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Morphological Evolution of Inorganic Crystal into Zigzag and Helical Architectures with an Exquisite Association of Polymer: A Novel Approach for Morphological Complexity Yuya Oaki and Hiroaki Imai* Department of Applied Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Received June 29, 2004. In Final Form: August 7, 2004 The morphology of potassium sulfate (K2SO4) crystals grown in a viscous polymer solution of poly(acrylic acid) (PAA) was remarkably changed from the tilted columnar assembly into zigzag and helical architectures with increasing PAA concentration. The habit modification of orthorhombic K2SO4 with adsorption of PAA molecules on a specified crystal face fundamentally led to the formation of tilted unit crystals. Concurrently with the habit modification, a diffusion-limited condition controlling the assembly of tilted units was achieved in the presence of PAA molecules in the matrix. Various complex morphologies, including zigzag and helical assembly, emerged through the formation of twinned crystals with the variation of the diffusion condition. Understanding the morphogenesis observed in this report would provide a novel approach for sophisticated crystal design by using an exquisite association of organic and inorganic materials.
Introduction Designing crystals with a tailored morphology promises to be a core technology toward the next stage in materials chemistry. Recently, much effort has been devoted to the biomimetic morphogenesis of inorganic materials.1 Various approaches for controlling crystal morphologies, such as particles, rods, wires, sheets, and more complex forms, have been explored in the past decade. Polymer-mediated crystallization is a typical technique for crystal design in ambient conditions. It is widely known that polymeric additives, including macromolecules, polyelectrolytes, and surfactants, induce the morphological evolution of inorganic crystals.2-28 For example, the morphological modi* To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +81 45 566 1556. Fax: +81 45 566 1551. (1) Examples of recent key reviews for biomimetic morphogenesis: (a) Antonietti, M.; Ozin, G. A. Chem.sEur. J. 2004, 10, 28-41. (b) Co¨lfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42, 2350-2365. (c) Dujardin, E.; Mann, S. Angew. Chem., Int. Ed. 2002, 13, 775-788. (d) Estroff, L. A.; Hamilton, A. D. Chem. Mater. 2001, 13, 3237-3235. (e) Mann, S. Angew. Chem., Int. Ed. 2000, 42, 3393-3406 and references therein. (2) Co¨lfen, H. Curr. Opin. Colloid Interface Sci. 2003, 8, 23-31 (recent review of CaCO3 morphology). (3) (a) Marentette, J. M.; Norwig, J.; Stockelmann, E.; Meyer, W. H.; Wegner, G. Adv. Mater. 1997, 9, 647-651. (b) Co¨lfen, H.; Antonietti, M. Langmuir 1998, 14, 582-589. (c) Co¨lfen, H.; Antonietti, M. Chem.s Eur. J. 2001, 7, 106-116. (e) Yu, S. H.; Co¨lfen, H.; Hartman, J.; Antonietti, M. Adv. Funct. Mater. 2002, 12, 541-545. (f) Rudloff, J.; Co¨lfen, H. Langmuir 2004, 20, 991-996 (double hydrophilic block copolymers). (4) (a) Walsh, D.; Mann, S. Nature 1995, 377, 320-323. (b) Walsh, D.; Lebeau, B.; Mann, S. Adv. Mater. 1999, 11, 324-328 (reverse micelle). (5) (a) Kato, T. Adv. Mater. 2000, 12, 1543-1546. (b) Hosoda, N.; Kato, T. Chem. Mater. 2001, 13, 688-693. (c) Kato, T.; Sugawara, A.; Hosoda, N. Adv. Mater. 2002, 13, 869-877. (d) Hosoda, N.; Sugawara, A.; Kato, T. Macromolecules 2003, 36, 6449-6452. (e) Sugawara, A.; Ishii, T.; Kato, T. Angew. Chem., Int. Ed. 2003, 42, 5299-5303. (f) Olszta, M. J.; Gajjeraman, S.; Kaufman, M.; Gower, L. B. Chem. Mater. 2004, 16, 2355-2362 (poly(acrylic acid) and modified substrate). (6) (a) Gower, L. A.; Tirrell, D. A. J. Cryst. Growth 1998, 191, 153160. (b) Donners, J. J. J. M.; Nolte, R. J. M.; Sommerdijik, A. J. M. J. Am. Chem. Soc. 2002, 124, 9700-9701. (c) Sugawara, T.; Suwa, Y.; Ohkawa, K.; Yamamoto, H. Macromol. Rapid Commun. 2003, 24, 847851. (d) MacKenzie, C. R.; Wilbanks, S. M.; McGrath, K. M. J. Mater. Chem. 2004, 14, 1238-1244 (poly(amino acids)). (7) (a) Grassmann, O.; Mu¨ller, G.; Lo¨bmann, P. Chem. Mater. 2002, 14, 4530-4535. (b) Yang, D.; Qi, L.; Ma, J. Chem. Commun. 2003, 11801181 (gelatin and agar hydrogel).
fication of carbonates,2-9 sulfates,10-15 chromates,16,17 phosphate,18-21 iron oxides,22-25 and zinc oxides26-28 has been extensively studied as a structural analogue of bioinorganic materials. The morphogenesis is generally (8) (a) Garcı´a-Ruiz, J. M. J. Cryst. Growth 1985, 73, 251-262. (b) Terada, T.; Yamabi, S.; Imai, H. J. Cryst. Growth 2003, 253, 435-444. (c) Imai, H.; Terada, T.; Yamabi, S. Chem. Commun. 2003, 484-485 (silica gel). (9) (a) Grassmann, O.; Lo¨bmann, P. Biomaterials 2004, 25, 277282. (b) Grassmann, O.; Lo¨bmann, P. Chem.sEur. J. 2003, 9, 13101316. (c) Grassmann, O.; Neder, R. B.; Putnis, A.; Lo¨bmann, P. Am. Mineral. 2003, 88, 647-652 (synthetic gel matrix). (10) (a) Qi, L.; Co¨lfen, H.; Antonietti, M. Angew. Chem., Int. Ed. 2000, 39, 604-607. (b) Qi, L.; Co¨lfen, H.; Antonietti, M.; Li, M.; Hopwood, J. D.; Ashley, A. J.; Mann, S. Chem.sEur. J. 2001, 7, 3526-3532 (double hydrophilic block copolymers). (11) Qi, L.; Co¨lfen, H.; Antonietti, M. Chem. Mater. 2000, 12, 23922403 (double hydrophilic block copolymers). (12) Li, M.; Mann, S. Langmuir 2000, 16, 7088-7094 (reverse micelle). (13) Hopwood, J. D.; Mann, S. Chem. Mater. 1997, 9, 1819-1828 (reverse micelle). (14) Rautaray, D.; Kumar, A.; Reddy, S.; Sainkar, S. R.; Sastry, M. Cryst. Growth Des. 2002, 2, 197-203. (15) Ress, G. D.; Evans-Gowing, R.; Hammond, S. J.; Robinson, B. H. Langmuir 1999, 15, 1993-2002 (calcium sulfate in reverse micelle). (16) (a) Li, M.; Schanblegger, H.; Mann, S. Nature 1999, 402, 393395. (b) Shi, H.; Qi, L.; Ma, J.; Cheng, H.; Zhu, B. Adv. Mater. 2003, 15, 1647-1651 (reverse micelle). (17) (a) Yu, S. H.; Antonietti, M.; Co¨lfen, H.; Hartmann, J. Nano. Lett. 2003, 3, 379-382. (b) Yu, S. H.; Co¨lfen, H.; Antonietti, M. Chem.s Eur. J. 2003, 8, 2937-2945 (double hydrophilic block copolymers). (18) (a) Zhang, W.; Liao, S. S.; Cui, F. Z. Chem. Mater. 2003, 15, 3221-3226. (b) Bradt, J. H.; Mertig, M.; Teresiak, A.; Pompe, W. Chem. Mater. 1999, 11, 2694-2701. (c) Girija, E. K.; Yokogawa, Y.; Nagata, F. Chem. Lett. 2002, 702-703 (collagen). (19) Bigi, A.; Boanini, E.; Walsh, D.; Mann, S. Angew. Chem., Int. Ed. 2002, 41, 2163-2166 (PAsp and PAA). (20) (a) Kneip, R.; Busch, S. Angew. Chem., Int. Ed. Engl. 1996, 35, 2624-2626. (b) Busch, S.; Dolhaine, H.; Duchesne, A.; Heinz, S.; Hochrein, O.; Laeri, F.; Podebrad, O.; Weiland, T.; Kniep, R. Eur. J. Inorg. Chem. 1999, 1643-1653 (gelatin). (21) Imai, H.; Tatara, S.; Furuichi, K.; Oaki, Y. Chem. Commun. 2003, 1952-1953 (PAA hydrogel). (22) Boal, A. K.; Headley, T. J.; Tissot, R. G.; Bunker, B. C. Adv. Funct. Mater. 2004, 14, 19-24 (protein). (23) (a) Reeves, N. J.; Mann, S. J. Chem. Soc., Faraday Trans. 1991, 87, 3875-3880. (b) Cheon, J.; Kang, N. J.; Lee, S. M.; Lee, J. H.; Yoon, J. H.; Oh, S. J. J. Am. Chem. Soc. 2004, 126, 1950-1951 (small organic molecules). (24) Xiong, Y.; Xie, Y.; Chen, S.; Li, Z. Chem.sEur. J. 2003, 9, 49914996 (1,10-phenanthroline). (25) Lian, S.; Kang, Z.; Wang, E.; Jiang, M.; Hu, G.; Xu, L. Solid State Commun. 2003, 127, 605-608 (PEG).
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ascribed to a habit modification with adsorption of polymeric additives on specified crystal faces. Appropriate design of polymers as a crystal modifier would lead to the tailored architecture of inorganic materials. Many researchers believe in the great potential of polymermediated crystallization as a novel technique for the morphological control of functional materials. However, the morphological complexity in recent reports is not always attributable to a simple habit modification. The sophisticated assembly of unit crystals resulted in various well-designed architectures in biomineralization.1 In our previous studies, we successfully demonstrated that the diffusion of solutes was a key aspect for the assembly and subsequent morphology of inorganic crystals in the presence of macromolecules.29-31 Polyhedral forms exhibiting specific habits were changed into various dendritic morphologies with the suppression of diffusion of the solutes in the presence of organic macromolecules.29 Typical diffusion-limited morphologies, including dendritic forms, were actually reported on the crystallization in the presence of polymeric additives.10b,15,32-34 Understanding the association between diffusion and morphogenesis constitutes a significant step toward the strict design of crystals. Twisted and helical morphologies that have chirality are one of the fascinating architectures in nature. The macroscopic chirality of the morphology is of central importance to a broad range of scientific and technological investigations. The formation of macroscopic helical forms of macromolecules and polymers is mainly ascribed to the twisted assembly of chiral molecules.35 Helical inorganic morphologies have also been reported in recent years; examples of such materials are carbonate,6a,8a,8b potassium dichromate,36 barium sulfate,11-13 silica-surfactant composite,37 manganese oxide,38 and carbon coils.39 The unique architectures are strongly expected for novel types of functional materials that have specific optical, electric, (26) (a) Oner, M.; Norwig, J.; Meyer, W. H.; Wegner, G. Chem. Mater. 1998, 10, 460-463. (b) Taubert, A.; Ku¨bel, C.; Martin, D. C. J. Phys. Chem. B 2003, 107, 2660-2666 (double hydrophilic block copolymers). (27) (a) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J. J. Am. Chem. Soc. 2002, 124, 12954-12955. (b) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J.; Rodriguez, M. A.; Konishi, H.; Xu, H. Nat. Mater. 2003, 2, 821-826 (citric acid). (28) Imai, H.; Iwai, S.; Yamabi, S. Chem. Lett. 2004, 33, 768-769 (phosphonate anion). (29) Oaki, Y.; Imai, H. Cryst. Growth Des. 2003, 3, 711-716 and references therein. (30) Imai, H.; Oaki, Y. Angew. Chem., Int. Ed. 2004, 43, 1363-1368. (31) Oaki, Y.; Imai, H. J. Am. Chem. Soc. 2004, 126, 9271-9275. (32) Leontidis, E.; Kleitou, K.; Kyprianidou-Leodidou, T.; Bekiari, V.; Lianos, P. Langmuir 2002, 18, 3659-3668 (Au). (33) (a) Zhou, Y.; Yu, S. H.; Wang, C. Y.; Li, X. G.; Zhu, Y. R.; Chen, Z. Y. Adv. Mater. 1999, 11, 850-852. (b) Wang, X.; Itoh, H.; Naka, K.; Chujo, Y. Langmuir 2003, 19, 6242-6246 (Ag). (34) Kuang, D.; Xu, A.; Fang, Y.; Liu, H.; Frommen, C.; Fenske, D. Adv. Mater. 2003, 14, 1747-1750 (PbS). (35) Helical architectures of various materials were categorized and summarized in our previous article, ref 31. (36) (a) Suda, J.; Nakayama, T.; Nakahara, A.; Matsushita, M. J. Phys. Soc. Jpn. 1996, 65, 771-777. (b) Suda, J.; Nakayama, T.; Matsushita, M. J. Phys. Soc. Jpn. 1998, 67, 2981-2983 (potassium dichromate). (37) (a) Yang, S. M.; Sokolov, I.; Coombs, N.; Kresge, C. T.; Ozin, G. A. Adv. Mater. 1999, 11, 1427-1431. (b) Kim, W. J.; Yang, S. M. Adv. Mater. 2001, 13, 1191-1194. (c) Seddon, A. M.; Patel, H. M.; Burkett, S. L.; Mann, S. Angew. Chem., Int. Ed. 2002, 41, 2988-2991. (d) Kim, W. J.; Yang, S. M. Adv. Mater. 2003, 13, 1191-1195. (38) (a) Giraldo, O.; Brock, S. L.; Marquez, M.; Suib, S. L.; Hillhouse, H.; Tsapatsis, M. Nature 2000, 405, 38. (b) Giraldo, O.; Marquez, M.; Brock, S. L.; Suib, S. L.; Hillhouse, H.; Tsapatsis, M. J. Am. Chem. Soc. 2000, 122, 12158-12163 (manganese oxide). (39) (a) Baker, R. T. K.; Harris, P. S.; Terry, S. Nature 1975, 253, 37-39. (b) Amelinckx, S.; Zhang, X. B.; Bernaerts, D.; Zhang, X. F.; Ivanov, V.; Nagy, J. B. Science 1994, 265, 635-639. (c) Yang, S.; Chen, X.; Motojima, S. Appl. Phys. Lett. 2002, 81, 3567-3569 (carbon coil).
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and catalytic behavior emergent from the chiral shape.40 However, the morphogenesis was barely grasped because these inorganic crystals were achiral components. Helical morphologies were obtained from the diffusion-limited growth of triclinic crystals, such as potassium dichromate and boric acid, in various kinds of gel matrixes without chiral molecules.30,31 We have proposed a “twisted twin model,” which was ascribed to the twinned assembly of tilted unit crystals with a slight rotation under a diffusionlimited condition.30 However, unsolved problems related to the growth condition and the formation mechanism remain. Here, we show a novel technique for the morphological control of an inorganic crystal in the presence of an organic polymer. Adsorption of poly(acrylic acid) (PAA) on certain crystal faces initially modified orthorhombic K2SO4 into a tilted platy unit. Since the polymer simultaneously formed a diffusion field around the tilted unit, the assembly of the units was controlled in a diffusion-limited condition according to the PAA concentration. As a result, various morphologies, including tilted columnar assembly, zigzag, curved, and helical shapes, emerged from the assembly process in the diffusion field. The habit modification and diffusion-limited growth were effectively incorporated into the crystal growth in the presence of a polymer. Remarkably, this is the first time that polymeric additives fulfill two important roles in crystallization at the same time. The results shown in this study are quite consistent with the twisted twin model that we have proposed.30 On the basis of this approach, appropriate tailoring of the polymeric additives would make the complex design of crystals possible. Experimental Section Materials and Procedures. Potassium sulfate (K2SO4; Kanto Chemical, 99.0%) was grown in a PAA (Mw ) 250 000, 35 wt % aqueous solutions; Aldrich Chemical) matrix to achieve both polymer-mediated and diffusion-controlled crystallization. Stock solutions containing 100 g dm-3 K2SO4 were prepared using purified water at room temperature. A specified amount of PAA aqueous solution was added into 20 cm3 of the stock solutions. The concentration of PAA (CPAA) was adjusted in the range from 0 to 20.0 g dm-3 (amount of 35 wt % PAA aqueous solution). After being completely dissolved, 10 cm3 of the precursor solution was poured into a polypropylene sample bottle (30 mm in diameter and 70 mm in height) and maintained at 25 °C for several days without sealing. Crystal growth proceeded as water evaporated. Characterization. The crystal morphology of the resulting materials was observed by using a field-emission scanning electron microscope (FESEM, Hitachi S-4700 and FEI Sirion). Other structural determinations for the products were performed using an X-ray powder diffractometer (XRD) with Cu KR radiation (Rigaku RAD-C), a field-emission transmission electron microscope (FETEM, Philips TECNAI-F20), thermogravimetrydifferential thermal analysis (TG-DTA, Seiko Instruments, TG-DTA 6200), and a Fourier transform infrared (FT-IR) spectrometer (BIO-RAD FTS-60A).
Results and Discussion Morphological Evolution of K2SO4 Crystal with Increasing PAA Concentration. Crystallization was completed in several days as water evaporated from the precursor solution. Totally mineralized material consisting of various morphologies of crystals, as shown in Figure 1, was obtained regardless of the initial PAA concentration (the macroscopic image is shown in Figure S1 in the (40) (a) Rowan, A. E.; Nolte, R. J. M. Angew. Chem., Int. Ed. 1998, 37, 63-68. (b) Hodgkinson, I.; Wu, Q. H. Adv. Mater. 2001, 13, 889897. (c) Sato, I.; Kadowaki, K.; Urabe, H.; Jung, J. H.; Ono, Y.; Shinkai, S.; Soai, K. Tetrahedron Lett. 2003, 44, 721-724.
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Figure 2. Schematic illustrations of the morphology of the K2SO4 unit crystal: (a) a single crystal grown from a 10 wt % K2SO4 aqueous solution without additives; (b) a tilted unit embedded in the unit cell; (c) a tilted platy unit with habit modification; (d,e) planar representation of the (021) and (110) faces, respectively. These crystallographic data were estimated from FESEM images, FETEM observations, XRD analysis, and the unit cell structure. The habit modification drawn in panel a induces the tilted unit in panels b and c. Figure 1. Morphological evolution of K2SO4 crystal with an increase in PAA concentration: (a) thin plates, (inset of panel a) embryos of growth perpendicular to the surface; (b) tilted columnar assembly; (c,d) columns with branches and bending; (e) zigzag architecture; (f) curved morphology; (g-i) helical morphologies and backbone. Habit modification through specific adsorption of PAA resulted in the plates and tilted units. The diffusion-limited condition in the presence of PAA controlled the assembly of tilted units and then induced morphological variation.
Supporting Information). We took some part of the mineralized material and then observed the morphology of crystals by using the FESEM without further washing. The morphology of crystals grown in a viscous PAA matrix remarkably changed as the density of the matrix was increased. Figure 1 shows the variation of the morphology of K2SO4 crystals grown with increasing PAA concentration. Thin plates were obtained in a relatively low PAA concentration (CPAA ) 3-5 g dm-3) through habit modification by PAA (Figure 1a). The alignment of the platy units in the specified crystallographic direction made up a tilted columnar assembly with CPAA ) 5-7 g dm-3 (Figure 1b). As the PAA concentration increased, the growth direction of the tilted assembly was turned and/or branched to a specified direction (CPAA ) 6-10 g dm-3, Figure 1c,d). Repetitive turns of the growth direction resulted in a zigzag architecture when the PAA concentration was increased to 10-16 g dm-3 (Figure 1e). Finally, curved and helical morphologies emerged from a high concentration of PAA (CPAA ) 10-20 g dm-3, Figure 1fh). Since crystal growth started with evaporation of about 5 cm3 of water from the precursor solution, the effective concentration of PAA was increased to twice the initial condition. In fact, crystal growth proceeded in viscous
polymer solution containing a high concentration of PAA. The interaction with PAA molecules reduced the diffusion rate of inorganic ions and produced a diffusion field around the crystal in the same manner that was previously demonstrated in gel and viscous polymer matrix.29 In this work, thus, PAA concentration changed the diffusioncontrolled condition and substantially influenced the architecture of the assembly of the tilted units. Detailed explanations related to the shape and the assembly of the tilted units will be discussed separately in the following sections. Formation of the Tilted Unit with Habit Modification. Figure 2a shows a typical habit of K2SO4 single crystals grown in aqueous solution without PAA. The habit is easily variable by the crystallization condition, such as the evaporation rate of water and adsorption of various additives. The architectures grown with PAA consisted of tilted platy units (Figure 1b-i), as illustrated in Figure 2b,c. The nonsymmetric morphology of the units was estimated from FESEM images and FETEM observations. The XRD measurement strongly indicated that the top face of the platy units was the (010). The remarkably enhanced (040) peak indicates that the top surface of the plate is the (010) face, because the plates were pressed and attached parallel to the holder plate in the measurement (see Figure S2 in the Supporting Information). The tilted angles on the (021) and (110) faces of the unit are 40° and 70°, respectively (Figure 2d,e). The directions of assembly and branching, as shown in Figure 1a-d, are reflected in the angles of the (021) and (110) faces. Fundamentally, adsorption of PAA on the (010) face changes the symmetric basic form (Figure 2a) into a thin plate with growth inhibition in the [010] direction (Figure 1a). The nonsymmetric unit could be formed by selective
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Figure 4. Schematic illustrations for the adsorption of PAA on the crystal face: (a) surface cleavage of the (010) face of K2SO4 (refs 41 and 42); (b,c) estimation for the distance of carboxy groups in the PAA molecule. Distance between potassium ions: OP ) 0.308 nm, OQ ) 0.577 nm, OR ) 1.16 nm, OS ) 0.937 nm, and OT ) 1.01 nm. Flexible PAA chains could interact with potassium ions on the (010) face through electrostatic interaction because the distance between the potassium ions and the carboxy groups is approximately matched. Figure 3. TG-DTA curves for (a) PAA, (b) K2SO4 with PAA, and (c) pure K2SO4 crystal. The weight loss and exothermal peaks around 500 °C indicate the thermal decomposition of a free PAA molecule. The later weight loss at 750 °C in panel b shows the presence of the PAA molecule with strong interaction with K2SO4. Since the pure K2SO4 crystal does not show any peaks except the endothermal peak with a crystal phase transition at 580 °C, the weight losses in panel b result from the thermal decomposition of PAA.
adsorption on the pair of (021) and (110) (or (02-1) and (1-10)) faces because the affinity of the PAA molecule to these pairs is different (Figure 2b,c). Finally, the titled plate surrounded by (010), (021), and (110) (or (010), (021), and (1-10)) faces is formed through habit modification (Figure 2a-c). The fundamental habit of the tilted unit mainly exhibiting the (010) face was not influenced by the change of PAA concentration, because the excess amount of PAA with increasing concentration mainly contributed to form a diffusion-controlled condition around the unit crystal, as described in the following section. TG-DTA strongly suggested the adsorption of PAA on the specified crystal faces (Figure 3). All the initially dissolved PAA was accompanied with the K2SO4 crystals during the evaporation of water, as shown in Figure 1. Although pure PAA was completely decomposed at 500 °C with an exothermal reaction (Figure 3a), the K2SO4 crystals containing PAA showed a two-step weight loss at 500 and 750 °C (Figure 3b). Pure K2SO4 crystals did not show any peaks, except the endothermal peak at 580 °C caused by the phase transition of K2SO4 crystal from orthorhombic to hexagonal (Figure 3c). Thus, we concluded that the weight losses in Figure 3b resulted from PAA.43 The results indicate that PAA molecules were included in two different styles in the architectures. The weight loss with an exothermal peak at 750 °C is assumed to be caused
by the decomposition of PAA strongly incorporated in the assembly of the crystals, while the decomposition at 500 °C is ascribed to the presence of PAA simply attached to the surface of the architectures. The cleavage of the (010) face,41,42 illustrated in Figure 4a, suggests that carboxy groups in PAA molecules could interact with potassium ions on the surface (Figure 4b,c). Thus, the PAA molecules exhibiting thermal degradation at 750 °C strongly adsorbed on the specified crystal face. In this way, PAA molecules that adsorbed on the specified faces play an important role in habit modification leading to the tilted unit and are incorporated in the assembly of the crystals. However, the detailed interaction and incorporation behavior of the PAA molecules with K2SO4 crystals are now under investigation. Formation of the Diffusion Field. The weight losses with thermal degradation at 500 and 750 °C corresponded to the amounts of PAA molecules simply attached to and strongly incorporated in the assembly of the crystals, respectively. About 20% of PAA molecules served as the incorporated PAA, which resulted in habit modification through specific adsorption. The whole morphology of the units was not fundamentally changed with increasing PAA concentration. The other PAA molecules (about 80%) would be attached to the surface of the crystals after evaporation of water from the precursor solution. An increase in the PAA concentration mainly contributed to an increase in the amount of simply attached PAA (41) Atomic arrangement of the potassium sulfate on the (010) plane in Figure 4a was illustrated by Crystal Designer (version 6.02, Crystal structure design AS, 1997). The atomic parameters were referred to ref 42. (42) Ojima, K.; Nishihata, Y.; Sawada, A. Acta Crystallogr. B 1995, 51, 287-293. (43) In addition, the melting point of pure K2SO4 crystal is 1067 °C.
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molecules. The attached PAA molecules provided a viscous matrix, reducing the diffusion rate of inorganic ions around the growing crystals in the precursor solution. Considering these facts, the diffusion-limited condition became more remarkable with increasing PAA concentration. As we previously proposed,29,30 the formation of a diffusion field with reduction of the diffusion rate in a gel and a viscous polymer matrix is essential for the morphological variation in Figure 1. Formation and Structure from a Tilted Columnar Assembly into a Zigzag Form. In a low concentration of PAA (CPAA ) 3-5 g dm-3), thin plates exhibiting (010) faces were obtained with habit modification by the specific adsorption of PAA (Figure 1a). Since an increase in the PAA concentration mainly increased the amount of attached PAA, a diffusion field was produced around the growing crystal with a reduction of the diffusion rate of the solutes. The formation of a steep diffusion field promotes crystal growth perpendicularly to the (010) face of the plate. A steep concentration gradient is produced on the (010) plane of the plate by the growth inhibition of PAA, whereas rapid intake of the solute provides a moderate diffusion field around the side faces (see Figure S3 in the Supporting Information). We found the embryo of the crystal growth on the surface (inset of Figure 1a). This fact suggests that growth in the [010] direction is not completely inhibited with the coverage of PAA on the (010) face. Consequently, an increase in the concentration gradient in a diffusion field with increasing PAA concentration strongly induces the vertical assembly of the platy units of K2SO4 crystals. Then, the tilted columnar assembly was observed in CPAA ) 5-7 g dm-3 (Figure 1b). A further increase in the PAA concentration resulted in a change in the growth direction with branching (Figure 1c,d) and then led to the formation of a zigzag architecture (Figure 1e). The tilted angles of the architectures, such as 40° and 70°, were naturally reflected to that of the tilted units (Figures 1b-e and 2c). The change in the direction of the assembly is ascribed to the presence of the reversed twins because the angles of bending are related to the specified angle at the edges of the platy unit, such as 40° and 70° (Figures 1 and 2). We previously demonstrated growth with the formation of twinned branches in a gel matrix.29,30 Dendrite of calcium oxalate was also reported with repeated twinning in a gel matrix.44 Evolution into Curved and Helical Forms with Twisted Twins. Curved and helical morphologies were obtained with a high PAA concentration (CPAA ) 10-20 g dm-3, Figure 1f-h). Under these conditions, an excessive amount of PAA would produce a steep concentration gradient in a diffusion field and promote further morphological evolution. In our previous study, we found that the backbone of helical morphology was composed of twisted twins of unit crystals with a specified rotating direction and angle.30 The coincident lattice was well matched on the twin plane at the angle. The helical morphology of K2SO4 as described here could be addressed in a similar manner. Figure 1h,i shows the presence of a backbone consisting of tilted units. From several FESEM observations (see Figure S4 in the Supporting Information), the tilted units in the backbone were aligned along the b axis with a rotation angle of about 4.5°. The pitch of the helix was varied by the size of the tilted units; the thicker units induced a longer pitch in the twisting. The backbone of these helical morphologies was easily reproduced by the assembly of replicas of the tilted unit with a rotation angle of 4.5° (Figure 5). We confirmed that the (44) Cody, A. M.; Cody, R. D. J. Cryst. Growth 1995, 151, 369-374.
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Figure 5. A simple model of a helical backbone. The replicas of the tilted platy unit (inset) connected with a rotation angle of about 4.5° reproduced the helical backbone.
Figure 6. Two-dimensional assembly of the tilted unit in a diffusion field: (a) Diffusion field around the tilted unit. (b) Formation of the twin. A nonsymmetric diffusion field is produced around the tilted unit. The concentration of PAA provides a specified concentration gradient or intensity of the diffusion field around the growing crystals. (c) Tilted columnar assembly grown in the original direction Z1. (d,e) Tilted column with branches and bending resulting from the formation of twins to achieve faster growth. (f) Zigzag architecture with accumulation of twins. This morphological evolution was fundamentally ascribed to the growth mechanism, as illustrated in panels a and b.
twin lattice is formed with a twisted angle of 4.5° on the (010) plane (see Figure S5 in the Supporting Information). These results strongly indicate that the backbone of the K2SO4 helices consists of the twisted twins of the tilted units. Assembly Control of the Tilted Unit in a Diffusion Field. Morphological evolution with increasing PAA concentration, as shown in Figure 1, is successfully explained by the assembly of the tilted unit in a diffusion field. When the diffusion process of the solute molecules plays an important role in crystal growth, the concentration gradient around the growing crystal is bound to have a strong influence on the growth behavior.30 If a tilted unit is placed in a two-dimensional diffusion field, the contour lines will be distributed nonsymmetrically, as shown in Figure 6a. The driving force of growth was anisotropic because of the nonsymmetric diffusion field around the tilted unit. Although the original growth direction of the tilted unit is direction Z1, the higher driving
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force for crystal growth is obtained along direction A, which is the higher concentration gradient (Figure 6a). Thus, a twin consisting of the reversed unit along direction Z2 would be introduced to achieve faster growth, and the original growth direction (Z1) would then be adjusted to direction Z (Figure 6b). In a diffusion field, faster growth with the formation of the twins is favorable rather than growth in the original direction even though excess interfacial energy is introduced at the interface. The whole morphology would be determined by the concentration gradient in a diffusion field around the growing crystals. In the present study, PAA molecules concurrently fulfill two important roles: one is a tilted subunit resulting from habit modification, and the other is a diffusion field reducing the diffusion, which is generated in the crystallization process. The concentration gradient or the intensity of a diffusion field is associated with the PAA concentration because only a part of PAA was utilized for habit modification. When a weak diffusion field is formed in a low PAA concentration, the tilted units are accumulated along the original growth direction Z1 and then make up a tilted column (Figure 6c). As the PAA concentration increases, the formation of reversed twins along direction Z2 is preferred to achieve faster growth in a relatively high concentration gradient. Consequently, branching and bending with the formation of a twinned connection occurred in the tilted columnar assembly (Figure 6d,e). The zigzag architecture is ascribed to the repetition of twinning with a steep gradient in a nonsymmetric diffusion field (Figure 6f). In this case, the orientation of the units (Z1, Z2, Z1′, Z2′, Z1′′, Z2′′, ...) was finally adjusted to the direction Z with repetitive twinnings because of the nonsymmetric diffusion field around the tilted unit. The growth behavior in a three-dimensional diffusion field is fundamentally the same as that of a twodimensional model (Figures 1 and 7). The evolution from a tilted column to a zigzag shape can be explained by similar mechanisms in a two-dimensional model. The most characteristic phenomenon in a three-dimensional diffusion field was the emergence of a curved and helical morphology in a relatively high PAA concentration. The nucleation probability of the completely reversed twin would be relatively small under a steep concentration gradient in a high PAA concentration. The formation of slightly rotated twins would be more favorable in such a diffusion field because faster growth is preferred to the most stable state under that condition. The angle of rotation is determined by lattice matching on the twin plane (see Figure S5 in the Supporting Information). The accumulation of slightly rotated twins adjusts the orientation of the growth on the units (Z1, Z2, Z3, ..., Z40, Z1′, ..., Z40′, Z1′′, ..., Z40′′, ..., as shown in Figure 7e,f) into the direction of the diffusion (direction Z). Therefore, the assembly of rotated twins induces the curved and helical morphologies that consisted of the tilted units. This twisted twin model is quite consistent with the experimental results, as shown in Figure 1. The handedness of the twisted shape is another interesting problem. We previously demonstrated that the macroscopic chirality in the morphology of crystals was kept in an achiral polymer matrix and in the presence of achiral molecules.30,31 Since the K2SO4 helices were grown in an achiral PAA matrix in this study, equal amounts of the right- and left-handed helices were naturally observed in many FESEM images.45 Precise tuning of the chirality, as reported in a previous study,31 is the next challenge for us. (45) As far as we counted, 11 right- and 9 left-handed helices were obtained.
Oaki and Imai
Figure 7. Three-dimensional assembly of the tilted unit: (a) tilted columnar assembly; (b,c) tilted column with branches and bending resulting from the formation of the twin; (d) zigzag architecture consisting of repetitive twins; (e,f) curved and helical morphology with the accumulation of slightly rotated twins, respectively. The formation mechanisms from the tilted assembly to the zigzag shape are the same as those of the twodimensional model. The key aspect of the curved and helical forms is the slightly rotated twins to achieve faster growth. Slightly rotated twins also adjust the original growth directions (Z1, Z2, Z3, ..., Z40) to direction Z. This model is quite consistent with the results in Figure 1.
The twisted twin model would be applicable for the crystallization of other systems in a diffusion-limited condition. Zigzag, curved, and helical morphologies of barium sulfate were reported to be synthesized in anionic reverse micelles using an anionic double-hydrophilic block copolymer.11-13 The formation of these morphologies may be attributed to a similar mechanism described in this work. Adsorption of polymeric additives would lead to a tilted unit from an orthorhombic barite crystal with habit modification. These complex morphologies are understandable by the assembly of the tilted units in a diffusion field. However, further investigations are needed for an overall understanding of the zigzag and helical architectures of various materials. Conclusions The habit modification of an orthorhombic K2SO4 crystal leads to tilted units through the specific adsorption of PAA molecules. As the PAA concentration increased, tilted columns, zigzags, and helices were generated from the assembly of units in a diffusion field with a specified concentration gradient. The morphogenesis is fundamentally ascribed to the twin formation to achieve faster growth in a steep concentration gradient formed around the crystals with an increase in the PAA concentration. The dual roles of PAA molecules in habit modification
Morphological Evolution of Inorganic Crystal
and the formation of a diffusion field were successfully demonstrated. This approach shows a great potential for precious crystal design of inorganic materials using macromolecules. Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research (No. 15560587) and the 21st Century COE program “KEIO Life Conjugate
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Chemistry” from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. Supporting Information Available: XRD pattern, FESEM images, and schematic models. This material is available free of charge via the Internet at http://pubs.acs.org. LA048400D
