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Articles Fabrication of Inverse Opal via Ordered Highly Charged Colloidal Spheres Fang Zeng,* Zaiwu Sun, Chaoyang Wang, Biye Ren, Xinxing Liu, and Zhen Tong Research Institute of Materials Science, South China University of Technology, Guangzhou 510640, People’s Republic of China Received February 4, 2002. In Final Form: August 19, 2002 A new and simple procedure for producing highly ordered three-dimensional colloidal crystalline template was developed, which employs latexes containing highly charged polystyrene spheres. The water on the surface of the latex was evaporated, causing the formation of close-packed colloidal crystals on the surface. The driving forces for the formation of close-packed crystal involve electrostatic interaction and lateral capillary force, and the ion diffusion during the evaporation is considered to be an important factor for the successful assembly of ordered structure on the latex surface. Then the sol-gel solution was infiltrated into the interstitial space of the prepared colloidal crystals through capillary effect, and ordered inverse opals were obtained by calcining the polystyrene beads. The inverse opal thus prepared has long-range ordering with its defect-free area extending to over 10 micrometers and covering thousands of pores.
1. Introduction Submicrometer and nanometer pores with long-ranged ordering, known as inverse opal, have a variety of potential applications in electro- and nonlinear optics, optical and magnetic information processing and storage, catalyst and sensors.1-4 The most exciting potential application of this structured material is the possibility to obtain 3D photonic crystals, which are periodic on an optical length scale and would allow us to inhibit unwanted spontaneous emission and manipulate the flow of light.5-7 One approach to fabricate the inverse opal is to use the ordered submicrometer colloidal spheressthe colloidal crystalssas the template and fill the interstitial spaces between the spheres with another materials and then selectively remove the spheres by chemical etching or calcination.8-10 Recently, significant progresses have been achieved in the selection of materials for fabricating inverse opals that may exhibit complete band gap, and a number of methods have been proved to be successful in filling the colloidal template with high refractive index phase, like sol-gel, electrodeposition, electroless deposition and chemical vapor deposition.11 However, one challenge in this field for materials science is the fabrica* To whom correspondence should be addressed. (1) Joannopoulos, J. D.; Villeneuve, P. R.; Fan, S. Nature 1997, 386, 143. (2) Velev, O. D.; Lenhoff, A. M. Curr. Opin. Colloid Interface Sci. 2000, 5, 56. (3) Temelkuran, B.; Ozbay, E. Appl. Phys. Lett. 1999, 74, 486. (4) Painter, O.; Lee, R. K.; Scherer, A.; Yariv, A.; O’Brien, J. D.; Dapkus, P. D.; Kim, I. Science 1999, 284, 1819. (5) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. (6) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals Molding the Flow of Light; Princeton University Press: Princeton, NJ, 1995. (7) Krauss, T. F.; De La Rue, R. M. Prog. Quantum Electron. 1999, 23, 51. (8) Velev, O. D.; Jede, T. A.; Lobo, R. F.; Lenhoff, A. M. Nature 1997, 389, 447. (9) Park, S. H.; Xia, Y. Chem. Mater. 1998, 10, 1745. (10) Holland, B. T.; Blanford, C. F.; Stein, A. Science 1998, 281, 538. (11) Norris, D. J.; Vlasov, Y. A. Adv. Mater. 2001, 13, 371.
tion of such 3D structures with simpler procedures and at reasonable cost. The assembly of solvent-free colloidal crystals with highly ordered structure is the basis for obtaining 3D inverse opal. For preparing solvent-free 3D colloidal crystals from meso- or nanoscale particles, several approaches such as solvent evaporation, sedimentation, physical confinement, filtration and pressing have all been proved to be feasible.2, 12-15 Assembly based on electrostatic interactions can produce 3D colloidal arrays with ordered structure over larger area, but the colloidal particles are usually separated by a certain distance due to electrostatic repulsive force. Concerning the preparation of colloidal crystals based on electrostatic repulsive interaction, previous studies16,17 indicate that the colloid with charged monodisperse particles will undergo phase transition between amorphous liquidlike structure and the ordered colloidal crystalline. The crystallization from amorphous structure into crystalline can be induced either by an decrease in stray ion (electrolyte) concentration, an increase in particle concentration or a decrease in temperature,16 or in other words, as specified by Robins and Grest,17 by increasing the volume fraction of colloidal spheres or increasing the electrostatic screening length (or Debye-Hu¨ckel length κ-1, a parameter that scales inversely with the square root of the ionic strength of the latex and measures the distance over which the repulsive Coulomb potential is canceled by the screen effect of the counterions). In this paper, we report an alternative and simple method for fabricating close-packed 3D colloidal crystals (12) Park, S. H.; Xia, Y. Langmuir 1999, 15, 266. (13) Park, S. H.; Qin, D.; Xia, Y. Adv. Mater. 1998, 10, 1028. (14) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (15) Donselaar, L. N.; Philipse, A. P.; Suurmond, J. Langmuir 1997, 13, 6018. (16) Young, D. A. Phase Diagrams of the Elements; University of California Press: Berkeley, CA, 1991. (17) Robbins, M. O.; Grest, G. S. J. Chem. Phys. 1988, 88, 3286.
10.1021/la020114j CCC: $22.00 © 2002 American Chemical Society Published on Web 11/01/2002
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based on above principle by using the latex containing highly charged monodisperse polystyrene spheres. In the study, we added a given amount of electrolytes into the latex to adjust the ion strength instead of removing the stray ions from the latex, then evaporated the water on the surface of the latex, and finally we obtained solventfree crystals. Based on the prepared colloidal crystal template, we also prepared the ordered inverse opals with their defect-free areas containing thousands of building blocks. 2. Experimental Section Materials. Styrene (Aldrich) was distilled under reduced pressure of nitrogen at 40 °C. Sodium styrene sulfonate (NaSS), potassium persulfate, and sodium bisulfite were all obtained from Aldrich and were used as received. Acetone, ethanol, hydrochloric acid, tertraethyl orthosilicate (TEOS), and sodium bicarbonate were commercial analytical reagents and used as received. The water was double-distilled and deionized with a Millipore water purification device and was used throughout. Colloid Preparation. The emulsifier-free polymerization was used to prepare monodisperse, highly charged poly(styrene/ sodium styrenesulfonate) latex at 40 °C with potassium persulfate/sodium bisulfite as initiators. We followed the procedure of Kim et al.18 for the emulsifier-free polymerization except that the water/acetone mixture was used as the reaction medium to reduce the polydispersity of the particle diameter of the final colloidal particles. The copolymerization of styrene and sodium styrenesulfonate was carried out in a 250-cm3 four-neck flask equipped with reflux condenser, nitrogen inlet, Teflon stirrer, and thermometer. The reactor was immersed in a water bath at 42 °C and charged with 15.0 g of styrene, 30.0 g of acetone, and 80.0 g of deionized water, and after 20 min of stirring and flushing with nitrogen, 0.070 g of NaSS, 0.124 g of K2S2O8, and 0.023 g of NaHSO3 were dissolved into 20 g of water and added into the reactor to initiate the reaction. The whole reaction lasted 12 h before the latex was used for colloidal crystal growth. The surface charge densities of the latex particles were determined by conductometric titration after the latexes were cleaned with the procedure proposed by de las Nieves et al.,19 and the particle size and polydispersity were measured on a JEM-100CXII transmission electron microscopy. The particle size was determined by measurement of about 200 particles. The polydispersity of the particles was defined by the uniformity ratio U:18
U ) Dw/Dn where Dw is the weight-average diameter of latex particles and Dn is the number-average diameter. Colloidal Crystal Fabrication and SEM Photographing. Right after the emulsifier-free polymerization, 10 mL of fresh latex was moved into a glass container and its concentration was adjusted to about 10 wt % by dilution; then a given amount of 1.0 wt % sodium chloride (NaCl) solution was added into the latex to adjust the total ion strength for the latex to ∼31-40 mM. After the latex was sonicated for 3 min, we moved the latex into an infrared lamp oven in which the infrared light irradiates from above and let the water evaporate from the latex. The evaporation speed was controlled by setting the latex temperature at around 70 °C. Eight minutes later, weak iridescence could be seen at the latex surface, and the iridescence became very bright after 10 min of evaporation, as seen in Figure 1 (top). When the latex was heated for about 30 min, a copper grid was used to take a part of the film from the surface of the latex, and the film (colloidal crystal) was further dried in air and then heated in a dry oven at 90 °C for 2 h. The colloidal crystal split into several smaller thin pieces with sizes ranging from 3 to 5 mm2 and thicknesses of about 200 µm. These pieces of crystal could be removed from the copper grid and then moved into a Philips XL 30 scanning electronic microscope to observe the crystal structure. (18) Kim, J. H.; Chainey, M.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 171. (19) de las Nieves, F. J.; Danials, E. S.; El-Aasser; M. S. Colloids Surf. 1991, 60, 107.
Langmuir, Vol. 18, No. 24, 2002 9117 To prepare a larger colloidal crystal for fabricating inverse opal, a 1 cm × 1 cm glass plate was used to take a piece of film with the same size from the surface of latex; then the sample was dried in air for 2 h and then moved into culture plate, and afterward the sample was covered with a piece of wet filter paper of similar size. The sample (with the wet filter paper) was then dried at room temperature for 3 days. The drying of sample at very low speed in the presence of moisture prevented the occurrence of cracking on the sample. The resultant colloidal crystal did not split; it firmly stuck on the glass plate and its size remained the same as its substrates. The thickness of the crystals prepared this way can range from about 80 µm to about 200 µm (about 300 to over 1000 layers), depending on the time of evaporation. Evaporating the latex for too long (longer than 1 h) would result in wrinkles on the crystal. The crystal could also be taken from its substrate, but it split into pieces with sizes ranging from about 10 to 20 mm2. Fabrication and SEM Photographing of Inverse Opal. HCl solution (0.05 M, 0.5 mL) was added into the mixture containing 1 mL of ethanol and 5 mL of TEOS. The resultant solution was rapidly stirred for 4 h at room temperature. The colloidal crystals with area of ca. 1 cm2 were prepared according to above-mentioned procedure and then moved into a glass container with its bottom covered with a piece of filter paper. The precursor liquid was then dropped onto the filter paper until it was completely wetted; the colloidal crystals on the filter paper were also filled with precursor liquid due to the capillary effect. After the colloidal crystals were dried in the air for 24 h, the filling process was repeated for 5 times to ensure a larger filling ratio of precursor in the interstitial space of the colloidal crystals. Then the colloidal crystals filled with precursor were first heated in a dry oven for 2 h at 90 °C and then calcined in air at 450-500 °C for 5 h. After the calcination, the samples split into freestanding pieces with sizes from 5 to 20 mm2 and with very low density. SEM photographs for the macroporous materials were recorded on a Philips XL 30 scanning electronic microscope.
3. Results and Discussion Fabrication of Colloidal Crystal. Emulsifier-free emulsion polymerization has long been applied to prepare monodisperse and “clean” polymer lattices. In this study, we prepared a series of monodisperse polystyrene spheres by using a redox initiator system at lower temperature; the conversion for those latexes was at about 85%. We chose one of the latexes for this study, of which the particle diameter is 180 nm, the polydispersity is 1.03, and the surface charge density is 15.6 µC/cm2. The fabrication of 3D colloidal crystals by self-assembly of charged particles with the help of electrostatic interaction is relatively simple and effective. However, the particles in colloidal crystals are usually separated by the dispersion medium. In this study, to overcome the problem, we first adjusted the concentration of stray electrolytes by adding a certain amount of electrolyte into the freshly prepared latex, which was subject to no treatment before, and then slowly reduced the center-to-center distances between the particles at the surface of the latex by evaporating the dispersion medium (water) at higher temperature. Finally, bright iridescence can be seen and highly ordered crystalline structures were formed on the surface of the latex, as shown in Figure 1, and the colloidal crystal film extended to the whole surface area of the container (the diameter of the container is about 2 cm). Figure 1 (top) shows a complete, liquidlike thin film formed on the surface of the latex after it was heated for 10 min. A glass rod was inserted into the middle of this thin film, and the film quickly dispersed outward due to surface tension, leaving a blank circular spot on the surface of the latex; the film substances could not be moved out for photographing, and the periphery of this blank circle is quite smooth, as shown in Figure 1 (middle). However,
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Figure 2. SEM photograph of colloidal crystal prepared through evaporation on latex surface. (a, top panel) Morphology of first layer of colloidal crystal. Scale bar (at top right corner) 1 µm. (b, bottom panel) Morphology of cross section. Scale bar 2 µm.
Figure 1. Film of colloidal crystal formed on the surface of latex containing highly charged polystyrene particles. (a, top panel) Thin film formed upon 10 min of evaporation. (b, middle panel) After being contacted by a glass rod, the thin film had a blank spot. (c, bottom panel) Thick film formed upon 30 min of evaporation and a part of it was taken out.
after the latex was heated for about 30 min, a thick, solid film was formed on the surface of the latex and a part of the film could be taken out with a copper grid, leaving behind a blank spot on the surface of the latex; but the periphery of this blank spot was not smooth, as shown in Figure 1 (bottom). This result indicates that the colloidal crystal was formed on the latex surface layer by layer and its thickness increases with the evaporation time. The colloidal crystals we prepared by this method are in a stacked solid state if the film on the surface was thick enough; it is different from the colloidal arrays formed in suspension, in which the colloidal particles are separated by dispersion medium. Figure 2 shows the structure of the prepared colloidal crystals. The surface morphology of the colloidal crystals is shown in the top panel, while the bottom panel shows the cross-sectional structure of the crystal, in which nearly 80 layers can be seen. The figures clearly indicate that the three-dimensional colloidal crystal was prepared by using this surface evaporation method. Considering our experimental observations, we propose the following two-stage mechanism of the 3D crystal formation. At the first stage the 3D crystalline structure (non-close-packed) formed as a result of the electrostatic interaction between the charged particles. First, as the
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ion strength was adjusted to a relatively large value, which was from 31 to 40 mM for this study, the screening length of charged particles was reduced to a small value, but it could not be calculated by using the classical formula because it was short compared with the particle diameter under this condition. Therefore, prior to the evaporation, the screening length was shorter than the center-to-center distance between adjacent charged particles; as the evaporation proceeded, the center-to-center distance on the surface of the latex gradually decreased, while the screen length was nearly unchanged due to the reason given below. Eventually, the screen length became longer than the center-to-center distance, causing the repulsive interaction between particles to begin to function. At this time, although the particle concentration and ion strength was high on the surface of the latex, the repulsive interaction could still assemble the particles into 3D crystalline. We believe the key to the formation of ordered 3D structure by using this method is that during the controlled evaporation of water, as the stray ions surrounding the particles on the surface become concentrated, they could diffuse downward into the dispersion medium below. Therefore, the stray ion concentration can be kept at a relatively constant level on the surface since the volume of the film on the surface was far smaller than that of the latex; this ensures the gradual decrease in center-to-center distance between the particles at the surface without remarkably changing the electrostatic screening length and makes it possible to fabricate 3D ordered structure. However, the crystal was not closely packed yet since there was repulsive force between the ordered particles and those particles were separated by the dispersion solvent, though the distance between the ordered particles was very short. Once the ordered colloidal crystal formed at the latex surface, the second stage started, during which the separated colloidal particles moved into contact with one another and formed a close-packed crystal. Our explanation on the close packing of the colloidal crystal is given below. Upon the formation of 3D colloidal crystal on the surface caused by evaporation, the Brownian motion of the particles significantly slowed since the particles were almost fixed at a given position by the electrostatic interaction system; besides, the density of polymer particles is relatively low (the density of polystyrene particles is about 1.05 g/cm3), and thus their sedimentation became very slow. As a result, the individual particle within the 3D ordered structure was unable to make longrange movement and hence the ordered domain on the surface could be kept afloat because the sedimentation of these particles could be offset by the high-speed evaporation. As the latex was further evaporated, the upper part of the particles of the first layer began to protrude out of the latex surface. At that time, there were several kinds of interparticle forces governing the movement of the ordered particles:20-23 lateral capillary forces (including floating lateral capillary forces and immersion lateral capillary forces), electrostatic repulsive forces, and convection forces. Among them, the floating lateral capillary force is acting on particles that are floating on the water surface; it can be negligible for our sample since it will disappear for particles with their radius less than 10 µm. The immersion lateral capillary force acts solely on particles protruding out of the water surface; the convec(20) Kralchevsky, P. A.; Nagayama, K. Langmuir 1994, 10, 23. (21) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183. (22) Dimitrov, A. S.; Nagayama, K. Chem. Phys. Lett. 1995, 243, 462. (23) Rakers, S.; Chi, L. F.; Fuchs, H. Langmuir 1997, 13, 7121.
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Figure 3. SEM photographs of inverse opals. (a, top panel) Typical image of inverse opal based on colloidal crystal prepared through evaporation on latex surface. Scale bar 5 µm. (b, middle panel) Region of defect-free area of inverse opal. Scale bar 1 µm. (c, bottom panel) Magnification of middle panel. Scale bar 200 nm.
tion force is similar to the lateral capillary force in its effect but is based on a different mechanism.20 As the particles of the first layer protruded out of the latex surface under the effect of both the evaporation and the electrostatic repulsive forces from the particles of the second layer, the water meniscus 20 could probably be formed between particles and thus a strong capillary force arose. The lateral projection of the capillary force dragged the particles together and the crystal nucleus formed on the surface, since the lateral capillary force is a kind of attractive force in this case and much stronger than the electrostatic repulsive force between the adjacent particles on the first layer. After that, driven by the lateral capillary force and convection force, the crystal grew on the first layer through directional motion of particles toward the closely packed nucleus.23 After the close packing of the first layer, the close packing occurred layer by layer with a similar process. Finally, the 3D colloidal crystal with a larger area could be obtained on the latex surface. We believe that, in stage 2, the close packing of the colloidal crystals involved only a very short motion for the particles because no obvious blue shift of the iridescence color was observed throughout the evaporation. We suppose the density of the colloids is critical to the formation of crystal on the latex surface, since only the
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light particles (such as some polymer particles or biological particles) can be kept afloat by the evaporation and the electrostatic repulsive forces. For the heavy particles such as silica beads, we have not conducted similar experiments on them, but we think our method is not applicable. Fabrication of Inverse Opal. The freshly prepared colloidal crystals contain highly charged particles; these particles are in physical contact with each other and will separate from each other when the crystal is infiltrated with a sol-gel precursor solution due to the repulsive force. Moreover, the emulsion polymerization for preparing the highly charged spheres was conducted at 40 °C, at which the conversion percentage could hardly exceed 85%. So it is necessary to anneal the colloidal crystals and make the polystyrene spheres closely contact with each other. We found that the colloidal crystals could withstand the infiltration of sol-gel solution upon 2 h of annealing at 90 °C. To prepare the inverse opal, the sol-gel precursor solutions should be infiltrated into the interstitial space of colloidal crystals, which can also be regarded as the porous materials. Liquids and gases have been found to exhibit characteristic transport behavior in different types of porous materials:24 for mesoporous materials (2 nm < pore diameter < 50 nm), the dominant transport processes are surface diffusion and capillary transport. The diameter of the particles of the colloidal crystals is 180 nm, and the size of most of their interstitial space is less than 50 nm. So the capillary transport of sol-gel solution is possible in the interstitial space of colloidal crystals. In this study, we put the colloidal crystals on a sheet of filter paper soaked with sol-gel precursor solution and let the colloidal crystal “suck up” the solution from below. To ensure the smooth transport of sol-gel solution, we kept the viscosity of the solution at a lower level. On the other hand, to ensure the higher final infiltration rate of SiO2, we infiltrate the colloidal crystals several times after each preceding infiltrated solution was dried. Figure 3 shows the SEM photographs of inverse opals. Figure 3 (top) is the photograph of typical surface pattern (24) Gates, B.; Yin, Y.; Xia, Y. Chem. Mater. 1999, 11, 2827.
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for the inverse opal based on the colloidal crystal prepared by the method described above. The middle and bottom panels of Figure 3 show a defect-free area of the inverse opal with different magnifications. Figure 3 (middle) shows an ordered region of inverse opal extending nearly 10 µm and containing over 1000 voids (pores). In Figure 3 (bottom), the pores are highly ordered in a hexagonal array with the SiO2 nanoparticles forming the walls of the pores. The holes connecting the pores are clearly visible and indicate the three-dimensional ordering of the structure of both the inverse opal and colloidal crystals. These holes are important to allow for complete removal of the template through burnout. The nanoparticles that form the walls, we suppose, were probably caused by cracking during the calcination. Conclusions We developed a new and simple procedure for producing close-packed colloidal crystals with larger area by evaporating the water on the surface of latex containing highly charged polystyrene spheres. The electrostatic interaction and lateral capillary forces as well as ion diffusion during the evaporation all contribute to the formation of the colloidal crystals. On the basis of the prepared colloidal crystals, we infiltrated the sol-gel solution into the interstitial space of the colloidal crystals through the capillary effect and obtained relatively highly ordered inverse opal by calcining the polystyrene beads. The defectfree area of the inverse opal extended to over 10 micrometers and covered thousands of voids. Although our method for preparing colloidal crystals cannot provide tight control over the thickness of crystal, it can be used in preparing solvent-free colloidal crystals as long as colloids are light and highly charged. Acknowledgment. This study is supported by National Natural Science Foundation of China (59903002), Guangdong Provincial Natural Science Foundation of China (990613), and the research fund granted by Guangzhou Municipal Science and Technology Bureau. LA020114J
