CRYSTAL GROWTH & DESIGN
Influence of Silicate Anions on the Morphology of Calcite Crystals
2003 VOL. 3, NO. 4 611-614
Rajamani Lakshminarayanan and Suresh Valiyaveettil* Department of Chemistry, 3 Science Drive 3, National University of Singapore, Singapore 117543 Received February 10, 2003;
Revised Manuscript Received May 14, 2003
ABSTRACT: Calcite crystals with interesting morphologies were synthesized using various oligomers of silicate in solution. The crystals were grown by adding an appropriate amount of sodium silicate to a supersaturated calcium bicarbonate solution at room temperature. The morphology of nucleated calcite crystals changed significantly with the concentrations of silicate anion in solution. At lower levels, trapezoidal shaped crystals exhibiting indented and terraced structures were formed. At a Ca2+/silicate ion ratio of 1:1, oriented crystals with pseudohexagonal shapes were nucleated in solution. A higher concentration of sodium silicate produced polycrystalline calcite crystal aggregates. Powder XRD data indicates significant expansion of the calcite lattice because of the incorporation of silicate anions. We hypothesize that the observed sequential changes in morphology of calcite crystals with added silicate anion concentration may be due to the incorporation of monomeric, oligomeric, and polymeric forms of silicate anion species into the calcite lattice. Introduction
Experimental Procedures
Synthesis of inorganic materials that are ordered beyond the molecular level is gaining importance owing to their potential applications in the areas of catalysis, separation technology, and biomedical engineering.1-3 Many of these developments are largely driven by understanding the fundamentals behind the natural processes. Biomineralization is one such important process in which the inorganic phase of a hard tissue is formed through a template-assisted process.4 Microorganisms tailor-make such hard materials (with the help of bioorganic macromolecules, such as proteins, polysaccharides, or glycoproteins) and use them for structural support,5 gravity sensors,6 and protection. The chemistry and structural complexity of biominerals was reviewed in the past by Mann7a and Ozin.7b Calcium carbonate (CaCO3) is one such biomineral known to exist abundantly in natural systems and has three polymorphic forms: calcite, aragonite, and vaterite. The first two are abundant in geological and biological systems, and the third one is a metastable polymorph, which is rarely seen. A number of approaches8,9 have been made to synthesize specific polymorphs of CaCO3 in various forms. These include the use of films,10 selfassembled monolayers,11 ligand-receptor complexes,12 block copolymers,13 microemulsions,14 synthetic polypeptides,15 and synthetic molecular assemblies.16 Recently, we established that the water-soluble polymer poly(vinyl alcohol) has a significant effect on controlling the selective nucleation of a specific polymorph of CaCO3 crystals.17 We report here the synthesis of calcite crystals using the complex solution properties of sodium silicate. Our work in this area is motivated by earlier reports that sodium silicate glass is able to induce the growth of apatite crystals owing to the influence of silanol groups.18
Commercially available sodium metasilicate solution (Merck) was diluted to give final SiO2 concentrations of 2.5, 5.0, 7.5, 10, and 12.5 mM. Calcium carbonate crystals were grown from supersaturated calcium bicarbonate (Ca(HCO3)2) solutions using a procedure reported elsewhere.19 Purified carbon dioxide, obtained from Soxal (Singapore) Pte. Ltd., was passed through a stirred suspension of calcium carbonate powder in water for 5 h. The white suspension was filtered twice, and CO2 gas was purged through the filtrate for an additional 1 h to dissolve residual crystal nuclei formed. To the resulting clear solution, a different amount of sodium silicate solution (6, 12, 18, 25.2, and 29 µL in 10 mL of Ca(HCO3)2 solution) was added to give a Ca/silicate ratio of 3:1, 1.5:1, 1:1, 1:1.4, and 1:1.6, respectively. No significant changes in pH were observed after the addition of silicate solution. The crystals formed after 96 h at the air-water interface were collected on a glass microscope slide and characterized using scanning electron microscopy (SEM), elemental analysis, energy-dispersive X-ray scattering (EDXS), and powder X-ray diffraction studies. For SEM and EDXS investigations, the collected crystals were carefully placed on copper stubs with double-sided carbon tape and then sputter coated with gold and examined with a JEOL JSTM 220A scanning electron microscope at 15 kV. Elemental analyses were carried out using an inductively coupled plasma (ICP) optical emission spectrometer model Thermo Jarrel Ash IRIS/AP. In a typical analysis, about 0.3 mg of the calcite crystals were dissolved in 1.5 mL of 3 M hydrochloric acid, diluted to 10 mL using Millipore water, and analyzed. The calcium content in the bicarbonate solution was estimated to be 7.4 mM. For powder X-ray diffraction studies, finely powdered samples were placed on a sample holder and wetted with a few drops of ethanol and slightly pressed to form a thin layer. Powder X-ray diffraction studies of the crystals were done using a D5005 Siemens X-ray diffractometer with CuΚR radiation at 40 kV and 40 mA. All the samples were scanned over a 2θ range of 20-60° at a step size of 0.008°. The lattice parameters were evaluated using a least-squares method.
* To whom correspondence should be addressed. Tel.: (65) 6874 4327. Fax: (65) 779 1691. E-mail:
[email protected].
Results and Discussion In all our experiments, only calcite crystals were nucleated at all concentrations of added silicate anions; however, a significant effect on the morphology of the crystals was observed (Figure 1). The crystals grown at
10.1021/cg034023b CCC: $25.00 © 2003 American Chemical Society Published on Web 05/29/2003
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Lakshminarayanan and Valiyaveettil Table 1. Crystal Lattice Parameters for the Calcite Crystals Grown in the Presence of Silicate Lattice parametersa
Ca/silicate
d104 (Å)
1:0 1:0.33 1:0.67 1:1 1:1.4 1:1.6
3.035 3.049 3.066 3.093 3.071 3.068
silicate b (Å) c (Å) content a (Å) (mol %)b ((0.01) ((0.01) ((0.02) 1.5 2.3 6.9 4.9 3.5
4.98 5.00 5.02 5.04 5.03 5.02
4.98 5.00 5.02 5.04 5.03 5.02
17.08 17.13 17.17 17.20 17.19 17.20
cell volume (Å3) 368.0(1.9) 371.4(1.9) 375.5(1.9) 378.4(2.9) 376.7(1.9) 376.0(1.9)
a Numbers in the parentheses indicates the estimated standard deviation (esd). b Calculated from elemental analysis.
Figure 1. Representative SEM images of the calcite crystals grown in the presence of different concentrations of sodium silicate. All the crystals were grown from a supersaturated calcium bicarbonate solution for 96 h.
a Ca/silicate ratio of 1:0.33 were showed to be hollow trapezoidal in shape. These trapezoids grew along the length forming an extended trapezoid as the concentration of sodium silicate was increased (1:0.67). The crystal surface appeared to be very rough with steps and wrinkles. They also exhibited spiral pits at the center of the faces. A closer examination of the surface of these elongated and hollow sides indicated that it consisted of an array of columnar rods. Each rod had a length of 10-35 µm and was highly oriented parallel to each other forming a wall thickness of ca. 8-10 µm. At a stoichiometric ratio of 1:1, the nucleated calcite crystals showed pseudohexagonal shaped morphology capped with rhombohedral faces oriented along the c axis. The surface of the crystals appeared smooth and flat indicating that the nucleation in the direction perpendicular to the c axis was inhibited. The mean aspect ratio (i.e., the length/width) of 30 crystals was found to be 2.1 ( 0.6. A further increase in the sodium silicate level resulted in the formation of calcite crystal aggregates with a mean size of ca. 48 µm. A thorough examination of these aggregates indicated that they are stacked crystals with (104) rhombohedral faces (Figure 1). In the absence of sodium silicate (Ca/silicate ratio of 1:0), calcite crystals with regular rhombohedral morphology were formed at the air-water interface. Elemental analysis of the crystals indicated that the silicate anion incorporation into the calcite lattice passed through a maximum at a Ca/silicate ratio of 1:1 (6.9 mol %, Table 1). Energy-dispersive X-ray scattering studies (EDXS) of all the crystals showed the presence of Si atoms on the surface of the crystals. The powder X-ray diffraction of the samples is shown in Figure 2, which indicates the presence of the calcite phase.
Figure 2. Representative X-ray diffraction of the calcite phase formed at various Ca/silicate ratios. Note the gradual shift in the peaks toward lower 2θ values. The shift is clearly visible in the I104 peak at 2θ ) 29.4°. Also note that the shift is maximum at Ca/silicate ratio of 1:1.
However, there is a significant shift in the peak positions toward a lower angle with an increase in silicate anion concentration, with a maximum shift at a Ca/ silicate ratio of 1:1. The shift of the peak position as compared to that of the control sample indicated an increase in lattice parameters (i.e., a, b, c, and unit-cell volume). Therefore, we evaluated lattice constants of all the calcite crystals grown at different concentration levels of added silicate anions. The results are given in Table 1. The observed increases in lattice parameters indicate that silicate anions enter into the calcite lattice by replacing the carbonate ions. It is also interesting to note that the lattice expansion via such ion exchange is seen only up to a particular concentration of silicate anions, above which the lattice parameters decrease. In addition to this, an increase in intensity of the (113) and (116) diffraction maxima was observed in the presence of sodium silicate indicating that the growth is oriented along the c axis. The solution behavior of sodium silicate is complex; it exists as polymeric ions in concentrated solutions and at higher SiO2/Na2O ratios.20 However, at high dilutions (∼1 mM), the predominantly monomeric silicate (SiO44-) species was expected to be present in solution. The hierarchical changes in the morphology of calcite crystals reflect the changes in silicate anion concentration. The formation of spiral pits and hollow crystals could be due to the incorporation of monomeric species. Gower
Influence of Silicate Anions on Calcite Crystals
Figure 3. Schematic representation of tetrasilicate oligomer adsorbed onto the (001) face of the calcite crystals. The larger circles represent calcium ions, and smaller ones belong to either carbon or oxygen.
et al.20 showed that highly charged polyaspartate anions induced similar pits at the center of the faces. Dimeric silicate may form as the concentration of silicate anions increases, thus inducing the nucleation of calcite crystals having an elongated trapezoidal shape. The formation of macroscopic steps indicates a nonspecific interaction between the monomeric/dimeric anions and the growing calcite crystals. The nucleation of smooth pseudohexagonal-shaped crystals is ascribed to the presence of oligomeric silicate anions. The calcium carbonate lattice has homocharged layers along the (001) plane. Stabilization of such a face requires stereochemical matching between the interacting species (polyanionic or polycationic) and the calcium or carbonate ions. Addition of malonate anion,22 synthetic block copolymers,23 and highly acidic proteins extracted from sea urchins5a have been found to stabilize this face. We propose that the oligomeric silicate anions could interact in the same way as malonate anions on the {1-10} plane of the calcite crystals22 as the distance between the two neighboring Si atoms in oligomeric silicate anions matches the distance between the adjacent carbonate layer23 as shown in Figure 3. The inclusion of silicate anions in the crystal lattice is supported by the expansion in the unit-cell volume. Mumtaz et al.24 has shown that aggregates of calcium oxalate monohydrate crystals are formed if the growth rate of the crystal is higher than the hydrodynamic forces. A similar mechanism may be plausible in which the polymeric network structure of each repeat unit of the silicate chain acts as the nucleation site and accelerates the growth of the crystallites. Owing to their bulky structure, the incorporation of silicate anions into the calcite lattice is hindered; consequently, the amount of silicate anions incorporated in the lattice is decreased. Therefore, at higher concentration ratios, only a smaller amount of silicate anions enters into the calcite lattice as is evident from the elemental analyses and powder X-ray diffraction studies. The changes in morphology of calcite crystals with the expected structure of silicate anion (based on concentration) are schematically represented in Figure 4. Similar morphogenesis was reported in the case of BaSO4 systems25 and CaCO3 systems13 using block copolymers.
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Figure 4. Morphology map showing the sequential changes in the morphology of calcite crystals at various levels of sodium silicate. The tentative structural changes in the silicate species are shown at the top of the diagram.
In conclusion, we have shown that significant crystalhabit modification of the calcite phase can be achieved through the addition of a silicate anion species at different concentrations. The calcite lattice parameters were effectively changed through an anion exchange mechanism. Here, we report the synthesis of a hybrid material by selective occlusion of silicate anions into the lattice of calcite crystals. The progressive changes in the calcite morphology with an increase in concentration of the silicate anions could be explained using the change in size and structure of silicate anions in solution. The use of readily available starting materials would enable one to synthesize complex structures of inorganic crystals (morphosynthesis) with interesting properties. Acknowledgment. S.V. acknowledges the National University of Singapore (NUS) for financial support. Technical and instrumental support from the Department of Chemistry is greatly appreciated. References (1) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892-898. (2) Sellinger, A.; Weiss, P. M.; Nguyen, A.; Lu, Y.; Assink, R. A.; Gong, W.; Brinker, C. J. Nature 1998, 394, 256-260. (3) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schuth, F. Science 1996, 273, 768-771. (4) Weiner, S.; Addadi, L. J. Mater. Chem. 1997, 7, 689-702. (5) (a) Albeck, S.; Aizenberg, J.; Addadi, L.; Weiner, S. J. Am. Chem. Soc. 1993, 115, 11691-11697. (b) Mann, S. Nature 1993, 365, 499-505. (6) Oukda, M.; Francois, M.; Membre, H.; Bautz, A.; Dournon, C. Hear. Res. 1999, 132, 85-93. (7) (a) Mann, S. Angew. Chem., Int. Ed. Engl. 2000, 39, 33923406. (b) Ozin, G. A. Acc. Chem. Res. 1997, 30, 17-27. (c) Mann, S.; Webb, J.; Williams, R. J. P., Eds. Biomineralization: Chemical and Biochemical Perspectives; VCH: Weinheim, 1989. (8) (a) Levi, Y.; Albeck, S.; Brack, A.; Weiner, S.; Addadi, L. Chem.sEur. J. 1998, 4, 389-396. (b) Falini, G.; Femani, S.; Gazzano, M.; Ripamonti, A. Chem.sEur. J. 1998, 4, 1048-1052.
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CG034023B
