Communication pubs.acs.org/crystal
Doping Incompatible Elements into Calcite through Amorphous Calcium Carbonate Satoshi Matsunuma,† Hiroyuki Kagi,*,† Kazuki Komatsu,† Koji Maruyama,† and Toru Yoshino‡ †
Geochemical Research Center, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan Tokyo Metropolitan Industrial Technology Research Institute, 2-4-10 Aomi, Koto-ku, Tokyo 135-0064, Japan
‡
ABSTRACT: Doping large amounts of a incompatible element into calcite, a polymorph of CaCO3, was achieved by pressurizing amorphous calcium carbonate (ACC) containing a dopant. Strontium-doped ACC samples were prepared from supersaturated solutions. ACC was transformed to calcite at 0.8 GPa and 25 °C. The lattice volume of calcite obtained by pressurizing ACC increased monotonically up to 372.5 Å3 (1.3% increase from a Sr-free sample) with increasing Sr content in ACC up to 0.15 in Sr/(Sr + Ca). For comparison, the lattice volumes of calcite samples precipitated from supersaturated CaCO3 solution containing Sr were obtained, but the lattice volume of calcite precipitated from supersaturated solutions showed no noteworthy increase. Results of this study indicate that pressure-induced crystallization is an efficient pathway to dope incompatible elements into crystals.
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Echinodermata as a precursor phase and in crayfish as gastroliths, which serve as a temporary CaCO3 storage for exoskeleton at the molt.12,13 Many organisms form skeletons of magnesium-rich calcite with magnesium contents greater than 10 mol %. The high magnesium content in biomineral calcite is explainable by the high-magnesium content in the precursor ACC.14 Inspired by biomineralization, other ions might be taken into the calcite lattice through ACC, which is thermodynamically metastable and which transforms to crystalline phases under high-temperature and high-humidity environments.15,16 Recently, Yoshino et al. (2012) reported pressure-induced phase transition from ACC to calcite and vaterite.17 Pressure-induced crystallization of ACC might result in the development of a new method for doping incompatible but functional elements efficiently to calcium carbonate. In this study, we investigated the possibility of Sr-doping into calcite. Experimental procedures. In this study, ACC was synthesized based on a previously reported method.15 Strontium-doped ACC samples were prepared by mixing 0.1 M Na2CO3 and 0.1 M blended solutions of CaCl2 and SrCl2 with varying Ca/Sr ratios at 0 °C. Precipitates were filtered immediately using a membrane filter (ϕ 0.45 μm), washed with acetone, and dried for 1 day at 25 °C in a vacuum desiccator that had been evacuated with a diaphragm pump. This procedure is equivalent to the method described by Yoshino et al. (2012),17 except for Sr-doping. Synthesized ACC samples were confirmed to contain no crystalline phase from powder X-ray diffraction (XRD) patterns. Water contents in the obtained ACC samples were estimated from weight loss in a temperature range from 25 to 250 °C in
alcium carbonate, an important resource for industrial material, is a major natural mineral in surface environments and in the global carbon cycle.1 Recently, its application to nanoscale delivery system has been studied intensively.2,3 Properties of calcium carbonate with various polymorphs and morphologies are the target of fundamental science. Calcium carbonate has three polymorphs: calcite, aragonite, and vaterite. Calcite has a trigonal structure. Its Ca2+ coordination number is six. Aragonite has an orthorhombic structure. Its coordination number is nine. Because of differences in their crystal structures, aragonite is much denser than calcite. In the P−T phase diagram of CaCO3, calcite is the stable phase at ambient conditions. Aragonite is a high-pressure phase.4 In contrast, vaterite is a metastable phase in the whole P−T region. Divalent ions of alkaline earth elements and transition metals with ionic radius smaller than that of Ca2+ form a carbonate with the calcite structure. Carbonates with the calcite structure tend to capture impurity elements of which the ionic radius is smaller than that of calcium ion (e.g., Mg2+, Fe2+, Cd2+, Mn2+, Zn2+, Cu2+, Ni2+).5 In contrast, divalent ions with an ionic radius larger than that of Ca2+ form a carbonate with the aragonite structure. Carbonates with the aragonite structure tend to capture impurity elements of which the ionic radius is larger than Ca2+ (e.g., Sr2+, Pb2+, Ba2+, Na2+, U2+).5 For example, Sr2+ is taken selectively into aragonite, but not into calcite. A quantum chemical calculation study revealed that these large ions are incompatible with calcite.6 In addition to the three polymorphs of calcium carbonate, amorphous calcium carbonate (ACC) has recently attracted attention from researchers in biology,7 material science,8−11 and other disciplines. Actually, ACC is known as a precursor material of biominerals. Taking the form of ACC for living organisms presents many benefits.7 In fact, ACC exists in larval spicules of © 2014 American Chemical Society
Received: June 30, 2014 Revised: October 6, 2014 Published: October 14, 2014 5344
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Communication
Figure 1. Sr/(Sr + Ca) molar ratios of starting solutions vs those of ACC samples prepared from the solutions.
thermogravimetric−differential thermal analysis (TG−DTA) measurements (TG-8120; Rigaku Corp.). Two representative samples showed similar water contents: CaCO3·1.48H2O for nondoped ACC and Ca0.82Sr0.18CO3·1.40H2O for Sr-doped ACC precipitated from a solution with Sr/(Sr + Ca) = 0.1. Synthesized ACC samples were pressurized using a hydraulic press in a tungsten carbide (WC) piston−cylinder of 4 mm inner diameter, the same equipment as that used by Yoshino et al.17 for 10 min at 25 °C. The applied pressure was 0.8 GPa. After decompression, the recovered samples were kept in a vacuum desiccator for removal of water emitted after pressureinduced phase transition. For comparison, calcium carbonate samples were precipitated from supersaturated CaCO3 solutions containing Sr components. Supersaturated solutions were obtained by mixing 0.1 M Na2CO3 and 0.1 M blended solutions of CaCl2 and SrCl2 with varying Ca/Sr ratios. The precipitates were aged in solutions for 3 days with stirring at 25 °C. Calcite was obtained under an equilibrated condition. Actually, ACC, a transient metastable precipitates, is known to crystallize to calcite through vaterite within 10 h in a supersaturated solution.18 Three days are sufficient to obtain the most stable phase, calcite, by aging. Obtained powders were treated in the same manner as the ACC samples described above, filtered, washed with acetone, and dried for 1 day in an evacuated vacuum desiccator. Hereinafter, the precipitates are designated as aged precipitates. Powder X-ray diffraction (XRD) patterns of the calcium carbonate samples were obtained on a silicon zero background plate using an X-ray diffractometer (MiniFlexII, Rigaku Corp.). Potassium chloride powder as an internal standard for a lattice constant was mixed with the samples. The measurement conditions for XRD were 0.02° step, scanned region from 10° to 70° in 2θ, a scan rate of 1° per minute, Cu Kα radiation operated at 15 mA and 30 kV. Lattice constants of calcite were refined using Rietveld analysis with GSAS software19 and EXPGUI.20
Figure 2. Powder X-ray diffraction patterns of calcium carbonate samples with various Sr contents. Strontium molar concentrations in starting solutions are denoted as x, where x is Sr/(Sr + Ca) × 100: (a) pressurized ACC and (b) aged precipitation.
Scanning electron microscopy (SEM) images were obtained with an accelerating voltage of 5 kV after Au coating (JSM6610LA; JEOL). The concentrations of Ca and Sr in ACC samples were measured using an ICP inductively coupled plasma mass spectrometer (ELAN DRCII; PerkinElmer Inc.). Results and discussion. Figure 1 presents Sr concentrations in starting solutions versus of those of ACC determined from ICP-MS measurements. The relation shows that ACC captures Sr2+ preferentially from the starting aqueous solutions. 5345
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Figure 2a displays powder XRD patterns of ACC samples recovered from pressurization treatment at 0.8 GPa. After pressurization, ACC crystallized to calcite. No other phase was detected in the range of x = 0 to x = 9, where x is a molar percentage of Sr2+ in a starting solution described as Sr/(Sr + Ca) × 100. Samples crystallized from solutions with higher Sr concentrations (x = 10 and 15) contained a crystalline phase of vaterite. The most intense reflection of calcite at 29.4° shifts slightly to a lower angle with increasing Sr concentrations, which suggests an increase of lattice volume with increasing Sr concentrations of starting solutions. Figure 2b portrays powder XRD patterns of aged precipitate samples of calcium carbonate. All samples include calcite. One sample precipitated from a high Sr concentration (x = 15) contained strontianite (SrCO3 with aragonite structure). Lattice parameters of calcite samples recovered from the high-pressure treatment on ACC and precipitated from the supersaturated solutions were estimated from X-ray diffraction patterns. The obtained lattice volume data are presented in Figure 3a. At least up to x = 15, the lattice volume of calcite obtained by pressurization of ACC increased monotonically with increasing Sr concentration in ACC. In contrast, the lattice volume of calcite obtained from aged precipitation from supersaturated solutions increased at least up to x = 8, subsequently reaching a plateau at x = 10. The increase in lattice
Figure 3. Plots of (a) lattice volume vs Sr concentration in the starting solutions. (b) c/a ratio of calcite samples obtained from ACC vs Sr concentration in the starting solutions.
Figure 4. SEM images showing calcite transformed from ACC at 0.8 GPa for (a) x = 0 and (b) x = 10 and showing aged precipitation obtained from a supersaturated calcium carbonate solution for (c) x = 0 and (d) x = 10. SEM images of pressurized ACC were obtained on a cracked cross section. Aged precipitate samples were attached to a carbon tape. Scale bars of (a) and (b) are 50 μm and those of (c) and (d) are 10 μm. 5346
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volume reflects the uptake of Sr ion into the calcite lattice because the ionic radius of Sr2+, for which the coordination number of six is 18% larger than that of Ca2+.21 The increase of the c-axis with increasing Sr content is more prominent than the increase of a-axis with increasing Sr content in the starting ACC samples (see Figure 3b). Incorporation of large cations into a calcite-structure compound increases the c/a axial ratio.22 Figure 3b shows the incorporation of Sr2+ into calcite lattice. Figure 4 portrays representative SEM images of calcium carbonate samples prepared by pressurizing ACC and aged precipitation obtained from supersaturated calcium carbonate solutions. As shown in Figure 4a,b, the texture of the pressurized samples can be grouped mainly into two regions: the edge of the sample attached to a WC piston and the internal part. Spherulitic grains, which aggregate inside of the samples, consist of crystal arrays radiating from a single nucleation site. Their texture is dendritic. Grain sizes differ between Sr-free samples and Sr-doped samples (see Figure 4a,b); the grains of Sr-free specimens are coarser than those of Sr-doped specimens. This contrast suggests a difference in nucleation density. In the Sr-free sample nucleation that occurred at the interface between ACC and a WC piston, a prismatic layer grew normal to the boundary layer (see Figure 4a). In contrast, the interface between ACC and the WC piston for the Sr-doped samples consists of spherulitic grains (see Figure 4b). These contrastive results suggest that Sr ion notably affects the crystallization of calcite from ACC. The grain size and morphology of pressurized ACC and aged precipitates differ greatly from each other. The grain sizes of calcite crystallized from ACC at high pressure were much larger than those of aged precipitations (cf., Figure 4, panel a vs c and Figure 4, panel b vs d). Results of this study suggest strongly that the pressure-induced crystallization process from ACC differed from that from a supersaturated solution. During crystal growth of calcite from a supersaturated aqueous solution, Sr2+ impurity is excluded from the calcite structure because of the incompatibility and mobility of Sr2+ in the solution. Calcite samples transformed from ACC at high pressure contained significantly higher concentrations of Sr compared with those of calcite precipitated from a supersaturated solution, which suggests that crystal growth of calcite kinetically predominates over the exclusion of Sr from calcite. High-pressure treatments on ACC induced calcite crystallization and caused the preferential incorporation of Sr2+ into the calcite lattice. Figure 5 presents a schematic illustration contrasting the two crystallization processes. Amorphous materials can accommodate elements that are structurally incompatible with the corresponding crystalline phases with high concentration. This study demonstrated that Sr, which is incompatible with calcite, was structurally incorporated into calcite through pressure-induced crystallization from Sr-doped ACC. Similar phenomena have been reported for the uptake of salt species into a high-pressure phase of ice.23 In general, salt is extremely incompatible with ice lattices. Reportedly, the significant concentrations of LiCl were captured structurally into ice VII (a high-pressure phase of ice) crystallized through an amorphous state. It is reasonable that an amorphous state significantly affects the distribution of incompatible elements into crystals. Results of this study are expected to present new avenues for the development of new materials by doping incompatible ions or functional molecules into a crystal phase through an amorphous state.
Figure 5. Schematic processes of pressure-induced crystallization from ACC and precipitation from supersaturated solutions.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Phone: +81(0)-3-58417625. Fax: +81(0)-3-5841-4119. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Prof. Kazumasa Sugiyama for valuable discussion. We are grateful to two anonymous reviewers whose comments improved the manuscript considerably. This study was performed under the Inter-university Cooperative Research Program of the Institute for Materials Research, Tohoku University (Proposals No. 13K0056 and 14K0046). This study was financially supported by a Grant-in-aid for Challenging Exploratory Research (25610162).
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