CRYSTAL GROWTH & DESIGN
Emergence of Acute Morphologies Consisting of Iso-Oriented Calcite Nanobricks in a Binary Poly(Acrylic Acid) System
2006 VOL. 6, NO. 2 612-615
Takashi Miura, Akiko Kotachi, Yuya Oaki, and Hiroaki Imai* Department of Applied Chemistry, Faculty of Science and Technology, Keio UniVersity, 3-14-1 Hiyoshi, Kohoku-ku,Yokohama 223-8522, Japan ReceiVed May 20, 2005; ReVised Manuscript ReceiVed October 22, 2005
ABSTRACT: Acute spines and cones of calcite similar to biominerals in mollusks were grown in an aqueous solution system containing low- and high-molecular-weight poly(acrylic acid). The tapering morphologies consisting of polymer-mediated calcite bricks with a preferred crystallographic orientation were produced by the self-organized crystal growth in a diffusion field. Introduction Biomineralization has inspired a novel route of material synthesis to realize functional microstructures in aqueous solutions because biominerals are elaborate materials composed of small units highly tailored with organic molecules. Sophisticated architectures of biominerals consisting of CaCO3 in mollusks, such as a laminated architecture of nacre, a porous network of spicules, and an array of microlenses,1 are prototypes of artificial materials exhibiting high performance. Thus, studies on the formation of carbonate crystals mediated by organic macromolecules with carboxy groups have been performed as an interesting approach to mimick biomineralization.2-20 The studies have also served as basic research on the morphological control of various functional materials in micro- and nanoscales. The formation of CaCO3 films was reported in a supersaturated solution containing a small amount of poly(acrylic acid) (PAA) on a suitable surface, such as a Langmuir monolayer with carboxy groups at the solution-air interface2 and polysaccharide films on a glass substrate.3-8 Spiral forms, porous projections, and self-similar stars of CaCO3 were prepared in a solution containing poly(aspartic acid)9 and silicate ions.10-13 The morphology of CaCO3 crystals grown in a supersaturated solution was influenced by the presence of a variety of electrolytes as a suppressant of crystal growth and a template for the nucleation on a substrate. Recent research has tended to focus on the complicated combination of ionic species, molecules, particles, and substrates for the fabrication of highly tailored microstructures.14 Co¨lfen et al. reported that polymermediated calcite consists of three-dimensionally, well-aligned nanocrystals that are scaffolded to a so-called mesocrystal.15-17 Loste et al. described that the transformation of amorphous calcium carbonate (ACC) provides an effective synthesis route to manipulating the complex morphology of calcite crystals.18-20 However, the self-organized formation of three-dimensional architectures is not sufficiently understood. The authors previously reported that lozenge-shaped calcite plates consisting of iso-oriented crystal units were produced in a binary polyelectrolyte system.21 Low- and high-molecularweight PAAs performed as a moderate suppressant and a template of the crystallization, respectively. This paper shows the emergence of structural analogues of echinoderm spines grown in the final stage of precipitation in the binary polyelectrolyte system. This is an inspiring example of the development of three-dimensional sophisticated architectures similar to * To whom correspondence should be addressed. Fax: +81 45 5661551. Tel: +81 45 5661556. E-mail:
[email protected].
biominerals through the self-organized crystal growth in a nonequilibrium system. Experimental Procedures Crystals of CaCO3 were produced using the system described in our previous report.21 Precipitation was induced by the dissolution of CO2 into a 20 mM CaCl2 aqueous solution containing two kinds of PAA molecules (Mw: 2000 (PAA2k) and 250000 (PAA250k)) (SigmaAldrich). The concentrations of PAA2k and PAA250k were independently varied in the range between 8.0 × 10-4 (the concentration of carboxy groups: 0.11 mM) and 1.1 × 10-1 wt % (15 mM). Glass slides and a piece of single calcite crystal as a substrate were treated in a mixture of ethanol and potassium hydroxide aqueous solution and then washed with purified water. Vessels containing 100 cm3 of the precursor solution and substrates were covered with a polymer film with several pinholes and were placed in a 5 dm3 desiccator filled with CO2 generated by the decomposition of (NH4)2CO3 at room temperature (∼20 °C). The amount of (NH4)2CO3 charged into the desiccator was varied to control the reaction rate, and long-term precipitation over 6 days with 1.7 g of (NH4)2CO3 was suitable for the growth of tapering morphologies.
Results and Discussion Deposition occurred in a few days on a glass substrate and on a cleavage of a calcite single crystal in the aqueous solution. According to X-ray diffraction data collected with a Rigaku RAD-C system, calcite was produced on the substrates in the solutions containing binary PAAs. Acute spines of calcite (Figure 1a) were observed on the {101h4} surfaces of a calcite single crystal in a binary system of 2.1 × 10-2 wt % (3.0 mM) PAA2k and 7.2 × 10-2 wt % (10 mM) PAA250k using a Hitachi S-4700 field-emission scanning electron microscope (SEM). In contrast, small rhombohedral units (Figure 1b) were formed on the surface in a supersaturated solution without PAA. Most of the spines were perpendicular to the {101h4} faces of the basal calcite. On a glass substrate, round rhombohedra of calcite were mainly produced in the initial stage. Then spines were grown on the previously deposited pedestals with long-term precipitation over 6 days (Figure 2). It appears that the spines were grown to the same orientations, which were perpendicular to the face assigned to the {101h4}. These facts suggest that the acute forms were a calcite crystal elongated in a specific direction, such as . This particular morphology was obtained by a combination of 10-12 mM PAA250k and 0.33-4.0 mM PAA2k. In contrast, planar films or granular aggregates were grown with higher or lower PAA concentrations, and the tapering spines were not observed on the basal structures. When
10.1021/cg0502237 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/10/2006
Emergence of Acute Morphologies
Crystal Growth & Design, Vol. 6, No. 2, 2006 613
Figure 1. SEM images of crystals grown on the (101h4) surface of a calcite single crystal in the presence (a) and in the absence (b) of a binary system of 2.1 × 10-2 (3.0 mM) wt % PAA2k and 7.2 × 10-2 wt % (10 mM) PAA250k for 8 days.
Figure 4. SEM images of typical cones obtained in a binary system of 2.4 × 10-3-2.4 × 10-2 wt % (0.33-3.3 mM) PAA2k and (7.28.6) × 10-2 wt % (10-12 mM) PAA250k for 8 days. (c) Enlargement of the surface of the cones. (d) A cone in the earlier stage.
Figure 2. SEM images of typical spines grown on round rhombohedral pedestals in the binary system of 2.4 × 10-3-2.4 × 10-2 wt % (0.333.3 mM) PAA2k and (7.2-8.6) × 10-2 wt % (10-12 mM) PAA250k for 8 days. Dotted lines divide the spines having the same orientation. Images (a-d) show pedestals and spines formed on different places of a glass substrate at different magnifications.
Figure 3. An enlarged SEM image (a) of the top and a TEM image (b) of the fracture of a spine.
the amount of (NH4)2CO3 was higher than 2.0 g, the spines were not obtained, because the precipitation reaction was completed in a couple of days. An enlarged SEM image (Figure 3a) of the top of a spine shows the presence of nanoscale bricks on the surface. Extremely fine grains 2.0 g) charged into a desiccator. A high reaction rate with a large amount of (NH4)2CO3 promotes precipitation via homogeneous nucleation and inhibits the formation of a concentration gradient. Very recently, the preparation of single crystalline calcite fibers using an aqueous system containing a small amount of PAA was reported.26 The formation of the fibrous morphology through one-dimensional growth induced with a precursor droplet was suggested. The fibrous forms and the formation mechanism are thought to be fundamentally different from the tapering morphologies consisting of the nanobricks described in this report. Mesocrystals, which are oriented assemblies of polymermediated nanocrystals, attracts much attention as examples of nonclassical crystallization.15-17 Similar cone-like structures were observed for BaSO4 and BaCrO4 precipitated in the presence of PAA.27,28 In these cases, however, the cones consisted of single crystalline fibers assembled by directed nanoparticle aggregation. Whereas the acute spines consisting of iso-oriented nanocrystals may be classified into a family of mesocrystals, the tapering morphology was constructed through the build-up construction from the basal crystals (Figure 2). The acute spines were easily grown on a cleaved surface of single crystalline calcite (Figure 1). The parabolic shape of the top (Figure 4d) suggested the strong influence of the diffusion field on the assembly. Because the crystallographic orientation of the spines was inherited from the basal calcite, the directed construction of the building units in the acute morphology is tentatively ascribed to the consecutive growth controlled with PAA on the underlying crystals (Figure 6). In this case, the building blocks would be connected to each other through nanoscale mineral bridges. However, further investigation is required to clarify the detailed microstructure with experimental evidence. Alternatively, the attachment of nanoscale units directed by the adsorbed polymers also provides the oriented assembly. Amorphous calcium carbonate (ACC) has been reported to play an important role in the formation of complex morphologies of crystalline calcium carbonate.18-20 Actually, the presence of ACC was shown at the growing top of sea urchin spines.29 Whereas we observed precipitation of ACC in the solution at the initial stage, particles were not detected during the growth of the acute spines and cones. Moreover, the previous formation of ACC was not found in this work, although the transient amorphous phase may be produced as a precursor of nanoscale calcite. Therefore, we should study the relationship between the formation of iso-oriented nanoscale calcite units and transient ACC as a precursor of the crystalline phase in the further investigation. Conclusion Characteristic forms, such as acute pines and cones, were found to consist of nanoscale calcite bricks and polymer mortar. Recently, it has been suggested that a real biomineral is composed of small grains.30,31 Therefore, our findings contribute to the clarification of biomineralization and the development of biomimetic materials processing.
Emergence of Acute Morphologies
Acknowledgment. This work was supported by Grant-inAid for Scientific Research (No. 15560587) and the 21st Century COE program “KEIO Life Conjugate Chemistry” from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. References (1) Simkiss, K.; Wilbur, K. M. Biomineralization; Academic Press: San Diego, 1989. (2) Xu, G.; Yao, N.; Aksay, I. A.; Groves, J. T. J. Am. Chem. Soc. 1998, 120, 11977. (3) Zhang, S.; Gonsalves, K. E. J. Appl. Polym. Sci. 1995, 56, 687. (4) Kato, T.; Amamiya, T. Chem. Lett. 1999, 28, 199. (5) Kato, T.; Suzuki, T.; Irie, T. Chem. Lett. 2000, 29, 186. (6) Sugawara, A.; Kato, T. Chem. Commun. 2000, 487. (7) Hosoda, N.; Kato, T. Chem. Mater. 2001, 13, 688. (8) Kato, T.; Suzuki, T.; Amamiya, T.; Irie, T.; Komiyama, M.; Yui, H. Supramol. Sci. 1998, 5, 411. (9) Gower, L. A.; Tirrell, D. A. J. Cryst. Growth 1998, 191, 153. (10) Garcı´a-Ruiz, J. M.; J. Cryst. Growth 1985, 73, 251. (11) Bella, S. D.; Garcı´a-Ruiz, J. M. J. Cryst. Growth 1986, 79, 236. (12) Imai, H.; Terada, T.; Miura, T.; Yamabi, S. J. Cryst. Growth 2002, 244, 200. (13) Imai, H.; Terada, T.; Yamabi, S. Chem. Commun. 2003, 484. (14) Co¨lfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42, 2350. (15) Co¨lfen, H.; Antonietti, M. Angew. Chem., Int. Ed. 2005, 44, 2.
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