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Colloidal Crystallization Induced by Capillary Force Hailin Cong and Weixiao Cao* College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, People’s Republic of China Received March 14, 2003. In Final Form: June 29, 2003 Two array fashions (hexagonal and square) of P(St-MMA-AA) monodispersed copolymer latex particles and two packing modes (hexagonal and face centered cubic) of the colloidal crystals formed by deposition of the particles on an inclined substrate were studied. The scanning electron microscopic observation for the formed crystals verified that the square array in the formation of colloidal crystals is closely related to the temperature and surface tension of the liquid in which the colloidal crystal forms. The influence of the conditions such as temperature and surfactant on the array fashion of the particles as well as on the packing modes of the crystals has been investigated.
Introduction Three-dimensional order structures from monodispersed spheres (particles), both organic and inorganic, have attracted a great deal of attention.1-10 The ordered array of the colloidal particles represents an important and interesting approach to fabricate colloidal crystals.11-17 The most important use of the colloidal crystals is considered to be as a template to prepare macroporous and photonic materials.18-25 In order to prepare a highquality template it is important to understand the array fashion of the spheres and the packing mode of the crystals. Although in recent years many papers have been published to report studies on colloidal crystals as templates for preparing macroporous and photonic ma* To whom correspondence should be addressed. E-mail: wxcao@ pku.edu.cn. (1) Goldenberg, L. M.; Jung, B. D.; Wagner, J.; Stumpe, J.; Paulke, B. R.; Gornitz, E. Langmuir 2003, 19, 205. (2) Velikov, K. P.; Christova, C. G.; Dullens, R. P. A.; Blaaderen, A. Science 2002, 296, 106. (3) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453. (4) Guo, Q.; Aronoux, C.; Palmer, R. E. Langmuir 2001, 17, 7150. (5) Zeng, F.; Sun, Z.; Wang, C.; Ren, B.; Liu, X.; Tong, Z. Langmuir 2002, 18, 9116. (6) Goldenberg, L. M.; Wagner, J.; Stumpe, J.; Paulke, B.; Gornitz, E. Langmuir 2002, 18, 3319. (7) Reculusa, S.; Ravaine, S. Chem. Mater. 2003, 15, 598. (8) Gu, Z.; Fujishima, A.; Sato, O. Chem. Mater. 2002, 14, 760. (9) Sakamoto, J. S.; Dunn, B. J. Mater. Chem. 2002, 12, 2859. (10) Duffel, B.; Ras, R. H. A.; Schryver, F. C. D.; Schoonheydt, R. A. J. Mater. Chem. 2001, 11, 3333. (11) Liang, Z.; Susha, A. S.; Caruso, F. Adv. Mater. 2002, 14, 1160. (12) Cardoso, A. H.; Leite, C. A. P.; Zaniquelli, M. E. D.; Galembeck, F. Colloids Surf., A 1998, 144, 207. (13) Bardosova, M.; Tredgold, R. H. J. Mater. Chem. 2002, 12, 2835. (14) Vlasov, Y. A.; Bo, X.; Sturm, J. C.; Norris, D. J. Nature 2001, 414, 289. (15) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (16) Yin, Y.; Li, Z.; Xia, Y. Langmuir 2003, 19, 622. (17) Romanov, S. G.; Maka, T.; Sotomayor Torres, C. M.; Muller, M.; Zentel, R. Synth. Met. 2001, 116, 475. (18) Velev, O. D.; Lenhoff, A. M.; Kaler, E. W. Science 2000, 287, 2240. (19) Yi, G.; Moon, J. H.; Yang, S. Chem. Mater. 2001, 13, 2613. (20) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006. (21) Meng, Q. B.; Fu, C. H.; Einaga, Y.; Gu, Z. Z.; Fujishima, A.; Sato, O. Chem. Mater. 2002, 14, 83. (22) Jaannopoulos, J. D. Nature 2001, 414, 257. (23) Wijnhoven, J. E. G. J.; Vos, W. L. Science 1998, 281, 802. (24) Schroden, R. C.; Daous, M. A.; Blanford, C. F.; Stein, A. Chem. Mater. 2002, 14, 3305. (25) Harkins, P.; Eustace, D.; Gallagher, J.; McComb, D. W. J. J. Mater. Chem. 2002, 12, 1247.
terials, investigations related to the array fashion of the particles and packing mode of the crystal are very limited. Dushkin et al.26,27 studied the effect of water evaporation rate, liquid meniscus at the boundary, particle size, etc. on circular-shaped crystals formed from a thin layer of a latex suspension. The authors mentioned that a hexagonal lattice from monodispersed colloids prevails in the formed crystal, but a square lattice can be observed in the transition regions between various hexagonal lattices. Also, the formation of square array is favorable by addition of glucose, which decreases the particle movement. Usually in most colloidal crystals the monodispersed spheres prefer to adopt face centered cubic (fcc) packing as compared to hexagonal packing.14-17 A large area of fcc packing of poly(methyl methacrylate) colloids on mercury surface has been recently reported by Zentel et al.28 In this article a series of organic colloidal crystals was prepared by deposition of monodispersed P(St-MMAAA) copolymer latex particles on an inclined substrate under different conditions. The effects of temperature and surfactant on the array fashions of the particles as well as the packing modes of the crystal have also been investigated. Experimental Section Synthesis of Monodispersed P(St-MMA-AA) Colloidal Particles. Styrene (St), methyl methacrylate (MMA), and acrylic acid (AA) were distilled before use. Ammonium persulfate and ammonium bicarbonate were chemical grade reagents and used as received. The monodispersed P(St-MMA-AA) colloidal particles were synthesized according to a literature method29 with some modifications. A typical sample was prepared as follows: 120 mL of aqueous solution (A), containing 0.4 g of (NH4)2S2O8 and 0.8 g of NH4HCO3 in a funnel, and 25 mL of monomer mixture (B), consisting of St/MMA/AA (90:5:5 v/v/v) in another funnel, were added at the same time into a 250 mL flask. The mixture was stirred at 70 °C in N2 atmosphere for 5 h to obtain a homogeneous latex with particle diameter of ∼380 nm. The latex particles were almost monodisperse. (26) Dushkin, C. D.; Yoshimura, H.; Nagayama, K. Chem. Phys. Lett. 1993, 204, 455. (27) Dushkin, C. D.; Lazarov, G. S.; Kotsev, S. N.; Yoshimura, H.; Nagayama, K. Colloid Polym. Sci. 1999, 277, 914. (28) Griesebock, B.; Egen, M.; Zentel, R. Chem. Mater. 2002, 14, 4023. (29) Sakota, K.; Okaya, T. J. Appl. Polym. Sci. 1977, 21, 1035. (30) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (31) North, J.; Neyndorff, H.; King, D.; Levy, J. G. Blood Cells 1992, 18, 129.
10.1021/la0344480 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/20/2003
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Cong and Cao Scheme 1. Illustration of the Formation of Ordered Array of the Colloidal Particles in Deposition Processa
Figure 1. Illustration of the inclined deposition method to make stairlike colloidal crystals. Preparation of Colloidal Crystal and Characterization. The colloidal crystals were prepared by deposition of the colloidal spheres on an inclined substrate. A normal procedure is depicted as follows: a silicon wafer was pretreated at 70-75 °C in a H2SO4/H2O2 (7:3 v/v) mixture for 30 min to create a clean and hydrophilic surface. The pretreated wafer with 45° incline was then dipped into a 10 mL vessel containing 8 mL of P(St-MMAAA) latex, which had been diluted 50 times by deionized water. The colloidal crystallization induced by the capillary force takes place as the water is vaporized.30,31 In this process the particles will be arranged in an orderly fashion on the wafer surface to form a multilayer colloidal crystal. It is a simpler and easier approach to create a three-dimensional periodic structure from submicrometer spheres in organic solvents or in water.27 As shown in Figure 1, the liquid meniscus along the wafer surface descends as the water is vaporized naturally at ambient temperature, and the particles deposit onto the wafer surface step by step spontaneously to create a continuously increasing thickness of the crystal from top to bottom. After deposition for 7-8 days (∼25 °C), a P(St-MMA-AA) colloidal crystal (∼2 cm2) with one particle layer on top and ∼35 layers on bottom was acquired on a silicon wafer (10 mm × 30 mm × 1 mm). A scanning electron microscope (SEM; HITACHI S-4200, Japan) was used to observe the structures and morphologies of the colloidal crystals. At least five times observations were performed on a 40 µm × 50 µm area, which was selected arbitrarily on the sample surface. Then the representative structure or morphology was determined as an image.
a Left: alignment motion along with the meniscus of the liquid. Right: order array of the particles along the surface of the substrate, driven by capillary force and affected by the rate of water evaporation, which was determined by the temperature.
Results and Discussion Stairlike Colloidal Crystals. Figure 2 shows the SEM section profiles of the formed stairlike colloidal crystal along the wafer. We can see there is only one layer at the top of the wafer (Figure 2a), and then the number of layers increases gradually from two-seven layers (Figure 2b,c) to more than 35 layers at the bottom of the wafer (Figure 2d). This means that the deposition of the colloidal spheres on the inclined substrate is a convenient way to obtain the stairlike colloidal crystal. Array Fashions of the Colloidal Particles. Figure 3 provides SEM planar profiles of the formed colloidal crystals. It shows that two different array fashions (hexagonal and square arrays) can be observed, which are similar to the results reported by Dushkin et al. for
Figure 2. SEM images of the stairlike colloidal crystal formed along the wafer at ∼25 °C: (a) 1 layer; (b) 2-3 layers; (c) 4-7 layers; (d) 34-36 layers.
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Figure 3. SEM images of hexagonal and square arrays in the formed colloidal crystals. (a, c) Hexagonal array. (b, d) Square array.
Figure 4. Square array observed in the colloidal crystals by SEM: (a) at the boundary of two hexagonal layers; (b) inside a hexagonal layer; (c and d) with a relatively larger area.
a colloidal crystal from polystyrene latex.26 The square array (Figure 3b) has more interspaces than the hexagonal array (Figure 3a). Figure 3c,d, we can see that the upper layer of the crystal possesses the same array as that of the layer just below it. That is, in the formation of colloidal crystal, the array fashion of the colloids has transitivity, and the upper layer is always the replica of the underside layer. Figure 4 displays SEM micrographs showing the square arrays existing in the boundary of two hexagonal layers (Figure 4a) and inside the hexagonal array layer (Figure 4b), and even detected in a relatively larger area (Figure 4c,d). Influence of the Conditions on the Square Array. It was confirmed that the array fashion will be obviously influenced by temperature. Figure 5 presents the SEM images showing the effect of temperature on the array
fashion. The proportion of the square array over the entire surface of the four colloidal crystals made at different temperatures (0-75 °C) decreases from ∼40% (deposited at 0 °C) and ∼20% (deposited at 25 °C) to ∼10% (deposited at 50 °C) and ∼0% (deposited at 75 °C). This shows that a lower deposition temperature is favorable to forming a square array. A suitable temperature such as 25 °C is preferable to arrange the colloids into a hexagonal array (Figure 5b), which is more thermodynamically stable and is always an overwhelming array in the formation of the colloidal crystals at room temperature. At a lower temperature, the free motion of the particles was restrained, and the square array induced by dislocations of the particles can be produced in a larger area (Figure 5a). At a higher temperature, the order of the array will be disturbed due to the quick evaporation of water and free motion of the
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Figure 5. Effect of temperature on the square array in formed colloidal crystals: (a) 0 °C; (b) 25 °C; (c) 50 °C; (d) 75 °C.
Figure 6. SEM images of colloidal crystals formed in the presence of SDS (0.1 mg‚mL-1) at 25 °C. (a) Normal profile. (b) Large area of square array.
Figure 7. Hexagonal and face centered cubic packing in the colloidal crystals. Insets show a schematic of two packing modes.
particles, resulting in the formation of disordered arrays (Figure 5c,d). The alignment motion of the particles on the substrate surface was mainly determined by the evaporation rate of water, free motion of the particles, and magnitude of the capillary force. It can be schematically illustrated in Scheme 1. The magnitude of the capillary force is related closely to the surface tension of the liquid: a higher capillary force will be favorable to arrange the particles closely to produce a less interspaced hexagonal array. This was supported by the fact that the addition of surfactants
affects the array fashion and is favorable to form a square array owing to the decreasing surface tension of the liquid. The SEM images of Figure 6 show the influence of sodium dodecyl sulfate (SDS, 0.1 mg‚mL-1 in the diluted latex) on the array fashion of the colloids. The proportion of the square array of the colloidal crystal formed in the presence of SDS increases to ∼30% (Figure 6a) as compared to that formed in the absence of SDS (∼20%, Figure 5b), and a larger area of square array can be observed (Figure 6b). Packing Modes of Colloidal Crystals. The SEM images of Figure 7 show the cross-sectional profiles of the
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formed colloidal crystals having two packing modes: hexagonal packing (Figure 7a) and face centered cubic packing (Figure 7b). Different array fashions will form different packing modes. From Figure 7a, we can see the hexagonal packing is fabricated only by the hexagonal array, and no square array is found. In Figure 7b, we can see the square array act as (100) and the hexagonal array act as (111) crystal planes in the face centered cubic (fcc) packing. Conclusions Colloidal crystals from monodispersed P(St-MMA-AA) copolymer latex particles were prepared by deposition of the particles on an inclined substrate. From SEM ex-
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aminations, two array fashions, i.e., hexagonal and square arrays of the colloids, and two packing modes, i.e., hexagonal and face centered cubic packing of the crystal, were observed and studied. The influences of the experimental conditions on the array fashion were also investigated and verified that the deposition temperature and surface tension of the latex solution, in which the colloidal crystal forms, play important roles in determining the structure of the formed colloidal crystals. Acknowledgment. The authors are grateful to the NSFC for financial support of this work (Grant 20274002). LA0344480
