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Fabrication of superhydrophobic surfaces on aluminum
Applied Surface Science 254 (2008) 5599–5601
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication of superhydrophobic surfaces on aluminum Hui Wang *, Dan Dai, Xuedong Wu Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbao Zhuangshi Road 519, Ningbo 315021, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 8 November 2007 Received in revised form 3 March 2008 Accepted 3 March 2008 Available online 12 March 2008
A superhydrophobic surface was prepared on aluminum substrate. Anodization and low-temperature plasma treatment were used to create micro–nano-structure and subsequently trichlorooctadecyl-silane modified the rough surface. The result shows that the water static contact of the aluminum surface after anodization and modification by trichlorooctadecyl-silane reaches to 152.18. A rougher surface with some micro–nano-pores and small mastoids along the edges of pores was generated when lowtemperature plasma treatment was applied to anodized aluminum film, resulting in water static contact angle up to 157.88. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Superhydrophobic surface Anodization Aluminum Low-temperature plasma treatment
1. Introduction Superhydrophobic surfaces, with a water contact angle (CA) greater than 1508, have recently attracted a great deal of interest due to their self-cleaning character. In order to form such a surface, micro- and/or nano-structures on the surface and a coating with low-surface energy materials on it should be considered. Much attention now focuses on the design of microstructure surface with the advancement of nano-technique, such as chemical vapor deposition, chemical etching, template-based extrusion, and electro-deposition [1–6]. Aluminum alloy possess much predominance, such as highspecific strength, excellent heat and electrical conductivities and low-specific weight. So aluminum alloy has an important status in the applications for the modern industry. Therefore, fabricating superhydrophobic surface on aluminum is of great significance. Some work has been reported on designing rough superhydrophobic surfaces on aluminum. Qian and Shen [7] and Guo et al. [8] obtained superhydrophobic surfaces by etching Al in acid or alkali solutions and coating low-surface energy materials. A patent (CN1814862A) reported that blast sanding to aluminum surface could increase surface roughness for preparing superhydrophobic surface. However, chemical etching usually decreases the corrosion resistance of Al substrate and blast sanding makes globule difficult to roll.
* Corresponding author. Tel.: +86 574 86685165; fax: +86 574 87910728. E-mail address: [email protected] (H. Wang). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.03.004
However, anodization on aluminum alloy can not only increases its corrosion resistance but also forms nano-pore structure film [9–11] which provides a microcosmic coarse structure of surperhydrophobic surface. Furthermore, low-temperature plasma treatment can release lots of high-energy particles such as electrons, ions and free radicals, which may etch surface or make it rough, leading to cross-linking reaction or chemical modification to vary surface physical or chemical properties. Therefore, low-temperature plasma treatment may have positive effects on the formation of microcosmic coarse structure and subsequent self-assembly of hydrophobic substance. In this paper, anodization combining with the low-temperature plasma treatment was applied to aluminum to design micro–nanostructure, after assembling low-surface energy substance on it, and a superhydrophobic surface was obtained. 2. Experimental 2.1. Porous rough surface Annealed Al foils were used to grow anodized porous layers. Prior to anodizing, the specimens were degreased and activated with 40 g/L sodium hydroxide at 60–70 8C for 3 min and then washed by distilled water. Anodization process was conducted in the solution with 50 g/L phosphoric acid. The anodizing parameters were 1.0–2.0 A/dm2, 10–20 8C, 120 min. After anodization, the specimens were put into the plasma reaction chamber (DT-01) to further modify the surface. The plasma equipment is composed of action chamber, high-frequency power, vacuum system and
H. Wang et al. / Applied Surface Science 254 (2008) 5599–5601
5600
Fig. 1. (a and b) Cross-sectional and superficial morphologies of aluminum anodized film.
Fig. 2. SEM images of the superhydrophobic surfaces modified with trichlorooctadecyl-silane after plasma treatment: (a) 10.0k; (b) 40.0k.
sampling system. Glow discharge method was used in this experiment. The air in the reaction chamber was extracted until the vacuum value decreased to less than 4 Pa, then charged the needed gas (in this experiment, the used gas is still the air) and maintained its pressure at 30–50 Pa for 2–5 min, with the power of 150–250 W. As a result, glow discharge is generated and modified the surface of the specimen.
Fig. 1b shows that the pores were uniformly distributed on the surface, most of which were circular or hexagonal. The pore diameter ranged from 0.15 to 0.2 mm, while the wall of hole was only 20–50 nm. This honey-combing surface affords a good microstructure for fabricating superhydrophobic surface.
2.2. Hydrophobization of the surfaces
As above introduction, low-temperature plasma treatment can generate high-energy active particles to modify the property of
3.2. Al anodized surface by plasma treatment
The Al foils after anodization and plasma treatment were quickly dipped into the 5 mol/L trichlorooctadecyl-silane/hexane solution for 2 h, then washed thoroughly with hexane. After drying in an oven at 60 8C, the superhydrophobic films were obtained. 2.3. Analysis The film surfaces were observed by FE-SEM (JEOL, JSM-7401F and S4800). Contact angles on coating films were measured with a contact angle meter (Dataphysics Instrument, Dataphysics OCA 20) at room temperature. Water droplets of 4 mL were placed at five positions for one sample and the averaged value was adopted as the contact angle. 3. Results and discussion 3.1. Al surface by anodization The cross-sectional and superficial morphologies of an anodized film on Al are shown in Fig. 1. It can be seen that the film thickness was homogeneous with the size of about 15 mm (Fig. 1a).
Fig. 3. EDAX spectrum of superhydrophobic surface on Al by anodizing and plasma treatment.
H. Wang et al. / Applied Surface Science 254 (2008) 5599–5601
5601
Fig. 4. Shape of water droplets on the aluminum substrates: (a) anodization; (b) anodization and plasma treatment.
surface, while not changing the property of the substrate. In this paper, low-temperature plasma treatment was also applied to the anodized aluminum to probe into its effect on the surface. Fig. 2 gives the SEM images of aluminum surface after plasma treatment and assembling trichlorooctadecyl-silane on it. Compared with Fig. 1b, the whole surface became rougher and some pores merged together to form larger micro-pores (pointed by arrow in Fig. 2a), besides, forming the amplified image (Fig. 2b). It can be seen that the edges of most of pores frosted and many smaller mastoids were generated, with the size of 10–30 nm. Therefore, it can be concluded that the surface was etched to become rougher by low-temperature plasma treatment in nano-scale. The EDAX spectrum of above superhydrophobic surface is shown in Fig. 3. The elements of C, Si and Cl were all found, but their contents were lower than those of Al and O. These may be due to a monolayer formation on the rough surface. The surface of the alumina film would be activated by plasma treatment which is easy to make the trichlorooctadecyl-silane act with hydroxy among anodized aluminum to form alkyl silica film with the lowsurface energy. 3.3. Hydrophobicity In this experiment, the aluminum anodized films with and without plasma treatment were hydrophilic with their CA less than 908 before the surface modification. But after surface modification of trichlorooctadecyl-silane, both these two surfaces became hydrophobic because of the monolayer formation with low energy, shown in Fig. 4. The CA of the former reached to 152.18 and the later was up to 157.88. This can be explained by Cassie model. When a water droplet drops on this surface, air and the solid rough surface forms a composite surface, according to the Cassie equation: cos uc ¼ f s cos us þ f v cos uv
(1)
where uc is the apparent CA, and fs and f v are the area fractions of the solid and air on the surface, respectively. Since f s þ f v ¼ 1, us = u, uv ¼ 180 , Eq. (1) can be written as Eq. (2): cos uc ¼ f s ðcos u þ 1Þ 1
(2)
The surface modification on anodized aluminum made by the plasma treatment enhanced its surface roughness (shown in
Fig. 2). Therefore, acting as Eq. (2), the CA also increased accordingly. Besides, because of the ordered structure of the anodized film, uc can be estimated before measured. In this experiment, the CA of glazed aluminum surface after modification with trichlorooctadecyl-silane is about 1208 and fs of anodized aluminum without and with plasma treatment are respectively 0.29 and 0.18. According to Eq. (2), the calculated values of uc are 1498 and 155.58, which are close to the measured values. Therefore, uc of the aluminum surface can be changed and estimated by controlling the microstructure of its anodized film in this preparing method. 4. Conclusion A superhydrophobic surface with good resistance to corrosion and wear was prepared on aluminum substrate. Anodizing method was used to create rough surface with nano-pores, and the CA can reach to 152.18 after modifying trichlorooctadecyl-silane on anodized aluminum surface. Moreover, the low-temperature plasma treatment on the porous anodized film can produce micro–nano-pores and some small mastoids on the edges of pores, which further enhances the surface roughness, as a result, leads to increase in the CA up to 157.88. Acknowledgement This work was supported by Ningbo Nature Science Foundation under Grant number 2007A610026.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
X.J. Feng, L. Jiang, Adv. Mater. 18 (2006) 3063–3078. Y.C. Hong, H.S. Uhm, Appl. Phys. Lett. 88 (2006) 44101. X.T. Zhang, M. Jin, Z.Y. Liu, et al. Langmuir 22 (2006) 9477–9479. J. Fresnais, J.P. Chapel, F.P. Epaillard, Surf. Coat. Technol. 200 (2006) 5296. N.J. Shirtcliffe, G. McHale, M.I. Newton, et al. Adv. Mater. 16 (2004) 1929–1932. S.T. Wang, L. Feng, L. Jiang, Adv. Mater. 18 (2006) 767–771. B.T. Qian, Z.Q. Shen, Langmuir 21 (2005) 9007–9009. Z.G. Guo, F. Zhou, J.C. Hao, et al. J. Colloid Interf. Sci. 303 (2006) 298–305. G.E. Thompson, G.C. Wood, Nature 290 (1981) 230. S.M. Chou, H. Leidheiser Jr., Ind. Eng. Chem. Prod. Res. 25 (1986) 473–478. K. Kobayashl, K. Shimizu, J. Electrochem. Soc. 135 (1988) 908.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication of superhydrophobic surfaces on aluminum Hui Wang *, Dan Dai, Xuedong Wu Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbao Zhuangshi Road 519, Ningbo 315021, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 8 November 2007 Received in revised form 3 March 2008 Accepted 3 March 2008 Available online 12 March 2008
A superhydrophobic surface was prepared on aluminum substrate. Anodization and low-temperature plasma treatment were used to create micro–nano-structure and subsequently trichlorooctadecyl-silane modified the rough surface. The result shows that the water static contact of the aluminum surface after anodization and modification by trichlorooctadecyl-silane reaches to 152.18. A rougher surface with some micro–nano-pores and small mastoids along the edges of pores was generated when lowtemperature plasma treatment was applied to anodized aluminum film, resulting in water static contact angle up to 157.88. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Superhydrophobic surface Anodization Aluminum Low-temperature plasma treatment
1. Introduction Superhydrophobic surfaces, with a water contact angle (CA) greater than 1508, have recently attracted a great deal of interest due to their self-cleaning character. In order to form such a surface, micro- and/or nano-structures on the surface and a coating with low-surface energy materials on it should be considered. Much attention now focuses on the design of microstructure surface with the advancement of nano-technique, such as chemical vapor deposition, chemical etching, template-based extrusion, and electro-deposition [1–6]. Aluminum alloy possess much predominance, such as highspecific strength, excellent heat and electrical conductivities and low-specific weight. So aluminum alloy has an important status in the applications for the modern industry. Therefore, fabricating superhydrophobic surface on aluminum is of great significance. Some work has been reported on designing rough superhydrophobic surfaces on aluminum. Qian and Shen [7] and Guo et al. [8] obtained superhydrophobic surfaces by etching Al in acid or alkali solutions and coating low-surface energy materials. A patent (CN1814862A) reported that blast sanding to aluminum surface could increase surface roughness for preparing superhydrophobic surface. However, chemical etching usually decreases the corrosion resistance of Al substrate and blast sanding makes globule difficult to roll.
* Corresponding author. Tel.: +86 574 86685165; fax: +86 574 87910728. E-mail address: [email protected] (H. Wang). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.03.004
However, anodization on aluminum alloy can not only increases its corrosion resistance but also forms nano-pore structure film [9–11] which provides a microcosmic coarse structure of surperhydrophobic surface. Furthermore, low-temperature plasma treatment can release lots of high-energy particles such as electrons, ions and free radicals, which may etch surface or make it rough, leading to cross-linking reaction or chemical modification to vary surface physical or chemical properties. Therefore, low-temperature plasma treatment may have positive effects on the formation of microcosmic coarse structure and subsequent self-assembly of hydrophobic substance. In this paper, anodization combining with the low-temperature plasma treatment was applied to aluminum to design micro–nanostructure, after assembling low-surface energy substance on it, and a superhydrophobic surface was obtained. 2. Experimental 2.1. Porous rough surface Annealed Al foils were used to grow anodized porous layers. Prior to anodizing, the specimens were degreased and activated with 40 g/L sodium hydroxide at 60–70 8C for 3 min and then washed by distilled water. Anodization process was conducted in the solution with 50 g/L phosphoric acid. The anodizing parameters were 1.0–2.0 A/dm2, 10–20 8C, 120 min. After anodization, the specimens were put into the plasma reaction chamber (DT-01) to further modify the surface. The plasma equipment is composed of action chamber, high-frequency power, vacuum system and
H. Wang et al. / Applied Surface Science 254 (2008) 5599–5601
5600
Fig. 1. (a and b) Cross-sectional and superficial morphologies of aluminum anodized film.
Fig. 2. SEM images of the superhydrophobic surfaces modified with trichlorooctadecyl-silane after plasma treatment: (a) 10.0k; (b) 40.0k.
sampling system. Glow discharge method was used in this experiment. The air in the reaction chamber was extracted until the vacuum value decreased to less than 4 Pa, then charged the needed gas (in this experiment, the used gas is still the air) and maintained its pressure at 30–50 Pa for 2–5 min, with the power of 150–250 W. As a result, glow discharge is generated and modified the surface of the specimen.
Fig. 1b shows that the pores were uniformly distributed on the surface, most of which were circular or hexagonal. The pore diameter ranged from 0.15 to 0.2 mm, while the wall of hole was only 20–50 nm. This honey-combing surface affords a good microstructure for fabricating superhydrophobic surface.
2.2. Hydrophobization of the surfaces
As above introduction, low-temperature plasma treatment can generate high-energy active particles to modify the property of
3.2. Al anodized surface by plasma treatment
The Al foils after anodization and plasma treatment were quickly dipped into the 5 mol/L trichlorooctadecyl-silane/hexane solution for 2 h, then washed thoroughly with hexane. After drying in an oven at 60 8C, the superhydrophobic films were obtained. 2.3. Analysis The film surfaces were observed by FE-SEM (JEOL, JSM-7401F and S4800). Contact angles on coating films were measured with a contact angle meter (Dataphysics Instrument, Dataphysics OCA 20) at room temperature. Water droplets of 4 mL were placed at five positions for one sample and the averaged value was adopted as the contact angle. 3. Results and discussion 3.1. Al surface by anodization The cross-sectional and superficial morphologies of an anodized film on Al are shown in Fig. 1. It can be seen that the film thickness was homogeneous with the size of about 15 mm (Fig. 1a).
Fig. 3. EDAX spectrum of superhydrophobic surface on Al by anodizing and plasma treatment.
H. Wang et al. / Applied Surface Science 254 (2008) 5599–5601
5601
Fig. 4. Shape of water droplets on the aluminum substrates: (a) anodization; (b) anodization and plasma treatment.
surface, while not changing the property of the substrate. In this paper, low-temperature plasma treatment was also applied to the anodized aluminum to probe into its effect on the surface. Fig. 2 gives the SEM images of aluminum surface after plasma treatment and assembling trichlorooctadecyl-silane on it. Compared with Fig. 1b, the whole surface became rougher and some pores merged together to form larger micro-pores (pointed by arrow in Fig. 2a), besides, forming the amplified image (Fig. 2b). It can be seen that the edges of most of pores frosted and many smaller mastoids were generated, with the size of 10–30 nm. Therefore, it can be concluded that the surface was etched to become rougher by low-temperature plasma treatment in nano-scale. The EDAX spectrum of above superhydrophobic surface is shown in Fig. 3. The elements of C, Si and Cl were all found, but their contents were lower than those of Al and O. These may be due to a monolayer formation on the rough surface. The surface of the alumina film would be activated by plasma treatment which is easy to make the trichlorooctadecyl-silane act with hydroxy among anodized aluminum to form alkyl silica film with the lowsurface energy. 3.3. Hydrophobicity In this experiment, the aluminum anodized films with and without plasma treatment were hydrophilic with their CA less than 908 before the surface modification. But after surface modification of trichlorooctadecyl-silane, both these two surfaces became hydrophobic because of the monolayer formation with low energy, shown in Fig. 4. The CA of the former reached to 152.18 and the later was up to 157.88. This can be explained by Cassie model. When a water droplet drops on this surface, air and the solid rough surface forms a composite surface, according to the Cassie equation: cos uc ¼ f s cos us þ f v cos uv
(1)
where uc is the apparent CA, and fs and f v are the area fractions of the solid and air on the surface, respectively. Since f s þ f v ¼ 1, us = u, uv ¼ 180 , Eq. (1) can be written as Eq. (2): cos uc ¼ f s ðcos u þ 1Þ 1
(2)
The surface modification on anodized aluminum made by the plasma treatment enhanced its surface roughness (shown in
Fig. 2). Therefore, acting as Eq. (2), the CA also increased accordingly. Besides, because of the ordered structure of the anodized film, uc can be estimated before measured. In this experiment, the CA of glazed aluminum surface after modification with trichlorooctadecyl-silane is about 1208 and fs of anodized aluminum without and with plasma treatment are respectively 0.29 and 0.18. According to Eq. (2), the calculated values of uc are 1498 and 155.58, which are close to the measured values. Therefore, uc of the aluminum surface can be changed and estimated by controlling the microstructure of its anodized film in this preparing method. 4. Conclusion A superhydrophobic surface with good resistance to corrosion and wear was prepared on aluminum substrate. Anodizing method was used to create rough surface with nano-pores, and the CA can reach to 152.18 after modifying trichlorooctadecyl-silane on anodized aluminum surface. Moreover, the low-temperature plasma treatment on the porous anodized film can produce micro–nano-pores and some small mastoids on the edges of pores, which further enhances the surface roughness, as a result, leads to increase in the CA up to 157.88. Acknowledgement This work was supported by Ningbo Nature Science Foundation under Grant number 2007A610026.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
X.J. Feng, L. Jiang, Adv. Mater. 18 (2006) 3063–3078. Y.C. Hong, H.S. Uhm, Appl. Phys. Lett. 88 (2006) 44101. X.T. Zhang, M. Jin, Z.Y. Liu, et al. Langmuir 22 (2006) 9477–9479. J. Fresnais, J.P. Chapel, F.P. Epaillard, Surf. Coat. Technol. 200 (2006) 5296. N.J. Shirtcliffe, G. McHale, M.I. Newton, et al. Adv. Mater. 16 (2004) 1929–1932. S.T. Wang, L. Feng, L. Jiang, Adv. Mater. 18 (2006) 767–771. B.T. Qian, Z.Q. Shen, Langmuir 21 (2005) 9007–9009. Z.G. Guo, F. Zhou, J.C. Hao, et al. J. Colloid Interf. Sci. 303 (2006) 298–305. G.E. Thompson, G.C. Wood, Nature 290 (1981) 230. S.M. Chou, H. Leidheiser Jr., Ind. Eng. Chem. Prod. Res. 25 (1986) 473–478. K. Kobayashl, K. Shimizu, J. Electrochem. Soc. 135 (1988) 908.