Article pubs.acs.org/Langmuir
Fabrication of Transparent Antifouling Thin Films with Fractal Structure by Atmospheric Pressure Cold Plasma Deposition Hayato Miyagawa,*,† Koji Yamauchi,† Yoon-Kee Kim,‡ Kazufumi Ogawa,† Kenzo Yamaguchi,† and Yoshifumi Suzaki† †
Department of Advanced Materials Science, Faculty of Engineering, Kagawa University, Hayashi-cho 2217-20, Takamatsu 761-0396, Japan ‡ Department of Production Science & Welding Engineering, Hanbat National University, San 16-1, Dukmyung-dong, Yuseong-gu, Daejeon 305-764, Korea S Supporting Information *
ABSTRACT: Antifouling surface with both superhydrophobicity and oil-repellency has been fabricated on glass substrate by forming fractal microstructure(s). The fractal microstructure was constituted by transparent silica particles of 100 nm diameter and transparent zinc-oxide columns grown on silica particles by atmospheric pressure cold plasma deposition. The sample surface was coated with a chemically adsorbed monomolecular layer. We found that one sample has the superhydrophobic ability with a water droplet contact angle of more than 150°, while another sample has a high transmittance of more than 85% in a wavelength range from 400 to 800 nm.
1. INTRODUCTION Fouling has been a significant problem in flat panel devices used as a cover on devices. For example, surface dirt on outdoormounted solar panels causes the reduction of power efficiency, and grease stains from our fingertips on electronic touch displays cause the deterioration of the image visibility. To overcome the problem, we need to fabricate antifouling transparent surfaces with superhydrophobicity and oil-repellency. The term “superhydrophobicity” is used to described a surface with a contact angle of water greater than 150°. Several papers reported that the repellency against both water and oil is attainable by forming microscopic fractal structures1 to mimic the surface of lotus leaves because such fractal surfaces have small effective contact areas and this relatively enhances the surface energy of water and oil droplets on them. This is the so-called “lotus effect”2,3. Wenzel4 reported that a flat surface (i.e., without any fractal microstructure) has the limitation of water-repellency with the maximum value of the water drop contact angle of 120°.5 This means that we can not realize the superhydrophobicity on flat surfaces. It was reported6,7 that when a thin ZnO film was deposited on a flat glass substrate, the film was formed with packed ZnO columns, with a 30 nm width and 300 nm length, oriented in a direction perpendicular to the surface plane. We expected that, if ZnO is deposited on the spherical surface of a SiO2 particle, then the ZnO columns grow radially on the particle. In this way, all the particles are surrounded by smaller ZnO columnar pillars, as shown in Figure 1. We named the microstructures, “raspberry structures”. In this paper, we aim to fabricate raspberry microstructures on glass substrates to attain superhydrophobicity. © 2012 American Chemical Society
Figure 1. Raspberry structure which is made of transparent silica particles of 100 nm in diameter and a transparent columnar structure of zinc oxide. The surface of this structure is coated with a chemically adsorbed monomolecular layer. R means the fluorocarbon group and/or alkyl group.
2. SAMPLE PREPARATION There were three processes for sample preparation of the transparent antifouling film. First, SiO2 particles of 100 nm in average diameter were arranged on glass substrates. Second, a thin zinc-oxide film with a columnar structure was fabricated on the SiO2 particles by atmospheric pressure cold plasma (APCP) deposition. In the end, the sample surface was coated with chemically adsorbed monomolecular layer (CAM), Received: January 4, 2012 Revised: November 23, 2012 Published: November 27, 2012 17761
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which enhances the antifouling property. Details of each process are described as following subsections. 2.1. Arrangement of SiO2 Particles. First of all, a glass substrate was treated with a corona discharge to achieve the hydrophilicity. After the treatment, SiO2 particles were arranged on substrate surface by a dipcoating technique, where the substrate was dipped in the SiO2 particle suspension and pulled up at a constant rate, as shown in Figure 2. In this
are mixed and placed into a torch of glow discharge plasma, generated by a pulsed voltage of 1.0 kV. More details of the APCP deposition is described in ref 6. As raw material for ZnO deposition, we used 2-methoxy-6-methyl-3,5-heptanedionato (Zn-MOPD, Ube Industries, Ltd.). The molecular structure of Zn-MOPD is shown in Figure 4.
Figure 4. Molecular structure of Zn-MOPD, which was used as a raw material for ZnO deposition. Zn-MOPD is decomposed in the plasma torch, and ZnO is deposited on the substrate. The substrate temperature during the deposition was set at 215 °C for sample A and 50 °C for the other samples. Table 1 shows the conditions of sample fabrication.
Figure 2. Dip coating technique to arrange SiO2 particles on a glass substrate. The substrate was dipped in the SiO2 particle suspension and pulled up at constant rates.
Table 1. Conditions of Sample Fabrication and Water Drop Contact Angles
technique, one can easily control the density of SiO2 particles [i.e., when the pulling speed is fast (slow), the density on the substrate becomes dense (sparse)]. The SiO2 particle suspension was made of a mixture of ethanol (99.5% purity, 34 mL), COLCOAT P (6 mL), and IPA-ST-ZL (20 mL). The average size of SiO2 particles in IPA-ST-ZL is 100 nm. We made several types of substrates with various pulling speeds, and found the optimal speed, 0.23 mm/s, for the closed packing arrangement of SiO2 particles. We chose three pulling speeds, 0.23 mm/s, 0.1, and 0.05 mm/s, for comparison in further processes. After dipping, substrates were sintered in an electric furnace at a temperature of 500 °C. 2.2. Deposition of Thin ZnO Films. The thin ZnO films were made on the SiO2-arranged substrates in an atmospheric pressure cold plasma (APCP) deposition system. Figure 3 shows a layout of the APCP system. In Figure 3, oxygen gas and helium gas with raw material
sample A sample B sample C sample D
pulling up velocity (mm/s)
substrate temperature (°C)
average value of contact angles (deg)
(S.D.)
0.10 0.23 0.10 0.05
215 50 50 50
151.8 143.6 138.3 143.7
(1.4) (0.4) (1.1) (1.3)
2.3. CAM coating. After ZnO deposition, the substrate was treated with a chemically adsorbed monomolecular layer (CAM) by a chemisorbing method, where samples were dipped in a molecular solution for one hour and dried in the air in one day. A molecule used for CAM coating has a terminal reactive site and a functional site, which were composed of the methoxy-silyl group and the fluorocarbon group, respectively, as shown in Figure 5. The fluorocarbon group provides a function of the surface energy reduction, which leads to the waterand oil-repellancy. In the chemisorbing process, CAM coating is selforganized due to the molecular interaction between the reactive site in
Figure 5. Molecular formula of a molecule used for CAM, which has a terminal reactive site of the methoxy-silyl group and a functional site of the fluorocarbon group.
Figure 3. Layout of APCP deposition system. Mixed gas of oxygen and helium with Zn-MOPD is placed into a plasma torch. 17762
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the molecule and the substrate surface. First, the methoxy-silyl group of the reactive site changes to the silanol group by the dealcoholization reaction with water in the solution. And next, the silanol group is adhered with the hydroxyl group of the substrate surface.8,9 In this reaction, siloxane bonds are formed at the interface, which causes strong adhesion of CAM on the substrate surface. The fluorocarbon group at the other end plays an important role in antifouling properties. The thickness of CAM is so thin (1−2 nm) that the surface morphology is kept without losing the original color and gloss of the substrate. According to the contact-angle measurements of water droplets on CAM-coated samples (which you can find in the Supporting Information), the quality of CAM coating and its effect on the hydrophobic property can be considered to be the same for all samples A−D.
3. EVALUATION METHOD In order to assess the antifouling ability of samples, we performed waterdrop contact-angle measurements, where 3 μL of water was dropped on the sample surface, and the contact angle was measured at the edge of the droplet (i.e., an angle between the water surface and the substrate), by means of the θ/2 method. Transmittance measurements using UV-3150 (Shimadzu Corporation) were carried out in the visible light range of wavelength from 400 to 780 nm. We obtained the information of the surface structure by FE-SEM observation using S-900S (Hitachi, Ltd.) with an accelerating voltage of 15 kV.
4. RESULTS In Table 1, we show the values of water-drop contact angles and the standard deviations (S.D.), which were obtained from measurements at several surface points on each sample. The average value of the contact angles of sample A, 151.8°, warrants the superhydrophobic surface. However, the average values of the other samples are less than 150°. Transmittance spectra were shown in Figure 6. Sample B has a high transmittance, 90%, as the average value through the
Figure 6. Transmittance spectra of samples in the visible light range.
measurement wavelength range, which is expected to be due to the dense packing of the SiO2 particles arrangement. Other samples with sparse arrangements have lower transmittance, especially in the low wavelength range of less than 500 nm. Figure 7(panels a−d′) shows FE-SEM images of all samples. In sample A, as shown in Figure 7 (panels a and a′), ZnO columns have grown surrounding the SiO2 particle and formed the raspberry-like structure successfully. However, we cannot find such a raspberry structure in the other samples, where it seems that ZnO deposition occurred just on the top of the SiO2 particle and/or glass substrate surface. In Figure 7 (panels a′, c′, and d′), there are some concave areas, which were formed due to the
Figure 7. FE-SEM images of surface structures of (a) sample A, (b) sample B, (c) sample C, and (d) sample D. Images labeled with the alphabetical symbol with a dash superscript are obtained from another angle in a wider field of view.
sparseness of the SiO2 particle arrangement. On the other hand, in Figure 7b′, sample B has a uniform surface with closed packing of the ZnO columns, which is due to the dense arrangement of SiO2 particles. Another noticeable feature we found from Figure 7 (panels b, c, and d) is that there are pebble lumps on the surface of ZnO columns of samples B and D, which is found less in sample C. 17763
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5. DISCUSSION Sample A has a superhydrophobic surface with a contact angle of more than 150°. This higher value comes from the raspberry structure as shown in Figure 7 (panels a and a′). The result is consistent with Wenzel’s or Cassie−Baxter’s10 theory, where uneven surfaces like the raspberry structure are supposed to be able to have the superhydrophobicity. In samples B, C, and D, we cannot find such raspberry structures. The reason for this difference of structure is explained by the substrate temperature during ZnO deposition. When the temperature is high enough to reach 200 °C, deposited Zn and O atoms on the surface of the SiO2 particle can easily move. This is due to the high thermal energy and migration toward the side of the spherical surface of the SiO2 particle that collide and couple with each other and form the ZnO nuclear cores, as shown in Figure 8a. The ZnO cores are
the effective contact area of a sample small and relatively enhance the surface energy of a water or an oil droplet on it. In sample A, we obtained higher superhydrophobic properties than those of H. M. Shang’s sample11 (150.0° as the maximum contact angle), which was a fabricated monolayer adsorption of tridecafluoro-1,1,2,2-tetrahydrooctyldimethylchlorosilane (TFCS) on a silica-based structure without ZnO deposition. However, our samples have a lower contact angle than those of L.Wang’s sample,12 where the monolayer of octadecyltrimethoxysilane (OTS) was adsorbed after the SiO2 shell deposition on a ZnO nanowire. We expect that the best antifouling property with the high contact angle will be attainable with the conditions where the pulling speed from the SiO2 particle suspension is 0.23 mm/s, and the substrate temperature of the ZnO deposition is 215 °C. However, in this paper we mainly focus on low temperature deposition in order to apply the process to plastic films and do not pursue the condition at high temperature. Transmittance spectrum in Figure 6 shows a distinct difference of transmittance between samples with sparse and dense state of SiO2 particles (i.e., the sparse samples, A, C, and D, have a lower transmittance than the dense sample B, especially in the low wavelength range). Figure 9 shows a schematic cross-sectional
Figure 8. Conceptual illustration of the growth process of the thin ZnO film. (a) A high substrate temperature of 215 °C during the deposition and (b) a low substrate temperature of 50 °C. Figure 9. A schematic cross-sectional image of sparse SiO2-state samples with paths of light waves.
formed not only on the top but also at side of the SiO2 particle. Subsequently, the cores grow into a columnar shape because of the shadowing effect, which enhances the growth rate at the convex portions of these cores. However, at the low substrate temperature, the migration of atoms does not occur because the thermal energy is insufficient. As a result, most of the atoms stay at the top of the SiO2 particle and make the nuclear cores only on the top, and they also grow vertically as shown in Figure 8b. In Figure 7b′, sample B has a uniform surface with the closed packing of ZnO columns (which came from the optimal pulling speed in the dipping process); in sample D, ZnO are not closed packed, and there are concave areas as shown in Figure 7d′. However, there is no significant difference of the contact angles between sample B and sample D. It may suggest that the surface unevenness, in a scale of more than a few hundreds nm, does not have any influence on the water-drop contact angle. Sample C has the minimum value of the water-drop contact angle among samples. We thought that the pebble lumps on the ZnO column surface, which were found less in Figure 7c, make
image of sparse samples with paths of light waves. For such samples, thickness is significantly different between the covered and the uncovered area by SiO2 particles. The light paths of both areas make some phase shifts of the transmission wavelength, which leads to the lower transmission.
6. CONCLUSION We fabricated antifouling surfaces on glass substrates, where 100 nm diameter SiO2 nanoparticles were arranged by the dipcoating technique, deposited with a ZnO columnar structure, and finally covered with chemisorbed CAM. Sample A, which was grown with the condition of a high substrate temperature of 215 °C, had the fractal microstructure-like raspberry and achieved the superhydrophobicity of the water-drop contact angle of more than 150°. The raspberry structure was formed by the migration of Zn and O atoms and the subsequent radial growth of ZnO columns on the surface of the SiO2 particles. Sample B had the highest transmittance, where the SiO2 particles 17764
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were arranged with the optimal pulling speed of 0.23 mm/s during the dip-coating process. The speed makes a dense closedpacked state of SiO2 particles on the glass substrate, which makes the light paths constant over the whole surface area and produces the high transmission.
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ASSOCIATED CONTENT
S Supporting Information *
Additional FE-SEM images of SiO2 nanoparticles, data of waterdroplet contact angles after CAM coating, details of Zn-MOPD, and external appearance of CPAP during ZnO deposition. This material is available free of charge via the Internet at http://pubs. acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +81-87-864-2396. Fax: +81-87-864-2438. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Center for International Exchange at Hanbat National University, Korea, and the Grantin-Aid for scientific research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.
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REFERENCES
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