the deposited films were anodized at 10 V in a hot phosphate glycerol electrolyte to form a nanoporous anodic oxide layer. To reduce the surface tension, the surfaces were coated with a monolayer of fluoroalkyl phosphate, CF3(CF2)7CH2CH2OPO(OH)2, (FAP).

’ EXPERIMENTAL SECTION Al Nb pillar films with an average pillar height of 1 μm were formed by dc magnetron sputtering at oblique angles of 70, 85, 95, and 110° with respect to the substrate normal on a scalloped aluminum substrate with a cellular concave structure for 1.8 ks, as in our previous report.20 The scalloped aluminum substrate was prepared by anodizing a 99.99% pure aluminum sheet in 0.1 mol dm 3 phosphoric acid solution at a constant voltage of 170 V at 293 K or in a 2 mass % citric acid solution at a constant voltage of 390 V at 283 K, followed by the dissolution of the resultant anodic oxide film by immersing the anodized specimen in a mixture of chromic acid and phosphoric acid at 343 K. During anodizing, self-ordered nanoporous anodic alumina films with a hexagonal cell structure were developed. The cell size increases almost linearly with the formation voltage. The resultant surfaces have a cellular morphology, as shown in Figure S1 in the Supporting Information. The cell sizes obtained in phosphoric acid and citric acid were approximately 400 and 900 nm, respectively. The films formed by OAD were anodized in a glycerol electrolyte containing 0.8 mol dm 3 K2HPO4 at a constant voltage of 10 V at 433 K for 0.3 ks, and then they were immersed in the same electrolyte for pore widening and chemical dissolution of the volume-expanded nanoporous anodic oxide films. The deposited films and the subsequent anodized films were immersed in ethanol containing 2 mass % mono[2-(perfluorooctyl)ethyl]phosphate, (CF3(CF2)7CH2CH2OPO(OH)2), (FAP), for 5 days at room temperature to reduce the surface free energy by coating with a FAP monolayer. This FAP was synthesized by the approach described in ref 11, and the chemical structure was checked by nuclear magnetic resonance analysis. The surface morphology of the films was observed with a field-emission scanning electron microscope (SEM) (JEOL Co., JSM-6500F). The wettability of the specimens obtained was evaluated by contact angle measurements for liquid droplets (2 μL) of water, rapeseed oil, and hexadecane using an optical contact angle meter

Figure 2. Scanning electron micrographs of the dual pillar specimens fabricated by OAD and anodizing. OAD was carried out at deposition angles of (a, e) 70°, (b, f) 85°, (c, g) 95°, and (d, h) 110° on scalloped aluminum substrates with average cell sizes of (a d) 400 and (e h) 900 nm. (Kyowa Interface Science Co., DM-CE1). For the dynamic advancing and receding contact angle measurements, liquids were pumped into/sucked from the drop with a syringe.

’ RESULTS AND DISCUSSION Figure 2 shows the SEM images of the dual pillar specimens obtained by OAD and anodizing. The Al Nb alloy was mainly deposited on the ridge regions of the scalloped aluminum substrate. Thus, the submicrometer pillar array followed the morphology of the scalloped substrate. The gaps between the submicrometer pillars increase with an increase in the deposition angle from 70 to 110°, with the pillar morphology also changing from polygonal to platelike. The increased shadow regions at higher deposition angles should induce such a morphological change. Obviously, the gaps between the submicrometer pillars are markedly enlarged by increasing the cell size of the scalloped substrate (Figure 2e h). A more detailed morphology of the Al Nb alloy specimens deposited at 85° on the substrate with an average cell size of 900 nm before and after anodizing is shown in Figure 3. The platelike morphology of the deposits is more obvious before anodizing (Figure 3a). The formation of platelike morphology was discussed in a separate paper.20 Cross-sectional high-magnification images (Figure 3b,c) show that the surface of the pillars, facing the incident flux of the depositing atoms, is rather smooth, whereas a fish-scale-like morphology is developed on the opposite side of the surface. The back-side surface might be a shadow region with respect to the incident deposition atoms; however, deposition actually occurred even on the back-side surface, owing to, for instance, the reflection of the incident atoms on the fore-side surface of the next pillar. Surface roughening on the back-side 11753

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Table 1. Static Contact Angles of Several FAP-Coated Flat and Pillar Specimens for Water (72.8 mN m 1), Rapeseed Oil (35 mN m 1), and Hexadecane (27.5 mN m 1) Droplets contact angle (deg) specimen

water

rapeseed oil

hexadecane

flat

116.6 ( 0.6

90.0 ( 0.7

79.1 ( 0.7

submicrometer pillarb

157.6 ( 1.4

144.5 ( 0.8

131.8 ( 1.0

dual pillarc

158.7 ( 0.3

155.9 ( 0.9

151.3 ( 2.8

a

Magnetron-sputtered Al Nb alloy film with a flat surface. b Al Nb film deposited at a deposition angle of 85° on the scalloped aluminum substrate with an average cell size of 900 nm. c Anodized specimen in b. a

Figure 3. Scanning electron micrographs of the OAD Al-43 atom % Nb specimens (a c) before and (d f) after anodizing at 10 V in the hot phosphate glycerol electrolyte for 300 s and subsequent immersion in the electrolyte for 600 s. OAD was carried out at a deposition angle of 85° on a scalloped aluminum substrate with an average cell size of 900 nm.

surface might be associated with an additional shadowing effect. Owing to the considerable amount of deposition on the back-side surface of the substrate, pillar films of similar thicknesses were deposited at angles of 95 and 110° (Figure 2c,d,g,h), which are the back sides of the substrates deposited at 85 and 70°, respectively. The platelike pillars become wider after anodizing in the hot phosphate glycerol electrolyte (Figure 3a,d) because the thickness of the anodic oxide film that is formed is usually larger than that of the metal consumed by anodizing.21 A nanopillar-type feature is developed on the platelike pillars, with the porosity being larger on the back-side surface. The increased porosity on the back-side surface is associated with the higher roughness before anodizing. Nanoporous anodic oxide films with a hexagonal cell structure are usually formed by anodizing valve metals.22 The formation of the nanopillar oxide layer might be associated with additional chemical dissolution during anodization and subsequent immersion in the electrolyte, leaving only the triple-point regions as nanopillars.16 FAP coating was carried out by immersing the pillar specimens in an ethanol solution containing 2 mass % FAP for 5 days at room temperature. Table 1 lists the static contact angles of the FAP-coated flat and pillar specimens for water, rapeseed oil, and hexadecane. The flat specimen was prepared by a FAP coating on the flat Al Nb alloy film covered with a thin air-formed oxide layer. Although the static contact angle for water exceeds 90° on the flat surface, those for rapeseed oil and hexadecane are equal to or less than 90° even on the FAP-coated surface. Thus, the third factor in a pinning effect via a re-entrant, overhanging, or dual pillar structure is really needed to achieve superoleophobicity. The FAP-coated submicrometer pillar specimen without nanopillars is not superoleophobic for both rapeseed oil and

Figure 4. Changes in the static contact angle and contact angle hysteresis of the FAP-coated dual pillar specimens for water (triangles), rapeseed oil (circles), and hexadecane (squares) droplets with the size of the gaps between submicrometer pillars.

hexadecane, though the contact angles increase to greater than 130°. However, the dual pillar specimen shows static contact angles of greater than 150° even for hexadecane with γl as low as 27.5 mN m 1 (Figure S2 in the Supporting Information). The dual pillar morphology effectively pins the droplet penetrating the gaps between the pillars. For water droplets, both submicrometer pillars with and without nanopillars are superhydrophobic because the surface is hydrophobic with a contact angle for water on a flat surface of greater than 90°. A more significant influence of the dual pillar structure appeared when the dynamic contact angles were measured. The dual pillar surface reveals a relatively small contact angle hysteresis (difference between advancing and receding contact angles) of as small as 2° for rapeseed oil and ∼6° for hexadecane, suggesting that this surface is superoleophobic (Figure S3a,b in the Supporting Information). In contrast, the submicrometer pillar surface without nanopillars shows very low receding contact angles that change continuously during the measurement (Figure S3c,d in the Supporting Information), although the static contact angle is rather high as shown in Table 1. Thus, it can be said that only the dual pillar surface is in the Cassie Baxter state. Although the dual pillar specimens exhibited superoleophobicity as shown in Table 1, the importance of gaps between the submicrometer pillars is found through an examination of the static and dynamic contact angles on the various specimens shown in Figure 2. Figure 4 shows the change in the static contact angles as well as the contact angle hysteresis for water, rapeseed 11754

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Figure 5. Schematic illustration showing the role of nanopillars on the submicrometer pillars in stabilizing a composite air liquid solid interface. (a) Three-dimensional image, (b) without nanopillars, and (c) with nanopillars.

oil, and hexadecane with gap size between submicrometer pillars. The gaps were changed by the deposition angle and cell size of the scalloped substrate. Obviously, the static contact angles for rapeseed oil and hexadecane increase with the gap size of the submicrometer pillars and become greater than 150° when the gaps are larger than 400 nm. In addition, small contact angle hysteresis of less than 3° and ∼6° are obtained for rapeseed oil and hexadecane, respectively, when the gaps are larger than 400 nm. For specimens with gaps of less than 400 nm, the receding contact angles decreased to less than 90°, showing very large contact angle hysteresis. The findings of this study demonstrate the necessity of dual pillar morphology for superoleophobicity. The role of nanopillars can be explained in terms of the geometrical air-trapped effect discussed in refs 23 and 24 or the re-entrant structure12 and is illustrated in Figure 5. The re-entrant structure produced by the nanopillar array may stabilize the air pocket in the gaps between submicrometer pillars. However, the Cassie Baxter state is obtained only when the gaps are larger. Roughness (i.e., the reduction of the contact area between an oil droplet and a solid surface) may not be sufficient for dual pillar surfaces with smaller gaps between submicrometer pillars.

’ CONCLUSIONS We fabricated superoleophobic dual pillar surfaces by a combined process of oblique angle physical vapor deposition and anodizing and subsequent surface coating with fluoroalkyl phosphate layers. By changing the deposition angle and cell size of a scalloped substrate for oblique angle deposition, it became possible to control the gap size between submicrometer pillars. Anodizing of submicrometer pillars generated the hierarchical morphology with nanopillars on submicrometer pillars. The FAP-coated dual pillar surfaces with submicrometer intervals of larger than 400 nm were super-oil-repellent for rapeseed oil and hexadecane, although the submicrometer pillar surfaces without nanopillars were not superoleophobic. In summary, this study demonstrates the importance of the pillar intervals (gap size between pillars) in realizing superoleophobicity. This should be one of the key criteria for the design of surfaces that show superrepellency for oils with low surface tension. ’ ASSOCIATED CONTENT

bS

Supporting Information. AFM images of scalloped aluminum substrates and digital camera images of droplets on the FAP-coated dual pillar specimens and submicrometer pillar specimens without nanopillars. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel/Fax: +81-11-7066575.

’ ACKNOWLEDGMENT We thank Dr. Masanori Yoshida, Division of Chemical Process Engineering, Faculty of Engineering, Hokkaido University, for his help with the synthesis of FAP. This work was supported by a grant-in-aid for exploratory research (no. 21656180) from the Japan Society for the Promotion of Science, the Global COE Program “Catalysis as the Basis for Innovation in Materials Science” from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and the Light Metal Educational Foundation, Inc. ’ REFERENCES (1) (a) Nakajima, A.; Hashimoto, K.; Watanabe, T.; Takai, K.; Yamauchi, G.; Fujishima, A. Langmuir 2000, 16, 7044–7047. (b) Furstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21, 956–961. (c) Quere, D. Rep. Prog. Phys. 2005, 68, 2495–2532. (d) Sun, T. L.; Feng, L.; Gao, X. F.; Jiang, L. Acc. Chem. Res. 2005, 38, 644–652. (2) Mishchenko, L.; Hatton, B.; Bahadur, V.; Taylor, J. A.; Krupenkin, T.; Aizenberg, J. ACS Nano 2010, 4, 7699–7707. (3) Saito, H.; Takai, K.; Takazawa, H.; Yamauchi, G. Mater. Sci. Res. Int. 1997, 3, 216–219. (4) (a) Ishizaki, T.; Hieda, J.; Saito, N.; Takai, O. Electrochim. Acta 2010, 55, 7094–7101. (b) Liu, H. Q.; Szunerits, S.; Xu, W. G.; Boukherroub, R. ACS Appl. Mater. Interfaces 2009, 1, 1150–1153. (c) Wang, P.; Qiu, R.; Zhang, D.; Lin, Z. F.; Hou, B. R. Electrochim. Acta 2010, 56, 517–522. (5) Scardino, A. J.; Zhang, H.; Cookson, D. J.; Lamb, R. N.; de Nys, R. Biofouling 2009, 25, 757–767. (6) Cheng, Y. T.; Rodak, D. E. Appl. Phys. Lett. 2005, 86. (7) Gao, X. F.; Jiang, L. Nature 2004, 432, 36–36. (8) Nishino, T.; Meguro, M.; Nakamae, K.; Matsushita, M.; Ueda, Y. Langmuir 1999, 15, 4321–4323. (9) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994. (10) Tsujii, K.; Yamamoto, T.; Onda, T.; Shibuichi, S. Angew. Chem., Int. Ed. Engl. 1997, 36, 1011–1012. (11) Shibuichi, S.; Yamamoto, T.; Onda, T.; Tsujii, K. J. Colloid Interface Sci. 1998, 208, 287–294. (12) Tuteja, A.; Choi, W.; Ma, M. L.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Science 2007, 318, 1618–1622. (13) Cao, L. L.; Price, T. P.; Weiss, M.; Gao, D. Langmuir 2008, 24, 1640–1643. (14) Wang, D.; Wang, X.; Liu, X.; Zhou, F. J. Phys. Chem. C 2010, 114, 9938–9944. 11755

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