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super-hydrophobic micropatterned template
Surface & Coatings Technology 202 (2008) 5535–5538
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Control of site-selective adsorption reaction on a biomimetic super-hydrophilic/super-hydrophobic micropatterned template Takahiro Ishizaki a,⁎, Hiroshi Sakurai b, Nagahiro Saito b, Osamu Takai c a
Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology (AIST), 2266-98, Anagahora, Shimo-Shidami, Moriyama-ku Nagoya 463-8560, Japan Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan c EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan b
A R T I C L E
I N F O
Available online 18 June 2008 Keywords Super-hydrophobicity Super-hydrophilicity Micropattern Biomimetic
A B S T R A C T A biomimetic super-hydrophobic/super-hydrophilic micropatterned template was successfully fabricated by microwave plasma-enhanced chemical vapor deposition (MPECVD) and vacuum ultraviolet (VUV) lithography. Water drops were selectively condensed on super-hydrophilic regions of the template. In the solution of pH 2.6, carboxyl-terminated fluorescent polystyrene spheres were sedimented on superhydrophobic regions due to electrostatic attractive forces between negative –COO− and positive –CH3 groups. Cu was also deposited densely on only super-hydrophobic regions through electroless plating. From these results, we revealed that the biomimetic super-hydrophobic/super-hydrophilic micropatterned template was used as site-selective chemical reaction field. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Materials processing based on biomimetics have attracted the attention from many researchers, since it is one of the green processes, and produces high functions and novel self-organized structures [1]. In nature, there are super-water repellent plant leaves such as lotus and taro. Studies on the leaves revealed that an ultra-water repellent surface with a large contact angle needs a cooperation of micro- and nano-structures [2]. These surfaces are actually covered with hydrophobic micropapilla. The presence of hydrophobic group on such a structure leads to ultra-water repellency [3–5]. These results obtained from the natural world would provide a guide for constructing artificial ultra-water repellent surfaces and designing surfaces with controllable wettability. A biomimetic approach is based on this concept. By using the biomimetic approach, we have succeeded in creating artificially ultra-water repellent film [1,6–8]. In addition, super-hydrophilic films had been also produced artificially. By combining the super-hydrophobic characteristic with super-hydrophobic one, it is possible to selectively control the way a water drop tends to move, since water droplets would be formed on only superhydrophilic regions when water was spilled over an artificial superhydrophobic/super-hydrophilic micropattern on substrates. In this study, we focused on the fabrication and application of biomimetic super-hydrophobic/super-hydrophilic micropattern using
⁎ Corresponding author. Materials Research Institute for Sustainable Development National Institute of Advanced Industrial Science and Technology (AIST) 2266-98, Anagahora, Shimo-Shidami, Moriyama-ku Nagoya 463-8560, Japan. E-mail address: [email protected] (T. Ishizaki). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.06.124
the microwave plasma-enhanced chemical vapor deposition (MPECVD) and vacuum ultra-violet (VUV) lithography. After fabrication of the micropattern, fluorescent polystyrene spheres were siteselectively assembled on super-hydrophobic region, and electroless plating of copper was also deposited site-selectively on superhydrophobic region. From these results, we showed that the biomimetic super-hydrophobic/hydrophilic micropatterned template was used as a site-selective chemical reaction field. 2. Experiment 2.1. Fabrication of super-hydrophobic/super-hydrophilic micropatterned template p-type Si(100) wafers were used as substrates. The substrates were ultrasonically cleaned in ethanol for 10 min. The cleaned substrate was put on the substrate stage in MPECVD system. Super-hydrophobic films were then deposited on the Si substrates using the MPECVD system. The details of the MPECVD system were described in elsewhere [1]. The chamber of the MPECVD system was evacuated down to 6.7 Pa prior to the deposition. A 2.45 GHz generator supplied microwave power of 300 W. The raw materials were a gas mixture of trimethylmethoxysilane (TMMOS) and Ar. Ar was needed in order to maintain the microwave discharge. The partial pressures of TMMOS and Ar were kept constant at 93 and 40 Pa, respectively. The substrate temperature was remained below 333 K during the deposition for 5 min. Fig. 1 shows the fabrication process of super-hydrophilic/hydrophobic micropatterned template. After deposition of super-hydrophobic film, vacuum ultraviolet (VUV) light in a wavelength of 172 nm was selectively irradiated for 20 min at
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Fig. 1. Fabrication processes of super-hydrophilic/hydrophobic micropatterned template.
the pressure of 10 Pa through a TEM mesh to fabricate super-hydrophilic region. The VUV irradiation in the presence of O2 and H2O produces atomic oxygen and OH radical. The atomic oxygen promotes oxidization and decomposition of organic compounds, and the OH radical introduces hydroxyl group onto sample surface as a termination reaction, leading to that the region irradiated by VUV becomes super-hydrophilic. 2.2. Characterization Behaviors of water droplets with micrometer scale on the superhydrophilic/hydrophobic micropattern were observed using an environmental scanning electron microscope (ESEM: Nikon Co., ESEM2700), equipped with a differential pumping system, and gaseous secondary electron detector [9–12]. The ESEM can image a hydrated sample, and even water drops condensed on a sample surface, without drying them. Water vapor was introduced into the specimen chamber of the ESEM to maintain a pressure of 650 Pa. The sample holder was cooled down to a temperature of 275.5 K, below the dew point (276.1 K) at this water vapor pressure. Consequently, water vapor condensed on the sample surface and formed a number of drops. The sample holder was tilted 60° from the normal. Water contact angles of sample surfaces were geometrically measured with a protractor on ESEM photographs. The super-hydrophilic/hydrophobic micropatterned template was immersed in a solution dispersed with carboxylate-terminated polystyrene fluorescent spheres with a mean diameter of 200 nm. In order to use the template as a site-selective chemical reaction field, we attempted to assemble site-selectively carboxyl-terminated fluorescent polystyrene spheres. The procedure was as follows. Firstly, we prepared a mixture aqueous solution of ultra-pure water and the fluorescent polystyrene spheres. The volume ratio mixed (fluorescence spheres/H2O) was 20. The mixed aqueous solution was adjusted to 2.6 with a HCl and then stirred
Fig. 2. ESEM image of water drops on super-hydrophilic/hydrophobic template micropatterned.
ultrasonically for 20 min to disperse the fluorescent polystyrene spheres. Next, the super-hydrophilic/hydrophobic micropatterned template was immersed for 90 min in the mixed solution. Finally, the template was stored for 30 min in air and dehydrated by air-drying. Assembling of the fluorescent polystyrene spheres on the template was observed with an optical fluorescence microscope (Olympus, IX71-ARCEVA). Zeta-potential of carboxylate-terminated polystyrene fluorescent sphere was measured by an electrophoretic light scattering spectrophotometer (ELSS; ELS-600, Otsuka Electronics). A solution containing 1 mM KCl as supporting electrolyte was used, adjusting its pH over the range of 2.5 to 8 by adding HCl or NaOH. Details of the zeta-potential measurements have been described in elsewhere [13]. Site-selective electroless deposition of copper on super-hydrophilic/hydrophobic micropatterned template was observed using a field emission scanning electron microscope (FE-SEM: JEOL Ltd., JSM6330F) with an accelerating voltage at 10 keV. 3. Results and discussion Water drops condensed on super-hydrophobic/hydrophilic micropatterned template was observed with an ESEM. The ESEM image of water drop behaviors is shown in Fig. 2. As clearly indicated by the water drops on the template, water vapor selectively condensed on only super-hydrophilic region and the super-hydrophilic regions fill with condensate and can form drops with half-spherical shape. Note that although on super-hydrophobic region several water drops are observed, the water drops are almost spherical. These results indicate that the micropatterned template composed of super-hydrophilic/
Fig. 3. The zeta-potentials of the carboxyl-terminated fluorescence polystyrene spheres in the pH range of 2.5 to 8.2.
T. Ishizaki et al. / Surface & Coatings Technology 202 (2008) 5535–5538
Fig. 4. A fluorescent microscopic image of fluorescence polystyrene spheres on superhydrophilic/hydrophobic template micropatterned. The spheres selectively assembled on super-hydrophobic regions.
hydrophobic regions was successfully fabricated. In order to use the template as a site-selective chemical reaction field, we attempted to assemble site-selectively carboxyl-terminated fluorescent polystyrene spheres. Functional groups on super-hydrophilic regions are silanol (– SiOH). It has been reported [14,15] that isoelectric point (IEP) of the silanol surface was in the ranges of 1.5 to 2.2. Thus, in the solution of pH 2.6, the silanol groups show negative zeta-potential, since they are partially ionized to –SiO− due to the proton dissociation from –SiOH groups. The surfaces of super-hydrophobic regions are mainly covered with methyl groups. It has been reported [15,16] that IEP of the methyl-terminated SAM surface was at approximately pH 4.0. IEP of nonpolar polymers without ionizable surface groups were also at about pH 4.0 [17–20]. Thus, the methyl-terminated SAM surface would become positive zeta-potential at the pH less than 4.0, which was attributed to proton adsorption in the double layer. Consequently, the micropatterned surface composed of super-hydrophilic and -hydrophobic regions would be separately charged with different polarity depended on the surface functional groups. Thus, the differences in chemical properties on the surface could allow us to utilize the micropatterned template composed of super-hydrophilic
5537
and -hydrophobic regions as site-selective chemical reaction field. To confirm this argument, assembling of carboxyl-modified fluorescence polystyrene spheres was performed on the template. The zetapotential of the carboxyl-terminated fluorescent polystyrene spheres was measured to investigate the surface charge. Fig. 3 shows the variation in zeta-potential of the carboxyl-terminated fluorescent polystyrene spheres. The zeta-potential was estimated from the values averaged three times. At the pH range of 2.5 to 8.2, all the zeta-potentials of the carboxyl-terminated fluorescent polystyrene spheres show negative values. This is due to partial ionization of carboxyl groups to –COO−. The carboxyl and silanol groups in the solution of pH 2.6 were converted into –COO− and –SiO− groups, respectively, so the selective adsorption of fluorescence spheres on the super-hydrophilic regions could not occurred due to their repulsive electrostatic interaction to the surface. On the contrary, the methyl groups on the super-hydrophobic regions were positively charged. Thus, the carboxyl-modified polystyrene fluorescence spheres would adsorb onto the super-hydrophobic regions due to electrostatic attractive forces. Fig. 4 shows an image acquired by fluorescent microscopy of the micropatterned template surface after immersion. The light areas and dark regions correspond to super-hydrophobic and hydrophilic regions. This fluorescent microscope image indicates that carboxyl-modified polystyrene fluorescence spheres selectively adsorbed on the super-hydrophobic region, that is, –CH3 groups regions since scattered light due to surface roughness can be observed. This indicates that we demonstrated site-selective chemical reaction due to the difference of surface functional group on super-hydrophilic/ hydrophobic micropatterned template. We confirmed that the super-hydrophilic/hydrophobic micropattern is a useful template for assembling of polystyrene spheres. In addition, the template can control a water behavior, i.e., aqueous solution, due to chemically different surface characteristics. In practical processes, there are many fabrication techniques using an aqueous solution such as sol– gel, electrodeposition, and electroless plating. Among these, electroless plating is most practical method, so the control of the deposition in micro- or nano-meter scale is expected to apply to various application fields. Thus, we attempted to selectively deposit Cu through electroless plating on the super-hydrophilic/hydrophobic template micropatterned. The micropatterned template was immersed in a pH =1.1 solution containing Sn (High Purity Chemicals, Co., Ltd.). Such a solution is commonly used in order to activate a surface. Due to this immersion, Sn particles were selectively immobilized on the super-hydrophobic regions. The adsorption mechanism of Sn particles to the super-hydrophobic regions is considered as follows. After immersion of the template, the –
Fig. 5. (a) FE-SEM image of Cu deposition on super-hydrophilic/hydrophobic template micropatterned. Cu was selectively deposited on super-hydrophobic regions through electroless plating. (b) FE-SEM image enlarged of the surface morphology.
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strength, the adhesion test was conducted with a Scotch tape. No peeling was observed on the sample surface. These results indicate that by employing the technique presented here, we were able to successfully fabricate super-hydrophilic/hydrophobic micropatterned template and use the template as site-selective chemical reaction field in micrometer scale. 4. Conclusions We successfully fabricated a biomimetic super-hydrophobic/ super-hydrophilic micropatterned template. By using the template, chemical reactions using aqueous solution were site-selectively governed. As a result, fluorescent polystyrene spheres were assembled on super-hydrophobic regions and Cu was also deposited on superhydrophobic regions through electroless plating. The biomimetic approach would be a key technology for fabricating template on which chemical reactions in micrometer scale can be controlled siteselectively. Acknowledgement
Fig. 6. (a) FE-SEM image of Cu deposition on super-hydrophilic/hydrophobic template micropatterned. (b) Cu distribution on line by EDAX.
This work has been supported in part by research program ‘Nagoya Nano-Technology Cluster of Innovative Production System’ of the Ministry of Education, Culture, Sports, Science and Technology. References
CH3-terminated regions are oxidized to –COOH terminated ones. This oxidation of the –CH3-terminated surface can be explained by the disproportionation of Sn as follows: 2Sn2+ =Sn0 +Sn4+. The Sn4+ ions in the acid solution act as a catalyst for oxidation. Due to their presence, the – CH3-terminated surface can be oxidized to –COOH terminated one by the Sn4+ ions. Saito et al confirmed by XPS analysis that carboxyl end groups had been introduced into the –CH3-terminated surface by the disproportionation of Sn [21]. Thus, Sn particles were selectively adsorbed on the carboxyl end groups. Next, the Sn adsorbed template was immersed in a pH =1.6 electroless plating solution containing Pd ions (High Purity Chemicals, Co., Ltd.). The sample was then immersed in a pH =13.0 electroless plating solution containing Cu ions (High Purity Chemicals, Co. Ltd.). Fig. 5 shows a FE-SEM image of the obtained template surface. The bright and dark regions correspond to metal and super-hydrophilic ones, respectively. The image indicates highly selective metal deposition onto the super-hydrophobic regions. The deposition consisted of uniformly distributed large grains, typically 500 nm in size. Fig. 6(a) and (b) show the Cu distribution based on energy dispersive X-ray analysis (EDAX) on a line and FE-SEM image of the template surface, respectively. The EDAX analysis revealed that the deposits on the super-hydrophobic regions were Cu particles. No Cu distribution was observed in the dark circular regions, that is, super-hydrophilic regions. In order to evaluate the
[1] S. Mann, J.P. Hannington, R.J.P. Williams, Nature 324 (1986) 565. [2] R.N. Wenzel, Ind. Eng. Chem. 28 (1936) 988. [3] L. Jiang, R. Wang, B. Yang, T.J. Li, D.A. Tryk, A. Fujishima, K. Hashimoto, D.B. Zhu, Pure Appl. Chem. 72 (2000) 73. [4] C. Guo, L. Zhai, G. Wang, Y. Song, L. Jiang, D. Zhu, Chem. Phys. Chem. 5 (2004) 750. [5] L. Jiang, Y. Zhao, J. Zhai, Angew. Chem. Int. Ed. 43 (2004) 4338. [6] Y. Wu, H. Sugimura, Y. Inoue, O. Takai, Thin Solid Films 435 (2003) 161. [7] Y. Wu, M. Kuroda, H. Sugimura, Y. Inoue, O. Takai, Surf. Coat. Technol. 174–175 (2003) 867. [8] Y. Wu, M. Bekke, Y. Inoue, H. Sugimura, H. Kitaguchi, C. Liu, O. Takai, Thin Solid Films 457 (2004) 122. [9] R. Durkin, J.S. Shah, J. Microsc. 169 (1993) 33. [10] R. Cameron, A. Donald, J. Microsc. 173 (1994) 227. [11] A. Fletcher, B. Thiel, A. Donald, J. Phys. D: Appl. Phys. 30 (1997) 2249. [12] H. Kobayashi, A.T. Bell, M. Shen, J. Appl. Polym. Sci. 17 (1973) 885. [13] A. Hozumi, H. Sugimura, Y. Yokogawa, T. Kameyama, O. Takai, Colloids Surf. A. 182 (2001) 257. [14] G.A. Parks, Chem. Rev. 65 (1965) 177. [15] H. Sugimura, A. Hozumi, T. Kameyama, O. Takai, Surf. Interface Anal. 34 (2002) 550. [16] R. Schweiss, P.B. Wezel, C. Werner, W. Knoll, Langmuir. 17 (2001) 4304. [17] K. Grundke, H.J. Jacobasch, F. Simon, S. Schneider, J. Adhes. Sci. Technol. 9 (1995) 327. [18] P. Weidenhammer, H.J. Jacobasch, J. Colloid Interface Sci. 180 (1996) 232. [19] H.J. Jacobasch, F. Simon, P. Weidenhammer, Colloid Polym. Sci. 276 (1998) 434. [20] A. Bismarck, M.E. Kumru, J. Springer, J. Colloid Interface Sci. 217 (1999) 377. [21] N. Saito, J. Hieda, O. Takai, Electrochem. Solid. State. Lett. 7 (2004) C140.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Control of site-selective adsorption reaction on a biomimetic super-hydrophilic/super-hydrophobic micropatterned template Takahiro Ishizaki a,⁎, Hiroshi Sakurai b, Nagahiro Saito b, Osamu Takai c a
Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology (AIST), 2266-98, Anagahora, Shimo-Shidami, Moriyama-ku Nagoya 463-8560, Japan Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan c EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan b
A R T I C L E
I N F O
Available online 18 June 2008 Keywords Super-hydrophobicity Super-hydrophilicity Micropattern Biomimetic
A B S T R A C T A biomimetic super-hydrophobic/super-hydrophilic micropatterned template was successfully fabricated by microwave plasma-enhanced chemical vapor deposition (MPECVD) and vacuum ultraviolet (VUV) lithography. Water drops were selectively condensed on super-hydrophilic regions of the template. In the solution of pH 2.6, carboxyl-terminated fluorescent polystyrene spheres were sedimented on superhydrophobic regions due to electrostatic attractive forces between negative –COO− and positive –CH3 groups. Cu was also deposited densely on only super-hydrophobic regions through electroless plating. From these results, we revealed that the biomimetic super-hydrophobic/super-hydrophilic micropatterned template was used as site-selective chemical reaction field. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Materials processing based on biomimetics have attracted the attention from many researchers, since it is one of the green processes, and produces high functions and novel self-organized structures [1]. In nature, there are super-water repellent plant leaves such as lotus and taro. Studies on the leaves revealed that an ultra-water repellent surface with a large contact angle needs a cooperation of micro- and nano-structures [2]. These surfaces are actually covered with hydrophobic micropapilla. The presence of hydrophobic group on such a structure leads to ultra-water repellency [3–5]. These results obtained from the natural world would provide a guide for constructing artificial ultra-water repellent surfaces and designing surfaces with controllable wettability. A biomimetic approach is based on this concept. By using the biomimetic approach, we have succeeded in creating artificially ultra-water repellent film [1,6–8]. In addition, super-hydrophilic films had been also produced artificially. By combining the super-hydrophobic characteristic with super-hydrophobic one, it is possible to selectively control the way a water drop tends to move, since water droplets would be formed on only superhydrophilic regions when water was spilled over an artificial superhydrophobic/super-hydrophilic micropattern on substrates. In this study, we focused on the fabrication and application of biomimetic super-hydrophobic/super-hydrophilic micropattern using
⁎ Corresponding author. Materials Research Institute for Sustainable Development National Institute of Advanced Industrial Science and Technology (AIST) 2266-98, Anagahora, Shimo-Shidami, Moriyama-ku Nagoya 463-8560, Japan. E-mail address: [email protected] (T. Ishizaki). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.06.124
the microwave plasma-enhanced chemical vapor deposition (MPECVD) and vacuum ultra-violet (VUV) lithography. After fabrication of the micropattern, fluorescent polystyrene spheres were siteselectively assembled on super-hydrophobic region, and electroless plating of copper was also deposited site-selectively on superhydrophobic region. From these results, we showed that the biomimetic super-hydrophobic/hydrophilic micropatterned template was used as a site-selective chemical reaction field. 2. Experiment 2.1. Fabrication of super-hydrophobic/super-hydrophilic micropatterned template p-type Si(100) wafers were used as substrates. The substrates were ultrasonically cleaned in ethanol for 10 min. The cleaned substrate was put on the substrate stage in MPECVD system. Super-hydrophobic films were then deposited on the Si substrates using the MPECVD system. The details of the MPECVD system were described in elsewhere [1]. The chamber of the MPECVD system was evacuated down to 6.7 Pa prior to the deposition. A 2.45 GHz generator supplied microwave power of 300 W. The raw materials were a gas mixture of trimethylmethoxysilane (TMMOS) and Ar. Ar was needed in order to maintain the microwave discharge. The partial pressures of TMMOS and Ar were kept constant at 93 and 40 Pa, respectively. The substrate temperature was remained below 333 K during the deposition for 5 min. Fig. 1 shows the fabrication process of super-hydrophilic/hydrophobic micropatterned template. After deposition of super-hydrophobic film, vacuum ultraviolet (VUV) light in a wavelength of 172 nm was selectively irradiated for 20 min at
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T. Ishizaki et al. / Surface & Coatings Technology 202 (2008) 5535–5538
Fig. 1. Fabrication processes of super-hydrophilic/hydrophobic micropatterned template.
the pressure of 10 Pa through a TEM mesh to fabricate super-hydrophilic region. The VUV irradiation in the presence of O2 and H2O produces atomic oxygen and OH radical. The atomic oxygen promotes oxidization and decomposition of organic compounds, and the OH radical introduces hydroxyl group onto sample surface as a termination reaction, leading to that the region irradiated by VUV becomes super-hydrophilic. 2.2. Characterization Behaviors of water droplets with micrometer scale on the superhydrophilic/hydrophobic micropattern were observed using an environmental scanning electron microscope (ESEM: Nikon Co., ESEM2700), equipped with a differential pumping system, and gaseous secondary electron detector [9–12]. The ESEM can image a hydrated sample, and even water drops condensed on a sample surface, without drying them. Water vapor was introduced into the specimen chamber of the ESEM to maintain a pressure of 650 Pa. The sample holder was cooled down to a temperature of 275.5 K, below the dew point (276.1 K) at this water vapor pressure. Consequently, water vapor condensed on the sample surface and formed a number of drops. The sample holder was tilted 60° from the normal. Water contact angles of sample surfaces were geometrically measured with a protractor on ESEM photographs. The super-hydrophilic/hydrophobic micropatterned template was immersed in a solution dispersed with carboxylate-terminated polystyrene fluorescent spheres with a mean diameter of 200 nm. In order to use the template as a site-selective chemical reaction field, we attempted to assemble site-selectively carboxyl-terminated fluorescent polystyrene spheres. The procedure was as follows. Firstly, we prepared a mixture aqueous solution of ultra-pure water and the fluorescent polystyrene spheres. The volume ratio mixed (fluorescence spheres/H2O) was 20. The mixed aqueous solution was adjusted to 2.6 with a HCl and then stirred
Fig. 2. ESEM image of water drops on super-hydrophilic/hydrophobic template micropatterned.
ultrasonically for 20 min to disperse the fluorescent polystyrene spheres. Next, the super-hydrophilic/hydrophobic micropatterned template was immersed for 90 min in the mixed solution. Finally, the template was stored for 30 min in air and dehydrated by air-drying. Assembling of the fluorescent polystyrene spheres on the template was observed with an optical fluorescence microscope (Olympus, IX71-ARCEVA). Zeta-potential of carboxylate-terminated polystyrene fluorescent sphere was measured by an electrophoretic light scattering spectrophotometer (ELSS; ELS-600, Otsuka Electronics). A solution containing 1 mM KCl as supporting electrolyte was used, adjusting its pH over the range of 2.5 to 8 by adding HCl or NaOH. Details of the zeta-potential measurements have been described in elsewhere [13]. Site-selective electroless deposition of copper on super-hydrophilic/hydrophobic micropatterned template was observed using a field emission scanning electron microscope (FE-SEM: JEOL Ltd., JSM6330F) with an accelerating voltage at 10 keV. 3. Results and discussion Water drops condensed on super-hydrophobic/hydrophilic micropatterned template was observed with an ESEM. The ESEM image of water drop behaviors is shown in Fig. 2. As clearly indicated by the water drops on the template, water vapor selectively condensed on only super-hydrophilic region and the super-hydrophilic regions fill with condensate and can form drops with half-spherical shape. Note that although on super-hydrophobic region several water drops are observed, the water drops are almost spherical. These results indicate that the micropatterned template composed of super-hydrophilic/
Fig. 3. The zeta-potentials of the carboxyl-terminated fluorescence polystyrene spheres in the pH range of 2.5 to 8.2.
T. Ishizaki et al. / Surface & Coatings Technology 202 (2008) 5535–5538
Fig. 4. A fluorescent microscopic image of fluorescence polystyrene spheres on superhydrophilic/hydrophobic template micropatterned. The spheres selectively assembled on super-hydrophobic regions.
hydrophobic regions was successfully fabricated. In order to use the template as a site-selective chemical reaction field, we attempted to assemble site-selectively carboxyl-terminated fluorescent polystyrene spheres. Functional groups on super-hydrophilic regions are silanol (– SiOH). It has been reported [14,15] that isoelectric point (IEP) of the silanol surface was in the ranges of 1.5 to 2.2. Thus, in the solution of pH 2.6, the silanol groups show negative zeta-potential, since they are partially ionized to –SiO− due to the proton dissociation from –SiOH groups. The surfaces of super-hydrophobic regions are mainly covered with methyl groups. It has been reported [15,16] that IEP of the methyl-terminated SAM surface was at approximately pH 4.0. IEP of nonpolar polymers without ionizable surface groups were also at about pH 4.0 [17–20]. Thus, the methyl-terminated SAM surface would become positive zeta-potential at the pH less than 4.0, which was attributed to proton adsorption in the double layer. Consequently, the micropatterned surface composed of super-hydrophilic and -hydrophobic regions would be separately charged with different polarity depended on the surface functional groups. Thus, the differences in chemical properties on the surface could allow us to utilize the micropatterned template composed of super-hydrophilic
5537
and -hydrophobic regions as site-selective chemical reaction field. To confirm this argument, assembling of carboxyl-modified fluorescence polystyrene spheres was performed on the template. The zetapotential of the carboxyl-terminated fluorescent polystyrene spheres was measured to investigate the surface charge. Fig. 3 shows the variation in zeta-potential of the carboxyl-terminated fluorescent polystyrene spheres. The zeta-potential was estimated from the values averaged three times. At the pH range of 2.5 to 8.2, all the zeta-potentials of the carboxyl-terminated fluorescent polystyrene spheres show negative values. This is due to partial ionization of carboxyl groups to –COO−. The carboxyl and silanol groups in the solution of pH 2.6 were converted into –COO− and –SiO− groups, respectively, so the selective adsorption of fluorescence spheres on the super-hydrophilic regions could not occurred due to their repulsive electrostatic interaction to the surface. On the contrary, the methyl groups on the super-hydrophobic regions were positively charged. Thus, the carboxyl-modified polystyrene fluorescence spheres would adsorb onto the super-hydrophobic regions due to electrostatic attractive forces. Fig. 4 shows an image acquired by fluorescent microscopy of the micropatterned template surface after immersion. The light areas and dark regions correspond to super-hydrophobic and hydrophilic regions. This fluorescent microscope image indicates that carboxyl-modified polystyrene fluorescence spheres selectively adsorbed on the super-hydrophobic region, that is, –CH3 groups regions since scattered light due to surface roughness can be observed. This indicates that we demonstrated site-selective chemical reaction due to the difference of surface functional group on super-hydrophilic/ hydrophobic micropatterned template. We confirmed that the super-hydrophilic/hydrophobic micropattern is a useful template for assembling of polystyrene spheres. In addition, the template can control a water behavior, i.e., aqueous solution, due to chemically different surface characteristics. In practical processes, there are many fabrication techniques using an aqueous solution such as sol– gel, electrodeposition, and electroless plating. Among these, electroless plating is most practical method, so the control of the deposition in micro- or nano-meter scale is expected to apply to various application fields. Thus, we attempted to selectively deposit Cu through electroless plating on the super-hydrophilic/hydrophobic template micropatterned. The micropatterned template was immersed in a pH =1.1 solution containing Sn (High Purity Chemicals, Co., Ltd.). Such a solution is commonly used in order to activate a surface. Due to this immersion, Sn particles were selectively immobilized on the super-hydrophobic regions. The adsorption mechanism of Sn particles to the super-hydrophobic regions is considered as follows. After immersion of the template, the –
Fig. 5. (a) FE-SEM image of Cu deposition on super-hydrophilic/hydrophobic template micropatterned. Cu was selectively deposited on super-hydrophobic regions through electroless plating. (b) FE-SEM image enlarged of the surface morphology.
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T. Ishizaki et al. / Surface & Coatings Technology 202 (2008) 5535–5538
strength, the adhesion test was conducted with a Scotch tape. No peeling was observed on the sample surface. These results indicate that by employing the technique presented here, we were able to successfully fabricate super-hydrophilic/hydrophobic micropatterned template and use the template as site-selective chemical reaction field in micrometer scale. 4. Conclusions We successfully fabricated a biomimetic super-hydrophobic/ super-hydrophilic micropatterned template. By using the template, chemical reactions using aqueous solution were site-selectively governed. As a result, fluorescent polystyrene spheres were assembled on super-hydrophobic regions and Cu was also deposited on superhydrophobic regions through electroless plating. The biomimetic approach would be a key technology for fabricating template on which chemical reactions in micrometer scale can be controlled siteselectively. Acknowledgement
Fig. 6. (a) FE-SEM image of Cu deposition on super-hydrophilic/hydrophobic template micropatterned. (b) Cu distribution on line by EDAX.
This work has been supported in part by research program ‘Nagoya Nano-Technology Cluster of Innovative Production System’ of the Ministry of Education, Culture, Sports, Science and Technology. References
CH3-terminated regions are oxidized to –COOH terminated ones. This oxidation of the –CH3-terminated surface can be explained by the disproportionation of Sn as follows: 2Sn2+ =Sn0 +Sn4+. The Sn4+ ions in the acid solution act as a catalyst for oxidation. Due to their presence, the – CH3-terminated surface can be oxidized to –COOH terminated one by the Sn4+ ions. Saito et al confirmed by XPS analysis that carboxyl end groups had been introduced into the –CH3-terminated surface by the disproportionation of Sn [21]. Thus, Sn particles were selectively adsorbed on the carboxyl end groups. Next, the Sn adsorbed template was immersed in a pH =1.6 electroless plating solution containing Pd ions (High Purity Chemicals, Co., Ltd.). The sample was then immersed in a pH =13.0 electroless plating solution containing Cu ions (High Purity Chemicals, Co. Ltd.). Fig. 5 shows a FE-SEM image of the obtained template surface. The bright and dark regions correspond to metal and super-hydrophilic ones, respectively. The image indicates highly selective metal deposition onto the super-hydrophobic regions. The deposition consisted of uniformly distributed large grains, typically 500 nm in size. Fig. 6(a) and (b) show the Cu distribution based on energy dispersive X-ray analysis (EDAX) on a line and FE-SEM image of the template surface, respectively. The EDAX analysis revealed that the deposits on the super-hydrophobic regions were Cu particles. No Cu distribution was observed in the dark circular regions, that is, super-hydrophilic regions. In order to evaluate the
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