Water-Soluble Terpolymer Directs the Hollow Triangular Cones of Packed Calcite Needles Ranjith Krishna Pai*,†,‡ and Saju Pillai†,§ Department of Experimental Physics, UniVersity of Ulm, D-89069 Ulm, Germany, Center for AdVanced Interdisciplinary Research in Materials (CIMAT) and Faculty of Veterinary and Animal Science, Department of Biological Science, UniVersity of Chile, Santa Rosa 11735, La Pintana, Casilla 2 Correo 15, Santiago, Chile, and Interdisciplinary Nanoscience Centre (iNANO), UniVersity of Aarhus, Ny Munkegade, 8000 Aarhus C, Denmark
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 2 215-217
ReceiVed July 24, 2006; ReVised Manuscript ReceiVed NoVember 9, 2006
ABSTRACT: Hollow triangular cones of packed calcite needles, which have never been seen before in natural biominerals, were prepared using a double-decomposition precipitation method. These are shown to be oriented attachments of inverted triangular cones toward crystalline superstructures in addition to “mesocrystal” formation. Calcium carbonate (CaCO3) is an important mineral because of its significance as a biomineral and its various industrial applications, for example, as a filler and an abrasive, and also because it is a major component in scale formation from water. Therefore, CaCO3 crystallization has been widely studied for decades, receiving both scientific and industrial interest. In recent years, CaCO3 has been intensely studied with the aim of understanding how crystal polymorph and structural features can be controlled by organic additives.1,2 It has been suggested that organisms control crystal morphology by forming crystals within a preformed matrix composed of either macromolecules or lipid membranes, and that the ultimate morphology of the crystal is defined by the form of the matrix.3 Alternatively, specialized organic macromolecules, which are occluded within the majority of biological CaCO3 crystals, have been shown to interact with specific crystal faces of growing crystal.4-7 This research is inspired by the fascinating mechanical and optical properties of biominerals, their complex forms, and the exquisite control of crystallization over several length scales in biomineralization processes.8 Usually, when CaCO3 precipitation is carried out by double-decomposition precipitation9 method without additives, thermodynamically stable calcite-rhombohedra is formed.10 In the early stages, other usual CaCO3 morphologies might be found, such as spherical amorphous calcium carbonate (ACC) or needlelike aragonite crystals,11 which subsequently transform to calcite rhombohedra. In general, the growth of crystals is well-established to follow two possible mechanisms: Ostwald ripening,12 which involves the growth of larger crystals at the expense of smaller crystals, or aggregation, which can be a random or epitaxial mechanism.13,14 The latter involves crystallographically directed fusion of the nanoparticles to produce single crystals. Here we report the formation of hollow triangular cones of packed needles, which have never been seen before in natural biominerals, prepared by a double-decomposition9 precipitation method in the presence of poly(acrylamide-co-acrylamido-2-methyl1-propane sodium sulfonate-co-vinyl alcohol) [poly(AM-NaAMPSVA)] (relative viscosity ∼2) with a monomer composition of 80: 10:10 as crystal modifier.10 Yu and co-workers15 have reported a more or less similar triangular-shaped superstructure on the spherical surface, but with a different crystal modifier. The present study has revealed a more detailed core structure of the microsphere, and a thorough examination made on a broken region revealed the formation of hollow inverted triangular cones consisting of packed needlelike structures. * Corresponding author. Tel: 56-2-9785642. Fax: 56-2-9785526. Email:
[email protected]. † University of Ulm. ‡ University of Chile. § University of Aarhus.
Figure 1. SEM micrographs of CaCO3 crystals. (A) Early stage (after 2 h) of CaCO3 crystallization in the presence of poly(AM-NaAMPS-VA) resulted in hollow spheres. inset (B) Higher magnification showing detailed textures and porosities.
Figure 2. SEM micrographs of spherical superstructure. (A) On aging for a month, CaCO3 crystallization resulted in a spherical superstructure. (B) Higher-magnification SEM images showing detailed core structure.
Figures 1-4 show scanning electron microscope (SEM) images of the as-prepared samples obtained in the presence of poly(AMNaAMPS-VA) (100 mL, 2.0 g L-1) after aging at room temperature for a month. The early stages (after 2 h) of CaCO3 crystallization resulted in crystals that exhibited hollow spheres in sizes between 70 and 350 nm (Figure 1) and spherical superstructures on aging for a month (Figure 2). Hollow spheres of CaCO3 particles, which are characteristics of unstable ACC, were observed initially, generated the final spherical superstructure form, and identified to
10.1021/cg0604780 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/20/2006
216 Crystal Growth & Design, Vol. 7, No. 2, 2007
Figure 3. Triangular-shaped cones of packed calcite needles. (C) SEM micrograph showing that the entire surface of the sphere consists of closely packed inverted cones of lengths ranging from 700 nm to 1.25 µm and (D) higher-magnification images showing cones of packed calcite needles.
Figure 4. Triangular apexes of CaCO3 crystals. (E) Each of these cones has a triangular apex of sizes varying from 300 to 500 nm. (F)Highermagnification SEM images show that the microsphere appears to comprise higher-order porous triangular-shaped cones of packed calcite needles.
be ACC using XRD (shown in the Supporting Information, Figure S1). Raman spectroscopy is a complementary technique used to confirm hydrated amorphous calcium carbonate in the sample.16 Therefore, we carried out Raman analysis to confirm ACC (see the Supporting Information, Figure S2), which shows a broad peak around 1061 cm-1 and a broad featureless hump around 154-390 cm-1. This is in contrast to the crystalline forms of CaCO3 that all have a series of relatively sharp peaks.17 Furthermore, ACC has a weak absorption band around at 725 and 872 cm-1; this is in contrast to calcite, which has sharp peaks at 711 cm-1.16,18,19 To investigate whether the ACC phase was necessary for the precipitation environment to impose its structure on the final morphology of the crystal, we repeated the experiments at room, rather than low, temperature. In the absence of polymer, calcite precipitated immediately from solution. Precipitation within the polymeric solution resulted in crystals of irregular, as opposed to spherical, superstructure form. The spherical superstructures are about 40-45 µm in size. The entire surface of the sphere consists of closely packed inverted cones of lengths ranging from 700 nm to 1.25 µm (Figure 3). Each of these cones has a triangular apex of sizes varying from 300 to 500 nm (Figure 4). The cones are lined with packed calcite needles. Higher-magnification SEM images show that the microsphere appears to be higher-order porous triangular-shaped cones of packed
Communications calcite needles (Figure 4F). The growth process from spherical porous ACC nanoparticles to the final spherical superstructure was investigated with transmission electron microscope (TEM) by examining the early stages of the crystal growth. As shown in the Supporting Information, Figure S4A, spherical porous ACC nanoparticles appear at the early stages of the crystallization (after 2 h). After aging for a month, the particles grow larger and become a spherical superstructure consisting of porous triangles (see the Supporting Information, Figure S4B). The FTIR analysis (see the Supporting Information, Table S5 and Figure S3) of the obtained spherical superstructure indicates that mixtures of calcite, aragonite, and CaCO3·6H2O (ACC) modifications are present. However, the corresponding XRD pattern (see the Supporting Information, Figure S1) exhibits only sharp calcite reflections. The discrepancy between FTIR and XRD methods might be due to the fact that the XRD will not detect the polymorph if the sample contains less than ca. 10 mass % of the according modification. Selected area electron diffraction, carried out in a TEM, was performed on the mature spherical superstructure in an attempt to obtain a diffraction pattern from a thinner edge area. Some diffraction patterns were obtained, all of which could be fitted to calcite. The formation of the unusual triangular-shaped cones of packed calcite needles could be due to aggregation of nanoparticles via an oriented attachment mechanism. This oriented attachment of inverted triangular cones toward crystalline superstructures is in addition to “mesocrystal” formation,20 which is currently identified to be relevant in a whole range of crystallization processes. Co¨lfen et al. reported more or less similar cones of BaSO4 fiber bundles,21 formed by the oriented attachment mechanism, where two nanoparticles combine and fuse to form single crystals. They observed stabilized amorphous nanoparticles in the early reaction stage and it remains at all stages of the experiment, although their concentrations decrease with time as a result of transformation into aggregates and finally fiber bundles. This is in contrast to the present study; at a very early reaction stage, anhydrous porous ACC nanoparticles were formed, which subsequently aggregate to spherical particles (see Figures 1 and 2). Later, a transformation into hydrated ACC occurs (supported by FTIR analysis; see the Supporting Information, Figures S3 and S5). Hydrated ACC is relatively stable compared with anhydrous ACC; the hydrated ACC then grew at the expense of the dissolving anhydrous ACC particles. These formation mechanism are in agreement with earlier published findings by Xu et al.22 Judat et al. reported that the formation of porous crystals and their rough surfaces strongly indicate a change in the crystallization mechanism from ionic growth to mesoscale assembly.23 Thus, the existence of ACC porous nanoparticles in the early crystallization stages (Figure 1 and the Supporting Information, Figure S4A) and triangularshaped spherical superstructure in the final crystals (Figure 2 and the Supporting Information, Figure S4B) provide further evidence for mesoscale assembly24 from amorphous primary particles25 to amorphous aggregates with subsequent crystallization and assembly to mesocrystals.35,36 We assume that a large amount of calcium and carbonate ions electrostatically accumulate on the poly(AM-NaAMPS-VA) chains, resulting in the sudden formation of a large number of ACC nanoparticles through nucleation. These particles aggregate by covalent interactions between Ca2+ ions on the surface of CaCO3 and functional groups (amide, hydroxyl, and sulfonic acid) on the polymer chains.26,27 When the growth of CaCO3 particles reaches a critical size, the inorganic/organic composite particles separate from solution due to phase separation. These particles generally tend to adopt a spherical morphology, as this gives the minimum total surface energy for a given volume. In the second step, the functional groups (amide, hydroxyl, and sulfonic acid) are adsorbed and self-assembled on the surface of the composite microspheres. Mann28 and Heywood et al.29 had proposed that this kind of environment might be more suitable for the growth of rhombohedral calcite crystals, which are often nucleated on a small triangular
Communications (001) face at a rigid substrate surface. Such secondary nucleation would result in the formation of many small calcite crystals with a trigonal pyramidal morphology.30 The deposition of the precise array of calcite crystals on the surface of composite microspheres is ascribed to the controlled nucleation at the interface between the crystals and the adsorbed functional groups (amide, hydroxyl, and sulfonic acid) of the terpolymer. Another explanation for the growth of these porous triangularshaped cones of packed calcite needles can be the initial formation of an unstable CaCO3 modification, such as ACC. This is support by XRD, Raman, and FTIR results (see the Supporting Information, Figures S1-S3). With time, unstable CaCO3 will be transformed to the more stable modifications, especially in the presence of moisture.9 Gutjahr et al.31 explained this transformation clearly by the Ostwald step rule. Under standard conditions, the unstable CaCO3 phase readily transforms into the stable aragonite9 or calcite phase in the presence of water through a solvent-mediated mechanism.32 This process involves dissolution of the unstable phase followed by nucleation of the calcite phase, and generally takes place within 80 h. The structural transformation from unstable CaCO3 to a more stable phase may lead to porous triangular-shaped cones of packed calcite needles. This is confirmed by comparing the images of the hollow spheres of CaCO3 particles at early stages (after 2 h; Figure 1 and the Supporting Information, Figure S4A), and after aging for a month time (Figure 2 and the Supporting Information, Figure S4B) shows that the structure of CaCO3 particles alters with time. To conclude, we reanalyzed the formation of hollow triangular cones of packed needles via a double-decomposition precipitation method by SEM and TEM under well-controlled conditions. Poly(AM-NaAMPS-VA) acts as an effective additive for the control of CaCO3 morphology. It was shown that the structures formed by a mesoscale assembly process, in which first small spheres (or nuclei) are formed, followed by overgrowth of the spheres by selforganization of smaller particles on their surface. The observation of such a mechanism of CaCO3 crystallization brings it into the discussion as a possible biomineralization mechanism, as it can explain the formation of mesocrystalline biomineral structures with complex shape. Calcium Carbonate Precipitation. In a typical synthesis, a solution of CaCl2 (2 M, 2 mL) was added to an aqueous solution of poly(AM-NaAMPS-VA) (100 mL, 2.0 g L-1) and the pH of the solution [CaCl2/poly(AM-NaAMPS-VA)/H2O] was adjusted to 7.5 using HCl or NaOH. A solution of K2CO3 (2 M, 2 mL) was then added dropwise into the pH-adjusted solution under vigorous stirring for 1 min, and the solution was kept under static conditions in the refrigerator at -20 °C. Acknowledgment. The work described in this paper was carried out during the framework of R.K.P.’s doctoral degree program at the University of Ulm, Germany. The work was financially supported by the Graduate College “Molecular Organization and Dynamics at Interfaces”, University of Ulm, and partially supported by Project FONDAP 11980002 granted by the Chilean Council for Science and Technology (FONDECYT). We acknowledge Dr. A. Ziegler, University of Ulm, for SEM analysis. The authors thank Prof. Dr. O. Marti, University of Ulm, for providing work space. Supporting Information Available: XRD patterns, Raman and FTIR spectra, TEM images, table of FTIR peaks, and additional experimental details. This material is available free of charge via the Internet at http:// pubs.acs.org.
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