Crystallization of Calcium Carbonate Mineral with Hierarchical

Crystallization of Calcium Carbonate Mineral with Hierarchical Structures in DMF Solution under Control of Poly(ethylene glycol)-b-poly(L-glutamic acid): Effects of Crystallization Temperature and Polymer Concentration

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 4 1233–1242

Xiao-Hui Guo, An-Wu Xu, and Shu-Hong Yu* DiVision of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, the School of Chemistry & Materials, UniVersity of Science and Technology of China, Hefei 230026, P. R. China ReceiVed September 1, 2007; ReVised Manuscript ReceiVed October 27, 2007

ABSTRACT: CaCO3 superstructures with complex forms and hierarchical surface textures were mineralized in the presence of peptide type block copolymer poly(ethylene glycol)-b-poly(L-glutamic acid) (PEG-b-pGlu) as a crystal growth modifier in dimethyl formamide (DMF) solution. Spindle-like CaCO3 crystals with a rather smooth surface and a size of 10 µm in length and a maximum diameter of 6 µm were mineralized at 24 ( 2 °C. Unique ellipsoid-like CaCO3 particles with many similar thorns distributed on the particle surface formed when the mineralization temperature was kept at 14 ( 2 °C. Complex column-like CaCO3 superstructures comprised of many tiny rods and irregular particles were observed at 4 ( 2 °C. In addition, when the reagent concentration is varied, the morphology of CaCO3 changes from a mixture of spherical and ellipsoidal structures to a kind of complex ellipsoidal superstructure whose surface is attached by many hierarchical tubular structures, and then to a complex columnar structure, when the concentration of Ca2+ ions was varied from 40 to 20 mM and finally decreased to 10 mM, respectively. Correspondingly, phase transition occurred from a mixture of aragonite and calcite to pure calcite, and then to a mixture of vaterite and calcite when the mineralization temperature increased. Moreover, changing the polymer concentration resulted in phase transition from calcite to a mixture of aragonite and calcite, and then to a mixture of vaterite and calcite. An emergent self-organization process and its combination with an aggregated mechanism have been proposed for the formation of the complex ellipsoidal superstructures. The results imply that the specific biomimetic synthesis strategy in a nonaqueous solution can provide a useful pathway to produce inorganic or inorganic/organic hybrid materials with a unique morphology and specific textures.

1. Introduction In recent years, emergent inorganic structures formed by organisms have provided unique inspirations for materials design.1,2 Calcium carbonate is one of the most abundant biominerals produced by organisms and also has industrial applications due to its wide use as filler in paints, plastics, rubber, or paper; it possesses three main crystalline phases: calcite, aragonite, and vaterite.3–5 Calcite and aragonite are by far the most common and stable forms, whereas vaterite, a less stable polymorph from the viewpoint of thermodynamics, also plays key roles in biological life and health, although it is not commonly formed by organisms.6 Organic additives or templates such as double-hydrophilic block copolymers (DHBCs),7,8 dendrimers,9 common polymer,10 synthetic peptides or amino acids,11 and biomacromolecules12 have been used to induce the controlled growth and crystallization of CaCO3 mineral, as well as for control of the polymorphs. It has been demonstrated that DHBC additives have specific and prominent influences on mediating the morphologies and polymorphs of inorganic minerals, for example, CaCO3 and BaCO3, including the formation of some unusual mineral structures such as superlong BaCO3 helices by tectonic arrangement of BaCO3 nanocrystals,8d CaCO3 pancakes,8e and CaCO3 microrings.8f The adjustment of the pH value and initial supersaturation of solution, molecule weight, polymer types with various functional groups, and concentration of polymer can also exert prominent influences on the polymorphic selection and morphology of CaCO3.13–16 In particular, it should be mentioned * To whom correspondence should be addressed. E-mail: [email protected].

that kinetic regimes such as the reaction temperature can play a key role in mediating the nucleation and crystallization kinetics of carbonate minerals.17 A complex vaterite with a multilayer structure and aragonite rods formed at a relatively higher reaction temperature (ca. 120 °C) in a mixed solvent of water and ethanol.18 The spherical aggregates of vaterite CaCO3 particles can be synthesized at 50 °C.19 Nanofibrous calcite crystals were synthesized via a solution-precursor-solid mechanism at a low temperature of 4 °C.20 In contrast, calcite crystals with irregular shapes were obtained at 6 °C in the presence of poly(aspartic acid).11a In addition, an amorphous calcium carbonate (ACC) film formed on a -OH modified matrix surface at 4 °C.21 Usually, the mineralization reaction of inorganic minerals has been carried out in aqueous solution. It has been reported that for the mineralization reactions in nonaqueous solutions, such as in alcohol, ethanol, isopropanol, and diethylene glycol, the as-prepared samples only displayed such structures as elongated spheres, rods, or inhomogeneous aggregated morphology.22 Recently, polymer-controlled crystallization in a mixed solvent can result in entirely distinct results due to the change in sample solubility, precipitation kinetics, solution property, and aggregate forms of the polymer in a mixed solution.23 Monodisperse vaterite microspheres can be fabricated in a mixed solvent made of N,N-dimethylformamide (DMF) and deionized water in the presence of a polypeptide-type polymer poly(ethylene glycol)b-poly(L-glutamic acid) (PEG-b-pGlu).23 In general, investigation of mineralization crystallization in incomplete aqueous solutions or organic solvents has been rarely studied as compared to that in aqueous solution until now.24

10.1021/cg7008368 CCC: $40.75  2008 American Chemical Society Published on Web 02/22/2008

1234 Crystal Growth & Design, Vol. 8, No. 4, 2008

In this paper, mineralization of CaCO3 crystals with complex and hierarchical structures has been reported by a slow gas–liquid diffusion reaction in the presence of a peptide-type copolymer so-called PEG-b-pGlu25 in DMF. The CaCO3 crystals with multiple thorn ellipsoidal-like shapes and more complex ellipsoidal superstructures can be mineralized at varied mineralization temperatures, respectively. The influence of temperature and reagent concentration on the mediation of the phase transformation and morphology change of CaCO3 crystals in the presence of PEG-b-pGlu has been investigated. The results demonstrate that the kinetic and thermodynamic regimes can prominently regulate the polymorphs and self-assembly process of building blocks of CaCO3 mineral in DMF solution under control of this peptide-type copolymer.

2. Experimental Section All chemicals are of analytical grade. Ammonium carbonate and CaCl2 were purchased from Shanghai Chemical Reagent Company and used as received without further purification. A block copolymer containing a poly(ethylene glycol)-b-poly(L-glutamic acid) (PEG ) 1680 g mol-1, pGlu ) 2060 g mol-1) was synthesized as described elsewhere.25 The polymer was purified exhaustively before use in the crystallization of calcium carbonate. DMF was obtained from Shanghai Chemical Reagent Company in China. Deionized water (DIW) (18.2 Ω cm-1) was obtained from Millipore simplicity 185 type. All glassware (glass bottles and small pieces of glass substrates) was cleaned and sonicated in ethanol for 5-10 min, then rinsed with deionized water (DIW) (18.2 Ω cm-1) and further soaked with a water/ HNO3 (65%)/H2O2 (1:1:1, v/v/v) solution, then rinsed with DIW, and finally dried with acetone. The mineralization experiments were carried out as described by Addadi et al.26 The precipitation of CaCO3 was carried out in glass bottles with a volume of 5-15 mL, which were put in a closed desiccator at different reaction temperatures. Stock aqueous solution of CaCl2 (0.1 M) was freshly prepared in DIW. Five milligrams of polymer was added into 5 mL of DMF in the glass bottle, which was continuously stirred and dissolved completely so as to contain 1 g L-1 PEG-b-pGlu in solution. After that, a 500 µL CaCl2 (0.1 M) solution was quickly injected into glass bottles containing 5 mL of PEG-b-pGlu solution under vigorous stirring. This gave a final CaCO3 concentration of 10 mM. The bottle was then covered with parafilm, which was punched with three needle holes, and placed in a larger desiccator. Three small glass bottles (10 mL) of crushed ammonium carbonate were also covered with parafilm punched with four needle holes and placed at the bottom of the desiccator. After different periods of time, the parafilm was removed, and the precipitate was rinsed with DIW and ethanol, respectively, and allowed to dry at ambient temperature. Time-dependent crystallization experiments were carried out by taking out the small pieces of glass substrates from the bottles in order to stop the reaction for examination. The precipitates were collected and washed with DIW and dried in air for further characterization. Herein, the concentration of PEG-b-pGlu was varied from 2.0 to 0.5 g L-1, the concentration of CaCl2 was varied from 10 to 40 mM, and the crystallization reaction was carried out at three different reaction temperatures, 4 ( 2, 14 ( 2, and 24 ( 2 °C, respectively. The small pieces of coverslips were examined by sputtering with gold for scanning electron microscopy (SEM) on a KYKY-1010B microscope and field emission scanning electron microscopy (FE-SEM) on a JSM-6700F microscope. The product’s structure was characterized by X-ray diffraction pattern (XRD), recorded on a (Philips X’Pert Pro Super) X-ray powder diffractometer with Cu KR radiation (λ ) 1.541874 Å). Infrared spectra were collected by using a Nicollet Impact 400 FT-IR spectrometer on KBr pellets. Transmission electron microscope (TEM) and selective area electronic diffraction (SAED) were performed on a Hitachi (Tokyo, Japan) H-800 TEM at 200 kV. For TEM analysis, copper grids were directly placed in the polymer solution in the desiccator, then they were taken out from the reaction solution at different time intervals and washed with DIW and ethanol, respectively; the sample deposited on the copper grid for TEM measurements. High-resolution transmission electron microscopy (HRTEM) performed on a JEOL-2011 HRTEM at an accelerating voltage of 200 kV. Thermo gravimetric analysis (TGA) was carried out on a

Guo et al. Diamond TG/DTA thermal analyzer (Perkin-Elmer Corporation) with a heat rate of 10 °C/min-1 in nitrogen atmosphere.

3. Results and Discussion 3.1. Morphogenesis of CaCO3 Mineral at Different Reaction Temperatures. CaCO3 crystals produced at various mineralization temperatures in the presence of PEG-b-pGlu exhibit distinct morphologies. When the reaction temperature was kept at 24 ( 2 °C, CaCO3 crystals formed mainly exhibited ellipsoid-like structures with a rather smooth surface (Figure 1a) and have a size of about 10 µm in length and a maximum diameter of about 6 µm; the particles are slightly shorter than those obtained at 14 ( 2 °C (Figure 1b), and a few irregular aggregates were also observed. When the reaction temperature was further lowered to 4 ( 2 °C, a similar complex ellipsoidlike CaCO3 structure with a very rough surface which is composed of many nanorods and irregular nanoparticles was produced (Figure 1c,d), which is somewhat different from that obtained at 14 ( 2 °C. These CaCO3 particles obtained in the case of 4 ( 2 °C have a broad size distribution as compared to those obtained at 14 ( 2 °C shown in Figure 1b. When the reaction temperature was increased, the surface of the CaCO3 crystals gradually transformed from very coarse to generally rough, and finally to rather smooth. However, the CaCO3 samples formed in DMF solution without any additives at different reaction temperatures display distinct morphology (Supporting Information, Figure S1). The sample was mainly composed of rhombohedral calcite and some spherical vaterite aggregates when the reaction temperature was 4 ( 2 °C and [Ca2+] ) 10 mM (Supporting Information, Figure S1a), which is obviously different from the morphology shown in Figure 1d. Meanwhile, a mixture of rhombohedral calcite and larger vaterite spheres was observed in the case of a reaction temperature of 14 ( 2 °C and [Ca2+] ) 10 mM (Supporting Information, Figure S1b). On the basis of these results, it can be seen that the polymer additive in DMF solution plays a crucial role in mediating the morphology and structure of CaCO3 crystals. In addition, the formation of different morphologies and phases of CaCO3 crystals at different temperatures could be relative to different nucleation rates and dissolution-diffusion kinetic processes mediated by reaction temperature. The modifications of the CaCO3 samples obtained at different mineralization temperatures were examined by the XRD patterns (Figure 2). The CaCO3 sample prepared at 14 ( 2 °C in the presence of PEG-b-pGlu is pure calcite as indexed in Figure 2a. However, the CaCO3 sample obtained at 24 ( 2 °C can be indexed as a mixture of calcite and vaterite (Figure 2c). The phase of the product was also confirmed by the FT-IR spectrum (Figure 3). The presence of peaks at 875 and 712 cm-1 can be assigned as the characteristic peaks for calcite as well as the presence of a weak peak at 745 cm-1 can also be indexed as the characteristic peak for vaterite.27 The calculated content of vaterite is about 37.3 wt % in the sample from IR absorption based on the previous reported method. It is also confirmed that CaCO3 samples formed at 14 ( 2 °C and in the presence of PEG-b-pGlu can be indexed as a mixture of calcite and vaterite based on the above FT-IR results, which is not contradicted with the XRD results (Figure 2a), because the preferential orientation degree and intensity of some diffraction peaks of calcite phase are high enough to mask the occurrence of characteristic peaks of the vaterite phase in the XRD pattern. In addition, the sample obtained at 4 ( 2 °C can be indexed as a mixture of a majority amount of calcite and a trace amount of aragonite (Figure 2b).

Crystallization of Calcium Carbonate

Crystal Growth & Design, Vol. 8, No. 4, 2008 1235

Figure 1. SEM images of CaCO3 particles obtained after crystallization for 7 days in the presence of 1.0 g L-1 PEG-b-pGlu at various reaction temperatures. (a) 24 ( 2 °C; (b) 14 ( 2 °C; (c, d) 4 ( 2 °C. The initial concentration of CaCl2 ) 10 mM.

Figure 2. XRD patterns of the CaCO3 crystals obtained in the presence of 1.0 g L-1 PEG-b-pGlu at different reaction temperatures. (a) 14 ( 2 °C; (b) 4 ( 2 °C; (c) 24 ( 2 °C. The initial concentration of [CaCl2] ) 10 mM. Note: c denotes calcite (JCPDS: 05-0586), and v denotes vaterite (JCPDS: 33-0268).

Uniform and well-defined CaCO3 crystals with an ellipsoidallike shape and multiple thorns on the surface were obtained at 14 ( 2 °C in DMF (Figure 4). The ellipsoidal-like particles have an average size of about 13 µm in length and a width of 8 µm (Figure 4a,b). Both shape and structures are different from those spherical structures formed in aqueous solution.23,25 The morphology of CaCO3 particles formed in aqueous solution in the presence of PEG-b-pGlu was nearly spherical. The pricky

Figure 3. FT-IR absorbance spectrum of the CaCO3 particles obtained in the presence of 1.0 g L-1 PEG-b-pGlu at 14 ( 2 °C. The sample was obtained by crystallization for 7 days.

structures can be observed on the surface of ellipsoidal particles (aggregates of calcite nanoparticles), and the similar rodlike structures tend to stretch out along different directions (Figure 4b). This type of hierarchical structure is similar to the weeds of seedlike fruit in shape, a kind of cocklebur plant. A high magnification SEM image (Figure 4c) indicated that the surface of the prickly aggregates is actually composed of many tiny rods of around 250 nm in diameter. The body of as-prepared prickly structures formed by aggregation of primary building units - calcite nanoparticles with a size of about 40-60 nm


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Figure 4. (a-d) SEM images of ellipsoid-like CaCO3 crystals obtained in the presence of 1 g L-1 PEG-b-pGlu after crystallization for 7 days at 14 ( 2 °C. The initial concentration [CaCl2] ) 10 mM.

(Figure 4d). The mode of self-assembly of nanoscale building blocks under control of a polymer additive agrees well with the growth model proposed by Cölfen et al.28 3.2. Effect of the Concentration of Polymer and Calcium Ions at Lower Temperature. The kinetic regime such as the reaction temperature can play a key role in mediating the nucleation and crystallization kinetics of carbonate minerals, so the investigation of the crystallization and polymorph transformation during the polymer concentration variation together with the temperature effect will be more interesting.17 Here, the influence of the concentration of Ca2+ ions and the polymer itself on the morphologies and phase transformation of CaCO3 at lower crystallization temperatures, for example, 4 ( 2 °C, was investigated. When the concentration of Ca2+ ions was up to 40 mM, the as-prepared CaCO3 sample was composed of both spheres of ca. 14–50 µm in diameter and similar ellipsoidal particles of 10–18 µm in length, and both two kinds of structures have relatively smooth surfaces (Figure 5a,b), which are different from that obtained in the case of [Ca2+] ) 10 mM (Figure 1c). However, when the calcium ion concentration was decreased to 20 mM, a kind of more complex ellipsoidal superstructure, on which some longer fibers attached, was observed (Figure 5c,d). The fibers are longer and more stretched outward as compared to the sample obtained at a lower calcium ion concentration of 10 mM (Figure 1c). In addition, the phase transformation of the three CaCO3 samples formed in the presence of different calcium ion concentrations was examined (Figure 6). The polymorphs of

CaCO3 product obtained when the calcium ion concentration were 20 mM and 10 mM, respectively, which can both be indexed as a mixture of aragonite and calcite (Figure 6b,c). However, the CaCO3 product formed in the case of [Ca2+] ) 20 mM contains more aragonite than the sample produced in the case of [Ca2+] ) 10 mM. The XRD patterns in Figure 6b,c indicate that increasing [Ca2+] from 10 mM to 20 mM results in the weakening of the diffraction peak (104) for calcite but an enhancing of the 121 diffraction peak for aragonite. Also the (104) diffraction peak becomes weaker in the case of 10 mM CaCl2 (Figure 6b) compared to that observed in the case of 20 mM (Figure 6c). Further increasing the concentration of calcium ions up to 40 mM leads to the formation of a mixture of vaterite and calcite (Figure 6a). In addition, the variation of polymer concentration can also effectively modulate the morphologies and phase modifications of CaCO3 mineral. Column-like aggregates of CaCO3 formed when the polymer concentration is 0.5 g L-1 (Figure 7a). With the concentration of polymer increased up to 2.0 g L-1, nearly spherical superstructures with some tiny disklike particles grown on the surface were observed (Figure 7b). Moreover, the case of phase transition of CaCO3 samples obtained at different polymer concentrations is shown in Figure 8. When a lower polymer concentration of 0.5 g L-1 was applied, pure calcite phase formed (Figure 8a). In contrast, when the polymer concentration increased up to 2.0 g L-1, the product was confirmed as a mixture of vaterite and a trace amount of calcite phase according to the XRD patterns (Figure 8b). To further



Figure 5. SEM images of CaCO3 crystals formed in DMF solution and in the presence of 1 g L-1 PEG-b-pGlu, using different concentrations of calcium ion. (a) 40 mM; (b) magnified SEM image of the particles shown in (a); (c) 20 mM; (d) higher magnification of an individual aggregated structure shown in (c). The reaction temperature is 4 ( 2 °C; the samples formed after crystallization for 7 days. The microrods are aragonite and calcite nanoparticles, which are located on the aggregated core.

investigate the phase transition behavior at different crystallization times, the CaCO3 sample obtained in the case of a polymer concentration of 2.0 g L-1 and crystallization for 7 days can be indexed as a mixture of calcite and vaterite (Supporting Information, Figure S2a). The XRD pattern showed a weak characteristic 112 peak for vaterite, but strong 104 peak for calcite (Supporting Information, Figure S2a) compared to the case that occurred in Figure 8b. In addition, the polymorph of the CaCO3 sample formed in the presence of a polymer concentration of 0.5 g L-1 and crystallization for 7 days is similar to that obtained at the same polymer concentration but crystallization for 10 days (Figure 8a and Supporting Information, Figure S2b). Above results indicate that when the concentration of the polymer or calcium ion is changed, the CaCO3 crystals formed undergo distinct variation in shape, size, and polymorph, accordingly. It can be seen that the high polymer concentration or calcium ions are favorable for the formation of the vaterite phase. Aragonite CaCO3 crystals with hierarchical tubular superstructures were obtained under an initial concentration of

calcium ion of 20 mM in the presence of PEG-b-pGlu and at a relative lower temperature of 4 ( 2 °C, which has not been reported up to now.8,29 The CaCO3 aggregates composed of aragonite microtubes formed in the presence of PEG-b-pGlu in DMF media, as clearly shown in Figure 9a (marked with arrows). In addition, there are many small aragonite nanorods grown on the surface of large microtubes, and some microtubes are parallel to each other and aligned into bundles as shown in Figure 9b. A typical TEM image for an individual large rod is shown in Figure 10a, showing that this rod has a length of 2.8 µm and a diameter of about 200 nm. Interestingly, some tiny nanorods can be observed on the backbone of this microtube (Figure 10a), and the nanorods grow perpendicularly on the backbone of a large rod, and are aligned in parallel to one another. HRTEM observation shows that there exists much tiny fibrous-like stuff coated on the surface of bigger rodlike structure (Figure 10e). HRTEM images taken from different selected areas of the complex tubular superstructure are shown in Figure 10b-g. Figure 10b represents a two-dimensional crystal lattice image with lattice spacings of 3.21 and 4.20 Å taken from the thick


Figure 6. XRD patterns of CaCO3 crystals obtained at different concentrations of calcium chloride and in the presence of 1 g L-1 PEGb-pGlu, (a) 40 mM; (b) 10 mM; (c) 20 mM. The samples formed after crystallization for 7 days at 4 ( 2 °C. Note: A denotes aragonite (JCPDS: 41-1475), C denotes calcite (JCPDS: 05-0586), and V denotes vaterite (JCPDS: 33-0268).

Figure 7. SEM images of CaCO3 samples obtained at different polymer concentrations and at a reaction temperature of 4 ( 2 °C, [Ca2+] ) 10 mM. The samples formed by crystallization for 10 days. (a) 0.5 g L-1; (b) 2.0 g L-1.

end of the tube. These can be ascribed to the (021) and (110) plane of aragonite, respectively. The corresponding selective area electronic diffraction (SAED) pattern shown in Figure 10c (the inset in Figure 10b) indicates that this rodlike aragonite is a single crystal. The lattice resolved image taken on the area of the central part of the complex tube is shown in Figure 10d, corresponding to that for the (111) plane with a lattice spacing of 3.38 Å and the (110) plane with a lattice spacing of 4.20 Å. Moreover, the lattice resolved image of a secondary rodlike structure with a smaller size attached to the surface of a thicker rod indicates that the lattice spacings are 2.62 and 2.51 Å (Figure 10f,g), corresponding to those for the (012) and (102) faces of aragonite, respectively. Hence, it can be concluded that the similar rodlike structures with a smaller size are aragonite phase. It should be mentioned that, herein, the aragonite complex rod or tubular-like superstructures formed in DMF solution can be stable for more than 30 days. On the surface of big rods, there exist some irregular calcite nanoparticles attached on the microrods, as shown in Figure 9b (indicated by arrows); this is further confirmed by HRTEM observation. Figure 10f taken from the surface of this microrod clearly shows a lattice. Spacing of 3.04 Å is ascribed to the (104) plane of calcite. In addition, the specific interaction of the crystal surface with the adsorbed polymer has to be sufficient to overbalance the

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Figure 8. XRD patterns of CaCO3 crystals obtained at different polymer concentrations and a reaction temperature of 4 ( 2 °C, [Ca2+] ) 10 mM. The samples formed by crystallization for 10 days, (a) 0.5 g L-1; (b) 2.0 g L-1. Note: c denotes calcite, v denotes vaterite.

lattice energy gained for the formation of the thermodynamically favored modification. A summary of the morphologies and modifications of the CaCO3 samples obtained at different experimental conditions is presented in Table 1. From Table 1, it is obvious that CaCO3 crystals obtained at various experimental conditions, including reaction temperature, variable reagent concentration, and reaction media, display several distinct morphologies and phase transition processes, which are also different from that reported in previous literature.23,25 A mineralization reaction carried out in pure aqueous solution in the presence of PEG-b-pGlu can only result in the formation of nearly spherical or irregular particles, when the other experimental conditions were kept unchanged. However, the mineralization reaction that occurred in DMF/tetrahydrofuran (THF) mixed solution can produce the particles that are mostly aggregates of oblong blocks with small cavities in the center of the crystal surfaces. More interestingly, particles with a multibranched hierarchical structure formed in DMF/ 1,4-dioxane mixed solution.23 In addition, altering the concentration of polymer can also mediate the morphology of CaCO3 from irregular bulk aggregates to nearly spherical and ellipsoidal mixed structure, while the concentration of polymer PEG-bpAsp was changed from 0.001 to 0.008 g L-1, and finally increased to 1 g L-1.25b According to these modifications listed in Table 1, it is necessary to further analyze the possible origin mechanism of various phase transition from the viewpoint of kinetics and thermodynamics. The energy of a crystal is comprised of surface energy and bulk lattice energy based on Gibbs theory.30 The Gibbs energy of transformation from vaterite to calcite is about -3 kJ · mol-1 when the solvent is pure water.31 In addition, the calculated lattice energies of the three polymorphs are very close to one another.32 Thus, the surface energies for the three polymorphs should play a dominant role in the total energy of a crystal, which is directly associated with the difference in solvent properties.18 Aragonite becomes a dominant phase at 4 ( 2 °C when the concentration of calcium ion is 20 mM. Then, the crystallization rate of smaller building blocks obviously slows down and keeps for longer period at a metastable state at a lower reaction temperature. These could result from the gain in the weight of surface energy in the total energy under thermodynamic control.



Figure 9. (a) FE-SEM images of a full overview of the complex prickly superstructures; (b) a magnified SEM image taken on the ends of a complex tubular structure. The sample was prepared in the presence of 1 g L-1 PEG-b-pGlu and [Ca2+] ) 20 mM at 4 ( 2 °C for 7 days.

Figure 10. TEM images and HRTEM images of CaCO3 tubes obtained in the presence of PEG-b-pGlu (1.0 g L-1), [Ca2+] ) 20 mM, at 4 ( 2 °C 7 days. (a) TEM image of the microtube; (b) a HRTEM image taken from the selected area marked in (a); (c) the inset in panel (b) is the SAED pattern; (d) a HRTEM image taken from the selected area marked in (a); (e) TEM image showing aragonite rodlike branches on the wall of the microtube; (f) a lattice resolved HRTEM image taken from the selected area marked in (e); (g) a HRTEM image taken from the tips of nanorods.

In addition, the presence of DMF solvent benefits the formation of vaterite phase, which is similar to that in the ethanol/water system.18 This phenomenon could be ascribed to the following reasons:31 (i) the Gibbs free energy of phase transition between the metastable phase and the stable phase is low; (ii) the vaterite product is usually composed of nanocrystals and thus has a high surface area and thus it is possible to dissolve and recrystallize to form a more stable phase; and (iii) the

kinetics in DMF mixture solution are too complex to be controlled precisely, and thus the product is always a mixture of two phases. Previous biomineralization reactions were usually carried out at ambient temperature higher than 20 °C;7 however, CaCO3 crystals with various morphologies and polymorphs can be produced when the reaction temperature is lowered. When the reaction temperature increases, the rate of kinetic nucleation


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Table 1. The summary of Morphologies and Modifications of the CaCO3 Crystals Obtained under Different Experimental Conditionsa sample number

reaction temperature (°C)

[polymer] [g L-1]

[CaCl2] [mM]

morphology

modifications

1 2 3 4 5 6 7

4(2 4(2 4(2 4(2 4(2 14 ( 2 24 ( 2

0.5 1.0 2.0 1.0 1.0 1.0 1.0

10 10 10 20 40 10 10

irregular particles columnar structure with smooth surface similar spherical complex structure hierarchical tubular structure spheres and ellipsoidals ellipsoidal and irregular aggregate shuttle-like complex structure

c A* + c v+c A+c v+c c+v v+c

a

Note: * shows sample 2 contains a little amount of A. v denotes vaterite, c denotes calcite, A denotes aragonite.

Figure 11. TEM images and electron diffraction patterns (ED) of the particles formed in DMF during the early reaction stage at 4 ( 2 °C. (a-c) 1 h; (d-f) 24 h. [Ca2+] ) 20 mM, [polymer] ) 1.0 g L-1.

and crystallization process can be accelerated, and it contributes to the dissolution of the CaCO3 crystals formed at an early reaction stage,18 which is helpful for the formation of CaCO3 crystals with a different surface texture. On the other hand, the initial concentration of polymer or calcium ions can also effectively inhibit the nucleation fashion and overgrowth along some certain crystal faces.8 As the polymer at higher concentrations is a strong nucleation inhibitor, it slows down the transformation rate from vaterite to calcite and stabilizes the formed vaterite phase; thus, higher polymer concentration is favorable for the formation of vaterite. The increase in reagent concentration can lead to a higher heterogeneity of the crystals until the specific crystal shape disappears.29b In addition, the influence of reagent concentration on the morphologies of CaCO3 could be interpreted in mediating the specific interaction of calcium ions with the carboxylic groups of the PEG-b-pGlu and affecting the aggregating conformation or secondary structures of the polymer formed

by self-assembly occurring in DMF incompletely aqueous solution, and finally resulting in different morphogenesis of CaCO3 crystals. The early stage of the formation of the superstructures has been examined. The time-dependent experiments showed that amorphous aggregates with a size of 100–150 nm formed at mineralization for 1 h (Figure 11a-c). After mineralization for 24 h, the crystalline ellipsoidal particles formed (Figure 11d-f). The property of the organic solvent (DMF) may also play a dominant pole in mediating secondary structures of peptide chains of PEG-b-pGlu, resulting in the formation of CaCO3 particles with fine and complex morphologies. Because of the polarity of DMF is higher than that of DIW, it is possible that the strength of hydrogen bonding formed between DMF and PEG-b-pGlu is greater than that formed between DIW and glutamaic acid; the intensity difference of hydrogen bonding formed between solvents (DMF, DIW) and PEG-b-pGlu could be favored to yield proper and desired secondary structures of



Figure 12. Scheme of the formation process of hierarchical rod-shaped CaCO3 superstructures obtained in the presence of PEG-b-pGlu (1.0 g L-1) and DMF. [Ca2+] ) 10 mM, and the reaction temperature is 4 ( 2 °C.

PEG-b-pGlu,23,33 which may contribute to the formation of CaCO3 with a complex and hierarchical tubular superstructure. According to the above analysis and discussion, the formation process of CaCO3 crystals with a complex hierarchical superstructure shown in Figure 5c,d was proposed as illustrated in Figure 12. First, amorphous calcium carbonate (ACC) nanoparticles formed at an early reaction stage (Figure 12a), and then aggregated primary building blocks formed (Figure 12b,c). When the reaction time is prolonged, nanostructured building blocks do not change in aggregated state and crystallization. Consequently, the ordered alignment and packing of building blocks occur via electrostatic interaction and van der Waals forces, and then form mesoscale prismatic like calcite nanoparticle aggregates stabilized by polymer molecules (Figure 12d,e). When the reaction process continues, second nucleation on the ellipsoidal surface occurs and then overgrows as a result of two crystal lattice geometric matching and soft-epitaxy effects;28 these so-called intermediate mesocrystals begin to selforganize and align in a certain fashion through electrostatic and dipole interactions and then fuse to one another so as to meet the minimum total surface energy, and thus form ellipsoidal aggregates with many corners and edges (Figure 12f,g). Afterward, some nanoscaled building blocks can selectively attach and bind to the surface of ellipsoidal aggregates through polymer absorption and in an oriented attachment fashion (Figure 12h);8,27 they further grow and aggregate by an Ostwald ripening process and finally produce a kind of rod-shaped superstructure having many tiny fiber-like irregular stuff on the large microtubes. The formation of the tubular structure could be related to the re-dissolution in the inner part of rodlike aggregates that result from the difference of polymer concentration distribution within the rodlike aggregates based on a previously proposed mechanism (Figure 12i,j).8d In addition, the weight amount of PEG-b-pGlu occluded in the final sample can be determined by thermal gravimetric analysis (TGA). The results indicated that the weight percent of PEG-b-pGlu is about 9.1 wt % remaining in the CaCO3 sample formed in DMF solution ([PEG-b-pGlu] ) 1.0 g L-1) (Supporting Information, Figure S3), revealing that the polymer can play a gluing role in the formation of the CaCO3 complex superstructures. Similar oriented attachment and overgrowth events continue to occur

on such complex superstructures, resulting in the formation of hierarchical complex superstructures with a size of more than 10 µm (Figures 9a and 11k). In principle, inorganic nuclei can be formed on organic surfaces by lowering the activation energy of nucleation (∆G) through interfacial recognition.28 Different interactions between organic surfaces and inorganic ions may then create an ensemble of nucleation profiles of ∆G such as to make kinetic control of crystal polymorphs possible.34 In light of this, gradually increasing interactions between the carboxylic groups of the polymer and a carbonate would presumably induce the formation of a less stable polymorph, here, aragonite. It can be concluded that ACC first formed and then crystallized into calcite nanoparticles; these particles assembled into ellipsoidal spheres, during which polymer concentration and calcium ions were decreased in the reaction system. Polymer molecules adsorbed on the surfaces of preformed calcite particles can provide conditions for heterogeneous nucleation of aragonite crystals, which is energetically more favorable than homogeneous nucleation of aragonite crystals in solution, and finally complex aragonite rodlike superstructure formed;35 this is similar to the microgel templates for aragonite reported earlier.36

4. Conclusion In summary, mineralization of CaCO3 mineral in DMF solution at different temperatures using PEG-b-pGlu as a crystal growth modifier results in the formation of the CaCO3 crystals with multiple thorn ellipsoidal-like shapes and more complex ellipsoidal superstructures. The influences of temperature, concentration of calcium ions, and polymer concentration on the mediation of phase transformation and morphology of CaCO3 crystals have been systematically investigated. Spindlelike CaCO3 crystals with a rather smooth surface and a size of 10 µm in length and a maximum diameter of 6 µm were observed at 24 ( 2 °C. Unique and well-defined ellipsoid-like CaCO3 particles with many similar thorn structures distributed on the particle surface formed when the mineralization temperature was kept at 14 ( 2 °C. Furthermore, complex columnlike CaCO3 superstructures comprised of many tiny rods and irregular particles were mineralized at 4 ( 2 °C. In addition,


when the reagent concentration was altered, the morphologies of CaCO3 changed from a mixture of sphere and ellipsoidal structures to a complex ellipsoidal superstructure on which was attached many hierarchical tubular structures. Correspondingly, phase transition occurred from a mixture of aragonite and calcite to pure calcite, and then to a mixture of vaterite and calcite with increased mineralization temperature. Changing the concentration of the polymer resulted in phase transition from calcite to a mixture of aragonite and calcite, and then to a mixture of vaterite and calcite. The results imply that the specific biomimetic mineralization strategy in a nonaqueous solution can provide a useful pathway to produce inorganic or inorganic/ organic hybrid materials with exquisite morphology and specific textures. Supporting Information Available: SEM images, XRD patterns, and TG data. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgment. S.H.Y. acknowledges the special funding support from the National Natural Science Foundation of China (NSFC, Nos. 50732006, 20325104, 20621061, 20671085), the 973 project (2005CB623601), the Centurial Program of the Chinese Academy of Sciences, Anhui Development Fund for Talent Personnel and Anhui Education Committee (2006Z027, ZD2007004-1), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, the Specialized Research Fund for the Doctoral Program (SRFDP) of Higher Education State Education Ministry, and the Partner-Group of the Chinese Academy of Sciences-the Max Planck Society.

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CG7008368

Crystallization of Calcium Carbonate Mineral with Hierarchical

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