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Mechanically stable and corrosion resistant superhydrophobic sol–gel coatings on copper substrate
Applied Surface Science 257 (2011) 5772–5776
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Mechanically stable and corrosion resistant superhydrophobic sol–gel coatings on copper substrate A. Venkateswara Rao a,∗ , Sanjay S. Latthe a , Satish A. Mahadik a , Charles Kappenstein b a b
Air Glass Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, Maharashtra, India University of Poitiers, Laboratory of Catalysis in Organic Chemistry, LA CCO, UMR CNRS 6503, Poitiers 86000, France
a r t i c l e
i n f o
Article history: Received 6 October 2010 Received in revised form 16 January 2011 Accepted 24 January 2011 Available online 31 January 2011 Keywords: Coatings Corrosion Superhydrophobic Wetting Contact angle
a b s t r a c t Development of the anticorrosion coatings on metals having both passive matrix functionality and active response to changes in the aggressive environment has raised tremendous interest in material science. Using a sol–gel deposition method, superhydrophobic copper substrate could be obtained. The best hydrophobic coating sol was prepared with methyltriethoxysilane (MTES), methanol (MeOH), and water (as 7 M NH4 OH) at a molar ratio of 1:19.1:4.31 respectively. The surface morphological study showed the ball like silica particles distributed on the copper substrate with particle sizes ranging from 8 to 12 m. The coatings showed the static water contact angle as high as 155◦ and the water sliding angle as low as 7◦ . The superhydrophobic nature was maintained even though the deposited copper substrate was soaked for 100 h in 50% of HCl solution. The coatings are stable against humidity and showed superhydrophobic behavior even after 90 days of exposure. The coatings are mechanically stable and water drops maintained the spherical shape on the bent copper substrate, which was bent more than 90◦ . © 2011 Elsevier B.V. All rights reserved.
1. Introduction Superhydrophobic coatings exhibit extreme water-repellency, with water droplets roll off with high contact angles. Over the last decade, superhydrophobic surfaces have attracted considerable interest from both the academic and industrial research communities because these surfaces exhibit a host of interesting and unusual properties, including extremely high water contact angles, very low flow resistance, and self-cleaning behavior. The control on surface wettability is a challenging project that requires comprehensive knowledge in geometry, chemistry, and physics, namely, in surface structures, chemical compositions, and physical laws [1]. The enormously high economic and environmental influence of corrosion of metallic structures raised large scientific attention to this problem and requires an interdisciplinary approach. The need for renewable materials with improved material properties is steadily increasing. Metals have great demand in the chemical and microelectronics industries due to their high thermal and electrical conductivities. A notable disadvantage in the use of metals is that they easily get corroded, especially in aqueous atmospheres. The development of coatings that provide the requisite protection of the metal surface is essential for its efficient use in these applications [2]. A great deal of research work in this area is aimed at increasing the life-
∗ Corresponding author. Tel.: +91 231 2609228; fax: +91 231 2609233. E-mail address: [email protected] (A.V. Rao). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.01.099
time of several material surfaces (glass, metal, alloy, ceramic, and wood) by coating [3]. Coatings based on organic, inorganic, and organic–inorganic hybrid systems have been developed to achieve water repellent surfaces [4–14]. Different approaches have been described up to now in the literature for the fabrication of superhydrophobic steel, copper, and titanium surfaces. Recently, Qu et al. have reported a novel mixed-solution system for the fabrication of superhydrophobic surface on steel, copper alloy and titanium alloy by a chemical etching method [15]. Wang et al. have fabricated a stable bionic superhydrophobic surface by immersing a copper plate into a solution of fatty acid [16]. The sol–gel coating on metals is relatively recent and has been not sufficiently investigated, in spite of its potential technological interest. The basic concept of chemical conversion of metal surfaces is based on deposition of a hydrophobic sol–gel barrier layer for surface protection and corrosion prevention. Herein, we used the organosilica sol–gel materials for coating copper substrate. Methyltriethoxysilane (MTES) precursor is used to prepare hydrophobic coatings on copper substrate, which not only provide improved adhesion but also act as a barrier protection layer for minimizing the permeability of corrosive species. The presence of organic groups also renders these materials hydrophobic [17] making them impermeable to ions, moisture, and other hydrophilic species as compared to pristine sol–gel-derived silica coatings. The studies are mainly aimed to test the capability of sol–gel coatings to improve mechanical and corrosion resistance of the copper substrate. It is found that the coatings are effective at
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preventing corrosion of copper substrate. The mechanical stability of the sol–gel coating was tested by bending the coated copper substrate for more than 90◦ . However, the mechanical stability of the film at bent condition, to our knowledge, has not been previously reported. Besides, the reproducibility is very good and we can get the similar result in every experiment under the preparation conditions. Overall, the strategy presented herein may provide a generic approach for fabrication of protective coatings on different metallic surfaces.
2. Experimental details 2.1. Materials The chemicals used were methyltriethoxysilane, (Sigma–Aldrich Chemie, Germany), methanol (S.D.Fine Chem Limited, Mumbai), and ammonia (NH3 , sp.gr.0.91, Qualigens Fine Chemicals, Mumbai). Double distilled water was used for all the experiments. All the reagents were used as received.
2.2. Sample preparation The superhydrophobic coatings were made as follows. Prior to the deposition of the hydrophobic coatings on copper substrates, the substrates were cleaned by the procedure described earlier in order to get uniform deposition [18]. The coating solution was prepared under basic condition from the MTES, CH3 OH, and H2 O in the molar ratio of 1:19.1:4.31 respectively with 7 M NH4 OH. The MeOH/MTES (M) molar ratio was varied from 9.5 to 19.1. Firstly, methanol and base catalyst (7 M NH4 OH) was stirred for 30 min and then MTES was added drop by drop while stirring. The final coating solution was stirred for approximately 15 min. After substrate cleaning and sol preparation, the film deposition on the copper substrates utilized a simple dip-coating process. The substrates were dipped in the sol at a constant rate of 6 mm/min, immersed in the sol for approximately 40 min, withdrawn at the same constant rate, and then air-dried for approximately 30 min. Following deposition, the substrates were sintered at 250 ◦ C for 3 h at a heating rate of 2 ◦ C/min to ensure densification of the gel network.
2.3. Characterization The microstructure of the films was observed by using scanning electron microscopy (JEOL, JEM-6360, Japan) and the transmission electron microscopy (Philips, Tecnai, F20 model, The Netherlands). The surface chemical modification of the films was studied using infrared spectroscopy, Perkin-Elmer (Model no. 783, USA), which gave the information about the various chemical bonds. The wettability of the films was evaluated by measuring the contact angle () of a water droplet of 10 mg placed on the film surface using the contact angle meter equipped with a CCD camera (Ramehart instrument Co., USA) at a ambient temperature. The static water contact angles were measured at five different positions for each sample, and the average value was adopted as the contact angle. The sliding angle of the water droplet was observed by putting the water droplet on horizontal surface and then slowly tilting the film surface until the droplet starts sliding. The effect of humidity on wetting properties of the films was studied by putting the films in a humidity chamber (REMI instruments Ltd., Mumbai) at relative humidity of 95% at 35 ◦ C temperature over 90 days and measured the contact angles before and after exposing to the humid surrounding.
Fig. 1. The surface morphology of the silica films prepared with (a) M = 12.7, (b) M = 19.1, and (c) magnified (5000×) SEM micrograph of M = 19.1.
3. Results and discussion 3.1. Surface morphological studies As the surface morphology is important parameter for the superhydrophobic properties, the coatings were characterized using scanning electron microscopy. Fig. 1(a) and (b) shows the surface morphology of the silica films prepared with M = 12.7 and M = 19.1, respectively. Fig. 1(a) shows the irregular shaped silica particles, which are non-homogeneously spread on the copper substrate. In the case of silica film prepared with M = 19.1 (Fig. 1(b)), the SEM micrograph shows ball like silica particles distributed on the cop-
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Fig. 3. FT-IR spectra of the silica films prepared with (a) M = 12.7 and (b) M = 19.1. Fig. 2. The transmission electron microscopy image of the silica films prepared with M = 19.1.
per substrate. The high magnified SEM micrograph of this film (Fig. 1(c)) shows ball like spherical shaped silica particles with each having diameter typically ranges from 8 to 12 m. This is due to the fact that the lower dilution of MTES (less M value) has high catalyst concentration in the sol during the hydrolysis and condensation reactions. Therefore, there is a rapid clusterification of siloxanes which give rise to dense and irregular network structure. However, an increase in the dilution of MTES in methanol reduces the base concentrations and forms well-tailored 3D network structure with bigger well shaped particle sizes.
at 847 cm−1 are due to the Si–C bonds [20]. The peak at around 1600 cm−1 and the broad absorption band at around 3400 cm−1 are due to the –OH groups [21]. The residual Si–OH groups are the main source of hydrophilic character. The OH peaks are quite visible for silica films prepared from M = 12.7. With an increase in M value at M = 19.1, the intensity of the peak at 1600 cm−1 and the broad OH absorption band at 3400 cm−1 decreased, whereas the intensities of the C–H absorption peak at around 2950 cm−1 and Si–C absorption peak at around 840 cm−1 increased. The Si–OH band seen in both the FT-IR spectra indicates that surface hydroxyls still exist, even though the materials show the strong water repellent properties.
3.2. Transmission electron microscopy studies
3.4. Static and dynamic water contact angle measurements
The microstructure and particle morphology of the film prepared from M value of 19.1 was obtained by transmission electron microscopy. As shown in Fig. 2, uniform and spherical shaped silica particles were observed.
The hydrophobicity of the resulting surfaces was assessed with water contact angle measurements. A low sliding angle renders the surface “nonsticky”, a property that is crucial for the fabrication of water-repellent and self-cleaning surfaces. The Young’s equation [22] for the contact angle () of a liquid droplet can be applied only to a flat surface and not to a rough one. The effect of surface roughness on wetting is accounted by the model developed by Wenzel [23], where it is assumed that the space between the protrusions on the surface is filled by the liquid. The apparent water contact angle ( rough ) and intrinsic water contact angle ( flat ) are then linked by,
3.3. Fourier transform infrared studies The wetting behavior of hydrophobic surfaces is governed mainly by chemical composition of the surface. The chemical composition of the films deposited on copper substrate was investigated by the FT-IR spectroscopy. The coating material on copper substrate was removed and powder of material exposed upto 100 ◦ C for one hour for removing the moisture. Then coating powder was milled with potassium bromide (KBr) to form a very fine powder. This powder is then compressed into a thin pellet for FTIR analysis in transmission mode, since KBr is transparent in the IR region. The heat treatment is helpful to remove the physically adsorbed water molecules. Several characteristic absorption peaks were observed in the range 450–4000 cm−1 indicating the presence of methyl groups in the sample. The FT-IR spectra of the silica films prepared from M = 12.7 and M = 19.1 are shown in Fig. 3(a) and (b) respectively. The peak at 1074 cm−1 corresponded to the Si–O–Si asymmetric stretching vibration [19]. The presence of this peak confirms the formation of a network structure inside the film. The absorption bands observed at around 2950 cm−1 and 1400 cm−1 are due to stretching and bending of C–H bonds and the peaks observed
cos rough = r cos flat
(1)
where r is the ratio between the true surface area and its horizontal projection. This regime provides hydrophobic interfaces with contact angles below 120◦ , however, it cannot give rise to superhydrophobicity. In Cassie state, the apparent water contact angle ( rough ) is related to the intrinsic water contact angle ( flat ) of the solid surface by the Cassie–Baxter equation [24], cos rough = ϕs cos flat − (1 − ϕs )
(2)
where ϕs is the area fraction of the solid surface that contacts water. From this equation, it is apparent that rough is greater than 90◦ (or cos rough < 0) if ϕ<
1 (1 + cos flat )
(3)
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Table 1 The influence of MeOH/MTES molar ratio (M) on static water contact angle, sliding angle and maximum frictional force required to slide the water droplet on film surface. MeOH/MTES molar ratio (M)
Water contact angle ()
Water sliding angle
Maximum frictional force fmax (N)
9.5 12.7 15.92 19.1
107◦ 123◦ 137◦ 155◦
56◦ 43◦ 32◦ 7◦
81.24 66.83 51.93 11.94
The Cassie–Baxter model assumes that a droplet is suspended on the rough structures and allows air trapping between the rough structures on a surface underneath the droplet. The Cassie–Baxter model suggests that the trapped air is a key for the superhydrophobic behavior. Sliding angle (SA) can be used to demonstrate the self-cleaning properties of superhydrophobic surfaces. Generally, superhydrophobic surfaces with SA less than 5◦ are needed for the self-cleaning property. The SA is the incline angle at which the tailing edge of a drop of known mass will just begin to move and is a manifestation of the force required to dislodge a liquid from a surface. The maximum frictional force (fmax ) required to dislodge a liquid from a surface can be calculated via the formula shown below; fmax = mg sin
(4)
where m and g are the mass of the water droplet and the acceleration due to gravity, respectively. Where represents the minimal sliding angle of the water droplet on the hydrophobic surface. The wetting behavior of superhydrophobic surfaces is governed by both their chemical composition and geometric microstructure. To evaluate the hydrophobic properties of the silica films, the contact angle () of the water droplet on the films prepared with various M values have been measured. The influence of MeOH/MTES molar ratio (M) on static water contact angle, sliding angle and maximum frictional force to slide the water droplet on film surface is shown in Table 1. The highest static water contact angle, 155◦ , was obtained for the surface prepared from the M value of 19.1. A low water sliding angle of 7◦ was also observed for this surface. The water droplet on the silica film prepared with M = 9.5, adhere on film surface resulting in a water contact angle of 107◦ and maximum frictional force required to slide the water droplet on film surface is 81.24 N at sliding angle of 56◦ . At lower M values, the silica film surface is covered with fewer silicon alkyl groups, leading to less water contact angle and high sliding angle. However, as the M value is increased, the silica film surface becomes more hydrophobic and hence large water contact angle and low sliding angle is resulted. Although, the maximum frictional force required to slide water droplet on a film surface is decreased with increasing M value. The water droplets easily roll off on the silica film surface (M = 19.1) for a small force of 11.94 N at sliding angle of 7◦ . This strongly suggests that the contact model of a water droplet on the film prepared from M = 19.1 is the Cassie–Baxter’s model. Whereas in the case of the silica film prepared with M = 9.5, satisfies the Wenzel’s model. The methyl groups enhanced the water repellency of the surface. Fig. 4 shows the image of the water droplets on the silica film prepared on copper substrate from M value of 19.1. All the three water drops on the superhydrophobic copper substrate shows the same contact angle of 155◦ , which confirms uniform deposition over the copper substrate. To evaluate the mechanical properties of the silica films, the copper substrates deposited from M = 19.1 was bent for more than 90◦ . The contact angle of the water droplet on the bent copper substrate was measured which shows the almost same contact angle as on
Fig. 4. Shape of water droplets on the superhydrophobic copper substrate.
the flat film. A water droplet rolls off from the bent area of the coating without pinning. Fig. 5 shows the image of the water droplet on the bent (>90◦ ) copper substrate deposited with M = 19.1. Thus, the superhydrophobic coatings prepared on copper substrate show good mechanical strength. The coatings demonstrated excellent adhesion and flexibility, which could be attributed to the formation of chemical bonding at the interface and the incorporation of organic components, respectively.
3.5. Chemical aging test The stability of the water contact angle over time is a very important factor for superhydrophobic surfaces, providing information about the long-time surface dynamics. Unfortunately, results from long-time stability measurements are seldom reported [25]. The anticorrosive performance of the sol–gel coating was tested by direct exposure of the coated copper substrates to corrosive media. Interestingly, chemical aging test demonstrated that the superhydrophobic nature is maintained even though the deposited film was soaked for 100 h in 50% of HCl solution. However, we found that the water contact angle decreased from 158◦ to 146◦ after 120 h of exposure to the acid environment.
Fig. 5. Shape of water droplet on the bent (>90◦ ) superhydrophobic copper substrate.
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3.6. Effect of humidity on the wetting properties
References
For artificial superhydrophobic surfaces, the water repellent capability gradually degrades during long-term outdoor exposure and accumulation of contamination. The effect of humidity on the wetting properties of silica film prepared on copper substrate with M = 19.1 was carried out at relative humidity of 95% at 35 ◦ C temperature over 90 days. It was observed that there was no any significant effect on the superhydrophobicity of the silica films. This reveals that the silica films prepared on copper substrate with M = 19.1 are strongly durable against humidity.
[1] L. Feng, S.H. Li, Y.S. Li, H.J. Li, L.J. Zhang, J. Zhai, Y.L. Song, B.Q. Liu, L. Jiang, D.B. Zhu, Adv. Mater. (Weinheim, Ger.) 14 (2002) 1857. [2] W. Song, J. Zhang, Y. Xie, Q. Cong, B. Zhao, J. Colloid Interface Sci. 329 (2009) 208. [3] H. Sayilkan, S. Sener, E. Sener, M. Sulu, Mater. Sci. 39 (2003) 733. [4] Z.G. Guo, W.M. Liu, B.L. Su, Appl. Phys. Lett. 92 (2008) 063104. [5] S.S. Latthe, Hiroaki Imai, V. Ganesan, A.V. Rao, J. Appl. Surf. Sci. 256 (2009) 217. [6] S.S. Latthe, D.Y. Nadargi, A.V. Rao, J. Appl. Surf. Sci. 255 (2009) 3600. [7] S.S. Latthe, H. Hirashima, A.V. Rao, J. Smart Mater. Struct. 18 (2009) 095017. [8] S.S. Latthe, Hiroaki Imai, V. Ganesan, C. Kappenstein, A.V. Rao, J. Sol-Gel Sci. Technol. 53 (2010) 208. [9] S.S. Latthe, Hiroaki Imai, V. Ganesan, A.V. Rao, J. Porous Mater., doi:10.1007/s10934-009-9325-0, in press. [10] S.S. Latthe, S.L. Dhere, C. Kappenstein, Hiroaki Imai, V. Ganesan, S.C. Gupta, P.B. Wagh, A.V. Rao, J. Appl. Surf. Sci. 256 (2010) 3259. [11] S.S. Latthe, Hiroaki Imai, V. Ganesan, A.V. Rao, J. Microporous Mesoporous Mater. 130 (2010) 115. [12] A.V. Rao, S.S. Latthe, S.L. Dhere, S.S. Pawar, Hiroaki Imai, V. Ganesan, S.C. Gupta, P.B. Wagh, J. Appl. Surf. Sci. 256 (2010) 2115. [13] S.L. Dhere, S.S. Latthe, C. Kappenstein, G.M. Pajonk, Hiroaki Imai, V. Ganesan, S.C. Gupta, P.B. Wagh, A.V. Rao, J. Appl. Surf. Sci. 256 (2010) 3624. [14] A.B. Gurav, S.S. Latthe, C. Kappenstein, S.K. Mukharjee, A.V. Rao, R.S. Vhatkar, J. Porous Mater., doi:10.1007/s10934-010-9386-0, in press. [15] M.N. Qu, B.W. Zhang, S.Y. Song, L. Chen, J.Y. Zhang, X.P. Cao, Adv. Funct. Mater. 17 (2007) 593. [16] S.T. Wang, L. Feng, L. Jiang, Adv. Mater. 18 (2006) 767. [17] M.S. Rao, B.C. Dave, Adv. Mater. 13 (2001) 274. [18] S.L. Dhere, S.S. Latthe, C. Kappenstein, S.K. Mukherjee, A.V. Rao, J. Appl. Surf. Sci. 256 (2010) 3967. [19] J.K. Hong, H.R. Kim, H.H. Park, Thin Solid Films 332 (1998) 449. [20] N. Hering, K. Schriber, R. Riedel, O. Lichtenberger, J. Woltersodorf, Appl. Organomet. Chem. 15 (2001) 879. [21] B.E. Yoldas, J. Non-Cryst. Solids 63 (1984) 145. [22] T. Young, Philos. Trans. R. Soc. London 95 (1805) 65. [23] R.N. Wenzel, Ind. Eng. Chem. 28 (1936) 988. [24] A.B.D. Cassie, S. Baxter, Trans. Faraday Soc. 40 (1944) 546. [25] D. Nystrom, J. Lindqvist, E. Ostmark, P. Antoni, A. Carlmark, A. Hult, E. Malmstrom, Appl. Mater. Interfaces 1 (2009) 816.
4. Conclusions Silica based coating, prepared by a single step sol–gel process using methyltriethoxysilane as a precursor was found to be uniform and relatively dense. Precise selection of precursor and sol–gel composition yielded coatings that were found to be adhesive, water-repellant, mechanically stable and effective at preventing corrosion of coated copper substrate. As such, the strategy can be used to prepare adhesive, stable, chemically resistant, inert, long lasting coatings for efficient prevention of corrosion. Finally, the approach outlined herein presents a novel alternative technology, which may be easily adapted for commercial and mass production of anticorrosion coatings for different metallic surfaces. Acknowledgement The authors are grateful to the Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy (DAE), Mumbai, Government of India, for the financial support for this work through a major research project on “Aerogels” (no. 2008/37/47/BRNS).
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Mechanically stable and corrosion resistant superhydrophobic sol–gel coatings on copper substrate A. Venkateswara Rao a,∗ , Sanjay S. Latthe a , Satish A. Mahadik a , Charles Kappenstein b a b
Air Glass Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, Maharashtra, India University of Poitiers, Laboratory of Catalysis in Organic Chemistry, LA CCO, UMR CNRS 6503, Poitiers 86000, France
a r t i c l e
i n f o
Article history: Received 6 October 2010 Received in revised form 16 January 2011 Accepted 24 January 2011 Available online 31 January 2011 Keywords: Coatings Corrosion Superhydrophobic Wetting Contact angle
a b s t r a c t Development of the anticorrosion coatings on metals having both passive matrix functionality and active response to changes in the aggressive environment has raised tremendous interest in material science. Using a sol–gel deposition method, superhydrophobic copper substrate could be obtained. The best hydrophobic coating sol was prepared with methyltriethoxysilane (MTES), methanol (MeOH), and water (as 7 M NH4 OH) at a molar ratio of 1:19.1:4.31 respectively. The surface morphological study showed the ball like silica particles distributed on the copper substrate with particle sizes ranging from 8 to 12 m. The coatings showed the static water contact angle as high as 155◦ and the water sliding angle as low as 7◦ . The superhydrophobic nature was maintained even though the deposited copper substrate was soaked for 100 h in 50% of HCl solution. The coatings are stable against humidity and showed superhydrophobic behavior even after 90 days of exposure. The coatings are mechanically stable and water drops maintained the spherical shape on the bent copper substrate, which was bent more than 90◦ . © 2011 Elsevier B.V. All rights reserved.
1. Introduction Superhydrophobic coatings exhibit extreme water-repellency, with water droplets roll off with high contact angles. Over the last decade, superhydrophobic surfaces have attracted considerable interest from both the academic and industrial research communities because these surfaces exhibit a host of interesting and unusual properties, including extremely high water contact angles, very low flow resistance, and self-cleaning behavior. The control on surface wettability is a challenging project that requires comprehensive knowledge in geometry, chemistry, and physics, namely, in surface structures, chemical compositions, and physical laws [1]. The enormously high economic and environmental influence of corrosion of metallic structures raised large scientific attention to this problem and requires an interdisciplinary approach. The need for renewable materials with improved material properties is steadily increasing. Metals have great demand in the chemical and microelectronics industries due to their high thermal and electrical conductivities. A notable disadvantage in the use of metals is that they easily get corroded, especially in aqueous atmospheres. The development of coatings that provide the requisite protection of the metal surface is essential for its efficient use in these applications [2]. A great deal of research work in this area is aimed at increasing the life-
∗ Corresponding author. Tel.: +91 231 2609228; fax: +91 231 2609233. E-mail address: [email protected] (A.V. Rao). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.01.099
time of several material surfaces (glass, metal, alloy, ceramic, and wood) by coating [3]. Coatings based on organic, inorganic, and organic–inorganic hybrid systems have been developed to achieve water repellent surfaces [4–14]. Different approaches have been described up to now in the literature for the fabrication of superhydrophobic steel, copper, and titanium surfaces. Recently, Qu et al. have reported a novel mixed-solution system for the fabrication of superhydrophobic surface on steel, copper alloy and titanium alloy by a chemical etching method [15]. Wang et al. have fabricated a stable bionic superhydrophobic surface by immersing a copper plate into a solution of fatty acid [16]. The sol–gel coating on metals is relatively recent and has been not sufficiently investigated, in spite of its potential technological interest. The basic concept of chemical conversion of metal surfaces is based on deposition of a hydrophobic sol–gel barrier layer for surface protection and corrosion prevention. Herein, we used the organosilica sol–gel materials for coating copper substrate. Methyltriethoxysilane (MTES) precursor is used to prepare hydrophobic coatings on copper substrate, which not only provide improved adhesion but also act as a barrier protection layer for minimizing the permeability of corrosive species. The presence of organic groups also renders these materials hydrophobic [17] making them impermeable to ions, moisture, and other hydrophilic species as compared to pristine sol–gel-derived silica coatings. The studies are mainly aimed to test the capability of sol–gel coatings to improve mechanical and corrosion resistance of the copper substrate. It is found that the coatings are effective at
A.V. Rao et al. / Applied Surface Science 257 (2011) 5772–5776
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preventing corrosion of copper substrate. The mechanical stability of the sol–gel coating was tested by bending the coated copper substrate for more than 90◦ . However, the mechanical stability of the film at bent condition, to our knowledge, has not been previously reported. Besides, the reproducibility is very good and we can get the similar result in every experiment under the preparation conditions. Overall, the strategy presented herein may provide a generic approach for fabrication of protective coatings on different metallic surfaces.
2. Experimental details 2.1. Materials The chemicals used were methyltriethoxysilane, (Sigma–Aldrich Chemie, Germany), methanol (S.D.Fine Chem Limited, Mumbai), and ammonia (NH3 , sp.gr.0.91, Qualigens Fine Chemicals, Mumbai). Double distilled water was used for all the experiments. All the reagents were used as received.
2.2. Sample preparation The superhydrophobic coatings were made as follows. Prior to the deposition of the hydrophobic coatings on copper substrates, the substrates were cleaned by the procedure described earlier in order to get uniform deposition [18]. The coating solution was prepared under basic condition from the MTES, CH3 OH, and H2 O in the molar ratio of 1:19.1:4.31 respectively with 7 M NH4 OH. The MeOH/MTES (M) molar ratio was varied from 9.5 to 19.1. Firstly, methanol and base catalyst (7 M NH4 OH) was stirred for 30 min and then MTES was added drop by drop while stirring. The final coating solution was stirred for approximately 15 min. After substrate cleaning and sol preparation, the film deposition on the copper substrates utilized a simple dip-coating process. The substrates were dipped in the sol at a constant rate of 6 mm/min, immersed in the sol for approximately 40 min, withdrawn at the same constant rate, and then air-dried for approximately 30 min. Following deposition, the substrates were sintered at 250 ◦ C for 3 h at a heating rate of 2 ◦ C/min to ensure densification of the gel network.
2.3. Characterization The microstructure of the films was observed by using scanning electron microscopy (JEOL, JEM-6360, Japan) and the transmission electron microscopy (Philips, Tecnai, F20 model, The Netherlands). The surface chemical modification of the films was studied using infrared spectroscopy, Perkin-Elmer (Model no. 783, USA), which gave the information about the various chemical bonds. The wettability of the films was evaluated by measuring the contact angle () of a water droplet of 10 mg placed on the film surface using the contact angle meter equipped with a CCD camera (Ramehart instrument Co., USA) at a ambient temperature. The static water contact angles were measured at five different positions for each sample, and the average value was adopted as the contact angle. The sliding angle of the water droplet was observed by putting the water droplet on horizontal surface and then slowly tilting the film surface until the droplet starts sliding. The effect of humidity on wetting properties of the films was studied by putting the films in a humidity chamber (REMI instruments Ltd., Mumbai) at relative humidity of 95% at 35 ◦ C temperature over 90 days and measured the contact angles before and after exposing to the humid surrounding.
Fig. 1. The surface morphology of the silica films prepared with (a) M = 12.7, (b) M = 19.1, and (c) magnified (5000×) SEM micrograph of M = 19.1.
3. Results and discussion 3.1. Surface morphological studies As the surface morphology is important parameter for the superhydrophobic properties, the coatings were characterized using scanning electron microscopy. Fig. 1(a) and (b) shows the surface morphology of the silica films prepared with M = 12.7 and M = 19.1, respectively. Fig. 1(a) shows the irregular shaped silica particles, which are non-homogeneously spread on the copper substrate. In the case of silica film prepared with M = 19.1 (Fig. 1(b)), the SEM micrograph shows ball like silica particles distributed on the cop-
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Fig. 3. FT-IR spectra of the silica films prepared with (a) M = 12.7 and (b) M = 19.1. Fig. 2. The transmission electron microscopy image of the silica films prepared with M = 19.1.
per substrate. The high magnified SEM micrograph of this film (Fig. 1(c)) shows ball like spherical shaped silica particles with each having diameter typically ranges from 8 to 12 m. This is due to the fact that the lower dilution of MTES (less M value) has high catalyst concentration in the sol during the hydrolysis and condensation reactions. Therefore, there is a rapid clusterification of siloxanes which give rise to dense and irregular network structure. However, an increase in the dilution of MTES in methanol reduces the base concentrations and forms well-tailored 3D network structure with bigger well shaped particle sizes.
at 847 cm−1 are due to the Si–C bonds [20]. The peak at around 1600 cm−1 and the broad absorption band at around 3400 cm−1 are due to the –OH groups [21]. The residual Si–OH groups are the main source of hydrophilic character. The OH peaks are quite visible for silica films prepared from M = 12.7. With an increase in M value at M = 19.1, the intensity of the peak at 1600 cm−1 and the broad OH absorption band at 3400 cm−1 decreased, whereas the intensities of the C–H absorption peak at around 2950 cm−1 and Si–C absorption peak at around 840 cm−1 increased. The Si–OH band seen in both the FT-IR spectra indicates that surface hydroxyls still exist, even though the materials show the strong water repellent properties.
3.2. Transmission electron microscopy studies
3.4. Static and dynamic water contact angle measurements
The microstructure and particle morphology of the film prepared from M value of 19.1 was obtained by transmission electron microscopy. As shown in Fig. 2, uniform and spherical shaped silica particles were observed.
The hydrophobicity of the resulting surfaces was assessed with water contact angle measurements. A low sliding angle renders the surface “nonsticky”, a property that is crucial for the fabrication of water-repellent and self-cleaning surfaces. The Young’s equation [22] for the contact angle () of a liquid droplet can be applied only to a flat surface and not to a rough one. The effect of surface roughness on wetting is accounted by the model developed by Wenzel [23], where it is assumed that the space between the protrusions on the surface is filled by the liquid. The apparent water contact angle ( rough ) and intrinsic water contact angle ( flat ) are then linked by,
3.3. Fourier transform infrared studies The wetting behavior of hydrophobic surfaces is governed mainly by chemical composition of the surface. The chemical composition of the films deposited on copper substrate was investigated by the FT-IR spectroscopy. The coating material on copper substrate was removed and powder of material exposed upto 100 ◦ C for one hour for removing the moisture. Then coating powder was milled with potassium bromide (KBr) to form a very fine powder. This powder is then compressed into a thin pellet for FTIR analysis in transmission mode, since KBr is transparent in the IR region. The heat treatment is helpful to remove the physically adsorbed water molecules. Several characteristic absorption peaks were observed in the range 450–4000 cm−1 indicating the presence of methyl groups in the sample. The FT-IR spectra of the silica films prepared from M = 12.7 and M = 19.1 are shown in Fig. 3(a) and (b) respectively. The peak at 1074 cm−1 corresponded to the Si–O–Si asymmetric stretching vibration [19]. The presence of this peak confirms the formation of a network structure inside the film. The absorption bands observed at around 2950 cm−1 and 1400 cm−1 are due to stretching and bending of C–H bonds and the peaks observed
cos rough = r cos flat
(1)
where r is the ratio between the true surface area and its horizontal projection. This regime provides hydrophobic interfaces with contact angles below 120◦ , however, it cannot give rise to superhydrophobicity. In Cassie state, the apparent water contact angle ( rough ) is related to the intrinsic water contact angle ( flat ) of the solid surface by the Cassie–Baxter equation [24], cos rough = ϕs cos flat − (1 − ϕs )
(2)
where ϕs is the area fraction of the solid surface that contacts water. From this equation, it is apparent that rough is greater than 90◦ (or cos rough < 0) if ϕ<
1 (1 + cos flat )
(3)
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Table 1 The influence of MeOH/MTES molar ratio (M) on static water contact angle, sliding angle and maximum frictional force required to slide the water droplet on film surface. MeOH/MTES molar ratio (M)
Water contact angle ()
Water sliding angle
Maximum frictional force fmax (N)
9.5 12.7 15.92 19.1
107◦ 123◦ 137◦ 155◦
56◦ 43◦ 32◦ 7◦
81.24 66.83 51.93 11.94
The Cassie–Baxter model assumes that a droplet is suspended on the rough structures and allows air trapping between the rough structures on a surface underneath the droplet. The Cassie–Baxter model suggests that the trapped air is a key for the superhydrophobic behavior. Sliding angle (SA) can be used to demonstrate the self-cleaning properties of superhydrophobic surfaces. Generally, superhydrophobic surfaces with SA less than 5◦ are needed for the self-cleaning property. The SA is the incline angle at which the tailing edge of a drop of known mass will just begin to move and is a manifestation of the force required to dislodge a liquid from a surface. The maximum frictional force (fmax ) required to dislodge a liquid from a surface can be calculated via the formula shown below; fmax = mg sin
(4)
where m and g are the mass of the water droplet and the acceleration due to gravity, respectively. Where represents the minimal sliding angle of the water droplet on the hydrophobic surface. The wetting behavior of superhydrophobic surfaces is governed by both their chemical composition and geometric microstructure. To evaluate the hydrophobic properties of the silica films, the contact angle () of the water droplet on the films prepared with various M values have been measured. The influence of MeOH/MTES molar ratio (M) on static water contact angle, sliding angle and maximum frictional force to slide the water droplet on film surface is shown in Table 1. The highest static water contact angle, 155◦ , was obtained for the surface prepared from the M value of 19.1. A low water sliding angle of 7◦ was also observed for this surface. The water droplet on the silica film prepared with M = 9.5, adhere on film surface resulting in a water contact angle of 107◦ and maximum frictional force required to slide the water droplet on film surface is 81.24 N at sliding angle of 56◦ . At lower M values, the silica film surface is covered with fewer silicon alkyl groups, leading to less water contact angle and high sliding angle. However, as the M value is increased, the silica film surface becomes more hydrophobic and hence large water contact angle and low sliding angle is resulted. Although, the maximum frictional force required to slide water droplet on a film surface is decreased with increasing M value. The water droplets easily roll off on the silica film surface (M = 19.1) for a small force of 11.94 N at sliding angle of 7◦ . This strongly suggests that the contact model of a water droplet on the film prepared from M = 19.1 is the Cassie–Baxter’s model. Whereas in the case of the silica film prepared with M = 9.5, satisfies the Wenzel’s model. The methyl groups enhanced the water repellency of the surface. Fig. 4 shows the image of the water droplets on the silica film prepared on copper substrate from M value of 19.1. All the three water drops on the superhydrophobic copper substrate shows the same contact angle of 155◦ , which confirms uniform deposition over the copper substrate. To evaluate the mechanical properties of the silica films, the copper substrates deposited from M = 19.1 was bent for more than 90◦ . The contact angle of the water droplet on the bent copper substrate was measured which shows the almost same contact angle as on
Fig. 4. Shape of water droplets on the superhydrophobic copper substrate.
the flat film. A water droplet rolls off from the bent area of the coating without pinning. Fig. 5 shows the image of the water droplet on the bent (>90◦ ) copper substrate deposited with M = 19.1. Thus, the superhydrophobic coatings prepared on copper substrate show good mechanical strength. The coatings demonstrated excellent adhesion and flexibility, which could be attributed to the formation of chemical bonding at the interface and the incorporation of organic components, respectively.
3.5. Chemical aging test The stability of the water contact angle over time is a very important factor for superhydrophobic surfaces, providing information about the long-time surface dynamics. Unfortunately, results from long-time stability measurements are seldom reported [25]. The anticorrosive performance of the sol–gel coating was tested by direct exposure of the coated copper substrates to corrosive media. Interestingly, chemical aging test demonstrated that the superhydrophobic nature is maintained even though the deposited film was soaked for 100 h in 50% of HCl solution. However, we found that the water contact angle decreased from 158◦ to 146◦ after 120 h of exposure to the acid environment.
Fig. 5. Shape of water droplet on the bent (>90◦ ) superhydrophobic copper substrate.
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3.6. Effect of humidity on the wetting properties
References
For artificial superhydrophobic surfaces, the water repellent capability gradually degrades during long-term outdoor exposure and accumulation of contamination. The effect of humidity on the wetting properties of silica film prepared on copper substrate with M = 19.1 was carried out at relative humidity of 95% at 35 ◦ C temperature over 90 days. It was observed that there was no any significant effect on the superhydrophobicity of the silica films. This reveals that the silica films prepared on copper substrate with M = 19.1 are strongly durable against humidity.
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4. Conclusions Silica based coating, prepared by a single step sol–gel process using methyltriethoxysilane as a precursor was found to be uniform and relatively dense. Precise selection of precursor and sol–gel composition yielded coatings that were found to be adhesive, water-repellant, mechanically stable and effective at preventing corrosion of coated copper substrate. As such, the strategy can be used to prepare adhesive, stable, chemically resistant, inert, long lasting coatings for efficient prevention of corrosion. Finally, the approach outlined herein presents a novel alternative technology, which may be easily adapted for commercial and mass production of anticorrosion coatings for different metallic surfaces. Acknowledgement The authors are grateful to the Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy (DAE), Mumbai, Government of India, for the financial support for this work through a major research project on “Aerogels” (no. 2008/37/47/BRNS).