CRYSTAL GROWTH & DESIGN
Magnesium-Mediated Nanocrystalline Mosaics of Calcite
2009 VOL. 9, NO. 1 223–226
Yuichi Nishino, 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 April 1, 2008; ReVised Manuscript ReceiVed September 16, 2008
ABSTRACT: Magnesium ions are widely found in calcium-based biominerals as an accessory component. In this report, the influence of magnesium ions on the crystal growth of CaCO3 was investigated on the basis of the nanostructure. The morphology of calcite grown in a supersaturated solution was drastically changed from a regular rhombohedron into a spherical architecture consisting of nanocrystalline mosaics in the presence of a large number of magnesium ions. While magnesium ions were substituted for ca. 6% of calcium ions in the crystal lattice at a maximum, an excess amount of magnesium produced an amorphous phase leading to the nanostructure with modulation of the crystal growth of calcite. Introduction Fascinating morphologies of biominerals are skillfully produced by life on a wide range of scales. Sophisticated architectures of CaCO3 crystals in mollusks, such as a laminated structure, a porous network, and a microlens array,1 have been thoroughly investigated. In recent years, various carbonate-based biominerals have been found to be composed of oriented architectures of nanometric units incorporated with organic polymers, although it has been reported that they consisted of micrometric single crystals.2-6 Nanometric crystalline units appear to play an important role in the formation of oriented architectures to achieve versatile macroscopic morphogeneses. The biomineralization producing the hierarchical architectures has inspired a novel route of material synthesis to realize functional microstructures in aqueous solutions. The formation of carbonate crystals mediated by a variety of electrolytes or organic macromolecules has been studied to mimic the biomineralization.7-25 The morphology of carbonate crystals was influenced by soluble and insoluble species as a suppressant of crystal growth and a template for the nucleation on a substrate, respectively. For instance, thin films of CaCO3 were prepared in a supersaturated solution containing a small amount of poly(acrylic acid) (PAA) on a suitable surface, such as a Langmuir monolayer at the solution-air interface7 and polysaccharide films on a glass substrate.8-13 Spiral forms, porous projections, and self-similar stars were produced in a solution containing poly(aspartic acid) and silicate ions.14-18 Recent research has tended to focus on the complicated combination of ionic species, molecules, particles, and substrates for the fabrication of highly tailored microstructures.19 Polymer-mediated calcite, consisting of three-dimensional, well-aligned nanocrystals, was reported.20-22 The presence of amorphous calcium carbonate (ACC) was recently recognized in studies regarding biomineralization and biomimetic processes.6,26,27 Elucidation of a biological strategy for the achievement of versatile morphologies is desired for an improved understanding of biomineralization and further development of materials science. The transformation of ACC was shown to provide an effective synthesis route to the manipulation of the complex morphology of calcite crystals. However, the crystallization process of ACC is not fully understood in terms of its crystal growth and phase transitions. Moreover, a similar * Corresponding author. E-mail:
[email protected]. Tel.: 81-45-5661556. Fax: 81-45-566-1551.
amorphous phase was found on aragonite plates of nacre.28 Thus, the presence of ACC would be essential for the morphological control of biomineralization. In seawater, the concentration of Mg2+ is about five times larger than that of Ca2+, and thus, CaCO3 in many biominerals contains a significant percentage of Mg2+.29 The alkali earth cation is substituted for Ca2+ in the lattice of calcite and decreases the lattice constant linearly with the substitution ratio. Davis et al. carefully studied the role of Mg2+ as an impurity in calcite growth with atomic force microscopy.30 The presence of Mg2+ was reported to suppress the crystal growth of calcium carbonate and to stabilize ACC.31 However, the influence of Mg2+ on the morphogenesis of CaCO3 has not been sufficiently clarified, whereas the macroscopic shapes of calcite grown on self-assembled monolayer (SAM) templates were modulated with Mg2+.32 The present report focuses on the effect of Mg2+ on the nanoscopic structures of calcite and shows oriented architectures of bridged nanocrystals incorporated with an amorphous phase containing Mg2+. The sequential growth of nanocrystals was suggested to lead to oriented structures and subsequent macroscopic morphologies. The bricklaying strategy using crystalline nanobricks and nanomortars of an amorphous phase helped realize a versatile morphological design of materials through a bottom-up approach. Experimental Procedures Crystals of Mg-doped CaCO3 were basically produced using the system described in our previous report.33,34 Precipitation occurred with the introduction of CO2 into a 20 mM CaCl2 aqueous solution. The concentrations of Mg2+ varied in the range of 0-100 mM by dissolution of MgCl2 · 6H2O into the mother solution. Glass plates coated with fluorine-doped SnO2 were used as a substrate for deposition through heterogeneous nucleation of CaCO3. Vessels containing 50 cm3 of the precursor solution and substrates were covered with a polymer film with several pinholes. Two vessels were placed in a 750 cm3 desiccator filled with CO2 generated by the decomposition of 4.0 g of (NH4)2CO3 at 25 °C. The crystal structure of the precipitates was identified using X-ray diffraction (XRD, Bruker AXS, D8 Advance with Cu KR radiation and graphite monochromator). The morphologies of the resultant materials were observed using a field-emission scanning electron microscope (FESEM, FEI Sirion) operated at 2.0 kV and a fieldemission transmission electron microscope (FETEM, FEI Tecnai F20) operated at 200 kV. High-resolution observation of FETEM (HRTEM) was performed with its fast Fourier transform image (FFT) and selected area electron diffraction (SAED). An amorphous phase was characterized by the KBr method using Fourier transform infrared spectroscopy
10.1021/cg800331a CCC: $40.75 2009 American Chemical Society Published on Web 11/19/2008
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Nishino et al.
Figure 1. XRD patterns of deposits formed with various Mg2+ concentrations. As Mg2+ concentration increased, the diffraction peak due to the (104) plane weakened (a) and shifted to a higher angle (b). Diffraction peaks assigned to SnO2 originated from the substrate.
Figure 2. d104 spacing (a) and the contents of Mg2+ (x in (Ca1-xMgx)CO3) estimated from the shift of d104 and ion chromatography (IC) (b) as a function of the Mg2+ concentration in the mother solutions.
(FT-IR, Bio-Rad, FTS 60A). The total content of Mg2+ was determined by high performance liquid chromatography/ion chromatography (HPLC/IC) after dissolution of the products with diluted nitric acid.
Results and Discussion Crystal Structure. According to XRD patterns shown in Figure 1a, crystalline deposits on a SnO2-coated glass substrate were predominantly identified to be calcite. Because the (104) peak was dominantly observed in the patterns, the (104) face of the deposited calcite was arranged parallel to the surface of the substrate. Because deposits on a bare glass plate and precipitates were a mixture of calcite, vaterite, and aragonite, the surface of crystalline SnO2 induced the nucleation of calcite rather than other polymorphs. The presence of an intense (200) peak of SnO2 indicates that the {100} plane was exposed on the surface of the substrate. The formation of oriented calcite crystals may be ascribed to the lattice matching between the (104) of calcite and the {100} of SnO2,35 but the detailed mechanism for selective nucleation is unclear. As the Mg2+ concentration increased, the diffraction peak due to the (104) plane shifted to a higher angle with a decrease in the amount of the crystalline products (Figure 1b). The shift and broadening of the diffraction peak indicate a decrease and disordering in the lattice constant with the substitution of Mg2+ for Ca2+. Thus, the substitution ratio can be associated with the shift of the (104) peak. Because no diffraction peaks were observed with 100 mM Mg2+, the deposit was identified as an amorphous phase. As shown in Figure 2(a), d104 decreased with increasing the Mg2+ concentration of the mother solution. The lattice constant of calcite-type (Ca1-xMgx)CO3 changes linearly with the varia-
Figure 3. SEM images of the products with (a) 0, (b) 100, (c, d) 20, (e, f) 40, and (g, h) 60 mM in the Mg2+ concentrations. As Mg2+ concentration increased, the morphology of calcite was drastically changed from regular rhombohedra (a) into spherical architectures (g, h) through elongated rhombohedra (c, d) and dumbbells (e, f). A large amount of Mg2+ above 100 mM induced the formation of a planar film of ACC with a smooth surface (b).
tion of the substitution ratio (see the Supporting Information, Figure S1) because the lattice structure of MgCO3 is the same as that of calcite. Thus, the substitution ratio could be estimated from the shift of d104 according to Vegard’s law. In Figure 2b, the substitution ratio obtained from the lattice constant is smaller than the total amount of Mg2+ determined by ion chromatography. The difference suggests that an excess amount of Mg2+ existed outside the crystalline lattice. Morphology. As shown in Figure 3, the presence of Mg2+ modulated the macroscopic morphology of calcite crystals grown on the substrate. Regular rhombohedral crystals bound by the {104} faces were observed in the absence of Mg2+ (Figure 3a). As reported above, the (104) face was arranged parallel to the surface. Elongated rhombohedra along the c axis of calcite were produced with the addition of 10-20 mM Mg2+ (images c and d in Figure 3). Triangular submicron units with a width of several hundred nanometers were arranged with the same orientation on the elongated crystals (images a and b in Figure 4). Moreover, nanocrystals smaller than 50 nm were observed on the submicron units. As the concentration of Mg2+ increased to 40 mM, dumbbell shapes were formed with oriented upgrowth at both ends of the elongated crystal (images e and f in Figure 3). Further development of the crystal growth induced the morphological variation from the dumbbells to stacked spheres with 50-60 mM Mg2+ (images g and h in Figure 3). The size of the nanocrystals composing the dumbbells and spheres was smaller than 20 nm (images c and d in Figure 4). These facts suggest that the presence of Mg2+ changed the macroscopic and microscopic morphologies of calcite with a
Mg-Mediated Nanocrystalline Mosaics of Calcite
Crystal Growth & Design, Vol. 9, No. 1, 2009 225
Figure 4. SEM images of the products with (a, b) 20, (c) 40, and (d) 60 mM in the Mg2+ concentrations. Dumbbells and spheres of calcite grown in the presence of Mg2+ were composed of submicrometer units arranged with the same orientation.
Figure 6. TEM (a) and HRTEM (b) images of the products with 20 mM Mg2+. The HRTEM (b) image was divided into several crystalline areas labeled with circled number and amorphous parts A, as shown in (c). The arrows indicating the direction of the (104) planes in the crystalline areas deviated slightly from each other.
Figure 5. SEM images of the products with 40 mM in the Mg2+ concentrations. A radial pattern consisting of submicrometer units was observed around the center of the semicircular crystal.
decrease in the unit size. On the other hand, the crystallographic direction of the dumbbells and stacked spheres seemed to be almost the same as that of the regular rhombohedra. From a cross-sectional view of a fractured dumbbell (Figure 5), a radial pattern consisting of submicron units was observed around the center of the semicircular crystal. Nanocrystals of 20-30 nm were also observed in the radial pattern of the submicron units. Thus, the specific morphologies are suggested to be produced by sequential growth of hierarchically organized units. A large amount of Mg2+ above 100 mM induced the formation of a planar film on the substrate (Figure 3b). This was identified to be ACC from the absence of the diffraction peak (Figure 1a) and the presence of specific FTIR bands (see the Supporting Information, Figure S2). Figure 6 shows TEM images of a triangular submicron unit of the dumbbell calcite grown with 20 mM Mg2+. The unit exhibits the habit of calcite (Figure 6a), and the lattice image of the (104) planes of calcite is clearly observed in the HRTEM images (Figure 6b). However, interestingly, the crystalline phase was divided by an amorphous phase and dislocations in the unit (Figure 6c). The direction of each crystalline area slightly deviated in the crystalline area, as demonstrated with arrows labeled with circled number. This means that the submicron calcite units had a mosaic texture of connected nanocrystals with a size of 5-20 nm with the amorphous phase. Mechanism. The addition of Mg2+ promoted the formation of hierarchically structured calcite consisting of nanocrystals.
The formation of submicrometer units consisting of nanocrystals changes the macroscopic morphology from rhombohedra into stacked spheres via dumbbells. The nanocrystals incorporated with an amorphous phase were connected to each other, but their crystallographic direction slightly deviated. Although Mg2+ ions were fundamentally substituted for Ca2+ in the lattice of calcite, a surplus amount of Mg2+ would stabilize ACC, suppressing the regular crystal growth of calcite. Our previous work has shown that the presence of PAA molecules suppressed the regular crystal growth through adsorption on specific surfaces of calcite and then induced the formation of small, bridged grains on the basal crystal.33 The small triangular grains were inferred to be surrounded by the {104} faces covered with the polymers.36The oriented architecture was achieved by the sequential growth through mineral bridges along specific directions in the diffusion field. Morphological variation of fluorapatite aggregates from the elongated hexagonal prism through dumbbell shapes to a sphere was achieved in gelatin as an organic matrix.37 In these cases, the composite structures of inorganic crystal and organic polymers would be essential for the formation of specific morphologies composed of nanoscale units. Soluble inorganic anions, such as silicates, also exhibited a capping effect in the morphogenesis of carbonate crystals. The formation of the small, bridged crystalline units with silicate anions was fundamentally similar to that of those formed with PAA, although the morphology depended on the specificity of the adsorption of the polymeric anions.36 In the present work, amorphous films were obtained with a large amount of Mg2+ (100 mM). The presence of ACC with calcite nanocrystals grown with 20-60 mM Mg2+ was suggested from the TEM image (Figure 6) and FTIR spectra (see the Supporting Information, Figure S2).31,38 Thus, Mg-stabilized ACC would suppress the regular crystal growth of calcite in a manner similar to PAA and silicate. The modulation of the crystal growth with the suppression induced the formation of connected crystallites whose crystallographic
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direction slightly deviated. Finally, the specific macroscopic morphologies could be formed via the radial, sequential growth of the small units from the nucleation site.
Nishino et al. (9) (10) (11) (12) (13)
Conclusions The morphological variation from rhombohedra to stacked spheres was achieved in the presence of Mg2+. Basically, Mg2+ was substituted for Ca2+ in the lattice of calcite grown in a supersaturated solution containing those cations. As the Mg2+ concentration increased, surplus Mg2+ stabilized the amorphous phase of calcium carbonate and then suppressed the regular crystal growth of calcite and miniaturized the crystallites. Dumbbell-like and spherical morphologies of calcite were produced with the construction of connected nanocrystals incorporated with the amorphous phase. Acknowledgment. This research was supported by Grantin-Aid for Scientific Research (No. 19655078) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. Supporting Information Available: Two additional figures (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org.
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