Superstructures of Calcium Carbonate Crystals by Oriented Attachment Nicole Gehrke,† Helmut Co¨lfen,*,† Nicola Pinna,‡ Markus Antonietti,† and Nadine Nassif*,† Max-Planck Institute of Colloids and Interfaces, Colloid Chemistry, Research Campus Golm, D-14424 Potsdam, Germany, and Martin-Luther-Universita¨ t Halle-Wittenberg, Institut fu¨ r Anorganische Chemie, Kurt-Mothes-Strasse 2, D-06120 Halle (Saale), Germany Received February 7, 2005;
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 4 1317-1319
Revised Manuscript Received May 13, 2005
ABSTRACT: Crystalline hexagonal-shaped superstructures of calcium carbonate, synthesized in the presence of ammonia, are shown to be assembled by a three-dimensional oriented attachment of vaterite nanoparticles. This unusual crystallographic lock-in mechanism enables the formation of complicated rounded structures with a crystallographic orientation from nanosized building blocks, which has so far only been found for transition metal systems. Calcium carbonate is an important mineral due to its significance as a biomineral and its various industrial applications, for example, as a filler and abrasive, and also because it is a major component in scale formation from water. Therefore, calcium carbonate crystallization has been widely studied for decades, receiving both scientific and industrial interest. In recent years, calcium carbonate has been intensely studied with the aim of understanding how crystal polymorph and structural features can be controlled by organic additives.1,2 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.3 Many different CaCO3 crystallization methods exist starting from very fast stopped flow techniques, in which crystallization occurs in the millisecond range,4 to very slow gas diffusion techniques such as the Kitano method,5 or the vapor diffusion method, which applies thermal decomposition of ammonium carbonate for the slow generation of CO2.6 The vapor diffusion method is popular for slow CaCO3 crystallization as it enables the production of very well-facetted calcite crystals. However, a drawback of the method is that it depends highly on the experimental setup.7 For example, in a recent study,8 we have obtained calcite rhombohedra instead of the vaterite structures reported here at similar CaCl2 concentrations and reaction times but for different solution and desiccator volumes. Furthermore, the pH increases steadily due to ammonia dissolution as a byproduct of the ammonium carbonate decomposition.9 Usually, when calcium carbonate precipitation is carried out by this method without additives, thermodynamically stable calcite-rhombohedra (JCPDS card 5-586) are formed within hours to days. In the early stages, other usual CaCO3 morphologies might be found such as spherical vaterite (JCPDS card 33-286) or needlelike aragonite crystals,7 which subsequently transform to calcite rhombohedra. In general, the growth of crystals is well established to follow two possible mechanisms: Ostwald ripening,10 which involves the growth of larger crystals at the expense of smaller crystals, or aggregation, which can be a random or epitaxial mechanism.11,12 The latter involves crystallographically directed fusion of the nanoparticles to produce single crystals. * To whom correspondence should be addressed. (H.C.) Tel: ++49-331567-9513; fax: ++49-331-567-9502; e-mail:
[email protected]; (N.N.) e-mail:
[email protected]. † Max-Planck Institute of Colloids and Interfaces. ‡ Martin-Luther-Universita¨t Halle-Wittenberg.
Figure 1. SEM pictures showing the lens-shaped (a) and two rosetta-shaped (b and c) crystals with hexagonal symmetry formed after 18 h from a 0.02 and a 10-3 M CaCl2 solution, respectively.
In this paper, we report on the mechanism of the formation of vaterite supercrystals with hexagonal symmetry13-19 via the vapor diffusion method under specific conditions in the absence of any additional additives, confirming the active influence of ammonium ions on calcium carbonate precipitation. Such an influence was already previously described in simulated gas diffusion conditions via the Kitano method with added ammonia.18 We reanalyzed the formation of the unusual hexagonalshaped vaterite morphologies and found that they are built by the aggregation of nanoparticles via an oriented attachment mechanism. This oriented attachment toward larger, single crystalline superstructures is in addition to “mesocrystal” formation,20,21 which is currently identified to be relevant in a whole range of crystallization processes. Three different experiments were carried out at room temperature (22 ( 1 °C) with varying calcium chloride concentrations, 10-3, 0.02, and 0.2 mol/L. Two flasks (30 mL, h ) 68 mm) covered by punched Parafilm (five holes) containing calcium chloride solutions (20 mL), and fresh ammonium carbonate (2 g) were placed into a closed chamber (1000 cm3). Reaction conditions except the CaCl2 concentration were the same for each experiment, and the experiments were observed over two weeks. Considering the similarities between results obtained with the samples with concentrations of 0.02 and 0.2 mol/L (Supporting Information), we will focus our discussion on sample concentrations of 10-3 and 0.02 mol/L. In the case of the 0.02 mol/L CaCl2 concentration after 18-40 h, sample observations by scanning electron microscopy (SEM) show well-defined hexagon-shaped crystals with a large range of diameters between approximately 0.5 and 20 µm (Figure 1a and Supporting Information). With subsequent growth, they lose their hexagonal character and become more lensshaped, both of these stages being visible in Figure 1a. These structures transform after some days (>3 days) to classical, thermodynamically stable rhombohedral calcite.
10.1021/cg050051d CCC: $30.25 © 2005 American Chemical Society Published on Web 06/07/2005
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Figure 3. SEM images showing the lens-shaped crystals of vaterite from a 0.02 M CaCl2 sample aged 18 h (a) and a higher magnification showing the surface roughness (b).
Figure 2. TEM pictures of hexagon crystals formed from a 0.02 M CaCl2 solution after 18 h (a); and selected area electron diffractions of the upper (b) and lower (c) particle, respectively. Both are characteristic of the vaterite structure confirmed by the HRTEM image (d) and the power spectrum (e) of the upper particle.
For the lower CaCl2 concentration sample (10-3 mol/L), remarkably well-expressed rosetta-shaped mesostructures with 6-fold symmetry were observed (Figures 1b,c). These superstructures are composed of truncated hexagonal crystalline platelets with slight morphological differences (Figures 1b,c). They were found to be stable in solution for at least two weeks. A related morphology was previously observed in a CaCl2-NH4HCO3 mixed solution and described as “overgrown calcite”. It was reported to be due to the presence of aragonite-specific polyanionic proteins.22 It is an open question whether these structures are indeed identical, i.e., the proteins are redundant and the general crystallization scheme is driven instead by the ammonium. In any case, for the vapor diffusion technique the experimental conditions should be chosen carefully, as it provides several simultaneous reaction pathways. In another study, vaterite crystals with the same morphology as in our experiments were formed under stearic acid monolayers due to stereochemical recognition of the monolayer.19 Transmission electronic microscopy (TEM) investigations were performed to investigate the formation mechanism of these unusual morphologies. Figure 2a shows two hexagon-shaped crystals of a sample from a CaCl2 concentration (0.02 M), aged 18 h (Figure 2a) in which the lower particle already shows increased electron density in the center indicative of increased thickness. The electron diffraction patterns of the upper (Figure 2b) and lower (Figure 2c) crystals are both characteristic of single crystalline vaterite particles (the expected kinetic crystallization product) oriented along the [001] direction. Additionally, high-resolution TEM investigations of the upper particle show its perfect crystallinity (Figure 2d), although the image quality is lowered due to the specimen thickness. Also the power spectrum presents several single spots characteristic of a vaterite single-crystal oriented along the [001] direction. Moreover, the upper thin crystal can clearly be recognized to consist of smaller building blocks, that is, a perfectly vectorially aligned stack of even thinner truncated trigonal vaterite plates. To understand how such vaterite crystal shapes can be formed, several factors in the vapor
diffusion method have to be considered. Indeed, both the pH and the ionic strength increase from the bottom to the top due to the continued diffusion and solvation of NH3 into the solution. Ammonia as well as CO2 dissolve and diffuse into the CaCl2 solution, leading to the increase of the pH from 5.8 to 9.5. As the pKa of ammonia is 9.25 (T ) 25 °C), the predominant species will be the positively charged ammonium ion, which interacts with the surface of the growing crystals. The influence of the ammonium ion is reminiscent of data from the literature in which calcium carbonate was crystallized with added Li+ 17 and via the Kitano method with added ammonia.18 With Fourier transformation infrared spectroscopy (FTIR) investigations (characteristic vibrational bands: 1083, 865 cm-1), it seems that the first species to form are amorphous calcium carbonate particles. This is in good agreement with the generally accepted process of calcium carbonate crystallization in the presence of additives.23,24 In a second step, vaterite nanocrystals are formed, which adopt the unusual, truncated trigonal morphology depicted in Figure 2. Considering the symmetry and the electron diffraction data, this morphology must expose the (001) faces. Vaterite usually does not expose the highly polar (001) faces as these would be composed of only carbonate or calcium ions in a hexagonal orientation, which would in the absence of modifiers result in a too high surface energy. The fact that this face now becomes dominant speaks for a sticking of the positively charged ammonium ions to the negatively charged (001/001 h ) plane, leading to a lowering of surface energy and inhibition of growth in this direction. It should be noted that ammonium adsorption can take place on one face only, as otherwise the internal Coulomb fields in the crystal would be too high. Considering that the hexagonshaped crystals are stable for days only, the stabilization due to ammonium ions seems to be more reversible than the one due to polymers,10,13,16 although the vaterite phase is still stabilized against the transformation into calcite. The primary platelets, due to the asymmetry of ion adsorption, exert strong dipolar fields onto each other and stack spontaneously to vectorially aligned multilayers, as seen in TEM (Figure 2), but also in the side view in SEM (Figure 3a). As the hexagons and flat lenses scatter X-rays and electrons as single crystals, they must have a perfect crystallographic alignment. The described scenario is also supported by FTIR. The spectrum of a sample aged 18 h shows the presence of amorphous calcium carbonate, as noted above, and vaterite (characteristic vibrational bands: 743 cm-1). With perpetuated growth and increasing size of the superstructures, the fields of the hexagonal crystal stacks become strong enough to attract also the amorphous intermediates. As observed by scanning electron microscopy, all particles beyond a certain size are rough (Figure 3b and Supporting Information) and a large number of nanoparticles can be identified to make up the lens surfaces (size ∼ 20 nm). This is presumably also the reason they are so homogeneously rounded.
Communications As electron diffraction proves that the hexagons with the rough surface are still composed of pure single crystalline vaterite, the growth of the superstructure seems to occur by the aggregation of the colloidal amorphous precursors onto the surface and their subsequent crystallization along the vectorial reference system of the superstructure. Such crystallographic lock-in by oriented attachment is a noteworthy growth mechanism found for transition metal systems25-27 but now also for CaCO3 indicating a broader validity than so far presumed. More precisely, the oriented attachment mechanism observed here follows a threedimensional assembly route previously described in the case of SnO2.28 The rosetta-shaped crystals formed at lower CaCl2 concentration (10-3 mol/L) are significantly larger, that is, the lower supersaturation of CaCO3 at the onset of precipitation leads to lower nucleation rates and a slower, more controlled structure formation, which allows this remarkable self-assembly of the building blocks. The X-ray powder diffraction spectrum (data not shown) of the whole precipitated sample after 18 h shows diffraction peaks characteristic for vaterite. Also few of the peaks characteristic for calcite are found, due to a low amount of rhombohedral calcite found in the collected precipitate. At long times and depending on the setup, these structures transform completely into calcite-rhombohedra via a dissolution reprecipitation mechanism, as vaterite still dissolves more easily than calcite. The (001) vaterite face stabilization by weakly adsorbed ammonium ions gets completely lost upon the transformation into calcite, which can show a similar highly polar (001) face as vaterite. Obviously, the lowest energy (104) faces are preferred on the thermodynamic calcite formation pathway. The isolated vaterite superstructures in absence of water however are stable with no transformation observed. To conclude, we reanalyzed by SEM and HRTEM the formation of hexagonal, lens- and rosetta-shaped superstructures via gas diffusion under well-controlled conditions. Simple ammonium ions act as effective additives for the control of CaCO3 morphology in a way that was previously described for other additives.13-16,19,22 It was shown that the structures form by a mesoscale assembly process, in which in a first step [001] oriented vaterite nanosheets presumably stabilized by ammonium are stacked to hexagonal superplates with subsequent fusion to a single crystal by oriented attachment. Exceeding a certain critical size, the stacks can also attract the primary amorphous nanoparticles, which then undergo lock-in crystallization toward a common single crystalline reference system, whereas the common lens shape is due to the force fields and the related aggregation probability. 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. In addition, our results contradict the discussed epitaxial model for the observed preferred CaCO3 nucleation faces under Langmuir monolayers,19 as the same morphologies can be formed in the presence of simple ions, as observed here. The nonrelevance of epitaxy under Langmuir monolayers was already suggested by Volkmer et al.29 and further confirmed by CaCO3 crystallization in the presence of form stable block copolymers.30 Also in the present case, mesoscopic transformations31 appear to be omnipresent at various levels, which give rise to an appealing structural complexity beyond the symmetry predetermined by the unit cell expressed in socalled mesocrystals.20,21
Crystal Growth & Design, Vol. 5, No. 4, 2005 1319 Acknowledgment. We thank Miles Page for useful discussions and the Max-Planck-Society and the DFG SPP 1117 priority program “Principles of Biomineralization” for financial support of this work. We thank the Fritz-Haber Institute and Prof. R. Schlo¨gl for the use of the electron microscope and Klaus Weiss for his technical assistance. Supporting Information Available: Additional figures showing the hexagonal-shaped crystals synthesized with a 0.2 M CaCl2 sample aged 18 h observed by SEM. This information is available via the Internet at http://pubs.acs.org.
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