CRYSTAL GROWTH & DESIGN
Morphosynthesis of Mixed Metal Carbonates Using Micellar Aggregation S.
Sindhu,†
S.
Jegadesan,‡
R. A. Edward
Leong,‡
and S.
2006 VOL. 6, NO. 6 1537-1541
Valiyaveettil*,†,‡
NUS Nanoscience and Nanotechnology InitiatiVe (NUSNNI) and Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543 ReceiVed August 12, 2005; ReVised Manuscript ReceiVed March 11, 2006
ABSTRACT: Understanding the nucleation of crystals of calcium salts is an important area of research owing to its biological significance. Roles of surfactants and added bivalent metal ions on the nucleation of calcium carbonate crystals were investigated. Different mixed metal carbonates from bivalent ions such as calcium, magnesium, zinc, cadmium, and barium ions were prepared in the presence of a cationic surfactant, cetyltrimethylammonium bromide, and fully characterized using different microscopic techniques. Crystals of calcium carbonate and calcium-cadmium carbonate with elongated rod-type superstructure were nucleated with high aspect ratios. The presence of different bivalent metal ions in the crystallization medium significantly influenced the composition and morphologies of the crystals. Synthetic parameters such as concentration of cations, anions, surfactant, aging temperature, and aging time were controlled to get highly ordered metal carbonate structures. Incorporation of high concentrations of cadmium in the calcium carbonate lattice may be the reason for high biological intake of cadmium in various tissues of animals living in pollutant environment. Introduction Shape- and size-controlled growth of inorganic materials using reverse micelles or microemulsions with complex morphology received considerable interest in the recent past owing to its diverse application potential in areas such as catalysis, medicine, pigments, cosmetics, and separation technology.1-4 The microemulsion technique was used for the synthesis of nanoparticles with well-defined particle size and size distribution. However, changes in reaction conditions also led these nanoparticles to aggregate to form highly ordered superstructures5,6. Templates such as self-assembled monolayers,7 block copolymers,8 synthetic polypeptides,9 or molecular assemblies10,11 were employed for the synthesis of inorganic materials, in particular calcium carbonate, with interesting morphologies. Surfactants molecules are known to give monolayers, microemulsions, or micelles, which can act as a soft functional template for the directed crystal growth to form ordered superstructures of metal carbonates (e.g., CaCO3, BaCO3, or BaSO4).12-21 These hierarchical arrangements of inorganic ions are affected by many parameters such as temperature, concentration, aging time, and added salts in the crystallization media. In a classical crystallization model, expansion of crystal lattice occurs as a result of the unit cell replication without any structural changes.22 In the case of aggregation-mediated crystal growth, colloidal nanoparticles of uniform size were formed first followed by internal restructuring.16 Sponge-type or spindleshaped aggregates of calcium carbonate were prepared using the microemulsion method.11,23 Since biomineralization of calcium carbonate occurs in the presence of various metal ions in the organism, it is important to understand the role of such ions in the crystallization process. Recently, Qi et al. reported the formation of pinecone-shaped calcite crystals from a mixed solution of surfactant and block copolymers.24 In this paper, we discuss the crystallization of metal carbonates in the presence of a cationic surfactant, cetyltrimethylammonium bromide * To whom the correspondence should be addressed. Tel: (65) 6516 4327. Fax: (65) 6779 1691. E-mail:
[email protected]. † NUS Nanoscience and Nanotechnology Initiative (NUSNNI). ‡ Department of Chemistry.
(CTAB). Mixed metal carbonate systems, such as Ca-CdCO3, Ca-MgCO3, and Ca-ZnCO3, were also studied to analyze the effect of organized media and the presence of bivalent metal ions on the crystal growth mechanism. Experimental Procedures All chemicals and reagents were purchased from commercial sources and used without further purification unless otherwise specified. Calcium chloride (CaCl2‚2H2O), sodium bicarbonate (NaHCO3), cadmium chloride (CdCl2‚2.5H2O), barium chloride (BaCl2‚2H2O), magnesium chloride (MgCl2‚6H2O), zinc chloride (ZnCl2), and N-N-cetyl trimethylammoniumbromide (CTAB) were obtained from commercial sources. Calcium carbonate crystals were grown in reverse microemulsions prepared using CTAB. In a typical procedure, a mixture of toluene (25 mL), CaCl2 solution (0.06 M, 5 mL), and CTAB (2.5 g) was taken in a 100 mL three neck round-bottom flask, stirred to form a turbid mixture, and heated to 80 °C, and 5 mL (0.54 M) of sodium bicarbonate solution was added dropwise. After 5 min of stirring under reflux, the system was kept undisturbed for 3 h at 80 °C. The same procedure was used for the synthesis of mixed metal carbonate crystals. The solutions were prepared in such a way that the molar concentration of Ca2+/M2+ varies as 1:0.5, 1:1, 1:2, and 1:3. The crystals formed after 3 h were collected by filtration followed by washing with absolute alcohol and water to remove traces of surfactants. The collected samples were dried in an oven at 60 °C for several hours. These powder samples were then characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy disperse X-ray scattering (EDXS), Fourier transform infrared spectroscopy (FTIR), and powder X-ray diffraction (XRD) studies. For SEM studies, the collected crystals were carefully placed on aluminum stubs with double-sided carbon tape, sputter coated with gold, and examined using a JEOL JSM 6700F scanning electron microscope. TEM and EDXS studies were done by dispersing the crystals in alcohol, transferring them to the carbon-coated copper grids, and examining them with a JEOL 3010 electron microscope. For X-ray diffraction studies, finely powdered samples were placed on a sample holder and pressed to form a thin layer. The diffractograms of the crystals were recorded using D5005 Siemens X-ray diffractometer with Cu ΚR (λ ) 1.54 Å) radiation at 40 kV and 40 mA. All samples were scanned over a 2θ range of 2070° at a step size of 0.02°. IR spectrum was recorded using a Bio-Rad FTS 165 FT-IR spectrophotometer with 4 cm-1 resolution using KBr disks. Attenuated total reflectance (ATR)-FTIR spectra were collected on a Shimadzu infrared microscope AIM 8800 spectrophotometer equipped with the supply of liquid nitrogen to a MCT (mercury,
10.1021/cg050411x CCC: $33.50 © 2006 American Chemical Society Published on Web 04/21/2006
1538 Crystal Growth & Design, Vol. 6, No. 6, 2006
Figure 1. X-ray diffraction pattern of spindle-like CaCO3 crystals formed in the presence of CTAB. Peaks due to aragonite (A) and vaterite (V) are marked in the figure. cadmium, tellurium) detector. A total of 64 scans were collected and averaged at a resolution of 4 cm-1.
Results and Discussion The polymorph characterization of calcium carbonate and mixed metal carbonates was done using powder X-ray diffraction studies and FTIR spectroscopy. X-ray diffraction patterns of calcium carbonate crystals showed peaks corresponding to aragonite and vaterite phases (Figure 1). But no peaks corresponding to the surfactant were observed in the XRD pattern (Figure 1) or in FTIR spectra (Figure 2) indicating the absence of surfactant molecules in the crystal lattice. In the FTIR spectrum, the peaks observed at 746 and 873 cm-1 correspond to the in-plane bending (ν4) and out-of-plane bending (ν2) modes of CO32-, the weak peak at 1087 cm-1 corresponds to symmetric C-O stretching (ν1), and the peaks at 1488 and 1455 cm-1 are assigned to the asymmetric C-O stretching (ν3a and ν3b).25 The peaks at 746, 873, 1488, and 1455 cm-1 confirm the presence of the vaterite phase. A weak peak at 712 cm-1 (Figure 2A) and the presence of a shoulder at 873 cm-1 indicate the existence of small amounts of the aragonite phase in the formed crystal aggregates. The presence of the aragonite phase was again confirmed by recording ATR-FITR spectra of the elongated crystal aggregates. The aperture for
Sindhu et al.
ATR-FTIR was selected randomly in such a way that different positions of the crystal aggregate surfaces were selected as the scan area by moving the sample holder in the proper direction. The spectra B and C in Figure 2 make a distinction between them by showing an increase in the 848 cm-1 peak intensity, which belongs to the aragonite phase. XRD analysis of the crystal aggregate also indicated the formation a mixture of vaterite and aragonite phases. In addition to this, the scanning electron micrographs of these crystals showed that spherical crystals were attached to a central rod-type structure (arrows in Figure 3A indicate the central portion of the crystal aggregates). Based on these analyses, we presume that the rod-type central portion nucleates first, which corresponds to the aragonite part, while the spherical aggregates corresponds to the vaterite polymorphs. Morphology of the formed crystal aggregates was investigated using SEM. Scanning electron micrograph of the crystal aggregates (Figure 3A) shows an elongated structure with spherical aggregates attached on the surface. The surfactant molecules are known to be organized in a tubular form at the concentration used in our experiments. We expect that the crystal nucleation and growth occurs at the interior part of the tubular micelles. Different aging times and temperatures were used, and an optimum condition of 80 °C and an aging time of 3 h were chosen for further experiments. The mean aspect ratio (length/ width) of the elongated crystal aggregate was ca. 7.5 ( 0.5. Other than calcium carbonate, mixed metal carbonates, such as Ca-CdCO3, Ca-MgCO3, and Ca-ZnCO3 were synthesized in the presence of CTAB under similar experimental conditions. The presence of bivalent cations (e.g., Mg2+, Zn2+, or Cd2+) showed significant influence on the final crystal morphology. Here the concentrations of metal chlorides were chosen in such a way that the final molar concentration ratio of calcium to the metal ion in the crystallization medium was 1:2. Mixed metal carbonates of Ca/Mg and Ca/Zn showed flower-type morphologies (Figure 3B,C). Experiments in the presence of barium ions showed no significant changes in the rhombohedral morphology typically observed for calcite crystals (Figure 4A). However, the crystals obtained in the presence of cadmium ions showed a common crystallographic axis (Figure 5A). Among these mixed metal carbonates, detailed study was carried out on Ca-CdCO3 due to its superstructure formation. Since these crystals were nucleated in the presence of various metal ions, optimization of the experimental conditions was done by changing different synthetic parameters (Table 1) such
Figure 2. FTIR spectra of the CaCO3 crystal aggregates using the KBr pellet method (A) and ATR-FTIR spectra (B,C) at two different regions on the sample dispersed over a glass plate.
Morphosynthesis of Mixed Metal Carbonates
Figure 3. SEM micrographs of CaCO3 (A), Ca-MgCO3 (B) and CaZnCO3 (C) formed in reverse microemulsion. Arrows in panel A indicate the tail-like portion of the crystal aggregates.
Figure 4. SEM micrographs of Ca-BaCO3 (A), Ca-CdCO3 (B) in the absence of surfactant, and pure cadmium carbonate with a Cd2+ ion concentration of 0.03 M (C) and 0.075 M (D).
as the concentrations of surfactants and cations (Figure 4BD) by keeping the bicarbonate anion concentration constant. Details of the samples prepared and the synthetic conditions used were summarized in Table 1. In the absence of the surfactants or using various concentrations of other bivalent cations, the morphologies of the crystals were changed signifi-
Crystal Growth & Design, Vol. 6, No. 6, 2006 1539
Figure 5. SEM micrograph of Ca-CdCO3 (A), electron diffraction pattern of these crystal aggregates (B), and the EDX data of the crystal surface (C).
cantly. These results indicate that both the cation and the surfactant play significant roles toward controlling the nucleation of crystals. A scanning electron micrograph of the calcium-cadmium carbonates is shown in Figure 5A. Figure 5B shows the electron diffraction pattern of the crystal aggregates obtained, which indicates a common crystallographic orientation for these crystals. All the reciprocal lattice nodes lying in the plane normal to this direction produce concentric rings in random crystal orientations around this direction, owing to the presence of a common crystallographic direction for the crystal aggregates.26 The size of the individual crystals in this aggregate is ca. 1 µm with an aspect ratio for the aggregate of ca. 10 ( 0.3, which is higher than that of pure CaCO3 crystal rods. The EDX spectrum (Figure 5C) indicates the presence of calcium and cadmium ions on the surface of these crystal aggregates. Furthermore elemental analysis of the crystals was done using the inductively coupled plasma ionization technique. Table 2 lists the individual metal composition in the mixed metal carbonate crystals. The structural analysis using XRD (Figure 6) indicates that the formed crystals have a mixed lattice of calcium and cadmium carbonate crystals. The diffraction patterns show peak broadening and a shift in diffraction peak position to the higher 2θ value with respect to pure calcium carbonate lattice. The changes in position of the maximum intensity peak (∆2θ) for the mixed metal carbonate with respect to pure calcium carbonate and cadmium carbonate lattice were ca. 0.52 and -0.20, respectively. The crystal growth mechanism observed for spindle-shaped calcium carbonate and rod-like calcium-cadmium carbonate
1540 Crystal Growth & Design, Vol. 6, No. 6, 2006
Sindhu et al.
Table 1. Details of Synthetic Conditions and Crystal Morphology Obtained for different metal carbonatesa molar concentration CaCl2
CdCl2
NaHCO3
CTAB
aging temp (°C)
crystal morph. spindle-shaped aggregates rod-like aggregates 95% cubes and 5% rod-like aggregatesb 96% cubes and 4% rod-like aggregatesb cubes cubes cubes cubes cubes cubes
0.03
0.0
0.27
0.68
80
0.03
0.06
0.27
0.68
80
0.03
0.06
0.27
0.54
80
0.03
0.06
0.27
0.82
80
0.03 0.03 0.03
0.06 0.03 0.015 0.03 0.075 0.06
0.27 0.27 0.27 0.27 0.27 0.27
0.68 0.68 0.68 0.68 0.68 0.0
27 80 80 80 80 80
0.03
molar concentration CaCl2
MgCl2
ZnCl2
BaCl2
NaHCO3
CTAB
aging temp (°C)
0.03
0.06
0.0
0.0
0.27
0.68
80
0.03
0.0
0.06
0.0
0.27
0.68
80
0.03
0.0
0.0
0.06
0.27
0.68
80
crystal morph. flower type flower type cubic crystals
a All crystallizations were carried out for 3 h. b Percentage of the rod type crystal aggregate was calculated manually from the SEM micrographs.
Table 2. ICP Results Showing the Proportion of Cations (mol %) Present in Different Mixed Metal Carbonate Systems metal carbonates
Ca
Mg
Cd
Zn
Ca-MgCO3 Ca-ZnCO3 Ca-CdCO3
58.5 36.9 50.07
41.5 a a
a a 49.9
a 63.0 a
a
Not detected.
aggregates were found to be interesting due to the higher order arrangement of crystals. Here the reverse micelles formed by the CTAB are considered to be acting as a crystallization chamber. This results in the formation of a filament or rod-like structures stabilized by the surfactant molecules on the surface.21 Introduction of a second bivalent metal ion (Cd2+) to the crystallization medium shows interesting morphology different from that of spindle-shaped calcium carbonate. The second bivalent metal ion influences the crystal growth process and final morphology of the mixed metal carbonate crystals. In the case of Ca-CdCO3 the individual single crystals were attached to each other at the obtuse step edges and form rod-type structures. Usually the crystal growth or dissolution is found to occur from the step edges, and the impurity ions accumulate at the growing crystal faces to inhibit the crystal growth.27 Since the ionic radius of Cd2+ (1.09) is close to that of Ca2+ (1.14), it is conceivable that the cadmium atoms may be able to replace Ca2+ ions or vice versa in the crystal lattice. Theoretical studies on calcium carbonate crystallization in the presence of divalent ions showed that, if the ionic radius of the competing ion is close to that of calcium or greater than that of calcium the competing ion will probably go to the kink sites on the obtuse step edge.27 From the SEM image (Figure 5A), it is clear that the obtuse step edges of the crystals were distorted more, and this may be due to the incorporation and accumulation of
Figure 6. X-ray diffraction patterns of CaCO3, Ca-CdCO3, and CdCO3 crystals.
cadmium ions on that edge of the crystal. Accumulation of ions on the growing step edge makes the interparticle interaction high at this area, which may lead to the superstructure formation. Colfen et al.12,21 discussed the formation of such hybrid nanostructures based on the higher order organization of molecules. In our control experiments carried out in the absence of surfactants, no elongated crystal aggregates were observed (Figure 4B). Hence both surfactants and mixed metal ions play a significant role in the formation of superstructures of mixed metal carbonates. In conclusion, we have shown that the highly organized calcium carbonate and mixed metal carbonate structures were crystallized inside a reverse microemulsion. The organization of these crystals depends on experimental conditions such as the cation concentration, cation/surfactant ratio, aging temperature, and aging time. Different cations such as Cd2+, Ba2+, Mg2+, and Zn2+ were used together with Ca2+ to produce mixed metal carbonates. The effect of these cations on the final morphology of the crystal was found to be different for different cations. Researchers have explored the cadmium uptake in different organisms,28 owing to its harmful biological activities and increased concentration in air, water, and soil due to industrialization. However, further studies are needed to identify various existing pathways. Even though, we used a high concentration of the bivalent ions, it is interesting to see the strong incorporation of these bivalent ions in the crystal lattice of CaCO3. The use of readily available starting materials would enable one to synthesize complex inorganic structures with interesting properties. Acknowledgment. The authors thank the NUS Nanoscience and Nanotechnology Initiative and National University of Singapore for the financial support and Department of Chemistry for technical support. References (1) Pileni, M. P.; Gulik-Krzywicki, T.; Tanori, J.; Filankembo, A.; Dedieu, J. C. Langmuir 1998, 14, 7359. (2) Hopwood, J. D.; Mann, S. Chem. Mater. 1997, 9, 1819. (3) Qi, L.; Ma, J.; Cheng, H.; Zhao, Z. J. Phys. Chem. B 1997, 101, 3460. (4) Rees, G. D.; Evans-Gowing, R.; Hammond, S. J.; Robinson, B. H. Langmuir 1999, 15, 1993.
Morphosynthesis of Mixed Metal Carbonates (5) Mohammed, M. B.; Ismail, K. Z.; Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 9370. (6) Esumi, K.; Matsuhisa, K.; Torigoe, K. Langmuir 1995, 11, 3285. (7) Berman, A.; Ahn, D. J.; Lio, A.; Salmeron, M.; Reichert, A.; Charych, D. Science 1995, 269, 515. (8) Colfen, H.; Qi, L. M. Chem.sEur. J. 2001, 7, 106. (9) DeOliveira, D. B.; Laursen, R. A. J. Am. Chem. Soc. 1997, 119, 10627. (10) Champ, S.; Dickinson, J. A.; Fallon, P. S.; Heywood, B. R.; Mascal, M. Angew. Chem., Int. Ed. 2000, 39, 2716. (11) Walsh, D.; Lebeau, B.; Mann, S. AdV. Mater. 1999, 11, 324. (12) Li, M.; Colfen, H.; Mann, S. J. Mater. Chem. 2004, 14, 2269. (13) (a) Kato, T.; Suzuki, T.; Irie, T. Chem. Lett. 2000, 186. (b) Hosoda, N.; Kato, T. Chem. Mater. 2001, 13, 688. (14) (a) Sugawara, A.; Ishii, T.; Kato, T. Angew. Chem., Int. Ed. 2003, 42, 5299. (b) Yu, S. H.; Colfen, H.; Xu, A. W.; Dong, W. Cryst. Growth Des. 2004, 4, 33. (15) (a) Park, H. K.; Lee, I.; Kim, K. Chem. Commun. 2004, 24. (b) Yu, S. H.; Colfen, H.; Antonietti, M. J. Phys. Chem. B 2003, 107, 7396. (16) (a) Keum, D. K.; Naka, K.; Chujo, Y. Chem. Lett. 2004, 33 (3), 310. (b) Wei, H.; Shen, Q.; Zhao, Y.; Wang, D.; Xu, D. J. Cryst. Growth 2004, 264, 424.
Crystal Growth & Design, Vol. 6, No. 6, 2006 1541 (17) (a) Ichikawa, K.; Shimomura, N.; Yamada, M.; Ohkubo, N. Chem.s Eur. J. 2003, 9, 3235. (b) Lahari, J.; Xu, G.; Dabbs, D. M.; Yao, N.; Aksay, I. A.; Groves, J. T. J. Am. Chem. Soc. 1997, 119, 5449. (18) Pai, R. K.; Hild, S.; Ziegler, A.; Marti, O. Langmuir 2004, 20, 3123. (19) Li, M.; Lebeau, B.; Mann, S. AdV. Mater. 2003, 15 (23), 2032. (20) Rock, M. L.; Tranchitella, L. J.; Pilato, R. S. Colloid Polym. Sci. 1997, 275 (9), 893. (21) (a) Colfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42, 2350. (b) Colfen, H.; Antonietti, M. Angew. Chem., Int. Ed. 2005, 44, 5576. (22) Jongen, N.; Bowen, P.; Lemaitre, J.; Valmalette, J. C.; Hofmann, H. J. Colloid Interface Sci. 2000, 226, 189. (23) Li, M.; Mann. S. AdV. Funct. Mater. 2002, 12, 773. (24) Qi, L.; Li, J.; Ma, J. AdV. Mater. 2002, 14, 300. (25) Andersen, F. A.; Brecevic, L. Acta Chem. Scand. 1991, 45, 1018. (26) Cowley, J. M. Electron Diffraction Techniques; International Union of Crystallography: Chester, England, 1992; Vol. I, pp 241-243. (27) De Leeuw, N. H. J. Phys. Chem. B 2002, 106, 5241. (28) Baldisserotto, B.; Kamunde, C.; Matsuo, A.; Wood, C. M. Comp. Biochem. Physiol., Part C: Toxicol. Pharmacol. 2004, 137, 363.
CG050411X
