Fabrication of Calcite Aggregates and Aragonite Rods in a Water/ Pyridine Solution Zhaodong Nan,*,† Bianqing Yan,† Xiuzhen Wang,‡ Rong Guo,† and Wanguo Hou*,§ College of Chemistry and Chemical Engineering, Yangzhou UniVersity, Yangzhou 225002, P.R. China, College of Chemical Science, Qufu Normal UniVersity, Qufu 273165, P.R. China, Key Laboratory of Colloid and Interface Chemistry of the State Education Ministry, Shandong UniVersity, Jinan 250100, P.R. China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 11 4026–4030
ReceiVed March 17, 2008; ReVised Manuscript ReceiVed July 20, 2008
ABSTRACT: Polymorphs of calcium carbonate (CaCO3) have been easily synthesized in a water/pyridine mixed solvent system under mild conditions without using any other organic additives. The phase transition from pure aragonite to almost pure vaterite, and then to pure calcite, can be nicely captured by the choice of a suitable water to pyridine ratio. The formation mechanisms of calcium carbonate crystals have been proposed. A novel morphology of calcite, wire-like shaped, was found in pure pyridine solvent. A self-assembly process for the formation of wire-like calcite in pure pyridine solution has been proposed. The effect of experimental time on morphology of calcite is investigated in pure pyridine solvent. The results obtained proved that such reaction media could give an alternative and versatile tool for controlling both the structure and the polymorphism of inorganic materials.
1. Introduction The biological processes lead to a wide variety of largely unsolved biomineral structures.1 Calcium carbonate (CaCO3), as one of the most ubiquitous existing biominerals, has received much attention in various fields. There are three anhydrous crystalline phases, calcite, aragonite, and vaterite. According to thermodynamics, calcite is the most stable phase at ambient temperature and pressure. Calcite and aragonite are the main forms of CaCO3 existing in organisms.2,3 Vaterite is expected to have potential applications for different purposes because of its properties such as high specific surface area, high solubility, high dispersion, and lower density compared with the other two crystalline phases.4 In organisms, these crystals have a wide range of naturally occurring crystal habits and are normally found assembled into hierarchical structures that result in differences with intriguing properties.5 Scientists inspired by these results have attempted to fabricate CaCO3 crystals with different morphologies and properties and have investigated the formation mechanisms outside of biological organisms by mimicking the biomineralization processes. Water-soluble additives have been selected to mediate CaCO3 morphologies through specific interactions between the faces of the growing crystals and the additives, such as metal ions,6,7 anion surfactants,8,9 polymers,10,11 and biomolecules.12 Complex CaCO3 superstructures were synthesized in reverse microemulsions.13 An ethanol/water mixed solution system was used to fabricate CaCO3 crystallines.14 Pure aragonite, almost pure vaterite, and a mixture of calcite and aragonite were synthesized without any organic additives. Crystalline hexagonal-shaped superstructures of calcium carbonate were synthesized in the presence of ammonia, which were assembled by a threedimensional oriented attachment of vaterite nanoparticles.15 Recently, a novel morphology of aragonite, “magnified” rhombohedral, was fabricated in our group, and phase transition from calcite to aragonite were found with PAM and CTAB as additives.16 However, the controlled preparation of calcium * To whom correspondence should be addressed. E-mail:
[email protected]. † Yangzhou University. ‡ Qufu Normal University. § Key Laboratory of Colloid and Interface Chemistry of the State Education Ministry.
carbonate with different morphologies and different polymorphs in pyridine solution has not been reported yet. Pyridine, a liquid with a disagreeable smell, is now used extensively in the production of pharmaceuticals such as sulfa-drugs and antihistamines, as a denaturant for ethyl alcohol, as a solvent for organic chemicals, and in the preparation of waterproofing agents for textiles. In this work, CaCO3 crystals with diverse morphologies and crystal polymorphs were synthesized by a simple precipitation of calcium acetate and urea at 393 K in a mixed pyridine/water solvent. The growth processes of calcium carbonate were also investigated in this study. The as-prepared products were characterized by FT-IR, XRPD, HRTEM, SEM, TEM, SAED, and TG techniques.
2. Experimental Section 2.1. Materials and Sample Synthesis. Analytical grade urea, calcium acetate, and pyridine were all used as received without further purification. Distilled water was used throughout the experiment. In a typical preparation, 40 mL of 0.05 M Ca(CH3COO)2 and 0.25 M CO(NH2)2 pyridine/water solutions were put into a Teflon-lined stainless-steel autoclave which was filled up to 80% of the total volume of the Teflon cylinder (50 mL). The autoclave was maintained at 120 °C for 24 h and subsequently quenched to room temperature. The obtained precipitate was filtered and rinsed several times with ethanol and double deionized water and dried at room temperature for at least 24 h in vacuum. 2.2. Characterization. X-ray powder diffraction (XRPD) patterns were recorded using a Bruker D8 Advanced XRD diffractometer with Cu KR radiation at a scanning rate of 0.04 degree · s-1. Scanning electron microscope (SEM) images were taken with a JEOL JSM6700FXIB, fitted with a field emission source, and working at 20 kV. All samples were mounted on copper stubs and sputter coated with gold prior to examination. Infrared spectroscopic analysis was performed in transmission mode (FT-IR) using a Nicolet Aexus 470, with scanning from 4000 to 500 cm-1 by using KBr pellets. Transmission electron microscopy (TEM) images were obtained on a 150 kV H-800 microscope. High-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) images were recorded by using a JEM-2010 UHR high-resolution transmission electron microscope (Japan Electron Company). Thermogravimetry (TG) curves were obtained with a NETZSCH STA 409 PC/PG.
10.1021/cg800282j CCC: $40.75 2008 American Chemical Society Published on Web 09/05/2008
Fabrication of Calcite Aggregates and Aragonite Rods
Crystal Growth & Design, Vol. 8, No. 11, 2008 4027
Scheme 1. Sketch of the Relationship between the Polymorph of CaCO3 and the Solvent Composition
3. Results and Discussion 3.1. Crystallization in Water/Pyridine Reaction System. The phase transition from pure aragonite to almost pure vaterite, and then to pure calcite, can be found by the choice of a suitable water to pyridine ratio, as given in Scheme 1. The XRPD patterns corresponding to the as-obtained samples are given in Figure 1. The Bragg reflections marked with “C” correspond to calcite, “V” to vaterite, and “A” to aragonite polymorphs, respectively (standard JCPDS files are as follows: aragonite, 5-0453; calcite, 83-0578; vaterite, 25-0127). Figure 1a shows that pure calcite was fabricated with pure pyridine as solvent. With the increase of the water/pyridine ratio (10% water + 90% pyridine), almost pure vaterite (about 97 wt %) was found, as given in Figure 1b. With a further increase of the volume ratio (50% water + 50% pyridine), pure aragonite was obtained (Figure 1c). Almost pure aragonite has been synthesized with pure distilled water as solvent.16 The above polymorphs are further confirmed by Fourier transform infrared spectroscopy (FT-IR). Spectrum a in Figure 2 shows that vibrational bands at about 876 and 713 cm-1 can be attributed to ν2 and ν4 modes of calcite, respectively. In spectrum b, the bands at about 1080 and 745 cm-1 can be attributed to the characteristic symmetric carbonate stretching (ν1 mode) and ν4 mode of vaterite, respectively; the bands at about 855 and 713 cm-1 can be attributed to out-of-plane bending vibrations (ν2 mode) and in-plane bending modes of aragonite, respectively.2,17 These results demonstrated that the as-obtained sample in the mixed solvent (10% water + 90% pyridine) is composed of vaterite and aragonite. In spectrum c, the bands at about 855, 713, and 710 cm-1 demonstrated that pure aragonite was fabricated. The SEM images show wire-like and irregular shapes (Figure 3A) and concern the sample formed in pure pyridine. This kind
Figure 1. XRD patterns of the samples obtained in water/pyridine mixed solvent system: (a) pure pyridine; (b) 10% water + 90% pyridine; (c) 50% water + 50% pyridine.
Figure 2. FT-IR spectra of CaCO3 crystals obtained in a pyridine/ water mixed solvent system: (a) pure pyridine; (b) 10% water + 90% pyridine; (c) 50% water + 50% pyridine.
of wire-like shape has not been reported, to our knowledge, and belongs to calcite, as it ensues from XRPD and FT-IR diagrams. Spherical shaped morphology of vaterite, corresponding to 10% water + 90% pyridine, is found in Figure 3B. The sample was composed of vaterite and a small amount of aragonite (about 3 wt %) as it follows from XRPD and FT-IR results. The uniform rod-like shape is shown in Figure 3C, corresponding to 50% water + 50% pyridine. This kind of rod-like morphology is typical of aragonite, according to the results obtained by XRPD and FT-IR. 3.2. Formation Mechanism of CaCO3 Polymorphs in the Mixed Solvent. The energy of a crystal is made up of two components: a surface component and a bulk component.18
E)
∑ γiAi + Elattice
(1)
i
The bulk lattice energies of calcite, aragonite, and vaterite were calculated to be -1.6876 × 108, -1.7851 × 108, and -1.6577 × 108 kJ · mol -1, respectively.19Thus, aragonite will be the most stable form of calcium carbonate when the surface component is insignificant. In our experimental conditions, with 50% water + 50% pyridine as solvent, the bulk component may be significant. Thus, pure aragonite was obtained. The surface energies of calcite, aragonite, and vaterite were calculated.19 In our experimental conditions with pure pyridine as solvent, the surface energy may be significant. Calcite is the most stable one among these three kinds of polymorphs. Thus, pure calcite was fabricated. The stability of calcite, aragonite, and vaterite needs further investigation by measuring (or calculating) the influence of a solvent (water, pyridine, water + pyridine) on those faces determining the equilibrium shape of the crystal. However, the morphology of the as-obtained calcite is not like that fabricated in pure water, which is made by single rhombohedral shaped crystals. A TEM image of the sample is given in Figure 4 that outlines that the wire-like structure is an assembly of micrometric rhombohedral shaped crystallites. Almost pure vaterite (97 wt %) was obtained when 10% water + 90% pyridine was used as the solvent. Figure 5 shows the TG curves of the as-synthesized samples in mixed solvents. To compare the obtained results, the curve of the sample fabricated in pure water is also given in Figure 5. Before about 600 °C, no mass loss is found for curves B and C in Figure 5, which correspond to the samples obtained in the mixed solvent (50%
4028 Crystal Growth & Design, Vol. 8, No. 11, 2008
Nan et al.
Figure 3. SEM images of the samples obtained in a water/pyridine mixed solvent system: (A) pure pyridine; (B) 10% water + 90% pyridine; (C) 50% water + 50% pyridine. The inset in A shows the irregular shape.
Figure 4. TEM image of the sample formed in pure pyridine system.
water + 50% pyridine) and pure water, respectively. For the curve A, the mass loss is found starting at about 150 °C, and is calculated to be about 4 wt % before 600 °C, which corresponds to the sample synthesized in the mixed solvent (10% water + 90% pyridine). The boiling point is 115 °C for pure pyridine. The vaporizing temperature of pyridine is increased because of the interaction between pyridine and Ca2+ in the system. Thus, the vaporization of pyridine can be thought of as the reason for the lost mass of the sample before 600 °C. The result obtained by the TG curves demonstrates that pyridine adsorbs and absorbs on the faces and in the crystal of the vaterite fabricated in the mixed solvent (10% water + 90% pyridine), respectively. These may be the reasons which vaterite was obtained in 10% water + 90% pyridine solvent. 3.3. Effect of Experimental Time on the Growth of CaCO3 in Pure Pyridine Solvent. To investigate the processes of CaCO3 growth in pure pyridine solvent, the samples are synthesized in 12 and 6 h, respectively, the other experimental conditions remaining the same as given in section 2. No
Figure 5. TG curves of the samples obtained in pyridine/water mixed solvent system: (A) 10% water + 90% pyridine; (B) 50% water + 50% pyridine, (C) pure water.
precipitation was found when the experimental time was shorter than 6 h. Figure 6 shows SEM images of CaCO3 crystals synthesized in pure pyridine solvent in 12 and 6 h, respectively. Figures 6A1 and 6A2 show the wire-like and irregular shapes, respectively, of the sample fabricated in 12 h. The results prove that these two kinds of aggregates are formed in 12 h. The XRPD patterns are given in Figure 7. To compare these results with those obtained from the sample fabricated in 24 h, the XRPD patterns of two samples fabricated in pure pyridine in 12 and 24 h are shown in Figure 7, as 7b and 7a, respectively. It follows that calcite can be synthesized in 12 h, even if the crystal growth is not so good as that of the sample obtained in 24 h. The results are further demonstrated by FT-IR, as given in Figure 8. The vibrational bands at about 876 and 711 cm-1
Fabrication of Calcite Aggregates and Aragonite Rods
Figure 6. SEM images of the samples obtained in pure pyridine solvent system: (A) 12 h; (B) 6 h. A1 and A2 show wire-like and irregular structures of the sample fabricated in 12 h. B2 is the magnified figure of B1.
Crystal Growth & Design, Vol. 8, No. 11, 2008 4029
Figure 8. FT-IR spectra of CaCO3 crystals obtained in pure pyridine solvent system: (A) 24 h; (B) 12 h; (C) 6 h.
Figure 9. HRTEM image and the electron diffraction patterns of the samples obtained in pure pyridine system: (A) 24 h; (B) 12 h; (C) 6 h. Figure 7. XRD patterns of the as-obtained samples in pure pyridine solvent: (a) 24 h; (b) 12 h.
can be attributed to the modes of calcite for curve B in Figure 8, while the vibrational bands at about 619 and 672 cm-1 can be attributed to pyridine. To compare the obtained results, the FT-IR curve for the sample obtained within 24 h is also given in Figure 8 (curve A). No bands at about 619 and 672 cm-1 can be found for the curve A. These results prove that the adsorbed pyridine decreases and crystal growth improves with increasing experimental time. For the experimental time decreasing to 6 h, the sample morphology is given in Figure 6B (see also the B2 magnification). The rod-like shapes are found, their length being shorter than those fabricated in 12 and 24 h. These results prove that the rod-like structure would evolve to a wire-like and irregular structure as obtained for the experimental times of 12 and 24 h. No clear X-ray diffraction peak is found for the as-obtained sample. The corresponding FT-IR curve is given in Figure 8 as curve C. The weak band at about 875 cm-1 can be attributed to the vibration mode of calcite. The vibrational bands at about 617 and 672 cm-1 can be attributed to pyridine. These results demonstrate that pyridine is adsorbed at the surfaces of the as-synthesized product, and the growth becomes worse when compared with the sample fabricated in 12 h.
More information can be provided by HRTEM images of the calcite obtained in 24, 12, and 6 h, as shown in panels A, B, and C of Figure 9, respectively. The SAED patterns are also given as inset figures. The perfect crystals are found for the as-obtained samples after 12 and 24 h as shown in Figure 9A,B. The values of the HRTEM measured lattice spacings are consistent with the (102), (006), (110) and (204) interplanar spacing of calcite, such as 0.3873 nm, 0.2845 nm, 0.2503 nm, and 0.1940 nm, respectively (Figure 9A). The lattice spacings of 0.3873 nm, 0.2294 nm, 0.2503 nm, and 0.2102 nm correspond to those of the (102), (113), (110) and (202) planes of calcite, respectively (Figure 9B). According to the SAED pattern of Figure 9C, the (104) planes are found to correspond to calcite. However, the corresponding lattice spacing can not be found on the HRTEM image, and points 2 and 3 in Figure 9C (inset figure) do not correspond to any diffracting family of calcite lattice planes. These results are the same as those obtained in Figures 7 and 8. When the experimental time takes 6 h, the precipitation shows a rod-like shape and poor crystalline structure since pyridine is adsorbed on the surfaces of the product. With increasing experimental time, the shape of the obtained sample becomes wire-like and irregular, the
4030 Crystal Growth & Design, Vol. 8, No. 11, 2008
crystallinity of calcite improves, and the adsorbed pyridine on the sample’s surfaces decreases.
4. Conclusions We found a novel route for CaCO3 polymorphs discrimination in a water/pyridine mixed solution without using any other organic additives. The phase transition from aragonite to almost vaterite and then to calcite can be nicely captured by the choice of a suitable water to pyridine ratio. Aragonite occurs with 50% water + 50% pyridine as solvent. Vaterite was obtained with 10% water + 90% pyridine as solvent. Calcite was fabricated with pure pyridine as solvent. The as-obtained calcite crystallizes in a wire-like shape self-assembled by rhombohedral micrometric individuals, in pure pyridine. The wire-shaped calcite becomes perfect with increasing experimental time in pure pyridine solvent from 6 to 24 h.
References (1) (a) Lowenstam, H. A.; Weiner, S. On Biomineralization; Oxford University Press: New York, 1989. (b) Mann, S. Biomineralization. Principles and Concepts in Bioinorganic materials Chemistry; Oxford University Press: New York, 2001. (2) Zhou, G. T.; Yu, J. C.; Wang, X. C.; Zhang, L. Z. New. J. Chem. 2004, 28, 1027–1031.
Nan et al. (3) Hou, W. T.; Feng, Q. L. Cryst. Growth Des. 2006, 6, 1086–1090. (4) Naka, K.; Tanaka, Y.; Chujo, Y. Langmuir 2002, 18, 3655–3658. (5) Yu, J. G.; Lei, M.; Cheng, B.; Zhao, X. J. J. Solid State Chem. 2004, 177, 681–689. (6) Yu, J. G.; Zhao, X. F.; Cheng, B.; Zhang, Q. J. J. Solid State Chem. 2005, 178, 861–867. (7) Shaw, W. H. R.; Bordeaux, J. J. J. Am. Chem. Soc. 1955, 77, 4729– 4733. (8) Shen, Q.; Wei, H.; Wang, L.; Zhou, Y.; Zhao, Y.; Zhang, Z.; Wang, D.; Xu, G.; Xu, D. J. Phys. Chem. B 2005, 109, 18342–18347. (9) Shen, Q.; Wang, L.; Huang, Y.; Sun, J.; Wang, H.; Zhou, Y.; Wang, D. J. Phys. Chem. B 2006, 110, 23148–23153. (10) Loges, N.; Graf, K.; Nasdala, L.; Tremel, W. Langmuir 2006, 22, 3037–3080. (11) Kotachi, A.; Miura, T.; Imai, H. Chem. Mater. 2004, 16, 3191–3196. (12) Liu, D.; Yates, M. Z. Langmuir 2006, 22, 5566–5569. (13) Thachepan, S.; Li, M.; Davis, S. A.; Mann, S. Chem. Mater. 2006, 18, 3557–3561. (14) Chen, S. F.; Yu, S. H.; Jiang, J.; Li, F.; Liu, Y. Chem. Mater. 2006, 18, 115–122. (15) Gehrke, N.; Co¨lfen, H.; Pinna, N.; Antonietti, M.; Nassif, N. Cryst. Growth Des. 2005, 5, 1317–1319. (16) Nan, Z.; Shi, Z.; Yan, B.; Guo, R.; Hou, W. J. Colloid Interface Sci. 2008, 317, 77–82. (17) Wang, L.; Sondi, I.; Matijevicˇ, E. J. Colloid Interface Sci. 1999, 218, 545–553. (18) Gibbs, J. W. Collected Works; Longman: New York, 1928. (19) de Leeuw, N. H.; Parker, S. C. J. Phys. Chem. B 1998, 102, 2914–2922.
CG800282J
