Phase Transformation of Magnesium Amorphous Calcium Carbonate

Article pubs.acs.org/crystal

Phase Transformation of Magnesium Amorphous Calcium Carbonate (Mg-ACC) in a Binary Solution of Ethanol and Water Yang-Yi Liu, Jun Jiang, Min-Rui Gao, Bo Yu, Li-Bo Mao, and Shu-Hong Yu* Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China S Supporting Information *

ABSTRACT: In this article, we report that the phase transformation of magnesium amorphous calcium carbonate (Mg-ACC) can be easily realized in an ethanol/water mixed solution free from organic additives under mild conditions, and the phase transformation can be nicely captured by the choice of a suitable concentration of Mg-ACC, the volume ratio of ethanol to water as well as the reaction temperature. Moreover, a complex selfassembly process for the production of aragonite aggregate with ellipsoid shape in this binary solvent has been proposed. These results demonstrate that controlled phase transition from Mg-ACC can be easily achieved by using a binary reaction media. This study may provide some useful clues for understanding the mineralization process of CaCO3 in nature and implies that this method could be scaled up for industrial production of CaCO3 with different polymorphs.

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INTRODUCTION Calcium carbonate is a very important mineral because of its low cost, low environmental impact, and high industrial efficiency.1−3 In parallel, CaCO3 can be used as a model mineral for biomimetic research, by which biogenic control over mineral orientation, polymorph and morphology is been understood.2 The polymorphs of calcium carbonate are composed of three anhydrous crystalline forms, i.e., aragonite, vaterite and calcite, as well as three metastable phases, i.e., monohydrate, hexahydrate and amorphous calcium carbonate (ACC), of which ACC is the least stable form.4,5 Many research results illustrate that ACC is a useful amorphous phase for many aspects and plays an important role in the biomineralization of CaCO3.6−8 For an example, ACC is found in various organisms such as mature sea urchin larvae8,9 and larvae of molluscan bivalves.10 For one thing, there are examples of organisms that form relatively large structural tissues that are strengthened with ACC.11,12 On the other hand, ACC has been discovered as a transient precursor to either calcite or aragonite.13,14 Therefore, it is important to understand formation mechanisms of ACC because researchers can apply that to other inorganic systems to modulate their physical and chemical properties for advanced functional materials. However, due to the unstable nature of ACC, the research on the preparation and characterization of ACC has been developing slowly.15 Recently, various additives have been functionalized to be used as stabilizers for ACC, such as magnesium ion,16−18 phosphate,19,20 polyaspartate, polyglutamates and DNA.21−23 Our group has reported previously 1,3-diamino-2-hydroxypropane-N,N,N′,N′-tetraacetic acid24 and poly(acrylic acid) (PAA)25could stabilize ACC. In particular, magnesium amorphous calcium carbonate (Mg-ACC) is more important © 2012 American Chemical Society

for the research of biomineralization and magnesium can influence the crystallization of biogenic ACC by inducing the synthesis of aragonite,26 incorporating into the lattice of calcite, stabilizing ACC and so on.27 Fortunately, magnesium exists in nearly all the known biogenic amorphous calcium carbonate and is present in large quantities in seawater.28,29 Up to now, many researchers have investigated the effect of solvent on the synthesis of crystalline carbonate thin films and particles via transformation of ACC.30−32 Mann et al. reported that the extent of water penetration into the ACC cores can affect the transformation pathways of ACC.31 Shen et al. demonstrated that organic solvents have profound effects on the crystallization of amorphous calcium carbonate films.32 Additionally, several research groups have studied the use of mixtures of different solvents to control the transformation of ACC or the crystallization of calcium carbonate.33−41 Tang et al. reported water-induced phase transformation of poly(4sodium styrene sulfonate) or poly(acrylic acid) stabilized ACC in water−ethanol solution at room temperature.33,36 Recently, on the basis of the synergistic effects of the block copolymer and a mixed solvent, highly monodisperse vaterite microspheres can be synthesized.40 By a double diffusion method, the mineralization and morphosynthesis of CaCO3 can occur on Allium f istulosum L. bulb inner membranes in a mixed solvent system.41 Moreover, our group has demonstrated that a binary solvent system could offer an alternative and versatile tool for controlling the polymorph and morphology of inorganic minerals through adjusting the thermodynamics and kiReceived: June 17, 2012 Revised: October 12, 2012 Published: November 16, 2012 59

dx.doi.org/10.1021/cg3008206 | Cryst. Growth Des. 2013, 13, 59−65

Crystal Growth & Design

Article

netics.37,38 However, the direct phase transformation of MgACC in a binary solution or a mixed solvent without using any additives has not been reported so far. Recently, we realized the gram scale synthesis of highly stable Mg-ACC nanoparticles.18 Herein, we further report an effective and environmentally friendly method for phase transformation of these Mg-ACC nanoparticles in an ethanol/water mixed solution under mild conditions without using any organic additives. By this method, the thermodynamic and kinetic control can be manipulated by simply regulating the temperature, the volume ratio of ethanol to water and the initial concentration of Mg-ACC. Moreover, complex aragonite ellipsoids can be selectively synthesized and their formation mechanism has been proposed.

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Figure 1. XRD pattern of the sample prepared when dispersing 2.5 mL of 0.18 mM Mg-ACC solution in ethanol−water binary solvent. The volume ratio of ethanol to water (R) is 3/2 at 55 °C for 24 h (JCPDS Card number: 41-1475).

EXPERIMENTAL SECTION

Chemicals. All chemicals are analytical grade. Calcium chloride (Mw = 110.99), anhydrous sodium carbonate (Mw = 105.99), magnesium chloride hexahydrate (Mw = 203.30), ethanol (Mw = 46.07) and acetone (Mw = 58.08) were obtained from Shanghai Chemical Reagent Company and used without further purification. Double-distilled water was used to prepare aqueous CaCl2 and Na2CO3 solutions. Synthesis of Magnesium Amorphous Calcium Carbonate (Mg-ACC). The Mg-ACC was prepared by a rapid mixing synthesis method. In a typical procedure, the solutions of calcium chloride (0.1 mol·dm−3, 72 mL) and magnesium chloride hexahydrate (0.5 mol·dm−3, 14.4 mL) were mixed with mechanic stirring for 5 min to obtain a mixed solution. Then, an anhydrous sodium carbonate solution (0.1 mol·dm−3, 72 mL) was added rapidly to the mixed solution with stirring under ambient condition. The precipitate was filtered off immediately and washed with ethanol and finally dispersed in 400 mL of ethanol to ensure the concentration of Mg-ACC is 18 mM. Phase Transformation of Mg-ACC. In a typical experimental procedure, double-distilled water and ethanol were mixed to obtain solution with a different R value (R is the volume ratio of ethanol to water). Then, 2.5 mL of Mg-ACC solution was added to the mixed solution to ensure the ultimate concentration of Mg-ACC solution is 4.5 mM. Then, the solution was sonicated for 10 s and placed at 55 °C for 24 h. After crystallization, the precipitates were rinsed with water and acetone, and dried at room temperature for further characterization. Characterization. The crystalline phase of the products was identified by X-ray powder diffraction (XRD) using an MXPAHF Xray diffractometer with Cu Kα (λ = 1.54056 Å) and FT-IR spectra measured on a Bruker Vector-22 FT-IR spectrometer from 4000 to 400 cm−1 at room temperature. Morphology and sizes of the particles were observed on a field-emission scanning electron microanalyzer (FESEM, JEOL-6700F). Transmission electron microscopy (TEM) was performed on JEOL-2010 with an acceleration voltage of 200 kV. The Mg/Ca molar ratio was analyzed by the coupled plasma atomicemission spectroscopy (ICP-AES) technology on an Atomscan Advantage (Thermo Jarrell Ash Corporation, USA) instrument.

ellipsoids with length of 2.5−3 μm, and some ellipsoids were cross-linked and some little nanotablets were attached around the ellipsoids. Each microellipsoid is composed of nanoneedles as confirmed by a transmission electron microscopy (TEM) image (Figure 2b). The selected area diffraction pattern (SAED) in Figure 2c indicated that the preferential growth of the nanoneedle is along the [100] direction. Moreover, a highresolution TEM (HRTEM) image taken on a typical nanoneedle in Figure 2d shows lattice spacings of 2.70 Å and 2.72 Å, which correspond to those for the (012) and (121) plane, respectively. To understand the formation process of aragonite microellipsoids at 55 °C when R = 3/2 and the ultimate concentration of Mg-ACC was 4.5 mM, time-dependent phase and shape evolution process was carefully examined (Figures 3 and 4). The sample obtained after 3 h was poor crystalline (Figure 3a). There existed a majority of aggregates with spherical shape but only a few spindle-like crystal aggregates (Figure 4a). The diameter and length of crystal are about 3 μm and 2−2.5 μm, respectively, and some spindlelike crystal aggregates are made up of larger cross-linked crystals (Figure 4a). Each spindle-like crystal aggregate is composed of little needle-like crystals (inset in Figure 4a). Addadi et al.42 reported that the growth in aragonite is preferred along the c axis to the other crystallographic directions. As a result of that, under normal temperature and pressure, aragonite constructs as thin needles that do not generally grow into large crystals. After 12 h, the diffraction peaks of aragonite phase were strengthened (Figure 3b), most spherical crystal aggregates turned to be ellipsoids and more spindle-like crystal aggregates formed (Figure 4b). With prolonged time, most spindle-like crystal aggregates disappeared and many tablet-like aggregates attached on the ellipsoids after reaction for 15 h (Figures 3c and 4c). After aging for 48 h, the crystal aggregates become uniform and the tablets attached on the ellipsoid-like crystal aggregates turned to be larger (Figures 3d and 4d). On the basis of the above analysis, we propose that the primary nanocrystals (aragonite needle-like crystals) joined together by aggregation to become larger spindle-like aggregates, some of them integrated into cross-linked aggregates. Then, the primary nanocrystals continued to be attracted on the surface of spindle-like or cross-linked crystal aggregates to produce uniform aragonite microellipsoid

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RESULTS AND DISCUSSION Synthesis of Microellipsoids Aragonite in an Ethanol− Water Binary Solvent System. Dispersing Mg-ACC solution in ethanol−water solvent when the ultimate concentration of Mg-ACC is 4.5 mM and R = 3/2 (R: the volume ratio of ethanol to water) at 55 °C for 24 h produces nearly pure aragonite as shown in the X-ray diffraction (XRD) pattern in Figure 1. All reflection peaks of the sample can be indexed as a nearly pure aragonite phase. Figure 2a presents a typical scanning electron microscopy (SEM) image of the aragonite sample, indicating that these aragonite particles are micro60



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Figure 2. (a) SEM image of the sample prepared by dispersing Mg-ACC solution in ethanol−water solvent with R = 3/2 (R: the volume ratio of ethanol to water) at 55 °C for 24 h when the ultimate concentration of Mg-ACC is 4.5 mM; (b) TEM image of a typical microellipsoid. (c) A lattice resolved HRTEM image of a typical nanoneedle; (d) Selected area diffraction pattern (SAED) in (c) taken along the ⟨600⟩ zone axis, showing the perfect single crystalline nature.

why the samples of aragonite phase can be obtained more easily in the presence of sufficient magnesium. It must be pointed out that the real mechanism is rather complicated and needs more systematic work in the future. Effect of the Volume Ratio of Ethanol to Water on the Phase Transformation of Mg-ACC. At 55 °C, varying the volume ratio of ethanol to double-distilled water can result in the formation of different phases of CaCO3 as confirmed by XRD patterns (Figure 5). The main results are summarized in Scheme 1. When R = 1/14, the samples contain two crystalline phases, namely, calcite and aragonite, and calcite was found to be dominant (Figure 5a). When R was increased to 1/2, the aragonite phase became dominant (Figure 5b). While increasing R to 3/2, only the aragonite phase was found in the sample. Further increasing R to 11/4, the XRD pattern indicates that the samples were aragonite phase, but its crystallinity was not as good as that synthesized at lower R values (Figure 5c). That was because the percentage of the volume of ethanol was so large that the phase transformation of Mg-ACC turned out to be difficult. Figure 6 shows the SEM images of the prepared samples. When R = 1/14, rhombohedral calcite and flower-like aragonite aggregates coexisted (Figure 6a). With the increase of R to 1/2, more and more aragonite microellipsoids were formed with a mixture of rhombohedral calcite (Figure 6b). Further increasing R to 3/2, almost pure aragonite ellipsoid aggregates were formed (Figure 2a). When R increased to 11/4, almost pure spherical aggregates were observed (Figure 6c). Effect of Temperature on the Phase Transformation of Mg-ACC. The phase transformation reaction at different

Figure 3. XRD patterns of the samples prepared when the ultimate concentration of Mg-ACC is kept as 4.5 mM. The Mg-ACC dispersion was dispersed in a mixed solvent with R = 3/2 at 55 °C and aged for (a) 3 h; (b) 12 h; (c) 15 h; (d) 48 h. The standard reference data shows aragonite phase (JCPDS Card No. 41-1475).

aggregates that do not generally grow into larger ones. As a result of that, the primary nanocrystals attached around aragonite microellipsoids to form some tablets that dispersed on the surface of microellipsoids. Also, the role that magnesium may play in the phase and morphology control of CaCO3 should be considered. The previous results have demonstrated that Mg2+, as a cosolute of calcium, has higher hydration energy and that magnesium can be incorporated in the calcite lattice where it causes a pronounced inhibition of crystal growth.5,43 This is the reason 61



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Figure 4. SEM images of the samples prepared when the ultimate concentration of Mg-ACC is kept as 4.5 mM. The Mg-ACC dispersion is dispersed in a mixed solvent with R = 3/2 at 55 °C and aged for (a) 3 h; (b) 12 h; (c) 15 h; (d) 48 h. Inset in (a) shows the TEM image of a typical cross-linked spindle-like crystals aggregate.

Scheme 1. Relationship between the Polymorph and the Solvent Compositiona

a

R = volume ratio of ethanol to double-distilled water (v/v).

mixture of aragonite cross-linked spindle aggregates and calcite rhombohedrons were obtained (Figures 7c and 8c). With further decreasing the temperature to 35 °C, a mixture of aragonite, vaterite and calcite phases appears (Figures 7b and Figure 8b). Then, calcite became dominant at 25 °C (Figures 7a and 8a). However, the size of aragonite crystals became larger, and some tablets grew around aragonite microellipsoids at 65 °C (Figures 7d and 8d). These results demonstrated that the phase transformation tends to change from dominant calcite phase to a mixture of aragonite, calcite, and vaterite to pure aragonite with increasing the temperature. It has to be emphasized that slight change in temperature can lead to dramatic difference in the crystalline phase while other experiment parameters are kept constant. The calcite phase was dominant at 25 °C and the pure aragonite phase was obtained above 55 °C. According to Chen et al.,44 temperature is inversely proportional to the supersaturation (S) of CaCO3 that is directly related to the polymorph of CaCO3. High temperatures favor the formation of the CaCO3 phase of low

Figure 5. XRD patterns of the samples obtained after aging for 24 h when the ultimate concentration of Mg-ACC is kept as 4.5 mM at 55 °C. The Mg-ACC solution was dispersed in a solution with (a) R = 1/ 14; (b) R = 1/2; (c) R = 11/4. Note: +, calcite phase (JCPDS Card No. 86-2340); *, aragonite phase (JCPDS Card No. 41−1475).

temperatures results in the formation of different polymorphs of CaCO3 as confirmed by the XRD patterns shown in Figure 7. Figures 2a and 8 present typical scanning electron microscopy (SEM) images of the CaCO3 samples obtained by resolving Mg-ACC in ethanol−water solvent (R = 3/2) at the ultimate concentration of 4.5 mM and at different temperatures. Nearly pure aragonite microellipsoids were obtained when phase transformation was carried out at 55 °C (Figures 1 and 2a). On decreasing the temperature to 45 °C, a 62



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Figure 6. SEM images of the samples obtained after aging for 24 h when the ultimate concentration of Mg-ACC is kept as 4.5 mM at 55 °C. The Mg-ACC solution was dispersed in a solution: (a) R = 1/14; (b) R = 1/2 (c) R = 11/4 (R: the volume ratio of ethanol to double-distilled water (v/ v)).

supersaturation. In addition, the supersaturation of calcite is greatest and that of aragonite is lowest in the polymorphs of CaCO3.44 At 55 °C, pure aragonite is obtained. When decreasing temperature to 35 °C, the supersaturation of CaCO3 is higher than that at 55 °C; therefore, a mixture of aragonite and vaterite and calcite is formed. When further decreasing the temperature to 25 °C, the supersaturation increases again, and calcite phase becomes dominant. Effect of the Concentration of Mg-ACC on the Phase Transformation of Mg-ACC. In order to study the effect of Mg-ACC solution with different concentrations on the polymorph discrimination of CaCO3, the Mg-ACC precipitate was dispersed in 200 and 300 mL of ethanol, respectively. The ultimate concentration of Mg-ACC is 9 and 6 mM, respectively. The phase transition from a mixture of calcite and aragonite to a mixture of calcite, aragonite and vaterite, and then to almost pure aragonite, can be captured by choosing a suitable concentration of Mg-ACC as precursors when other experiment conditions were kept constant. The XRD pattern in Figure 9a shows that a mixture of calcite and aragonite is obtained when the concentration of Mg-ACC

Figure 7. XRD patterns of the samples obtained after aging for 24 h when the ultimate concentration of Mg-ACC was 4.5 mM. The MgACC solution was dispersed in a solution with R = 3/2 at (a) 25 °C; (b) 35 °C; (c) 45 °C ; (d) 65 °C . Note: +, calcite phase (JCPDS Card No. 86-2340); *, aragonite phase (JCPDS Card No. 41-1475) ; Δ, vaterite phase (JCPDS Card No. 33-0268).

Figure 8. SEM images of the samples obtained after aging for 24 h when the ultimate concentration of Mg-ACC is kept as 4.5 mM. The Mg-ACC solution was dispersed in a solution with R = 3/2 at (a) 25 °C; (b) 35 °C; (c) 45 °C; (d) 65 °C. 63



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CONCLUSIONS In summary, we have demonstrated a facile route for phase transition of Mg-ACC in an ethanol/water mixed solvent under mild conditions without using any organic additives for the first time. Aragonite microellipsoids composed of stacked nanoneedles can be synthesized by dispersing Mg-ACC in ethanol− water solvent with R = 3/2 at 55 °C for 24 h. When regulating the volume ratio of ethanol to water, phase switching between a mixture of calcite and aragonite can be achieved. In addition, a slight change in temperature can lead to a dramatic change in polymorph. Moreover, the phase transition from a mixture of calcite and aragonite and vaterite, and then to nearly pure aragonite, can be nicely captured by adjusting the concentration of Mg-ACC. We conclude that the thermodynamic and kinetic regimes, which contribute to the polymorph control, can be nicely manipulated in this binary solution system by simply tuning the volume ratio of ethanol to water, the reaction temperature and the concentration of Mg-ACC. This study may give some useful clues for understanding the mineralization process of CaCO3 in nature. Furthermore, this method can be possibly scaled up for industrial production of CaCO3 with different polymorphs.

Figure 9. XRD patterns of the samples obtained after aging at 55 °C for 24 h when R is 3/2. The Mg-ACC solution was dispersed in (a) 200 mL of ethanol; (b) 300 mL of ethanol. Note: +, Calcite phase (JCPDS Card No. 86-2340); *, aragonite phase (JCPDS Card No. 411475); Δ, vaterite phase (JCPDS Card No. 33-0268).

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powder is 9 mM at 55 °C (R = 3/2). Rhombohedral calcite and dendrimerlike aragonite were observed, as shown in Figure 10a. With the decrease of the concentration of Mg-ACC to 6 mM, the aragonite phase become dominant (Figures 9b and 10b). Pure aragonite microellipsoids are obtained through the choice of a suitable concentration of 4.5 mM, as shown in Figures 1 and 2a. The results indicated that the formation of pure aragonite is favored by the decrease the concentration of MgACC. The concentration of Mg-ACC has a significant influence on the phases and morphology of crystals formed because it determines the supersaturation. The higher the concentration of amorphous cluster that are formed, the higher the supersaturation is. As mentioned before, the supersaturation of calcite is greatest and that of aragonite is lowest in the polymorphs of CaCO3, which is nearly the same as our results. When the ultimate concentration of Mg-ACC is 4.5 mM, only aragonite is obtained. Increasing the concentration to 6 mM, the supersaturation is higher than that at 4.5 mM, and a mixture of aragonite, calcite and vaterite will form. When further increasing the concentration to 9 mM, the supersaturation increases again, and the calcite phase is dominant.

ASSOCIATED CONTENT

S Supporting Information *

Characterization of the samples and the preparation of MgACC. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: + 86 551 3603040. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Ministry of Science and Techology of China (MOST) (Grant 2012BAD32B05-4), the National Basic Research Program of China (2010CB934700), the National Natural Science Foundation of China (Grants 91022032, 21061160492, J1030412), Chinese Academy of Sciences (Grant KJZD-EW-M01-1), International Science & Technology Cooperation Program of China (Grant 2010DFA41170), and the Principal Investigator Award by the

Figure 10. SEM images of the samples obtained after aging at 55 °C for 24 h which R is 3/2. The Mg-ACC solution was dispersed in (a) 200 mL of ethanol; (b) 300 mL of ethanol, respectively. 64



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National Synchrotron Radiation Laboratory at the University of Science and Technology of China.

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Phase Transformation of Magnesium Amorphous Calcium Carbonate

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