Langmuir 2006, 22, 9957-9961
9957
Fabrication of Flexible Gold Films with Periodic Sub-Micrometer Roughness and Their Wettability Control by Modification of SAM Takeshi Kawai,* Maho Suzuki, and Takeshi Kondo Department of Industrial Chemistry, Faculty of Engineering, Tokyo UniVersity of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan ReceiVed June 20, 2006. In Final Form: August 14, 2006 Flexible honeycomb gold films supported by polymer sheets are fabricated by using polystyrene particle monolayers. The surfaces of the flexible gold films are covered with self-assembled monolayers (SAMs) of hydrophobic or hydrophilic thiol compounds, and the wettability of the modified surface is evaluated by measurements of the contact angles of water droplets. The contact angle of the film covered with hydrophobic SAM is ca. 150°, which is greater than the value of 112° for a flat gold surface, while the values for hydrophilic SAM are below 10°.
I. Introduction Wettability control of surfaces has been receiving a great deal of recent interest, because wettability is a critical factor for applications of self-cleaning materials, low flow resistance coatings in microfluidic systems, high performance thermal conduction materials, and antifogging materials. The control of wettability has been accomplished thus far with light-induced thin TiO2 films,1,2 and by using well-defined heterogeneous surfaces of hydrophobic and hydrophilic regions by patterning a self-assembled monolayer (SAM) using micro-contact printing3 and with fractal surfaces of wax.4 Recently, to form a superhydrophobic surface, periodically nanopatterned surfaces have been fabricated by various techniques,5-23 such as the particle template method,5-8 * Corresponding author. Tel.: +81 3 5228 8312. Fax: +81 3 5261 4631. E-mail:
[email protected]. (1) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature (London) 1997, 388, 431. (2) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 1999, 103, 2188. (3) Drelich, J.; Millar, J. D.; Kumar, A.; Whitesides, G. M. Colloids Surf., A 1994, 93, 1. (4) Onda, T.; Shibuichi, S.; Tsujii, K. Langmuir 1996, 12, 2125. (5) Shiu, J.-Y.; Kuo, C.-W.; Chen, P.; Mou, C.-Y. Chem. Mater. 2004, 16, 4, 561. (6) Abdelsalam, M. E.; Bartlett, P. N.; Kelf, T.; Baumberg, J. Langmuir 2005, 21, 1753. (7) Nakae, H.; Inui, R.; Hirata, Y.; Saito, H. Acta Mater. 1998, 46, 2313. (8) Zhang, J.; Xue, L.; Han, Y. Langmuir 2005, 21, 5. (9) Uelzen, Y.; Muller, J. Thin Solid Films 2003, 434, 311. (10) Martines, E.; Seunarine, K.; Morgan, H.; Gadegaard, N.; Wilkinson, C. D. W.; Riehle, M. O. Nano Lett. 2005, 5, 2097. (11) Suh, K. Y.; Jon, S. Langmuir 2005, 21, 6836. (12) Lee, W.; Jin, M.; Yoo, W.; Lee, J. Langmuir 2004, 20, 7665. (13) O ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (14) Bico, J.; Marzolin, C.; Quere, D. Europhys. Lett. 1999, 47, 220. (15) Sugimura, H.; Ushiyama, K.; Hozumi, A.; Takai, O. Langmuir 2000, 16, 885. (16) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2005, 21, 937. (17) Fu¨tstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21, 956. (18) Zheng, Z.; Azzaroni, O.; Zhou, F.; Huck, W. T. S. J. Am. Chem. Soc. 2006, 128, 7730. (19) Lee, K.; Pan, F.; Carroll, G. T.; Turro, N. J.; Koberstein, J. T. Langmuir 2004, 20, 1812. (20) Delamarche, E.; Geissler, M.; Wolf, H.; Michel, B. J. Am. Chem. Soc. 2002, 124, 3834. (21) McHale, G.; Shirtcliffe, N. J.; Newton, M. I. Langmuir 2004, 20, 10146. (22) Sun, T.; Wang, G.; Liu, H.; Feng, L.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2003, 125, 14996. (23) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; O ¨ ner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395.
lithography,10-17 and micro-contact printing.18-21 Shiu et al.5 have fabricated periodic rough surfaces by using oxygen plasma etching of polystyrene (PS) particle monolayers and have demonstrated the effect of the etching time, that is, the roughness of PS particles on water contact angles. Abdelsalam et al.6 have fabricated regularly structured gold films on gold electrodes by the electrodeposition of gold through a PS latex template. They showed the relation between the water contact angle and the thickness of the deposited gold. Uelzen et al.9 have measured the water contact angle of tetrafluoroethylene, gold, and hexamethyldisiloxane layers deposited on smooth silicon wafers and on rough tin. They showed that wettability, hydrophobicity or hydrophilicity, was enhanced by rough surfaces. Many of these periodically nanopatterned surfaces have been fabricated on rigid substrates. If flexible films of periodically nanopatterned surfaces were available, the applications of wettability control of surfaces would be broadened. In this study, we have fabricated flexible honeycomb films of gold supported by polymer sheets, which made a mold of periodic PS particle arrays in the particle monolayer, and then modified the surfaces of the flexible gold films with SAM of hydrophobic or hydrophilic thiol compounds. We show that the modification of the gold surface with thiol compounds brings about enhanced hydrophobicity or hydrophilicity. II. Experimental Section Materials. Styrene (Kanto Chemicals) was purified by distillation under reduced pressure in a nitrogen atmosphere. Reagent grade potassium persulfate (KPS) was used as an initiator without further purification. Octadecanethiol (Merck, ODT), 16-mercaptohexadecanoic acid (Aldrich, MHA), 3-mercaptopropanoic acid (Aldrich, MPA), 1H,1H,2H,2H-perfluorooctanethiol (Apollo scientific LTD, FOT), and nonionic surfactant of Triton X-100 (Aldrich) were used as received. Preparation of Monodispersed PS Particles. Monodispersed PS particles (three types) with mean diameters of 147, 183, and 200 nm were synthesized following standard emulsion polymerization. The emulsion polymerization was carried out at 75 °C in a 300 mL glass reactor under an argon atmosphere. The reactor was first charged with 120 mL of water, NaOH (adjusted to pH 10), and 0.16 g of sodium dodecyl sulfate, followed by 14.4 g of styrene monomer. The initiator solution of KPS (0.08 g) was added to the reactor after the temperature had reached 75 °C. In addition, PS particles with mean diameters of 560 and 1120 nm were produced by emulsifier-free emulsion polymerization with
10.1021/la061780d CCC: $33.50 © 2006 American Chemical Society Published on Web 10/21/2006
9958 Langmuir, Vol. 22, No. 24, 2006
Kawai et al. Scheme 1. Schematic Illustration of the Experimental Procedure Used for the Fabrication of the Flexible Honeycomb Gold Film Supported by a Polymer Sheet
Figure 1. Typical SEM image of PS particle monolayer on glass substrates. The particle size is 200 nm. KPS as the initiator in water.24,25 Styrene monomer (15 g) and 180 mL of water were added to a 300 mL glass reactor, and the temperature was raised to 70 °C under vigorous stirring. An aqueous solution (20 mL) of KPS (0.35 g) was added to the reactor. Preparation of PS Particles Monolayer. The PS particles dispersion solution was diluted with a 1.25% ethanol solution of nonionic surfactant Triton X-100 (2:1 by volume).26 Two-dimensional colloid arrays of PS particles on glass substrates were prepared by spin-coating of the dispersed PS particles. PS particles were spincoated over the entire substrate surface at 4500 rpm, and the particle density was 0.33 g/mL. A representative scanning electron microscopy (SEM) micrograph of the particles is shown in Figure 1. SAM Formation on Gold Film. The deposition of gold on the PS particles monolayer was performed using a thermal vacuum evaporator (SVC-700Turbo-TM, SANYU, Japan) at a rate of 0.1 nm/s. The thickness of the evaporated Au film was the same as the diameter of the PS particles. Surface modification of the gold film was performed by SAM formation of thiol compounds. The gold films were immersed in a 3 mM hexane solution of ODT, MPA, MAH, or FOT for 8 h after the gold films were subjected to UV/ ozone treatment to obtain a clean surface. The UV/ozone treatment was performed using a 25 W low-pressure Hg lamp under O2 atmosphere for 30 min.27-30 To verify the cleaning of the gold film after UV/ozone treatment, infrared ATR spectra were observed using an FT-IR spectrometer (Nicolet 510M) with a resolution of 4 cm-1. In the ATR spectrum, there was no CH stretching peaks or benzene ring peaks for PS. For the ATR measurements, a Harrick single reflection ATR attachment (GATR) was used; the angle of incidence of the infrared beam was 60°, and the ATR crystal used was germanium. Contact Angle Measurements. Contact angles were determined by the sessile drop technique with a Face model CA-DT contact angle meter (Kyowa Interface Science). A 1 µL water drop was formed using a micro-pipet and was placed directly onto the sample. The liquid drops were observed with an optical microscope with 5× magnification. We used the mean values of the angle of five measurement values or more at different locations. Analysis of Gold Film Structure. The structures of the PS particle monolayer and gold films were examined by atomic force microscopy (24) Okubo, M.; Izumi, J. Colloids Surf., A 1999, 153, 297. (25) Okubo, M.; Ise, T.; Yamashita, T. J. Appl. Polym. Sci. 1999, 74, 278. (26) Romero-Cano, M. S.; Martin-Rodriguez, A.; Chauveteau, G.; Nieves, F. J. Colloids Surf., A 1998, 140, 347. (27) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (28) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626. (29) Poirier, G. E.; Herne, T. M.; Miller, C. C.; Tarlov, M. J. J. Am. Chem. Soc. 1999, 121, 9703. (30) Chan, K. C.; Kim, T.; Schoer, J. K.; Crooks, R. M. J. Am. Chem. Soc. 1995, 117, 5875.
(AFM) and scanning electron microscopy (SEM). SEM images were obtained with an S-5000 scanning electron microscope (HITACHI). AFM images of gold films were recorded with a Nanoscope III (Digital Instruments) operated in the tapping mode at a resonance frequency of about 300 kHz. The measurements were performed under ambient conditions using Si probes with a spring constant of 40 N/m.
III. Results and Discussion Fabrication of Honeycomb Au Film. Fabrication of a honeycomb Au film was carried out according to Scheme 1. For the fabrication of a honeycomb Au film, important properties for the polymer support are (i) low elasticity for the peeling process from the substrate, (ii) high solvent-resistance during SAM formation, and (iii) water resistance for contact angle measurements. First, gold was deposited onto the PS particle monolayer by a vacuum evaporation method, and then a 1.5 mL aqueous solution of poly(vinyl alcohol) (PVA, 20 wt %) or 2 mL of a chloroform solution of polyvinylformal (PVF, 20 wt %) was cast on the gold film. After drying of the solvent, the polymer film was peeled off the glass substrate. Figure 2 shows SEM images of the peeled gold/PVA film (Figure 2a) and the gold/PVF film (Figure 2b) after the PS particles were removed by gently rinsing with toluene. Because the gold/PVA film was easily elongated and the gold/PVF film ripped during the peeling process, a number of cracks as well as rolling marks in both films are observed in the SEM images in Figure 2. We tested other polymers such as poly(dimethylsiloxane) (PDMS) and polyvinyl acetate copolymer as the supporting material to make honeycomb gold films. However, all of the films were easily deformed by tensile stress during the peeling process. We then used two kinds of polymers; that is, after preparing the first layer of polymer on the Au/PS particles, a second polymer layer was cast on the first one, as shown in Scheme 1. From various tests, a combination of PVF and PVA was found to provide a good supporting polymer. Thus, a 1.5 mL aqueous solution of PVA was deposited on a PVF film, and after the PVA layer was dried, the composite film consisting of PS particles and gold supported by the PVA+PVF layers was peeled off the glass substrate. The peeled composite film was
Fabrication of Flexible Gold Films
Figure 2. SEM micrographs of the honeycomb film supported by (a) a PVA and (b) a PVF layer.
Figure 3. SEM image of a honeycomb film of Au/(PVF+PVA) obtained by using 200 nm PS particles. The inset shows a magnified view.
gently rinsed with toluene to remove PS particles. Figure 3 shows an SEM image of the resulting Au film using 200 nm PS particles. These images show that there is no elongation in the peeling direction and no cracks were formed in the film. From cross-section profiles of AFM images for 200 nm PS particles (not shown), the average diameter of the holes was evaluated to be 163 nm. Although a precise evaluation of the hole depths was difficult because of miss position of the cantilever in AFM measurements of the hole, the depth of the holes is probably smaller than 100 nm as estimated from the SEM images. Thus, the size of the holes is smaller than the hemispherical size of the PS particles, because the diameter of PS particles is 200 nm, which is the same as the thickness of the evaporated Au.
Langmuir, Vol. 22, No. 24, 2006 9959
Figure 4. Schematic illustrations of cross-sections of evaporated gold film. The rim of gold among the PS particles (a) remains and (b) is cut off during the process of removal of PS particles. (c) AFM image of an evaporated gold film supported by a PVF+PVA layer after the removal of PS particles.
This result indicates that the brim of the evaporated Au among the particles should be flat as shown in Figure 4b rather than Figure 4a. Thus, in the removal process of PS particles in Scheme 1, the tips of the brim of the gold film are cut. This was confirmed by AFM measurements as shown in Figure 4c. In the case of other sized PS particles, the depths of the holes were also smaller that the corresponding radii of the PS particles, and the tips of the brims of evaporated Au came off during the process of removal of PS particles or detachment from the glass substrate. Modification of SAM on Gold Film. The resulting Au/ (PVF+PVA) film was immersed into various solvents containing thiol compounds. We could not find good solvents that did not deform the film structure. The PVA layer was then peeled from the Au/PVF layer (see Scheme 1), before the gold films were subjected to immersion into various solutions of thiol compounds. As shown in Figure 5, the structure of the Au/PVF film was not deformed from the peeling process of the PVA layer. Although the honeycomb structure was heterogeneous in a microscopic scale as can be seen in Figure 5b, the area of a few square centimeters appears homogeneous to the naked eye. When the Au/PVF films were immersed into various solvents, it was found that methanol, ethanol, and chloroform caused a decrease in the flexibility, curling, and deformation of the films, respectively, while hexane had no effect on the nature of the films. Contact Angle of Water. Sessile water contact angles of the honeycomb Au film were measured, and the resulting average contact angles are listed in Table 1. The contact angles of surfaces covered with FOT and ODT, which yield a hydrophobic surface, were about 150° and greater than the values of 112° and 105°
9960 Langmuir, Vol. 22, No. 24, 2006
Kawai et al.
Figure 6. Contact angles of the honeycomb surface (θR) for a water droplet are plotted as a function of the corresponding angles of the flat surface (θS). 0; PS-200, [; PS-560. The theoretical predictions by Wenzel (solid) and Cassie-Baxter (dashed line) equations used a roughness factor r ) 1.2 and surface fraction of gold φ1 ) 0.20.
the calculated curves of the Wenzel equation for r ) 1.2. For the hydrophilic SAM, the observed angles were close to the calculated angles, while those for the hydrophobic SAM were not close and the contact angles were much larger as compared to the expected values. We assumed the reason for the results to be the presence of air under the water droplet,6-14 that is, the water droplet contact with gold at the rim of the honeycomb and with air at the center of the honeycomb. Under this condition, the apparent contact angle can be estimated by the following Cassie-Baxter equation:6-14 Figure 5. (a) Photograph and (b) AFM image of the honeycomb gold film after peeling off the PVF layer. The higher regions in AFM are boundaries between the domains of the particle array. Table 1. Contact Angles of Water Droplets for Honeycomb Surfaces and a Flat Surface, Modified with Various SAMs contact angles/deg samples
FOT
ODT
MHdA
MPA
no treatment
PS-200 (θR) PS-560 (θR) flat (θS)
155 150 112
149 146 105
68 76 75
8 13 45
54 56 63
for the flat surface. This indicates that surface roughness enhances the hydrophobicity. On the other hand, the contact angle of the hydrophilic MPA was 8°, much smaller than that for the flat surface. Thus, the hydrophilicity was also emphasized by the surface roughness.4,7,9-11,31 In addition, the enhancement of hydrophobicity and hydrophilicity for PS-200 was larger than that for PS-560. The apparent contact angle, θR, of rough surfaces can be expressed by Wenzel’s formulation as:32
cos θR ) r cos θS
(1)
where r is the roughness factor (the ratio of total surface area to the projected area in the horizontal plane) and θS is the contact angle measured on the flat surface. The values of r were evaluated on the basis of the AFM images33 and were found to be 1.22 for PS-200 and 1.19 for PS-560. The solid line in Figure 6 shows (31) Yu¨ce, M. Y.; Demirel, A. L.; Menzel, F. Langmuir 2005, 21, 5073. (32) Wenzel, T. N. J. Phys. Colloid Chem. 1949, 53, 1455.
cos θR ) φ1 cos θ1 + φ2 cos θ2
(2)
where φ1 and φ2 are the surface fraction of gold and air, respectively, and θ1 and θ2 are the corresponding contact angles. Because the contact angle of water on air is 180° and φ1 + φ2 ) 1, eq 2 should be
cos θR ) φ1(cos θS + 1) - 1
(3)
As shown in Figure 4b and c, the honeycomb film consists of edges and holes. We assume that the water droplet for measuring the contact angle only contacts the edges and air is present in the holes. The areas of the edge and the hole for the honeycomb film were then measured from the AFM images. The evaluated values of φ1 were 0.18 for PS-200 and 0.22 for PS-560. The dashed line in Figure 6 shows the calculated results of the Cassie-Baxter equation. The observed values for the FOT and ODT SAMs were consistent with the predicted values from the Cassie-Baxter equation. Thus, for the hydrophobic SAM system, air is present in a honeycomb hole. Although the observed values were close to the calculated values, they were slightly lower than the corresponding calculated values. This may be due to the fact that the air pockets in the holes are smaller; that is, the water droplet may be in contact with a portion of a hole of the honeycomb.6 (33) The actual surface areas (SAC) of honeycomb film are evaluated by the following. The radius of the pore mouth was evaluated from AFM images (for example, dark part of Figure 4c), and then the area of the pores was calculated from simple arithmetic. By assuming that the brims of the honeycomb film are flat, we calculated SAC, and then we obtained the roughness factor r ) SAC/SAP (SAP: apparent surface area).
Fabrication of Flexible Gold Films
Figure 7. Influence of PS particle diameter on the contact angle for water droplets, and the surface fraction of gold (φ1) on the honeycomb surface.
On the other hand, the contact angles of honeycomb surface without SAM were 54° and 56°, smaller than that for the flat surface. When θS ) 63° and r ) 1.2 are substituted in the Wenzel equation, we obtained θR ) 56°, which is almost equal to the observed values. Thus, it was proved that in the case of bare gold the wettability obeys the Wenzel equation and there is no air pocket in the holes. This result is different from the result previously reported by Abdelsalam et al.,6 which shows that the honeycomb surface of bare gold is hydrophobic because of the presence of air pockets in the holes. In the present process, the evaporated gold film is torn and the tips of the brims of gold come off as mentioned above, while in the reported process,6 the thickness of gold film is controlled by electrodeposition time. Thus, the contradiction between the present work and the previous work is probably caused by the difference in surface structure of gold film. Unfortunately, however, the details of the difference are unclear because it was difficult to check the detailed structure of gold film from the article by Abdelsalam et al. Relationship between the Particle Size and Hydrophobicity. Figure 7 shows the particle size dependence of the contact angles of the honeycomb film covered with FOT. If the cutting position of the Au film during the removal process of the PS particles is the same regardless of the size of the PS particle, the area ratio of the brim to the hole of the honeycomb Au/PVF film should be independent of the periodical array size of the honeycomb.6 Thus, we expected that the contact angles should be constant for
Langmuir, Vol. 22, No. 24, 2006 9961
various sized PS-particles. However, as can be seen in Figure 7, the contact angles increased with particle size in the range of 150-200 nm. This may be because the above assumption is incorrect. That is to say, the cutting position of the Au film probably varies with the particle size, because the tips of the brim of gold film (Figure 4a) for small particle may have a low tensile strength. We then evaluated φ1 for the different sizes of PS particles from AFM images and plotted these values. As shown in Figure 7, φ1, the area fraction of the flat part of the brims (Figure 4b), for small particles is larger than that for large particles. The profile of φ1 is inversely related to that of the contact angles (θR), as can be seen from eq 3. Thus, a larger φ1 resulted in smaller contact angles for small PS particles. However, when φ1 ) 0.24 (PS size: 186 nm) and θS ) 112° (Table 1) are substituted in eq 3, we obtain θR ) 148°, which is larger than the observed value of 137°. The disagreement may be attributed to the contact between water droplet and a portion of a hole of honeycomb gold film. Because for small particles the honeycomb gold film has shallow holes, water droplets for contact angle measurements penetrate easily into the holes. Thus, for small particles, air pockets in the holes are smaller and actual values of φ1 are probably much larger than the values evaluated from AFM images. Accordingly, smaller contact angles were observed for small particles.
IV. Conclusions In this paper, we have demonstrated the fabrication of flexible gold honeycomb structures supported with polymer layers by using a periodic latex monolayer. The present method can be extended to other metals, such as Pt, Cr, Ag, and Al, as well as organic molecules, pentacene, etc. The modification of the gold surface with thiol compounds brought about enhanced hydrophobicity or hydrophilicity. In particular, the contact angle of the honeycomb surface modified with FOT was more than 150°. Acknowledgment. This study was partially supported by a Grant-in-Aid for Scientific Research (No. 17510091) and by the “High-Tech Research Center” project for private Universities (2002-2006) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. LA061780D
