LETTER pubs.acs.org/Langmuir
Facile Fabrication of Colored Superhydrophobic Coatings by Spraying a Pigment Nanoparticle Suspension Hitoshi Ogihara,* Jun Okagaki, and Tetsuo Saji Department of Chemistry & Materials Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan
Langmuir 2011.27:9069-9072. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/08/18. For personal use only.
bS Supporting Information ABSTRACT: Superhydrophobic coatings were prepared by spraying a pigment nanoparticle suspension. By changing the type of pigment nanoparticles, the colors of the coating could be controlled. The particle size of the pigments, which determines the surface structure of the coatings, played an important role in exhibiting superhydrophobicity. The spray-coating process is applicable to a variety of materials (e.g., copper, glass, paper, coiled wire, and tied thread), and the superhydrophobicity was repairable.
1. INTRODUCTION Water-repellent surfaces with a water contact angle higher than 150° are known as superhydrophobic surfaces. A water droplet on a superhydrophobic surface rolls off at a small sliding angle.1 4 After the discovery of superhydrophobic surfaces in nature (e.g., lotus leaves), many artificial superhydrophobic coatings have been fabricated by mimicking nature. According to previous research, superhydrophobic coatings must satisfy the following two conditions: (1) a low surface energy of the constituent materials in the coating and (2) the presence of surface roughness, especially hierarchical micrometer/ nanometer-sized roughness. In artificial superhydrophobic coatings, such roughness has been introduced using top-down or bottom-up processes.5 7 From the viewpoint of potential applications of superhydrophobic coatings (e.g., paints, cloth, self-cleaning building materials, glass, and interior fabrics), the diverse appearance (e.g., color variation) of superhydrophobic coatings is an important property, but it is difficult to design their appearance because most components of superhydrophobic coatings are metals, metal oxides, polymers, or carbonaceous materials, which exhibit limited color variation. So far, colored superhydrophobic coatings have been prepared on the basis of a structural color effect.8 Although many fabrication processes used to exhibit the structural color effect involved multiple steps because the surfaces must be periodically patterned on the nanometer scale, Ishizaki and Sakamoto recently reported the facile formation of a colortuned superhydrophobic magnesium alloy by immersion in water at a temperature of 120 °C.9 Another simple approach to the preparation of colored superhydrophobic coating is to use pigments or dyes as raw materials, but there are few reports on such colored superhydrophobic coatings. Soler et al. fabricated very hydrophobic colored materials (i.e., cotton fabric and glass surfaces) by grafting with polyfluorinated azo dyes.10 Borras and Gr€oning prepared colored superhydrophobic surfaces based on r 2011 American Chemical Society
supported organic nanowires by the sublimation of dye molecules in an Ar atmosphere.11 We have reported the preparation of colored superhydrophobic coatings by the one-step electrophoretic deposition of pigments; however, the resulting coatings had poor durability.12 Durability is a serious and common problem in superhydrophobic coatings because the surface roughness, which is an essential requirement of superhydrophobicity, is fragile. Some researchers have reported a spray-coating process in which coatings were formed by simply spraying a solution or suspension that imparted superhydrophobicity to the surfaces of various materials.13 15 Stepien et al. prepared superhydrophobic TiO2 nanoparticle coatings by a liquid flame spraying process.16,17 The spray-coating method is a simple one-step, lowcost process that can be nonhazardous by the selection of appropriate raw materials. In this study, we report the facile fabrication of colored superhydrophobic coatings using a spraycoating method. Colored superhydrophobic coatings could be prepared simply by spraying a pigment nanoparticle suspension. We discuss why this simple method can control the color and hydrophobicity of coatings. In addition, the spray-coating method was applied to a variety of materials (e.g., copper plates, glass plates, paper, wire, and thread). The durability of the superhydrophobicity in the colored coatings was also examined.
2. EXPERIMENTAL SECTION 2.1. Materials. Nanometer-sized β-type copper phthalocyanine (Dainichiseika Color & Chemicals Co., Ltd.), micrometer-sized copper phthalocyanine (Kanto Kagaku Co., Ltd.), phthalocyanine green (BASF Ltd.), disazo yellow (Holbein Art Materials Co., Ltd.), carbon black Received: March 10, 2011 Revised: June 27, 2011 Published: July 01, 2011 9069
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Table 1. Contact and Sliding Angles of Pigment Particle Coatings Formed on Copper Plates and Particle Sizes of the Pigments particle size/
contact angle /
sliding angle/
nma
deg
deg
cromophtal DPP red
50 200
156 ( 1
49 ( 8
copper phthalocyanine
50 200
156 ( 1
5
disazo yellow
50 100
159 ( 1
39 ( 4
phthalocyanine green carbon black
20 100 ca. 100
156 154 ( 3
3 8(4
copper phthalocyanine
>1000
130 ( 2
b
copper phthalocyanine
50 200
130 ( 4
b
pigments
(without TMSS) a
The particle size of the pigment was estimated by using its TEM image. b The sliding angles of these coatings could not be measured because a water droplet did not roll off at any angle (i.e., a water droplet was pinned). Figure 1. TEM images of pigment nanoparticles (a) cromophtal DPP red, (b) copper phthalocyanine, (c) disazo yellow, (d) phthalocyanine green, and (e) carbon black. Scale bars correspond to 100 nm. Insets show their photographs. (f) SEM image of micrometer-sized copper phthalocyanine. The scale bar corresponds to 5 μm.
Figure 3. (a) Low-magnification and (b) high-magnification SEM images of copper phthalocyanine nanoparticle coatings. Figure 2. Colored coatings formed on glass substrates by spray coating cromophtal DPP red, copper phthalocyanine, phthalocyanine green, disazo yellow, and carbon black. (Denki Kagaku Kogyo Co., Ltd.), cromophtal DPP red (Nihon CibaGeigy Co., Ltd.), trimethylsiloxysilicate (TMSS)/cyclopentasiloxane (50/50 v/v, KF-7312J, Shin-Etsu Chemical Co., Ltd.), and ethanol (99.5%, Kanto Kagaku Co., Ltd.) were used without further purification.
2.2. Preparation of Colored Superhydrophobic Coatings. Pigment nanoparticles (0.5 g of β-type copper phthalocyanine, phthalocyanine green, or disazo yellow; 0.165 g of carbon black; and 0.375 g of cromophtal DPP red) and 0.5 mL of trimethylsiloxysilicate (TMSS)/ cyclopentasiloxane (50/50 v/v) were added to 4.5 mL of ethanol. After being stirred for 1 h, the suspension was sprayed over substrates (copper plates, glass plates, paper, wire, and thread) 20 cm away using a vaporizer (Tokyo Glass Kikai Co., Ltd.) and then dried at room temperature. Micrometer-sized β-type copper phthalocyanine particles (0.5 g) were spray coated in the same way. The spray-coating process was operated manually. As soon as air was supplied into the vaporizer by pushing a rubber bulb, pressure in the vaporizer was suddenly and instantaneously increased, leading to spraying the mist of the suspensions. The spray coating was repeated 10 20 times to prepare colored coatings. The resulting coatings were placed horizontally and dried at room temperature. SiO2 particle coatings were prepared according to the following procedure. SiO2 particles (mean particle sizes: 25 nm (C.I. Kasei Co., Ltd.), 250 nm (Admatechs Co., Ltd.), 500 nm (Admatechs Co., Ltd.), and 1500 nm (Nippon Shokubai)) were refluxed in toluene containing dodecyltrichlorosilane to impart hydrophobicity to the SiO2 particles.12 The hydrophobic SiO2 particles were dispersed in ethanol (70 g(SiO2)/L) and sprayed onto copper substrates.
2.3. Characterization. The contact angles of water droplets (3 μL) on the pigment particle coatings were measured using a contact angle meter (Kyowa Interfacial Science Japan, CA-D). The angle at which a water droplet rolled off when a sample was tilted at 0.5°/s was measured as the sliding angle. SEM images were obtained using a Keyence VE-8800 operated at 2 or 3 kV. TEM images were obtained using a Hitachi H-800 operated at 200 kV. Dynamic-mode AFM images were taken using an SPA 400 with an SPI 3800N probe station (SII NanoTechnology).
3. RESULTS AND DISCUSSION TEM and SEM images of the pigments are shown in Figure 1, and their molecular structures are shown in Figure S1. We can estimate their shapes and sizes on the basis of the images. Figure 2 shows the pigment nanoparticle coatings on glass plates. These coatings have distinct, uniform colors such as red, blue, yellow, green, and black, covering the glass plates. Table 1 lists the contact and sliding angles of the coatings. Coatings prepared with pigment nanoparticles and TMSS had contact angles higher than 150° and low sliding angles. Although the sliding angles of the cromophtal DPP red and disazo yellow coatings were higher than those of the other pigments, these colored coatings exhibited high water repellency. The high sliding angle for the cromophtal DPP red and disazo yellow coatings is attributed to the petal effect, which is observed on a superhydrophobic surface with high adhesion.18 20 It is reported that the petal effect is explained by several factors such as nano- and micrometer-sized surface structures, attractive van der Waals forces, and capillary 9070
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Figure 4. Photographs of spray-coated (a) paper, (b) a coiled wire, and (c) a tied thread in contact with water.
Figure 5. Photographs of a copper phthalocyanine coating (a) before and (b) after scratching and (c) the repaired coating.
adhesion. The cromophtal DPP red and disazo yellow coatings fulfill the requirements of the petal effect. The results in Figure 2 and Table 1 confirm that colored superhydrophobic coatings can be fabricated by a simple spray-coating process using pigment nanoparticles. In contrast, micrometer-sized copper phthalocyanine did not exhibit superhydrophobicity (its contact angle was 130°), which clearly indicates that the pigment particle size plays an important role in the hydrophobicity of the coatings. SEM images of copper phthalocyanine nanoparticle coatings are shown in Figure 3. The low-magnification SEM image (Figure 3a) shows that the coating surface is not smooth but has micrometer-scale roughness. In the high-magnification SEM image (Figure 3b), numerous void spaces among individual nanoparticles are observed, and nanometer-scale roughness is evident. The AFM image showed that the nanometer-scale roughness was attributed to the aggregation of nanoparticles (Figure S2). The hierarchical micrometer/nanometer-sized roughness was also observed in other pigment coatings (Figure S3). Such hierarchical micrometer/nanometer-sized roughness is known as a typical structure of superhydrophobic surfaces.21 In contrast, the micrometer-sized copper phthalocyanine particle coating that did not exhibit superhydrophobicity had only micrometer-scale roughness (Figure S4) because micrometer-sized particles could not form nanometer-sized void spaces. To confirm the effect of particle size on the wettability in more detail, hydrophobic SiO2 particles with different sizes (25, 250, 500, and 1500 nm) were used as model materials, and contact and sliding angles of spray-coated hydrophobic SiO2 particles were measured (Table S1). It is clear that hydrophobicity increases with decreasing particle size: SiO2 (25 and 250 nm) coatings exhibited superhydrophobicity, but superhydrophobicity was not observed in SiO2 (500 and 1500 nm) coatings. Because the aggregates of large SiO2 particles cannot form nanometer-sized void spaces, they would not show superhydrophobicity. As shown in Table 1, the particle size of the pigments ranges from 50 to 200 nm; therefore, it is considered that these pigments have appropriate particle sizes for exhibiting superhydrophobicity. In the study by Borras and Gr€oning, 1D nanostructure (i.e., nanowires) of metalloporphyrins and metallophthalocyanines contributed to the formation of the roughness
structure.11 In our study, both the particle size of pigments and the aggregation of nanoparticles would be important for exhibiting superhydrophobicity. The presence of TMSS is also indispensable in the preparation of superhydrophobic coatings in this process. For example, when copper phthalocyanine nanoparticle coatings were prepared without TMSS, the contact angle was 130 ( 4° (Table 1). It is possible that TMSS contributes to making the coating more hydrophobic because much research on superhydrophobic polymer surfaces has been reported,22 24 but the role of TMSS is unclear at the present moment. It is desirable that the coating process be applicable to a variety of substrates, independent of substrate properties such as chemical composition, shape, and electrical conductivity. Therefore, three different types of materials (paper, coiled wire, and a tied thread) were spray coated with copper phthalocyanine nanoparticles to assess a variety of coating substrates. Figure 4a,b shows that water droplets on the coated paper and coiled wire substrates were almost spherical and the water droplets easily rolled off the coating. Figure 4c shows that the coated thread did not become wet because water was repelled from its surface. These results suggest that the spray-coating method is applicable to a wide range of materials. Advantages of the spray-coating method are limited not only to the simplicity of the process, the colorability, and the diversity of applicable materials but also to its repairability. One major drawback that restricts the practical application of superhydrophobic coatings is that the superhydrophobic property can be easily lost by mechanical contact such as abrasion because the roughness of the superhydrophobic coatings is extremely fragile. Although certain methods could be used to restore the superhydrophobicity following the physical removal of damaged coatings, this is very impractical and an in situ restoration method is much more desirable. Wu et al. demonstrated that superhydrophobicity could be repaired by the repeated spray coating of metal alkylcarboxylates.14 We also examined the repairability of superhydrophobicity in pigment nanoparticle coatings (Figure 5). After being scratched with a pencil, the colored superhydrophobic coating lost both its color and superhydrophobicity (i.e., a water droplet on the scratched surface was pinned) because 9071
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Langmuir scratching with a pencil was such a strong treatment that the surface of the glass substrate was exposed. However, the superhydrophobicity was repaired by repeated spray coatings on the scratched part. The simple repairability of colored superhydrophobicity is an attractive feature from the viewpoint of the utilization of superhydrophobic coatings.
4. CONCLUSIONS Colored superhydrophobic coatings can be obtained by spraycoating a pigment nanoparticle suspension. The particle size of the pigments affects the superhydrophobicity of the coatings, because hierarchical micrometer/nanometer-sized roughness is necessary for superhydrophobicity. This simple one-step, costeffective, repairable superhydrophobic coating process is applicable to a variety of materials (e.g., copper, glass, paper, coiled wire, and tied thread).
LETTER
(16) Stepien, M.; Saarinen, J. J.; Teisala, H.; Tuominen, M.; Aromaa, M.; Kuusipalo, J.; M€akel€a, J. M.; Toivakka, M. Appl. Surf. Sci. 2011, 257, 1911–1917. (17) Teisala, H.; Tuominen, M.; Aromaa, M.; M€akel€a, J. M.; Stepien, M.; Saarinen, J. J.; Toivakka, M.; Kuusipalo, J. Surf. Coat. Technol. 2010, 205, 436–445. (18) Feng, L.; Zhang, Y.; Xi, J.; Zhu, Y.; Wang, N.; Xia, F.; Jiang, L. Langmuir 2008, 24, 4114–4119. (19) Bhushan, B.; Her, E. K. Langmuir 2010, 26, 8207–8217. (20) Dawood, M. K.; Zheng, H.; Liew, T. H.; Leong, K. C.; Foo, Y. L.; Rajagopalan, R.; Khan, S. A.; Choi, W. K. Langmuir 2011, 27, 4126–4133. (21) Barthlott, W.; Neinhuis, C. Planta 1977, 201, 1–8. (22) Nicolas, M.; Guittard, F.; Geribaldi, S. Langmuir 2006, 22 3081–3088. (23) Han, J. T.; Kim, J. S.; Kim, S. H.; Lim, H. S.; Jeong, H. J.; Jeong, S. Y.; Lee, G.-W. ACS Appl. Mater. Interfaces 2010, 2, 3378–3383. (24) Wang, J.; Chen, X.; Kang, Y.; Yang, G.; Yu, L.; Zhang, P. Appl. Surf. Sci. 2010, 257, 1473–1477.
’ ASSOCIATED CONTENT
bS
Supporting Information. Molecular structure of the pigments, SEM and AFM images of pigment particle coatings, and contact and sliding angles of hydrophobic SiO2 particle coatings. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work is supported by the Mizuho Foundation for the Promotion of Sciences. We thank Mr. Y. Nodasaka for recording TEM images and Prof. Ohtomo and Dr. Oshima for recording AFM images. ’ REFERENCES (1) Li, X.-M.; Reinhoudt, D.; Crego-Calama, M. Chem. Soc. Rev. 2007, 36, 1350–1368. (2) Guo, Z.; Liu, W.; Su, B.-L. J. Colloid Interface Sci. 2011, 353, 335–355. (3) Voronov, R. S.; Papavassiliou, D. V.; Lee, L. L. Ind. Eng. Chem. Res. 2008, 47, 2455–2477. (4) Gao, L.; MaCarthy, T. J. Langmuir 2009, 25, 14105–14115. (5) Martines, E.; Seunarine, K.; Morgan, H.; Gadegaard, N.; Wilkinson, C. D. W.; Riehle, M. O. Nano Lett. 2005, 5, 2097–2103. (6) F€urstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21, 956–961. (7) Minko, S.; Muller, M.; Motornov, M.; Nitschke, M.; Grundke, K.; Stamm, M. J. Am. Chem. Soc. 2003, 125, 3896–3900. (8) Sato, O.; Kubo, S.; Gu, Z.-Z. Acc. Chem. Res. 2009, 42, 1–10. (9) Ishizaki, T.; Sakamoto, M. Langmuir 2011, 27, 2375–2381. (10) Soler, R.; Salabert, J.; Sebastian, R. M.; Vallribera, A.; Roma, N.; Ricartand, S.; Molins, E. Chem. Commun. 2011, 47, 2889–2891. (11) Borras, A.; Gr€oning, P. Langmuir 2010, 26, 1478–1492. (12) Ogihara, H.; Okagaki, J.; Saji, T. Chem. Lett. 2009, 38, 132–133. (13) Levkin, P. A.; Svec, F.; Frechet, J. M. J. Adv. Funct. Mater. 2009, 19, 1993–1998. (14) Wu, W.; Wang, X.; Liu, X.; Zhou, F. ACS Appl. Mater. Interfaces 2009, 1, 1656–1661. (15) Manoudis, P. N.; Karapanagiotis, I.; Tsakalof, A.; Zuburtikudis, I.; Panayiotou, C. Langmuir 2008, 24, 11225–11232. 9072
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