Binary Colloidal Crystals Fabricated with a Horizontal Deposition

Apr 1, 2009 - Qingdao University, Qingdao 266071, P. R. China, and. ) Max Planck ..... force and the mobility of small particles at the very top of th...
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Binary Colloidal Crystals Fabricated with a Horizontal Deposition Method )

Likui Wang,†,‡ Yong Wan,§ Yanqiang Li,§ Zhongyu Cai,† Hong-Liang Li,§ X. S. Zhao,*,†,§ and Qin Li*,‡, †

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Department of Chemical and Biomolecular Engineering, National University of Singapore, Engineering Drive 4, Singapore 117576, ‡ Department of Chemical Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia, § Institute of Multifunctional Materials (IMM), Laboratory of New Fiber Materials and Modern Textile, College of Chemistry, Chemical Engineering and Environment, Qingdao University, Qingdao 266071, P. R. China, and Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Received January 22, 2009. Revised Manuscript Received March 13, 2009 We describe the use of a horizontal deposition method to prepare large-area binary colloidal crystals (bCCs). Two different sets of binary polystyrene spheres were employed to demonstrate the validity of this method. By varying the number ratios of small spheres with respect to large spheres, the stoichiometric configuration of the bCCs can be altered. Stable corresponding replica structures of the bCCs were also prepared, and the cross-sectional images of the binary inverse opals were obtained. Optical characterization demonstrated the presence of pseudostop bands, which were in agreement with the compositions of the material. The formation of the bCC by such a simple self-assembly method was attributed to the cooperative effect of interparticle electrostatic interactions and geometrical constrictions. This facile fabrication method further enhances the application potential of the bCCs and their inverse porous replicas with a binary pore system in the fields of photonics, solar cells, separations, catalysis, and biosensing.

Introduction With defined size distributions of either monodisperse distribution, or bimodal distribution, or trimodal distribution, colloidal spheres can be fabricated as unary colloidal crystals (CCs), binary colloidal crystals (bCCs), and ternary colloidal crystals (tCCs). With the colloidal crystals as templates, their inverse structures can be readily prepared1,2 for various applications, such as photonics,3,4 separation,5 catalysis,6,7 and tissue engineering,8 among others. A number of self-assembly methods have been reported in the literature for fabricating colloidal crystals. Examples include gravitational sedimentation,9,10 vertical deposition (VD),11 electrophoretic deposition,12 spin-coating,13,14 confined cell method,15 Langmuir-Blodgett technique,16 floating self-assembly,17 and *Corresponding authors. E-mail: [email protected]; [email protected]. (1) Li, Q.; Retsch, M.; Wang, J.; Knoll, W.; Jonas, U., Porous networks through colloidal templates. In Topics in Current Chemistry - Templates in Chemistry, 2008, DOI: 10.1007/128_2008_3. (2) Stein, A.; Li, F.; Denny, N. R. Chem. Mater. 2008, 20, 649. (3) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. (4) John, S. Phys. Rev. Lett. 1987, 58, 2486. (5) Zeng, Y.; Harrison, D. J. Electrophoresis 2006, 27, 3747. (6) Velev, O. D.; Tessier, P. M.; Lenhoff, A. M.; Kaler, E. W. Nature (London) 1999, 401, 548. (7) Wang, Z. Y.; Kiesel, E. R.; Stein, A. J. Mater. Chem. 2008, 18, 2194. (8) Kotov, N. A.; Liu, Y.; Wang, S.; Cumming, C.; Eghtedari, M.; Vargas, G.; Motamedi, M.; Nichols, J.; Cortiella, J. Langmuir 2004, 20, 7887. (9) Zhu, J. X.; Li, M.; Rogers, R.; Meyer, W.; Ottewill, R. H.; Russell, W. B.; Chaikin, P. M. Nature (London) 1997, 387, 883. (10) Miguez, H.; Meseguer, F.; Lopez, C.; Blanco, A.; Moya, J. S.; Requena, J.; Mifsud, A.; Fornes, V. Adv. Mater. 1998, 10, 480. (11) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (12) Holgado, M.; Garcia-Santamaria, F.; Blanco, A.; Ibisate, M.; Cintas, A.; Miguez, H.; Serna, C. J.; Molpeceres, C.; Requena, J.; Mifsud, A.; Meseguer, F.; Lopez, C. Langmuir 1999, 15, 4701. (13) Jiang, P.; McFarland, M. J. J. Am. Chem. Soc. 2004, 126, 13778. (14) Mihi, A.; Ocana, M.; Miguez, H. Adv. Mater. 2006, 18, 2244. (15) Park, S. H.; Gates, B.; Xia, Y. N. Adv. Mater. 1999, 11, 462. (16) Reculusa, S.; Ravaine, S. Chem. Mater. 2003, 15, 598. (17) Im, S. H.; Park, O. O. Langmuir 2002, 18, 9642.

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horizontal deposition.18,19 While most of these methods offer the formation of CCs in large domains with a controllable thickness, the horizontal deposition method is considered as a simple yet effective method for fabricating CCs. In this method, a drop of colloidal suspension is spread over a substrate, followed by a drying process. The concentration of the colloid, thickness of the colloidal liquid film on the substrate, temperature, and humidity all influence the structure of the resultant CC thin films, thus the morphology and quality. It has been shown 19 that by selecting an optimal concentration of the colloidal particles, under ambient conditions, high-quality CCs in large domains can be fabricated, and the crystallization time can be significantly shortened from days to a couple of hours. As such, this method is superior in terms of the simplicity and efficiency of colloidal self-assembly. Over the past few years, novel binary structures fabricated using the self-assembly methods have attracted a great deal of attention for the benefit of crystal growth research as well as novel functional material fabrications.20-23 It has been demonstrated that bCCs can be fabricated by layer-by-layer growth strategy,24,25 vertical lifting,26-28 and contact printing.29 Wang and (18) Muller, M.; Zentel, R.; Maka, T.; Romanov, S. G.; Torres, C. M. S. Adv. Mater. 2000, 12, 1499. (19) Yan, Q. F.; Zhou, Z. C.; Zhao, X. S. Langmuir 2005, 21, 3158. (20) Bartlett, P.; Ottewill, R. H.; Pusey, P. N. J. Chem. Phys. 1990, 93, 1299. (21) Bartlett, P.; Ottewill, R. H.; Pusey, P. N. Phys. Rev. Lett. 1992, 68, 3801. (22) Larsen, A. E.; Grier, D. G. Nature (London) 1997, 385, 230. (23) Hynninen, A. P.; Thijssen, J. H. J.; Vermolen, E. C. M.; Dijkstra, M.; Van Blaaderen, A. Nat. Mater. 2007, 6, 202. (24) Velikov, K. P.; Christova, C. G.; Dullens, R. P. A.; van Blaaderen, A. Science 2002, 296, 106. (25) Wang, D. Y.; Mohwald, H. Adv. Mater. 2004, 16, 244. (26) Kitaev, V.; Ozin, G. A. Adv. Mater. 2003, 15, 75. (27) Cong, H. L.; Cao, W. X. J. Phys. Chem. B 2005, 109, 1695. (28) Gu, Z. Z.; Uetsuka, H.; Takahashi, K.; Nakajima, R.; Onishi, H.; Fujishima, A.; Sato, O. Angew. Chem., Int. Ed. 2003, 42, 894. (29) Burkert, K.; Neumann, T.; Wang, J. J.; Jonas, U.; Knoll, W.; Ottleben, H. Langmuir 2007, 23, 3478.

Published on Web 04/01/2009

DOI: 10.1021/la9002737

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Wang et al. Table 1. Parameters of Binary Colloidal Samples B1-B4 large spheres

small spheres

diameter ratio

volume fraction ratio

particle number ratio

sample

DL (nm)

VF (%)

DS (nm)

VF (%)

(DS/L)

(VFS/L)

(NS/L)

B1 B2

789 789

8.59 7.88

154 154

0.16 0.23

0.195

0.015 0.03

2.5 3.9

B3 B4

1000 1000

15.39 13.19

154 154

0.23 0.39

0.154

0.015 0.03

4.1 8.1

co-workers30,31 described for the first time a three-dimensional (3D) structure of tCC and its inverse binary opal fabricated by the vertical lifting technique, where the small particles were considered as having packed in an ordered manner to the maximum density within the large-sphere network.30 There have been a few limitations associated with the fabrication of bCCs via the self-assembly approach. These include the size of colloidal spheres, the speed of the self-assembly process, and the stability and effective size of the bCC films and their inverse structures. For instance, the diameter of the large spheres needs to be smaller than about 800 nm in vertical lifting method; the self-assembly process often takes days to months; and the bCC films particularly their inverse structures are often too small and fragile to be investigated further. Recently it has been reported that an employment of infrared technique has expedited the growth rate and quality of bCCs.32 However, for practical applications, more rapid and cost-effective methods are still desired. In this paper, we demonstrate the horizontal deposition method as a facile approach to the growth of bCCs. It is the first time to show that large-area bCCs can be fabricated by using such a simple yet highly effective method without involving elaborated instrumentation. We also discuss the formation mechanism of the bCCs by this method and the feasible operational regimes.

Experimental Section Synthesis of PS Spheres and Preparation of Binary Colloids. PS spheres of 154, 789, and 1000 nm in diameter with a polydispersity of less than 3% were synthesized using an emulsifier-free emulsion polymerization method.33 Binary PS colloids B1 and B2, containing both small spheres (S) of 154 nm in diameter and large spheres (L) of 789 nm in diameter, were prepared by mixing the two sizes of spheres with given volume fractions (VF). Binary colloids B3 and B4 are mixtures of the 154 nm small spheres and the 1000 nm large spheres. All parameters were tabulated in Table 1. Fabrication of PS bCCs. The PS bCCs were fabricated on a clean glass slide (22  22  0.3 mm, Marienfeld, Germany, treated with a piranha solution before use34) using the horizontal deposition method.19 A drop of 30 μL of binary colloidal suspension was applied on the glass substrate, followed by careful spreading with a pipet tip to allow the suspension to fully cover the substrate. The suspension-covered glass slides were then kept on a horizontal bench (with minimum disturbances) for drying under the ambient conditions (20 °C, humidity = 28%). As the suspension drying proceeded, the bCCs were formed. A complete drying usually took about 2 h under the experimental conditions. The shorthand of these PS bCC (30) Wang, J.; Li, Q.; Knoll, W.; Jonas, U. J. Am. Chem. Soc. 2006, 128, 15606. (31) Wang, J.; Ahl, S.; Li, Q.; Kreiter, M.; Neumann, T.; Burkert, K.; Knoll, W.; Jonas, U. J. Mater. Chem. 2008, 18, 981. (32) Zheng, Z.; Gao, K.; Luo, Y.; Li, D.; Meng, Q.; Wang, Y.; Zhang, D. J. Am. Chem. Soc. 2008, 130, 9785. (33) Shim, S. E.; Cha, Y. J.; Byun, J. M.; Choe, S. J. Appl. Polym. Sci. 1999, 71, 2259. (34) Wang, L. K.; Yan, Q. F.; Zhao, X. S. Langmuir 2006, 22, 3481.

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samples was based on their original binary colloidal suspension labels, that is bCC B1 to bCC B4. Fabrication of Silica Binary Inverse Opals. A silicate solution was prepared by mixing tetraethyl orthosilicate (98%, Acros Organics), ethanol (99.95%, Aldrich), and 0.1 M HCl (Merck) solution (volume ratio = 1:10:1) under stirring for 4 h. Then the solution was infiltrated into the voids of the bCCs using a Laurell spin coater (WS-400B-6NPP/LITE) operated at 1200 RPM for 15 s. The silicate-infiltrated bCC was dried at room temperature for 4 h.34 Three cycles of the infiltration and drying were performed. Subsequently, the PS spheres were removed by immersing the sample in toluene (99.5%, Merck) at 60 °C for 4 h. This was repeated three times to ensure complete removal of the PS beads. The shorthand of these silica binary inverse opals was given as inv-B1 to inv-B4 in accordance to their parent bCCs. Characterizations. Field-emission scanning electron microscope (FESEM) images were obtained on a JEOL JSM-6700F. For cross-sectional images, samples were sliced using a diamond cutter. Optical transmission spectra were measured on a UVvis-near-infrared spectrophotometer (Shimadzu UV-3101PC).

Results and Discussions Horizontal Deposition of bCCs. A simple geometrical analysis showed that the upper limit of the diameter ratio of small spheres over large ones in fabricating bCCs is about 0.225 to allow the small spheres to fit in the tetrahedral voids (the limiting case) of the large spheres.31 In this work, the DS/L were selected to be less than 0.2 as depicted in Table 1. The binary colloidal suspension was dropped on a clean glass substrate, followed by careful spreading with a pipet tip to allow for full coverage of the liquid on the substrate. It was observed that after a few minutes the edge of the substrate began to exhibit iridescent colors. The iridescence gradually spread from the periphery to the center of the substrate. It took about 2 h for the process to complete under the ambient condition (temperature: 24 °C, humidity: 28%). As shown in Figure 1a, the iridescent color can be seen across the whole sample area (2.2  2.2 cm). Similar to the self-assembly of unary CCs,19 a void also formed in the center of the film. This can be explained by the “coffee ring” phenomenon.35 The evaporation of the solvent resulted in a convective flow, which drove the entrainment of the colloidal particles from the center area toward the periphery area because of the higher evaporation rate of solvent along the periphery. Because of the high concentration of the colloids used in this work, a large-area and relatively uniform CC film formed, instead of a ringlike structure in the case of diluted systems.35 Figures 1b-e show the FESEM images of samples B1 and B2, whose DS/L values are both 0.195, and the NS/L values are 2.5 and 3.9, respectively (see Table 1). It can be seen from the FESEM images that the addition of small spheres in the colloids did not alter the self-assembly process of the large spheres since the large spheres still invariably formed a face-centered cubic (fcc) lattice. (35) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature (London) 1997, 389, 827.

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Figure 1. (a) Photograph of bCC B1 showing iridescent colors. (b) A low-magnification SEM image of a large area of the binary lattice structure of bCC B1. (c) A close-up SEM image of bCC B1 (inset: a higher-magnification view). (d) SEM cross-sectional view of bCC B1. (e) A SEM top view of sample bCC B2. (f) A higher-magnification top view of bCC B2. All scale bars = 1 μm unless otherwise specified.

The arrangement of the small spheres is also highly regular in the interstitial voids of the large spheres in most of the areas. As shown in Figure 1b, the low-magnification SEM image shows that the single crystal is in the range of 40-50 μm, which is of the same length scale as that of the unary CC films fabricated using the same method19 and the bCCs fabricated using the vertical lifting deposition method.30,31 Figure 1c and the inset show the structural details of the top layer of sample B1. It is seen that most of the 3-fold voids are occupied by one small sphere, while sporadically some voids accommodate two or three small spheres. The increase of VFS/L from 0.015 to 0.03 led to an increase in the number of the small spheres residing in the voids of the large spheres (see Figure 1e). Sample B2 presents a large area of binary crystal structure as is seen from Figures 1e and f, where each 3-fold void is regularly filled by three small spheres and on average each large sphere is surrounded by six small spheres from the planar viewpoint. It is interesting to note that although the overall NS/L of B2, i.e. 3.9, is barely 2-fold of the NS/L of B1, i. e. 2.5, the population of the small spheres on the bCC top surface of bCC B2 is nearly 3-fold of that on the top surface of bCC B1. It suggests that with this horizontal deposition method the top layer of the bCC may have the priority in forming thermodynamically stable binary lattice structures in comparison to the inner bulk crystal. To observe the distribution of small spheres inside the crystal, bCC B1 was broken into halves, and its cross sections were viewed by the FESEM. Figure 1d reveals that the large spheres Langmuir 2009, 25(12), 6753–6759

are clearly in the form of the fcc lattice; however, the distribution of the small spheres is not as ordered as those on the bCC B1 top surface. In some areas, the small particles evenly distribute over the surface of the large sphere as individuals (as exemplified in the red-circled areas), while some of the small particles aggregate in the interstitial void (as exemplified in the blue-circled areas). On the other hand, it is also noted that the invasive SEM sample preparation method, i.e., breaking with a diamond cutter, may have disrupted the arrangement of the small particles which are weakly adhered on the large particles. It is reasonable to assume that some of the small particles have fallen out or dislocated during the sample preparation. The SEM images shown in Figure 2 exhibit the top surface morphologies of bCC B3 and bCC B4. It is seen that the surfaces of both bCCs B3 and B4 are also of highly ordered binary crystal structures. Due to the smaller DS/L, i.e., 0.154 and greater NS/L, more small spheres have been fitted in the interstitial voids among the large spheres compared to bCC B1 and bCC B2. The SEM images in Figure 2a and 2b demonstrate that the top layer of bCC B3 is of the highly ordered binary lattice structure where each large sphere is surrounded by six small spheres from the planar viewpoint. This structure is identical to sample bCC B2, despite that the DS/L of B2 is 27% greater than that of B3. It appears that the similar NS/L values in B2 and B3, i.e., 3.9 and 4.1, respectively, may have been the determinant factor in such binary crystal structure formation. An increase of NS/L value to 8.1 in sample B4 has resulted in a very different binary structure as shown by the SEM images in DOI: 10.1021/la9002737

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Figure 2. (a) SEM top view of bCC B3. (b) High magnification of (a). (c) SEM top view of bCC B4. (d) High magnification of c. Scale bars in (a) and (c) = 1 μm; scale bars in (b) and (d) = 300 nm.

Figure 2c and 2d. It is observed that in the bCC B4 small spheres are densely packed in almost every available void space among the large spheres, not only residing in the 3-fold void sites but also in between the neighboring large spheres. Each large sphere is approximately surrounded by 15 small spheres from the planar viewpoint. The ordered arrangement of the small spheres in bCC B4 also appears more prone to be defective. Representative types of defects are marked by the red circles in Figure 2C. This may be caused by the competition between the dense crystal structure formation versus the hydrodynamic pressure when water evaporates from inside through the top surface during the crystal formation. Like their counterpart monodisperse colloidal systems,19 the bCCs self-assembled by the horizontal deposition method are also influenced by the colloidal concentration (see Supporting Information S1). In general, if the particle concentration is too dilute, the evaporation would be too slow compared to the speed of beads fluxing into the substrate edge where the three phase contact line lies. This would result in most of the spheres assembling around the edge of the substrate, and only a very limited area of colloidal crystals is formed. The extreme case is the “coffee ring” phenomenon.35 On the other hand, if the particle concentration is too high, the evaporation would be too fast to allow the self-assembly to complete, which requires water as the mediator. Hence it would result in noncrystal formation in the central area. By tuning the colloidal concentration and the evaporation rate, the method can be further optimized to improve the quality of the formed bCCs. Binary Inverse Opal. After annealing, infiltration of silica, and the subsequent removal of PS spheres, large-area binary inverse opals were obtained. Similar to their corresponding bCCs, iridescent color can be clearly seen across the film, as shown in the Supporting Information (Figure S2). The resultant replicas reveal two sets of pore structures with defined sizes. In addition to the feature of being a hierarchical porous material,1,36 the beauty of such a replica system also lies in its preservation of the original CC structures. (36) Sen, T.; Tiddy, G. J. T.; Casci, J. L.; Anderson, M. W. Chem. Mater. 2004, 16, 2044.

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The SEM images in Figure 3 illustrate detailed binary inverse opal structures. It can be clearly observed that besides the large and small spherical pores originated from the templating large and small latex particles there are large and small windows (the dark circles on the images) connecting the neighboring pores in the porous structure. The large windows are results of the original contact points between the large particles, while the small windows reveal the previous contract points between the large and small spheres. Therefore, by examining the large and small windows shown in the top view SEM images of the inverse opals, insight of the 3D architecture in the first layer of this hierarchical structure can be obtained. Figures 3a, 3b, and 3c show the top view SEM images of the inv-B1, inv-B4, and invB2, respectively. From Figure 3b, it was measured that for sample inv-B4 the large window size was about 250 nm in diameter and the small window size was about 60 nm. Similarly, the large window diameter on inv-B2 was measured as about 150 nm, and the small window was also around 60 nm. The regular distributions of large windows in Figure 3a, 3b, and 3c demonstrate that the original large spheres in the top two layers were packed in the fcc configuration, and they were not separated apart by the small spheres. On the other hand, the distribution of small windows does not present a consistent, regular pattern in these SEM images, which suggests that the small spheres may not pack in a highly ordered fashion inside the bCC. Due to the high quality, large area of the parent bCCs fabricated by the horizontal deposition, the resultant silica replica structure was robust enough to allow for cutting into halves. The cross-sectional view of inv-B2 as shown in Figure 3d further demonstrates the internal architecture of the inverse binary opal, hence, the configuration of the original bCC. This information is more reliable compared to the cross-section view taken directly from the bCCs as presented in Figure 1d owing to the improved structural stability. By examining the distribution of the large and small windows presented in the cross-sectional images, it is deduced that the general distribution of the small spheres in the bCC system appears to be uniform; however, inside the bCCs the arrangement of the small spheres in the interstitial voids among the large spheres is not as highly ordered as that on the top layer Langmuir 2009, 25(12), 6753–6759

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Figure 3. SEM images of inverse bCCs: (a) top view of inv-B1; (b) top view of inv-B4; (c) and (d) are the top view and cross-sectional view of inv-B2, respectively. Scale bars in (a) and (d) = 1 μm; scale bars in (b) and (c) = 300 nm.

(more cross-sectional SEM images of inv-B1 and inv-B2 are available in the Supporting Information, Figure S3). This information also raises doubts on the assumed internal configurations of bCCs obtained from the vertical lifting method where the cross-sectional SEM images of the binary inverse opals were not examined.30 Nevertheless, such a hierarchical porous material clearly possesses two defined pore sizes with regularity as well as enhanced surface area and interconnectivity. The double macropore system is particularly interesting for controlling the transport and retention of biomacromolecules by adjusting the sizes of the templating large and small spheres. Such a feature presents a potential advantage in bioseparation, microreactors, and biosensing. Spectral Analysis. The vis-NIR transmission spectra of the samples were obtained, as shown in Figure 4. The distinctive dips in the transmittance spectra, i.e., the pseudo stop bands, confirm the long-range ordering of the binary opals and binary inverse opals. The less ordered distribution of the small particles within the sample bulk does not appear to affect the structural optical property that is evidently determined by the spatial configuration of the large spheres, although the small spheres also contribute to the effective refractive index. It is known that bCCs exhibit a red shift in their stop band positions compared to their counterpart unary CCs due to the increased effective refractive index as a result of the interstitial small particles, while the binary inverse opals show a noticeable blue shift compared to their counterpart unary inverse CCs because of the increased porosity.30 From the transmission spectra in Figure 4, slight red shifts of stop bands of bCC B2 versus bCC B1 and bCC B4 versus bCC B3 were observed. This is due to the increased small sphere fractions from B1 to B2 and from B3 to B4. For the binary inverse opals, respective blue shifts were observed for inv-B2 from inv-B1 and for inv-B4 from inv-B3. The stop band positions are consistent with the constitutions of the binary colloidal systems. These characteristic absorption peaks have potential important usages in bio/chemosensing, photoreactive microreactors, and solar cell platforms. Table 2 provides a comparison between the estimated peak wavelengths versus the measured ones. The peak wavelength Langmuir 2009, 25(12), 6753–6759

Figure 4. IR spectra of the bCCs and their inverse structures.

calculation is based on the Bragg’s equation for the absorption maxima with regard to the (1 1 1) crystal face λmax ¼ ð8=3Þ1=2 Dðn2eff -sin2 θÞ1=2 P 2 where n2eff = N i=1 ni φi, n is the reflective index, φ is the volume fraction, subscript i refers to the component, and D is the large sphere diameter. In the calculation, we assumed that the large sphere volume fraction was 0.74 according to the fcc crystal lattice. The calculated data show a similar trend in terms of the relative positions of the absorption maxima. As shown in Table 2, generally the measured peak wavelengths for the bCCs are larger than the calculated peak wavelengths, while the measured values for the inverse bCCs are smaller than the calculated values. Hence, it indicates the deviation is not just caused by a system error. It is noticed that measured stop band wavelengths for bCCs B3 and B4 are significantly higher than the calculated values. This may be caused by the denser packing of the small spheres in the first couple of layers (from the top DOI: 10.1021/la9002737

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Table 2. Comparison between the Calculated and the Measured Stop Band Wavelengthsa

sample

assumed assumed VF of large VF of small sphere in sphere in calculated peak measured peak bCC bCC wavelength/nm wavelength/nm

bCC B1 0.74 0.0138 1890 1906 bCC B2 0.74 0.0216 1895 1916 inv-B1 0.74 0.0138 1432 1316 inv-B2 0.74 0.0216 1428 1286 bCC B3 0.74 0.0111 2393 2540 bCC B4 0.74 0.0219 2402 2560 inv-B3 0.74 0.0111 1818 1810 inv-B4 0.74 0.0219 1810 1750 a The calculations are based on: the refractive index of PS is taken as 1.59; the refractive index of silica network is taken as 1.4; and the refractive index of air is 1.

surface) on bCCs B3 and B4, compared to the average packing density of the small spheres throughout the structure, which is used in the calculation. The pronounced ripples on the absorption peaks of bCC B3, bCC B4, inv-B3, and inv-B4 may be regarded as the Fabry-Perot fringes, which suggests the refractive index variations among the crystal layers.16 Formation Mechanism. When the suspensions of large and small PS particles were mixed together, the binary mixture did not aggregate despite the high volume fractions of the solid phase. On the contrary, both negatively charged (both zeta potentials about -30 mV) large and small spheres were uniformly distributed in the suspension with the small sphere “halos” around each large sphere.37 Since in the final bCC structures as shown in Figures 1, 2, 3 the small spheres are uniformly distributed throughout the crystal film, demonstrating binary crystal lattices particularly on the top layers, it can be deduced that in the drying process of such a binary colloidal suspension the large and small particles are entrained toward the edge of the substrate by the convective flow at the same velocity. Evidence of such a halo effect can be found in the Supporting Information, Figure S1-2(4). Because of their higher volume fraction and larger size, the large spheres consolidate first into a close-packed network that has a constriction pore size of a = 0.155DL (a is the constriction pore diameter, and DL is the large sphere diameter), as illustrated in Figure 5a. When the small sphere size is less than a, they then have the possibility to travel through the constriction pores. Otherwise, they are confined in the voids. In samples B1 and B2, the small particle size (DS = 0.195DL) is greater than the constriction pore size, and the small particles are restrained within the interstitial voids maintaining almost the same relative positions as in the binary suspension. Therefore, the distributions of the small spheres throughout the bCCs are uniform, as an inherited attribute from the binary suspension. Although samples B3 and B4 have smaller particle size ratio (DS = 0.154DL), which is almost the same as the constriction pore size, the surface charge effect, hydrodynamic radius, and the strong interfacial interactions among the particles may have significantly discounted the marginal size advantage; therefore, the small particles in samples B3 and B4 appear to be as well confined within the vicinity of their original positions around the large spheres. This is starkly different from what has been observed by Harris et al.38 where the small particles segregate owing to the evaporation-induced convective flow. In the work (37) Tohver, V.; Smay, J. E.; Braem, A.; Braun, P. V.; Lewis, J. A. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 8950. (38) Harris, D. J.; Hu, H.; Conrad, J. C.; Lewis, J. A. Phys. Rev. Lett. 2007, 98.

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carried out by Lewis, Braun, and co-workers,37-39 the small particles are much smaller than the constriction pore size (DS/L ≈ 1/100), therefore they can be easily transported and redistributed in the microsphere network by the convective fluid flow. Since in a fcc structure-based bCC 0.225 is the largest relative size of the small spheres that can be accommodated by a tetrahedral site as illustrated in Figure 4b, we can conclude that when the DS/L is in the range from 0.154 to 0.225 horizontal deposition is feasible in fabrications of bCCs. Generally, when the size ratio is greater than the upper limit, the small spheres obstruct the fcc lattice formation of the large spheres (see Supporting Information, Figure S3); when the size ratio is less than the lower limit, the small spheres segregate due to the freedom in moving through the void network among the large spheres. This phenomenon has been elegantly illustrated by a sequence of cryo-SEM images which demonstrate the temporal evolution of the small sphere migration in the large sphere network.40 The majority of reported bCC and binary inverse opal studies has only shown the top views of the fabricated binary crystal structures. In this work, we have obtained similar binary lattice results through horizontal deposition but also images of complete cut-through cross-sectional views of the binary inverse opals. The latter have revealed, although the top surfaces have highly ordered hierarchical configurations, inside the binary crystals the small spheres are not necessarily packed in an ordered manner. The structural difference between the top surface and the internal bulk may be attributed to the capillary force and the mobility of small particles at the very top of the sample, which are unconstrained. The solvent evaporation first forms menisci between the large spheres that protrude out of the liquid film, while the small spheres may still be completely immersed in the liquid film and move according to the convective flow coupled with Brownian motion. Even in the case if we assume all the particles at the top of the film are protruding out of the liquid film, given the same separation distance, the lateral capillary attraction force between the large spheres is greater than that between a large sphere and a small sphere, which is again greater than that between a pair of small spheres, as described by the established equation: FC_ = -2πγr2k(sin2 θs)/ L (where rk is the radius of contact line, θs is the mean slope angle of the meniscus on the sphere with respect to the plane of the contact line between two spheres; and L is the separation distance of the sphere-to-sphere centers as illustrated in Figure 4c).41,42 Therefore, the capillary force first packs the large particles into the hexagonal configuration, while the small spheres are still mobile in the thin solvent film at the top layer following the evaporation-induced convective liquid flow. As drying further proceeds, the small particles are gradually packed into the most energy favorable position due to the work of immersion capillary force. Hence, they are packed in order, residing in the position that has the lowest energy.

Conclusions In this work, we demonstrated the fabrication of bCCs using a horizontal deposition method. The significant advantages of such a fabrication method lie in its simplicity (requiring no elaborated (39) Lee, W.; Chan, A.; Bevan, M. A.; Lewis, J. A.; Braun, P. V. Langmuir 2004, 20, 5262. (40) Luo, H.; Cardinal, C. M.; Scriven, L. E.; Francis, L. F. Langmuir 2008, 24, 5552. (41) Li, Q.; Jonas, U.; Zhao, X. S.; Kappl, M. Asia Pacific J. Chem. Eng. 2008, 3, 255. (42) Kralchevsky, P. A.; Nagayama, K. Adv. Colloid Interface Sci. 2000, 85, 145.

Langmuir 2009, 25(12), 6753–6759

Wang et al.

Article

Figure 5. Geometry-imposed upper and lower bounds for the constriction pore size effect. (a) The lower limit as = 0.155DL which is the smallest small particle size that can be retained by the tetrahedral pores. (b) The upper limit as = 0.225DL which is the largest small particle that can be fitted in a tetrahedral site. (c) An illustration of the parameters used in lateral capillary force calculations.

equipment), the fast production rate (about 2 h over an area of 2.2  2.2 cm2), and the ability in handling large colloids (>600 nm). The examination of the crystal structure of the top surface showed that binary crystal lattice structures were formed by this simple fabrication method. By varying the number ratios of small spheres with respect to large spheres, the stoichiometric configurations of the bCCs can be altered. Owing to the large area and high quality of the formed bCCs, stable replica, silica-based binary inverse opals were prepared, which enabled a close examination of the internal architecture of the hierarchical material by a cross-sectional exposure. The replica internal structures, however, reveal that in the bulk of the bCCs the arrangement of small spheres is uniform but not necessarily ordered. This horizontal deposition bCC formation is attributed to the following. (1) The nanoparticle halo formation: the small spheres are uniformly distributed around large spheres in a binary colloidal suspension. (2) The void constriction effect: due to the void constriction, small particles are confined within their original relative positions, instead of being entrained within the microsphere void network by the convective flows and forming uneven distributions. From the experimental evidence and geometrical analysis, it was found that the operational window of utilizing the void constriction effect in forming bCCs is approximately within 0.154 e DS/L e 0.225. Within this size ratio range, bCCs can be conveniently fabricated by the horizontal deposition method. Current undertaking includes optimizing the volume fractions of the small and large spheres to enhance the ordering of the small spheres inside the bCCs.

Langmuir 2009, 25(12), 6753–6759

The inverse structure of bCCs possesses two sets of pores with respective defined sizes. Such a configuration considerably increases the porosity of the material, which can enhance the mass transfer within the network. The flexibility of the infiltration material (i.e., the replica material) choice further enhances the versatility of this hierarchically ordered structure. The spectra of all the samples confirm the long-range ordering of the binary system. The stop band positions also agree with the volumetric constitutions of the binary mixtures. These features are particularly favored by sensing platform development and photoreactive microreactors. The simple fabrication method, the large area ordered double macropore system, as well as the optical property promise that these horizontal deposited bCCs have considerable potential in the applications of optoelectronic devices, separations, catalysis, microreactions, solar cells, and biosensing. Acknowledgment. This work was supported by the Australian Research Council (ARC) under DP 0558727, Ministry of Education of Singapore AcRF Tier 1 funding (RP279000224112), and Qingdao University. Supporting Information Available: The influence of colloidal concentration on the quality of bCC has been investigated. The photographs of the inverse binary opal films, additional SEM images of the cross-sectional view of the inverse bCCs, and conditions forming non-ordered binary structures are presented. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la9002737

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