pubs.acs.org/Langmuir © 2009 American Chemical Society
Fabrication of Hierarchical Structures by Wetting Porous Templates with Polymer Microspheres Jiun-Tai Chen, Dian Chen, and Thomas P. Russell* Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003 Received January 29, 2009. Revised Manuscript Received February 16, 2009 We present a simple route to prepare hierarchical structures by allowing the partial diffusion of polymer microspheres into the nanopores of anodic aluminum oxide (AAO) templates. Polystyrene (PS) microspheres were first spread onto a silicon substrate and allowed to self-assemble into well-ordered monolayers of the microspheres. Upon heating above the glass-transition temperature, diffusion of the PS into the pores of the AAO occurred only at pores of the microspheres in contact with the membrane. After the removal of the AAO membrane, ordered arrays of microspheres capped with nanorods were produced, yielding surfaces with topographies spanning multiple length scales. Control over the nanoscopic and microscopic length scales can be trivially achieved by changing the size of the microspheres and the diameter of the pores in the membranes.
Introduction In recent years, the fabrication of hierarchical structures has been of great interest because of their unique properties and potential applications.1,2 If the synthesis of simple nanostructures and hierarchical architecture can be controlled, then rational methods for patterning and assembling nanoscaled building blocks need to be developed to generate advanced nanodevices.3 For example, inspired by lotus leaves, many studies have focused on the superhydrophobic behavior of surfaces that possess dual-sized roughness. Micro- and nanometer-scale hierarchical structures have been shown to be essential in generating the self-cleaning superhydrophobic properties of such surfaces.4 Various techniques have been developed for the generation of hierarchical structures.2,5-8 Among these techniques, nanosphere lithography, which uses self-assembled microspheres, is probably the most established one for patterning periodic nanoarrays. For example, Li et al. have decorated polystyrene microsphere monolayers with single-walled and multiwalled carbon nanotubes and have modified the surfaces with fluoroalkylsilane.7 A superhydrophobic surface was created, and a contact angle of 165 ° was observed. In addition, Wu et al. have used a dual latex-surfactant method to fabricate raspberry-like silica spheres.8 Structure-directing templates such as cationic cetyltrimethylammonium and polystyrene micropsheres were utilized in their studies, and hierarchical hollow silica spheres were formed. *To whom correspondence should be addressed. E-mail: russell@mail. pse.umass.edu. Phone: +1 (413) 577-1516. Fax: +1 (413) 577-1510. (1) Stein, A.; Li, F.; Denny, N. R. Chem. Mater. 2008, 20, 649–666. (2) Ikkala, O.; ten Brinke, G. Chem. Commun. 2004, 2131–2137. (3) Yu, T.; Varghese, B.; Shen, Z. X.; Lim, C. T.; Sow, C. H. Mater. Lett. 2008, 62, 389–393. (4) Feng, X. J.; Jiang, L. Adv. Mater. 2006, 18, 3063–3078. (5) Keizer, H. M.; Sijbesma, R. P. Chem. Soc. Rev. 2005, 34, 226–234. (6) Li, Y.; Cai, W.; Duan, G. Chem. Mater. 2008, 20, 615–624. (7) Li, Y.; Huang, X. J.; Heo, S. H.; Li, C. C.; Choi, Y. K.; Cai, W. P.; Cho, S. O. Langmuir 2007, 23, 2169–2174. (8) Wu, X. F.; Tian, Y. J.; Cui, Y. B.; Wei, L. Q.; Wang, Q.; Chen, Y. F. J. Phys. Chem. C 2007, 111, 9704–9708.
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Figure 1. Schematic procedure for the preparation of hierarchical structures by wetting anodic aluminum oxide membranes with polystyrene microspheres. Here, we used polystyrene microspheres to generate the first length scale of ordering. These microspheres self-assembled into ordered hexagonal arrays using various methods such as spin coating, dip coating, drop casting, and electrophoretic deposition.6 Microspheres of polystyrene or silica can be easily synthesized by different methods, and their size deviation is usually less than 5%.6 Several attempts have been made to achieve well-ordered microsphere monolayers for their applications as lithography masks.9,10 For monolayer fabrication, Langmuir-Blodgett (L-B) techniques and the thin liquid film method are considered to be two of the most effective methods.11,12 Anodic aluminum oxide (AAO) templates were then used to generate patterns on the microspheres, giving rise to a second length scale. AAO, produced electrochemically from aluminum, can be prepared with a range of pore diameters (5-267 nm) following the two-step anodization process established by Masuda and co-workers.13 AAO templates are (9) Deckman, H. W.; Dunsmuir, J. H. Appl. Phys. Lett. 1982, 41, 377–379. (10) Bullen, H. A.; Garrett, S. J. Nano Lett. 2002, 2, 739–745. (11) Goldenberg, L. M.; Wagner, J.; Stumpe, J.; Paulke, B. R.; Gornitz, E. Langmuir 2002, 18, 5627–5629. (12) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303–1311. (13) Masuda, H.; Fukuda, K. Science 1995, 268, 1466–1468.
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Figure 2. Self-assembled microsphere monolayers on a silicon wafer: (a) optical microscopy image of the microsphere monolayer and (b) SEM image of the microsphere monolayer. The diameter of the polystyrene microsphere is around 2 μm.
Figure 3. SEM images of synthesized AAO templates: (a) top view and (b) tilt view. The pore size is around 40 nm, and the pore length is around 100 μm. quite versatile14 and are widely used to generate metallic and semiconductor nanowires, carbon nanotube transistors,15,16 and polymeric nanorods and nanotubes.17-20 Stacked-disk or toroidal-type structures, for example, have been obtained inside the pores of the AAO templates because of the high degree of curvature and confinement-induced entropy loss.20 Although different hierarchical structures can be generated by various techniques, the ability to control the surface properties of these structures precisely has been relatively limited because of the poorly defined patterns on the surface at different length scales. To overcome this limitation, we present a novel approach to prepare hierarchical structures by wetting polymer microspheres on the surfaces of AAO templates. We combined the self-assembly of microspheres into a hexagonal array with the capillary force that draws polymers into nanopores. Polystyrene (PS) microspheres were first spread and self-assembled into well-ordered monolayers on a planar (14) Martin, C. R. Science 1994, 266, 1961–1966. (15) Davydov, D. N.; Sattari, P. A.; AlMawlawi, D.; Osika, A.; Haslett, T. L.; Moskovits, M. J. Appl. Phys. 1999, 86, 3983–3987. (16) Jeong, S. H.; Hwang, H. Y.; Lee, K. H.; Jeong, Y. Appl. Phys. Lett. 2001, 78, 2052–2054. (17) Steinhart, M.; Wendorff, J. H.; Greiner, A.; Wehrspohn, R. B.; Nielsch, K.; Schilling, J.; Choi, J.; Gosele, U. Science 2002, 296, 1997–1997. (18) Chen, J. T.; Zhang, M. F.; Russell, T. P. Nano Lett. 2007, 7, 183–187. (19) Zhang, M. F.; Dobriyal, P.; Chen, J. T.; Russell, T. P.; Olmo, J.; Merry, A. Nano Lett. 2006, 6, 1075–1079. (20) Shin, K.; Xiang, H. Q.; Moon, S. I.; Kim, T.; McCarthy, T. J.; Russell, T. P. Science 2004, 306, 76–76.
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silicon wafer. By bringing a porous AAO template into contact with the monolayer assembly, thermal annealing drew the PS into the nanopores of the AAO membrane, producing short nanorods on top of the microspheres.
Experimental Section Polystyrene microspheres were purchased from Polysciences, Inc. Different sizes (300 nm, 500 nm, 700 nm, 1 μm, and 2 μm) of microspheres with narrow size distributions were used. A 2.5 wt % solid (w/v) aqueous suspension was diluted with ethanol (1:1) before being spread onto the substrate. Silicon wafers were cleaned by O2 plasma etching. The AAO templates were synthesized according to the two-step anodization method developed by Masuda and co-workers.13,21 At first, a high-purity aluminum sheet (Sigma-Aldrich, 99.99%, 0.5 mm thick) was degreased in acetone and rinsed in an ethanol solution. Subsequently, the aluminum sheet was electropolished in a perchloric acid/ethanol mixture at 4 °C. The aluminum sheet was anodized at 40 V in 0.3 M oxalic acid at 17 °C for 12 h or 25 V in 0.3 M sulfuric acid at 4 °C for 12 h. After the resultant aluminum oxide film was chemically etched in a mixture with phosphochromic acid, a second anodization under the same conditions as for the first anodization was performed for 24 h. The resultant template contains regular and hexagonally packed pores. For the AAO templates prepared with 0.3 M oxalic acid at 40 V, the pore-to-pore distance is around 100 nm, and the (21) Masuda, H.; Satoh, M. Jpn. J. Appl. Phys. Part 2 - Lett. 1996, 35, L126–L129.
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Figure 4. (a) SEM image of a single microsphere on an AAO template. The diameter of the microsphere is 2 μm, and the pore size of the AAO template is 40 nm. (b, c) SEM images of the tilt view and top view of hierarchical microspheres, where the size of the microsphere is 2 μm and that of the nanorod is around 40 nm. (d) Top-view image of a microsphere that is 2 μm in diameter with nanorods of around 15 nm on the top of the microsphere. The annealing was all done at 120 °C for 2 h. pore size is around 50 nm after the pore-widening process using phosphoric acid (5 wt %) at 40 °C. The microscopic features of the samples were investigated using a JEOL 6320 model scanning electron microscope (SEM) at an accelerating voltage of 5 kV. Specimens were coated with gold to reduce charging. Scanning force microscopy (SFM) measurements were made using a Dimension 3100 Nanoscope III SFM (Digital Instrument) in tapping mode.
Results and Discussion Figure 1 shows the schematic illustration for the preparation of hierarchical structures by wetting polystyrene microspheres into the nanopores of AAO templates. A 2.5% (w/v) solid suspension of polystyrene microspheres in water that had been diluted with ethanol (1:1) was spread onto a cleaned silicon wafer. After drying at room temperature for 24 h, polystyrene microspheres were self-assembled to form monolayers. Subsequently, an AAO template with the desired pore diameter (15 or 40 nm) was placed on top of the self-assembled microspheres. This assembly was heated above the glasstransition temperature (Tg) of polystyrene (PS) (Tg = 105 °C) for different periods of time. The PS was drawn into the nanopores by capillary forces, forming nanorods whose length was governed by the time allowed for the PS to be drawn into the nanopores.19 After short nanorods were formed within the nanopores of the AAO templates, they were released by dissolving the alumina template with a 5% NaOH(aq) solution. Langmuir 2009, 25(8), 4331–4335
After the microsphere solution was spread onto the surface of a silicon wafer and dried at room temperature, the microspheres self-assembled to form monolayers or multilayers. Shown in Figure 2a is an optical micrograph of self-assembled polystyrene microsphere monolayer. As shown, the microspheres self-assembled into a 2D hexagonal array as a result of capillary forces arising from the evaporation of the solvents.22 As described in detail by Nagayama et al.,23,24 the attractive capillary forces between microspheres cause them to close pack into a hexagonal 2D array on solid supports or in thin films of liquids. Nagayama et al. also found that a nucleus consisting of many microspheres was first generated when the thickness of the liquid layer approached the diameter of the microspheres. Thus, convective transport draws the microspheres toward and organizes them around this nucleus as a result of capillary forces.23,24 To achieve a highly ordered monolayer of microspheres with large grain sizes, a clean, flat, and chemically homogeneous surface was used. As can be seen in Figure 2a, there were large grains (several tens of micrometers in size) produced with some defects using this preparation condition as a result of impurities and the distribution in the size of the microspheres. Nonetheless, a monolayer of (22) Xia, Y. N.; Gates, B.; Yin, Y. D.; Lu, Y. Adv. Mater. 2000, 12, 693–713. (23) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183–3190. (24) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Nature 1993, 361, 26–26.
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Figure 5. SEM images of hierarchical polymer structures from closely packed microspheres: (a) lower magnification of 2 μm microspheres
wetting 40 nm AAO templates, (b) higher magnification of 2 μm microspheres wetting 40 nm AAO templates, (c) lower magnification of 700 nm microspheres wetting 40 nm AAO templates, and (d) higher magnification of 700 nm microspheres wetting 40 nm AAO templates. The annealing was done at 120 °C for 2 h.
highly ordered arrays of hexagonally packed microspheres could be produced. Figure 2b shows the SEM image of a selfassembled microspheres monolayer. From the SEM, the local hexagonal packing of the microspheres is evident. If higher concentrations of microspheres were used, then multilayers, unsuitable for the studies here, were produced. Anodic aluminum oxide (AAO) templates were used to generate a second length scale of ordering on the selfassembled microspheres. The AAO membranes, consisting of a hexagonal array of nanopores in alumina, have pore densities of as high as 1011 pores/cm2 and pore lengths from 10 to 100 μm.25 Different pore diameters from 5 to 300 nm were produced by varying the composition and concentration of the acidic electrolyte solution and the temperature and voltage of the anodization.26 Figure 3a,b shows the SEM images of the top view and the side view of a synthesized AAO template that was anodized in oxalic acid at 40 V. This template contains hexagonally packed 40-nmdiameter pores. Unlike the commercially available membranes that have tapered pores and polydisperse pore diameters, the in-house-synthesized membranes have uniform pore diameters, as can be seen. Shown in Figure 4a is an SEM image of a 2 μm microsphere on an AAO template with 40-nm-diameter nanopores. (25) Almawlawi, D.; Coombs, N.; Moskovits, M. J. Appl. Phys. 1991, 70, 4421–4425. (26) Li, A. P.; Muller, F.; Birner, A.; Nielsch, K.; Gosele, U. J. Appl. Phys. 1998, 84, 6023–6026.
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When the microspheres were spread on the AAO template, a very limited area at the bottom of the microsphere was in contact with the template. When the sample was heated above the glass-transition temperature of the polystyrene, the bottoms of the microspheres in contact with the AAO membrane were initially drawn into the nanopores. With time, as the PS was drawn into the pores, more of the microsphere came into contact with the membrane, causing a distribution in the length of the nanorods produced in the AAO membrane. Upon heating the samples above the Tg of the PS, the PS is drawn into the pores of the AAO template by wetting the surface of the pore walls. The rate of the flow of polymer melt in the cylindrical nanopores can be estimated by27 dz Rγ cos θ ¼ dt 4ηz
ð1Þ
where z is the height, t is the time, R is the radius of the nanopores, γ is the surface tension, and η is the viscosity. Zhang et al. studied the wetting behavior of polystyrene melts using nanoporous alumina membranes as templates by systematically changing the annealing temperature and polymer molecular weight.19 Figure 4b-d shows the SEM images of microspheres having nanorods on their surfaces. Figure 4b,c contains several views of 2 μm microspheres having 40 nm (27) Kim, E.; Xia, Y. N.; Whitesides, G. M. Nature 1995, 376, 581–584.
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nanorods on their surfaces. AAO templates with smallerdiameter (15 nm) pores were also used. Figure 4d shows 2 μm PS microspheres with ∼15-nm-diameter nanorods on their surfaces. Because the diameters and the center-to-center distances of these nanorods are smaller, the density of nanorods on the surface is greater. Figure 5 shows SEM images of microsphere arrays with different microsphere sizes. The microspheres are closely packed, and hierarchical features can been seen. Figure 5a,b shows SEM images of 2-μm-diameter microspheres with 40 nm nanorods on their surfaces at lower and higher magnifications, respectively. The microspheres effectively sector or separate the arrays of nanorods. Figure 5c,d shows SEM images with lower and higher magnification, showing 0.7-μmdiameter microspheres with 40-nm-diameter nanorods on their surfaces. From these images, the two length scales of ordering are evident. One length is dictated by the size of the
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microspheres, and the second is dictated by the diameter of the nanopores in the AAO membranes.
Conclusions We have developed a novel method to fabricate hierarchical structures by placing self-assembled polystyrene microspheres in contact with the nanopores of AAO membranes. The ordering on two different size scales can be controlled by changing the size of the microsphere and the pore sizes of the AAO templates. Studies are currently underway to investigate the surface properties of these unique structures. Supporting Information Available: To examine the surface features of the nanorods on the microspheres further, scanning force microscopy (SFM) was also used. This material is available free of charge via the Internet at http://pubs. acs.org.
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