Photoinduced Formation of Superhydrophobic Surface on Which

Dec 2, 2012 - Department of Arts and Sciences, Faculty of Education, Osaka Kyoiku University, 4-698-1 Asahigaoka, Kashiwara, Osaka 582-8582,. Japan. â...
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Photoinduced Formation of Superhydrophobic Surface on Which Contact Angle of a Water Droplet Exceeds 170° by Reversible Topographical Changes on a Diarylethene Microcrystalline Surface Naoki Nishikawa,† Hiroyuki Kiyohara,† Shingo Sakiyama,† Seiji Yamazoe,†,○ Hiroyuki Mayama,‡,# Tsuyoshi Tsujioka,§ Yuko Kojima,∥ Satoshi Yokojima,⊥,⊗ Shinichiro Nakamura,⊗ and Kingo Uchida*,† †

Department of Materials Chemistry, Faculty of Science and Technology, Ryukoku University, Seta, Otsu 520-2194, Japan Research Institute for Electronic Science, Hokkaido University, N21, W10 Kita-ku, Sapporo 001-0021, Japan § Department of Arts and Sciences, Faculty of Education, Osaka Kyoiku University, 4-698-1 Asahigaoka, Kashiwara, Osaka 582-8582, Japan ∥ Mitsubishi Chemical Group Science and Technology Research Center, Inc., 1000 Kamoshida, Yokohama 227-8502, Japan ⊥ School of Pharmacy, Tokyo University of Pharmacy and Life Sciences, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan ⊗ RIKEN Research Cluster for Innovation, Nakamura Laboratory, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ‡

S Supporting Information *

ABSTRACT: A superhydrophobic surface on which the contact angle of a water droplet exceeds 170° was reversibly produced by alternate irradiation with UV and visible light. Superhydrophobicity is due to the formation of densely generated submicrometer sized needle-shaped crystals (less than 0.2−0.3 μm diameter and 2.2−2.5 μm long) at 30 °C, which is much lower than the eutectic temperature of either isomers of the diarylethene. Below the eutectic temperature, the generated crystals were much smaller than those generated above the eutectic temperature. These smaller crystals more effectively enhanced the superhydrophobicity.

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INTRODUCTION Surface roughness profoundly influences the wetting properties of a material, especially for the wetting of superhydrophobic surfaces. Studies of the superhydrophobic surfaces of various materials, for example, even such natural materials as lotus and taro leaves, have revealed that fascinating functions were manifested by combining micro- and nanoscaled hierarchical structures with low-surface-energy materials.1−10 To date, superhydrophobic coatings promise a wide range of applications such as self-cleaning surfaces, corrosion-resistant, antiadhesive, and drag-reducing coatings.11−14 The robustness of superhydrophobicity is a fundamental requisite for the applications of water-repellant materials. Recently, superhydrophobic materials are well designed, and contact angles (CAs) larger than 170° have been reported.15−18 Typically, fabrication of superhydrophobic materials with a water CA of 178° was performed using a perpendicular nanopin film,15 and CA of 172° was performed on densely packed polyacrylonitrile (PAN) nanofibers made by template synthesis using an anodic aluminum oxide membrane.16 Recently, photocontrollable systems with surface superhydrophobic properties were reported. Such systems were prepared by the introduction of photoinduced polarity changeable materials on rough surfaces.19−21 In such systems, © 2012 American Chemical Society

photochromic molecules are also introduced whose polarity changed accompanied with photoisomerization. Photochromism is defined as the reversible transformation of a single molecule between two states having different absorption spectra. Hence, photochromic molecules often hold considerable potential toward applications as molecular switches and control elements in molecular devices.22,23 Diarylethenes are among the most promising photochromic compounds,24−28 not only as memory materials but also as switching units for molecular devices and in supramolecular systems.29,30 Recently the surface properties, i.e., surface topographical changes accompanied with wettability changes31−38 and metal deposition capability,39−41 were controlled by light. Such photoresponse was not considered in past photochromic systems in which the reversible formation of steps and valleys on a diarylethene single crystal surface was observed.42 In their study, photoirradiation was carried out below the eutectic temperature of open- and closed-ring isomers of diarylethene 1. In contrast, we found the formation of needle-shaped crystals of 3c on the microcrystalline surface of 3o by UV irradiation Received: November 5, 2012 Revised: November 29, 2012 Published: December 2, 2012 17817

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Scheme 1. Molecular Structures of Diarylethenes

measurements. The heating and cooling rates were 5 °C/min and were measured between temperature ranges of 20−220 °C. Crystal Data of 4o and 4c. The intensity data of the 4o and 4c crystals were collected by ω scan on a Bruker SMART APEX CCD diffractometer with graphite-monochromatized Mo Kα radiation (λ = 0.710 73 Å) at 93 K. The structure was solved by direct methods using the program SHELXS9744 and refined by full-matrix least-squares against F2 of the observed reflections with SHELXL97.45 All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were located at ideal positions and refined in isotropic approximation. The following are the crystal data for the plate-shaped crystals of 4o. (C29H30F6S4Si2) crystal system, monoclinic; space group, P21/c; a = 15.8171(11) Å; b = 17.8379(12) Å; c = 11.9561(8) Å; α = 90°; β = 102.1840(10)°; γ = 90°; V = 3297.36(3) Å3; Z = 4; Dcalcd = 1.633 Mg m−3; R((I) > 2σ(I)) = R1 = 0.0796; wR2 = 0.2048, T = 103(2) K; CCDC 626732. The following are the crystal data for the rod-shaped crystals of 4c. (C29H30F6S4Si2) crystal system, monoclinic; space group, P2/c; a = 25.378(3) Å; b = 6.1561(8) Å; c = 23.542(3) Å; α = 90°; β = 116.779(2)°; γ = 90°; V = 3283.5(3) Å3; Z = 4; Dcalcd = 1.623 Mg m−3; R((I) > 2σ(I)) = R1 = 0.0626; wR2 = 0.1681; T = 103(2) K; CCDC 626731. These data can be obtained free from The Cambridge Crystallographic Data Center at www.ccdc.cam.ac.uk/data_request/ cif. Surface Structure Observations and Fractal Analysis. For the fractal analysis, the sample on a cover glass was set on the electron microscope stage with conductive carbon tape and the Au−Pd alloy was evaporated onto the sample surface. The cross-section was observed for the sample that was cracked with the cover glass and set perpendicularly on the stage. The fractal dimension of the cross-section of the rough solid surfaces was calculated from the trace curves of the surfaces by the box-counting method. A two-dimensional space containing the above trace curve was divided by identical boxes of side size r like a piece of cross section paper. The number of boxes containing trace curve N(r) was counted, and then side size r was changed. The number of boxes was counted again for new side size r, and the above process was repeated. On the basis of the box-counting method, the fractal dimension can be calculated from the following relationship:

followed by storage at 30 °C, which is the eutectic temperature of 3o and 3c.31 Herein we report a new system of a microcrystalline surface of diarylethene 4o, which forms smaller fibrils below the eutectic temperature and generates a superwater-repellant surface whose CA of a water droplet exceeds 170°.

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MATERIALS AND METHODS

Preparation of Film and Characterization. Diarylethene 4o was prepared according to a previous paper.43 The film was prepared by coating a chloroform solution containing 4o (300 mg/mL) on the slide glass substrate (10 mm × 10 mm). The substrate was stored at room temperature to evaporate the solvent and placed in a desiccator; the residual solvent was removed under 58 mmHg for 30 min for scanning electron microscopy (SEM) observation. Residual chloroform was not observed in 1H NMR in C6D6. The film thickness was approximately 20 μm by laser microscope (KEYENCE VK-8550) after scratching the surface. A scanning electron microscope (KEYENCE VE-8800) and an optical microscope (Leica DMLP) were used to study the surface microstructure. Photoirradiation (visible light, λ > 500 nm) was carried out using a Ushio 500-W xenon lamp with a cutoff filter (Toshiba color filter Y-50), and UV irradiation was carried out with a Spectroline Hand-Held UV lamp, E-series (λ = 313 nm, 8 W). Photoirradiation experiments at the eutectic temperature were carried out on a Mettler Toledo FP90 to which a FP82HT hot stage was attached. The static CA and SA were measured on an optical contact angle meter (Kyowa Interface Science Co., Ltd., Drop Master 500) with capillary o.d. 30 μm (straight) at ambient temperature. Deionized water (1.5 μL) was dropped carefully onto the surface. An average CA value was obtained by measuring the samples at eight different positions. An atomic force microscope (AFM, Shimadzu SPM-9600) was used for measuring the force curve on the single crystalline surfaces. Differential Scanning Calorimetry (DSC) Measurement of Different Contents of Mixtures of 4o and 4c. A phase diagram was prepared according to the DSC measurements of the mixtures of 4o and 4c with different components. The 4o and 4c crystals with different ratios were measured and ground in an agate mortar and pestle for 1 h. In total 2 mg of the mixture was used for the DSC

N (r ) ∝ r −D 17818

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was pinned on the surface, showing a petal effect.47 The CA profile of a water droplet depends on storage periods at 135 °C (Figure 3d). The CA decrease after 15 min is attributed to the enlargement of the needle-shaped crystals by Ostwaldripening.34 Such surface topographical changes due to the crystal growth of each isomer through the melted eutectic mixture were already reported for the diarylethene derivatives of 1 and 3 only above the eutectic temperature.31−35 No topographical changes were observed for these derivatives below the eutectic temperatures. In contrast, photoinduced step and valley formation were reported for single crystals of 1,2-bis(2,4dimethyl-5-phenylthien-3-yl)perfluorocyclopentene (2).42 The observed changes were proceeded in the crystalline state at room temperature, which is less than the eutectic temperature, and those are attributed to the changes in the molecular volume between 2o and 2c.42 To clarify the differences of the systems, we measured the force curve using an atomic force microscope (AFM) to estimate the surface hardness. The force curves obtained by AFM at 22 °C on the surfaces before and after UV irradiation of the single crystalline surfaces of 2o and 4o are shown in Figure 4. The speed of the approach and the release of the cantilever were both 100 nm/s. The spring constant was 42 N/ m. For a single crystalline surface of 2o, no hysteresis between the approach and release processes was observed. This is attributed to the surface hardness. In contrast, hysteresis was observed for the surface of a single crystal of 4o, indicating that the surface was softened after UV light irradiation. This is due to the loss of the degree of crystallization of the 4o film, which is ascertained by weakened X-ray diffraction (XRD) peaks of 4o upon UV irradiation to the deposited film by vacuum evaporation (Figure S2 in the Supporting Information). The results inspired us to generate needle-shaped microcrystals of 4c at even a lower temperature, at room temperature. The UV-irradiated microcrystalline surface was also stored in the dark at 30 °C, and CA changes were monitored. The results are summarized in Figure 5. The CA dramatically increased and reached 154° after 4 h even at 30 °C. At a time 12 h later, it reached 162°, which is almost the same CA as a water droplet on a lotus leaf. After prolonged storage at this temperature, the CA gradually increased and finally reached 172° (Figure 5d). The surface shows a lotus effect. The sliding angle (SA) of a water droplet was less than 1°. The SEM images of the surface are shown in Figure 5a,b. The surface is covered with needle-shaped crystals whose diameters and lengths are around 0.20−0.35 μm and 2.2−2.5 μm, respectively. The size is much smaller than that observed on the surface stored at 135 °C, as shown in Figure 3, (0.46− 0.71 μm width and 2.1−3.2 μm long), and the 3c fibrils (around 1−2 μm diameters and 10 μm long) whose CA and SA were 162.9° and less than 2°, respectively. 31,34 The extraordinary high CA of the film of 4 is attributed to the fine structures of the surface.48,49 The fractal analysis of the surfaces, which were stored for 9 days at 30 °C in the dark, was carried out by box counting for the cross-sectional trace curves of the surface’s cross-section (Figure S3 in the Supporting Information). Figure 6 shows the result. The slope of log N(r) vs log r plot was −1.4, so the fractal dimension of cross-section Dcross was determined to be 1.4. Fractal dimension D of the surface was evaluated as D = Dcross + 1 = 2.4. Upper limit scale L and lower limit scale l of the fractal behavior could be determined from the log N(r) vs log r plot in Figure 6. The

where D is the fractal dimension. Dimension D in eq 1 is the fractal dimension of the cross-section, and the dimension of surface Ds is approximately obtained by Ds = D + 1.

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RESULTS AND DISCUSSION Diarylethene 4o, which was synthesized according to a previous paper,43 underwent cyclization and cycloreversion reactions in the crystalline state as well as in the solutions, because the distance between the reactive carbon atoms of 4o in the crystalline state was 3.634 Å. The distance must be less than 4 Å for the cyclization reaction to proceed upon UV irradiation in the crystalline state.46 The phase diagrams of the open-ring isomer 4o (cubic shaped-crystals; mp, 162 °C) and the closed-ring isomer 4c (needle-shaped crystals; 202 °C) were obtained by measuring the DSC curves at different component ratios of the 4o and 4c mixtures (Figure 1). Photoinduced topographical changes were

Figure 1. Phase diagram of open- and closed-ring isomers of diarylethene 4 by DSC.

observed at 141 °C of the eutectic temperature. After UV irradiation (5 min at room temperature) to the microcrystalline film of 4o, the film was stored at 141 °C in the dark and the surface was monitored by SEM and CA measurements. After being stored at 20 min, rod-shaped crystals appeared on the surface (Figure 2a,b) and the CA reached 138°. The crystals were 1.5 μm in diameter and 20 μm long on average. Their shapes agreed well with those estimated from single crystal analysis (see the Supporting Information, Figure S1). In this case, they lie on the surface. Therefore the CA was not enhanced, and superhydrophobicity was not achieved. The CA gradually decreased (Figure 2d) with prolonged heating due to the melting of the edge of the crystals. At high temperature, the thermal cycloreversion reaction from 4c to 4o proceeded, and the crystal started to melt by forming an eutectic mixture. The half-life period of 4c at 141 °C due to the thermal recovery to 4o was 55 min in the decalin solution. Next we monitored the crystal growth of 4c at 135 °C, which is lower than the eutectic temperature. UV irradiation was also carried out for 5 min at room temperature followed by storage of the film at 135 °C. Then needle-shaped crystals of 4c appeared on the surface whose diameters and lengths were less than 1 and 10 μm, and superhydrophobicity was observed. After 15 min, the surface was covered with needle-shaped crystals of 4c (Figure 3a,b), and the CA of a water droplet was 160 ± 0.9° (Figure 3c). When the surface was tilted, the droplet 17819

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Figure 2. SEM images of the microcrystalline surface of 4 stored at 141 °C for 20 min after UV irradiation (a, b) and water droplet on the surface (c). CA profile after UV irradiation by maintaining the eutectic temperature (d).

values for L and l were found to be 10 and 0.3 μm, respectively, which correspond to the length and diameters of the needleshaped crystals. The formation of such fine structures can be explained by considering the temperature dependence of the population of the nuclear formation of the crystals and the rate of the crystal growths. At low temperature, the population of the nuclear formation is larger than that of the higher temperature, while the rate is much faster at higher temperature. Therefore small crystal formation is observed at lower temperature. Next we measured the activation energy to form the needleshaped crystals of 4c below the eutectic temperature. The activation energy to form the lying crystals above the eutectic temperature was not measured because dramatic CA changes were not observed. We followed the method reported by Tsujii et al. The activation energy to form a fractal surface of an alkylketene dimer or a triglyceride was obtained by measuring the periods required for the CA to reach 150° at different storage temperatures, followed by application of the Arrhenius equation.50 In our system, the CAs of a water droplet increased with the growth of the needle-shaped crystals, and CA profiles at five different temperatures were obtained (Figure 7). At lower temperatures, the CA increased slowly indicating slower crystal growth. The obtained activation energy was 58 kJ/mol, which is much lower than that of diarylethene 3 (143 kJ/ mol).36 Such low activation energy probably reflects the low crystallinity of 4, since the molecular structure has bulky 5trimethylsilylthienyl groups. We would like to point out that such low activation energy in bulky molecules is essentially correlated to the easiness of phase transformation of crystalline structure or change in molecular packing. Actually, we have found that bulky molecules such as triacylglycerides, which have three long alkyl chains on a triglycerol chain, essentially have

several metastable states in the crystalline phase.51−53 Also, we found that the formation of the rough surface with excess surface area can be achieved when the deformation energy from a crystalline phase to another one in the phase transformation becomes equal to the excess surface energy.54 Roughly speaking, this is described as

(1/2)Eε 2V = nγS

(2)

where E and ε are the Young modulus and deformation of elastic body (now, a crystalline phase), respectively, V and S are the volume of a minimum surface structure (a needle or a flake) and the surface area of a newly formed surface, respectively, n is the number of new surfaces and γ is the surface energy density (surface tension). Here, it should be noted that E and ε reflect the interaction between molecules and the change in the molecular packing, respectively, while γ is the molecular structure which appears in the gas−solid interface. Therefore the fibril formation of 4c could proceed below the eutectic temperature. Above it, the surface was so soft that needleshaped crystals could not keep standing during their growing process. All the rough surfaces, which were covered with needleshaped crystals of 4c (Figures 2, 3, and 5), reverted to an original surface covered with cubic crystals of 4o by visible light irradiation for 4 h followed by 11 h of storage at 141 °C in the dark (Figure S4 in the Supporting Information). The reversible formation of the needle-shaped crystals were monitored by XRD. The reversible intensity changes of a diffraction peak at 16.9° (2θ) for the standing-needle-shaped 4c crystals, attributed to 012 diffraction of 4c, was observed (Figure S5 in the Supporting Information) accompanied with topographical changes (Figure S6 in the Supporting Information), while a diffraction peak at 17.1° (2θ), attributed to 40-4 17820

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Figure 3. SEM images of microcrystalline surface of 4 after storage at 135 °C for 15 min after UV irradiation (a, b), water droplet on surface (c), and CA profile of the surface of storing periods at the eutectic temperature (d).

Figure 4. Force curves obtained by AFM on single crystalline surfaces of 4o ((a) before and (b) after UV irradiation) and 2o ((c) before and (d) after UV irradiation) at 22 °C. Approach and release processes are depicted as pink and blue lines.

irradiated for 5 min and stored in the dark at 30 °C for 9 days to obtain the film with standing needle-shaped crystals. The SEM image of the surface (Figure S9a in the Supporting Information) was similar to that in Figure 5a,b. The XRD peak at 16.9° was observed for the film having standing 4c crystals, reproducibly (Figure S9b in the Supporting Information).

diffraction of 4c, was observed for the lying 4c crystals (Figures S7 and S8 in the Supporting Information). These diffraction peaks (012 and 40-4) showed the orientation directions of the standing needle-shaped 4c and the lying 4c crystals. To the melted film surface by visible light irradiation to the microcrystalline surface having lying 4c crystals, UV light was 17821

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Figure 5. SEM images of the microcrystalline surface of 4 after 5 min of UV irradiation followed by storage at 30 °C in the dark for 9 days (a, b), water droplet on the surface (c), and CA profile of surface of storing periods at 30 °C (d).

Figure 7. CA profiles of the water droplet on the microcrystalline surface of 4 at each temperature during storage after UV irradiation.

bulky substituent of the diarylethene, which induces low crystallinity.55−58

Figure 6. Plots of log N(r) versus log r for cross-sectional trace curves of the microcrystalline surface of 4 after 5 min of UV irradiation followed by storage at 30 °C in the dark for 9 days.

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ASSOCIATED CONTENT

S Supporting Information *

Estimated crystal shapes from the crystal units of 4o and 4c, SEM image of a surface covered with cubic crystals of 4o, reversible intensity changes of a diffraction peak around 17° (2θ). This material is available free of charge via the Internet at http://pubs.acs.org.

CONCLUSION In conclusion, a superhydrophobic surface whose CA is larger than 170° and whose SA is less than 1° was formed on the microcrystalline surface of diarylethene 4. Such extraordinary hydrophobic character is attributed to the surface structure, where submicroscale needle-shaped crystals are standing in a densely packed situation. A rough surface was produced below the eutectic temperature of the 4o and 4c mixture, indicating that the growth mechanism of the crystal is different from our previous results.31−38 This difference is considered due to the

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81-77-543-7462. Fax: +81-77-543-7483. E-mail: [email protected]. 17822

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Present Addresses

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Department of Chemistry, School of Science, the University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo, 221-0033, Japan. # Department of Chemistry, Asahikawa Medical University, 2-11-1 Midorigaoka-higashi, Asahikawa, Hokkaido 078-8510, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by Ryukoku University Science and Technology Fund, Izumi Science and Technology Foundation, and Grants-in-Aids for Scientific Research on Priority Area ‘‘New Frontiers in Photochromism (Grant No. 471)’’ from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.

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