pubs.acs.org/Langmuir © 2009 American Chemical Society
New Route to Produce Dry Colloidal Crystals without Cracks Toshimitsu Kanai†,‡ and Tsutomu Sawada*,† †
National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and ‡Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama, Kanagawa 240-8501, Japan Received September 9, 2009. Revised Manuscript Received October 26, 2009
We report a new route for fabricating opal films in which the particles are bounded by polymer gel. A loosely packed colloidal crystal immobilized in a hydrogel is homogeneously shrunk and dried and converted into a crack-free opal film. The obtained film shows a uniform diffraction color and excellent transmission spectral properties inherited from the starting state. The optical stop band wavelength is adjustable by varying the amount of gel-polymer.
Colloidal crystals, three-dimensional periodic arrays of colloidal particles,1 have attracted considerable attention with respect to their potential for optical applications such as photonic crystals,2 optical sensors,3 refractive flat-panel displays,4 and nonbleachable color materials.5 In most cases, the particle arrays in colloidal crystals are formed in a liquid medium, and the liquid is eventually removed to stabilize the material and make it convenient for practical applications. Such a dried colloidal crystal is often called a synthetic opal. One of the serious problems for synthetic opals is the existence of crack networks, ranging in size to up to a few hundred micrometers, generated during the drying process, which significantly deteriorates the optical properties of the material. Various fabrication methods such as substrate dipping,6 cell packing,7 centrifugation,8 grapho-epitaxy,9 and slow evaporation10 have been proposed. In our best knowledge, however, an effective solution for the crack problem has not yet been found except for an elegant way using core-shell particles reported by Ruhl et al. and Pursiainen et al., where the soft shell deforms and fills the space, preventing generation of cracks over large areas.11 In the present paper, we report a new, different route for fabricating crack-free films of dry colloidal crystals. For the starting material, we use a gel-immobilized colloidal crystal film with a charged colloidal system to obtain a loosely packed colloidal crystal with a single-crystalline-like *To whom correspondence should be addressed. Telephone: þ81-29-8604359. Fax: þ81-29-851-6159. E-mail:
[email protected].
(1) (a) Pieranski, P. Contemp. Phys. 1983, 24, 25. (b) Ito, K.; Sumaru, K.; Ise, N. Phys. Rev. B 1992, 46, 3105. (c) Ordering and Phase Transitions in Charged Colloids; Arora, A. K., Tata, B. V. R., Eds.; VCH: New York, 1996. (2) (a) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. (b) John, S. Phys. Rev. Lett. 1987, 58, 2486. (c) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals; Princeton University Press: Princeton, NJ, 1995. (d) Sakoda, K. Optical Properties of Photonic Crystals; Springer-Verlag: Berlin, 2001. (e) Special Issue on Photonic Crystals. Adv. Mater. 2001, 13, 369. (3) (a) Holtz, J.; Asher, S. A. Nature 1997, 389, 829. (b) Saito, H.; Takeoka, Y.; Watanabe, M. Chem. Commun. 2003, 2126. (c) Fudouzi, H.; Sawada, T. Langmuir 2006, 22, 1365. (4) Arsenault, A. C.; Puzzo, D. P.; Manners, I.; Ozin, G. A. Nat. Photonics 2007, 1, 468. (5) Fudouzi, H.; Xia, Y. Adv. Mater. 2003, 15, 892. (6) (a) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (b) Gu, Z.-Z.; Fujishima, A.; Sato, O. Chem. Mater. 2002, 14, 760. (7) Park, S. H.; Qin, D.; Xia, Y. Adv. Mater. 1998, 10, 1028. (8) Wijnhoven, J. E. G. L.; Vos, W. L. Science 1998, 281, 802. (9) Matsuo, S.; Fujine, T.; Fukuda, K.; Juodkazis, S.; Misawa, H. Appl. Phys. Lett. 2003, 82, 4283. (10) Fudouzi, H. J. Colloid Interface Sci. 2004, 275, 277. (11) (a) Ruhl, T.; Spahn, P.; Winkler, H.; Hellmann, G. P. Macromol. Chem. Phys. 2004, 205, 1385. (b) Pursiainen, O. L. J.; Baumberg, J. J.; Winkler, H.; Viel, B.; Spahn, P.; Ruhl, T. Adv. Mater. 2008, 20, 1484.
Langmuir 2009, 25(23), 13315–13317
texture over a large area through a fast and simple process.12,13 We show that such a gel film, which includes a liquid (water in the present case), can be converted into a dry film of densely packed colloidal crystal without cracks while maintaining the excellent optical qualities inherited from the starting state. We also show that the optical stop band wavelength of the present synthetic opal can be adjusted without changing the particle diameter, which is a great advantage in practical aspects. The fabrication process with loosely packed colloidal crystal gel film as the starting material is outlined as follows.12,13 The aqueous suspension of charged polystyrene particles (198 nm in diameter) in an ordered state (i.e., colloidal crystal) was tailored into a single-crystalline-like texture in a flat flow cell by using the shear-flow effect.12 The obtained single-domain crystal was immobilized through the photopolymerization of gelation reagents (a monomer, cross-linker, and polymerization initiator) added in advance to the starting suspension.13 Thus, we obtained a selfstanding gel film including the loosely packed colloidal crystal (about 10% in particle volume fraction) with dimensions of 50 9 0.1 mm3, but it contained a large amount of water. Since particles are bounded by polymer networks in the gelled colloidal crystal, it seems possible to obtain dry colloidal crystals without destroying the particle arrangement structure by removing the water. However, simply drying the gelled crystals in air results in a loss of iridescence, indicating that the regular particle arrays were destroyed. This must be caused by the inhomogeneous shrinkage of the gel, especially at the final stage of the drying process, and is probably a consequence of the gel tissue being unevenly pulled in different directions by the capillary force of the finishing water. To suppress the inhomogeneous shrinkage and deformation of the gel, we conceived of the plan shown in Scheme 1. The first two processes of Scheme 1 are for the fabrication of a loosely packed colloidal crystal gel film, as mentioned above. The third process is for homogeneous shrinkage of the gel. In this process, the specimen is soaked in aqueous solutions with increasing concentrations from 0% to 100% (in increments of 1-2 vol%) ethanol (EtOH), a solvent with poor (12) (a) Sawada, T.; Suzuki, Y.; Toyotama, A.; Iyi, N. Jpn. J. Appl. Phys. 2001, 40, L1226. (b) Kanai, T.; Sawada, T.; Kitamura, K. Langmuir 2003, 19, 1984. (c) Kanai, T.; Sawada, T.; Maki, I.; Kitamura, K. Jpn. J. Appl. Phys. 2003, 42, L655. (d) Kanai, T.; Sawada, T.; Toyotama, A.; Kitamura, K. Adv. Funct. Mater. 2005, 15, 25. (e) Kanai, T.; Sawada, T.; Kitamura, K. Chem. Lett. 2005, 34, 904. (13) (a) Toyotama, A.; Kanai, T.; Sawada, T.; Yamanaka, J.; Ito, K.; Kitamura, K. Langmuir 2005, 21, 10268. (b) Kanai, T.; Sawada, T.; Toyotama, A.; Yamanaka, J.; Kitamura, K. Langmuir 2007, 23, 3503.
Published on Web 10/29/2009
DOI: 10.1021/la9033854
13315
Letter
Kanai and Sawada Scheme 1. Fabrication Plan for Crack-Free Colloidal Crystal Film with a Large Area
Figure 1. Photograph of dried colloidal crystal gel film. Diameter of the film is about 4 mm.
affinity to the gel-polymer. Since the gel size is a function of EtOH concentration, a gel surrounded by a homogeneous solution with a slightly higher EtOH concentration than that in the preceding step can be shrunk gradually and homogeneously. After the gel is processed with 100% EtOH solution and shrunk to the densely packed state, the fourth process evaporates the EtOH from the shrunken gel; the capillary force problem is expected to be less serious because EtOH has a much lower surface tension than water (EtOH, 23 mN m-1; water, 73 mN m-1). Following the above scheme, we first prepared a wet gel film of loosely packed colloidal crystal (particle concentration: 10%). Polystyrene latex (Duke Scientific Corp., Palo Alto, CA; particle diameter, 198 nm; standard deviation, 3%) was deionized using mixed-bed ion-exchange resin (AG501-X8, Bio-Rad, Hercules, CA) in vials until the suspension showed iridescence indicative of a crystal phase. This colloidal crystal was mixed with aqueous solutions of N-methylol-acrylamide (N-MAM) as a monomer, N,N’-methylene-bisacrylamide (BIS) as a cross-linker, and camphorquinone (CQ) as a polymerization initiator for the visible light; its absorption maximum is 468 nm. Here, the N-MAM and BIS concentrations were 1200 and 30 mM, respectively; the CQ concentration was 0.4 mM. The reaction solution was bubbled with Ar gas for 10 min to remove dissolved O2 and CO2; it was then injected into a flat capillary cell (interior dimensions: 0.1 mm thick, 9 mm wide, and 50 mm long) made of glass and processed with a momentary strong shear flow to obtain a single-crystallinelike domain for the whole capillary. The glass capillary cell was uniformly irradiated by high-brightness blue LED arrays (Moritex Corp., Tokyo, Japan; peak wavelength emission of 470 nm) to polymerize the gelation reagents. After removing the gel film from the fabrication cell, we obtained the self-standing wet gel film including the loosely packed colloidal crystal. A circular fragment (diameter, 7 mm; thickness, 0.1 mm) was cut from the wet gel film and converted to a dried gel following steps 3 and 4 of Scheme 1. The circular gel was finally contracted down to 52% in linear scale, corresponding to a particle volume fraction of about 73% that almost achieves the sphere-touching condition. Figure 1 is a photograph of the resultant dried sample; it shows an even, round shape with a remarkable diffraction color, indicating that isotropic contraction preserving the crystalline order is achieved after completing all of the Scheme 1 processes. 13316 DOI: 10.1021/la9033854
Figure 2. Images of dried colloidal crystal gel film at various scale sizes, observed by using an optical microscope (A, B) and a scanning electron microscope (C-F).
Figure 3. Transmission spectra of colloidal crystal gel film swelled in water, shrunk in 100% EtOH, and dried. The dark level of the transmittance increases by a few percent because of the characteristics of the charge-coupled device spectrometer.
The regularity of the particle arrays was directly confirmed through scanning electron microscopy and is shown in Figure 2E and F. It should be noted that the dry colloidal crystal has no cracks at any scale; this can be confirmed in Figure 2, where images of the dried sample are shown at various scale sizes observed by using an optical microscope (A and B) and a scanning electron microscope (C-F). Figure 3 shows the transmission spectra for the normal incidence of a probe light for a sample of the gelled colloidal crystal at representative stages in Scheme 1: before step 3 (swelled in water), after step 3 (shrunk in 100% EtOH), and after step 4 (dried). All of the spectra show a sharp dip at the stop band and high transmittance at the pass band wavelength, suggesting that, Langmuir 2009, 25(23), 13315–13317
Kanai and Sawada
throughout Scheme 1, the crystallinity of the gel sample is preserved not only at the surface but also across its thickness. (Deterioration of the crystallinity through the sample results in degradation of the transmission characteristics, such as deformation of the dip shape and a reduction in the transmittance in the pass band.12,13) Here, the shift in the dip wavelength is caused by the contraction or reduction in the lattice spacing and the change in the refractive index of the medium because it is determined by the Bragg condition λ = 2nd, where λ is the dip wavelength, n is the refractive index of the colloidal crystal, and d is the spacing of the lattice planes parallel to the gel surface (111 planes of the face-centered cubic structure in the present case). The reduction in the dip width and depth should be caused by changes in the refractive index contrast between the particles and background. In conventional synthetic opals, the particles touch each other; since the lattice spacing of the crystal is uniquely determined as a function of particle diameter, the stop band wavelength cannot be varied unless the refractive index changes. Therefore, in order to make crystals with significantly different photonic band properties, it is required to use particles with different diameters or to develop additional processes to widen the interparticle spacing.14 For the present dry colloidal crystal, however, we found that the stop band wavelength could be adjusted over a wide range simply by varying the amount of gelation reagent without changing the particle diameter. Figure 4A shows the normal reflection spectra of dry colloidal crystal gels obtained from wet ones prepared with different monomer concentrations, and Figure 4B shows the plots of the peak wavelength read from Figure 4A as a function of monomer concentration. The data show that the stop band wavelength can be elongated by as much as 14%. The dependence (14) Fudouzi, H.; Sawada, T. Langmuir 2006, 22, 1365. (15) The solid line in Figure 4B is calculated using the Bragg equation by assuming that the space among particles is filled with gel-polymer: λ = 2na(2π/ √ 9 3φPS)1/3, where λ is the Bragg wavelength for normal incidence to the 111 lattice plane of the face-centered cubic lattice, a is the particle diameter (a = 198 nm), φPS is the volume fraction of the polystyrene particles in the dried sample, and n is the refractive index of the colloidal crystal given by the volume-weighted average for the refractive indices of the components n = nPSφPS þ nGel(1 - φPS); nPS and nGel are the refractive indices of the polystyrene particle (nPS = 1.59) and fully densified gel-polymer (nGel = 1.41, which is approximated by the index of N-MAM16), respectively. φPS is given by the particle and gel-polymer volume fractions φPS-wet and φGel-wet for the sample before drying (indicated with the suffix wet) as φPS = φPS-wet/(φPS-wet þ φGel-wet). φGel-wet would be calculated as φGel-wet = (MGel-monomerCGel-monomer þ MGel-cross-linkerCGel-cross-linker)/FGel, where MGel-monomer and MGel-cross-linker are the molecular weights of the gel-monomer and gel-cross-linker (MGel-monomer = 101.11, MGel-cross-linker = 154.17), respectively, CGel-monomer and CGel-cross-linker are the concentrations of the gel-monomer and gelcross-linker upon preparing the wet gels, respectively, and FGel is the density of the fully densified gel-polymer (FGel = 1082 g L-1, which is approximated by the density of N-MAM16).
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Figure 4. (A) Normal reflection spectra of dried colloidal crystal gel films prepared with different gel-monomer concentrations. (B) Plots of the wavelength peaks read from (A) as a function of the gel-monomer concentration. The solid line was calculated using the Bragg equation, assuming that the space among particles is filled with gel-polymer.15
of the stop band wavelength on the monomer concentration can be explained by assuming that the space between nontouching particles is filled with gel-polymer, as shown by the solid curve in Figure 4B.15 The adjustability of the stop band wavelength without changing the particle diameter is a great advantage, especially for industrial applications. In conclusion, we obtained synthetic opals via loosely packed colloidal crystals formed in a charged colloidal dispersion; this new route is fairly different from other known methods. Since in charged systems there is a possibility of fabricating good crystals, though loosely packed, on a scale as large as a centimeter order,13,17 the present findings open a new route for producing large high-quality synthetic opals. (16) http://www.chemicalbook.com/ChemicalProductProperty_EN.aspx? CBNumber=CB3182194. (17) (a) Wakabayashi, N.; Yamanaka, J.; Murai, M.; Ito, K.; Sawada, T.; Yonese, M. Langmuir 2006, 22, 7936. (b) Toyotama, A.; Yamanaka, J.; Yonese, M.; Sawada, T.; Uchida, F. J. Am. Chem. Soc. 2007, 129, 3044.
DOI: 10.1021/la9033854
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