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Facile Method to Fabricate Raspberry-like Particulate Films for Superhydrophobic Surfaces Hui-Jung Tsai and Yuh-Lang Lee* Department of Chemical Engineering, National Cheng Kung UniVersity, Tainan 70101, Taiwan ReceiVed August 15, 2007. In Final Form: September 18, 2007 A facile method using layer-by-layer assembly of silica particles is proposed to prepare raspberry-like particulate films for the fabrication of superhydrophobic surfaces. Silica particles 0.5 µm in diameter were used to prepare a surface with a microscale roughness. Nanosized silica particles were then assembled on the particulate film to construct a finer structure on top of the coarse one. After surface modification with dodecyltrichlorosilane, the advancing and receding contact angles of water on the dual-sized structured surface were 169 and 165°, respectively. The scale ratio of the micro/nano surface structure and the regularity of the particulate films on the superhydrophobic surface performance are discussed.
Introduction In recent years, much effort has been devoted to the study of the superhydrophobic behavior of a surface, not only for fundamental research but also for practical applications in selfcleaning surfaces. A self-cleaning mechanism modeled after lotus leaves suggests that a combination of low surface energy materials and a specific surface topography based on dual-sized roughness are required to accomplish a superhydrophobic surface. Microand nanoscale hierarchical structures have proven to be vital in generating the self-cleaning superhydrophobic property of a surface.1,2 Various methods have been proposed to fabricate superhydrophobic surfaces mimicking the lotus leaf surface structure, including lithographic patterning,3,4 plasma etching,5 electrochemical deposition,6-10 chemical vapor deposition,11 sol-gel methods,12 layer-by-layer (LBL) assembly plus phase separation,13,14 utilization of templates, etc. Materials such as silicon,6,15 silica,16 polymers,17,18 polyelectrolyte/silica,14 poly* Corresponding author. Tel.: 886-6-2757575, ext. 62693; fax: 886-62344496; e-mail:
[email protected]. (1) Feng, X. J.; Jiang, L. AdV. Mater. 2006, 18, 3063. (2) Gorb, E.; Haas, K.; Henrich, A.; Enders, S.; Barbakadze, N.; Gorb, S. J. Exp. Biol. 2005, 208, 4561. (3) O ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (4) Fu¨rstner, R.; Barthlott, W.; Neinhuius, C.; Walzel, P. Langmuir 2005, 21, 956. (5) Jin, M. H.; Feng, X. J.; Xi, J. M.; Zhai, J.; Cho, K.; Feng, L.; Jiang, L. Macromol. Rapid Commun. 2005, 26, 1805. (6) Wang, M. F.; Raghunathan, N.; Ziaie, B. Langmuir 2007, 23, 2300. (7) Zhao, N.; Shi, F.; Wang, Z.; Zhang, X. Langmuir 2005, 21, 4713. (8) Jiang, Y.; Wang, Z.; Yu, X.; Shi, F.; Xu, H.; Zhang, X.; Smet, M.; Dehaen, W. Langmuir 2005, 21, 1986. (9) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338. (10) Shi, F.; Wang, Z.; Zhang, X. AdV. Mater. 2005, 8, 1005. (11) Huan, L.; Lin, F.; Jin, Z.; Lei, J.; Daoben, Z. Langmuir 2004, 20, 5659. (12) Ebril, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science (Washington, DC, U.S.) 2003, 299, 1377. (13) Lin, J. J.; Chu, C. C.; Chiang, M. L.; Tsai, W. C. AdV. Mater. 2006, 18, 3248. (14) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (15) Verplanck, N.; Galopin, E.; Camart, J. C.; Thomy, V.; Coffinier, Y.; Boukherroub, R. Nano Lett. 2007, 7, 813. (16) Zhang, G.; Wang, D.; Gu, Z. Z.; Mohwald, H. Langmuir 2005, 21, 9143. (17) Xie, Q.; Xu, J.; Feng, L.; Jiang, L.; Tang, W.; Luo, X.; Han, C. C. AdV. Mater. 2004, 16, 302. (18) Zhu, Y.; Zhang, J.; Zheng, Y.; Huang, Z.; Feng, L.; Jiang, L. AdV. Funct. Mater. 2006, 16, 568.
electrolyte/metal,7,8,19 carbon nanotubes,20-22 and raspberry-like particles23 have been utilized to prepare a surface with a doublestructured roughness. Among the various techniques employed to fabricate a superhydrophobic surface, LBL assembly is a simple and inexpensive method to prepare a micro/nano dual-scale surface structure. Using the LBL assembly technique, Rubner and coworkers14 prepared a honeycomb-like polyelectrolyte multilayer using a 100.5 bilayer of a poly(allylamine hydrochloride) (PAH)/ poly(acrylic acid) (PAA) film. A lotus leaf structure was mimicked by overcoating the microporous structure with silica nanoparticles. Zhang et al.7,8 fabricated a superhydrophobic surface by electrodeposition of silver or gold aggregates on a polyelectrolyte multilayer matrix. Sun et al.24 prepared a dualscale structure by LBL deposition of 5-bilayer poly(diallyldimethylammonium chloride) (PDDA)/sodium silicate on a silicasphere-coatedsubstrateanduseditforpreparingasuperhydrophobic surface. Other strategies using LBL assembly of polyelectrolyte/ nanoparticle hybrid films have also been reported in the literature.19,25,26 Particles of a single size19,25 or various sizes26 were employed in this approach. For the LBL assembly methods reported in the literature, the polyelectrolyte is an important material either as a microscale porous matrix or as a buffer layer to incorporate nanoparticles using electrostatic force. However, tens, or even hundreds, of polyelectrolyte bilayers are required, which would make fabricating a superhydrophobic surface time-consuming. Moreover, the electrostatic force of a polyelectrolyte layer is not strong enough to adsorb particles of microscale sizes. In another report, Ming et al.23 prepared a superhydrophobic film by synthesizing silica-based raspberry-like particles and (19) Zhang, X.; Feng, S.; Yu, X.; Liu, H.; Wang, Z.; Jiang, L.; Li, X. J. Am. Chem. Soc. 2004, 126, 3064. (20) Wang, Z.; Ci, L.; Chen, L.; Nayak, S.; Ajayan, P. M.; Koratkar, N. Nano Lett. 2007, 7, 697. (21) Liu, Y.; Tang, J.; Wang, R.; Lu, H.; Li, L.; Kong, Y.; Qi, K.; Xin, J. H. J. Mater. Chem. 2007, 17, 1071. (22) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701. (23) Ming, W.; Wu, D.; van Benthem, R.; de With, G. Nano Lett. 2005, 5, 2298. (24) Zhang, L.; Chen, H.; Sun, J.; Shen, J. Chem. Mater. 2007, 19, 948. (25) Han, J. T.; Zheng, Y.; Cho, J. H.; Xu, X.; Cho, K. J. Phys. Chem. B 2005, 109, 20773. (26) Bravo, J.; Zhai, L.; Wu, Z.; Cohen, R. E.; Rubner, M. F. Langmuir 2007, 23, 2723.
10.1021/la702521u CCC: $37.00 © 2007 American Chemical Society Published on Web 11/07/2007
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Scheme 1. Schematic Illustration of Procedure for Preparation of Raspberry-like Particulate Films
binding them on an epoxy-based polymer matrix. In their study, a complex procedure was required to prepare the raspberry-like particles, including the preparation of amino-functionalized nanoparticles and epoxy-functionalized microparticles, a reaction between the nano- and microparticles, and a long reaction and separation process. A film forming reaction was then required to assemble the particles on a substrate surface, which took another 20 h. In the present work, a simple and highly efficient method to prepare raspberry-like particulate films of silica using commercially bare silica particles is presented. As shown in Scheme 1, a glass substrate was surface modified with 3-aminopropyldiethoxymethyl silane (3-AMDS), rendering an amine-terminated surface. By treating with hydrochloric acid, the surface was positively charged and was able to adsorb negatively charged silica particles through electrostatic interactions. Large silica particles (0.5 µm in diameter) were assembled first on the protonated 3-AMDS modified glass surface to produce a surface with microscale roughness. After a subsequent modification of the particulate film with 3-AMDS and acid treatment, small silica particles (20, 35, 70, or 90 nm in diameter) were adsorbed on the particulate film to construct a finer structure on the coarse one, leading to a surface with a raspberry-like morphology. Heat treatment was then performed to remove possible amine groups left on the surface and to enhance the mechanical property of the particulate film. A superhydrophobic surface was obtained after modification of the surface with dodecyltrichlorosilane. Experimental Procedures The main component required to construct the dual-sized surface structure is silica particles. Commercially available silica particles, 500 nm in diameter (Lancaster, 99.9%), were used directly in this work. For the nanosized silica particles, the Sto¨ber et al. method27 was used to prepare silica particles with mean diameters of 20, 35, 70, or 90 nm. Optical glass was used as the substrate for preparing the particulate films. For the LBL assembly method, a glass plate was carefully cleaned and then surface modified by immersion in a 3-AMDS toluene solution (1 vol %) for 30 min. After protonization of the 3-AMDS modified substrate with 0.1 N hydrochloric acid, the positively charged glass plate was dipped into an isopropyl alcohol dispersion of 0.5 µm silica particles (1 mg/mL) for approximately 30 min. The assembling cycle can be repeated to increase the surface coverage and to incorporate the large silica particles. To incorporate the nanoscale silica particles, the as-prepared silica colloidal solution can be used directly without a further purification process. (27) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62.
The Langmuir-Blodgett (LB) technique was also used as an alternative method to assemble the particulate films of large silica particle (0.5 µm). The detailed process for LB deposition is contained in a previous paper.28,29 The silica particles were dispersed in chloroform in the presence of 1000 ppm octyltrimethylammonium bromide. An appropriate amount of the silica suspension was spread on a pure water subphase and compressed after solvent evaporation. The particulate monolayer at the air/water interface was transferred onto a glass plate by vertical deposition at a selected surface pressure of 10 mN/m. A glass plate was first immersed in the subphase before monolayer compression, and the particulate film was deposited in the upstroke direction using a transfer rate of 1 mm/min. A transfer ratio close to 1 was obtained, and only one layer of the LB particulate film was prepared and used in this study. Particulate films prepared using the LBL or LB assembling processes have weak interactions between particles and between particle and substrate. During the measurement of dynamic contact angles, the assembled particles may be peeled off from the substrate surface due to the movement of the three phase contact line. To solve this problem, the particulate films were heat treated at 500 °C for 1 h, not only to increase the mechanical endurance for contact angle measurements but also to degrade the possible organic contaminants left on the film surface. Finally, the particulate films were chemically modified by being dipped in 1 wt % dodecyltrichlorosilane in chloroform for 30 min. The surface morphologies of the particulate films were examined by scanning electron microscope supplied by JEOL (JSM-6700F). The surface wettability of the particulate films was expressed in terms of the advancing and receding contact angles of water on a film surface. The contact angles were measured by the Wilhelmy plate technique using a dynamic contact angle analyzer (Thermo Cahn, WinDCA 300).28,30 Doubly distilled water with a measured surface tension of 72 mN/m was used in this analysis. For each experimental condition, three specimens were analyzed, and the mean value was taken as the final result. The deviation of the measured contact angles between various specimens of the same condition was less than 3°.
Results and Discussion A surface with microscale roughness was prepared by assembling large silica particles (0.5 µm in diameter) on a protonated 3-AMDS modified glass plate. The adsorption rate and adsorbed number of silica particles on the substrate were found to be affected by the particle size and by the concentration of the particles dispersed in the solution. For the 500 nm silica, equilibrium adsorption was reached in 30 min, with the (28) Tsai, P. S.; Yang, Y. M.; Lee, Y. L. Langmuir 2006, 22, 5660. (29) Lee, Y. L.; Du, Z. C.; Lin, W. X.; Yang, Y. M. J. Colloid Interface Sci. 2006, 296, 233. (30) Lee, Y. L. Langmuir 1999, 15, 1796.
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Figure 1. SEM micrographs showing the surface morphology of large silica particulate films prepared using LBL assembling of 1 (a), 2 (b), and 3 (c) layers of 0.5 µm particles. The particulate film prepared using LB deposition is shown in panel d.
Figure 2. SEM micrographs showing the surface morphology of raspberry-like particulate films fabricated by assembling one layer of 35 nm silica particles on large silica particulate films prepared using LBL assembling of 1 (a), 2 (b), and 3 (c) layers of particles and using LB deposition (d).
morphology shown in Figure 1a. For the particle concentration controlled here (1 mg/mL), the substrate surface was only partially covered by the particles. All silica particles were adsorbed as a particulate monolayer, indicating that the incorporation is controlled by the electrostatic interaction between substrate and particle. Although the monolayer coverage can be increased by elevating the particle concentration in the solution, this strategy was not adopted on consideration of preparing a rougher surface. The as-prepared particulate film after the first assembling cycle was further treated by 3-AMDS and hydrochloric acid in sequence, followed by dipping in the particle solution to increase the
incorporated amount of silica particles. The surface morphology of the particulate films after two and three assembling cycles was examined by SEM and is shown in Figure 1b,c, respectively. Apparently, the surface coverage of the particles increased due to the increase of assembly cycles. A multilayer particulate structure can be observed in Figure 1b,c, indicating that a silica particle can adsorb on the uncovered substrate or on top of the particulate layer, which is believed to be advantageous for constructing a rougher surface. For the present system, the substrate surface was covered completely by the silica particles after three assembly cycles (Figure 1c). To compare properties of particulate films with ordered and non-ordered arrangements,
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a particulate film was also prepared by the LB technique using the procedure described in a previous paper.28,29 The surface morphology of the particulate film prepared by the LB method (Figure 1d) exhibits ordered and closely packed domains, which is contrary to the morphology prepared by the LBL assembly technique. The results shown in Figure 1 clearly demonstrate that a rough surface with a microscale structure, regular or irregular, can be generated by assembling particles on a substrate and that the scale of the roughness can be controlled by the size of the assembled particles. Moreover, it is also possible to prepare a dual-sized surface structure by using particles of different sizes in succeeding assembly cycles. This strategy was performed by assembling nanoscale silica particles (35 nm in mean diameter) on the microscale particulate films. The surface morphology after one assembly cycle is shown in Figure 2. The nanosized particles uniformly covered the particulate films. For the substrate partially covered by the large SiO2 particles, the small particles adsorbed both on the uncovered substrate and on the surface of the large particles (Figure 2a,b). For the particulate film with a multilayer structure (Figure 1c), not only the outmost layer but also the inner layer(s) can be decorated by the nanosized SiO2 particles, as is clearly demonstrated in Figure 2c. For the particulate film prepared using the LB technique, an ordered array of raspberry-like particles was fabricated using the present strategy (Figure 2d). Apparently, a raspberry-like particulate film with hierarchical micro- and nanostructures was prepared after incorporation of the small particles on a large-scale particulate film, no matter whether the large-scale film was prepared using the LBL assembly or the LB method. Adsorption of the small particles occurred very quickly, reaching an equilibrium adsorption within 5 min, with only one assembly cycle being required to obtain a well-covered state. To enhance the mechanical endurance of the particulate films, the films were heat treated at 500 °C for 1 h. After this treatment, a lengthy immersion of the films in water could not remove the particles from the substrate. Meanwhile, possible organic contaminants, including surface modifying agents, can be degraded during the heat treatment, leaving a bare silica surface for the following surface hydrophobization. It is noteworthy that this strategy cannot only be applied for particles of identical material but also for particles of different materials between succeeding layers. At the present stage, a hierarchically structured surface comprised of raspberry-like silica particles was successively fabricated. To give the surface a superhydrophobic property, the films were surface modified with dodecyltrichlorosilane. The surface wettability was evaluated by measuring the advancing and receding contact angles of water on the particulate films using a dynamic contact angle analyzer. Figure 3a shows the advancing and receding contact angles measured from the particulate films consisting of only 500 nm SiO2 particles. The hydrophobilized glass substrate had advancing (θa) and receding (θr) contact angles of 110 and 89°, respectively. After assembling the large SiO2 particles, both advancing and receding contacts angles increased with assembly cycles, which can be attributed to the increase of surface roughness due to the coverage of particles. The superhydrophobic character (θa > 150°) was observed after two LBL assembly cycles. However, the receding contact angle is still small (less than 115°), indicating a high contact angle hysteresis for a single-sized surface structure. It is noteworthy that the contact angles measured for the ordered LB particulate film (θa ) 149° and θr ) 100°) were smaller than those measured for the particulate film prepared
Tsai and Lee
Figure 3. Advancing and receding contact angles of water measured from the microscale particulate films prepared by 0.5 µm silica particles (a) and from dual-scale raspberry-like particulate films fabricated by assembling 35 nm silica particles on large silica particulate films (b).
using the LBL assembly (two or three cycles) (θa ) 165° and θr ) 115°). This fact implies that an ordered structure is not a dominant factor in determining the hydrophobic character of a surface. When the nanoparticles were introduced on the large-scale particulate film, a significant increase of the receding contact angle was observed, as well as a slight increase of the advancing contact angle (Figure 3b). For the particulate film with three assembly cycles of 500 nm SiO2 and one layer of 35 nm SiO2, the advancing and receding contact angles were 169 and 165°, respectively. Apparently, a superhydrophobic surface with a high contact angle and very small contact angle hysteresis was prepared, ascribed to the two-level surface structure of the raspberry-like particulate film. Figure 3 also shows that the dualscale morphology is especially important in obtaining a high receding contact angle. For the single-scale structure (Figure 3a) or a poorly covered dual-scale structure (Figure 3b), the receding contact angle cannot attain a superhydrophobic character, although an advancing contact angle as high as 160° can be accomplished. For the regular raspberry-like particulate film prepared by coupling LB and LBL techniques, the advancing and receding contact angles were 162 and 131°, respectively. The small receding contact angle indicates once again that the ordered microscale structure prepared using the LB technique is not rough enough for fabricating a superhydropholic surface.
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Figure 4. SEM micrographs showing the surface morphology of the particulate films with a dual-scale surface structure prepared by assembling 90 (a), 70 (b), 20 (c), and mixed 35/70 nm (d) silica nanoparticles on 3 layers of large silica particulate films.
Two well-known models, the Wenzel model31 and the Cassie and Baxter model,32 are commonly used to describe the effect of surface roughness on surface hydrophobicity. In Wenzel’s model, the contact angle of a liquid droplet on a rough surface (θ′) is related to the roughness factor (r) of the surface and the contact angle on a smooth surface (θ) of the same material
cos θ′ ) r cos θ
(1)
Wenzel’s model describes a roughness regime in which the water penetrates into the surface cavity and the hydrophobicity of a rough surface is augmented by the increase of the solidliquid contact area due to the roughness. As the roughness factor increases in the Wenzel regime, the water advancing contact angle increases, but the receding contact angle remains small, leading to a high contact angle hysteresis.33,34 As the roughness factor increases to a critical value, the water cannot penetrate into the cavity anymore, and air is trapped in the hollows between the water droplet and the rough surface. In such a case, Cassie and Baxter’s model can be used to describe the contact angle (θ′) on the composite surface comprised of solid-liquid and airliquid interfaces. When the contact angle between water and air interface is taken to be 180°, Cassie and Baxter’s equation can be expressed as
cos θ′ ) f1 cos θ - f2
(2)
where f1 and f2 represent the surface area fractions of solidliquid and air-liquid contact areas, respectively. According to Johnson and Dettre’s simulation for an idealized sinusoidal surface,33,34 the receding contact angle increases dramatically with an increase of roughness factor in the Cassie and Baxter regime. For particulate films containing only large particles (Figure 3a), although an advancing contact angle higher than 160° can be obtained by increasing the assembling layer, the receding (31) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (32) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (33) Johnson, R. E., Jr.; Dettre, R. H. AdV. Chem. Ser. 1963, 43, 112. (34) Johnson, R. E., Jr.; Dettre, R. H. In Surface and Colloid Science; Matijevic, E., Ed.; Wiley: New York, 1969; Vol. 2, pp 85-153.
contact angle is still small (less than 115°). This result indicates that the surface roughness factors of these particulate films are at a level where the Wenzel regime dominates. In these films, the water droplet will penetrate into the surface cavity when the surface is comprised of only a microscale structure. After introducing the nanoparticles on the microscale particulate films, the significant increase of the receding contact angles shows that the roughness factor is increased over a critical level and that the dominant hydrophobicity mode switches from the Wenzel to the Cassie and Baxter regime. That is, the penetration of a water droplet into the surface cavity is inhibited due to the incorporation of a finer structure on a coarser one. The increasing receding contact angle with the increase of LBL cycles is ascribed to the increase of the roughness factor. Although we have demonstrated that a dual-sized structure for a superhydrophobic surface can be fabricated by assembling 35 nm SiO2 on a 500 nm SiO2 particulate film, it is also interesting to know the performance of other particulate films composed of different particle pairs. Using the present strategy, nanosilica particles of various sizes (90, 70, and 20 nm) were also used to construct finer structures by being assembled on 500 nm SiO2 particulate films. The surface morphology of these particulate films, shown in Figure 4, demonstrates that dual-scale surface structures can also be constructed using these fine particles. However, the film morphology departs from the raspberry-like structure for larger-sized fine particles (90 and 70 nm). The corresponding performance of these particulate films in contact angle measurements is shown in Figure 5. It was found that a low contact angle hysteresis can only be accomplished by using a particle size smaller than 35 nm. When larger nanoparticles were used to construct the fine structure, the receding contact angle decreased to 143° for 70 nm particles and 135° for 90 nm particles. It was also found that the receding contact angle cannot be increased by increasing the coverage ratio of the fine particles using multiple assembly cycles. Therefore, the low contact angle is attributed to the particle size effect but not to a lower coverage ratio of fine particles (as shown in Figure 4a,b). Although the value of the receding contact angle is located in the Cassie and Baxter dominant regime, the small receding contact angle
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film. A more irregularly fine structure is created due to the coassembling of the differently sized particles. It is noteworthy that the smaller particles adsorb more easily than the larger ones, which can be attributed to the lower adhesion force required for a smaller particle to conquer its gravitational force. However, it is possible to tune the area ratio of the two particles by controlling the density ratio of the two particles in the colloidal solution. The advancing and receding contact angles measured for this particulate film were 165 and 159°, respectively, which also shows high performance in superhydrophobicity.
Conclusion
Figure 5. Advancing and receding contact angles of water measured from the particulate films with dual-scale surface structure prepared by assembling various sizes of silica nanoparticles on 3 LBL layers of large silica particulate films.
indicates that the roughness factor of the particulate films prepared using larger nanoparticles is not high enough to obtain a superhydrophobic surface with low contact angle hysteresis. Using the present LBL method, it is also possible to construct a fine structure using particles of mixed sizes. Figure 4c shows the morphology of a film prepared by co-adsorption of mixed 35 nm/90 nm silica colloidal particles onto a 500 nm particulate
In summary, a raspberry-like particulate film with a controlled two-level surface structure was prepared using LBL assembly of silica particles of different sizes. The dual-sized structure is able to achieve a superhydrophobic property with advancing and receding contact angles of 169 and 165°, respectively. The present strategy for preparing the raspberry-like particulate films offers the advantage of simple fabrication, easy availability of materials, and large-scale area production, which makes it suitable for a variety of superhydrophobic applications. Acknowledgment. The support of this research by the National Science Council of Taiwan through Grant NSC-952221-E-006-324 is acknowledged. LA702521U
