Article pubs.acs.org/Langmuir
Recrystallization and Zone Melting of Charged Colloids by Thermally Induced Crystallization Mariko Shinohara,† Akiko Toyotama,† Misaki Suzuki,† Yukihiro Sugao,† Tohru Okuzono,† Fumio Uchida,‡ and Junpei Yamanaka*,† †
Graduate School of Pharmaceutical Sciences, Nagoya City University, 3-1 Tanabe, Mizuho, Nagoya, Aichi 467-8603, Japan Fuji Chemicals Company, Ltd., 1-35-1 Deyashiki-Nishi, Hirakata, Osaka 573-0003, Japan
‡
S Supporting Information *
ABSTRACT: We examined the application of recrystallization and zone-melting crystallization methods, which have been used widely to fabricate large, high-purity crystals of atomic and molecular systems, to charged colloidal crystals. Our samples were aqueous dispersions of colloidal silica (with particle diameters of d = 108 or 121 nm and particle volume fractions of ϕ = 0.035−0.05) containing the weak base pyridine. The samples crystallized upon heating because of increases in the particle charge numbers, and they melted reversibly on cooling. During the recrystallization experiments, the polycrystalline colloids were partially melted in a Peltier cooling device and then were crystallized by stopping the cooling and allowing the system to return to ambient temperature. The zone-melting crystallization was carried out by melting a narrow zone (millimeter-sized in width) of the polycrystalline colloid samples and then moving the sample slowly over a cooling device to recrystallize the molten region. Using both methods, we fabricated a few centimeter-sized crystals, starting from millimeter-sized original polycrystals when the crystallization rates were sufficiently slow (33 μm/s). Furthermore, the optical quality of the colloidal crystals, such as the half-band widths of the diffraction peaks, was significantly improved. These methods were also useful for refining. Small amounts of impurity particles (fluorescent polystyrene particles, d = 333 nm, ϕ = 5 × 10−5), added to the colloidal crystals, were excluded from the crystals when the crystallization rates were sufficiently slow (∼0.1 μm/s). We expect that the present findings will be useful for fabricating large, high-purity colloidal crystals.
1. INTRODUCTION Submicrometer-sized charged colloidal particles dispersed in liquid media self-assemble to form ordered “crystal” structures as a result of strong electrostatic interactions between the particles.1−6 Over the last few decades, charged colloidal crystals have received considerable attention as novel optical materials, such as photonic crystals,7−10 because their Bragg wavelengths usually lie in the visible to near-infrared region. Generally, colloidal crystals are polycrystals composed of millimeter- to submillimeter-sized crystal grains that contain various lattice defects. To produce the large, high-quality crystals that are often required for material applications, various ingenious approaches have been devised. These include shear annealing of the polycrystals confined in submillimeter gaps,11,12 epitaxial growth from 2D templates,13 and crystallization under external fields.14 All of these methods have provided large-area, thin ( 30 °C; such peaks were not detected at the lower T values. The crystallization was thermoreversible (i.e., the crystals that formed at T > 30 °C melted when cooled to T < 30 °C). Kossel line diffraction measurements17 showed that the crystals had a body-centeredD
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upper element (T = 5 °C) was brought into contact with the cell so that these two Peltier elements cooled the same region of the sample from the upper and lower surfaces of the cell. Figure 5a displays photographs that show the change in the
crystal/melt boundaries (X*) from the photographs (Figure 5a). In Figure 5c, variations in x* and X* over time are shown by filled and open circles. During the melting process, x* and X* were in close agreement. This means that the sample melted instantaneously when the temperature of the sample reached T*. However, there existed a time lag of a few hundred seconds for the recrystallization process. This is probably due to the presence of a supercooled state because the temperature increased very rapidly after we stopped the cooling. We frequently observed that crystal melting was initiated at the crystal grain boundaries. This was presumably because the grain boundaries were more or less disordered. Alsayed et al.29 reported that melting was initiated from the grain boundaries for noncharged microgel colloidal crystals. The present observation for charged colloids is in accordance with their finding. In addition, we sometimes observed that thin crystal layers were formed from the container wall, before bulk crystal formation. Heterogeneous nucleation of small crystals from the wall occurs when the temperature variation was fast, but it was not observed at sufficiently slow temperature changes. The grain size of the recrystallized sample was significantly dependent on the recrystallization rate, and it was sometimes much larger than the original size. Figure 6a presents an overview of the partially recrystallized sample (S1, [Py] = 50
Figure 5. (a) Photographs showing the local melting and recrystallization process of the colloidal silica (S2, [Py] = 54 μM, ϕ = 0.05). (b) Temperature distributions on the cell surface determined by thermography. (c) Locations of the crystal/melt boundaries determined from the video images of the crystals (x = x*, filled circles) plotted against time t. A region of the sample located at x = 20−25 mm was cooled at t = 0−900 s. Open symbols are the locations satisfying T(x) = T* (x = X*) as determined from thermography.
overviews of the samples with time t. (The upper element was removed for photographic purposes.) The original crystal exhibited a uniform greenish Bragg diffraction color. Within t = 300 s, the cooled portion of the crystals (width about 5 mm) began to melt. Then, the molten area extended over time, but it was not significantly enlarged after t ≈ 600 s. Subsequently, we set all of the Peltier elements to T = 24 °C at t = 900 s, which caused recrystallization. The entire sample was restored to the crystal state by t = 1380 s. Figure 5b presents the temperature distribution on the upper cell surface obtained by thermography, during the abovementioned melting/recrystallization processes. The data points satisfying T = n × (5 ± 0.1) °C (n is an integer) are indicated by black points in Figure 5b. From these thermographs, we determined the temperature profiles along the central line in the longitudinal direction of the cell (shown in Figure S-1) and estimated the location x (distance from the left end of the cell) for T(x) = T* = 17 °C (denoted as x*, with two x* values existing in each profile). We also estimated the x value at the
Figure 6. (a) Overview of the partially recrystallized sample (S1). [Py] = 50 μM and ϕ = 0.035. (b) Transmission spectra taken at various locations in original (top) and recrystallized (bottom) regions. The measured locations are indicated by in part a. (c) Half-dip width Δ of the spectra in the original and recrystallized regions. Data are given for samples prepared at three values of [Py]. E
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μM, ϕ = 0.035; T* = 10 °C). The original crystals, prepared at TR = 20 °C, were composed of submillimeter-sized grains. The sample was then melted from the right end by cooling. After the portion within 20 mm from the right end was melted (the crystal/melt boundary is shown by a dashed line in Figure 6a), the sample was kept at T = 20 °C to allow recrystallization. After 20 min, a centimeter-sized crystal grain was formed by recrystallization. Figure 6b shows transmission spectra of the sample at the several points shown in Figure 6a (1−4, original part; 5−7, recrystallized part). A number of small dips, resulting from the randomly oriented polycrystal structure, were seen in the original portion. However, a sharp single dip was observed in the recrystallized region. Thus, the optical properties of the crystals were improved by the recrystallization. In Figure 6c, the half-dip widths of the transmission spectra Δ in the original and recrystallized regions are shown as blue and red circles for samples with three values of [Py] (bars represent the experimental error as the standard deviations). The Δ values were reduced by about 1 nm after the recrystallizations, presumably as a result of reductions in the areas of the grain boundaries. The observed Δ values were somewhat smaller at lower [Py] (i.e., at lower values of Z). This might be due to the slower growth rate at the lower [Py], which results in more regular crystal arrangements. The smallest Δ value obtained by recrystallization was about 4.5 nm, which was smaller than that obtained in our study using unidirectional crystallization under a temperature gradient (6 nm).17 3.3. Zone Melting. On the basis of the above-mentioned findings, we then examined zone melting. Figure 7 shows
Figure 8. Overviews of the colloidal crystal sample (S1) before and after zone melting at various values of v. [Py] = 50 μM and ϕ = 0.05.
after zone melting at v = 700 μm/s, but at v = 500 μm/s, the grain size increased to 1 to 2 mm. At v = 300 μm/s, large centimeter-sized crystals were formed; however, the sample still contained crystal grains of a few millimeters in size. At v = 33 μm/s, the resultant crystal exhibited an even diffraction color, which suggested that a large crystal, with a size as large as that of the cell (1 × 10 × 40 mm3), was formed. The larger crystal sizes observed at the lower values of v can be explained in terms of incomplete melting at large v. When the zone-melting rate was too fast, some of the crystals remained (presumably as islandlike crystals) and acted as crystallization nuclei during recrystallization. Because the crystal grain size was larger at lower nuclei density, larger crystals should have been formed at lower v. The Bragg wavelength of the crystals was 554 nm, which did not change significantly upon zone melting. The Δ value decreased from 6.3 to 5.3 nm by zone melting at v = 33 μm/s. As mentioned above, we could improve the size and optical quality of the colloidal crystals by zone melting. Furthermore, we also succeeded in immobilizing the crystal obtained by zone melting in a polymer (poly(N-methylolacrylamide)) gel matrix16,18,42,43 using a previously reported method.16 A photograph and the transmission spectra of the gelled crystals are presented in Figure S-2. 3.4. Refining Due to Recrystallization and Zone Melting. We recently reported that impurity particles added to colloidal crystals were excluded from the crystals during crystal growth33,34 and grain growth34 processes. Here, we examined the exclusion of impurity particles during recrystallization and zone melting. The sample used was S2 silica (ϕ = 0.035, [Py] = 65 μM; T* = 20 °C), which contained a small number of impurity particles (fluorescent polystyrene particles, diameter = 333 nm, ϕ = 5 × 10−5). The original polycrystals were made at 25 °C. Then, all of the Peltier elements were set at 25 °C, and at t = 0, the central Peltier unit was cooled to 5 °C. Simultaneously, the upper part of the cell was cooled by another Peltier unit to −10 °C. The time evolutions of the impurity distributions were examined by fluorescence microscopy.
Figure 7. Temperature profiles of the cell surface during zone melting. The cooled zone moved from right to left. Movement rate: v = 50 μm/ s.
examples of thermographs obtained during the zone-melting process. Here, we moved the melted zone in the longitudinal direction of the sample (apparatus 1, moved from right to left at v = 50 μm/s). We first focused on the influence of the movement rate on the crystal size using apparatus 2. The silica sample was S1 ([Py] = 50 μM, ϕ = 0.05) with T* = 10 °C. The sample was locally melted at 5 °C. Figure 8 displays overviews of the samples before and after zone melting at v = 700, 500, 300, and 33 μm/s. The original crystals consisted of submillimeter-sized fine crystal grains. The grain sizes did not change significantly F
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Figure 9a shows micrographs that reveal the variation in the distribution of impurity particles with time. The crystal region
Figure 10. (a) Fluorescence micrographs showing distributions of impurity particles after zone melting. (b) Concentration profiles of the impurity particles before and after zone melting.
4. CONCLUSIONS In this Article, we studied the recrystallization and zone melting of colloidal silica based on the thermally induced crystallization of silica + Py colloids. The crystallization mechanism was attributable to an increase in the particle charge number on heating. The crystal size and optical quality were significantly improved by zone melting. When the movement rate was sufficiently slow (33 μm/s), a few centimeter-sized crystals were formed from millimeter-sized original polycrystals, whereas the crystal sizes were smaller at faster movement rates. The resulting crystals had narrower half-band widths in the reflection spectra compared to those of the large crystals obtained in our previous studies (e.g., unidirectional crystallization under temperature gradients). The impurity particles, having different sizes from the majority, were excluded during the recrystallization and zone melting at a very slow movement rate (about 0.1 μm/s). We expect that the present findings will be useful for the fabrication of large, high-purity colloidal crystals.
Figure 9. (a) Fluorescence micrographs showing the spatial distributions of the impurities (fluorescent polystyrene particles, ϕ = 5 × 10−5) in the partially recrystallized sample (S2). [Py] = 65 μM and ϕ = 0.05. (b) Time dependence of the impurity particle concentration profiles.
was formed within t < 10 min and grew from left to right, as shown in Figure 9a. The impurity particles were swept toward the crystal/melt boundary and accumulated in the boundary regions. In Figure 9b, we demonstrate the impurity concentration estimated as a function of location x, which was taken along the crystal growth direction. The impurity concentrations were calculated from the fluorescence intensities averaged perpendicularly in the x direction in each micrograph. We confirmed that the integrated particle concentration thus determined was conserved within experimental error (less than 5%) during recrystallization. From Figure 9b, it was clear that the impurity particles, which were almost homogeneously distributed in the original sample, were excluded by recrystallization. From the rate of migration of the peak position in Figure 9b, the exclusion rate was estimated to be about 0.2 μm/s. We also examined the exclusion of impurities by zone melting. Although an increase in the crystal size was distinctly observed at v = 33 μm/s, the exclusion of impurity particles was not observed at v ≥ 0.5 μm/s. When we reduced the value of v to 0.1 μm/s, the impurity particles were excluded. We note that the recrystallization rate at which the exclusions were observed (0.2 μm/s) was close to this v value. Figure 10a shows confocal laser scanning microscopy (LSM) images that reveal the distributions of impurity particles obtained by zone melting. A narrow region of the original sample (width = 5 mm) was melted. The LSM image was obtained at t = 4 h, when the width of the recrystallized region was about 1000 μm. Figure 10b shows the concentration profiles of the impurity particles before (blue) and after (red) zone melting. Obviously, the impurity particles had accumulated in the recrystallization/melt boundary. Further systematic studies, including an effect of multiple melting, are in progress concerning refining by zone melting.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details on the temperature dependence of the degree of dissociation of Py and the charge number of silica particles. Additional data on temperature profiles of the recrystallization process. Reflection spectra of the gelled crystals. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest. G
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materials are molten, and a seed crystal rod is dipped into the melt. The seed crystal is then slowly pulled upwards (and usually rotated simultaneously) typically at a rate of few millimeters per second with precise control of the ambient temperature. Large ingots of a single crystal, more than a few tens of centimeters in diameter and 2 m in length, are obtainable by this method. Various applications of the CZ method are described in ref 19. (22) In metallurgy and crystal growth science, the term recrystallization implies a process by which deformed crystal grains are replaced by a new set of undeformed grains. (23) Zhang, K.-Q.; Liu, X. Y. Controlled Formation of Colloidal Structures by an Alternating Electric Field and its Mechanisms. J. Chem. Phys. 2009, 130, 184901. (24) Grier, D. G. On the Points of Melting. Nature 1996, 379, 773− 775. (25) Villanova-Vidal, E.; Palberg, T.; Schöpe, H. J.; Löwen, H. NoneEquilibrium Melting of Colloidal Crystals in Confinement. Philos. Mag. 2009, 89, 1695−1714. (26) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1976; Chapter 6. (27) Yamanaka, J.; Yoshida, H.; Koga, T.; Ise, N.; Hashimoto, T. Reentrant Solid-Liquid Transition in Ionic Colloidal Dispersions by Varying Particle Charged Density. Phys. Rev. Lett. 1998, 29, 5806− 5809. (28) One can control the crystallization through changes in the ionic strength and particle volume fraction. Thermally induced crystallizations of thermoresponsive microgel colloids have been reported.29 In addition, the crystallization of hard sphere colloids was reportedly controllable by changing T through a variation in the osmotic pressure.30 Crystallization in colloid + polymer mixtures also exhibited a temperature dependence.31,32 (29) Alsayed, A. M.; Islam, M. F.; Zhang, J.; Collings, P. J.; Yodh, A. G. Premelting at Defects within Bulk Colloidal Crystals. Science 2005, 309, 1207−1210. (30) Chen, Z.; Russel, W. B.; Chaikin, P. M. Controlled Growth of Hard-Sphere Colloidal Crystals. Nature 1999, 401, 893−895. (31) Vrij, A.; Penders, M. H. G.; Rouw, P. W.; de Kruif, C. G.; Dhont, J. K. G.; Smits, C.; Lekkerkerker, H. N. W. Phase-Transition Phenomena in Colloidal Systems with Attractive and Repulsive Particle Interactions. Faraday Discuss. Chem. Soc. 1990, 90, 31−40. (32) Savage, J. R.; Blair, D. W.; Levine, A. J.; Guyer, R. A.; Dinsmore, A. D. Imaging the Sublimation Dynamics of Colloidal Crystallites. Science 2006, 314, 795−798. (33) Sugao, Y.; Yoshizawa, K.; Toyotama, A.; Okuzono, T.; Yamanaka, J. Striation Pattern of Impurity Particles in Charged Colloidal Crystals Formed by Stepwise Thermally Induced Crystallization. Chem. Lett. 2012, 41, 1163−1165. (34) Yoshizawa, K.; Okuzono, T.; Koga, T.; Taniji, T.; Yamanaka, J. Exclusion of Impurity Particles during Grain Growth in Charged Colloidal Crystals. Langmuir 2011, 27, 13420. (35) Nozawa, J.; Uda, S.; Naradate, Y.; Koizumi, H.; Fujiwara, K.; Toyotama, A.; Yamanaka, J. Impurity Partitioning During Colloidal Crystallization. J. Phys. Chem. B 2013, 117, 5289−5295. (36) Ghofraniha, N.; Tamborini, E.; Oberdisse, J.; Cipelletti, L.; Ramos, L. Grain Refinement and Partitioning of Impurities in the Grain Boundaries of a Colloidal Polycrystal. Soft Matter 2012, 8, 6214−6219. (37) Ashton, L. A.; Bullock, J. I. Effect of Temperature on the Protonation Constants of Some Aroatic, Heterocyclic Nitrogen Bases and the Anion of 8-Hydroxyquinoline. J. Chem. Soc., Faraday Trans. 1 1982, 78, 1961−1970. (38) In ref 37, it was reported that the pKa of Py decreased with increasing T (i.e., Ka was larger at higher T). This does not imply that the dissociation of Py in the aqueous solution decreased with increasing T because when T increases the dissociation constant of water molecules increases more rapidly than does that of Py. Calculations based on mass action show that the base dissociation constant of Py, pKb (= pKw − pKa; here pKw = −log10 Kw and Kw is the
ACKNOWLEDGMENTS We express sincere gratitude to Mr. Yasuharu Kakehashi, Mr. Yuki Watanabe, and Mr. Kyoichi Arakane, Advanced Engineering Services Co., Ltd., Japan, for discussions and the construction of the zone-melting apparatus. Sincere thanks are due to Dr. Tsutomu Sawada, National Institute of Materials Sciences, and Professor Satoshi Uda and Assistant Professor Jun Nozawa, Tohoku University, for their helpful discussions. A part of the present work was performed at Practical Application Research, Plaza Tokai, Japan Science and Technology Agency (JST). This work was partially supported by Kakenhi (20550165 to J.Y.).
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