Influence of Silicate Anions on the Morphology of Calcite Crystals 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 Ca 2+ /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 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 Microor- ganisms tailor-make such hard materials (with the help of bioorganic macromolecules, such as proteins, polysac- charides, or glycoproteins) and use them for structural support, 5 gravity sensors, 6 and protection. The chemis- try and structural complexity of biominerals was re- viewed in the past by Mann 7a and Ozin. 7b Calcium carbonate (CaCO 3 ) 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 approaches 8,9 have been made to synthesize specific polymorphs of CaCO 3 in various forms. These include the use of films, 10 self- assembled monolayers, 11 ligand-receptor complexes, 12 block copolymers, 13 microemulsions, 14 synthetic polypep- tides, 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 CaCO 3 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 Experimental Procedures Commercially available sodium metasilicate solution (Mer- ck) 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) solu- tions 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 CO 2 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(HCO 3)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. 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 * To whom correspondence should be addressed. Tel.: (65) 6874 4327. Fax: (65) 779 1691. E-mail: chmsv@nus.edu.sg. CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 4 611 - 614 10.1021/cg034023b CCC: $25.00 © 2003 American Chemical Society Published on Web 05/29/2003